diff --git a/.github/ISSUE_TEMPLATE/00-bug_report.yml b/.github/ISSUE_TEMPLATE/00-bug_report.yml
new file mode 100644
index 00000000000..32cebe15a8c
--- /dev/null
+++ b/.github/ISSUE_TEMPLATE/00-bug_report.yml
@@ -0,0 +1,53 @@
+name: Bug Report
+description: Let us know that something does not work as expected.
+title: "[Bug]: Please title this bug report"
+body:
+  - type: textarea
+    id: what-happened
+    attributes:
+      label: Describe the issue
+      description: What happened, and what did you expect to happen?
+    validations:
+      required: true
+  - type: textarea
+    id: steps
+    attributes:
+      label: Steps to reproduce the problem
+      description: It is important that we are able to reproduce the problem that you are experiencing. Please provide all code and relevant steps to reproduce the problem, including your `BUILD`/`CMakeLists.txt` file and build commands. Links to a GitHub branch or [godbolt.org](https://godbolt.org/) that demonstrate the problem are also helpful.
+    validations:
+      required: true
+  - type: textarea
+    id: version
+    attributes:
+      label: What version of Abseil are you using?
+      description: Please include the output of `git rev-parse HEAD` or the name of the LTS release that you are using.
+    validations:
+      required: true
+  - type: textarea
+    id: os
+    attributes:
+      label: What operating system and version are you using?
+      description: If you are using a Linux distribution please include the name and version of the distribution as well.
+    validations:
+      required: true
+  - type: textarea
+    id: compiler
+    attributes:
+      label: What compiler and version are you using?
+      description: Please include the output of `gcc -v` or `clang -v`, or the equivalent for your compiler.
+    validations:
+      required: true
+  - type: textarea
+    id: buildsystem
+    attributes:
+      label: What build system are you using?
+      description: Please include the output of `bazel --version` or `cmake --version`, or the equivalent for your build system.
+    validations:
+      required: true
+  - type: textarea
+    id: additional
+    attributes:
+      label: Additional context
+      description: Add any other context about the problem here.
+    validations:
+      required: false
diff --git a/.github/ISSUE_TEMPLATE/config.yml b/.github/ISSUE_TEMPLATE/config.yml
new file mode 100644
index 00000000000..c690fd9a51b
--- /dev/null
+++ b/.github/ISSUE_TEMPLATE/config.yml
@@ -0,0 +1,5 @@
+blank_issues_enabled: false
+contact_links:
+    - name: Question
+      url: https://github.com/abseil/abseil-cpp/discussions
+      about: Have a question? Ask us anything! :-)
diff --git a/.github/PULL_REQUEST_TEMPLATE.md b/.github/PULL_REQUEST_TEMPLATE.md
new file mode 100644
index 00000000000..ff55e352945
--- /dev/null
+++ b/.github/PULL_REQUEST_TEMPLATE.md
@@ -0,0 +1,8 @@
+Thank you for your contribution to Abseil!
+
+Before submitting this PR, please be sure to read our [contributing
+guidelines](https://github.com/abseil/abseil-cpp/blob/master/CONTRIBUTING.md).
+
+If you are a Googler, please also note that it is required that you send us a
+Piper CL instead of using the GitHub pull-request process. The code propagation
+process will deliver the change to GitHub.
diff --git a/.gitignore b/.gitignore
index d54fa5a91fe..3a729f416b6 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,3 +1,5 @@
+# Bzlmod lockfile
+MODULE.bazel.lock
 # Ignore all bazel-* symlinks.
 /bazel-*
 # Ignore Bazel verbose explanations
diff --git a/BUILD.bazel b/BUILD.bazel
new file mode 100644
index 00000000000..03122a958e5
--- /dev/null
+++ b/BUILD.bazel
@@ -0,0 +1,35 @@
+# Copyright 2020 The Abseil Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+package(default_visibility = ["//visibility:public"])
+
+licenses(["notice"])  # Apache 2.0
+
+# Expose license for external usage through bazel.
+exports_files([
+    "AUTHORS",
+    "LICENSE",
+])
+
+# For building with clang-cl.
+# https://bazel.build/configure/windows#clang
+platform(
+    name = "x64_windows-clang-cl",
+    constraint_values = [
+        "@platforms//cpu:x86_64",
+        "@platforms//os:windows",
+        "@bazel_tools//tools/cpp:clang-cl",
+    ],
+)
diff --git a/CMake/AbseilDll.cmake b/CMake/AbseilDll.cmake
new file mode 100644
index 00000000000..2e3a2f22830
--- /dev/null
+++ b/CMake/AbseilDll.cmake
@@ -0,0 +1,949 @@
+include(CMakeParseArguments)
+include(GNUInstallDirs)
+
+set(ABSL_INTERNAL_DLL_FILES
+  "algorithm/algorithm.h"
+  "algorithm/container.h"
+  "base/attributes.h"
+  "base/call_once.h"
+  "base/casts.cc"
+  "base/casts.h"
+  "base/config.h"
+  "base/const_init.h"
+  "base/dynamic_annotations.h"
+  "base/fast_type_id.h"
+  "base/internal/atomic_hook.h"
+  "base/internal/cpu_detect.cc"
+  "base/internal/cpu_detect.h"
+  "base/internal/cycleclock.cc"
+  "base/internal/cycleclock.h"
+  "base/internal/cycleclock_config.h"
+  "base/internal/direct_mmap.h"
+  "base/internal/dynamic_annotations.h"
+  "base/internal/endian.h"
+  "base/internal/errno_saver.h"
+  "base/internal/hardening.h"
+  "base/internal/hide_ptr.h"
+  "base/internal/iterator_traits.h"
+  "base/internal/low_level_alloc.cc"
+  "base/internal/low_level_alloc.h"
+  "base/internal/low_level_scheduling.h"
+  "base/internal/nullability_traits.h"
+  "base/internal/per_thread_tls.h"
+  "base/internal/poison.cc"
+  "base/internal/poison.h"
+  "base/internal/pretty_function.h"
+  "base/internal/raw_logging.cc"
+  "base/internal/raw_logging.h"
+  "base/internal/scheduling_mode.h"
+  "base/internal/scoped_set_env.cc"
+  "base/internal/scoped_set_env.h"
+  "base/internal/spinlock.cc"
+  "base/internal/spinlock.h"
+  "base/internal/spinlock_wait.cc"
+  "base/internal/spinlock_wait.h"
+  "base/internal/strerror.cc"
+  "base/internal/strerror.h"
+  "base/internal/sysinfo.cc"
+  "base/internal/sysinfo.h"
+  "base/internal/thread_identity.cc"
+  "base/internal/thread_identity.h"
+  "base/internal/tracing.cc"
+  "base/internal/tracing.h"
+  "base/internal/tsan_mutex_interface.h"
+  "base/internal/unaligned_access.h"
+  "base/internal/unscaledcycleclock.cc"
+  "base/internal/unscaledcycleclock.h"
+  "base/internal/unscaledcycleclock_config.h"
+  "base/log_severity.cc"
+  "base/log_severity.h"
+  "base/macros.h"
+  "base/no_destructor.h"
+  "base/nullability.h"
+  "base/optimization.h"
+  "base/options.h"
+  "base/policy_checks.h"
+  "base/port.h"
+  "base/prefetch.h"
+  "base/thread_annotations.h"
+  "base/throw_delegate.cc"
+  "base/throw_delegate.h"
+  "cleanup/cleanup.h"
+  "cleanup/internal/cleanup.h"
+  "container/btree_map.h"
+  "container/btree_set.h"
+  "container/chunked_queue.h"
+  "container/fixed_array.h"
+  "container/flat_hash_map.h"
+  "container/flat_hash_set.h"
+  "container/hash_container_defaults.h"
+  "container/inlined_vector.h"
+  "container/internal/btree.h"
+  "container/internal/btree_container.h"
+  "container/internal/chunked_queue.h"
+  "container/internal/common.h"
+  "container/internal/common_policy_traits.h"
+  "container/internal/compressed_tuple.h"
+  "container/internal/container_memory.h"
+  "container/internal/hash_function_defaults.h"
+  "container/internal/hash_policy_traits.h"
+  "container/internal/hashtable_control_bytes.h"
+  "container/internal/hashtable_debug.h"
+  "container/internal/hashtable_debug_hooks.h"
+  "container/internal/hashtablez_sampler.cc"
+  "container/internal/hashtablez_sampler.h"
+  "container/internal/hashtablez_sampler_force_weak_definition.cc"
+  "container/internal/inlined_vector.h"
+  "container/internal/layout.h"
+  "container/internal/node_slot_policy.h"
+  "container/internal/raw_hash_map.h"
+  "container/internal/raw_hash_set.cc"
+  "container/internal/raw_hash_set.h"
+  "container/internal/raw_hash_set_resize_impl.h"
+  "container/internal/tracked.h"
+  "container/linked_hash_map.h"
+  "container/linked_hash_set.h"
+  "container/node_hash_map.h"
+  "container/node_hash_set.h"
+  "crc/crc32c.cc"
+  "crc/crc32c.h"
+  "crc/internal/crc.cc"
+  "crc/internal/crc.h"
+  "crc/internal/crc32_x86_arm_combined_simd.h"
+  "crc/internal/crc32c.h"
+  "crc/internal/crc32c_inline.h"
+  "crc/internal/crc_cord_state.cc"
+  "crc/internal/crc_cord_state.h"
+  "crc/internal/crc_internal.h"
+  "crc/internal/crc_memcpy.h"
+  "crc/internal/crc_memcpy_fallback.cc"
+  "crc/internal/crc_memcpy_x86_arm_combined.cc"
+  "crc/internal/crc_non_temporal_memcpy.cc"
+  "crc/internal/crc_x86_arm_combined.cc"
+  "crc/internal/non_temporal_arm_intrinsics.h"
+  "crc/internal/non_temporal_memcpy.h"
+  "debugging/failure_signal_handler.cc"
+  "debugging/failure_signal_handler.h"
+  "debugging/internal/address_is_readable.cc"
+  "debugging/internal/address_is_readable.h"
+  "debugging/internal/addresses.h"
+  "debugging/internal/bounded_utf8_length_sequence.h"
+  "debugging/internal/decode_rust_punycode.cc"
+  "debugging/internal/decode_rust_punycode.h"
+  "debugging/internal/demangle.cc"
+  "debugging/internal/demangle.h"
+  "debugging/internal/demangle_rust.cc"
+  "debugging/internal/demangle_rust.h"
+  "debugging/internal/elf_mem_image.cc"
+  "debugging/internal/elf_mem_image.h"
+  "debugging/internal/examine_stack.cc"
+  "debugging/internal/examine_stack.h"
+  "debugging/internal/stack_consumption.cc"
+  "debugging/internal/stack_consumption.h"
+  "debugging/internal/stacktrace_config.h"
+  "debugging/internal/symbolize.h"
+  "debugging/internal/utf8_for_code_point.cc"
+  "debugging/internal/utf8_for_code_point.h"
+  "debugging/internal/vdso_support.cc"
+  "debugging/internal/vdso_support.h"
+  "debugging/leak_check.h"
+  "debugging/stacktrace.cc"
+  "debugging/stacktrace.h"
+  "debugging/symbolize.cc"
+  "debugging/symbolize.h"
+  "functional/any_invocable.h"
+  "functional/bind_back.h"
+  "functional/bind_front.h"
+  "functional/function_ref.h"
+  "functional/internal/any_invocable.h"
+  "functional/internal/back_binder.h"
+  "functional/internal/front_binder.h"
+  "functional/internal/function_ref.h"
+  "functional/overload.h"
+  "hash/hash.h"
+  "hash/internal/city.cc"
+  "hash/internal/city.h"
+  "hash/internal/hash.cc"
+  "hash/internal/hash.h"
+  "hash/internal/spy_hash_state.h"
+  "hash/internal/weakly_mixed_integer.h"
+  "log/absl_check.h"
+  "log/absl_log.h"
+  "log/absl_vlog_is_on.h"
+  "log/check.h"
+  "log/die_if_null.cc"
+  "log/die_if_null.h"
+  "log/globals.cc"
+  "log/globals.h"
+  "log/initialize.cc"
+  "log/initialize.h"
+  "log/internal/append_truncated.h"
+  "log/internal/check_impl.h"
+  "log/internal/check_op.cc"
+  "log/internal/check_op.h"
+  "log/internal/conditions.cc"
+  "log/internal/conditions.h"
+  "log/internal/config.h"
+  "log/internal/container.h"
+  "log/internal/fnmatch.cc"
+  "log/internal/fnmatch.h"
+  "log/internal/globals.cc"
+  "log/internal/globals.h"
+  "log/internal/log_format.cc"
+  "log/internal/log_format.h"
+  "log/internal/log_impl.h"
+  "log/internal/log_message.cc"
+  "log/internal/log_message.h"
+  "log/internal/log_sink_set.cc"
+  "log/internal/log_sink_set.h"
+  "log/internal/nullguard.cc"
+  "log/internal/nullguard.h"
+  "log/internal/nullstream.h"
+  "log/internal/proto.cc"
+  "log/internal/proto.h"
+  "log/internal/strip.h"
+  "log/internal/structured.h"
+  "log/internal/structured_proto.cc"
+  "log/internal/structured_proto.h"
+  "log/internal/vlog_config.cc"
+  "log/internal/vlog_config.h"
+  "log/internal/voidify.h"
+  "log/log.h"
+  "log/log_entry.cc"
+  "log/log_entry.h"
+  "log/log_sink.cc"
+  "log/log_sink.h"
+  "log/log_sink_registry.h"
+  "log/log_streamer.h"
+  "log/structured.h"
+  "log/vlog_is_on.h"
+  "memory/memory.h"
+  "meta/internal/requires.h"
+  "meta/type_traits.h"
+  "numeric/bits.h"
+  "numeric/int128.cc"
+  "numeric/int128.h"
+  "numeric/internal/bits.h"
+  "numeric/internal/representation.h"
+  "profiling/hashtable.cc"
+  "profiling/hashtable.h"
+  "profiling/internal/exponential_biased.cc"
+  "profiling/internal/exponential_biased.h"
+  "profiling/internal/periodic_sampler.cc"
+  "profiling/internal/periodic_sampler.h"
+  "profiling/internal/profile_builder.cc"
+  "profiling/internal/profile_builder.h"
+  "profiling/internal/sample_recorder.h"
+  "random/bernoulli_distribution.h"
+  "random/beta_distribution.h"
+  "random/bit_gen_ref.h"
+  "random/discrete_distribution.cc"
+  "random/discrete_distribution.h"
+  "random/distributions.h"
+  "random/exponential_distribution.h"
+  "random/gaussian_distribution.cc"
+  "random/gaussian_distribution.h"
+  "random/internal/distribution_caller.h"
+  "random/internal/entropy_pool.cc"
+  "random/internal/entropy_pool.h"
+  "random/internal/fast_uniform_bits.h"
+  "random/internal/fastmath.h"
+  "random/internal/generate_real.h"
+  "random/internal/iostream_state_saver.h"
+  "random/internal/nonsecure_base.h"
+  "random/internal/pcg_engine.h"
+  "random/internal/platform.h"
+  "random/internal/randen.cc"
+  "random/internal/randen.h"
+  "random/internal/randen_detect.cc"
+  "random/internal/randen_detect.h"
+  "random/internal/randen_engine.h"
+  "random/internal/randen_hwaes.cc"
+  "random/internal/randen_hwaes.h"
+  "random/internal/randen_round_keys.cc"
+  "random/internal/randen_slow.cc"
+  "random/internal/randen_slow.h"
+  "random/internal/randen_traits.h"
+  "random/internal/salted_seed_seq.h"
+  "random/internal/seed_material.cc"
+  "random/internal/seed_material.h"
+  "random/internal/sequence_urbg.h"
+  "random/internal/traits.h"
+  "random/internal/uniform_helper.h"
+  "random/internal/wide_multiply.h"
+  "random/log_uniform_int_distribution.h"
+  "random/poisson_distribution.h"
+  "random/random.h"
+  "random/seed_gen_exception.cc"
+  "random/seed_gen_exception.h"
+  "random/seed_sequences.cc"
+  "random/seed_sequences.h"
+  "random/uniform_int_distribution.h"
+  "random/uniform_real_distribution.h"
+  "random/zipf_distribution.h"
+  "status/internal/status_internal.cc"
+  "status/internal/status_internal.h"
+  "status/internal/statusor_internal.h"
+  "status/status.cc"
+  "status/status.h"
+  "status/status_builder.cc"
+  "status/status_builder.h"
+  "status/status_macros.h"
+  "status/status_payload_printer.cc"
+  "status/status_payload_printer.h"
+  "status/statusor.cc"
+  "status/statusor.h"
+  "strings/ascii.cc"
+  "strings/ascii.h"
+  "strings/charconv.cc"
+  "strings/charconv.h"
+  "strings/charset.h"
+  "strings/cord.cc"
+  "strings/cord.h"
+  "strings/cord_analysis.cc"
+  "strings/cord_analysis.h"
+  "strings/cord_buffer.h"
+  "strings/escaping.cc"
+  "strings/escaping.h"
+  "strings/has_absl_stringify.h"
+  "strings/has_ostream_operator.h"
+  "strings/internal/append_and_overwrite.h"
+  "strings/internal/charconv_bigint.cc"
+  "strings/internal/charconv_bigint.h"
+  "strings/internal/charconv_parse.cc"
+  "strings/internal/charconv_parse.h"
+  "strings/internal/cord_data_edge.h"
+  "strings/internal/cord_internal.cc"
+  "strings/internal/cord_internal.h"
+  "strings/internal/cord_rep_btree.cc"
+  "strings/internal/cord_rep_btree.h"
+  "strings/internal/cord_rep_btree_navigator.cc"
+  "strings/internal/cord_rep_btree_navigator.h"
+  "strings/internal/cord_rep_btree_reader.cc"
+  "strings/internal/cord_rep_btree_reader.h"
+  "strings/internal/cord_rep_consume.cc"
+  "strings/internal/cord_rep_consume.h"
+  "strings/internal/cord_rep_crc.cc"
+  "strings/internal/cord_rep_crc.h"
+  "strings/internal/cord_rep_flat.h"
+  "strings/internal/cordz_functions.cc"
+  "strings/internal/cordz_functions.h"
+  "strings/internal/cordz_handle.cc"
+  "strings/internal/cordz_handle.h"
+  "strings/internal/cordz_info.cc"
+  "strings/internal/cordz_info.h"
+  "strings/internal/cordz_sample_token.cc"
+  "strings/internal/cordz_sample_token.h"
+  "strings/internal/cordz_statistics.h"
+  "strings/internal/cordz_update_scope.h"
+  "strings/internal/cordz_update_tracker.h"
+  "strings/internal/damerau_levenshtein_distance.cc"
+  "strings/internal/damerau_levenshtein_distance.h"
+  "strings/internal/escaping.cc"
+  "strings/internal/escaping.h"
+  "strings/internal/generic_printer.cc"
+  "strings/internal/generic_printer.h"
+  "strings/internal/generic_printer_internal.h"
+  "strings/internal/memutil.cc"
+  "strings/internal/memutil.h"
+  "strings/internal/ostringstream.cc"
+  "strings/internal/ostringstream.h"
+  "strings/internal/pow10_helper.cc"
+  "strings/internal/pow10_helper.h"
+  "strings/internal/resize_uninitialized.h"
+  "strings/internal/str_format/arg.cc"
+  "strings/internal/str_format/arg.h"
+  "strings/internal/str_format/bind.cc"
+  "strings/internal/str_format/bind.h"
+  "strings/internal/str_format/checker.h"
+  "strings/internal/str_format/constexpr_parser.h"
+  "strings/internal/str_format/extension.cc"
+  "strings/internal/str_format/extension.h"
+  "strings/internal/str_format/float_conversion.cc"
+  "strings/internal/str_format/float_conversion.h"
+  "strings/internal/str_format/output.cc"
+  "strings/internal/str_format/output.h"
+  "strings/internal/str_format/parser.cc"
+  "strings/internal/str_format/parser.h"
+  "strings/internal/str_join_internal.h"
+  "strings/internal/str_split_internal.h"
+  "strings/internal/string_constant.h"
+  "strings/internal/stringify_sink.cc"
+  "strings/internal/stringify_sink.h"
+  "strings/internal/stringify_stream.h"
+  "strings/internal/utf8.cc"
+  "strings/internal/utf8.h"
+  "strings/match.cc"
+  "strings/match.h"
+  "strings/numbers.cc"
+  "strings/numbers.h"
+  "strings/resize_and_overwrite.h"
+  "strings/str_cat.cc"
+  "strings/str_cat.h"
+  "strings/str_format.h"
+  "strings/str_join.h"
+  "strings/str_replace.cc"
+  "strings/str_replace.h"
+  "strings/str_split.cc"
+  "strings/str_split.h"
+  "strings/strip.h"
+  "strings/substitute.cc"
+  "strings/substitute.h"
+  "synchronization/barrier.cc"
+  "synchronization/barrier.h"
+  "synchronization/blocking_counter.cc"
+  "synchronization/blocking_counter.h"
+  "synchronization/internal/create_thread_identity.cc"
+  "synchronization/internal/create_thread_identity.h"
+  "synchronization/internal/futex.h"
+  "synchronization/internal/futex_waiter.cc"
+  "synchronization/internal/futex_waiter.h"
+  "synchronization/internal/graphcycles.cc"
+  "synchronization/internal/graphcycles.h"
+  "synchronization/internal/kernel_timeout.cc"
+  "synchronization/internal/kernel_timeout.h"
+  "synchronization/internal/per_thread_sem.cc"
+  "synchronization/internal/per_thread_sem.h"
+  "synchronization/internal/pthread_waiter.cc"
+  "synchronization/internal/pthread_waiter.h"
+  "synchronization/internal/sem_waiter.cc"
+  "synchronization/internal/sem_waiter.h"
+  "synchronization/internal/stdcpp_waiter.cc"
+  "synchronization/internal/stdcpp_waiter.h"
+  "synchronization/internal/thread_pool.h"
+  "synchronization/internal/waiter.h"
+  "synchronization/internal/waiter_base.cc"
+  "synchronization/internal/waiter_base.h"
+  "synchronization/internal/win32_waiter.cc"
+  "synchronization/internal/win32_waiter.h"
+  "synchronization/mutex.cc"
+  "synchronization/mutex.h"
+  "synchronization/notification.cc"
+  "synchronization/notification.h"
+  "time/civil_time.cc"
+  "time/civil_time.h"
+  "time/clock.cc"
+  "time/clock.h"
+  "time/clock_interface.cc"
+  "time/clock_interface.h"
+  "time/duration.cc"
+  "time/format.cc"
+  "time/internal/cctz/include/cctz/civil_time.h"
+  "time/internal/cctz/include/cctz/civil_time_detail.h"
+  "time/internal/cctz/include/cctz/time_zone.h"
+  "time/internal/cctz/include/cctz/zone_info_source.h"
+  "time/internal/cctz/src/civil_time_detail.cc"
+  "time/internal/cctz/src/time_zone_fixed.cc"
+  "time/internal/cctz/src/time_zone_fixed.h"
+  "time/internal/cctz/src/time_zone_format.cc"
+  "time/internal/cctz/src/time_zone_if.cc"
+  "time/internal/cctz/src/time_zone_if.h"
+  "time/internal/cctz/src/time_zone_impl.cc"
+  "time/internal/cctz/src/time_zone_impl.h"
+  "time/internal/cctz/src/time_zone_info.cc"
+  "time/internal/cctz/src/time_zone_info.h"
+  "time/internal/cctz/src/time_zone_libc.cc"
+  "time/internal/cctz/src/time_zone_libc.h"
+  "time/internal/cctz/src/time_zone_lookup.cc"
+  "time/internal/cctz/src/time_zone_posix.cc"
+  "time/internal/cctz/src/time_zone_posix.h"
+  "time/internal/cctz/src/tzfile.h"
+  "time/internal/cctz/src/zone_info_source.cc"
+  "time/simulated_clock.cc"
+  "time/simulated_clock.h"
+  "time/time.cc"
+  "time/time.h"
+  "types/any.h"
+  "types/any_span.h"
+  "types/compare.h"
+  "types/internal/any_span.h"
+  "types/internal/span.h"
+  "types/optional.h"
+  "types/optional_ref.h"
+  "types/source_location.cc"
+  "types/source_location.h"
+  "types/span.h"
+  "types/variant.h"
+  "utility/utility.h"
+  "debugging/leak_check.cc"
+  "strings/string_view.h"
+)
+
+if(WIN32)
+  list(APPEND ABSL_INTERNAL_DLL_FILES
+    "time/internal/cctz/src/time_zone_name_win.cc"
+    "time/internal/cctz/src/time_zone_name_win.h"
+  )
+endif()
+
+if(NOT MSVC)
+  list(APPEND ABSL_INTERNAL_DLL_FILES
+    "flags/commandlineflag.cc"
+    "flags/commandlineflag.h"
+    "flags/config.h"
+    "flags/declare.h"
+    "flags/flag.h"
+    "flags/internal/commandlineflag.cc"
+    "flags/internal/commandlineflag.h"
+    "flags/internal/flag.cc"
+    "flags/internal/flag.h"
+    "flags/internal/parse.h"
+    "flags/internal/path_util.h"
+    "flags/internal/private_handle_accessor.cc"
+    "flags/internal/private_handle_accessor.h"
+    "flags/internal/program_name.cc"
+    "flags/internal/program_name.h"
+    "flags/internal/registry.h"
+    "flags/internal/sequence_lock.h"
+    "flags/internal/usage.cc"
+    "flags/internal/usage.h"
+    "flags/marshalling.cc"
+    "flags/marshalling.h"
+    "flags/parse.cc"
+    "flags/parse.h"
+    "flags/reflection.cc"
+    "flags/reflection.h"
+    "flags/usage.cc"
+    "flags/usage.h"
+    "flags/usage_config.cc"
+    "flags/usage_config.h"
+    "log/flags.cc"
+    "log/flags.h"
+    "log/internal/flags.h"
+  )
+endif()
+
+set(ABSL_INTERNAL_DLL_TARGETS
+  "absl_check"
+  "absl_log"
+  "absl_vlog_is_on"
+  "algorithm"
+  "algorithm_container"
+  "any"
+  "any_invocable"
+  "atomic_hook"
+  "base"
+  "base_internal"
+  "bind_front"
+  "bits"
+  "btree"
+  "check"
+  "city"
+  "civil_time"
+  "compare"
+  "compressed_tuple"
+  "config"
+  "container"
+  "container_common"
+  "container_memory"
+  "cord"
+  "cord_internal"
+  "cordz_functions"
+  "cordz_handle"
+  "cordz_info"
+  "cordz_sample_token"
+  "core_headers"
+  "counting_allocator"
+  "crc_cord_state"
+  "crc_cpu_detect"
+  "crc_internal"
+  "crc32c"
+  "debugging"
+  "debugging_internal"
+  "demangle_internal"
+  "die_if_null"
+  "dynamic_annotations"
+  "endian"
+  "examine_stack"
+  "exponential_biased"
+  "failure_signal_handler"
+  "fixed_array"
+  "flat_hash_map"
+  "flat_hash_set"
+  "function_ref"
+  "graphcycles_internal"
+  "hash"
+  "hash_function_defaults"
+  "hash_policy_traits"
+  "hashtable_debug"
+  "hashtable_debug_hooks"
+  "hashtablez_sampler"
+  "inlined_vector"
+  "inlined_vector_internal"
+  "int128"
+  "kernel_timeout_internal"
+  "layout"
+  "leak_check"
+  "log_internal_check_impl"
+  "log_internal_check_op"
+  "log_internal_conditions"
+  "log_internal_config"
+  "log_internal_fnmatch"
+  "log_internal_format"
+  "log_internal_globals"
+  "log_internal_log_impl"
+  "log_internal_proto"
+  "log_internal_message"
+  "log_internal_log_sink_set"
+  "log_internal_nullguard"
+  "log_internal_nullstream"
+  "log_internal_strip"
+  "log_internal_voidify"
+  "log_internal_append_truncated"
+  "log_globals"
+  "log_initialize"
+  "log"
+  "log_entry"
+  "log_sink"
+  "log_sink_registry"
+  "log_streamer"
+  "log_internal_structured"
+  "log_severity"
+  "log_structured"
+  "low_level_hash"
+  "malloc_internal"
+  "memory"
+  "meta"
+  "node_hash_map"
+  "node_hash_set"
+  "node_slot_policy"
+  "non_temporal_arm_intrinsics"
+  "non_temporal_memcpy"
+  "numeric"
+  "optional"
+  "periodic_sampler"
+  "pow10_helper"
+  "pretty_function"
+  "random_bit_gen_ref"
+  "random_distributions"
+  "random_internal_distribution_caller"
+  "random_internal_distributions"
+  "random_internal_explicit_seed_seq"
+  "random_internal_fastmath"
+  "random_internal_fast_uniform_bits"
+  "random_internal_generate_real"
+  "random_internal_iostream_state_saver"
+  "random_internal_nonsecure_base"
+  "random_internal_pcg_engine"
+  "random_internal_platform"
+  "random_internal_pool_urbg"
+  "random_internal_randen"
+  "random_internal_randen_engine"
+  "random_internal_randen_hwaes"
+  "random_internal_randen_hwaes_impl"
+  "random_internal_randen_slow"
+  "random_internal_salted_seed_seq"
+  "random_internal_seed_material"
+  "random_internal_sequence_urbg"
+  "random_internal_traits"
+  "random_internal_uniform_helper"
+  "random_internal_wide_multiply"
+  "random_random"
+  "random_seed_gen_exception"
+  "random_seed_sequences"
+  "raw_hash_map"
+  "raw_hash_set"
+  "raw_logging_internal"
+  "sample_recorder"
+  "scoped_set_env"
+  "span"
+  "spinlock_wait"
+  "spy_hash_state"
+  "stack_consumption"
+  "stacktrace"
+  "status"
+  "statusor"
+  "str_format"
+  "str_format_internal"
+  "strerror"
+  "strings"
+  "strings_internal"
+  "string_view"
+  "symbolize"
+  "synchronization"
+  "thread_pool"
+  "throw_delegate"
+  "time"
+  "time_zone"
+  "tracked"
+  "type_traits"
+  "utility"
+  "variant"
+  "vlog_config_internal"
+  "vlog_is_on"
+)
+
+if(NOT MSVC)
+  list(APPEND ABSL_INTERNAL_DLL_TARGETS
+    "flags"
+    "flags_commandlineflag"
+    "flags_commandlineflag_internal"
+    "flags_config"
+    "flags_internal"
+    "flags_marshalling"
+    "flags_parse"
+    "flags_path_util"
+    "flags_private_handle_accessor"
+    "flags_program_name"
+    "flags_reflection"
+    "flags_usage"
+    "flags_usage_internal"
+    "log_internal_flags"
+    "log_flags"
+  )
+endif()
+
+set(ABSL_INTERNAL_TEST_DLL_FILES
+  "hash/hash_testing.h"
+  "log/scoped_mock_log.cc"
+  "log/scoped_mock_log.h"
+  "random/internal/chi_square.cc"
+  "random/internal/chi_square.h"
+  "random/internal/distribution_test_util.cc"
+  "random/internal/distribution_test_util.h"
+  "random/internal/mock_helpers.h"
+  "random/internal/mock_overload_set.h"
+  "random/mocking_bit_gen.h"
+  "random/mock_distributions.h"
+  "status/status_matchers.h"
+  "status/internal/status_matchers.cc"
+  "status/internal/status_matchers.h"
+  "strings/cordz_test_helpers.h"
+  "strings/cord_test_helpers.h"
+)
+
+set(ABSL_INTERNAL_TEST_DLL_TARGETS
+  "cord_test_helpers"
+  "cordz_test_helpers"
+  "hash_testing"
+  "random_mocking_bit_gen"
+  "random_internal_distribution_test_util"
+  "random_internal_mock_overload_set"
+  "scoped_mock_log"
+  "status_matchers"
+)
+
+if(DEFINED CMAKE_CXX_STANDARD AND CMAKE_CXX_STANDARD_REQUIRED)
+  if(CMAKE_CXX_STANDARD GREATER_EQUAL 20)
+    set(ABSL_INTERNAL_AT_LEAST_CXX20 ON)
+    set(ABSL_INTERNAL_AT_LEAST_CXX17 ON)
+  elseif(CMAKE_CXX_STANDARD GREATER_EQUAL 17)
+    set(ABSL_INTERNAL_AT_LEAST_CXX17 ON)
+  endif()
+else()
+  include(CheckCXXSourceCompiles)
+
+  check_cxx_source_compiles(
+    [==[
+#ifdef _MSC_VER
+#  if _MSVC_LANG < 201703L
+#    error "The compiler defaults or is configured for C++ < 17"
+#  endif
+#elif __cplusplus < 201703L
+#  error "The compiler defaults or is configured for C++ < 17"
+#endif
+int main() { return 0; }
+]==]
+    ABSL_INTERNAL_AT_LEAST_CXX17)
+
+  check_cxx_source_compiles(
+    [==[
+#ifdef _MSC_VER
+#  if _MSVC_LANG < 202002L
+#    error "The compiler defaults or is configured for C++ < 20"
+#  endif
+#elif __cplusplus < 202002L
+#  error "The compiler defaults or is configured for C++ < 20"
+#endif
+int main() { return 0; }
+]==]
+    ABSL_INTERNAL_AT_LEAST_CXX20)
+endif()
+
+if(ABSL_INTERNAL_AT_LEAST_CXX20)
+  set(ABSL_INTERNAL_CXX_STD_FEATURE cxx_std_20)
+elseif(ABSL_INTERNAL_AT_LEAST_CXX17)
+  set(ABSL_INTERNAL_CXX_STD_FEATURE cxx_std_17)
+else()
+  message(FATAL_ERROR "The compiler defaults to or is configured for C++ < 17. C++ >= 17 is required and Abseil and all libraries that use Abseil must use the same C++ language standard")
+endif()
+
+function(absl_internal_dll_contains)
+  cmake_parse_arguments(ABSL_INTERNAL_DLL
+    ""
+    "OUTPUT;TARGET"
+    ""
+    ${ARGN}
+  )
+
+  STRING(REGEX REPLACE "^absl::" "" _target ${ABSL_INTERNAL_DLL_TARGET})
+
+  if (_target IN_LIST ABSL_INTERNAL_DLL_TARGETS)
+    set(${ABSL_INTERNAL_DLL_OUTPUT} 1 PARENT_SCOPE)
+  else()
+    set(${ABSL_INTERNAL_DLL_OUTPUT} 0 PARENT_SCOPE)
+  endif()
+endfunction()
+
+function(absl_internal_test_dll_contains)
+  cmake_parse_arguments(ABSL_INTERNAL_TEST_DLL
+    ""
+    "OUTPUT;TARGET"
+    ""
+    ${ARGN}
+  )
+
+  STRING(REGEX REPLACE "^absl::" "" _target ${ABSL_INTERNAL_TEST_DLL_TARGET})
+
+  if (_target IN_LIST ABSL_INTERNAL_TEST_DLL_TARGETS)
+    set(${ABSL_INTERNAL_TEST_DLL_OUTPUT} 1 PARENT_SCOPE)
+  else()
+    set(${ABSL_INTERNAL_TEST_DLL_OUTPUT} 0 PARENT_SCOPE)
+  endif()
+endfunction()
+
+function(absl_internal_dll_targets)
+  cmake_parse_arguments(ABSL_INTERNAL_DLL
+  ""
+  "OUTPUT"
+  "DEPS"
+  ${ARGN}
+  )
+
+  set(_deps "")
+  foreach(dep IN LISTS ABSL_INTERNAL_DLL_DEPS)
+    absl_internal_dll_contains(TARGET ${dep} OUTPUT _dll_contains)
+    absl_internal_test_dll_contains(TARGET ${dep} OUTPUT _test_dll_contains)
+    if (_dll_contains)
+      list(APPEND _deps abseil_dll)
+    elseif (_test_dll_contains)
+      list(APPEND _deps abseil_test_dll)
+    else()
+      list(APPEND _deps ${dep})
+    endif()
+  endforeach()
+
+  # Because we may have added the DLL multiple times
+  list(REMOVE_DUPLICATES _deps)
+  set(${ABSL_INTERNAL_DLL_OUTPUT} "${_deps}" PARENT_SCOPE)
+endfunction()
+
+function(absl_make_dll)
+  cmake_parse_arguments(ABSL_INTERNAL_MAKE_DLL
+  ""
+  "TEST"
+  ""
+  ${ARGN}
+  )
+
+  if (ABSL_INTERNAL_MAKE_DLL_TEST)
+    set(_dll "abseil_test_dll")
+    set(_dll_files ${ABSL_INTERNAL_TEST_DLL_FILES})
+    set(_dll_libs "abseil_dll" "GTest::gtest" "GTest::gmock")
+    set(_dll_compile_definitions "GTEST_LINKED_AS_SHARED_LIBRARY=1")
+    set(_dll_includes ${absl_gtest_src_dir}/googletest/include ${absl_gtest_src_dir}/googlemock/include)
+    set(_dll_consume "ABSL_CONSUME_TEST_DLL")
+    set(_dll_build "ABSL_BUILD_TEST_DLL")
+  else()
+    set(_dll "abseil_dll")
+    set(_dll_files ${ABSL_INTERNAL_DLL_FILES})
+    set(_dll_libs
+      Threads::Threads
+      # TODO(#1495): Use $<LINK_LIBRARY:FRAMEWORK,CoreFoundation> once our
+      # minimum CMake version >= 3.24
+      $<$<PLATFORM_ID:Darwin>:-Wl,-framework,CoreFoundation>
+    )
+    set(_dll_compile_definitions "")
+    set(_dll_includes "")
+    set(_dll_consume "ABSL_CONSUME_DLL")
+    set(_dll_build "ABSL_BUILD_DLL")
+  endif()
+
+  add_library(
+    ${_dll}
+    SHARED
+      ${_dll_files}
+  )
+  target_link_libraries(
+    ${_dll}
+    PRIVATE
+      ${_dll_libs}
+      ${ABSL_DEFAULT_LINKOPTS}
+      $<$<BOOL:${ANDROID}>:-llog>
+      $<$<BOOL:${MINGW}>:-ladvapi32>
+      $<$<BOOL:${MINGW}>:-ldbghelp>
+      $<$<BOOL:${MINGW}>:-lbcrypt>
+  )
+  set_target_properties(${_dll} PROPERTIES
+    LINKER_LANGUAGE "CXX"
+    SOVERSION ${ABSL_SOVERSION}
+  )
+  target_include_directories(
+    ${_dll}
+    PUBLIC
+      "$<BUILD_INTERFACE:${ABSL_COMMON_INCLUDE_DIRS}>"
+      $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>
+    PRIVATE
+      ${_dll_includes}
+  )
+
+  target_compile_options(
+    ${_dll}
+    PRIVATE
+      ${ABSL_DEFAULT_COPTS}
+  )
+
+  foreach(cflag ${ABSL_CC_LIB_COPTS})
+    if(${cflag} MATCHES "^(-Wno|/wd)")
+      # These flags are needed to suppress warnings that might fire in our headers.
+      set(PC_CFLAGS "${PC_CFLAGS} ${cflag}")
+    elseif(${cflag} MATCHES "^(-W|/w[1234eo])")
+      # Don't impose our warnings on others.
+    else()
+      set(PC_CFLAGS "${PC_CFLAGS} ${cflag}")
+    endif()
+  endforeach()
+  string(REPLACE ";" " " PC_LINKOPTS "${ABSL_CC_LIB_LINKOPTS}")
+
+  FILE(GENERATE OUTPUT "${CMAKE_BINARY_DIR}/lib/pkgconfig/${_dll}.pc" CONTENT "\
+prefix=${CMAKE_INSTALL_PREFIX}\n\
+exec_prefix=\${prefix}\n\
+libdir=${CMAKE_INSTALL_FULL_LIBDIR}\n\
+includedir=${CMAKE_INSTALL_FULL_INCLUDEDIR}\n\
+\n\
+Name: ${_dll}\n\
+Description: Abseil DLL library\n\
+URL: https://abseil.io/\n\
+Version: ${absl_VERSION}\n\
+Libs: -L\${libdir} $<$<NOT:$<BOOL:${ABSL_CC_LIB_IS_INTERFACE}>>:-l${_dll}> ${PC_LINKOPTS}\n\
+Cflags: -I\${includedir}${PC_CFLAGS}\n")
+  INSTALL(FILES "${CMAKE_BINARY_DIR}/lib/pkgconfig/${_dll}.pc"
+    DESTINATION "${CMAKE_INSTALL_LIBDIR}/pkgconfig")
+
+  target_compile_definitions(
+    ${_dll}
+    PUBLIC
+      ${_dll_compile_definitions}
+    PRIVATE
+      ${_dll_build}
+      NOMINMAX
+    INTERFACE
+      ${ABSL_CC_LIB_DEFINES}
+      ${_dll_consume}
+  )
+
+  if(ABSL_PROPAGATE_CXX_STD)
+    # Abseil libraries require C++17 as the current minimum standard. When
+    # compiled with a higher minimum (either because it is the compiler's
+    # default or explicitly requested), then Abseil requires that standard.
+    target_compile_features(${_dll} PUBLIC ${ABSL_INTERNAL_CXX_STD_FEATURE})
+  endif()
+
+  install(TARGETS ${_dll} EXPORT ${PROJECT_NAME}Targets
+        RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR}
+        LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}
+        ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR}
+  )
+
+  add_library(absl::${_dll} ALIAS ${_dll})
+endfunction()
diff --git a/CMake/AbseilHelpers.cmake b/CMake/AbseilHelpers.cmake
index 67b97fe14df..61e1ae42116 100644
--- a/CMake/AbseilHelpers.cmake
+++ b/CMake/AbseilHelpers.cmake
@@ -16,13 +16,21 @@
 
 include(CMakeParseArguments)
 include(AbseilConfigureCopts)
-include(AbseilInstallDirs)
+include(AbseilDll)
 
 # The IDE folder for Abseil that will be used if Abseil is included in a CMake
 # project that sets
 #    set_property(GLOBAL PROPERTY USE_FOLDERS ON)
 # For example, Visual Studio supports folders.
-set(ABSL_IDE_FOLDER Abseil)
+if(NOT DEFINED ABSL_IDE_FOLDER)
+  set(ABSL_IDE_FOLDER Abseil)
+endif()
+
+if(ABSL_USE_SYSTEM_INCLUDES)
+  set(ABSL_INTERNAL_INCLUDE_WARNING_GUARD SYSTEM)
+else()
+  set(ABSL_INTERNAL_INCLUDE_WARNING_GUARD "")
+endif()
 
 # absl_cc_library()
 #
@@ -38,7 +46,8 @@ set(ABSL_IDE_FOLDER Abseil)
 # LINKOPTS: List of link options
 # PUBLIC: Add this so that this library will be exported under absl::
 # Also in IDE, target will appear in Abseil folder while non PUBLIC will be in Abseil/internal.
-# TESTONLY: When added, this target will only be built if user passes -DABSL_RUN_TESTS=ON to CMake.
+# TESTONLY: When added, this target will only be built if both
+#           BUILD_TESTING=ON and ABSL_BUILD_TESTING=ON.
 #
 # Note:
 # By default, absl_cc_library will always create a library named absl_${NAME},
@@ -71,7 +80,7 @@ set(ABSL_IDE_FOLDER Abseil)
 #     absl::fantastic_lib
 # )
 #
-# TODO: Implement "ALWAYSLINK"
+# TODO(b/320467376): Implement "ALWAYSLINK".
 function(absl_cc_library)
   cmake_parse_arguments(ABSL_CC_LIB
     "DISABLE_INSTALL;PUBLIC;TESTONLY"
@@ -80,95 +89,276 @@ function(absl_cc_library)
     ${ARGN}
   )
 
-  if(NOT ABSL_CC_LIB_TESTONLY OR ABSL_RUN_TESTS)
+  if(ABSL_CC_LIB_TESTONLY AND
+      NOT ((BUILD_TESTING AND ABSL_BUILD_TESTING) OR
+        (ABSL_BUILD_TEST_HELPERS AND ABSL_CC_LIB_PUBLIC)))
+    return()
+  endif()
+
+  if(ABSL_ENABLE_INSTALL)
+    set(_NAME "${ABSL_CC_LIB_NAME}")
+  else()
+    set(_NAME "absl_${ABSL_CC_LIB_NAME}")
+  endif()
+
+  # Check if this is a header-only library
+  # Note that as of February 2019, many popular OS's (for example, Ubuntu
+  # 16.04 LTS) only come with cmake 3.5 by default.  For this reason, we can't
+  # use list(FILTER...)
+  set(ABSL_CC_SRCS "${ABSL_CC_LIB_SRCS}")
+  foreach(src_file IN LISTS ABSL_CC_SRCS)
+    if(${src_file} MATCHES ".*\\.(h|inc)")
+      list(REMOVE_ITEM ABSL_CC_SRCS "${src_file}")
+    endif()
+  endforeach()
+
+  if(ABSL_CC_SRCS STREQUAL "")
+    set(ABSL_CC_LIB_IS_INTERFACE 1)
+  else()
+    set(ABSL_CC_LIB_IS_INTERFACE 0)
+  endif()
+
+  # Determine this build target's relationship to the DLL. It's one of four things:
+  # 1. "dll"     -- This target is part of the DLL
+  # 2. "dll_dep" -- This target is not part of the DLL, but depends on the DLL.
+  #                 Note that we assume any target not in the DLL depends on the
+  #                 DLL. This is not a technical necessity but a convenience
+  #                 which happens to be true, because nearly every target is
+  #                 part of the DLL.
+  # 3. "shared"  -- This is a shared library, perhaps on a non-windows platform
+  #                 where DLL doesn't make sense.
+  # 4. "static"  -- This target does not depend on the DLL and should be built
+  #                 statically.
+  if (${ABSL_BUILD_DLL})
     if(ABSL_ENABLE_INSTALL)
-      set(_NAME "${ABSL_CC_LIB_NAME}")
+      absl_internal_dll_contains(TARGET ${_NAME} OUTPUT _in_dll)
+      absl_internal_test_dll_contains(TARGET ${_NAME} OUTPUT _in_test_dll)
     else()
-      set(_NAME "absl_${ABSL_CC_LIB_NAME}")
+      absl_internal_dll_contains(TARGET ${ABSL_CC_LIB_NAME} OUTPUT _in_dll)
+      absl_internal_test_dll_contains(TARGET ${ABSL_CC_LIB_NAME} OUTPUT _in_test_dll)
     endif()
-
-    # Check if this is a header-only library
-    # Note that as of February 2019, many popular OS's (for example, Ubuntu
-    # 16.04 LTS) only come with cmake 3.5 by default.  For this reason, we can't
-    # use list(FILTER...)
-    set(ABSL_CC_SRCS "${ABSL_CC_LIB_SRCS}")
-    foreach(src_file IN LISTS ABSL_CC_SRCS)
-      if(${src_file} MATCHES ".*\\.(h|inc)")
-        list(REMOVE_ITEM ABSL_CC_SRCS "${src_file}")
-      endif()
-    endforeach()
-    if("${ABSL_CC_SRCS}" STREQUAL "")
+    if (${_in_dll} OR ${_in_test_dll})
+      # This target should be replaced by the DLL
+      set(_build_type "dll")
       set(ABSL_CC_LIB_IS_INTERFACE 1)
     else()
-      set(ABSL_CC_LIB_IS_INTERFACE 0)
+      # Building a DLL, but this target is not part of the DLL
+      set(_build_type "dll_dep")
+    endif()
+  elseif(BUILD_SHARED_LIBS)
+    set(_build_type "shared")
+  else()
+    set(_build_type "static")
+  endif()
+
+  # Generate a pkg-config file for every library:
+  if(ABSL_ENABLE_INSTALL)
+    if(absl_VERSION)
+      set(PC_VERSION "${absl_VERSION}")
+    else()
+      set(PC_VERSION "head")
+    endif()
+    if(NOT _build_type STREQUAL "dll")
+      set(LNK_LIB "${LNK_LIB} -labsl_${_NAME}")
     endif()
+    foreach(dep ${ABSL_CC_LIB_DEPS})
+      if(${dep} MATCHES "^absl::(.*)")
+        # for DLL builds many libs are not created, but add
+        # the pkgconfigs nevertheless, pointing to the dll.
+        if(_build_type STREQUAL "dll")
+          # hide this MATCHES in an if-clause so it doesn't overwrite
+          # the CMAKE_MATCH_1 from (${dep} MATCHES "^absl::(.*)")
+          if(NOT PC_DEPS MATCHES "abseil_dll")
+            # Join deps with commas.
+            if(PC_DEPS)
+              set(PC_DEPS "${PC_DEPS},")
+            endif()
+            # don't duplicate dll-dep if it exists already
+            set(PC_DEPS "${PC_DEPS} abseil_dll = ${PC_VERSION}")
+            set(LNK_LIB "${LNK_LIB} -labseil_dll")
+          endif()
+        else()
+          # Join deps with commas.
+          if(PC_DEPS)
+            set(PC_DEPS "${PC_DEPS},")
+          endif()
+          set(PC_DEPS "${PC_DEPS} absl_${CMAKE_MATCH_1} = ${PC_VERSION}")
+        endif()
+      endif()
+    endforeach()
+    foreach(cflag ${ABSL_CC_LIB_COPTS})
+      # Strip out the CMake-specific `SHELL:` prefix, which is used to construct
+      # a group of space-separated options.
+      # https://cmake.org/cmake/help/v3.30/command/target_compile_options.html#option-de-duplication
+      string(REGEX REPLACE "^SHELL:" "" cflag "${cflag}")
+      if(${cflag} MATCHES "^-Xarch_")
+        # An -Xarch_ flag implies that its successor only applies to the
+        # specified platform. Such option groups are each specified in a single
+        # `SHELL:`-prefixed string in the COPTS list, which we simply ignore.
+      elseif(${cflag} MATCHES "^(-Wno-|/wd)")
+        # These flags are needed to suppress warnings that might fire in our headers.
+        set(PC_CFLAGS "${PC_CFLAGS} ${cflag}")
+      elseif(${cflag} MATCHES "^(-W|/w[1234eo])")
+        # Don't impose our warnings on others.
+      elseif(${cflag} MATCHES "^-m")
+        # Don't impose CPU instruction requirements on others, as
+        # the code performs feature detection on runtime.
+      else()
+        set(PC_CFLAGS "${PC_CFLAGS} ${cflag}")
+      endif()
+    endforeach()
+    string(REPLACE ";" " " PC_LINKOPTS "${ABSL_CC_LIB_LINKOPTS}")
+    FILE(GENERATE OUTPUT "${CMAKE_BINARY_DIR}/lib/pkgconfig/absl_${_NAME}.pc" CONTENT "\
+prefix=${CMAKE_INSTALL_PREFIX}\n\
+exec_prefix=\${prefix}\n\
+libdir=${CMAKE_INSTALL_FULL_LIBDIR}\n\
+includedir=${CMAKE_INSTALL_FULL_INCLUDEDIR}\n\
+\n\
+Name: absl_${_NAME}\n\
+Description: Abseil ${_NAME} library\n\
+URL: https://abseil.io/\n\
+Version: ${PC_VERSION}\n\
+Requires:${PC_DEPS}\n\
+Libs: -L\${libdir} $<$<NOT:$<BOOL:${ABSL_CC_LIB_IS_INTERFACE}>>:${LNK_LIB}> ${PC_LINKOPTS}\n\
+Cflags: -I\${includedir}${PC_CFLAGS}\n")
+    INSTALL(FILES "${CMAKE_BINARY_DIR}/lib/pkgconfig/absl_${_NAME}.pc"
+            DESTINATION "${CMAKE_INSTALL_LIBDIR}/pkgconfig")
+  endif()
 
-    if(NOT ABSL_CC_LIB_IS_INTERFACE)
+  if(NOT ABSL_CC_LIB_IS_INTERFACE)
+    if(_build_type STREQUAL "dll_dep")
+      # This target depends on the DLL. When adding dependencies to this target,
+      # any depended-on-target which is contained inside the DLL is replaced
+      # with a dependency on the DLL.
       add_library(${_NAME} STATIC "")
       target_sources(${_NAME} PRIVATE ${ABSL_CC_LIB_SRCS} ${ABSL_CC_LIB_HDRS})
-      target_include_directories(${_NAME}
-        PUBLIC
-          "$<BUILD_INTERFACE:${ABSL_COMMON_INCLUDE_DIRS}>"
-          $<INSTALL_INTERFACE:${ABSL_INSTALL_INCLUDEDIR}>
+      absl_internal_dll_targets(
+        DEPS ${ABSL_CC_LIB_DEPS}
+        OUTPUT _dll_deps
       )
-      target_compile_options(${_NAME}
-        PRIVATE ${ABSL_CC_LIB_COPTS})
       target_link_libraries(${_NAME}
-        PUBLIC ${ABSL_CC_LIB_DEPS}
+        PUBLIC ${_dll_deps}
         PRIVATE
           ${ABSL_CC_LIB_LINKOPTS}
           ${ABSL_DEFAULT_LINKOPTS}
       )
-      target_compile_definitions(${_NAME} PUBLIC ${ABSL_CC_LIB_DEFINES})
 
-      # Add all Abseil targets to a a folder in the IDE for organization.
-      if(ABSL_CC_LIB_PUBLIC)
-        set_property(TARGET ${_NAME} PROPERTY FOLDER ${ABSL_IDE_FOLDER})
-      elseif(ABSL_CC_LIB_TESTONLY)
-        set_property(TARGET ${_NAME} PROPERTY FOLDER ${ABSL_IDE_FOLDER}/test)
+      if (ABSL_CC_LIB_TESTONLY)
+        set(_gtest_link_define "GTEST_LINKED_AS_SHARED_LIBRARY=1")
       else()
-        set_property(TARGET ${_NAME} PROPERTY FOLDER ${ABSL_IDE_FOLDER}/internal)
+        set(_gtest_link_define)
+      endif()
+
+      target_compile_definitions(${_NAME}
+        PUBLIC
+          ABSL_CONSUME_DLL
+          "${_gtest_link_define}"
+      )
+
+    elseif(_build_type STREQUAL "static" OR _build_type STREQUAL "shared")
+      add_library(${_NAME} "")
+      target_sources(${_NAME} PRIVATE ${ABSL_CC_LIB_SRCS} ${ABSL_CC_LIB_HDRS})
+      if(APPLE)
+        set_target_properties(${_NAME} PROPERTIES
+          INSTALL_RPATH "@loader_path")
+      elseif(UNIX)
+        set_target_properties(${_NAME} PROPERTIES
+          INSTALL_RPATH "$ORIGIN")
       endif()
+      target_link_libraries(${_NAME}
+      PUBLIC ${ABSL_CC_LIB_DEPS}
+      PRIVATE
+        ${ABSL_CC_LIB_LINKOPTS}
+        ${ABSL_DEFAULT_LINKOPTS}
+      )
+    else()
+      message(FATAL_ERROR "Invalid build type: ${_build_type}")
+    endif()
+
+    # Linker language can be inferred from sources, but in the case of DLLs we
+    # don't have any .cc files so it would be ambiguous. We could set it
+    # explicitly only in the case of DLLs but, because "CXX" is always the
+    # correct linker language for static or for shared libraries, we set it
+    # unconditionally.
+    set_property(TARGET ${_NAME} PROPERTY LINKER_LANGUAGE "CXX")
+
+    target_include_directories(${_NAME} ${ABSL_INTERNAL_INCLUDE_WARNING_GUARD}
+      PUBLIC
+        "$<BUILD_INTERFACE:${ABSL_COMMON_INCLUDE_DIRS}>"
+        $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>
+    )
+    target_compile_options(${_NAME}
+      PRIVATE ${ABSL_CC_LIB_COPTS})
+    target_compile_definitions(${_NAME} PUBLIC ${ABSL_CC_LIB_DEFINES})
+
+    # Add all Abseil targets to a a folder in the IDE for organization.
+    if(ABSL_CC_LIB_PUBLIC)
+      set_property(TARGET ${_NAME} PROPERTY FOLDER ${ABSL_IDE_FOLDER})
+    elseif(ABSL_CC_LIB_TESTONLY)
+      set_property(TARGET ${_NAME} PROPERTY FOLDER ${ABSL_IDE_FOLDER}/test)
+    else()
+      set_property(TARGET ${_NAME} PROPERTY FOLDER ${ABSL_IDE_FOLDER}/internal)
+    endif()
 
-      # INTERFACE libraries can't have the CXX_STANDARD property set
-      set_property(TARGET ${_NAME} PROPERTY CXX_STANDARD ${ABSL_CXX_STANDARD})
-      set_property(TARGET ${_NAME} PROPERTY CXX_STANDARD_REQUIRED ON)
+    if(ABSL_PROPAGATE_CXX_STD)
+      # Abseil libraries require C++17 as the current minimum standard. When
+      # compiled with a higher standard (either because it is the compiler's
+      # default or explicitly requested), then Abseil requires that standard.
+      target_compile_features(${_NAME} PUBLIC ${ABSL_INTERNAL_CXX_STD_FEATURE})
+    endif()
 
-      # When being installed, we lose the absl_ prefix.  We want to put it back
-      # to have properly named lib files.  This is a no-op when we are not being
-      # installed.
+    # When being installed, we lose the absl_ prefix.  We want to put it back
+    # to have properly named lib files.  This is a no-op when we are not being
+    # installed.
+    if(ABSL_ENABLE_INSTALL)
       set_target_properties(${_NAME} PROPERTIES
         OUTPUT_NAME "absl_${_NAME}"
+        SOVERSION "${ABSL_SOVERSION}"
       )
-    else()
-      # Generating header-only library
-      add_library(${_NAME} INTERFACE)
-      target_include_directories(${_NAME}
-        INTERFACE
-          "$<BUILD_INTERFACE:${ABSL_COMMON_INCLUDE_DIRS}>"
-          $<INSTALL_INTERFACE:${ABSL_INSTALL_INCLUDEDIR}>
-        )
-      target_link_libraries(${_NAME}
-        INTERFACE
-          ${ABSL_CC_LIB_DEPS}
-          ${ABSL_CC_LIB_LINKOPTS}
-          ${ABSL_DEFAULT_LINKOPTS}
+    endif()
+  else()
+    # Generating header-only library
+    add_library(${_NAME} INTERFACE)
+    target_include_directories(${_NAME} ${ABSL_INTERNAL_INCLUDE_WARNING_GUARD}
+      INTERFACE
+        "$<BUILD_INTERFACE:${ABSL_COMMON_INCLUDE_DIRS}>"
+        $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>
       )
-      target_compile_definitions(${_NAME} INTERFACE ${ABSL_CC_LIB_DEFINES})
+
+    if (_build_type STREQUAL "dll")
+         if(${_in_dll})
+           set(ABSL_CC_LIB_DEPS abseil_dll)
+         endif()
+         if(${_in_test_dll})
+           set(ABSL_CC_LIB_DEPS abseil_test_dll)
+         endif()
     endif()
 
-    # TODO currently we don't install googletest alongside abseil sources, so
-    # installed abseil can't be tested.
-    if(NOT ABSL_CC_LIB_TESTONLY AND ABSL_ENABLE_INSTALL)
-      install(TARGETS ${_NAME} EXPORT ${PROJECT_NAME}Targets
-            RUNTIME DESTINATION ${ABSL_INSTALL_BINDIR}
-            LIBRARY DESTINATION ${ABSL_INSTALL_LIBDIR}
-            ARCHIVE DESTINATION ${ABSL_INSTALL_LIBDIR}
-      )
+    target_link_libraries(${_NAME}
+      INTERFACE
+        ${ABSL_CC_LIB_DEPS}
+        ${ABSL_CC_LIB_LINKOPTS}
+        ${ABSL_DEFAULT_LINKOPTS}
+    )
+    target_compile_definitions(${_NAME} INTERFACE ${ABSL_CC_LIB_DEFINES})
+
+    if(ABSL_PROPAGATE_CXX_STD)
+      # Abseil libraries require C++17 as the current minimum standard.
+      # Top-level application CMake projects should ensure a consistent C++
+      # standard for all compiled sources by setting CMAKE_CXX_STANDARD.
+      target_compile_features(${_NAME} INTERFACE ${ABSL_INTERNAL_CXX_STD_FEATURE})
     endif()
+  endif()
 
-    add_library(absl::${ABSL_CC_LIB_NAME} ALIAS ${_NAME})
+  if(ABSL_ENABLE_INSTALL)
+    install(TARGETS ${_NAME} EXPORT ${PROJECT_NAME}Targets
+          RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR}
+          LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}
+          ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR}
+    )
   endif()
+
+    add_library(absl::${ABSL_CC_LIB_NAME} ALIAS ${_NAME})
 endfunction()
 
 # absl_cc_test()
@@ -205,11 +395,11 @@ endfunction()
 #     "awesome_test.cc"
 #   DEPS
 #     absl::awesome
-#     gmock
-#     gtest_main
+#     GTest::gmock
+#     GTest::gtest_main
 # )
 function(absl_cc_test)
-  if(NOT ABSL_RUN_TESTS)
+  if(NOT (BUILD_TESTING AND ABSL_BUILD_TESTING))
     return()
   endif()
 
@@ -221,35 +411,55 @@ function(absl_cc_test)
   )
 
   set(_NAME "absl_${ABSL_CC_TEST_NAME}")
+
   add_executable(${_NAME} "")
   target_sources(${_NAME} PRIVATE ${ABSL_CC_TEST_SRCS})
   target_include_directories(${_NAME}
     PUBLIC ${ABSL_COMMON_INCLUDE_DIRS}
-    PRIVATE ${GMOCK_INCLUDE_DIRS} ${GTEST_INCLUDE_DIRS}
-  )
-  target_compile_definitions(${_NAME}
-    PUBLIC ${ABSL_CC_TEST_DEFINES}
+    PRIVATE ${absl_gtest_src_dir}/googletest/include ${absl_gtest_src_dir}/googlemock/include
   )
+
+  if (${ABSL_BUILD_DLL})
+    target_compile_definitions(${_NAME}
+      PUBLIC
+        ${ABSL_CC_TEST_DEFINES}
+        ABSL_CONSUME_DLL
+        ABSL_CONSUME_TEST_DLL
+        GTEST_LINKED_AS_SHARED_LIBRARY=1
+    )
+
+    # Replace dependencies on targets inside the DLL with abseil_dll itself.
+    absl_internal_dll_targets(
+      DEPS ${ABSL_CC_TEST_DEPS}
+      OUTPUT ABSL_CC_TEST_DEPS
+    )
+    absl_internal_dll_targets(
+      DEPS ${ABSL_CC_TEST_LINKOPTS}
+      OUTPUT ABSL_CC_TEST_LINKOPTS
+    )
+  else()
+    target_compile_definitions(${_NAME}
+      PUBLIC
+        ${ABSL_CC_TEST_DEFINES}
+    )
+  endif()
   target_compile_options(${_NAME}
     PRIVATE ${ABSL_CC_TEST_COPTS}
   )
+
   target_link_libraries(${_NAME}
     PUBLIC ${ABSL_CC_TEST_DEPS}
     PRIVATE ${ABSL_CC_TEST_LINKOPTS}
   )
-  # Add all Abseil targets to a a folder in the IDE for organization.
+  # Add all Abseil targets to a folder in the IDE for organization.
   set_property(TARGET ${_NAME} PROPERTY FOLDER ${ABSL_IDE_FOLDER}/test)
 
-  set_property(TARGET ${_NAME} PROPERTY CXX_STANDARD ${ABSL_CXX_STANDARD})
-  set_property(TARGET ${_NAME} PROPERTY CXX_STANDARD_REQUIRED ON)
+  if(ABSL_PROPAGATE_CXX_STD)
+    # Abseil libraries require C++17 as the current minimum standard.
+    # Top-level application CMake projects should ensure a consistent C++
+    # standard for all compiled sources by setting CMAKE_CXX_STANDARD.
+    target_compile_features(${_NAME} PUBLIC ${ABSL_INTERNAL_CXX_STD_FEATURE})
+  endif()
 
   add_test(NAME ${_NAME} COMMAND ${_NAME})
 endfunction()
-
-
-function(check_target my_target)
-  if(NOT TARGET ${my_target})
-    message(FATAL_ERROR " ABSL: compiling absl requires a ${my_target} CMake target in your project,
-                   see CMake/README.md for more details")
-  endif(NOT TARGET ${my_target})
-endfunction()
diff --git a/CMake/AbseilInstallDirs.cmake b/CMake/AbseilInstallDirs.cmake
deleted file mode 100644
index b67272f830c..00000000000
--- a/CMake/AbseilInstallDirs.cmake
+++ /dev/null
@@ -1,20 +0,0 @@
-include(GNUInstallDirs)
-
-# absl_VERSION is only set if we are an LTS release being installed, in which
-# case it may be into a system directory and so we need to make subdirectories
-# for each installed version of Abseil.  This mechanism is implemented in
-# Abseil's internal Copybara (https://github.com/google/copybara) workflows and
-# isn't visible in the CMake buildsystem itself.
-
-if(absl_VERSION)
-  set(ABSL_SUBDIR "${PROJECT_NAME}_${PROJECT_VERSION}")
-  set(ABSL_INSTALL_BINDIR "${CMAKE_INSTALL_BINDIR}/${ABSL_SUBDIR}")
-  set(ABSL_INSTALL_CONFIGDIR "${CMAKE_INSTALL_LIBDIR}/cmake/${ABSL_SUBDIR}")
-  set(ABSL_INSTALL_INCLUDEDIR "${CMAKE_INSTALL_INCLUDEDIR}/{ABSL_SUBDIR}")
-  set(ABSL_INSTALL_LIBDIR "${CMAKE_INSTALL_LIBDIR}/${ABSL_SUBDIR}")
-else()
-  set(ABSL_INSTALL_BINDIR "${CMAKE_INSTALL_BINDIR}")
-  set(ABSL_INSTALL_CONFIGDIR "${CMAKE_INSTALL_LIBDIR}/cmake/${PROJECT_NAME}")
-  set(ABSL_INSTALL_INCLUDEDIR "${CMAKE_INSTALL_INCLUDEDIR}")
-  set(ABSL_INSTALL_LIBDIR "${CMAKE_INSTALL_LIBDIR}")
-endif()
\ No newline at end of file
diff --git a/CMake/Googletest/CMakeLists.txt.in b/CMake/Googletest/CMakeLists.txt.in
index d60a33e9ac6..3db48341a25 100644
--- a/CMake/Googletest/CMakeLists.txt.in
+++ b/CMake/Googletest/CMakeLists.txt.in
@@ -1,15 +1,14 @@
-cmake_minimum_required(VERSION 2.8.2)
+cmake_minimum_required(VERSION 3.16)
 
-project(googletest-download NONE)
+project(googletest-external NONE)
 
 include(ExternalProject)
 ExternalProject_Add(googletest
-  GIT_REPOSITORY    https://github.com/google/googletest.git
-  GIT_TAG           master
-  SOURCE_DIR        "${CMAKE_BINARY_DIR}/googletest-src"
-  BINARY_DIR        "${CMAKE_BINARY_DIR}/googletest-build"
+  URL               "${absl_gtest_download_url}"  # May be empty
+  SOURCE_DIR        "${absl_gtest_src_dir}"
+  BINARY_DIR        "${absl_gtest_build_dir}"
   CONFIGURE_COMMAND ""
   BUILD_COMMAND     ""
   INSTALL_COMMAND   ""
   TEST_COMMAND      ""
-)
\ No newline at end of file
+)
diff --git a/CMake/Googletest/DownloadGTest.cmake b/CMake/Googletest/DownloadGTest.cmake
index 3c682aef07f..9d071c91701 100644
--- a/CMake/Googletest/DownloadGTest.cmake
+++ b/CMake/Googletest/DownloadGTest.cmake
@@ -1,32 +1,41 @@
-# Downloads and unpacks googletest at configure time.  Based on the instructions
-# at https://github.com/google/googletest/tree/master/googletest#incorporating-into-an-existing-cmake-project
+# Integrates googletest at configure time.  Based on the instructions at
+# https://github.com/google/googletest/tree/master/googletest#incorporating-into-an-existing-cmake-project
 
-# Download the latest googletest from Github master
+# Set up the external googletest project, downloading the latest from Github
+# master if requested.
 configure_file(
   ${CMAKE_CURRENT_LIST_DIR}/CMakeLists.txt.in
-  ${CMAKE_BINARY_DIR}/googletest-download/CMakeLists.txt
+  ${CMAKE_BINARY_DIR}/googletest-external/CMakeLists.txt
 )
 
-# Configure and build the downloaded googletest source
+set(ABSL_SAVE_CMAKE_CXX_FLAGS ${CMAKE_CXX_FLAGS})
+set(ABSL_SAVE_CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_RUNTIME_OUTPUT_DIRECTORY})
+if (BUILD_SHARED_LIBS)
+  set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/bin)
+  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DGTEST_CREATE_SHARED_LIBRARY=1")
+endif()
+
+# Configure and build the googletest source.
 execute_process(COMMAND ${CMAKE_COMMAND} -G "${CMAKE_GENERATOR}" .
   RESULT_VARIABLE result
-  WORKING_DIRECTORY ${CMAKE_BINARY_DIR}/googletest-download )
+  WORKING_DIRECTORY ${CMAKE_BINARY_DIR}/googletest-external )
 if(result)
   message(FATAL_ERROR "CMake step for googletest failed: ${result}")
 endif()
 
 execute_process(COMMAND ${CMAKE_COMMAND} --build .
   RESULT_VARIABLE result
-  WORKING_DIRECTORY ${CMAKE_BINARY_DIR}/googletest-download)
+  WORKING_DIRECTORY ${CMAKE_BINARY_DIR}/googletest-external)
 if(result)
   message(FATAL_ERROR "Build step for googletest failed: ${result}")
 endif()
 
+set(CMAKE_CXX_FLAGS ${ABSL_SAVE_CMAKE_CXX_FLAGS})
+set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${ABSL_SAVE_CMAKE_RUNTIME_OUTPUT_DIRECTORY})
+
 # Prevent overriding the parent project's compiler/linker settings on Windows
 set(gtest_force_shared_crt ON CACHE BOOL "" FORCE)
 
 # Add googletest directly to our build. This defines the gtest and gtest_main
 # targets.
-add_subdirectory(${CMAKE_BINARY_DIR}/googletest-src
-                 ${CMAKE_BINARY_DIR}/googletest-build
-                 EXCLUDE_FROM_ALL)
+add_subdirectory(${absl_gtest_src_dir} ${absl_gtest_build_dir} EXCLUDE_FROM_ALL)
diff --git a/CMake/README.md b/CMake/README.md
index 04d5df3ab09..ecaeaf067e4 100644
--- a/CMake/README.md
+++ b/CMake/README.md
@@ -20,8 +20,10 @@ googletest framework
 ### Step-by-Step Instructions
 
 1. If you want to build the Abseil tests, integrate the Abseil dependency
-[Google Test](https://github.com/google/googletest) into your CMake project. To disable Abseil tests, you have to pass
-`-DBUILD_TESTING=OFF` when configuring your project with CMake.
+[Google Test](https://github.com/google/googletest) into your CMake
+project. To disable Abseil tests, you have to pass either
+`-DBUILD_TESTING=OFF` or `-DABSL_BUILD_TESTING=OFF` when configuring your
+project with CMake.
 
 2. Download Abseil and copy it into a subdirectory in your CMake project or add
 Abseil as a [git submodule](https://git-scm.com/docs/git-submodule) in your
@@ -34,15 +36,16 @@ to include Abseil directly in your CMake project.
 4. Add the **absl::** target you wish to use to the
 [`target_link_libraries()`](https://cmake.org/cmake/help/latest/command/target_link_libraries.html)
 section of your executable or of your library.<br>
-Here is a short CMakeLists.txt example of a project file using Abseil.
+Here is a short CMakeLists.txt example of an application project using Abseil.
 
 ```cmake
-cmake_minimum_required(VERSION 3.5)
-project(my_project)
+cmake_minimum_required(VERSION 3.16)
+project(my_app_project)
 
 # Pick the C++ standard to compile with.
-# Abseil currently supports C++11, C++14, and C++17.
-set(CMAKE_CXX_STANDARD 11)
+# Abseil currently supports C++17 and C++20.
+set(CMAKE_CXX_STANDARD 17)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
 
 add_subdirectory(abseil-cpp)
 
@@ -50,9 +53,48 @@ add_executable(my_exe source.cpp)
 target_link_libraries(my_exe absl::base absl::synchronization absl::strings)
 ```
 
+Note that if you are developing a library designed for use by other clients, you
+should instead leave `CMAKE_CXX_STANDARD` unset (or only set if being built as
+the current top-level CMake project) and configure the minimum required C++
+standard at the target level. If you require a later minimum C++ standard than
+Abseil does, it's a good idea to also enforce that `CMAKE_CXX_STANDARD` (which
+will control Abseil library targets) is set to at least that minimum. For
+example:
+
+```cmake
+cmake_minimum_required(VERSION 3.16)
+project(my_lib_project)
+
+# Leave C++ standard up to the root application, so set it only if this is the
+# current top-level CMake project.
+if(CMAKE_SOURCE_DIR STREQUAL my_lib_project_SOURCE_DIR)
+  set(CMAKE_CXX_STANDARD 17)
+  set(CMAKE_CXX_STANDARD_REQUIRED ON)
+endif()
+
+add_subdirectory(abseil-cpp)
+
+add_library(my_lib source.cpp)
+target_link_libraries(my_lib absl::base absl::synchronization absl::strings)
+
+# Enforce that my_lib requires C++17. Important to document for clients that they
+# must set CMAKE_CXX_STANDARD to 17 or higher for proper Abseil ABI compatibility
+# (since otherwise, Abseil library targets could be compiled with a lower C++
+# standard than my_lib).
+target_compile_features(my_lib PUBLIC cxx_std_17)
+if(CMAKE_CXX_STANDARD LESS 17)
+  message(FATAL_ERROR
+      "my_lib_project requires CMAKE_CXX_STANDARD >= 17 (got: ${CMAKE_CXX_STANDARD})")
+endif()
+```
+
+Then the top-level application project that uses your library is responsible for
+setting a consistent `CMAKE_CXX_STANDARD` that is sufficiently high.
+
 ### Running Abseil Tests with CMake
 
-Use the `-DABSL_RUN_TESTS=ON` flag to run Abseil tests.  Note that if the `-DBUILD_TESTING=OFF` flag is passed then Abseil tests will not be run.
+Use the `-DABSL_BUILD_TESTING=ON` flag to run Abseil tests.  Note that
+BUILD_TESTING must also be on (the default).
 
 You will need to provide Abseil with a Googletest dependency.  There are two
 options for how to do this:
@@ -70,7 +112,7 @@ For example, to run just the Abseil tests, you could use this script:
 cd path/to/abseil-cpp
 mkdir build
 cd build
-cmake -DABSL_USE_GOOGLETEST_HEAD=ON -DABSL_RUN_TESTS=ON ..
+cmake -DABSL_BUILD_TESTING=ON -DABSL_USE_GOOGLETEST_HEAD=ON ..
 make -j
 ctest
 ```
@@ -93,9 +135,54 @@ absl::flags
 absl::memory
 absl::meta
 absl::numeric
-absl::random
+absl::random_random
 absl::strings
 absl::synchronization
 absl::time
 absl::utility
 ```
+
+## Traditional CMake Set-Up
+
+For larger projects, it may make sense to use the traditional CMake set-up where you build and install projects separately.
+
+First, you'd need to build and install Google Test:
+```
+cmake -S /source/googletest -B /build/googletest -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=/installation/dir -DBUILD_GMOCK=ON
+cmake --build /build/googletest --target install
+```
+
+Then you need to configure and build Abseil. Make sure you enable `ABSL_USE_EXTERNAL_GOOGLETEST` and `ABSL_FIND_GOOGLETEST`. You also need to enable `ABSL_ENABLE_INSTALL` so that you can install Abseil itself.
+```
+cmake -S /source/abseil-cpp -B /build/abseil-cpp -DCMAKE_PREFIX_PATH=/installation/dir -DCMAKE_INSTALL_PREFIX=/installation/dir -DABSL_ENABLE_INSTALL=ON -DABSL_USE_EXTERNAL_GOOGLETEST=ON -DABSL_FIND_GOOGLETEST=ON
+cmake --build /temporary/build/abseil-cpp
+```
+
+(`CMAKE_PREFIX_PATH` is where you already have Google Test installed; `CMAKE_INSTALL_PREFIX` is where you want to have Abseil installed; they can be different.)
+
+Run the tests:
+```
+ctest --test-dir /temporary/build/abseil-cpp
+```
+
+And finally install:
+```
+cmake --build /temporary/build/abseil-cpp --target install
+```
+
+# CMake Option Synopsis
+
+## Enable Standard CMake Installation
+
+`-DABSL_ENABLE_INSTALL=ON`
+
+## Google Test Options
+
+`-DABSL_BUILD_TESTING=ON` must be set to enable testing
+
+- Have Abseil download and build Google Test for you: `-DABSL_USE_EXTERNAL_GOOGLETEST=OFF` (default)
+  - Download and build latest Google Test: `-DABSL_USE_GOOGLETEST_HEAD=ON`
+  - Download specific Google Test version (ZIP archive): `-DABSL_GOOGLETEST_DOWNLOAD_URL=https://.../version.zip`
+  - Use Google Test from specific local directory: `-DABSL_LOCAL_GOOGLETEST_DIR=/path/to/googletest`
+- Use Google Test included elsewhere in your project: `-DABSL_USE_EXTERNAL_GOOGLETEST=ON`
+- Use standard CMake `find_package(CTest)` to find installed Google Test: `-DABSL_USE_EXTERNAL_GOOGLETEST=ON -DABSL_FIND_GOOGLETEST=ON`
diff --git a/CMake/abslConfig.cmake.in b/CMake/abslConfig.cmake.in
index 60847fa7723..62d246d01e0 100644
--- a/CMake/abslConfig.cmake.in
+++ b/CMake/abslConfig.cmake.in
@@ -1,7 +1,8 @@
 # absl CMake configuration file.
 
-include(FindThreads)
+include(CMakeFindDependencyMacro)
+find_dependency(Threads)
 
 @PACKAGE_INIT@
 
-include ("${CMAKE_CURRENT_LIST_DIR}/@PROJECT_NAME@Targets.cmake")
\ No newline at end of file
+include ("${CMAKE_CURRENT_LIST_DIR}/@PROJECT_NAME@Targets.cmake")
diff --git a/CMake/install_test_project/CMakeLists.txt b/CMake/install_test_project/CMakeLists.txt
index 06b797e9edd..3229887c8d6 100644
--- a/CMake/install_test_project/CMakeLists.txt
+++ b/CMake/install_test_project/CMakeLists.txt
@@ -15,13 +15,11 @@
 
 # A simple CMakeLists.txt for testing cmake installation
 
-cmake_minimum_required(VERSION 3.5)
+cmake_minimum_required(VERSION 3.16)
 project(absl_cmake_testing CXX)
 
-set(CMAKE_CXX_STANDARD 11)
-
 add_executable(simple simple.cc)
 
 find_package(absl REQUIRED)
 
-target_link_libraries(simple absl::strings)
+target_link_libraries(simple absl::strings absl::config)
diff --git a/CMake/install_test_project/simple.cc b/CMake/install_test_project/simple.cc
index e9e352912ba..7daa7f09017 100644
--- a/CMake/install_test_project/simple.cc
+++ b/CMake/install_test_project/simple.cc
@@ -14,8 +14,17 @@
 // limitations under the License.
 
 #include <iostream>
+#include "absl/base/config.h"
 #include "absl/strings/substitute.h"
 
+#if !defined(ABSL_LTS_RELEASE_VERSION) || ABSL_LTS_RELEASE_VERSION != 99998877
+#error ABSL_LTS_RELEASE_VERSION is not set correctly.
+#endif
+
+#if !defined(ABSL_LTS_RELEASE_PATCH_LEVEL) || ABSL_LTS_RELEASE_PATCH_LEVEL != 0
+#error ABSL_LTS_RELEASE_PATCH_LEVEL is not set correctly.
+#endif
+
 int main(int argc, char** argv) {
   for (int i = 0; i < argc; ++i) {
     std::cout << absl::Substitute("Arg $0: $1\n", i, argv[i]);
diff --git a/CMake/install_test_project/test.sh b/CMake/install_test_project/test.sh
index 99989b031d3..962bc8d2d83 100755
--- a/CMake/install_test_project/test.sh
+++ b/CMake/install_test_project/test.sh
@@ -13,70 +13,61 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
-
-# "Unit" and integration tests for Absl CMake installation
-
-# TODO(absl-team): This script isn't fully hermetic because
-# -DABSL_USE_GOOGLETEST_HEAD=ON means that this script isn't pinned to a fixed
-# version of GoogleTest. This means that an upstream change to GoogleTest could
-# break this test. Fix this by allowing this script to pin to a known-good
-# version of GoogleTest.
+#
+# Unit and integration tests for Abseil LTS CMake installation
 
 # Fail on any error. Treat unset variables an error. Print commands as executed.
 set -euox pipefail
 
-install_absl() {
-  pushd "${absl_build_dir}"
-  if [[ "${#}" -eq 1 ]]; then
-    cmake -DCMAKE_INSTALL_PREFIX="${1}" "${absl_dir}"
-  else
-    cmake "${absl_dir}"
-  fi
-  cmake --build . --target install -- -j
-  popd
-}
-
-uninstall_absl() {
-  xargs rm < "${absl_build_dir}"/install_manifest.txt
-  rm -rf "${absl_build_dir}"
-  mkdir -p "${absl_build_dir}"
-}
-
-lts_install=""
-
-while getopts ":l" lts; do
-  case "${lts}" in
-    l )
-      lts_install="true"
-      ;;
-  esac
-done
-
 absl_dir=/abseil-cpp
-absl_build_dir=/buildfs/absl-build
-project_dir="${absl_dir}"/CMake/install_test_project
+absl_build_dir=/buildfs
+googletest_builddir=/googletest_builddir
+googletest_archive="googletest-${ABSL_GOOGLETEST_VERSION}.tar.gz"
+project_dir="${absl_dir}/CMake/install_test_project"
 project_build_dir=/buildfs/project-build
 
-mkdir -p "${absl_build_dir}"
-mkdir -p "${project_build_dir}"
-
-if [[ "${lts_install}" ]]; then
-  install_dir="/usr/local"
-else
-  install_dir="${project_build_dir}"/install
+build_shared_libs="OFF"
+if [ "${LINK_TYPE:-}" = "DYNAMIC" ]; then
+  build_shared_libs="ON"
 fi
-mkdir -p "${install_dir}"
 
-# Test build, install, and link against installed abseil
-pushd "${project_build_dir}"
-if [[ "${lts_install}" ]]; then
-  install_absl
-  cmake "${project_dir}"
-else
-  install_absl "${install_dir}"
-  cmake "${project_dir}" -DCMAKE_PREFIX_PATH="${install_dir}"
-fi
+# Build and install GoogleTest
+mkdir "${googletest_builddir}"
+pushd "${googletest_builddir}"
+curl -L "${ABSL_GOOGLETEST_DOWNLOAD_URL}" --output "${googletest_archive}"
+tar -xz -f "${googletest_archive}"
+pushd "googletest-${ABSL_GOOGLETEST_VERSION}"
+mkdir build
+pushd build
+cmake -DCMAKE_BUILD_TYPE=Release -DBUILD_SHARED_LIBS="${build_shared_libs}" ..
+make -j $(nproc)
+make install
+ldconfig
+popd
+popd
+popd
 
+# Run the LTS transformations
+./create_lts.py 99998877
+
+# Build and install Abseil
+pushd "${absl_build_dir}"
+cmake "${absl_dir}" \
+  -DABSL_USE_EXTERNAL_GOOGLETEST=ON \
+  -DABSL_FIND_GOOGLETEST=ON  \
+  -DCMAKE_BUILD_TYPE=Release \
+  -DABSL_BUILD_TESTING=ON \
+  -DBUILD_SHARED_LIBS="${build_shared_libs}"
+make -j $(nproc)
+ctest -j $(nproc) --output-on-failure
+make install
+ldconfig
+popd
+
+# Test the project against the installed Abseil
+mkdir -p "${project_build_dir}"
+pushd "${project_build_dir}"
+cmake "${project_dir}"
 cmake --build . --target simple
 
 output="$(${project_build_dir}/simple "printme" 2>&1)"
@@ -88,57 +79,35 @@ fi
 
 popd
 
-# Test that we haven't accidentally made absl::abslblah
-pushd "${install_dir}"
-
-# Starting in CMake 3.12 the default install dir is lib$bit_width
-if [[ -d lib64 ]]; then
-  libdir="lib64"
-elif [[ -d lib ]]; then
-  libdir="lib"
-else
-  echo "ls *, */*, */*/*:"
-  ls *
-  ls */*
-  ls */*/*
-  echo "unknown lib dir"
-fi
-
-if [[ "${lts_install}" ]]; then
-  # LTS versions append the date of the release to the subdir.
-  # 9999/99/99 is the dummy date used in the local_lts workflow.
-  absl_subdir="absl_99999999"
-else
-  absl_subdir="absl"
-fi
-
-if ! grep absl::strings "${libdir}/cmake/${absl_subdir}/abslTargets.cmake"; then
-  cat "${libdir}"/cmake/absl/abslTargets.cmake
+if ! grep absl::strings "/usr/local/lib/cmake/absl/abslTargets.cmake"; then
+  cat "/usr/local/lib/cmake/absl/abslTargets.cmake"
   echo "CMake targets named incorrectly"
   exit 1
 fi
 
-uninstall_absl
-popd
+pushd "${HOME}"
+cat > hello-abseil.cc << EOF
+#include <cstdlib>
 
-if [[ ! "${lts_install}" ]]; then
-  # Test that we warn if installed without a prefix or a system prefix
-  output="$(install_absl 2>&1)"
-  if [[ "${output}" != *"Please set CMAKE_INSTALL_PREFIX"* ]]; then
-    echo "Install without prefix didn't warn as expected. Output:"
-    echo "${output}"
-    exit 1
-  fi
-  uninstall_absl
-
-  output="$(install_absl /usr 2>&1)"
-  if [[ "${output}" != *"Please set CMAKE_INSTALL_PREFIX"* ]]; then
-    echo "Install with /usr didn't warn as expected. Output:"
-    echo "${output}"
-    exit 1
-  fi
-  uninstall_absl
+#include "absl/strings/str_format.h"
+
+int main(int argc, char **argv) {
+  absl::PrintF("Hello Abseil!\n");
+  return EXIT_SUCCESS;
+}
+EOF
+
+if [ "${LINK_TYPE:-}" != "DYNAMIC" ]; then
+  pc_args=($(pkg-config --cflags --libs --static absl_str_format))
+  g++ -static -o hello-abseil hello-abseil.cc "${pc_args[@]}"
+else
+  pc_args=($(pkg-config --cflags --libs absl_str_format))
+  g++ -o hello-abseil hello-abseil.cc "${pc_args[@]}"
 fi
+hello="$(./hello-abseil)"
+[[ "${hello}" == "Hello Abseil!" ]]
+
+popd
 
 echo "Install test complete!"
 exit 0
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 86f56344e74..3583e3dbe46 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -14,39 +14,62 @@
 # limitations under the License.
 #
 
-# Most widely used distributions have cmake 3.5 or greater available as of March
-# 2019.  A notable exception is RHEL-7 (CentOS7).  You can install a current
-# version of CMake by first installing Extra Packages for Enterprise Linux
-# (https://fedoraproject.org/wiki/EPEL#Extra_Packages_for_Enterprise_Linux_.28EPEL.29)
-# and then issuing `yum install cmake3` on the command line.
-cmake_minimum_required(VERSION 3.5)
+# https://github.com/google/oss-policies-info/blob/main/foundational-cxx-support-matrix.md
+# As of 2024-07-01, CMake 3.16 is the minimum supported version.
+cmake_minimum_required(VERSION 3.16)
 
-# Compiler id for Apple Clang is now AppleClang.
-cmake_policy(SET CMP0025 NEW)
+# Allow the user to specify the CMAKE_MSVC_DEBUG_INFORMATION_FORMAT
+if (POLICY CMP0141)
+  cmake_policy(SET CMP0141 NEW)
+endif (POLICY CMP0141)
 
-# if command can use IN_LIST
-cmake_policy(SET CMP0057 NEW)
+project(absl LANGUAGES CXX)
+set(ABSL_SOVERSION 0)
+include(CTest)
 
-# Project version variables are the empty std::string if version is unspecified
-cmake_policy(SET CMP0048 NEW)
-
-project(absl CXX)
+# Output directory is correct by default for most build setups. However, when
+# building Abseil as a DLL, it is important to have the DLL in the same
+# directory as the executable using it. Thus, we put all executables in a single
+# /bin directory.
+set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/bin)
 
 # when absl is included as subproject (i.e. using add_subdirectory(abseil-cpp))
 # in the source tree of a project that uses it, install rules are disabled.
-if(NOT "^${CMAKE_SOURCE_DIR}$" STREQUAL "^${PROJECT_SOURCE_DIR}$")
-  set(ABSL_ENABLE_INSTALL FALSE)
+if(NOT CMAKE_SOURCE_DIR STREQUAL PROJECT_SOURCE_DIR)
+  option(ABSL_ENABLE_INSTALL "Enable install rule" OFF)
 else()
-  set(ABSL_ENABLE_INSTALL TRUE)
+  option(ABSL_ENABLE_INSTALL "Enable install rule" ON)
 endif()
 
+set(CMAKE_INSTALL_RPATH "$ORIGIN")
+set(CMAKE_INSTALL_RPATH_USE_LINK_PATH ON)
+set(CMAKE_BUILD_RPATH_USE_ORIGIN ON)
+
+option(ABSL_PROPAGATE_CXX_STD
+  "Use CMake C++ standard meta features (e.g. cxx_std_17) that propagate to targets that link to Abseil"
+  ON)
+
+option(ABSL_USE_SYSTEM_INCLUDES
+  "Silence warnings in Abseil headers by marking them as SYSTEM includes"
+  OFF)
+
 list(APPEND CMAKE_MODULE_PATH
   ${CMAKE_CURRENT_LIST_DIR}/CMake
   ${CMAKE_CURRENT_LIST_DIR}/absl/copts
 )
 
-include(AbseilInstallDirs)
+option(ABSL_MSVC_STATIC_RUNTIME
+  "Link static runtime libraries"
+  OFF)
+if(ABSL_MSVC_STATIC_RUNTIME)
+  set(CMAKE_MSVC_RUNTIME_LIBRARY "MultiThreaded$<$<CONFIG:Debug>:Debug>")
+else()
+  set(CMAKE_MSVC_RUNTIME_LIBRARY "MultiThreaded$<$<CONFIG:Debug>:Debug>DLL")
+endif()
+
 include(CMakePackageConfigHelpers)
+include(GNUInstallDirs)
+include(AbseilDll)
 include(AbseilHelpers)
 
 
@@ -74,37 +97,74 @@ endif()
 ## pthread
 find_package(Threads REQUIRED)
 
-option(ABSL_USE_GOOGLETEST_HEAD
-  "If ON, abseil will download HEAD from googletest at config time." OFF)
+include(CMakeDependentOption)
 
-option(ABSL_RUN_TESTS "If ON, Abseil tests will be run." OFF)
+option(ABSL_BUILD_TESTING
+  "If ON, Abseil will build all of Abseil's own tests." OFF)
 
-if(${ABSL_RUN_TESTS})
-  # enable CTest.  This will set BUILD_TESTING to ON unless otherwise specified
-  # on the command line
-  include(CTest)
-  enable_testing()
-endif()
+option(ABSL_BUILD_TEST_HELPERS
+  "If ON, Abseil will build libraries that you can use to write tests against Abseil code. This option requires that Abseil is configured to use GoogleTest."
+  OFF)
 
-## check targets
-if(BUILD_TESTING)
+option(ABSL_USE_EXTERNAL_GOOGLETEST
+  "If ON, Abseil will assume that the targets for GoogleTest are already provided by the including project. This makes sense when Abseil is used with add_subdirectory." OFF)
 
-  if(${ABSL_USE_GOOGLETEST_HEAD})
-    include(CMake/Googletest/DownloadGTest.cmake)
-    set(absl_gtest_src_dir ${CMAKE_BINARY_DIR}/googletest-src)
-    set(absl_gtest_build_dir ${CMAKE_BINARY_DIR}/googletest-build)
-  endif()
+cmake_dependent_option(ABSL_FIND_GOOGLETEST
+  "If ON, Abseil will use find_package(GTest) rather than assuming that GoogleTest is already provided by the including project."
+  ON
+  "ABSL_USE_EXTERNAL_GOOGLETEST"
+  OFF)
 
-  check_target(gtest)
-  check_target(gtest_main)
-  check_target(gmock)
 
-  list(APPEND ABSL_TEST_COMMON_LIBRARIES
-    gtest_main
-    gtest
-    gmock
-    ${CMAKE_THREAD_LIBS_INIT}
+option(ABSL_USE_GOOGLETEST_HEAD
+  "If ON, abseil will download HEAD from GoogleTest at config time." OFF)
+
+set(ABSL_GOOGLETEST_DOWNLOAD_URL "" CACHE STRING "If set, download GoogleTest from this URL")
+
+set(ABSL_LOCAL_GOOGLETEST_DIR "/usr/src/googletest" CACHE PATH
+  "If ABSL_USE_GOOGLETEST_HEAD is OFF and ABSL_GOOGLETEST_URL is not set, specifies the directory of a local GoogleTest checkout."
   )
+
+option(ABSL_BUILD_MONOLITHIC_SHARED_LIBS
+  "Build Abseil as a single shared library (always enabled for Windows)"
+  OFF
+)
+if(NOT BUILD_SHARED_LIBS AND ABSL_BUILD_MONOLITHIC_SHARED_LIBS)
+  message(WARNING "Not building a shared library because BUILD_SHARED_LIBS is not set. Ignoring ABSL_BUILD_MONOLITHIC_SHARED_LIBS.")
+endif()
+
+if((BUILD_TESTING AND ABSL_BUILD_TESTING) OR ABSL_BUILD_TEST_HELPERS)
+  if (ABSL_USE_EXTERNAL_GOOGLETEST)
+    if (ABSL_FIND_GOOGLETEST)
+      find_package(GTest REQUIRED)
+    elseif(NOT TARGET GTest::gtest)
+      if(TARGET gtest)
+        # When Google Test is included directly rather than through find_package, the aliases are missing.
+        add_library(GTest::gtest ALIAS gtest)
+        add_library(GTest::gtest_main ALIAS gtest_main)
+        add_library(GTest::gmock ALIAS gmock)
+        add_library(GTest::gmock_main ALIAS gmock_main)
+      else()
+        message(FATAL_ERROR "ABSL_USE_EXTERNAL_GOOGLETEST is ON and ABSL_FIND_GOOGLETEST is OFF, which means that the top-level project must build the Google Test project. However, the target gtest was not found.")
+      endif()
+    endif()
+  else()
+    set(absl_gtest_build_dir ${CMAKE_BINARY_DIR}/googletest-build)
+    if(ABSL_USE_GOOGLETEST_HEAD AND ABSL_GOOGLETEST_DOWNLOAD_URL)
+      message(FATAL_ERROR "Do not set both ABSL_USE_GOOGLETEST_HEAD and ABSL_GOOGLETEST_DOWNLOAD_URL")
+    endif()
+    if(ABSL_USE_GOOGLETEST_HEAD)
+      set(absl_gtest_download_url "https://github.com/google/googletest/archive/main.zip")
+    elseif(ABSL_GOOGLETEST_DOWNLOAD_URL)
+      set(absl_gtest_download_url ${ABSL_GOOGLETEST_DOWNLOAD_URL})
+    endif()
+    if(absl_gtest_download_url)
+      set(absl_gtest_src_dir ${CMAKE_BINARY_DIR}/googletest-src)
+    else()
+      set(absl_gtest_src_dir ${ABSL_LOCAL_GOOGLETEST_DIR})
+    endif()
+    include(CMake/Googletest/DownloadGTest.cmake)
+  endif()
 endif()
 
 add_subdirectory(absl)
@@ -125,16 +185,16 @@ if(ABSL_ENABLE_INSTALL)
   # install as a subdirectory only
   install(EXPORT ${PROJECT_NAME}Targets
     NAMESPACE absl::
-    DESTINATION "${ABSL_INSTALL_CONFIGDIR}"
+    DESTINATION "${CMAKE_INSTALL_LIBDIR}/cmake/${PROJECT_NAME}"
   )
 
   configure_package_config_file(
     CMake/abslConfig.cmake.in
     "${PROJECT_BINARY_DIR}/${PROJECT_NAME}Config.cmake"
-    INSTALL_DESTINATION "${ABSL_INSTALL_CONFIGDIR}"
+    INSTALL_DESTINATION "${CMAKE_INSTALL_LIBDIR}/cmake/${PROJECT_NAME}"
   )
   install(FILES "${PROJECT_BINARY_DIR}/${PROJECT_NAME}Config.cmake"
-    DESTINATION "${ABSL_INSTALL_CONFIGDIR}"
+    DESTINATION "${CMAKE_INSTALL_LIBDIR}/cmake/${PROJECT_NAME}"
   )
 
   # Abseil only has a version in LTS releases.  This mechanism is accomplished
@@ -147,14 +207,74 @@ if(ABSL_ENABLE_INSTALL)
     )
 
     install(FILES "${PROJECT_BINARY_DIR}/${PROJECT_NAME}ConfigVersion.cmake"
-      DESTINATION ${ABSL_INSTALL_CONFIGDIR}
+      DESTINATION "${CMAKE_INSTALL_LIBDIR}/cmake/${PROJECT_NAME}"
     )
   endif()  # absl_VERSION
 
+  # Install the headers except for "options.h" which is installed separately.
   install(DIRECTORY absl
-    DESTINATION ${ABSL_INSTALL_INCLUDEDIR}
+    DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}
     FILES_MATCHING
       PATTERN "*.inc"
       PATTERN "*.h"
-  )
+      PATTERN "options.h" EXCLUDE
+      PATTERN "copts" EXCLUDE
+      PATTERN "testdata" EXCLUDE
+    )
+
+  # Rewrite options.h to use the compiled ABI.
+  file(READ "absl/base/options.h" ABSL_INTERNAL_OPTIONS_H_CONTENTS)
+
+  # Handle features that require at least C++20.
+  if (ABSL_INTERNAL_AT_LEAST_CXX20)
+    foreach(FEATURE "ORDERING" "SOURCE_LOCATION")
+      string(REPLACE
+      "#define ABSL_OPTION_USE_STD_${FEATURE} 2"
+      "#define ABSL_OPTION_USE_STD_${FEATURE} 1"
+      ABSL_INTERNAL_OPTIONS_H_PINNED
+      "${ABSL_INTERNAL_OPTIONS_H_CONTENTS}")
+      set(ABSL_INTERNAL_OPTIONS_H_CONTENTS "${ABSL_INTERNAL_OPTIONS_H_PINNED}")
+    endforeach()
+  endif()
+
+  # Handle features that require at least C++17.
+  if (ABSL_INTERNAL_AT_LEAST_CXX17)
+    foreach(FEATURE "ANY" "OPTIONAL" "STRING_VIEW" "VARIANT")
+      string(REPLACE
+      "#define ABSL_OPTION_USE_STD_${FEATURE} 2"
+      "#define ABSL_OPTION_USE_STD_${FEATURE} 1"
+      ABSL_INTERNAL_OPTIONS_H_PINNED
+      "${ABSL_INTERNAL_OPTIONS_H_CONTENTS}")
+      set(ABSL_INTERNAL_OPTIONS_H_CONTENTS "${ABSL_INTERNAL_OPTIONS_H_PINNED}")
+    endforeach()
+  endif()
+
+  # Any feature that still has the value of 2 (because it was not handled above)
+  # should be set to 0.
+  string(REGEX REPLACE
+    "#define ABSL_OPTION_USE_STD_([^ ]*) 2"
+    "#define ABSL_OPTION_USE_STD_\\1 0"
+    ABSL_INTERNAL_OPTIONS_H_PINNED
+    "${ABSL_INTERNAL_OPTIONS_H_CONTENTS}")
+
+  # If the file already exists, check if it matches the new contents.
+  # This avoids writing the file if it is already up-to-date when the CMake
+  # generation is triggered and triggering unnecessary rebuilds.
+  set(ABSL_INTERNAL_OPTIONS_H_PINNED_NEEDS_UPDATE TRUE)
+  if (EXISTS "${CMAKE_BINARY_DIR}/options-pinned.h")
+    file(READ "${CMAKE_BINARY_DIR}/options-pinned.h" ABSL_INTERNAL_OPTIONS_PINNED_H_CONTENTS)
+    if ("${ABSL_INTERNAL_OPTIONS_H_PINNED}" STREQUAL "${ABSL_INTERNAL_OPTIONS_PINNED_H_CONTENTS}")
+      set(ABSL_INTERNAL_OPTIONS_H_PINNED_NEEDS_UPDATE FALSE)
+    endif()
+  endif()
+
+  # If the file needs an update, generate it.
+  if (ABSL_INTERNAL_OPTIONS_H_PINNED_NEEDS_UPDATE)
+    file(GENERATE OUTPUT "${CMAKE_BINARY_DIR}/options-pinned.h" CONTENT "${ABSL_INTERNAL_OPTIONS_H_PINNED}")
+  endif()
+
+  install(FILES "${CMAKE_BINARY_DIR}/options-pinned.h"
+         DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/absl/base
+         RENAME "options.h")
+
 endif()  # ABSL_ENABLE_INSTALL
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index f4cb4a29ed0..a87254c5035 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -75,9 +75,9 @@ will be expected to conform to the style outlined
 
 ## Guidelines for Pull Requests
 
-*   If you are a Googler, it is preferable to first create an internal CL and
-    have it reviewed and submitted. The code propagation process will deliver
-    the change to GitHub.
+*   If you are a Googler, it is required that you send us a Piper CL instead of
+    using the GitHub pull-request process. The code propagation process will
+    deliver the change to GitHub.
 
 *   Create **small PRs** that are narrowly focused on **addressing a single
     concern**. We often receive PRs that are trying to fix several things at a
@@ -123,10 +123,13 @@ will be expected to conform to the style outlined
 
 ## Running Tests
 
-Use "bazel test <>" functionality to run the unit tests.
+If you have [Bazel](https://bazel.build/) installed, use `bazel test
+--test_tag_filters="-benchmark" ...` to run the unit tests.
 
-Prerequisites for building and running tests are listed in
-[README.md](README.md)
+If you are running the Linux operating system and have
+[Docker](https://www.docker.com/) installed, you can also run the `linux_*.sh`
+scripts under the `ci/`(https://github.com/abseil/abseil-cpp/tree/master/ci)
+directory to test Abseil under a variety of conditions.
 
 ## Abseil Committers
 
diff --git a/FAQ.md b/FAQ.md
new file mode 100644
index 00000000000..30291fe4f18
--- /dev/null
+++ b/FAQ.md
@@ -0,0 +1,218 @@
+# Abseil FAQ
+
+## Is Abseil the right home for my utility library?
+
+Most often the answer to the question is "no." As both the
+[About Abseil](https://abseil.io/about/) page and our
+[contributing guidelines](https://github.com/abseil/abseil-cpp/blob/master/CONTRIBUTING.md#contribution-guidelines)
+explain, Abseil contains a variety of core C++ library code that is widely used
+at [Google](https://www.google.com/). As such, Abseil's primary purpose is to be
+used as a dependency by Google's open source C++ projects. While we do hope that
+Abseil is also useful to the C++ community at large, this added constraint also
+means that we are unlikely to accept a contribution of utility code that isn't
+already widely used by Google.
+
+## How do I set the C++ dialect used to build Abseil?
+
+The short answer is that whatever mechanism you choose, you need to make sure
+that you set this option consistently at the global level for your entire
+project. If, for example, you want to set the C++ dialect to C++17, with
+[Bazel](https://bazel.build/) as the build system and `gcc` or `clang` as the
+compiler, there are several ways to do this: * Pass `--cxxopt=-std=c++17` on the
+command line (for example, `bazel build --cxxopt=-std=c++17 ...`) * Set the
+environment variable `BAZEL_CXXOPTS` (for example, `BAZEL_CXXOPTS=-std=c++17`) *
+Add `build --cxxopt=-std=c++17` to your
+[`.bazelrc` file](https://docs.bazel.build/versions/master/guide.html#bazelrc)
+
+If you are using CMake as the build system, you'll need to add a line like
+`set(CMAKE_CXX_STANDARD 17)` to your top level `CMakeLists.txt` file. If you are
+developing a library designed to be used by other clients, you should instead
+leave `CMAKE_CXX_STANDARD` unset and configure the minimum C++ standard required
+by each of your library targets via `target_compile_features`. See the
+[CMake build instructions](https://github.com/abseil/abseil-cpp/blob/master/CMake/README.md)
+for more information.
+
+For a longer answer to this question and to understand why some other approaches
+don't work, see the answer to ["What is ABI and why don't you recommend using a
+pre-compiled version of
+Abseil?"](#what-is-abi-and-why-dont-you-recommend-using-a-pre-compiled-version-of-abseil)
+
+## What is ABI and why don't you recommend using a pre-compiled version of Abseil?
+
+For the purposes of this discussion,
+[ABI](https://en.wikipedia.org/wiki/Application_binary_interface) refers to the
+compiled representation of code interfaces. This contrasts with
+[API](https://en.wikipedia.org/wiki/Application_programming_interface), which
+refers to the interfaces defined in the source code. [Abseil has a strong
+promise of API compatibility, but does not make any promise of ABI
+compatibility](https://abseil.io/about/compatibility). Let's take a look at what
+this means in practice.
+
+You might be tempted to do something like this in a
+[Bazel](https://bazel.build/) `BUILD` file:
+
+```
+# DON'T DO THIS!!!
+cc_library(
+    name = "my_library",
+    srcs = ["my_library.cc"],
+    copts = ["-std=c++17"],  # May create a mixed-mode compile!
+    deps = ["@com_google_absl//absl/strings"],
+)
+```
+
+Applying `-std=c++17` to an individual target in your `BUILD` file is going to
+compile that specific target in C++17 mode, but it isn't going to ensure the
+Abseil library is built in C++17 mode, since the Abseil library itself is a
+different build target. If your code includes an Abseil header, then your
+program may contain conflicting definitions of the same
+class/function/variable/enum, etc. As a rule, all compile options that affect
+the ABI of a program need to be applied to the entire build on a global basis.
+
+C++ has something called the
+[One Definition Rule](https://en.wikipedia.org/wiki/One_Definition_Rule) (ODR).
+C++ doesn't allow multiple definitions of the same class/function/variable/enum,
+etc. ODR violations sometimes result in linker errors, but linkers do not always
+catch violations. Uncaught ODR violations can result in strange runtime
+behaviors or crashes that can be hard to debug.
+
+If you build the Abseil library and your code using different compile options
+that affect ABI, there is a good chance you will run afoul of the One Definition
+Rule. Examples of GCC compile options that affect ABI include (but aren't
+limited to) language dialect (e.g. `-std=`), optimization level (e.g. `-O2`),
+code generation flags (e.g. `-fexceptions`), and preprocessor defines (e.g.
+`-DNDEBUG`).
+
+If you use a pre-compiled version of Abseil, (for example, from your Linux
+distribution package manager or from something like
+[vcpkg](https://github.com/microsoft/vcpkg)) you have to be very careful to
+ensure ABI compatibility across the components of your program. The only way you
+can be sure your program is going to be correct regarding ABI is to ensure
+you've used the exact same compile options as were used to build the
+pre-compiled library. This does not mean that Abseil cannot work as part of a
+Linux distribution since a knowledgeable binary packager will have ensured that
+all packages have been built with consistent compile options. This is one of the
+reasons we warn against - though do not outright reject - using Abseil as a
+pre-compiled library.
+
+Another possible way that you might run afoul of ABI issues is if you
+accidentally include two versions of Abseil in your program. Multiple versions
+of Abseil can end up within the same binary if your program uses the Abseil
+library and another library also transitively depends on Abseil (resulting in
+what is sometimes called the diamond dependency problem). In cases such as this
+you must structure your build so that all libraries use the same version of
+Abseil.
+[Abseil's strong promise of API compatibility between releases](https://abseil.io/about/compatibility)
+means the latest "HEAD" release of Abseil is almost certainly the right choice
+if you are doing as we recommend and building all of your code from source.
+
+For these reasons we recommend you avoid pre-compiled code and build the Abseil
+library yourself in a consistent manner with the rest of your code.
+
+## What is "live at head" and how do I do it?
+
+From Abseil's point-of-view, "live at head" means that every Abseil source
+release (which happens on an almost daily basis) is either API compatible with
+the previous release, or comes with an automated tool that you can run over code
+to make it compatible. In practice, the need to use an automated tool is
+extremely rare. This means that upgrading from one source release to another
+should be a routine practice that can and should be performed often.
+
+We recommend you update to the
+[latest commit in the `master` branch of Abseil](https://github.com/abseil/abseil-cpp/commits/master)
+as often as possible. Not only will you pick up bug fixes more quickly, but if
+you have good automated testing, you will catch and be able to fix any
+[Hyrum's Law](https://www.hyrumslaw.com/) dependency problems on an incremental
+basis instead of being overwhelmed by them and having difficulty isolating them
+if you wait longer between updates.
+
+If you are using the [Bazel](https://bazel.build/) build system with
+[Bzlmod](https://bazel.build/external/overview#bzlmod), you can use a
+`git_override` in your `MODULE.bazel` file to track the latest commit.
+
+For example, to update to the latest commit, you would add (or update) the
+following snippet in your `MODULE.bazel` file:
+
+```starlark
+bazel_dep(name = "abseil-cpp", version = "20260107.1")
+
+git_override(
+    module_name = "abseil-cpp",
+    remote = "https://github.com/abseil/abseil-cpp.git",
+    # Replace the following line with the latest commit.
+    commit = "6ec9964c325db0610a376b3cb81de073ea6ada90",
+)
+```
+
+You can commit the updated `MODULE.bazel` file to your source control every time
+you update, and if you have good automated testing, you might even consider
+automating this.
+
+## Why do I see strange behaviors when I use Abseil hash tables?
+
+Abseil's hash function uses a random seed.
+
+Many programmers believe incorrectly that this is a defense against
+[hash flooding](https://en.wikipedia.org/wiki/Collision_attack#Hash_flooding).
+While it does make a hash flooding attack more difficult, Abseil's hash function
+prioritizes speed over thorough mixing (and thus is not cryptographically
+secure), and the current seed implementation is also not cryptographically
+secure. If you are storing a large amount of attacker-controlled data, the most
+reliable defense against hash-flooding is to use a container that does not have
+`O(n)` worst-case behavior.
+
+The real reason for hash randomization is to prevent
+[Hyrum's Law](https://www.hyrumslaw.com/) dependencies on iteration order. This
+has allowed us to roll out steady improvements to the implementation without
+breaking users who may have otherwise written code that was dependent on
+ordering or other characteristics.
+
+The current implementation uses a global seed, which, if linked incorrectly
+(e.g., static Abseil in multiple DSOs), can cause the ODR violations. If more
+than one seed is linked, different calls to the hash function may return
+different values, rendering hash elements inaccessible, causing crashes, or
+other arbitrarily bad behaviors.
+
+We are often asked for a knob to disable hash randomization. The answer is a
+hard "no", even under test or under a flag, because people will find a way to
+force it and allow their code or tests to depend on it. At Google-scale, the
+compute costs that are saved by preserving the ability to improve the
+implementation far outweigh the inconvenience of learning how to write code
+resilient to change.
+
+## How do I use the LLVM Sanitizers with Abseil?
+
+LLVM Sanitizers are a suite of powerful, dynamic analysis tools that
+automatically detect various critical bugs during program execution. They work
+by instrumenting the compiled binary and linking a runtime library to intercept
+operations and report issues.
+
+We receive many incorrect bug reports from users trying to use the sanitizers.
+The most common cause of these issues is ODR violations in the form of an
+instrumentation mismatch. This happens when users try to link libraries compiled
+with sanitizer instrumentation with uninstrumented libraries. It is important
+that all code in the application is built with the same sanitizer configuration.
+
+The easiest way to do this is to use [Bazel](https://bazel.build) and pass the
+sanitizer options on the commandline, but it is important not to overlook the
+importance of avoiding precompiled system libraries, including the C++ standard
+library. For instance,
+[MemorySanitizer requires an instrumented `libc++`](https://github.com/google/sanitizers/wiki/MemorySanitizerLibcxxHowTo).
+
+Since most users are not going to build `libc++` with Bazel, here is a
+MemorySanitizer recipe that currently works (and could easily be tweaked for
+ThreadSanitizer and friends):
+
+```shell
+# From the root of the LLVM source tree, configure libc++ to be instrumented with MSAN:
+cmake -G Ninja -S runtimes -B build_msan -DLLVM_ENABLE_RUNTIMES="libcxx;libcxxabi;libunwind" -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX="${HOME}/llvm-msan" -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ -DLLVM_USE_SANITIZER=MemoryWithOrigins -DLLVM_TARGETS_TO_BUILD="host"
+
+# Build and install it into ${HOME}/llvm-msan
+ninja -C build_msan install-cxx install-cxxabi install-unwind
+
+# Then build (or test) your code like this:
+bazel test --repo_env=CC=clang --repo_env=BAZEL_CXXOPTS=nostdinc++ --repo_env=BAZEL_LINKOPTS=-L${HOME}/llvm-msan/lib:-lc++:-lc++abi:-lgcc_s:-lm:-Wl,-rpath=${HOME}/llvm-msan/lib --repo_env=CPLUS_INCLUDE_PATH=${HOME}/llvm-msan/include/c++/v1 --copt=-fsanitize=memory --linkopt=-fsanitize=memory --linkopt=-fsanitize-link-c++-runtime ...
+```
+
+You should consider adding these options to a
+[`.bazelrc`](https://bazel.build/run/bazelrc) file to avoid retyping them.
diff --git a/LTS.md b/LTS.md
deleted file mode 100644
index 94363b6a36f..00000000000
--- a/LTS.md
+++ /dev/null
@@ -1,15 +0,0 @@
-# Long Term Support (LTS) Branches
-
-This repository contains periodic snapshots of the Abseil codebase that are
-Long Term Support (LTS) branches. An LTS branch allows you to use a known
-version of Abseil without interfering with other projects which may also, in
-turn, use Abseil. (For more information about our releases, see the
-[Abseil Release Management](https://abseil.io/about/releases) guide.)
-
-## LTS Branches
-
-The following lists LTS branches and the dates on which they have been released:
-
-* [LTS Branch December 18, 2018](https://github.com/abseil/abseil-cpp/tree/lts_2018_12_18/)
-* [LTS Branch June 20, 2018](https://github.com/abseil/abseil-cpp/tree/lts_2018_06_20/)
-* [LTS Branch August 8, 2019](https://github.com/abseil/abseil-cpp/tree/lts_2019_08_08/)
diff --git a/MODULE.bazel b/MODULE.bazel
new file mode 100644
index 00000000000..00ab4c35dda
--- /dev/null
+++ b/MODULE.bazel
@@ -0,0 +1,52 @@
+# Copyright 2024 The Abseil Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# https://bazel.build/external/overview#bzlmod
+
+module(
+    name = "abseil-cpp",
+    version = "head",
+)
+
+cc_configure = use_extension(
+    "@rules_cc//cc:extensions.bzl",
+    "cc_configure_extension",
+    dev_dependency = True,
+)
+use_repo(cc_configure, "local_config_cc")
+
+bazel_dep(name = "rules_cc", version = "0.2.22")
+bazel_dep(name = "bazel_skylib", version = "1.9.2")
+bazel_dep(name = "platforms", version = "1.1.0")
+
+bazel_dep(
+    name = "google_benchmark",
+    version = "1.9.5",
+    dev_dependency = True,
+)
+
+# Note: Googletest is NOT a dev_dependency. Some Abseil test utilities
+# intended to be used by Abseil users depend on GoogleTest.
+bazel_dep(
+    name = "googletest",
+    version = "1.18.0",
+)
+
+# Note: Gloop is NOT a dev_dependency, but should never be used directly.  It is only included here
+# so that we can give visibility into Abseil internals.
+bazel_dep(
+    name = "gloop",
+    version = "20260708.rc1",
+    repo_name = "do_not_use_for_gloop_visibility_only",
+)
diff --git a/PrivacyInfo.xcprivacy b/PrivacyInfo.xcprivacy
new file mode 100644
index 00000000000..3ff4a9d98b1
--- /dev/null
+++ b/PrivacyInfo.xcprivacy
@@ -0,0 +1,14 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+  <key>NSPrivacyTracking</key>
+  <false/>
+  <key>NSPrivacyCollectedDataTypes</key>
+  <array/>
+  <key>NSPrivacyTrackingDomains</key>
+  <array/>
+  <key>NSPrivacyAccessedAPITypes</key>
+  <array/>
+</dict>
+</plist>
diff --git a/README.md b/README.md
index 85de569658c..c2c851ec35b 100644
--- a/README.md
+++ b/README.md
@@ -1,7 +1,7 @@
 # Abseil - C++ Common Libraries
 
 The repository contains the Abseil C++ library code. Abseil is an open-source
-collection of C++ code (compliant to C++11) designed to augment the C++
+collection of C++ code (compliant to C++17) designed to augment the C++
 standard library.
 
 ## Table of Contents
@@ -9,7 +9,9 @@ standard library.
 - [About Abseil](#about)
 - [Quickstart](#quickstart)
 - [Building Abseil](#build)
+- [Support](#support)
 - [Codemap](#codemap)
+- [Releases](#releases)
 - [License](#license)
 - [Links](#links)
 
@@ -42,47 +44,78 @@ the Abseil code, running tests, and getting a simple binary working.
 <a name="build"></a>
 ## Building Abseil
 
-[Bazel](https://bazel.build) is the official build system for Abseil,
-which is supported on most major platforms (Linux, Windows, macOS, for example)
-and compilers. See the [quickstart](https://abseil.io/docs/cpp/quickstart) for
-more information on building Abseil using the Bazel build system.
-
-<a name="cmake"></a>
-If you require CMake support, please check the
-[CMake build instructions](CMake/README.md).
-
+[Bazel](https://bazel.build) and [CMake](https://cmake.org/) are the official
+build systems for Abseil.
+See the [quickstart](https://abseil.io/docs/cpp/quickstart) for more information
+on building Abseil using the Bazel build system.
+If you require CMake support, please check the [CMake build
+instructions](CMake/README.md) and [CMake
+Quickstart](https://abseil.io/docs/cpp/quickstart-cmake).
+
+<a name="support"></a>
+## Support
+
+Abseil follows Google's [Foundational C++ Support
+Policy](https://opensource.google/documentation/policies/cplusplus-support). See
+[this
+table](https://github.com/google/oss-policies-info/blob/main/foundational-cxx-support-matrix.md)
+for a list of currently supported versions compilers, platforms, and build
+tools.
+
+<a name="codemap"></a>
 ## Codemap
 
 Abseil contains the following C++ library components:
 
-* [`base`](absl/base/) Abseil Fundamentals
+* [`base`](absl/base/)
   <br /> The `base` library contains initialization code and other code which
   all other Abseil code depends on. Code within `base` may not depend on any
   other code (other than the C++ standard library).
 * [`algorithm`](absl/algorithm/)
   <br /> The `algorithm` library contains additions to the C++ `<algorithm>`
   library and container-based versions of such algorithms.
+* [`cleanup`](absl/cleanup/)
+  <br /> The `cleanup` library contains the control-flow-construct-like type
+  `absl::Cleanup` which is used for executing a callback on scope exit.
 * [`container`](absl/container/)
   <br /> The `container` library contains additional STL-style containers,
   including Abseil's unordered "Swiss table" containers.
+* [`crc`](absl/crc/) The `crc` library contains code for
+  computing error-detecting cyclic redundancy checks on data.
 * [`debugging`](absl/debugging/)
   <br /> The `debugging` library contains code useful for enabling leak
   checks, and stacktrace and symbolization utilities.
+* [`flags`](absl/flags/)
+  <br /> The `flags` library contains code for handling command line flags for
+  libraries and binaries built with Abseil.
 * [`hash`](absl/hash/)
   <br /> The `hash` library contains the hashing framework and default hash
   functor implementations for hashable types in Abseil.
+* [`log`](absl/log/)
+  <br /> The `log` library contains `LOG` and `CHECK` macros and facilities
+  for writing logged messages out to disk, `stderr`, or user-extensible
+  destinations.
 * [`memory`](absl/memory/)
-  <br /> The `memory` library contains C++11-compatible versions of
-  `std::make_unique()` and related memory management facilities.
+  <br /> The `memory` library contains memory management facilities that augment
+  C++'s `<memory>` library.
 * [`meta`](absl/meta/)
-  <br /> The `meta` library contains C++11-compatible versions of type checks
-  available within C++14 and C++17 versions of the C++ `<type_traits>` library.
+  <br /> The `meta` library contains type checks
+  similar to those available in the C++ `<type_traits>` library.
 * [`numeric`](absl/numeric/)
-  <br /> The `numeric` library contains C++11-compatible 128-bit integers.
+  <br /> The `numeric` library contains 128-bit integer types as well as
+  implementations of C++20's bitwise math functions.
+* [`profiling`](absl/profiling/)
+  <br /> The `profiling` library contains utility code for profiling C++
+  entities.  It is currently a private dependency of other Abseil libraries.
+* [`random`](absl/random/)
+  <br /> The `random` library contains functions for generating pseudorandom
+  values.
+* [`status`](absl/status/)
+  <br /> The `status` library contains abstractions for error handling,
+  specifically `absl::Status` and `absl::StatusOr<T>`.
 * [`strings`](absl/strings/)
   <br /> The `strings` library contains a variety of strings routines and
-  utilities, including a C++11-compatible version of the C++17
-  `std::string_view` type.
+  utilities.
 * [`synchronization`](absl/synchronization/)
   <br /> The `synchronization` library contains concurrency primitives (Abseil's
   `absl::Mutex` class, an alternative to `std::mutex`) and a variety of
@@ -92,16 +125,27 @@ Abseil contains the following C++ library components:
   points in time, durations of time, and formatting and parsing time within
   time zones.
 * [`types`](absl/types/)
-  <br /> The `types` library contains non-container utility types, like a
-  C++11-compatible version of the C++17 `std::optional` type.
+  <br /> The `types` library contains non-container utility types.
 * [`utility`](absl/utility/)
   <br /> The `utility` library contains utility and helper code.
 
+<a name="releases"></a>
+## Releases
+
+Abseil recommends users "live-at-head" (update to the latest commit from the
+master branch as often as possible). However, we realize this philosophy doesn't
+work for every project, so we also provide [Long Term Support
+Releases](https://github.com/abseil/abseil-cpp/releases) to which we backport
+fixes for severe bugs. See our [release
+management](https://abseil.io/about/releases) document for more details.
+
+<a name="license"></a>
 ## License
 
 The Abseil C++ library is licensed under the terms of the Apache
 license. See [LICENSE](LICENSE) for more information.
 
+<a name="links"></a>
 ## Links
 
 For more information about Abseil:
diff --git a/UPGRADES.md b/UPGRADES.md
index 35599d0878d..3cac141d0f4 100644
--- a/UPGRADES.md
+++ b/UPGRADES.md
@@ -1,6 +1,6 @@
 # C++ Upgrade Tools
 
-Abseil may occassionally release API-breaking changes. As noted in our
+Abseil may occasionally release API-breaking changes. As noted in our
 [Compatibility Guidelines][compatibility-guide], we will aim to provide a tool
 to do the work of effecting such API-breaking changes, when absolutely
 necessary.
diff --git a/WORKSPACE b/WORKSPACE
deleted file mode 100644
index 572f5b1f7fa..00000000000
--- a/WORKSPACE
+++ /dev/null
@@ -1,45 +0,0 @@
-#
-# Copyright 2019 The Abseil Authors.
-#
-# Licensed under the Apache License, Version 2.0 (the "License");
-# you may not use this file except in compliance with the License.
-# You may obtain a copy of the License at
-#
-#      https://www.apache.org/licenses/LICENSE-2.0
-#
-# Unless required by applicable law or agreed to in writing, software
-# distributed under the License is distributed on an "AS IS" BASIS,
-# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-# See the License for the specific language governing permissions and
-# limitations under the License.
-#
-
-workspace(name = "com_google_absl")
-load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
-
-# GoogleTest/GoogleMock framework. Used by most unit-tests.
-http_archive(
-     name = "com_google_googletest",
-     urls = ["https://github.com/google/googletest/archive/b6cd405286ed8635ece71c72f118e659f4ade3fb.zip"],  # 2019-01-07
-     strip_prefix = "googletest-b6cd405286ed8635ece71c72f118e659f4ade3fb",
-     sha256 = "ff7a82736e158c077e76188232eac77913a15dac0b22508c390ab3f88e6d6d86",
-)
-
-# Google benchmark.
-http_archive(
-    name = "com_github_google_benchmark",
-    urls = ["https://github.com/google/benchmark/archive/16703ff83c1ae6d53e5155df3bb3ab0bc96083be.zip"],
-    strip_prefix = "benchmark-16703ff83c1ae6d53e5155df3bb3ab0bc96083be",
-    sha256 = "59f918c8ccd4d74b6ac43484467b500f1d64b40cc1010daa055375b322a43ba3",
-)
-
-# C++ rules for Bazel.
-http_archive(
-    name = "rules_cc",
-    sha256 = "67412176974bfce3f4cf8bdaff39784a72ed709fc58def599d1f68710b58d68b",
-    strip_prefix = "rules_cc-b7fe9697c0c76ab2fd431a891dbb9a6a32ed7c3e",
-    urls = [
-        "https://mirror.bazel.build/github.com/bazelbuild/rules_cc/archive/b7fe9697c0c76ab2fd431a891dbb9a6a32ed7c3e.zip",
-        "https://github.com/bazelbuild/rules_cc/archive/b7fe9697c0c76ab2fd431a891dbb9a6a32ed7c3e.zip",
-    ],
-)
diff --git a/absl/BUILD.bazel b/absl/BUILD.bazel
index 853330d447e..980563a9f12 100644
--- a/absl/BUILD.bazel
+++ b/absl/BUILD.bazel
@@ -14,43 +14,49 @@
 # limitations under the License.
 #
 
-package(default_visibility = ["//visibility:public"])
+load("@bazel_skylib//lib:selects.bzl", "selects")
 
-licenses(["notice"])  # Apache 2.0
+package(default_visibility = ["//visibility:private"])
 
-load(":compiler_config_setting.bzl", "create_llvm_config")
+licenses(["notice"])
 
-create_llvm_config(
-    name = "llvm_compiler",
+config_setting(
+    name = "clang_compiler",
+    flag_values = {
+        "@bazel_tools//tools/cpp:compiler": "clang",
+    },
     visibility = [":__subpackages__"],
 )
 
 config_setting(
-    name = "osx",
-    constraint_values = [
-        "@bazel_tools//platforms:osx",
-    ],
+    name = "mingw_unspecified_compiler",
+    flag_values = {
+        "@bazel_tools//tools/cpp:compiler": "mingw",
+    },
+    visibility = [":__subpackages__"],
 )
 
 config_setting(
-    name = "ios",
-    constraint_values = [
-        "@bazel_tools//platforms:ios",
-    ],
+    name = "mingw-gcc_compiler",
+    flag_values = {
+        "@bazel_tools//tools/cpp:compiler": "mingw-gcc",
+    },
+    visibility = [":__subpackages__"],
 )
 
 config_setting(
-    name = "windows",
-    values = {
-        "cpu": "x64_windows",
-    },
+    name = "fuchsia",
+    constraint_values = [
+        "@platforms//os:fuchsia",
+    ],
     visibility = [":__subpackages__"],
 )
 
-config_setting(
-    name = "ppc",
-    values = {
-        "cpu": "ppc",
-    },
+selects.config_setting_group(
+    name = "mingw_compiler",
+    match_any = [
+        ":mingw_unspecified_compiler",
+        ":mingw-gcc_compiler",
+    ],
     visibility = [":__subpackages__"],
 )
diff --git a/absl/CMakeLists.txt b/absl/CMakeLists.txt
index 3e78397c976..810d7f31b9a 100644
--- a/absl/CMakeLists.txt
+++ b/absl/CMakeLists.txt
@@ -14,20 +14,31 @@
 # limitations under the License.
 #
 
-
-
 add_subdirectory(base)
 add_subdirectory(algorithm)
+add_subdirectory(cleanup)
 add_subdirectory(container)
+add_subdirectory(crc)
 add_subdirectory(debugging)
 add_subdirectory(flags)
+add_subdirectory(functional)
 add_subdirectory(hash)
+add_subdirectory(log)
 add_subdirectory(memory)
 add_subdirectory(meta)
 add_subdirectory(numeric)
+add_subdirectory(profiling)
 add_subdirectory(random)
+add_subdirectory(status)
 add_subdirectory(strings)
 add_subdirectory(synchronization)
 add_subdirectory(time)
 add_subdirectory(types)
 add_subdirectory(utility)
+
+if (ABSL_BUILD_DLL)
+  absl_make_dll()
+  if ((BUILD_TESTING AND ABSL_BUILD_TESTING) OR ABSL_BUILD_TEST_HELPERS)
+    absl_make_dll(TEST ON)
+  endif()
+endif()
diff --git a/absl/abseil.podspec.gen.py b/absl/abseil.podspec.gen.py
new file mode 100755
index 00000000000..e19f951193c
--- /dev/null
+++ b/absl/abseil.podspec.gen.py
@@ -0,0 +1,245 @@
+#!/usr/bin/env python3
+# -*- coding: utf-8 -*-
+"""This script generates abseil.podspec from all BUILD.bazel files.
+
+This is expected to run on abseil git repository with Bazel 1.0 on Linux.
+It recursively analyzes BUILD.bazel files using query command of Bazel to
+dump its build rules in XML format. From these rules, it constructs podspec
+structure.
+"""
+
+import argparse
+import collections
+import os
+import re
+import subprocess
+import xml.etree.ElementTree
+
+# Template of root podspec.
+SPEC_TEMPLATE = """
+# This file has been automatically generated from a script.
+# Please make modifications to `abseil.podspec.gen.py` instead.
+Pod::Spec.new do |s|
+  s.name     = 'abseil'
+  s.version  = '${version}'
+  s.summary  = 'Abseil Common Libraries (C++) from Google'
+  s.homepage = 'https://abseil.io'
+  s.license  = 'Apache License, Version 2.0'
+  s.authors  = { 'Abseil Team' => 'abseil-io@googlegroups.com' }
+  s.source = {
+    :git => 'https://github.com/abseil/abseil-cpp.git',
+    :tag => '${tag}',
+  }
+  s.resource_bundles = {
+    s.module_name => 'PrivacyInfo.xcprivacy',
+  }
+  s.module_name = 'absl'
+  s.header_mappings_dir = 'absl'
+  s.header_dir = 'absl'
+  s.libraries = 'c++'
+  s.compiler_flags = '-Wno-everything'
+  s.pod_target_xcconfig = {
+    'USER_HEADER_SEARCH_PATHS' => '$(inherited) "$(PODS_TARGET_SRCROOT)"',
+    'USE_HEADERMAP' => 'NO',
+    'ALWAYS_SEARCH_USER_PATHS' => 'NO',
+    'CLANG_CXX_LANGUAGE_STANDARD' => 'c++17',
+  }
+  s.ios.deployment_target = '12.0'
+  s.osx.deployment_target = '10.13'
+  s.tvos.deployment_target = '12.0'
+  s.watchos.deployment_target = '4.0'
+  s.visionos.deployment_target = '1.0'
+  s.subspec 'xcprivacy' do |ss|
+    ss.resource_bundles = {
+      ss.module_name => 'PrivacyInfo.xcprivacy',
+    }
+  end
+"""
+
+# Rule object representing the rule of Bazel BUILD.
+Rule = collections.namedtuple(
+    "Rule", "type name package srcs hdrs textual_hdrs deps visibility testonly")
+
+
+def get_elem_value(elem, name):
+  """Returns the value of XML element with the given name."""
+  for child in elem:
+    if child.attrib.get("name") != name:
+      continue
+    if child.tag == "string":
+      return child.attrib.get("value")
+    if child.tag == "boolean":
+      return child.attrib.get("value") == "true"
+    if child.tag == "list":
+      return [nested_child.attrib.get("value") for nested_child in child]
+    raise "Cannot recognize tag: " + child.tag
+  return None
+
+
+def normalize_paths(paths):
+  """Returns the list of normalized path."""
+  # e.g. ["//absl/strings:dir/header.h"] -> ["absl/strings/dir/header.h"]
+  return [path.lstrip("/").replace(":", "/") for path in paths]
+
+
+def parse_rule(elem, package):
+  """Returns a rule from bazel XML rule."""
+  return Rule(
+      type=elem.attrib["class"],
+      name=get_elem_value(elem, "name"),
+      package=package,
+      srcs=normalize_paths(get_elem_value(elem, "srcs") or []),
+      hdrs=normalize_paths(get_elem_value(elem, "hdrs") or []),
+      textual_hdrs=normalize_paths(get_elem_value(elem, "textual_hdrs") or []),
+      deps=get_elem_value(elem, "deps") or [],
+      visibility=get_elem_value(elem, "visibility") or [],
+      testonly=get_elem_value(elem, "testonly") or False)
+
+
+def read_build(package):
+  """Runs bazel query on given package file and returns all cc rules."""
+  result = subprocess.check_output(
+      ["bazel", "query", package + ":all", "--output", "xml"])
+  root = xml.etree.ElementTree.fromstring(result)
+  return [
+      parse_rule(elem, package)
+      for elem in root
+      if elem.tag == "rule" and elem.attrib["class"].startswith("cc_")
+  ]
+
+
+def collect_rules(root_path):
+  """Collects and returns all rules from root path recursively."""
+  rules = []
+  for cur, _, _ in os.walk(root_path):
+    build_path = os.path.join(cur, "BUILD.bazel")
+    if os.path.exists(build_path):
+      rules.extend(read_build("//" + cur))
+  return rules
+
+
+def relevant_rule(rule):
+  """Returns true if a given rule is relevant when generating a podspec."""
+  return (
+      # cc_library only (ignore cc_test, cc_binary)
+      rule.type == "cc_library" and
+      # ignore empty rule
+      (rule.hdrs + rule.textual_hdrs + rule.srcs) and
+      # ignore test-only rule
+      not rule.testonly)
+
+
+def get_spec_var(depth):
+  """Returns the name of variable for spec with given depth."""
+  return "s" if depth == 0 else "s{}".format(depth)
+
+
+def get_spec_name(label):
+  """Converts the label of bazel rule to the name of podspec."""
+  assert label.startswith("//absl/"), "{} doesn't start with //absl/".format(
+      label)
+  # e.g. //absl/apple/banana -> abseil/apple/banana
+  return "abseil/" + label[7:]
+
+
+def write_podspec(f, rules, args):
+  """Writes a podspec from given rules and args."""
+  rule_dir = build_rule_directory(rules)["abseil"]
+  # Write root part with given arguments
+  spec = re.sub(r"\$\{(\w+)\}", lambda x: args[x.group(1)],
+                SPEC_TEMPLATE).lstrip()
+  f.write(spec)
+  # Write all target rules
+  write_podspec_map(f, rule_dir, 0)
+  f.write("end\n")
+
+
+def build_rule_directory(rules):
+  """Builds a tree-style rule directory from given rules."""
+  rule_dir = {}
+  for rule in rules:
+    cur = rule_dir
+    for frag in get_spec_name(rule.package).split("/"):
+      cur = cur.setdefault(frag, {})
+    cur[rule.name] = rule
+  return rule_dir
+
+
+def write_podspec_map(f, cur_map, depth):
+  """Writes podspec from rule map recursively."""
+  for key, value in sorted(cur_map.items()):
+    indent = "  " * (depth + 1)
+    f.write("{indent}{var0}.subspec '{key}' do |{var1}|\n".format(
+        indent=indent,
+        key=key,
+        var0=get_spec_var(depth),
+        var1=get_spec_var(depth + 1)))
+    if isinstance(value, dict):
+      write_podspec_map(f, value, depth + 1)
+    else:
+      write_podspec_rule(f, value, depth + 1)
+    f.write("{indent}end\n".format(indent=indent))
+
+
+def write_podspec_rule(f, rule, depth):
+  """Writes podspec from given rule."""
+  indent = "  " * (depth + 1)
+  spec_var = get_spec_var(depth)
+  # Puts all files in hdrs, textual_hdrs, and srcs into source_files.
+  # Since CocoaPods treats header_files a bit differently from bazel,
+  # this won't generate a header_files field so that all source_files
+  # are considered as header files.
+  srcs = sorted(set(rule.hdrs + rule.textual_hdrs + rule.srcs))
+  write_indented_list(
+      f, "{indent}{var}.source_files = ".format(indent=indent, var=spec_var),
+      srcs)
+  # Writes dependencies of this rule.
+  for dep in sorted(rule.deps):
+    name = get_spec_name(dep.replace(":", "/"))
+    f.write("{indent}{var}.dependency '{dep}'\n".format(
+        indent=indent, var=spec_var, dep=name))
+  # Writes dependency to xcprivacy
+  f.write(
+      "{indent}{var}.dependency '{dep}'\n".format(
+          indent=indent, var=spec_var, dep="abseil/xcprivacy"
+      )
+  )
+
+
+def write_indented_list(f, leading, values):
+  """Writes leading values in an indented style."""
+  f.write(leading)
+  f.write((",\n" + " " * len(leading)).join("'{}'".format(v) for v in values))
+  f.write("\n")
+
+
+def generate(args):
+  """Generates a podspec file from all BUILD files under absl directory."""
+  rules = filter(relevant_rule, collect_rules("absl"))
+  with open(args.output, "wt") as f:
+    write_podspec(f, rules, vars(args))
+
+
+def main():
+  parser = argparse.ArgumentParser(
+      description="Generates abseil.podspec from BUILD.bazel")
+  parser.add_argument(
+      "-v", "--version", help="The version of podspec", required=True)
+  parser.add_argument(
+      "-t",
+      "--tag",
+      default=None,
+      help="The name of git tag (default: version)")
+  parser.add_argument(
+      "-o",
+      "--output",
+      default="abseil.podspec",
+      help="The name of output file (default: abseil.podspec)")
+  args = parser.parse_args()
+  if args.tag is None:
+    args.tag = args.version
+  generate(args)
+
+
+if __name__ == "__main__":
+  main()
diff --git a/absl/algorithm/BUILD.bazel b/absl/algorithm/BUILD.bazel
index 2ee8c09e626..10b9400bb55 100644
--- a/absl/algorithm/BUILD.bazel
+++ b/absl/algorithm/BUILD.bazel
@@ -14,7 +14,8 @@
 # limitations under the License.
 #
 
-load("@rules_cc//cc:defs.bzl", "cc_library", "cc_test")
+load("@rules_cc//cc:cc_library.bzl", "cc_library")
+load("@rules_cc//cc:cc_test.bzl", "cc_test")
 load(
     "//absl:copts/configure_copts.bzl",
     "ABSL_DEFAULT_COPTS",
@@ -22,15 +23,27 @@ load(
     "ABSL_TEST_COPTS",
 )
 
-package(default_visibility = ["//visibility:public"])
+package(
+    default_visibility = ["//visibility:private"],
+    features = [
+        "header_modules",
+        "layering_check",
+        "parse_headers",
+    ],
+)
 
-licenses(["notice"])  # Apache 2.0
+licenses(["notice"])
 
 cc_library(
     name = "algorithm",
     hdrs = ["algorithm.h"],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
+    deps = [
+        "//absl/base:config",
+        "//absl/base:core_headers",
+    ],
 )
 
 cc_test(
@@ -41,20 +54,9 @@ cc_test(
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":algorithm",
-        "@com_google_googletest//:gtest_main",
-    ],
-)
-
-cc_test(
-    name = "algorithm_benchmark",
-    srcs = ["equal_benchmark.cc"],
-    copts = ABSL_TEST_COPTS,
-    linkopts = ABSL_DEFAULT_LINKOPTS,
-    tags = ["benchmark"],
-    deps = [
-        ":algorithm",
-        "//absl/base:core_headers",
-        "@com_github_google_benchmark//:benchmark_main",
+        "//absl/base:config",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -65,9 +67,13 @@ cc_library(
     ],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
     deps = [
         ":algorithm",
+        "//absl/base:config",
         "//absl/base:core_headers",
+        "//absl/base:hardening",
+        "//absl/base:iterator_traits_internal",
         "//absl/meta:type_traits",
     ],
 )
@@ -80,9 +86,13 @@ cc_test(
     deps = [
         ":container",
         "//absl/base",
+        "//absl/base:config",
         "//absl/base:core_headers",
         "//absl/memory",
+        "//absl/meta:type_traits",
+        "//absl/random",
         "//absl/types:span",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
diff --git a/absl/algorithm/CMakeLists.txt b/absl/algorithm/CMakeLists.txt
index 9fbe36f698f..cd7fa221ef7 100644
--- a/absl/algorithm/CMakeLists.txt
+++ b/absl/algorithm/CMakeLists.txt
@@ -21,6 +21,9 @@ absl_cc_library(
     "algorithm.h"
   COPTS
     ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::config
+    absl::core_headers
   PUBLIC
 )
 
@@ -33,7 +36,8 @@ absl_cc_test(
     ${ABSL_TEST_COPTS}
   DEPS
     absl::algorithm
-    gmock_main
+    absl::config
+    GTest::gmock_main
 )
 
 absl_cc_library(
@@ -45,6 +49,9 @@ absl_cc_library(
     ${ABSL_DEFAULT_COPTS}
   DEPS
     absl::algorithm
+    absl::hardening
+    absl::config
+    absl::iterator_traits_internal
     absl::core_headers
     absl::meta
   PUBLIC
@@ -59,9 +66,11 @@ absl_cc_test(
     ${ABSL_TEST_COPTS}
   DEPS
     absl::algorithm_container
-    absl::base
+    absl::config
     absl::core_headers
     absl::memory
+    absl::meta
+    absl::random_random
     absl::span
-    gmock_main
+    GTest::gmock_main
 )
diff --git a/absl/algorithm/algorithm.h b/absl/algorithm/algorithm.h
index bb90215d80d..0b75908a053 100644
--- a/absl/algorithm/algorithm.h
+++ b/absl/algorithm/algorithm.h
@@ -26,93 +26,57 @@
 #include <iterator>
 #include <type_traits>
 
-namespace absl {
+#include "absl/base/config.h"
+#include "absl/base/macros.h"
 
-namespace algorithm_internal {
+namespace absl {
+ABSL_NAMESPACE_BEGIN
 
-// Performs comparisons with operator==, similar to C++14's `std::equal_to<>`.
-struct EqualTo {
-  template <typename T, typename U>
-  bool operator()(const T& a, const U& b) const {
-    return a == b;
-  }
-};
+// equal()
+// rotate()
+//
+// Historical note: Abseil once provided implementations of these algorithms
+// prior to their adoption in C++14. New code should prefer to use the std
+// variants.
+//
+// See the documentation for the STL <algorithm> header for more information:
+// https://en.cppreference.com/w/cpp/header/algorithm
 
-template <typename InputIter1, typename InputIter2, typename Pred>
-bool EqualImpl(InputIter1 first1, InputIter1 last1, InputIter2 first2,
-               InputIter2 last2, Pred pred, std::input_iterator_tag,
-               std::input_iterator_tag) {
-  while (true) {
-    if (first1 == last1) return first2 == last2;
-    if (first2 == last2) return false;
-    if (!pred(*first1, *first2)) return false;
-    ++first1;
-    ++first2;
-  }
+template <class InputIt1, class InputIt2>
+ABSL_DEPRECATE_AND_INLINE()
+constexpr bool equal(InputIt1 first1, InputIt1 last1, InputIt2 first2) {
+  return std::equal(first1, last1, first2);
 }
 
-template <typename InputIter1, typename InputIter2, typename Pred>
-bool EqualImpl(InputIter1 first1, InputIter1 last1, InputIter2 first2,
-               InputIter2 last2, Pred&& pred, std::random_access_iterator_tag,
-               std::random_access_iterator_tag) {
-  return (last1 - first1 == last2 - first2) &&
-         std::equal(first1, last1, first2, std::forward<Pred>(pred));
+template <class InputIt1, class InputIt2, class BinaryPredicate>
+ABSL_DEPRECATE_AND_INLINE()
+constexpr bool equal(InputIt1 first1, InputIt1 last1, InputIt2 first2,
+                     BinaryPredicate p) {
+  return std::equal(first1, last1, first2, p);
 }
 
-// When we are using our own internal predicate that just applies operator==, we
-// forward to the non-predicate form of std::equal. This enables an optimization
-// in libstdc++ that can result in std::memcmp being used for integer types.
-template <typename InputIter1, typename InputIter2>
-bool EqualImpl(InputIter1 first1, InputIter1 last1, InputIter2 first2,
-               InputIter2 last2, algorithm_internal::EqualTo /* unused */,
-               std::random_access_iterator_tag,
-               std::random_access_iterator_tag) {
-  return (last1 - first1 == last2 - first2) &&
-         std::equal(first1, last1, first2);
+template <class InputIt1, class InputIt2>
+ABSL_DEPRECATE_AND_INLINE()
+constexpr bool equal(InputIt1 first1, InputIt1 last1, InputIt2 first2,
+                     InputIt2 last2) {
+  return std::equal(first1, last1, first2, last2);
 }
 
-template <typename It>
-It RotateImpl(It first, It middle, It last, std::true_type) {
-  return std::rotate(first, middle, last);
+template <class InputIt1, class InputIt2, class BinaryPredicate>
+ABSL_DEPRECATE_AND_INLINE()
+constexpr bool equal(InputIt1 first1, InputIt1 last1, InputIt2 first2,
+                     InputIt2 last2, BinaryPredicate p) {
+  return std::equal(first1, last1, first2, last2, p);
 }
 
-template <typename It>
-It RotateImpl(It first, It middle, It last, std::false_type) {
-  std::rotate(first, middle, last);
-  return std::next(first, std::distance(middle, last));
+template <class ForwardIt>
+ABSL_DEPRECATE_AND_INLINE()
+constexpr ForwardIt rotate(ForwardIt first, ForwardIt n_first, ForwardIt last) {
+  return std::rotate(first, n_first, last);
 }
 
-}  // namespace algorithm_internal
-
-// Compares the equality of two ranges specified by pairs of iterators, using
-// the given predicate, returning true iff for each corresponding iterator i1
-// and i2 in the first and second range respectively, pred(*i1, *i2) == true
-//
-// This comparison takes at most min(`last1` - `first1`, `last2` - `first2`)
-// invocations of the predicate. Additionally, if InputIter1 and InputIter2 are
-// both random-access iterators, and `last1` - `first1` != `last2` - `first2`,
-// then the predicate is never invoked and the function returns false.
+// linear_search()
 //
-// This is a C++11-compatible implementation of C++14 `std::equal`.  See
-// https://en.cppreference.com/w/cpp/algorithm/equal for more information.
-template <typename InputIter1, typename InputIter2, typename Pred>
-bool equal(InputIter1 first1, InputIter1 last1, InputIter2 first2,
-           InputIter2 last2, Pred&& pred) {
-  return algorithm_internal::EqualImpl(
-      first1, last1, first2, last2, std::forward<Pred>(pred),
-      typename std::iterator_traits<InputIter1>::iterator_category{},
-      typename std::iterator_traits<InputIter2>::iterator_category{});
-}
-
-// Performs comparison of two ranges specified by pairs of iterators using
-// operator==.
-template <typename InputIter1, typename InputIter2>
-bool equal(InputIter1 first1, InputIter1 last1, InputIter2 first2,
-           InputIter2 last2) {
-  return absl::equal(first1, last1, first2, last2,
-                     algorithm_internal::EqualTo{});
-}
-
 // Performs a linear search for `value` using the iterator `first` up to
 // but not including `last`, returning true if [`first`, `last`) contains an
 // element equal to `value`.
@@ -121,30 +85,12 @@ bool equal(InputIter1 first1, InputIter1 last1, InputIter2 first2,
 // n = (`last` - `first`) comparisons. A linear search over short containers
 // may be faster than a binary search, even when the container is sorted.
 template <typename InputIterator, typename EqualityComparable>
-bool linear_search(InputIterator first, InputIterator last,
-                   const EqualityComparable& value) {
+constexpr bool linear_search(InputIterator first, InputIterator last,
+                             const EqualityComparable& value) {
   return std::find(first, last, value) != last;
 }
 
-// Performs a left rotation on a range of elements (`first`, `last`) such that
-// `middle` is now the first element. `rotate()` returns an iterator pointing to
-// the first element before rotation. This function is exactly the same as
-// `std::rotate`, but fixes a bug in gcc
-// <= 4.9 where `std::rotate` returns `void` instead of an iterator.
-//
-// The complexity of this algorithm is the same as that of `std::rotate`, but if
-// `ForwardIterator` is not a random-access iterator, then `absl::rotate`
-// performs an additional pass over the range to construct the return value.
-
-template <typename ForwardIterator>
-ForwardIterator rotate(ForwardIterator first, ForwardIterator middle,
-                       ForwardIterator last) {
-  return algorithm_internal::RotateImpl(
-      first, middle, last,
-      std::is_same<decltype(std::rotate(first, middle, last)),
-                   ForwardIterator>());
-}
-
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_ALGORITHM_ALGORITHM_H_
diff --git a/absl/algorithm/algorithm_test.cc b/absl/algorithm/algorithm_test.cc
index 81fccb61353..1c1a3079f8a 100644
--- a/absl/algorithm/algorithm_test.cc
+++ b/absl/algorithm/algorithm_test.cc
@@ -14,138 +14,14 @@
 
 #include "absl/algorithm/algorithm.h"
 
-#include <algorithm>
-#include <list>
+#include <array>
 #include <vector>
 
-#include "gmock/gmock.h"
 #include "gtest/gtest.h"
+#include "absl/base/config.h"
 
 namespace {
 
-TEST(EqualTest, DefaultComparisonRandomAccess) {
-  std::vector<int> v1{1, 2, 3};
-  std::vector<int> v2 = v1;
-  std::vector<int> v3 = {1, 2};
-  std::vector<int> v4 = {1, 2, 4};
-
-  EXPECT_TRUE(absl::equal(v1.begin(), v1.end(), v2.begin(), v2.end()));
-  EXPECT_FALSE(absl::equal(v1.begin(), v1.end(), v3.begin(), v3.end()));
-  EXPECT_FALSE(absl::equal(v1.begin(), v1.end(), v4.begin(), v4.end()));
-}
-
-TEST(EqualTest, DefaultComparison) {
-  std::list<int> lst1{1, 2, 3};
-  std::list<int> lst2 = lst1;
-  std::list<int> lst3{1, 2};
-  std::list<int> lst4{1, 2, 4};
-
-  EXPECT_TRUE(absl::equal(lst1.begin(), lst1.end(), lst2.begin(), lst2.end()));
-  EXPECT_FALSE(absl::equal(lst1.begin(), lst1.end(), lst3.begin(), lst3.end()));
-  EXPECT_FALSE(absl::equal(lst1.begin(), lst1.end(), lst4.begin(), lst4.end()));
-}
-
-TEST(EqualTest, EmptyRange) {
-  std::vector<int> v1{1, 2, 3};
-  std::vector<int> empty1;
-  std::vector<int> empty2;
-
-  EXPECT_FALSE(absl::equal(v1.begin(), v1.end(), empty1.begin(), empty1.end()));
-  EXPECT_FALSE(absl::equal(empty1.begin(), empty1.end(), v1.begin(), v1.end()));
-  EXPECT_TRUE(
-      absl::equal(empty1.begin(), empty1.end(), empty2.begin(), empty2.end()));
-}
-
-TEST(EqualTest, MixedIterTypes) {
-  std::vector<int> v1{1, 2, 3};
-  std::list<int> lst1{v1.begin(), v1.end()};
-  std::list<int> lst2{1, 2, 4};
-  std::list<int> lst3{1, 2};
-
-  EXPECT_TRUE(absl::equal(v1.begin(), v1.end(), lst1.begin(), lst1.end()));
-  EXPECT_FALSE(absl::equal(v1.begin(), v1.end(), lst2.begin(), lst2.end()));
-  EXPECT_FALSE(absl::equal(v1.begin(), v1.end(), lst3.begin(), lst3.end()));
-}
-
-TEST(EqualTest, MixedValueTypes) {
-  std::vector<int> v1{1, 2, 3};
-  std::vector<char> v2{1, 2, 3};
-  std::vector<char> v3{1, 2};
-  std::vector<char> v4{1, 2, 4};
-
-  EXPECT_TRUE(absl::equal(v1.begin(), v1.end(), v2.begin(), v2.end()));
-  EXPECT_FALSE(absl::equal(v1.begin(), v1.end(), v3.begin(), v3.end()));
-  EXPECT_FALSE(absl::equal(v1.begin(), v1.end(), v4.begin(), v4.end()));
-}
-
-TEST(EqualTest, WeirdIterators) {
-  std::vector<bool> v1{true, false};
-  std::vector<bool> v2 = v1;
-  std::vector<bool> v3{true};
-  std::vector<bool> v4{true, true, true};
-
-  EXPECT_TRUE(absl::equal(v1.begin(), v1.end(), v2.begin(), v2.end()));
-  EXPECT_FALSE(absl::equal(v1.begin(), v1.end(), v3.begin(), v3.end()));
-  EXPECT_FALSE(absl::equal(v1.begin(), v1.end(), v4.begin(), v4.end()));
-}
-
-TEST(EqualTest, CustomComparison) {
-  int n[] = {1, 2, 3, 4};
-  std::vector<int*> v1{&n[0], &n[1], &n[2]};
-  std::vector<int*> v2 = v1;
-  std::vector<int*> v3{&n[0], &n[1], &n[3]};
-  std::vector<int*> v4{&n[0], &n[1]};
-
-  auto eq = [](int* a, int* b) { return *a == *b; };
-
-  EXPECT_TRUE(absl::equal(v1.begin(), v1.end(), v2.begin(), v2.end(), eq));
-  EXPECT_FALSE(absl::equal(v1.begin(), v1.end(), v3.begin(), v3.end(), eq));
-  EXPECT_FALSE(absl::equal(v1.begin(), v1.end(), v4.begin(), v4.end(), eq));
-}
-
-TEST(EqualTest, MoveOnlyPredicate) {
-  std::vector<int> v1{1, 2, 3};
-  std::vector<int> v2{4, 5, 6};
-
-  // move-only equality predicate
-  struct Eq {
-    Eq() = default;
-    Eq(Eq &&) = default;
-    Eq(const Eq &) = delete;
-    Eq &operator=(const Eq &) = delete;
-    bool operator()(const int a, const int b) const { return a == b; }
-  };
-
-  EXPECT_TRUE(absl::equal(v1.begin(), v1.end(), v1.begin(), v1.end(), Eq()));
-  EXPECT_FALSE(absl::equal(v1.begin(), v1.end(), v2.begin(), v2.end(), Eq()));
-}
-
-struct CountingTrivialPred {
-  int* count;
-  bool operator()(int, int) const {
-    ++*count;
-    return true;
-  }
-};
-
-TEST(EqualTest, RandomAccessComplexity) {
-  std::vector<int> v1{1, 1, 3};
-  std::vector<int> v2 = v1;
-  std::vector<int> v3{1, 2};
-
-  do {
-    int count = 0;
-    absl::equal(v1.begin(), v1.end(), v2.begin(), v2.end(),
-                CountingTrivialPred{&count});
-    EXPECT_LE(count, 3);
-  } while (std::next_permutation(v2.begin(), v2.end()));
-
-  int count = 0;
-  absl::equal(v1.begin(), v1.end(), v3.begin(), v3.end(),
-              CountingTrivialPred{&count});
-  EXPECT_EQ(count, 0);
-}
-
 class LinearSearchTest : public testing::Test {
  protected:
   LinearSearchTest() : container_{1, 2, 3} {}
@@ -169,14 +45,16 @@ TEST_F(LinearSearchTest, linear_searchConst) {
       absl::linear_search(const_container->begin(), const_container->end(), 4));
 }
 
-TEST(RotateTest, Rotate) {
-  std::vector<int> v{0, 1, 2, 3, 4};
-  EXPECT_EQ(*absl::rotate(v.begin(), v.begin() + 2, v.end()), 0);
-  EXPECT_THAT(v, testing::ElementsAreArray({2, 3, 4, 0, 1}));
+#if defined(ABSL_INTERNAL_CPLUSPLUS_LANG) && \
+    ABSL_INTERNAL_CPLUSPLUS_LANG >= 202002L
 
-  std::list<int> l{0, 1, 2, 3, 4};
-  EXPECT_EQ(*absl::rotate(l.begin(), std::next(l.begin(), 3), l.end()), 0);
-  EXPECT_THAT(l, testing::ElementsAreArray({3, 4, 0, 1, 2}));
+TEST_F(LinearSearchTest, Constexpr) {
+  static constexpr std::array<int, 3> kArray = {1, 2, 3};
+  static_assert(absl::linear_search(kArray.begin(), kArray.end(), 3));
+  static_assert(!absl::linear_search(kArray.begin(), kArray.end(), 4));
 }
 
+#endif  // defined(ABSL_INTERNAL_CPLUSPLUS_LANG) &&
+        //  ABSL_INTERNAL_CPLUSPLUS_LANG >= 202002L
+
 }  // namespace
diff --git a/absl/algorithm/container.h b/absl/algorithm/container.h
index c84de461ac2..9d2b48e9f14 100644
--- a/absl/algorithm/container.h
+++ b/absl/algorithm/container.h
@@ -42,6 +42,7 @@
 
 #include <algorithm>
 #include <cassert>
+#include <cstddef>
 #include <iterator>
 #include <numeric>
 #include <type_traits>
@@ -51,10 +52,22 @@
 #include <vector>
 
 #include "absl/algorithm/algorithm.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/hardening.h"
+#include "absl/base/internal/iterator_traits.h"
 #include "absl/base/macros.h"
 #include "absl/meta/type_traits.h"
 
+#ifdef __cpp_lib_span
+#include <span>  // NOLINT(build/c++20)
+#endif
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+template <typename T>
+class Span;
+
 namespace container_algorithm_internal {
 
 // NOTE: it is important to defer to ADL lookup for building with C++ modules,
@@ -72,13 +85,12 @@ using ContainerIter = decltype(begin(std::declval<C&>()));
 // An MSVC bug involving template parameter substitution requires us to use
 // decltype() here instead of just std::pair.
 template <typename C1, typename C2>
-using ContainerIterPairType =
-    decltype(std::make_pair(ContainerIter<C1>(), ContainerIter<C2>()));
+using ContainerIterPairType = decltype(std::make_pair(
+    std::declval<ContainerIter<C1>>(), std::declval<ContainerIter<C2>>()));
 
 template <typename C>
-using ContainerDifferenceType =
-    decltype(std::distance(std::declval<ContainerIter<C>>(),
-                           std::declval<ContainerIter<C>>()));
+using ContainerDifferenceType = decltype(std::distance(
+    std::declval<ContainerIter<C>>(), std::declval<ContainerIter<C>>()));
 
 template <typename C>
 using ContainerPointerType =
@@ -89,17 +101,55 @@ using ContainerPointerType =
 // lookup of std::begin and std::end, i.e.
 //   using std::begin;
 //   using std::end;
-//   std::foo(begin(c), end(c);
+//   std::foo(begin(c), end(c));
 // becomes
-//   std::foo(container_algorithm_internal::begin(c),
-//   container_algorithm_internal::end(c));
+//   std::foo(container_algorithm_internal::c_begin(c),
+//            container_algorithm_internal::c_end(c));
 // These are meant for internal use only.
 
 template <typename C>
-ContainerIter<C> c_begin(C& c) { return begin(c); }
+constexpr ContainerIter<C> c_begin(C& c) {
+  return begin(c);
+}
 
 template <typename C>
-ContainerIter<C> c_end(C& c) { return end(c); }
+constexpr ContainerIter<C> c_end(C& c) {
+  return end(c);
+}
+
+// Helper to check that the `OutputRange` has enough space.
+// Only performs the check if the iterators are ForwardIterators or better.
+template <typename InputSequence, typename Size, typename OutputRange>
+constexpr void AssertCopyNSize(InputSequence& input, Size n,
+                               OutputRange& output) {
+  using InputIter = ContainerIter<InputSequence>;
+  using OutputIter = ContainerIter<OutputRange>;
+
+  if constexpr (base_internal::IsAtLeastForwardIterator<InputIter>::value) {
+    base_internal::HardeningAssertLE(
+        n, std::distance(container_algorithm_internal::c_begin(input),
+                         container_algorithm_internal::c_end(input)));
+  }
+  if constexpr (base_internal::IsAtLeastForwardIterator<OutputIter>::value) {
+    base_internal::HardeningAssertLE(
+        n, std::distance(container_algorithm_internal::c_begin(output),
+                         container_algorithm_internal::c_end(output)));
+  }
+}
+
+template <typename InputSequence, typename OutputRange>
+constexpr void AssertCopySize(InputSequence& input, OutputRange& output) {
+  using InputIter = ContainerIter<InputSequence>;
+  using OutputIter = ContainerIter<OutputRange>;
+  if constexpr (base_internal::IsAtLeastForwardIterator<InputIter>::value &&
+                base_internal::IsAtLeastForwardIterator<OutputIter>::value) {
+    base_internal::HardeningAssertLE(
+        std::distance(container_algorithm_internal::c_begin(input),
+                      container_algorithm_internal::c_end(input)),
+        std::distance(container_algorithm_internal::c_begin(output),
+                      container_algorithm_internal::c_end(output)));
+  }
+}
 
 template <typename T>
 struct IsUnorderedContainer : std::false_type {};
@@ -112,6 +162,77 @@ template <class Key, class Hash, class KeyEqual, class Allocator>
 struct IsUnorderedContainer<std::unordered_set<Key, Hash, KeyEqual, Allocator>>
     : std::true_type {};
 
+template <typename T, typename = void>
+struct HasBeginEnd : std::false_type {};
+
+template <typename T>
+struct HasBeginEnd<T, std::void_t<decltype(container_algorithm_internal::begin(
+                                      std::declval<T (*)()>()())),
+                                  decltype(container_algorithm_internal::end(
+                                      std::declval<T (*)()>()()))>>
+    : std::true_type {};
+
+// We don't support multidimensional arrays yet
+template <class T>
+using IsMultidimensionalArray = std::is_array<std::remove_extent_t<T>>;
+
+template <typename Iter, typename = void>
+struct IsIterator : std::false_type {};
+
+template <typename Iter>
+struct IsIterator<
+    Iter, std::void_t<typename std::iterator_traits<Iter>::iterator_category>>
+    : std::true_type {};
+
+template <typename C, typename OutputIterator>
+using ResultOfRangeToIteratorTransfer =
+    std::enable_if_t<container_algorithm_internal::IsIterator<
+                         absl::remove_cvref_t<OutputIterator>>::value &&
+                         !container_algorithm_internal::IsMultidimensionalArray<
+                             std::remove_reference_t<C>>::value,
+                     std::decay_t<OutputIterator>>;
+
+// Similar to std::is_pointer, but for testing if a type is a span.
+//
+// Note that subclasses of spans do not automatically qualify as spans, as they
+// may deviate from the ownership assumption of a span.
+template <typename T>
+struct IsSpan
+    : std::conditional_t<std::is_same_v<T, std::remove_cv_t<T>>,
+                         std::false_type, IsSpan<std::remove_cv_t<T>>> {};
+
+template <typename T>
+struct IsSpan<absl::Span<T>> : std::true_type {};
+
+#ifdef __cpp_lib_span
+template <typename T, size_t Extent>
+struct IsSpan<std::span<T, Extent>> : std::true_type {};
+#endif
+
+// Indicates whether the given type is safe to pass as a sink to a function such
+// as absl::c_fill(). Similar idea as std::ranges::borrowed_range.
+//
+// We are deliberately conservative here and only support lvalues and spans for
+// now, in order to avoid divergence from C++17 or potentially unforeseen
+// consequences. If needed in the future, we can probably extend this to all
+// types that satisfy std::ranges::borrowed_range.
+template <typename C>
+using IsPermissibleDestinationRange =
+    std::conditional_t<std::is_lvalue_reference_v<C>, std::true_type,
+                       IsSpan<C>>;
+
+template <typename C, typename OutputRange>
+using ResultOfRangeToRangeTransfer =
+    std::enable_if_t<container_algorithm_internal::HasBeginEnd<
+                         std::add_lvalue_reference_t<OutputRange>>::value &&
+                         !container_algorithm_internal::IsMultidimensionalArray<
+                             std::remove_reference_t<OutputRange>>::value &&
+                         !container_algorithm_internal::IsMultidimensionalArray<
+                             std::remove_reference_t<C>>::value &&
+                         container_algorithm_internal::
+                             IsPermissibleDestinationRange<OutputRange>::value,
+                     void>;
+
 }  // namespace container_algorithm_internal
 
 // PUBLIC API
@@ -124,11 +245,13 @@ struct IsUnorderedContainer<std::unordered_set<Key, Hash, KeyEqual, Allocator>>
 //
 // Container-based version of absl::linear_search() for performing a linear
 // search within a container.
+//
+// For a generalization that uses a predicate, see absl::c_any_of().
 template <typename C, typename EqualityComparable>
-bool c_linear_search(const C& c, EqualityComparable&& value) {
-  return linear_search(container_algorithm_internal::c_begin(c),
-                       container_algorithm_internal::c_end(c),
-                       std::forward<EqualityComparable>(value));
+constexpr bool c_linear_search(const C& c, EqualityComparable&& value) {
+  return absl::linear_search(container_algorithm_internal::c_begin(c),
+                             container_algorithm_internal::c_end(c),
+                             std::forward<EqualityComparable>(value));
 }
 
 //------------------------------------------------------------------------------
@@ -140,8 +263,8 @@ bool c_linear_search(const C& c, EqualityComparable&& value) {
 // Container-based version of the <iterator> `std::distance()` function to
 // return the number of elements within a container.
 template <typename C>
-container_algorithm_internal::ContainerDifferenceType<const C> c_distance(
-    const C& c) {
+constexpr container_algorithm_internal::ContainerDifferenceType<const C>
+c_distance(const C& c) {
   return std::distance(container_algorithm_internal::c_begin(c),
                        container_algorithm_internal::c_end(c));
 }
@@ -153,9 +276,9 @@ container_algorithm_internal::ContainerDifferenceType<const C> c_distance(
 // c_all_of()
 //
 // Container-based version of the <algorithm> `std::all_of()` function to
-// test a condition on all elements within a container.
+// test if all elements within a container satisfy a condition.
 template <typename C, typename Pred>
-bool c_all_of(const C& c, Pred&& pred) {
+constexpr bool c_all_of(const C& c, Pred&& pred) {
   return std::all_of(container_algorithm_internal::c_begin(c),
                      container_algorithm_internal::c_end(c),
                      std::forward<Pred>(pred));
@@ -166,7 +289,7 @@ bool c_all_of(const C& c, Pred&& pred) {
 // Container-based version of the <algorithm> `std::any_of()` function to
 // test if any element in a container fulfills a condition.
 template <typename C, typename Pred>
-bool c_any_of(const C& c, Pred&& pred) {
+constexpr bool c_any_of(const C& c, Pred&& pred) {
   return std::any_of(container_algorithm_internal::c_begin(c),
                      container_algorithm_internal::c_end(c),
                      std::forward<Pred>(pred));
@@ -175,9 +298,9 @@ bool c_any_of(const C& c, Pred&& pred) {
 // c_none_of()
 //
 // Container-based version of the <algorithm> `std::none_of()` function to
-// test if no elements in a container fulfil a condition.
+// test if no elements in a container fulfill a condition.
 template <typename C, typename Pred>
-bool c_none_of(const C& c, Pred&& pred) {
+constexpr bool c_none_of(const C& c, Pred&& pred) {
   return std::none_of(container_algorithm_internal::c_begin(c),
                       container_algorithm_internal::c_end(c),
                       std::forward<Pred>(pred));
@@ -188,7 +311,7 @@ bool c_none_of(const C& c, Pred&& pred) {
 // Container-based version of the <algorithm> `std::for_each()` function to
 // apply a function to a container's elements.
 template <typename C, typename Function>
-decay_t<Function> c_for_each(C&& c, Function&& f) {
+constexpr std::decay_t<Function> c_for_each(C&& c, Function&& f) {
   return std::for_each(container_algorithm_internal::c_begin(c),
                        container_algorithm_internal::c_end(c),
                        std::forward<Function>(f));
@@ -199,18 +322,30 @@ decay_t<Function> c_for_each(C&& c, Function&& f) {
 // Container-based version of the <algorithm> `std::find()` function to find
 // the first element containing the passed value within a container value.
 template <typename C, typename T>
-container_algorithm_internal::ContainerIter<C> c_find(C& c, T&& value) {
+constexpr container_algorithm_internal::ContainerIter<C> c_find(C& c,
+                                                                T&& value) {
   return std::find(container_algorithm_internal::c_begin(c),
                    container_algorithm_internal::c_end(c),
                    std::forward<T>(value));
 }
 
+// c_contains()
+//
+// Container-based version of the <algorithm> `std::ranges::contains()` C++23
+// function to search a container for a value.
+template <typename Sequence, typename T>
+constexpr bool c_contains(const Sequence& sequence, T&& value) {
+  return absl::c_find(sequence, std::forward<T>(value)) !=
+         container_algorithm_internal::c_end(sequence);
+}
+
 // c_find_if()
 //
 // Container-based version of the <algorithm> `std::find_if()` function to find
 // the first element in a container matching the given condition.
 template <typename C, typename Pred>
-container_algorithm_internal::ContainerIter<C> c_find_if(C& c, Pred&& pred) {
+constexpr container_algorithm_internal::ContainerIter<C> c_find_if(
+    C& c, Pred&& pred) {
   return std::find_if(container_algorithm_internal::c_begin(c),
                       container_algorithm_internal::c_end(c),
                       std::forward<Pred>(pred));
@@ -221,8 +356,8 @@ container_algorithm_internal::ContainerIter<C> c_find_if(C& c, Pred&& pred) {
 // Container-based version of the <algorithm> `std::find_if_not()` function to
 // find the first element in a container not matching the given condition.
 template <typename C, typename Pred>
-container_algorithm_internal::ContainerIter<C> c_find_if_not(C& c,
-                                                             Pred&& pred) {
+constexpr container_algorithm_internal::ContainerIter<C> c_find_if_not(
+    C& c, Pred&& pred) {
   return std::find_if_not(container_algorithm_internal::c_begin(c),
                           container_algorithm_internal::c_end(c),
                           std::forward<Pred>(pred));
@@ -233,7 +368,7 @@ container_algorithm_internal::ContainerIter<C> c_find_if_not(C& c,
 // Container-based version of the <algorithm> `std::find_end()` function to
 // find the last subsequence within a container.
 template <typename Sequence1, typename Sequence2>
-container_algorithm_internal::ContainerIter<Sequence1> c_find_end(
+constexpr container_algorithm_internal::ContainerIter<Sequence1> c_find_end(
     Sequence1& sequence, Sequence2& subsequence) {
   return std::find_end(container_algorithm_internal::c_begin(sequence),
                        container_algorithm_internal::c_end(sequence),
@@ -244,7 +379,7 @@ container_algorithm_internal::ContainerIter<Sequence1> c_find_end(
 // Overload of c_find_end() for using a predicate evaluation other than `==` as
 // the function's test condition.
 template <typename Sequence1, typename Sequence2, typename BinaryPredicate>
-container_algorithm_internal::ContainerIter<Sequence1> c_find_end(
+constexpr container_algorithm_internal::ContainerIter<Sequence1> c_find_end(
     Sequence1& sequence, Sequence2& subsequence, BinaryPredicate&& pred) {
   return std::find_end(container_algorithm_internal::c_begin(sequence),
                        container_algorithm_internal::c_end(sequence),
@@ -256,10 +391,11 @@ container_algorithm_internal::ContainerIter<Sequence1> c_find_end(
 // c_find_first_of()
 //
 // Container-based version of the <algorithm> `std::find_first_of()` function to
-// find the first elements in an ordered set within a container.
+// find the first element within the container that is also within the options
+// container.
 template <typename C1, typename C2>
-container_algorithm_internal::ContainerIter<C1> c_find_first_of(C1& container,
-                                                                C2& options) {
+constexpr container_algorithm_internal::ContainerIter<C1> c_find_first_of(
+    C1& container, const C2& options) {
   return std::find_first_of(container_algorithm_internal::c_begin(container),
                             container_algorithm_internal::c_end(container),
                             container_algorithm_internal::c_begin(options),
@@ -269,8 +405,8 @@ container_algorithm_internal::ContainerIter<C1> c_find_first_of(C1& container,
 // Overload of c_find_first_of() for using a predicate evaluation other than
 // `==` as the function's test condition.
 template <typename C1, typename C2, typename BinaryPredicate>
-container_algorithm_internal::ContainerIter<C1> c_find_first_of(
-    C1& container, C2& options, BinaryPredicate&& pred) {
+constexpr container_algorithm_internal::ContainerIter<C1> c_find_first_of(
+    C1& container, const C2& options, BinaryPredicate&& pred) {
   return std::find_first_of(container_algorithm_internal::c_begin(container),
                             container_algorithm_internal::c_end(container),
                             container_algorithm_internal::c_begin(options),
@@ -283,7 +419,7 @@ container_algorithm_internal::ContainerIter<C1> c_find_first_of(
 // Container-based version of the <algorithm> `std::adjacent_find()` function to
 // find equal adjacent elements within a container.
 template <typename Sequence>
-container_algorithm_internal::ContainerIter<Sequence> c_adjacent_find(
+constexpr container_algorithm_internal::ContainerIter<Sequence> c_adjacent_find(
     Sequence& sequence) {
   return std::adjacent_find(container_algorithm_internal::c_begin(sequence),
                             container_algorithm_internal::c_end(sequence));
@@ -292,7 +428,7 @@ container_algorithm_internal::ContainerIter<Sequence> c_adjacent_find(
 // Overload of c_adjacent_find() for using a predicate evaluation other than
 // `==` as the function's test condition.
 template <typename Sequence, typename BinaryPredicate>
-container_algorithm_internal::ContainerIter<Sequence> c_adjacent_find(
+constexpr container_algorithm_internal::ContainerIter<Sequence> c_adjacent_find(
     Sequence& sequence, BinaryPredicate&& pred) {
   return std::adjacent_find(container_algorithm_internal::c_begin(sequence),
                             container_algorithm_internal::c_end(sequence),
@@ -304,8 +440,8 @@ container_algorithm_internal::ContainerIter<Sequence> c_adjacent_find(
 // Container-based version of the <algorithm> `std::count()` function to count
 // values that match within a container.
 template <typename C, typename T>
-container_algorithm_internal::ContainerDifferenceType<const C> c_count(
-    const C& c, T&& value) {
+constexpr container_algorithm_internal::ContainerDifferenceType<const C>
+c_count(const C& c, T&& value) {
   return std::count(container_algorithm_internal::c_begin(c),
                     container_algorithm_internal::c_end(c),
                     std::forward<T>(value));
@@ -316,8 +452,8 @@ container_algorithm_internal::ContainerDifferenceType<const C> c_count(
 // Container-based version of the <algorithm> `std::count_if()` function to
 // count values matching a condition within a container.
 template <typename C, typename Pred>
-container_algorithm_internal::ContainerDifferenceType<const C> c_count_if(
-    const C& c, Pred&& pred) {
+constexpr container_algorithm_internal::ContainerDifferenceType<const C>
+c_count_if(const C& c, Pred&& pred) {
   return std::count_if(container_algorithm_internal::c_begin(c),
                        container_algorithm_internal::c_end(c),
                        std::forward<Pred>(pred));
@@ -326,60 +462,50 @@ container_algorithm_internal::ContainerDifferenceType<const C> c_count_if(
 // c_mismatch()
 //
 // Container-based version of the <algorithm> `std::mismatch()` function to
-// return the first element where two ordered containers differ.
+// return the first element where two ordered containers differ. Applies `==` to
+// the first N elements of `c1` and `c2`, where N = min(size(c1), size(c2)).
 template <typename C1, typename C2>
-container_algorithm_internal::ContainerIterPairType<C1, C2>
+constexpr container_algorithm_internal::ContainerIterPairType<C1, C2>
 c_mismatch(C1& c1, C2& c2) {
   return std::mismatch(container_algorithm_internal::c_begin(c1),
                        container_algorithm_internal::c_end(c1),
-                       container_algorithm_internal::c_begin(c2));
+                       container_algorithm_internal::c_begin(c2),
+                       container_algorithm_internal::c_end(c2));
 }
 
 // Overload of c_mismatch() for using a predicate evaluation other than `==` as
-// the function's test condition.
+// the function's test condition. Applies `pred`to the first N elements of `c1`
+// and `c2`, where N = min(size(c1), size(c2)).
 template <typename C1, typename C2, typename BinaryPredicate>
-container_algorithm_internal::ContainerIterPairType<C1, C2>
-c_mismatch(C1& c1, C2& c2, BinaryPredicate&& pred) {
+constexpr container_algorithm_internal::ContainerIterPairType<C1, C2>
+c_mismatch(C1& c1, C2& c2, BinaryPredicate pred) {
   return std::mismatch(container_algorithm_internal::c_begin(c1),
                        container_algorithm_internal::c_end(c1),
                        container_algorithm_internal::c_begin(c2),
-                       std::forward<BinaryPredicate>(pred));
+                       container_algorithm_internal::c_end(c2), pred);
 }
 
 // c_equal()
 //
 // Container-based version of the <algorithm> `std::equal()` function to
 // test whether two containers are equal.
-//
-// NOTE: the semantics of c_equal() are slightly different than those of
-// equal(): while the latter iterates over the second container only up to the
-// size of the first container, c_equal() also checks whether the container
-// sizes are equal.  This better matches expectations about c_equal() based on
-// its signature.
-//
-// Example:
-//   vector v1 = <1, 2, 3>;
-//   vector v2 = <1, 2, 3, 4>;
-//   equal(std::begin(v1), std::end(v1), std::begin(v2)) returns true
-//   c_equal(v1, v2) returns false
-
 template <typename C1, typename C2>
-bool c_equal(const C1& c1, const C2& c2) {
-  return ((c1.size() == c2.size()) &&
-          std::equal(container_algorithm_internal::c_begin(c1),
-                     container_algorithm_internal::c_end(c1),
-                     container_algorithm_internal::c_begin(c2)));
+constexpr bool c_equal(const C1& c1, const C2& c2) {
+  return std::equal(container_algorithm_internal::c_begin(c1),
+                    container_algorithm_internal::c_end(c1),
+                    container_algorithm_internal::c_begin(c2),
+                    container_algorithm_internal::c_end(c2));
 }
 
 // Overload of c_equal() for using a predicate evaluation other than `==` as
 // the function's test condition.
 template <typename C1, typename C2, typename BinaryPredicate>
-bool c_equal(const C1& c1, const C2& c2, BinaryPredicate&& pred) {
-  return ((c1.size() == c2.size()) &&
-          std::equal(container_algorithm_internal::c_begin(c1),
-                     container_algorithm_internal::c_end(c1),
-                     container_algorithm_internal::c_begin(c2),
-                     std::forward<BinaryPredicate>(pred)));
+constexpr bool c_equal(const C1& c1, const C2& c2, BinaryPredicate&& pred) {
+  return std::equal(container_algorithm_internal::c_begin(c1),
+                    container_algorithm_internal::c_end(c1),
+                    container_algorithm_internal::c_begin(c2),
+                    container_algorithm_internal::c_end(c2),
+                    std::forward<BinaryPredicate>(pred));
 }
 
 // c_is_permutation()
@@ -387,21 +513,22 @@ bool c_equal(const C1& c1, const C2& c2, BinaryPredicate&& pred) {
 // Container-based version of the <algorithm> `std::is_permutation()` function
 // to test whether a container is a permutation of another.
 template <typename C1, typename C2>
-bool c_is_permutation(const C1& c1, const C2& c2) {
-  using std::begin;
-  using std::end;
-  return c1.size() == c2.size() &&
-         std::is_permutation(begin(c1), end(c1), begin(c2));
+constexpr bool c_is_permutation(const C1& c1, const C2& c2) {
+  return std::is_permutation(container_algorithm_internal::c_begin(c1),
+                             container_algorithm_internal::c_end(c1),
+                             container_algorithm_internal::c_begin(c2),
+                             container_algorithm_internal::c_end(c2));
 }
 
 // Overload of c_is_permutation() for using a predicate evaluation other than
 // `==` as the function's test condition.
 template <typename C1, typename C2, typename BinaryPredicate>
-bool c_is_permutation(const C1& c1, const C2& c2, BinaryPredicate&& pred) {
-  using std::begin;
-  using std::end;
-  return c1.size() == c2.size() &&
-         std::is_permutation(begin(c1), end(c1), begin(c2),
+constexpr bool c_is_permutation(const C1& c1, const C2& c2,
+                                BinaryPredicate&& pred) {
+  return std::is_permutation(container_algorithm_internal::c_begin(c1),
+                             container_algorithm_internal::c_end(c1),
+                             container_algorithm_internal::c_begin(c2),
+                             container_algorithm_internal::c_end(c2),
                              std::forward<BinaryPredicate>(pred));
 }
 
@@ -410,7 +537,7 @@ bool c_is_permutation(const C1& c1, const C2& c2, BinaryPredicate&& pred) {
 // Container-based version of the <algorithm> `std::search()` function to search
 // a container for a subsequence.
 template <typename Sequence1, typename Sequence2>
-container_algorithm_internal::ContainerIter<Sequence1> c_search(
+constexpr container_algorithm_internal::ContainerIter<Sequence1> c_search(
     Sequence1& sequence, Sequence2& subsequence) {
   return std::search(container_algorithm_internal::c_begin(sequence),
                      container_algorithm_internal::c_end(sequence),
@@ -421,7 +548,7 @@ container_algorithm_internal::ContainerIter<Sequence1> c_search(
 // Overload of c_search() for using a predicate evaluation other than
 // `==` as the function's test condition.
 template <typename Sequence1, typename Sequence2, typename BinaryPredicate>
-container_algorithm_internal::ContainerIter<Sequence1> c_search(
+constexpr container_algorithm_internal::ContainerIter<Sequence1> c_search(
     Sequence1& sequence, Sequence2& subsequence, BinaryPredicate&& pred) {
   return std::search(container_algorithm_internal::c_begin(sequence),
                      container_algorithm_internal::c_end(sequence),
@@ -430,12 +557,33 @@ container_algorithm_internal::ContainerIter<Sequence1> c_search(
                      std::forward<BinaryPredicate>(pred));
 }
 
+// c_contains_subrange()
+//
+// Container-based version of the <algorithm> `std::ranges::contains_subrange()`
+// C++23 function to search a container for a subsequence.
+template <typename Sequence1, typename Sequence2>
+constexpr bool c_contains_subrange(Sequence1& sequence,
+                                   Sequence2& subsequence) {
+  return absl::c_search(sequence, subsequence) !=
+         container_algorithm_internal::c_end(sequence);
+}
+
+// Overload of c_contains_subrange() for using a predicate evaluation other than
+// `==` as the function's test condition.
+template <typename Sequence1, typename Sequence2, typename BinaryPredicate>
+constexpr bool c_contains_subrange(Sequence1& sequence, Sequence2& subsequence,
+                                   BinaryPredicate&& pred) {
+  return absl::c_search(sequence, subsequence,
+                        std::forward<BinaryPredicate>(pred)) !=
+         container_algorithm_internal::c_end(sequence);
+}
+
 // c_search_n()
 //
 // Container-based version of the <algorithm> `std::search_n()` function to
 // search a container for the first sequence of N elements.
 template <typename Sequence, typename Size, typename T>
-container_algorithm_internal::ContainerIter<Sequence> c_search_n(
+constexpr container_algorithm_internal::ContainerIter<Sequence> c_search_n(
     Sequence& sequence, Size count, T&& value) {
   return std::search_n(container_algorithm_internal::c_begin(sequence),
                        container_algorithm_internal::c_end(sequence), count,
@@ -446,7 +594,7 @@ container_algorithm_internal::ContainerIter<Sequence> c_search_n(
 // `==` as the function's test condition.
 template <typename Sequence, typename Size, typename T,
           typename BinaryPredicate>
-container_algorithm_internal::ContainerIter<Sequence> c_search_n(
+constexpr container_algorithm_internal::ContainerIter<Sequence> c_search_n(
     Sequence& sequence, Size count, T&& value, BinaryPredicate&& pred) {
   return std::search_n(container_algorithm_internal::c_begin(sequence),
                        container_algorithm_internal::c_end(sequence), count,
@@ -463,9 +611,30 @@ container_algorithm_internal::ContainerIter<Sequence> c_search_n(
 // Container-based version of the <algorithm> `std::copy()` function to copy a
 // container's elements into an iterator.
 template <typename InputSequence, typename OutputIterator>
-OutputIterator c_copy(const InputSequence& input, OutputIterator output) {
+constexpr container_algorithm_internal::ResultOfRangeToIteratorTransfer<
+    InputSequence, OutputIterator>
+c_copy(const InputSequence& input, OutputIterator&& output) {
   return std::copy(container_algorithm_internal::c_begin(input),
-                   container_algorithm_internal::c_end(input), output);
+                   container_algorithm_internal::c_end(input),
+                   std::forward<OutputIterator>(output));
+}
+
+// Copies elements from `input` to `output`. `absl::c_copy(input, output)` is
+// equivalent to `std::copy(std::begin(input), std::end(input),
+// std::begin(output))`.
+//
+// The `output` container must be large enough to hold all elements of `input`;
+// this function does not resize `output`.
+
+// If `std::size(input) > std::size(output)`, behavior is undefined.
+// If `std::size(output) > std::size(input)`, only `std::size(input)` elements
+// are copied, and `output` is not truncated.
+template <typename InputSequence, typename OutputRange>
+constexpr container_algorithm_internal::ResultOfRangeToRangeTransfer<
+    InputSequence, OutputRange>
+c_copy(const InputSequence& input, OutputRange&& output) {
+  container_algorithm_internal::AssertCopySize(input, output);
+  absl::c_copy(input, container_algorithm_internal::c_begin(output));
 }
 
 // c_copy_n()
@@ -473,8 +642,30 @@ OutputIterator c_copy(const InputSequence& input, OutputIterator output) {
 // Container-based version of the <algorithm> `std::copy_n()` function to copy a
 // container's first N elements into an iterator.
 template <typename C, typename Size, typename OutputIterator>
-OutputIterator c_copy_n(const C& input, Size n, OutputIterator output) {
-  return std::copy_n(container_algorithm_internal::c_begin(input), n, output);
+constexpr container_algorithm_internal::ResultOfRangeToIteratorTransfer<
+    C, OutputIterator>
+c_copy_n(const C& input, Size n, OutputIterator&& output) {
+  return std::copy_n(container_algorithm_internal::c_begin(input), n,
+                     std::forward<OutputIterator>(output));
+}
+
+// Copies the first `n` elements from `input` to `output`.
+// `absl::c_copy_n(input, n, output)` is equivalent to
+// `std::copy_n(std::begin(input), n, std::begin(output))`.
+//
+// The `output` container must be large enough to hold N elements; this function
+// does not resize `output`.
+//
+// If `n > std::size(output)` or `n > std::size(input)`, behavior is
+// undefined.
+// If `std::size(output) > n`, only `n` elements are copied, and `output` is not
+// truncated.
+template <typename C, typename Size, typename OutputRange>
+constexpr container_algorithm_internal::ResultOfRangeToRangeTransfer<
+    C, OutputRange>
+c_copy_n(const C& input, Size n, OutputRange&& output) {
+  container_algorithm_internal::AssertCopyNSize(input, n, output);
+  absl::c_copy_n(input, n, container_algorithm_internal::c_begin(output));
 }
 
 // c_copy_if()
@@ -482,8 +673,8 @@ OutputIterator c_copy_n(const C& input, Size n, OutputIterator output) {
 // Container-based version of the <algorithm> `std::copy_if()` function to copy
 // a container's elements satisfying some condition into an iterator.
 template <typename InputSequence, typename OutputIterator, typename Pred>
-OutputIterator c_copy_if(const InputSequence& input, OutputIterator output,
-                         Pred&& pred) {
+constexpr OutputIterator c_copy_if(const InputSequence& input,
+                                   OutputIterator output, Pred&& pred) {
   return std::copy_if(container_algorithm_internal::c_begin(input),
                       container_algorithm_internal::c_end(input), output,
                       std::forward<Pred>(pred));
@@ -494,8 +685,8 @@ OutputIterator c_copy_if(const InputSequence& input, OutputIterator output,
 // Container-based version of the <algorithm> `std::copy_backward()` function to
 // copy a container's elements in reverse order into an iterator.
 template <typename C, typename BidirectionalIterator>
-BidirectionalIterator c_copy_backward(const C& src,
-                                      BidirectionalIterator dest) {
+constexpr BidirectionalIterator c_copy_backward(const C& src,
+                                                BidirectionalIterator dest) {
   return std::copy_backward(container_algorithm_internal::c_begin(src),
                             container_algorithm_internal::c_end(src), dest);
 }
@@ -505,9 +696,26 @@ BidirectionalIterator c_copy_backward(const C& src,
 // Container-based version of the <algorithm> `std::move()` function to move
 // a container's elements into an iterator.
 template <typename C, typename OutputIterator>
-OutputIterator c_move(C&& src, OutputIterator dest) {
+constexpr container_algorithm_internal::ResultOfRangeToIteratorTransfer<
+    C, OutputIterator>
+c_move(C&& src, OutputIterator&& dest) {
   return std::move(container_algorithm_internal::c_begin(src),
-                   container_algorithm_internal::c_end(src), dest);
+                   container_algorithm_internal::c_end(src),
+                   std::forward<OutputIterator>(dest));
+}
+
+// Moves elements from `src` to `dest`. `absl::c_move(src, dest)` is
+// equivalent to `std::move(std::begin(src), std::end(src), std::begin(dest))`.
+//
+// The `dest` container must be large enough to hold all elements of `src`;
+// this function does not resize `dest`.
+template <typename C, typename OutputRange>
+constexpr container_algorithm_internal::ResultOfRangeToRangeTransfer<
+    C, OutputRange>
+c_move(C&& src, OutputRange&& dest) {
+  container_algorithm_internal::AssertCopySize(src, dest);
+  absl::c_move(std::forward<C>(src),
+               container_algorithm_internal::c_begin(dest));
 }
 
 // c_move_backward()
@@ -515,7 +723,8 @@ OutputIterator c_move(C&& src, OutputIterator dest) {
 // Container-based version of the <algorithm> `std::move_backward()` function to
 // move a container's elements into an iterator in reverse order.
 template <typename C, typename BidirectionalIterator>
-BidirectionalIterator c_move_backward(C&& src, BidirectionalIterator dest) {
+constexpr BidirectionalIterator c_move_backward(C&& src,
+                                                BidirectionalIterator dest) {
   return std::move_backward(container_algorithm_internal::c_begin(src),
                             container_algorithm_internal::c_end(src), dest);
 }
@@ -523,12 +732,21 @@ BidirectionalIterator c_move_backward(C&& src, BidirectionalIterator dest) {
 // c_swap_ranges()
 //
 // Container-based version of the <algorithm> `std::swap_ranges()` function to
-// swap a container's elements with another container's elements.
+// swap a container's elements with another container's elements. Swaps the
+// first N elements of `c1` and `c2`, where N = min(size(c1), size(c2)).
 template <typename C1, typename C2>
-container_algorithm_internal::ContainerIter<C2> c_swap_ranges(C1& c1, C2& c2) {
-  return std::swap_ranges(container_algorithm_internal::c_begin(c1),
-                          container_algorithm_internal::c_end(c1),
-                          container_algorithm_internal::c_begin(c2));
+constexpr container_algorithm_internal::ContainerIter<C2> c_swap_ranges(
+    C1& c1, C2& c2) {
+  auto first1 = container_algorithm_internal::c_begin(c1);
+  auto last1 = container_algorithm_internal::c_end(c1);
+  auto first2 = container_algorithm_internal::c_begin(c2);
+  auto last2 = container_algorithm_internal::c_end(c2);
+
+  using std::swap;
+  for (; first1 != last1 && first2 != last2; ++first1, (void)++first2) {
+    swap(*first1, *first2);
+  }
+  return first2;
 }
 
 // c_transform()
@@ -538,24 +756,109 @@ container_algorithm_internal::ContainerIter<C2> c_swap_ranges(C1& c1, C2& c2) {
 // result in an iterator pointing to the last transformed element in the output
 // range.
 template <typename InputSequence, typename OutputIterator, typename UnaryOp>
-OutputIterator c_transform(const InputSequence& input, OutputIterator output,
-                           UnaryOp&& unary_op) {
+constexpr container_algorithm_internal::ResultOfRangeToIteratorTransfer<
+    InputSequence, OutputIterator>
+c_transform(const InputSequence& input, OutputIterator&& output,
+            UnaryOp&& unary_op) {
   return std::transform(container_algorithm_internal::c_begin(input),
-                        container_algorithm_internal::c_end(input), output,
+                        container_algorithm_internal::c_end(input),
+                        std::forward<OutputIterator>(output),
                         std::forward<UnaryOp>(unary_op));
 }
 
+// Performs a transformation using a unary predicate. Stores the result in
+// `output`. `absl::c_transform(input, output, unary_op)` is equivalent to
+// `std::transform(std::begin(input), std::end(input), std::begin(output),
+// unary_op)`.
+//
+// The `output` container must be large enough to hold all elements of `input`;
+// this function does not resize `output`.
+template <typename InputSequence, typename OutputRange, typename UnaryOp>
+constexpr container_algorithm_internal::ResultOfRangeToRangeTransfer<
+    InputSequence, OutputRange>
+c_transform(const InputSequence& input, OutputRange&& output,
+            UnaryOp&& unary_op) {
+  container_algorithm_internal::AssertCopySize(input, output);
+  absl::c_transform(
+      input,
+      container_algorithm_internal::c_begin(std::forward<OutputRange>(output)),
+      std::forward<UnaryOp>(unary_op));
+}
+
 // Overload of c_transform() for performing a transformation using a binary
-// predicate.
+// predicate. Applies `binary_op` to the first N elements of `c1` and `c2`,
+// where N = min(size(c1), size(c2)).
 template <typename InputSequence1, typename InputSequence2,
           typename OutputIterator, typename BinaryOp>
-OutputIterator c_transform(const InputSequence1& input1,
-                           const InputSequence2& input2, OutputIterator output,
-                           BinaryOp&& binary_op) {
-  return std::transform(container_algorithm_internal::c_begin(input1),
-                        container_algorithm_internal::c_end(input1),
-                        container_algorithm_internal::c_begin(input2), output,
-                        std::forward<BinaryOp>(binary_op));
+constexpr container_algorithm_internal::ResultOfRangeToIteratorTransfer<
+    InputSequence1, OutputIterator>
+c_transform(const InputSequence1& input1, const InputSequence2& input2,
+            OutputIterator&& output, BinaryOp&& binary_op) {
+  auto first1 = container_algorithm_internal::c_begin(input1);
+  auto last1 = container_algorithm_internal::c_end(input1);
+  auto first2 = container_algorithm_internal::c_begin(input2);
+  auto last2 = container_algorithm_internal::c_end(input2);
+  std::decay_t<OutputIterator> out = std::forward<OutputIterator>(output);
+  for (; first1 != last1 && first2 != last2; ++first1, (void)++first2, ++out) {
+    *out = binary_op(*first1, *first2);
+  }
+  return out;
+}
+
+// Performs a transformation using a binary predicate. Stores the result in
+// `output`. Applies `binary_op` to the first N elements of `input1` and
+// `input2`, where N = min(size(input1), size(input2)).
+//
+// The `output` container must be large enough to hold all N elements;
+// this function does not resize `output`.
+template <typename InputSequence1, typename InputSequence2,
+          typename OutputRange, typename BinaryOp>
+constexpr std::common_type_t<
+    container_algorithm_internal::ResultOfRangeToRangeTransfer<InputSequence1,
+                                                               OutputRange>,
+    container_algorithm_internal::ResultOfRangeToRangeTransfer<InputSequence2,
+                                                               OutputRange>>
+c_transform(const InputSequence1& input1, const InputSequence2& input2,
+            OutputRange&& output, BinaryOp&& binary_op) {
+  using InputIter1 =
+      container_algorithm_internal::ContainerIter<InputSequence1>;
+  using InputIter2 =
+      container_algorithm_internal::ContainerIter<InputSequence2>;
+  using OutputIter = container_algorithm_internal::ContainerIter<OutputRange>;
+  if constexpr (base_internal::IsAtLeastForwardIterator<OutputIter>::value) {
+    constexpr bool input1_has_size =
+        base_internal::IsAtLeastForwardIterator<InputIter1>::value;
+    constexpr bool input2_has_size =
+        base_internal::IsAtLeastForwardIterator<InputIter2>::value;
+    auto output_size =
+        std::distance(container_algorithm_internal::c_begin(output),
+                      container_algorithm_internal::c_end(output));
+
+    if constexpr (input1_has_size && input2_has_size) {
+      base_internal::HardeningAssertLE(
+          (std::min)(std::distance(
+                         container_algorithm_internal::c_begin(input1),
+                         container_algorithm_internal::c_end(input1)),
+                     std::distance(
+                         container_algorithm_internal::c_begin(input2),
+                         container_algorithm_internal::c_end(input2))),
+          output_size);
+    } else if constexpr (input1_has_size) {
+      base_internal::HardeningAssertLE(
+          std::distance(container_algorithm_internal::c_begin(input1),
+                        container_algorithm_internal::c_end(input1)),
+          output_size);
+    } else if constexpr (input2_has_size) {
+      base_internal::HardeningAssertLE(
+          std::distance(container_algorithm_internal::c_begin(input2),
+                        container_algorithm_internal::c_end(input2)),
+          output_size);
+    }
+  }
+  absl::c_transform(
+      input1, input2,
+      container_algorithm_internal::c_begin(std::forward<OutputRange>(output)),
+      std::forward<BinaryOp>(binary_op));
 }
 
 // c_replace()
@@ -564,7 +867,8 @@ OutputIterator c_transform(const InputSequence1& input1,
 // replace a container's elements of some value with a new value. The container
 // is modified in place.
 template <typename Sequence, typename T>
-void c_replace(Sequence& sequence, const T& old_value, const T& new_value) {
+constexpr void c_replace(Sequence& sequence, const T& old_value,
+                         const T& new_value) {
   std::replace(container_algorithm_internal::c_begin(sequence),
                container_algorithm_internal::c_end(sequence), old_value,
                new_value);
@@ -576,7 +880,7 @@ void c_replace(Sequence& sequence, const T& old_value, const T& new_value) {
 // replace a container's elements of some value with a new value based on some
 // condition. The container is modified in place.
 template <typename C, typename Pred, typename T>
-void c_replace_if(C& c, Pred&& pred, T&& new_value) {
+constexpr void c_replace_if(C& c, Pred&& pred, T&& new_value) {
   std::replace_if(container_algorithm_internal::c_begin(c),
                   container_algorithm_internal::c_end(c),
                   std::forward<Pred>(pred), std::forward<T>(new_value));
@@ -588,8 +892,8 @@ void c_replace_if(C& c, Pred&& pred, T&& new_value) {
 // replace a container's elements of some value with a new value  and return the
 // results within an iterator.
 template <typename C, typename OutputIterator, typename T>
-OutputIterator c_replace_copy(const C& c, OutputIterator result, T&& old_value,
-                              T&& new_value) {
+constexpr OutputIterator c_replace_copy(const C& c, OutputIterator result,
+                                        T&& old_value, T&& new_value) {
   return std::replace_copy(container_algorithm_internal::c_begin(c),
                            container_algorithm_internal::c_end(c), result,
                            std::forward<T>(old_value),
@@ -602,12 +906,11 @@ OutputIterator c_replace_copy(const C& c, OutputIterator result, T&& old_value,
 // to replace a container's elements of some value with a new value based on
 // some condition, and return the results within an iterator.
 template <typename C, typename OutputIterator, typename Pred, typename T>
-OutputIterator c_replace_copy_if(const C& c, OutputIterator result, Pred&& pred,
-                                 T&& new_value) {
+constexpr OutputIterator c_replace_copy_if(const C& c, OutputIterator result,
+                                           Pred&& pred, const T& new_value) {
   return std::replace_copy_if(container_algorithm_internal::c_begin(c),
                               container_algorithm_internal::c_end(c), result,
-                              std::forward<Pred>(pred),
-                              std::forward<T>(new_value));
+                              std::forward<Pred>(pred), new_value);
 }
 
 // c_fill()
@@ -615,9 +918,11 @@ OutputIterator c_replace_copy_if(const C& c, OutputIterator result, Pred&& pred,
 // Container-based version of the <algorithm> `std::fill()` function to fill a
 // container with some value.
 template <typename C, typename T>
-void c_fill(C& c, T&& value) {
+constexpr std::enable_if_t<
+    container_algorithm_internal::IsPermissibleDestinationRange<C>::value, void>
+c_fill(C&& c, const T& value) {
   std::fill(container_algorithm_internal::c_begin(c),
-            container_algorithm_internal::c_end(c), std::forward<T>(value));
+            container_algorithm_internal::c_end(c), value);
 }
 
 // c_fill_n()
@@ -625,9 +930,10 @@ void c_fill(C& c, T&& value) {
 // Container-based version of the <algorithm> `std::fill_n()` function to fill
 // the first N elements in a container with some value.
 template <typename C, typename Size, typename T>
-void c_fill_n(C& c, Size n, T&& value) {
-  std::fill_n(container_algorithm_internal::c_begin(c), n,
-              std::forward<T>(value));
+constexpr std::enable_if_t<
+    container_algorithm_internal::IsPermissibleDestinationRange<C>::value, void>
+c_fill_n(C&& c, Size n, const T& value) {
+  std::fill_n(container_algorithm_internal::c_begin(c), n, value);
 }
 
 // c_generate()
@@ -635,7 +941,7 @@ void c_fill_n(C& c, Size n, T&& value) {
 // Container-based version of the <algorithm> `std::generate()` function to
 // assign a container's elements to the values provided by the given generator.
 template <typename C, typename Generator>
-void c_generate(C& c, Generator&& gen) {
+constexpr void c_generate(C& c, Generator&& gen) {
   std::generate(container_algorithm_internal::c_begin(c),
                 container_algorithm_internal::c_end(c),
                 std::forward<Generator>(gen));
@@ -647,8 +953,8 @@ void c_generate(C& c, Generator&& gen) {
 // assign a container's first N elements to the values provided by the given
 // generator.
 template <typename C, typename Size, typename Generator>
-container_algorithm_internal::ContainerIter<C> c_generate_n(C& c, Size n,
-                                                            Generator&& gen) {
+constexpr container_algorithm_internal::ContainerIter<C> c_generate_n(
+    C& c, Size n, Generator&& gen) {
   return std::generate_n(container_algorithm_internal::c_begin(c), n,
                          std::forward<Generator>(gen));
 }
@@ -664,10 +970,11 @@ container_algorithm_internal::ContainerIter<C> c_generate_n(C& c, Size n,
 // copy a container's elements while removing any elements matching the given
 // `value`.
 template <typename C, typename OutputIterator, typename T>
-OutputIterator c_remove_copy(const C& c, OutputIterator result, T&& value) {
+constexpr OutputIterator c_remove_copy(const C& c, OutputIterator result,
+                                       const T& value) {
   return std::remove_copy(container_algorithm_internal::c_begin(c),
                           container_algorithm_internal::c_end(c), result,
-                          std::forward<T>(value));
+                          value);
 }
 
 // c_remove_copy_if()
@@ -676,8 +983,8 @@ OutputIterator c_remove_copy(const C& c, OutputIterator result, T&& value) {
 // to copy a container's elements while removing any elements matching the given
 // condition.
 template <typename C, typename OutputIterator, typename Pred>
-OutputIterator c_remove_copy_if(const C& c, OutputIterator result,
-                                Pred&& pred) {
+constexpr OutputIterator c_remove_copy_if(const C& c, OutputIterator result,
+                                          Pred&& pred) {
   return std::remove_copy_if(container_algorithm_internal::c_begin(c),
                              container_algorithm_internal::c_end(c), result,
                              std::forward<Pred>(pred));
@@ -689,7 +996,7 @@ OutputIterator c_remove_copy_if(const C& c, OutputIterator result,
 // copy a container's elements while removing any elements containing duplicate
 // values.
 template <typename C, typename OutputIterator>
-OutputIterator c_unique_copy(const C& c, OutputIterator result) {
+constexpr OutputIterator c_unique_copy(const C& c, OutputIterator result) {
   return std::unique_copy(container_algorithm_internal::c_begin(c),
                           container_algorithm_internal::c_end(c), result);
 }
@@ -697,8 +1004,8 @@ OutputIterator c_unique_copy(const C& c, OutputIterator result) {
 // Overload of c_unique_copy() for using a predicate evaluation other than
 // `==` for comparing uniqueness of the element values.
 template <typename C, typename OutputIterator, typename BinaryPredicate>
-OutputIterator c_unique_copy(const C& c, OutputIterator result,
-                             BinaryPredicate&& pred) {
+constexpr OutputIterator c_unique_copy(const C& c, OutputIterator result,
+                                       BinaryPredicate&& pred) {
   return std::unique_copy(container_algorithm_internal::c_begin(c),
                           container_algorithm_internal::c_end(c), result,
                           std::forward<BinaryPredicate>(pred));
@@ -709,7 +1016,7 @@ OutputIterator c_unique_copy(const C& c, OutputIterator result,
 // Container-based version of the <algorithm> `std::reverse()` function to
 // reverse a container's elements.
 template <typename Sequence>
-void c_reverse(Sequence& sequence) {
+constexpr void c_reverse(Sequence& sequence) {
   std::reverse(container_algorithm_internal::c_begin(sequence),
                container_algorithm_internal::c_end(sequence));
 }
@@ -719,7 +1026,8 @@ void c_reverse(Sequence& sequence) {
 // Container-based version of the <algorithm> `std::reverse()` function to
 // reverse a container's elements and write them to an iterator range.
 template <typename C, typename OutputIterator>
-OutputIterator c_reverse_copy(const C& sequence, OutputIterator result) {
+constexpr OutputIterator c_reverse_copy(const C& sequence,
+                                        OutputIterator result) {
   return std::reverse_copy(container_algorithm_internal::c_begin(sequence),
                            container_algorithm_internal::c_end(sequence),
                            result);
@@ -732,9 +1040,9 @@ OutputIterator c_reverse_copy(const C& sequence, OutputIterator result) {
 // the first element in the container.
 template <typename C,
           typename Iterator = container_algorithm_internal::ContainerIter<C>>
-Iterator c_rotate(C& sequence, Iterator middle) {
-  return absl::rotate(container_algorithm_internal::c_begin(sequence), middle,
-                      container_algorithm_internal::c_end(sequence));
+constexpr Iterator c_rotate(C& sequence, Iterator middle) {
+  return std::rotate(container_algorithm_internal::c_begin(sequence), middle,
+                     container_algorithm_internal::c_end(sequence));
 }
 
 // c_rotate_copy()
@@ -743,7 +1051,7 @@ Iterator c_rotate(C& sequence, Iterator middle) {
 // shift a container's elements leftward such that the `middle` element becomes
 // the first element in a new iterator range.
 template <typename C, typename OutputIterator>
-OutputIterator c_rotate_copy(
+constexpr OutputIterator c_rotate_copy(
     const C& sequence,
     container_algorithm_internal::ContainerIter<const C> middle,
     OutputIterator result) {
@@ -764,6 +1072,20 @@ void c_shuffle(RandomAccessContainer& c, UniformRandomBitGenerator&& gen) {
                std::forward<UniformRandomBitGenerator>(gen));
 }
 
+// c_sample()
+//
+// Container-based version of the <algorithm> `std::sample()` function to
+// randomly sample elements from the container without replacement using a
+// `gen()` uniform random number generator and write them to an iterator range.
+template <typename C, typename OutputIterator, typename Distance,
+          typename UniformRandomBitGenerator>
+OutputIterator c_sample(const C& c, OutputIterator result, Distance n,
+                        UniformRandomBitGenerator&& gen) {
+  return std::sample(container_algorithm_internal::c_begin(c),
+                     container_algorithm_internal::c_end(c), result, n,
+                     std::forward<UniformRandomBitGenerator>(gen));
+}
+
 //------------------------------------------------------------------------------
 // <algorithm> Partition functions
 //------------------------------------------------------------------------------
@@ -774,7 +1096,7 @@ void c_shuffle(RandomAccessContainer& c, UniformRandomBitGenerator&& gen) {
 // to test whether all elements in the container for which `pred` returns `true`
 // precede those for which `pred` is `false`.
 template <typename C, typename Pred>
-bool c_is_partitioned(const C& c, Pred&& pred) {
+constexpr bool c_is_partitioned(const C& c, Pred&& pred) {
   return std::is_partitioned(container_algorithm_internal::c_begin(c),
                              container_algorithm_internal::c_end(c),
                              std::forward<Pred>(pred));
@@ -787,7 +1109,8 @@ bool c_is_partitioned(const C& c, Pred&& pred) {
 // which `pred` returns `true` precede all those for which it returns `false`,
 // returning an iterator to the first element of the second group.
 template <typename C, typename Pred>
-container_algorithm_internal::ContainerIter<C> c_partition(C& c, Pred&& pred) {
+constexpr container_algorithm_internal::ContainerIter<C> c_partition(
+    C& c, Pred&& pred) {
   return std::partition(container_algorithm_internal::c_begin(c),
                         container_algorithm_internal::c_end(c),
                         std::forward<Pred>(pred));
@@ -816,7 +1139,7 @@ container_algorithm_internal::ContainerIter<C> c_stable_partition(C& c,
 
 template <typename C, typename OutputIterator1, typename OutputIterator2,
           typename Pred>
-std::pair<OutputIterator1, OutputIterator2> c_partition_copy(
+constexpr std::pair<OutputIterator1, OutputIterator2> c_partition_copy(
     const C& c, OutputIterator1 out_true, OutputIterator2 out_false,
     Pred&& pred) {
   return std::partition_copy(container_algorithm_internal::c_begin(c),
@@ -830,8 +1153,8 @@ std::pair<OutputIterator1, OutputIterator2> c_partition_copy(
 // to return the first element of an already partitioned container for which
 // the given `pred` is not `true`.
 template <typename C, typename Pred>
-container_algorithm_internal::ContainerIter<C> c_partition_point(C& c,
-                                                                 Pred&& pred) {
+constexpr container_algorithm_internal::ContainerIter<C> c_partition_point(
+    C& c, Pred&& pred) {
   return std::partition_point(container_algorithm_internal::c_begin(c),
                               container_algorithm_internal::c_end(c),
                               std::forward<Pred>(pred));
@@ -846,18 +1169,18 @@ container_algorithm_internal::ContainerIter<C> c_partition_point(C& c,
 // Container-based version of the <algorithm> `std::sort()` function
 // to sort elements in ascending order of their values.
 template <typename C>
-void c_sort(C& c) {
+constexpr void c_sort(C& c) {
   std::sort(container_algorithm_internal::c_begin(c),
             container_algorithm_internal::c_end(c));
 }
 
 // Overload of c_sort() for performing a `comp` comparison other than the
 // default `operator<`.
-template <typename C, typename Compare>
-void c_sort(C& c, Compare&& comp) {
+template <typename C, typename LessThan>
+constexpr void c_sort(C& c, LessThan&& comp) {
   std::sort(container_algorithm_internal::c_begin(c),
             container_algorithm_internal::c_end(c),
-            std::forward<Compare>(comp));
+            std::forward<LessThan>(comp));
 }
 
 // c_stable_sort()
@@ -873,11 +1196,11 @@ void c_stable_sort(C& c) {
 
 // Overload of c_stable_sort() for performing a `comp` comparison other than the
 // default `operator<`.
-template <typename C, typename Compare>
-void c_stable_sort(C& c, Compare&& comp) {
+template <typename C, typename LessThan>
+void c_stable_sort(C& c, LessThan&& comp) {
   std::stable_sort(container_algorithm_internal::c_begin(c),
                    container_algorithm_internal::c_end(c),
-                   std::forward<Compare>(comp));
+                   std::forward<LessThan>(comp));
 }
 
 // c_is_sorted()
@@ -885,18 +1208,18 @@ void c_stable_sort(C& c, Compare&& comp) {
 // Container-based version of the <algorithm> `std::is_sorted()` function
 // to evaluate whether the given container is sorted in ascending order.
 template <typename C>
-bool c_is_sorted(const C& c) {
+constexpr bool c_is_sorted(const C& c) {
   return std::is_sorted(container_algorithm_internal::c_begin(c),
                         container_algorithm_internal::c_end(c));
 }
 
 // c_is_sorted() overload for performing a `comp` comparison other than the
 // default `operator<`.
-template <typename C, typename Compare>
-bool c_is_sorted(const C& c, Compare&& comp) {
+template <typename C, typename LessThan>
+constexpr bool c_is_sorted(const C& c, LessThan&& comp) {
   return std::is_sorted(container_algorithm_internal::c_begin(c),
                         container_algorithm_internal::c_end(c),
-                        std::forward<Compare>(comp));
+                        std::forward<LessThan>(comp));
 }
 
 // c_partial_sort()
@@ -905,7 +1228,7 @@ bool c_is_sorted(const C& c, Compare&& comp) {
 // to rearrange elements within a container such that elements before `middle`
 // are sorted in ascending order.
 template <typename RandomAccessContainer>
-void c_partial_sort(
+constexpr void c_partial_sort(
     RandomAccessContainer& sequence,
     container_algorithm_internal::ContainerIter<RandomAccessContainer> middle) {
   std::partial_sort(container_algorithm_internal::c_begin(sequence), middle,
@@ -914,24 +1237,25 @@ void c_partial_sort(
 
 // Overload of c_partial_sort() for performing a `comp` comparison other than
 // the default `operator<`.
-template <typename RandomAccessContainer, typename Compare>
-void c_partial_sort(
+template <typename RandomAccessContainer, typename LessThan>
+constexpr void c_partial_sort(
     RandomAccessContainer& sequence,
     container_algorithm_internal::ContainerIter<RandomAccessContainer> middle,
-    Compare&& comp) {
+    LessThan&& comp) {
   std::partial_sort(container_algorithm_internal::c_begin(sequence), middle,
                     container_algorithm_internal::c_end(sequence),
-                    std::forward<Compare>(comp));
+                    std::forward<LessThan>(comp));
 }
 
 // c_partial_sort_copy()
 //
 // Container-based version of the <algorithm> `std::partial_sort_copy()`
-// function to sort elements within a container such that elements before
-// `middle` are sorted in ascending order, and return the result within an
-// iterator.
+// function to sort the elements in the given range `result` within the larger
+// `sequence` in ascending order (and using `result` as the output parameter).
+// At most min(result.last - result.first, sequence.last - sequence.first)
+// elements from the sequence will be stored in the result.
 template <typename C, typename RandomAccessContainer>
-container_algorithm_internal::ContainerIter<RandomAccessContainer>
+constexpr container_algorithm_internal::ContainerIter<RandomAccessContainer>
 c_partial_sort_copy(const C& sequence, RandomAccessContainer& result) {
   return std::partial_sort_copy(container_algorithm_internal::c_begin(sequence),
                                 container_algorithm_internal::c_end(sequence),
@@ -941,15 +1265,15 @@ c_partial_sort_copy(const C& sequence, RandomAccessContainer& result) {
 
 // Overload of c_partial_sort_copy() for performing a `comp` comparison other
 // than the default `operator<`.
-template <typename C, typename RandomAccessContainer, typename Compare>
-container_algorithm_internal::ContainerIter<RandomAccessContainer>
+template <typename C, typename RandomAccessContainer, typename LessThan>
+constexpr container_algorithm_internal::ContainerIter<RandomAccessContainer>
 c_partial_sort_copy(const C& sequence, RandomAccessContainer& result,
-                    Compare&& comp) {
+                    LessThan&& comp) {
   return std::partial_sort_copy(container_algorithm_internal::c_begin(sequence),
                                 container_algorithm_internal::c_end(sequence),
                                 container_algorithm_internal::c_begin(result),
                                 container_algorithm_internal::c_end(result),
-                                std::forward<Compare>(comp));
+                                std::forward<LessThan>(comp));
 }
 
 // c_is_sorted_until()
@@ -958,19 +1282,20 @@ c_partial_sort_copy(const C& sequence, RandomAccessContainer& result,
 // to return the first element within a container that is not sorted in
 // ascending order as an iterator.
 template <typename C>
-container_algorithm_internal::ContainerIter<C> c_is_sorted_until(C& c) {
+constexpr container_algorithm_internal::ContainerIter<C> c_is_sorted_until(
+    C& c) {
   return std::is_sorted_until(container_algorithm_internal::c_begin(c),
                               container_algorithm_internal::c_end(c));
 }
 
 // Overload of c_is_sorted_until() for performing a `comp` comparison other than
 // the default `operator<`.
-template <typename C, typename Compare>
-container_algorithm_internal::ContainerIter<C> c_is_sorted_until(
-    C& c, Compare&& comp) {
+template <typename C, typename LessThan>
+constexpr container_algorithm_internal::ContainerIter<C> c_is_sorted_until(
+    C& c, LessThan&& comp) {
   return std::is_sorted_until(container_algorithm_internal::c_begin(c),
                               container_algorithm_internal::c_end(c),
-                              std::forward<Compare>(comp));
+                              std::forward<LessThan>(comp));
 }
 
 // c_nth_element()
@@ -981,7 +1306,7 @@ container_algorithm_internal::ContainerIter<C> c_is_sorted_until(
 // any order, except that all preceding `nth` will be less than that element,
 // and all following `nth` will be greater than that element.
 template <typename RandomAccessContainer>
-void c_nth_element(
+constexpr void c_nth_element(
     RandomAccessContainer& sequence,
     container_algorithm_internal::ContainerIter<RandomAccessContainer> nth) {
   std::nth_element(container_algorithm_internal::c_begin(sequence), nth,
@@ -990,14 +1315,14 @@ void c_nth_element(
 
 // Overload of c_nth_element() for performing a `comp` comparison other than
 // the default `operator<`.
-template <typename RandomAccessContainer, typename Compare>
-void c_nth_element(
+template <typename RandomAccessContainer, typename LessThan>
+constexpr void c_nth_element(
     RandomAccessContainer& sequence,
     container_algorithm_internal::ContainerIter<RandomAccessContainer> nth,
-    Compare&& comp) {
+    LessThan&& comp) {
   std::nth_element(container_algorithm_internal::c_begin(sequence), nth,
                    container_algorithm_internal::c_end(sequence),
-                   std::forward<Compare>(comp));
+                   std::forward<LessThan>(comp));
 }
 
 //------------------------------------------------------------------------------
@@ -1010,21 +1335,20 @@ void c_nth_element(
 // to return an iterator pointing to the first element in a sorted container
 // which does not compare less than `value`.
 template <typename Sequence, typename T>
-container_algorithm_internal::ContainerIter<Sequence> c_lower_bound(
-    Sequence& sequence, T&& value) {
+constexpr container_algorithm_internal::ContainerIter<Sequence> c_lower_bound(
+    Sequence& sequence, const T& value) {
   return std::lower_bound(container_algorithm_internal::c_begin(sequence),
-                          container_algorithm_internal::c_end(sequence),
-                          std::forward<T>(value));
+                          container_algorithm_internal::c_end(sequence), value);
 }
 
 // Overload of c_lower_bound() for performing a `comp` comparison other than
 // the default `operator<`.
-template <typename Sequence, typename T, typename Compare>
-container_algorithm_internal::ContainerIter<Sequence> c_lower_bound(
-    Sequence& sequence, T&& value, Compare&& comp) {
+template <typename Sequence, typename T, typename LessThan>
+constexpr container_algorithm_internal::ContainerIter<Sequence> c_lower_bound(
+    Sequence& sequence, const T& value, LessThan&& comp) {
   return std::lower_bound(container_algorithm_internal::c_begin(sequence),
-                          container_algorithm_internal::c_end(sequence),
-                          std::forward<T>(value), std::forward<Compare>(comp));
+                          container_algorithm_internal::c_end(sequence), value,
+                          std::forward<LessThan>(comp));
 }
 
 // c_upper_bound()
@@ -1033,21 +1357,20 @@ container_algorithm_internal::ContainerIter<Sequence> c_lower_bound(
 // to return an iterator pointing to the first element in a sorted container
 // which is greater than `value`.
 template <typename Sequence, typename T>
-container_algorithm_internal::ContainerIter<Sequence> c_upper_bound(
-    Sequence& sequence, T&& value) {
+constexpr container_algorithm_internal::ContainerIter<Sequence> c_upper_bound(
+    Sequence& sequence, const T& value) {
   return std::upper_bound(container_algorithm_internal::c_begin(sequence),
-                          container_algorithm_internal::c_end(sequence),
-                          std::forward<T>(value));
+                          container_algorithm_internal::c_end(sequence), value);
 }
 
 // Overload of c_upper_bound() for performing a `comp` comparison other than
 // the default `operator<`.
-template <typename Sequence, typename T, typename Compare>
-container_algorithm_internal::ContainerIter<Sequence> c_upper_bound(
-    Sequence& sequence, T&& value, Compare&& comp) {
+template <typename Sequence, typename T, typename LessThan>
+constexpr container_algorithm_internal::ContainerIter<Sequence> c_upper_bound(
+    Sequence& sequence, const T& value, LessThan&& comp) {
   return std::upper_bound(container_algorithm_internal::c_begin(sequence),
-                          container_algorithm_internal::c_end(sequence),
-                          std::forward<T>(value), std::forward<Compare>(comp));
+                          container_algorithm_internal::c_end(sequence), value,
+                          std::forward<LessThan>(comp));
 }
 
 // c_equal_range()
@@ -1056,21 +1379,22 @@ container_algorithm_internal::ContainerIter<Sequence> c_upper_bound(
 // to return an iterator pair pointing to the first and last elements in a
 // sorted container which compare equal to `value`.
 template <typename Sequence, typename T>
-container_algorithm_internal::ContainerIterPairType<Sequence, Sequence>
-c_equal_range(Sequence& sequence, T&& value) {
+constexpr container_algorithm_internal::ContainerIterPairType<Sequence,
+                                                              Sequence>
+c_equal_range(Sequence& sequence, const T& value) {
   return std::equal_range(container_algorithm_internal::c_begin(sequence),
-                          container_algorithm_internal::c_end(sequence),
-                          std::forward<T>(value));
+                          container_algorithm_internal::c_end(sequence), value);
 }
 
 // Overload of c_equal_range() for performing a `comp` comparison other than
 // the default `operator<`.
-template <typename Sequence, typename T, typename Compare>
-container_algorithm_internal::ContainerIterPairType<Sequence, Sequence>
-c_equal_range(Sequence& sequence, T&& value, Compare&& comp) {
+template <typename Sequence, typename T, typename LessThan>
+constexpr container_algorithm_internal::ContainerIterPairType<Sequence,
+                                                              Sequence>
+c_equal_range(Sequence& sequence, const T& value, LessThan&& comp) {
   return std::equal_range(container_algorithm_internal::c_begin(sequence),
-                          container_algorithm_internal::c_end(sequence),
-                          std::forward<T>(value), std::forward<Compare>(comp));
+                          container_algorithm_internal::c_end(sequence), value,
+                          std::forward<LessThan>(comp));
 }
 
 // c_binary_search()
@@ -1079,20 +1403,20 @@ c_equal_range(Sequence& sequence, T&& value, Compare&& comp) {
 // to test if any element in the sorted container contains a value equivalent to
 // 'value'.
 template <typename Sequence, typename T>
-bool c_binary_search(Sequence&& sequence, T&& value) {
+constexpr bool c_binary_search(const Sequence& sequence, const T& value) {
   return std::binary_search(container_algorithm_internal::c_begin(sequence),
                             container_algorithm_internal::c_end(sequence),
-                            std::forward<T>(value));
+                            value);
 }
 
 // Overload of c_binary_search() for performing a `comp` comparison other than
 // the default `operator<`.
-template <typename Sequence, typename T, typename Compare>
-bool c_binary_search(Sequence&& sequence, T&& value, Compare&& comp) {
+template <typename Sequence, typename T, typename LessThan>
+constexpr bool c_binary_search(const Sequence& sequence, const T& value,
+                               LessThan&& comp) {
   return std::binary_search(container_algorithm_internal::c_begin(sequence),
                             container_algorithm_internal::c_end(sequence),
-                            std::forward<T>(value),
-                            std::forward<Compare>(comp));
+                            value, std::forward<LessThan>(comp));
 }
 
 //------------------------------------------------------------------------------
@@ -1104,7 +1428,8 @@ bool c_binary_search(Sequence&& sequence, T&& value, Compare&& comp) {
 // Container-based version of the <algorithm> `std::merge()` function
 // to merge two sorted containers into a single sorted iterator.
 template <typename C1, typename C2, typename OutputIterator>
-OutputIterator c_merge(const C1& c1, const C2& c2, OutputIterator result) {
+constexpr OutputIterator c_merge(const C1& c1, const C2& c2,
+                                 OutputIterator result) {
   return std::merge(container_algorithm_internal::c_begin(c1),
                     container_algorithm_internal::c_end(c1),
                     container_algorithm_internal::c_begin(c2),
@@ -1113,14 +1438,14 @@ OutputIterator c_merge(const C1& c1, const C2& c2, OutputIterator result) {
 
 // Overload of c_merge() for performing a `comp` comparison other than
 // the default `operator<`.
-template <typename C1, typename C2, typename OutputIterator, typename Compare>
-OutputIterator c_merge(const C1& c1, const C2& c2, OutputIterator result,
-                       Compare&& comp) {
+template <typename C1, typename C2, typename OutputIterator, typename LessThan>
+constexpr OutputIterator c_merge(const C1& c1, const C2& c2,
+                                 OutputIterator result, LessThan&& comp) {
   return std::merge(container_algorithm_internal::c_begin(c1),
                     container_algorithm_internal::c_end(c1),
                     container_algorithm_internal::c_begin(c2),
                     container_algorithm_internal::c_end(c2), result,
-                    std::forward<Compare>(comp));
+                    std::forward<LessThan>(comp));
 }
 
 // c_inplace_merge()
@@ -1136,13 +1461,13 @@ void c_inplace_merge(C& c,
 
 // Overload of c_inplace_merge() for performing a merge using a `comp` other
 // than `operator<`.
-template <typename C, typename Compare>
+template <typename C, typename LessThan>
 void c_inplace_merge(C& c,
                      container_algorithm_internal::ContainerIter<C> middle,
-                     Compare&& comp) {
+                     LessThan&& comp) {
   std::inplace_merge(container_algorithm_internal::c_begin(c), middle,
                      container_algorithm_internal::c_end(c),
-                     std::forward<Compare>(comp));
+                     std::forward<LessThan>(comp));
 }
 
 // c_includes()
@@ -1151,7 +1476,7 @@ void c_inplace_merge(C& c,
 // to test whether a sorted container `c1` entirely contains another sorted
 // container `c2`.
 template <typename C1, typename C2>
-bool c_includes(const C1& c1, const C2& c2) {
+constexpr bool c_includes(const C1& c1, const C2& c2) {
   return std::includes(container_algorithm_internal::c_begin(c1),
                        container_algorithm_internal::c_end(c1),
                        container_algorithm_internal::c_begin(c2),
@@ -1160,13 +1485,13 @@ bool c_includes(const C1& c1, const C2& c2) {
 
 // Overload of c_includes() for performing a merge using a `comp` other than
 // `operator<`.
-template <typename C1, typename C2, typename Compare>
-bool c_includes(const C1& c1, const C2& c2, Compare&& comp) {
+template <typename C1, typename C2, typename LessThan>
+constexpr bool c_includes(const C1& c1, const C2& c2, LessThan&& comp) {
   return std::includes(container_algorithm_internal::c_begin(c1),
                        container_algorithm_internal::c_end(c1),
                        container_algorithm_internal::c_begin(c2),
                        container_algorithm_internal::c_end(c2),
-                       std::forward<Compare>(comp));
+                       std::forward<LessThan>(comp));
 }
 
 // c_set_union()
@@ -1174,14 +1499,14 @@ bool c_includes(const C1& c1, const C2& c2, Compare&& comp) {
 // Container-based version of the <algorithm> `std::set_union()` function
 // to return an iterator containing the union of two containers; duplicate
 // values are not copied into the output.
-template <typename C1, typename C2, typename OutputIterator,
-          typename = typename std::enable_if<
-              !container_algorithm_internal::IsUnorderedContainer<C1>::value,
-              void>::type,
-          typename = typename std::enable_if<
-              !container_algorithm_internal::IsUnorderedContainer<C2>::value,
-              void>::type>
-OutputIterator c_set_union(const C1& c1, const C2& c2, OutputIterator output) {
+template <
+    typename C1, typename C2, typename OutputIterator,
+    typename = std::enable_if_t<
+        !container_algorithm_internal::IsUnorderedContainer<C1>::value, void>,
+    typename = std::enable_if_t<
+        !container_algorithm_internal::IsUnorderedContainer<C2>::value, void>>
+constexpr OutputIterator c_set_union(const C1& c1, const C2& c2,
+                                     OutputIterator output) {
   return std::set_union(container_algorithm_internal::c_begin(c1),
                         container_algorithm_internal::c_end(c1),
                         container_algorithm_internal::c_begin(c2),
@@ -1190,35 +1515,38 @@ OutputIterator c_set_union(const C1& c1, const C2& c2, OutputIterator output) {
 
 // Overload of c_set_union() for performing a merge using a `comp` other than
 // `operator<`.
-template <typename C1, typename C2, typename OutputIterator, typename Compare,
-          typename = typename std::enable_if<
-              !container_algorithm_internal::IsUnorderedContainer<C1>::value,
-              void>::type,
-          typename = typename std::enable_if<
-              !container_algorithm_internal::IsUnorderedContainer<C2>::value,
-              void>::type>
-OutputIterator c_set_union(const C1& c1, const C2& c2, OutputIterator output,
-                           Compare&& comp) {
+template <
+    typename C1, typename C2, typename OutputIterator, typename LessThan,
+    typename = std::enable_if_t<
+        !container_algorithm_internal::IsUnorderedContainer<C1>::value, void>,
+    typename = std::enable_if_t<
+        !container_algorithm_internal::IsUnorderedContainer<C2>::value, void>>
+constexpr OutputIterator c_set_union(const C1& c1, const C2& c2,
+                                     OutputIterator output, LessThan&& comp) {
   return std::set_union(container_algorithm_internal::c_begin(c1),
                         container_algorithm_internal::c_end(c1),
                         container_algorithm_internal::c_begin(c2),
                         container_algorithm_internal::c_end(c2), output,
-                        std::forward<Compare>(comp));
+                        std::forward<LessThan>(comp));
 }
 
 // c_set_intersection()
 //
 // Container-based version of the <algorithm> `std::set_intersection()` function
-// to return an iterator containing the intersection of two containers.
-template <typename C1, typename C2, typename OutputIterator,
-          typename = typename std::enable_if<
-              !container_algorithm_internal::IsUnorderedContainer<C1>::value,
-              void>::type,
-          typename = typename std::enable_if<
-              !container_algorithm_internal::IsUnorderedContainer<C2>::value,
-              void>::type>
-OutputIterator c_set_intersection(const C1& c1, const C2& c2,
-                                  OutputIterator output) {
+// to return an iterator containing the intersection of two sorted containers.
+template <
+    typename C1, typename C2, typename OutputIterator,
+    typename = std::enable_if_t<
+        !container_algorithm_internal::IsUnorderedContainer<C1>::value, void>,
+    typename = std::enable_if_t<
+        !container_algorithm_internal::IsUnorderedContainer<C2>::value, void>>
+constexpr OutputIterator c_set_intersection(const C1& c1, const C2& c2,
+                                            OutputIterator output) {
+  // In debug builds, ensure that both containers are sorted with respect to the
+  // default comparator. std::set_intersection requires the containers be sorted
+  // using operator<.
+  ABSL_ASSERT(absl::c_is_sorted(c1));
+  ABSL_ASSERT(absl::c_is_sorted(c2));
   return std::set_intersection(container_algorithm_internal::c_begin(c1),
                                container_algorithm_internal::c_end(c1),
                                container_algorithm_internal::c_begin(c2),
@@ -1227,20 +1555,25 @@ OutputIterator c_set_intersection(const C1& c1, const C2& c2,
 
 // Overload of c_set_intersection() for performing a merge using a `comp` other
 // than `operator<`.
-template <typename C1, typename C2, typename OutputIterator, typename Compare,
-          typename = typename std::enable_if<
-              !container_algorithm_internal::IsUnorderedContainer<C1>::value,
-              void>::type,
-          typename = typename std::enable_if<
-              !container_algorithm_internal::IsUnorderedContainer<C2>::value,
-              void>::type>
-OutputIterator c_set_intersection(const C1& c1, const C2& c2,
-                                  OutputIterator output, Compare&& comp) {
+template <
+    typename C1, typename C2, typename OutputIterator, typename LessThan,
+    typename = std::enable_if_t<
+        !container_algorithm_internal::IsUnorderedContainer<C1>::value, void>,
+    typename = std::enable_if_t<
+        !container_algorithm_internal::IsUnorderedContainer<C2>::value, void>>
+constexpr OutputIterator c_set_intersection(const C1& c1, const C2& c2,
+                                            OutputIterator output,
+                                            LessThan&& comp) {
+  // In debug builds, ensure that both containers are sorted with respect to the
+  // default comparator. std::set_intersection requires the containers be sorted
+  // using the same comparator.
+  ABSL_ASSERT(absl::c_is_sorted(c1, comp));
+  ABSL_ASSERT(absl::c_is_sorted(c2, comp));
   return std::set_intersection(container_algorithm_internal::c_begin(c1),
                                container_algorithm_internal::c_end(c1),
                                container_algorithm_internal::c_begin(c2),
                                container_algorithm_internal::c_end(c2), output,
-                               std::forward<Compare>(comp));
+                               std::forward<LessThan>(comp));
 }
 
 // c_set_difference()
@@ -1248,15 +1581,14 @@ OutputIterator c_set_intersection(const C1& c1, const C2& c2,
 // Container-based version of the <algorithm> `std::set_difference()` function
 // to return an iterator containing elements present in the first container but
 // not in the second.
-template <typename C1, typename C2, typename OutputIterator,
-          typename = typename std::enable_if<
-              !container_algorithm_internal::IsUnorderedContainer<C1>::value,
-              void>::type,
-          typename = typename std::enable_if<
-              !container_algorithm_internal::IsUnorderedContainer<C2>::value,
-              void>::type>
-OutputIterator c_set_difference(const C1& c1, const C2& c2,
-                                OutputIterator output) {
+template <
+    typename C1, typename C2, typename OutputIterator,
+    typename = std::enable_if_t<
+        !container_algorithm_internal::IsUnorderedContainer<C1>::value, void>,
+    typename = std::enable_if_t<
+        !container_algorithm_internal::IsUnorderedContainer<C2>::value, void>>
+constexpr OutputIterator c_set_difference(const C1& c1, const C2& c2,
+                                          OutputIterator output) {
   return std::set_difference(container_algorithm_internal::c_begin(c1),
                              container_algorithm_internal::c_end(c1),
                              container_algorithm_internal::c_begin(c2),
@@ -1265,20 +1597,20 @@ OutputIterator c_set_difference(const C1& c1, const C2& c2,
 
 // Overload of c_set_difference() for performing a merge using a `comp` other
 // than `operator<`.
-template <typename C1, typename C2, typename OutputIterator, typename Compare,
-          typename = typename std::enable_if<
-              !container_algorithm_internal::IsUnorderedContainer<C1>::value,
-              void>::type,
-          typename = typename std::enable_if<
-              !container_algorithm_internal::IsUnorderedContainer<C2>::value,
-              void>::type>
-OutputIterator c_set_difference(const C1& c1, const C2& c2,
-                                OutputIterator output, Compare&& comp) {
+template <
+    typename C1, typename C2, typename OutputIterator, typename LessThan,
+    typename = std::enable_if_t<
+        !container_algorithm_internal::IsUnorderedContainer<C1>::value, void>,
+    typename = std::enable_if_t<
+        !container_algorithm_internal::IsUnorderedContainer<C2>::value, void>>
+constexpr OutputIterator c_set_difference(const C1& c1, const C2& c2,
+                                          OutputIterator output,
+                                          LessThan&& comp) {
   return std::set_difference(container_algorithm_internal::c_begin(c1),
                              container_algorithm_internal::c_end(c1),
                              container_algorithm_internal::c_begin(c2),
                              container_algorithm_internal::c_end(c2), output,
-                             std::forward<Compare>(comp));
+                             std::forward<LessThan>(comp));
 }
 
 // c_set_symmetric_difference()
@@ -1286,15 +1618,14 @@ OutputIterator c_set_difference(const C1& c1, const C2& c2,
 // Container-based version of the <algorithm> `std::set_symmetric_difference()`
 // function to return an iterator containing elements present in either one
 // container or the other, but not both.
-template <typename C1, typename C2, typename OutputIterator,
-          typename = typename std::enable_if<
-              !container_algorithm_internal::IsUnorderedContainer<C1>::value,
-              void>::type,
-          typename = typename std::enable_if<
-              !container_algorithm_internal::IsUnorderedContainer<C2>::value,
-              void>::type>
-OutputIterator c_set_symmetric_difference(const C1& c1, const C2& c2,
-                                          OutputIterator output) {
+template <
+    typename C1, typename C2, typename OutputIterator,
+    typename = std::enable_if_t<
+        !container_algorithm_internal::IsUnorderedContainer<C1>::value, void>,
+    typename = std::enable_if_t<
+        !container_algorithm_internal::IsUnorderedContainer<C2>::value, void>>
+constexpr OutputIterator c_set_symmetric_difference(const C1& c1, const C2& c2,
+                                                    OutputIterator output) {
   return std::set_symmetric_difference(
       container_algorithm_internal::c_begin(c1),
       container_algorithm_internal::c_end(c1),
@@ -1304,22 +1635,21 @@ OutputIterator c_set_symmetric_difference(const C1& c1, const C2& c2,
 
 // Overload of c_set_symmetric_difference() for performing a merge using a
 // `comp` other than `operator<`.
-template <typename C1, typename C2, typename OutputIterator, typename Compare,
-          typename = typename std::enable_if<
-              !container_algorithm_internal::IsUnorderedContainer<C1>::value,
-              void>::type,
-          typename = typename std::enable_if<
-              !container_algorithm_internal::IsUnorderedContainer<C2>::value,
-              void>::type>
-OutputIterator c_set_symmetric_difference(const C1& c1, const C2& c2,
-                                          OutputIterator output,
-                                          Compare&& comp) {
+template <
+    typename C1, typename C2, typename OutputIterator, typename LessThan,
+    typename = std::enable_if_t<
+        !container_algorithm_internal::IsUnorderedContainer<C1>::value, void>,
+    typename = std::enable_if_t<
+        !container_algorithm_internal::IsUnorderedContainer<C2>::value, void>>
+constexpr OutputIterator c_set_symmetric_difference(const C1& c1, const C2& c2,
+                                                    OutputIterator output,
+                                                    LessThan&& comp) {
   return std::set_symmetric_difference(
       container_algorithm_internal::c_begin(c1),
       container_algorithm_internal::c_end(c1),
       container_algorithm_internal::c_begin(c2),
       container_algorithm_internal::c_end(c2), output,
-      std::forward<Compare>(comp));
+      std::forward<LessThan>(comp));
 }
 
 //------------------------------------------------------------------------------
@@ -1331,18 +1661,18 @@ OutputIterator c_set_symmetric_difference(const C1& c1, const C2& c2,
 // Container-based version of the <algorithm> `std::push_heap()` function
 // to push a value onto a container heap.
 template <typename RandomAccessContainer>
-void c_push_heap(RandomAccessContainer& sequence) {
+constexpr void c_push_heap(RandomAccessContainer& sequence) {
   std::push_heap(container_algorithm_internal::c_begin(sequence),
                  container_algorithm_internal::c_end(sequence));
 }
 
 // Overload of c_push_heap() for performing a push operation on a heap using a
 // `comp` other than `operator<`.
-template <typename RandomAccessContainer, typename Compare>
-void c_push_heap(RandomAccessContainer& sequence, Compare&& comp) {
+template <typename RandomAccessContainer, typename LessThan>
+constexpr void c_push_heap(RandomAccessContainer& sequence, LessThan&& comp) {
   std::push_heap(container_algorithm_internal::c_begin(sequence),
                  container_algorithm_internal::c_end(sequence),
-                 std::forward<Compare>(comp));
+                 std::forward<LessThan>(comp));
 }
 
 // c_pop_heap()
@@ -1350,18 +1680,18 @@ void c_push_heap(RandomAccessContainer& sequence, Compare&& comp) {
 // Container-based version of the <algorithm> `std::pop_heap()` function
 // to pop a value from a heap container.
 template <typename RandomAccessContainer>
-void c_pop_heap(RandomAccessContainer& sequence) {
+constexpr void c_pop_heap(RandomAccessContainer& sequence) {
   std::pop_heap(container_algorithm_internal::c_begin(sequence),
                 container_algorithm_internal::c_end(sequence));
 }
 
 // Overload of c_pop_heap() for performing a pop operation on a heap using a
 // `comp` other than `operator<`.
-template <typename RandomAccessContainer, typename Compare>
-void c_pop_heap(RandomAccessContainer& sequence, Compare&& comp) {
+template <typename RandomAccessContainer, typename LessThan>
+constexpr void c_pop_heap(RandomAccessContainer& sequence, LessThan&& comp) {
   std::pop_heap(container_algorithm_internal::c_begin(sequence),
                 container_algorithm_internal::c_end(sequence),
-                std::forward<Compare>(comp));
+                std::forward<LessThan>(comp));
 }
 
 // c_make_heap()
@@ -1369,18 +1699,18 @@ void c_pop_heap(RandomAccessContainer& sequence, Compare&& comp) {
 // Container-based version of the <algorithm> `std::make_heap()` function
 // to make a container a heap.
 template <typename RandomAccessContainer>
-void c_make_heap(RandomAccessContainer& sequence) {
+constexpr void c_make_heap(RandomAccessContainer& sequence) {
   std::make_heap(container_algorithm_internal::c_begin(sequence),
                  container_algorithm_internal::c_end(sequence));
 }
 
 // Overload of c_make_heap() for performing heap comparisons using a
 // `comp` other than `operator<`
-template <typename RandomAccessContainer, typename Compare>
-void c_make_heap(RandomAccessContainer& sequence, Compare&& comp) {
+template <typename RandomAccessContainer, typename LessThan>
+constexpr void c_make_heap(RandomAccessContainer& sequence, LessThan&& comp) {
   std::make_heap(container_algorithm_internal::c_begin(sequence),
                  container_algorithm_internal::c_end(sequence),
-                 std::forward<Compare>(comp));
+                 std::forward<LessThan>(comp));
 }
 
 // c_sort_heap()
@@ -1388,18 +1718,18 @@ void c_make_heap(RandomAccessContainer& sequence, Compare&& comp) {
 // Container-based version of the <algorithm> `std::sort_heap()` function
 // to sort a heap into ascending order (after which it is no longer a heap).
 template <typename RandomAccessContainer>
-void c_sort_heap(RandomAccessContainer& sequence) {
+constexpr void c_sort_heap(RandomAccessContainer& sequence) {
   std::sort_heap(container_algorithm_internal::c_begin(sequence),
                  container_algorithm_internal::c_end(sequence));
 }
 
 // Overload of c_sort_heap() for performing heap comparisons using a
 // `comp` other than `operator<`
-template <typename RandomAccessContainer, typename Compare>
-void c_sort_heap(RandomAccessContainer& sequence, Compare&& comp) {
+template <typename RandomAccessContainer, typename LessThan>
+constexpr void c_sort_heap(RandomAccessContainer& sequence, LessThan&& comp) {
   std::sort_heap(container_algorithm_internal::c_begin(sequence),
                  container_algorithm_internal::c_end(sequence),
-                 std::forward<Compare>(comp));
+                 std::forward<LessThan>(comp));
 }
 
 // c_is_heap()
@@ -1407,18 +1737,19 @@ void c_sort_heap(RandomAccessContainer& sequence, Compare&& comp) {
 // Container-based version of the <algorithm> `std::is_heap()` function
 // to check whether the given container is a heap.
 template <typename RandomAccessContainer>
-bool c_is_heap(const RandomAccessContainer& sequence) {
+constexpr bool c_is_heap(const RandomAccessContainer& sequence) {
   return std::is_heap(container_algorithm_internal::c_begin(sequence),
                       container_algorithm_internal::c_end(sequence));
 }
 
 // Overload of c_is_heap() for performing heap comparisons using a
 // `comp` other than `operator<`
-template <typename RandomAccessContainer, typename Compare>
-bool c_is_heap(const RandomAccessContainer& sequence, Compare&& comp) {
+template <typename RandomAccessContainer, typename LessThan>
+constexpr bool c_is_heap(const RandomAccessContainer& sequence,
+                         LessThan&& comp) {
   return std::is_heap(container_algorithm_internal::c_begin(sequence),
                       container_algorithm_internal::c_end(sequence),
-                      std::forward<Compare>(comp));
+                      std::forward<LessThan>(comp));
 }
 
 // c_is_heap_until()
@@ -1426,7 +1757,7 @@ bool c_is_heap(const RandomAccessContainer& sequence, Compare&& comp) {
 // Container-based version of the <algorithm> `std::is_heap_until()` function
 // to find the first element in a given container which is not in heap order.
 template <typename RandomAccessContainer>
-container_algorithm_internal::ContainerIter<RandomAccessContainer>
+constexpr container_algorithm_internal::ContainerIter<RandomAccessContainer>
 c_is_heap_until(RandomAccessContainer& sequence) {
   return std::is_heap_until(container_algorithm_internal::c_begin(sequence),
                             container_algorithm_internal::c_end(sequence));
@@ -1434,12 +1765,12 @@ c_is_heap_until(RandomAccessContainer& sequence) {
 
 // Overload of c_is_heap_until() for performing heap comparisons using a
 // `comp` other than `operator<`
-template <typename RandomAccessContainer, typename Compare>
-container_algorithm_internal::ContainerIter<RandomAccessContainer>
-c_is_heap_until(RandomAccessContainer& sequence, Compare&& comp) {
+template <typename RandomAccessContainer, typename LessThan>
+constexpr container_algorithm_internal::ContainerIter<RandomAccessContainer>
+c_is_heap_until(RandomAccessContainer& sequence, LessThan&& comp) {
   return std::is_heap_until(container_algorithm_internal::c_begin(sequence),
                             container_algorithm_internal::c_end(sequence),
-                            std::forward<Compare>(comp));
+                            std::forward<LessThan>(comp));
 }
 
 //------------------------------------------------------------------------------
@@ -1452,7 +1783,7 @@ c_is_heap_until(RandomAccessContainer& sequence, Compare&& comp) {
 // to return an iterator pointing to the element with the smallest value, using
 // `operator<` to make the comparisons.
 template <typename Sequence>
-container_algorithm_internal::ContainerIter<Sequence> c_min_element(
+constexpr container_algorithm_internal::ContainerIter<Sequence> c_min_element(
     Sequence& sequence) {
   return std::min_element(container_algorithm_internal::c_begin(sequence),
                           container_algorithm_internal::c_end(sequence));
@@ -1460,12 +1791,12 @@ container_algorithm_internal::ContainerIter<Sequence> c_min_element(
 
 // Overload of c_min_element() for performing a `comp` comparison other than
 // `operator<`.
-template <typename Sequence, typename Compare>
-container_algorithm_internal::ContainerIter<Sequence> c_min_element(
-    Sequence& sequence, Compare&& comp) {
+template <typename Sequence, typename LessThan>
+constexpr container_algorithm_internal::ContainerIter<Sequence> c_min_element(
+    Sequence& sequence, LessThan&& comp) {
   return std::min_element(container_algorithm_internal::c_begin(sequence),
                           container_algorithm_internal::c_end(sequence),
-                          std::forward<Compare>(comp));
+                          std::forward<LessThan>(comp));
 }
 
 // c_max_element()
@@ -1474,7 +1805,7 @@ container_algorithm_internal::ContainerIter<Sequence> c_min_element(
 // to return an iterator pointing to the element with the largest value, using
 // `operator<` to make the comparisons.
 template <typename Sequence>
-container_algorithm_internal::ContainerIter<Sequence> c_max_element(
+constexpr container_algorithm_internal::ContainerIter<Sequence> c_max_element(
     Sequence& sequence) {
   return std::max_element(container_algorithm_internal::c_begin(sequence),
                           container_algorithm_internal::c_end(sequence));
@@ -1482,12 +1813,12 @@ container_algorithm_internal::ContainerIter<Sequence> c_max_element(
 
 // Overload of c_max_element() for performing a `comp` comparison other than
 // `operator<`.
-template <typename Sequence, typename Compare>
-container_algorithm_internal::ContainerIter<Sequence> c_max_element(
-    Sequence& sequence, Compare&& comp) {
+template <typename Sequence, typename LessThan>
+constexpr container_algorithm_internal::ContainerIter<Sequence> c_max_element(
+    Sequence& sequence, LessThan&& comp) {
   return std::max_element(container_algorithm_internal::c_begin(sequence),
                           container_algorithm_internal::c_end(sequence),
-                          std::forward<Compare>(comp));
+                          std::forward<LessThan>(comp));
 }
 
 // c_minmax_element()
@@ -1497,7 +1828,7 @@ container_algorithm_internal::ContainerIter<Sequence> c_max_element(
 // smallest and largest values, respectively, using `operator<` to make the
 // comparisons.
 template <typename C>
-container_algorithm_internal::ContainerIterPairType<C, C>
+constexpr container_algorithm_internal::ContainerIterPairType<C, C>
 c_minmax_element(C& c) {
   return std::minmax_element(container_algorithm_internal::c_begin(c),
                              container_algorithm_internal::c_end(c));
@@ -1505,12 +1836,12 @@ c_minmax_element(C& c) {
 
 // Overload of c_minmax_element() for performing `comp` comparisons other than
 // `operator<`.
-template <typename C, typename Compare>
-container_algorithm_internal::ContainerIterPairType<C, C>
-c_minmax_element(C& c, Compare&& comp) {
+template <typename C, typename LessThan>
+constexpr container_algorithm_internal::ContainerIterPairType<C, C>
+c_minmax_element(C& c, LessThan&& comp) {
   return std::minmax_element(container_algorithm_internal::c_begin(c),
                              container_algorithm_internal::c_end(c),
-                             std::forward<Compare>(comp));
+                             std::forward<LessThan>(comp));
 }
 
 //------------------------------------------------------------------------------
@@ -1525,7 +1856,8 @@ c_minmax_element(C& c, Compare&& comp) {
 // that capital letters ("A-Z") have ASCII values less than lowercase letters
 // ("a-z").
 template <typename Sequence1, typename Sequence2>
-bool c_lexicographical_compare(Sequence1&& sequence1, Sequence2&& sequence2) {
+constexpr bool c_lexicographical_compare(const Sequence1& sequence1,
+                                         const Sequence2& sequence2) {
   return std::lexicographical_compare(
       container_algorithm_internal::c_begin(sequence1),
       container_algorithm_internal::c_end(sequence1),
@@ -1535,15 +1867,16 @@ bool c_lexicographical_compare(Sequence1&& sequence1, Sequence2&& sequence2) {
 
 // Overload of c_lexicographical_compare() for performing a lexicographical
 // comparison using a `comp` operator instead of `operator<`.
-template <typename Sequence1, typename Sequence2, typename Compare>
-bool c_lexicographical_compare(Sequence1&& sequence1, Sequence2&& sequence2,
-                               Compare&& comp) {
+template <typename Sequence1, typename Sequence2, typename LessThan>
+constexpr bool c_lexicographical_compare(const Sequence1& sequence1,
+                                         const Sequence2& sequence2,
+                                         LessThan&& comp) {
   return std::lexicographical_compare(
       container_algorithm_internal::c_begin(sequence1),
       container_algorithm_internal::c_end(sequence1),
       container_algorithm_internal::c_begin(sequence2),
       container_algorithm_internal::c_end(sequence2),
-      std::forward<Compare>(comp));
+      std::forward<LessThan>(comp));
 }
 
 // c_next_permutation()
@@ -1552,18 +1885,18 @@ bool c_lexicographical_compare(Sequence1&& sequence1, Sequence2&& sequence2,
 // to rearrange a container's elements into the next lexicographically greater
 // permutation.
 template <typename C>
-bool c_next_permutation(C& c) {
+constexpr bool c_next_permutation(C& c) {
   return std::next_permutation(container_algorithm_internal::c_begin(c),
                                container_algorithm_internal::c_end(c));
 }
 
 // Overload of c_next_permutation() for performing a lexicographical
 // comparison using a `comp` operator instead of `operator<`.
-template <typename C, typename Compare>
-bool c_next_permutation(C& c, Compare&& comp) {
+template <typename C, typename LessThan>
+constexpr bool c_next_permutation(C& c, LessThan&& comp) {
   return std::next_permutation(container_algorithm_internal::c_begin(c),
                                container_algorithm_internal::c_end(c),
-                               std::forward<Compare>(comp));
+                               std::forward<LessThan>(comp));
 }
 
 // c_prev_permutation()
@@ -1572,18 +1905,18 @@ bool c_next_permutation(C& c, Compare&& comp) {
 // to rearrange a container's elements into the next lexicographically lesser
 // permutation.
 template <typename C>
-bool c_prev_permutation(C& c) {
+constexpr bool c_prev_permutation(C& c) {
   return std::prev_permutation(container_algorithm_internal::c_begin(c),
                                container_algorithm_internal::c_end(c));
 }
 
 // Overload of c_prev_permutation() for performing a lexicographical
 // comparison using a `comp` operator instead of `operator<`.
-template <typename C, typename Compare>
-bool c_prev_permutation(C& c, Compare&& comp) {
+template <typename C, typename LessThan>
+constexpr bool c_prev_permutation(C& c, LessThan&& comp) {
   return std::prev_permutation(container_algorithm_internal::c_begin(c),
                                container_algorithm_internal::c_end(c),
-                               std::forward<Compare>(comp));
+                               std::forward<LessThan>(comp));
 }
 
 //------------------------------------------------------------------------------
@@ -1592,26 +1925,26 @@ bool c_prev_permutation(C& c, Compare&& comp) {
 
 // c_iota()
 //
-// Container-based version of the <algorithm> `std::iota()` function
+// Container-based version of the <numeric> `std::iota()` function
 // to compute successive values of `value`, as if incremented with `++value`
-// after each element is written. and write them to the container.
+// after each element is written, and write them to the container.
 template <typename Sequence, typename T>
-void c_iota(Sequence& sequence, T&& value) {
+constexpr void c_iota(Sequence& sequence, const T& value) {
   std::iota(container_algorithm_internal::c_begin(sequence),
-            container_algorithm_internal::c_end(sequence),
-            std::forward<T>(value));
+            container_algorithm_internal::c_end(sequence), value);
 }
+
 // c_accumulate()
 //
-// Container-based version of the <algorithm> `std::accumulate()` function
+// Container-based version of the <numeric> `std::accumulate()` function
 // to accumulate the element values of a container to `init` and return that
 // accumulation by value.
 //
 // Note: Due to a language technicality this function has return type
-// absl::decay_t<T>. As a user of this function you can casually read
+// std::decay_t<T>. As a user of this function you can casually read
 // this as "returns T by value" and assume it does the right thing.
 template <typename Sequence, typename T>
-decay_t<T> c_accumulate(const Sequence& sequence, T&& init) {
+constexpr std::decay_t<T> c_accumulate(const Sequence& sequence, T&& init) {
   return std::accumulate(container_algorithm_internal::c_begin(sequence),
                          container_algorithm_internal::c_end(sequence),
                          std::forward<T>(init));
@@ -1620,8 +1953,8 @@ decay_t<T> c_accumulate(const Sequence& sequence, T&& init) {
 // Overload of c_accumulate() for using a binary operations other than
 // addition for computing the accumulation.
 template <typename Sequence, typename T, typename BinaryOp>
-decay_t<T> c_accumulate(const Sequence& sequence, T&& init,
-                        BinaryOp&& binary_op) {
+constexpr std::decay_t<T> c_accumulate(const Sequence& sequence, T&& init,
+                                       BinaryOp&& binary_op) {
   return std::accumulate(container_algorithm_internal::c_begin(sequence),
                          container_algorithm_internal::c_end(sequence),
                          std::forward<T>(init),
@@ -1630,15 +1963,15 @@ decay_t<T> c_accumulate(const Sequence& sequence, T&& init,
 
 // c_inner_product()
 //
-// Container-based version of the <algorithm> `std::inner_product()` function
+// Container-based version of the <numeric> `std::inner_product()` function
 // to compute the cumulative inner product of container element pairs.
 //
 // Note: Due to a language technicality this function has return type
-// absl::decay_t<T>. As a user of this function you can casually read
+// std::decay_t<T>. As a user of this function you can casually read
 // this as "returns T by value" and assume it does the right thing.
 template <typename Sequence1, typename Sequence2, typename T>
-decay_t<T> c_inner_product(const Sequence1& factors1, const Sequence2& factors2,
-                           T&& sum) {
+constexpr std::decay_t<T> c_inner_product(const Sequence1& factors1,
+                                          const Sequence2& factors2, T&& sum) {
   return std::inner_product(container_algorithm_internal::c_begin(factors1),
                             container_algorithm_internal::c_end(factors1),
                             container_algorithm_internal::c_begin(factors2),
@@ -1650,8 +1983,9 @@ decay_t<T> c_inner_product(const Sequence1& factors1, const Sequence2& factors2,
 // the product between the two container's element pair).
 template <typename Sequence1, typename Sequence2, typename T,
           typename BinaryOp1, typename BinaryOp2>
-decay_t<T> c_inner_product(const Sequence1& factors1, const Sequence2& factors2,
-                           T&& sum, BinaryOp1&& op1, BinaryOp2&& op2) {
+constexpr std::decay_t<T> c_inner_product(const Sequence1& factors1,
+                                          const Sequence2& factors2, T&& sum,
+                                          BinaryOp1&& op1, BinaryOp2&& op2) {
   return std::inner_product(container_algorithm_internal::c_begin(factors1),
                             container_algorithm_internal::c_end(factors1),
                             container_algorithm_internal::c_begin(factors2),
@@ -1661,12 +1995,12 @@ decay_t<T> c_inner_product(const Sequence1& factors1, const Sequence2& factors2,
 
 // c_adjacent_difference()
 //
-// Container-based version of the <algorithm> `std::adjacent_difference()`
+// Container-based version of the <numeric> `std::adjacent_difference()`
 // function to compute the difference between each element and the one preceding
 // it and write it to an iterator.
 template <typename InputSequence, typename OutputIt>
-OutputIt c_adjacent_difference(const InputSequence& input,
-                               OutputIt output_first) {
+constexpr OutputIt c_adjacent_difference(const InputSequence& input,
+                                         OutputIt output_first) {
   return std::adjacent_difference(container_algorithm_internal::c_begin(input),
                                   container_algorithm_internal::c_end(input),
                                   output_first);
@@ -1675,8 +2009,8 @@ OutputIt c_adjacent_difference(const InputSequence& input,
 // Overload of c_adjacent_difference() for using a binary operation other than
 // subtraction to compute the adjacent difference.
 template <typename InputSequence, typename OutputIt, typename BinaryOp>
-OutputIt c_adjacent_difference(const InputSequence& input,
-                               OutputIt output_first, BinaryOp&& op) {
+constexpr OutputIt c_adjacent_difference(const InputSequence& input,
+                                         OutputIt output_first, BinaryOp&& op) {
   return std::adjacent_difference(container_algorithm_internal::c_begin(input),
                                   container_algorithm_internal::c_end(input),
                                   output_first, std::forward<BinaryOp>(op));
@@ -1684,12 +2018,13 @@ OutputIt c_adjacent_difference(const InputSequence& input,
 
 // c_partial_sum()
 //
-// Container-based version of the <algorithm> `std::partial_sum()` function
+// Container-based version of the <numeric> `std::partial_sum()` function
 // to compute the partial sum of the elements in a sequence and write them
 // to an iterator. The partial sum is the sum of all element values so far in
 // the sequence.
 template <typename InputSequence, typename OutputIt>
-OutputIt c_partial_sum(const InputSequence& input, OutputIt output_first) {
+constexpr OutputIt c_partial_sum(const InputSequence& input,
+                                 OutputIt output_first) {
   return std::partial_sum(container_algorithm_internal::c_begin(input),
                           container_algorithm_internal::c_end(input),
                           output_first);
@@ -1698,13 +2033,14 @@ OutputIt c_partial_sum(const InputSequence& input, OutputIt output_first) {
 // Overload of c_partial_sum() for using a binary operation other than addition
 // to compute the "partial sum".
 template <typename InputSequence, typename OutputIt, typename BinaryOp>
-OutputIt c_partial_sum(const InputSequence& input, OutputIt output_first,
-                       BinaryOp&& op) {
+constexpr OutputIt c_partial_sum(const InputSequence& input,
+                                 OutputIt output_first, BinaryOp&& op) {
   return std::partial_sum(container_algorithm_internal::c_begin(input),
                           container_algorithm_internal::c_end(input),
                           output_first, std::forward<BinaryOp>(op));
 }
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_ALGORITHM_CONTAINER_H_
diff --git a/absl/algorithm/container_test.cc b/absl/algorithm/container_test.cc
index 86bf9d3ea32..e0253827a32 100644
--- a/absl/algorithm/container_test.cc
+++ b/absl/algorithm/container_test.cc
@@ -14,6 +14,10 @@
 
 #include "absl/algorithm/container.h"
 
+#include <algorithm>
+#include <array>
+#include <cstddef>
+#include <forward_list>
 #include <functional>
 #include <initializer_list>
 #include <iterator>
@@ -22,6 +26,7 @@
 #include <ostream>
 #include <random>
 #include <set>
+#include <type_traits>
 #include <unordered_set>
 #include <utility>
 #include <valarray>
@@ -30,18 +35,27 @@
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
 #include "absl/base/casts.h"
+#include "absl/base/config.h"
 #include "absl/base/macros.h"
 #include "absl/memory/memory.h"
+#include "absl/meta/type_traits.h"
+#include "absl/random/random.h"
 #include "absl/types/span.h"
 
+#ifdef __cpp_lib_span
+#include <span>  // NOLINT(build/c++20)
+#endif
+
 namespace {
 
 using ::testing::Each;
 using ::testing::ElementsAre;
 using ::testing::Gt;
 using ::testing::IsNull;
+using ::testing::IsSubsetOf;
 using ::testing::Lt;
 using ::testing::Pointee;
+using ::testing::SizeIs;
 using ::testing::Truly;
 using ::testing::UnorderedElementsAre;
 
@@ -57,9 +71,7 @@ class NonMutatingTest : public testing::Test {
 };
 
 struct AccumulateCalls {
-  void operator()(int value) {
-    calls.push_back(value);
-  }
+  void operator()(int value) { calls.push_back(value); }
   std::vector<int> calls;
 };
 
@@ -68,15 +80,41 @@ bool BinPredicate(int v1, int v2) { return v1 < v2; }
 bool Equals(int v1, int v2) { return v1 == v2; }
 bool IsOdd(int x) { return x % 2 != 0; }
 
+TEST(Span, IsSpan) {
+  static_assert(
+      absl::container_algorithm_internal::IsSpan<absl::Span<int>>::value);
+  static_assert(
+      absl::container_algorithm_internal::IsSpan<const absl::Span<int>>::value);
+  static_assert(absl::container_algorithm_internal::IsSpan<
+                volatile absl::Span<int>>::value);
+  static_assert(absl::container_algorithm_internal::IsSpan<
+                const volatile absl::Span<int>>::value);
+
+  static_assert(
+      !absl::container_algorithm_internal::IsSpan<absl::Span<int>&>::value);
+  static_assert(
+      !absl::container_algorithm_internal::IsSpan<absl::Span<int>&&>::value);
+
+#ifdef __cpp_lib_span
+  static_assert(
+      absl::container_algorithm_internal::IsSpan<std::span<int>>::value);
+  static_assert(
+      !absl::container_algorithm_internal::IsSpan<std::span<int>&>::value);
+  static_assert(
+      !absl::container_algorithm_internal::IsSpan<std::span<int>&&>::value);
+#endif
+}
 
 TEST_F(NonMutatingTest, Distance) {
-  EXPECT_EQ(container_.size(), absl::c_distance(container_));
-  EXPECT_EQ(sequence_.size(), absl::c_distance(sequence_));
-  EXPECT_EQ(vector_.size(), absl::c_distance(vector_));
-  EXPECT_EQ(ABSL_ARRAYSIZE(array_), absl::c_distance(array_));
+  EXPECT_EQ(container_.size(),
+            static_cast<size_t>(absl::c_distance(container_)));
+  EXPECT_EQ(sequence_.size(), static_cast<size_t>(absl::c_distance(sequence_)));
+  EXPECT_EQ(vector_.size(), static_cast<size_t>(absl::c_distance(vector_)));
+  EXPECT_EQ(std::size(array_), static_cast<size_t>(absl::c_distance(array_)));
 
   // Works with a temporary argument.
-  EXPECT_EQ(vector_.size(), absl::c_distance(std::vector<int>(vector_)));
+  EXPECT_EQ(vector_.size(),
+            static_cast<size_t>(absl::c_distance(std::vector<int>(vector_))));
 }
 
 TEST_F(NonMutatingTest, Distance_OverloadedBeginEnd) {
@@ -110,6 +148,11 @@ TEST_F(NonMutatingTest, FindReturnsCorrectType) {
   absl::c_find(absl::implicit_cast<const std::list<int>&>(sequence_), 3);
 }
 
+TEST_F(NonMutatingTest, Contains) {
+  EXPECT_TRUE(absl::c_contains(container_, 3));
+  EXPECT_FALSE(absl::c_contains(container_, 4));
+}
+
 TEST_F(NonMutatingTest, FindIf) { absl::c_find_if(container_, Predicate); }
 
 TEST_F(NonMutatingTest, FindIfNot) {
@@ -129,11 +172,13 @@ TEST_F(NonMutatingTest, FindEndWithPredicate) {
 TEST_F(NonMutatingTest, FindFirstOf) {
   absl::c_find_first_of(container_, sequence_);
   absl::c_find_first_of(sequence_, container_);
+  absl::c_find_first_of(sequence_, std::array<int, 2>{1, 2});
 }
 
 TEST_F(NonMutatingTest, FindFirstOfWithPredicate) {
   absl::c_find_first_of(container_, sequence_, BinPredicate);
   absl::c_find_first_of(sequence_, container_, BinPredicate);
+  absl::c_find_first_of(sequence_, std::array<int, 2>{1, 2}, BinPredicate);
 }
 
 TEST_F(NonMutatingTest, AdjacentFind) { absl::c_adjacent_find(sequence_); }
@@ -151,35 +196,118 @@ TEST_F(NonMutatingTest, CountIf) {
 }
 
 TEST_F(NonMutatingTest, Mismatch) {
-  absl::c_mismatch(container_, sequence_);
-  absl::c_mismatch(sequence_, container_);
+  // Testing necessary as absl::c_mismatch executes logic.
+  {
+    auto result = absl::c_mismatch(vector_, sequence_);
+    EXPECT_EQ(result.first, vector_.end());
+    EXPECT_EQ(result.second, sequence_.end());
+  }
+  {
+    auto result = absl::c_mismatch(sequence_, vector_);
+    EXPECT_EQ(result.first, sequence_.end());
+    EXPECT_EQ(result.second, vector_.end());
+  }
+
+  sequence_.back() = 5;
+  {
+    auto result = absl::c_mismatch(vector_, sequence_);
+    EXPECT_EQ(result.first, std::prev(vector_.end()));
+    EXPECT_EQ(result.second, std::prev(sequence_.end()));
+  }
+  {
+    auto result = absl::c_mismatch(sequence_, vector_);
+    EXPECT_EQ(result.first, std::prev(sequence_.end()));
+    EXPECT_EQ(result.second, std::prev(vector_.end()));
+  }
+
+  sequence_.pop_back();
+  {
+    auto result = absl::c_mismatch(vector_, sequence_);
+    EXPECT_EQ(result.first, std::prev(vector_.end()));
+    EXPECT_EQ(result.second, sequence_.end());
+  }
+  {
+    auto result = absl::c_mismatch(sequence_, vector_);
+    EXPECT_EQ(result.first, sequence_.end());
+    EXPECT_EQ(result.second, std::prev(vector_.end()));
+  }
+  {
+    struct NoNotEquals {
+      constexpr bool operator==(NoNotEquals) const { return true; }
+      constexpr bool operator!=(NoNotEquals) const = delete;
+    };
+    std::vector<NoNotEquals> first;
+    std::list<NoNotEquals> second;
+
+    // Check this still compiles.
+    absl::c_mismatch(first, second);
+  }
 }
 
 TEST_F(NonMutatingTest, MismatchWithPredicate) {
-  absl::c_mismatch(container_, sequence_, BinPredicate);
-  absl::c_mismatch(sequence_, container_, BinPredicate);
+  // Testing necessary as absl::c_mismatch executes logic.
+  {
+    auto result = absl::c_mismatch(vector_, sequence_, BinPredicate);
+    EXPECT_EQ(result.first, vector_.begin());
+    EXPECT_EQ(result.second, sequence_.begin());
+  }
+  {
+    auto result = absl::c_mismatch(sequence_, vector_, BinPredicate);
+    EXPECT_EQ(result.first, sequence_.begin());
+    EXPECT_EQ(result.second, vector_.begin());
+  }
+
+  sequence_.front() = 0;
+  {
+    auto result = absl::c_mismatch(vector_, sequence_, BinPredicate);
+    EXPECT_EQ(result.first, vector_.begin());
+    EXPECT_EQ(result.second, sequence_.begin());
+  }
+  {
+    auto result = absl::c_mismatch(sequence_, vector_, BinPredicate);
+    EXPECT_EQ(result.first, std::next(sequence_.begin()));
+    EXPECT_EQ(result.second, std::next(vector_.begin()));
+  }
+
+  sequence_.clear();
+  {
+    auto result = absl::c_mismatch(vector_, sequence_, BinPredicate);
+    EXPECT_EQ(result.first, vector_.begin());
+    EXPECT_EQ(result.second, sequence_.end());
+  }
+  {
+    auto result = absl::c_mismatch(sequence_, vector_, BinPredicate);
+    EXPECT_EQ(result.first, sequence_.end());
+    EXPECT_EQ(result.second, vector_.begin());
+  }
 }
 
 TEST_F(NonMutatingTest, Equal) {
   EXPECT_TRUE(absl::c_equal(vector_, sequence_));
   EXPECT_TRUE(absl::c_equal(sequence_, vector_));
+  EXPECT_TRUE(absl::c_equal(sequence_, array_));
+  EXPECT_TRUE(absl::c_equal(array_, vector_));
 
   // Test that behavior appropriately differs from that of equal().
   std::vector<int> vector_plus = {1, 2, 3};
   vector_plus.push_back(4);
   EXPECT_FALSE(absl::c_equal(vector_plus, sequence_));
   EXPECT_FALSE(absl::c_equal(sequence_, vector_plus));
+  EXPECT_FALSE(absl::c_equal(array_, vector_plus));
 }
 
 TEST_F(NonMutatingTest, EqualWithPredicate) {
   EXPECT_TRUE(absl::c_equal(vector_, sequence_, Equals));
   EXPECT_TRUE(absl::c_equal(sequence_, vector_, Equals));
+  EXPECT_TRUE(absl::c_equal(array_, sequence_, Equals));
+  EXPECT_TRUE(absl::c_equal(vector_, array_, Equals));
 
   // Test that behavior appropriately differs from that of equal().
   std::vector<int> vector_plus = {1, 2, 3};
   vector_plus.push_back(4);
   EXPECT_FALSE(absl::c_equal(vector_plus, sequence_, Equals));
   EXPECT_FALSE(absl::c_equal(sequence_, vector_plus, Equals));
+  EXPECT_FALSE(absl::c_equal(vector_plus, array_, Equals));
 }
 
 TEST_F(NonMutatingTest, IsPermutation) {
@@ -219,6 +347,17 @@ TEST_F(NonMutatingTest, SearchWithPredicate) {
   absl::c_search(vector_, sequence_, BinPredicate);
 }
 
+TEST_F(NonMutatingTest, ContainsSubrange) {
+  EXPECT_TRUE(absl::c_contains_subrange(sequence_, vector_));
+  EXPECT_TRUE(absl::c_contains_subrange(vector_, sequence_));
+  EXPECT_TRUE(absl::c_contains_subrange(array_, sequence_));
+}
+
+TEST_F(NonMutatingTest, ContainsSubrangeWithPredicate) {
+  EXPECT_TRUE(absl::c_contains_subrange(sequence_, vector_, Equals));
+  EXPECT_TRUE(absl::c_contains_subrange(vector_, sequence_, Equals));
+}
+
 TEST_F(NonMutatingTest, SearchN) { absl::c_search_n(sequence_, 3, 1); }
 
 TEST_F(NonMutatingTest, SearchNWithPredicate) {
@@ -513,11 +652,9 @@ TEST_F(SortingTest, IsSortedUntil) {
 TEST_F(SortingTest, NthElement) {
   std::vector<int> unsorted = {2, 4, 1, 3};
   absl::c_nth_element(unsorted, unsorted.begin() + 2);
-  EXPECT_THAT(unsorted,
-              ElementsAre(Lt(3), Lt(3), 3, Gt(3)));
+  EXPECT_THAT(unsorted, ElementsAre(Lt(3), Lt(3), 3, Gt(3)));
   absl::c_nth_element(unsorted, unsorted.begin() + 2, std::greater<int>());
-  EXPECT_THAT(unsorted,
-              ElementsAre(Gt(2), Gt(2), 2, Lt(2)));
+  EXPECT_THAT(unsorted, ElementsAre(Gt(2), Gt(2), 2, Lt(2)));
 }
 
 TEST(MutatingTest, IsPartitioned) {
@@ -606,6 +743,222 @@ TEST(MutatingTest, CopyN) {
   EXPECT_EQ(expected, actual);
 }
 
+TEST(MutatingTest, CopyNWithNegativeN) {
+#ifdef _LIBCPP_VERSION
+  GTEST_SKIP() << "libc++ does not handle negative counts correctly";
+#else
+  const std::vector<int> input = {1, 2, 3};
+  std::vector<int> actual = {0, 0, 0};
+  absl::c_copy_n(input, -1, actual.begin());
+  EXPECT_THAT(actual, ElementsAre(0, 0, 0));
+#endif
+}
+
+TEST(MutatingTest, CopyToContainer) {
+  const std::vector<int> input = {1, 2, 3};
+  std::vector<int> actual = {0, 0, 0, 4, 5};
+  absl::c_copy(input, actual);
+  EXPECT_THAT(actual, ElementsAre(1, 2, 3, 4, 5));
+}
+
+TEST(MutatingTest, CopyToSpanRvalue) {
+  const std::vector<int> input = {1, 2, 3};
+  std::vector<int> actual = {0, 0, 0, 4, 5};
+  absl::c_copy(input, absl::MakeSpan(actual));
+  EXPECT_THAT(actual, ElementsAre(1, 2, 3, 4, 5));
+}
+
+TEST(MutatingTest, CopyNToContainer) {
+  const std::vector<int> input = {1, 2, 3, 4, 5};
+  std::vector<int> actual = {0, 0, 0, 0, 0};
+  absl::c_copy_n(input, 2, actual);
+  EXPECT_THAT(actual, ElementsAre(1, 2, 0, 0, 0));
+}
+
+TEST(MutatingTest, CopyNToSpanRvalue) {
+  const std::vector<int> input = {1, 2, 3, 4, 5};
+  std::vector<int> actual = {0, 0, 0, 0, 0};
+  absl::c_copy_n(input, 2, absl::MakeSpan(actual));
+  EXPECT_THAT(actual, ElementsAre(1, 2, 0, 0, 0));
+}
+
+TEST(MutatingTest, CopyNToContainerWithZeroN) {
+  const std::vector<int> input = {1, 2, 3, 4, 5};
+  std::vector<int> actual = {0, 0, 0, 0, 0};
+  absl::c_copy_n(input, 0, actual);
+  EXPECT_THAT(actual, ElementsAre(0, 0, 0, 0, 0));
+}
+
+TEST(MutatingTest, CopyNToContainerWithNegativeN) {
+#ifdef _LIBCPP_VERSION
+  GTEST_SKIP() << "libc++ does not handle negative counts correctly";
+#else
+  const std::vector<int> input = {1, 2, 3};
+  std::vector<int> actual = {0, 0, 0};
+  absl::c_copy_n(input, -1, actual);
+  EXPECT_THAT(actual, ElementsAre(0, 0, 0));
+#endif
+}
+
+TEST(MutatingTest, CopyToDifferentContainerType) {
+  const std::list<int> input = {1, 2, 3};
+  std::array<int, 5> actual = {0, 0, 0, 4, 5};
+  absl::c_copy(input, actual);
+  EXPECT_THAT(actual, ElementsAre(1, 2, 3, 4, 5));
+}
+
+TEST(MutatingTest, CopyNToDifferentContainerType) {
+  const std::list<int> input = {1, 2, 3, 4, 5};
+  std::array<int, 5> actual = {0, 0, 0, 0, 0};
+  absl::c_copy_n(input, 2, actual);
+  EXPECT_THAT(actual, ElementsAre(1, 2, 0, 0, 0));
+}
+
+TEST(MutatingTest, CopyToCArray) {
+  const std::vector<int> input = {1, 2, 3};
+  int actual[5] = {0, 0, 0, 4, 5};
+  absl::c_copy(input, actual);
+  EXPECT_THAT(actual, ElementsAre(1, 2, 3, 4, 5));
+}
+
+TEST(MutatingTest, CopyNToCArray) {
+  const std::vector<int> input = {1, 2, 3, 4, 5};
+  int actual[5] = {0, 0, 0, 0, 0};
+  absl::c_copy_n(input, 2, actual);
+  EXPECT_THAT(actual, ElementsAre(1, 2, 0, 0, 0));
+}
+
+TEST(MutatingTest, CopyFromCArray) {
+  const int input[5] = {1, 2, 3, 4, 5};
+  std::vector<int> actual = {0, 0, 0, 0, 0};
+  absl::c_copy(input, actual);
+  EXPECT_THAT(actual, ElementsAre(1, 2, 3, 4, 5));
+}
+
+TEST(MutatingTest, CopyNFromCArray) {
+  const int input[5] = {1, 2, 3, 4, 5};
+  std::vector<int> actual = {0, 0, 0, 0, 0};
+  absl::c_copy_n(input, 2, actual);
+  EXPECT_THAT(actual, ElementsAre(1, 2, 0, 0, 0));
+}
+
+TEST(MutatingTest, CopyContainerWithNoSizeMethod) {
+  const std::forward_list<int> input = {1, 2, 3};
+  std::forward_list<int> actual = {0, 0, 0, 4, 5};
+  absl::c_copy(input, actual);
+  EXPECT_THAT(actual, ElementsAre(1, 2, 3, 4, 5));
+}
+
+TEST(MutatingTest, CopyNContainerWithNoSizeMethod) {
+  const std::forward_list<int> input = {1, 2, 3, 4, 5};
+  std::forward_list<int> actual = {0, 0, 0, 0, 0};
+  absl::c_copy_n(input, 2, actual);
+  EXPECT_THAT(actual, ElementsAre(1, 2, 0, 0, 0));
+}
+
+
+bool IsHardened() {
+  bool hardened = false;
+  ABSL_HARDENING_ASSERT([&hardened]() {
+    hardened = true;
+    return true;
+  }());
+  return hardened;
+}
+
+TEST(MutatingTest, CopyToCArrayInvalidSize) {
+  const std::vector<int> input = {1, 2, 3};
+  int actual[2] = {0, 0};
+  (void)actual;
+#if GTEST_HAS_DEATH_TEST
+  if (IsHardened()) {
+    EXPECT_DEATH(absl::c_copy(input, actual), "");
+  }
+#endif
+}
+
+TEST(MutatingTest, CopyNToCArrayInvalidSize) {
+  const std::vector<int> input = {1, 2, 3};
+  int actual[2] = {0, 0};
+  (void)actual;
+#if GTEST_HAS_DEATH_TEST
+  if (IsHardened()) {
+    EXPECT_DEATH(absl::c_copy_n(input, 3, actual), "");
+  }
+#endif
+}
+
+TEST(MutatingTest, CopyNToCArrayNGreaterThanInput) {
+  const std::vector<int> input = {1, 2, 3};
+  int actual[4] = {0, 0, 0, 0};
+  (void)actual;
+#if GTEST_HAS_DEATH_TEST
+  if (IsHardened()) {
+    EXPECT_DEATH(absl::c_copy_n(input, 4, actual), "");
+  }
+#endif
+}
+
+TEST(MutatingTest, CopyToContainerInvalidSize) {
+  const std::list<int> input = {1, 2, 3, 4, 5};
+  std::list<int> actual = {0, 0, 0};
+#if GTEST_HAS_DEATH_TEST
+  if (IsHardened()) {
+    EXPECT_DEATH(absl::c_copy(input, actual), "");
+  }
+#endif
+}
+
+TEST(MutatingTest, CopyNToContainerNGreaterThanInput) {
+  const std::vector<int> input = {1, 2, 3};
+  std::vector<int> actual = {0, 0, 0, 0};
+#if GTEST_HAS_DEATH_TEST
+  if (IsHardened()) {
+    EXPECT_DEATH(absl::c_copy_n(input, 4, actual), "");
+  }
+#endif
+}
+
+TEST(MutatingTest, CopyNToContainerNGreaterThanOutput) {
+  const std::vector<int> input = {1, 2, 3};
+  std::vector<int> actual = {0, 0};
+#if GTEST_HAS_DEATH_TEST
+  if (IsHardened()) {
+    EXPECT_DEATH(absl::c_copy_n(input, 3, actual), "");
+  }
+#endif
+}
+
+TEST(MutatingTest, CopyToForwardListInvalidSize) {
+  const std::forward_list<int> input = {1, 2, 3, 4, 5};
+  std::forward_list<int> actual = {0, 0, 0};
+#if GTEST_HAS_DEATH_TEST
+  if (IsHardened()) {
+    EXPECT_DEATH(absl::c_copy(input, actual), "");
+  }
+#endif
+}
+
+TEST(MutatingTest, CopyNToForwardListNGreaterThanInput) {
+  const std::forward_list<int> input = {1, 2, 3};
+  std::forward_list<int> actual = {0, 0, 0, 0};
+#if GTEST_HAS_DEATH_TEST
+  if (IsHardened()) {
+    EXPECT_DEATH(absl::c_copy_n(input, 4, actual), "");
+  }
+#endif
+}
+
+TEST(MutatingTest, CopyNToForwardListNGreaterThanOutput) {
+  const std::forward_list<int> input = {1, 2, 3};
+  std::forward_list<int> actual = {0, 0};
+#if GTEST_HAS_DEATH_TEST
+  if (IsHardened()) {
+    EXPECT_DEATH(absl::c_copy_n(input, 3, actual), "");
+  }
+#endif
+}
+
 TEST(MutatingTest, CopyIf) {
   const std::list<int> input = {1, 2, 3};
   std::vector<int> output;
@@ -623,11 +976,11 @@ TEST(MutatingTest, CopyBackward) {
 
 TEST(MutatingTest, Move) {
   std::vector<std::unique_ptr<int>> src;
-  src.emplace_back(absl::make_unique<int>(1));
-  src.emplace_back(absl::make_unique<int>(2));
-  src.emplace_back(absl::make_unique<int>(3));
-  src.emplace_back(absl::make_unique<int>(4));
-  src.emplace_back(absl::make_unique<int>(5));
+  src.emplace_back(std::make_unique<int>(1));
+  src.emplace_back(std::make_unique<int>(2));
+  src.emplace_back(std::make_unique<int>(3));
+  src.emplace_back(std::make_unique<int>(4));
+  src.emplace_back(std::make_unique<int>(5));
 
   std::vector<std::unique_ptr<int>> dest = {};
   absl::c_move(src, std::back_inserter(dest));
@@ -638,11 +991,11 @@ TEST(MutatingTest, Move) {
 
 TEST(MutatingTest, MoveBackward) {
   std::vector<std::unique_ptr<int>> actual;
-  actual.emplace_back(absl::make_unique<int>(1));
-  actual.emplace_back(absl::make_unique<int>(2));
-  actual.emplace_back(absl::make_unique<int>(3));
-  actual.emplace_back(absl::make_unique<int>(4));
-  actual.emplace_back(absl::make_unique<int>(5));
+  actual.emplace_back(std::make_unique<int>(1));
+  actual.emplace_back(std::make_unique<int>(2));
+  actual.emplace_back(std::make_unique<int>(3));
+  actual.emplace_back(std::make_unique<int>(4));
+  actual.emplace_back(std::make_unique<int>(5));
   auto subrange = absl::MakeSpan(actual.data(), 3);
   absl::c_move_backward(subrange, actual.end());
   EXPECT_THAT(actual, ElementsAre(IsNull(), IsNull(), Pointee(1), Pointee(2),
@@ -652,9 +1005,9 @@ TEST(MutatingTest, MoveBackward) {
 TEST(MutatingTest, MoveWithRvalue) {
   auto MakeRValueSrc = [] {
     std::vector<std::unique_ptr<int>> src;
-    src.emplace_back(absl::make_unique<int>(1));
-    src.emplace_back(absl::make_unique<int>(2));
-    src.emplace_back(absl::make_unique<int>(3));
+    src.emplace_back(std::make_unique<int>(1));
+    src.emplace_back(std::make_unique<int>(2));
+    src.emplace_back(std::make_unique<int>(3));
     return src;
   };
 
@@ -664,12 +1017,136 @@ TEST(MutatingTest, MoveWithRvalue) {
                                 Pointee(2), Pointee(3)));
 }
 
+TEST(MutatingTest, MoveToContainer) {
+  std::vector<std::unique_ptr<int>> input;
+  input.push_back(std::make_unique<int>(1));
+  input.push_back(std::make_unique<int>(2));
+  input.push_back(std::make_unique<int>(3));
+
+  std::vector<std::unique_ptr<int>> actual(5);
+  absl::c_move(input, actual);
+
+  EXPECT_EQ(input[0], nullptr);
+  EXPECT_EQ(input[1], nullptr);
+  EXPECT_EQ(input[2], nullptr);
+
+  ASSERT_NE(actual[0], nullptr);
+  EXPECT_EQ(*actual[0], 1);
+  ASSERT_NE(actual[1], nullptr);
+  EXPECT_EQ(*actual[1], 2);
+  ASSERT_NE(actual[2], nullptr);
+  EXPECT_EQ(*actual[2], 3);
+  EXPECT_EQ(actual[3], nullptr);
+}
+
+TEST(MutatingTest, MoveToSpanRvalue) {
+  std::vector<std::unique_ptr<int>> input;
+  input.push_back(std::make_unique<int>(1));
+  input.push_back(std::make_unique<int>(2));
+  input.push_back(std::make_unique<int>(3));
+
+  std::vector<std::unique_ptr<int>> actual(5);
+  absl::c_move(input, absl::MakeSpan(actual));
+
+  EXPECT_EQ(input[0], nullptr);
+  EXPECT_EQ(input[1], nullptr);
+  EXPECT_EQ(input[2], nullptr);
+
+  ASSERT_NE(actual[0], nullptr);
+  EXPECT_EQ(*actual[0], 1);
+  ASSERT_NE(actual[1], nullptr);
+  EXPECT_EQ(*actual[1], 2);
+  ASSERT_NE(actual[2], nullptr);
+  EXPECT_EQ(*actual[2], 3);
+  EXPECT_EQ(actual[3], nullptr);
+}
+
+TEST(MutatingTest, MoveToDifferentContainerType) {
+  std::list<std::unique_ptr<int>> input;
+  input.push_back(std::make_unique<int>(1));
+  input.push_back(std::make_unique<int>(2));
+
+  std::array<std::unique_ptr<int>, 3> actual;
+  absl::c_move(input, actual);
+
+  EXPECT_EQ(input.front(), nullptr);
+  ASSERT_NE(actual[0], nullptr);
+  EXPECT_EQ(*actual[0], 1);
+}
+
+TEST(MutatingTest, MoveToCArray) {
+  std::vector<std::unique_ptr<int>> input;
+  input.push_back(std::make_unique<int>(1));
+  input.push_back(std::make_unique<int>(2));
+
+  std::unique_ptr<int> actual[3];
+  absl::c_move(input, actual);
+
+  EXPECT_EQ(input.front(), nullptr);
+  ASSERT_NE(actual[0], nullptr);
+  EXPECT_EQ(*actual[0], 1);
+}
+
+TEST(MutatingTest, MoveFromCArray) {
+  std::unique_ptr<int> input[2];
+  input[0] = std::make_unique<int>(1);
+  input[1] = std::make_unique<int>(2);
+
+  std::vector<std::unique_ptr<int>> actual(3);
+  absl::c_move(input, actual);
+
+  EXPECT_EQ(input[0], nullptr);
+  ASSERT_NE(actual[0], nullptr);
+  EXPECT_EQ(*actual[0], 1);
+}
+
+
+TEST(MutatingTest, MoveToCArrayInvalidSize) {
+  std::vector<int> input = {1, 2, 3};
+  int actual[2] = {0, 0};
+  (void)actual;
+#if GTEST_HAS_DEATH_TEST
+  if (IsHardened()) {
+    EXPECT_DEATH(absl::c_move(input, actual), "");
+  }
+#endif
+}
+
+TEST(MutatingTest, MoveToContainerInvalidSize) {
+  std::list<int> input = {1, 2, 3, 4, 5};
+  std::list<int> actual = {0, 0, 0};
+#if GTEST_HAS_DEATH_TEST
+  if (IsHardened()) {
+    EXPECT_DEATH(absl::c_move(input, actual), "");
+  }
+#endif
+}
+
+TEST(MutatingTest, MoveToForwardListInvalidSize) {
+  std::forward_list<int> input = {1, 2, 3, 4, 5};
+  std::forward_list<int> actual = {0, 0, 0};
+#if GTEST_HAS_DEATH_TEST
+  if (IsHardened()) {
+    EXPECT_DEATH(absl::c_move(input, actual), "");
+  }
+#endif
+}
+
 TEST(MutatingTest, SwapRanges) {
   std::vector<int> odds = {2, 4, 6};
   std::vector<int> evens = {1, 3, 5};
   absl::c_swap_ranges(odds, evens);
   EXPECT_THAT(odds, ElementsAre(1, 3, 5));
   EXPECT_THAT(evens, ElementsAre(2, 4, 6));
+
+  odds.pop_back();
+  absl::c_swap_ranges(odds, evens);
+  EXPECT_THAT(odds, ElementsAre(2, 4));
+  EXPECT_THAT(evens, ElementsAre(1, 3, 6));
+
+  absl::c_swap_ranges(evens, odds);
+  EXPECT_THAT(odds, ElementsAre(1, 3));
+  EXPECT_THAT(evens, ElementsAre(2, 4, 6));
 }
 
 TEST_F(NonMutatingTest, Transform) {
@@ -684,6 +1161,139 @@ TEST_F(NonMutatingTest, Transform) {
   EXPECT_EQ(std::vector<int>({1, 5, 4}), z);
   *end = 7;
   EXPECT_EQ(std::vector<int>({1, 5, 4, 7}), z);
+
+  std::vector<int> x2{1, 2, 3};
+  std::vector<int> y2{10, 20, 30};
+  std::vector<int> z2(3);
+  auto out_it = z2.begin();
+  absl::c_transform(x2, y2, out_it, std::plus<int>());
+  // The caller's iterator should not be modified.
+  EXPECT_EQ(out_it, z2.begin());
+  EXPECT_EQ(z2, std::vector<int>({11, 22, 33}));
+}
+
+TEST(MutatingTest, TransformToContainer) {
+  const std::vector<int> input = {1, 2, 3};
+  std::vector<int> actual = {0, 0, 0, 4, 5};
+  absl::c_transform(input, actual, [](int x) { return x * 2; });
+  EXPECT_THAT(actual, ElementsAre(2, 4, 6, 4, 5));
+}
+
+TEST(MutatingTest, BinaryTransformToContainer) {
+  {
+    const std::vector<int> input1 = {1, 2, 3};
+    const std::vector<int> input2 = {10, 20, 30};
+    std::vector<int> actual = {0, 0, 0, 4, 5};
+    absl::c_transform(input1, input2, actual, std::plus<int>());
+    EXPECT_THAT(actual, ElementsAre(11, 22, 33, 4, 5));
+  }
+  {
+    const std::vector<int> input1 = {1, 2, 3, 4};
+    const std::vector<int> input2 = {10, 20};
+    std::vector<int> actual = {0, 0, 0, 4, 5};
+    absl::c_transform(input1, input2, actual, std::plus<int>());
+    EXPECT_THAT(actual, ElementsAre(11, 22, 0, 4, 5));
+  }
+  {
+    const std::vector<int> input1 = {1, 2};
+    const std::vector<int> input2 = {10, 20, 30, 40};
+    std::vector<int> actual = {0, 0, 0, 4, 5};
+    absl::c_transform(input1, input2, actual, std::plus<int>());
+    EXPECT_THAT(actual, ElementsAre(11, 22, 0, 4, 5));
+  }
+}
+
+TEST(MutatingTest, TransformToDifferentContainerType) {
+  const std::list<int> input = {1, 2, 3};
+  std::array<int, 5> actual = {0, 0, 0, 4, 5};
+  absl::c_transform(input, actual, [](int x) { return x * 2; });
+  EXPECT_THAT(actual, ElementsAre(2, 4, 6, 4, 5));
+}
+
+TEST(MutatingTest, TransformToCArray) {
+  const std::vector<int> input = {1, 2, 3};
+  int actual[5] = {0, 0, 0, 4, 5};
+  absl::c_transform(input, actual, [](int x) { return x * 2; });
+  EXPECT_THAT(actual, ElementsAre(2, 4, 6, 4, 5));
+}
+
+TEST(MutatingTest, TransformFromCArray) {
+  const int input[5] = {1, 2, 3, 4, 5};
+  std::vector<int> actual = {0, 0, 0, 0, 0};
+  absl::c_transform(input, actual, [](int x) { return x * 2; });
+  EXPECT_THAT(actual, ElementsAre(2, 4, 6, 8, 10));
+}
+
+TEST(MutatingTest, TransformToCArrayInvalidSize) {
+  const std::vector<int> input = {1, 2, 3};
+  int actual[2] = {0, 0};
+  (void)actual;
+#if GTEST_HAS_DEATH_TEST
+  if (IsHardened()) {
+    EXPECT_DEATH(absl::c_transform(input, actual, [](int x) { return x * 2; }),
+                 "");
+  }
+#endif
+}
+
+TEST(MutatingTest, BinaryTransformToCArrayInvalidSize) {
+  const std::vector<int> input1 = {1, 2, 3};
+  const std::vector<int> input2 = {10, 20, 30};
+  int actual[2] = {0, 0};
+  (void)actual;
+#if GTEST_HAS_DEATH_TEST
+  if (IsHardened()) {
+    EXPECT_DEATH(absl::c_transform(input1, input2, actual, std::plus<int>()),
+                 "");
+  }
+#endif
+}
+
+TEST(MutatingTest, TransformToContainerInvalidSize) {
+  const std::list<int> input = {1, 2, 3, 4, 5};
+  std::list<int> actual = {0, 0, 0};
+#if GTEST_HAS_DEATH_TEST
+  if (IsHardened()) {
+    EXPECT_DEATH(absl::c_transform(input, actual, [](int x) { return x * 2; }),
+                 "");
+  }
+#endif
+}
+
+TEST(MutatingTest, BinaryTransformToContainerInvalidSize) {
+  const std::vector<int> input1 = {1, 2, 3, 4, 5};
+  const std::vector<int> input2 = {10, 20, 30, 40, 50};
+  std::vector<int> actual = {0, 0, 0};
+#if GTEST_HAS_DEATH_TEST
+  if (IsHardened()) {
+    EXPECT_DEATH(absl::c_transform(input1, input2, actual, std::plus<int>()),
+                 "");
+  }
+#endif
+}
+
+TEST(MutatingTest, BinaryTransformInput2InvalidSize) {
+  const std::vector<int> input1 = {1, 2, 3};
+  const std::vector<int> input2 = {10, 20};
+  std::vector<int> actual = {0};
+#if GTEST_HAS_DEATH_TEST
+  if (IsHardened()) {
+    EXPECT_DEATH(absl::c_transform(input1, input2, actual, std::plus<int>()),
+                 "");
+  }
+#endif
+}
+
+TEST(MutatingTest, BinaryTransformInput1InvalidSize) {
+  const std::vector<int> input1 = {1, 2};
+  const std::vector<int> input2 = {10, 20, 30};
+  std::vector<int> actual = {0};
+#if GTEST_HAS_DEATH_TEST
+  if (IsHardened()) {
+    EXPECT_DEATH(absl::c_transform(input1, input2, actual, std::plus<int>()),
+                 "");
+  }
+#endif
 }
 
 TEST(MutatingTest, Replace) {
@@ -749,10 +1359,9 @@ MATCHER_P2(IsElement, key, value, "") {
 TEST(MutatingTest, StableSort) {
   std::vector<Element> test_vector = {{1, 1}, {2, 1}, {2, 0}, {1, 0}, {2, 2}};
   absl::c_stable_sort(test_vector);
-  EXPECT_THAT(
-      test_vector,
-      ElementsAre(IsElement(1, 1), IsElement(1, 0), IsElement(2, 1),
-                  IsElement(2, 0), IsElement(2, 2)));
+  EXPECT_THAT(test_vector,
+              ElementsAre(IsElement(1, 1), IsElement(1, 0), IsElement(2, 1),
+                          IsElement(2, 0), IsElement(2, 2)));
 }
 
 TEST(MutatingTest, StableSortWithPredicate) {
@@ -760,10 +1369,9 @@ TEST(MutatingTest, StableSortWithPredicate) {
   absl::c_stable_sort(test_vector, [](const Element& e1, const Element& e2) {
     return e2 < e1;
   });
-  EXPECT_THAT(
-      test_vector,
-      ElementsAre(IsElement(2, 1), IsElement(2, 0), IsElement(2, 2),
-                  IsElement(1, 1), IsElement(1, 0)));
+  EXPECT_THAT(test_vector,
+              ElementsAre(IsElement(2, 1), IsElement(2, 0), IsElement(2, 2),
+                          IsElement(1, 1), IsElement(1, 0)));
 }
 
 TEST(MutatingTest, ReplaceCopyIf) {
@@ -781,12 +1389,24 @@ TEST(MutatingTest, Fill) {
   EXPECT_THAT(actual, ElementsAre(1, 1, 1, 1, 1));
 }
 
+TEST(MutatingTest, FillWithRvalue) {
+  std::vector<int> actual(5);
+  absl::c_fill(absl::MakeSpan(actual), 1);
+  EXPECT_THAT(actual, ElementsAre(1, 1, 1, 1, 1));
+}
+
 TEST(MutatingTest, FillN) {
   std::vector<int> actual(5, 0);
   absl::c_fill_n(actual, 2, 1);
   EXPECT_THAT(actual, ElementsAre(1, 1, 0, 0, 0));
 }
 
+TEST(MutatingTest, FillNWithRvalue) {
+  std::vector<int> actual(5, 0);
+  absl::c_fill_n(absl::MakeSpan(actual), 2, 1);
+  EXPECT_THAT(actual, ElementsAre(1, 1, 0, 0, 0));
+}
+
 TEST(MutatingTest, Generate) {
   std::vector<int> actual(5);
   int x = 0;
@@ -863,10 +1483,18 @@ TEST(MutatingTest, RotateCopy) {
 
 TEST(MutatingTest, Shuffle) {
   std::vector<int> actual = {1, 2, 3, 4, 5};
-  absl::c_shuffle(actual, std::random_device());
+  absl::c_shuffle(actual, absl::InsecureBitGen());
   EXPECT_THAT(actual, UnorderedElementsAre(1, 2, 3, 4, 5));
 }
 
+TEST(MutatingTest, Sample) {
+  std::vector<int> actual;
+  absl::c_sample(std::vector<int>{1, 2, 3, 4, 5}, std::back_inserter(actual), 3,
+                 absl::InsecureBitGen());
+  EXPECT_THAT(actual, IsSubsetOf({1, 2, 3, 4, 5}));
+  EXPECT_THAT(actual, SizeIs(3));
+}
+
 TEST(MutatingTest, PartialSort) {
   std::vector<int> sequence{5, 3, 42, 0};
   absl::c_partial_sort(sequence, sequence.begin() + 2);
@@ -1022,4 +1650,1286 @@ TEST(MutatingTest, PermutationOperations) {
   EXPECT_EQ(initial, permuted);
 }
 
+#if defined(ABSL_INTERNAL_CPLUSPLUS_LANG) && \
+    ABSL_INTERNAL_CPLUSPLUS_LANG >= 201703L
+
+TEST(ConstexprTest, Distance) {
+  // Works at compile time with constexpr containers.
+  static_assert(absl::c_distance(std::array<int, 3>()) == 3);
+}
+
+TEST(ConstexprTest, MinElement) {
+  constexpr std::array<int, 3> kArray = {1, 2, 3};
+  static_assert(*absl::c_min_element(kArray) == 1);
+}
+
+TEST(ConstexprTest, MinElementWithPredicate) {
+  constexpr std::array<int, 3> kArray = {1, 2, 3};
+  static_assert(*absl::c_min_element(kArray, std::greater<int>()) == 3);
+}
+
+TEST(ConstexprTest, MaxElement) {
+  constexpr std::array<int, 3> kArray = {1, 2, 3};
+  static_assert(*absl::c_max_element(kArray) == 3);
+}
+
+TEST(ConstexprTest, MaxElementWithPredicate) {
+  constexpr std::array<int, 3> kArray = {1, 2, 3};
+  static_assert(*absl::c_max_element(kArray, std::greater<int>()) == 1);
+}
+
+TEST(ConstexprTest, MinMaxElement) {
+  static constexpr std::array<int, 3> kArray = {1, 2, 3};
+  constexpr auto kMinMaxPair = absl::c_minmax_element(kArray);
+  static_assert(*kMinMaxPair.first == 1);
+  static_assert(*kMinMaxPair.second == 3);
+}
+
+TEST(ConstexprTest, MinMaxElementWithPredicate) {
+  static constexpr std::array<int, 3> kArray = {1, 2, 3};
+  constexpr auto kMinMaxPair =
+      absl::c_minmax_element(kArray, std::greater<int>());
+  static_assert(*kMinMaxPair.first == 3);
+  static_assert(*kMinMaxPair.second == 1);
+}
+#endif  // defined(ABSL_INTERNAL_CPLUSPLUS_LANG) &&
+        //  ABSL_INTERNAL_CPLUSPLUS_LANG >= 201703L
+
+#if defined(ABSL_INTERNAL_CPLUSPLUS_LANG) && \
+    ABSL_INTERNAL_CPLUSPLUS_LANG >= 202002L
+
+TEST(ConstexprTest, LinearSearch) {
+  static constexpr std::array<int, 3> kArray = {1, 2, 3};
+  static_assert(absl::c_linear_search(kArray, 3));
+  static_assert(!absl::c_linear_search(kArray, 4));
+}
+
+TEST(ConstexprTest, AllOf) {
+  static constexpr std::array<int, 3> kArray = {1, 2, 3};
+  static_assert(!absl::c_all_of(kArray, [](int x) { return x > 1; }));
+  static_assert(absl::c_all_of(kArray, [](int x) { return x > 0; }));
+}
+
+TEST(ConstexprTest, AnyOf) {
+  static constexpr std::array<int, 3> kArray = {1, 2, 3};
+  static_assert(absl::c_any_of(kArray, [](int x) { return x > 2; }));
+  static_assert(!absl::c_any_of(kArray, [](int x) { return x > 5; }));
+}
+
+TEST(ConstexprTest, NoneOf) {
+  static constexpr std::array<int, 3> kArray = {1, 2, 3};
+  static_assert(!absl::c_none_of(kArray, [](int x) { return x > 2; }));
+  static_assert(absl::c_none_of(kArray, [](int x) { return x > 5; }));
+}
+
+TEST(ConstexprTest, ForEach) {
+  static constexpr std::array<int, 3> kArray = [] {
+    std::array<int, 3> array = {1, 2, 3};
+    absl::c_for_each(array, [](int& x) { x += 1; });
+    return array;
+  }();
+  static_assert(kArray == std::array{2, 3, 4});
+}
+
+TEST(ConstexprTest, Find) {
+  static constexpr std::array<int, 3> kArray = {1, 2, 3};
+  static_assert(absl::c_find(kArray, 1) == kArray.begin());
+  static_assert(absl::c_find(kArray, 4) == kArray.end());
+}
+
+TEST(ConstexprTest, Contains) {
+  static constexpr std::array<int, 3> kArray = {1, 2, 3};
+  static_assert(absl::c_contains(kArray, 1));
+  static_assert(!absl::c_contains(kArray, 4));
+}
+
+TEST(ConstexprTest, FindIf) {
+  static constexpr std::array<int, 3> kArray = {1, 2, 3};
+  static_assert(absl::c_find_if(kArray, [](int x) { return x > 2; }) ==
+                kArray.begin() + 2);
+  static_assert(absl::c_find_if(kArray, [](int x) { return x > 5; }) ==
+                kArray.end());
+}
+
+TEST(ConstexprTest, FindIfNot) {
+  static constexpr std::array<int, 3> kArray = {1, 2, 3};
+  static_assert(absl::c_find_if_not(kArray, [](int x) { return x > 1; }) ==
+                kArray.begin());
+  static_assert(absl::c_find_if_not(kArray, [](int x) { return x > 0; }) ==
+                kArray.end());
+}
+
+TEST(ConstexprTest, FindEnd) {
+  static constexpr std::array<int, 5> kHaystack = {1, 2, 3, 2, 3};
+  static constexpr std::array<int, 2> kNeedle = {2, 3};
+  static_assert(absl::c_find_end(kHaystack, kNeedle) == kHaystack.begin() + 3);
+}
+
+TEST(ConstexprTest, FindFirstOf) {
+  static constexpr std::array<int, 3> kArray = {1, 2, 3};
+  static_assert(absl::c_find_first_of(kArray, kArray) == kArray.begin());
+}
+
+TEST(ConstexprTest, AdjacentFind) {
+  static constexpr std::array<int, 4> kArray = {1, 2, 2, 3};
+  static_assert(absl::c_adjacent_find(kArray) == kArray.begin() + 1);
+}
+
+TEST(ConstexprTest, AdjacentFindWithPredicate) {
+  static constexpr std::array<int, 3> kArray = {1, 2, 3};
+  static_assert(absl::c_adjacent_find(kArray, std::less<int>()) ==
+                kArray.begin());
+}
+
+TEST(ConstexprTest, Count) {
+  static constexpr std::array<int, 3> kArray = {1, 2, 3};
+  static_assert(absl::c_count(kArray, 1) == 1);
+  static_assert(absl::c_count(kArray, 2) == 1);
+  static_assert(absl::c_count(kArray, 3) == 1);
+  static_assert(absl::c_count(kArray, 4) == 0);
+}
+
+TEST(ConstexprTest, CountIf) {
+  static constexpr std::array<int, 3> kArray = {1, 2, 3};
+  static_assert(absl::c_count_if(kArray, [](int x) { return x > 0; }) == 3);
+  static_assert(absl::c_count_if(kArray, [](int x) { return x > 1; }) == 2);
+}
+
+TEST(ConstexprTest, Mismatch) {
+  static constexpr std::array<int, 3> kArray1 = {1, 2, 3};
+  static constexpr std::array<int, 3> kArray2 = {1, 2, 3};
+  static constexpr std::array<int, 3> kArray3 = {2, 3, 4};
+  static_assert(absl::c_mismatch(kArray1, kArray2) ==
+                std::pair{kArray1.end(), kArray2.end()});
+  static_assert(absl::c_mismatch(kArray1, kArray3) ==
+                std::pair{kArray1.begin(), kArray3.begin()});
+}
+
+TEST(ConstexprTest, MismatchWithPredicate) {
+  static constexpr std::array<int, 3> kArray1 = {1, 2, 3};
+  static constexpr std::array<int, 3> kArray2 = {1, 2, 3};
+  static constexpr std::array<int, 3> kArray3 = {2, 3, 4};
+  static_assert(absl::c_mismatch(kArray1, kArray2, std::not_equal_to<int>()) ==
+                std::pair{kArray1.begin(), kArray2.begin()});
+  static_assert(absl::c_mismatch(kArray1, kArray3, std::not_equal_to<int>()) ==
+                std::pair{kArray1.end(), kArray3.end()});
+}
+
+TEST(ConstexprTest, Equal) {
+  static constexpr std::array<int, 3> kArray1 = {1, 2, 3};
+  static constexpr std::array<int, 3> kArray2 = {1, 2, 3};
+  static constexpr std::array<int, 3> kArray3 = {2, 3, 4};
+  static_assert(absl::c_equal(kArray1, kArray2));
+  static_assert(!absl::c_equal(kArray1, kArray3));
+}
+
+TEST(ConstexprTest, EqualWithPredicate) {
+  static constexpr std::array<int, 3> kArray1 = {1, 2, 3};
+  static constexpr std::array<int, 3> kArray2 = {1, 2, 3};
+  static constexpr std::array<int, 3> kArray3 = {2, 3, 4};
+  static_assert(!absl::c_equal(kArray1, kArray2, std::not_equal_to<int>()));
+  static_assert(absl::c_equal(kArray1, kArray3, std::not_equal_to<int>()));
+}
+
+TEST(ConstexprTest, IsPermutation) {
+  static constexpr std::array<int, 3> kArray1 = {1, 2, 3};
+  static constexpr std::array<int, 3> kArray2 = {3, 2, 1};
+  static constexpr std::array<int, 3> kArray3 = {2, 3, 4};
+  static_assert(absl::c_is_permutation(kArray1, kArray2));
+  static_assert(!absl::c_is_permutation(kArray1, kArray3));
+}
+
+TEST(ConstexprTest, IsPermutationWithPredicate) {
+  static constexpr std::array<int, 3> kArray1 = {1, 2, 3};
+  static constexpr std::array<int, 3> kArray2 = {3, 2, 1};
+  static constexpr std::array<int, 3> kArray3 = {2, 3, 4};
+  static_assert(absl::c_is_permutation(kArray1, kArray2, std::equal_to<int>()));
+  static_assert(
+      !absl::c_is_permutation(kArray1, kArray3, std::equal_to<int>()));
+}
+
+TEST(ConstexprTest, Search) {
+  static constexpr std::array<int, 3> kArray1 = {1, 2, 3};
+  static constexpr std::array<int, 3> kArray2 = {1, 2, 3};
+  static constexpr std::array<int, 3> kArray3 = {2, 3, 4};
+  static_assert(absl::c_search(kArray1, kArray2) == kArray1.begin());
+  static_assert(absl::c_search(kArray1, kArray3) == kArray1.end());
+}
+
+TEST(ConstexprTest, SearchWithPredicate) {
+  static constexpr std::array<int, 3> kArray1 = {1, 2, 3};
+  static constexpr std::array<int, 3> kArray2 = {1, 2, 3};
+  static constexpr std::array<int, 3> kArray3 = {2, 3, 4};
+  static_assert(absl::c_search(kArray1, kArray2, std::not_equal_to<int>()) ==
+                kArray1.end());
+  static_assert(absl::c_search(kArray1, kArray3, std::not_equal_to<int>()) ==
+                kArray1.begin());
+}
+
+TEST(ConstexprTest, ContainsSubrange) {
+  static constexpr std::array<int, 3> kArray1 = {1, 2, 3};
+  static constexpr std::array<int, 3> kArray2 = {1, 2, 3};
+  static constexpr std::array<int, 3> kArray3 = {2, 3, 4};
+  static_assert(absl::c_contains_subrange(kArray1, kArray2));
+  static_assert(!absl::c_contains_subrange(kArray1, kArray3));
+}
+
+TEST(ConstexprTest, ContainsSubrangeWithPredicate) {
+  static constexpr std::array<int, 3> kArray1 = {1, 2, 3};
+  static constexpr std::array<int, 3> kArray2 = {1, 2, 3};
+  static constexpr std::array<int, 3> kArray3 = {2, 3, 4};
+  static_assert(
+      !absl::c_contains_subrange(kArray1, kArray2, std::not_equal_to<>()));
+  static_assert(
+      absl::c_contains_subrange(kArray1, kArray3, std::not_equal_to<>()));
+}
+
+TEST(ConstexprTest, SearchN) {
+  static constexpr std::array<int, 4> kArray = {1, 2, 2, 3};
+  static_assert(absl::c_search_n(kArray, 1, 1) == kArray.begin());
+  static_assert(absl::c_search_n(kArray, 2, 2) == kArray.begin() + 1);
+  static_assert(absl::c_search_n(kArray, 1, 4) == kArray.end());
+}
+
+TEST(ConstexprTest, SearchNWithPredicate) {
+  static constexpr std::array<int, 4> kArray = {1, 2, 2, 3};
+  static_assert(absl::c_search_n(kArray, 1, 1, std::not_equal_to<int>()) ==
+                kArray.begin() + 1);
+  static_assert(absl::c_search_n(kArray, 2, 2, std::not_equal_to<int>()) ==
+                kArray.end());
+  static_assert(absl::c_search_n(kArray, 1, 4, std::not_equal_to<int>()) ==
+                kArray.begin());
+}
+
+TEST(ConstexprTest, Copy) {
+  static constexpr std::array kArray = {1, 2, 3};
+  static constexpr auto kArrayCopy = [] {
+    std::array<int, 3> array;
+    absl::c_copy(kArray, array.begin());
+    return array;
+  }();
+  static_assert(kArrayCopy == kArray);
+}
+
+TEST(ConstexprTest, CopyN) {
+  static constexpr std::array kArray = {1, 2, 3};
+  static constexpr auto kArrayCopy = [] {
+    std::array<int, 2> array;
+    absl::c_copy_n(kArray, 2, array.begin());
+    return array;
+  }();
+  static_assert(kArrayCopy == std::array{1, 2});
+}
+
+TEST(ConstexprTest, CopyIf) {
+  static constexpr std::array kArray = {1, 2, 3, 4};
+  static constexpr auto kArrayCopy = [] {
+    std::array<int, 3> array;
+    absl::c_copy_if(kArray, array.begin(), [](int x) { return x > 1; });
+    return array;
+  }();
+  static_assert(kArrayCopy == std::array{2, 3, 4});
+}
+
+TEST(ConstexprTest, CopyBackward) {
+  static constexpr std::array kArray = {1, 2, 3};
+  static constexpr auto kArrayCopy = [] {
+    std::array<int, 3> array;
+    absl::c_copy_backward(kArray, array.end());
+    return array;
+  }();
+  static_assert(kArrayCopy == kArray);
+}
+
+TEST(ConstexprTest, Move) {
+  static constexpr std::array kArray = {1, 2, 3};
+  static constexpr auto kArrayMove = [] {
+    std::array<int, 3> array;
+    absl::c_move(kArray, array.begin());
+    return array;
+  }();
+  static_assert(kArrayMove == kArray);
+}
+
+TEST(ConstexprTest, MoveBackward) {
+  static constexpr std::array kArray = {1, 2, 3};
+  static constexpr auto kArrayMove = [] {
+    std::array<int, 3> array;
+    absl::c_move_backward(kArray, array.end());
+    return array;
+  }();
+  static_assert(kArrayMove == kArray);
+}
+
+TEST(ConstexprTest, SwapRanges) {
+  static constexpr std::array kArray1 = {1, 2, 3};
+  static constexpr std::array kArray2 = {4, 5, 6};
+
+  static constexpr auto kSwapped = [] {
+    std::array arr1 = kArray1;
+    std::array arr2 = kArray2;
+    absl::c_swap_ranges(arr1, arr2);
+    return std::make_pair(arr1, arr2);
+  }();
+
+  static_assert(kSwapped.first == kArray2);
+  static_assert(kSwapped.second == kArray1);
+}
+
+TEST(ConstexprTest, Transform) {
+  static constexpr std::array kArray = {1, 2, 3};
+  static constexpr auto kArrayTransform = [] {
+    std::array<int, 3> array;
+    absl::c_transform(kArray, array.begin(), [](int x) { return x + 1; });
+    return array;
+  }();
+  static_assert(kArrayTransform == std::array{2, 3, 4});
+}
+
+TEST(ConstexprTest, Replace) {
+  static constexpr std::array kArray = {1, 2, 3};
+  static constexpr auto kArrayReplace = [] {
+    std::array array = kArray;
+    absl::c_replace(array, 1, 4);
+    return array;
+  }();
+  static_assert(kArrayReplace == std::array{4, 2, 3});
+}
+
+TEST(ConstexprTest, ReplaceIf) {
+  static constexpr std::array kArray = {1, 2, 3};
+  static constexpr auto kArrayReplaceIf = [] {
+    std::array array = kArray;
+    absl::c_replace_if(array, [](int x) { return x == 1; }, 4);
+    return array;
+  }();
+  static_assert(kArrayReplaceIf == std::array{4, 2, 3});
+}
+
+TEST(ConstexprTest, ReplaceCopy) {
+  static constexpr std::array kArray = {1, 2, 3};
+  static constexpr auto kArrayReplaceCopy = [] {
+    std::array<int, 3> array;
+    absl::c_replace_copy(kArray, array.begin(), 1, 4);
+    return array;
+  }();
+  static_assert(kArrayReplaceCopy == std::array{4, 2, 3});
+}
+
+TEST(ConstexprTest, ReplaceCopyIf) {
+  static constexpr std::array kArray = {1, 2, 3};
+  static constexpr auto kArrayReplaceCopyIf = [] {
+    std::array<int, 3> array;
+    absl::c_replace_copy_if(
+        kArray, array.begin(), [](int x) { return x == 1; }, 4);
+    return array;
+  }();
+  static_assert(kArrayReplaceCopyIf == std::array{4, 2, 3});
+}
+
+TEST(ConstexprTest, Fill) {
+  static constexpr auto kArrayFill = [] {
+    std::array<int, 3> array;
+    absl::c_fill(array, 4);
+    return array;
+  }();
+  static_assert(kArrayFill == std::array{4, 4, 4});
+}
+
+TEST(ConstexprTest, FillN) {
+  static constexpr auto kArrayFillN = [] {
+    std::array array = {0, 0, 0};
+    absl::c_fill_n(array, 2, 4);
+    return array;
+  }();
+  static_assert(kArrayFillN == std::array{4, 4, 0});
+}
+
+TEST(ConstexprTest, Generate) {
+  static constexpr auto kArrayGenerate = [] {
+    std::array<int, 3> array;
+    absl::c_generate(array, []() { return 4; });
+    return array;
+  }();
+  static_assert(kArrayGenerate == std::array{4, 4, 4});
+}
+
+TEST(ConstexprTest, GenerateN) {
+  static constexpr auto kArrayGenerateN = [] {
+    std::array array = {0, 0, 0};
+    absl::c_generate_n(array, 2, []() { return 4; });
+    return array;
+  }();
+  static_assert(kArrayGenerateN == std::array{4, 4, 0});
+}
+
+TEST(ConstexprTest, RemoveCopy) {
+  static constexpr std::array kArray = {1, 2, 3};
+  static constexpr auto kArrayRemoveCopy = [] {
+    std::array<int, 2> array;
+    absl::c_remove_copy(kArray, array.begin(), 1);
+    return array;
+  }();
+  static_assert(kArrayRemoveCopy == std::array{2, 3});
+}
+
+TEST(ConstexprTest, RemoveCopyIf) {
+  static constexpr std::array kArray = {1, 2, 3};
+  static constexpr auto kArrayRemoveCopyIf = [] {
+    std::array<int, 2> array;
+    absl::c_remove_copy_if(kArray, array.begin(), [](int x) { return x == 1; });
+    return array;
+  }();
+  static_assert(kArrayRemoveCopyIf == std::array{2, 3});
+}
+
+TEST(ConstexprTest, UniqueCopy) {
+  static constexpr std::array kArray = {1, 2, 2, 3};
+  static constexpr auto kArrayUniqueCopy = [] {
+    std::array<int, 3> array;
+    absl::c_unique_copy(kArray, array.begin());
+    return array;
+  }();
+  static_assert(kArrayUniqueCopy == std::array{1, 2, 3});
+}
+
+TEST(ConstexprTest, UniqueCopyWithPredicate) {
+  static constexpr std::array kArray = {1, 2, 2, 3};
+  static constexpr auto kArrayUniqueCopy = [] {
+    std::array<int, 3> array;
+    absl::c_unique_copy(kArray, array.begin(), std::equal_to<>());
+    return array;
+  }();
+  static_assert(kArrayUniqueCopy == std::array{1, 2, 3});
+}
+
+TEST(ConstexprTest, Reverse) {
+  static constexpr std::array kArray = {1, 2, 3};
+  static constexpr auto kArrayReverse = [] {
+    std::array array = kArray;
+    absl::c_reverse(array);
+    return array;
+  }();
+  static_assert(kArrayReverse == std::array{3, 2, 1});
+}
+
+TEST(ConstexprTest, ReverseCopy) {
+  static constexpr std::array kArray = {1, 2, 3};
+  static constexpr auto kArrayReverseCopy = [] {
+    std::array<int, 3> array;
+    absl::c_reverse_copy(kArray, array.begin());
+    return array;
+  }();
+  static_assert(kArrayReverseCopy == std::array{3, 2, 1});
+}
+
+TEST(ConstexprTest, Rotate) {
+  static constexpr std::array kArray = {1, 2, 3};
+  static constexpr auto kArrayRotate = [] {
+    std::array array = kArray;
+    absl::c_rotate(array, array.begin() + 1);
+    return array;
+  }();
+  static_assert(kArrayRotate == std::array{2, 3, 1});
+}
+
+TEST(ConstexprTest, RotateCopy) {
+  static constexpr std::array kArray = {1, 2, 3};
+  static constexpr auto kArrayRotateCopy = [] {
+    std::array<int, 3> array;
+    absl::c_rotate_copy(kArray, kArray.begin() + 1, array.begin());
+    return array;
+  }();
+  static_assert(kArrayRotateCopy == std::array{2, 3, 1});
+}
+
+TEST(ConstexprTest, IsPartitioned) {
+  static constexpr std::array kArray = {1, 2, 3};
+  static_assert(!absl::c_is_partitioned(kArray, [](int x) { return x > 1; }));
+
+  static constexpr std::array kPartitionedArray = {2, 3, 1};
+  static_assert(
+      absl::c_is_partitioned(kPartitionedArray, [](int x) { return x > 1; }));
+}
+
+TEST(ConstexprTest, Partition) {
+  static constexpr std::array kArray = {1, 2, 3};
+  static constexpr auto kArrayPartition = [] {
+    std::array array = kArray;
+    absl::c_partition(array, [](int x) { return x > 1; });
+    return array;
+  }();
+  static_assert(
+      absl::c_is_partitioned(kArrayPartition, [](int x) { return x > 1; }));
+}
+
+TEST(ConstexprTest, PartitionCopy) {
+  static constexpr std::array kArray = {1, 2, 3};
+  static constexpr auto kPartitioned = [] {
+    std::array<int, 2> true_part;
+    std::array<int, 1> false_part;
+    absl::c_partition_copy(kArray, true_part.begin(), false_part.begin(),
+                           [](int x) { return x > 1; });
+    return std::make_pair(true_part, false_part);
+  }();
+  static_assert(kPartitioned.first == std::array{2, 3});
+  static_assert(kPartitioned.second == std::array{1});
+}
+
+TEST(ConstexprTest, PartitionPoint) {
+  static constexpr std::array kArray = {1, 2, 3};
+  static constexpr auto kPartitionPoint =
+      absl::c_partition_point(kArray, [](int x) { return x > 1; });
+  static_assert(kPartitionPoint == kArray.end());
+}
+
+TEST(ConstexprTest, Sort) {
+  static constexpr std::array kArray = {2, 1, 3};
+  static constexpr auto kArraySort = [] {
+    std::array array = kArray;
+    absl::c_sort(array);
+    return array;
+  }();
+  static_assert(kArraySort == std::array{1, 2, 3});
+}
+
+TEST(ConstexprTest, SortWithPredicate) {
+  static constexpr std::array kArray = {1, 2, 3};
+  static constexpr auto kArraySort = [] {
+    std::array array = kArray;
+    absl::c_sort(array, std::greater<>());
+    return array;
+  }();
+  static_assert(kArraySort == std::array{3, 2, 1});
+}
+
+TEST(ConstexprTest, IsSorted) {
+  static constexpr std::array kSortedArray = {1, 2, 3};
+  static_assert(absl::c_is_sorted(kSortedArray));
+  static constexpr std::array kUnsortedArray = {1, 3, 2};
+  static_assert(!absl::c_is_sorted(kUnsortedArray));
+}
+
+TEST(ConstexprTest, IsSortedWithPredicate) {
+  static constexpr std::array kSortedArray = {3, 2, 1};
+  static_assert(absl::c_is_sorted(kSortedArray, std::greater<>()));
+  static constexpr std::array kUnsortedArray = {1, 3, 2};
+  static_assert(!absl::c_is_sorted(kUnsortedArray, std::greater<>()));
+}
+
+TEST(ConstexprTest, PartialSort) {
+  static constexpr std::array kArray = {3, 1, 4, 2};
+  static constexpr auto kArrayPartialSort = [] {
+    std::array array = kArray;
+    absl::c_partial_sort(array, array.begin() + 2);
+    return array;
+  }();
+  static_assert(kArrayPartialSort[0] == 1);
+  static_assert(kArrayPartialSort[1] == 2);
+}
+
+TEST(ConstexprTest, PartialSortWithPredicate) {
+  static constexpr std::array kArray = {3, 1, 4, 2};
+  static constexpr auto kArrayPartialSort = [] {
+    std::array array = kArray;
+    absl::c_partial_sort(array, array.begin() + 2, std::greater<>());
+    return array;
+  }();
+  static_assert(kArrayPartialSort[0] == 4);
+  static_assert(kArrayPartialSort[1] == 3);
+}
+
+TEST(ConstexprTest, PartialSortCopy) {
+  static constexpr std::array kArray = {3, 1, 4, 2};
+  static constexpr auto kArrayPartialSort = [] {
+    std::array<int, 4> array;
+    absl::c_partial_sort_copy(kArray, array);
+    return array;
+  }();
+  static_assert(kArrayPartialSort[0] == 1);
+  static_assert(kArrayPartialSort[1] == 2);
+}
+
+TEST(ConstexprTest, PartialSortCopyWithPredicate) {
+  static constexpr std::array kArray = {3, 1, 4, 2};
+  static constexpr auto kArrayPartialSort = [] {
+    std::array<int, 4> array;
+    absl::c_partial_sort_copy(kArray, array, std::greater<>());
+    return array;
+  }();
+  static_assert(kArrayPartialSort[0] == 4);
+  static_assert(kArrayPartialSort[1] == 3);
+}
+
+TEST(ConstexprTest, IsSortedUntil) {
+  static constexpr std::array kSortedArray = {1, 2, 3};
+  static_assert(absl::c_is_sorted_until(kSortedArray) == kSortedArray.end());
+  static constexpr std::array kUnsortedArray = {1, 3, 2};
+  static_assert(absl::c_is_sorted_until(kUnsortedArray) ==
+                kUnsortedArray.begin() + 2);
+}
+
+TEST(ConstexprTest, IsSortedUntilWithPredicate) {
+  static constexpr std::array kSortedArray = {3, 2, 1};
+  static_assert(absl::c_is_sorted_until(kSortedArray, std::greater<>()) ==
+                kSortedArray.end());
+  static constexpr std::array kUnsortedArray = {1, 3, 2};
+  static_assert(absl::c_is_sorted_until(kUnsortedArray, std::greater<>()) ==
+                kUnsortedArray.begin() + 1);
+}
+
+TEST(ConstexprTest, NthElement) {
+  static constexpr std::array kArray = {2, 1, 3, 4};
+  static constexpr auto kArrayNthElement = [] {
+    std::array array = kArray;
+    absl::c_nth_element(array, array.begin() + 2);
+    return array;
+  }();
+  static_assert(kArrayNthElement[2] == 3);
+  static_assert(kArrayNthElement[0] <= kArrayNthElement[2]);
+  static_assert(kArrayNthElement[1] <= kArrayNthElement[2]);
+  static_assert(kArrayNthElement[3] >= kArrayNthElement[2]);
+}
+
+TEST(ConstexprTest, NthElementWithPredicate) {
+  static constexpr std::array kArray = {1, 2, 3, 4};
+  static constexpr auto kArrayNthElement = [] {
+    std::array array = kArray;
+    absl::c_nth_element(array, array.begin() + 2, std::greater<>());
+    return array;
+  }();
+  static_assert(kArrayNthElement[2] == 2);
+  static_assert(std::greater<>()(kArrayNthElement[0], kArrayNthElement[2]) ||
+                kArrayNthElement[0] == kArrayNthElement[2]);
+  static_assert(std::greater<>()(kArrayNthElement[1], kArrayNthElement[2]) ||
+                kArrayNthElement[1] == kArrayNthElement[2]);
+  static_assert(std::greater<>()(kArrayNthElement[2], kArrayNthElement[3]) ||
+                kArrayNthElement[2] == kArrayNthElement[3]);
+}
+
+TEST(ConstexprTest, LowerBound) {
+  static constexpr std::array kArray = {1, 2, 3, 4};
+  static constexpr auto kLowerBound = absl::c_lower_bound(kArray, 2);
+  static_assert(kLowerBound == kArray.begin() + 1);
+}
+
+TEST(ConstexprTest, LowerBoundWithPredicate) {
+  static constexpr std::array kArray = {4, 3, 2, 1};
+  static constexpr auto kLowerBound =
+      absl::c_lower_bound(kArray, 2, std::greater<>());
+  static_assert(kLowerBound == kArray.begin() + 2);
+}
+
+TEST(ConstexprTest, UpperBound) {
+  static constexpr std::array kArray = {1, 2, 3, 4};
+  static constexpr auto kUpperBound = absl::c_upper_bound(kArray, 2);
+  static_assert(kUpperBound == kArray.begin() + 2);
+}
+
+TEST(ConstexprTest, UpperBoundWithPredicate) {
+  static constexpr std::array kArray = {4, 3, 2, 1};
+  static constexpr auto kUpperBound =
+      absl::c_upper_bound(kArray, 2, std::greater<>());
+  static_assert(kUpperBound == kArray.begin() + 3);
+}
+
+TEST(ConstexprTest, EqualRange) {
+  static constexpr std::array kArray = {1, 2, 3, 4};
+  static constexpr auto kEqualRange = absl::c_equal_range(kArray, 2);
+  static_assert(kEqualRange.first == kArray.begin() + 1);
+  static_assert(kEqualRange.second == kArray.begin() + 2);
+}
+
+TEST(ConstexprTest, EqualRangeWithPredicate) {
+  static constexpr std::array kArray = {4, 3, 2, 1};
+  static constexpr auto kEqualRange =
+      absl::c_equal_range(kArray, 2, std::greater<>());
+  static_assert(kEqualRange.first == kArray.begin() + 2);
+  static_assert(kEqualRange.second == kArray.begin() + 3);
+}
+
+TEST(ConstexprTest, BinarySearch) {
+  static constexpr std::array kArray = {1, 2, 3, 4};
+  static constexpr bool kBinarySearch = absl::c_binary_search(kArray, 2);
+  static_assert(kBinarySearch);
+}
+
+TEST(ConstexprTest, BinarySearchWithPredicate) {
+  static constexpr std::array kArray = {4, 3, 2, 1};
+  static constexpr bool kBinarySearch =
+      absl::c_binary_search(kArray, 2, std::greater<>());
+  static_assert(kBinarySearch);
+}
+
+TEST(ConstexprTest, Merge) {
+  static constexpr std::array kArray1 = {1, 2, 3};
+  static constexpr std::array kArray2 = {4, 5, 6};
+  static constexpr auto kArrayMerge = [] {
+    std::array<int, 6> array;
+    absl::c_merge(kArray1, kArray2, array.begin());
+    return array;
+  }();
+  static_assert(kArrayMerge == std::array{1, 2, 3, 4, 5, 6});
+}
+
+TEST(ConstexprTest, MergeWithPredicate) {
+  static constexpr std::array kArray1 = {3, 2, 1};
+  static constexpr std::array kArray2 = {6, 5, 4};
+  static constexpr auto kArrayMerge = [] {
+    std::array<int, 6> array;
+    absl::c_merge(kArray1, kArray2, array.begin(), std::greater<>());
+    return array;
+  }();
+  static_assert(kArrayMerge == std::array{6, 5, 4, 3, 2, 1});
+}
+
+TEST(ConstexprTest, Includes) {
+  static constexpr std::array kArray1 = {1, 2, 3, 4, 5, 6};
+  static constexpr std::array kArray2 = {2, 3, 5};
+  static constexpr bool kIncludes = absl::c_includes(kArray1, kArray2);
+  static_assert(kIncludes);
+}
+
+TEST(ConstexprTest, IncludesWithPredicate) {
+  static constexpr std::array kArray1 = {6, 5, 4, 3, 2, 1};
+  static constexpr std::array kArray2 = {5, 3, 2};
+  static constexpr bool kIncludes =
+      absl::c_includes(kArray1, kArray2, std::greater<>());
+  static_assert(kIncludes);
+}
+
+TEST(ConstexprTest, SetUnion) {
+  static constexpr std::array kArray1 = {1, 2, 3};
+  static constexpr std::array kArray2 = {1, 3, 4};
+  static constexpr auto kArraySetUnion = [] {
+    std::array<int, 4> array;
+    absl::c_set_union(kArray1, kArray2, array.begin());
+    return array;
+  }();
+  static_assert(kArraySetUnion == std::array{1, 2, 3, 4});
+}
+
+TEST(ConstexprTest, SetUnionWithPredicate) {
+  static constexpr std::array kArray1 = {3, 2, 1};
+  static constexpr std::array kArray2 = {4, 3, 1};
+  static constexpr auto kArraySetUnion = [] {
+    std::array<int, 4> array;
+    absl::c_set_union(kArray1, kArray2, array.begin(), std::greater<>());
+    return array;
+  }();
+  static_assert(kArraySetUnion == std::array{4, 3, 2, 1});
+}
+
+TEST(ConstexprTest, SetIntersection) {
+  static constexpr std::array kArray1 = {1, 2, 3};
+  static constexpr std::array kArray2 = {1, 3, 4};
+  static constexpr auto kArraySetIntersection = [] {
+    std::array<int, 2> array;
+    absl::c_set_intersection(kArray1, kArray2, array.begin());
+    return array;
+  }();
+  static_assert(kArraySetIntersection == std::array{1, 3});
+}
+
+TEST(ConstexprTest, SetIntersectionWithPredicate) {
+  static constexpr std::array kArray1 = {3, 2, 1};
+  static constexpr std::array kArray2 = {4, 3, 1};
+  static constexpr auto kArraySetIntersection = [] {
+    std::array<int, 2> array;
+    absl::c_set_intersection(kArray1, kArray2, array.begin(), std::greater<>());
+    return array;
+  }();
+  static_assert(kArraySetIntersection == std::array{3, 1});
+}
+
+TEST(ConstexprTest, SetDifference) {
+  static constexpr std::array kArray1 = {1, 2, 3};
+  static constexpr std::array kArray2 = {1, 3, 4};
+  static constexpr auto kArraySetDifference = [] {
+    std::array<int, 1> array;
+    absl::c_set_difference(kArray1, kArray2, array.begin());
+    return array;
+  }();
+  static_assert(kArraySetDifference == std::array{2});
+}
+
+TEST(ConstexprTest, SetDifferenceWithPredicate) {
+  static constexpr std::array kArray1 = {3, 2, 1};
+  static constexpr std::array kArray2 = {4, 3, 1};
+  static constexpr auto kArraySetDifference = [] {
+    std::array<int, 1> array;
+    absl::c_set_difference(kArray1, kArray2, array.begin(), std::greater<>());
+    return array;
+  }();
+  static_assert(kArraySetDifference == std::array{2});
+}
+
+TEST(ConstexprTest, SetSymmetricDifference) {
+  static constexpr std::array kArray1 = {1, 2, 3};
+  static constexpr std::array kArray2 = {1, 3, 4};
+  static constexpr auto kArraySetSymmetricDifference = [] {
+    std::array<int, 2> array;
+    absl::c_set_symmetric_difference(kArray1, kArray2, array.begin());
+    return array;
+  }();
+  static_assert(kArraySetSymmetricDifference == std::array{2, 4});
+}
+
+TEST(ConstexprTest, SetSymmetricDifferenceWithPredicate) {
+  static constexpr std::array kArray1 = {3, 2, 1};
+  static constexpr std::array kArray2 = {4, 3, 1};
+  static constexpr auto kArraySetSymmetricDifference = [] {
+    std::array<int, 2> array;
+    absl::c_set_symmetric_difference(kArray1, kArray2, array.begin(),
+                                     std::greater<>());
+    return array;
+  }();
+  static_assert(kArraySetSymmetricDifference == std::array{4, 2});
+}
+
+TEST(ConstexprTest, PushHeap) {
+  static constexpr auto kArray = [] {
+    std::array array = {1, 2, 3, 4};
+    absl::c_push_heap(array);
+    return array;
+  }();
+  static_assert(kArray[0] == 4);
+}
+
+TEST(ConstexprTest, PushHeapWithPredicate) {
+  static constexpr auto kArray = [] {
+    std::array array = {4, 3, 2, 1};
+    absl::c_push_heap(array, std::greater<>());
+    return array;
+  }();
+  static_assert(kArray[0] == 1);
+}
+
+TEST(ConstexprTest, PopHeap) {
+  static constexpr auto kArray = [] {
+    std::array array = {4, 3, 2, 1};
+    absl::c_pop_heap(array);
+    return array;
+  }();
+  static_assert(kArray[3] == 4);
+}
+
+TEST(ConstexprTest, PopHeapWithPredicate) {
+  static constexpr auto kArray = [] {
+    std::array array = {1, 2, 3, 4};
+    absl::c_pop_heap(array, std::greater<>());
+    return array;
+  }();
+  static_assert(kArray[3] == 1);
+}
+
+TEST(ConstexprTest, MakeHeap) {
+  static constexpr auto kArray = [] {
+    std::array array = {1, 2, 3, 4};
+    absl::c_make_heap(array);
+    return array;
+  }();
+  static_assert(absl::c_is_heap(kArray));
+}
+
+TEST(ConstexprTest, MakeHeapWithPredicate) {
+  static constexpr auto kArray = [] {
+    std::array array = {4, 3, 2, 1};
+    absl::c_make_heap(array, std::greater<>());
+    return array;
+  }();
+  static_assert(absl::c_is_heap(kArray, std::greater<>()));
+}
+
+TEST(ConstexprTest, SortHeap) {
+  static constexpr auto kArray = [] {
+    std::array array = {1, 2, 3, 4};
+    absl::c_make_heap(array);
+    absl::c_sort_heap(array);
+    return array;
+  }();
+  static_assert(kArray == std::array{1, 2, 3, 4});
+}
+
+TEST(ConstexprTest, SortHeapWithPredicate) {
+  static constexpr auto kArray = [] {
+    std::array array = {4, 3, 2, 1};
+    absl::c_make_heap(array, std::greater<>());
+    absl::c_sort_heap(array, std::greater<>());
+    return array;
+  }();
+  static_assert(kArray == std::array{4, 3, 2, 1});
+}
+
+TEST(ConstexprTest, IsHeap) {
+  static constexpr std::array kHeap = {4, 2, 3, 1};
+  static_assert(absl::c_is_heap(kHeap));
+  static constexpr std::array kNotHeap = {1, 2, 3, 4};
+  static_assert(!absl::c_is_heap(kNotHeap));
+}
+
+TEST(ConstexprTest, IsHeapWithPredicate) {
+  static constexpr std::array kHeap = {1, 2, 3, 4};
+  static_assert(absl::c_is_heap(kHeap, std::greater<>()));
+  static constexpr std::array kNotHeap = {4, 3, 2, 1};
+  static_assert(!absl::c_is_heap(kNotHeap, std::greater<>()));
+}
+
+TEST(ConstexprTest, IsHeapUntil) {
+  static constexpr std::array kHeap = {4, 2, 3, 1};
+  static_assert(absl::c_is_heap_until(kHeap) == kHeap.end());
+  static constexpr std::array kNotHeap = {4, 2, 3, 5};
+  static_assert(absl::c_is_heap_until(kNotHeap) == kNotHeap.begin() + 3);
+}
+
+TEST(ConstexprTest, IsHeapUntilWithPredicate) {
+  static constexpr std::array kHeap = {1, 2, 3, 4};
+  static_assert(absl::c_is_heap_until(kHeap, std::greater<>()) == kHeap.end());
+  static constexpr std::array kNotHeap = {1, 2, 3, 0};
+  static_assert(absl::c_is_heap_until(kNotHeap, std::greater<>()) ==
+                kNotHeap.begin() + 3);
+}
+
+TEST(ConstexprTest, LexicographicalCompare) {
+  static constexpr std::array kArray1 = {1, 2, 3};
+  static constexpr std::array kArray2 = {1, 2, 4};
+  static constexpr std::array kArray3 = {1, 2, 3};
+  static_assert(absl::c_lexicographical_compare(kArray1, kArray2));
+  static_assert(!absl::c_lexicographical_compare(kArray2, kArray1));
+  static_assert(!absl::c_lexicographical_compare(kArray1, kArray3));
+}
+
+TEST(ConstexprTest, LexicographicalCompareWithPredicate) {
+  static constexpr std::array kArray1 = {1, 2, 3};
+  static constexpr std::array kArray2 = {1, 2, 4};
+  static constexpr std::array kArray3 = {1, 2, 3};
+  static_assert(
+      !absl::c_lexicographical_compare(kArray1, kArray2, std::greater<>()));
+  static_assert(
+      absl::c_lexicographical_compare(kArray2, kArray1, std::greater<>()));
+  static_assert(
+      !absl::c_lexicographical_compare(kArray1, kArray3, std::greater<>()));
+}
+
+TEST(ConstexprTest, NextPermutation) {
+  static constexpr auto kArray = [] {
+    std::array array = {1, 2, 3};
+    absl::c_next_permutation(array);
+    return array;
+  }();
+  static_assert(kArray == std::array{1, 3, 2});
+}
+
+TEST(ConstexprTest, NextPermutationWithPredicate) {
+  static constexpr auto kArray = [] {
+    std::array array = {3, 2, 1};
+    absl::c_next_permutation(array, std::greater<>());
+    return array;
+  }();
+  static_assert(kArray == std::array{3, 1, 2});
+}
+
+TEST(ConstexprTest, PrevPermutation) {
+  static constexpr auto kArray = [] {
+    std::array array = {1, 3, 2};
+    absl::c_prev_permutation(array);
+    return array;
+  }();
+  static_assert(kArray == std::array{1, 2, 3});
+}
+
+TEST(ConstexprTest, PrevPermutationWithPredicate) {
+  static constexpr auto kArray = [] {
+    std::array array = {1, 2, 3};
+    absl::c_prev_permutation(array, std::greater<>());
+    return array;
+  }();
+  static_assert(kArray == std::array{1, 3, 2});
+}
+
+TEST(ConstexprTest, Iota) {
+  static constexpr auto kArray = [] {
+    std::array<int, 3> array;
+    absl::c_iota(array, 1);
+    return array;
+  }();
+  static_assert(kArray == std::array{1, 2, 3});
+}
+
+TEST(ConstexprTest, Accumulate) {
+  static constexpr std::array kArray = {1, 2, 3};
+  static_assert(absl::c_accumulate(kArray, 0) == 6);
+}
+
+TEST(ConstexprTest, AccumulateWithPredicate) {
+  static constexpr std::array kArray = {1, 2, 3};
+  static_assert(absl::c_accumulate(kArray, 1, std::multiplies<>()) == 6);
+}
+
+TEST(ConstexprTest, InnerProduct) {
+  static constexpr std::array kArray1 = {1, 2, 3};
+  static constexpr std::array kArray2 = {4, 5, 6};
+  static_assert(absl::c_inner_product(kArray1, kArray2, 0) == 32);
+}
+
+TEST(ConstexprTest, InnerProductWithPredicate) {
+  static constexpr std::array kArray1 = {1, 2, 3};
+  static constexpr std::array kArray2 = {4, 5, 6};
+  static_assert(absl::c_inner_product(kArray1, kArray2, 1, std::multiplies<>(),
+                                      std::plus<>()) == 315);
+}
+
+TEST(ConstexprTest, AdjacentDifference) {
+  static constexpr std::array kArray = {1, 2, 4};
+  static constexpr auto kArrayAdjacentDifference = [] {
+    std::array<int, 3> array;
+    absl::c_adjacent_difference(kArray, array.begin());
+    return array;
+  }();
+  static_assert(kArrayAdjacentDifference == std::array{1, 1, 2});
+}
+
+TEST(ConstexprTest, AdjacentDifferenceWithPredicate) {
+  static constexpr std::array kArray = {1, 2, 4};
+  static constexpr auto kArrayAdjacentDifference = [] {
+    std::array<int, 3> array;
+    absl::c_adjacent_difference(kArray, array.begin(), std::multiplies<>());
+    return array;
+  }();
+  static_assert(kArrayAdjacentDifference == std::array{1, 2, 8});
+}
+
+TEST(ConstexprTest, PartialSum) {
+  static constexpr std::array kArray = {1, 2, 3};
+  static constexpr auto kArrayPartialSum = [] {
+    std::array<int, 3> array;
+    absl::c_partial_sum(kArray, array.begin());
+    return array;
+  }();
+  static_assert(kArrayPartialSum == std::array{1, 3, 6});
+}
+
+TEST(ConstexprTest, PartialSumWithPredicate) {
+  static constexpr std::array kArray = {1, 2, 3};
+  static constexpr auto kArrayPartialSum = [] {
+    std::array<int, 3> array;
+    absl::c_partial_sum(kArray, array.begin(), std::multiplies<>());
+    return array;
+  }();
+  static_assert(kArrayPartialSum == std::array{1, 2, 6});
+}
+
+#endif  // defined(ABSL_INTERNAL_CPLUSPLUS_LANG) &&
+        //  ABSL_INTERNAL_CPLUSPLUS_LANG >= 202002L
+
+// A type that acts as both a container and an iterator.
+struct AmbiguousType {
+  // Container requirements
+  int* begin() { return nullptr; }
+  int* end() { return nullptr; }
+
+  // Iterator requirements
+  using iterator_category = std::input_iterator_tag;
+  using value_type = int;
+  using difference_type = std::ptrdiff_t;
+  using pointer = int*;
+  using reference = int&;
+
+  int& operator*() {
+    static int x;
+    return x;
+  }
+  AmbiguousType& operator++() { return *this; }
+  AmbiguousType operator++(int) { return *this; }
+  friend bool operator==(const AmbiguousType&, const AmbiguousType&) {
+    return true;
+  }
+  friend bool operator!=(const AmbiguousType&, const AmbiguousType&) {
+    return false;
+  }
+};
+
+template <typename Container, typename Output, typename = void>
+struct CanCopy : std::false_type {};
+template <typename Container, typename Output>
+struct CanCopy<Container, Output,
+               std::void_t<decltype(absl::c_copy(std::declval<Container>(),
+                                                 std::declval<Output>()))>>
+    : std::true_type {};
+
+template <typename Container, typename Output, typename = void>
+struct CanCopyN : std::false_type {};
+template <typename Container, typename Output>
+struct CanCopyN<Container, Output,
+                std::void_t<decltype(absl::c_copy_n(
+                    std::declval<Container>(), std::declval<ptrdiff_t>(),
+                    std::declval<Output>()))>> : std::true_type {};
+
+template <typename Container, typename Output, typename = void>
+struct CanMove : std::false_type {};
+template <typename Container, typename Output>
+struct CanMove<Container, Output,
+               std::void_t<decltype(absl::c_move(std::declval<Container>(),
+                                                 std::declval<Output>()))>>
+    : std::true_type {};
+
+template <typename Container, typename Output, typename UnaryOp,
+          typename = void>
+struct CanTransformUnary : std::false_type {};
+template <typename Container, typename Output, typename UnaryOp>
+struct CanTransformUnary<Container, Output, UnaryOp,
+                         absl::void_t<decltype(absl::c_transform(
+                             std::declval<Container>(), std::declval<Output>(),
+                             std::declval<UnaryOp>()))>> : std::true_type {};
+
+template <typename Container1, typename Container2, typename Output,
+          typename BinaryOp, typename = void>
+struct CanTransformBinary : std::false_type {};
+template <typename Container1, typename Container2, typename Output,
+          typename BinaryOp>
+struct CanTransformBinary<
+    Container1, Container2, Output, BinaryOp,
+    absl::void_t<decltype(absl::c_transform(
+        std::declval<Container1>(), std::declval<Container2>(),
+        std::declval<Output>(), std::declval<BinaryOp>()))>> : std::true_type {
+};
+
+TEST(CanCopyTest, CopyToMultiDimArray) {
+  static_assert(CanCopy<std::vector<int>, int (&)[10]>::value);
+  static_assert(!CanCopy<std::vector<int>, int (&)[2][2]>::value);
+  static_assert(CanCopyN<std::vector<int>, int (&)[10]>::value);
+  static_assert(!CanCopyN<std::vector<int>, int (&)[2][2]>::value);
+
+  static_assert(CanCopy<int[10], int (&)[10]>::value);
+  static_assert(!CanCopy<int[10], int (&)[2][2]>::value);
+  static_assert(CanCopyN<int[10], int (&)[10]>::value);
+  static_assert(!CanCopyN<int[10], int (&)[2][2]>::value);
+  static_assert(!CanCopy<int[2][2], int (&)[4]>::value);
+  static_assert(!CanCopy<int[2][2], int (&)[2][2]>::value);
+  static_assert(!CanCopyN<int[2][2], int (&)[4]>::value);
+  static_assert(!CanCopyN<int[2][2], int (&)[2][2]>::value);
+}
+
+TEST(CanCopyTest, BlockNonWritableIterators) {
+  using Vec = std::vector<int>;
+
+  static_assert(CanCopy<Vec, Vec::iterator>::value);
+  static_assert(CanCopy<Vec, std::back_insert_iterator<Vec>>::value);
+}
+
+TEST(CanCopyTest, AmbiguousTypeFailsToCompile) {
+  using Vec = std::vector<int>;
+  // Because AmbiguousType is both an iterator and a container,
+  // the compiler should fail to resolve the c_copy overload.
+  static_assert(!CanCopy<Vec, AmbiguousType&>::value,
+                "Ambiguous types should not compile!");
+  static_assert(!CanCopyN<Vec, AmbiguousType&>::value,
+                "Ambiguous types should not compile!");
+}
+
+TEST(CanCopyTest, CopyToRValue) {
+  using Vec = std::vector<int>;
+  using Span = absl::Span<int>;
+  static_assert(CanCopy<Vec, Span>::value,
+                "Should be able to copy to rvalue Span");
+  static_assert(!CanCopy<Vec, Vec>::value,
+                "Should not be able to copy to rvalue vector");
+  static_assert(CanCopyN<Vec, Span>::value,
+                "Should be able to copy_n to rvalue Span");
+  static_assert(!CanCopyN<Vec, Vec>::value,
+                "Should not be able to copy_n to rvalue vector");
+}
+
+TEST(CanMoveTest, MoveToMultiDimArray) {
+  static_assert(CanMove<std::vector<int>, int (&)[10]>::value);
+  static_assert(!CanMove<std::vector<int>, int (&)[2][2]>::value);
+
+  static_assert(CanMove<int[10], int (&)[10]>::value);
+  static_assert(!CanMove<int[10], int (&)[2][2]>::value);
+  static_assert(!CanMove<int[2][2], int (&)[4]>::value);
+  static_assert(!CanMove<int[2][2], int (&)[2][2]>::value);
+}
+
+TEST(CanMoveTest, AmbiguousTypeFailsToCompile) {
+  using Vec = std::vector<int>;
+  // Because AmbiguousType is both an iterator and a container,
+  // the compiler should fail to resolve the c_move overload.
+  static_assert(!CanMove<Vec, AmbiguousType&>::value,
+                "Ambiguous types should not compile!");
+}
+
+TEST(CanMoveTest, MoveToRValue) {
+  using Vec = std::vector<std::unique_ptr<int>>;
+  using Span = absl::Span<std::unique_ptr<int>>;
+  static_assert(CanMove<Vec, Span>::value,
+                "Should be able to move to rvalue Span");
+  static_assert(!CanMove<Vec, Vec>::value,
+                "Should not be able to move to rvalue vector");
+}
+
+TEST(CanTransformTest, TransformToMultiDimArray) {
+  using Negate = std::negate<int>;
+  using Plus = std::plus<int>;
+
+  static_assert(
+      CanTransformUnary<std::vector<int>, int (&)[10], Negate>::value);
+  static_assert(
+      !CanTransformUnary<std::vector<int>, int (&)[2][2], Negate>::value);
+
+  static_assert(CanTransformUnary<int[10], int (&)[10], Negate>::value);
+  static_assert(!CanTransformUnary<int[10], int (&)[2][2], Negate>::value);
+  static_assert(!CanTransformUnary<int[2][2], int (&)[4], Negate>::value);
+  static_assert(!CanTransformUnary<int[2][2], int (&)[2][2], Negate>::value);
+
+  static_assert(CanTransformBinary<std::vector<int>, std::vector<int>,
+                                   int (&)[10], Plus>::value);
+  static_assert(!CanTransformBinary<std::vector<int>, std::vector<int>,
+                                    int (&)[2][2], Plus>::value);
+
+  static_assert(CanTransformBinary<int[10], int[10], int (&)[10], Plus>::value);
+  static_assert(
+      !CanTransformBinary<int[10], int[10], int (&)[2][2], Plus>::value);
+  static_assert(
+      !CanTransformBinary<int[2][2], int[4], int (&)[4], Plus>::value);
+  static_assert(
+      !CanTransformBinary<int[2][2], int[2][2], int (&)[2][2], Plus>::value);
+}
+
+TEST(CanTransformTest, AmbiguousTypeFailsToCompile) {
+  using Vec = std::vector<int>;
+  using Negate = std::negate<int>;
+  using Plus = std::plus<int>;
+
+  // Because AmbiguousType is both an iterator and a container,
+  // the compiler should fail to resolve the c_transform overload.
+  static_assert(!CanTransformUnary<Vec, AmbiguousType&, Negate>::value,
+                "Ambiguous types should not compile!");
+  static_assert(!CanTransformBinary<Vec, Vec, AmbiguousType&, Plus>::value,
+                "Ambiguous types should not compile!");
+}
+
+template <typename C, typename T, typename = void>
+struct CanFill : std::false_type {};
+
+template <typename C, typename T>
+struct CanFill<C, T,
+               std::void_t<decltype(absl::c_fill(std::declval<C (*)()>()(),
+                                                 std::declval<T (*)()>()()))>>
+    : std::true_type {};
+
+TEST(CanFillTest, NonSpans) {
+  using T = int;
+
+  struct AbslSpanSubclass : absl::Span<T> {};
+  static_assert(!CanFill<std::vector<T>, T>::value,
+                "non-spans must not be allowed");
+  static_assert(!CanFill<AbslSpanSubclass, T>::value,
+                "subclasses must not be allowed");
+
+#ifdef __cpp_lib_span
+  struct StdSpanSubclass : std::span<T> {};
+  static_assert(!CanFill<StdSpanSubclass, T>::value,
+                "std::span must not be allowed");
+#endif
+}
 }  // namespace
diff --git a/absl/algorithm/equal_benchmark.cc b/absl/algorithm/equal_benchmark.cc
deleted file mode 100644
index 7bf62c9a7f5..00000000000
--- a/absl/algorithm/equal_benchmark.cc
+++ /dev/null
@@ -1,126 +0,0 @@
-// Copyright 2017 The Abseil Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-#include <cstdint>
-#include <cstring>
-
-#include "benchmark/benchmark.h"
-#include "absl/algorithm/algorithm.h"
-
-namespace {
-
-// The range of sequence sizes to benchmark.
-constexpr int kMinBenchmarkSize = 1024;
-constexpr int kMaxBenchmarkSize = 8 * 1024 * 1024;
-
-// A user-defined type for use in equality benchmarks. Note that we expect
-// std::memcmp to win for this type: libstdc++'s std::equal only defers to
-// memcmp for integral types. This is because it is not straightforward to
-// guarantee that std::memcmp would produce a result "as-if" compared by
-// operator== for other types (example gotchas: NaN floats, structs with
-// padding).
-struct EightBits {
-  explicit EightBits(int /* unused */) : data(0) {}
-  bool operator==(const EightBits& rhs) const { return data == rhs.data; }
-  uint8_t data;
-};
-
-template <typename T>
-void BM_absl_equal_benchmark(benchmark::State& state) {
-  std::vector<T> xs(state.range(0), T(0));
-  std::vector<T> ys = xs;
-  while (state.KeepRunning()) {
-    const bool same = absl::equal(xs.begin(), xs.end(), ys.begin(), ys.end());
-    benchmark::DoNotOptimize(same);
-  }
-}
-
-template <typename T>
-void BM_std_equal_benchmark(benchmark::State& state) {
-  std::vector<T> xs(state.range(0), T(0));
-  std::vector<T> ys = xs;
-  while (state.KeepRunning()) {
-    const bool same = std::equal(xs.begin(), xs.end(), ys.begin());
-    benchmark::DoNotOptimize(same);
-  }
-}
-
-template <typename T>
-void BM_memcmp_benchmark(benchmark::State& state) {
-  std::vector<T> xs(state.range(0), T(0));
-  std::vector<T> ys = xs;
-  while (state.KeepRunning()) {
-    const bool same =
-        std::memcmp(xs.data(), ys.data(), xs.size() * sizeof(T)) == 0;
-    benchmark::DoNotOptimize(same);
-  }
-}
-
-// The expectation is that the compiler should be able to elide the equality
-// comparison altogether for sufficiently simple types.
-template <typename T>
-void BM_absl_equal_self_benchmark(benchmark::State& state) {
-  std::vector<T> xs(state.range(0), T(0));
-  while (state.KeepRunning()) {
-    const bool same = absl::equal(xs.begin(), xs.end(), xs.begin(), xs.end());
-    benchmark::DoNotOptimize(same);
-  }
-}
-
-BENCHMARK_TEMPLATE(BM_absl_equal_benchmark, uint8_t)
-    ->Range(kMinBenchmarkSize, kMaxBenchmarkSize);
-BENCHMARK_TEMPLATE(BM_std_equal_benchmark, uint8_t)
-    ->Range(kMinBenchmarkSize, kMaxBenchmarkSize);
-BENCHMARK_TEMPLATE(BM_memcmp_benchmark, uint8_t)
-    ->Range(kMinBenchmarkSize, kMaxBenchmarkSize);
-BENCHMARK_TEMPLATE(BM_absl_equal_self_benchmark, uint8_t)
-    ->Range(kMinBenchmarkSize, kMaxBenchmarkSize);
-
-BENCHMARK_TEMPLATE(BM_absl_equal_benchmark, uint16_t)
-    ->Range(kMinBenchmarkSize, kMaxBenchmarkSize);
-BENCHMARK_TEMPLATE(BM_std_equal_benchmark, uint16_t)
-    ->Range(kMinBenchmarkSize, kMaxBenchmarkSize);
-BENCHMARK_TEMPLATE(BM_memcmp_benchmark, uint16_t)
-    ->Range(kMinBenchmarkSize, kMaxBenchmarkSize);
-BENCHMARK_TEMPLATE(BM_absl_equal_self_benchmark, uint16_t)
-    ->Range(kMinBenchmarkSize, kMaxBenchmarkSize);
-
-BENCHMARK_TEMPLATE(BM_absl_equal_benchmark, uint32_t)
-    ->Range(kMinBenchmarkSize, kMaxBenchmarkSize);
-BENCHMARK_TEMPLATE(BM_std_equal_benchmark, uint32_t)
-    ->Range(kMinBenchmarkSize, kMaxBenchmarkSize);
-BENCHMARK_TEMPLATE(BM_memcmp_benchmark, uint32_t)
-    ->Range(kMinBenchmarkSize, kMaxBenchmarkSize);
-BENCHMARK_TEMPLATE(BM_absl_equal_self_benchmark, uint32_t)
-    ->Range(kMinBenchmarkSize, kMaxBenchmarkSize);
-
-BENCHMARK_TEMPLATE(BM_absl_equal_benchmark, uint64_t)
-    ->Range(kMinBenchmarkSize, kMaxBenchmarkSize);
-BENCHMARK_TEMPLATE(BM_std_equal_benchmark, uint64_t)
-    ->Range(kMinBenchmarkSize, kMaxBenchmarkSize);
-BENCHMARK_TEMPLATE(BM_memcmp_benchmark, uint64_t)
-    ->Range(kMinBenchmarkSize, kMaxBenchmarkSize);
-BENCHMARK_TEMPLATE(BM_absl_equal_self_benchmark, uint64_t)
-    ->Range(kMinBenchmarkSize, kMaxBenchmarkSize);
-
-BENCHMARK_TEMPLATE(BM_absl_equal_benchmark, EightBits)
-    ->Range(kMinBenchmarkSize, kMaxBenchmarkSize);
-BENCHMARK_TEMPLATE(BM_std_equal_benchmark, EightBits)
-    ->Range(kMinBenchmarkSize, kMaxBenchmarkSize);
-BENCHMARK_TEMPLATE(BM_memcmp_benchmark, EightBits)
-    ->Range(kMinBenchmarkSize, kMaxBenchmarkSize);
-BENCHMARK_TEMPLATE(BM_absl_equal_self_benchmark, EightBits)
-    ->Range(kMinBenchmarkSize, kMaxBenchmarkSize);
-
-}  // namespace
diff --git a/absl/base/BUILD.bazel b/absl/base/BUILD.bazel
index 57caa0656e5..c0f276c16bd 100644
--- a/absl/base/BUILD.bazel
+++ b/absl/base/BUILD.bazel
@@ -14,7 +14,9 @@
 # limitations under the License.
 #
 
-load("@rules_cc//cc:defs.bzl", "cc_binary", "cc_library", "cc_test")
+load("@rules_cc//cc:cc_binary.bzl", "cc_binary")
+load("@rules_cc//cc:cc_library.bzl", "cc_library")
+load("@rules_cc//cc:cc_test.bzl", "cc_test")
 load(
     "//absl:copts/configure_copts.bzl",
     "ABSL_DEFAULT_COPTS",
@@ -22,9 +24,16 @@ load(
     "ABSL_TEST_COPTS",
 )
 
-package(default_visibility = ["//visibility:public"])
+package(
+    default_visibility = ["//visibility:private"],
+    features = [
+        "header_modules",
+        "layering_check",
+        "parse_headers",
+    ],
+)
 
-licenses(["notice"])  # Apache 2.0
+licenses(["notice"])
 
 cc_library(
     name = "atomic_hook",
@@ -34,6 +43,56 @@ cc_library(
     visibility = [
         "//absl:__subpackages__",
     ],
+    deps = [
+        ":config",
+        ":core_headers",
+    ],
+)
+
+cc_library(
+    name = "errno_saver",
+    hdrs = ["internal/errno_saver.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl:__subpackages__",
+    ],
+    deps = [":config"],
+)
+
+cc_library(
+    name = "cpu_detect",
+    srcs = [
+        "internal/cpu_detect.cc",
+    ],
+    hdrs = ["internal/cpu_detect.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl:__subpackages__",
+        "@do_not_use_for_gloop_visibility_only//gloop/util/sysinfo/topology:__subpackages__",
+    ],
+    deps = [
+        ":config",
+    ],
+)
+
+cc_library(
+    name = "hardening",
+    hdrs = [
+        "internal/hardening.h",
+    ],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl:__subpackages__",
+        "@do_not_use_for_gloop_visibility_only//gloop/util/coding:__subpackages__",
+        "@do_not_use_for_gloop_visibility_only//gloop/util/gtl:__subpackages__",
+    ],
+    deps = [
+        ":config",
+        ":core_headers",
+    ],
 )
 
 cc_library(
@@ -42,7 +101,46 @@ cc_library(
     hdrs = ["log_severity.h"],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
-    deps = [":core_headers"],
+    visibility = ["//visibility:public"],
+    deps = [
+        ":config",
+        ":core_headers",
+    ],
+)
+
+cc_library(
+    name = "no_destructor",
+    hdrs = ["no_destructor.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
+    deps = [
+        ":config",
+        ":nullability",
+    ],
+)
+
+cc_library(
+    name = "nullability",
+    hdrs = ["nullability.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
+    deps = [":config"],
+)
+
+cc_library(
+    name = "nullability_traits_internal",
+    hdrs = ["internal/nullability_traits.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl:__subpackages__",
+    ],
+    deps = [
+        ":config",
+        ":nullability",
+    ],
 )
 
 cc_library(
@@ -53,11 +151,19 @@ cc_library(
     linkopts = ABSL_DEFAULT_LINKOPTS,
     visibility = [
         "//absl:__subpackages__",
+        "@do_not_use_for_gloop_visibility_only//gloop/base:__subpackages__",
+        "@do_not_use_for_gloop_visibility_only//gloop/concurrent/percpu:__pkg__",
+        "@do_not_use_for_gloop_visibility_only//gloop/concurrent/rcu:__subpackages__",
+        "@do_not_use_for_gloop_visibility_only//gloop/strings:__pkg__",
+        "@do_not_use_for_gloop_visibility_only//gloop/thread/fiber:__subpackages__",
+        "@do_not_use_for_gloop_visibility_only//gloop/util/hash:__subpackages__",
+        "@do_not_use_for_gloop_visibility_only//gloop/util/process:__subpackages__",
     ],
     deps = [
         ":atomic_hook",
         ":config",
         ":core_headers",
+        ":errno_saver",
         ":log_severity",
     ],
 )
@@ -71,42 +177,71 @@ cc_library(
         "internal/spinlock_wait.cc",
         "internal/spinlock_win32.inc",
     ],
-    hdrs = [
-        "internal/scheduling_mode.h",
-        "internal/spinlock_wait.h",
-    ],
+    hdrs = ["internal/spinlock_wait.h"],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     visibility = [
         "//absl/base:__pkg__",
+        "@do_not_use_for_gloop_visibility_only//gloop/base:__pkg__",
+        "@do_not_use_for_gloop_visibility_only//gloop/thread/fiber:__pkg__",
+    ],
+    deps = [
+        ":base_internal",
+        ":core_headers",
+        ":errno_saver",
     ],
-    deps = [":core_headers"],
 )
 
 cc_library(
     name = "config",
     hdrs = [
         "config.h",
+        "options.h",
         "policy_checks.h",
     ],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
 )
 
 cc_library(
-    name = "dynamic_annotations",
-    srcs = ["dynamic_annotations.cc"],
-    hdrs = ["dynamic_annotations.h"],
+    name = "cycleclock_internal",
+    hdrs = [
+        "internal/cycleclock_config.h",
+        "internal/unscaledcycleclock_config.h",
+    ],
     copts = ABSL_DEFAULT_COPTS,
-    defines = ["__CLANG_SUPPORT_DYN_ANNOTATION__"],
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl:__subpackages__",
+        "@do_not_use_for_gloop_visibility_only//gloop/base:__subpackages__",
+        "@do_not_use_for_gloop_visibility_only//gloop/util/random:__pkg__",
+    ],
+    deps = [
+        ":base_internal",
+        ":config",
+    ],
 )
 
 cc_library(
-    name = "core_headers",
+    name = "dynamic_annotations",
     srcs = [
-        "internal/thread_annotations.h",
+        "internal/dynamic_annotations.h",
+    ],
+    hdrs = [
+        "dynamic_annotations.h",
+    ],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
+    deps = [
+        ":config",
+        ":core_headers",
     ],
+)
+
+cc_library(
+    name = "core_headers",
     hdrs = [
         "attributes.h",
         "const_init.h",
@@ -117,6 +252,7 @@ cc_library(
     ],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
     deps = [
         ":config",
     ],
@@ -131,21 +267,25 @@ cc_library(
         "internal/direct_mmap.h",
         "internal/low_level_alloc.h",
     ],
-    copts = ABSL_DEFAULT_COPTS,
+    copts = ABSL_DEFAULT_COPTS + select({
+        "//conditions:default": [],
+    }),
     linkopts = select({
-        "//absl:windows": [],
+        "@rules_cc//cc/compiler:msvc-cl": [],
+        "@rules_cc//cc/compiler:clang-cl": [],
+        "@rules_cc//cc/compiler:emscripten": [],
         "//conditions:default": ["-pthread"],
     }) + ABSL_DEFAULT_LINKOPTS,
     visibility = [
-        "//absl:__subpackages__",
+        "//visibility:public",
     ],
     deps = [
         ":base",
+        ":base_internal",
         ":config",
         ":core_headers",
         ":dynamic_annotations",
         ":raw_logging_internal",
-        ":spinlock_wait",
     ],
 )
 
@@ -153,17 +293,16 @@ cc_library(
     name = "base_internal",
     hdrs = [
         "internal/hide_ptr.h",
-        "internal/identity.h",
-        "internal/inline_variable.h",
-        "internal/invoke.h",
         "internal/scheduling_mode.h",
     ],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     visibility = [
         "//absl:__subpackages__",
+        "@do_not_use_for_gloop_visibility_only//gloop/base:__subpackages__",
     ],
     deps = [
+        ":config",
         "//absl/meta:type_traits",
     ],
 )
@@ -171,6 +310,7 @@ cc_library(
 cc_library(
     name = "base",
     srcs = [
+        "casts.cc",
         "internal/cycleclock.cc",
         "internal/spinlock.cc",
         "internal/sysinfo.cc",
@@ -191,22 +331,46 @@ cc_library(
     ],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = select({
-        "//absl:windows": [],
+        "@rules_cc//cc/compiler:msvc-cl": [
+            "-DEFAULTLIB:advapi32.lib",
+        ],
+        "@rules_cc//cc/compiler:clang-cl": [
+            "-DEFAULTLIB:advapi32.lib",
+        ],
+        "//absl:mingw_compiler": [
+            "-DEFAULTLIB:advapi32.lib",
+            "-ladvapi32",
+        ],
+        "@rules_cc//cc/compiler:emscripten": [],
         "//conditions:default": ["-pthread"],
     }) + ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
     deps = [
         ":atomic_hook",
         ":base_internal",
         ":config",
         ":core_headers",
-        ":dynamic_annotations",
-        ":log_severity",
+        ":cycleclock_internal",
+        ":nullability",
         ":raw_logging_internal",
         ":spinlock_wait",
         "//absl/meta:type_traits",
     ],
 )
 
+cc_library(
+    name = "atomic_hook_test_helper",
+    testonly = True,
+    srcs = ["internal/atomic_hook_test_helper.cc"],
+    hdrs = ["internal/atomic_hook_test_helper.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":atomic_hook",
+        ":core_headers",
+    ],
+)
+
 cc_test(
     name = "atomic_hook_test",
     size = "small",
@@ -215,8 +379,10 @@ cc_test(
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":atomic_hook",
+        ":atomic_hook_test_helper",
         ":core_headers",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -231,19 +397,49 @@ cc_test(
     deps = [
         ":base",
         ":core_headers",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "attributes_test",
+    srcs = [
+        "attributes_test.cc",
+    ],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":config",
+        ":core_headers",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "c_header_test",
+    srcs = ["c_header_test.c"],
+    tags = [
+        "no_test_wasm",
+    ],
+    deps = [
+        ":config",
+        ":core_headers",
     ],
 )
 
 cc_library(
     name = "throw_delegate",
-    srcs = ["internal/throw_delegate.cc"],
-    hdrs = ["internal/throw_delegate.h"],
+    srcs = [
+        "throw_delegate.cc",
+    ],
+    hdrs = [
+        "throw_delegate.h",
+    ],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
-    visibility = [
-        "//absl:__subpackages__",
-    ],
+    visibility = ["//visibility:public"],
     deps = [
         ":config",
         ":raw_logging_internal",
@@ -258,22 +454,38 @@ cc_test(
     deps = [
         ":config",
         ":throw_delegate",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "errno_saver_test",
+    size = "small",
+    srcs = ["internal/errno_saver_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":errno_saver",
+        ":strerror",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
 cc_library(
     name = "exception_testing",
-    testonly = 1,
+    testonly = True,
     hdrs = ["internal/exception_testing.h"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     visibility = [
         "//absl:__subpackages__",
+        "@do_not_use_for_gloop_visibility_only//gloop/util/gtl:__subpackages__",
     ],
     deps = [
         ":config",
-        "@com_google_googletest//:gtest",
+        "@googletest//:gtest",
     ],
 )
 
@@ -281,24 +493,28 @@ cc_library(
     name = "pretty_function",
     hdrs = ["internal/pretty_function.h"],
     linkopts = ABSL_DEFAULT_LINKOPTS,
-    visibility = ["//absl:__subpackages__"],
+    visibility = [
+        "//absl:__subpackages__",
+    ],
 )
 
 cc_library(
     name = "exception_safety_testing",
-    testonly = 1,
+    testonly = True,
     srcs = ["internal/exception_safety_testing.cc"],
     hdrs = ["internal/exception_safety_testing.h"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//absl:__subpackages__"],
     deps = [
         ":config",
         ":pretty_function",
         "//absl/memory",
         "//absl/meta:type_traits",
         "//absl/strings",
+        "//absl/strings:string_view",
         "//absl/utility",
-        "@com_google_googletest//:gtest",
+        "@googletest//:gtest",
     ],
 )
 
@@ -310,38 +526,8 @@ cc_test(
     deps = [
         ":exception_safety_testing",
         "//absl/memory",
-        "@com_google_googletest//:gtest_main",
-    ],
-)
-
-cc_test(
-    name = "inline_variable_test",
-    size = "small",
-    srcs = [
-        "inline_variable_test.cc",
-        "inline_variable_test_a.cc",
-        "inline_variable_test_b.cc",
-        "internal/inline_variable_testing.h",
-    ],
-    copts = ABSL_TEST_COPTS,
-    linkopts = ABSL_DEFAULT_LINKOPTS,
-    deps = [
-        ":base_internal",
-        "@com_google_googletest//:gtest_main",
-    ],
-)
-
-cc_test(
-    name = "invoke_test",
-    size = "small",
-    srcs = ["invoke_test.cc"],
-    copts = ABSL_TEST_COPTS,
-    linkopts = ABSL_DEFAULT_LINKOPTS,
-    deps = [
-        ":base_internal",
-        "//absl/memory",
-        "//absl/strings",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -349,16 +535,20 @@ cc_test(
 # AbslInternalSpinLockDelay and AbslInternalSpinLockWake.
 cc_library(
     name = "spinlock_test_common",
-    testonly = 1,
+    testonly = True,
     srcs = ["spinlock_test_common.cc"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "@do_not_use_for_gloop_visibility_only//gloop/base:__pkg__",
+    ],
     deps = [
         ":base",
+        ":base_internal",
+        ":config",
         ":core_headers",
-        ":spinlock_wait",
         "//absl/synchronization",
-        "@com_google_googletest//:gtest",
+        "@googletest//:gtest",
     ],
     alwayslink = 1,
 )
@@ -369,18 +559,23 @@ cc_test(
     srcs = ["spinlock_test_common.cc"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    tags = [
+        "no_test_wasm",
+    ],
     deps = [
         ":base",
+        ":base_internal",
+        ":config",
         ":core_headers",
-        ":spinlock_wait",
         "//absl/synchronization",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
 cc_library(
     name = "spinlock_benchmark_common",
-    testonly = 1,
+    testonly = True,
     srcs = ["internal/spinlock_benchmark.cc"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
@@ -390,18 +585,20 @@ cc_library(
     deps = [
         ":base",
         ":base_internal",
+        ":no_destructor",
         ":raw_logging_internal",
         "//absl/synchronization",
-        "@com_github_google_benchmark//:benchmark_main",
+        "@google_benchmark//:benchmark_main",
     ],
     alwayslink = 1,
 )
 
 cc_binary(
     name = "spinlock_benchmark",
-    testonly = 1,
+    testonly = True,
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    tags = ["benchmark"],
     visibility = ["//visibility:private"],
     deps = [
         ":spinlock_benchmark_common",
@@ -416,9 +613,14 @@ cc_library(
     ],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl:__subpackages__",
+    ],
     deps = [
+        ":base",
         ":config",
         ":core_headers",
+        ":nullability",
     ],
 )
 
@@ -429,7 +631,8 @@ cc_test(
     deps = [
         ":config",
         ":endian",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -441,7 +644,8 @@ cc_test(
     deps = [
         ":config",
         "//absl/synchronization:thread_pool",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -454,7 +658,102 @@ cc_test(
         ":base",
         ":core_headers",
         "//absl/synchronization",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "casts_test",
+    size = "small",
+    srcs = [
+        "casts_test.cc",
+    ],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":base",
+        ":config",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "hardening_test",
+    size = "small",
+    srcs = [
+        "internal/hardening_test.cc",
+    ],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":config",
+        ":core_headers",
+        ":hardening",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "no_destructor_test",
+    srcs = ["no_destructor_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":config",
+        ":no_destructor",
+        ":raw_logging_internal",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_binary(
+    name = "no_destructor_benchmark",
+    testonly = True,
+    srcs = ["no_destructor_benchmark.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    tags = ["benchmark"],
+    visibility = ["//visibility:private"],
+    deps = [
+        ":no_destructor",
+        ":raw_logging_internal",
+        "@google_benchmark//:benchmark_main",
+    ],
+)
+
+cc_test(
+    name = "nullability_test",
+    srcs = ["nullability_test.cc"],
+    deps = [
+        ":nullability",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "nullability_default_nonnull_test",
+    srcs = ["nullability_default_nonnull_test.cc"],
+    deps = [
+        ":nullability",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "nullability_traits_test",
+    srcs = ["internal/nullability_traits_test.cc"],
+    deps = [
+        ":config",
+        ":nullability",
+        ":nullability_traits_internal",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -466,7 +765,8 @@ cc_test(
     deps = [
         ":raw_logging_internal",
         "//absl/strings",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -479,7 +779,22 @@ cc_test(
     deps = [
         ":base",
         "//absl/synchronization",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "cpu_detect_test",
+    size = "small",
+    srcs = ["internal/cpu_detect_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":config",
+        ":cpu_detect",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -489,26 +804,36 @@ cc_test(
     srcs = ["internal/low_level_alloc_test.cc"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
-    tags = ["no_test_ios_x86_64"],
-    deps = [":malloc_internal"],
+    tags = [
+        "no_test_ios_x86_64",
+        "no_test_wasm",
+    ],
+    deps = [
+        ":malloc_internal",
+        "//absl/container:node_hash_map",
+    ],
 )
 
 cc_test(
     name = "thread_identity_test",
-    size = "small",
     srcs = ["internal/thread_identity_test.cc"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    tags = [
+        "no_test_wasm",
+    ],
     deps = [
         ":base",
         ":core_headers",
         "//absl/synchronization",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
-cc_test(
+cc_binary(
     name = "thread_identity_benchmark",
+    testonly = True,
     srcs = ["internal/thread_identity_benchmark.cc"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
@@ -517,66 +842,296 @@ cc_test(
     deps = [
         ":base",
         "//absl/synchronization",
-        "@com_github_google_benchmark//:benchmark_main",
+        "@google_benchmark//:benchmark_main",
     ],
 )
 
 cc_library(
-    name = "bits",
-    hdrs = ["internal/bits.h"],
+    name = "scoped_set_env",
+    testonly = True,
+    srcs = ["internal/scoped_set_env.cc"],
+    hdrs = ["internal/scoped_set_env.h"],
+    copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     visibility = [
         "//absl:__subpackages__",
     ],
-    deps = [":core_headers"],
+    deps = [
+        ":config",
+        ":raw_logging_internal",
+    ],
 )
 
 cc_test(
-    name = "bits_test",
+    name = "scoped_set_env_test",
     size = "small",
-    srcs = ["internal/bits_test.cc"],
+    srcs = ["internal/scoped_set_env_test.cc"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
-        ":bits",
-        "@com_google_googletest//:gtest_main",
+        ":scoped_set_env",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "log_severity_test",
+    size = "small",
+    srcs = ["log_severity_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":log_severity",
+        "//absl/flags:flag_internal",
+        "//absl/flags:marshalling",
+        "//absl/strings",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
 cc_library(
-    name = "scoped_set_env",
-    testonly = 1,
-    srcs = ["internal/scoped_set_env.cc"],
-    hdrs = ["internal/scoped_set_env.h"],
+    name = "strerror",
+    srcs = ["internal/strerror.cc"],
+    hdrs = ["internal/strerror.h"],
+    copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     visibility = [
         "//absl:__subpackages__",
+        "@do_not_use_for_gloop_visibility_only//gloop/base:__subpackages__",
+    ],
+    deps = [
+        ":config",
+        ":core_headers",
+        ":errno_saver",
     ],
-    deps = [":raw_logging_internal"],
 )
 
 cc_test(
-    name = "scoped_set_env_test",
+    name = "strerror_test",
     size = "small",
-    srcs = ["internal/scoped_set_env_test.cc"],
+    srcs = ["internal/strerror_test.cc"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
-        ":scoped_set_env",
-        "@com_google_googletest//:gtest_main",
+        ":strerror",
+        "//absl/strings",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_binary(
+    name = "strerror_benchmark",
+    testonly = True,
+    srcs = ["internal/strerror_benchmark.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    tags = ["benchmark"],
+    visibility = ["//visibility:private"],
+    deps = [
+        ":strerror",
+        "@google_benchmark//:benchmark_main",
+    ],
+)
+
+cc_library(
+    name = "fast_type_id",
+    hdrs = ["fast_type_id.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
+    deps = [
+        ":config",
     ],
 )
 
 cc_test(
-    name = "log_severity_test",
+    name = "fast_type_id_test",
     size = "small",
-    srcs = ["log_severity_test.cc"],
+    srcs = ["fast_type_id_test.cc"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
-        ":log_severity",
-        "//absl/flags:marshalling",
-        "//absl/strings",
-        "@com_google_googletest//:gtest_main",
+        ":core_headers",
+        ":fast_type_id",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_library(
+    name = "prefetch",
+    hdrs = [
+        "prefetch.h",
+    ],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
+    deps = [
+        ":config",
+        ":core_headers",
+    ],
+)
+
+cc_test(
+    name = "prefetch_test",
+    size = "small",
+    srcs = [
+        "prefetch_test.cc",
+    ],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":prefetch",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_library(
+    name = "poison",
+    srcs = [
+        "internal/poison.cc",
+    ],
+    hdrs = ["internal/poison.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl:__subpackages__",
+    ],
+    deps = [
+        ":config",
+        ":core_headers",
+        ":malloc_internal",
+    ],
+)
+
+cc_test(
+    name = "poison_test",
+    size = "small",
+    timeout = "short",
+    srcs = [
+        "internal/poison_test.cc",
+    ],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":config",
+        ":poison",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "unique_small_name_test",
+    size = "small",
+    srcs = ["internal/unique_small_name_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    linkstatic = 1,
+    deps = [
+        ":core_headers",
+        "//absl/strings:string_view",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "optimization_test",
+    size = "small",
+    srcs = ["optimization_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":core_headers",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_library(
+    name = "iterator_traits_internal",
+    hdrs = ["internal/iterator_traits.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl:__subpackages__",
+        "@do_not_use_for_gloop_visibility_only//gloop/util/gtl:__subpackages__",
+    ],
+    deps = [
+        ":config",
+        "//absl/meta:type_traits",
+    ],
+)
+
+cc_test(
+    name = "iterator_traits_test",
+    srcs = ["internal/iterator_traits_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":config",
+        ":iterator_traits_internal",
+        ":iterator_traits_test_helper",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_library(
+    name = "tracing_internal",
+    srcs = ["internal/tracing.cc"],
+    hdrs = ["internal/tracing.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl:__subpackages__",
+        "@do_not_use_for_gloop_visibility_only//gloop/perftools/tracing:__subpackages__",
+    ],
+    deps = [
+        ":config",
+        ":core_headers",
+    ],
+)
+
+cc_library(
+    name = "iterator_traits_test_helper",
+    hdrs = ["internal/iterator_traits_test_helper.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl:__subpackages__",
+        "//cloud/blockstore/metallica/util:__pkg__",
+    ],
+    deps = [":config"],
+)
+
+cc_test(
+    name = "tracing_internal_weak_test",
+    srcs = ["internal/tracing_weak_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":tracing_internal",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "tracing_internal_strong_test",
+    srcs = ["internal/tracing_strong_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":config",
+        ":core_headers",
+        ":tracing_internal",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
diff --git a/absl/base/CMakeLists.txt b/absl/base/CMakeLists.txt
index 2f11ef86556..20c09ef982d 100644
--- a/absl/base/CMakeLists.txt
+++ b/absl/base/CMakeLists.txt
@@ -14,11 +14,56 @@
 # limitations under the License.
 #
 
+find_library(LIBRT rt)
+
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     atomic_hook
   HDRS
     "internal/atomic_hook.h"
+  DEPS
+    absl::config
+    absl::core_headers
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+)
+
+# Internal-only target, do not depend on directly.
+absl_cc_library(
+  NAME
+    base_cpu_detect
+  HDRS
+    "internal/cpu_detect.h"
+  SRCS
+    "internal/cpu_detect.cc"
+  DEPS
+    absl::base
+    absl::config
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+)
+
+# Internal-only target, do not depend on directly.
+absl_cc_library(
+  NAME
+    errno_saver
+  HDRS
+    "internal/errno_saver.h"
+  DEPS
+    absl::config
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+)
+
+absl_cc_library(
+  NAME
+    hardening
+  HDRS
+    "internal/hardening.h"
+  DEPS
+    absl::config
+    absl::core_headers
   COPTS
     ${ABSL_DEFAULT_COPTS}
 )
@@ -31,11 +76,88 @@ absl_cc_library(
   SRCS
     "log_severity.cc"
   DEPS
+    absl::config
     absl::core_headers
   COPTS
     ${ABSL_DEFAULT_COPTS}
 )
 
+absl_cc_library(
+  NAME
+    no_destructor
+  HDRS
+    "no_destructor.h"
+  DEPS
+    absl::config
+    absl::nullability
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+)
+
+absl_cc_library(
+  NAME
+    nullability
+  HDRS
+    "nullability.h"
+  DEPS
+    absl::config
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+)
+
+absl_cc_test(
+  NAME
+    nullability_test
+  SRCS
+    "nullability_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::nullability
+    GTest::gtest_main
+)
+
+absl_cc_test(
+  NAME
+    nullability_default_nonnull_test
+  SRCS
+    "nullability_default_nonnull_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::nullability
+    GTest::gtest_main
+)
+
+# Internal-only target, do not depend on directly.
+absl_cc_library(
+  NAME
+    nullability_traits_internal
+  HDRS
+    "internal/nullability_traits.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::config
+    absl::nullability
+  PUBLIC
+)
+
+absl_cc_test(
+  NAME
+    nullability_traits_test
+  SRCS
+    "internal/nullability_traits_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::config
+    absl::nullability
+    absl::nullability_traits_internal
+    GTest::gtest_main
+)
+
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     raw_logging_internal
@@ -47,16 +169,17 @@ absl_cc_library(
     absl::atomic_hook
     absl::config
     absl::core_headers
+    absl::errno_saver
     absl::log_severity
   COPTS
     ${ABSL_DEFAULT_COPTS}
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     spinlock_wait
   HDRS
-    "internal/scheduling_mode.h"
     "internal/spinlock_wait.h"
   SRCS
     "internal/spinlock_akaros.inc"
@@ -67,7 +190,9 @@ absl_cc_library(
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
+    absl::base_internal
     absl::core_headers
+    absl::errno_saver
 )
 
 absl_cc_library(
@@ -75,6 +200,7 @@ absl_cc_library(
     config
   HDRS
     "config.h"
+    "options.h"
     "policy_checks.h"
   COPTS
     ${ABSL_DEFAULT_COPTS}
@@ -87,11 +213,11 @@ absl_cc_library(
   HDRS
     "dynamic_annotations.h"
   SRCS
-    "dynamic_annotations.cc"
+    "internal/dynamic_annotations.h"
   COPTS
     ${ABSL_DEFAULT_COPTS}
-  DEFINES
-    "__CLANG_SUPPORT_DYN_ANNOTATION__"
+  DEPS
+    absl::config
   PUBLIC
 )
 
@@ -105,7 +231,6 @@ absl_cc_library(
     "optimization.h"
     "port.h"
     "thread_annotations.h"
-    "internal/thread_annotations.h"
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
@@ -113,6 +238,7 @@ absl_cc_library(
   PUBLIC
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     malloc_internal
@@ -125,25 +251,25 @@ absl_cc_library(
     ${ABSL_DEFAULT_COPTS}
   DEPS
     absl::base
+    absl::base_internal
     absl::config
     absl::core_headers
     absl::dynamic_annotations
     absl::raw_logging_internal
-    absl::spinlock_wait
     Threads::Threads
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     base_internal
   HDRS
     "internal/hide_ptr.h"
-    "internal/identity.h"
-    "internal/inline_variable.h"
-    "internal/invoke.h"
+    "internal/scheduling_mode.h"
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
+    absl::config
     absl::type_traits
 )
 
@@ -154,6 +280,7 @@ absl_cc_library(
     "call_once.h"
     "casts.h"
     "internal/cycleclock.h"
+    "internal/cycleclock_config.h"
     "internal/low_level_scheduling.h"
     "internal/per_thread_tls.h"
     "internal/spinlock.h"
@@ -161,16 +288,20 @@ absl_cc_library(
     "internal/thread_identity.h"
     "internal/tsan_mutex_interface.h"
     "internal/unscaledcycleclock.h"
-    "log_severity.h"
+    "internal/unscaledcycleclock_config.h"
   SRCS
+    "casts.cc"
     "internal/cycleclock.cc"
     "internal/spinlock.cc"
     "internal/sysinfo.cc"
     "internal/thread_identity.cc"
     "internal/unscaledcycleclock.cc"
-    "log_severity.cc"
   COPTS
     ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+    $<$<BOOL:${LIBRT}>:-lrt>
+    $<$<BOOL:${MINGW}>:-ladvapi32>
   DEPS
     absl::atomic_hook
     absl::base_internal
@@ -178,6 +309,7 @@ absl_cc_library(
     absl::core_headers
     absl::dynamic_annotations
     absl::log_severity
+    absl::nullability
     absl::raw_logging_internal
     absl::spinlock_wait
     absl::type_traits
@@ -189,9 +321,9 @@ absl_cc_library(
   NAME
     throw_delegate
   HDRS
-    "internal/throw_delegate.h"
+    "throw_delegate.h"
   SRCS
-    "internal/throw_delegate.cc"
+    "throw_delegate.cc"
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
@@ -199,6 +331,7 @@ absl_cc_library(
     absl::raw_logging_internal
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     exception_testing
@@ -208,10 +341,11 @@ absl_cc_library(
     ${ABSL_DEFAULT_COPTS}
   DEPS
     absl::config
-    gtest
+    GTest::gtest
   TESTONLY
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     pretty_function
@@ -221,6 +355,7 @@ absl_cc_library(
     ${ABSL_DEFAULT_COPTS}
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     exception_safety_testing
@@ -235,9 +370,10 @@ absl_cc_library(
     absl::pretty_function
     absl::memory
     absl::meta
+    absl::string_view
     absl::strings
     absl::utility
-    gtest
+    GTest::gtest
   TESTONLY
 )
 
@@ -251,7 +387,21 @@ absl_cc_test(
   DEPS
     absl::exception_safety_testing
     absl::memory
-    gtest_main
+    GTest::gtest_main
+)
+
+# Internal-only target, do not depend on directly.
+absl_cc_library(
+  NAME
+    atomic_hook_test_helper
+  SRCS
+    "internal/atomic_hook_test_helper.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::atomic_hook
+    absl::core_headers
+  TESTONLY
 )
 
 absl_cc_test(
@@ -262,9 +412,38 @@ absl_cc_test(
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
+    absl::atomic_hook_test_helper
     absl::atomic_hook
     absl::core_headers
-    gtest_main
+    GTest::gmock
+    GTest::gtest_main
+)
+
+absl_cc_test(
+  NAME
+    attributes_test
+  SRCS
+    "attributes_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::config
+    absl::core_headers
+    GTest::gtest_main
+)
+
+absl_cc_test(
+  NAME
+    hardening_test
+  SRCS
+    "internal/hardening_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::config
+    absl::core_headers
+    absl::hardening
+    GTest::gtest_main
 )
 
 absl_cc_test(
@@ -277,53 +456,51 @@ absl_cc_test(
   DEPS
     absl::base
     absl::core_headers
-    gtest_main
+    GTest::gtest_main
 )
 
 absl_cc_test(
   NAME
-    throw_delegate_test
+    casts_test
   SRCS
-    "throw_delegate_test.cc"
+    "casts_test.cc"
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
     absl::base
-    absl::config
-    absl::throw_delegate
-    gtest_main
+    absl::core_headers
+    GTest::gtest_main
 )
 
 absl_cc_test(
   NAME
-    inline_variable_test
+    errno_saver_test
   SRCS
-    "internal/inline_variable_testing.h"
-    "inline_variable_test.cc"
-    "inline_variable_test_a.cc"
-    "inline_variable_test_b.cc"
+    "internal/errno_saver_test.cc"
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
-    absl::base_internal
-    gtest_main
+    absl::errno_saver
+    absl::strerror
+    GTest::gmock
+    GTest::gtest_main
 )
 
 absl_cc_test(
   NAME
-    invoke_test
+    throw_delegate_test
   SRCS
-    "invoke_test.cc"
+    "throw_delegate_test.cc"
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
-    absl::base_internal
-    absl::memory
-    absl::strings
-    gmock
-    gtest_main
+    absl::base
+    absl::config
+    absl::throw_delegate
+    GTest::gtest_main
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     spinlock_test_common
@@ -333,10 +510,11 @@ absl_cc_library(
     ${ABSL_TEST_COPTS}
   DEPS
     absl::base
+    absl::config
+    absl::base_internal
     absl::core_headers
-    absl::spinlock_wait
     absl::synchronization
-    gtest
+    GTest::gtest
   TESTONLY
 )
 
@@ -350,12 +528,14 @@ absl_cc_test(
     ${ABSL_TEST_COPTS}
   DEPS
     absl::base
+    absl::base_internal
+    absl::config
     absl::core_headers
-    absl::spinlock_wait
     absl::synchronization
-    gtest_main
+    GTest::gtest_main
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     endian
@@ -365,8 +545,10 @@ absl_cc_library(
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
+    absl::base
     absl::config
     absl::core_headers
+    absl::nullability
   PUBLIC
 )
 
@@ -381,7 +563,7 @@ absl_cc_test(
     absl::base
     absl::config
     absl::endian
-    gtest_main
+    GTest::gtest_main
 )
 
 absl_cc_test(
@@ -394,7 +576,7 @@ absl_cc_test(
   DEPS
     absl::config
     absl::synchronization
-    gtest_main
+    GTest::gtest_main
 )
 
 absl_cc_test(
@@ -408,7 +590,22 @@ absl_cc_test(
     absl::base
     absl::core_headers
     absl::synchronization
-    gtest_main
+    GTest::gtest_main
+)
+
+absl_cc_test(
+  NAME
+    no_destructor_test
+  SRCS
+    "no_destructor_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::no_destructor
+    absl::config
+    absl::raw_logging_internal
+    GTest::gmock
+    GTest::gtest_main
 )
 
 absl_cc_test(
@@ -421,7 +618,7 @@ absl_cc_test(
   DEPS
     absl::raw_logging_internal
     absl::strings
-    gtest_main
+    GTest::gtest_main
 )
 
 absl_cc_test(
@@ -434,7 +631,20 @@ absl_cc_test(
   DEPS
     absl::base
     absl::synchronization
-    gtest_main
+    GTest::gtest_main
+)
+
+absl_cc_test(
+  NAME
+    cpu_detect_test
+  SRCS
+    "internal/cpu_detect_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::base_cpu_detect
+    absl::config
+    GTest::gtest_main
 )
 
 absl_cc_test(
@@ -446,6 +656,7 @@ absl_cc_test(
     ${ABSL_TEST_COPTS}
   DEPS
     absl::malloc_internal
+    absl::node_hash_map
     Threads::Threads
 )
 
@@ -461,77 +672,263 @@ absl_cc_test(
     absl::core_headers
     absl::synchronization
     Threads::Threads
-    gtest_main
+    GTest::gtest_main
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
-    bits
+    scoped_set_env
+  SRCS
+    "internal/scoped_set_env.cc"
   HDRS
-    "internal/bits.h"
+    "internal/scoped_set_env.h"
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
+    absl::config
+    absl::raw_logging_internal
+)
+
+absl_cc_test(
+  NAME
+    scoped_set_env_test
+  SRCS
+    "internal/scoped_set_env_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::scoped_set_env
+    GTest::gtest_main
+)
+
+absl_cc_test(
+  NAME
+    cmake_thread_test
+  SRCS
+    "internal/cmake_thread_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::base
+)
+
+absl_cc_test(
+  NAME
+    log_severity_test
+  SRCS
+    "log_severity_test.cc"
+  DEPS
+    absl::flags_internal
+    absl::flags_marshalling
+    absl::log_severity
+    absl::strings
+    GTest::gmock
+    GTest::gtest_main
+)
+
+# Internal-only target, do not depend on directly.
+absl_cc_library(
+  NAME
+    strerror
+  SRCS
+    "internal/strerror.cc"
+  HDRS
+    "internal/strerror.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::config
     absl::core_headers
+    absl::errno_saver
 )
 
 absl_cc_test(
   NAME
-    bits_test
+    strerror_test
   SRCS
-    "internal/bits_test.cc"
+    "internal/strerror_test.cc"
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
-    absl::bits
-    gtest_main
+    absl::strerror
+    absl::strings
+    GTest::gmock
+    GTest::gtest_main
 )
 
 absl_cc_library(
   NAME
-    scoped_set_env
+    fast_type_id
+  HDRS
+    "fast_type_id.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::config
+)
+
+absl_cc_test(
+  NAME
+    fast_type_id_test
   SRCS
-    "internal/scoped_set_env.cc"
+    "fast_type_id_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::core_headers
+    absl::fast_type_id
+    GTest::gtest_main
+)
+
+absl_cc_library(
+  NAME
+    prefetch
   HDRS
-    "internal/scoped_set_env.h"
+    "prefetch.h"
   COPTS
     ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
   DEPS
-    absl::raw_logging_internal
+    absl::config
+    absl::core_headers
 )
 
 absl_cc_test(
   NAME
-    scoped_set_env_test
+    prefetch_test
   SRCS
-    "internal/scoped_set_env_test.cc"
+    "prefetch_test.cc"
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
-    absl::scoped_set_env
-    gtest_main
+    absl::prefetch
+    GTest::gtest_main
 )
 
 absl_cc_test(
   NAME
-    cmake_thread_test
+    optimization_test
   SRCS
-    "internal/cmake_thread_test.cc"
+    "optimization_test.cc"
   COPTS
     ${ABSL_TEST_COPTS}
+  DEPS
+    absl::core_headers
+    GTest::gtest_main
+)
+
+absl_cc_library(
+  NAME
+    poison
+  SRCS
+    "internal/poison.cc"
+  HDRS
+    "internal/poison.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::config
+    absl::core_headers
+    absl::malloc_internal
+)
+
+absl_cc_test(
+  NAME
+    poison_test
+  SRCS
+    "internal/poison_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::config
+    absl::poison
+    GTest::gtest_main
+)
+
+absl_cc_library(
+  NAME
+    tracing_internal
+  HDRS
+    "internal/tracing.h"
+  SRCS
+    "internal/tracing.cc"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
   DEPS
     absl::base
 )
 
 absl_cc_test(
   NAME
-    log_severity_test
+    tracing_internal_weak_test
   SRCS
-    "log_severity_test.cc"
+    "internal/tracing_weak_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
   DEPS
-    absl::flags_marshalling
-    absl::log_severity
-    absl::strings
-    gmock
-    gtest_main
+    absl::base
+    absl::tracing_internal
+    GTest::gtest_main
+)
+
+absl_cc_test(
+  NAME
+    tracing_internal_strong_test
+  SRCS
+    "internal/tracing_strong_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::base
+    absl::tracing_internal
+    GTest::gtest_main
+)
+
+# Internal-only target, do not depend on directly.
+absl_cc_library(
+  NAME
+    iterator_traits_internal
+  HDRS
+    "internal/iterator_traits.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::config
+    absl::type_traits
+  PUBLIC
+)
+
+absl_cc_test(
+  NAME
+    iterator_traits_test
+  SRCS
+    "internal/iterator_traits_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::config
+    absl::iterator_traits_internal
+    absl::iterator_traits_test_helper_internal
+    GTest::gtest_main
+)
+
+# Internal-only target, do not depend on directly.
+absl_cc_library(
+  NAME
+    iterator_traits_test_helper_internal
+  HDRS
+    "internal/iterator_traits_test_helper.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::config
+  PUBLIC
 )
diff --git a/absl/base/attributes.h b/absl/base/attributes.h
index f298297e994..53e440e68fe 100644
--- a/absl/base/attributes.h
+++ b/absl/base/attributes.h
@@ -18,8 +18,6 @@
 // These macros are used within Abseil and allow the compiler to optimize, where
 // applicable, certain function calls.
 //
-// This file is used for both C and C++!
-//
 // Most macros here are exposing GCC or Clang features, and are stubbed out for
 // other compilers.
 //
@@ -32,34 +30,14 @@
 // of them are not supported in older version of Clang. Thus, we check
 // `__has_attribute()` first. If the check fails, we check if we are on GCC and
 // assume the attribute exists on GCC (which is verified on GCC 4.7).
-//
-// -----------------------------------------------------------------------------
-// Sanitizer Attributes
-// -----------------------------------------------------------------------------
-//
-// Sanitizer-related attributes are not "defined" in this file (and indeed
-// are not defined as such in any file). To utilize the following
-// sanitizer-related attributes within your builds, define the following macros
-// within your build using a `-D` flag, along with the given value for
-// `-fsanitize`:
-//
-//   * `ADDRESS_SANITIZER` + `-fsanitize=address` (Clang, GCC 4.8)
-//   * `MEMORY_SANITIZER` + `-fsanitize=memory` (Clang-only)
-//   * `THREAD_SANITIZER + `-fsanitize=thread` (Clang, GCC 4.8+)
-//   * `UNDEFINED_BEHAVIOR_SANITIZER` + `-fsanitize=undefined` (Clang, GCC 4.9+)
-//   * `CONTROL_FLOW_INTEGRITY` + -fsanitize=cfi (Clang-only)
-//
-// Example:
-//
-//   // Enable branches in the Abseil code that are tagged for ASan:
-//   $ bazel build --copt=-DADDRESS_SANITIZER --copt=-fsanitize=address
-//     --linkopt=-fsanitize=address *target*
-//
-// Since these macro names are only supported by GCC and Clang, we only check
-// for `__GNUC__` (GCC or Clang) and the above macros.
+
+// SKIP_ABSL_INLINE_NAMESPACE_CHECK
+
 #ifndef ABSL_BASE_ATTRIBUTES_H_
 #define ABSL_BASE_ATTRIBUTES_H_
 
+#include "absl/base/config.h"
+
 // ABSL_HAVE_ATTRIBUTE
 //
 // A function-like feature checking macro that is a wrapper around
@@ -143,7 +121,7 @@
 #if ABSL_HAVE_ATTRIBUTE(disable_tail_calls)
 #define ABSL_HAVE_ATTRIBUTE_NO_TAIL_CALL 1
 #define ABSL_ATTRIBUTE_NO_TAIL_CALL __attribute__((disable_tail_calls))
-#elif defined(__GNUC__) && !defined(__clang__)
+#elif defined(__GNUC__) && !defined(__clang__) && !defined(__e2k__)
 #define ABSL_HAVE_ATTRIBUTE_NO_TAIL_CALL 1
 #define ABSL_ATTRIBUTE_NO_TAIL_CALL \
   __attribute__((optimize("no-optimize-sibling-calls")))
@@ -155,12 +133,17 @@
 // ABSL_ATTRIBUTE_WEAK
 //
 // Tags a function as weak for the purposes of compilation and linking.
-// Weak attributes currently do not work properly in LLVM's Windows backend,
-// so disable them there. See https://bugs.llvm.org/show_bug.cgi?id=37598
-// for further information.
-#if (ABSL_HAVE_ATTRIBUTE(weak) || \
-     (defined(__GNUC__) && !defined(__clang__))) && \
-    !(defined(__llvm__) && defined(_WIN32))
+// Weak attributes did not work properly in LLVM's Windows backend before
+// 9.0.0, so disable them there. See https://bugs.llvm.org/show_bug.cgi?id=37598
+// for further information. Weak attributes do not work across DLL boundary.
+// The MinGW compiler doesn't complain about the weak attribute until the link
+// step, presumably because Windows doesn't use ELF binaries.
+#if (ABSL_HAVE_ATTRIBUTE(weak) ||                                 \
+     (defined(__GNUC__) && !defined(__clang__))) &&               \
+    (!defined(_WIN32) ||                                          \
+     (defined(__clang__) && __clang_major__ >= 9 &&               \
+      !defined(ABSL_BUILD_DLL) && !defined(ABSL_CONSUME_DLL))) && \
+    !defined(__MINGW32__)
 #undef ABSL_ATTRIBUTE_WEAK
 #define ABSL_ATTRIBUTE_WEAK __attribute__((weak))
 #define ABSL_HAVE_ATTRIBUTE_WEAK 1
@@ -216,6 +199,9 @@
 // ABSL_ATTRIBUTE_NORETURN
 //
 // Tells the compiler that a given function never returns.
+//
+// Deprecated: Prefer the `[[noreturn]]` attribute standardized by C++11 over
+// this macro.
 #if ABSL_HAVE_ATTRIBUTE(noreturn) || (defined(__GNUC__) && !defined(__clang__))
 #define ABSL_ATTRIBUTE_NORETURN __attribute__((noreturn))
 #elif defined(_MSC_VER)
@@ -232,21 +218,33 @@
 // out of bounds or does other scary things with memory.
 // NOTE: GCC supports AddressSanitizer(asan) since 4.8.
 // https://gcc.gnu.org/gcc-4.8/changes.html
-#if defined(__GNUC__)
+#if defined(ABSL_HAVE_ADDRESS_SANITIZER) && \
+    ABSL_HAVE_ATTRIBUTE(no_sanitize_address)
 #define ABSL_ATTRIBUTE_NO_SANITIZE_ADDRESS __attribute__((no_sanitize_address))
+#elif defined(ABSL_HAVE_ADDRESS_SANITIZER) && defined(_MSC_VER) && \
+    _MSC_VER >= 1928
+// https://docs.microsoft.com/en-us/cpp/cpp/no-sanitize-address
+#define ABSL_ATTRIBUTE_NO_SANITIZE_ADDRESS __declspec(no_sanitize_address)
+#elif defined(ABSL_HAVE_HWADDRESS_SANITIZER) && ABSL_HAVE_ATTRIBUTE(no_sanitize)
+// HWAddressSanitizer is a sanitizer similar to AddressSanitizer, which uses CPU
+// features to detect similar bugs with less CPU and memory overhead.
+// NOTE: GCC supports HWAddressSanitizer(hwasan) since 11.
+// https://gcc.gnu.org/gcc-11/changes.html
+#define ABSL_ATTRIBUTE_NO_SANITIZE_ADDRESS \
+  __attribute__((no_sanitize("hwaddress")))
 #else
 #define ABSL_ATTRIBUTE_NO_SANITIZE_ADDRESS
 #endif
 
 // ABSL_ATTRIBUTE_NO_SANITIZE_MEMORY
 //
-// Tells the  MemorySanitizer to relax the handling of a given function. All
-// "Use of uninitialized value" warnings from such functions will be suppressed,
-// and all values loaded from memory will be considered fully initialized.
-// This attribute is similar to the ADDRESS_SANITIZER attribute above, but deals
-// with initialized-ness rather than addressability issues.
+// Tells the MemorySanitizer to relax the handling of a given function. All "Use
+// of uninitialized value" warnings from such functions will be suppressed, and
+// all values loaded from memory will be considered fully initialized.  This
+// attribute is similar to the ABSL_ATTRIBUTE_NO_SANITIZE_ADDRESS attribute
+// above, but deals with initialized-ness rather than addressability issues.
 // NOTE: MemorySanitizer(msan) is supported by Clang but not GCC.
-#if defined(__clang__)
+#if ABSL_HAVE_ATTRIBUTE(no_sanitize_memory)
 #define ABSL_ATTRIBUTE_NO_SANITIZE_MEMORY __attribute__((no_sanitize_memory))
 #else
 #define ABSL_ATTRIBUTE_NO_SANITIZE_MEMORY
@@ -257,7 +255,7 @@
 // Tells the ThreadSanitizer to not instrument a given function.
 // NOTE: GCC supports ThreadSanitizer(tsan) since 4.8.
 // https://gcc.gnu.org/gcc-4.8/changes.html
-#if defined(__GNUC__)
+#if ABSL_HAVE_ATTRIBUTE(no_sanitize_thread)
 #define ABSL_ATTRIBUTE_NO_SANITIZE_THREAD __attribute__((no_sanitize_thread))
 #else
 #define ABSL_ATTRIBUTE_NO_SANITIZE_THREAD
@@ -269,8 +267,10 @@
 // where certain behavior (eg. division by zero) is being used intentionally.
 // NOTE: GCC supports UndefinedBehaviorSanitizer(ubsan) since 4.9.
 // https://gcc.gnu.org/gcc-4.9/changes.html
-#if defined(__GNUC__) && \
-    (defined(UNDEFINED_BEHAVIOR_SANITIZER) || defined(ADDRESS_SANITIZER))
+#if ABSL_HAVE_ATTRIBUTE(no_sanitize_undefined)
+#define ABSL_ATTRIBUTE_NO_SANITIZE_UNDEFINED \
+  __attribute__((no_sanitize_undefined))
+#elif ABSL_HAVE_ATTRIBUTE(no_sanitize)
 #define ABSL_ATTRIBUTE_NO_SANITIZE_UNDEFINED \
   __attribute__((no_sanitize("undefined")))
 #else
@@ -281,7 +281,7 @@
 //
 // Tells the ControlFlowIntegrity sanitizer to not instrument a given function.
 // See https://clang.llvm.org/docs/ControlFlowIntegrity.html for details.
-#if defined(__GNUC__) && defined(CONTROL_FLOW_INTEGRITY)
+#if ABSL_HAVE_ATTRIBUTE(no_sanitize) && defined(__llvm__)
 #define ABSL_ATTRIBUTE_NO_SANITIZE_CFI __attribute__((no_sanitize("cfi")))
 #else
 #define ABSL_ATTRIBUTE_NO_SANITIZE_CFI
@@ -291,7 +291,7 @@
 //
 // Tells the SafeStack to not instrument a given function.
 // See https://clang.llvm.org/docs/SafeStack.html for details.
-#if defined(__GNUC__) && defined(SAFESTACK_SANITIZER)
+#if ABSL_HAVE_ATTRIBUTE(no_sanitize)
 #define ABSL_ATTRIBUTE_NO_SANITIZE_SAFESTACK \
   __attribute__((no_sanitize("safe-stack")))
 #else
@@ -301,10 +301,7 @@
 // ABSL_ATTRIBUTE_RETURNS_NONNULL
 //
 // Tells the compiler that a particular function never returns a null pointer.
-#if ABSL_HAVE_ATTRIBUTE(returns_nonnull) || \
-    (defined(__GNUC__) && \
-     (__GNUC__ > 5 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 9)) && \
-     !defined(__clang__))
+#if ABSL_HAVE_ATTRIBUTE(returns_nonnull)
 #define ABSL_ATTRIBUTE_RETURNS_NONNULL __attribute__((returns_nonnull))
 #else
 #define ABSL_ATTRIBUTE_RETURNS_NONNULL
@@ -334,15 +331,22 @@
   __attribute__((section(#name))) __attribute__((noinline))
 #endif
 
-
 // ABSL_ATTRIBUTE_SECTION_VARIABLE
 //
 // Tells the compiler/linker to put a given variable into a section and define
 // `__start_ ## name` and `__stop_ ## name` symbols to bracket the section.
 // This functionality is supported by GNU linker.
 #ifndef ABSL_ATTRIBUTE_SECTION_VARIABLE
+#ifdef _AIX
+// __attribute__((section(#name))) on AIX is achieved by using the `.csect`
+// pseudo op which includes an additional integer as part of its syntax
+// indicating alignment. If data fall under different alignments then you might
+// get a compilation error indicating a `Section type conflict`.
+#define ABSL_ATTRIBUTE_SECTION_VARIABLE(name)
+#else
 #define ABSL_ATTRIBUTE_SECTION_VARIABLE(name) __attribute__((section(#name)))
 #endif
+#endif
 
 // ABSL_DECLARE_ATTRIBUTE_SECTION_VARS
 //
@@ -353,8 +357,8 @@
 // a no-op on ELF but not on Mach-O.
 //
 #ifndef ABSL_DECLARE_ATTRIBUTE_SECTION_VARS
-#define ABSL_DECLARE_ATTRIBUTE_SECTION_VARS(name) \
-  extern char __start_##name[] ABSL_ATTRIBUTE_WEAK;    \
+#define ABSL_DECLARE_ATTRIBUTE_SECTION_VARS(name)   \
+  extern char __start_##name[] ABSL_ATTRIBUTE_WEAK; \
   extern char __stop_##name[] ABSL_ATTRIBUTE_WEAK
 #endif
 #ifndef ABSL_DEFINE_ATTRIBUTE_SECTION_VARS
@@ -415,6 +419,9 @@
 //
 // Tells the compiler to warn about unused results.
 //
+// For code or headers that are assured to only build with C++17 and up, prefer
+// just using the standard `[[nodiscard]]` directly over this macro.
+//
 // When annotating a function, it must appear as the first part of the
 // declaration or definition. The compiler will warn if the return value from
 // such a function is unused:
@@ -441,9 +448,10 @@
 // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66425
 //
 // Note: past advice was to place the macro after the argument list.
-#if ABSL_HAVE_ATTRIBUTE(nodiscard)
-#define ABSL_MUST_USE_RESULT [[nodiscard]]
-#elif defined(__clang__) && ABSL_HAVE_ATTRIBUTE(warn_unused_result)
+//
+// TODO(b/176172494): Use ABSL_HAVE_CPP_ATTRIBUTE(nodiscard) when all code is
+// compliant with the stricter [[nodiscard]].
+#if defined(__clang__) && ABSL_HAVE_ATTRIBUTE(warn_unused_result)
 #define ABSL_MUST_USE_RESULT __attribute__((warn_unused_result))
 #else
 #define ABSL_MUST_USE_RESULT
@@ -486,8 +494,8 @@
 //
 // These attributes only take effect when the following conditions are met:
 //
-//   * The file/target is built in at least C++11 mode, with a Clang compiler
-//     that supports XRay attributes.
+//   * The file/target is built with a Clang compiler that supports XRay
+//     attributes.
 //   * The file/target is built with the -fxray-instrument flag set for the
 //     Clang/LLVM compiler.
 //   * The function is defined in the translation unit (the compiler honors the
@@ -505,13 +513,15 @@
 // packages/targets, as this may lead to conflicting definitions of functions at
 // link-time.
 //
+// XRay isn't currently supported on Android:
+// https://github.com/android/ndk/issues/368
 #if ABSL_HAVE_CPP_ATTRIBUTE(clang::xray_always_instrument) && \
-    !defined(ABSL_NO_XRAY_ATTRIBUTES)
+    !defined(ABSL_NO_XRAY_ATTRIBUTES) && !defined(__ANDROID__)
 #define ABSL_XRAY_ALWAYS_INSTRUMENT [[clang::xray_always_instrument]]
 #define ABSL_XRAY_NEVER_INSTRUMENT [[clang::xray_never_instrument]]
 #if ABSL_HAVE_CPP_ATTRIBUTE(clang::xray_log_args)
 #define ABSL_XRAY_LOG_ARGS(N) \
-    [[clang::xray_always_instrument, clang::xray_log_args(N)]]
+  [[clang::xray_always_instrument, clang::xray_log_args(N)]]
 #else
 #define ABSL_XRAY_LOG_ARGS(N) [[clang::xray_always_instrument]]
 #endif
@@ -521,6 +531,25 @@
 #define ABSL_XRAY_LOG_ARGS(N)
 #endif
 
+// ABSL_ATTRIBUTE_NULL_AFTER_MOVE
+//
+// Indicates that a user-defined smart-pointer-like type makes guarantees on the
+// state of a moved-from object, leaving it in a null state, where it can be
+// used as long as it is not dereferenced. In other words, these are the same
+// semantics that smart pointers from the standard library provide.
+//
+// The clang-tidy check bugprone-use-after-move allows member functions of types
+// marked with this attribute to be called on objects that have been moved from;
+// without the attribute, this would result in a use-after-move warning.
+#if ABSL_HAVE_CPP_ATTRIBUTE(clang::annotate) && defined(__clang__) && \
+    __clang_major__ >= 12
+#define ABSL_ATTRIBUTE_NULL_AFTER_MOVE                       \
+  [[clang::annotate("clang-tidy", "bugprone-use-after-move", \
+                    "null_after_move")]]
+#else
+#define ABSL_ATTRIBUTE_NULL_AFTER_MOVE
+#endif
+
 // ABSL_ATTRIBUTE_REINITIALIZES
 //
 // Indicates that a member function reinitializes the entire object to a known
@@ -542,6 +571,12 @@
 // ABSL_ATTRIBUTE_UNUSED
 //
 // Prevents the compiler from complaining about variables that appear unused.
+//
+// Deprecated: Use the standard C++17 `[[maybe_unused]]` instead.
+//
+// Due to differences in positioning requirements between the old, compiler
+// specific __attribute__ syntax and the now standard `[[maybe_unused]]`, this
+// macro does not attempt to take advantage of `[[maybe_unused]]`.
 #if ABSL_HAVE_ATTRIBUTE(unused) || (defined(__GNUC__) && !defined(__clang__))
 #undef ABSL_ATTRIBUTE_UNUSED
 #define ABSL_ATTRIBUTE_UNUSED __attribute__((__unused__))
@@ -561,7 +596,29 @@
 
 // ABSL_ATTRIBUTE_PACKED
 //
-// Prevents the compiler from padding a structure to natural alignment
+// Instructs the compiler not to use natural alignment for a tagged data
+// structure, but instead to reduce its alignment to 1.
+//
+// Use of this attribute is HIGHLY DISCOURAGED. Taking the address of or
+// binding a reference to any unaligned member is UB, and it is very easy to
+// do so unintentionally when passing such members as function arguments.
+//
+// DO NOT APPLY THIS ATTRIBUTE TO STRUCTS CONTAINING ATOMICS. Doing
+// so can cause atomic variables to be mis-aligned and silently violate
+// atomicity on x86.
+//
+// This attribute can either be applied to members of a structure or to a
+// structure in its entirety. Applying this attribute (judiciously) to a
+// structure in its entirety to optimize the memory footprint of very
+// commonly-used structs is fine. Do not apply this attribute to a structure in
+// its entirety if the purpose is to control the offsets of the members in the
+// structure. Instead, apply this attribute only to structure members that need
+// it.
+//
+// When applying ABSL_ATTRIBUTE_PACKED only to specific structure members the
+// natural alignment of structure members not annotated is preserved. Aligned
+// member accesses are faster than non-aligned member accesses even if the
+// targeted microprocessor supports non-aligned accesses.
 #if ABSL_HAVE_ATTRIBUTE(packed) || (defined(__GNUC__) && !defined(__clang__))
 #define ABSL_ATTRIBUTE_PACKED __attribute__((__packed__))
 #else
@@ -578,15 +635,167 @@
 #define ABSL_ATTRIBUTE_FUNC_ALIGN(bytes)
 #endif
 
+// ABSL_FALLTHROUGH_INTENDED
+//
+// Annotates implicit fall-through between switch labels, allowing a case to
+// indicate intentional fallthrough and turn off warnings about any lack of a
+// `break` statement.
+//
+// Deprecated: Use the standard C++17 `[[fallthrough]]` instead.
+//
+// This macro has no effect on runtime behavior and performance of code.
+
+#ifdef ABSL_FALLTHROUGH_INTENDED
+#error "ABSL_FALLTHROUGH_INTENDED should not be defined."
+#endif
+#define ABSL_FALLTHROUGH_INTENDED [[fallthrough]]
+
+// ABSL_DEPRECATED()
+//
+// Marks a deprecated class, struct, enum, function, method and variable
+// declarations. The macro argument is used as a custom diagnostic message (e.g.
+// suggestion of a better alternative).
+//
+// For code or headers that are assured to only build with C++14 and up, prefer
+// just using the standard `[[deprecated("message")]]` directly over this macro.
+//
+// Examples:
+//
+//   class ABSL_DEPRECATED("Use Bar instead") Foo {...};
+//
+//   ABSL_DEPRECATED("Use Baz() instead") void Bar() {...}
+//
+//   template <typename T>
+//   ABSL_DEPRECATED("Use DoThat() instead")
+//   void DoThis();
+//
+//   enum FooEnum {
+//     kBar ABSL_DEPRECATED("Use kBaz instead"),
+//   };
+//
+// Every usage of a deprecated entity will trigger a warning when compiled with
+// GCC/Clang's `-Wdeprecated-declarations` option. Google's production toolchain
+// turns this warning off by default, instead relying on clang-tidy to report
+// new uses of deprecated code.
+#if ABSL_HAVE_ATTRIBUTE(deprecated)
+#define ABSL_DEPRECATED(message) __attribute__((deprecated(message)))
+#else
+#define ABSL_DEPRECATED(message)
+#endif
+
+// When deprecating Abseil code, it is sometimes necessary to turn off the
+// warning within Abseil, until the deprecated code is actually removed. The
+// deprecated code can be surrounded with these directives to achieve that
+// result.
+//
+// class ABSL_DEPRECATED("Use Bar instead") Foo;
+//
+// ABSL_INTERNAL_DISABLE_DEPRECATED_DECLARATION_WARNING
+// Baz ComputeBazFromFoo(Foo f);
+// ABSL_INTERNAL_RESTORE_DEPRECATED_DECLARATION_WARNING
+#if defined(__GNUC__) || defined(__clang__)
+// Clang also supports these GCC pragmas.
+#define ABSL_INTERNAL_DISABLE_DEPRECATED_DECLARATION_WARNING \
+  _Pragma("GCC diagnostic push")             \
+  _Pragma("GCC diagnostic ignored \"-Wdeprecated-declarations\"")
+#define ABSL_INTERNAL_RESTORE_DEPRECATED_DECLARATION_WARNING \
+  _Pragma("GCC diagnostic pop")
+#elif defined(_MSC_VER)
+#define ABSL_INTERNAL_DISABLE_DEPRECATED_DECLARATION_WARNING \
+  _Pragma("warning(push)") _Pragma("warning(disable: 4996)")
+#define ABSL_INTERNAL_RESTORE_DEPRECATED_DECLARATION_WARNING \
+  _Pragma("warning(pop)")
+#else
+#define ABSL_INTERNAL_DISABLE_DEPRECATED_DECLARATION_WARNING
+#define ABSL_INTERNAL_RESTORE_DEPRECATED_DECLARATION_WARNING
+#endif  // defined(__GNUC__) || defined(__clang__)
+
+// ABSL_REQUIRE_EXPLICIT_INIT
+//
+// ABSL_REQUIRE_EXPLICIT_INIT is placed *after* the data members of an aggregate
+// type to indicate that the annotated member must be explicitly initialized by
+// the user whenever the aggregate is constructed. For example:
+//
+//   struct Coord {
+//     int x ABSL_REQUIRE_EXPLICIT_INIT;
+//     int y ABSL_REQUIRE_EXPLICIT_INIT;
+//   };
+//   Coord coord = {1};  // warning: field 'y' is not explicitly initialized
+//
+// Note that usage on C arrays is not supported in C++.
+// Use a struct (such as std::array) to wrap the array member instead.
+//
+// Avoid applying this attribute to the members of non-aggregate types.
+// The behavior within non-aggregates is unspecified and subject to change.
+//
+// Do NOT attempt to suppress or demote the error generated by this attribute.
+// Just like with a missing function argument, it is a hard error by design.
+//
+// See the upstream documentation for more details:
+// https://clang.llvm.org/docs/AttributeReference.html#require-explicit-initialization
+#ifdef __cplusplus
+#if ABSL_HAVE_CPP_ATTRIBUTE(clang::require_explicit_initialization)
+// clang-format off
+#define ABSL_REQUIRE_EXPLICIT_INIT \
+  [[clang::require_explicit_initialization]] = \
+    AbslInternal_YouForgotToExplicitlyInitializeAField::v
+#else
+#define ABSL_REQUIRE_EXPLICIT_INIT \
+  = AbslInternal_YouForgotToExplicitlyInitializeAField::v
+#endif
+// clang-format on
+#else
+// clang-format off
+#if ABSL_HAVE_ATTRIBUTE(require_explicit_initialization)
+#define ABSL_REQUIRE_EXPLICIT_INIT \
+  __attribute__((require_explicit_initialization))
+#else
+#define ABSL_REQUIRE_EXPLICIT_INIT \
+  /* No portable fallback for C is available */
+#endif
+// clang-format on
+#endif
+
+#ifdef __cplusplus
+struct AbslInternal_YouForgotToExplicitlyInitializeAField {
+  // A portable version of [[clang::require_explicit_initialization]] that
+  // never builds, as a last resort for all toolchains.
+  // The error messages are poor, so we don't rely on this unless we have to.
+  template <class T>
+#if !defined(SWIG)
+  constexpr
+#endif
+  operator T() const /* NOLINT */ {
+    const void *volatile deliberately_volatile_ptr = nullptr;
+    // Infinite loop to prevent constexpr compilation
+    for (;;) {
+      // This assignment ensures the 'this' pointer is not optimized away, so
+      // that linking always fails.
+      deliberately_volatile_ptr = this;  // Deliberately not constexpr
+      (void)deliberately_volatile_ptr;
+    }
+  }
+  // This is deliberately left undefined to prevent linking
+  static AbslInternal_YouForgotToExplicitlyInitializeAField v;
+};
+#endif
+
 // ABSL_CONST_INIT
 //
-// A variable declaration annotated with the `ABSL_CONST_INIT` attribute will
-// not compile (on supported platforms) unless the variable has a constant
-// initializer. This is useful for variables with static and thread storage
-// duration, because it guarantees that they will not suffer from the so-called
-// "static init order fiasco".  Prefer to put this attribute on the most visible
-// declaration of the variable, if there's more than one, because code that
-// accesses the variable can then use the attribute for optimization.
+// A variable declared with `ABSL_CONST_INIT` will not compile (on supported
+// platforms) unless the variable has a constant initializer. This is useful for
+// variables with static and thread storage duration, because it guarantees that
+// they will not suffer from the so-called "static init order fiasco".
+//
+// `ABSL_CONST_INIT` must be placed on the initializing declaration
+// (i.e. definition) of the variable. Some compilers will give a
+// `-Wmissing-constinit` warning when it is placed on some other
+// declaration but missing from the initializing declaration.
+//
+// For thread_local variables, placing `ABSL_CONST_INIT` on the non-initializing
+// declaration tells the compiler that the variable is already initialized,
+// reducing overhead that would otherwise be incurred by a hidden guard
+// variable.
 //
 // Example:
 //
@@ -595,14 +804,282 @@
 //     ABSL_CONST_INIT static MyType my_var;
 //   };
 //
-//   MyType MyClass::my_var = MakeMyType(...);
+//   ABSL_CONST_INIT MyType MyClass::my_var = MakeMyType(...);
 //
-// Note that this attribute is redundant if the variable is declared constexpr.
-#if ABSL_HAVE_CPP_ATTRIBUTE(clang::require_constant_initialization)
-// NOLINTNEXTLINE(whitespace/braces)
+// For code or headers that are assured to only build with C++20 and up, prefer
+// using the standard `constinit` keyword directly over this macro.
+//
+// Note that `ABSL_CONST_INIT` must not be used on a variable declared
+// constexpr.
+#if defined(__cpp_constinit) && __cpp_constinit >= 201907L
+#define ABSL_CONST_INIT constinit
+#elif ABSL_HAVE_CPP_ATTRIBUTE(clang::require_constant_initialization)
 #define ABSL_CONST_INIT [[clang::require_constant_initialization]]
 #else
 #define ABSL_CONST_INIT
-#endif  // ABSL_HAVE_CPP_ATTRIBUTE(clang::require_constant_initialization)
+#endif
+
+// ABSL_ATTRIBUTE_PURE_FUNCTION
+//
+// ABSL_ATTRIBUTE_PURE_FUNCTION is used to annotate declarations of "pure"
+// functions. A function is pure if its return value is only a function of its
+// arguments. The pure attribute prohibits a function from modifying the state
+// of the program that is observable by means other than inspecting the
+// function's return value. Declaring such functions with the pure attribute
+// allows the compiler to avoid emitting some calls in repeated invocations of
+// the function with the same argument values.
+//
+// Example:
+//
+//  ABSL_ATTRIBUTE_PURE_FUNCTION std::string FormatTime(Time t);
+#if ABSL_HAVE_CPP_ATTRIBUTE(gnu::pure)
+#define ABSL_ATTRIBUTE_PURE_FUNCTION [[gnu::pure]]
+#elif ABSL_HAVE_ATTRIBUTE(pure)
+#define ABSL_ATTRIBUTE_PURE_FUNCTION __attribute__((pure))
+#else
+// If the attribute isn't defined, we'll fallback to ABSL_MUST_USE_RESULT since
+// pure functions are useless if its return is ignored.
+#define ABSL_ATTRIBUTE_PURE_FUNCTION ABSL_MUST_USE_RESULT
+#endif
+
+// ABSL_ATTRIBUTE_CONST_FUNCTION
+//
+// ABSL_ATTRIBUTE_CONST_FUNCTION is used to annotate declarations of "const"
+// functions. A const function is similar to a pure function, with one
+// exception: Pure functions may return value that depend on a non-volatile
+// object that isn't provided as a function argument, while the const function
+// is guaranteed to return the same result given the same arguments.
+//
+// Example:
+//
+//  ABSL_ATTRIBUTE_CONST_FUNCTION int64_t ToInt64Milliseconds(Duration d);
+#if defined(_MSC_VER) && !defined(__clang__)
+// Put the MSVC case first since MSVC seems to parse const as a C++ keyword.
+#define ABSL_ATTRIBUTE_CONST_FUNCTION ABSL_ATTRIBUTE_PURE_FUNCTION
+#elif ABSL_HAVE_CPP_ATTRIBUTE(gnu::const)
+#define ABSL_ATTRIBUTE_CONST_FUNCTION [[gnu::const]]
+#elif ABSL_HAVE_ATTRIBUTE(const)
+#define ABSL_ATTRIBUTE_CONST_FUNCTION __attribute__((const))
+#else
+// Since const functions are more restrictive pure function, we'll fallback to a
+// pure function if the const attribute is not handled.
+#define ABSL_ATTRIBUTE_CONST_FUNCTION ABSL_ATTRIBUTE_PURE_FUNCTION
+#endif
+
+// ABSL_ATTRIBUTE_LIFETIME_BOUND indicates that a resource owned by a function
+// parameter or implicit object parameter is retained by the return value of the
+// annotated function (or, for a parameter of a constructor, in the value of the
+// constructed object). This attribute causes warnings to be produced if a
+// temporary object does not live long enough.
+//
+// When applied to a reference parameter, the referenced object is assumed to be
+// retained by the return value of the function. When applied to a non-reference
+// parameter (for example, a pointer or a class type), all temporaries
+// referenced by the parameter are assumed to be retained by the return value of
+// the function.
+//
+// See also the upstream documentation:
+// https://clang.llvm.org/docs/AttributeReference.html#lifetimebound
+// https://learn.microsoft.com/en-us/cpp/code-quality/c26816?view=msvc-170
+#if ABSL_HAVE_CPP_ATTRIBUTE(clang::lifetimebound)
+#define ABSL_ATTRIBUTE_LIFETIME_BOUND [[clang::lifetimebound]]
+#elif ABSL_HAVE_CPP_ATTRIBUTE(msvc::lifetimebound)
+#define ABSL_ATTRIBUTE_LIFETIME_BOUND [[msvc::lifetimebound]]
+#elif ABSL_HAVE_ATTRIBUTE(lifetimebound)
+#define ABSL_ATTRIBUTE_LIFETIME_BOUND __attribute__((lifetimebound))
+#else
+#define ABSL_ATTRIBUTE_LIFETIME_BOUND
+#endif
+
+// Internal attribute; name and documentation TBD.
+//
+// See the upstream documentation:
+// https://clang.llvm.org/docs/AttributeReference.html#lifetime_capture_by
+#if ABSL_HAVE_CPP_ATTRIBUTE(clang::lifetime_capture_by)
+#define ABSL_INTERNAL_ATTRIBUTE_CAPTURED_BY(Owner) \
+  [[clang::lifetime_capture_by(Owner)]]
+#else
+#define ABSL_INTERNAL_ATTRIBUTE_CAPTURED_BY(Owner)
+#endif
+
+// Internal attribute; name and documentation TBD.
+//
+// See the upstream documentation:
+// https://clang.llvm.org/docs/AttributeReference.html#lifetime_capture_by_this
+//
+// Note: ABSL_INTERNAL_ATTRIBUTE_CAPTURED_BY(this) is deprecated. Use
+// ABSL_INTERNAL_ATTRIBUTE_CAPTURED_BY_THIS instead.
+#if ABSL_HAVE_CPP_ATTRIBUTE(clang::lifetime_capture_by_this)
+#define ABSL_INTERNAL_ATTRIBUTE_CAPTURED_BY_THIS \
+  [[clang::lifetime_capture_by_this]]
+#elif ABSL_HAVE_CPP_ATTRIBUTE(clang::lifetime_capture_by)
+#define ABSL_INTERNAL_ATTRIBUTE_CAPTURED_BY_THIS \
+  ABSL_INTERNAL_ATTRIBUTE_CAPTURED_BY(this)
+#else
+#define ABSL_INTERNAL_ATTRIBUTE_CAPTURED_BY_THIS
+#endif
+
+// ABSL_ATTRIBUTE_VIEW indicates that a type is solely a "view" of data that it
+// points to, similarly to a span, string_view, or other non-owning reference
+// type.
+// This enables diagnosing certain lifetime issues similar to those enabled by
+// ABSL_ATTRIBUTE_LIFETIME_BOUND, such as:
+//
+//   struct ABSL_ATTRIBUTE_VIEW StringView {
+//     template<class R>
+//     StringView(const R&);
+//   };
+//
+//   StringView f(std::string s) {
+//     return s;  // warning: address of stack memory returned
+//   }
+//
+// We disable this on Clang versions < 13 because of the following
+// false-positive:
+//
+//   absl::string_view f(std::optional<absl::string_view> sv) { return *sv; }
+//
+// See the following links for details:
+// https://reviews.llvm.org/D64448
+// https://lists.llvm.org/pipermail/cfe-dev/2018-November/060355.html
+#if ABSL_HAVE_CPP_ATTRIBUTE(gsl::Pointer) && \
+    (!defined(__clang_major__) || __clang_major__ >= 13)
+#define ABSL_ATTRIBUTE_VIEW [[gsl::Pointer]]
+#else
+#define ABSL_ATTRIBUTE_VIEW
+#endif
+
+// ABSL_ATTRIBUTE_OWNER indicates that a type is a container, smart pointer, or
+// similar class that owns all the data that it points to.
+// This enables diagnosing certain lifetime issues similar to those enabled by
+// ABSL_ATTRIBUTE_LIFETIME_BOUND, such as:
+//
+//   struct ABSL_ATTRIBUTE_VIEW StringView {
+//     template<class R>
+//     StringView(const R&);
+//   };
+//
+//   struct ABSL_ATTRIBUTE_OWNER String {};
+//
+//   StringView f(String s) {
+//     return s;  // warning: address of stack memory returned
+//   }
+//
+// We disable this on Clang versions < 13 because of the following
+// false-positive:
+//
+//   absl::string_view f(std::optional<absl::string_view> sv) { return *sv; }
+//
+// See the following links for details:
+// https://reviews.llvm.org/D64448
+// https://lists.llvm.org/pipermail/cfe-dev/2018-November/060355.html
+#if ABSL_HAVE_CPP_ATTRIBUTE(gsl::Owner) && \
+    (!defined(__clang_major__) || __clang_major__ >= 13)
+#define ABSL_ATTRIBUTE_OWNER [[gsl::Owner]]
+#else
+#define ABSL_ATTRIBUTE_OWNER
+#endif
+
+// ABSL_ATTRIBUTE_TRIVIAL_ABI
+// Indicates that a type is "trivially relocatable" -- meaning it can be
+// relocated without invoking the constructor/destructor, using a form of move
+// elision.
+//
+// From a memory safety point of view, putting aside destructor ordering, it's
+// safe to apply ABSL_ATTRIBUTE_TRIVIAL_ABI if an object's location
+// can change over the course of its lifetime: if a constructor can be run one
+// place, and then the object magically teleports to another place where some
+// methods are run, and then the object teleports to yet another place where it
+// is destroyed. This is notably not true for self-referential types, where the
+// move-constructor must keep the self-reference up to date. If the type changed
+// location without invoking the move constructor, it would have a dangling
+// self-reference.
+//
+// The use of this teleporting machinery means that the number of paired
+// move/destroy operations can change, and so it is a bad idea to apply this to
+// a type meant to count the number of moves.
+//
+// Warning: applying this can, rarely, break callers. Objects passed by value
+// will be destroyed at the end of the call, instead of the end of the
+// full-expression containing the call. In addition, it changes the ABI
+// of functions accepting this type by value (e.g. to pass in registers).
+//
+// See also the upstream documentation:
+// https://clang.llvm.org/docs/AttributeReference.html#trivial-abi
+//
+// b/321691395 - This is currently disabled in open-source builds since
+// compiler support differs. If system libraries compiled with GCC are mixed
+// with libraries compiled with Clang, types will have different ideas about
+// their ABI, leading to hard to debug crashes.
+#define ABSL_ATTRIBUTE_TRIVIAL_ABI
+
+// ABSL_ATTRIBUTE_NO_UNIQUE_ADDRESS
+//
+// Indicates a data member can be optimized to occupy no space (if it is empty)
+// and/or its tail padding can be used for other members.
+//
+// For code that is assured to only build with C++20 or later, prefer using
+// the standard attribute `[[no_unique_address]]` directly instead of this
+// macro.
+//
+// https://devblogs.microsoft.com/cppblog/msvc-cpp20-and-the-std-cpp20-switch/#c20-no_unique_address
+// Current versions of MSVC have disabled `[[no_unique_address]]` since it
+// breaks ABI compatibility, but offers `[[msvc::no_unique_address]]` for
+// situations when it can be assured that it is desired. Since Abseil does not
+// claim ABI compatibility in mixed builds, we can offer it unconditionally.
+#if defined(_MSC_VER) && _MSC_VER >= 1929
+#define ABSL_ATTRIBUTE_NO_UNIQUE_ADDRESS [[msvc::no_unique_address]]
+#elif ABSL_HAVE_CPP_ATTRIBUTE(no_unique_address)
+#define ABSL_ATTRIBUTE_NO_UNIQUE_ADDRESS [[no_unique_address]]
+#else
+#define ABSL_ATTRIBUTE_NO_UNIQUE_ADDRESS
+#endif
+
+// ABSL_ATTRIBUTE_UNINITIALIZED
+//
+// GCC and Clang support a flag `-ftrivial-auto-var-init=<option>` (<option>
+// can be "zero" or "pattern") that can be used to initialize automatic stack
+// variables. Variables with this attribute will be left uninitialized,
+// overriding the compiler flag.
+//
+// See https://clang.llvm.org/docs/AttributeReference.html#uninitialized
+// and https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html#index-uninitialized-variable-attribute
+#if ABSL_HAVE_CPP_ATTRIBUTE(clang::uninitialized)
+#define ABSL_ATTRIBUTE_UNINITIALIZED [[clang::uninitialized]]
+#elif ABSL_HAVE_CPP_ATTRIBUTE(gnu::uninitialized)
+#define ABSL_ATTRIBUTE_UNINITIALIZED [[gnu::uninitialized]]
+#elif ABSL_HAVE_ATTRIBUTE(uninitialized)
+#define ABSL_ATTRIBUTE_UNINITIALIZED __attribute__((uninitialized))
+#else
+#define ABSL_ATTRIBUTE_UNINITIALIZED
+#endif
+
+// ABSL_ATTRIBUTE_WARN_UNUSED
+//
+// Compilers routinely warn about trivial variables that are unused.  For
+// non-trivial types, this warning is suppressed since the
+// constructor/destructor may be intentional and load-bearing, for example, with
+// a RAII scoped lock.
+//
+// For example:
+//
+// class ABSL_ATTRIBUTE_WARN_UNUSED MyType {
+//  public:
+//   MyType();
+//   ~MyType();
+// };
+//
+// void foo() {
+//   // Warns with ABSL_ATTRIBUTE_WARN_UNUSED attribute present.
+//   MyType unused;
+// }
+//
+// See https://clang.llvm.org/docs/AttributeReference.html#warn-unused and
+// https://gcc.gnu.org/onlinedocs/gcc/C_002b_002b-Attributes.html#index-warn_005funused-type-attribute
+#if ABSL_HAVE_CPP_ATTRIBUTE(gnu::warn_unused)
+#define ABSL_ATTRIBUTE_WARN_UNUSED [[gnu::warn_unused]]
+#else
+#define ABSL_ATTRIBUTE_WARN_UNUSED
+#endif
 
 #endif  // ABSL_BASE_ATTRIBUTES_H_
diff --git a/absl/base/attributes_test.cc b/absl/base/attributes_test.cc
new file mode 100644
index 00000000000..5d213a4263a
--- /dev/null
+++ b/absl/base/attributes_test.cc
@@ -0,0 +1,43 @@
+// Copyright 2025 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/base/attributes.h"
+
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+
+namespace {
+
+TEST(Attributes, RequireExplicitInit) {
+  struct Agg {
+    int f1;
+    int f2 ABSL_REQUIRE_EXPLICIT_INIT;
+  };
+  Agg good1 ABSL_ATTRIBUTE_UNUSED = {1, 2};
+#if ABSL_INTERNAL_CPLUSPLUS_LANG >= 202002L
+  Agg good2 ABSL_ATTRIBUTE_UNUSED(1, 2);
+#endif
+  Agg good3 ABSL_ATTRIBUTE_UNUSED{1, 2};
+  Agg good4 ABSL_ATTRIBUTE_UNUSED = {1, 2};
+  Agg good5 ABSL_ATTRIBUTE_UNUSED = Agg{1, 2};
+  Agg good6[1] ABSL_ATTRIBUTE_UNUSED = {{1, 2}};
+  Agg good7[1] ABSL_ATTRIBUTE_UNUSED = {Agg{1, 2}};
+  union {
+    Agg agg;
+  } good8 ABSL_ATTRIBUTE_UNUSED = {{1, 2}};
+  constexpr Agg good9 ABSL_ATTRIBUTE_UNUSED = {1, 2};
+  constexpr Agg good10 ABSL_ATTRIBUTE_UNUSED{1, 2};
+}
+
+}  // namespace
diff --git a/absl/base/bit_cast_test.cc b/absl/base/bit_cast_test.cc
index 4846add435c..8bded1dcfa6 100644
--- a/absl/base/bit_cast_test.cc
+++ b/absl/base/bit_cast_test.cc
@@ -16,12 +16,13 @@
 
 #include <cstdint>
 #include <cstring>
+#include <iterator>
 
 #include "gtest/gtest.h"
 #include "absl/base/casts.h"
-#include "absl/base/macros.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace {
 
 template <int N>
@@ -61,25 +62,25 @@ void TestIntegral(const T values[], int num_values) {
 
 TEST(BitCast, Bool) {
   static const bool bool_list[] = { false, true };
-  TestMarshall<bool>(bool_list, ABSL_ARRAYSIZE(bool_list));
+  TestMarshall<bool>(bool_list, std::size(bool_list));
 }
 
 TEST(BitCast, Int32) {
   static const int32_t int_list[] =
     { 0, 1, 100, 2147483647, -1, -100, -2147483647, -2147483647-1 };
-  TestMarshall<int32_t>(int_list, ABSL_ARRAYSIZE(int_list));
+  TestMarshall<int32_t>(int_list, std::size(int_list));
 }
 
 TEST(BitCast, Int64) {
   static const int64_t int64_list[] =
     { 0, 1, 1LL << 40, -1, -(1LL<<40) };
-  TestMarshall<int64_t>(int64_list, ABSL_ARRAYSIZE(int64_list));
+  TestMarshall<int64_t>(int64_list, std::size(int64_list));
 }
 
 TEST(BitCast, Uint64) {
   static const uint64_t uint64_list[] =
     { 0, 1, 1LLU << 40, 1LLU << 63 };
-  TestMarshall<uint64_t>(uint64_list, ABSL_ARRAYSIZE(uint64_list));
+  TestMarshall<uint64_t>(uint64_list, std::size(uint64_list));
 }
 
 TEST(BitCast, Float) {
@@ -87,9 +88,9 @@ TEST(BitCast, Float) {
     { 0.0f, 1.0f, -1.0f, 10.0f, -10.0f,
       1e10f, 1e20f, 1e-10f, 1e-20f,
       2.71828f, 3.14159f };
-  TestMarshall<float>(float_list, ABSL_ARRAYSIZE(float_list));
-  TestIntegral<float, int>(float_list, ABSL_ARRAYSIZE(float_list));
-  TestIntegral<float, unsigned>(float_list, ABSL_ARRAYSIZE(float_list));
+  TestMarshall<float>(float_list, std::size(float_list));
+  TestIntegral<float, int>(float_list, std::size(float_list));
+  TestIntegral<float, unsigned>(float_list, std::size(float_list));
 }
 
 TEST(BitCast, Double) {
@@ -98,10 +99,11 @@ TEST(BitCast, Double) {
       1e10, 1e100, 1e-10, 1e-100,
       2.718281828459045,
       3.141592653589793238462643383279502884197169399375105820974944 };
-  TestMarshall<double>(double_list, ABSL_ARRAYSIZE(double_list));
-  TestIntegral<double, int64_t>(double_list, ABSL_ARRAYSIZE(double_list));
-  TestIntegral<double, uint64_t>(double_list, ABSL_ARRAYSIZE(double_list));
+  TestMarshall<double>(double_list, std::size(double_list));
+  TestIntegral<double, int64_t>(double_list, std::size(double_list));
+  TestIntegral<double, uint64_t>(double_list, std::size(double_list));
 }
 
 }  // namespace
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/base/c_header_test.c b/absl/base/c_header_test.c
new file mode 100644
index 00000000000..3cd0177848f
--- /dev/null
+++ b/absl/base/c_header_test.c
@@ -0,0 +1,30 @@
+// Copyright 2024 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifdef __cplusplus
+#error This is a C compile test
+#endif
+
+// This test ensures that headers that are included in legacy C code are
+// compatible with C. Abseil is a C++ library. We do not desire to expand C
+// compatibility or keep C compatibility forever. This test only exists to
+// ensure C compatibility until it is no longer required. Do not add new code
+// that requires C compatibility.
+#include "absl/base/attributes.h"     // IWYU pragma: keep
+#include "absl/base/config.h"         // IWYU pragma: keep
+#include "absl/base/optimization.h"   // IWYU pragma: keep
+#include "absl/base/policy_checks.h"  // IWYU pragma: keep
+#include "absl/base/port.h"           // IWYU pragma: keep
+
+int main() { return 0; }
diff --git a/absl/base/call_once.h b/absl/base/call_once.h
index 4aa6360cffc..77852d124fc 100644
--- a/absl/base/call_once.h
+++ b/absl/base/call_once.h
@@ -28,24 +28,29 @@
 #include <algorithm>
 #include <atomic>
 #include <cstdint>
+#include <functional>
+#include <iterator>
 #include <type_traits>
 #include <utility>
 
-#include "absl/base/internal/invoke.h"
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
 #include "absl/base/internal/low_level_scheduling.h"
 #include "absl/base/internal/raw_logging.h"
 #include "absl/base/internal/scheduling_mode.h"
 #include "absl/base/internal/spinlock_wait.h"
-#include "absl/base/macros.h"
+#include "absl/base/nullability.h"
 #include "absl/base/optimization.h"
 #include "absl/base/port.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 class once_flag;
 
 namespace base_internal {
-std::atomic<uint32_t>* ControlWord(absl::once_flag* flag);
+std::atomic<uint32_t>* absl_nonnull ControlWord(
+    absl::once_flag* absl_nonnull flag);
 }  // namespace base_internal
 
 // call_once()
@@ -88,7 +93,8 @@ class once_flag {
   once_flag& operator=(const once_flag&) = delete;
 
  private:
-  friend std::atomic<uint32_t>* base_internal::ControlWord(once_flag* flag);
+  friend std::atomic<uint32_t>* absl_nonnull base_internal::ControlWord(
+      once_flag* absl_nonnull flag);
   std::atomic<uint32_t> control_;
 };
 
@@ -102,7 +108,8 @@ namespace base_internal {
 // Like call_once, but uses KERNEL_ONLY scheduling. Intended to be used to
 // initialize entities used by the scheduler implementation.
 template <typename Callable, typename... Args>
-void LowLevelCallOnce(absl::once_flag* flag, Callable&& fn, Args&&... args);
+void LowLevelCallOnce(absl::once_flag* absl_nonnull flag, Callable&& fn,
+                      Args&&... args);
 
 // Disables scheduling while on stack when scheduling mode is non-cooperative.
 // No effect for cooperative scheduling modes.
@@ -122,7 +129,7 @@ class SchedulingHelper {
 
  private:
   base_internal::SchedulingMode mode_;
-  bool guard_result_;
+  bool guard_result_ = false;
 };
 
 // Bit patterns for call_once state machine values.  Internal implementation
@@ -142,17 +149,18 @@ enum {
 };
 
 template <typename Callable, typename... Args>
-ABSL_ATTRIBUTE_NOINLINE
-void CallOnceImpl(std::atomic<uint32_t>* control,
-                  base_internal::SchedulingMode scheduling_mode, Callable&& fn,
-                  Args&&... args) {
+    void
+    CallOnceImpl(std::atomic<uint32_t>* absl_nonnull control,
+                 base_internal::SchedulingMode scheduling_mode, Callable&& fn,
+                 Args&&... args) {
 #ifndef NDEBUG
   {
-    uint32_t old_control = control->load(std::memory_order_acquire);
+    uint32_t old_control = control->load(std::memory_order_relaxed);
     if (old_control != kOnceInit &&
         old_control != kOnceRunning &&
         old_control != kOnceWaiter &&
         old_control != kOnceDone) {
+      // Memory corruption may cause this error.
       ABSL_RAW_LOG(FATAL, "Unexpected value for control word: 0x%lx",
                    static_cast<unsigned long>(old_control));  // NOLINT
     }
@@ -166,28 +174,31 @@ void CallOnceImpl(std::atomic<uint32_t>* control,
   // Must do this before potentially modifying control word's state.
   base_internal::SchedulingHelper maybe_disable_scheduling(scheduling_mode);
   // Short circuit the simplest case to avoid procedure call overhead.
+  // The base_internal::SpinLockWait() call returns either kOnceInit or
+  // kOnceDone. If it returns kOnceDone, it must have loaded the control word
+  // with std::memory_order_acquire and seen a value of kOnceDone.
   uint32_t old_control = kOnceInit;
   if (control->compare_exchange_strong(old_control, kOnceRunning,
-                                       std::memory_order_acquire,
                                        std::memory_order_relaxed) ||
-      base_internal::SpinLockWait(control, ABSL_ARRAYSIZE(trans), trans,
+      base_internal::SpinLockWait(control, std::size(trans), trans,
                                   scheduling_mode) == kOnceInit) {
-    base_internal::Invoke(std::forward<Callable>(fn),
-                          std::forward<Args>(args)...);
-    old_control = control->load(std::memory_order_relaxed);
-    control->store(base_internal::kOnceDone, std::memory_order_release);
+    std::invoke(std::forward<Callable>(fn), std::forward<Args>(args)...);
+    old_control =
+        control->exchange(base_internal::kOnceDone, std::memory_order_release);
     if (old_control == base_internal::kOnceWaiter) {
       base_internal::SpinLockWake(control, true);
     }
   }  // else *control is already kOnceDone
 }
 
-inline std::atomic<uint32_t>* ControlWord(once_flag* flag) {
+inline std::atomic<uint32_t>* absl_nonnull ControlWord(
+    once_flag* absl_nonnull flag) {
   return &flag->control_;
 }
 
 template <typename Callable, typename... Args>
-void LowLevelCallOnce(absl::once_flag* flag, Callable&& fn, Args&&... args) {
+void LowLevelCallOnce(absl::once_flag* absl_nonnull flag, Callable&& fn,
+                      Args&&... args) {
   std::atomic<uint32_t>* once = base_internal::ControlWord(flag);
   uint32_t s = once->load(std::memory_order_acquire);
   if (ABSL_PREDICT_FALSE(s != base_internal::kOnceDone)) {
@@ -200,7 +211,8 @@ void LowLevelCallOnce(absl::once_flag* flag, Callable&& fn, Args&&... args) {
 }  // namespace base_internal
 
 template <typename Callable, typename... Args>
-void call_once(absl::once_flag& flag, Callable&& fn, Args&&... args) {
+    void
+    call_once(absl::once_flag& flag, Callable&& fn, Args&&... args) {
   std::atomic<uint32_t>* once = base_internal::ControlWord(&flag);
   uint32_t s = once->load(std::memory_order_acquire);
   if (ABSL_PREDICT_FALSE(s != base_internal::kOnceDone)) {
@@ -210,6 +222,7 @@ void call_once(absl::once_flag& flag, Callable&& fn, Args&&... args) {
   }
 }
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_BASE_CALL_ONCE_H_
diff --git a/absl/base/call_once_test.cc b/absl/base/call_once_test.cc
index 9a5a5c13be7..630e7ce4dfd 100644
--- a/absl/base/call_once_test.cc
+++ b/absl/base/call_once_test.cc
@@ -24,6 +24,7 @@
 #include "absl/synchronization/mutex.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace {
 
 absl::once_flag once;
@@ -38,25 +39,25 @@ bool done_blocking ABSL_GUARDED_BY(counters_mu) = false;
 
 // Function to be called from absl::call_once.  Waits for a notification.
 void WaitAndIncrement() {
-  counters_mu.Lock();
+  counters_mu.lock();
   ++call_once_invoke_count;
-  counters_mu.Unlock();
+  counters_mu.unlock();
 
   counters_mu.LockWhen(Condition(&done_blocking));
   ++call_once_finished_count;
-  counters_mu.Unlock();
+  counters_mu.unlock();
 }
 
 void ThreadBody() {
-  counters_mu.Lock();
+  counters_mu.lock();
   ++running_thread_count;
-  counters_mu.Unlock();
+  counters_mu.unlock();
 
   absl::call_once(once, WaitAndIncrement);
 
-  counters_mu.Lock();
+  counters_mu.lock();
   ++call_once_return_count;
-  counters_mu.Unlock();
+  counters_mu.unlock();
 }
 
 // Returns true if all threads are set up for the test.
@@ -88,18 +89,19 @@ TEST(CallOnceTest, ExecutionCount) {
   // Allow WaitAndIncrement to finish executing.  Once it does, the other
   // call_once waiters will be unblocked.
   done_blocking = true;
-  counters_mu.Unlock();
+  counters_mu.unlock();
 
   for (std::thread& thread : threads) {
     thread.join();
   }
 
-  counters_mu.Lock();
+  counters_mu.lock();
   EXPECT_EQ(call_once_invoke_count, 1);
   EXPECT_EQ(call_once_finished_count, 1);
   EXPECT_EQ(call_once_return_count, 10);
-  counters_mu.Unlock();
+  counters_mu.unlock();
 }
 
 }  // namespace
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/base/casts.cc b/absl/base/casts.cc
new file mode 100644
index 00000000000..d864a8c55cc
--- /dev/null
+++ b/absl/base/casts.cc
@@ -0,0 +1,61 @@
+// Copyright 2025 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/base/casts.h"
+
+#include <cstdlib>
+#include <string>
+
+#include "absl/base/config.h"
+#include "absl/base/internal/raw_logging.h"
+
+#ifdef ABSL_INTERNAL_HAS_CXA_DEMANGLE
+#include <cxxabi.h>
+#endif
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+namespace base_internal {
+
+namespace {
+
+std::string DemangleCppString(const char* mangled) {
+  std::string out;
+  int status = 0;
+  char* demangled = nullptr;
+#ifdef ABSL_INTERNAL_HAS_CXA_DEMANGLE
+  demangled = abi::__cxa_demangle(mangled, nullptr, nullptr, &status);
+#endif
+  if (status == 0 && demangled != nullptr) {
+    out.append(demangled);
+    free(demangled);
+  } else {
+    out.append(mangled);
+  }
+  return out;
+}
+
+}  // namespace
+
+void BadDownCastCrash(const char* source_type, const char* target_type) {
+  ABSL_RAW_LOG(FATAL, "down cast from %s to %s failed",
+               DemangleCppString(source_type).c_str(),
+               DemangleCppString(target_type).c_str());
+}
+
+}  // namespace base_internal
+
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/base/casts.h b/absl/base/casts.h
index aba0178219c..17cc0b9dede 100644
--- a/absl/base/casts.h
+++ b/absl/base/casts.h
@@ -27,26 +27,26 @@
 #include <cstring>
 #include <memory>
 #include <type_traits>
+#include <typeinfo>
 #include <utility>
 
-#include "absl/base/internal/identity.h"
+#if __has_include(<version>)
+#include <version>  // For __cpp_lib_bit_cast.
+#endif
+
+#if defined(__cpp_lib_bit_cast) && __cpp_lib_bit_cast >= 201806L
+#include <bit>  // For std::bit_cast.
+#endif  // defined(__cpp_lib_bit_cast) && __cpp_lib_bit_cast >= 201806L
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
 #include "absl/base/macros.h"
+#include "absl/base/optimization.h"
+#include "absl/base/options.h"
 #include "absl/meta/type_traits.h"
 
 namespace absl {
-
-namespace internal_casts {
-
-template <class Dest, class Source>
-struct is_bitcastable
-    : std::integral_constant<
-          bool,
-          sizeof(Dest) == sizeof(Source) &&
-              type_traits_internal::is_trivially_copyable<Source>::value &&
-              type_traits_internal::is_trivially_copyable<Dest>::value &&
-              std::is_default_constructible<Dest>::value> {};
-
-}  // namespace internal_casts
+ABSL_NAMESPACE_BEGIN
 
 // implicit_cast()
 //
@@ -98,85 +98,213 @@ struct is_bitcastable
 //
 // Such implicit cast chaining may be useful within template logic.
 template <typename To>
-constexpr To implicit_cast(typename absl::internal::identity_t<To> to) {
+constexpr std::enable_if_t<
+    !type_traits_internal::IsView<std::enable_if_t<
+        !std::is_reference_v<To>, std::remove_cv_t<To>>>::value,
+    To>
+implicit_cast(absl::type_identity_t<To> to) {
   return to;
 }
+template <typename To>
+constexpr std::enable_if_t<
+    type_traits_internal::IsView<std::enable_if_t<!std::is_reference_v<To>,
+                                                  std::remove_cv_t<To>>>::value,
+    To>
+implicit_cast(absl::type_identity_t<To> to ABSL_ATTRIBUTE_LIFETIME_BOUND) {
+  return to;
+}
+template <typename To>
+constexpr std::enable_if_t<std::is_reference_v<To>, To> implicit_cast(
+    absl::type_identity_t<To> to ABSL_ATTRIBUTE_LIFETIME_BOUND) {
+  return std::forward<absl::type_identity_t<To>>(to);
+}
 
 // bit_cast()
 //
-// Performs a bitwise cast on a type without changing the underlying bit
-// representation of that type's value. The two types must be of the same size
-// and both types must be trivially copyable. As with most casts, use with
-// caution. A `bit_cast()` might be needed when you need to temporarily treat a
-// type as some other type, such as in the following cases:
+// Creates a value of the new type `Dest` whose representation is the same as
+// that of the argument, which is of (deduced) type `Source` (a "bitwise cast";
+// every bit in the value representation of the result is equal to the
+// corresponding bit in the object representation of the source). Source and
+// destination types must be of the same size, and both types must be trivially
+// copyable.
 //
-//    * Serialization (casting temporarily to `char *` for those purposes is
-//      always allowed by the C++ standard)
-//    * Managing the individual bits of a type within mathematical operations
-//      that are not normally accessible through that type
-//    * Casting non-pointer types to pointer types (casting the other way is
-//      allowed by `reinterpret_cast()` but round-trips cannot occur the other
-//      way).
-//
-// Example:
+// As with most casts, use with caution. A `bit_cast()` might be needed when you
+// need to treat a value as the value of some other type, for example, to access
+// the individual bits of an object which are not normally accessible through
+// the object's type, such as for working with the binary representation of a
+// floating point value:
 //
 //   float f = 3.14159265358979;
-//   int i = bit_cast<int32_t>(f);
+//   int i = bit_cast<int>(f);
 //   // i = 0x40490fdb
 //
-// Casting non-pointer types to pointer types and then dereferencing them
-// traditionally produces undefined behavior.
+// Reinterpreting and accessing a value directly as a different type (as shown
+// below) usually results in undefined behavior.
 //
 // Example:
 //
 //   // WRONG
-//   float f = 3.14159265358979;            // WRONG
-//   int i = * reinterpret_cast<int*>(&f);  // WRONG
+//   float f = 3.14159265358979;
+//   int i = reinterpret_cast<int&>(f);    // Wrong
+//   int j = *reinterpret_cast<int*>(&f);  // Equally wrong
+//   int k = *bit_cast<int*>(&f);          // Equally wrong
 //
-// The address-casting method produces undefined behavior according to the ISO
-// C++ specification section [basic.lval]. Roughly, this section says: if an
-// object in memory has one type, and a program accesses it with a different
-// type, the result is undefined behavior for most values of "different type".
+// Reinterpret-casting results in undefined behavior according to the ISO C++
+// specification, section [basic.lval]. Roughly, this section says: if an object
+// in memory has one type, and a program accesses it with a different type, the
+// result is undefined behavior for most "different type".
+//
+// Using bit_cast on a pointer and then dereferencing it is no better than using
+// reinterpret_cast. You should only use bit_cast on the value itself.
 //
 // Such casting results in type punning: holding an object in memory of one type
 // and reading its bits back using a different type. A `bit_cast()` avoids this
-// issue by implementing its casts using `memcpy()`, which avoids introducing
-// this undefined behavior.
-//
-// NOTE: The requirements here are more strict than the bit_cast of standard
-// proposal p0476 due to the need for workarounds and lack of intrinsics.
-// Specifically, this implementation also requires `Dest` to be
-// default-constructible.
-template <
-    typename Dest, typename Source,
-    typename std::enable_if<internal_casts::is_bitcastable<Dest, Source>::value,
-                            int>::type = 0>
+// issue by copying the object representation to a new value, which avoids
+// introducing this undefined behavior (since the original value is never
+// accessed in the wrong way).
+//
+// The requirements of `absl::bit_cast` are more strict than that of
+// `std::bit_cast` unless compiler support is available. Specifically, without
+// compiler support, this implementation also requires `Dest` to be
+// default-constructible. In C++20, `absl::bit_cast` is replaced by
+// `std::bit_cast`.
+#if defined(__cpp_lib_bit_cast) && __cpp_lib_bit_cast >= 201806L
+
+using std::bit_cast;
+
+#else  // defined(__cpp_lib_bit_cast) && __cpp_lib_bit_cast >= 201806L
+
+template <typename Dest, typename Source,
+          std::enable_if_t<sizeof(Dest) == sizeof(Source) &&
+                               std::is_trivially_copyable_v<Source> &&
+                               std::is_trivially_copyable_v<Dest>
+#if !ABSL_HAVE_BUILTIN(__builtin_bit_cast)
+                               && std::is_default_constructible_v<Dest>
+#endif  // !ABSL_HAVE_BUILTIN(__builtin_bit_cast)
+                           ,
+                           int> = 0>
+#if ABSL_HAVE_BUILTIN(__builtin_bit_cast)
+inline constexpr Dest bit_cast(const Source& source) {
+  return __builtin_bit_cast(Dest, source);
+}
+#else  // ABSL_HAVE_BUILTIN(__builtin_bit_cast)
 inline Dest bit_cast(const Source& source) {
   Dest dest;
   memcpy(static_cast<void*>(std::addressof(dest)),
          static_cast<const void*>(std::addressof(source)), sizeof(dest));
   return dest;
 }
+#endif  // ABSL_HAVE_BUILTIN(__builtin_bit_cast)
 
-// NOTE: This overload is only picked if the requirements of bit_cast are not
-// met. It is therefore UB, but is provided temporarily as previous versions of
-// this function template were unchecked. Do not use this in new code.
-template <
-    typename Dest, typename Source,
-    typename std::enable_if<
-        !internal_casts::is_bitcastable<Dest, Source>::value, int>::type = 0>
-ABSL_DEPRECATED(
-    "absl::bit_cast type requirements were violated. Update the types being "
-    "used such that they are the same size and are both TriviallyCopyable.")
-inline Dest bit_cast(const Source& source) {
-  static_assert(sizeof(Dest) == sizeof(Source),
-                "Source and destination types should have equal sizes.");
+#endif  // defined(__cpp_lib_bit_cast) && __cpp_lib_bit_cast >= 201806L
 
-  Dest dest;
-  memcpy(&dest, &source, sizeof(dest));
-  return dest;
+namespace base_internal {
+
+[[noreturn]] ABSL_ATTRIBUTE_NOINLINE void BadDownCastCrash(
+    const char* source_type, const char* target_type);
+
+template <typename To, typename From>
+inline void ValidateDownCast(From* f) {
+  // Assert only if RTTI is enabled and in debug mode or hardened asserts are
+  // enabled.
+#if defined(ABSL_INTERNAL_HAS_RTTI) && \
+    (!defined(NDEBUG) || (ABSL_OPTION_HARDENED == 1))
+  // Suppress erroneous nonnull comparison warning on older GCC.
+#if defined(__GNUC__) && !defined(__clang__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wnonnull-compare"
+#endif
+  if (ABSL_PREDICT_FALSE(f != nullptr && dynamic_cast<To>(f) == nullptr)) {
+#if defined(__GNUC__) && !defined(__clang__)
+#pragma GCC diagnostic pop
+#endif
+    absl::base_internal::BadDownCastCrash(
+        typeid(*f).name(), typeid(std::remove_pointer_t<To>).name());
+  }
+#else
+  (void)f;  // Denote this as "used" to avoid warning
+#endif
+}
+
+}  // namespace base_internal
+
+// An "upcast", i.e. a conversion from a pointer to an object to a pointer to a
+// base subobject, always succeeds if the base is unambiguous and accessible,
+// and so it's fine to use implicit_cast.
+//
+// A "downcast", i.e. a conversion from a pointer to an object to a pointer
+// to a more-derived object that may contain the original object as a base
+// subobject, cannot safely be done using static_cast, because you do not
+// generally know whether the source object is really the base subobject of
+// a containing, more-derived object of the target type. Thus, when you
+// downcast in a polymorphic type hierarchy, you should use the following
+// function template.
+//
+// This function only returns null when the input is null. In debug mode, we
+// use dynamic_cast to double-check whether the downcast is legal (we die if
+// it's not). In normal mode, we do the efficient static_cast instead. Because
+// the process will die in debug mode, it's important to test to make sure the
+// cast is legal before calling this function!
+//
+// dynamic_cast should be avoided except as allowed by the style guide
+// (https://google.github.io/styleguide/cppguide.html#Run-Time_Type_Information__RTTI_).
+
+template <typename To, typename From>  // use like this: down_cast<T*>(foo);
+[[nodiscard]]
+inline To down_cast(From* f) {  // so we only accept pointers
+  static_assert(std::is_pointer_v<To>, "target type not a pointer");
+  // dynamic_cast allows casting to the same type or a more cv-qualified
+  // version of the same type without them being polymorphic.
+  if constexpr (!std::is_same_v<std::remove_cv_t<std::remove_pointer_t<To>>,
+                                std::remove_cv_t<From>>) {
+    static_assert(std::is_polymorphic_v<From>,
+                  "source type must be polymorphic");
+    static_assert(std::is_polymorphic_v<std::remove_pointer_t<To>>,
+                  "target type must be polymorphic");
+  }
+  static_assert(
+      std::is_convertible_v<std::remove_cv_t<std::remove_pointer_t<To>>*,
+                            std::remove_cv_t<From>*>,
+      "target type not derived from source type");
+
+  absl::base_internal::ValidateDownCast<To>(f);
+
+  return static_cast<To>(f);
+}
+
+// Overload of down_cast for references. Use like this:
+// absl::down_cast<T&>(foo). The code is slightly convoluted because we're still
+// using the pointer form of dynamic cast. (The reference form throws an
+// exception if it fails.)
+//
+// There's no need for a special const overload either for the pointer
+// or the reference form. If you call down_cast with a const T&, the
+// compiler will just bind From to const T.
+template <typename To, typename From>
+[[nodiscard]]
+inline To down_cast(From& f) {
+  static_assert(std::is_lvalue_reference_v<To>, "target type not a reference");
+  // dynamic_cast allows casting to the same type or a more cv-qualified
+  // version of the same type without them being polymorphic.
+  if constexpr (!std::is_same_v<std::remove_cv_t<std::remove_reference_t<To>>,
+                                std::remove_cv_t<From>>) {
+    static_assert(std::is_polymorphic_v<From>,
+                  "source type must be polymorphic");
+    static_assert(std::is_polymorphic_v<std::remove_reference_t<To>>,
+                  "target type must be polymorphic");
+  }
+  static_assert(
+      std::is_convertible_v<std::remove_cv_t<std::remove_reference_t<To>>*,
+                            std::remove_cv_t<From>*>,
+      "target type not derived from source type");
+
+  absl::base_internal::ValidateDownCast<std::remove_reference_t<To>*>(
+      std::addressof(f));
+
+  return static_cast<To>(f);
 }
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_BASE_CASTS_H_
diff --git a/absl/base/casts_test.cc b/absl/base/casts_test.cc
new file mode 100644
index 00000000000..dda3ae6edbe
--- /dev/null
+++ b/absl/base/casts_test.cc
@@ -0,0 +1,148 @@
+// Copyright 2025 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/base/casts.h"
+
+#include <type_traits>
+#include <utility>
+
+#include "gtest/gtest.h"
+#include "absl/base/options.h"
+
+namespace {
+
+struct BaseForImplicitCast {
+  explicit BaseForImplicitCast(int value) : x(value) {}
+  BaseForImplicitCast(const BaseForImplicitCast& other) = delete;
+  BaseForImplicitCast& operator=(const BaseForImplicitCast& other) = delete;
+  int x;
+};
+struct DerivedForImplicitCast : BaseForImplicitCast {
+  explicit DerivedForImplicitCast(int value) : BaseForImplicitCast(value) {}
+};
+
+static_assert(std::is_same_v<decltype(absl::implicit_cast<BaseForImplicitCast&>(
+                                 std::declval<DerivedForImplicitCast&>())),
+                             BaseForImplicitCast&>);
+static_assert(
+    std::is_same_v<decltype(absl::implicit_cast<const BaseForImplicitCast&>(
+                       std::declval<DerivedForImplicitCast>())),
+                   const BaseForImplicitCast&>);
+
+TEST(ImplicitCastTest, LValueReference) {
+  DerivedForImplicitCast derived(5);
+  EXPECT_EQ(&absl::implicit_cast<BaseForImplicitCast&>(derived), &derived);
+  EXPECT_EQ(&absl::implicit_cast<const BaseForImplicitCast&>(derived),
+            &derived);
+}
+
+TEST(ImplicitCastTest, RValueReference) {
+  DerivedForImplicitCast derived(5);
+  BaseForImplicitCast&& base =
+      absl::implicit_cast<BaseForImplicitCast&&>(std::move(derived));
+  EXPECT_EQ(&base, &derived);
+
+  const DerivedForImplicitCast cderived(6);
+  const BaseForImplicitCast&& cbase =
+      absl::implicit_cast<const BaseForImplicitCast&&>(std::move(cderived));
+  EXPECT_EQ(&cbase, &cderived);
+}
+
+class BaseForDownCast {
+ public:
+  virtual ~BaseForDownCast() = default;
+};
+
+class DerivedForDownCast : public BaseForDownCast {};
+class Derived2ForDownCast : public BaseForDownCast {};
+
+TEST(DownCastTest, Pointer) {
+  DerivedForDownCast derived;
+  BaseForDownCast* const base_ptr = &derived;
+
+  // Tests casting a BaseForDownCast* to a DerivedForDownCast*.
+  EXPECT_EQ(&derived, absl::down_cast<DerivedForDownCast*>(base_ptr));
+
+  // Tests casting a const BaseForDownCast* to a const DerivedForDownCast*.
+  const BaseForDownCast* const_base_ptr = base_ptr;
+  EXPECT_EQ(&derived,
+            absl::down_cast<const DerivedForDownCast*>(const_base_ptr));
+
+  // Tests casting a BaseForDownCast* to a const DerivedForDownCast*.
+  EXPECT_EQ(&derived, absl::down_cast<const DerivedForDownCast*>(base_ptr));
+
+  // Tests casting a BaseForDownCast* to a BaseForDownCast* (an identity cast).
+  EXPECT_EQ(base_ptr, absl::down_cast<BaseForDownCast*>(base_ptr));
+
+  // Tests down casting NULL.
+  EXPECT_EQ(nullptr,
+            (absl::down_cast<DerivedForDownCast*, BaseForDownCast>(nullptr)));
+
+  // Tests a bad downcast. We have to disguise the badness just enough
+  // that the compiler doesn't warn about it at compile time.
+  BaseForDownCast* base2 = new BaseForDownCast();
+#if GTEST_HAS_DEATH_TEST && (!defined(NDEBUG) || (ABSL_OPTION_HARDENED == 1))
+  EXPECT_DEATH(static_cast<void>(absl::down_cast<DerivedForDownCast*>(base2)),
+               ".*down cast from .*BaseForDownCast.* to "
+               ".*DerivedForDownCast.* failed.*");
+#endif
+  delete base2;
+}
+
+TEST(DownCastTest, Reference) {
+  DerivedForDownCast derived;
+  BaseForDownCast& base_ref = derived;
+
+  // Tests casting a BaseForDownCast& to a DerivedForDownCast&.
+  // NOLINTNEXTLINE(runtime/casting)
+  EXPECT_EQ(&derived, &absl::down_cast<DerivedForDownCast&>(base_ref));
+
+  // Tests casting a const BaseForDownCast& to a const DerivedForDownCast&.
+  const BaseForDownCast& const_base_ref = base_ref;
+  // NOLINTNEXTLINE(runtime/casting)
+  EXPECT_EQ(&derived,
+            &absl::down_cast<const DerivedForDownCast&>(const_base_ref));
+
+  // Tests casting a BaseForDownCast& to a const DerivedForDownCast&.
+  // NOLINTNEXTLINE(runtime/casting)
+  EXPECT_EQ(&derived, &absl::down_cast<const DerivedForDownCast&>(base_ref));
+
+  // Tests casting a BaseForDownCast& to a BaseForDownCast& (an identity cast).
+  // NOLINTNEXTLINE(runtime/casting)
+  EXPECT_EQ(&base_ref, &absl::down_cast<BaseForDownCast&>(base_ref));
+
+  // Tests a bad downcast. We have to disguise the badness just enough
+  // that the compiler doesn't warn about it at compile time.
+  BaseForDownCast& base2 = *new BaseForDownCast();
+#if GTEST_HAS_DEATH_TEST && (!defined(NDEBUG) || (ABSL_OPTION_HARDENED == 1))
+  EXPECT_DEATH(static_cast<void>(absl::down_cast<DerivedForDownCast&>(base2)),
+               ".*down cast from .*BaseForDownCast.* to "
+               ".*DerivedForDownCast.* failed.*");
+#endif
+  delete &base2;
+}
+
+TEST(DownCastTest, ErrorMessage) {
+  DerivedForDownCast derived;
+  BaseForDownCast& base = derived;
+  (void)base;
+
+#if GTEST_HAS_DEATH_TEST && (!defined(NDEBUG) || (ABSL_OPTION_HARDENED == 1))
+  EXPECT_DEATH(static_cast<void>(absl::down_cast<Derived2ForDownCast&>(base)),
+               ".*down cast from .*DerivedForDownCast.* to "
+               ".*Derived2ForDownCast.* failed.*");
+#endif
+}
+
+}  // namespace
diff --git a/absl/base/config.h b/absl/base/config.h
index 1cb69b0f700..e3e2c96a39c 100644
--- a/absl/base/config.h
+++ b/absl/base/config.h
@@ -56,6 +56,31 @@
 #include <cstddef>
 #endif  // __cplusplus
 
+// ABSL_INTERNAL_CPLUSPLUS_LANG
+//
+// MSVC does not set the value of __cplusplus correctly, but instead uses
+// _MSVC_LANG as a stand-in.
+// https://docs.microsoft.com/en-us/cpp/preprocessor/predefined-macros
+//
+// However, there are reports that MSVC even sets _MSVC_LANG incorrectly at
+// times, for example:
+// https://github.com/microsoft/vscode-cpptools/issues/1770
+// https://reviews.llvm.org/D70996
+//
+// For this reason, this symbol is considered INTERNAL and code outside of
+// Abseil must not use it.
+#if defined(_MSVC_LANG)
+#define ABSL_INTERNAL_CPLUSPLUS_LANG _MSVC_LANG
+#elif defined(__cplusplus)
+#define ABSL_INTERNAL_CPLUSPLUS_LANG __cplusplus
+#endif
+
+#if defined(ABSL_INTERNAL_CPLUSPLUS_LANG) && \
+    ABSL_INTERNAL_CPLUSPLUS_LANG >= 202002L
+// Include library feature test macros.
+#include <version>
+#endif
+
 #if defined(__APPLE__)
 // Included for TARGET_OS_IPHONE, __IPHONE_OS_VERSION_MIN_REQUIRED,
 // __IPHONE_8_0.
@@ -63,8 +88,107 @@
 #include <TargetConditionals.h>
 #endif
 
+#include "absl/base/options.h"
 #include "absl/base/policy_checks.h"
 
+// Abseil long-term support (LTS) releases will define
+// `ABSL_LTS_RELEASE_VERSION` to the integer representing the date string of the
+// LTS release version, and will define `ABSL_LTS_RELEASE_PATCH_LEVEL` to the
+// integer representing the patch-level for that release.
+//
+// For example, for LTS release version "20300401.2", this would give us
+// ABSL_LTS_RELEASE_VERSION == 20300401 && ABSL_LTS_RELEASE_PATCH_LEVEL == 2
+//
+// These symbols will not be defined in non-LTS code.
+//
+// Abseil recommends that clients live-at-head. Therefore, if you are using
+// these symbols to assert a minimum version requirement, we recommend you do it
+// as
+//
+// #if defined(ABSL_LTS_RELEASE_VERSION) && ABSL_LTS_RELEASE_VERSION < 20300401
+// #error Project foo requires Abseil LTS version >= 20300401
+// #endif
+//
+// The `defined(ABSL_LTS_RELEASE_VERSION)` part of the check excludes
+// live-at-head clients from the minimum version assertion.
+//
+// See https://abseil.io/about/releases for more information on Abseil release
+// management.
+//
+// LTS releases can be obtained from
+// https://github.com/abseil/abseil-cpp/releases.
+#undef ABSL_LTS_RELEASE_VERSION
+#undef ABSL_LTS_RELEASE_PATCH_LEVEL
+
+// Helper macro to convert a CPP variable to a string literal.
+#define ABSL_INTERNAL_DO_TOKEN_STR(x) #x
+#define ABSL_INTERNAL_TOKEN_STR(x) ABSL_INTERNAL_DO_TOKEN_STR(x)
+
+// -----------------------------------------------------------------------------
+// Abseil namespace annotations
+// -----------------------------------------------------------------------------
+
+// ABSL_NAMESPACE_BEGIN/ABSL_NAMESPACE_END
+//
+// An annotation placed at the beginning/end of each `namespace absl` scope.
+// This is used to inject an inline namespace.
+//
+// The proper way to write Abseil code in the `absl` namespace is:
+//
+// namespace absl {
+// ABSL_NAMESPACE_BEGIN
+//
+// void Foo();  // absl::Foo().
+//
+// ABSL_NAMESPACE_END
+// }  // namespace absl
+//
+// Users of Abseil should not use these macros, because users of Abseil should
+// not write `namespace absl {` in their own code for any reason.  (Abseil does
+// not support forward declarations of its own types, nor does it support
+// user-provided specialization of Abseil templates.  Code that violates these
+// rules may be broken without warning.)
+#if !defined(ABSL_OPTION_USE_INLINE_NAMESPACE) || \
+    !defined(ABSL_OPTION_INLINE_NAMESPACE_NAME)
+#error options.h is misconfigured.
+#endif
+
+// Check that ABSL_OPTION_INLINE_NAMESPACE_NAME is neither "head" nor ""
+#if defined(__cplusplus) && ABSL_OPTION_USE_INLINE_NAMESPACE == 1
+
+#define ABSL_INTERNAL_INLINE_NAMESPACE_STR \
+  ABSL_INTERNAL_TOKEN_STR(ABSL_OPTION_INLINE_NAMESPACE_NAME)
+
+static_assert(ABSL_INTERNAL_INLINE_NAMESPACE_STR[0] != '\0',
+              "options.h misconfigured: ABSL_OPTION_INLINE_NAMESPACE_NAME must "
+              "not be empty.");
+static_assert(ABSL_INTERNAL_INLINE_NAMESPACE_STR[0] != 'h' ||
+                  ABSL_INTERNAL_INLINE_NAMESPACE_STR[1] != 'e' ||
+                  ABSL_INTERNAL_INLINE_NAMESPACE_STR[2] != 'a' ||
+                  ABSL_INTERNAL_INLINE_NAMESPACE_STR[3] != 'd' ||
+                  ABSL_INTERNAL_INLINE_NAMESPACE_STR[4] != '\0',
+              "options.h misconfigured: ABSL_OPTION_INLINE_NAMESPACE_NAME must "
+              "be changed to a new, unique identifier name.");
+
+#endif
+
+#if ABSL_OPTION_USE_INLINE_NAMESPACE == 0
+#define ABSL_NAMESPACE_BEGIN
+#define ABSL_NAMESPACE_END
+#define ABSL_INTERNAL_C_SYMBOL(x) x
+#elif ABSL_OPTION_USE_INLINE_NAMESPACE == 1
+#define ABSL_NAMESPACE_BEGIN \
+  inline namespace ABSL_OPTION_INLINE_NAMESPACE_NAME {
+#define ABSL_NAMESPACE_END }
+#define ABSL_INTERNAL_C_SYMBOL_HELPER_2(x, v) x##_##v
+#define ABSL_INTERNAL_C_SYMBOL_HELPER_1(x, v) \
+  ABSL_INTERNAL_C_SYMBOL_HELPER_2(x, v)
+#define ABSL_INTERNAL_C_SYMBOL(x) \
+  ABSL_INTERNAL_C_SYMBOL_HELPER_1(x, ABSL_OPTION_INLINE_NAMESPACE_NAME)
+#else
+#error options.h is misconfigured.
+#endif
+
 // -----------------------------------------------------------------------------
 // Compiler Feature Checks
 // -----------------------------------------------------------------------------
@@ -84,109 +208,47 @@
 #define ABSL_HAVE_BUILTIN(x) 0
 #endif
 
-// ABSL_HAVE_TLS is defined to 1 when __thread should be supported.
-// We assume __thread is supported on Linux when compiled with Clang or compiled
-// against libstdc++ with _GLIBCXX_HAVE_TLS defined.
-#ifdef ABSL_HAVE_TLS
-#error ABSL_HAVE_TLS cannot be directly set
-#elif defined(__linux__) && (defined(__clang__) || defined(_GLIBCXX_HAVE_TLS))
-#define ABSL_HAVE_TLS 1
+#ifdef __has_feature
+#define ABSL_HAVE_FEATURE(f) __has_feature(f)
+#else
+#define ABSL_HAVE_FEATURE(f) 0
 #endif
 
-// ABSL_HAVE_STD_IS_TRIVIALLY_DESTRUCTIBLE
-//
-// Checks whether `std::is_trivially_destructible<T>` is supported.
-//
-// Notes: All supported compilers using libc++ support this feature, as does
-// gcc >= 4.8.1 using libstdc++, and Visual Studio.
-#ifdef ABSL_HAVE_STD_IS_TRIVIALLY_DESTRUCTIBLE
-#error ABSL_HAVE_STD_IS_TRIVIALLY_DESTRUCTIBLE cannot be directly set
-#elif defined(_LIBCPP_VERSION) ||                                        \
-    (!defined(__clang__) && defined(__GNUC__) && defined(__GLIBCXX__) && \
-     (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 8))) ||        \
-    defined(_MSC_VER)
-#define ABSL_HAVE_STD_IS_TRIVIALLY_DESTRUCTIBLE 1
+// Portable check for GCC minimum version:
+// https://gcc.gnu.org/onlinedocs/cpp/Common-Predefined-Macros.html
+#if defined(__GNUC__) && defined(__GNUC_MINOR__)
+#define ABSL_INTERNAL_HAVE_MIN_GNUC_VERSION(x, y) \
+  (__GNUC__ > (x) || __GNUC__ == (x) && __GNUC_MINOR__ >= (y))
+#else
+#define ABSL_INTERNAL_HAVE_MIN_GNUC_VERSION(x, y) 0
 #endif
 
-// ABSL_HAVE_STD_IS_TRIVIALLY_CONSTRUCTIBLE
-//
-// Checks whether `std::is_trivially_default_constructible<T>` and
-// `std::is_trivially_copy_constructible<T>` are supported.
-
-// ABSL_HAVE_STD_IS_TRIVIALLY_ASSIGNABLE
-//
-// Checks whether `std::is_trivially_copy_assignable<T>` is supported.
+#if defined(__clang__) && defined(__clang_major__) && defined(__clang_minor__)
+#define ABSL_INTERNAL_HAVE_MIN_CLANG_VERSION(x, y) \
+  (__clang_major__ > (x) || __clang_major__ == (x) && __clang_minor__ >= (y))
+#else
+#define ABSL_INTERNAL_HAVE_MIN_CLANG_VERSION(x, y) 0
+#endif
 
-// Notes: Clang with libc++ supports these features, as does gcc >= 5.1 with
-// either libc++ or libstdc++, and Visual Studio (but not NVCC).
-#if defined(ABSL_HAVE_STD_IS_TRIVIALLY_CONSTRUCTIBLE)
-#error ABSL_HAVE_STD_IS_TRIVIALLY_CONSTRUCTIBLE cannot be directly set
-#elif defined(ABSL_HAVE_STD_IS_TRIVIALLY_ASSIGNABLE)
-#error ABSL_HAVE_STD_IS_TRIVIALLY_ASSIGNABLE cannot directly set
-#elif (defined(__clang__) && defined(_LIBCPP_VERSION)) ||        \
-    (!defined(__clang__) && defined(__GNUC__) &&                 \
-     (__GNUC__ > 7 || (__GNUC__ == 7 && __GNUC_MINOR__ >= 4)) && \
-     (defined(_LIBCPP_VERSION) || defined(__GLIBCXX__))) ||      \
-    (defined(_MSC_VER) && !defined(__NVCC__))
-#define ABSL_HAVE_STD_IS_TRIVIALLY_CONSTRUCTIBLE 1
-#define ABSL_HAVE_STD_IS_TRIVIALLY_ASSIGNABLE 1
+// ABSL_HAVE_TLS is defined to 1 when __thread should be supported.
+// We assume __thread is supported on Linux when compiled with Clang or
+// compiled against libstdc++ with _GLIBCXX_HAVE_TLS defined.
+#ifdef ABSL_HAVE_TLS
+#error ABSL_HAVE_TLS cannot be directly set
+#elif (defined(__linux__)) && (defined(__clang__) || defined(_GLIBCXX_HAVE_TLS))
+#define ABSL_HAVE_TLS 1
+#elif defined(__INTEL_LLVM_COMPILER)
+#define ABSL_HAVE_TLS 1
 #endif
 
 // ABSL_HAVE_THREAD_LOCAL
 //
-// Checks whether C++11's `thread_local` storage duration specifier is
-// supported.
+// Checks whether the `thread_local` storage duration specifier is supported.
 #ifdef ABSL_HAVE_THREAD_LOCAL
 #error ABSL_HAVE_THREAD_LOCAL cannot be directly set
-#elif defined(__APPLE__)
-// Notes:
-// * Xcode's clang did not support `thread_local` until version 8, and
-//   even then not for all iOS < 9.0.
-// * Xcode 9.3 started disallowing `thread_local` for 32-bit iOS simulator
-//   targeting iOS 9.x.
-// * Xcode 10 moves the deployment target check for iOS < 9.0 to link time
-//   making __has_feature unreliable there.
-//
-// Otherwise, `__has_feature` is only supported by Clang so it has be inside
-// `defined(__APPLE__)` check.
-#if __has_feature(cxx_thread_local) && \
-    !(TARGET_OS_IPHONE && __IPHONE_OS_VERSION_MIN_REQUIRED < __IPHONE_9_0)
+#elif !defined(__XTENSA__)
 #define ABSL_HAVE_THREAD_LOCAL 1
 #endif
-#else  // !defined(__APPLE__)
-#define ABSL_HAVE_THREAD_LOCAL 1
-#endif
-
-// There are platforms for which TLS should not be used even though the compiler
-// makes it seem like it's supported (Android NDK < r12b for example).
-// This is primarily because of linker problems and toolchain misconfiguration:
-// Abseil does not intend to support this indefinitely. Currently, the newest
-// toolchain that we intend to support that requires this behavior is the
-// r11 NDK - allowing for a 5 year support window on that means this option
-// is likely to be removed around June of 2021.
-// TLS isn't supported until NDK r12b per
-// https://developer.android.com/ndk/downloads/revision_history.html
-// Since NDK r16, `__NDK_MAJOR__` and `__NDK_MINOR__` are defined in
-// <android/ndk-version.h>. For NDK < r16, users should define these macros,
-// e.g. `-D__NDK_MAJOR__=11 -D__NKD_MINOR__=0` for NDK r11.
-#if defined(__ANDROID__) && defined(__clang__)
-#if __has_include(<android/ndk-version.h>)
-#include <android/ndk-version.h>
-#endif  // __has_include(<android/ndk-version.h>)
-#if defined(__ANDROID__) && defined(__clang__) && defined(__NDK_MAJOR__) && \
-    defined(__NDK_MINOR__) &&                                               \
-    ((__NDK_MAJOR__ < 12) || ((__NDK_MAJOR__ == 12) && (__NDK_MINOR__ < 1)))
-#undef ABSL_HAVE_TLS
-#undef ABSL_HAVE_THREAD_LOCAL
-#endif
-#endif  // defined(__ANDROID__) && defined(__clang__)
-
-// Emscripten doesn't yet support `thread_local` or `__thread`.
-// https://github.com/emscripten-core/emscripten/issues/3502
-#if defined(__EMSCRIPTEN__)
-#undef ABSL_HAVE_TLS
-#undef ABSL_HAVE_THREAD_LOCAL
-#endif  // defined(__EMSCRIPTEN__)
 
 // ABSL_HAVE_INTRINSIC_INT128
 //
@@ -204,8 +266,8 @@
 #ifdef ABSL_HAVE_INTRINSIC_INT128
 #error ABSL_HAVE_INTRINSIC_INT128 cannot be directly set
 #elif defined(__SIZEOF_INT128__)
-#if (defined(__clang__) && !defined(_WIN32)) || \
-    (defined(__CUDACC__) && __CUDACC_VER_MAJOR__ >= 9) ||                \
+#if (defined(__clang__) && !defined(_WIN32)) ||           \
+    (defined(__CUDACC__) && __CUDACC_VER_MAJOR__ >= 9) || \
     (defined(__GNUC__) && !defined(__clang__) && !defined(__CUDACC__))
 #define ABSL_HAVE_INTRINSIC_INT128 1
 #elif defined(__CUDACC__)
@@ -233,19 +295,19 @@
 // For further details, consult the compiler's documentation.
 #ifdef ABSL_HAVE_EXCEPTIONS
 #error ABSL_HAVE_EXCEPTIONS cannot be directly set.
-
+#elif ABSL_INTERNAL_HAVE_MIN_CLANG_VERSION(3, 6)
+// Clang >= 3.6
+#if ABSL_HAVE_FEATURE(cxx_exceptions)
+#define ABSL_HAVE_EXCEPTIONS 1
+#endif  // ABSL_HAVE_FEATURE(cxx_exceptions)
 #elif defined(__clang__)
-// TODO(calabrese)
-// Switch to using __cpp_exceptions when we no longer support versions < 3.6.
-// For details on this check, see:
-//   http://releases.llvm.org/3.6.0/tools/clang/docs/ReleaseNotes.html#the-exceptions-macro
-#if defined(__EXCEPTIONS) && __has_feature(cxx_exceptions)
+// Clang < 3.6
+// http://releases.llvm.org/3.6.0/tools/clang/docs/ReleaseNotes.html#the-exceptions-macro
+#if defined(__EXCEPTIONS) && ABSL_HAVE_FEATURE(cxx_exceptions)
 #define ABSL_HAVE_EXCEPTIONS 1
-#endif  // defined(__EXCEPTIONS) && __has_feature(cxx_exceptions)
-
+#endif  // defined(__EXCEPTIONS) && ABSL_HAVE_FEATURE(cxx_exceptions)
 // Handle remaining special cases and default to exceptions being supported.
-#elif !(defined(__GNUC__) && (__GNUC__ < 5) && !defined(__EXCEPTIONS)) &&    \
-    !(defined(__GNUC__) && (__GNUC__ >= 5) && !defined(__cpp_exceptions)) && \
+#elif !(defined(__GNUC__) && !defined(__cpp_exceptions)) && \
     !(defined(_MSC_VER) && !defined(_CPPUNWIND))
 #define ABSL_HAVE_EXCEPTIONS 1
 #endif
@@ -263,10 +325,10 @@
 //   Darwin (macOS and iOS)            __APPLE__
 //   Akaros (http://akaros.org)        __ros__
 //   Windows                           _WIN32
-//   NaCL                              __native_client__
 //   AsmJS                             __asmjs__
-//   WebAssembly                       __wasm__
+//   WebAssembly (Emscripten)          __EMSCRIPTEN__
 //   Fuchsia                           __Fuchsia__
+//   WebAssembly (WASI)                _WASI_EMULATED_MMAN (implies __wasi__)
 //
 // Note that since Android defines both __ANDROID__ and __linux__, one
 // may probe for either Linux or Android by simply testing for __linux__.
@@ -277,10 +339,13 @@
 // POSIX.1-2001.
 #ifdef ABSL_HAVE_MMAP
 #error ABSL_HAVE_MMAP cannot be directly set
-#elif defined(__linux__) || defined(__APPLE__) || defined(__FreeBSD__) ||   \
-    defined(__ros__) || defined(__native_client__) || defined(__asmjs__) || \
-    defined(__wasm__) || defined(__Fuchsia__) || defined(__sun) || \
-    defined(__ASYLO__)
+#elif defined(__linux__) || defined(__APPLE__) || defined(__FreeBSD__) || \
+    defined(_AIX) || defined(__ros__) || defined(__asmjs__) ||            \
+    defined(__EMSCRIPTEN__) || defined(__Fuchsia__) || defined(__sun) ||  \
+    defined(__myriad2__) || defined(__HAIKU__) || defined(__OpenBSD__) || \
+    defined(__NetBSD__) || defined(__QNX__) || defined(__VXWORKS__) ||    \
+    defined(__hexagon__) || defined(__XTENSA__) ||                        \
+    defined(_WASI_EMULATED_MMAN)
 #define ABSL_HAVE_MMAP 1
 #endif
 
@@ -291,23 +356,34 @@
 #ifdef ABSL_HAVE_PTHREAD_GETSCHEDPARAM
 #error ABSL_HAVE_PTHREAD_GETSCHEDPARAM cannot be directly set
 #elif defined(__linux__) || defined(__APPLE__) || defined(__FreeBSD__) || \
-    defined(__ros__)
+    defined(_AIX) || defined(__ros__) || defined(__OpenBSD__) ||          \
+    defined(__NetBSD__) || defined(__VXWORKS__)
 #define ABSL_HAVE_PTHREAD_GETSCHEDPARAM 1
 #endif
 
+// ABSL_HAVE_SCHED_GETCPU
+//
+// Checks whether sched_getcpu is available.
+#ifdef ABSL_HAVE_SCHED_GETCPU
+#error ABSL_HAVE_SCHED_GETCPU cannot be directly set
+#elif defined(__linux__)
+#define ABSL_HAVE_SCHED_GETCPU 1
+#endif
+
 // ABSL_HAVE_SCHED_YIELD
 //
 // Checks whether the platform implements sched_yield(2) as defined in
 // POSIX.1-2001.
 #ifdef ABSL_HAVE_SCHED_YIELD
 #error ABSL_HAVE_SCHED_YIELD cannot be directly set
-#elif defined(__linux__) || defined(__ros__) || defined(__native_client__)
+#elif defined(__linux__) || defined(__ros__) || defined(__native_client__) || \
+    defined(__VXWORKS__)
 #define ABSL_HAVE_SCHED_YIELD 1
 #endif
 
 // ABSL_HAVE_SEMAPHORE_H
 //
-// Checks whether the platform supports the <semaphore.h> header and sem_open(3)
+// Checks whether the platform supports the <semaphore.h> header and sem_init(3)
 // family of functions as standardized in POSIX.1-2001.
 //
 // Note: While Apple provides <semaphore.h> for both iOS and macOS, it is
@@ -316,7 +392,7 @@
 // platforms.
 #ifdef ABSL_HAVE_SEMAPHORE_H
 #error ABSL_HAVE_SEMAPHORE_H cannot be directly set
-#elif defined(__linux__) || defined(__ros__)
+#elif defined(__linux__) || defined(__ros__) || defined(__VXWORKS__)
 #define ABSL_HAVE_SEMAPHORE_H 1
 #endif
 
@@ -334,11 +410,18 @@
 #define ABSL_HAVE_ALARM 1
 #elif defined(_MSC_VER)
 // feature tests for Microsoft's library
+#elif defined(__MINGW32__)
+// mingw32 doesn't provide alarm(2):
+// https://osdn.net/projects/mingw/scm/git/mingw-org-wsl/blobs/5.2-trunk/mingwrt/include/unistd.h
+// mingw-w64 provides a no-op implementation:
+// https://sourceforge.net/p/mingw-w64/mingw-w64/ci/master/tree/mingw-w64-crt/misc/alarm.c
 #elif defined(__EMSCRIPTEN__)
 // emscripten doesn't support signals
+#elif defined(__wasi__)
+// WASI doesn't support signals
 #elif defined(__Fuchsia__)
 // Signals don't exist on fuchsia.
-#elif defined(__native_client__)
+#elif defined(__hexagon__)
 #else
 // other standard libraries
 #define ABSL_HAVE_ALARM 1
@@ -349,6 +432,9 @@
 //
 // Checks the endianness of the platform.
 //
+// Prefer using `std::endian` in C++20, or `absl::endian` from
+// absl/numeric/bits.h prior to C++20.
+//
 // Notes: uses the built in endian macros provided by GCC (since 4.6) and
 // Clang (since 3.2); see
 // https://gcc.gnu.org/onlinedocs/cpp/Common-Predefined-Macros.html.
@@ -372,103 +458,341 @@
 #error "absl endian detection needs to be set up for your compiler"
 #endif
 
-// macOS 10.13 and iOS 10.11 don't let you use <any>, <optional>, or <variant>
-// even though the headers exist and are publicly noted to work.  See
-// https://github.com/abseil/abseil-cpp/issues/207 and
-// https://developer.apple.com/documentation/xcode_release_notes/xcode_10_release_notes
-// libc++ spells out the availability requirements in the file
-// llvm-project/libcxx/include/__config via the #define
-// _LIBCPP_AVAILABILITY_BAD_OPTIONAL_ACCESS.
-#if defined(__APPLE__) && defined(_LIBCPP_VERSION) && \
-  ((defined(__ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__) && \
-   __ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__ < 101400) || \
-  (defined(__ENVIRONMENT_IPHONE_OS_VERSION_MIN_REQUIRED__) && \
-   __ENVIRONMENT_IPHONE_OS_VERSION_MIN_REQUIRED__ < 120000) || \
-  (defined(__ENVIRONMENT_WATCH_OS_VERSION_MIN_REQUIRED__) && \
-   __ENVIRONMENT_WATCH_OS_VERSION_MIN_REQUIRED__ < 120000) || \
-  (defined(__ENVIRONMENT_TV_OS_VERSION_MIN_REQUIRED__) && \
-   __ENVIRONMENT_TV_OS_VERSION_MIN_REQUIRED__ < 50000))
-#define ABSL_INTERNAL_APPLE_CXX17_TYPES_UNAVAILABLE 1
+// Deprecated macros for polyfill detection.
+#define ABSL_HAVE_STD_ANY 1
+#define ABSL_USES_STD_ANY 1
+#define ABSL_HAVE_STD_OPTIONAL 1
+#define ABSL_USES_STD_OPTIONAL 1
+#define ABSL_HAVE_STD_STRING_VIEW 1
+#define ABSL_USES_STD_STRING_VIEW 1
+#define ABSL_HAVE_STD_VARIANT 1
+#define ABSL_USES_STD_VARIANT 1
+
+// ABSL_HAVE_STD_SOURCE_LOCATION
+//
+// Checks whether C++20 std::source_location is available.
+#ifdef ABSL_HAVE_STD_SOURCE_LOCATION
+#error "ABSL_HAVE_STD_SOURCE_LOCATION cannot be directly set."
+#elif (defined(__cpp_lib_source_location) &&    \
+       __cpp_lib_source_location >= 201907L) || \
+    (defined(ABSL_INTERNAL_CPLUSPLUS_LANG) &&   \
+     ABSL_INTERNAL_CPLUSPLUS_LANG >= 202002L)
+#if __has_include(<source_location>)
+#define ABSL_HAVE_STD_SOURCE_LOCATION 1
+#endif
+#endif
+
+// ABSL_USES_STD_SOURCE_LOCATION
+//
+// Indicates whether absl::SourceLocation is an alias for std::source_location.
+#if !defined(ABSL_OPTION_USE_STD_SOURCE_LOCATION)
+#error options.h is misconfigured.
+#elif ABSL_OPTION_USE_STD_SOURCE_LOCATION == 0 || \
+    (ABSL_OPTION_USE_STD_SOURCE_LOCATION == 2 &&  \
+     !defined(ABSL_HAVE_STD_SOURCE_LOCATION))
+#undef ABSL_USES_STD_SOURCE_LOCATION
+#elif ABSL_OPTION_USE_STD_SOURCE_LOCATION == 1 || \
+    (ABSL_OPTION_USE_STD_SOURCE_LOCATION == 2 &&  \
+     defined(ABSL_HAVE_STD_SOURCE_LOCATION))
+#define ABSL_USES_STD_SOURCE_LOCATION 1
 #else
-#define ABSL_INTERNAL_APPLE_CXX17_TYPES_UNAVAILABLE 0
+#error options.h is misconfigured.
 #endif
 
-// ABSL_HAVE_STD_ANY
+// ABSL_HAVE_STD_ORDERING
 //
-// Checks whether C++17 std::any is available by checking whether <any> exists.
-#ifdef ABSL_HAVE_STD_ANY
-#error "ABSL_HAVE_STD_ANY cannot be directly set."
-#endif
-
-#ifdef __has_include
-#if __has_include(<any>) && __cplusplus >= 201703L && \
-    !ABSL_INTERNAL_APPLE_CXX17_TYPES_UNAVAILABLE
-#define ABSL_HAVE_STD_ANY 1
-#endif
-#endif
-
-// ABSL_HAVE_STD_OPTIONAL
+// Checks whether C++20 std::{partial,weak,strong}_ordering are available.
 //
-// Checks whether C++17 std::optional is available.
-#ifdef ABSL_HAVE_STD_OPTIONAL
-#error "ABSL_HAVE_STD_OPTIONAL cannot be directly set."
-#endif
-
-#ifdef __has_include
-#if __has_include(<optional>) && __cplusplus >= 201703L && \
-    !ABSL_INTERNAL_APPLE_CXX17_TYPES_UNAVAILABLE
-#define ABSL_HAVE_STD_OPTIONAL 1
-#endif
+// __cpp_lib_three_way_comparison is missing on libc++
+// (https://github.com/llvm/llvm-project/issues/73953) so treat it as defined
+// when building in C++20 mode.
+#ifdef ABSL_HAVE_STD_ORDERING
+#error "ABSL_HAVE_STD_ORDERING cannot be directly set."
+#elif (defined(__cpp_lib_three_way_comparison) &&    \
+       __cpp_lib_three_way_comparison >= 201907L) || \
+    (defined(ABSL_INTERNAL_CPLUSPLUS_LANG) &&        \
+     ABSL_INTERNAL_CPLUSPLUS_LANG >= 202002L)
+#define ABSL_HAVE_STD_ORDERING 1
 #endif
 
-// ABSL_HAVE_STD_VARIANT
+// ABSL_USES_STD_ORDERING
 //
-// Checks whether C++17 std::variant is available.
-#ifdef ABSL_HAVE_STD_VARIANT
-#error "ABSL_HAVE_STD_VARIANT cannot be directly set."
+// Indicates whether absl::{partial,weak,strong}_ordering are aliases for the
+// std:: ordering types.
+#if !defined(ABSL_OPTION_USE_STD_ORDERING)
+#error options.h is misconfigured.
+#elif ABSL_OPTION_USE_STD_ORDERING == 0 || \
+    (ABSL_OPTION_USE_STD_ORDERING == 2 && !defined(ABSL_HAVE_STD_ORDERING))
+#undef ABSL_USES_STD_ORDERING
+#elif ABSL_OPTION_USE_STD_ORDERING == 1 || \
+    (ABSL_OPTION_USE_STD_ORDERING == 2 && defined(ABSL_HAVE_STD_ORDERING))
+#define ABSL_USES_STD_ORDERING 1
+#else
+#error options.h is misconfigured.
 #endif
 
-#ifdef __has_include
-#if __has_include(<variant>) && __cplusplus >= 201703L && \
-    !ABSL_INTERNAL_APPLE_CXX17_TYPES_UNAVAILABLE
-#define ABSL_HAVE_STD_VARIANT 1
+// ABSL_INTERNAL_MANGLED_NS
+// ABSL_INTERNAL_MANGLED_BACKREFERENCE
+//
+// Internal macros for building up mangled names in our internal fork of CCTZ.
+// This implementation detail is only needed and provided for the MSVC build.
+//
+// These macros both expand to string literals.  ABSL_INTERNAL_MANGLED_NS is
+// the mangled spelling of the `absl` namespace, and
+// ABSL_INTERNAL_MANGLED_BACKREFERENCE is a back-reference integer representing
+// the proper count to skip past the CCTZ fork namespace names.  (This number
+// is one larger when there is an inline namespace name to skip.)
+#if defined(_MSC_VER)
+#if ABSL_OPTION_USE_INLINE_NAMESPACE == 0
+#define ABSL_INTERNAL_MANGLED_NS "absl"
+#define ABSL_INTERNAL_MANGLED_BACKREFERENCE "5"
+#else
+#define ABSL_INTERNAL_MANGLED_NS \
+  ABSL_INTERNAL_TOKEN_STR(ABSL_OPTION_INLINE_NAMESPACE_NAME) "@absl"
+#define ABSL_INTERNAL_MANGLED_BACKREFERENCE "6"
 #endif
 #endif
 
-// ABSL_HAVE_STD_STRING_VIEW
+// ABSL_DLL
 //
-// Checks whether C++17 std::string_view is available.
-#ifdef ABSL_HAVE_STD_STRING_VIEW
-#error "ABSL_HAVE_STD_STRING_VIEW cannot be directly set."
-#endif
-
-#ifdef __has_include
-#if __has_include(<string_view>) && __cplusplus >= 201703L
-#define ABSL_HAVE_STD_STRING_VIEW 1
+// When building Abseil as a DLL, this macro expands to `__declspec(dllexport)`
+// so we can annotate symbols appropriately as being exported. When used in
+// headers consuming a DLL, this macro expands to `__declspec(dllimport)` so
+// that consumers know the symbol is defined inside the DLL. In all other cases,
+// the macro expands to nothing.
+#if defined(_MSC_VER)
+#if defined(ABSL_BUILD_DLL)
+#define ABSL_DLL __declspec(dllexport)
+#elif defined(ABSL_CONSUME_DLL)
+#define ABSL_DLL __declspec(dllimport)
+#else
+#define ABSL_DLL
 #endif
+#else
+#define ABSL_DLL
+#endif  // defined(_MSC_VER)
+
+#if defined(_MSC_VER)
+#if defined(ABSL_BUILD_TEST_DLL)
+#define ABSL_TEST_DLL __declspec(dllexport)
+#elif defined(ABSL_CONSUME_TEST_DLL)
+#define ABSL_TEST_DLL __declspec(dllimport)
+#else
+#define ABSL_TEST_DLL
 #endif
-
-// For MSVC, `__has_include` is supported in VS 2017 15.3, which is later than
-// the support for <optional>, <any>, <string_view>, <variant>. So we use
-// _MSC_VER to check whether we have VS 2017 RTM (when <optional>, <any>,
-// <string_view>, <variant> is implemented) or higher. Also, `__cplusplus` is
-// not correctly set by MSVC, so we use `_MSVC_LANG` to check the language
-// version.
-// TODO(zhangxy): fix tests before enabling aliasing for `std::any`.
-#if defined(_MSC_VER) && _MSC_VER >= 1910 && \
-    ((defined(_MSVC_LANG) && _MSVC_LANG > 201402) || __cplusplus > 201402)
-// #define ABSL_HAVE_STD_ANY 1
-#define ABSL_HAVE_STD_OPTIONAL 1
-#define ABSL_HAVE_STD_VARIANT 1
-#define ABSL_HAVE_STD_STRING_VIEW 1
+#else
+#define ABSL_TEST_DLL
+#endif  // defined(_MSC_VER)
+
+// ABSL_HAVE_MEMORY_SANITIZER
+//
+// MemorySanitizer (MSan) is a detector of uninitialized reads. It consists of
+// a compiler instrumentation module and a run-time library.
+#ifdef ABSL_HAVE_MEMORY_SANITIZER
+#error "ABSL_HAVE_MEMORY_SANITIZER cannot be directly set."
+#elif !defined(__native_client__) && ABSL_HAVE_FEATURE(memory_sanitizer)
+#define ABSL_HAVE_MEMORY_SANITIZER 1
+#endif
+
+// ABSL_HAVE_THREAD_SANITIZER
+//
+// ThreadSanitizer (TSan) is a fast data race detector.
+#ifdef ABSL_HAVE_THREAD_SANITIZER
+#error "ABSL_HAVE_THREAD_SANITIZER cannot be directly set."
+#elif defined(__SANITIZE_THREAD__)
+#define ABSL_HAVE_THREAD_SANITIZER 1
+#elif ABSL_HAVE_FEATURE(thread_sanitizer)
+#define ABSL_HAVE_THREAD_SANITIZER 1
+#endif
+
+// ABSL_HAVE_ADDRESS_SANITIZER
+//
+// AddressSanitizer (ASan) is a fast memory error detector.
+#ifdef ABSL_HAVE_ADDRESS_SANITIZER
+#error "ABSL_HAVE_ADDRESS_SANITIZER cannot be directly set."
+#elif defined(__SANITIZE_ADDRESS__)
+#define ABSL_HAVE_ADDRESS_SANITIZER 1
+#elif ABSL_HAVE_FEATURE(address_sanitizer)
+#define ABSL_HAVE_ADDRESS_SANITIZER 1
+#endif
+
+// ABSL_HAVE_HWADDRESS_SANITIZER
+//
+// Hardware-Assisted AddressSanitizer (or HWASAN) is even faster than asan
+// memory error detector which can use CPU features like ARM TBI, Intel LAM or
+// AMD UAI.
+#ifdef ABSL_HAVE_HWADDRESS_SANITIZER
+#error "ABSL_HAVE_HWADDRESS_SANITIZER cannot be directly set."
+#elif defined(__SANITIZE_HWADDRESS__)
+#define ABSL_HAVE_HWADDRESS_SANITIZER 1
+#elif ABSL_HAVE_FEATURE(hwaddress_sanitizer)
+#define ABSL_HAVE_HWADDRESS_SANITIZER 1
+#endif
+
+// ABSL_HAVE_DATAFLOW_SANITIZER
+//
+// Dataflow Sanitizer (or DFSAN) is a generalised dynamic data flow analysis.
+#ifdef ABSL_HAVE_DATAFLOW_SANITIZER
+#error "ABSL_HAVE_DATAFLOW_SANITIZER cannot be directly set."
+#elif defined(DATAFLOW_SANITIZER)
+// GCC provides no method for detecting the presence of the standalone
+// DataFlowSanitizer (-fsanitize=dataflow), so GCC users of -fsanitize=dataflow
+// should also use -DDATAFLOW_SANITIZER.
+#define ABSL_HAVE_DATAFLOW_SANITIZER 1
+#elif ABSL_HAVE_FEATURE(dataflow_sanitizer)
+#define ABSL_HAVE_DATAFLOW_SANITIZER 1
+#endif
+
+// ABSL_HAVE_LEAK_SANITIZER
+//
+// LeakSanitizer (or lsan) is a detector of memory leaks.
+// https://clang.llvm.org/docs/LeakSanitizer.html
+// https://github.com/google/sanitizers/wiki/AddressSanitizerLeakSanitizer
+//
+// The macro ABSL_HAVE_LEAK_SANITIZER can be used to detect at compile-time
+// whether the LeakSanitizer is potentially available. However, just because the
+// LeakSanitizer is available does not mean it is active. Use the
+// always-available run-time interface in //absl/debugging/leak_check.h for
+// interacting with LeakSanitizer.
+#ifdef ABSL_HAVE_LEAK_SANITIZER
+#error "ABSL_HAVE_LEAK_SANITIZER cannot be directly set."
+#elif defined(LEAK_SANITIZER)
+// GCC provides no method for detecting the presence of the standalone
+// LeakSanitizer (-fsanitize=leak), so GCC users of -fsanitize=leak should also
+// use -DLEAK_SANITIZER.
+#define ABSL_HAVE_LEAK_SANITIZER 1
+// Clang standalone LeakSanitizer (-fsanitize=leak)
+#elif ABSL_HAVE_FEATURE(leak_sanitizer)
+#define ABSL_HAVE_LEAK_SANITIZER 1
+#elif defined(ABSL_HAVE_ADDRESS_SANITIZER) && !defined(_WIN32)
+// GCC or Clang using the LeakSanitizer integrated into AddressSanitizer.
+#define ABSL_HAVE_LEAK_SANITIZER 1
+#endif
+
+// `ABSL_INTERNAL_HAS_RTTI` determines whether abseil is being compiled with
+// RTTI support.
+#ifdef ABSL_INTERNAL_HAS_RTTI
+#error ABSL_INTERNAL_HAS_RTTI cannot be directly set
+#elif ABSL_HAVE_FEATURE(cxx_rtti)
+#define ABSL_INTERNAL_HAS_RTTI 1
+#elif defined(__GNUC__) && defined(__GXX_RTTI)
+#define ABSL_INTERNAL_HAS_RTTI 1
+#elif defined(_MSC_VER) && defined(_CPPRTTI)
+#define ABSL_INTERNAL_HAS_RTTI 1
+#elif !defined(__GNUC__) && !defined(_MSC_VER)
+// Unknown compiler, default to RTTI
+#define ABSL_INTERNAL_HAS_RTTI 1
+#endif
+
+// `ABSL_INTERNAL_HAS_CXA_DEMANGLE` determines whether `abi::__cxa_demangle` is
+// available.
+#ifdef ABSL_INTERNAL_HAS_CXA_DEMANGLE
+#error ABSL_INTERNAL_HAS_CXA_DEMANGLE cannot be directly set
+#elif defined(OS_ANDROID) && (defined(__i386__) || defined(__x86_64__))
+#undef ABSL_INTERNAL_HAS_CXA_DEMANGLE
+#elif defined(__GNUC__)
+#define ABSL_INTERNAL_HAS_CXA_DEMANGLE 1
+#elif defined(__clang__) && !defined(_MSC_VER)
+#define ABSL_INTERNAL_HAS_CXA_DEMANGLE 1
+#endif
+
+// ABSL_INTERNAL_HAVE_SSE is used for compile-time detection of SSE support.
+// See https://gcc.gnu.org/onlinedocs/gcc/x86-Options.html for an overview of
+// which architectures support the various x86 instruction sets.
+#ifdef ABSL_INTERNAL_HAVE_SSE
+#error ABSL_INTERNAL_HAVE_SSE cannot be directly set
+#elif defined(__SSE__)
+#define ABSL_INTERNAL_HAVE_SSE 1
+#elif (defined(_M_X64) || (defined(_M_IX86_FP) && _M_IX86_FP >= 1)) && \
+    !defined(_M_ARM64EC)
+// MSVC only defines _M_IX86_FP for x86 32-bit code, and _M_IX86_FP >= 1
+// indicates that at least SSE was targeted with the /arch:SSE option.
+// All x86-64 processors support SSE, so support can be assumed.
+// https://docs.microsoft.com/en-us/cpp/preprocessor/predefined-macros
+#define ABSL_INTERNAL_HAVE_SSE 1
+#endif
+
+// ABSL_INTERNAL_HAVE_SSE2 is used for compile-time detection of SSE2 support.
+// See https://gcc.gnu.org/onlinedocs/gcc/x86-Options.html for an overview of
+// which architectures support the various x86 instruction sets.
+#ifdef ABSL_INTERNAL_HAVE_SSE2
+#error ABSL_INTERNAL_HAVE_SSE2 cannot be directly set
+#elif defined(__SSE2__)
+#define ABSL_INTERNAL_HAVE_SSE2 1
+#elif (defined(_M_X64) || (defined(_M_IX86_FP) && _M_IX86_FP >= 2)) && \
+    !defined(_M_ARM64EC)
+// MSVC only defines _M_IX86_FP for x86 32-bit code, and _M_IX86_FP >= 2
+// indicates that at least SSE2 was targeted with the /arch:SSE2 option.
+// All x86-64 processors support SSE2, so support can be assumed.
+// https://docs.microsoft.com/en-us/cpp/preprocessor/predefined-macros
+#define ABSL_INTERNAL_HAVE_SSE2 1
+#endif
+
+// ABSL_INTERNAL_HAVE_SSSE3 is used for compile-time detection of SSSE3 support.
+// See https://gcc.gnu.org/onlinedocs/gcc/x86-Options.html for an overview of
+// which architectures support the various x86 instruction sets.
+//
+// MSVC does not have a mode that targets SSSE3 at compile-time. To use SSSE3
+// with MSVC requires either assuming that the code will only every run on CPUs
+// that support SSSE3, otherwise __cpuid() can be used to detect support at
+// runtime and fallback to a non-SSSE3 implementation when SSSE3 is unsupported
+// by the CPU.
+#ifdef ABSL_INTERNAL_HAVE_SSSE3
+#error ABSL_INTERNAL_HAVE_SSSE3 cannot be directly set
+#elif defined(__SSSE3__)
+#define ABSL_INTERNAL_HAVE_SSSE3 1
+#endif
+
+// ABSL_INTERNAL_HAVE_ARM_NEON is used for compile-time detection of NEON (ARM
+// SIMD).
+//
+// If __CUDA_ARCH__ is defined, then we are compiling CUDA code in device mode.
+// In device mode, NEON intrinsics are not available, regardless of host
+// platform.
+// https://llvm.org/docs/CompileCudaWithLLVM.html#detecting-clang-vs-nvcc-from-code
+#ifdef ABSL_INTERNAL_HAVE_ARM_NEON
+#error ABSL_INTERNAL_HAVE_ARM_NEON cannot be directly set
+#elif defined(__ARM_NEON) && !(defined(__NVCC__) && defined(__CUDACC__))
+#define ABSL_INTERNAL_HAVE_ARM_NEON 1
+#endif
+
+#if ABSL_HAVE_BUILTIN(__builtin_LINE) && ABSL_HAVE_BUILTIN(__builtin_FILE)
+#define ABSL_INTERNAL_HAVE_BUILTIN_LINE_FILE 1
+#elif defined(__GNUC__) && !defined(__clang__) && 5 <= __GNUC__ && __GNUC__ < 10
+#define ABSL_INTERNAL_HAVE_BUILTIN_LINE_FILE 1
+#elif defined(_MSC_VER) && _MSC_VER >= 1926
+#define ABSL_INTERNAL_HAVE_BUILTIN_LINE_FILE 1
+#endif
+
+// ABSL_HAVE_CONSTANT_EVALUATED is used for compile-time detection of
+// constant evaluation support through `absl::is_constant_evaluated`.
+#ifdef ABSL_HAVE_CONSTANT_EVALUATED
+#error ABSL_HAVE_CONSTANT_EVALUATED cannot be directly set
+#endif
+#ifdef __cpp_lib_is_constant_evaluated
+#define ABSL_HAVE_CONSTANT_EVALUATED 1
+#elif ABSL_HAVE_BUILTIN(__builtin_is_constant_evaluated)
+#define ABSL_HAVE_CONSTANT_EVALUATED 1
+#endif
+
+// ABSL_INTERNAL_EMSCRIPTEN_VERSION combines Emscripten's three version macros
+// into an integer that can be compared against.
+#ifdef ABSL_INTERNAL_EMSCRIPTEN_VERSION
+#error ABSL_INTERNAL_EMSCRIPTEN_VERSION cannot be directly set
+#endif
+#ifdef __EMSCRIPTEN__
+#include <emscripten/version.h>
+#ifdef __EMSCRIPTEN_MAJOR__
+#if __EMSCRIPTEN_MINOR__ >= 1000
+#error __EMSCRIPTEN_MINOR__ is too big to fit in ABSL_INTERNAL_EMSCRIPTEN_VERSION
+#endif
+#if __EMSCRIPTEN_TINY__ >= 1000
+#error __EMSCRIPTEN_TINY__ is too big to fit in ABSL_INTERNAL_EMSCRIPTEN_VERSION
+#endif
+#define ABSL_INTERNAL_EMSCRIPTEN_VERSION                              \
+  ((__EMSCRIPTEN_MAJOR__) * 1000000 + (__EMSCRIPTEN_MINOR__) * 1000 + \
+   (__EMSCRIPTEN_TINY__))
 #endif
-
-// In debug mode, MSVC 2017's std::variant throws a EXCEPTION_ACCESS_VIOLATION
-// SEH exception from emplace for variant<SomeStruct> when constructing the
-// struct can throw. This defeats some of variant_test and
-// variant_exception_safety_test.
-#if defined(_MSC_VER) && _MSC_VER >= 1700 && defined(_DEBUG)
-#define ABSL_INTERNAL_MSVC_2017_DBG_MODE
 #endif
 
 #endif  // ABSL_BASE_CONFIG_H_
diff --git a/absl/base/const_init.h b/absl/base/const_init.h
index 17858a7778a..28dee6f3d19 100644
--- a/absl/base/const_init.h
+++ b/absl/base/const_init.h
@@ -22,14 +22,16 @@
 #ifndef ABSL_BASE_CONST_INIT_H_
 #define ABSL_BASE_CONST_INIT_H_
 
+#include "absl/base/config.h"
+
 // In general, objects with static storage duration (such as global variables)
 // can trigger tricky object lifetime situations.  Attempting to access them
 // from the constructors or destructors of other global objects can result in
 // undefined behavior, unless their constructors and destructors are designed
 // with this issue in mind.
 //
-// The normal way to deal with this issue in C++11 is to use constant
-// initialization and trivial destructors.
+// The normal way to deal with this issue is to use constant initialization and
+// trivial destructors.
 //
 // Constant initialization is guaranteed to occur before any other code
 // executes.  Constructors that are declared 'constexpr' are eligible for
@@ -62,11 +64,13 @@
 // or thread_local storage duration.
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 enum ConstInitType {
   kConstInit,
 };
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_BASE_CONST_INIT_H_
diff --git a/absl/base/dynamic_annotations.cc b/absl/base/dynamic_annotations.cc
deleted file mode 100644
index 21e822e53cf..00000000000
--- a/absl/base/dynamic_annotations.cc
+++ /dev/null
@@ -1,129 +0,0 @@
-// Copyright 2017 The Abseil Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-#include <stdlib.h>
-#include <string.h>
-
-#include "absl/base/dynamic_annotations.h"
-
-#ifndef __has_feature
-#define __has_feature(x) 0
-#endif
-
-/* Compiler-based ThreadSanitizer defines
-   DYNAMIC_ANNOTATIONS_EXTERNAL_IMPL = 1
-   and provides its own definitions of the functions. */
-
-#ifndef DYNAMIC_ANNOTATIONS_EXTERNAL_IMPL
-# define DYNAMIC_ANNOTATIONS_EXTERNAL_IMPL 0
-#endif
-
-/* Each function is empty and called (via a macro) only in debug mode.
-   The arguments are captured by dynamic tools at runtime. */
-
-#if DYNAMIC_ANNOTATIONS_EXTERNAL_IMPL == 0 && !defined(__native_client__)
-
-#if __has_feature(memory_sanitizer)
-#include <sanitizer/msan_interface.h>
-#endif
-
-#ifdef __cplusplus
-extern "C" {
-#endif
-
-void AnnotateRWLockCreate(const char *, int,
-                          const volatile void *){}
-void AnnotateRWLockDestroy(const char *, int,
-                           const volatile void *){}
-void AnnotateRWLockAcquired(const char *, int,
-                            const volatile void *, long){}
-void AnnotateRWLockReleased(const char *, int,
-                            const volatile void *, long){}
-void AnnotateBenignRace(const char *, int,
-                        const volatile void *,
-                        const char *){}
-void AnnotateBenignRaceSized(const char *, int,
-                             const volatile void *,
-                             size_t,
-                             const char *) {}
-void AnnotateThreadName(const char *, int,
-                        const char *){}
-void AnnotateIgnoreReadsBegin(const char *, int){}
-void AnnotateIgnoreReadsEnd(const char *, int){}
-void AnnotateIgnoreWritesBegin(const char *, int){}
-void AnnotateIgnoreWritesEnd(const char *, int){}
-void AnnotateEnableRaceDetection(const char *, int, int){}
-void AnnotateMemoryIsInitialized(const char *, int,
-                                 const volatile void *mem, size_t size) {
-#if __has_feature(memory_sanitizer)
-  __msan_unpoison(mem, size);
-#else
-  (void)mem;
-  (void)size;
-#endif
-}
-
-void AnnotateMemoryIsUninitialized(const char *, int,
-                                   const volatile void *mem, size_t size) {
-#if __has_feature(memory_sanitizer)
-  __msan_allocated_memory(mem, size);
-#else
-  (void)mem;
-  (void)size;
-#endif
-}
-
-static int GetRunningOnValgrind(void) {
-#ifdef RUNNING_ON_VALGRIND
-  if (RUNNING_ON_VALGRIND) return 1;
-#endif
-  char *running_on_valgrind_str = getenv("RUNNING_ON_VALGRIND");
-  if (running_on_valgrind_str) {
-    return strcmp(running_on_valgrind_str, "0") != 0;
-  }
-  return 0;
-}
-
-/* See the comments in dynamic_annotations.h */
-int RunningOnValgrind(void) {
-  static volatile int running_on_valgrind = -1;
-  int local_running_on_valgrind = running_on_valgrind;
-  /* C doesn't have thread-safe initialization of statics, and we
-     don't want to depend on pthread_once here, so hack it. */
-  ANNOTATE_BENIGN_RACE(&running_on_valgrind, "safe hack");
-  if (local_running_on_valgrind == -1)
-    running_on_valgrind = local_running_on_valgrind = GetRunningOnValgrind();
-  return local_running_on_valgrind;
-}
-
-/* See the comments in dynamic_annotations.h */
-double ValgrindSlowdown(void) {
-  /* Same initialization hack as in RunningOnValgrind(). */
-  static volatile double slowdown = 0.0;
-  double local_slowdown = slowdown;
-  ANNOTATE_BENIGN_RACE(&slowdown, "safe hack");
-  if (RunningOnValgrind() == 0) {
-    return 1.0;
-  }
-  if (local_slowdown == 0.0) {
-    char *env = getenv("VALGRIND_SLOWDOWN");
-    slowdown = local_slowdown = env ? atof(env) : 50.0;
-  }
-  return local_slowdown;
-}
-
-#ifdef __cplusplus
-}  // extern "C"
-#endif
-#endif  /* DYNAMIC_ANNOTATIONS_EXTERNAL_IMPL == 0 */
diff --git a/absl/base/dynamic_annotations.h b/absl/base/dynamic_annotations.h
index 65a54b447f8..f18b5e0a8fa 100644
--- a/absl/base/dynamic_annotations.h
+++ b/absl/base/dynamic_annotations.h
@@ -1,389 +1,480 @@
-/*
- *  Copyright 2017 The Abseil Authors.
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- *      https://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-/* This file defines dynamic annotations for use with dynamic analysis
-   tool such as valgrind, PIN, etc.
-
-   Dynamic annotation is a source code annotation that affects
-   the generated code (that is, the annotation is not a comment).
-   Each such annotation is attached to a particular
-   instruction and/or to a particular object (address) in the program.
-
-   The annotations that should be used by users are macros in all upper-case
-   (e.g., ANNOTATE_THREAD_NAME).
-
-   Actual implementation of these macros may differ depending on the
-   dynamic analysis tool being used.
-
-   This file supports the following configurations:
-   - Dynamic Annotations enabled (with static thread-safety warnings disabled).
-     In this case, macros expand to functions implemented by Thread Sanitizer,
-     when building with TSan. When not provided an external implementation,
-     dynamic_annotations.cc provides no-op implementations.
-
-   - Static Clang thread-safety warnings enabled.
-     When building with a Clang compiler that supports thread-safety warnings,
-     a subset of annotations can be statically-checked at compile-time. We
-     expand these macros to static-inline functions that can be analyzed for
-     thread-safety, but afterwards elided when building the final binary.
-
-   - All annotations are disabled.
-     If neither Dynamic Annotations nor Clang thread-safety warnings are
-     enabled, then all annotation-macros expand to empty. */
+// Copyright 2017 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// This file defines dynamic annotations for use with dynamic analysis tool
+// such as valgrind, PIN, etc.
+//
+// Dynamic annotation is a source code annotation that affects the generated
+// code (that is, the annotation is not a comment). Each such annotation is
+// attached to a particular instruction and/or to a particular object (address)
+// in the program.
+//
+// The annotations that should be used by users are macros in all upper-case
+// (e.g., ABSL_ANNOTATE_THREAD_NAME).
+//
+// Actual implementation of these macros may differ depending on the dynamic
+// analysis tool being used.
+//
+// This file supports the following configurations:
+// - Dynamic Annotations enabled (with static thread-safety warnings disabled).
+//   In this case, macros expand to functions implemented by Thread Sanitizer,
+//   when building with TSan. When not provided an external implementation,
+//   dynamic_annotations.cc provides no-op implementations.
+//
+// - Static Clang thread-safety warnings enabled.
+//   When building with a Clang compiler that supports thread-safety warnings,
+//   a subset of annotations can be statically-checked at compile-time. We
+//   expand these macros to static-inline functions that can be analyzed for
+//   thread-safety, but afterwards elided when building the final binary.
+//
+// - All annotations are disabled.
+//   If neither Dynamic Annotations nor Clang thread-safety warnings are
+//   enabled, then all annotation-macros expand to empty.
 
 #ifndef ABSL_BASE_DYNAMIC_ANNOTATIONS_H_
 #define ABSL_BASE_DYNAMIC_ANNOTATIONS_H_
 
-#ifndef DYNAMIC_ANNOTATIONS_ENABLED
-# define DYNAMIC_ANNOTATIONS_ENABLED 0
+#include <stddef.h>
+#include <stdint.h>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#ifdef __cplusplus
+#include "absl/base/macros.h"
 #endif
 
-#if DYNAMIC_ANNOTATIONS_ENABLED != 0
-
-  /* -------------------------------------------------------------
-     Annotations that suppress errors.  It is usually better to express the
-     program's synchronization using the other annotations, but these can
-     be used when all else fails. */
-
-  /* Report that we may have a benign race at "pointer", with size
-     "sizeof(*(pointer))". "pointer" must be a non-void* pointer.  Insert at the
-     point where "pointer" has been allocated, preferably close to the point
-     where the race happens.  See also ANNOTATE_BENIGN_RACE_STATIC. */
-  #define ANNOTATE_BENIGN_RACE(pointer, description) \
-    AnnotateBenignRaceSized(__FILE__, __LINE__, pointer, \
-                            sizeof(*(pointer)), description)
-
-  /* Same as ANNOTATE_BENIGN_RACE(address, description), but applies to
-     the memory range [address, address+size). */
-  #define ANNOTATE_BENIGN_RACE_SIZED(address, size, description) \
-    AnnotateBenignRaceSized(__FILE__, __LINE__, address, size, description)
-
-  /* Enable (enable!=0) or disable (enable==0) race detection for all threads.
-     This annotation could be useful if you want to skip expensive race analysis
-     during some period of program execution, e.g. during initialization. */
-  #define ANNOTATE_ENABLE_RACE_DETECTION(enable) \
-    AnnotateEnableRaceDetection(__FILE__, __LINE__, enable)
-
-  /* -------------------------------------------------------------
-     Annotations useful for debugging. */
-
-  /* Report the current thread name to a race detector. */
-  #define ANNOTATE_THREAD_NAME(name) \
-    AnnotateThreadName(__FILE__, __LINE__, name)
-
-  /* -------------------------------------------------------------
-     Annotations useful when implementing locks.  They are not
-     normally needed by modules that merely use locks.
-     The "lock" argument is a pointer to the lock object. */
-
-  /* Report that a lock has been created at address "lock". */
-  #define ANNOTATE_RWLOCK_CREATE(lock) \
-    AnnotateRWLockCreate(__FILE__, __LINE__, lock)
-
-  /* Report that a linker initialized lock has been created at address "lock".
-   */
-#ifdef THREAD_SANITIZER
-  #define ANNOTATE_RWLOCK_CREATE_STATIC(lock) \
-    AnnotateRWLockCreateStatic(__FILE__, __LINE__, lock)
-#else
-  #define ANNOTATE_RWLOCK_CREATE_STATIC(lock) ANNOTATE_RWLOCK_CREATE(lock)
+#ifdef ABSL_HAVE_HWADDRESS_SANITIZER
+#include <sanitizer/hwasan_interface.h>
 #endif
 
-  /* Report that the lock at address "lock" is about to be destroyed. */
-  #define ANNOTATE_RWLOCK_DESTROY(lock) \
-    AnnotateRWLockDestroy(__FILE__, __LINE__, lock)
+// TODO(rogeeff): Remove after the backward compatibility period.
+#include "absl/base/internal/dynamic_annotations.h"  // IWYU pragma: export
 
-  /* Report that the lock at address "lock" has been acquired.
-     is_w=1 for writer lock, is_w=0 for reader lock. */
-  #define ANNOTATE_RWLOCK_ACQUIRED(lock, is_w) \
-    AnnotateRWLockAcquired(__FILE__, __LINE__, lock, is_w)
+// -------------------------------------------------------------------------
+// Decide which features are enabled.
 
-  /* Report that the lock at address "lock" is about to be released. */
-  #define ANNOTATE_RWLOCK_RELEASED(lock, is_w) \
-    AnnotateRWLockReleased(__FILE__, __LINE__, lock, is_w)
+#ifdef ABSL_HAVE_THREAD_SANITIZER
 
-#else  /* DYNAMIC_ANNOTATIONS_ENABLED == 0 */
+#define ABSL_INTERNAL_RACE_ANNOTATIONS_ENABLED 1
+#define ABSL_INTERNAL_READS_ANNOTATIONS_ENABLED 1
+#define ABSL_INTERNAL_WRITES_ANNOTATIONS_ENABLED 1
+#define ABSL_INTERNAL_ANNOTALYSIS_ENABLED 0
+#define ABSL_INTERNAL_READS_WRITES_ANNOTATIONS_ENABLED 1
 
-  #define ANNOTATE_RWLOCK_CREATE(lock) /* empty */
-  #define ANNOTATE_RWLOCK_CREATE_STATIC(lock) /* empty */
-  #define ANNOTATE_RWLOCK_DESTROY(lock) /* empty */
-  #define ANNOTATE_RWLOCK_ACQUIRED(lock, is_w) /* empty */
-  #define ANNOTATE_RWLOCK_RELEASED(lock, is_w) /* empty */
-  #define ANNOTATE_BENIGN_RACE(address, description) /* empty */
-  #define ANNOTATE_BENIGN_RACE_SIZED(address, size, description) /* empty */
-  #define ANNOTATE_THREAD_NAME(name) /* empty */
-  #define ANNOTATE_ENABLE_RACE_DETECTION(enable) /* empty */
+#else
 
-#endif  /* DYNAMIC_ANNOTATIONS_ENABLED */
+#define ABSL_INTERNAL_RACE_ANNOTATIONS_ENABLED 0
+#define ABSL_INTERNAL_READS_ANNOTATIONS_ENABLED 0
+#define ABSL_INTERNAL_WRITES_ANNOTATIONS_ENABLED 0
 
-/* These annotations are also made available to LLVM's Memory Sanitizer */
-#if DYNAMIC_ANNOTATIONS_ENABLED == 1 || defined(MEMORY_SANITIZER)
-  #define ANNOTATE_MEMORY_IS_INITIALIZED(address, size) \
-    AnnotateMemoryIsInitialized(__FILE__, __LINE__, address, size)
+// Clang provides limited support for static thread-safety analysis through a
+// feature called Annotalysis. We configure macro-definitions according to
+// whether Annotalysis support is available. When running in opt-mode, GCC
+// will issue a warning, if these attributes are compiled. Only include them
+// when compiling using Clang.
 
-  #define ANNOTATE_MEMORY_IS_UNINITIALIZED(address, size) \
-    AnnotateMemoryIsUninitialized(__FILE__, __LINE__, address, size)
-#else
-  #define ANNOTATE_MEMORY_IS_INITIALIZED(address, size) /* empty */
-  #define ANNOTATE_MEMORY_IS_UNINITIALIZED(address, size) /* empty */
-#endif  /* DYNAMIC_ANNOTATIONS_ENABLED || MEMORY_SANITIZER */
-
-/* TODO(delesley) -- Replace __CLANG_SUPPORT_DYN_ANNOTATION__ with the
-   appropriate feature ID. */
-#if defined(__clang__) && (!defined(SWIG)) \
-    && defined(__CLANG_SUPPORT_DYN_ANNOTATION__)
-
-  #if DYNAMIC_ANNOTATIONS_ENABLED == 0
-    #define ANNOTALYSIS_ENABLED
-  #endif
-
-  /* When running in opt-mode, GCC will issue a warning, if these attributes are
-     compiled. Only include them when compiling using Clang. */
-  #define ATTRIBUTE_IGNORE_READS_BEGIN \
-      __attribute((exclusive_lock_function("*")))
-  #define ATTRIBUTE_IGNORE_READS_END \
-      __attribute((unlock_function("*")))
+#if defined(__clang__)
+#define ABSL_INTERNAL_ANNOTALYSIS_ENABLED 1
+#if !defined(SWIG)
+#define ABSL_INTERNAL_IGNORE_READS_ATTRIBUTE_ENABLED 1
+#endif
 #else
-  #define ATTRIBUTE_IGNORE_READS_BEGIN  /* empty */
-  #define ATTRIBUTE_IGNORE_READS_END  /* empty */
-#endif  /* defined(__clang__) && ... */
-
-#if (DYNAMIC_ANNOTATIONS_ENABLED != 0) || defined(ANNOTALYSIS_ENABLED)
-  #define ANNOTATIONS_ENABLED
+#define ABSL_INTERNAL_ANNOTALYSIS_ENABLED 0
 #endif
 
-#if (DYNAMIC_ANNOTATIONS_ENABLED != 0)
-
-  /* Request the analysis tool to ignore all reads in the current thread
-     until ANNOTATE_IGNORE_READS_END is called.
-     Useful to ignore intentional racey reads, while still checking
-     other reads and all writes.
-     See also ANNOTATE_UNPROTECTED_READ. */
-  #define ANNOTATE_IGNORE_READS_BEGIN() \
-    AnnotateIgnoreReadsBegin(__FILE__, __LINE__)
+// Read/write annotations are enabled in Annotalysis mode; disabled otherwise.
+#define ABSL_INTERNAL_READS_WRITES_ANNOTATIONS_ENABLED \
+  ABSL_INTERNAL_ANNOTALYSIS_ENABLED
 
-  /* Stop ignoring reads. */
-  #define ANNOTATE_IGNORE_READS_END() \
-    AnnotateIgnoreReadsEnd(__FILE__, __LINE__)
+#endif  // ABSL_HAVE_THREAD_SANITIZER
 
-  /* Similar to ANNOTATE_IGNORE_READS_BEGIN, but ignore writes instead. */
-  #define ANNOTATE_IGNORE_WRITES_BEGIN() \
-    AnnotateIgnoreWritesBegin(__FILE__, __LINE__)
+#ifdef __cplusplus
+#define ABSL_INTERNAL_BEGIN_EXTERN_C extern "C" {
+#define ABSL_INTERNAL_END_EXTERN_C }  // extern "C"
+#define ABSL_INTERNAL_GLOBAL_SCOPED(F) ::F
+#define ABSL_INTERNAL_STATIC_INLINE inline
+#else
+#define ABSL_INTERNAL_BEGIN_EXTERN_C  // empty
+#define ABSL_INTERNAL_END_EXTERN_C    // empty
+#define ABSL_INTERNAL_GLOBAL_SCOPED(F) F
+#define ABSL_INTERNAL_STATIC_INLINE static inline
+#endif
 
-  /* Stop ignoring writes. */
-  #define ANNOTATE_IGNORE_WRITES_END() \
-    AnnotateIgnoreWritesEnd(__FILE__, __LINE__)
+// -------------------------------------------------------------------------
+// Define race annotations.
+
+#if ABSL_INTERNAL_RACE_ANNOTATIONS_ENABLED == 1
+// Some of the symbols used in this section (e.g. AnnotateBenignRaceSized) are
+// defined by the compiler-based sanitizer implementation, not by the Abseil
+// library. Therefore they do not use ABSL_INTERNAL_C_SYMBOL.
+
+// -------------------------------------------------------------
+// Annotations that suppress errors. It is usually better to express the
+// program's synchronization using the other annotations, but these can be used
+// when all else fails.
+
+// Report that we may have a benign race at `pointer`, with size
+// "sizeof(*(pointer))". `pointer` must be a non-void* pointer. Insert at the
+// point where `pointer` has been allocated, preferably close to the point
+// where the race happens. See also ABSL_ANNOTATE_BENIGN_RACE_STATIC.
+#define ABSL_ANNOTATE_BENIGN_RACE(pointer, description) \
+  ABSL_INTERNAL_GLOBAL_SCOPED(AnnotateBenignRaceSized)  \
+  (__FILE__, __LINE__, pointer, sizeof(*(pointer)), description)
+
+// Same as ABSL_ANNOTATE_BENIGN_RACE(`address`, `description`), but applies to
+// the memory range [`address`, `address`+`size`).
+#define ABSL_ANNOTATE_BENIGN_RACE_SIZED(address, size, description) \
+  ABSL_INTERNAL_GLOBAL_SCOPED(AnnotateBenignRaceSized)              \
+  (__FILE__, __LINE__, address, size, description)
+
+// Enable (`enable`!=0) or disable (`enable`==0) race detection for all threads.
+// This annotation could be useful if you want to skip expensive race analysis
+// during some period of program execution, e.g. during initialization.
+#define ABSL_ANNOTATE_ENABLE_RACE_DETECTION(enable)        \
+  ABSL_INTERNAL_GLOBAL_SCOPED(AnnotateEnableRaceDetection) \
+  (__FILE__, __LINE__, enable)
+
+// -------------------------------------------------------------
+// Annotations useful for debugging.
+
+// Report the current thread `name` to a race detector.
+#define ABSL_ANNOTATE_THREAD_NAME(name) \
+  ABSL_INTERNAL_GLOBAL_SCOPED(AnnotateThreadName)(__FILE__, __LINE__, name)
+
+// -------------------------------------------------------------
+// Annotations useful when implementing locks. They are not normally needed by
+// modules that merely use locks. The `lock` argument is a pointer to the lock
+// object.
+
+// Report that a lock has been created at address `lock`.
+#define ABSL_ANNOTATE_RWLOCK_CREATE(lock) \
+  ABSL_INTERNAL_GLOBAL_SCOPED(AnnotateRWLockCreate)(__FILE__, __LINE__, lock)
+
+// Report that a linker initialized lock has been created at address `lock`.
+#ifdef ABSL_HAVE_THREAD_SANITIZER
+#define ABSL_ANNOTATE_RWLOCK_CREATE_STATIC(lock)          \
+  ABSL_INTERNAL_GLOBAL_SCOPED(AnnotateRWLockCreateStatic) \
+  (__FILE__, __LINE__, lock)
+#else
+#define ABSL_ANNOTATE_RWLOCK_CREATE_STATIC(lock) \
+  ABSL_ANNOTATE_RWLOCK_CREATE(lock)
+#endif
 
-/* Clang provides limited support for static thread-safety analysis
-   through a feature called Annotalysis. We configure macro-definitions
-   according to whether Annotalysis support is available. */
-#elif defined(ANNOTALYSIS_ENABLED)
+// Report that the lock at address `lock` is about to be destroyed.
+#define ABSL_ANNOTATE_RWLOCK_DESTROY(lock) \
+  ABSL_INTERNAL_GLOBAL_SCOPED(AnnotateRWLockDestroy)(__FILE__, __LINE__, lock)
+
+// Report that the lock at address `lock` has been acquired.
+// `is_w`=1 for writer lock, `is_w`=0 for reader lock.
+#define ABSL_ANNOTATE_RWLOCK_ACQUIRED(lock, is_w)     \
+  ABSL_INTERNAL_GLOBAL_SCOPED(AnnotateRWLockAcquired) \
+  (__FILE__, __LINE__, lock, is_w)
+
+// Report that the lock at address `lock` is about to be released.
+// `is_w`=1 for writer lock, `is_w`=0 for reader lock.
+#define ABSL_ANNOTATE_RWLOCK_RELEASED(lock, is_w)     \
+  ABSL_INTERNAL_GLOBAL_SCOPED(AnnotateRWLockReleased) \
+  (__FILE__, __LINE__, lock, is_w)
+
+// Apply ABSL_ANNOTATE_BENIGN_RACE_SIZED to a static variable `static_var`.
+#define ABSL_ANNOTATE_BENIGN_RACE_STATIC(static_var, description)      \
+  namespace {                                                          \
+  class static_var##_annotator {                                       \
+   public:                                                             \
+    static_var##_annotator() {                                         \
+      ABSL_ANNOTATE_BENIGN_RACE_SIZED(&static_var, sizeof(static_var), \
+                                      #static_var ": " description);   \
+    }                                                                  \
+  };                                                                   \
+  static static_var##_annotator the##static_var##_annotator;           \
+  }  // namespace
+
+// Function prototypes of annotations provided by the compiler-based sanitizer
+// implementation.
+ABSL_INTERNAL_BEGIN_EXTERN_C
+void AnnotateRWLockCreate(const char* file, int line,
+                          const volatile void* lock);
+void AnnotateRWLockCreateStatic(const char* file, int line,
+                                const volatile void* lock);
+void AnnotateRWLockDestroy(const char* file, int line,
+                           const volatile void* lock);
+void AnnotateRWLockAcquired(const char* file, int line,
+                            const volatile void* lock, long is_w);  // NOLINT
+void AnnotateRWLockReleased(const char* file, int line,
+                            const volatile void* lock, long is_w);  // NOLINT
+void AnnotateBenignRace(const char* file, int line,
+                        const volatile void* address, const char* description);
+void AnnotateBenignRaceSized(const char* file, int line,
+                             const volatile void* address, size_t size,
+                             const char* description);
+void AnnotateThreadName(const char* file, int line, const char* name);
+void AnnotateEnableRaceDetection(const char* file, int line, int enable);
+ABSL_INTERNAL_END_EXTERN_C
+
+#else  // ABSL_INTERNAL_RACE_ANNOTATIONS_ENABLED == 0
+
+#define ABSL_ANNOTATE_RWLOCK_CREATE(lock)                            // empty
+#define ABSL_ANNOTATE_RWLOCK_CREATE_STATIC(lock)                     // empty
+#define ABSL_ANNOTATE_RWLOCK_DESTROY(lock)                           // empty
+#define ABSL_ANNOTATE_RWLOCK_ACQUIRED(lock, is_w)                    // empty
+#define ABSL_ANNOTATE_RWLOCK_RELEASED(lock, is_w)                    // empty
+#define ABSL_ANNOTATE_BENIGN_RACE(address, description)              // empty
+#define ABSL_ANNOTATE_BENIGN_RACE_SIZED(address, size, description)  // empty
+#define ABSL_ANNOTATE_THREAD_NAME(name)                              // empty
+#define ABSL_ANNOTATE_ENABLE_RACE_DETECTION(enable)                  // empty
+#define ABSL_ANNOTATE_BENIGN_RACE_STATIC(static_var, description)    // empty
+
+#endif  // ABSL_INTERNAL_RACE_ANNOTATIONS_ENABLED
+
+// -------------------------------------------------------------------------
+// Define memory annotations.
+
+#ifdef ABSL_HAVE_MEMORY_SANITIZER
+
+#include <sanitizer/msan_interface.h>
+
+#define ABSL_ANNOTATE_MEMORY_IS_INITIALIZED(address, size) \
+  __msan_unpoison(address, size)
+
+#define ABSL_ANNOTATE_MEMORY_IS_UNINITIALIZED(address, size) \
+  __msan_allocated_memory(address, size)
+
+#else  // !defined(ABSL_HAVE_MEMORY_SANITIZER)
+
+#define ABSL_ANNOTATE_MEMORY_IS_INITIALIZED(address, size)    // empty
+#define ABSL_ANNOTATE_MEMORY_IS_UNINITIALIZED(address, size)  // empty
+
+#endif  // ABSL_HAVE_MEMORY_SANITIZER
+
+// -------------------------------------------------------------------------
+// Define IGNORE_READS_BEGIN/_END attributes.
+
+#if defined(ABSL_INTERNAL_IGNORE_READS_ATTRIBUTE_ENABLED)
+
+#define ABSL_INTERNAL_IGNORE_READS_BEGIN_ATTRIBUTE \
+  __attribute((exclusive_lock_function("*")))
+#define ABSL_INTERNAL_IGNORE_READS_END_ATTRIBUTE \
+  __attribute((unlock_function("*")))
+
+#else  // !defined(ABSL_INTERNAL_IGNORE_READS_ATTRIBUTE_ENABLED)
+
+#define ABSL_INTERNAL_IGNORE_READS_BEGIN_ATTRIBUTE  // empty
+#define ABSL_INTERNAL_IGNORE_READS_END_ATTRIBUTE    // empty
+
+#endif  // defined(ABSL_INTERNAL_IGNORE_READS_ATTRIBUTE_ENABLED)
+
+// -------------------------------------------------------------------------
+// Define IGNORE_READS_BEGIN/_END annotations.
+
+#if ABSL_INTERNAL_READS_ANNOTATIONS_ENABLED == 1
+// Some of the symbols used in this section (e.g. AnnotateIgnoreReadsBegin) are
+// defined by the compiler-based implementation, not by the Abseil
+// library. Therefore they do not use ABSL_INTERNAL_C_SYMBOL.
+
+// Request the analysis tool to ignore all reads in the current thread until
+// ABSL_ANNOTATE_IGNORE_READS_END is called. Useful to ignore intentional racey
+// reads, while still checking other reads and all writes.
+// See also ABSL_ANNOTATE_UNPROTECTED_READ.
+#define ABSL_ANNOTATE_IGNORE_READS_BEGIN()              \
+  ABSL_INTERNAL_GLOBAL_SCOPED(AnnotateIgnoreReadsBegin) \
+  (__FILE__, __LINE__)
+
+// Stop ignoring reads.
+#define ABSL_ANNOTATE_IGNORE_READS_END()              \
+  ABSL_INTERNAL_GLOBAL_SCOPED(AnnotateIgnoreReadsEnd) \
+  (__FILE__, __LINE__)
+
+// Function prototypes of annotations provided by the compiler-based sanitizer
+// implementation.
+ABSL_INTERNAL_BEGIN_EXTERN_C
+void AnnotateIgnoreReadsBegin(const char* file, int line)
+    ABSL_INTERNAL_IGNORE_READS_BEGIN_ATTRIBUTE;
+void AnnotateIgnoreReadsEnd(const char* file,
+                            int line) ABSL_INTERNAL_IGNORE_READS_END_ATTRIBUTE;
+ABSL_INTERNAL_END_EXTERN_C
+
+#elif defined(ABSL_INTERNAL_ANNOTALYSIS_ENABLED)
+
+// When Annotalysis is enabled without Dynamic Annotations, the use of
+// static-inline functions allows the annotations to be read at compile-time,
+// while still letting the compiler elide the functions from the final build.
+//
+// TODO(delesley) -- The exclusive lock here ignores writes as well, but
+// allows IGNORE_READS_AND_WRITES to work properly.
 
-  #define ANNOTATE_IGNORE_READS_BEGIN() \
-    StaticAnnotateIgnoreReadsBegin(__FILE__, __LINE__)
+#define ABSL_ANNOTATE_IGNORE_READS_BEGIN()                          \
+  ABSL_INTERNAL_GLOBAL_SCOPED(                                      \
+      ABSL_INTERNAL_C_SYMBOL(AbslInternalAnnotateIgnoreReadsBegin)) \
+  ()
 
-  #define ANNOTATE_IGNORE_READS_END() \
-    StaticAnnotateIgnoreReadsEnd(__FILE__, __LINE__)
+#define ABSL_ANNOTATE_IGNORE_READS_END()                          \
+  ABSL_INTERNAL_GLOBAL_SCOPED(                                    \
+      ABSL_INTERNAL_C_SYMBOL(AbslInternalAnnotateIgnoreReadsEnd)) \
+  ()
 
-  #define ANNOTATE_IGNORE_WRITES_BEGIN() \
-    StaticAnnotateIgnoreWritesBegin(__FILE__, __LINE__)
+ABSL_INTERNAL_STATIC_INLINE void ABSL_INTERNAL_C_SYMBOL(
+    AbslInternalAnnotateIgnoreReadsBegin)()
+    ABSL_INTERNAL_IGNORE_READS_BEGIN_ATTRIBUTE {}
 
-  #define ANNOTATE_IGNORE_WRITES_END() \
-    StaticAnnotateIgnoreWritesEnd(__FILE__, __LINE__)
+ABSL_INTERNAL_STATIC_INLINE void ABSL_INTERNAL_C_SYMBOL(
+    AbslInternalAnnotateIgnoreReadsEnd)()
+    ABSL_INTERNAL_IGNORE_READS_END_ATTRIBUTE {}
 
 #else
-  #define ANNOTATE_IGNORE_READS_BEGIN()  /* empty */
-  #define ANNOTATE_IGNORE_READS_END()  /* empty */
-  #define ANNOTATE_IGNORE_WRITES_BEGIN()  /* empty */
-  #define ANNOTATE_IGNORE_WRITES_END()  /* empty */
+
+#define ABSL_ANNOTATE_IGNORE_READS_BEGIN()  // empty
+#define ABSL_ANNOTATE_IGNORE_READS_END()    // empty
+
 #endif
 
-/* Implement the ANNOTATE_IGNORE_READS_AND_WRITES_* annotations using the more
-   primitive annotations defined above. */
-#if defined(ANNOTATIONS_ENABLED)
+// -------------------------------------------------------------------------
+// Define IGNORE_WRITES_BEGIN/_END annotations.
+
+#if ABSL_INTERNAL_WRITES_ANNOTATIONS_ENABLED == 1
+
+// Similar to ABSL_ANNOTATE_IGNORE_READS_BEGIN, but ignore writes instead.
+#define ABSL_ANNOTATE_IGNORE_WRITES_BEGIN() \
+  ABSL_INTERNAL_GLOBAL_SCOPED(AnnotateIgnoreWritesBegin)(__FILE__, __LINE__)
 
-  /* Start ignoring all memory accesses (both reads and writes). */
-  #define ANNOTATE_IGNORE_READS_AND_WRITES_BEGIN() \
-    do {                                           \
-      ANNOTATE_IGNORE_READS_BEGIN();               \
-      ANNOTATE_IGNORE_WRITES_BEGIN();              \
-    }while (0)
+// Stop ignoring writes.
+#define ABSL_ANNOTATE_IGNORE_WRITES_END() \
+  ABSL_INTERNAL_GLOBAL_SCOPED(AnnotateIgnoreWritesEnd)(__FILE__, __LINE__)
 
-  /* Stop ignoring both reads and writes. */
-  #define ANNOTATE_IGNORE_READS_AND_WRITES_END()   \
-    do {                                           \
-      ANNOTATE_IGNORE_WRITES_END();                \
-      ANNOTATE_IGNORE_READS_END();                 \
-    }while (0)
+// Function prototypes of annotations provided by the compiler-based sanitizer
+// implementation.
+ABSL_INTERNAL_BEGIN_EXTERN_C
+void AnnotateIgnoreWritesBegin(const char* file, int line);
+void AnnotateIgnoreWritesEnd(const char* file, int line);
+ABSL_INTERNAL_END_EXTERN_C
 
 #else
-  #define ANNOTATE_IGNORE_READS_AND_WRITES_BEGIN()  /* empty */
-  #define ANNOTATE_IGNORE_READS_AND_WRITES_END()  /* empty */
-#endif
 
-/* Use the macros above rather than using these functions directly. */
-#include <stddef.h>
-#ifdef __cplusplus
-extern "C" {
-#endif
-void AnnotateRWLockCreate(const char *file, int line,
-                          const volatile void *lock);
-void AnnotateRWLockCreateStatic(const char *file, int line,
-                          const volatile void *lock);
-void AnnotateRWLockDestroy(const char *file, int line,
-                           const volatile void *lock);
-void AnnotateRWLockAcquired(const char *file, int line,
-                            const volatile void *lock, long is_w);  /* NOLINT */
-void AnnotateRWLockReleased(const char *file, int line,
-                            const volatile void *lock, long is_w);  /* NOLINT */
-void AnnotateBenignRace(const char *file, int line,
-                        const volatile void *address,
-                        const char *description);
-void AnnotateBenignRaceSized(const char *file, int line,
-                        const volatile void *address,
-                        size_t size,
-                        const char *description);
-void AnnotateThreadName(const char *file, int line,
-                        const char *name);
-void AnnotateEnableRaceDetection(const char *file, int line, int enable);
-void AnnotateMemoryIsInitialized(const char *file, int line,
-                                 const volatile void *mem, size_t size);
-void AnnotateMemoryIsUninitialized(const char *file, int line,
-                                   const volatile void *mem, size_t size);
-
-/* Annotations expand to these functions, when Dynamic Annotations are enabled.
-   These functions are either implemented as no-op calls, if no Sanitizer is
-   attached, or provided with externally-linked implementations by a library
-   like ThreadSanitizer. */
-void AnnotateIgnoreReadsBegin(const char *file, int line)
-    ATTRIBUTE_IGNORE_READS_BEGIN;
-void AnnotateIgnoreReadsEnd(const char *file, int line)
-    ATTRIBUTE_IGNORE_READS_END;
-void AnnotateIgnoreWritesBegin(const char *file, int line);
-void AnnotateIgnoreWritesEnd(const char *file, int line);
-
-#if defined(ANNOTALYSIS_ENABLED)
-/* When Annotalysis is enabled without Dynamic Annotations, the use of
-   static-inline functions allows the annotations to be read at compile-time,
-   while still letting the compiler elide the functions from the final build.
-
-   TODO(delesley) -- The exclusive lock here ignores writes as well, but
-   allows IGNORE_READS_AND_WRITES to work properly. */
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Wunused-function"
-static inline void StaticAnnotateIgnoreReadsBegin(const char *file, int line)
-    ATTRIBUTE_IGNORE_READS_BEGIN { (void)file; (void)line; }
-static inline void StaticAnnotateIgnoreReadsEnd(const char *file, int line)
-    ATTRIBUTE_IGNORE_READS_END { (void)file; (void)line; }
-static inline void StaticAnnotateIgnoreWritesBegin(
-    const char *file, int line) { (void)file; (void)line; }
-static inline void StaticAnnotateIgnoreWritesEnd(
-    const char *file, int line) { (void)file; (void)line; }
-#pragma GCC diagnostic pop
+#define ABSL_ANNOTATE_IGNORE_WRITES_BEGIN()  // empty
+#define ABSL_ANNOTATE_IGNORE_WRITES_END()    // empty
+
 #endif
 
-/* Return non-zero value if running under valgrind.
-
-  If "valgrind.h" is included into dynamic_annotations.cc,
-  the regular valgrind mechanism will be used.
-  See http://valgrind.org/docs/manual/manual-core-adv.html about
-  RUNNING_ON_VALGRIND and other valgrind "client requests".
-  The file "valgrind.h" may be obtained by doing
-     svn co svn://svn.valgrind.org/valgrind/trunk/include
-
-  If for some reason you can't use "valgrind.h" or want to fake valgrind,
-  there are two ways to make this function return non-zero:
-    - Use environment variable: export RUNNING_ON_VALGRIND=1
-    - Make your tool intercept the function RunningOnValgrind() and
-      change its return value.
- */
-int RunningOnValgrind(void);
-
-/* ValgrindSlowdown returns:
-    * 1.0, if (RunningOnValgrind() == 0)
-    * 50.0, if (RunningOnValgrind() != 0 && getenv("VALGRIND_SLOWDOWN") == NULL)
-    * atof(getenv("VALGRIND_SLOWDOWN")) otherwise
-   This function can be used to scale timeout values:
-   EXAMPLE:
-   for (;;) {
-     DoExpensiveBackgroundTask();
-     SleepForSeconds(5 * ValgrindSlowdown());
-   }
- */
-double ValgrindSlowdown(void);
+// -------------------------------------------------------------------------
+// Define the ABSL_ANNOTATE_IGNORE_READS_AND_WRITES_* annotations using the more
+// primitive annotations defined above.
+//
+//     Instead of doing
+//        ABSL_ANNOTATE_IGNORE_READS_BEGIN();
+//        ... = x;
+//        ABSL_ANNOTATE_IGNORE_READS_END();
+//     one can use
+//        ... = ABSL_ANNOTATE_UNPROTECTED_READ(x);
+
+#if defined(ABSL_INTERNAL_READS_WRITES_ANNOTATIONS_ENABLED)
+
+// Start ignoring all memory accesses (both reads and writes).
+#define ABSL_ANNOTATE_IGNORE_READS_AND_WRITES_BEGIN() \
+  do {                                                \
+    ABSL_ANNOTATE_IGNORE_READS_BEGIN();               \
+    ABSL_ANNOTATE_IGNORE_WRITES_BEGIN();              \
+  } while (0)
+
+// Stop ignoring both reads and writes.
+#define ABSL_ANNOTATE_IGNORE_READS_AND_WRITES_END() \
+  do {                                              \
+    ABSL_ANNOTATE_IGNORE_WRITES_END();              \
+    ABSL_ANNOTATE_IGNORE_READS_END();               \
+  } while (0)
 
 #ifdef __cplusplus
-}
-#endif
+// ABSL_ANNOTATE_UNPROTECTED_READ is the preferred way to annotate racey reads.
+#define ABSL_ANNOTATE_UNPROTECTED_READ(x) \
+  absl::base_internal::AnnotateUnprotectedRead(x)
 
-/* ANNOTATE_UNPROTECTED_READ is the preferred way to annotate racey reads.
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace base_internal {
 
-     Instead of doing
-        ANNOTATE_IGNORE_READS_BEGIN();
-        ... = x;
-        ANNOTATE_IGNORE_READS_END();
-     one can use
-        ... = ANNOTATE_UNPROTECTED_READ(x); */
-#if defined(__cplusplus) && defined(ANNOTATIONS_ENABLED)
 template <typename T>
-inline T ANNOTATE_UNPROTECTED_READ(const volatile T &x) { /* NOLINT */
-  ANNOTATE_IGNORE_READS_BEGIN();
+inline T AnnotateUnprotectedRead(const volatile T& x) {  // NOLINT
+  ABSL_ANNOTATE_IGNORE_READS_BEGIN();
   T res = x;
-  ANNOTATE_IGNORE_READS_END();
+  ABSL_ANNOTATE_IGNORE_READS_END();
   return res;
-  }
+}
+
+}  // namespace base_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+#endif
+
 #else
-  #define ANNOTATE_UNPROTECTED_READ(x) (x)
+
+#define ABSL_ANNOTATE_IGNORE_READS_AND_WRITES_BEGIN()  // empty
+#define ABSL_ANNOTATE_IGNORE_READS_AND_WRITES_END()    // empty
+#define ABSL_ANNOTATE_UNPROTECTED_READ(x) (x)
+
 #endif
 
-#if DYNAMIC_ANNOTATIONS_ENABLED != 0 && defined(__cplusplus)
-  /* Apply ANNOTATE_BENIGN_RACE_SIZED to a static variable. */
-  #define ANNOTATE_BENIGN_RACE_STATIC(static_var, description)        \
-    namespace {                                                       \
-      class static_var ## _annotator {                                \
-       public:                                                        \
-        static_var ## _annotator() {                                  \
-          ANNOTATE_BENIGN_RACE_SIZED(&static_var,                     \
-                                      sizeof(static_var),             \
-            # static_var ": " description);                           \
-        }                                                             \
-      };                                                              \
-      static static_var ## _annotator the ## static_var ## _annotator;\
-    }  // namespace
-#else /* DYNAMIC_ANNOTATIONS_ENABLED == 0 */
-  #define ANNOTATE_BENIGN_RACE_STATIC(static_var, description)  /* empty */
-#endif /* DYNAMIC_ANNOTATIONS_ENABLED */
-
-#ifdef ADDRESS_SANITIZER
-/* Describe the current state of a contiguous container such as e.g.
- * std::vector or std::string. For more details see
- * sanitizer/common_interface_defs.h, which is provided by the compiler. */
+// -------------------------------------------------------------------------
+// Address sanitizer annotations
+
+#ifdef ABSL_HAVE_ADDRESS_SANITIZER
+// Describe the current state of a contiguous container such as e.g.
+// std::vector or std::string. For more details see
+// sanitizer/common_interface_defs.h, which is provided by the compiler.
 #include <sanitizer/common_interface_defs.h>
-#define ANNOTATE_CONTIGUOUS_CONTAINER(beg, end, old_mid, new_mid) \
+
+#define ABSL_ANNOTATE_CONTIGUOUS_CONTAINER(beg, end, old_mid, new_mid) \
   __sanitizer_annotate_contiguous_container(beg, end, old_mid, new_mid)
-#define ADDRESS_SANITIZER_REDZONE(name)         \
-  struct { char x[8] __attribute__ ((aligned (8))); } name
+#define ABSL_ADDRESS_SANITIZER_REDZONE(name) \
+  struct {                                   \
+    alignas(8) char x[8];                    \
+  } name
+
 #else
-#define ANNOTATE_CONTIGUOUS_CONTAINER(beg, end, old_mid, new_mid)
-#define ADDRESS_SANITIZER_REDZONE(name) static_assert(true, "")
-#endif  // ADDRESS_SANITIZER
 
-/* Undefine the macros intended only in this file. */
-#undef ANNOTALYSIS_ENABLED
-#undef ANNOTATIONS_ENABLED
-#undef ATTRIBUTE_IGNORE_READS_BEGIN
-#undef ATTRIBUTE_IGNORE_READS_END
+#define ABSL_ANNOTATE_CONTIGUOUS_CONTAINER(beg, end, old_mid, new_mid)  // empty
+#define ABSL_ADDRESS_SANITIZER_REDZONE(name) static_assert(true, "")
+
+#endif  // ABSL_HAVE_ADDRESS_SANITIZER
+
+// -------------------------------------------------------------------------
+// HWAddress sanitizer annotations
+
+#ifdef __cplusplus
+namespace absl {
+#ifdef ABSL_HAVE_HWADDRESS_SANITIZER
+// Under HWASAN changes the tag of the pointer.
+template <typename T>
+T* HwasanTagPointer(T* ptr, uintptr_t tag) {
+  return reinterpret_cast<T*>(__hwasan_tag_pointer(ptr, tag));
+}
+#else
+template <typename T>
+T* HwasanTagPointer(T* ptr, uintptr_t) {
+  return ptr;
+}
+#endif
+}  // namespace absl
+#endif
+
+// -------------------------------------------------------------------------
+// Undefine the macros intended only for this file.
+
+#undef ABSL_INTERNAL_RACE_ANNOTATIONS_ENABLED
+#undef ABSL_INTERNAL_READS_ANNOTATIONS_ENABLED
+#undef ABSL_INTERNAL_WRITES_ANNOTATIONS_ENABLED
+#undef ABSL_INTERNAL_ANNOTALYSIS_ENABLED
+#undef ABSL_INTERNAL_READS_WRITES_ANNOTATIONS_ENABLED
+#undef ABSL_INTERNAL_BEGIN_EXTERN_C
+#undef ABSL_INTERNAL_END_EXTERN_C
+#undef ABSL_INTERNAL_STATIC_INLINE
 
-#endif  /* ABSL_BASE_DYNAMIC_ANNOTATIONS_H_ */
+#endif  // ABSL_BASE_DYNAMIC_ANNOTATIONS_H_
diff --git a/absl/base/exception_safety_testing_test.cc b/absl/base/exception_safety_testing_test.cc
index 575b535d6b5..7cf8c36fe35 100644
--- a/absl/base/exception_safety_testing_test.cc
+++ b/absl/base/exception_safety_testing_test.cc
@@ -148,7 +148,7 @@ TEST(ThrowingValueTest, ThrowingBitwiseOps) {
   ThrowingValue<> bomb1, bomb2;
 
   TestOp([&bomb1]() { ~bomb1; });
-  TestOp([&]() { bomb1& bomb2; });
+  TestOp([&]() { bomb1 & bomb2; });
   TestOp([&]() { bomb1 | bomb2; });
   TestOp([&]() { bomb1 ^ bomb2; });
 }
@@ -236,8 +236,8 @@ void TestAllocatingOp(const F& f) {
 TEST(ThrowingValueTest, ThrowingAllocatingOps) {
   // make_unique calls unqualified operator new, so these exercise the
   // ThrowingValue overloads.
-  TestAllocatingOp([]() { return absl::make_unique<ThrowingValue<>>(1); });
-  TestAllocatingOp([]() { return absl::make_unique<ThrowingValue<>[]>(2); });
+  TestAllocatingOp([]() { return std::make_unique<ThrowingValue<>>(1); });
+  TestAllocatingOp([]() { return std::make_unique<ThrowingValue<>[]>(2); });
 }
 
 TEST(ThrowingValueTest, NonThrowingMoveCtor) {
@@ -328,19 +328,20 @@ TEST(ThrowingValueTest, NonThrowingDelete) {
   UnsetCountdown();
 }
 
-using Storage =
-    absl::aligned_storage_t<sizeof(ThrowingValue<>), alignof(ThrowingValue<>)>;
-
 TEST(ThrowingValueTest, NonThrowingPlacementDelete) {
   constexpr int kArrayLen = 2;
   // We intentionally create extra space to store the tag allocated by placement
   // new[].
-  constexpr int kStorageLen = 4;
+  constexpr size_t kExtraSpaceLen = sizeof(size_t) * 2;
 
-  Storage buf;
-  Storage array_buf[kStorageLen];
+  alignas(ThrowingValue<>) unsigned char buf[sizeof(ThrowingValue<>)];
+  alignas(ThrowingValue<>) unsigned char
+      array_buf[kExtraSpaceLen + sizeof(ThrowingValue<>[kArrayLen])];
   auto* placed = new (&buf) ThrowingValue<>(1);
   auto placed_array = new (&array_buf) ThrowingValue<>[kArrayLen];
+  auto* placed_array_end = reinterpret_cast<unsigned char*>(placed_array) +
+                           sizeof(ThrowingValue<>[kArrayLen]);
+  EXPECT_LE(placed_array_end, array_buf + sizeof(array_buf));
 
   SetCountdown();
   ExpectNoThrow([placed, &buf]() {
@@ -517,7 +518,7 @@ struct NullaryTestValidator : public std::false_type {};
 template <typename TesterInstance>
 struct NullaryTestValidator<
     TesterInstance,
-    absl::void_t<decltype(std::declval<TesterInstance>().Test())>>
+    std::void_t<decltype(std::declval<TesterInstance>().Test())>>
     : public std::true_type {};
 
 template <typename TesterInstance>
@@ -533,7 +534,7 @@ struct UnaryTestValidator : public std::false_type {};
 template <typename TesterInstance>
 struct UnaryTestValidator<
     TesterInstance,
-    absl::void_t<decltype(std::declval<TesterInstance>().Test(DummyOp))>>
+    std::void_t<decltype(std::declval<TesterInstance>().Test(DummyOp))>>
     : public std::true_type {};
 
 template <typename TesterInstance>
@@ -545,7 +546,7 @@ TEST(ExceptionSafetyTesterTest, IncompleteTypesAreNotTestable) {
   using T = exceptions_internal::UninitializedT;
   auto op = [](T* t) {};
   auto inv = [](T*) { return testing::AssertionSuccess(); };
-  auto fac = []() { return absl::make_unique<T>(); };
+  auto fac = []() { return std::make_unique<T>(); };
 
   // Test that providing operation and inveriants still does not allow for the
   // the invocation of .Test() and .Test(op) because it lacks a factory
@@ -574,7 +575,7 @@ TEST(ExceptionSafetyTesterTest, IncompleteTypesAreNotTestable) {
 struct ExampleStruct {};
 
 std::unique_ptr<ExampleStruct> ExampleFunctionFactory() {
-  return absl::make_unique<ExampleStruct>();
+  return std::make_unique<ExampleStruct>();
 }
 
 void ExampleFunctionOperation(ExampleStruct*) {}
@@ -703,7 +704,6 @@ struct BasicGuaranteeWithExtraContracts : public NonNegative {
 
   static constexpr int kExceptionSentinel = 9999;
 };
-constexpr int BasicGuaranteeWithExtraContracts::kExceptionSentinel;
 
 TEST(ExceptionCheckTest, BasicGuaranteeWithExtraContracts) {
   auto tester_with_val =
@@ -792,7 +792,7 @@ struct NonCopyable : public NonNegative {
 };
 
 TEST(ExceptionCheckTest, NonCopyable) {
-  auto factory = []() { return absl::make_unique<NonCopyable>(); };
+  auto factory = []() { return std::make_unique<NonCopyable>(); };
   EXPECT_TRUE(tester.WithFactory(factory).Test());
   EXPECT_TRUE(strong_tester.WithFactory(factory).Test());
 }
@@ -869,14 +869,14 @@ TEST(ExceptionCheckTest, Exhaustiveness) {
 }
 
 struct LeaksIfCtorThrows : private exceptions_internal::TrackedObject {
-  LeaksIfCtorThrows() : TrackedObject(ABSL_PRETTY_FUNCTION) {
+  LeaksIfCtorThrows() : TrackedObject(ABSL_INTERNAL_PRETTY_FUNCTION) {
     ++counter;
     ThrowingValue<> v;
     static_cast<void>(v);
     --counter;
   }
   LeaksIfCtorThrows(const LeaksIfCtorThrows&) noexcept
-      : TrackedObject(ABSL_PRETTY_FUNCTION) {}
+      : TrackedObject(ABSL_INTERNAL_PRETTY_FUNCTION) {}
   static int counter;
 };
 int LeaksIfCtorThrows::counter = 0;
@@ -888,7 +888,7 @@ TEST(ExceptionCheckTest, TestLeakyCtor) {
 }
 
 struct Tracked : private exceptions_internal::TrackedObject {
-  Tracked() : TrackedObject(ABSL_PRETTY_FUNCTION) {}
+  Tracked() : TrackedObject(ABSL_INTERNAL_PRETTY_FUNCTION) {}
 };
 
 TEST(ConstructorTrackerTest, CreatedBefore) {
@@ -902,12 +902,12 @@ TEST(ConstructorTrackerTest, CreatedAfter) {
 }
 
 TEST(ConstructorTrackerTest, NotDestroyedAfter) {
-  absl::aligned_storage_t<sizeof(Tracked), alignof(Tracked)> storage;
+  alignas(Tracked) unsigned char storage[sizeof(Tracked)];
   EXPECT_NONFATAL_FAILURE(
       {
         exceptions_internal::ConstructorTracker ct(
             exceptions_internal::countdown);
-        new (&storage) Tracked;
+        new (&storage) Tracked();
       },
       "not destroyed");
 }
@@ -924,11 +924,11 @@ TEST(ConstructorTrackerTest, DestroyedTwice) {
 
 TEST(ConstructorTrackerTest, ConstructedTwice) {
   exceptions_internal::ConstructorTracker ct(exceptions_internal::countdown);
-  absl::aligned_storage_t<sizeof(Tracked), alignof(Tracked)> storage;
+  alignas(Tracked) unsigned char storage[sizeof(Tracked)];
   EXPECT_NONFATAL_FAILURE(
       {
-        new (&storage) Tracked;
-        new (&storage) Tracked;
+        new (&storage) Tracked();
+        new (&storage) Tracked();
         reinterpret_cast<Tracked*>(&storage)->~Tracked();
       },
       "re-constructed");
@@ -936,8 +936,8 @@ TEST(ConstructorTrackerTest, ConstructedTwice) {
 
 TEST(ThrowingValueTraitsTest, RelationalOperators) {
   ThrowingValue<> a, b;
-  EXPECT_TRUE((std::is_convertible<decltype(a == b), bool>::value));
-  EXPECT_TRUE((std::is_convertible<decltype(a != b), bool>::value));
+  EXPECT_TRUE((std::is_convertible_v<decltype(a == b), bool>));
+  EXPECT_TRUE((std::is_convertible_v<decltype(a != b), bool>));
   EXPECT_TRUE((std::is_convertible<decltype(a < b), bool>::value));
   EXPECT_TRUE((std::is_convertible<decltype(a <= b), bool>::value));
   EXPECT_TRUE((std::is_convertible<decltype(a > b), bool>::value));
@@ -945,10 +945,10 @@ TEST(ThrowingValueTraitsTest, RelationalOperators) {
 }
 
 TEST(ThrowingAllocatorTraitsTest, Assignablility) {
-  EXPECT_TRUE(absl::is_move_assignable<ThrowingAllocator<int>>::value);
-  EXPECT_TRUE(absl::is_copy_assignable<ThrowingAllocator<int>>::value);
-  EXPECT_TRUE(std::is_nothrow_move_assignable<ThrowingAllocator<int>>::value);
-  EXPECT_TRUE(std::is_nothrow_copy_assignable<ThrowingAllocator<int>>::value);
+  EXPECT_TRUE(std::is_move_assignable_v<ThrowingAllocator<int>>);
+  EXPECT_TRUE(std::is_copy_assignable_v<ThrowingAllocator<int>>);
+  EXPECT_TRUE(std::is_nothrow_move_assignable_v<ThrowingAllocator<int>>);
+  EXPECT_TRUE(std::is_nothrow_copy_assignable_v<ThrowingAllocator<int>>);
 }
 
 }  // namespace
diff --git a/absl/base/fast_type_id.h b/absl/base/fast_type_id.h
new file mode 100644
index 00000000000..6e4c573af98
--- /dev/null
+++ b/absl/base/fast_type_id.h
@@ -0,0 +1,73 @@
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+#ifndef ABSL_BASE_FAST_TYPE_ID_H_
+#define ABSL_BASE_FAST_TYPE_ID_H_
+
+#include <utility>
+
+#include "absl/base/config.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+namespace base_internal {
+template <typename Type>
+struct FastTypeTag {
+  static constexpr char kDummyVar = 0;
+};
+}  // namespace base_internal
+
+// The type returned by `absl::FastTypeId<T>()`.
+class FastTypeIdType final {
+ public:
+  // Creates a value that does not correspond to any type. This value is
+  // distinct from any value returned by `FastTypeId<T>()`.
+  constexpr FastTypeIdType() = default;
+
+  template <typename H>
+  friend H AbslHashValue(H h, FastTypeIdType x) {
+    return H::combine(std::move(h), x.ptr_);
+  }
+
+  friend constexpr bool operator==(FastTypeIdType a, FastTypeIdType b) {
+    return a.ptr_ == b.ptr_;
+  }
+  friend constexpr bool operator!=(FastTypeIdType a, FastTypeIdType b) {
+    return a.ptr_ != b.ptr_;
+  }
+
+ private:
+  // `FastTypeId<T>()` is the generator method for FastTypeIdType values.
+  template <typename T>
+  friend constexpr FastTypeIdType FastTypeId();
+
+  explicit constexpr FastTypeIdType(const void* ptr) : ptr_(ptr) {}
+
+  const void* ptr_ = nullptr;
+};
+
+// `absl::FastTypeId<Type>()` evaluates at compile-time to a unique id for the
+// passed-in type. These are meant to be good match for keys into maps or
+// straight up comparisons.
+template <typename Type>
+constexpr FastTypeIdType FastTypeId() {
+  return FastTypeIdType(&base_internal::FastTypeTag<Type>::kDummyVar);
+}
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_BASE_FAST_TYPE_ID_H_
diff --git a/absl/base/fast_type_id_test.cc b/absl/base/fast_type_id_test.cc
new file mode 100644
index 00000000000..a957d70b99b
--- /dev/null
+++ b/absl/base/fast_type_id_test.cc
@@ -0,0 +1,131 @@
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/base/fast_type_id.h"
+
+#include <cstddef>
+#include <cstdint>
+#include <iterator>
+#include <map>
+#include <vector>
+
+#include "gtest/gtest.h"
+#include "absl/base/macros.h"
+
+namespace {
+
+// Ensure that absl::bit_cast works between FastTypeIdType and void*.
+static_assert(sizeof(absl::FastTypeIdType) == sizeof(void*));
+
+// NOLINTBEGIN(runtime/int)
+#define PRIM_TYPES(A)   \
+  A(bool)               \
+  A(short)              \
+  A(unsigned short)     \
+  A(int)                \
+  A(unsigned int)       \
+  A(long)               \
+  A(unsigned long)      \
+  A(long long)          \
+  A(unsigned long long) \
+  A(float)              \
+  A(double)             \
+  A(long double)
+// NOLINTEND(runtime/int)
+
+TEST(FastTypeIdTest, PrimitiveTypes) {
+  // clang-format off
+  constexpr absl::FastTypeIdType kTypeIds[] = {
+#define A(T) absl::FastTypeId<T>(),
+    PRIM_TYPES(A)
+#undef A
+#define A(T) absl::FastTypeId<const T>(),
+    PRIM_TYPES(A)
+#undef A
+#define A(T) absl::FastTypeId<volatile T>(),
+    PRIM_TYPES(A)
+#undef A
+#define A(T) absl::FastTypeId<const volatile T>(),
+    PRIM_TYPES(A)
+#undef A
+  };
+  // clang-format on
+
+  for (size_t i = 0; i < std::size(kTypeIds); ++i) {
+    EXPECT_EQ(kTypeIds[i], kTypeIds[i]);
+    for (size_t j = 0; j < i; ++j) {
+      EXPECT_NE(kTypeIds[i], kTypeIds[j]);
+    }
+  }
+}
+
+#define FIXED_WIDTH_TYPES(A) \
+  A(int8_t)                  \
+  A(uint8_t)                 \
+  A(int16_t)                 \
+  A(uint16_t)                \
+  A(int32_t)                 \
+  A(uint32_t)                \
+  A(int64_t)                 \
+  A(uint64_t)
+
+TEST(FastTypeIdTest, FixedWidthTypes) {
+  // clang-format off
+  constexpr absl::FastTypeIdType kTypeIds[] = {
+#define A(T) absl::FastTypeId<T>(),
+    FIXED_WIDTH_TYPES(A)
+#undef A
+#define A(T) absl::FastTypeId<const T>(),
+    FIXED_WIDTH_TYPES(A)
+#undef A
+#define A(T) absl::FastTypeId<volatile T>(),
+    FIXED_WIDTH_TYPES(A)
+#undef A
+#define A(T) absl::FastTypeId<const volatile T>(),
+    FIXED_WIDTH_TYPES(A)
+#undef A
+  };
+  // clang-format on
+
+  for (size_t i = 0; i < std::size(kTypeIds); ++i) {
+    EXPECT_EQ(kTypeIds[i], kTypeIds[i]);
+    for (size_t j = 0; j < i; ++j) {
+      EXPECT_NE(kTypeIds[i], kTypeIds[j]);
+    }
+  }
+}
+
+TEST(FastTypeIdTest, AliasTypes) {
+  using int_alias = int;
+  EXPECT_EQ(absl::FastTypeId<int_alias>(), absl::FastTypeId<int>());
+}
+
+TEST(FastTypeIdTest, TemplateSpecializations) {
+  EXPECT_NE(absl::FastTypeId<std::vector<int>>(),
+            absl::FastTypeId<std::vector<long>>());  // NOLINT(runtime/int)
+
+  EXPECT_NE((absl::FastTypeId<std::map<int, float>>()),
+            (absl::FastTypeId<std::map<int, double>>()));
+}
+
+struct Base {};
+struct Derived : Base {};
+struct PDerived : private Base {};
+
+TEST(FastTypeIdTest, Inheritance) {
+  EXPECT_NE(absl::FastTypeId<Base>(), absl::FastTypeId<Derived>());
+  EXPECT_NE(absl::FastTypeId<Base>(), absl::FastTypeId<PDerived>());
+}
+
+}  // namespace
diff --git a/absl/base/inline_variable_test.cc b/absl/base/inline_variable_test.cc
deleted file mode 100644
index 471f70633ca..00000000000
--- a/absl/base/inline_variable_test.cc
+++ /dev/null
@@ -1,62 +0,0 @@
-// Copyright 2017 The Abseil Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-#include <type_traits>
-
-#include "absl/base/internal/inline_variable.h"
-#include "absl/base/internal/inline_variable_testing.h"
-
-#include "gtest/gtest.h"
-
-namespace absl {
-namespace inline_variable_testing_internal {
-namespace {
-
-TEST(InlineVariableTest, Constexpr) {
-  static_assert(inline_variable_foo.value == 5, "");
-  static_assert(other_inline_variable_foo.value == 5, "");
-  static_assert(inline_variable_int == 5, "");
-  static_assert(other_inline_variable_int == 5, "");
-}
-
-TEST(InlineVariableTest, DefaultConstructedIdentityEquality) {
-  EXPECT_EQ(get_foo_a().value, 5);
-  EXPECT_EQ(get_foo_b().value, 5);
-  EXPECT_EQ(&get_foo_a(), &get_foo_b());
-}
-
-TEST(InlineVariableTest, DefaultConstructedIdentityInequality) {
-  EXPECT_NE(&inline_variable_foo, &other_inline_variable_foo);
-}
-
-TEST(InlineVariableTest, InitializedIdentityEquality) {
-  EXPECT_EQ(get_int_a(), 5);
-  EXPECT_EQ(get_int_b(), 5);
-  EXPECT_EQ(&get_int_a(), &get_int_b());
-}
-
-TEST(InlineVariableTest, InitializedIdentityInequality) {
-  EXPECT_NE(&inline_variable_int, &other_inline_variable_int);
-}
-
-TEST(InlineVariableTest, FunPtrType) {
-  static_assert(
-      std::is_same<void(*)(),
-                   std::decay<decltype(inline_variable_fun_ptr)>::type>::value,
-      "");
-}
-
-}  // namespace
-}  // namespace inline_variable_testing_internal
-}  // namespace absl
diff --git a/absl/base/internal/atomic_hook.h b/absl/base/internal/atomic_hook.h
index 803e9059761..ae21cd7fe50 100644
--- a/absl/base/internal/atomic_hook.h
+++ b/absl/base/internal/atomic_hook.h
@@ -11,7 +11,6 @@
 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 // See the License for the specific language governing permissions and
 // limitations under the License.
-//
 
 #ifndef ABSL_BASE_INTERNAL_ATOMIC_HOOK_H_
 #define ABSL_BASE_INTERNAL_ATOMIC_HOOK_H_
@@ -21,28 +20,51 @@
 #include <cstdint>
 #include <utility>
 
-#ifdef _MSC_FULL_VER
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+
+#if defined(_MSC_VER) && !defined(__clang__)
+#define ABSL_HAVE_WORKING_CONSTEXPR_STATIC_INIT 0
+#else
+#define ABSL_HAVE_WORKING_CONSTEXPR_STATIC_INIT 1
+#endif
+
+#if defined(_MSC_VER)
 #define ABSL_HAVE_WORKING_ATOMIC_POINTER 0
 #else
 #define ABSL_HAVE_WORKING_ATOMIC_POINTER 1
 #endif
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 
 template <typename T>
 class AtomicHook;
 
-// AtomicHook is a helper class, templatized on a raw function pointer type, for
-// implementing Abseil customization hooks.  It is a callable object that
-// dispatches to the registered hook.
+// To workaround AtomicHook not being constant-initializable on some platforms,
+// prefer to annotate instances with `ABSL_INTERNAL_ATOMIC_HOOK_ATTRIBUTES`
+// instead of `ABSL_CONST_INIT`.
+#if ABSL_HAVE_WORKING_CONSTEXPR_STATIC_INIT
+#define ABSL_INTERNAL_ATOMIC_HOOK_ATTRIBUTES ABSL_CONST_INIT
+#else
+#define ABSL_INTERNAL_ATOMIC_HOOK_ATTRIBUTES
+#endif
+
+// `AtomicHook` is a helper class, templatized on a raw function pointer type,
+// for implementing Abseil customization hooks.  It is a callable object that
+// dispatches to the registered hook.  Objects of type `AtomicHook` must have
+// static or thread storage duration.
 //
 // A default constructed object performs a no-op (and returns a default
 // constructed object) if no hook has been registered.
 //
-// Hooks can be pre-registered via constant initialization, for example,
-// ABSL_CONST_INIT static AtomicHook<void(*)()> my_hook(DefaultAction);
-// and then changed at runtime via a call to Store().
+// Hooks can be pre-registered via constant initialization, for example:
+//
+// ABSL_INTERNAL_ATOMIC_HOOK_ATTRIBUTES static AtomicHook<void(*)()>
+//     my_hook(DefaultAction);
+//
+// and then changed at runtime via a call to `Store()`.
 //
 // Reads and writes guarantee memory_order_acquire/memory_order_release
 // semantics.
@@ -57,12 +79,23 @@ class AtomicHook<ReturnType (*)(Args...)> {
 
   // Constructs an object that by default dispatches to/returns the
   // pre-registered default_fn when no hook has been registered at runtime.
-#if ABSL_HAVE_WORKING_ATOMIC_POINTER
+#if ABSL_HAVE_WORKING_ATOMIC_POINTER && ABSL_HAVE_WORKING_CONSTEXPR_STATIC_INIT
   explicit constexpr AtomicHook(FnPtr default_fn)
       : hook_(default_fn), default_fn_(default_fn) {}
-#else
+#elif ABSL_HAVE_WORKING_CONSTEXPR_STATIC_INIT
   explicit constexpr AtomicHook(FnPtr default_fn)
       : hook_(kUninitialized), default_fn_(default_fn) {}
+#else
+  // As of January 2020, on all known versions of MSVC this constructor runs in
+  // the global constructor sequence.  If `Store()` is called by a dynamic
+  // initializer, we want to preserve the value, even if this constructor runs
+  // after the call to `Store()`.  If not, `hook_` will be
+  // zero-initialized by the linker and we have no need to set it.
+  // https://developercommunity.visualstudio.com/content/problem/336946/class-with-constexpr-constructor-not-using-static.html
+  explicit constexpr AtomicHook(FnPtr default_fn)
+      : /* hook_(deliberately omitted), */ default_fn_(default_fn) {
+    static_assert(kUninitialized == 0, "here we rely on zero-initialization");
+  }
 #endif
 
   // Stores the provided function pointer as the value for this hook.
@@ -158,8 +191,10 @@ class AtomicHook<ReturnType (*)(Args...)> {
 };
 
 #undef ABSL_HAVE_WORKING_ATOMIC_POINTER
+#undef ABSL_HAVE_WORKING_CONSTEXPR_STATIC_INIT
 
 }  // namespace base_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_BASE_INTERNAL_ATOMIC_HOOK_H_
diff --git a/absl/base/internal/atomic_hook_test.cc b/absl/base/internal/atomic_hook_test.cc
index ecc80406b00..e577a8fd930 100644
--- a/absl/base/internal/atomic_hook_test.cc
+++ b/absl/base/internal/atomic_hook_test.cc
@@ -14,16 +14,22 @@
 
 #include "absl/base/internal/atomic_hook.h"
 
+#include "gmock/gmock.h"
 #include "gtest/gtest.h"
 #include "absl/base/attributes.h"
+#include "absl/base/internal/atomic_hook_test_helper.h"
 
 namespace {
 
+using ::testing::Eq;
+
 int value = 0;
 void TestHook(int x) { value = x; }
 
 TEST(AtomicHookTest, NoDefaultFunction) {
-  ABSL_CONST_INIT static absl::base_internal::AtomicHook<void(*)(int)> hook;
+  ABSL_INTERNAL_ATOMIC_HOOK_ATTRIBUTES static absl::base_internal::AtomicHook<
+      void (*)(int)>
+      hook;
   value = 0;
 
   // Test the default DummyFunction.
@@ -49,8 +55,9 @@ TEST(AtomicHookTest, NoDefaultFunction) {
 
 TEST(AtomicHookTest, WithDefaultFunction) {
   // Set the default value to TestHook at compile-time.
-  ABSL_CONST_INIT static absl::base_internal::AtomicHook<void (*)(int)> hook(
-      TestHook);
+  ABSL_INTERNAL_ATOMIC_HOOK_ATTRIBUTES static absl::base_internal::AtomicHook<
+      void (*)(int)>
+      hook(TestHook);
   value = 0;
 
   // Test the default value is TestHook.
@@ -67,4 +74,24 @@ TEST(AtomicHookTest, WithDefaultFunction) {
   EXPECT_EQ(value, 2);
 }
 
+ABSL_CONST_INIT int override_func_calls = 0;
+void OverrideFunc() { override_func_calls++; }
+static struct OverrideInstaller {
+  OverrideInstaller() { absl::atomic_hook_internal::func.Store(OverrideFunc); }
+} override_installer;
+
+TEST(AtomicHookTest, DynamicInitFromAnotherTU) {
+  // MSVC 14.2 doesn't do constexpr static init correctly; in particular it
+  // tends to sequence static init (i.e. defaults) of `AtomicHook` objects
+  // after their dynamic init (i.e. overrides), overwriting whatever value was
+  // written during dynamic init.  This regression test validates the fix.
+  // https://developercommunity.visualstudio.com/content/problem/336946/class-with-constexpr-constructor-not-using-static.html
+  EXPECT_THAT(absl::atomic_hook_internal::default_func_calls, Eq(0));
+  EXPECT_THAT(override_func_calls, Eq(0));
+  absl::atomic_hook_internal::func();
+  EXPECT_THAT(absl::atomic_hook_internal::default_func_calls, Eq(0));
+  EXPECT_THAT(override_func_calls, Eq(1));
+  EXPECT_THAT(absl::atomic_hook_internal::func.Load(), Eq(OverrideFunc));
+}
+
 }  // namespace
diff --git a/absl/base/internal/identity.h b/absl/base/internal/atomic_hook_test_helper.cc
similarity index 57%
rename from absl/base/internal/identity.h
rename to absl/base/internal/atomic_hook_test_helper.cc
index 086447c6039..537d47cd2d2 100644
--- a/absl/base/internal/identity.h
+++ b/absl/base/internal/atomic_hook_test_helper.cc
@@ -11,23 +11,22 @@
 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 // See the License for the specific language governing permissions and
 // limitations under the License.
-//
 
-#ifndef ABSL_BASE_INTERNAL_IDENTITY_H_
-#define ABSL_BASE_INTERNAL_IDENTITY_H_
+#include "absl/base/internal/atomic_hook_test_helper.h"
 
-namespace absl {
-namespace internal {
+#include "absl/base/attributes.h"
+#include "absl/base/internal/atomic_hook.h"
 
-template <typename T>
-struct identity {
-  typedef T type;
-};
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace atomic_hook_internal {
 
-template <typename T>
-using identity_t = typename identity<T>::type;
+ABSL_INTERNAL_ATOMIC_HOOK_ATTRIBUTES absl::base_internal::AtomicHook<VoidF>
+    func(DefaultFunc);
+ABSL_CONST_INIT int default_func_calls = 0;
+void DefaultFunc() { default_func_calls++; }
+void RegisterFunc(VoidF f) { func.Store(f); }
 
-}  // namespace internal
+}  // namespace atomic_hook_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
-
-#endif  // ABSL_BASE_INTERNAL_IDENTITY_H_
diff --git a/absl/base/internal/atomic_hook_test_helper.h b/absl/base/internal/atomic_hook_test_helper.h
new file mode 100644
index 00000000000..c72015ef966
--- /dev/null
+++ b/absl/base/internal/atomic_hook_test_helper.h
@@ -0,0 +1,34 @@
+// Copyright 2017 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_BASE_INTERNAL_ATOMIC_HOOK_TEST_HELPER_H_
+#define ABSL_BASE_INTERNAL_ATOMIC_HOOK_TEST_HELPER_H_
+
+#include "absl/base/internal/atomic_hook.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace atomic_hook_internal {
+
+using VoidF = void (*)();
+extern absl::base_internal::AtomicHook<VoidF> func;
+extern int default_func_calls;
+void DefaultFunc();
+void RegisterFunc(VoidF func);
+
+}  // namespace atomic_hook_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_BASE_INTERNAL_ATOMIC_HOOK_TEST_HELPER_H_
diff --git a/absl/base/internal/bits.h b/absl/base/internal/bits.h
deleted file mode 100644
index b0780f2d121..00000000000
--- a/absl/base/internal/bits.h
+++ /dev/null
@@ -1,193 +0,0 @@
-// Copyright 2018 The Abseil Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-#ifndef ABSL_BASE_INTERNAL_BITS_H_
-#define ABSL_BASE_INTERNAL_BITS_H_
-
-// This file contains bitwise ops which are implementation details of various
-// absl libraries.
-
-#include <cstdint>
-
-// Clang on Windows has __builtin_clzll; otherwise we need to use the
-// windows intrinsic functions.
-#if defined(_MSC_VER)
-#include <intrin.h>
-#if defined(_M_X64)
-#pragma intrinsic(_BitScanReverse64)
-#pragma intrinsic(_BitScanForward64)
-#endif
-#pragma intrinsic(_BitScanReverse)
-#pragma intrinsic(_BitScanForward)
-#endif
-
-#include "absl/base/attributes.h"
-
-#if defined(_MSC_VER)
-// We can achieve something similar to attribute((always_inline)) with MSVC by
-// using the __forceinline keyword, however this is not perfect. MSVC is
-// much less aggressive about inlining, and even with the __forceinline keyword.
-#define ABSL_BASE_INTERNAL_FORCEINLINE __forceinline
-#else
-// Use default attribute inline.
-#define ABSL_BASE_INTERNAL_FORCEINLINE inline ABSL_ATTRIBUTE_ALWAYS_INLINE
-#endif
-
-
-namespace absl {
-namespace base_internal {
-
-ABSL_BASE_INTERNAL_FORCEINLINE int CountLeadingZeros64Slow(uint64_t n) {
-  int zeroes = 60;
-  if (n >> 32) zeroes -= 32, n >>= 32;
-  if (n >> 16) zeroes -= 16, n >>= 16;
-  if (n >> 8) zeroes -= 8, n >>= 8;
-  if (n >> 4) zeroes -= 4, n >>= 4;
-  return "\4\3\2\2\1\1\1\1\0\0\0\0\0\0\0"[n] + zeroes;
-}
-
-ABSL_BASE_INTERNAL_FORCEINLINE int CountLeadingZeros64(uint64_t n) {
-#if defined(_MSC_VER) && defined(_M_X64)
-  // MSVC does not have __buitin_clzll. Use _BitScanReverse64.
-  unsigned long result = 0;  // NOLINT(runtime/int)
-  if (_BitScanReverse64(&result, n)) {
-    return 63 - result;
-  }
-  return 64;
-#elif defined(_MSC_VER)
-  // MSVC does not have __buitin_clzll. Compose two calls to _BitScanReverse
-  unsigned long result = 0;  // NOLINT(runtime/int)
-  if ((n >> 32) && _BitScanReverse(&result, n >> 32)) {
-    return 31 - result;
-  }
-  if (_BitScanReverse(&result, n)) {
-    return 63 - result;
-  }
-  return 64;
-#elif defined(__GNUC__)
-  // Use __builtin_clzll, which uses the following instructions:
-  //  x86: bsr
-  //  ARM64: clz
-  //  PPC: cntlzd
-  static_assert(sizeof(unsigned long long) == sizeof(n),  // NOLINT(runtime/int)
-                "__builtin_clzll does not take 64-bit arg");
-
-  // Handle 0 as a special case because __builtin_clzll(0) is undefined.
-  if (n == 0) {
-    return 64;
-  }
-  return __builtin_clzll(n);
-#else
-  return CountLeadingZeros64Slow(n);
-#endif
-}
-
-ABSL_BASE_INTERNAL_FORCEINLINE int CountLeadingZeros32Slow(uint64_t n) {
-  int zeroes = 28;
-  if (n >> 16) zeroes -= 16, n >>= 16;
-  if (n >> 8) zeroes -= 8, n >>= 8;
-  if (n >> 4) zeroes -= 4, n >>= 4;
-  return "\4\3\2\2\1\1\1\1\0\0\0\0\0\0\0"[n] + zeroes;
-}
-
-ABSL_BASE_INTERNAL_FORCEINLINE int CountLeadingZeros32(uint32_t n) {
-#if defined(_MSC_VER)
-  unsigned long result = 0;  // NOLINT(runtime/int)
-  if (_BitScanReverse(&result, n)) {
-    return 31 - result;
-  }
-  return 32;
-#elif defined(__GNUC__)
-  // Use __builtin_clz, which uses the following instructions:
-  //  x86: bsr
-  //  ARM64: clz
-  //  PPC: cntlzd
-  static_assert(sizeof(int) == sizeof(n),
-                "__builtin_clz does not take 32-bit arg");
-
-  // Handle 0 as a special case because __builtin_clz(0) is undefined.
-  if (n == 0) {
-    return 32;
-  }
-  return __builtin_clz(n);
-#else
-  return CountLeadingZeros32Slow(n);
-#endif
-}
-
-ABSL_BASE_INTERNAL_FORCEINLINE int CountTrailingZerosNonZero64Slow(uint64_t n) {
-  int c = 63;
-  n &= ~n + 1;
-  if (n & 0x00000000FFFFFFFF) c -= 32;
-  if (n & 0x0000FFFF0000FFFF) c -= 16;
-  if (n & 0x00FF00FF00FF00FF) c -= 8;
-  if (n & 0x0F0F0F0F0F0F0F0F) c -= 4;
-  if (n & 0x3333333333333333) c -= 2;
-  if (n & 0x5555555555555555) c -= 1;
-  return c;
-}
-
-ABSL_BASE_INTERNAL_FORCEINLINE int CountTrailingZerosNonZero64(uint64_t n) {
-#if defined(_MSC_VER) && defined(_M_X64)
-  unsigned long result = 0;  // NOLINT(runtime/int)
-  _BitScanForward64(&result, n);
-  return result;
-#elif defined(_MSC_VER)
-  unsigned long result = 0;  // NOLINT(runtime/int)
-  if (static_cast<uint32_t>(n) == 0) {
-    _BitScanForward(&result, n >> 32);
-    return result + 32;
-  }
-  _BitScanForward(&result, n);
-  return result;
-#elif defined(__GNUC__)
-  static_assert(sizeof(unsigned long long) == sizeof(n),  // NOLINT(runtime/int)
-                "__builtin_ctzll does not take 64-bit arg");
-  return __builtin_ctzll(n);
-#else
-  return CountTrailingZerosNonZero64Slow(n);
-#endif
-}
-
-ABSL_BASE_INTERNAL_FORCEINLINE int CountTrailingZerosNonZero32Slow(uint32_t n) {
-  int c = 31;
-  n &= ~n + 1;
-  if (n & 0x0000FFFF) c -= 16;
-  if (n & 0x00FF00FF) c -= 8;
-  if (n & 0x0F0F0F0F) c -= 4;
-  if (n & 0x33333333) c -= 2;
-  if (n & 0x55555555) c -= 1;
-  return c;
-}
-
-ABSL_BASE_INTERNAL_FORCEINLINE int CountTrailingZerosNonZero32(uint32_t n) {
-#if defined(_MSC_VER)
-  unsigned long result = 0;  // NOLINT(runtime/int)
-  _BitScanForward(&result, n);
-  return result;
-#elif defined(__GNUC__)
-  static_assert(sizeof(int) == sizeof(n),
-                "__builtin_ctz does not take 32-bit arg");
-  return __builtin_ctz(n);
-#else
-  return CountTrailingZerosNonZero32Slow(n);
-#endif
-}
-
-#undef ABSL_BASE_INTERNAL_FORCEINLINE
-
-}  // namespace base_internal
-}  // namespace absl
-
-#endif  // ABSL_BASE_INTERNAL_BITS_H_
diff --git a/absl/base/internal/bits_test.cc b/absl/base/internal/bits_test.cc
deleted file mode 100644
index 7855fa62972..00000000000
--- a/absl/base/internal/bits_test.cc
+++ /dev/null
@@ -1,97 +0,0 @@
-// Copyright 2018 The Abseil Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-#include "absl/base/internal/bits.h"
-
-#include "gtest/gtest.h"
-
-namespace {
-
-int CLZ64(uint64_t n) {
-  int fast = absl::base_internal::CountLeadingZeros64(n);
-  int slow = absl::base_internal::CountLeadingZeros64Slow(n);
-  EXPECT_EQ(fast, slow) << n;
-  return fast;
-}
-
-TEST(BitsTest, CountLeadingZeros64) {
-  EXPECT_EQ(64, CLZ64(uint64_t{}));
-  EXPECT_EQ(0, CLZ64(~uint64_t{}));
-
-  for (int index = 0; index < 64; index++) {
-    uint64_t x = static_cast<uint64_t>(1) << index;
-    const auto cnt = 63 - index;
-    ASSERT_EQ(cnt, CLZ64(x)) << index;
-    ASSERT_EQ(cnt, CLZ64(x + x - 1)) << index;
-  }
-}
-
-int CLZ32(uint32_t n) {
-  int fast = absl::base_internal::CountLeadingZeros32(n);
-  int slow = absl::base_internal::CountLeadingZeros32Slow(n);
-  EXPECT_EQ(fast, slow) << n;
-  return fast;
-}
-
-TEST(BitsTest, CountLeadingZeros32) {
-  EXPECT_EQ(32, CLZ32(uint32_t{}));
-  EXPECT_EQ(0, CLZ32(~uint32_t{}));
-
-  for (int index = 0; index < 32; index++) {
-    uint32_t x = static_cast<uint32_t>(1) << index;
-    const auto cnt = 31 - index;
-    ASSERT_EQ(cnt, CLZ32(x)) << index;
-    ASSERT_EQ(cnt, CLZ32(x + x - 1)) << index;
-    ASSERT_EQ(CLZ64(x), CLZ32(x) + 32);
-  }
-}
-
-int CTZ64(uint64_t n) {
-  int fast = absl::base_internal::CountTrailingZerosNonZero64(n);
-  int slow = absl::base_internal::CountTrailingZerosNonZero64Slow(n);
-  EXPECT_EQ(fast, slow) << n;
-  return fast;
-}
-
-TEST(BitsTest, CountTrailingZerosNonZero64) {
-  EXPECT_EQ(0, CTZ64(~uint64_t{}));
-
-  for (int index = 0; index < 64; index++) {
-    uint64_t x = static_cast<uint64_t>(1) << index;
-    const auto cnt = index;
-    ASSERT_EQ(cnt, CTZ64(x)) << index;
-    ASSERT_EQ(cnt, CTZ64(~(x - 1))) << index;
-  }
-}
-
-int CTZ32(uint32_t n) {
-  int fast = absl::base_internal::CountTrailingZerosNonZero32(n);
-  int slow = absl::base_internal::CountTrailingZerosNonZero32Slow(n);
-  EXPECT_EQ(fast, slow) << n;
-  return fast;
-}
-
-TEST(BitsTest, CountTrailingZerosNonZero32) {
-  EXPECT_EQ(0, CTZ32(~uint32_t{}));
-
-  for (int index = 0; index < 32; index++) {
-    uint32_t x = static_cast<uint32_t>(1) << index;
-    const auto cnt = index;
-    ASSERT_EQ(cnt, CTZ32(x)) << index;
-    ASSERT_EQ(cnt, CTZ32(~(x - 1))) << index;
-  }
-}
-
-
-}  // namespace
diff --git a/absl/base/internal/cpu_detect.cc b/absl/base/internal/cpu_detect.cc
new file mode 100644
index 00000000000..a939bbfaa78
--- /dev/null
+++ b/absl/base/internal/cpu_detect.cc
@@ -0,0 +1,438 @@
+// Copyright 2022 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/base/internal/cpu_detect.h"
+
+#include <cstdint>
+#include <optional>  // IWYU pragma: keep
+#include <string>
+
+#include "absl/base/config.h"
+
+#if defined(__aarch64__) && defined(__linux__)
+#include <asm/hwcap.h>
+#include <sys/auxv.h>
+#endif
+
+#if defined(__aarch64__) && defined(__APPLE__)
+#if __has_include(<arm/cpu_capabilities_public.h>)
+#include <arm/cpu_capabilities_public.h>
+#endif
+#include <sys/sysctl.h>
+#include <sys/types.h>
+#endif
+
+#if defined(_WIN32) || defined(_WIN64)
+#include <intrin.h>
+#endif
+
+#if defined(__x86_64__) || defined(_M_X64)
+#if ABSL_HAVE_BUILTIN(__cpuid)
+// MSVC-equivalent __cpuid intrinsic declaration for clang-like compilers
+// for non-Windows build environments.
+extern void __cpuid(int[4], int);
+extern void __cpuidex(int[4], int, int);
+#elif !defined(_WIN32) && !defined(_WIN64)
+// MSVC defines this function for us.
+// https://learn.microsoft.com/en-us/cpp/intrinsics/cpuid-cpuidex
+static void __cpuid(int cpu_info[4], int info_type) {
+  __asm__ volatile("cpuid \n\t"
+                   : "=a"(cpu_info[0]), "=b"(cpu_info[1]), "=c"(cpu_info[2]),
+                     "=d"(cpu_info[3])
+                   : "a"(info_type), "c"(0));
+}
+static void __cpuidex(int cpu_info[4], int info_type, int ecx) {
+  __asm__ volatile("cpuid \n\t"
+                   : "=a"(cpu_info[0]), "=b"(cpu_info[1]), "=c"(cpu_info[2]),
+                     "=d"(cpu_info[3])
+                   : "a"(info_type), "c"(ecx));
+}
+#endif  // !defined(_WIN32) && !defined(_WIN64)
+#endif  // defined(__x86_64__) || defined(_M_X64)
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace base_internal {
+
+#if defined(__x86_64__) || defined(_M_X64)
+
+namespace {
+
+enum class Vendor {
+  kUnknown,
+  kIntel,
+  kAmd,
+};
+
+Vendor GetVendor() {
+  // Get the vendor string (issue CPUID with eax = 0).
+  int cpu_info[4];
+  __cpuid(cpu_info, 0);
+
+  std::string vendor;
+  vendor.append(reinterpret_cast<char*>(&cpu_info[1]), 4);
+  vendor.append(reinterpret_cast<char*>(&cpu_info[3]), 4);
+  vendor.append(reinterpret_cast<char*>(&cpu_info[2]), 4);
+  if (vendor == "GenuineIntel") {
+    return Vendor::kIntel;
+  } else if (vendor == "AuthenticAMD") {
+    return Vendor::kAmd;
+  } else {
+    return Vendor::kUnknown;
+  }
+}
+
+CpuType GetIntelCpuType() {
+  // To get general information and extended features we send eax = 1 and
+  // ecx = 0 to cpuid.  The response is returned in eax, ebx, ecx and edx.
+  // (See Intel 64 and IA-32 Architectures Software Developer's Manual
+  // Volume 2A: Instruction Set Reference, A-M CPUID).
+  // https://www.intel.com/content/www/us/en/architecture-and-technology/64-ia-32-architectures-software-developer-vol-2a-manual.html
+  // https://learn.microsoft.com/en-us/cpp/intrinsics/cpuid-cpuidex
+  int cpu_info[4];
+  __cpuid(cpu_info, 1);
+
+  // Response in eax bits as follows:
+  // 0-3 (stepping id)
+  // 4-7 (model number),
+  // 8-11 (family code),
+  // 12-13 (processor type),
+  // 16-19 (extended model)
+  // 20-27 (extended family)
+
+  int family = (cpu_info[0] >> 8) & 0x0f;
+  int model_num = (cpu_info[0] >> 4) & 0x0f;
+  int ext_family = (cpu_info[0] >> 20) & 0xff;
+  int ext_model_num = (cpu_info[0] >> 16) & 0x0f;
+
+  int brand_id = cpu_info[1] & 0xff;
+
+  // Process the extended family and model info if necessary
+  if (family == 0x0f) {
+    family += ext_family;
+  }
+
+  if (family == 0x0f || family == 0x6) {
+    model_num += (ext_model_num << 4);
+  }
+
+  switch (brand_id) {
+    case 0:  // no brand ID, so parse CPU family/model
+      switch (family) {
+        case 6:  // Most PentiumIII processors are in this category
+          switch (model_num) {
+            case 0x2c:  // Westmere: Gulftown
+              return CpuType::kIntelWestmere;
+            case 0x2d:  // Sandybridge
+              return CpuType::kIntelSandybridge;
+            case 0x3e:  // Ivybridge
+              return CpuType::kIntelIvybridge;
+            case 0x3c:  // Haswell (client)
+            case 0x3f:  // Haswell
+              return CpuType::kIntelHaswell;
+            case 0x4f:  // Broadwell
+            case 0x56:  // BroadwellDE
+              return CpuType::kIntelBroadwell;
+            case 0x55:                         // Skylake Xeon
+              if ((cpu_info[0] & 0x0f) < 5) {  // stepping < 5 is skylake
+                return CpuType::kIntelSkylakeXeon;
+              } else {  // stepping >= 5 is cascadelake
+                return CpuType::kIntelCascadelakeXeon;
+              }
+            case 0x5e:  // Skylake (client)
+              return CpuType::kIntelSkylake;
+            case 0x6a:  // Ice Lake
+              return CpuType::kIntelIcelake;
+            case 0x8f:  // Sapphire Rapids
+              return CpuType::kIntelSapphirerapids;
+            case 0xcf:  // Emerald Rapids
+              return CpuType::kIntelEmeraldrapids;
+            case 0xad:  // Granite Rapids
+              return CpuType::kIntelGraniterapids;
+            default:
+              return CpuType::kUnknown;
+          }
+        default:
+          return CpuType::kUnknown;
+      }
+    default:
+      return CpuType::kUnknown;
+  }
+}
+
+CpuType GetAmdCpuType() {
+  // To get general information and extended features we send eax = 1 and
+  // ecx = 0 to cpuid.  The response is returned in eax, ebx, ecx and edx.
+  // (See Intel 64 and IA-32 Architectures Software Developer's Manual
+  // Volume 2A: Instruction Set Reference, A-M CPUID).
+  // https://learn.microsoft.com/en-us/cpp/intrinsics/cpuid-cpuidex
+  int cpu_info[4];
+  __cpuid(cpu_info, 1);
+
+  // Response in eax bits as follows:
+  // 0-3 (stepping id)
+  // 4-7 (model number),
+  // 8-11 (family code),
+  // 12-13 (processor type),
+  // 16-19 (extended model)
+  // 20-27 (extended family)
+
+  int family = (cpu_info[0] >> 8) & 0x0f;
+  int model_num = (cpu_info[0] >> 4) & 0x0f;
+  int ext_family = (cpu_info[0] >> 20) & 0xff;
+  int ext_model_num = (cpu_info[0] >> 16) & 0x0f;
+
+  if (family == 0x0f) {
+    family += ext_family;
+    model_num += (ext_model_num << 4);
+  }
+
+  switch (family) {
+    case 0x17:
+      switch (model_num) {
+        case 0x0:  // Stepping Ax
+        case 0x1:  // Stepping Bx
+          return CpuType::kAmdNaples;
+        case 0x30:  // Stepping Ax
+        case 0x31:  // Stepping Bx
+          return CpuType::kAmdRome;
+        default:
+          return CpuType::kUnknown;
+      }
+      break;
+    case 0x19:
+      switch (model_num) {
+        case 0x0:  // Stepping Ax
+        case 0x1:  // Stepping B0
+          return CpuType::kAmdMilan;
+        case 0x10:  // Stepping A0
+        case 0x11:  // Stepping B0
+          return CpuType::kAmdGenoa;
+        case 0x44:  // Stepping A0
+          return CpuType::kAmdRyzenV3000;
+        default:
+          return CpuType::kUnknown;
+      }
+      break;
+    case 0x1A:
+      switch (model_num) {
+        case 0x2:
+          return CpuType::kAmdTurin;
+        default:
+          return CpuType::kUnknown;
+      }
+      break;
+    default:
+      return CpuType::kUnknown;
+  }
+}
+
+}  // namespace
+
+CpuType GetCpuType() {
+  switch (GetVendor()) {
+    case Vendor::kIntel:
+      return GetIntelCpuType();
+    case Vendor::kAmd:
+      return GetAmdCpuType();
+    default:
+      return CpuType::kUnknown;
+  }
+}
+
+bool SupportsArmCRC32PMULL() { return false; }
+
+bool SupportsBmi2() {
+  int cpu_info[4];
+  __cpuid(cpu_info, 0);
+  if (cpu_info[0] < 7) {
+    return false;
+  }
+  __cpuidex(cpu_info, 7, 0);
+  return (cpu_info[1] & (1 << 8)) != 0;
+}
+
+#elif defined(__aarch64__) && defined(__linux__)
+
+#ifndef HWCAP_CPUID
+#define HWCAP_CPUID (1 << 11)
+#endif
+
+#define ABSL_INTERNAL_AARCH64_ID_REG_READ(id, val) \
+  asm("mrs %0, " #id : "=r"(val))
+
+CpuType GetCpuType() {
+  // MIDR_EL1 is not visible to EL0, however the access will be emulated by
+  // linux if AT_HWCAP has HWCAP_CPUID set.
+  //
+  // This method will be unreliable on heterogeneous computing systems (ex:
+  // big.LITTLE) since the value of MIDR_EL1 will change based on the calling
+  // thread.
+  uint64_t hwcaps = getauxval(AT_HWCAP);
+  if (hwcaps & HWCAP_CPUID) {
+    uint64_t midr = 0;
+    ABSL_INTERNAL_AARCH64_ID_REG_READ(MIDR_EL1, midr);
+    uint32_t implementer = (midr >> 24) & 0xff;
+    uint32_t part_number = (midr >> 4) & 0xfff;
+    switch (implementer) {
+      case 0x41:
+        switch (part_number) {
+          case 0xd0c:
+            return CpuType::kArmNeoverseN1;
+          case 0xd40:
+            return CpuType::kArmNeoverseV1;
+          case 0xd49:
+            return CpuType::kArmNeoverseN2;
+          case 0xd4f: {
+            uint64_t isar0 = 0;
+            ABSL_INTERNAL_AARCH64_ID_REG_READ(ID_AA64ISAR0_EL1, isar0);
+            if (((isar0 >> 60) & 0xf) == 0x0) {
+              return CpuType::kNvidiaGrace;
+            }
+            return CpuType::kArmNeoverseV2;
+          }
+          case 0xd8e:
+            return CpuType::kArmNeoverseN3;
+          default:
+            return CpuType::kUnknown;
+        }
+        break;
+      case 0xc0:
+        switch (part_number) {
+          case 0xac3:
+            return CpuType::kAmpereSiryn;
+          default:
+            return CpuType::kUnknown;
+        }
+        break;
+      default:
+        return CpuType::kUnknown;
+    }
+  }
+  return CpuType::kUnknown;
+}
+
+bool SupportsArmCRC32PMULL() {
+#if defined(HWCAP_CRC32) && defined(HWCAP_PMULL)
+  uint64_t hwcaps = getauxval(AT_HWCAP);
+  return (hwcaps & HWCAP_CRC32) && (hwcaps & HWCAP_PMULL);
+#else
+  return false;
+#endif
+}
+
+bool SupportsBmi2() { return false; }
+
+#elif defined(__aarch64__) && defined(__APPLE__)
+
+CpuType GetCpuType() { return CpuType::kUnknown; }
+
+template <typename T>
+static std::optional<T> ReadSysctlByName(const char* name) {
+  T val;
+  size_t val_size = sizeof(T);
+  int ret = sysctlbyname(name, &val, &val_size, nullptr, 0);
+  if (ret == -1) {
+    return std::nullopt;
+  }
+  return val;
+}
+
+bool SupportsArmCRC32PMULL() {
+  static const bool supported = []() {
+  // Newer XNU kernels support querying all capabilities in a single
+  // sysctlbyname.
+#if defined(CAP_BIT_CRC32) && defined(CAP_BIT_FEAT_PMULL)
+    const std::optional<uint64_t> caps =
+        ReadSysctlByName<uint64_t>("hw.optional.arm.caps");
+    if (caps.has_value()) {
+      constexpr uint64_t kCrc32AndPmullCaps =
+          (uint64_t{1} << CAP_BIT_CRC32) | (uint64_t{1} << CAP_BIT_FEAT_PMULL);
+      return (*caps & kCrc32AndPmullCaps) == kCrc32AndPmullCaps;
+    }
+#endif
+
+    // https://developer.apple.com/documentation/kernel/1387446-sysctlbyname/determining_instruction_set_characteristics#3915619
+    const std::optional<int> armv8_crc32 =
+        ReadSysctlByName<int>("hw.optional.armv8_crc32");
+    if (armv8_crc32.value_or(0) == 0) {
+      return false;
+    }
+    // https://developer.apple.com/documentation/kernel/1387446-sysctlbyname/determining_instruction_set_characteristics#3918855
+    const std::optional<int> feat_pmull =
+        ReadSysctlByName<int>("hw.optional.arm.FEAT_PMULL");
+    if (feat_pmull.value_or(0) == 0) {
+      return false;
+    }
+    return true;
+  }();
+  return supported;
+}
+
+bool SupportsBmi2() { return false; }
+
+#else
+
+CpuType GetCpuType() { return CpuType::kUnknown; }
+
+bool SupportsArmCRC32PMULL() { return false; }
+
+bool SupportsBmi2() { return false; }
+
+#endif
+
+// Returns how many hardware contexts per CPU exist. Note: AMD CPUs prior to Zen
+// 2 (Rome, 2019) do not support CPUID leaf 0xb. We intentionally avoid falling
+// back to leaf 1 ebx[23:16] because it reports total logical processors per
+// package (not threads per core), which risks false positives on older
+// multi-core non-SMT chips. Pre-Zen 2 AMD safely defaults to 1.
+int NumContextsPerCPU() {
+#if defined(__x86_64__) || defined(_M_X64)
+  int info[4];
+  __cpuid(info, 0);
+  if (info[0] < 0xb) {
+    return 1;
+  }
+
+  __cpuid(info, 1);
+  bool has_ht = (info[3] & (1 << 28)) != 0;
+  if (!has_ht) {
+    return 1;
+  }
+
+  for (int sub_leaf = 0; sub_leaf < 4; ++sub_leaf) {
+    __cpuidex(info, 0xb, sub_leaf);
+    int level_type = (info[2] >> 8) & 0xff;
+    if (level_type == 0) {
+      break;
+    }
+    if (level_type == 1) {
+      int num_threads = info[1] & 0x0ffff;
+      if (num_threads >= 1) {
+        return num_threads;
+      }
+    }
+  }
+
+  return 1;
+#else
+  return 1;
+#endif
+}
+
+bool IsSMTEnabled() { return NumContextsPerCPU() > 1; }
+
+}  // namespace base_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/base/internal/cpu_detect.h b/absl/base/internal/cpu_detect.h
new file mode 100644
index 00000000000..43c7c382d1d
--- /dev/null
+++ b/absl/base/internal/cpu_detect.h
@@ -0,0 +1,80 @@
+// Copyright 2022 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_BASE_INTERNAL_CPU_DETECT_H_
+#define ABSL_BASE_INTERNAL_CPU_DETECT_H_
+
+#include "absl/base/config.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace base_internal {
+
+// Enumeration of architectures that we have special-case tuning parameters for.
+// This set may change over time.
+enum class CpuType {
+  kUnknown,
+  kIntelHaswell,
+  kAmdRome,
+  kAmdNaples,
+  kAmdMilan,
+  kAmdGenoa,
+  kAmdTurin,
+  kAmdRyzenV3000,
+  kIntelCascadelakeXeon,
+  kIntelSkylakeXeon,
+  kIntelBroadwell,
+  kIntelIcelake,
+  kIntelSapphirerapids,
+  kIntelEmeraldrapids,
+  kIntelGraniterapids,
+  kIntelSkylake,
+  kIntelIvybridge,
+  kIntelSandybridge,
+  kIntelWestmere,
+  kArmNeoverseN1,
+  kArmNeoverseV1,
+  kAmpereSiryn,
+  kArmNeoverseN2,
+  kArmNeoverseV2,
+  kArmNeoverseN3,
+  kNvidiaGrace,
+};
+
+// Returns the type of host CPU this code is running on.  Returns kUnknown if
+// the host CPU is of unknown type, or if detection otherwise fails.
+CpuType GetCpuType();
+
+// Returns whether the host CPU supports the CPU features needed for our
+// accelerated implementations. The CpuTypes enumerated above apart from
+// kUnknown support the required features. On unknown CPUs, we can use
+// this to see if it's safe to use hardware acceleration, though without any
+// tuning.
+bool SupportsArmCRC32PMULL();
+
+// Returns whether the host CPU supports BMI2 instructions.
+bool SupportsBmi2();
+
+// Returns whether the host CPU supports simultaneous multithreading (SMT) and
+// if it is enabled.
+bool IsSMTEnabled();
+
+// Returns how many hardware contexts per CPU exist.
+int NumContextsPerCPU();
+
+}  // namespace base_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_BASE_INTERNAL_CPU_DETECT_H_
diff --git a/absl/base/internal/cpu_detect_test.cc b/absl/base/internal/cpu_detect_test.cc
new file mode 100644
index 00000000000..80b8b87f7ac
--- /dev/null
+++ b/absl/base/internal/cpu_detect_test.cc
@@ -0,0 +1,43 @@
+// Copyright 2026 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/base/internal/cpu_detect.h"
+
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace base_internal {
+namespace {
+
+TEST(CpuDetectTest, SupportsBmi2) {
+#if defined(__x86_64__) || defined(_M_X64)
+#if ABSL_HAVE_BUILTIN(__builtin_cpu_supports) && defined(__linux__)
+  EXPECT_EQ(SupportsBmi2(), __builtin_cpu_supports("bmi2") != 0);
+#else
+  // If __builtin_cpu_supports is not available, we just verify it doesn't
+  // crash.
+  (void)SupportsBmi2();
+#endif
+#else
+  // On non-x86, SupportsBmi2() must return false.
+  EXPECT_FALSE(SupportsBmi2());
+#endif
+}
+
+}  // namespace
+}  // namespace base_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/base/internal/cycleclock.cc b/absl/base/internal/cycleclock.cc
index e9844b7177a..af6babf6a93 100644
--- a/absl/base/internal/cycleclock.cc
+++ b/absl/base/internal/cycleclock.cc
@@ -25,51 +25,26 @@
 #include <atomic>
 #include <chrono>  // NOLINT(build/c++11)
 
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
 #include "absl/base/internal/unscaledcycleclock.h"
+#include "absl/base/internal/unscaledcycleclock_config.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 
 #if ABSL_USE_UNSCALED_CYCLECLOCK
 
-namespace {
+ABSL_CONST_INIT std::atomic<CycleClockSourceFunc>
+    CycleClock::cycle_clock_source_{nullptr};
 
-#ifdef NDEBUG
-#ifdef ABSL_INTERNAL_UNSCALED_CYCLECLOCK_FREQUENCY_IS_CPU_FREQUENCY
-// Not debug mode and the UnscaledCycleClock frequency is the CPU
-// frequency.  Scale the CycleClock to prevent overflow if someone
-// tries to represent the time as cycles since the Unix epoch.
-static constexpr int32_t kShift = 1;
-#else
-// Not debug mode and the UnscaledCycleClock isn't operating at the
-// raw CPU frequency. There is no need to do any scaling, so don't
-// needlessly sacrifice precision.
-static constexpr int32_t kShift = 0;
-#endif
-#else
-// In debug mode use a different shift to discourage depending on a
-// particular shift value.
-static constexpr int32_t kShift = 2;
-#endif
-
-static constexpr double kFrequencyScale = 1.0 / (1 << kShift);
-static std::atomic<CycleClockSourceFunc> cycle_clock_source;
-
-CycleClockSourceFunc LoadCycleClockSource() {
-  // Optimize for the common case (no callback) by first doing a relaxed load;
-  // this is significantly faster on non-x86 platforms.
-  if (cycle_clock_source.load(std::memory_order_relaxed) == nullptr) {
-    return nullptr;
-  }
-  // This corresponds to the store(std::memory_order_release) in
-  // CycleClockSource::Register, and makes sure that any updates made prior to
-  // registering the callback are visible to this thread before the callback is
-  // invoked.
-  return cycle_clock_source.load(std::memory_order_acquire);
+void CycleClockSource::Register(CycleClockSourceFunc source) {
+  // Corresponds to the load(std::memory_order_acquire) in LoadCycleClockSource.
+  CycleClock::cycle_clock_source_.store(source, std::memory_order_release);
 }
 
-}  // namespace
-
+#ifdef _WIN32
 int64_t CycleClock::Now() {
   auto fn = LoadCycleClockSource();
   if (fn == nullptr) {
@@ -77,15 +52,7 @@ int64_t CycleClock::Now() {
   }
   return fn() >> kShift;
 }
-
-double CycleClock::Frequency() {
-  return kFrequencyScale * base_internal::UnscaledCycleClock::Frequency();
-}
-
-void CycleClockSource::Register(CycleClockSourceFunc source) {
-  // Corresponds to the load(std::memory_order_acquire) in LoadCycleClockSource.
-  cycle_clock_source.store(source, std::memory_order_release);
-}
+#endif
 
 #else
 
@@ -102,4 +69,5 @@ double CycleClock::Frequency() {
 #endif
 
 }  // namespace base_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/base/internal/cycleclock.h b/absl/base/internal/cycleclock.h
index 794564e12f1..5abffa566aa 100644
--- a/absl/base/internal/cycleclock.h
+++ b/absl/base/internal/cycleclock.h
@@ -42,11 +42,21 @@
 #ifndef ABSL_BASE_INTERNAL_CYCLECLOCK_H_
 #define ABSL_BASE_INTERNAL_CYCLECLOCK_H_
 
+#include <atomic>
 #include <cstdint>
 
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/cycleclock_config.h"
+#include "absl/base/internal/unscaledcycleclock.h"
+#include "absl/base/internal/unscaledcycleclock_config.h"
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 
+using CycleClockSourceFunc = int64_t (*)();
+
 // -----------------------------------------------------------------------------
 // CycleClock
 // -----------------------------------------------------------------------------
@@ -65,12 +75,21 @@ class CycleClock {
   static double Frequency();
 
  private:
+#if ABSL_USE_UNSCALED_CYCLECLOCK
+  static CycleClockSourceFunc LoadCycleClockSource();
+
+  static constexpr int32_t kShift = kCycleClockShift;
+  static constexpr double kFrequencyScale = kCycleClockFrequencyScale;
+
+  ABSL_CONST_INIT static std::atomic<CycleClockSourceFunc> cycle_clock_source_;
+#endif  //  ABSL_USE_UNSCALED_CYCLECLOC
+
   CycleClock() = delete;  // no instances
   CycleClock(const CycleClock&) = delete;
   CycleClock& operator=(const CycleClock&) = delete;
-};
 
-using CycleClockSourceFunc = int64_t (*)();
+  friend class CycleClockSource;
+};
 
 class CycleClockSource {
  private:
@@ -84,7 +103,43 @@ class CycleClockSource {
   static void Register(CycleClockSourceFunc source);
 };
 
+#if ABSL_USE_UNSCALED_CYCLECLOCK
+
+inline CycleClockSourceFunc CycleClock::LoadCycleClockSource() {
+#if !defined(__x86_64__)
+  // Optimize for the common case (no callback) by first doing a relaxed load;
+  // this is significantly faster on non-x86 platforms.
+  if (cycle_clock_source_.load(std::memory_order_relaxed) == nullptr) {
+    return nullptr;
+  }
+#endif  // !defined(__x86_64__)
+
+  // This corresponds to the store(std::memory_order_release) in
+  // CycleClockSource::Register, and makes sure that any updates made prior to
+  // registering the callback are visible to this thread before the callback
+  // is invoked.
+  return cycle_clock_source_.load(std::memory_order_acquire);
+}
+
+// Accessing globals in inlined code in Window DLLs is problematic.
+#ifndef _WIN32
+inline int64_t CycleClock::Now() {
+  auto fn = LoadCycleClockSource();
+  if (fn == nullptr) {
+    return base_internal::UnscaledCycleClock::Now() >> kShift;
+  }
+  return fn() >> kShift;
+}
+#endif
+
+inline double CycleClock::Frequency() {
+  return kFrequencyScale * base_internal::UnscaledCycleClock::Frequency();
+}
+
+#endif  // ABSL_USE_UNSCALED_CYCLECLOCK
+
 }  // namespace base_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_BASE_INTERNAL_CYCLECLOCK_H_
diff --git a/absl/base/internal/cycleclock_config.h b/absl/base/internal/cycleclock_config.h
new file mode 100644
index 00000000000..50a46978687
--- /dev/null
+++ b/absl/base/internal/cycleclock_config.h
@@ -0,0 +1,55 @@
+// Copyright 2022 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_BASE_INTERNAL_CYCLECLOCK_CONFIG_H_
+#define ABSL_BASE_INTERNAL_CYCLECLOCK_CONFIG_H_
+
+#include <cstdint>
+
+#include "absl/base/config.h"
+#include "absl/base/internal/unscaledcycleclock_config.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace base_internal {
+
+#if ABSL_USE_UNSCALED_CYCLECLOCK
+#ifdef NDEBUG
+#ifdef ABSL_INTERNAL_UNSCALED_CYCLECLOCK_FREQUENCY_IS_CPU_FREQUENCY
+// Not debug mode and the UnscaledCycleClock frequency is the CPU
+// frequency.  Scale the CycleClock to prevent overflow if someone
+// tries to represent the time as cycles since the Unix epoch.
+inline constexpr int32_t kCycleClockShift = 1;
+#else
+// Not debug mode and the UnscaledCycleClock isn't operating at the
+// raw CPU frequency. There is no need to do any scaling, so don't
+// needlessly sacrifice precision.
+inline constexpr int32_t kCycleClockShift = 0;
+#endif
+#else   // NDEBUG
+// In debug mode use a different shift to discourage depending on a
+// particular shift value.
+inline constexpr int32_t kCycleClockShift = 2;
+#endif  // NDEBUG
+
+inline constexpr double kCycleClockFrequencyScale =
+    1.0 / (1 << kCycleClockShift);
+
+#endif  // ABSL_USE_UNSCALED_CYCLECLOCK
+
+}  // namespace base_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_BASE_INTERNAL_CYCLECLOCK_CONFIG_H_
diff --git a/absl/base/internal/direct_mmap.h b/absl/base/internal/direct_mmap.h
index 0401ddfa7e6..f56ba239873 100644
--- a/absl/base/internal/direct_mmap.h
+++ b/absl/base/internal/direct_mmap.h
@@ -20,7 +20,7 @@
 
 #include "absl/base/config.h"
 
-#if ABSL_HAVE_MMAP
+#ifdef ABSL_HAVE_MMAP
 
 #include <sys/mman.h>
 
@@ -35,19 +35,21 @@
 
 #include <linux/unistd.h>
 #include <unistd.h>
+
 #include <cerrno>
 #include <cstdarg>
+#include <cstddef>
 #include <cstdint>
 
 #ifdef __mips__
 // Include definitions of the ABI currently in use.
-#ifdef __BIONIC__
+#if defined(__BIONIC__) || !defined(__GLIBC__)
 // Android doesn't have sgidefs.h, but does have asm/sgidefs.h, which has the
 // definitions we need.
 #include <asm/sgidefs.h>
 #else
 #include <sgidefs.h>
-#endif  // __BIONIC__
+#endif  // __BIONIC__ || !__GLIBC__
 #endif  // __mips__
 
 // SYS_mmap and SYS_munmap are not defined in Android.
@@ -61,17 +63,26 @@ extern "C" void* __mmap2(void*, size_t, int, int, int, size_t);
 #endif
 #endif  // __BIONIC__
 
+#if defined(__NR_mmap2) && !defined(SYS_mmap2)
+#define SYS_mmap2 __NR_mmap2
+#endif
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 
 // Platform specific logic extracted from
 // https://chromium.googlesource.com/linux-syscall-support/+/master/linux_syscall_support.h
 inline void* DirectMmap(void* start, size_t length, int prot, int flags, int fd,
-                        off64_t offset) noexcept {
+                        off_t offset) noexcept {
 #if defined(__i386__) || defined(__ARM_ARCH_3__) || defined(__ARM_EABI__) || \
+    defined(__m68k__) || defined(__sh__) ||                                  \
+    (defined(__hppa__) && !defined(__LP64__)) ||                             \
     (defined(__mips__) && _MIPS_SIM == _MIPS_SIM_ABI32) ||                   \
     (defined(__PPC__) && !defined(__PPC64__)) ||                             \
-    (defined(__s390__) && !defined(__s390x__))
+    (defined(__riscv) && __riscv_xlen == 32) ||                              \
+    (defined(__s390__) && !defined(__s390x__)) ||                            \
+    (defined(__sparc__) && !defined(__arch64__))
   // On these architectures, implement mmap with mmap2.
   static int pagesize = 0;
   if (pagesize == 0) {
@@ -88,11 +99,12 @@ inline void* DirectMmap(void* start, size_t length, int prot, int flags, int fd,
 #ifdef __BIONIC__
   // SYS_mmap2 has problems on Android API level <= 16.
   // Workaround by invoking __mmap2() instead.
-  return __mmap2(start, length, prot, flags, fd, offset / pagesize);
+  return __mmap2(start, length, prot, flags, fd,
+                 static_cast<size_t>(offset / pagesize));
 #else
   return reinterpret_cast<void*>(
       syscall(SYS_mmap2, start, length, prot, flags, fd,
-              static_cast<off_t>(offset / pagesize)));
+              static_cast<unsigned long>(offset / pagesize)));  // NOLINT
 #endif
 #elif defined(__s390x__)
   // On s390x, mmap() arguments are passed in memory.
@@ -128,6 +140,7 @@ inline int DirectMunmap(void* start, size_t length) {
 }
 
 }  // namespace base_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #else  // !__linux__
@@ -136,6 +149,7 @@ inline int DirectMunmap(void* start, size_t length) {
 // actual mmap()/munmap() methods.
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 
 inline void* DirectMmap(void* start, size_t length, int prot, int flags, int fd,
@@ -148,6 +162,7 @@ inline int DirectMunmap(void* start, size_t length) {
 }
 
 }  // namespace base_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // __linux__
diff --git a/absl/base/internal/dynamic_annotations.h b/absl/base/internal/dynamic_annotations.h
new file mode 100644
index 00000000000..537a2fe677e
--- /dev/null
+++ b/absl/base/internal/dynamic_annotations.h
@@ -0,0 +1,398 @@
+// Copyright 2017 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// This file defines dynamic annotations for use with dynamic analysis tool
+// such as valgrind, PIN, etc.
+//
+// Dynamic annotation is a source code annotation that affects the generated
+// code (that is, the annotation is not a comment). Each such annotation is
+// attached to a particular instruction and/or to a particular object (address)
+// in the program.
+//
+// The annotations that should be used by users are macros in all upper-case
+// (e.g., ANNOTATE_THREAD_NAME).
+//
+// Actual implementation of these macros may differ depending on the dynamic
+// analysis tool being used.
+//
+// This file supports the following configurations:
+// - Dynamic Annotations enabled (with static thread-safety warnings disabled).
+//   In this case, macros expand to functions implemented by Thread Sanitizer,
+//   when building with TSan. When not provided an external implementation,
+//   dynamic_annotations.cc provides no-op implementations.
+//
+// - Static Clang thread-safety warnings enabled.
+//   When building with a Clang compiler that supports thread-safety warnings,
+//   a subset of annotations can be statically-checked at compile-time. We
+//   expand these macros to static-inline functions that can be analyzed for
+//   thread-safety, but afterwards elided when building the final binary.
+//
+// - All annotations are disabled.
+//   If neither Dynamic Annotations nor Clang thread-safety warnings are
+//   enabled, then all annotation-macros expand to empty.
+
+#ifndef ABSL_BASE_INTERNAL_DYNAMIC_ANNOTATIONS_H_
+#define ABSL_BASE_INTERNAL_DYNAMIC_ANNOTATIONS_H_
+
+#include <stddef.h>
+
+#include "absl/base/config.h"
+
+// -------------------------------------------------------------------------
+// Decide which features are enabled
+
+#ifndef DYNAMIC_ANNOTATIONS_ENABLED
+#define DYNAMIC_ANNOTATIONS_ENABLED 0
+#endif
+
+#if defined(__clang__) && !defined(SWIG)
+#define ABSL_INTERNAL_IGNORE_READS_ATTRIBUTE_ENABLED 1
+#endif
+
+#if DYNAMIC_ANNOTATIONS_ENABLED != 0
+
+#define ABSL_INTERNAL_RACE_ANNOTATIONS_ENABLED 1
+#define ABSL_INTERNAL_READS_ANNOTATIONS_ENABLED 1
+#define ABSL_INTERNAL_WRITES_ANNOTATIONS_ENABLED 1
+#define ABSL_INTERNAL_ANNOTALYSIS_ENABLED 0
+#define ABSL_INTERNAL_READS_WRITES_ANNOTATIONS_ENABLED 1
+
+#else
+
+#define ABSL_INTERNAL_RACE_ANNOTATIONS_ENABLED 0
+#define ABSL_INTERNAL_READS_ANNOTATIONS_ENABLED 0
+#define ABSL_INTERNAL_WRITES_ANNOTATIONS_ENABLED 0
+
+// Clang provides limited support for static thread-safety analysis through a
+// feature called Annotalysis. We configure macro-definitions according to
+// whether Annotalysis support is available. When running in opt-mode, GCC
+// will issue a warning, if these attributes are compiled. Only include them
+// when compiling using Clang.
+
+// ANNOTALYSIS_ENABLED == 1 when IGNORE_READ_ATTRIBUTE_ENABLED == 1
+#define ABSL_INTERNAL_ANNOTALYSIS_ENABLED \
+  defined(ABSL_INTERNAL_IGNORE_READS_ATTRIBUTE_ENABLED)
+// Read/write annotations are enabled in Annotalysis mode; disabled otherwise.
+#define ABSL_INTERNAL_READS_WRITES_ANNOTATIONS_ENABLED \
+  ABSL_INTERNAL_ANNOTALYSIS_ENABLED
+#endif
+
+// Memory annotations are also made available to LLVM's Memory Sanitizer
+#if defined(ABSL_HAVE_MEMORY_SANITIZER)
+#define ABSL_INTERNAL_MEMORY_ANNOTATIONS_ENABLED 1
+#endif
+
+#ifndef ABSL_INTERNAL_MEMORY_ANNOTATIONS_ENABLED
+#define ABSL_INTERNAL_MEMORY_ANNOTATIONS_ENABLED 0
+#endif
+
+#ifdef __cplusplus
+#define ABSL_INTERNAL_BEGIN_EXTERN_C extern "C" {
+#define ABSL_INTERNAL_END_EXTERN_C }  // extern "C"
+#define ABSL_INTERNAL_GLOBAL_SCOPED(F) ::F
+#define ABSL_INTERNAL_STATIC_INLINE inline
+#else
+#define ABSL_INTERNAL_BEGIN_EXTERN_C  // empty
+#define ABSL_INTERNAL_END_EXTERN_C    // empty
+#define ABSL_INTERNAL_GLOBAL_SCOPED(F) F
+#define ABSL_INTERNAL_STATIC_INLINE static inline
+#endif
+
+// -------------------------------------------------------------------------
+// Define race annotations.
+
+#if ABSL_INTERNAL_RACE_ANNOTATIONS_ENABLED == 1
+
+// -------------------------------------------------------------
+// Annotations that suppress errors. It is usually better to express the
+// program's synchronization using the other annotations, but these can be used
+// when all else fails.
+
+// Report that we may have a benign race at `pointer`, with size
+// "sizeof(*(pointer))". `pointer` must be a non-void* pointer. Insert at the
+// point where `pointer` has been allocated, preferably close to the point
+// where the race happens. See also ANNOTATE_BENIGN_RACE_STATIC.
+#define ANNOTATE_BENIGN_RACE(pointer, description)     \
+  ABSL_INTERNAL_GLOBAL_SCOPED(AnnotateBenignRaceSized) \
+  (__FILE__, __LINE__, pointer, sizeof(*(pointer)), description)
+
+// Same as ANNOTATE_BENIGN_RACE(`address`, `description`), but applies to
+// the memory range [`address`, `address`+`size`).
+#define ANNOTATE_BENIGN_RACE_SIZED(address, size, description) \
+  ABSL_INTERNAL_GLOBAL_SCOPED(AnnotateBenignRaceSized)         \
+  (__FILE__, __LINE__, address, size, description)
+
+// Enable (`enable`!=0) or disable (`enable`==0) race detection for all threads.
+// This annotation could be useful if you want to skip expensive race analysis
+// during some period of program execution, e.g. during initialization.
+#define ANNOTATE_ENABLE_RACE_DETECTION(enable)             \
+  ABSL_INTERNAL_GLOBAL_SCOPED(AnnotateEnableRaceDetection) \
+  (__FILE__, __LINE__, enable)
+
+// -------------------------------------------------------------
+// Annotations useful for debugging.
+
+// Report the current thread `name` to a race detector.
+#define ANNOTATE_THREAD_NAME(name) \
+  ABSL_INTERNAL_GLOBAL_SCOPED(AnnotateThreadName)(__FILE__, __LINE__, name)
+
+// -------------------------------------------------------------
+// Annotations useful when implementing locks. They are not normally needed by
+// modules that merely use locks. The `lock` argument is a pointer to the lock
+// object.
+
+// Report that a lock has been created at address `lock`.
+#define ANNOTATE_RWLOCK_CREATE(lock) \
+  ABSL_INTERNAL_GLOBAL_SCOPED(AnnotateRWLockCreate)(__FILE__, __LINE__, lock)
+
+// Report that a linker initialized lock has been created at address `lock`.
+#ifdef ABSL_HAVE_THREAD_SANITIZER
+#define ANNOTATE_RWLOCK_CREATE_STATIC(lock)               \
+  ABSL_INTERNAL_GLOBAL_SCOPED(AnnotateRWLockCreateStatic) \
+  (__FILE__, __LINE__, lock)
+#else
+#define ANNOTATE_RWLOCK_CREATE_STATIC(lock) ANNOTATE_RWLOCK_CREATE(lock)
+#endif
+
+// Report that the lock at address `lock` is about to be destroyed.
+#define ANNOTATE_RWLOCK_DESTROY(lock) \
+  ABSL_INTERNAL_GLOBAL_SCOPED(AnnotateRWLockDestroy)(__FILE__, __LINE__, lock)
+
+// Report that the lock at address `lock` has been acquired.
+// `is_w`=1 for writer lock, `is_w`=0 for reader lock.
+#define ANNOTATE_RWLOCK_ACQUIRED(lock, is_w)          \
+  ABSL_INTERNAL_GLOBAL_SCOPED(AnnotateRWLockAcquired) \
+  (__FILE__, __LINE__, lock, is_w)
+
+// Report that the lock at address `lock` is about to be released.
+// `is_w`=1 for writer lock, `is_w`=0 for reader lock.
+#define ANNOTATE_RWLOCK_RELEASED(lock, is_w)          \
+  ABSL_INTERNAL_GLOBAL_SCOPED(AnnotateRWLockReleased) \
+  (__FILE__, __LINE__, lock, is_w)
+
+// Apply ANNOTATE_BENIGN_RACE_SIZED to a static variable `static_var`.
+#define ANNOTATE_BENIGN_RACE_STATIC(static_var, description)      \
+  namespace {                                                     \
+  class static_var##_annotator {                                  \
+   public:                                                        \
+    static_var##_annotator() {                                    \
+      ANNOTATE_BENIGN_RACE_SIZED(&static_var, sizeof(static_var), \
+                                 #static_var ": " description);   \
+    }                                                             \
+  };                                                              \
+  static static_var##_annotator the##static_var##_annotator;      \
+  }  // namespace
+
+#else  // ABSL_INTERNAL_RACE_ANNOTATIONS_ENABLED == 0
+
+#define ANNOTATE_RWLOCK_CREATE(lock)                            // empty
+#define ANNOTATE_RWLOCK_CREATE_STATIC(lock)                     // empty
+#define ANNOTATE_RWLOCK_DESTROY(lock)                           // empty
+#define ANNOTATE_RWLOCK_ACQUIRED(lock, is_w)                    // empty
+#define ANNOTATE_RWLOCK_RELEASED(lock, is_w)                    // empty
+#define ANNOTATE_BENIGN_RACE(address, description)              // empty
+#define ANNOTATE_BENIGN_RACE_SIZED(address, size, description)  // empty
+#define ANNOTATE_THREAD_NAME(name)                              // empty
+#define ANNOTATE_ENABLE_RACE_DETECTION(enable)                  // empty
+#define ANNOTATE_BENIGN_RACE_STATIC(static_var, description)    // empty
+
+#endif  // ABSL_INTERNAL_RACE_ANNOTATIONS_ENABLED
+
+// -------------------------------------------------------------------------
+// Define memory annotations.
+
+#if ABSL_INTERNAL_MEMORY_ANNOTATIONS_ENABLED == 1
+
+#include <sanitizer/msan_interface.h>
+
+#define ANNOTATE_MEMORY_IS_INITIALIZED(address, size) \
+  __msan_unpoison(address, size)
+
+#define ANNOTATE_MEMORY_IS_UNINITIALIZED(address, size) \
+  __msan_allocated_memory(address, size)
+
+#else  // ABSL_INTERNAL_MEMORY_ANNOTATIONS_ENABLED == 0
+
+#if DYNAMIC_ANNOTATIONS_ENABLED == 1
+#define ANNOTATE_MEMORY_IS_INITIALIZED(address, size) \
+  do {                                                \
+    (void)(address);                                  \
+    (void)(size);                                     \
+  } while (0)
+#define ANNOTATE_MEMORY_IS_UNINITIALIZED(address, size) \
+  do {                                                  \
+    (void)(address);                                    \
+    (void)(size);                                       \
+  } while (0)
+#else
+#define ANNOTATE_MEMORY_IS_INITIALIZED(address, size)    // empty
+#define ANNOTATE_MEMORY_IS_UNINITIALIZED(address, size)  // empty
+#endif
+
+#endif  // ABSL_INTERNAL_MEMORY_ANNOTATIONS_ENABLED
+
+// -------------------------------------------------------------------------
+// Define IGNORE_READS_BEGIN/_END attributes.
+
+#if defined(ABSL_INTERNAL_IGNORE_READS_ATTRIBUTE_ENABLED)
+
+#define ABSL_INTERNAL_IGNORE_READS_BEGIN_ATTRIBUTE \
+  __attribute((exclusive_lock_function("*")))
+#define ABSL_INTERNAL_IGNORE_READS_END_ATTRIBUTE \
+  __attribute((unlock_function("*")))
+
+#else  // !defined(ABSL_INTERNAL_IGNORE_READS_ATTRIBUTE_ENABLED)
+
+#define ABSL_INTERNAL_IGNORE_READS_BEGIN_ATTRIBUTE  // empty
+#define ABSL_INTERNAL_IGNORE_READS_END_ATTRIBUTE    // empty
+
+#endif  // defined(ABSL_INTERNAL_IGNORE_READS_ATTRIBUTE_ENABLED)
+
+// -------------------------------------------------------------------------
+// Define IGNORE_READS_BEGIN/_END annotations.
+
+#if ABSL_INTERNAL_READS_ANNOTATIONS_ENABLED == 1
+
+// Request the analysis tool to ignore all reads in the current thread until
+// ANNOTATE_IGNORE_READS_END is called. Useful to ignore intentional racey
+// reads, while still checking other reads and all writes.
+// See also ANNOTATE_UNPROTECTED_READ.
+#define ANNOTATE_IGNORE_READS_BEGIN() \
+  ABSL_INTERNAL_GLOBAL_SCOPED(AnnotateIgnoreReadsBegin)(__FILE__, __LINE__)
+
+// Stop ignoring reads.
+#define ANNOTATE_IGNORE_READS_END() \
+  ABSL_INTERNAL_GLOBAL_SCOPED(AnnotateIgnoreReadsEnd)(__FILE__, __LINE__)
+
+#elif defined(ABSL_INTERNAL_ANNOTALYSIS_ENABLED)
+
+// When Annotalysis is enabled without Dynamic Annotations, the use of
+// static-inline functions allows the annotations to be read at compile-time,
+// while still letting the compiler elide the functions from the final build.
+//
+// TODO(delesley) -- The exclusive lock here ignores writes as well, but
+// allows IGNORE_READS_AND_WRITES to work properly.
+
+#define ANNOTATE_IGNORE_READS_BEGIN() \
+  ABSL_INTERNAL_GLOBAL_SCOPED(AbslInternalAnnotateIgnoreReadsBegin)()
+
+#define ANNOTATE_IGNORE_READS_END() \
+  ABSL_INTERNAL_GLOBAL_SCOPED(AbslInternalAnnotateIgnoreReadsEnd)()
+
+#else
+
+#define ANNOTATE_IGNORE_READS_BEGIN()  // empty
+#define ANNOTATE_IGNORE_READS_END()    // empty
+
+#endif
+
+// -------------------------------------------------------------------------
+// Define IGNORE_WRITES_BEGIN/_END annotations.
+
+#if ABSL_INTERNAL_WRITES_ANNOTATIONS_ENABLED == 1
+
+// Similar to ANNOTATE_IGNORE_READS_BEGIN, but ignore writes instead.
+#define ANNOTATE_IGNORE_WRITES_BEGIN() \
+  ABSL_INTERNAL_GLOBAL_SCOPED(AnnotateIgnoreWritesBegin)(__FILE__, __LINE__)
+
+// Stop ignoring writes.
+#define ANNOTATE_IGNORE_WRITES_END() \
+  ABSL_INTERNAL_GLOBAL_SCOPED(AnnotateIgnoreWritesEnd)(__FILE__, __LINE__)
+
+#else
+
+#define ANNOTATE_IGNORE_WRITES_BEGIN()  // empty
+#define ANNOTATE_IGNORE_WRITES_END()    // empty
+
+#endif
+
+// -------------------------------------------------------------------------
+// Define the ANNOTATE_IGNORE_READS_AND_WRITES_* annotations using the more
+// primitive annotations defined above.
+//
+//     Instead of doing
+//        ANNOTATE_IGNORE_READS_BEGIN();
+//        ... = x;
+//        ANNOTATE_IGNORE_READS_END();
+//     one can use
+//        ... = ANNOTATE_UNPROTECTED_READ(x);
+
+#if defined(ABSL_INTERNAL_READS_WRITES_ANNOTATIONS_ENABLED)
+
+// Start ignoring all memory accesses (both reads and writes).
+#define ANNOTATE_IGNORE_READS_AND_WRITES_BEGIN() \
+  do {                                           \
+    ANNOTATE_IGNORE_READS_BEGIN();               \
+    ANNOTATE_IGNORE_WRITES_BEGIN();              \
+  } while (0)
+
+// Stop ignoring both reads and writes.
+#define ANNOTATE_IGNORE_READS_AND_WRITES_END() \
+  do {                                         \
+    ANNOTATE_IGNORE_WRITES_END();              \
+    ANNOTATE_IGNORE_READS_END();               \
+  } while (0)
+
+#ifdef __cplusplus
+// ANNOTATE_UNPROTECTED_READ is the preferred way to annotate racey reads.
+#define ANNOTATE_UNPROTECTED_READ(x) \
+  absl::base_internal::AnnotateUnprotectedRead(x)
+
+#endif
+
+#else
+
+#define ANNOTATE_IGNORE_READS_AND_WRITES_BEGIN()  // empty
+#define ANNOTATE_IGNORE_READS_AND_WRITES_END()    // empty
+#define ANNOTATE_UNPROTECTED_READ(x) (x)
+
+#endif
+
+// -------------------------------------------------------------------------
+// Address sanitizer annotations
+
+#ifdef ABSL_HAVE_ADDRESS_SANITIZER
+// Describe the current state of a contiguous container such as e.g.
+// std::vector or std::string. For more details see
+// sanitizer/common_interface_defs.h, which is provided by the compiler.
+#include <sanitizer/common_interface_defs.h>
+
+#define ANNOTATE_CONTIGUOUS_CONTAINER(beg, end, old_mid, new_mid) \
+  __sanitizer_annotate_contiguous_container(beg, end, old_mid, new_mid)
+#define ADDRESS_SANITIZER_REDZONE(name)    \
+  struct {                                 \
+    char x[8] __attribute__((aligned(8))); \
+  } name
+
+#else
+
+#define ANNOTATE_CONTIGUOUS_CONTAINER(beg, end, old_mid, new_mid)
+#define ADDRESS_SANITIZER_REDZONE(name) static_assert(true, "")
+
+#endif  // ABSL_HAVE_ADDRESS_SANITIZER
+
+// -------------------------------------------------------------------------
+// Undefine the macros intended only for this file.
+
+#undef ABSL_INTERNAL_RACE_ANNOTATIONS_ENABLED
+#undef ABSL_INTERNAL_MEMORY_ANNOTATIONS_ENABLED
+#undef ABSL_INTERNAL_READS_ANNOTATIONS_ENABLED
+#undef ABSL_INTERNAL_WRITES_ANNOTATIONS_ENABLED
+#undef ABSL_INTERNAL_ANNOTALYSIS_ENABLED
+#undef ABSL_INTERNAL_READS_WRITES_ANNOTATIONS_ENABLED
+#undef ABSL_INTERNAL_BEGIN_EXTERN_C
+#undef ABSL_INTERNAL_END_EXTERN_C
+#undef ABSL_INTERNAL_STATIC_INLINE
+
+#endif  // ABSL_BASE_INTERNAL_DYNAMIC_ANNOTATIONS_H_
diff --git a/absl/base/internal/endian.h b/absl/base/internal/endian.h
index b43c9a46ee3..fb38f602470 100644
--- a/absl/base/internal/endian.h
+++ b/absl/base/internal/endian.h
@@ -12,126 +12,72 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 //
+// This file is for Abseil internal use only.
+// See //absl/numeric/bits.h for supported functions related to endian-ness.
 
 #ifndef ABSL_BASE_INTERNAL_ENDIAN_H_
 #define ABSL_BASE_INTERNAL_ENDIAN_H_
 
-// The following guarantees declaration of the byte swap functions
-#ifdef _MSC_VER
-#include <stdlib.h>  // NOLINT(build/include)
-#elif defined(__APPLE__)
-// macOS / Darwin features
-#include <libkern/OSByteOrder.h>
-#elif defined(__FreeBSD__)
-#include <sys/endian.h>
-#elif defined(__GLIBC__)
-#include <byteswap.h>  // IWYU pragma: export
-#endif
-
 #include <cstdint>
+#include <cstdlib>
+
+#include "absl/base/casts.h"
 #include "absl/base/config.h"
 #include "absl/base/internal/unaligned_access.h"
+#include "absl/base/nullability.h"
 #include "absl/base/port.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
-// Use compiler byte-swapping intrinsics if they are available.  32-bit
-// and 64-bit versions are available in Clang and GCC as of GCC 4.3.0.
-// The 16-bit version is available in Clang and GCC only as of GCC 4.8.0.
-// For simplicity, we enable them all only for GCC 4.8.0 or later.
-#if defined(__clang__) || \
-    (defined(__GNUC__) && \
-     ((__GNUC__ == 4 && __GNUC_MINOR__ >= 8) || __GNUC__ >= 5))
-inline uint64_t gbswap_64(uint64_t host_int) {
-  return __builtin_bswap64(host_int);
-}
-inline uint32_t gbswap_32(uint32_t host_int) {
-  return __builtin_bswap32(host_int);
-}
-inline uint16_t gbswap_16(uint16_t host_int) {
-  return __builtin_bswap16(host_int);
-}
-
-#elif defined(_MSC_VER)
-inline uint64_t gbswap_64(uint64_t host_int) {
-  return _byteswap_uint64(host_int);
-}
-inline uint32_t gbswap_32(uint32_t host_int) {
-  return _byteswap_ulong(host_int);
-}
-inline uint16_t gbswap_16(uint16_t host_int) {
-  return _byteswap_ushort(host_int);
-}
-
-#elif defined(__APPLE__)
-inline uint64_t gbswap_64(uint64_t host_int) { return OSSwapInt16(host_int); }
-inline uint32_t gbswap_32(uint32_t host_int) { return OSSwapInt32(host_int); }
-inline uint16_t gbswap_16(uint16_t host_int) { return OSSwapInt64(host_int); }
-
+constexpr uint64_t gbswap_64(uint64_t x) {
+#if ABSL_HAVE_BUILTIN(__builtin_bswap64) || defined(__GNUC__)
+  return __builtin_bswap64(x);
 #else
-inline uint64_t gbswap_64(uint64_t host_int) {
-#if defined(__GNUC__) && defined(__x86_64__) && !defined(__APPLE__)
-  // Adapted from /usr/include/byteswap.h.  Not available on Mac.
-  if (__builtin_constant_p(host_int)) {
-    return __bswap_constant_64(host_int);
-  } else {
-    uint64_t result;
-    __asm__("bswap %0" : "=r"(result) : "0"(host_int));
-    return result;
-  }
-#elif defined(__GLIBC__)
-  return bswap_64(host_int);
-#else
-  return (((host_int & uint64_t{0xFF}) << 56) |
-          ((host_int & uint64_t{0xFF00}) << 40) |
-          ((host_int & uint64_t{0xFF0000}) << 24) |
-          ((host_int & uint64_t{0xFF000000}) << 8) |
-          ((host_int & uint64_t{0xFF00000000}) >> 8) |
-          ((host_int & uint64_t{0xFF0000000000}) >> 24) |
-          ((host_int & uint64_t{0xFF000000000000}) >> 40) |
-          ((host_int & uint64_t{0xFF00000000000000}) >> 56));
-#endif  // bswap_64
+  return (((x & uint64_t{0xFF}) << 56) |
+          ((x & uint64_t{0xFF00}) << 40) |
+          ((x & uint64_t{0xFF0000}) << 24) |
+          ((x & uint64_t{0xFF000000}) << 8) |
+          ((x & uint64_t{0xFF00000000}) >> 8) |
+          ((x & uint64_t{0xFF0000000000}) >> 24) |
+          ((x & uint64_t{0xFF000000000000}) >> 40) |
+          ((x & uint64_t{0xFF00000000000000}) >> 56));
+#endif
 }
 
-inline uint32_t gbswap_32(uint32_t host_int) {
-#if defined(__GLIBC__)
-  return bswap_32(host_int);
+constexpr uint32_t gbswap_32(uint32_t x) {
+#if ABSL_HAVE_BUILTIN(__builtin_bswap32) || defined(__GNUC__)
+  return __builtin_bswap32(x);
 #else
-  return (((host_int & uint32_t{0xFF}) << 24) |
-          ((host_int & uint32_t{0xFF00}) << 8) |
-          ((host_int & uint32_t{0xFF0000}) >> 8) |
-          ((host_int & uint32_t{0xFF000000}) >> 24));
+  return (((x & uint32_t{0xFF}) << 24) |
+          ((x & uint32_t{0xFF00}) << 8) |
+          ((x & uint32_t{0xFF0000}) >> 8) |
+          ((x & uint32_t{0xFF000000}) >> 24));
 #endif
 }
 
-inline uint16_t gbswap_16(uint16_t host_int) {
-#if defined(__GLIBC__)
-  return bswap_16(host_int);
+constexpr uint16_t gbswap_16(uint16_t x) {
+#if ABSL_HAVE_BUILTIN(__builtin_bswap16) || defined(__GNUC__)
+  return __builtin_bswap16(x);
 #else
-  return (((host_int & uint16_t{0xFF}) << 8) |
-          ((host_int & uint16_t{0xFF00}) >> 8));
+  return (((x & uint16_t{0xFF}) << 8) |
+          ((x & uint16_t{0xFF00}) >> 8));
 #endif
 }
 
-#endif  // intrinsics available
-
 #ifdef ABSL_IS_LITTLE_ENDIAN
 
-// Definitions for ntohl etc. that don't require us to include
-// netinet/in.h. We wrap gbswap_32 and gbswap_16 in functions rather
-// than just #defining them because in debug mode, gcc doesn't
-// correctly handle the (rather involved) definitions of bswap_32.
-// gcc guarantees that inline functions are as fast as macros, so
-// this isn't a performance hit.
+// Portable definitions for htonl (host-to-network) and friends on little-endian
+// architectures.
 inline uint16_t ghtons(uint16_t x) { return gbswap_16(x); }
 inline uint32_t ghtonl(uint32_t x) { return gbswap_32(x); }
 inline uint64_t ghtonll(uint64_t x) { return gbswap_64(x); }
 
 #elif defined ABSL_IS_BIG_ENDIAN
 
-// These definitions are simpler on big-endian machines
-// These are functions instead of macros to avoid self-assignment warnings
-// on calls such as "i = ghtnol(i);".  This also provides type checking.
+// Portable definitions for htonl (host-to-network) etc on big-endian
+// architectures. These definitions are simpler since the host byte order is the
+// same as network byte order.
 inline uint16_t ghtons(uint16_t x) { return x; }
 inline uint32_t ghtonl(uint32_t x) { return x; }
 inline uint64_t ghtonll(uint64_t x) { return x; }
@@ -180,28 +126,58 @@ inline constexpr bool IsLittleEndian() { return false; }
 
 #endif /* ENDIAN */
 
+inline uint8_t FromHost(uint8_t x) { return x; }
+inline uint16_t FromHost(uint16_t x) { return FromHost16(x); }
+inline uint32_t FromHost(uint32_t x) { return FromHost32(x); }
+inline uint64_t FromHost(uint64_t x) { return FromHost64(x); }
+inline uint8_t ToHost(uint8_t x) { return x; }
+inline uint16_t ToHost(uint16_t x) { return ToHost16(x); }
+inline uint32_t ToHost(uint32_t x) { return ToHost32(x); }
+inline uint64_t ToHost(uint64_t x) { return ToHost64(x); }
+
+inline int8_t FromHost(int8_t x) { return x; }
+inline int16_t FromHost(int16_t x) {
+  return bit_cast<int16_t>(FromHost16(bit_cast<uint16_t>(x)));
+}
+inline int32_t FromHost(int32_t x) {
+  return bit_cast<int32_t>(FromHost32(bit_cast<uint32_t>(x)));
+}
+inline int64_t FromHost(int64_t x) {
+  return bit_cast<int64_t>(FromHost64(bit_cast<uint64_t>(x)));
+}
+inline int8_t ToHost(int8_t x) { return x; }
+inline int16_t ToHost(int16_t x) {
+  return bit_cast<int16_t>(ToHost16(bit_cast<uint16_t>(x)));
+}
+inline int32_t ToHost(int32_t x) {
+  return bit_cast<int32_t>(ToHost32(bit_cast<uint32_t>(x)));
+}
+inline int64_t ToHost(int64_t x) {
+  return bit_cast<int64_t>(ToHost64(bit_cast<uint64_t>(x)));
+}
+
 // Functions to do unaligned loads and stores in little-endian order.
-inline uint16_t Load16(const void *p) {
+inline uint16_t Load16(const void* absl_nonnull p) {
   return ToHost16(ABSL_INTERNAL_UNALIGNED_LOAD16(p));
 }
 
-inline void Store16(void *p, uint16_t v) {
+inline void Store16(void* absl_nonnull p, uint16_t v) {
   ABSL_INTERNAL_UNALIGNED_STORE16(p, FromHost16(v));
 }
 
-inline uint32_t Load32(const void *p) {
+inline uint32_t Load32(const void* absl_nonnull p) {
   return ToHost32(ABSL_INTERNAL_UNALIGNED_LOAD32(p));
 }
 
-inline void Store32(void *p, uint32_t v) {
+inline void Store32(void* absl_nonnull p, uint32_t v) {
   ABSL_INTERNAL_UNALIGNED_STORE32(p, FromHost32(v));
 }
 
-inline uint64_t Load64(const void *p) {
+inline uint64_t Load64(const void* absl_nonnull p) {
   return ToHost64(ABSL_INTERNAL_UNALIGNED_LOAD64(p));
 }
 
-inline void Store64(void *p, uint64_t v) {
+inline void Store64(void* absl_nonnull p, uint64_t v) {
   ABSL_INTERNAL_UNALIGNED_STORE64(p, FromHost64(v));
 }
 
@@ -240,33 +216,64 @@ inline constexpr bool IsLittleEndian() { return false; }
 
 #endif /* ENDIAN */
 
+inline uint8_t FromHost(uint8_t x) { return x; }
+inline uint16_t FromHost(uint16_t x) { return FromHost16(x); }
+inline uint32_t FromHost(uint32_t x) { return FromHost32(x); }
+inline uint64_t FromHost(uint64_t x) { return FromHost64(x); }
+inline uint8_t ToHost(uint8_t x) { return x; }
+inline uint16_t ToHost(uint16_t x) { return ToHost16(x); }
+inline uint32_t ToHost(uint32_t x) { return ToHost32(x); }
+inline uint64_t ToHost(uint64_t x) { return ToHost64(x); }
+
+inline int8_t FromHost(int8_t x) { return x; }
+inline int16_t FromHost(int16_t x) {
+  return bit_cast<int16_t>(FromHost16(bit_cast<uint16_t>(x)));
+}
+inline int32_t FromHost(int32_t x) {
+  return bit_cast<int32_t>(FromHost32(bit_cast<uint32_t>(x)));
+}
+inline int64_t FromHost(int64_t x) {
+  return bit_cast<int64_t>(FromHost64(bit_cast<uint64_t>(x)));
+}
+inline int8_t ToHost(int8_t x) { return x; }
+inline int16_t ToHost(int16_t x) {
+  return bit_cast<int16_t>(ToHost16(bit_cast<uint16_t>(x)));
+}
+inline int32_t ToHost(int32_t x) {
+  return bit_cast<int32_t>(ToHost32(bit_cast<uint32_t>(x)));
+}
+inline int64_t ToHost(int64_t x) {
+  return bit_cast<int64_t>(ToHost64(bit_cast<uint64_t>(x)));
+}
+
 // Functions to do unaligned loads and stores in big-endian order.
-inline uint16_t Load16(const void *p) {
+inline uint16_t Load16(const void* absl_nonnull p) {
   return ToHost16(ABSL_INTERNAL_UNALIGNED_LOAD16(p));
 }
 
-inline void Store16(void *p, uint16_t v) {
+inline void Store16(void* absl_nonnull p, uint16_t v) {
   ABSL_INTERNAL_UNALIGNED_STORE16(p, FromHost16(v));
 }
 
-inline uint32_t Load32(const void *p) {
+inline uint32_t Load32(const void* absl_nonnull p) {
   return ToHost32(ABSL_INTERNAL_UNALIGNED_LOAD32(p));
 }
 
-inline void Store32(void *p, uint32_t v) {
+inline void Store32(void* absl_nonnull p, uint32_t v) {
   ABSL_INTERNAL_UNALIGNED_STORE32(p, FromHost32(v));
 }
 
-inline uint64_t Load64(const void *p) {
+inline uint64_t Load64(const void* absl_nonnull p) {
   return ToHost64(ABSL_INTERNAL_UNALIGNED_LOAD64(p));
 }
 
-inline void Store64(void *p, uint64_t v) {
+inline void Store64(void* absl_nonnull p, uint64_t v) {
   ABSL_INTERNAL_UNALIGNED_STORE64(p, FromHost64(v));
 }
 
 }  // namespace big_endian
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_BASE_INTERNAL_ENDIAN_H_
diff --git a/absl/base/internal/endian_test.cc b/absl/base/internal/endian_test.cc
index 98a099e43e7..0671951f228 100644
--- a/absl/base/internal/endian_test.cc
+++ b/absl/base/internal/endian_test.cc
@@ -15,15 +15,19 @@
 #include "absl/base/internal/endian.h"
 
 #include <algorithm>
+#include <cstddef>
 #include <cstdint>
+#include <ios>
 #include <limits>
 #include <random>
 #include <vector>
 
 #include "gtest/gtest.h"
 #include "absl/base/config.h"
+#include "absl/base/internal/unaligned_access.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace {
 
 const uint64_t kInitialNumber{0x0123456789abcdef};
@@ -53,24 +57,22 @@ const uint32_t k32ValueBE{0x67452301};
 const uint16_t k16ValueBE{0x2301};
 #endif
 
-template<typename T>
-std::vector<T> GenerateAllValuesForType() {
-  std::vector<T> result;
-  T next = std::numeric_limits<T>::min();
-  while (true) {
-    result.push_back(next);
-    if (next == std::numeric_limits<T>::max()) {
-      return result;
-    }
-    ++next;
+std::vector<uint16_t> GenerateAllUint16Values() {
+  std::vector<uint16_t> result;
+  result.reserve(size_t{1} << (sizeof(uint16_t) * 8));
+  for (uint32_t i = std::numeric_limits<uint16_t>::min();
+       i <= std::numeric_limits<uint16_t>::max(); ++i) {
+    result.push_back(static_cast<uint16_t>(i));
   }
+  return result;
 }
 
 template<typename T>
-std::vector<T> GenerateRandomIntegers(size_t numValuesToTest) {
+std::vector<T> GenerateRandomIntegers(size_t num_values_to_test) {
   std::vector<T> result;
+  result.reserve(num_values_to_test);
   std::mt19937_64 rng(kRandomSeed);
-  for (size_t i = 0; i < numValuesToTest; ++i) {
+  for (size_t i = 0; i < num_values_to_test; ++i) {
     result.push_back(rng());
   }
   return result;
@@ -147,7 +149,7 @@ void Swap64(char* bytes) {
 }
 
 TEST(EndianessTest, Uint16) {
-  GBSwapHelper(GenerateAllValuesForType<uint16_t>(), &Swap16);
+  GBSwapHelper(GenerateAllUint16Values(), &Swap16);
 }
 
 TEST(EndianessTest, Uint32) {
@@ -260,4 +262,5 @@ TEST(EndianessTest, big_endian) {
 }
 
 }  // namespace
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/base/internal/errno_saver.h b/absl/base/internal/errno_saver.h
new file mode 100644
index 00000000000..251de510fc9
--- /dev/null
+++ b/absl/base/internal/errno_saver.h
@@ -0,0 +1,43 @@
+// Copyright 2017 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_BASE_INTERNAL_ERRNO_SAVER_H_
+#define ABSL_BASE_INTERNAL_ERRNO_SAVER_H_
+
+#include <cerrno>
+
+#include "absl/base/config.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace base_internal {
+
+// `ErrnoSaver` captures the value of `errno` upon construction and restores it
+// upon deletion.  It is used in low-level code and must be super fast.  Do not
+// add instrumentation, even in debug modes.
+class ErrnoSaver {
+ public:
+  ErrnoSaver() : saved_errno_(errno) {}
+  ~ErrnoSaver() { errno = saved_errno_; }
+  int operator()() const { return saved_errno_; }
+
+ private:
+  const int saved_errno_;
+};
+
+}  // namespace base_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_BASE_INTERNAL_ERRNO_SAVER_H_
diff --git a/absl/base/internal/errno_saver_test.cc b/absl/base/internal/errno_saver_test.cc
new file mode 100644
index 00000000000..06be72036bd
--- /dev/null
+++ b/absl/base/internal/errno_saver_test.cc
@@ -0,0 +1,46 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/base/internal/errno_saver.h"
+
+#include <cerrno>
+#include <ostream>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/internal/strerror.h"
+
+namespace {
+using ::testing::Eq;
+
+struct ErrnoPrinter {
+  int no;
+};
+std::ostream &operator<<(std::ostream &os, ErrnoPrinter ep) {
+  return os << absl::base_internal::StrError(ep.no) << " [" << ep.no << "]";
+}
+bool operator==(ErrnoPrinter one, ErrnoPrinter two) { return one.no == two.no; }
+
+TEST(ErrnoSaverTest, Works) {
+  errno = EDOM;
+  {
+    absl::base_internal::ErrnoSaver errno_saver;
+    EXPECT_THAT(ErrnoPrinter{errno}, Eq(ErrnoPrinter{EDOM}));
+    errno = ERANGE;
+    EXPECT_THAT(ErrnoPrinter{errno}, Eq(ErrnoPrinter{ERANGE}));
+    EXPECT_THAT(ErrnoPrinter{errno_saver()}, Eq(ErrnoPrinter{EDOM}));
+  }
+  EXPECT_THAT(ErrnoPrinter{errno}, Eq(ErrnoPrinter{EDOM}));
+}
+}  // namespace
diff --git a/absl/base/internal/exception_safety_testing.h b/absl/base/internal/exception_safety_testing.h
index 6ba89d05dfc..7082da78a86 100644
--- a/absl/base/internal/exception_safety_testing.h
+++ b/absl/base/internal/exception_safety_testing.h
@@ -14,6 +14,8 @@
 
 // Utilities for testing exception-safety
 
+// SKIP_ABSL_INLINE_NAMESPACE_CHECK
+
 #ifndef ABSL_BASE_INTERNAL_EXCEPTION_SAFETY_TESTING_H_
 #define ABSL_BASE_INTERNAL_EXCEPTION_SAFETY_TESTING_H_
 
@@ -38,28 +40,29 @@
 #include "absl/strings/substitute.h"
 #include "absl/utility/utility.h"
 
+// TODO(b/500018833): Update the namespace as appropriate.
 namespace testing {
 
 enum class TypeSpec;
 enum class AllocSpec;
 
 constexpr TypeSpec operator|(TypeSpec a, TypeSpec b) {
-  using T = absl::underlying_type_t<TypeSpec>;
+  using T = std::underlying_type_t<TypeSpec>;
   return static_cast<TypeSpec>(static_cast<T>(a) | static_cast<T>(b));
 }
 
 constexpr TypeSpec operator&(TypeSpec a, TypeSpec b) {
-  using T = absl::underlying_type_t<TypeSpec>;
+  using T = std::underlying_type_t<TypeSpec>;
   return static_cast<TypeSpec>(static_cast<T>(a) & static_cast<T>(b));
 }
 
 constexpr AllocSpec operator|(AllocSpec a, AllocSpec b) {
-  using T = absl::underlying_type_t<AllocSpec>;
+  using T = std::underlying_type_t<AllocSpec>;
   return static_cast<AllocSpec>(static_cast<T>(a) | static_cast<T>(b));
 }
 
 constexpr AllocSpec operator&(AllocSpec a, AllocSpec b) {
-  using T = absl::underlying_type_t<AllocSpec>;
+  using T = std::underlying_type_t<AllocSpec>;
   return static_cast<AllocSpec>(static_cast<T>(a) & static_cast<T>(b));
 }
 
@@ -214,7 +217,7 @@ class ThrowingBool {
  public:
   ThrowingBool(bool b) noexcept : b_(b) {}  // NOLINT(runtime/explicit)
   operator bool() const {                   // NOLINT
-    exceptions_internal::MaybeThrow(ABSL_PRETTY_FUNCTION);
+    exceptions_internal::MaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION);
     return b_;
   }
 
@@ -264,7 +267,7 @@ class ThrowingValue : private exceptions_internal::TrackedObject {
 
  public:
   ThrowingValue() : TrackedObject(GetInstanceString(kDefaultValue)) {
-    exceptions_internal::MaybeThrow(ABSL_PRETTY_FUNCTION);
+    exceptions_internal::MaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION);
     dummy_ = kDefaultValue;
   }
 
@@ -272,7 +275,7 @@ class ThrowingValue : private exceptions_internal::TrackedObject {
       IsSpecified(TypeSpec::kNoThrowCopy))
       : TrackedObject(GetInstanceString(other.dummy_)) {
     if (!IsSpecified(TypeSpec::kNoThrowCopy)) {
-      exceptions_internal::MaybeThrow(ABSL_PRETTY_FUNCTION);
+      exceptions_internal::MaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION);
     }
     dummy_ = other.dummy_;
   }
@@ -281,13 +284,13 @@ class ThrowingValue : private exceptions_internal::TrackedObject {
       IsSpecified(TypeSpec::kNoThrowMove))
       : TrackedObject(GetInstanceString(other.dummy_)) {
     if (!IsSpecified(TypeSpec::kNoThrowMove)) {
-      exceptions_internal::MaybeThrow(ABSL_PRETTY_FUNCTION);
+      exceptions_internal::MaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION);
     }
     dummy_ = other.dummy_;
   }
 
   explicit ThrowingValue(int i) : TrackedObject(GetInstanceString(i)) {
-    exceptions_internal::MaybeThrow(ABSL_PRETTY_FUNCTION);
+    exceptions_internal::MaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION);
     dummy_ = i;
   }
 
@@ -301,7 +304,7 @@ class ThrowingValue : private exceptions_internal::TrackedObject {
       IsSpecified(TypeSpec::kNoThrowCopy)) {
     dummy_ = kBadValue;
     if (!IsSpecified(TypeSpec::kNoThrowCopy)) {
-      exceptions_internal::MaybeThrow(ABSL_PRETTY_FUNCTION);
+      exceptions_internal::MaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION);
     }
     dummy_ = other.dummy_;
     return *this;
@@ -311,7 +314,7 @@ class ThrowingValue : private exceptions_internal::TrackedObject {
       IsSpecified(TypeSpec::kNoThrowMove)) {
     dummy_ = kBadValue;
     if (!IsSpecified(TypeSpec::kNoThrowMove)) {
-      exceptions_internal::MaybeThrow(ABSL_PRETTY_FUNCTION);
+      exceptions_internal::MaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION);
     }
     dummy_ = other.dummy_;
     return *this;
@@ -319,73 +322,73 @@ class ThrowingValue : private exceptions_internal::TrackedObject {
 
   // Arithmetic Operators
   ThrowingValue operator+(const ThrowingValue& other) const {
-    exceptions_internal::MaybeThrow(ABSL_PRETTY_FUNCTION);
+    exceptions_internal::MaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION);
     return ThrowingValue(dummy_ + other.dummy_, nothrow_ctor);
   }
 
   ThrowingValue operator+() const {
-    exceptions_internal::MaybeThrow(ABSL_PRETTY_FUNCTION);
+    exceptions_internal::MaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION);
     return ThrowingValue(dummy_, nothrow_ctor);
   }
 
   ThrowingValue operator-(const ThrowingValue& other) const {
-    exceptions_internal::MaybeThrow(ABSL_PRETTY_FUNCTION);
+    exceptions_internal::MaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION);
     return ThrowingValue(dummy_ - other.dummy_, nothrow_ctor);
   }
 
   ThrowingValue operator-() const {
-    exceptions_internal::MaybeThrow(ABSL_PRETTY_FUNCTION);
+    exceptions_internal::MaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION);
     return ThrowingValue(-dummy_, nothrow_ctor);
   }
 
   ThrowingValue& operator++() {
-    exceptions_internal::MaybeThrow(ABSL_PRETTY_FUNCTION);
+    exceptions_internal::MaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION);
     ++dummy_;
     return *this;
   }
 
   ThrowingValue operator++(int) {
-    exceptions_internal::MaybeThrow(ABSL_PRETTY_FUNCTION);
+    exceptions_internal::MaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION);
     auto out = ThrowingValue(dummy_, nothrow_ctor);
     ++dummy_;
     return out;
   }
 
   ThrowingValue& operator--() {
-    exceptions_internal::MaybeThrow(ABSL_PRETTY_FUNCTION);
+    exceptions_internal::MaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION);
     --dummy_;
     return *this;
   }
 
   ThrowingValue operator--(int) {
-    exceptions_internal::MaybeThrow(ABSL_PRETTY_FUNCTION);
+    exceptions_internal::MaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION);
     auto out = ThrowingValue(dummy_, nothrow_ctor);
     --dummy_;
     return out;
   }
 
   ThrowingValue operator*(const ThrowingValue& other) const {
-    exceptions_internal::MaybeThrow(ABSL_PRETTY_FUNCTION);
+    exceptions_internal::MaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION);
     return ThrowingValue(dummy_ * other.dummy_, nothrow_ctor);
   }
 
   ThrowingValue operator/(const ThrowingValue& other) const {
-    exceptions_internal::MaybeThrow(ABSL_PRETTY_FUNCTION);
+    exceptions_internal::MaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION);
     return ThrowingValue(dummy_ / other.dummy_, nothrow_ctor);
   }
 
   ThrowingValue operator%(const ThrowingValue& other) const {
-    exceptions_internal::MaybeThrow(ABSL_PRETTY_FUNCTION);
+    exceptions_internal::MaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION);
     return ThrowingValue(dummy_ % other.dummy_, nothrow_ctor);
   }
 
   ThrowingValue operator<<(int shift) const {
-    exceptions_internal::MaybeThrow(ABSL_PRETTY_FUNCTION);
+    exceptions_internal::MaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION);
     return ThrowingValue(dummy_ << shift, nothrow_ctor);
   }
 
   ThrowingValue operator>>(int shift) const {
-    exceptions_internal::MaybeThrow(ABSL_PRETTY_FUNCTION);
+    exceptions_internal::MaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION);
     return ThrowingValue(dummy_ >> shift, nothrow_ctor);
   }
 
@@ -394,129 +397,129 @@ class ThrowingValue : private exceptions_internal::TrackedObject {
   // types/containers requires T to be convertible to bool.
   friend ThrowingBool operator==(const ThrowingValue& a,
                                  const ThrowingValue& b) {
-    exceptions_internal::MaybeThrow(ABSL_PRETTY_FUNCTION);
+    exceptions_internal::MaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION);
     return a.dummy_ == b.dummy_;
   }
   friend ThrowingBool operator!=(const ThrowingValue& a,
                                  const ThrowingValue& b) {
-    exceptions_internal::MaybeThrow(ABSL_PRETTY_FUNCTION);
+    exceptions_internal::MaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION);
     return a.dummy_ != b.dummy_;
   }
   friend ThrowingBool operator<(const ThrowingValue& a,
                                 const ThrowingValue& b) {
-    exceptions_internal::MaybeThrow(ABSL_PRETTY_FUNCTION);
+    exceptions_internal::MaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION);
     return a.dummy_ < b.dummy_;
   }
   friend ThrowingBool operator<=(const ThrowingValue& a,
                                  const ThrowingValue& b) {
-    exceptions_internal::MaybeThrow(ABSL_PRETTY_FUNCTION);
+    exceptions_internal::MaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION);
     return a.dummy_ <= b.dummy_;
   }
   friend ThrowingBool operator>(const ThrowingValue& a,
                                 const ThrowingValue& b) {
-    exceptions_internal::MaybeThrow(ABSL_PRETTY_FUNCTION);
+    exceptions_internal::MaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION);
     return a.dummy_ > b.dummy_;
   }
   friend ThrowingBool operator>=(const ThrowingValue& a,
                                  const ThrowingValue& b) {
-    exceptions_internal::MaybeThrow(ABSL_PRETTY_FUNCTION);
+    exceptions_internal::MaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION);
     return a.dummy_ >= b.dummy_;
   }
 
   // Logical Operators
   ThrowingBool operator!() const {
-    exceptions_internal::MaybeThrow(ABSL_PRETTY_FUNCTION);
+    exceptions_internal::MaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION);
     return !dummy_;
   }
 
   ThrowingBool operator&&(const ThrowingValue& other) const {
-    exceptions_internal::MaybeThrow(ABSL_PRETTY_FUNCTION);
+    exceptions_internal::MaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION);
     return dummy_ && other.dummy_;
   }
 
   ThrowingBool operator||(const ThrowingValue& other) const {
-    exceptions_internal::MaybeThrow(ABSL_PRETTY_FUNCTION);
+    exceptions_internal::MaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION);
     return dummy_ || other.dummy_;
   }
 
   // Bitwise Logical Operators
   ThrowingValue operator~() const {
-    exceptions_internal::MaybeThrow(ABSL_PRETTY_FUNCTION);
+    exceptions_internal::MaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION);
     return ThrowingValue(~dummy_, nothrow_ctor);
   }
 
   ThrowingValue operator&(const ThrowingValue& other) const {
-    exceptions_internal::MaybeThrow(ABSL_PRETTY_FUNCTION);
+    exceptions_internal::MaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION);
     return ThrowingValue(dummy_ & other.dummy_, nothrow_ctor);
   }
 
   ThrowingValue operator|(const ThrowingValue& other) const {
-    exceptions_internal::MaybeThrow(ABSL_PRETTY_FUNCTION);
+    exceptions_internal::MaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION);
     return ThrowingValue(dummy_ | other.dummy_, nothrow_ctor);
   }
 
   ThrowingValue operator^(const ThrowingValue& other) const {
-    exceptions_internal::MaybeThrow(ABSL_PRETTY_FUNCTION);
+    exceptions_internal::MaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION);
     return ThrowingValue(dummy_ ^ other.dummy_, nothrow_ctor);
   }
 
   // Compound Assignment operators
   ThrowingValue& operator+=(const ThrowingValue& other) {
-    exceptions_internal::MaybeThrow(ABSL_PRETTY_FUNCTION);
+    exceptions_internal::MaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION);
     dummy_ += other.dummy_;
     return *this;
   }
 
   ThrowingValue& operator-=(const ThrowingValue& other) {
-    exceptions_internal::MaybeThrow(ABSL_PRETTY_FUNCTION);
+    exceptions_internal::MaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION);
     dummy_ -= other.dummy_;
     return *this;
   }
 
   ThrowingValue& operator*=(const ThrowingValue& other) {
-    exceptions_internal::MaybeThrow(ABSL_PRETTY_FUNCTION);
+    exceptions_internal::MaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION);
     dummy_ *= other.dummy_;
     return *this;
   }
 
   ThrowingValue& operator/=(const ThrowingValue& other) {
-    exceptions_internal::MaybeThrow(ABSL_PRETTY_FUNCTION);
+    exceptions_internal::MaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION);
     dummy_ /= other.dummy_;
     return *this;
   }
 
   ThrowingValue& operator%=(const ThrowingValue& other) {
-    exceptions_internal::MaybeThrow(ABSL_PRETTY_FUNCTION);
+    exceptions_internal::MaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION);
     dummy_ %= other.dummy_;
     return *this;
   }
 
   ThrowingValue& operator&=(const ThrowingValue& other) {
-    exceptions_internal::MaybeThrow(ABSL_PRETTY_FUNCTION);
+    exceptions_internal::MaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION);
     dummy_ &= other.dummy_;
     return *this;
   }
 
   ThrowingValue& operator|=(const ThrowingValue& other) {
-    exceptions_internal::MaybeThrow(ABSL_PRETTY_FUNCTION);
+    exceptions_internal::MaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION);
     dummy_ |= other.dummy_;
     return *this;
   }
 
   ThrowingValue& operator^=(const ThrowingValue& other) {
-    exceptions_internal::MaybeThrow(ABSL_PRETTY_FUNCTION);
+    exceptions_internal::MaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION);
     dummy_ ^= other.dummy_;
     return *this;
   }
 
   ThrowingValue& operator<<=(int shift) {
-    exceptions_internal::MaybeThrow(ABSL_PRETTY_FUNCTION);
+    exceptions_internal::MaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION);
     dummy_ <<= shift;
     return *this;
   }
 
   ThrowingValue& operator>>=(int shift) {
-    exceptions_internal::MaybeThrow(ABSL_PRETTY_FUNCTION);
+    exceptions_internal::MaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION);
     dummy_ >>= shift;
     return *this;
   }
@@ -526,22 +529,37 @@ class ThrowingValue : private exceptions_internal::TrackedObject {
 
   // Stream operators
   friend std::ostream& operator<<(std::ostream& os, const ThrowingValue& tv) {
-    exceptions_internal::MaybeThrow(ABSL_PRETTY_FUNCTION);
+    exceptions_internal::MaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION);
     return os << GetInstanceString(tv.dummy_);
   }
 
   friend std::istream& operator>>(std::istream& is, const ThrowingValue&) {
-    exceptions_internal::MaybeThrow(ABSL_PRETTY_FUNCTION);
+    exceptions_internal::MaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION);
     return is;
   }
 
   // Memory management operators
-  // Args.. allows us to overload regular and placement new in one shot
+  static void* operator new(size_t s) noexcept(
+      IsSpecified(TypeSpec::kNoThrowNew)) {
+    if (!IsSpecified(TypeSpec::kNoThrowNew)) {
+      exceptions_internal::MaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION, true);
+    }
+    return ::operator new(s);
+  }
+
+  static void* operator new[](size_t s) noexcept(
+      IsSpecified(TypeSpec::kNoThrowNew)) {
+    if (!IsSpecified(TypeSpec::kNoThrowNew)) {
+      exceptions_internal::MaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION, true);
+    }
+    return ::operator new[](s);
+  }
+
   template <typename... Args>
   static void* operator new(size_t s, Args&&... args) noexcept(
       IsSpecified(TypeSpec::kNoThrowNew)) {
     if (!IsSpecified(TypeSpec::kNoThrowNew)) {
-      exceptions_internal::MaybeThrow(ABSL_PRETTY_FUNCTION, true);
+      exceptions_internal::MaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION, true);
     }
     return ::operator new(s, std::forward<Args>(args)...);
   }
@@ -550,19 +568,13 @@ class ThrowingValue : private exceptions_internal::TrackedObject {
   static void* operator new[](size_t s, Args&&... args) noexcept(
       IsSpecified(TypeSpec::kNoThrowNew)) {
     if (!IsSpecified(TypeSpec::kNoThrowNew)) {
-      exceptions_internal::MaybeThrow(ABSL_PRETTY_FUNCTION, true);
+      exceptions_internal::MaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION, true);
     }
     return ::operator new[](s, std::forward<Args>(args)...);
   }
 
   // Abseil doesn't support throwing overloaded operator delete.  These are
   // provided so a throwing operator-new can clean up after itself.
-  //
-  // We provide both regular and templated operator delete because if only the
-  // templated version is provided as we did with operator new, the compiler has
-  // no way of knowing which overload of operator delete to call. See
-  // https://en.cppreference.com/w/cpp/memory/new/operator_delete and
-  // https://en.cppreference.com/w/cpp/language/delete for the gory details.
   void operator delete(void* p) noexcept { ::operator delete(p); }
 
   template <typename... Args>
@@ -634,15 +646,14 @@ class ThrowingAllocator : private exceptions_internal::TrackedObject {
   using size_type = size_t;
   using difference_type = ptrdiff_t;
 
-  using is_nothrow =
-      std::integral_constant<bool, Spec == AllocSpec::kNoThrowAllocate>;
+  using is_nothrow = std::bool_constant<Spec == AllocSpec::kNoThrowAllocate>;
   using propagate_on_container_copy_assignment = std::true_type;
   using propagate_on_container_move_assignment = std::true_type;
   using propagate_on_container_swap = std::true_type;
   using is_always_equal = std::false_type;
 
   ThrowingAllocator() : TrackedObject(GetInstanceString(next_id_)) {
-    exceptions_internal::MaybeThrow(ABSL_PRETTY_FUNCTION);
+    exceptions_internal::MaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION);
     dummy_ = std::make_shared<const int>(next_id_++);
   }
 
@@ -693,7 +704,7 @@ class ThrowingAllocator : private exceptions_internal::TrackedObject {
 
   pointer allocate(size_type n) noexcept(
       IsSpecified(AllocSpec::kNoThrowAllocate)) {
-    ReadStateAndMaybeThrow(ABSL_PRETTY_FUNCTION);
+    ReadStateAndMaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION);
     return static_cast<pointer>(::operator new(n * sizeof(T)));
   }
 
@@ -710,7 +721,7 @@ class ThrowingAllocator : private exceptions_internal::TrackedObject {
   template <typename U, typename... Args>
   void construct(U* ptr, Args&&... args) noexcept(
       IsSpecified(AllocSpec::kNoThrowAllocate)) {
-    ReadStateAndMaybeThrow(ABSL_PRETTY_FUNCTION);
+    ReadStateAndMaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION);
     ::new (static_cast<void*>(ptr)) U(std::forward<Args>(args)...);
   }
 
@@ -726,9 +737,8 @@ class ThrowingAllocator : private exceptions_internal::TrackedObject {
 
   ThrowingAllocator select_on_container_copy_construction() noexcept(
       IsSpecified(AllocSpec::kNoThrowAllocate)) {
-    auto& out = *this;
-    ReadStateAndMaybeThrow(ABSL_PRETTY_FUNCTION);
-    return out;
+    ReadStateAndMaybeThrow(ABSL_INTERNAL_PRETTY_FUNCTION);
+    return *this;
   }
 
   template <typename U>
@@ -826,7 +836,7 @@ template <typename T>
 class DefaultFactory {
  public:
   explicit DefaultFactory(const T& t) : t_(t) {}
-  std::unique_ptr<T> operator()() const { return absl::make_unique<T>(t_); }
+  std::unique_ptr<T> operator()() const { return std::make_unique<T>(t_); }
 
  private:
   T t_;
@@ -834,10 +844,10 @@ class DefaultFactory {
 
 template <size_t LazyContractsCount, typename LazyFactory,
           typename LazyOperation>
-using EnableIfTestable = typename absl::enable_if_t<
-    LazyContractsCount != 0 &&
-    !std::is_same<LazyFactory, UninitializedT>::value &&
-    !std::is_same<LazyOperation, UninitializedT>::value>;
+using EnableIfTestable =
+    typename std::enable_if_t<LazyContractsCount != 0 &&
+                              !std::is_same_v<LazyFactory, UninitializedT> &&
+                              !std::is_same_v<LazyOperation, UninitializedT>>;
 
 template <typename Factory = UninitializedT,
           typename Operation = UninitializedT, typename... Contracts>
@@ -938,7 +948,7 @@ class ExceptionSafetyTest {
  *   `std::unique_ptr<T> operator()() const` where T is the type being tested.
  *   It is used for reliably creating identical T instances to test on.
  *
- * - Operation: The operation object (passsed in via tester.WithOperation(...)
+ * - Operation: The operation object (passed in via tester.WithOperation(...)
  *   or tester.Test(...)) must be invocable with the signature
  *   `void operator()(T*) const` where T is the type being tested. It is used
  *   for performing steps on a T instance that may throw and that need to be
@@ -986,7 +996,7 @@ class ExceptionSafetyTestBuilder {
    * method tester.WithInitialValue(...).
    */
   template <typename NewFactory>
-  ExceptionSafetyTestBuilder<absl::decay_t<NewFactory>, Operation, Contracts...>
+  ExceptionSafetyTestBuilder<std::decay_t<NewFactory>, Operation, Contracts...>
   WithFactory(const NewFactory& new_factory) const {
     return {new_factory, operation_, contracts_};
   }
@@ -997,7 +1007,7 @@ class ExceptionSafetyTestBuilder {
    * newly created tester.
    */
   template <typename NewOperation>
-  ExceptionSafetyTestBuilder<Factory, absl::decay_t<NewOperation>, Contracts...>
+  ExceptionSafetyTestBuilder<Factory, std::decay_t<NewOperation>, Contracts...>
   WithOperation(const NewOperation& new_operation) const {
     return {factory_, new_operation, contracts_};
   }
@@ -1017,11 +1027,11 @@ class ExceptionSafetyTestBuilder {
    */
   template <typename... MoreContracts>
   ExceptionSafetyTestBuilder<Factory, Operation, Contracts...,
-                             absl::decay_t<MoreContracts>...>
+                             std::decay_t<MoreContracts>...>
   WithContracts(const MoreContracts&... more_contracts) const {
     return {
         factory_, operation_,
-        std::tuple_cat(contracts_, std::tuple<absl::decay_t<MoreContracts>...>(
+        std::tuple_cat(contracts_, std::tuple<std::decay_t<MoreContracts>...>(
                                        more_contracts...))};
   }
 
@@ -1045,7 +1055,7 @@ class ExceptionSafetyTestBuilder {
       typename NewOperation,
       typename = EnableIfTestable<sizeof...(Contracts), Factory, NewOperation>>
   testing::AssertionResult Test(const NewOperation& new_operation) const {
-    return TestImpl(new_operation, absl::index_sequence_for<Contracts...>());
+    return TestImpl(new_operation, std::index_sequence_for<Contracts...>());
   }
 
   /*
@@ -1081,7 +1091,7 @@ class ExceptionSafetyTestBuilder {
 
   template <typename SelectedOperation, size_t... Indices>
   testing::AssertionResult TestImpl(SelectedOperation selected_operation,
-                                    absl::index_sequence<Indices...>) const {
+                                    std::index_sequence<Indices...>) const {
     return ExceptionSafetyTest<FactoryElementType<Factory>>(
                factory_, selected_operation, std::get<Indices>(contracts_)...)
         .Test();
diff --git a/absl/base/internal/hardening.h b/absl/base/internal/hardening.h
new file mode 100644
index 00000000000..26957a3ada0
--- /dev/null
+++ b/absl/base/internal/hardening.h
@@ -0,0 +1,138 @@
+//
+// Copyright 2026 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: hardening.h
+// -----------------------------------------------------------------------------
+//
+// This header file defines macros and functions for performing Abseil
+// hardening checks and aborts.
+
+#ifndef ABSL_BASE_INTERNAL_HARDENING_H_
+#define ABSL_BASE_INTERNAL_HARDENING_H_
+
+#include <cstddef>
+
+#include "absl/base/config.h"
+#include "absl/base/macros.h"
+#include "absl/base/options.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+namespace base_internal {
+
+// `HardeningAssert` performs runtime checks when Abseil Hardening is enabled,
+// even if `NDEBUG` is defined.
+//
+// When `NDEBUG` is not defined, `HardeningAssert`'s behavior is identical to
+// `ABSL_ASSERT`.
+//
+// Prefer a more specific assertion function over this more general one,
+// as assertion functions which perform the comparison themselves
+// can have the cost of the comparison attributed to them.
+constexpr void HardeningAssert(bool cond) {
+  ABSL_ASSERT(cond);
+#if (ABSL_OPTION_HARDENED == 1 || ABSL_OPTION_HARDENED == 2) && defined(NDEBUG)
+  if (ABSL_PREDICT_FALSE(!cond)) {
+    ABSL_INTERNAL_HARDENING_ABORT();
+  }
+#endif
+}
+
+// `HardeningAssertSlow` is used to perform runtime checks which are too
+// computationally expensive to enable widely by default.
+//
+// When `NDEBUG` is not defined, `HardeningAssertSlow`'s behavior is identical
+// to `ABSL_ASSERT`.
+constexpr void HardeningAssertSlow(bool cond) {
+  ABSL_ASSERT(cond);
+#if (ABSL_OPTION_HARDENED == 1) && defined(NDEBUG)
+  if (ABSL_PREDICT_FALSE(!cond)) {
+    ABSL_INTERNAL_HARDENING_ABORT();
+  }
+#endif
+}
+
+template <typename T1, typename T2>
+constexpr void HardeningAssertGT(T1 val1, T2 val2) {
+  ABSL_ASSERT(val1 > val2);
+#if (ABSL_OPTION_HARDENED == 1 || ABSL_OPTION_HARDENED == 2) && defined(NDEBUG)
+  if (!ABSL_PREDICT_TRUE(val1 > val2)) {
+    ABSL_INTERNAL_HARDENING_ABORT();
+  }
+#endif
+}
+
+template <typename T1, typename T2>
+constexpr void HardeningAssertGE(T1 val1, T2 val2) {
+  ABSL_ASSERT(val1 >= val2);
+#if (ABSL_OPTION_HARDENED == 1 || ABSL_OPTION_HARDENED == 2) && defined(NDEBUG)
+  if (!ABSL_PREDICT_TRUE(val1 >= val2)) {
+    ABSL_INTERNAL_HARDENING_ABORT();
+  }
+#endif
+}
+
+template <typename T1, typename T2>
+constexpr void HardeningAssertLT(T1 val1, T2 val2) {
+  ABSL_ASSERT(val1 < val2);
+#if (ABSL_OPTION_HARDENED == 1 || ABSL_OPTION_HARDENED == 2) && defined(NDEBUG)
+  if (!ABSL_PREDICT_TRUE(val1 < val2)) {
+    ABSL_INTERNAL_HARDENING_ABORT();
+  }
+#endif
+}
+
+template <typename T1, typename T2>
+constexpr void HardeningAssertLE(T1 val1, T2 val2) {
+  ABSL_ASSERT(val1 <= val2);
+#if (ABSL_OPTION_HARDENED == 1 || ABSL_OPTION_HARDENED == 2) && defined(NDEBUG)
+  if (!ABSL_PREDICT_TRUE(val1 <= val2)) {
+    ABSL_INTERNAL_HARDENING_ABORT();
+  }
+#endif
+}
+
+constexpr void HardeningAssertInBounds(size_t index, size_t size) {
+  HardeningAssertLT(index, size);
+}
+
+template <typename T>
+constexpr void HardeningAssertNonEmpty(const T& container) {
+  ABSL_ASSERT(!container.empty());
+#if (ABSL_OPTION_HARDENED == 1 || ABSL_OPTION_HARDENED == 2) && defined(NDEBUG)
+  if (ABSL_PREDICT_FALSE(container.empty())) {
+    ABSL_INTERNAL_HARDENING_ABORT();
+  }
+#endif
+}
+
+template <typename T>
+constexpr void HardeningAssertNonNull(T ptr) {
+  ABSL_ASSERT(ptr != nullptr);
+#if (ABSL_OPTION_HARDENED == 1 || ABSL_OPTION_HARDENED == 2) && defined(NDEBUG)
+  if (ABSL_PREDICT_FALSE(ptr == nullptr)) {
+    ABSL_INTERNAL_HARDENING_ABORT();
+  }
+#endif
+}
+
+}  // namespace base_internal
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_BASE_INTERNAL_HARDENING_H_
diff --git a/absl/base/internal/hardening_test.cc b/absl/base/internal/hardening_test.cc
new file mode 100644
index 00000000000..cffa87cabc9
--- /dev/null
+++ b/absl/base/internal/hardening_test.cc
@@ -0,0 +1,168 @@
+// Copyright 2026 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/base/internal/hardening.h"
+
+#include <vector>
+
+#include "gtest/gtest.h"
+#include "absl/base/macros.h"
+#include "absl/base/options.h"
+
+namespace {
+
+bool IsHardened() {
+  bool hardened = false;
+  ABSL_HARDENING_ASSERT([&hardened]() {
+      hardened = true;
+      return true;
+    }()
+  );
+  return hardened;
+}
+
+bool IsHardenedSlow() {
+  bool hardened = false;
+  ABSL_HARDENING_ASSERT_SLOW([&hardened]() {
+      hardened = true;
+      return true;
+    }()
+  );
+  return hardened;
+}
+
+TEST(HardeningTest, HardeningAssertSlow) {
+  absl::base_internal::HardeningAssertSlow(true);
+  if (!IsHardenedSlow()) {
+    absl::base_internal::HardeningAssertSlow(false);
+  }
+}
+
+TEST(HardeningDeathTest, HardeningAssertSlow) {
+#if GTEST_HAS_DEATH_TEST
+  if (IsHardenedSlow()) {
+    // The underlying mechanism of termination varies, and may include SIGILL
+    // or SIGABRT.
+    EXPECT_DEATH(absl::base_internal::HardeningAssertSlow(false), "");
+  }
+#endif
+}
+
+TEST(HardeningTest, HardeningAssertGT) {
+  absl::base_internal::HardeningAssertGT(1, 0);
+}
+
+TEST(HardeningDeathTest, HardeningAssertGT) {
+#if GTEST_HAS_DEATH_TEST
+  if (IsHardened()) {
+    // The underlying mechanism of termination varies, and may include SIGILL
+    // or SIGABRT.
+    EXPECT_DEATH(absl::base_internal::HardeningAssertGT(1, 1), "");
+    EXPECT_DEATH(absl::base_internal::HardeningAssertGT(0, 1), "");
+  }
+#endif
+}
+
+TEST(HardeningTest, HardeningAssertGE) {
+  absl::base_internal::HardeningAssertGE(1, 0);
+  absl::base_internal::HardeningAssertGE(1, 1);
+}
+
+TEST(HardeningDeathTest, HardeningAssertGE) {
+#if GTEST_HAS_DEATH_TEST
+  if (IsHardened()) {
+    // The underlying mechanism of termination varies, and may include SIGILL
+    // or SIGABRT.
+    EXPECT_DEATH(absl::base_internal::HardeningAssertGE(0, 1), "");
+  }
+#endif
+}
+
+TEST(HardeningTest, HardeningAssertLT) {
+  absl::base_internal::HardeningAssertLT(0, 1);
+}
+
+TEST(HardeningDeathTest, HardeningAssertLT) {
+#if GTEST_HAS_DEATH_TEST
+  if (IsHardened()) {
+    // The underlying mechanism of termination varies, and may include SIGILL
+    // or SIGABRT.
+    EXPECT_DEATH(absl::base_internal::HardeningAssertLT(1, 1), "");
+    EXPECT_DEATH(absl::base_internal::HardeningAssertLT(1, 0), "");
+  }
+#endif
+}
+
+TEST(HardeningTest, HardeningAssertLE) {
+  absl::base_internal::HardeningAssertLE(0, 1);
+  absl::base_internal::HardeningAssertLE(1, 1);
+}
+
+TEST(HardeningDeathTest, HardeningAssertLE) {
+#if GTEST_HAS_DEATH_TEST
+  if (IsHardened()) {
+    // The underlying mechanism of termination varies, and may include SIGILL
+    // or SIGABRT.
+    EXPECT_DEATH(absl::base_internal::HardeningAssertLE(1, 0), "");
+  }
+#endif
+}
+
+TEST(HardeningTest, HardeningAssertInBounds) {
+  absl::base_internal::HardeningAssertInBounds(0, 10);
+}
+
+TEST(HardeningDeathTest, HardeningAssertInBounds) {
+#if GTEST_HAS_DEATH_TEST
+  if (IsHardened()) {
+    // The underlying mechanism of termination varies, and may include SIGILL
+    // or SIGABRT.
+    EXPECT_DEATH(absl::base_internal::HardeningAssertInBounds(10, 10), "");
+  }
+#endif
+}
+
+TEST(HardeningTest, HardeningAssertNonEmpty) {
+  std::vector<int> v = {1};
+  absl::base_internal::HardeningAssertNonEmpty(v);
+}
+
+TEST(HardeningDeathTest, HardeningAssertNonEmpty) {
+#if GTEST_HAS_DEATH_TEST
+  if (IsHardened()) {
+    // The underlying mechanism of termination varies, and may include SIGILL
+    // or SIGABRT.
+    std::vector<int> v = {};
+    EXPECT_DEATH(absl::base_internal::HardeningAssertNonEmpty(v), "");
+  }
+#endif
+}
+
+TEST(HardeningTest, HardeningAssertNonNull) {
+  int x = 1;
+  absl::base_internal::HardeningAssertNonNull(&x);
+}
+
+TEST(HardeningDeathTest, HardeningAssertNonNull) {
+#if GTEST_HAS_DEATH_TEST
+  if (IsHardened()) {
+    // The underlying mechanism of termination varies, and may include SIGILL
+    // or SIGABRT.
+    int *x = nullptr;
+    EXPECT_DEATH(absl::base_internal::HardeningAssertNonNull(x), "");
+  }
+#endif
+}
+
+}  // namespace
diff --git a/absl/base/internal/hide_ptr.h b/absl/base/internal/hide_ptr.h
index cf8f4080246..1dba80909a9 100644
--- a/absl/base/internal/hide_ptr.h
+++ b/absl/base/internal/hide_ptr.h
@@ -17,7 +17,10 @@
 
 #include <cstdint>
 
+#include "absl/base/config.h"
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 
 // Arbitrary value with high bits set. Xor'ing with it is unlikely
@@ -42,6 +45,7 @@ inline T* UnhidePtr(uintptr_t hidden) {
 }
 
 }  // namespace base_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_BASE_INTERNAL_HIDE_PTR_H_
diff --git a/absl/base/internal/inline_variable.h b/absl/base/internal/inline_variable.h
deleted file mode 100644
index 130d8c2476d..00000000000
--- a/absl/base/internal/inline_variable.h
+++ /dev/null
@@ -1,107 +0,0 @@
-// Copyright 2017 The Abseil Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-#ifndef ABSL_BASE_INTERNAL_INLINE_VARIABLE_EMULATION_H_
-#define ABSL_BASE_INTERNAL_INLINE_VARIABLE_EMULATION_H_
-
-#include <type_traits>
-
-#include "absl/base/internal/identity.h"
-
-// File:
-//   This file define a macro that allows the creation of or emulation of C++17
-//   inline variables based on whether or not the feature is supported.
-
-////////////////////////////////////////////////////////////////////////////////
-// Macro: ABSL_INTERNAL_INLINE_CONSTEXPR(type, name, init)
-//
-// Description:
-//   Expands to the equivalent of an inline constexpr instance of the specified
-//   `type` and `name`, initialized to the value `init`. If the compiler being
-//   used is detected as supporting actual inline variables as a language
-//   feature, then the macro expands to an actual inline variable definition.
-//
-// Requires:
-//   `type` is a type that is usable in an extern variable declaration.
-//
-// Requires: `name` is a valid identifier
-//
-// Requires:
-//   `init` is an expression that can be used in the following definition:
-//     constexpr type name = init;
-//
-// Usage:
-//
-//   // Equivalent to: `inline constexpr size_t variant_npos = -1;`
-//   ABSL_INTERNAL_INLINE_CONSTEXPR(size_t, variant_npos, -1);
-//
-// Differences in implementation:
-//   For a direct, language-level inline variable, decltype(name) will be the
-//   type that was specified along with const qualification, whereas for
-//   emulated inline variables, decltype(name) may be different (in practice
-//   it will likely be a reference type).
-////////////////////////////////////////////////////////////////////////////////
-
-#ifdef __cpp_inline_variables
-
-// Clang's -Wmissing-variable-declarations option erroneously warned that
-// inline constexpr objects need to be pre-declared. This has now been fixed,
-// but we will need to support this workaround for people building with older
-// versions of clang.
-//
-// Bug: https://bugs.llvm.org/show_bug.cgi?id=35862
-//
-// Note:
-//   identity_t is used here so that the const and name are in the
-//   appropriate place for pointer types, reference types, function pointer
-//   types, etc..
-#if defined(__clang__)
-#define ABSL_INTERNAL_EXTERN_DECL(type, name) \
-  extern const ::absl::internal::identity_t<type> name;
-#else  // Otherwise, just define the macro to do nothing.
-#define ABSL_INTERNAL_EXTERN_DECL(type, name)
-#endif  // defined(__clang__)
-
-// See above comment at top of file for details.
-#define ABSL_INTERNAL_INLINE_CONSTEXPR(type, name, init) \
-  ABSL_INTERNAL_EXTERN_DECL(type, name)                  \
-  inline constexpr ::absl::internal::identity_t<type> name = init
-
-#else
-
-// See above comment at top of file for details.
-//
-// Note:
-//   identity_t is used here so that the const and name are in the
-//   appropriate place for pointer types, reference types, function pointer
-//   types, etc..
-#define ABSL_INTERNAL_INLINE_CONSTEXPR(var_type, name, init)                  \
-  template <class /*AbslInternalDummy*/ = void>                               \
-  struct AbslInternalInlineVariableHolder##name {                             \
-    static constexpr ::absl::internal::identity_t<var_type> kInstance = init; \
-  };                                                                          \
-                                                                              \
-  template <class AbslInternalDummy>                                          \
-  constexpr ::absl::internal::identity_t<var_type>                            \
-      AbslInternalInlineVariableHolder##name<AbslInternalDummy>::kInstance;   \
-                                                                              \
-  static constexpr const ::absl::internal::identity_t<var_type>&              \
-      name = /* NOLINT */                                                     \
-      AbslInternalInlineVariableHolder##name<>::kInstance;                    \
-  static_assert(sizeof(void (*)(decltype(name))) != 0,                        \
-                "Silence unused variable warnings.")
-
-#endif  // __cpp_inline_variables
-
-#endif  // ABSL_BASE_INTERNAL_INLINE_VARIABLE_EMULATION_H_
diff --git a/absl/base/internal/inline_variable_testing.h b/absl/base/internal/inline_variable_testing.h
deleted file mode 100644
index 15dc481e101..00000000000
--- a/absl/base/internal/inline_variable_testing.h
+++ /dev/null
@@ -1,44 +0,0 @@
-// Copyright 2017 The Abseil Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-#ifndef ABSL_BASE_INLINE_VARIABLE_TESTING_H_
-#define ABSL_BASE_INLINE_VARIABLE_TESTING_H_
-
-#include "absl/base/internal/inline_variable.h"
-
-namespace absl {
-namespace inline_variable_testing_internal {
-
-struct Foo {
-  int value = 5;
-};
-
-ABSL_INTERNAL_INLINE_CONSTEXPR(Foo, inline_variable_foo, {});
-ABSL_INTERNAL_INLINE_CONSTEXPR(Foo, other_inline_variable_foo, {});
-
-ABSL_INTERNAL_INLINE_CONSTEXPR(int, inline_variable_int, 5);
-ABSL_INTERNAL_INLINE_CONSTEXPR(int, other_inline_variable_int, 5);
-
-ABSL_INTERNAL_INLINE_CONSTEXPR(void(*)(), inline_variable_fun_ptr, nullptr);
-
-const Foo& get_foo_a();
-const Foo& get_foo_b();
-
-const int& get_int_a();
-const int& get_int_b();
-
-}  // namespace inline_variable_testing_internal
-}  // namespace absl
-
-#endif  // ABSL_BASE_INLINE_VARIABLE_TESTING_H_
diff --git a/absl/base/internal/invoke.h b/absl/base/internal/invoke.h
deleted file mode 100644
index 44f13302f43..00000000000
--- a/absl/base/internal/invoke.h
+++ /dev/null
@@ -1,185 +0,0 @@
-// Copyright 2017 The Abseil Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-//
-// absl::base_internal::Invoke(f, args...) is an implementation of
-// INVOKE(f, args...) from section [func.require] of the C++ standard.
-//
-// [func.require]
-// Define INVOKE (f, t1, t2, ..., tN) as follows:
-// 1. (t1.*f)(t2, ..., tN) when f is a pointer to a member function of a class T
-//    and t1 is an object of type T or a reference to an object of type T or a
-//    reference to an object of a type derived from T;
-// 2. ((*t1).*f)(t2, ..., tN) when f is a pointer to a member function of a
-//    class T and t1 is not one of the types described in the previous item;
-// 3. t1.*f when N == 1 and f is a pointer to member data of a class T and t1 is
-//    an object of type T or a reference to an object of type T or a reference
-//    to an object of a type derived from T;
-// 4. (*t1).*f when N == 1 and f is a pointer to member data of a class T and t1
-//    is not one of the types described in the previous item;
-// 5. f(t1, t2, ..., tN) in all other cases.
-//
-// The implementation is SFINAE-friendly: substitution failure within Invoke()
-// isn't an error.
-
-#ifndef ABSL_BASE_INTERNAL_INVOKE_H_
-#define ABSL_BASE_INTERNAL_INVOKE_H_
-
-#include <algorithm>
-#include <type_traits>
-#include <utility>
-
-#include "absl/meta/type_traits.h"
-
-// The following code is internal implementation detail.  See the comment at the
-// top of this file for the API documentation.
-
-namespace absl {
-namespace base_internal {
-
-// The five classes below each implement one of the clauses from the definition
-// of INVOKE. The inner class template Accept<F, Args...> checks whether the
-// clause is applicable; static function template Invoke(f, args...) does the
-// invocation.
-//
-// By separating the clause selection logic from invocation we make sure that
-// Invoke() does exactly what the standard says.
-
-template <typename Derived>
-struct StrippedAccept {
-  template <typename... Args>
-  struct Accept : Derived::template AcceptImpl<typename std::remove_cv<
-                      typename std::remove_reference<Args>::type>::type...> {};
-};
-
-// (t1.*f)(t2, ..., tN) when f is a pointer to a member function of a class T
-// and t1 is an object of type T or a reference to an object of type T or a
-// reference to an object of a type derived from T.
-struct MemFunAndRef : StrippedAccept<MemFunAndRef> {
-  template <typename... Args>
-  struct AcceptImpl : std::false_type {};
-
-  template <typename MemFunType, typename C, typename Obj, typename... Args>
-  struct AcceptImpl<MemFunType C::*, Obj, Args...>
-      : std::integral_constant<bool, std::is_base_of<C, Obj>::value &&
-                                         absl::is_function<MemFunType>::value> {
-  };
-
-  template <typename MemFun, typename Obj, typename... Args>
-  static decltype((std::declval<Obj>().*
-                   std::declval<MemFun>())(std::declval<Args>()...))
-  Invoke(MemFun&& mem_fun, Obj&& obj, Args&&... args) {
-    return (std::forward<Obj>(obj).*
-            std::forward<MemFun>(mem_fun))(std::forward<Args>(args)...);
-  }
-};
-
-// ((*t1).*f)(t2, ..., tN) when f is a pointer to a member function of a
-// class T and t1 is not one of the types described in the previous item.
-struct MemFunAndPtr : StrippedAccept<MemFunAndPtr> {
-  template <typename... Args>
-  struct AcceptImpl : std::false_type {};
-
-  template <typename MemFunType, typename C, typename Ptr, typename... Args>
-  struct AcceptImpl<MemFunType C::*, Ptr, Args...>
-      : std::integral_constant<bool, !std::is_base_of<C, Ptr>::value &&
-                                         absl::is_function<MemFunType>::value> {
-  };
-
-  template <typename MemFun, typename Ptr, typename... Args>
-  static decltype(((*std::declval<Ptr>()).*
-                   std::declval<MemFun>())(std::declval<Args>()...))
-  Invoke(MemFun&& mem_fun, Ptr&& ptr, Args&&... args) {
-    return ((*std::forward<Ptr>(ptr)).*
-            std::forward<MemFun>(mem_fun))(std::forward<Args>(args)...);
-  }
-};
-
-// t1.*f when N == 1 and f is a pointer to member data of a class T and t1 is
-// an object of type T or a reference to an object of type T or a reference
-// to an object of a type derived from T.
-struct DataMemAndRef : StrippedAccept<DataMemAndRef> {
-  template <typename... Args>
-  struct AcceptImpl : std::false_type {};
-
-  template <typename R, typename C, typename Obj>
-  struct AcceptImpl<R C::*, Obj>
-      : std::integral_constant<bool, std::is_base_of<C, Obj>::value &&
-                                         !absl::is_function<R>::value> {};
-
-  template <typename DataMem, typename Ref>
-  static decltype(std::declval<Ref>().*std::declval<DataMem>()) Invoke(
-      DataMem&& data_mem, Ref&& ref) {
-    return std::forward<Ref>(ref).*std::forward<DataMem>(data_mem);
-  }
-};
-
-// (*t1).*f when N == 1 and f is a pointer to member data of a class T and t1
-// is not one of the types described in the previous item.
-struct DataMemAndPtr : StrippedAccept<DataMemAndPtr> {
-  template <typename... Args>
-  struct AcceptImpl : std::false_type {};
-
-  template <typename R, typename C, typename Ptr>
-  struct AcceptImpl<R C::*, Ptr>
-      : std::integral_constant<bool, !std::is_base_of<C, Ptr>::value &&
-                                         !absl::is_function<R>::value> {};
-
-  template <typename DataMem, typename Ptr>
-  static decltype((*std::declval<Ptr>()).*std::declval<DataMem>()) Invoke(
-      DataMem&& data_mem, Ptr&& ptr) {
-    return (*std::forward<Ptr>(ptr)).*std::forward<DataMem>(data_mem);
-  }
-};
-
-// f(t1, t2, ..., tN) in all other cases.
-struct Callable {
-  // Callable doesn't have Accept because it's the last clause that gets picked
-  // when none of the previous clauses are applicable.
-  template <typename F, typename... Args>
-  static decltype(std::declval<F>()(std::declval<Args>()...)) Invoke(
-      F&& f, Args&&... args) {
-    return std::forward<F>(f)(std::forward<Args>(args)...);
-  }
-};
-
-// Resolves to the first matching clause.
-template <typename... Args>
-struct Invoker {
-  typedef typename std::conditional<
-      MemFunAndRef::Accept<Args...>::value, MemFunAndRef,
-      typename std::conditional<
-          MemFunAndPtr::Accept<Args...>::value, MemFunAndPtr,
-          typename std::conditional<
-              DataMemAndRef::Accept<Args...>::value, DataMemAndRef,
-              typename std::conditional<DataMemAndPtr::Accept<Args...>::value,
-                                        DataMemAndPtr, Callable>::type>::type>::
-          type>::type type;
-};
-
-// The result type of Invoke<F, Args...>.
-template <typename F, typename... Args>
-using InvokeT = decltype(Invoker<F, Args...>::type::Invoke(
-    std::declval<F>(), std::declval<Args>()...));
-
-// Invoke(f, args...) is an implementation of INVOKE(f, args...) from section
-// [func.require] of the C++ standard.
-template <typename F, typename... Args>
-InvokeT<F, Args...> Invoke(F&& f, Args&&... args) {
-  return Invoker<F, Args...>::type::Invoke(std::forward<F>(f),
-                                           std::forward<Args>(args)...);
-}
-}  // namespace base_internal
-}  // namespace absl
-
-#endif  // ABSL_BASE_INTERNAL_INVOKE_H_
diff --git a/absl/base/internal/iterator_traits.h b/absl/base/internal/iterator_traits.h
new file mode 100644
index 00000000000..51132b573e5
--- /dev/null
+++ b/absl/base/internal/iterator_traits.h
@@ -0,0 +1,75 @@
+// Copyright 2025 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: internal/iterator_traits.h
+// -----------------------------------------------------------------------------
+//
+// Helpers for querying traits of iterators, for implementing containers, etc.
+
+#ifndef ABSL_BASE_INTERNAL_ITERATOR_TRAITS_H_
+#define ABSL_BASE_INTERNAL_ITERATOR_TRAITS_H_
+
+#include <iterator>
+#include <type_traits>
+
+#include "absl/base/config.h"
+#include "absl/meta/type_traits.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace base_internal {
+
+template <typename Iterator, typename = void>
+struct IteratorCategory {};
+
+template <typename Iterator>
+struct IteratorCategory<
+    Iterator,
+    std::void_t<typename std::iterator_traits<Iterator>::iterator_category>> {
+  using type = typename std::iterator_traits<Iterator>::iterator_category;
+};
+
+template <typename Iterator, typename = void>
+struct IteratorConceptImpl : IteratorCategory<Iterator> {};
+
+template <typename Iterator>
+struct IteratorConceptImpl<
+    Iterator,
+    std::void_t<typename std::iterator_traits<Iterator>::iterator_concept>> {
+  using type = typename std::iterator_traits<Iterator>::iterator_concept;
+};
+
+// The newer `std::iterator_traits<Iterator>::iterator_concept` if available,
+// else `std::iterator_traits<Iterator>::iterator_category`.
+template <typename Iterator>
+using IteratorConcept = typename IteratorConceptImpl<Iterator>::type;
+
+template <typename IteratorTag, typename Iterator>
+using IsAtLeastIterator =
+    std::is_convertible<IteratorConcept<Iterator>, IteratorTag>;
+
+template <typename Iterator>
+using IsAtLeastInputIterator =
+    IsAtLeastIterator<std::input_iterator_tag, Iterator>;
+
+template <typename Iterator>
+using IsAtLeastForwardIterator =
+    IsAtLeastIterator<std::forward_iterator_tag, Iterator>;
+
+}  // namespace base_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_BASE_INTERNAL_ITERATOR_TRAITS_H_
diff --git a/absl/base/internal/iterator_traits_test.cc b/absl/base/internal/iterator_traits_test.cc
new file mode 100644
index 00000000000..cb2044fe001
--- /dev/null
+++ b/absl/base/internal/iterator_traits_test.cc
@@ -0,0 +1,85 @@
+// Copyright 2025 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/base/internal/iterator_traits.h"
+
+#include <forward_list>
+#include <iterator>
+#include <list>
+#include <vector>
+
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/iterator_traits_test_helper.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace base_internal {
+namespace {
+
+TEST(IsAtLeastIteratorTest, IsAtLeastIterator) {
+  EXPECT_TRUE((IsAtLeastIterator<std::input_iterator_tag, int*>()));
+  EXPECT_TRUE((IsAtLeastIterator<std::forward_iterator_tag, int*>()));
+  EXPECT_TRUE((IsAtLeastIterator<std::bidirectional_iterator_tag, int*>()));
+  EXPECT_TRUE((IsAtLeastIterator<std::random_access_iterator_tag, int*>()));
+  EXPECT_TRUE((IsAtLeastIterator<std::input_iterator_tag,
+                                 std::vector<int>::iterator>()));
+  EXPECT_TRUE((IsAtLeastIterator<std::forward_iterator_tag,
+                                 std::vector<int>::iterator>()));
+  EXPECT_TRUE((IsAtLeastIterator<std::bidirectional_iterator_tag,
+                                 std::vector<int>::iterator>()));
+  EXPECT_TRUE((IsAtLeastIterator<std::random_access_iterator_tag,
+                                 std::vector<int>::iterator>()));
+
+  EXPECT_TRUE(
+      (IsAtLeastIterator<std::input_iterator_tag, std::list<int>::iterator>()));
+  EXPECT_TRUE((IsAtLeastIterator<std::forward_iterator_tag,
+                                 std::list<int>::iterator>()));
+  EXPECT_TRUE((IsAtLeastIterator<std::bidirectional_iterator_tag,
+                                 std::list<int>::iterator>()));
+  EXPECT_FALSE((IsAtLeastIterator<std::random_access_iterator_tag,
+                                  std::list<int>::iterator>()));
+
+  EXPECT_TRUE((IsAtLeastIterator<std::input_iterator_tag,
+                                 std::forward_list<int>::iterator>()));
+  EXPECT_TRUE((IsAtLeastIterator<std::forward_iterator_tag,
+                                 std::forward_list<int>::iterator>()));
+  EXPECT_FALSE((IsAtLeastIterator<std::bidirectional_iterator_tag,
+                                  std::forward_list<int>::iterator>()));
+  EXPECT_FALSE((IsAtLeastIterator<std::random_access_iterator_tag,
+                                  std::forward_list<int>::iterator>()));
+
+  EXPECT_TRUE((IsAtLeastIterator<std::input_iterator_tag,
+                                 std::istream_iterator<int>>()));
+  EXPECT_FALSE((IsAtLeastIterator<std::forward_iterator_tag,
+                                  std::istream_iterator<int>>()));
+  EXPECT_FALSE((IsAtLeastIterator<std::bidirectional_iterator_tag,
+                                  std::istream_iterator<int>>()));
+  EXPECT_FALSE((IsAtLeastIterator<std::random_access_iterator_tag,
+                                  std::istream_iterator<int>>()));
+
+  EXPECT_TRUE((IsAtLeastIterator<std::input_iterator_tag,
+                                 Cpp20ForwardZipIterator<int*>>()));
+  EXPECT_TRUE((IsAtLeastIterator<std::forward_iterator_tag,
+                                 Cpp20ForwardZipIterator<int*>>()));
+  EXPECT_FALSE((IsAtLeastIterator<std::bidirectional_iterator_tag,
+                                  Cpp20ForwardZipIterator<int*>>()));
+  EXPECT_FALSE((IsAtLeastIterator<std::random_access_iterator_tag,
+                                  Cpp20ForwardZipIterator<int*>>()));
+}
+
+}  // namespace
+}  // namespace base_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/base/internal/iterator_traits_test_helper.h b/absl/base/internal/iterator_traits_test_helper.h
new file mode 100644
index 00000000000..707612d6621
--- /dev/null
+++ b/absl/base/internal/iterator_traits_test_helper.h
@@ -0,0 +1,97 @@
+// Copyright 2025 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_BASE_INTERNAL_ITERATOR_TRAITS_TEST_HELPER_H_
+#define ABSL_BASE_INTERNAL_ITERATOR_TRAITS_TEST_HELPER_H_
+
+#include <iterator>
+#include <utility>
+
+#include "absl/base/config.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace base_internal {
+
+// This would be a forward_iterator in C++20, but it's only an input iterator
+// before that, since it has a non-reference `reference`.
+template <typename Iterator>
+class Cpp20ForwardZipIterator {
+  using IteratorReference = typename std::iterator_traits<Iterator>::reference;
+
+ public:
+  Cpp20ForwardZipIterator() = default;
+  explicit Cpp20ForwardZipIterator(Iterator left, Iterator right)
+      : left_(left), right_(right) {}
+
+  Cpp20ForwardZipIterator& operator++() {
+    ++left_;
+    ++right_;
+    return *this;
+  }
+
+  Cpp20ForwardZipIterator operator++(int) {
+    Cpp20ForwardZipIterator tmp(*this);
+    ++*this;
+    return *this;
+  }
+
+  std::pair<IteratorReference, IteratorReference> operator*() const {
+    return {*left_, *right_};
+  }
+
+  // C++17 input iterators require `operator->`, but this isn't  possible to
+  // implement. C++20 dropped the requirement.
+
+  friend bool operator==(const Cpp20ForwardZipIterator& lhs,
+                         const Cpp20ForwardZipIterator& rhs) {
+    return lhs.left_ == rhs.left_ && lhs.right_ == rhs.right_;
+  }
+
+  friend bool operator!=(const Cpp20ForwardZipIterator& lhs,
+                         const Cpp20ForwardZipIterator& rhs) {
+    return !(lhs == rhs);
+  }
+
+ private:
+  Iterator left_{};
+  Iterator right_{};
+};
+
+}  // namespace base_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+template <typename Iterator>
+struct std::iterator_traits<
+    absl::base_internal::Cpp20ForwardZipIterator<Iterator>> {
+ private:
+  using IteratorReference = typename std::iterator_traits<Iterator>::reference;
+
+ public:
+  using iterator_category = std::input_iterator_tag;
+  using iterator_concept = std::forward_iterator_tag;
+  using value_type = std::pair<IteratorReference, IteratorReference>;
+  using difference_type =
+      typename std::iterator_traits<Iterator>::difference_type;
+  using reference = value_type;
+  using pointer = void;
+};
+
+#if defined(__cpp_lib_concepts)
+static_assert(
+    std::forward_iterator<absl::base_internal::Cpp20ForwardZipIterator<int*>>);
+#endif  // defined(__cpp_lib_concepts)
+
+#endif  // ABSL_BASE_INTERNAL_ITERATOR_TRAITS_TEST_HELPER_H_
diff --git a/absl/base/internal/low_level_alloc.cc b/absl/base/internal/low_level_alloc.cc
index 64d7aa80b13..3887a92f412 100644
--- a/absl/base/internal/low_level_alloc.cc
+++ b/absl/base/internal/low_level_alloc.cc
@@ -19,6 +19,9 @@
 
 #include "absl/base/internal/low_level_alloc.h"
 
+#include <stdint.h>
+
+#include <optional>
 #include <type_traits>
 
 #include "absl/base/call_once.h"
@@ -42,27 +45,34 @@
 #include <windows.h>
 #endif
 
+#ifdef __linux__
+#include <sys/prctl.h>
+#ifndef PR_SET_VMA
+#define PR_SET_VMA 0x53564d41
+#endif
+#ifndef PR_SET_VMA_ANON_NAME
+#define PR_SET_VMA_ANON_NAME 0
+#endif
+#endif
+
 #include <string.h>
+
 #include <algorithm>
 #include <atomic>
 #include <cerrno>
 #include <cstddef>
-#include <new>                   // for placement-new
+#include <new>  // for placement-new
 
 #include "absl/base/dynamic_annotations.h"
 #include "absl/base/internal/raw_logging.h"
 #include "absl/base/internal/spinlock.h"
 
-// MAP_ANONYMOUS
-#if defined(__APPLE__)
-// For mmap, Linux defines both MAP_ANONYMOUS and MAP_ANON and says MAP_ANON is
-// deprecated. In Darwin, MAP_ANON is all there is.
-#if !defined MAP_ANONYMOUS
+#if defined(MAP_ANON) && !defined(MAP_ANONYMOUS)
 #define MAP_ANONYMOUS MAP_ANON
-#endif  // !MAP_ANONYMOUS
-#endif  // __APPLE__
+#endif
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 
 // A first-fit allocator with amortized logarithmic free() time.
@@ -121,7 +131,7 @@ static int IntLog2(size_t size, size_t base) {
 static int Random(uint32_t *state) {
   uint32_t r = *state;
   int result = 1;
-  while ((((r = r*1103515245 + 12345) >> 30) & 1) == 0) {
+  while ((((r = r * 1103515245 + 12345) >> 30) & 1) == 0) {
     result++;
   }
   *state = r;
@@ -143,7 +153,7 @@ static int LLA_SkiplistLevels(size_t size, size_t base, uint32_t *random) {
   size_t max_fit = (size - offsetof(AllocList, next)) / sizeof(AllocList *);
   int level = IntLog2(size, base) + (random != nullptr ? Random(random) : 1);
   if (static_cast<size_t>(level) > max_fit) level = static_cast<int>(max_fit);
-  if (level > kMaxLevel-1) level = kMaxLevel - 1;
+  if (level > kMaxLevel - 1) level = kMaxLevel - 1;
   ABSL_RAW_CHECK(level >= 1, "block not big enough for even one level");
   return level;
 }
@@ -152,8 +162,8 @@ static int LLA_SkiplistLevels(size_t size, size_t base, uint32_t *random) {
 // For 0 <= i < head->levels, set prev[i] to "no_greater", where no_greater
 // points to the last element at level i in the AllocList less than *e, or is
 // head if no such element exists.
-static AllocList *LLA_SkiplistSearch(AllocList *head,
-                                     AllocList *e, AllocList **prev) {
+static AllocList *LLA_SkiplistSearch(AllocList *head, AllocList *e,
+                                     AllocList **prev) {
   AllocList *p = head;
   for (int level = head->levels - 1; level >= 0; level--) {
     for (AllocList *n; (n = p->next[level]) != nullptr && n < e; p = n) {
@@ -189,7 +199,7 @@ static void LLA_SkiplistDelete(AllocList *head, AllocList *e,
     prev[i]->next[i] = e->next[i];
   }
   while (head->levels > 0 && head->next[head->levels - 1] == nullptr) {
-    head->levels--;   // reduce head->levels if level unused
+    head->levels--;  // reduce head->levels if level unused
   }
 }
 
@@ -211,24 +221,51 @@ struct LowLevelAlloc::Arena {
   // Result of sysconf(_SC_PAGESIZE)
   const size_t pagesize;
   // Lowest power of two >= max(16, sizeof(AllocList))
-  const size_t roundup;
+  const size_t round_up;
   // Smallest allocation block size
   const size_t min_size;
   // PRNG state
   uint32_t random ABSL_GUARDED_BY(mu);
 };
 
-namespace {
-using ArenaStorage = std::aligned_storage<sizeof(LowLevelAlloc::Arena),
-                                          alignof(LowLevelAlloc::Arena)>::type;
+// ---------------------------------------------------------------
+// An async-signal-safe arena for LowLevelAlloc
+static std::atomic<base_internal::LowLevelAlloc::Arena *> g_sig_safe_arena;
+
+base_internal::LowLevelAlloc::Arena *SigSafeArena() {
+  return g_sig_safe_arena.load(std::memory_order_acquire);
+}
 
+void InitSigSafeArena() {
+  if (SigSafeArena() == nullptr) {
+    uint32_t flags = 0;
+#ifndef ABSL_LOW_LEVEL_ALLOC_ASYNC_SIGNAL_SAFE_MISSING
+    flags |= base_internal::LowLevelAlloc::kAsyncSignalSafe;
+#endif
+    base_internal::LowLevelAlloc::Arena *new_arena =
+        base_internal::LowLevelAlloc::NewArena(flags);
+    base_internal::LowLevelAlloc::Arena *old_value = nullptr;
+    if (!g_sig_safe_arena.compare_exchange_strong(old_value, new_arena,
+                                                  std::memory_order_release,
+                                                  std::memory_order_relaxed)) {
+      // We lost a race to allocate an arena; deallocate.
+      base_internal::LowLevelAlloc::DeleteArena(new_arena);
+    }
+  }
+}
+
+namespace {
 // Static storage space for the lazily-constructed, default global arena
 // instances.  We require this space because the whole point of LowLevelAlloc
 // is to avoid relying on malloc/new.
-ArenaStorage default_arena_storage;
-ArenaStorage unhooked_arena_storage;
+alignas(LowLevelAlloc::Arena) unsigned char default_arena_storage[sizeof(
+    LowLevelAlloc::Arena)];
+alignas(LowLevelAlloc::Arena) unsigned char unhooked_arena_storage[sizeof(
+    LowLevelAlloc::Arena)];
 #ifndef ABSL_LOW_LEVEL_ALLOC_ASYNC_SIGNAL_SAFE_MISSING
-ArenaStorage unhooked_async_sig_safe_arena_storage;
+alignas(
+    LowLevelAlloc::Arena) unsigned char unhooked_async_sig_safe_arena_storage
+    [sizeof(LowLevelAlloc::Arena)];
 #endif
 
 // We must use LowLevelCallOnce here to construct the global arenas, rather than
@@ -247,9 +284,9 @@ void CreateGlobalArenas() {
 
 // Returns a global arena that does not call into hooks.  Used by NewArena()
 // when kCallMallocHook is not set.
-LowLevelAlloc::Arena* UnhookedArena() {
+LowLevelAlloc::Arena *UnhookedArena() {
   base_internal::LowLevelCallOnce(&create_globals_once, CreateGlobalArenas);
-  return reinterpret_cast<LowLevelAlloc::Arena*>(&unhooked_arena_storage);
+  return reinterpret_cast<LowLevelAlloc::Arena *>(&unhooked_arena_storage);
 }
 
 #ifndef ABSL_LOW_LEVEL_ALLOC_ASYNC_SIGNAL_SAFE_MISSING
@@ -267,7 +304,7 @@ LowLevelAlloc::Arena *UnhookedAsyncSigSafeArena() {
 // Returns the default arena, as used by LowLevelAlloc::Alloc() and friends.
 LowLevelAlloc::Arena *LowLevelAlloc::DefaultArena() {
   base_internal::LowLevelCallOnce(&create_globals_once, CreateGlobalArenas);
-  return reinterpret_cast<LowLevelAlloc::Arena*>(&default_arena_storage);
+  return reinterpret_cast<LowLevelAlloc::Arena *>(&default_arena_storage);
 }
 
 // magic numbers to identify allocated and unallocated blocks
@@ -287,11 +324,11 @@ class ABSL_SCOPED_LOCKABLE ArenaLock {
       mask_valid_ = pthread_sigmask(SIG_BLOCK, &all, &mask_) == 0;
     }
 #endif
-    arena_->mu.Lock();
+    arena_->mu.lock();
   }
   ~ArenaLock() { ABSL_RAW_CHECK(left_, "haven't left Arena region"); }
   void Leave() ABSL_UNLOCK_FUNCTION() {
-    arena_->mu.Unlock();
+    arena_->mu.unlock();
 #ifndef ABSL_LOW_LEVEL_ALLOC_ASYNC_SIGNAL_SAFE_MISSING
     if (mask_valid_) {
       const int err = pthread_sigmask(SIG_SETMASK, &mask_, nullptr);
@@ -327,20 +364,20 @@ size_t GetPageSize() {
   SYSTEM_INFO system_info;
   GetSystemInfo(&system_info);
   return std::max(system_info.dwPageSize, system_info.dwAllocationGranularity);
-#elif defined(__wasm__) || defined(__asmjs__)
-  return getpagesize();
+#elif defined(__wasm__) || defined(__asmjs__) || defined(__hexagon__)
+  return static_cast<size_t>(getpagesize());
 #else
-  return sysconf(_SC_PAGESIZE);
+  return static_cast<size_t>(sysconf(_SC_PAGESIZE));
 #endif
 }
 
 size_t RoundedUpBlockSize() {
   // Round up block sizes to a power of two close to the header size.
-  size_t roundup = 16;
-  while (roundup < sizeof(AllocList::Header)) {
-    roundup += roundup;
+  size_t round_up = 16;
+  while (round_up < sizeof(AllocList::Header)) {
+    round_up += round_up;
   }
-  return roundup;
+  return round_up;
 }
 
 }  // namespace
@@ -350,19 +387,18 @@ LowLevelAlloc::Arena::Arena(uint32_t flags_value)
       allocation_count(0),
       flags(flags_value),
       pagesize(GetPageSize()),
-      roundup(RoundedUpBlockSize()),
-      min_size(2 * roundup),
+      round_up(RoundedUpBlockSize()),
+      min_size(2 * round_up),
       random(0) {
   freelist.header.size = 0;
-  freelist.header.magic =
-      Magic(kMagicUnallocated, &freelist.header);
+  freelist.header.magic = Magic(kMagicUnallocated, &freelist.header);
   freelist.header.arena = this;
   freelist.levels = 0;
   memset(freelist.next, 0, sizeof(freelist.next));
 }
 
 // L < meta_data_arena->mu
-LowLevelAlloc::Arena *LowLevelAlloc::NewArena(int32_t flags) {
+LowLevelAlloc::Arena *LowLevelAlloc::NewArena(uint32_t flags) {
   Arena *meta_data_arena = DefaultArena();
 #ifndef ABSL_LOW_LEVEL_ALLOC_ASYNC_SIGNAL_SAFE_MISSING
   if ((flags & LowLevelAlloc::kAsyncSignalSafe) != 0) {
@@ -373,7 +409,7 @@ LowLevelAlloc::Arena *LowLevelAlloc::NewArena(int32_t flags) {
     meta_data_arena = UnhookedArena();
   }
   Arena *result =
-    new (AllocWithArena(sizeof (*result), meta_data_arena)) Arena(flags);
+      new (AllocWithArena(sizeof(*result), meta_data_arena)) Arena(flags);
   return result;
 }
 
@@ -447,8 +483,8 @@ static inline uintptr_t RoundUp(uintptr_t addr, uintptr_t align) {
 // that the freelist is in the correct order, that it
 // consists of regions marked "unallocated", and that no two regions
 // are adjacent in memory (they should have been coalesced).
-// L < arena->mu
-static AllocList *Next(int i, AllocList *prev, LowLevelAlloc::Arena *arena) {
+static AllocList *Next(int i, AllocList *prev, LowLevelAlloc::Arena *arena)
+    ABSL_EXCLUSIVE_LOCKS_REQUIRED(arena->mu) {
   ABSL_RAW_CHECK(i < prev->levels, "too few levels in Next()");
   AllocList *next = prev->next[i];
   if (next != nullptr) {
@@ -472,44 +508,43 @@ static void Coalesce(AllocList *a) {
   if (n != nullptr && reinterpret_cast<char *>(a) + a->header.size ==
                           reinterpret_cast<char *>(n)) {
     LowLevelAlloc::Arena *arena = a->header.arena;
+    arena->mu.AssertHeld();
     a->header.size += n->header.size;
     n->header.magic = 0;
     n->header.arena = nullptr;
     AllocList *prev[kMaxLevel];
     LLA_SkiplistDelete(&arena->freelist, n, prev);
     LLA_SkiplistDelete(&arena->freelist, a, prev);
-    a->levels = LLA_SkiplistLevels(a->header.size, arena->min_size,
-                                   &arena->random);
+    a->levels =
+        LLA_SkiplistLevels(a->header.size, arena->min_size, &arena->random);
     LLA_SkiplistInsert(&arena->freelist, a, prev);
   }
 }
 
 // Adds block at location "v" to the free list
-// L >= arena->mu
-static void AddToFreelist(void *v, LowLevelAlloc::Arena *arena) {
-  AllocList *f = reinterpret_cast<AllocList *>(
-                        reinterpret_cast<char *>(v) - sizeof (f->header));
+static void AddToFreelist(void *v, LowLevelAlloc::Arena *arena)
+    ABSL_EXCLUSIVE_LOCKS_REQUIRED(arena->mu) {
+  AllocList *f = reinterpret_cast<AllocList *>(reinterpret_cast<char *>(v) -
+                                               sizeof(f->header));
   ABSL_RAW_CHECK(f->header.magic == Magic(kMagicAllocated, &f->header),
                  "bad magic number in AddToFreelist()");
   ABSL_RAW_CHECK(f->header.arena == arena,
                  "bad arena pointer in AddToFreelist()");
-  f->levels = LLA_SkiplistLevels(f->header.size, arena->min_size,
-                                 &arena->random);
+  f->levels =
+      LLA_SkiplistLevels(f->header.size, arena->min_size, &arena->random);
   AllocList *prev[kMaxLevel];
   LLA_SkiplistInsert(&arena->freelist, f, prev);
   f->header.magic = Magic(kMagicUnallocated, &f->header);
-  Coalesce(f);                  // maybe coalesce with successor
-  Coalesce(prev[0]);            // maybe coalesce with predecessor
+  Coalesce(f);        // maybe coalesce with successor
+  Coalesce(prev[0]);  // maybe coalesce with predecessor
 }
 
 // Frees storage allocated by LowLevelAlloc::Alloc().
 // L < arena->mu
 void LowLevelAlloc::Free(void *v) {
   if (v != nullptr) {
-    AllocList *f = reinterpret_cast<AllocList *>(
-                        reinterpret_cast<char *>(v) - sizeof (f->header));
-    ABSL_RAW_CHECK(f->header.magic == Magic(kMagicAllocated, &f->header),
-                   "bad magic number in Free()");
+    AllocList *f = reinterpret_cast<AllocList *>(reinterpret_cast<char *>(v) -
+                                                 sizeof(f->header));
     LowLevelAlloc::Arena *arena = f->header.arena;
     ArenaLock section(arena);
     AddToFreelist(v, arena);
@@ -524,33 +559,33 @@ void LowLevelAlloc::Free(void *v) {
 static void *DoAllocWithArena(size_t request, LowLevelAlloc::Arena *arena) {
   void *result = nullptr;
   if (request != 0) {
-    AllocList *s;       // will point to region that satisfies request
+    AllocList *s;  // will point to region that satisfies request
     ArenaLock section(arena);
     // round up with header
-    size_t req_rnd = RoundUp(CheckedAdd(request, sizeof (s->header)),
-                             arena->roundup);
-    for (;;) {      // loop until we find a suitable region
+    size_t req_rnd =
+        RoundUp(CheckedAdd(request, sizeof(s->header)), arena->round_up);
+    for (;;) {  // loop until we find a suitable region
       // find the minimum levels that a block of this size must have
       int i = LLA_SkiplistLevels(req_rnd, arena->min_size, nullptr) - 1;
-      if (i < arena->freelist.levels) {   // potential blocks exist
+      if (i < arena->freelist.levels) {        // potential blocks exist
         AllocList *before = &arena->freelist;  // predecessor of s
         while ((s = Next(i, before, arena)) != nullptr &&
                s->header.size < req_rnd) {
           before = s;
         }
-        if (s != nullptr) {       // we found a region
+        if (s != nullptr) {  // we found a region
           break;
         }
       }
       // we unlock before mmap() both because mmap() may call a callback hook,
       // and because it may be slow.
-      arena->mu.Unlock();
+      arena->mu.unlock();
       // mmap generous 64K chunks to decrease
       // the chances/impact of fragmentation:
       size_t new_pages_size = RoundUp(req_rnd, arena->pagesize * 16);
       void *new_pages;
 #ifdef _WIN32
-      new_pages = VirtualAlloc(0, new_pages_size,
+      new_pages = VirtualAlloc(nullptr, new_pages_size,
                                MEM_RESERVE | MEM_COMMIT, PAGE_READWRITE);
       ABSL_RAW_CHECK(new_pages != nullptr, "VirtualAlloc failed");
 #else
@@ -570,8 +605,20 @@ static void *DoAllocWithArena(size_t request, LowLevelAlloc::Arena *arena) {
         ABSL_RAW_LOG(FATAL, "mmap error: %d", errno);
       }
 
+#ifdef __linux__
+#if defined(PR_SET_VMA) && defined(PR_SET_VMA_ANON_NAME)
+      // Attempt to name the allocated address range in /proc/$PID/smaps on
+      // Linux.
+      //
+      // This invocation of prctl() may fail if the Linux kernel was not
+      // configured with the CONFIG_ANON_VMA_NAME option.  This is OK since
+      // the naming of arenas is primarily a debugging aid.
+      prctl(PR_SET_VMA, PR_SET_VMA_ANON_NAME, new_pages, new_pages_size,
+            "absl");
+#endif
+#endif  // __linux__
 #endif  // _WIN32
-      arena->mu.Lock();
+      arena->mu.lock();
       s = reinterpret_cast<AllocList *>(new_pages);
       s->header.size = new_pages_size;
       // Pretend the block is allocated; call AddToFreelist() to free it.
@@ -580,12 +627,12 @@ static void *DoAllocWithArena(size_t request, LowLevelAlloc::Arena *arena) {
       AddToFreelist(&s->levels, arena);  // insert new region into free list
     }
     AllocList *prev[kMaxLevel];
-    LLA_SkiplistDelete(&arena->freelist, s, prev);    // remove from free list
+    LLA_SkiplistDelete(&arena->freelist, s, prev);  // remove from free list
     // s points to the first free region that's big enough
     if (CheckedAdd(req_rnd, arena->min_size) <= s->header.size) {
       // big enough to split
-      AllocList *n = reinterpret_cast<AllocList *>
-                        (req_rnd + reinterpret_cast<char *>(s));
+      AllocList *n =
+          reinterpret_cast<AllocList *>(req_rnd + reinterpret_cast<char *>(s));
       n->header.size = s->header.size - req_rnd;
       n->header.magic = Magic(kMagicAllocated, &n->header);
       n->header.arena = arena;
@@ -598,7 +645,7 @@ static void *DoAllocWithArena(size_t request, LowLevelAlloc::Arena *arena) {
     section.Leave();
     result = &s->levels;
   }
-  ANNOTATE_MEMORY_IS_UNINITIALIZED(result, request);
+  ABSL_ANNOTATE_MEMORY_IS_UNINITIALIZED(result, request);
   return result;
 }
 
@@ -614,6 +661,7 @@ void *LowLevelAlloc::AllocWithArena(size_t request, Arena *arena) {
 }
 
 }  // namespace base_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_LOW_LEVEL_ALLOC_MISSING
diff --git a/absl/base/internal/low_level_alloc.h b/absl/base/internal/low_level_alloc.h
index c98cdb30235..23218dd5a6f 100644
--- a/absl/base/internal/low_level_alloc.h
+++ b/absl/base/internal/low_level_alloc.h
@@ -46,7 +46,8 @@
 // for more information.
 #ifdef ABSL_LOW_LEVEL_ALLOC_ASYNC_SIGNAL_SAFE_MISSING
 #error ABSL_LOW_LEVEL_ALLOC_ASYNC_SIGNAL_SAFE_MISSING cannot be directly set
-#elif defined(_WIN32) || defined(__asmjs__) || defined(__wasm__)
+#elif defined(_WIN32) || defined(__asmjs__) || defined(__wasm__) || \
+    defined(__hexagon__)
 #define ABSL_LOW_LEVEL_ALLOC_ASYNC_SIGNAL_SAFE_MISSING 1
 #endif
 
@@ -55,6 +56,7 @@
 #include "absl/base/port.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 
 class LowLevelAlloc {
@@ -102,7 +104,7 @@ class LowLevelAlloc {
   // the provided flags.  For example, the call NewArena(kAsyncSignalSafe)
   // is itself async-signal-safe, as well as generatating an arena that provides
   // async-signal-safe Alloc/Free.
-  static Arena *NewArena(int32_t flags);
+  static Arena *NewArena(uint32_t flags);
 
   // Destroys an arena allocated by NewArena and returns true,
   // provided no allocated blocks remain in the arena.
@@ -118,7 +120,14 @@ class LowLevelAlloc {
   LowLevelAlloc();      // no instances
 };
 
+// Returns a global async-signal-safe arena for LowLevelAlloc.
+LowLevelAlloc::Arena *SigSafeArena();
+
+// Ensures the global async-signal-safe arena for LowLevelAlloc is initialized.
+void InitSigSafeArena();
+
 }  // namespace base_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_BASE_INTERNAL_LOW_LEVEL_ALLOC_H_
diff --git a/absl/base/internal/low_level_alloc_test.cc b/absl/base/internal/low_level_alloc_test.cc
index 34a080cb15a..3282076f492 100644
--- a/absl/base/internal/low_level_alloc_test.cc
+++ b/absl/base/internal/low_level_alloc_test.cc
@@ -17,11 +17,19 @@
 #include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>
+
 #include <thread>  // NOLINT(build/c++11)
 #include <unordered_map>
 #include <utility>
 
+#include "absl/container/node_hash_map.h"
+
+#ifdef __EMSCRIPTEN__
+#include <emscripten.h>
+#endif
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 namespace {
 
@@ -74,12 +82,12 @@ static bool using_low_level_alloc = false;
 // allocations and deallocations are reported via the MallocHook
 // interface.
 static void Test(bool use_new_arena, bool call_malloc_hook, int n) {
-  typedef std::unordered_map<int, BlockDesc> AllocMap;
+  typedef absl::node_hash_map<int, BlockDesc> AllocMap;
   AllocMap allocated;
   AllocMap::iterator it;
   BlockDesc block_desc;
   int rnd;
-  LowLevelAlloc::Arena *arena = 0;
+  LowLevelAlloc::Arena *arena = nullptr;
   if (use_new_arena) {
     int32_t flags = call_malloc_hook ? LowLevelAlloc::kCallMallocHook : 0;
     arena = LowLevelAlloc::NewArena(flags);
@@ -94,11 +102,10 @@ static void Test(bool use_new_arena, bool call_malloc_hook, int n) {
     case 0:     // coin came up heads: add a block
       using_low_level_alloc = true;
       block_desc.len = rand() & 0x3fff;
-      block_desc.ptr =
-        reinterpret_cast<char *>(
-                        arena == 0
-                        ? LowLevelAlloc::Alloc(block_desc.len)
-                        : LowLevelAlloc::AllocWithArena(block_desc.len, arena));
+      block_desc.ptr = reinterpret_cast<char *>(
+          arena == nullptr
+              ? LowLevelAlloc::Alloc(block_desc.len)
+              : LowLevelAlloc::AllocWithArena(block_desc.len, arena));
       using_low_level_alloc = false;
       RandomizeBlockDesc(&block_desc);
       rnd = rand();
@@ -149,10 +156,26 @@ static struct BeforeMain {
 
 }  // namespace
 }  // namespace base_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 int main(int argc, char *argv[]) {
   // The actual test runs in the global constructor of `before_main`.
   printf("PASS\n");
+#ifdef __EMSCRIPTEN__
+  // clang-format off
+// This is JS here. Don't try to format it.
+    MAIN_THREAD_EM_ASM({
+      if (ENVIRONMENT_IS_WEB) {
+        if (typeof TEST_FINISH === 'function') {
+          TEST_FINISH($0);
+        } else {
+          console.error('Attempted to exit with status ' + $0);
+          console.error('But TEST_FINSIHED is not a function.');
+        }
+      }
+    }, 0);
+// clang-format on
+#endif
   return 0;
 }
diff --git a/absl/base/internal/low_level_scheduling.h b/absl/base/internal/low_level_scheduling.h
index 0fcc8d3bed9..04e44e94fa2 100644
--- a/absl/base/internal/low_level_scheduling.h
+++ b/absl/base/internal/low_level_scheduling.h
@@ -18,7 +18,12 @@
 #ifndef ABSL_BASE_INTERNAL_LOW_LEVEL_SCHEDULING_H_
 #define ABSL_BASE_INTERNAL_LOW_LEVEL_SCHEDULING_H_
 
+#include <atomic>
+#include <cstdint>
+
+#include "absl/base/internal/raw_logging.h"
 #include "absl/base/internal/scheduling_mode.h"
+#include "absl/base/internal/thread_identity.h"
 #include "absl/base/macros.h"
 
 // The following two declarations exist so SchedulingGuard may friend them with
@@ -28,6 +33,14 @@ extern "C" bool __google_disable_rescheduling(void);
 extern "C" void __google_enable_rescheduling(bool disable_result);
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
+class CondVar;
+class Mutex;
+
+namespace synchronization_internal {
+int MutexDelay(int32_t c, int mode);
+}  // namespace synchronization_internal
+
 namespace base_internal {
 
 class SchedulingHelper;  // To allow use of SchedulingGuard.
@@ -52,8 +65,9 @@ class SchedulingGuard {
  public:
   // Returns true iff the calling thread may be cooperatively rescheduled.
   static bool ReschedulingIsAllowed();
+  SchedulingGuard(const SchedulingGuard&) = delete;
+  SchedulingGuard& operator=(const SchedulingGuard&) = delete;
 
- private:
   // Disable cooperative rescheduling of the calling thread.  It may still
   // initiate scheduling operations (e.g. wake-ups), however, it may not itself
   // reschedule.  Nestable.  The returned result is opaque, clients should not
@@ -75,12 +89,16 @@ class SchedulingGuard {
     bool disabled;
   };
 
-  // Access to SchedulingGuard is explicitly white-listed.
-  friend class SchedulingHelper;
-  friend class SpinLock;
+  // A scoped helper to enable rescheduling temporarily.
+  // REQUIRES: destructor must run in same thread as constructor.
+  class ScopedEnable {
+   public:
+    ScopedEnable();
+    ~ScopedEnable();
 
-  SchedulingGuard(const SchedulingGuard&) = delete;
-  SchedulingGuard& operator=(const SchedulingGuard&) = delete;
+   private:
+    int scheduling_disabled_depth_;
+  };
 };
 
 //------------------------------------------------------------------------------
@@ -88,18 +106,92 @@ class SchedulingGuard {
 //------------------------------------------------------------------------------
 
 inline bool SchedulingGuard::ReschedulingIsAllowed() {
-  return false;
+  ThreadIdentity* identity = CurrentThreadIdentityIfPresent();
+  if (identity != nullptr) {
+    ThreadIdentity::SchedulerState* state = &identity->scheduler_state;
+    // For a thread to be eligible for re-scheduling it must have a bound
+    // schedulable (otherwise it's not cooperative) and not be within a
+    // SchedulerGuard region.
+    return state->bound_schedulable.load(std::memory_order_relaxed) !=
+               nullptr &&
+           state->scheduling_disabled_depth.load(std::memory_order_relaxed) ==
+               0;
+  } else {
+    // Cooperative threads always have a ThreadIdentity.
+    return false;
+  }
 }
 
+// We don't use [[nodiscard]] here as some clients (e.g.
+// FinishPotentiallyBlockingRegion()) cannot yet properly consume it.
 inline bool SchedulingGuard::DisableRescheduling() {
-  return false;
+  ThreadIdentity* identity;
+  identity = CurrentThreadIdentityIfPresent();
+  if (identity != nullptr) {
+    // The depth is accessed concurrently from other threads, so it must be
+    // atomic, but it's only mutated from this thread, so we don't need an
+    // atomic increment.
+    int old_val = identity->scheduler_state.scheduling_disabled_depth.load(
+        std::memory_order_relaxed);
+    identity->scheduler_state.scheduling_disabled_depth.store(
+        old_val + 1, std::memory_order_relaxed);
+    return true;
+  } else {
+    return false;
+  }
+}
+
+inline void SchedulingGuard::EnableRescheduling(bool disable_result) {
+  if (!disable_result) {
+    // There was no installed thread identity at the time that scheduling was
+    // disabled, so we have nothing to do.  This is an implementation detail
+    // that may change in the future, clients may not depend on it.
+    // EnableRescheduling() must always be called.
+    return;
+  }
+
+  ThreadIdentity* identity;
+  // A thread identity exists, see above
+  identity = CurrentThreadIdentityIfPresent();
+  // The depth is accessed concurrently from other threads, so it must be
+  // atomic, but it's only mutated from this thread, so we don't need an atomic
+  // decrement.
+  int old_val = identity->scheduler_state.scheduling_disabled_depth.load(
+      std::memory_order_relaxed);
+  identity->scheduler_state.scheduling_disabled_depth.store(
+      old_val - 1, std::memory_order_relaxed);
+}
+
+inline SchedulingGuard::ScopedEnable::ScopedEnable() {
+  ThreadIdentity* identity;
+  identity = CurrentThreadIdentityIfPresent();
+  if (identity != nullptr) {
+    scheduling_disabled_depth_ =
+        identity->scheduler_state.scheduling_disabled_depth.load(
+            std::memory_order_relaxed);
+    if (scheduling_disabled_depth_ != 0) {
+      // The store below does not need to be compare_exchange because
+      // the value is never modified concurrently (only accessed).
+      identity->scheduler_state.scheduling_disabled_depth.store(
+          0, std::memory_order_relaxed);
+    }
+  } else {
+    scheduling_disabled_depth_ = 0;
+  }
 }
 
-inline void SchedulingGuard::EnableRescheduling(bool /* disable_result */) {
-  return;
+inline SchedulingGuard::ScopedEnable::~ScopedEnable() {
+  if (scheduling_disabled_depth_ == 0) {
+    return;
+  }
+  ThreadIdentity* identity = CurrentThreadIdentityIfPresent();
+  // itentity is guaranteed to exist, see the constructor above.
+  identity->scheduler_state.scheduling_disabled_depth.store(
+      scheduling_disabled_depth_, std::memory_order_relaxed);
 }
 
 }  // namespace base_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_BASE_INTERNAL_LOW_LEVEL_SCHEDULING_H_
diff --git a/absl/base/internal/nullability_traits.h b/absl/base/internal/nullability_traits.h
new file mode 100644
index 00000000000..790ec909643
--- /dev/null
+++ b/absl/base/internal/nullability_traits.h
@@ -0,0 +1,71 @@
+// Copyright 2025 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_BASE_INTERNAL_NULLABILITY_TRAITS_H_
+#define ABSL_BASE_INTERNAL_NULLABILITY_TRAITS_H_
+
+#include <type_traits>
+
+#include "absl/base/config.h"
+#include "absl/base/nullability.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace base_internal {
+
+// `value` is true if the type `T` is compatible with nullability annotations
+// (is a raw pointer, a smart pointer, or marked with
+// ABSL_NULLABILITY_COMPATIBLE). Prefer to use the higher-level
+// `AddNonnullIfCompatible` if that is sufficient.
+//
+// NOTE: This should not be used to detect if the compiler is Clang (since
+// Clang is the only compiler that supports nullability annotations).
+#if defined(__clang__) && !defined(__OBJC__) && \
+    ABSL_HAVE_FEATURE(nullability_on_classes)
+template <class T, class = void>
+struct IsNullabilityCompatibleType {
+  constexpr static bool value = false;
+};
+
+template <class T>
+struct IsNullabilityCompatibleType<T, std::void_t<absl_nullable T>> {
+  constexpr static bool value = true;
+};
+#else
+// False when absl_nullable is a no-op (for non-Clang compilers or Objective-C.)
+template <class T, class = void>
+struct IsNullabilityCompatibleType {
+  constexpr static bool value = false;
+};
+#endif
+
+// A trait to add `absl_nonnull` to a type if it is compatible with nullability
+// annotations.
+template <typename T, bool ShouldAdd = IsNullabilityCompatibleType<T>::value>
+struct AddNonnullIfCompatible;
+
+template <typename T>
+struct AddNonnullIfCompatible<T, false> {
+  using type = T;
+};
+template <typename T>
+struct AddNonnullIfCompatible<T, true> {
+  using type = absl_nonnull T;
+};
+
+}  // namespace base_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_BASE_INTERNAL_NULLABILITY_TRAITS_H_
diff --git a/absl/base/internal/nullability_traits_test.cc b/absl/base/internal/nullability_traits_test.cc
new file mode 100644
index 00000000000..2451239c0a6
--- /dev/null
+++ b/absl/base/internal/nullability_traits_test.cc
@@ -0,0 +1,98 @@
+// Copyright 2025 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/base/internal/nullability_traits.h"
+
+#include <memory>
+#include <type_traits>
+
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+#include "absl/base/nullability.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace base_internal {
+namespace {
+
+struct NotSmartPtr {
+  int* x;
+};
+
+class ABSL_NULLABILITY_COMPATIBLE MySmartPtr : std::unique_ptr<int> {};
+
+// The IsNullabilityCompatibleType trait value can only be true when we define
+// `absl_nullable` (isn't a no-op).
+#if defined(__clang__) && !defined(__OBJC__) && \
+    ABSL_HAVE_FEATURE(nullability_on_classes)
+#define EXPECT_TRUE_IF_SUPPORTED EXPECT_TRUE
+#else
+#define EXPECT_TRUE_IF_SUPPORTED EXPECT_FALSE
+#endif
+
+TEST(NullabilityTraitsTest, IsNullabilityEligibleTypePrimitives) {
+  EXPECT_FALSE(IsNullabilityCompatibleType<int>::value);
+  EXPECT_FALSE(IsNullabilityCompatibleType<int*&>::value);
+
+  EXPECT_TRUE_IF_SUPPORTED(IsNullabilityCompatibleType<int*>::value);
+  EXPECT_TRUE_IF_SUPPORTED(IsNullabilityCompatibleType<int**>::value);
+  EXPECT_TRUE_IF_SUPPORTED(IsNullabilityCompatibleType<int* const>::value);
+  EXPECT_TRUE_IF_SUPPORTED(IsNullabilityCompatibleType<const int*>::value);
+  EXPECT_TRUE_IF_SUPPORTED(IsNullabilityCompatibleType<void (*)(int)>::value);
+}
+
+TEST(NullabilityTraitsTest, IsNullabilityCompatibleTypeAliases) {
+  using MyInt = int;
+  using MyIntPtr = int*;
+  EXPECT_FALSE(IsNullabilityCompatibleType<MyInt>::value);
+  EXPECT_TRUE_IF_SUPPORTED(IsNullabilityCompatibleType<MyIntPtr>::value);
+}
+
+TEST(NullabilityTraitsTest, IsNullabilityCompatibleTypeSmartPointers) {
+  EXPECT_TRUE_IF_SUPPORTED(
+      IsNullabilityCompatibleType<std::unique_ptr<int>>::value);
+  EXPECT_TRUE_IF_SUPPORTED(
+      IsNullabilityCompatibleType<std::shared_ptr<int>>::value);
+
+  EXPECT_FALSE(IsNullabilityCompatibleType<NotSmartPtr>::value);
+  EXPECT_TRUE_IF_SUPPORTED(IsNullabilityCompatibleType<NotSmartPtr*>::value);
+  EXPECT_TRUE_IF_SUPPORTED(IsNullabilityCompatibleType<MySmartPtr>::value);
+}
+
+#undef EXPECT_TRUE_IF_SUPPORTED
+
+TEST(NullabilityTraitsTest, AddNonnullIfCompatiblePassThroughPrimitives) {
+  EXPECT_TRUE((std::is_same_v<AddNonnullIfCompatible<int>::type, int>));
+  EXPECT_TRUE((std::is_same_v<AddNonnullIfCompatible<int*>::type, int*>));
+  EXPECT_TRUE(
+      (std::is_same_v<AddNonnullIfCompatible<int* const>::type, int* const>));
+}
+
+TEST(NullabilityTraitsTest, AddNonnullIfCompatiblePassThroughSmartPointers) {
+  EXPECT_TRUE(
+      (std::is_same_v<AddNonnullIfCompatible<std::unique_ptr<int>>::type,
+                      std::unique_ptr<int>>));
+  EXPECT_TRUE(
+      (std::is_same_v<AddNonnullIfCompatible<std::shared_ptr<int>>::type,
+                      std::shared_ptr<int>>));
+  EXPECT_TRUE(
+      (std::is_same_v<AddNonnullIfCompatible<NotSmartPtr>::type, NotSmartPtr>));
+  EXPECT_TRUE(
+      (std::is_same_v<AddNonnullIfCompatible<MySmartPtr>::type, MySmartPtr>));
+}
+
+}  // namespace
+}  // namespace base_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/base/internal/poison.cc b/absl/base/internal/poison.cc
new file mode 100644
index 00000000000..c639c966628
--- /dev/null
+++ b/absl/base/internal/poison.cc
@@ -0,0 +1,85 @@
+// Copyright 2024 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/base/internal/poison.h"
+
+#include <cstdlib>
+
+#include "absl/base/config.h"
+#include "absl/base/internal/direct_mmap.h"
+
+#ifndef _WIN32
+#include <unistd.h>
+#endif
+
+#if defined(ABSL_HAVE_ADDRESS_SANITIZER)
+#include <sanitizer/asan_interface.h>
+#elif defined(ABSL_HAVE_MEMORY_SANITIZER)
+#include <sanitizer/msan_interface.h>
+#elif defined(ABSL_HAVE_MMAP)
+#include <sys/mman.h>
+#endif
+
+#if defined(_WIN32)
+#include <windows.h>
+#endif
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace base_internal {
+
+namespace {
+
+size_t GetPageSize() {
+#ifdef _WIN32
+  SYSTEM_INFO system_info;
+  GetSystemInfo(&system_info);
+  return system_info.dwPageSize;
+#elif defined(__wasm__) || defined(__asmjs__) || defined(__hexagon__)
+  return getpagesize();
+#else
+  return static_cast<size_t>(sysconf(_SC_PAGESIZE));
+#endif
+}
+
+}  // namespace
+
+void* InitializePoisonedPointerInternal() {
+  const size_t block_size = GetPageSize();
+  void* data = nullptr;
+#if defined(ABSL_HAVE_ADDRESS_SANITIZER)
+  data = malloc(block_size);
+  ASAN_POISON_MEMORY_REGION(data, block_size);
+#elif defined(ABSL_HAVE_MEMORY_SANITIZER)
+  data = malloc(block_size);
+  __msan_poison(data, block_size);
+#elif defined(ABSL_HAVE_MMAP)
+  data = DirectMmap(nullptr, block_size, PROT_NONE, MAP_PRIVATE | MAP_ANONYMOUS,
+                    -1, 0);
+  if (data == MAP_FAILED) return GetBadPointerInternal();
+#elif defined(_WIN32)
+  data = VirtualAlloc(nullptr, block_size, MEM_RESERVE | MEM_COMMIT,
+                      PAGE_NOACCESS);
+  if (data == nullptr) return GetBadPointerInternal();
+#else
+  return GetBadPointerInternal();
+#endif
+  // Return the middle of the block so that dereferences before and after the
+  // pointer will both crash.
+  return static_cast<char*>(data) + block_size / 2;
+}
+
+}  // namespace base_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/base/internal/poison.h b/absl/base/internal/poison.h
new file mode 100644
index 00000000000..28113bdd71a
--- /dev/null
+++ b/absl/base/internal/poison.h
@@ -0,0 +1,59 @@
+// Copyright 2024 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_BASE_INTERNAL_POISON_H_
+#define ABSL_BASE_INTERNAL_POISON_H_
+
+#include <cstdint>
+
+#include "absl/base/config.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace base_internal {
+
+inline void* GetBadPointerInternal() {
+  // A likely bad pointer. Pointers are required to have high bits that are all
+  // zero or all one for certain 64-bit CPUs. This pointer value will hopefully
+  // cause a crash on dereference and also be clearly recognizable as invalid.
+  constexpr uint64_t kBadPtr = 0xBAD0BAD0BAD0BAD0;
+  auto ret = reinterpret_cast<void*>(static_cast<uintptr_t>(kBadPtr));
+#ifndef _MSC_VER  // MSVC doesn't support inline asm with `volatile`.
+  // Try to prevent the compiler from optimizing out the undefined behavior.
+  asm volatile("" : : "r"(ret) :);  // NOLINT
+#endif
+  return ret;
+}
+
+void* InitializePoisonedPointerInternal();
+
+inline void* get_poisoned_pointer() {
+#if defined(NDEBUG) && !defined(ABSL_HAVE_ADDRESS_SANITIZER) && \
+    !defined(ABSL_HAVE_MEMORY_SANITIZER)
+  // In optimized non-sanitized builds, avoid the function-local static because
+  // of the codegen and runtime cost.
+  return GetBadPointerInternal();
+#else
+  // Non-optimized builds may use more robust implementation. Note that we can't
+  // use a static global because Chromium doesn't allow non-constinit globals.
+  static void* ptr = InitializePoisonedPointerInternal();
+  return ptr;
+#endif
+}
+
+}  // namespace base_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_BASE_INTERNAL_POISON_H_
diff --git a/absl/base/internal/poison_test.cc b/absl/base/internal/poison_test.cc
new file mode 100644
index 00000000000..6596b45434b
--- /dev/null
+++ b/absl/base/internal/poison_test.cc
@@ -0,0 +1,41 @@
+// Copyright 2024 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/base/internal/poison.h"
+
+#include <iostream>
+
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace base_internal {
+namespace {
+
+TEST(PoisonTest, CrashesOnDereference) {
+#ifdef __ANDROID__
+  GTEST_SKIP() << "On Android, poisoned pointer dereference times out instead "
+                  "of crashing.";
+#endif
+  int* poisoned_ptr = static_cast<int*>(get_poisoned_pointer());
+  EXPECT_DEATH_IF_SUPPORTED(std::cout << *poisoned_ptr, "");
+  EXPECT_DEATH_IF_SUPPORTED(std::cout << *(poisoned_ptr - 10), "");
+  EXPECT_DEATH_IF_SUPPORTED(std::cout << *(poisoned_ptr + 10), "");
+}
+
+}  // namespace
+}  // namespace base_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/base/internal/pretty_function.h b/absl/base/internal/pretty_function.h
index 35d51676dc2..fbc997a5e8b 100644
--- a/absl/base/internal/pretty_function.h
+++ b/absl/base/internal/pretty_function.h
@@ -15,17 +15,18 @@
 #ifndef ABSL_BASE_INTERNAL_PRETTY_FUNCTION_H_
 #define ABSL_BASE_INTERNAL_PRETTY_FUNCTION_H_
 
-// ABSL_PRETTY_FUNCTION
+// ABSL_INTERNAL_PRETTY_FUNCTION
 //
 // In C++11, __func__ gives the undecorated name of the current function.  That
 // is, "main", not "int main()".  Various compilers give extra macros to get the
 // decorated function name, including return type and arguments, to
-// differentiate between overload sets.  ABSL_PRETTY_FUNCTION is a portable
-// version of these macros which forwards to the correct macro on each compiler.
+// differentiate between overload sets.  ABSL_INTERNAL_PRETTY_FUNCTION is a
+// portable version of these macros which forwards to the correct macro on each
+// compiler.
 #if defined(_MSC_VER)
-#define ABSL_PRETTY_FUNCTION __FUNCSIG__
+#define ABSL_INTERNAL_PRETTY_FUNCTION __FUNCSIG__
 #elif defined(__GNUC__)
-#define ABSL_PRETTY_FUNCTION __PRETTY_FUNCTION__
+#define ABSL_INTERNAL_PRETTY_FUNCTION __PRETTY_FUNCTION__
 #else
 #error "Unsupported compiler"
 #endif
diff --git a/absl/base/internal/raw_logging.cc b/absl/base/internal/raw_logging.cc
index 878fe6c6c2c..7c39f6d3ce7 100644
--- a/absl/base/internal/raw_logging.cc
+++ b/absl/base/internal/raw_logging.cc
@@ -14,17 +14,23 @@
 
 #include "absl/base/internal/raw_logging.h"
 
-#include <stddef.h>
 #include <cstdarg>
+#include <cstddef>
 #include <cstdio>
 #include <cstdlib>
 #include <cstring>
+#include <string>
 
 #include "absl/base/attributes.h"
 #include "absl/base/config.h"
 #include "absl/base/internal/atomic_hook.h"
+#include "absl/base/internal/errno_saver.h"
 #include "absl/base/log_severity.h"
 
+#ifdef __EMSCRIPTEN__
+#include <emscripten/console.h>
+#endif
+
 // We know how to perform low-level writes to stderr in POSIX and Windows.  For
 // these platforms, we define the token ABSL_LOW_LEVEL_WRITE_SUPPORTED.
 // Much of raw_logging.cc becomes a no-op when we can't output messages,
@@ -35,11 +41,11 @@
 //
 // This preprocessor token is also defined in raw_io.cc.  If you need to copy
 // this, consider moving both to config.h instead.
-#if defined(__linux__) || defined(__APPLE__) || defined(__FreeBSD__) || \
-    defined(__Fuchsia__) || defined(__native_client__) || \
-    defined(__EMSCRIPTEN__)
-#include <unistd.h>
+#if defined(__linux__) || defined(__APPLE__) || defined(__FreeBSD__) ||     \
+    defined(__hexagon__) || defined(__Fuchsia__) || defined(__OpenBSD__) || \
+    defined(__EMSCRIPTEN__) || defined(__ASYLO__)
 
+#include <unistd.h>
 
 #define ABSL_HAVE_POSIX_WRITE 1
 #define ABSL_LOW_LEVEL_WRITE_SUPPORTED 1
@@ -67,34 +73,34 @@
 #undef ABSL_HAVE_RAW_IO
 #endif
 
-// TODO(gfalcon): We want raw-logging to work on as many platforms as possible.
-// Explicitly #error out when not ABSL_LOW_LEVEL_WRITE_SUPPORTED, except for a
-// whitelisted set of platforms for which we expect not to be able to raw log.
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace raw_log_internal {
+namespace {
 
-ABSL_CONST_INIT static absl::base_internal::AtomicHook<
-    absl::raw_logging_internal::LogPrefixHook> log_prefix_hook;
-ABSL_CONST_INIT static absl::base_internal::AtomicHook<
-    absl::raw_logging_internal::AbortHook> abort_hook;
+// TODO(gfalcon): We want raw-logging to work on as many platforms as possible.
+// Explicitly `#error` out when not `ABSL_LOW_LEVEL_WRITE_SUPPORTED`, except for
+// a selected set of platforms for which we expect not to be able to raw log.
 
 #ifdef ABSL_LOW_LEVEL_WRITE_SUPPORTED
-static const char kTruncated[] = " ... (message truncated)\n";
+constexpr char kTruncated[] = " ... (message truncated)\n";
 
 // sprintf the format to the buffer, adjusting *buf and *size to reflect the
 // consumed bytes, and return whether the message fit without truncation.  If
 // truncation occurred, if possible leave room in the buffer for the message
 // kTruncated[].
-inline static bool VADoRawLog(char** buf, int* size, const char* format,
-                              va_list ap) ABSL_PRINTF_ATTRIBUTE(3, 0);
-inline static bool VADoRawLog(char** buf, int* size,
-                              const char* format, va_list ap) {
-  int n = vsnprintf(*buf, *size, format, ap);
+bool VADoRawLog(char** buf, int* size, const char* format, va_list ap)
+    ABSL_PRINTF_ATTRIBUTE(3, 0);
+bool VADoRawLog(char** buf, int* size, const char* format, va_list ap) {
+  if (*size < 0) return false;
+  int n = vsnprintf(*buf, static_cast<size_t>(*size), format, ap);
   bool result = true;
   if (n < 0 || n > *size) {
     result = false;
     if (static_cast<size_t>(*size) > sizeof(kTruncated)) {
-      n = *size - sizeof(kTruncated);  // room for truncation message
+      n = *size - static_cast<int>(sizeof(kTruncated));
     } else {
-      n = 0;                           // no room for truncation message
+      n = 0;  // no room for truncation message
     }
   }
   *size -= n;
@@ -103,9 +109,7 @@ inline static bool VADoRawLog(char** buf, int* size,
 }
 #endif  // ABSL_LOW_LEVEL_WRITE_SUPPORTED
 
-static constexpr int kLogBufSize = 3000;
-
-namespace {
+constexpr int kLogBufSize = 3000;
 
 // CAVEAT: vsnprintf called from *DoRawLog below has some (exotic) code paths
 // that invoke malloc() and getenv() that might acquire some locks.
@@ -116,9 +120,10 @@ namespace {
 bool DoRawLog(char** buf, int* size, const char* format, ...)
     ABSL_PRINTF_ATTRIBUTE(3, 4);
 bool DoRawLog(char** buf, int* size, const char* format, ...) {
+  if (*size < 0) return false;
   va_list ap;
   va_start(ap, format);
-  int n = vsnprintf(*buf, *size, format, ap);
+  int n = vsnprintf(*buf, static_cast<size_t>(*size), format, ap);
   va_end(ap);
   if (n < 0 || n > *size) return false;
   *size -= n;
@@ -126,6 +131,18 @@ bool DoRawLog(char** buf, int* size, const char* format, ...) {
   return true;
 }
 
+bool DefaultLogFilterAndPrefix(absl::LogSeverity, const char* file, int line,
+                               char** buf, int* buf_size) {
+  DoRawLog(buf, buf_size, "[%s : %d] RAW: ", file, line);
+  return true;
+}
+
+ABSL_INTERNAL_ATOMIC_HOOK_ATTRIBUTES
+absl::base_internal::AtomicHook<LogFilterAndPrefixHook>
+    log_filter_and_prefix_hook(DefaultLogFilterAndPrefix);
+ABSL_INTERNAL_ATOMIC_HOOK_ATTRIBUTES
+absl::base_internal::AtomicHook<AbortHook> abort_hook;
+
 void RawLogVA(absl::LogSeverity severity, const char* file, int line,
               const char* format, va_list ap) ABSL_PRINTF_ATTRIBUTE(4, 0);
 void RawLogVA(absl::LogSeverity severity, const char* file, int line,
@@ -140,20 +157,13 @@ void RawLogVA(absl::LogSeverity severity, const char* file, int line,
 #endif
 
 #ifdef ABSL_MIN_LOG_LEVEL
-  if (severity < static_cast<absl::LogSeverity>(ABSL_MIN_LOG_LEVEL) &&
+  if (severity < static_cast<absl::LogSeverityAtLeast>(ABSL_MIN_LOG_LEVEL) &&
       severity < absl::LogSeverity::kFatal) {
     enabled = false;
   }
 #endif
 
-  auto log_prefix_hook_ptr = log_prefix_hook.Load();
-  if (log_prefix_hook_ptr) {
-    enabled = log_prefix_hook_ptr(severity, file, line, &buf, &size);
-  } else {
-    if (enabled) {
-      DoRawLog(&buf, &size, "[%s : %d] RAW: ", file, line);
-    }
-  }
+  enabled = log_filter_and_prefix_hook(severity, file, line, &buf, &size);
   const char* const prefix_end = buf;
 
 #ifdef ABSL_LOW_LEVEL_WRITE_SUPPORTED
@@ -164,11 +174,12 @@ void RawLogVA(absl::LogSeverity severity, const char* file, int line,
     } else {
       DoRawLog(&buf, &size, "%s", kTruncated);
     }
-    absl::raw_logging_internal::SafeWriteToStderr(buffer, strlen(buffer));
+    AsyncSignalSafeWriteError(buffer, static_cast<size_t>(buf - buffer));
   }
 #else
   static_cast<void>(format);
   static_cast<void>(ap);
+  static_cast<void>(enabled);
 #endif
 
   // Abort the process after logging a FATAL message, even if the output itself
@@ -179,26 +190,60 @@ void RawLogVA(absl::LogSeverity severity, const char* file, int line,
   }
 }
 
+// Non-formatting version of RawLog().
+//
+// TODO(gfalcon): When string_view no longer depends on base, change this
+// interface to take its message as a string_view instead.
+void DefaultInternalLog(absl::LogSeverity severity, const char* file, int line,
+                        const std::string& message) {
+  RawLog(severity, file, line, "%.*s", static_cast<int>(message.size()),
+         message.data());
+}
+
 }  // namespace
 
-namespace absl {
-namespace raw_logging_internal {
-void SafeWriteToStderr(const char *s, size_t len) {
-#if defined(ABSL_HAVE_SYSCALL_WRITE)
+void AsyncSignalSafeWriteError(const char* s, size_t len) {
+  if (!len) return;
+  absl::base_internal::ErrnoSaver errno_saver;
+#if defined(__EMSCRIPTEN__)
+  // In WebAssembly, bypass filesystem emulation via fwrite.
+  if (s[len - 1] == '\n') {
+    // Skip a trailing newline character as emscripten_errn adds one itself.
+    len--;
+  }
+  // emscripten_errn was introduced in 3.1.41 but broken in standalone mode
+  // until 3.1.43.
+#if ABSL_INTERNAL_EMSCRIPTEN_VERSION >= 3001043
+  emscripten_errn(s, len);
+#else
+  char buf[kLogBufSize];
+  if (len >= kLogBufSize) {
+    len = kLogBufSize - 1;
+    constexpr size_t trunc_len = sizeof(kTruncated) - 2;
+    memcpy(buf + len - trunc_len, kTruncated, trunc_len);
+    buf[len] = '\0';
+    len -= trunc_len;
+  } else {
+    buf[len] = '\0';
+  }
+  memcpy(buf, s, len);
+  _emscripten_err(buf);
+#endif
+#elif defined(ABSL_HAVE_SYSCALL_WRITE)
+  // We prefer calling write via `syscall` to minimize the risk of libc doing
+  // something "helpful".
   syscall(SYS_write, STDERR_FILENO, s, len);
 #elif defined(ABSL_HAVE_POSIX_WRITE)
   write(STDERR_FILENO, s, len);
 #elif defined(ABSL_HAVE_RAW_IO)
-  _write(/* stderr */ 2, s, len);
+  _write(/* stderr */ 2, s, static_cast<unsigned>(len));
 #else
   // stderr logging unsupported on this platform
-  (void) s;
-  (void) len;
+  (void)s;
+  (void)len;
 #endif
 }
 
-void RawLog(absl::LogSeverity severity, const char* file, int line,
-            const char* format, ...) ABSL_PRINTF_ATTRIBUTE(4, 5);
 void RawLog(absl::LogSeverity severity, const char* file, int line,
             const char* format, ...) {
   va_list ap;
@@ -207,29 +252,28 @@ void RawLog(absl::LogSeverity severity, const char* file, int line,
   va_end(ap);
 }
 
-// Non-formatting version of RawLog().
-//
-// TODO(gfalcon): When string_view no longer depends on base, change this
-// interface to take its message as a string_view instead.
-static void DefaultInternalLog(absl::LogSeverity severity, const char* file,
-                               int line, const std::string& message) {
-  RawLog(severity, file, line, "%s", message.c_str());
-}
-
 bool RawLoggingFullySupported() {
 #ifdef ABSL_LOW_LEVEL_WRITE_SUPPORTED
   return true;
-#else  // !ABSL_LOW_LEVEL_WRITE_SUPPORTED
+#else   // !ABSL_LOW_LEVEL_WRITE_SUPPORTED
   return false;
 #endif  // !ABSL_LOW_LEVEL_WRITE_SUPPORTED
 }
 
-ABSL_CONST_INIT absl::base_internal::AtomicHook<InternalLogFunction>
-    internal_log_function(DefaultInternalLog);
+ABSL_INTERNAL_ATOMIC_HOOK_ATTRIBUTES ABSL_DLL
+    absl::base_internal::AtomicHook<InternalLogFunction>
+        internal_log_function(DefaultInternalLog);
+
+void RegisterLogFilterAndPrefixHook(LogFilterAndPrefixHook func) {
+  log_filter_and_prefix_hook.Store(func);
+}
+
+void RegisterAbortHook(AbortHook func) { abort_hook.Store(func); }
 
 void RegisterInternalLogFunction(InternalLogFunction func) {
   internal_log_function.Store(func);
 }
 
-}  // namespace raw_logging_internal
+}  // namespace raw_log_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/base/internal/raw_logging.h b/absl/base/internal/raw_logging.h
index 6a4c093603f..8637030f70f 100644
--- a/absl/base/internal/raw_logging.h
+++ b/absl/base/internal/raw_logging.h
@@ -19,12 +19,15 @@
 #ifndef ABSL_BASE_INTERNAL_RAW_LOGGING_H_
 #define ABSL_BASE_INTERNAL_RAW_LOGGING_H_
 
+#include <cstddef>
 #include <string>
 
 #include "absl/base/attributes.h"
+#include "absl/base/config.h"
 #include "absl/base/internal/atomic_hook.h"
 #include "absl/base/log_severity.h"
 #include "absl/base/macros.h"
+#include "absl/base/optimization.h"
 #include "absl/base/port.h"
 
 // This is similar to LOG(severity) << format..., but
@@ -41,12 +44,12 @@
 
 #define ABSL_RAW_LOG(severity, ...)                                            \
   do {                                                                         \
-    constexpr const char* absl_raw_logging_internal_basename =                 \
-        ::absl::raw_logging_internal::Basename(__FILE__,                       \
-                                               sizeof(__FILE__) - 1);          \
-    ::absl::raw_logging_internal::RawLog(ABSL_RAW_LOGGING_INTERNAL_##severity, \
-                                         absl_raw_logging_internal_basename,   \
-                                         __LINE__, __VA_ARGS__);               \
+    constexpr const char* absl_raw_log_internal_basename =                     \
+        ::absl::raw_log_internal::Basename(__FILE__, sizeof(__FILE__) - 1);    \
+    ::absl::raw_log_internal::RawLog(ABSL_RAW_LOG_INTERNAL_##severity,         \
+                                     absl_raw_log_internal_basename, __LINE__, \
+                                     __VA_ARGS__);                             \
+    ABSL_RAW_LOG_INTERNAL_MAYBE_UNREACHABLE_##severity;                        \
   } while (0)
 
 // Similar to CHECK(condition) << message, but for low-level modules:
@@ -70,14 +73,13 @@
 //
 // The API is a subset of the above: each macro only takes two arguments.  Use
 // StrCat if you need to build a richer message.
-#define ABSL_INTERNAL_LOG(severity, message)                          \
-  do {                                                                \
-    constexpr const char* absl_raw_logging_internal_basename =        \
-        ::absl::raw_logging_internal::Basename(__FILE__,              \
-                                               sizeof(__FILE__) - 1); \
-    ::absl::raw_logging_internal::internal_log_function(              \
-        ABSL_RAW_LOGGING_INTERNAL_##severity,                         \
-        absl_raw_logging_internal_basename, __LINE__, message);       \
+#define ABSL_INTERNAL_LOG(severity, message)                              \
+  do {                                                                    \
+    constexpr const char* absl_raw_log_internal_filename = __FILE__;      \
+    ::absl::raw_log_internal::internal_log_function(                      \
+        ABSL_RAW_LOG_INTERNAL_##severity, absl_raw_log_internal_filename, \
+        __LINE__, message);                                               \
+    ABSL_RAW_LOG_INTERNAL_MAYBE_UNREACHABLE_##severity;                   \
   } while (0)
 
 #define ABSL_INTERNAL_CHECK(condition, message)                    \
@@ -89,15 +91,38 @@
     }                                                              \
   } while (0)
 
-#define ABSL_RAW_LOGGING_INTERNAL_INFO ::absl::LogSeverity::kInfo
-#define ABSL_RAW_LOGGING_INTERNAL_WARNING ::absl::LogSeverity::kWarning
-#define ABSL_RAW_LOGGING_INTERNAL_ERROR ::absl::LogSeverity::kError
-#define ABSL_RAW_LOGGING_INTERNAL_FATAL ::absl::LogSeverity::kFatal
-#define ABSL_RAW_LOGGING_INTERNAL_LEVEL(severity) \
+#ifndef NDEBUG
+
+#define ABSL_RAW_DLOG(severity, ...) ABSL_RAW_LOG(severity, __VA_ARGS__)
+#define ABSL_RAW_DCHECK(condition, message) ABSL_RAW_CHECK(condition, message)
+
+#else  // NDEBUG
+
+#define ABSL_RAW_DLOG(severity, ...)                   \
+  while (false) ABSL_RAW_LOG(severity, __VA_ARGS__)
+#define ABSL_RAW_DCHECK(condition, message) \
+  while (false) ABSL_RAW_CHECK(condition, message)
+
+#endif  // NDEBUG
+
+#define ABSL_RAW_LOG_INTERNAL_INFO ::absl::LogSeverity::kInfo
+#define ABSL_RAW_LOG_INTERNAL_WARNING ::absl::LogSeverity::kWarning
+#define ABSL_RAW_LOG_INTERNAL_ERROR ::absl::LogSeverity::kError
+#define ABSL_RAW_LOG_INTERNAL_FATAL ::absl::LogSeverity::kFatal
+#define ABSL_RAW_LOG_INTERNAL_DFATAL ::absl::kLogDebugFatal
+#define ABSL_RAW_LOG_INTERNAL_LEVEL(severity) \
   ::absl::NormalizeLogSeverity(severity)
 
+#define ABSL_RAW_LOG_INTERNAL_MAYBE_UNREACHABLE_INFO
+#define ABSL_RAW_LOG_INTERNAL_MAYBE_UNREACHABLE_WARNING
+#define ABSL_RAW_LOG_INTERNAL_MAYBE_UNREACHABLE_ERROR
+#define ABSL_RAW_LOG_INTERNAL_MAYBE_UNREACHABLE_FATAL ABSL_UNREACHABLE()
+#define ABSL_RAW_LOG_INTERNAL_MAYBE_UNREACHABLE_DFATAL
+#define ABSL_RAW_LOG_INTERNAL_MAYBE_UNREACHABLE_LEVEL(severity)
+
 namespace absl {
-namespace raw_logging_internal {
+ABSL_NAMESPACE_BEGIN
+namespace raw_log_internal {
 
 // Helper function to implement ABSL_RAW_LOG
 // Logs format... at "severity" level, reporting it
@@ -106,12 +131,9 @@ namespace raw_logging_internal {
 void RawLog(absl::LogSeverity severity, const char* file, int line,
             const char* format, ...) ABSL_PRINTF_ATTRIBUTE(4, 5);
 
-// Writes the provided buffer directly to stderr, in a safe, low-level manner.
-//
-// In POSIX this means calling write(), which is async-signal safe and does
-// not malloc.  If the platform supports the SYS_write syscall, we invoke that
-// directly to side-step any libc interception.
-void SafeWriteToStderr(const char *s, size_t len);
+// Writes the provided buffer directly to stderr, in a signal-safe, low-level
+// manner.  Preserves errno.
+void AsyncSignalSafeWriteError(const char* s, size_t len);
 
 // compile-time function to get the "base" filename, that is, the part of
 // a filename after the last "/" or "\" path separator.  The search starts at
@@ -130,7 +152,7 @@ constexpr const char* Basename(const char* fname, int offset) {
 // TODO(gfalcon): Come up with a better name for this method.
 bool RawLoggingFullySupported();
 
-// Function type for a raw_logging customization hook for suppressing messages
+// Function type for a raw_log customization hook for suppressing messages
 // by severity, and for writing custom prefixes on non-suppressed messages.
 //
 // The installed hook is called for every raw log invocation.  The message will
@@ -139,27 +161,31 @@ bool RawLoggingFullySupported();
 // also provided with an output buffer, where it can write a custom log message
 // prefix.
 //
-// The raw_logging system does not allocate memory or grab locks.  User-provided
+// The raw_log system does not allocate memory or grab locks.  User-provided
 // hooks must avoid these operations, and must not throw exceptions.
 //
 // 'severity' is the severity level of the message being written.
 // 'file' and 'line' are the file and line number where the ABSL_RAW_LOG macro
 // was located.
-// 'buffer' and 'buf_size' are pointers to the buffer and buffer size.  If the
-// hook writes a prefix, it must increment *buffer and decrement *buf_size
+// 'buf' and 'buf_size' are pointers to the buffer and buffer size.  If the
+// hook writes a prefix, it must increment *buf and decrement *buf_size
 // accordingly.
-using LogPrefixHook = bool (*)(absl::LogSeverity severity, const char* file,
-                               int line, char** buffer, int* buf_size);
+using LogFilterAndPrefixHook = bool (*)(absl::LogSeverity severity,
+                                        const char* file, int line, char** buf,
+                                        int* buf_size);
 
-// Function type for a raw_logging customization hook called to abort a process
+// Function type for a raw_log customization hook called to abort a process
 // when a FATAL message is logged.  If the provided AbortHook() returns, the
 // logging system will call abort().
 //
 // 'file' and 'line' are the file and line number where the ABSL_RAW_LOG macro
 // was located.
-// The null-terminated logged message lives in the buffer between 'buf_start'
+// The NUL-terminated logged message lives in the buffer between 'buf_start'
 // and 'buf_end'.  'prefix_end' points to the first non-prefix character of the
-// buffer (as written by the LogPrefixHook.)
+// buffer (as written by the LogFilterAndPrefixHook.)
+//
+// The lifetime of the filename and message buffers will not end while the
+// process remains alive.
 using AbortHook = void (*)(const char* file, int line, const char* buf_start,
                            const char* prefix_end, const char* buf_end);
 
@@ -171,11 +197,22 @@ using InternalLogFunction = void (*)(absl::LogSeverity severity,
                                      const char* file, int line,
                                      const std::string& message);
 
-extern base_internal::AtomicHook<InternalLogFunction> internal_log_function;
+ABSL_INTERNAL_ATOMIC_HOOK_ATTRIBUTES ABSL_DLL extern base_internal::AtomicHook<
+    InternalLogFunction>
+    internal_log_function;
 
+// Registers hooks of the above types.  Only a single hook of each type may be
+// registered.  It is an error to call these functions multiple times with
+// different input arguments.
+//
+// These functions are safe to call at any point during initialization; they do
+// not block or malloc, and are async-signal safe.
+void RegisterLogFilterAndPrefixHook(LogFilterAndPrefixHook func);
+void RegisterAbortHook(AbortHook func);
 void RegisterInternalLogFunction(InternalLogFunction func);
 
-}  // namespace raw_logging_internal
+}  // namespace raw_log_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_BASE_INTERNAL_RAW_LOGGING_H_
diff --git a/absl/base/internal/scheduling_mode.h b/absl/base/internal/scheduling_mode.h
index d5b4b7fd54f..8be5ab6dd3c 100644
--- a/absl/base/internal/scheduling_mode.h
+++ b/absl/base/internal/scheduling_mode.h
@@ -18,7 +18,10 @@
 #ifndef ABSL_BASE_INTERNAL_SCHEDULING_MODE_H_
 #define ABSL_BASE_INTERNAL_SCHEDULING_MODE_H_
 
+#include "absl/base/config.h"
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 
 // Used to describe how a thread may be scheduled.  Typically associated with
@@ -49,6 +52,7 @@ enum SchedulingMode {
 };
 
 }  // namespace base_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_BASE_INTERNAL_SCHEDULING_MODE_H_
diff --git a/absl/base/internal/scoped_set_env.cc b/absl/base/internal/scoped_set_env.cc
index 3ac3f68ddf9..b13e7a2410b 100644
--- a/absl/base/internal/scoped_set_env.cc
+++ b/absl/base/internal/scoped_set_env.cc
@@ -14,15 +14,16 @@
 
 #include "absl/base/internal/scoped_set_env.h"
 
-#ifdef _WIN32
-#include <windows.h>
-#endif
-
 #include <cstdlib>
 
 #include "absl/base/internal/raw_logging.h"
 
+#ifdef _WIN32
+#include <windows.h>
+#endif
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 
 namespace {
@@ -76,4 +77,5 @@ ScopedSetEnv::~ScopedSetEnv() {
 }
 
 }  // namespace base_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/base/internal/scoped_set_env.h b/absl/base/internal/scoped_set_env.h
index 855b22fd41f..19ec7b5d8a0 100644
--- a/absl/base/internal/scoped_set_env.h
+++ b/absl/base/internal/scoped_set_env.h
@@ -19,7 +19,10 @@
 
 #include <string>
 
+#include "absl/base/config.h"
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 
 class ScopedSetEnv {
@@ -36,6 +39,7 @@ class ScopedSetEnv {
 };
 
 }  // namespace base_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_BASE_INTERNAL_SCOPED_SET_ENV_H_
diff --git a/absl/base/internal/scoped_set_env_test.cc b/absl/base/internal/scoped_set_env_test.cc
index 5cbad246c64..6d6775a8777 100644
--- a/absl/base/internal/scoped_set_env_test.cc
+++ b/absl/base/internal/scoped_set_env_test.cc
@@ -12,13 +12,17 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+#include "absl/base/internal/scoped_set_env.h"
+
+#include <cstdlib>
+#include <string>
+
+#include "gtest/gtest.h"
+
 #ifdef _WIN32
 #include <windows.h>
 #endif
 
-#include "gtest/gtest.h"
-#include "absl/base/internal/scoped_set_env.h"
-
 namespace {
 
 using absl::base_internal::ScopedSetEnv;
diff --git a/absl/base/internal/spinlock.cc b/absl/base/internal/spinlock.cc
index 7354438fbac..41d2b482f64 100644
--- a/absl/base/internal/spinlock.cc
+++ b/absl/base/internal/spinlock.cc
@@ -16,14 +16,18 @@
 
 #include <algorithm>
 #include <atomic>
+#include <cstdint>
 #include <limits>
 
 #include "absl/base/attributes.h"
+#include "absl/base/call_once.h"
+#include "absl/base/config.h"
 #include "absl/base/internal/atomic_hook.h"
 #include "absl/base/internal/cycleclock.h"
+#include "absl/base/internal/scheduling_mode.h"
 #include "absl/base/internal/spinlock_wait.h"
 #include "absl/base/internal/sysinfo.h" /* For NumCPUs() */
-#include "absl/base/call_once.h"
+#include "absl/base/internal/tsan_mutex_interface.h"
 
 // Description of lock-word:
 //  31..00: [............................3][2][1][0]
@@ -54,10 +58,11 @@
 //          holder to acquire the lock.  There may be outstanding waiter(s).
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 
-ABSL_CONST_INIT static base_internal::AtomicHook<void (*)(const void *lock,
-                                                          int64_t wait_cycles)>
+ABSL_INTERNAL_ATOMIC_HOOK_ATTRIBUTES static AtomicHook<void (*)(
+    const void *lock, int64_t wait_cycles)>
     submit_profile_data;
 
 void RegisterSpinLockProfiler(void (*fn)(const void *contendedlock,
@@ -65,48 +70,24 @@ void RegisterSpinLockProfiler(void (*fn)(const void *contendedlock,
   submit_profile_data.Store(fn);
 }
 
-// Uncommon constructors.
-SpinLock::SpinLock(base_internal::SchedulingMode mode)
-    : lockword_(IsCooperative(mode) ? kSpinLockCooperative : 0) {
-  ABSL_TSAN_MUTEX_CREATE(this, __tsan_mutex_not_static);
-}
-
-SpinLock::SpinLock(base_internal::LinkerInitialized,
-                   base_internal::SchedulingMode mode) {
-  ABSL_TSAN_MUTEX_CREATE(this, 0);
-  if (IsCooperative(mode)) {
-    InitLinkerInitializedAndCooperative();
-  }
-  // Otherwise, lockword_ is already initialized.
-}
-
-// Static (linker initialized) spinlocks always start life as functional
-// non-cooperative locks.  When their static constructor does run, it will call
-// this initializer to augment the lockword with the cooperative bit.  By
-// actually taking the lock when we do this we avoid the need for an atomic
-// operation in the regular unlock path.
-//
-// SlowLock() must be careful to re-test for this bit so that any outstanding
-// waiters may be upgraded to cooperative status.
-void SpinLock::InitLinkerInitializedAndCooperative() {
-  Lock();
-  lockword_.fetch_or(kSpinLockCooperative, std::memory_order_relaxed);
-  Unlock();
-}
-
 // Monitor the lock to see if its value changes within some time period
-// (adaptive_spin_count loop iterations). The last value read from the lock
+// (adaptive_spin_count_ loop iterations). The last value read from the lock
 // is returned from the method.
+ABSL_CONST_INIT std::atomic<int> SpinLock::adaptive_spin_count_{0};
 uint32_t SpinLock::SpinLoop() {
   // We are already in the slow path of SpinLock, initialize the
   // adaptive_spin_count here.
-  ABSL_CONST_INIT static absl::once_flag init_adaptive_spin_count;
-  ABSL_CONST_INIT static int adaptive_spin_count = 0;
-  base_internal::LowLevelCallOnce(&init_adaptive_spin_count, []() {
-    adaptive_spin_count = base_internal::NumCPUs() > 1 ? 1000 : 1;
-  });
-
-  int c = adaptive_spin_count;
+  if (adaptive_spin_count_.load(std::memory_order_relaxed) == 0) {
+    int current_spin_count = 0;
+    int new_spin_count = NumCPUs() > 1 ? 1000 : 1;
+    // If this fails, the value will remain unchanged. We may not spin for the
+    // intended duration, but that is still safe. We will try again on the next
+    // call to SpinLoop.
+    adaptive_spin_count_.compare_exchange_weak(
+        current_spin_count, new_spin_count, std::memory_order_relaxed,
+        std::memory_order_relaxed);
+  }
+  int c = adaptive_spin_count_.load(std::memory_order_relaxed);
   uint32_t lock_value;
   do {
     lock_value = lockword_.load(std::memory_order_relaxed);
@@ -120,6 +101,14 @@ void SpinLock::SlowLock() {
   if ((lock_value & kSpinLockHeld) == 0) {
     return;
   }
+
+  SchedulingMode scheduling_mode;
+  if ((lock_value & kSpinLockCooperative) != 0) {
+    scheduling_mode = SCHEDULE_COOPERATIVE_AND_KERNEL;
+  } else {
+    scheduling_mode = SCHEDULE_KERNEL_ONLY;
+  }
+
   // The lock was not obtained initially, so this thread needs to wait for
   // it.  Record the current timestamp in the local variable wait_start_time
   // so the total wait time can be stored in the lockword once this thread
@@ -132,8 +121,9 @@ void SpinLock::SlowLock() {
     // it as having a sleeper.
     if ((lock_value & kWaitTimeMask) == 0) {
       // Here, just "mark" that the thread is going to sleep.  Don't store the
-      // lock wait time in the lock as that will cause the current lock
-      // owner to think it experienced contention.
+      // lock wait time in the lock -- the lock word stores the amount of time
+      // that the current holder waited before acquiring the lock, not the wait
+      // time of any thread currently waiting to acquire it.
       if (lockword_.compare_exchange_strong(
               lock_value, lock_value | kSpinLockSleeper,
               std::memory_order_relaxed, std::memory_order_relaxed)) {
@@ -146,22 +136,24 @@ void SpinLock::SlowLock() {
         // new lock state will be the number of cycles this thread waited if
         // this thread obtains the lock.
         lock_value = TryLockInternal(lock_value, wait_cycles);
-        continue;   // Skip the delay at the end of the loop.
+        continue;  // Skip the delay at the end of the loop.
+      } else if ((lock_value & kWaitTimeMask) == 0) {
+        // The lock is still held, without a waiter being marked, but something
+        // else about the lock word changed, causing our CAS to fail. For
+        // example, a new lock holder may have acquired the lock with
+        // kSpinLockDisabledScheduling set, whereas the previous holder had not
+        // set that flag. In this case, attempt again to mark ourselves as a
+        // waiter.
+        continue;
       }
     }
 
-    base_internal::SchedulingMode scheduling_mode;
-    if ((lock_value & kSpinLockCooperative) != 0) {
-      scheduling_mode = base_internal::SCHEDULE_COOPERATIVE_AND_KERNEL;
-    } else {
-      scheduling_mode = base_internal::SCHEDULE_KERNEL_ONLY;
-    }
     // SpinLockDelay() calls into fiber scheduler, we need to see
     // synchronization there to avoid false positives.
     ABSL_TSAN_MUTEX_PRE_DIVERT(this, 0);
     // Wait for an OS specific delay.
-    base_internal::SpinLockDelay(&lockword_, lock_value, ++lock_wait_call_count,
-                                 scheduling_mode);
+    SpinLockDelay(&lockword_, lock_value, ++lock_wait_call_count,
+                  scheduling_mode);
     ABSL_TSAN_MUTEX_POST_DIVERT(this, 0);
     // Spin again after returning from the wait routine to give this thread
     // some chance of obtaining the lock.
@@ -172,14 +164,14 @@ void SpinLock::SlowLock() {
 }
 
 void SpinLock::SlowUnlock(uint32_t lock_value) {
-  base_internal::SpinLockWake(&lockword_,
-                              false);  // wake waiter if necessary
+  SpinLockWake(&lockword_,
+               false);  // wake waiter if necessary
 
   // If our acquisition was contended, collect contentionz profile info.  We
   // reserve a unitary wait time to represent that a waiter exists without our
   // own acquisition having been contended.
   if ((lock_value & kWaitTimeMask) != kSpinLockSleeper) {
-    const uint64_t wait_cycles = DecodeWaitCycles(lock_value);
+    const int64_t wait_cycles = DecodeWaitCycles(lock_value);
     ABSL_TSAN_MUTEX_PRE_DIVERT(this, 0);
     submit_profile_data(this, wait_cycles);
     ABSL_TSAN_MUTEX_POST_DIVERT(this, 0);
@@ -189,43 +181,45 @@ void SpinLock::SlowUnlock(uint32_t lock_value) {
 // We use the upper 29 bits of the lock word to store the time spent waiting to
 // acquire this lock.  This is reported by contentionz profiling.  Since the
 // lower bits of the cycle counter wrap very quickly on high-frequency
-// processors we divide to reduce the granularity to 2^PROFILE_TIMESTAMP_SHIFT
+// processors we divide to reduce the granularity to 2^kProfileTimestampShift
 // sized units.  On a 4Ghz machine this will lose track of wait times greater
 // than (2^29/4 Ghz)*128 =~ 17.2 seconds.  Such waits should be extremely rare.
-enum { PROFILE_TIMESTAMP_SHIFT = 7 };
-enum { LOCKWORD_RESERVED_SHIFT = 3 };  // We currently reserve the lower 3 bits.
+static constexpr int kProfileTimestampShift = 7;
+
+// We currently reserve the lower 3 bits.
+static constexpr int kLockwordReservedShift = 3;
 
 uint32_t SpinLock::EncodeWaitCycles(int64_t wait_start_time,
                                     int64_t wait_end_time) {
   static const int64_t kMaxWaitTime =
-      std::numeric_limits<uint32_t>::max() >> LOCKWORD_RESERVED_SHIFT;
+      std::numeric_limits<uint32_t>::max() >> kLockwordReservedShift;
   int64_t scaled_wait_time =
-      (wait_end_time - wait_start_time) >> PROFILE_TIMESTAMP_SHIFT;
+      (wait_end_time - wait_start_time) >> kProfileTimestampShift;
 
   // Return a representation of the time spent waiting that can be stored in
   // the lock word's upper bits.
   uint32_t clamped = static_cast<uint32_t>(
-      std::min(scaled_wait_time, kMaxWaitTime) << LOCKWORD_RESERVED_SHIFT);
+      std::min(scaled_wait_time, kMaxWaitTime) << kLockwordReservedShift);
 
   if (clamped == 0) {
     return kSpinLockSleeper;  // Just wake waiters, but don't record contention.
   }
   // Bump up value if necessary to avoid returning kSpinLockSleeper.
   const uint32_t kMinWaitTime =
-      kSpinLockSleeper + (1 << LOCKWORD_RESERVED_SHIFT);
+      kSpinLockSleeper + (1 << kLockwordReservedShift);
   if (clamped == kSpinLockSleeper) {
     return kMinWaitTime;
   }
   return clamped;
 }
 
-uint64_t SpinLock::DecodeWaitCycles(uint32_t lock_value) {
+int64_t SpinLock::DecodeWaitCycles(uint32_t lock_value) {
   // Cast to uint32_t first to ensure bits [63:32] are cleared.
-  const uint64_t scaled_wait_time =
+  const int64_t scaled_wait_time =
       static_cast<uint32_t>(lock_value & kWaitTimeMask);
-  return scaled_wait_time
-      << (PROFILE_TIMESTAMP_SHIFT - LOCKWORD_RESERVED_SHIFT);
+  return scaled_wait_time << (kProfileTimestampShift - kLockwordReservedShift);
 }
 
 }  // namespace base_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/base/internal/spinlock.h b/absl/base/internal/spinlock.h
index 1d1bd6cfcf5..81210ca30ca 100644
--- a/absl/base/internal/spinlock.h
+++ b/absl/base/internal/spinlock.h
@@ -15,71 +15,87 @@
 //
 
 //  Most users requiring mutual exclusion should use Mutex.
-//  SpinLock is provided for use in three situations:
-//   - for use in code that Mutex itself depends on
-//   - to get a faster fast-path release under low contention (without an
-//     atomic read-modify-write) In return, SpinLock has worse behaviour under
-//     contention, which is why Mutex is preferred in most situations.
+//  SpinLock is provided for use in two situations:
+//   - for use by Abseil internal code that Mutex itself depends on
 //   - for async signal safety (see below)
 
-// SpinLock is async signal safe.  If a spinlock is used within a signal
-// handler, all code that acquires the lock must ensure that the signal cannot
-// arrive while they are holding the lock.  Typically, this is done by blocking
-// the signal.
+// SpinLock with a SchedulingMode::SCHEDULE_KERNEL_ONLY is async
+// signal safe. If a spinlock is used within a signal handler, all code that
+// acquires the lock must ensure that the signal cannot arrive while they are
+// holding the lock. Typically, this is done by blocking the signal.
+//
+// Threads waiting on a SpinLock may be woken in an arbitrary order.
 
 #ifndef ABSL_BASE_INTERNAL_SPINLOCK_H_
 #define ABSL_BASE_INTERNAL_SPINLOCK_H_
 
-#include <stdint.h>
-#include <sys/types.h>
-
 #include <atomic>
+#include <cstdint>
+#include <mutex>
+#include <type_traits>
 
 #include "absl/base/attributes.h"
-#include "absl/base/dynamic_annotations.h"
+#include "absl/base/config.h"
+#include "absl/base/const_init.h"
 #include "absl/base/internal/low_level_scheduling.h"
 #include "absl/base/internal/raw_logging.h"
 #include "absl/base/internal/scheduling_mode.h"
 #include "absl/base/internal/tsan_mutex_interface.h"
 #include "absl/base/macros.h"
-#include "absl/base/port.h"
 #include "absl/base/thread_annotations.h"
 
+namespace tcmalloc {
+namespace tcmalloc_internal {
+
+class AllocationGuardSpinLockHolder;
+class Static;
+
+}  // namespace tcmalloc_internal
+}  // namespace tcmalloc
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 
-class ABSL_LOCKABLE SpinLock {
+class ABSL_LOCKABLE ABSL_ATTRIBUTE_WARN_UNUSED SpinLock {
  public:
-  SpinLock() : lockword_(kSpinLockCooperative) {
-    ABSL_TSAN_MUTEX_CREATE(this, __tsan_mutex_not_static);
-  }
-
-  // Special constructor for use with static SpinLock objects.  E.g.,
-  //
-  //    static SpinLock lock(base_internal::kLinkerInitialized);
-  //
-  // When initialized using this constructor, we depend on the fact
-  // that the linker has already initialized the memory appropriately. The lock
-  // is initialized in non-cooperative mode.
-  //
-  // A SpinLock constructed like this can be freely used from global
-  // initializers without worrying about the order in which global
-  // initializers run.
-  explicit SpinLock(base_internal::LinkerInitialized) {
-    // Does nothing; lockword_ is already initialized
-    ABSL_TSAN_MUTEX_CREATE(this, 0);
-  }
+  constexpr SpinLock() : lockword_(kSpinLockCooperative) { RegisterWithTsan(); }
 
   // Constructors that allow non-cooperative spinlocks to be created for use
   // inside thread schedulers.  Normal clients should not use these.
-  explicit SpinLock(base_internal::SchedulingMode mode);
-  SpinLock(base_internal::LinkerInitialized,
-           base_internal::SchedulingMode mode);
+  constexpr explicit SpinLock(SchedulingMode mode)
+      : lockword_(IsCooperative(mode) ? kSpinLockCooperative : 0) {
+    RegisterWithTsan();
+  }
 
+#if ABSL_HAVE_ATTRIBUTE(enable_if) && !defined(_WIN32)
+  // Constructor to inline users of the default scheduling mode.
+  //
+  // This only needs to exists for inliner runs, but doesn't work correctly in
+  // clang+windows builds, likely due to mangling differences.
+  ABSL_DEPRECATE_AND_INLINE()
+  constexpr explicit SpinLock(SchedulingMode mode)
+      __attribute__((enable_if(mode == SCHEDULE_COOPERATIVE_AND_KERNEL,
+                               "Cooperative use default constructor")))
+      : SpinLock() {}
+#endif
+
+  // Constructor for global SpinLock instances.  See absl/base/const_init.h.
+  ABSL_DEPRECATE_AND_INLINE()
+  constexpr SpinLock(absl::ConstInitType, SchedulingMode mode)
+      : SpinLock(mode) {}
+
+  // For global SpinLock instances prefer trivial destructor when possible.
+  // Default but non-trivial destructor in some build configurations causes an
+  // extra static initializer.
+#ifdef ABSL_INTERNAL_HAVE_TSAN_INTERFACE
   ~SpinLock() { ABSL_TSAN_MUTEX_DESTROY(this, __tsan_mutex_not_static); }
+#else
+  ~SpinLock() = default;
+#endif
 
   // Acquire this SpinLock.
-  inline void Lock() ABSL_EXCLUSIVE_LOCK_FUNCTION() {
+  inline void lock() ABSL_EXCLUSIVE_LOCK_FUNCTION() {
     ABSL_TSAN_MUTEX_PRE_LOCK(this, 0);
     if (!TryLockImpl()) {
       SlowLock();
@@ -87,11 +103,14 @@ class ABSL_LOCKABLE SpinLock {
     ABSL_TSAN_MUTEX_POST_LOCK(this, 0, 0);
   }
 
+  ABSL_DEPRECATE_AND_INLINE()
+  inline void Lock() ABSL_EXCLUSIVE_LOCK_FUNCTION() { return lock(); }
+
   // Try to acquire this SpinLock without blocking and return true if the
   // acquisition was successful.  If the lock was not acquired, false is
-  // returned.  If this SpinLock is free at the time of the call, TryLock
-  // will return true with high probability.
-  inline bool TryLock() ABSL_EXCLUSIVE_TRYLOCK_FUNCTION(true) {
+  // returned.  If this SpinLock is free at the time of the call, try_lock will
+  // return true with high probability.
+  [[nodiscard]] inline bool try_lock() ABSL_EXCLUSIVE_TRYLOCK_FUNCTION(true) {
     ABSL_TSAN_MUTEX_PRE_LOCK(this, __tsan_mutex_try_lock);
     bool res = TryLockImpl();
     ABSL_TSAN_MUTEX_POST_LOCK(
@@ -100,15 +119,20 @@ class ABSL_LOCKABLE SpinLock {
     return res;
   }
 
+  ABSL_DEPRECATE_AND_INLINE()
+  [[nodiscard]] inline bool TryLock() ABSL_EXCLUSIVE_TRYLOCK_FUNCTION(true) {
+    return try_lock();
+  }
+
   // Release this SpinLock, which must be held by the calling thread.
-  inline void Unlock() ABSL_UNLOCK_FUNCTION() {
+  inline void unlock() ABSL_UNLOCK_FUNCTION() {
     ABSL_TSAN_MUTEX_PRE_UNLOCK(this, 0);
     uint32_t lock_value = lockword_.load(std::memory_order_relaxed);
     lock_value = lockword_.exchange(lock_value & kSpinLockCooperative,
                                     std::memory_order_release);
 
     if ((lock_value & kSpinLockDisabledScheduling) != 0) {
-      base_internal::SchedulingGuard::EnableRescheduling(true);
+      SchedulingGuard::EnableRescheduling(true);
     }
     if ((lock_value & kWaitTimeMask) != 0) {
       // Collect contentionz profile info, and speed the wakeup of any waiter.
@@ -119,13 +143,24 @@ class ABSL_LOCKABLE SpinLock {
     ABSL_TSAN_MUTEX_POST_UNLOCK(this, 0);
   }
 
+  ABSL_DEPRECATE_AND_INLINE()
+  inline void Unlock() ABSL_UNLOCK_FUNCTION() { unlock(); }
+
   // Determine if the lock is held.  When the lock is held by the invoking
   // thread, true will always be returned. Intended to be used as
   // CHECK(lock.IsHeld()).
-  inline bool IsHeld() const {
+  [[nodiscard]] inline bool IsHeld() const {
     return (lockword_.load(std::memory_order_relaxed) & kSpinLockHeld) != 0;
   }
 
+  // Return immediately if this thread holds the SpinLock exclusively.
+  // Otherwise, report an error by crashing with a diagnostic.
+  inline void AssertHeld() const ABSL_ASSERT_EXCLUSIVE_LOCK() {
+    if (!IsHeld()) {
+      ABSL_RAW_LOG(FATAL, "thread should hold the lock on SpinLock");
+    }
+  }
+
  protected:
   // These should not be exported except for testing.
 
@@ -135,33 +170,67 @@ class ABSL_LOCKABLE SpinLock {
                                    int64_t wait_end_time);
 
   // Extract number of wait cycles in a lock value.
-  static uint64_t DecodeWaitCycles(uint32_t lock_value);
+  static int64_t DecodeWaitCycles(uint32_t lock_value);
 
   // Provide access to protected method above.  Use for testing only.
   friend struct SpinLockTest;
+  friend class tcmalloc::tcmalloc_internal::AllocationGuardSpinLockHolder;
+  friend class tcmalloc::tcmalloc_internal::Static;
+
+  static int GetAdaptiveSpinCount() {
+    return adaptive_spin_count_.load(std::memory_order_relaxed);
+  }
+  static void SetAdaptiveSpinCount(int count) {
+    adaptive_spin_count_.store(count, std::memory_order_relaxed);
+  }
+
+  static std::atomic<int> adaptive_spin_count_;
 
  private:
   // lockword_ is used to store the following:
   //
   // bit[0] encodes whether a lock is being held.
   // bit[1] encodes whether a lock uses cooperative scheduling.
-  // bit[2] encodes whether a lock disables scheduling.
+  // bit[2] encodes whether the current lock holder disabled scheduling when
+  //        acquiring the lock. Only set when kSpinLockHeld is also set.
   // bit[3:31] encodes time a lock spent on waiting as a 29-bit unsigned int.
-  enum { kSpinLockHeld = 1 };
-  enum { kSpinLockCooperative = 2 };
-  enum { kSpinLockDisabledScheduling = 4 };
-  enum { kSpinLockSleeper = 8 };
-  enum { kWaitTimeMask =                      // Includes kSpinLockSleeper.
-    ~(kSpinLockHeld | kSpinLockCooperative | kSpinLockDisabledScheduling) };
+  //        This is set by the lock holder to indicate how long it waited on
+  //        the lock before eventually acquiring it. The number of cycles is
+  //        encoded as a 29-bit unsigned int, or in the case that the current
+  //        holder did not wait but another waiter is queued, the LSB
+  //        (kSpinLockSleeper) is set. The implementation does not explicitly
+  //        track the number of queued waiters beyond this. It must always be
+  //        assumed that waiters may exist if the current holder was required to
+  //        queue.
+  //
+  // Invariant: if the lock is not held, the value is either 0 or
+  // kSpinLockCooperative.
+  static constexpr uint32_t kSpinLockHeld = 1;
+  static constexpr uint32_t kSpinLockCooperative = 2;
+  static constexpr uint32_t kSpinLockDisabledScheduling = 4;
+  static constexpr uint32_t kSpinLockSleeper = 8;
+  // Includes kSpinLockSleeper.
+  static constexpr uint32_t kWaitTimeMask =
+      ~(kSpinLockHeld | kSpinLockCooperative | kSpinLockDisabledScheduling);
 
   // Returns true if the provided scheduling mode is cooperative.
-  static constexpr bool IsCooperative(
-      base_internal::SchedulingMode scheduling_mode) {
-    return scheduling_mode == base_internal::SCHEDULE_COOPERATIVE_AND_KERNEL;
+  static constexpr bool IsCooperative(SchedulingMode scheduling_mode) {
+    return scheduling_mode == SCHEDULE_COOPERATIVE_AND_KERNEL;
+  }
+
+  constexpr void RegisterWithTsan() {
+#if ABSL_HAVE_BUILTIN(__builtin_is_constant_evaluated)
+    if (!__builtin_is_constant_evaluated()) {
+      ABSL_TSAN_MUTEX_CREATE(this, __tsan_mutex_not_static);
+    }
+#endif
+  }
+
+  bool IsCooperative() const {
+    return lockword_.load(std::memory_order_relaxed) & kSpinLockCooperative;
   }
 
   uint32_t TryLockInternal(uint32_t lock_value, uint32_t wait_cycles);
-  void InitLinkerInitializedAndCooperative();
   void SlowLock() ABSL_ATTRIBUTE_COLD;
   void SlowUnlock(uint32_t lock_value) ABSL_ATTRIBUTE_COLD;
   uint32_t SpinLoop();
@@ -179,19 +248,18 @@ class ABSL_LOCKABLE SpinLock {
 
 // Corresponding locker object that arranges to acquire a spinlock for
 // the duration of a C++ scope.
-class ABSL_SCOPED_LOCKABLE SpinLockHolder {
+class ABSL_SCOPED_LOCKABLE [[nodiscard]] SpinLockHolder
+    : public std::lock_guard<SpinLock> {
  public:
+  inline explicit SpinLockHolder(
+      SpinLock& l ABSL_INTERNAL_ATTRIBUTE_CAPTURED_BY_THIS)
+      ABSL_EXCLUSIVE_LOCK_FUNCTION(l)
+      : std::lock_guard<SpinLock>(l) {}
+  ABSL_DEPRECATE_AND_INLINE()
   inline explicit SpinLockHolder(SpinLock* l) ABSL_EXCLUSIVE_LOCK_FUNCTION(l)
-      : lock_(l) {
-    l->Lock();
-  }
-  inline ~SpinLockHolder() ABSL_UNLOCK_FUNCTION() { lock_->Unlock(); }
+      : SpinLockHolder(*l) {}
 
-  SpinLockHolder(const SpinLockHolder&) = delete;
-  SpinLockHolder& operator=(const SpinLockHolder&) = delete;
-
- private:
-  SpinLock* lock_;
+  inline ~SpinLockHolder() ABSL_UNLOCK_FUNCTION() = default;
 };
 
 // Register a hook for profiling support.
@@ -220,7 +288,7 @@ inline uint32_t SpinLock::TryLockInternal(uint32_t lock_value,
   if ((lock_value & kSpinLockCooperative) == 0) {
     // For non-cooperative locks we must make sure we mark ourselves as
     // non-reschedulable before we attempt to CompareAndSwap.
-    if (base_internal::SchedulingGuard::DisableRescheduling()) {
+    if (SchedulingGuard::DisableRescheduling()) {
       sched_disabled_bit = kSpinLockDisabledScheduling;
     }
   }
@@ -229,13 +297,14 @@ inline uint32_t SpinLock::TryLockInternal(uint32_t lock_value,
           lock_value,
           kSpinLockHeld | lock_value | wait_cycles | sched_disabled_bit,
           std::memory_order_acquire, std::memory_order_relaxed)) {
-    base_internal::SchedulingGuard::EnableRescheduling(sched_disabled_bit != 0);
+    SchedulingGuard::EnableRescheduling(sched_disabled_bit != 0);
   }
 
   return lock_value;
 }
 
 }  // namespace base_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_BASE_INTERNAL_SPINLOCK_H_
diff --git a/absl/base/internal/spinlock_akaros.inc b/absl/base/internal/spinlock_akaros.inc
index bc468940fc5..7b0cada4f13 100644
--- a/absl/base/internal/spinlock_akaros.inc
+++ b/absl/base/internal/spinlock_akaros.inc
@@ -20,7 +20,7 @@
 
 extern "C" {
 
-ABSL_ATTRIBUTE_WEAK void AbslInternalSpinLockDelay(
+ABSL_ATTRIBUTE_WEAK void ABSL_INTERNAL_C_SYMBOL(AbslInternalSpinLockDelay)(
     std::atomic<uint32_t>* /* lock_word */, uint32_t /* value */,
     int /* loop */, absl::base_internal::SchedulingMode /* mode */) {
   // In Akaros, one must take care not to call anything that could cause a
@@ -29,7 +29,7 @@ ABSL_ATTRIBUTE_WEAK void AbslInternalSpinLockDelay(
   // arbitrary code.
 }
 
-ABSL_ATTRIBUTE_WEAK void AbslInternalSpinLockWake(
+ABSL_ATTRIBUTE_WEAK void ABSL_INTERNAL_C_SYMBOL(AbslInternalSpinLockWake)(
     std::atomic<uint32_t>* /* lock_word */, bool /* all */) {}
 
 }  // extern "C"
diff --git a/absl/base/internal/spinlock_benchmark.cc b/absl/base/internal/spinlock_benchmark.cc
index 0451c65f955..c79f49fefb0 100644
--- a/absl/base/internal/spinlock_benchmark.cc
+++ b/absl/base/internal/spinlock_benchmark.cc
@@ -18,22 +18,39 @@
 #include "absl/base/internal/raw_logging.h"
 #include "absl/base/internal/scheduling_mode.h"
 #include "absl/base/internal/spinlock.h"
+#include "absl/base/no_destructor.h"
 #include "absl/synchronization/internal/create_thread_identity.h"
 #include "benchmark/benchmark.h"
 
 namespace {
 
+template <absl::base_internal::SchedulingMode scheduling_mode>
+static void BM_TryLock(benchmark::State& state) {
+  // Ensure a ThreadIdentity is installed so that KERNEL_ONLY has an effect.
+  ABSL_INTERNAL_CHECK(
+      absl::synchronization_internal::GetOrCreateCurrentThreadIdentity() !=
+          nullptr,
+      "GetOrCreateCurrentThreadIdentity() failed");
+
+  static absl::NoDestructor<absl::base_internal::SpinLock> spinlock(
+      scheduling_mode);
+  for (auto _ : state) {
+    if (spinlock->try_lock()) spinlock->unlock();
+  }
+}
+
 template <absl::base_internal::SchedulingMode scheduling_mode>
 static void BM_SpinLock(benchmark::State& state) {
-  // Ensure a ThreadIdentity is installed.
+  // Ensure a ThreadIdentity is installed so that KERNEL_ONLY has an effect.
   ABSL_INTERNAL_CHECK(
       absl::synchronization_internal::GetOrCreateCurrentThreadIdentity() !=
           nullptr,
       "GetOrCreateCurrentThreadIdentity() failed");
 
-  static auto* spinlock = new absl::base_internal::SpinLock(scheduling_mode);
+  static absl::NoDestructor<absl::base_internal::SpinLock> spinlock(
+      scheduling_mode);
   for (auto _ : state) {
-    absl::base_internal::SpinLockHolder holder(spinlock);
+    absl::base_internal::SpinLockHolder holder(*spinlock.get());
   }
 }
 
@@ -49,4 +66,15 @@ BENCHMARK_TEMPLATE(BM_SpinLock,
     ->Threads(1)
     ->ThreadPerCpu();
 
+BENCHMARK_TEMPLATE(BM_TryLock, absl::base_internal::SCHEDULE_KERNEL_ONLY)
+    ->UseRealTime()
+    ->Threads(1)
+    ->ThreadPerCpu();
+
+BENCHMARK_TEMPLATE(BM_TryLock,
+                   absl::base_internal::SCHEDULE_COOPERATIVE_AND_KERNEL)
+    ->UseRealTime()
+    ->Threads(1)
+    ->ThreadPerCpu();
+
 }  // namespace
diff --git a/absl/base/internal/spinlock_linux.inc b/absl/base/internal/spinlock_linux.inc
index 28e29d19190..fe8ba674f50 100644
--- a/absl/base/internal/spinlock_linux.inc
+++ b/absl/base/internal/spinlock_linux.inc
@@ -19,12 +19,12 @@
 #include <unistd.h>
 
 #include <atomic>
-#include <cerrno>
 #include <climits>
 #include <cstdint>
 #include <ctime>
 
 #include "absl/base/attributes.h"
+#include "absl/base/internal/errno_saver.h"
 
 // The SpinLock lockword is `std::atomic<uint32_t>`. Here we assert that
 // `std::atomic<uint32_t>` is bitwise equivalent of the `int` expected
@@ -46,21 +46,25 @@ static_assert(sizeof(std::atomic<uint32_t>) == sizeof(int),
 #endif
 #endif
 
+#if defined(__NR_futex_time64) && !defined(SYS_futex_time64)
+#define SYS_futex_time64 __NR_futex_time64
+#endif
+
+#if defined(SYS_futex_time64) && !defined(SYS_futex)
+#define SYS_futex SYS_futex_time64
+#endif
+
 extern "C" {
 
-ABSL_ATTRIBUTE_WEAK void AbslInternalSpinLockDelay(
-    std::atomic<uint32_t> *w, uint32_t value, int loop,
+ABSL_ATTRIBUTE_WEAK void ABSL_INTERNAL_C_SYMBOL(AbslInternalSpinLockDelay)(
+    std::atomic<uint32_t> *w, uint32_t value, int,
     absl::base_internal::SchedulingMode) {
-  int save_errno = errno;
-  struct timespec tm;
-  tm.tv_sec = 0;
-  tm.tv_nsec = absl::base_internal::SpinLockSuggestedDelayNS(loop);
-  syscall(SYS_futex, w, FUTEX_WAIT | FUTEX_PRIVATE_FLAG, value, &tm);
-  errno = save_errno;
+  absl::base_internal::ErrnoSaver errno_saver;
+  syscall(SYS_futex, w, FUTEX_WAIT | FUTEX_PRIVATE_FLAG, value, nullptr);
 }
 
-ABSL_ATTRIBUTE_WEAK void AbslInternalSpinLockWake(std::atomic<uint32_t> *w,
-                                                  bool all) {
+ABSL_ATTRIBUTE_WEAK void ABSL_INTERNAL_C_SYMBOL(AbslInternalSpinLockWake)(
+    std::atomic<uint32_t> *w, bool all) {
   syscall(SYS_futex, w, FUTEX_WAKE | FUTEX_PRIVATE_FLAG, all ? INT_MAX : 1, 0);
 }
 
diff --git a/absl/base/internal/spinlock_posix.inc b/absl/base/internal/spinlock_posix.inc
index f025b5f8a37..4f6f887d993 100644
--- a/absl/base/internal/spinlock_posix.inc
+++ b/absl/base/internal/spinlock_posix.inc
@@ -15,19 +15,20 @@
 // This file is a Posix-specific part of spinlock_wait.cc
 
 #include <sched.h>
+
 #include <atomic>
 #include <ctime>
-#include <cerrno>
 
+#include "absl/base/internal/errno_saver.h"
 #include "absl/base/internal/scheduling_mode.h"
 #include "absl/base/port.h"
 
 extern "C" {
 
-ABSL_ATTRIBUTE_WEAK void AbslInternalSpinLockDelay(
+ABSL_ATTRIBUTE_WEAK void ABSL_INTERNAL_C_SYMBOL(AbslInternalSpinLockDelay)(
     std::atomic<uint32_t>* /* lock_word */, uint32_t /* value */, int loop,
     absl::base_internal::SchedulingMode /* mode */) {
-  int save_errno = errno;
+  absl::base_internal::ErrnoSaver errno_saver;
   if (loop == 0) {
   } else if (loop == 1) {
     sched_yield();
@@ -37,10 +38,9 @@ ABSL_ATTRIBUTE_WEAK void AbslInternalSpinLockDelay(
     tm.tv_nsec = absl::base_internal::SpinLockSuggestedDelayNS(loop);
     nanosleep(&tm, nullptr);
   }
-  errno = save_errno;
 }
 
-ABSL_ATTRIBUTE_WEAK void AbslInternalSpinLockWake(
+ABSL_ATTRIBUTE_WEAK void ABSL_INTERNAL_C_SYMBOL(AbslInternalSpinLockWake)(
     std::atomic<uint32_t>* /* lock_word */, bool /* all */) {}
 
 }  // extern "C"
diff --git a/absl/base/internal/spinlock_wait.cc b/absl/base/internal/spinlock_wait.cc
index fac8a21db4c..fa824be1c00 100644
--- a/absl/base/internal/spinlock_wait.cc
+++ b/absl/base/internal/spinlock_wait.cc
@@ -32,6 +32,7 @@
 #endif
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 
 // See spinlock_wait.h for spec.
@@ -76,4 +77,5 @@ int SpinLockSuggestedDelayNS(int loop) {
 }
 
 }  // namespace base_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/base/internal/spinlock_wait.h b/absl/base/internal/spinlock_wait.h
index 6642ce1ab31..9a1adcda5ea 100644
--- a/absl/base/internal/spinlock_wait.h
+++ b/absl/base/internal/spinlock_wait.h
@@ -24,6 +24,7 @@
 #include "absl/base/internal/scheduling_mode.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 
 // SpinLockWait() waits until it can perform one of several transitions from
@@ -38,22 +39,22 @@ struct SpinLockWaitTransition {
 // satisfying 0<=i<n && trans[i].done, atomically make the transition,
 // then return the old value of *w.   Make any other atomic transitions
 // where !trans[i].done, but continue waiting.
+//
+// Wakeups for threads blocked on SpinLockWait do not respect priorities.
 uint32_t SpinLockWait(std::atomic<uint32_t> *w, int n,
                       const SpinLockWaitTransition trans[],
                       SchedulingMode scheduling_mode);
 
-// If possible, wake some thread that has called SpinLockDelay(w, ...). If
-// "all" is true, wake all such threads.  This call is a hint, and on some
-// systems it may be a no-op; threads calling SpinLockDelay() will always wake
-// eventually even if SpinLockWake() is never called.
+// If possible, wake some thread that has called SpinLockDelay(w, ...). If `all`
+// is true, wake all such threads. On some systems, this may be a no-op; on
+// those systems, threads calling SpinLockDelay() will always wake eventually
+// even if SpinLockWake() is never called.
 void SpinLockWake(std::atomic<uint32_t> *w, bool all);
 
 // Wait for an appropriate spin delay on iteration "loop" of a
 // spin loop on location *w, whose previously observed value was "value".
 // SpinLockDelay() may do nothing, may yield the CPU, may sleep a clock tick,
-// or may wait for a delay that can be truncated by a call to SpinLockWake(w).
-// In all cases, it must return in bounded time even if SpinLockWake() is not
-// called.
+// or may wait for a call to SpinLockWake(w).
 void SpinLockDelay(std::atomic<uint32_t> *w, uint32_t value, int loop,
                    base_internal::SchedulingMode scheduling_mode);
 
@@ -62,6 +63,7 @@ void SpinLockDelay(std::atomic<uint32_t> *w, uint32_t value, int loop,
 int SpinLockSuggestedDelayNS(int loop);
 
 }  // namespace base_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 // In some build configurations we pass --detect-odr-violations to the
@@ -71,21 +73,23 @@ int SpinLockSuggestedDelayNS(int loop);
 // By changing our extension points to be extern "C", we dodge this
 // check.
 extern "C" {
-void AbslInternalSpinLockWake(std::atomic<uint32_t> *w, bool all);
-void AbslInternalSpinLockDelay(
+void ABSL_INTERNAL_C_SYMBOL(AbslInternalSpinLockWake)(std::atomic<uint32_t> *w,
+                                                      bool all);
+void ABSL_INTERNAL_C_SYMBOL(AbslInternalSpinLockDelay)(
     std::atomic<uint32_t> *w, uint32_t value, int loop,
     absl::base_internal::SchedulingMode scheduling_mode);
 }
 
 inline void absl::base_internal::SpinLockWake(std::atomic<uint32_t> *w,
                                               bool all) {
-  AbslInternalSpinLockWake(w, all);
+  ABSL_INTERNAL_C_SYMBOL(AbslInternalSpinLockWake)(w, all);
 }
 
 inline void absl::base_internal::SpinLockDelay(
     std::atomic<uint32_t> *w, uint32_t value, int loop,
     absl::base_internal::SchedulingMode scheduling_mode) {
-  AbslInternalSpinLockDelay(w, value, loop, scheduling_mode);
+  ABSL_INTERNAL_C_SYMBOL(AbslInternalSpinLockDelay)
+  (w, value, loop, scheduling_mode);
 }
 
 #endif  // ABSL_BASE_INTERNAL_SPINLOCK_WAIT_H_
diff --git a/absl/base/internal/spinlock_win32.inc b/absl/base/internal/spinlock_win32.inc
index 78654b5b596..934c2016fb6 100644
--- a/absl/base/internal/spinlock_win32.inc
+++ b/absl/base/internal/spinlock_win32.inc
@@ -20,18 +20,21 @@
 
 extern "C" {
 
-void AbslInternalSpinLockDelay(std::atomic<uint32_t>* /* lock_word */,
-                               uint32_t /* value */, int loop,
-                               absl::base_internal::SchedulingMode /* mode */) {
+void ABSL_INTERNAL_C_SYMBOL(AbslInternalSpinLockDelay)(
+    std::atomic<uint32_t>* /* lock_word */, uint32_t /* value */, int loop,
+    absl::base_internal::SchedulingMode /* mode */) {
   if (loop == 0) {
   } else if (loop == 1) {
     Sleep(0);
   } else {
-    Sleep(absl::base_internal::SpinLockSuggestedDelayNS(loop) / 1000000);
+    // SpinLockSuggestedDelayNS() always returns a positive integer, so this
+    // static_cast is safe.
+    Sleep(static_cast<DWORD>(
+        absl::base_internal::SpinLockSuggestedDelayNS(loop) / 1000000));
   }
 }
 
-void AbslInternalSpinLockWake(std::atomic<uint32_t>* /* lock_word */,
-                              bool /* all */) {}
+void ABSL_INTERNAL_C_SYMBOL(AbslInternalSpinLockWake)(
+    std::atomic<uint32_t>* /* lock_word */, bool /* all */) {}
 
 }  // extern "C"
diff --git a/absl/base/internal/strerror.cc b/absl/base/internal/strerror.cc
new file mode 100644
index 00000000000..e9c4c23b8fa
--- /dev/null
+++ b/absl/base/internal/strerror.cc
@@ -0,0 +1,88 @@
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/base/internal/strerror.h"
+
+#include <array>
+#include <cerrno>
+#include <cstddef>
+#include <cstdio>
+#include <cstring>
+#include <string>
+#include <type_traits>
+
+#include "absl/base/internal/errno_saver.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace base_internal {
+namespace {
+
+const char* StrErrorAdaptor(int errnum, char* buf, size_t buflen) {
+#if defined(_WIN32)
+  int rc = strerror_s(buf, buflen, errnum);
+  buf[buflen - 1] = '\0';  // guarantee NUL termination
+  if (rc == 0 && strncmp(buf, "Unknown error", buflen) == 0) *buf = '\0';
+  return buf;
+#else
+  // The type of `ret` is platform-specific; both of these branches must compile
+  // either way but only one will execute on any given platform:
+  auto ret = strerror_r(errnum, buf, buflen);
+  if (std::is_same_v<decltype(ret), int>) {
+    // XSI `strerror_r`; `ret` is `int`:
+    if (ret) *buf = '\0';
+    return buf;
+  } else {
+    // GNU `strerror_r`; `ret` is `char *`:
+    return reinterpret_cast<const char*>(ret);
+  }
+#endif
+}
+
+std::string StrErrorInternal(int errnum) {
+  char buf[100];
+  const char* str = StrErrorAdaptor(errnum, buf, sizeof buf);
+  if (*str == '\0') {
+    snprintf(buf, sizeof buf, "Unknown error %d", errnum);
+    str = buf;
+  }
+  return str;
+}
+
+// kSysNerr is the number of errors from a recent glibc. `StrError()` falls back
+// to `StrErrorInternal()` if the value is larger than this.
+constexpr int kSysNerr = 135;
+
+std::array<std::string, kSysNerr>* NewStrErrorTable() {
+  auto* table = new std::array<std::string, kSysNerr>;
+  for (size_t i = 0; i < table->size(); ++i) {
+    (*table)[i] = StrErrorInternal(static_cast<int>(i));
+  }
+  return table;
+}
+
+}  // namespace
+
+std::string StrError(int errnum) {
+  absl::base_internal::ErrnoSaver errno_saver;
+  static const auto* table = NewStrErrorTable();
+  if (errnum >= 0 && static_cast<size_t>(errnum) < table->size()) {
+    return (*table)[static_cast<size_t>(errnum)];
+  }
+  return StrErrorInternal(errnum);
+}
+
+}  // namespace base_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/base/internal/strerror.h b/absl/base/internal/strerror.h
new file mode 100644
index 00000000000..350097366ee
--- /dev/null
+++ b/absl/base/internal/strerror.h
@@ -0,0 +1,39 @@
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_BASE_INTERNAL_STRERROR_H_
+#define ABSL_BASE_INTERNAL_STRERROR_H_
+
+#include <string>
+
+#include "absl/base/config.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace base_internal {
+
+// A portable and thread-safe alternative to C89's `strerror`.
+//
+// The C89 specification of `strerror` is not suitable for use in a
+// multi-threaded application as the returned string may be changed by calls to
+// `strerror` from another thread.  The many non-stdlib alternatives differ
+// enough in their names, availability, and semantics to justify this wrapper
+// around them.  `errno` will not be modified by a call to `absl::StrError`.
+std::string StrError(int errnum);
+
+}  // namespace base_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_BASE_INTERNAL_STRERROR_H_
diff --git a/absl/random/internal/seed_salting_sequence_generator.cc b/absl/base/internal/strerror_benchmark.cc
similarity index 60%
rename from absl/random/internal/seed_salting_sequence_generator.cc
rename to absl/base/internal/strerror_benchmark.cc
index 31fdcfe1db4..c9ab14a89db 100644
--- a/absl/random/internal/seed_salting_sequence_generator.cc
+++ b/absl/base/internal/strerror_benchmark.cc
@@ -1,4 +1,4 @@
-// Copyright 2017 The Abseil Authors.
+// Copyright 2020 The Abseil Authors.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
@@ -12,19 +12,18 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-#include <iostream>
-#include <random>
+#include <cerrno>
+#include <cstdio>
+#include <string>
 
-#include "absl/random/random.h"
+#include "absl/base/internal/strerror.h"
+#include "benchmark/benchmark.h"
 
-// This program is used in integration tests.
-
-int main() {
-  std::seed_seq seed_seq{1234};
-  absl::BitGen rng(seed_seq);
-  constexpr size_t kSequenceLength = 8;
-  for (size_t i = 0; i < kSequenceLength; i++) {
-    std::cout << rng() << "\n";
+namespace {
+void BM_AbslStrError(benchmark::State& state) {
+  for (auto _ : state) {
+    benchmark::DoNotOptimize(absl::base_internal::StrError(ERANGE));
   }
-  return 0;
 }
+BENCHMARK(BM_AbslStrError);
+}  // namespace
diff --git a/absl/base/internal/strerror_test.cc b/absl/base/internal/strerror_test.cc
new file mode 100644
index 00000000000..d12c537b99d
--- /dev/null
+++ b/absl/base/internal/strerror_test.cc
@@ -0,0 +1,89 @@
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/base/internal/strerror.h"
+
+#include <atomic>
+#include <cerrno>
+#include <cstdio>
+#include <cstring>
+#include <string>
+#include <thread>  // NOLINT(build/c++11)
+#include <vector>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/strings/match.h"
+
+namespace {
+using ::testing::AnyOf;
+using ::testing::Eq;
+
+TEST(StrErrorTest, ValidErrorCode) {
+  errno = ERANGE;
+  EXPECT_THAT(absl::base_internal::StrError(EDOM), Eq(strerror(EDOM)));
+  EXPECT_THAT(errno, Eq(ERANGE));
+}
+
+TEST(StrErrorTest, InvalidErrorCode) {
+  errno = ERANGE;
+  EXPECT_THAT(absl::base_internal::StrError(-1),
+              AnyOf(Eq("No error information"), Eq("Unknown error -1"),
+                    Eq("Unknown error")));
+  EXPECT_THAT(errno, Eq(ERANGE));
+}
+
+TEST(StrErrorTest, MultipleThreads) {
+  // In this test, we will start up 2 threads and have each one call
+  // StrError 1000 times, each time with a different errnum.  We
+  // expect that StrError(errnum) will return a string equal to the
+  // one returned by strerror(errnum), if the code is known.  Since
+  // strerror is known to be thread-hostile, collect all the expected
+  // strings up front.
+  const int kNumCodes = 1000;
+  std::vector<std::string> expected_strings(kNumCodes);
+  for (int i = 0; i < kNumCodes; ++i) {
+    expected_strings[i] = strerror(i);
+  }
+
+  std::atomic_int counter(0);
+  auto thread_fun = [&]() {
+    for (int i = 0; i < kNumCodes; ++i) {
+      ++counter;
+      errno = ERANGE;
+      const std::string value = absl::base_internal::StrError(i);
+      // EXPECT_* could change errno. Stash it first.
+      int check_err = errno;
+      EXPECT_THAT(check_err, Eq(ERANGE));
+      // Only the GNU implementation is guaranteed to provide the
+      // string "Unknown error nnn". POSIX doesn't say anything.
+      if (!absl::StartsWith(value, "Unknown error ")) {
+        EXPECT_THAT(value, Eq(expected_strings[i]));
+      }
+    }
+  };
+
+  const int kNumThreads = 100;
+  std::vector<std::thread> threads;
+  for (int i = 0; i < kNumThreads; ++i) {
+    threads.push_back(std::thread(thread_fun));
+  }
+  for (auto& thread : threads) {
+    thread.join();
+  }
+
+  EXPECT_THAT(counter, Eq(kNumThreads * kNumCodes));
+}
+
+}  // namespace
diff --git a/absl/base/internal/sysinfo.cc b/absl/base/internal/sysinfo.cc
index 4dd3adda46e..1f380bcc60b 100644
--- a/absl/base/internal/sysinfo.cc
+++ b/absl/base/internal/sysinfo.cc
@@ -14,10 +14,28 @@
 
 #include "absl/base/internal/sysinfo.h"
 
+#include <string.h>
+
+#include <cassert>
+#include <cerrno>
+#include <cstdint>
+#include <cstdio>
+#include <cstdlib>
+#include <ctime>
+#include <limits>
+#include <thread>  // NOLINT(build/c++11)
+#include <utility>
+#include <vector>
+
 #include "absl/base/attributes.h"
+#include "absl/base/call_once.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/base/internal/spinlock.h"
+#include "absl/base/internal/unscaledcycleclock.h"
+#include "absl/base/thread_annotations.h"
 
 #ifdef _WIN32
-#include <shlwapi.h>
 #include <windows.h>
 #else
 #include <fcntl.h>
@@ -35,57 +53,134 @@
 #include <sys/sysctl.h>
 #endif
 
+#ifdef __FreeBSD__
+#include <pthread_np.h>
+#endif
+
+#ifdef __NetBSD__
+#include <lwp.h>
+#endif
+
 #if defined(__myriad2__)
 #include <rtems.h>
 #endif
 
-#include <string.h>
-#include <cassert>
-#include <cstdint>
-#include <cstdio>
-#include <cstdlib>
-#include <ctime>
-#include <limits>
-#include <thread>  // NOLINT(build/c++11)
-#include <utility>
-#include <vector>
-
-#include "absl/base/call_once.h"
-#include "absl/base/internal/raw_logging.h"
-#include "absl/base/internal/spinlock.h"
-#include "absl/base/internal/unscaledcycleclock.h"
+#if defined(__Fuchsia__)
+#include <zircon/process.h>
+#endif
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 
-static once_flag init_system_info_once;
-static int num_cpus = 0;
-static double nominal_cpu_frequency = 1.0;  // 0.0 might be dangerous.
+namespace {
+
+#if defined(_WIN32)
+
+// Returns number of bits set in `bitMask`
+DWORD Win32CountSetBits(ULONG_PTR bitMask) {
+  for (DWORD bitSetCount = 0; ; ++bitSetCount) {
+    if (bitMask == 0) return bitSetCount;
+    bitMask &= bitMask - 1;
+  }
+}
+
+// Returns the number of logical CPUs using GetLogicalProcessorInformation(), or
+// 0 if the number of processors is not available or can not be computed.
+// https://docs.microsoft.com/en-us/windows/win32/api/sysinfoapi/nf-sysinfoapi-getlogicalprocessorinformation
+int Win32NumCPUs() {
+#pragma comment(lib, "kernel32.lib")
+  using Info = SYSTEM_LOGICAL_PROCESSOR_INFORMATION;
+
+  DWORD info_size = sizeof(Info);
+  Info* info(static_cast<Info*>(malloc(info_size)));
+  if (info == nullptr) return 0;
+
+  bool success = GetLogicalProcessorInformation(info, &info_size);
+  if (!success && GetLastError() == ERROR_INSUFFICIENT_BUFFER) {
+    free(info);
+    info = static_cast<Info*>(malloc(info_size));
+    if (info == nullptr) return 0;
+    success = GetLogicalProcessorInformation(info, &info_size);
+  }
+
+  DWORD logicalProcessorCount = 0;
+  if (success) {
+    Info* ptr = info;
+    DWORD byteOffset = 0;
+    while (byteOffset + sizeof(Info) <= info_size) {
+      switch (ptr->Relationship) {
+        case RelationProcessorCore:
+          logicalProcessorCount += Win32CountSetBits(ptr->ProcessorMask);
+          break;
+
+        case RelationNumaNode:
+        case RelationCache:
+        case RelationProcessorPackage:
+          // Ignore other entries
+          break;
+
+        default:
+          // Ignore unknown entries
+          break;
+      }
+      byteOffset += sizeof(Info);
+      ptr++;
+    }
+  }
+  free(info);
+  return static_cast<int>(logicalProcessorCount);
+}
+
+#endif
+
+}  // namespace
 
 static int GetNumCPUs() {
 #if defined(__myriad2__)
   return 1;
+#elif defined(_WIN32)
+  const int hardware_concurrency = Win32NumCPUs();
+  return hardware_concurrency ? hardware_concurrency : 1;
+#elif defined(_AIX)
+  return sysconf(_SC_NPROCESSORS_ONLN);
 #else
   // Other possibilities:
   //  - Read /sys/devices/system/cpu/online and use cpumask_parse()
   //  - sysconf(_SC_NPROCESSORS_ONLN)
-  return std::thread::hardware_concurrency();
+  return static_cast<int>(std::thread::hardware_concurrency());
 #endif
 }
 
 #if defined(_WIN32)
 
 static double GetNominalCPUFrequency() {
-  DWORD data;
-  DWORD data_size = sizeof(data);
-  #pragma comment(lib, "shlwapi.lib")  // For SHGetValue().
-  if (SUCCEEDED(
-          SHGetValueA(HKEY_LOCAL_MACHINE,
-                      "HARDWARE\\DESCRIPTION\\System\\CentralProcessor\\0",
-                      "~MHz", nullptr, &data, &data_size))) {
-    return data * 1e6;  // Value is MHz.
+#if WINAPI_FAMILY_PARTITION(WINAPI_PARTITION_APP) && \
+    !WINAPI_FAMILY_PARTITION(WINAPI_PARTITION_DESKTOP)
+  // UWP apps don't have access to the registry and currently don't provide an
+  // API informing about CPU nominal frequency.
+  return 1.0;
+#else
+#pragma comment(lib, "advapi32.lib")  // For Reg* functions.
+  HKEY key;
+  // Use the Reg* functions rather than the SH functions because shlwapi.dll
+  // pulls in gdi32.dll which makes process destruction much more costly.
+  if (RegOpenKeyExA(HKEY_LOCAL_MACHINE,
+                    "HARDWARE\\DESCRIPTION\\System\\CentralProcessor\\0", 0,
+                    KEY_READ, &key) == ERROR_SUCCESS) {
+    DWORD type = 0;
+    DWORD data = 0;
+    DWORD data_size = sizeof(data);
+    auto result = RegQueryValueExA(key, "~MHz", nullptr, &type,
+                                   reinterpret_cast<LPBYTE>(&data), &data_size);
+    RegCloseKey(key);
+    if (result == ERROR_SUCCESS && type == REG_DWORD &&
+        data_size == sizeof(data)) {
+      return data * 1e6;  // Value is MHz.
+    }
   }
   return 1.0;
+#endif  // WINAPI_PARTITION_APP && !WINAPI_PARTITION_DESKTOP
 }
 
 #elif defined(CTL_HW) && defined(HW_CPU_FREQ)
@@ -106,12 +201,21 @@ static double GetNominalCPUFrequency() {
 // and the memory location pointed to by value is set to the value read.
 static bool ReadLongFromFile(const char *file, long *value) {
   bool ret = false;
-  int fd = open(file, O_RDONLY);
+#if defined(_POSIX_C_SOURCE)
+  const int file_mode = (O_RDONLY | O_CLOEXEC);
+#else
+  const int file_mode = O_RDONLY;
+#endif
+
+  int fd = open(file, file_mode);
   if (fd != -1) {
     char line[1024];
     char *err;
     memset(line, '\0', sizeof(line));
-    int len = read(fd, line, sizeof(line) - 1);
+    ssize_t len;
+    do {
+      len = read(fd, line, sizeof(line) - 1);
+    } while (len < 0 && errno == EINTR);
     if (len <= 0) {
       ret = false;
     } else {
@@ -139,8 +243,7 @@ static int64_t ReadMonotonicClockNanos() {
   int rc = clock_gettime(CLOCK_MONOTONIC, &t);
 #endif
   if (rc != 0) {
-    perror("clock_gettime() failed");
-    abort();
+    ABSL_RAW_LOG(FATAL, "clock_gettime() failed: (%d)", errno);
   }
   return int64_t{t.tv_sec} * 1000000000 + t.tv_nsec;
 }
@@ -256,40 +359,54 @@ static double GetNominalCPUFrequency() {
 
 #endif
 
-// InitializeSystemInfo() may be called before main() and before
-// malloc is properly initialized, therefore this must not allocate
-// memory.
-static void InitializeSystemInfo() {
-  num_cpus = GetNumCPUs();
-  nominal_cpu_frequency = GetNominalCPUFrequency();
-}
+ABSL_CONST_INIT static once_flag init_num_cpus_once;
+ABSL_CONST_INIT static int num_cpus = 0;
 
+// NumCPUs() may be called before main() and before malloc is properly
+// initialized, therefore this must not allocate memory.
 int NumCPUs() {
-  base_internal::LowLevelCallOnce(&init_system_info_once, InitializeSystemInfo);
+  base_internal::LowLevelCallOnce(
+      &init_num_cpus_once, []() { num_cpus = GetNumCPUs(); });
   return num_cpus;
 }
 
+// A default frequency of 0.0 might be dangerous if it is used in division.
+ABSL_CONST_INIT static once_flag init_nominal_cpu_frequency_once;
+ABSL_CONST_INIT static double nominal_cpu_frequency = 1.0;
+
+// NominalCPUFrequency() may be called before main() and before malloc is
+// properly initialized, therefore this must not allocate memory.
 double NominalCPUFrequency() {
-  base_internal::LowLevelCallOnce(&init_system_info_once, InitializeSystemInfo);
+  base_internal::LowLevelCallOnce(&init_nominal_cpu_frequency_once, []() {
+    nominal_cpu_frequency = GetNominalCPUFrequency();
+  });
   return nominal_cpu_frequency;
 }
 
 #if defined(_WIN32)
 
-pid_t GetTID() {
-  return GetCurrentThreadId();
-}
+pid_t GetTID() { return pid_t{GetCurrentThreadId()}; }
 
 #elif defined(__linux__)
+#ifdef __ANDROID__
+#if __ANDROID_API__ >= 21
+#define ABSL_INTERNAL_HAVE_GETTID 1
+#endif
+#endif
 
+#ifdef ABSL_INTERNAL_HAVE_GETTID
+pid_t GetTID() {
+  return static_cast<pid_t>(gettid());
+}
+#else
 #ifndef SYS_gettid
 #define SYS_gettid __NR_gettid
 #endif
 
-pid_t GetTID() {
-  return syscall(SYS_gettid);
-}
+pid_t GetTID() { return static_cast<pid_t>(syscall(SYS_gettid)); }
 
+#endif
+#undef ABSL_INTERNAL_HAVE_GETTID
 #elif defined(__akaros__)
 
 pid_t GetTID() {
@@ -309,9 +426,8 @@ pid_t GetTID() {
   // TODO(dcross): Akaros anticipates moving the thread ID to the uthread
   // structure at some point. We should modify this code to remove the cast
   // when that happens.
-  if (in_vcore_context())
-    return 0;
-  return reinterpret_cast<struct pthread_tcb *>(current_uthread)->id;
+  if (in_vcore_context()) return 0;
+  return reinterpret_cast<struct pthread_tcb*>(current_uthread)->id;
 }
 
 #elif defined(__myriad2__)
@@ -322,83 +438,73 @@ pid_t GetTID() {
   return tid;
 }
 
-#else
+#elif defined(__APPLE__)
 
-// Fallback implementation of GetTID using pthread_getspecific.
-static once_flag tid_once;
-static pthread_key_t tid_key;
-static absl::base_internal::SpinLock tid_lock(
-    absl::base_internal::kLinkerInitialized);
-
-// We set a bit per thread in this array to indicate that an ID is in
-// use. ID 0 is unused because it is the default value returned by
-// pthread_getspecific().
-static std::vector<uint32_t>* tid_array GUARDED_BY(tid_lock) = nullptr;
-static constexpr int kBitsPerWord = 32;  // tid_array is uint32_t.
-
-// Returns the TID to tid_array.
-static void FreeTID(void *v) {
-  intptr_t tid = reinterpret_cast<intptr_t>(v);
-  int word = tid / kBitsPerWord;
-  uint32_t mask = ~(1u << (tid % kBitsPerWord));
-  absl::base_internal::SpinLockHolder lock(&tid_lock);
-  assert(0 <= word && static_cast<size_t>(word) < tid_array->size());
-  (*tid_array)[word] &= mask;
+pid_t GetTID() {
+  uint64_t tid;
+  // `nullptr` here implies this thread.  This only fails if the specified
+  // thread is invalid or the pointer-to-tid is null, so we needn't worry about
+  // it.
+  pthread_threadid_np(nullptr, &tid);
+  return static_cast<pid_t>(tid);
 }
 
-static void InitGetTID() {
-  if (pthread_key_create(&tid_key, FreeTID) != 0) {
-    // The logging system calls GetTID() so it can't be used here.
-    perror("pthread_key_create failed");
-    abort();
-  }
+#elif defined(__FreeBSD__)
 
-  // Initialize tid_array.
-  absl::base_internal::SpinLockHolder lock(&tid_lock);
-  tid_array = new std::vector<uint32_t>(1);
-  (*tid_array)[0] = 1;  // ID 0 is never-allocated.
-}
+pid_t GetTID() { return static_cast<pid_t>(pthread_getthreadid_np()); }
 
-// Return a per-thread small integer ID from pthread's thread-specific data.
-pid_t GetTID() {
-  absl::call_once(tid_once, InitGetTID);
+#elif defined(__OpenBSD__)
 
-  intptr_t tid = reinterpret_cast<intptr_t>(pthread_getspecific(tid_key));
-  if (tid != 0) {
-    return tid;
-  }
+pid_t GetTID() { return getthrid(); }
 
-  int bit;  // tid_array[word] = 1u << bit;
-  size_t word;
-  {
-    // Search for the first unused ID.
-    absl::base_internal::SpinLockHolder lock(&tid_lock);
-    // First search for a word in the array that is not all ones.
-    word = 0;
-    while (word < tid_array->size() && ~(*tid_array)[word] == 0) {
-      ++word;
-    }
-    if (word == tid_array->size()) {
-      tid_array->push_back(0);  // No space left, add kBitsPerWord more IDs.
-    }
-    // Search for a zero bit in the word.
-    bit = 0;
-    while (bit < kBitsPerWord && (((*tid_array)[word] >> bit) & 1) != 0) {
-      ++bit;
-    }
-    tid = (word * kBitsPerWord) + bit;
-    (*tid_array)[word] |= 1u << bit;  // Mark the TID as allocated.
-  }
+#elif defined(__NetBSD__)
 
-  if (pthread_setspecific(tid_key, reinterpret_cast<void *>(tid)) != 0) {
-    perror("pthread_setspecific failed");
-    abort();
-  }
+pid_t GetTID() { return static_cast<pid_t>(_lwp_self()); }
 
-  return static_cast<pid_t>(tid);
+#elif defined(__Fuchsia__)
+
+pid_t GetTID() {
+  // Use our thread handle as the TID, which should be unique within this
+  // process (but may not be globally unique). The handle value was chosen over
+  // a kernel object ID (KOID) because zx_handle_t (32-bits) can be cast to a
+  // pid_t type without loss of precision, but a zx_koid_t (64-bits) cannot.
+  return static_cast<pid_t>(zx_thread_self());
+}
+
+#else
+
+// Fallback implementation of `GetTID` using `pthread_self`.
+pid_t GetTID() {
+  // `pthread_t` need not be arithmetic per POSIX; platforms where it isn't
+  // should be handled above.
+  return static_cast<pid_t>(pthread_self());
 }
 
 #endif
 
+// GetCachedTID() caches the thread ID in thread-local storage (which is a
+// userspace construct) to avoid unnecessary system calls. Without this caching,
+// it can take roughly 98ns, while it takes roughly 1ns with this caching.
+pid_t GetCachedTID() {
+#ifdef __ANDROID__
+// NDK defaults to emulated TLS for API < 29, and native ELF TLS for API >= 29.
+// Emulated TLS is slower than bionic's internal caching.
+#if __ANDROID_API__ < 29
+#define ABSL_INTERNAL_USING_EMULATED_TLS 1
+#endif
+#endif
+
+#if defined(ABSL_HAVE_THREAD_LOCAL) && \
+    !defined(ABSL_INTERNAL_USING_EMULATED_TLS)
+  static thread_local pid_t thread_id = GetTID();
+  return thread_id;
+#else
+  return GetTID();
+#endif  // defined(ABSL_HAVE_THREAD_LOCAL) &&
+        // !defined(ABSL_INTERNAL_USING_EMULATED_TLS)
+#undef ABSL_INTERNAL_USING_EMULATED_TLS
+}
+
 }  // namespace base_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/base/internal/sysinfo.h b/absl/base/internal/sysinfo.h
index b864a59741f..94df348a45b 100644
--- a/absl/base/internal/sysinfo.h
+++ b/absl/base/internal/sysinfo.h
@@ -24,15 +24,17 @@
 #ifndef ABSL_BASE_INTERNAL_SYSINFO_H_
 #define ABSL_BASE_INTERNAL_SYSINFO_H_
 
+#include <cstdint>
+
+#include "absl/base/config.h"
+#include "absl/base/port.h"
+
 #ifndef _WIN32
 #include <sys/types.h>
-#else
-#include <intsafe.h>
 #endif
 
-#include "absl/base/port.h"
-
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 
 // Nominal core processor cycles per second of each processor.   This is _not_
@@ -51,13 +53,22 @@ int NumCPUs();
 // On Linux, you may send a signal to the resulting ID with kill().  However,
 // it is recommended for portability that you use pthread_kill() instead.
 #ifdef _WIN32
-// On Windows, process id and thread id are of the same type according to
-// the return types of GetProcessId() and GetThreadId() are both DWORD.
-using pid_t = DWORD;
+// On Windows, process id and thread id are of the same type according to the
+// return types of GetProcessId() and GetThreadId() are both DWORD, an unsigned
+// 32-bit type.
+using pid_t = uint32_t;
 #endif
 pid_t GetTID();
 
+// Like GetTID(), but caches the result in thread-local storage in order
+// to avoid unnecessary system calls. Note that there are some cases where
+// one must call through to GetTID directly, which is why this exists as a
+// separate function. For example, GetCachedTID() is not safe to call in
+// an asynchronous signal-handling context nor right after a call to fork().
+pid_t GetCachedTID();
+
 }  // namespace base_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_BASE_INTERNAL_SYSINFO_H_
diff --git a/absl/base/internal/sysinfo_test.cc b/absl/base/internal/sysinfo_test.cc
index 82bbcc2b3bd..7248fde9add 100644
--- a/absl/base/internal/sysinfo_test.cc
+++ b/absl/base/internal/sysinfo_test.cc
@@ -14,11 +14,6 @@
 
 #include "absl/base/internal/sysinfo.h"
 
-#ifndef _WIN32
-#include <sys/types.h>
-#include <unistd.h>
-#endif
-
 #include <thread>  // NOLINT(build/c++11)
 #include <unordered_set>
 #include <vector>
@@ -27,7 +22,13 @@
 #include "absl/synchronization/barrier.h"
 #include "absl/synchronization/mutex.h"
 
+#ifndef _WIN32
+#include <sys/types.h>
+#include <unistd.h>
+#endif
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 namespace {
 
@@ -36,30 +37,12 @@ TEST(SysinfoTest, NumCPUs) {
       << "NumCPUs() should not have the default value of 0";
 }
 
-TEST(SysinfoTest, NominalCPUFrequency) {
-#if !(defined(__aarch64__) && defined(__linux__)) && !defined(__EMSCRIPTEN__)
-  EXPECT_GE(NominalCPUFrequency(), 1000.0)
-      << "NominalCPUFrequency() did not return a reasonable value";
-#else
-  // Aarch64 cannot read the CPU frequency from sysfs, so we get back 1.0.
-  // Emscripten does not have a sysfs to read from at all.
-  EXPECT_EQ(NominalCPUFrequency(), 1.0)
-      << "CPU frequency detection was fixed! Please update unittest.";
-#endif
-}
-
 TEST(SysinfoTest, GetTID) {
   EXPECT_EQ(GetTID(), GetTID());  // Basic compile and equality test.
-#ifdef __native_client__
-  // Native Client has a race condition bug that leads to memory
-  // exaustion when repeatedly creating and joining threads.
-  // https://bugs.chromium.org/p/nativeclient/issues/detail?id=1027
-  return;
-#endif
   // Test that TIDs are unique to each thread.
   // Uses a few loops to exercise implementations that reallocate IDs.
-  for (int i = 0; i < 32; ++i) {
-    constexpr int kNumThreads = 64;
+  for (int i = 0; i < 10; ++i) {
+    constexpr int kNumThreads = 10;
     Barrier all_threads_done(kNumThreads);
     std::vector<std::thread> threads;
 
@@ -70,7 +53,7 @@ TEST(SysinfoTest, GetTID) {
       threads.push_back(std::thread([&]() {
         pid_t id = GetTID();
         {
-          MutexLock lock(&mutex);
+          MutexLock lock(mutex);
           ASSERT_TRUE(tids.find(id) == tids.end());
           tids.insert(id);
         }
@@ -95,4 +78,5 @@ TEST(SysinfoTest, LinuxGetTID) {
 
 }  // namespace
 }  // namespace base_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/base/internal/thread_annotations.h b/absl/base/internal/thread_annotations.h
deleted file mode 100644
index 4dab6a9c150..00000000000
--- a/absl/base/internal/thread_annotations.h
+++ /dev/null
@@ -1,271 +0,0 @@
-// Copyright 2019 The Abseil Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-//
-// -----------------------------------------------------------------------------
-// File: thread_annotations.h
-// -----------------------------------------------------------------------------
-//
-// WARNING: This is a backwards compatible header and it will be removed after
-// the migration to prefixed thread annotations is finished; please include
-// "absl/base/thread_annotations.h".
-//
-// This header file contains macro definitions for thread safety annotations
-// that allow developers to document the locking policies of multi-threaded
-// code. The annotations can also help program analysis tools to identify
-// potential thread safety issues.
-//
-// These annotations are implemented using compiler attributes. Using the macros
-// defined here instead of raw attributes allow for portability and future
-// compatibility.
-//
-// When referring to mutexes in the arguments of the attributes, you should
-// use variable names or more complex expressions (e.g. my_object->mutex_)
-// that evaluate to a concrete mutex object whenever possible. If the mutex
-// you want to refer to is not in scope, you may use a member pointer
-// (e.g. &MyClass::mutex_) to refer to a mutex in some (unknown) object.
-
-#ifndef ABSL_BASE_INTERNAL_THREAD_ANNOTATIONS_H_
-#define ABSL_BASE_INTERNAL_THREAD_ANNOTATIONS_H_
-
-#if defined(__clang__)
-#define THREAD_ANNOTATION_ATTRIBUTE__(x)   __attribute__((x))
-#else
-#define THREAD_ANNOTATION_ATTRIBUTE__(x)   // no-op
-#endif
-
-// GUARDED_BY()
-//
-// Documents if a shared field or global variable needs to be protected by a
-// mutex. GUARDED_BY() allows the user to specify a particular mutex that
-// should be held when accessing the annotated variable.
-//
-// Although this annotation (and PT_GUARDED_BY, below) cannot be applied to
-// local variables, a local variable and its associated mutex can often be
-// combined into a small class or struct, thereby allowing the annotation.
-//
-// Example:
-//
-//   class Foo {
-//     Mutex mu_;
-//     int p1_ GUARDED_BY(mu_);
-//     ...
-//   };
-#define GUARDED_BY(x) THREAD_ANNOTATION_ATTRIBUTE__(guarded_by(x))
-
-// PT_GUARDED_BY()
-//
-// Documents if the memory location pointed to by a pointer should be guarded
-// by a mutex when dereferencing the pointer.
-//
-// Example:
-//   class Foo {
-//     Mutex mu_;
-//     int *p1_ PT_GUARDED_BY(mu_);
-//     ...
-//   };
-//
-// Note that a pointer variable to a shared memory location could itself be a
-// shared variable.
-//
-// Example:
-//
-//   // `q_`, guarded by `mu1_`, points to a shared memory location that is
-//   // guarded by `mu2_`:
-//   int *q_ GUARDED_BY(mu1_) PT_GUARDED_BY(mu2_);
-#define PT_GUARDED_BY(x) THREAD_ANNOTATION_ATTRIBUTE__(pt_guarded_by(x))
-
-// ACQUIRED_AFTER() / ACQUIRED_BEFORE()
-//
-// Documents the acquisition order between locks that can be held
-// simultaneously by a thread. For any two locks that need to be annotated
-// to establish an acquisition order, only one of them needs the annotation.
-// (i.e. You don't have to annotate both locks with both ACQUIRED_AFTER
-// and ACQUIRED_BEFORE.)
-//
-// As with GUARDED_BY, this is only applicable to mutexes that are shared
-// fields or global variables.
-//
-// Example:
-//
-//   Mutex m1_;
-//   Mutex m2_ ACQUIRED_AFTER(m1_);
-#define ACQUIRED_AFTER(...) \
-  THREAD_ANNOTATION_ATTRIBUTE__(acquired_after(__VA_ARGS__))
-
-#define ACQUIRED_BEFORE(...) \
-  THREAD_ANNOTATION_ATTRIBUTE__(acquired_before(__VA_ARGS__))
-
-// EXCLUSIVE_LOCKS_REQUIRED() / SHARED_LOCKS_REQUIRED()
-//
-// Documents a function that expects a mutex to be held prior to entry.
-// The mutex is expected to be held both on entry to, and exit from, the
-// function.
-//
-// An exclusive lock allows read-write access to the guarded data member(s), and
-// only one thread can acquire a lock exclusively at any one time. A shared lock
-// allows read-only access, and any number of threads can acquire a shared lock
-// concurrently.
-//
-// Generally, non-const methods should be annotated with
-// EXCLUSIVE_LOCKS_REQUIRED, while const methods should be annotated with
-// SHARED_LOCKS_REQUIRED.
-//
-// Example:
-//
-//   Mutex mu1, mu2;
-//   int a GUARDED_BY(mu1);
-//   int b GUARDED_BY(mu2);
-//
-//   void foo() EXCLUSIVE_LOCKS_REQUIRED(mu1, mu2) { ... }
-//   void bar() const SHARED_LOCKS_REQUIRED(mu1, mu2) { ... }
-#define EXCLUSIVE_LOCKS_REQUIRED(...) \
-  THREAD_ANNOTATION_ATTRIBUTE__(exclusive_locks_required(__VA_ARGS__))
-
-#define SHARED_LOCKS_REQUIRED(...) \
-  THREAD_ANNOTATION_ATTRIBUTE__(shared_locks_required(__VA_ARGS__))
-
-// LOCKS_EXCLUDED()
-//
-// Documents the locks acquired in the body of the function. These locks
-// cannot be held when calling this function (as Abseil's `Mutex` locks are
-// non-reentrant).
-#define LOCKS_EXCLUDED(...) \
-  THREAD_ANNOTATION_ATTRIBUTE__(locks_excluded(__VA_ARGS__))
-
-// LOCK_RETURNED()
-//
-// Documents a function that returns a mutex without acquiring it.  For example,
-// a public getter method that returns a pointer to a private mutex should
-// be annotated with LOCK_RETURNED.
-#define LOCK_RETURNED(x) \
-  THREAD_ANNOTATION_ATTRIBUTE__(lock_returned(x))
-
-// LOCKABLE
-//
-// Documents if a class/type is a lockable type (such as the `Mutex` class).
-#define LOCKABLE \
-  THREAD_ANNOTATION_ATTRIBUTE__(lockable)
-
-// SCOPED_LOCKABLE
-//
-// Documents if a class does RAII locking (such as the `MutexLock` class).
-// The constructor should use `LOCK_FUNCTION()` to specify the mutex that is
-// acquired, and the destructor should use `UNLOCK_FUNCTION()` with no
-// arguments; the analysis will assume that the destructor unlocks whatever the
-// constructor locked.
-#define SCOPED_LOCKABLE \
-  THREAD_ANNOTATION_ATTRIBUTE__(scoped_lockable)
-
-// EXCLUSIVE_LOCK_FUNCTION()
-//
-// Documents functions that acquire a lock in the body of a function, and do
-// not release it.
-#define EXCLUSIVE_LOCK_FUNCTION(...) \
-  THREAD_ANNOTATION_ATTRIBUTE__(exclusive_lock_function(__VA_ARGS__))
-
-// SHARED_LOCK_FUNCTION()
-//
-// Documents functions that acquire a shared (reader) lock in the body of a
-// function, and do not release it.
-#define SHARED_LOCK_FUNCTION(...) \
-  THREAD_ANNOTATION_ATTRIBUTE__(shared_lock_function(__VA_ARGS__))
-
-// UNLOCK_FUNCTION()
-//
-// Documents functions that expect a lock to be held on entry to the function,
-// and release it in the body of the function.
-#define UNLOCK_FUNCTION(...) \
-  THREAD_ANNOTATION_ATTRIBUTE__(unlock_function(__VA_ARGS__))
-
-// EXCLUSIVE_TRYLOCK_FUNCTION() / SHARED_TRYLOCK_FUNCTION()
-//
-// Documents functions that try to acquire a lock, and return success or failure
-// (or a non-boolean value that can be interpreted as a boolean).
-// The first argument should be `true` for functions that return `true` on
-// success, or `false` for functions that return `false` on success. The second
-// argument specifies the mutex that is locked on success. If unspecified, this
-// mutex is assumed to be `this`.
-#define EXCLUSIVE_TRYLOCK_FUNCTION(...) \
-  THREAD_ANNOTATION_ATTRIBUTE__(exclusive_trylock_function(__VA_ARGS__))
-
-#define SHARED_TRYLOCK_FUNCTION(...) \
-  THREAD_ANNOTATION_ATTRIBUTE__(shared_trylock_function(__VA_ARGS__))
-
-// ASSERT_EXCLUSIVE_LOCK() / ASSERT_SHARED_LOCK()
-//
-// Documents functions that dynamically check to see if a lock is held, and fail
-// if it is not held.
-#define ASSERT_EXCLUSIVE_LOCK(...) \
-  THREAD_ANNOTATION_ATTRIBUTE__(assert_exclusive_lock(__VA_ARGS__))
-
-#define ASSERT_SHARED_LOCK(...) \
-  THREAD_ANNOTATION_ATTRIBUTE__(assert_shared_lock(__VA_ARGS__))
-
-// NO_THREAD_SAFETY_ANALYSIS
-//
-// Turns off thread safety checking within the body of a particular function.
-// This annotation is used to mark functions that are known to be correct, but
-// the locking behavior is more complicated than the analyzer can handle.
-#define NO_THREAD_SAFETY_ANALYSIS \
-  THREAD_ANNOTATION_ATTRIBUTE__(no_thread_safety_analysis)
-
-//------------------------------------------------------------------------------
-// Tool-Supplied Annotations
-//------------------------------------------------------------------------------
-
-// TS_UNCHECKED should be placed around lock expressions that are not valid
-// C++ syntax, but which are present for documentation purposes.  These
-// annotations will be ignored by the analysis.
-#define TS_UNCHECKED(x) ""
-
-// TS_FIXME is used to mark lock expressions that are not valid C++ syntax.
-// It is used by automated tools to mark and disable invalid expressions.
-// The annotation should either be fixed, or changed to TS_UNCHECKED.
-#define TS_FIXME(x) ""
-
-// Like NO_THREAD_SAFETY_ANALYSIS, this turns off checking within the body of
-// a particular function.  However, this attribute is used to mark functions
-// that are incorrect and need to be fixed.  It is used by automated tools to
-// avoid breaking the build when the analysis is updated.
-// Code owners are expected to eventually fix the routine.
-#define NO_THREAD_SAFETY_ANALYSIS_FIXME  NO_THREAD_SAFETY_ANALYSIS
-
-// Similar to NO_THREAD_SAFETY_ANALYSIS_FIXME, this macro marks a GUARDED_BY
-// annotation that needs to be fixed, because it is producing thread safety
-// warning.  It disables the GUARDED_BY.
-#define GUARDED_BY_FIXME(x)
-
-// Disables warnings for a single read operation.  This can be used to avoid
-// warnings when it is known that the read is not actually involved in a race,
-// but the compiler cannot confirm that.
-#define TS_UNCHECKED_READ(x) thread_safety_analysis::ts_unchecked_read(x)
-
-
-namespace thread_safety_analysis {
-
-// Takes a reference to a guarded data member, and returns an unguarded
-// reference.
-template <typename T>
-inline const T& ts_unchecked_read(const T& v) NO_THREAD_SAFETY_ANALYSIS {
-  return v;
-}
-
-template <typename T>
-inline T& ts_unchecked_read(T& v) NO_THREAD_SAFETY_ANALYSIS {
-  return v;
-}
-
-}  // namespace thread_safety_analysis
-
-#endif  // ABSL_BASE_INTERNAL_THREAD_ANNOTATIONS_H_
diff --git a/absl/base/internal/thread_identity.cc b/absl/base/internal/thread_identity.cc
index 91273a6b1a1..27b5816394a 100644
--- a/absl/base/internal/thread_identity.cc
+++ b/absl/base/internal/thread_identity.cc
@@ -14,20 +14,26 @@
 
 #include "absl/base/internal/thread_identity.h"
 
-#ifndef _WIN32
-#include <pthread.h>
-#include <signal.h>
-#endif
-
 #include <atomic>
 #include <cassert>
 #include <memory>
 
+#include "absl/base/attributes.h"
 #include "absl/base/call_once.h"
 #include "absl/base/internal/raw_logging.h"
 #include "absl/base/internal/spinlock.h"
 
+#if !defined(_WIN32) || defined(__MINGW32__)
+#include <pthread.h>
+#ifndef __wasi__
+// WASI does not provide this header, either way we disable use
+// of signals with it below.
+#include <signal.h>
+#endif
+#endif
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 
 #if ABSL_THREAD_IDENTITY_MODE != ABSL_THREAD_IDENTITY_MODE_USE_CPP11
@@ -52,14 +58,23 @@ void AllocateThreadIdentityKey(ThreadIdentityReclaimerFunction reclaimer) {
 // exist within a process (via dlopen() or similar), references to
 // thread_identity_ptr from each instance of the code will refer to
 // *different* instances of this ptr.
-#ifdef __GNUC__
-__attribute__((visibility("protected")))
-#endif  // __GNUC__
-  ABSL_PER_THREAD_TLS_KEYWORD ThreadIdentity* thread_identity_ptr;
+// Apple platforms have the visibility attribute, but issue a compile warning
+// that protected visibility is unsupported.
+ABSL_CONST_INIT  // Must come before __attribute__((visibility("protected")))
+#if ABSL_HAVE_ATTRIBUTE(visibility) && !defined(__APPLE__)
+    __attribute__((visibility("protected")))
+#endif  // ABSL_HAVE_ATTRIBUTE(visibility) && !defined(__APPLE__)
+#if ABSL_PER_THREAD_TLS
+    // Prefer __thread to thread_local as benchmarks indicate it is a bit
+    // faster.
+    ABSL_PER_THREAD_TLS_KEYWORD ThreadIdentity* thread_identity_ptr = nullptr;
+#elif defined(ABSL_HAVE_THREAD_LOCAL)
+    thread_local ThreadIdentity* thread_identity_ptr = nullptr;
+#endif  // ABSL_PER_THREAD_TLS
 #endif  // TLS or CPP11
 
-void SetCurrentThreadIdentity(
-    ThreadIdentity* identity, ThreadIdentityReclaimerFunction reclaimer) {
+void SetCurrentThreadIdentity(ThreadIdentity* identity,
+                              ThreadIdentityReclaimerFunction reclaimer) {
   assert(CurrentThreadIdentityIfPresent() == nullptr);
   // Associate our destructor.
   // NOTE: This call to pthread_setspecific is currently the only immovable
@@ -69,10 +84,12 @@ void SetCurrentThreadIdentity(
   absl::call_once(init_thread_identity_key_once, AllocateThreadIdentityKey,
                   reclaimer);
 
-#ifdef __EMSCRIPTEN__
-  // Emscripten PThread implementation does not support signals.
-  // See https://kripken.github.io/emscripten-site/docs/porting/pthreads.html
-  // for more information.
+#if defined(__wasi__) || defined(__EMSCRIPTEN__) || defined(__MINGW32__) || \
+    defined(__hexagon__)
+  // Emscripten, WASI and MinGW pthread implementations does not support
+  // signals. See
+  // https://kripken.github.io/emscripten-site/docs/porting/pthreads.html for
+  // more information.
   pthread_setspecific(thread_identity_pthread_key,
                       reinterpret_cast<void*>(identity));
 #else
@@ -89,7 +106,7 @@ void SetCurrentThreadIdentity(
   pthread_setspecific(thread_identity_pthread_key,
                       reinterpret_cast<void*>(identity));
   pthread_sigmask(SIG_SETMASK, &curr_signals, nullptr);
-#endif  // !__EMSCRIPTEN__
+#endif  // !__EMSCRIPTEN__ && !__MINGW32__
 
 #elif ABSL_THREAD_IDENTITY_MODE == ABSL_THREAD_IDENTITY_MODE_USE_TLS
   // NOTE: Not async-safe.  But can be open-coded.
@@ -107,12 +124,24 @@ void SetCurrentThreadIdentity(
 #endif
 }
 
+#if ABSL_THREAD_IDENTITY_MODE == ABSL_THREAD_IDENTITY_MODE_USE_TLS || \
+    ABSL_THREAD_IDENTITY_MODE == ABSL_THREAD_IDENTITY_MODE_USE_CPP11
+
+// Please see the comment on `CurrentThreadIdentityIfPresent` in
+// thread_identity.h. When we cannot expose thread_local variables in
+// headers, we opt for the correct-but-slower option of not inlining this
+// function.
+#ifndef ABSL_INTERNAL_INLINE_CURRENT_THREAD_IDENTITY_IF_PRESENT
+ThreadIdentity* CurrentThreadIdentityIfPresent() { return thread_identity_ptr; }
+#endif
+#endif
+
 void ClearCurrentThreadIdentity() {
 #if ABSL_THREAD_IDENTITY_MODE == ABSL_THREAD_IDENTITY_MODE_USE_TLS || \
     ABSL_THREAD_IDENTITY_MODE == ABSL_THREAD_IDENTITY_MODE_USE_CPP11
   thread_identity_ptr = nullptr;
 #elif ABSL_THREAD_IDENTITY_MODE == \
-      ABSL_THREAD_IDENTITY_MODE_USE_POSIX_SETSPECIFIC
+    ABSL_THREAD_IDENTITY_MODE_USE_POSIX_SETSPECIFIC
   // pthread_setspecific expected to clear value on destruction
   assert(CurrentThreadIdentityIfPresent() == nullptr);
 #endif
@@ -130,4 +159,5 @@ ThreadIdentity* CurrentThreadIdentityIfPresent() {
 #endif
 
 }  // namespace base_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/base/internal/thread_identity.h b/absl/base/internal/thread_identity.h
index b34674a62d4..a9f26d06557 100644
--- a/absl/base/internal/thread_identity.h
+++ b/absl/base/internal/thread_identity.h
@@ -20,6 +20,15 @@
 #ifndef ABSL_BASE_INTERNAL_THREAD_IDENTITY_H_
 #define ABSL_BASE_INTERNAL_THREAD_IDENTITY_H_
 
+#include <atomic>
+#include <cstddef>
+#include <cstdint>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/per_thread_tls.h"
+#include "absl/base/optimization.h"
+
 #ifndef _WIN32
 #include <pthread.h>
 // Defines __GOOGLE_GRTE_VERSION__ (via glibc-specific features.h) when
@@ -27,12 +36,8 @@
 #include <unistd.h>
 #endif
 
-#include <atomic>
-#include <cstdint>
-
-#include "absl/base/internal/per_thread_tls.h"
-
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 struct SynchLocksHeld;
 struct SynchWaitParams;
@@ -59,38 +64,36 @@ struct PerThreadSynch {
     return reinterpret_cast<ThreadIdentity*>(this);
   }
 
-  PerThreadSynch *next;  // Circular waiter queue; initialized to 0.
-  PerThreadSynch *skip;  // If non-zero, all entries in Mutex queue
+  PerThreadSynch* next;  // Circular waiter queue; initialized to 0.
+  PerThreadSynch* skip;  // If non-zero, all entries in Mutex queue
                          // up to and including "skip" have same
                          // condition as this, and will be woken later
   bool may_skip;         // if false while on mutex queue, a mutex unlocker
                          // is using this PerThreadSynch as a terminator.  Its
                          // skip field must not be filled in because the loop
                          // might then skip over the terminator.
-
-  // The wait parameters of the current wait.  waitp is null if the
-  // thread is not waiting. Transitions from null to non-null must
-  // occur before the enqueue commit point (state = kQueued in
-  // Enqueue() and CondVarEnqueue()). Transitions from non-null to
-  // null must occur after the wait is finished (state = kAvailable in
-  // Mutex::Block() and CondVar::WaitCommon()). This field may be
-  // changed only by the thread that describes this PerThreadSynch.  A
-  // special case is Fer(), which calls Enqueue() on another thread,
-  // but with an identical SynchWaitParams pointer, thus leaving the
-  // pointer unchanged.
-  SynchWaitParams *waitp;
-
-  bool suppress_fatal_errors;  // If true, try to proceed even in the face of
-                               // broken invariants.  This is used within fatal
-                               // signal handlers to improve the chances of
-                               // debug logging information being output
-                               // successfully.
-
-  intptr_t readers;     // Number of readers in mutex.
-  int priority;         // Priority of thread (updated every so often).
-
-  // When priority will next be read (cycles).
-  int64_t next_priority_read_cycles;
+  bool wake;             // This thread is to be woken from a Mutex.
+  // If "x" is on a waiter list for a mutex, "x->cond_waiter" is true iff the
+  // waiter is waiting on the mutex as part of a CV Wait or Mutex Await.
+  //
+  // The value of "x->cond_waiter" is meaningless if "x" is not on a
+  // Mutex waiter list.
+  bool cond_waiter;
+  bool maybe_unlocking;  // Valid at head of Mutex waiter queue;
+                         // true if UnlockSlow could be searching
+                         // for a waiter to wake.  Used for an optimization
+                         // in Enqueue().  true is always a valid value.
+                         // Can be reset to false when the unlocker or any
+                         // writer releases the lock, or a reader fully
+                         // releases the lock.  It may not be set to false
+                         // by a reader that decrements the count to
+                         // non-zero. protected by mutex spinlock
+  bool suppress_fatal_errors;  // If true, try to proceed even in the face
+                               // of broken invariants.  This is used within
+                               // fatal signal handlers to improve the
+                               // chances of debug logging information being
+                               // output successfully.
+  int priority;                // Priority of thread (updated every so often).
 
   // State values:
   //   kAvailable: This PerThreadSynch is available.
@@ -103,36 +106,41 @@ struct PerThreadSynch {
   //
   // Transitions from kAvailable to kQueued require no barrier, they
   // are externally ordered by the Mutex.
-  enum State {
-    kAvailable,
-    kQueued
-  };
+  enum State { kAvailable, kQueued };
   std::atomic<State> state;
 
-  bool maybe_unlocking;  // Valid at head of Mutex waiter queue;
-                         // true if UnlockSlow could be searching
-                         // for a waiter to wake.  Used for an optimization
-                         // in Enqueue().  true is always a valid value.
-                         // Can be reset to false when the unlocker or any
-                         // writer releases the lock, or a reader fully releases
-                         // the lock.  It may not be set to false by a reader
-                         // that decrements the count to non-zero.
-                         // protected by mutex spinlock
+  // The wait parameters of the current wait.  waitp is null if the
+  // thread is not waiting. Transitions from null to non-null must
+  // occur before the enqueue commit point (state = kQueued in
+  // Enqueue() and CondVarEnqueue()). Transitions from non-null to
+  // null must occur after the wait is finished (state = kAvailable in
+  // Mutex::Block() and CondVar::WaitCommon()). This field may be
+  // changed only by the thread that describes this PerThreadSynch.  A
+  // special case is Fer(), which calls Enqueue() on another thread,
+  // but with an identical SynchWaitParams pointer, thus leaving the
+  // pointer unchanged.
+  SynchWaitParams* waitp;
 
-  bool wake;  // This thread is to be woken from a Mutex.
+  intptr_t readers;  // Number of readers in mutex.
 
-  // If "x" is on a waiter list for a mutex, "x->cond_waiter" is true iff the
-  // waiter is waiting on the mutex as part of a CV Wait or Mutex Await.
-  //
-  // The value of "x->cond_waiter" is meaningless if "x" is not on a
-  // Mutex waiter list.
-  bool cond_waiter;
+  // When priority will next be read (cycles).
+  int64_t next_priority_read_cycles;
 
   // Locks held; used during deadlock detection.
   // Allocated in Synch_GetAllLocks() and freed in ReclaimThreadIdentity().
-  SynchLocksHeld *all_locks;
+  SynchLocksHeld* all_locks;
 };
 
+// The instances of this class are allocated in NewThreadIdentity() with an
+// alignment of PerThreadSynch::kAlignment and never destroyed. Initialization
+// should happen in OneTimeInitThreadIdentity().
+//
+// Instances may be reused by new threads - fields should be reset in
+// ResetThreadIdentityBetweenReuse().
+//
+// NOTE: The layout of fields in this structure is critical, please do not
+//       add, remove, or modify the field placements without fully auditing the
+//       layout.
 struct ThreadIdentity {
   // Must be the first member.  The Mutex implementation requires that
   // the PerThreadSynch object associated with each thread is
@@ -140,9 +148,57 @@ struct ThreadIdentity {
   // ThreadIdentity itself.
   PerThreadSynch per_thread_synch;
 
+  struct SchedulerState {
+    std::atomic<void*> bound_schedulable{nullptr};
+    // Storage space for a SpinLock, which is created through a placement new to
+    // break a dependency cycle.
+    uint32_t association_lock_word;
+    std::atomic<int> scheduling_disabled_depth;
+    int potentially_blocking_depth;
+    uint32_t schedule_next_state;
+
+    // When true, current thread is unlocking a mutex and actively waking a
+    // thread that was previously waiting, but that lock has yet more waiters.
+    // Used to signal to schedulers that work being woken should get an
+    // elevated priority.
+    bool waking_designated_waker;
+
+    inline SpinLock* association_lock() {
+      return reinterpret_cast<SpinLock*>(&association_lock_word);
+    }
+  } scheduler_state;  // Private: Reserved for use in Gloop
+
+  // For worker threads that may not be doing any interesting user work, this
+  // tracks the current state of the worker. This is used to handle those
+  // threads differently e.g. when printing stacktraces.
+  //
+  // It should only be written to by the thread itself.
+  //
+  // Note that this is different from the mutex idle bit - threads running user
+  // work can be waiting but still be active.
+  //
+  // Note: not all parts of the code-base may maintain this field correctly and
+  // therefore this field should only be used to improve debugging/monitoring.
+  //
+  // Put it here to reuse some of the padding space.
+  enum class WaitState : uint8_t {
+    kActive = 0,
+    kWaitingForWork = 1,
+  };
+  std::atomic<WaitState> wait_state;
+  static_assert(std::atomic<WaitState>::is_always_lock_free);
+
+  // Add a padding such that scheduler_state is on a different cache line than
+  // waiter state.  We use padding here, so that the size of the structure does
+  // not substantially grow due to the added padding.
+  static constexpr size_t kToBePaddedSize =
+      sizeof(SchedulerState) + sizeof(std::atomic<WaitState>);
+  static_assert(ABSL_CACHELINE_SIZE >= kToBePaddedSize);
+  char padding[ABSL_CACHELINE_SIZE - kToBePaddedSize];
+
   // Private: Reserved for absl::synchronization_internal::Waiter.
   struct WaiterState {
-    char data[128];
+    alignas(void*) char data[256];
   } waiter_state;
 
   // Used by PerThreadSem::{Get,Set}ThreadBlockedCounter().
@@ -155,6 +211,10 @@ struct ThreadIdentity {
   std::atomic<int> wait_start;  // Ticker value when thread started waiting.
   std::atomic<bool> is_idle;    // Has thread become idle yet?
 
+  // For tracking depth of __cxa_guard_acquire.  This used to recognize heap
+  // allocations for function static objects.
+  int static_initialization_depth;
+
   ThreadIdentity* next;
 };
 
@@ -165,7 +225,10 @@ struct ThreadIdentity {
 //
 // Does not malloc(*), and is async-signal safe.
 // [*] Technically pthread_setspecific() does malloc on first use; however this
-// is handled internally within tcmalloc's initialization already.
+// is handled internally within tcmalloc's initialization already. Note that
+// darwin does *not* use tcmalloc, so this can catch you if using MallocHooks
+// on Apple platforms. Whatever function is calling your MallocHooks will need
+// to watch for recursion on Apple platforms.
 //
 // New ThreadIdentity objects can be constructed and associated with a thread
 // by calling GetOrCreateCurrentThreadIdentity() in per-thread-sem.h.
@@ -187,28 +250,30 @@ void ClearCurrentThreadIdentity();
 // May be chosen at compile time via: -DABSL_FORCE_THREAD_IDENTITY_MODE=<mode
 // index>
 #ifdef ABSL_THREAD_IDENTITY_MODE_USE_POSIX_SETSPECIFIC
-#error ABSL_THREAD_IDENTITY_MODE_USE_POSIX_SETSPECIFIC cannot be direcly set
+#error ABSL_THREAD_IDENTITY_MODE_USE_POSIX_SETSPECIFIC cannot be directly set
 #else
 #define ABSL_THREAD_IDENTITY_MODE_USE_POSIX_SETSPECIFIC 0
 #endif
 
 #ifdef ABSL_THREAD_IDENTITY_MODE_USE_TLS
-#error ABSL_THREAD_IDENTITY_MODE_USE_TLS cannot be direcly set
+#error ABSL_THREAD_IDENTITY_MODE_USE_TLS cannot be directly set
 #else
 #define ABSL_THREAD_IDENTITY_MODE_USE_TLS 1
 #endif
 
 #ifdef ABSL_THREAD_IDENTITY_MODE_USE_CPP11
-#error ABSL_THREAD_IDENTITY_MODE_USE_CPP11 cannot be direcly set
+#error ABSL_THREAD_IDENTITY_MODE_USE_CPP11 cannot be directly set
 #else
 #define ABSL_THREAD_IDENTITY_MODE_USE_CPP11 2
 #endif
 
 #ifdef ABSL_THREAD_IDENTITY_MODE
-#error ABSL_THREAD_IDENTITY_MODE cannot be direcly set
+#error ABSL_THREAD_IDENTITY_MODE cannot be directly set
 #elif defined(ABSL_FORCE_THREAD_IDENTITY_MODE)
 #define ABSL_THREAD_IDENTITY_MODE ABSL_FORCE_THREAD_IDENTITY_MODE
-#elif defined(_WIN32)
+#elif defined(_WIN32) && !defined(__MINGW32__)
+#define ABSL_THREAD_IDENTITY_MODE ABSL_THREAD_IDENTITY_MODE_USE_CPP11
+#elif defined(__APPLE__) && defined(ABSL_HAVE_THREAD_LOCAL)
 #define ABSL_THREAD_IDENTITY_MODE ABSL_THREAD_IDENTITY_MODE_USE_CPP11
 #elif ABSL_PER_THREAD_TLS && defined(__GOOGLE_GRTE_VERSION__) && \
     (__GOOGLE_GRTE_VERSION__ >= 20140228L)
@@ -224,11 +289,31 @@ void ClearCurrentThreadIdentity();
 #if ABSL_THREAD_IDENTITY_MODE == ABSL_THREAD_IDENTITY_MODE_USE_TLS || \
     ABSL_THREAD_IDENTITY_MODE == ABSL_THREAD_IDENTITY_MODE_USE_CPP11
 
-extern ABSL_PER_THREAD_TLS_KEYWORD ThreadIdentity* thread_identity_ptr;
+#if ABSL_PER_THREAD_TLS
+ABSL_CONST_INIT extern ABSL_PER_THREAD_TLS_KEYWORD ThreadIdentity*
+    thread_identity_ptr;
+#elif defined(ABSL_HAVE_THREAD_LOCAL)
+ABSL_CONST_INIT extern thread_local ThreadIdentity* thread_identity_ptr;
+#else
+#error Thread-local storage not detected on this platform
+#endif
+
+// thread_local variables cannot be in headers exposed by DLLs or in certain
+// build configurations on Apple platforms. However, it is important for
+// performance reasons in general that `CurrentThreadIdentityIfPresent` be
+// inlined. In the other cases we opt to have the function not be inlined. Note
+// that `CurrentThreadIdentityIfPresent` is declared above so we can exclude
+// this entire inline definition.
+#if !defined(__APPLE__) && !defined(ABSL_BUILD_DLL) && \
+    !defined(ABSL_CONSUME_DLL)
+#define ABSL_INTERNAL_INLINE_CURRENT_THREAD_IDENTITY_IF_PRESENT 1
+#endif
 
+#ifdef ABSL_INTERNAL_INLINE_CURRENT_THREAD_IDENTITY_IF_PRESENT
 inline ThreadIdentity* CurrentThreadIdentityIfPresent() {
   return thread_identity_ptr;
 }
+#endif
 
 #elif ABSL_THREAD_IDENTITY_MODE != \
     ABSL_THREAD_IDENTITY_MODE_USE_POSIX_SETSPECIFIC
@@ -236,6 +321,7 @@ inline ThreadIdentity* CurrentThreadIdentityIfPresent() {
 #endif
 
 }  // namespace base_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_BASE_INTERNAL_THREAD_IDENTITY_H_
diff --git a/absl/base/internal/thread_identity_benchmark.cc b/absl/base/internal/thread_identity_benchmark.cc
index 0ae10f2b1e8..419e82dbac1 100644
--- a/absl/base/internal/thread_identity_benchmark.cc
+++ b/absl/base/internal/thread_identity_benchmark.cc
@@ -12,10 +12,10 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-#include "benchmark/benchmark.h"
 #include "absl/base/internal/thread_identity.h"
 #include "absl/synchronization/internal/create_thread_identity.h"
 #include "absl/synchronization/internal/per_thread_sem.h"
+#include "benchmark/benchmark.h"
 
 namespace {
 
diff --git a/absl/base/internal/thread_identity_test.cc b/absl/base/internal/thread_identity_test.cc
index 13bfbe3bdbc..94beaa836bf 100644
--- a/absl/base/internal/thread_identity_test.cc
+++ b/absl/base/internal/thread_identity_test.cc
@@ -14,6 +14,7 @@
 
 #include "absl/base/internal/thread_identity.h"
 
+#include <cstdint>
 #include <thread>  // NOLINT(build/c++11)
 #include <vector>
 
@@ -21,17 +22,19 @@
 #include "absl/base/attributes.h"
 #include "absl/base/internal/spinlock.h"
 #include "absl/base/macros.h"
+#include "absl/base/thread_annotations.h"
+#include "absl/synchronization/internal/create_thread_identity.h"
 #include "absl/synchronization/internal/per_thread_sem.h"
 #include "absl/synchronization/mutex.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 namespace {
 
-// protects num_identities_reused
-static absl::base_internal::SpinLock map_lock(
-    absl::base_internal::kLinkerInitialized);
-static int num_identities_reused;
+ABSL_CONST_INIT static absl::base_internal::SpinLock map_lock(
+    base_internal::SCHEDULE_KERNEL_ONLY);
+ABSL_CONST_INIT static int num_identities_reused ABSL_GUARDED_BY(map_lock);
 
 static const void* const kCheckNoIdentity = reinterpret_cast<void*>(1);
 
@@ -57,7 +60,7 @@ static void TestThreadIdentityCurrent(const void* assert_no_identity) {
                    PerThreadSynch::kAlignment);
   EXPECT_EQ(identity, identity->per_thread_synch.thread_identity());
 
-  absl::base_internal::SpinLockHolder l(&map_lock);
+  absl::base_internal::SpinLockHolder l(map_lock);
   num_identities_reused++;
 }
 
@@ -74,7 +77,7 @@ TEST(ThreadIdentityTest, BasicIdentityWorksThreaded) {
   // - If a thread implementation chooses to recycle threads, that
   //   correct re-initialization occurs.
   static const int kNumLoops = 3;
-  static const int kNumThreads = 400;
+  static const int kNumThreads = 32;
   for (int iter = 0; iter < kNumLoops; iter++) {
     std::vector<std::thread> threads;
     for (int i = 0; i < kNumThreads; ++i) {
@@ -89,11 +92,12 @@ TEST(ThreadIdentityTest, BasicIdentityWorksThreaded) {
   // We should have recycled ThreadIdentity objects above; while (external)
   // library threads allocating their own identities may preclude some
   // reuse, we should have sufficient repetitions to exclude this.
+  absl::base_internal::SpinLockHolder l(map_lock);
   EXPECT_LT(kNumThreads, num_identities_reused);
 }
 
 TEST(ThreadIdentityTest, ReusedThreadIdentityMutexTest) {
-  // This test repeatly creates and joins a series of threads, each of
+  // This test repeatedly creates and joins a series of threads, each of
   // which acquires and releases shared Mutex locks. This verifies
   // Mutex operations work correctly under a reused
   // ThreadIdentity. Note that the most likely failure mode of this
@@ -110,7 +114,7 @@ TEST(ThreadIdentityTest, ReusedThreadIdentityMutexTest) {
       threads.push_back(std::thread([&]() {
         for (int l = 0; l < kNumLockLoops; ++l) {
           for (int m = 0; m < kNumMutexes; ++m) {
-            MutexLock lock(&mutexes[m]);
+            MutexLock lock(mutexes[m]);
           }
         }
       }));
@@ -123,4 +127,5 @@ TEST(ThreadIdentityTest, ReusedThreadIdentityMutexTest) {
 
 }  // namespace
 }  // namespace base_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/base/internal/throw_delegate.cc b/absl/base/internal/throw_delegate.cc
deleted file mode 100644
index 8e928b8a257..00000000000
--- a/absl/base/internal/throw_delegate.cc
+++ /dev/null
@@ -1,106 +0,0 @@
-// Copyright 2017 The Abseil Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-#include "absl/base/internal/throw_delegate.h"
-
-#include <cstdlib>
-#include <functional>
-#include <new>
-#include <stdexcept>
-#include "absl/base/config.h"
-#include "absl/base/internal/raw_logging.h"
-
-namespace absl {
-namespace base_internal {
-
-namespace {
-template <typename T>
-[[noreturn]] void Throw(const T& error) {
-#ifdef ABSL_HAVE_EXCEPTIONS
-  throw error;
-#else
-  ABSL_RAW_LOG(FATAL, "%s", error.what());
-  std::abort();
-#endif
-}
-}  // namespace
-
-void ThrowStdLogicError(const std::string& what_arg) {
-  Throw(std::logic_error(what_arg));
-}
-void ThrowStdLogicError(const char* what_arg) {
-  Throw(std::logic_error(what_arg));
-}
-void ThrowStdInvalidArgument(const std::string& what_arg) {
-  Throw(std::invalid_argument(what_arg));
-}
-void ThrowStdInvalidArgument(const char* what_arg) {
-  Throw(std::invalid_argument(what_arg));
-}
-
-void ThrowStdDomainError(const std::string& what_arg) {
-  Throw(std::domain_error(what_arg));
-}
-void ThrowStdDomainError(const char* what_arg) {
-  Throw(std::domain_error(what_arg));
-}
-
-void ThrowStdLengthError(const std::string& what_arg) {
-  Throw(std::length_error(what_arg));
-}
-void ThrowStdLengthError(const char* what_arg) {
-  Throw(std::length_error(what_arg));
-}
-
-void ThrowStdOutOfRange(const std::string& what_arg) {
-  Throw(std::out_of_range(what_arg));
-}
-void ThrowStdOutOfRange(const char* what_arg) {
-  Throw(std::out_of_range(what_arg));
-}
-
-void ThrowStdRuntimeError(const std::string& what_arg) {
-  Throw(std::runtime_error(what_arg));
-}
-void ThrowStdRuntimeError(const char* what_arg) {
-  Throw(std::runtime_error(what_arg));
-}
-
-void ThrowStdRangeError(const std::string& what_arg) {
-  Throw(std::range_error(what_arg));
-}
-void ThrowStdRangeError(const char* what_arg) {
-  Throw(std::range_error(what_arg));
-}
-
-void ThrowStdOverflowError(const std::string& what_arg) {
-  Throw(std::overflow_error(what_arg));
-}
-void ThrowStdOverflowError(const char* what_arg) {
-  Throw(std::overflow_error(what_arg));
-}
-
-void ThrowStdUnderflowError(const std::string& what_arg) {
-  Throw(std::underflow_error(what_arg));
-}
-void ThrowStdUnderflowError(const char* what_arg) {
-  Throw(std::underflow_error(what_arg));
-}
-
-void ThrowStdBadFunctionCall() { Throw(std::bad_function_call()); }
-
-void ThrowStdBadAlloc() { Throw(std::bad_alloc()); }
-
-}  // namespace base_internal
-}  // namespace absl
diff --git a/absl/base/internal/tracing.cc b/absl/base/internal/tracing.cc
new file mode 100644
index 00000000000..d304e6a29a6
--- /dev/null
+++ b/absl/base/internal/tracing.cc
@@ -0,0 +1,39 @@
+// Copyright 2024 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/base/internal/tracing.h"
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace base_internal {
+
+extern "C" {
+
+ABSL_ATTRIBUTE_WEAK void ABSL_INTERNAL_C_SYMBOL(AbslInternalTraceWait)(
+    const void*, ObjectKind) {}
+ABSL_ATTRIBUTE_WEAK void ABSL_INTERNAL_C_SYMBOL(AbslInternalTraceContinue)(
+    const void*, ObjectKind) {}
+ABSL_ATTRIBUTE_WEAK void ABSL_INTERNAL_C_SYMBOL(AbslInternalTraceSignal)(
+    const void*, ObjectKind) {}
+ABSL_ATTRIBUTE_WEAK void ABSL_INTERNAL_C_SYMBOL(AbslInternalTraceObserved)(
+    const void*, ObjectKind) {}
+
+}  // extern "C"
+
+}  // namespace base_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/base/internal/tracing.h b/absl/base/internal/tracing.h
new file mode 100644
index 00000000000..e7ab7758fe0
--- /dev/null
+++ b/absl/base/internal/tracing.h
@@ -0,0 +1,81 @@
+// Copyright 2024 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_BASE_INTERNAL_TRACING_H_
+#define ABSL_BASE_INTERNAL_TRACING_H_
+
+#include "absl/base/config.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace base_internal {
+
+// Well known Abseil object types that have causality.
+enum class ObjectKind { kUnknown, kBlockingCounter, kNotification };
+
+// `TraceWait` and `TraceContinue` record the start and end of a potentially
+// blocking wait operation on `object`. `object` typically represents a higher
+// level synchronization object such as `absl::Notification`.
+void TraceWait(const void* object, ObjectKind kind);
+void TraceContinue(const void* object, ObjectKind kind);
+
+// `TraceSignal` records a signal on `object`.
+void TraceSignal(const void* object, ObjectKind kind);
+
+// `TraceObserved` records the non-blocking observation of a signaled object.
+void TraceObserved(const void* object, ObjectKind kind);
+
+// ---------------------------------------------------------------------------
+// Weak implementation detail:
+//
+// We define the weak API as extern "C": in some build configurations we pass
+// `--detect-odr-violations` to the gold linker. This causes it to flag weak
+// symbol overrides as ODR violations. Because ODR only applies to C++ and not
+// C, `--detect-odr-violations` ignores symbols not mangled with C++ names.
+// By changing our extension points to be extern "C", we dodge this check.
+// ---------------------------------------------------------------------------
+extern "C" {
+
+  void ABSL_INTERNAL_C_SYMBOL(AbslInternalTraceWait)(const void* object,
+                                                     ObjectKind kind);
+  void ABSL_INTERNAL_C_SYMBOL(AbslInternalTraceContinue)(const void* object,
+                                                         ObjectKind kind);
+  void ABSL_INTERNAL_C_SYMBOL(AbslInternalTraceSignal)(const void* object,
+                                                       ObjectKind kind);
+  void ABSL_INTERNAL_C_SYMBOL(AbslInternalTraceObserved)(const void* object,
+                                                         ObjectKind kind);
+
+}  // extern "C"
+
+inline void TraceWait(const void* object, ObjectKind kind) {
+  ABSL_INTERNAL_C_SYMBOL(AbslInternalTraceWait)(object, kind);
+}
+
+inline void TraceContinue(const void* object, ObjectKind kind) {
+  ABSL_INTERNAL_C_SYMBOL(AbslInternalTraceContinue)(object, kind);
+}
+
+inline void TraceSignal(const void* object, ObjectKind kind) {
+  ABSL_INTERNAL_C_SYMBOL(AbslInternalTraceSignal)(object, kind);
+}
+
+inline void TraceObserved(const void* object, ObjectKind kind) {
+  ABSL_INTERNAL_C_SYMBOL(AbslInternalTraceObserved)(object, kind);
+}
+
+}  // namespace base_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_BASE_INTERNAL_TRACING_H_
diff --git a/absl/base/internal/tracing_strong_test.cc b/absl/base/internal/tracing_strong_test.cc
new file mode 100644
index 00000000000..979f1c577ab
--- /dev/null
+++ b/absl/base/internal/tracing_strong_test.cc
@@ -0,0 +1,117 @@
+// Copyright 2024 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+#include <tuple>
+#include <vector>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/tracing.h"
+
+#if ABSL_HAVE_ATTRIBUTE_WEAK
+
+namespace {
+
+using ::testing::ElementsAre;
+
+using ::absl::base_internal::ObjectKind;
+
+enum Function { kWait, kContinue, kSignal, kObserved };
+
+using Record = std::tuple<Function, const void*, ObjectKind>;
+
+thread_local std::vector<Record>* tls_records = nullptr;
+
+}  // namespace
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace base_internal {
+
+// Strong extern "C" implementation.
+extern "C" {
+
+void ABSL_INTERNAL_C_SYMBOL(AbslInternalTraceWait)(const void* object,
+                                                   ObjectKind kind) {
+  if (tls_records != nullptr) {
+    tls_records->push_back({kWait, object, kind});
+  }
+}
+
+void ABSL_INTERNAL_C_SYMBOL(AbslInternalTraceContinue)(const void* object,
+                                                       ObjectKind kind) {
+  if (tls_records != nullptr) {
+    tls_records->push_back({kContinue, object, kind});
+  }
+}
+
+void ABSL_INTERNAL_C_SYMBOL(AbslInternalTraceSignal)(const void* object,
+                                                     ObjectKind kind) {
+  if (tls_records != nullptr) {
+    tls_records->push_back({kSignal, object, kind});
+  }
+}
+
+void ABSL_INTERNAL_C_SYMBOL(AbslInternalTraceObserved)(const void* object,
+                                                       ObjectKind kind) {
+  if (tls_records != nullptr) {
+    tls_records->push_back({kObserved, object, kind});
+  }
+}
+
+}  // extern "C"
+
+}  // namespace base_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+namespace {
+
+TEST(TracingInternal, InvokesStrongFunctionWithNullptr) {
+  std::vector<Record> records;
+  tls_records = &records;
+  auto kind = absl::base_internal::ObjectKind::kUnknown;
+  absl::base_internal::TraceWait(nullptr, kind);
+  absl::base_internal::TraceContinue(nullptr, kind);
+  absl::base_internal::TraceSignal(nullptr, kind);
+  absl::base_internal::TraceObserved(nullptr, kind);
+  tls_records = nullptr;
+
+  EXPECT_THAT(records, ElementsAre(Record{kWait, nullptr, kind},
+                                   Record{kContinue, nullptr, kind},
+                                   Record{kSignal, nullptr, kind},
+                                   Record{kObserved, nullptr, kind}));
+}
+
+TEST(TracingInternal, InvokesStrongFunctionWithObjectAddress) {
+  int object = 0;
+  std::vector<Record> records;
+  tls_records = &records;
+  auto kind = absl::base_internal::ObjectKind::kUnknown;
+  absl::base_internal::TraceWait(&object, kind);
+  absl::base_internal::TraceContinue(&object, kind);
+  absl::base_internal::TraceSignal(&object, kind);
+  absl::base_internal::TraceObserved(&object, kind);
+  tls_records = nullptr;
+
+  EXPECT_THAT(records, ElementsAre(Record{kWait, &object, kind},
+                                   Record{kContinue, &object, kind},
+                                   Record{kSignal, &object, kind},
+                                   Record{kObserved, &object, kind}));
+}
+
+}  // namespace
+
+#endif  // ABSL_HAVE_ATTRIBUTE_WEAK
diff --git a/absl/base/internal/tracing_weak_test.cc b/absl/base/internal/tracing_weak_test.cc
new file mode 100644
index 00000000000..6d7553fd6aa
--- /dev/null
+++ b/absl/base/internal/tracing_weak_test.cc
@@ -0,0 +1,34 @@
+// Copyright 2024 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "gtest/gtest.h"
+#include "absl/base/internal/tracing.h"
+
+namespace {
+
+TEST(TracingInternal, HasDefaultImplementation) {
+  auto kind = absl::base_internal::ObjectKind::kUnknown;
+  absl::base_internal::TraceWait(nullptr, kind);
+  absl::base_internal::TraceContinue(nullptr, kind);
+  absl::base_internal::TraceSignal(nullptr, kind);
+  absl::base_internal::TraceObserved(nullptr, kind);
+
+  int object = 0;
+  absl::base_internal::TraceWait(&object, kind);
+  absl::base_internal::TraceContinue(&object, kind);
+  absl::base_internal::TraceSignal(&object, kind);
+  absl::base_internal::TraceObserved(&object, kind);
+}
+
+}  // namespace
diff --git a/absl/base/internal/tsan_mutex_interface.h b/absl/base/internal/tsan_mutex_interface.h
index 2a510603bc8..e2d3d931913 100644
--- a/absl/base/internal/tsan_mutex_interface.h
+++ b/absl/base/internal/tsan_mutex_interface.h
@@ -16,9 +16,13 @@
 // It provides ThreadSanitizer annotations for custom mutexes.
 // See <sanitizer/tsan_interface.h> for meaning of these annotations.
 
+// SKIP_ABSL_INLINE_NAMESPACE_CHECK
+
 #ifndef ABSL_BASE_INTERNAL_TSAN_MUTEX_INTERFACE_H_
 #define ABSL_BASE_INTERNAL_TSAN_MUTEX_INTERFACE_H_
 
+#include "absl/base/config.h"
+
 // ABSL_INTERNAL_HAVE_TSAN_INTERFACE
 // Macro intended only for internal use.
 //
@@ -28,11 +32,10 @@
 #error "ABSL_INTERNAL_HAVE_TSAN_INTERFACE cannot be directly set."
 #endif
 
-#if defined(THREAD_SANITIZER) && defined(__has_include)
-#if __has_include(<sanitizer/tsan_interface.h>)
+#if defined(ABSL_HAVE_THREAD_SANITIZER) && \
+    __has_include(<sanitizer/tsan_interface.h>)
 #define ABSL_INTERNAL_HAVE_TSAN_INTERFACE 1
 #endif
-#endif
 
 #ifdef ABSL_INTERNAL_HAVE_TSAN_INTERFACE
 #include <sanitizer/tsan_interface.h>
diff --git a/absl/base/internal/unaligned_access.h b/absl/base/internal/unaligned_access.h
index 2cf7c1d45b3..3f5dd6f9ff9 100644
--- a/absl/base/internal/unaligned_access.h
+++ b/absl/base/internal/unaligned_access.h
@@ -18,9 +18,12 @@
 #define ABSL_BASE_INTERNAL_UNALIGNED_ACCESS_H_
 
 #include <string.h>
+
 #include <cstdint>
 
 #include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/nullability.h"
 
 // unaligned APIs
 
@@ -29,106 +32,42 @@
 // The unaligned API is C++ only.  The declarations use C++ features
 // (namespaces, inline) which are absent or incompatible in C.
 #if defined(__cplusplus)
-
-#if defined(ADDRESS_SANITIZER) || defined(THREAD_SANITIZER) ||\
-    defined(MEMORY_SANITIZER)
-// Consider we have an unaligned load/store of 4 bytes from address 0x...05.
-// AddressSanitizer will treat it as a 3-byte access to the range 05:07 and
-// will miss a bug if 08 is the first unaddressable byte.
-// ThreadSanitizer will also treat this as a 3-byte access to 05:07 and will
-// miss a race between this access and some other accesses to 08.
-// MemorySanitizer will correctly propagate the shadow on unaligned stores
-// and correctly report bugs on unaligned loads, but it may not properly
-// update and report the origin of the uninitialized memory.
-// For all three tools, replacing an unaligned access with a tool-specific
-// callback solves the problem.
-
-// Make sure uint16_t/uint32_t/uint64_t are defined.
-#include <stdint.h>
-
-extern "C" {
-uint16_t __sanitizer_unaligned_load16(const void *p);
-uint32_t __sanitizer_unaligned_load32(const void *p);
-uint64_t __sanitizer_unaligned_load64(const void *p);
-void __sanitizer_unaligned_store16(void *p, uint16_t v);
-void __sanitizer_unaligned_store32(void *p, uint32_t v);
-void __sanitizer_unaligned_store64(void *p, uint64_t v);
-}  // extern "C"
-
-namespace absl {
-namespace base_internal {
-
-inline uint16_t UnalignedLoad16(const void *p) {
-  return __sanitizer_unaligned_load16(p);
-}
-
-inline uint32_t UnalignedLoad32(const void *p) {
-  return __sanitizer_unaligned_load32(p);
-}
-
-inline uint64_t UnalignedLoad64(const void *p) {
-  return __sanitizer_unaligned_load64(p);
-}
-
-inline void UnalignedStore16(void *p, uint16_t v) {
-  __sanitizer_unaligned_store16(p, v);
-}
-
-inline void UnalignedStore32(void *p, uint32_t v) {
-  __sanitizer_unaligned_store32(p, v);
-}
-
-inline void UnalignedStore64(void *p, uint64_t v) {
-  __sanitizer_unaligned_store64(p, v);
-}
-
-}  // namespace base_internal
-}  // namespace absl
-
-#define ABSL_INTERNAL_UNALIGNED_LOAD16(_p) \
-  (absl::base_internal::UnalignedLoad16(_p))
-#define ABSL_INTERNAL_UNALIGNED_LOAD32(_p) \
-  (absl::base_internal::UnalignedLoad32(_p))
-#define ABSL_INTERNAL_UNALIGNED_LOAD64(_p) \
-  (absl::base_internal::UnalignedLoad64(_p))
-
-#define ABSL_INTERNAL_UNALIGNED_STORE16(_p, _val) \
-  (absl::base_internal::UnalignedStore16(_p, _val))
-#define ABSL_INTERNAL_UNALIGNED_STORE32(_p, _val) \
-  (absl::base_internal::UnalignedStore32(_p, _val))
-#define ABSL_INTERNAL_UNALIGNED_STORE64(_p, _val) \
-  (absl::base_internal::UnalignedStore64(_p, _val))
-
-#else
-
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 
-inline uint16_t UnalignedLoad16(const void *p) {
+inline uint16_t UnalignedLoad16(const void* absl_nonnull p) {
   uint16_t t;
   memcpy(&t, p, sizeof t);
   return t;
 }
 
-inline uint32_t UnalignedLoad32(const void *p) {
+inline uint32_t UnalignedLoad32(const void* absl_nonnull p) {
   uint32_t t;
   memcpy(&t, p, sizeof t);
   return t;
 }
 
-inline uint64_t UnalignedLoad64(const void *p) {
+inline uint64_t UnalignedLoad64(const void* absl_nonnull p) {
   uint64_t t;
   memcpy(&t, p, sizeof t);
   return t;
 }
 
-inline void UnalignedStore16(void *p, uint16_t v) { memcpy(p, &v, sizeof v); }
+inline void UnalignedStore16(void* absl_nonnull p, uint16_t v) {
+  memcpy(p, &v, sizeof v);
+}
 
-inline void UnalignedStore32(void *p, uint32_t v) { memcpy(p, &v, sizeof v); }
+inline void UnalignedStore32(void* absl_nonnull p, uint32_t v) {
+  memcpy(p, &v, sizeof v);
+}
 
-inline void UnalignedStore64(void *p, uint64_t v) { memcpy(p, &v, sizeof v); }
+inline void UnalignedStore64(void* absl_nonnull p, uint64_t v) {
+  memcpy(p, &v, sizeof v);
+}
 
 }  // namespace base_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #define ABSL_INTERNAL_UNALIGNED_LOAD16(_p) \
@@ -145,8 +84,6 @@ inline void UnalignedStore64(void *p, uint64_t v) { memcpy(p, &v, sizeof v); }
 #define ABSL_INTERNAL_UNALIGNED_STORE64(_p, _val) \
   (absl::base_internal::UnalignedStore64(_p, _val))
 
-#endif
-
 #endif  // defined(__cplusplus), end of unaligned API
 
 #endif  // ABSL_BASE_INTERNAL_UNALIGNED_ACCESS_H_
diff --git a/absl/base/internal/unique_small_name_test.cc b/absl/base/internal/unique_small_name_test.cc
new file mode 100644
index 00000000000..ff8c2b3fb42
--- /dev/null
+++ b/absl/base/internal/unique_small_name_test.cc
@@ -0,0 +1,77 @@
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "gtest/gtest.h"
+#include "absl/base/optimization.h"
+#include "absl/strings/string_view.h"
+
+// This test by itself does not do anything fancy, but it serves as binary I can
+// query in shell test.
+
+namespace {
+
+template <class T>
+void DoNotOptimize(const T& var) {
+#ifdef __GNUC__
+  asm volatile("" : "+m"(const_cast<T&>(var)));
+#else
+  std::cout << (void*)&var;
+#endif
+}
+
+int very_long_int_variable_name ABSL_INTERNAL_UNIQUE_SMALL_NAME() = 0;
+char very_long_str_variable_name[] ABSL_INTERNAL_UNIQUE_SMALL_NAME() = "abc";
+
+TEST(UniqueSmallName, NonAutomaticVar) {
+  EXPECT_EQ(very_long_int_variable_name, 0);
+  EXPECT_EQ(absl::string_view(very_long_str_variable_name), "abc");
+}
+
+int VeryLongFreeFunctionName() ABSL_INTERNAL_UNIQUE_SMALL_NAME();
+
+TEST(UniqueSmallName, FreeFunction) {
+  DoNotOptimize(&VeryLongFreeFunctionName);
+
+  EXPECT_EQ(VeryLongFreeFunctionName(), 456);
+}
+
+int VeryLongFreeFunctionName() { return 456; }
+
+struct VeryLongStructName {
+  explicit VeryLongStructName(int i);
+
+  int VeryLongMethodName() ABSL_INTERNAL_UNIQUE_SMALL_NAME();
+
+  static int VeryLongStaticMethodName() ABSL_INTERNAL_UNIQUE_SMALL_NAME();
+
+ private:
+  int fld;
+};
+
+TEST(UniqueSmallName, Struct) {
+  VeryLongStructName var(10);
+
+  DoNotOptimize(var);
+  DoNotOptimize(&VeryLongStructName::VeryLongMethodName);
+  DoNotOptimize(&VeryLongStructName::VeryLongStaticMethodName);
+
+  EXPECT_EQ(var.VeryLongMethodName(), 10);
+  EXPECT_EQ(VeryLongStructName::VeryLongStaticMethodName(), 123);
+}
+
+VeryLongStructName::VeryLongStructName(int i) : fld(i) {}
+int VeryLongStructName::VeryLongMethodName() { return fld; }
+int VeryLongStructName::VeryLongStaticMethodName() { return 123; }
+
+}  // namespace
diff --git a/absl/base/internal/unscaledcycleclock.cc b/absl/base/internal/unscaledcycleclock.cc
index 593762bc8b8..9e931808412 100644
--- a/absl/base/internal/unscaledcycleclock.cc
+++ b/absl/base/internal/unscaledcycleclock.cc
@@ -14,6 +14,8 @@
 
 #include "absl/base/internal/unscaledcycleclock.h"
 
+#include "absl/base/internal/unscaledcycleclock_config.h"
+
 #if ABSL_USE_UNSCALED_CYCLECLOCK
 
 #if defined(_WIN32)
@@ -21,12 +23,23 @@
 #endif
 
 #if defined(__powerpc__) || defined(__ppc__)
+#ifdef __GLIBC__
 #include <sys/platform/ppc.h>
+#elif defined(__FreeBSD__)
+// clang-format off
+// This order does actually matter =(.
+#include <sys/types.h>
+#include <sys/sysctl.h>
+// clang-format on
+
+#include "absl/base/call_once.h"
+#endif
 #endif
 
 #include "absl/base/internal/sysinfo.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 
 #if defined(__i386__)
@@ -43,12 +56,6 @@ double UnscaledCycleClock::Frequency() {
 
 #elif defined(__x86_64__)
 
-int64_t UnscaledCycleClock::Now() {
-  uint64_t low, high;
-  __asm__ volatile("rdtsc" : "=a"(low), "=d"(high));
-  return (high << 32) | low;
-}
-
 double UnscaledCycleClock::Frequency() {
   return base_internal::NominalCPUFrequency();
 }
@@ -56,25 +63,54 @@ double UnscaledCycleClock::Frequency() {
 #elif defined(__powerpc__) || defined(__ppc__)
 
 int64_t UnscaledCycleClock::Now() {
-  return __ppc_get_timebase();
+#ifdef __GLIBC__
+  return static_cast<int64_t>(__ppc_get_timebase());
+#else
+#ifdef __powerpc64__
+  int64_t tbr;
+  asm volatile("mfspr %0, 268" : "=r"(tbr));
+  return tbr;
+#else
+  int32_t tbu, tbl, tmp;
+  asm volatile(
+      "mftbu %[hi32]\n"
+      "mftb %[lo32]\n"
+      "mftbu %[tmp]\n"
+      "cmpw %[tmp],%[hi32]\n"
+      "bne $-16\n"  // Retry on failure.
+      : [hi32] "=r"(tbu), [lo32] "=r"(tbl), [tmp] "=r"(tmp));
+  return (static_cast<int64_t>(tbu) << 32) | tbl;
+#endif
+#endif
 }
 
 double UnscaledCycleClock::Frequency() {
+#ifdef __GLIBC__
   return __ppc_get_timebase_freq();
+#elif defined(__linux__)
+  // Fallback for musl + ppc64le: use constant timebase frequency (512 MHz)
+  // Must come after __GLIBC__.
+  return static_cast<double>(512000000);
+#elif defined(_AIX)
+  // This is the same constant value as returned by
+  // __ppc_get_timebase_freq().
+  return static_cast<double>(512000000);
+#elif defined(__FreeBSD__)
+  static once_flag init_timebase_frequency_once;
+  static double timebase_frequency = 0.0;
+  base_internal::LowLevelCallOnce(&init_timebase_frequency_once, [&]() {
+    size_t length = sizeof(timebase_frequency);
+    sysctlbyname("kern.timecounter.tc.timebase.frequency", &timebase_frequency,
+                 &length, nullptr, 0);
+  });
+  return timebase_frequency;
+#else
+#error Must implement UnscaledCycleClock::Frequency()
+#endif
 }
 
 #elif defined(__aarch64__)
 
-// System timer of ARMv8 runs at a different frequency than the CPU's.
-// The frequency is fixed, typically in the range 1-50MHz.  It can be
-// read at CNTFRQ special register.  We assume the OS has set up
-// the virtual timer properly.
-int64_t UnscaledCycleClock::Now() {
-  int64_t virtual_timer_value;
-  asm volatile("mrs %0, cntvct_el0" : "=r"(virtual_timer_value));
-  return virtual_timer_value;
-}
-
 double UnscaledCycleClock::Frequency() {
   uint64_t aarch64_timer_frequency;
   asm volatile("mrs %0, cntfrq_el0" : "=r"(aarch64_timer_frequency));
@@ -85,9 +121,7 @@ double UnscaledCycleClock::Frequency() {
 
 #pragma intrinsic(__rdtsc)
 
-int64_t UnscaledCycleClock::Now() {
-  return __rdtsc();
-}
+int64_t UnscaledCycleClock::Now() { return __rdtsc(); }
 
 double UnscaledCycleClock::Frequency() {
   return base_internal::NominalCPUFrequency();
@@ -96,6 +130,7 @@ double UnscaledCycleClock::Frequency() {
 #endif
 
 }  // namespace base_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_USE_UNSCALED_CYCLECLOCK
diff --git a/absl/base/internal/unscaledcycleclock.h b/absl/base/internal/unscaledcycleclock.h
index 2d361e96284..c8a4e4b7e0d 100644
--- a/absl/base/internal/unscaledcycleclock.h
+++ b/absl/base/internal/unscaledcycleclock.h
@@ -15,8 +15,8 @@
 // UnscaledCycleClock
 //    An UnscaledCycleClock yields the value and frequency of a cycle counter
 //    that increments at a rate that is approximately constant.
-//    This class is for internal / whitelisted use only, you should consider
-//    using CycleClock instead.
+//    This class is for internal use only, you should consider using CycleClock
+//    instead.
 //
 // Notes:
 // The cycle counter frequency is not necessarily the core clock frequency.
@@ -38,54 +38,21 @@
 
 #include <cstdint>
 
+#include "absl/base/config.h"
+#include "absl/base/internal/unscaledcycleclock_config.h"
+
 #if defined(__APPLE__)
 #include <TargetConditionals.h>
 #endif
 
-#include "absl/base/port.h"
-
-// The following platforms have an implementation of a hardware counter.
-#if defined(__i386__) || defined(__x86_64__) || defined(__aarch64__) || \
-  defined(__powerpc__) || defined(__ppc__) || \
-  defined(_M_IX86) || defined(_M_X64)
-#define ABSL_HAVE_UNSCALED_CYCLECLOCK_IMPLEMENTATION 1
-#else
-#define ABSL_HAVE_UNSCALED_CYCLECLOCK_IMPLEMENTATION 0
-#endif
-
-// The following platforms often disable access to the hardware
-// counter (through a sandbox) even if the underlying hardware has a
-// usable counter. The CycleTimer interface also requires a *scaled*
-// CycleClock that runs at atleast 1 MHz. We've found some Android
-// ARM64 devices where this is not the case, so we disable it by
-// default on Android ARM64.
-#if defined(__native_client__) ||                      \
-    (defined(TARGET_OS_IPHONE) && TARGET_OS_IPHONE) || \
-    (defined(__ANDROID__) && defined(__aarch64__))
-#define ABSL_USE_UNSCALED_CYCLECLOCK_DEFAULT 0
-#else
-#define ABSL_USE_UNSCALED_CYCLECLOCK_DEFAULT 1
-#endif
-
-// UnscaledCycleClock is an optional internal feature.
-// Use "#if ABSL_USE_UNSCALED_CYCLECLOCK" to test for its presence.
-// Can be overridden at compile-time via -DABSL_USE_UNSCALED_CYCLECLOCK=0|1
-#if !defined(ABSL_USE_UNSCALED_CYCLECLOCK)
-#define ABSL_USE_UNSCALED_CYCLECLOCK               \
-  (ABSL_HAVE_UNSCALED_CYCLECLOCK_IMPLEMENTATION && \
-   ABSL_USE_UNSCALED_CYCLECLOCK_DEFAULT)
-#endif
-
 #if ABSL_USE_UNSCALED_CYCLECLOCK
 
-// This macro can be used to test if UnscaledCycleClock::Frequency()
-// is NominalCPUFrequency() on a particular platform.
-#if  (defined(__i386__) || defined(__x86_64__) || \
-      defined(_M_IX86) || defined(_M_X64))
-#define ABSL_INTERNAL_UNSCALED_CYCLECLOCK_FREQUENCY_IS_CPU_FREQUENCY
-#endif
+namespace gloop_do_not_use {
+class UnscaledCycleClockWrapperForPerCpuTest;
+}  // namespace gloop_do_not_use
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 class UnscaledCycleClockWrapperForGetCurrentTime;
 }  // namespace time_internal
@@ -108,13 +75,42 @@ class UnscaledCycleClock {
   // value.
   static double Frequency();
 
-  // Whitelisted friends.
+  // Allowed users
   friend class base_internal::CycleClock;
   friend class time_internal::UnscaledCycleClockWrapperForGetCurrentTime;
   friend class base_internal::UnscaledCycleClockWrapperForInitializeFrequency;
+  friend class gloop_do_not_use::UnscaledCycleClockWrapperForPerCpuTest;
 };
 
+#if defined(__x86_64__)
+
+inline int64_t UnscaledCycleClock::Now() {
+  uint64_t low, high;
+  __asm__ volatile("rdtsc" : "=a"(low), "=d"(high));
+  return static_cast<int64_t>((high << 32) | low);
+}
+
+#elif defined(__aarch64__)
+
+// System timer of ARMv8 runs at a different frequency than the CPU's.
+//
+// Frequency is fixed. From Armv8.6-A and Armv9.1-A on, the frequency is 1GHz.
+// Pre-Armv8.6-A, the frequency was a system design choice, typically in the
+// range of 1MHz to 50MHz. See also:
+// https://developer.arm.com/documentation/102379/0101/What-is-the-Generic-Timer-
+//
+// It can be read at CNTFRQ special register.  We assume the OS has set up the
+// virtual timer properly.
+inline int64_t UnscaledCycleClock::Now() {
+  int64_t virtual_timer_value;
+  asm volatile("mrs %0, cntvct_el0" : "=r"(virtual_timer_value));
+  return virtual_timer_value;
+}
+
+#endif
+
 }  // namespace base_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_USE_UNSCALED_CYCLECLOCK
diff --git a/absl/base/internal/unscaledcycleclock_config.h b/absl/base/internal/unscaledcycleclock_config.h
new file mode 100644
index 00000000000..9a0841dfa79
--- /dev/null
+++ b/absl/base/internal/unscaledcycleclock_config.h
@@ -0,0 +1,62 @@
+// Copyright 2022 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_BASE_INTERNAL_UNSCALEDCYCLECLOCK_CONFIG_H_
+#define ABSL_BASE_INTERNAL_UNSCALEDCYCLECLOCK_CONFIG_H_
+
+#if defined(__APPLE__)
+#include <TargetConditionals.h>
+#endif
+
+// The following platforms have an implementation of a hardware counter.
+#if defined(__i386__) || defined(__x86_64__) || defined(__aarch64__) || \
+    defined(__powerpc__) || defined(__ppc__) || defined(_M_IX86) ||     \
+    (defined(_M_X64) && !defined(_M_ARM64EC))
+#define ABSL_HAVE_UNSCALED_CYCLECLOCK_IMPLEMENTATION 1
+#else
+#define ABSL_HAVE_UNSCALED_CYCLECLOCK_IMPLEMENTATION 0
+#endif
+
+// The following platforms often disable access to the hardware
+// counter (through a sandbox) even if the underlying hardware has a
+// usable counter. The CycleTimer interface also requires a *scaled*
+// CycleClock that runs at atleast 1 MHz. We've found some Android
+// ARM64 devices where this is not the case, so we disable it by
+// default on Android ARM64.
+#if defined(__APPLE__) || \
+    (defined(__ANDROID__) && defined(__aarch64__))
+#define ABSL_USE_UNSCALED_CYCLECLOCK_DEFAULT 0
+#else
+#define ABSL_USE_UNSCALED_CYCLECLOCK_DEFAULT 1
+#endif
+
+// UnscaledCycleClock is an optional internal feature.
+// Use "#if ABSL_USE_UNSCALED_CYCLECLOCK" to test for its presence.
+// Can be overridden at compile-time via -DABSL_USE_UNSCALED_CYCLECLOCK=0|1
+#if !defined(ABSL_USE_UNSCALED_CYCLECLOCK)
+#define ABSL_USE_UNSCALED_CYCLECLOCK               \
+  (ABSL_HAVE_UNSCALED_CYCLECLOCK_IMPLEMENTATION && \
+   ABSL_USE_UNSCALED_CYCLECLOCK_DEFAULT)
+#endif
+
+#if ABSL_USE_UNSCALED_CYCLECLOCK
+// This macro can be used to test if UnscaledCycleClock::Frequency()
+// is NominalCPUFrequency() on a particular platform.
+#if (defined(__i386__) || defined(__x86_64__) || defined(_M_IX86) || \
+     defined(_M_X64))
+#define ABSL_INTERNAL_UNSCALED_CYCLECLOCK_FREQUENCY_IS_CPU_FREQUENCY
+#endif
+#endif
+
+#endif  // ABSL_BASE_INTERNAL_UNSCALEDCYCLECLOCK_CONFIG_H_
diff --git a/absl/base/invoke_test.cc b/absl/base/invoke_test.cc
deleted file mode 100644
index 9074443b3cb..00000000000
--- a/absl/base/invoke_test.cc
+++ /dev/null
@@ -1,221 +0,0 @@
-// Copyright 2017 The Abseil Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-#include "absl/base/internal/invoke.h"
-
-#include <functional>
-#include <memory>
-#include <string>
-#include <utility>
-
-#include "gmock/gmock.h"
-#include "gtest/gtest.h"
-#include "absl/memory/memory.h"
-#include "absl/strings/str_cat.h"
-
-namespace absl {
-namespace base_internal {
-namespace {
-
-int Function(int a, int b) { return a - b; }
-
-int Sink(std::unique_ptr<int> p) {
-  return *p;
-}
-
-std::unique_ptr<int> Factory(int n) {
-  return make_unique<int>(n);
-}
-
-void NoOp() {}
-
-struct ConstFunctor {
-  int operator()(int a, int b) const { return a - b; }
-};
-
-struct MutableFunctor {
-  int operator()(int a, int b) { return a - b; }
-};
-
-struct EphemeralFunctor {
-  int operator()(int a, int b) && { return a - b; }
-};
-
-struct OverloadedFunctor {
-  template <typename... Args>
-  std::string operator()(const Args&... args) & {
-    return StrCat("&", args...);
-  }
-  template <typename... Args>
-  std::string operator()(const Args&... args) const& {
-    return StrCat("const&", args...);
-  }
-  template <typename... Args>
-  std::string operator()(const Args&... args) && {
-    return StrCat("&&", args...);
-  }
-};
-
-struct Class {
-  int Method(int a, int b) { return a - b; }
-  int ConstMethod(int a, int b) const { return a - b; }
-  int RefMethod(int a, int b) & { return a - b; }
-  int RefRefMethod(int a, int b) && { return a - b; }
-  int NoExceptMethod(int a, int b) noexcept { return a - b; }
-  int VolatileMethod(int a, int b) volatile { return a - b; }
-
-  int member;
-};
-
-struct FlipFlop {
-  int ConstMethod() const { return member; }
-  FlipFlop operator*() const { return {-member}; }
-
-  int member;
-};
-
-// CallMaybeWithArg(f) resolves either to Invoke(f) or Invoke(f, 42), depending
-// on which one is valid.
-template <typename F>
-decltype(Invoke(std::declval<const F&>())) CallMaybeWithArg(const F& f) {
-  return Invoke(f);
-}
-
-template <typename F>
-decltype(Invoke(std::declval<const F&>(), 42)) CallMaybeWithArg(const F& f) {
-  return Invoke(f, 42);
-}
-
-TEST(InvokeTest, Function) {
-  EXPECT_EQ(1, Invoke(Function, 3, 2));
-  EXPECT_EQ(1, Invoke(&Function, 3, 2));
-}
-
-TEST(InvokeTest, NonCopyableArgument) {
-  EXPECT_EQ(42, Invoke(Sink, make_unique<int>(42)));
-}
-
-TEST(InvokeTest, NonCopyableResult) {
-  EXPECT_THAT(Invoke(Factory, 42), ::testing::Pointee(42));
-}
-
-TEST(InvokeTest, VoidResult) {
-  Invoke(NoOp);
-}
-
-TEST(InvokeTest, ConstFunctor) {
-  EXPECT_EQ(1, Invoke(ConstFunctor(), 3, 2));
-}
-
-TEST(InvokeTest, MutableFunctor) {
-  MutableFunctor f;
-  EXPECT_EQ(1, Invoke(f, 3, 2));
-  EXPECT_EQ(1, Invoke(MutableFunctor(), 3, 2));
-}
-
-TEST(InvokeTest, EphemeralFunctor) {
-  EphemeralFunctor f;
-  EXPECT_EQ(1, Invoke(std::move(f), 3, 2));
-  EXPECT_EQ(1, Invoke(EphemeralFunctor(), 3, 2));
-}
-
-TEST(InvokeTest, OverloadedFunctor) {
-  OverloadedFunctor f;
-  const OverloadedFunctor& cf = f;
-
-  EXPECT_EQ("&", Invoke(f));
-  EXPECT_EQ("& 42", Invoke(f, " 42"));
-
-  EXPECT_EQ("const&", Invoke(cf));
-  EXPECT_EQ("const& 42", Invoke(cf, " 42"));
-
-  EXPECT_EQ("&&", Invoke(std::move(f)));
-  EXPECT_EQ("&& 42", Invoke(std::move(f), " 42"));
-}
-
-TEST(InvokeTest, ReferenceWrapper) {
-  ConstFunctor cf;
-  MutableFunctor mf;
-  EXPECT_EQ(1, Invoke(std::cref(cf), 3, 2));
-  EXPECT_EQ(1, Invoke(std::ref(cf), 3, 2));
-  EXPECT_EQ(1, Invoke(std::ref(mf), 3, 2));
-}
-
-TEST(InvokeTest, MemberFunction) {
-  std::unique_ptr<Class> p(new Class);
-  std::unique_ptr<const Class> cp(new Class);
-  std::unique_ptr<volatile Class> vp(new Class);
-
-  EXPECT_EQ(1, Invoke(&Class::Method, p, 3, 2));
-  EXPECT_EQ(1, Invoke(&Class::Method, p.get(), 3, 2));
-  EXPECT_EQ(1, Invoke(&Class::Method, *p, 3, 2));
-  EXPECT_EQ(1, Invoke(&Class::RefMethod, p, 3, 2));
-  EXPECT_EQ(1, Invoke(&Class::RefMethod, p.get(), 3, 2));
-  EXPECT_EQ(1, Invoke(&Class::RefMethod, *p, 3, 2));
-  EXPECT_EQ(1, Invoke(&Class::RefRefMethod, std::move(*p), 3, 2));  // NOLINT
-  EXPECT_EQ(1, Invoke(&Class::NoExceptMethod, p, 3, 2));
-  EXPECT_EQ(1, Invoke(&Class::NoExceptMethod, p.get(), 3, 2));
-  EXPECT_EQ(1, Invoke(&Class::NoExceptMethod, *p, 3, 2));
-
-  EXPECT_EQ(1, Invoke(&Class::ConstMethod, p, 3, 2));
-  EXPECT_EQ(1, Invoke(&Class::ConstMethod, p.get(), 3, 2));
-  EXPECT_EQ(1, Invoke(&Class::ConstMethod, *p, 3, 2));
-
-  EXPECT_EQ(1, Invoke(&Class::ConstMethod, cp, 3, 2));
-  EXPECT_EQ(1, Invoke(&Class::ConstMethod, cp.get(), 3, 2));
-  EXPECT_EQ(1, Invoke(&Class::ConstMethod, *cp, 3, 2));
-
-  EXPECT_EQ(1, Invoke(&Class::VolatileMethod, p, 3, 2));
-  EXPECT_EQ(1, Invoke(&Class::VolatileMethod, p.get(), 3, 2));
-  EXPECT_EQ(1, Invoke(&Class::VolatileMethod, *p, 3, 2));
-  EXPECT_EQ(1, Invoke(&Class::VolatileMethod, vp, 3, 2));
-  EXPECT_EQ(1, Invoke(&Class::VolatileMethod, vp.get(), 3, 2));
-  EXPECT_EQ(1, Invoke(&Class::VolatileMethod, *vp, 3, 2));
-
-  EXPECT_EQ(1, Invoke(&Class::Method, make_unique<Class>(), 3, 2));
-  EXPECT_EQ(1, Invoke(&Class::ConstMethod, make_unique<Class>(), 3, 2));
-  EXPECT_EQ(1, Invoke(&Class::ConstMethod, make_unique<const Class>(), 3, 2));
-}
-
-TEST(InvokeTest, DataMember) {
-  std::unique_ptr<Class> p(new Class{42});
-  std::unique_ptr<const Class> cp(new Class{42});
-  EXPECT_EQ(42, Invoke(&Class::member, p));
-  EXPECT_EQ(42, Invoke(&Class::member, *p));
-  EXPECT_EQ(42, Invoke(&Class::member, p.get()));
-
-  Invoke(&Class::member, p) = 42;
-  Invoke(&Class::member, p.get()) = 42;
-
-  EXPECT_EQ(42, Invoke(&Class::member, cp));
-  EXPECT_EQ(42, Invoke(&Class::member, *cp));
-  EXPECT_EQ(42, Invoke(&Class::member, cp.get()));
-}
-
-TEST(InvokeTest, FlipFlop) {
-  FlipFlop obj = {42};
-  // This call could resolve to (obj.*&FlipFlop::ConstMethod)() or
-  // ((*obj).*&FlipFlop::ConstMethod)(). We verify that it's the former.
-  EXPECT_EQ(42, Invoke(&FlipFlop::ConstMethod, obj));
-  EXPECT_EQ(42, Invoke(&FlipFlop::member, obj));
-}
-
-TEST(InvokeTest, SfinaeFriendly) {
-  CallMaybeWithArg(NoOp);
-  EXPECT_THAT(CallMaybeWithArg(Factory), ::testing::Pointee(42));
-}
-
-}  // namespace
-}  // namespace base_internal
-}  // namespace absl
diff --git a/absl/base/log_severity.cc b/absl/base/log_severity.cc
index 02a2a4852a5..8e7bbbc9e0e 100644
--- a/absl/base/log_severity.cc
+++ b/absl/base/log_severity.cc
@@ -16,10 +16,41 @@
 
 #include <ostream>
 
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 std::ostream& operator<<(std::ostream& os, absl::LogSeverity s) {
   if (s == absl::NormalizeLogSeverity(s)) return os << absl::LogSeverityName(s);
   return os << "absl::LogSeverity(" << static_cast<int>(s) << ")";
 }
+
+std::ostream& operator<<(std::ostream& os, absl::LogSeverityAtLeast s) {
+  switch (s) {
+    case absl::LogSeverityAtLeast::kInfo:
+    case absl::LogSeverityAtLeast::kWarning:
+    case absl::LogSeverityAtLeast::kError:
+    case absl::LogSeverityAtLeast::kFatal:
+      return os << ">=" << static_cast<absl::LogSeverity>(s);
+    case absl::LogSeverityAtLeast::kInfinity:
+      return os << "INFINITY";
+  }
+  return os;
+}
+
+std::ostream& operator<<(std::ostream& os, absl::LogSeverityAtMost s) {
+  switch (s) {
+    case absl::LogSeverityAtMost::kInfo:
+    case absl::LogSeverityAtMost::kWarning:
+    case absl::LogSeverityAtMost::kError:
+    case absl::LogSeverityAtMost::kFatal:
+      return os << "<=" << static_cast<absl::LogSeverity>(s);
+    case absl::LogSeverityAtMost::kNegativeInfinity:
+      return os << "NEGATIVE_INFINITY";
+  }
+  return os;
+}
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/base/log_severity.h b/absl/base/log_severity.h
index f5dc7d26612..de9702ab684 100644
--- a/absl/base/log_severity.h
+++ b/absl/base/log_severity.h
@@ -12,29 +12,59 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-#ifndef ABSL_BASE_INTERNAL_LOG_SEVERITY_H_
-#define ABSL_BASE_INTERNAL_LOG_SEVERITY_H_
+#ifndef ABSL_BASE_LOG_SEVERITY_H_
+#define ABSL_BASE_LOG_SEVERITY_H_
 
 #include <array>
 #include <ostream>
 
 #include "absl/base/attributes.h"
+#include "absl/base/config.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
-// Four severity levels are defined.  Logging APIs should terminate the program
+// absl::LogSeverity
+//
+// Four severity levels are defined. Logging APIs should terminate the program
 // when a message is logged at severity `kFatal`; the other levels have no
 // special semantics.
 //
-// Abseil flags may be defined with type `LogSeverity`.  Dependency layering
-// constraints require that the `AbslParseFlag` overload be declared and defined
-// in the flags module rather than here.  The `AbslUnparseFlag` overload is
-// defined there too for consistency.
+// Values other than the four defined levels (e.g. produced by `static_cast`)
+// are valid, but their semantics when passed to a function, macro, or flag
+// depend on the function, macro, or flag. The usual behavior is to normalize
+// such values to a defined severity level, however in some cases values other
+// than the defined levels are useful for comparison.
+//
+// Example:
+//
+//   // Effectively disables all logging:
+//   SetMinLogLevel(static_cast<absl::LogSeverity>(100));
+//
+// Abseil flags may be defined with type `LogSeverity`. Dependency layering
+// constraints require that the `AbslParseFlag()` overload be declared and
+// defined in the flags library itself rather than here. The `AbslUnparseFlag()`
+// overload is defined there as well for consistency.
+//
+// absl::LogSeverity Flag String Representation
+//
+// An `absl::LogSeverity` has a string representation used for parsing
+// command-line flags based on the enumerator name (e.g. `kFatal`) or
+// its unprefixed name (without the `k`) in any case-insensitive form. (E.g.
+// "FATAL", "fatal" or "Fatal" are all valid.) Unparsing such flags produces an
+// unprefixed string representation in all caps (e.g. "FATAL") or an integer.
 //
-// The parser accepts arbitrary integers (as if the type were `int`).  It also
-// accepts each named enumerator, without regard for case, with or without the
-// leading 'k'.  For example: "kInfo", "INFO", and "info" all parse to the value
-// `absl::LogSeverity::kInfo`.
+// Additionally, the parser accepts arbitrary integers (as if the type were
+// `int`).
+//
+// Examples:
+//
+//   --my_log_level=kInfo
+//   --my_log_level=INFO
+//   --my_log_level=info
+//   --my_log_level=0
+//
+// `DFATAL` and `kLogDebugFatal` are similarly accepted.
 //
 // Unparsing a flag produces the same result as `absl::LogSeverityName()` for
 // the standard levels and a base-ten integer otherwise.
@@ -45,6 +75,8 @@ enum class LogSeverity : int {
   kFatal = 3,
 };
 
+// LogSeverities()
+//
 // Returns an iterable of all standard `absl::LogSeverity` values, ordered from
 // least to most severe.
 constexpr std::array<absl::LogSeverity, 4> LogSeverities() {
@@ -52,33 +84,102 @@ constexpr std::array<absl::LogSeverity, 4> LogSeverities() {
            absl::LogSeverity::kError, absl::LogSeverity::kFatal}};
 }
 
+// `absl::kLogDebugFatal` equals `absl::LogSeverity::kFatal` in debug builds
+// (i.e. when `NDEBUG` is not defined) and `absl::LogSeverity::kError`
+// otherwise.  Avoid ODR-using this variable as it has internal linkage and thus
+// distinct storage in different TUs.
+#ifdef NDEBUG
+static constexpr absl::LogSeverity kLogDebugFatal = absl::LogSeverity::kError;
+#else
+static constexpr absl::LogSeverity kLogDebugFatal = absl::LogSeverity::kFatal;
+#endif
+
+// LogSeverityName()
+//
 // Returns the all-caps string representation (e.g. "INFO") of the specified
 // severity level if it is one of the standard levels and "UNKNOWN" otherwise.
 constexpr const char* LogSeverityName(absl::LogSeverity s) {
-  return s == absl::LogSeverity::kInfo
-             ? "INFO"
-             : s == absl::LogSeverity::kWarning
-                   ? "WARNING"
-                   : s == absl::LogSeverity::kError
-                         ? "ERROR"
-                         : s == absl::LogSeverity::kFatal ? "FATAL" : "UNKNOWN";
+  switch (s) {
+    case absl::LogSeverity::kInfo: return "INFO";
+    case absl::LogSeverity::kWarning: return "WARNING";
+    case absl::LogSeverity::kError: return "ERROR";
+    case absl::LogSeverity::kFatal: return "FATAL";
+  }
+  return "UNKNOWN";
 }
 
+// NormalizeLogSeverity()
+//
 // Values less than `kInfo` normalize to `kInfo`; values greater than `kFatal`
 // normalize to `kError` (**NOT** `kFatal`).
 constexpr absl::LogSeverity NormalizeLogSeverity(absl::LogSeverity s) {
-  return s < absl::LogSeverity::kInfo
-             ? absl::LogSeverity::kInfo
-             : s > absl::LogSeverity::kFatal ? absl::LogSeverity::kError : s;
+  absl::LogSeverity n = s;
+  if (n < absl::LogSeverity::kInfo) n = absl::LogSeverity::kInfo;
+  if (n > absl::LogSeverity::kFatal) n = absl::LogSeverity::kError;
+  return n;
 }
 constexpr absl::LogSeverity NormalizeLogSeverity(int s) {
   return absl::NormalizeLogSeverity(static_cast<absl::LogSeverity>(s));
 }
 
+// operator<<
+//
 // The exact representation of a streamed `absl::LogSeverity` is deliberately
 // unspecified; do not rely on it.
 std::ostream& operator<<(std::ostream& os, absl::LogSeverity s);
 
+// Enums representing a lower bound for LogSeverity. APIs that only operate on
+// messages of at least a certain level (for example, `SetMinLogLevel()`) use
+// this type to specify that level. absl::LogSeverityAtLeast::kInfinity is
+// a level above all threshold levels and therefore no log message will
+// ever meet this threshold.
+enum class LogSeverityAtLeast : int {
+  kInfo = static_cast<int>(absl::LogSeverity::kInfo),
+  kWarning = static_cast<int>(absl::LogSeverity::kWarning),
+  kError = static_cast<int>(absl::LogSeverity::kError),
+  kFatal = static_cast<int>(absl::LogSeverity::kFatal),
+  kInfinity = 1000,
+};
+
+std::ostream& operator<<(std::ostream& os, absl::LogSeverityAtLeast s);
+
+// Enums representing an upper bound for LogSeverity. APIs that only operate on
+// messages of at most a certain level (for example, buffer all messages at or
+// below a certain level) use this type to specify that level.
+// absl::LogSeverityAtMost::kNegativeInfinity is a level below all threshold
+// levels and therefore will exclude all log messages.
+enum class LogSeverityAtMost : int {
+  kNegativeInfinity = -1000,
+  kInfo = static_cast<int>(absl::LogSeverity::kInfo),
+  kWarning = static_cast<int>(absl::LogSeverity::kWarning),
+  kError = static_cast<int>(absl::LogSeverity::kError),
+  kFatal = static_cast<int>(absl::LogSeverity::kFatal),
+};
+
+std::ostream& operator<<(std::ostream& os, absl::LogSeverityAtMost s);
+
+#define COMPOP(op1, op2, T)                                         \
+  constexpr bool operator op1(absl::T lhs, absl::LogSeverity rhs) { \
+    return static_cast<absl::LogSeverity>(lhs) op1 rhs;             \
+  }                                                                 \
+  constexpr bool operator op2(absl::LogSeverity lhs, absl::T rhs) { \
+    return lhs op2 static_cast<absl::LogSeverity>(rhs);             \
+  }
+
+// Comparisons between `LogSeverity` and `LogSeverityAtLeast`/
+// `LogSeverityAtMost` are only supported in one direction.
+// Valid checks are:
+//   LogSeverity >= LogSeverityAtLeast
+//   LogSeverity < LogSeverityAtLeast
+//   LogSeverity <= LogSeverityAtMost
+//   LogSeverity > LogSeverityAtMost
+COMPOP(>, <, LogSeverityAtLeast)
+COMPOP(<=, >=, LogSeverityAtLeast)
+COMPOP(<, >, LogSeverityAtMost)
+COMPOP(>=, <=, LogSeverityAtMost)
+#undef COMPOP
+
+ABSL_NAMESPACE_END
 }  // namespace absl
 
-#endif  // ABSL_BASE_INTERNAL_LOG_SEVERITY_H_
+#endif  // ABSL_BASE_LOG_SEVERITY_H_
diff --git a/absl/base/log_severity_test.cc b/absl/base/log_severity_test.cc
index 1e3aafa5661..7b92b4fbfca 100644
--- a/absl/base/log_severity_test.cc
+++ b/absl/base/log_severity_test.cc
@@ -24,8 +24,10 @@
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
+#include "absl/flags/internal/flag.h"
 #include "absl/flags/marshalling.h"
 #include "absl/strings/str_cat.h"
+#include "absl/strings/string_view.h"
 
 namespace {
 using ::testing::Eq;
@@ -34,7 +36,8 @@ using ::testing::IsTrue;
 using ::testing::TestWithParam;
 using ::testing::Values;
 
-std::string StreamHelper(absl::LogSeverity value) {
+template <typename T>
+std::string StreamHelper(T value) {
   std::ostringstream stream;
   stream << value;
   return stream.str();
@@ -51,6 +54,10 @@ TEST(StreamTest, Works) {
               Eq("absl::LogSeverity(4)"));
 }
 
+static_assert(absl::flags_internal::FlagUseValueAndInitBitStorage<
+                  absl::LogSeverity>::value,
+              "Flags of type absl::LogSeverity ought to be lock-free.");
+
 using ParseFlagFromOutOfRangeIntegerTest = TestWithParam<int64_t>;
 INSTANTIATE_TEST_SUITE_P(
     Instantiation, ParseFlagFromOutOfRangeIntegerTest,
@@ -140,7 +147,12 @@ INSTANTIATE_TEST_SUITE_P(
            std::make_tuple("fatal", absl::LogSeverity::kFatal),
            std::make_tuple("kFatal", absl::LogSeverity::kFatal),
            std::make_tuple("FaTaL", absl::LogSeverity::kFatal),
-           std::make_tuple("KfAtAl", absl::LogSeverity::kFatal)));
+           std::make_tuple("KfAtAl", absl::LogSeverity::kFatal),
+           std::make_tuple("DFATAL", absl::kLogDebugFatal),
+           std::make_tuple("dfatal", absl::kLogDebugFatal),
+           std::make_tuple("kLogDebugFatal", absl::kLogDebugFatal),
+           std::make_tuple("dFaTaL", absl::kLogDebugFatal),
+           std::make_tuple("kLoGdEbUgFaTaL", absl::kLogDebugFatal)));
 TEST_P(ParseFlagFromEnumeratorTest, YieldsExpectedValue) {
   const absl::string_view to_parse = std::get<0>(GetParam());
   const absl::LogSeverity expected = std::get<1>(GetParam());
@@ -152,7 +164,8 @@ TEST_P(ParseFlagFromEnumeratorTest, YieldsExpectedValue) {
 
 using ParseFlagFromGarbageTest = TestWithParam<absl::string_view>;
 INSTANTIATE_TEST_SUITE_P(Instantiation, ParseFlagFromGarbageTest,
-                         Values("", "\0", " ", "garbage", "kkinfo", "I"));
+                         Values("", "\0", " ", "garbage", "kkinfo", "I",
+                                "kDFATAL", "LogDebugFatal", "lOgDeBuGfAtAl"));
 TEST_P(ParseFlagFromGarbageTest, ReturnsError) {
   const absl::string_view to_parse = GetParam();
   absl::LogSeverity value;
@@ -196,4 +209,44 @@ TEST_P(UnparseFlagToOtherIntegerTest, ReturnsExpectedValueAndRoundTrips) {
               IsTrue());
   EXPECT_THAT(reparsed_value, Eq(to_unparse));
 }
+
+TEST(LogThresholdTest, LogSeverityAtLeastTest) {
+  EXPECT_LT(absl::LogSeverity::kError, absl::LogSeverityAtLeast::kFatal);
+  EXPECT_GT(absl::LogSeverityAtLeast::kError, absl::LogSeverity::kInfo);
+
+  EXPECT_LE(absl::LogSeverityAtLeast::kInfo, absl::LogSeverity::kError);
+  EXPECT_GE(absl::LogSeverity::kError, absl::LogSeverityAtLeast::kInfo);
+}
+
+TEST(LogThresholdTest, LogSeverityAtMostTest) {
+  EXPECT_GT(absl::LogSeverity::kError, absl::LogSeverityAtMost::kWarning);
+  EXPECT_LT(absl::LogSeverityAtMost::kError, absl::LogSeverity::kFatal);
+
+  EXPECT_GE(absl::LogSeverityAtMost::kFatal, absl::LogSeverity::kError);
+  EXPECT_LE(absl::LogSeverity::kWarning, absl::LogSeverityAtMost::kError);
+}
+
+TEST(LogThresholdTest, Extremes) {
+  EXPECT_LT(absl::LogSeverity::kFatal, absl::LogSeverityAtLeast::kInfinity);
+  EXPECT_GT(absl::LogSeverity::kInfo,
+            absl::LogSeverityAtMost::kNegativeInfinity);
+}
+
+TEST(LogThresholdTest, Output) {
+  EXPECT_THAT(StreamHelper(absl::LogSeverityAtLeast::kInfo), Eq(">=INFO"));
+  EXPECT_THAT(StreamHelper(absl::LogSeverityAtLeast::kWarning),
+              Eq(">=WARNING"));
+  EXPECT_THAT(StreamHelper(absl::LogSeverityAtLeast::kError), Eq(">=ERROR"));
+  EXPECT_THAT(StreamHelper(absl::LogSeverityAtLeast::kFatal), Eq(">=FATAL"));
+  EXPECT_THAT(StreamHelper(absl::LogSeverityAtLeast::kInfinity),
+              Eq("INFINITY"));
+
+  EXPECT_THAT(StreamHelper(absl::LogSeverityAtMost::kInfo), Eq("<=INFO"));
+  EXPECT_THAT(StreamHelper(absl::LogSeverityAtMost::kWarning), Eq("<=WARNING"));
+  EXPECT_THAT(StreamHelper(absl::LogSeverityAtMost::kError), Eq("<=ERROR"));
+  EXPECT_THAT(StreamHelper(absl::LogSeverityAtMost::kFatal), Eq("<=FATAL"));
+  EXPECT_THAT(StreamHelper(absl::LogSeverityAtMost::kNegativeInfinity),
+              Eq("NEGATIVE_INFINITY"));
+}
+
 }  // namespace
diff --git a/absl/base/macros.h b/absl/base/macros.h
index d481288f35f..0d7a0486f8a 100644
--- a/absl/base/macros.h
+++ b/absl/base/macros.h
@@ -28,10 +28,14 @@
 #ifndef ABSL_BASE_MACROS_H_
 #define ABSL_BASE_MACROS_H_
 
+#include <atomic>
 #include <cassert>
 #include <cstddef>
 
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
 #include "absl/base/optimization.h"
+#include "absl/base/options.h"
 #include "absl/base/port.h"
 
 // ABSL_ARRAYSIZE()
@@ -39,123 +43,52 @@
 // Returns the number of elements in an array as a compile-time constant, which
 // can be used in defining new arrays. If you use this macro on a pointer by
 // mistake, you will get a compile-time error.
+//
+// NOTE: Avoid using this macro. Instead, use std::size(a) if possible, or
+// std::extent_v<decltype(a)> otherwise.
 #define ABSL_ARRAYSIZE(array) \
   (sizeof(::absl::macros_internal::ArraySizeHelper(array)))
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace macros_internal {
 // Note: this internal template function declaration is used by ABSL_ARRAYSIZE.
 // The function doesn't need a definition, as we only use its type.
 template <typename T, size_t N>
 auto ArraySizeHelper(const T (&array)[N]) -> char (&)[N];
 }  // namespace macros_internal
-}  // namespace absl
 
-// kLinkerInitialized
-//
-// An enum used only as a constructor argument to indicate that a variable has
-// static storage duration, and that the constructor should do nothing to its
-// state. Use of this macro indicates to the reader that it is legal to
-// declare a static instance of the class, provided the constructor is given
-// the absl::base_internal::kLinkerInitialized argument.
-//
-// Normally, it is unsafe to declare a static variable that has a constructor or
-// a destructor because invocation order is undefined. However, if the type can
-// be zero-initialized (which the loader does for static variables) into a valid
-// state and the type's destructor does not affect storage, then a constructor
-// for static initialization can be declared.
-//
-// Example:
-//       // Declaration
-//       explicit MyClass(absl::base_internal:LinkerInitialized x) {}
-//
-//       // Invocation
-//       static MyClass my_global(absl::base_internal::kLinkerInitialized);
-namespace absl {
 namespace base_internal {
-enum LinkerInitialized {
-  kLinkerInitialized = 0,
-};
-}  // namespace base_internal
-}  // namespace absl
-
-// ABSL_FALLTHROUGH_INTENDED
-//
-// Annotates implicit fall-through between switch labels, allowing a case to
-// indicate intentional fallthrough and turn off warnings about any lack of a
-// `break` statement. The ABSL_FALLTHROUGH_INTENDED macro should be followed by
-// a semicolon and can be used in most places where `break` can, provided that
-// no statements exist between it and the next switch label.
-//
-// Example:
-//
-//  switch (x) {
-//    case 40:
-//    case 41:
-//      if (truth_is_out_there) {
-//        ++x;
-//        ABSL_FALLTHROUGH_INTENDED;  // Use instead of/along with annotations
-//                                    // in comments
-//      } else {
-//        return x;
-//      }
-//    case 42:
-//      ...
-//
-// Notes: when compiled with clang in C++11 mode, the ABSL_FALLTHROUGH_INTENDED
-// macro is expanded to the [[clang::fallthrough]] attribute, which is analysed
-// when  performing switch labels fall-through diagnostic
-// (`-Wimplicit-fallthrough`). See clang documentation on language extensions
-// for details:
-// http://clang.llvm.org/docs/AttributeReference.html#fallthrough-clang-fallthrough
-//
-// When used with unsupported compilers, the ABSL_FALLTHROUGH_INTENDED macro
-// has no effect on diagnostics. In any case this macro has no effect on runtime
-// behavior and performance of code.
-#ifdef ABSL_FALLTHROUGH_INTENDED
-#error "ABSL_FALLTHROUGH_INTENDED should not be defined."
+#if ABSL_HAVE_CPP_ATTRIBUTE(clang::nomerge)
+[[clang::nomerge]]  // Needed when this function is not inlined
 #endif
-
-// TODO(zhangxy): Use c++17 standard [[fallthrough]] macro, when supported.
-#if defined(__clang__) && defined(__has_warning)
-#if __has_feature(cxx_attributes) && __has_warning("-Wimplicit-fallthrough")
-#define ABSL_FALLTHROUGH_INTENDED [[clang::fallthrough]]
-#endif
-#elif defined(__GNUC__) && __GNUC__ >= 7
-#define ABSL_FALLTHROUGH_INTENDED [[gnu::fallthrough]]
-#endif
-
-#ifndef ABSL_FALLTHROUGH_INTENDED
-#define ABSL_FALLTHROUGH_INTENDED \
-  do {                            \
-  } while (0)
+[[noreturn]] inline void HardeningAbort() {
+#if ABSL_HAVE_CPP_ATTRIBUTE(clang::nomerge)
+  [[clang::nomerge]]  // Needed when this function is inlined
 #endif
+  ABSL_INTERNAL_IMMEDIATE_ABORT_IMPL();
+  ABSL_INTERNAL_UNREACHABLE_IMPL();
+}
+}  // namespace base_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
 
-// ABSL_DEPRECATED()
-//
-// Marks a deprecated class, struct, enum, function, method and variable
-// declarations. The macro argument is used as a custom diagnostic message (e.g.
-// suggestion of a better alternative).
-//
-// Examples:
+// ABSL_INTERNAL_UNEVALUATED()
 //
-//   class ABSL_DEPRECATED("Use Bar instead") Foo {...};
-//
-//   ABSL_DEPRECATED("Use Baz() instead") void Bar() {...}
-//
-//   template <typename T>
-//   ABSL_DEPRECATED("Use DoThat() instead")
-//   void DoThis();
-//
-// Every usage of a deprecated entity will trigger a warning when compiled with
-// clang's `-Wdeprecated-declarations` option. This option is turned off by
-// default, but the warnings will be reported by clang-tidy.
-#if defined(__clang__) && __cplusplus >= 201103L
-#define ABSL_DEPRECATED(message) __attribute__((deprecated(message)))
-#endif
-
-#ifndef ABSL_DEPRECATED
-#define ABSL_DEPRECATED(message)
+// Expands into a no-op expression that contains the given expression. Used to
+// avoid unused-variable warnings in configurations that don't need to evaluate
+// the given expression (e.g., NDEBUG).
+#if ABSL_INTERNAL_CPLUSPLUS_LANG >= 202002L
+// We use `decltype` here to avoid generating unnecessary code that the
+// optimizer then has to optimize away.
+// This not only improves compilation performance by reducing codegen bloat
+// and optimization work, but also guarantees fast run-time performance without
+// having to rely on the optimizer.
+#define ABSL_INTERNAL_UNEVALUATED(expr) (decltype((void)(expr))())
+#else
+// Pre-C++20, lambdas can't be inside unevaluated operands, so we're forced to
+// rely on the optimizer.
+#define ABSL_INTERNAL_UNEVALUATED(expr) (false ? (void)(expr) : void())
 #endif
 
 // ABSL_BAD_CALL_IF()
@@ -163,7 +96,7 @@ enum LinkerInitialized {
 // Used on a function overload to trap bad calls: any call that matches the
 // overload will cause a compile-time error. This macro uses a clang-specific
 // "enable_if" attribute, as described at
-// http://clang.llvm.org/docs/AttributeReference.html#enable-if
+// https://clang.llvm.org/docs/AttributeReference.html#enable-if
 //
 // Overloads which use this macro should be bracketed by
 // `#ifdef ABSL_BAD_CALL_IF`.
@@ -176,19 +109,16 @@ enum LinkerInitialized {
 //     ABSL_BAD_CALL_IF(c <= -1 || c > 255,
 //                       "'c' must have the value of an unsigned char or EOF");
 //   #endif // ABSL_BAD_CALL_IF
-
-#if defined(__clang__)
-# if __has_attribute(enable_if)
-#  define ABSL_BAD_CALL_IF(expr, msg) \
-    __attribute__((enable_if(expr, "Bad call trap"), unavailable(msg)))
-# endif
+#if ABSL_HAVE_ATTRIBUTE(enable_if)
+#define ABSL_BAD_CALL_IF(expr, msg) \
+  __attribute__((enable_if(expr, "Bad call trap"), unavailable(msg)))
 #endif
 
 // ABSL_ASSERT()
 //
-// In C++11, `assert` can't be used portably within constexpr functions.
-// ABSL_ASSERT functions as a runtime assert but works in C++11 constexpr
-// functions.  Example:
+// `assert` generates spurious unused-symbol warnings when NDEBUG is defined.
+// ABSL_ASSERT functions as a runtime assert but maintains references to
+// symbols even under NDEBUG.  Example:
 //
 // constexpr double Divide(double a, double b) {
 //   return ABSL_ASSERT(b != 0), a / b;
@@ -197,12 +127,64 @@ enum LinkerInitialized {
 // This macro is inspired by
 // https://akrzemi1.wordpress.com/2017/05/18/asserts-in-constexpr-functions/
 #if defined(NDEBUG)
-#define ABSL_ASSERT(expr) \
-  (false ? static_cast<void>(expr) : static_cast<void>(0))
+#define ABSL_ASSERT(expr) ABSL_INTERNAL_UNEVALUATED((expr) ? void() : void())
 #else
 #define ABSL_ASSERT(expr)                           \
   (ABSL_PREDICT_TRUE((expr)) ? static_cast<void>(0) \
-                             : [] { assert(false && #expr); }())  // NOLINT
+                             : assert(false && #expr))  // NOLINT
+#endif
+
+// `ABSL_INTERNAL_HARDENING_ABORT()` controls how `ABSL_HARDENING_ASSERT()`
+// aborts the program in release mode (when NDEBUG is defined). The
+// implementation should abort the program as quickly as possible and ideally it
+// should not be possible to ignore the abort request.
+#if defined(__CUDACC__) || defined(__CUDA_ARCH__) || defined(__CUDA__)
+#define ABSL_INTERNAL_HARDENING_ABORT()   \
+  do {                                    \
+    ABSL_INTERNAL_IMMEDIATE_ABORT_IMPL(); \
+    ABSL_INTERNAL_UNREACHABLE_IMPL();     \
+  } while (false)
+#else
+#define ABSL_INTERNAL_HARDENING_ABORT() ::absl::base_internal::HardeningAbort()
+#endif
+
+// ABSL_HARDENING_ASSERT()
+//
+// `ABSL_HARDENING_ASSERT()` is like `ABSL_ASSERT()`, but used to implement
+// runtime assertions that should be enabled in hardened builds even when
+// `NDEBUG` is defined.
+//
+// When `NDEBUG` is not defined, `ABSL_HARDENING_ASSERT()` is identical to
+// `ABSL_ASSERT()`.
+//
+// See `ABSL_OPTION_HARDENED` in `absl/base/options.h` for more information on
+// hardened mode.
+#if (ABSL_OPTION_HARDENED == 1 || ABSL_OPTION_HARDENED == 2) && defined(NDEBUG)
+#define ABSL_HARDENING_ASSERT(expr)    \
+  do {                                 \
+    if (!ABSL_PREDICT_TRUE((expr))) {  \
+      ABSL_INTERNAL_HARDENING_ABORT(); \
+    }                                  \
+  } while (false)
+#else
+#define ABSL_HARDENING_ASSERT(expr) ABSL_ASSERT(expr)
+#endif
+
+// ABSL_HARDENING_ASSERT_SLOW()
+//
+// `ABSL_HARDENING_ASSERT()` is like `ABSL_HARDENING_ASSERT()`,
+//  but specifically for assertions whose predicates are too slow
+//  to be enabled in many applications.
+//
+// When `NDEBUG` is not defined, `ABSL_HARDENING_ASSERT_SLOW()` is identical to
+// `ABSL_ASSERT()`.
+//
+// See `ABSL_OPTION_HARDENED` in `absl/base/options.h` for more information on
+// hardened mode.
+#if ABSL_OPTION_HARDENED == 1 && defined(NDEBUG)
+#define ABSL_HARDENING_ASSERT_SLOW(expr) ABSL_HARDENING_ASSERT(expr)
+#else
+#define ABSL_HARDENING_ASSERT_SLOW(expr) ABSL_ASSERT(expr)
 #endif
 
 #ifdef ABSL_HAVE_EXCEPTIONS
@@ -215,4 +197,92 @@ enum LinkerInitialized {
 #define ABSL_INTERNAL_RETHROW do {} while (false)
 #endif  // ABSL_HAVE_EXCEPTIONS
 
+// ABSL_REFACTOR_INLINE
+//
+// Marks a function or type for automated refactoring by go/cpp-inliner. It can
+// be used on inline function definitions or type aliases in header files and
+// should be combined with the `[[deprecated]]` attribute.
+//
+// Using `ABSL_REFACTOR_INLINE` differs from using the `[[deprecated]]` alone in
+// the following ways:
+//
+// 1. New uses of the function or type will be discouraged via Tricorder
+//    warnings.
+// 2. If enabled via `METADATA`, automated changes will be sent out inlining the
+//    functions's body or replacing the type where it is used.
+//
+// Examples:
+//
+// [[deprecated("Use NewFunc() instead")]] ABSL_REFACTOR_INLINE
+// inline int OldFunc(int x) {
+//   return NewFunc(x, 0);
+// }
+//
+// using OldType [[deprecated("Use NewType instead")]] ABSL_REFACTOR_INLINE =
+//     NewType;
+//
+// will mark `OldFunc` and `OldType` as deprecated, and the go/cpp-inliner
+// service will replace calls to `OldFunc(x)` with calls to `NewFunc(x, 0)` and
+// `OldType` with `NewType`. Once all replacements have been completed, the old
+// function or type can be deleted.
+//
+// Internal note: Clang also allows `ABSL_REFACTOR_INLINE` to be used on
+// using-declarations, but attributes on using-declarations are invalid in C++.
+// (NOTE: This note refers to `using a::b ABSL_REFACTOR_INLINE;` and not
+// `using b ABSL_REFACTOR_INLINE = a::b;`, which is OK.) Therefore:
+//
+// 1. In OSS: Do not use this on using-declarations. Such usage is invalid and
+//    unsupported usage, and may break at any time.
+// 2. In Google: Avoid such usage except as a last resort. Instead, prefer other
+//    inlining approaches (such as type aliases or forwarding functions,
+//    illustrated above) whenever possible. This is because Clang (currently)
+//    does not honor the [[deprecated]] attribute on using-declarations, and
+//    therefore cannot surface the deprecation to users in the middle of a
+//    migration.
+//
+// See go/cpp-inliner for more information.
+//
+// Note: go/cpp-inliner is Google-internal service for automated refactoring.
+// While open-source users do not have access to this service, the macro is
+// provided for compatibility.
+#if ABSL_HAVE_CPP_ATTRIBUTE(clang::annotate) && !defined(__NVCC__)
+#define ABSL_REFACTOR_INLINE                                                \
+  _Pragma("clang diagnostic push") /* Avoid errors on using-declarations */ \
+      _Pragma("clang diagnostic ignored \"-Wcxx-attribute-extension\"")     \
+          [[clang::annotate("inline-me")]] _Pragma("clang diagnostic pop")
+#else
+#define ABSL_REFACTOR_INLINE
+#endif
+
+// ABSL_DEPRECATE_AND_INLINE()
+//
+// This is the original macro used by go/cpp-inliner that combines
+// [[deprecated]] and ABSL_REFACTOR_INLINE.
+//
+// Examples:
+//
+// ABSL_DEPRECATE_AND_INLINE() inline int OldFunc(int x) {
+//   return NewFunc(x, 0);
+// }
+//
+// using OldType ABSL_DEPRECATE_AND_INLINE() = NewType;
+//
+// The combination of `[[deprecated("Use X instead")]]` and
+// `ABSL_REFACTOR_INLINE` is preferred because it provides a more informative
+// deprecation message to developers, especially those that do not have access
+// to the automated refactoring capabilities of go/cpp-inliner.
+#define ABSL_DEPRECATE_AND_INLINE() [[deprecated]] ABSL_REFACTOR_INLINE
+
+// Requires the compiler to prove that the size of the given object is at least
+// the expected amount.
+#if ABSL_HAVE_ATTRIBUTE(diagnose_if) && ABSL_HAVE_BUILTIN(__builtin_object_size)
+#define ABSL_INTERNAL_NEED_MIN_SIZE(Obj, N)                     \
+  __attribute__((diagnose_if(__builtin_object_size(Obj, 0) < N, \
+                             "object size provably too small "  \
+                             "(this would corrupt memory)",     \
+                             "error")))
+#else
+#define ABSL_INTERNAL_NEED_MIN_SIZE(Obj, N)
+#endif
+
 #endif  // ABSL_BASE_MACROS_H_
diff --git a/absl/base/no_destructor.h b/absl/base/no_destructor.h
new file mode 100644
index 00000000000..a0e990e025b
--- /dev/null
+++ b/absl/base/no_destructor.h
@@ -0,0 +1,191 @@
+// Copyright 2023 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: no_destructor.h
+// -----------------------------------------------------------------------------
+//
+// This header file defines the absl::NoDestructor<T> wrapper for defining a
+// static type that does not need to be destructed upon program exit. Instead,
+// such an object survives during program exit (and can be safely accessed at
+// any time).
+//
+// absl::NoDestructor<T> is useful when when a variable has static storage
+// duration but its type has a non-trivial destructor. Global constructors are
+// not recommended because of the C++'s static initialization order fiasco (See
+// https://en.cppreference.com/w/cpp/language/siof). Global destructors are not
+// allowed due to similar concerns about destruction ordering. Using
+// absl::NoDestructor<T> as a function-local static prevents both of these
+// issues.
+//
+// See below for complete details.
+
+
+#ifndef ABSL_BASE_NO_DESTRUCTOR_H_
+#define ABSL_BASE_NO_DESTRUCTOR_H_
+
+#include <new>
+#include <type_traits>
+#include <utility>
+
+#include "absl/base/config.h"
+#include "absl/base/nullability.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+// absl::NoDestructor<T>
+//
+// NoDestructor<T> is a wrapper around an object of type T that behaves as an
+// object of type T but never calls T's destructor. NoDestructor<T> makes it
+// safer and/or more efficient to use such objects in static storage contexts,
+// ideally as function scope static variables.
+//
+// An instance of absl::NoDestructor<T> has similar type semantics to an
+// instance of T:
+//
+// * Constructs in the same manner as an object of type T through perfect
+//   forwarding.
+// * Provides pointer/reference semantic access to the object of type T via
+//   `->`, `*`, and `get()`.
+//   (Note that `const NoDestructor<T>` works like a pointer to const `T`.)
+//
+// Additionally, NoDestructor<T> provides the following benefits:
+//
+// * Never calls T's destructor for the object
+// * If the object is a function-local static variable, the type can be
+//   lazily constructed.
+//
+// An object of type NoDestructor<T> is "trivially destructible" in the notion
+// that its destructor is never run.
+//
+// Usage as Function Scope Static Variables
+//
+// Function static objects will be lazily initialized within static storage:
+//
+//    // Function scope.
+//    const std::string& MyString() {
+//      static const absl::NoDestructor<std::string> x("foo");
+//      return *x;
+//    }
+//
+// For function static variables, NoDestructor avoids heap allocation and can be
+// inlined in static storage, resulting in exactly-once, thread-safe
+// construction of an object, and very fast access thereafter (the cost is a few
+// extra cycles).
+//
+// Using NoDestructor<T> in this manner is generally better than other patterns
+// which require pointer chasing:
+//
+//   // Prefer using absl::NoDestructor<T> instead for the static variable.
+//   const std::string& MyString() {
+//     static const std::string* x = new std::string("foo");
+//     return *x;
+//   }
+//
+// Usage as Global Static Variables
+//
+// NoDestructor<T> allows declaration of a global object of type T that has a
+// non-trivial destructor since its destructor is never run. However, such
+// objects still need to worry about initialization order, so such use is not
+// recommended, strongly discouraged by the Google C++ Style Guide, and outright
+// banned in Chromium.
+// See https://google.github.io/styleguide/cppguide.html#Static_and_Global_Variables
+//
+//    // Global or namespace scope.
+//    absl::NoDestructor<MyRegistry> reg{"foo", "bar", 8008};
+//
+// Note that if your object already has a trivial destructor, you don't need to
+// use NoDestructor<T>.
+//
+template <typename T>
+class NoDestructor {
+ public:
+  // Forwards arguments to the T's constructor: calls T(args...).
+  template <typename... Ts,
+            // Disable this overload when it might collide with copy/move.
+            std::enable_if_t<!std::is_same_v<void(std::decay_t<Ts>&...),
+                                             void(NoDestructor&)>,
+                             int> = 0>
+  explicit constexpr NoDestructor(Ts&&... args)
+      : impl_(std::forward<Ts>(args)...) {}
+
+  // Forwards copy and move construction for T. Enables usage like this:
+  //   static NoDestructor<std::array<string, 3>> x{{{"1", "2", "3"}}};
+  //   static NoDestructor<std::vector<int>> x{{1, 2, 3}};
+  explicit constexpr NoDestructor(const T& x) : impl_(x) {}
+  explicit constexpr NoDestructor(T&& x)
+      : impl_(std::move(x)) {}
+
+  // No copying.
+  NoDestructor(const NoDestructor&) = delete;
+  NoDestructor& operator=(const NoDestructor&) = delete;
+
+  // Pretend to be a smart pointer to T with deep constness.
+  // Never returns a null pointer.
+  T& operator*() { return *get(); }
+  T* absl_nonnull operator->() { return get(); }
+  T* absl_nonnull get() { return impl_.get(); }
+  const T& operator*() const { return *get(); }
+  const T* absl_nonnull operator->() const { return get(); }
+  const T* absl_nonnull get() const { return impl_.get(); }
+
+ private:
+  class DirectImpl {
+   public:
+    template <typename... Args>
+    explicit constexpr DirectImpl(Args&&... args)
+        : value_(std::forward<Args>(args)...) {}
+    const T* absl_nonnull get() const { return &value_; }
+    T* absl_nonnull get() { return &value_; }
+
+   private:
+    T value_;
+  };
+
+  class PlacementImpl {
+   public:
+    template <typename... Args>
+    explicit PlacementImpl(Args&&... args) {
+      new (&space_) T(std::forward<Args>(args)...);
+    }
+    const T* absl_nonnull get() const {
+      return std::launder(reinterpret_cast<const T*>(&space_));
+    }
+    T* absl_nonnull get() {
+      return std::launder(reinterpret_cast<T*>(&space_));
+    }
+
+   private:
+    alignas(T) unsigned char space_[sizeof(T)];
+  };
+
+  // If the object is trivially destructible we use a member directly to avoid
+  // potential once-init runtime initialization. It somewhat defeats the
+  // purpose of NoDestructor in this case, but this makes the class more
+  // friendly to generic code.
+  std::conditional_t<std::is_trivially_destructible_v<T>, DirectImpl,
+                     PlacementImpl>
+      impl_;
+};
+
+// Provide 'Class Template Argument Deduction': the type of NoDestructor's T
+// will be the same type as the argument passed to NoDestructor's constructor.
+template <typename T>
+NoDestructor(T) -> NoDestructor<T>;
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_BASE_NO_DESTRUCTOR_H_
diff --git a/absl/base/no_destructor_benchmark.cc b/absl/base/no_destructor_benchmark.cc
new file mode 100644
index 00000000000..5fc88f1dfe8
--- /dev/null
+++ b/absl/base/no_destructor_benchmark.cc
@@ -0,0 +1,165 @@
+// Copyright 2023 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <cstdint>
+
+#include "absl/base/internal/raw_logging.h"
+#include "absl/base/no_destructor.h"
+#include "benchmark/benchmark.h"
+
+namespace {
+
+// Number of static-NoDestructor-in-a-function to exercise.
+// This must be low enough not to hit template instantiation limits
+// (happens around 1000).
+constexpr int kNumObjects = 1;  // set to 512 when doing benchmarks
+                                // 1 is faster to compile: just one templated
+                                // function instantiation
+
+// Size of individual objects to benchmark static-NoDestructor-in-a-function
+// usage with.
+constexpr int kObjSize = sizeof(void*)*1;
+
+// Simple object of kObjSize bytes (rounded to int).
+// We benchmark complete reading of its state via Verify().
+class BM_Blob {
+ public:
+  BM_Blob(int val) { for (auto& d : data_) d = val; }
+  BM_Blob() : BM_Blob(-1) {}
+  void Verify(int val) const {  // val must be the c-tor argument
+    for (auto& d : data_) ABSL_INTERNAL_CHECK(d == val, "");
+  }
+ private:
+  int data_[kObjSize / sizeof(int) > 0 ? kObjSize / sizeof(int) : 1];
+};
+
+// static-NoDestructor-in-a-function pattern instances.
+// We'll instantiate kNumObjects of them.
+template<int i>
+const BM_Blob& NoDestrBlobFunc() {
+  static absl::NoDestructor<BM_Blob> x(i);
+  return *x;
+}
+
+// static-heap-ptr-in-a-function pattern instances
+// We'll instantiate kNumObjects of them.
+template<int i>
+const BM_Blob& OnHeapBlobFunc() {
+  static BM_Blob* x = new BM_Blob(i);
+  return *x;
+}
+
+// Type for NoDestrBlobFunc or OnHeapBlobFunc.
+typedef const BM_Blob& (*FuncType)();
+
+// ========================================================================= //
+// Simple benchmarks that read a single BM_Blob over and over, hence
+// all they touch fits into L1 CPU cache:
+
+// Direct non-POD global variable (style guide violation) as a baseline.
+static BM_Blob direct_blob(0);
+
+void BM_Direct(benchmark::State& state) {
+  for (auto s : state) {
+    direct_blob.Verify(0);
+  }
+}
+BENCHMARK(BM_Direct);
+
+void BM_NoDestr(benchmark::State& state) {
+  for (auto s : state) {
+    NoDestrBlobFunc<0>().Verify(0);
+  }
+}
+BENCHMARK(BM_NoDestr);
+
+void BM_OnHeap(benchmark::State& state) {
+  for (auto s : state) {
+    OnHeapBlobFunc<0>().Verify(0);
+  }
+}
+BENCHMARK(BM_OnHeap);
+
+// ========================================================================= //
+// Benchmarks that read kNumObjects of BM_Blob over and over, hence with
+// appropriate values of sizeof(BM_Blob) and kNumObjects their working set
+// can exceed a given layer of CPU cache.
+
+// Type of benchmark to select between NoDestrBlobFunc and OnHeapBlobFunc.
+enum BM_Type { kNoDestr, kOnHeap, kDirect };
+
+// BlobFunc<n>(t, i) returns the i-th function of type t.
+// n must be larger than i (we'll use kNumObjects for n).
+template<int n>
+FuncType BlobFunc(BM_Type t, int i) {
+  if (i == n) {
+    switch (t) {
+      case kNoDestr:  return &NoDestrBlobFunc<n>;
+      case kOnHeap:   return &OnHeapBlobFunc<n>;
+      case kDirect:   return nullptr;
+    }
+  }
+  return BlobFunc<n-1>(t, i);
+}
+
+template<>
+FuncType BlobFunc<0>(BM_Type t, int i) {
+  ABSL_INTERNAL_CHECK(i == 0, "");
+  switch (t) {
+    case kNoDestr:  return &NoDestrBlobFunc<0>;
+    case kOnHeap:   return &OnHeapBlobFunc<0>;
+    case kDirect:   return nullptr;
+  }
+  return nullptr;
+}
+
+// Direct non-POD global variables (style guide violation) as a baseline.
+static BM_Blob direct_blobs[kNumObjects];
+
+// Helper that cheaply maps benchmark iteration to randomish index in
+// [0, kNumObjects).
+int RandIdx(int i) {
+  // int64 is to avoid overflow and generating negative return values:
+  return (static_cast<int64_t>(i) * 13) % kNumObjects;
+}
+
+// Generic benchmark working with kNumObjects for any of the possible BM_Type.
+template <BM_Type t>
+void BM_Many(benchmark::State& state) {
+  FuncType funcs[kNumObjects];
+  for (int i = 0; i < kNumObjects; ++i) {
+    funcs[i] = BlobFunc<kNumObjects-1>(t, i);
+  }
+  if (t == kDirect) {
+    for (auto s : state) {
+      int idx = RandIdx(state.iterations());
+      direct_blobs[idx].Verify(-1);
+    }
+  } else {
+    for (auto s : state) {
+      int idx = RandIdx(state.iterations());
+      funcs[idx]().Verify(idx);
+    }
+  }
+}
+
+void BM_DirectMany(benchmark::State& state) { BM_Many<kDirect>(state); }
+void BM_NoDestrMany(benchmark::State& state) { BM_Many<kNoDestr>(state); }
+void BM_OnHeapMany(benchmark::State& state) { BM_Many<kOnHeap>(state); }
+
+BENCHMARK(BM_DirectMany);
+BENCHMARK(BM_NoDestrMany);
+BENCHMARK(BM_OnHeapMany);
+
+}  // namespace
diff --git a/absl/base/no_destructor_test.cc b/absl/base/no_destructor_test.cc
new file mode 100644
index 00000000000..161d86faeaf
--- /dev/null
+++ b/absl/base/no_destructor_test.cc
@@ -0,0 +1,202 @@
+// Copyright 2023 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/base/no_destructor.h"
+
+#include <array>
+#include <initializer_list>
+#include <string>
+#include <type_traits>
+#include <vector>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/raw_logging.h"
+
+namespace {
+
+struct Blob {
+  Blob() : val(42) {}
+  Blob(int x, int y) : val(x + y) {}
+  Blob(std::initializer_list<int> xs) {
+    val = 0;
+    for (auto& x : xs) val += x;
+  }
+
+  Blob(const Blob& /*b*/) = delete;
+  Blob(Blob&& b) noexcept : val(b.val) {
+    b.moved_out = true;
+  }  // moving is fine
+
+  // no crash: NoDestructor indeed does not destruct (the moved-out Blob
+  // temporaries do get destroyed though)
+  ~Blob() { ABSL_INTERNAL_CHECK(moved_out, "~Blob"); }
+
+  int val;
+  bool moved_out = false;
+};
+
+struct TypeWithDeletedDestructor {
+  ~TypeWithDeletedDestructor() = delete;
+};
+
+TEST(NoDestructorTest, DestructorNeverCalled) {
+  absl::NoDestructor<TypeWithDeletedDestructor> a;
+  (void)a;
+}
+
+TEST(NoDestructorTest, Noncopyable) {
+  using T = absl::NoDestructor<int>;
+
+  EXPECT_FALSE((std::is_constructible_v<T, T>));
+  EXPECT_FALSE((std::is_constructible_v<T, const T>));
+  EXPECT_FALSE((std::is_constructible_v<T, T&>));
+  EXPECT_FALSE((std::is_constructible_v<T, const T&>));
+
+  EXPECT_FALSE((std::is_assignable_v<T&, T>));
+  EXPECT_FALSE((std::is_assignable_v<T&, const T>));
+  EXPECT_FALSE((std::is_assignable_v<T&, T&>));
+  EXPECT_FALSE((std::is_assignable_v<T&, const T&>));
+}
+
+TEST(NoDestructorTest, Interface) {
+  EXPECT_TRUE(std::is_trivially_destructible_v<absl::NoDestructor<Blob>>);
+  EXPECT_TRUE(std::is_trivially_destructible_v<absl::NoDestructor<const Blob>>);
+  {
+    absl::NoDestructor<Blob> b;  // default c-tor
+    // access: *, ->, get()
+    EXPECT_EQ(42, (*b).val);
+    (*b).val = 55;
+    EXPECT_EQ(55, b->val);
+    b->val = 66;
+    EXPECT_EQ(66, b.get()->val);
+    b.get()->val = 42;  // NOLINT
+    EXPECT_EQ(42, (*b).val);
+  }
+  {
+    absl::NoDestructor<const Blob> b(70, 7);  // regular c-tor, const
+    EXPECT_EQ(77, (*b).val);
+    EXPECT_EQ(77, b->val);
+    EXPECT_EQ(77, b.get()->val);
+  }
+  {
+    const absl::NoDestructor<Blob> b{
+        {20, 28, 40}};  // init-list c-tor, deep const
+    // This only works in clang, not in gcc:
+    // const absl::NoDestructor<Blob> b({20, 28, 40});
+    EXPECT_EQ(88, (*b).val);
+    EXPECT_EQ(88, b->val);
+    EXPECT_EQ(88, b.get()->val);
+  }
+}
+
+TEST(NoDestructorTest, SfinaeRegressionAbstractArg) {
+  struct Abstract {
+    virtual ~Abstract() = default;
+    virtual int foo() const = 0;
+  };
+
+  struct Concrete : Abstract {
+    int foo() const override { return 17; }
+  };
+
+  struct UsesAbstractInConstructor {
+    explicit UsesAbstractInConstructor(const Abstract& abstract)
+        : i(abstract.foo()) {}
+    int i;
+  };
+
+  Concrete input;
+  absl::NoDestructor<UsesAbstractInConstructor> foo1(input);
+  EXPECT_EQ(foo1->i, 17);
+  absl::NoDestructor<UsesAbstractInConstructor> foo2(
+      static_cast<const Abstract&>(input));
+  EXPECT_EQ(foo2->i, 17);
+}
+
+// ========================================================================= //
+
+std::string* Str0() {
+  static absl::NoDestructor<std::string> x;
+  return x.get();
+}
+
+extern const std::string& Str2();
+
+const char* Str1() {
+  static absl::NoDestructor<std::string> x(Str2() + "_Str1");
+  return x->c_str();
+}
+
+const std::string& Str2() {
+  static absl::NoDestructor<std::string> x("Str2");
+  return *x;
+}
+
+const std::string& Str2Copy() {
+  // Exercise copy construction
+  static absl::NoDestructor<std::string> x(Str2());
+  return *x;
+}
+
+typedef std::array<std::string, 3> MyArray;
+const MyArray& Array() {
+  static absl::NoDestructor<MyArray> x{{{"foo", "bar", "baz"}}};
+  // This only works in clang, not in gcc:
+  // static absl::NoDestructor<MyArray> x({{"foo", "bar", "baz"}});
+  return *x;
+}
+
+typedef std::vector<int> MyVector;
+const MyVector& Vector() {
+  static absl::NoDestructor<MyVector> x{{1, 2, 3}};
+  return *x;
+}
+
+const int& Int() {
+  static absl::NoDestructor<int> x;
+  return *x;
+}
+
+TEST(NoDestructorTest, StaticPattern) {
+  EXPECT_TRUE(
+      std::is_trivially_destructible_v<absl::NoDestructor<std::string>>);
+  EXPECT_TRUE(std::is_trivially_destructible_v<absl::NoDestructor<MyArray>>);
+  EXPECT_TRUE(std::is_trivially_destructible_v<absl::NoDestructor<MyVector>>);
+  EXPECT_TRUE(std::is_trivially_destructible_v<absl::NoDestructor<int>>);
+
+  EXPECT_EQ(*Str0(), "");
+  Str0()->append("foo");
+  EXPECT_EQ(*Str0(), "foo");
+
+  EXPECT_EQ(std::string(Str1()), "Str2_Str1");
+
+  EXPECT_EQ(Str2(), "Str2");
+  EXPECT_EQ(Str2Copy(), "Str2");
+
+  EXPECT_THAT(Array(), testing::ElementsAre("foo", "bar", "baz"));
+
+  EXPECT_THAT(Vector(), testing::ElementsAre(1, 2, 3));
+
+  EXPECT_EQ(0, Int());  // should get zero-initialized
+}
+
+TEST(NoDestructorTest, ClassTemplateArgumentDeduction) {
+  absl::NoDestructor i(1);
+  static_assert(std::is_same_v<decltype(i), absl::NoDestructor<int>>,
+                "Expected deduced type to be int.");
+}
+
+}  // namespace
diff --git a/absl/base/nullability.h b/absl/base/nullability.h
new file mode 100644
index 00000000000..ad80178adc1
--- /dev/null
+++ b/absl/base/nullability.h
@@ -0,0 +1,318 @@
+// Copyright 2023 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: nullability.h
+// -----------------------------------------------------------------------------
+//
+// This header file defines a set of annotations for designating the expected
+// nullability of pointers. These annotations allow you to designate pointers in
+// one of three classification states:
+//
+//  * "Non-null" (for pointers annotated `absl_nonnull`), indicating that it is
+//    invalid for the given pointer to ever be null.
+//  * "Nullable" (for pointers annotated `absl_nullable`), indicating that it is
+//    valid for the given pointer to be null.
+//  * "Unknown" (for pointers annotated `absl_nullability_unknown`), indicating
+//    that the given pointer has not yet been classified as either nullable or
+//    non-null. This is the default state of unannotated pointers.
+//
+// NOTE: Unannotated pointers implicitly bear the annotation
+// `absl_nullability_unknown`; you should rarely, if ever, see this annotation
+// used in the codebase explicitly.
+//
+// -----------------------------------------------------------------------------
+// Nullability and Contracts
+// -----------------------------------------------------------------------------
+//
+// These nullability annotations allow you to more clearly specify contracts on
+// software components by narrowing the *preconditions*, *postconditions*, and
+// *invariants* of pointer state(s) in any given interface. It then depends on
+// context who is responsible for fulfilling the annotation's requirements.
+//
+// For example, a function may receive a pointer argument. Designating that
+// pointer argument as "non-null" tightens the precondition of the contract of
+// that function. It is then the responsibility of anyone calling such a
+// function to ensure that the passed pointer is not null.
+//
+// Similarly, a function may have a pointer as a return value. Designating that
+// return value as "non-null" tightens the postcondition of the contract of that
+// function. In this case, however, it is the responsibility of the function
+// itself to ensure that the returned pointer is not null.
+//
+// Clearly defining these contracts allows providers (and consumers) of such
+// pointers to have more confidence in their null state. If a function declares
+// a return value as "non-null", for example, the caller should not need to
+// check whether the returned value is `nullptr`; it can simply assume the
+// pointer is valid.
+//
+// Of course most interfaces already have expectations on the nullability state
+// of pointers, and these expectations are, in effect, a contract; often,
+// however, those contracts are either poorly or partially specified, assumed,
+// or misunderstood. These nullability annotations are designed to allow you to
+// formalize those contracts within the codebase.
+//
+// -----------------------------------------------------------------------------
+// Annotation Syntax
+// -----------------------------------------------------------------------------
+//
+// The annotations should be positioned as a qualifier for the pointer type. For
+// example, the position of `const` when declaring a const pointer (not a
+// pointer to a const type) is the position you should also use for these
+// annotations.
+//
+// Example:
+//
+// // A const non-null pointer to an `Employee`.
+// Employee* absl_nonnull const e;
+//
+// // A non-null pointer to a const `Employee`.
+// const Employee* absl_nonnull e;
+//
+// // A non-null pointer to a const nullable pointer to an `Employee`.
+// Employee* absl_nullable const* absl_nonnull e;
+//
+// // A non-null function pointer.
+// void (*absl_nonnull func)(int, double);
+//
+// // A non-null array of `Employee`s as a parameter.
+// void func(Employee employees[absl_nonnull]);
+//
+// // A non-null std::unique_ptr to an `Employee`.
+// // As with `const`, it is possible to place the annotation on either side of
+// // a named type not ending in `*`, but placing it before the type it
+// // describes is preferred, unless inconsistent with surrounding code.
+// absl_nonnull std::unique_ptr<Employee> employee;
+//
+// // Invalid annotation usage - this attempts to declare a pointer to a
+// // nullable `Employee`, which is meaningless.
+// absl_nullable Employee* e;
+//
+// -----------------------------------------------------------------------------
+// Using Nullability Annotations
+// -----------------------------------------------------------------------------
+//
+// Each annotation acts as a form of documentation about the contract for the
+// given pointer. Each annotation requires providers or consumers of these
+// pointers across API boundaries to take appropriate steps when setting or
+// using these pointers:
+//
+// * "Non-null" pointers should never be null. It is the responsibility of the
+//   provider of this pointer to ensure that the pointer may never be set to
+//   null. Consumers of such pointers can treat such pointers as non-null.
+// * "Nullable" pointers may or may not be null. Consumers of such pointers
+//   should precede any usage of that pointer (e.g. a dereference operation)
+//   with a a `nullptr` check.
+// * "Unknown" pointers may be either "non-null" or "nullable" but have not been
+//   definitively determined to be in either classification state. Providers of
+//   such pointers across API boundaries should determine --  over time -- to
+//   annotate the pointer in either of the above two states. Consumers of such
+//   pointers across an API boundary should continue to treat such pointers as
+//   they currently do.
+//
+// Example:
+//
+// // PaySalary() requires the passed pointer to an `Employee` to be non-null.
+// void PaySalary(Employee* absl_nonnull e) {
+//   pay(e->salary);  // OK to dereference
+// }
+//
+// // CompleteTransaction() guarantees the returned pointer to an `Account` to
+// // be non-null.
+// Account* absl_nonnull CompleteTransaction(double fee) {
+// ...
+// }
+//
+// // Note that specifying a nullability annotation does not prevent someone
+// // from violating the contract:
+//
+// Employee* absl_nullable find(Map& employees, std::string_view name);
+//
+// void g(Map& employees) {
+//   Employee *e = find(employees, "Pat");
+//   // `e` can now be null.
+//   PaySalary(e); // Violates contract, but compiles!
+// }
+//
+// Nullability annotations, in other words, are useful for defining and
+// narrowing contracts; *enforcement* of those contracts depends on use and any
+// additional (static or dynamic analysis) tooling.
+//
+// NOTE: The "unknown" annotation state indicates that a pointer's contract has
+// not yet been positively identified. The unknown state therefore acts as a
+// form of documentation of your technical debt, and a codebase that adopts
+// nullability annotations should aspire to annotate every pointer as either
+// "non-null" or "nullable".
+//
+// -----------------------------------------------------------------------------
+// Applicability of Nullability Annotations
+// -----------------------------------------------------------------------------
+//
+// By default, nullability annotations are applicable to raw and smart
+// pointers. User-defined types can indicate compatibility with nullability
+// annotations by adding the ABSL_NULLABILITY_COMPATIBLE attribute.
+//
+// // Example:
+// struct ABSL_NULLABILITY_COMPATIBLE MyPtr {
+//   ...
+// };
+//
+// Note: Compilers that don't support the `nullability_on_classes` feature will
+// allow nullability annotations to be applied to any type, not just ones
+// marked with `ABSL_NULLABILITY_COMPATIBLE`.
+//
+// DISCLAIMER:
+// ===========================================================================
+// These nullability annotations are primarily a human readable signal about the
+// intended contract of the pointer. They are not *types* and do not currently
+// provide any correctness guarantees. For example, a pointer annotated as
+// `absl_nonnull` is *not guaranteed* to be non-null, and the compiler won't
+// alert or prevent assignment of a `T* absl_nullable` to a `T* absl_nonnull`.
+// ===========================================================================
+// SKIP_ABSL_INLINE_NAMESPACE_CHECK
+#ifndef ABSL_BASE_NULLABILITY_H_
+#define ABSL_BASE_NULLABILITY_H_
+
+#include "absl/base/config.h"
+
+// ABSL_POINTERS_DEFAULT_NONNULL
+//
+// This macro specifies that all unannotated pointer types within the given
+// file are designated as nonnull (instead of the default "unknown"). This macro
+// exists as a standalone statement and applies default nonnull behavior to all
+// subsequent pointers; as a result, place this macro as the first non-comment,
+// non-`#include` line in a file.
+//
+// Example:
+//
+//     #include "absl/base/nullability.h"
+//
+//     ABSL_POINTERS_DEFAULT_NONNULL
+//
+//     void FillMessage(Message *m);                  // implicitly non-null
+//     T* absl_nullable GetNullablePtr();           // explicitly nullable
+//     T* absl_nullability_unknown GetUnknownPtr();  // explicitly unknown
+//
+// The macro can be safely used in header files - it will not affect any files
+// that include it.
+//
+// In files with the macro, plain `T*` syntax means `T* absl_nonnull`, and the
+// exceptions (`absl_nullable` and `absl_nullability_unknown`) must be marked
+// explicitly. The same holds, correspondingly, for smart pointer types.
+//
+// For comparison, without the macro, all unannotated pointers would default to
+// unknown, and otherwise require explicit annotations to change this behavior:
+//
+//     #include "absl/base/nullability.h"
+//
+//     void FillMessage(Message* absl_nonnull m);  // explicitly non-null
+//     T* absl_nullable GetNullablePtr();          // explicitly nullable
+//     T* GetUnknownPtr();                           // implicitly unknown
+//
+// No-op except for being a human readable signal.
+#define ABSL_POINTERS_DEFAULT_NONNULL
+
+#if defined(__clang__) && !defined(__OBJC__) && \
+    ABSL_HAVE_FEATURE(nullability_on_classes)
+// absl_nonnull (default with `ABSL_POINTERS_DEFAULT_NONNULL`)
+//
+// The indicated pointer is never null. It is the responsibility of the provider
+// of this pointer across an API boundary to ensure that the pointer is never
+// set to null. Consumers of this pointer across an API boundary may safely
+// dereference the pointer.
+//
+// Example:
+//
+// // `employee` is designated as not null.
+// void PaySalary(Employee* absl_nonnull employee) {
+//   pay(*employee);  // OK to dereference
+// }
+#define absl_nonnull _Nonnull
+
+// absl_nullable
+//
+// The indicated pointer may, by design, be either null or non-null. Consumers
+// of this pointer across an API boundary should perform a `nullptr` check
+// before performing any operation using the pointer.
+//
+// Example:
+//
+// // `employee` may  be null.
+// void PaySalary(Employee* absl_nullable employee) {
+//   if (employee != nullptr) {
+//     Pay(*employee);  // OK to dereference
+//   }
+// }
+#define absl_nullable _Nullable
+
+// absl_nullability_unknown  (default without `ABSL_POINTERS_DEFAULT_NONNULL`)
+//
+// The indicated pointer has not yet been determined to be definitively
+// "non-null" or "nullable." Providers of such pointers across API boundaries
+// should, over time, annotate such pointers as either "non-null" or "nullable."
+// Consumers of these pointers across an API boundary should treat such pointers
+// with the same caution they treat currently unannotated pointers. Most
+// existing code will have "unknown"  pointers, which should eventually be
+// migrated into one of the above two nullability states: `absl_nonnull` or
+//  `absl_nullable`.
+//
+// NOTE: For files that do not specify `ABSL_POINTERS_DEFAULT_NONNULL`,
+// because this annotation is the global default state, unannotated pointers are
+// are assumed to have "unknown" semantics. This assumption is designed to
+// minimize churn and reduce clutter within the codebase.
+//
+// Example:
+//
+// // `employee`s nullability state is unknown.
+// void PaySalary(Employee* absl_nullability_unknown employee) {
+//   Pay(*employee); // Potentially dangerous. API provider should investigate.
+// }
+//
+// Note that a pointer without an annotation, by default, is assumed to have the
+// annotation `NullabilityUnknown`.
+//
+// // `employee`s nullability state is unknown.
+// void PaySalary(Employee* employee) {
+//   Pay(*employee); // Potentially dangerous. API provider should investigate.
+// }
+#define absl_nullability_unknown _Null_unspecified
+#else
+// No-op for non-Clang compilers or Objective-C.
+#define absl_nonnull
+// No-op for non-Clang compilers or Objective-C.
+#define absl_nullable
+// No-op for non-Clang compilers or Objective-C.
+#define absl_nullability_unknown
+#endif
+
+// ABSL_NULLABILITY_COMPATIBLE
+//
+// Indicates that a class is compatible with nullability annotations.
+//
+// For example:
+//
+// struct ABSL_NULLABILITY_COMPATIBLE MyPtr {
+//   ...
+// };
+//
+// Note: Compilers that don't support the `nullability_on_classes` feature will
+// allow nullability annotations to be applied to any type, not just ones marked
+// with `ABSL_NULLABILITY_COMPATIBLE`.
+#if ABSL_HAVE_FEATURE(nullability_on_classes)
+#define ABSL_NULLABILITY_COMPATIBLE _Nullable
+#else
+#define ABSL_NULLABILITY_COMPATIBLE
+#endif
+
+#endif  // ABSL_BASE_NULLABILITY_H_
diff --git a/absl/base/nullability_default_nonnull_test.cc b/absl/base/nullability_default_nonnull_test.cc
new file mode 100644
index 00000000000..bd5b483429a
--- /dev/null
+++ b/absl/base/nullability_default_nonnull_test.cc
@@ -0,0 +1,44 @@
+// Copyright 2024 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <cassert>
+
+#include "gtest/gtest.h"
+#include "absl/base/nullability.h"
+
+ABSL_POINTERS_DEFAULT_NONNULL
+
+namespace {
+
+void FuncWithDefaultNonnullArg(int* /*arg*/) {}
+template <typename T>
+void FuncWithDeducedDefaultNonnullArg(T* /*arg*/) {}
+
+TEST(DefaultNonnullTest, NonnullArgument) {
+  int var = 0;
+  FuncWithDefaultNonnullArg(&var);
+  FuncWithDeducedDefaultNonnullArg<int>(&var);
+}
+
+int* FuncWithDefaultNonnullReturn() {
+  static int var = 0;
+  return &var;
+}
+
+TEST(DefaultNonnullTest, NonnullReturn) {
+  auto var = FuncWithDefaultNonnullReturn();
+  (void)var;
+}
+
+}  // namespace
diff --git a/absl/base/nullability_test.cc b/absl/base/nullability_test.cc
new file mode 100644
index 00000000000..648b4be1bfe
--- /dev/null
+++ b/absl/base/nullability_test.cc
@@ -0,0 +1,90 @@
+// Copyright 2023 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/base/nullability.h"
+
+#include <memory>
+#include <type_traits>
+#include <utility>
+
+#include "gtest/gtest.h"
+
+namespace {
+void funcWithNonnullArg(int* absl_nonnull /*arg*/) {}
+template <typename T>
+void funcWithDeducedNonnullArg(T* absl_nonnull /*arg*/) {}
+
+TEST(NonnullTest, NonnullArgument) {
+  int var = 0;
+  funcWithNonnullArg(&var);
+  funcWithDeducedNonnullArg(&var);
+}
+
+int* absl_nonnull funcWithNonnullReturn() {
+  static int var = 0;
+  return &var;
+}
+
+TEST(NonnullTest, NonnullReturn) {
+  auto var = funcWithNonnullReturn();
+  (void)var;
+}
+
+TEST(PassThroughTest, PassesThroughRawPointerToInt) {
+  EXPECT_TRUE((std::is_same_v<int* absl_nonnull, int*>));
+  EXPECT_TRUE((std::is_same_v<int* absl_nullable, int*>));
+  EXPECT_TRUE((std::is_same_v<int* absl_nullability_unknown, int*>));
+}
+
+TEST(PassThroughTest, PassesThroughRawPointerToVoid) {
+  EXPECT_TRUE((std::is_same_v<void* absl_nonnull, void*>));
+  EXPECT_TRUE((std::is_same_v<void* absl_nullable, void*>));
+  EXPECT_TRUE((std::is_same_v<void* absl_nullability_unknown, void*>));
+}
+
+TEST(PassThroughTest, PassesThroughUniquePointerToInt) {
+  using T = std::unique_ptr<int>;
+  EXPECT_TRUE((std::is_same_v<absl_nonnull T, T>));
+  EXPECT_TRUE((std::is_same_v<absl_nullable T, T>));
+  EXPECT_TRUE((std::is_same_v<absl_nullability_unknown T, T>));
+}
+
+TEST(PassThroughTest, PassesThroughSharedPointerToInt) {
+  using T = std::shared_ptr<int>;
+  EXPECT_TRUE((std::is_same_v<absl_nonnull T, T>));
+  EXPECT_TRUE((std::is_same_v<absl_nullable T, T>));
+  EXPECT_TRUE((std::is_same_v<absl_nullability_unknown T, T>));
+}
+
+TEST(PassThroughTest, PassesThroughSharedPointerToVoid) {
+  using T = std::shared_ptr<void>;
+  EXPECT_TRUE((std::is_same_v<absl_nonnull T, T>));
+  EXPECT_TRUE((std::is_same_v<absl_nullable T, T>));
+  EXPECT_TRUE((std::is_same_v<absl_nullability_unknown T, T>));
+}
+
+TEST(PassThroughTest, PassesThroughPointerToMemberObject) {
+  using T = decltype(&std::pair<int, int>::first);
+  EXPECT_TRUE((std::is_same_v<absl_nonnull T, T>));
+  EXPECT_TRUE((std::is_same_v<absl_nullable T, T>));
+  EXPECT_TRUE((std::is_same_v<absl_nullability_unknown T, T>));
+}
+
+TEST(PassThroughTest, PassesThroughPointerToMemberFunction) {
+  using T = decltype(&std::unique_ptr<int>::reset);
+  EXPECT_TRUE((std::is_same_v<absl_nonnull T, T>));
+  EXPECT_TRUE((std::is_same_v<absl_nullable T, T>));
+  EXPECT_TRUE((std::is_same_v<absl_nullability_unknown T, T>));
+}
+}  // namespace
diff --git a/absl/base/optimization.h b/absl/base/optimization.h
index 0dcbef32a3c..8247ef29a6b 100644
--- a/absl/base/optimization.h
+++ b/absl/base/optimization.h
@@ -18,17 +18,34 @@
 // -----------------------------------------------------------------------------
 //
 // This header file defines portable macros for performance optimization.
+//
+// This header is included in both C++ code and legacy C code and thus must
+// remain compatible with both C and C++. C compatibility will be removed if
+// the legacy code is removed or converted to C++. Do not include this header in
+// new code that requires C compatibility or assume C compatibility will remain
+// indefinitely.
+
+// SKIP_ABSL_INLINE_NAMESPACE_CHECK
 
 #ifndef ABSL_BASE_OPTIMIZATION_H_
 #define ABSL_BASE_OPTIMIZATION_H_
 
+#include <assert.h>
+#include <stdlib.h>
+
+#ifdef __cplusplus
+// Included for std::unreachable()
+#include <utility>
+#endif  // __cplusplus
+
 #include "absl/base/config.h"
+#include "absl/base/options.h"
 
 // ABSL_BLOCK_TAIL_CALL_OPTIMIZATION
 //
-// Instructs the compiler to avoid optimizing tail-call recursion. Use of this
-// macro is useful when you wish to preserve the existing function order within
-// a stack trace for logging, debugging, or profiling purposes.
+// Instructs the compiler to avoid optimizing tail-call recursion. This macro is
+// useful when you wish to preserve the existing function order within a stack
+// trace for logging, debugging, or profiling purposes.
 //
 // Example:
 //
@@ -37,9 +54,7 @@
 //     ABSL_BLOCK_TAIL_CALL_OPTIMIZATION();
 //     return result;
 //   }
-#if defined(__pnacl__)
-#define ABSL_BLOCK_TAIL_CALL_OPTIMIZATION() if (volatile int x = 0) { (void)x; }
-#elif defined(__clang__)
+#if defined(__clang__)
 // Clang will not tail call given inline volatile assembly.
 #define ABSL_BLOCK_TAIL_CALL_OPTIMIZATION() __asm__ __volatile__("")
 #elif defined(__GNUC__)
@@ -89,6 +104,7 @@
 #define ABSL_CACHELINE_SIZE 64
 #endif
 #endif
+#endif
 
 #ifndef ABSL_CACHELINE_SIZE
 // A reasonable default guess.  Note that overestimates tend to waste more
@@ -104,9 +120,10 @@
 // Cacheline aligning objects properly allows constructive memory sharing and
 // prevents destructive (or "false") memory sharing.
 //
-// NOTE: this macro should be replaced with usage of `alignas()` using
+// NOTE: callers should replace uses of this macro with `alignas()` using
 // `std::hardware_constructive_interference_size` and/or
-// `std::hardware_destructive_interference_size` when available within C++17.
+// `std::hardware_destructive_interference_size` when C++17 becomes available to
+// them.
 //
 // See http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0154r1.html
 // for more information.
@@ -138,12 +155,11 @@
 //    the generated machine code.
 // 3) Prefer applying this attribute to individual variables. Avoid
 //    applying it to types. This tends to localize the effect.
+#if defined(__clang__) || defined(__GNUC__)
 #define ABSL_CACHELINE_ALIGNED __attribute__((aligned(ABSL_CACHELINE_SIZE)))
 #elif defined(_MSC_VER)
-#define ABSL_CACHELINE_SIZE 64
 #define ABSL_CACHELINE_ALIGNED __declspec(align(ABSL_CACHELINE_SIZE))
 #else
-#define ABSL_CACHELINE_SIZE 64
 #define ABSL_CACHELINE_ALIGNED
 #endif
 
@@ -171,11 +187,126 @@
 // to yield performance improvements.
 #if ABSL_HAVE_BUILTIN(__builtin_expect) || \
     (defined(__GNUC__) && !defined(__clang__))
-#define ABSL_PREDICT_FALSE(x) (__builtin_expect(x, 0))
-#define ABSL_PREDICT_TRUE(x) (__builtin_expect(!!(x), 1))
+#define ABSL_PREDICT_FALSE(x) (__builtin_expect(false || (x), false))
+#define ABSL_PREDICT_TRUE(x) (__builtin_expect(false || (x), true))
 #else
 #define ABSL_PREDICT_FALSE(x) (x)
 #define ABSL_PREDICT_TRUE(x) (x)
 #endif
 
+// `ABSL_INTERNAL_IMMEDIATE_ABORT_IMPL()` aborts the program in the fastest
+// possible way, with no attempt at logging. One use is to implement hardening
+// aborts with ABSL_OPTION_HARDENED.  Since this is an internal symbol, it
+// should not be used directly outside of Abseil.
+#if ABSL_HAVE_BUILTIN(__builtin_trap) || \
+    (defined(__GNUC__) && !defined(__clang__))
+#define ABSL_INTERNAL_IMMEDIATE_ABORT_IMPL() __builtin_trap()
+#else
+#define ABSL_INTERNAL_IMMEDIATE_ABORT_IMPL() abort()
+#endif
+
+// `ABSL_INTERNAL_UNREACHABLE_IMPL()` is the platform specific directive to
+// indicate that a statement is unreachable, and to allow the compiler to
+// optimize accordingly. Clients should use `ABSL_UNREACHABLE()`, which is
+// defined below.
+#if defined(__cpp_lib_unreachable) && __cpp_lib_unreachable >= 202202L
+#define ABSL_INTERNAL_UNREACHABLE_IMPL() std::unreachable()
+#elif defined(__GNUC__) || ABSL_HAVE_BUILTIN(__builtin_unreachable)
+#define ABSL_INTERNAL_UNREACHABLE_IMPL() __builtin_unreachable()
+#elif ABSL_HAVE_BUILTIN(__builtin_assume)
+#define ABSL_INTERNAL_UNREACHABLE_IMPL() __builtin_assume(false)
+#elif defined(_MSC_VER)
+#define ABSL_INTERNAL_UNREACHABLE_IMPL() __assume(false)
+#else
+#define ABSL_INTERNAL_UNREACHABLE_IMPL() ((void)0)
+#endif
+
+// `ABSL_UNREACHABLE()` is an unreachable statement.  A program which reaches
+// one has undefined behavior, and the compiler may optimize accordingly.
+#if (ABSL_OPTION_HARDENED == 1 || ABSL_OPTION_HARDENED == 2) && defined(NDEBUG)
+// Abort in hardened mode to avoid dangerous undefined behavior.
+#define ABSL_UNREACHABLE()                \
+  do {                                    \
+    ABSL_INTERNAL_IMMEDIATE_ABORT_IMPL(); \
+    ABSL_INTERNAL_UNREACHABLE_IMPL();     \
+  } while (false)
+#else
+// The assert only fires in debug mode to aid in debugging.
+// When NDEBUG is defined, reaching ABSL_UNREACHABLE() is undefined behavior.
+#define ABSL_UNREACHABLE()                       \
+  do {                                           \
+    /* NOLINTNEXTLINE: misc-static-assert */     \
+    assert(false && "ABSL_UNREACHABLE reached"); \
+    ABSL_INTERNAL_UNREACHABLE_IMPL();            \
+  } while (false)
+#endif
+
+// ABSL_ASSUME(cond)
+//
+// Informs the compiler that a condition is always true and that it can assume
+// it to be true for optimization purposes.
+//
+// WARNING: If the condition is false, the program can produce undefined and
+// potentially dangerous behavior.
+//
+// In !NDEBUG mode, the condition is checked with an assert().
+//
+// NOTE: The expression must not have side effects, as it may only be evaluated
+// in some compilation modes and not others. Some compilers may issue a warning
+// if the compiler cannot prove the expression has no side effects. For example,
+// the expression should not use a function call since the compiler cannot prove
+// that a function call does not have side effects.
+//
+// Example:
+//
+//   int x = ...;
+//   ABSL_ASSUME(x >= 0);
+//   // The compiler can optimize the division to a simple right shift using the
+//   // assumption specified above.
+//   int y = x / 16;
+//
+#if !defined(NDEBUG)
+#define ABSL_ASSUME(cond) \
+  (ABSL_PREDICT_TRUE((cond)) ? void() : assert(false && #cond))  // NOLINT
+#elif ABSL_HAVE_BUILTIN(__builtin_assume)
+#define ABSL_ASSUME(cond) __builtin_assume(cond)
+#elif defined(_MSC_VER)
+#define ABSL_ASSUME(cond) __assume(cond)
+#elif defined(__cpp_lib_unreachable) && __cpp_lib_unreachable >= 202202L
+#define ABSL_ASSUME(cond) ((cond) ? void() : std::unreachable())
+#elif defined(__GNUC__) || ABSL_HAVE_BUILTIN(__builtin_unreachable)
+#define ABSL_ASSUME(cond) ((cond) ? void() : __builtin_unreachable())
+#elif ABSL_INTERNAL_CPLUSPLUS_LANG >= 202002L
+// Unimplemented. Uses the same definition as ABSL_ASSERT in the NDEBUG case.
+#define ABSL_ASSUME(expr) (decltype((expr) ? void() : void())())
+#else
+#define ABSL_ASSUME(expr) (false ? ((expr) ? void() : void()) : void())
+#endif
+
+// ABSL_INTERNAL_UNIQUE_SMALL_NAME(cond)
+// This macro forces small unique name on a static file level symbols like
+// static local variables or static functions. This is intended to be used in
+// macro definitions to optimize the cost of generated code. Do NOT use it on
+// symbols exported from translation unit since it may cause a link time
+// conflict.
+//
+// Example:
+//
+// #define MY_MACRO(txt)
+// namespace {
+//  char VeryVeryLongVarName[] ABSL_INTERNAL_UNIQUE_SMALL_NAME() = txt;
+//  const char* VeryVeryLongFuncName() ABSL_INTERNAL_UNIQUE_SMALL_NAME();
+//  const char* VeryVeryLongFuncName() { return txt; }
+// }
+//
+
+#if defined(__GNUC__)
+#define ABSL_INTERNAL_UNIQUE_SMALL_NAME2(x) #x
+#define ABSL_INTERNAL_UNIQUE_SMALL_NAME1(x) ABSL_INTERNAL_UNIQUE_SMALL_NAME2(x)
+#define ABSL_INTERNAL_UNIQUE_SMALL_NAME() \
+  asm(ABSL_INTERNAL_UNIQUE_SMALL_NAME1(.absl.__COUNTER__))
+#else
+#define ABSL_INTERNAL_UNIQUE_SMALL_NAME()
+#endif
+
 #endif  // ABSL_BASE_OPTIMIZATION_H_
diff --git a/absl/base/optimization_test.cc b/absl/base/optimization_test.cc
new file mode 100644
index 00000000000..11d22fe97b6
--- /dev/null
+++ b/absl/base/optimization_test.cc
@@ -0,0 +1,142 @@
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/base/optimization.h"
+
+#include <optional>
+
+#include "gtest/gtest.h"
+
+namespace {
+
+// Tests for the ABSL_PREDICT_TRUE and ABSL_PREDICT_FALSE macros.
+// The tests only verify that the macros are functionally correct - i.e. code
+// behaves as if they weren't used. They don't try to check their impact on
+// optimization.
+
+TEST(PredictTest, PredictTrue) {
+  EXPECT_TRUE(ABSL_PREDICT_TRUE(true));
+  EXPECT_FALSE(ABSL_PREDICT_TRUE(false));
+  EXPECT_TRUE(ABSL_PREDICT_TRUE(1 == 1));
+  EXPECT_FALSE(ABSL_PREDICT_TRUE(1 == 2));
+
+  if (ABSL_PREDICT_TRUE(false)) ADD_FAILURE();
+  if (!ABSL_PREDICT_TRUE(true)) ADD_FAILURE();
+
+  EXPECT_TRUE(ABSL_PREDICT_TRUE(true) && true);
+  EXPECT_TRUE(ABSL_PREDICT_TRUE(true) || false);
+}
+
+TEST(PredictTest, PredictFalse) {
+  EXPECT_TRUE(ABSL_PREDICT_FALSE(true));
+  EXPECT_FALSE(ABSL_PREDICT_FALSE(false));
+  EXPECT_TRUE(ABSL_PREDICT_FALSE(1 == 1));
+  EXPECT_FALSE(ABSL_PREDICT_FALSE(1 == 2));
+
+  if (ABSL_PREDICT_FALSE(false)) ADD_FAILURE();
+  if (!ABSL_PREDICT_FALSE(true)) ADD_FAILURE();
+
+  EXPECT_TRUE(ABSL_PREDICT_FALSE(true) && true);
+  EXPECT_TRUE(ABSL_PREDICT_FALSE(true) || false);
+}
+
+TEST(PredictTest, OneEvaluation) {
+  // Verify that the expression is only evaluated once.
+  int x = 0;
+  if (ABSL_PREDICT_TRUE((++x) == 0)) ADD_FAILURE();
+  EXPECT_EQ(x, 1);
+  if (ABSL_PREDICT_FALSE((++x) == 0)) ADD_FAILURE();
+  EXPECT_EQ(x, 2);
+}
+
+TEST(PredictTest, OperatorOrder) {
+  // Verify that operator order inside and outside the macro behaves well.
+  // These would fail for a naive '#define ABSL_PREDICT_TRUE(x) x'
+  EXPECT_TRUE(ABSL_PREDICT_TRUE(1 && 2) == true);
+  EXPECT_TRUE(ABSL_PREDICT_FALSE(1 && 2) == true);
+  EXPECT_TRUE(!ABSL_PREDICT_TRUE(1 == 2));
+  EXPECT_TRUE(!ABSL_PREDICT_FALSE(1 == 2));
+}
+
+TEST(PredictTest, Pointer) {
+  const int x = 3;
+  const int *good_intptr = &x;
+  const int *null_intptr = nullptr;
+  EXPECT_TRUE(ABSL_PREDICT_TRUE(good_intptr));
+  EXPECT_FALSE(ABSL_PREDICT_TRUE(null_intptr));
+  EXPECT_TRUE(ABSL_PREDICT_FALSE(good_intptr));
+  EXPECT_FALSE(ABSL_PREDICT_FALSE(null_intptr));
+}
+
+TEST(PredictTest, Optional) {
+  // Note: An optional's truth value is the value's existence, not its truth.
+  std::optional<bool> has_value(false);
+  std::optional<bool> no_value;
+  EXPECT_TRUE(ABSL_PREDICT_TRUE(has_value));
+  EXPECT_FALSE(ABSL_PREDICT_TRUE(no_value));
+  EXPECT_TRUE(ABSL_PREDICT_FALSE(has_value));
+  EXPECT_FALSE(ABSL_PREDICT_FALSE(no_value));
+}
+
+class ImplicitlyConvertibleToBool {
+ public:
+  explicit ImplicitlyConvertibleToBool(bool value) : value_(value) {}
+  operator bool() const {  // NOLINT(google-explicit-constructor)
+    return value_;
+  }
+
+ private:
+  bool value_;
+};
+
+TEST(PredictTest, ImplicitBoolConversion) {
+  const ImplicitlyConvertibleToBool is_true(true);
+  const ImplicitlyConvertibleToBool is_false(false);
+  if (!ABSL_PREDICT_TRUE(is_true)) ADD_FAILURE();
+  if (ABSL_PREDICT_TRUE(is_false)) ADD_FAILURE();
+  if (!ABSL_PREDICT_FALSE(is_true)) ADD_FAILURE();
+  if (ABSL_PREDICT_FALSE(is_false)) ADD_FAILURE();
+}
+
+class ExplicitlyConvertibleToBool {
+ public:
+  explicit ExplicitlyConvertibleToBool(bool value) : value_(value) {}
+  explicit operator bool() const { return value_; }
+
+ private:
+  bool value_;
+};
+
+TEST(PredictTest, ExplicitBoolConversion) {
+  const ExplicitlyConvertibleToBool is_true(true);
+  const ExplicitlyConvertibleToBool is_false(false);
+  if (!ABSL_PREDICT_TRUE(is_true)) ADD_FAILURE();
+  if (ABSL_PREDICT_TRUE(is_false)) ADD_FAILURE();
+  if (!ABSL_PREDICT_FALSE(is_true)) ADD_FAILURE();
+  if (ABSL_PREDICT_FALSE(is_false)) ADD_FAILURE();
+}
+
+// This verifies that ABSL_ASSUME compiles in a variety of contexts.
+// It does not test optimization.
+TEST(AbslAssume, Compiles) {
+  int x = 0;
+  ABSL_ASSUME(x >= 0);
+  EXPECT_EQ(x, 0);
+
+  // https://github.com/abseil/abseil-cpp/issues/1814
+  ABSL_ASSUME(x >= 0), (x >= 0) ? ++x : --x;
+  EXPECT_EQ(x, 1);
+}
+
+}  // namespace
diff --git a/absl/base/options.h b/absl/base/options.h
new file mode 100644
index 00000000000..01cb80a8661
--- /dev/null
+++ b/absl/base/options.h
@@ -0,0 +1,212 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: options.h
+// -----------------------------------------------------------------------------
+//
+// This file contains Abseil configuration options for setting specific
+// implementations instead of letting Abseil determine which implementation to
+// use at compile-time. Setting these options may be useful for package or build
+// managers who wish to guarantee ABI stability within binary builds (which are
+// otherwise difficult to enforce).
+//
+// *** IMPORTANT NOTICE FOR PACKAGE MANAGERS:  It is important that
+// maintainers of package managers who wish to package Abseil read and
+// understand this file! ***
+//
+// Abseil contains a number of possible configuration endpoints, based on
+// parameters such as the detected platform, language version, or command-line
+// flags used to invoke the underlying binary. As is the case with all
+// libraries, binaries which contain Abseil code must ensure that separate
+// packages use the same compiled copy of Abseil to avoid a diamond dependency
+// problem, which can occur if two packages built with different Abseil
+// configuration settings are linked together. Diamond dependency problems in
+// C++ may manifest as violations to the One Definition Rule (ODR) (resulting in
+// linker errors), or undefined behavior (resulting in crashes).
+//
+// Diamond dependency problems can be avoided if all packages utilize the same
+// exact version of Abseil. Building from source code with the same compilation
+// parameters is the easiest way to avoid such dependency problems. However, for
+// package managers who cannot control such compilation parameters, we are
+// providing the file to allow you to inject ABI (Application Binary Interface)
+// stability across builds. Settings options in this file will neither change
+// API nor ABI, providing a stable copy of Abseil between packages.
+//
+// Care must be taken to keep options within these configurations isolated
+// from any other dynamic settings, such as command-line flags which could alter
+// these options. This file is provided specifically to help build and package
+// managers provide a stable copy of Abseil within their libraries and binaries;
+// other developers should not have need to alter the contents of this file.
+//
+// -----------------------------------------------------------------------------
+// Usage
+// -----------------------------------------------------------------------------
+//
+// For any particular package release, set the appropriate definitions within
+// this file to whatever value makes the most sense for your package(s). Note
+// that, by default, most of these options, at the moment, affect the
+// implementation of types; future options may affect other implementation
+// details.
+//
+// NOTE: the defaults within this file all assume that Abseil can select the
+// proper Abseil implementation at compile-time, which will not be sufficient
+// to guarantee ABI stability to package managers.
+
+// SKIP_ABSL_INLINE_NAMESPACE_CHECK
+
+#ifndef ABSL_BASE_OPTIONS_H_
+#define ABSL_BASE_OPTIONS_H_
+
+// -----------------------------------------------------------------------------
+// Type Compatibility Options
+// -----------------------------------------------------------------------------
+
+// ABSL_OPTION_USE_STD_SOURCE_LOCATION
+//
+// This option controls whether absl::SourceLocation is implemented as an alias
+// to the std::source_location type, or as an independent implementation.
+//
+// A value of 0 means to use Abseil's implementation.  This requires only C++17
+// support, and is expected to run on every toolchain we support, and to
+// properly capture source location information on every toolchain that supports
+// the necessary built-ins (such as `__builtin_LINE`).
+//
+// A value of 1 means to use aliases.  This requires that all code using Abseil
+// is built in C++20 mode or later.
+//
+// A value of 2 means to detect the C++ version being used to compile Abseil,
+// and use an alias only if working std::source_location types are available.
+// This option is useful when you are building your program from source.  It
+// should not be used otherwise -- for example, if you are distributing Abseil
+// in a binary package manager -- since in mode 2, they will name different
+// types, with different mangled names and binary layout, depending on the
+// compiler flags passed by the end user.  For more info, see
+// https://abseil.io/about/design/dropin-types.
+//
+// User code should not inspect this macro.  To check in the preprocessor if
+// the source location type is an alias of std::source_location type, use the
+// feature macro ABSL_USES_STD_SOURCE_LOCATION.
+//
+#define ABSL_OPTION_USE_STD_SOURCE_LOCATION 2
+
+// ABSL_OPTION_USE_STD_ORDERING
+//
+// This option controls whether absl::{partial,weak,strong}_ordering are
+// implemented as aliases to the std:: ordering types, or as an independent
+// implementation.
+//
+// A value of 0 means to use Abseil's implementation.  This is expected to
+// work on every toolchain we support.
+//
+// A value of 1 means to use aliases.  This requires that all code using Abseil
+// is built in C++20 mode or later.
+//
+// A value of 2 means to detect the C++ version being used to compile Abseil,
+// and use an alias only if working std:: ordering types are available.  This
+// option is useful when you are building your program from source.  It should
+// not be used otherwise -- for example, if you are distributing Abseil in a
+// binary package manager -- since in mode 2, they will name different types,
+// with different mangled names and binary layout, depending on the compiler
+// flags passed by the end user.  For more info, see
+// https://abseil.io/about/design/dropin-types.
+//
+// User code should not inspect this macro.  To check in the preprocessor if
+// the ordering types are aliases of std:: ordering types, use the feature macro
+// ABSL_USES_STD_ORDERING.
+
+#define ABSL_OPTION_USE_STD_ORDERING 2
+
+// ABSL_OPTION_USE_INLINE_NAMESPACE
+// ABSL_OPTION_INLINE_NAMESPACE_NAME
+//
+// These options controls whether all entities in the absl namespace are
+// contained within an inner inline namespace.  This does not affect the
+// user-visible API of Abseil, but it changes the mangled names of all symbols.
+//
+// This can be useful as a version tag if you are distributing Abseil in
+// precompiled form.  This will prevent a binary library build of Abseil with
+// one inline namespace being used with headers configured with a different
+// inline namespace name.  Binary packagers are reminded that Abseil does not
+// guarantee any ABI stability in Abseil, so any update of Abseil or
+// configuration change in such a binary package should be combined with a
+// new, unique value for the inline namespace name.
+//
+// A value of 0 means not to use inline namespaces.
+//
+// A value of 1 means to use an inline namespace with the given name inside
+// namespace absl.  If this is set, ABSL_OPTION_INLINE_NAMESPACE_NAME must also
+// be changed to a new, unique identifier name.  In particular "head" is not
+// allowed.
+
+#define ABSL_OPTION_USE_INLINE_NAMESPACE 0
+#define ABSL_OPTION_INLINE_NAMESPACE_NAME head
+
+// ABSL_OPTION_HARDENED
+//
+// This option enables a "hardened" build in release mode (in this context,
+// release mode is defined as a build where the `NDEBUG` macro is defined).
+//
+// A value of 0 means that "hardened" mode is not enabled.
+//
+// A value of 1 means that "hardened" mode is enabled with all checks.
+//
+// A value of 2 means that "hardened" mode is partially enabled, with
+// only a subset of checks chosen to minimize performance impact.
+//
+// Hardened builds have additional security checks enabled when `NDEBUG` is
+// defined. Defining `NDEBUG` is normally used to turn `assert()` macro into a
+// no-op, as well as disabling other bespoke program consistency checks. By
+// defining ABSL_OPTION_HARDENED to 1, a select set of checks remain enabled in
+// release mode. These checks guard against programming errors that may lead to
+// security vulnerabilities. In release mode, when one of these programming
+// errors is encountered, the program will immediately abort, possibly without
+// any attempt at logging.
+//
+// The checks enabled by this option are not free; they do incur runtime cost.
+//
+// The checks enabled by this option are always active when `NDEBUG` is not
+// defined, even in the case when ABSL_OPTION_HARDENED is defined to 0. The
+// checks enabled by this option may abort the program in a different way and
+// log additional information when `NDEBUG` is not defined.
+
+#define ABSL_OPTION_HARDENED 0
+
+// ABSL_OPTION_INLINE_HW_ACCEL_STRATEGY
+//
+// This option controls whether Abseil is allowed to use non-portable
+// hardware-accelerated implementations in headers (where they are typically
+// inlined into the caller's translation unit).
+//
+// Using such optimizations in headers can lead to One Definition Rule (ODR)
+// violations if different translation units are built with different CPU
+// architecture flags (e.g., -march=native vs -march=generic) and linked
+// together.
+//
+// A value of 0 means to use the portable software implementation in headers.
+// This provides the best ODR guarantees when linking code built with
+// inconsistent flags, but may be slower.
+//
+// A value of 1 means that the implementation requires the use of a
+// hardware-accelerated implementation. This requires the compiler environment
+// to support these instructions; otherwise, the build will fail.
+//
+// A value of 2 means to select the best available implementation based on
+// the compiler flags, but can't guarantee ODR safety if translation units are
+// built with inconsistent flags.
+//
+// User code should not inspect this macro.
+#define ABSL_OPTION_INLINE_HW_ACCEL_STRATEGY 0
+
+#endif  // ABSL_BASE_OPTIONS_H_
diff --git a/absl/base/policy_checks.h b/absl/base/policy_checks.h
index 699fb1a2e0f..45fe3d5b847 100644
--- a/absl/base/policy_checks.h
+++ b/absl/base/policy_checks.h
@@ -21,6 +21,8 @@
 // reported with `#error`. This enforcement is best effort, so successfully
 // compiling this header does not guarantee a supported configuration.
 
+// SKIP_ABSL_INLINE_NAMESPACE_CHECK
+
 #ifndef ABSL_BASE_POLICY_CHECKS_H_
 #define ABSL_BASE_POLICY_CHECKS_H_
 
@@ -41,20 +43,20 @@
 #endif
 
 // -----------------------------------------------------------------------------
-// Compiler Check
+// Toolchain Check
 // -----------------------------------------------------------------------------
 
-// We support MSVC++ 14.0 update 2 and later.
+// We support Visual Studio 2022 (MSVC++ 17.0) and later.
 // This minimum will go up.
-#if defined(_MSC_FULL_VER) && _MSC_FULL_VER < 190023918 && !defined(__clang__)
-#error "This package requires Visual Studio 2015 Update 2 or higher."
+#if defined(_MSC_VER) && _MSC_VER < 1930 && !defined(__clang__)
+#error "This package requires Visual Studio 2022 (MSVC++ 17.0) or higher."
 #endif
 
-// We support gcc 4.7 and later.
+// We support GCC 10 and later.
 // This minimum will go up.
 #if defined(__GNUC__) && !defined(__clang__)
-#if __GNUC__ < 4 || (__GNUC__ == 4 && __GNUC_MINOR__ < 7)
-#error "This package requires gcc 4.7 or higher."
+#if __GNUC__ < 10
+#error "This package requires GCC 10 or higher."
 #endif
 #endif
 
@@ -69,29 +71,21 @@
 // C++ Version Check
 // -----------------------------------------------------------------------------
 
-// Enforce C++11 as the minimum.  Note that Visual Studio has not
-// advanced __cplusplus despite being good enough for our purposes, so
-// so we exempt it from the check.
-#if defined(__cplusplus) && !defined(_MSC_VER)
-#if __cplusplus < 201103L
-#error "C++ versions less than C++11 are not supported."
-#endif
+// Enforce C++17 as the minimum.
+#if defined(_MSVC_LANG)
+#if _MSVC_LANG < 201703L
+#error "C++ versions less than C++17 are not supported."
+#endif  // _MSVC_LANG < 201703L
+#elif defined(__cplusplus)
+#if __cplusplus < 201703L
+#error "C++ versions less than C++17 are not supported."
+#endif  // __cplusplus < 201703L
 #endif
 
 // -----------------------------------------------------------------------------
 // Standard Library Check
 // -----------------------------------------------------------------------------
 
-// We have chosen glibc 2.12 as the minimum as it was tagged for release
-// in May, 2010 and includes some functionality used in Google software
-// (for instance pthread_setname_np):
-// https://sourceware.org/ml/libc-alpha/2010-05/msg00000.html
-#if defined(__GLIBC__) && defined(__GLIBC_PREREQ)
-#if !__GLIBC_PREREQ(2, 12)
-#error "Minimum required version of glibc is 2.12."
-#endif
-#endif
-
 #if defined(_STLPORT_VERSION)
 #error "STLPort is not supported."
 #endif
diff --git a/absl/base/port.h b/absl/base/port.h
index 6c28068d4f4..485d2c04d51 100644
--- a/absl/base/port.h
+++ b/absl/base/port.h
@@ -14,7 +14,8 @@
 //
 // This files is a forwarding header for other headers containing various
 // portability macros and functions.
-// This file is used for both C and C++!
+
+// SKIP_ABSL_INLINE_NAMESPACE_CHECK
 
 #ifndef ABSL_BASE_PORT_H_
 #define ABSL_BASE_PORT_H_
diff --git a/absl/base/prefetch.h b/absl/base/prefetch.h
new file mode 100644
index 00000000000..482cde3a016
--- /dev/null
+++ b/absl/base/prefetch.h
@@ -0,0 +1,209 @@
+// Copyright 2023 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: prefetch.h
+// -----------------------------------------------------------------------------
+//
+// This header file defines prefetch functions to prefetch memory contents
+// into the first level cache (L1) for the current CPU. The prefetch logic
+// offered in this header is limited to prefetching first level cachelines
+// only, and is aimed at relatively 'simple' prefetching logic.
+//
+#ifndef ABSL_BASE_PREFETCH_H_
+#define ABSL_BASE_PREFETCH_H_
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+
+#if defined(ABSL_INTERNAL_HAVE_SSE)
+#include <xmmintrin.h>
+#endif
+
+#if defined(_MSC_VER)
+#include <intrin.h>
+#if defined(ABSL_INTERNAL_HAVE_SSE)
+#pragma intrinsic(_mm_prefetch)
+#endif
+#endif
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+// Moves data into the L1 cache before it is read, or "prefetches" it.
+//
+// The value of `addr` is the address of the memory to prefetch. If
+// the target and compiler support it, data prefetch instructions are
+// generated. If the prefetch is done some time before the memory is
+// read, it may be in the cache by the time the read occurs.
+//
+// This method prefetches data with the highest degree of temporal locality;
+// data is prefetched where possible into all levels of the cache.
+//
+// Incorrect or gratuitous use of this function can degrade performance.
+// Use this function only when representative benchmarks show an improvement.
+//
+// Example:
+//
+//  // Computes incremental checksum for `data`.
+//  int ComputeChecksum(int sum, absl::string_view data);
+//
+//  // Computes cumulative checksum for all values in `data`
+//  int ComputeChecksum(absl::Span<const std::string> data) {
+//    int sum = 0;
+//    auto it = data.begin();
+//    auto pit = data.begin();
+//    auto end = data.end();
+//    for (int dist = 8; dist > 0 && pit != data.end(); --dist, ++pit) {
+//      absl::PrefetchToLocalCache(pit->data());
+//    }
+//    for (; pit != end; ++pit, ++it) {
+//      sum = ComputeChecksum(sum, *it);
+//      absl::PrefetchToLocalCache(pit->data());
+//    }
+//    for (; it != end; ++it) {
+//      sum = ComputeChecksum(sum, *it);
+//    }
+//    return sum;
+//  }
+//
+void PrefetchToLocalCache(const void* addr);
+
+// Moves data into the L1 cache before it is read, or "prefetches" it.
+//
+// This function is identical to `PrefetchToLocalCache()` except that it has
+// non-temporal locality: the fetched data should not be left in any of the
+// cache tiers. This is useful for cases where the data is used only once /
+// short term, for example, invoking a destructor on an object.
+//
+// Incorrect or gratuitous use of this function can degrade performance.
+// Use this function only when representative benchmarks show an improvement.
+//
+// Example:
+//
+//  template <typename Iterator>
+//  void DestroyPointers(Iterator begin, Iterator end) {
+//    size_t distance = std::min(8U, bars.size());
+//
+//    int dist = 8;
+//    auto prefetch_it = begin;
+//    while (prefetch_it != end && --dist;) {
+//      absl::PrefetchToLocalCacheNta(*prefetch_it++);
+//    }
+//    while (prefetch_it != end) {
+//      delete *begin++;
+//      absl::PrefetchToLocalCacheNta(*prefetch_it++);
+//    }
+//    while (begin != end) {
+//      delete *begin++;
+//    }
+//  }
+//
+void PrefetchToLocalCacheNta(const void* addr);
+
+// Moves data into the L1 cache with the intent to modify it.
+//
+// This function is similar to `PrefetchToLocalCache()` except that it
+// prefetches cachelines with an 'intent to modify' This typically includes
+// invalidating cache entries for this address in all other cache tiers, and an
+// exclusive access intent.
+//
+// Incorrect or gratuitous use of this function can degrade performance. As this
+// function can invalidate cached cachelines on other caches and computer cores,
+// incorrect usage of this function can have an even greater negative impact
+// than incorrect regular prefetches.
+// Use this function only when representative benchmarks show an improvement.
+//
+// Example:
+//
+//  void* Arena::Allocate(size_t size) {
+//    void* ptr = AllocateBlock(size);
+//    absl::PrefetchToLocalCacheForWrite(ptr);
+//    return ptr;
+//  }
+//
+void PrefetchToLocalCacheForWrite(const void* addr);
+
+#if ABSL_HAVE_BUILTIN(__builtin_prefetch) || defined(__GNUC__)
+
+#define ABSL_HAVE_PREFETCH 1
+
+// See __builtin_prefetch:
+// https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html.
+//
+ABSL_ATTRIBUTE_ALWAYS_INLINE inline void PrefetchToLocalCache(
+    const void* addr) {
+  __builtin_prefetch(addr, 0, 3);
+}
+
+ABSL_ATTRIBUTE_ALWAYS_INLINE inline void PrefetchToLocalCacheNta(
+    const void* addr) {
+  __builtin_prefetch(addr, 0, 0);
+}
+
+ABSL_ATTRIBUTE_ALWAYS_INLINE inline void PrefetchToLocalCacheForWrite(
+    const void* addr) {
+  // [x86] gcc/clang don't generate PREFETCHW for __builtin_prefetch(.., 1)
+  // unless -march=broadwell or newer; this is not generally the default, so we
+  // manually emit prefetchw. PREFETCHW is recognized as a no-op on older Intel
+  // processors and has been present on AMD processors since the K6-2.
+#if defined(__x86_64__) && !defined(__PRFCHW__)
+  asm("prefetchw %0" : : "m"(*reinterpret_cast<const char*>(addr)));
+#else
+  __builtin_prefetch(addr, 1, 3);
+#endif
+}
+
+#elif defined(ABSL_INTERNAL_HAVE_SSE)
+
+#define ABSL_HAVE_PREFETCH 1
+
+ABSL_ATTRIBUTE_ALWAYS_INLINE inline void PrefetchToLocalCache(
+    const void* addr) {
+  _mm_prefetch(reinterpret_cast<const char*>(addr), _MM_HINT_T0);
+}
+
+ABSL_ATTRIBUTE_ALWAYS_INLINE inline void PrefetchToLocalCacheNta(
+    const void* addr) {
+  _mm_prefetch(reinterpret_cast<const char*>(addr), _MM_HINT_NTA);
+}
+
+ABSL_ATTRIBUTE_ALWAYS_INLINE inline void PrefetchToLocalCacheForWrite(
+    const void* addr) {
+#if defined(_MM_HINT_ET0)
+  _mm_prefetch(reinterpret_cast<const char*>(addr), _MM_HINT_ET0);
+#elif !defined(_MSC_VER) && defined(__x86_64__)
+  // _MM_HINT_ET0 is not universally supported. As we commented further
+  // up, PREFETCHW is recognized as a no-op on older Intel processors
+  // and has been present on AMD processors since the K6-2. We have this
+  // disabled for MSVC compilers as this miscompiles on older MSVC compilers.
+  asm("prefetchw %0" : : "m"(*reinterpret_cast<const char*>(addr)));
+#endif
+}
+
+#else
+
+ABSL_ATTRIBUTE_ALWAYS_INLINE inline void PrefetchToLocalCache(
+    const void* addr) {}
+ABSL_ATTRIBUTE_ALWAYS_INLINE inline void PrefetchToLocalCacheNta(
+    const void* addr) {}
+ABSL_ATTRIBUTE_ALWAYS_INLINE inline void PrefetchToLocalCacheForWrite(
+    const void* addr) {}
+
+#endif
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_BASE_PREFETCH_H_
diff --git a/absl/base/prefetch_test.cc b/absl/base/prefetch_test.cc
new file mode 100644
index 00000000000..4e50bd17670
--- /dev/null
+++ b/absl/base/prefetch_test.cc
@@ -0,0 +1,65 @@
+// Copyright 2023 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/base/prefetch.h"
+
+#include <cstring>
+#include <memory>
+
+#include "gtest/gtest.h"
+
+namespace {
+
+// Below tests exercise the functions only to guarantee they compile and execute
+// correctly. We make no attempt at verifying any prefetch instructions being
+// generated and executed: we assume the various implementation in terms of
+// __builtin_prefetch() or x86 intrinsics to be correct and well tested.
+
+TEST(PrefetchTest, PrefetchToLocalCache_StackA) {
+  char buf[100] = {};
+  absl::PrefetchToLocalCache(buf);
+  absl::PrefetchToLocalCacheNta(buf);
+  absl::PrefetchToLocalCacheForWrite(buf);
+}
+
+TEST(PrefetchTest, PrefetchToLocalCache_Heap) {
+  auto memory = std::make_unique<char[]>(200 << 10);
+  memset(memory.get(), 0, 200 << 10);
+  absl::PrefetchToLocalCache(memory.get());
+  absl::PrefetchToLocalCacheNta(memory.get());
+  absl::PrefetchToLocalCacheForWrite(memory.get());
+  absl::PrefetchToLocalCache(memory.get() + (50 << 10));
+  absl::PrefetchToLocalCacheNta(memory.get() + (50 << 10));
+  absl::PrefetchToLocalCacheForWrite(memory.get() + (50 << 10));
+  absl::PrefetchToLocalCache(memory.get() + (100 << 10));
+  absl::PrefetchToLocalCacheNta(memory.get() + (100 << 10));
+  absl::PrefetchToLocalCacheForWrite(memory.get() + (100 << 10));
+  absl::PrefetchToLocalCache(memory.get() + (150 << 10));
+  absl::PrefetchToLocalCacheNta(memory.get() + (150 << 10));
+  absl::PrefetchToLocalCacheForWrite(memory.get() + (150 << 10));
+}
+
+TEST(PrefetchTest, PrefetchToLocalCache_Nullptr) {
+  absl::PrefetchToLocalCache(nullptr);
+  absl::PrefetchToLocalCacheNta(nullptr);
+  absl::PrefetchToLocalCacheForWrite(nullptr);
+}
+
+TEST(PrefetchTest, PrefetchToLocalCache_InvalidPtr) {
+  absl::PrefetchToLocalCache(reinterpret_cast<const void*>(0x785326532L));
+  absl::PrefetchToLocalCacheNta(reinterpret_cast<const void*>(0x785326532L));
+  absl::PrefetchToLocalCacheForWrite(reinterpret_cast<const void*>(0x78532L));
+}
+
+}  // namespace
diff --git a/absl/base/raw_logging_test.cc b/absl/base/raw_logging_test.cc
index 3d30bd38611..48158c024e5 100644
--- a/absl/base/raw_logging_test.cc
+++ b/absl/base/raw_logging_test.cc
@@ -18,6 +18,7 @@
 
 #include "absl/base/internal/raw_logging.h"
 
+#include <string>
 #include <tuple>
 
 #include "gtest/gtest.h"
@@ -35,16 +36,35 @@ TEST(RawLoggingCompilationTest, Log) {
   ABSL_RAW_LOG(ERROR, "RAW ERROR: %d", 1);
 }
 
+TEST(RawLoggingCompilationTest, LogWithNulls) {
+  ABSL_RAW_LOG(INFO, "RAW INFO: %s%c%s", "Hello", 0, "World");
+}
+
 TEST(RawLoggingCompilationTest, PassingCheck) {
   ABSL_RAW_CHECK(true, "RAW CHECK");
 }
 
+TEST(RawLoggingCompilationTest, DebugLog) {
+  ABSL_RAW_DLOG(INFO, "RAW DLOG: %d", 1);
+}
+
+TEST(RawLoggingCompilationTest, PassingDebugCheck) {
+  ABSL_RAW_DCHECK(true, "failure message");
+}
+
 // Not all platforms support output from raw log, so we don't verify any
 // particular output for RAW check failures (expecting the empty string
 // accomplishes this).  This test is primarily a compilation test, but we
 // are verifying process death when EXPECT_DEATH works for a platform.
 const char kExpectedDeathOutput[] = "";
 
+#if !defined(NDEBUG)  // if debug build
+TEST(RawLoggingDeathTest, FailingDebugCheck) {
+  EXPECT_DEATH_IF_SUPPORTED(ABSL_RAW_DCHECK(1 == 0, "explanation"),
+                            kExpectedDeathOutput);
+}
+#endif  // if debug build
+
 TEST(RawLoggingDeathTest, FailingCheck) {
   EXPECT_DEATH_IF_SUPPORTED(ABSL_RAW_CHECK(1 == 0, "explanation"),
                             kExpectedDeathOutput);
diff --git a/absl/base/spinlock_test_common.cc b/absl/base/spinlock_test_common.cc
index 84fc4dac0be..e198fafe682 100644
--- a/absl/base/spinlock_test_common.cc
+++ b/absl/base/spinlock_test_common.cc
@@ -16,14 +16,18 @@
 // spinlock.  If the spinlock is working properly, all elements of the
 // array should be equal at the end of the test.
 
+#include <cstddef>
 #include <cstdint>
 #include <limits>
+#include <mutex>  // NOLINT(build/c++11)
 #include <random>
 #include <thread>  // NOLINT(build/c++11)
+#include <type_traits>
 #include <vector>
 
 #include "gtest/gtest.h"
 #include "absl/base/attributes.h"
+#include "absl/base/config.h"
 #include "absl/base/internal/low_level_scheduling.h"
 #include "absl/base/internal/scheduling_mode.h"
 #include "absl/base/internal/spinlock.h"
@@ -32,10 +36,11 @@
 #include "absl/synchronization/blocking_counter.h"
 #include "absl/synchronization/notification.h"
 
-constexpr int32_t kNumThreads = 10;
+constexpr uint32_t kNumThreads = 10;
 constexpr int32_t kIters = 1000;
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 
 // This is defined outside of anonymous namespace so that it can be
@@ -45,44 +50,61 @@ struct SpinLockTest {
                                    int64_t wait_end_time) {
     return SpinLock::EncodeWaitCycles(wait_start_time, wait_end_time);
   }
-  static uint64_t DecodeWaitCycles(uint32_t lock_value) {
+  static int64_t DecodeWaitCycles(uint32_t lock_value) {
     return SpinLock::DecodeWaitCycles(lock_value);
   }
+
+  static bool IsCooperative(const SpinLock& l) { return l.IsCooperative(); }
 };
 
 namespace {
 
-static constexpr int kArrayLength = 10;
+static constexpr size_t kArrayLength = 10;
 static uint32_t values[kArrayLength];
 
-static SpinLock static_spinlock(base_internal::kLinkerInitialized);
-static SpinLock static_cooperative_spinlock(
-    base_internal::kLinkerInitialized,
-    base_internal::SCHEDULE_COOPERATIVE_AND_KERNEL);
-static SpinLock static_noncooperative_spinlock(
-    base_internal::kLinkerInitialized, base_internal::SCHEDULE_KERNEL_ONLY);
+ABSL_CONST_INIT static SpinLock static_cooperative_spinlock;
+ABSL_CONST_INIT static SpinLock static_noncooperative_spinlock(
+    base_internal::SCHEDULE_KERNEL_ONLY);
 
 // Simple integer hash function based on the public domain lookup2 hash.
 // http://burtleburtle.net/bob/c/lookup2.c
 static uint32_t Hash32(uint32_t a, uint32_t c) {
   uint32_t b = 0x9e3779b9UL;  // The golden ratio; an arbitrary value.
-  a -= b; a -= c; a ^= (c >> 13);
-  b -= c; b -= a; b ^= (a << 8);
-  c -= a; c -= b; c ^= (b >> 13);
-  a -= b; a -= c; a ^= (c >> 12);
-  b -= c; b -= a; b ^= (a << 16);
-  c -= a; c -= b; c ^= (b >> 5);
-  a -= b; a -= c; a ^= (c >> 3);
-  b -= c; b -= a; b ^= (a << 10);
-  c -= a; c -= b; c ^= (b >> 15);
+  a -= b;
+  a -= c;
+  a ^= (c >> 13);
+  b -= c;
+  b -= a;
+  b ^= (a << 8);
+  c -= a;
+  c -= b;
+  c ^= (b >> 13);
+  a -= b;
+  a -= c;
+  a ^= (c >> 12);
+  b -= c;
+  b -= a;
+  b ^= (a << 16);
+  c -= a;
+  c -= b;
+  c ^= (b >> 5);
+  a -= b;
+  a -= c;
+  a ^= (c >> 3);
+  b -= c;
+  b -= a;
+  b ^= (a << 10);
+  c -= a;
+  c -= b;
+  c ^= (b >> 15);
   return c;
 }
 
-static void TestFunction(int thread_salt, SpinLock* spinlock) {
+static void TestFunction(uint32_t thread_salt, SpinLock* spinlock) {
   for (int i = 0; i < kIters; i++) {
-    SpinLockHolder h(spinlock);
-    for (int j = 0; j < kArrayLength; j++) {
-      const int index = (j + thread_salt) % kArrayLength;
+    SpinLockHolder h(*spinlock);
+    for (size_t j = 0; j < kArrayLength; j++) {
+      const size_t index = (j + thread_salt) % kArrayLength;
       values[index] = Hash32(values[index], thread_salt);
       std::this_thread::yield();
     }
@@ -91,30 +113,33 @@ static void TestFunction(int thread_salt, SpinLock* spinlock) {
 
 static void ThreadedTest(SpinLock* spinlock) {
   std::vector<std::thread> threads;
-  for (int i = 0; i < kNumThreads; ++i) {
+  threads.reserve(kNumThreads);
+  for (uint32_t i = 0; i < kNumThreads; ++i) {
     threads.push_back(std::thread(TestFunction, i, spinlock));
   }
   for (auto& thread : threads) {
     thread.join();
   }
 
-  SpinLockHolder h(spinlock);
-  for (int i = 1; i < kArrayLength; i++) {
+  SpinLockHolder h(*spinlock);
+  for (size_t i = 1; i < kArrayLength; i++) {
     EXPECT_EQ(values[0], values[i]);
   }
 }
 
+#ifndef ABSL_HAVE_THREAD_SANITIZER
+static_assert(std::is_trivially_destructible<SpinLock>());
+#endif
+
 TEST(SpinLock, StackNonCooperativeDisablesScheduling) {
   SpinLock spinlock(base_internal::SCHEDULE_KERNEL_ONLY);
-  spinlock.Lock();
+  SpinLockHolder l(spinlock);
   EXPECT_FALSE(base_internal::SchedulingGuard::ReschedulingIsAllowed());
-  spinlock.Unlock();
 }
 
 TEST(SpinLock, StaticNonCooperativeDisablesScheduling) {
-  static_noncooperative_spinlock.Lock();
+  SpinLockHolder l(static_noncooperative_spinlock);
   EXPECT_FALSE(base_internal::SchedulingGuard::ReschedulingIsAllowed());
-  static_noncooperative_spinlock.Unlock();
 }
 
 TEST(SpinLock, WaitCyclesEncoding) {
@@ -126,29 +151,29 @@ TEST(SpinLock, WaitCyclesEncoding) {
   // We should be able to encode up to (1^kMaxCycleBits - 1) without clamping
   // but the lower kProfileTimestampShift will be dropped.
   const int kMaxCyclesShift =
-    32 - kLockwordReservedShift + kProfileTimestampShift;
-  const uint64_t kMaxCycles = (int64_t{1} << kMaxCyclesShift) - 1;
+      32 - kLockwordReservedShift + kProfileTimestampShift;
+  const int64_t kMaxCycles = (int64_t{1} << kMaxCyclesShift) - 1;
 
   // These bits should be zero after encoding.
   const uint32_t kLockwordReservedMask = (1 << kLockwordReservedShift) - 1;
 
   // These bits are dropped when wait cycles are encoded.
-  const uint64_t kProfileTimestampMask = (1 << kProfileTimestampShift) - 1;
+  const int64_t kProfileTimestampMask = (1 << kProfileTimestampShift) - 1;
 
   // Test a bunch of random values
   std::default_random_engine generator;
   // Shift to avoid overflow below.
-  std::uniform_int_distribution<uint64_t> time_distribution(
-      0, std::numeric_limits<uint64_t>::max() >> 4);
-  std::uniform_int_distribution<uint64_t> cycle_distribution(0, kMaxCycles);
+  std::uniform_int_distribution<int64_t> time_distribution(
+      0, std::numeric_limits<int64_t>::max() >> 3);
+  std::uniform_int_distribution<int64_t> cycle_distribution(0, kMaxCycles);
 
   for (int i = 0; i < 100; i++) {
     int64_t start_time = time_distribution(generator);
     int64_t cycles = cycle_distribution(generator);
     int64_t end_time = start_time + cycles;
     uint32_t lock_value = SpinLockTest::EncodeWaitCycles(start_time, end_time);
-    EXPECT_EQ(0, lock_value & kLockwordReservedMask);
-    uint64_t decoded = SpinLockTest::DecodeWaitCycles(lock_value);
+    EXPECT_EQ(0u, lock_value & kLockwordReservedMask);
+    int64_t decoded = SpinLockTest::DecodeWaitCycles(lock_value);
     EXPECT_EQ(0, decoded & kProfileTimestampMask);
     EXPECT_EQ(cycles & ~kProfileTimestampMask, decoded);
   }
@@ -163,44 +188,40 @@ TEST(SpinLock, WaitCyclesEncoding) {
             SpinLockTest::DecodeWaitCycles(~kLockwordReservedMask));
 
   // Check that we cannot produce kSpinLockSleeper during encoding.
-  int64_t sleeper_cycles =
-      kSpinLockSleeper << (kProfileTimestampShift - kLockwordReservedShift);
+  int64_t sleeper_cycles = kSpinLockSleeper
+                           << (kProfileTimestampShift - kLockwordReservedShift);
   uint32_t sleeper_value =
       SpinLockTest::EncodeWaitCycles(start_time, start_time + sleeper_cycles);
   EXPECT_NE(sleeper_value, kSpinLockSleeper);
 
   // Test clamping
   uint32_t max_value =
-    SpinLockTest::EncodeWaitCycles(start_time, start_time + kMaxCycles);
-  uint64_t max_value_decoded = SpinLockTest::DecodeWaitCycles(max_value);
-  uint64_t expected_max_value_decoded = kMaxCycles & ~kProfileTimestampMask;
+      SpinLockTest::EncodeWaitCycles(start_time, start_time + kMaxCycles);
+  int64_t max_value_decoded = SpinLockTest::DecodeWaitCycles(max_value);
+  int64_t expected_max_value_decoded = kMaxCycles & ~kProfileTimestampMask;
   EXPECT_EQ(expected_max_value_decoded, max_value_decoded);
 
   const int64_t step = (1 << kProfileTimestampShift);
-  uint32_t after_max_value =
-    SpinLockTest::EncodeWaitCycles(start_time, start_time + kMaxCycles + step);
-  uint64_t after_max_value_decoded =
+  uint32_t after_max_value = SpinLockTest::EncodeWaitCycles(
+      start_time, start_time + kMaxCycles + step);
+  int64_t after_max_value_decoded =
       SpinLockTest::DecodeWaitCycles(after_max_value);
   EXPECT_EQ(expected_max_value_decoded, after_max_value_decoded);
 
   uint32_t before_max_value = SpinLockTest::EncodeWaitCycles(
       start_time, start_time + kMaxCycles - step);
-  uint64_t before_max_value_decoded =
-    SpinLockTest::DecodeWaitCycles(before_max_value);
+  int64_t before_max_value_decoded =
+      SpinLockTest::DecodeWaitCycles(before_max_value);
   EXPECT_GT(expected_max_value_decoded, before_max_value_decoded);
 }
 
-TEST(SpinLockWithThreads, StaticSpinLock) {
-  ThreadedTest(&static_spinlock);
-}
-
 TEST(SpinLockWithThreads, StackSpinLock) {
   SpinLock spinlock;
   ThreadedTest(&spinlock);
 }
 
 TEST(SpinLockWithThreads, StackCooperativeSpinLock) {
-  SpinLock spinlock(base_internal::SCHEDULE_COOPERATIVE_AND_KERNEL);
+  SpinLock spinlock;
   ThreadedTest(&spinlock);
 }
 
@@ -223,12 +244,12 @@ TEST(SpinLockWithThreads, DoesNotDeadlock) {
                                BlockingCounter* b) {
       locked->WaitForNotification();  // Wait for LockThenWait() to hold "s".
       b->DecrementCount();
-      SpinLockHolder l(spinlock);
+      SpinLockHolder l(*spinlock);
     }
 
     static void LockThenWait(Notification* locked, SpinLock* spinlock,
                              BlockingCounter* b) {
-      SpinLockHolder l(spinlock);
+      SpinLockHolder l(*spinlock);
       locked->Notify();
       b->Wait();
     }
@@ -251,19 +272,32 @@ TEST(SpinLockWithThreads, DoesNotDeadlock) {
     }
   };
 
-  SpinLock stack_cooperative_spinlock(
-      base_internal::SCHEDULE_COOPERATIVE_AND_KERNEL);
+  int num_threads = base_internal::NumCPUs() * 2;
+  SpinLock stack_cooperative_spinlock;
   SpinLock stack_noncooperative_spinlock(base_internal::SCHEDULE_KERNEL_ONLY);
-  Helper::DeadlockTest(&stack_cooperative_spinlock,
-                       base_internal::NumCPUs() * 2);
-  Helper::DeadlockTest(&stack_noncooperative_spinlock,
-                       base_internal::NumCPUs() * 2);
-  Helper::DeadlockTest(&static_cooperative_spinlock,
-                       base_internal::NumCPUs() * 2);
-  Helper::DeadlockTest(&static_noncooperative_spinlock,
-                       base_internal::NumCPUs() * 2);
+  Helper::DeadlockTest(&stack_cooperative_spinlock, num_threads);
+  Helper::DeadlockTest(&stack_noncooperative_spinlock, num_threads);
+  Helper::DeadlockTest(&static_cooperative_spinlock, num_threads);
+  Helper::DeadlockTest(&static_noncooperative_spinlock, num_threads);
+}
+
+TEST(SpinLockTest, IsCooperative) {
+  SpinLock default_constructor;
+  EXPECT_TRUE(SpinLockTest::IsCooperative(default_constructor));
+
+  SpinLock cooperative;
+  EXPECT_TRUE(SpinLockTest::IsCooperative(cooperative));
+
+  SpinLock kernel_only(base_internal::SCHEDULE_KERNEL_ONLY);
+  EXPECT_FALSE(SpinLockTest::IsCooperative(kernel_only));
+}
+
+TEST(SpinLockTest, ScopedLock) {
+  SpinLock s;
+  std::scoped_lock l(s);
 }
 
 }  // namespace
 }  // namespace base_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/base/thread_annotations.h b/absl/base/thread_annotations.h
index f98af9f93d0..4a3f3e33e9e 100644
--- a/absl/base/thread_annotations.h
+++ b/absl/base/thread_annotations.h
@@ -34,14 +34,8 @@
 #ifndef ABSL_BASE_THREAD_ANNOTATIONS_H_
 #define ABSL_BASE_THREAD_ANNOTATIONS_H_
 
-// TODO(mbonadei): Remove after the backward compatibility period.
-#include "absl/base/internal/thread_annotations.h"  // IWYU pragma: export
-
-#if defined(__clang__)
-#define ABSL_INTERNAL_THREAD_ANNOTATION_ATTRIBUTE(x) __attribute__((x))
-#else
-#define ABSL_INTERNAL_THREAD_ANNOTATION_ATTRIBUTE(x)  // no-op
-#endif
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
 
 // ABSL_GUARDED_BY()
 //
@@ -60,8 +54,11 @@
 //     int p1_ ABSL_GUARDED_BY(mu_);
 //     ...
 //   };
-#define ABSL_GUARDED_BY(x) \
-  ABSL_INTERNAL_THREAD_ANNOTATION_ATTRIBUTE(guarded_by(x))
+#if ABSL_HAVE_ATTRIBUTE(guarded_by)
+#define ABSL_GUARDED_BY(x) __attribute__((guarded_by(x)))
+#else
+#define ABSL_GUARDED_BY(x)
+#endif
 
 // ABSL_PT_GUARDED_BY()
 //
@@ -83,8 +80,11 @@
 //   // `q_`, guarded by `mu1_`, points to a shared memory location that is
 //   // guarded by `mu2_`:
 //   int *q_ ABSL_GUARDED_BY(mu1_) ABSL_PT_GUARDED_BY(mu2_);
-#define ABSL_PT_GUARDED_BY(x) \
-  ABSL_INTERNAL_THREAD_ANNOTATION_ATTRIBUTE(pt_guarded_by(x))
+#if ABSL_HAVE_ATTRIBUTE(pt_guarded_by)
+#define ABSL_PT_GUARDED_BY(x) __attribute__((pt_guarded_by(x)))
+#else
+#define ABSL_PT_GUARDED_BY(x)
+#endif
 
 // ABSL_ACQUIRED_AFTER() / ABSL_ACQUIRED_BEFORE()
 //
@@ -101,11 +101,17 @@
 //
 //   Mutex m1_;
 //   Mutex m2_ ABSL_ACQUIRED_AFTER(m1_);
-#define ABSL_ACQUIRED_AFTER(...) \
-  ABSL_INTERNAL_THREAD_ANNOTATION_ATTRIBUTE(acquired_after(__VA_ARGS__))
+#if ABSL_HAVE_ATTRIBUTE(acquired_after)
+#define ABSL_ACQUIRED_AFTER(...) __attribute__((acquired_after(__VA_ARGS__)))
+#else
+#define ABSL_ACQUIRED_AFTER(...)
+#endif
 
-#define ABSL_ACQUIRED_BEFORE(...) \
-  ABSL_INTERNAL_THREAD_ANNOTATION_ATTRIBUTE(acquired_before(__VA_ARGS__))
+#if ABSL_HAVE_ATTRIBUTE(acquired_before)
+#define ABSL_ACQUIRED_BEFORE(...) __attribute__((acquired_before(__VA_ARGS__)))
+#else
+#define ABSL_ACQUIRED_BEFORE(...)
+#endif
 
 // ABSL_EXCLUSIVE_LOCKS_REQUIRED() / ABSL_SHARED_LOCKS_REQUIRED()
 //
@@ -130,33 +136,50 @@
 //
 //   void foo() ABSL_EXCLUSIVE_LOCKS_REQUIRED(mu1, mu2) { ... }
 //   void bar() const ABSL_SHARED_LOCKS_REQUIRED(mu1, mu2) { ... }
-#define ABSL_EXCLUSIVE_LOCKS_REQUIRED(...)   \
-  ABSL_INTERNAL_THREAD_ANNOTATION_ATTRIBUTE( \
-      exclusive_locks_required(__VA_ARGS__))
+#if ABSL_HAVE_ATTRIBUTE(exclusive_locks_required)
+#define ABSL_EXCLUSIVE_LOCKS_REQUIRED(...) \
+  __attribute__((exclusive_locks_required(__VA_ARGS__)))
+#else
+#define ABSL_EXCLUSIVE_LOCKS_REQUIRED(...)
+#endif
 
+#if ABSL_HAVE_ATTRIBUTE(shared_locks_required)
 #define ABSL_SHARED_LOCKS_REQUIRED(...) \
-  ABSL_INTERNAL_THREAD_ANNOTATION_ATTRIBUTE(shared_locks_required(__VA_ARGS__))
+  __attribute__((shared_locks_required(__VA_ARGS__)))
+#else
+#define ABSL_SHARED_LOCKS_REQUIRED(...)
+#endif
 
 // ABSL_LOCKS_EXCLUDED()
 //
-// Documents the locks acquired in the body of the function. These locks
-// cannot be held when calling this function (as Abseil's `Mutex` locks are
+// Documents the locks that cannot be held by callers of this function, as they
+// might be acquired by this function (Abseil's `Mutex` locks are
 // non-reentrant).
-#define ABSL_LOCKS_EXCLUDED(...) \
-  ABSL_INTERNAL_THREAD_ANNOTATION_ATTRIBUTE(locks_excluded(__VA_ARGS__))
+#if ABSL_HAVE_ATTRIBUTE(locks_excluded)
+#define ABSL_LOCKS_EXCLUDED(...) __attribute__((locks_excluded(__VA_ARGS__)))
+#else
+#define ABSL_LOCKS_EXCLUDED(...)
+#endif
 
 // ABSL_LOCK_RETURNED()
 //
 // Documents a function that returns a mutex without acquiring it.  For example,
 // a public getter method that returns a pointer to a private mutex should
 // be annotated with ABSL_LOCK_RETURNED.
-#define ABSL_LOCK_RETURNED(x) \
-  ABSL_INTERNAL_THREAD_ANNOTATION_ATTRIBUTE(lock_returned(x))
+#if ABSL_HAVE_ATTRIBUTE(lock_returned)
+#define ABSL_LOCK_RETURNED(x) __attribute__((lock_returned(x)))
+#else
+#define ABSL_LOCK_RETURNED(x)
+#endif
 
 // ABSL_LOCKABLE
 //
 // Documents if a class/type is a lockable type (such as the `Mutex` class).
-#define ABSL_LOCKABLE ABSL_INTERNAL_THREAD_ANNOTATION_ATTRIBUTE(lockable)
+#if ABSL_HAVE_ATTRIBUTE(lockable)
+#define ABSL_LOCKABLE __attribute__((lockable))
+#else
+#define ABSL_LOCKABLE
+#endif
 
 // ABSL_SCOPED_LOCKABLE
 //
@@ -165,30 +188,43 @@
 // acquired, and the destructor should use `UNLOCK_FUNCTION()` with no
 // arguments; the analysis will assume that the destructor unlocks whatever the
 // constructor locked.
-#define ABSL_SCOPED_LOCKABLE \
-  ABSL_INTERNAL_THREAD_ANNOTATION_ATTRIBUTE(scoped_lockable)
+#if ABSL_HAVE_ATTRIBUTE(scoped_lockable)
+#define ABSL_SCOPED_LOCKABLE __attribute__((scoped_lockable))
+#else
+#define ABSL_SCOPED_LOCKABLE
+#endif
 
 // ABSL_EXCLUSIVE_LOCK_FUNCTION()
 //
 // Documents functions that acquire a lock in the body of a function, and do
 // not release it.
-#define ABSL_EXCLUSIVE_LOCK_FUNCTION(...)    \
-  ABSL_INTERNAL_THREAD_ANNOTATION_ATTRIBUTE( \
-      exclusive_lock_function(__VA_ARGS__))
+#if ABSL_HAVE_ATTRIBUTE(exclusive_lock_function)
+#define ABSL_EXCLUSIVE_LOCK_FUNCTION(...) \
+  __attribute__((exclusive_lock_function(__VA_ARGS__)))
+#else
+#define ABSL_EXCLUSIVE_LOCK_FUNCTION(...)
+#endif
 
 // ABSL_SHARED_LOCK_FUNCTION()
 //
 // Documents functions that acquire a shared (reader) lock in the body of a
 // function, and do not release it.
+#if ABSL_HAVE_ATTRIBUTE(shared_lock_function)
 #define ABSL_SHARED_LOCK_FUNCTION(...) \
-  ABSL_INTERNAL_THREAD_ANNOTATION_ATTRIBUTE(shared_lock_function(__VA_ARGS__))
+  __attribute__((shared_lock_function(__VA_ARGS__)))
+#else
+#define ABSL_SHARED_LOCK_FUNCTION(...)
+#endif
 
 // ABSL_UNLOCK_FUNCTION()
 //
 // Documents functions that expect a lock to be held on entry to the function,
 // and release it in the body of the function.
-#define ABSL_UNLOCK_FUNCTION(...) \
-  ABSL_INTERNAL_THREAD_ANNOTATION_ATTRIBUTE(unlock_function(__VA_ARGS__))
+#if ABSL_HAVE_ATTRIBUTE(unlock_function)
+#define ABSL_UNLOCK_FUNCTION(...) __attribute__((unlock_function(__VA_ARGS__)))
+#else
+#define ABSL_UNLOCK_FUNCTION(...)
+#endif
 
 // ABSL_EXCLUSIVE_TRYLOCK_FUNCTION() / ABSL_SHARED_TRYLOCK_FUNCTION()
 //
@@ -198,31 +234,49 @@
 // success, or `false` for functions that return `false` on success. The second
 // argument specifies the mutex that is locked on success. If unspecified, this
 // mutex is assumed to be `this`.
+#if ABSL_HAVE_ATTRIBUTE(exclusive_trylock_function)
 #define ABSL_EXCLUSIVE_TRYLOCK_FUNCTION(...) \
-  ABSL_INTERNAL_THREAD_ANNOTATION_ATTRIBUTE( \
-      exclusive_trylock_function(__VA_ARGS__))
+  __attribute__((exclusive_trylock_function(__VA_ARGS__)))
+#else
+#define ABSL_EXCLUSIVE_TRYLOCK_FUNCTION(...)
+#endif
 
-#define ABSL_SHARED_TRYLOCK_FUNCTION(...)    \
-  ABSL_INTERNAL_THREAD_ANNOTATION_ATTRIBUTE( \
-      shared_trylock_function(__VA_ARGS__))
+#if ABSL_HAVE_ATTRIBUTE(shared_trylock_function)
+#define ABSL_SHARED_TRYLOCK_FUNCTION(...) \
+  __attribute__((shared_trylock_function(__VA_ARGS__)))
+#else
+#define ABSL_SHARED_TRYLOCK_FUNCTION(...)
+#endif
 
 // ABSL_ASSERT_EXCLUSIVE_LOCK() / ABSL_ASSERT_SHARED_LOCK()
 //
 // Documents functions that dynamically check to see if a lock is held, and fail
 // if it is not held.
+#if ABSL_HAVE_ATTRIBUTE(assert_exclusive_lock)
 #define ABSL_ASSERT_EXCLUSIVE_LOCK(...) \
-  ABSL_INTERNAL_THREAD_ANNOTATION_ATTRIBUTE(assert_exclusive_lock(__VA_ARGS__))
+  __attribute__((assert_exclusive_lock(__VA_ARGS__)))
+#else
+#define ABSL_ASSERT_EXCLUSIVE_LOCK(...)
+#endif
 
+#if ABSL_HAVE_ATTRIBUTE(assert_shared_lock)
 #define ABSL_ASSERT_SHARED_LOCK(...) \
-  ABSL_INTERNAL_THREAD_ANNOTATION_ATTRIBUTE(assert_shared_lock(__VA_ARGS__))
+  __attribute__((assert_shared_lock(__VA_ARGS__)))
+#else
+#define ABSL_ASSERT_SHARED_LOCK(...)
+#endif
 
 // ABSL_NO_THREAD_SAFETY_ANALYSIS
 //
 // Turns off thread safety checking within the body of a particular function.
 // This annotation is used to mark functions that are known to be correct, but
 // the locking behavior is more complicated than the analyzer can handle.
+#if ABSL_HAVE_ATTRIBUTE(no_thread_safety_analysis)
 #define ABSL_NO_THREAD_SAFETY_ANALYSIS \
-  ABSL_INTERNAL_THREAD_ANNOTATION_ATTRIBUTE(no_thread_safety_analysis)
+  __attribute__((no_thread_safety_analysis))
+#else
+#define ABSL_NO_THREAD_SAFETY_ANALYSIS
+#endif
 
 //------------------------------------------------------------------------------
 // Tool-Supplied Annotations
@@ -256,11 +310,12 @@
 #define ABSL_TS_UNCHECKED_READ(x) absl::base_internal::ts_unchecked_read(x)
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 
 // Takes a reference to a guarded data member, and returns an unguarded
 // reference.
-// Do not used this function directly, use ABSL_TS_UNCHECKED_READ instead.
+// Do not use this function directly, use ABSL_TS_UNCHECKED_READ instead.
 template <typename T>
 inline const T& ts_unchecked_read(const T& v) ABSL_NO_THREAD_SAFETY_ANALYSIS {
   return v;
@@ -272,6 +327,7 @@ inline T& ts_unchecked_read(T& v) ABSL_NO_THREAD_SAFETY_ANALYSIS {
 }
 
 }  // namespace base_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_BASE_THREAD_ANNOTATIONS_H_
diff --git a/absl/base/throw_delegate.cc b/absl/base/throw_delegate.cc
new file mode 100644
index 00000000000..3ac399cc6e5
--- /dev/null
+++ b/absl/base/throw_delegate.cc
@@ -0,0 +1,206 @@
+// Copyright 2017 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/base/throw_delegate.h"
+
+#include <cstdlib>
+#include <functional>
+#include <new>
+#include <stdexcept>
+#include <string>
+
+#include "absl/base/config.h"
+#include "absl/base/internal/raw_logging.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+void ThrowStdLogicError(const std::string& what_arg) {
+#ifdef ABSL_HAVE_EXCEPTIONS
+  throw std::logic_error(what_arg);
+#else
+  ABSL_RAW_LOG(FATAL, "%s", what_arg.c_str());
+  std::abort();
+#endif
+}
+void ThrowStdLogicError(const char* what_arg) {
+#ifdef ABSL_HAVE_EXCEPTIONS
+  throw std::logic_error(what_arg);
+#else
+  ABSL_RAW_LOG(FATAL, "%s", what_arg);
+  std::abort();
+#endif
+}
+void ThrowStdInvalidArgument(const std::string& what_arg) {
+#ifdef ABSL_HAVE_EXCEPTIONS
+  throw std::invalid_argument(what_arg);
+#else
+  ABSL_RAW_LOG(FATAL, "%s", what_arg.c_str());
+  std::abort();
+#endif
+}
+void ThrowStdInvalidArgument(const char* what_arg) {
+#ifdef ABSL_HAVE_EXCEPTIONS
+  throw std::invalid_argument(what_arg);
+#else
+  ABSL_RAW_LOG(FATAL, "%s", what_arg);
+  std::abort();
+#endif
+}
+
+void ThrowStdDomainError(const std::string& what_arg) {
+#ifdef ABSL_HAVE_EXCEPTIONS
+  throw std::domain_error(what_arg);
+#else
+  ABSL_RAW_LOG(FATAL, "%s", what_arg.c_str());
+  std::abort();
+#endif
+}
+void ThrowStdDomainError(const char* what_arg) {
+#ifdef ABSL_HAVE_EXCEPTIONS
+  throw std::domain_error(what_arg);
+#else
+  ABSL_RAW_LOG(FATAL, "%s", what_arg);
+  std::abort();
+#endif
+}
+
+void ThrowStdLengthError(const std::string& what_arg) {
+#ifdef ABSL_HAVE_EXCEPTIONS
+  throw std::length_error(what_arg);
+#else
+  ABSL_RAW_LOG(FATAL, "%s", what_arg.c_str());
+  std::abort();
+#endif
+}
+void ThrowStdLengthError(const char* what_arg) {
+#ifdef ABSL_HAVE_EXCEPTIONS
+  throw std::length_error(what_arg);
+#else
+  ABSL_RAW_LOG(FATAL, "%s", what_arg);
+  std::abort();
+#endif
+}
+
+void ThrowStdOutOfRange(const std::string& what_arg) {
+#ifdef ABSL_HAVE_EXCEPTIONS
+  throw std::out_of_range(what_arg);
+#else
+  ABSL_RAW_LOG(FATAL, "%s", what_arg.c_str());
+  std::abort();
+#endif
+}
+void ThrowStdOutOfRange(const char* what_arg) {
+#ifdef ABSL_HAVE_EXCEPTIONS
+  throw std::out_of_range(what_arg);
+#else
+  ABSL_RAW_LOG(FATAL, "%s", what_arg);
+  std::abort();
+#endif
+}
+
+void ThrowStdRuntimeError(const std::string& what_arg) {
+#ifdef ABSL_HAVE_EXCEPTIONS
+  throw std::runtime_error(what_arg);
+#else
+  ABSL_RAW_LOG(FATAL, "%s", what_arg.c_str());
+  std::abort();
+#endif
+}
+void ThrowStdRuntimeError(const char* what_arg) {
+#ifdef ABSL_HAVE_EXCEPTIONS
+  throw std::runtime_error(what_arg);
+#else
+  ABSL_RAW_LOG(FATAL, "%s", what_arg);
+  std::abort();
+#endif
+}
+
+void ThrowStdRangeError(const std::string& what_arg) {
+#ifdef ABSL_HAVE_EXCEPTIONS
+  throw std::range_error(what_arg);
+#else
+  ABSL_RAW_LOG(FATAL, "%s", what_arg.c_str());
+  std::abort();
+#endif
+}
+void ThrowStdRangeError(const char* what_arg) {
+#ifdef ABSL_HAVE_EXCEPTIONS
+  throw std::range_error(what_arg);
+#else
+  ABSL_RAW_LOG(FATAL, "%s", what_arg);
+  std::abort();
+#endif
+}
+
+void ThrowStdOverflowError(const std::string& what_arg) {
+#ifdef ABSL_HAVE_EXCEPTIONS
+  throw std::overflow_error(what_arg);
+#else
+  ABSL_RAW_LOG(FATAL, "%s", what_arg.c_str());
+  std::abort();
+#endif
+}
+void ThrowStdOverflowError(const char* what_arg) {
+#ifdef ABSL_HAVE_EXCEPTIONS
+  throw std::overflow_error(what_arg);
+#else
+  ABSL_RAW_LOG(FATAL, "%s", what_arg);
+  std::abort();
+#endif
+}
+
+void ThrowStdUnderflowError(const std::string& what_arg) {
+#ifdef ABSL_HAVE_EXCEPTIONS
+  throw std::underflow_error(what_arg);
+#else
+  ABSL_RAW_LOG(FATAL, "%s", what_arg.c_str());
+  std::abort();
+#endif
+}
+void ThrowStdUnderflowError(const char* what_arg) {
+#ifdef ABSL_HAVE_EXCEPTIONS
+  throw std::underflow_error(what_arg);
+#else
+  ABSL_RAW_LOG(FATAL, "%s", what_arg);
+  std::abort();
+#endif
+}
+
+void ThrowStdBadFunctionCall() {
+#ifdef ABSL_HAVE_EXCEPTIONS
+  throw std::bad_function_call();
+#else
+  std::abort();
+#endif
+}
+
+void ThrowStdBadAlloc() {
+#ifdef ABSL_HAVE_EXCEPTIONS
+  throw std::bad_alloc();
+#else
+  std::abort();
+#endif
+}
+
+void ThrowStdBadArrayNewLength() {
+#ifdef ABSL_HAVE_EXCEPTIONS
+  throw std::bad_array_new_length();
+#else
+  std::abort();
+#endif
+}
+
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/base/internal/throw_delegate.h b/absl/base/throw_delegate.h
similarity index 81%
rename from absl/base/internal/throw_delegate.h
rename to absl/base/throw_delegate.h
index 03c700b5ee0..e89712ab79c 100644
--- a/absl/base/internal/throw_delegate.h
+++ b/absl/base/throw_delegate.h
@@ -1,4 +1,3 @@
-//
 // Copyright 2017 The Abseil Authors.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
@@ -12,15 +11,16 @@
 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 // See the License for the specific language governing permissions and
 // limitations under the License.
-//
 
-#ifndef ABSL_BASE_INTERNAL_THROW_DELEGATE_H_
-#define ABSL_BASE_INTERNAL_THROW_DELEGATE_H_
+#ifndef ABSL_BASE_THROW_DELEGATE_H_
+#define ABSL_BASE_THROW_DELEGATE_H_
 
 #include <string>
 
+#include "absl/base/config.h"
+
 namespace absl {
-namespace base_internal {
+ABSL_NAMESPACE_BEGIN
 
 // Helper functions that allow throwing exceptions consistently from anywhere.
 // The main use case is for header-based libraries (eg templates), as they will
@@ -57,15 +57,9 @@ namespace base_internal {
 
 [[noreturn]] void ThrowStdBadFunctionCall();
 [[noreturn]] void ThrowStdBadAlloc();
+[[noreturn]] void ThrowStdBadArrayNewLength();
 
-// ThrowStdBadArrayNewLength() cannot be consistently supported because
-// std::bad_array_new_length is missing in libstdc++ until 4.9.0.
-// https://gcc.gnu.org/onlinedocs/gcc-4.8.3/libstdc++/api/a01379_source.html
-// https://gcc.gnu.org/onlinedocs/gcc-4.9.0/libstdc++/api/a01327_source.html
-// libcxx (as of 3.2) and msvc (as of 2015) both have it.
-// [[noreturn]] void ThrowStdBadArrayNewLength();
-
-}  // namespace base_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
-#endif  // ABSL_BASE_INTERNAL_THROW_DELEGATE_H_
+#endif  // ABSL_BASE_THROW_DELEGATE_H_
diff --git a/absl/base/throw_delegate_test.cc b/absl/base/throw_delegate_test.cc
index 5ba4ce55e6d..c8fc1cdd2a8 100644
--- a/absl/base/throw_delegate_test.cc
+++ b/absl/base/throw_delegate_test.cc
@@ -12,28 +12,30 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-#include "absl/base/internal/throw_delegate.h"
+#include "absl/base/throw_delegate.h"
 
 #include <functional>
 #include <new>
 #include <stdexcept>
+#include <string>
 
-#include "absl/base/config.h"
 #include "gtest/gtest.h"
+#include "absl/base/config.h"
 
 namespace {
 
-using absl::base_internal::ThrowStdLogicError;
-using absl::base_internal::ThrowStdInvalidArgument;
-using absl::base_internal::ThrowStdDomainError;
-using absl::base_internal::ThrowStdLengthError;
-using absl::base_internal::ThrowStdOutOfRange;
-using absl::base_internal::ThrowStdRuntimeError;
-using absl::base_internal::ThrowStdRangeError;
-using absl::base_internal::ThrowStdOverflowError;
-using absl::base_internal::ThrowStdUnderflowError;
-using absl::base_internal::ThrowStdBadFunctionCall;
-using absl::base_internal::ThrowStdBadAlloc;
+using absl::ThrowStdBadAlloc;
+using absl::ThrowStdBadArrayNewLength;
+using absl::ThrowStdBadFunctionCall;
+using absl::ThrowStdDomainError;
+using absl::ThrowStdInvalidArgument;
+using absl::ThrowStdLengthError;
+using absl::ThrowStdLogicError;
+using absl::ThrowStdOutOfRange;
+using absl::ThrowStdOverflowError;
+using absl::ThrowStdRangeError;
+using absl::ThrowStdRuntimeError;
+using absl::ThrowStdUnderflowError;
 
 constexpr const char* what_arg = "The quick brown fox jumps over the lazy dog";
 
@@ -78,30 +80,88 @@ void ExpectThrowNoWhat(void (*f)()) {
 #endif
 }
 
-TEST(ThrowHelper, Test) {
-  // Not using EXPECT_THROW because we want to check the .what() message too.
+TEST(ThrowDelegate, ThrowStdLogicErrorChar) {
   ExpectThrowChar<std::logic_error>(ThrowStdLogicError);
+}
+
+TEST(ThrowDelegate, ThrowStdInvalidArgumentChar) {
   ExpectThrowChar<std::invalid_argument>(ThrowStdInvalidArgument);
+}
+
+TEST(ThrowDelegate, ThrowStdDomainErrorChar) {
   ExpectThrowChar<std::domain_error>(ThrowStdDomainError);
+}
+
+TEST(ThrowDelegate, ThrowStdLengthErrorChar) {
   ExpectThrowChar<std::length_error>(ThrowStdLengthError);
+}
+
+TEST(ThrowDelegate, ThrowStdOutOfRangeChar) {
   ExpectThrowChar<std::out_of_range>(ThrowStdOutOfRange);
+}
+
+TEST(ThrowDelegate, ThrowStdRuntimeErrorChar) {
   ExpectThrowChar<std::runtime_error>(ThrowStdRuntimeError);
+}
+
+TEST(ThrowDelegate, ThrowStdRangeErrorChar) {
   ExpectThrowChar<std::range_error>(ThrowStdRangeError);
+}
+
+TEST(ThrowDelegate, ThrowStdOverflowErrorChar) {
   ExpectThrowChar<std::overflow_error>(ThrowStdOverflowError);
+}
+
+TEST(ThrowDelegate, ThrowStdUnderflowErrorChar) {
   ExpectThrowChar<std::underflow_error>(ThrowStdUnderflowError);
+}
 
+TEST(ThrowDelegate, ThrowStdLogicErrorString) {
   ExpectThrowString<std::logic_error>(ThrowStdLogicError);
+}
+
+TEST(ThrowDelegate, ThrowStdInvalidArgumentString) {
   ExpectThrowString<std::invalid_argument>(ThrowStdInvalidArgument);
+}
+
+TEST(ThrowDelegate, ThrowStdDomainErrorString) {
   ExpectThrowString<std::domain_error>(ThrowStdDomainError);
+}
+
+TEST(ThrowDelegate, ThrowStdLengthErrorString) {
   ExpectThrowString<std::length_error>(ThrowStdLengthError);
+}
+
+TEST(ThrowDelegate, ThrowStdOutOfRangeString) {
   ExpectThrowString<std::out_of_range>(ThrowStdOutOfRange);
+}
+
+TEST(ThrowDelegate, ThrowStdRuntimeErrorString) {
   ExpectThrowString<std::runtime_error>(ThrowStdRuntimeError);
+}
+
+TEST(ThrowDelegate, ThrowStdRangeErrorString) {
   ExpectThrowString<std::range_error>(ThrowStdRangeError);
+}
+
+TEST(ThrowDelegate, ThrowStdOverflowErrorString) {
   ExpectThrowString<std::overflow_error>(ThrowStdOverflowError);
+}
+
+TEST(ThrowDelegate, ThrowStdUnderflowErrorString) {
   ExpectThrowString<std::underflow_error>(ThrowStdUnderflowError);
+}
 
+TEST(ThrowDelegate, ThrowStdBadFunctionCallNoWhat) {
   ExpectThrowNoWhat<std::bad_function_call>(ThrowStdBadFunctionCall);
+}
+
+TEST(ThrowDelegate, ThrowStdBadAllocNoWhat) {
   ExpectThrowNoWhat<std::bad_alloc>(ThrowStdBadAlloc);
 }
 
+TEST(ThrowDelegate, ThrowStdBadArrayNewLength) {
+  ExpectThrowNoWhat<std::bad_array_new_length>(ThrowStdBadArrayNewLength);
+}
+
 }  // namespace
diff --git a/absl/cleanup/BUILD.bazel b/absl/cleanup/BUILD.bazel
new file mode 100644
index 00000000000..dad0d6b85a6
--- /dev/null
+++ b/absl/cleanup/BUILD.bazel
@@ -0,0 +1,78 @@
+# Copyright 2021 The Abseil Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+load("@rules_cc//cc:cc_library.bzl", "cc_library")
+load("@rules_cc//cc:cc_test.bzl", "cc_test")
+load(
+    "//absl:copts/configure_copts.bzl",
+    "ABSL_DEFAULT_COPTS",
+    "ABSL_DEFAULT_LINKOPTS",
+    "ABSL_TEST_COPTS",
+)
+
+package(
+    default_visibility = ["//visibility:private"],
+    features = [
+        "header_modules",
+        "layering_check",
+        "parse_headers",
+    ],
+)
+
+licenses(["notice"])
+
+cc_library(
+    name = "cleanup_internal",
+    hdrs = ["internal/cleanup.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],  # Should be private, but cleanup is required.
+    deps = [
+        "//absl/base:core_headers",
+        "//absl/base:hardening",
+        "//absl/utility",
+    ],
+)
+
+cc_library(
+    name = "cleanup",
+    hdrs = [
+        "cleanup.h",
+    ],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
+    deps = [
+        ":cleanup_internal",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:hardening",
+    ],
+)
+
+cc_test(
+    name = "cleanup_test",
+    size = "small",
+    srcs = [
+        "cleanup_test.cc",
+    ],
+    copts = ABSL_TEST_COPTS,
+    deps = [
+        ":cleanup",
+        "//absl/base:config",
+        "//absl/utility",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
diff --git a/absl/cleanup/CMakeLists.txt b/absl/cleanup/CMakeLists.txt
new file mode 100644
index 00000000000..47460c9a900
--- /dev/null
+++ b/absl/cleanup/CMakeLists.txt
@@ -0,0 +1,57 @@
+# Copyright 2021 The Abseil Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# Internal-only target, do not depend on directly.
+absl_cc_library(
+  NAME
+    cleanup_internal
+  HDRS
+    "internal/cleanup.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::core_headers
+    absl::hardening
+    absl::utility
+  PUBLIC
+)
+
+absl_cc_library(
+  NAME
+    cleanup
+  HDRS
+    "cleanup.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::cleanup_internal
+    absl::config
+    absl::core_headers
+    absl::hardening
+  PUBLIC
+)
+
+absl_cc_test(
+  NAME
+    cleanup_test
+  SRCS
+    "cleanup_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::cleanup
+    absl::config
+    absl::utility
+    GTest::gmock_main
+)
diff --git a/absl/cleanup/cleanup.h b/absl/cleanup/cleanup.h
new file mode 100644
index 00000000000..078674afb39
--- /dev/null
+++ b/absl/cleanup/cleanup.h
@@ -0,0 +1,143 @@
+// Copyright 2021 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: cleanup.h
+// -----------------------------------------------------------------------------
+//
+// `absl::Cleanup` implements the scope guard idiom, invoking the contained
+// callback's `operator()() &&` on scope exit.
+//
+// This class doesn't allocate or take any locks, and is safe to use in a signal
+// handler. Of course the callback with which it is constructed also must be
+// signal safe in order for this to be useful.
+//
+// Example:
+//
+// ```
+//   absl::Status CopyGoodData(const char* source_path, const char* sink_path) {
+//     FILE* source_file = fopen(source_path, "r");
+//     if (source_file == nullptr) {
+//       return absl::NotFoundError("No source file");  // No cleanups execute
+//     }
+//
+//     // C++17 style cleanup using class template argument deduction
+//     absl::Cleanup source_closer = [source_file] { fclose(source_file); };
+//
+//     FILE* sink_file = fopen(sink_path, "w");
+//     if (sink_file == nullptr) {
+//       return absl::NotFoundError("No sink file");  // First cleanup executes
+//     }
+//
+//     // C++11 style cleanup using the factory function
+//     auto sink_closer = absl::MakeCleanup([sink_file] { fclose(sink_file); });
+//
+//     Data data;
+//     while (ReadData(source_file, &data)) {
+//       if (!data.IsGood()) {
+//         absl::Status result = absl::FailedPreconditionError("Read bad data");
+//         return result;  // Both cleanups execute
+//       }
+//       SaveData(sink_file, &data);
+//     }
+//
+//     return absl::OkStatus();  // Both cleanups execute
+//   }
+// ```
+//
+// Methods:
+//
+// `std::move(cleanup).Cancel()` will prevent the callback from executing.
+//
+// `std::move(cleanup).Invoke()` will execute the callback early, before
+// destruction, and prevent the callback from executing in the destructor.
+//
+// Usage:
+//
+// `absl::Cleanup` is not an interface type. It is only intended to be used
+// within the body of a function. It is not a value type and instead models a
+// control flow construct. Check out `defer` in Golang for something similar.
+
+#ifndef ABSL_CLEANUP_CLEANUP_H_
+#define ABSL_CLEANUP_CLEANUP_H_
+
+#include <utility>
+
+#include "absl/base/config.h"
+#include "absl/base/internal/hardening.h"
+#include "absl/base/macros.h"
+#include "absl/cleanup/internal/cleanup.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+template <typename Arg, typename Callback = void()>
+class [[nodiscard]] Cleanup final {
+  static_assert(cleanup_internal::WasDeduced<Arg>(),
+                "Explicit template parameters are not supported.");
+
+  static_assert(cleanup_internal::ReturnsVoid<Callback>(),
+                "Callbacks that return values are not supported.");
+
+ public:
+  Cleanup(Callback callback) : storage_(std::move(callback)) {}  // NOLINT
+
+  Cleanup(Cleanup&& other) = default;
+
+  void Cancel() && {
+    absl::base_internal::HardeningAssert(storage_.IsCallbackEngaged());
+    storage_.DestroyCallback();
+  }
+
+  void Invoke() && {
+    absl::base_internal::HardeningAssert(storage_.IsCallbackEngaged());
+    storage_.InvokeCallback();
+    storage_.DestroyCallback();
+  }
+
+  ~Cleanup() {
+    if (storage_.IsCallbackEngaged()) {
+      storage_.InvokeCallback();
+      storage_.DestroyCallback();
+    }
+  }
+
+ private:
+  cleanup_internal::Storage<Callback> storage_;
+};
+
+// `absl::Cleanup c = /* callback */;`
+//
+// C++17 type deduction API for creating an instance of `absl::Cleanup`
+template <typename Callback>
+Cleanup(Callback callback) -> Cleanup<cleanup_internal::Tag, Callback>;
+
+// `auto c = absl::MakeCleanup(/* callback */);`
+//
+// C++11 type deduction API for creating an instance of `absl::Cleanup`
+template <typename... Args, typename Callback>
+absl::Cleanup<cleanup_internal::Tag, Callback> MakeCleanup(Callback callback) {
+  static_assert(cleanup_internal::WasDeduced<cleanup_internal::Tag, Args...>(),
+                "Explicit template parameters are not supported.");
+
+  static_assert(cleanup_internal::ReturnsVoid<Callback>(),
+                "Callbacks that return values are not supported.");
+
+  return {std::move(callback)};
+}
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_CLEANUP_CLEANUP_H_
diff --git a/absl/cleanup/cleanup_test.cc b/absl/cleanup/cleanup_test.cc
new file mode 100644
index 00000000000..a413b7efeb2
--- /dev/null
+++ b/absl/cleanup/cleanup_test.cc
@@ -0,0 +1,303 @@
+// Copyright 2021 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/cleanup/cleanup.h"
+
+#include <functional>
+#include <type_traits>
+#include <utility>
+
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+#include "absl/utility/utility.h"
+
+namespace {
+
+using Tag = absl::cleanup_internal::Tag;
+
+template <typename Type1, typename Type2>
+constexpr bool IsSame() {
+  return (std::is_same_v<Type1, Type2>);
+}
+
+struct IdentityFactory {
+  template <typename Callback>
+  static Callback AsCallback(Callback callback) {
+    return Callback(std::move(callback));
+  }
+};
+
+// `FunctorClass` is a type used for testing `absl::Cleanup`. It is intended to
+// represent users that make their own move-only callback types outside of
+// `std::function` and lambda literals.
+class FunctorClass {
+  using Callback = std::function<void()>;
+
+ public:
+  explicit FunctorClass(Callback callback) : callback_(std::move(callback)) {}
+
+  FunctorClass(FunctorClass&& other)
+      : callback_(std::exchange(other.callback_, Callback())) {}
+
+  FunctorClass(const FunctorClass&) = delete;
+
+  FunctorClass& operator=(const FunctorClass&) = delete;
+
+  FunctorClass& operator=(FunctorClass&&) = delete;
+
+  void operator()() const& = delete;
+
+  void operator()() && {
+    ASSERT_TRUE(callback_);
+    callback_();
+    callback_ = nullptr;
+  }
+
+ private:
+  Callback callback_;
+};
+
+struct FunctorClassFactory {
+  template <typename Callback>
+  static FunctorClass AsCallback(Callback callback) {
+    return FunctorClass(std::move(callback));
+  }
+};
+
+struct StdFunctionFactory {
+  template <typename Callback>
+  static std::function<void()> AsCallback(Callback callback) {
+    return std::function<void()>(std::move(callback));
+  }
+};
+
+using CleanupTestParams =
+    ::testing::Types<IdentityFactory, FunctorClassFactory, StdFunctionFactory>;
+template <typename>
+struct CleanupTest : public ::testing::Test {};
+TYPED_TEST_SUITE(CleanupTest, CleanupTestParams);
+
+bool fn_ptr_called = false;
+void FnPtrFunction() { fn_ptr_called = true; }
+
+TYPED_TEST(CleanupTest, FactoryProducesCorrectType) {
+  {
+    auto callback = TypeParam::AsCallback([] {});
+    auto cleanup = absl::MakeCleanup(std::move(callback));
+
+    static_assert(
+        IsSame<absl::Cleanup<Tag, decltype(callback)>, decltype(cleanup)>());
+  }
+
+  {
+    auto cleanup = absl::MakeCleanup(&FnPtrFunction);
+
+    static_assert(IsSame<absl::Cleanup<Tag, void (*)()>, decltype(cleanup)>());
+  }
+
+  {
+    auto cleanup = absl::MakeCleanup(FnPtrFunction);
+
+    static_assert(IsSame<absl::Cleanup<Tag, void (*)()>, decltype(cleanup)>());
+  }
+}
+
+TYPED_TEST(CleanupTest, CTADProducesCorrectType) {
+  {
+    auto callback = TypeParam::AsCallback([] {});
+    absl::Cleanup cleanup = std::move(callback);
+
+    static_assert(
+        IsSame<absl::Cleanup<Tag, decltype(callback)>, decltype(cleanup)>());
+  }
+
+  {
+    absl::Cleanup cleanup = &FnPtrFunction;
+
+    static_assert(IsSame<absl::Cleanup<Tag, void (*)()>, decltype(cleanup)>());
+  }
+
+  {
+    absl::Cleanup cleanup = FnPtrFunction;
+
+    static_assert(IsSame<absl::Cleanup<Tag, void (*)()>, decltype(cleanup)>());
+  }
+}
+
+TYPED_TEST(CleanupTest, FactoryAndCTADProduceSameType) {
+  {
+    auto callback = IdentityFactory::AsCallback([] {});
+    auto factory_cleanup = absl::MakeCleanup(callback);
+    absl::Cleanup deduction_cleanup = callback;
+
+    static_assert(
+        IsSame<decltype(factory_cleanup), decltype(deduction_cleanup)>());
+  }
+
+  {
+    auto factory_cleanup =
+        absl::MakeCleanup(FunctorClassFactory::AsCallback([] {}));
+    absl::Cleanup deduction_cleanup = FunctorClassFactory::AsCallback([] {});
+
+    static_assert(
+        IsSame<decltype(factory_cleanup), decltype(deduction_cleanup)>());
+  }
+
+  {
+    auto factory_cleanup =
+        absl::MakeCleanup(StdFunctionFactory::AsCallback([] {}));
+    absl::Cleanup deduction_cleanup = StdFunctionFactory::AsCallback([] {});
+
+    static_assert(
+        IsSame<decltype(factory_cleanup), decltype(deduction_cleanup)>());
+  }
+
+  {
+    auto factory_cleanup = absl::MakeCleanup(&FnPtrFunction);
+    absl::Cleanup deduction_cleanup = &FnPtrFunction;
+
+    static_assert(
+        IsSame<decltype(factory_cleanup), decltype(deduction_cleanup)>());
+  }
+
+  {
+    auto factory_cleanup = absl::MakeCleanup(FnPtrFunction);
+    absl::Cleanup deduction_cleanup = FnPtrFunction;
+
+    static_assert(
+        IsSame<decltype(factory_cleanup), decltype(deduction_cleanup)>());
+  }
+}
+
+TYPED_TEST(CleanupTest, BasicUsage) {
+  bool called = false;
+
+  {
+    auto cleanup =
+        absl::MakeCleanup(TypeParam::AsCallback([&called] { called = true; }));
+    EXPECT_FALSE(called);  // Constructor shouldn't invoke the callback
+  }
+
+  EXPECT_TRUE(called);  // Destructor should invoke the callback
+}
+
+TYPED_TEST(CleanupTest, BasicUsageWithFunctionPointer) {
+  fn_ptr_called = false;
+
+  {
+    auto cleanup = absl::MakeCleanup(TypeParam::AsCallback(&FnPtrFunction));
+    EXPECT_FALSE(fn_ptr_called);  // Constructor shouldn't invoke the callback
+  }
+
+  EXPECT_TRUE(fn_ptr_called);  // Destructor should invoke the callback
+}
+
+TYPED_TEST(CleanupTest, Cancel) {
+  bool called = false;
+
+  {
+    auto cleanup =
+        absl::MakeCleanup(TypeParam::AsCallback([&called] { called = true; }));
+    EXPECT_FALSE(called);  // Constructor shouldn't invoke the callback
+
+    std::move(cleanup).Cancel();
+    EXPECT_FALSE(called);  // Cancel shouldn't invoke the callback
+  }
+
+  EXPECT_FALSE(called);  // Destructor shouldn't invoke the callback
+}
+
+TYPED_TEST(CleanupTest, Invoke) {
+  bool called = false;
+
+  {
+    auto cleanup =
+        absl::MakeCleanup(TypeParam::AsCallback([&called] { called = true; }));
+    EXPECT_FALSE(called);  // Constructor shouldn't invoke the callback
+
+    std::move(cleanup).Invoke();
+    EXPECT_TRUE(called);  // Invoke should invoke the callback
+
+    called = false;  // Reset tracker before destructor runs
+  }
+
+  EXPECT_FALSE(called);  // Destructor shouldn't invoke the callback
+}
+
+TYPED_TEST(CleanupTest, Move) {
+  bool called = false;
+
+  {
+    auto moved_from_cleanup =
+        absl::MakeCleanup(TypeParam::AsCallback([&called] { called = true; }));
+    EXPECT_FALSE(called);  // Constructor shouldn't invoke the callback
+
+    {
+      auto moved_to_cleanup = std::move(moved_from_cleanup);
+      EXPECT_FALSE(called);  // Move shouldn't invoke the callback
+    }
+
+    EXPECT_TRUE(called);  // Destructor should invoke the callback
+
+    called = false;  // Reset tracker before destructor runs
+  }
+
+  EXPECT_FALSE(called);  // Destructor shouldn't invoke the callback
+}
+
+int DestructionCount = 0;
+
+struct DestructionCounter {
+  void operator()() {}
+
+  ~DestructionCounter() { ++DestructionCount; }
+};
+
+TYPED_TEST(CleanupTest, DestructorDestroys) {
+  {
+    auto cleanup =
+        absl::MakeCleanup(TypeParam::AsCallback(DestructionCounter()));
+    DestructionCount = 0;
+  }
+
+  EXPECT_EQ(DestructionCount, 1);  // Engaged cleanup destroys
+}
+
+TYPED_TEST(CleanupTest, CancelDestroys) {
+  {
+    auto cleanup =
+        absl::MakeCleanup(TypeParam::AsCallback(DestructionCounter()));
+    DestructionCount = 0;
+
+    std::move(cleanup).Cancel();
+    EXPECT_EQ(DestructionCount, 1);  // Cancel destroys
+  }
+
+  EXPECT_EQ(DestructionCount, 1);  // Canceled cleanup does not double destroy
+}
+
+TYPED_TEST(CleanupTest, InvokeDestroys) {
+  {
+    auto cleanup =
+        absl::MakeCleanup(TypeParam::AsCallback(DestructionCounter()));
+    DestructionCount = 0;
+
+    std::move(cleanup).Invoke();
+    EXPECT_EQ(DestructionCount, 1);  // Invoke destroys
+  }
+
+  EXPECT_EQ(DestructionCount, 1);  // Invoked cleanup does not double destroy
+}
+
+}  // namespace
diff --git a/absl/cleanup/internal/cleanup.h b/absl/cleanup/internal/cleanup.h
new file mode 100644
index 00000000000..f078323d99d
--- /dev/null
+++ b/absl/cleanup/internal/cleanup.h
@@ -0,0 +1,99 @@
+// Copyright 2021 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_CLEANUP_INTERNAL_CLEANUP_H_
+#define ABSL_CLEANUP_INTERNAL_CLEANUP_H_
+
+#include <new>
+#include <type_traits>
+#include <utility>
+
+#include "absl/base/internal/hardening.h"
+#include "absl/base/macros.h"
+#include "absl/base/thread_annotations.h"
+#include "absl/utility/utility.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+namespace cleanup_internal {
+
+struct Tag {};
+
+template <typename Arg, typename... Args>
+constexpr bool WasDeduced() {
+  return (std::is_same_v<cleanup_internal::Tag, Arg>) && (sizeof...(Args) == 0);
+}
+
+template <typename Callback>
+constexpr bool ReturnsVoid() {
+  return (std::is_same_v<std::invoke_result_t<Callback>, void>);
+}
+
+template <typename Callback>
+class Storage {
+ public:
+  Storage() = delete;
+
+  explicit Storage(Callback callback) {
+    // Placement-new into a character buffer is used for eager destruction when
+    // the cleanup is invoked or cancelled. To ensure this optimizes well, the
+    // behavior is implemented locally instead of using a std::optional.
+    ::new (GetCallbackBuffer()) Callback(std::move(callback));
+    is_callback_engaged_ = true;
+  }
+
+  Storage(Storage&& other) {
+    absl::base_internal::HardeningAssert(other.IsCallbackEngaged());
+
+    ::new (GetCallbackBuffer()) Callback(std::move(other.GetCallback()));
+    is_callback_engaged_ = true;
+
+    other.DestroyCallback();
+  }
+
+  Storage(const Storage& other) = delete;
+
+  Storage& operator=(Storage&& other) = delete;
+
+  Storage& operator=(const Storage& other) = delete;
+
+  void* GetCallbackBuffer() { return static_cast<void*>(callback_buffer_); }
+
+  Callback& GetCallback() {
+    return *reinterpret_cast<Callback*>(GetCallbackBuffer());
+  }
+
+  bool IsCallbackEngaged() const { return is_callback_engaged_; }
+
+  void DestroyCallback() {
+    is_callback_engaged_ = false;
+    GetCallback().~Callback();
+  }
+
+  void InvokeCallback() ABSL_NO_THREAD_SAFETY_ANALYSIS {
+    std::move(GetCallback())();
+  }
+
+ private:
+  bool is_callback_engaged_;
+  alignas(Callback) unsigned char callback_buffer_[sizeof(Callback)];
+};
+
+}  // namespace cleanup_internal
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_CLEANUP_INTERNAL_CLEANUP_H_
diff --git a/absl/compiler_config_setting.bzl b/absl/compiler_config_setting.bzl
deleted file mode 100644
index 66962294d09..00000000000
--- a/absl/compiler_config_setting.bzl
+++ /dev/null
@@ -1,38 +0,0 @@
-#
-# Copyright 2018 The Abseil Authors.
-#
-# Licensed under the Apache License, Version 2.0 (the "License");
-# you may not use this file except in compliance with the License.
-# You may obtain a copy of the License at
-#
-#      https://www.apache.org/licenses/LICENSE-2.0
-#
-# Unless required by applicable law or agreed to in writing, software
-# distributed under the License is distributed on an "AS IS" BASIS,
-# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-# See the License for the specific language governing permissions and
-# limitations under the License.
-
-"""Creates config_setting that allows selecting based on 'compiler' value."""
-
-def create_llvm_config(name, visibility):
-    # The "do_not_use_tools_cpp_compiler_present" attribute exists to
-    # distinguish between older versions of Bazel that do not support
-    # "@bazel_tools//tools/cpp:compiler" flag_value, and newer ones that do.
-    # In the future, the only way to select on the compiler will be through
-    # flag_values{"@bazel_tools//tools/cpp:compiler"} and the else branch can
-    # be removed.
-    if hasattr(cc_common, "do_not_use_tools_cpp_compiler_present"):
-        native.config_setting(
-            name = name,
-            flag_values = {
-                "@bazel_tools//tools/cpp:compiler": "llvm",
-            },
-            visibility = visibility,
-        )
-    else:
-        native.config_setting(
-            name = name,
-            values = {"compiler": "llvm"},
-            visibility = visibility,
-        )
diff --git a/absl/container/BUILD.bazel b/absl/container/BUILD.bazel
index 0894bb207c6..f9b0c5cea66 100644
--- a/absl/container/BUILD.bazel
+++ b/absl/container/BUILD.bazel
@@ -14,7 +14,9 @@
 # limitations under the License.
 #
 
-load("@rules_cc//cc:defs.bzl", "cc_library", "cc_test")
+load("@rules_cc//cc:cc_binary.bzl", "cc_binary")
+load("@rules_cc//cc:cc_library.bzl", "cc_library")
+load("@rules_cc//cc:cc_test.bzl", "cc_test")
 load(
     "//absl:copts/configure_copts.bzl",
     "ABSL_DEFAULT_COPTS",
@@ -22,15 +24,23 @@ load(
     "ABSL_TEST_COPTS",
 )
 
-package(default_visibility = ["//visibility:public"])
+package(
+    default_visibility = ["//visibility:private"],
+    features = [
+        "header_modules",
+        "layering_check",
+        "parse_headers",
+    ],
+)
 
-licenses(["notice"])  # Apache 2.0
+licenses(["notice"])
 
 cc_library(
     name = "compressed_tuple",
     hdrs = ["internal/compressed_tuple.h"],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],  # Should be private, but cleanup is required.
     deps = [
         "//absl/utility",
     ],
@@ -45,10 +55,9 @@ cc_test(
         ":compressed_tuple",
         ":test_instance_tracker",
         "//absl/memory",
-        "//absl/types:any",
-        "//absl/types:optional",
         "//absl/utility",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -57,12 +66,17 @@ cc_library(
     hdrs = ["fixed_array.h"],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
     deps = [
         ":compressed_tuple",
         "//absl/algorithm",
+        "//absl/base:config",
         "//absl/base:core_headers",
         "//absl/base:dynamic_annotations",
+        "//absl/base:hardening",
+        "//absl/base:iterator_traits_internal",
         "//absl/base:throw_delegate",
+        "//absl/hash:weakly_mixed_integer",
         "//absl/memory",
     ],
 )
@@ -74,10 +88,14 @@ cc_test(
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":fixed_array",
+        ":test_allocator",
+        "//absl/base:config",
         "//absl/base:exception_testing",
+        "//absl/base:iterator_traits_test_helper",
         "//absl/hash:hash_testing",
         "//absl/memory",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -90,19 +108,21 @@ cc_test(
         ":fixed_array",
         "//absl/base:config",
         "//absl/base:exception_safety_testing",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
-cc_test(
+cc_binary(
     name = "fixed_array_benchmark",
+    testonly = True,
     srcs = ["fixed_array_benchmark.cc"],
     copts = ABSL_TEST_COPTS + ["$(STACK_FRAME_UNLIMITED)"],
     linkopts = ABSL_DEFAULT_LINKOPTS,
     tags = ["benchmark"],
     deps = [
         ":fixed_array",
-        "@com_github_google_benchmark//:benchmark_main",
+        "@google_benchmark//:benchmark_main",
     ],
 )
 
@@ -113,7 +133,10 @@ cc_library(
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":compressed_tuple",
+        "//absl/base:base_internal",
+        "//absl/base:config",
         "//absl/base:core_headers",
+        "//absl/base:hardening",
         "//absl/memory",
         "//absl/meta:type_traits",
         "//absl/types:span",
@@ -125,21 +148,26 @@ cc_library(
     hdrs = ["inlined_vector.h"],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
     deps = [
         ":inlined_vector_internal",
         "//absl/algorithm",
         "//absl/base:core_headers",
+        "//absl/base:hardening",
+        "//absl/base:iterator_traits_internal",
         "//absl/base:throw_delegate",
+        "//absl/hash:weakly_mixed_integer",
         "//absl/memory",
+        "//absl/meta:type_traits",
     ],
 )
 
 cc_library(
-    name = "counting_allocator",
-    testonly = 1,
-    hdrs = ["internal/counting_allocator.h"],
-    copts = ABSL_DEFAULT_COPTS,
+    name = "test_allocator",
+    testonly = True,
+    copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    textual_hdrs = ["internal/test_allocator.h"],
     visibility = ["//visibility:private"],
 )
 
@@ -149,21 +177,25 @@ cc_test(
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
-        ":counting_allocator",
         ":inlined_vector",
+        ":test_allocator",
         ":test_instance_tracker",
+        "//absl/base:config",
         "//absl/base:core_headers",
         "//absl/base:exception_testing",
-        "//absl/base:raw_logging_internal",
+        "//absl/base:iterator_traits_test_helper",
         "//absl/hash:hash_testing",
+        "//absl/log:check",
         "//absl/memory",
         "//absl/strings",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
-cc_test(
+cc_binary(
     name = "inlined_vector_benchmark",
+    testonly = True,
     srcs = ["inlined_vector_benchmark.cc"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
@@ -173,7 +205,7 @@ cc_test(
         "//absl/base:core_headers",
         "//absl/base:raw_logging_internal",
         "//absl/strings",
-        "@com_github_google_benchmark//:benchmark_main",
+        "@google_benchmark//:benchmark_main",
     ],
 )
 
@@ -185,19 +217,21 @@ cc_test(
         ":inlined_vector",
         "//absl/base:config",
         "//absl/base:exception_safety_testing",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
 cc_library(
     name = "test_instance_tracker",
-    testonly = 1,
+    testonly = True,
     srcs = ["internal/test_instance_tracker.cc"],
     hdrs = ["internal/test_instance_tracker.h"],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     visibility = [
         "//absl:__subpackages__",
+        "@do_not_use_for_gloop_visibility_only//gloop/util/gtl:__subpackages__",
     ],
     deps = ["//absl/types:compare"],
 )
@@ -209,15 +243,11 @@ cc_test(
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":test_instance_tracker",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
-NOTEST_TAGS_NONMOBILE = [
-    "no_test_darwin_x86_64",
-    "no_test_loonix",
-]
-
 NOTEST_TAGS_MOBILE = [
     "no_test_android_arm",
     "no_test_android_arm64",
@@ -225,19 +255,19 @@ NOTEST_TAGS_MOBILE = [
     "no_test_ios_x86_64",
 ]
 
-NOTEST_TAGS = NOTEST_TAGS_MOBILE + NOTEST_TAGS_NONMOBILE
-
 cc_library(
     name = "flat_hash_map",
     hdrs = ["flat_hash_map.h"],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
     deps = [
         ":container_memory",
-        ":hash_function_defaults",
+        ":hash_container_defaults",
         ":raw_hash_map",
         "//absl/algorithm:container",
-        "//absl/memory",
+        "//absl/base:core_headers",
+        "//absl/meta:type_traits",
     ],
 )
 
@@ -246,16 +276,21 @@ cc_test(
     srcs = ["flat_hash_map_test.cc"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
-    tags = NOTEST_TAGS_NONMOBILE,
+    tags = ["no_test_loonix"],
     deps = [
         ":flat_hash_map",
         ":hash_generator_testing",
+        ":hash_policy_testing",
+        ":test_allocator",
         ":unordered_map_constructor_test",
         ":unordered_map_lookup_test",
         ":unordered_map_members_test",
         ":unordered_map_modifiers_test",
-        "//absl/types:any",
-        "@com_google_googletest//:gtest_main",
+        "//absl/base:config",
+        "//absl/log:check",
+        "//absl/meta:type_traits",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -264,32 +299,41 @@ cc_library(
     hdrs = ["flat_hash_set.h"],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
     deps = [
         ":container_memory",
-        ":hash_function_defaults",
+        ":hash_container_defaults",
         ":raw_hash_set",
         "//absl/algorithm:container",
         "//absl/base:core_headers",
         "//absl/memory",
+        "//absl/meta:type_traits",
     ],
 )
 
 cc_test(
     name = "flat_hash_set_test",
     srcs = ["flat_hash_set_test.cc"],
-    copts = ABSL_TEST_COPTS + ["-DUNORDERED_SET_CXX17"],
+    copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
-    tags = NOTEST_TAGS_NONMOBILE,
+    tags = ["no_test_loonix"],
     deps = [
+        ":container_memory",
         ":flat_hash_set",
+        ":hash_container_defaults",
         ":hash_generator_testing",
+        ":test_allocator",
         ":unordered_set_constructor_test",
         ":unordered_set_lookup_test",
         ":unordered_set_members_test",
         ":unordered_set_modifiers_test",
+        "//absl/base:config",
+        "//absl/hash",
+        "//absl/log:check",
         "//absl/memory",
         "//absl/strings",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -298,13 +342,16 @@ cc_library(
     hdrs = ["node_hash_map.h"],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
     deps = [
         ":container_memory",
-        ":hash_function_defaults",
-        ":node_hash_policy",
+        ":hash_container_defaults",
+        ":node_slot_policy",
         ":raw_hash_map",
         "//absl/algorithm:container",
+        "//absl/base:core_headers",
         "//absl/memory",
+        "//absl/meta:type_traits",
     ],
 )
 
@@ -313,16 +360,18 @@ cc_test(
     srcs = ["node_hash_map_test.cc"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
-    tags = NOTEST_TAGS_NONMOBILE,
+    tags = ["no_test_loonix"],
     deps = [
-        ":hash_generator_testing",
+        ":hash_policy_testing",
         ":node_hash_map",
         ":tracked",
         ":unordered_map_constructor_test",
         ":unordered_map_lookup_test",
         ":unordered_map_members_test",
         ":unordered_map_modifiers_test",
-        "@com_google_googletest//:gtest_main",
+        "//absl/base:config",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -331,28 +380,37 @@ cc_library(
     hdrs = ["node_hash_set.h"],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
     deps = [
-        ":hash_function_defaults",
-        ":node_hash_policy",
+        ":container_memory",
+        ":hash_container_defaults",
+        ":node_slot_policy",
         ":raw_hash_set",
         "//absl/algorithm:container",
+        "//absl/base:core_headers",
         "//absl/memory",
+        "//absl/meta:type_traits",
     ],
 )
 
 cc_test(
     name = "node_hash_set_test",
     srcs = ["node_hash_set_test.cc"],
-    copts = ABSL_TEST_COPTS + ["-DUNORDERED_SET_CXX17"],
+    copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
-    tags = NOTEST_TAGS_NONMOBILE,
+    tags = ["no_test_loonix"],
     deps = [
+        ":hash_generator_testing",
+        ":hash_policy_testing",
         ":node_hash_set",
         ":unordered_set_constructor_test",
         ":unordered_set_lookup_test",
         ":unordered_set_members_test",
         ":unordered_set_modifiers_test",
-        "@com_google_googletest//:gtest_main",
+        "//absl/base:config",
+        "//absl/memory",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -361,8 +419,16 @@ cc_library(
     hdrs = ["internal/container_memory.h"],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl:__subpackages__",
+        "@do_not_use_for_gloop_visibility_only//gloop/strings:__pkg__",
+        "@do_not_use_for_gloop_visibility_only//gloop/util/gtl:__pkg__",
+    ],
     deps = [
+        "//absl/base:config",
+        "//absl/hash",
         "//absl/memory",
+        "//absl/meta:type_traits",
         "//absl/utility",
     ],
 )
@@ -372,11 +438,16 @@ cc_test(
     srcs = ["internal/container_memory_test.cc"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
-    tags = NOTEST_TAGS_NONMOBILE,
+    tags = ["no_test_loonix"],
     deps = [
         ":container_memory",
+        ":test_instance_tracker",
+        "//absl/base:config",
+        "//absl/base:no_destructor",
+        "//absl/meta:type_traits",
         "//absl/strings",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -385,10 +456,28 @@ cc_library(
     hdrs = ["internal/hash_function_defaults.h"],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "@do_not_use_for_gloop_visibility_only//gloop/util/gtl:__subpackages__",
+    ],
     deps = [
+        ":common",
         "//absl/base:config",
         "//absl/hash",
+        "//absl/meta:type_traits",
         "//absl/strings",
+        "//absl/strings:cord",
+    ],
+)
+
+cc_library(
+    name = "hash_container_defaults",
+    hdrs = ["hash_container_defaults.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
+    deps = [
+        ":hash_function_defaults",
+        "//absl/base:config",
     ],
 )
 
@@ -397,36 +486,48 @@ cc_test(
     srcs = ["internal/hash_function_defaults_test.cc"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
-    tags = NOTEST_TAGS,
+    tags = NOTEST_TAGS_MOBILE + ["no_test_loonix"],
     deps = [
+        ":flat_hash_map",
+        ":flat_hash_set",
         ":hash_function_defaults",
         "//absl/hash",
+        "//absl/random",
         "//absl/strings",
-        "@com_google_googletest//:gtest_main",
+        "//absl/strings:cord",
+        "//absl/strings:cord_test_helpers",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
 cc_library(
     name = "hash_generator_testing",
-    testonly = 1,
+    testonly = True,
     srcs = ["internal/hash_generator_testing.cc"],
     hdrs = ["internal/hash_generator_testing.h"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":hash_policy_testing",
+        "//absl/base:config",
+        "//absl/base:no_destructor",
         "//absl/memory",
         "//absl/meta:type_traits",
+        "//absl/random",
         "//absl/strings",
     ],
 )
 
 cc_library(
     name = "hash_policy_testing",
-    testonly = 1,
+    testonly = True,
     hdrs = ["internal/hash_policy_testing.h"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "@do_not_use_for_gloop_visibility_only//gloop/util/gtl:__subpackages__",
+    ],
     deps = [
         "//absl/hash",
         "//absl/strings",
@@ -440,7 +541,8 @@ cc_test(
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":hash_policy_testing",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -449,7 +551,11 @@ cc_library(
     hdrs = ["internal/hash_policy_traits.h"],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
-    deps = ["//absl/meta:type_traits"],
+    deps = [
+        ":common_policy_traits",
+        ":container_memory",
+        "//absl/meta:type_traits",
+    ],
 )
 
 cc_test(
@@ -458,8 +564,32 @@ cc_test(
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
+        ":container_memory",
         ":hash_policy_traits",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_library(
+    name = "common_policy_traits",
+    hdrs = ["internal/common_policy_traits.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:private"],
+    deps = ["//absl/meta:type_traits"],
+)
+
+cc_test(
+    name = "common_policy_traits_test",
+    srcs = ["internal/common_policy_traits_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":common_policy_traits",
+        "//absl/base:config",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -478,6 +608,12 @@ cc_library(
     hdrs = ["internal/hashtable_debug_hooks.h"],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl/container:__pkg__",
+    ],
+    deps = [
+        "//absl/base:config",
+    ],
 )
 
 cc_library(
@@ -489,13 +625,21 @@ cc_library(
     hdrs = ["internal/hashtablez_sampler.h"],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl:__subpackages__",
+    ],
     deps = [
-        ":have_sse",
         "//absl/base",
+        "//absl/base:config",
         "//absl/base:core_headers",
+        "//absl/base:no_destructor",
+        "//absl/base:raw_logging_internal",
         "//absl/debugging:stacktrace",
         "//absl/memory",
+        "//absl/profiling:exponential_biased",
+        "//absl/profiling:sample_recorder",
         "//absl/synchronization",
+        "//absl/time",
         "//absl/utility",
     ],
 )
@@ -504,33 +648,46 @@ cc_test(
     name = "hashtablez_sampler_test",
     srcs = ["internal/hashtablez_sampler_test.cc"],
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    tags = [
+        "no_test_wasm",
+    ],
     deps = [
         ":hashtablez_sampler",
-        ":have_sse",
+        "//absl/base:config",
         "//absl/base:core_headers",
+        "//absl/profiling:sample_recorder",
+        "//absl/random",
         "//absl/synchronization",
         "//absl/synchronization:thread_pool",
         "//absl/time",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
 cc_library(
-    name = "node_hash_policy",
-    hdrs = ["internal/node_hash_policy.h"],
+    name = "node_slot_policy",
+    hdrs = ["internal/node_slot_policy.h"],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl:__subpackages__",
+        "@do_not_use_for_gloop_visibility_only//gloop/util/gtl:__pkg__",
+    ],
+    deps = ["//absl/base:config"],
 )
 
 cc_test(
-    name = "node_hash_policy_test",
-    srcs = ["internal/node_hash_policy_test.cc"],
+    name = "node_slot_policy_test",
+    srcs = ["internal/node_slot_policy_test.cc"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":hash_policy_traits",
-        ":node_hash_policy",
-        "@com_google_googletest//:gtest_main",
+        ":node_slot_policy",
+        "//absl/base:config",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -539,29 +696,83 @@ cc_library(
     hdrs = ["internal/raw_hash_map.h"],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl:__subpackages__",
+        "@do_not_use_for_gloop_visibility_only//gloop/util/gtl:__pkg__",
+    ],
     deps = [
+        ":common_policy_traits",
         ":container_memory",
         ":raw_hash_set",
+        "//absl/base:config",
+        "//absl/base:core_headers",
         "//absl/base:throw_delegate",
+        "//absl/meta:type_traits",
     ],
 )
 
 cc_library(
-    name = "have_sse",
-    hdrs = ["internal/have_sse.h"],
+    name = "common",
+    hdrs = ["internal/common.h"],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
-    visibility = ["//visibility:private"],
+    visibility = [
+        "//absl/container:__pkg__",
+        "@do_not_use_for_gloop_visibility_only//gloop/util/gtl:__pkg__",
+    ],
+    deps = [
+        "//absl/meta:type_traits",
+        "//absl/types:optional",
+    ],
 )
 
 cc_library(
-    name = "common",
-    hdrs = ["internal/common.h"],
+    name = "hashtable_control_bytes",
+    hdrs = ["internal/hashtable_control_bytes.h"],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl/container:__pkg__",
+    ],
     deps = [
-        "//absl/meta:type_traits",
-        "//absl/types:optional",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:endian",
+        "//absl/numeric:bits",
+    ],
+)
+
+cc_test(
+    name = "hashtable_control_bytes_test",
+    srcs = ["internal/hashtable_control_bytes_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":hashtable_control_bytes",
+        "//absl/base:config",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_library(
+    name = "raw_hash_set_resize_impl",
+    hdrs = ["internal/raw_hash_set_resize_impl.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = ["//absl/base:config"],
+)
+
+cc_test(
+    name = "raw_hash_set_resize_impl_test",
+    srcs = ["internal/raw_hash_set_resize_impl_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":raw_hash_set_resize_impl",
+        "//absl/base:config",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -571,21 +782,32 @@ cc_library(
     hdrs = ["internal/raw_hash_set.h"],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],  # Should be private, but requires cleanup
     deps = [
         ":common",
+        ":common_policy_traits",
         ":compressed_tuple",
         ":container_memory",
+        ":hash_function_defaults",
         ":hash_policy_traits",
+        ":hashtable_control_bytes",
         ":hashtable_debug_hooks",
         ":hashtablez_sampler",
-        ":have_sse",
-        ":layout",
-        "//absl/base:bits",
+        ":raw_hash_set_resize_impl",
+        "//absl/base",
         "//absl/base:config",
         "//absl/base:core_headers",
+        "//absl/base:dynamic_annotations",
         "//absl/base:endian",
+        "//absl/base:iterator_traits_internal",
+        "//absl/base:prefetch",
+        "//absl/base:raw_logging_internal",
+        "//absl/functional:function_ref",
+        "//absl/hash",
+        "//absl/hash:weakly_mixed_integer",
         "//absl/memory",
         "//absl/meta:type_traits",
+        "//absl/numeric:bits",
         "//absl/utility",
     ],
 )
@@ -595,18 +817,85 @@ cc_test(
     srcs = ["internal/raw_hash_set_test.cc"],
     copts = ABSL_TEST_COPTS,
     linkstatic = 1,
-    tags = NOTEST_TAGS,
+    tags = NOTEST_TAGS_MOBILE + [
+        "no_test_chromiumos_arm",
+        "no_test_chromiumos_armv7",
+        "no_test_chromiumos_x86_64",
+        "no_test_loonix",
+        # TODO(b/237097643): investigate race and remove
+        "noarm_gemu",
+    ],
     deps = [
         ":container_memory",
+        ":flat_hash_map",
+        ":flat_hash_set",
         ":hash_function_defaults",
         ":hash_policy_testing",
+        ":hashtable_control_bytes",
         ":hashtable_debug",
+        ":hashtablez_sampler",
+        ":node_hash_set",
         ":raw_hash_set",
+        ":raw_hash_set_resize_impl",
+        ":test_allocator",
+        ":test_instance_tracker",
         "//absl/base",
+        "//absl/base:config",
         "//absl/base:core_headers",
+        "//absl/base:prefetch",
+        "//absl/functional:function_ref",
+        "//absl/hash",
+        "//absl/log",
+        "//absl/log:check",
+        "//absl/memory",
+        "//absl/meta:type_traits",
+        "//absl/numeric:int128",
+        "//absl/random",
+        "//absl/strings",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_binary(
+    name = "raw_hash_set_benchmark",
+    testonly = True,
+    srcs = ["internal/raw_hash_set_benchmark.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    tags = ["benchmark"],
+    visibility = ["//visibility:private"],
+    deps = [
+        ":container_memory",
+        ":hash_function_defaults",
+        ":raw_hash_set",
         "//absl/base:raw_logging_internal",
+        "//absl/random",
+        "//absl/strings:str_format",
+        "@google_benchmark//:benchmark_main",
+    ],
+)
+
+cc_binary(
+    name = "raw_hash_set_probe_benchmark",
+    testonly = True,
+    srcs = ["internal/raw_hash_set_probe_benchmark.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = select({
+        "//conditions:default": [],
+    }) + ABSL_DEFAULT_LINKOPTS,
+    tags = ["benchmark"],
+    visibility = ["//visibility:private"],
+    deps = [
+        ":flat_hash_map",
+        ":hash_function_defaults",
+        ":hashtable_debug",
+        ":raw_hash_set",
+        "//absl/base:no_destructor",
+        "//absl/random",
+        "//absl/random:distributions",
         "//absl/strings",
-        "@com_google_googletest//:gtest_main",
+        "//absl/strings:str_format",
     ],
 )
 
@@ -617,10 +906,12 @@ cc_test(
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
+        ":container_memory",
         ":raw_hash_set",
         ":tracked",
-        "//absl/base:core_headers",
-        "@com_google_googletest//:gtest_main",
+        "//absl/base:config",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -629,8 +920,11 @@ cc_library(
     hdrs = ["internal/layout.h"],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],  # Should be private, requires cleanup
     deps = [
+        "//absl/base:config",
         "//absl/base:core_headers",
+        "//absl/debugging:demangle_internal",
         "//absl/meta:type_traits",
         "//absl/strings",
         "//absl/types:span",
@@ -644,125 +938,178 @@ cc_test(
     srcs = ["internal/layout_test.cc"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
-    tags = NOTEST_TAGS,
+    tags = NOTEST_TAGS_MOBILE + ["no_test_loonix"],
+    visibility = ["//visibility:private"],
+    deps = [
+        ":layout",
+        "//absl/base:config",
+        "//absl/log:check",
+        "//absl/types:span",
+        "//absl/utility",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_binary(
+    name = "layout_benchmark",
+    testonly = True,
+    srcs = ["internal/layout_benchmark.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    tags = ["benchmark"],
     visibility = ["//visibility:private"],
     deps = [
         ":layout",
         "//absl/base:core_headers",
         "//absl/base:raw_logging_internal",
-        "//absl/types:span",
-        "@com_google_googletest//:gtest_main",
+        "@google_benchmark//:benchmark_main",
     ],
 )
 
 cc_library(
     name = "tracked",
-    testonly = 1,
+    testonly = True,
     hdrs = ["internal/tracked.h"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        "//absl/base:config",
+    ],
 )
 
 cc_library(
     name = "unordered_map_constructor_test",
-    testonly = 1,
+    testonly = True,
     hdrs = ["internal/unordered_map_constructor_test.h"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl/container:__pkg__",
+        "@do_not_use_for_gloop_visibility_only//gloop/util/gtl:__subpackages__",
+    ],
     deps = [
         ":hash_generator_testing",
         ":hash_policy_testing",
-        "@com_google_googletest//:gtest",
+        "//absl/base:config",
+        "@googletest//:gtest",
     ],
 )
 
 cc_library(
     name = "unordered_map_lookup_test",
-    testonly = 1,
+    testonly = True,
     hdrs = ["internal/unordered_map_lookup_test.h"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl/container:__pkg__",
+        "@do_not_use_for_gloop_visibility_only//gloop/util/gtl:__subpackages__",
+    ],
     deps = [
         ":hash_generator_testing",
         ":hash_policy_testing",
-        "@com_google_googletest//:gtest",
+        "@googletest//:gtest",
     ],
 )
 
 cc_library(
     name = "unordered_map_modifiers_test",
-    testonly = 1,
+    testonly = True,
     hdrs = ["internal/unordered_map_modifiers_test.h"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl/container:__pkg__",
+        "@do_not_use_for_gloop_visibility_only//gloop/util/gtl:__subpackages__",
+    ],
     deps = [
         ":hash_generator_testing",
         ":hash_policy_testing",
-        "@com_google_googletest//:gtest",
+        "@googletest//:gtest",
     ],
 )
 
 cc_library(
     name = "unordered_set_constructor_test",
-    testonly = 1,
+    testonly = True,
     hdrs = ["internal/unordered_set_constructor_test.h"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl/container:__pkg__",
+        "@do_not_use_for_gloop_visibility_only//gloop/util/gtl:__pkg__",
+    ],
     deps = [
         ":hash_generator_testing",
         ":hash_policy_testing",
+        "//absl/base:config",
         "//absl/meta:type_traits",
-        "@com_google_googletest//:gtest",
+        "@googletest//:gtest",
     ],
 )
 
 cc_library(
     name = "unordered_set_members_test",
-    testonly = 1,
+    testonly = True,
     hdrs = ["internal/unordered_set_members_test.h"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl/container:__pkg__",
+    ],
     deps = [
         "//absl/meta:type_traits",
-        "@com_google_googletest//:gtest",
+        "@googletest//:gtest",
     ],
 )
 
 cc_library(
     name = "unordered_map_members_test",
-    testonly = 1,
+    testonly = True,
     hdrs = ["internal/unordered_map_members_test.h"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl/container:__pkg__",
+    ],
     deps = [
         "//absl/meta:type_traits",
-        "@com_google_googletest//:gtest",
+        "@googletest//:gtest",
     ],
 )
 
 cc_library(
     name = "unordered_set_lookup_test",
-    testonly = 1,
+    testonly = True,
     hdrs = ["internal/unordered_set_lookup_test.h"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl/container:__pkg__",
+        "@do_not_use_for_gloop_visibility_only//gloop/util/gtl:__pkg__",
+    ],
     deps = [
         ":hash_generator_testing",
         ":hash_policy_testing",
-        "@com_google_googletest//:gtest",
+        "@googletest//:gtest",
     ],
 )
 
 cc_library(
     name = "unordered_set_modifiers_test",
-    testonly = 1,
+    testonly = True,
     hdrs = ["internal/unordered_set_modifiers_test.h"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl/container:__pkg__",
+        "@do_not_use_for_gloop_visibility_only//gloop/util/gtl:__pkg__",
+    ],
     deps = [
         ":hash_generator_testing",
         ":hash_policy_testing",
-        "@com_google_googletest//:gtest",
+        "@googletest//:gtest",
     ],
 )
 
@@ -771,13 +1118,14 @@ cc_test(
     srcs = ["internal/unordered_set_test.cc"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
-    tags = NOTEST_TAGS_NONMOBILE,
+    tags = ["no_test_loonix"],
     deps = [
         ":unordered_set_constructor_test",
         ":unordered_set_lookup_test",
         ":unordered_set_members_test",
         ":unordered_set_modifiers_test",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -786,13 +1134,32 @@ cc_test(
     srcs = ["internal/unordered_map_test.cc"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
-    tags = NOTEST_TAGS_NONMOBILE,
+    tags = ["no_test_loonix"],
     deps = [
         ":unordered_map_constructor_test",
         ":unordered_map_lookup_test",
         ":unordered_map_members_test",
         ":unordered_map_modifiers_test",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "sample_element_size_test",
+    srcs = ["sample_element_size_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    tags = ["no_test_loonix"],
+    visibility = ["//visibility:private"],
+    deps = [
+        ":flat_hash_map",
+        ":flat_hash_set",
+        ":hashtablez_sampler",
+        ":node_hash_map",
+        ":node_hash_set",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -811,22 +1178,27 @@ cc_library(
     visibility = ["//visibility:public"],
     deps = [
         ":common",
+        ":common_policy_traits",
         ":compressed_tuple",
         ":container_memory",
         ":layout",
+        "//absl/base:config",
         "//absl/base:core_headers",
+        "//absl/base:hardening",
+        "//absl/base:raw_logging_internal",
         "//absl/base:throw_delegate",
+        "//absl/hash:weakly_mixed_integer",
         "//absl/memory",
         "//absl/meta:type_traits",
         "//absl/strings",
+        "//absl/strings:cord",
         "//absl/types:compare",
-        "//absl/utility",
     ],
 )
 
 cc_library(
     name = "btree_test_common",
-    testonly = 1,
+    testonly = True,
     hdrs = ["btree_test.h"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
@@ -835,6 +1207,7 @@ cc_library(
         ":btree",
         ":flat_hash_set",
         "//absl/strings",
+        "//absl/strings:cord",
         "//absl/time",
     ],
 )
@@ -848,20 +1221,228 @@ cc_test(
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     shard_count = 10,
+    tags = [
+        "no_test:os:ios",
+        "no_test_ios",
+        "no_test_wasm",
+    ],
     visibility = ["//visibility:private"],
     deps = [
         ":btree",
         ":btree_test_common",
-        ":counting_allocator",
+        ":test_allocator",
         ":test_instance_tracker",
+        "//absl/algorithm:container",
         "//absl/base:core_headers",
         "//absl/base:raw_logging_internal",
         "//absl/flags:flag",
         "//absl/hash:hash_testing",
         "//absl/memory",
-        "//absl/meta:type_traits",
+        "//absl/random",
         "//absl/strings",
         "//absl/types:compare",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_binary(
+    name = "btree_benchmark",
+    testonly = True,
+    srcs = [
+        "btree_benchmark.cc",
+    ],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    tags = ["benchmark"],
+    visibility = ["//visibility:private"],
+    deps = [
+        ":btree",
+        ":btree_test_common",
+        ":flat_hash_map",
+        ":flat_hash_set",
+        ":hashtable_debug",
+        "//absl/algorithm:container",
+        "//absl/base:raw_logging_internal",
+        "//absl/hash",
+        "//absl/log",
+        "//absl/memory",
+        "//absl/random",
+        "//absl/strings:cord",
+        "//absl/strings:str_format",
+        "//absl/time",
+        "@google_benchmark//:benchmark_main",
+    ],
+)
+
+cc_library(
+    name = "heterogeneous_lookup_testing",
+    testonly = True,
+    hdrs = ["internal/heterogeneous_lookup_testing.h"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl/container:__pkg__",
+    ],
+    deps = [
+        ":test_instance_tracker",
+        "//absl/base:config",
+        "@googletest//:gtest",
+    ],
+)
+
+cc_library(
+    name = "linked_hash_set",
+    hdrs = ["linked_hash_set.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
+    deps = [
+        ":common",
+        ":flat_hash_set",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+    ],
+)
+
+cc_test(
+    name = "linked_hash_set_test",
+    srcs = ["linked_hash_set_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":hash_generator_testing",
+        ":hash_policy_testing",
+        ":heterogeneous_lookup_testing",
+        ":linked_hash_set",
+        ":test_allocator",
+        ":test_instance_tracker",
+        ":unordered_set_constructor_test",
+        ":unordered_set_lookup_test",
+        ":unordered_set_members_test",
+        ":unordered_set_modifiers_test",
+        "//absl/base:config",
+        "//absl/strings:string_view",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_binary(
+    name = "linked_hash_set_benchmark",
+    testonly = True,
+    srcs = ["linked_hash_set_benchmark.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    tags = ["benchmark"],
+    visibility = ["//visibility:private"],
+    deps = [
+        ":linked_hash_set",
+        "//absl/functional:function_ref",
+        "//absl/strings:str_format",
+        "//absl/strings:string_view",
+        "@google_benchmark//:benchmark_main",
+    ],
+)
+
+cc_library(
+    name = "linked_hash_map",
+    hdrs = ["linked_hash_map.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
+    deps = [
+        ":common",
+        ":flat_hash_set",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:throw_delegate",
+    ],
+)
+
+cc_test(
+    name = "linked_hash_map_test",
+    srcs = ["linked_hash_map_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":hash_generator_testing",
+        ":hash_policy_testing",
+        ":heterogeneous_lookup_testing",
+        ":linked_hash_map",
+        ":test_allocator",
+        ":test_instance_tracker",
+        ":unordered_map_constructor_test",
+        ":unordered_map_lookup_test",
+        ":unordered_map_members_test",
+        ":unordered_map_modifiers_test",
+        "//absl/base:config",
+        "//absl/base:exception_testing",
+        "//absl/strings:string_view",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_binary(
+    name = "linked_hash_map_benchmark",
+    testonly = True,
+    srcs = ["linked_hash_map_benchmark.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    tags = ["benchmark"],
+    visibility = ["//visibility:private"],
+    deps = [
+        ":linked_hash_map",
+        "//absl/functional:function_ref",
+        "//absl/strings:str_format",
+        "//absl/strings:string_view",
+        "@google_benchmark//:benchmark_main",
+    ],
+)
+
+cc_library(
+    name = "chunked_queue",
+    srcs = ["internal/chunked_queue.h"],
+    hdrs = ["chunked_queue.h"],
+    visibility = ["//visibility:public"],
+    deps = [
+        ":layout",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:hardening",
+        "//absl/base:iterator_traits_internal",
+    ],
+)
+
+cc_test(
+    name = "chunked_queue_test",
+    size = "small",
+    srcs = ["chunked_queue_test.cc"],
+    deps = [
+        ":chunked_queue",
+        ":test_allocator",
+        "//absl/base:core_headers",
+        "//absl/base:throw_delegate",
+        "//absl/strings",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_binary(
+    name = "chunked_queue_benchmark",
+    testonly = True,
+    srcs = ["chunked_queue_benchmark.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    tags = ["benchmark"],
+    visibility = ["//visibility:private"],
+    deps = [
+        ":chunked_queue",
+        "//absl/random",
+        "//absl/status",
+        "//absl/strings:cord",
+        "@google_benchmark//:benchmark_main",
     ],
 )
diff --git a/absl/container/CMakeLists.txt b/absl/container/CMakeLists.txt
index 933c7a8c48a..effe790f7e3 100644
--- a/absl/container/CMakeLists.txt
+++ b/absl/container/CMakeLists.txt
@@ -14,15 +14,6 @@
 # limitations under the License.
 #
 
-# This is deprecated and will be removed in the future.  It also doesn't do
-# anything anyways.  Prefer to use the library associated with the API you are
-# using.
-absl_cc_library(
-  NAME
-    container
-  PUBLIC
-)
-
 absl_cc_library(
   NAME
     btree
@@ -36,19 +27,25 @@ absl_cc_library(
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
-    absl::container_common
+    absl::common_policy_traits
     absl::compare
     absl::compressed_tuple
+    absl::config
+    absl::container_common
     absl::container_memory
+    absl::cord
     absl::core_headers
+    absl::hardening
     absl::layout
     absl::memory
+    absl::raw_logging_internal
     absl::strings
     absl::throw_delegate
     absl::type_traits
-    absl::utility
+    absl::weakly_mixed_integer
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     btree_test_common
@@ -60,6 +57,7 @@ absl_cc_library(
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
     absl::btree
+    absl::cord
     absl::flat_hash_set
     absl::strings
     absl::time
@@ -76,20 +74,22 @@ absl_cc_test(
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
+    absl::algorithm_container
     absl::btree
     absl::btree_test_common
     absl::compare
     absl::core_headers
-    absl::counting_allocator
     absl::flags
     absl::hash_testing
+    absl::random_random
     absl::raw_logging_internal
     absl::strings
+    absl::test_allocator
     absl::test_instance_tracker
-    absl::type_traits
-    gmock_main
+    GTest::gmock_main
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     compressed_tuple
@@ -110,13 +110,11 @@ absl_cc_test(
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
-    absl::any
     absl::compressed_tuple
     absl::memory
-    absl::optional
     absl::test_instance_tracker
     absl::utility
-    gmock_main
+    GTest::gmock_main
 )
 
 absl_cc_library(
@@ -129,10 +127,14 @@ absl_cc_library(
   DEPS
     absl::compressed_tuple
     absl::algorithm
+    absl::config
     absl::core_headers
     absl::dynamic_annotations
+    absl::hardening
+    absl::iterator_traits_internal
     absl::throw_delegate
     absl::memory
+    absl::weakly_mixed_integer
   PUBLIC
 )
 
@@ -145,10 +147,13 @@ absl_cc_test(
     ${ABSL_TEST_COPTS}
   DEPS
     absl::fixed_array
+    absl::config
     absl::exception_testing
     absl::hash_testing
+    absl::iterator_traits_test_helper_internal
     absl::memory
-    gmock_main
+    absl::test_allocator
+    GTest::gmock_main
 )
 
 absl_cc_test(
@@ -162,9 +167,10 @@ absl_cc_test(
     absl::fixed_array
     absl::config
     absl::exception_safety_testing
-    gmock_main
+    GTest::gmock_main
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     inlined_vector_internal
@@ -173,8 +179,11 @@ absl_cc_library(
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
+    absl::base_internal
     absl::compressed_tuple
+    absl::config
     absl::core_headers
+    absl::hardening
     absl::memory
     absl::span
     absl::type_traits
@@ -191,19 +200,27 @@ absl_cc_library(
   DEPS
     absl::algorithm
     absl::core_headers
+    absl::hardening
     absl::inlined_vector_internal
     absl::throw_delegate
     absl::memory
+    absl::type_traits
+    absl::weakly_mixed_integer
   PUBLIC
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
-    counting_allocator
+    test_allocator
   HDRS
-    "internal/counting_allocator.h"
+    "internal/test_allocator.h"
   COPTS
     ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::config
+    GTest::gmock
+  TESTONLY
 )
 
 absl_cc_test(
@@ -214,16 +231,17 @@ absl_cc_test(
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
-    absl::counting_allocator
-    absl::inlined_vector
-    absl::test_instance_tracker
+    absl::check
+    absl::config
     absl::core_headers
     absl::exception_testing
     absl::hash_testing
+    absl::inlined_vector
     absl::memory
-    absl::raw_logging_internal
     absl::strings
-    gmock_main
+    absl::test_allocator
+    absl::test_instance_tracker
+    GTest::gmock_main
 )
 
 absl_cc_test(
@@ -237,9 +255,10 @@ absl_cc_test(
     absl::inlined_vector
     absl::config
     absl::exception_safety_testing
-    gmock_main
+    GTest::gmock_main
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     test_instance_tracker
@@ -263,7 +282,7 @@ absl_cc_test(
     ${ABSL_TEST_COPTS}
   DEPS
     absl::test_instance_tracker
-    gmock_main
+    GTest::gmock_main
 )
 
 absl_cc_library(
@@ -275,10 +294,11 @@ absl_cc_library(
     ${ABSL_DEFAULT_COPTS}
   DEPS
     absl::container_memory
-    absl::hash_function_defaults
+    absl::core_headers
+    absl::hash_container_defaults
     absl::raw_hash_map
     absl::algorithm_container
-    absl::memory
+    absl::type_traits
   PUBLIC
 )
 
@@ -290,14 +310,18 @@ absl_cc_test(
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
+    absl::check
+    absl::config
     absl::flat_hash_map
     absl::hash_generator_testing
+    absl::hash_policy_testing
+    absl::test_allocator
+    absl::type_traits
     absl::unordered_map_constructor_test
     absl::unordered_map_lookup_test
     absl::unordered_map_members_test
     absl::unordered_map_modifiers_test
-    absl::any
-    gmock_main
+    GTest::gmock_main
 )
 
 absl_cc_library(
@@ -309,11 +333,12 @@ absl_cc_library(
     ${ABSL_DEFAULT_COPTS}
   DEPS
     absl::container_memory
-    absl::hash_function_defaults
+    absl::hash_container_defaults
     absl::raw_hash_set
     absl::algorithm_container
     absl::core_headers
     absl::memory
+    absl::type_traits
   PUBLIC
 )
 
@@ -324,17 +349,22 @@ absl_cc_test(
     "flat_hash_set_test.cc"
   COPTS
     ${ABSL_TEST_COPTS}
-    "-DUNORDERED_SET_CXX17"
   DEPS
+    absl::check
+    absl::config
+    absl::container_memory
     absl::flat_hash_set
+    absl::hash
+    absl::hash_container_defaults
     absl::hash_generator_testing
+    absl::memory
+    absl::strings
+    absl::test_allocator
     absl::unordered_set_constructor_test
     absl::unordered_set_lookup_test
     absl::unordered_set_members_test
     absl::unordered_set_modifiers_test
-    absl::memory
-    absl::strings
-    gmock_main
+    GTest::gmock_main
 )
 
 absl_cc_library(
@@ -346,11 +376,13 @@ absl_cc_library(
     ${ABSL_DEFAULT_COPTS}
   DEPS
     absl::container_memory
-    absl::hash_function_defaults
-    absl::node_hash_policy
+    absl::core_headers
+    absl::hash_container_defaults
+    absl::node_slot_policy
     absl::raw_hash_map
     absl::algorithm_container
     absl::memory
+    absl::type_traits
   PUBLIC
 )
 
@@ -362,14 +394,15 @@ absl_cc_test(
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
-    absl::hash_generator_testing
+    absl::config
+    absl::hash_policy_testing
     absl::node_hash_map
     absl::tracked
     absl::unordered_map_constructor_test
     absl::unordered_map_lookup_test
     absl::unordered_map_members_test
     absl::unordered_map_modifiers_test
-    gmock_main
+    GTest::gmock_main
 )
 
 absl_cc_library(
@@ -380,11 +413,14 @@ absl_cc_library(
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
-    absl::hash_function_defaults
-    absl::node_hash_policy
+    absl::container_memory
+    absl::core_headers
+    absl::hash_container_defaults
+    absl::node_slot_policy
     absl::raw_hash_set
     absl::algorithm_container
     absl::memory
+    absl::type_traits
   PUBLIC
 )
 
@@ -395,17 +431,33 @@ absl_cc_test(
     "node_hash_set_test.cc"
   COPTS
     ${ABSL_TEST_COPTS}
-    "-DUNORDERED_SET_CXX17"
   DEPS
     absl::hash_generator_testing
+    absl::hash_policy_testing
+    absl::memory
     absl::node_hash_set
+    absl::type_traits
     absl::unordered_set_constructor_test
     absl::unordered_set_lookup_test
     absl::unordered_set_members_test
     absl::unordered_set_modifiers_test
-    gmock_main
+    GTest::gmock_main
 )
 
+absl_cc_library(
+  NAME
+    hash_container_defaults
+  HDRS
+    "hash_container_defaults.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::config
+    absl::hash_function_defaults
+  PUBLIC
+)
+
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     container_memory
@@ -414,7 +466,10 @@ absl_cc_library(
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
+    absl::config
+    absl::hash
     absl::memory
+    absl::type_traits
     absl::utility
   PUBLIC
 )
@@ -427,11 +482,16 @@ absl_cc_test(
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
+    absl::config
     absl::container_memory
+    absl::no_destructor
     absl::strings
-    gmock_main
+    absl::test_instance_tracker
+    absl::type_traits
+    GTest::gmock_main
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     hash_function_defaults
@@ -441,8 +501,11 @@ absl_cc_library(
     ${ABSL_DEFAULT_COPTS}
   DEPS
     absl::config
+    absl::container_common
+    absl::cord
     absl::hash
     absl::strings
+    absl::type_traits
   PUBLIC
 )
 
@@ -454,12 +517,18 @@ absl_cc_test(
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
+    absl::cord
+    absl::cord_test_helpers
+    absl::flat_hash_map
+    absl::flat_hash_set
     absl::hash_function_defaults
     absl::hash
+    absl::random_random
     absl::strings
-    gmock_main
+    GTest::gmock_main
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     hash_generator_testing
@@ -470,13 +539,17 @@ absl_cc_library(
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
+    absl::config
     absl::hash_policy_testing
     absl::memory
     absl::meta
+    absl::no_destructor
+    absl::random_random
     absl::strings
   TESTONLY
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     hash_policy_testing
@@ -499,9 +572,10 @@ absl_cc_test(
     ${ABSL_TEST_COPTS}
   DEPS
     absl::hash_policy_testing
-    gmock_main
+    GTest::gmock_main
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     hash_policy_traits
@@ -510,6 +584,8 @@ absl_cc_library(
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
+    absl::container_memory
+    absl::common_policy_traits
     absl::meta
   PUBLIC
 )
@@ -522,10 +598,38 @@ absl_cc_test(
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
+    absl::container_memory
     absl::hash_policy_traits
-    gmock_main
+    GTest::gmock_main
 )
 
+# Internal-only target, do not depend on directly.
+absl_cc_library(
+  NAME
+    common_policy_traits
+  HDRS
+    "internal/common_policy_traits.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::meta
+  PUBLIC
+)
+
+absl_cc_test(
+  NAME
+    common_policy_traits_test
+  SRCS
+    "internal/common_policy_traits_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::common_policy_traits
+    absl::config
+    GTest::gmock_main
+)
+
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     hashtablez_sampler
@@ -538,8 +642,13 @@ absl_cc_library(
     ${ABSL_DEFAULT_COPTS}
   DEPS
     absl::base
-    absl::have_sse
+    absl::config
+    absl::exponential_biased
+    absl::no_destructor
+    absl::raw_logging_internal
+    absl::sample_recorder
     absl::synchronization
+    absl::time
 )
 
 absl_cc_test(
@@ -550,11 +659,13 @@ absl_cc_test(
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
+    absl::config
     absl::hashtablez_sampler
-    absl::have_sse
-    gmock_main
+    absl::random_random
+    GTest::gmock_main
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     hashtable_debug
@@ -566,6 +677,7 @@ absl_cc_library(
     absl::hashtable_debug_hooks
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     hashtable_debug_hooks
@@ -573,41 +685,39 @@ absl_cc_library(
     "internal/hashtable_debug_hooks.h"
   COPTS
     ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::config
   PUBLIC
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
-    have_sse
+    node_slot_policy
   HDRS
-    "internal/have_sse.h"
-  COPTS
-    ${ABSL_DEFAULT_COPTS}
-)
-
-absl_cc_library(
-  NAME
-    node_hash_policy
-  HDRS
-    "internal/node_hash_policy.h"
+    "internal/node_slot_policy.h"
   COPTS
     ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::config
   PUBLIC
 )
 
 absl_cc_test(
   NAME
-    node_hash_policy_test
+    node_slot_policy_test
   SRCS
-    "internal/node_hash_policy_test.cc"
+    "internal/node_slot_policy_test.cc"
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
+    absl::config
     absl::hash_policy_traits
-    absl::node_hash_policy
-    gmock_main
+    absl::node_slot_policy
+    GTest::gmock_main
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     raw_hash_map
@@ -616,23 +726,57 @@ absl_cc_library(
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
+    absl::config
+    absl::common_policy_traits
     absl::container_memory
+    absl::core_headers
     absl::raw_hash_set
+    absl::type_traits
     absl::throw_delegate
   PUBLIC
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     container_common
   HDRS
-    "internal/commom.h"
+    "internal/common.h"
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
     absl::type_traits
 )
 
+# Internal-only target, do not depend on directly.
+absl_cc_library(
+  NAME
+    hashtable_control_bytes
+  HDRS
+    "internal/hashtable_control_bytes.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::bits
+    absl::config
+    absl::core_headers
+    absl::endian
+)
+
+absl_cc_test(
+  NAME
+    hashtable_control_bytes_test
+  SRCS
+    "internal/hashtable_control_bytes_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::config
+    absl::hashtable_control_bytes
+    GTest::gmock_main
+)
+
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     raw_hash_set
@@ -643,22 +787,31 @@ absl_cc_library(
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
+    absl::base
     absl::bits
+    absl::common_policy_traits
     absl::compressed_tuple
     absl::config
     absl::container_common
     absl::container_memory
     absl::core_headers
+    absl::dynamic_annotations
     absl::endian
+    absl::function_ref
+    absl::hash
+    absl::hash_function_defaults
     absl::hash_policy_traits
+    absl::hashtable_control_bytes
     absl::hashtable_debug_hooks
-    absl::have_sse
-    absl::layout
+    absl::hashtablez_sampler
+    absl::iterator_traits_internal
     absl::memory
     absl::meta
     absl::optional
+    absl::prefetch
+    absl::raw_logging_internal
     absl::utility
-    absl::hashtablez_sampler
+    absl::weakly_mixed_integer
   PUBLIC
 )
 
@@ -670,16 +823,57 @@ absl_cc_test(
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
+    absl::base
+    absl::check
+    absl::config
     absl::container_memory
+    absl::core_headers
+    absl::flat_hash_map
+    absl::flat_hash_set
+    absl::function_ref
+    absl::hash
     absl::hash_function_defaults
     absl::hash_policy_testing
     absl::hashtable_debug
+    absl::hashtablez_sampler
+    absl::int128
+    absl::log
+    absl::memory
+    absl::node_hash_set
+    absl::prefetch
+    absl::random_random
     absl::raw_hash_set
-    absl::base
-    absl::core_headers
-    absl::raw_logging_internal
     absl::strings
-    gmock_main
+    absl::test_allocator
+    absl::test_instance_tracker
+    absl::type_traits
+    GTest::gmock_main
+)
+
+# Internal-only target, do not depend on directly.
+absl_cc_library(
+  NAME
+    raw_hash_set_resize_impl
+  HDRS
+    "internal/raw_hash_set_resize_impl.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::config
+  PUBLIC
+)
+
+absl_cc_test(
+  NAME
+    raw_hash_set_resize_impl_test
+  SRCS
+    "internal/raw_hash_set_resize_impl_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::config
+    absl::raw_hash_set_resize_impl
+    GTest::gmock_main
 )
 
 absl_cc_test(
@@ -690,12 +884,14 @@ absl_cc_test(
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
+    absl::config
+    absl::container_memory
     absl::raw_hash_set
     absl::tracked
-    absl::core_headers
-    gmock_main
+    GTest::gmock_main
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     layout
@@ -704,7 +900,9 @@ absl_cc_library(
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
+    absl::config
     absl::core_headers
+    absl::debugging_internal
     absl::meta
     absl::strings
     absl::span
@@ -721,12 +919,14 @@ absl_cc_test(
     ${ABSL_TEST_COPTS}
   DEPS
     absl::layout
-    absl::core_headers
-    absl::raw_logging_internal
+    absl::check
+    absl::config
     absl::span
-    gmock_main
+    absl::utility
+    GTest::gmock_main
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     tracked
@@ -734,9 +934,12 @@ absl_cc_library(
     "internal/tracked.h"
   COPTS
     ${ABSL_TEST_COPTS}
+  DEPS
+    absl::config
   TESTONLY
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     unordered_map_constructor_test
@@ -745,12 +948,14 @@ absl_cc_library(
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
+    absl::config
     absl::hash_generator_testing
     absl::hash_policy_testing
-    gmock
+    GTest::gmock
   TESTONLY
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     unordered_map_lookup_test
@@ -761,10 +966,11 @@ absl_cc_library(
   DEPS
     absl::hash_generator_testing
     absl::hash_policy_testing
-    gmock
+    GTest::gmock
   TESTONLY
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     unordered_map_members_test
@@ -774,10 +980,11 @@ absl_cc_library(
     ${ABSL_TEST_COPTS}
   DEPS
     absl::type_traits
-    gmock
+    GTest::gmock
   TESTONLY
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     unordered_map_modifiers_test
@@ -788,10 +995,11 @@ absl_cc_library(
   DEPS
     absl::hash_generator_testing
     absl::hash_policy_testing
-    gmock
+    GTest::gmock
   TESTONLY
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     unordered_set_constructor_test
@@ -800,12 +1008,14 @@ absl_cc_library(
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
+    absl::config
     absl::hash_generator_testing
     absl::hash_policy_testing
-    gmock
+    GTest::gmock
   TESTONLY
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     unordered_set_lookup_test
@@ -816,10 +1026,11 @@ absl_cc_library(
   DEPS
     absl::hash_generator_testing
     absl::hash_policy_testing
-    gmock
+    GTest::gmock
   TESTONLY
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     unordered_set_members_test
@@ -829,10 +1040,11 @@ absl_cc_library(
     ${ABSL_TEST_COPTS}
   DEPS
     absl::type_traits
-    gmock
+    GTest::gmock
   TESTONLY
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     unordered_set_modifiers_test
@@ -843,7 +1055,7 @@ absl_cc_library(
   DEPS
     absl::hash_generator_testing
     absl::hash_policy_testing
-    gmock
+    GTest::gmock
   TESTONLY
 )
 
@@ -859,7 +1071,7 @@ absl_cc_test(
     absl::unordered_set_lookup_test
     absl::unordered_set_members_test
     absl::unordered_set_modifiers_test
-    gmock_main
+    GTest::gmock_main
 )
 
 absl_cc_test(
@@ -874,5 +1086,158 @@ absl_cc_test(
     absl::unordered_map_lookup_test
     absl::unordered_map_members_test
     absl::unordered_map_modifiers_test
-    gmock_main
+    GTest::gmock_main
+)
+
+absl_cc_test(
+  NAME
+    sample_element_size_test
+  SRCS
+    "sample_element_size_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::flat_hash_map
+    absl::flat_hash_set
+    absl::node_hash_map
+    absl::node_hash_set
+    absl::hashtablez_sampler
+    GTest::gmock_main
+)
+
+absl_cc_library(
+  NAME
+    heterogeneous_lookup_testing
+  HDRS
+    "internal/heterogeneous_lookup_testing.h"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::config
+    absl::test_instance_tracker
+    GTest::gmock
+  TESTONLY
+)
+
+absl_cc_library(
+  NAME
+    linked_hash_set
+  HDRS
+    "linked_hash_set.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+     absl::container_common
+     absl::config
+     absl::core_headers
+     absl::flat_hash_set
+)
+
+absl_cc_test(
+  NAME
+    linked_hash_set_test
+  SRCS
+    "linked_hash_set_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::linked_hash_set
+    absl::config
+    absl::heterogeneous_lookup_testing
+    absl::hash_generator_testing
+    absl::hash_policy_testing
+    absl::string_view
+    absl::test_allocator
+    absl::test_instance_tracker
+    absl::unordered_set_constructor_test
+    absl::unordered_set_lookup_test
+    absl::unordered_set_members_test
+    absl::unordered_set_modifiers_test
+    GTest::gmock_main
+)
+
+absl_cc_library(
+  NAME
+    linked_hash_map
+  HDRS
+    "linked_hash_map.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+     absl::container_common
+     absl::config
+     absl::core_headers
+     absl::flat_hash_set
+     absl::throw_delegate
+)
+
+absl_cc_test(
+  NAME
+    linked_hash_map_test
+  SRCS
+    "linked_hash_map_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::linked_hash_map
+    absl::config
+    absl::exception_testing
+    absl::heterogeneous_lookup_testing
+    absl::hash_generator_testing
+    absl::hash_policy_testing
+    absl::string_view
+    absl::test_allocator
+    absl::test_instance_tracker
+    absl::unordered_set_constructor_test
+    absl::unordered_set_lookup_test
+    absl::unordered_set_members_test
+    absl::unordered_set_modifiers_test
+    GTest::gmock_main
+)
+
+absl_cc_library(
+  NAME
+    chunked_queue
+  HDRS
+    "chunked_queue.h"
+    "internal/chunked_queue.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+     absl::config
+     absl::core_headers
+     absl::hardening
+     absl::iterator_traits_internal
+     absl::layout
+)
+
+absl_cc_test(
+  NAME
+    chunked_queue_test
+  SRCS
+    "chunked_queue_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::chunked_queue
+    absl::config
+    absl::core_headers
+    absl::strings
+    absl::test_allocator
+    absl::throw_delegate
+    GTest::gmock_main
 )
diff --git a/absl/container/btree_benchmark.cc b/absl/container/btree_benchmark.cc
new file mode 100644
index 00000000000..69a81959285
--- /dev/null
+++ b/absl/container/btree_benchmark.cc
@@ -0,0 +1,804 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <stdint.h>
+
+#include <algorithm>
+#include <functional>
+#include <map>
+#include <numeric>
+#include <random>
+#include <set>
+#include <string>
+#include <type_traits>
+#include <unordered_map>
+#include <unordered_set>
+#include <vector>
+
+#include "absl/algorithm/container.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/container/btree_map.h"
+#include "absl/container/btree_set.h"
+#include "absl/container/btree_test.h"
+#include "absl/container/flat_hash_map.h"
+#include "absl/container/flat_hash_set.h"
+#include "absl/container/internal/hashtable_debug.h"
+#include "absl/hash/hash.h"
+#include "absl/log/log.h"
+#include "absl/memory/memory.h"
+#include "absl/random/random.h"
+#include "absl/strings/cord.h"
+#include "absl/strings/str_format.h"
+#include "absl/time/time.h"
+#include "benchmark/benchmark.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace container_internal {
+namespace {
+
+constexpr size_t kBenchmarkValues = 1 << 20;
+
+// How many times we add and remove sub-batches in one batch of *AddRem
+// benchmarks.
+constexpr size_t kAddRemBatchSize = 1 << 2;
+
+// Generates n values in the range [0, 4 * n].
+template <typename V>
+std::vector<V> GenerateValues(int n) {
+  constexpr int kSeed = 23;
+  return GenerateValuesWithSeed<V>(n, 4 * n, kSeed);
+}
+
+// Benchmark insertion of values into a container.
+template <typename T>
+void BM_InsertImpl(benchmark::State& state, bool sorted) {
+  using V = typename remove_pair_const<typename T::value_type>::type;
+  typename KeyOfValue<typename T::key_type, V>::type key_of_value;
+
+  std::vector<V> values = GenerateValues<V>(kBenchmarkValues);
+  if (sorted) {
+    std::sort(values.begin(), values.end());
+  }
+  T container(values.begin(), values.end());
+
+  // Remove and re-insert 10% of the keys per batch.
+  const int batch_size = (kBenchmarkValues + 9) / 10;
+  while (state.KeepRunningBatch(batch_size)) {
+    state.PauseTiming();
+    const auto i = static_cast<int>(state.iterations());
+
+    for (int j = i; j < i + batch_size; j++) {
+      int x = j % kBenchmarkValues;
+      container.erase(key_of_value(values[x]));
+    }
+
+    state.ResumeTiming();
+
+    for (int j = i; j < i + batch_size; j++) {
+      int x = j % kBenchmarkValues;
+      container.insert(values[x]);
+    }
+  }
+}
+
+template <typename T>
+void BM_Insert(benchmark::State& state) {
+  BM_InsertImpl<T>(state, false);
+}
+
+template <typename T>
+void BM_InsertSorted(benchmark::State& state) {
+  BM_InsertImpl<T>(state, true);
+}
+
+// Benchmark inserting the first few elements in a container. In b-tree, this is
+// when the root node grows.
+template <typename T>
+void BM_InsertSmall(benchmark::State& state) {
+  using V = typename remove_pair_const<typename T::value_type>::type;
+
+  const int kSize = 8;
+  std::vector<V> values = GenerateValues<V>(kSize);
+  T container;
+
+  while (state.KeepRunningBatch(kSize)) {
+    for (int i = 0; i < kSize; ++i) {
+      benchmark::DoNotOptimize(container.insert(values[i]));
+    }
+    state.PauseTiming();
+    // Do not measure the time it takes to clear the container.
+    container.clear();
+    state.ResumeTiming();
+  }
+}
+
+template <typename T>
+void BM_LookupImpl(benchmark::State& state, bool sorted) {
+  using V = typename remove_pair_const<typename T::value_type>::type;
+  typename KeyOfValue<typename T::key_type, V>::type key_of_value;
+
+  std::vector<V> values = GenerateValues<V>(kBenchmarkValues);
+  if (sorted) {
+    std::sort(values.begin(), values.end());
+  }
+  T container(values.begin(), values.end());
+
+  while (state.KeepRunning()) {
+    int idx = state.iterations() % kBenchmarkValues;
+    benchmark::DoNotOptimize(container.find(key_of_value(values[idx])));
+  }
+}
+
+// Benchmark lookup of values in a container.
+template <typename T>
+void BM_Lookup(benchmark::State& state) {
+  BM_LookupImpl<T>(state, false);
+}
+
+// Benchmark lookup of values in a full container, meaning that values
+// are inserted in-order to take advantage of biased insertion, which
+// yields a full tree.
+template <typename T>
+void BM_FullLookup(benchmark::State& state) {
+  BM_LookupImpl<T>(state, true);
+}
+
+// Benchmark erasing values from a container.
+template <typename T>
+void BM_Erase(benchmark::State& state) {
+  using V = typename remove_pair_const<typename T::value_type>::type;
+  typename KeyOfValue<typename T::key_type, V>::type key_of_value;
+  std::vector<V> values = GenerateValues<V>(kBenchmarkValues);
+  T container(values.begin(), values.end());
+
+  // Remove and re-insert 10% of the keys per batch.
+  const int batch_size = (kBenchmarkValues + 9) / 10;
+  while (state.KeepRunningBatch(batch_size)) {
+    const int i = state.iterations();
+
+    for (int j = i; j < i + batch_size; j++) {
+      int x = j % kBenchmarkValues;
+      container.erase(key_of_value(values[x]));
+    }
+
+    state.PauseTiming();
+    for (int j = i; j < i + batch_size; j++) {
+      int x = j % kBenchmarkValues;
+      container.insert(values[x]);
+    }
+    state.ResumeTiming();
+  }
+}
+
+// Benchmark erasing multiple values from a container.
+template <typename T>
+void BM_EraseRange(benchmark::State& state) {
+  using V = typename remove_pair_const<typename T::value_type>::type;
+  typename KeyOfValue<typename T::key_type, V>::type key_of_value;
+  std::vector<V> values = GenerateValues<V>(kBenchmarkValues);
+  T container(values.begin(), values.end());
+
+  // Remove and re-insert 10% of the keys per batch.
+  const int batch_size = (kBenchmarkValues + 9) / 10;
+  while (state.KeepRunningBatch(batch_size)) {
+    const int i = state.iterations();
+
+    const int start_index = i % kBenchmarkValues;
+
+    state.PauseTiming();
+    {
+      std::vector<V> removed;
+      removed.reserve(batch_size);
+      auto itr = container.find(key_of_value(values[start_index]));
+      auto start = itr;
+      for (int j = 0; j < batch_size; j++) {
+        if (itr == container.end()) {
+          state.ResumeTiming();
+          container.erase(start, itr);
+          state.PauseTiming();
+          itr = container.begin();
+          start = itr;
+        }
+        removed.push_back(*itr++);
+      }
+
+      state.ResumeTiming();
+      container.erase(start, itr);
+      state.PauseTiming();
+
+      container.insert(removed.begin(), removed.end());
+    }
+    state.ResumeTiming();
+  }
+}
+
+// Predicate that erases every other element. We can't use a lambda because
+// C++11 doesn't support generic lambdas.
+// TODO(b/207389011): consider adding benchmarks that remove different fractions
+// of keys (e.g. 10%, 90%).
+struct EraseIfPred {
+  uint64_t i = 0;
+  template <typename T>
+  bool operator()(const T&) {
+    return ++i % 2;
+  }
+};
+
+// Benchmark erasing multiple values from a container with a predicate.
+template <typename T>
+void BM_EraseIf(benchmark::State& state) {
+  using V = typename remove_pair_const<typename T::value_type>::type;
+  std::vector<V> values = GenerateValues<V>(kBenchmarkValues);
+
+  // Removes half of the keys per batch.
+  const int batch_size = (kBenchmarkValues + 1) / 2;
+  EraseIfPred pred;
+  while (state.KeepRunningBatch(batch_size)) {
+    state.PauseTiming();
+    {
+      T container(values.begin(), values.end());
+      state.ResumeTiming();
+      erase_if(container, pred);
+      benchmark::DoNotOptimize(container);
+      state.PauseTiming();
+    }
+    state.ResumeTiming();
+  }
+}
+
+// Benchmark steady-state insert (into first half of range) and remove (from
+// second half of range), treating the container approximately like a queue with
+// log-time access for all elements. This benchmark does not test the case where
+// insertion and removal happen in the same region of the tree.  This benchmark
+// counts two value constructors.
+template <typename T>
+void BM_QueueAddRem(benchmark::State& state) {
+  using V = typename remove_pair_const<typename T::value_type>::type;
+  typename KeyOfValue<typename T::key_type, V>::type key_of_value;
+
+  ABSL_RAW_CHECK(kBenchmarkValues % 2 == 0, "for performance");
+
+  T container;
+
+  const size_t half = kBenchmarkValues / 2;
+  std::vector<int> remove_keys(half);
+  std::vector<int> add_keys(half);
+
+  // We want to do the exact same work repeatedly, and the benchmark can end
+  // after a different number of iterations depending on the speed of the
+  // individual run so we use a large batch size here and ensure that we do
+  // deterministic work every batch.
+  while (state.KeepRunningBatch(half * kAddRemBatchSize)) {
+    state.PauseTiming();
+
+    container.clear();
+
+    for (size_t i = 0; i < half; ++i) {
+      remove_keys[i] = i;
+      add_keys[i] = i;
+    }
+    constexpr int kSeed = 5;
+    std::mt19937_64 rand(kSeed);
+    std::shuffle(remove_keys.begin(), remove_keys.end(), rand);
+    std::shuffle(add_keys.begin(), add_keys.end(), rand);
+
+    // Note needs lazy generation of values.
+    Generator<V> g(kBenchmarkValues * kAddRemBatchSize);
+
+    for (size_t i = 0; i < half; ++i) {
+      container.insert(g(add_keys[i]));
+      container.insert(g(half + remove_keys[i]));
+    }
+
+    // There are three parts each of size "half":
+    // 1 is being deleted from  [offset - half, offset)
+    // 2 is standing            [offset, offset + half)
+    // 3 is being inserted into [offset + half, offset + 2 * half)
+    size_t offset = 0;
+
+    for (size_t i = 0; i < kAddRemBatchSize; ++i) {
+      std::shuffle(remove_keys.begin(), remove_keys.end(), rand);
+      std::shuffle(add_keys.begin(), add_keys.end(), rand);
+      offset += half;
+
+      state.ResumeTiming();
+      for (size_t idx = 0; idx < half; ++idx) {
+        container.erase(key_of_value(g(offset - half + remove_keys[idx])));
+        container.insert(g(offset + half + add_keys[idx]));
+      }
+      state.PauseTiming();
+    }
+    state.ResumeTiming();
+  }
+}
+
+// Mixed insertion and deletion in the same range using pre-constructed values.
+template <typename T>
+void BM_MixedAddRem(benchmark::State& state) {
+  using V = typename remove_pair_const<typename T::value_type>::type;
+  typename KeyOfValue<typename T::key_type, V>::type key_of_value;
+
+  ABSL_RAW_CHECK(kBenchmarkValues % 2 == 0, "for performance");
+
+  T container;
+
+  // Create two random shuffles
+  std::vector<int> remove_keys(kBenchmarkValues);
+  std::vector<int> add_keys(kBenchmarkValues);
+
+  // We want to do the exact same work repeatedly, and the benchmark can end
+  // after a different number of iterations depending on the speed of the
+  // individual run so we use a large batch size here and ensure that we do
+  // deterministic work every batch.
+  while (state.KeepRunningBatch(kBenchmarkValues * kAddRemBatchSize)) {
+    state.PauseTiming();
+
+    container.clear();
+
+    constexpr int kSeed = 7;
+    std::mt19937_64 rand(kSeed);
+
+    std::vector<V> values = GenerateValues<V>(kBenchmarkValues * 2);
+
+    // Insert the first half of the values (already in random order)
+    container.insert(values.begin(), values.begin() + kBenchmarkValues);
+
+    // Insert the first half of the values (already in random order)
+    for (size_t i = 0; i < kBenchmarkValues; ++i) {
+      // remove_keys and add_keys will be swapped before each round,
+      // therefore fill add_keys here w/ the keys being inserted, so
+      // they'll be the first to be removed.
+      remove_keys[i] = i + kBenchmarkValues;
+      add_keys[i] = i;
+    }
+
+    for (size_t i = 0; i < kAddRemBatchSize; ++i) {
+      remove_keys.swap(add_keys);
+      std::shuffle(remove_keys.begin(), remove_keys.end(), rand);
+      std::shuffle(add_keys.begin(), add_keys.end(), rand);
+
+      state.ResumeTiming();
+      for (size_t idx = 0; idx < kBenchmarkValues; ++idx) {
+        container.erase(key_of_value(values[remove_keys[idx]]));
+        container.insert(values[add_keys[idx]]);
+      }
+      state.PauseTiming();
+    }
+    state.ResumeTiming();
+  }
+}
+
+// Insertion at end, removal from the beginning.  This benchmark
+// counts two value constructors.
+// TODO(ezb): we could add a GenerateNext version of generator that could reduce
+// noise for string-like types.
+template <typename T>
+void BM_Fifo(benchmark::State& state) {
+  using V = typename remove_pair_const<typename T::value_type>::type;
+
+  T container;
+  // Need lazy generation of values as state.max_iterations is large.
+  Generator<V> g(kBenchmarkValues + state.max_iterations);
+
+  for (int i = 0; i < kBenchmarkValues; i++) {
+    container.insert(g(i));
+  }
+
+  while (state.KeepRunning()) {
+    container.erase(container.begin());
+    container.insert(container.end(), g(state.iterations() + kBenchmarkValues));
+  }
+}
+
+// Iteration (forward) through the tree
+template <typename T>
+void BM_FwdIter(benchmark::State& state) {
+  using V = typename remove_pair_const<typename T::value_type>::type;
+
+  std::vector<V> values = GenerateValues<V>(kBenchmarkValues);
+  T container(values.begin(), values.end());
+
+  auto iter = container.end();
+
+  while (state.KeepRunning()) {
+    if (iter == container.end()) iter = container.begin();
+    auto *v = &(*iter);
+    benchmark::DoNotOptimize(v);
+    benchmark::DoNotOptimize(++iter);
+  }
+}
+
+// Benchmark random range-construction of a container.
+template <typename T>
+void BM_RangeConstructionImpl(benchmark::State& state, bool sorted) {
+  using V = typename remove_pair_const<typename T::value_type>::type;
+
+  std::vector<V> values = GenerateValues<V>(kBenchmarkValues);
+  if (sorted) {
+    std::sort(values.begin(), values.end());
+  }
+  {
+    T container(values.begin(), values.end());
+  }
+
+  while (state.KeepRunning()) {
+    T container(values.begin(), values.end());
+    benchmark::DoNotOptimize(container);
+  }
+}
+
+template <typename T>
+void BM_InsertRangeRandom(benchmark::State& state) {
+  BM_RangeConstructionImpl<T>(state, false);
+}
+
+template <typename T>
+void BM_InsertRangeSorted(benchmark::State& state) {
+  BM_RangeConstructionImpl<T>(state, true);
+}
+
+#define STL_ORDERED_TYPES(value)                     \
+  using stl_set_##value = std::set<value>;           \
+  using stl_map_##value = std::map<value, intptr_t>; \
+  using stl_multiset_##value = std::multiset<value>; \
+  using stl_multimap_##value = std::multimap<value, intptr_t>
+
+using StdString = std::string;
+STL_ORDERED_TYPES(int32_t);
+STL_ORDERED_TYPES(int64_t);
+STL_ORDERED_TYPES(StdString);
+STL_ORDERED_TYPES(Cord);
+STL_ORDERED_TYPES(Time);
+
+#define STL_UNORDERED_TYPES(value)                                       \
+  using stl_unordered_set_##value = std::unordered_set<value>;           \
+  using stl_unordered_map_##value = std::unordered_map<value, intptr_t>; \
+  using flat_hash_set_##value = flat_hash_set<value>;                    \
+  using flat_hash_map_##value = flat_hash_map<value, intptr_t>;          \
+  using stl_unordered_multiset_##value = std::unordered_multiset<value>; \
+  using stl_unordered_multimap_##value =                                 \
+      std::unordered_multimap<value, intptr_t>
+
+#define STL_UNORDERED_TYPES_CUSTOM_HASH(value, hash)                           \
+  using stl_unordered_set_##value = std::unordered_set<value, hash>;           \
+  using stl_unordered_map_##value = std::unordered_map<value, intptr_t, hash>; \
+  using flat_hash_set_##value = flat_hash_set<value, hash>;                    \
+  using flat_hash_map_##value = flat_hash_map<value, intptr_t, hash>;          \
+  using stl_unordered_multiset_##value = std::unordered_multiset<value, hash>; \
+  using stl_unordered_multimap_##value =                                       \
+      std::unordered_multimap<value, intptr_t, hash>
+
+STL_UNORDERED_TYPES_CUSTOM_HASH(Cord, absl::Hash<absl::Cord>);
+
+STL_UNORDERED_TYPES(int32_t);
+STL_UNORDERED_TYPES(int64_t);
+STL_UNORDERED_TYPES(StdString);
+STL_UNORDERED_TYPES_CUSTOM_HASH(Time, absl::Hash<absl::Time>);
+
+#define BTREE_TYPES(value)                                            \
+  using btree_256_set_##value =                                       \
+      btree_set<value, std::less<value>, std::allocator<value>>;      \
+  using btree_256_map_##value =                                       \
+      btree_map<value, intptr_t, std::less<value>,                    \
+                std::allocator<std::pair<const value, intptr_t>>>;    \
+  using btree_256_multiset_##value =                                  \
+      btree_multiset<value, std::less<value>, std::allocator<value>>; \
+  using btree_256_multimap_##value =                                  \
+      btree_multimap<value, intptr_t, std::less<value>,               \
+                     std::allocator<std::pair<const value, intptr_t>>>
+
+BTREE_TYPES(int32_t);
+BTREE_TYPES(int64_t);
+BTREE_TYPES(StdString);
+BTREE_TYPES(Cord);
+BTREE_TYPES(Time);
+
+#define MY_BENCHMARK4(type, func)                                              \
+  void BM_##type##_##func(benchmark::State& state) { BM_##func<type>(state); } \
+  BENCHMARK(BM_##type##_##func)
+
+#define MY_BENCHMARK3_STL(type)           \
+  MY_BENCHMARK4(type, Insert);            \
+  MY_BENCHMARK4(type, InsertSorted);      \
+  MY_BENCHMARK4(type, InsertSmall);       \
+  MY_BENCHMARK4(type, Lookup);            \
+  MY_BENCHMARK4(type, FullLookup);        \
+  MY_BENCHMARK4(type, Erase);             \
+  MY_BENCHMARK4(type, EraseRange);        \
+  MY_BENCHMARK4(type, QueueAddRem);       \
+  MY_BENCHMARK4(type, MixedAddRem);       \
+  MY_BENCHMARK4(type, Fifo);              \
+  MY_BENCHMARK4(type, FwdIter);           \
+  MY_BENCHMARK4(type, InsertRangeRandom); \
+  MY_BENCHMARK4(type, InsertRangeSorted)
+
+#define MY_BENCHMARK3(type)     \
+  MY_BENCHMARK4(type, EraseIf); \
+  MY_BENCHMARK3_STL(type)
+
+#define MY_BENCHMARK2_SUPPORTS_MULTI_ONLY(type) \
+  MY_BENCHMARK3_STL(stl_##type);                \
+  MY_BENCHMARK3_STL(stl_unordered_##type);      \
+  MY_BENCHMARK3(btree_256_##type)
+
+#define MY_BENCHMARK2(type)                \
+  MY_BENCHMARK2_SUPPORTS_MULTI_ONLY(type); \
+  MY_BENCHMARK3(flat_hash_##type)
+
+// Define MULTI_TESTING to see benchmarks for multi-containers also.
+//
+// You can use --copt=-DMULTI_TESTING.
+#ifdef MULTI_TESTING
+#define MY_BENCHMARK(type)                            \
+  MY_BENCHMARK2(set_##type);                          \
+  MY_BENCHMARK2(map_##type);                          \
+  MY_BENCHMARK2_SUPPORTS_MULTI_ONLY(multiset_##type); \
+  MY_BENCHMARK2_SUPPORTS_MULTI_ONLY(multimap_##type)
+#else
+#define MY_BENCHMARK(type)   \
+  MY_BENCHMARK2(set_##type); \
+  MY_BENCHMARK2(map_##type)
+#endif
+
+MY_BENCHMARK(int32_t);
+MY_BENCHMARK(int64_t);
+MY_BENCHMARK(StdString);
+MY_BENCHMARK(Cord);
+MY_BENCHMARK(Time);
+
+// Define a type whose size and cost of moving are independently customizable.
+// When sizeof(value_type) increases, we expect btree to no longer have as much
+// cache-locality advantage over STL. When cost of moving increases, we expect
+// btree to actually do more work than STL because it has to move values around
+// and STL doesn't have to.
+template <int Size, int Copies>
+struct BigType {
+  BigType() : BigType(0) {}
+  explicit BigType(int x) { std::iota(values.begin(), values.end(), x); }
+
+  void Copy(const BigType& other) {
+    for (int i = 0; i < Size && i < Copies; ++i) values[i] = other.values[i];
+    // If Copies > Size, do extra copies.
+    for (int i = Size, idx = 0; i < Copies; ++i) {
+      int64_t tmp = other.values[idx];
+      benchmark::DoNotOptimize(tmp);
+      idx = idx + 1 == Size ? 0 : idx + 1;
+    }
+  }
+
+  BigType(const BigType& other) { Copy(other); }
+  BigType& operator=(const BigType& other) {
+    Copy(other);
+    return *this;
+  }
+
+  // Compare only the first Copies elements if Copies is less than Size.
+  bool operator<(const BigType& other) const {
+    return std::lexicographical_compare(
+        values.begin(), values.begin() + std::min(Size, Copies),
+        other.values.begin(), other.values.begin() + std::min(Size, Copies));
+  }
+  bool operator==(const BigType& other) const {
+    return std::equal(values.begin(), values.begin() + std::min(Size, Copies),
+                      other.values.begin());
+  }
+
+  // Support absl::Hash.
+  template <typename State>
+  friend State AbslHashValue(State h, const BigType& b) {
+    for (int i = 0; i < Size && i < Copies; ++i)
+      h = State::combine(std::move(h), b.values[i]);
+    return h;
+  }
+
+  std::array<int64_t, Size> values;
+};
+
+#define BIG_TYPE_BENCHMARKS(SIZE, COPIES)                                     \
+  using stl_set_size##SIZE##copies##COPIES = std::set<BigType<SIZE, COPIES>>; \
+  using stl_map_size##SIZE##copies##COPIES =                                  \
+      std::map<BigType<SIZE, COPIES>, intptr_t>;                              \
+  using stl_multiset_size##SIZE##copies##COPIES =                             \
+      std::multiset<BigType<SIZE, COPIES>>;                                   \
+  using stl_multimap_size##SIZE##copies##COPIES =                             \
+      std::multimap<BigType<SIZE, COPIES>, intptr_t>;                         \
+  using stl_unordered_set_size##SIZE##copies##COPIES =                        \
+      std::unordered_set<BigType<SIZE, COPIES>,                               \
+                         absl::Hash<BigType<SIZE, COPIES>>>;                  \
+  using stl_unordered_map_size##SIZE##copies##COPIES =                        \
+      std::unordered_map<BigType<SIZE, COPIES>, intptr_t,                     \
+                         absl::Hash<BigType<SIZE, COPIES>>>;                  \
+  using flat_hash_set_size##SIZE##copies##COPIES =                            \
+      flat_hash_set<BigType<SIZE, COPIES>>;                                   \
+  using flat_hash_map_size##SIZE##copies##COPIES =                            \
+      flat_hash_map<BigType<SIZE, COPIES>, intptr_t>;                         \
+  using stl_unordered_multiset_size##SIZE##copies##COPIES =                   \
+      std::unordered_multiset<BigType<SIZE, COPIES>,                          \
+                              absl::Hash<BigType<SIZE, COPIES>>>;             \
+  using stl_unordered_multimap_size##SIZE##copies##COPIES =                   \
+      std::unordered_multimap<BigType<SIZE, COPIES>, intptr_t,                \
+                              absl::Hash<BigType<SIZE, COPIES>>>;             \
+  using btree_256_set_size##SIZE##copies##COPIES =                            \
+      btree_set<BigType<SIZE, COPIES>>;                                       \
+  using btree_256_map_size##SIZE##copies##COPIES =                            \
+      btree_map<BigType<SIZE, COPIES>, intptr_t>;                             \
+  using btree_256_multiset_size##SIZE##copies##COPIES =                       \
+      btree_multiset<BigType<SIZE, COPIES>>;                                  \
+  using btree_256_multimap_size##SIZE##copies##COPIES =                       \
+      btree_multimap<BigType<SIZE, COPIES>, intptr_t>;                        \
+  MY_BENCHMARK(size##SIZE##copies##COPIES)
+
+// Define BIG_TYPE_TESTING to see benchmarks for more big types.
+//
+// You can use --copt=-DBIG_TYPE_TESTING.
+#ifndef NODESIZE_TESTING
+#ifdef BIG_TYPE_TESTING
+BIG_TYPE_BENCHMARKS(1, 4);
+BIG_TYPE_BENCHMARKS(4, 1);
+BIG_TYPE_BENCHMARKS(4, 4);
+BIG_TYPE_BENCHMARKS(1, 8);
+BIG_TYPE_BENCHMARKS(8, 1);
+BIG_TYPE_BENCHMARKS(8, 8);
+BIG_TYPE_BENCHMARKS(1, 16);
+BIG_TYPE_BENCHMARKS(16, 1);
+BIG_TYPE_BENCHMARKS(16, 16);
+BIG_TYPE_BENCHMARKS(1, 32);
+BIG_TYPE_BENCHMARKS(32, 1);
+BIG_TYPE_BENCHMARKS(32, 32);
+#else
+BIG_TYPE_BENCHMARKS(32, 32);
+#endif
+#endif
+
+// Benchmark using unique_ptrs to large value types. In order to be able to use
+// the same benchmark code as the other types, use a type that holds a
+// unique_ptr and has a copy constructor.
+template <int Size>
+struct BigTypePtr {
+  BigTypePtr() : BigTypePtr(0) {}
+  explicit BigTypePtr(int x) { ptr = std::make_unique<BigType<Size, Size>>(x); }
+  BigTypePtr(const BigTypePtr& other) {
+    ptr = std::make_unique<BigType<Size, Size>>(*other.ptr);
+  }
+  BigTypePtr(BigTypePtr&& other) noexcept = default;
+  BigTypePtr& operator=(const BigTypePtr& other) {
+    ptr = std::make_unique<BigType<Size, Size>>(*other.ptr);
+  }
+  BigTypePtr& operator=(BigTypePtr&& other) noexcept = default;
+
+  bool operator<(const BigTypePtr& other) const { return *ptr < *other.ptr; }
+  bool operator==(const BigTypePtr& other) const { return *ptr == *other.ptr; }
+
+  std::unique_ptr<BigType<Size, Size>> ptr;
+};
+
+template <int Size>
+double ContainerInfo(const btree_set<BigTypePtr<Size>>& b) {
+  const double bytes_used =
+      b.bytes_used() + b.size() * sizeof(BigType<Size, Size>);
+  const double bytes_per_value = bytes_used / b.size();
+  BtreeContainerInfoLog(b, bytes_used, bytes_per_value);
+  return bytes_per_value;
+}
+template <int Size>
+double ContainerInfo(const btree_map<int, BigTypePtr<Size>>& b) {
+  const double bytes_used =
+      b.bytes_used() + b.size() * sizeof(BigType<Size, Size>);
+  const double bytes_per_value = bytes_used / b.size();
+  BtreeContainerInfoLog(b, bytes_used, bytes_per_value);
+  return bytes_per_value;
+}
+
+#define BIG_TYPE_PTR_BENCHMARKS(SIZE)                                          \
+  using stl_set_size##SIZE##copies##SIZE##ptr = std::set<BigType<SIZE, SIZE>>; \
+  using stl_map_size##SIZE##copies##SIZE##ptr =                                \
+      std::map<int, BigType<SIZE, SIZE>>;                                      \
+  using stl_unordered_set_size##SIZE##copies##SIZE##ptr =                      \
+      std::unordered_set<BigType<SIZE, SIZE>,                                  \
+                         absl::Hash<BigType<SIZE, SIZE>>>;                     \
+  using stl_unordered_map_size##SIZE##copies##SIZE##ptr =                      \
+      std::unordered_map<int, BigType<SIZE, SIZE>>;                            \
+  using flat_hash_set_size##SIZE##copies##SIZE##ptr =                          \
+      flat_hash_set<BigType<SIZE, SIZE>>;                                      \
+  using flat_hash_map_size##SIZE##copies##SIZE##ptr =                          \
+      flat_hash_map<int, BigTypePtr<SIZE>>;                                    \
+  using btree_256_set_size##SIZE##copies##SIZE##ptr =                          \
+      btree_set<BigTypePtr<SIZE>>;                                             \
+  using btree_256_map_size##SIZE##copies##SIZE##ptr =                          \
+      btree_map<int, BigTypePtr<SIZE>>;                                        \
+  MY_BENCHMARK3_STL(stl_set_size##SIZE##copies##SIZE##ptr);                    \
+  MY_BENCHMARK3_STL(stl_unordered_set_size##SIZE##copies##SIZE##ptr);          \
+  MY_BENCHMARK3(flat_hash_set_size##SIZE##copies##SIZE##ptr);                  \
+  MY_BENCHMARK3(btree_256_set_size##SIZE##copies##SIZE##ptr);                  \
+  MY_BENCHMARK3_STL(stl_map_size##SIZE##copies##SIZE##ptr);                    \
+  MY_BENCHMARK3_STL(stl_unordered_map_size##SIZE##copies##SIZE##ptr);          \
+  MY_BENCHMARK3(flat_hash_map_size##SIZE##copies##SIZE##ptr);                  \
+  MY_BENCHMARK3(btree_256_map_size##SIZE##copies##SIZE##ptr)
+
+BIG_TYPE_PTR_BENCHMARKS(32);
+
+void BM_BtreeSet_IteratorDifference(benchmark::State& state) {
+  absl::InsecureBitGen bitgen;
+  std::vector<int> vec;
+  // Randomize the set's insertion order so the nodes aren't all full.
+  vec.reserve(state.range(0));
+  for (int i = 0; i < state.range(0); ++i) vec.push_back(i);
+  absl::c_shuffle(vec, bitgen);
+
+  absl::btree_set<int> set;
+  for (int i : vec) set.insert(i);
+
+  size_t distance = absl::Uniform(bitgen, 0u, set.size());
+  while (state.KeepRunningBatch(distance)) {
+    size_t end = absl::Uniform(bitgen, distance, set.size());
+    size_t begin = end - distance;
+    benchmark::DoNotOptimize(set.find(static_cast<int>(end)) -
+                             set.find(static_cast<int>(begin)));
+    distance = absl::Uniform(bitgen, 0u, set.size());
+  }
+}
+
+BENCHMARK(BM_BtreeSet_IteratorDifference)->Range(1 << 10, 1 << 20);
+
+void BM_BtreeSet_IteratorAddition(benchmark::State& state) {
+  absl::InsecureBitGen bitgen;
+  std::vector<int> vec;
+  // Randomize the set's insertion order so the nodes aren't all full.
+  vec.reserve(static_cast<size_t>(state.range(0)));
+  for (int i = 0; i < state.range(0); ++i) vec.push_back(i);
+  absl::c_shuffle(vec, bitgen);
+
+  absl::btree_set<int> set;
+  for (int i : vec) set.insert(i);
+
+  size_t distance = absl::Uniform(bitgen, 0u, set.size());
+  while (state.KeepRunningBatch(distance)) {
+    // Let the increment go all the way to the `end` iterator.
+    const size_t begin =
+        absl::Uniform(absl::IntervalClosed, bitgen, 0u, set.size() - distance);
+    auto it = set.find(static_cast<int>(begin));
+    benchmark::DoNotOptimize(it += static_cast<int>(distance));
+    distance = absl::Uniform(bitgen, 0u, set.size());
+  }
+}
+
+BENCHMARK(BM_BtreeSet_IteratorAddition)->Range(1 << 10, 1 << 20);
+
+void BM_BtreeSet_IteratorSubtraction(benchmark::State& state) {
+  absl::InsecureBitGen bitgen;
+  std::vector<int> vec;
+  // Randomize the set's insertion order so the nodes aren't all full.
+  vec.reserve(static_cast<size_t>(state.range(0)));
+  for (int i = 0; i < state.range(0); ++i) vec.push_back(i);
+  absl::c_shuffle(vec, bitgen);
+
+  absl::btree_set<int> set;
+  for (int i : vec) set.insert(i);
+
+  size_t distance = absl::Uniform(bitgen, 0u, set.size());
+  while (state.KeepRunningBatch(distance)) {
+    size_t end = absl::Uniform(bitgen, distance, set.size());
+    auto it = set.find(static_cast<int>(end));
+    benchmark::DoNotOptimize(it -= static_cast<int>(distance));
+    distance = absl::Uniform(bitgen, 0u, set.size());
+  }
+}
+
+BENCHMARK(BM_BtreeSet_IteratorSubtraction)->Range(1 << 10, 1 << 20);
+
+}  // namespace
+}  // namespace container_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/container/btree_map.h b/absl/container/btree_map.h
index 9f35f63949d..257a6246de8 100644
--- a/absl/container/btree_map.h
+++ b/absl/container/btree_map.h
@@ -35,22 +35,68 @@
 //
 // However, these types should not be considered drop-in replacements for
 // `std::map` and `std::multimap` as there are some API differences, which are
-// noted in this header file.
+// noted in this header file. The most consequential differences with respect to
+// migrating to b-tree from the STL types are listed in the next paragraph.
+// Other API differences are minor.
 //
 // Importantly, insertions and deletions may invalidate outstanding iterators,
 // pointers, and references to elements. Such invalidations are typically only
 // an issue if insertion and deletion operations are interleaved with the use of
-// more than one iterator, pointer, or reference simultaneously. For this
-// reason, `insert()` and `erase()` return a valid iterator at the current
-// position.
+// more than one iterator, pointer, or reference simultaneously.  For this
+// reason, `insert()`, `erase()`, and `extract_and_get_next()` return a valid
+// iterator at the current position. Another important difference is that
+// key-types must be copy-constructible.
+//
+// There are other API differences: first, btree iterators can be subtracted,
+// and this is faster than using `std::distance`. Additionally, btree
+// iterators can be advanced via `operator+=` and `operator-=`, which is faster
+// than using `std::advance`.
+//
+// B-tree maps are not exception-safe.
 
 #ifndef ABSL_CONTAINER_BTREE_MAP_H_
 #define ABSL_CONTAINER_BTREE_MAP_H_
 
+#include <functional>
+#include <memory>
+#include <type_traits>
+#include <utility>
+
+#include "absl/base/attributes.h"
 #include "absl/container/internal/btree.h"  // IWYU pragma: export
 #include "absl/container/internal/btree_container.h"  // IWYU pragma: export
+#include "absl/container/internal/common.h"
+#include "absl/container/internal/container_memory.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+namespace container_internal {
+
+template <typename Key, typename Data, typename... Params>
+struct map_params_impl;
+
+template <typename Key, typename Data>
+struct btree_map_defaults {
+  using Compare = std::less<Key>;
+  using Alloc = std::allocator<std::pair<const Key, Data>>;
+  using TargetNodeSize = std::integral_constant<int, 256>;
+  using IsMulti = std::false_type;
+};
+
+template <typename Key, typename Data, typename Compare, typename Alloc,
+          int TargetNodeSize, bool IsMulti>
+using map_params = typename ApplyWithoutDefaultSuffix<
+    map_params_impl,
+    TypeList<void, void, typename btree_map_defaults<Key, Data>::Compare,
+             typename btree_map_defaults<Key, Data>::Alloc,
+             typename btree_map_defaults<Key, Data>::TargetNodeSize,
+             typename btree_map_defaults<Key, Data>::IsMulti>,
+    TypeList<Key, Data, Compare, Alloc,
+             std::integral_constant<int, TargetNodeSize>,
+             std::bool_constant<IsMulti>>>::type;
+
+}  // namespace container_internal
 
 // absl::btree_map<>
 //
@@ -69,11 +115,11 @@ namespace absl {
 //
 template <typename Key, typename Value, typename Compare = std::less<Key>,
           typename Alloc = std::allocator<std::pair<const Key, Value>>>
-class btree_map
+class ABSL_ATTRIBUTE_OWNER btree_map
     : public container_internal::btree_map_container<
           container_internal::btree<container_internal::map_params<
               Key, Value, Compare, Alloc, /*TargetNodeSize=*/256,
-              /*Multi=*/false>>> {
+              /*IsMulti=*/false>>> {
   using Base = typename btree_map::btree_map_container;
 
  public:
@@ -97,8 +143,8 @@ class btree_map
   //
   // * Copy assignment operator
   //
-  //  absl::btree_map<int, std::string> map4;
-  //  map4 = map3;
+  //   absl::btree_map<int, std::string> map4;
+  //   map4 = map3;
   //
   // * Move constructor
   //
@@ -184,7 +230,7 @@ class btree_map
   // template <typename K> size_type erase(const K& key):
   //
   //   Erases the element with the matching key, if it exists, returning the
-  //   number of elements erased.
+  //   number of elements erased (0 or 1).
   using Base::erase;
 
   // btree_map::insert()
@@ -225,6 +271,30 @@ class btree_map
   //   Inserts the elements within the initializer list `ilist`.
   using Base::insert;
 
+  // btree_map::insert_or_assign()
+  //
+  // Inserts an element of the specified value into the `btree_map` provided
+  // that a value with the given key does not already exist, or replaces the
+  // corresponding mapped type with the forwarded `obj` argument if a key for
+  // that value already exists, returning an iterator pointing to the newly
+  // inserted element. Overloads are listed below.
+  //
+  // pair<iterator, bool> insert_or_assign(const key_type& k, M&& obj):
+  // pair<iterator, bool> insert_or_assign(key_type&& k, M&& obj):
+  //
+  //   Inserts/Assigns (or moves) the element of the specified key into the
+  //   `btree_map`. If the returned bool is true, insertion took place, and if
+  //   it's false, assignment took place.
+  //
+  // iterator insert_or_assign(const_iterator hint,
+  //                           const key_type& k, M&& obj):
+  // iterator insert_or_assign(const_iterator hint, key_type&& k, M&& obj):
+  //
+  //   Inserts/Assigns (or moves) the element of the specified key into the
+  //   `btree_map` using the position of `hint` as a non-binding suggestion
+  //   for where to begin the insertion search.
+  using Base::insert_or_assign;
+
   // btree_map::emplace()
   //
   // Inserts an element of the specified value by constructing it in-place
@@ -286,14 +356,15 @@ class btree_map
   // btree_map::extract()
   //
   // Extracts the indicated element, erasing it in the process, and returns it
-  // as a C++17-compatible node handle. Overloads are listed below.
+  // as a C++17-compatible node handle. Any references, pointers, or iterators
+  // are invalidated. Overloads are listed below.
   //
   // node_type extract(const_iterator position):
   //
   //   Extracts the element at the indicated position and returns a node handle
   //   owning that extracted data.
   //
-  // template <typename K> node_type extract(const K& x):
+  // template <typename K> node_type extract(const K& k):
   //
   //   Extracts the element with the key matching the passed key value and
   //   returns a node handle owning that extracted data. If the `btree_map`
@@ -306,6 +377,21 @@ class btree_map
   // It does NOT refer to the data layout of the underlying btree.
   using Base::extract;
 
+  // btree_map::extract_and_get_next()
+  //
+  // Extracts the indicated element, erasing it in the process, and returns it
+  // as a C++17-compatible node handle along with an iterator to the next
+  // element.
+  //
+  // extract_and_get_next_return_type extract_and_get_next(
+  //     const_iterator position):
+  //
+  //   Extracts the element at the indicated position, returns a struct
+  //   containing a member named `node`: a node handle owning that extracted
+  //   data and a member named `next`: an iterator pointing to the next element
+  //   in the btree.
+  using Base::extract_and_get_next;
+
   // btree_map::merge()
   //
   // Extracts elements from a given `source` btree_map into this
@@ -336,8 +422,8 @@ class btree_map
   // Determines whether an element comparing equal to the given `key` exists
   // within the `btree_map`, returning `true` if so or `false` otherwise.
   //
-  // Supports heterogeneous lookup, provided that the map is provided a
-  // compatible heterogeneous comparator.
+  // Supports heterogeneous lookup, provided that the map has a compatible
+  // heterogeneous comparator.
   using Base::contains;
 
   // btree_map::count()
@@ -348,15 +434,14 @@ class btree_map
   // the `btree_map`. Note that this function will return either `1` or `0`
   // since duplicate elements are not allowed within a `btree_map`.
   //
-  // Supports heterogeneous lookup, provided that the map is provided a
-  // compatible heterogeneous comparator.
+  // Supports heterogeneous lookup, provided that the map has a compatible
+  // heterogeneous comparator.
   using Base::count;
 
   // btree_map::equal_range()
   //
-  // Returns a closed range [first, last], defined by a `std::pair` of two
-  // iterators, containing all elements with the passed key in the
-  // `btree_map`.
+  // Returns a half-open range [first, last), defined by a `std::pair` of two
+  // iterators, containing all elements with the passed key in the `btree_map`.
   using Base::equal_range;
 
   // btree_map::find()
@@ -366,10 +451,34 @@ class btree_map
   //
   // Finds an element with the passed `key` within the `btree_map`.
   //
-  // Supports heterogeneous lookup, provided that the map is provided a
-  // compatible heterogeneous comparator.
+  // Supports heterogeneous lookup, provided that the map has a compatible
+  // heterogeneous comparator.
   using Base::find;
 
+  // btree_map::lower_bound()
+  //
+  // template <typename K> iterator lower_bound(const K& key):
+  // template <typename K> const_iterator lower_bound(const K& key) const:
+  //
+  // Finds the first element with a key that is not less than `key` within the
+  // `btree_map`.
+  //
+  // Supports heterogeneous lookup, provided that the map has a compatible
+  // heterogeneous comparator.
+  using Base::lower_bound;
+
+  // btree_map::upper_bound()
+  //
+  // template <typename K> iterator upper_bound(const K& key):
+  // template <typename K> const_iterator upper_bound(const K& key) const:
+  //
+  // Finds the first element with a key that is greater than `key` within the
+  // `btree_map`.
+  //
+  // Supports heterogeneous lookup, provided that the map has a compatible
+  // heterogeneous comparator.
+  using Base::upper_bound;
+
   // btree_map::operator[]()
   //
   // Returns a reference to the value mapped to the passed key within the
@@ -411,6 +520,16 @@ void swap(btree_map<K, V, C, A> &x, btree_map<K, V, C, A> &y) {
   return x.swap(y);
 }
 
+// absl::erase_if(absl::btree_map<>, Pred)
+//
+// Erases all elements that satisfy the predicate pred from the container.
+// Returns the number of erased elements.
+template <typename K, typename V, typename C, typename A, typename Pred>
+typename btree_map<K, V, C, A>::size_type erase_if(
+    btree_map<K, V, C, A> &map, Pred pred) {
+  return container_internal::btree_access::erase_if(map, std::move(pred));
+}
+
 // absl::btree_multimap
 //
 // An `absl::btree_multimap<K, V>` is an ordered associative container of
@@ -429,11 +548,11 @@ void swap(btree_map<K, V, C, A> &x, btree_map<K, V, C, A> &y) {
 //
 template <typename Key, typename Value, typename Compare = std::less<Key>,
           typename Alloc = std::allocator<std::pair<const Key, Value>>>
-class btree_multimap
+class ABSL_ATTRIBUTE_OWNER btree_multimap
     : public container_internal::btree_multimap_container<
           container_internal::btree<container_internal::map_params<
               Key, Value, Compare, Alloc, /*TargetNodeSize=*/256,
-              /*Multi=*/true>>> {
+              /*IsMulti=*/true>>> {
   using Base = typename btree_multimap::btree_multimap_container;
 
  public:
@@ -457,8 +576,8 @@ class btree_multimap
   //
   // * Copy assignment operator
   //
-  //  absl::btree_multimap<int, std::string> map4;
-  //  map4 = map3;
+  //   absl::btree_multimap<int, std::string> map4;
+  //   map4 = map3;
   //
   // * Move constructor
   //
@@ -606,7 +725,7 @@ class btree_multimap
   //   Extracts the element at the indicated position and returns a node handle
   //   owning that extracted data.
   //
-  // template <typename K> node_type extract(const K& x):
+  // template <typename K> node_type extract(const K& k):
   //
   //   Extracts the element with the key matching the passed key value and
   //   returns a node handle owning that extracted data. If the `btree_multimap`
@@ -619,11 +738,25 @@ class btree_multimap
   // It does NOT refer to the data layout of the underlying btree.
   using Base::extract;
 
+  // btree_multimap::extract_and_get_next()
+  //
+  // Extracts the indicated element, erasing it in the process, and returns it
+  // as a C++17-compatible node handle along with an iterator to the next
+  // element.
+  //
+  // extract_and_get_next_return_type extract_and_get_next(
+  //     const_iterator position):
+  //
+  //   Extracts the element at the indicated position, returns a struct
+  //   containing a member named `node`: a node handle owning that extracted
+  //   data and a member named `next`: an iterator pointing to the next element
+  //   in the btree.
+  using Base::extract_and_get_next;
+
   // btree_multimap::merge()
   //
-  // Extracts elements from a given `source` btree_multimap into this
-  // `btree_multimap`. If the destination `btree_multimap` already contains an
-  // element with an equivalent key, that element is not extracted.
+  // Extracts all elements from a given `source` btree_multimap into this
+  // `btree_multimap`.
   using Base::merge;
 
   // btree_multimap::swap(btree_multimap& other)
@@ -643,8 +776,8 @@ class btree_multimap
   // Determines whether an element comparing equal to the given `key` exists
   // within the `btree_multimap`, returning `true` if so or `false` otherwise.
   //
-  // Supports heterogeneous lookup, provided that the map is provided a
-  // compatible heterogeneous comparator.
+  // Supports heterogeneous lookup, provided that the map has a compatible
+  // heterogeneous comparator.
   using Base::contains;
 
   // btree_multimap::count()
@@ -654,13 +787,13 @@ class btree_multimap
   // Returns the number of elements comparing equal to the given `key` within
   // the `btree_multimap`.
   //
-  // Supports heterogeneous lookup, provided that the map is provided a
-  // compatible heterogeneous comparator.
+  // Supports heterogeneous lookup, provided that the map has a compatible
+  // heterogeneous comparator.
   using Base::count;
 
   // btree_multimap::equal_range()
   //
-  // Returns a closed range [first, last], defined by a `std::pair` of two
+  // Returns a half-open range [first, last), defined by a `std::pair` of two
   // iterators, containing all elements with the passed key in the
   // `btree_multimap`.
   using Base::equal_range;
@@ -672,10 +805,34 @@ class btree_multimap
   //
   // Finds an element with the passed `key` within the `btree_multimap`.
   //
-  // Supports heterogeneous lookup, provided that the map is provided a
-  // compatible heterogeneous comparator.
+  // Supports heterogeneous lookup, provided that the map has a compatible
+  // heterogeneous comparator.
   using Base::find;
 
+  // btree_multimap::lower_bound()
+  //
+  // template <typename K> iterator lower_bound(const K& key):
+  // template <typename K> const_iterator lower_bound(const K& key) const:
+  //
+  // Finds the first element with a key that is not less than `key` within the
+  // `btree_multimap`.
+  //
+  // Supports heterogeneous lookup, provided that the map has a compatible
+  // heterogeneous comparator.
+  using Base::lower_bound;
+
+  // btree_multimap::upper_bound()
+  //
+  // template <typename K> iterator upper_bound(const K& key):
+  // template <typename K> const_iterator upper_bound(const K& key) const:
+  //
+  // Finds the first element with a key that is greater than `key` within the
+  // `btree_multimap`.
+  //
+  // Supports heterogeneous lookup, provided that the map has a compatible
+  // heterogeneous comparator.
+  using Base::upper_bound;
+
   // btree_multimap::get_allocator()
   //
   // Returns the allocator function associated with this `btree_multimap`.
@@ -700,6 +857,75 @@ void swap(btree_multimap<K, V, C, A> &x, btree_multimap<K, V, C, A> &y) {
   return x.swap(y);
 }
 
+// absl::erase_if(absl::btree_multimap<>, Pred)
+//
+// Erases all elements that satisfy the predicate pred from the container.
+// Returns the number of erased elements.
+template <typename K, typename V, typename C, typename A, typename Pred>
+typename btree_multimap<K, V, C, A>::size_type erase_if(
+    btree_multimap<K, V, C, A> &map, Pred pred) {
+  return container_internal::btree_access::erase_if(map, std::move(pred));
+}
+
+namespace container_internal {
+
+// A parameters structure for holding the type parameters for a btree_map.
+// Compare and Alloc should be nothrow copy-constructible.
+template <typename Key, typename Data, typename... Params>
+struct map_params_impl
+    : common_params<
+          Key,
+          GetFromListOr<typename btree_map_defaults<Key, Data>::Compare, 0,
+                        Params...>,
+          GetFromListOr<typename btree_map_defaults<Key, Data>::Alloc, 1,
+                        Params...>,
+          GetFromListOr<typename btree_map_defaults<Key, Data>::TargetNodeSize,
+                        2, Params...>::value,
+          GetFromListOr<typename btree_map_defaults<Key, Data>::IsMulti, 3,
+                        Params...>::value,
+          /*IsMap=*/true, map_slot_policy<Key, Data>> {
+  using super_type = typename map_params_impl::common_params;
+  using mapped_type = Data;
+  // This type allows us to move keys when it is safe to do so. It is safe
+  // for maps in which value_type and mutable_value_type are layout compatible.
+  using slot_policy = typename super_type::slot_policy;
+  using slot_type = typename super_type::slot_type;
+  using value_type = typename super_type::value_type;
+  using init_type = typename super_type::init_type;
+
+  static_assert(
+      std::is_same_v<
+          map_params<
+              Key, Data,
+              GetFromListOr<typename btree_map_defaults<Key, Data>::Compare, 0,
+                            Params...>,
+              GetFromListOr<typename btree_map_defaults<Key, Data>::Alloc, 1,
+                            Params...>,
+              GetFromListOr<
+                  typename btree_map_defaults<Key, Data>::TargetNodeSize, 2,
+                  Params...>::value,
+              GetFromListOr<typename btree_map_defaults<Key, Data>::IsMulti, 3,
+                            Params...>::value>,
+          map_params_impl>);
+
+  template <typename V>
+  static auto key(const V &value ABSL_ATTRIBUTE_LIFETIME_BOUND)
+      -> decltype((value.first)) {
+    return value.first;
+  }
+  static const Key &key(const slot_type *s) { return slot_policy::key(s); }
+  static const Key &key(slot_type *s) { return slot_policy::key(s); }
+  // For use in node handle.
+  static auto mutable_key(slot_type *s)
+      -> decltype(slot_policy::mutable_key(s)) {
+    return slot_policy::mutable_key(s);
+  }
+  static mapped_type &value(value_type *value) { return value->second; }
+};
+
+}  // namespace container_internal
+
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_BTREE_MAP_H_
diff --git a/absl/container/btree_set.h b/absl/container/btree_set.h
index 6e47b4aae65..368c6a7b9e6 100644
--- a/absl/container/btree_set.h
+++ b/absl/container/btree_set.h
@@ -35,22 +35,68 @@
 //
 // However, these types should not be considered drop-in replacements for
 // `std::set` and `std::multiset` as there are some API differences, which are
-// noted in this header file.
+// noted in this header file. The most consequential differences with respect to
+// migrating to b-tree from the STL types are listed in the next paragraph.
+// Other API differences are minor.
 //
 // Importantly, insertions and deletions may invalidate outstanding iterators,
 // pointers, and references to elements. Such invalidations are typically only
 // an issue if insertion and deletion operations are interleaved with the use of
 // more than one iterator, pointer, or reference simultaneously. For this
-// reason, `insert()` and `erase()` return a valid iterator at the current
-// position.
+// reason, `insert()`, `erase()`, and `extract_and_get_next()` return a valid
+// iterator at the current position.
+//
+// There are other API differences: first, btree iterators can be subtracted,
+// and this is faster than using `std::distance`. Additionally, btree
+// iterators can be advanced via `operator+=` and `operator-=`, which is faster
+// than using `std::advance`.
+//
+// B-tree sets are not exception-safe.
 
 #ifndef ABSL_CONTAINER_BTREE_SET_H_
 #define ABSL_CONTAINER_BTREE_SET_H_
 
+#include <functional>
+#include <memory>
+#include <type_traits>
+#include <utility>
+
+#include "absl/base/attributes.h"
 #include "absl/container/internal/btree.h"  // IWYU pragma: export
 #include "absl/container/internal/btree_container.h"  // IWYU pragma: export
+#include "absl/container/internal/common.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+namespace container_internal {
+
+template <typename Key>
+struct set_slot_policy;
+
+template <typename Key, typename...Params>
+struct set_params_impl;
+
+template <typename Key>
+struct btree_set_defaults {
+  using Compare = std::less<Key>;
+  using Alloc = std::allocator<Key>;
+  using TargetNodeSize = std::integral_constant<int, 256>;
+  using IsMulti = std::false_type;
+};
+
+template <typename Key, typename Compare, typename Alloc, int TargetNodeSize,
+          bool IsMulti>
+using set_params = typename ApplyWithoutDefaultSuffix<
+    set_params_impl,
+    TypeList<void, typename btree_set_defaults<Key>::Compare,
+             typename btree_set_defaults<Key>::Alloc,
+             typename btree_set_defaults<Key>::TargetNodeSize,
+             typename btree_set_defaults<Key>::IsMulti>,
+    TypeList<Key, Compare, Alloc, std::integral_constant<int, TargetNodeSize>,
+             std::bool_constant<IsMulti>>>::type;
+
+}  // namespace container_internal
 
 // absl::btree_set<>
 //
@@ -69,11 +115,11 @@ namespace absl {
 //
 template <typename Key, typename Compare = std::less<Key>,
           typename Alloc = std::allocator<Key>>
-class btree_set
+class ABSL_ATTRIBUTE_OWNER btree_set
     : public container_internal::btree_set_container<
           container_internal::btree<container_internal::set_params<
               Key, Compare, Alloc, /*TargetNodeSize=*/256,
-              /*Multi=*/false>>> {
+              /*IsMulti=*/false>>> {
   using Base = typename btree_set::btree_set_container;
 
  public:
@@ -97,8 +143,8 @@ class btree_set
   //
   // * Copy assignment operator
   //
-  //  absl::btree_set<std::string> set4;
-  //  set4 = set3;
+  //   absl::btree_set<std::string> set4;
+  //   set4 = set3;
   //
   // * Move constructor
   //
@@ -182,7 +228,7 @@ class btree_set
   // template <typename K> size_type erase(const K& key):
   //
   //   Erases the element with the matching key, if it exists, returning the
-  //   number of elements erased.
+  //   number of elements erased (0 or 1).
   using Base::erase;
 
   // btree_set::insert()
@@ -255,14 +301,15 @@ class btree_set
   // btree_set::extract()
   //
   // Extracts the indicated element, erasing it in the process, and returns it
-  // as a C++17-compatible node handle. Overloads are listed below.
+  // as a C++17-compatible node handle. Any references, pointers, or iterators
+  // are invalidated. Overloads are listed below.
   //
   // node_type extract(const_iterator position):
   //
   //   Extracts the element at the indicated position and returns a node handle
   //   owning that extracted data.
   //
-  // template <typename K> node_type extract(const K& x):
+  // template <typename K> node_type extract(const K& k):
   //
   //   Extracts the element with the key matching the passed key value and
   //   returns a node handle owning that extracted data. If the `btree_set`
@@ -275,6 +322,21 @@ class btree_set
   // It does NOT refer to the data layout of the underlying btree.
   using Base::extract;
 
+  // btree_set::extract_and_get_next()
+  //
+  // Extracts the indicated element, erasing it in the process, and returns it
+  // as a C++17-compatible node handle along with an iterator to the next
+  // element.
+  //
+  // extract_and_get_next_return_type extract_and_get_next(
+  //     const_iterator position):
+  //
+  //   Extracts the element at the indicated position, returns a struct
+  //   containing a member named `node`: a node handle owning that extracted
+  //   data and a member named `next`: an iterator pointing to the next element
+  //   in the btree.
+  using Base::extract_and_get_next;
+
   // btree_set::merge()
   //
   // Extracts elements from a given `source` btree_set into this
@@ -299,8 +361,8 @@ class btree_set
   // Determines whether an element comparing equal to the given `key` exists
   // within the `btree_set`, returning `true` if so or `false` otherwise.
   //
-  // Supports heterogeneous lookup, provided that the set is provided a
-  // compatible heterogeneous comparator.
+  // Supports heterogeneous lookup, provided that the set has a compatible
+  // heterogeneous comparator.
   using Base::contains;
 
   // btree_set::count()
@@ -311,8 +373,8 @@ class btree_set
   // the `btree_set`. Note that this function will return either `1` or `0`
   // since duplicate elements are not allowed within a `btree_set`.
   //
-  // Supports heterogeneous lookup, provided that the set is provided a
-  // compatible heterogeneous comparator.
+  // Supports heterogeneous lookup, provided that the set has a compatible
+  // heterogeneous comparator.
   using Base::count;
 
   // btree_set::equal_range()
@@ -329,10 +391,32 @@ class btree_set
   //
   // Finds an element with the passed `key` within the `btree_set`.
   //
-  // Supports heterogeneous lookup, provided that the set is provided a
-  // compatible heterogeneous comparator.
+  // Supports heterogeneous lookup, provided that the set has a compatible
+  // heterogeneous comparator.
   using Base::find;
 
+  // btree_set::lower_bound()
+  //
+  // template <typename K> iterator lower_bound(const K& key):
+  // template <typename K> const_iterator lower_bound(const K& key) const:
+  //
+  // Finds the first element that is not less than `key` within the `btree_set`.
+  //
+  // Supports heterogeneous lookup, provided that the set has a compatible
+  // heterogeneous comparator.
+  using Base::lower_bound;
+
+  // btree_set::upper_bound()
+  //
+  // template <typename K> iterator upper_bound(const K& key):
+  // template <typename K> const_iterator upper_bound(const K& key) const:
+  //
+  // Finds the first element that is greater than `key` within the `btree_set`.
+  //
+  // Supports heterogeneous lookup, provided that the set has a compatible
+  // heterogeneous comparator.
+  using Base::upper_bound;
+
   // btree_set::get_allocator()
   //
   // Returns the allocator function associated with this `btree_set`.
@@ -359,6 +443,16 @@ void swap(btree_set<K, C, A> &x, btree_set<K, C, A> &y) {
   return x.swap(y);
 }
 
+// absl::erase_if(absl::btree_set<>, Pred)
+//
+// Erases all elements that satisfy the predicate pred from the container.
+// Returns the number of erased elements.
+template <typename K, typename C, typename A, typename Pred>
+typename btree_set<K, C, A>::size_type erase_if(btree_set<K, C, A> &set,
+                                                Pred pred) {
+  return container_internal::btree_access::erase_if(set, std::move(pred));
+}
+
 // absl::btree_multiset<>
 //
 // An `absl::btree_multiset<K>` is an ordered associative container of
@@ -377,11 +471,11 @@ void swap(btree_set<K, C, A> &x, btree_set<K, C, A> &y) {
 //
 template <typename Key, typename Compare = std::less<Key>,
           typename Alloc = std::allocator<Key>>
-class btree_multiset
+class ABSL_ATTRIBUTE_OWNER btree_multiset
     : public container_internal::btree_multiset_container<
           container_internal::btree<container_internal::set_params<
               Key, Compare, Alloc, /*TargetNodeSize=*/256,
-              /*Multi=*/true>>> {
+              /*IsMulti=*/true>>> {
   using Base = typename btree_multiset::btree_multiset_container;
 
  public:
@@ -405,8 +499,8 @@ class btree_multiset
   //
   // * Copy assignment operator
   //
-  //  absl::btree_multiset<std::string> set4;
-  //  set4 = set3;
+  //   absl::btree_multiset<std::string> set4;
+  //   set4 = set3;
   //
   // * Move constructor
   //
@@ -552,7 +646,7 @@ class btree_multiset
   //   Extracts the element at the indicated position and returns a node handle
   //   owning that extracted data.
   //
-  // template <typename K> node_type extract(const K& x):
+  // template <typename K> node_type extract(const K& k):
   //
   //   Extracts the element with the key matching the passed key value and
   //   returns a node handle owning that extracted data. If the `btree_multiset`
@@ -565,11 +659,25 @@ class btree_multiset
   // It does NOT refer to the data layout of the underlying btree.
   using Base::extract;
 
+  // btree_multiset::extract_and_get_next()
+  //
+  // Extracts the indicated element, erasing it in the process, and returns it
+  // as a C++17-compatible node handle along with an iterator to the next
+  // element.
+  //
+  // extract_and_get_next_return_type extract_and_get_next(
+  //     const_iterator position):
+  //
+  //   Extracts the element at the indicated position, returns a struct
+  //   containing a member named `node`: a node handle owning that extracted
+  //   data and a member named `next`: an iterator pointing to the next element
+  //   in the btree.
+  using Base::extract_and_get_next;
+
   // btree_multiset::merge()
   //
-  // Extracts elements from a given `source` btree_multiset into this
-  // `btree_multiset`. If the destination `btree_multiset` already contains an
-  // element with an equivalent key, that element is not extracted.
+  // Extracts all elements from a given `source` btree_multiset into this
+  // `btree_multiset`.
   using Base::merge;
 
   // btree_multiset::swap(btree_multiset& other)
@@ -589,8 +697,8 @@ class btree_multiset
   // Determines whether an element comparing equal to the given `key` exists
   // within the `btree_multiset`, returning `true` if so or `false` otherwise.
   //
-  // Supports heterogeneous lookup, provided that the set is provided a
-  // compatible heterogeneous comparator.
+  // Supports heterogeneous lookup, provided that the set has a compatible
+  // heterogeneous comparator.
   using Base::contains;
 
   // btree_multiset::count()
@@ -600,8 +708,8 @@ class btree_multiset
   // Returns the number of elements comparing equal to the given `key` within
   // the `btree_multiset`.
   //
-  // Supports heterogeneous lookup, provided that the set is provided a
-  // compatible heterogeneous comparator.
+  // Supports heterogeneous lookup, provided that the set has a compatible
+  // heterogeneous comparator.
   using Base::count;
 
   // btree_multiset::equal_range()
@@ -618,10 +726,34 @@ class btree_multiset
   //
   // Finds an element with the passed `key` within the `btree_multiset`.
   //
-  // Supports heterogeneous lookup, provided that the set is provided a
-  // compatible heterogeneous comparator.
+  // Supports heterogeneous lookup, provided that the set has a compatible
+  // heterogeneous comparator.
   using Base::find;
 
+  // btree_multiset::lower_bound()
+  //
+  // template <typename K> iterator lower_bound(const K& key):
+  // template <typename K> const_iterator lower_bound(const K& key) const:
+  //
+  // Finds the first element that is not less than `key` within the
+  // `btree_multiset`.
+  //
+  // Supports heterogeneous lookup, provided that the set has a compatible
+  // heterogeneous comparator.
+  using Base::lower_bound;
+
+  // btree_multiset::upper_bound()
+  //
+  // template <typename K> iterator upper_bound(const K& key):
+  // template <typename K> const_iterator upper_bound(const K& key) const:
+  //
+  // Finds the first element that is greater than `key` within the
+  // `btree_multiset`.
+  //
+  // Supports heterogeneous lookup, provided that the set has a compatible
+  // heterogeneous comparator.
+  using Base::upper_bound;
+
   // btree_multiset::get_allocator()
   //
   // Returns the allocator function associated with this `btree_multiset`.
@@ -648,6 +780,93 @@ void swap(btree_multiset<K, C, A> &x, btree_multiset<K, C, A> &y) {
   return x.swap(y);
 }
 
+// absl::erase_if(absl::btree_multiset<>, Pred)
+//
+// Erases all elements that satisfy the predicate pred from the container.
+// Returns the number of erased elements.
+template <typename K, typename C, typename A, typename Pred>
+typename btree_multiset<K, C, A>::size_type erase_if(
+   btree_multiset<K, C, A> & set, Pred pred) {
+  return container_internal::btree_access::erase_if(set, std::move(pred));
+}
+
+namespace container_internal {
+
+// This type implements the necessary functions from the
+// absl::container_internal::slot_type interface for btree_(multi)set.
+template <typename Key>
+struct set_slot_policy {
+  using slot_type = Key;
+  using value_type = Key;
+  using mutable_value_type = Key;
+
+  static value_type &element(slot_type *slot) { return *slot; }
+  static const value_type &element(const slot_type *slot) { return *slot; }
+
+  template <typename Alloc, class... Args>
+  static void construct(Alloc *alloc, slot_type *slot, Args &&...args) {
+    std::allocator_traits<Alloc>::construct(*alloc, slot,
+                                            std::forward<Args>(args)...);
+  }
+
+  template <typename Alloc>
+  static void construct(Alloc *alloc, slot_type *slot, slot_type *other) {
+    std::allocator_traits<Alloc>::construct(*alloc, slot, std::move(*other));
+  }
+
+  template <typename Alloc>
+  static void construct(Alloc *alloc, slot_type *slot, const slot_type *other) {
+    std::allocator_traits<Alloc>::construct(*alloc, slot, *other);
+  }
+
+  template <typename Alloc>
+  static void destroy(Alloc *alloc, slot_type *slot) {
+    std::allocator_traits<Alloc>::destroy(*alloc, slot);
+  }
+};
+
+// A parameters structure for holding the type parameters for a btree_set.
+// Compare and Alloc should be nothrow copy-constructible.
+template <typename Key, typename... Params>
+struct set_params_impl
+    : common_params<
+          Key,
+          GetFromListOr<typename btree_set_defaults<Key>::Compare, 0,
+                        Params...>,
+          GetFromListOr<typename btree_set_defaults<Key>::Alloc, 1, Params...>,
+          GetFromListOr<typename btree_set_defaults<Key>::TargetNodeSize, 2,
+                        Params...>::value,
+          GetFromListOr<typename btree_set_defaults<Key>::IsMulti, 3,
+                        Params...>::value,
+          /*IsMap=*/false, set_slot_policy<Key>> {
+  using value_type = Key;
+  using slot_type = typename set_params_impl::common_params::slot_type;
+
+  static_assert(
+      std::is_same_v<
+          set_params<
+              Key,
+              GetFromListOr<typename btree_set_defaults<Key>::Compare, 0,
+                            Params...>,
+              GetFromListOr<typename btree_set_defaults<Key>::Alloc, 1,
+                            Params...>,
+              GetFromListOr<typename btree_set_defaults<Key>::TargetNodeSize, 2,
+                            Params...>::value,
+              GetFromListOr<typename btree_set_defaults<Key>::IsMulti, 3,
+                            Params...>::value>,
+          set_params_impl>);
+
+  template <typename V>
+  static const V &key(const V &value) {
+    return value;
+  }
+  static const Key &key(const slot_type *slot) { return *slot; }
+  static const Key &key(slot_type *slot) { return *slot; }
+};
+
+}  // namespace container_internal
+
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_BTREE_SET_H_
diff --git a/absl/container/btree_test.cc b/absl/container/btree_test.cc
index 48f70cb0a5c..5b76c34a0f4 100644
--- a/absl/container/btree_test.cc
+++ b/absl/container/btree_test.cc
@@ -14,26 +14,36 @@
 
 #include "absl/container/btree_test.h"
 
+#include <algorithm>
+#include <array>
 #include <cstdint>
+#include <functional>
+#include <iostream>
+#include <iterator>
+#include <limits>
 #include <map>
 #include <memory>
+#include <numeric>
+#include <optional>
 #include <stdexcept>
 #include <string>
 #include <type_traits>
 #include <utility>
+#include <vector>
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
+#include "absl/algorithm/container.h"
 #include "absl/base/internal/raw_logging.h"
 #include "absl/base/macros.h"
 #include "absl/container/btree_map.h"
 #include "absl/container/btree_set.h"
-#include "absl/container/internal/counting_allocator.h"
+#include "absl/container/internal/test_allocator.h"
 #include "absl/container/internal/test_instance_tracker.h"
 #include "absl/flags/flag.h"
 #include "absl/hash/hash_testing.h"
 #include "absl/memory/memory.h"
-#include "absl/meta/type_traits.h"
+#include "absl/random/random.h"
 #include "absl/strings/str_cat.h"
 #include "absl/strings/str_split.h"
 #include "absl/strings/string_view.h"
@@ -42,14 +52,19 @@
 ABSL_FLAG(int, test_values, 10000, "The number of values to use for tests");
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 namespace {
 
+using ::absl::test_internal::CopyableMovableInstance;
 using ::absl::test_internal::InstanceTracker;
 using ::absl::test_internal::MovableOnlyInstance;
 using ::testing::ElementsAre;
 using ::testing::ElementsAreArray;
+using ::testing::IsEmpty;
+using ::testing::IsNull;
 using ::testing::Pair;
+using ::testing::SizeIs;
 
 template <typename T, typename U>
 void CheckPairEquals(const T &x, const U &y) {
@@ -86,8 +101,8 @@ class base_checker {
 
  public:
   base_checker() : const_tree_(tree_) {}
-  base_checker(const base_checker &x)
-      : tree_(x.tree_), const_tree_(tree_), checker_(x.checker_) {}
+  base_checker(const base_checker &other)
+      : tree_(other.tree_), const_tree_(tree_), checker_(other.checker_) {}
   template <typename InputIterator>
   base_checker(InputIterator b, InputIterator e)
       : tree_(b, e), const_tree_(tree_), checker_(b, e) {}
@@ -121,11 +136,11 @@ class base_checker {
     }
     return tree_iter;
   }
-  void value_check(const value_type &x) {
+  void value_check(const value_type &v) {
     typename KeyOfValue<typename TreeType::key_type,
                         typename TreeType::value_type>::type key_of_value;
-    const key_type &key = key_of_value(x);
-    CheckPairEquals(*find(key), x);
+    const key_type &key = key_of_value(v);
+    CheckPairEquals(*find(key), v);
     lower_bound(key);
     upper_bound(key);
     equal_range(key);
@@ -177,18 +192,16 @@ class base_checker {
   const_iterator find(const key_type &key) const {
     return iter_check(tree_.find(key), checker_.find(key));
   }
-  bool contains(const key_type &key) const {
-    return find(key) != end();
-  }
+  bool contains(const key_type &key) const { return find(key) != end(); }
   size_type count(const key_type &key) const {
     size_type res = checker_.count(key);
     EXPECT_EQ(res, tree_.count(key));
     return res;
   }
 
-  base_checker &operator=(const base_checker &x) {
-    tree_ = x.tree_;
-    checker_ = x.checker_;
+  base_checker &operator=(const base_checker &other) {
+    tree_ = other.tree_;
+    checker_ = other.checker_;
     return *this;
   }
 
@@ -237,8 +250,10 @@ class base_checker {
         ++checker_end;
       }
     }
-    checker_.erase(checker_begin, checker_end);
-    tree_.erase(begin, end);
+    const auto checker_ret = checker_.erase(checker_begin, checker_end);
+    const auto tree_ret = tree_.erase(begin, end);
+    EXPECT_EQ(std::distance(checker_.begin(), checker_ret),
+              std::distance(tree_.begin(), tree_ret));
     EXPECT_EQ(tree_.size(), checker_.size());
     EXPECT_EQ(tree_.size(), size - count);
   }
@@ -247,9 +262,9 @@ class base_checker {
     tree_.clear();
     checker_.clear();
   }
-  void swap(base_checker &x) {
-    tree_.swap(x.tree_);
-    checker_.swap(x.checker_);
+  void swap(base_checker &other) {
+    tree_.swap(other.tree_);
+    checker_.swap(other.checker_);
   }
 
   void verify() const {
@@ -320,28 +335,28 @@ class unique_checker : public base_checker<TreeType, CheckerType> {
 
  public:
   unique_checker() : super_type() {}
-  unique_checker(const unique_checker &x) : super_type(x) {}
+  unique_checker(const unique_checker &other) : super_type(other) {}
   template <class InputIterator>
   unique_checker(InputIterator b, InputIterator e) : super_type(b, e) {}
-  unique_checker& operator=(const unique_checker&) = default;
+  unique_checker &operator=(const unique_checker &) = default;
 
   // Insertion routines.
-  std::pair<iterator, bool> insert(const value_type &x) {
+  std::pair<iterator, bool> insert(const value_type &v) {
     int size = this->tree_.size();
     std::pair<typename CheckerType::iterator, bool> checker_res =
-        this->checker_.insert(x);
-    std::pair<iterator, bool> tree_res = this->tree_.insert(x);
+        this->checker_.insert(v);
+    std::pair<iterator, bool> tree_res = this->tree_.insert(v);
     CheckPairEquals(*tree_res.first, *checker_res.first);
     EXPECT_EQ(tree_res.second, checker_res.second);
     EXPECT_EQ(this->tree_.size(), this->checker_.size());
     EXPECT_EQ(this->tree_.size(), size + tree_res.second);
     return tree_res;
   }
-  iterator insert(iterator position, const value_type &x) {
+  iterator insert(iterator position, const value_type &v) {
     int size = this->tree_.size();
     std::pair<typename CheckerType::iterator, bool> checker_res =
-        this->checker_.insert(x);
-    iterator tree_res = this->tree_.insert(position, x);
+        this->checker_.insert(v);
+    iterator tree_res = this->tree_.insert(position, v);
     CheckPairEquals(*tree_res, *checker_res.first);
     EXPECT_EQ(this->tree_.size(), this->checker_.size());
     EXPECT_EQ(this->tree_.size(), size + checker_res.second);
@@ -368,25 +383,25 @@ class multi_checker : public base_checker<TreeType, CheckerType> {
 
  public:
   multi_checker() : super_type() {}
-  multi_checker(const multi_checker &x) : super_type(x) {}
+  multi_checker(const multi_checker &other) : super_type(other) {}
   template <class InputIterator>
   multi_checker(InputIterator b, InputIterator e) : super_type(b, e) {}
-  multi_checker& operator=(const multi_checker&) = default;
+  multi_checker &operator=(const multi_checker &) = default;
 
   // Insertion routines.
-  iterator insert(const value_type &x) {
+  iterator insert(const value_type &v) {
     int size = this->tree_.size();
-    auto checker_res = this->checker_.insert(x);
-    iterator tree_res = this->tree_.insert(x);
+    auto checker_res = this->checker_.insert(v);
+    iterator tree_res = this->tree_.insert(v);
     CheckPairEquals(*tree_res, *checker_res);
     EXPECT_EQ(this->tree_.size(), this->checker_.size());
     EXPECT_EQ(this->tree_.size(), size + 1);
     return tree_res;
   }
-  iterator insert(iterator position, const value_type &x) {
+  iterator insert(iterator position, const value_type &v) {
     int size = this->tree_.size();
-    auto checker_res = this->checker_.insert(x);
-    iterator tree_res = this->tree_.insert(position, x);
+    auto checker_res = this->checker_.insert(v);
+    iterator tree_res = this->tree_.insert(position, v);
     CheckPairEquals(*tree_res, *checker_res);
     EXPECT_EQ(this->tree_.size(), this->checker_.size());
     EXPECT_EQ(this->tree_.size(), size + 1);
@@ -589,7 +604,7 @@ void BtreeTest() {
   using V = typename remove_pair_const<typename T::value_type>::type;
   const std::vector<V> random_values = GenerateValuesWithSeed<V>(
       absl::GetFlag(FLAGS_test_values), 4 * absl::GetFlag(FLAGS_test_values),
-      testing::GTEST_FLAG(random_seed));
+      GTEST_FLAG_GET(random_seed));
 
   unique_checker<T, C> container;
 
@@ -613,7 +628,7 @@ void BtreeMultiTest() {
   using V = typename remove_pair_const<typename T::value_type>::type;
   const std::vector<V> random_values = GenerateValuesWithSeed<V>(
       absl::GetFlag(FLAGS_test_values), 4 * absl::GetFlag(FLAGS_test_values),
-      testing::GTEST_FLAG(random_seed));
+      GTEST_FLAG_GET(random_seed));
 
   multi_checker<T, C> container;
 
@@ -642,111 +657,6 @@ void BtreeMultiTest() {
   DoTest("identical: ", &container, identical_values);
 }
 
-template <typename T>
-struct PropagatingCountingAlloc : public CountingAllocator<T> {
-  using propagate_on_container_copy_assignment = std::true_type;
-  using propagate_on_container_move_assignment = std::true_type;
-  using propagate_on_container_swap = std::true_type;
-
-  using Base = CountingAllocator<T>;
-  using Base::Base;
-
-  template <typename U>
-  explicit PropagatingCountingAlloc(const PropagatingCountingAlloc<U> &other)
-      : Base(other.bytes_used_) {}
-
-  template <typename U>
-  struct rebind {
-    using other = PropagatingCountingAlloc<U>;
-  };
-};
-
-template <typename T>
-void BtreeAllocatorTest() {
-  using value_type = typename T::value_type;
-
-  int64_t bytes1 = 0, bytes2 = 0;
-  PropagatingCountingAlloc<T> allocator1(&bytes1);
-  PropagatingCountingAlloc<T> allocator2(&bytes2);
-  Generator<value_type> generator(1000);
-
-  // Test that we allocate properly aligned memory. If we don't, then Layout
-  // will assert fail.
-  auto unused1 = allocator1.allocate(1);
-  auto unused2 = allocator2.allocate(1);
-
-  // Test copy assignment
-  {
-    T b1(typename T::key_compare(), allocator1);
-    T b2(typename T::key_compare(), allocator2);
-
-    int64_t original_bytes1 = bytes1;
-    b1.insert(generator(0));
-    EXPECT_GT(bytes1, original_bytes1);
-
-    // This should propagate the allocator.
-    b1 = b2;
-    EXPECT_EQ(b1.size(), 0);
-    EXPECT_EQ(b2.size(), 0);
-    EXPECT_EQ(bytes1, original_bytes1);
-
-    for (int i = 1; i < 1000; i++) {
-      b1.insert(generator(i));
-    }
-
-    // We should have allocated out of allocator2.
-    EXPECT_GT(bytes2, bytes1);
-  }
-
-  // Test move assignment
-  {
-    T b1(typename T::key_compare(), allocator1);
-    T b2(typename T::key_compare(), allocator2);
-
-    int64_t original_bytes1 = bytes1;
-    b1.insert(generator(0));
-    EXPECT_GT(bytes1, original_bytes1);
-
-    // This should propagate the allocator.
-    b1 = std::move(b2);
-    EXPECT_EQ(b1.size(), 0);
-    EXPECT_EQ(bytes1, original_bytes1);
-
-    for (int i = 1; i < 1000; i++) {
-      b1.insert(generator(i));
-    }
-
-    // We should have allocated out of allocator2.
-    EXPECT_GT(bytes2, bytes1);
-  }
-
-  // Test swap
-  {
-    T b1(typename T::key_compare(), allocator1);
-    T b2(typename T::key_compare(), allocator2);
-
-    int64_t original_bytes1 = bytes1;
-    b1.insert(generator(0));
-    EXPECT_GT(bytes1, original_bytes1);
-
-    // This should swap the allocators.
-    swap(b1, b2);
-    EXPECT_EQ(b1.size(), 0);
-    EXPECT_EQ(b2.size(), 1);
-    EXPECT_GT(bytes1, original_bytes1);
-
-    for (int i = 1; i < 1000; i++) {
-      b1.insert(generator(i));
-    }
-
-    // We should have allocated out of allocator2.
-    EXPECT_GT(bytes2, bytes1);
-  }
-
-  allocator1.deallocate(unused1, 1);
-  allocator2.deallocate(unused2, 1);
-}
-
 template <typename T>
 void BtreeMapTest() {
   using value_type = typename T::value_type;
@@ -786,10 +696,7 @@ void SetTest() {
       sizeof(absl::btree_set<K>),
       2 * sizeof(void *) + sizeof(typename absl::btree_set<K>::size_type));
   using BtreeSet = absl::btree_set<K>;
-  using CountingBtreeSet =
-      absl::btree_set<K, std::less<K>, PropagatingCountingAlloc<K>>;
   BtreeTest<BtreeSet, std::set<K>>();
-  BtreeAllocatorTest<CountingBtreeSet>();
 }
 
 template <typename K, int N = 256>
@@ -798,22 +705,16 @@ void MapTest() {
       sizeof(absl::btree_map<K, K>),
       2 * sizeof(void *) + sizeof(typename absl::btree_map<K, K>::size_type));
   using BtreeMap = absl::btree_map<K, K>;
-  using CountingBtreeMap =
-      absl::btree_map<K, K, std::less<K>,
-                      PropagatingCountingAlloc<std::pair<const K, K>>>;
   BtreeTest<BtreeMap, std::map<K, K>>();
-  BtreeAllocatorTest<CountingBtreeMap>();
   BtreeMapTest<BtreeMap>();
 }
 
 TEST(Btree, set_int32) { SetTest<int32_t>(); }
-TEST(Btree, set_int64) { SetTest<int64_t>(); }
 TEST(Btree, set_string) { SetTest<std::string>(); }
-TEST(Btree, set_pair) { SetTest<std::pair<int, int>>(); }
+TEST(Btree, set_cord) { SetTest<absl::Cord>(); }
 TEST(Btree, map_int32) { MapTest<int32_t>(); }
-TEST(Btree, map_int64) { MapTest<int64_t>(); }
 TEST(Btree, map_string) { MapTest<std::string>(); }
-TEST(Btree, map_pair) { MapTest<std::pair<int, int>>(); }
+TEST(Btree, map_cord) { MapTest<absl::Cord>(); }
 
 template <typename K, int N = 256>
 void MultiSetTest() {
@@ -821,10 +722,7 @@ void MultiSetTest() {
       sizeof(absl::btree_multiset<K>),
       2 * sizeof(void *) + sizeof(typename absl::btree_multiset<K>::size_type));
   using BtreeMSet = absl::btree_multiset<K>;
-  using CountingBtreeMSet =
-      absl::btree_multiset<K, std::less<K>, PropagatingCountingAlloc<K>>;
   BtreeMultiTest<BtreeMSet, std::multiset<K>>();
-  BtreeAllocatorTest<CountingBtreeMSet>();
 }
 
 template <typename K, int N = 256>
@@ -833,22 +731,16 @@ void MultiMapTest() {
             2 * sizeof(void *) +
                 sizeof(typename absl::btree_multimap<K, K>::size_type));
   using BtreeMMap = absl::btree_multimap<K, K>;
-  using CountingBtreeMMap =
-      absl::btree_multimap<K, K, std::less<K>,
-                           PropagatingCountingAlloc<std::pair<const K, K>>>;
   BtreeMultiTest<BtreeMMap, std::multimap<K, K>>();
   BtreeMultiMapTest<BtreeMMap>();
-  BtreeAllocatorTest<CountingBtreeMMap>();
 }
 
 TEST(Btree, multiset_int32) { MultiSetTest<int32_t>(); }
-TEST(Btree, multiset_int64) { MultiSetTest<int64_t>(); }
 TEST(Btree, multiset_string) { MultiSetTest<std::string>(); }
-TEST(Btree, multiset_pair) { MultiSetTest<std::pair<int, int>>(); }
+TEST(Btree, multiset_cord) { MultiSetTest<absl::Cord>(); }
 TEST(Btree, multimap_int32) { MultiMapTest<int32_t>(); }
-TEST(Btree, multimap_int64) { MultiMapTest<int64_t>(); }
 TEST(Btree, multimap_string) { MultiMapTest<std::string>(); }
-TEST(Btree, multimap_pair) { MultiMapTest<std::pair<int, int>>(); }
+TEST(Btree, multimap_cord) { MultiMapTest<absl::Cord>(); }
 
 struct CompareIntToString {
   bool operator()(const std::string &a, const std::string &b) const {
@@ -865,7 +757,7 @@ struct CompareIntToString {
 
 struct NonTransparentCompare {
   template <typename T, typename U>
-  bool operator()(const T& t, const U& u) const {
+  bool operator()(const T &t, const U &u) const {
     // Treating all comparators as transparent can cause inefficiencies (see
     // N3657 C++ proposal). Test that for comparators without 'is_transparent'
     // alias (like this one), we do not attempt heterogeneous lookup.
@@ -874,15 +766,12 @@ struct NonTransparentCompare {
   }
 };
 
-template <typename T>
-bool CanEraseWithEmptyBrace(T t, decltype(t.erase({})) *) {
-  return true;
-}
+template <class T, class = void>
+struct CanEraseWithEmptyBrace : std::false_type {};
 
-template <typename T>
-bool CanEraseWithEmptyBrace(T, ...) {
-  return false;
-}
+template <class T>
+struct CanEraseWithEmptyBrace<
+    T, std::void_t<decltype(std::declval<T>().erase({}))*>> : std::true_type {};
 
 template <typename T>
 void TestHeterogeneous(T table) {
@@ -927,7 +816,7 @@ void TestHeterogeneous(T table) {
   EXPECT_EQ(table.size() - 1, copy.size());
   copy.erase({"5"});
   EXPECT_EQ(table.size() - 2, copy.size());
-  EXPECT_FALSE(CanEraseWithEmptyBrace(table, nullptr));
+  EXPECT_FALSE(CanEraseWithEmptyBrace<T>::value);
 
   // Also run it with const T&.
   if (std::is_class<T>()) TestHeterogeneous<const T &>(table);
@@ -1002,21 +891,15 @@ class StringLike {
  public:
   StringLike() = default;
 
-  StringLike(const char* s) : s_(s) {  // NOLINT
+  StringLike(const char *s) : s_(s) {  // NOLINT
     ++constructor_calls_;
   }
 
-  bool operator<(const StringLike& a) const {
-    return s_ < a.s_;
-  }
+  bool operator<(const StringLike &a) const { return s_ < a.s_; }
 
-  static void clear_constructor_call_count() {
-    constructor_calls_ = 0;
-  }
+  static void clear_constructor_call_count() { constructor_calls_ = 0; }
 
-  static int constructor_calls() {
-    return constructor_calls_;
-  }
+  static int constructor_calls() { return constructor_calls_; }
 
  private:
   static int constructor_calls_;
@@ -1179,12 +1062,116 @@ TEST(Btree, RangeCtorSanity) {
   EXPECT_EQ(1, tmap.size());
 }
 
+}  // namespace
+
+class BtreeNodePeer {
+ public:
+  // Yields the size of a leaf node with a specific number of values.
+  template <typename ValueType>
+  constexpr static size_t GetTargetNodeSize(size_t target_values_per_node) {
+    return btree_node<
+        set_params<ValueType, std::less<ValueType>, std::allocator<ValueType>,
+                   /*TargetNodeSize=*/256,  // This parameter isn't used here.
+                   /*Multi=*/false>>::SizeWithNSlots(target_values_per_node);
+  }
+
+  // Yields the number of slots in a (non-root) leaf node for this btree.
+  template <typename Btree>
+  constexpr static size_t GetNumSlotsPerNode() {
+    return btree_node<typename Btree::params_type>::kNodeSlots;
+  }
+
+  template <typename Btree>
+  constexpr static size_t GetMaxFieldType() {
+    return std::numeric_limits<
+        typename btree_node<typename Btree::params_type>::field_type>::max();
+  }
+
+  template <typename Btree>
+  constexpr static bool UsesLinearNodeSearch() {
+    return btree_node<typename Btree::params_type>::use_linear_search::value;
+  }
+
+  template <typename Btree>
+  constexpr static bool FieldTypeEqualsSlotType() {
+    return std::is_same_v<
+        typename btree_node<typename Btree::params_type>::field_type,
+        typename btree_node<typename Btree::params_type>::slot_type>;
+  }
+};
+
+namespace {
+
+class BtreeMapTest : public ::testing::Test {
+ public:
+  struct Key {};
+  struct Cmp {
+    template <typename T>
+    [[maybe_unused]] bool operator()(T, T) const {
+      return false;
+    }
+  };
+
+  struct KeyLin {
+    using absl_btree_prefer_linear_node_search = std::true_type;
+  };
+  struct CmpLin : Cmp {
+    using absl_btree_prefer_linear_node_search = std::true_type;
+  };
+
+  struct KeyBin {
+    using absl_btree_prefer_linear_node_search = std::false_type;
+  };
+  struct CmpBin : Cmp {
+    using absl_btree_prefer_linear_node_search = std::false_type;
+  };
+
+  template <typename K, typename C>
+  static bool IsLinear() {
+    return BtreeNodePeer::UsesLinearNodeSearch<absl::btree_map<K, int, C>>();
+  }
+};
+
+TEST_F(BtreeMapTest, TestLinearSearchPreferredForKeyLinearViaAlias) {
+  // Test requesting linear search by directly exporting an alias.
+  EXPECT_FALSE((IsLinear<Key, Cmp>()));
+  EXPECT_TRUE((IsLinear<KeyLin, Cmp>()));
+  EXPECT_TRUE((IsLinear<Key, CmpLin>()));
+  EXPECT_TRUE((IsLinear<KeyLin, CmpLin>()));
+}
+
+TEST_F(BtreeMapTest, LinearChoiceTree) {
+  // Cmp has precedence, and is forcing binary
+  EXPECT_FALSE((IsLinear<Key, CmpBin>()));
+  EXPECT_FALSE((IsLinear<KeyLin, CmpBin>()));
+  EXPECT_FALSE((IsLinear<KeyBin, CmpBin>()));
+  EXPECT_FALSE((IsLinear<int, CmpBin>()));
+  EXPECT_FALSE((IsLinear<std::string, CmpBin>()));
+  // Cmp has precedence, and is forcing linear
+  EXPECT_TRUE((IsLinear<Key, CmpLin>()));
+  EXPECT_TRUE((IsLinear<KeyLin, CmpLin>()));
+  EXPECT_TRUE((IsLinear<KeyBin, CmpLin>()));
+  EXPECT_TRUE((IsLinear<int, CmpLin>()));
+  EXPECT_TRUE((IsLinear<std::string, CmpLin>()));
+  // Cmp has no preference, Key determines linear vs binary.
+  EXPECT_FALSE((IsLinear<Key, Cmp>()));
+  EXPECT_TRUE((IsLinear<KeyLin, Cmp>()));
+  EXPECT_FALSE((IsLinear<KeyBin, Cmp>()));
+  // arithmetic key w/ std::less or std::greater: linear
+  EXPECT_TRUE((IsLinear<int, std::less<int>>()));
+  EXPECT_TRUE((IsLinear<double, std::greater<double>>()));
+  // arithmetic key w/ custom compare: binary
+  EXPECT_FALSE((IsLinear<int, Cmp>()));
+  // non-arithmetic key: binary
+  EXPECT_FALSE((IsLinear<std::string, std::less<std::string>>()));
+}
+
 TEST(Btree, BtreeMapCanHoldMoveOnlyTypes) {
   absl::btree_map<std::string, std::unique_ptr<std::string>> m;
 
   std::unique_ptr<std::string> &v = m["A"];
   EXPECT_TRUE(v == nullptr);
-  v.reset(new std::string("X"));
+  v = std::make_unique<std::string>("X");
 
   auto iter = m.find("A");
   EXPECT_EQ("X", *iter->second);
@@ -1243,36 +1230,34 @@ TEST(Btree, InitializerListInsert) {
   EXPECT_EQ(++it, range.second);
 }
 
-template <typename Compare, typename K>
-void AssertKeyCompareToAdapted() {
-  using Adapted = typename key_compare_to_adapter<Compare>::type;
-  static_assert(!std::is_same<Adapted, Compare>::value,
-                "key_compare_to_adapter should have adapted this comparator.");
+template <typename Compare, typename Key>
+void AssertKeyCompareStringAdapted() {
+  using Adapted = typename key_compare_adapter<Compare, Key>::type;
   static_assert(
-      std::is_same<absl::weak_ordering,
-                   absl::result_of_t<Adapted(const K &, const K &)>>::value,
-      "Adapted comparator should be a key-compare-to comparator.");
+      std::is_same_v<Adapted, StringBtreeDefaultLess> ||
+          std::is_same_v<Adapted, StringBtreeDefaultGreater>,
+      "key_compare_adapter should have string-adapted this comparator.");
 }
-template <typename Compare, typename K>
-void AssertKeyCompareToNotAdapted() {
-  using Unadapted = typename key_compare_to_adapter<Compare>::type;
-  static_assert(
-      std::is_same<Unadapted, Compare>::value,
-      "key_compare_to_adapter shouldn't have adapted this comparator.");
+template <typename Compare, typename Key>
+void AssertKeyCompareNotStringAdapted() {
+  using Adapted = typename key_compare_adapter<Compare, Key>::type;
   static_assert(
-      std::is_same<bool,
-                   absl::result_of_t<Unadapted(const K &, const K &)>>::value,
-      "Un-adapted comparator should return bool.");
+      !std::is_same_v<Adapted, StringBtreeDefaultLess> &&
+          !std::is_same_v<Adapted, StringBtreeDefaultGreater>,
+      "key_compare_adapter shouldn't have string-adapted this comparator.");
 }
 
-TEST(Btree, KeyCompareToAdapter) {
-  AssertKeyCompareToAdapted<std::less<std::string>, std::string>();
-  AssertKeyCompareToAdapted<std::greater<std::string>, std::string>();
-  AssertKeyCompareToAdapted<std::less<absl::string_view>, absl::string_view>();
-  AssertKeyCompareToAdapted<std::greater<absl::string_view>,
-                            absl::string_view>();
-  AssertKeyCompareToNotAdapted<std::less<int>, int>();
-  AssertKeyCompareToNotAdapted<std::greater<int>, int>();
+TEST(Btree, KeyCompareAdapter) {
+  AssertKeyCompareStringAdapted<std::less<std::string>, std::string>();
+  AssertKeyCompareStringAdapted<std::greater<std::string>, std::string>();
+  AssertKeyCompareStringAdapted<std::less<absl::string_view>,
+                                absl::string_view>();
+  AssertKeyCompareStringAdapted<std::greater<absl::string_view>,
+                                absl::string_view>();
+  AssertKeyCompareStringAdapted<std::less<absl::Cord>, absl::Cord>();
+  AssertKeyCompareStringAdapted<std::greater<absl::Cord>, absl::Cord>();
+  AssertKeyCompareNotStringAdapted<std::less<int>, int>();
+  AssertKeyCompareNotStringAdapted<std::greater<int>, int>();
 }
 
 TEST(Btree, RValueInsert) {
@@ -1322,39 +1307,25 @@ TEST(Btree, RValueInsert) {
   EXPECT_EQ(tracker.swaps(), 0);
 }
 
-}  // namespace
-
-class BtreeNodePeer {
- public:
-  // Yields the size of a leaf node with a specific number of values.
-  template <typename ValueType>
-  constexpr static size_t GetTargetNodeSize(size_t target_values_per_node) {
-    return btree_node<
-        set_params<ValueType, std::less<ValueType>, std::allocator<ValueType>,
-                   /*TargetNodeSize=*/256,  // This parameter isn't used here.
-                   /*Multi=*/false>>::SizeWithNValues(target_values_per_node);
-  }
-
-  // Yields the number of values in a (non-root) leaf node for this set.
-  template <typename Set>
-  constexpr static size_t GetNumValuesPerNode() {
-    return btree_node<typename Set::params_type>::kNodeValues;
-  }
+template <typename Cmp>
+struct CheckedCompareOptedOutCmp : Cmp, BtreeTestOnlyCheckedCompareOptOutBase {
+  using Cmp::Cmp;
+  CheckedCompareOptedOutCmp() {}
+  CheckedCompareOptedOutCmp(Cmp cmp) : Cmp(std::move(cmp)) {}  // NOLINT
 };
 
-namespace {
-
-// A btree set with a specific number of values per node.
+// A btree set with a specific number of values per node. Opt out of
+// checked_compare so that we can expect exact numbers of comparisons.
 template <typename Key, int TargetValuesPerNode, typename Cmp = std::less<Key>>
 class SizedBtreeSet
     : public btree_set_container<btree<
-          set_params<Key, Cmp, std::allocator<Key>,
+          set_params<Key, CheckedCompareOptedOutCmp<Cmp>, std::allocator<Key>,
                      BtreeNodePeer::GetTargetNodeSize<Key>(TargetValuesPerNode),
                      /*Multi=*/false>>> {
   using Base = typename SizedBtreeSet::btree_set_container;
 
  public:
-  SizedBtreeSet() {}
+  SizedBtreeSet() = default;
   using Base::Base;
 };
 
@@ -1372,12 +1343,21 @@ void ExpectOperationCounts(const int expected_moves,
   tracker->ResetCopiesMovesSwaps();
 }
 
+#if defined(ABSL_HAVE_ADDRESS_SANITIZER) || \
+    defined(ABSL_HAVE_HWADDRESS_SANITIZER)
+constexpr bool kAsan = true;
+#else
+constexpr bool kAsan = false;
+#endif
+
 // Note: when the values in this test change, it is expected to have an impact
 // on performance.
 TEST(Btree, MovesComparisonsCopiesSwapsTracking) {
+  if (kAsan) GTEST_SKIP() << "We do extra operations in ASan mode.";
+
   InstanceTracker tracker;
   // Note: this is minimum number of values per node.
-  SizedBtreeSet<MovableOnlyInstance, /*TargetValuesPerNode=*/3> set3;
+  SizedBtreeSet<MovableOnlyInstance, /*TargetValuesPerNode=*/4> set4;
   // Note: this is the default number of values per node for a set of int32s
   // (with 64-bit pointers).
   SizedBtreeSet<MovableOnlyInstance, /*TargetValuesPerNode=*/61> set61;
@@ -1388,28 +1368,29 @@ TEST(Btree, MovesComparisonsCopiesSwapsTracking) {
   std::vector<int> values =
       GenerateValuesWithSeed<int>(10000, 1 << 22, /*seed=*/23);
 
-  EXPECT_EQ(BtreeNodePeer::GetNumValuesPerNode<decltype(set3)>(), 3);
-  EXPECT_EQ(BtreeNodePeer::GetNumValuesPerNode<decltype(set61)>(), 61);
-  EXPECT_EQ(BtreeNodePeer::GetNumValuesPerNode<decltype(set100)>(), 100);
+  EXPECT_EQ(BtreeNodePeer::GetNumSlotsPerNode<decltype(set4)>(), 4);
+  EXPECT_EQ(BtreeNodePeer::GetNumSlotsPerNode<decltype(set61)>(), 61);
+  EXPECT_EQ(BtreeNodePeer::GetNumSlotsPerNode<decltype(set100)>(), 100);
   if (sizeof(void *) == 8) {
-    EXPECT_EQ(BtreeNodePeer::GetNumValuesPerNode<absl::btree_set<int32_t>>(),
-              BtreeNodePeer::GetNumValuesPerNode<decltype(set61)>());
+    EXPECT_EQ(BtreeNodePeer::GetNumSlotsPerNode<absl::btree_set<int32_t>>(),
+              // When we have generations, there is one fewer slot.
+              BtreeGenerationsEnabled() ? 60 : 61);
   }
 
   // Test key insertion/deletion in random order.
-  ExpectOperationCounts(45281, 132551, values, &tracker, &set3);
+  ExpectOperationCounts(56540, 134212, values, &tracker, &set4);
   ExpectOperationCounts(386718, 129807, values, &tracker, &set61);
   ExpectOperationCounts(586761, 130310, values, &tracker, &set100);
 
   // Test key insertion/deletion in sorted order.
   std::sort(values.begin(), values.end());
-  ExpectOperationCounts(26638, 92134, values, &tracker, &set3);
+  ExpectOperationCounts(24972, 85563, values, &tracker, &set4);
   ExpectOperationCounts(20208, 87757, values, &tracker, &set61);
   ExpectOperationCounts(20124, 96583, values, &tracker, &set100);
 
   // Test key insertion/deletion in reverse sorted order.
   std::reverse(values.begin(), values.end());
-  ExpectOperationCounts(49951, 119325, values, &tracker, &set3);
+  ExpectOperationCounts(54949, 127531, values, &tracker, &set4);
   ExpectOperationCounts(338813, 118266, values, &tracker, &set61);
   ExpectOperationCounts(534529, 125279, values, &tracker, &set100);
 }
@@ -1424,11 +1405,13 @@ struct MovableOnlyInstanceThreeWayCompare {
 // Note: when the values in this test change, it is expected to have an impact
 // on performance.
 TEST(Btree, MovesComparisonsCopiesSwapsTrackingThreeWayCompare) {
+  if (kAsan) GTEST_SKIP() << "We do extra operations in ASan mode.";
+
   InstanceTracker tracker;
   // Note: this is minimum number of values per node.
-  SizedBtreeSet<MovableOnlyInstance, /*TargetValuesPerNode=*/3,
+  SizedBtreeSet<MovableOnlyInstance, /*TargetValuesPerNode=*/4,
                 MovableOnlyInstanceThreeWayCompare>
-      set3;
+      set4;
   // Note: this is the default number of values per node for a set of int32s
   // (with 64-bit pointers).
   SizedBtreeSet<MovableOnlyInstance, /*TargetValuesPerNode=*/61,
@@ -1443,28 +1426,29 @@ TEST(Btree, MovesComparisonsCopiesSwapsTrackingThreeWayCompare) {
   std::vector<int> values =
       GenerateValuesWithSeed<int>(10000, 1 << 22, /*seed=*/23);
 
-  EXPECT_EQ(BtreeNodePeer::GetNumValuesPerNode<decltype(set3)>(), 3);
-  EXPECT_EQ(BtreeNodePeer::GetNumValuesPerNode<decltype(set61)>(), 61);
-  EXPECT_EQ(BtreeNodePeer::GetNumValuesPerNode<decltype(set100)>(), 100);
+  EXPECT_EQ(BtreeNodePeer::GetNumSlotsPerNode<decltype(set4)>(), 4);
+  EXPECT_EQ(BtreeNodePeer::GetNumSlotsPerNode<decltype(set61)>(), 61);
+  EXPECT_EQ(BtreeNodePeer::GetNumSlotsPerNode<decltype(set100)>(), 100);
   if (sizeof(void *) == 8) {
-    EXPECT_EQ(BtreeNodePeer::GetNumValuesPerNode<absl::btree_set<int32_t>>(),
-              BtreeNodePeer::GetNumValuesPerNode<decltype(set61)>());
+    EXPECT_EQ(BtreeNodePeer::GetNumSlotsPerNode<absl::btree_set<int32_t>>(),
+              // When we have generations, there is one fewer slot.
+              BtreeGenerationsEnabled() ? 60 : 61);
   }
 
   // Test key insertion/deletion in random order.
-  ExpectOperationCounts(45281, 122560, values, &tracker, &set3);
+  ExpectOperationCounts(56540, 124221, values, &tracker, &set4);
   ExpectOperationCounts(386718, 119816, values, &tracker, &set61);
   ExpectOperationCounts(586761, 120319, values, &tracker, &set100);
 
   // Test key insertion/deletion in sorted order.
   std::sort(values.begin(), values.end());
-  ExpectOperationCounts(26638, 92134, values, &tracker, &set3);
+  ExpectOperationCounts(24972, 85563, values, &tracker, &set4);
   ExpectOperationCounts(20208, 87757, values, &tracker, &set61);
   ExpectOperationCounts(20124, 96583, values, &tracker, &set100);
 
   // Test key insertion/deletion in reverse sorted order.
   std::reverse(values.begin(), values.end());
-  ExpectOperationCounts(49951, 109326, values, &tracker, &set3);
+  ExpectOperationCounts(54949, 117532, values, &tracker, &set4);
   ExpectOperationCounts(338813, 108267, values, &tracker, &set61);
   ExpectOperationCounts(534529, 115280, values, &tracker, &set100);
 }
@@ -1473,7 +1457,7 @@ struct NoDefaultCtor {
   int num;
   explicit NoDefaultCtor(int i) : num(i) {}
 
-  friend bool operator<(const NoDefaultCtor& a, const NoDefaultCtor& b) {
+  friend bool operator<(const NoDefaultCtor &a, const NoDefaultCtor &b) {
     return a.num < b.num;
   }
 };
@@ -1540,7 +1524,7 @@ TEST(Btree, MapAt) {
 #ifdef ABSL_HAVE_EXCEPTIONS
   EXPECT_THROW(map.at(3), std::out_of_range);
 #else
-  EXPECT_DEATH(map.at(3), "absl::btree_map::at");
+  EXPECT_DEATH_IF_SUPPORTED(map.at(3), "absl::btree_map::at");
 #endif
 }
 
@@ -1550,10 +1534,9 @@ TEST(Btree, BtreeMultisetEmplace) {
   auto iter = s.emplace(value_to_insert);
   ASSERT_NE(iter, s.end());
   EXPECT_EQ(*iter, value_to_insert);
-  auto iter2 = s.emplace(value_to_insert);
-  EXPECT_NE(iter2, iter);
-  ASSERT_NE(iter2, s.end());
-  EXPECT_EQ(*iter2, value_to_insert);
+  iter = s.emplace(value_to_insert);
+  ASSERT_NE(iter, s.end());
+  EXPECT_EQ(*iter, value_to_insert);
   auto result = s.equal_range(value_to_insert);
   EXPECT_EQ(std::distance(result.first, result.second), 2);
 }
@@ -1564,44 +1547,45 @@ TEST(Btree, BtreeMultisetEmplaceHint) {
   auto iter = s.emplace(value_to_insert);
   ASSERT_NE(iter, s.end());
   EXPECT_EQ(*iter, value_to_insert);
-  auto emplace_iter = s.emplace_hint(iter, value_to_insert);
-  EXPECT_NE(emplace_iter, iter);
-  ASSERT_NE(emplace_iter, s.end());
-  EXPECT_EQ(*emplace_iter, value_to_insert);
+  iter = s.emplace_hint(iter, value_to_insert);
+  // The new element should be before the previously inserted one.
+  EXPECT_EQ(iter, s.lower_bound(value_to_insert));
+  ASSERT_NE(iter, s.end());
+  EXPECT_EQ(*iter, value_to_insert);
 }
 
 TEST(Btree, BtreeMultimapEmplace) {
   const int key_to_insert = 123456;
   const char value0[] = "a";
-  absl::btree_multimap<int, std::string> s;
-  auto iter = s.emplace(key_to_insert, value0);
-  ASSERT_NE(iter, s.end());
+  absl::btree_multimap<int, std::string> m;
+  auto iter = m.emplace(key_to_insert, value0);
+  ASSERT_NE(iter, m.end());
   EXPECT_EQ(iter->first, key_to_insert);
   EXPECT_EQ(iter->second, value0);
   const char value1[] = "b";
-  auto iter2 = s.emplace(key_to_insert, value1);
-  EXPECT_NE(iter2, iter);
-  ASSERT_NE(iter2, s.end());
-  EXPECT_EQ(iter2->first, key_to_insert);
-  EXPECT_EQ(iter2->second, value1);
-  auto result = s.equal_range(key_to_insert);
+  iter = m.emplace(key_to_insert, value1);
+  ASSERT_NE(iter, m.end());
+  EXPECT_EQ(iter->first, key_to_insert);
+  EXPECT_EQ(iter->second, value1);
+  auto result = m.equal_range(key_to_insert);
   EXPECT_EQ(std::distance(result.first, result.second), 2);
 }
 
 TEST(Btree, BtreeMultimapEmplaceHint) {
   const int key_to_insert = 123456;
   const char value0[] = "a";
-  absl::btree_multimap<int, std::string> s;
-  auto iter = s.emplace(key_to_insert, value0);
-  ASSERT_NE(iter, s.end());
+  absl::btree_multimap<int, std::string> m;
+  auto iter = m.emplace(key_to_insert, value0);
+  ASSERT_NE(iter, m.end());
   EXPECT_EQ(iter->first, key_to_insert);
   EXPECT_EQ(iter->second, value0);
   const char value1[] = "b";
-  auto emplace_iter = s.emplace_hint(iter, key_to_insert, value1);
-  EXPECT_NE(emplace_iter, iter);
-  ASSERT_NE(emplace_iter, s.end());
-  EXPECT_EQ(emplace_iter->first, key_to_insert);
-  EXPECT_EQ(emplace_iter->second, value1);
+  iter = m.emplace_hint(iter, key_to_insert, value1);
+  // The new element should be before the previously inserted one.
+  EXPECT_EQ(iter, m.lower_bound(key_to_insert));
+  ASSERT_NE(iter, m.end());
+  EXPECT_EQ(iter->first, key_to_insert);
+  EXPECT_EQ(iter->second, value1);
 }
 
 TEST(Btree, ConstIteratorAccessors) {
@@ -1627,10 +1611,25 @@ TEST(Btree, StrSplitCompatible) {
   EXPECT_EQ(split_set, expected_set);
 }
 
-// We can't use EXPECT_EQ/etc. to compare absl::weak_ordering because they
-// convert literal 0 to int and absl::weak_ordering can only be compared with
-// literal 0. Defining this function allows for avoiding ClangTidy warnings.
-bool Identity(const bool b) { return b; }
+TEST(Btree, KeyComp) {
+  absl::btree_set<int> s;
+  EXPECT_TRUE(s.key_comp()(1, 2));
+  EXPECT_FALSE(s.key_comp()(2, 2));
+  EXPECT_FALSE(s.key_comp()(2, 1));
+
+  absl::btree_map<int, int> m1;
+  EXPECT_TRUE(m1.key_comp()(1, 2));
+  EXPECT_FALSE(m1.key_comp()(2, 2));
+  EXPECT_FALSE(m1.key_comp()(2, 1));
+
+  // Even though we internally adapt the comparator of `m2` to be three-way and
+  // heterogeneous, the comparator we expose through key_comp() is the original
+  // unadapted comparator.
+  absl::btree_map<std::string, int> m2;
+  EXPECT_TRUE(m2.key_comp()("a", "b"));
+  EXPECT_FALSE(m2.key_comp()("b", "b"));
+  EXPECT_FALSE(m2.key_comp()("b", "a"));
+}
 
 TEST(Btree, ValueComp) {
   absl::btree_set<int> s;
@@ -1643,13 +1642,29 @@ TEST(Btree, ValueComp) {
   EXPECT_FALSE(m1.value_comp()(std::make_pair(2, 0), std::make_pair(2, 0)));
   EXPECT_FALSE(m1.value_comp()(std::make_pair(2, 0), std::make_pair(1, 0)));
 
+  // Even though we internally adapt the comparator of `m2` to be three-way and
+  // heterogeneous, the comparator we expose through value_comp() is based on
+  // the original unadapted comparator.
   absl::btree_map<std::string, int> m2;
-  EXPECT_TRUE(Identity(
-      m2.value_comp()(std::make_pair("a", 0), std::make_pair("b", 0)) < 0));
-  EXPECT_TRUE(Identity(
-      m2.value_comp()(std::make_pair("b", 0), std::make_pair("b", 0)) == 0));
-  EXPECT_TRUE(Identity(
-      m2.value_comp()(std::make_pair("b", 0), std::make_pair("a", 0)) > 0));
+  EXPECT_TRUE(m2.value_comp()(std::make_pair("a", 0), std::make_pair("b", 0)));
+  EXPECT_FALSE(m2.value_comp()(std::make_pair("b", 0), std::make_pair("b", 0)));
+  EXPECT_FALSE(m2.value_comp()(std::make_pair("b", 0), std::make_pair("a", 0)));
+}
+
+// Test that we have the protected members from the std::map::value_compare API.
+// See https://en.cppreference.com/w/cpp/container/map/value_compare.
+TEST(Btree, MapValueCompProtected) {
+  struct key_compare {
+    bool operator()(int l, int r) const { return l < r; }
+    int id;
+  };
+  using value_compare = absl::btree_map<int, int, key_compare>::value_compare;
+  struct value_comp_child : public value_compare {
+    explicit value_comp_child(key_compare kc) : value_compare(kc) {}
+    int GetId() const { return comp.id; }
+  };
+  value_comp_child c(key_compare{10});
+  EXPECT_EQ(c.GetId(), 10);
 }
 
 TEST(Btree, DefaultConstruction) {
@@ -1823,25 +1838,80 @@ TEST(Btree, ExtractAndInsertNodeHandleSet) {
   EXPECT_EQ(res.node.value(), 3);
 }
 
-struct Deref {
-  bool operator()(const std::unique_ptr<int> &lhs,
-                  const std::unique_ptr<int> &rhs) const {
-    return *lhs < *rhs;
+template <typename Set>
+void TestExtractWithTrackingForSet() {
+  InstanceTracker tracker;
+  {
+    Set s;
+    // Add enough elements to make sure we test internal nodes too.
+    const size_t kSize = 1000;
+    while (s.size() < kSize) {
+      s.insert(MovableOnlyInstance(s.size()));
+    }
+    for (int i = 0; i < kSize; ++i) {
+      // Extract with key
+      auto nh = s.extract(MovableOnlyInstance(i));
+      EXPECT_EQ(s.size(), kSize - 1);
+      EXPECT_EQ(nh.value().value(), i);
+      // Insert with node
+      s.insert(std::move(nh));
+      EXPECT_EQ(s.size(), kSize);
+
+      // Extract with iterator
+      auto it = s.find(MovableOnlyInstance(i));
+      nh = s.extract(it);
+      EXPECT_EQ(s.size(), kSize - 1);
+      EXPECT_EQ(nh.value().value(), i);
+      // Insert with node and hint
+      s.insert(s.begin(), std::move(nh));
+      EXPECT_EQ(s.size(), kSize);
+    }
   }
-};
+  EXPECT_EQ(0, tracker.instances());
+}
+
+template <typename Map>
+void TestExtractWithTrackingForMap() {
+  InstanceTracker tracker;
+  {
+    Map m;
+    // Add enough elements to make sure we test internal nodes too.
+    const size_t kSize = 1000;
+    while (m.size() < kSize) {
+      m.insert(
+          {CopyableMovableInstance(m.size()), MovableOnlyInstance(m.size())});
+    }
+    for (int i = 0; i < kSize; ++i) {
+      // Extract with key
+      auto nh = m.extract(CopyableMovableInstance(i));
+      EXPECT_EQ(m.size(), kSize - 1);
+      EXPECT_EQ(nh.key().value(), i);
+      EXPECT_EQ(nh.mapped().value(), i);
+      // Insert with node
+      m.insert(std::move(nh));
+      EXPECT_EQ(m.size(), kSize);
+
+      // Extract with iterator
+      auto it = m.find(CopyableMovableInstance(i));
+      nh = m.extract(it);
+      EXPECT_EQ(m.size(), kSize - 1);
+      EXPECT_EQ(nh.key().value(), i);
+      EXPECT_EQ(nh.mapped().value(), i);
+      // Insert with node and hint
+      m.insert(m.begin(), std::move(nh));
+      EXPECT_EQ(m.size(), kSize);
+    }
+  }
+  EXPECT_EQ(0, tracker.instances());
+}
 
-TEST(Btree, ExtractWithUniquePtr) {
-  absl::btree_set<std::unique_ptr<int>, Deref> s;
-  s.insert(absl::make_unique<int>(1));
-  s.insert(absl::make_unique<int>(2));
-  s.insert(absl::make_unique<int>(3));
-  s.insert(absl::make_unique<int>(4));
-  s.insert(absl::make_unique<int>(5));
-  auto nh = s.extract(s.find(absl::make_unique<int>(3)));
-  EXPECT_EQ(s.size(), 4);
-  EXPECT_EQ(*nh.value(), 3);
-  s.insert(std::move(nh));
-  EXPECT_EQ(s.size(), 5);
+TEST(Btree, ExtractTracking) {
+  TestExtractWithTrackingForSet<absl::btree_set<MovableOnlyInstance>>();
+  TestExtractWithTrackingForSet<absl::btree_multiset<MovableOnlyInstance>>();
+  TestExtractWithTrackingForMap<
+      absl::btree_map<CopyableMovableInstance, MovableOnlyInstance>>();
+  TestExtractWithTrackingForMap<
+      absl::btree_multimap<CopyableMovableInstance, MovableOnlyInstance>>();
 }
 
 TEST(Btree, ExtractAndInsertNodeHandleMultiSet) {
@@ -1902,6 +1972,106 @@ TEST(Btree, ExtractAndInsertNodeHandleMultiMap) {
   EXPECT_EQ(res, ++other.begin());
 }
 
+TEST(Btree, ExtractMultiMapEquivalentKeys) {
+  // Note: using string keys means a three-way comparator.
+  absl::btree_multimap<std::string, int> map;
+  for (int i = 0; i < 100; ++i) {
+    for (int j = 0; j < 100; ++j) {
+      map.insert({absl::StrCat(i), j});
+    }
+  }
+
+  for (int i = 0; i < 100; ++i) {
+    const std::string key = absl::StrCat(i);
+    auto node_handle = map.extract(key);
+    EXPECT_EQ(node_handle.key(), key);
+    EXPECT_EQ(node_handle.mapped(), 0) << i;
+  }
+
+  for (int i = 0; i < 100; ++i) {
+    const std::string key = absl::StrCat(i);
+    auto node_handle = map.extract(key);
+    EXPECT_EQ(node_handle.key(), key);
+    EXPECT_EQ(node_handle.mapped(), 1) << i;
+  }
+}
+
+TEST(Btree, ExtractAndGetNextSet) {
+  absl::btree_set<int> src = {1, 2, 3, 4, 5};
+  auto it = src.find(3);
+  auto extracted_and_next = src.extract_and_get_next(it);
+  EXPECT_THAT(src, ElementsAre(1, 2, 4, 5));
+  EXPECT_EQ(extracted_and_next.node.value(), 3);
+  EXPECT_EQ(*extracted_and_next.next, 4);
+}
+
+TEST(Btree, ExtractAndGetNextMultiSet) {
+  absl::btree_multiset<int> src = {1, 2, 3, 4, 5};
+  auto it = src.find(3);
+  auto extracted_and_next = src.extract_and_get_next(it);
+  EXPECT_THAT(src, ElementsAre(1, 2, 4, 5));
+  EXPECT_EQ(extracted_and_next.node.value(), 3);
+  EXPECT_EQ(*extracted_and_next.next, 4);
+}
+
+TEST(Btree, ExtractAndGetNextMap) {
+  absl::btree_map<int, int> src = {{1, 2}, {3, 4}, {5, 6}};
+  auto it = src.find(3);
+  auto extracted_and_next = src.extract_and_get_next(it);
+  EXPECT_THAT(src, ElementsAre(Pair(1, 2), Pair(5, 6)));
+  EXPECT_EQ(extracted_and_next.node.key(), 3);
+  EXPECT_EQ(extracted_and_next.node.mapped(), 4);
+  EXPECT_THAT(*extracted_and_next.next, Pair(5, 6));
+}
+
+TEST(Btree, ExtractAndGetNextMultiMap) {
+  absl::btree_multimap<int, int> src = {{1, 2}, {3, 4}, {5, 6}};
+  auto it = src.find(3);
+  auto extracted_and_next = src.extract_and_get_next(it);
+  EXPECT_THAT(src, ElementsAre(Pair(1, 2), Pair(5, 6)));
+  EXPECT_EQ(extracted_and_next.node.key(), 3);
+  EXPECT_EQ(extracted_and_next.node.mapped(), 4);
+  EXPECT_THAT(*extracted_and_next.next, Pair(5, 6));
+}
+
+TEST(Btree, ExtractAndGetNextEndIter) {
+  absl::btree_set<int> src = {1, 2, 3, 4, 5};
+  auto it = src.find(5);
+  auto extracted_and_next = src.extract_and_get_next(it);
+  EXPECT_THAT(src, ElementsAre(1, 2, 3, 4));
+  EXPECT_EQ(extracted_and_next.node.value(), 5);
+  EXPECT_EQ(extracted_and_next.next, src.end());
+}
+
+TEST(Btree, ExtractDoesntCauseExtraMoves) {
+#ifdef _MSC_VER
+  // This conditional is to avoid an unreachable code warning.
+  if (_MSC_VER > 0) {
+    GTEST_SKIP() << "This test fails on MSVC.";
+  }
+#endif
+
+  using Set = absl::btree_set<MovableOnlyInstance>;
+  std::array<std::function<void(Set &)>, 3> extracters = {
+      [](Set &s) { auto node = s.extract(s.begin()); },
+      [](Set &s) { auto ret = s.extract_and_get_next(s.begin()); },
+      [](Set &s) { auto node = s.extract(MovableOnlyInstance(0)); }};
+
+  InstanceTracker tracker;
+  for (int i = 0; i < 3; ++i) {
+    Set s;
+    s.insert(MovableOnlyInstance(0));
+    tracker.ResetCopiesMovesSwaps();
+
+    extracters[i](s);
+    // We expect to see exactly 1 move: from the original slot into the
+    // extracted node.
+    EXPECT_EQ(tracker.copies(), 0) << i;
+    EXPECT_EQ(tracker.moves(), 1) << i;
+    EXPECT_EQ(tracker.swaps(), 0) << i;
+  }
+}
+
 // For multisets, insert with hint also affects correctness because we need to
 // insert immediately before the hint if possible.
 struct InsertMultiHintData {
@@ -2043,6 +2213,31 @@ TEST(Btree, MergeIntoMultiMapsWithDifferentComparators) {
                                Pair(4, 1), Pair(4, 4), Pair(5, 5)));
 }
 
+TEST(Btree, MergeIntoSetMovableOnly) {
+  absl::btree_set<MovableOnlyInstance> src;
+  src.insert(MovableOnlyInstance(1));
+  absl::btree_multiset<MovableOnlyInstance> dst1;
+  dst1.insert(MovableOnlyInstance(2));
+  absl::btree_set<MovableOnlyInstance> dst2;
+
+  // Test merge into multiset.
+  dst1.merge(src);
+
+  EXPECT_TRUE(src.empty());
+  // ElementsAre/ElementsAreArray don't work with move-only types.
+  ASSERT_THAT(dst1, SizeIs(2));
+  EXPECT_EQ(*dst1.begin(), MovableOnlyInstance(1));
+  EXPECT_EQ(*std::next(dst1.begin()), MovableOnlyInstance(2));
+
+  // Test merge into set.
+  dst2.merge(dst1);
+
+  EXPECT_TRUE(dst1.empty());
+  ASSERT_THAT(dst2, SizeIs(2));
+  EXPECT_EQ(*dst2.begin(), MovableOnlyInstance(1));
+  EXPECT_EQ(*std::next(dst2.begin()), MovableOnlyInstance(2));
+}
+
 struct KeyCompareToWeakOrdering {
   template <typename T>
   absl::weak_ordering operator()(const T &a, const T &b) const {
@@ -2074,11 +2269,11 @@ TEST(Btree, UserProvidedKeyCompareToComparators) {
 TEST(Btree, TryEmplaceBasicTest) {
   absl::btree_map<int, std::string> m;
 
-  // Should construct a std::string from the literal.
+  // Should construct a string from the literal.
   m.try_emplace(1, "one");
   EXPECT_EQ(1, m.size());
 
-  // Try other std::string constructors and const lvalue key.
+  // Try other string constructors and const lvalue key.
   const int key(42);
   m.try_emplace(key, 3, 'a');
   m.try_emplace(2, std::string("two"));
@@ -2097,7 +2292,9 @@ TEST(Btree, TryEmplaceWithHintWorks) {
   };
   using Cmp = decltype(cmp);
 
-  absl::btree_map<int, int, Cmp> m(cmp);
+  // Use a map that is opted out of key_compare being adapted so we can expect
+  // strict comparison call limits.
+  absl::btree_map<int, int, CheckedCompareOptedOutCmp<Cmp>> m(cmp);
   for (int i = 0; i < 128; ++i) {
     m.emplace(i, i);
   }
@@ -2193,59 +2390,1254 @@ TEST(Btree, TryEmplaceWithHintAndMultipleValueArgsWorks) {
   EXPECT_EQ(std::string(10, 'a'), m[1]);
 }
 
-TEST(Btree, MoveAssignmentAllocatorPropagation) {
-  InstanceTracker tracker;
+template <typename Alloc>
+using BtreeSetAlloc = absl::btree_set<int, std::less<int>, Alloc>;
+
+TEST(Btree, AllocatorPropagation) {
+  TestAllocPropagation<BtreeSetAlloc>();
+}
+
+TEST(Btree, MinimumAlignmentAllocator) {
+  absl::btree_set<int8_t, std::less<int8_t>, MinimumAlignmentAlloc<int8_t>> set;
 
-  int64_t bytes1 = 0, bytes2 = 0;
-  PropagatingCountingAlloc<MovableOnlyInstance> allocator1(&bytes1);
-  PropagatingCountingAlloc<MovableOnlyInstance> allocator2(&bytes2);
-  std::less<MovableOnlyInstance> cmp;
+  // Do some basic operations. Test that everything is fine when allocator uses
+  // minimal alignment.
+  for (int8_t i = 0; i < 100; ++i) set.insert(i);
+  set.erase(set.find(50), set.end());
+  for (int8_t i = 51; i < 101; ++i) set.insert(i);
 
-  // Test propagating allocator_type.
+  EXPECT_EQ(set.size(), 100);
+}
+
+TEST(Btree, EmptyTree) {
+  absl::btree_set<int> s;
+  EXPECT_TRUE(s.empty());
+  EXPECT_EQ(s.size(), 0);
+  EXPECT_GT(s.max_size(), 0);
+}
+
+bool IsEven(int k) { return k % 2 == 0; }
+
+TEST(Btree, EraseIf) {
+  // Test that erase_if works with all the container types and supports lambdas.
+  {
+    absl::btree_set<int> s = {1, 3, 5, 6, 100};
+    EXPECT_EQ(erase_if(s, [](int k) { return k > 3; }), 3);
+    EXPECT_THAT(s, ElementsAre(1, 3));
+  }
+  {
+    absl::btree_multiset<int> s = {1, 3, 3, 5, 6, 6, 100};
+    EXPECT_EQ(erase_if(s, [](int k) { return k <= 3; }), 3);
+    EXPECT_THAT(s, ElementsAre(5, 6, 6, 100));
+  }
+  {
+    absl::btree_map<int, int> m = {{1, 1}, {3, 3}, {6, 6}, {100, 100}};
+    EXPECT_EQ(
+        erase_if(m, [](std::pair<const int, int> kv) { return kv.first > 3; }),
+        2);
+    EXPECT_THAT(m, ElementsAre(Pair(1, 1), Pair(3, 3)));
+  }
+  {
+    absl::btree_multimap<int, int> m = {{1, 1}, {3, 3}, {3, 6},
+                                        {6, 6}, {6, 7}, {100, 6}};
+    EXPECT_EQ(
+        erase_if(m,
+                 [](std::pair<const int, int> kv) { return kv.second == 6; }),
+        3);
+    EXPECT_THAT(m, ElementsAre(Pair(1, 1), Pair(3, 3), Pair(6, 7)));
+  }
+  // Test that erasing all elements from a large set works and test support for
+  // function pointers.
+  {
+    absl::btree_set<int> s;
+    for (int i = 0; i < 1000; ++i) s.insert(2 * i);
+    EXPECT_EQ(erase_if(s, IsEven), 1000);
+    EXPECT_THAT(s, IsEmpty());
+  }
+  // Test that erase_if supports other format of function pointers.
   {
-    absl::btree_set<MovableOnlyInstance, std::less<MovableOnlyInstance>,
-                    PropagatingCountingAlloc<MovableOnlyInstance>>
-        set1(cmp, allocator1), set2(cmp, allocator2);
+    absl::btree_set<int> s = {1, 3, 5, 6, 100};
+    EXPECT_EQ(erase_if(s, &IsEven), 2);
+    EXPECT_THAT(s, ElementsAre(1, 3, 5));
+  }
+  // Test that erase_if invokes the predicate once per element.
+  {
+    absl::btree_set<int> s;
+    for (int i = 0; i < 1000; ++i) s.insert(i);
+    int pred_calls = 0;
+    EXPECT_EQ(erase_if(s,
+                       [&pred_calls](int k) {
+                         ++pred_calls;
+                         return k % 2;
+                       }),
+              500);
+    EXPECT_THAT(s, SizeIs(500));
+    EXPECT_EQ(pred_calls, 1000);
+  }
+}
+
+TEST(Btree, InsertOrAssign) {
+  absl::btree_map<int, int> m = {{1, 1}, {3, 3}};
+  using value_type = typename decltype(m)::value_type;
+
+  auto ret = m.insert_or_assign(4, 4);
+  EXPECT_EQ(*ret.first, value_type(4, 4));
+  EXPECT_TRUE(ret.second);
+  ret = m.insert_or_assign(3, 100);
+  EXPECT_EQ(*ret.first, value_type(3, 100));
+  EXPECT_FALSE(ret.second);
+
+  auto hint_ret = m.insert_or_assign(ret.first, 3, 200);
+  EXPECT_EQ(*hint_ret, value_type(3, 200));
+  hint_ret = m.insert_or_assign(m.find(1), 0, 1);
+  EXPECT_EQ(*hint_ret, value_type(0, 1));
+  // Test with bad hint.
+  hint_ret = m.insert_or_assign(m.end(), -1, 1);
+  EXPECT_EQ(*hint_ret, value_type(-1, 1));
+
+  EXPECT_THAT(m, ElementsAre(Pair(-1, 1), Pair(0, 1), Pair(1, 1), Pair(3, 200),
+                             Pair(4, 4)));
+}
+
+TEST(Btree, InsertOrAssignMovableOnly) {
+  absl::btree_map<int, MovableOnlyInstance> m;
+  using value_type = typename decltype(m)::value_type;
+
+  auto ret = m.insert_or_assign(4, MovableOnlyInstance(4));
+  EXPECT_EQ(*ret.first, value_type(4, MovableOnlyInstance(4)));
+  EXPECT_TRUE(ret.second);
+  ret = m.insert_or_assign(4, MovableOnlyInstance(100));
+  EXPECT_EQ(*ret.first, value_type(4, MovableOnlyInstance(100)));
+  EXPECT_FALSE(ret.second);
+
+  auto hint_ret = m.insert_or_assign(ret.first, 3, MovableOnlyInstance(200));
+  EXPECT_EQ(*hint_ret, value_type(3, MovableOnlyInstance(200)));
+
+  EXPECT_EQ(m.size(), 2);
+}
+
+TEST(Btree, BitfieldArgument) {
+  union {
+    int n : 1;
+  };
+  n = 0;
+  absl::btree_map<int, int> m;
+  m.erase(n);
+  m.count(n);
+  m.find(n);
+  m.contains(n);
+  m.equal_range(n);
+  m.insert_or_assign(n, n);
+  m.insert_or_assign(m.end(), n, n);
+  m.try_emplace(n);
+  m.try_emplace(m.end(), n);
+  m.at(n);
+  m[n];
+}
 
-    for (int i = 0; i < 100; ++i) set1.insert(MovableOnlyInstance(i));
+TEST(Btree, SetRangeConstructorAndInsertSupportExplicitConversionComparable) {
+  const absl::string_view names[] = {"n1", "n2"};
 
-    tracker.ResetCopiesMovesSwaps();
-    set2 = std::move(set1);
-    EXPECT_EQ(tracker.moves(), 0);
+  absl::btree_set<std::string> name_set1{std::begin(names), std::end(names)};
+  EXPECT_THAT(name_set1, ElementsAreArray(names));
+
+  absl::btree_set<std::string> name_set2;
+  name_set2.insert(std::begin(names), std::end(names));
+  EXPECT_THAT(name_set2, ElementsAreArray(names));
+}
+
+// A type that is explicitly convertible from int and counts constructor calls.
+struct ConstructorCounted {
+  explicit ConstructorCounted(int i) : i(i) { ++constructor_calls; }
+  bool operator==(int other) const { return i == other; }
+
+  int i;
+  static int constructor_calls;
+};
+int ConstructorCounted::constructor_calls = 0;
+
+struct ConstructorCountedCompare {
+  bool operator()(int a, const ConstructorCounted &b) const { return a < b.i; }
+  bool operator()(const ConstructorCounted &a, int b) const { return a.i < b; }
+  bool operator()(const ConstructorCounted &a,
+                  const ConstructorCounted &b) const {
+    return a.i < b.i;
   }
-  // Test non-propagating allocator_type with equal allocators.
+  using is_transparent = void;
+};
+
+TEST(Btree,
+     SetRangeConstructorAndInsertExplicitConvComparableLimitConstruction) {
+  const int i[] = {0, 1, 1};
+  ConstructorCounted::constructor_calls = 0;
+
+  absl::btree_set<ConstructorCounted, ConstructorCountedCompare> set{
+      std::begin(i), std::end(i)};
+  EXPECT_THAT(set, ElementsAre(0, 1));
+  EXPECT_EQ(ConstructorCounted::constructor_calls, 2);
+
+  set.insert(std::begin(i), std::end(i));
+  EXPECT_THAT(set, ElementsAre(0, 1));
+  EXPECT_EQ(ConstructorCounted::constructor_calls, 2);
+}
+
+TEST(Btree,
+     SetRangeConstructorAndInsertSupportExplicitConversionNonComparable) {
+  const int i[] = {0, 1};
+
+  absl::btree_set<std::vector<void *>> s1{std::begin(i), std::end(i)};
+  EXPECT_THAT(s1, ElementsAre(IsEmpty(), ElementsAre(IsNull())));
+
+  absl::btree_set<std::vector<void *>> s2;
+  s2.insert(std::begin(i), std::end(i));
+  EXPECT_THAT(s2, ElementsAre(IsEmpty(), ElementsAre(IsNull())));
+}
+
+// libstdc++ included with GCC 4.9 has a bug in the std::pair constructors that
+// prevents explicit conversions between pair types.
+// We only run this test for the libstdc++ from GCC 7 or newer because we can't
+// reliably check the libstdc++ version prior to that release.
+#if !defined(__GLIBCXX__) || \
+    (defined(_GLIBCXX_RELEASE) && _GLIBCXX_RELEASE >= 7)
+TEST(Btree, MapRangeConstructorAndInsertSupportExplicitConversionComparable) {
+  const std::pair<absl::string_view, int> names[] = {{"n1", 1}, {"n2", 2}};
+
+  absl::btree_map<std::string, int> name_map1{std::begin(names),
+                                              std::end(names)};
+  EXPECT_THAT(name_map1, ElementsAre(Pair("n1", 1), Pair("n2", 2)));
+
+  absl::btree_map<std::string, int> name_map2;
+  name_map2.insert(std::begin(names), std::end(names));
+  EXPECT_THAT(name_map2, ElementsAre(Pair("n1", 1), Pair("n2", 2)));
+}
+
+TEST(Btree,
+     MapRangeConstructorAndInsertExplicitConvComparableLimitConstruction) {
+  const std::pair<int, int> i[] = {{0, 1}, {1, 2}, {1, 3}};
+  ConstructorCounted::constructor_calls = 0;
+
+  absl::btree_map<ConstructorCounted, int, ConstructorCountedCompare> map{
+      std::begin(i), std::end(i)};
+  EXPECT_THAT(map, ElementsAre(Pair(0, 1), Pair(1, 2)));
+  EXPECT_EQ(ConstructorCounted::constructor_calls, 2);
+
+  map.insert(std::begin(i), std::end(i));
+  EXPECT_THAT(map, ElementsAre(Pair(0, 1), Pair(1, 2)));
+  EXPECT_EQ(ConstructorCounted::constructor_calls, 2);
+}
+
+TEST(Btree,
+     MapRangeConstructorAndInsertSupportExplicitConversionNonComparable) {
+  const std::pair<int, int> i[] = {{0, 1}, {1, 2}};
+
+  absl::btree_map<std::vector<void *>, int> m1{std::begin(i), std::end(i)};
+  EXPECT_THAT(m1,
+              ElementsAre(Pair(IsEmpty(), 1), Pair(ElementsAre(IsNull()), 2)));
+
+  absl::btree_map<std::vector<void *>, int> m2;
+  m2.insert(std::begin(i), std::end(i));
+  EXPECT_THAT(m2,
+              ElementsAre(Pair(IsEmpty(), 1), Pair(ElementsAre(IsNull()), 2)));
+}
+
+TEST(Btree, HeterogeneousTryEmplace) {
+  absl::btree_map<std::string, int> m;
+  std::string s = "key";
+  absl::string_view sv = s;
+  m.try_emplace(sv, 1);
+  EXPECT_EQ(m[s], 1);
+
+  m.try_emplace(m.end(), sv, 2);
+  EXPECT_EQ(m[s], 1);
+}
+
+TEST(Btree, HeterogeneousOperatorMapped) {
+  absl::btree_map<std::string, int> m;
+  std::string s = "key";
+  absl::string_view sv = s;
+  m[sv] = 1;
+  EXPECT_EQ(m[s], 1);
+
+  m[sv] = 2;
+  EXPECT_EQ(m[s], 2);
+}
+
+TEST(Btree, HeterogeneousInsertOrAssign) {
+  absl::btree_map<std::string, int> m;
+  std::string s = "key";
+  absl::string_view sv = s;
+  m.insert_or_assign(sv, 1);
+  EXPECT_EQ(m[s], 1);
+
+  m.insert_or_assign(m.end(), sv, 2);
+  EXPECT_EQ(m[s], 2);
+}
+#endif
+
+TEST(Btree, NodeHandleMutableKeyAccess) {
   {
-    absl::btree_set<MovableOnlyInstance, std::less<MovableOnlyInstance>,
-                    CountingAllocator<MovableOnlyInstance>>
-        set1(cmp, allocator1), set2(cmp, allocator1);
+    absl::btree_map<std::string, std::string> map;
 
-    for (int i = 0; i < 100; ++i) set1.insert(MovableOnlyInstance(i));
+    map["key1"] = "mapped";
 
-    tracker.ResetCopiesMovesSwaps();
-    set2 = std::move(set1);
-    EXPECT_EQ(tracker.moves(), 0);
+    auto nh = map.extract(map.begin());
+    nh.key().resize(3);
+    map.insert(std::move(nh));
+
+    EXPECT_THAT(map, ElementsAre(Pair("key", "mapped")));
   }
-  // Test non-propagating allocator_type with different allocators.
+  // Also for multimap.
   {
-    absl::btree_set<MovableOnlyInstance, std::less<MovableOnlyInstance>,
-                    CountingAllocator<MovableOnlyInstance>>
-        set1(cmp, allocator1), set2(cmp, allocator2);
+    absl::btree_multimap<std::string, std::string> map;
 
-    for (int i = 0; i < 100; ++i) set1.insert(MovableOnlyInstance(i));
+    map.emplace("key1", "mapped");
 
-    tracker.ResetCopiesMovesSwaps();
-    set2 = std::move(set1);
-    EXPECT_GE(tracker.moves(), 100);
+    auto nh = map.extract(map.begin());
+    nh.key().resize(3);
+    map.insert(std::move(nh));
+
+    EXPECT_THAT(map, ElementsAre(Pair("key", "mapped")));
   }
 }
 
-TEST(Btree, EmptyTree) {
-  absl::btree_set<int> s;
-  EXPECT_TRUE(s.empty());
-  EXPECT_EQ(s.size(), 0);
-  EXPECT_GT(s.max_size(), 0);
+struct MultiKey {
+  int i1;
+  int i2;
+};
+
+bool operator==(const MultiKey a, const MultiKey b) {
+  return a.i1 == b.i1 && a.i2 == b.i2;
+}
+
+// A heterogeneous comparator that has different equivalence classes for
+// different lookup types.
+struct MultiKeyComp {
+  using is_transparent = void;
+  bool operator()(const MultiKey a, const MultiKey b) const {
+    if (a.i1 != b.i1) return a.i1 < b.i1;
+    return a.i2 < b.i2;
+  }
+  bool operator()(const int a, const MultiKey b) const { return a < b.i1; }
+  bool operator()(const MultiKey a, const int b) const { return a.i1 < b; }
+};
+
+// A heterogeneous, three-way comparator that has different equivalence classes
+// for different lookup types.
+struct MultiKeyThreeWayComp {
+  using is_transparent = void;
+  absl::weak_ordering operator()(const MultiKey a, const MultiKey b) const {
+    if (a.i1 < b.i1) return absl::weak_ordering::less;
+    if (a.i1 > b.i1) return absl::weak_ordering::greater;
+    if (a.i2 < b.i2) return absl::weak_ordering::less;
+    if (a.i2 > b.i2) return absl::weak_ordering::greater;
+    return absl::weak_ordering::equivalent;
+  }
+  absl::weak_ordering operator()(const int a, const MultiKey b) const {
+    if (a < b.i1) return absl::weak_ordering::less;
+    if (a > b.i1) return absl::weak_ordering::greater;
+    return absl::weak_ordering::equivalent;
+  }
+  absl::weak_ordering operator()(const MultiKey a, const int b) const {
+    if (a.i1 < b) return absl::weak_ordering::less;
+    if (a.i1 > b) return absl::weak_ordering::greater;
+    return absl::weak_ordering::equivalent;
+  }
+};
+
+template <typename Compare>
+class BtreeMultiKeyTest : public ::testing::Test {};
+using MultiKeyComps = ::testing::Types<MultiKeyComp, MultiKeyThreeWayComp>;
+TYPED_TEST_SUITE(BtreeMultiKeyTest, MultiKeyComps);
+
+TYPED_TEST(BtreeMultiKeyTest, EqualRange) {
+  absl::btree_set<MultiKey, TypeParam> set;
+  for (int i = 0; i < 100; ++i) {
+    for (int j = 0; j < 100; ++j) {
+      set.insert({i, j});
+    }
+  }
+
+  for (int i = 0; i < 100; ++i) {
+    auto equal_range = set.equal_range(i);
+    EXPECT_EQ(equal_range.first->i1, i);
+    EXPECT_EQ(equal_range.first->i2, 0) << i;
+    EXPECT_EQ(std::distance(equal_range.first, equal_range.second), 100) << i;
+  }
+}
+
+TYPED_TEST(BtreeMultiKeyTest, Extract) {
+  absl::btree_set<MultiKey, TypeParam> set;
+  for (int i = 0; i < 100; ++i) {
+    for (int j = 0; j < 100; ++j) {
+      set.insert({i, j});
+    }
+  }
+
+  for (int i = 0; i < 100; ++i) {
+    auto node_handle = set.extract(i);
+    EXPECT_EQ(node_handle.value().i1, i);
+    EXPECT_EQ(node_handle.value().i2, 0) << i;
+  }
+
+  for (int i = 0; i < 100; ++i) {
+    auto node_handle = set.extract(i);
+    EXPECT_EQ(node_handle.value().i1, i);
+    EXPECT_EQ(node_handle.value().i2, 1) << i;
+  }
+}
+
+TYPED_TEST(BtreeMultiKeyTest, Erase) {
+  absl::btree_set<MultiKey, TypeParam> set = {
+      {1, 1}, {2, 1}, {2, 2}, {3, 1}};
+  EXPECT_EQ(set.erase(2), 2);
+  EXPECT_THAT(set, ElementsAre(MultiKey{1, 1}, MultiKey{3, 1}));
+}
+
+TYPED_TEST(BtreeMultiKeyTest, Count) {
+  const absl::btree_set<MultiKey, TypeParam> set = {
+      {1, 1}, {2, 1}, {2, 2}, {3, 1}};
+  EXPECT_EQ(set.count(2), 2);
+}
+
+TEST(Btree, SetIteratorsAreConst) {
+  using Set = absl::btree_set<int>;
+  EXPECT_TRUE((std::is_same_v<typename Set::iterator::reference, const int&>));
+  EXPECT_TRUE((std::is_same_v<typename Set::iterator::pointer, const int*>));
+
+  using MSet = absl::btree_multiset<int>;
+  EXPECT_TRUE((std::is_same_v<typename MSet::iterator::reference, const int&>));
+  EXPECT_TRUE((std::is_same_v<typename MSet::iterator::pointer, const int*>));
+}
+
+TEST(Btree, AllocConstructor) {
+  using Alloc = CountingAllocator<int>;
+  using Set = absl::btree_set<int, std::less<int>, Alloc>;
+  int64_t bytes_used = 0;
+  Alloc alloc(&bytes_used);
+  Set set(alloc);
+
+  set.insert({1, 2, 3});
+
+  EXPECT_THAT(set, ElementsAre(1, 2, 3));
+  EXPECT_GT(bytes_used, set.size() * sizeof(int));
+}
+
+TEST(Btree, AllocInitializerListConstructor) {
+  using Alloc = CountingAllocator<int>;
+  using Set = absl::btree_set<int, std::less<int>, Alloc>;
+  int64_t bytes_used = 0;
+  Alloc alloc(&bytes_used);
+  Set set({1, 2, 3}, alloc);
+
+  EXPECT_THAT(set, ElementsAre(1, 2, 3));
+  EXPECT_GT(bytes_used, set.size() * sizeof(int));
+}
+
+TEST(Btree, AllocRangeConstructor) {
+  using Alloc = CountingAllocator<int>;
+  using Set = absl::btree_set<int, std::less<int>, Alloc>;
+  int64_t bytes_used = 0;
+  Alloc alloc(&bytes_used);
+  std::vector<int> v = {1, 2, 3};
+  Set set(v.begin(), v.end(), alloc);
+
+  EXPECT_THAT(set, ElementsAre(1, 2, 3));
+  EXPECT_GT(bytes_used, set.size() * sizeof(int));
+}
+
+TEST(Btree, AllocCopyConstructor) {
+  using Alloc = CountingAllocator<int>;
+  using Set = absl::btree_set<int, std::less<int>, Alloc>;
+  int64_t bytes_used1 = 0;
+  Alloc alloc1(&bytes_used1);
+  Set set1(alloc1);
+
+  set1.insert({1, 2, 3});
+
+  int64_t bytes_used2 = 0;
+  Alloc alloc2(&bytes_used2);
+  Set set2(set1, alloc2);
+
+  EXPECT_THAT(set1, ElementsAre(1, 2, 3));
+  EXPECT_THAT(set2, ElementsAre(1, 2, 3));
+  EXPECT_GT(bytes_used1, set1.size() * sizeof(int));
+  EXPECT_EQ(bytes_used1, bytes_used2);
+}
+
+TEST(Btree, AllocMoveConstructor_SameAlloc) {
+  using Alloc = CountingAllocator<int>;
+  using Set = absl::btree_set<int, std::less<int>, Alloc>;
+  int64_t bytes_used = 0;
+  Alloc alloc(&bytes_used);
+  Set set1(alloc);
+
+  set1.insert({1, 2, 3});
+
+  const int64_t original_bytes_used = bytes_used;
+  EXPECT_GT(original_bytes_used, set1.size() * sizeof(int));
+
+  Set set2(std::move(set1), alloc);
+
+  EXPECT_THAT(set2, ElementsAre(1, 2, 3));
+  EXPECT_EQ(bytes_used, original_bytes_used);
+}
+
+TEST(Btree, AllocMoveConstructor_DifferentAlloc) {
+  using Alloc = CountingAllocator<int>;
+  using Set = absl::btree_set<int, std::less<int>, Alloc>;
+  int64_t bytes_used1 = 0;
+  Alloc alloc1(&bytes_used1);
+  Set set1(alloc1);
+
+  set1.insert({1, 2, 3});
+
+  const int64_t original_bytes_used = bytes_used1;
+  EXPECT_GT(original_bytes_used, set1.size() * sizeof(int));
+
+  int64_t bytes_used2 = 0;
+  Alloc alloc2(&bytes_used2);
+  Set set2(std::move(set1), alloc2);
+
+  EXPECT_THAT(set2, ElementsAre(1, 2, 3));
+  // We didn't free these bytes allocated by `set1` yet.
+  EXPECT_EQ(bytes_used1, original_bytes_used);
+  EXPECT_EQ(bytes_used2, original_bytes_used);
+}
+
+bool IntCmp(const int a, const int b) { return a < b; }
+
+TEST(Btree, SupportsFunctionPtrComparator) {
+  absl::btree_set<int, decltype(IntCmp) *> set(IntCmp);
+  set.insert({1, 2, 3});
+  EXPECT_THAT(set, ElementsAre(1, 2, 3));
+  EXPECT_TRUE(set.key_comp()(1, 2));
+  EXPECT_TRUE(set.value_comp()(1, 2));
+
+  absl::btree_map<int, int, decltype(IntCmp) *> map(&IntCmp);
+  map[1] = 1;
+  EXPECT_THAT(map, ElementsAre(Pair(1, 1)));
+  EXPECT_TRUE(map.key_comp()(1, 2));
+  EXPECT_TRUE(map.value_comp()(std::make_pair(1, 1), std::make_pair(2, 2)));
+}
+
+template <typename Compare>
+struct TransparentPassThroughComp {
+  using is_transparent = void;
+
+  // This will fail compilation if we attempt a comparison that Compare does not
+  // support, and the failure will happen inside the function implementation so
+  // it can't be avoided by using SFINAE on this comparator.
+  template <typename T, typename U>
+  bool operator()(const T &lhs, const U &rhs) const {
+    return Compare()(lhs, rhs);
+  }
+};
+
+TEST(Btree,
+     SupportsTransparentComparatorThatDoesNotImplementAllVisibleOperators) {
+  absl::btree_set<MultiKey, TransparentPassThroughComp<MultiKeyComp>> set;
+  set.insert(MultiKey{1, 2});
+  EXPECT_TRUE(set.contains(1));
+}
+
+TEST(Btree, ConstructImplicitlyWithUnadaptedComparator) {
+  absl::btree_set<MultiKey, MultiKeyComp> set = {{}, MultiKeyComp{}};
+  EXPECT_TRUE(set.empty());
+}
+
+TEST(Btree, InvalidComparatorsCaught) {
+  if (!IsAssertEnabled()) GTEST_SKIP() << "Assertions not enabled.";
+
+  {
+    struct ZeroAlwaysLessCmp {
+      bool operator()(int lhs, int rhs) const {
+        if (lhs == 0) return true;
+        return lhs < rhs;
+      }
+    };
+    absl::btree_set<int, ZeroAlwaysLessCmp> set;
+    EXPECT_DEATH(set.insert({0, 1, 2}), "is_self_equivalent");
+  }
+  {
+    struct ThreeWayAlwaysLessCmp {
+      absl::weak_ordering operator()(int, int) const {
+        return absl::weak_ordering::less;
+      }
+    };
+    absl::btree_set<int, ThreeWayAlwaysLessCmp> set;
+    EXPECT_DEATH(set.insert({0, 1, 2}), "is_self_equivalent");
+  }
+  {
+    struct SumGreaterZeroCmp {
+      bool operator()(int lhs, int rhs) const {
+        // First, do equivalence correctly - so we can test later condition.
+        if (lhs == rhs) return false;
+        return lhs + rhs > 0;
+      }
+    };
+    absl::btree_set<int, SumGreaterZeroCmp> set;
+    // Note: '!' only needs to be escaped when it's the first character.
+    EXPECT_DEATH(set.insert({0, 1, 2}),
+                 R"regex(\!lhs_comp_rhs \|\| !comp\(\)\(rhs, lhs\))regex");
+  }
+  {
+    struct ThreeWaySumGreaterZeroCmp {
+      absl::weak_ordering operator()(int lhs, int rhs) const {
+        // First, do equivalence correctly - so we can test later condition.
+        if (lhs == rhs) return absl::weak_ordering::equivalent;
+
+        if (lhs + rhs > 0) return absl::weak_ordering::less;
+        if (lhs + rhs == 0) return absl::weak_ordering::equivalent;
+        return absl::weak_ordering::greater;
+      }
+    };
+    absl::btree_set<int, ThreeWaySumGreaterZeroCmp> set;
+    EXPECT_DEATH(set.insert({0, 1, 2}), "lhs_comp_rhs < 0 -> rhs_comp_lhs > 0");
+  }
+  // Verify that we detect cases of comparators that violate transitivity.
+  // When the comparators below check for the presence of an optional field,
+  // they violate transitivity because instances that have the optional field
+  // compare differently with each other from how they compare with instances
+  // that don't have the optional field.
+  struct ClockTime {
+    std::optional<int> hour;
+    int minute;
+  };
+  // `comp(a,b) && comp(b,c) && !comp(a,c)` violates transitivity.
+  ClockTime a = {std::nullopt, 1};
+  ClockTime b = {2, 5};
+  ClockTime c = {6, 0};
+  {
+    struct NonTransitiveTimeCmp {
+      bool operator()(ClockTime lhs, ClockTime rhs) const {
+        if (lhs.hour.has_value() && rhs.hour.has_value() &&
+            *lhs.hour != *rhs.hour) {
+          return *lhs.hour < *rhs.hour;
+        }
+        return lhs.minute < rhs.minute;
+      }
+    };
+    NonTransitiveTimeCmp cmp;
+    ASSERT_TRUE(cmp(a, b) && cmp(b, c) && !cmp(a, c));
+    absl::btree_set<ClockTime, NonTransitiveTimeCmp> set;
+    EXPECT_DEATH(set.insert({a, b, c}), "is_ordered_correctly");
+    absl::btree_multiset<ClockTime, NonTransitiveTimeCmp> mset;
+    EXPECT_DEATH(mset.insert({a, a, b, b, c, c}), "is_ordered_correctly");
+  }
+  {
+    struct ThreeWayNonTransitiveTimeCmp {
+      absl::weak_ordering operator()(ClockTime lhs, ClockTime rhs) const {
+        if (lhs.hour.has_value() && rhs.hour.has_value() &&
+            *lhs.hour != *rhs.hour) {
+          return *lhs.hour < *rhs.hour ? absl::weak_ordering::less
+                                       : absl::weak_ordering::greater;
+        }
+        return lhs.minute < rhs.minute    ? absl::weak_ordering::less
+               : lhs.minute == rhs.minute ? absl::weak_ordering::equivalent
+                                          : absl::weak_ordering::greater;
+      }
+    };
+    ThreeWayNonTransitiveTimeCmp cmp;
+    ASSERT_TRUE(cmp(a, b) < 0 && cmp(b, c) < 0 && cmp(a, c) > 0);
+    absl::btree_set<ClockTime, ThreeWayNonTransitiveTimeCmp> set;
+    EXPECT_DEATH(set.insert({a, b, c}), "is_ordered_correctly");
+    absl::btree_multiset<ClockTime, ThreeWayNonTransitiveTimeCmp> mset;
+    EXPECT_DEATH(mset.insert({a, a, b, b, c, c}), "is_ordered_correctly");
+  }
+}
+
+TEST(Btree, MutatedKeysCaught) {
+  if (!IsAssertEnabled()) GTEST_SKIP() << "Assertions not enabled.";
+
+  struct IntPtrCmp {
+    bool operator()(int *lhs, int *rhs) const { return *lhs < *rhs; }
+  };
+  {
+    absl::btree_set<int *, IntPtrCmp> set;
+    int arr[] = {0, 1, 2};
+    set.insert({&arr[0], &arr[1], &arr[2]});
+    arr[0] = 100;
+    EXPECT_DEATH(set.insert(&arr[0]), "is_ordered_correctly");
+  }
+  {
+    absl::btree_multiset<int *, IntPtrCmp> set;
+    int arr[] = {0, 1, 2};
+    set.insert({&arr[0], &arr[0], &arr[1], &arr[1], &arr[2], &arr[2]});
+    arr[0] = 100;
+    EXPECT_DEATH(set.insert(&arr[0]), "is_ordered_correctly");
+  }
+}
+
+#ifndef _MSC_VER
+// This test crashes on MSVC.
+TEST(Btree, InvalidIteratorUse) {
+  if (!BtreeGenerationsEnabled())
+    GTEST_SKIP() << "Generation validation for iterators is disabled.";
+
+  // Invalid memory use can trigger use-after-free in ASan, HWASAN or
+  // invalidated iterator assertions.
+  constexpr const char *kInvalidMemoryDeathMessage =
+      "use-after-free|invalidated iterator";
+
+  {
+    absl::btree_set<int> set;
+    for (int i = 0; i < 10; ++i) set.insert(i);
+    auto it = set.begin();
+    set.erase(it++);
+    EXPECT_DEATH(set.erase(it++), kInvalidMemoryDeathMessage);
+  }
+  {
+    absl::btree_set<int> set;
+    for (int i = 0; i < 10; ++i) set.insert(i);
+    auto it = set.insert(20).first;
+    set.insert(30);
+    EXPECT_DEATH(*it, kInvalidMemoryDeathMessage);
+  }
+  {
+    absl::btree_set<int> set;
+    for (int i = 0; i < 10000; ++i) set.insert(i);
+    auto it = set.find(5000);
+    ASSERT_NE(it, set.end());
+    set.erase(1);
+    EXPECT_DEATH(*it, kInvalidMemoryDeathMessage);
+  }
+  {
+    absl::btree_set<int> set;
+    for (int i = 0; i < 10; ++i) set.insert(i);
+    auto it = set.insert(20).first;
+    set.insert(30);
+    EXPECT_DEATH(void(it == set.begin()), kInvalidMemoryDeathMessage);
+    EXPECT_DEATH(void(set.begin() == it), kInvalidMemoryDeathMessage);
+  }
+}
+#endif
+
+class OnlyConstructibleByAllocator {
+  explicit OnlyConstructibleByAllocator(int i) : i_(i) {}
+
+ public:
+  OnlyConstructibleByAllocator(const OnlyConstructibleByAllocator &other)
+      : i_(other.i_) {}
+  OnlyConstructibleByAllocator &operator=(
+      const OnlyConstructibleByAllocator &other) {
+    i_ = other.i_;
+    return *this;
+  }
+  int Get() const { return i_; }
+  bool operator==(int i) const { return i_ == i; }
+
+ private:
+  template <typename T>
+  friend class OnlyConstructibleAllocator;
+
+  int i_;
+};
+
+template <typename T = OnlyConstructibleByAllocator>
+class OnlyConstructibleAllocator : public std::allocator<T> {
+ public:
+  OnlyConstructibleAllocator() = default;
+  template <class U>
+  explicit OnlyConstructibleAllocator(const OnlyConstructibleAllocator<U> &) {}
+
+  void construct(OnlyConstructibleByAllocator *p, int i) {
+    new (p) OnlyConstructibleByAllocator(i);
+  }
+  template <typename Pair>
+  void construct(Pair *p, const int i) {
+    OnlyConstructibleByAllocator only(i);
+    new (p) Pair(std::move(only), i);
+  }
+
+  template <class U>
+  struct rebind {
+    using other = OnlyConstructibleAllocator<U>;
+  };
+};
+
+struct OnlyConstructibleByAllocatorComp {
+  using is_transparent = void;
+  bool operator()(OnlyConstructibleByAllocator a,
+                  OnlyConstructibleByAllocator b) const {
+    return a.Get() < b.Get();
+  }
+  bool operator()(int a, OnlyConstructibleByAllocator b) const {
+    return a < b.Get();
+  }
+  bool operator()(OnlyConstructibleByAllocator a, int b) const {
+    return a.Get() < b;
+  }
+};
+
+TEST(Btree, OnlyConstructibleByAllocatorType) {
+  const std::array<int, 2> arr = {3, 4};
+  {
+    absl::btree_set<OnlyConstructibleByAllocator,
+                    OnlyConstructibleByAllocatorComp,
+                    OnlyConstructibleAllocator<>>
+        set;
+    set.emplace(1);
+    set.emplace_hint(set.end(), 2);
+    set.insert(arr.begin(), arr.end());
+    EXPECT_THAT(set, ElementsAre(1, 2, 3, 4));
+  }
+  {
+    absl::btree_multiset<OnlyConstructibleByAllocator,
+                         OnlyConstructibleByAllocatorComp,
+                         OnlyConstructibleAllocator<>>
+        set;
+    set.emplace(1);
+    set.emplace_hint(set.end(), 2);
+    // TODO(ezb): fix insert_multi to allow this to compile.
+    // set.insert(arr.begin(), arr.end());
+    EXPECT_THAT(set, ElementsAre(1, 2));
+  }
+  {
+    absl::btree_map<OnlyConstructibleByAllocator, int,
+                    OnlyConstructibleByAllocatorComp,
+                    OnlyConstructibleAllocator<>>
+        map;
+    map.emplace(1);
+    map.emplace_hint(map.end(), 2);
+    map.insert(arr.begin(), arr.end());
+    EXPECT_THAT(map,
+                ElementsAre(Pair(1, 1), Pair(2, 2), Pair(3, 3), Pair(4, 4)));
+  }
+  {
+    absl::btree_multimap<OnlyConstructibleByAllocator, int,
+                         OnlyConstructibleByAllocatorComp,
+                         OnlyConstructibleAllocator<>>
+        map;
+    map.emplace(1);
+    map.emplace_hint(map.end(), 2);
+    // TODO(ezb): fix insert_multi to allow this to compile.
+    // map.insert(arr.begin(), arr.end());
+    EXPECT_THAT(map, ElementsAre(Pair(1, 1), Pair(2, 2)));
+  }
+}
+
+class NotAssignable {
+ public:
+  explicit NotAssignable(int i) : i_(i) {}
+  NotAssignable(const NotAssignable &other) : i_(other.i_) {}
+  NotAssignable &operator=(NotAssignable &&other) = delete;
+  int Get() const { return i_; }
+  bool operator==(int i) const { return i_ == i; }
+  friend bool operator<(NotAssignable a, NotAssignable b) {
+    return a.i_ < b.i_;
+  }
+
+ private:
+  int i_;
+};
+
+TEST(Btree, NotAssignableType) {
+  {
+    absl::btree_set<NotAssignable> set;
+    set.emplace(1);
+    set.emplace_hint(set.end(), 2);
+    set.insert(NotAssignable(3));
+    set.insert(set.end(), NotAssignable(4));
+    EXPECT_THAT(set, ElementsAre(1, 2, 3, 4));
+    set.erase(set.begin());
+    EXPECT_THAT(set, ElementsAre(2, 3, 4));
+  }
+  {
+    absl::btree_multiset<NotAssignable> set;
+    set.emplace(1);
+    set.emplace_hint(set.end(), 2);
+    set.insert(NotAssignable(2));
+    set.insert(set.end(), NotAssignable(3));
+    EXPECT_THAT(set, ElementsAre(1, 2, 2, 3));
+    set.erase(set.begin());
+    EXPECT_THAT(set, ElementsAre(2, 2, 3));
+  }
+  {
+    absl::btree_map<NotAssignable, int> map;
+    map.emplace(NotAssignable(1), 1);
+    map.emplace_hint(map.end(), NotAssignable(2), 2);
+    map.insert({NotAssignable(3), 3});
+    map.insert(map.end(), {NotAssignable(4), 4});
+    EXPECT_THAT(map,
+                ElementsAre(Pair(1, 1), Pair(2, 2), Pair(3, 3), Pair(4, 4)));
+    map.erase(map.begin());
+    EXPECT_THAT(map, ElementsAre(Pair(2, 2), Pair(3, 3), Pair(4, 4)));
+  }
+  {
+    absl::btree_multimap<NotAssignable, int> map;
+    map.emplace(NotAssignable(1), 1);
+    map.emplace_hint(map.end(), NotAssignable(2), 2);
+    map.insert({NotAssignable(2), 3});
+    map.insert(map.end(), {NotAssignable(3), 3});
+    EXPECT_THAT(map,
+                ElementsAre(Pair(1, 1), Pair(2, 2), Pair(2, 3), Pair(3, 3)));
+    map.erase(map.begin());
+    EXPECT_THAT(map, ElementsAre(Pair(2, 2), Pair(2, 3), Pair(3, 3)));
+  }
+}
+
+struct ArenaLike {
+  void* recycled = nullptr;
+  size_t recycled_size = 0;
+};
+
+// A very simple implementation of arena allocation.
+template <typename T>
+class ArenaLikeAllocator : public std::allocator<T> {
+ public:
+  // Standard library containers require the ability to allocate objects of
+  // different types which they can do so via rebind.other.
+  template <typename U>
+  struct rebind {
+    using other = ArenaLikeAllocator<U>;
+  };
+
+  explicit ArenaLikeAllocator(ArenaLike* arena) noexcept : arena_(arena) {}
+
+  ~ArenaLikeAllocator() {
+    if (arena_->recycled != nullptr) {
+      delete [] static_cast<T*>(arena_->recycled);
+      arena_->recycled = nullptr;
+    }
+  }
+
+  template<typename U>
+  explicit ArenaLikeAllocator(const ArenaLikeAllocator<U>& other) noexcept
+      : arena_(other.arena_) {}
+
+  T* allocate(size_t num_objects, const void* = nullptr) {
+    size_t size = num_objects * sizeof(T);
+    if (arena_->recycled != nullptr && arena_->recycled_size == size) {
+      T* result = static_cast<T*>(arena_->recycled);
+      arena_->recycled = nullptr;
+      return result;
+    }
+    return new T[num_objects];
+  }
+
+  void deallocate(T* p, size_t num_objects) {
+    size_t size = num_objects * sizeof(T);
+
+    // Simulate writing to the freed memory as an actual arena allocator might
+    // do. This triggers an error report if the memory is poisoned.
+    memset(p, 0xde, size);
+
+    if (arena_->recycled == nullptr) {
+      arena_->recycled = p;
+      arena_->recycled_size = size;
+    } else {
+      delete [] p;
+    }
+  }
+
+  ArenaLike* arena_;
+};
+
+// This test verifies that an arena allocator that reuses memory will not be
+// asked to free poisoned BTree memory.
+TEST(Btree, ReusePoisonMemory) {
+  using Alloc = ArenaLikeAllocator<int64_t>;
+  using Set = absl::btree_set<int64_t, std::less<int64_t>, Alloc>;
+  ArenaLike arena;
+  Alloc alloc(&arena);
+  Set set(alloc);
+
+  set.insert(0);
+  set.erase(0);
+  set.insert(0);
+}
+
+TEST(Btree, IteratorDifference) {
+  absl::BitGen bitgen;
+  std::vector<int> vec;
+  // Randomize the set's insertion order so the nodes aren't all full.
+  for (int i = 0; i < 1000000; ++i) vec.push_back(i);
+  absl::c_shuffle(vec, bitgen);
+
+  absl::btree_set<int> set;
+  for (int i : vec) set.insert(i);
+
+  for (int i = 0; i < 1000; ++i) {
+    size_t begin = absl::Uniform(bitgen, 0u, set.size());
+    size_t end = absl::Uniform(bitgen, begin, set.size());
+    ASSERT_EQ(end - begin, set.find(end) - set.find(begin))
+        << begin << " " << end;
+  }
+}
+
+TEST(Btree, IteratorAddition) {
+  absl::BitGen bitgen;
+  std::vector<int> vec;
+
+  // Randomize the set's insertion order so the nodes aren't all full.
+  constexpr int kSetSize = 1000000;
+  for (int i = 0; i < kSetSize; ++i) vec.push_back(i);
+  absl::c_shuffle(vec, bitgen);
+
+  absl::btree_set<int> set;
+  for (int i : vec) set.insert(i);
+
+  for (int i = 0; i < 1000; ++i) {
+    int begin = absl::Uniform(bitgen, 0, kSetSize);
+    int end = absl::Uniform(bitgen, begin, kSetSize);
+    ASSERT_LE(begin, end);
+
+    auto it = set.find(begin);
+    it += end - begin;
+    ASSERT_EQ(it, set.find(end)) << end;
+
+    it += begin - end;
+    ASSERT_EQ(it, set.find(begin)) << begin;
+  }
+}
+
+TEST(Btree, IteratorAdditionOutOfBounds) {
+  const absl::btree_set<int> set({5});
+
+  auto it = set.find(5);
+
+  auto forward = it;
+  forward += 1;
+  EXPECT_EQ(forward, set.end());
+
+  auto backward = it;
+  EXPECT_EQ(backward, set.begin());
+
+  if (IsAssertEnabled()) {
+    EXPECT_DEATH(forward += 1, "");
+    EXPECT_DEATH(backward += -1, "");
+  }
+}
+
+TEST(Btree, IteratorSubtraction) {
+  absl::BitGen bitgen;
+  std::vector<int> vec;
+
+  // Randomize the set's insertion order so the nodes aren't all full.
+  constexpr int kSetSize = 1000000;
+  for (int i = 0; i < kSetSize; ++i) vec.push_back(i);
+  absl::c_shuffle(vec, bitgen);
+
+  absl::btree_set<int> set;
+  for (int i : vec) set.insert(i);
+
+  for (int i = 0; i < 1000; ++i) {
+    int begin = absl::Uniform(bitgen, 0, kSetSize);
+    int end = absl::Uniform(bitgen, begin, kSetSize);
+    ASSERT_LE(begin, end);
+
+    auto it = set.find(end);
+    it -= end - begin;
+    ASSERT_EQ(it, set.find(begin)) << begin;
+
+    it -= begin - end;
+    ASSERT_EQ(it, set.find(end)) << end;
+  }
+}
+
+TEST(Btree, IteratorSubtractionOutOfBounds) {
+  const absl::btree_set<int> set({5});
+
+  auto it = set.find(5);
+
+  auto backward = it;
+  EXPECT_EQ(backward, set.begin());
+
+  auto forward = it;
+  forward -= -1;
+  EXPECT_EQ(forward, set.end());
+
+  if (IsAssertEnabled()) {
+    EXPECT_DEATH(backward -= 1, "");
+    EXPECT_DEATH(forward -= -1, "");
+  }
+}
+
+TEST(Btree, DereferencingEndIterator) {
+  if (!IsAssertEnabled()) GTEST_SKIP() << "Assertions not enabled.";
+
+  absl::btree_set<int> set;
+  for (int i = 0; i < 1000; ++i) set.insert(i);
+  EXPECT_DEATH(*set.end(), "");
+}
+
+TEST(Btree, InvalidIteratorComparison) {
+  if (!IsAssertEnabled()) GTEST_SKIP() << "Assertions not enabled.";
+
+  absl::btree_set<int> set1, set2;
+  for (int i = 0; i < 1000; ++i) {
+    set1.insert(i);
+    set2.insert(i);
+  }
+
+  typename absl::btree_set<int>::iterator iter1, iter2;
+  EXPECT_EQ(iter1, iter2);
+  EXPECT_DEATH(void(set1.begin() == iter1), "");
+  EXPECT_DEATH(void(iter1 == set1.begin()), "");
+
+  constexpr const char *kDifferentContainerDeathMessage =
+      "Comparing iterators from different containers";
+  iter1 = set1.begin();
+  iter2 = set2.begin();
+  EXPECT_DEATH(void(iter1 == iter2), kDifferentContainerDeathMessage);
+  EXPECT_DEATH(void(iter2 == iter1), kDifferentContainerDeathMessage);
+}
+
+TEST(Btree, InvalidPointerUse) {
+  if (!kAsan)
+    GTEST_SKIP() << "We only detect invalid pointer use in ASan mode.";
+
+  absl::btree_set<int> set;
+  set.insert(0);
+  const int *ptr = &*set.begin();
+  set.insert(1);
+  EXPECT_DEATH(std::cout << *ptr, "use-after-free");
+  size_t slots_per_node = BtreeNodePeer::GetNumSlotsPerNode<decltype(set)>();
+  for (int i = 2; i < slots_per_node - 1; ++i) set.insert(i);
+  ptr = &*set.begin();
+  set.insert(static_cast<int>(slots_per_node));
+  EXPECT_DEATH(std::cout << *ptr, "use-after-free");
+}
+
+template<typename Set>
+void TestBasicFunctionality(Set set) {
+  using value_type = typename Set::value_type;
+  for (int i = 0; i < 100; ++i) { set.insert(value_type(i)); }
+  for (int i = 50; i < 100; ++i) { set.erase(value_type(i)); }
+  auto it = set.begin();
+  for (int i = 0; i < 50; ++i, ++it) {
+    ASSERT_EQ(set.find(value_type(i)), it) << i;
+  }
+}
+
+template<size_t align>
+struct alignas(align) OveralignedKey {
+  explicit OveralignedKey(int i) : key(i) {}
+  bool operator<(const OveralignedKey &other) const { return key < other.key; }
+  int key = 0;
+};
+
+TEST(Btree, OveralignedKey) {
+  // Test basic functionality with both even and odd numbers of slots per node.
+  // The goal here is to detect cases where alignment may be incorrect.
+  TestBasicFunctionality(
+      SizedBtreeSet<OveralignedKey<16>, /*TargetValuesPerNode=*/8>());
+  TestBasicFunctionality(
+      SizedBtreeSet<OveralignedKey<16>, /*TargetValuesPerNode=*/9>());
+}
+
+TEST(Btree, FieldTypeEqualsSlotType) {
+  // This breaks if we try to do layout_type::Pointer<slot_type> because
+  // slot_type is the same as field_type.
+  using set_type = absl::btree_set<uint8_t>;
+  static_assert(BtreeNodePeer::FieldTypeEqualsSlotType<set_type>());
+  TestBasicFunctionality(set_type());
+}
+
+// Alias whose only purpose is to have the same length as set_params for better
+// alignment in the test below.
+template <typename... T>
+using set_p_impl = set_params_impl<T...>;
+
+TEST(BtreeTest, SetParamsStripsDefaults) {
+  using K = int;
+  using DA = btree_set_defaults<int>::Compare;
+  using DB = btree_set_defaults<int>::Alloc;
+  using DC = btree_set_defaults<int>::TargetNodeSize;
+  using DD = btree_set_defaults<int>::IsMulti;
+
+  using XA = std::greater<int>;
+  struct XB {};
+  using XC = std::integral_constant<int, 100>;
+  using XD = std::true_type;
+
+  EXPECT_TRUE((std::is_same_v<set_params<K, XA, XB, XC{}, XD{}>,
+                              set_p_impl<K, XA, XB, XC, XD>>));
+  EXPECT_TRUE((std::is_same_v<set_params<K, XA, XB, XC{}, DD{}>,
+                              set_p_impl<K, XA, XB, XC>>));
+  EXPECT_TRUE((std::is_same_v<set_params<K, XA, XB, DC{}, XD{}>,
+                              set_p_impl<K, XA, XB, DC, XD>>));
+  EXPECT_TRUE((std::is_same_v<set_params<K, XA, XB, DC{}, DD{}>,
+                              set_p_impl<K, XA, XB>>));
+  EXPECT_TRUE((std::is_same_v<set_params<K, XA, DB, XC{}, XD{}>,
+                              set_p_impl<K, XA, DB, XC, XD>>));
+  EXPECT_TRUE((std::is_same_v<set_params<K, XA, DB, XC{}, DD{}>,
+                              set_p_impl<K, XA, DB, XC>>));
+  EXPECT_TRUE((std::is_same_v<set_params<K, XA, DB, DC{}, XD{}>,
+                              set_p_impl<K, XA, DB, DC, XD>>));
+  EXPECT_TRUE((std::is_same_v<set_params<K, XA, DB, DC{}, DD{}>,
+                              set_p_impl<K, XA>>));
+  EXPECT_TRUE((std::is_same_v<set_params<K, DA, XB, XC{}, XD{}>,
+                              set_p_impl<K, DA, XB, XC, XD>>));
+  EXPECT_TRUE((std::is_same_v<set_params<K, DA, XB, XC{}, DD{}>,
+                              set_p_impl<K, DA, XB, XC>>));
+  EXPECT_TRUE((std::is_same_v<set_params<K, DA, XB, DC{}, XD{}>,
+                              set_p_impl<K, DA, XB, DC, XD>>));
+  EXPECT_TRUE((std::is_same_v<set_params<K, DA, XB, DC{}, DD{}>,
+                              set_p_impl<K, DA, XB>>));
+  EXPECT_TRUE((std::is_same_v<set_params<K, DA, DB, XC{}, XD{}>,
+                              set_p_impl<K, DA, DB, XC, XD>>));
+  EXPECT_TRUE((std::is_same_v<set_params<K, DA, DB, XC{}, DD{}>,
+                              set_p_impl<K, DA, DB, XC>>));
+  EXPECT_TRUE((std::is_same_v<set_params<K, DA, DB, DC{}, XD{}>,
+                              set_p_impl<K, DA, DB, DC, XD>>));
+  EXPECT_TRUE((std::is_same_v<set_params<K, DA, DB, DC{}, DD{}>,
+                              set_p_impl<K>>));
+}
+
+// Alias whose only purpose is to have the same length as map_params for better
+// alignment in the test below.
+template <typename... T>
+using map_p_impl = map_params_impl<T...>;
+
+TEST(BtreeTest, MapParamsStripsDefaults) {
+  using K = int;
+  using V = double;
+  using DA = btree_map_defaults<int, double>::Compare;
+  using DB = btree_map_defaults<int, double>::Alloc;
+  using DC = btree_map_defaults<int, double>::TargetNodeSize;
+  using DD = btree_map_defaults<int, double>::IsMulti;
+
+  using XA = std::greater<int>;
+  struct XB {};
+  using XC = std::integral_constant<int, 100>;
+  using XD = std::true_type;
+
+  EXPECT_TRUE((std::is_same_v<map_params<K, V, XA, XB, XC{}, XD{}>,
+                              map_p_impl<K, V, XA, XB, XC, XD>>));
+  EXPECT_TRUE((std::is_same_v<map_params<K, V, XA, XB, XC{}, DD{}>,
+                              map_p_impl<K, V, XA, XB, XC>>));
+  EXPECT_TRUE((std::is_same_v<map_params<K, V, XA, XB, DC{}, XD{}>,
+                              map_p_impl<K, V, XA, XB, DC, XD>>));
+  EXPECT_TRUE((std::is_same_v<map_params<K, V, XA, XB, DC{}, DD{}>,
+                              map_p_impl<K, V, XA, XB>>));
+  EXPECT_TRUE((std::is_same_v<map_params<K, V, XA, DB, XC{}, XD{}>,
+                              map_p_impl<K, V, XA, DB, XC, XD>>));
+  EXPECT_TRUE((std::is_same_v<map_params<K, V, XA, DB, XC{}, DD{}>,
+                              map_p_impl<K, V, XA, DB, XC>>));
+  EXPECT_TRUE((std::is_same_v<map_params<K, V, XA, DB, DC{}, XD{}>,
+                              map_p_impl<K, V, XA, DB, DC, XD>>));
+  EXPECT_TRUE((std::is_same_v<map_params<K, V, XA, DB, DC{}, DD{}>,
+                              map_p_impl<K, V, XA>>));
+  EXPECT_TRUE((std::is_same_v<map_params<K, V, DA, XB, XC{}, XD{}>,
+                              map_p_impl<K, V, DA, XB, XC, XD>>));
+  EXPECT_TRUE((std::is_same_v<map_params<K, V, DA, XB, XC{}, DD{}>,
+                              map_p_impl<K, V, DA, XB, XC>>));
+  EXPECT_TRUE((std::is_same_v<map_params<K, V, DA, XB, DC{}, XD{}>,
+                              map_p_impl<K, V, DA, XB, DC, XD>>));
+  EXPECT_TRUE((std::is_same_v<map_params<K, V, DA, XB, DC{}, DD{}>,
+                              map_p_impl<K, V, DA, XB>>));
+  EXPECT_TRUE((std::is_same_v<map_params<K, V, DA, DB, XC{}, XD{}>,
+                              map_p_impl<K, V, DA, DB, XC, XD>>));
+  EXPECT_TRUE((std::is_same_v<map_params<K, V, DA, DB, XC{}, DD{}>,
+                              map_p_impl<K, V, DA, DB, XC>>));
+  EXPECT_TRUE((std::is_same_v<map_params<K, V, DA, DB, DC{}, XD{}>,
+                              map_p_impl<K, V, DA, DB, DC, XD>>));
+  EXPECT_TRUE((std::is_same_v<map_params<K, V, DA, DB, DC{}, DD{}>,
+                              map_p_impl<K, V>>));
 }
 
 }  // namespace
 }  // namespace container_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/container/btree_test.h b/absl/container/btree_test.h
index 5ecf43ceebe..79c3d95bf07 100644
--- a/absl/container/btree_test.h
+++ b/absl/container/btree_test.h
@@ -19,21 +19,24 @@
 #include <cassert>
 #include <random>
 #include <string>
+#include <type_traits>
 #include <utility>
 #include <vector>
 
 #include "absl/container/btree_map.h"
 #include "absl/container/btree_set.h"
 #include "absl/container/flat_hash_set.h"
+#include "absl/strings/cord.h"
 #include "absl/time/time.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
 // Like remove_const but propagates the removal through std::pair.
 template <typename T>
 struct remove_pair_const {
-  using type = typename std::remove_const<T>::type;
+  using type = std::remove_const_t<T>;
 };
 template <typename T, typename U>
 struct remove_pair_const<std::pair<T, U> > {
@@ -99,6 +102,16 @@ struct Generator<std::string> {
   }
 };
 
+template <>
+struct Generator<Cord> {
+  int maxval;
+  explicit Generator(int m) : maxval(m) {}
+  Cord operator()(int i) const {
+    char buf[16];
+    return Cord(GenerateDigits(buf, i, maxval));
+  }
+};
+
 template <typename T, typename U>
 struct Generator<std::pair<T, U> > {
   Generator<typename remove_pair_const<T>::type> tgen;
@@ -148,6 +161,7 @@ std::vector<V> GenerateValuesWithSeed(int n, int maxval, int seed) {
 }
 
 }  // namespace container_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_BTREE_TEST_H_
diff --git a/absl/container/chunked_queue.h b/absl/container/chunked_queue.h
new file mode 100644
index 00000000000..8ada20853fa
--- /dev/null
+++ b/absl/container/chunked_queue.h
@@ -0,0 +1,752 @@
+// Copyright 2025 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: chunked_queue.h
+// -----------------------------------------------------------------------------
+//
+// `std::deque` provides random access and fast push/pop back/front. It is
+// implemented as an array of fixed blocks. It provides no control of block size
+// and implementations differ; libstdc++ tries to allocate blocks of ~512 bytes
+// and libc++ tries for blocks of ~4k bytes.
+//
+// `absl::chunked_queue` provides the same minus random access. It is
+// implemented as a double-linked list of fixed or variable sized blocks.
+//
+// `absl::chunked_queue` is useful when memory usage is paramount as it provides
+//  finegrained and configurable block sizing.
+//
+// The interface supported by this class is limited to:
+//
+//   empty()
+//   size()
+//   max_size()
+//   shrink_to_fit()
+//   resize()
+//   assign()
+//   push_back()
+//   emplace_back()
+//   pop_front()
+//   front()
+//   back()
+//   swap()
+//   clear()
+//   begin(), end()
+//   cbegin(), cend()
+//
+// === ADVANCED USAGE
+//
+// == clear()
+//
+// As an optimization clear() leaves the first block of the chunked_queue
+// allocated (but empty). So clear will not delete all memory of the container.
+// In order to do so, call shrink_to_fit() or swap the container with an empty
+// one.
+//
+//   absl::chunked_queue<int64> q = {1, 2, 3};
+//   q.clear();
+//   q.shrink_to_fit();
+//
+// == block size customization
+//
+// chunked_queue allows customization of the block size for each block. By
+// default the block size is set to 1 element and the size doubles for the next
+// block until it reaches the default max block size, which is 128 elements.
+//
+// = fixed size
+//
+// When only the first block size parameter is specified, it sets a fixed block
+// size for all blocks:
+//
+//   chunked_queue<T, 32>: 32 elements per block
+//
+// The smaller the block size, the less the memory usage for small queues at the
+// cost of performance. Caveat: For large queues, a smaller block size will
+// increase memory usage, and reduce performance.
+//
+// = variable size
+//
+// When both block size parameters are specified, they set the min and max block
+// sizes for the blocks. Initially the queue starts with the min block size and
+// as it grows, the size of each block grows until it reaches the max block
+// size.
+// New blocks are double the size of the tail block (so they at least
+// double the size of the queue).
+//
+//   chunked_queue<T, 4, 64>: first block 4 elements, second block 8 elements,
+//                            third block 16 elements, fourth block 32 elements,
+//                            all other blocks 64 elements
+//
+// One can specify a min and max such that small queues will not waste memory
+// and large queues will not have too many blocks.
+
+#ifndef ABSL_CONTAINER_CHUNKED_QUEUE_H_
+#define ABSL_CONTAINER_CHUNKED_QUEUE_H_
+
+#include <algorithm>
+#include <cstddef>
+#include <cstdint>
+#include <initializer_list>
+#include <iterator>
+#include <memory>
+#include <new>
+#include <tuple>
+#include <type_traits>
+#include <utility>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/hardening.h"
+#include "absl/base/internal/iterator_traits.h"
+#include "absl/base/macros.h"
+#include "absl/container/internal/chunked_queue.h"
+#include "absl/container/internal/layout.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+template <typename T, size_t BLo = 0, size_t BHi = BLo,
+          typename Allocator = std::allocator<T>>
+class chunked_queue {
+ public:
+  static constexpr size_t kBlockSizeMin = (BLo == 0 && BHi == 0) ? 1 : BLo;
+  static constexpr size_t kBlockSizeMax = (BLo == 0 && BHi == 0) ? 128 : BHi;
+
+ private:
+  static_assert(kBlockSizeMin > 0, "Min block size cannot be zero");
+  static_assert(kBlockSizeMin <= kBlockSizeMax, "Invalid block size bounds");
+
+  using Block = container_internal::ChunkedQueueBlock<T, Allocator>;
+  using AllocatorTraits = std::allocator_traits<Allocator>;
+
+  class iterator_common {
+   public:
+    friend bool operator==(const iterator_common& a, const iterator_common& b) {
+      return a.ptr == b.ptr;
+    }
+
+    friend bool operator!=(const iterator_common& a, const iterator_common& b) {
+      return !(a == b);
+    }
+
+   protected:
+    iterator_common() = default;
+    explicit iterator_common(Block* b)
+        : block(b), ptr(b->start()), limit(b->limit()) {}
+
+    void Incr() {
+      // If we do not have a next block, make ptr point one past the end of this
+      // block. If we do have a next block, make ptr point to the first element
+      // of the next block.
+      ++ptr;
+      if (ptr == limit && block->next()) *this = iterator_common(block->next());
+    }
+
+    void IncrBy(size_t n) {
+      while (n > static_cast<size_t>(limit - ptr)) {
+        n -= static_cast<size_t>(limit - ptr);
+        *this = iterator_common(block->next());
+      }
+      ptr += n;
+    }
+
+    Block* block = nullptr;
+    T* ptr = nullptr;
+    T* limit = nullptr;
+  };
+
+  // CT can be either T or const T.
+  template <typename CT>
+  class basic_iterator : public iterator_common {
+   public:
+    using iterator_category = std::forward_iterator_tag;
+    using value_type = typename AllocatorTraits::value_type;
+    using difference_type = typename AllocatorTraits::difference_type;
+    using pointer = std::conditional_t<std::is_const_v<CT>,
+                                       typename AllocatorTraits::const_pointer,
+                                       typename AllocatorTraits::pointer>;
+    using reference = CT&;
+
+    basic_iterator() = default;
+
+    // Copy ctor if CT is T.
+    // Otherwise it's a conversion of iterator to const_iterator.
+    basic_iterator(const basic_iterator<T>& it)  // NOLINT(runtime/explicit)
+        : iterator_common(it) {}
+
+    basic_iterator& operator=(const basic_iterator& other) = default;
+
+    reference operator*() const { return *this->ptr; }
+    pointer operator->() const { return this->ptr; }
+    basic_iterator& operator++() {
+      this->Incr();
+      return *this;
+    }
+    basic_iterator operator++(int) {
+      basic_iterator t = *this;
+      ++*this;
+      return t;
+    }
+
+   private:
+    explicit basic_iterator(Block* b) : iterator_common(b) {}
+
+    friend chunked_queue;
+  };
+
+ public:
+  using allocator_type = typename AllocatorTraits::allocator_type;
+  using value_type = typename AllocatorTraits::value_type;
+  using size_type = typename AllocatorTraits::size_type;
+  using difference_type = typename AllocatorTraits::difference_type;
+  using reference = value_type&;
+  using const_reference = const value_type&;
+  using iterator = basic_iterator<T>;
+  using const_iterator = basic_iterator<const T>;
+
+  // Constructs an empty queue.
+  chunked_queue() : chunked_queue(allocator_type()) {}
+
+  // Constructs an empty queue with a custom allocator.
+  explicit chunked_queue(const allocator_type& alloc)
+      : alloc_and_size_(alloc) {}
+
+  // Constructs a queue with `count` default-inserted elements.
+  explicit chunked_queue(size_type count,
+                         const allocator_type& alloc = allocator_type())
+      : alloc_and_size_(alloc) {
+    resize(count);
+  }
+
+  // Constructs a queue with `count` copies of `value`.
+  chunked_queue(size_type count, const T& value,
+                const allocator_type& alloc = allocator_type())
+      : alloc_and_size_(alloc) {
+    assign(count, value);
+  }
+
+  // Constructs a queue with the contents of the range [first, last).
+  template <typename Iter,
+            typename = std::enable_if_t<
+                base_internal::IsAtLeastInputIterator<Iter>::value>>
+  chunked_queue(Iter first, Iter last,
+                const allocator_type& alloc = allocator_type())
+      : alloc_and_size_(alloc) {
+    using Tag = typename std::iterator_traits<Iter>::iterator_category;
+    RangeInit(first, last, Tag());
+  }
+
+  // Constructs a queue with the contents of the initializer list `list`.
+  chunked_queue(std::initializer_list<T> list,
+                const allocator_type& alloc = allocator_type())
+      : chunked_queue(list.begin(), list.end(), alloc) {}
+
+  ~chunked_queue();
+
+  // Copy constructor.
+  chunked_queue(const chunked_queue& other)
+      : chunked_queue(other,
+                      AllocatorTraits::select_on_container_copy_construction(
+                          other.alloc_and_size_.allocator())) {}
+
+  // Copy constructor with specific allocator.
+  chunked_queue(const chunked_queue& other, const allocator_type& alloc)
+      : alloc_and_size_(alloc) {
+    for (const_reference item : other) {
+      push_back(item);
+    }
+  }
+
+  // Move constructor.
+  chunked_queue(chunked_queue&& other) noexcept
+      : head_(other.head_),
+        tail_(other.tail_),
+        alloc_and_size_(std::move(other.alloc_and_size_)) {
+    other.head_ = {};
+    other.tail_ = {};
+    other.alloc_and_size_.size = 0;
+  }
+
+  // Replaces contents with those from initializer list `il`.
+  chunked_queue& operator=(std::initializer_list<T> il) {
+    assign(il.begin(), il.end());
+    return *this;
+  }
+
+  // Copy assignment operator.
+  chunked_queue& operator=(const chunked_queue& other) {
+    if (this == &other) {
+      return *this;
+    }
+    if (AllocatorTraits::propagate_on_container_copy_assignment::value &&
+        (alloc_and_size_.allocator() != other.alloc_and_size_.allocator())) {
+      // Destroy all current elements and blocks with the current allocator,
+      // before switching this to use the allocator propagated from "other".
+      DestroyAndDeallocateAll();
+      alloc_and_size_ = AllocatorAndSize(other.alloc_and_size_.allocator());
+    }
+    assign(other.begin(), other.end());
+    return *this;
+  }
+
+  // Move assignment operator.
+  chunked_queue& operator=(chunked_queue&& other) noexcept;
+
+  // Returns true if the queue contains no elements.
+  bool empty() const { return alloc_and_size_.size == 0; }
+
+  // Returns the number of elements in the queue.
+  size_t size() const { return alloc_and_size_.size; }
+
+  // Returns the maximum number of elements the queue is able to hold.
+  size_type max_size() const noexcept {
+    return AllocatorTraits::max_size(alloc_and_size_.allocator());
+  }
+
+  // Resizes the container to contain `new_size` elements.
+  // If `new_size > size()`, additional default-inserted elements are appended.
+  // If `new_size < size()`, elements are removed from the end.
+  void resize(size_t new_size);
+
+  // Resizes the container to contain `new_size` elements.
+  // If `new_size > size()`, additional copies of `value` are appended.
+  // If `new_size < size()`, elements are removed from the end.
+  void resize(size_type new_size, const T& value) {
+    if (new_size > size()) {
+      size_t to_add = new_size - size();
+      for (size_t i = 0; i < to_add; ++i) {
+        push_back(value);
+      }
+    } else {
+      resize(new_size);
+    }
+  }
+
+  // Requests the removal of unused capacity.
+  void shrink_to_fit() {
+    // As an optimization clear() leaves the first block of the chunked_queue
+    // allocated (but empty). When empty, shrink_to_fit() deallocates the first
+    // block by swapping it a newly constructed container that has no first
+    // block.
+    if (empty()) {
+      chunked_queue(alloc_and_size_.allocator()).swap(*this);
+    }
+  }
+
+  // Replaces the contents with copies of those in the range [first, last).
+  template <typename Iter,
+            typename = std::enable_if_t<
+                base_internal::IsAtLeastInputIterator<Iter>::value>>
+  void assign(Iter first, Iter last) {
+    auto out = begin();
+    Block* prev_block = nullptr;
+
+    // Overwrite existing elements.
+    for (; out != end() && first != last; ++first) {
+      // Track the previous block so we can correctly update tail_ if we stop
+      // exactly at a block boundary.
+      if (out.ptr + 1 == out.block->limit()) {
+        prev_block = out.block;
+      }
+      *out = *first;
+      ++out;
+    }
+
+    // If we stopped exactly at the start of a block (meaning the previous block
+    // was full), we must ensure tail_ points to the end of the previous block,
+    // not the start of the current (now empty and to be deleted) block.
+    // This maintains the invariant required by back() which assumes tail_
+    // never points to the start of a block (unless it's the only block).
+    if (!empty() && out.block != nullptr && out.ptr == out.block->start() &&
+        prev_block != nullptr) {
+      // Delete the current block and all subsequent blocks.
+      //
+      // NOTE: Calling EraseAllFrom on an iterator that points to the limit of
+      // the previous block will not delete any element from the previous block.
+      iterator prev_block_end(prev_block);
+      prev_block_end.ptr = prev_block->limit();
+      EraseAllFrom(prev_block_end);
+
+      // Update tail_ to point to the end of the previous block.
+      tail_ = prev_block_end;
+      prev_block->set_next(nullptr);
+    } else {
+      // Standard erase from the current position to the end.
+      EraseAllFrom(out);
+    }
+
+    // Append any remaining new elements.
+    for (; first != last; ++first) {
+      push_back(*first);
+    }
+  }
+
+  // Replaces the contents with `count` copies of `value`.
+  void assign(size_type count, const T& value) {
+    clear();
+    for (size_type i = 0; i < count; ++i) {
+      push_back(value);
+    }
+  }
+
+  // Replaces the contents with the elements from the initializer list `il`.
+  void assign(std::initializer_list<T> il) { assign(il.begin(), il.end()); }
+
+  // Appends the given element value to the end of the container.
+  // Invalidates `end()` iterator. References to other elements remain valid.
+  void push_back(const T& val) { emplace_back(val); }
+  void push_back(T&& val) { emplace_back(std::move(val)); }
+
+  // Appends a new element to the end of the container.
+  // The element is constructed in-place with `args`.
+  // Returns a reference to the new element.
+  // Invalidates `end()` iterator. References to other elements remain valid.
+  template <typename... A>
+  T& emplace_back(A&&... args) {
+    T* storage = AllocateBack();
+    AllocatorTraits::construct(alloc_and_size_.allocator(), storage,
+                               std::forward<A>(args)...);
+    ++alloc_and_size_.size;
+    ++tail_.ptr;
+    return *storage;
+  }
+
+  // Removes the first element of the container.
+  // Invalidates iterators to the removed element.
+  // REQUIRES: !empty()
+  void pop_front();
+
+  // Returns a reference to the first element in the container.
+  // REQUIRES: !empty()
+  T& front() {
+    absl::base_internal::HardeningAssertNonEmpty(*this);
+    return *head_;
+  }
+  const T& front() const {
+    absl::base_internal::HardeningAssertNonEmpty(*this);
+    return *head_;
+  }
+
+  // Returns a reference to the last element in the container.
+  // REQUIRES: !empty()
+  T& back() {
+    absl::base_internal::HardeningAssertNonEmpty(*this);
+    return *(&*tail_ - 1);
+  }
+  const T& back() const {
+    absl::base_internal::HardeningAssertNonEmpty(*this);
+    return *(&*tail_ - 1);
+  }
+
+  // Swaps the contents of this queue with `other`.
+  void swap(chunked_queue& other) noexcept {
+    using std::swap;
+    swap(head_, other.head_);
+    swap(tail_, other.tail_);
+    if (AllocatorTraits::propagate_on_container_swap::value) {
+      swap(alloc_and_size_, other.alloc_and_size_);
+    } else {
+      // Swap only the sizes; each object keeps its allocator.
+      //
+      // (It is undefined behavior to swap between two containers with unequal
+      // allocators if propagate_on_container_swap is false, so we don't have to
+      // handle that here like we do in the move-assignment operator.)
+      absl::base_internal::HardeningAssert(get_allocator() ==
+                                           other.get_allocator());
+      swap(alloc_and_size_.size, other.alloc_and_size_.size);
+    }
+  }
+
+  // Erases all elements from the container.
+  // Note: Leaves one empty block allocated as an optimization.
+  // To free all memory, call shrink_to_fit() after calling clear().
+  void clear();
+
+  iterator begin() { return head_; }
+  iterator end() { return tail_; }
+
+  const_iterator begin() const { return head_; }
+  const_iterator end() const { return tail_; }
+
+  const_iterator cbegin() const { return head_; }
+  const_iterator cend() const { return tail_; }
+
+  // Returns the allocator associated with the container.
+  allocator_type get_allocator() const { return alloc_and_size_.allocator(); }
+
+ private:
+  // Empty base-class optimization: bundle storage for our allocator together
+  // with a field we had to store anyway (size), via inheriting from the
+  // allocator, so this allocator instance doesn't consume any storage
+  // when its type has no data members.
+  struct AllocatorAndSize : private allocator_type {
+    explicit AllocatorAndSize(const allocator_type& alloc)
+        : allocator_type(alloc) {}
+    const allocator_type& allocator() const { return *this; }
+    allocator_type& allocator() { return *this; }
+    size_t size = 0;
+  };
+
+  template <typename Iter>
+  void RangeInit(Iter first, Iter last, std::input_iterator_tag) {
+    while (first != last) {
+      AddTailBlock();
+      for (; first != last && tail_.ptr != tail_.limit;
+           ++alloc_and_size_.size, ++tail_.ptr, ++first) {
+        AllocatorTraits::construct(alloc_and_size_.allocator(), tail_.ptr,
+                                   *first);
+      }
+    }
+  }
+
+  void Construct(T* start, T* limit) {
+    ABSL_ASSERT(start <= limit);
+    for (; start != limit; ++start) {
+      AllocatorTraits::construct(alloc_and_size_.allocator(), start);
+    }
+  }
+
+  size_t Destroy(T* start, T* limit) {
+    ABSL_ASSERT(start <= limit);
+    const size_t n = limit - start;
+    for (; start != limit; ++start) {
+      AllocatorTraits::destroy(alloc_and_size_.allocator(), start);
+    }
+    return n;
+  }
+
+  T* block_begin(Block* b) const {
+    return b == head_.block ? head_.ptr : b->start();
+  }
+  T* block_end(Block* b) const {
+    // We have the choice of !b->next or b == tail_.block to determine if b is
+    // the tail or not. !b->next is usually faster because the caller of
+    // block_end() is most likely traversing the list of blocks so b->next is
+    // already fetched into some register.
+    return !b->next() ? tail_.ptr : b->limit();
+  }
+
+  void AddTailBlock();
+  size_t NewBlockSize() {
+    // Double the last block size and bound to [kBlockSizeMin, kBlockSizeMax].
+    if (!tail_.block) return kBlockSizeMin;
+    return (std::min)(kBlockSizeMax, 2 * tail_.block->size());
+  }
+
+  T* AllocateBack();
+  void EraseAllFrom(iterator i);
+
+  // Destroys any contained elements and destroys all allocated storage.
+  // (Like clear(), except this doesn't leave any empty blocks behind.)
+  void DestroyAndDeallocateAll();
+
+  // The set of elements in the queue is the following:
+  //
+  // (1) When we have just one block:
+  //      [head_.ptr .. tail_.ptr-1]
+  // (2) When we have multiple blocks:
+  //      [head_.ptr .. head_.limit-1]
+  //      ... concatenation of all elements from interior blocks ...
+  //      [tail_.ptr .. tail_.limit-1]
+  //
+  // Rep invariants:
+  // When have just one block:
+  //   head_.limit == tail_.limit == &head_.block->element[kBlockSize]
+  // Always:
+  //   head_.ptr <= head_.limit
+  //   tail_.ptr <= tail_.limit
+
+  iterator head_;
+  iterator tail_;
+  AllocatorAndSize alloc_and_size_;
+};
+
+template <typename T, size_t BLo, size_t BHi, typename Allocator>
+inline void swap(chunked_queue<T, BLo, BHi, Allocator>& a,
+                 chunked_queue<T, BLo, BHi, Allocator>& b) noexcept {
+  a.swap(b);
+}
+
+template <typename T, size_t BLo, size_t BHi, typename Allocator>
+chunked_queue<T, BLo, BHi, Allocator>&
+chunked_queue<T, BLo, BHi, Allocator>::operator=(
+    chunked_queue&& other) noexcept {
+  if (this == &other) {
+    return *this;
+  }
+  DestroyAndDeallocateAll();
+
+  if constexpr (AllocatorTraits::propagate_on_container_move_assignment::
+                    value) {
+    // Take over the storage of "other", along with its allocator.
+    head_ = other.head_;
+    tail_ = other.tail_;
+    alloc_and_size_ = std::move(other.alloc_and_size_);
+    other.head_ = {};
+    other.tail_ = {};
+    other.alloc_and_size_.size = 0;
+  } else if (get_allocator() == other.get_allocator()) {
+    // Take over the storage of "other", with which we share an allocator.
+    head_ = other.head_;
+    tail_ = other.tail_;
+    alloc_and_size_.size = other.alloc_and_size_.size;
+    other.head_ = {};
+    other.tail_ = {};
+    other.alloc_and_size_.size = 0;
+  } else {
+    // We cannot take over of the storage from "other", since it has a different
+    // allocator; we're stuck move-assigning elements individually.
+    for (auto& elem : other) {
+      push_back(std::move(elem));
+    }
+  }
+  return *this;
+}
+
+template <typename T, size_t BLo, size_t BHi, typename Allocator>
+inline chunked_queue<T, BLo, BHi, Allocator>::~chunked_queue() {
+  Block* b = head_.block;
+  while (b) {
+    Block* next = b->next();
+    Destroy(block_begin(b), block_end(b));
+    Block::Delete(b, &alloc_and_size_.allocator());
+    b = next;
+  }
+}
+
+template <typename T, size_t BLo, size_t BHi, typename Allocator>
+void chunked_queue<T, BLo, BHi, Allocator>::resize(size_t new_size) {
+  while (new_size > size()) {
+    if (tail_.ptr == tail_.limit) {
+      AddTailBlock();
+    }
+    size_t to_add = (std::min)(new_size - size(),
+                               static_cast<size_t>(tail_.limit - tail_.ptr));
+    T* start = tail_.ptr;
+    T* limit = start + to_add;
+    Construct(start, limit);
+    tail_.ptr = limit;
+    alloc_and_size_.size += to_add;
+  }
+  if (size() == new_size) {
+    return;
+  }
+  ABSL_ASSERT(new_size < size());
+  auto new_end = begin();
+  new_end.IncrBy(new_size);
+  ABSL_ASSERT(new_end != end());
+  EraseAllFrom(new_end);
+}
+
+template <typename T, size_t BLo, size_t BHi, typename Allocator>
+inline void chunked_queue<T, BLo, BHi, Allocator>::AddTailBlock() {
+  ABSL_ASSERT(tail_.ptr == tail_.limit);
+  auto* b = Block::New(NewBlockSize(), &alloc_and_size_.allocator());
+  if (!head_.block) {
+    ABSL_ASSERT(!tail_.block);
+    head_ = iterator(b);
+  } else {
+    ABSL_ASSERT(tail_.block);
+    tail_.block->set_next(b);
+  }
+  tail_ = iterator(b);
+}
+
+template <typename T, size_t BLo, size_t BHi, typename Allocator>
+inline T* chunked_queue<T, BLo, BHi, Allocator>::AllocateBack() {
+  if (tail_.ptr == tail_.limit) {
+    AddTailBlock();
+  }
+  return tail_.ptr;
+}
+
+template <typename T, size_t BLo, size_t BHi, typename Allocator>
+inline void chunked_queue<T, BLo, BHi, Allocator>::EraseAllFrom(iterator i) {
+  if (!i.block) {
+    return;
+  }
+  ABSL_ASSERT(i.ptr);
+  ABSL_ASSERT(i.limit);
+  alloc_and_size_.size -= Destroy(i.ptr, block_end(i.block));
+  Block* b = i.block->next();
+  while (b) {
+    Block* next = b->next();
+    alloc_and_size_.size -= Destroy(b->start(), block_end(b));
+    Block::Delete(b, &alloc_and_size_.allocator());
+    b = next;
+  }
+  tail_ = i;
+  tail_.block->set_next(nullptr);
+}
+
+template <typename T, size_t BLo, size_t BHi, typename Allocator>
+inline void chunked_queue<T, BLo, BHi, Allocator>::DestroyAndDeallocateAll() {
+  Block* b = head_.block;
+  while (b) {
+    Block* next = b->next();
+    Destroy(block_begin(b), block_end(b));
+    Block::Delete(b, &alloc_and_size_.allocator());
+    b = next;
+  }
+  head_ = iterator();
+  tail_ = iterator();
+  alloc_and_size_.size = 0;
+}
+
+template <typename T, size_t BLo, size_t BHi, typename Allocator>
+inline void chunked_queue<T, BLo, BHi, Allocator>::pop_front() {
+  absl::base_internal::HardeningAssertNonEmpty(*this);
+  ABSL_ASSERT(head_.block);
+  AllocatorTraits::destroy(alloc_and_size_.allocator(), head_.ptr);
+  ++head_.ptr;
+  --alloc_and_size_.size;
+  if (empty()) {
+    // Reset head and tail to the start of the (only) block.
+    ABSL_ASSERT(head_.block == tail_.block);
+    head_.ptr = tail_.ptr = head_.block->start();
+    return;
+  }
+  if (head_.ptr == head_.limit) {
+    Block* n = head_.block->next();
+    Block::Delete(head_.block, &alloc_and_size_.allocator());
+    head_ = iterator(n);
+  }
+}
+
+template <typename T, size_t BLo, size_t BHi, typename Allocator>
+void chunked_queue<T, BLo, BHi, Allocator>::clear() {
+  // NOTE: As an optimization we leave one block allocated.
+  Block* b = head_.block;
+  if (!b) {
+    ABSL_ASSERT(empty());
+    return;
+  }
+  while (b) {
+    Block* next = b->next();
+    Destroy(block_begin(b), block_end(b));
+    if (head_.block != b) {
+      Block::Delete(b, &alloc_and_size_.allocator());
+    }
+    b = next;
+  }
+  b = head_.block;
+  b->set_next(nullptr);
+  head_ = tail_ = iterator(b);
+  alloc_and_size_.size = 0;
+}
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_CONTAINER_CHUNKED_QUEUE_H_
diff --git a/absl/container/chunked_queue_benchmark.cc b/absl/container/chunked_queue_benchmark.cc
new file mode 100644
index 00000000000..ee4d3c150f7
--- /dev/null
+++ b/absl/container/chunked_queue_benchmark.cc
@@ -0,0 +1,386 @@
+// Copyright 2025 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <cstddef>
+#include <cstdint>
+#include <deque>
+#include <forward_list>
+#include <list>
+#include <random>
+
+#include "absl/container/chunked_queue.h"
+#include "absl/random/random.h"
+#include "absl/status/status.h"
+#include "absl/strings/cord.h"
+#include "benchmark/benchmark.h"
+
+namespace {
+
+// Queue implementation using std::forward_list. Used to benchmark
+// absl::chunked_queue against another plausable implementation.
+template <typename T>
+class forward_list_queue {
+ public:
+  using iterator = typename std::forward_list<T>::iterator;
+
+  forward_list_queue() = default;
+  ~forward_list_queue() = default;
+
+  template <typename... Args>
+  void emplace_back(Args&&... args) {
+    if (list_.empty()) {
+      list_.emplace_front(std::forward<Args>(args)...);
+      tail_ = list_.begin();
+    } else {
+      list_.emplace_after(tail_, std::forward<Args>(args)...);
+      ++tail_;
+    }
+  }
+
+  void push_back(const T& value) { emplace_back(value); }
+  iterator begin() { return list_.begin(); }
+  iterator end() { return list_.end(); }
+  T& front() { return list_.front(); }
+  const T& front() const { return list_.front(); }
+  void pop_front() { list_.pop_front(); }
+  bool empty() const { return list_.empty(); }
+  void clear() { list_.clear(); }
+
+ private:
+  std::forward_list<T> list_;
+  typename std::forward_list<T>::iterator tail_;
+};
+
+template <class T>
+using Deque = std::deque<T>;
+template <class T>
+using List = std::list<T>;
+template <class T>
+using FwdList = forward_list_queue<T>;
+template <class T>
+using Chunked = absl::chunked_queue<T>;
+template <class T>
+using ExpChunked = absl::chunked_queue<T, 2, 64>;
+
+class Element {
+ public:
+  Element() : Element(-1) {}
+  Element(int type) : type_(type) {}      // NOLINT
+  operator int() const { return type_; }  // NOLINT
+
+ private:
+  int type_;
+  absl::Cord item_;
+  absl::Status status_;
+};
+
+template <class Q>
+Q MakeQueue(int64_t num_elements) {
+  Q q;
+  for (int64_t i = 0; i < num_elements; i++) {
+    q.push_back(static_cast<int>(i));
+  }
+  return q;
+}
+
+void CustomArgs(benchmark::internal::Benchmark* b) {
+  b->Arg(1 << 4);
+  b->Arg(1 << 10);
+  b->Arg(1 << 17);
+}
+
+template <class Q>
+void BM_construct(benchmark::State& state) {
+  for (auto s : state) {
+    Q q;
+    benchmark::DoNotOptimize(q);
+  }
+}
+
+BENCHMARK_TEMPLATE(BM_construct, Deque<int64_t>);
+BENCHMARK_TEMPLATE(BM_construct, List<int64_t>);
+BENCHMARK_TEMPLATE(BM_construct, FwdList<int64_t>);
+BENCHMARK_TEMPLATE(BM_construct, Chunked<int64_t>);
+BENCHMARK_TEMPLATE(BM_construct, ExpChunked<int64_t>);
+BENCHMARK_TEMPLATE(BM_construct, Deque<Element>);
+BENCHMARK_TEMPLATE(BM_construct, List<Element>);
+BENCHMARK_TEMPLATE(BM_construct, FwdList<Element>);
+BENCHMARK_TEMPLATE(BM_construct, Chunked<Element>);
+BENCHMARK_TEMPLATE(BM_construct, ExpChunked<Element>);
+
+template <class Q>
+void BM_destroy(benchmark::State& state) {
+  const int64_t num_elements = state.range(0);
+
+  for (auto s : state) {
+    state.PauseTiming();
+    {
+      Q q = MakeQueue<Q>(num_elements);
+      benchmark::DoNotOptimize(q);
+      state.ResumeTiming();
+    }
+  }
+  state.SetItemsProcessed(state.iterations() * num_elements);
+}
+
+BENCHMARK_TEMPLATE(BM_destroy, Deque<int64_t>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_destroy, List<int64_t>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_destroy, FwdList<int64_t>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_destroy, Chunked<int64_t>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_destroy, ExpChunked<int64_t>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_destroy, Deque<Element>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_destroy, List<Element>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_destroy, FwdList<Element>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_destroy, Chunked<Element>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_destroy, ExpChunked<Element>)->Apply(CustomArgs);
+
+template <class Q>
+void BM_push_back(benchmark::State& state) {
+  const int64_t num_elements = state.range(0);
+
+  state.SetItemsProcessed(state.max_iterations * num_elements);
+  for (auto s : state) {
+    state.PauseTiming();
+    Q q;
+    state.ResumeTiming();
+    for (int j = 0; j < num_elements; j++) q.push_back(j);
+    benchmark::DoNotOptimize(q);
+  }
+}
+
+BENCHMARK_TEMPLATE(BM_push_back, Deque<int64_t>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_push_back, List<int64_t>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_push_back, FwdList<int64_t>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_push_back, Chunked<int64_t>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_push_back, ExpChunked<int64_t>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_push_back, Deque<Element>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_push_back, List<Element>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_push_back, FwdList<Element>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_push_back, Chunked<Element>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_push_back, ExpChunked<Element>)->Apply(CustomArgs);
+
+template <class Q>
+void BM_pop_front(benchmark::State& state) {
+  const int64_t num_elements = state.range(0);
+
+  state.SetItemsProcessed(state.max_iterations * num_elements);
+  for (auto s : state) {
+    state.PauseTiming();
+    Q q = MakeQueue<Q>(num_elements);
+    state.ResumeTiming();
+    for (int j = 0; j < num_elements; j++) q.pop_front();
+    benchmark::DoNotOptimize(q);
+  }
+}
+
+BENCHMARK_TEMPLATE(BM_pop_front, Deque<int64_t>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_pop_front, List<int64_t>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_pop_front, FwdList<int64_t>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_pop_front, Chunked<int64_t>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_pop_front, ExpChunked<int64_t>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_pop_front, Deque<Element>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_pop_front, List<Element>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_pop_front, FwdList<Element>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_pop_front, Chunked<Element>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_pop_front, ExpChunked<Element>)->Apply(CustomArgs);
+
+template <class Q>
+void BM_clear(benchmark::State& state) {
+  const int64_t num_elements = state.range(0);
+
+  state.SetItemsProcessed(state.max_iterations * num_elements);
+  for (auto s : state) {
+    state.PauseTiming();
+    Q q = MakeQueue<Q>(num_elements);
+    state.ResumeTiming();
+    q.clear();
+    benchmark::DoNotOptimize(q);
+  }
+}
+
+BENCHMARK_TEMPLATE(BM_clear, Deque<int64_t>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_clear, List<int64_t>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_clear, FwdList<int64_t>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_clear, Chunked<int64_t>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_clear, ExpChunked<int64_t>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_clear, Deque<Element>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_clear, List<Element>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_clear, FwdList<Element>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_clear, Chunked<Element>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_clear, ExpChunked<Element>)->Apply(CustomArgs);
+
+template <class Q>
+void BM_iter(benchmark::State& state) {
+  const int64_t num_elements = state.range(0);
+
+  state.SetItemsProcessed(state.max_iterations * num_elements);
+  for (auto s : state) {
+    state.PauseTiming();
+    Q q = MakeQueue<Q>(state.max_iterations);
+    int sum = 0;
+    state.ResumeTiming();
+    for (const auto& v : q) sum += v;
+    benchmark::DoNotOptimize(sum);
+  }
+}
+
+BENCHMARK_TEMPLATE(BM_iter, Deque<int64_t>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_iter, List<int64_t>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_iter, FwdList<int64_t>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_iter, Chunked<int64_t>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_iter, ExpChunked<int64_t>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_iter, Deque<Element>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_iter, List<Element>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_iter, FwdList<Element>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_iter, Chunked<Element>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_iter, ExpChunked<Element>)->Apply(CustomArgs);
+
+template <class Q>
+void BM_resize_shrink(benchmark::State& state) {
+  const int64_t num_elements = state.range(0);
+
+  state.SetItemsProcessed(state.max_iterations * num_elements);
+  for (auto s : state) {
+    state.PauseTiming();
+    Q q = MakeQueue<Q>(num_elements * 2);
+    state.ResumeTiming();
+    q.resize(num_elements);
+    benchmark::DoNotOptimize(q);
+  }
+}
+
+// FwdList does not support resize.
+BENCHMARK_TEMPLATE(BM_resize_shrink, Deque<int64_t>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_resize_shrink, List<int64_t>)->Apply(CustomArgs);
+// BENCHMARK_TEMPLATE(BM_resize_shrink, FwdList<int64>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_resize_shrink, Chunked<int64_t>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_resize_shrink, ExpChunked<int64_t>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_resize_shrink, Deque<Element>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_resize_shrink, List<Element>)->Apply(CustomArgs);
+// BENCHMARK_TEMPLATE(BM_resize_shrink, FwdList<Element>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_resize_shrink, Chunked<Element>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_resize_shrink, ExpChunked<Element>)->Apply(CustomArgs);
+
+template <class Q>
+void BM_resize_grow(benchmark::State& state) {
+  const int64_t num_elements = state.range(0);
+
+  state.SetItemsProcessed(state.max_iterations * num_elements);
+  for (auto s : state) {
+    state.PauseTiming();
+    Q q = MakeQueue<Q>(num_elements);
+    state.ResumeTiming();
+    q.resize(static_cast<size_t>(num_elements) * 2);
+    benchmark::DoNotOptimize(q);
+  }
+}
+
+// FwdList does not support resize.
+BENCHMARK_TEMPLATE(BM_resize_grow, Deque<int64_t>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_resize_grow, List<int64_t>)->Apply(CustomArgs);
+// BENCHMARK_TEMPLATE(BM_resize_grow, FwdList<int64>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_resize_grow, Chunked<int64_t>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_resize_grow, ExpChunked<int64_t>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_resize_grow, Deque<Element>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_resize_grow, List<Element>)->Apply(CustomArgs);
+// BENCHMARK_TEMPLATE(BM_resize_grow, FwdList<Element>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_resize_grow, Chunked<Element>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_resize_grow, ExpChunked<Element>)->Apply(CustomArgs);
+
+template <class Q>
+void BM_assign_shrink(benchmark::State& state) {
+  const int64_t num_elements = state.range(0);
+
+  state.SetItemsProcessed(state.max_iterations * num_elements);
+  for (auto s : state) {
+    state.PauseTiming();
+    const Q src = MakeQueue<Q>(num_elements);
+    Q dst = MakeQueue<Q>(num_elements * 2);
+    state.ResumeTiming();
+    dst = src;
+    benchmark::DoNotOptimize(dst);
+  }
+}
+
+BENCHMARK_TEMPLATE(BM_assign_shrink, Deque<int64_t>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_assign_shrink, List<int64_t>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_assign_shrink, FwdList<int64_t>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_assign_shrink, Chunked<int64_t>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_assign_shrink, ExpChunked<int64_t>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_assign_shrink, Deque<Element>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_assign_shrink, List<Element>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_assign_shrink, FwdList<Element>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_assign_shrink, Chunked<Element>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_assign_shrink, ExpChunked<Element>)->Apply(CustomArgs);
+
+template <class Q>
+void BM_assign_grow(benchmark::State& state) {
+  const int64_t num_elements = state.range(0);
+
+  state.SetItemsProcessed(state.max_iterations * num_elements);
+  for (auto s : state) {
+    state.PauseTiming();
+    const Q src = MakeQueue<Q>(num_elements * 2);
+    Q dst = MakeQueue<Q>(num_elements);
+    state.ResumeTiming();
+    dst = src;
+    benchmark::DoNotOptimize(dst);
+  }
+}
+
+BENCHMARK_TEMPLATE(BM_assign_grow, Deque<int64_t>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_assign_grow, List<int64_t>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_assign_grow, FwdList<int64_t>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_assign_grow, Chunked<int64_t>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_assign_grow, ExpChunked<int64_t>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_assign_grow, Deque<Element>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_assign_grow, List<Element>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_assign_grow, FwdList<Element>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_assign_grow, Chunked<Element>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_assign_grow, ExpChunked<Element>)->Apply(CustomArgs);
+
+template <class Q>
+void BM_push_pop(benchmark::State& state) {
+  const int64_t num_elements = state.range(0);
+
+  state.SetItemsProcessed(state.max_iterations * num_elements);
+
+  std::mt19937 rnd;
+  for (auto s : state) {
+    state.PauseTiming();
+    Q q;
+    state.ResumeTiming();
+    for (int j = 0; j < num_elements; j++) {
+      if (q.empty() || absl::Bernoulli(rnd, 0.5)) {
+        q.push_back(state.iterations());
+      } else {
+        q.pop_front();
+      }
+    }
+    benchmark::DoNotOptimize(q);
+  }
+}
+
+BENCHMARK_TEMPLATE(BM_push_pop, Deque<int64_t>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_push_pop, List<int64_t>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_push_pop, FwdList<int64_t>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_push_pop, Chunked<int64_t>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_push_pop, ExpChunked<int64_t>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_push_pop, Deque<Element>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_push_pop, List<Element>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_push_pop, FwdList<Element>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_push_pop, Chunked<Element>)->Apply(CustomArgs);
+BENCHMARK_TEMPLATE(BM_push_pop, ExpChunked<Element>)->Apply(CustomArgs);
+
+}  // namespace
diff --git a/absl/container/chunked_queue_test.cc b/absl/container/chunked_queue_test.cc
new file mode 100644
index 00000000000..22ce9442786
--- /dev/null
+++ b/absl/container/chunked_queue_test.cc
@@ -0,0 +1,869 @@
+// Copyright 2025 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/container/chunked_queue.h"
+
+#include <algorithm>
+#include <cstddef>
+#include <cstdint>
+#include <deque>
+#include <forward_list>
+#include <iterator>
+#include <limits>
+#include <list>
+#include <memory>
+#include <new>
+#include <string>
+#include <type_traits>
+#include <utility>
+#include <vector>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/macros.h"
+#include "absl/base/throw_delegate.h"
+#include "absl/container/internal/test_allocator.h"
+#include "absl/strings/str_cat.h"
+
+using ::testing::ElementsAre;
+using ::testing::Pair;
+using ::testing::Pointee;
+using ::testing::SizeIs;
+
+// Hide in a namespace to make sure swap is found via ADL.
+namespace adl_namespace {
+namespace {
+TEST(ChunkedQueueADLTest, Swap) {
+  absl::chunked_queue<int64_t> q1;
+  absl::chunked_queue<int64_t> q2;
+  q1.push_back(4);
+  q2.push_back(5);
+  q2.push_back(6);
+  swap(q1, q2);
+  EXPECT_THAT(q1, ElementsAre(5, 6));
+  EXPECT_THAT(q2, ElementsAre(4));
+}
+}  // namespace
+}  // namespace adl_namespace
+
+namespace {
+
+template <class T>
+using ChunkedQueueBlock =
+    absl::container_internal::ChunkedQueueBlock<T, std::allocator<T>>;
+
+TEST(Internal, elements_in_bytes) {
+  EXPECT_EQ(size_t{1}, ChunkedQueueBlock<int>::block_size_from_bytes(0));
+  EXPECT_EQ(size_t{1}, ChunkedQueueBlock<int>::block_size_from_bytes(
+                           sizeof(ChunkedQueueBlock<int>)));
+  EXPECT_EQ(size_t{1},
+            ChunkedQueueBlock<int>::block_size_from_bytes(sizeof(int)));
+  EXPECT_EQ(size_t{2}, ChunkedQueueBlock<int>::block_size_from_bytes(
+                           sizeof(ChunkedQueueBlock<int>) + 2 * sizeof(int)));
+}
+
+TEST(Internal, BlockSizedDelete) {
+  struct Item {
+    int i;
+    char c;
+  };
+  std::allocator<Item> allocator;
+  auto* block = ChunkedQueueBlock<Item>::New(3, &allocator);
+  ChunkedQueueBlock<Item>::Delete(block, &allocator);
+}
+
+template <size_t elem_size>
+void BlockSizeRounding() {
+  struct Elem {
+    char data[elem_size];
+  };
+  typedef ChunkedQueueBlock<Elem> Block;
+  for (size_t n = 1; n < 100; ++n) {
+    SCOPED_TRACE(n);
+    std::allocator<Elem> allocator;
+    Block* b = Block::New(n, &allocator);
+    EXPECT_GE(b->size(), n);
+    Block::Delete(b, &allocator);
+  }
+}
+
+TEST(Internal, BlockSizeRounding1) { BlockSizeRounding<1>(); }
+TEST(Internal, BlockSizeRounding17) { BlockSizeRounding<17>(); }
+TEST(Internal, BlockSizeRounding101) { BlockSizeRounding<101>(); }
+TEST(Internal, BlockSizeRounding528) { BlockSizeRounding<528>(); }
+
+TEST(ChunkedQueue, MinMaxBlockSize) {
+  absl::chunked_queue<int64_t, 1, 2> q = {1, 2, 3};
+  EXPECT_THAT(q, ElementsAre(1, 2, 3));
+}
+
+TEST(ChunkedQueue, Empty) {
+  absl::chunked_queue<int64_t> q;
+  EXPECT_TRUE(q.empty());
+  q.push_back(10);
+  EXPECT_FALSE(q.empty());
+  EXPECT_EQ(q.front(), 10);
+  EXPECT_EQ(q.back(), 10);
+  q.pop_front();
+  EXPECT_TRUE(q.empty());
+  q.clear();
+  EXPECT_TRUE(q.empty());
+}
+
+TEST(ChunkedQueue, CopyConstruct) {
+  absl::chunked_queue<int64_t> q;
+  q.push_back(1);
+  absl::chunked_queue<int64_t> r(q);
+  EXPECT_THAT(r, ElementsAre(1));
+  EXPECT_EQ(1, r.size());
+}
+
+TEST(ChunkedQueue, CopyConstructMultipleChunks) {
+  absl::chunked_queue<int64_t, 2> q;
+  q.push_back(1);
+  q.push_back(2);
+  q.push_back(3);
+  absl::chunked_queue<int64_t, 2> r(q);
+  EXPECT_THAT(r, ElementsAre(1, 2, 3));
+  EXPECT_EQ(3, r.size());
+}
+
+TEST(ChunkedQueue, BeginEndConstruct) {
+  std::vector<int64_t> src = {1, 2, 3, 4, 5};
+  absl::chunked_queue<int64_t, 2> q(src.begin(), src.end());
+  EXPECT_THAT(q, ElementsAre(1, 2, 3, 4, 5));
+  EXPECT_EQ(5, q.size());
+}
+
+TEST(ChunkedQueue, InitializerListConstruct) {
+  absl::chunked_queue<int64_t, 2> q = {1, 2, 3, 4, 5};
+  EXPECT_THAT(q, ElementsAre(1, 2, 3, 4, 5));
+  EXPECT_EQ(5, q.size());
+}
+
+TEST(ChunkedQueue, CountConstruct) {
+  absl::chunked_queue<int64_t> q(3);
+  EXPECT_THAT(q, ElementsAre(0, 0, 0));
+  EXPECT_EQ(3, q.size());
+}
+
+TEST(ChunkedQueue, CountValueConstruct) {
+  absl::chunked_queue<int64_t> q(3, 10);
+  EXPECT_THAT(q, ElementsAre(10, 10, 10));
+  EXPECT_EQ(3, q.size());
+}
+
+TEST(ChunkedQueue, InitializerListAssign) {
+  absl::chunked_queue<int64_t, 2> q;
+  q = {1, 2, 3, 4, 5};
+  EXPECT_THAT(q, ElementsAre(1, 2, 3, 4, 5));
+  EXPECT_EQ(5, q.size());
+}
+
+TEST(ChunkedQueue, CopyAssign) {
+  absl::chunked_queue<int64_t> q;
+  q.push_back(1);
+  absl::chunked_queue<int64_t> r = q;
+  EXPECT_THAT(r, ElementsAre(1));
+}
+
+TEST(ChunkedQueue, CopyAssignSelf) {
+  absl::chunked_queue<int64_t> q;
+  q.push_back(1);
+  q = *&q;  // Avoid -Wself-assign.
+  EXPECT_THAT(q, ElementsAre(1));
+  EXPECT_EQ(1, q.size());
+}
+
+TEST(ChunkedQueue, CopyAssignDestinationBigger) {
+  absl::chunked_queue<int64_t> q;
+  q.push_back(1);
+  absl::chunked_queue<int64_t> r;
+  r.push_back(9);
+  r.push_back(9);
+  r.push_back(9);
+  r = q;
+  EXPECT_THAT(r, ElementsAre(1));
+  EXPECT_EQ(1, r.size());
+}
+
+TEST(ChunkedQueue, CopyAssignSourceBiggerMultipleChunks) {
+  absl::chunked_queue<int64_t, 2> q;
+  q.push_back(1);
+  q.push_back(2);
+  q.push_back(3);
+  absl::chunked_queue<int64_t, 2> r;
+  r.push_back(9);
+  r = q;
+  EXPECT_THAT(r, ElementsAre(1, 2, 3));
+  EXPECT_EQ(3, r.size());
+}
+
+TEST(ChunkedQueue, CopyAssignDestinationBiggerMultipleChunks) {
+  absl::chunked_queue<int64_t, 2> q;
+  q.push_back(1);
+  absl::chunked_queue<int64_t, 2> r;
+  r.push_back(9);
+  r.push_back(9);
+  r.push_back(9);
+  r = q;
+  EXPECT_THAT(r, ElementsAre(1));
+  EXPECT_EQ(1, r.size());
+}
+
+TEST(ChunkedQueue, AssignCountValue) {
+  absl::chunked_queue<int64_t> q;
+  q.assign(3, 10);
+  EXPECT_THAT(q, ElementsAre(10, 10, 10));
+  EXPECT_EQ(3, q.size());
+
+  q.assign(2, 20);
+  EXPECT_THAT(q, ElementsAre(20, 20));
+  EXPECT_EQ(2, q.size());
+}
+
+TEST(ChunkedQueue, MoveConstruct) {
+  absl::chunked_queue<int64_t> q;
+  q.push_back(1);
+  absl::chunked_queue<int64_t> r(std::move(q));
+  EXPECT_THAT(r, ElementsAre(1));
+  EXPECT_EQ(1, r.size());
+}
+
+TEST(ChunkedQueue, MoveAssign) {
+  absl::chunked_queue<int64_t> q;
+  q.push_back(1);
+  absl::chunked_queue<int64_t> r;
+  r = std::move(q);
+  EXPECT_THAT(r, ElementsAre(1));
+  EXPECT_EQ(1, r.size());
+}
+
+TEST(ChunkedQueue, MoveAssignImmovable) {
+  struct Immovable {
+    Immovable() = default;
+
+    Immovable(const Immovable&) = delete;
+    Immovable& operator=(const Immovable&) = delete;
+    Immovable(Immovable&&) = delete;
+    Immovable& operator=(Immovable&&) = delete;
+  };
+  absl::chunked_queue<Immovable> q;
+  q.emplace_back();
+  absl::chunked_queue<Immovable> r;
+  r = std::move(q);
+  EXPECT_THAT(r, SizeIs(1));
+}
+
+TEST(ChunkedQueue, MoveAssignSelf) {
+  absl::chunked_queue<int64_t> q;
+  absl::chunked_queue<int64_t>& q2 = q;
+  q.push_back(1);
+  q = std::move(q2);
+  EXPECT_THAT(q, ElementsAre(1));
+  EXPECT_EQ(1, q.size());
+}
+
+TEST(ChunkedQueue, MoveAssignDestinationBigger) {
+  absl::chunked_queue<int64_t> q;
+  q.push_back(1);
+  absl::chunked_queue<int64_t> r;
+  r.push_back(9);
+  r.push_back(9);
+  r.push_back(9);
+  r = std::move(q);
+  EXPECT_THAT(r, ElementsAre(1));
+  EXPECT_EQ(1, r.size());
+}
+
+TEST(ChunkedQueue, MoveAssignDestinationBiggerMultipleChunks) {
+  absl::chunked_queue<int64_t, 2> q;
+  q.push_back(1);
+  absl::chunked_queue<int64_t, 2> r;
+  r.push_back(9);
+  r.push_back(9);
+  r.push_back(9);
+  r = std::move(q);
+  EXPECT_THAT(r, ElementsAre(1));
+  EXPECT_EQ(1, r.size());
+}
+
+TEST(ChunkedQueue, ConstFrontBack) {
+  absl::chunked_queue<int64_t> q;
+  q.push_back(10);
+  EXPECT_EQ(q.front(), 10);
+  EXPECT_EQ(q.back(), 10);
+  q.front() = 12;
+  EXPECT_EQ(q.front(), 12);
+  EXPECT_EQ(q.back(), 12);
+
+  const absl::chunked_queue<int64_t>& qref = q;
+  EXPECT_EQ(qref.front(), 12);
+  EXPECT_EQ(qref.back(), 12);
+
+  q.pop_front();
+
+  // Test at block bloundary and beyond
+  for (int i = 0; i < 64; ++i) q.push_back(i + 10);
+  EXPECT_EQ(q.front(), 10);
+  EXPECT_EQ(q.back(), 73);
+
+  for (int i = 64; i < 128; ++i) q.push_back(i + 10);
+  EXPECT_EQ(q.front(), 10);
+  EXPECT_EQ(q.back(), 137);
+  q.clear();
+  EXPECT_TRUE(q.empty());
+}
+
+TEST(ChunkedQueue, PushAndPop) {
+  absl::chunked_queue<int64_t> q;
+  EXPECT_TRUE(q.empty());
+  EXPECT_EQ(0, q.size());
+  for (int i = 0; i < 10000; i++) {
+    q.push_back(i);
+    EXPECT_EQ(q.front(), 0) << ": iteration " << i;
+    EXPECT_FALSE(q.empty());
+    EXPECT_EQ(i + 1, q.size());
+  }
+  for (int i = 0; i < 10000; i++) {
+    EXPECT_FALSE(q.empty());
+    EXPECT_EQ(10000 - i, q.size());
+    EXPECT_EQ(q.front(), i);
+    q.pop_front();
+  }
+  EXPECT_TRUE(q.empty());
+  EXPECT_EQ(0, q.size());
+}
+
+TEST(ChunkedQueue, Swap) {
+  absl::chunked_queue<int64_t> q1;
+  absl::chunked_queue<int64_t> q2;
+  q1.push_back(4);
+  q2.push_back(5);
+  q2.push_back(6);
+  q2.swap(q1);
+  EXPECT_EQ(2, q1.size());
+  EXPECT_EQ(5, q1.front());
+  EXPECT_EQ(1, q2.size());
+  EXPECT_EQ(4, q2.front());
+  q1.pop_front();
+  q1.swap(q2);
+  EXPECT_EQ(1, q1.size());
+  EXPECT_EQ(4, q1.front());
+  EXPECT_EQ(1, q2.size());
+  EXPECT_EQ(6, q2.front());
+  q1.pop_front();
+  q1.swap(q2);
+  EXPECT_EQ(1, q1.size());
+  EXPECT_EQ(6, q1.front());
+  EXPECT_EQ(0, q2.size());
+  q1.clear();
+  EXPECT_TRUE(q1.empty());
+}
+
+TEST(ChunkedQueue, ShrinkToFit) {
+  absl::chunked_queue<int64_t> q;
+  q.shrink_to_fit();  // Should work on empty
+  EXPECT_TRUE(q.empty());
+
+  q.push_back(1);
+  q.shrink_to_fit();  // Should work on non-empty
+  EXPECT_THAT(q, ElementsAre(1));
+
+  q.clear();
+  // We know clear leaves a block and shrink_to_fit should remove it.
+  // Hard to test internal memory state without mocks or inspection.
+  // But at least we verify it doesn't crash or corrupt.
+  q.shrink_to_fit();
+  EXPECT_TRUE(q.empty());
+}
+
+TEST(ChunkedQueue, ResizeExtends) {
+  absl::chunked_queue<int64_t> q;
+  q.resize(2);
+  EXPECT_THAT(q, ElementsAre(0, 0));
+  EXPECT_EQ(2, q.size());
+}
+
+TEST(ChunkedQueue, ResizeShrinks) {
+  absl::chunked_queue<int64_t> q;
+  q.push_back(1);
+  q.push_back(2);
+  q.resize(1);
+  EXPECT_THAT(q, ElementsAre(1));
+  EXPECT_EQ(1, q.size());
+}
+
+TEST(ChunkedQueue, ResizeExtendsMultipleBlocks) {
+  absl::chunked_queue<int64_t, 2> q;
+  q.resize(3);
+  EXPECT_THAT(q, ElementsAre(0, 0, 0));
+  EXPECT_EQ(3, q.size());
+}
+
+TEST(ChunkedQueue, ResizeShrinksMultipleBlocks) {
+  absl::chunked_queue<int64_t, 2> q;
+  q.push_back(1);
+  q.push_back(2);
+  q.push_back(3);
+  q.resize(1);
+  EXPECT_THAT(q, ElementsAre(1));
+  EXPECT_EQ(1, q.size());
+}
+
+TEST(ChunkedQueue, ResizeValue) {
+  absl::chunked_queue<int64_t> q;
+  q.resize(3, 10);
+  EXPECT_THAT(q, ElementsAre(10, 10, 10));
+  EXPECT_EQ(3, q.size());
+
+  q.resize(5, 20);
+  EXPECT_THAT(q, ElementsAre(10, 10, 10, 20, 20));
+  EXPECT_EQ(5, q.size());
+
+  q.resize(2, 30);
+  EXPECT_THAT(q, ElementsAre(10, 10));
+  EXPECT_EQ(2, q.size());
+}
+
+template <class T>
+struct LimitedAllocator {
+  using value_type = T;
+  int* alloc_count;
+  int max_allocs;
+
+  explicit LimitedAllocator(int* count, int max)
+      : alloc_count(count), max_allocs(max) {}
+  template <class U>
+  LimitedAllocator(const LimitedAllocator<U>& other)
+      : alloc_count(other.alloc_count), max_allocs(other.max_allocs) {}
+
+  T* allocate(size_t n) {
+    if (*alloc_count >= max_allocs) {
+      absl::ThrowStdBadAlloc();
+    }
+    ++*alloc_count;
+    return std::allocator<T>().allocate(n);
+  }
+
+  void deallocate(T* p, size_t n) {
+    std::allocator<T>().deallocate(p, n);
+  }
+
+  template <class U>
+  bool operator==(const LimitedAllocator<U>& other) const {
+    return alloc_count == other.alloc_count;
+  }
+  template <class U>
+  bool operator!=(const LimitedAllocator<U>& other) const {
+    return !(*this == other);
+  }
+};
+
+TEST(ChunkedQueue, ResizeOverflowSafe) {
+  int alloc_count = 0;
+  absl::chunked_queue<int64_t, 0, 0, LimitedAllocator<int64_t>> q(
+      LimitedAllocator<int64_t>(&alloc_count, 5));
+#ifdef ABSL_HAVE_EXCEPTIONS
+  EXPECT_THROW(q.resize(std::numeric_limits<size_t>::max()), std::bad_alloc);
+#else
+  EXPECT_DEATH_IF_SUPPORTED(q.resize(std::numeric_limits<size_t>::max()), "");
+#endif
+}
+
+TEST(ChunkedQueue, MaxSize) {
+  absl::chunked_queue<int64_t> q;
+  EXPECT_GE(q.max_size(),
+            size_t{1} << (sizeof(size_t) * 8 - sizeof(int64_t) - 4));
+}
+
+TEST(ChunkedQueue, AssignExtends) {
+  absl::chunked_queue<int64_t, 2> q;
+  std::vector<int64_t> v = {1, 2, 3, 4, 5};
+  q.assign(v.begin(), v.end());
+  EXPECT_THAT(q, ElementsAre(1, 2, 3, 4, 5));
+  EXPECT_EQ(5, q.size());
+}
+
+TEST(ChunkedQueue, AssignShrinks) {
+  absl::chunked_queue<int64_t, 2> q = {1, 2, 3, 4, 5};
+  std::vector<int64_t> v = {1};
+  q.assign(v.begin(), v.end());
+  EXPECT_THAT(q, ElementsAre(1));
+  EXPECT_EQ(1, q.size());
+}
+
+TEST(ChunkedQueue, AssignBoundaryCondition) {
+  // Create a queue with fixed block size of 4.
+  // 3 blocks: [1, 2, 3, 4], [5, 6, 7, 8], [9, 10, 11, 12]
+  absl::chunked_queue<int, 4> q = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12};
+
+  // Assign a range that fills exactly the first block (4 elements).
+  // This triggers the boundary condition where the assignment loop ends
+  // exactly at the limit of the first block.
+  std::vector<int> v = {101, 102, 103, 104};
+  q.assign(v.begin(), v.end());
+
+  EXPECT_EQ(q.size(), 4);
+  EXPECT_EQ(q.front(), 101);
+  // Verify back() is valid. If tail_ was incorrectly pointing to the start
+  // of the (now deleted) second block, this might access invalid memory
+  // or fail assertions.
+  EXPECT_EQ(q.back(), 104);
+
+  // Verify we can continue to push elements correctly.
+  q.push_back(105);
+  EXPECT_EQ(q.size(), 5);
+  EXPECT_EQ(q.back(), 105);
+}
+
+TEST(ChunkedQueue, Iterator) {
+  absl::chunked_queue<int64_t> q;
+  EXPECT_TRUE(q.begin() == q.end());
+
+  q.push_back(1);
+  absl::chunked_queue<int64_t>::const_iterator iter = q.begin();
+  ASSERT_FALSE(iter == q.end());
+  ASSERT_EQ(*iter, 1);
+  ++iter;
+  ASSERT_TRUE(iter == q.end());
+
+  q.push_back(2);
+  iter = q.begin();
+  ASSERT_EQ(*iter, 1);
+  ++iter;
+  absl::chunked_queue<int64_t>::const_iterator copy_iter = iter;
+  ASSERT_FALSE(copy_iter == q.end());
+  ASSERT_EQ(*copy_iter, 2);
+  ++copy_iter;
+  ASSERT_TRUE(copy_iter == q.end());
+
+  copy_iter = iter;
+  ASSERT_FALSE(iter == q.end());
+  ASSERT_EQ(*iter, 2);
+  ++iter;
+  ASSERT_TRUE(iter == q.end());
+
+  ASSERT_FALSE(copy_iter == q.end());
+  ASSERT_EQ(*copy_iter, 2);
+  ++copy_iter;
+  ASSERT_TRUE(copy_iter == q.end());
+}
+
+TEST(ChunkedQueue, IteratorDefaultConstructor) {
+  using ConstIter = absl::chunked_queue<int64_t>::const_iterator;
+  using Iter = absl::chunked_queue<int64_t>::iterator;
+  ConstIter const_iter;
+  EXPECT_TRUE(const_iter == ConstIter());
+  Iter iter;
+  EXPECT_TRUE(iter == Iter());
+}
+
+TEST(ChunkedQueue, IteratorConversion) {
+  using ConstIter = absl::chunked_queue<int64_t>::const_iterator;
+  using Iter = absl::chunked_queue<int64_t>::iterator;
+  EXPECT_FALSE((std::is_convertible_v<ConstIter, Iter>));
+  EXPECT_TRUE((std::is_convertible_v<Iter, ConstIter>));
+  absl::chunked_queue<int64_t> q;
+  ConstIter it1 = q.begin();
+  ConstIter it2 = q.cbegin();
+  Iter it3 = q.begin();
+  it1 = q.end();
+  it2 = q.cend();
+  it3 = q.end();
+  EXPECT_FALSE((std::is_assignable_v<Iter, ConstIter>));
+}
+
+struct TestEntry {
+  int x, y;
+};
+
+TEST(ChunkedQueue, Iterator2) {
+  absl::chunked_queue<TestEntry> q;
+  TestEntry e;
+  e.x = 1;
+  e.y = 2;
+  q.push_back(e);
+  e.x = 3;
+  e.y = 4;
+  q.push_back(e);
+
+  absl::chunked_queue<TestEntry>::const_iterator iter = q.begin();
+  EXPECT_EQ(iter->x, 1);
+  EXPECT_EQ(iter->y, 2);
+  ++iter;
+  EXPECT_EQ(iter->x, 3);
+  EXPECT_EQ(iter->y, 4);
+  ++iter;
+  EXPECT_TRUE(iter == q.end());
+}
+
+TEST(ChunkedQueue, Iterator_MultipleBlocks) {
+  absl::chunked_queue<int64_t> q;
+  for (int i = 0; i < 130; ++i) {
+    absl::chunked_queue<int64_t>::const_iterator iter = q.begin();
+    for (int j = 0; j < i; ++j) {
+      ASSERT_FALSE(iter == q.end());
+      EXPECT_EQ(*iter, j);
+      ++iter;
+    }
+    ASSERT_TRUE(iter == q.end());
+    q.push_back(i);
+  }
+
+  for (int i = 0; i < 130; ++i) {
+    absl::chunked_queue<int64_t>::const_iterator iter = q.begin();
+    for (int j = i; j < 130; ++j) {
+      ASSERT_FALSE(iter == q.end());
+      EXPECT_EQ(*iter, j);
+      ++iter;
+    }
+    q.pop_front();
+  }
+  EXPECT_TRUE(q.empty());
+  EXPECT_TRUE(q.begin() == q.end());
+}
+
+TEST(ChunkedQueue, Iterator_PopFrontInvalidate) {
+  absl::chunked_queue<int64_t> q;
+  for (int i = 0; i < 130; ++i) {
+    q.push_back(i);
+  }
+
+  auto iter = q.begin();
+  for (int i = 0; i < 130; ++i) {
+    auto prev = iter++;
+    ASSERT_FALSE(prev == q.end());
+    EXPECT_EQ(*prev, i);
+    q.pop_front();
+  }
+  ASSERT_TRUE(q.empty());
+}
+
+TEST(ChunkedQueue, Iterator_PushBackInvalidate) {
+  absl::chunked_queue<int64_t, 2> q;
+  q.push_back(0);
+  auto i = q.begin();
+  EXPECT_EQ(*i, 0);
+  q.push_back(1);
+  EXPECT_EQ(*++i, 1);
+  q.push_back(2);
+  EXPECT_EQ(*++i, 2);
+}
+
+struct MyType {
+  static int constructor_calls;
+  static int destructor_calls;
+
+  explicit MyType(int x) : val(x) { constructor_calls++; }
+  MyType(const MyType& t) : val(t.val) { constructor_calls++; }
+  ~MyType() { destructor_calls++; }
+
+  int val;
+};
+
+int MyType::constructor_calls = 0;
+int MyType::destructor_calls = 0;
+
+TEST(ChunkedQueue, ConstructorDestructorCalls) {
+  for (int i = 0; i < 100; i++) {
+    std::vector<MyType> vals;
+    for (int j = 0; j < i; j++) {
+      vals.push_back(MyType(j));
+    }
+    MyType::constructor_calls = 0;
+    MyType::destructor_calls = 0;
+    {
+      absl::chunked_queue<MyType> q;
+      for (int j = 0; j < i; j++) {
+        q.push_back(vals[j]);
+      }
+      if (i % 10 == 0) {
+        q.clear();
+      } else {
+        for (int j = 0; j < i; j++) {
+          EXPECT_EQ(q.front().val, j);
+          q.pop_front();
+        }
+      }
+    }
+    EXPECT_EQ(MyType::constructor_calls, i);
+    EXPECT_EQ(MyType::destructor_calls, i);
+  }
+}
+
+TEST(ChunkedQueue, MoveObjects) {
+  absl::chunked_queue<std::unique_ptr<int>> q;
+  q.push_back(std::make_unique<int>(10));
+  q.push_back(std::make_unique<int>(11));
+
+  EXPECT_EQ(10, *q.front());
+  q.pop_front();
+  EXPECT_EQ(11, *q.front());
+  q.pop_front();
+}
+
+TEST(ChunkedQueue, EmplaceBack1) {
+  absl::chunked_queue<std::pair<int, int>> q;
+  auto& v = q.emplace_back(1, 2);
+  EXPECT_THAT(v, Pair(1, 2));
+  EXPECT_THAT(q.front(), Pair(1, 2));
+  EXPECT_EQ(&v, &q.back());
+}
+
+TEST(ChunkedQueue, EmplaceBack2) {
+  absl::chunked_queue<std::pair<std::unique_ptr<int>, std::string>> q;
+  auto& v = q.emplace_back(std::make_unique<int>(11), "val12");
+  EXPECT_THAT(v, Pair(Pointee(11), "val12"));
+  EXPECT_THAT(q.front(), Pair(Pointee(11), "val12"));
+}
+
+TEST(ChunkedQueue, OveralignmentEmplaceBack) {
+  struct alignas(64) Overaligned {
+    int x;
+    int y;
+  };
+  absl::chunked_queue<Overaligned, 1, 8> q;
+  for (int i = 0; i < 10; ++i) {
+    auto& v = q.emplace_back(Overaligned{i, i});
+    EXPECT_EQ(reinterpret_cast<uintptr_t>(&v) % 64, 0);
+  }
+}
+
+TEST(ChunkedQueue, StatelessAllocatorDoesntAffectObjectSizes) {
+  // When a stateless allocator type is used -- such as when no explicit
+  // allocator type is given, and the stateless default is used -- it does not
+  // increase the object sizes from what they used to be before allocator
+  // support was added.  (In practice this verifies that allocator support makes
+  // use of the empty base-class optimization.)
+  //
+  // These "Mock*" structs model the data members of absl::chunked_queue<> and
+  // its internal ChunkedQueueBlock<> type, without any extra storage for
+  // allocator state.  (We use these to generate expected stateless-allocator
+  // object sizes in a portable way.)
+  struct MockQueue {
+    struct MockIterator {
+      void* block;
+      void* ptr;
+      void* limit;
+    };
+    MockIterator head;
+    MockIterator tail;
+    size_t size;
+  };
+  struct MockBlock {
+    void* next;
+    void* limit;
+  };
+  using TestQueueType = absl::chunked_queue<int64_t, 1, 16>;
+  EXPECT_EQ(sizeof(TestQueueType), sizeof(MockQueue));
+  EXPECT_EQ(sizeof(absl::container_internal::ChunkedQueueBlock<
+                   TestQueueType::value_type, TestQueueType::allocator_type>),
+            sizeof(MockBlock));
+}
+
+TEST(ChunkedQueue, DoesNotRoundBlockSizesUpWithNonDefaultAllocator) {
+  using OneByte = uint8_t;
+  using CustomAllocator = absl::container_internal::CountingAllocator<OneByte>;
+  using Block =
+      absl::container_internal::ChunkedQueueBlock<OneByte, CustomAllocator>;
+  int64_t allocator_live_bytes = 0;
+  CustomAllocator allocator(&allocator_live_bytes);
+  // Create a Block big enough to accomodate at least 1 OneByte.
+  Block* b = Block::New(1, &allocator);
+  ASSERT_TRUE(b != nullptr);
+  // With a non-default allocator in play, the resulting block should have
+  // capacity for exactly 1 element -- the implementation should not round the
+  // allocation size up, which may be inappropriate for non-default allocators.
+  //
+  // (Note that we don't always round up even with the default allocator in use,
+  // e.g. when compiling for ASAN analysis.)
+  EXPECT_EQ(b->size(), 1);
+  Block::Delete(b, &allocator);
+}
+
+TEST(ChunkedQueue, Hardening) {
+  bool hardened = false;
+  ABSL_HARDENING_ASSERT([&hardened]() {
+    hardened = true;
+    return true;
+  }());
+  if (!hardened) {
+    GTEST_SKIP() << "Not a hardened build";
+  }
+
+  absl::chunked_queue<int> q;
+  EXPECT_DEATH_IF_SUPPORTED(q.front(), "");
+  EXPECT_DEATH_IF_SUPPORTED(q.back(), "");
+  EXPECT_DEATH_IF_SUPPORTED(q.pop_front(), "");
+
+  const absl::chunked_queue<int> cq;
+  EXPECT_DEATH_IF_SUPPORTED(cq.front(), "");
+  EXPECT_DEATH_IF_SUPPORTED(cq.back(), "");
+}
+
+#ifdef ABSL_HAVE_EXCEPTIONS
+struct ThrowingCtor {
+  int* ctor_count;
+  int* dtor_count;
+  int value;
+
+  explicit ThrowingCtor(int v, bool should_throw, int* c_count, int* d_count)
+      : ctor_count(c_count), dtor_count(d_count), value(v) {
+    if (should_throw) {
+      throw 0;
+    }
+    ++*ctor_count;
+  }
+  ThrowingCtor(const ThrowingCtor& other)
+      : ctor_count(other.ctor_count),
+        dtor_count(other.dtor_count),
+        value(other.value) {
+    ++*ctor_count;
+  }
+  ThrowingCtor(ThrowingCtor&& other) noexcept
+      : ctor_count(other.ctor_count),
+        dtor_count(other.dtor_count),
+        value(other.value) {
+    ++*ctor_count;
+  }
+  ~ThrowingCtor() { ++*dtor_count; }
+};
+
+TEST(ChunkedQueue, StrongExceptionSafetyEmplaceBack) {
+  int ctor_count = 0;
+  int dtor_count = 0;
+  absl::chunked_queue<ThrowingCtor> q;
+  q.emplace_back(10, false, &ctor_count, &dtor_count);
+  q.emplace_back(20, false, &ctor_count, &dtor_count);
+  ASSERT_EQ(q.size(), 2);
+  ASSERT_EQ(ctor_count, 2);
+  ASSERT_EQ(dtor_count, 0);
+
+  try {
+    q.emplace_back(30, true, &ctor_count, &dtor_count);
+  } catch (...) {
+  }
+
+  EXPECT_EQ(q.size(), 2);
+  EXPECT_EQ(ctor_count, 2);
+  EXPECT_EQ(dtor_count, 0);
+
+  q.clear();
+  EXPECT_EQ(q.size(), 0);
+  EXPECT_EQ(dtor_count, 2);
+}
+#endif
+
+}  // namespace
diff --git a/absl/container/fixed_array.h b/absl/container/fixed_array.h
index 70e94ad5e18..713c68528b2 100644
--- a/absl/container/fixed_array.h
+++ b/absl/container/fixed_array.h
@@ -41,15 +41,21 @@
 #include <type_traits>
 
 #include "absl/algorithm/algorithm.h"
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
 #include "absl/base/dynamic_annotations.h"
-#include "absl/base/internal/throw_delegate.h"
+#include "absl/base/internal/hardening.h"
+#include "absl/base/internal/iterator_traits.h"
 #include "absl/base/macros.h"
 #include "absl/base/optimization.h"
 #include "absl/base/port.h"
+#include "absl/base/throw_delegate.h"
 #include "absl/container/internal/compressed_tuple.h"
+#include "absl/hash/internal/weakly_mixed_integer.h"
 #include "absl/memory/memory.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 constexpr static auto kFixedArrayUseDefault = static_cast<size_t>(-1);
 
@@ -60,58 +66,49 @@ constexpr static auto kFixedArrayUseDefault = static_cast<size_t>(-1);
 // A `FixedArray` provides a run-time fixed-size array, allocating a small array
 // inline for efficiency.
 //
-// Most users should not specify an `inline_elements` argument and let
-// `FixedArray` automatically determine the number of elements
-// to store inline based on `sizeof(T)`. If `inline_elements` is specified, the
-// `FixedArray` implementation will use inline storage for arrays with a
-// length <= `inline_elements`.
+// Most users should not specify the `N` template parameter and let `FixedArray`
+// automatically determine the number of elements to store inline based on
+// `sizeof(T)`. If `N` is specified, the `FixedArray` implementation will use
+// inline storage for arrays with a length <= `N`.
 //
 // Note that a `FixedArray` constructed with a `size_type` argument will
 // default-initialize its values by leaving trivially constructible types
 // uninitialized (e.g. int, int[4], double), and others default-constructed.
 // This matches the behavior of c-style arrays and `std::array`, but not
 // `std::vector`.
-//
-// Note that `FixedArray` does not provide a public allocator; if it requires a
-// heap allocation, it will do so with global `::operator new[]()` and
-// `::operator delete[]()`, even if T provides class-scope overrides for these
-// operators.
 template <typename T, size_t N = kFixedArrayUseDefault,
           typename A = std::allocator<T>>
-class FixedArray {
-  static_assert(!std::is_array<T>::value || std::extent<T>::value > 0,
+class ABSL_ATTRIBUTE_WARN_UNUSED FixedArray {
+  static_assert(!std::is_array_v<T> || std::extent_v<T> > 0,
                 "Arrays with unknown bounds cannot be used with FixedArray.");
 
   static constexpr size_t kInlineBytesDefault = 256;
 
   using AllocatorTraits = std::allocator_traits<A>;
-  // std::iterator_traits isn't guaranteed to be SFINAE-friendly until C++17,
-  // but this seems to be mostly pedantic.
   template <typename Iterator>
-  using EnableIfForwardIterator = absl::enable_if_t<std::is_convertible<
-      typename std::iterator_traits<Iterator>::iterator_category,
-      std::forward_iterator_tag>::value>;
+  using EnableIfInputIterator =
+      std::enable_if_t<base_internal::IsAtLeastInputIterator<Iterator>::value>;
   static constexpr bool NoexceptCopyable() {
-    return std::is_nothrow_copy_constructible<StorageElement>::value &&
+    return std::is_nothrow_copy_constructible_v<StorageElement> &&
            absl::allocator_is_nothrow<allocator_type>::value;
   }
   static constexpr bool NoexceptMovable() {
-    return std::is_nothrow_move_constructible<StorageElement>::value &&
+    return std::is_nothrow_move_constructible_v<StorageElement> &&
            absl::allocator_is_nothrow<allocator_type>::value;
   }
   static constexpr bool DefaultConstructorIsNonTrivial() {
-    return !absl::is_trivially_default_constructible<StorageElement>::value;
+    return !std::is_trivially_default_constructible_v<StorageElement>;
   }
 
  public:
   using allocator_type = typename AllocatorTraits::allocator_type;
-  using value_type = typename allocator_type::value_type;
-  using pointer = typename allocator_type::pointer;
-  using const_pointer = typename allocator_type::const_pointer;
-  using reference = typename allocator_type::reference;
-  using const_reference = typename allocator_type::const_reference;
-  using size_type = typename allocator_type::size_type;
-  using difference_type = typename allocator_type::difference_type;
+  using value_type = typename AllocatorTraits::value_type;
+  using pointer = typename AllocatorTraits::pointer;
+  using const_pointer = typename AllocatorTraits::const_pointer;
+  using reference = value_type&;
+  using const_reference = const value_type&;
+  using size_type = typename AllocatorTraits::size_type;
+  using difference_type = typename AllocatorTraits::difference_type;
   using iterator = pointer;
   using const_iterator = const_pointer;
   using reverse_iterator = std::reverse_iterator<iterator>;
@@ -121,14 +118,20 @@ class FixedArray {
       (N == kFixedArrayUseDefault ? kInlineBytesDefault / sizeof(value_type)
                                   : static_cast<size_type>(N));
 
-  FixedArray(
-      const FixedArray& other,
-      const allocator_type& a = allocator_type()) noexcept(NoexceptCopyable())
+  FixedArray(const FixedArray& other) noexcept(NoexceptCopyable())
+      : FixedArray(other,
+                   AllocatorTraits::select_on_container_copy_construction(
+                       other.storage_.alloc())) {}
+
+  FixedArray(const FixedArray& other,
+             const allocator_type& a) noexcept(NoexceptCopyable())
       : FixedArray(other.begin(), other.end(), a) {}
 
-  FixedArray(
-      FixedArray&& other,
-      const allocator_type& a = allocator_type()) noexcept(NoexceptMovable())
+  FixedArray(FixedArray&& other) noexcept(NoexceptMovable())
+      : FixedArray(std::move(other), other.storage_.alloc()) {}
+
+  FixedArray(FixedArray&& other,
+             const allocator_type& a) noexcept(NoexceptMovable())
       : FixedArray(std::make_move_iterator(other.begin()),
                    std::make_move_iterator(other.end()), a) {}
 
@@ -157,8 +160,8 @@ class FixedArray {
 
   // Creates an array initialized with the elements from the input
   // range. The array's size will always be `std::distance(first, last)`.
-  // REQUIRES: Iterator must be a forward_iterator or better.
-  template <typename Iterator, EnableIfForwardIterator<Iterator>* = nullptr>
+  // REQUIRES: Iterator must be a input_iterator or better.
+  template <typename Iterator, EnableIfInputIterator<Iterator>* = nullptr>
   FixedArray(Iterator first, Iterator last,
              const allocator_type& a = allocator_type())
       : storage_(std::distance(first, last), a) {
@@ -204,46 +207,50 @@ class FixedArray {
   //
   // Returns a const T* pointer to elements of the `FixedArray`. This pointer
   // can be used to access (but not modify) the contained elements.
-  const_pointer data() const { return AsValueType(storage_.begin()); }
+  const_pointer data() const ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return AsValueType(storage_.begin());
+  }
 
   // Overload of FixedArray::data() to return a T* pointer to elements of the
   // fixed array. This pointer can be used to access and modify the contained
   // elements.
-  pointer data() { return AsValueType(storage_.begin()); }
+  pointer data() ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return AsValueType(storage_.begin());
+  }
 
   // FixedArray::operator[]
   //
   // Returns a reference the ith element of the fixed array.
   // REQUIRES: 0 <= i < size()
-  reference operator[](size_type i) {
-    assert(i < size());
+  reference operator[](size_type i) ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    absl::base_internal::HardeningAssertLT(i, size());
     return data()[i];
   }
 
   // Overload of FixedArray::operator()[] to return a const reference to the
   // ith element of the fixed array.
   // REQUIRES: 0 <= i < size()
-  const_reference operator[](size_type i) const {
-    assert(i < size());
+  const_reference operator[](size_type i) const ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    absl::base_internal::HardeningAssertLT(i, size());
     return data()[i];
   }
 
   // FixedArray::at
   //
-  // Bounds-checked access.  Returns a reference to the ith element of the
-  // fiexed array, or throws std::out_of_range
-  reference at(size_type i) {
+  // Bounds-checked access.  Returns a reference to the ith element of the fixed
+  // array, or throws std::out_of_range
+  reference at(size_type i) ABSL_ATTRIBUTE_LIFETIME_BOUND {
     if (ABSL_PREDICT_FALSE(i >= size())) {
-      base_internal::ThrowStdOutOfRange("FixedArray::at failed bounds check");
+      ThrowStdOutOfRange("FixedArray::at failed bounds check");
     }
     return data()[i];
   }
 
   // Overload of FixedArray::at() to return a const reference to the ith element
   // of the fixed array.
-  const_reference at(size_type i) const {
+  const_reference at(size_type i) const ABSL_ATTRIBUTE_LIFETIME_BOUND {
     if (ABSL_PREDICT_FALSE(i >= size())) {
-      base_internal::ThrowStdOutOfRange("FixedArray::at failed bounds check");
+      ThrowStdOutOfRange("FixedArray::at failed bounds check");
     }
     return data()[i];
   }
@@ -251,80 +258,104 @@ class FixedArray {
   // FixedArray::front()
   //
   // Returns a reference to the first element of the fixed array.
-  reference front() { return *begin(); }
+  reference front() ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    absl::base_internal::HardeningAssertNonEmpty(*this);
+    return data()[0];
+  }
 
   // Overload of FixedArray::front() to return a reference to the first element
   // of a fixed array of const values.
-  const_reference front() const { return *begin(); }
+  const_reference front() const ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    absl::base_internal::HardeningAssertNonEmpty(*this);
+    return data()[0];
+  }
 
   // FixedArray::back()
   //
   // Returns a reference to the last element of the fixed array.
-  reference back() { return *(end() - 1); }
+  reference back() ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    absl::base_internal::HardeningAssertNonEmpty(*this);
+    return data()[size() - 1];
+  }
 
   // Overload of FixedArray::back() to return a reference to the last element
   // of a fixed array of const values.
-  const_reference back() const { return *(end() - 1); }
+  const_reference back() const ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    absl::base_internal::HardeningAssertNonEmpty(*this);
+    return data()[size() - 1];
+  }
 
   // FixedArray::begin()
   //
   // Returns an iterator to the beginning of the fixed array.
-  iterator begin() { return data(); }
+  iterator begin() ABSL_ATTRIBUTE_LIFETIME_BOUND { return data(); }
 
   // Overload of FixedArray::begin() to return a const iterator to the
   // beginning of the fixed array.
-  const_iterator begin() const { return data(); }
+  const_iterator begin() const ABSL_ATTRIBUTE_LIFETIME_BOUND { return data(); }
 
   // FixedArray::cbegin()
   //
   // Returns a const iterator to the beginning of the fixed array.
-  const_iterator cbegin() const { return begin(); }
+  const_iterator cbegin() const ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return begin();
+  }
 
   // FixedArray::end()
   //
   // Returns an iterator to the end of the fixed array.
-  iterator end() { return data() + size(); }
+  iterator end() ABSL_ATTRIBUTE_LIFETIME_BOUND { return data() + size(); }
 
   // Overload of FixedArray::end() to return a const iterator to the end of the
   // fixed array.
-  const_iterator end() const { return data() + size(); }
+  const_iterator end() const ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return data() + size();
+  }
 
   // FixedArray::cend()
   //
   // Returns a const iterator to the end of the fixed array.
-  const_iterator cend() const { return end(); }
+  const_iterator cend() const ABSL_ATTRIBUTE_LIFETIME_BOUND { return end(); }
 
   // FixedArray::rbegin()
   //
   // Returns a reverse iterator from the end of the fixed array.
-  reverse_iterator rbegin() { return reverse_iterator(end()); }
+  reverse_iterator rbegin() ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return reverse_iterator(end());
+  }
 
   // Overload of FixedArray::rbegin() to return a const reverse iterator from
   // the end of the fixed array.
-  const_reverse_iterator rbegin() const {
+  const_reverse_iterator rbegin() const ABSL_ATTRIBUTE_LIFETIME_BOUND {
     return const_reverse_iterator(end());
   }
 
   // FixedArray::crbegin()
   //
   // Returns a const reverse iterator from the end of the fixed array.
-  const_reverse_iterator crbegin() const { return rbegin(); }
+  const_reverse_iterator crbegin() const ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return rbegin();
+  }
 
   // FixedArray::rend()
   //
   // Returns a reverse iterator from the beginning of the fixed array.
-  reverse_iterator rend() { return reverse_iterator(begin()); }
+  reverse_iterator rend() ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return reverse_iterator(begin());
+  }
 
   // Overload of FixedArray::rend() for returning a const reverse iterator
   // from the beginning of the fixed array.
-  const_reverse_iterator rend() const {
+  const_reverse_iterator rend() const ABSL_ATTRIBUTE_LIFETIME_BOUND {
     return const_reverse_iterator(begin());
   }
 
   // FixedArray::crend()
   //
   // Returns a reverse iterator from the beginning of the fixed array.
-  const_reverse_iterator crend() const { return rend(); }
+  const_reverse_iterator crend() const ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return rend();
+  }
 
   // FixedArray::fill()
   //
@@ -334,7 +365,7 @@ class FixedArray {
   // Relational operators. Equality operators are elementwise using
   // `operator==`, while order operators order FixedArrays lexicographically.
   friend bool operator==(const FixedArray& lhs, const FixedArray& rhs) {
-    return absl::equal(lhs.begin(), lhs.end(), rhs.begin(), rhs.end());
+    return std::equal(lhs.begin(), lhs.end(), rhs.begin(), rhs.end());
   }
 
   friend bool operator!=(const FixedArray& lhs, const FixedArray& rhs) {
@@ -360,8 +391,7 @@ class FixedArray {
 
   template <typename H>
   friend H AbslHashValue(H h, const FixedArray& v) {
-    return H::combine(H::combine_contiguous(std::move(h), v.data(), v.size()),
-                      v.size());
+    return H::combine_contiguous(std::move(h), v.data(), v.size());
   }
 
  private:
@@ -385,23 +415,23 @@ class FixedArray {
   //     error: call to int __builtin___sprintf_chk(etc...)
   //     will always overflow destination buffer [-Werror]
   //
-  template <typename OuterT, typename InnerT = absl::remove_extent_t<OuterT>,
-            size_t InnerN = std::extent<OuterT>::value>
+  template <typename OuterT, typename InnerT = std::remove_extent_t<OuterT>,
+            size_t InnerN = std::extent_v<OuterT>>
   struct StorageElementWrapper {
     InnerT array[InnerN];
   };
 
   using StorageElement =
-      absl::conditional_t<std::is_array<value_type>::value,
-                          StorageElementWrapper<value_type>, value_type>;
+      std::conditional_t<std::is_array_v<value_type>,
+                         StorageElementWrapper<value_type>, value_type>;
 
   static pointer AsValueType(pointer ptr) { return ptr; }
   static pointer AsValueType(StorageElementWrapper<value_type>* ptr) {
     return std::addressof(ptr->array);
   }
 
-  static_assert(sizeof(StorageElement) == sizeof(value_type), "");
-  static_assert(alignof(StorageElement) == alignof(value_type), "");
+  static_assert(sizeof(StorageElement) == sizeof(value_type));
+  static_assert(alignof(StorageElement) == alignof(value_type));
 
   class NonEmptyInlinedStorage {
    public:
@@ -409,15 +439,16 @@ class FixedArray {
     void AnnotateConstruct(size_type n);
     void AnnotateDestruct(size_type n);
 
-#ifdef ADDRESS_SANITIZER
+#ifdef ABSL_HAVE_ADDRESS_SANITIZER
     void* RedzoneBegin() { return &redzone_begin_; }
     void* RedzoneEnd() { return &redzone_end_ + 1; }
-#endif  // ADDRESS_SANITIZER
+#endif  // ABSL_HAVE_ADDRESS_SANITIZER
 
    private:
-    ADDRESS_SANITIZER_REDZONE(redzone_begin_);
-    alignas(StorageElement) char buff_[sizeof(StorageElement[inline_elements])];
-    ADDRESS_SANITIZER_REDZONE(redzone_end_);
+    ABSL_ADDRESS_SANITIZER_REDZONE(redzone_begin_);
+    alignas(StorageElement) unsigned char buff_[sizeof(
+        StorageElement[inline_elements])];
+    ABSL_ADDRESS_SANITIZER_REDZONE(redzone_end_);
   };
 
   class EmptyInlinedStorage {
@@ -428,7 +459,7 @@ class FixedArray {
   };
 
   using InlinedStorage =
-      absl::conditional_t<inline_elements == 0, EmptyInlinedStorage,
+      std::conditional_t<inline_elements == 0, EmptyInlinedStorage,
                           NonEmptyInlinedStorage>;
 
   // Storage
@@ -456,12 +487,18 @@ class FixedArray {
     StorageElement* begin() const { return data_; }
     StorageElement* end() const { return begin() + size(); }
     allocator_type& alloc() { return size_alloc_.template get<1>(); }
+    const allocator_type& alloc() const {
+      return size_alloc_.template get<1>();
+    }
 
    private:
     static bool UsingInlinedStorage(size_type n) {
       return n <= inline_elements;
     }
 
+#ifdef ABSL_HAVE_ADDRESS_SANITIZER
+    ABSL_ATTRIBUTE_NOINLINE
+#endif  // ABSL_HAVE_ADDRESS_SANITIZER
     StorageElement* InitializeData() {
       if (UsingInlinedStorage(size())) {
         InlinedStorage::AnnotateConstruct(size());
@@ -480,34 +517,32 @@ class FixedArray {
   Storage storage_;
 };
 
-template <typename T, size_t N, typename A>
-constexpr size_t FixedArray<T, N, A>::kInlineBytesDefault;
-
-template <typename T, size_t N, typename A>
-constexpr typename FixedArray<T, N, A>::size_type
-    FixedArray<T, N, A>::inline_elements;
-
 template <typename T, size_t N, typename A>
 void FixedArray<T, N, A>::NonEmptyInlinedStorage::AnnotateConstruct(
     typename FixedArray<T, N, A>::size_type n) {
-#ifdef ADDRESS_SANITIZER
+#ifdef ABSL_HAVE_ADDRESS_SANITIZER
   if (!n) return;
-  ANNOTATE_CONTIGUOUS_CONTAINER(data(), RedzoneEnd(), RedzoneEnd(), data() + n);
-  ANNOTATE_CONTIGUOUS_CONTAINER(RedzoneBegin(), data(), data(), RedzoneBegin());
-#endif                   // ADDRESS_SANITIZER
+  ABSL_ANNOTATE_CONTIGUOUS_CONTAINER(data(), RedzoneEnd(), RedzoneEnd(),
+                                     data() + n);
+  ABSL_ANNOTATE_CONTIGUOUS_CONTAINER(RedzoneBegin(), data(), data(),
+                                     RedzoneBegin());
+#endif  // ABSL_HAVE_ADDRESS_SANITIZER
   static_cast<void>(n);  // Mark used when not in asan mode
 }
 
 template <typename T, size_t N, typename A>
 void FixedArray<T, N, A>::NonEmptyInlinedStorage::AnnotateDestruct(
     typename FixedArray<T, N, A>::size_type n) {
-#ifdef ADDRESS_SANITIZER
+#ifdef ABSL_HAVE_ADDRESS_SANITIZER
   if (!n) return;
-  ANNOTATE_CONTIGUOUS_CONTAINER(data(), RedzoneEnd(), data() + n, RedzoneEnd());
-  ANNOTATE_CONTIGUOUS_CONTAINER(RedzoneBegin(), data(), RedzoneBegin(), data());
-#endif                   // ADDRESS_SANITIZER
+  ABSL_ANNOTATE_CONTIGUOUS_CONTAINER(data(), RedzoneEnd(), data() + n,
+                                     RedzoneEnd());
+  ABSL_ANNOTATE_CONTIGUOUS_CONTAINER(RedzoneBegin(), data(), RedzoneBegin(),
+                                     data());
+#endif  // ABSL_HAVE_ADDRESS_SANITIZER
   static_cast<void>(n);  // Mark used when not in asan mode
 }
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_FIXED_ARRAY_H_
diff --git a/absl/container/fixed_array_benchmark.cc b/absl/container/fixed_array_benchmark.cc
index 3c7a5a72345..db6663e60d2 100644
--- a/absl/container/fixed_array_benchmark.cc
+++ b/absl/container/fixed_array_benchmark.cc
@@ -16,8 +16,8 @@
 
 #include <string>
 
-#include "benchmark/benchmark.h"
 #include "absl/container/fixed_array.h"
+#include "benchmark/benchmark.h"
 
 namespace {
 
diff --git a/absl/container/fixed_array_exception_safety_test.cc b/absl/container/fixed_array_exception_safety_test.cc
index 9aabfd5c95a..e5f59299b35 100644
--- a/absl/container/fixed_array_exception_safety_test.cc
+++ b/absl/container/fixed_array_exception_safety_test.cc
@@ -12,9 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-#include "absl/container/fixed_array.h"
-
 #include "absl/base/config.h"
+#include "absl/container/fixed_array.h"
 
 #ifdef ABSL_HAVE_EXCEPTIONS
 
@@ -24,6 +23,7 @@
 #include "absl/base/internal/exception_safety_testing.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 namespace {
 
@@ -37,10 +37,19 @@ constexpr int kUpdatedValue = 10;
 using ::testing::TestThrowingCtor;
 
 using Thrower = testing::ThrowingValue<testing::TypeSpec::kEverythingThrows>;
+using ThrowAlloc =
+    testing::ThrowingAllocator<Thrower, testing::AllocSpec::kEverythingThrows>;
+using MoveThrower = testing::ThrowingValue<testing::TypeSpec::kNoThrowMove>;
+using MoveThrowAlloc =
+    testing::ThrowingAllocator<MoveThrower,
+                               testing::AllocSpec::kEverythingThrows>;
+
 using FixedArr = absl::FixedArray<Thrower, kInlined>;
+using FixedArrWithAlloc = absl::FixedArray<Thrower, kInlined, ThrowAlloc>;
 
-using MoveThrower = testing::ThrowingValue<testing::TypeSpec::kNoThrowMove>;
 using MoveFixedArr = absl::FixedArray<MoveThrower, kInlined>;
+using MoveFixedArrWithAlloc =
+    absl::FixedArray<MoveThrower, kInlined, MoveThrowAlloc>;
 
 TEST(FixedArrayExceptionSafety, CopyConstructor) {
   auto small = FixedArr(kSmallSize);
@@ -50,6 +59,14 @@ TEST(FixedArrayExceptionSafety, CopyConstructor) {
   TestThrowingCtor<FixedArr>(large);
 }
 
+TEST(FixedArrayExceptionSafety, CopyConstructorWithAlloc) {
+  auto small = FixedArrWithAlloc(kSmallSize);
+  TestThrowingCtor<FixedArrWithAlloc>(small);
+
+  auto large = FixedArrWithAlloc(kLargeSize);
+  TestThrowingCtor<FixedArrWithAlloc>(large);
+}
+
 TEST(FixedArrayExceptionSafety, MoveConstructor) {
   TestThrowingCtor<FixedArr>(FixedArr(kSmallSize));
   TestThrowingCtor<FixedArr>(FixedArr(kLargeSize));
@@ -59,16 +76,35 @@ TEST(FixedArrayExceptionSafety, MoveConstructor) {
   TestThrowingCtor<MoveFixedArr>(MoveFixedArr(kLargeSize));
 }
 
+TEST(FixedArrayExceptionSafety, MoveConstructorWithAlloc) {
+  TestThrowingCtor<FixedArrWithAlloc>(FixedArrWithAlloc(kSmallSize));
+  TestThrowingCtor<FixedArrWithAlloc>(FixedArrWithAlloc(kLargeSize));
+
+  // TypeSpec::kNoThrowMove
+  TestThrowingCtor<MoveFixedArrWithAlloc>(MoveFixedArrWithAlloc(kSmallSize));
+  TestThrowingCtor<MoveFixedArrWithAlloc>(MoveFixedArrWithAlloc(kLargeSize));
+}
+
 TEST(FixedArrayExceptionSafety, SizeConstructor) {
   TestThrowingCtor<FixedArr>(kSmallSize);
   TestThrowingCtor<FixedArr>(kLargeSize);
 }
 
+TEST(FixedArrayExceptionSafety, SizeConstructorWithAlloc) {
+  TestThrowingCtor<FixedArrWithAlloc>(kSmallSize);
+  TestThrowingCtor<FixedArrWithAlloc>(kLargeSize);
+}
+
 TEST(FixedArrayExceptionSafety, SizeValueConstructor) {
   TestThrowingCtor<FixedArr>(kSmallSize, Thrower());
   TestThrowingCtor<FixedArr>(kLargeSize, Thrower());
 }
 
+TEST(FixedArrayExceptionSafety, SizeValueConstructorWithAlloc) {
+  TestThrowingCtor<FixedArrWithAlloc>(kSmallSize, Thrower());
+  TestThrowingCtor<FixedArrWithAlloc>(kLargeSize, Thrower());
+}
+
 TEST(FixedArrayExceptionSafety, IteratorConstructor) {
   auto small = FixedArr(kSmallSize);
   TestThrowingCtor<FixedArr>(small.begin(), small.end());
@@ -77,6 +113,14 @@ TEST(FixedArrayExceptionSafety, IteratorConstructor) {
   TestThrowingCtor<FixedArr>(large.begin(), large.end());
 }
 
+TEST(FixedArrayExceptionSafety, IteratorConstructorWithAlloc) {
+  auto small = FixedArrWithAlloc(kSmallSize);
+  TestThrowingCtor<FixedArrWithAlloc>(small.begin(), small.end());
+
+  auto large = FixedArrWithAlloc(kLargeSize);
+  TestThrowingCtor<FixedArrWithAlloc>(large.begin(), large.end());
+}
+
 TEST(FixedArrayExceptionSafety, InitListConstructor) {
   constexpr int small_inlined = 3;
   using SmallFixedArr = absl::FixedArray<Thrower, small_inlined>;
@@ -90,9 +134,23 @@ TEST(FixedArrayExceptionSafety, InitListConstructor) {
       Thrower{}, Thrower{}, Thrower{}, Thrower{}, Thrower{}});
 }
 
-testing::AssertionResult ReadMemory(FixedArr* fixed_arr) {
-  // Marked volatile to prevent optimization. Used for running asan tests.
-  volatile int sum = 0;
+TEST(FixedArrayExceptionSafety, InitListConstructorWithAlloc) {
+  constexpr int small_inlined = 3;
+  using SmallFixedArrWithAlloc =
+      absl::FixedArray<Thrower, small_inlined, ThrowAlloc>;
+
+  TestThrowingCtor<SmallFixedArrWithAlloc>(std::initializer_list<Thrower>{});
+  // Test inlined allocation
+  TestThrowingCtor<SmallFixedArrWithAlloc>(
+      std::initializer_list<Thrower>{Thrower{}, Thrower{}});
+  // Test out of line allocation
+  TestThrowingCtor<SmallFixedArrWithAlloc>(std::initializer_list<Thrower>{
+      Thrower{}, Thrower{}, Thrower{}, Thrower{}, Thrower{}});
+}
+
+template <typename FixedArrT>
+testing::AssertionResult ReadMemory(FixedArrT* fixed_arr) {
+  int sum = 0;
   for (const auto& thrower : *fixed_arr) {
     sum += thrower.Get();
   }
@@ -101,7 +159,7 @@ testing::AssertionResult ReadMemory(FixedArr* fixed_arr) {
 
 TEST(FixedArrayExceptionSafety, Fill) {
   auto test_fill = testing::MakeExceptionSafetyTester()
-                       .WithContracts(ReadMemory)
+                       .WithContracts(ReadMemory<FixedArr>)
                        .WithOperation([&](FixedArr* fixed_arr_ptr) {
                          auto thrower =
                              Thrower(kUpdatedValue, testing::nothrow_ctor);
@@ -116,8 +174,28 @@ TEST(FixedArrayExceptionSafety, Fill) {
           .Test());
 }
 
+TEST(FixedArrayExceptionSafety, FillWithAlloc) {
+  auto test_fill = testing::MakeExceptionSafetyTester()
+                       .WithContracts(ReadMemory<FixedArrWithAlloc>)
+                       .WithOperation([&](FixedArrWithAlloc* fixed_arr_ptr) {
+                         auto thrower =
+                             Thrower(kUpdatedValue, testing::nothrow_ctor);
+                         fixed_arr_ptr->fill(thrower);
+                       });
+
+  EXPECT_TRUE(test_fill
+                  .WithInitialValue(
+                      FixedArrWithAlloc(kSmallSize, Thrower(kInitialValue)))
+                  .Test());
+  EXPECT_TRUE(test_fill
+                  .WithInitialValue(
+                      FixedArrWithAlloc(kLargeSize, Thrower(kInitialValue)))
+                  .Test());
+}
+
 }  // namespace
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_HAVE_EXCEPTIONS
diff --git a/absl/container/fixed_array_test.cc b/absl/container/fixed_array_test.cc
index 2b1cf47eacb..71c6819bbdf 100644
--- a/absl/container/fixed_array_test.cc
+++ b/absl/container/fixed_array_test.cc
@@ -17,17 +17,24 @@
 #include <stdio.h>
 
 #include <cstring>
+#include <forward_list>
+#include <iterator>
 #include <list>
 #include <memory>
 #include <numeric>
 #include <scoped_allocator>
 #include <stdexcept>
 #include <string>
+#include <utility>
 #include <vector>
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
+#include "absl/base/config.h"
 #include "absl/base/internal/exception_testing.h"
+#include "absl/base/internal/iterator_traits_test_helper.h"
+#include "absl/base/options.h"
+#include "absl/container/internal/test_allocator.h"
 #include "absl/hash/hash_testing.h"
 #include "absl/memory/memory.h"
 
@@ -106,7 +113,7 @@ TEST(FixedArrayTest, CopyCtor) {
 TEST(FixedArrayTest, MoveCtor) {
   absl::FixedArray<std::unique_ptr<int>, 10> on_stack(5);
   for (int i = 0; i < 5; ++i) {
-    on_stack[i] = absl::make_unique<int>(i);
+    on_stack[i] = std::make_unique<int>(i);
   }
 
   absl::FixedArray<std::unique_ptr<int>, 10> stack_copy = std::move(on_stack);
@@ -115,7 +122,7 @@ TEST(FixedArrayTest, MoveCtor) {
 
   absl::FixedArray<std::unique_ptr<int>, 10> allocated(15);
   for (int i = 0; i < 15; ++i) {
-    allocated[i] = absl::make_unique<int>(i);
+    allocated[i] = std::make_unique<int>(i);
   }
 
   absl::FixedArray<std::unique_ptr<int>, 10> alloced_copy =
@@ -188,6 +195,21 @@ TEST(FixedArrayTest, AtThrows) {
                                  "failed bounds check");
 }
 
+TEST(FixedArrayTest, Hardened) {
+#if !defined(NDEBUG) || ABSL_OPTION_HARDENED
+  absl::FixedArray<int> a = {1, 2, 3};
+  EXPECT_EQ(a[2], 3);
+  EXPECT_DEATH_IF_SUPPORTED(a[3], "");
+  EXPECT_DEATH_IF_SUPPORTED(a[-1], "");
+
+  absl::FixedArray<int> empty(0);
+  EXPECT_DEATH_IF_SUPPORTED(empty[0], "");
+  EXPECT_DEATH_IF_SUPPORTED(empty[-1], "");
+  EXPECT_DEATH_IF_SUPPORTED(empty.front(), "");
+  EXPECT_DEATH_IF_SUPPORTED(empty.back(), "");
+#endif
+}
+
 TEST(FixedArrayRelationalsTest, EqualArrays) {
   for (int i = 0; i < 10; ++i) {
     absl::FixedArray<int, 5> a1(i);
@@ -286,7 +308,7 @@ static void TestArrayOfArrays(int n) {
     using InnerArray = ConstructionTester[elements_per_inner_array];
     // Heap-allocate the FixedArray to avoid blowing the stack frame.
     auto array_ptr =
-        absl::make_unique<absl::FixedArray<InnerArray, inline_elements>>(n);
+        std::make_unique<absl::FixedArray<InnerArray, inline_elements>>(n);
     auto& array = *array_ptr;
 
     ASSERT_EQ(array.size(), n);
@@ -338,20 +360,20 @@ static void TestArrayOfArrays(int n) {
 
 TEST(IteratorConstructorTest, NonInline) {
   int const kInput[] = {2, 3, 5, 7, 11, 13, 17};
-  absl::FixedArray<int, ABSL_ARRAYSIZE(kInput) - 1> const fixed(
-      kInput, kInput + ABSL_ARRAYSIZE(kInput));
-  ASSERT_EQ(ABSL_ARRAYSIZE(kInput), fixed.size());
-  for (size_t i = 0; i < ABSL_ARRAYSIZE(kInput); ++i) {
+  absl::FixedArray<int, std::size(kInput) - 1> const fixed(
+      kInput, kInput + std::size(kInput));
+  ASSERT_EQ(std::size(kInput), fixed.size());
+  for (size_t i = 0; i < std::size(kInput); ++i) {
     ASSERT_EQ(kInput[i], fixed[i]);
   }
 }
 
 TEST(IteratorConstructorTest, Inline) {
   int const kInput[] = {2, 3, 5, 7, 11, 13, 17};
-  absl::FixedArray<int, ABSL_ARRAYSIZE(kInput)> const fixed(
-      kInput, kInput + ABSL_ARRAYSIZE(kInput));
-  ASSERT_EQ(ABSL_ARRAYSIZE(kInput), fixed.size());
-  for (size_t i = 0; i < ABSL_ARRAYSIZE(kInput); ++i) {
+  absl::FixedArray<int, std::size(kInput)> const fixed(
+      kInput, kInput + std::size(kInput));
+  ASSERT_EQ(std::size(kInput), fixed.size());
+  for (size_t i = 0; i < std::size(kInput); ++i) {
     ASSERT_EQ(kInput[i], fixed[i]);
   }
 }
@@ -359,10 +381,9 @@ TEST(IteratorConstructorTest, Inline) {
 TEST(IteratorConstructorTest, NonPod) {
   char const* kInput[] = {"red",  "orange", "yellow", "green",
                           "blue", "indigo", "violet"};
-  absl::FixedArray<std::string> const fixed(kInput,
-                                            kInput + ABSL_ARRAYSIZE(kInput));
-  ASSERT_EQ(ABSL_ARRAYSIZE(kInput), fixed.size());
-  for (size_t i = 0; i < ABSL_ARRAYSIZE(kInput); ++i) {
+  absl::FixedArray<std::string> const fixed(kInput, kInput + std::size(kInput));
+  ASSERT_EQ(std::size(kInput), fixed.size());
+  for (size_t i = 0; i < std::size(kInput); ++i) {
     ASSERT_EQ(kInput[i], fixed[i]);
   }
 }
@@ -376,7 +397,7 @@ TEST(IteratorConstructorTest, FromEmptyVector) {
 
 TEST(IteratorConstructorTest, FromNonEmptyVector) {
   int const kInput[] = {2, 3, 5, 7, 11, 13, 17};
-  std::vector<int> const items(kInput, kInput + ABSL_ARRAYSIZE(kInput));
+  std::vector<int> const items(kInput, kInput + std::size(kInput));
   absl::FixedArray<int> const fixed(items.begin(), items.end());
   ASSERT_EQ(items.size(), fixed.size());
   for (size_t i = 0; i < items.size(); ++i) {
@@ -386,11 +407,25 @@ TEST(IteratorConstructorTest, FromNonEmptyVector) {
 
 TEST(IteratorConstructorTest, FromBidirectionalIteratorRange) {
   int const kInput[] = {2, 3, 5, 7, 11, 13, 17};
-  std::list<int> const items(kInput, kInput + ABSL_ARRAYSIZE(kInput));
+  std::list<int> const items(kInput, kInput + std::size(kInput));
   absl::FixedArray<int> const fixed(items.begin(), items.end());
   EXPECT_THAT(fixed, testing::ElementsAreArray(kInput));
 }
 
+TEST(IteratorConstructorTest, FromCpp20ForwardIteratorRange) {
+  std::forward_list<int> const kUnzippedInput = {2, 3, 5, 7, 11, 13, 17};
+  absl::base_internal::Cpp20ForwardZipIterator<
+      std::forward_list<int>::const_iterator> const
+      begin(std::begin(kUnzippedInput), std::begin(kUnzippedInput));
+  absl::base_internal::
+      Cpp20ForwardZipIterator<std::forward_list<int>::const_iterator> const end(
+          std::end(kUnzippedInput), std::end(kUnzippedInput));
+
+  std::forward_list<std::pair<int, int>> const items(begin, end);
+  absl::FixedArray<std::pair<int, int>> const fixed(begin, end);
+  EXPECT_THAT(fixed, testing::ElementsAreArray(items));
+}
+
 TEST(InitListConstructorTest, InitListConstruction) {
   absl::FixedArray<int> fixed = {1, 2, 3};
   EXPECT_THAT(fixed, testing::ElementsAreArray({1, 2, 3}));
@@ -401,7 +436,7 @@ TEST(FillConstructorTest, NonEmptyArrays) {
   EXPECT_THAT(stack_array, testing::ElementsAreArray({1, 1, 1, 1}));
 
   absl::FixedArray<int, 0> heap_array(4, 1);
-  EXPECT_THAT(stack_array, testing::ElementsAreArray({1, 1, 1, 1}));
+  EXPECT_THAT(heap_array, testing::ElementsAreArray({1, 1, 1, 1}));
 }
 
 TEST(FillConstructorTest, EmptyArray) {
@@ -500,7 +535,10 @@ struct PickyDelete {
   }
 };
 
-TEST(FixedArrayTest, UsesGlobalAlloc) { absl::FixedArray<PickyDelete, 0> a(5); }
+TEST(FixedArrayTest, UsesGlobalAlloc) {
+  absl::FixedArray<PickyDelete, 0> a(5);
+  EXPECT_EQ(a.size(), 5);
+}
 
 TEST(FixedArrayTest, Data) {
   static const int kInput[] = {2, 3, 5, 7, 11, 13, 17};
@@ -604,19 +642,16 @@ TEST(FixedArrayTest, Fill) {
   empty.fill(fill_val);
 }
 
-// TODO(johnsoncj): Investigate InlinedStorage default initialization in GCC 4.x
 #ifndef __GNUC__
 TEST(FixedArrayTest, DefaultCtorDoesNotValueInit) {
   using T = char;
   constexpr auto capacity = 10;
   using FixedArrType = absl::FixedArray<T, capacity>;
-  using FixedArrBuffType =
-      absl::aligned_storage_t<sizeof(FixedArrType), alignof(FixedArrType)>;
   constexpr auto scrubbed_bits = 0x95;
   constexpr auto length = capacity / 2;
 
-  FixedArrBuffType buff;
-  std::memset(std::addressof(buff), scrubbed_bits, sizeof(FixedArrBuffType));
+  alignas(FixedArrType) unsigned char buff[sizeof(FixedArrType)];
+  std::memset(std::addressof(buff), scrubbed_bits, sizeof(FixedArrType));
 
   FixedArrType* arr =
       ::new (static_cast<void*>(std::addressof(buff))) FixedArrType(length);
@@ -625,70 +660,9 @@ TEST(FixedArrayTest, DefaultCtorDoesNotValueInit) {
 }
 #endif  // __GNUC__
 
-// This is a stateful allocator, but the state lives outside of the
-// allocator (in whatever test is using the allocator). This is odd
-// but helps in tests where the allocator is propagated into nested
-// containers - that chain of allocators uses the same state and is
-// thus easier to query for aggregate allocation information.
-template <typename T>
-class CountingAllocator : public std::allocator<T> {
- public:
-  using Alloc = std::allocator<T>;
-  using pointer = typename Alloc::pointer;
-  using size_type = typename Alloc::size_type;
-
-  CountingAllocator() : bytes_used_(nullptr), instance_count_(nullptr) {}
-  explicit CountingAllocator(int64_t* b)
-      : bytes_used_(b), instance_count_(nullptr) {}
-  CountingAllocator(int64_t* b, int64_t* a)
-      : bytes_used_(b), instance_count_(a) {}
-
-  template <typename U>
-  explicit CountingAllocator(const CountingAllocator<U>& x)
-      : Alloc(x),
-        bytes_used_(x.bytes_used_),
-        instance_count_(x.instance_count_) {}
-
-  pointer allocate(size_type n, const void* const hint = nullptr) {
-    assert(bytes_used_ != nullptr);
-    *bytes_used_ += n * sizeof(T);
-    return Alloc::allocate(n, hint);
-  }
-
-  void deallocate(pointer p, size_type n) {
-    Alloc::deallocate(p, n);
-    assert(bytes_used_ != nullptr);
-    *bytes_used_ -= n * sizeof(T);
-  }
-
-  template <typename... Args>
-  void construct(pointer p, Args&&... args) {
-    Alloc::construct(p, absl::forward<Args>(args)...);
-    if (instance_count_) {
-      *instance_count_ += 1;
-    }
-  }
-
-  void destroy(pointer p) {
-    Alloc::destroy(p);
-    if (instance_count_) {
-      *instance_count_ -= 1;
-    }
-  }
-
-  template <typename U>
-  class rebind {
-   public:
-    using other = CountingAllocator<U>;
-  };
-
-  int64_t* bytes_used_;
-  int64_t* instance_count_;
-};
-
 TEST(AllocatorSupportTest, CountInlineAllocations) {
   constexpr size_t inlined_size = 4;
-  using Alloc = CountingAllocator<int>;
+  using Alloc = absl::container_internal::CountingAllocator<int>;
   using AllocFxdArr = absl::FixedArray<int, inlined_size, Alloc>;
 
   int64_t allocated = 0;
@@ -709,7 +683,7 @@ TEST(AllocatorSupportTest, CountInlineAllocations) {
 
 TEST(AllocatorSupportTest, CountOutoflineAllocations) {
   constexpr size_t inlined_size = 4;
-  using Alloc = CountingAllocator<int>;
+  using Alloc = absl::container_internal::CountingAllocator<int>;
   using AllocFxdArr = absl::FixedArray<int, inlined_size, Alloc>;
 
   int64_t allocated = 0;
@@ -719,7 +693,7 @@ TEST(AllocatorSupportTest, CountOutoflineAllocations) {
     const int ia[] = {0, 1, 2, 3, 4, 5, 6, 7};
     Alloc alloc(&allocated, &active_instances);
 
-    AllocFxdArr arr(ia, ia + ABSL_ARRAYSIZE(ia), alloc);
+    AllocFxdArr arr(ia, ia + std::size(ia), alloc);
 
     EXPECT_EQ(allocated, arr.size() * sizeof(int));
     static_cast<void>(arr);
@@ -730,7 +704,7 @@ TEST(AllocatorSupportTest, CountOutoflineAllocations) {
 
 TEST(AllocatorSupportTest, CountCopyInlineAllocations) {
   constexpr size_t inlined_size = 4;
-  using Alloc = CountingAllocator<int>;
+  using Alloc = absl::container_internal::CountingAllocator<int>;
   using AllocFxdArr = absl::FixedArray<int, inlined_size, Alloc>;
 
   int64_t allocated1 = 0;
@@ -758,7 +732,7 @@ TEST(AllocatorSupportTest, CountCopyInlineAllocations) {
 
 TEST(AllocatorSupportTest, CountCopyOutoflineAllocations) {
   constexpr size_t inlined_size = 4;
-  using Alloc = CountingAllocator<int>;
+  using Alloc = absl::container_internal::CountingAllocator<int>;
   using AllocFxdArr = absl::FixedArray<int, inlined_size, Alloc>;
 
   int64_t allocated1 = 0;
@@ -790,7 +764,7 @@ TEST(AllocatorSupportTest, SizeValAllocConstructor) {
   using testing::SizeIs;
 
   constexpr size_t inlined_size = 4;
-  using Alloc = CountingAllocator<int>;
+  using Alloc = absl::container_internal::CountingAllocator<int>;
   using AllocFxdArr = absl::FixedArray<int, inlined_size, Alloc>;
 
   {
@@ -814,16 +788,33 @@ TEST(AllocatorSupportTest, SizeValAllocConstructor) {
   }
 }
 
-#ifdef ADDRESS_SANITIZER
+TEST(AllocatorSupportTest, PropagatesStatefulAllocator) {
+  constexpr size_t inlined_size = 4;
+  using Alloc = absl::container_internal::CountingAllocator<int>;
+  using AllocFxdArr = absl::FixedArray<int, inlined_size, Alloc>;
+
+  auto len = inlined_size * 2;
+  auto val = 0;
+  int64_t allocated = 0;
+  AllocFxdArr arr(len, val, Alloc(&allocated));
+
+  EXPECT_EQ(allocated, len * sizeof(int));
+
+  AllocFxdArr copy = arr;
+  EXPECT_EQ(allocated, len * sizeof(int) * 2);
+  EXPECT_EQ(copy, arr);
+}
+
+#ifdef ABSL_HAVE_ADDRESS_SANITIZER
 TEST(FixedArrayTest, AddressSanitizerAnnotations1) {
   absl::FixedArray<int, 32> a(10);
   int* raw = a.data();
   raw[0] = 0;
   raw[9] = 0;
-  EXPECT_DEATH(raw[-2] = 0, "container-overflow");
-  EXPECT_DEATH(raw[-1] = 0, "container-overflow");
-  EXPECT_DEATH(raw[10] = 0, "container-overflow");
-  EXPECT_DEATH(raw[31] = 0, "container-overflow");
+  EXPECT_DEATH_IF_SUPPORTED(raw[-2] = 0, "container-overflow");
+  EXPECT_DEATH_IF_SUPPORTED(raw[-1] = 0, "container-overflow");
+  EXPECT_DEATH_IF_SUPPORTED(raw[10] = 0, "container-overflow");
+  EXPECT_DEATH_IF_SUPPORTED(raw[31] = 0, "container-overflow");
 }
 
 TEST(FixedArrayTest, AddressSanitizerAnnotations2) {
@@ -831,10 +822,10 @@ TEST(FixedArrayTest, AddressSanitizerAnnotations2) {
   char* raw = a.data();
   raw[0] = 0;
   raw[11] = 0;
-  EXPECT_DEATH(raw[-7] = 0, "container-overflow");
-  EXPECT_DEATH(raw[-1] = 0, "container-overflow");
-  EXPECT_DEATH(raw[12] = 0, "container-overflow");
-  EXPECT_DEATH(raw[17] = 0, "container-overflow");
+  EXPECT_DEATH_IF_SUPPORTED(raw[-7] = 0, "container-overflow");
+  EXPECT_DEATH_IF_SUPPORTED(raw[-1] = 0, "container-overflow");
+  EXPECT_DEATH_IF_SUPPORTED(raw[12] = 0, "container-overflow");
+  EXPECT_DEATH_IF_SUPPORTED(raw[17] = 0, "container-overflow");
 }
 
 TEST(FixedArrayTest, AddressSanitizerAnnotations3) {
@@ -842,8 +833,8 @@ TEST(FixedArrayTest, AddressSanitizerAnnotations3) {
   uint64_t* raw = a.data();
   raw[0] = 0;
   raw[19] = 0;
-  EXPECT_DEATH(raw[-1] = 0, "container-overflow");
-  EXPECT_DEATH(raw[20] = 0, "container-overflow");
+  EXPECT_DEATH_IF_SUPPORTED(raw[-1] = 0, "container-overflow");
+  EXPECT_DEATH_IF_SUPPORTED(raw[20] = 0, "container-overflow");
 }
 
 TEST(FixedArrayTest, AddressSanitizerAnnotations4) {
@@ -855,13 +846,13 @@ TEST(FixedArrayTest, AddressSanitizerAnnotations4) {
   // there is only a 8-byte red zone before the container range, so we only
   // access the last 4 bytes of the struct to make sure it stays within the red
   // zone.
-  EXPECT_DEATH(raw[-1].z_ = 0, "container-overflow");
-  EXPECT_DEATH(raw[10] = ThreeInts(), "container-overflow");
+  EXPECT_DEATH_IF_SUPPORTED(raw[-1].z_ = 0, "container-overflow");
+  EXPECT_DEATH_IF_SUPPORTED(raw[10] = ThreeInts(), "container-overflow");
   // The actual size of storage is kDefaultBytes=256, 21*12 = 252,
   // so reading raw[21] should still trigger the correct warning.
-  EXPECT_DEATH(raw[21] = ThreeInts(), "container-overflow");
+  EXPECT_DEATH_IF_SUPPORTED(raw[21] = ThreeInts(), "container-overflow");
 }
-#endif  // ADDRESS_SANITIZER
+#endif  // ABSL_HAVE_ADDRESS_SANITIZER
 
 TEST(FixedArrayTest, AbslHashValueWorks) {
   using V = absl::FixedArray<int>;
diff --git a/absl/container/flat_hash_map.h b/absl/container/flat_hash_map.h
index 5c16ac88fbb..e11d7e2e796 100644
--- a/absl/container/flat_hash_map.h
+++ b/absl/container/flat_hash_map.h
@@ -26,22 +26,27 @@
 //
 // In most cases, your default choice for a hash map should be a map of type
 // `flat_hash_map`.
+//
+// `flat_hash_map` is not exception-safe.
 
 #ifndef ABSL_CONTAINER_FLAT_HASH_MAP_H_
 #define ABSL_CONTAINER_FLAT_HASH_MAP_H_
 
 #include <cstddef>
-#include <new>
+#include <memory>
 #include <type_traits>
 #include <utility>
 
 #include "absl/algorithm/container.h"
+#include "absl/base/attributes.h"
+#include "absl/base/macros.h"
+#include "absl/container/hash_container_defaults.h"
 #include "absl/container/internal/container_memory.h"
-#include "absl/container/internal/hash_function_defaults.h"  // IWYU pragma: export
 #include "absl/container/internal/raw_hash_map.h"  // IWYU pragma: export
-#include "absl/memory/memory.h"
+#include "absl/meta/type_traits.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 template <class K, class V>
 struct FlatHashMapPolicy;
@@ -60,12 +65,21 @@ struct FlatHashMapPolicy;
 // * Requires values that are MoveConstructible
 // * Supports heterogeneous lookup, through `find()`, `operator[]()` and
 //   `insert()`, provided that the map is provided a compatible heterogeneous
-//   hashing function and equality operator.
+//   hashing function and equality operator. See below for details.
 // * Invalidates any references and pointers to elements within the table after
-//   `rehash()`.
+//   `rehash()` and when the table is moved.
 // * Contains a `capacity()` member function indicating the number of element
 //   slots (open, deleted, and empty) within the hash map.
 // * Returns `void` from the `erase(iterator)` overload.
+// * Constructors accept reservation size as an optional argument instead of
+//   bucket count. Reservation size is the number of elements that fits in the
+//   map before rehash.
+// * insert/emplace and other modification functions return special iterator
+//   that doesn't support iteration. std::next(it) for such iterators would
+//   always point to the end().
+//
+// TODO(b/519468416): copy redacted version of notable differences to
+// node_hash_* files.
 //
 // By default, `flat_hash_map` uses the `absl::Hash` hashing framework.
 // All fundamental and Abseil types that support the `absl::Hash` framework have
@@ -74,6 +88,23 @@ struct FlatHashMapPolicy;
 // absl/hash/hash.h for information on extending Abseil hashing to user-defined
 // types.
 //
+// Using `absl::flat_hash_map` at interface boundaries in dynamically loaded
+// libraries (e.g. .dll, .so) is unsupported due to way `absl::Hash` values may
+// be randomized across dynamically loaded libraries.
+//
+// To achieve heterogeneous lookup for custom types either `Hash` and `Eq` type
+// parameters can be used or `T` should have public inner types
+// `absl_container_hash` and (optionally) `absl_container_eq`. In either case,
+// `typename Hash::is_transparent` and `typename Eq::is_transparent` should be
+// well-formed. Both types are basically functors:
+// * `Hash` should support `size_t operator()(U val) const` that returns a hash
+// for the given `val`.
+// * `Eq` should support `bool operator()(U lhs, V rhs) const` that returns true
+// if `lhs` is equal to `rhs`.
+//
+// In most cases `T` needs only to provide the `absl_container_hash`. In this
+// case `std::equal_to<void>` will be used instead of `eq` part.
+//
 // NOTE: A `flat_hash_map` stores its value types directly inside its
 // implementation array to avoid memory indirection. Because a `flat_hash_map`
 // is designed to move data when rehashed, map values will not retain pointer
@@ -82,31 +113,40 @@ struct FlatHashMapPolicy;
 // If your types are not moveable or you require pointer stability for keys,
 // consider `absl::node_hash_map`.
 //
+// PERFORMANCE WARNING: Erasure & sparsity can negatively affect performance:
+//  * Iteration takes O(capacity) time, not O(size).
+//  * erase() slows down begin() and ++iterator.
+//  * Capacity only shrinks on rehash() or clear() -- not on erase().
+//
 // Example:
 //
 //   // Create a flat hash map of three strings (that map to strings)
 //   absl::flat_hash_map<std::string, std::string> ducks =
 //     {{"a", "huey"}, {"b", "dewey"}, {"c", "louie"}};
 //
-//  // Insert a new element into the flat hash map
-//  ducks.insert({"d", "donald"});
+//   // Insert a new element into the flat hash map
+//   ducks.insert({"d", "donald"});
 //
-//  // Force a rehash of the flat hash map
-//  ducks.rehash(0);
+//   // Force a rehash of the flat hash map
+//   ducks.rehash(0);
 //
-//  // Find the element with the key "b"
-//  std::string search_key = "b";
-//  auto result = ducks.find(search_key);
-//  if (result != ducks.end()) {
-//    std::cout << "Result: " << result->second << std::endl;
-//  }
-template <class K, class V,
-          class Hash = absl::container_internal::hash_default_hash<K>,
-          class Eq = absl::container_internal::hash_default_eq<K>,
-          class Allocator = std::allocator<std::pair<const K, V>>>
-class flat_hash_map : public absl::container_internal::raw_hash_map<
-                          absl::container_internal::FlatHashMapPolicy<K, V>,
-                          Hash, Eq, Allocator> {
+//   // Find the element with the key "b"
+//   std::string search_key = "b";
+//   auto result = ducks.find(search_key);
+//   if (result != ducks.end()) {
+//     std::cout << "Result: " << result->second << std::endl;
+//   }
+template <
+    class K, class V,
+    class Hash =
+        typename container_internal::FlatHashMapPolicy<K, V>::DefaultHash,
+    class Eq = typename container_internal::FlatHashMapPolicy<K, V>::DefaultEq,
+    class Allocator =
+        typename container_internal::FlatHashMapPolicy<K, V>::DefaultAlloc>
+class ABSL_ATTRIBUTE_OWNER flat_hash_map
+    : public absl::container_internal::InstantiateRawHashMap<
+          absl::container_internal::FlatHashMapPolicy<K, V>, Hash, Eq,
+          Allocator>::type {
   using Base = typename flat_hash_map::raw_hash_map;
 
  public:
@@ -131,25 +171,37 @@ class flat_hash_map : public absl::container_internal::raw_hash_map<
   //
   // * Copy assignment operator
   //
-  //  // Hash functor and Comparator are copied as well
-  //  absl::flat_hash_map<int, std::string> map4;
-  //  map4 = map3;
+  //   // Hash functor and Comparator are copied as well
+  //   absl::flat_hash_map<int, std::string> map4;
+  //   map4 = map3;
   //
   // * Move constructor
   //
   //   // Move is guaranteed efficient
   //   absl::flat_hash_map<int, std::string> map5(std::move(map4));
   //
+  //   // After the move, map4 is in a valid but unspecified state. The only
+  //   // operations guaranteed to be safe on a moved-from map are destruction,
+  //   // assignment, and clear(). Any other operation (e.g. size(), empty(),
+  //   // iteration) results in undefined behavior.
+  //
   // * Move assignment operator
   //
   //   // May be efficient if allocators are compatible
   //   absl::flat_hash_map<int, std::string> map6;
   //   map6 = std::move(map5);
   //
+  //   // Same moved-from guarantees apply to map5 after this operation.
+  //
   // * Range constructor
   //
   //   std::vector<std::pair<int, std::string>> v = {{1, "a"}, {2, "b"}};
   //   absl::flat_hash_map<int, std::string> map7(v.begin(), v.end());
+  //
+  // * from_range constructor (C++23)
+  //
+  //   std::vector<std::pair<int, std::string>> v = {{1, "a"}, {2, "b"}};
+  //   absl::flat_hash_map<int, std::string> map8(std::from_range, v);
   flat_hash_map() {}
   using Base::Base;
 
@@ -229,11 +281,16 @@ class flat_hash_map : public absl::container_internal::raw_hash_map<
   // iterator erase(const_iterator first, const_iterator last):
   //
   //   Erases the elements in the open interval [`first`, `last`), returning an
-  //   iterator pointing to `last`.
+  //   iterator pointing to `last`. The special case of calling
+  //   `erase(begin(), end())` resets the reserved growth such that if
+  //   `reserve(N)` has previously been called and there has been no intervening
+  //   call to `clear()`, then after calling `erase(begin(), end())`, it is safe
+  //   to assume that inserting N elements will not cause a rehash.
   //
   // size_type erase(const key_type& key):
   //
-  //   Erases the element with the matching key, if it exists.
+  //   Erases the element with the matching key, if it exists, returning the
+  //   number of elements erased (0 or 1).
   using Base::erase;
 
   // flat_hash_map::insert()
@@ -354,8 +411,8 @@ class flat_hash_map : public absl::container_internal::raw_hash_map<
   // `flat_hash_map`.
   //
   //   iterator try_emplace(const_iterator hint,
-  //                        const init_type& k, Args&&... args):
-  //   iterator try_emplace(const_iterator hint, init_type&& k, Args&&... args):
+  //                        const key_type& k, Args&&... args):
+  //   iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args):
   //
   // Inserts (via copy or move) the element of the specified key into the
   // `flat_hash_map` using the position of `hint` as a non-binding suggestion
@@ -394,14 +451,13 @@ class flat_hash_map : public absl::container_internal::raw_hash_map<
   // flat_hash_map::swap(flat_hash_map& other)
   //
   // Exchanges the contents of this `flat_hash_map` with those of the `other`
-  // flat hash map, avoiding invocation of any move, copy, or swap operations on
-  // individual elements.
+  // flat hash map.
   //
   // All iterators and references on the `flat_hash_map` remain valid, excepting
   // for the past-the-end iterator, which is invalidated.
   //
   // `swap()` requires that the flat hash map's hashing and key equivalence
-  // functions be Swappable, and are exchaged using unqualified calls to
+  // functions be Swappable, and are exchanged using unqualified calls to
   // non-member `swap()`. If the map's allocator has
   // `std::allocator_traits<allocator_type>::propagate_on_container_swap::value`
   // set to `true`, the allocators are also exchanged using an unqualified call
@@ -426,7 +482,9 @@ class flat_hash_map : public absl::container_internal::raw_hash_map<
   //
   // Sets the number of slots in the `flat_hash_map` to the number needed to
   // accommodate at least `count` total elements without exceeding the current
-  // maximum load factor, and may rehash the container if needed.
+  // maximum load factor, and may rehash the container if needed. After this
+  // returns, it is guaranteed that `count - size()` elements can be inserted
+  // into the `flat_hash_map` without another rehash.
   using Base::reserve;
 
   // flat_hash_map::at()
@@ -531,6 +589,64 @@ class flat_hash_map : public absl::container_internal::raw_hash_map<
   using Base::key_eq;
 };
 
+// erase_if(flat_hash_map<>, Pred)
+//
+// Erases all elements that satisfy the predicate `pred` from the container `c`.
+// Returns the number of erased elements.
+template <typename K, typename V, typename H, typename E, typename A,
+          typename Predicate>
+typename flat_hash_map<K, V, H, E, A>::size_type erase_if(
+    flat_hash_map<K, V, H, E, A>& c, Predicate pred) {
+  return container_internal::EraseIf(pred, &c);
+}
+
+// swap(flat_hash_map<>, flat_hash_map<>)
+//
+// Swaps the contents of two `flat_hash_map` containers.
+//
+// NOTE: we need to define this function template in order for
+// `flat_hash_set::swap` to be called instead of `std::swap`. Even though we
+// have `swap(raw_hash_set&, raw_hash_set&)` defined, that function requires a
+// derived-to-base conversion, whereas `std::swap` is a function template so
+// `std::swap` will be preferred by compiler.
+template <typename K, typename V, typename H, typename E, typename A>
+void swap(flat_hash_map<K, V, H, E, A>& x,
+          flat_hash_map<K, V, H, E, A>& y) noexcept(noexcept(x.swap(y))) {
+  x.swap(y);
+}
+
+namespace container_internal {
+
+// c_for_each_fast(flat_hash_map<>, Function)
+//
+// Container-based version of the <algorithm> `std::for_each()` function to
+// apply a function to a container's elements.
+// There is no guarantees on the order of the function calls.
+// Erasure and/or insertion of elements in the function is not allowed.
+template <typename K, typename V, typename H, typename E, typename A,
+          typename Function>
+std::decay_t<Function> c_for_each_fast(const flat_hash_map<K, V, H, E, A>& c,
+                                       Function&& f) {
+  container_internal::ForEach(f, &c);
+  return f;
+}
+template <typename K, typename V, typename H, typename E, typename A,
+          typename Function>
+std::decay_t<Function> c_for_each_fast(flat_hash_map<K, V, H, E, A>& c,
+                                       Function&& f) {
+  container_internal::ForEach(f, &c);
+  return f;
+}
+template <typename K, typename V, typename H, typename E, typename A,
+          typename Function>
+std::decay_t<Function> c_for_each_fast(flat_hash_map<K, V, H, E, A>&& c,
+                                       Function&& f) {
+  container_internal::ForEach(f, &c);
+  return f;
+}
+
+}  // namespace container_internal
+
 namespace container_internal {
 
 template <class K, class V>
@@ -541,20 +657,25 @@ struct FlatHashMapPolicy {
   using mapped_type = V;
   using init_type = std::pair</*non const*/ key_type, mapped_type>;
 
+  using DefaultHash = DefaultHashContainerHash<K>;
+  using DefaultEq = DefaultHashContainerEq<K>;
+  using DefaultAlloc = std::allocator<std::pair<const K, V>>;
+
   template <class Allocator, class... Args>
   static void construct(Allocator* alloc, slot_type* slot, Args&&... args) {
     slot_policy::construct(alloc, slot, std::forward<Args>(args)...);
   }
 
+  // Returns std::true_type in case destroy is trivial.
   template <class Allocator>
-  static void destroy(Allocator* alloc, slot_type* slot) {
-    slot_policy::destroy(alloc, slot);
+  static auto destroy(Allocator* alloc, slot_type* slot) {
+    return slot_policy::destroy(alloc, slot);
   }
 
   template <class Allocator>
-  static void transfer(Allocator* alloc, slot_type* new_slot,
+  static auto transfer(Allocator* alloc, slot_type* new_slot,
                        slot_type* old_slot) {
-    slot_policy::transfer(alloc, new_slot, old_slot);
+    return slot_policy::transfer(alloc, new_slot, old_slot);
   }
 
   template <class F, class... Args>
@@ -565,6 +686,13 @@ struct FlatHashMapPolicy {
                                                    std::forward<Args>(args)...);
   }
 
+  template <class Hash, bool kIsDefault, size_t kSeedShift>
+  static constexpr HashSlotFn get_hash_slot_fn() {
+    return memory_internal::IsLayoutCompatible<K, V>::value
+               ? &TypeErasedApplyToSlotFn<Hash, K, kIsDefault, kSeedShift>
+               : nullptr;
+  }
+
   static size_t space_used(const slot_type*) { return 0; }
 
   static std::pair<const K, V>& element(slot_type* slot) { return slot->value; }
@@ -584,6 +712,7 @@ struct IsUnorderedContainer<
 
 }  // namespace container_algorithm_internal
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_FLAT_HASH_MAP_H_
diff --git a/absl/container/flat_hash_map_test.cc b/absl/container/flat_hash_map_test.cc
index 6cff1a25716..213cb70301a 100644
--- a/absl/container/flat_hash_map_test.cc
+++ b/absl/container/flat_hash_map_test.cc
@@ -14,29 +14,59 @@
 
 #include "absl/container/flat_hash_map.h"
 
+#include <cstddef>
 #include <memory>
-
+#include <string>
+#include <type_traits>
+#include <utility>
+#include <vector>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
 #include "absl/container/internal/hash_generator_testing.h"
+#include "absl/container/internal/hash_policy_testing.h"
+#include "absl/container/internal/test_allocator.h"
 #include "absl/container/internal/unordered_map_constructor_test.h"
 #include "absl/container/internal/unordered_map_lookup_test.h"
 #include "absl/container/internal/unordered_map_members_test.h"
 #include "absl/container/internal/unordered_map_modifiers_test.h"
-#include "absl/types/any.h"
+#include "absl/log/check.h"
+#include "absl/meta/type_traits.h"
+
+#if ABSL_INTERNAL_CPLUSPLUS_LANG >= 202002L
+#include <ranges>  // NOLINT(build/c++20)
+#endif
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 namespace {
-using ::absl::container_internal::hash_internal::Enum;
-using ::absl::container_internal::hash_internal::EnumClass;
+
 using ::testing::_;
+using ::testing::IsEmpty;
 using ::testing::Pair;
 using ::testing::UnorderedElementsAre;
+using ::testing::UnorderedElementsAreArray;
+
+// Check that absl::flat_hash_map works in a global constructor.
+struct BeforeMain {
+  BeforeMain() {
+    absl::flat_hash_map<int, int> x;
+    x.insert({1, 1});
+    CHECK(x.find(0) == x.end()) << "x should not contain 0";
+    auto it = x.find(1);
+    CHECK(it != x.end()) << "x should contain 1";
+    CHECK(it->second) << "1 should map to 1";
+  }
+};
+const BeforeMain before_main;
 
 template <class K, class V>
 using Map = flat_hash_map<K, V, StatefulTestingHash, StatefulTestingEqual,
                           Alloc<std::pair<const K, V>>>;
 
-static_assert(!std::is_standard_layout<NonStandardLayout>(), "");
+static_assert(!std::is_standard_layout<NonStandardLayout>());
 
 using MapTypes =
     ::testing::Types<Map<int, int>, Map<std::string, int>,
@@ -62,7 +92,7 @@ TEST(FlatHashMap, StandardLayout) {
     bool operator==(const Int& other) const { return value == other.value; }
     size_t value;
   };
-  static_assert(std::is_standard_layout<Int>(), "");
+  static_assert(std::is_standard_layout<Int>());
 
   struct Hash {
     size_t operator()(const Int& obj) const { return obj.value; }
@@ -86,6 +116,31 @@ TEST(FlatHashMap, StandardLayout) {
   }
 }
 
+TEST(FlatHashMap, Relocatability) {
+  static_assert(absl::is_trivially_relocatable<int>::value);
+  static_assert(
+      std::is_same_v<
+          decltype(absl::container_internal::FlatHashMapPolicy<
+                   int, int>::transfer<std::allocator<char>>(nullptr, nullptr,
+                                                             nullptr)),
+          std::true_type>);
+
+  struct NonRelocatable {
+    NonRelocatable() = default;
+    NonRelocatable(NonRelocatable&&) {}
+    NonRelocatable& operator=(NonRelocatable&&) { return *this; }
+    void* self = nullptr;
+  };
+
+  EXPECT_FALSE(absl::is_trivially_relocatable<NonRelocatable>::value);
+  EXPECT_TRUE(
+      (std::is_same_v<decltype(absl::container_internal::FlatHashMapPolicy<
+                               int, NonRelocatable>::
+                                   transfer<std::allocator<char>>(
+                                       nullptr, nullptr, nullptr)),
+                      std::false_type>));
+}
+
 // gcc becomes unhappy if this is inside the method, so pull it out here.
 struct balast {};
 
@@ -134,9 +189,7 @@ struct Hash {
 
 struct Eq {
   using is_transparent = void;
-  bool operator()(size_t lhs, size_t rhs) const {
-    return lhs == rhs;
-  }
+  bool operator()(size_t lhs, size_t rhs) const { return lhs == rhs; }
   bool operator()(size_t lhs, const LazyInt& rhs) const {
     return lhs == rhs.value;
   }
@@ -214,41 +267,218 @@ TEST(FlatHashMap, MergeExtractInsert) {
   EXPECT_THAT(m, UnorderedElementsAre(Pair(1, 17), Pair(2, 9)));
 }
 
-#if (defined(ABSL_HAVE_STD_ANY) || !defined(_LIBCPP_VERSION)) && \
-    !defined(__EMSCRIPTEN__)
-TEST(FlatHashMap, Any) {
-  absl::flat_hash_map<int, absl::any> m;
-  m.emplace(1, 7);
-  auto it = m.find(1);
-  ASSERT_NE(it, m.end());
-  EXPECT_EQ(7, absl::any_cast<int>(it->second));
-
-  m.emplace(std::piecewise_construct, std::make_tuple(2), std::make_tuple(8));
-  it = m.find(2);
-  ASSERT_NE(it, m.end());
-  EXPECT_EQ(8, absl::any_cast<int>(it->second));
-
-  m.emplace(std::piecewise_construct, std::make_tuple(3),
-            std::make_tuple(absl::any(9)));
-  it = m.find(3);
-  ASSERT_NE(it, m.end());
-  EXPECT_EQ(9, absl::any_cast<int>(it->second));
-
-  struct H {
-    size_t operator()(const absl::any&) const { return 0; }
+bool FirstIsEven(std::pair<const int, int> p) { return p.first % 2 == 0; }
+
+TEST(FlatHashMap, EraseIf) {
+  // Erase all elements.
+  {
+    flat_hash_map<int, int> s = {{1, 1}, {2, 2}, {3, 3}, {4, 4}, {5, 5}};
+    EXPECT_EQ(erase_if(s, [](std::pair<const int, int>) { return true; }), 5);
+    EXPECT_THAT(s, IsEmpty());
+  }
+  // Erase no elements.
+  {
+    flat_hash_map<int, int> s = {{1, 1}, {2, 2}, {3, 3}, {4, 4}, {5, 5}};
+    EXPECT_EQ(erase_if(s, [](std::pair<const int, int>) { return false; }), 0);
+    EXPECT_THAT(s, UnorderedElementsAre(Pair(1, 1), Pair(2, 2), Pair(3, 3),
+                                        Pair(4, 4), Pair(5, 5)));
+  }
+  // Erase specific elements.
+  {
+    flat_hash_map<int, int> s = {{1, 1}, {2, 2}, {3, 3}, {4, 4}, {5, 5}};
+    EXPECT_EQ(erase_if(s,
+                       [](std::pair<const int, int> kvp) {
+                         return kvp.first % 2 == 1;
+                       }),
+              3);
+    EXPECT_THAT(s, UnorderedElementsAre(Pair(2, 2), Pair(4, 4)));
+  }
+  // Predicate is function reference.
+  {
+    flat_hash_map<int, int> s = {{1, 1}, {2, 2}, {3, 3}, {4, 4}, {5, 5}};
+    EXPECT_EQ(erase_if(s, FirstIsEven), 2);
+    EXPECT_THAT(s, UnorderedElementsAre(Pair(1, 1), Pair(3, 3), Pair(5, 5)));
+  }
+  // Predicate is function pointer.
+  {
+    flat_hash_map<int, int> s = {{1, 1}, {2, 2}, {3, 3}, {4, 4}, {5, 5}};
+    EXPECT_EQ(erase_if(s, &FirstIsEven), 2);
+    EXPECT_THAT(s, UnorderedElementsAre(Pair(1, 1), Pair(3, 3), Pair(5, 5)));
+  }
+}
+
+TEST(FlatHashMap, CForEach) {
+  flat_hash_map<int, int> m;
+  std::vector<std::pair<int, int>> expected;
+  for (int i = 0; i < 100; ++i) {
+    {
+      SCOPED_TRACE("mutable object iteration");
+      std::vector<std::pair<int, int>> v;
+      absl::container_internal::c_for_each_fast(
+          m, [&v](std::pair<const int, int>& p) { v.push_back(p); });
+      EXPECT_THAT(v, UnorderedElementsAreArray(expected));
+    }
+    {
+      SCOPED_TRACE("const object iteration");
+      std::vector<std::pair<int, int>> v;
+      const flat_hash_map<int, int>& cm = m;
+      absl::container_internal::c_for_each_fast(
+          cm, [&v](const std::pair<const int, int>& p) { v.push_back(p); });
+      EXPECT_THAT(v, UnorderedElementsAreArray(expected));
+    }
+    {
+      SCOPED_TRACE("const object iteration");
+      std::vector<std::pair<int, int>> v;
+      absl::container_internal::c_for_each_fast(
+          flat_hash_map<int, int>(m),
+          [&v](std::pair<const int, int>& p) { v.push_back(p); });
+      EXPECT_THAT(v, UnorderedElementsAreArray(expected));
+    }
+    m[i] = i;
+    expected.emplace_back(i, i);
+  }
+}
+
+TEST(FlatHashMap, CForEachMutate) {
+  flat_hash_map<int, int> s;
+  std::vector<std::pair<int, int>> expected;
+  for (int i = 0; i < 100; ++i) {
+    std::vector<std::pair<int, int>> v;
+    absl::container_internal::c_for_each_fast(
+        s, [&v](std::pair<const int, int>& p) {
+          v.push_back(p);
+          p.second++;
+        });
+    EXPECT_THAT(v, UnorderedElementsAreArray(expected));
+    for (auto& p : expected) {
+      p.second++;
+    }
+    EXPECT_THAT(s, UnorderedElementsAreArray(expected));
+    s[i] = i;
+    expected.emplace_back(i, i);
+  }
+}
+
+TEST(FlatHashMap, NodeHandleMutableKeyAccess) {
+  flat_hash_map<std::string, std::string> map;
+
+  map["key1"] = "mapped";
+
+  auto nh = map.extract(map.begin());
+  nh.key().resize(3);
+  map.insert(std::move(nh));
+
+  EXPECT_THAT(map, testing::ElementsAre(Pair("key", "mapped")));
+}
+
+TEST(FlatHashMap, Reserve) {
+  // Verify that if we reserve(size() + n) then we can perform n insertions
+  // without a rehash, i.e., without invalidating any references.
+  for (size_t trial = 0; trial < 20; ++trial) {
+    for (size_t initial = 3; initial < 100; ++initial) {
+      // Fill in `initial` entries, then erase 2 of them, then reserve space for
+      // two inserts and check for reference stability while doing the inserts.
+      flat_hash_map<size_t, size_t> map;
+      for (size_t i = 0; i < initial; ++i) {
+        map[i] = i;
+      }
+      map.erase(0);
+      map.erase(1);
+      map.reserve(map.size() + 2);
+      size_t& a2 = map[2];
+      // In the event of a failure, asan will complain in one of these two
+      // assignments.
+      map[initial] = a2;
+      map[initial + 1] = a2;
+      // Fail even when not under asan:
+      size_t& a2new = map[2];
+      EXPECT_EQ(&a2, &a2new);
+    }
+  }
+}
+
+TEST(FlatHashMap, RecursiveTypeCompiles) {
+  struct RecursiveType {
+    flat_hash_map<int, RecursiveType> m;
   };
-  struct E {
-    bool operator()(const absl::any&, const absl::any&) const { return true; }
+  RecursiveType t;
+  t.m[0] = RecursiveType{};
+}
+
+TEST(FlatHashMap, FlatHashMapPolicyDestroyReturnsTrue) {
+  EXPECT_TRUE(
+      (decltype(FlatHashMapPolicy<int, char>::destroy<std::allocator<char>>(
+          nullptr, nullptr))()));
+  EXPECT_FALSE(
+      (decltype(FlatHashMapPolicy<int, char>::destroy<CountingAllocator<char>>(
+          nullptr, nullptr))()));
+  EXPECT_FALSE((decltype(FlatHashMapPolicy<int, std::unique_ptr<int>>::destroy<
+                         std::allocator<char>>(nullptr, nullptr))()));
+}
+
+struct InconsistentHashEqType {
+  InconsistentHashEqType(int v1, int v2) : v1(v1), v2(v2) {}
+  template <typename H>
+  friend H AbslHashValue(H h, InconsistentHashEqType t) {
+    return H::combine(std::move(h), t.v1);
+  }
+  bool operator==(InconsistentHashEqType t) const { return v2 == t.v2; }
+  int v1, v2;
+};
+
+TEST(Iterator, InconsistentHashEqFunctorsValidation) {
+  if (!IsAssertEnabled()) GTEST_SKIP() << "Assertions not enabled.";
+
+  absl::flat_hash_map<InconsistentHashEqType, int> m;
+  for (int i = 0; i < 10; ++i) m[{i, i}] = 1;
+  // We need to insert multiple times to guarantee that we get the assertion
+  // because it's possible for the hash to collide with the inserted element
+  // that has v2==0. In those cases, the new element won't be inserted.
+  auto insert_conflicting_elems = [&] {
+    for (int i = 100; i < 20000; ++i) {
+      EXPECT_EQ((m[{i, 0}]), 1);
+    }
   };
-  absl::flat_hash_map<absl::any, int, H, E> m2;
-  m2.emplace(1, 7);
-  auto it2 = m2.find(1);
-  ASSERT_NE(it2, m2.end());
-  EXPECT_EQ(7, it2->second);
+
+  const char* crash_message = "hash/eq functors are inconsistent.";
+#if defined(__arm__) || defined(__aarch64__)
+  // On ARM, the crash message is garbled so don't expect a specific message.
+  crash_message = "";
+#endif
+  EXPECT_DEATH_IF_SUPPORTED(insert_conflicting_elems(), crash_message);
 }
-#endif  // (defined(ABSL_HAVE_STD_ANY) || !defined(_LIBCPP_VERSION)) &&
-        // !defined(__EMSCRIPTEN__)
+
+#if defined(__cpp_lib_containers_ranges) && \
+    __cpp_lib_containers_ranges >= 202202L
+TEST(FlatHashMap, FromRange) {
+  std::vector<std::pair<int, int>> v = {{1, 2}, {3, 4}, {5, 6}};
+  absl::flat_hash_map<int, int> m(std::from_range, v);
+  EXPECT_THAT(m, UnorderedElementsAre(Pair(1, 2), Pair(3, 4), Pair(5, 6)));
+}
+
+TEST(FlatHashMap, FromRangeWithAllocator) {
+  std::vector<std::pair<int, int>> v = {{1, 2}, {3, 4}, {5, 6}};
+  absl::flat_hash_map<int, int,
+                      absl::container_internal::hash_default_hash<int>,
+                      absl::container_internal::hash_default_eq<int>,
+                      Alloc<std::pair<const int, int>>>
+      m(std::from_range, v, 0, Alloc<std::pair<const int, int>>());
+  EXPECT_THAT(m, UnorderedElementsAre(Pair(1, 2), Pair(3, 4), Pair(5, 6)));
+}
+
+TEST(FlatHashMap, FromRangeWithHasherAndAllocator) {
+  std::vector<std::pair<int, int>> v = {{1, 2}, {3, 4}, {5, 6}};
+  using TestingHash = absl::container_internal::StatefulTestingHash;
+  absl::flat_hash_map<int, int, TestingHash,
+                      absl::container_internal::hash_default_eq<int>,
+                      Alloc<std::pair<const int, int>>>
+      m(std::from_range, v, 0, TestingHash{},
+        Alloc<std::pair<const int, int>>());
+  EXPECT_THAT(m, UnorderedElementsAre(Pair(1, 2), Pair(3, 4), Pair(5, 6)));
+}
+#endif
 
 }  // namespace
 }  // namespace container_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/container/flat_hash_set.h b/absl/container/flat_hash_set.h
index 2a51c3411d6..ed977431cc6 100644
--- a/absl/container/flat_hash_set.h
+++ b/absl/container/flat_hash_set.h
@@ -26,20 +26,28 @@
 //
 // In most cases, your default choice for a hash set should be a set of type
 // `flat_hash_set`.
+//
+// `flat_hash_set` is not exception-safe.
+
 #ifndef ABSL_CONTAINER_FLAT_HASH_SET_H_
 #define ABSL_CONTAINER_FLAT_HASH_SET_H_
 
+#include <cstddef>
+#include <memory>
 #include <type_traits>
 #include <utility>
 
 #include "absl/algorithm/container.h"
+#include "absl/base/attributes.h"
 #include "absl/base/macros.h"
+#include "absl/container/hash_container_defaults.h"
 #include "absl/container/internal/container_memory.h"
-#include "absl/container/internal/hash_function_defaults.h"  // IWYU pragma: export
 #include "absl/container/internal/raw_hash_set.h"  // IWYU pragma: export
 #include "absl/memory/memory.h"
+#include "absl/meta/type_traits.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 template <typename T>
 struct FlatHashSetPolicy;
@@ -54,23 +62,46 @@ struct FlatHashSetPolicy;
 // Its interface is similar to that of `std::unordered_set<T>` with the
 // following notable differences:
 //
-// * Requires keys that are CopyConstructible
+// * Requires keys that are MoveConstructible
 // * Supports heterogeneous lookup, through `find()` and `insert()`, provided
 //   that the set is provided a compatible heterogeneous hashing function and
-//   equality operator.
+//   equality operator. See below for details.
 // * Invalidates any references and pointers to elements within the table after
-//   `rehash()`.
+//   `rehash()` and when the table is moved.
 // * Contains a `capacity()` member function indicating the number of element
 //   slots (open, deleted, and empty) within the hash set.
 // * Returns `void` from the `erase(iterator)` overload.
+// * Constructors accept reservation size as an optional argument instead of
+//   bucket count. Reservation size is the number of elements that fits in the
+//   set before rehash.
+// * insert/emplace and other modification functions return special iterator
+//   that doesn't support iteration. std::next(it) for such iterators would
+//   always point to the end().
 //
 // By default, `flat_hash_set` uses the `absl::Hash` hashing framework. All
 // fundamental and Abseil types that support the `absl::Hash` framework have a
-// compatible equality operator for comparing insertions into `flat_hash_map`.
+// compatible equality operator for comparing insertions into `flat_hash_set`.
 // If your type is not yet supported by the `absl::Hash` framework, see
 // absl/hash/hash.h for information on extending Abseil hashing to user-defined
 // types.
 //
+// Using `absl::flat_hash_set` at interface boundaries in dynamically loaded
+// libraries (e.g. .dll, .so) is unsupported due to way `absl::Hash` values may
+// be randomized across dynamically loaded libraries.
+//
+// To achieve heterogeneous lookup for custom types either `Hash` and `Eq` type
+// parameters can be used or `T` should have public inner types
+// `absl_container_hash` and (optionally) `absl_container_eq`. In either case,
+// `typename Hash::is_transparent` and `typename Eq::is_transparent` should be
+// well-formed. Both types are basically functors:
+// * `Hash` should support `size_t operator()(U val) const` that returns a hash
+// for the given `val`.
+// * `Eq` should support `bool operator()(U lhs, V rhs) const` that returns true
+// if `lhs` is equal to `rhs`.
+//
+// In most cases `T` needs only to provide the `absl_container_hash`. In this
+// case `std::equal_to<void>` will be used instead of `eq` part.
+//
 // NOTE: A `flat_hash_set` stores its keys directly inside its implementation
 // array to avoid memory indirection. Because a `flat_hash_set` is designed to
 // move data when rehashed, set keys will not retain pointer stability. If you
@@ -78,34 +109,43 @@ struct FlatHashSetPolicy;
 // `absl::flat_hash_set<std::unique_ptr<T>>`. If your type is not moveable and
 // you require pointer stability, consider `absl::node_hash_set` instead.
 //
+// PERFORMANCE WARNING: Erasure & sparsity can negatively affect performance:
+//  * Iteration takes O(capacity) time, not O(size).
+//  * erase() slows down begin() and ++iterator.
+//  * Capacity only shrinks on rehash() or clear() -- not on erase().
+//
 // Example:
 //
 //   // Create a flat hash set of three strings
 //   absl::flat_hash_set<std::string> ducks =
 //     {"huey", "dewey", "louie"};
 //
-//  // Insert a new element into the flat hash set
-//  ducks.insert("donald");
+//   // Insert a new element into the flat hash set
+//   ducks.insert("donald");
 //
-//  // Force a rehash of the flat hash set
-//  ducks.rehash(0);
+//   // Force a rehash of the flat hash set
+//   ducks.rehash(0);
 //
-//  // See if "dewey" is present
-//  if (ducks.contains("dewey")) {
-//    std::cout << "We found dewey!" << std::endl;
-//  }
-template <class T, class Hash = absl::container_internal::hash_default_hash<T>,
-          class Eq = absl::container_internal::hash_default_eq<T>,
-          class Allocator = std::allocator<T>>
-class flat_hash_set
-    : public absl::container_internal::raw_hash_set<
-          absl::container_internal::FlatHashSetPolicy<T>, Hash, Eq, Allocator> {
+//   // See if "dewey" is present
+//   if (ducks.contains("dewey")) {
+//     std::cout << "We found dewey!" << std::endl;
+//   }
+template <
+    class T,
+    class Hash = typename container_internal::FlatHashSetPolicy<T>::DefaultHash,
+    class Eq = typename container_internal::FlatHashSetPolicy<T>::DefaultEq,
+    class Allocator =
+        typename container_internal::FlatHashSetPolicy<T>::DefaultAlloc>
+class ABSL_ATTRIBUTE_OWNER flat_hash_set
+    : public absl::container_internal::InstantiateRawHashSet<
+          absl::container_internal::FlatHashSetPolicy<T>, Hash, Eq,
+          Allocator>::type {
   using Base = typename flat_hash_set::raw_hash_set;
 
  public:
   // Constructors and Assignment Operators
   //
-  // A flat_hash_set supports the same overload set as `std::unordered_map`
+  // A flat_hash_set supports the same overload set as `std::unordered_set`
   // for construction and assignment:
   //
   // *  Default constructor
@@ -124,25 +164,37 @@ class flat_hash_set
   //
   // * Copy assignment operator
   //
-  //  // Hash functor and Comparator are copied as well
-  //  absl::flat_hash_set<std::string> set4;
-  //  set4 = set3;
+  //   // Hash functor and Comparator are copied as well
+  //   absl::flat_hash_set<std::string> set4;
+  //   set4 = set3;
   //
   // * Move constructor
   //
   //   // Move is guaranteed efficient
   //   absl::flat_hash_set<std::string> set5(std::move(set4));
   //
+  //   // After the move, set4 is in a valid but unspecified state. The only
+  //   // operations guaranteed to be safe on a moved-from set are destruction,
+  //   // assignment, and clear(). Any other operation (e.g. size(), empty(),
+  //   // iteration) results in undefined behavior.
+  //
   // * Move assignment operator
   //
   //   // May be efficient if allocators are compatible
   //   absl::flat_hash_set<std::string> set6;
   //   set6 = std::move(set5);
   //
+  //   // Same moved-from guarantees apply to set5 after this operation.
+  //
   // * Range constructor
   //
   //   std::vector<std::string> v = {"a", "b"};
   //   absl::flat_hash_set<std::string> set7(v.begin(), v.end());
+  //
+  // * from_range constructor (C++23)
+  //
+  //   std::vector<std::string> v = {"a", "b"};
+  //   absl::flat_hash_set<std::string> set8(std::from_range, v);
   flat_hash_set() {}
   using Base::Base;
 
@@ -172,7 +224,7 @@ class flat_hash_set
   // available within the `flat_hash_set`.
   //
   // NOTE: this member function is particular to `absl::flat_hash_set` and is
-  // not provided in the `std::unordered_map` API.
+  // not provided in the `std::unordered_set` API.
   using Base::capacity;
 
   // flat_hash_set::empty()
@@ -222,11 +274,16 @@ class flat_hash_set
   // iterator erase(const_iterator first, const_iterator last):
   //
   //   Erases the elements in the open interval [`first`, `last`), returning an
-  //   iterator pointing to `last`.
+  //   iterator pointing to `last`. The special case of calling
+  //   `erase(begin(), end())` resets the reserved growth such that if
+  //   `reserve(N)` has previously been called and there has been no intervening
+  //   call to `clear()`, then after calling `erase(begin(), end())`, it is safe
+  //   to assume that inserting N elements will not cause a rehash.
   //
   // size_type erase(const key_type& key):
   //
-  //   Erases the element with the matching key, if it exists.
+  //   Erases the element with the matching key, if it exists, returning the
+  //   number of elements erased (0 or 1).
   using Base::erase;
 
   // flat_hash_set::insert()
@@ -322,7 +379,7 @@ class flat_hash_set
 
   // flat_hash_set::merge()
   //
-  // Extracts elements from a given `source` flat hash map into this
+  // Extracts elements from a given `source` flat hash set into this
   // `flat_hash_set`. If the destination `flat_hash_set` already contains an
   // element with an equivalent key, that element is not extracted.
   using Base::merge;
@@ -330,15 +387,14 @@ class flat_hash_set
   // flat_hash_set::swap(flat_hash_set& other)
   //
   // Exchanges the contents of this `flat_hash_set` with those of the `other`
-  // flat hash map, avoiding invocation of any move, copy, or swap operations on
-  // individual elements.
+  // flat hash set.
   //
   // All iterators and references on the `flat_hash_set` remain valid, excepting
   // for the past-the-end iterator, which is invalidated.
   //
   // `swap()` requires that the flat hash set's hashing and key equivalence
-  // functions be Swappable, and are exchaged using unqualified calls to
-  // non-member `swap()`. If the map's allocator has
+  // functions be Swappable, and are exchanged using unqualified calls to
+  // non-member `swap()`. If the set's allocator has
   // `std::allocator_traits<allocator_type>::propagate_on_container_swap::value`
   // set to `true`, the allocators are also exchanged using an unqualified call
   // to non-member `swap()`; otherwise, the allocators are not swapped.
@@ -362,7 +418,9 @@ class flat_hash_set
   //
   // Sets the number of slots in the `flat_hash_set` to the number needed to
   // accommodate at least `count` total elements without exceeding the current
-  // maximum load factor, and may rehash the container if needed.
+  // maximum load factor, and may rehash the container if needed. After this
+  // returns, it is guaranteed that `count - size()` elements can be inserted
+  // into the `flat_hash_set` without another rehash.
   using Base::reserve;
 
   // flat_hash_set::contains()
@@ -393,14 +451,14 @@ class flat_hash_set
   // flat_hash_set::bucket_count()
   //
   // Returns the number of "buckets" within the `flat_hash_set`. Note that
-  // because a flat hash map contains all elements within its internal storage,
+  // because a flat hash set contains all elements within its internal storage,
   // this value simply equals the current capacity of the `flat_hash_set`.
   using Base::bucket_count;
 
   // flat_hash_set::load_factor()
   //
   // Returns the current load factor of the `flat_hash_set` (the average number
-  // of slots occupied with a value within the hash map).
+  // of slots occupied with a value within the hash set).
   using Base::load_factor;
 
   // flat_hash_set::max_load_factor()
@@ -438,6 +496,60 @@ class flat_hash_set
   using Base::key_eq;
 };
 
+// erase_if(flat_hash_set<>, Pred)
+//
+// Erases all elements that satisfy the predicate `pred` from the container `c`.
+// Returns the number of erased elements.
+template <typename T, typename H, typename E, typename A, typename Predicate>
+typename flat_hash_set<T, H, E, A>::size_type erase_if(
+    flat_hash_set<T, H, E, A>& c, Predicate pred) {
+  return container_internal::EraseIf(pred, &c);
+}
+
+// swap(flat_hash_set<>, flat_hash_set<>)
+//
+// Swaps the contents of two `flat_hash_set` containers.
+//
+// NOTE: we need to define this function template in order for
+// `flat_hash_set::swap` to be called instead of `std::swap`. Even though we
+// have `swap(raw_hash_set&, raw_hash_set&)` defined, that function requires a
+// derived-to-base conversion, whereas `std::swap` is a function template so
+// `std::swap` will be preferred by compiler.
+template <typename T, typename H, typename E, typename A>
+void swap(flat_hash_set<T, H, E, A>& x,
+          flat_hash_set<T, H, E, A>& y) noexcept(noexcept(x.swap(y))) {
+  return x.swap(y);
+}
+
+namespace container_internal {
+
+// c_for_each_fast(flat_hash_set<>, Function)
+//
+// Container-based version of the <algorithm> `std::for_each()` function to
+// apply a function to a container's elements.
+// There is no guarantees on the order of the function calls.
+// Erasure and/or insertion of elements in the function is not allowed.
+template <typename T, typename H, typename E, typename A, typename Function>
+std::decay_t<Function> c_for_each_fast(const flat_hash_set<T, H, E, A>& c,
+                                       Function&& f) {
+  container_internal::ForEach(f, &c);
+  return f;
+}
+template <typename T, typename H, typename E, typename A, typename Function>
+std::decay_t<Function> c_for_each_fast(flat_hash_set<T, H, E, A>& c,
+                                       Function&& f) {
+  container_internal::ForEach(f, &c);
+  return f;
+}
+template <typename T, typename H, typename E, typename A, typename Function>
+std::decay_t<Function> c_for_each_fast(flat_hash_set<T, H, E, A>&& c,
+                                       Function&& f) {
+  container_internal::ForEach(f, &c);
+  return f;
+}
+
+}  // namespace container_internal
+
 namespace container_internal {
 
 template <class T>
@@ -447,22 +559,21 @@ struct FlatHashSetPolicy {
   using init_type = T;
   using constant_iterators = std::true_type;
 
+  using DefaultHash = DefaultHashContainerHash<T>;
+  using DefaultEq = DefaultHashContainerEq<T>;
+  using DefaultAlloc = std::allocator<T>;
+
   template <class Allocator, class... Args>
   static void construct(Allocator* alloc, slot_type* slot, Args&&... args) {
-    absl::allocator_traits<Allocator>::construct(*alloc, slot,
-                                                 std::forward<Args>(args)...);
-  }
-
-  template <class Allocator>
-  static void destroy(Allocator* alloc, slot_type* slot) {
-    absl::allocator_traits<Allocator>::destroy(*alloc, slot);
+    std::allocator_traits<Allocator>::construct(*alloc, slot,
+                                                std::forward<Args>(args)...);
   }
 
+  // Return std::true_type in case destroy is trivial.
   template <class Allocator>
-  static void transfer(Allocator* alloc, slot_type* new_slot,
-                       slot_type* old_slot) {
-    construct(alloc, new_slot, std::move(*old_slot));
-    destroy(alloc, old_slot);
+  static auto destroy(Allocator* alloc, slot_type* slot) {
+    std::allocator_traits<Allocator>::destroy(*alloc, slot);
+    return IsDestructionTrivial<Allocator, slot_type>();
   }
 
   static T& element(slot_type* slot) { return *slot; }
@@ -476,6 +587,11 @@ struct FlatHashSetPolicy {
   }
 
   static size_t space_used(const T*) { return 0; }
+
+  template <class Hash, bool kIsDefault, size_t kSeedShift>
+  static constexpr HashSlotFn get_hash_slot_fn() {
+    return &TypeErasedApplyToSlotFn<Hash, T, kIsDefault, kSeedShift>;
+  }
 };
 }  // namespace container_internal
 
@@ -488,6 +604,7 @@ struct IsUnorderedContainer<absl::flat_hash_set<Key, Hash, KeyEqual, Allocator>>
 
 }  // namespace container_algorithm_internal
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_FLAT_HASH_SET_H_
diff --git a/absl/container/flat_hash_set_test.cc b/absl/container/flat_hash_set_test.cc
index b55be59b04c..95ead482a8f 100644
--- a/absl/container/flat_hash_set_test.cc
+++ b/absl/container/flat_hash_set_test.cc
@@ -14,26 +14,55 @@
 
 #include "absl/container/flat_hash_set.h"
 
+#include <cstddef>
+#include <cstdint>
+#include <memory>
+#include <string>
+#include <type_traits>
+#include <utility>
 #include <vector>
 
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+#include "absl/container/hash_container_defaults.h"
+#include "absl/container/internal/container_memory.h"
 #include "absl/container/internal/hash_generator_testing.h"
+#include "absl/container/internal/test_allocator.h"
 #include "absl/container/internal/unordered_set_constructor_test.h"
 #include "absl/container/internal/unordered_set_lookup_test.h"
 #include "absl/container/internal/unordered_set_members_test.h"
 #include "absl/container/internal/unordered_set_modifiers_test.h"
+#include "absl/hash/hash.h"
+#include "absl/log/check.h"
 #include "absl/memory/memory.h"
 #include "absl/strings/string_view.h"
 
+#if ABSL_INTERNAL_CPLUSPLUS_LANG >= 202002L
+#include <ranges>  // NOLINT(build/c++20)
+#endif
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 namespace {
 
-using ::absl::container_internal::hash_internal::Enum;
-using ::absl::container_internal::hash_internal::EnumClass;
+using ::testing::IsEmpty;
 using ::testing::Pointee;
 using ::testing::UnorderedElementsAre;
 using ::testing::UnorderedElementsAreArray;
 
+// Check that absl::flat_hash_set works in a global constructor.
+struct BeforeMain {
+  BeforeMain() {
+    absl::flat_hash_set<int> x;
+    x.insert(1);
+    CHECK(!x.contains(0)) << "x should not contain 0";
+    CHECK(x.contains(1)) << "x should contain 1";
+  }
+};
+const BeforeMain before_main;
+
 template <class T>
 using Set =
     absl::flat_hash_set<T, StatefulTestingHash, StatefulTestingEqual, Alloc<T>>;
@@ -80,11 +109,11 @@ TEST(FlatHashSet, MergeExtractInsert) {
     }
   };
   absl::flat_hash_set<std::unique_ptr<int>, Hash, Eq> set1, set2;
-  set1.insert(absl::make_unique<int>(7));
-  set1.insert(absl::make_unique<int>(17));
+  set1.insert(std::make_unique<int>(7));
+  set1.insert(std::make_unique<int>(17));
 
-  set2.insert(absl::make_unique<int>(7));
-  set2.insert(absl::make_unique<int>(19));
+  set2.insert(std::make_unique<int>(7));
+  set2.insert(std::make_unique<int>(19));
 
   EXPECT_THAT(set1, UnorderedElementsAre(Pointee(7), Pointee(17)));
   EXPECT_THAT(set2, UnorderedElementsAre(Pointee(7), Pointee(19)));
@@ -94,7 +123,7 @@ TEST(FlatHashSet, MergeExtractInsert) {
   EXPECT_THAT(set1, UnorderedElementsAre(Pointee(7), Pointee(17), Pointee(19)));
   EXPECT_THAT(set2, UnorderedElementsAre(Pointee(7)));
 
-  auto node = set1.extract(absl::make_unique<int>(7));
+  auto node = set1.extract(std::make_unique<int>(7));
   EXPECT_TRUE(node);
   EXPECT_THAT(node.value(), Pointee(7));
   EXPECT_THAT(set1, UnorderedElementsAre(Pointee(17), Pointee(19)));
@@ -108,12 +137,12 @@ TEST(FlatHashSet, MergeExtractInsert) {
   EXPECT_NE(insert_result.position->get(), insert_result.node.value().get());
   EXPECT_THAT(set2, UnorderedElementsAre(Pointee(7)));
 
-  node = set1.extract(absl::make_unique<int>(17));
+  node = set1.extract(std::make_unique<int>(17));
   EXPECT_TRUE(node);
   EXPECT_THAT(node.value(), Pointee(17));
   EXPECT_THAT(set1, UnorderedElementsAre(Pointee(19)));
 
-  node.value() = absl::make_unique<int>(23);
+  node.value() = std::make_unique<int>(23);
 
   insert_result = set2.insert(std::move(node));
   EXPECT_FALSE(node);
@@ -123,6 +152,282 @@ TEST(FlatHashSet, MergeExtractInsert) {
   EXPECT_THAT(set2, UnorderedElementsAre(Pointee(7), Pointee(23)));
 }
 
+bool IsEven(int k) { return k % 2 == 0; }
+
+TEST(FlatHashSet, EraseIf) {
+  // Erase all elements.
+  {
+    flat_hash_set<int> s = {1, 2, 3, 4, 5};
+    EXPECT_EQ(erase_if(s, [](int) { return true; }), 5);
+    EXPECT_THAT(s, IsEmpty());
+  }
+  // Erase no elements.
+  {
+    flat_hash_set<int> s = {1, 2, 3, 4, 5};
+    EXPECT_EQ(erase_if(s, [](int) { return false; }), 0);
+    EXPECT_THAT(s, UnorderedElementsAre(1, 2, 3, 4, 5));
+  }
+  // Erase specific elements.
+  {
+    flat_hash_set<int> s = {1, 2, 3, 4, 5};
+    EXPECT_EQ(erase_if(s, [](int k) { return k % 2 == 1; }), 3);
+    EXPECT_THAT(s, UnorderedElementsAre(2, 4));
+  }
+  // Predicate is function reference.
+  {
+    flat_hash_set<int> s = {1, 2, 3, 4, 5};
+    EXPECT_EQ(erase_if(s, IsEven), 2);
+    EXPECT_THAT(s, UnorderedElementsAre(1, 3, 5));
+  }
+  // Predicate is function pointer.
+  {
+    flat_hash_set<int> s = {1, 2, 3, 4, 5};
+    EXPECT_EQ(erase_if(s, &IsEven), 2);
+    EXPECT_THAT(s, UnorderedElementsAre(1, 3, 5));
+  }
+}
+
+TEST(FlatHashSet, CForEach) {
+  using ValueType = std::pair<int, int>;
+  flat_hash_set<ValueType> s;
+  std::vector<ValueType> expected;
+  for (int i = 0; i < 100; ++i) {
+    {
+      SCOPED_TRACE("mutable object iteration");
+      std::vector<ValueType> v;
+      absl::container_internal::c_for_each_fast(
+          s, [&v](const ValueType& p) { v.push_back(p); });
+      ASSERT_THAT(v, UnorderedElementsAreArray(expected));
+    }
+    {
+      SCOPED_TRACE("const object iteration");
+      std::vector<ValueType> v;
+      const flat_hash_set<ValueType>& cs = s;
+      absl::container_internal::c_for_each_fast(
+          cs, [&v](const ValueType& p) { v.push_back(p); });
+      ASSERT_THAT(v, UnorderedElementsAreArray(expected));
+    }
+    {
+      SCOPED_TRACE("temporary object iteration");
+      std::vector<ValueType> v;
+      absl::container_internal::c_for_each_fast(
+          flat_hash_set<ValueType>(s),
+          [&v](const ValueType& p) { v.push_back(p); });
+      ASSERT_THAT(v, UnorderedElementsAreArray(expected));
+    }
+    s.emplace(i, i);
+    expected.emplace_back(i, i);
+  }
+}
+
+class PoisonSoo {
+  int64_t data_;
+
+ public:
+  explicit PoisonSoo(int64_t d) : data_(d) { SanitizerPoisonObject(&data_); }
+  PoisonSoo(const PoisonSoo& that) : PoisonSoo(*that) {}
+  ~PoisonSoo() { SanitizerUnpoisonObject(&data_); }
+
+  int64_t operator*() const {
+    SanitizerUnpoisonObject(&data_);
+    const int64_t ret = data_;
+    SanitizerPoisonObject(&data_);
+    return ret;
+  }
+  template <typename H>
+  friend H AbslHashValue(H h, const PoisonSoo& pi) {
+    return H::combine(std::move(h), *pi);
+  }
+  bool operator==(const PoisonSoo& rhs) const { return **this == *rhs; }
+};
+
+TEST(FlatHashSet, PoisonSooBasic) {
+  PoisonSoo a(0), b(1);
+  flat_hash_set<PoisonSoo> set;
+  set.insert(a);
+  EXPECT_THAT(set, UnorderedElementsAre(a));
+  set.insert(b);
+  EXPECT_THAT(set, UnorderedElementsAre(a, b));
+  set.erase(a);
+  EXPECT_THAT(set, UnorderedElementsAre(b));
+  set.rehash(0);  // Shrink to SOO.
+  EXPECT_THAT(set, UnorderedElementsAre(b));
+}
+
+TEST(FlatHashSet, PoisonSooMoveConstructSooToSoo) {
+  PoisonSoo a(0);
+  flat_hash_set<PoisonSoo> set;
+  set.insert(a);
+  flat_hash_set<PoisonSoo> set2(std::move(set));
+  EXPECT_THAT(set2, UnorderedElementsAre(a));
+}
+
+TEST(FlatHashSet, PoisonSooAllocMoveConstructSooToSoo) {
+  PoisonSoo a(0);
+  flat_hash_set<PoisonSoo> set;
+  set.insert(a);
+  flat_hash_set<PoisonSoo> set2(std::move(set), std::allocator<PoisonSoo>());
+  EXPECT_THAT(set2, UnorderedElementsAre(a));
+}
+
+TEST(FlatHashSet, PoisonSooMoveAssignFullSooToEmptySoo) {
+  PoisonSoo a(0);
+  flat_hash_set<PoisonSoo> set, set2;
+  set.insert(a);
+  set2 = std::move(set);
+  EXPECT_THAT(set2, UnorderedElementsAre(a));
+}
+
+TEST(FlatHashSet, PoisonSooMoveAssignFullSooToFullSoo) {
+  PoisonSoo a(0), b(1);
+  flat_hash_set<PoisonSoo> set, set2;
+  set.insert(a);
+  set2.insert(b);
+  set2 = std::move(set);
+  EXPECT_THAT(set2, UnorderedElementsAre(a));
+}
+
+TEST(FlatHashSet, FlatHashSetPolicyDestroyReturnsTrue) {
+  EXPECT_TRUE((decltype(FlatHashSetPolicy<int>::destroy<std::allocator<int>>(
+      nullptr, nullptr))()));
+  EXPECT_FALSE(
+      (decltype(FlatHashSetPolicy<int>::destroy<CountingAllocator<int>>(
+          nullptr, nullptr))()));
+  EXPECT_FALSE((decltype(FlatHashSetPolicy<std::unique_ptr<int>>::destroy<
+                         std::allocator<int>>(nullptr, nullptr))()));
+}
+
+struct HashEqInvalidOnMove {
+  HashEqInvalidOnMove() = default;
+  HashEqInvalidOnMove(const HashEqInvalidOnMove& rhs) = default;
+  HashEqInvalidOnMove(HashEqInvalidOnMove&& rhs) { rhs.moved = true; }
+  HashEqInvalidOnMove& operator=(const HashEqInvalidOnMove& rhs) = default;
+  HashEqInvalidOnMove& operator=(HashEqInvalidOnMove&& rhs) {
+    rhs.moved = true;
+    return *this;
+  }
+
+  size_t operator()(int x) const {
+    CHECK(!moved);
+    return absl::HashOf(x);
+  }
+
+  bool operator()(int x, int y) const {
+    CHECK(!moved);
+    return x == y;
+  }
+
+  bool moved = false;
+};
+
+TEST(FlatHashSet, MovedFromCleared_HashMustBeValid) {
+  flat_hash_set<int, HashEqInvalidOnMove> s1, s2;
+  // Moving the hashtable must not move the hasher because we need to support
+  // this behavior.
+  s2 = std::move(s1);
+  s1.clear();
+  s1.insert(2);
+  EXPECT_THAT(s1, UnorderedElementsAre(2));
+}
+
+TEST(FlatHashSet, MovedFromCleared_EqMustBeValid) {
+  flat_hash_set<int, DefaultHashContainerHash<int>, HashEqInvalidOnMove> s1, s2;
+  // Moving the hashtable must not move the equality functor because we need to
+  // support this behavior.
+  s2 = std::move(s1);
+  s1.clear();
+  s1.insert(2);
+  EXPECT_THAT(s1, UnorderedElementsAre(2));
+}
+
+TEST(FlatHashSet, Equality) {
+  {
+    flat_hash_set<int> s1 = {1, 2, 3};
+    flat_hash_set<int> s2 = {1, 2, 3};
+    EXPECT_EQ(s1, s2);
+  }
+  {
+    flat_hash_set<std::string> s1 = {"a", "b", "c"};
+    flat_hash_set<std::string> s2 = {"a", "b", "c"};
+    EXPECT_EQ(s1, s2);
+  }
+}
+
+class MoveOnlyInt {
+ public:
+  explicit MoveOnlyInt(int value) : value_(value) {}
+
+  MoveOnlyInt(const MoveOnlyInt& other) = delete;
+  MoveOnlyInt& operator=(const MoveOnlyInt& other) = delete;
+
+  MoveOnlyInt(MoveOnlyInt&& other) = default;
+  MoveOnlyInt& operator=(MoveOnlyInt&& other) = default;
+
+  bool operator==(const MoveOnlyInt& other) const {
+    return value_ == other.value_;
+  }
+  bool operator==(int other) const { return value_ == other; }
+
+ private:
+  template <typename H>
+  friend H AbslHashValue(H h, const MoveOnlyInt& m) {
+    return H::combine(std::move(h), m.value_);
+  }
+
+  int value_;
+};
+
+TEST(FlatHashSet, MoveOnlyKey) {
+  flat_hash_set<MoveOnlyInt> s;
+  s.insert(MoveOnlyInt(1));
+  s.insert(MoveOnlyInt(2));
+  s.insert(MoveOnlyInt(3));
+  EXPECT_THAT(s, UnorderedElementsAre(1, 2, 3));
+}
+
+TEST(FlatHashSet, IsDefaultHash) {
+  using absl::container_internal::hashtable_debug_internal::
+      HashtableDebugAccess;
+  EXPECT_EQ(HashtableDebugAccess<flat_hash_set<int>>::kIsDefaultHash, true);
+  EXPECT_EQ(HashtableDebugAccess<flat_hash_set<std::string>>::kIsDefaultHash,
+            true);
+
+  struct Hash {
+    size_t operator()(size_t i) const { return i; }
+  };
+  EXPECT_EQ((HashtableDebugAccess<flat_hash_set<size_t, Hash>>::kIsDefaultHash),
+            false);
+}
+
+#if defined(__cpp_lib_containers_ranges) && \
+    __cpp_lib_containers_ranges >= 202202L
+TEST(FlatHashSet, FromRange) {
+  std::vector<int> v = {1, 2, 3, 4, 5};
+  absl::flat_hash_set<int> s(std::from_range, v);
+  EXPECT_THAT(s, UnorderedElementsAre(1, 2, 3, 4, 5));
+}
+
+TEST(FlatHashSet, FromRangeWithAllocator) {
+  std::vector<int> v = {1, 2, 3, 4, 5};
+  absl::flat_hash_set<int, absl::container_internal::hash_default_hash<int>,
+                      absl::container_internal::hash_default_eq<int>,
+                      Alloc<int>>
+      s(std::from_range, v, 0, Alloc<int>());
+  EXPECT_THAT(s, UnorderedElementsAre(1, 2, 3, 4, 5));
+}
+
+TEST(FlatHashSet, FromRangeWithHasherAndAllocator) {
+  std::vector<int> v = {1, 2, 3, 4, 5};
+  using TestingHash = absl::container_internal::StatefulTestingHash;
+  absl::flat_hash_set<int, TestingHash,
+                      absl::container_internal::hash_default_eq<int>,
+                      Alloc<int>>
+      s(std::from_range, v, 0, TestingHash{}, Alloc<int>());
+  EXPECT_THAT(s, UnorderedElementsAre(1, 2, 3, 4, 5));
+}
+#endif
+
 }  // namespace
 }  // namespace container_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/container/hash_container_defaults.h b/absl/container/hash_container_defaults.h
new file mode 100644
index 00000000000..eb944a7c817
--- /dev/null
+++ b/absl/container/hash_container_defaults.h
@@ -0,0 +1,45 @@
+// Copyright 2024 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_CONTAINER_HASH_CONTAINER_DEFAULTS_H_
+#define ABSL_CONTAINER_HASH_CONTAINER_DEFAULTS_H_
+
+#include "absl/base/config.h"
+#include "absl/container/internal/hash_function_defaults.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+// DefaultHashContainerHash is a convenience alias for the functor that is used
+// by default by Abseil hash-based (unordered) containers for hashing when
+// `Hash` type argument is not explicitly specified.
+//
+// This type alias can be used by generic code that wants to provide more
+// flexibility for defining underlying containers.
+template <typename T>
+using DefaultHashContainerHash = absl::container_internal::hash_default_hash<T>;
+
+// DefaultHashContainerEq is a convenience alias for the functor that is used by
+// default by Abseil hash-based (unordered) containers for equality check when
+// `Eq` type argument is not explicitly specified.
+//
+// This type alias can be used by generic code that wants to provide more
+// flexibility for defining underlying containers.
+template <typename T>
+using DefaultHashContainerEq = absl::container_internal::hash_default_eq<T>;
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_CONTAINER_HASH_CONTAINER_DEFAULTS_H_
diff --git a/absl/container/inlined_vector.h b/absl/container/inlined_vector.h
index 5138348a15e..8dfbf03c64c 100644
--- a/absl/container/inlined_vector.h
+++ b/absl/container/inlined_vector.h
@@ -36,7 +36,6 @@
 #define ABSL_CONTAINER_INLINED_VECTOR_H_
 
 #include <algorithm>
-#include <cassert>
 #include <cstddef>
 #include <cstdlib>
 #include <cstring>
@@ -47,13 +46,20 @@
 #include <utility>
 
 #include "absl/algorithm/algorithm.h"
-#include "absl/base/internal/throw_delegate.h"
+#include "absl/base/attributes.h"
+#include "absl/base/internal/hardening.h"
+#include "absl/base/internal/iterator_traits.h"
+#include "absl/base/macros.h"
 #include "absl/base/optimization.h"
 #include "absl/base/port.h"
+#include "absl/base/throw_delegate.h"
 #include "absl/container/internal/inlined_vector.h"
+#include "absl/hash/internal/weakly_mixed_integer.h"
 #include "absl/memory/memory.h"
+#include "absl/meta/type_traits.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 // -----------------------------------------------------------------------------
 // InlinedVector
 // -----------------------------------------------------------------------------
@@ -62,44 +68,57 @@ namespace absl {
 // `std::vector` for use cases where the vector's size is sufficiently small
 // that it can be inlined. If the inlined vector does grow beyond its estimated
 // capacity, it will trigger an initial allocation on the heap, and will behave
-// as a `std:vector`. The API of the `absl::InlinedVector` within this file is
+// as a `std::vector`. The API of the `absl::InlinedVector` within this file is
 // designed to cover the same API footprint as covered by `std::vector`.
 template <typename T, size_t N, typename A = std::allocator<T>>
-class InlinedVector {
-  static_assert(N > 0, "`absl::InlinedVector` requires an inlined capacity.");
+class ABSL_ATTRIBUTE_WARN_UNUSED InlinedVector {
+  static_assert(N > 0, "absl::InlinedVector requires an inlined capacity.");
 
   using Storage = inlined_vector_internal::Storage<T, N, A>;
-  using rvalue_reference = typename Storage::rvalue_reference;
-  using MoveIterator = typename Storage::MoveIterator;
-  using AllocatorTraits = typename Storage::AllocatorTraits;
-  using IsMemcpyOk = typename Storage::IsMemcpyOk;
 
-  template <typename Iterator>
+  template <typename TheA>
+  using AllocatorTraits = inlined_vector_internal::AllocatorTraits<TheA>;
+  template <typename TheA>
+  using MoveIterator = inlined_vector_internal::MoveIterator<TheA>;
+  template <typename TheA>
+  using IsMoveAssignOk = inlined_vector_internal::IsMoveAssignOk<TheA>;
+
+  template <typename TheA, typename Iterator>
   using IteratorValueAdapter =
-      typename Storage::template IteratorValueAdapter<Iterator>;
-  using CopyValueAdapter = typename Storage::CopyValueAdapter;
-  using DefaultValueAdapter = typename Storage::DefaultValueAdapter;
+      inlined_vector_internal::IteratorValueAdapter<TheA, Iterator>;
+  template <typename TheA>
+  using CopyValueAdapter = inlined_vector_internal::CopyValueAdapter<TheA>;
+  template <typename TheA>
+  using DefaultValueAdapter =
+      inlined_vector_internal::DefaultValueAdapter<TheA>;
 
   template <typename Iterator>
-  using EnableIfAtLeastForwardIterator = absl::enable_if_t<
-      inlined_vector_internal::IsAtLeastForwardIterator<Iterator>::value>;
+  using EnableIfAtLeastForwardIterator = std::enable_if_t<
+      base_internal::IsAtLeastForwardIterator<Iterator>::value, int>;
   template <typename Iterator>
-  using DisableIfAtLeastForwardIterator = absl::enable_if_t<
-      !inlined_vector_internal::IsAtLeastForwardIterator<Iterator>::value>;
+  using DisableIfAtLeastForwardIterator = std::enable_if_t<
+      !base_internal::IsAtLeastForwardIterator<Iterator>::value, int>;
+
+  using MemcpyPolicy = typename Storage::MemcpyPolicy;
+  using ElementwiseAssignPolicy = typename Storage::ElementwiseAssignPolicy;
+  using ElementwiseConstructPolicy =
+      typename Storage::ElementwiseConstructPolicy;
+  using MoveAssignmentPolicy = typename Storage::MoveAssignmentPolicy;
 
  public:
-  using allocator_type = typename Storage::allocator_type;
-  using value_type = typename Storage::value_type;
-  using pointer = typename Storage::pointer;
-  using const_pointer = typename Storage::const_pointer;
-  using reference = typename Storage::reference;
-  using const_reference = typename Storage::const_reference;
-  using size_type = typename Storage::size_type;
-  using difference_type = typename Storage::difference_type;
-  using iterator = typename Storage::iterator;
-  using const_iterator = typename Storage::const_iterator;
-  using reverse_iterator = typename Storage::reverse_iterator;
-  using const_reverse_iterator = typename Storage::const_reverse_iterator;
+  using allocator_type = A;
+  using value_type = inlined_vector_internal::ValueType<A>;
+  using pointer = inlined_vector_internal::Pointer<A>;
+  using const_pointer = inlined_vector_internal::ConstPointer<A>;
+  using size_type = inlined_vector_internal::SizeType<A>;
+  using difference_type = inlined_vector_internal::DifferenceType<A>;
+  using reference = inlined_vector_internal::Reference<A>;
+  using const_reference = inlined_vector_internal::ConstReference<A>;
+  using iterator = inlined_vector_internal::Iterator<A>;
+  using const_iterator = inlined_vector_internal::ConstIterator<A>;
+  using reverse_iterator = inlined_vector_internal::ReverseIterator<A>;
+  using const_reverse_iterator =
+      inlined_vector_internal::ConstReverseIterator<A>;
 
   // ---------------------------------------------------------------------------
   // InlinedVector Constructors and Destructor
@@ -108,28 +127,34 @@ class InlinedVector {
   // Creates an empty inlined vector with a value-initialized allocator.
   InlinedVector() noexcept(noexcept(allocator_type())) : storage_() {}
 
-  // Creates an empty inlined vector with a copy of `alloc`.
-  explicit InlinedVector(const allocator_type& alloc) noexcept
-      : storage_(alloc) {}
+  // Creates an empty inlined vector with a copy of `allocator`.
+  explicit InlinedVector(const allocator_type& allocator) noexcept
+      : storage_(allocator) {}
 
   // Creates an inlined vector with `n` copies of `value_type()`.
   explicit InlinedVector(size_type n,
-                         const allocator_type& alloc = allocator_type())
-      : storage_(alloc) {
-    storage_.Initialize(DefaultValueAdapter(), n);
+                         const allocator_type& allocator = allocator_type())
+      : storage_(allocator) {
+    if (ABSL_PREDICT_FALSE(n > max_size())) {
+      ThrowStdLengthError("InlinedVector::InlinedVector failed length check");
+    }
+    storage_.Initialize(DefaultValueAdapter<A>(), n);
   }
 
   // Creates an inlined vector with `n` copies of `v`.
   InlinedVector(size_type n, const_reference v,
-                const allocator_type& alloc = allocator_type())
-      : storage_(alloc) {
-    storage_.Initialize(CopyValueAdapter(v), n);
+                const allocator_type& allocator = allocator_type())
+      : storage_(allocator) {
+    if (ABSL_PREDICT_FALSE(n > max_size())) {
+      ThrowStdLengthError("InlinedVector::InlinedVector failed length check");
+    }
+    storage_.Initialize(CopyValueAdapter<A>(std::addressof(v)), n);
   }
 
   // Creates an inlined vector with copies of the elements of `list`.
   InlinedVector(std::initializer_list<value_type> list,
-                const allocator_type& alloc = allocator_type())
-      : InlinedVector(list.begin(), list.end(), alloc) {}
+                const allocator_type& allocator = allocator_type())
+      : InlinedVector(list.begin(), list.end(), allocator) {}
 
   // Creates an inlined vector with elements constructed from the provided
   // forward iterator range [`first`, `last`).
@@ -138,38 +163,53 @@ class InlinedVector {
   // this constructor with two integral arguments and a call to the above
   // `InlinedVector(size_type, const_reference)` constructor.
   template <typename ForwardIterator,
-            EnableIfAtLeastForwardIterator<ForwardIterator>* = nullptr>
+            EnableIfAtLeastForwardIterator<ForwardIterator> = 0>
   InlinedVector(ForwardIterator first, ForwardIterator last,
-                const allocator_type& alloc = allocator_type())
-      : storage_(alloc) {
-    storage_.Initialize(IteratorValueAdapter<ForwardIterator>(first),
-                        std::distance(first, last));
+                const allocator_type& allocator = allocator_type())
+      : storage_(allocator) {
+    const size_type s = static_cast<size_type>(std::distance(first, last));
+    if (ABSL_PREDICT_FALSE(s > max_size())) {
+      ThrowStdLengthError("InlinedVector::InlinedVector failed length check");
+    }
+    storage_.Initialize(IteratorValueAdapter<A, ForwardIterator>(first), s);
   }
 
   // Creates an inlined vector with elements constructed from the provided input
   // iterator range [`first`, `last`).
   template <typename InputIterator,
-            DisableIfAtLeastForwardIterator<InputIterator>* = nullptr>
+            DisableIfAtLeastForwardIterator<InputIterator> = 0>
   InlinedVector(InputIterator first, InputIterator last,
-                const allocator_type& alloc = allocator_type())
-      : storage_(alloc) {
+                const allocator_type& allocator = allocator_type())
+      : storage_(allocator) {
     std::copy(first, last, std::back_inserter(*this));
   }
 
   // Creates an inlined vector by copying the contents of `other` using
   // `other`'s allocator.
   InlinedVector(const InlinedVector& other)
-      : InlinedVector(other, *other.storage_.GetAllocPtr()) {}
+      : InlinedVector(other, other.storage_.GetAllocator()) {}
+
+  // Creates an inlined vector by copying the contents of `other` using the
+  // provided `allocator`.
+  InlinedVector(const InlinedVector& other, const allocator_type& allocator)
+      : storage_(allocator) {
+    // Fast path: if the other vector is empty, there's nothing for us to do.
+    if (other.empty()) {
+      return;
+    }
 
-  // Creates an inlined vector by copying the contents of `other` using `alloc`.
-  InlinedVector(const InlinedVector& other, const allocator_type& alloc)
-      : storage_(alloc) {
-    if (IsMemcpyOk::value && !other.storage_.GetIsAllocated()) {
+    // Fast path: if the value type is trivially copy constructible, we know the
+    // allocator doesn't do anything fancy, and there is nothing on the heap
+    // then we know it is legal for us to simply memcpy the other vector's
+    // inlined bytes to form our copy of its elements.
+    if (std::is_trivially_copy_constructible_v<value_type> &&
+        std::is_same_v<A, std::allocator<value_type>> &&
+        !other.storage_.GetIsAllocated()) {
       storage_.MemcpyFrom(other.storage_);
-    } else {
-      storage_.Initialize(IteratorValueAdapter<const_pointer>(other.data()),
-                          other.size());
+      return;
     }
+
+    storage_.InitFrom(other.storage_);
   }
 
   // Creates an inlined vector by moving in the contents of `other` without
@@ -188,56 +228,82 @@ class InlinedVector {
   // or `value_type`'s move constructor is specified as `noexcept`.
   InlinedVector(InlinedVector&& other) noexcept(
       absl::allocator_is_nothrow<allocator_type>::value ||
-      std::is_nothrow_move_constructible<value_type>::value)
-      : storage_(*other.storage_.GetAllocPtr()) {
-    if (IsMemcpyOk::value) {
+      std::is_nothrow_move_constructible_v<value_type>)
+      : storage_(other.storage_.GetAllocator()) {
+    // Fast path: if the value type can be trivially relocated (i.e. moved from
+    // and destroyed), and we know the allocator doesn't do anything fancy, then
+    // it's safe for us to simply adopt the contents of the storage for `other`
+    // and remove its own reference to them. It's as if we had individually
+    // move-constructed each value and then destroyed the original.
+    if (absl::is_trivially_relocatable<value_type>::value &&
+        std::is_same_v<A, std::allocator<value_type>>) {
       storage_.MemcpyFrom(other.storage_);
-
       other.storage_.SetInlinedSize(0);
-    } else if (other.storage_.GetIsAllocated()) {
-      storage_.SetAllocatedData(other.storage_.GetAllocatedData(),
-                                other.storage_.GetAllocatedCapacity());
+      return;
+    }
+
+    // Fast path: if the other vector is on the heap, we can simply take over
+    // its allocation.
+    if (other.storage_.GetIsAllocated()) {
+      storage_.SetAllocation({other.storage_.GetAllocatedData(),
+                              other.storage_.GetAllocatedCapacity()});
       storage_.SetAllocatedSize(other.storage_.GetSize());
 
       other.storage_.SetInlinedSize(0);
-    } else {
-      IteratorValueAdapter<MoveIterator> other_values(
-          MoveIterator(other.storage_.GetInlinedData()));
+      return;
+    }
 
-      inlined_vector_internal::ConstructElements(
-          storage_.GetAllocPtr(), storage_.GetInlinedData(), &other_values,
-          other.storage_.GetSize());
+    // Otherwise we must move each element individually.
+    IteratorValueAdapter<A, MoveIterator<A>> other_values(
+        MoveIterator<A>(other.storage_.GetInlinedData()));
 
-      storage_.SetInlinedSize(other.storage_.GetSize());
-    }
+    inlined_vector_internal::ConstructElements<A>(
+        storage_.GetAllocator(), storage_.GetInlinedData(), other_values,
+        other.storage_.GetSize());
+
+    storage_.SetInlinedSize(other.storage_.GetSize());
   }
 
   // Creates an inlined vector by moving in the contents of `other` with a copy
-  // of `alloc`.
+  // of `allocator`.
   //
-  // NOTE: if `other`'s allocator is not equal to `alloc`, even if `other`
+  // NOTE: if `other`'s allocator is not equal to `allocator`, even if `other`
   // contains allocated memory, this move constructor will still allocate. Since
   // allocation is performed, this constructor can only be `noexcept` if the
   // specified allocator is also `noexcept`.
-  InlinedVector(InlinedVector&& other, const allocator_type& alloc) noexcept(
-      absl::allocator_is_nothrow<allocator_type>::value)
-      : storage_(alloc) {
-    if (IsMemcpyOk::value) {
+  InlinedVector(
+      InlinedVector&& other,
+      const allocator_type&
+          allocator) noexcept(absl::allocator_is_nothrow<allocator_type>::value)
+      : storage_(allocator) {
+    // Fast path: if the value type can be trivially relocated (i.e. moved from
+    // and destroyed), and we know the allocator doesn't do anything fancy, then
+    // it's safe for us to simply adopt the contents of the storage for `other`
+    // and remove its own reference to them. It's as if we had individually
+    // move-constructed each value and then destroyed the original.
+    if (absl::is_trivially_relocatable<value_type>::value &&
+        std::is_same_v<A, std::allocator<value_type>>) {
       storage_.MemcpyFrom(other.storage_);
-
       other.storage_.SetInlinedSize(0);
-    } else if ((*storage_.GetAllocPtr() == *other.storage_.GetAllocPtr()) &&
-               other.storage_.GetIsAllocated()) {
-      storage_.SetAllocatedData(other.storage_.GetAllocatedData(),
-                                other.storage_.GetAllocatedCapacity());
+      return;
+    }
+
+    // Fast path: if the other vector is on the heap and shared the same
+    // allocator, we can simply take over its allocation.
+    if ((storage_.GetAllocator() == other.storage_.GetAllocator()) &&
+        other.storage_.GetIsAllocated()) {
+      storage_.SetAllocation({other.storage_.GetAllocatedData(),
+                              other.storage_.GetAllocatedCapacity()});
       storage_.SetAllocatedSize(other.storage_.GetSize());
 
       other.storage_.SetInlinedSize(0);
-    } else {
-      storage_.Initialize(
-          IteratorValueAdapter<MoveIterator>(MoveIterator(other.data())),
-          other.size());
+      return;
     }
+
+    // Otherwise we must move each element individually.
+    storage_.Initialize(
+        IteratorValueAdapter<A, MoveIterator<A>>(MoveIterator<A>(other.data())),
+        other.size());
   }
 
   ~InlinedVector() {}
@@ -262,8 +328,10 @@ class InlinedVector {
   size_type max_size() const noexcept {
     // One bit of the size storage is used to indicate whether the inlined
     // vector contains allocated memory. As a result, the maximum size that the
-    // inlined vector can express is half of the max for `size_type`.
-    return (std::numeric_limits<size_type>::max)() / 2;
+    // inlined vector can express is the minimum of the limit of how many
+    // objects we can allocate and std::numeric_limits<size_type>::max() / 2.
+    return (std::min)(AllocatorTraits<A>::max_size(storage_.GetAllocator()),
+                      (std::numeric_limits<size_type>::max)() / 2);
   }
 
   // `InlinedVector::capacity()`
@@ -286,7 +354,7 @@ class InlinedVector {
   // can be used to access and modify the contained elements.
   //
   // NOTE: only elements within [`data()`, `data() + size()`) are valid.
-  pointer data() noexcept {
+  pointer data() noexcept ABSL_ATTRIBUTE_LIFETIME_BOUND {
     return storage_.GetIsAllocated() ? storage_.GetAllocatedData()
                                      : storage_.GetInlinedData();
   }
@@ -296,7 +364,7 @@ class InlinedVector {
   // modify the contained elements.
   //
   // NOTE: only elements within [`data()`, `data() + size()`) are valid.
-  const_pointer data() const noexcept {
+  const_pointer data() const noexcept ABSL_ATTRIBUTE_LIFETIME_BOUND {
     return storage_.GetIsAllocated() ? storage_.GetAllocatedData()
                                      : storage_.GetInlinedData();
   }
@@ -304,17 +372,15 @@ class InlinedVector {
   // `InlinedVector::operator[](...)`
   //
   // Returns a `reference` to the `i`th element of the inlined vector.
-  reference operator[](size_type i) {
-    assert(i < size());
-
+  reference operator[](size_type i) ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    absl::base_internal::HardeningAssertLT(i, size());
     return data()[i];
   }
 
   // Overload of `InlinedVector::operator[](...)` that returns a
   // `const_reference` to the `i`th element of the inlined vector.
-  const_reference operator[](size_type i) const {
-    assert(i < size());
-
+  const_reference operator[](size_type i) const ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    absl::base_internal::HardeningAssertLT(i, size());
     return data()[i];
   }
 
@@ -324,12 +390,10 @@ class InlinedVector {
   //
   // NOTE: if `i` is not within the required range of `InlinedVector::at(...)`,
   // in both debug and non-debug builds, `std::out_of_range` will be thrown.
-  reference at(size_type i) {
+  reference at(size_type i) ABSL_ATTRIBUTE_LIFETIME_BOUND {
     if (ABSL_PREDICT_FALSE(i >= size())) {
-      base_internal::ThrowStdOutOfRange(
-          "`InlinedVector::at(size_type)` failed bounds check");
+      ThrowStdOutOfRange("InlinedVector::at(size_type) failed bounds check");
     }
-
     return data()[i];
   }
 
@@ -338,114 +402,127 @@ class InlinedVector {
   //
   // NOTE: if `i` is not within the required range of `InlinedVector::at(...)`,
   // in both debug and non-debug builds, `std::out_of_range` will be thrown.
-  const_reference at(size_type i) const {
+  const_reference at(size_type i) const ABSL_ATTRIBUTE_LIFETIME_BOUND {
     if (ABSL_PREDICT_FALSE(i >= size())) {
-      base_internal::ThrowStdOutOfRange(
-          "`InlinedVector::at(size_type) const` failed bounds check");
+      ThrowStdOutOfRange("InlinedVector::at(size_type) failed bounds check");
     }
-
     return data()[i];
   }
 
   // `InlinedVector::front()`
   //
   // Returns a `reference` to the first element of the inlined vector.
-  reference front() {
-    assert(!empty());
-
-    return at(0);
+  reference front() ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    absl::base_internal::HardeningAssertNonEmpty(*this);
+    return data()[0];
   }
 
   // Overload of `InlinedVector::front()` that returns a `const_reference` to
   // the first element of the inlined vector.
-  const_reference front() const {
-    assert(!empty());
-
-    return at(0);
+  const_reference front() const ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    absl::base_internal::HardeningAssertNonEmpty(*this);
+    return data()[0];
   }
 
   // `InlinedVector::back()`
   //
   // Returns a `reference` to the last element of the inlined vector.
-  reference back() {
-    assert(!empty());
-
-    return at(size() - 1);
+  reference back() ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    absl::base_internal::HardeningAssertNonEmpty(*this);
+    return data()[size() - 1];
   }
 
   // Overload of `InlinedVector::back()` that returns a `const_reference` to the
   // last element of the inlined vector.
-  const_reference back() const {
-    assert(!empty());
-
-    return at(size() - 1);
+  const_reference back() const ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    absl::base_internal::HardeningAssertNonEmpty(*this);
+    return data()[size() - 1];
   }
 
   // `InlinedVector::begin()`
   //
   // Returns an `iterator` to the beginning of the inlined vector.
-  iterator begin() noexcept { return data(); }
+  iterator begin() noexcept ABSL_ATTRIBUTE_LIFETIME_BOUND { return data(); }
 
   // Overload of `InlinedVector::begin()` that returns a `const_iterator` to
   // the beginning of the inlined vector.
-  const_iterator begin() const noexcept { return data(); }
+  const_iterator begin() const noexcept ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return data();
+  }
 
   // `InlinedVector::end()`
   //
   // Returns an `iterator` to the end of the inlined vector.
-  iterator end() noexcept { return data() + size(); }
+  iterator end() noexcept ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return data() + size();
+  }
 
   // Overload of `InlinedVector::end()` that returns a `const_iterator` to the
   // end of the inlined vector.
-  const_iterator end() const noexcept { return data() + size(); }
+  const_iterator end() const noexcept ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return data() + size();
+  }
 
   // `InlinedVector::cbegin()`
   //
   // Returns a `const_iterator` to the beginning of the inlined vector.
-  const_iterator cbegin() const noexcept { return begin(); }
+  const_iterator cbegin() const noexcept ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return begin();
+  }
 
   // `InlinedVector::cend()`
   //
   // Returns a `const_iterator` to the end of the inlined vector.
-  const_iterator cend() const noexcept { return end(); }
+  const_iterator cend() const noexcept ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return end();
+  }
 
   // `InlinedVector::rbegin()`
   //
   // Returns a `reverse_iterator` from the end of the inlined vector.
-  reverse_iterator rbegin() noexcept { return reverse_iterator(end()); }
+  reverse_iterator rbegin() noexcept ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return reverse_iterator(end());
+  }
 
   // Overload of `InlinedVector::rbegin()` that returns a
   // `const_reverse_iterator` from the end of the inlined vector.
-  const_reverse_iterator rbegin() const noexcept {
+  const_reverse_iterator rbegin() const noexcept ABSL_ATTRIBUTE_LIFETIME_BOUND {
     return const_reverse_iterator(end());
   }
 
   // `InlinedVector::rend()`
   //
   // Returns a `reverse_iterator` from the beginning of the inlined vector.
-  reverse_iterator rend() noexcept { return reverse_iterator(begin()); }
+  reverse_iterator rend() noexcept ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return reverse_iterator(begin());
+  }
 
   // Overload of `InlinedVector::rend()` that returns a `const_reverse_iterator`
   // from the beginning of the inlined vector.
-  const_reverse_iterator rend() const noexcept {
+  const_reverse_iterator rend() const noexcept ABSL_ATTRIBUTE_LIFETIME_BOUND {
     return const_reverse_iterator(begin());
   }
 
   // `InlinedVector::crbegin()`
   //
   // Returns a `const_reverse_iterator` from the end of the inlined vector.
-  const_reverse_iterator crbegin() const noexcept { return rbegin(); }
+  const_reverse_iterator crbegin() const noexcept
+      ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return rbegin();
+  }
 
   // `InlinedVector::crend()`
   //
   // Returns a `const_reverse_iterator` from the beginning of the inlined
   // vector.
-  const_reverse_iterator crend() const noexcept { return rend(); }
+  const_reverse_iterator crend() const noexcept ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return rend();
+  }
 
   // `InlinedVector::get_allocator()`
   //
   // Returns a copy of the inlined vector's allocator.
-  allocator_type get_allocator() const { return *storage_.GetAllocPtr(); }
+  allocator_type get_allocator() const { return storage_.GetAllocator(); }
 
   // ---------------------------------------------------------------------------
   // InlinedVector Member Mutators
@@ -479,18 +556,7 @@ class InlinedVector {
   // unspecified state.
   InlinedVector& operator=(InlinedVector&& other) {
     if (ABSL_PREDICT_TRUE(this != std::addressof(other))) {
-      if (IsMemcpyOk::value || other.storage_.GetIsAllocated()) {
-        inlined_vector_internal::DestroyElements(storage_.GetAllocPtr(), data(),
-                                                 size());
-        storage_.DeallocateIfAllocated();
-        storage_.MemcpyFrom(other.storage_);
-
-        other.storage_.SetInlinedSize(0);
-      } else {
-        storage_.Assign(IteratorValueAdapter<MoveIterator>(
-                            MoveIterator(other.storage_.GetInlinedData())),
-                        other.size());
-      }
+      MoveAssignment(MoveAssignmentPolicy{}, std::move(other));
     }
 
     return *this;
@@ -500,7 +566,10 @@ class InlinedVector {
   //
   // Replaces the contents of the inlined vector with `n` copies of `v`.
   void assign(size_type n, const_reference v) {
-    storage_.Assign(CopyValueAdapter(v), n);
+    if (ABSL_PREDICT_FALSE(n > max_size())) {
+      ThrowStdLengthError("InlinedVector::assign failed length check");
+    }
+    storage_.Assign(CopyValueAdapter<A>(std::addressof(v)), n);
   }
 
   // Overload of `InlinedVector::assign(...)` that replaces the contents of the
@@ -514,10 +583,13 @@ class InlinedVector {
   //
   // NOTE: this overload is for iterators that are "forward" category or better.
   template <typename ForwardIterator,
-            EnableIfAtLeastForwardIterator<ForwardIterator>* = nullptr>
+            EnableIfAtLeastForwardIterator<ForwardIterator> = 0>
   void assign(ForwardIterator first, ForwardIterator last) {
-    storage_.Assign(IteratorValueAdapter<ForwardIterator>(first),
-                    std::distance(first, last));
+    const size_type s = static_cast<size_type>(std::distance(first, last));
+    if (ABSL_PREDICT_FALSE(s > max_size())) {
+      ThrowStdLengthError("InlinedVector::assign failed length check");
+    }
+    storage_.Assign(IteratorValueAdapter<A, ForwardIterator>(first), s);
   }
 
   // Overload of `InlinedVector::assign(...)` to replace the contents of the
@@ -525,11 +597,11 @@ class InlinedVector {
   //
   // NOTE: this overload is for iterators that are "input" category.
   template <typename InputIterator,
-            DisableIfAtLeastForwardIterator<InputIterator>* = nullptr>
+            DisableIfAtLeastForwardIterator<InputIterator> = 0>
   void assign(InputIterator first, InputIterator last) {
     size_type i = 0;
     for (; i < size() && first != last; ++i, static_cast<void>(++first)) {
-      at(i) = *first;
+      data()[i] = *first;
     }
 
     erase(data() + i, data() + size());
@@ -540,9 +612,14 @@ class InlinedVector {
   //
   // Resizes the inlined vector to contain `n` elements.
   //
-  // NOTE: if `n` is smaller than `size()`, extra elements are destroyed. If `n`
+  // NOTE: If `n` is smaller than `size()`, extra elements are destroyed. If `n`
   // is larger than `size()`, new elements are value-initialized.
-  void resize(size_type n) { storage_.Resize(DefaultValueAdapter(), n); }
+  void resize(size_type n) {
+    if (ABSL_PREDICT_FALSE(n > max_size())) {
+      ThrowStdLengthError("InlinedVector::resize failed length check");
+    }
+    storage_.Resize(DefaultValueAdapter<A>(), n);
+  }
 
   // Overload of `InlinedVector::resize(...)` that resizes the inlined vector to
   // contain `n` elements.
@@ -550,33 +627,55 @@ class InlinedVector {
   // NOTE: if `n` is smaller than `size()`, extra elements are destroyed. If `n`
   // is larger than `size()`, new elements are copied-constructed from `v`.
   void resize(size_type n, const_reference v) {
-    storage_.Resize(CopyValueAdapter(v), n);
+    if (ABSL_PREDICT_FALSE(n > max_size())) {
+      ThrowStdLengthError("InlinedVector::resize failed length check");
+    }
+    storage_.Resize(CopyValueAdapter<A>(std::addressof(v)), n);
   }
 
   // `InlinedVector::insert(...)`
   //
   // Inserts a copy of `v` at `pos`, returning an `iterator` to the newly
   // inserted element.
-  iterator insert(const_iterator pos, const_reference v) {
+  iterator insert(const_iterator pos,
+                  const_reference v) ABSL_ATTRIBUTE_LIFETIME_BOUND {
     return emplace(pos, v);
   }
 
   // Overload of `InlinedVector::insert(...)` that inserts `v` at `pos` using
   // move semantics, returning an `iterator` to the newly inserted element.
-  iterator insert(const_iterator pos, rvalue_reference v) {
+  iterator insert(const_iterator pos,
+                  value_type&& v) ABSL_ATTRIBUTE_LIFETIME_BOUND {
     return emplace(pos, std::move(v));
   }
 
   // Overload of `InlinedVector::insert(...)` that inserts `n` contiguous copies
   // of `v` starting at `pos`, returning an `iterator` pointing to the first of
   // the newly inserted elements.
-  iterator insert(const_iterator pos, size_type n, const_reference v) {
-    assert(pos >= begin());
-    assert(pos <= end());
+  iterator insert(const_iterator pos, size_type n,
+                  const_reference v) ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    absl::base_internal::HardeningAssertGE(pos, cbegin());
+    absl::base_internal::HardeningAssertLE(pos, cend());
+    if (ABSL_PREDICT_FALSE(n > max_size() - size())) {
+      ThrowStdLengthError("InlinedVector::insert failed length check");
+    }
 
     if (ABSL_PREDICT_TRUE(n != 0)) {
       value_type dealias = v;
-      return storage_.Insert(pos, CopyValueAdapter(dealias), n);
+      // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102329#c2
+      // It appears that GCC thinks that since `pos` is a const pointer and may
+      // point to uninitialized memory at this point, a warning should be
+      // issued. But `pos` is actually only used to compute an array index to
+      // write to.
+#if !defined(__clang__) && defined(__GNUC__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
+#endif
+      return storage_.Insert(pos, CopyValueAdapter<A>(std::addressof(dealias)),
+                             n);
+#if !defined(__clang__) && defined(__GNUC__)
+#pragma GCC diagnostic pop
+#endif
     } else {
       return const_cast<iterator>(pos);
     }
@@ -585,7 +684,8 @@ class InlinedVector {
   // Overload of `InlinedVector::insert(...)` that inserts copies of the
   // elements of `list` starting at `pos`, returning an `iterator` pointing to
   // the first of the newly inserted elements.
-  iterator insert(const_iterator pos, std::initializer_list<value_type> list) {
+  iterator insert(const_iterator pos, std::initializer_list<value_type> list)
+      ABSL_ATTRIBUTE_LIFETIME_BOUND {
     return insert(pos, list.begin(), list.end());
   }
 
@@ -595,15 +695,19 @@ class InlinedVector {
   //
   // NOTE: this overload is for iterators that are "forward" category or better.
   template <typename ForwardIterator,
-            EnableIfAtLeastForwardIterator<ForwardIterator>* = nullptr>
+            EnableIfAtLeastForwardIterator<ForwardIterator> = 0>
   iterator insert(const_iterator pos, ForwardIterator first,
-                  ForwardIterator last) {
-    assert(pos >= begin());
-    assert(pos <= end());
+                  ForwardIterator last) ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    absl::base_internal::HardeningAssertGE(pos, cbegin());
+    absl::base_internal::HardeningAssertLE(pos, cend());
+    const size_type s = static_cast<size_type>(std::distance(first, last));
+    if (ABSL_PREDICT_FALSE(s > max_size() - size())) {
+      ThrowStdLengthError("InlinedVector::insert failed length check");
+    }
 
     if (ABSL_PREDICT_TRUE(first != last)) {
-      return storage_.Insert(pos, IteratorValueAdapter<ForwardIterator>(first),
-                             std::distance(first, last));
+      return storage_.Insert(
+          pos, IteratorValueAdapter<A, ForwardIterator>(first), s);
     } else {
       return const_cast<iterator>(pos);
     }
@@ -615,12 +719,13 @@ class InlinedVector {
   //
   // NOTE: this overload is for iterators that are "input" category.
   template <typename InputIterator,
-            DisableIfAtLeastForwardIterator<InputIterator>* = nullptr>
-  iterator insert(const_iterator pos, InputIterator first, InputIterator last) {
-    assert(pos >= begin());
-    assert(pos <= end());
+            DisableIfAtLeastForwardIterator<InputIterator> = 0>
+  iterator insert(const_iterator pos, InputIterator first,
+                  InputIterator last) ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    absl::base_internal::HardeningAssertGE(pos, cbegin());
+    absl::base_internal::HardeningAssertLE(pos, cend());
 
-    size_type index = std::distance(cbegin(), pos);
+    size_type index = static_cast<size_type>(std::distance(cbegin(), pos));
     for (size_type i = index; first != last; ++i, static_cast<void>(++first)) {
       insert(data() + i, *first);
     }
@@ -633,15 +738,31 @@ class InlinedVector {
   // Constructs and inserts an element using `args...` in the inlined vector at
   // `pos`, returning an `iterator` pointing to the newly emplaced element.
   template <typename... Args>
-  iterator emplace(const_iterator pos, Args&&... args) {
-    assert(pos >= begin());
-    assert(pos <= end());
+  iterator emplace(const_iterator pos,
+                   Args&&... args) ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    absl::base_internal::HardeningAssertGE(pos, cbegin());
+    absl::base_internal::HardeningAssertLE(pos, cend());
+    if (ABSL_PREDICT_FALSE(size() == max_size())) {
+      ThrowStdLengthError("InlinedVector::emplace failed length check");
+    }
 
     value_type dealias(std::forward<Args>(args)...);
+    // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102329#c2
+    // It appears that GCC thinks that since `pos` is a const pointer and may
+    // point to uninitialized memory at this point, a warning should be
+    // issued. But `pos` is actually only used to compute an array index to
+    // write to.
+#if !defined(__clang__) && defined(__GNUC__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
+#endif
     return storage_.Insert(pos,
-                           IteratorValueAdapter<MoveIterator>(
-                               MoveIterator(std::addressof(dealias))),
+                           IteratorValueAdapter<A, MoveIterator<A>>(
+                               MoveIterator<A>(std::addressof(dealias))),
                            1);
+#if !defined(__clang__) && defined(__GNUC__)
+#pragma GCC diagnostic pop
+#endif
   }
 
   // `InlinedVector::emplace_back(...)`
@@ -649,7 +770,10 @@ class InlinedVector {
   // Constructs and inserts an element using `args...` in the inlined vector at
   // `end()`, returning a `reference` to the newly emplaced element.
   template <typename... Args>
-  reference emplace_back(Args&&... args) {
+  reference emplace_back(Args&&... args) ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    if (ABSL_PREDICT_FALSE(size() == max_size())) {
+      ThrowStdLengthError("InlinedVector::emplace_back failed length check");
+    }
     return storage_.EmplaceBack(std::forward<Args>(args)...);
   }
 
@@ -660,7 +784,7 @@ class InlinedVector {
 
   // Overload of `InlinedVector::push_back(...)` for inserting `v` at `end()`
   // using move semantics.
-  void push_back(rvalue_reference v) {
+  void push_back(value_type&& v) {
     static_cast<void>(emplace_back(std::move(v)));
   }
 
@@ -668,9 +792,9 @@ class InlinedVector {
   //
   // Destroys the element at `back()`, reducing the size by `1`.
   void pop_back() noexcept {
-    assert(!empty());
+    absl::base_internal::HardeningAssertNonEmpty(*this);
 
-    AllocatorTraits::destroy(*storage_.GetAllocPtr(), data() + (size() - 1));
+    AllocatorTraits<A>::destroy(storage_.GetAllocator(), data() + (size() - 1));
     storage_.SubtractSize(1);
   }
 
@@ -679,23 +803,37 @@ class InlinedVector {
   // Erases the element at `pos`, returning an `iterator` pointing to where the
   // erased element was located.
   //
-  // NOTE: may return `end()`, which is not dereferencable.
-  iterator erase(const_iterator pos) {
-    assert(pos >= begin());
-    assert(pos < end());
-
+  // NOTE: may return `end()`, which is not dereferenceable.
+  iterator erase(const_iterator pos) ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    absl::base_internal::HardeningAssertGE(pos, cbegin());
+    absl::base_internal::HardeningAssertLT(pos, cend());
+
+    // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102329#c2
+    // It appears that GCC thinks that since `pos` is a const pointer and may
+    // point to uninitialized memory at this point, a warning should be
+    // issued. But `pos` is actually only used to compute an array index to
+    // write to.
+#if !defined(__clang__) && defined(__GNUC__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
+#pragma GCC diagnostic ignored "-Wuninitialized"
+#endif
     return storage_.Erase(pos, pos + 1);
+#if !defined(__clang__) && defined(__GNUC__)
+#pragma GCC diagnostic pop
+#endif
   }
 
   // Overload of `InlinedVector::erase(...)` that erases every element in the
   // range [`from`, `to`), returning an `iterator` pointing to where the first
   // erased element was located.
   //
-  // NOTE: may return `end()`, which is not dereferencable.
-  iterator erase(const_iterator from, const_iterator to) {
-    assert(from >= begin());
-    assert(from <= to);
-    assert(to <= end());
+  // NOTE: may return `end()`, which is not dereferenceable.
+  iterator erase(const_iterator from,
+                 const_iterator to) ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    absl::base_internal::HardeningAssertGE(from, cbegin());
+    absl::base_internal::HardeningAssertLE(from, to);
+    absl::base_internal::HardeningAssertLE(to, cend());
 
     if (ABSL_PREDICT_TRUE(from != to)) {
       return storage_.Erase(from, to);
@@ -707,31 +845,31 @@ class InlinedVector {
   // `InlinedVector::clear()`
   //
   // Destroys all elements in the inlined vector, setting the size to `0` and
-  // deallocating any held memory.
+  // preserving capacity.
   void clear() noexcept {
-    inlined_vector_internal::DestroyElements(storage_.GetAllocPtr(), data(),
-                                             size());
-    storage_.DeallocateIfAllocated();
-
-    storage_.SetInlinedSize(0);
+    inlined_vector_internal::DestroyAdapter<A>::DestroyElements(
+        storage_.GetAllocator(), data(), size());
+    storage_.SetSize(0);
   }
 
   // `InlinedVector::reserve(...)`
   //
   // Ensures that there is enough room for at least `n` elements.
-  void reserve(size_type n) { storage_.Reserve(n); }
+  void reserve(size_type n) {
+    if (ABSL_PREDICT_FALSE(n > max_size())) {
+      ThrowStdLengthError("InlinedVector::reserve failed length check");
+    }
+    storage_.Reserve(n);
+  }
 
   // `InlinedVector::shrink_to_fit()`
   //
-  // Reduces memory usage by freeing unused memory. After being called, calls to
-  // `capacity()` will be equal to `max(N, size())`.
-  //
-  // If `size() <= N` and the inlined vector contains allocated memory, the
-  // elements will all be moved to the inlined space and the allocated memory
-  // will be deallocated.
+  // Attempts to reduce memory usage by moving elements to (or keeping elements
+  // in) the smallest available buffer sufficient for containing `size()`
+  // elements.
   //
-  // If `size() > N` and `size() < capacity()`, the elements will be moved to a
-  // smaller allocation.
+  // If `size()` is sufficiently small, the elements will be moved into (or kept
+  // in) the inlined space.
   void shrink_to_fit() {
     if (storage_.GetIsAllocated()) {
       storage_.ShrinkToFit();
@@ -751,6 +889,79 @@ class InlinedVector {
   template <typename H, typename TheT, size_t TheN, typename TheA>
   friend H AbslHashValue(H h, const absl::InlinedVector<TheT, TheN, TheA>& a);
 
+  void MoveAssignment(MemcpyPolicy, InlinedVector&& other) {
+    // Assumption check: we shouldn't be told to use memcpy to implement move
+    // assignment unless we have trivially destructible elements and an
+    // allocator that does nothing fancy.
+    static_assert(std::is_trivially_destructible_v<value_type>);
+    static_assert(std::is_same_v<A, std::allocator<value_type>>);
+
+    // Throw away our existing heap allocation, if any. There is no need to
+    // destroy the existing elements one by one because we know they are
+    // trivially destructible.
+    storage_.DeallocateIfAllocated();
+
+    // Adopt the other vector's inline elements or heap allocation.
+    storage_.MemcpyFrom(other.storage_);
+    other.storage_.SetInlinedSize(0);
+  }
+
+  // Destroy our existing elements, if any, and adopt the heap-allocated
+  // elements of the other vector.
+  //
+  // REQUIRES: other.storage_.GetIsAllocated()
+  void DestroyExistingAndAdopt(InlinedVector&& other) {
+    absl::base_internal::HardeningAssert(other.storage_.GetIsAllocated());
+
+    inlined_vector_internal::DestroyAdapter<A>::DestroyElements(
+        storage_.GetAllocator(), data(), size());
+    storage_.DeallocateIfAllocated();
+
+    storage_.MemcpyFrom(other.storage_);
+    other.storage_.SetInlinedSize(0);
+  }
+
+  void MoveAssignment(ElementwiseAssignPolicy, InlinedVector&& other) {
+    // Fast path: if the other vector is on the heap then we don't worry about
+    // actually move-assigning each element. Instead we only throw away our own
+    // existing elements and adopt the heap allocation of the other vector.
+    if (other.storage_.GetIsAllocated()) {
+      DestroyExistingAndAdopt(std::move(other));
+      return;
+    }
+
+    storage_.Assign(IteratorValueAdapter<A, MoveIterator<A>>(
+                        MoveIterator<A>(other.storage_.GetInlinedData())),
+                    other.size());
+  }
+
+  void MoveAssignment(ElementwiseConstructPolicy, InlinedVector&& other) {
+    // Fast path: if the other vector is on the heap then we don't worry about
+    // actually move-assigning each element. Instead we only throw away our own
+    // existing elements and adopt the heap allocation of the other vector.
+    if (other.storage_.GetIsAllocated()) {
+      DestroyExistingAndAdopt(std::move(other));
+      return;
+    }
+
+    inlined_vector_internal::DestroyAdapter<A>::DestroyElements(
+        storage_.GetAllocator(), data(), size());
+    storage_.DeallocateIfAllocated();
+
+    if constexpr (!std::is_nothrow_move_constructible_v<value_type>) {
+      // Reset the size to zero before moving to avoid leaking freed memory if
+      // an exception is thrown.
+      storage_.SetInlinedSize(0);
+    }
+
+    IteratorValueAdapter<A, MoveIterator<A>> other_values(
+        MoveIterator<A>(other.storage_.GetInlinedData()));
+    inlined_vector_internal::ConstructElements<A>(
+        storage_.GetAllocator(), storage_.GetInlinedData(), other_values,
+        other.storage_.GetSize());
+    storage_.SetInlinedSize(other.storage_.GetSize());
+  }
+
   Storage storage_;
 };
 
@@ -775,7 +986,7 @@ bool operator==(const absl::InlinedVector<T, N, A>& a,
                 const absl::InlinedVector<T, N, A>& b) {
   auto a_data = a.data();
   auto b_data = b.data();
-  return absl::equal(a_data, a_data + a.size(), b_data, b_data + b.size());
+  return std::equal(a_data, a_data + a.size(), b_data, b_data + b.size());
 }
 
 // `operator!=(...)`
@@ -836,10 +1047,20 @@ bool operator>=(const absl::InlinedVector<T, N, A>& a,
 // call this directly.
 template <typename H, typename T, size_t N, typename A>
 H AbslHashValue(H h, const absl::InlinedVector<T, N, A>& a) {
-  auto size = a.size();
-  return H::combine(H::combine_contiguous(std::move(h), a.data(), size), size);
+  return H::combine_contiguous(std::move(h), a.data(), a.size());
+}
+
+template <typename T, size_t N, typename A, typename Predicate>
+constexpr typename InlinedVector<T, N, A>::size_type erase_if(
+    InlinedVector<T, N, A>& v, Predicate pred) {
+  const auto it = std::remove_if(v.begin(), v.end(), std::move(pred));
+  const auto removed = static_cast<typename InlinedVector<T, N, A>::size_type>(
+      std::distance(it, v.end()));
+  v.erase(it, v.end());
+  return removed;
 }
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_INLINED_VECTOR_H_
diff --git a/absl/container/inlined_vector_benchmark.cc b/absl/container/inlined_vector_benchmark.cc
index 3f2b4ed28a5..5a04277c6fb 100644
--- a/absl/container/inlined_vector_benchmark.cc
+++ b/absl/container/inlined_vector_benchmark.cc
@@ -16,11 +16,11 @@
 #include <string>
 #include <vector>
 
-#include "benchmark/benchmark.h"
 #include "absl/base/internal/raw_logging.h"
 #include "absl/base/macros.h"
 #include "absl/container/inlined_vector.h"
 #include "absl/strings/str_cat.h"
+#include "benchmark/benchmark.h"
 
 namespace {
 
@@ -66,7 +66,7 @@ void BM_StdVectorFill(benchmark::State& state) {
 BENCHMARK(BM_StdVectorFill)->Range(1, 256);
 
 // The purpose of the next two benchmarks is to verify that
-// absl::InlinedVector is efficient when moving is more efficent than
+// absl::InlinedVector is efficient when moving is more efficient than
 // copying. To do so, we use strings that are larger than the short
 // string optimization.
 bool StringRepresentedInline(std::string s) {
@@ -83,7 +83,7 @@ int GetNonShortStringOptimizationSize() {
   }
   ABSL_RAW_LOG(
       FATAL,
-      "Failed to find a std::string larger than the short std::string optimization");
+      "Failed to find a string larger than the short string optimization");
   return -1;
 }
 
@@ -534,6 +534,28 @@ void BM_ConstructFromMove(benchmark::State& state) {
 ABSL_INTERNAL_BENCHMARK_ONE_SIZE(BM_ConstructFromMove, TrivialType);
 ABSL_INTERNAL_BENCHMARK_ONE_SIZE(BM_ConstructFromMove, NontrivialType);
 
+// Measure cost of copy-constructor+destructor.
+void BM_CopyTrivial(benchmark::State& state) {
+  const int n = state.range(0);
+  InlVec<int64_t> src(n);
+  for (auto s : state) {
+    InlVec<int64_t> copy(src);
+    benchmark::DoNotOptimize(copy);
+  }
+}
+BENCHMARK(BM_CopyTrivial)->Arg(0)->Arg(1)->Arg(kLargeSize);
+
+// Measure cost of copy-constructor+destructor.
+void BM_CopyNonTrivial(benchmark::State& state) {
+  const int n = state.range(0);
+  InlVec<InlVec<int64_t>> src(n);
+  for (auto s : state) {
+    InlVec<InlVec<int64_t>> copy(src);
+    benchmark::DoNotOptimize(copy);
+  }
+}
+BENCHMARK(BM_CopyNonTrivial)->Arg(0)->Arg(1)->Arg(kLargeSize);
+
 template <typename T, size_t FromSize, size_t ToSize>
 void BM_AssignSizeRef(benchmark::State& state) {
   auto size = ToSize;
diff --git a/absl/container/inlined_vector_exception_safety_test.cc b/absl/container/inlined_vector_exception_safety_test.cc
index 25994f1685a..0e6a05b5f6a 100644
--- a/absl/container/inlined_vector_exception_safety_test.cc
+++ b/absl/container/inlined_vector_exception_safety_test.cc
@@ -16,7 +16,7 @@
 
 #include "absl/base/config.h"
 
-#ifdef ABSL_HAVE_EXCEPTIONS
+#if defined(ABSL_HAVE_EXCEPTIONS)
 
 #include <array>
 #include <initializer_list>
@@ -364,9 +364,11 @@ TYPED_TEST(OneSizeTest, EmplaceBack) {
   using VecT = typename TypeParam::VecT;
   constexpr static auto size = TypeParam::GetSizeAt(0);
 
+  // For testing calls to `emplace_back(...)` that reallocate.
   VecT full_vec{size};
   full_vec.resize(full_vec.capacity());
 
+  // For testing calls to `emplace_back(...)` that don't reallocate.
   VecT nonfull_vec{size};
   nonfull_vec.reserve(size + 1);
 
@@ -374,12 +376,11 @@ TYPED_TEST(OneSizeTest, EmplaceBack) {
       InlinedVectorInvariants<VecT>);
 
   EXPECT_TRUE(tester.WithInitialValue(nonfull_vec).Test([](VecT* vec) {
-    vec->emplace_back();  //
+    vec->emplace_back();
   }));
 
-  EXPECT_TRUE(tester.WithInitialValue(full_vec).Test([](VecT* vec) {
-    vec->emplace_back();  //
-  }));
+  EXPECT_TRUE(tester.WithInitialValue(full_vec).Test(
+      [](VecT* vec) { vec->emplace_back(); }));
 }
 
 TYPED_TEST(OneSizeTest, PopBack) {
@@ -416,6 +417,19 @@ TYPED_TEST(OneSizeTest, Erase) {
     vec->erase(it);
   }));
 
+  EXPECT_TRUE(tester.Test([](VecT* vec) {
+    auto it = vec->begin();
+    vec->erase(it, it);
+  }));
+  EXPECT_TRUE(tester.Test([](VecT* vec) {
+    auto it = vec->begin() + (vec->size() / 2);
+    vec->erase(it, it);
+  }));
+  EXPECT_TRUE(tester.Test([](VecT* vec) {
+    auto it = vec->begin() + (vec->size() - 1);
+    vec->erase(it, it);
+  }));
+
   EXPECT_TRUE(tester.Test([](VecT* vec) {
     auto it = vec->begin();
     vec->erase(it, it + 1);
@@ -452,9 +466,7 @@ TYPED_TEST(TwoSizeTest, Reserve) {
                     .WithInitialValue(VecT{from_size})
                     .WithContracts(InlinedVectorInvariants<VecT>);
 
-  EXPECT_TRUE(tester.Test([](VecT* vec) {
-    vec->reserve(to_capacity);  //
-  }));
+  EXPECT_TRUE(tester.Test([](VecT* vec) { vec->reserve(to_capacity); }));
 }
 
 TYPED_TEST(OneSizeTest, ShrinkToFit) {
@@ -493,4 +505,4 @@ TYPED_TEST(TwoSizeTest, Swap) {
 
 }  // namespace
 
-#endif  // ABSL_HAVE_EXCEPTIONS
+#endif  // defined(ABSL_HAVE_EXCEPTIONS)
diff --git a/absl/container/inlined_vector_test.cc b/absl/container/inlined_vector_test.cc
index 080ea95665d..73584edde34 100644
--- a/absl/container/inlined_vector_test.cc
+++ b/absl/container/inlined_vector_test.cc
@@ -15,24 +15,30 @@
 #include "absl/container/inlined_vector.h"
 
 #include <algorithm>
+#include <cstddef>
 #include <forward_list>
+#include <iterator>
 #include <list>
 #include <memory>
 #include <scoped_allocator>
 #include <sstream>
 #include <stdexcept>
 #include <string>
+#include <type_traits>
+#include <utility>
 #include <vector>
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
 #include "absl/base/attributes.h"
 #include "absl/base/internal/exception_testing.h"
-#include "absl/base/internal/raw_logging.h"
+#include "absl/base/internal/iterator_traits_test_helper.h"
 #include "absl/base/macros.h"
-#include "absl/container/internal/counting_allocator.h"
+#include "absl/base/options.h"
+#include "absl/container/internal/test_allocator.h"
 #include "absl/container/internal/test_instance_tracker.h"
 #include "absl/hash/hash_testing.h"
+#include "absl/log/check.h"
 #include "absl/memory/memory.h"
 #include "absl/strings/str_cat.h"
 
@@ -46,16 +52,16 @@ using testing::AllOf;
 using testing::Each;
 using testing::ElementsAre;
 using testing::ElementsAreArray;
+using testing::IsEmpty;
 using testing::Eq;
 using testing::Gt;
+using testing::Pointee;
+using testing::Pointwise;
 using testing::PrintToString;
+using testing::SizeIs;
 
 using IntVec = absl::InlinedVector<int, 8>;
 
-MATCHER_P(SizeIs, n, "") {
-  return testing::ExplainMatchResult(n, arg.size(), result_listener);
-}
-
 MATCHER_P(CapacityIs, n, "") {
   return testing::ExplainMatchResult(n, arg.capacity(), result_listener);
 }
@@ -100,13 +106,13 @@ class RefCounted {
   }
 
   void Ref() const {
-    ABSL_RAW_CHECK(count_ != nullptr, "");
+    CHECK_NE(count_, nullptr);
     ++(*count_);
   }
 
   void Unref() const {
     --(*count_);
-    ABSL_RAW_CHECK(*count_ >= 0, "");
+    CHECK_GE(*count_, 0);
   }
 
   int value_;
@@ -125,20 +131,20 @@ using DynamicVec = absl::InlinedVector<Dynamic, 8>;
 
 // Append 0..len-1 to *v
 template <typename Container>
-static void Fill(Container* v, int len, int offset = 0) {
-  for (int i = 0; i < len; i++) {
-    v->push_back(i + offset);
+static void Fill(Container* v, size_t len, int offset = 0) {
+  for (size_t i = 0; i < len; i++) {
+    v->push_back(static_cast<int>(i) + offset);
   }
 }
 
-static IntVec Fill(int len, int offset = 0) {
+static IntVec Fill(size_t len, int offset = 0) {
   IntVec v;
   Fill(&v, len, offset);
   return v;
 }
 
 TEST(IntVec, SimpleOps) {
-  for (int len = 0; len < 20; len++) {
+  for (size_t len = 0; len < 20; len++) {
     IntVec v;
     const IntVec& cv = v;  // const alias
 
@@ -146,42 +152,42 @@ TEST(IntVec, SimpleOps) {
     EXPECT_EQ(len, v.size());
     EXPECT_LE(len, v.capacity());
 
-    for (int i = 0; i < len; i++) {
-      EXPECT_EQ(i, v[i]);
-      EXPECT_EQ(i, v.at(i));
+    for (size_t i = 0; i < len; i++) {
+      EXPECT_EQ(static_cast<int>(i), v[i]);
+      EXPECT_EQ(static_cast<int>(i), v.at(i));
     }
     EXPECT_EQ(v.begin(), v.data());
     EXPECT_EQ(cv.begin(), cv.data());
 
-    int counter = 0;
+    size_t counter = 0;
     for (IntVec::iterator iter = v.begin(); iter != v.end(); ++iter) {
-      EXPECT_EQ(counter, *iter);
+      EXPECT_EQ(static_cast<int>(counter), *iter);
       counter++;
     }
     EXPECT_EQ(counter, len);
 
     counter = 0;
     for (IntVec::const_iterator iter = v.begin(); iter != v.end(); ++iter) {
-      EXPECT_EQ(counter, *iter);
+      EXPECT_EQ(static_cast<int>(counter), *iter);
       counter++;
     }
     EXPECT_EQ(counter, len);
 
     counter = 0;
     for (IntVec::const_iterator iter = v.cbegin(); iter != v.cend(); ++iter) {
-      EXPECT_EQ(counter, *iter);
+      EXPECT_EQ(static_cast<int>(counter), *iter);
       counter++;
     }
     EXPECT_EQ(counter, len);
 
     if (len > 0) {
       EXPECT_EQ(0, v.front());
-      EXPECT_EQ(len - 1, v.back());
+      EXPECT_EQ(static_cast<int>(len - 1), v.back());
       v.pop_back();
       EXPECT_EQ(len - 1, v.size());
-      for (int i = 0; i < v.size(); ++i) {
-        EXPECT_EQ(i, v[i]);
-        EXPECT_EQ(i, v.at(i));
+      for (size_t i = 0; i < v.size(); ++i) {
+        EXPECT_EQ(static_cast<int>(i), v[i]);
+        EXPECT_EQ(static_cast<int>(i), v.at(i));
       }
     }
   }
@@ -190,7 +196,7 @@ TEST(IntVec, SimpleOps) {
 TEST(IntVec, PopBackNoOverflow) {
   IntVec v = {1};
   v.pop_back();
-  EXPECT_EQ(v.size(), 0);
+  EXPECT_EQ(v.size(), 0u);
 }
 
 TEST(IntVec, AtThrows) {
@@ -200,94 +206,263 @@ TEST(IntVec, AtThrows) {
                                  "failed bounds check");
 }
 
+TEST(IntVec, LengthThrows) {
+  IntVec v = {1, 2, 3};
+  ABSL_BASE_INTERNAL_EXPECT_FAIL(v.insert(v.begin(), v.max_size() + 1, 0),
+                                 std::length_error, "failed length check");
+  ABSL_BASE_INTERNAL_EXPECT_FAIL(
+      v.insert(v.begin(), static_cast<size_t>(-1), 0), std::length_error,
+      "failed length check");
+  ABSL_BASE_INTERNAL_EXPECT_FAIL(v.resize(v.max_size() + 1), std::length_error,
+                                 "failed length check");
+  ABSL_BASE_INTERNAL_EXPECT_FAIL(v.reserve(v.max_size() + 1), std::length_error,
+                                 "failed length check");
+  ABSL_BASE_INTERNAL_EXPECT_FAIL(static_cast<void>(IntVec(v.max_size() + 1, 0)),
+                                 std::length_error, "failed length check");
+}
+
+template <typename T>
+struct SmallMaxAllocator : std::allocator<T> {
+  template <typename U>
+  struct rebind {
+    using other = SmallMaxAllocator<U>;
+  };
+  size_t max_size() const noexcept { return 2; }
+};
+
+TEST(IntVec, EmplaceLengthThrows) {
+  absl::InlinedVector<int, 1, SmallMaxAllocator<int>> v = {1, 2};
+  ABSL_BASE_INTERNAL_EXPECT_FAIL(v.push_back(3), std::length_error,
+                                 "failed length check");
+  ABSL_BASE_INTERNAL_EXPECT_FAIL(v.emplace_back(3), std::length_error,
+                                 "failed length check");
+  ABSL_BASE_INTERNAL_EXPECT_FAIL(v.insert(v.begin(), 3), std::length_error,
+                                 "failed length check");
+  ABSL_BASE_INTERNAL_EXPECT_FAIL(v.emplace(v.begin(), 3), std::length_error,
+                                 "failed length check");
+}
+
 TEST(IntVec, ReverseIterator) {
-  for (int len = 0; len < 20; len++) {
+  for (size_t len = 0; len < 20; len++) {
     IntVec v;
     Fill(&v, len);
 
-    int counter = len;
+    size_t counter = len;
     for (IntVec::reverse_iterator iter = v.rbegin(); iter != v.rend(); ++iter) {
       counter--;
-      EXPECT_EQ(counter, *iter);
+      EXPECT_EQ(static_cast<int>(counter), *iter);
     }
-    EXPECT_EQ(counter, 0);
+    EXPECT_EQ(counter, 0u);
 
     counter = len;
     for (IntVec::const_reverse_iterator iter = v.rbegin(); iter != v.rend();
          ++iter) {
       counter--;
-      EXPECT_EQ(counter, *iter);
+      EXPECT_EQ(static_cast<int>(counter), *iter);
     }
-    EXPECT_EQ(counter, 0);
+    EXPECT_EQ(counter, 0u);
 
     counter = len;
     for (IntVec::const_reverse_iterator iter = v.crbegin(); iter != v.crend();
          ++iter) {
       counter--;
-      EXPECT_EQ(counter, *iter);
+      EXPECT_EQ(static_cast<int>(counter), *iter);
     }
-    EXPECT_EQ(counter, 0);
+    EXPECT_EQ(counter, 0u);
   }
 }
 
 TEST(IntVec, Erase) {
-  for (int len = 1; len < 20; len++) {
-    for (int i = 0; i < len; ++i) {
+  for (size_t len = 1; len < 20; len++) {
+    for (size_t i = 0; i < len; ++i) {
       IntVec v;
       Fill(&v, len);
       v.erase(v.begin() + i);
       EXPECT_EQ(len - 1, v.size());
-      for (int j = 0; j < i; ++j) {
-        EXPECT_EQ(j, v[j]);
+      for (size_t j = 0; j < i; ++j) {
+        EXPECT_EQ(static_cast<int>(j), v[j]);
       }
-      for (int j = i; j < len - 1; ++j) {
-        EXPECT_EQ(j + 1, v[j]);
+      for (size_t j = i; j < len - 1; ++j) {
+        EXPECT_EQ(static_cast<int>(j + 1), v[j]);
       }
     }
   }
 }
 
+TEST(IntVec, Hardened) {
+  IntVec v;
+  Fill(&v, 10);
+  EXPECT_EQ(v[9], 9);
+#if !defined(NDEBUG) || ABSL_OPTION_HARDENED
+  EXPECT_DEATH_IF_SUPPORTED(v[10], "");
+  EXPECT_DEATH_IF_SUPPORTED(v[static_cast<size_t>(-1)], "");
+#endif
+}
+
+// Move construction of a container of unique pointers should work fine, with no
+// leaks, despite the fact that unique pointers are trivially relocatable but
+// not trivially destructible.
+TEST(UniquePtr, MoveConstruct) {
+  for (size_t size = 0; size < 16; ++size) {
+    SCOPED_TRACE(size);
+
+    absl::InlinedVector<std::unique_ptr<size_t>, 2> a;
+    for (size_t i = 0; i < size; ++i) {
+      a.push_back(std::make_unique<size_t>(i));
+    }
+
+    absl::InlinedVector<std::unique_ptr<size_t>, 2> b(std::move(a));
+
+    ASSERT_THAT(b, SizeIs(size));
+    for (size_t i = 0; i < size; ++i) {
+      ASSERT_THAT(b[i], Pointee(i));
+    }
+  }
+}
+
+// Move assignment of a container of unique pointers should work fine, with no
+// leaks, despite the fact that unique pointers are trivially relocatable but
+// not trivially destructible.
+TEST(UniquePtr, MoveAssign) {
+  for (size_t size = 0; size < 16; ++size) {
+    SCOPED_TRACE(size);
+
+    absl::InlinedVector<std::unique_ptr<size_t>, 2> a;
+    for (size_t i = 0; i < size; ++i) {
+      a.push_back(std::make_unique<size_t>(i));
+    }
+
+    absl::InlinedVector<std::unique_ptr<size_t>, 2> b;
+    b = std::move(a);
+
+    ASSERT_THAT(b, SizeIs(size));
+    for (size_t i = 0; i < size; ++i) {
+      ASSERT_THAT(b[i], Pointee(i));
+    }
+  }
+}
+
+// Swapping containers of unique pointers should work fine, with no
+// leaks, despite the fact that unique pointers are trivially relocatable but
+// not trivially destructible.
+// TODO(absl-team): Using unique_ptr here is technically correct, but
+// a trivially relocatable struct would be less semantically confusing.
+TEST(UniquePtr, Swap) {
+  for (size_t size1 = 0; size1 < 5; ++size1) {
+    for (size_t size2 = 0; size2 < 5; ++size2) {
+      absl::InlinedVector<std::unique_ptr<size_t>, 2> a;
+      absl::InlinedVector<std::unique_ptr<size_t>, 2> b;
+      for (size_t i = 0; i < size1; ++i) {
+        a.push_back(std::make_unique<size_t>(i + 10));
+      }
+      for (size_t i = 0; i < size2; ++i) {
+        b.push_back(std::make_unique<size_t>(i + 20));
+      }
+      a.swap(b);
+      ASSERT_THAT(a, SizeIs(size2));
+      ASSERT_THAT(b, SizeIs(size1));
+      for (size_t i = 0; i < a.size(); ++i) {
+        ASSERT_THAT(a[i], Pointee(i + 20));
+      }
+      for (size_t i = 0; i < b.size(); ++i) {
+        ASSERT_THAT(b[i], Pointee(i + 10));
+      }
+    }
+  }
+}
+
+// Erasing from a container of unique pointers should work fine, with no
+// leaks, despite the fact that unique pointers are trivially relocatable but
+// not trivially destructible.
+// TODO(absl-team): Using unique_ptr here is technically correct, but
+// a trivially relocatable struct would be less semantically confusing.
+TEST(UniquePtr, EraseSingle) {
+  for (size_t size = 4; size < 16; ++size) {
+    absl::InlinedVector<std::unique_ptr<size_t>, 8> a;
+    for (size_t i = 0; i < size; ++i) {
+      a.push_back(std::make_unique<size_t>(i));
+    }
+    a.erase(a.begin());
+    ASSERT_THAT(a, SizeIs(size - 1));
+    for (size_t i = 0; i < size - 1; ++i) {
+      ASSERT_THAT(a[i], Pointee(i + 1));
+    }
+    a.erase(a.begin() + 2);
+    ASSERT_THAT(a, SizeIs(size - 2));
+    ASSERT_THAT(a[0], Pointee(1));
+    ASSERT_THAT(a[1], Pointee(2));
+    for (size_t i = 2; i < size - 2; ++i) {
+      ASSERT_THAT(a[i], Pointee(i + 2));
+    }
+  }
+}
+
+// Erasing from a container of unique pointers should work fine, with no
+// leaks, despite the fact that unique pointers are trivially relocatable but
+// not trivially destructible.
+// TODO(absl-team): Using unique_ptr here is technically correct, but
+// a trivially relocatable struct would be less semantically confusing.
+TEST(UniquePtr, EraseMulti) {
+  for (size_t size = 5; size < 16; ++size) {
+    absl::InlinedVector<std::unique_ptr<size_t>, 8> a;
+    for (size_t i = 0; i < size; ++i) {
+      a.push_back(std::make_unique<size_t>(i));
+    }
+    a.erase(a.begin(), a.begin() + 2);
+    ASSERT_THAT(a, SizeIs(size - 2));
+    for (size_t i = 0; i < size - 2; ++i) {
+      ASSERT_THAT(a[i], Pointee(i + 2));
+    }
+    a.erase(a.begin() + 1, a.begin() + 3);
+    ASSERT_THAT(a, SizeIs(size - 4));
+    ASSERT_THAT(a[0], Pointee(2));
+    for (size_t i = 1; i < size - 4; ++i) {
+      ASSERT_THAT(a[i], Pointee(i + 4));
+    }
+  }
+}
+
 // At the end of this test loop, the elements between [erase_begin, erase_end)
 // should have reference counts == 0, and all others elements should have
 // reference counts == 1.
 TEST(RefCountedVec, EraseBeginEnd) {
-  for (int len = 1; len < 20; ++len) {
-    for (int erase_begin = 0; erase_begin < len; ++erase_begin) {
-      for (int erase_end = erase_begin; erase_end <= len; ++erase_end) {
+  for (size_t len = 1; len < 20; ++len) {
+    for (size_t erase_begin = 0; erase_begin < len; ++erase_begin) {
+      for (size_t erase_end = erase_begin; erase_end <= len; ++erase_end) {
         std::vector<int> counts(len, 0);
         RefCountedVec v;
-        for (int i = 0; i < len; ++i) {
-          v.push_back(RefCounted(i, &counts[i]));
+        for (size_t i = 0; i < len; ++i) {
+          v.push_back(RefCounted(static_cast<int>(i), &counts[i]));
         }
 
-        int erase_len = erase_end - erase_begin;
+        size_t erase_len = erase_end - erase_begin;
 
         v.erase(v.begin() + erase_begin, v.begin() + erase_end);
 
         EXPECT_EQ(len - erase_len, v.size());
 
         // Check the elements before the first element erased.
-        for (int i = 0; i < erase_begin; ++i) {
-          EXPECT_EQ(i, v[i].value_);
+        for (size_t i = 0; i < erase_begin; ++i) {
+          EXPECT_EQ(static_cast<int>(i), v[i].value_);
         }
 
         // Check the elements after the first element erased.
-        for (int i = erase_begin; i < v.size(); ++i) {
-          EXPECT_EQ(i + erase_len, v[i].value_);
+        for (size_t i = erase_begin; i < v.size(); ++i) {
+          EXPECT_EQ(static_cast<int>(i + erase_len), v[i].value_);
         }
 
         // Check that the elements at the beginning are preserved.
-        for (int i = 0; i < erase_begin; ++i) {
+        for (size_t i = 0; i < erase_begin; ++i) {
           EXPECT_EQ(1, counts[i]);
         }
 
         // Check that the erased elements are destroyed
-        for (int i = erase_begin; i < erase_end; ++i) {
+        for (size_t i = erase_begin; i < erase_end; ++i) {
           EXPECT_EQ(0, counts[i]);
         }
 
         // Check that the elements at the end are preserved.
-        for (int i = erase_end; i < len; ++i) {
+        for (size_t i = erase_end; i < len; ++i) {
           EXPECT_EQ(1, counts[i]);
         }
       }
@@ -336,16 +511,16 @@ TEST(InlinedVectorTest, MoveOnly) {
   v.emplace(v.begin(), MoveOnly{});
 }
 TEST(InlinedVectorTest, Noexcept) {
-  EXPECT_TRUE(std::is_nothrow_move_constructible<IntVec>::value);
-  EXPECT_TRUE((std::is_nothrow_move_constructible<
-               absl::InlinedVector<MoveOnly, 2>>::value));
+  EXPECT_TRUE(std::is_nothrow_move_constructible_v<IntVec>);
+  EXPECT_TRUE(
+      (std::is_nothrow_move_constructible_v<absl::InlinedVector<MoveOnly, 2>>));
 
   struct MoveCanThrow {
     MoveCanThrow(MoveCanThrow&&) {}
   };
   EXPECT_EQ(absl::default_allocator_is_nothrow::value,
-            (std::is_nothrow_move_constructible<
-                absl::InlinedVector<MoveCanThrow, 2>>::value));
+            (std::is_nothrow_move_constructible_v<
+                absl::InlinedVector<MoveCanThrow, 2>>));
 }
 
 TEST(InlinedVectorTest, EmplaceBack) {
@@ -366,21 +541,21 @@ TEST(InlinedVectorTest, ShrinkToFitGrowingVector) {
   absl::InlinedVector<std::pair<std::string, int>, 1> v;
 
   v.shrink_to_fit();
-  EXPECT_EQ(v.capacity(), 1);
+  EXPECT_EQ(v.capacity(), 1u);
 
   v.emplace_back("answer", 42);
   v.shrink_to_fit();
-  EXPECT_EQ(v.capacity(), 1);
+  EXPECT_EQ(v.capacity(), 1u);
 
   v.emplace_back("taxicab", 1729);
-  EXPECT_GE(v.capacity(), 2);
+  EXPECT_GE(v.capacity(), 2u);
   v.shrink_to_fit();
-  EXPECT_EQ(v.capacity(), 2);
+  EXPECT_EQ(v.capacity(), 2u);
 
   v.reserve(100);
-  EXPECT_GE(v.capacity(), 100);
+  EXPECT_GE(v.capacity(), 100u);
   v.shrink_to_fit();
-  EXPECT_EQ(v.capacity(), 2);
+  EXPECT_EQ(v.capacity(), 2u);
 }
 
 TEST(InlinedVectorTest, ShrinkToFitEdgeCases) {
@@ -388,10 +563,10 @@ TEST(InlinedVectorTest, ShrinkToFitEdgeCases) {
     absl::InlinedVector<std::pair<std::string, int>, 1> v;
     v.emplace_back("answer", 42);
     v.emplace_back("taxicab", 1729);
-    EXPECT_GE(v.capacity(), 2);
+    EXPECT_GE(v.capacity(), 2u);
     v.pop_back();
     v.shrink_to_fit();
-    EXPECT_EQ(v.capacity(), 1);
+    EXPECT_EQ(v.capacity(), 1u);
     EXPECT_EQ(v[0].first, "answer");
     EXPECT_EQ(v[0].second, 42);
   }
@@ -400,34 +575,34 @@ TEST(InlinedVectorTest, ShrinkToFitEdgeCases) {
     absl::InlinedVector<std::string, 2> v(100);
     v.resize(0);
     v.shrink_to_fit();
-    EXPECT_EQ(v.capacity(), 2);  // inlined capacity
+    EXPECT_EQ(v.capacity(), 2u);  // inlined capacity
   }
 
   {
     absl::InlinedVector<std::string, 2> v(100);
     v.resize(1);
     v.shrink_to_fit();
-    EXPECT_EQ(v.capacity(), 2);  // inlined capacity
+    EXPECT_EQ(v.capacity(), 2u);  // inlined capacity
   }
 
   {
     absl::InlinedVector<std::string, 2> v(100);
     v.resize(2);
     v.shrink_to_fit();
-    EXPECT_EQ(v.capacity(), 2);
+    EXPECT_EQ(v.capacity(), 2u);
   }
 
   {
     absl::InlinedVector<std::string, 2> v(100);
     v.resize(3);
     v.shrink_to_fit();
-    EXPECT_EQ(v.capacity(), 3);
+    EXPECT_EQ(v.capacity(), 3u);
   }
 }
 
 TEST(IntVec, Insert) {
-  for (int len = 0; len < 20; len++) {
-    for (int pos = 0; pos <= len; pos++) {
+  for (size_t len = 0; len < 20; len++) {
+    for (ptrdiff_t pos = 0; pos <= static_cast<ptrdiff_t>(len); pos++) {
       {
         // Single element
         std::vector<int> std_v;
@@ -512,19 +687,46 @@ TEST(IntVec, Insert) {
   }
 }
 
+TEST(IntPairVec, Insert) {
+  for (size_t len = 0; len < 20; len++) {
+    for (ptrdiff_t pos = 0; pos <= static_cast<ptrdiff_t>(len); pos++) {
+      // Iterator range (C++20 forward iterator)
+      const std::forward_list<int> unzipped_input = {9999, 8888, 7777};
+      const absl::base_internal::Cpp20ForwardZipIterator<
+          std::forward_list<int>::const_iterator>
+          begin(unzipped_input.cbegin(), unzipped_input.cbegin());
+      const absl::base_internal::Cpp20ForwardZipIterator<
+          std::forward_list<int>::const_iterator>
+          end(unzipped_input.cend(), unzipped_input.cend());
+
+      std::vector<std::pair<int, int>> std_v;
+      absl::InlinedVector<std::pair<int, int>, 8> v;
+      for (size_t i = 0; i < len; ++i) {
+        std_v.emplace_back(i, i);
+        v.emplace_back(i, i);
+      }
+
+      std_v.insert(std_v.begin() + pos, begin, end);
+      auto it = v.insert(v.cbegin() + pos, begin, end);
+      EXPECT_THAT(v, ElementsAreArray(std_v));
+      EXPECT_EQ(it, v.cbegin() + pos);
+    }
+  }
+}
+
 TEST(RefCountedVec, InsertConstructorDestructor) {
   // Make sure the proper construction/destruction happen during insert
   // operations.
-  for (int len = 0; len < 20; len++) {
+  for (size_t len = 0; len < 20; len++) {
     SCOPED_TRACE(len);
-    for (int pos = 0; pos <= len; pos++) {
+    for (size_t pos = 0; pos <= len; pos++) {
       SCOPED_TRACE(pos);
       std::vector<int> counts(len, 0);
       int inserted_count = 0;
       RefCountedVec v;
-      for (int i = 0; i < len; ++i) {
+      for (size_t i = 0; i < len; ++i) {
         SCOPED_TRACE(i);
-        v.push_back(RefCounted(i, &counts[i]));
+        v.push_back(RefCounted(static_cast<int>(i), &counts[i]));
       }
 
       EXPECT_THAT(counts, Each(Eq(1)));
@@ -541,20 +743,20 @@ TEST(RefCountedVec, InsertConstructorDestructor) {
 }
 
 TEST(IntVec, Resize) {
-  for (int len = 0; len < 20; len++) {
+  for (size_t len = 0; len < 20; len++) {
     IntVec v;
     Fill(&v, len);
 
     // Try resizing up and down by k elements
     static const int kResizeElem = 1000000;
-    for (int k = 0; k < 10; k++) {
+    for (size_t k = 0; k < 10; k++) {
       // Enlarging resize
       v.resize(len + k, kResizeElem);
       EXPECT_EQ(len + k, v.size());
       EXPECT_LE(len + k, v.capacity());
-      for (int i = 0; i < len + k; i++) {
+      for (size_t i = 0; i < len + k; i++) {
         if (i < len) {
-          EXPECT_EQ(i, v[i]);
+          EXPECT_EQ(static_cast<int>(i), v[i]);
         } else {
           EXPECT_EQ(kResizeElem, v[i]);
         }
@@ -564,26 +766,26 @@ TEST(IntVec, Resize) {
       v.resize(len, kResizeElem);
       EXPECT_EQ(len, v.size());
       EXPECT_LE(len, v.capacity());
-      for (int i = 0; i < len; i++) {
-        EXPECT_EQ(i, v[i]);
+      for (size_t i = 0; i < len; i++) {
+        EXPECT_EQ(static_cast<int>(i), v[i]);
       }
     }
   }
 }
 
 TEST(IntVec, InitWithLength) {
-  for (int len = 0; len < 20; len++) {
+  for (size_t len = 0; len < 20; len++) {
     IntVec v(len, 7);
     EXPECT_EQ(len, v.size());
     EXPECT_LE(len, v.capacity());
-    for (int i = 0; i < len; i++) {
+    for (size_t i = 0; i < len; i++) {
       EXPECT_EQ(7, v[i]);
     }
   }
 }
 
 TEST(IntVec, CopyConstructorAndAssignment) {
-  for (int len = 0; len < 20; len++) {
+  for (size_t len = 0; len < 20; len++) {
     IntVec v;
     Fill(&v, len);
     EXPECT_EQ(len, v.size());
@@ -592,7 +794,7 @@ TEST(IntVec, CopyConstructorAndAssignment) {
     IntVec v2(v);
     EXPECT_TRUE(v == v2) << PrintToString(v) << PrintToString(v2);
 
-    for (int start_len = 0; start_len < 20; start_len++) {
+    for (size_t start_len = 0; start_len < 20; start_len++) {
       IntVec v3;
       Fill(&v3, start_len, 99);  // Add dummy elements that should go away
       v3 = v;
@@ -602,7 +804,7 @@ TEST(IntVec, CopyConstructorAndAssignment) {
 }
 
 TEST(IntVec, AliasingCopyAssignment) {
-  for (int len = 0; len < 20; ++len) {
+  for (size_t len = 0; len < 20; ++len) {
     IntVec original;
     Fill(&original, len);
     IntVec dup = original;
@@ -612,9 +814,9 @@ TEST(IntVec, AliasingCopyAssignment) {
 }
 
 TEST(IntVec, MoveConstructorAndAssignment) {
-  for (int len = 0; len < 20; len++) {
+  for (size_t len = 0; len < 20; len++) {
     IntVec v_in;
-    const int inlined_capacity = v_in.capacity();
+    const size_t inlined_capacity = v_in.capacity();
     Fill(&v_in, len);
     EXPECT_EQ(len, v_in.size());
     EXPECT_LE(len, v_in.capacity());
@@ -631,7 +833,7 @@ TEST(IntVec, MoveConstructorAndAssignment) {
         EXPECT_FALSE(v_out.data() == old_data);
       }
     }
-    for (int start_len = 0; start_len < 20; start_len++) {
+    for (size_t start_len = 0; start_len < 20; start_len++) {
       IntVec v_out;
       Fill(&v_out, start_len, 99);  // Add dummy elements that should go away
       IntVec v_temp(v_in);
@@ -657,7 +859,7 @@ class NotTriviallyDestructible {
       : p_(new int(*other.p_)) {}
 
   NotTriviallyDestructible& operator=(const NotTriviallyDestructible& other) {
-    p_ = absl::make_unique<int>(*other.p_);
+    p_ = std::make_unique<int>(*other.p_);
     return *this;
   }
 
@@ -670,10 +872,10 @@ class NotTriviallyDestructible {
 };
 
 TEST(AliasingTest, Emplace) {
-  for (int i = 2; i < 20; ++i) {
+  for (size_t i = 2; i < 20; ++i) {
     absl::InlinedVector<NotTriviallyDestructible, 10> vec;
-    for (int j = 0; j < i; ++j) {
-      vec.push_back(NotTriviallyDestructible(j));
+    for (size_t j = 0; j < i; ++j) {
+      vec.push_back(NotTriviallyDestructible(static_cast<int>(j)));
     }
     vec.emplace(vec.begin(), vec[0]);
     EXPECT_EQ(vec[0], vec[1]);
@@ -685,12 +887,12 @@ TEST(AliasingTest, Emplace) {
 }
 
 TEST(AliasingTest, InsertWithCount) {
-  for (int i = 1; i < 20; ++i) {
+  for (size_t i = 1; i < 20; ++i) {
     absl::InlinedVector<NotTriviallyDestructible, 10> vec;
-    for (int j = 0; j < i; ++j) {
-      vec.push_back(NotTriviallyDestructible(j));
+    for (size_t j = 0; j < i; ++j) {
+      vec.push_back(NotTriviallyDestructible(static_cast<int>(j)));
     }
-    for (int n = 0; n < 5; ++n) {
+    for (size_t n = 0; n < 5; ++n) {
       // We use back where we can because it's guaranteed to become invalidated
       vec.insert(vec.begin(), n, vec.back());
       auto b = vec.begin();
@@ -725,41 +927,49 @@ TEST(OverheadTest, Storage) {
   // In particular, ensure that std::allocator doesn't cost anything to store.
   // The union should be absorbing some of the allocation bookkeeping overhead
   // in the larger vectors, leaving only the size_ field as overhead.
-  EXPECT_EQ(2 * sizeof(int*),
-            sizeof(absl::InlinedVector<int*, 1>) - 1 * sizeof(int*));
-  EXPECT_EQ(1 * sizeof(int*),
-            sizeof(absl::InlinedVector<int*, 2>) - 2 * sizeof(int*));
-  EXPECT_EQ(1 * sizeof(int*),
-            sizeof(absl::InlinedVector<int*, 3>) - 3 * sizeof(int*));
-  EXPECT_EQ(1 * sizeof(int*),
-            sizeof(absl::InlinedVector<int*, 4>) - 4 * sizeof(int*));
-  EXPECT_EQ(1 * sizeof(int*),
-            sizeof(absl::InlinedVector<int*, 5>) - 5 * sizeof(int*));
-  EXPECT_EQ(1 * sizeof(int*),
-            sizeof(absl::InlinedVector<int*, 6>) - 6 * sizeof(int*));
-  EXPECT_EQ(1 * sizeof(int*),
-            sizeof(absl::InlinedVector<int*, 7>) - 7 * sizeof(int*));
-  EXPECT_EQ(1 * sizeof(int*),
-            sizeof(absl::InlinedVector<int*, 8>) - 8 * sizeof(int*));
+
+  struct T {
+    void* val;
+  };
+  size_t expected_overhead = sizeof(T);
+
+  EXPECT_EQ((2 * expected_overhead),
+            sizeof(absl::InlinedVector<T, 1>) - sizeof(T[1]));
+  EXPECT_EQ(expected_overhead,
+            sizeof(absl::InlinedVector<T, 2>) - sizeof(T[2]));
+  EXPECT_EQ(expected_overhead,
+            sizeof(absl::InlinedVector<T, 3>) - sizeof(T[3]));
+  EXPECT_EQ(expected_overhead,
+            sizeof(absl::InlinedVector<T, 4>) - sizeof(T[4]));
+  EXPECT_EQ(expected_overhead,
+            sizeof(absl::InlinedVector<T, 5>) - sizeof(T[5]));
+  EXPECT_EQ(expected_overhead,
+            sizeof(absl::InlinedVector<T, 6>) - sizeof(T[6]));
+  EXPECT_EQ(expected_overhead,
+            sizeof(absl::InlinedVector<T, 7>) - sizeof(T[7]));
+  EXPECT_EQ(expected_overhead,
+            sizeof(absl::InlinedVector<T, 8>) - sizeof(T[8]));
 }
 
 TEST(IntVec, Clear) {
-  for (int len = 0; len < 20; len++) {
+  for (size_t len = 0; len < 20; len++) {
     SCOPED_TRACE(len);
     IntVec v;
     Fill(&v, len);
+    size_t capacity_before_clear = v.capacity();
     v.clear();
-    EXPECT_EQ(0, v.size());
+    EXPECT_EQ(v.capacity(), capacity_before_clear);
+    EXPECT_EQ(0u, v.size());
     EXPECT_EQ(v.begin(), v.end());
   }
 }
 
 TEST(IntVec, Reserve) {
-  for (int len = 0; len < 20; len++) {
+  for (size_t len = 0; len < 20; len++) {
     IntVec v;
     Fill(&v, len);
 
-    for (int newlen = 0; newlen < 100; newlen++) {
+    for (size_t newlen = 0; newlen < 100; newlen++) {
       const int* start_rep = v.data();
       v.reserve(newlen);
       const int* final_rep = v.data();
@@ -780,7 +990,7 @@ TEST(IntVec, Reserve) {
 TEST(StringVec, SelfRefPushBack) {
   std::vector<std::string> std_v;
   absl::InlinedVector<std::string, 4> v;
-  const std::string s = "A quite long std::string to ensure heap.";
+  const std::string s = "A quite long string to ensure heap.";
   std_v.push_back(s);
   v.push_back(s);
   for (int i = 0; i < 20; ++i) {
@@ -795,7 +1005,7 @@ TEST(StringVec, SelfRefPushBack) {
 TEST(StringVec, SelfRefPushBackWithMove) {
   std::vector<std::string> std_v;
   absl::InlinedVector<std::string, 4> v;
-  const std::string s = "A quite long std::string to ensure heap.";
+  const std::string s = "A quite long string to ensure heap.";
   std_v.push_back(s);
   v.push_back(s);
   for (int i = 0; i < 20; ++i) {
@@ -808,7 +1018,7 @@ TEST(StringVec, SelfRefPushBackWithMove) {
 }
 
 TEST(StringVec, SelfMove) {
-  const std::string s = "A quite long std::string to ensure heap.";
+  const std::string s = "A quite long string to ensure heap.";
   for (int len = 0; len < 20; len++) {
     SCOPED_TRACE(len);
     absl::InlinedVector<std::string, 8> v;
@@ -826,9 +1036,9 @@ TEST(StringVec, SelfMove) {
 }
 
 TEST(IntVec, Swap) {
-  for (int l1 = 0; l1 < 20; l1++) {
+  for (size_t l1 = 0; l1 < 20; l1++) {
     SCOPED_TRACE(l1);
-    for (int l2 = 0; l2 < 20; l2++) {
+    for (size_t l2 = 0; l2 < 20; l2++) {
       SCOPED_TRACE(l2);
       IntVec a = Fill(l1, 0);
       IntVec b = Fill(l2, 100);
@@ -838,13 +1048,13 @@ TEST(IntVec, Swap) {
       }
       EXPECT_EQ(l1, b.size());
       EXPECT_EQ(l2, a.size());
-      for (int i = 0; i < l1; i++) {
+      for (size_t i = 0; i < l1; i++) {
         SCOPED_TRACE(i);
-        EXPECT_EQ(i, b[i]);
+        EXPECT_EQ(static_cast<int>(i), b[i]);
       }
-      for (int i = 0; i < l2; i++) {
+      for (size_t i = 0; i < l2; i++) {
         SCOPED_TRACE(i);
-        EXPECT_EQ(100 + i, a[i]);
+        EXPECT_EQ(100 + static_cast<int>(i), a[i]);
       }
     }
   }
@@ -853,46 +1063,48 @@ TEST(IntVec, Swap) {
 TYPED_TEST_P(InstanceTest, Swap) {
   using Instance = TypeParam;
   using InstanceVec = absl::InlinedVector<Instance, 8>;
-  for (int l1 = 0; l1 < 20; l1++) {
+  for (size_t l1 = 0; l1 < 20; l1++) {
     SCOPED_TRACE(l1);
-    for (int l2 = 0; l2 < 20; l2++) {
+    for (size_t l2 = 0; l2 < 20; l2++) {
       SCOPED_TRACE(l2);
       InstanceTracker tracker;
       InstanceVec a, b;
       const size_t inlined_capacity = a.capacity();
       auto min_len = std::min(l1, l2);
       auto max_len = std::max(l1, l2);
-      for (int i = 0; i < l1; i++) a.push_back(Instance(i));
-      for (int i = 0; i < l2; i++) b.push_back(Instance(100 + i));
-      EXPECT_EQ(tracker.instances(), l1 + l2);
+      for (size_t i = 0; i < l1; i++)
+        a.push_back(Instance(static_cast<int>(i)));
+      for (size_t i = 0; i < l2; i++)
+        b.push_back(Instance(100 + static_cast<int>(i)));
+      EXPECT_EQ(tracker.instances(), static_cast<int>(l1 + l2));
       tracker.ResetCopiesMovesSwaps();
       {
         using std::swap;
         swap(a, b);
       }
-      EXPECT_EQ(tracker.instances(), l1 + l2);
+      EXPECT_EQ(tracker.instances(), static_cast<int>(l1 + l2));
       if (a.size() > inlined_capacity && b.size() > inlined_capacity) {
         EXPECT_EQ(tracker.swaps(), 0);  // Allocations are swapped.
         EXPECT_EQ(tracker.moves(), 0);
       } else if (a.size() <= inlined_capacity && b.size() <= inlined_capacity) {
-        EXPECT_EQ(tracker.swaps(), min_len);
+        EXPECT_EQ(tracker.swaps(), static_cast<int>(min_len));
         EXPECT_EQ((tracker.moves() ? tracker.moves() : tracker.copies()),
-                  max_len - min_len);
+                  static_cast<int>(max_len - min_len));
       } else {
         // One is allocated and the other isn't. The allocation is transferred
         // without copying elements, and the inlined instances are copied/moved.
         EXPECT_EQ(tracker.swaps(), 0);
         EXPECT_EQ((tracker.moves() ? tracker.moves() : tracker.copies()),
-                  min_len);
+                  static_cast<int>(min_len));
       }
 
       EXPECT_EQ(l1, b.size());
       EXPECT_EQ(l2, a.size());
-      for (int i = 0; i < l1; i++) {
-        EXPECT_EQ(i, b[i].value());
+      for (size_t i = 0; i < l1; i++) {
+        EXPECT_EQ(static_cast<int>(i), b[i].value());
       }
-      for (int i = 0; i < l2; i++) {
-        EXPECT_EQ(100 + i, a[i].value());
+      for (size_t i = 0; i < l2; i++) {
+        EXPECT_EQ(100 + static_cast<int>(i), a[i].value());
       }
     }
   }
@@ -921,9 +1133,9 @@ TEST(IntVec, EqualAndNotEqual) {
 
   a.clear();
   b.clear();
-  for (int i = 0; i < 100; i++) {
-    a.push_back(i);
-    b.push_back(i);
+  for (size_t i = 0; i < 100; i++) {
+    a.push_back(static_cast<int>(i));
+    b.push_back(static_cast<int>(i));
     EXPECT_TRUE(a == b);
     EXPECT_FALSE(a != b);
 
@@ -962,26 +1174,26 @@ TYPED_TEST_P(InstanceTest, CountConstructorsDestructors) {
   using Instance = TypeParam;
   using InstanceVec = absl::InlinedVector<Instance, 8>;
   InstanceTracker tracker;
-  for (int len = 0; len < 20; len++) {
+  for (size_t len = 0; len < 20; len++) {
     SCOPED_TRACE(len);
     tracker.ResetCopiesMovesSwaps();
 
     InstanceVec v;
     const size_t inlined_capacity = v.capacity();
-    for (int i = 0; i < len; i++) {
-      v.push_back(Instance(i));
+    for (size_t i = 0; i < len; i++) {
+      v.push_back(Instance(static_cast<int>(i)));
     }
-    EXPECT_EQ(tracker.instances(), len);
+    EXPECT_EQ(tracker.instances(), static_cast<int>(len));
     EXPECT_GE(tracker.copies() + tracker.moves(),
-              len);  // More due to reallocation.
+              static_cast<int>(len));  // More due to reallocation.
     tracker.ResetCopiesMovesSwaps();
 
     // Enlarging resize() must construct some objects
     tracker.ResetCopiesMovesSwaps();
     v.resize(len + 10, Instance(100));
-    EXPECT_EQ(tracker.instances(), len + 10);
+    EXPECT_EQ(tracker.instances(), static_cast<int>(len) + 10);
     if (len <= inlined_capacity && len + 10 > inlined_capacity) {
-      EXPECT_EQ(tracker.copies() + tracker.moves(), 10 + len);
+      EXPECT_EQ(tracker.copies() + tracker.moves(), 10 + static_cast<int>(len));
     } else {
       // Only specify a minimum number of copies + moves. We don't want to
       // depend on the reallocation policy here.
@@ -992,29 +1204,30 @@ TYPED_TEST_P(InstanceTest, CountConstructorsDestructors) {
     // Shrinking resize() must destroy some objects
     tracker.ResetCopiesMovesSwaps();
     v.resize(len, Instance(100));
-    EXPECT_EQ(tracker.instances(), len);
+    EXPECT_EQ(tracker.instances(), static_cast<int>(len));
     EXPECT_EQ(tracker.copies(), 0);
     EXPECT_EQ(tracker.moves(), 0);
 
     // reserve() must not increase the number of initialized objects
     SCOPED_TRACE("reserve");
     v.reserve(len + 1000);
-    EXPECT_EQ(tracker.instances(), len);
-    EXPECT_EQ(tracker.copies() + tracker.moves(), len);
+    EXPECT_EQ(tracker.instances(), static_cast<int>(len));
+    EXPECT_EQ(tracker.copies() + tracker.moves(), static_cast<int>(len));
 
     // pop_back() and erase() must destroy one object
     if (len > 0) {
       tracker.ResetCopiesMovesSwaps();
       v.pop_back();
-      EXPECT_EQ(tracker.instances(), len - 1);
+      EXPECT_EQ(tracker.instances(), static_cast<int>(len) - 1);
       EXPECT_EQ(tracker.copies(), 0);
       EXPECT_EQ(tracker.moves(), 0);
 
       if (!v.empty()) {
         tracker.ResetCopiesMovesSwaps();
         v.erase(v.begin());
-        EXPECT_EQ(tracker.instances(), len - 2);
-        EXPECT_EQ(tracker.copies() + tracker.moves(), len - 2);
+        EXPECT_EQ(tracker.instances(), static_cast<int>(len) - 2);
+        EXPECT_EQ(tracker.copies() + tracker.moves(),
+                  static_cast<int>(len) - 2);
       }
     }
 
@@ -1044,6 +1257,7 @@ TYPED_TEST_P(InstanceTest, CountConstructorsDestructorsOnCopyConstruction) {
     tracker.ResetCopiesMovesSwaps();
     {  // Copy constructor should create 'len' more instances.
       InstanceVec v_copy(v);
+      EXPECT_EQ(v_copy.size(), v.size());
       EXPECT_EQ(tracker.instances(), len + len);
       EXPECT_EQ(tracker.copies(), len);
       EXPECT_EQ(tracker.moves(), 0);
@@ -1071,12 +1285,13 @@ TYPED_TEST_P(InstanceTest, CountConstructorsDestructorsOnMoveConstruction) {
     tracker.ResetCopiesMovesSwaps();
     {
       InstanceVec v_copy(std::move(v));
-      if (len > inlined_capacity) {
+      EXPECT_EQ(v_copy.size(), len);
+      if (static_cast<size_t>(len) > inlined_capacity) {
         // Allocation is moved as a whole.
         EXPECT_EQ(tracker.instances(), len);
         EXPECT_EQ(tracker.live_instances(), len);
         // Tests an implementation detail, don't rely on this in your code.
-        EXPECT_EQ(v.size(), 0);  // NOLINT misc-use-after-move
+        EXPECT_EQ(v.size(), 0u);  // NOLINT misc-use-after-move
         EXPECT_EQ(tracker.copies(), 0);
         EXPECT_EQ(tracker.moves(), 0);
       } else {
@@ -1142,7 +1357,7 @@ TYPED_TEST_P(InstanceTest, CountConstructorsDestructorsOnMoveAssignment) {
       tracker.ResetCopiesMovesSwaps();
 
       InstanceVec longer, shorter;
-      const int inlined_capacity = longer.capacity();
+      const size_t inlined_capacity = longer.capacity();
       for (int i = 0; i < len; i++) {
         longer.push_back(Instance(i));
         shorter.push_back(Instance(i));
@@ -1161,7 +1376,7 @@ TYPED_TEST_P(InstanceTest, CountConstructorsDestructorsOnMoveAssignment) {
         src_len = len;
         longer = std::move(shorter);
       }
-      if (src_len > inlined_capacity) {
+      if (static_cast<size_t>(src_len) > inlined_capacity) {
         // Allocation moved as a whole.
         EXPECT_EQ(tracker.instances(), src_len);
         EXPECT_EQ(tracker.live_instances(), src_len);
@@ -1186,6 +1401,8 @@ TYPED_TEST_P(InstanceTest, CountConstructorsDestructorsOnMoveAssignment) {
 }
 
 TEST(CountElemAssign, SimpleTypeWithInlineBacking) {
+  const size_t inlined_capacity = absl::InlinedVector<int, 2>().capacity();
+
   for (size_t original_size = 0; original_size <= 5; ++original_size) {
     SCOPED_TRACE(original_size);
     // Original contents are [12345, 12345, ...]
@@ -1194,10 +1411,10 @@ TEST(CountElemAssign, SimpleTypeWithInlineBacking) {
     absl::InlinedVector<int, 2> v(original_contents.begin(),
                                   original_contents.end());
     v.assign(2, 123);
-    EXPECT_THAT(v, AllOf(SizeIs(2), ElementsAre(123, 123)));
-    if (original_size <= 2) {
+    EXPECT_THAT(v, AllOf(SizeIs(2u), ElementsAre(123, 123)));
+    if (original_size <= inlined_capacity) {
       // If the original had inline backing, it should stay inline.
-      EXPECT_EQ(2, v.capacity());
+      EXPECT_EQ(v.capacity(), inlined_capacity);
     }
   }
 }
@@ -1211,7 +1428,7 @@ TEST(CountElemAssign, SimpleTypeWithAllocation) {
     absl::InlinedVector<int, 2> v(original_contents.begin(),
                                   original_contents.end());
     v.assign(3, 123);
-    EXPECT_THAT(v, AllOf(SizeIs(3), ElementsAre(123, 123, 123)));
+    EXPECT_THAT(v, AllOf(SizeIs(3u), ElementsAre(123, 123, 123)));
     EXPECT_LE(v.size(), v.capacity());
   }
 }
@@ -1226,10 +1443,10 @@ TYPED_TEST_P(InstanceTest, CountElemAssignInlineBacking) {
     absl::InlinedVector<Instance, 2> v(original_contents.begin(),
                                        original_contents.end());
     v.assign(2, Instance(123));
-    EXPECT_THAT(v, AllOf(SizeIs(2), ElementsAre(ValueIs(123), ValueIs(123))));
+    EXPECT_THAT(v, AllOf(SizeIs(2u), ElementsAre(ValueIs(123), ValueIs(123))));
     if (original_size <= 2) {
       // If the original had inline backing, it should stay inline.
-      EXPECT_EQ(2, v.capacity());
+      EXPECT_EQ(2u, v.capacity());
     }
   }
 }
@@ -1244,8 +1461,8 @@ void InstanceCountElemAssignWithAllocationTest() {
     absl::InlinedVector<Instance, 2> v(original_contents.begin(),
                                        original_contents.end());
     v.assign(3, Instance(123));
-    EXPECT_THAT(v, AllOf(SizeIs(3), ElementsAre(ValueIs(123), ValueIs(123),
-                                                ValueIs(123))));
+    EXPECT_THAT(v, AllOf(SizeIs(3u), ElementsAre(ValueIs(123), ValueIs(123),
+                                                 ValueIs(123))));
     EXPECT_LE(v.size(), v.capacity());
   }
 }
@@ -1260,16 +1477,17 @@ TEST(RangedConstructor, SimpleType) {
   std::vector<int> source_v = {4, 5, 6};
   // First try to fit in inline backing
   absl::InlinedVector<int, 4> v(source_v.begin(), source_v.end());
-  EXPECT_EQ(3, v.size());
-  EXPECT_EQ(4, v.capacity());  // Indication that we're still on inlined storage
+  EXPECT_EQ(3u, v.size());
+  EXPECT_EQ(4u,
+            v.capacity());  // Indication that we're still on inlined storage
   EXPECT_EQ(4, v[0]);
   EXPECT_EQ(5, v[1]);
   EXPECT_EQ(6, v[2]);
 
   // Now, force a re-allocate
   absl::InlinedVector<int, 2> realloc_v(source_v.begin(), source_v.end());
-  EXPECT_EQ(3, realloc_v.size());
-  EXPECT_LT(2, realloc_v.capacity());
+  EXPECT_EQ(3u, realloc_v.size());
+  EXPECT_LT(2u, realloc_v.capacity());
   EXPECT_EQ(4, realloc_v[0]);
   EXPECT_EQ(5, realloc_v[1]);
   EXPECT_EQ(6, realloc_v[2]);
@@ -1284,8 +1502,8 @@ void InstanceRangedConstructorTestForContainer() {
   tracker.ResetCopiesMovesSwaps();
   absl::InlinedVector<Instance, inlined_capacity> v(source_v.begin(),
                                                     source_v.end());
-  EXPECT_EQ(2, v.size());
-  EXPECT_LT(1, v.capacity());
+  EXPECT_EQ(2u, v.size());
+  EXPECT_LT(1u, v.capacity());
   EXPECT_EQ(0, v[0].value());
   EXPECT_EQ(1, v[1].value());
   EXPECT_EQ(tracker.copies(), 2);
@@ -1337,6 +1555,8 @@ TEST(RangedConstructor, ElementsAreConstructed) {
 }
 
 TEST(RangedAssign, SimpleType) {
+  const size_t inlined_capacity = absl::InlinedVector<int, 3>().capacity();
+
   // Test for all combinations of original sizes (empty and non-empty inline,
   // and out of line) and target sizes.
   for (size_t original_size = 0; original_size <= 5; ++original_size) {
@@ -1350,7 +1570,7 @@ TEST(RangedAssign, SimpleType) {
       // New contents are [3, 4, ...]
       std::vector<int> new_contents;
       for (size_t i = 0; i < target_size; ++i) {
-        new_contents.push_back(i + 3);
+        new_contents.push_back(static_cast<int>(i + 3));
       }
 
       absl::InlinedVector<int, 3> v(original_contents.begin(),
@@ -1359,9 +1579,10 @@ TEST(RangedAssign, SimpleType) {
 
       EXPECT_EQ(new_contents.size(), v.size());
       EXPECT_LE(new_contents.size(), v.capacity());
-      if (target_size <= 3 && original_size <= 3) {
+      if (target_size <= inlined_capacity &&
+          original_size <= inlined_capacity) {
         // Storage should stay inline when target size is small.
-        EXPECT_EQ(3, v.capacity());
+        EXPECT_EQ(v.capacity(), inlined_capacity);
       }
       EXPECT_THAT(v, ElementsAreArray(new_contents));
     }
@@ -1394,7 +1615,7 @@ void InstanceRangedAssignTestForContainer() {
       // TODO(bsamwel): Test with an input iterator.
       std::vector<Instance> new_contents_in;
       for (size_t i = 0; i < target_size; ++i) {
-        new_contents_in.push_back(Instance(i + 3));
+        new_contents_in.push_back(Instance(static_cast<int>(i) + 3));
       }
       SourceContainer new_contents(new_contents_in.begin(),
                                    new_contents_in.end());
@@ -1407,7 +1628,7 @@ void InstanceRangedAssignTestForContainer() {
       EXPECT_LE(new_contents.size(), v.capacity());
       if (target_size <= 3 && original_size <= 3) {
         // Storage should stay inline when target size is small.
-        EXPECT_EQ(3, v.capacity());
+        EXPECT_EQ(3u, v.capacity());
       }
       EXPECT_TRUE(std::equal(v.begin(), v.end(), new_contents.begin(),
                              InstanceValuesEqual<Instance>));
@@ -1431,12 +1652,12 @@ TYPED_TEST_P(InstanceTest, RangedAssign) {
 
 TEST(InitializerListConstructor, SimpleTypeWithInlineBacking) {
   EXPECT_THAT((absl::InlinedVector<int, 4>{4, 5, 6}),
-              AllOf(SizeIs(3), CapacityIs(4), ElementsAre(4, 5, 6)));
+              AllOf(SizeIs(3u), CapacityIs(4u), ElementsAre(4, 5, 6)));
 }
 
 TEST(InitializerListConstructor, SimpleTypeWithReallocationRequired) {
   EXPECT_THAT((absl::InlinedVector<int, 2>{4, 5, 6}),
-              AllOf(SizeIs(3), CapacityIs(Gt(2)), ElementsAre(4, 5, 6)));
+              AllOf(SizeIs(3u), CapacityIs(Gt(2u)), ElementsAre(4, 5, 6)));
 }
 
 TEST(InitializerListConstructor, DisparateTypesInList) {
@@ -1447,16 +1668,19 @@ TEST(InitializerListConstructor, DisparateTypesInList) {
 }
 
 TEST(InitializerListConstructor, ComplexTypeWithInlineBacking) {
-  EXPECT_THAT((absl::InlinedVector<CopyableMovableInstance, 1>{
-                  CopyableMovableInstance(0)}),
-              AllOf(SizeIs(1), CapacityIs(1), ElementsAre(ValueIs(0))));
+  const size_t inlined_capacity =
+      absl::InlinedVector<CopyableMovableInstance, 1>().capacity();
+  EXPECT_THAT(
+      (absl::InlinedVector<CopyableMovableInstance, 1>{
+          CopyableMovableInstance(0)}),
+      AllOf(SizeIs(1u), CapacityIs(inlined_capacity), ElementsAre(ValueIs(0))));
 }
 
 TEST(InitializerListConstructor, ComplexTypeWithReallocationRequired) {
-  EXPECT_THAT(
-      (absl::InlinedVector<CopyableMovableInstance, 1>{
-          CopyableMovableInstance(0), CopyableMovableInstance(1)}),
-      AllOf(SizeIs(2), CapacityIs(Gt(1)), ElementsAre(ValueIs(0), ValueIs(1))));
+  EXPECT_THAT((absl::InlinedVector<CopyableMovableInstance, 1>{
+                  CopyableMovableInstance(0), CopyableMovableInstance(1)}),
+              AllOf(SizeIs(2u), CapacityIs(Gt(1u)),
+                    ElementsAre(ValueIs(0), ValueIs(1))));
 }
 
 TEST(InitializerListAssign, SimpleTypeFitsInlineBacking) {
@@ -1466,14 +1690,14 @@ TEST(InitializerListAssign, SimpleTypeFitsInlineBacking) {
     absl::InlinedVector<int, 2> v1(original_size, 12345);
     const size_t original_capacity_v1 = v1.capacity();
     v1.assign({3});
-    EXPECT_THAT(
-        v1, AllOf(SizeIs(1), CapacityIs(original_capacity_v1), ElementsAre(3)));
+    EXPECT_THAT(v1, AllOf(SizeIs(1u), CapacityIs(original_capacity_v1),
+                          ElementsAre(3)));
 
     absl::InlinedVector<int, 2> v2(original_size, 12345);
     const size_t original_capacity_v2 = v2.capacity();
     v2 = {3};
-    EXPECT_THAT(
-        v2, AllOf(SizeIs(1), CapacityIs(original_capacity_v2), ElementsAre(3)));
+    EXPECT_THAT(v2, AllOf(SizeIs(1u), CapacityIs(original_capacity_v2),
+                          ElementsAre(3)));
   }
 }
 
@@ -1482,13 +1706,13 @@ TEST(InitializerListAssign, SimpleTypeDoesNotFitInlineBacking) {
     SCOPED_TRACE(original_size);
     absl::InlinedVector<int, 2> v1(original_size, 12345);
     v1.assign({3, 4, 5});
-    EXPECT_THAT(v1, AllOf(SizeIs(3), ElementsAre(3, 4, 5)));
-    EXPECT_LE(3, v1.capacity());
+    EXPECT_THAT(v1, AllOf(SizeIs(3u), ElementsAre(3, 4, 5)));
+    EXPECT_LE(3u, v1.capacity());
 
     absl::InlinedVector<int, 2> v2(original_size, 12345);
     v2 = {3, 4, 5};
-    EXPECT_THAT(v2, AllOf(SizeIs(3), ElementsAre(3, 4, 5)));
-    EXPECT_LE(3, v2.capacity());
+    EXPECT_THAT(v2, AllOf(SizeIs(3u), ElementsAre(3, 4, 5)));
+    EXPECT_LE(3u, v2.capacity());
   }
 }
 
@@ -1517,7 +1741,7 @@ TYPED_TEST_P(InstanceTest, InitializerListAssign) {
     absl::InlinedVector<Instance, 2> v(original_size, Instance(12345));
     const size_t original_capacity = v.capacity();
     v.assign({Instance(3)});
-    EXPECT_THAT(v, AllOf(SizeIs(1), CapacityIs(original_capacity),
+    EXPECT_THAT(v, AllOf(SizeIs(1u), CapacityIs(original_capacity),
                          ElementsAre(ValueIs(3))));
   }
   for (size_t original_size = 0; original_size <= 4; ++original_size) {
@@ -1525,28 +1749,53 @@ TYPED_TEST_P(InstanceTest, InitializerListAssign) {
     absl::InlinedVector<Instance, 2> v(original_size, Instance(12345));
     v.assign({Instance(3), Instance(4), Instance(5)});
     EXPECT_THAT(
-        v, AllOf(SizeIs(3), ElementsAre(ValueIs(3), ValueIs(4), ValueIs(5))));
-    EXPECT_LE(3, v.capacity());
+        v, AllOf(SizeIs(3u), ElementsAre(ValueIs(3), ValueIs(4), ValueIs(5))));
+    EXPECT_LE(3u, v.capacity());
   }
 }
 
-REGISTER_TYPED_TEST_CASE_P(InstanceTest, Swap, CountConstructorsDestructors,
-                           CountConstructorsDestructorsOnCopyConstruction,
-                           CountConstructorsDestructorsOnMoveConstruction,
-                           CountConstructorsDestructorsOnAssignment,
-                           CountConstructorsDestructorsOnMoveAssignment,
-                           CountElemAssignInlineBacking, RangedConstructor,
-                           RangedAssign, InitializerListAssign);
+REGISTER_TYPED_TEST_SUITE_P(InstanceTest, Swap, CountConstructorsDestructors,
+                            CountConstructorsDestructorsOnCopyConstruction,
+                            CountConstructorsDestructorsOnMoveConstruction,
+                            CountConstructorsDestructorsOnAssignment,
+                            CountConstructorsDestructorsOnMoveAssignment,
+                            CountElemAssignInlineBacking, RangedConstructor,
+                            RangedAssign, InitializerListAssign);
 
 using InstanceTypes =
     ::testing::Types<CopyableOnlyInstance, CopyableMovableInstance>;
-INSTANTIATE_TYPED_TEST_CASE_P(InstanceTestOnTypes, InstanceTest, InstanceTypes);
+INSTANTIATE_TYPED_TEST_SUITE_P(InstanceTestOnTypes, InstanceTest,
+                               InstanceTypes);
 
 TEST(DynamicVec, DynamicVecCompiles) {
   DynamicVec v;
   (void)v;
 }
 
+TEST(DynamicVec, CreateNonEmptyDynamicVec) {
+  DynamicVec v(1);
+  EXPECT_EQ(v.size(), 1u);
+}
+
+TEST(DynamicVec, EmplaceBack) {
+  DynamicVec v;
+  v.emplace_back(Dynamic{});
+  EXPECT_EQ(v.size(), 1u);
+}
+
+TEST(DynamicVec, EmplaceBackAfterHeapAllocation) {
+  DynamicVec v;
+  v.reserve(10);
+  v.emplace_back(Dynamic{});
+  EXPECT_EQ(v.size(), 1u);
+}
+
+TEST(DynamicVec, EmptyIteratorComparison) {
+  DynamicVec v;
+  EXPECT_EQ(v.begin(), v.end());
+  EXPECT_EQ(v.cbegin(), v.cend());
+}
+
 TEST(AllocatorSupportTest, Constructors) {
   using MyAlloc = CountingAllocator<int>;
   using AllocVec = absl::InlinedVector<int, 4, MyAlloc>;
@@ -1555,7 +1804,9 @@ TEST(AllocatorSupportTest, Constructors) {
   MyAlloc alloc(&allocated);
   { AllocVec ABSL_ATTRIBUTE_UNUSED v; }
   { AllocVec ABSL_ATTRIBUTE_UNUSED v(alloc); }
-  { AllocVec ABSL_ATTRIBUTE_UNUSED v(ia, ia + ABSL_ARRAYSIZE(ia), alloc); }
+  {
+    AllocVec ABSL_ATTRIBUTE_UNUSED v(ia, ia + std::size(ia), alloc);
+  }
   { AllocVec ABSL_ATTRIBUTE_UNUSED v({1, 2, 3}, alloc); }
 
   AllocVec v2;
@@ -1567,61 +1818,80 @@ TEST(AllocatorSupportTest, CountAllocations) {
   using MyAlloc = CountingAllocator<int>;
   using AllocVec = absl::InlinedVector<int, 4, MyAlloc>;
   const int ia[] = {0, 1, 2, 3, 4, 5, 6, 7};
-  int64_t allocated = 0;
-  MyAlloc alloc(&allocated);
+  int64_t bytes_allocated = 0;
+  int64_t instance_count = 0;
+  MyAlloc alloc(&bytes_allocated, &instance_count);
   {
     AllocVec ABSL_ATTRIBUTE_UNUSED v(ia, ia + 4, alloc);
-    EXPECT_THAT(allocated, 0);
+    EXPECT_THAT(bytes_allocated, Eq(0));
+    EXPECT_THAT(instance_count, Eq(4));
   }
-  EXPECT_THAT(allocated, 0);
+  EXPECT_THAT(bytes_allocated, Eq(0));
+  EXPECT_THAT(instance_count, Eq(0));
   {
-    AllocVec ABSL_ATTRIBUTE_UNUSED v(ia, ia + ABSL_ARRAYSIZE(ia), alloc);
-    EXPECT_THAT(allocated, v.size() * sizeof(int));
+    AllocVec ABSL_ATTRIBUTE_UNUSED v(ia, ia + std::size(ia), alloc);
+    EXPECT_THAT(bytes_allocated,
+                Eq(static_cast<int64_t>(v.size() * sizeof(int))));
+    EXPECT_THAT(instance_count, Eq(static_cast<int64_t>(v.size())));
   }
-  EXPECT_THAT(allocated, 0);
+  EXPECT_THAT(bytes_allocated, Eq(0));
+  EXPECT_THAT(instance_count, Eq(0));
   {
     AllocVec v(4, 1, alloc);
-    EXPECT_THAT(allocated, 0);
-
-    int64_t allocated2 = 0;
-    MyAlloc alloc2(&allocated2);
-    AllocVec v2(v, alloc2);
-    EXPECT_THAT(allocated2, 0);
-
-    int64_t allocated3 = 0;
-    MyAlloc alloc3(&allocated3);
-    AllocVec v3(std::move(v), alloc3);
-    EXPECT_THAT(allocated3, 0);
+    EXPECT_THAT(bytes_allocated, Eq(0));
+    EXPECT_THAT(instance_count, Eq(4));
+
+    int64_t bytes_allocated2 = 0;
+    MyAlloc alloc2(&bytes_allocated2);
+    ABSL_ATTRIBUTE_UNUSED AllocVec v2(v, alloc2);
+    EXPECT_THAT(bytes_allocated2, Eq(0));
+
+    int64_t bytes_allocated3 = 0;
+    MyAlloc alloc3(&bytes_allocated3);
+    ABSL_ATTRIBUTE_UNUSED AllocVec v3(std::move(v), alloc3);
+    EXPECT_THAT(bytes_allocated3, Eq(0));
   }
-  EXPECT_THAT(allocated, 0);
+  EXPECT_THAT(bytes_allocated, Eq(0));
+  EXPECT_THAT(instance_count, Eq(0));
   {
     AllocVec v(8, 2, alloc);
-    EXPECT_THAT(allocated, v.size() * sizeof(int));
+    EXPECT_THAT(bytes_allocated,
+                Eq(static_cast<int64_t>(v.size() * sizeof(int))));
+    EXPECT_THAT(instance_count, Eq(static_cast<int64_t>(v.size())));
 
-    int64_t allocated2 = 0;
-    MyAlloc alloc2(&allocated2);
+    int64_t bytes_allocated2 = 0;
+    MyAlloc alloc2(&bytes_allocated2);
     AllocVec v2(v, alloc2);
-    EXPECT_THAT(allocated2, v2.size() * sizeof(int));
+    EXPECT_THAT(bytes_allocated2,
+                Eq(static_cast<int64_t>(v2.size() * sizeof(int))));
 
-    int64_t allocated3 = 0;
-    MyAlloc alloc3(&allocated3);
+    int64_t bytes_allocated3 = 0;
+    MyAlloc alloc3(&bytes_allocated3);
     AllocVec v3(std::move(v), alloc3);
-    EXPECT_THAT(allocated3, v3.size() * sizeof(int));
+    EXPECT_THAT(bytes_allocated3,
+                Eq(static_cast<int64_t>(v3.size() * sizeof(int))));
   }
-  EXPECT_EQ(allocated, 0);
+  EXPECT_EQ(bytes_allocated, 0);
+  EXPECT_EQ(instance_count, 0);
   {
     // Test shrink_to_fit deallocations.
     AllocVec v(8, 2, alloc);
-    EXPECT_EQ(allocated, 8 * sizeof(int));
+    EXPECT_EQ(bytes_allocated, static_cast<int64_t>(8 * sizeof(int)));
+    EXPECT_EQ(instance_count, 8);
     v.resize(5);
-    EXPECT_EQ(allocated, 8 * sizeof(int));
+    EXPECT_EQ(bytes_allocated, static_cast<int64_t>(8 * sizeof(int)));
+    EXPECT_EQ(instance_count, 5);
     v.shrink_to_fit();
-    EXPECT_EQ(allocated, 5 * sizeof(int));
+    EXPECT_EQ(bytes_allocated, static_cast<int64_t>(5 * sizeof(int)));
+    EXPECT_EQ(instance_count, 5);
     v.resize(4);
-    EXPECT_EQ(allocated, 5 * sizeof(int));
+    EXPECT_EQ(bytes_allocated, static_cast<int64_t>(5 * sizeof(int)));
+    EXPECT_EQ(instance_count, 4);
     v.shrink_to_fit();
-    EXPECT_EQ(allocated, 0);
+    EXPECT_EQ(bytes_allocated, 0);
+    EXPECT_EQ(instance_count, 4);
   }
+  EXPECT_EQ(instance_count, 0);
 }
 
 TEST(AllocatorSupportTest, SwapBothAllocated) {
@@ -1634,16 +1904,20 @@ TEST(AllocatorSupportTest, SwapBothAllocated) {
     const int ia2[] = {0, 1, 2, 3, 4, 5, 6, 7, 8};
     MyAlloc a1(&allocated1);
     MyAlloc a2(&allocated2);
-    AllocVec v1(ia1, ia1 + ABSL_ARRAYSIZE(ia1), a1);
-    AllocVec v2(ia2, ia2 + ABSL_ARRAYSIZE(ia2), a2);
+    AllocVec v1(ia1, ia1 + std::size(ia1), a1);
+    AllocVec v2(ia2, ia2 + std::size(ia2), a2);
     EXPECT_LT(v1.capacity(), v2.capacity());
-    EXPECT_THAT(allocated1, v1.capacity() * sizeof(int));
-    EXPECT_THAT(allocated2, v2.capacity() * sizeof(int));
+    EXPECT_THAT(allocated1,
+                Eq(static_cast<int64_t>(v1.capacity() * sizeof(int))));
+    EXPECT_THAT(allocated2,
+                Eq(static_cast<int64_t>(v2.capacity() * sizeof(int))));
     v1.swap(v2);
     EXPECT_THAT(v1, ElementsAreArray(ia2));
     EXPECT_THAT(v2, ElementsAreArray(ia1));
-    EXPECT_THAT(allocated1, v2.capacity() * sizeof(int));
-    EXPECT_THAT(allocated2, v1.capacity() * sizeof(int));
+    EXPECT_THAT(allocated1,
+                Eq(static_cast<int64_t>(v2.capacity() * sizeof(int))));
+    EXPECT_THAT(allocated2,
+                Eq(static_cast<int64_t>(v1.capacity() * sizeof(int))));
   }
   EXPECT_THAT(allocated1, 0);
   EXPECT_THAT(allocated2, 0);
@@ -1659,15 +1933,17 @@ TEST(AllocatorSupportTest, SwapOneAllocated) {
     const int ia2[] = {0, 1, 2, 3};
     MyAlloc a1(&allocated1);
     MyAlloc a2(&allocated2);
-    AllocVec v1(ia1, ia1 + ABSL_ARRAYSIZE(ia1), a1);
-    AllocVec v2(ia2, ia2 + ABSL_ARRAYSIZE(ia2), a2);
-    EXPECT_THAT(allocated1, v1.capacity() * sizeof(int));
-    EXPECT_THAT(allocated2, 0);
+    AllocVec v1(ia1, ia1 + std::size(ia1), a1);
+    AllocVec v2(ia2, ia2 + std::size(ia2), a2);
+    EXPECT_THAT(allocated1,
+                Eq(static_cast<int64_t>(v1.capacity() * sizeof(int))));
+    EXPECT_THAT(allocated2, Eq(0));
     v1.swap(v2);
     EXPECT_THAT(v1, ElementsAreArray(ia2));
     EXPECT_THAT(v2, ElementsAreArray(ia1));
-    EXPECT_THAT(allocated1, v2.capacity() * sizeof(int));
-    EXPECT_THAT(allocated2, 0);
+    EXPECT_THAT(allocated1,
+                Eq(static_cast<int64_t>(v2.capacity() * sizeof(int))));
+    EXPECT_THAT(allocated2, Eq(0));
     EXPECT_TRUE(v2.get_allocator() == a1);
     EXPECT_TRUE(v1.get_allocator() == a2);
   }
@@ -1729,7 +2005,7 @@ TEST(AllocatorSupportTest, ScopedAllocatorWorksAllocated) {
 }
 
 TEST(AllocatorSupportTest, SizeAllocConstructor) {
-  constexpr int inlined_size = 4;
+  constexpr size_t inlined_size = 4;
   using Alloc = CountingAllocator<int>;
   using AllocVec = absl::InlinedVector<int, inlined_size, Alloc>;
 
@@ -1739,7 +2015,7 @@ TEST(AllocatorSupportTest, SizeAllocConstructor) {
     auto v = AllocVec(len, Alloc(&allocated));
 
     // Inline storage used; allocator should not be invoked
-    EXPECT_THAT(allocated, 0);
+    EXPECT_THAT(allocated, Eq(0));
     EXPECT_THAT(v, AllOf(SizeIs(len), Each(0)));
   }
 
@@ -1749,20 +2025,44 @@ TEST(AllocatorSupportTest, SizeAllocConstructor) {
     auto v = AllocVec(len, Alloc(&allocated));
 
     // Out of line storage used; allocation of 8 elements expected
-    EXPECT_THAT(allocated, len * sizeof(int));
+    EXPECT_THAT(allocated, Eq(static_cast<int64_t>(len * sizeof(int))));
     EXPECT_THAT(v, AllOf(SizeIs(len), Each(0)));
   }
 }
 
+TEST(InlinedVectorTest, MinimumAllocatorCompilesUsingTraits) {
+  using T = int;
+  using A = std::allocator<T>;
+  using ATraits = std::allocator_traits<A>;
+
+  struct MinimumAllocator {
+    using value_type = T;
+
+    value_type* allocate(size_t n) {
+      A a;
+      return ATraits::allocate(a, n);
+    }
+
+    void deallocate(value_type* p, size_t n) {
+      A a;
+      ATraits::deallocate(a, p, n);
+    }
+  };
+
+  absl::InlinedVector<T, 1, MinimumAllocator> vec;
+  vec.emplace_back();
+  vec.resize(0);
+}
+
 TEST(InlinedVectorTest, AbslHashValueWorks) {
   using V = absl::InlinedVector<int, 4>;
   std::vector<V> cases;
 
   // Generate a variety of vectors some of these are small enough for the inline
   // space but are stored out of line.
-  for (int i = 0; i < 10; ++i) {
+  for (size_t i = 0; i < 10; ++i) {
     V v;
-    for (int j = 0; j < i; ++j) {
+    for (int j = 0; j < static_cast<int>(i); ++j) {
       v.push_back(j);
     }
     cases.push_back(v);
@@ -1773,4 +2073,314 @@ TEST(InlinedVectorTest, AbslHashValueWorks) {
   EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(cases));
 }
 
+class MoveConstructibleOnlyInstance
+    : public absl::test_internal::BaseCountedInstance {
+ public:
+  explicit MoveConstructibleOnlyInstance(int x) : BaseCountedInstance(x) {}
+  MoveConstructibleOnlyInstance(MoveConstructibleOnlyInstance&& other) =
+      default;
+  MoveConstructibleOnlyInstance& operator=(
+      MoveConstructibleOnlyInstance&& other) = delete;
+};
+
+MATCHER(HasValue, "") {
+  return ::testing::get<0>(arg).value() == ::testing::get<1>(arg);
+}
+
+TEST(NonAssignableMoveAssignmentTest, AllocatedToInline) {
+  using X = MoveConstructibleOnlyInstance;
+  InstanceTracker tracker;
+  absl::InlinedVector<X, 2> inlined;
+  inlined.emplace_back(1);
+  absl::InlinedVector<X, 2> allocated;
+  allocated.emplace_back(1);
+  allocated.emplace_back(2);
+  allocated.emplace_back(3);
+  tracker.ResetCopiesMovesSwaps();
+
+  inlined = std::move(allocated);
+  // passed ownership of the allocated storage
+  EXPECT_EQ(tracker.moves(), 0);
+  EXPECT_EQ(tracker.live_instances(), 3);
+
+  EXPECT_THAT(inlined, Pointwise(HasValue(), {1, 2, 3}));
+}
+
+TEST(NonAssignableMoveAssignmentTest, InlineToAllocated) {
+  using X = MoveConstructibleOnlyInstance;
+  InstanceTracker tracker;
+  absl::InlinedVector<X, 2> inlined;
+  inlined.emplace_back(1);
+  absl::InlinedVector<X, 2> allocated;
+  allocated.emplace_back(1);
+  allocated.emplace_back(2);
+  allocated.emplace_back(3);
+  tracker.ResetCopiesMovesSwaps();
+
+  allocated = std::move(inlined);
+  // Moved elements
+  EXPECT_EQ(tracker.moves(), 1);
+  EXPECT_EQ(tracker.live_instances(), 1);
+
+  EXPECT_THAT(allocated, Pointwise(HasValue(), {1}));
+}
+
+TEST(NonAssignableMoveAssignmentTest, InlineToInline) {
+  using X = MoveConstructibleOnlyInstance;
+  InstanceTracker tracker;
+  absl::InlinedVector<X, 2> inlined_a;
+  inlined_a.emplace_back(1);
+  absl::InlinedVector<X, 2> inlined_b;
+  inlined_b.emplace_back(1);
+  tracker.ResetCopiesMovesSwaps();
+
+  inlined_a = std::move(inlined_b);
+  // Moved elements
+  EXPECT_EQ(tracker.moves(), 1);
+  EXPECT_EQ(tracker.live_instances(), 1);
+
+  EXPECT_THAT(inlined_a, Pointwise(HasValue(), {1}));
+}
+
+TEST(NonAssignableMoveAssignmentTest, AllocatedToAllocated) {
+  using X = MoveConstructibleOnlyInstance;
+  InstanceTracker tracker;
+  absl::InlinedVector<X, 2> allocated_a;
+  allocated_a.emplace_back(1);
+  allocated_a.emplace_back(2);
+  allocated_a.emplace_back(3);
+  absl::InlinedVector<X, 2> allocated_b;
+  allocated_b.emplace_back(4);
+  allocated_b.emplace_back(5);
+  allocated_b.emplace_back(6);
+  allocated_b.emplace_back(7);
+  tracker.ResetCopiesMovesSwaps();
+
+  allocated_a = std::move(allocated_b);
+  // passed ownership of the allocated storage
+  EXPECT_EQ(tracker.moves(), 0);
+  EXPECT_EQ(tracker.live_instances(), 4);
+
+  EXPECT_THAT(allocated_a, Pointwise(HasValue(), {4, 5, 6, 7}));
+}
+
+TEST(NonAssignableMoveAssignmentTest, AssignThis) {
+  using X = MoveConstructibleOnlyInstance;
+  InstanceTracker tracker;
+  absl::InlinedVector<X, 2> v;
+  v.emplace_back(1);
+  v.emplace_back(2);
+  v.emplace_back(3);
+
+  tracker.ResetCopiesMovesSwaps();
+
+  // Obfuscated in order to pass -Wself-move.
+  v = std::move(*std::addressof(v));
+  // nothing happens
+  EXPECT_EQ(tracker.moves(), 0);
+  EXPECT_EQ(tracker.live_instances(), 3);
+
+  EXPECT_THAT(v, Pointwise(HasValue(), {1, 2, 3}));
+}
+
+class NonSwappableInstance : public absl::test_internal::BaseCountedInstance {
+ public:
+  explicit NonSwappableInstance(int x) : BaseCountedInstance(x) {}
+  NonSwappableInstance(const NonSwappableInstance& other) = default;
+  NonSwappableInstance& operator=(const NonSwappableInstance& other) = default;
+  NonSwappableInstance(NonSwappableInstance&& other) = default;
+  NonSwappableInstance& operator=(NonSwappableInstance&& other) = default;
+};
+
+void swap(NonSwappableInstance&, NonSwappableInstance&) = delete;
+
+TEST(NonSwappableSwapTest, InlineAndAllocatedTransferStorageAndMove) {
+  using X = NonSwappableInstance;
+  InstanceTracker tracker;
+  absl::InlinedVector<X, 2> inlined;
+  inlined.emplace_back(1);
+  absl::InlinedVector<X, 2> allocated;
+  allocated.emplace_back(1);
+  allocated.emplace_back(2);
+  allocated.emplace_back(3);
+  tracker.ResetCopiesMovesSwaps();
+
+  inlined.swap(allocated);
+  EXPECT_EQ(tracker.moves(), 1);
+  EXPECT_EQ(tracker.live_instances(), 4);
+
+  EXPECT_THAT(inlined, Pointwise(HasValue(), {1, 2, 3}));
+}
+
+TEST(NonSwappableSwapTest, InlineAndInlineMoveIndividualElements) {
+  using X = NonSwappableInstance;
+  InstanceTracker tracker;
+  absl::InlinedVector<X, 2> inlined_a;
+  inlined_a.emplace_back(1);
+  absl::InlinedVector<X, 2> inlined_b;
+  inlined_b.emplace_back(2);
+  tracker.ResetCopiesMovesSwaps();
+
+  inlined_a.swap(inlined_b);
+  EXPECT_EQ(tracker.moves(), 3);
+  EXPECT_EQ(tracker.live_instances(), 2);
+
+  EXPECT_THAT(inlined_a, Pointwise(HasValue(), {2}));
+  EXPECT_THAT(inlined_b, Pointwise(HasValue(), {1}));
+}
+
+TEST(NonSwappableSwapTest, AllocatedAndAllocatedOnlyTransferStorage) {
+  using X = NonSwappableInstance;
+  InstanceTracker tracker;
+  absl::InlinedVector<X, 2> allocated_a;
+  allocated_a.emplace_back(1);
+  allocated_a.emplace_back(2);
+  allocated_a.emplace_back(3);
+  absl::InlinedVector<X, 2> allocated_b;
+  allocated_b.emplace_back(4);
+  allocated_b.emplace_back(5);
+  allocated_b.emplace_back(6);
+  allocated_b.emplace_back(7);
+  tracker.ResetCopiesMovesSwaps();
+
+  allocated_a.swap(allocated_b);
+  EXPECT_EQ(tracker.moves(), 0);
+  EXPECT_EQ(tracker.live_instances(), 7);
+
+  EXPECT_THAT(allocated_a, Pointwise(HasValue(), {4, 5, 6, 7}));
+  EXPECT_THAT(allocated_b, Pointwise(HasValue(), {1, 2, 3}));
+}
+
+TEST(NonSwappableSwapTest, SwapThis) {
+  using X = NonSwappableInstance;
+  InstanceTracker tracker;
+  absl::InlinedVector<X, 2> v;
+  v.emplace_back(1);
+  v.emplace_back(2);
+  v.emplace_back(3);
+
+  tracker.ResetCopiesMovesSwaps();
+
+  v.swap(v);
+  EXPECT_EQ(tracker.moves(), 0);
+  EXPECT_EQ(tracker.live_instances(), 3);
+
+  EXPECT_THAT(v, Pointwise(HasValue(), {1, 2, 3}));
+}
+
+template <size_t N>
+using CharVec = absl::InlinedVector<char, N>;
+
+// Warning: This struct "simulates" the type `InlinedVector::Storage::Allocated`
+// to make reasonable expectations for inlined storage capacity optimization. If
+// implementation changes `Allocated`, then `MySpan` and tests that use it need
+// to be updated accordingly.
+template <typename T>
+struct MySpan {
+  T* data;
+  size_t size;
+};
+
+TEST(StorageTest, InlinedCapacityAutoIncrease) {
+  // The requested capacity is auto increased to `sizeof(MySpan<char>)`.
+  EXPECT_GT(CharVec<1>().capacity(), 1);
+  EXPECT_EQ(CharVec<1>().capacity(), sizeof(MySpan<char>));
+  EXPECT_EQ(CharVec<1>().capacity(), CharVec<2>().capacity());
+  EXPECT_EQ(sizeof(CharVec<1>), sizeof(CharVec<2>));
+
+  // The requested capacity is auto increased to
+  // `sizeof(MySpan<int>) / sizeof(int)`.
+  EXPECT_GT((absl::InlinedVector<int, 1>().capacity()), 1);
+  EXPECT_EQ((absl::InlinedVector<int, 1>().capacity()),
+            sizeof(MySpan<int>) / sizeof(int));
+}
+
+TEST(IntVec, EraseIf) {
+  IntVec v = {3, 1, 2, 0};
+  EXPECT_EQ(absl::erase_if(v, [](int i) { return i > 1; }), 2u);
+  EXPECT_THAT(v, ElementsAre(1, 0));
+}
+
+TEST(IntVec, EraseIfMatchesNone) {
+  IntVec v = {1, 2, 3};
+  EXPECT_EQ(absl::erase_if(v, [](int i) { return i > 10; }), 0u);;
+  EXPECT_THAT(v, ElementsAre(1, 2, 3));
+}
+
+TEST(IntVec, EraseIfMatchesAll) {
+  IntVec v = {1, 2, 3};
+  EXPECT_EQ(absl::erase_if(v, [](int i) { return i > 0; }), 3u);
+  EXPECT_THAT(v, IsEmpty());
+}
+
+#ifdef ABSL_HAVE_EXCEPTIONS
+struct ThrowOnMove {
+  static constexpr uint32_t kAlive = 0xA11FE123;
+  static constexpr uint32_t kDestroyed = 0xDEADBEEF;
+
+  explicit ThrowOnMove(int* count) : alive_count(count), sentinel(kAlive) {
+    if (alive_count) ++(*alive_count);
+  }
+  ThrowOnMove(const ThrowOnMove&) = delete;
+  ThrowOnMove& operator=(const ThrowOnMove&) = delete;
+  ThrowOnMove(ThrowOnMove&& other)
+      : alive_count(other.alive_count), sentinel(kAlive) {
+    if (other.should_throw) throw std::runtime_error("ThrowOnMove");
+    if (alive_count) ++(*alive_count);
+  }
+  ~ThrowOnMove() {
+    EXPECT_EQ(sentinel, kAlive)
+        << "Double destroy detected: destructor called twice on memory slot!";
+    sentinel = kDestroyed;
+    if (alive_count) {
+      EXPECT_GT(*alive_count, 0)
+          << "More destructors called than constructors!";
+      --(*alive_count);
+    }
+  }
+
+  int* alive_count = nullptr;
+  uint32_t sentinel = kDestroyed;
+  bool should_throw = false;
+};
+
+TEST(InlinedVectorTest, SwapExceptionSafety) {
+  int alive_count = 0;
+  try {
+    absl::InlinedVector<ThrowOnMove, 2> a;
+    a.emplace_back(&alive_count);
+    absl::InlinedVector<ThrowOnMove, 2> b;
+    b.emplace_back(&alive_count);
+    b[0].should_throw = true;
+    a.swap(b);
+    FAIL() << "Expected swap to throw std::runtime_error";
+  } catch (const std::runtime_error&) {
+    // Expected exception from throwing move-constructor inside swap.
+  }
+  EXPECT_EQ(alive_count, 0)
+      << "Mismatch between constructor and destructor calls!";
+}
+
+// Ensures that an exception thrown during move construction doesn't leave the
+// inlined vector in a bad state. Needs ASAN for reliable detection.
+TEST(InlinedVectorTest, TruncatesBeforeThrowingMoveConstruction) {
+  using Vec = absl::InlinedVector<ThrowOnMove, 1>;
+  int count = 0;
+  {
+    Vec dest;
+    dest.reserve(dest.capacity() + 1);  // force heap allocation
+    dest.emplace_back(&count);
+
+    Vec src;
+    src.emplace_back(&count).should_throw = true;
+
+    try {
+      dest = std::move(src);
+    } catch (const std::runtime_error&) {
+    }
+  }
+  EXPECT_EQ(count, 0);
+}
+#endif
+
 }  // anonymous namespace
diff --git a/absl/container/internal/btree.h b/absl/container/internal/btree.h
index cf7ef02cccc..55ff45731be 100644
--- a/absl/container/internal/btree.h
+++ b/absl/container/internal/btree.h
@@ -53,31 +53,58 @@
 #include <functional>
 #include <iterator>
 #include <limits>
-#include <new>
+#include <memory>
 #include <string>
 #include <type_traits>
 #include <utility>
 
+#include "absl/base/config.h"
+#include "absl/base/internal/hardening.h"
+#include "absl/base/internal/raw_logging.h"
 #include "absl/base/macros.h"
+#include "absl/base/optimization.h"
 #include "absl/container/internal/common.h"
+#include "absl/container/internal/common_policy_traits.h"
 #include "absl/container/internal/compressed_tuple.h"
 #include "absl/container/internal/container_memory.h"
 #include "absl/container/internal/layout.h"
 #include "absl/memory/memory.h"
 #include "absl/meta/type_traits.h"
+#include "absl/strings/cord.h"
 #include "absl/strings/string_view.h"
 #include "absl/types/compare.h"
-#include "absl/utility/utility.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
+#ifdef ABSL_BTREE_ENABLE_GENERATIONS
+#error ABSL_BTREE_ENABLE_GENERATIONS cannot be directly set
+#elif (defined(ABSL_HAVE_ADDRESS_SANITIZER) ||   \
+       defined(ABSL_HAVE_HWADDRESS_SANITIZER) || \
+       defined(ABSL_HAVE_MEMORY_SANITIZER)) &&   \
+    !defined(NDEBUG_SANITIZER)  // If defined, performance is important.
+// When compiled in sanitizer mode, we add generation integers to the nodes and
+// iterators. When iterators are used, we validate that the container has not
+// been mutated since the iterator was constructed.
+#define ABSL_BTREE_ENABLE_GENERATIONS
+#endif
+
+#ifdef ABSL_BTREE_ENABLE_GENERATIONS
+constexpr bool BtreeGenerationsEnabled() { return true; }
+#else
+constexpr bool BtreeGenerationsEnabled() { return false; }
+#endif
+
+template <typename Compare, typename T, typename U>
+using compare_result_t =
+    std::invoke_result_t<const Compare, const T &, const U &>;
+
 // A helper class that indicates if the Compare parameter is a key-compare-to
 // comparator.
 template <typename Compare, typename T>
 using btree_is_key_compare_to =
-    std::is_convertible<absl::result_of_t<Compare(const T &, const T &)>,
-                        absl::weak_ordering>;
+    std::is_convertible<compare_result_t<Compare, T, T>, absl::weak_ordering>;
 
 struct StringBtreeDefaultLess {
   using is_transparent = void;
@@ -85,13 +112,31 @@ struct StringBtreeDefaultLess {
   StringBtreeDefaultLess() = default;
 
   // Compatibility constructor.
-  StringBtreeDefaultLess(std::less<std::string>) {}  // NOLINT
-  StringBtreeDefaultLess(std::less<string_view>) {}  // NOLINT
+  StringBtreeDefaultLess(std::less<std::string>) {}        // NOLINT
+  StringBtreeDefaultLess(std::less<absl::string_view>) {}  // NOLINT
+
+  // Allow converting to std::less for use in key_comp()/value_comp().
+  explicit operator std::less<std::string>() const { return {}; }
+  explicit operator std::less<absl::string_view>() const { return {}; }
+  explicit operator std::less<absl::Cord>() const { return {}; }
 
   absl::weak_ordering operator()(absl::string_view lhs,
                                  absl::string_view rhs) const {
     return compare_internal::compare_result_as_ordering(lhs.compare(rhs));
   }
+  StringBtreeDefaultLess(std::less<absl::Cord>) {}  // NOLINT
+  absl::weak_ordering operator()(const absl::Cord &lhs,
+                                 const absl::Cord &rhs) const {
+    return compare_internal::compare_result_as_ordering(lhs.Compare(rhs));
+  }
+  absl::weak_ordering operator()(const absl::Cord &lhs,
+                                 absl::string_view rhs) const {
+    return compare_internal::compare_result_as_ordering(lhs.Compare(rhs));
+  }
+  absl::weak_ordering operator()(absl::string_view lhs,
+                                 const absl::Cord &rhs) const {
+    return compare_internal::compare_result_as_ordering(-rhs.Compare(lhs));
+  }
 };
 
 struct StringBtreeDefaultGreater {
@@ -99,69 +144,261 @@ struct StringBtreeDefaultGreater {
 
   StringBtreeDefaultGreater() = default;
 
-  StringBtreeDefaultGreater(std::greater<std::string>) {}  // NOLINT
-  StringBtreeDefaultGreater(std::greater<string_view>) {}  // NOLINT
+  StringBtreeDefaultGreater(std::greater<std::string>) {}        // NOLINT
+  StringBtreeDefaultGreater(std::greater<absl::string_view>) {}  // NOLINT
+
+  // Allow converting to std::greater for use in key_comp()/value_comp().
+  explicit operator std::greater<std::string>() const { return {}; }
+  explicit operator std::greater<absl::string_view>() const { return {}; }
+  explicit operator std::greater<absl::Cord>() const { return {}; }
 
   absl::weak_ordering operator()(absl::string_view lhs,
                                  absl::string_view rhs) const {
     return compare_internal::compare_result_as_ordering(rhs.compare(lhs));
   }
+  StringBtreeDefaultGreater(std::greater<absl::Cord>) {}  // NOLINT
+  absl::weak_ordering operator()(const absl::Cord &lhs,
+                                 const absl::Cord &rhs) const {
+    return compare_internal::compare_result_as_ordering(rhs.Compare(lhs));
+  }
+  absl::weak_ordering operator()(const absl::Cord &lhs,
+                                 absl::string_view rhs) const {
+    return compare_internal::compare_result_as_ordering(-lhs.Compare(rhs));
+  }
+  absl::weak_ordering operator()(absl::string_view lhs,
+                                 const absl::Cord &rhs) const {
+    return compare_internal::compare_result_as_ordering(rhs.Compare(lhs));
+  }
 };
 
-// A helper class to convert a boolean comparison into a three-way "compare-to"
-// comparison that returns a negative value to indicate less-than, zero to
-// indicate equality and a positive value to indicate greater-than. This helper
-// class is specialized for less<std::string>, greater<std::string>,
-// less<string_view>, and greater<string_view>.
-//
-// key_compare_to_adapter is provided so that btree users
-// automatically get the more efficient compare-to code when using common
-// google string types with common comparison functors.
-// These string-like specializations also turn on heterogeneous lookup by
-// default.
+// See below comments for checked_compare.
+template <typename Compare, bool is_class = std::is_class_v<Compare>>
+struct checked_compare_base : Compare {
+  using Compare::Compare;
+  explicit checked_compare_base(Compare c) : Compare(std::move(c)) {}
+  const Compare &comp() const { return *this; }
+};
 template <typename Compare>
-struct key_compare_to_adapter {
-  using type = Compare;
+struct checked_compare_base<Compare, false> {
+  explicit checked_compare_base(Compare c) : compare(std::move(c)) {}
+  const Compare &comp() const { return compare; }
+  Compare compare;
+};
+
+// A mechanism for opting out of checked_compare for use only in btree_test.cc.
+struct BtreeTestOnlyCheckedCompareOptOutBase {};
+
+// A helper class to adapt the specified comparator for two use cases:
+// (1) When using common Abseil string types with common comparison functors,
+// convert a boolean comparison into a three-way comparison that returns an
+// `absl::weak_ordering`. This helper class is specialized for
+// less<std::string>, greater<std::string>, less<string_view>,
+// greater<string_view>, less<absl::Cord>, and greater<absl::Cord>.
+// (2) Adapt the comparator to diagnose cases of non-strict-weak-ordering (see
+// https://en.cppreference.com/w/cpp/named_req/Compare) in debug mode. Whenever
+// a comparison is made, we will make assertions to verify that the comparator
+// is valid.
+template <typename Compare, typename Key>
+struct key_compare_adapter {
+  // Inherit from checked_compare_base to support function pointers and also
+  // keep empty-base-optimization (EBO) support for classes.
+  // Note: we can't use CompressedTuple here because that would interfere
+  // with the EBO for `btree::rightmost_`. `btree::rightmost_` is itself a
+  // CompressedTuple and nested `CompressedTuple`s don't support EBO.
+  // TODO(b/214288561): use CompressedTuple instead once it supports EBO for
+  // nested `CompressedTuple`s.
+  struct checked_compare : checked_compare_base<Compare> {
+   private:
+    using Base = typename checked_compare::checked_compare_base;
+    using Base::comp;
+
+    // If possible, returns whether `t` is equivalent to itself. We can only do
+    // this for `Key`s because we can't be sure that it's safe to call
+    // `comp()(k, k)` otherwise. Even if SFINAE allows it, there could be a
+    // compilation failure inside the implementation of the comparison operator.
+    bool is_self_equivalent(const Key &k) const {
+      // Note: this works for both boolean and three-way comparators.
+      return comp()(k, k) == 0;
+    }
+    // If we can't compare `t` with itself, returns true unconditionally.
+    template <typename T>
+    bool is_self_equivalent(const T &) const {
+      return true;
+    }
+
+   public:
+    using Base::Base;
+    checked_compare(Compare cmp) : Base(std::move(cmp)) {}  // NOLINT
+
+    // Allow converting to Compare for use in key_comp()/value_comp().
+    explicit operator Compare() const { return comp(); }
+
+    template <
+        typename T, typename U,
+        std::enable_if_t<std::is_same_v<bool, compare_result_t<Compare, T, U>>,
+                         int> = 0>
+    bool operator()(const T& lhs, const U& rhs) const {
+      // NOTE: if any of these assertions fail, then the comparator does not
+      // establish a strict-weak-ordering (see
+      // https://en.cppreference.com/w/cpp/named_req/Compare).
+      assert(is_self_equivalent(lhs));
+      assert(is_self_equivalent(rhs));
+      const bool lhs_comp_rhs = comp()(lhs, rhs);
+      assert(!lhs_comp_rhs || !comp()(rhs, lhs));
+      return lhs_comp_rhs;
+    }
+
+    template <
+        typename T, typename U,
+        std::enable_if_t<std::is_convertible_v<compare_result_t<Compare, T, U>,
+                                               absl::weak_ordering>,
+                         int> = 0>
+    absl::weak_ordering operator()(const T& lhs, const U& rhs) const {
+      // NOTE: if any of these assertions fail, then the comparator does not
+      // establish a strict-weak-ordering (see
+      // https://en.cppreference.com/w/cpp/named_req/Compare).
+      assert(is_self_equivalent(lhs));
+      assert(is_self_equivalent(rhs));
+      const absl::weak_ordering lhs_comp_rhs = comp()(lhs, rhs);
+#ifndef NDEBUG
+      const absl::weak_ordering rhs_comp_lhs = comp()(rhs, lhs);
+      if (lhs_comp_rhs > 0) {
+        assert(rhs_comp_lhs < 0 && "lhs_comp_rhs > 0 -> rhs_comp_lhs < 0");
+      } else if (lhs_comp_rhs == 0) {
+        assert(rhs_comp_lhs == 0 && "lhs_comp_rhs == 0 -> rhs_comp_lhs == 0");
+      } else {
+        assert(rhs_comp_lhs > 0 && "lhs_comp_rhs < 0 -> rhs_comp_lhs > 0");
+      }
+#endif
+      return lhs_comp_rhs;
+    }
+  };
+  using type = std::conditional_t<
+      std::is_base_of_v<BtreeTestOnlyCheckedCompareOptOutBase, Compare>,
+      Compare, checked_compare>;
+};
+
+template <>
+struct key_compare_adapter<std::less<std::string>, std::string> {
+  using type = StringBtreeDefaultLess;
+};
+
+template <>
+struct key_compare_adapter<std::greater<std::string>, std::string> {
+  using type = StringBtreeDefaultGreater;
 };
 
 template <>
-struct key_compare_to_adapter<std::less<std::string>> {
+struct key_compare_adapter<std::less<absl::string_view>, absl::string_view> {
   using type = StringBtreeDefaultLess;
 };
 
 template <>
-struct key_compare_to_adapter<std::greater<std::string>> {
+struct key_compare_adapter<std::greater<absl::string_view>, absl::string_view> {
   using type = StringBtreeDefaultGreater;
 };
 
 template <>
-struct key_compare_to_adapter<std::less<absl::string_view>> {
+struct key_compare_adapter<std::less<absl::Cord>, absl::Cord> {
   using type = StringBtreeDefaultLess;
 };
 
 template <>
-struct key_compare_to_adapter<std::greater<absl::string_view>> {
+struct key_compare_adapter<std::greater<absl::Cord>, absl::Cord> {
   using type = StringBtreeDefaultGreater;
 };
 
+// Detects an 'absl_btree_prefer_linear_node_search' member. This is
+// a protocol used as an opt-in or opt-out of linear search.
+//
+//  For example, this would be useful for key types that wrap an integer
+//  and define their own cheap operator<(). For example:
+//
+//   class K {
+//    public:
+//     using absl_btree_prefer_linear_node_search = std::true_type;
+//     ...
+//    private:
+//     friend bool operator<(K a, K b) { return a.k_ < b.k_; }
+//     int k_;
+//   };
+//
+//   btree_map<K, V> m;  // Uses linear search
+//
+// If T has the preference tag, then it has a preference.
+// Btree will use the tag's truth value.
+template <typename T, typename = void>
+struct has_linear_node_search_preference : std::false_type {};
+template <typename T, typename = void>
+struct prefers_linear_node_search : std::false_type {};
+template <typename T>
+struct has_linear_node_search_preference<
+    T, std::void_t<typename T::absl_btree_prefer_linear_node_search>>
+    : std::true_type {};
+template <typename T>
+struct prefers_linear_node_search<
+    T, std::void_t<typename T::absl_btree_prefer_linear_node_search>>
+    : T::absl_btree_prefer_linear_node_search {};
+
+template <typename Compare, typename Key>
+constexpr bool compare_has_valid_result_type() {
+  using compare_result_type = compare_result_t<Compare, Key, Key>;
+  return std::is_same_v<compare_result_type, bool> ||
+         std::is_convertible_v<compare_result_type, absl::weak_ordering>;
+}
+
+template <typename original_key_compare, typename value_type>
+class map_value_compare {
+  template <typename Params>
+  friend class btree;
+
+  // Note: this `protected` is part of the API of std::map::value_compare. See
+  // https://en.cppreference.com/w/cpp/container/map/value_compare.
+ protected:
+  explicit map_value_compare(original_key_compare c) : comp(std::move(c)) {}
+
+  original_key_compare comp;  // NOLINT
+
+ public:
+  auto operator()(const value_type &lhs, const value_type &rhs) const
+      -> decltype(comp(lhs.first, rhs.first)) {
+    return comp(lhs.first, rhs.first);
+  }
+};
+
 template <typename Key, typename Compare, typename Alloc, int TargetNodeSize,
-          bool Multi, typename SlotPolicy>
-struct common_params {
-  // If Compare is a common comparator for a std::string-like type, then we adapt it
+          bool IsMulti, bool IsMap, typename SlotPolicy>
+struct common_params : common_policy_traits<SlotPolicy> {
+  using original_key_compare = Compare;
+
+  // If Compare is a common comparator for a string-like type, then we adapt it
   // to use heterogeneous lookup and to be a key-compare-to comparator.
-  using key_compare = typename key_compare_to_adapter<Compare>::type;
+  // We also adapt the comparator to diagnose invalid comparators in debug mode.
+  // We disable this when `Compare` is invalid in a way that will cause
+  // adaptation to fail (having invalid return type) so that we can give a
+  // better compilation failure in static_assert_validation. If we don't do
+  // this, then there will be cascading compilation failures that are confusing
+  // for users.
+  using key_compare =
+      std::conditional_t<!compare_has_valid_result_type<Compare, Key>(),
+                          Compare,
+                          typename key_compare_adapter<Compare, Key>::type>;
+
+  static constexpr bool kIsKeyCompareStringAdapted =
+      std::is_same_v<key_compare, StringBtreeDefaultLess> ||
+      std::is_same_v<key_compare, StringBtreeDefaultGreater>;
+  static constexpr bool kIsKeyCompareTransparent =
+      IsTransparent<original_key_compare>::value || kIsKeyCompareStringAdapted;
+
   // A type which indicates if we have a key-compare-to functor or a plain old
   // key-compare functor.
   using is_key_compare_to = btree_is_key_compare_to<key_compare, Key>;
 
   using allocator_type = Alloc;
   using key_type = Key;
-  using size_type = std::make_signed<size_t>::type;
+  using size_type = size_t;
   using difference_type = ptrdiff_t;
 
-  // True if this is a multiset or multimap.
-  using is_multi_container = std::integral_constant<bool, Multi>;
-
   using slot_policy = SlotPolicy;
   using slot_type = typename slot_policy::slot_type;
   using value_type = typename slot_policy::value_type;
@@ -171,151 +408,42 @@ struct common_params {
   using reference = value_type &;
   using const_reference = const value_type &;
 
+  using value_compare =
+      std::conditional_t<IsMap,
+                          map_value_compare<original_key_compare, value_type>,
+                          original_key_compare>;
+  using is_map_container = std::bool_constant<IsMap>;
+
+  // For the given lookup key type, returns whether we can have multiple
+  // equivalent keys in the btree. If this is a multi-container, then we can.
+  // Otherwise, we can have multiple equivalent keys only if all of the
+  // following conditions are met:
+  // - The comparator is transparent.
+  // - The lookup key type is not the same as key_type.
+  // - The comparator is not a StringBtreeDefault{Less,Greater} comparator
+  //   that we know has the same equivalence classes for all lookup types.
+  template <typename LookupKey>
+  constexpr static bool can_have_multiple_equivalent_keys() {
+    return IsMulti ||
+           (IsTransparent<key_compare>::value &&
+            !std::is_same_v<LookupKey, Key> && !kIsKeyCompareStringAdapted);
+  }
+
   enum {
     kTargetNodeSize = TargetNodeSize,
 
-    // Upper bound for the available space for values. This is largest for leaf
+    // Upper bound for the available space for slots. This is largest for leaf
     // nodes, which have overhead of at least a pointer + 4 bytes (for storing
     // 3 field_types and an enum).
-    kNodeValueSpace =
-        TargetNodeSize - /*minimum overhead=*/(sizeof(void *) + 4),
+    kNodeSlotSpace = TargetNodeSize - /*minimum overhead=*/(sizeof(void *) + 4),
   };
 
-  // This is an integral type large enough to hold as many
-  // ValueSize-values as will fit a node of TargetNodeSize bytes.
+  // This is an integral type large enough to hold as many slots as will fit a
+  // node of TargetNodeSize bytes.
   using node_count_type =
-      absl::conditional_t<(kNodeValueSpace / sizeof(value_type) >
+      std::conditional_t<(kNodeSlotSpace / sizeof(slot_type) >
                            (std::numeric_limits<uint8_t>::max)()),
                           uint16_t, uint8_t>;  // NOLINT
-
-  // The following methods are necessary for passing this struct as PolicyTraits
-  // for node_handle and/or are used within btree.
-  static value_type &element(slot_type *slot) {
-    return slot_policy::element(slot);
-  }
-  static const value_type &element(const slot_type *slot) {
-    return slot_policy::element(slot);
-  }
-  template <class... Args>
-  static void construct(Alloc *alloc, slot_type *slot, Args &&... args) {
-    slot_policy::construct(alloc, slot, std::forward<Args>(args)...);
-  }
-  static void construct(Alloc *alloc, slot_type *slot, slot_type *other) {
-    slot_policy::construct(alloc, slot, other);
-  }
-  static void destroy(Alloc *alloc, slot_type *slot) {
-    slot_policy::destroy(alloc, slot);
-  }
-  static void transfer(Alloc *alloc, slot_type *new_slot, slot_type *old_slot) {
-    construct(alloc, new_slot, old_slot);
-    destroy(alloc, old_slot);
-  }
-  static void swap(Alloc *alloc, slot_type *a, slot_type *b) {
-    slot_policy::swap(alloc, a, b);
-  }
-  static void move(Alloc *alloc, slot_type *src, slot_type *dest) {
-    slot_policy::move(alloc, src, dest);
-  }
-  static void move(Alloc *alloc, slot_type *first, slot_type *last,
-                   slot_type *result) {
-    slot_policy::move(alloc, first, last, result);
-  }
-};
-
-// A parameters structure for holding the type parameters for a btree_map.
-// Compare and Alloc should be nothrow copy-constructible.
-template <typename Key, typename Data, typename Compare, typename Alloc,
-          int TargetNodeSize, bool Multi>
-struct map_params : common_params<Key, Compare, Alloc, TargetNodeSize, Multi,
-                                  map_slot_policy<Key, Data>> {
-  using super_type = typename map_params::common_params;
-  using mapped_type = Data;
-  // This type allows us to move keys when it is safe to do so. It is safe
-  // for maps in which value_type and mutable_value_type are layout compatible.
-  using slot_policy = typename super_type::slot_policy;
-  using slot_type = typename super_type::slot_type;
-  using value_type = typename super_type::value_type;
-  using init_type = typename super_type::init_type;
-
-  using key_compare = typename super_type::key_compare;
-  // Inherit from key_compare for empty base class optimization.
-  struct value_compare : private key_compare {
-    value_compare() = default;
-    explicit value_compare(const key_compare &cmp) : key_compare(cmp) {}
-
-    template <typename T, typename U>
-    auto operator()(const T &left, const U &right) const
-        -> decltype(std::declval<key_compare>()(left.first, right.first)) {
-      return key_compare::operator()(left.first, right.first);
-    }
-  };
-  using is_map_container = std::true_type;
-
-  static const Key &key(const value_type &x) { return x.first; }
-  static const Key &key(const init_type &x) { return x.first; }
-  static const Key &key(const slot_type *x) { return slot_policy::key(x); }
-  static mapped_type &value(value_type *value) { return value->second; }
-};
-
-// This type implements the necessary functions from the
-// absl::container_internal::slot_type interface.
-template <typename Key>
-struct set_slot_policy {
-  using slot_type = Key;
-  using value_type = Key;
-  using mutable_value_type = Key;
-
-  static value_type &element(slot_type *slot) { return *slot; }
-  static const value_type &element(const slot_type *slot) { return *slot; }
-
-  template <typename Alloc, class... Args>
-  static void construct(Alloc *alloc, slot_type *slot, Args &&... args) {
-    absl::allocator_traits<Alloc>::construct(*alloc, slot,
-                                             std::forward<Args>(args)...);
-  }
-
-  template <typename Alloc>
-  static void construct(Alloc *alloc, slot_type *slot, slot_type *other) {
-    absl::allocator_traits<Alloc>::construct(*alloc, slot, std::move(*other));
-  }
-
-  template <typename Alloc>
-  static void destroy(Alloc *alloc, slot_type *slot) {
-    absl::allocator_traits<Alloc>::destroy(*alloc, slot);
-  }
-
-  template <typename Alloc>
-  static void swap(Alloc * /*alloc*/, slot_type *a, slot_type *b) {
-    using std::swap;
-    swap(*a, *b);
-  }
-
-  template <typename Alloc>
-  static void move(Alloc * /*alloc*/, slot_type *src, slot_type *dest) {
-    *dest = std::move(*src);
-  }
-
-  template <typename Alloc>
-  static void move(Alloc *alloc, slot_type *first, slot_type *last,
-                   slot_type *result) {
-    for (slot_type *src = first, *dest = result; src != last; ++src, ++dest)
-      move(alloc, src, dest);
-  }
-};
-
-// A parameters structure for holding the type parameters for a btree_set.
-// Compare and Alloc should be nothrow copy-constructible.
-template <typename Key, typename Compare, typename Alloc, int TargetNodeSize,
-          bool Multi>
-struct set_params : common_params<Key, Compare, Alloc, TargetNodeSize, Multi,
-                                  set_slot_policy<Key>> {
-  using value_type = Key;
-  using slot_type = typename set_params::common_params::slot_type;
-  using value_compare = typename set_params::common_params::key_compare;
-  using is_map_container = std::false_type;
-
-  static const Key &key(const value_type &x) { return x; }
-  static const Key &key(const slot_type *x) { return *x; }
 };
 
 // An adapter class that converts a lower-bound compare into an upper-bound
@@ -325,8 +453,8 @@ struct set_params : common_params<Key, Compare, Alloc, TargetNodeSize, Multi,
 template <typename Compare>
 struct upper_bound_adapter {
   explicit upper_bound_adapter(const Compare &c) : comp(c) {}
-  template <typename K, typename LK>
-  bool operator()(const K &a, const LK &b) const {
+  template <typename K1, typename K2>
+  bool operator()(const K1 &a, const K2 &b) const {
     // Returns true when a is not greater than b.
     return !compare_internal::compare_result_as_less_than(comp(b, a));
   }
@@ -351,6 +479,10 @@ struct SearchResult {
 // useful information.
 template <typename V>
 struct SearchResult<V, false> {
+  SearchResult() = default;
+  explicit SearchResult(V v) : value(v) {}
+  SearchResult(V v, MatchKind /*match*/) : value(v) {}
+
   V value;
 
   static constexpr bool HasMatch() { return false; }
@@ -363,10 +495,10 @@ struct SearchResult<V, false> {
 template <typename Params>
 class btree_node {
   using is_key_compare_to = typename Params::is_key_compare_to;
-  using is_multi_container = typename Params::is_multi_container;
   using field_type = typename Params::node_count_type;
   using allocator_type = typename Params::allocator_type;
   using slot_type = typename Params::slot_type;
+  using original_key_compare = typename Params::original_key_compare;
 
  public:
   using params_type = Params;
@@ -381,47 +513,61 @@ class btree_node {
   using difference_type = typename Params::difference_type;
 
   // Btree decides whether to use linear node search as follows:
+  //   - If the comparator expresses a preference, use that.
+  //   - If the key expresses a preference, use that.
   //   - If the key is arithmetic and the comparator is std::less or
   //     std::greater, choose linear.
   //   - Otherwise, choose binary.
   // TODO(ezb): Might make sense to add condition(s) based on node-size.
   using use_linear_search = std::integral_constant<
       bool,
-                std::is_arithmetic<key_type>::value &&
-                    (std::is_same<std::less<key_type>, key_compare>::value ||
-                     std::is_same<std::greater<key_type>, key_compare>::value)>;
-
-  // This class is organized by gtl::Layout as if it had the following
-  // structure:
+      has_linear_node_search_preference<original_key_compare>::value
+          ? prefers_linear_node_search<original_key_compare>::value
+      : has_linear_node_search_preference<key_type>::value
+          ? prefers_linear_node_search<key_type>::value
+          : std::is_arithmetic_v<key_type> &&
+                (std::is_same_v<std::less<key_type>, original_key_compare> ||
+                 std::is_same_v<std::greater<key_type>, original_key_compare>)>;
+
+  // This class is organized by absl::container_internal::Layout as if it had
+  // the following structure:
   //   // A pointer to the node's parent.
   //   btree_node *parent;
   //
+  //   // When ABSL_BTREE_ENABLE_GENERATIONS is defined, we also have a
+  //   // generation integer in order to check that when iterators are
+  //   // used, they haven't been invalidated already. Only the generation on
+  //   // the root is used, but we have one on each node because whether a node
+  //   // is root or not can change.
+  //   uint32_t generation;
+  //
   //   // The position of the node in the node's parent.
   //   field_type position;
   //   // The index of the first populated value in `values`.
   //   // TODO(ezb): right now, `start` is always 0. Update insertion/merge
   //   // logic to allow for floating storage within nodes.
   //   field_type start;
-  //   // The count of the number of populated values in the node.
-  //   field_type count;
+  //   // The index after the last populated value in `values`. Currently, this
+  //   // is the same as the count of values.
+  //   field_type finish;
   //   // The maximum number of values the node can hold. This is an integer in
-  //   // [1, kNodeValues] for root leaf nodes, kNodeValues for non-root leaf
+  //   // [1, kNodeSlots] for root leaf nodes, kNodeSlots for non-root leaf
   //   // nodes, and kInternalNodeMaxCount (as a sentinel value) for internal
-  //   // nodes (even though there are still kNodeValues values in the node).
+  //   // nodes (even though there are still kNodeSlots values in the node).
   //   // TODO(ezb): make max_count use only 4 bits and record log2(capacity)
   //   // to free extra bits for is_root, etc.
   //   field_type max_count;
   //
   //   // The array of values. The capacity is `max_count` for leaf nodes and
-  //   // kNodeValues for internal nodes. Only the values in
-  //   // [start, start + count) have been initialized and are valid.
+  //   // kNodeSlots for internal nodes. Only the values in
+  //   // [start, finish) have been initialized and are valid.
   //   slot_type values[max_count];
   //
   //   // The array of child pointers. The keys in children[i] are all less
   //   // than key(i). The keys in children[i + 1] are all greater than key(i).
-  //   // There are 0 children for leaf nodes and kNodeValues + 1 children for
+  //   // There are 0 children for leaf nodes and kNodeSlots + 1 children for
   //   // internal nodes.
-  //   btree_node *children[kNodeValues + 1];
+  //   btree_node *children[kNodeSlots + 1];
   //
   // This class is only constructed by EmptyNodeType. Normally, pointers to the
   // layout above are allocated, cast to btree_node*, and de-allocated within
@@ -430,187 +576,237 @@ class btree_node {
   btree_node(btree_node const &) = delete;
   btree_node &operator=(btree_node const &) = delete;
 
-  // Public for EmptyNodeType.
-  constexpr static size_type Alignment() {
-    static_assert(LeafLayout(1).Alignment() == InternalLayout().Alignment(),
-                  "Alignment of all nodes must be equal.");
-    return InternalLayout().Alignment();
-  }
-
  protected:
   btree_node() = default;
 
  private:
-  using layout_type = absl::container_internal::Layout<btree_node *, field_type,
-                                                       slot_type, btree_node *>;
-  constexpr static size_type SizeWithNValues(size_type n) {
-    return layout_type(/*parent*/ 1,
-                       /*position, start, count, max_count*/ 4,
-                       /*values*/ n,
-                       /*children*/ 0)
-        .AllocSize();
-  }
-  // A lower bound for the overhead of fields other than values in a leaf node.
+  using layout_type =
+      absl::container_internal::Layout<btree_node *, uint32_t, field_type,
+                                       slot_type, btree_node *>;
+  using leaf_layout_type = typename layout_type::template WithStaticSizes<
+      /*parent*/ 1,
+      /*generation*/ BtreeGenerationsEnabled() ? 1 : 0,
+      /*position, start, finish, max_count*/ 4>;
+  constexpr static size_type SizeWithNSlots(size_type n) {
+    return leaf_layout_type(/*slots*/ n, /*children*/ 0).AllocSize();
+  }
+  // A lower bound for the overhead of fields other than slots in a leaf node.
   constexpr static size_type MinimumOverhead() {
-    return SizeWithNValues(1) - sizeof(value_type);
+    return SizeWithNSlots(1) - sizeof(slot_type);
   }
 
   // Compute how many values we can fit onto a leaf node taking into account
   // padding.
-  constexpr static size_type NodeTargetValues(const int begin, const int end) {
+  constexpr static size_type NodeTargetSlots(const size_type begin,
+                                             const size_type end) {
     return begin == end ? begin
-                        : SizeWithNValues((begin + end) / 2 + 1) >
-                                  params_type::kTargetNodeSize
-                              ? NodeTargetValues(begin, (begin + end) / 2)
-                              : NodeTargetValues((begin + end) / 2 + 1, end);
+           : SizeWithNSlots((begin + end) / 2 + 1) >
+                   params_type::kTargetNodeSize
+               ? NodeTargetSlots(begin, (begin + end) / 2)
+               : NodeTargetSlots((begin + end) / 2 + 1, end);
   }
 
-  enum {
-    kTargetNodeSize = params_type::kTargetNodeSize,
-    kNodeTargetValues = NodeTargetValues(0, params_type::kTargetNodeSize),
+  constexpr static size_type kTargetNodeSize = params_type::kTargetNodeSize;
+  constexpr static size_type kNodeTargetSlots =
+      NodeTargetSlots(0, kTargetNodeSize);
 
-    // We need a minimum of 3 values per internal node in order to perform
-    // splitting (1 value for the two nodes involved in the split and 1 value
-    // propagated to the parent as the delimiter for the split).
-    kNodeValues = kNodeTargetValues >= 3 ? kNodeTargetValues : 3,
+  // We need a minimum of 3 slots per internal node in order to perform
+  // splitting (1 value for the two nodes involved in the split and 1 value
+  // propagated to the parent as the delimiter for the split). For performance
+  // reasons, we don't allow 3 slots-per-node due to bad worst case occupancy of
+  // 1/3 (for a node, not a b-tree).
+  constexpr static size_type kMinNodeSlots = 4;
 
-    // The node is internal (i.e. is not a leaf node) if and only if `max_count`
-    // has this value.
-    kInternalNodeMaxCount = 0,
-  };
+  constexpr static size_type kNodeSlots =
+      kNodeTargetSlots >= kMinNodeSlots ? kNodeTargetSlots : kMinNodeSlots;
 
-  // Leaves can have less than kNodeValues values.
-  constexpr static layout_type LeafLayout(const int max_values = kNodeValues) {
-    return layout_type(/*parent*/ 1,
-                       /*position, start, count, max_count*/ 4,
-                       /*values*/ max_values,
-                       /*children*/ 0);
-  }
-  constexpr static layout_type InternalLayout() {
-    return layout_type(/*parent*/ 1,
-                       /*position, start, count, max_count*/ 4,
-                       /*values*/ kNodeValues,
-                       /*children*/ kNodeValues + 1);
+  using internal_layout_type = typename layout_type::template WithStaticSizes<
+      /*parent*/ 1,
+      /*generation*/ BtreeGenerationsEnabled() ? 1 : 0,
+      /*position, start, finish, max_count*/ 4, /*slots*/ kNodeSlots,
+      /*children*/ kNodeSlots + 1>;
+
+  // The node is internal (i.e. is not a leaf node) if and only if `max_count`
+  // has this value.
+  constexpr static field_type kInternalNodeMaxCount = 0;
+
+  // Leaves can have less than kNodeSlots values.
+  constexpr static leaf_layout_type LeafLayout(
+      const size_type slot_count = kNodeSlots) {
+    return leaf_layout_type(slot_count, 0);
   }
-  constexpr static size_type LeafSize(const int max_values = kNodeValues) {
-    return LeafLayout(max_values).AllocSize();
+  constexpr static auto InternalLayout() { return internal_layout_type(); }
+  constexpr static size_type LeafSize(const size_type slot_count = kNodeSlots) {
+    return LeafLayout(slot_count).AllocSize();
   }
   constexpr static size_type InternalSize() {
     return InternalLayout().AllocSize();
   }
 
+  constexpr static size_type Alignment() {
+    static_assert(LeafLayout(1).Alignment() == InternalLayout().Alignment(),
+                  "Alignment of all nodes must be equal.");
+    return InternalLayout().Alignment();
+  }
+
   // N is the index of the type in the Layout definition.
   // ElementType<N> is the Nth type in the Layout definition.
   template <size_type N>
   inline typename layout_type::template ElementType<N> *GetField() {
     // We assert that we don't read from values that aren't there.
-    assert(N < 3 || !leaf());
+    assert(N < 4 || is_internal());
     return InternalLayout().template Pointer<N>(reinterpret_cast<char *>(this));
   }
   template <size_type N>
   inline const typename layout_type::template ElementType<N> *GetField() const {
-    assert(N < 3 || !leaf());
+    assert(N < 4 || is_internal());
     return InternalLayout().template Pointer<N>(
         reinterpret_cast<const char *>(this));
   }
   void set_parent(btree_node *p) { *GetField<0>() = p; }
-  field_type &mutable_count() { return GetField<1>()[2]; }
-  slot_type *slot(int i) { return &GetField<2>()[i]; }
-  const slot_type *slot(int i) const { return &GetField<2>()[i]; }
-  void set_position(field_type v) { GetField<1>()[0] = v; }
-  void set_start(field_type v) { GetField<1>()[1] = v; }
-  void set_count(field_type v) { GetField<1>()[2] = v; }
+  field_type &mutable_finish() { return GetField<2>()[2]; }
+  slot_type *slot(size_type i) { return &GetField<3>()[i]; }
+  slot_type *start_slot() { return slot(start()); }
+  slot_type *finish_slot() { return slot(finish()); }
+  const slot_type *slot(size_type i) const { return &GetField<3>()[i]; }
+  void set_position(field_type v) { GetField<2>()[0] = v; }
+  void set_start(field_type v) { GetField<2>()[1] = v; }
+  void set_finish(field_type v) { GetField<2>()[2] = v; }
   // This method is only called by the node init methods.
-  void set_max_count(field_type v) { GetField<1>()[3] = v; }
+  void set_max_count(field_type v) { GetField<2>()[3] = v; }
 
  public:
   // Whether this is a leaf node or not. This value doesn't change after the
   // node is created.
-  bool leaf() const { return GetField<1>()[3] != kInternalNodeMaxCount; }
+  bool is_leaf() const { return GetField<2>()[3] != kInternalNodeMaxCount; }
+  // Whether this is an internal node or not. This value doesn't change after
+  // the node is created.
+  bool is_internal() const { return !is_leaf(); }
 
   // Getter for the position of this node in its parent.
-  field_type position() const { return GetField<1>()[0]; }
+  field_type position() const { return GetField<2>()[0]; }
 
   // Getter for the offset of the first value in the `values` array.
-  field_type start() const { return GetField<1>()[1]; }
+  field_type start() const {
+    // TODO(ezb): when floating storage is implemented, return GetField<2>()[1];
+    assert(GetField<2>()[1] == 0);
+    return 0;
+  }
+
+  // Getter for the offset after the last value in the `values` array.
+  field_type finish() const { return GetField<2>()[2]; }
 
   // Getters for the number of values stored in this node.
-  field_type count() const { return GetField<1>()[2]; }
+  field_type count() const {
+    assert(finish() >= start());
+    return finish() - start();
+  }
   field_type max_count() const {
     // Internal nodes have max_count==kInternalNodeMaxCount.
-    // Leaf nodes have max_count in [1, kNodeValues].
-    const field_type max_count = GetField<1>()[3];
+    // Leaf nodes have max_count in [1, kNodeSlots].
+    const field_type max_count = GetField<2>()[3];
     return max_count == field_type{kInternalNodeMaxCount}
-               ? field_type{kNodeValues}
+               ? field_type{kNodeSlots}
                : max_count;
   }
 
   // Getter for the parent of this node.
+  // TODO(ezb): assert that the child of the returned node at position
+  // `node_->position()` maps to the current node.
   btree_node *parent() const { return *GetField<0>(); }
   // Getter for whether the node is the root of the tree. The parent of the
   // root of the tree is the leftmost node in the tree which is guaranteed to
   // be a leaf.
-  bool is_root() const { return parent()->leaf(); }
+  bool is_root() const { return parent()->is_leaf(); }
   void make_root() {
     assert(parent()->is_root());
+    set_generation(parent()->generation());
     set_parent(parent()->parent());
   }
 
+  // Gets the root node's generation integer, which is the one used by the tree.
+  uint32_t *get_root_generation() const {
+    assert(BtreeGenerationsEnabled());
+    const btree_node *curr = this;
+    for (; !curr->is_root(); curr = curr->parent()) continue;
+    return const_cast<uint32_t *>(&curr->GetField<1>()[0]);
+  }
+
+  // Returns the generation for iterator validation.
+  uint32_t generation() const {
+    return BtreeGenerationsEnabled() ? *get_root_generation() : 0;
+  }
+  // Updates generation. Should only be called on a root node or during node
+  // initialization.
+  void set_generation(uint32_t generation) {
+    if (BtreeGenerationsEnabled()) GetField<1>()[0] = generation;
+  }
+  // Updates the generation. We do this whenever the node is mutated.
+  void next_generation() {
+    if (BtreeGenerationsEnabled()) ++*get_root_generation();
+  }
+
   // Getters for the key/value at position i in the node.
-  const key_type &key(int i) const { return params_type::key(slot(i)); }
-  reference value(int i) { return params_type::element(slot(i)); }
-  const_reference value(int i) const { return params_type::element(slot(i)); }
+  const key_type &key(size_type i) const { return params_type::key(slot(i)); }
+  reference value(size_type i) { return params_type::element(slot(i)); }
+  const_reference value(size_type i) const {
+    return params_type::element(slot(i));
+  }
 
   // Getters/setter for the child at position i in the node.
-  btree_node *child(int i) const { return GetField<3>()[i]; }
-  btree_node *&mutable_child(int i) { return GetField<3>()[i]; }
-  void clear_child(int i) {
+  btree_node *child(field_type i) const { return GetField<4>()[i]; }
+  btree_node *start_child() const { return child(start()); }
+  btree_node *&mutable_child(field_type i) { return GetField<4>()[i]; }
+  void clear_child(field_type i) {
     absl::container_internal::SanitizerPoisonObject(&mutable_child(i));
   }
-  void set_child(int i, btree_node *c) {
+  void set_child_noupdate_position(field_type i, btree_node *c) {
     absl::container_internal::SanitizerUnpoisonObject(&mutable_child(i));
     mutable_child(i) = c;
+  }
+  void set_child(field_type i, btree_node *c) {
+    set_child_noupdate_position(i, c);
     c->set_position(i);
   }
-  void init_child(int i, btree_node *c) {
+  void init_child(field_type i, btree_node *c) {
     set_child(i, c);
     c->set_parent(this);
   }
 
   // Returns the position of the first value whose key is not less than k.
   template <typename K>
-  SearchResult<int, is_key_compare_to::value> lower_bound(
+  SearchResult<size_type, is_key_compare_to::value> lower_bound(
       const K &k, const key_compare &comp) const {
     return use_linear_search::value ? linear_search(k, comp)
                                     : binary_search(k, comp);
   }
   // Returns the position of the first value whose key is greater than k.
   template <typename K>
-  int upper_bound(const K &k, const key_compare &comp) const {
+  size_type upper_bound(const K &k, const key_compare &comp) const {
     auto upper_compare = upper_bound_adapter<key_compare>(comp);
     return use_linear_search::value ? linear_search(k, upper_compare).value
                                     : binary_search(k, upper_compare).value;
   }
 
   template <typename K, typename Compare>
-  SearchResult<int, btree_is_key_compare_to<Compare, key_type>::value>
+  SearchResult<size_type, btree_is_key_compare_to<Compare, key_type>::value>
   linear_search(const K &k, const Compare &comp) const {
-    return linear_search_impl(k, 0, count(), comp,
+    return linear_search_impl(k, start(), finish(), comp,
                               btree_is_key_compare_to<Compare, key_type>());
   }
 
   template <typename K, typename Compare>
-  SearchResult<int, btree_is_key_compare_to<Compare, key_type>::value>
+  SearchResult<size_type, btree_is_key_compare_to<Compare, key_type>::value>
   binary_search(const K &k, const Compare &comp) const {
-    return binary_search_impl(k, 0, count(), comp,
+    return binary_search_impl(k, start(), finish(), comp,
                               btree_is_key_compare_to<Compare, key_type>());
   }
 
   // Returns the position of the first value whose key is not less than k using
   // linear search performed using plain compare.
   template <typename K, typename Compare>
-  SearchResult<int, false> linear_search_impl(
-      const K &k, int s, const int e, const Compare &comp,
+  SearchResult<size_type, false> linear_search_impl(
+      const K &k, size_type s, const size_type e, const Compare &comp,
       std::false_type /* IsCompareTo */) const {
     while (s < e) {
       if (!comp(key(s), k)) {
@@ -618,14 +814,14 @@ class btree_node {
       }
       ++s;
     }
-    return {s};
+    return SearchResult<size_type, false>{s};
   }
 
   // Returns the position of the first value whose key is not less than k using
   // linear search performed using compare-to.
   template <typename K, typename Compare>
-  SearchResult<int, true> linear_search_impl(
-      const K &k, int s, const int e, const Compare &comp,
+  SearchResult<size_type, true> linear_search_impl(
+      const K &k, size_type s, const size_type e, const Compare &comp,
       std::true_type /* IsCompareTo */) const {
     while (s < e) {
       const absl::weak_ordering c = comp(key(s), k);
@@ -642,30 +838,30 @@ class btree_node {
   // Returns the position of the first value whose key is not less than k using
   // binary search performed using plain compare.
   template <typename K, typename Compare>
-  SearchResult<int, false> binary_search_impl(
-      const K &k, int s, int e, const Compare &comp,
+  SearchResult<size_type, false> binary_search_impl(
+      const K &k, size_type s, size_type e, const Compare &comp,
       std::false_type /* IsCompareTo */) const {
     while (s != e) {
-      const int mid = (s + e) >> 1;
+      const size_type mid = (s + e) >> 1;
       if (comp(key(mid), k)) {
         s = mid + 1;
       } else {
         e = mid;
       }
     }
-    return {s};
+    return SearchResult<size_type, false>{s};
   }
 
   // Returns the position of the first value whose key is not less than k using
   // binary search performed using compare-to.
   template <typename K, typename CompareTo>
-  SearchResult<int, true> binary_search_impl(
-      const K &k, int s, int e, const CompareTo &comp,
+  SearchResult<size_type, true> binary_search_impl(
+      const K &k, size_type s, size_type e, const CompareTo &comp,
       std::true_type /* IsCompareTo */) const {
-    if (is_multi_container::value) {
+    if (params_type::template can_have_multiple_equivalent_keys<K>()) {
       MatchKind exact_match = MatchKind::kNe;
       while (s != e) {
-        const int mid = (s + e) >> 1;
+        const size_type mid = (s + e) >> 1;
         const absl::weak_ordering c = comp(key(mid), k);
         if (c < 0) {
           s = mid + 1;
@@ -673,16 +869,16 @@ class btree_node {
           e = mid;
           if (c == 0) {
             // Need to return the first value whose key is not less than k,
-            // which requires continuing the binary search if this is a
-            // multi-container.
+            // which requires continuing the binary search if there could be
+            // multiple equivalent keys.
             exact_match = MatchKind::kEq;
           }
         }
       }
       return {s, exact_match};
-    } else {  // Not a multi-container.
+    } else {  // Can't have multiple equivalent keys.
       while (s != e) {
-        const int mid = (s + e) >> 1;
+        const size_type mid = (s + e) >> 1;
         const absl::weak_ordering c = comp(key(mid), k);
         if (c < 0) {
           s = mid + 1;
@@ -696,117 +892,226 @@ class btree_node {
     }
   }
 
+  // Returns whether key i is ordered correctly with respect to the other keys
+  // in the node. The motivation here is to detect comparators that violate
+  // transitivity. Note: we only do comparisons of keys on this node rather than
+  // the whole tree so that this is constant time.
+  template <typename Compare>
+  bool is_ordered_correctly(field_type i, const Compare &comp) const {
+    if (std::is_base_of_v<BtreeTestOnlyCheckedCompareOptOutBase, Compare> ||
+        params_type::kIsKeyCompareStringAdapted) {
+      return true;
+    }
+
+    const auto compare = [&](field_type a, field_type b) {
+      const absl::weak_ordering cmp =
+          compare_internal::do_three_way_comparison(comp, key(a), key(b));
+      return cmp < 0 ? -1 : cmp > 0 ? 1 : 0;
+    };
+    int cmp = -1;
+    constexpr bool kCanHaveEquivKeys =
+        params_type::template can_have_multiple_equivalent_keys<key_type>();
+    for (field_type j = start(); j < finish(); ++j) {
+      if (j == i) {
+        if (cmp > 0) return false;
+        continue;
+      }
+      int new_cmp = compare(j, i);
+      if (new_cmp < cmp || (!kCanHaveEquivKeys && new_cmp == 0)) return false;
+      cmp = new_cmp;
+    }
+    return true;
+  }
+
   // Emplaces a value at position i, shifting all existing values and
   // children at positions >= i to the right by 1.
   template <typename... Args>
-  void emplace_value(size_type i, allocator_type *alloc, Args &&... args);
-
-  // Removes the value at position i, shifting all existing values and children
-  // at positions > i to the left by 1.
-  void remove_value(int i, allocator_type *alloc);
+  void emplace_value(field_type i, allocator_type *alloc, Args &&...args);
 
-  // Removes the values at positions [i, i + to_erase), shifting all values
-  // after that range to the left by to_erase. Does not change children at all.
-  void remove_values_ignore_children(int i, int to_erase,
-                                     allocator_type *alloc);
+  // Removes the values at positions [i, i + to_erase), shifting all existing
+  // values and children after that range to the left by to_erase. Clears all
+  // children between [i, i + to_erase).
+  void remove_values(field_type i, field_type to_erase, allocator_type *alloc);
 
   // Rebalances a node with its right sibling.
-  void rebalance_right_to_left(int to_move, btree_node *right,
+  void rebalance_right_to_left(field_type to_move, btree_node *right,
                                allocator_type *alloc);
-  void rebalance_left_to_right(int to_move, btree_node *right,
+  void rebalance_left_to_right(field_type to_move, btree_node *right,
                                allocator_type *alloc);
 
   // Splits a node, moving a portion of the node's values to its right sibling.
   void split(int insert_position, btree_node *dest, allocator_type *alloc);
 
   // Merges a node with its right sibling, moving all of the values and the
-  // delimiting key in the parent node onto itself.
-  void merge(btree_node *sibling, allocator_type *alloc);
-
-  // Swap the contents of "this" and "src".
-  void swap(btree_node *src, allocator_type *alloc);
+  // delimiting key in the parent node onto itself, and deleting the src node.
+  void merge(btree_node *src, allocator_type *alloc);
 
   // Node allocation/deletion routines.
-  static btree_node *init_leaf(btree_node *n, btree_node *parent,
-                               int max_count) {
-    n->set_parent(parent);
-    n->set_position(0);
-    n->set_start(0);
-    n->set_count(0);
-    n->set_max_count(max_count);
+  void init_leaf(field_type position, field_type max_count,
+                 btree_node *parent) {
+    set_generation(0);
+    set_parent(parent);
+    set_position(position);
+    set_start(0);
+    set_finish(0);
+    set_max_count(max_count);
     absl::container_internal::SanitizerPoisonMemoryRegion(
-        n->slot(0), max_count * sizeof(slot_type));
-    return n;
+        start_slot(), max_count * sizeof(slot_type));
   }
-  static btree_node *init_internal(btree_node *n, btree_node *parent) {
-    init_leaf(n, parent, kNodeValues);
+  void init_internal(field_type position, btree_node *parent) {
+    init_leaf(position, kNodeSlots, parent);
     // Set `max_count` to a sentinel value to indicate that this node is
     // internal.
-    n->set_max_count(kInternalNodeMaxCount);
+    set_max_count(kInternalNodeMaxCount);
     absl::container_internal::SanitizerPoisonMemoryRegion(
-        &n->mutable_child(0), (kNodeValues + 1) * sizeof(btree_node *));
-    return n;
-  }
-  void destroy(allocator_type *alloc) {
-    for (int i = 0; i < count(); ++i) {
-      value_destroy(i, alloc);
-    }
+        &mutable_child(start()), (kNodeSlots + 1) * sizeof(btree_node *));
   }
 
- public:
-  // Exposed only for tests.
-  static bool testonly_uses_linear_node_search() {
-    return use_linear_search::value;
+  static void deallocate(const size_type size, btree_node *node,
+                         allocator_type *alloc) {
+    absl::container_internal::SanitizerUnpoisonMemoryRegion(node, size);
+    absl::container_internal::Deallocate<Alignment()>(alloc, node, size);
   }
 
+  // Deletes a node and all of its children.
+  static void clear_and_delete(btree_node *node, allocator_type *alloc);
+
  private:
   template <typename... Args>
-  void value_init(const size_type i, allocator_type *alloc, Args &&... args) {
+  void value_init(const field_type i, allocator_type *alloc, Args &&...args) {
+    next_generation();
     absl::container_internal::SanitizerUnpoisonObject(slot(i));
     params_type::construct(alloc, slot(i), std::forward<Args>(args)...);
   }
-  void value_destroy(const size_type i, allocator_type *alloc) {
+  void value_destroy(const field_type i, allocator_type *alloc) {
+    next_generation();
     params_type::destroy(alloc, slot(i));
     absl::container_internal::SanitizerPoisonObject(slot(i));
   }
+  void value_destroy_n(const field_type i, const field_type n,
+                       allocator_type *alloc) {
+    next_generation();
+    for (slot_type *s = slot(i), *end = slot(i + n); s != end; ++s) {
+      params_type::destroy(alloc, s);
+      absl::container_internal::SanitizerPoisonObject(s);
+    }
+  }
+
+  static void transfer(slot_type *dest, slot_type *src, allocator_type *alloc) {
+    absl::container_internal::SanitizerUnpoisonObject(dest);
+    params_type::transfer(alloc, dest, src);
+    absl::container_internal::SanitizerPoisonObject(src);
+  }
+
+  // Transfers value from slot `src_i` in `src_node` to slot `dest_i` in `this`.
+  void transfer(const size_type dest_i, const size_type src_i,
+                btree_node *src_node, allocator_type *alloc) {
+    next_generation();
+    transfer(slot(dest_i), src_node->slot(src_i), alloc);
+  }
 
-  // Move n values starting at value i in this node into the values starting at
-  // value j in node x.
-  void uninitialized_move_n(const size_type n, const size_type i,
-                            const size_type j, btree_node *x,
-                            allocator_type *alloc) {
-    absl::container_internal::SanitizerUnpoisonMemoryRegion(
-        x->slot(j), n * sizeof(slot_type));
-    for (slot_type *src = slot(i), *end = src + n, *dest = x->slot(j);
+  // Transfers `n` values starting at value `src_i` in `src_node` into the
+  // values starting at value `dest_i` in `this`.
+  void transfer_n(const size_type n, const size_type dest_i,
+                  const size_type src_i, btree_node *src_node,
+                  allocator_type *alloc) {
+    next_generation();
+    for (slot_type *src = src_node->slot(src_i), *end = src + n,
+                   *dest = slot(dest_i);
          src != end; ++src, ++dest) {
-      params_type::construct(alloc, dest, src);
+      transfer(dest, src, alloc);
     }
   }
 
-  // Destroys a range of n values, starting at index i.
-  void value_destroy_n(const size_type i, const size_type n,
-                       allocator_type *alloc) {
-    for (int j = 0; j < n; ++j) {
-      value_destroy(i + j, alloc);
+  // Same as above, except that we start at the end and work our way to the
+  // beginning.
+  void transfer_n_backward(const size_type n, const size_type dest_i,
+                           const size_type src_i, btree_node *src_node,
+                           allocator_type *alloc) {
+    next_generation();
+    for (slot_type *src = src_node->slot(src_i + n), *end = src - n,
+                   *dest = slot(dest_i + n);
+         src != end; --src, --dest) {
+      // If we modified the loop index calculations above to avoid the -1s here,
+      // it would result in UB in the computation of `end` (and possibly `src`
+      // as well, if n == 0), since slot() is effectively an array index and it
+      // is UB to compute the address of any out-of-bounds array element except
+      // for one-past-the-end.
+      transfer(dest - 1, src - 1, alloc);
     }
   }
 
   template <typename P>
   friend class btree;
   template <typename N, typename R, typename P>
-  friend struct btree_iterator;
+  friend class btree_iterator;
   friend class BtreeNodePeer;
+  friend struct btree_access;
 };
 
-template <typename Node, typename Reference, typename Pointer>
-struct btree_iterator {
+template <typename Node>
+bool AreNodesFromSameContainer(const Node *node_a, const Node *node_b) {
+  // If either node is null, then give up on checking whether they're from the
+  // same container. (If exactly one is null, then we'll trigger the
+  // default-constructed assert in Equals.)
+  if (node_a == nullptr || node_b == nullptr) return true;
+  while (!node_a->is_root()) node_a = node_a->parent();
+  while (!node_b->is_root()) node_b = node_b->parent();
+  return node_a == node_b;
+}
+
+class btree_iterator_generation_info_enabled {
+ public:
+  explicit btree_iterator_generation_info_enabled(uint32_t g)
+      : generation_(g) {}
+
+  // Updates the generation. For use internally right before we return an
+  // iterator to the user.
+  template <typename Node>
+  void update_generation(const Node *node) {
+    if (node != nullptr) generation_ = node->generation();
+  }
+  uint32_t generation() const { return generation_; }
+
+  template <typename Node>
+  void assert_valid_generation(const Node *node) const {
+    if (node != nullptr && node->generation() != generation_) {
+      ABSL_INTERNAL_LOG(
+          FATAL,
+          "Attempting to use an invalidated iterator. The corresponding b-tree "
+          "container has been mutated since this iterator was constructed.");
+    }
+  }
+
  private:
+  // Used to check that the iterator hasn't been invalidated.
+  uint32_t generation_;
+};
+
+class btree_iterator_generation_info_disabled {
+ public:
+  explicit btree_iterator_generation_info_disabled(uint32_t) {}
+  static void update_generation(const void *) {}
+  static uint32_t generation() { return 0; }
+  static void assert_valid_generation(const void *) {}
+};
+
+#ifdef ABSL_BTREE_ENABLE_GENERATIONS
+using btree_iterator_generation_info = btree_iterator_generation_info_enabled;
+#else
+using btree_iterator_generation_info = btree_iterator_generation_info_disabled;
+#endif
+
+template <typename Node, typename Reference, typename Pointer>
+class btree_iterator : private btree_iterator_generation_info {
+  using field_type = typename Node::field_type;
   using key_type = typename Node::key_type;
   using size_type = typename Node::size_type;
   using params_type = typename Node::params_type;
+  using is_map_container = typename params_type::is_map_container;
 
   using node_type = Node;
-  using normal_node = typename std::remove_const<Node>::type;
+  using normal_node = std::remove_const_t<Node>;
   using const_node = const Node;
   using normal_pointer = typename params_type::pointer;
   using normal_reference = typename params_type::reference;
@@ -814,8 +1119,11 @@ struct btree_iterator {
   using const_reference = typename params_type::const_reference;
   using slot_type = typename params_type::slot_type;
 
-  using iterator =
-      btree_iterator<normal_node, normal_reference, normal_pointer>;
+  // In sets, all iterators are const.
+  using iterator = std::conditional_t<
+      is_map_container::value,
+      btree_iterator<normal_node, normal_reference, normal_pointer>,
+      btree_iterator<normal_node, const_reference, const_pointer>>;
   using const_iterator =
       btree_iterator<const_node, const_reference, const_pointer>;
 
@@ -827,72 +1135,92 @@ struct btree_iterator {
   using reference = Reference;
   using iterator_category = std::bidirectional_iterator_tag;
 
-  btree_iterator() : node(nullptr), position(-1) {}
-  btree_iterator(Node *n, int p) : node(n), position(p) {}
+  btree_iterator() : btree_iterator(nullptr, -1) {}
+  explicit btree_iterator(Node *n) : btree_iterator(n, n->start()) {}
+  btree_iterator(Node *n, int p)
+      : btree_iterator_generation_info(n != nullptr ? n->generation()
+                                                    : ~uint32_t{}),
+        node_(n),
+        position_(p) {}
 
   // NOTE: this SFINAE allows for implicit conversions from iterator to
-  // const_iterator, but it specifically avoids defining copy constructors so
-  // that btree_iterator can be trivially copyable. This is for performance and
-  // binary size reasons.
-  template <typename N, typename R, typename P,
-            absl::enable_if_t<
-                std::is_same<btree_iterator<N, R, P>, iterator>::value &&
-                    std::is_same<btree_iterator, const_iterator>::value,
-                int> = 0>
-  btree_iterator(const btree_iterator<N, R, P> &x)  // NOLINT
-      : node(x.node), position(x.position) {}
-
- private:
-  // This SFINAE allows explicit conversions from const_iterator to
-  // iterator, but also avoids defining a copy constructor.
-  // NOTE: the const_cast is safe because this constructor is only called by
-  // non-const methods and the container owns the nodes.
-  template <typename N, typename R, typename P,
-            absl::enable_if_t<
-                std::is_same<btree_iterator<N, R, P>, const_iterator>::value &&
-                    std::is_same<btree_iterator, iterator>::value,
-                int> = 0>
-  explicit btree_iterator(const btree_iterator<N, R, P> &x)
-      : node(const_cast<node_type *>(x.node)), position(x.position) {}
+  // const_iterator, but it specifically avoids hiding the copy constructor so
+  // that the trivial one will be used when possible.
+  template <
+      typename N, typename R, typename P,
+      std::enable_if_t<std::is_same_v<btree_iterator<N, R, P>, iterator> &&
+                           std::is_same_v<btree_iterator, const_iterator>,
+                       int> = 0>
+  btree_iterator(const btree_iterator<N, R, P> other)  // NOLINT
+      : btree_iterator_generation_info(other),
+        node_(other.node_),
+        position_(other.position_) {}
 
-  // Increment/decrement the iterator.
-  void increment() {
-    if (node->leaf() && ++position < node->count()) {
-      return;
-    }
-    increment_slow();
+  bool operator==(const iterator &other) const {
+    return Equals(other);
+  }
+  bool operator==(const const_iterator &other) const {
+    return Equals(other);
+  }
+  bool operator!=(const iterator &other) const {
+    return !Equals(other);
+  }
+  bool operator!=(const const_iterator &other) const {
+    return !Equals(other);
   }
-  void increment_slow();
 
-  void decrement() {
-    if (node->leaf() && --position >= 0) {
-      return;
+  // Returns n such that n calls to ++other yields *this.
+  // Precondition: n exists.
+  difference_type operator-(const_iterator other) const {
+    if (node_ == other.node_) {
+      if (node_->is_leaf()) return position_ - other.position_;
+      if (position_ == other.position_) return 0;
     }
-    decrement_slow();
+    return distance_slow(other);
   }
-  void decrement_slow();
 
- public:
-  bool operator==(const const_iterator &x) const {
-    return node == x.node && position == x.position;
+  // Advances the iterator by `n`. Values of `n` must not result in going past
+  // the `end` iterator (for a positive `n`) or before the `begin` iterator (for
+  // a negative `n`).
+  btree_iterator &operator+=(difference_type n) {
+    assert_valid_generation(node_);
+    if (n == 0) return *this;
+    if (n < 0) return decrement_n_slow(-n);
+    return increment_n_slow(n);
   }
-  bool operator!=(const const_iterator &x) const {
-    return node != x.node || position != x.position;
+
+  // Moves the iterator by `n` positions backwards. Values of `n` must not
+  // result in going before the `begin` iterator (for a positive `n`) or past
+  // the `end` iterator (for a negative `n`).
+  btree_iterator &operator-=(difference_type n) {
+    assert_valid_generation(node_);
+    if (n == 0) return *this;
+    if (n < 0) return increment_n_slow(-n);
+    return decrement_n_slow(n);
   }
 
   // Accessors for the key/value the iterator is pointing at.
   reference operator*() const {
-    return node->value(position);
-  }
-  pointer operator->() const {
-    return &node->value(position);
-  }
-
-  btree_iterator& operator++() {
+    absl::base_internal::HardeningAssertNonNull(node_);
+    assert_valid_generation(node_);
+    absl::base_internal::HardeningAssertGE(position_,
+                                           static_cast<int>(node_->start()));
+    if (position_ >= node_->finish()) {
+      // If this assertion fails, we have tried to dereference an end()
+      // iterator.
+      absl::base_internal::HardeningAssert(!IsEndIterator());
+      absl::base_internal::HardeningAssertLT(position_,
+                                             static_cast<int>(node_->finish()));
+    }
+    return node_->value(static_cast<field_type>(position_));
+  }
+  pointer operator->() const { return &operator*(); }
+
+  btree_iterator &operator++() {
     increment();
     return *this;
   }
-  btree_iterator& operator--() {
+  btree_iterator &operator--() {
     decrement();
     return *this;
   }
@@ -908,6 +1236,8 @@ struct btree_iterator {
   }
 
  private:
+  friend iterator;
+  friend const_iterator;
   template <typename Params>
   friend class btree;
   template <typename Tree>
@@ -918,75 +1248,137 @@ struct btree_iterator {
   friend class btree_map_container;
   template <typename Tree>
   friend class btree_multiset_container;
-  template <typename N, typename R, typename P>
-  friend struct btree_iterator;
   template <typename TreeType, typename CheckerType>
   friend class base_checker;
+  friend struct btree_access;
+
+  // This SFINAE allows explicit conversions from const_iterator to
+  // iterator, but also avoids hiding the copy constructor.
+  // NOTE: the const_cast is safe because this constructor is only called by
+  // non-const methods and the container owns the nodes.
+  template <typename N, typename R, typename P,
+            std::enable_if_t<
+                std::is_same_v<btree_iterator<N, R, P>, const_iterator> &&
+                    std::is_same_v<btree_iterator, iterator>,
+                int> = 0>
+  explicit btree_iterator(const btree_iterator<N, R, P> other)
+      : btree_iterator_generation_info(other.generation()),
+        node_(const_cast<node_type*>(other.node_)),
+        position_(other.position_) {}
+
+  bool Equals(const const_iterator other) const {
+    absl::base_internal::HardeningAssert(
+        ((node_ == nullptr && other.node_ == nullptr) ||
+         (node_ != nullptr && other.node_ != nullptr)) &&
+        "Comparing default-constructed iterator with "
+        "non-default-constructed iterator.");
+    // Note: we use assert instead of ABSL_HARDENING_ASSERT here because this
+    // changes the complexity of Equals from O(1) to O(log(N) + log(M)) where
+    // N/M are sizes of the containers containing node_/other.node_.
+    assert(AreNodesFromSameContainer(node_, other.node_) &&
+           "Comparing iterators from different containers.");
+    assert_valid_generation(node_);
+    other.assert_valid_generation(other.node_);
+    return node_ == other.node_ && position_ == other.position_;
+  }
+
+  bool IsEndIterator() const {
+    if (position_ != node_->finish()) return false;
+    node_type *node = node_;
+    while (!node->is_root()) {
+      if (node->position() != node->parent()->finish()) return false;
+      node = node->parent();
+    }
+    return true;
+  }
 
-  const key_type &key() const { return node->key(position); }
-  slot_type *slot() { return node->slot(position); }
+  // Returns n such that n calls to ++other yields *this.
+  // Precondition: n exists && (this->node_ != other.node_ ||
+  // !this->node_->is_leaf() || this->position_ != other.position_).
+  difference_type distance_slow(const_iterator other) const;
+
+  // Increment/decrement the iterator.
+  void increment() {
+    assert_valid_generation(node_);
+    if (node_->is_leaf() && ++position_ < node_->finish()) {
+      return;
+    }
+    increment_slow();
+  }
+  void increment_slow();
+  btree_iterator &increment_n_slow(difference_type n);
+
+  void decrement() {
+    assert_valid_generation(node_);
+    if (node_->is_leaf() && --position_ >= node_->start()) {
+      return;
+    }
+    decrement_slow();
+  }
+  void decrement_slow();
+  btree_iterator &decrement_n_slow(difference_type n);
+
+  const key_type &key() const {
+    return node_->key(static_cast<size_type>(position_));
+  }
+  decltype(std::declval<Node *>()->slot(0)) slot() {
+    return node_->slot(static_cast<size_type>(position_));
+  }
+
+  void update_generation() {
+    btree_iterator_generation_info::update_generation(node_);
+  }
 
   // The node in the tree the iterator is pointing at.
-  Node *node;
+  Node *node_;
   // The position within the node of the tree the iterator is pointing at.
-  // TODO(ezb): make this a field_type
-  int position;
+  // NOTE: this is an int rather than a field_type because iterators can point
+  // to invalid positions (such as -1) in certain circumstances.
+  int position_;
 };
 
 template <typename Params>
 class btree {
   using node_type = btree_node<Params>;
   using is_key_compare_to = typename Params::is_key_compare_to;
+  using field_type = typename node_type::field_type;
 
   // We use a static empty node for the root/leftmost/rightmost of empty btrees
   // in order to avoid branching in begin()/end().
-  struct alignas(node_type::Alignment()) EmptyNodeType : node_type {
+  struct EmptyNodeType : node_type {
     using field_type = typename node_type::field_type;
     node_type *parent;
+#ifdef ABSL_BTREE_ENABLE_GENERATIONS
+    uint32_t generation = 0;
+#endif
     field_type position = 0;
     field_type start = 0;
-    field_type count = 0;
+    field_type finish = 0;
     // max_count must be != kInternalNodeMaxCount (so that this node is regarded
     // as a leaf node). max_count() is never called when the tree is empty.
     field_type max_count = node_type::kInternalNodeMaxCount + 1;
 
-#ifdef _MSC_VER
-    // MSVC has constexpr code generations bugs here.
-    EmptyNodeType() : parent(this) {}
-#else
-    constexpr EmptyNodeType(node_type *p) : parent(p) {}
-#endif
+    constexpr EmptyNodeType() : parent(this) {}
   };
 
   static node_type *EmptyNode() {
-#ifdef _MSC_VER
-    static EmptyNodeType* empty_node = new EmptyNodeType;
-    // This assert fails on some other construction methods.
-    assert(empty_node->parent == empty_node);
-    return empty_node;
-#else
-    static constexpr EmptyNodeType empty_node(
-        const_cast<EmptyNodeType *>(&empty_node));
+    alignas(node_type::Alignment()) static constexpr EmptyNodeType empty_node;
     return const_cast<EmptyNodeType *>(&empty_node);
-#endif
   }
 
-  enum {
-    kNodeValues = node_type::kNodeValues,
-    kMinNodeValues = kNodeValues / 2,
+  enum : uint32_t {
+    kNodeSlots = node_type::kNodeSlots,
+    kMinNodeValues = kNodeSlots / 2,
   };
 
   struct node_stats {
     using size_type = typename Params::size_type;
 
-    node_stats(size_type l, size_type i)
-        : leaf_nodes(l),
-          internal_nodes(i) {
-    }
+    node_stats(size_type l, size_type i) : leaf_nodes(l), internal_nodes(i) {}
 
-    node_stats& operator+=(const node_stats &x) {
-      leaf_nodes += x.leaf_nodes;
-      internal_nodes += x.internal_nodes;
+    node_stats &operator+=(const node_stats &other) {
+      leaf_nodes += other.leaf_nodes;
+      internal_nodes += other.internal_nodes;
       return *this;
     }
 
@@ -1000,13 +1392,15 @@ class btree {
   using size_type = typename Params::size_type;
   using difference_type = typename Params::difference_type;
   using key_compare = typename Params::key_compare;
+  using original_key_compare = typename Params::original_key_compare;
   using value_compare = typename Params::value_compare;
   using allocator_type = typename Params::allocator_type;
   using reference = typename Params::reference;
   using const_reference = typename Params::const_reference;
   using pointer = typename Params::pointer;
   using const_pointer = typename Params::const_pointer;
-  using iterator = btree_iterator<node_type, reference, pointer>;
+  using iterator =
+      typename btree_iterator<node_type, reference, pointer>::iterator;
   using const_iterator = typename iterator::const_iterator;
   using reverse_iterator = std::reverse_iterator<iterator>;
   using const_reverse_iterator = std::reverse_iterator<const_iterator>;
@@ -1017,29 +1411,39 @@ class btree {
   using slot_type = typename Params::slot_type;
 
  private:
-  // For use in copy_or_move_values_in_order.
-  const value_type &maybe_move_from_iterator(const_iterator x) { return *x; }
-  value_type &&maybe_move_from_iterator(iterator x) { return std::move(*x); }
-
   // Copies or moves (depending on the template parameter) the values in
-  // x into this btree in their order in x. This btree must be empty before this
-  // method is called. This method is used in copy construction, copy
-  // assignment, and move assignment.
+  // other into this btree in their order in other. This btree must be empty
+  // before this method is called. This method is used in copy construction,
+  // copy assignment, and move assignment.
   template <typename Btree>
-  void copy_or_move_values_in_order(Btree *x);
+  void copy_or_move_values_in_order(Btree &other);
 
   // Validates that various assumptions/requirements are true at compile time.
   constexpr static bool static_assert_validation();
 
  public:
-  btree(const key_compare &comp, const allocator_type &alloc);
-
-  btree(const btree &x);
-  btree(btree &&x) noexcept
-      : root_(std::move(x.root_)),
-        rightmost_(absl::exchange(x.rightmost_, EmptyNode())),
-        size_(absl::exchange(x.size_, 0)) {
-    x.mutable_root() = EmptyNode();
+  btree(const key_compare &comp, const allocator_type &alloc)
+      : root_(EmptyNode()), rightmost_(comp, alloc, EmptyNode()), size_(0) {}
+
+  btree(const btree &other) : btree(other, other.allocator()) {}
+  btree(const btree &other, const allocator_type &alloc)
+      : btree(other.key_comp(), alloc) {
+    copy_or_move_values_in_order(other);
+  }
+  btree(btree &&other) noexcept
+      : root_(std::exchange(other.root_, EmptyNode())),
+        rightmost_(std::move(other.rightmost_)),
+        size_(std::exchange(other.size_, 0u)) {
+    other.mutable_rightmost() = EmptyNode();
+  }
+  btree(btree &&other, const allocator_type &alloc)
+      : btree(other.key_comp(), alloc) {
+    if (alloc == other.allocator()) {
+      swap(other);
+    } else {
+      // Move values from `other` one at a time when allocators are different.
+      copy_or_move_values_in_order(other);
+    }
   }
 
   ~btree() {
@@ -1049,44 +1453,41 @@ class btree {
     clear();
   }
 
-  // Assign the contents of x to *this.
-  btree &operator=(const btree &x);
-  btree &operator=(btree &&x) noexcept;
+  // Assign the contents of other to *this.
+  btree &operator=(const btree &other);
+  btree &operator=(btree &&other) noexcept;
 
-  iterator begin() {
-    return iterator(leftmost(), 0);
-  }
-  const_iterator begin() const {
-    return const_iterator(leftmost(), 0);
-  }
-  iterator end() { return iterator(rightmost_, rightmost_->count()); }
+  iterator begin() { return iterator(leftmost()); }
+  const_iterator begin() const { return const_iterator(leftmost()); }
+  iterator end() { return iterator(rightmost(), rightmost()->finish()); }
   const_iterator end() const {
-    return const_iterator(rightmost_, rightmost_->count());
-  }
-  reverse_iterator rbegin() {
-    return reverse_iterator(end());
+    return const_iterator(rightmost(), rightmost()->finish());
   }
+  reverse_iterator rbegin() { return reverse_iterator(end()); }
   const_reverse_iterator rbegin() const {
     return const_reverse_iterator(end());
   }
-  reverse_iterator rend() {
-    return reverse_iterator(begin());
-  }
+  reverse_iterator rend() { return reverse_iterator(begin()); }
   const_reverse_iterator rend() const {
     return const_reverse_iterator(begin());
   }
 
-  // Finds the first element whose key is not less than key.
+  // Finds the first element whose key is not less than `key`.
   template <typename K>
   iterator lower_bound(const K &key) {
-    return internal_end(internal_lower_bound(key));
+    return internal_end(internal_lower_bound(key).value);
   }
   template <typename K>
   const_iterator lower_bound(const K &key) const {
-    return internal_end(internal_lower_bound(key));
+    return internal_end(internal_lower_bound(key).value);
   }
 
-  // Finds the first element whose key is greater than key.
+  // Finds the first element whose key is not less than `key` and also returns
+  // whether that element is equal to `key`.
+  template <typename K>
+  std::pair<iterator, bool> lower_bound_equal(const K &key) const;
+
+  // Finds the first element whose key is greater than `key`.
   template <typename K>
   iterator upper_bound(const K &key) {
     return internal_end(internal_upper_bound(key));
@@ -1097,23 +1498,21 @@ class btree {
   }
 
   // Finds the range of values which compare equal to key. The first member of
-  // the returned pair is equal to lower_bound(key). The second member pair of
-  // the pair is equal to upper_bound(key).
+  // the returned pair is equal to lower_bound(key). The second member of the
+  // pair is equal to upper_bound(key).
   template <typename K>
-  std::pair<iterator, iterator> equal_range(const K &key) {
-    return {lower_bound(key), upper_bound(key)};
-  }
+  std::pair<iterator, iterator> equal_range(const K &key);
   template <typename K>
   std::pair<const_iterator, const_iterator> equal_range(const K &key) const {
-    return {lower_bound(key), upper_bound(key)};
+    return const_cast<btree *>(this)->equal_range(key);
   }
 
   // Inserts a value into the btree only if it does not already exist. The
   // boolean return value indicates whether insertion succeeded or failed.
   // Requirement: if `key` already exists in the btree, does not consume `args`.
   // Requirement: `key` is never referenced after consuming `args`.
-  template <typename... Args>
-  std::pair<iterator, bool> insert_unique(const key_type &key, Args &&... args);
+  template <typename K, typename... Args>
+  std::pair<iterator, bool> insert_unique(const K &key, Args &&...args);
 
   // Inserts with hint. Checks to see if the value should be placed immediately
   // before `position` in the tree. If so, then the insertion will take
@@ -1121,14 +1520,22 @@ class btree {
   // logarithmic time as if a call to insert_unique() were made.
   // Requirement: if `key` already exists in the btree, does not consume `args`.
   // Requirement: `key` is never referenced after consuming `args`.
-  template <typename... Args>
-  std::pair<iterator, bool> insert_hint_unique(iterator position,
-                                               const key_type &key,
-                                               Args &&... args);
+  template <typename K, typename... Args>
+  std::pair<iterator, bool> insert_hint_unique(iterator position, const K &key,
+                                               Args &&...args);
 
   // Insert a range of values into the btree.
+  // Note: the first overload avoids constructing a value_type if the key
+  // already exists in the btree.
+  template <typename InputIterator,
+            typename = decltype(std::declval<const key_compare &>()(
+                params_type::key(*std::declval<InputIterator>()),
+                std::declval<const key_type &>()))>
+  void insert_iterator_unique(InputIterator b, InputIterator e, int);
+  // We need the second overload for cases in which we need to construct a
+  // value_type in order to compare it with the keys already in the btree.
   template <typename InputIterator>
-  void insert_iterator_unique(InputIterator b, InputIterator e);
+  void insert_iterator_unique(InputIterator b, InputIterator e, char);
 
   // Inserts a value into the btree.
   template <typename ValueType>
@@ -1149,7 +1556,8 @@ class btree {
 
   // Insert a range of values into the btree.
   template <typename InputIterator>
-  void insert_iterator_multi(InputIterator b, InputIterator e);
+  void insert_iterator_multi(InputIterator b,
+                             InputIterator e);
 
   // Erase the specified iterator from the btree. The iterator must be valid
   // (i.e. not equal to end()).  Return an iterator pointing to the node after
@@ -1159,20 +1567,10 @@ class btree {
 
   // Erases range. Returns the number of keys erased and an iterator pointing
   // to the element after the last erased element.
-  std::pair<size_type, iterator> erase(iterator begin, iterator end);
-
-  // Erases the specified key from the btree. Returns 1 if an element was
-  // erased and 0 otherwise.
-  template <typename K>
-  size_type erase_unique(const K &key);
-
-  // Erases all of the entries matching the specified key from the
-  // btree. Returns the number of elements erased.
-  template <typename K>
-  size_type erase_multi(const K &key);
+  std::pair<size_type, iterator> erase_range(iterator begin, iterator end);
 
-  // Finds the iterator corresponding to a key or returns end() if the key is
-  // not present.
+  // Finds an element with key equivalent to `key` or returns `end()` if `key`
+  // is not present.
   template <typename K>
   iterator find(const K &key) {
     return internal_end(internal_find(key));
@@ -1182,38 +1580,23 @@ class btree {
     return internal_end(internal_find(key));
   }
 
-  // Returns a count of the number of times the key appears in the btree.
-  template <typename K>
-  size_type count_unique(const K &key) const {
-    const iterator begin = internal_find(key);
-    if (begin.node == nullptr) {
-      // The key doesn't exist in the tree.
-      return 0;
-    }
-    return 1;
-  }
-  // Returns a count of the number of times the key appears in the btree.
-  template <typename K>
-  size_type count_multi(const K &key) const {
-    const auto range = equal_range(key);
-    return std::distance(range.first, range.second);
-  }
-
   // Clear the btree, deleting all of the values it contains.
   void clear();
 
-  // Swap the contents of *this and x.
-  void swap(btree &x);
+  // Swaps the contents of `this` and `other`.
+  void swap(btree &other);
 
   const key_compare &key_comp() const noexcept {
-    return root_.template get<0>();
+    return rightmost_.template get<0>();
   }
-  template <typename K, typename LK>
-  bool compare_keys(const K &x, const LK &y) const {
-    return compare_internal::compare_result_as_less_than(key_comp()(x, y));
+  template <typename K1, typename K2>
+  bool compare_keys(const K1 &a, const K2 &b) const {
+    return compare_internal::compare_result_as_less_than(key_comp()(a, b));
   }
 
-  value_compare value_comp() const { return value_compare(key_comp()); }
+  value_compare value_comp() const {
+    return value_compare(original_key_compare(key_comp()));
+  }
 
   // Verifies the structure of the btree.
   void verify() const;
@@ -1241,9 +1624,7 @@ class btree {
   }
 
   // The number of internal, leaf and total nodes used by the btree.
-  size_type leaf_nodes() const {
-    return internal_stats(root()).leaf_nodes;
-  }
+  size_type leaf_nodes() const { return internal_stats(root()).leaf_nodes; }
   size_type internal_nodes() const {
     return internal_stats(root()).internal_nodes;
   }
@@ -1253,24 +1634,24 @@ class btree {
   }
 
   // The total number of bytes used by the btree.
+  // TODO(b/169338300): update to support node_btree_*.
   size_type bytes_used() const {
     node_stats stats = internal_stats(root());
     if (stats.leaf_nodes == 1 && stats.internal_nodes == 0) {
-      return sizeof(*this) +
-             node_type::LeafSize(root()->max_count());
+      return sizeof(*this) + node_type::LeafSize(root()->max_count());
     } else {
-      return sizeof(*this) +
-             stats.leaf_nodes * node_type::LeafSize() +
+      return sizeof(*this) + stats.leaf_nodes * node_type::LeafSize() +
              stats.internal_nodes * node_type::InternalSize();
     }
   }
 
-  // The average number of bytes used per value stored in the btree.
+  // The average number of bytes used per value stored in the btree assuming
+  // random insertion order.
   static double average_bytes_per_value() {
-    // Returns the number of bytes per value on a leaf node that is 75%
-    // full. Experimentally, this matches up nicely with the computed number of
-    // bytes per value in trees that had their values inserted in random order.
-    return node_type::LeafSize() / (kNodeValues * 0.75);
+    // The expected number of values per node with random insertion order is the
+    // average of the maximum and minimum numbers of values per node.
+    const double expected_values_per_node = (kNodeSlots + kMinNodeValues) / 2.0;
+    return node_type::LeafSize() / expected_values_per_node;
   }
 
   // The fullness of the btree. Computed as the number of elements in the btree
@@ -1280,7 +1661,7 @@ class btree {
   // Returns 0 for empty trees.
   double fullness() const {
     if (empty()) return 0.0;
-    return static_cast<double>(size()) / (nodes() * kNodeValues);
+    return static_cast<double>(size()) / (nodes() * kNodeSlots);
   }
   // The overhead of the btree structure in bytes per node. Computed as the
   // total number of bytes used by the btree minus the number of bytes used for
@@ -1293,16 +1674,23 @@ class btree {
   }
 
   // The allocator used by the btree.
-  allocator_type get_allocator() const {
-    return allocator();
-  }
+  allocator_type get_allocator() const { return allocator(); }
 
  private:
+  friend struct btree_access;
+
   // Internal accessor routines.
-  node_type *root() { return root_.template get<2>(); }
-  const node_type *root() const { return root_.template get<2>(); }
-  node_type *&mutable_root() noexcept { return root_.template get<2>(); }
-  key_compare *mutable_key_comp() noexcept { return &root_.template get<0>(); }
+  node_type *root() { return root_; }
+  const node_type *root() const { return root_; }
+  node_type *&mutable_root() noexcept { return root_; }
+  node_type *rightmost() { return rightmost_.template get<2>(); }
+  const node_type *rightmost() const { return rightmost_.template get<2>(); }
+  node_type *&mutable_rightmost() noexcept {
+    return rightmost_.template get<2>();
+  }
+  key_compare *mutable_key_comp() noexcept {
+    return &rightmost_.template get<0>();
+  }
 
   // The leftmost node is stored as the parent of the root node.
   node_type *leftmost() { return root()->parent(); }
@@ -1310,54 +1698,40 @@ class btree {
 
   // Allocator routines.
   allocator_type *mutable_allocator() noexcept {
-    return &root_.template get<1>();
+    return &rightmost_.template get<1>();
   }
   const allocator_type &allocator() const noexcept {
-    return root_.template get<1>();
+    return rightmost_.template get<1>();
   }
 
   // Allocates a correctly aligned node of at least size bytes using the
   // allocator.
-  node_type *allocate(const size_type size) {
+  node_type *allocate(size_type size) {
     return reinterpret_cast<node_type *>(
         absl::container_internal::Allocate<node_type::Alignment()>(
             mutable_allocator(), size));
   }
 
   // Node creation/deletion routines.
-  node_type* new_internal_node(node_type *parent) {
-    node_type *p = allocate(node_type::InternalSize());
-    return node_type::init_internal(p, parent);
+  node_type *new_internal_node(field_type position, node_type *parent) {
+    node_type *n = allocate(node_type::InternalSize());
+    n->init_internal(position, parent);
+    return n;
   }
-  node_type* new_leaf_node(node_type *parent) {
-    node_type *p = allocate(node_type::LeafSize());
-    return node_type::init_leaf(p, parent, kNodeValues);
+  node_type *new_leaf_node(field_type position, node_type *parent) {
+    node_type *n = allocate(node_type::LeafSize());
+    n->init_leaf(position, kNodeSlots, parent);
+    return n;
   }
-  node_type *new_leaf_root_node(const int max_count) {
-    node_type *p = allocate(node_type::LeafSize(max_count));
-    return node_type::init_leaf(p, p, max_count);
+  node_type *new_leaf_root_node(field_type max_count) {
+    node_type *n = allocate(node_type::LeafSize(max_count));
+    n->init_leaf(/*position=*/0, max_count, /*parent=*/n);
+    return n;
   }
 
   // Deletion helper routines.
-  void erase_same_node(iterator begin, iterator end);
-  iterator erase_from_leaf_node(iterator begin, size_type to_erase);
   iterator rebalance_after_delete(iterator iter);
 
-  // Deallocates a node of a certain size in bytes using the allocator.
-  void deallocate(const size_type size, node_type *node) {
-    absl::container_internal::Deallocate<node_type::Alignment()>(
-        mutable_allocator(), node, size);
-  }
-
-  void delete_internal_node(node_type *node) {
-    node->destroy(mutable_allocator());
-    deallocate(node_type::InternalSize(), node);
-  }
-  void delete_leaf_node(node_type *node) {
-    node->destroy(mutable_allocator());
-    deallocate(node_type::LeafSize(node->max_count()), node);
-  }
-
   // Rebalances or splits the node iter points to.
   void rebalance_or_split(iterator *iter);
 
@@ -1375,48 +1749,38 @@ class btree {
   void try_shrink();
 
   iterator internal_end(iterator iter) {
-    return iter.node != nullptr ? iter : end();
+    return iter.node_ != nullptr ? iter : end();
   }
   const_iterator internal_end(const_iterator iter) const {
-    return iter.node != nullptr ? iter : end();
+    return iter.node_ != nullptr ? iter : end();
   }
 
   // Emplaces a value into the btree immediately before iter. Requires that
   // key(v) <= iter.key() and (--iter).key() <= key(v).
   template <typename... Args>
-  iterator internal_emplace(iterator iter, Args &&... args);
+  iterator internal_emplace(iterator iter, Args &&...args);
 
   // Returns an iterator pointing to the first value >= the value "iter" is
-  // pointing at. Note that "iter" might be pointing to an invalid location as
-  // iter.position == iter.node->count(). This routine simply moves iter up in
-  // the tree to a valid location.
-  // Requires: iter.node is non-null.
+  // pointing at. Note that "iter" might be pointing to an invalid location such
+  // as iter.position_ == iter.node_->finish(). This routine simply moves iter
+  // up in the tree to a valid location. Requires: iter.node_ is non-null.
   template <typename IterType>
   static IterType internal_last(IterType iter);
 
   // Returns an iterator pointing to the leaf position at which key would
-  // reside in the tree. We provide 2 versions of internal_locate. The first
-  // version uses a less-than comparator and is incapable of distinguishing when
-  // there is an exact match. The second version is for the key-compare-to
-  // specialization and distinguishes exact matches. The key-compare-to
-  // specialization allows the caller to avoid a subsequent comparison to
-  // determine if an exact match was made, which is important for keys with
-  // expensive comparison, such as strings.
+  // reside in the tree, unless there is an exact match - in which case, the
+  // result may not be on a leaf. When there's a three-way comparator, we can
+  // return whether there was an exact match. This allows the caller to avoid a
+  // subsequent comparison to determine if an exact match was made, which is
+  // important for keys with expensive comparison, such as strings.
   template <typename K>
   SearchResult<iterator, is_key_compare_to::value> internal_locate(
       const K &key) const;
 
-  template <typename K>
-  SearchResult<iterator, false> internal_locate_impl(
-      const K &key, std::false_type /* IsCompareTo */) const;
-
-  template <typename K>
-  SearchResult<iterator, true> internal_locate_impl(
-      const K &key, std::true_type /* IsCompareTo */) const;
-
   // Internal routine which implements lower_bound().
   template <typename K>
-  iterator internal_lower_bound(const K &key) const;
+  SearchResult<iterator, is_key_compare_to::value> internal_lower_bound(
+      const K &key) const;
 
   // Internal routine which implements upper_bound().
   template <typename K>
@@ -1426,44 +1790,33 @@ class btree {
   template <typename K>
   iterator internal_find(const K &key) const;
 
-  // Deletes a node and all of its children.
-  void internal_clear(node_type *node);
-
   // Verifies the tree structure of node.
-  int internal_verify(const node_type *node,
-                      const key_type *lo, const key_type *hi) const;
+  size_type internal_verify(const node_type *node, const key_type *lo,
+                            const key_type *hi) const;
 
   node_stats internal_stats(const node_type *node) const {
     // The root can be a static empty node.
     if (node == nullptr || (node == root() && empty())) {
       return node_stats(0, 0);
     }
-    if (node->leaf()) {
+    if (node->is_leaf()) {
       return node_stats(1, 0);
     }
     node_stats res(0, 1);
-    for (int i = 0; i <= node->count(); ++i) {
+    for (int i = node->start(); i <= node->finish(); ++i) {
       res += internal_stats(node->child(i));
     }
     return res;
   }
 
- public:
-  // Exposed only for tests.
-  static bool testonly_uses_linear_node_search() {
-    return node_type::testonly_uses_linear_node_search();
-  }
+  node_type *root_;
 
- private:
-  // We use compressed tuple in order to save space because key_compare and
-  // allocator_type are usually empty.
+  // A pointer to the rightmost node. Note that the leftmost node is stored as
+  // the root's parent. We use compressed tuple in order to save space because
+  // key_compare and allocator_type are usually empty.
   absl::container_internal::CompressedTuple<key_compare, allocator_type,
                                             node_type *>
-      root_;
-
-  // A pointer to the rightmost node. Note that the leftmost node is stored as
-  // the root's parent.
-  node_type *rightmost_;
+      rightmost_;
 
   // Number of values.
   size_type size_;
@@ -1473,23 +1826,22 @@ class btree {
 // btree_node methods
 template <typename P>
 template <typename... Args>
-inline void btree_node<P>::emplace_value(const size_type i,
+inline void btree_node<P>::emplace_value(const field_type i,
                                          allocator_type *alloc,
-                                         Args &&... args) {
-  assert(i <= count());
+                                         Args &&...args) {
+  assert(i >= start());
+  assert(i <= finish());
   // Shift old values to create space for new value and then construct it in
   // place.
-  if (i < count()) {
-    value_init(count(), alloc, slot(count() - 1));
-    for (size_type j = count() - 1; j > i; --j)
-      params_type::move(alloc, slot(j - 1), slot(j));
-    value_destroy(i, alloc);
+  if (i < finish()) {
+    transfer_n_backward(finish() - i, /*dest_i=*/i + 1, /*src_i=*/i, this,
+                        alloc);
   }
-  value_init(i, alloc, std::forward<Args>(args)...);
-  set_count(count() + 1);
+  value_init(static_cast<field_type>(i), alloc, std::forward<Args>(args)...);
+  set_finish(finish() + 1);
 
-  if (!leaf() && count() > i + 1) {
-    for (int j = count(); j > i + 1; --j) {
+  if (is_internal() && finish() > i + 1) {
+    for (field_type j = finish(); j > i + 1; --j) {
       set_child(j, child(j - 1));
     }
     clear_child(i + 1);
@@ -1497,28 +1849,31 @@ inline void btree_node<P>::emplace_value(const size_type i,
 }
 
 template <typename P>
-inline void btree_node<P>::remove_value(const int i, allocator_type *alloc) {
-  if (!leaf() && count() > i + 1) {
-    assert(child(i + 1)->count() == 0);
-    for (size_type j = i + 1; j < count(); ++j) {
-      set_child(j, child(j + 1));
+inline void btree_node<P>::remove_values(const field_type i,
+                                         const field_type to_erase,
+                                         allocator_type *alloc) {
+  // Transfer values after the removed range into their new places.
+  value_destroy_n(i, to_erase, alloc);
+  const field_type orig_finish = finish();
+  const field_type src_i = i + to_erase;
+  transfer_n(orig_finish - src_i, i, src_i, this, alloc);
+
+  if (is_internal()) {
+    // Delete all children between begin and end.
+    for (field_type j = 0; j < to_erase; ++j) {
+      clear_and_delete(child(i + j + 1), alloc);
+    }
+    // Rotate children after end into new positions.
+    for (field_type j = i + to_erase + 1; j <= orig_finish; ++j) {
+      set_child(j - to_erase, child(j));
+      clear_child(j);
     }
-    clear_child(count());
   }
-
-  remove_values_ignore_children(i, /*to_erase=*/1, alloc);
+  set_finish(orig_finish - to_erase);
 }
 
 template <typename P>
-inline void btree_node<P>::remove_values_ignore_children(
-    const int i, const int to_erase, allocator_type *alloc) {
-  params_type::move(alloc, slot(i + to_erase), slot(count()), slot(i));
-  value_destroy_n(count() - to_erase, to_erase, alloc);
-  set_count(count() - to_erase);
-}
-
-template <typename P>
-void btree_node<P>::rebalance_right_to_left(const int to_move,
+void btree_node<P>::rebalance_right_to_left(field_type to_move,
                                             btree_node *right,
                                             allocator_type *alloc) {
   assert(parent() == right->parent());
@@ -1528,41 +1883,37 @@ void btree_node<P>::rebalance_right_to_left(const int to_move,
   assert(to_move <= right->count());
 
   // 1) Move the delimiting value in the parent to the left node.
-  value_init(count(), alloc, parent()->slot(position()));
+  transfer(finish(), position(), parent(), alloc);
 
   // 2) Move the (to_move - 1) values from the right node to the left node.
-  right->uninitialized_move_n(to_move - 1, 0, count() + 1, this, alloc);
+  transfer_n(to_move - 1, finish() + 1, right->start(), right, alloc);
 
   // 3) Move the new delimiting value to the parent from the right node.
-  params_type::move(alloc, right->slot(to_move - 1),
-                    parent()->slot(position()));
-
-  // 4) Shift the values in the right node to their correct position.
-  params_type::move(alloc, right->slot(to_move), right->slot(right->count()),
-                    right->slot(0));
+  parent()->transfer(position(), right->start() + to_move - 1, right, alloc);
 
-  // 5) Destroy the now-empty to_move entries in the right node.
-  right->value_destroy_n(right->count() - to_move, to_move, alloc);
+  // 4) Shift the values in the right node to their correct positions.
+  right->transfer_n(right->count() - to_move, right->start(),
+                    right->start() + to_move, right, alloc);
 
-  if (!leaf()) {
+  if (is_internal()) {
     // Move the child pointers from the right to the left node.
-    for (int i = 0; i < to_move; ++i) {
-      init_child(count() + i + 1, right->child(i));
+    for (field_type i = 0; i < to_move; ++i) {
+      init_child(finish() + i + 1, right->child(i));
     }
-    for (int i = 0; i <= right->count() - to_move; ++i) {
+    for (field_type i = right->start(); i <= right->finish() - to_move; ++i) {
       assert(i + to_move <= right->max_count());
       right->init_child(i, right->child(i + to_move));
       right->clear_child(i + to_move);
     }
   }
 
-  // Fixup the counts on the left and right nodes.
-  set_count(count() + to_move);
-  right->set_count(right->count() - to_move);
+  // Fixup `finish` on the left and right nodes.
+  set_finish(finish() + to_move);
+  right->set_finish(right->finish() - to_move);
 }
 
 template <typename P>
-void btree_node<P>::rebalance_left_to_right(const int to_move,
+void btree_node<P>::rebalance_left_to_right(field_type to_move,
                                             btree_node *right,
                                             allocator_type *alloc) {
   assert(parent() == right->parent());
@@ -1577,106 +1928,74 @@ void btree_node<P>::rebalance_left_to_right(const int to_move,
   // Lastly, a new delimiting value is moved from the left node into the
   // parent, and the remaining empty left node entries are destroyed.
 
-  if (right->count() >= to_move) {
-    // The original location of the right->count() values are sufficient to hold
-    // the new to_move entries from the parent and left node.
+  // 1) Shift existing values in the right node to their correct positions.
+  right->transfer_n_backward(right->count(), right->start() + to_move,
+                             right->start(), right, alloc);
 
-    // 1) Shift existing values in the right node to their correct positions.
-    right->uninitialized_move_n(to_move, right->count() - to_move,
-                                right->count(), right, alloc);
-    for (slot_type *src = right->slot(right->count() - to_move - 1),
-                   *dest = right->slot(right->count() - 1),
-                   *end = right->slot(0);
-         src >= end; --src, --dest) {
-      params_type::move(alloc, src, dest);
-    }
-
-    // 2) Move the delimiting value in the parent to the right node.
-    params_type::move(alloc, parent()->slot(position()),
-                      right->slot(to_move - 1));
-
-    // 3) Move the (to_move - 1) values from the left node to the right node.
-    params_type::move(alloc, slot(count() - (to_move - 1)), slot(count()),
-                      right->slot(0));
-  } else {
-    // The right node does not have enough initialized space to hold the new
-    // to_move entries, so part of them will move to uninitialized space.
+  // 2) Move the delimiting value in the parent to the right node.
+  right->transfer(right->start() + to_move - 1, position(), parent(), alloc);
 
-    // 1) Shift existing values in the right node to their correct positions.
-    right->uninitialized_move_n(right->count(), 0, to_move, right, alloc);
-
-    // 2) Move the delimiting value in the parent to the right node.
-    right->value_init(to_move - 1, alloc, parent()->slot(position()));
-
-    // 3) Move the (to_move - 1) values from the left node to the right node.
-    const size_type uninitialized_remaining = to_move - right->count() - 1;
-    uninitialized_move_n(uninitialized_remaining,
-                         count() - uninitialized_remaining, right->count(),
-                         right, alloc);
-    params_type::move(alloc, slot(count() - (to_move - 1)),
-                      slot(count() - uninitialized_remaining), right->slot(0));
-  }
+  // 3) Move the (to_move - 1) values from the left node to the right node.
+  right->transfer_n(to_move - 1, right->start(), finish() - (to_move - 1), this,
+                    alloc);
 
   // 4) Move the new delimiting value to the parent from the left node.
-  params_type::move(alloc, slot(count() - to_move), parent()->slot(position()));
+  parent()->transfer(position(), finish() - to_move, this, alloc);
 
-  // 5) Destroy the now-empty to_move entries in the left node.
-  value_destroy_n(count() - to_move, to_move, alloc);
-
-  if (!leaf()) {
+  if (is_internal()) {
     // Move the child pointers from the left to the right node.
-    for (int i = right->count(); i >= 0; --i) {
-      right->init_child(i + to_move, right->child(i));
-      right->clear_child(i);
+    for (field_type i = right->finish() + 1; i > right->start(); --i) {
+      right->init_child(i - 1 + to_move, right->child(i - 1));
+      right->clear_child(i - 1);
     }
-    for (int i = 1; i <= to_move; ++i) {
-      right->init_child(i - 1, child(count() - to_move + i));
-      clear_child(count() - to_move + i);
+    for (field_type i = 1; i <= to_move; ++i) {
+      right->init_child(i - 1, child(finish() - to_move + i));
+      clear_child(finish() - to_move + i);
     }
   }
 
   // Fixup the counts on the left and right nodes.
-  set_count(count() - to_move);
-  right->set_count(right->count() + to_move);
+  set_finish(finish() - to_move);
+  right->set_finish(right->finish() + to_move);
 }
 
 template <typename P>
 void btree_node<P>::split(const int insert_position, btree_node *dest,
                           allocator_type *alloc) {
   assert(dest->count() == 0);
-  assert(max_count() == kNodeValues);
+  assert(max_count() == kNodeSlots);
+  assert(position() + 1 == dest->position());
+  assert(parent() == dest->parent());
 
   // We bias the split based on the position being inserted. If we're
   // inserting at the beginning of the left node then bias the split to put
   // more values on the right node. If we're inserting at the end of the
   // right node then bias the split to put more values on the left node.
-  if (insert_position == 0) {
-    dest->set_count(count() - 1);
-  } else if (insert_position == kNodeValues) {
-    dest->set_count(0);
+  if (insert_position == start()) {
+    dest->set_finish(dest->start() + finish() - 1);
+  } else if (insert_position == kNodeSlots) {
+    dest->set_finish(dest->start());
   } else {
-    dest->set_count(count() / 2);
+    dest->set_finish(dest->start() + count() / 2);
   }
-  set_count(count() - dest->count());
+  set_finish(finish() - dest->count());
   assert(count() >= 1);
 
   // Move values from the left sibling to the right sibling.
-  uninitialized_move_n(dest->count(), count(), 0, dest, alloc);
-
-  // Destroy the now-empty entries in the left node.
-  value_destroy_n(count(), dest->count(), alloc);
+  dest->transfer_n(dest->count(), dest->start(), finish(), this, alloc);
 
   // The split key is the largest value in the left sibling.
-  set_count(count() - 1);
-  parent()->emplace_value(position(), alloc, slot(count()));
-  value_destroy(count(), alloc);
-  parent()->init_child(position() + 1, dest);
+  --mutable_finish();
+  parent()->emplace_value(position(), alloc, finish_slot());
+  value_destroy(finish(), alloc);
+  parent()->set_child_noupdate_position(position() + 1, dest);
 
-  if (!leaf()) {
-    for (int i = 0; i <= dest->count(); ++i) {
-      assert(child(count() + i + 1) != nullptr);
-      dest->init_child(i, child(count() + i + 1));
-      clear_child(count() + i + 1);
+  if (is_internal()) {
+    for (field_type i = dest->start(), j = finish() + 1; i <= dest->finish();
+         ++i, ++j) {
+      assert(child(j) != nullptr);
+      dest->init_child(i, child(j));
+      clear_child(j);
     }
   }
 }
@@ -1687,172 +2006,327 @@ void btree_node<P>::merge(btree_node *src, allocator_type *alloc) {
   assert(position() + 1 == src->position());
 
   // Move the delimiting value to the left node.
-  value_init(count(), alloc, parent()->slot(position()));
+  value_init(finish(), alloc, parent()->slot(position()));
 
   // Move the values from the right to the left node.
-  src->uninitialized_move_n(src->count(), 0, count() + 1, this, alloc);
+  transfer_n(src->count(), finish() + 1, src->start(), src, alloc);
 
-  // Destroy the now-empty entries in the right node.
-  src->value_destroy_n(0, src->count(), alloc);
-
-  if (!leaf()) {
+  if (is_internal()) {
     // Move the child pointers from the right to the left node.
-    for (int i = 0; i <= src->count(); ++i) {
-      init_child(count() + i + 1, src->child(i));
+    for (field_type i = src->start(), j = finish() + 1; i <= src->finish();
+         ++i, ++j) {
+      init_child(j, src->child(i));
       src->clear_child(i);
     }
   }
 
-  // Fixup the counts on the src and dest nodes.
-  set_count(1 + count() + src->count());
-  src->set_count(0);
+  // Fixup `finish` on the src and dest nodes.
+  set_finish(start() + 1 + count() + src->count());
+  src->set_finish(src->start());
 
-  // Remove the value on the parent node.
-  parent()->remove_value(position(), alloc);
+  // Remove the value on the parent node and delete the src node.
+  parent()->remove_values(position(), /*to_erase=*/1, alloc);
 }
 
 template <typename P>
-void btree_node<P>::swap(btree_node *x, allocator_type *alloc) {
-  using std::swap;
-  assert(leaf() == x->leaf());
-
-  // Determine which is the smaller/larger node.
-  btree_node *smaller = this, *larger = x;
-  if (smaller->count() > larger->count()) {
-    swap(smaller, larger);
+void btree_node<P>::clear_and_delete(btree_node *node, allocator_type *alloc) {
+  if (node->is_leaf()) {
+    node->value_destroy_n(node->start(), node->count(), alloc);
+    deallocate(LeafSize(node->max_count()), node, alloc);
+    return;
   }
-
-  // Swap the values.
-  for (slot_type *a = smaller->slot(0), *b = larger->slot(0),
-                 *end = a + smaller->count();
-       a != end; ++a, ++b) {
-    params_type::swap(alloc, a, b);
+  if (node->count() == 0) {
+    deallocate(InternalSize(), node, alloc);
+    return;
   }
 
-  // Move values that can't be swapped.
-  const size_type to_move = larger->count() - smaller->count();
-  larger->uninitialized_move_n(to_move, smaller->count(), smaller->count(),
-                               smaller, alloc);
-  larger->value_destroy_n(smaller->count(), to_move, alloc);
+  // The parent of the root of the subtree we are deleting.
+  btree_node *delete_root_parent = node->parent();
 
-  if (!leaf()) {
-    // Swap the child pointers.
-    std::swap_ranges(&smaller->mutable_child(0),
-                     &smaller->mutable_child(smaller->count() + 1),
-                     &larger->mutable_child(0));
-    // Update swapped children's parent pointers.
-    int i = 0;
-    for (; i <= smaller->count(); ++i) {
-      smaller->child(i)->set_parent(smaller);
-      larger->child(i)->set_parent(larger);
-    }
-    // Move the child pointers that couldn't be swapped.
-    for (; i <= larger->count(); ++i) {
-      smaller->init_child(i, larger->child(i));
-      larger->clear_child(i);
-    }
+  // Navigate to the leftmost leaf under node, and then delete upwards.
+  while (node->is_internal()) node = node->start_child();
+#ifdef ABSL_BTREE_ENABLE_GENERATIONS
+  // When generations are enabled, we delete the leftmost leaf last in case it's
+  // the parent of the root and we need to check whether it's a leaf before we
+  // can update the root's generation.
+  // TODO(ezb): if we change btree_node::is_root to check a bool inside the node
+  // instead of checking whether the parent is a leaf, we can remove this logic.
+  btree_node *leftmost_leaf = node;
+#endif
+  // Use `size_type` because `pos` needs to be able to hold `kNodeSlots+1`,
+  // which isn't guaranteed to be a valid `field_type`.
+  size_type pos = node->position();
+  btree_node *parent = node->parent();
+  for (;;) {
+    // In each iteration of the next loop, we delete one leaf node and go right.
+    assert(pos <= parent->finish());
+    do {
+      node = parent->child(static_cast<field_type>(pos));
+      if (node->is_internal()) {
+        // Navigate to the leftmost leaf under node.
+        while (node->is_internal()) node = node->start_child();
+        pos = node->position();
+        parent = node->parent();
+      }
+      node->value_destroy_n(node->start(), node->count(), alloc);
+#ifdef ABSL_BTREE_ENABLE_GENERATIONS
+      if (leftmost_leaf != node)
+#endif
+        deallocate(LeafSize(node->max_count()), node, alloc);
+      ++pos;
+    } while (pos <= parent->finish());
+
+    // Once we've deleted all children of parent, delete parent and go up/right.
+    assert(pos > parent->finish());
+    do {
+      node = parent;
+      pos = node->position();
+      parent = node->parent();
+      node->value_destroy_n(node->start(), node->count(), alloc);
+      deallocate(InternalSize(), node, alloc);
+      if (parent == delete_root_parent) {
+#ifdef ABSL_BTREE_ENABLE_GENERATIONS
+        deallocate(LeafSize(leftmost_leaf->max_count()), leftmost_leaf, alloc);
+#endif
+        return;
+      }
+      ++pos;
+    } while (pos > parent->finish());
   }
-
-  // Swap the counts.
-  swap(mutable_count(), x->mutable_count());
 }
 
 ////
 // btree_iterator methods
+
+// Note: the implementation here is based on btree_node::clear_and_delete.
+template <typename N, typename R, typename P>
+auto btree_iterator<N, R, P>::distance_slow(const_iterator other) const
+    -> difference_type {
+  const_iterator begin = other;
+  const_iterator end = *this;
+  assert(begin.node_ != end.node_ || !begin.node_->is_leaf() ||
+         begin.position_ != end.position_);
+
+  const node_type *node = begin.node_;
+  // We need to compensate for double counting if begin.node_ is a leaf node.
+  difference_type count = node->is_leaf() ? -begin.position_ : 0;
+
+  // First navigate to the leftmost leaf node past begin.
+  if (node->is_internal()) {
+    ++count;
+    node = node->child(begin.position_ + 1);
+  }
+  while (node->is_internal()) node = node->start_child();
+
+  // Use `size_type` because `pos` needs to be able to hold `kNodeSlots+1`,
+  // which isn't guaranteed to be a valid `field_type`.
+  size_type pos = node->position();
+  const node_type *parent = node->parent();
+  for (;;) {
+    // In each iteration of the next loop, we count one leaf node and go right.
+    assert(pos <= parent->finish());
+    do {
+      node = parent->child(static_cast<field_type>(pos));
+      if (node->is_internal()) {
+        // Navigate to the leftmost leaf under node.
+        while (node->is_internal()) node = node->start_child();
+        pos = node->position();
+        parent = node->parent();
+      }
+      if (node == end.node_) return count + end.position_;
+      if (parent == end.node_ && pos == static_cast<size_type>(end.position_))
+        return count + node->count();
+      // +1 is for the next internal node value.
+      count += node->count() + 1;
+      ++pos;
+    } while (pos <= parent->finish());
+
+    // Once we've counted all children of parent, go up/right.
+    assert(pos > parent->finish());
+    do {
+      node = parent;
+      pos = node->position();
+      parent = node->parent();
+      // -1 because we counted the value at end and shouldn't.
+      if (parent == end.node_ && pos == static_cast<size_type>(end.position_))
+        return count - 1;
+      ++pos;
+    } while (pos > parent->finish());
+  }
+}
+
 template <typename N, typename R, typename P>
 void btree_iterator<N, R, P>::increment_slow() {
-  if (node->leaf()) {
-    assert(position >= node->count());
-    btree_iterator save(*this);
-    while (position == node->count() && !node->is_root()) {
+  N* node = node_;
+  int position = position_;
+  if (node->is_leaf()) {
+    assert(position >= node->finish());
+    while (position == node->finish() && !node->is_root()) {
       assert(node->parent()->child(node->position()) == node);
       position = node->position();
       node = node->parent();
     }
-    if (position == node->count()) {
-      *this = save;
+    // TODO(ezb): assert we aren't incrementing end() instead of handling.
+    if (position == node->finish()) {
+      return;
     }
   } else {
-    assert(position < node->count());
-    node = node->child(position + 1);
-    while (!node->leaf()) {
-      node = node->child(0);
+    assert(position < node->finish());
+    node = node->child(static_cast<field_type>(position + 1));
+    while (node->is_internal()) {
+      node = node->start_child();
     }
-    position = 0;
+    position = node->start();
   }
+  *this = {node, position};
 }
 
 template <typename N, typename R, typename P>
 void btree_iterator<N, R, P>::decrement_slow() {
-  if (node->leaf()) {
+  N* node = node_;
+  int position = position_;
+  if (node->is_leaf()) {
     assert(position <= -1);
-    btree_iterator save(*this);
-    while (position < 0 && !node->is_root()) {
+    while (position < node->start() && !node->is_root()) {
       assert(node->parent()->child(node->position()) == node);
       position = node->position() - 1;
       node = node->parent();
     }
-    if (position < 0) {
-      *this = save;
+    // TODO(ezb): assert we aren't decrementing begin() instead of handling.
+    if (position < node->start()) {
+      return;
     }
   } else {
-    assert(position >= 0);
-    node = node->child(position);
-    while (!node->leaf()) {
-      node = node->child(node->count());
+    assert(position >= node->start());
+    node = node->child(static_cast<field_type>(position));
+    while (node->is_internal()) {
+      node = node->child(node->finish());
+    }
+    position = node->finish() - 1;
+  }
+  *this = {node, position};
+}
+
+template <typename N, typename R, typename P>
+btree_iterator<N, R, P> &btree_iterator<N, R, P>::increment_n_slow(
+    difference_type n) {
+  N *node = node_;
+  int position = position_;
+  ABSL_ASSUME(n > 0);
+  while (n > 0) {
+    if (node->is_leaf()) {
+      if (position + n < node->finish()) {
+        position += n;
+        break;
+      } else {
+        n -= node->finish() - position;
+        position = node->finish();
+        btree_iterator save = {node, position};
+        while (position == node->finish() && !node->is_root()) {
+          position = node->position();
+          node = node->parent();
+        }
+        if (position == node->finish()) {
+          absl::base_internal::HardeningAssert(n == 0);
+          return *this = save;
+        }
+      }
+    } else {
+      --n;
+      assert(position < node->finish());
+      node = node->child(static_cast<field_type>(position + 1));
+      while (node->is_internal()) {
+        node = node->start_child();
+      }
+      position = node->start();
+    }
+  }
+  node_ = node;
+  position_ = position;
+  return *this;
+}
+
+template <typename N, typename R, typename P>
+btree_iterator<N, R, P> &btree_iterator<N, R, P>::decrement_n_slow(
+    difference_type n) {
+  N *node = node_;
+  int position = position_;
+  ABSL_ASSUME(n > 0);
+  while (n > 0) {
+    if (node->is_leaf()) {
+      if (position - n >= node->start()) {
+        position -= n;
+        break;
+      } else {
+        n -= 1 + position - node->start();
+        position = node->start() - 1;
+        while (position < node->start() && !node->is_root()) {
+          position = node->position() - 1;
+          node = node->parent();
+        }
+        absl::base_internal::HardeningAssertGE(position,
+                                               static_cast<int>(node->start()));
+      }
+    } else {
+      --n;
+      assert(position >= node->start());
+      node = node->child(static_cast<field_type>(position));
+      while (node->is_internal()) {
+        node = node->child(node->finish());
+      }
+      position = node->finish() - 1;
     }
-    position = node->count() - 1;
   }
+  node_ = node;
+  position_ = position;
+  return *this;
 }
 
 ////
 // btree methods
 template <typename P>
 template <typename Btree>
-void btree<P>::copy_or_move_values_in_order(Btree *x) {
-  static_assert(std::is_same<btree, Btree>::value ||
-                    std::is_same<const btree, Btree>::value,
-                "Btree type must be same or const.");
+void btree<P>::copy_or_move_values_in_order(Btree &other) {
+  static_assert(
+      std::is_same_v<btree, Btree> || std::is_same_v<const btree, Btree>,
+      "Btree type must be same or const.");
   assert(empty());
 
   // We can avoid key comparisons because we know the order of the
   // values is the same order we'll store them in.
-  auto iter = x->begin();
-  if (iter == x->end()) return;
-  insert_multi(maybe_move_from_iterator(iter));
+  auto iter = other.begin();
+  if (iter == other.end()) return;
+  insert_multi(iter.slot());
   ++iter;
-  for (; iter != x->end(); ++iter) {
+  for (; iter != other.end(); ++iter) {
     // If the btree is not empty, we can just insert the new value at the end
     // of the tree.
-    internal_emplace(end(), maybe_move_from_iterator(iter));
+    internal_emplace(end(), iter.slot());
   }
 }
 
 template <typename P>
 constexpr bool btree<P>::static_assert_validation() {
-  static_assert(std::is_nothrow_copy_constructible<key_compare>::value,
+  static_assert(std::is_nothrow_copy_constructible_v<key_compare>,
                 "Key comparison must be nothrow copy constructible");
-  static_assert(std::is_nothrow_copy_constructible<allocator_type>::value,
+  static_assert(std::is_nothrow_copy_constructible_v<allocator_type>,
                 "Allocator must be nothrow copy constructible");
-  static_assert(type_traits_internal::is_trivially_copyable<iterator>::value,
+  static_assert(std::is_trivially_copyable_v<iterator>,
                 "iterator not trivially copyable.");
 
   // Note: We assert that kTargetValues, which is computed from
   // Params::kTargetNodeSize, must fit the node_type::field_type.
   static_assert(
-      kNodeValues < (1 << (8 * sizeof(typename node_type::field_type))),
+      kNodeSlots < (1 << (8 * sizeof(typename node_type::field_type))),
       "target node size too large");
 
   // Verify that key_compare returns an absl::{weak,strong}_ordering or bool.
-  using compare_result_type =
-      absl::result_of_t<key_compare(key_type, key_type)>;
   static_assert(
-      std::is_same<compare_result_type, bool>::value ||
-          std::is_convertible<compare_result_type, absl::weak_ordering>::value,
+      compare_has_valid_result_type<key_compare, key_type>(),
       "key comparison function must return absl::{weak,strong}_ordering or "
       "bool.");
 
-  // Test the assumption made in setting kNodeValueSpace.
+  // Test the assumption made in setting kNodeSlotSpace.
   static_assert(node_type::MinimumOverhead() >= sizeof(void *) + 4,
                 "node space assumption incorrect");
 
@@ -1860,24 +2334,57 @@ constexpr bool btree<P>::static_assert_validation() {
 }
 
 template <typename P>
-btree<P>::btree(const key_compare &comp, const allocator_type &alloc)
-    : root_(comp, alloc, EmptyNode()), rightmost_(EmptyNode()), size_(0) {}
+template <typename K>
+auto btree<P>::lower_bound_equal(const K &key) const
+    -> std::pair<iterator, bool> {
+  const SearchResult<iterator, is_key_compare_to::value> res =
+      internal_lower_bound(key);
+  const iterator lower = iterator(internal_end(res.value));
+  const bool equal = res.HasMatch()
+                         ? res.IsEq()
+                         : lower != end() && !compare_keys(key, lower.key());
+  return {lower, equal};
+}
 
 template <typename P>
-btree<P>::btree(const btree &x) : btree(x.key_comp(), x.allocator()) {
-  copy_or_move_values_in_order(&x);
+template <typename K>
+auto btree<P>::equal_range(const K &key) -> std::pair<iterator, iterator> {
+  const std::pair<iterator, bool> lower_and_equal = lower_bound_equal(key);
+  const iterator lower = lower_and_equal.first;
+  if (!lower_and_equal.second) {
+    return {lower, lower};
+  }
+
+  const iterator next = std::next(lower);
+  if (!params_type::template can_have_multiple_equivalent_keys<K>()) {
+    // The next iterator after lower must point to a key greater than `key`.
+    // Note: if this assert fails, then it may indicate that the comparator does
+    // not meet the equivalence requirements for Compare
+    // (see https://en.cppreference.com/w/cpp/named_req/Compare).
+    assert(next == end() || compare_keys(key, next.key()));
+    return {lower, next};
+  }
+  // Try once more to avoid the call to upper_bound() if there's only one
+  // equivalent key. This should prevent all calls to upper_bound() in cases of
+  // unique-containers with heterogeneous comparators in which all comparison
+  // operators have the same equivalence classes.
+  if (next == end() || compare_keys(key, next.key())) return {lower, next};
+
+  // In this case, we need to call upper_bound() to avoid worst case O(N)
+  // behavior if we were to iterate over equal keys.
+  return {lower, upper_bound(key)};
 }
 
 template <typename P>
-template <typename... Args>
-auto btree<P>::insert_unique(const key_type &key, Args &&... args)
+template <typename K, typename... Args>
+auto btree<P>::insert_unique(const K &key, Args &&...args)
     -> std::pair<iterator, bool> {
   if (empty()) {
-    mutable_root() = rightmost_ = new_leaf_root_node(1);
+    mutable_root() = mutable_rightmost() = new_leaf_root_node(1);
   }
 
-  auto res = internal_locate(key);
-  iterator &iter = res.value;
+  SearchResult<iterator, is_key_compare_to::value> res = internal_locate(key);
+  iterator iter = res.value;
 
   if (res.HasMatch()) {
     if (res.IsEq()) {
@@ -1886,7 +2393,7 @@ auto btree<P>::insert_unique(const key_type &key, Args &&... args)
     }
   } else {
     iterator last = internal_last(iter);
-    if (last.node && !compare_keys(key, last.key())) {
+    if (last.node_ && !compare_keys(key, last.key())) {
       // The key already exists in the tree, do nothing.
       return {last, false};
     }
@@ -1895,14 +2402,13 @@ auto btree<P>::insert_unique(const key_type &key, Args &&... args)
 }
 
 template <typename P>
-template <typename... Args>
-inline auto btree<P>::insert_hint_unique(iterator position, const key_type &key,
-                                         Args &&... args)
+template <typename K, typename... Args>
+inline auto btree<P>::insert_hint_unique(iterator position, const K &key,
+                                         Args &&...args)
     -> std::pair<iterator, bool> {
   if (!empty()) {
     if (position == end() || compare_keys(key, position.key())) {
-      iterator prev = position;
-      if (position == begin() || compare_keys((--prev).key(), key)) {
+      if (position == begin() || compare_keys(std::prev(position).key(), key)) {
         // prev.key() < key < position.key()
         return {internal_emplace(position, std::forward<Args>(args)...), true};
       }
@@ -1921,22 +2427,34 @@ inline auto btree<P>::insert_hint_unique(iterator position, const key_type &key,
 }
 
 template <typename P>
-template <typename InputIterator>
-void btree<P>::insert_iterator_unique(InputIterator b, InputIterator e) {
+template <typename InputIterator, typename>
+void btree<P>::insert_iterator_unique(InputIterator b, InputIterator e, int) {
   for (; b != e; ++b) {
     insert_hint_unique(end(), params_type::key(*b), *b);
   }
 }
 
+template <typename P>
+template <typename InputIterator>
+void btree<P>::insert_iterator_unique(InputIterator b, InputIterator e, char) {
+  for (; b != e; ++b) {
+    // Use a node handle to manage a temp slot.
+    auto node_handle =
+        CommonAccess::Construct<node_handle_type>(get_allocator(), *b);
+    slot_type *slot = CommonAccess::GetSlot(node_handle);
+    insert_hint_unique(end(), params_type::key(slot), slot);
+  }
+}
+
 template <typename P>
 template <typename ValueType>
 auto btree<P>::insert_multi(const key_type &key, ValueType &&v) -> iterator {
   if (empty()) {
-    mutable_root() = rightmost_ = new_leaf_root_node(1);
+    mutable_root() = mutable_rightmost() = new_leaf_root_node(1);
   }
 
   iterator iter = internal_upper_bound(key);
-  if (iter.node == nullptr) {
+  if (iter.node_ == nullptr) {
     iter = end();
   }
   return internal_emplace(iter, std::forward<ValueType>(v));
@@ -1948,17 +2466,16 @@ auto btree<P>::insert_hint_multi(iterator position, ValueType &&v) -> iterator {
   if (!empty()) {
     const key_type &key = params_type::key(v);
     if (position == end() || !compare_keys(position.key(), key)) {
-      iterator prev = position;
-      if (position == begin() || !compare_keys(key, (--prev).key())) {
+      if (position == begin() ||
+          !compare_keys(key, std::prev(position).key())) {
         // prev.key() <= key <= position.key()
         return internal_emplace(position, std::forward<ValueType>(v));
       }
     } else {
-      iterator next = position;
-      ++next;
-      if (next == end() || !compare_keys(next.key(), key)) {
-        // position.key() < key <= next.key()
-        return internal_emplace(next, std::forward<ValueType>(v));
+      ++position;
+      if (position == end() || !compare_keys(position.key(), key)) {
+        // {original `position`}.key() < key < {current `position`}.key()
+        return internal_emplace(position, std::forward<ValueType>(v));
       }
     }
   }
@@ -1974,46 +2491,47 @@ void btree<P>::insert_iterator_multi(InputIterator b, InputIterator e) {
 }
 
 template <typename P>
-auto btree<P>::operator=(const btree &x) -> btree & {
-  if (this != &x) {
+auto btree<P>::operator=(const btree &other) -> btree & {
+  if (this != &other) {
     clear();
 
-    *mutable_key_comp() = x.key_comp();
-    if (absl::allocator_traits<
+    *mutable_key_comp() = other.key_comp();
+    if (std::allocator_traits<
             allocator_type>::propagate_on_container_copy_assignment::value) {
-      *mutable_allocator() = x.allocator();
+      *mutable_allocator() = other.allocator();
     }
 
-    copy_or_move_values_in_order(&x);
+    copy_or_move_values_in_order(other);
   }
   return *this;
 }
 
 template <typename P>
-auto btree<P>::operator=(btree &&x) noexcept -> btree & {
-  if (this != &x) {
+auto btree<P>::operator=(btree &&other) noexcept -> btree & {
+  if (this != &other) {
     clear();
 
     using std::swap;
-    if (absl::allocator_traits<
-            allocator_type>::propagate_on_container_copy_assignment::value) {
-      // Note: `root_` also contains the allocator and the key comparator.
-      swap(root_, x.root_);
-      swap(rightmost_, x.rightmost_);
-      swap(size_, x.size_);
+    if (std::allocator_traits<
+            allocator_type>::propagate_on_container_move_assignment::value) {
+      swap(root_, other.root_);
+      // Note: `rightmost_` also contains the allocator and the key comparator.
+      swap(rightmost_, other.rightmost_);
+      swap(size_, other.size_);
     } else {
-      if (allocator() == x.allocator()) {
-        swap(mutable_root(), x.mutable_root());
-        swap(*mutable_key_comp(), *x.mutable_key_comp());
-        swap(rightmost_, x.rightmost_);
-        swap(size_, x.size_);
+      if (allocator() == other.allocator()) {
+        swap(mutable_root(), other.mutable_root());
+        swap(*mutable_key_comp(), *other.mutable_key_comp());
+        swap(mutable_rightmost(), other.mutable_rightmost());
+        swap(size_, other.size_);
       } else {
         // We aren't allowed to propagate the allocator and the allocator is
         // different so we can't take over its memory. We must move each element
-        // individually. We need both `x` and `this` to have `x`s key comparator
-        // while moving the values so we can't swap the key comparators.
-        *mutable_key_comp() = x.key_comp();
-        copy_or_move_values_in_order(&x);
+        // individually. We need both `other` and `this` to have `other`s key
+        // comparator while moving the values so we can't swap the key
+        // comparators.
+        *mutable_key_comp() = other.key_comp();
+        copy_or_move_values_in_order(other);
       }
     }
   }
@@ -2022,23 +2540,34 @@ auto btree<P>::operator=(btree &&x) noexcept -> btree & {
 
 template <typename P>
 auto btree<P>::erase(iterator iter) -> iterator {
-  bool internal_delete = false;
-  if (!iter.node->leaf()) {
-    // Deletion of a value on an internal node. First, move the largest value
-    // from our left child here, then delete that position (in remove_value()
-    // below). We can get to the largest value from our left child by
-    // decrementing iter.
+  iter.node_->value_destroy(static_cast<field_type>(iter.position_),
+                            mutable_allocator());
+  iter.update_generation();
+
+  const bool internal_delete = iter.node_->is_internal();
+  if (internal_delete) {
+    // Deletion of a value on an internal node. First, transfer the largest
+    // value from our left child here, then erase/rebalance from that position.
+    // We can get to the largest value from our left child by decrementing iter.
     iterator internal_iter(iter);
     --iter;
-    assert(iter.node->leaf());
-    assert(!compare_keys(internal_iter.key(), iter.key()));
-    params_type::move(mutable_allocator(), iter.node->slot(iter.position),
-                      internal_iter.node->slot(internal_iter.position));
-    internal_delete = true;
-  }
-
-  // Delete the key from the leaf.
-  iter.node->remove_value(iter.position, mutable_allocator());
+    assert(iter.node_->is_leaf());
+    internal_iter.node_->transfer(
+        static_cast<size_type>(internal_iter.position_),
+        static_cast<size_type>(iter.position_), iter.node_,
+        mutable_allocator());
+  } else {
+    // Shift values after erased position in leaf. In the internal case, we
+    // don't need to do this because the leaf position is the end of the node.
+    const field_type transfer_from =
+        static_cast<field_type>(iter.position_ + 1);
+    const field_type num_to_transfer = iter.node_->finish() - transfer_from;
+    iter.node_->transfer_n(num_to_transfer,
+                           static_cast<size_type>(iter.position_),
+                           transfer_from, iter.node_, mutable_allocator());
+  }
+  // Update node finish and container size.
+  iter.node_->set_finish(iter.node_->finish() - 1);
   --size_;
 
   // We want to return the next value after the one we just erased. If we
@@ -2046,7 +2575,7 @@ auto btree<P>::erase(iterator iter) -> iterator {
   // value is ++(++iter). If we erased from a leaf node (internal_delete ==
   // false) then the next value is ++iter. Note that ++iter may point to an
   // internal node and the value in the internal node may move to a leaf node
-  // (iter.node) when rebalancing is performed at the leaf level.
+  // (iter.node_) when rebalancing is performed at the leaf level.
 
   iterator res = rebalance_after_delete(iter);
 
@@ -2063,14 +2592,14 @@ auto btree<P>::rebalance_after_delete(iterator iter) -> iterator {
   iterator res(iter);
   bool first_iteration = true;
   for (;;) {
-    if (iter.node == root()) {
+    if (iter.node_ == root()) {
       try_shrink();
       if (empty()) {
         return end();
       }
       break;
     }
-    if (iter.node->count() >= kMinNodeValues) {
+    if (iter.node_->count() >= kMinNodeValues) {
       break;
     }
     bool merged = try_merge_or_rebalance(&iter);
@@ -2083,192 +2612,140 @@ auto btree<P>::rebalance_after_delete(iterator iter) -> iterator {
     if (!merged) {
       break;
     }
-    iter.position = iter.node->position();
-    iter.node = iter.node->parent();
+    iter.position_ = iter.node_->position();
+    iter.node_ = iter.node_->parent();
   }
+  res.update_generation();
 
   // Adjust our return value. If we're pointing at the end of a node, advance
   // the iterator.
-  if (res.position == res.node->count()) {
-    res.position = res.node->count() - 1;
+  if (res.position_ == res.node_->finish()) {
+    res.position_ = res.node_->finish() - 1;
     ++res;
   }
 
   return res;
 }
 
+// Note: we tried implementing this more efficiently by erasing all of the
+// elements in [begin, end) at once and then doing rebalancing once at the end
+// (rather than interleaving deletion and rebalancing), but that adds a lot of
+// complexity, which seems to outweigh the performance win.
 template <typename P>
-auto btree<P>::erase(iterator begin, iterator end)
+auto btree<P>::erase_range(iterator begin, iterator end)
     -> std::pair<size_type, iterator> {
-  difference_type count = std::distance(begin, end);
+  size_type count = static_cast<size_type>(end - begin);
   assert(count >= 0);
 
   if (count == 0) {
     return {0, begin};
   }
 
-  if (count == size_) {
+  if (static_cast<size_type>(count) == size_) {
     clear();
     return {count, this->end()};
   }
 
-  if (begin.node == end.node) {
-    erase_same_node(begin, end);
+  if (begin.node_ == end.node_) {
+    assert(end.position_ > begin.position_);
+    begin.node_->remove_values(
+        static_cast<field_type>(begin.position_),
+        static_cast<field_type>(end.position_ - begin.position_),
+        mutable_allocator());
     size_ -= count;
     return {count, rebalance_after_delete(begin)};
   }
 
   const size_type target_size = size_ - count;
   while (size_ > target_size) {
-    if (begin.node->leaf()) {
+    if (begin.node_->is_leaf()) {
       const size_type remaining_to_erase = size_ - target_size;
-      const size_type remaining_in_node = begin.node->count() - begin.position;
-      begin = erase_from_leaf_node(
-          begin, (std::min)(remaining_to_erase, remaining_in_node));
+      const size_type remaining_in_node =
+          static_cast<size_type>(begin.node_->finish() - begin.position_);
+      const field_type to_erase = static_cast<field_type>(
+          (std::min)(remaining_to_erase, remaining_in_node));
+      begin.node_->remove_values(static_cast<field_type>(begin.position_),
+                                 to_erase, mutable_allocator());
+      size_ -= to_erase;
+      begin = rebalance_after_delete(begin);
     } else {
       begin = erase(begin);
     }
   }
+  begin.update_generation();
   return {count, begin};
 }
 
-template <typename P>
-void btree<P>::erase_same_node(iterator begin, iterator end) {
-  assert(begin.node == end.node);
-  assert(end.position > begin.position);
-
-  node_type *node = begin.node;
-  size_type to_erase = end.position - begin.position;
-  if (!node->leaf()) {
-    // Delete all children between begin and end.
-    for (size_type i = 0; i < to_erase; ++i) {
-      internal_clear(node->child(begin.position + i + 1));
-    }
-    // Rotate children after end into new positions.
-    for (size_type i = begin.position + to_erase + 1; i <= node->count(); ++i) {
-      node->set_child(i - to_erase, node->child(i));
-      node->clear_child(i);
-    }
-  }
-  node->remove_values_ignore_children(begin.position, to_erase,
-                                      mutable_allocator());
-
-  // Do not need to update rightmost_, because
-  // * either end == this->end(), and therefore node == rightmost_, and still
-  //   exists
-  // * or end != this->end(), and therefore rightmost_ hasn't been erased, since
-  //   it wasn't covered in [begin, end)
-}
-
-template <typename P>
-auto btree<P>::erase_from_leaf_node(iterator begin, size_type to_erase)
-    -> iterator {
-  node_type *node = begin.node;
-  assert(node->leaf());
-  assert(node->count() > begin.position);
-  assert(begin.position + to_erase <= node->count());
-
-  node->remove_values_ignore_children(begin.position, to_erase,
-                                      mutable_allocator());
-
-  size_ -= to_erase;
-
-  return rebalance_after_delete(begin);
-}
-
-template <typename P>
-template <typename K>
-auto btree<P>::erase_unique(const K &key) -> size_type {
-  const iterator iter = internal_find(key);
-  if (iter.node == nullptr) {
-    // The key doesn't exist in the tree, return nothing done.
-    return 0;
-  }
-  erase(iter);
-  return 1;
-}
-
-template <typename P>
-template <typename K>
-auto btree<P>::erase_multi(const K &key) -> size_type {
-  const iterator begin = internal_lower_bound(key);
-  if (begin.node == nullptr) {
-    // The key doesn't exist in the tree, return nothing done.
-    return 0;
-  }
-  // Delete all of the keys between begin and upper_bound(key).
-  const iterator end = internal_end(internal_upper_bound(key));
-  return erase(begin, end).first;
-}
-
 template <typename P>
 void btree<P>::clear() {
   if (!empty()) {
-    internal_clear(root());
+    node_type::clear_and_delete(root(), mutable_allocator());
   }
-  mutable_root() = EmptyNode();
-  rightmost_ = EmptyNode();
+  mutable_root() = mutable_rightmost() = EmptyNode();
   size_ = 0;
 }
 
 template <typename P>
-void btree<P>::swap(btree &x) {
+void btree<P>::swap(btree &other) {
   using std::swap;
-  if (absl::allocator_traits<
+  if (std::allocator_traits<
           allocator_type>::propagate_on_container_swap::value) {
-    // Note: `root_` also contains the allocator and the key comparator.
-    swap(root_, x.root_);
+    // Note: `rightmost_` also contains the allocator and the key comparator.
+    swap(rightmost_, other.rightmost_);
   } else {
     // It's undefined behavior if the allocators are unequal here.
-    assert(allocator() == x.allocator());
-    swap(mutable_root(), x.mutable_root());
-    swap(*mutable_key_comp(), *x.mutable_key_comp());
+    assert(allocator() == other.allocator());
+    swap(mutable_rightmost(), other.mutable_rightmost());
+    swap(*mutable_key_comp(), *other.mutable_key_comp());
   }
-  swap(rightmost_, x.rightmost_);
-  swap(size_, x.size_);
+  swap(mutable_root(), other.mutable_root());
+  swap(size_, other.size_);
 }
 
 template <typename P>
 void btree<P>::verify() const {
   assert(root() != nullptr);
   assert(leftmost() != nullptr);
-  assert(rightmost_ != nullptr);
+  assert(rightmost() != nullptr);
   assert(empty() || size() == internal_verify(root(), nullptr, nullptr));
-  assert(leftmost() == (++const_iterator(root(), -1)).node);
-  assert(rightmost_ == (--const_iterator(root(), root()->count())).node);
-  assert(leftmost()->leaf());
-  assert(rightmost_->leaf());
+  assert(leftmost() == (++const_iterator(root(), -1)).node_);
+  assert(rightmost() == (--const_iterator(root(), root()->finish())).node_);
+  assert(leftmost()->is_leaf());
+  assert(rightmost()->is_leaf());
 }
 
 template <typename P>
 void btree<P>::rebalance_or_split(iterator *iter) {
-  node_type *&node = iter->node;
-  int &insert_position = iter->position;
+  node_type *&node = iter->node_;
+  int &insert_position = iter->position_;
   assert(node->count() == node->max_count());
-  assert(kNodeValues == node->max_count());
+  assert(kNodeSlots == node->max_count());
 
   // First try to make room on the node by rebalancing.
   node_type *parent = node->parent();
   if (node != root()) {
-    if (node->position() > 0) {
+    if (node->position() > parent->start()) {
       // Try rebalancing with our left sibling.
       node_type *left = parent->child(node->position() - 1);
-      assert(left->max_count() == kNodeValues);
-      if (left->count() < kNodeValues) {
+      assert(left->max_count() == kNodeSlots);
+      if (left->count() < kNodeSlots) {
         // We bias rebalancing based on the position being inserted. If we're
         // inserting at the end of the right node then we bias rebalancing to
         // fill up the left node.
-        int to_move = (kNodeValues - left->count()) /
-                      (1 + (insert_position < kNodeValues));
-        to_move = (std::max)(1, to_move);
-
-        if (((insert_position - to_move) >= 0) ||
-            ((left->count() + to_move) < kNodeValues)) {
+        field_type to_move =
+            (kNodeSlots - left->count()) /
+            (1 + (static_cast<field_type>(insert_position) < kNodeSlots));
+        to_move = (std::max)(field_type{1}, to_move);
+
+        if (static_cast<field_type>(insert_position) - to_move >=
+                node->start() ||
+            left->count() + to_move < kNodeSlots) {
           left->rebalance_right_to_left(to_move, node, mutable_allocator());
 
           assert(node->max_count() - node->count() == to_move);
-          insert_position = insert_position - to_move;
-          if (insert_position < 0) {
+          insert_position = static_cast<int>(
+              static_cast<field_type>(insert_position) - to_move);
+          if (insert_position < node->start()) {
             insert_position = insert_position + left->count() + 1;
             node = left;
           }
@@ -2279,23 +2756,24 @@ void btree<P>::rebalance_or_split(iterator *iter) {
       }
     }
 
-    if (node->position() < parent->count()) {
+    if (node->position() < parent->finish()) {
       // Try rebalancing with our right sibling.
       node_type *right = parent->child(node->position() + 1);
-      assert(right->max_count() == kNodeValues);
-      if (right->count() < kNodeValues) {
+      assert(right->max_count() == kNodeSlots);
+      if (right->count() < kNodeSlots) {
         // We bias rebalancing based on the position being inserted. If we're
         // inserting at the beginning of the left node then we bias rebalancing
         // to fill up the right node.
-        int to_move =
-            (kNodeValues - right->count()) / (1 + (insert_position > 0));
-        to_move = (std::max)(1, to_move);
+        field_type to_move = (kNodeSlots - right->count()) /
+                             (1 + (insert_position > node->start()));
+        to_move = (std::max)(field_type{1}, to_move);
 
-        if ((insert_position <= (node->count() - to_move)) ||
-            ((right->count() + to_move) < kNodeValues)) {
+        if (static_cast<field_type>(insert_position) <=
+                node->finish() - to_move ||
+            right->count() + to_move < kNodeSlots) {
           node->rebalance_left_to_right(to_move, right, mutable_allocator());
 
-          if (insert_position > node->count()) {
+          if (insert_position > node->finish()) {
             insert_position = insert_position - node->count() - 1;
             node = right;
           }
@@ -2308,34 +2786,37 @@ void btree<P>::rebalance_or_split(iterator *iter) {
 
     // Rebalancing failed, make sure there is room on the parent node for a new
     // value.
-    assert(parent->max_count() == kNodeValues);
-    if (parent->count() == kNodeValues) {
-      iterator parent_iter(node->parent(), node->position());
+    assert(parent->max_count() == kNodeSlots);
+    if (parent->count() == kNodeSlots) {
+      iterator parent_iter(parent, node->position());
       rebalance_or_split(&parent_iter);
+      parent = node->parent();
     }
   } else {
     // Rebalancing not possible because this is the root node.
     // Create a new root node and set the current root node as the child of the
     // new root.
-    parent = new_internal_node(parent);
-    parent->init_child(0, root());
+    parent = new_internal_node(/*position=*/0, parent);
+    parent->set_generation(root()->generation());
+    parent->init_child(parent->start(), node);
     mutable_root() = parent;
     // If the former root was a leaf node, then it's now the rightmost node.
-    assert(!parent->child(0)->leaf() || parent->child(0) == rightmost_);
+    assert(parent->start_child()->is_internal() ||
+           parent->start_child() == rightmost());
   }
 
   // Split the node.
   node_type *split_node;
-  if (node->leaf()) {
-    split_node = new_leaf_node(parent);
+  if (node->is_leaf()) {
+    split_node = new_leaf_node(node->position() + 1, parent);
     node->split(insert_position, split_node, mutable_allocator());
-    if (rightmost_ == node) rightmost_ = split_node;
+    if (rightmost() == node) mutable_rightmost() = split_node;
   } else {
-    split_node = new_internal_node(parent);
+    split_node = new_internal_node(node->position() + 1, parent);
     node->split(insert_position, split_node, mutable_allocator());
   }
 
-  if (insert_position > node->count()) {
+  if (insert_position > node->finish()) {
     insert_position = insert_position - node->count() - 1;
     node = split_node;
   }
@@ -2344,62 +2825,57 @@ void btree<P>::rebalance_or_split(iterator *iter) {
 template <typename P>
 void btree<P>::merge_nodes(node_type *left, node_type *right) {
   left->merge(right, mutable_allocator());
-  if (right->leaf()) {
-    if (rightmost_ == right) rightmost_ = left;
-    delete_leaf_node(right);
-  } else {
-    delete_internal_node(right);
-  }
+  if (rightmost() == right) mutable_rightmost() = left;
 }
 
 template <typename P>
 bool btree<P>::try_merge_or_rebalance(iterator *iter) {
-  node_type *parent = iter->node->parent();
-  if (iter->node->position() > 0) {
+  node_type *parent = iter->node_->parent();
+  if (iter->node_->position() > parent->start()) {
     // Try merging with our left sibling.
-    node_type *left = parent->child(iter->node->position() - 1);
-    assert(left->max_count() == kNodeValues);
-    if ((1 + left->count() + iter->node->count()) <= kNodeValues) {
-      iter->position += 1 + left->count();
-      merge_nodes(left, iter->node);
-      iter->node = left;
+    node_type *left = parent->child(iter->node_->position() - 1);
+    assert(left->max_count() == kNodeSlots);
+    if (1U + left->count() + iter->node_->count() <= kNodeSlots) {
+      iter->position_ += 1 + left->count();
+      merge_nodes(left, iter->node_);
+      iter->node_ = left;
       return true;
     }
   }
-  if (iter->node->position() < parent->count()) {
+  if (iter->node_->position() < parent->finish()) {
     // Try merging with our right sibling.
-    node_type *right = parent->child(iter->node->position() + 1);
-    assert(right->max_count() == kNodeValues);
-    if ((1 + iter->node->count() + right->count()) <= kNodeValues) {
-      merge_nodes(iter->node, right);
+    node_type *right = parent->child(iter->node_->position() + 1);
+    assert(right->max_count() == kNodeSlots);
+    if (1U + iter->node_->count() + right->count() <= kNodeSlots) {
+      merge_nodes(iter->node_, right);
       return true;
     }
     // Try rebalancing with our right sibling. We don't perform rebalancing if
-    // we deleted the first element from iter->node and the node is not
+    // we deleted the first element from iter->node_ and the node is not
     // empty. This is a small optimization for the common pattern of deleting
     // from the front of the tree.
-    if ((right->count() > kMinNodeValues) &&
-        ((iter->node->count() == 0) ||
-         (iter->position > 0))) {
-      int to_move = (right->count() - iter->node->count()) / 2;
-      to_move = (std::min)(to_move, right->count() - 1);
-      iter->node->rebalance_right_to_left(to_move, right, mutable_allocator());
+    if (right->count() > kMinNodeValues &&
+        (iter->node_->count() == 0 || iter->position_ > iter->node_->start())) {
+      field_type to_move = (right->count() - iter->node_->count()) / 2;
+      to_move =
+          (std::min)(to_move, static_cast<field_type>(right->count() - 1));
+      iter->node_->rebalance_right_to_left(to_move, right, mutable_allocator());
       return false;
     }
   }
-  if (iter->node->position() > 0) {
+  if (iter->node_->position() > parent->start()) {
     // Try rebalancing with our left sibling. We don't perform rebalancing if
-    // we deleted the last element from iter->node and the node is not
+    // we deleted the last element from iter->node_ and the node is not
     // empty. This is a small optimization for the common pattern of deleting
     // from the back of the tree.
-    node_type *left = parent->child(iter->node->position() - 1);
-    if ((left->count() > kMinNodeValues) &&
-        ((iter->node->count() == 0) ||
-         (iter->position < iter->node->count()))) {
-      int to_move = (left->count() - iter->node->count()) / 2;
-      to_move = (std::min)(to_move, left->count() - 1);
-      left->rebalance_left_to_right(to_move, iter->node, mutable_allocator());
-      iter->position += to_move;
+    node_type *left = parent->child(iter->node_->position() - 1);
+    if (left->count() > kMinNodeValues &&
+        (iter->node_->count() == 0 ||
+         iter->position_ < iter->node_->finish())) {
+      field_type to_move = (left->count() - iter->node_->count()) / 2;
+      to_move = (std::min)(to_move, static_cast<field_type>(left->count() - 1));
+      left->rebalance_left_to_right(to_move, iter->node_, mutable_allocator());
+      iter->position_ += to_move;
       return false;
     }
   }
@@ -2408,68 +2884,114 @@ bool btree<P>::try_merge_or_rebalance(iterator *iter) {
 
 template <typename P>
 void btree<P>::try_shrink() {
-  if (root()->count() > 0) {
+  node_type *orig_root = root();
+  if (orig_root->count() > 0) {
     return;
   }
   // Deleted the last item on the root node, shrink the height of the tree.
-  if (root()->leaf()) {
+  if (orig_root->is_leaf()) {
     assert(size() == 0);
-    delete_leaf_node(root());
-    mutable_root() = EmptyNode();
-    rightmost_ = EmptyNode();
+    mutable_root() = mutable_rightmost() = EmptyNode();
   } else {
-    node_type *child = root()->child(0);
+    node_type *child = orig_root->start_child();
     child->make_root();
-    delete_internal_node(root());
     mutable_root() = child;
   }
+  node_type::clear_and_delete(orig_root, mutable_allocator());
 }
 
 template <typename P>
 template <typename IterType>
 inline IterType btree<P>::internal_last(IterType iter) {
-  assert(iter.node != nullptr);
-  while (iter.position == iter.node->count()) {
-    iter.position = iter.node->position();
-    iter.node = iter.node->parent();
-    if (iter.node->leaf()) {
-      iter.node = nullptr;
+  assert(iter.node_ != nullptr);
+  while (iter.position_ == iter.node_->finish()) {
+    iter.position_ = iter.node_->position();
+    iter.node_ = iter.node_->parent();
+    if (iter.node_->is_leaf()) {
+      iter.node_ = nullptr;
       break;
     }
   }
+  iter.update_generation();
   return iter;
 }
 
 template <typename P>
 template <typename... Args>
-inline auto btree<P>::internal_emplace(iterator iter, Args &&... args)
+inline auto btree<P>::internal_emplace(iterator iter, Args &&...args)
     -> iterator {
-  if (!iter.node->leaf()) {
+  if (iter.node_->is_internal()) {
     // We can't insert on an internal node. Instead, we'll insert after the
     // previous value which is guaranteed to be on a leaf node.
     --iter;
-    ++iter.position;
-  }
-  const int max_count = iter.node->max_count();
-  if (iter.node->count() == max_count) {
+    ++iter.position_;
+  }
+  const field_type max_count = iter.node_->max_count();
+  allocator_type *alloc = mutable_allocator();
+
+  const auto transfer_and_delete = [&](node_type *old_node,
+                                       node_type *new_node) {
+    new_node->transfer_n(old_node->count(), new_node->start(),
+                         old_node->start(), old_node, alloc);
+    new_node->set_finish(old_node->finish());
+    old_node->set_finish(old_node->start());
+    new_node->set_generation(old_node->generation());
+    node_type::clear_and_delete(old_node, alloc);
+  };
+  const auto replace_leaf_root_node = [&](field_type new_node_size) {
+    assert(iter.node_ == root());
+    node_type *old_root = iter.node_;
+    node_type *new_root = iter.node_ = new_leaf_root_node(new_node_size);
+    transfer_and_delete(old_root, new_root);
+    mutable_root() = mutable_rightmost() = new_root;
+  };
+
+  bool replaced_node = false;
+  if (iter.node_->count() == max_count) {
     // Make room in the leaf for the new item.
-    if (max_count < kNodeValues) {
+    if (max_count < kNodeSlots) {
       // Insertion into the root where the root is smaller than the full node
       // size. Simply grow the size of the root node.
-      assert(iter.node == root());
-      iter.node =
-          new_leaf_root_node((std::min<int>)(kNodeValues, 2 * max_count));
-      iter.node->swap(root(), mutable_allocator());
-      delete_leaf_node(root());
-      mutable_root() = iter.node;
-      rightmost_ = iter.node;
+      replace_leaf_root_node(static_cast<field_type>(
+          (std::min)(static_cast<int>(kNodeSlots), 2 * max_count)));
+      replaced_node = true;
     } else {
       rebalance_or_split(&iter);
     }
   }
-  iter.node->emplace_value(iter.position, mutable_allocator(),
-                           std::forward<Args>(args)...);
+  (void)replaced_node;
+#if defined(ABSL_HAVE_ADDRESS_SANITIZER) || \
+    defined(ABSL_HAVE_HWADDRESS_SANITIZER)
+  if (!replaced_node) {
+    assert(iter.node_->is_leaf());
+    if (iter.node_->is_root()) {
+      replace_leaf_root_node(max_count);
+    } else {
+      node_type *old_node = iter.node_;
+      const bool was_rightmost = rightmost() == old_node;
+      const bool was_leftmost = leftmost() == old_node;
+      node_type *parent = old_node->parent();
+      const field_type position = old_node->position();
+      node_type *new_node = iter.node_ = new_leaf_node(position, parent);
+      parent->set_child_noupdate_position(position, new_node);
+      transfer_and_delete(old_node, new_node);
+      if (was_rightmost) mutable_rightmost() = new_node;
+      // The leftmost node is stored as the parent of the root node.
+      if (was_leftmost) root()->set_parent(new_node);
+    }
+  }
+#endif
+  iter.node_->emplace_value(static_cast<field_type>(iter.position_), alloc,
+                            std::forward<Args>(args)...);
+  assert(
+      iter.node_->is_ordered_correctly(static_cast<field_type>(iter.position_),
+                                       original_key_compare(key_comp())) &&
+      "If this assert fails, then either (1) the comparator may violate "
+      "transitivity, i.e. comp(a,b) && comp(b,c) -> comp(a,c) (see "
+      "https://en.cppreference.com/w/cpp/named_req/Compare), or (2) a "
+      "key may have been mutated after it was inserted into the tree.");
   ++size_;
+  iter.update_generation();
   return iter;
 }
 
@@ -2477,73 +2999,63 @@ template <typename P>
 template <typename K>
 inline auto btree<P>::internal_locate(const K &key) const
     -> SearchResult<iterator, is_key_compare_to::value> {
-  return internal_locate_impl(key, is_key_compare_to());
-}
-
-template <typename P>
-template <typename K>
-inline auto btree<P>::internal_locate_impl(
-    const K &key, std::false_type /* IsCompareTo */) const
-    -> SearchResult<iterator, false> {
-  iterator iter(const_cast<node_type *>(root()), 0);
+  iterator iter(const_cast<node_type *>(root()));
   for (;;) {
-    iter.position = iter.node->lower_bound(key, key_comp()).value;
-    // NOTE: we don't need to walk all the way down the tree if the keys are
-    // equal, but determining equality would require doing an extra comparison
-    // on each node on the way down, and we will need to go all the way to the
-    // leaf node in the expected case.
-    if (iter.node->leaf()) {
-      break;
-    }
-    iter.node = iter.node->child(iter.position);
-  }
-  return {iter};
-}
-
-template <typename P>
-template <typename K>
-inline auto btree<P>::internal_locate_impl(
-    const K &key, std::true_type /* IsCompareTo */) const
-    -> SearchResult<iterator, true> {
-  iterator iter(const_cast<node_type *>(root()), 0);
-  for (;;) {
-    SearchResult<int, true> res = iter.node->lower_bound(key, key_comp());
-    iter.position = res.value;
-    if (res.match == MatchKind::kEq) {
+    SearchResult<size_type, is_key_compare_to::value> res =
+        iter.node_->lower_bound(key, key_comp());
+    iter.position_ = static_cast<int>(res.value);
+    if (res.IsEq()) {
       return {iter, MatchKind::kEq};
     }
-    if (iter.node->leaf()) {
+    // Note: in the non-key-compare-to case, we don't need to walk all the way
+    // down the tree if the keys are equal, but determining equality would
+    // require doing an extra comparison on each node on the way down, and we
+    // will need to go all the way to the leaf node in the expected case.
+    if (iter.node_->is_leaf()) {
       break;
     }
-    iter.node = iter.node->child(iter.position);
+    iter.node_ = iter.node_->child(static_cast<field_type>(iter.position_));
   }
+  // Note: in the non-key-compare-to case, the key may actually be equivalent
+  // here (and the MatchKind::kNe is ignored).
   return {iter, MatchKind::kNe};
 }
 
 template <typename P>
 template <typename K>
-auto btree<P>::internal_lower_bound(const K &key) const -> iterator {
-  iterator iter(const_cast<node_type *>(root()), 0);
+auto btree<P>::internal_lower_bound(const K &key) const
+    -> SearchResult<iterator, is_key_compare_to::value> {
+  if (!params_type::template can_have_multiple_equivalent_keys<K>()) {
+    SearchResult<iterator, is_key_compare_to::value> ret = internal_locate(key);
+    ret.value = internal_last(ret.value);
+    return ret;
+  }
+  iterator iter(const_cast<node_type *>(root()));
+  SearchResult<size_type, is_key_compare_to::value> res;
+  bool seen_eq = false;
   for (;;) {
-    iter.position = iter.node->lower_bound(key, key_comp()).value;
-    if (iter.node->leaf()) {
+    res = iter.node_->lower_bound(key, key_comp());
+    iter.position_ = static_cast<int>(res.value);
+    if (iter.node_->is_leaf()) {
       break;
     }
-    iter.node = iter.node->child(iter.position);
+    seen_eq = seen_eq || res.IsEq();
+    iter.node_ = iter.node_->child(static_cast<field_type>(iter.position_));
   }
-  return internal_last(iter);
+  if (res.IsEq()) return {iter, MatchKind::kEq};
+  return {internal_last(iter), seen_eq ? MatchKind::kEq : MatchKind::kNe};
 }
 
 template <typename P>
 template <typename K>
 auto btree<P>::internal_upper_bound(const K &key) const -> iterator {
-  iterator iter(const_cast<node_type *>(root()), 0);
+  iterator iter(const_cast<node_type *>(root()));
   for (;;) {
-    iter.position = iter.node->upper_bound(key, key_comp());
-    if (iter.node->leaf()) {
+    iter.position_ = static_cast<int>(iter.node_->upper_bound(key, key_comp()));
+    if (iter.node_->is_leaf()) {
       break;
     }
-    iter.node = iter.node->child(iter.position);
+    iter.node_ = iter.node_->child(static_cast<field_type>(iter.position_));
   }
   return internal_last(iter);
 }
@@ -2551,14 +3063,14 @@ auto btree<P>::internal_upper_bound(const K &key) const -> iterator {
 template <typename P>
 template <typename K>
 auto btree<P>::internal_find(const K &key) const -> iterator {
-  auto res = internal_locate(key);
+  SearchResult<iterator, is_key_compare_to::value> res = internal_locate(key);
   if (res.HasMatch()) {
     if (res.IsEq()) {
       return res.value;
     }
   } else {
     const iterator iter = internal_last(res.value);
-    if (iter.node != nullptr && !compare_keys(key, iter.key())) {
+    if (iter.node_ != nullptr && !compare_keys(key, iter.key())) {
       return iter;
     }
   }
@@ -2566,47 +3078,79 @@ auto btree<P>::internal_find(const K &key) const -> iterator {
 }
 
 template <typename P>
-void btree<P>::internal_clear(node_type *node) {
-  if (!node->leaf()) {
-    for (int i = 0; i <= node->count(); ++i) {
-      internal_clear(node->child(i));
-    }
-    delete_internal_node(node);
-  } else {
-    delete_leaf_node(node);
-  }
-}
-
-template <typename P>
-int btree<P>::internal_verify(
+typename btree<P>::size_type btree<P>::internal_verify(
     const node_type *node, const key_type *lo, const key_type *hi) const {
   assert(node->count() > 0);
   assert(node->count() <= node->max_count());
   if (lo) {
-    assert(!compare_keys(node->key(0), *lo));
+    assert(!compare_keys(node->key(node->start()), *lo));
   }
   if (hi) {
-    assert(!compare_keys(*hi, node->key(node->count() - 1)));
+    assert(!compare_keys(*hi, node->key(node->finish() - 1)));
   }
-  for (int i = 1; i < node->count(); ++i) {
+  for (int i = node->start() + 1; i < node->finish(); ++i) {
     assert(!compare_keys(node->key(i), node->key(i - 1)));
   }
-  int count = node->count();
-  if (!node->leaf()) {
-    for (int i = 0; i <= node->count(); ++i) {
+  size_type count = node->count();
+  if (node->is_internal()) {
+    for (field_type i = node->start(); i <= node->finish(); ++i) {
       assert(node->child(i) != nullptr);
       assert(node->child(i)->parent() == node);
       assert(node->child(i)->position() == i);
-      count += internal_verify(
-          node->child(i),
-          (i == 0) ? lo : &node->key(i - 1),
-          (i == node->count()) ? hi : &node->key(i));
+      count += internal_verify(node->child(i),
+                               i == node->start() ? lo : &node->key(i - 1),
+                               i == node->finish() ? hi : &node->key(i));
     }
   }
   return count;
 }
 
+struct btree_access {
+  template <typename BtreeContainer, typename Pred>
+  static auto erase_if(BtreeContainer &container, Pred pred) ->
+      typename BtreeContainer::size_type {
+    const auto initial_size = container.size();
+    auto &tree = container.tree_;
+    auto *alloc = tree.mutable_allocator();
+    for (auto it = container.begin(); it != container.end();) {
+      if (!pred(*it)) {
+        ++it;
+        continue;
+      }
+      auto *node = it.node_;
+      if (node->is_internal()) {
+        // Handle internal nodes normally.
+        it = container.erase(it);
+        continue;
+      }
+      // If this is a leaf node, then we do all the erases from this node
+      // at once before doing rebalancing.
+
+      // The current position to transfer slots to.
+      int to_pos = it.position_;
+      node->value_destroy(it.position_, alloc);
+      while (++it.position_ < node->finish()) {
+        it.update_generation();
+        if (pred(*it)) {
+          node->value_destroy(it.position_, alloc);
+        } else {
+          node->transfer(node->slot(to_pos++), node->slot(it.position_), alloc);
+        }
+      }
+      const int num_deleted = node->finish() - to_pos;
+      tree.size_ -= num_deleted;
+      node->set_finish(to_pos);
+      it.position_ = to_pos;
+      it = tree.rebalance_after_delete(it);
+    }
+    return initial_size - container.size();
+  }
+};
+
+#undef ABSL_BTREE_ENABLE_GENERATIONS
+
 }  // namespace container_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_INTERNAL_BTREE_H_
diff --git a/absl/container/internal/btree_container.h b/absl/container/internal/btree_container.h
index 726861d5bbb..8a4582f22aa 100644
--- a/absl/container/internal/btree_container.h
+++ b/absl/container/internal/btree_container.h
@@ -18,14 +18,20 @@
 #include <algorithm>
 #include <initializer_list>
 #include <iterator>
+#include <memory>
+#include <type_traits>
 #include <utility>
 
-#include "absl/base/internal/throw_delegate.h"
+#include "absl/base/attributes.h"
+#include "absl/base/throw_delegate.h"
 #include "absl/container/internal/btree.h"  // IWYU pragma: export
 #include "absl/container/internal/common.h"
+#include "absl/hash/internal/weakly_mixed_integer.h"
+#include "absl/memory/memory.h"
 #include "absl/meta/type_traits.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
 // A common base class for btree_set, btree_map, btree_multiset, and
@@ -41,15 +47,15 @@ class btree_container {
   // transparent case.
   template <class K>
   using key_arg =
-      typename KeyArg<IsTransparent<typename Tree::key_compare>::value>::
-          template type<K, typename Tree::key_type>;
+      typename KeyArg<params_type::kIsKeyCompareTransparent>::template type<
+          K, typename Tree::key_type>;
 
  public:
   using key_type = typename Tree::key_type;
   using value_type = typename Tree::value_type;
   using size_type = typename Tree::size_type;
   using difference_type = typename Tree::difference_type;
-  using key_compare = typename Tree::key_compare;
+  using key_compare = typename Tree::original_key_compare;
   using value_compare = typename Tree::value_compare;
   using allocator_type = typename Tree::allocator_type;
   using reference = typename Tree::reference;
@@ -62,38 +68,80 @@ class btree_container {
   using const_reverse_iterator = typename Tree::const_reverse_iterator;
   using node_type = typename Tree::node_handle_type;
 
+  struct extract_and_get_next_return_type {
+    node_type node;
+    iterator next;
+  };
+
   // Constructors/assignments.
   btree_container() : tree_(key_compare(), allocator_type()) {}
   explicit btree_container(const key_compare &comp,
                            const allocator_type &alloc = allocator_type())
       : tree_(comp, alloc) {}
-  btree_container(const btree_container &x) = default;
-  btree_container(btree_container &&x) noexcept = default;
-  btree_container &operator=(const btree_container &x) = default;
-  btree_container &operator=(btree_container &&x) noexcept(
-      std::is_nothrow_move_assignable<Tree>::value) = default;
+  explicit btree_container(const allocator_type &alloc)
+      : tree_(key_compare(), alloc) {}
+
+  btree_container(const btree_container& other)
+      : btree_container(other, std::allocator_traits<allocator_type>::
+                                   select_on_container_copy_construction(
+                                       other.get_allocator())) {}
+  btree_container(const btree_container &other, const allocator_type &alloc)
+      : tree_(other.tree_, alloc) {}
+
+  btree_container(btree_container&& other) noexcept(
+      std::is_nothrow_move_constructible_v<Tree>) = default;
+  btree_container(btree_container &&other, const allocator_type &alloc)
+      : tree_(std::move(other.tree_), alloc) {}
+
+  btree_container &operator=(const btree_container &other) = default;
+  btree_container& operator=(btree_container&& other) noexcept(
+      std::is_nothrow_move_assignable_v<Tree>) = default;
 
   // Iterator routines.
-  iterator begin() { return tree_.begin(); }
-  const_iterator begin() const { return tree_.begin(); }
-  const_iterator cbegin() const { return tree_.begin(); }
-  iterator end() { return tree_.end(); }
-  const_iterator end() const { return tree_.end(); }
-  const_iterator cend() const { return tree_.end(); }
-  reverse_iterator rbegin() { return tree_.rbegin(); }
-  const_reverse_iterator rbegin() const { return tree_.rbegin(); }
-  const_reverse_iterator crbegin() const { return tree_.rbegin(); }
-  reverse_iterator rend() { return tree_.rend(); }
-  const_reverse_iterator rend() const { return tree_.rend(); }
-  const_reverse_iterator crend() const { return tree_.rend(); }
+  iterator begin() ABSL_ATTRIBUTE_LIFETIME_BOUND { return tree_.begin(); }
+  const_iterator begin() const ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return tree_.begin();
+  }
+  const_iterator cbegin() const ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return tree_.begin();
+  }
+  iterator end() ABSL_ATTRIBUTE_LIFETIME_BOUND { return tree_.end(); }
+  const_iterator end() const ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return tree_.end();
+  }
+  const_iterator cend() const ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return tree_.end();
+  }
+  reverse_iterator rbegin() ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return tree_.rbegin();
+  }
+  const_reverse_iterator rbegin() const ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return tree_.rbegin();
+  }
+  const_reverse_iterator crbegin() const ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return tree_.rbegin();
+  }
+  reverse_iterator rend() ABSL_ATTRIBUTE_LIFETIME_BOUND { return tree_.rend(); }
+  const_reverse_iterator rend() const ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return tree_.rend();
+  }
+  const_reverse_iterator crend() const ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return tree_.rend();
+  }
 
   // Lookup routines.
   template <typename K = key_type>
-  iterator find(const key_arg<K> &key) {
+  size_type count(const key_arg<K> &key) const {
+    auto equal_range = this->equal_range(key);
+    return equal_range.second - equal_range.first;
+  }
+  template <typename K = key_type>
+  iterator find(const key_arg<K> &key) ABSL_ATTRIBUTE_LIFETIME_BOUND {
     return tree_.find(key);
   }
   template <typename K = key_type>
-  const_iterator find(const key_arg<K> &key) const {
+  const_iterator find(const key_arg<K> &key) const
+      ABSL_ATTRIBUTE_LIFETIME_BOUND {
     return tree_.find(key);
   }
   template <typename K = key_type>
@@ -101,28 +149,31 @@ class btree_container {
     return find(key) != end();
   }
   template <typename K = key_type>
-  iterator lower_bound(const key_arg<K> &key) {
+  iterator lower_bound(const key_arg<K> &key) ABSL_ATTRIBUTE_LIFETIME_BOUND {
     return tree_.lower_bound(key);
   }
   template <typename K = key_type>
-  const_iterator lower_bound(const key_arg<K> &key) const {
+  const_iterator lower_bound(const key_arg<K> &key) const
+      ABSL_ATTRIBUTE_LIFETIME_BOUND {
     return tree_.lower_bound(key);
   }
   template <typename K = key_type>
-  iterator upper_bound(const key_arg<K> &key) {
+  iterator upper_bound(const key_arg<K> &key) ABSL_ATTRIBUTE_LIFETIME_BOUND {
     return tree_.upper_bound(key);
   }
   template <typename K = key_type>
-  const_iterator upper_bound(const key_arg<K> &key) const {
+  const_iterator upper_bound(const key_arg<K> &key) const
+      ABSL_ATTRIBUTE_LIFETIME_BOUND {
     return tree_.upper_bound(key);
   }
   template <typename K = key_type>
-  std::pair<iterator, iterator> equal_range(const key_arg<K> &key) {
+  std::pair<iterator, iterator> equal_range(const key_arg<K> &key)
+      ABSL_ATTRIBUTE_LIFETIME_BOUND {
     return tree_.equal_range(key);
   }
   template <typename K = key_type>
   std::pair<const_iterator, const_iterator> equal_range(
-      const key_arg<K> &key) const {
+      const key_arg<K> &key) const ABSL_ATTRIBUTE_LIFETIME_BOUND {
     return tree_.equal_range(key);
   }
 
@@ -132,17 +183,37 @@ class btree_container {
   // Erase the specified iterator from the btree. The iterator must be valid
   // (i.e. not equal to end()).  Return an iterator pointing to the node after
   // the one that was erased (or end() if none exists).
-  iterator erase(const_iterator iter) { return tree_.erase(iterator(iter)); }
-  iterator erase(iterator iter) { return tree_.erase(iter); }
-  iterator erase(const_iterator first, const_iterator last) {
-    return tree_.erase(iterator(first), iterator(last)).second;
+  iterator erase(const_iterator iter) ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return tree_.erase(iterator(iter));
+  }
+  iterator erase(iterator iter) ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return tree_.erase(iter);
+  }
+  iterator erase(const_iterator first,
+                 const_iterator last) ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return tree_.erase_range(iterator(first), iterator(last)).second;
+  }
+  template <typename K = key_type>
+  size_type erase(const key_arg<K> &key) {
+    auto equal_range = this->equal_range(key);
+    return tree_.erase_range(equal_range.first, equal_range.second).first;
   }
 
   // Extract routines.
+  extract_and_get_next_return_type extract_and_get_next(const_iterator position)
+      ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    // Use Construct instead of Transfer because the rebalancing code will
+    // destroy the slot later.
+    // Note: we rely on erase() taking place after Construct().
+    return {CommonAccess::Construct<node_type>(get_allocator(),
+                                               iterator(position).slot()),
+            erase(position)};
+  }
   node_type extract(iterator position) {
-    // Use Move instead of Transfer, because the rebalancing code expects to
-    // have a valid object to scribble metadata bits on top of.
-    auto node = CommonAccess::Move<node_type>(get_allocator(), position.slot());
+    // Use Construct instead of Transfer because the rebalancing code will
+    // destroy the slot later.
+    auto node =
+        CommonAccess::Construct<node_type>(get_allocator(), position.slot());
     erase(position);
     return node;
   }
@@ -150,10 +221,9 @@ class btree_container {
     return extract(iterator(position));
   }
 
- public:
   // Utility routines.
-  void clear() { tree_.clear(); }
-  void swap(btree_container &x) { tree_.swap(x.tree_); }
+  ABSL_ATTRIBUTE_REINITIALIZES void clear() { tree_.clear(); }
+  void swap(btree_container &other) { tree_.swap(other.tree_); }
   void verify() const { tree_.verify(); }
 
   // Size routines.
@@ -190,7 +260,7 @@ class btree_container {
   allocator_type get_allocator() const { return tree_.get_allocator(); }
 
   // The key comparator used by the btree.
-  key_compare key_comp() const { return tree_.key_comp(); }
+  key_compare key_comp() const { return key_compare(tree_.key_comp()); }
   value_compare value_comp() const { return tree_.value_comp(); }
 
   // Support absl::Hash.
@@ -199,10 +269,12 @@ class btree_container {
     for (const auto &v : b) {
       h = State::combine(std::move(h), v);
     }
-    return State::combine(std::move(h), b.size());
+    return State::combine(std::move(h),
+                          hash_internal::WeaklyMixedInteger{b.size()});
   }
 
  protected:
+  friend struct btree_access;
   Tree tree_;
 };
 
@@ -223,7 +295,7 @@ class btree_set_container : public btree_container<Tree> {
   using key_type = typename Tree::key_type;
   using value_type = typename Tree::value_type;
   using size_type = typename Tree::size_type;
-  using key_compare = typename Tree::key_compare;
+  using key_compare = typename Tree::original_key_compare;
   using allocator_type = typename Tree::allocator_type;
   using iterator = typename Tree::iterator;
   using const_iterator = typename Tree::const_iterator;
@@ -234,7 +306,7 @@ class btree_set_container : public btree_container<Tree> {
   using super_type::super_type;
   btree_set_container() {}
 
-  // Range constructor.
+  // Range constructors.
   template <class InputIterator>
   btree_set_container(InputIterator b, InputIterator e,
                       const key_compare &comp = key_compare(),
@@ -242,107 +314,114 @@ class btree_set_container : public btree_container<Tree> {
       : super_type(comp, alloc) {
     insert(b, e);
   }
+  template <class InputIterator>
+  btree_set_container(InputIterator b, InputIterator e,
+                      const allocator_type &alloc)
+      : btree_set_container(b, e, key_compare(), alloc) {}
 
-  // Initializer list constructor.
+  // Initializer list constructors.
   btree_set_container(std::initializer_list<init_type> init,
                       const key_compare &comp = key_compare(),
                       const allocator_type &alloc = allocator_type())
       : btree_set_container(init.begin(), init.end(), comp, alloc) {}
-
-  // Lookup routines.
-  template <typename K = key_type>
-  size_type count(const key_arg<K> &key) const {
-    return this->tree_.count_unique(key);
-  }
+  btree_set_container(std::initializer_list<init_type> init,
+                      const allocator_type &alloc)
+      : btree_set_container(init.begin(), init.end(), alloc) {}
 
   // Insertion routines.
-  std::pair<iterator, bool> insert(const value_type &x) {
-    return this->tree_.insert_unique(params_type::key(x), x);
-  }
-  std::pair<iterator, bool> insert(value_type &&x) {
-    return this->tree_.insert_unique(params_type::key(x), std::move(x));
+  std::pair<iterator, bool> insert(const value_type &v)
+      ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return this->tree_.insert_unique(params_type::key(v), v);
   }
-  template <typename... Args>
-  std::pair<iterator, bool> emplace(Args &&... args) {
-    init_type v(std::forward<Args>(args)...);
+  std::pair<iterator, bool> insert(value_type &&v)
+      ABSL_ATTRIBUTE_LIFETIME_BOUND {
     return this->tree_.insert_unique(params_type::key(v), std::move(v));
   }
-  iterator insert(const_iterator position, const value_type &x) {
+  template <typename... Args>
+  std::pair<iterator, bool> emplace(Args &&...args)
+      ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    // Use a node handle to manage a temp slot.
+    auto node = CommonAccess::Construct<node_type>(this->get_allocator(),
+                                                   std::forward<Args>(args)...);
+    auto *slot = CommonAccess::GetSlot(node);
+    return this->tree_.insert_unique(params_type::key(slot), slot);
+  }
+  iterator insert(const_iterator hint,
+                  const value_type &v) ABSL_ATTRIBUTE_LIFETIME_BOUND {
     return this->tree_
-        .insert_hint_unique(iterator(position), params_type::key(x), x)
+        .insert_hint_unique(iterator(hint), params_type::key(v), v)
         .first;
   }
-  iterator insert(const_iterator position, value_type &&x) {
+  iterator insert(const_iterator hint,
+                  value_type &&v) ABSL_ATTRIBUTE_LIFETIME_BOUND {
     return this->tree_
-        .insert_hint_unique(iterator(position), params_type::key(x),
-                            std::move(x))
+        .insert_hint_unique(iterator(hint), params_type::key(v), std::move(v))
         .first;
   }
   template <typename... Args>
-  iterator emplace_hint(const_iterator position, Args &&... args) {
-    init_type v(std::forward<Args>(args)...);
+  iterator emplace_hint(const_iterator hint,
+                        Args &&...args) ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    // Use a node handle to manage a temp slot.
+    auto node = CommonAccess::Construct<node_type>(this->get_allocator(),
+                                                   std::forward<Args>(args)...);
+    auto *slot = CommonAccess::GetSlot(node);
     return this->tree_
-        .insert_hint_unique(iterator(position), params_type::key(v),
-                            std::move(v))
+        .insert_hint_unique(iterator(hint), params_type::key(slot), slot)
         .first;
   }
   template <typename InputIterator>
   void insert(InputIterator b, InputIterator e) {
-    this->tree_.insert_iterator_unique(b, e);
+    this->tree_.insert_iterator_unique(b, e, 0);
   }
   void insert(std::initializer_list<init_type> init) {
-    this->tree_.insert_iterator_unique(init.begin(), init.end());
+    this->tree_.insert_iterator_unique(init.begin(), init.end(), 0);
   }
-  insert_return_type insert(node_type &&node) {
+  insert_return_type insert(node_type &&node) ABSL_ATTRIBUTE_LIFETIME_BOUND {
     if (!node) return {this->end(), false, node_type()};
     std::pair<iterator, bool> res =
-        insert(std::move(params_type::element(CommonAccess::GetSlot(node))));
+        this->tree_.insert_unique(params_type::key(CommonAccess::GetSlot(node)),
+                                  CommonAccess::GetSlot(node));
     if (res.second) {
-      CommonAccess::Reset(&node);
+      CommonAccess::Destroy(&node);
       return {res.first, true, node_type()};
     } else {
       return {res.first, false, std::move(node)};
     }
   }
-  iterator insert(const_iterator hint, node_type &&node) {
+  iterator insert(const_iterator hint,
+                  node_type &&node) ABSL_ATTRIBUTE_LIFETIME_BOUND {
     if (!node) return this->end();
     std::pair<iterator, bool> res = this->tree_.insert_hint_unique(
         iterator(hint), params_type::key(CommonAccess::GetSlot(node)),
-        std::move(params_type::element(CommonAccess::GetSlot(node))));
-    if (res.second) CommonAccess::Reset(&node);
+        CommonAccess::GetSlot(node));
+    if (res.second) CommonAccess::Destroy(&node);
     return res.first;
   }
 
-  // Deletion routines.
-  template <typename K = key_type>
-  size_type erase(const key_arg<K> &key) {
-    return this->tree_.erase_unique(key);
-  }
-  using super_type::erase;
-
   // Node extraction routines.
   template <typename K = key_type>
   node_type extract(const key_arg<K> &key) {
-    auto it = find(key);
-    return it == this->end() ? node_type() : extract(it);
+    const std::pair<iterator, bool> lower_and_equal =
+        this->tree_.lower_bound_equal(key);
+    return lower_and_equal.second ? extract(lower_and_equal.first)
+                                  : node_type();
   }
   using super_type::extract;
 
   // Merge routines.
   // Moves elements from `src` into `this`. If the element already exists in
   // `this`, it is left unmodified in `src`.
-  template <
-      typename T,
-      typename absl::enable_if_t<
-          absl::conjunction<
-              std::is_same<value_type, typename T::value_type>,
-              std::is_same<allocator_type, typename T::allocator_type>,
-              std::is_same<typename params_type::is_map_container,
-                           typename T::params_type::is_map_container>>::value,
-          int> = 0>
-  void merge(btree_container<T> &src) {  // NOLINT
+  template <typename T,
+            typename std::enable_if_t<
+                std::conjunction_v<
+                    std::is_same<value_type, typename T::value_type>,
+                    std::is_same<allocator_type, typename T::allocator_type>,
+                    std::is_same<typename params_type::is_map_container,
+                                 typename T::params_type::is_map_container>>,
+                int> = 0>
+  void merge(btree_container<T>& src) {  // NOLINT
     for (auto src_it = src.begin(); src_it != src.end();) {
-      if (insert(std::move(*src_it)).second) {
+      if (insert(std::move(params_type::element(src_it.slot()))).second) {
         src_it = src.erase(src_it);
       } else {
         ++src_it;
@@ -350,16 +429,15 @@ class btree_set_container : public btree_container<Tree> {
     }
   }
 
-  template <
-      typename T,
-      typename absl::enable_if_t<
-          absl::conjunction<
-              std::is_same<value_type, typename T::value_type>,
-              std::is_same<allocator_type, typename T::allocator_type>,
-              std::is_same<typename params_type::is_map_container,
-                           typename T::params_type::is_map_container>>::value,
-          int> = 0>
-  void merge(btree_container<T> &&src) {
+  template <typename T,
+            typename std::enable_if_t<
+                std::conjunction_v<
+                    std::is_same<value_type, typename T::value_type>,
+                    std::is_same<allocator_type, typename T::allocator_type>,
+                    std::is_same<typename params_type::is_map_container,
+                                 typename T::params_type::is_map_container>>,
+                int> = 0>
+  void merge(btree_container<T>&& src) {
     merge(src);
   }
 };
@@ -369,16 +447,40 @@ template <typename Tree>
 class btree_map_container : public btree_set_container<Tree> {
   using super_type = btree_set_container<Tree>;
   using params_type = typename Tree::params_type;
+  friend class BtreeNodePeer;
 
- protected:
+ private:
   template <class K>
   using key_arg = typename super_type::template key_arg<K>;
 
+  // NOTE: The mess here is to shorten the code for the (very repetitive)
+  // function overloads, and to allow the lifetime-bound overloads to dispatch
+  // to the non-lifetime-bound overloads, to ensure there is a single source of
+  // truth for each overload set.
+  //
+  // Enabled if an assignment from the given type would require the
+  // source object to remain alive for the life of the element.
+  //
+  // TODO(b/402804213): Remove these traits and simplify the overloads whenever
+  // we have a better mechanism available to handle lifetime analysis.
+  template <class K, bool Value, typename = void>
+  using LifetimeBoundK =
+      HasValue<Value, type_traits_internal::IsLifetimeBoundAssignment<
+                          typename Tree::key_type, K>>;
+  template <class M, bool Value, typename = void>
+  using LifetimeBoundV =
+      HasValue<Value, type_traits_internal::IsLifetimeBoundAssignment<
+                          typename Tree::params_type::mapped_type, M>>;
+  template <class K, bool KValue, class M, bool MValue, typename... Dummy>
+  using LifetimeBoundKV =
+      std::conjunction<LifetimeBoundK<K, KValue, std::void_t<Dummy...>>,
+                       LifetimeBoundV<M, MValue>>;
+
  public:
   using key_type = typename Tree::key_type;
   using mapped_type = typename params_type::mapped_type;
   using value_type = typename Tree::value_type;
-  using key_compare = typename Tree::key_compare;
+  using key_compare = typename Tree::original_key_compare;
   using allocator_type = typename Tree::allocator_type;
   using iterator = typename Tree::iterator;
   using const_iterator = typename Tree::const_iterator;
@@ -387,67 +489,220 @@ class btree_map_container : public btree_set_container<Tree> {
   using super_type::super_type;
   btree_map_container() {}
 
+  // TODO(b/402804213): Remove these macros whenever we have a better mechanism
+  // available to handle lifetime analysis.
+#define ABSL_INTERNAL_X(Func, Callee, KQual, MQual, KValue, MValue, ...)     \
+  template <                                                                 \
+      typename K = key_type, class M,                                        \
+      ABSL_INTERNAL_IF_##KValue##_NOR_##MValue(                              \
+          int = (EnableIf<LifetimeBoundKV<K, KValue, M, MValue,              \
+                                          IfRRef<int KQual>::AddPtr<K>,      \
+                                          IfRRef<int MQual>::AddPtr<M>>>()), \
+          ABSL_INTERNAL_SINGLE_ARG(                                          \
+              int&...,                                                       \
+              decltype(EnableIf<LifetimeBoundKV<K, KValue, M, MValue>>()) =  \
+                  0))>                                                       \
+  decltype(auto) Func(                                                       \
+      __VA_ARGS__ key_arg<K> KQual k ABSL_INTERNAL_IF_##KValue(              \
+          ABSL_INTERNAL_ATTRIBUTE_CAPTURED_BY_THIS),                         \
+      M MQual obj ABSL_INTERNAL_IF_##MValue(                                 \
+          ABSL_INTERNAL_ATTRIBUTE_CAPTURED_BY_THIS))                         \
+      ABSL_ATTRIBUTE_LIFETIME_BOUND {                                        \
+    return ABSL_INTERNAL_IF_##KValue##_OR_##MValue(                          \
+        (this->template Func<K, M, 0>), Callee)(                             \
+        __VA_ARGS__ std::forward<decltype(k)>(k),                            \
+        std::forward<decltype(obj)>(obj));                                   \
+  }                                                                          \
+  static_assert(true, "this assertion forces a semicolon")
   // Insertion routines.
-  template <typename... Args>
-  std::pair<iterator, bool> try_emplace(const key_type &k, Args &&... args) {
-    return this->tree_.insert_unique(
-        k, std::piecewise_construct, std::forward_as_tuple(k),
-        std::forward_as_tuple(std::forward<Args>(args)...));
-  }
-  template <typename... Args>
-  std::pair<iterator, bool> try_emplace(key_type &&k, Args &&... args) {
-    // Note: `key_ref` exists to avoid a ClangTidy warning about moving from `k`
-    // and then using `k` unsequenced. This is safe because the move is into a
-    // forwarding reference and insert_unique guarantees that `key` is never
-    // referenced after consuming `args`.
-    const key_type& key_ref = k;
-    return this->tree_.insert_unique(
-        key_ref, std::piecewise_construct, std::forward_as_tuple(std::move(k)),
-        std::forward_as_tuple(std::forward<Args>(args)...));
+  // Note: the nullptr template arguments and extra `const M&` overloads allow
+  // for supporting bitfield arguments.
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_impl, const &, const &,
+                  false, false);
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_impl, const &, const &,
+                  false, true);
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_impl, const &, const &,
+                  true, false);
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_impl, const &, const &,
+                  true, true);
+
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_impl, const &, &&, false,
+                  false);
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_impl, const &, &&, false,
+                  true);
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_impl, const &, &&, true,
+                  false);
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_impl, const &, &&, true,
+                  true);
+
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_impl, &&, const &, false,
+                  false);
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_impl, &&, const &, false,
+                  true);
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_impl, &&, const &, true,
+                  false);
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_impl, &&, const &, true,
+                  true);
+
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_impl, &&, &&, false,
+                  false);
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_impl, &&, &&, false, true);
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_impl, &&, &&, true, false);
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_impl, &&, &&, true, true);
+
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_hint_impl, const &,
+                  const &, false, false,
+                  const_iterator(hint) ABSL_INTERNAL_COMMA);
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_hint_impl, const &,
+                  const &, false, true,
+                  const_iterator(hint) ABSL_INTERNAL_COMMA);
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_hint_impl, const &,
+                  const &, true, false,
+                  const_iterator(hint) ABSL_INTERNAL_COMMA);
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_hint_impl, const &,
+                  const &, true, true,
+                  const_iterator(hint) ABSL_INTERNAL_COMMA);
+
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_hint_impl, const &, &&,
+                  false, false, const_iterator(hint) ABSL_INTERNAL_COMMA);
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_hint_impl, const &, &&,
+                  false, true, const_iterator(hint) ABSL_INTERNAL_COMMA);
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_hint_impl, const &, &&,
+                  true, false, const_iterator(hint) ABSL_INTERNAL_COMMA);
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_hint_impl, const &, &&,
+                  true, true, const_iterator(hint) ABSL_INTERNAL_COMMA);
+
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_hint_impl, &&, const &,
+                  false, false, const_iterator(hint) ABSL_INTERNAL_COMMA);
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_hint_impl, &&, const &,
+                  false, true, const_iterator(hint) ABSL_INTERNAL_COMMA);
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_hint_impl, &&, const &,
+                  true, false, const_iterator(hint) ABSL_INTERNAL_COMMA);
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_hint_impl, &&, const &,
+                  true, true, const_iterator(hint) ABSL_INTERNAL_COMMA);
+
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_hint_impl, &&, &&, false,
+                  false, const_iterator(hint) ABSL_INTERNAL_COMMA);
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_hint_impl, &&, &&, false,
+                  true, const_iterator(hint) ABSL_INTERNAL_COMMA);
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_hint_impl, &&, &&, true,
+                  false, const_iterator(hint) ABSL_INTERNAL_COMMA);
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_hint_impl, &&, &&, true,
+                  true, const_iterator(hint) ABSL_INTERNAL_COMMA);
+#undef ABSL_INTERNAL_X
+
+#define ABSL_INTERNAL_X(Func, Callee, KQual, KValue, ...)                      \
+  template <                                                                   \
+      class K = key_type,                                                      \
+      ABSL_INTERNAL_IF_##KValue(                                               \
+          class... Args,                                                       \
+          int = (EnableIf<                                                     \
+                 LifetimeBoundK<K, KValue, IfRRef<int KQual>::AddPtr<K>>>())), \
+      ABSL_INTERNAL_IF_##KValue(                                               \
+          decltype(EnableIf<LifetimeBoundK<                                    \
+                       K, KValue, IfRRef<int KQual>::AddPtr<K>>>()) = 0,       \
+          class... Args),                                                      \
+      std::enable_if_t<!std::is_convertible_v<K, const_iterator>, int> = 0>    \
+  decltype(auto) Func(                                                         \
+      __VA_ARGS__ key_arg<K> KQual k ABSL_INTERNAL_IF_##KValue(                \
+          ABSL_INTERNAL_ATTRIBUTE_CAPTURED_BY_THIS),                           \
+      Args&&... args) ABSL_ATTRIBUTE_LIFETIME_BOUND {                          \
+    return ABSL_INTERNAL_IF_##KValue((this->template Func<K, 0>), Callee)(     \
+        __VA_ARGS__ std::forward<decltype(k)>(k),                              \
+        std::forward<decltype(args)>(args)...);                                \
+  }                                                                            \
+  static_assert(true, "this assertion forces a semicolon")
+  ABSL_INTERNAL_X(try_emplace, try_emplace_impl, const &, false);
+  ABSL_INTERNAL_X(try_emplace, try_emplace_impl, const &, true);
+  ABSL_INTERNAL_X(try_emplace, try_emplace_impl, &&, false);
+  ABSL_INTERNAL_X(try_emplace, try_emplace_impl, &&, true);
+  ABSL_INTERNAL_X(try_emplace, try_emplace_hint_impl, const &, false,
+                  const_iterator(hint) ABSL_INTERNAL_COMMA);
+  ABSL_INTERNAL_X(try_emplace, try_emplace_hint_impl, const &, true,
+                  const_iterator(hint) ABSL_INTERNAL_COMMA);
+  ABSL_INTERNAL_X(try_emplace, try_emplace_hint_impl, &&, false,
+                  const_iterator(hint) ABSL_INTERNAL_COMMA);
+  ABSL_INTERNAL_X(try_emplace, try_emplace_hint_impl, &&, true,
+                  const_iterator(hint) ABSL_INTERNAL_COMMA);
+#undef ABSL_INTERNAL_X
+
+  template <class K = key_type, int = EnableIf<LifetimeBoundK<K, false>>()>
+  mapped_type &operator[](const key_arg<K> &k) ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return try_emplace(k).first->second;
   }
-  template <typename... Args>
-  iterator try_emplace(const_iterator hint, const key_type &k,
-                       Args &&... args) {
-    return this->tree_
-        .insert_hint_unique(iterator(hint), k, std::piecewise_construct,
-                            std::forward_as_tuple(k),
-                            std::forward_as_tuple(std::forward<Args>(args)...))
-        .first;
+  template <class K = key_type, int &..., EnableIf<LifetimeBoundK<K, true>> = 0>
+  mapped_type &operator[](
+      const key_arg<K> &k ABSL_INTERNAL_ATTRIBUTE_CAPTURED_BY_THIS)
+      ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return this->template operator[]<K, 0>(k);
   }
-  template <typename... Args>
-  iterator try_emplace(const_iterator hint, key_type &&k, Args &&... args) {
-    // Note: `key_ref` exists to avoid a ClangTidy warning about moving from `k`
-    // and then using `k` unsequenced. This is safe because the move is into a
-    // forwarding reference and insert_hint_unique guarantees that `key` is
-    // never referenced after consuming `args`.
-    const key_type& key_ref = k;
-    return this->tree_
-        .insert_hint_unique(iterator(hint), key_ref, std::piecewise_construct,
-                            std::forward_as_tuple(std::move(k)),
-                            std::forward_as_tuple(std::forward<Args>(args)...))
-        .first;
-  }
-  mapped_type &operator[](const key_type &k) {
-    return try_emplace(k).first->second;
+  template <class K = key_type, int = EnableIf<LifetimeBoundK<K, false>>()>
+  mapped_type &operator[](key_arg<K> &&k) ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return try_emplace(std::forward<key_arg<K>>(k)).first->second;
   }
-  mapped_type &operator[](key_type &&k) {
-    return try_emplace(std::move(k)).first->second;
+  template <class K = key_type, int &..., EnableIf<LifetimeBoundK<K, true>> = 0>
+  mapped_type &operator[](
+      key_arg<K> &&k ABSL_INTERNAL_ATTRIBUTE_CAPTURED_BY_THIS)
+      ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return this->template operator[]<K, 0>(std::forward<key_arg<K>>(k));
   }
 
   template <typename K = key_type>
-  mapped_type &at(const key_arg<K> &key) {
+  mapped_type &at(const key_arg<K> &key) ABSL_ATTRIBUTE_LIFETIME_BOUND {
     auto it = this->find(key);
-    if (it == this->end())
-      base_internal::ThrowStdOutOfRange("absl::btree_map::at");
+    if (it == this->end()) ThrowStdOutOfRange("absl::btree_map::at");
     return it->second;
   }
   template <typename K = key_type>
-  const mapped_type &at(const key_arg<K> &key) const {
+  const mapped_type &at(const key_arg<K> &key) const
+      ABSL_ATTRIBUTE_LIFETIME_BOUND {
     auto it = this->find(key);
-    if (it == this->end())
-      base_internal::ThrowStdOutOfRange("absl::btree_map::at");
+    if (it == this->end()) ThrowStdOutOfRange("absl::btree_map::at");
     return it->second;
   }
+
+ private:
+  // Note: when we call `std::forward<M>(obj)` twice, it's safe because
+  // insert_unique/insert_hint_unique are guaranteed to not consume `obj` when
+  // `ret.second` is false.
+  template <class K, class M>
+  std::pair<iterator, bool> insert_or_assign_impl(K &&k, M &&obj) {
+    const std::pair<iterator, bool> ret =
+        this->tree_.insert_unique(k, std::forward<K>(k), std::forward<M>(obj));
+    if (!ret.second) {
+      // NOLINTNEXTLINE(bugprone-use-after-move)
+      ret.first->second = std::forward<M>(obj);
+    }
+    return ret;
+  }
+  template <class K, class M>
+  iterator insert_or_assign_hint_impl(const_iterator hint, K &&k, M &&obj) {
+    const std::pair<iterator, bool> ret = this->tree_.insert_hint_unique(
+        iterator(hint), k, std::forward<K>(k), std::forward<M>(obj));
+    if (!ret.second) {
+      // NOLINTNEXTLINE(bugprone-use-after-move)
+      ret.first->second = std::forward<M>(obj);
+    }
+    return ret.first;
+  }
+
+  template <class K, class... Args>
+  std::pair<iterator, bool> try_emplace_impl(K &&k, Args &&... args) {
+    return this->tree_.insert_unique(
+        // NOLINTNEXTLINE(bugprone-use-after-move)
+        k, std::piecewise_construct, std::forward_as_tuple(std::forward<K>(k)),
+        std::forward_as_tuple(std::forward<Args>(args)...));
+  }
+  template <class K, class... Args>
+  iterator try_emplace_hint_impl(const_iterator hint, K &&k, Args &&... args) {
+    return this->tree_
+        .insert_hint_unique(iterator(hint),
+                            // NOLINTNEXTLINE(bugprone-use-after-move)
+                            k, std::piecewise_construct,
+                            std::forward_as_tuple(std::forward<K>(k)),
+                            std::forward_as_tuple(std::forward<Args>(args)...))
+        .first;
+  }
 };
 
 // A common base class for btree_multiset and btree_multimap.
@@ -457,6 +712,7 @@ class btree_multiset_container : public btree_container<Tree> {
   using params_type = typename Tree::params_type;
   using init_type = typename params_type::init_type;
   using is_key_compare_to = typename params_type::is_key_compare_to;
+  friend class BtreeNodePeer;
 
   template <class K>
   using key_arg = typename super_type::template key_arg<K>;
@@ -465,7 +721,7 @@ class btree_multiset_container : public btree_container<Tree> {
   using key_type = typename Tree::key_type;
   using value_type = typename Tree::value_type;
   using size_type = typename Tree::size_type;
-  using key_compare = typename Tree::key_compare;
+  using key_compare = typename Tree::original_key_compare;
   using allocator_type = typename Tree::allocator_type;
   using iterator = typename Tree::iterator;
   using const_iterator = typename Tree::const_iterator;
@@ -475,7 +731,7 @@ class btree_multiset_container : public btree_container<Tree> {
   using super_type::super_type;
   btree_multiset_container() {}
 
-  // Range constructor.
+  // Range constructors.
   template <class InputIterator>
   btree_multiset_container(InputIterator b, InputIterator e,
                            const key_compare &comp = key_compare(),
@@ -483,29 +739,34 @@ class btree_multiset_container : public btree_container<Tree> {
       : super_type(comp, alloc) {
     insert(b, e);
   }
+  template <class InputIterator>
+  btree_multiset_container(InputIterator b, InputIterator e,
+                           const allocator_type &alloc)
+      : btree_multiset_container(b, e, key_compare(), alloc) {}
 
-  // Initializer list constructor.
+  // Initializer list constructors.
   btree_multiset_container(std::initializer_list<init_type> init,
                            const key_compare &comp = key_compare(),
                            const allocator_type &alloc = allocator_type())
       : btree_multiset_container(init.begin(), init.end(), comp, alloc) {}
-
-  // Lookup routines.
-  template <typename K = key_type>
-  size_type count(const key_arg<K> &key) const {
-    return this->tree_.count_multi(key);
-  }
+  btree_multiset_container(std::initializer_list<init_type> init,
+                           const allocator_type &alloc)
+      : btree_multiset_container(init.begin(), init.end(), alloc) {}
 
   // Insertion routines.
-  iterator insert(const value_type &x) { return this->tree_.insert_multi(x); }
-  iterator insert(value_type &&x) {
-    return this->tree_.insert_multi(std::move(x));
+  iterator insert(const value_type &v) ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return this->tree_.insert_multi(v);
+  }
+  iterator insert(value_type &&v) ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return this->tree_.insert_multi(std::move(v));
   }
-  iterator insert(const_iterator position, const value_type &x) {
-    return this->tree_.insert_hint_multi(iterator(position), x);
+  iterator insert(const_iterator hint,
+                  const value_type &v) ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return this->tree_.insert_hint_multi(iterator(hint), v);
   }
-  iterator insert(const_iterator position, value_type &&x) {
-    return this->tree_.insert_hint_multi(iterator(position), std::move(x));
+  iterator insert(const_iterator hint,
+                  value_type &&v) ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return this->tree_.insert_hint_multi(iterator(hint), std::move(v));
   }
   template <typename InputIterator>
   void insert(InputIterator b, InputIterator e) {
@@ -515,76 +776,75 @@ class btree_multiset_container : public btree_container<Tree> {
     this->tree_.insert_iterator_multi(init.begin(), init.end());
   }
   template <typename... Args>
-  iterator emplace(Args &&... args) {
-    return this->tree_.insert_multi(init_type(std::forward<Args>(args)...));
-  }
-  template <typename... Args>
-  iterator emplace_hint(const_iterator position, Args &&... args) {
-    return this->tree_.insert_hint_multi(
-        iterator(position), init_type(std::forward<Args>(args)...));
+  iterator emplace(Args &&...args) ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    // Use a node handle to manage a temp slot.
+    auto node = CommonAccess::Construct<node_type>(this->get_allocator(),
+                                                   std::forward<Args>(args)...);
+    return this->tree_.insert_multi(CommonAccess::GetSlot(node));
   }
-
- private:
   template <typename... Args>
-  iterator insert_node_helper(node_type &&node, Args &&... args) {
+  iterator emplace_hint(const_iterator hint,
+                        Args &&...args) ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    // Use a node handle to manage a temp slot.
+    auto node = CommonAccess::Construct<node_type>(this->get_allocator(),
+                                                   std::forward<Args>(args)...);
+    return this->tree_.insert_hint_multi(iterator(hint),
+                                         CommonAccess::GetSlot(node));
+  }
+  iterator insert(node_type &&node) ABSL_ATTRIBUTE_LIFETIME_BOUND {
     if (!node) return this->end();
     iterator res =
-        insert(std::forward<Args>(args)...,
-               std::move(params_type::element(CommonAccess::GetSlot(node))));
-    CommonAccess::Reset(&node);
+        this->tree_.insert_multi(params_type::key(CommonAccess::GetSlot(node)),
+                                 CommonAccess::GetSlot(node));
+    CommonAccess::Destroy(&node);
     return res;
   }
-
- public:
-  iterator insert(node_type &&node) {
-    return insert_node_helper(std::move(node));
-  }
-  iterator insert(const_iterator hint, node_type &&node) {
-    return insert_node_helper(std::move(node), hint);
-  }
-
-  // Deletion routines.
-  template <typename K = key_type>
-  size_type erase(const key_arg<K> &key) {
-    return this->tree_.erase_multi(key);
+  iterator insert(const_iterator hint,
+                  node_type &&node) ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    if (!node) return this->end();
+    iterator res = this->tree_.insert_hint_multi(
+        iterator(hint),
+        std::move(params_type::element(CommonAccess::GetSlot(node))));
+    CommonAccess::Destroy(&node);
+    return res;
   }
-  using super_type::erase;
 
   // Node extraction routines.
   template <typename K = key_type>
   node_type extract(const key_arg<K> &key) {
-    auto it = find(key);
-    return it == this->end() ? node_type() : extract(it);
+    const std::pair<iterator, bool> lower_and_equal =
+        this->tree_.lower_bound_equal(key);
+    return lower_and_equal.second ? extract(lower_and_equal.first)
+                                  : node_type();
   }
   using super_type::extract;
 
   // Merge routines.
   // Moves all elements from `src` into `this`.
-  template <
-      typename T,
-      typename absl::enable_if_t<
-          absl::conjunction<
-              std::is_same<value_type, typename T::value_type>,
-              std::is_same<allocator_type, typename T::allocator_type>,
-              std::is_same<typename params_type::is_map_container,
-                           typename T::params_type::is_map_container>>::value,
-          int> = 0>
-  void merge(btree_container<T> &src) {  // NOLINT
-    insert(std::make_move_iterator(src.begin()),
-           std::make_move_iterator(src.end()));
+  template <typename T,
+            typename std::enable_if_t<
+                std::conjunction_v<
+                    std::is_same<value_type, typename T::value_type>,
+                    std::is_same<allocator_type, typename T::allocator_type>,
+                    std::is_same<typename params_type::is_map_container,
+                                 typename T::params_type::is_map_container>>,
+                int> = 0>
+  void merge(btree_container<T>& src) {  // NOLINT
+    for (auto src_it = src.begin(), end = src.end(); src_it != end; ++src_it) {
+      insert(std::move(params_type::element(src_it.slot())));
+    }
     src.clear();
   }
 
-  template <
-      typename T,
-      typename absl::enable_if_t<
-          absl::conjunction<
-              std::is_same<value_type, typename T::value_type>,
-              std::is_same<allocator_type, typename T::allocator_type>,
-              std::is_same<typename params_type::is_map_container,
-                           typename T::params_type::is_map_container>>::value,
-          int> = 0>
-  void merge(btree_container<T> &&src) {
+  template <typename T,
+            typename std::enable_if_t<
+                std::conjunction_v<
+                    std::is_same<value_type, typename T::value_type>,
+                    std::is_same<allocator_type, typename T::allocator_type>,
+                    std::is_same<typename params_type::is_map_container,
+                                 typename T::params_type::is_map_container>>,
+                int> = 0>
+  void merge(btree_container<T>&& src) {
     merge(src);
   }
 };
@@ -594,6 +854,7 @@ template <typename Tree>
 class btree_multimap_container : public btree_multiset_container<Tree> {
   using super_type = btree_multiset_container<Tree>;
   using params_type = typename Tree::params_type;
+  friend class BtreeNodePeer;
 
  public:
   using mapped_type = typename params_type::mapped_type;
@@ -604,6 +865,7 @@ class btree_multimap_container : public btree_multiset_container<Tree> {
 };
 
 }  // namespace container_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_INTERNAL_BTREE_CONTAINER_H_
diff --git a/absl/container/internal/chunked_queue.h b/absl/container/internal/chunked_queue.h
new file mode 100644
index 00000000000..c3718ac3bba
--- /dev/null
+++ b/absl/container/internal/chunked_queue.h
@@ -0,0 +1,173 @@
+// Copyright 2025 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_CONTAINER_INTERNAL_CHUNKED_QUEUE_H_
+#define ABSL_CONTAINER_INTERNAL_CHUNKED_QUEUE_H_
+
+#include <algorithm>
+#include <cstddef>
+#include <cstdint>
+#include <initializer_list>
+#include <iterator>
+#include <memory>
+#include <new>
+#include <tuple>
+#include <type_traits>
+#include <utility>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/macros.h"
+#include "absl/container/internal/layout.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace container_internal {
+
+// ChunkedQueueBlock defines a node in a forward list of uninitialized storage
+// of size T's. The user is responsible for constructing and destroying T's in
+// said storage.
+//
+// ChunkedQueueBlock::New(size) returns said node, with at least size_hint T's
+// of uninitialized storage.
+template <typename T, typename Allocator>
+class ChunkedQueueBlock {
+ private:
+  using ChunkedQueueBlockAllocator = typename std::allocator_traits<
+      Allocator>::template rebind_alloc<ChunkedQueueBlock>;
+  using ByteAllocator =
+      typename std::allocator_traits<Allocator>::template rebind_alloc<char>;
+
+ public:
+  // NB, instances of this must not be created or destroyed directly, only via
+  // the New() and Delete() methods.  (This notionally-private constructor is
+  // public only to allow access from allocator types used by New().)
+  explicit ChunkedQueueBlock(size_t size)
+      : next_(nullptr), limit_(start() + size) {}
+
+  // Must be deleted by ChunkedQueueBlock::Delete.
+  static ChunkedQueueBlock* New(size_t size_hint, Allocator* alloc) {  // NOLINT
+    ABSL_ASSERT(size_hint >= size_t{1});
+    size_t allocation_bytes = AllocSize(size_hint);
+    void* mem;
+    std::tie(mem, allocation_bytes) = Allocate(allocation_bytes, alloc);
+    const size_t element_count =
+        (allocation_bytes - start_offset()) / sizeof(T);
+    ChunkedQueueBlock* as_block = static_cast<ChunkedQueueBlock*>(mem);
+    ChunkedQueueBlockAllocator block_alloc(*alloc);
+    std::allocator_traits<ChunkedQueueBlockAllocator>::construct(
+        block_alloc, as_block, element_count);
+    return as_block;
+  }
+
+  static void Delete(ChunkedQueueBlock* ptr, Allocator* alloc) {
+    const size_t allocation_bytes = AllocSize(ptr->size());
+    ChunkedQueueBlockAllocator block_alloc(*alloc);
+    std::allocator_traits<ChunkedQueueBlockAllocator>::destroy(block_alloc,
+                                                               ptr);
+    if constexpr (std::is_same_v<ByteAllocator, std::allocator<char>>) {
+#ifdef __STDCPP_DEFAULT_NEW_ALIGNMENT__
+      if (alignment() > __STDCPP_DEFAULT_NEW_ALIGNMENT__) {
+        ::operator delete(ptr
+#ifdef __cpp_sized_deallocation
+                          ,
+                          allocation_bytes
+#endif
+                          ,
+                          std::align_val_t(alignment()));
+        return;
+      }
+#endif
+      ::operator delete(ptr);
+    } else {
+      void* mem = ptr;
+      ByteAllocator byte_alloc(*alloc);
+      std::allocator_traits<ByteAllocator>::deallocate(
+          byte_alloc, static_cast<char*>(mem), allocation_bytes);
+    }
+  }
+
+  ChunkedQueueBlock* next() const { return next_; }
+  void set_next(ChunkedQueueBlock* next) { next_ = next; }
+  T* start() {
+    return reinterpret_cast<T*>(reinterpret_cast<uintptr_t>(this) +
+                                start_offset());
+  }
+  T* limit() { return limit_; }
+  size_t size() { return limit() - start(); }
+
+  static constexpr size_t block_size_from_bytes(size_t bytes) {
+    return bytes <= static_cast<size_t>(start_offset())
+               ? size_t{1}
+               : elements_in_bytes(bytes - start_offset());
+  }
+
+ private:
+  ChunkedQueueBlock(const ChunkedQueueBlock&) = delete;
+  ChunkedQueueBlock& operator=(const ChunkedQueueBlock&) = delete;
+
+  // The byte size to allocate to ensure space for `min_element_count` elements.
+  static constexpr size_t AllocSize(size_t min_element_count) {
+    return absl::container_internal::Layout<ChunkedQueueBlock, T>(
+               1, min_element_count)
+        .AllocSize();
+  }
+
+  static constexpr ptrdiff_t start_offset() {
+    return absl::container_internal::Layout<ChunkedQueueBlock, T>(1, 1)
+        .template Offset<1>();
+  }
+
+  static constexpr size_t alignment() {
+    return absl::container_internal::Layout<ChunkedQueueBlock, T>(1, 1)
+        .Alignment();
+  }
+
+  static constexpr size_t elements_in_bytes(size_t bytes) {
+    return (bytes + sizeof(T) - 1) / sizeof(T);
+  }
+
+  static std::pair<void*, size_t> Allocate(size_t allocation_bytes,
+                                           Allocator* alloc) {
+    // If we're using the default allocator, then we can use new.
+    void* mem;
+    if constexpr (std::is_same_v<ByteAllocator, std::allocator<char>>) {
+      // Older GCC versions have an unused variable warning on `alloc` inside
+      // this constexpr branch.
+      static_cast<void>(alloc);
+#ifdef __STDCPP_DEFAULT_NEW_ALIGNMENT__
+      if (alignment() > __STDCPP_DEFAULT_NEW_ALIGNMENT__) {
+        // Align the allocation to respect alignof(T).
+        mem = ::operator new(allocation_bytes, std::align_val_t(alignment()));
+        return {mem, allocation_bytes};
+      }
+#endif
+      mem = ::operator new(allocation_bytes);
+    } else {
+      ByteAllocator byte_alloc(*alloc);
+      mem = std::allocator_traits<ByteAllocator>::allocate(byte_alloc,
+                                                           allocation_bytes);
+    }
+    return {mem, allocation_bytes};
+  }
+
+  ChunkedQueueBlock* next_;
+  T* limit_;
+};
+
+}  // namespace container_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_CONTAINER_INTERNAL_CHUNKED_QUEUE_H_
diff --git a/absl/container/internal/common.h b/absl/container/internal/common.h
index 591d3ea11fa..b2839e6e24d 100644
--- a/absl/container/internal/common.h
+++ b/absl/container/internal/common.h
@@ -12,22 +12,70 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-#ifndef ABSL_CONTAINER_INTERNAL_CONTAINER_H_
-#define ABSL_CONTAINER_INTERNAL_CONTAINER_H_
+#ifndef ABSL_CONTAINER_INTERNAL_COMMON_H_
+#define ABSL_CONTAINER_INTERNAL_COMMON_H_
 
+#include <algorithm>
 #include <cassert>
+#include <cstddef>
+#include <optional>
+#include <tuple>
 #include <type_traits>
 
 #include "absl/meta/type_traits.h"
 #include "absl/types/optional.h"
 
+// TODO(b/402804213): Clean up these macros when no longer needed.
+#define ABSL_INTERNAL_SINGLE_ARG(...) __VA_ARGS__
+
+#define ABSL_INTERNAL_IF_true(if_satisfied, ...) if_satisfied
+#define ABSL_INTERNAL_IF_false(if_satisfied, ...) __VA_ARGS__
+
+#define ABSL_INTERNAL_IF_true_AND_true ABSL_INTERNAL_IF_true
+#define ABSL_INTERNAL_IF_false_AND_false ABSL_INTERNAL_IF_false
+#define ABSL_INTERNAL_IF_true_AND_false ABSL_INTERNAL_IF_false_AND_false
+#define ABSL_INTERNAL_IF_false_AND_true ABSL_INTERNAL_IF_false_AND_false
+
+#define ABSL_INTERNAL_IF_true_OR_true ABSL_INTERNAL_IF_true
+#define ABSL_INTERNAL_IF_false_OR_false ABSL_INTERNAL_IF_false
+#define ABSL_INTERNAL_IF_true_OR_false ABSL_INTERNAL_IF_true_OR_true
+#define ABSL_INTERNAL_IF_false_OR_true ABSL_INTERNAL_IF_true_OR_true
+
+#define ABSL_INTERNAL_IF_true_NOR_true ABSL_INTERNAL_IF_false_AND_false
+#define ABSL_INTERNAL_IF_false_NOR_false ABSL_INTERNAL_IF_true_AND_true
+#define ABSL_INTERNAL_IF_true_NOR_false ABSL_INTERNAL_IF_false_AND_true
+#define ABSL_INTERNAL_IF_false_NOR_true ABSL_INTERNAL_IF_true_AND_false
+
+#define ABSL_INTERNAL_COMMA ,
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
+// TODO(b/402804213): Clean up these traits when no longer needed or
+// deduplicate them with absl::functional_internal::EnableIf.
+template <class Cond>
+using EnableIf = std::enable_if_t<Cond::value, int>;
+
+template <bool Value, class T>
+using HasValue = std::conditional_t<Value, T, std::negation<T>>;
+
+template <class T>
+struct IfRRef {
+  template <class Other>
+  using AddPtr = Other;
+};
+
+template <class T>
+struct IfRRef<T&&> {
+  template <class Other>
+  using AddPtr = Other*;
+};
+
 template <class, class = void>
 struct IsTransparent : std::false_type {};
 template <class T>
-struct IsTransparent<T, absl::void_t<typename T::is_transparent>>
+struct IsTransparent<T, std::void_t<typename T::is_transparent>>
     : std::true_type {};
 
 template <bool is_transparent>
@@ -55,7 +103,7 @@ class node_handle_base {
  public:
   using allocator_type = Alloc;
 
-  constexpr node_handle_base() {}
+  constexpr node_handle_base() = default;
   node_handle_base(node_handle_base&& other) noexcept {
     *this = std::move(other);
   }
@@ -83,10 +131,11 @@ class node_handle_base {
     PolicyTraits::transfer(alloc(), slot(), s);
   }
 
-  struct move_tag_t {};
-  node_handle_base(move_tag_t, const allocator_type& a, slot_type* s)
+  struct construct_tag_t {};
+  template <typename... Args>
+  node_handle_base(construct_tag_t, const allocator_type& a, Args&&... args)
       : alloc_(a) {
-    PolicyTraits::construct(alloc(), slot(), s);
+    PolicyTraits::construct(alloc(), slot(), std::forward<Args>(args)...);
   }
 
   void destroy() {
@@ -98,7 +147,7 @@ class node_handle_base {
 
   void reset() {
     assert(alloc_.has_value());
-    alloc_ = absl::nullopt;
+    alloc_ = std::nullopt;
   }
 
   slot_type* slot() const {
@@ -108,16 +157,15 @@ class node_handle_base {
   allocator_type* alloc() { return std::addressof(*alloc_); }
 
  private:
-  absl::optional<allocator_type> alloc_;
-  mutable absl::aligned_storage_t<sizeof(slot_type), alignof(slot_type)>
-      slot_space_;
+  std::optional<allocator_type> alloc_ = {};
+  alignas(slot_type) mutable unsigned char slot_space_[sizeof(slot_type)] = {};
 };
 
 // For sets.
 template <typename Policy, typename PolicyTraits, typename Alloc,
           typename = void>
 class node_handle : public node_handle_base<PolicyTraits, Alloc> {
-  using Base = typename node_handle::node_handle_base;
+  using Base = node_handle_base<PolicyTraits, Alloc>;
 
  public:
   using value_type = typename PolicyTraits::value_type;
@@ -135,9 +183,10 @@ class node_handle : public node_handle_base<PolicyTraits, Alloc> {
 // For maps.
 template <typename Policy, typename PolicyTraits, typename Alloc>
 class node_handle<Policy, PolicyTraits, Alloc,
-                  absl::void_t<typename Policy::mapped_type>>
+                  std::void_t<typename Policy::mapped_type>>
     : public node_handle_base<PolicyTraits, Alloc> {
-  using Base = typename node_handle::node_handle_base;
+  using Base = node_handle_base<PolicyTraits, Alloc>;
+  using slot_type = typename PolicyTraits::slot_type;
 
  public:
   using key_type = typename Policy::key_type;
@@ -145,8 +194,9 @@ class node_handle<Policy, PolicyTraits, Alloc,
 
   constexpr node_handle() {}
 
-  auto key() const -> decltype(PolicyTraits::key(this->slot())) {
-    return PolicyTraits::key(this->slot());
+  auto key() const
+      -> decltype(PolicyTraits::mutable_key(std::declval<slot_type*>())) {
+    return PolicyTraits::mutable_key(this->slot());
   }
 
   mapped_type& mapped() const {
@@ -166,6 +216,11 @@ struct CommonAccess {
     return node.slot();
   }
 
+  template <typename Node>
+  static void Destroy(Node* node) {
+    node->destroy();
+  }
+
   template <typename Node>
   static void Reset(Node* node) {
     node->reset();
@@ -177,8 +232,8 @@ struct CommonAccess {
   }
 
   template <typename T, typename... Args>
-  static T Move(Args&&... args) {
-    return T(typename T::move_tag_t{}, std::forward<Args>(args)...);
+  static T Construct(Args&&... args) {
+    return T(typename T::construct_tag_t{}, std::forward<Args>(args)...);
   }
 };
 
@@ -190,7 +245,56 @@ struct InsertReturnType {
   NodeType node;
 };
 
+// Utilities to strip redundant template parameters from the underlying
+// implementation types.
+// We use a variadic pack (ie Params...) to specify required prefix of types for
+// non-default types, and then we use GetFromListOr to select the provided types
+// or the default ones otherwise.
+//
+// These default types do not contribute information for debugging and just
+// bloat the binary.
+// Removing the redundant tail types reduces mangled names and stringified
+// function names like __PRETTY_FUNCTION__.
+//
+// How to use:
+//  1. Define a template with `typename ...Params`
+//  2. Instantiate it via `ApplyWithoutDefaultSuffix<>` to only pass the minimal
+//     set of types.
+//  3. Inside the template use `GetFromListOr` to map back from the existing
+//     `Params` list to the actual types, filling the gaps when types are
+//     missing.
+
+template <typename Or, size_t N, typename... Params>
+using GetFromListOr = std::tuple_element_t<(std::min)(N, sizeof...(Params)),
+                                           std::tuple<Params..., Or>>;
+
+template <typename... T>
+struct TypeList {
+  template <template <typename...> class Template>
+  using Apply = Template<T...>;
+};
+
+// Evaluate to `Template<TPrefix...>` where the last type in the list (if any)
+// is different from the corresponding one in the default list.
+// Eg
+//   ApplyWithoutDefaultSuffix<Template, TypeList<a, b, c>, TypeList<a, X, c>>
+// evaluates to
+//   Template<a, X>
+template <template <typename...> class Template, typename D, typename T,
+          typename L = TypeList<>, typename = void>
+struct ApplyWithoutDefaultSuffix {
+  using type = typename L::template Apply<Template>;
+};
+template <template <typename...> class Template, typename D, typename... Ds,
+          typename T, typename... Ts, typename... L>
+struct ApplyWithoutDefaultSuffix<
+    Template, TypeList<D, Ds...>, TypeList<T, Ts...>, TypeList<L...>,
+    std::enable_if_t<!std::is_same_v<TypeList<D, Ds...>, TypeList<T, Ts...>>>>
+    : ApplyWithoutDefaultSuffix<Template, TypeList<Ds...>, TypeList<Ts...>,
+                       TypeList<L..., T>> {};
+
 }  // namespace container_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
-#endif  // ABSL_CONTAINER_INTERNAL_CONTAINER_H_
+#endif  // ABSL_CONTAINER_INTERNAL_COMMON_H_
diff --git a/absl/container/internal/common_policy_traits.h b/absl/container/internal/common_policy_traits.h
new file mode 100644
index 00000000000..0eb05427ca9
--- /dev/null
+++ b/absl/container/internal/common_policy_traits.h
@@ -0,0 +1,151 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_CONTAINER_INTERNAL_COMMON_POLICY_TRAITS_H_
+#define ABSL_CONTAINER_INTERNAL_COMMON_POLICY_TRAITS_H_
+
+#include <cstddef>
+#include <cstring>
+#include <memory>
+#include <new>
+#include <type_traits>
+#include <utility>
+
+#include "absl/meta/type_traits.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace container_internal {
+
+template <class Policy, class = void>
+struct policy_trait_element_is_owner : std::false_type {};
+
+template <class Policy>
+struct policy_trait_element_is_owner<
+    Policy,
+    std::enable_if_t<!std::is_void_v<typename Policy::element_is_owner>>>
+    : Policy::element_is_owner {};
+
+// Defines how slots are initialized/destroyed/moved.
+template <class Policy, class = void>
+struct common_policy_traits {
+  // The actual object stored in the container.
+  using slot_type = typename Policy::slot_type;
+  using reference = decltype(Policy::element(std::declval<slot_type*>()));
+  using value_type = std::remove_reference_t<reference>;
+
+  // PRECONDITION: `slot` is UNINITIALIZED
+  // POSTCONDITION: `slot` is INITIALIZED
+  template <class Alloc, class... Args>
+  static void construct(Alloc* alloc, slot_type* slot, Args&&... args) {
+    Policy::construct(alloc, slot, std::forward<Args>(args)...);
+  }
+
+  // PRECONDITION: `slot` is INITIALIZED
+  // POSTCONDITION: `slot` is UNINITIALIZED
+  // Returns std::true_type in case destroy is trivial.
+  template <class Alloc>
+  static auto destroy(Alloc* alloc, slot_type* slot) {
+    return Policy::destroy(alloc, slot);
+  }
+
+  // Transfers the `old_slot` to `new_slot`. Any memory allocated by the
+  // allocator inside `old_slot` to `new_slot` can be transferred.
+  //
+  // OPTIONAL: defaults to:
+  //
+  //     clone(new_slot, std::move(*old_slot));
+  //     destroy(old_slot);
+  //
+  // PRECONDITION: `new_slot` is UNINITIALIZED and `old_slot` is INITIALIZED
+  // POSTCONDITION: `new_slot` is INITIALIZED and `old_slot` is
+  //                UNINITIALIZED
+  template <class Alloc>
+  static void transfer(Alloc* alloc, slot_type* new_slot, slot_type* old_slot) {
+    transfer_impl(alloc, new_slot, old_slot, Rank2{});
+  }
+
+  // PRECONDITION: `slot` is INITIALIZED
+  // POSTCONDITION: `slot` is INITIALIZED
+  // Note: we use remove_const_t so that the two overloads have different args
+  // in the case of sets with explicitly const value_types.
+  template <class P = Policy>
+  static auto element(std::remove_const_t<slot_type>* slot)
+      -> decltype(P::element(slot)) {
+    return P::element(slot);
+  }
+  template <class P = Policy>
+  static auto element(const slot_type* slot) -> decltype(P::element(slot)) {
+    return P::element(slot);
+  }
+
+  static constexpr bool transfer_uses_memcpy() {
+    return std::is_same_v<decltype(transfer_impl<std::allocator<char>>(
+                              nullptr, nullptr, nullptr, Rank2{})),
+                          std::true_type>;
+  }
+
+  // Returns true if destroy is trivial and can be omitted.
+  template <class Alloc>
+  static constexpr bool destroy_is_trivial() {
+    return std::is_same_v<decltype(destroy<Alloc>(nullptr, nullptr)),
+                          std::true_type>;
+  }
+
+ private:
+  // Use go/ranked-overloads for dispatching.
+  struct Rank0 {};
+  struct Rank1 : Rank0 {};
+  struct Rank2 : Rank1 {};
+
+  // Use auto -> decltype as an enabler.
+  // P::transfer returns std::true_type if transfer uses memcpy (e.g. in
+  // node_slot_policy).
+  template <class Alloc, class P = Policy>
+  static auto transfer_impl(Alloc* alloc, slot_type* new_slot,
+                            slot_type* old_slot,
+                            Rank2) -> decltype(P::transfer(alloc, new_slot,
+                                                           old_slot)) {
+    return P::transfer(alloc, new_slot, old_slot);
+  }
+
+  // This overload returns true_type for the trait below.
+  // The conditional_t is to make the enabler type dependent.
+  template <class Alloc,
+            typename = std::enable_if_t<absl::is_trivially_relocatable<
+                std::conditional_t<false, Alloc, value_type>>::value>>
+  static std::true_type transfer_impl(Alloc*, slot_type* new_slot,
+                                      slot_type* old_slot, Rank1) {
+    // TODO(b/247130232): remove casts after fixing warnings.
+    // TODO(b/251814870): remove casts after fixing warnings.
+    std::memcpy(
+        static_cast<void*>(std::launder(
+            const_cast<std::remove_const_t<value_type>*>(&element(new_slot)))),
+        static_cast<const void*>(&element(old_slot)), sizeof(value_type));
+    return {};
+  }
+
+  template <class Alloc>
+  static void transfer_impl(Alloc* alloc, slot_type* new_slot,
+                            slot_type* old_slot, Rank0) {
+    construct(alloc, new_slot, std::move(element(old_slot)));
+    destroy(alloc, old_slot);
+  }
+};
+
+}  // namespace container_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_CONTAINER_INTERNAL_COMMON_POLICY_TRAITS_H_
diff --git a/absl/container/internal/common_policy_traits_test.cc b/absl/container/internal/common_policy_traits_test.cc
new file mode 100644
index 00000000000..8d8f8baa05f
--- /dev/null
+++ b/absl/container/internal/common_policy_traits_test.cc
@@ -0,0 +1,157 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/container/internal/common_policy_traits.h"
+
+#include <functional>
+#include <memory>
+#include <type_traits>
+#include <utility>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace container_internal {
+namespace {
+
+using ::testing::MockFunction;
+using ::testing::AnyNumber;
+using ::testing::ReturnRef;
+
+using Slot = int;
+
+struct PolicyWithoutOptionalOps {
+  using slot_type = Slot;
+  using key_type = Slot;
+  using init_type = Slot;
+
+  struct PolicyFunctions {
+    std::function<void(void*, Slot*, Slot)> construct;
+    std::function<void(void*, Slot*)> destroy;
+    std::function<Slot&(Slot*)> element;
+  };
+
+  static PolicyFunctions* functions() {
+    static PolicyFunctions* functions = new PolicyFunctions();
+    return functions;
+  }
+
+  static void construct(void* a, Slot* b, Slot c) {
+    functions()->construct(a, b, c);
+  }
+  static void destroy(void* a, Slot* b) { functions()->destroy(a, b); }
+  static Slot& element(Slot* b) { return functions()->element(b); }
+};
+
+struct PolicyWithOptionalOps : PolicyWithoutOptionalOps {
+  struct TransferFunctions {
+    std::function<void(void*, Slot*, Slot*)> transfer;
+  };
+
+  static TransferFunctions* transfer_fn() {
+    static TransferFunctions* transfer_fn = new TransferFunctions();
+    return transfer_fn;
+  }
+  static void transfer(void* a, Slot* b, Slot* c) {
+    transfer_fn()->transfer(a, b, c);
+  }
+};
+
+struct PolicyWithMemcpyTransferAndTrivialDestroy : PolicyWithoutOptionalOps {
+  static std::true_type transfer(void*, Slot*, Slot*) { return {}; }
+  static std::true_type destroy(void*, Slot*) { return {}; }
+};
+
+struct Test : ::testing::Test {
+  Test() {
+    PolicyWithoutOptionalOps::functions()->construct = [&](void* a1, Slot* a2,
+                                                           Slot a3) {
+      construct.Call(a1, a2, std::move(a3));
+    };
+    PolicyWithoutOptionalOps::functions()->destroy = [&](void* a1, Slot* a2) {
+      destroy.Call(a1, a2);
+    };
+
+    PolicyWithoutOptionalOps::functions()->element = [&](Slot* a1) -> Slot& {
+      return element.Call(a1);
+    };
+
+    PolicyWithOptionalOps::transfer_fn()->transfer =
+        [&](void* a1, Slot* a2, Slot* a3) { return transfer.Call(a1, a2, a3); };
+  }
+
+  std::allocator<Slot> alloc;
+  int a = 53;
+
+  MockFunction<void(void*, Slot*, Slot)> construct;
+  MockFunction<void(void*, Slot*)> destroy;
+
+  MockFunction<Slot&(Slot*)> element;
+
+  MockFunction<void(void*, Slot*, Slot*)> transfer;
+};
+
+TEST_F(Test, construct) {
+  EXPECT_CALL(construct, Call(&alloc, &a, 53));
+  common_policy_traits<PolicyWithoutOptionalOps>::construct(&alloc, &a, 53);
+}
+
+TEST_F(Test, destroy) {
+  EXPECT_CALL(destroy, Call(&alloc, &a));
+  common_policy_traits<PolicyWithoutOptionalOps>::destroy(&alloc, &a);
+}
+
+TEST_F(Test, element) {
+  int b = 0;
+  EXPECT_CALL(element, Call(&a)).WillOnce(ReturnRef(b));
+  EXPECT_EQ(&b, &common_policy_traits<PolicyWithoutOptionalOps>::element(&a));
+}
+
+TEST_F(Test, without_transfer) {
+  int b = 42;
+  EXPECT_CALL(element, Call(&a)).Times(AnyNumber()).WillOnce(ReturnRef(a));
+  EXPECT_CALL(element, Call(&b)).WillOnce(ReturnRef(b));
+  EXPECT_CALL(construct, Call(&alloc, &a, b)).Times(AnyNumber());
+  EXPECT_CALL(destroy, Call(&alloc, &b)).Times(AnyNumber());
+  common_policy_traits<PolicyWithoutOptionalOps>::transfer(&alloc, &a, &b);
+}
+
+TEST_F(Test, with_transfer) {
+  int b = 42;
+  EXPECT_CALL(transfer, Call(&alloc, &a, &b));
+  common_policy_traits<PolicyWithOptionalOps>::transfer(&alloc, &a, &b);
+}
+
+TEST(TransferUsesMemcpy, Basic) {
+  EXPECT_FALSE(
+      common_policy_traits<PolicyWithOptionalOps>::transfer_uses_memcpy());
+  EXPECT_TRUE(
+      common_policy_traits<
+          PolicyWithMemcpyTransferAndTrivialDestroy>::transfer_uses_memcpy());
+}
+
+TEST(DestroyIsTrivial, Basic) {
+  EXPECT_FALSE(common_policy_traits<PolicyWithOptionalOps>::destroy_is_trivial<
+               std::allocator<char>>());
+  EXPECT_TRUE(common_policy_traits<PolicyWithMemcpyTransferAndTrivialDestroy>::
+                  destroy_is_trivial<std::allocator<char>>());
+}
+
+}  // namespace
+}  // namespace container_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/container/internal/compressed_tuple.h b/absl/container/internal/compressed_tuple.h
index 7d08e370dec..4d0fb2a08e2 100644
--- a/absl/container/internal/compressed_tuple.h
+++ b/absl/container/internal/compressed_tuple.h
@@ -48,6 +48,7 @@
 #endif
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
 template <typename... Ts>
@@ -63,96 +64,81 @@ struct Elem<CompressedTuple<B...>, I>
 template <typename D, size_t I>
 using ElemT = typename Elem<D, I>::type;
 
-// Use the __is_final intrinsic if available. Where it's not available, classes
-// declared with the 'final' specifier cannot be used as CompressedTuple
-// elements.
-// TODO(sbenza): Replace this with std::is_final in C++14.
-template <typename T>
-constexpr bool IsFinal() {
-#if defined(__clang__) || defined(__GNUC__)
-  return __is_final(T);
-#else
-  return false;
-#endif
-}
-
-// We can't use EBCO on other CompressedTuples because that would mean that we
-// derive from multiple Storage<> instantiations with the same I parameter,
-// and potentially from multiple identical Storage<> instantiations.  So anytime
-// we use type inheritance rather than encapsulation, we mark
-// CompressedTupleImpl, to make this easy to detect.
-struct uses_inheritance {};
 
 template <typename T>
 constexpr bool ShouldUseBase() {
-  return std::is_class<T>::value && std::is_empty<T>::value && !IsFinal<T>() &&
-         !std::is_base_of<uses_inheritance, T>::value;
+  return std::is_class_v<T> && std::is_empty_v<T> && !std::is_final_v<T>;
 }
 
+// Tag type used to disambiguate Storage types for different CompresseedTuples.
+// Without it, CompressedTuple<T, CompressedTuple<T>> would inherit from
+// Storage<T, 0> twice.
+template <typename... Ts>
+struct StorageTag;
+
 // The storage class provides two specializations:
 //  - For empty classes, it stores T as a base class.
 //  - For everything else, it stores T as a member.
-template <typename T, size_t I,
-#if defined(_MSC_VER)
-          bool UseBase =
-              ShouldUseBase<typename std::enable_if<true, T>::type>()>
-#else
-          bool UseBase = ShouldUseBase<T>()>
-#endif
+// Tag should be set to StorageTag<Ts...>.
+template <typename T, size_t I, typename Tag, bool UseBase = ShouldUseBase<T>()>
 struct Storage {
   T value;
   constexpr Storage() = default;
   template <typename V>
-  explicit constexpr Storage(absl::in_place_t, V&& v)
-      : value(absl::forward<V>(v)) {}
+  explicit constexpr Storage(std::in_place_t, V&& v)
+      : value(std::forward<V>(v)) {}
   constexpr const T& get() const& { return value; }
-  T& get() & { return value; }
-  constexpr const T&& get() const&& { return absl::move(*this).value; }
-  T&& get() && { return std::move(*this).value; }
+  constexpr T& get() & { return value; }
+  constexpr const T&& get() const&& { return std::move(*this).value; }
+  constexpr T&& get() && { return std::move(*this).value; }
 };
 
-template <typename T, size_t I>
-struct ABSL_INTERNAL_COMPRESSED_TUPLE_DECLSPEC Storage<T, I, true> : T {
+template <typename T, size_t I, typename Tag>
+struct ABSL_INTERNAL_COMPRESSED_TUPLE_DECLSPEC Storage<T, I, Tag, true> : T {
   constexpr Storage() = default;
 
   template <typename V>
-  explicit constexpr Storage(absl::in_place_t, V&& v)
-      : T(absl::forward<V>(v)) {}
+  explicit constexpr Storage(std::in_place_t, V&& v) : T(std::forward<V>(v)) {}
 
   constexpr const T& get() const& { return *this; }
-  T& get() & { return *this; }
-  constexpr const T&& get() const&& { return absl::move(*this); }
-  T&& get() && { return std::move(*this); }
+  constexpr T& get() & { return *this; }
+  constexpr const T&& get() const&& { return std::move(*this); }
+  constexpr T&& get() && { return std::move(*this); }
 };
 
 template <typename D, typename I, bool ShouldAnyUseBase>
 struct ABSL_INTERNAL_COMPRESSED_TUPLE_DECLSPEC CompressedTupleImpl;
 
 template <typename... Ts, size_t... I, bool ShouldAnyUseBase>
-struct ABSL_INTERNAL_COMPRESSED_TUPLE_DECLSPEC CompressedTupleImpl<
-    CompressedTuple<Ts...>, absl::index_sequence<I...>, ShouldAnyUseBase>
+struct ABSL_INTERNAL_COMPRESSED_TUPLE_DECLSPEC
+    CompressedTupleImpl<CompressedTuple<Ts...>, std::index_sequence<I...>,
+                        ShouldAnyUseBase>
     // We use the dummy identity function through std::integral_constant to
     // convince MSVC of accepting and expanding I in that context. Without it
     // you would get:
     //   error C3548: 'I': parameter pack cannot be used in this context
-    : uses_inheritance,
-      Storage<Ts, std::integral_constant<size_t, I>::value>... {
+    : Storage<Ts, std::integral_constant<size_t, I>::value,
+              StorageTag<Ts...>>... {
   constexpr CompressedTupleImpl() = default;
   template <typename... Vs>
-  explicit constexpr CompressedTupleImpl(absl::in_place_t, Vs&&... args)
-      : Storage<Ts, I>(absl::in_place, absl::forward<Vs>(args))... {}
+  explicit constexpr CompressedTupleImpl(std::in_place_t, Vs&&... args)
+      : Storage<Ts, I, StorageTag<Ts...>>(std::in_place,
+                                          std::forward<Vs>(args))... {}
   friend CompressedTuple<Ts...>;
 };
 
 template <typename... Ts, size_t... I>
-struct ABSL_INTERNAL_COMPRESSED_TUPLE_DECLSPEC CompressedTupleImpl<
-    CompressedTuple<Ts...>, absl::index_sequence<I...>, false>
+struct ABSL_INTERNAL_COMPRESSED_TUPLE_DECLSPEC
+    CompressedTupleImpl<CompressedTuple<Ts...>, std::index_sequence<I...>,
+                        false>
     // We use the dummy identity function as above...
-    : Storage<Ts, std::integral_constant<size_t, I>::value, false>... {
+    : Storage<Ts, std::integral_constant<size_t, I>::value, StorageTag<Ts...>,
+              false>... {
   constexpr CompressedTupleImpl() = default;
   template <typename... Vs>
-  explicit constexpr CompressedTupleImpl(absl::in_place_t, Vs&&... args)
-      : Storage<Ts, I, false>(absl::in_place, absl::forward<Vs>(args))... {}
+  explicit constexpr CompressedTupleImpl(std::in_place_t, Vs&&... args)
+      : Storage<Ts, I, StorageTag<Ts...>, false>(std::in_place,
+                                                 std::forward<Vs>(args))... {}
   friend CompressedTuple<Ts...>;
 };
 
@@ -163,23 +149,42 @@ std::true_type Or(std::initializer_list<bool>);
 // of CompressedTuple below.
 template <typename... Ts>
 constexpr bool ShouldAnyUseBase() {
-  return decltype(
-      Or({std::integral_constant<bool, ShouldUseBase<Ts>()>()...})){};
+  return decltype(Or({std::bool_constant<ShouldUseBase<Ts>()>()...})){};
 }
 
 template <typename T, typename V>
-using TupleMoveConstructible = typename std::conditional<
-      std::is_reference<T>::value, std::is_convertible<V, T>,
-      std::is_constructible<T, V&&>>::type;
+using TupleElementMoveConstructible =
+    std::conditional_t<std::is_reference_v<T>, std::is_convertible<V, T>,
+                       std::is_constructible<T, V&&>>;
+
+template <bool SizeMatches, class T, class... Vs>
+struct TupleMoveConstructible : std::false_type {};
+
+template <class... Ts, class... Vs>
+struct TupleMoveConstructible<true, CompressedTuple<Ts...>, Vs...>
+    : std::integral_constant<
+          bool,
+          std::conjunction_v<TupleElementMoveConstructible<Ts, Vs&&>...>> {};
+
+template <typename T>
+struct compressed_tuple_size;
+
+template <typename... Es>
+struct compressed_tuple_size<CompressedTuple<Es...>>
+    : public std::integral_constant<std::size_t, sizeof...(Es)> {};
+
+template <class T, class... Vs>
+struct TupleItemsMoveConstructible
+    : std::bool_constant<TupleMoveConstructible<
+          compressed_tuple_size<T>::value == sizeof...(Vs), T, Vs...>::value> {
+};
 
 }  // namespace internal_compressed_tuple
 
 // Helper class to perform the Empty Base Class Optimization.
 // Ts can contain classes and non-classes, empty or not. For the ones that
 // are empty classes, we perform the CompressedTuple. If all types in Ts are
-// empty classes, then CompressedTuple<Ts...> is itself an empty class.  (This
-// does not apply when one or more of those empty classes is itself an empty
-// CompressedTuple.)
+// empty classes, then CompressedTuple<Ts...> is itself an empty class.
 //
 // To access the members, use member .get<N>() function.
 //
@@ -195,14 +200,15 @@ using TupleMoveConstructible = typename std::conditional<
 template <typename... Ts>
 class ABSL_INTERNAL_COMPRESSED_TUPLE_DECLSPEC CompressedTuple
     : private internal_compressed_tuple::CompressedTupleImpl<
-          CompressedTuple<Ts...>, absl::index_sequence_for<Ts...>,
+          CompressedTuple<Ts...>, std::index_sequence_for<Ts...>,
           internal_compressed_tuple::ShouldAnyUseBase<Ts...>()> {
  private:
   template <int I>
   using ElemT = internal_compressed_tuple::ElemT<CompressedTuple, I>;
 
   template <int I>
-  using StorageT = internal_compressed_tuple::Storage<ElemT<I>, I>;
+  using StorageT = internal_compressed_tuple::Storage<
+      ElemT<I>, I, internal_compressed_tuple::StorageTag<Ts...>>;
 
  public:
   // There seems to be a bug in MSVC dealing in which using '=default' here will
@@ -214,24 +220,25 @@ class ABSL_INTERNAL_COMPRESSED_TUPLE_DECLSPEC CompressedTuple
   constexpr CompressedTuple() = default;
 #endif
   explicit constexpr CompressedTuple(const Ts&... base)
-      : CompressedTuple::CompressedTupleImpl(absl::in_place, base...) {}
+      : CompressedTuple::CompressedTupleImpl(std::in_place, base...) {}
 
-  template <typename... Vs,
-            absl::enable_if_t<
-                absl::conjunction<
+  template <typename First, typename... Vs,
+            std::enable_if_t<
+                std::conjunction_v<
                     // Ensure we are not hiding default copy/move constructors.
-                    absl::negation<std::is_same<void(CompressedTuple),
-                                                void(absl::decay_t<Vs>...)>>,
-                    internal_compressed_tuple::TupleMoveConstructible<
-                        Ts, Vs&&>...>::value,
+                    std::negation<std::is_same<void(CompressedTuple),
+                                               void(std::decay_t<First>)>>,
+                    internal_compressed_tuple::TupleItemsMoveConstructible<
+                        CompressedTuple<Ts...>, First, Vs...>>,
                 bool> = true>
-  explicit constexpr CompressedTuple(Vs&&... base)
-      : CompressedTuple::CompressedTupleImpl(absl::in_place,
-                                             absl::forward<Vs>(base)...) {}
+  explicit constexpr CompressedTuple(First&& first, Vs&&... base)
+      : CompressedTuple::CompressedTupleImpl(std::in_place,
+                                             std::forward<First>(first),
+                                             std::forward<Vs>(base)...) {}
 
   template <int I>
-  ElemT<I>& get() & {
-    return internal_compressed_tuple::Storage<ElemT<I>, I>::get();
+  constexpr ElemT<I>& get() & {
+    return StorageT<I>::get();
   }
 
   template <int I>
@@ -240,13 +247,13 @@ class ABSL_INTERNAL_COMPRESSED_TUPLE_DECLSPEC CompressedTuple
   }
 
   template <int I>
-  ElemT<I>&& get() && {
+  constexpr ElemT<I>&& get() && {
     return std::move(*this).StorageT<I>::get();
   }
 
   template <int I>
   constexpr const ElemT<I>&& get() const&& {
-    return absl::move(*this).StorageT<I>::get();
+    return std::move(*this).StorageT<I>::get();
   }
 };
 
@@ -256,6 +263,7 @@ template <>
 class ABSL_INTERNAL_COMPRESSED_TUPLE_DECLSPEC CompressedTuple<> {};
 
 }  // namespace container_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #undef ABSL_INTERNAL_COMPRESSED_TUPLE_DECLSPEC
diff --git a/absl/container/internal/compressed_tuple_test.cc b/absl/container/internal/compressed_tuple_test.cc
index 19af8f10aa6..2331fc4463e 100644
--- a/absl/container/internal/compressed_tuple_test.cc
+++ b/absl/container/internal/compressed_tuple_test.cc
@@ -14,27 +14,39 @@
 
 #include "absl/container/internal/compressed_tuple.h"
 
+#include <any>
 #include <memory>
+#include <optional>
+#include <set>
 #include <string>
+#include <type_traits>
+#include <utility>
+#include <vector>
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
 #include "absl/container/internal/test_instance_tracker.h"
 #include "absl/memory/memory.h"
-#include "absl/types/any.h"
-#include "absl/types/optional.h"
 #include "absl/utility/utility.h"
 
 // These are declared at global scope purely so that error messages
 // are smaller and easier to understand.
-enum class CallType { kConstRef, kConstMove };
+enum class CallType { kMutableRef, kConstRef, kMutableMove, kConstMove };
 
 template <int>
 struct Empty {
+  constexpr CallType value() & { return CallType::kMutableRef; }
   constexpr CallType value() const& { return CallType::kConstRef; }
+  constexpr CallType value() && { return CallType::kMutableMove; }
   constexpr CallType value() const&& { return CallType::kConstMove; }
 };
 
+// Unconditionally return an lvalue reference to `t`.
+template <typename T>
+constexpr T& AsLValue(T&& t) {
+  return t;
+}
+
 template <typename T>
 struct NotEmpty {
   T value;
@@ -48,11 +60,13 @@ struct TwoValues {
 
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 namespace {
 
 using absl::test_internal::CopyableMovableInstance;
 using absl::test_internal::InstanceTracker;
+using ::testing::Each;
 
 TEST(CompressedTupleTest, Sizeof) {
   EXPECT_EQ(sizeof(int), sizeof(CompressedTuple<int>));
@@ -69,6 +83,30 @@ TEST(CompressedTupleTest, Sizeof) {
             sizeof(CompressedTuple<int, Empty<0>, NotEmpty<double>, Empty<1>>));
 }
 
+TEST(CompressedTupleTest, PointerToEmpty) {
+  auto to_void_ptrs = [](const auto&... objs) {
+    return std::vector<const void*>{static_cast<const void*>(&objs)...};
+  };
+  {
+    using Tuple = CompressedTuple<int, Empty<0>>;
+    EXPECT_EQ(sizeof(int), sizeof(Tuple));
+    Tuple t;
+    EXPECT_THAT(to_void_ptrs(t.get<1>()), Each(&t));
+  }
+  {
+    using Tuple = CompressedTuple<int, Empty<0>, Empty<1>>;
+    EXPECT_EQ(sizeof(int), sizeof(Tuple));
+    Tuple t;
+    EXPECT_THAT(to_void_ptrs(t.get<1>(), t.get<2>()), Each(&t));
+  }
+  {
+    using Tuple = CompressedTuple<int, Empty<0>, Empty<1>, Empty<2>>;
+    EXPECT_EQ(sizeof(int), sizeof(Tuple));
+    Tuple t;
+    EXPECT_THAT(to_void_ptrs(t.get<1>(), t.get<2>(), t.get<3>()), Each(&t));
+  }
+}
+
 TEST(CompressedTupleTest, OneMoveOnRValueConstructionTemp) {
   InstanceTracker tracker;
   CompressedTuple<CopyableMovableInstance> x1(CopyableMovableInstance(1));
@@ -264,23 +302,23 @@ TEST(CompressedTupleTest, Nested) {
   EXPECT_EQ(4 * sizeof(char),
             sizeof(CompressedTuple<CompressedTuple<char, char>,
                                    CompressedTuple<char, char>>));
-  EXPECT_TRUE((std::is_empty<CompressedTuple<Empty<0>, Empty<1>>>::value));
+  EXPECT_TRUE((std::is_empty_v<CompressedTuple<Empty<0>, Empty<1>>>));
 
   // Make sure everything still works when things are nested.
   struct CT_Empty : CompressedTuple<Empty<0>> {};
   CompressedTuple<Empty<0>, CT_Empty> nested_empty;
   auto contained = nested_empty.get<0>();
   auto nested = nested_empty.get<1>().get<0>();
-  EXPECT_TRUE((std::is_same<decltype(contained), decltype(nested)>::value));
+  EXPECT_TRUE((std::is_same_v<decltype(contained), decltype(nested)>));
 }
 
 TEST(CompressedTupleTest, Reference) {
   int i = 7;
-  std::string s = "Very long std::string that goes in the heap";
+  std::string s = "Very long string that goes in the heap";
   CompressedTuple<int, int&, std::string, std::string&> x(i, i, s, s);
 
   // Sanity check. We should have not moved from `s`
-  EXPECT_EQ(s, "Very long std::string that goes in the heap");
+  EXPECT_EQ(s, "Very long string that goes in the heap");
 
   EXPECT_EQ(x.get<0>(), x.get<1>());
   EXPECT_NE(&x.get<0>(), &x.get<1>());
@@ -294,16 +332,16 @@ TEST(CompressedTupleTest, Reference) {
 TEST(CompressedTupleTest, NoElements) {
   CompressedTuple<> x;
   static_cast<void>(x);  // Silence -Wunused-variable.
-  EXPECT_TRUE(std::is_empty<CompressedTuple<>>::value);
+  EXPECT_TRUE(std::is_empty_v<CompressedTuple<>>);
 }
 
 TEST(CompressedTupleTest, MoveOnlyElements) {
   CompressedTuple<std::unique_ptr<std::string>> str_tup(
-      absl::make_unique<std::string>("str"));
+      std::make_unique<std::string>("str"));
 
   CompressedTuple<CompressedTuple<std::unique_ptr<std::string>>,
                   std::unique_ptr<int>>
-  x(std::move(str_tup), absl::make_unique<int>(5));
+      x(std::move(str_tup), std::make_unique<int>(5));
 
   EXPECT_EQ(*x.get<0>().get<0>(), "str");
   EXPECT_EQ(*x.get<1>(), 5);
@@ -317,7 +355,7 @@ TEST(CompressedTupleTest, MoveOnlyElements) {
 
 TEST(CompressedTupleTest, MoveConstructionMoveOnlyElements) {
   CompressedTuple<std::unique_ptr<std::string>> base(
-      absl::make_unique<std::string>("str"));
+      std::make_unique<std::string>("str"));
   EXPECT_EQ(*base.get<0>(), "str");
 
   CompressedTuple<std::unique_ptr<std::string>> copy(std::move(base));
@@ -325,17 +363,13 @@ TEST(CompressedTupleTest, MoveConstructionMoveOnlyElements) {
 }
 
 TEST(CompressedTupleTest, AnyElements) {
-  any a(std::string("str"));
-  CompressedTuple<any, any&> x(any(5), a);
-  EXPECT_EQ(absl::any_cast<int>(x.get<0>()), 5);
-  EXPECT_EQ(absl::any_cast<std::string>(x.get<1>()), "str");
+  std::any a(std::string("str"));
+  CompressedTuple<std::any, std::any&> x(std::any(5), a);
+  EXPECT_EQ(std::any_cast<int>(x.get<0>()), 5);
+  EXPECT_EQ(std::any_cast<std::string>(x.get<1>()), "str");
 
   a = 0.5f;
-  EXPECT_EQ(absl::any_cast<float>(x.get<1>()), 0.5);
-
-  // Ensure copy construction work in the face of a type with a universal
-  // implicit constructor;
-  CompressedTuple<absl::any> c{}, d(c);  // NOLINT
+  EXPECT_EQ(std::any_cast<float>(x.get<1>()), 0.5);
 }
 
 TEST(CompressedTupleTest, Constexpr) {
@@ -349,8 +383,10 @@ TEST(CompressedTupleTest, Constexpr) {
     constexpr int value() const { return v; }
     int v;
   };
-  constexpr CompressedTuple<int, double, CompressedTuple<int>, Empty<0>> x(
-      7, 1.25, CompressedTuple<int>(5), {});
+
+  using Tuple = CompressedTuple<int, double, CompressedTuple<int>, Empty<0>>;
+
+  constexpr Tuple x(7, 1.25, CompressedTuple<int>(5), {});
   constexpr int x0 = x.get<0>();
   constexpr double x1 = x.get<1>();
   constexpr int x2 = x.get<2>().get<0>();
@@ -361,7 +397,18 @@ TEST(CompressedTupleTest, Constexpr) {
   EXPECT_EQ(x2, 5);
   EXPECT_EQ(x3, CallType::kConstRef);
 
-#if !defined(__GNUC__) || defined(__clang__) || __GNUC__ > 4
+  constexpr int m0 = Tuple(5, 0.25, CompressedTuple<int>(3), {}).get<0>();
+  constexpr double m1 = Tuple(5, 0.25, CompressedTuple<int>(3), {}).get<1>();
+  constexpr int m2 =
+      Tuple(5, 0.25, CompressedTuple<int>(3), {}).get<2>().get<0>();
+  constexpr CallType m3 =
+      Tuple(5, 0.25, CompressedTuple<int>(3), {}).get<3>().value();
+
+  EXPECT_EQ(m0, 5);
+  EXPECT_EQ(m1, 0.25);
+  EXPECT_EQ(m2, 3);
+  EXPECT_EQ(m3, CallType::kMutableMove);
+
   constexpr CompressedTuple<Empty<0>, TrivialStruct, int> trivial = {};
   constexpr CallType trivial0 = trivial.get<0>().value();
   constexpr int trivial1 = trivial.get<1>().value();
@@ -370,17 +417,16 @@ TEST(CompressedTupleTest, Constexpr) {
   EXPECT_EQ(trivial0, CallType::kConstRef);
   EXPECT_EQ(trivial1, 0);
   EXPECT_EQ(trivial2, 0);
-#endif
 
-  constexpr CompressedTuple<Empty<0>, NonTrivialStruct, absl::optional<int>>
+  constexpr CompressedTuple<Empty<0>, NonTrivialStruct, std::optional<int>>
       non_trivial = {};
   constexpr CallType non_trivial0 = non_trivial.get<0>().value();
   constexpr int non_trivial1 = non_trivial.get<1>().value();
-  constexpr absl::optional<int> non_trivial2 = non_trivial.get<2>();
+  constexpr std::optional<int> non_trivial2 = non_trivial.get<2>();
 
   EXPECT_EQ(non_trivial0, CallType::kConstRef);
   EXPECT_EQ(non_trivial1, 5);
-  EXPECT_EQ(non_trivial2, absl::nullopt);
+  EXPECT_EQ(non_trivial2, std::nullopt);
 
   static constexpr char data[] = "DEF";
   constexpr CompressedTuple<const char*> z(data);
@@ -389,8 +435,8 @@ TEST(CompressedTupleTest, Constexpr) {
 
 #if defined(__clang__)
   // An apparent bug in earlier versions of gcc claims these are ambiguous.
-  constexpr int x2m = absl::move(x.get<2>()).get<0>();
-  constexpr CallType x3m = absl::move(x).get<3>().value();
+  constexpr int x2m = std::move(x.get<2>()).get<0>();
+  constexpr CallType x3m = std::move(x).get<3>().value();
   EXPECT_EQ(x2m, 5);
   EXPECT_EQ(x3m, CallType::kConstMove);
 #endif
@@ -406,6 +452,18 @@ TEST(CompressedTupleTest, EmptyFinalClass) {
 }
 #endif
 
+TEST(CompressedTupleTest, NestedEbo) {
+  struct Empty1 {};
+  struct Empty2 {};
+  CompressedTuple<Empty1, CompressedTuple<Empty2>, int> x;
+  CompressedTuple<Empty1, Empty2, int> y;
+  EXPECT_EQ(sizeof(x), sizeof(y));
+
+  using NestedEmpty = CompressedTuple<Empty1, CompressedTuple<Empty2>>;
+  EXPECT_TRUE(std::is_empty_v<NestedEmpty>);
+}
+
 }  // namespace
 }  // namespace container_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/container/internal/container_memory.h b/absl/container/internal/container_memory.h
index e5bb977355e..fceb4ebaae1 100644
--- a/absl/container/internal/container_memory.h
+++ b/absl/container/internal/container_memory.h
@@ -15,27 +15,42 @@
 #ifndef ABSL_CONTAINER_INTERNAL_CONTAINER_MEMORY_H_
 #define ABSL_CONTAINER_INTERNAL_CONTAINER_MEMORY_H_
 
-#ifdef ADDRESS_SANITIZER
-#include <sanitizer/asan_interface.h>
-#endif
-
-#ifdef MEMORY_SANITIZER
-#include <sanitizer/msan_interface.h>
-#endif
-
 #include <cassert>
 #include <cstddef>
+#include <cstdint>
+#include <cstring>
 #include <memory>
+#include <new>
 #include <tuple>
 #include <type_traits>
 #include <utility>
 
+#include "absl/base/config.h"
+#include "absl/hash/hash.h"
 #include "absl/memory/memory.h"
+#include "absl/meta/type_traits.h"
 #include "absl/utility/utility.h"
 
+#ifdef ABSL_HAVE_ADDRESS_SANITIZER
+#include <sanitizer/asan_interface.h>
+#endif
+
+#ifdef ABSL_HAVE_MEMORY_SANITIZER
+#include <sanitizer/msan_interface.h>
+#endif
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
+template <size_t Alignment>
+struct alignas(Alignment) AlignedType {
+  // When alignment is sufficient for the allocated memory to store pointers,
+  // include a pointer member so that swisstable backing arrays end up in the
+  // pointer-containing partition for heap partitioning.
+  std::conditional_t<(Alignment < alignof(void*)), char, void*> pointer;
+};
+
 // Allocates at least n bytes aligned to the specified alignment.
 // Alignment must be a power of 2. It must be positive.
 //
@@ -45,29 +60,47 @@ namespace container_internal {
 // returns insufficiently alignment pointer, that's what you are going to get.
 template <size_t Alignment, class Alloc>
 void* Allocate(Alloc* alloc, size_t n) {
-  static_assert(Alignment > 0, "");
+  static_assert(Alignment > 0);
   assert(n && "n must be positive");
-  struct alignas(Alignment) M {};
-  using A = typename absl::allocator_traits<Alloc>::template rebind_alloc<M>;
-  using AT = typename absl::allocator_traits<Alloc>::template rebind_traits<M>;
-  A mem_alloc(*alloc);
-  void* p = AT::allocate(mem_alloc, (n + sizeof(M) - 1) / sizeof(M));
+  using M = AlignedType<Alignment>;
+  using A = typename std::allocator_traits<Alloc>::template rebind_alloc<M>;
+  using AT = typename std::allocator_traits<Alloc>::template rebind_traits<M>;
+  // On macOS, "mem_alloc" is a #define with one argument defined in
+  // rpc/types.h, so we can't name the variable "mem_alloc" and initialize it
+  // with the "foo(bar)" syntax.
+  A my_mem_alloc(*alloc);
+  void* p = AT::allocate(my_mem_alloc, (n + sizeof(M) - 1) / sizeof(M));
   assert(reinterpret_cast<uintptr_t>(p) % Alignment == 0 &&
          "allocator does not respect alignment");
   return p;
 }
 
+// Returns true if the destruction of the value with given Allocator will be
+// trivial.
+template <class Allocator, class ValueType>
+constexpr auto IsDestructionTrivial() {
+  constexpr bool result =
+      std::is_trivially_destructible_v<ValueType> &&
+      std::is_same_v<typename std::allocator_traits<
+                         Allocator>::template rebind_alloc<char>,
+                     std::allocator<char>>;
+  return std::bool_constant<result>();
+}
+
 // The pointer must have been previously obtained by calling
 // Allocate<Alignment>(alloc, n).
 template <size_t Alignment, class Alloc>
 void Deallocate(Alloc* alloc, void* p, size_t n) {
-  static_assert(Alignment > 0, "");
+  static_assert(Alignment > 0);
   assert(n && "n must be positive");
-  struct alignas(Alignment) M {};
-  using A = typename absl::allocator_traits<Alloc>::template rebind_alloc<M>;
-  using AT = typename absl::allocator_traits<Alloc>::template rebind_traits<M>;
-  A mem_alloc(*alloc);
-  AT::deallocate(mem_alloc, static_cast<M*>(p),
+  using M = AlignedType<Alignment>;
+  using A = typename std::allocator_traits<Alloc>::template rebind_alloc<M>;
+  using AT = typename std::allocator_traits<Alloc>::template rebind_traits<M>;
+  // On macOS, "mem_alloc" is a #define with one argument defined in
+  // rpc/types.h, so we can't name the variable "mem_alloc" and initialize it
+  // with the "foo(bar)" syntax.
+  A my_mem_alloc(*alloc);
+  AT::deallocate(my_mem_alloc, static_cast<M*>(p),
                  (n + sizeof(M) - 1) / sizeof(M));
 }
 
@@ -77,8 +110,8 @@ namespace memory_internal {
 // specified in the tuple.
 template <class Alloc, class T, class Tuple, size_t... I>
 void ConstructFromTupleImpl(Alloc* alloc, T* ptr, Tuple&& t,
-                            absl::index_sequence<I...>) {
-  absl::allocator_traits<Alloc>::construct(
+                            std::index_sequence<I...>) {
+  std::allocator_traits<Alloc>::construct(
       *alloc, ptr, std::get<I>(std::forward<Tuple>(t))...);
 }
 
@@ -94,28 +127,27 @@ struct WithConstructedImplF {
 
 template <class T, class Tuple, size_t... Is, class F>
 decltype(std::declval<F>()(std::declval<T>())) WithConstructedImpl(
-    Tuple&& t, absl::index_sequence<Is...>, F&& f) {
+    Tuple&& t, std::index_sequence<Is...>, F&& f) {
   return WithConstructedImplF<T, F>{std::forward<F>(f)}(
       std::get<Is>(std::forward<Tuple>(t))...);
 }
 
 template <class T, size_t... Is>
-auto TupleRefImpl(T&& t, absl::index_sequence<Is...>)
+auto TupleRefImpl(T&& t, std::index_sequence<Is...>)
     -> decltype(std::forward_as_tuple(std::get<Is>(std::forward<T>(t))...)) {
+  // NOLINTNEXTLINE(bugprone-use-after-move)
   return std::forward_as_tuple(std::get<Is>(std::forward<T>(t))...);
 }
 
 // Returns a tuple of references to the elements of the input tuple. T must be a
 // tuple.
 template <class T>
-auto TupleRef(T&& t) -> decltype(
-    TupleRefImpl(std::forward<T>(t),
-                 absl::make_index_sequence<
-                     std::tuple_size<typename std::decay<T>::type>::value>())) {
+auto TupleRef(T&& t) -> decltype(TupleRefImpl(
+    std::forward<T>(t),
+    std::make_index_sequence<std::tuple_size_v<std::decay_t<T>>>())) {
   return TupleRefImpl(
       std::forward<T>(t),
-      absl::make_index_sequence<
-          std::tuple_size<typename std::decay<T>::type>::value>());
+      std::make_index_sequence<std::tuple_size_v<std::decay_t<T>>>());
 }
 
 template <class F, class K, class V>
@@ -135,23 +167,21 @@ template <class Alloc, class T, class Tuple>
 void ConstructFromTuple(Alloc* alloc, T* ptr, Tuple&& t) {
   memory_internal::ConstructFromTupleImpl(
       alloc, ptr, std::forward<Tuple>(t),
-      absl::make_index_sequence<
-          std::tuple_size<typename std::decay<Tuple>::type>::value>());
+      std::make_index_sequence<std::tuple_size_v<std::decay_t<Tuple>>>());
 }
 
 // Constructs T using the args specified in the tuple and calls F with the
 // constructed value.
 template <class T, class Tuple, class F>
-decltype(std::declval<F>()(std::declval<T>())) WithConstructed(
-    Tuple&& t, F&& f) {
+decltype(std::declval<F>()(std::declval<T>())) WithConstructed(Tuple&& t,
+                                                               F&& f) {
   return memory_internal::WithConstructedImpl<T>(
       std::forward<Tuple>(t),
-      absl::make_index_sequence<
-          std::tuple_size<typename std::decay<Tuple>::type>::value>(),
+      std::make_index_sequence<std::tuple_size_v<std::decay_t<Tuple>>>(),
       std::forward<F>(f));
 }
 
-// Given arguments of an std::pair's consructor, PairArgs() returns a pair of
+// Given arguments of an std::pair's constructor, PairArgs() returns a pair of
 // tuples with references to the passed arguments. The tuples contain
 // constructor arguments for the first and the second elements of the pair.
 //
@@ -161,7 +191,7 @@ decltype(std::declval<F>()(std::declval<T>())) WithConstructed(
 //
 // 2. auto a = PairArgs(args...);
 //    std::pair<F, S> p(std::piecewise_construct,
-//                      std::move(p.first), std::move(p.second));
+//                      std::move(a.first), std::move(a.second));
 inline std::pair<std::tuple<>, std::tuple<>> PairArgs() { return {}; }
 template <class F, class S>
 std::pair<std::tuple<F&&>, std::tuple<S&&>> PairArgs(F&& f, S&& s) {
@@ -204,10 +234,10 @@ DecomposeValue(F&& f, Arg&& arg) {
 
 // Helper functions for asan and msan.
 inline void SanitizerPoisonMemoryRegion(const void* m, size_t s) {
-#ifdef ADDRESS_SANITIZER
+#ifdef ABSL_HAVE_ADDRESS_SANITIZER
   ASAN_POISON_MEMORY_REGION(m, s);
 #endif
-#ifdef MEMORY_SANITIZER
+#ifdef ABSL_HAVE_MEMORY_SANITIZER
   __msan_poison(m, s);
 #endif
   (void)m;
@@ -215,10 +245,10 @@ inline void SanitizerPoisonMemoryRegion(const void* m, size_t s) {
 }
 
 inline void SanitizerUnpoisonMemoryRegion(const void* m, size_t s) {
-#ifdef ADDRESS_SANITIZER
+#ifdef ABSL_HAVE_ADDRESS_SANITIZER
   ASAN_UNPOISON_MEMORY_REGION(m, s);
 #endif
-#ifdef MEMORY_SANITIZER
+#ifdef ABSL_HAVE_MEMORY_SANITIZER
   __msan_unpoison(m, s);
 #endif
   (void)m;
@@ -245,8 +275,8 @@ namespace memory_internal {
 // type, which is non-portable.
 template <class Pair, class = std::true_type>
 struct OffsetOf {
-  static constexpr size_t kFirst = -1;
-  static constexpr size_t kSecond = -1;
+  static constexpr size_t kFirst = static_cast<size_t>(-1);
+  static constexpr size_t kSecond = static_cast<size_t>(-1);
 };
 
 template <class Pair>
@@ -315,18 +345,20 @@ union map_slot_type {
   map_slot_type() {}
   ~map_slot_type() = delete;
   using value_type = std::pair<const K, V>;
-  using mutable_value_type = std::pair<K, V>;
+  using mutable_value_type =
+      std::pair<std::remove_const_t<K>, std::remove_const_t<V>>;
 
   value_type value;
   mutable_value_type mutable_value;
-  K key;
+  std::remove_const_t<K> key;
 };
 
 template <class K, class V>
 struct map_slot_policy {
   using slot_type = map_slot_type<K, V>;
   using value_type = std::pair<const K, V>;
-  using mutable_value_type = std::pair<K, V>;
+  using mutable_value_type =
+      std::pair<std::remove_const_t<K>, std::remove_const_t<V>>;
 
  private:
   static void emplace(slot_type* slot) {
@@ -345,6 +377,14 @@ struct map_slot_policy {
     return slot->value;
   }
 
+  static K& mutable_key(slot_type* slot) {
+    // Still check for kMutableKeys so that we can avoid calling std::launder
+    // unless necessary because it can interfere with optimizations.
+    return kMutableKeys::value ? slot->key
+                               : *std::launder(const_cast<K*>(
+                                     std::addressof(slot->value.first)));
+  }
+
   static const K& key(const slot_type* slot) {
     return kMutableKeys::value ? slot->key : slot->value.first;
   }
@@ -353,11 +393,11 @@ struct map_slot_policy {
   static void construct(Allocator* alloc, slot_type* slot, Args&&... args) {
     emplace(slot);
     if (kMutableKeys::value) {
-      absl::allocator_traits<Allocator>::construct(*alloc, &slot->mutable_value,
-                                                   std::forward<Args>(args)...);
+      std::allocator_traits<Allocator>::construct(*alloc, &slot->mutable_value,
+                                                  std::forward<Args>(args)...);
     } else {
-      absl::allocator_traits<Allocator>::construct(*alloc, &slot->value,
-                                                   std::forward<Args>(args)...);
+      std::allocator_traits<Allocator>::construct(*alloc, &slot->value,
+                                                  std::forward<Args>(args)...);
     }
   }
 
@@ -366,73 +406,163 @@ struct map_slot_policy {
   static void construct(Allocator* alloc, slot_type* slot, slot_type* other) {
     emplace(slot);
     if (kMutableKeys::value) {
-      absl::allocator_traits<Allocator>::construct(
+      std::allocator_traits<Allocator>::construct(
           *alloc, &slot->mutable_value, std::move(other->mutable_value));
     } else {
-      absl::allocator_traits<Allocator>::construct(*alloc, &slot->value,
-                                                   std::move(other->value));
+      std::allocator_traits<Allocator>::construct(*alloc, &slot->value,
+                                                  std::move(other->value));
     }
   }
 
+  // Construct this slot by copying from another slot.
   template <class Allocator>
-  static void destroy(Allocator* alloc, slot_type* slot) {
+  static void construct(Allocator* alloc, slot_type* slot,
+                        const slot_type* other) {
+    emplace(slot);
+    std::allocator_traits<Allocator>::construct(*alloc, &slot->value,
+                                                other->value);
+  }
+
+  template <class Allocator>
+  static auto destroy(Allocator* alloc, slot_type* slot) {
     if (kMutableKeys::value) {
-      absl::allocator_traits<Allocator>::destroy(*alloc, &slot->mutable_value);
+      std::allocator_traits<Allocator>::destroy(*alloc, &slot->mutable_value);
     } else {
-      absl::allocator_traits<Allocator>::destroy(*alloc, &slot->value);
+      std::allocator_traits<Allocator>::destroy(*alloc, &slot->value);
     }
+    return IsDestructionTrivial<Allocator, value_type>();
   }
 
   template <class Allocator>
-  static void transfer(Allocator* alloc, slot_type* new_slot,
+  static auto transfer(Allocator* alloc, slot_type* new_slot,
                        slot_type* old_slot) {
+    // This should really just be
+    // typename absl::is_trivially_relocatable<value_type>::type()
+    // but std::pair is not trivially copyable in C++23 in some standard
+    // library versions.
+    // See https://github.com/llvm/llvm-project/pull/95444 for instance.
+    auto is_relocatable = typename std::conjunction<
+        absl::is_trivially_relocatable<typename value_type::first_type>,
+        absl::is_trivially_relocatable<typename value_type::second_type>>::
+        type();
+
     emplace(new_slot);
+    if (is_relocatable) {
+      // TODO(b/247130232,b/251814870): remove casts after fixing warnings.
+      std::memcpy(static_cast<void*>(std::launder(&new_slot->value)),
+                  static_cast<const void*>(&old_slot->value),
+                  sizeof(value_type));
+      return is_relocatable;
+    }
+
     if (kMutableKeys::value) {
-      absl::allocator_traits<Allocator>::construct(
+      std::allocator_traits<Allocator>::construct(
           *alloc, &new_slot->mutable_value, std::move(old_slot->mutable_value));
     } else {
-      absl::allocator_traits<Allocator>::construct(*alloc, &new_slot->value,
-                                                   std::move(old_slot->value));
+      std::allocator_traits<Allocator>::construct(*alloc, &new_slot->value,
+                                                  std::move(old_slot->value));
     }
     destroy(alloc, old_slot);
+    return is_relocatable;
   }
+};
 
-  template <class Allocator>
-  static void swap(Allocator* alloc, slot_type* a, slot_type* b) {
-    if (kMutableKeys::value) {
-      using std::swap;
-      swap(a->mutable_value, b->mutable_value);
-    } else {
-      value_type tmp = std::move(a->value);
-      absl::allocator_traits<Allocator>::destroy(*alloc, &a->value);
-      absl::allocator_traits<Allocator>::construct(*alloc, &a->value,
-                                                   std::move(b->value));
-      absl::allocator_traits<Allocator>::destroy(*alloc, &b->value);
-      absl::allocator_traits<Allocator>::construct(*alloc, &b->value,
-                                                   std::move(tmp));
+// Suppress erroneous uninitialized memory errors on GCC. For example, GCC
+// thinks that the call to slot_array() in find_or_prepare_insert() is reading
+// uninitialized memory, but slot_array is only called there when the table is
+// non-empty and this memory is initialized when the table is non-empty.
+#if !defined(__clang__) && defined(__GNUC__)
+#define ABSL_SWISSTABLE_IGNORE_UNINITIALIZED(x)                    \
+  _Pragma("GCC diagnostic push")                                   \
+      _Pragma("GCC diagnostic ignored \"-Wmaybe-uninitialized\"")  \
+          _Pragma("GCC diagnostic ignored \"-Wuninitialized\"") x; \
+  _Pragma("GCC diagnostic pop")
+#define ABSL_SWISSTABLE_IGNORE_UNINITIALIZED_RETURN(x) \
+  ABSL_SWISSTABLE_IGNORE_UNINITIALIZED(return x)
+#else
+#define ABSL_SWISSTABLE_IGNORE_UNINITIALIZED(x) x
+#define ABSL_SWISSTABLE_IGNORE_UNINITIALIZED_RETURN(x) return x
+#endif
+
+// Variadic arguments hash function that ignore the rest of the arguments.
+// Useful for usage with policy traits.
+template <class Hash, bool kIsDefault, size_t kSeedShift>
+struct HashElement {
+  HashElement(const Hash& h, size_t s) : hash(h), seed(s >> kSeedShift) {}
+
+  template <class K, class... Args>
+  size_t operator()(const K& key, Args&&...) const {
+    if constexpr (kIsDefault) {
+      // TODO(b/384509507): resolve `no header providing
+      // "absl::hash_internal::SupportsHashWithSeed" is directly included`.
+      // Maybe we should make "internal/hash.h" be a separate library.
+      return absl::hash_internal::HashWithSeed().hash(hash, key, seed);
     }
+    // NOLINTNEXTLINE(clang-diagnostic-sign-conversion)
+    return hash(key) ^ seed;
   }
+  const Hash& hash;
+  size_t seed;
+};
 
-  template <class Allocator>
-  static void move(Allocator* alloc, slot_type* src, slot_type* dest) {
-    if (kMutableKeys::value) {
-      dest->mutable_value = std::move(src->mutable_value);
-    } else {
-      absl::allocator_traits<Allocator>::destroy(*alloc, &dest->value);
-      absl::allocator_traits<Allocator>::construct(*alloc, &dest->value,
-                                                   std::move(src->value));
-    }
+// No arguments function hash function for a specific key.
+template <class Hash, class Key, bool kIsDefault, size_t kSeedShift>
+struct HashKey {
+  HashKey(const Hash& h, const Key& k) : hash(h), key(k) {}
+
+  size_t operator()(size_t seed) const {
+    return HashElement<Hash, kIsDefault, kSeedShift>{hash, seed}(key);
   }
+  const Hash& hash;
+  const Key& key;
+};
 
-  template <class Allocator>
-  static void move(Allocator* alloc, slot_type* first, slot_type* last,
-                   slot_type* result) {
-    for (slot_type *src = first, *dest = result; src != last; ++src, ++dest)
-      move(alloc, src, dest);
+// Variadic arguments equality function that ignore the rest of the arguments.
+// Useful for usage with policy traits.
+template <class K1, class KeyEqual>
+struct EqualElement {
+  template <class K2, class... Args>
+  bool operator()(const K2& lhs, Args&&...) const {
+    ABSL_SWISSTABLE_IGNORE_UNINITIALIZED_RETURN(eq(lhs, rhs));
   }
+  const K1& rhs;
+  const KeyEqual& eq;
 };
 
+// Type erased function for computing hash of the slot.
+using HashSlotFn = size_t (*)(const void* hash_fn, void* slot, size_t seed);
+
+// Type erased function to apply `Fn` to data inside of the `slot`.
+// The data is expected to have type `T`.
+template <class Fn, class T, bool kIsDefault, size_t kSeedShift>
+size_t TypeErasedApplyToSlotFn(const void* fn, void* slot, size_t seed) {
+  const auto* f = static_cast<const Fn*>(fn);
+  return HashElement<Fn, kIsDefault, kSeedShift>{
+      *f, seed}(*static_cast<const T*>(slot));
+}
+
+// Type erased function to apply `Fn` to data inside of the `*slot_ptr`.
+// The data is expected to have type `T`.
+template <class Fn, class T, bool kIsDefault, size_t kSeedShift>
+size_t TypeErasedDerefAndApplyToSlotFn(const void* fn, void* slot_ptr,
+                                       size_t seed) {
+  const auto* f = static_cast<const Fn*>(fn);
+  const T* slot = *static_cast<T**>(slot_ptr);
+  return HashElement<Fn, kIsDefault, kSeedShift>{*f, seed}(*slot);
+}
+
+// Type erased function to apply `Fn` to data inside of the `slot_ptr->first`.
+// The data is expected to have type `T`.
+template <class Fn, class T, bool kIsDefault, size_t kSeedShift>
+size_t TypeErasedDerefAndApplyToSlotFirstFn(const void* fn, void* slot_ptr,
+                                            size_t seed) {
+  const auto* f = static_cast<const Fn*>(fn);
+  const T* slot = *static_cast<T**>(slot_ptr);
+  return HashElement<Fn, kIsDefault, kSeedShift>{*f, seed}(slot->first);
+}
+
 }  // namespace container_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_INTERNAL_CONTAINER_MEMORY_H_
diff --git a/absl/container/internal/container_memory_test.cc b/absl/container/internal/container_memory_test.cc
index d6b0495f173..1aa459c2c8a 100644
--- a/absl/container/internal/container_memory_test.cc
+++ b/absl/container/internal/container_memory_test.cc
@@ -14,20 +14,45 @@
 
 #include "absl/container/internal/container_memory.h"
 
+#include <cstddef>
 #include <cstdint>
+#include <memory>
 #include <tuple>
+#include <type_traits>
+#include <typeindex>
+#include <typeinfo>
 #include <utility>
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
+#include "absl/base/config.h"
+#include "absl/base/no_destructor.h"
+#include "absl/container/internal/test_instance_tracker.h"
+#include "absl/meta/type_traits.h"
 #include "absl/strings/string_view.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 namespace {
 
+using ::absl::test_internal::CopyableMovableInstance;
+using ::absl::test_internal::InstanceTracker;
+using ::testing::_;
+using ::testing::ElementsAre;
+using ::testing::Gt;
 using ::testing::Pair;
 
+#if ABSL_HAVE_BUILTIN(__builtin_infer_alloc_token)
+TEST(Memory, AlignedTypeAllocToken) {
+#if defined(__wasm__)
+  GTEST_SKIP() << "Fails on wasm due to lack of heap partitioning support.";
+#endif
+  EXPECT_GT(__builtin_infer_alloc_token(sizeof(AlignedType<alignof(void*)>)),
+            __builtin_infer_alloc_token(sizeof(int)));
+}
+#endif
+
 TEST(Memory, AlignmentLargerThanBase) {
   std::allocator<int8_t> alloc;
   void* mem = Allocate<2>(&alloc, 3);
@@ -44,6 +69,39 @@ TEST(Memory, AlignmentSmallerThanBase) {
   Deallocate<2>(&alloc, mem, 3);
 }
 
+std::map<std::type_index, int>& AllocationMap() {
+  static absl::NoDestructor<std::map<std::type_index, int>> map;
+  return *map;
+}
+
+template <typename T>
+struct TypeCountingAllocator {
+  TypeCountingAllocator() = default;
+  template <typename U>
+  TypeCountingAllocator(const TypeCountingAllocator<U>&) {}  // NOLINT
+
+  using value_type = T;
+
+  T* allocate(size_t n, const void* = nullptr) {
+    AllocationMap()[typeid(T)] += n;
+    return std::allocator<T>().allocate(n);
+  }
+  void deallocate(T* p, std::size_t n) {
+    AllocationMap()[typeid(T)] -= n;
+    return std::allocator<T>().deallocate(p, n);
+  }
+};
+
+TEST(Memory, AllocateDeallocateMatchType) {
+  TypeCountingAllocator<int> alloc;
+  void* mem = Allocate<1>(&alloc, 1);
+  // Verify that it was allocated
+  EXPECT_THAT(AllocationMap(), ElementsAre(Pair(_, Gt(0))));
+  Deallocate<1>(&alloc, mem, 1);
+  // Verify that the deallocation matched.
+  EXPECT_THAT(AllocationMap(), ElementsAre(Pair(_, 0)));
+}
+
 class Fixture : public ::testing::Test {
   using Alloc = std::allocator<std::string>;
 
@@ -124,7 +182,7 @@ TryDecomposeValue(F&& f, Arg&& arg) {
 }
 
 TEST(DecomposeValue, Decomposable) {
-  auto f = [](const int& x, int&& y) {
+  auto f = [](const int& x, int&& y) {  // NOLINT
     EXPECT_EQ(&x, &y);
     EXPECT_EQ(42, x);
     return 'A';
@@ -158,7 +216,8 @@ TryDecomposePair(F&& f, Args&&... args) {
 }
 
 TEST(DecomposePair, Decomposable) {
-  auto f = [](const int& x, std::piecewise_construct_t, std::tuple<int&&> k,
+  auto f = [](const int& x,  // NOLINT
+              std::piecewise_construct_t, std::tuple<int&&> k,
               std::tuple<double>&& v) {
     EXPECT_EQ(&x, &std::get<0>(k));
     EXPECT_EQ(42, x);
@@ -176,13 +235,138 @@ TEST(DecomposePair, NotDecomposable) {
     ADD_FAILURE() << "Must not be called";
     return 'A';
   };
-  EXPECT_STREQ("not decomposable",
-               TryDecomposePair(f));
+  EXPECT_STREQ("not decomposable", TryDecomposePair(f));
   EXPECT_STREQ("not decomposable",
                TryDecomposePair(f, std::piecewise_construct, std::make_tuple(),
                                 std::make_tuple(0.5)));
 }
 
+TEST(MapSlotPolicy, ConstKeyAndValue) {
+  using slot_policy = map_slot_policy<const CopyableMovableInstance,
+                                      const CopyableMovableInstance>;
+  using slot_type = typename slot_policy::slot_type;
+
+  union Slots {
+    Slots() {}
+    ~Slots() {}
+    slot_type slots[100];
+  } slots;
+
+  std::allocator<
+      std::pair<const CopyableMovableInstance, const CopyableMovableInstance>>
+      alloc;
+  InstanceTracker tracker;
+  slot_policy::construct(&alloc, &slots.slots[0], CopyableMovableInstance(1),
+                         CopyableMovableInstance(1));
+  for (int i = 0; i < 99; ++i) {
+    slot_policy::transfer(&alloc, &slots.slots[i + 1], &slots.slots[i]);
+  }
+  slot_policy::destroy(&alloc, &slots.slots[99]);
+
+  EXPECT_EQ(tracker.copies(), 0);
+}
+
+TEST(MapSlotPolicy, TransferReturnsTrue) {
+  {
+    using slot_policy = map_slot_policy<int, float>;
+    EXPECT_TRUE(
+        (std::is_same_v<decltype(slot_policy::transfer<std::allocator<char>>(
+                            nullptr, nullptr, nullptr)),
+                        std::true_type>));
+  }
+  {
+    struct NonRelocatable {
+      NonRelocatable() = default;
+      NonRelocatable(NonRelocatable&&) {}
+      NonRelocatable& operator=(NonRelocatable&&) { return *this; }
+      void* self = nullptr;
+    };
+
+    EXPECT_FALSE(absl::is_trivially_relocatable<NonRelocatable>::value);
+    using slot_policy = map_slot_policy<int, NonRelocatable>;
+    EXPECT_TRUE(
+        (std::is_same_v<decltype(slot_policy::transfer<std::allocator<char>>(
+                            nullptr, nullptr, nullptr)),
+                        std::false_type>));
+  }
+}
+
+TEST(MapSlotPolicy, DestroyReturnsTrue) {
+  {
+    using slot_policy = map_slot_policy<int, float>;
+    EXPECT_TRUE(
+        (std::is_same_v<decltype(slot_policy::destroy<std::allocator<char>>(
+                            nullptr, nullptr)),
+                        std::true_type>));
+  }
+  {
+    EXPECT_FALSE(std::is_trivially_destructible_v<std::unique_ptr<int>>);
+    using slot_policy = map_slot_policy<int, std::unique_ptr<int>>;
+    EXPECT_TRUE(
+        (std::is_same_v<decltype(slot_policy::destroy<std::allocator<char>>(
+                            nullptr, nullptr)),
+                        std::false_type>));
+  }
+}
+
+TEST(ApplyTest, TypeErasedApplyToSlotFn) {
+  size_t x = 7;
+  size_t seed = 100;
+  auto fn = [](size_t v) { return v * 2; };
+  EXPECT_EQ((TypeErasedApplyToSlotFn<decltype(fn), size_t, /*kIsDefault=*/false,
+                                     /*kSeedShift=*/0>(&fn, &x, seed)),
+            (HashElement<decltype(fn), /*kIsDefault=*/false, /*kSeedShift=*/0>(
+                fn, seed)(x)));
+}
+
+TEST(ApplyTest, TypeErasedDerefAndApplyToSlotFn) {
+  size_t x = 7;
+  size_t seed = 100;
+  auto fn = [](size_t v) { return v * 2; };
+  size_t* x_ptr = &x;
+  EXPECT_EQ(
+      (TypeErasedDerefAndApplyToSlotFn<decltype(fn), size_t,
+                                       /*kIsDefault=*/false,
+                                       /*kSeedShift=*/0>(&fn, &x_ptr, seed)),
+      (HashElement<decltype(fn), /*kIsDefault=*/false, /*kSeedShift=*/0>(
+          fn, seed)(x)));
+}
+
+TEST(HashElement, DefaultHash) {
+  size_t x = 7;
+  size_t seed = 100;
+  struct HashWithSeed {
+    size_t operator()(size_t v) const { return v * 2; }
+    size_t hash_with_seed(size_t v, size_t seed) const {
+      return v * 2 + seed * 3;
+    }
+  } hash;
+  EXPECT_EQ((HashElement<HashWithSeed, /*kIsDefault=*/true,
+                         /*kSeedShift=*/0>(hash, seed)(x)),
+            hash.hash_with_seed(x, seed));
+}
+
+TEST(HashElement, NonDefaultHash) {
+  size_t x = 7;
+  size_t seed = 100;
+  auto fn = [](size_t v) { return v * 2; };
+  EXPECT_EQ(
+      (HashElement<decltype(fn), /*kIsDefault=*/false, /*kSeedShift=*/0>(
+          fn, seed)(x)),
+      fn(x) ^ seed);
+}
+
+TEST(HashElement, NonDefaultHashWithSeedShift) {
+  size_t x = 7;
+  size_t seed = 100;
+  auto fn = [](size_t v) { return v * 2; };
+  EXPECT_EQ(
+      (HashElement<decltype(fn), /*kIsDefault=*/false, /*kSeedShift=*/1>(
+          fn, seed)(x)),
+      fn(x) ^ (seed >> 1));
+}
+
 }  // namespace
 }  // namespace container_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/container/internal/counting_allocator.h b/absl/container/internal/counting_allocator.h
deleted file mode 100644
index 4e717bef678..00000000000
--- a/absl/container/internal/counting_allocator.h
+++ /dev/null
@@ -1,79 +0,0 @@
-// Copyright 2018 The Abseil Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-#ifndef ABSL_CONTAINER_INTERNAL_COUNTING_ALLOCATOR_H_
-#define ABSL_CONTAINER_INTERNAL_COUNTING_ALLOCATOR_H_
-
-#include <cassert>
-#include <cstdint>
-#include <memory>
-
-namespace absl {
-namespace container_internal {
-
-// This is a stateful allocator, but the state lives outside of the
-// allocator (in whatever test is using the allocator). This is odd
-// but helps in tests where the allocator is propagated into nested
-// containers - that chain of allocators uses the same state and is
-// thus easier to query for aggregate allocation information.
-template <typename T>
-class CountingAllocator : public std::allocator<T> {
- public:
-  using Alloc = std::allocator<T>;
-  using pointer = typename Alloc::pointer;
-  using size_type = typename Alloc::size_type;
-
-  CountingAllocator() : bytes_used_(nullptr) {}
-  explicit CountingAllocator(int64_t* b) : bytes_used_(b) {}
-
-  template <typename U>
-  CountingAllocator(const CountingAllocator<U>& x)
-      : Alloc(x), bytes_used_(x.bytes_used_) {}
-
-  pointer allocate(size_type n,
-                   std::allocator<void>::const_pointer hint = nullptr) {
-    assert(bytes_used_ != nullptr);
-    *bytes_used_ += n * sizeof(T);
-    return Alloc::allocate(n, hint);
-  }
-
-  void deallocate(pointer p, size_type n) {
-    Alloc::deallocate(p, n);
-    assert(bytes_used_ != nullptr);
-    *bytes_used_ -= n * sizeof(T);
-  }
-
-  template<typename U>
-  class rebind {
-   public:
-    using other = CountingAllocator<U>;
-  };
-
-  friend bool operator==(const CountingAllocator& a,
-                         const CountingAllocator& b) {
-    return a.bytes_used_ == b.bytes_used_;
-  }
-
-  friend bool operator!=(const CountingAllocator& a,
-                         const CountingAllocator& b) {
-    return !(a == b);
-  }
-
-  int64_t* bytes_used_;
-};
-
-}  // namespace container_internal
-}  // namespace absl
-
-#endif  // ABSL_CONTAINER_INTERNAL_COUNTING_ALLOCATOR_H_
diff --git a/absl/container/internal/hash_function_defaults.h b/absl/container/internal/hash_function_defaults.h
index cb8f03c87aa..dcf7dbd5c51 100644
--- a/absl/container/internal/hash_function_defaults.h
+++ b/absl/container/internal/hash_function_defaults.h
@@ -45,17 +45,22 @@
 #ifndef ABSL_CONTAINER_INTERNAL_HASH_FUNCTION_DEFAULTS_H_
 #define ABSL_CONTAINER_INTERNAL_HASH_FUNCTION_DEFAULTS_H_
 
-#include <stdint.h>
 #include <cstddef>
+#include <functional>
 #include <memory>
 #include <string>
+#include <string_view>
 #include <type_traits>
 
 #include "absl/base/config.h"
+#include "absl/container/internal/common.h"
 #include "absl/hash/hash.h"
+#include "absl/meta/type_traits.h"
+#include "absl/strings/cord.h"
 #include "absl/strings/string_view.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
 // The hash of an object of type T is computed by using absl::Hash.
@@ -71,23 +76,95 @@ struct StringHash {
   size_t operator()(absl::string_view v) const {
     return absl::Hash<absl::string_view>{}(v);
   }
+  size_t operator()(const absl::Cord& v) const {
+    return absl::Hash<absl::Cord>{}(v);
+  }
+
+ private:
+  friend struct absl::hash_internal::HashWithSeed;
+
+  size_t hash_with_seed(absl::string_view v, size_t seed) const {
+    return absl::hash_internal::HashWithSeed().hash(
+        absl::Hash<absl::string_view>{}, v, seed);
+  }
+  size_t hash_with_seed(const absl::Cord& v, size_t seed) const {
+    return absl::hash_internal::HashWithSeed().hash(absl::Hash<absl::Cord>{}, v,
+                                                    seed);
+  }
+};
+
+struct StringEq {
+  using is_transparent = void;
+  bool operator()(absl::string_view lhs, absl::string_view rhs) const {
+    return lhs == rhs;
+  }
+  bool operator()(const absl::Cord& lhs, const absl::Cord& rhs) const {
+    return lhs == rhs;
+  }
+  bool operator()(const absl::Cord& lhs, absl::string_view rhs) const {
+    return lhs == rhs;
+  }
+  bool operator()(absl::string_view lhs, const absl::Cord& rhs) const {
+    return lhs == rhs;
+  }
 };
 
 // Supports heterogeneous lookup for string-like elements.
 struct StringHashEq {
   using Hash = StringHash;
-  struct Eq {
-    using is_transparent = void;
-    bool operator()(absl::string_view lhs, absl::string_view rhs) const {
-      return lhs == rhs;
-    }
-  };
+  using Eq = StringEq;
 };
 
 template <>
 struct HashEq<std::string> : StringHashEq {};
 template <>
 struct HashEq<absl::string_view> : StringHashEq {};
+template <>
+struct HashEq<absl::Cord> : StringHashEq {};
+
+template <typename TChar>
+struct BasicStringHash {
+  using is_transparent = void;
+
+  size_t operator()(std::basic_string_view<TChar> v) const {
+    return absl::Hash<std::basic_string_view<TChar>>{}(v);
+  }
+};
+
+template <typename TChar>
+struct BasicStringEq {
+  using is_transparent = void;
+  bool operator()(std::basic_string_view<TChar> lhs,
+                  std::basic_string_view<TChar> rhs) const {
+    return lhs == rhs;
+  }
+};
+
+// Supports heterogeneous lookup for w/u8/u16/u32 string + string_view + char*.
+template <typename TChar>
+struct BasicStringHashEq {
+  using Hash = BasicStringHash<TChar>;
+  using Eq = BasicStringEq<TChar>;
+};
+
+template <>
+struct HashEq<std::wstring> : BasicStringHashEq<wchar_t> {};
+template <>
+struct HashEq<std::wstring_view> : BasicStringHashEq<wchar_t> {};
+#ifdef __cpp_char8_t
+template <>
+struct HashEq<std::u8string> : BasicStringHashEq<char8_t> {};
+template <>
+struct HashEq<std::u8string_view> : BasicStringHashEq<char8_t> {};
+#endif
+template <>
+struct HashEq<std::u16string> : BasicStringHashEq<char16_t> {};
+template <>
+struct HashEq<std::u16string_view> : BasicStringHashEq<char16_t> {};
+template <>
+struct HashEq<std::u32string> : BasicStringHashEq<char32_t> {};
+template <>
+struct HashEq<std::u32string_view> : BasicStringHashEq<char32_t> {};
 
 // Supports heterogeneous lookup for pointers and smart pointers.
 template <class T>
@@ -124,6 +201,71 @@ struct HashEq<std::unique_ptr<T, D>> : HashEq<T*> {};
 template <class T>
 struct HashEq<std::shared_ptr<T>> : HashEq<T*> {};
 
+template <typename T, typename E = void>
+struct HasAbslContainerHash : std::false_type {};
+
+template <typename T>
+struct HasAbslContainerHash<T, std::void_t<typename T::absl_container_hash>>
+    : std::true_type {};
+
+template <typename T, typename E = void>
+struct HasAbslContainerEq : std::false_type {};
+
+template <typename T>
+struct HasAbslContainerEq<T, std::void_t<typename T::absl_container_eq>>
+    : std::true_type {};
+
+template <typename T, typename E = void>
+struct AbslContainerEq {
+  using type = std::equal_to<>;
+};
+
+template <typename T>
+struct AbslContainerEq<
+    T, typename std::enable_if_t<HasAbslContainerEq<T>::value>> {
+  using type = typename T::absl_container_eq;
+};
+
+template <typename T, typename E = void>
+struct AbslContainerHash {
+  using type = void;
+};
+
+template <typename T>
+struct AbslContainerHash<
+    T, typename std::enable_if_t<HasAbslContainerHash<T>::value>> {
+  using type = typename T::absl_container_hash;
+};
+
+// HashEq specialization for user types that provide `absl_container_hash` and
+// (optionally) `absl_container_eq`. This specialization allows user types to
+// provide heterogeneous lookup without requiring to explicitly specify Hash/Eq
+// type arguments in unordered Abseil containers.
+//
+// Both `absl_container_hash` and `absl_container_eq` should be transparent
+// (have inner is_transparent type). While there is no technical reason to
+// restrict to transparent-only types, there is also no feasible use case when
+// it shouldn't be transparent - it is easier to relax the requirement later if
+// such a case arises rather than restricting it.
+//
+// If type provides only `absl_container_hash` then `eq` part will be
+// `std::equal_to<void>`.
+//
+// User types are not allowed to provide only a `Eq` part as there is no
+// feasible use case for this behavior - if Hash should be a default one then Eq
+// should be an equivalent to the `std::equal_to<T>`.
+template <typename T>
+struct HashEq<T, typename std::enable_if_t<HasAbslContainerHash<T>::value>> {
+  using Hash = typename AbslContainerHash<T>::type;
+  using Eq = typename AbslContainerEq<T>::type;
+  static_assert(IsTransparent<Hash>::value,
+                "absl_container_hash must be transparent. To achieve it add a "
+                "`using is_transparent = void;` clause to this type.");
+  static_assert(IsTransparent<Eq>::value,
+                "absl_container_eq must be transparent. To achieve it add a "
+                "`using is_transparent = void;` clause to this type.");
+};
+
 // This header's visibility is restricted.  If you need to access the default
 // hasher please use the container's ::hasher alias instead.
 //
@@ -139,6 +281,7 @@ template <class T>
 using hash_default_eq = typename container_internal::HashEq<T>::Eq;
 
 }  // namespace container_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_INTERNAL_HASH_FUNCTION_DEFAULTS_H_
diff --git a/absl/container/internal/hash_function_defaults_test.cc b/absl/container/internal/hash_function_defaults_test.cc
index 82708dbe953..67c853cfdf4 100644
--- a/absl/container/internal/hash_function_defaults_test.cc
+++ b/absl/container/internal/hash_function_defaults_test.cc
@@ -14,14 +14,24 @@
 
 #include "absl/container/internal/hash_function_defaults.h"
 
+#include <cstddef>
 #include <functional>
+#include <string>
+#include <string_view>
 #include <type_traits>
 #include <utility>
 
 #include "gtest/gtest.h"
+#include "absl/container/flat_hash_map.h"
+#include "absl/container/flat_hash_set.h"
+#include "absl/hash/hash.h"
+#include "absl/random/random.h"
+#include "absl/strings/cord.h"
+#include "absl/strings/cord_test_helpers.h"
 #include "absl/strings/string_view.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 namespace {
 
@@ -105,6 +115,162 @@ TYPED_TEST(HashString, Works) {
   EXPECT_NE(h, hash(std::string("b")));
 }
 
+TEST(BasicStringViewTest, WStringEqWorks) {
+  hash_default_eq<std::wstring> eq;
+  EXPECT_TRUE(eq(L"a", L"a"));
+  EXPECT_TRUE(eq(L"a", std::wstring_view(L"a")));
+  EXPECT_TRUE(eq(L"a", std::wstring(L"a")));
+  EXPECT_FALSE(eq(L"a", L"b"));
+  EXPECT_FALSE(eq(L"a", std::wstring_view(L"b")));
+  EXPECT_FALSE(eq(L"a", std::wstring(L"b")));
+}
+
+TEST(BasicStringViewTest, WStringViewEqWorks) {
+  hash_default_eq<std::wstring_view> eq;
+  EXPECT_TRUE(eq(L"a", L"a"));
+  EXPECT_TRUE(eq(L"a", std::wstring_view(L"a")));
+  EXPECT_TRUE(eq(L"a", std::wstring(L"a")));
+  EXPECT_FALSE(eq(L"a", L"b"));
+  EXPECT_FALSE(eq(L"a", std::wstring_view(L"b")));
+  EXPECT_FALSE(eq(L"a", std::wstring(L"b")));
+}
+
+#ifdef __cpp_char8_t
+TEST(BasicStringViewTest, U8StringEqWorks) {
+  hash_default_eq<std::u8string> eq;
+  EXPECT_TRUE(eq(u8"a", u8"a"));
+  EXPECT_TRUE(eq(u8"a", std::u8string_view(u8"a")));
+  EXPECT_TRUE(eq(u8"a", std::u8string(u8"a")));
+  EXPECT_FALSE(eq(u8"a", u8"b"));
+  EXPECT_FALSE(eq(u8"a", std::u8string_view(u8"b")));
+  EXPECT_FALSE(eq(u8"a", std::u8string(u8"b")));
+}
+
+TEST(BasicStringViewTest, U8StringViewEqWorks) {
+  hash_default_eq<std::u8string_view> eq;
+  EXPECT_TRUE(eq(u8"a", u8"a"));
+  EXPECT_TRUE(eq(u8"a", std::u8string_view(u8"a")));
+  EXPECT_TRUE(eq(u8"a", std::u8string(u8"a")));
+  EXPECT_FALSE(eq(u8"a", u8"b"));
+  EXPECT_FALSE(eq(u8"a", std::u8string_view(u8"b")));
+  EXPECT_FALSE(eq(u8"a", std::u8string(u8"b")));
+}
+#endif
+
+TEST(BasicStringViewTest, U16StringEqWorks) {
+  hash_default_eq<std::u16string> eq;
+  EXPECT_TRUE(eq(u"a", u"a"));
+  EXPECT_TRUE(eq(u"a", std::u16string_view(u"a")));
+  EXPECT_TRUE(eq(u"a", std::u16string(u"a")));
+  EXPECT_FALSE(eq(u"a", u"b"));
+  EXPECT_FALSE(eq(u"a", std::u16string_view(u"b")));
+  EXPECT_FALSE(eq(u"a", std::u16string(u"b")));
+}
+
+TEST(BasicStringViewTest, U16StringViewEqWorks) {
+  hash_default_eq<std::u16string_view> eq;
+  EXPECT_TRUE(eq(u"a", u"a"));
+  EXPECT_TRUE(eq(u"a", std::u16string_view(u"a")));
+  EXPECT_TRUE(eq(u"a", std::u16string(u"a")));
+  EXPECT_FALSE(eq(u"a", u"b"));
+  EXPECT_FALSE(eq(u"a", std::u16string_view(u"b")));
+  EXPECT_FALSE(eq(u"a", std::u16string(u"b")));
+}
+
+TEST(BasicStringViewTest, U32StringEqWorks) {
+  hash_default_eq<std::u32string> eq;
+  EXPECT_TRUE(eq(U"a", U"a"));
+  EXPECT_TRUE(eq(U"a", std::u32string_view(U"a")));
+  EXPECT_TRUE(eq(U"a", std::u32string(U"a")));
+  EXPECT_FALSE(eq(U"a", U"b"));
+  EXPECT_FALSE(eq(U"a", std::u32string_view(U"b")));
+  EXPECT_FALSE(eq(U"a", std::u32string(U"b")));
+}
+
+TEST(BasicStringViewTest, U32StringViewEqWorks) {
+  hash_default_eq<std::u32string_view> eq;
+  EXPECT_TRUE(eq(U"a", U"a"));
+  EXPECT_TRUE(eq(U"a", std::u32string_view(U"a")));
+  EXPECT_TRUE(eq(U"a", std::u32string(U"a")));
+  EXPECT_FALSE(eq(U"a", U"b"));
+  EXPECT_FALSE(eq(U"a", std::u32string_view(U"b")));
+  EXPECT_FALSE(eq(U"a", std::u32string(U"b")));
+}
+
+TEST(BasicStringViewTest, WStringHashWorks) {
+  hash_default_hash<std::wstring> hash;
+  auto h = hash(L"a");
+  EXPECT_EQ(h, hash(std::wstring_view(L"a")));
+  EXPECT_EQ(h, hash(std::wstring(L"a")));
+  EXPECT_NE(h, hash(std::wstring_view(L"b")));
+  EXPECT_NE(h, hash(std::wstring(L"b")));
+}
+
+TEST(BasicStringViewTest, WStringViewHashWorks) {
+  hash_default_hash<std::wstring_view> hash;
+  auto h = hash(L"a");
+  EXPECT_EQ(h, hash(std::wstring_view(L"a")));
+  EXPECT_EQ(h, hash(std::wstring(L"a")));
+  EXPECT_NE(h, hash(std::wstring_view(L"b")));
+  EXPECT_NE(h, hash(std::wstring(L"b")));
+}
+
+#ifdef __cpp_char8_t
+TEST(BasicStringViewTest, U8StringHashWorks) {
+  hash_default_hash<std::u8string> hash;
+  auto h = hash(u8"a");
+  EXPECT_EQ(h, hash(std::u8string_view(u8"a")));
+  EXPECT_EQ(h, hash(std::u8string(u8"a")));
+  EXPECT_NE(h, hash(std::u8string_view(u8"b")));
+  EXPECT_NE(h, hash(std::u8string(u8"b")));
+}
+
+TEST(BasicStringViewTest, U8StringViewHashWorks) {
+  hash_default_hash<std::u8string_view> hash;
+  auto h = hash(u8"a");
+  EXPECT_EQ(h, hash(std::u8string_view(u8"a")));
+  EXPECT_EQ(h, hash(std::u8string(u8"a")));
+  EXPECT_NE(h, hash(std::u8string_view(u8"b")));
+  EXPECT_NE(h, hash(std::u8string(u8"b")));
+}
+#endif
+
+TEST(BasicStringViewTest, U16StringHashWorks) {
+  hash_default_hash<std::u16string> hash;
+  auto h = hash(u"a");
+  EXPECT_EQ(h, hash(std::u16string_view(u"a")));
+  EXPECT_EQ(h, hash(std::u16string(u"a")));
+  EXPECT_NE(h, hash(std::u16string_view(u"b")));
+  EXPECT_NE(h, hash(std::u16string(u"b")));
+}
+
+TEST(BasicStringViewTest, U16StringViewHashWorks) {
+  hash_default_hash<std::u16string_view> hash;
+  auto h = hash(u"a");
+  EXPECT_EQ(h, hash(std::u16string_view(u"a")));
+  EXPECT_EQ(h, hash(std::u16string(u"a")));
+  EXPECT_NE(h, hash(std::u16string_view(u"b")));
+  EXPECT_NE(h, hash(std::u16string(u"b")));
+}
+
+TEST(BasicStringViewTest, U32StringHashWorks) {
+  hash_default_hash<std::u32string> hash;
+  auto h = hash(U"a");
+  EXPECT_EQ(h, hash(std::u32string_view(U"a")));
+  EXPECT_EQ(h, hash(std::u32string(U"a")));
+  EXPECT_NE(h, hash(std::u32string_view(U"b")));
+  EXPECT_NE(h, hash(std::u32string(U"b")));
+}
+
+TEST(BasicStringViewTest, U32StringViewHashWorks) {
+  hash_default_hash<std::u32string_view> hash;
+  auto h = hash(U"a");
+  EXPECT_EQ(h, hash(std::u32string_view(U"a")));
+  EXPECT_EQ(h, hash(std::u32string(U"a")));
+  EXPECT_NE(h, hash(std::u32string_view(U"b")));
+  EXPECT_NE(h, hash(std::u32string(U"b")));
+}
+
 struct NoDeleter {
   template <class T>
   void operator()(const T* ptr) const {}
@@ -202,10 +368,91 @@ TYPED_TEST(HashPointer, Works) {
   EXPECT_NE(hash(&dummy), hash(cuptr));
 }
 
+TEST(EqCord, Works) {
+  hash_default_eq<absl::Cord> eq;
+  const absl::string_view a_string_view = "a";
+  const absl::Cord a_cord(a_string_view);
+  const absl::string_view b_string_view = "b";
+  const absl::Cord b_cord(b_string_view);
+
+  EXPECT_TRUE(eq(a_cord, a_cord));
+  EXPECT_TRUE(eq(a_cord, a_string_view));
+  EXPECT_TRUE(eq(a_string_view, a_cord));
+  EXPECT_FALSE(eq(a_cord, b_cord));
+  EXPECT_FALSE(eq(a_cord, b_string_view));
+  EXPECT_FALSE(eq(b_string_view, a_cord));
+}
+
+TEST(HashCord, Works) {
+  hash_default_hash<absl::Cord> hash;
+  const absl::string_view a_string_view = "a";
+  const absl::Cord a_cord(a_string_view);
+  const absl::string_view b_string_view = "b";
+  const absl::Cord b_cord(b_string_view);
+
+  EXPECT_EQ(hash(a_cord), hash(a_cord));
+  EXPECT_EQ(hash(b_cord), hash(b_cord));
+  EXPECT_EQ(hash(a_string_view), hash(a_cord));
+  EXPECT_EQ(hash(b_string_view), hash(b_cord));
+  EXPECT_EQ(hash(absl::Cord("")), hash(""));
+  EXPECT_EQ(hash(absl::Cord()), hash(absl::string_view()));
+
+  EXPECT_NE(hash(a_cord), hash(b_cord));
+  EXPECT_NE(hash(a_cord), hash(b_string_view));
+  EXPECT_NE(hash(a_string_view), hash(b_cord));
+  EXPECT_NE(hash(a_string_view), hash(b_string_view));
+}
+
+void NoOpReleaser(absl::string_view data, void* arg) {}
+
+TEST(HashCord, FragmentedCordWorks) {
+  hash_default_hash<absl::Cord> hash;
+  absl::Cord c = absl::MakeFragmentedCord({"a", "b", "c"});
+  EXPECT_FALSE(c.TryFlat().has_value());
+  EXPECT_EQ(hash(c), hash("abc"));
+}
+
+TEST(HashCord, FragmentedLongCordWorks) {
+  hash_default_hash<absl::Cord> hash;
+  // Crete some large strings which do not fit on the stack.
+  std::string a(65536, 'a');
+  std::string b(65536, 'b');
+  absl::Cord c = absl::MakeFragmentedCord({a, b});
+  EXPECT_FALSE(c.TryFlat().has_value());
+  EXPECT_EQ(hash(c), hash(a + b));
+}
+
+TEST(HashCord, RandomCord) {
+  hash_default_hash<absl::Cord> hash;
+  auto bitgen = absl::BitGen();
+  for (int i = 0; i < 1000; ++i) {
+    const int number_of_segments = absl::Uniform(bitgen, 0, 10);
+    std::vector<std::string> pieces;
+    for (size_t s = 0; s < number_of_segments; ++s) {
+      std::string str;
+      str.resize(absl::Uniform(bitgen, 0, 4096));
+      // MSVC needed the explicit return type in the lambda.
+      std::generate(str.begin(), str.end(), [&]() -> char {
+        return static_cast<char>(absl::Uniform<unsigned char>(bitgen));
+      });
+      pieces.push_back(str);
+    }
+    absl::Cord c = absl::MakeFragmentedCord(pieces);
+    EXPECT_EQ(hash(c), hash(std::string(c)));
+  }
+}
+
 // Cartesian product of (std::string, absl::string_view)
-// with (std::string, absl::string_view, const char*).
+// with (std::string, absl::string_view, const char*, absl::Cord).
 using StringTypesCartesianProduct = Types<
     // clang-format off
+    std::pair<absl::Cord, std::string>,
+    std::pair<absl::Cord, absl::string_view>,
+    std::pair<absl::Cord, absl::Cord>,
+    std::pair<absl::Cord, const char*>,
+
+    std::pair<std::string, absl::Cord>,
+    std::pair<absl::string_view, absl::Cord>,
 
     std::pair<absl::string_view, std::string>,
     std::pair<absl::string_view, absl::string_view>,
@@ -225,37 +472,169 @@ struct StringLikeTest : public ::testing::Test {
   hash_default_hash<typename T::first_type> hash;
 };
 
-TYPED_TEST_CASE_P(StringLikeTest);
+TYPED_TEST_SUITE(StringLikeTest, StringTypesCartesianProduct);
 
-TYPED_TEST_P(StringLikeTest, Eq) {
+TYPED_TEST(StringLikeTest, Eq) {
   EXPECT_TRUE(this->eq(this->a1, this->b1));
   EXPECT_TRUE(this->eq(this->b1, this->a1));
 }
 
-TYPED_TEST_P(StringLikeTest, NotEq) {
+TYPED_TEST(StringLikeTest, NotEq) {
   EXPECT_FALSE(this->eq(this->a1, this->b2));
   EXPECT_FALSE(this->eq(this->b2, this->a1));
 }
 
-TYPED_TEST_P(StringLikeTest, HashEq) {
+TYPED_TEST(StringLikeTest, HashEq) {
   EXPECT_EQ(this->hash(this->a1), this->hash(this->b1));
   EXPECT_EQ(this->hash(this->a2), this->hash(this->b2));
   // It would be a poor hash function which collides on these strings.
   EXPECT_NE(this->hash(this->a1), this->hash(this->b2));
 }
 
-TYPED_TEST_SUITE(StringLikeTest, StringTypesCartesianProduct);
+struct TypeWithAbslContainerHash {
+  struct absl_container_hash {
+    using is_transparent = void;
+
+    size_t operator()(const TypeWithAbslContainerHash& foo) const {
+      return absl::HashOf(foo.value);
+    }
+
+    // Extra overload to test that heterogeneity works for this hasher.
+    size_t operator()(int value) const { return absl::HashOf(value); }
+  };
+
+  friend bool operator==(const TypeWithAbslContainerHash& lhs,
+                         const TypeWithAbslContainerHash& rhs) {
+    return lhs.value == rhs.value;
+  }
+
+  friend bool operator==(const TypeWithAbslContainerHash& lhs, int rhs) {
+    return lhs.value == rhs;
+  }
+
+  int value;
+  int noise;
+};
+
+struct TypeWithAbslContainerHashAndEq {
+  struct absl_container_hash {
+    using is_transparent = void;
+
+    size_t operator()(const TypeWithAbslContainerHashAndEq& foo) const {
+      return absl::HashOf(foo.value);
+    }
+
+    // Extra overload to test that heterogeneity works for this hasher.
+    size_t operator()(int value) const { return absl::HashOf(value); }
+  };
+
+  struct absl_container_eq {
+    using is_transparent = void;
+
+    bool operator()(const TypeWithAbslContainerHashAndEq& lhs,
+                    const TypeWithAbslContainerHashAndEq& rhs) const {
+      return lhs.value == rhs.value;
+    }
+
+    // Extra overload to test that heterogeneity works for this eq.
+    bool operator()(const TypeWithAbslContainerHashAndEq& lhs, int rhs) const {
+      return lhs.value == rhs;
+    }
+  };
+
+  template <typename T>
+  bool operator==(T&& other) const = delete;
+
+  int value;
+  int noise;
+};
+
+using AbslContainerHashTypes =
+    Types<TypeWithAbslContainerHash, TypeWithAbslContainerHashAndEq>;
+
+template <typename T>
+using AbslContainerHashTest = ::testing::Test;
+
+TYPED_TEST_SUITE(AbslContainerHashTest, AbslContainerHashTypes);
+
+TYPED_TEST(AbslContainerHashTest, HasherWorks) {
+  hash_default_hash<TypeParam> hasher;
+
+  TypeParam foo1{/*value=*/1, /*noise=*/100};
+  TypeParam foo1_copy{/*value=*/1, /*noise=*/20};
+  TypeParam foo2{/*value=*/2, /*noise=*/100};
+
+  EXPECT_EQ(hasher(foo1), absl::HashOf(1));
+  EXPECT_EQ(hasher(foo2), absl::HashOf(2));
+  EXPECT_EQ(hasher(foo1), hasher(foo1_copy));
+
+  // Heterogeneity works.
+  EXPECT_EQ(hasher(foo1), hasher(1));
+  EXPECT_EQ(hasher(foo2), hasher(2));
+}
+
+TYPED_TEST(AbslContainerHashTest, EqWorks) {
+  hash_default_eq<TypeParam> eq;
+
+  TypeParam foo1{/*value=*/1, /*noise=*/100};
+  TypeParam foo1_copy{/*value=*/1, /*noise=*/20};
+  TypeParam foo2{/*value=*/2, /*noise=*/100};
+
+  EXPECT_TRUE(eq(foo1, foo1_copy));
+  EXPECT_FALSE(eq(foo1, foo2));
+
+  // Heterogeneity works.
+  EXPECT_TRUE(eq(foo1, 1));
+  EXPECT_FALSE(eq(foo1, 2));
+}
+
+TYPED_TEST(AbslContainerHashTest, HeterogeneityInMapWorks) {
+  absl::flat_hash_map<TypeParam, int> map;
+
+  TypeParam foo1{/*value=*/1, /*noise=*/100};
+  TypeParam foo1_copy{/*value=*/1, /*noise=*/20};
+  TypeParam foo2{/*value=*/2, /*noise=*/100};
+  TypeParam foo3{/*value=*/3, /*noise=*/100};
+
+  map[foo1] = 1;
+  map[foo2] = 2;
+
+  EXPECT_TRUE(map.contains(foo1_copy));
+  EXPECT_EQ(map.at(foo1_copy), 1);
+  EXPECT_TRUE(map.contains(1));
+  EXPECT_EQ(map.at(1), 1);
+  EXPECT_TRUE(map.contains(2));
+  EXPECT_EQ(map.at(2), 2);
+  EXPECT_FALSE(map.contains(foo3));
+  EXPECT_FALSE(map.contains(3));
+}
+
+TYPED_TEST(AbslContainerHashTest, HeterogeneityInSetWorks) {
+  absl::flat_hash_set<TypeParam> set;
+
+  TypeParam foo1{/*value=*/1, /*noise=*/100};
+  TypeParam foo1_copy{/*value=*/1, /*noise=*/20};
+  TypeParam foo2{/*value=*/2, /*noise=*/100};
+
+  set.insert(foo1);
+
+  EXPECT_TRUE(set.contains(foo1_copy));
+  EXPECT_TRUE(set.contains(1));
+  EXPECT_FALSE(set.contains(foo2));
+  EXPECT_FALSE(set.contains(2));
+}
 
 }  // namespace
 }  // namespace container_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 enum Hash : size_t {
-  kStd = 0x2,       // std::hash
+  kStd = 0x1,  // std::hash
 #ifdef _MSC_VER
   kExtension = kStd,  // In MSVC, std::hash == ::hash
 #else                 // _MSC_VER
-  kExtension = 0x4,  // ::hash (GCC extension)
+  kExtension = 0x2,  // ::hash (GCC extension)
 #endif                // _MSC_VER
 };
 
@@ -270,7 +649,7 @@ namespace std {
 template <int H>
 struct hash<Hashable<H>> {
   template <class E = Hashable<H>,
-            class = typename std::enable_if<E::HashableBy(kStd)>::type>
+            class = std::enable_if_t<E::HashableBy(kStd)>>
   size_t operator()(E) const {
     return kStd;
   }
@@ -278,6 +657,7 @@ struct hash<Hashable<H>> {
 }  // namespace std
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 namespace {
 
@@ -292,4 +672,5 @@ TEST(Delegate, HashDispatch) {
 
 }  // namespace
 }  // namespace container_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/container/internal/hash_generator_testing.cc b/absl/container/internal/hash_generator_testing.cc
index 37a23d6091d..4ae58da1feb 100644
--- a/absl/container/internal/hash_generator_testing.cc
+++ b/absl/container/internal/hash_generator_testing.cc
@@ -14,59 +14,41 @@
 
 #include "absl/container/internal/hash_generator_testing.h"
 
+#include <algorithm>
 #include <deque>
+#include <random>
+#include <string>
+
+#include "absl/base/no_destructor.h"
+#include "absl/random/random.h"
+#include "absl/strings/string_view.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
-namespace hash_internal {
-namespace {
-
-class RandomDeviceSeedSeq {
- public:
-  using result_type = typename std::random_device::result_type;
-
-  template <class Iterator>
-  void generate(Iterator start, Iterator end) {
-    while (start != end) {
-      *start = gen_();
-      ++start;
-    }
-  }
-
- private:
-  std::random_device gen_;
-};
-
-}  // namespace
-
-std::mt19937_64* GetSharedRng() {
-  RandomDeviceSeedSeq seed_seq;
-  static auto* rng = new std::mt19937_64(seed_seq);
-  return rng;
-}
 
 std::string Generator<std::string>::operator()() const {
+  absl::InsecureBitGen gen;
   // NOLINTNEXTLINE(runtime/int)
   std::uniform_int_distribution<short> chars(0x20, 0x7E);
   std::string res;
   res.resize(32);
-  std::generate(res.begin(), res.end(),
-                [&]() { return chars(*GetSharedRng()); });
+  std::generate(res.begin(), res.end(), [&]() { return chars(gen); });
   return res;
 }
 
 absl::string_view Generator<absl::string_view>::operator()() const {
-  static auto* arena = new std::deque<std::string>();
+  static absl::NoDestructor<std::deque<std::string>> arena;
+  absl::InsecureBitGen gen;
   // NOLINTNEXTLINE(runtime/int)
   std::uniform_int_distribution<short> chars(0x20, 0x7E);
   arena->emplace_back();
   auto& res = arena->back();
   res.resize(32);
-  std::generate(res.begin(), res.end(),
-                [&]() { return chars(*GetSharedRng()); });
+  std::generate(res.begin(), res.end(), [&]() { return chars(gen); });
   return res;
 }
 
-}  // namespace hash_internal
 }  // namespace container_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/container/internal/hash_generator_testing.h b/absl/container/internal/hash_generator_testing.h
index 477215cdba2..79bff7c1001 100644
--- a/absl/container/internal/hash_generator_testing.h
+++ b/absl/container/internal/hash_generator_testing.h
@@ -21,33 +21,37 @@
 #include <stdint.h>
 
 #include <algorithm>
+#include <cassert>
 #include <iosfwd>
+#include <memory>
 #include <random>
+#include <string>
 #include <tuple>
 #include <type_traits>
 #include <utility>
+#include <vector>
 
+#include "absl/base/config.h"
 #include "absl/container/internal/hash_policy_testing.h"
 #include "absl/memory/memory.h"
 #include "absl/meta/type_traits.h"
+#include "absl/random/random.h"
 #include "absl/strings/string_view.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
-namespace hash_internal {
 namespace generator_internal {
 
 template <class Container, class = void>
 struct IsMap : std::false_type {};
 
 template <class Map>
-struct IsMap<Map, absl::void_t<typename Map::mapped_type>> : std::true_type {};
+struct IsMap<Map, std::void_t<typename Map::mapped_type>> : std::true_type {};
 
 }  // namespace generator_internal
 
-std::mt19937_64* GetSharedRng();
-
-enum Enum {
+enum Enum : uint64_t {
   kEnumEmpty,
   kEnumDeleted,
 };
@@ -65,38 +69,24 @@ template <class T, class E = void>
 struct Generator;
 
 template <class T>
-struct Generator<T, typename std::enable_if<std::is_integral<T>::value>::type> {
-  T operator()() const {
-    std::uniform_int_distribution<T> dist;
-    return dist(*GetSharedRng());
-  }
+struct Generator<T, std::enable_if_t<std::is_integral_v<T>>> {
+  T operator()() const { return dist(gen); }
+  mutable absl::InsecureBitGen gen;
+  mutable std::uniform_int_distribution<T> dist;
 };
 
 template <>
 struct Generator<Enum> {
-  Enum operator()() const {
-    std::uniform_int_distribution<typename std::underlying_type<Enum>::type>
-        dist;
-    while (true) {
-      auto variate = dist(*GetSharedRng());
-      if (variate != kEnumEmpty && variate != kEnumDeleted)
-        return static_cast<Enum>(variate);
-    }
-  }
+  Enum operator()() const { return static_cast<Enum>(dist(gen)); }
+  mutable absl::InsecureBitGen gen;
+  mutable std::uniform_int_distribution<std::underlying_type_t<Enum>> dist;
 };
 
 template <>
 struct Generator<EnumClass> {
-  EnumClass operator()() const {
-    std::uniform_int_distribution<
-        typename std::underlying_type<EnumClass>::type>
-        dist;
-    while (true) {
-      EnumClass variate = static_cast<EnumClass>(dist(*GetSharedRng()));
-      if (variate != EnumClass::kEmpty && variate != EnumClass::kDeleted)
-        return static_cast<EnumClass>(variate);
-    }
-  }
+  EnumClass operator()() const { return static_cast<EnumClass>(dist(gen)); }
+  mutable absl::InsecureBitGen gen;
+  mutable std::uniform_int_distribution<std::underlying_type_t<EnumClass>> dist;
 };
 
 template <>
@@ -119,41 +109,60 @@ struct Generator<NonStandardLayout> {
 template <class K, class V>
 struct Generator<std::pair<K, V>> {
   std::pair<K, V> operator()() const {
-    return std::pair<K, V>(Generator<typename std::decay<K>::type>()(),
-                           Generator<typename std::decay<V>::type>()());
+    return std::pair<K, V>(Generator<std::decay_t<K>>()(),
+                           Generator<std::decay_t<V>>()());
   }
 };
 
 template <class... Ts>
 struct Generator<std::tuple<Ts...>> {
   std::tuple<Ts...> operator()() const {
-    return std::tuple<Ts...>(Generator<typename std::decay<Ts>::type>()()...);
+    return std::tuple<Ts...>(Generator<std::decay_t<Ts>>()()...);
   }
 };
 
 template <class T>
 struct Generator<std::unique_ptr<T>> {
   std::unique_ptr<T> operator()() const {
-    return absl::make_unique<T>(Generator<T>()());
+    return std::make_unique<T>(Generator<T>()());
   }
 };
 
 template <class U>
-struct Generator<U, absl::void_t<decltype(std::declval<U&>().key()),
+struct Generator<U, std::void_t<decltype(std::declval<U&>().key()),
                                 decltype(std::declval<U&>().value())>>
-    : Generator<std::pair<
-          typename std::decay<decltype(std::declval<U&>().key())>::type,
-          typename std::decay<decltype(std::declval<U&>().value())>::type>> {};
+    : Generator<std::pair<std::decay_t<decltype(std::declval<U&>().key())>,
+                          std::decay_t<decltype(std::declval<U&>().value())>>> {
+};
 
 template <class Container>
-using GeneratedType = decltype(
-    std::declval<const Generator<
-        typename std::conditional<generator_internal::IsMap<Container>::value,
-                                  typename Container::value_type,
-                                  typename Container::key_type>::type>&>()());
+using GeneratedType =
+    decltype(std::declval<const Generator<
+                 std::conditional_t<generator_internal::IsMap<Container>::value,
+                                    typename Container::value_type,
+                                    typename Container::key_type>>&>()());
+
+// Naive wrapper that performs a linear search of previous values.
+// Beware this is O(SQR), which is reasonable for smaller kMaxValues.
+template <class T, size_t kMaxValues = 64, class E = void>
+struct UniqueGenerator {
+  Generator<T, E> gen;
+  std::vector<T> values;
+
+  T operator()() {
+    assert(values.size() < kMaxValues);
+    for (;;) {
+      T value = gen();
+      if (std::find(values.begin(), values.end(), value) == values.end()) {
+        values.push_back(value);
+        return value;
+      }
+    }
+  }
+};
 
-}  // namespace hash_internal
 }  // namespace container_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_INTERNAL_HASH_GENERATOR_TESTING_H_
diff --git a/absl/container/internal/hash_policy_testing.h b/absl/container/internal/hash_policy_testing.h
index c57407a02b2..5eea528f313 100644
--- a/absl/container/internal/hash_policy_testing.h
+++ b/absl/container/internal/hash_policy_testing.h
@@ -30,6 +30,7 @@
 #include "absl/strings/string_view.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 namespace hash_testing_internal {
 
@@ -118,7 +119,11 @@ struct Alloc : std::allocator<T> {
   using propagate_on_container_swap = std::true_type;
 
   // Using old paradigm for this to ensure compatibility.
-  explicit Alloc(size_t id = 0) : id_(id) {}
+  //
+  // NOTE: As of 2025-05, this constructor cannot be explicit in order to work
+  // with the libstdc++ that ships with GCC15.
+  // NOLINTNEXTLINE(google-explicit-constructor)
+  Alloc(size_t id = 0) : id_(id) {}
 
   Alloc(const Alloc&) = default;
   Alloc& operator=(const Alloc&) = default;
@@ -153,30 +158,15 @@ auto items(const Map& m) -> std::vector<
 }
 
 template <class Set>
-auto keys(const Set& s)
-    -> std::vector<typename std::decay<typename Set::key_type>::type> {
-  std::vector<typename std::decay<typename Set::key_type>::type> res;
+auto keys(const Set& s) -> std::vector<std::decay_t<typename Set::key_type>> {
+  std::vector<std::decay_t<typename Set::key_type>> res;
   res.reserve(s.size());
   for (const auto& v : s) res.emplace_back(v);
   return res;
 }
 
 }  // namespace container_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
-// ABSL_UNORDERED_SUPPORTS_ALLOC_CTORS is false for glibcxx versions
-// where the unordered containers are missing certain constructors that
-// take allocator arguments. This test is defined ad-hoc for the platforms
-// we care about (notably Crosstool 17) because libstdcxx's useless
-// versioning scheme precludes a more principled solution.
-// From GCC-4.9 Changelog: (src: https://gcc.gnu.org/gcc-4.9/changes.html)
-// "the unordered associative containers in <unordered_map> and <unordered_set>
-// meet the allocator-aware container requirements;"
-#if (defined(__GLIBCXX__) && __GLIBCXX__ <= 20140425 ) || \
-( __GNUC__ < 4 || (__GNUC__ == 4 && __GNUC_MINOR__ < 9 ))
-#define ABSL_UNORDERED_SUPPORTS_ALLOC_CTORS 0
-#else
-#define ABSL_UNORDERED_SUPPORTS_ALLOC_CTORS 1
-#endif
-
 #endif  // ABSL_CONTAINER_INTERNAL_HASH_POLICY_TESTING_H_
diff --git a/absl/container/internal/hash_policy_testing_test.cc b/absl/container/internal/hash_policy_testing_test.cc
index 0c95eb5eaa5..f0b20fe345e 100644
--- a/absl/container/internal/hash_policy_testing_test.cc
+++ b/absl/container/internal/hash_policy_testing_test.cc
@@ -17,6 +17,7 @@
 #include "gtest/gtest.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 namespace {
 
@@ -40,4 +41,5 @@ TEST(_, Hash) {
 
 }  // namespace
 }  // namespace container_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/container/internal/hash_policy_traits.h b/absl/container/internal/hash_policy_traits.h
index fd007de7c0d..beed150b1d1 100644
--- a/absl/container/internal/hash_policy_traits.h
+++ b/absl/container/internal/hash_policy_traits.h
@@ -17,26 +17,45 @@
 
 #include <cstddef>
 #include <memory>
+#include <new>
 #include <type_traits>
 #include <utility>
 
+#include "absl/container/internal/common_policy_traits.h"
+#include "absl/container/internal/container_memory.h"
 #include "absl/meta/type_traits.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
 // Defines how slots are initialized/destroyed/moved.
 template <class Policy, class = void>
-struct hash_policy_traits {
+  struct hash_policy_traits : common_policy_traits<Policy> {
+  // The type of the keys stored in the hashtable.
+  using key_type = typename Policy::key_type;
+
  private:
   struct ReturnKey {
-    // We return `Key` here.
+    template <class Key,
+              std::enable_if_t<std::is_lvalue_reference_v<Key>, int> = 0>
+    static key_type& Impl(Key&& k, int) {
+      return *std::launder(
+          const_cast<key_type*>(std::addressof(std::forward<Key>(k))));
+    }
+
+    template <class Key>
+    static Key Impl(Key&& k, char) {
+      return std::forward<Key>(k);
+    }
+
     // When Key=T&, we forward the lvalue reference.
     // When Key=T, we return by value to avoid a dangling reference.
     // eg, for string_hash_map.
     template <class Key, class... Args>
-    Key operator()(Key&& k, const Args&...) const {
-      return std::forward<Key>(k);
+    auto operator()(Key&& k, const Args&...) const
+        -> decltype(Impl(std::forward<Key>(k), 0)) {
+      return Impl(std::forward<Key>(k), 0);
     }
   };
 
@@ -44,24 +63,21 @@ struct hash_policy_traits {
   struct ConstantIteratorsImpl : std::false_type {};
 
   template <class P>
-  struct ConstantIteratorsImpl<P, absl::void_t<typename P::constant_iterators>>
+  struct ConstantIteratorsImpl<P, std::void_t<typename P::constant_iterators>>
       : P::constant_iterators {};
 
  public:
   // The actual object stored in the hash table.
   using slot_type = typename Policy::slot_type;
 
-  // The type of the keys stored in the hashtable.
-  using key_type = typename Policy::key_type;
-
   // The argument type for insertions into the hashtable. This is different
   // from value_type for increased performance. See initializer_list constructor
   // and insert() member functions for more details.
   using init_type = typename Policy::init_type;
 
   using reference = decltype(Policy::element(std::declval<slot_type*>()));
-  using pointer = typename std::remove_reference<reference>::type*;
-  using value_type = typename std::remove_reference<reference>::type;
+  using pointer = std::remove_reference_t<reference>*;
+  using value_type = std::remove_reference_t<reference>;
 
   // Policies can set this variable to tell raw_hash_set that all iterators
   // should be constant, even `iterator`. This is useful for set-like
@@ -69,43 +85,6 @@ struct hash_policy_traits {
   // Defaults to false if not provided by the policy.
   using constant_iterators = ConstantIteratorsImpl<>;
 
-  // PRECONDITION: `slot` is UNINITIALIZED
-  // POSTCONDITION: `slot` is INITIALIZED
-  template <class Alloc, class... Args>
-  static void construct(Alloc* alloc, slot_type* slot, Args&&... args) {
-    Policy::construct(alloc, slot, std::forward<Args>(args)...);
-  }
-
-  // PRECONDITION: `slot` is INITIALIZED
-  // POSTCONDITION: `slot` is UNINITIALIZED
-  template <class Alloc>
-  static void destroy(Alloc* alloc, slot_type* slot) {
-    Policy::destroy(alloc, slot);
-  }
-
-  // Transfers the `old_slot` to `new_slot`. Any memory allocated by the
-  // allocator inside `old_slot` to `new_slot` can be transferred.
-  //
-  // OPTIONAL: defaults to:
-  //
-  //     clone(new_slot, std::move(*old_slot));
-  //     destroy(old_slot);
-  //
-  // PRECONDITION: `new_slot` is UNINITIALIZED and `old_slot` is INITIALIZED
-  // POSTCONDITION: `new_slot` is INITIALIZED and `old_slot` is
-  //                UNINITIALIZED
-  template <class Alloc>
-  static void transfer(Alloc* alloc, slot_type* new_slot, slot_type* old_slot) {
-    transfer_impl(alloc, new_slot, old_slot, 0);
-  }
-
-  // PRECONDITION: `slot` is INITIALIZED
-  // POSTCONDITION: `slot` is INITIALIZED
-  template <class P = Policy>
-  static auto element(slot_type* slot) -> decltype(P::element(slot)) {
-    return P::element(slot);
-  }
-
   // Returns the amount of memory owned by `slot`, exclusive of `sizeof(*slot)`.
   //
   // If `slot` is nullptr, returns the constant amount of memory owned by any
@@ -155,9 +134,9 @@ struct hash_policy_traits {
   // Returns the "key" portion of the slot.
   // Used for node handle manipulation.
   template <class P = Policy>
-  static auto key(slot_type* slot)
-      -> decltype(P::apply(ReturnKey(), element(slot))) {
-    return P::apply(ReturnKey(), element(slot));
+  static auto mutable_key(slot_type* slot)
+      -> decltype(P::apply(ReturnKey(), hash_policy_traits::element(slot))) {
+    return P::apply(ReturnKey(), hash_policy_traits::element(slot));
   }
 
   // Returns the "value" (as opposed to the "key") portion of the element. Used
@@ -167,23 +146,53 @@ struct hash_policy_traits {
     return P::value(elem);
   }
 
+  template <class Hash, bool kIsDefault, size_t kSeedShift>
+  static constexpr HashSlotFn get_hash_slot_fn() {
+// get_hash_slot_fn may return nullptr to signal that non type erased function
+// should be used. GCC warns against comparing function address with nullptr.
+#if defined(__GNUC__) && !defined(__clang__)
+#pragma GCC diagnostic push
+// silent error: the address of * will never be NULL [-Werror=address]
+#pragma GCC diagnostic ignored "-Waddress"
+#endif
+    return Policy::template get_hash_slot_fn<Hash, kIsDefault, kSeedShift>() ==
+                   nullptr
+               ? &hash_slot_fn_non_type_erased<Hash, kIsDefault, kSeedShift>
+               : Policy::template get_hash_slot_fn<Hash, kIsDefault,
+                                                   kSeedShift>();
+#if defined(__GNUC__) && !defined(__clang__)
+#pragma GCC diagnostic pop
+#endif
+  }
+
+  // Whether small object optimization is enabled. True by default.
+  static constexpr bool soo_enabled() { return soo_enabled_impl(Rank1{}); }
+
  private:
-  // Use auto -> decltype as an enabler.
-  template <class Alloc, class P = Policy>
-  static auto transfer_impl(Alloc* alloc, slot_type* new_slot,
-                            slot_type* old_slot, int)
-      -> decltype((void)P::transfer(alloc, new_slot, old_slot)) {
-    P::transfer(alloc, new_slot, old_slot);
+  template <class Hash, bool kIsDefault, size_t kSeedShift>
+  static size_t hash_slot_fn_non_type_erased(const void* hash_fn, void* slot,
+                                             size_t seed) {
+    return Policy::apply(
+        HashElement<Hash, kIsDefault, kSeedShift>{
+            *static_cast<const Hash*>(hash_fn), seed},
+        Policy::element(static_cast<slot_type*>(slot)));
   }
-  template <class Alloc>
-  static void transfer_impl(Alloc* alloc, slot_type* new_slot,
-                            slot_type* old_slot, char) {
-    construct(alloc, new_slot, std::move(element(old_slot)));
-    destroy(alloc, old_slot);
+
+  // Use go/ranked-overloads for dispatching. Rank1 is preferred.
+  struct Rank0 {};
+  struct Rank1 : Rank0 {};
+
+  // Use auto -> decltype as an enabler.
+  template <class P = Policy>
+  static constexpr auto soo_enabled_impl(Rank1) -> decltype(P::soo_enabled()) {
+    return P::soo_enabled();
   }
+
+  static constexpr bool soo_enabled_impl(Rank0) { return true; }
 };
 
 }  // namespace container_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_INTERNAL_HASH_POLICY_TRAITS_H_
diff --git a/absl/container/internal/hash_policy_traits_test.cc b/absl/container/internal/hash_policy_traits_test.cc
index e643d189913..8d498e0ff0f 100644
--- a/absl/container/internal/hash_policy_traits_test.cc
+++ b/absl/container/internal/hash_policy_traits_test.cc
@@ -14,14 +14,17 @@
 
 #include "absl/container/internal/hash_policy_traits.h"
 
+#include <cstddef>
 #include <functional>
 #include <memory>
 #include <new>
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
+#include "absl/container/internal/container_memory.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 namespace {
 
@@ -37,81 +40,36 @@ struct PolicyWithoutOptionalOps {
   using key_type = Slot;
   using init_type = Slot;
 
-  static std::function<void(void*, Slot*, Slot)> construct;
-  static std::function<void(void*, Slot*)> destroy;
-
   static std::function<Slot&(Slot*)> element;
   static int apply(int v) { return apply_impl(v); }
   static std::function<int(int)> apply_impl;
   static std::function<Slot&(Slot*)> value;
-};
 
-std::function<void(void*, Slot*, Slot)> PolicyWithoutOptionalOps::construct;
-std::function<void(void*, Slot*)> PolicyWithoutOptionalOps::destroy;
+  template <class Hash, bool kIsDefault, size_t kSeedShift>
+  static constexpr HashSlotFn get_hash_slot_fn() {
+    return nullptr;
+  }
+};
 
-std::function<Slot&(Slot*)> PolicyWithoutOptionalOps::element;
 std::function<int(int)> PolicyWithoutOptionalOps::apply_impl;
 std::function<Slot&(Slot*)> PolicyWithoutOptionalOps::value;
 
-struct PolicyWithOptionalOps : PolicyWithoutOptionalOps {
-  static std::function<void(void*, Slot*, Slot*)> transfer;
-};
-
-std::function<void(void*, Slot*, Slot*)> PolicyWithOptionalOps::transfer;
-
 struct Test : ::testing::Test {
   Test() {
-    PolicyWithoutOptionalOps::construct = [&](void* a1, Slot* a2, Slot a3) {
-      construct.Call(a1, a2, std::move(a3));
-    };
-    PolicyWithoutOptionalOps::destroy = [&](void* a1, Slot* a2) {
-      destroy.Call(a1, a2);
-    };
-
-    PolicyWithoutOptionalOps::element = [&](Slot* a1) -> Slot& {
-      return element.Call(a1);
-    };
     PolicyWithoutOptionalOps::apply_impl = [&](int a1) -> int {
       return apply.Call(a1);
     };
     PolicyWithoutOptionalOps::value = [&](Slot* a1) -> Slot& {
       return value.Call(a1);
     };
-
-    PolicyWithOptionalOps::transfer = [&](void* a1, Slot* a2, Slot* a3) {
-      return transfer.Call(a1, a2, a3);
-    };
   }
 
   std::allocator<int> alloc;
   int a = 53;
-
-  MockFunction<void(void*, Slot*, Slot)> construct;
-  MockFunction<void(void*, Slot*)> destroy;
-
-  MockFunction<Slot&(Slot*)> element;
   MockFunction<int(int)> apply;
   MockFunction<Slot&(Slot*)> value;
-
-  MockFunction<void(void*, Slot*, Slot*)> transfer;
 };
 
-TEST_F(Test, construct) {
-  EXPECT_CALL(construct, Call(&alloc, &a, 53));
-  hash_policy_traits<PolicyWithoutOptionalOps>::construct(&alloc, &a, 53);
-}
-
-TEST_F(Test, destroy) {
-  EXPECT_CALL(destroy, Call(&alloc, &a));
-  hash_policy_traits<PolicyWithoutOptionalOps>::destroy(&alloc, &a);
-}
-
-TEST_F(Test, element) {
-  int b = 0;
-  EXPECT_CALL(element, Call(&a)).WillOnce(ReturnRef(b));
-  EXPECT_EQ(&b, &hash_policy_traits<PolicyWithoutOptionalOps>::element(&a));
-}
-
 TEST_F(Test, apply) {
   EXPECT_CALL(apply, Call(42)).WillOnce(Return(1337));
   EXPECT_EQ(1337, (hash_policy_traits<PolicyWithoutOptionalOps>::apply(42)));
@@ -123,20 +81,72 @@ TEST_F(Test, value) {
   EXPECT_EQ(&b, &hash_policy_traits<PolicyWithoutOptionalOps>::value(&a));
 }
 
-TEST_F(Test, without_transfer) {
-  int b = 42;
-  EXPECT_CALL(element, Call(&b)).WillOnce(::testing::ReturnRef(b));
-  EXPECT_CALL(construct, Call(&alloc, &a, b));
-  EXPECT_CALL(destroy, Call(&alloc, &b));
-  hash_policy_traits<PolicyWithoutOptionalOps>::transfer(&alloc, &a, &b);
+struct Hash {
+  size_t operator()(Slot a) const { return static_cast<size_t>(a) * 5; }
+};
+
+struct PolicyNoHashFn {
+  using slot_type = Slot;
+  using key_type = Slot;
+  using init_type = Slot;
+
+  static size_t* apply_called_count;
+
+  static Slot& element(Slot* slot) { return *slot; }
+  template <typename Fn>
+  static size_t apply(const Fn& fn, int v) {
+    ++(*apply_called_count);
+    return fn(v);
+  }
+
+  template <class Hash, bool kIsDefault, size_t kSeedShift>
+  static constexpr HashSlotFn get_hash_slot_fn() {
+    return nullptr;
+  }
+};
+
+size_t* PolicyNoHashFn::apply_called_count;
+
+struct PolicyCustomHashFn : PolicyNoHashFn {
+  template <class Hash, bool kIsDefault, size_t kSeedShift>
+  static constexpr HashSlotFn get_hash_slot_fn() {
+    return &TypeErasedApplyToSlotFn<Hash, int, kIsDefault, kSeedShift>;
+  }
+};
+
+TEST(HashTest, PolicyNoHashFn_get_hash_slot_fn) {
+  size_t apply_called_count = 0;
+  PolicyNoHashFn::apply_called_count = &apply_called_count;
+
+  Hash hasher;
+  Slot value = 7;
+  auto* fn = hash_policy_traits<PolicyNoHashFn>::get_hash_slot_fn<
+      Hash, /*kIsDefault=*/false, /*kSeedShift=*/6>();
+  EXPECT_NE(fn, nullptr);
+  EXPECT_EQ(fn(&hasher, &value, 100),
+            (HashElement<Hash, /*kIsDefault=*/false, /*kSeedShift=*/6>(
+                hasher, 100)(value)));
+  EXPECT_EQ(apply_called_count, 1);
 }
 
-TEST_F(Test, with_transfer) {
-  int b = 42;
-  EXPECT_CALL(transfer, Call(&alloc, &a, &b));
-  hash_policy_traits<PolicyWithOptionalOps>::transfer(&alloc, &a, &b);
+TEST(HashTest, PolicyCustomHashFn_get_hash_slot_fn) {
+  size_t apply_called_count = 0;
+  PolicyNoHashFn::apply_called_count = &apply_called_count;
+
+  Hash hasher;
+  Slot value = 7;
+  auto* fn = hash_policy_traits<PolicyCustomHashFn>::get_hash_slot_fn<
+      Hash, /*kIsDefault=*/false, /*kSeedShift=*/6>();
+  EXPECT_EQ(fn,
+            (PolicyCustomHashFn::get_hash_slot_fn<Hash, /*kIsDefault=*/false,
+                                                  /*kSeedShift=*/6>()));
+  EXPECT_EQ(fn(&hasher, &value, 100),
+            (HashElement<Hash, /*kIsDefault=*/false, /*kSeedShift=*/6>(
+                hasher, 100)(value)));
+  EXPECT_EQ(apply_called_count, 0);
 }
 
 }  // namespace
 }  // namespace container_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/container/internal/hashtable_control_bytes.h b/absl/container/internal/hashtable_control_bytes.h
new file mode 100644
index 00000000000..b70ac8c827d
--- /dev/null
+++ b/absl/container/internal/hashtable_control_bytes.h
@@ -0,0 +1,528 @@
+// Copyright 2025 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// This file contains the implementation of the hashtable control bytes
+// manipulation.
+
+#ifndef ABSL_CONTAINER_INTERNAL_HASHTABLE_CONTROL_BYTES_H_
+#define ABSL_CONTAINER_INTERNAL_HASHTABLE_CONTROL_BYTES_H_
+
+#include <cassert>
+#include <cstddef>
+#include <cstdint>
+#include <type_traits>
+
+#include "absl/base/config.h"
+#include "absl/base/internal/endian.h"
+#include "absl/base/optimization.h"
+#include "absl/numeric/bits.h"
+
+#ifdef ABSL_INTERNAL_HAVE_SSE2
+#include <emmintrin.h>
+#endif
+
+#ifdef ABSL_INTERNAL_HAVE_SSSE3
+#include <tmmintrin.h>
+#endif
+
+#ifdef _MSC_VER
+#include <intrin.h>
+#endif
+
+#ifdef ABSL_INTERNAL_HAVE_ARM_NEON
+#include <arm_neon.h>
+#endif
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace container_internal {
+
+#ifdef ABSL_SWISSTABLE_ASSERT
+#error ABSL_SWISSTABLE_ASSERT cannot be directly set
+#else
+// We use this macro for assertions that users may see when the table is in an
+// invalid state that sanitizers may help diagnose.
+#define ABSL_SWISSTABLE_ASSERT(CONDITION) \
+  assert((CONDITION) && "Try enabling sanitizers.")
+#endif
+
+
+template <typename T>
+uint32_t TrailingZeros(T x) {
+  ABSL_ASSUME(x != 0);
+  return static_cast<uint32_t>(countr_zero(x));
+}
+
+// 8 bytes bitmask with most significant bit set for every byte.
+constexpr uint64_t kMsbs8Bytes = 0x8080808080808080ULL;
+// 8 kEmpty bytes that is useful for small table initialization.
+constexpr uint64_t k8EmptyBytes = kMsbs8Bytes;
+
+// An abstract bitmask, such as that emitted by a SIMD instruction.
+//
+// Specifically, this type implements a simple bitset whose representation is
+// controlled by `SignificantBits` and `Shift`. `SignificantBits` is the number
+// of abstract bits in the bitset, while `Shift` is the log-base-two of the
+// width of an abstract bit in the representation.
+// This mask provides operations for any number of real bits set in an abstract
+// bit. To add iteration on top of that, implementation must guarantee no more
+// than the most significant real bit is set in a set abstract bit.
+template <class T, int SignificantBits, int Shift = 0>
+class NonIterableBitMask {
+ public:
+  explicit NonIterableBitMask(T mask) : mask_(mask) {}
+
+  explicit operator bool() const { return mask_ != 0; }
+
+  // Returns the index of the lowest *abstract* bit set in `self`.
+  uint32_t LowestBitSet() const {
+    return container_internal::TrailingZeros(mask_) >> Shift;
+  }
+
+  // Returns the number of trailing zero *abstract* bits.
+  uint32_t TrailingZeros() const {
+    return container_internal::TrailingZeros(mask_) >> Shift;
+  }
+
+  // Returns the number of leading zero *abstract* bits.
+  uint32_t LeadingZeros() const {
+    constexpr int total_significant_bits = SignificantBits << Shift;
+    constexpr int extra_bits = sizeof(T) * 8 - total_significant_bits;
+    return static_cast<uint32_t>(
+               countl_zero(static_cast<T>(mask_ << extra_bits))) >>
+           Shift;
+  }
+
+  T mask_;
+};
+
+// Mask that can be iterable
+//
+// For example, when `SignificantBits` is 16 and `Shift` is zero, this is just
+// an ordinary 16-bit bitset occupying the low 16 bits of `mask`. When
+// `SignificantBits` is 8 and `Shift` is 3, abstract bits are represented as
+// the bytes `0x00` and `0x80`, and it occupies all 64 bits of the bitmask.
+// If NullifyBitsOnIteration is true (only allowed for Shift == 3),
+// non zero abstract bit is allowed to have additional bits
+// (e.g., `0xff`, `0x83` and `0x9c` are ok, but `0x6f` is not).
+//
+// For example:
+//   for (int i : BitMask<uint32_t, 16>(0b101)) -> yields 0, 2
+//   for (int i : BitMask<uint64_t, 8, 3>(0x0000000080800000)) -> yields 2, 3
+template <class T, int SignificantBits, int Shift = 0,
+          bool NullifyBitsOnIteration = false>
+class BitMask : public NonIterableBitMask<T, SignificantBits, Shift> {
+  using Base = NonIterableBitMask<T, SignificantBits, Shift>;
+  static_assert(std::is_unsigned_v<T>);
+  static_assert(Shift == 0 || Shift == 3);
+  static_assert(!NullifyBitsOnIteration || Shift == 3);
+
+ public:
+  explicit BitMask(T mask) : Base(mask) {
+    if (Shift == 3 && !NullifyBitsOnIteration) {
+      ABSL_SWISSTABLE_ASSERT(this->mask_ == (this->mask_ & kMsbs8Bytes));
+    }
+  }
+  // BitMask is an iterator over the indices of its abstract bits.
+  using value_type = int;
+  using iterator = BitMask;
+  using const_iterator = BitMask;
+
+  BitMask& operator++() {
+    if (Shift == 3 && NullifyBitsOnIteration) {
+      this->mask_ &= kMsbs8Bytes;
+    }
+    this->mask_ &= (this->mask_ - 1);
+    return *this;
+  }
+
+  uint32_t operator*() const { return Base::LowestBitSet(); }
+
+  BitMask begin() const { return *this; }
+  BitMask end() const { return BitMask(0); }
+
+ private:
+  friend bool operator==(const BitMask& a, const BitMask& b) {
+    return a.mask_ == b.mask_;
+  }
+  friend bool operator!=(const BitMask& a, const BitMask& b) {
+    return a.mask_ != b.mask_;
+  }
+};
+
+using h2_t = uint8_t;
+
+// The values here are selected for maximum performance. See the static asserts
+// below for details.
+
+// A `ctrl_t` is a single control byte, which can have one of four
+// states: empty, deleted, full (which has an associated seven-bit h2_t value)
+// and the sentinel. They have the following bit patterns:
+//
+//      empty: 1 0 0 0 0 0 0 0
+//    deleted: 1 1 1 1 1 1 1 0
+//       full: 0 h h h h h h h  // h represents the hash bits.
+//   sentinel: 1 1 1 1 1 1 1 1
+//
+// These values are specifically tuned for SSE-flavored SIMD.
+// The static_asserts below detail the source of these choices.
+//
+// We use an enum class so that when strict aliasing is enabled, the compiler
+// knows ctrl_t doesn't alias other types.
+enum class ctrl_t : int8_t {
+  kEmpty = -128,   // 0b10000000
+  kDeleted = -2,   // 0b11111110
+  kSentinel = -1,  // 0b11111111
+  // Special value used in the slow path of resizing.
+  kMarkedForSlowTransfer = -3,
+};
+static_assert(
+    (static_cast<int8_t>(ctrl_t::kEmpty) &
+     static_cast<int8_t>(ctrl_t::kDeleted) &
+     static_cast<int8_t>(ctrl_t::kSentinel) & 0x80) != 0,
+    "Special markers need to have the MSB to make checking for them efficient");
+static_assert(
+    ctrl_t::kEmpty < ctrl_t::kSentinel && ctrl_t::kDeleted < ctrl_t::kSentinel,
+    "ctrl_t::kEmpty and ctrl_t::kDeleted must be smaller than "
+    "ctrl_t::kSentinel to make the SIMD test of IsEmptyOrDeleted() efficient");
+static_assert(
+    ctrl_t::kSentinel == static_cast<ctrl_t>(-1),
+    "ctrl_t::kSentinel must be -1 to elide loading it from memory into SIMD "
+    "registers (pcmpeqd xmm, xmm)");
+static_assert(ctrl_t::kEmpty == static_cast<ctrl_t>(-128),
+              "ctrl_t::kEmpty must be -128 to make the SIMD check for its "
+              "existence efficient (psignb xmm, xmm)");
+static_assert(
+    (~static_cast<int8_t>(ctrl_t::kEmpty) &
+     ~static_cast<int8_t>(ctrl_t::kDeleted) &
+     static_cast<int8_t>(ctrl_t::kSentinel) & 0x7F) != 0,
+    "ctrl_t::kEmpty and ctrl_t::kDeleted must share an unset bit that is not "
+    "shared by ctrl_t::kSentinel to make the scalar test for "
+    "MaskEmptyOrDeleted() efficient");
+static_assert(ctrl_t::kDeleted == static_cast<ctrl_t>(-2),
+              "ctrl_t::kDeleted must be -2 to make the implementation of "
+              "ConvertSpecialToEmptyAndFullToDeleted efficient");
+static_assert(ctrl_t::kEmpty == static_cast<ctrl_t>(-128),
+              "ctrl_t::kEmpty must be -128 to use saturated subtraction in"
+              " ConvertSpecialToEmptyAndFullToDeleted");
+
+// Helpers for checking the state of a control byte.
+inline bool IsEmpty(ctrl_t c) { return c == ctrl_t::kEmpty; }
+inline bool IsFull(ctrl_t c) {
+  // Cast `c` to the underlying type instead of casting `0` to `ctrl_t` as `0`
+  // is not a value in the enum. Both ways are equivalent, but this way makes
+  // linters happier.
+  return static_cast<std::underlying_type_t<ctrl_t>>(c) >= 0;
+}
+inline bool IsDeleted(ctrl_t c) { return c == ctrl_t::kDeleted; }
+inline bool IsEmptyOrDeleted(ctrl_t c) { return c < ctrl_t::kSentinel; }
+
+#ifdef ABSL_INTERNAL_HAVE_SSE2
+// Quick reference guide for intrinsics used below:
+//
+// * __m128i: An XMM (128-bit) word.
+//
+// * _mm_setzero_si128: Returns a zero vector.
+// * _mm_set1_epi8:     Returns a vector with the same i8 in each lane.
+//
+// * _mm_subs_epi8:    Saturating-subtracts two i8 vectors.
+// * _mm_and_si128:    Ands two i128s together.
+// * _mm_or_si128:     Ors two i128s together.
+// * _mm_andnot_si128: And-nots two i128s together.
+//
+// * _mm_cmpeq_epi8: Component-wise compares two i8 vectors for equality,
+//                   filling each lane with 0x00 or 0xff.
+// * _mm_cmpgt_epi8: Same as above, but using > rather than ==.
+//
+// * _mm_loadu_si128:  Performs an unaligned load of an i128.
+// * _mm_storeu_si128: Performs an unaligned store of an i128.
+//
+// * _mm_sign_epi8:     Retains, negates, or zeroes each i8 lane of the first
+//                      argument if the corresponding lane of the second
+//                      argument is positive, negative, or zero, respectively.
+// * _mm_movemask_epi8: Selects the sign bit out of each i8 lane and produces a
+//                      bitmask consisting of those bits.
+// * _mm_shuffle_epi8:  Selects i8s from the first argument, using the low
+//                      four bits of each i8 lane in the second argument as
+//                      indices.
+
+// https://github.com/abseil/abseil-cpp/issues/209
+// https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87853
+// _mm_cmpgt_epi8 is broken under GCC with -funsigned-char
+// Work around this by using the portable implementation of Group
+// when using -funsigned-char under GCC.
+inline __m128i _mm_cmpgt_epi8_fixed(__m128i a, __m128i b) {
+#if defined(__GNUC__) && !defined(__clang__)
+  if (std::is_unsigned_v<char>) {
+    const __m128i mask = _mm_set1_epi8(0x80);
+    const __m128i diff = _mm_subs_epi8(b, a);
+    return _mm_cmpeq_epi8(_mm_and_si128(diff, mask), mask);
+  }
+#endif
+  return _mm_cmpgt_epi8(a, b);
+}
+
+struct GroupSse2Impl {
+  static constexpr size_t kWidth = 16;  // the number of slots per group
+  // There are only 16 bits, but using uint32_t instead of uint16_t allows for
+  // better codegen. In particular, there is no blsr instruction for a 16 bit
+  // register, but there is for a 32 bit register (used in BitMask::operator++).
+  using MaskInt = uint32_t;
+  using BitMaskType = BitMask<MaskInt, kWidth>;
+  using NonIterableBitMaskType = NonIterableBitMask<MaskInt, kWidth>;
+
+  explicit GroupSse2Impl(const ctrl_t* pos) {
+    ctrl = _mm_loadu_si128(reinterpret_cast<const __m128i*>(pos));
+  }
+
+  // Returns a bitmask representing the positions of slots that match hash.
+  BitMaskType Match(h2_t hash) const {
+    auto match = _mm_set1_epi8(static_cast<char>(hash));
+    return BitMaskType(MoveMask(_mm_cmpeq_epi8(match, ctrl)));
+  }
+
+  // Returns a bitmask representing the positions of empty slots.
+  NonIterableBitMaskType MaskEmpty() const {
+#ifdef ABSL_INTERNAL_HAVE_SSSE3
+    // This only works because ctrl_t::kEmpty is -128.
+    return NonIterableBitMaskType(MoveMask(_mm_sign_epi8(ctrl, ctrl)));
+#else
+    auto match = _mm_set1_epi8(static_cast<char>(ctrl_t::kEmpty));
+    return NonIterableBitMaskType(MoveMask(_mm_cmpeq_epi8(match, ctrl)));
+#endif
+  }
+
+  // Returns a bitmask representing the positions of full slots.
+  // Note: for `is_small()` tables group may contain the "same" slot twice:
+  // original and mirrored.
+  BitMaskType MaskFull() const { return BitMaskType(MoveMask(ctrl) ^ 0xffff); }
+
+  // Returns a bitmask representing the positions of non full slots.
+  // Note: this includes: kEmpty, kDeleted, kSentinel.
+  // It is useful in contexts when kSentinel is not present.
+  auto MaskNonFull() const { return BitMaskType(MoveMask(ctrl)); }
+
+  // Returns a bitmask representing the positions of empty or deleted slots.
+  NonIterableBitMaskType MaskEmptyOrDeleted() const {
+    auto special = _mm_set1_epi8(static_cast<char>(ctrl_t::kSentinel));
+    return NonIterableBitMaskType(
+        MoveMask(_mm_cmpgt_epi8_fixed(special, ctrl)));
+  }
+
+  // Returns a bitmask representing the positions of full or sentinel slots.
+  // Note: for `is_small()` tables group may contain the "same" slot twice:
+  // original and mirrored.
+  NonIterableBitMaskType MaskFullOrSentinel() const {
+    auto special = _mm_set1_epi8(static_cast<char>(ctrl_t::kSentinel) - 1);
+    return NonIterableBitMaskType(
+        MoveMask(_mm_cmpgt_epi8_fixed(ctrl, special)));
+  }
+
+  void ConvertSpecialToEmptyAndFullToDeleted(ctrl_t* dst) const {
+    // Take advantage of the fact that kEmpty is already the smallest signed
+    // char value, and using a saturated subtraction will not affect it.
+    // All special values have the MSB set, so after an AND with MSBS, we
+    // are left with -128 for special values and 0 for full. After applying
+    // subs 2, we arrive at the result of -128(kEmpty) for special and
+    // -2(kDeleted) for full.
+    auto msbs = _mm_set1_epi8(static_cast<char>(-128));
+    auto twos = _mm_set1_epi8(static_cast<char>(2));
+    auto res = _mm_subs_epi8(_mm_and_si128(msbs, ctrl), twos);
+    _mm_storeu_si128(reinterpret_cast<__m128i*>(dst), res);
+  }
+
+  static MaskInt MoveMask(__m128i xmm) {
+    auto mask = static_cast<MaskInt>(_mm_movemask_epi8(xmm));
+#ifdef __clang__
+    // TODO(b/472522597): Without the inline asm, clang ends up generating an
+    // unnecessary movzx to zero the upper bits of the output, but those bits
+    // are already zero. See https://godbolt.org/z/G6xW1Ecbx.
+    asm("" : "+r"(mask));  // NOLINT
+#endif
+    return mask;
+  }
+
+  __m128i ctrl;
+};
+#endif  // ABSL_INTERNAL_RAW_HASH_SET_HAVE_SSE2
+
+#if defined(ABSL_INTERNAL_HAVE_ARM_NEON) && defined(ABSL_IS_LITTLE_ENDIAN)
+struct GroupAArch64Impl {
+  static constexpr size_t kWidth = 8;
+  using BitMaskType = BitMask<uint64_t, kWidth, /*Shift=*/3,
+                              /*NullifyBitsOnIteration=*/true>;
+  using NonIterableBitMaskType =
+      NonIterableBitMask<uint64_t, kWidth, /*Shift=*/3>;
+
+  explicit GroupAArch64Impl(const ctrl_t* pos) {
+    ctrl = vld1_u8(reinterpret_cast<const uint8_t*>(pos));
+  }
+
+  auto Match(h2_t hash) const {
+    uint8x8_t dup = vdup_n_u8(hash);
+    auto mask = vceq_u8(ctrl, dup);
+    return BitMaskType(vget_lane_u64(vreinterpret_u64_u8(mask), 0));
+  }
+
+  auto MaskEmpty() const {
+    uint64_t mask =
+        vget_lane_u64(vreinterpret_u64_u8(vceq_s8(
+                          vdup_n_s8(static_cast<int8_t>(ctrl_t::kEmpty)),
+                          vreinterpret_s8_u8(ctrl))),
+                      0);
+    return NonIterableBitMaskType(mask);
+  }
+
+  // Returns a bitmask representing the positions of full slots.
+  // Note: for `is_small()` tables group may contain the "same" slot twice:
+  // original and mirrored.
+  auto MaskFull() const {
+    uint64_t mask = vget_lane_u64(
+        vreinterpret_u64_u8(vcge_s8(vreinterpret_s8_u8(ctrl),
+                                    vdup_n_s8(static_cast<int8_t>(0)))),
+        0);
+    return BitMaskType(mask);
+  }
+
+  // Returns a bitmask representing the positions of non full slots.
+  // Note: this includes: kEmpty, kDeleted, kSentinel.
+  // It is useful in contexts when kSentinel is not present.
+  auto MaskNonFull() const {
+    uint64_t mask = vget_lane_u64(
+        vreinterpret_u64_u8(vclt_s8(vreinterpret_s8_u8(ctrl),
+                                    vdup_n_s8(static_cast<int8_t>(0)))),
+        0);
+    return BitMaskType(mask);
+  }
+
+  auto MaskEmptyOrDeleted() const {
+    uint64_t mask =
+        vget_lane_u64(vreinterpret_u64_u8(vcgt_s8(
+                          vdup_n_s8(static_cast<int8_t>(ctrl_t::kSentinel)),
+                          vreinterpret_s8_u8(ctrl))),
+                      0);
+    return NonIterableBitMaskType(mask);
+  }
+
+  NonIterableBitMaskType MaskFullOrSentinel() const {
+    uint64_t mask = vget_lane_u64(
+        vreinterpret_u64_u8(
+            vcgt_s8(vreinterpret_s8_u8(ctrl),
+                    vdup_n_s8(static_cast<int8_t>(ctrl_t::kSentinel) - 1))),
+        0);
+    return NonIterableBitMaskType(mask);
+  }
+
+  void ConvertSpecialToEmptyAndFullToDeleted(ctrl_t* dst) const {
+    uint64_t mask = vget_lane_u64(vreinterpret_u64_u8(ctrl), 0);
+    constexpr uint64_t slsbs = 0x0202020202020202ULL;
+    constexpr uint64_t midbs = 0x7e7e7e7e7e7e7e7eULL;
+    auto x = slsbs & (mask >> 6);
+    auto res = (x + midbs) | kMsbs8Bytes;
+    little_endian::Store64(dst, res);
+  }
+
+  uint8x8_t ctrl;
+};
+#endif  // ABSL_INTERNAL_HAVE_ARM_NEON && ABSL_IS_LITTLE_ENDIAN
+
+struct GroupPortableImpl {
+  static constexpr size_t kWidth = 8;
+  using BitMaskType = BitMask<uint64_t, kWidth, /*Shift=*/3,
+                              /*NullifyBitsOnIteration=*/false>;
+  using NonIterableBitMaskType =
+      NonIterableBitMask<uint64_t, kWidth, /*Shift=*/3>;
+
+  explicit GroupPortableImpl(const ctrl_t* pos)
+      : ctrl(little_endian::Load64(pos)) {}
+
+  BitMaskType Match(h2_t hash) const {
+    // For the technique, see:
+    // http://graphics.stanford.edu/~seander/bithacks.html##ValueInWord
+    // (Determine if a word has a byte equal to n).
+    //
+    // Caveat: there are false positives but:
+    // - they only occur if there is a real match
+    // - they never occur on ctrl_t::kEmpty, ctrl_t::kDeleted, ctrl_t::kSentinel
+    // - they will be handled gracefully by subsequent checks in code
+    //
+    // Example:
+    //   v = 0x1716151413121110
+    //   hash = 0x12
+    //   retval = (v - lsbs) & ~v & msbs = 0x0000000080800000
+    constexpr uint64_t lsbs = 0x0101010101010101ULL;
+    auto x = ctrl ^ (lsbs * hash);
+    return BitMaskType((x - lsbs) & ~x & kMsbs8Bytes);
+  }
+
+  auto MaskEmpty() const {
+    return NonIterableBitMaskType((ctrl & ~(ctrl << 6)) & kMsbs8Bytes);
+  }
+
+  // Returns a bitmask representing the positions of full slots.
+  // Note: for `is_small()` tables group may contain the "same" slot twice:
+  // original and mirrored.
+  auto MaskFull() const {
+    return BitMaskType((ctrl ^ kMsbs8Bytes) & kMsbs8Bytes);
+  }
+
+  // Returns a bitmask representing the positions of non full slots.
+  // Note: this includes: kEmpty, kDeleted, kSentinel.
+  // It is useful in contexts when kSentinel is not present.
+  auto MaskNonFull() const { return BitMaskType(ctrl & kMsbs8Bytes); }
+
+  auto MaskEmptyOrDeleted() const {
+    return NonIterableBitMaskType((ctrl & ~(ctrl << 7)) & kMsbs8Bytes);
+  }
+
+  auto MaskFullOrSentinel() const {
+    return NonIterableBitMaskType((~ctrl | (ctrl << 7)) & kMsbs8Bytes);
+  }
+
+  void ConvertSpecialToEmptyAndFullToDeleted(ctrl_t* dst) const {
+    constexpr uint64_t lsbs = 0x0101010101010101ULL;
+    auto x = ctrl & kMsbs8Bytes;
+    auto res = (~x + (x >> 7)) & ~lsbs;
+    little_endian::Store64(dst, res);
+  }
+
+  uint64_t ctrl;
+};
+
+#ifdef ABSL_INTERNAL_HAVE_SSE2
+using Group = GroupSse2Impl;
+using GroupFullEmptyOrDeleted = GroupSse2Impl;
+#elif defined(ABSL_INTERNAL_HAVE_ARM_NEON) && defined(ABSL_IS_LITTLE_ENDIAN)
+using Group = GroupAArch64Impl;
+// For Aarch64, we use the portable implementation for counting and masking
+// full, empty or deleted group elements. This is to avoid the latency of moving
+// between data GPRs and Neon registers when it does not provide a benefit.
+// Using Neon is profitable when we call Match(), but is not when we don't,
+// which is the case when we do *EmptyOrDeleted and MaskFull operations.
+// It is difficult to make a similar approach beneficial on other architectures
+// such as x86 since they have much lower GPR <-> vector register transfer
+// latency and 16-wide Groups.
+using GroupFullEmptyOrDeleted = GroupPortableImpl;
+#else
+using Group = GroupPortableImpl;
+using GroupFullEmptyOrDeleted = GroupPortableImpl;
+#endif
+
+}  // namespace container_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#undef ABSL_SWISSTABLE_ASSERT
+
+#endif  // ABSL_CONTAINER_INTERNAL_HASHTABLE_CONTROL_BYTES_H_
diff --git a/absl/container/internal/hashtable_control_bytes_test.cc b/absl/container/internal/hashtable_control_bytes_test.cc
new file mode 100644
index 00000000000..a4aa3a9669a
--- /dev/null
+++ b/absl/container/internal/hashtable_control_bytes_test.cc
@@ -0,0 +1,259 @@
+// Copyright 2025 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/container/internal/hashtable_control_bytes.h"
+
+#include <array>
+#include <cstddef>
+#include <cstdint>
+#include <vector>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace container_internal {
+namespace {
+
+using ::testing::ElementsAre;
+using ::testing::ElementsAreArray;
+
+// Convenience function to static cast to ctrl_t.
+ctrl_t CtrlT(int i) { return static_cast<ctrl_t>(i); }
+
+TEST(BitMask, Smoke) {
+  EXPECT_FALSE((BitMask<uint8_t, 8>(0)));
+  EXPECT_TRUE((BitMask<uint8_t, 8>(5)));
+
+  EXPECT_THAT((BitMask<uint8_t, 8>(0)), ElementsAre());
+  EXPECT_THAT((BitMask<uint8_t, 8>(0x1)), ElementsAre(0));
+  EXPECT_THAT((BitMask<uint8_t, 8>(0x2)), ElementsAre(1));
+  EXPECT_THAT((BitMask<uint8_t, 8>(0x3)), ElementsAre(0, 1));
+  EXPECT_THAT((BitMask<uint8_t, 8>(0x4)), ElementsAre(2));
+  EXPECT_THAT((BitMask<uint8_t, 8>(0x5)), ElementsAre(0, 2));
+  EXPECT_THAT((BitMask<uint8_t, 8>(0x55)), ElementsAre(0, 2, 4, 6));
+  EXPECT_THAT((BitMask<uint8_t, 8>(0xAA)), ElementsAre(1, 3, 5, 7));
+}
+
+TEST(BitMask, WithShift_MatchPortable) {
+  // See the non-SSE version of Group for details on what this math is for.
+  uint64_t ctrl = 0x1716151413121110;
+  uint64_t hash = 0x12;
+  constexpr uint64_t lsbs = 0x0101010101010101ULL;
+  auto x = ctrl ^ (lsbs * hash);
+  uint64_t mask = (x - lsbs) & ~x & kMsbs8Bytes;
+  EXPECT_EQ(0x0000000080800000, mask);
+
+  BitMask<uint64_t, 8, 3> b(mask);
+  EXPECT_EQ(*b, 2);
+}
+
+constexpr uint64_t kSome8BytesMask = /*  */ 0x8000808080008000ULL;
+constexpr uint64_t kSome8BytesMaskAllOnes = 0xff00ffffff00ff00ULL;
+constexpr auto kSome8BytesMaskBits = std::array<int, 5>{1, 3, 4, 5, 7};
+
+TEST(BitMask, WithShift_FullMask) {
+  EXPECT_THAT((BitMask<uint64_t, 8, 3>(kMsbs8Bytes)),
+              ElementsAre(0, 1, 2, 3, 4, 5, 6, 7));
+  EXPECT_THAT(
+      (BitMask<uint64_t, 8, 3, /*NullifyBitsOnIteration=*/true>(kMsbs8Bytes)),
+      ElementsAre(0, 1, 2, 3, 4, 5, 6, 7));
+  EXPECT_THAT(
+      (BitMask<uint64_t, 8, 3, /*NullifyBitsOnIteration=*/true>(~uint64_t{0})),
+      ElementsAre(0, 1, 2, 3, 4, 5, 6, 7));
+}
+
+TEST(BitMask, WithShift_EmptyMask) {
+  EXPECT_THAT((BitMask<uint64_t, 8, 3>(0)), ElementsAre());
+  EXPECT_THAT((BitMask<uint64_t, 8, 3, /*NullifyBitsOnIteration=*/true>(0)),
+              ElementsAre());
+}
+
+TEST(BitMask, WithShift_SomeMask) {
+  EXPECT_THAT((BitMask<uint64_t, 8, 3>(kSome8BytesMask)),
+              ElementsAreArray(kSome8BytesMaskBits));
+  EXPECT_THAT((BitMask<uint64_t, 8, 3, /*NullifyBitsOnIteration=*/true>(
+                  kSome8BytesMask)),
+              ElementsAreArray(kSome8BytesMaskBits));
+  EXPECT_THAT((BitMask<uint64_t, 8, 3, /*NullifyBitsOnIteration=*/true>(
+                  kSome8BytesMaskAllOnes)),
+              ElementsAreArray(kSome8BytesMaskBits));
+}
+
+TEST(BitMask, WithShift_SomeMaskExtraBitsForNullify) {
+  // Verify that adding extra bits into non zero bytes is fine.
+  uint64_t extra_bits = 77;
+  for (int i = 0; i < 100; ++i) {
+    // Add extra bits, but keep zero bytes untouched.
+    uint64_t extra_mask = extra_bits & kSome8BytesMaskAllOnes;
+    EXPECT_THAT((BitMask<uint64_t, 8, 3, /*NullifyBitsOnIteration=*/true>(
+                    kSome8BytesMask | extra_mask)),
+                ElementsAreArray(kSome8BytesMaskBits))
+        << i << " " << extra_mask;
+    extra_bits = (extra_bits + 1) * 3;
+  }
+}
+
+TEST(BitMask, LeadingTrailing) {
+  EXPECT_EQ((BitMask<uint32_t, 16>(0x00001a40).LeadingZeros()), 3);
+  EXPECT_EQ((BitMask<uint32_t, 16>(0x00001a40).TrailingZeros()), 6);
+
+  EXPECT_EQ((BitMask<uint32_t, 16>(0x00000001).LeadingZeros()), 15);
+  EXPECT_EQ((BitMask<uint32_t, 16>(0x00000001).TrailingZeros()), 0);
+
+  EXPECT_EQ((BitMask<uint32_t, 16>(0x00008000).LeadingZeros()), 0);
+  EXPECT_EQ((BitMask<uint32_t, 16>(0x00008000).TrailingZeros()), 15);
+
+  EXPECT_EQ((BitMask<uint64_t, 8, 3>(0x0000008080808000).LeadingZeros()), 3);
+  EXPECT_EQ((BitMask<uint64_t, 8, 3>(0x0000008080808000).TrailingZeros()), 1);
+
+  EXPECT_EQ((BitMask<uint64_t, 8, 3>(0x0000000000000080).LeadingZeros()), 7);
+  EXPECT_EQ((BitMask<uint64_t, 8, 3>(0x0000000000000080).TrailingZeros()), 0);
+
+  EXPECT_EQ((BitMask<uint64_t, 8, 3>(0x8000000000000000).LeadingZeros()), 0);
+  EXPECT_EQ((BitMask<uint64_t, 8, 3>(0x8000000000000000).TrailingZeros()), 7);
+}
+
+template <class GroupTypeParam>
+class GroupTest : public testing::Test {};
+using GroupTypes =
+    ::testing::Types<Group, GroupPortableImpl, GroupFullEmptyOrDeleted>;
+TYPED_TEST_SUITE(GroupTest, GroupTypes);
+
+TYPED_TEST(GroupTest, Match) {
+  using GroupType = TypeParam;
+  if (GroupType::kWidth == 16) {
+    ctrl_t group[] = {ctrl_t::kEmpty, CtrlT(1), ctrl_t::kDeleted,  CtrlT(3),
+                      ctrl_t::kEmpty, CtrlT(5), ctrl_t::kSentinel, CtrlT(7),
+                      CtrlT(7),       CtrlT(5), CtrlT(3),          CtrlT(1),
+                      CtrlT(1),       CtrlT(1), CtrlT(1),          CtrlT(1)};
+    EXPECT_THAT(GroupType{group}.Match(0), ElementsAre());
+    EXPECT_THAT(GroupType{group}.Match(1), ElementsAre(1, 11, 12, 13, 14, 15));
+    EXPECT_THAT(GroupType{group}.Match(3), ElementsAre(3, 10));
+    EXPECT_THAT(GroupType{group}.Match(5), ElementsAre(5, 9));
+    EXPECT_THAT(GroupType{group}.Match(7), ElementsAre(7, 8));
+  } else if (GroupType::kWidth == 8) {
+    ctrl_t group[] = {ctrl_t::kEmpty,    CtrlT(1), CtrlT(2),
+                      ctrl_t::kDeleted,  CtrlT(2), CtrlT(1),
+                      ctrl_t::kSentinel, CtrlT(1)};
+    EXPECT_THAT(GroupType{group}.Match(0), ElementsAre());
+    EXPECT_THAT(GroupType{group}.Match(1), ElementsAre(1, 5, 7));
+    EXPECT_THAT(GroupType{group}.Match(2), ElementsAre(2, 4));
+  } else {
+    FAIL() << "No test coverage for Group::kWidth==" << GroupType::kWidth;
+  }
+}
+
+TYPED_TEST(GroupTest, MaskEmpty) {
+  using GroupType = TypeParam;
+  if (GroupType::kWidth == 16) {
+    ctrl_t group[] = {ctrl_t::kEmpty, CtrlT(1), ctrl_t::kDeleted,  CtrlT(3),
+                      ctrl_t::kEmpty, CtrlT(5), ctrl_t::kSentinel, CtrlT(7),
+                      CtrlT(7),       CtrlT(5), CtrlT(3),          CtrlT(1),
+                      CtrlT(1),       CtrlT(1), CtrlT(1),          CtrlT(1)};
+    EXPECT_THAT(GroupType{group}.MaskEmpty().LowestBitSet(), 0);
+  } else if (GroupType::kWidth == 8) {
+    ctrl_t group[] = {ctrl_t::kEmpty,    CtrlT(1), CtrlT(2),
+                      ctrl_t::kDeleted,  CtrlT(2), CtrlT(1),
+                      ctrl_t::kSentinel, CtrlT(1)};
+    EXPECT_THAT(GroupType{group}.MaskEmpty().LowestBitSet(), 0);
+  } else {
+    FAIL() << "No test coverage for Group::kWidth==" << GroupType::kWidth;
+  }
+}
+
+TYPED_TEST(GroupTest, MaskFull) {
+  using GroupType = TypeParam;
+  if (GroupType::kWidth == 16) {
+    ctrl_t group[] = {
+        ctrl_t::kEmpty, CtrlT(1),          ctrl_t::kDeleted,  CtrlT(3),
+        ctrl_t::kEmpty, CtrlT(5),          ctrl_t::kSentinel, CtrlT(7),
+        CtrlT(7),       CtrlT(5),          ctrl_t::kDeleted,  CtrlT(1),
+        CtrlT(1),       ctrl_t::kSentinel, ctrl_t::kEmpty,    CtrlT(1)};
+    EXPECT_THAT(GroupType{group}.MaskFull(),
+                ElementsAre(1, 3, 5, 7, 8, 9, 11, 12, 15));
+  } else if (GroupType::kWidth == 8) {
+    ctrl_t group[] = {ctrl_t::kEmpty,    CtrlT(1), ctrl_t::kEmpty,
+                      ctrl_t::kDeleted,  CtrlT(2), ctrl_t::kSentinel,
+                      ctrl_t::kSentinel, CtrlT(1)};
+    EXPECT_THAT(GroupType{group}.MaskFull(), ElementsAre(1, 4, 7));
+  } else {
+    FAIL() << "No test coverage for Group::kWidth==" << GroupType::kWidth;
+  }
+}
+
+TYPED_TEST(GroupTest, MaskNonFull) {
+  using GroupType = TypeParam;
+  if (GroupType::kWidth == 16) {
+    ctrl_t group[] = {
+        ctrl_t::kEmpty, CtrlT(1),          ctrl_t::kDeleted,  CtrlT(3),
+        ctrl_t::kEmpty, CtrlT(5),          ctrl_t::kSentinel, CtrlT(7),
+        CtrlT(7),       CtrlT(5),          ctrl_t::kDeleted,  CtrlT(1),
+        CtrlT(1),       ctrl_t::kSentinel, ctrl_t::kEmpty,    CtrlT(1)};
+    EXPECT_THAT(GroupType{group}.MaskNonFull(),
+                ElementsAre(0, 2, 4, 6, 10, 13, 14));
+  } else if (GroupType::kWidth == 8) {
+    ctrl_t group[] = {ctrl_t::kEmpty,    CtrlT(1), ctrl_t::kEmpty,
+                      ctrl_t::kDeleted,  CtrlT(2), ctrl_t::kSentinel,
+                      ctrl_t::kSentinel, CtrlT(1)};
+    EXPECT_THAT(GroupType{group}.MaskNonFull(), ElementsAre(0, 2, 3, 5, 6));
+  } else {
+    FAIL() << "No test coverage for Group::kWidth==" << GroupType::kWidth;
+  }
+}
+
+TYPED_TEST(GroupTest, MaskEmptyOrDeleted) {
+  using GroupType = TypeParam;
+  if (GroupType::kWidth == 16) {
+    ctrl_t group[] = {ctrl_t::kEmpty,   CtrlT(1), ctrl_t::kEmpty,    CtrlT(3),
+                      ctrl_t::kDeleted, CtrlT(5), ctrl_t::kSentinel, CtrlT(7),
+                      CtrlT(7),         CtrlT(5), CtrlT(3),          CtrlT(1),
+                      CtrlT(1),         CtrlT(1), CtrlT(1),          CtrlT(1)};
+    EXPECT_THAT(GroupType{group}.MaskEmptyOrDeleted().LowestBitSet(), 0);
+  } else if (GroupType::kWidth == 8) {
+    ctrl_t group[] = {ctrl_t::kEmpty,    CtrlT(1), CtrlT(2),
+                      ctrl_t::kDeleted,  CtrlT(2), CtrlT(1),
+                      ctrl_t::kSentinel, CtrlT(1)};
+    EXPECT_THAT(GroupType{group}.MaskEmptyOrDeleted().LowestBitSet(), 0);
+  } else {
+    FAIL() << "No test coverage for Group::kWidth==" << GroupType::kWidth;
+  }
+}
+
+TYPED_TEST(GroupTest, MaskFullOrSentinel) {
+  using GroupType = TypeParam;
+  if (GroupType::kWidth == 16) {
+    ctrl_t group[] = {
+        ctrl_t::kEmpty,   ctrl_t::kDeleted, ctrl_t::kEmpty,    CtrlT(3),
+        ctrl_t::kDeleted, CtrlT(5),         ctrl_t::kSentinel, ctrl_t::kEmpty,
+        ctrl_t::kEmpty,   ctrl_t::kDeleted, ctrl_t::kDeleted,  ctrl_t::kDeleted,
+        ctrl_t::kEmpty,   ctrl_t::kDeleted, ctrl_t::kDeleted,  ctrl_t::kDeleted,
+    };
+    EXPECT_THAT(GroupType{group}.MaskFullOrSentinel().LowestBitSet(), 3);
+  } else if (GroupType::kWidth == 8) {
+    ctrl_t group[] = {ctrl_t::kEmpty,   ctrl_t::kDeleted, CtrlT(2),
+                      ctrl_t::kDeleted, CtrlT(2),         ctrl_t::kSentinel,
+                      ctrl_t::kDeleted, ctrl_t::kEmpty};
+    EXPECT_THAT(GroupType{group}.MaskFullOrSentinel().LowestBitSet(), 2);
+  } else {
+    FAIL() << "No test coverage for Group::kWidth==" << GroupType::kWidth;
+  }
+}
+
+}  // namespace
+}  // namespace container_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/container/internal/hashtable_debug.h b/absl/container/internal/hashtable_debug.h
index 71930004607..c79c1a98912 100644
--- a/absl/container/internal/hashtable_debug.h
+++ b/absl/container/internal/hashtable_debug.h
@@ -38,6 +38,7 @@
 #include "absl/container/internal/hashtable_debug_hooks.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
 // Returns the number of probes required to lookup `key`.  Returns 0 for a
@@ -94,15 +95,8 @@ size_t AllocatedByteSize(const C& c) {
       HashtableDebugAccess<C>::AllocatedByteSize(c);
 }
 
-// Returns a tight lower bound for AllocatedByteSize(c) where `c` is of type `C`
-// and `c.size()` is equal to `num_elements`.
-template <typename C>
-size_t LowerBoundAllocatedByteSize(size_t num_elements) {
-  return absl::container_internal::hashtable_debug_internal::
-      HashtableDebugAccess<C>::LowerBoundAllocatedByteSize(num_elements);
-}
-
 }  // namespace container_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_INTERNAL_HASHTABLE_DEBUG_H_
diff --git a/absl/container/internal/hashtable_debug_hooks.h b/absl/container/internal/hashtable_debug_hooks.h
index 371ce81f55c..3e9ea5954e0 100644
--- a/absl/container/internal/hashtable_debug_hooks.h
+++ b/absl/container/internal/hashtable_debug_hooks.h
@@ -23,7 +23,10 @@
 #include <type_traits>
 #include <vector>
 
+#include "absl/base/config.h"
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 namespace hashtable_debug_internal {
 
@@ -76,6 +79,7 @@ struct HashtableDebugAccess {
 
 }  // namespace hashtable_debug_internal
 }  // namespace container_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_INTERNAL_HASHTABLE_DEBUG_HOOKS_H_
diff --git a/absl/container/internal/hashtablez_sampler.cc b/absl/container/internal/hashtablez_sampler.cc
index d03dd82e7fd..8942cc306ca 100644
--- a/absl/container/internal/hashtablez_sampler.cc
+++ b/absl/container/internal/hashtablez_sampler.cc
@@ -14,230 +14,164 @@
 
 #include "absl/container/internal/hashtablez_sampler.h"
 
+#include <algorithm>
 #include <atomic>
 #include <cassert>
 #include <cmath>
+#include <cstddef>
+#include <cstdint>
 #include <functional>
 #include <limits>
+#include <utility>
 
 #include "absl/base/attributes.h"
-#include "absl/container/internal/have_sse.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/per_thread_tls.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/base/macros.h"
+#include "absl/base/no_destructor.h"
+#include "absl/base/optimization.h"
 #include "absl/debugging/stacktrace.h"
 #include "absl/memory/memory.h"
+#include "absl/profiling/internal/exponential_biased.h"
+#include "absl/profiling/internal/sample_recorder.h"
 #include "absl/synchronization/mutex.h"
+#include "absl/time/clock.h"
+#include "absl/utility/utility.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
-constexpr int HashtablezInfo::kMaxStackDepth;
 
 namespace {
 ABSL_CONST_INIT std::atomic<bool> g_hashtablez_enabled{
     false
 };
 ABSL_CONST_INIT std::atomic<int32_t> g_hashtablez_sample_parameter{1 << 10};
-ABSL_CONST_INIT std::atomic<int32_t> g_hashtablez_max_samples{1 << 20};
-
-// Returns the next pseudo-random value.
-// pRNG is: aX+b mod c with a = 0x5DEECE66D, b =  0xB, c = 1<<48
-// This is the lrand64 generator.
-uint64_t NextRandom(uint64_t rnd) {
-  const uint64_t prng_mult = uint64_t{0x5DEECE66D};
-  const uint64_t prng_add = 0xB;
-  const uint64_t prng_mod_power = 48;
-  const uint64_t prng_mod_mask = ~(~uint64_t{0} << prng_mod_power);
-  return (prng_mult * rnd + prng_add) & prng_mod_mask;
-}
+std::atomic<HashtablezConfigListener> g_hashtablez_config_listener{nullptr};
 
-// Generates a geometric variable with the specified mean.
-// This is done by generating a random number between 0 and 1 and applying
-// the inverse cumulative distribution function for an exponential.
-// Specifically: Let m be the inverse of the sample period, then
-// the probability distribution function is m*exp(-mx) so the CDF is
-// p = 1 - exp(-mx), so
-// q = 1 - p = exp(-mx)
-// log_e(q) = -mx
-// -log_e(q)/m = x
-// log_2(q) * (-log_e(2) * 1/m) = x
-// In the code, q is actually in the range 1 to 2**26, hence the -26 below
-//
-int64_t GetGeometricVariable(int64_t mean) {
-#if ABSL_HAVE_THREAD_LOCAL
-  thread_local
-#else   // ABSL_HAVE_THREAD_LOCAL
-  // SampleSlow and hence GetGeometricVariable is guarded by a single mutex when
-  // there are not thread locals.  Thus, a single global rng is acceptable for
-  // that case.
-  static
-#endif  // ABSL_HAVE_THREAD_LOCAL
-      uint64_t rng = []() {
-        // We don't get well distributed numbers from this so we call
-        // NextRandom() a bunch to mush the bits around.  We use a global_rand
-        // to handle the case where the same thread (by memory address) gets
-        // created and destroyed repeatedly.
-        ABSL_CONST_INIT static std::atomic<uint32_t> global_rand(0);
-        uint64_t r = reinterpret_cast<uint64_t>(&rng) +
-                   global_rand.fetch_add(1, std::memory_order_relaxed);
-        for (int i = 0; i < 20; ++i) {
-          r = NextRandom(r);
-        }
-        return r;
-      }();
-
-  rng = NextRandom(rng);
-
-  // Take the top 26 bits as the random number
-  // (This plus the 1<<58 sampling bound give a max possible step of
-  // 5194297183973780480 bytes.)
-  const uint64_t prng_mod_power = 48;  // Number of bits in prng
-  // The uint32_t cast is to prevent a (hard-to-reproduce) NAN
-  // under piii debug for some binaries.
-  double q = static_cast<uint32_t>(rng >> (prng_mod_power - 26)) + 1.0;
-  // Put the computed p-value through the CDF of a geometric.
-  double interval = (log2(q) - 26) * (-std::log(2.0) * mean);
-
-  // Very large values of interval overflow int64_t. If we happen to
-  // hit such improbable condition, we simply cheat and clamp interval
-  // to largest supported value.
-  if (interval > static_cast<double>(std::numeric_limits<int64_t>::max() / 2)) {
-    return std::numeric_limits<int64_t>::max() / 2;
-  }
+#if defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
+ABSL_PER_THREAD_TLS_KEYWORD absl::profiling_internal::ExponentialBiased
+    g_exponential_biased_generator;
+#endif
 
-  // Small values of interval are equivalent to just sampling next time.
-  if (interval < 1) {
-    return 1;
-  }
-  return static_cast<int64_t>(interval);
+void TriggerHashtablezConfigListener() {
+  auto* listener = g_hashtablez_config_listener.load(std::memory_order_acquire);
+  if (listener != nullptr) listener();
 }
 
 }  // namespace
 
-HashtablezSampler& HashtablezSampler::Global() {
-  static auto* sampler = new HashtablezSampler();
-  return *sampler;
-}
+#if defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
+ABSL_PER_THREAD_TLS_KEYWORD SamplingState global_next_sample = {0, 0};
+#endif  // defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
 
-HashtablezSampler::DisposeCallback HashtablezSampler::SetDisposeCallback(
-    DisposeCallback f) {
-  return dispose_.exchange(f, std::memory_order_relaxed);
+HashtablezSampler& GlobalHashtablezSampler() {
+  static absl::NoDestructor<HashtablezSampler> sampler;
+  return *sampler;
 }
 
-HashtablezInfo::HashtablezInfo() { PrepareForSampling(); }
+HashtablezInfo::HashtablezInfo() = default;
 HashtablezInfo::~HashtablezInfo() = default;
 
-void HashtablezInfo::PrepareForSampling() {
+void HashtablezInfo::PrepareForSampling(int64_t stride,
+                                        size_t inline_element_size_value,
+                                        size_t key_size_value,
+                                        size_t value_size_value,
+                                        uint16_t soo_capacity_value) {
   capacity.store(0, std::memory_order_relaxed);
   size.store(0, std::memory_order_relaxed);
   num_erases.store(0, std::memory_order_relaxed);
+  num_rehashes.store(0, std::memory_order_relaxed);
   max_probe_length.store(0, std::memory_order_relaxed);
   total_probe_length.store(0, std::memory_order_relaxed);
   hashes_bitwise_or.store(0, std::memory_order_relaxed);
   hashes_bitwise_and.store(~size_t{}, std::memory_order_relaxed);
+  max_reserve.store(0, std::memory_order_relaxed);
 
   create_time = absl::Now();
+  weight = stride;
   // The inliner makes hardcoded skip_count difficult (especially when combined
   // with LTO).  We use the ability to exclude stacks by regex when encoding
   // instead.
-  depth = absl::GetStackTrace(stack, HashtablezInfo::kMaxStackDepth,
-                              /* skip_count= */ 0);
-  dead = nullptr;
-}
-
-HashtablezSampler::HashtablezSampler()
-    : dropped_samples_(0), size_estimate_(0), all_(nullptr), dispose_(nullptr) {
-  absl::MutexLock l(&graveyard_.init_mu);
-  graveyard_.dead = &graveyard_;
+  depth = static_cast<unsigned>(
+      absl::GetStackTrace(stack, HashtablezInfo::kMaxStackDepth,
+                          /* skip_count= */ 0));
+  inline_element_size = inline_element_size_value;
+  key_size = key_size_value;
+  value_size = value_size_value;
+  soo_capacity = soo_capacity_value;
 }
 
-HashtablezSampler::~HashtablezSampler() {
-  HashtablezInfo* s = all_.load(std::memory_order_acquire);
-  while (s != nullptr) {
-    HashtablezInfo* next = s->next;
-    delete s;
-    s = next;
+static bool ShouldForceSampling() {
+  enum ForceState {
+    kDontForce,
+    kForce,
+    kUninitialized
+  };
+  ABSL_CONST_INIT static std::atomic<ForceState> global_state{
+      kUninitialized};
+  ForceState state = global_state.load(std::memory_order_relaxed);
+  if (ABSL_PREDICT_TRUE(state == kDontForce)) return false;
+
+  if (state == kUninitialized) {
+    state = ABSL_INTERNAL_C_SYMBOL(AbslContainerInternalSampleEverything)()
+                ? kForce
+                : kDontForce;
+    global_state.store(state, std::memory_order_relaxed);
   }
+  return state == kForce;
 }
 
-void HashtablezSampler::PushNew(HashtablezInfo* sample) {
-  sample->next = all_.load(std::memory_order_relaxed);
-  while (!all_.compare_exchange_weak(sample->next, sample,
-                                     std::memory_order_release,
-                                     std::memory_order_relaxed)) {
-  }
+#if defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
+HashtablezInfoHandle ForcedTrySample(size_t inline_element_size,
+                                     size_t key_size, size_t value_size,
+                                     uint16_t soo_capacity) {
+  return HashtablezInfoHandle(SampleSlow(global_next_sample,
+                                         inline_element_size, key_size,
+                                         value_size, soo_capacity));
 }
-
-void HashtablezSampler::PushDead(HashtablezInfo* sample) {
-  if (auto* dispose = dispose_.load(std::memory_order_relaxed)) {
-    dispose(*sample);
-  }
-
-  absl::MutexLock graveyard_lock(&graveyard_.init_mu);
-  absl::MutexLock sample_lock(&sample->init_mu);
-  sample->dead = graveyard_.dead;
-  graveyard_.dead = sample;
+void TestOnlyRefreshSamplingStateForCurrentThread() {
+  global_next_sample.next_sample =
+      g_hashtablez_sample_parameter.load(std::memory_order_relaxed);
+  global_next_sample.sample_stride = global_next_sample.next_sample;
 }
-
-HashtablezInfo* HashtablezSampler::PopDead() {
-  absl::MutexLock graveyard_lock(&graveyard_.init_mu);
-
-  // The list is circular, so eventually it collapses down to
-  //   graveyard_.dead == &graveyard_
-  // when it is empty.
-  HashtablezInfo* sample = graveyard_.dead;
-  if (sample == &graveyard_) return nullptr;
-
-  absl::MutexLock sample_lock(&sample->init_mu);
-  graveyard_.dead = sample->dead;
-  sample->PrepareForSampling();
-  return sample;
-}
-
-HashtablezInfo* HashtablezSampler::Register() {
-  int64_t size = size_estimate_.fetch_add(1, std::memory_order_relaxed);
-  if (size > g_hashtablez_max_samples.load(std::memory_order_relaxed)) {
-    size_estimate_.fetch_sub(1, std::memory_order_relaxed);
-    dropped_samples_.fetch_add(1, std::memory_order_relaxed);
-    return nullptr;
-  }
-
-  HashtablezInfo* sample = PopDead();
-  if (sample == nullptr) {
-    // Resurrection failed.  Hire a new warlock.
-    sample = new HashtablezInfo();
-    PushNew(sample);
-  }
-
-  return sample;
-}
-
-void HashtablezSampler::Unregister(HashtablezInfo* sample) {
-  PushDead(sample);
-  size_estimate_.fetch_sub(1, std::memory_order_relaxed);
+#else
+HashtablezInfoHandle ForcedTrySample(size_t, size_t, size_t, uint16_t) {
+  return HashtablezInfoHandle{nullptr};
 }
-
-int64_t HashtablezSampler::Iterate(
-    const std::function<void(const HashtablezInfo& stack)>& f) {
-  HashtablezInfo* s = all_.load(std::memory_order_acquire);
-  while (s != nullptr) {
-    absl::MutexLock l(&s->init_mu);
-    if (s->dead == nullptr) {
-      f(*s);
-    }
-    s = s->next;
+void TestOnlyRefreshSamplingStateForCurrentThread() {}
+#endif  // ABSL_INTERNAL_HASHTABLEZ_SAMPLE
+
+HashtablezInfo* SampleSlow(SamplingState& next_sample,
+                           size_t inline_element_size, size_t key_size,
+                           size_t value_size, uint16_t soo_capacity) {
+  if (ABSL_PREDICT_FALSE(ShouldForceSampling())) {
+    next_sample.next_sample = 1;
+    const int64_t old_stride = std::exchange(next_sample.sample_stride, 1);
+    HashtablezInfo* result = GlobalHashtablezSampler().Register(
+        old_stride, inline_element_size, key_size, value_size, soo_capacity);
+    return result;
   }
 
-  return dropped_samples_.load(std::memory_order_relaxed);
-}
-
-HashtablezInfo* SampleSlow(int64_t* next_sample) {
-  if (kAbslContainerInternalSampleEverything) {
-    *next_sample = 1;
-    return HashtablezSampler::Global().Register();
-  }
+#if !defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
+  next_sample = {
+      std::numeric_limits<int64_t>::max(),
+      std::numeric_limits<int64_t>::max(),
+  };
+  return nullptr;
+#else
+  bool first = next_sample.next_sample < 0;
 
-  bool first = *next_sample < 0;
-  *next_sample = GetGeometricVariable(
+  const int64_t next_stride = g_exponential_biased_generator.GetStride(
       g_hashtablez_sample_parameter.load(std::memory_order_relaxed));
 
+  next_sample.next_sample = next_stride;
+  const int64_t old_stride =
+      std::exchange(next_sample.sample_stride, next_stride);
+  // Small values of interval are equivalent to just sampling next time.
+  ABSL_ASSERT(next_stride >= 1);
+
   // g_hashtablez_enabled can be dynamically flipped, we need to set a threshold
   // low enough that we will start sampling in a reasonable time, so we just use
   // the default sampling rate.
@@ -246,27 +180,63 @@ HashtablezInfo* SampleSlow(int64_t* next_sample) {
   // We will only be negative on our first count, so we should just retry in
   // that case.
   if (first) {
-    if (ABSL_PREDICT_TRUE(--*next_sample > 0)) return nullptr;
-    return SampleSlow(next_sample);
+    if (ABSL_PREDICT_TRUE(--next_sample.next_sample > 0)) return nullptr;
+    return SampleSlow(next_sample, inline_element_size, key_size, value_size,
+                      soo_capacity);
   }
 
-  return HashtablezSampler::Global().Register();
+  return GlobalHashtablezSampler().Register(old_stride, inline_element_size,
+                                            key_size, value_size, soo_capacity);
+#endif
 }
 
-#if ABSL_PER_THREAD_TLS == 1
-ABSL_PER_THREAD_TLS_KEYWORD int64_t global_next_sample = 0;
-#endif  // ABSL_PER_THREAD_TLS == 1
-
 void UnsampleSlow(HashtablezInfo* info) {
-  HashtablezSampler::Global().Unregister(info);
+  GlobalHashtablezSampler().Unregister(info);
+}
+
+void RecordRehashSlow(HashtablezInfo* info, size_t total_probe_length) {
+#ifdef ABSL_INTERNAL_HAVE_SSE2
+  total_probe_length /= 16;
+#else
+  total_probe_length /= 8;
+#endif
+  info->total_probe_length.store(total_probe_length, std::memory_order_relaxed);
+  info->num_erases.store(0, std::memory_order_relaxed);
+  // There is only one concurrent writer, so `load` then `store` is sufficient
+  // instead of using `fetch_add`.
+  info->num_rehashes.store(
+      1 + info->num_rehashes.load(std::memory_order_relaxed),
+      std::memory_order_relaxed);
+}
+
+void RecordReservationSlow(HashtablezInfo* info, size_t target_capacity) {
+  info->max_reserve.store(
+      (std::max)(info->max_reserve.load(std::memory_order_relaxed),
+                 target_capacity),
+      std::memory_order_relaxed);
+}
+
+void RecordClearedReservationSlow(HashtablezInfo* info) {
+  info->max_reserve.store(0, std::memory_order_relaxed);
+}
+
+void RecordStorageChangedSlow(HashtablezInfo* info, size_t size,
+                              size_t capacity) {
+  info->size.store(size, std::memory_order_relaxed);
+  info->capacity.store(capacity, std::memory_order_relaxed);
+  if (size == 0) {
+    // This is a clear, reset the total/num_erases too.
+    info->total_probe_length.store(0, std::memory_order_relaxed);
+    info->num_erases.store(0, std::memory_order_relaxed);
+  }
 }
 
-void RecordInsertSlow(HashtablezInfo* info, size_t hash,
-                      size_t distance_from_desired) {
+void RecordInsertMissSlow(HashtablezInfo* info, size_t hash,
+                          size_t distance_from_desired) {
   // SwissTables probe in groups of 16, so scale this to count items probes and
   // not offset from desired.
   size_t probe_length = distance_from_desired;
-#if SWISSTABLE_HAVE_SSE2
+#ifdef ABSL_INTERNAL_HAVE_SSE2
   probe_length /= 16;
 #else
   probe_length /= 8;
@@ -282,11 +252,41 @@ void RecordInsertSlow(HashtablezInfo* info, size_t hash,
   info->size.fetch_add(1, std::memory_order_relaxed);
 }
 
+void RecordEraseSlow(HashtablezInfo* info) {
+  info->size.fetch_sub(1, std::memory_order_relaxed);
+  // There is only one concurrent writer, so `load` then `store` is sufficient
+  // instead of using `fetch_add`.
+  info->num_erases.store(1 + info->num_erases.load(std::memory_order_relaxed),
+                         std::memory_order_relaxed);
+}
+
+void SetHashtablezConfigListener(HashtablezConfigListener l) {
+  g_hashtablez_config_listener.store(l, std::memory_order_release);
+}
+
+bool IsHashtablezEnabled() {
+  return g_hashtablez_enabled.load(std::memory_order_acquire);
+}
+
 void SetHashtablezEnabled(bool enabled) {
+  SetHashtablezEnabledInternal(enabled);
+  TriggerHashtablezConfigListener();
+}
+
+void SetHashtablezEnabledInternal(bool enabled) {
   g_hashtablez_enabled.store(enabled, std::memory_order_release);
 }
 
+int32_t GetHashtablezSampleParameter() {
+  return g_hashtablez_sample_parameter.load(std::memory_order_acquire);
+}
+
 void SetHashtablezSampleParameter(int32_t rate) {
+  SetHashtablezSampleParameterInternal(rate);
+  TriggerHashtablezConfigListener();
+}
+
+void SetHashtablezSampleParameterInternal(int32_t rate) {
   if (rate > 0) {
     g_hashtablez_sample_parameter.store(rate, std::memory_order_release);
   } else {
@@ -295,14 +295,23 @@ void SetHashtablezSampleParameter(int32_t rate) {
   }
 }
 
-void SetHashtablezMaxSamples(int32_t max) {
+size_t GetHashtablezMaxSamples() {
+  return GlobalHashtablezSampler().GetMaxSamples();
+}
+
+void SetHashtablezMaxSamples(size_t max) {
+  SetHashtablezMaxSamplesInternal(max);
+  TriggerHashtablezConfigListener();
+}
+
+void SetHashtablezMaxSamplesInternal(size_t max) {
   if (max > 0) {
-    g_hashtablez_max_samples.store(max, std::memory_order_release);
+    GlobalHashtablezSampler().SetMaxSamples(max);
   } else {
-    ABSL_RAW_LOG(ERROR, "Invalid hashtablez max samples: %lld",
-                 static_cast<long long>(max));  // NOLINT(runtime/int)
+    ABSL_RAW_LOG(ERROR, "Invalid hashtablez max samples: 0");
   }
 }
 
 }  // namespace container_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/container/internal/hashtablez_sampler.h b/absl/container/internal/hashtablez_sampler.h
index 4112175449e..eff44bf674d 100644
--- a/absl/container/internal/hashtablez_sampler.h
+++ b/absl/container/internal/hashtablez_sampler.h
@@ -40,23 +40,30 @@
 #define ABSL_CONTAINER_INTERNAL_HASHTABLEZ_SAMPLER_H_
 
 #include <atomic>
+#include <cstddef>
+#include <cstdint>
 #include <functional>
 #include <memory>
 #include <vector>
 
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
 #include "absl/base/internal/per_thread_tls.h"
 #include "absl/base/optimization.h"
-#include "absl/container/internal/have_sse.h"
+#include "absl/base/thread_annotations.h"
+#include "absl/profiling/internal/sample_recorder.h"
 #include "absl/synchronization/mutex.h"
+#include "absl/time/time.h"
 #include "absl/utility/utility.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
 // Stores information about a sampled hashtable.  All mutations to this *must*
 // be made through `Record*` functions below.  All reads from this *must* only
 // occur in the callback to `HashtablezSampler::Iterate`.
-struct HashtablezInfo {
+struct HashtablezInfo : public profiling_internal::Sample<HashtablezInfo> {
   // Constructs the object but does not fill in any fields.
   HashtablezInfo();
   ~HashtablezInfo();
@@ -65,89 +72,86 @@ struct HashtablezInfo {
 
   // Puts the object into a clean state, fills in the logically `const` members,
   // blocking for any readers that are currently sampling the object.
-  void PrepareForSampling() ABSL_EXCLUSIVE_LOCKS_REQUIRED(init_mu);
+  void PrepareForSampling(int64_t stride, size_t inline_element_size_value,
+                          size_t key_size, size_t value_size,
+                          uint16_t soo_capacity_value)
+      ABSL_EXCLUSIVE_LOCKS_REQUIRED(init_mu);
 
   // These fields are mutated by the various Record* APIs and need to be
   // thread-safe.
   std::atomic<size_t> capacity;
   std::atomic<size_t> size;
   std::atomic<size_t> num_erases;
+  std::atomic<size_t> num_rehashes;
   std::atomic<size_t> max_probe_length;
   std::atomic<size_t> total_probe_length;
   std::atomic<size_t> hashes_bitwise_or;
   std::atomic<size_t> hashes_bitwise_and;
-
-  // `HashtablezSampler` maintains intrusive linked lists for all samples.  See
-  // comments on `HashtablezSampler::all_` for details on these.  `init_mu`
-  // guards the ability to restore the sample to a pristine state.  This
-  // prevents races with sampling and resurrecting an object.
-  absl::Mutex init_mu;
-  HashtablezInfo* next;
-  HashtablezInfo* dead ABSL_GUARDED_BY(init_mu);
+  std::atomic<size_t> max_reserve;
 
   // All of the fields below are set by `PrepareForSampling`, they must not be
   // mutated in `Record*` functions.  They are logically `const` in that sense.
-  // These are guarded by init_mu, but that is not externalized to clients, who
-  // can only read them during `HashtablezSampler::Iterate` which will hold the
-  // lock.
+  // These are guarded by init_mu, but that is not externalized to clients,
+  // which can read them only during `SampleRecorder::Iterate` which will hold
+  // the lock.
   static constexpr int kMaxStackDepth = 64;
   absl::Time create_time;
-  int32_t depth;
+  uint32_t depth;
+  // The SOO capacity for this table in elements (not bytes). Note that sampled
+  // tables are never SOO because we need to store the infoz handle on the heap.
+  // Tables that would be SOO if not sampled should have: soo_capacity > 0 &&
+  // size <= soo_capacity && max_reserve <= soo_capacity.
+  uint16_t soo_capacity;
   void* stack[kMaxStackDepth];
+  size_t inline_element_size;  // How big is the slot in bytes?
+  size_t key_size;             // sizeof(key_type)
+  size_t value_size;           // sizeof(value_type)
 };
 
-inline void RecordRehashSlow(HashtablezInfo* info, size_t total_probe_length) {
-#if SWISSTABLE_HAVE_SSE2
-  total_probe_length /= 16;
-#else
-  total_probe_length /= 8;
-#endif
-  info->total_probe_length.store(total_probe_length, std::memory_order_relaxed);
-  info->num_erases.store(0, std::memory_order_relaxed);
-}
+void RecordRehashSlow(HashtablezInfo* info, size_t total_probe_length);
 
-inline void RecordStorageChangedSlow(HashtablezInfo* info, size_t size,
-                                     size_t capacity) {
-  info->size.store(size, std::memory_order_relaxed);
-  info->capacity.store(capacity, std::memory_order_relaxed);
-  if (size == 0) {
-    // This is a clear, reset the total/num_erases too.
-    RecordRehashSlow(info, 0);
-  }
-}
+void RecordReservationSlow(HashtablezInfo* info, size_t target_capacity);
 
-void RecordInsertSlow(HashtablezInfo* info, size_t hash,
-                      size_t distance_from_desired);
+void RecordClearedReservationSlow(HashtablezInfo* info);
 
-inline void RecordEraseSlow(HashtablezInfo* info) {
-  info->size.fetch_sub(1, std::memory_order_relaxed);
-  info->num_erases.fetch_add(1, std::memory_order_relaxed);
-}
+void RecordStorageChangedSlow(HashtablezInfo* info, size_t size,
+                              size_t capacity);
+
+void RecordInsertMissSlow(HashtablezInfo* info, size_t hash,
+                          size_t distance_from_desired);
+
+void RecordEraseSlow(HashtablezInfo* info);
+
+struct SamplingState {
+  int64_t next_sample;
+  // When we make a sampling decision, we record that distance so we can weight
+  // each sample.
+  int64_t sample_stride;
+};
 
-HashtablezInfo* SampleSlow(int64_t* next_sample);
+HashtablezInfo* SampleSlow(SamplingState& next_sample,
+                           size_t inline_element_size, size_t key_size,
+                           size_t value_size, uint16_t soo_capacity);
 void UnsampleSlow(HashtablezInfo* info);
 
+#if defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
+#error ABSL_INTERNAL_HASHTABLEZ_SAMPLE cannot be directly set
+#endif  // defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
+
+#if defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
 class HashtablezInfoHandle {
  public:
   explicit HashtablezInfoHandle() : info_(nullptr) {}
   explicit HashtablezInfoHandle(HashtablezInfo* info) : info_(info) {}
-  ~HashtablezInfoHandle() {
+
+  // We do not have a destructor. Caller is responsible for calling Unregister
+  // before destroying the handle.
+  void Unregister() {
     if (ABSL_PREDICT_TRUE(info_ == nullptr)) return;
     UnsampleSlow(info_);
   }
 
-  HashtablezInfoHandle(const HashtablezInfoHandle&) = delete;
-  HashtablezInfoHandle& operator=(const HashtablezInfoHandle&) = delete;
-
-  HashtablezInfoHandle(HashtablezInfoHandle&& o) noexcept
-      : info_(absl::exchange(o.info_, nullptr)) {}
-  HashtablezInfoHandle& operator=(HashtablezInfoHandle&& o) noexcept {
-    if (ABSL_PREDICT_FALSE(info_ != nullptr)) {
-      UnsampleSlow(info_);
-    }
-    info_ = absl::exchange(o.info_, nullptr);
-    return *this;
-  }
+  inline bool IsSampled() const { return ABSL_PREDICT_FALSE(info_ != nullptr); }
 
   inline void RecordStorageChanged(size_t size, size_t capacity) {
     if (ABSL_PREDICT_TRUE(info_ == nullptr)) return;
@@ -159,9 +163,19 @@ class HashtablezInfoHandle {
     RecordRehashSlow(info_, total_probe_length);
   }
 
-  inline void RecordInsert(size_t hash, size_t distance_from_desired) {
+  inline void RecordReservation(size_t target_capacity) {
+    if (ABSL_PREDICT_TRUE(info_ == nullptr)) return;
+    RecordReservationSlow(info_, target_capacity);
+  }
+
+  inline void RecordClearedReservation() {
+    if (ABSL_PREDICT_TRUE(info_ == nullptr)) return;
+    RecordClearedReservationSlow(info_);
+  }
+
+  inline void RecordInsertMiss(size_t hash, size_t distance_from_desired) {
     if (ABSL_PREDICT_TRUE(info_ == nullptr)) return;
-    RecordInsertSlow(info_, hash, distance_from_desired);
+    RecordInsertMissSlow(info_, hash, distance_from_desired);
   }
 
   inline void RecordErase() {
@@ -178,111 +192,103 @@ class HashtablezInfoHandle {
   friend class HashtablezInfoHandlePeer;
   HashtablezInfo* info_;
 };
+#else
+// Ensure that when Hashtablez is turned off at compile time, HashtablezInfo can
+// be removed by the linker, in order to reduce the binary size.
+class HashtablezInfoHandle {
+ public:
+  explicit HashtablezInfoHandle() = default;
+  explicit HashtablezInfoHandle(std::nullptr_t) {}
+
+  inline void Unregister() {}
+  inline bool IsSampled() const { return false; }
+  inline void RecordStorageChanged(size_t /*size*/, size_t /*capacity*/) {}
+  inline void RecordRehash(size_t /*total_probe_length*/) {}
+  inline void RecordReservation(size_t /*target_capacity*/) {}
+  inline void RecordClearedReservation() {}
+  inline void RecordInsertMiss(size_t /*hash*/,
+                               size_t /*distance_from_desired*/) {}
+  inline void RecordErase() {}
+
+  friend inline void swap(HashtablezInfoHandle& /*lhs*/,
+                          HashtablezInfoHandle& /*rhs*/) {}
+};
+#endif  // defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
+
+#if defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
+extern ABSL_PER_THREAD_TLS_KEYWORD SamplingState global_next_sample;
+#endif  // defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
+
+// Returns true if the next table should be sampled.
+// This function updates the global state.
+// If the function returns true, actual sampling should be done by calling
+// ForcedTrySample().
+inline bool ShouldSampleNextTable() {
+#if defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
+  if (ABSL_PREDICT_TRUE(--global_next_sample.next_sample > 0)) {
+    return false;
+  }
+  return true;
+#else
+  return false;
+#endif  // ABSL_INTERNAL_HASHTABLEZ_SAMPLE
+}
 
-#if ABSL_PER_THREAD_TLS == 1
-extern ABSL_PER_THREAD_TLS_KEYWORD int64_t global_next_sample;
-#endif  // ABSL_PER_THREAD_TLS
-
-// Returns an RAII sampling handle that manages registration and unregistation
-// with the global sampler.
-inline HashtablezInfoHandle Sample() {
-#if ABSL_PER_THREAD_TLS == 0
-  static auto* mu = new absl::Mutex;
-  static int64_t global_next_sample = 0;
-  absl::MutexLock l(mu);
-#endif  // !ABSL_HAVE_THREAD_LOCAL
-
-  if (ABSL_PREDICT_TRUE(--global_next_sample > 0)) {
+// Returns a sampling handle.
+// Must be called only if HashSetShouldBeSampled() returned true.
+// Returned handle still can be unsampled if sampling is not possible.
+HashtablezInfoHandle ForcedTrySample(size_t inline_element_size,
+                                     size_t key_size, size_t value_size,
+                                     uint16_t soo_capacity);
+
+// In case sampling needs to be disabled and re-enabled in tests, this function
+// can be used to reset the sampling state for the current thread.
+// It is useful to avoid sampling attempts and sampling delays in tests.
+void TestOnlyRefreshSamplingStateForCurrentThread();
+
+// Returns a sampling handle.
+inline HashtablezInfoHandle Sample(size_t inline_element_size, size_t key_size,
+                                   size_t value_size, uint16_t soo_capacity) {
+  if (ABSL_PREDICT_TRUE(!ShouldSampleNextTable())) {
     return HashtablezInfoHandle(nullptr);
   }
-  return HashtablezInfoHandle(SampleSlow(&global_next_sample));
+  return ForcedTrySample(inline_element_size, key_size, value_size,
+                         soo_capacity);
 }
 
-// Holds samples and their associated stack traces with a soft limit of
-// `SetHashtablezMaxSamples()`.
-//
-// Thread safe.
-class HashtablezSampler {
- public:
-  // Returns a global Sampler.
-  static HashtablezSampler& Global();
-
-  HashtablezSampler();
-  ~HashtablezSampler();
-
-  // Registers for sampling.  Returns an opaque registration info.
-  HashtablezInfo* Register();
+using HashtablezSampler =
+    ::absl::profiling_internal::SampleRecorder<HashtablezInfo>;
 
-  // Unregisters the sample.
-  void Unregister(HashtablezInfo* sample);
+// Returns a global Sampler.
+HashtablezSampler& GlobalHashtablezSampler();
 
-  // The dispose callback will be called on all samples the moment they are
-  // being unregistered. Only affects samples that are unregistered after the
-  // callback has been set.
-  // Returns the previous callback.
-  using DisposeCallback = void (*)(const HashtablezInfo&);
-  DisposeCallback SetDisposeCallback(DisposeCallback f);
-
-  // Iterates over all the registered `StackInfo`s.  Returning the number of
-  // samples that have been dropped.
-  int64_t Iterate(const std::function<void(const HashtablezInfo& stack)>& f);
-
- private:
-  void PushNew(HashtablezInfo* sample);
-  void PushDead(HashtablezInfo* sample);
-  HashtablezInfo* PopDead();
-
-  std::atomic<size_t> dropped_samples_;
-  std::atomic<size_t> size_estimate_;
-
-  // Intrusive lock free linked lists for tracking samples.
-  //
-  // `all_` records all samples (they are never removed from this list) and is
-  // terminated with a `nullptr`.
-  //
-  // `graveyard_.dead` is a circular linked list.  When it is empty,
-  // `graveyard_.dead == &graveyard`.  The list is circular so that
-  // every item on it (even the last) has a non-null dead pointer.  This allows
-  // `Iterate` to determine if a given sample is live or dead using only
-  // information on the sample itself.
-  //
-  // For example, nodes [A, B, C, D, E] with [A, C, E] alive and [B, D] dead
-  // looks like this (G is the Graveyard):
-  //
-  //           +---+    +---+    +---+    +---+    +---+
-  //    all -->| A |--->| B |--->| C |--->| D |--->| E |
-  //           |   |    |   |    |   |    |   |    |   |
-  //   +---+   |   | +->|   |-+  |   | +->|   |-+  |   |
-  //   | G |   +---+ |  +---+ |  +---+ |  +---+ |  +---+
-  //   |   |         |        |        |        |
-  //   |   | --------+        +--------+        |
-  //   +---+                                    |
-  //     ^                                      |
-  //     +--------------------------------------+
-  //
-  std::atomic<HashtablezInfo*> all_;
-  HashtablezInfo graveyard_;
-
-  std::atomic<DisposeCallback> dispose_;
-};
+using HashtablezConfigListener = void (*)();
+void SetHashtablezConfigListener(HashtablezConfigListener l);
 
 // Enables or disables sampling for Swiss tables.
+bool IsHashtablezEnabled();
 void SetHashtablezEnabled(bool enabled);
+void SetHashtablezEnabledInternal(bool enabled);
 
 // Sets the rate at which Swiss tables will be sampled.
+int32_t GetHashtablezSampleParameter();
 void SetHashtablezSampleParameter(int32_t rate);
+void SetHashtablezSampleParameterInternal(int32_t rate);
 
 // Sets a soft max for the number of samples that will be kept.
-void SetHashtablezMaxSamples(int32_t max);
+size_t GetHashtablezMaxSamples();
+void SetHashtablezMaxSamples(size_t max);
+void SetHashtablezMaxSamplesInternal(size_t max);
 
 // Configuration override.
 // This allows process-wide sampling without depending on order of
 // initialization of static storage duration objects.
 // The definition of this constant is weak, which allows us to inject a
 // different value for it at link time.
-extern "C" const bool kAbslContainerInternalSampleEverything;
+extern "C" bool ABSL_INTERNAL_C_SYMBOL(AbslContainerInternalSampleEverything)();
 
 }  // namespace container_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_INTERNAL_HASHTABLEZ_SAMPLER_H_
diff --git a/absl/container/internal/hashtablez_sampler_force_weak_definition.cc b/absl/container/internal/hashtablez_sampler_force_weak_definition.cc
index 4ca6ffdadd7..ed35a7eec3d 100644
--- a/absl/container/internal/hashtablez_sampler_force_weak_definition.cc
+++ b/absl/container/internal/hashtablez_sampler_force_weak_definition.cc
@@ -17,11 +17,15 @@
 #include "absl/base/attributes.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
 // See hashtablez_sampler.h for details.
-extern "C" ABSL_ATTRIBUTE_WEAK const bool
-    kAbslContainerInternalSampleEverything = false;
+extern "C" ABSL_ATTRIBUTE_WEAK bool ABSL_INTERNAL_C_SYMBOL(
+    AbslContainerInternalSampleEverything)() {
+  return false;
+}
 
 }  // namespace container_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/container/internal/hashtablez_sampler_test.cc b/absl/container/internal/hashtablez_sampler_test.cc
index 7f9e8dd21ff..ae89c023244 100644
--- a/absl/container/internal/hashtablez_sampler_test.cc
+++ b/absl/container/internal/hashtablez_sampler_test.cc
@@ -15,13 +15,19 @@
 #include "absl/container/internal/hashtablez_sampler.h"
 
 #include <atomic>
+#include <cassert>
+#include <cstddef>
+#include <cstdint>
 #include <limits>
 #include <random>
+#include <vector>
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
 #include "absl/base/attributes.h"
-#include "absl/container/internal/have_sse.h"
+#include "absl/base/config.h"
+#include "absl/profiling/internal/sample_recorder.h"
+#include "absl/random/random.h"
 #include "absl/synchronization/blocking_counter.h"
 #include "absl/synchronization/internal/thread_pool.h"
 #include "absl/synchronization/mutex.h"
@@ -29,22 +35,26 @@
 #include "absl/time/clock.h"
 #include "absl/time/time.h"
 
-#if SWISSTABLE_HAVE_SSE2
+#ifdef ABSL_INTERNAL_HAVE_SSE2
 constexpr int kProbeLength = 16;
 #else
 constexpr int kProbeLength = 8;
 #endif
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
+#if defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
 class HashtablezInfoHandlePeer {
  public:
-  static bool IsSampled(const HashtablezInfoHandle& h) {
-    return h.info_ != nullptr;
-  }
-
   static HashtablezInfo* GetInfo(HashtablezInfoHandle* h) { return h->info_; }
 };
+#else
+class HashtablezInfoHandlePeer {
+ public:
+  static HashtablezInfo* GetInfo(HashtablezInfoHandle*) { return nullptr; }
+};
+#endif  // defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
 
 namespace {
 using ::absl::synchronization_internal::ThreadPool;
@@ -53,14 +63,21 @@ using ::testing::UnorderedElementsAre;
 
 std::vector<size_t> GetSizes(HashtablezSampler* s) {
   std::vector<size_t> res;
-  s->Iterate([&](const HashtablezInfo& info) {
+  EXPECT_EQ(s->Iterate([&](const HashtablezInfo& info) {
     res.push_back(info.size.load(std::memory_order_acquire));
-  });
+  }),
+            0);
   return res;
 }
 
 HashtablezInfo* Register(HashtablezSampler* s, size_t size) {
-  auto* info = s->Register();
+  const int64_t test_stride = 123;
+  const size_t test_element_size = 17;
+  const size_t test_key_size = 3;
+  const size_t test_value_size = 5;
+  auto* info =
+      s->Register(test_stride, test_element_size, /*key_size=*/test_key_size,
+                  /*value_size=*/test_value_size, /*soo_capacity=*/0);
   assert(info != nullptr);
   info->size.store(size);
   return info;
@@ -68,18 +85,33 @@ HashtablezInfo* Register(HashtablezSampler* s, size_t size) {
 
 TEST(HashtablezInfoTest, PrepareForSampling) {
   absl::Time test_start = absl::Now();
+  const int64_t test_stride = 123;
+  const size_t test_element_size = 17;
+  const size_t test_key_size = 15;
+  const size_t test_value_size = 13;
+
   HashtablezInfo info;
-  absl::MutexLock l(&info.init_mu);
-  info.PrepareForSampling();
+  absl::MutexLock l(info.init_mu);
+  info.PrepareForSampling(test_stride, test_element_size,
+                          /*key_size=*/test_key_size,
+                          /*value_size=*/test_value_size,
+                          /*soo_capacity_value=*/1);
 
   EXPECT_EQ(info.capacity.load(), 0);
   EXPECT_EQ(info.size.load(), 0);
   EXPECT_EQ(info.num_erases.load(), 0);
+  EXPECT_EQ(info.num_rehashes.load(), 0);
   EXPECT_EQ(info.max_probe_length.load(), 0);
   EXPECT_EQ(info.total_probe_length.load(), 0);
   EXPECT_EQ(info.hashes_bitwise_or.load(), 0);
   EXPECT_EQ(info.hashes_bitwise_and.load(), ~size_t{});
+  EXPECT_EQ(info.max_reserve.load(), 0);
   EXPECT_GE(info.create_time, test_start);
+  EXPECT_EQ(info.weight, test_stride);
+  EXPECT_EQ(info.inline_element_size, test_element_size);
+  EXPECT_EQ(info.key_size, test_key_size);
+  EXPECT_EQ(info.value_size, test_value_size);
+  EXPECT_EQ(info.soo_capacity, 1);
 
   info.capacity.store(1, std::memory_order_relaxed);
   info.size.store(1, std::memory_order_relaxed);
@@ -88,23 +120,42 @@ TEST(HashtablezInfoTest, PrepareForSampling) {
   info.total_probe_length.store(1, std::memory_order_relaxed);
   info.hashes_bitwise_or.store(1, std::memory_order_relaxed);
   info.hashes_bitwise_and.store(1, std::memory_order_relaxed);
+  info.max_reserve.store(1, std::memory_order_relaxed);
   info.create_time = test_start - absl::Hours(20);
 
-  info.PrepareForSampling();
+  info.PrepareForSampling(test_stride * 2, test_element_size,
+                          /*key_size=*/test_key_size,
+                          /*value_size=*/test_value_size,
+                          /*soo_capacity_value=*/0);
   EXPECT_EQ(info.capacity.load(), 0);
   EXPECT_EQ(info.size.load(), 0);
   EXPECT_EQ(info.num_erases.load(), 0);
+  EXPECT_EQ(info.num_rehashes.load(), 0);
   EXPECT_EQ(info.max_probe_length.load(), 0);
   EXPECT_EQ(info.total_probe_length.load(), 0);
   EXPECT_EQ(info.hashes_bitwise_or.load(), 0);
   EXPECT_EQ(info.hashes_bitwise_and.load(), ~size_t{});
+  EXPECT_EQ(info.max_reserve.load(), 0);
+  EXPECT_EQ(info.weight, 2 * test_stride);
+  EXPECT_EQ(info.inline_element_size, test_element_size);
+  EXPECT_EQ(info.key_size, test_key_size);
+  EXPECT_EQ(info.value_size, test_value_size);
   EXPECT_GE(info.create_time, test_start);
+  EXPECT_EQ(info.soo_capacity, 0);
 }
 
 TEST(HashtablezInfoTest, RecordStorageChanged) {
   HashtablezInfo info;
-  absl::MutexLock l(&info.init_mu);
-  info.PrepareForSampling();
+  absl::MutexLock l(info.init_mu);
+  const int64_t test_stride = 21;
+  const size_t test_element_size = 19;
+  const size_t test_key_size = 17;
+  const size_t test_value_size = 15;
+
+  info.PrepareForSampling(test_stride, test_element_size,
+                          /*key_size=*/test_key_size,
+                          /*value_size=*/test_value_size,
+                          /*soo_capacity_value=*/0);
   RecordStorageChangedSlow(&info, 17, 47);
   EXPECT_EQ(info.size.load(), 17);
   EXPECT_EQ(info.capacity.load(), 47);
@@ -113,46 +164,74 @@ TEST(HashtablezInfoTest, RecordStorageChanged) {
   EXPECT_EQ(info.capacity.load(), 20);
 }
 
-TEST(HashtablezInfoTest, RecordInsert) {
+TEST(HashtablezInfoTest, RecordInsertMiss) {
   HashtablezInfo info;
-  absl::MutexLock l(&info.init_mu);
-  info.PrepareForSampling();
+  absl::MutexLock l(info.init_mu);
+  const int64_t test_stride = 25;
+  const size_t test_element_size = 23;
+  const size_t test_key_size = 21;
+  const size_t test_value_size = 19;
+
+  info.PrepareForSampling(test_stride, test_element_size,
+                          /*key_size=*/test_key_size,
+                          /*value_size=*/test_value_size,
+                          /*soo_capacity_value=*/0);
   EXPECT_EQ(info.max_probe_length.load(), 0);
-  RecordInsertSlow(&info, 0x0000FF00, 6 * kProbeLength);
+  RecordInsertMissSlow(&info, 0x0000FF00, 6 * kProbeLength);
   EXPECT_EQ(info.max_probe_length.load(), 6);
   EXPECT_EQ(info.hashes_bitwise_and.load(), 0x0000FF00);
   EXPECT_EQ(info.hashes_bitwise_or.load(), 0x0000FF00);
-  RecordInsertSlow(&info, 0x000FF000, 4 * kProbeLength);
+  RecordInsertMissSlow(&info, 0x000FF000, 4 * kProbeLength);
   EXPECT_EQ(info.max_probe_length.load(), 6);
   EXPECT_EQ(info.hashes_bitwise_and.load(), 0x0000F000);
   EXPECT_EQ(info.hashes_bitwise_or.load(), 0x000FFF00);
-  RecordInsertSlow(&info, 0x00FF0000, 12 * kProbeLength);
+  RecordInsertMissSlow(&info, 0x00FF0000, 12 * kProbeLength);
   EXPECT_EQ(info.max_probe_length.load(), 12);
   EXPECT_EQ(info.hashes_bitwise_and.load(), 0x00000000);
   EXPECT_EQ(info.hashes_bitwise_or.load(), 0x00FFFF00);
 }
 
 TEST(HashtablezInfoTest, RecordErase) {
+  const int64_t test_stride = 31;
+  const size_t test_element_size = 29;
+  const size_t test_key_size = 27;
+  const size_t test_value_size = 25;
+
   HashtablezInfo info;
-  absl::MutexLock l(&info.init_mu);
-  info.PrepareForSampling();
+  absl::MutexLock l(info.init_mu);
+  info.PrepareForSampling(test_stride, test_element_size,
+                          /*key_size=*/test_key_size,
+                          /*value_size=*/test_value_size,
+                          /*soo_capacity_value=*/1);
   EXPECT_EQ(info.num_erases.load(), 0);
   EXPECT_EQ(info.size.load(), 0);
-  RecordInsertSlow(&info, 0x0000FF00, 6 * kProbeLength);
+  RecordInsertMissSlow(&info, 0x0000FF00, 6 * kProbeLength);
   EXPECT_EQ(info.size.load(), 1);
   RecordEraseSlow(&info);
   EXPECT_EQ(info.size.load(), 0);
   EXPECT_EQ(info.num_erases.load(), 1);
+  EXPECT_EQ(info.inline_element_size, test_element_size);
+  EXPECT_EQ(info.key_size, test_key_size);
+  EXPECT_EQ(info.value_size, test_value_size);
+  EXPECT_EQ(info.soo_capacity, 1);
 }
 
 TEST(HashtablezInfoTest, RecordRehash) {
+  const int64_t test_stride = 33;
+  const size_t test_element_size = 31;
+  const size_t test_key_size = 29;
+  const size_t test_value_size = 27;
   HashtablezInfo info;
-  absl::MutexLock l(&info.init_mu);
-  info.PrepareForSampling();
-  RecordInsertSlow(&info, 0x1, 0);
-  RecordInsertSlow(&info, 0x2, kProbeLength);
-  RecordInsertSlow(&info, 0x4, kProbeLength);
-  RecordInsertSlow(&info, 0x8, 2 * kProbeLength);
+  absl::MutexLock l(info.init_mu);
+  info.PrepareForSampling(test_stride, test_element_size,
+                          /*key_size=*/test_key_size,
+                          /*value_size=*/test_value_size,
+
+                          /*soo_capacity_value=*/0);
+  RecordInsertMissSlow(&info, 0x1, 0);
+  RecordInsertMissSlow(&info, 0x2, kProbeLength);
+  RecordInsertMissSlow(&info, 0x4, kProbeLength);
+  RecordInsertMissSlow(&info, 0x8, 2 * kProbeLength);
   EXPECT_EQ(info.size.load(), 4);
   EXPECT_EQ(info.total_probe_length.load(), 4);
 
@@ -166,44 +245,99 @@ TEST(HashtablezInfoTest, RecordRehash) {
   EXPECT_EQ(info.size.load(), 2);
   EXPECT_EQ(info.total_probe_length.load(), 3);
   EXPECT_EQ(info.num_erases.load(), 0);
+  EXPECT_EQ(info.num_rehashes.load(), 1);
+  EXPECT_EQ(info.inline_element_size, test_element_size);
+  EXPECT_EQ(info.key_size, test_key_size);
+  EXPECT_EQ(info.value_size, test_value_size);
+  EXPECT_EQ(info.soo_capacity, 0);
 }
 
+TEST(HashtablezInfoTest, RecordReservation) {
+  HashtablezInfo info;
+  absl::MutexLock l(info.init_mu);
+  const int64_t test_stride = 35;
+  const size_t test_element_size = 33;
+  const size_t test_key_size = 31;
+  const size_t test_value_size = 29;
+
+  info.PrepareForSampling(test_stride, test_element_size,
+                          /*key_size=*/test_key_size,
+                          /*value_size=*/test_value_size,
+
+                          /*soo_capacity_value=*/0);
+  RecordReservationSlow(&info, 3);
+  EXPECT_EQ(info.max_reserve.load(), 3);
+
+  RecordReservationSlow(&info, 2);
+  // High watermark does not change
+  EXPECT_EQ(info.max_reserve.load(), 3);
+
+  RecordReservationSlow(&info, 10);
+  // High watermark does change
+  EXPECT_EQ(info.max_reserve.load(), 10);
+}
+
+#if defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
 TEST(HashtablezSamplerTest, SmallSampleParameter) {
+  const size_t test_element_size = 31;
+  const size_t test_key_size = 33;
+  const size_t test_value_size = 35;
+
   SetHashtablezEnabled(true);
   SetHashtablezSampleParameter(100);
 
   for (int i = 0; i < 1000; ++i) {
-    int64_t next_sample = 0;
-    HashtablezInfo* sample = SampleSlow(&next_sample);
-    EXPECT_GT(next_sample, 0);
+    SamplingState next_sample = {0, 0};
+    HashtablezInfo* sample =
+        SampleSlow(next_sample, test_element_size,
+                   /*key_size=*/test_key_size, /*value_size=*/test_value_size,
+
+                   /*soo_capacity=*/0);
+    EXPECT_GT(next_sample.next_sample, 0);
+    EXPECT_EQ(next_sample.next_sample, next_sample.sample_stride);
     EXPECT_NE(sample, nullptr);
     UnsampleSlow(sample);
   }
 }
 
 TEST(HashtablezSamplerTest, LargeSampleParameter) {
+  const size_t test_element_size = 31;
+  const size_t test_key_size = 33;
+  const size_t test_value_size = 35;
   SetHashtablezEnabled(true);
   SetHashtablezSampleParameter(std::numeric_limits<int32_t>::max());
 
   for (int i = 0; i < 1000; ++i) {
-    int64_t next_sample = 0;
-    HashtablezInfo* sample = SampleSlow(&next_sample);
-    EXPECT_GT(next_sample, 0);
+    SamplingState next_sample = {0, 0};
+    HashtablezInfo* sample =
+        SampleSlow(next_sample, test_element_size,
+                   /*key_size=*/test_key_size, /*value_size=*/test_value_size,
+                   /*soo_capacity=*/0);
+    EXPECT_GT(next_sample.next_sample, 0);
+    EXPECT_EQ(next_sample.next_sample, next_sample.sample_stride);
     EXPECT_NE(sample, nullptr);
     UnsampleSlow(sample);
   }
 }
 
 TEST(HashtablezSamplerTest, Sample) {
+  const size_t test_element_size = 31;
+  const size_t test_key_size = 33;
+  const size_t test_value_size = 35;
   SetHashtablezEnabled(true);
   SetHashtablezSampleParameter(100);
   int64_t num_sampled = 0;
   int64_t total = 0;
   double sample_rate = 0.0;
   for (int i = 0; i < 1000000; ++i) {
-    HashtablezInfoHandle h = Sample();
+    HashtablezInfoHandle h =
+        Sample(test_element_size,
+               /*key_size=*/test_key_size, /*value_size=*/test_value_size,
+
+               /*soo_capacity=*/0);
+
     ++total;
-    if (HashtablezInfoHandlePeer::IsSampled(h)) {
+    if (h.IsSampled()) {
       ++num_sampled;
     }
     sample_rate = static_cast<double>(num_sampled) / total;
@@ -213,23 +347,33 @@ TEST(HashtablezSamplerTest, Sample) {
 }
 
 TEST(HashtablezSamplerTest, Handle) {
-  auto& sampler = HashtablezSampler::Global();
-  HashtablezInfoHandle h(sampler.Register());
+  auto& sampler = GlobalHashtablezSampler();
+  const int64_t test_stride = 41;
+  const size_t test_element_size = 39;
+  const size_t test_key_size = 37;
+  const size_t test_value_size = 35;
+  HashtablezInfoHandle h(sampler.Register(test_stride, test_element_size,
+                                          /*key_size=*/test_key_size,
+                                          /*value_size=*/test_value_size,
+                                          /*soo_capacity=*/0));
   auto* info = HashtablezInfoHandlePeer::GetInfo(&h);
   info->hashes_bitwise_and.store(0x12345678, std::memory_order_relaxed);
 
   bool found = false;
-  sampler.Iterate([&](const HashtablezInfo& h) {
+  EXPECT_EQ(sampler.Iterate([&](const HashtablezInfo& h) {
     if (&h == info) {
+      EXPECT_EQ(h.weight, test_stride);
       EXPECT_EQ(h.hashes_bitwise_and.load(), 0x12345678);
       found = true;
     }
-  });
+  }),
+            0);
   EXPECT_TRUE(found);
 
+  h.Unregister();
   h = HashtablezInfoHandle();
   found = false;
-  sampler.Iterate([&](const HashtablezInfo& h) {
+  EXPECT_EQ(sampler.Iterate([&](const HashtablezInfo& h) {
     if (&h == info) {
       // this will only happen if some other thread has resurrected the info
       // the old handle was using.
@@ -237,9 +381,12 @@ TEST(HashtablezSamplerTest, Handle) {
         found = true;
       }
     }
-  });
+  }),
+            0);
   EXPECT_FALSE(found);
 }
+#endif
+
 
 TEST(HashtablezSamplerTest, Registration) {
   HashtablezSampler sampler;
@@ -284,18 +431,29 @@ TEST(HashtablezSamplerTest, MultiThreaded) {
   ThreadPool pool(10);
 
   for (int i = 0; i < 10; ++i) {
-    pool.Schedule([&sampler, &stop]() {
-      std::random_device rd;
-      std::mt19937 gen(rd());
+    const int64_t sampling_stride = 11 + i % 3;
+    const size_t elt_size = 10 + i % 2;
+    const size_t key_size = 12 + i % 4;
+    const size_t value_size = 13 + i % 5;
+    pool.Schedule([&sampler, &stop, sampling_stride, elt_size, key_size,
+                   value_size]() {
+      absl::InsecureBitGen gen;
 
       std::vector<HashtablezInfo*> infoz;
       while (!stop.HasBeenNotified()) {
         if (infoz.empty()) {
-          infoz.push_back(sampler.Register());
+          infoz.push_back(sampler.Register(sampling_stride, elt_size,
+                                           /*key_size=*/key_size,
+                                           /*value_size=*/value_size,
+                                           /*soo_capacity=*/0));
         }
         switch (std::uniform_int_distribution<>(0, 2)(gen)) {
           case 0: {
-            infoz.push_back(sampler.Register());
+            infoz.push_back(sampler.Register(sampling_stride, elt_size,
+                                             /*key_size=*/key_size,
+                                             /*value_size=*/value_size,
+
+                                             /*soo_capacity=*/0));
             break;
           }
           case 1: {
@@ -304,14 +462,16 @@ TEST(HashtablezSamplerTest, MultiThreaded) {
             HashtablezInfo* info = infoz[p];
             infoz[p] = infoz.back();
             infoz.pop_back();
+            EXPECT_EQ(info->weight, sampling_stride);
             sampler.Unregister(info);
             break;
           }
           case 2: {
             absl::Duration oldest = absl::ZeroDuration();
-            sampler.Iterate([&](const HashtablezInfo& info) {
+            EXPECT_EQ(sampler.Iterate([&](const HashtablezInfo& info) {
               oldest = std::max(oldest, absl::Now() - info.create_time);
-            });
+            }),
+                      0);
             ASSERT_GE(oldest, absl::ZeroDuration());
             break;
           }
@@ -352,4 +512,5 @@ TEST(HashtablezSamplerTest, Callback) {
 
 }  // namespace
 }  // namespace container_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/container/internal/have_sse.h b/absl/container/internal/have_sse.h
deleted file mode 100644
index 43414418dba..00000000000
--- a/absl/container/internal/have_sse.h
+++ /dev/null
@@ -1,49 +0,0 @@
-// Copyright 2018 The Abseil Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-//
-// Shared config probing for SSE instructions used in Swiss tables.
-#ifndef ABSL_CONTAINER_INTERNAL_HAVE_SSE_H_
-#define ABSL_CONTAINER_INTERNAL_HAVE_SSE_H_
-
-#ifndef SWISSTABLE_HAVE_SSE2
-#if defined(__SSE2__) ||  \
-    (defined(_MSC_VER) && \
-     (defined(_M_X64) || (defined(_M_IX86) && _M_IX86_FP >= 2)))
-#define SWISSTABLE_HAVE_SSE2 1
-#else
-#define SWISSTABLE_HAVE_SSE2 0
-#endif
-#endif
-
-#ifndef SWISSTABLE_HAVE_SSSE3
-#ifdef __SSSE3__
-#define SWISSTABLE_HAVE_SSSE3 1
-#else
-#define SWISSTABLE_HAVE_SSSE3 0
-#endif
-#endif
-
-#if SWISSTABLE_HAVE_SSSE3 && !SWISSTABLE_HAVE_SSE2
-#error "Bad configuration!"
-#endif
-
-#if SWISSTABLE_HAVE_SSE2
-#include <emmintrin.h>
-#endif
-
-#if SWISSTABLE_HAVE_SSSE3
-#include <tmmintrin.h>
-#endif
-
-#endif  // ABSL_CONTAINER_INTERNAL_HAVE_SSE_H_
diff --git a/absl/container/internal/heterogeneous_lookup_testing.h b/absl/container/internal/heterogeneous_lookup_testing.h
new file mode 100644
index 00000000000..b8cfae32149
--- /dev/null
+++ b/absl/container/internal/heterogeneous_lookup_testing.h
@@ -0,0 +1,80 @@
+// Copyright 2025 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_CONTAINER_INTERNAL_HETEROGENEOUS_LOOKUP_TESTING_H_
+#define ABSL_CONTAINER_INTERNAL_HETEROGENEOUS_LOOKUP_TESTING_H_
+
+#include <cstddef>
+#include <ostream>
+
+#include "gmock/gmock.h"
+#include "absl/base/config.h"
+#include "absl/container/internal/test_instance_tracker.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace container_internal {
+
+// An expensive class that is convertible to CheapType to demonstrate
+// heterogeneous lookups.
+class ExpensiveType : public absl::test_internal::CopyableMovableInstance {
+ public:
+  explicit ExpensiveType(int value)
+      : absl::test_internal::CopyableMovableInstance(value) {}
+
+  friend std::ostream& operator<<(std::ostream& os, const ExpensiveType& a) {
+    return os << a.value();
+  }
+};
+
+class CheapType {
+ public:
+  explicit CheapType(const int value) : value_(value) {}
+
+  explicit operator ExpensiveType() const { return ExpensiveType(value_); }
+
+  int value() const { return value_; }
+
+ private:
+  int value_;
+};
+
+struct HeterogeneousHash {
+  using is_transparent = void;
+  size_t operator()(const ExpensiveType& a) const { return a.value(); }
+  size_t operator()(const CheapType& a) const { return a.value(); }
+};
+
+struct HeterogeneousEqual {
+  using is_transparent = void;
+  bool operator()(const ExpensiveType& a, const ExpensiveType& b) const {
+    return a.value() == b.value();
+  }
+  bool operator()(const ExpensiveType& a, const CheapType& b) const {
+    return a.value() == b.value();
+  }
+  bool operator()(const CheapType& a, const ExpensiveType& b) const {
+    return a.value() == b.value();
+  }
+};
+
+MATCHER_P(HasExpensiveValue, n, "") {
+  return ::testing::ExplainMatchResult(n, arg.value(), result_listener);
+}
+
+}  // namespace container_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_CONTAINER_INTERNAL_HETEROGENEOUS_LOOKUP_TESTING_H_
diff --git a/absl/container/internal/inlined_vector.h b/absl/container/internal/inlined_vector.h
index 54369c85682..5b9763e9738 100644
--- a/absl/container/internal/inlined_vector.h
+++ b/absl/container/internal/inlined_vector.h
@@ -12,8 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-#ifndef ABSL_CONTAINER_INTERNAL_INLINED_VECTOR_INTERNAL_H_
-#define ABSL_CONTAINER_INTERNAL_INLINED_VECTOR_INTERNAL_H_
+#ifndef ABSL_CONTAINER_INTERNAL_INLINED_VECTOR_H_
+#define ABSL_CONTAINER_INTERNAL_INLINED_VECTOR_H_
 
 #include <algorithm>
 #include <cstddef>
@@ -21,8 +21,13 @@
 #include <iterator>
 #include <limits>
 #include <memory>
+#include <new>
+#include <type_traits>
 #include <utility>
 
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/hardening.h"
 #include "absl/base/macros.h"
 #include "absl/container/internal/compressed_tuple.h"
 #include "absl/memory/memory.h"
@@ -30,91 +35,135 @@
 #include "absl/types/span.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace inlined_vector_internal {
 
-template <typename Iterator>
-using IsAtLeastForwardIterator = std::is_convertible<
-    typename std::iterator_traits<Iterator>::iterator_category,
-    std::forward_iterator_tag>;
-
-template <typename AllocatorType>
-using IsMemcpyOk = absl::conjunction<
-    std::is_same<std::allocator<typename AllocatorType::value_type>,
-                 AllocatorType>,
-    absl::is_trivially_copy_constructible<typename AllocatorType::value_type>,
-    absl::is_trivially_copy_assignable<typename AllocatorType::value_type>,
-    absl::is_trivially_destructible<typename AllocatorType::value_type>>;
-
-template <typename AllocatorType, typename ValueType, typename SizeType>
-void DestroyElements(AllocatorType* alloc_ptr, ValueType* destroy_first,
-                     SizeType destroy_size) {
-  using AllocatorTraits = absl::allocator_traits<AllocatorType>;
-
-  if (destroy_first != nullptr) {
-    for (auto i = destroy_size; i != 0;) {
+// GCC does not deal very well with the below code
+#if !defined(__clang__) && defined(__GNUC__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Warray-bounds"
+#endif
+
+template <typename A>
+using AllocatorTraits = std::allocator_traits<A>;
+template <typename A>
+using ValueType = typename AllocatorTraits<A>::value_type;
+template <typename A>
+using SizeType = typename AllocatorTraits<A>::size_type;
+template <typename A>
+using Pointer = typename AllocatorTraits<A>::pointer;
+template <typename A>
+using ConstPointer = typename AllocatorTraits<A>::const_pointer;
+template <typename A>
+using SizeType = typename AllocatorTraits<A>::size_type;
+template <typename A>
+using DifferenceType = typename AllocatorTraits<A>::difference_type;
+template <typename A>
+using Reference = ValueType<A>&;
+template <typename A>
+using ConstReference = const ValueType<A>&;
+template <typename A>
+using Iterator = Pointer<A>;
+template <typename A>
+using ConstIterator = ConstPointer<A>;
+template <typename A>
+using ReverseIterator = typename std::reverse_iterator<Iterator<A>>;
+template <typename A>
+using ConstReverseIterator = typename std::reverse_iterator<ConstIterator<A>>;
+template <typename A>
+using MoveIterator = typename std::move_iterator<Iterator<A>>;
+
+template <typename A>
+using IsMoveAssignOk = std::is_move_assignable<ValueType<A>>;
+template <typename A>
+using IsSwapOk = std::is_swappable<ValueType<A>>;
+
+template <typename A, bool IsTriviallyDestructible =
+                          std::is_trivially_destructible_v<ValueType<A>> &&
+                          std::is_same_v<A, std::allocator<ValueType<A>>>>
+struct DestroyAdapter;
+
+template <typename A>
+struct DestroyAdapter<A, /* IsTriviallyDestructible */ false> {
+  static void DestroyElements(A& allocator, Pointer<A> destroy_first,
+                              SizeType<A> destroy_size) {
+    for (SizeType<A> i = destroy_size; i != 0;) {
       --i;
-      AllocatorTraits::destroy(*alloc_ptr, destroy_first + i);
+      AllocatorTraits<A>::destroy(allocator, destroy_first + i);
     }
+  }
+};
 
-#if !defined(NDEBUG)
-    // Overwrite unused memory with `0xab` so we can catch uninitialized usage.
-    //
-    // Cast to `void*` to tell the compiler that we don't care that we might be
-    // scribbling on a vtable pointer.
-    auto* memory_ptr = static_cast<void*>(destroy_first);
-    auto memory_size = sizeof(ValueType) * destroy_size;
-    std::memset(memory_ptr, 0xab, memory_size);
-#endif  // !defined(NDEBUG)
+template <typename A>
+struct DestroyAdapter<A, /* IsTriviallyDestructible */ true> {
+  static void DestroyElements(A& allocator, Pointer<A> destroy_first,
+                              SizeType<A> destroy_size) {
+    static_cast<void>(allocator);
+    static_cast<void>(destroy_first);
+    static_cast<void>(destroy_size);
   }
-}
+};
 
-template <typename AllocatorType, typename ValueType, typename ValueAdapter,
-          typename SizeType>
-void ConstructElements(AllocatorType* alloc_ptr, ValueType* construct_first,
-                       ValueAdapter* values_ptr, SizeType construct_size) {
-  for (SizeType i = 0; i < construct_size; ++i) {
-    ABSL_INTERNAL_TRY {
-      values_ptr->ConstructNext(alloc_ptr, construct_first + i);
-    }
+template <typename A>
+struct Allocation {
+  Pointer<A> data = nullptr;
+  SizeType<A> capacity = 0;
+};
+
+template <typename A,
+          bool IsOverAligned =
+              (alignof(ValueType<A>) > ABSL_INTERNAL_DEFAULT_NEW_ALIGNMENT)>
+struct MallocAdapter {
+  static Allocation<A> Allocate(A& allocator, SizeType<A> requested_capacity) {
+    return {AllocatorTraits<A>::allocate(allocator, requested_capacity),
+            requested_capacity};
+  }
+
+  static void Deallocate(A& allocator, Pointer<A> pointer,
+                         SizeType<A> capacity) {
+    AllocatorTraits<A>::deallocate(allocator, pointer, capacity);
+  }
+};
+
+template <typename A, typename ValueAdapter>
+void ConstructElements(absl::type_identity_t<A>& allocator,
+                       Pointer<A> construct_first, ValueAdapter& values,
+                       SizeType<A> construct_size) {
+  for (SizeType<A> i = 0; i < construct_size; ++i) {
+    ABSL_INTERNAL_TRY { values.ConstructNext(allocator, construct_first + i); }
     ABSL_INTERNAL_CATCH_ANY {
-      inlined_vector_internal::DestroyElements(alloc_ptr, construct_first, i);
+      DestroyAdapter<A>::DestroyElements(allocator, construct_first, i);
       ABSL_INTERNAL_RETHROW;
     }
   }
 }
 
-template <typename ValueType, typename ValueAdapter, typename SizeType>
-void AssignElements(ValueType* assign_first, ValueAdapter* values_ptr,
-                    SizeType assign_size) {
-  for (SizeType i = 0; i < assign_size; ++i) {
-    values_ptr->AssignNext(assign_first + i);
+template <typename A, typename ValueAdapter>
+void AssignElements(Pointer<A> assign_first, ValueAdapter& values,
+                    SizeType<A> assign_size) {
+  for (SizeType<A> i = 0; i < assign_size; ++i) {
+    values.AssignNext(assign_first + i);
   }
 }
 
-template <typename AllocatorType>
+template <typename A>
 struct StorageView {
-  using pointer = typename AllocatorType::pointer;
-  using size_type = typename AllocatorType::size_type;
-
-  pointer data;
-  size_type size;
-  size_type capacity;
+  Pointer<A> data;
+  SizeType<A> size;
+  SizeType<A> capacity;
 };
 
-template <typename AllocatorType, typename Iterator>
+template <typename A, typename Iterator>
 class IteratorValueAdapter {
-  using pointer = typename AllocatorType::pointer;
-  using AllocatorTraits = absl::allocator_traits<AllocatorType>;
-
  public:
   explicit IteratorValueAdapter(const Iterator& it) : it_(it) {}
 
-  void ConstructNext(AllocatorType* alloc_ptr, pointer construct_at) {
-    AllocatorTraits::construct(*alloc_ptr, construct_at, *it_);
+  void ConstructNext(A& allocator, Pointer<A> construct_at) {
+    AllocatorTraits<A>::construct(allocator, construct_at, *it_);
     ++it_;
   }
 
-  void AssignNext(pointer assign_at) {
+  void AssignNext(Pointer<A> assign_at) {
     *assign_at = *it_;
     ++it_;
   }
@@ -123,165 +172,161 @@ class IteratorValueAdapter {
   Iterator it_;
 };
 
-template <typename AllocatorType>
+template <typename A>
 class CopyValueAdapter {
-  using pointer = typename AllocatorType::pointer;
-  using const_pointer = typename AllocatorType::const_pointer;
-  using const_reference = typename AllocatorType::const_reference;
-  using AllocatorTraits = absl::allocator_traits<AllocatorType>;
-
  public:
-  explicit CopyValueAdapter(const_reference v) : ptr_(std::addressof(v)) {}
+  explicit CopyValueAdapter(ConstPointer<A> p) : ptr_(p) {}
 
-  void ConstructNext(AllocatorType* alloc_ptr, pointer construct_at) {
-    AllocatorTraits::construct(*alloc_ptr, construct_at, *ptr_);
+  void ConstructNext(A& allocator, Pointer<A> construct_at) {
+    AllocatorTraits<A>::construct(allocator, construct_at, *ptr_);
   }
 
-  void AssignNext(pointer assign_at) { *assign_at = *ptr_; }
+  void AssignNext(Pointer<A> assign_at) { *assign_at = *ptr_; }
 
  private:
-  const_pointer ptr_;
+  ConstPointer<A> ptr_;
 };
 
-template <typename AllocatorType>
+template <typename A>
 class DefaultValueAdapter {
-  using pointer = typename AllocatorType::pointer;
-  using value_type = typename AllocatorType::value_type;
-  using AllocatorTraits = absl::allocator_traits<AllocatorType>;
-
  public:
   explicit DefaultValueAdapter() {}
 
-  void ConstructNext(AllocatorType* alloc_ptr, pointer construct_at) {
-    AllocatorTraits::construct(*alloc_ptr, construct_at);
+  void ConstructNext(A& allocator, Pointer<A> construct_at) {
+    AllocatorTraits<A>::construct(allocator, construct_at);
   }
 
-  void AssignNext(pointer assign_at) { *assign_at = value_type(); }
+  void AssignNext(Pointer<A> assign_at) { *assign_at = ValueType<A>(); }
 };
 
-template <typename AllocatorType>
+template <typename A>
 class AllocationTransaction {
-  using value_type = typename AllocatorType::value_type;
-  using pointer = typename AllocatorType::pointer;
-  using size_type = typename AllocatorType::size_type;
-  using AllocatorTraits = absl::allocator_traits<AllocatorType>;
-
  public:
-  explicit AllocationTransaction(AllocatorType* alloc_ptr)
-      : alloc_data_(*alloc_ptr, nullptr) {}
+  explicit AllocationTransaction(A& allocator)
+      : allocator_data_(allocator, nullptr), capacity_(0) {}
 
   ~AllocationTransaction() {
     if (DidAllocate()) {
-      AllocatorTraits::deallocate(GetAllocator(), GetData(), GetCapacity());
+      MallocAdapter<A>::Deallocate(GetAllocator(), GetData(), GetCapacity());
     }
   }
 
   AllocationTransaction(const AllocationTransaction&) = delete;
   void operator=(const AllocationTransaction&) = delete;
 
-  AllocatorType& GetAllocator() { return alloc_data_.template get<0>(); }
-  pointer& GetData() { return alloc_data_.template get<1>(); }
-  size_type& GetCapacity() { return capacity_; }
+  A& GetAllocator() { return allocator_data_.template get<0>(); }
+  Pointer<A>& GetData() { return allocator_data_.template get<1>(); }
+  SizeType<A>& GetCapacity() { return capacity_; }
 
   bool DidAllocate() { return GetData() != nullptr; }
-  pointer Allocate(size_type capacity) {
-    GetData() = AllocatorTraits::allocate(GetAllocator(), capacity);
-    GetCapacity() = capacity;
-    return GetData();
+
+  Pointer<A> Allocate(SizeType<A> requested_capacity) {
+    Allocation<A> result =
+        MallocAdapter<A>::Allocate(GetAllocator(), requested_capacity);
+    GetData() = result.data;
+    GetCapacity() = result.capacity;
+    return result.data;
   }
 
+  [[nodiscard]] Allocation<A> Release() && {
+    Allocation<A> result = {GetData(), GetCapacity()};
+    Reset();
+    return result;
+  }
+
+ private:
   void Reset() {
     GetData() = nullptr;
     GetCapacity() = 0;
   }
 
- private:
-  container_internal::CompressedTuple<AllocatorType, pointer> alloc_data_;
-  size_type capacity_ = 0;
+  container_internal::CompressedTuple<A, Pointer<A>> allocator_data_;
+  SizeType<A> capacity_;
 };
 
-template <typename AllocatorType>
+template <typename A>
 class ConstructionTransaction {
-  using pointer = typename AllocatorType::pointer;
-  using size_type = typename AllocatorType::size_type;
-
  public:
-  explicit ConstructionTransaction(AllocatorType* alloc_ptr)
-      : alloc_data_(*alloc_ptr, nullptr) {}
+  explicit ConstructionTransaction(A& allocator)
+      : allocator_data_(allocator, nullptr), size_(0) {}
 
   ~ConstructionTransaction() {
     if (DidConstruct()) {
-      inlined_vector_internal::DestroyElements(std::addressof(GetAllocator()),
-                                               GetData(), GetSize());
+      DestroyAdapter<A>::DestroyElements(GetAllocator(), GetData(), GetSize());
     }
   }
 
   ConstructionTransaction(const ConstructionTransaction&) = delete;
   void operator=(const ConstructionTransaction&) = delete;
 
-  AllocatorType& GetAllocator() { return alloc_data_.template get<0>(); }
-  pointer& GetData() { return alloc_data_.template get<1>(); }
-  size_type& GetSize() { return size_; }
+  A& GetAllocator() { return allocator_data_.template get<0>(); }
+  Pointer<A>& GetData() { return allocator_data_.template get<1>(); }
+  SizeType<A>& GetSize() { return size_; }
 
   bool DidConstruct() { return GetData() != nullptr; }
   template <typename ValueAdapter>
-  void Construct(pointer data, ValueAdapter* values_ptr, size_type size) {
-    inlined_vector_internal::ConstructElements(std::addressof(GetAllocator()),
-                                               data, values_ptr, size);
+  void Construct(Pointer<A> data, ValueAdapter& values, SizeType<A> size) {
+    ConstructElements<A>(GetAllocator(), data, values, size);
     GetData() = data;
     GetSize() = size;
   }
-  void Commit() {
+  void Commit() && {
     GetData() = nullptr;
     GetSize() = 0;
   }
 
  private:
-  container_internal::CompressedTuple<AllocatorType, pointer> alloc_data_;
-  size_type size_ = 0;
+  container_internal::CompressedTuple<A, Pointer<A>> allocator_data_;
+  SizeType<A> size_;
 };
 
 template <typename T, size_t N, typename A>
 class Storage {
  public:
-  using allocator_type = A;
-  using value_type = typename allocator_type::value_type;
-  using pointer = typename allocator_type::pointer;
-  using const_pointer = typename allocator_type::const_pointer;
-  using reference = typename allocator_type::reference;
-  using const_reference = typename allocator_type::const_reference;
-  using rvalue_reference = typename allocator_type::value_type&&;
-  using size_type = typename allocator_type::size_type;
-  using difference_type = typename allocator_type::difference_type;
-  using iterator = pointer;
-  using const_iterator = const_pointer;
-  using reverse_iterator = std::reverse_iterator<iterator>;
-  using const_reverse_iterator = std::reverse_iterator<const_iterator>;
-  using MoveIterator = std::move_iterator<iterator>;
-  using AllocatorTraits = absl::allocator_traits<allocator_type>;
-  using IsMemcpyOk = inlined_vector_internal::IsMemcpyOk<allocator_type>;
-
-  using StorageView = inlined_vector_internal::StorageView<allocator_type>;
-
-  template <typename Iterator>
-  using IteratorValueAdapter =
-      inlined_vector_internal::IteratorValueAdapter<allocator_type, Iterator>;
-  using CopyValueAdapter =
-      inlined_vector_internal::CopyValueAdapter<allocator_type>;
-  using DefaultValueAdapter =
-      inlined_vector_internal::DefaultValueAdapter<allocator_type>;
-
-  using AllocationTransaction =
-      inlined_vector_internal::AllocationTransaction<allocator_type>;
-  using ConstructionTransaction =
-      inlined_vector_internal::ConstructionTransaction<allocator_type>;
-
-  static size_type NextCapacity(size_type current_capacity) {
+  struct MemcpyPolicy {};
+  struct ElementwiseAssignPolicy {};
+  struct ElementwiseSwapPolicy {};
+  struct ElementwiseConstructPolicy {};
+
+  using MoveAssignmentPolicy = std::conditional_t<
+      // Fast path: if the value type can be trivially move assigned and
+      // destroyed, and we know the allocator doesn't do anything fancy, then
+      // it's safe for us to simply adopt the contents of the storage for
+      // `other` and remove its own reference to them. It's as if we had
+      // individually move-assigned each value and then destroyed the original.
+      std::conjunction_v<std::is_trivially_move_assignable<ValueType<A>>,
+                         std::is_trivially_destructible<ValueType<A>>,
+                         std::is_same<A, std::allocator<ValueType<A>>>>,
+      MemcpyPolicy,
+      // Otherwise we use move assignment if possible. If not, we simulate
+      // move assignment using move construction.
+      //
+      // Note that this is in contrast to e.g. std::vector and std::optional,
+      // which are themselves not move-assignable when their contained type is
+      // not.
+      std::conditional_t<IsMoveAssignOk<A>::value, ElementwiseAssignPolicy,
+                         ElementwiseConstructPolicy>>;
+
+  // The policy to be used specifically when swapping inlined elements.
+  using SwapInlinedElementsPolicy = std::conditional_t<
+      // Fast path: if the value type can be trivially relocated, and we
+      // know the allocator doesn't do anything fancy, then it's safe for us
+      // to simply swap the bytes in the inline storage. It's as if we had
+      // relocated the first vector's elements into temporary storage,
+      // relocated the second's elements into the (now-empty) first's,
+      // and then relocated from temporary storage into the second.
+      std::conjunction_v<absl::is_trivially_relocatable<ValueType<A>>,
+                         std::is_same<A, std::allocator<ValueType<A>>>>,
+      MemcpyPolicy,
+      std::conditional_t<IsSwapOk<A>::value, ElementwiseSwapPolicy,
+                         ElementwiseConstructPolicy>>;
+
+  static SizeType<A> NextCapacity(SizeType<A> current_capacity) {
     return current_capacity * 2;
   }
 
-  static size_type ComputeCapacity(size_type current_capacity,
-                                   size_type requested_capacity) {
+  static SizeType<A> ComputeCapacity(SizeType<A> current_capacity,
+                                     SizeType<A> requested_capacity) {
     return (std::max)(NextCapacity(current_capacity), requested_capacity);
   }
 
@@ -289,140 +334,184 @@ class Storage {
   // Storage Constructors and Destructor
   // ---------------------------------------------------------------------------
 
-  Storage() : metadata_() {}
+  Storage() : metadata_(A(), /* size and is_allocated */ 0u) {}
 
-  explicit Storage(const allocator_type& alloc) : metadata_(alloc, {}) {}
+  explicit Storage(const A& allocator)
+      : metadata_(allocator, /* size and is_allocated */ 0u) {}
 
   ~Storage() {
-    pointer data = GetIsAllocated() ? GetAllocatedData() : GetInlinedData();
-    inlined_vector_internal::DestroyElements(GetAllocPtr(), data, GetSize());
-    DeallocateIfAllocated();
+    // Fast path: if we are empty and not allocated, there's nothing to do.
+    if (GetSizeAndIsAllocated() == 0) {
+      return;
+    }
+
+    // Fast path: if no destructors need to be run and we know the allocator
+    // doesn't do anything fancy, then all we need to do is deallocate (and
+    // maybe not even that).
+    if (std::is_trivially_destructible_v<ValueType<A>> &&
+        std::is_same_v<A, std::allocator<ValueType<A>>>) {
+      DeallocateIfAllocated();
+      return;
+    }
+
+    DestroyContents();
   }
 
   // ---------------------------------------------------------------------------
   // Storage Member Accessors
   // ---------------------------------------------------------------------------
 
-  size_type& GetSizeAndIsAllocated() { return metadata_.template get<1>(); }
+  SizeType<A>& GetSizeAndIsAllocated() { return metadata_.template get<1>(); }
 
-  const size_type& GetSizeAndIsAllocated() const {
+  const SizeType<A>& GetSizeAndIsAllocated() const {
     return metadata_.template get<1>();
   }
 
-  size_type GetSize() const { return GetSizeAndIsAllocated() >> 1; }
+  SizeType<A> GetSize() const { return GetSizeAndIsAllocated() >> 1; }
 
   bool GetIsAllocated() const { return GetSizeAndIsAllocated() & 1; }
 
-  pointer GetAllocatedData() { return data_.allocated.allocated_data; }
+  Pointer<A> GetAllocatedData() {
+    // GCC 12 has a false-positive -Wmaybe-uninitialized warning here.
+#if ABSL_INTERNAL_HAVE_MIN_GNUC_VERSION(12, 0)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
+#endif
+    return data_.allocated.allocated_data;
+#if ABSL_INTERNAL_HAVE_MIN_GNUC_VERSION(12, 0)
+#pragma GCC diagnostic pop
+#endif
+  }
 
-  const_pointer GetAllocatedData() const {
+  ConstPointer<A> GetAllocatedData() const {
     return data_.allocated.allocated_data;
   }
 
-  pointer GetInlinedData() {
-    return reinterpret_cast<pointer>(
-        std::addressof(data_.inlined.inlined_data[0]));
+  // ABSL_ATTRIBUTE_NO_SANITIZE_CFI is used because the memory pointed to may be
+  // uninitialized, a common pattern in allocate()+construct() APIs.
+  // https://clang.llvm.org/docs/ControlFlowIntegrity.html#bad-cast-checking
+  // NOTE: When this was written, LLVM documentation did not explicitly
+  // mention that casting `char*` and using `reinterpret_cast` qualifies
+  // as a bad cast.
+  ABSL_ATTRIBUTE_NO_SANITIZE_CFI Pointer<A> GetInlinedData() {
+    return reinterpret_cast<Pointer<A>>(data_.inlined.inlined_data);
   }
 
-  const_pointer GetInlinedData() const {
-    return reinterpret_cast<const_pointer>(
-        std::addressof(data_.inlined.inlined_data[0]));
+  ABSL_ATTRIBUTE_NO_SANITIZE_CFI ConstPointer<A> GetInlinedData() const {
+    return reinterpret_cast<ConstPointer<A>>(data_.inlined.inlined_data);
   }
 
-  size_type GetAllocatedCapacity() const {
+  SizeType<A> GetAllocatedCapacity() const {
     return data_.allocated.allocated_capacity;
   }
 
-  size_type GetInlinedCapacity() const { return static_cast<size_type>(N); }
-
-  StorageView MakeStorageView() {
-    return GetIsAllocated()
-               ? StorageView{GetAllocatedData(), GetSize(),
-                             GetAllocatedCapacity()}
-               : StorageView{GetInlinedData(), GetSize(), GetInlinedCapacity()};
+  SizeType<A> GetInlinedCapacity() const {
+    return static_cast<SizeType<A>>(kOptimalInlinedSize);
   }
 
-  allocator_type* GetAllocPtr() {
-    return std::addressof(metadata_.template get<0>());
+  StorageView<A> MakeStorageView() {
+    return GetIsAllocated() ? StorageView<A>{GetAllocatedData(), GetSize(),
+                                             GetAllocatedCapacity()}
+                            : StorageView<A>{GetInlinedData(), GetSize(),
+                                             GetInlinedCapacity()};
   }
 
-  const allocator_type* GetAllocPtr() const {
-    return std::addressof(metadata_.template get<0>());
-  }
+  A& GetAllocator() { return metadata_.template get<0>(); }
+
+  const A& GetAllocator() const { return metadata_.template get<0>(); }
 
   // ---------------------------------------------------------------------------
   // Storage Member Mutators
   // ---------------------------------------------------------------------------
 
+  ABSL_ATTRIBUTE_NOINLINE void InitFrom(const Storage& other);
+
   template <typename ValueAdapter>
-  void Initialize(ValueAdapter values, size_type new_size);
+  void Initialize(ValueAdapter values, SizeType<A> new_size);
 
   template <typename ValueAdapter>
-  void Assign(ValueAdapter values, size_type new_size);
+  void Assign(ValueAdapter values, SizeType<A> new_size);
 
   template <typename ValueAdapter>
-  void Resize(ValueAdapter values, size_type new_size);
+  void Resize(ValueAdapter values, SizeType<A> new_size);
 
   template <typename ValueAdapter>
-  iterator Insert(const_iterator pos, ValueAdapter values,
-                  size_type insert_count);
+  Iterator<A> Insert(ConstIterator<A> pos, ValueAdapter values,
+                     SizeType<A> insert_count);
 
   template <typename... Args>
-  reference EmplaceBack(Args&&... args);
+  Reference<A> EmplaceBack(Args&&... args);
 
-  iterator Erase(const_iterator from, const_iterator to);
+  Iterator<A> Erase(ConstIterator<A> from, ConstIterator<A> to);
 
-  void Reserve(size_type requested_capacity);
+  void Reserve(SizeType<A> requested_capacity);
 
   void ShrinkToFit();
 
   void Swap(Storage* other_storage_ptr);
 
   void SetIsAllocated() {
-    GetSizeAndIsAllocated() |= static_cast<size_type>(1);
+    GetSizeAndIsAllocated() |= static_cast<SizeType<A>>(1);
   }
 
   void UnsetIsAllocated() {
-    GetSizeAndIsAllocated() &= ((std::numeric_limits<size_type>::max)() - 1);
+    GetSizeAndIsAllocated() &= ((std::numeric_limits<SizeType<A>>::max)() - 1);
   }
 
-  void SetSize(size_type size) {
+  void SetSize(SizeType<A> size) {
     GetSizeAndIsAllocated() =
-        (size << 1) | static_cast<size_type>(GetIsAllocated());
+        (size << 1) | static_cast<SizeType<A>>(GetIsAllocated());
   }
 
-  void SetAllocatedSize(size_type size) {
-    GetSizeAndIsAllocated() = (size << 1) | static_cast<size_type>(1);
+  void SetAllocatedSize(SizeType<A> size) {
+    GetSizeAndIsAllocated() = (size << 1) | static_cast<SizeType<A>>(1);
   }
 
-  void SetInlinedSize(size_type size) {
-    GetSizeAndIsAllocated() = size << static_cast<size_type>(1);
+  void SetInlinedSize(SizeType<A> size) {
+    GetSizeAndIsAllocated() = size << static_cast<SizeType<A>>(1);
   }
 
-  void AddSize(size_type count) {
-    GetSizeAndIsAllocated() += count << static_cast<size_type>(1);
+  void AddSize(SizeType<A> count) {
+    GetSizeAndIsAllocated() += count << static_cast<SizeType<A>>(1);
   }
 
-  void SubtractSize(size_type count) {
-    assert(count <= GetSize());
+  void SubtractSize(SizeType<A> count) {
+    absl::base_internal::HardeningAssertLE(count, GetSize());
 
-    GetSizeAndIsAllocated() -= count << static_cast<size_type>(1);
+    GetSizeAndIsAllocated() -= count << static_cast<SizeType<A>>(1);
   }
 
-  void SetAllocatedData(pointer data, size_type capacity) {
-    data_.allocated.allocated_data = data;
-    data_.allocated.allocated_capacity = capacity;
-  }
-
-  void AcquireAllocatedData(AllocationTransaction* allocation_tx_ptr) {
-    SetAllocatedData(allocation_tx_ptr->GetData(),
-                     allocation_tx_ptr->GetCapacity());
-
-    allocation_tx_ptr->Reset();
+  void SetAllocation(Allocation<A> allocation) {
+    data_.allocated.allocated_data = allocation.data;
+    data_.allocated.allocated_capacity = allocation.capacity;
   }
 
   void MemcpyFrom(const Storage& other_storage) {
-    assert(IsMemcpyOk::value || other_storage.GetIsAllocated());
+    // Assumption check: it doesn't make sense to memcpy inlined elements unless
+    // we know the allocator doesn't do anything fancy, and one of the following
+    // holds:
+    //
+    //  *  The elements are trivially relocatable.
+    //
+    //  *  It's possible to trivially assign the elements and then destroy the
+    //     source.
+    //
+    //  *  It's possible to trivially copy construct/assign the elements.
+    //
+    {
+      using V = ValueType<A>;
+      ABSL_ASSERT(other_storage.GetIsAllocated() ||
+                  (std::is_same_v<A, std::allocator<V>> &&
+                   (
+                       // First case above
+                       absl::is_trivially_relocatable<V>::value ||
+                       // Second case above
+                       (std::is_trivially_move_assignable_v<V> &&
+                        std::is_trivially_destructible_v<V>) ||
+                       // Third case above
+                       (std::is_trivially_copy_constructible_v<V> ||
+                        std::is_trivially_copy_assignable_v<V>))));
+    }
 
     GetSizeAndIsAllocated() = other_storage.GetSizeAndIsAllocated();
     data_ = other_storage.data_;
@@ -430,22 +519,30 @@ class Storage {
 
   void DeallocateIfAllocated() {
     if (GetIsAllocated()) {
-      AllocatorTraits::deallocate(*GetAllocPtr(), GetAllocatedData(),
-                                  GetAllocatedCapacity());
+      MallocAdapter<A>::Deallocate(GetAllocator(), GetAllocatedData(),
+                                   GetAllocatedCapacity());
     }
   }
 
  private:
-  using Metadata =
-      container_internal::CompressedTuple<allocator_type, size_type>;
+  ABSL_ATTRIBUTE_NOINLINE void DestroyContents();
+
+  using Metadata = container_internal::CompressedTuple<A, SizeType<A>>;
 
   struct Allocated {
-    pointer allocated_data;
-    size_type allocated_capacity;
+    Pointer<A> allocated_data;
+    SizeType<A> allocated_capacity;
   };
 
+  // `kOptimalInlinedSize` is an automatically adjusted inlined capacity of the
+  // `InlinedVector`. Sometimes, it is possible to increase the capacity (from
+  // the user requested `N`) without increasing the size of the `InlinedVector`.
+  static constexpr size_t kOptimalInlinedSize =
+      (std::max)(N, sizeof(Allocated) / sizeof(ValueType<A>));
+
   struct Inlined {
-    alignas(value_type) char inlined_data[sizeof(value_type[N])];
+    alignas(ValueType<A>) unsigned char inlined_data[sizeof(
+        ValueType<A>[kOptimalInlinedSize])];
   };
 
   union Data {
@@ -453,36 +550,88 @@ class Storage {
     Inlined inlined;
   };
 
+  void SwapN(ElementwiseSwapPolicy, Storage* other, SizeType<A> n);
+  void SwapN(ElementwiseConstructPolicy, Storage* other, SizeType<A> n);
+
+  void SwapInlinedElements(MemcpyPolicy, Storage* other);
+  template <typename NotMemcpyPolicy>
+  void SwapInlinedElements(NotMemcpyPolicy, Storage* other);
+
+  template <typename... Args>
+  ABSL_ATTRIBUTE_NOINLINE Reference<A> EmplaceBackSlow(Args&&... args);
+
   Metadata metadata_;
   Data data_;
 };
 
+template <typename T, size_t N, typename A>
+void Storage<T, N, A>::DestroyContents() {
+  Pointer<A> data = GetIsAllocated() ? GetAllocatedData() : GetInlinedData();
+  DestroyAdapter<A>::DestroyElements(GetAllocator(), data, GetSize());
+  DeallocateIfAllocated();
+}
+
+template <typename T, size_t N, typename A>
+void Storage<T, N, A>::InitFrom(const Storage& other) {
+  const SizeType<A> n = other.GetSize();
+  ABSL_ASSERT(n > 0);  // Empty sources handled in caller.
+  ConstPointer<A> src;
+  Pointer<A> dst;
+  if (!other.GetIsAllocated()) {
+    dst = GetInlinedData();
+    src = other.GetInlinedData();
+  } else {
+    // Because this is only called from the `InlinedVector` constructors, it's
+    // safe to take on the allocation with size `0`. If `ConstructElements(...)`
+    // throws, deallocation will be automatically handled by `~Storage()`.
+    SizeType<A> requested_capacity = ComputeCapacity(GetInlinedCapacity(), n);
+    Allocation<A> allocation =
+        MallocAdapter<A>::Allocate(GetAllocator(), requested_capacity);
+    SetAllocation(allocation);
+    dst = allocation.data;
+    src = other.GetAllocatedData();
+  }
+
+  // Fast path: if the value type is trivially copy constructible and we know
+  // the allocator doesn't do anything fancy, then we know it is legal for us to
+  // simply memcpy the other vector's elements.
+  if (std::is_trivially_copy_constructible_v<ValueType<A>> &&
+      std::is_same_v<A, std::allocator<ValueType<A>>>) {
+    std::memcpy(reinterpret_cast<char*>(dst),
+                reinterpret_cast<const char*>(src), n * sizeof(ValueType<A>));
+  } else {
+    auto values = IteratorValueAdapter<A, ConstPointer<A>>(src);
+    ConstructElements<A>(GetAllocator(), dst, values, n);
+  }
+
+  GetSizeAndIsAllocated() = other.GetSizeAndIsAllocated();
+}
+
 template <typename T, size_t N, typename A>
 template <typename ValueAdapter>
-auto Storage<T, N, A>::Initialize(ValueAdapter values, size_type new_size)
-    -> void {
+auto Storage<T, N, A>::Initialize(ValueAdapter values,
+                                  SizeType<A> new_size) -> void {
   // Only callable from constructors!
-  assert(!GetIsAllocated());
-  assert(GetSize() == 0);
+  ABSL_ASSERT(!GetIsAllocated());
+  ABSL_ASSERT(GetSize() == 0);
 
-  pointer construct_data;
+  Pointer<A> construct_data;
   if (new_size > GetInlinedCapacity()) {
     // Because this is only called from the `InlinedVector` constructors, it's
     // safe to take on the allocation with size `0`. If `ConstructElements(...)`
     // throws, deallocation will be automatically handled by `~Storage()`.
-    size_type new_capacity = ComputeCapacity(GetInlinedCapacity(), new_size);
-    pointer new_data = AllocatorTraits::allocate(*GetAllocPtr(), new_capacity);
-
-    SetAllocatedData(new_data, new_capacity);
+    SizeType<A> requested_capacity =
+        ComputeCapacity(GetInlinedCapacity(), new_size);
+    Allocation<A> allocation =
+        MallocAdapter<A>::Allocate(GetAllocator(), requested_capacity);
+    construct_data = allocation.data;
+    SetAllocation(allocation);
     SetIsAllocated();
-
-    construct_data = new_data;
   } else {
     construct_data = GetInlinedData();
   }
 
-  inlined_vector_internal::ConstructElements(GetAllocPtr(), construct_data,
-                                             &values, new_size);
+  ConstructElements<A>(GetAllocator(), construct_data, values, new_size);
 
   // Since the initial size was guaranteed to be `0` and the allocated bit is
   // already correct for either case, *adding* `new_size` gives us the correct
@@ -492,20 +641,20 @@ auto Storage<T, N, A>::Initialize(ValueAdapter values, size_type new_size)
 
 template <typename T, size_t N, typename A>
 template <typename ValueAdapter>
-auto Storage<T, N, A>::Assign(ValueAdapter values, size_type new_size) -> void {
-  StorageView storage_view = MakeStorageView();
+auto Storage<T, N, A>::Assign(ValueAdapter values,
+                              SizeType<A> new_size) -> void {
+  StorageView<A> storage_view = MakeStorageView();
 
-  AllocationTransaction allocation_tx(GetAllocPtr());
+  AllocationTransaction<A> allocation_tx(GetAllocator());
 
-  absl::Span<value_type> assign_loop;
-  absl::Span<value_type> construct_loop;
-  absl::Span<value_type> destroy_loop;
+  absl::Span<ValueType<A>> assign_loop;
+  absl::Span<ValueType<A>> construct_loop;
+  absl::Span<ValueType<A>> destroy_loop;
 
   if (new_size > storage_view.capacity) {
-    size_type new_capacity = ComputeCapacity(storage_view.capacity, new_size);
-    pointer new_data = allocation_tx.Allocate(new_capacity);
-
-    construct_loop = {new_data, new_size};
+    SizeType<A> requested_capacity =
+        ComputeCapacity(storage_view.capacity, new_size);
+    construct_loop = {allocation_tx.Allocate(requested_capacity), new_size};
     destroy_loop = {storage_view.data, storage_view.size};
   } else if (new_size > storage_view.size) {
     assign_loop = {storage_view.data, storage_view.size};
@@ -516,18 +665,17 @@ auto Storage<T, N, A>::Assign(ValueAdapter values, size_type new_size) -> void {
     destroy_loop = {storage_view.data + new_size, storage_view.size - new_size};
   }
 
-  inlined_vector_internal::AssignElements(assign_loop.data(), &values,
-                                          assign_loop.size());
+  AssignElements<A>(assign_loop.data(), values, assign_loop.size());
 
-  inlined_vector_internal::ConstructElements(
-      GetAllocPtr(), construct_loop.data(), &values, construct_loop.size());
+  ConstructElements<A>(GetAllocator(), construct_loop.data(), values,
+                       construct_loop.size());
 
-  inlined_vector_internal::DestroyElements(GetAllocPtr(), destroy_loop.data(),
-                                           destroy_loop.size());
+  DestroyAdapter<A>::DestroyElements(GetAllocator(), destroy_loop.data(),
+                                     destroy_loop.size());
 
   if (allocation_tx.DidAllocate()) {
     DeallocateIfAllocated();
-    AcquireAllocatedData(&allocation_tx);
+    SetAllocation(std::move(allocation_tx).Release());
     SetIsAllocated();
   }
 
@@ -536,289 +684,292 @@ auto Storage<T, N, A>::Assign(ValueAdapter values, size_type new_size) -> void {
 
 template <typename T, size_t N, typename A>
 template <typename ValueAdapter>
-auto Storage<T, N, A>::Resize(ValueAdapter values, size_type new_size) -> void {
-  StorageView storage_view = MakeStorageView();
-
-  AllocationTransaction allocation_tx(GetAllocPtr());
-  ConstructionTransaction construction_tx(GetAllocPtr());
-
-  IteratorValueAdapter<MoveIterator> move_values(
-      MoveIterator(storage_view.data));
-
-  absl::Span<value_type> construct_loop;
-  absl::Span<value_type> move_construct_loop;
-  absl::Span<value_type> destroy_loop;
-
-  if (new_size > storage_view.capacity) {
-    size_type new_capacity = ComputeCapacity(storage_view.capacity, new_size);
-    pointer new_data = allocation_tx.Allocate(new_capacity);
-
-    construct_loop = {new_data + storage_view.size,
-                      new_size - storage_view.size};
-    move_construct_loop = {new_data, storage_view.size};
-    destroy_loop = {storage_view.data, storage_view.size};
-  } else if (new_size > storage_view.size) {
-    construct_loop = {storage_view.data + storage_view.size,
-                      new_size - storage_view.size};
+auto Storage<T, N, A>::Resize(ValueAdapter values,
+                              SizeType<A> new_size) -> void {
+  StorageView<A> storage_view = MakeStorageView();
+  Pointer<A> const base = storage_view.data;
+  const SizeType<A> size = storage_view.size;
+  A& alloc = GetAllocator();
+  if (new_size <= size) {
+    // Destroy extra old elements.
+    DestroyAdapter<A>::DestroyElements(alloc, base + new_size, size - new_size);
+  } else if (new_size <= storage_view.capacity) {
+    // Construct new elements in place.
+    ConstructElements<A>(alloc, base + size, values, new_size - size);
   } else {
-    destroy_loop = {storage_view.data + new_size, storage_view.size - new_size};
-  }
-
-  construction_tx.Construct(construct_loop.data(), &values,
-                            construct_loop.size());
-
-  inlined_vector_internal::ConstructElements(
-      GetAllocPtr(), move_construct_loop.data(), &move_values,
-      move_construct_loop.size());
-
-  inlined_vector_internal::DestroyElements(GetAllocPtr(), destroy_loop.data(),
-                                           destroy_loop.size());
-
-  construction_tx.Commit();
-  if (allocation_tx.DidAllocate()) {
+    // Steps:
+    //  a. Allocate new backing store.
+    //  b. Construct new elements in new backing store.
+    //  c. Move existing elements from old backing store to new backing store.
+    //  d. Destroy all elements in old backing store.
+    // Use transactional wrappers for the first two steps so we can roll
+    // back if necessary due to exceptions.
+    AllocationTransaction<A> allocation_tx(alloc);
+    SizeType<A> requested_capacity =
+        ComputeCapacity(storage_view.capacity, new_size);
+    Pointer<A> new_data = allocation_tx.Allocate(requested_capacity);
+
+    ConstructionTransaction<A> construction_tx(alloc);
+    construction_tx.Construct(new_data + size, values, new_size - size);
+
+    IteratorValueAdapter<A, MoveIterator<A>> move_values(
+        (MoveIterator<A>(base)));
+    ConstructElements<A>(alloc, new_data, move_values, size);
+
+    DestroyAdapter<A>::DestroyElements(alloc, base, size);
+    std::move(construction_tx).Commit();
     DeallocateIfAllocated();
-    AcquireAllocatedData(&allocation_tx);
+    SetAllocation(std::move(allocation_tx).Release());
     SetIsAllocated();
   }
-
   SetSize(new_size);
 }
 
 template <typename T, size_t N, typename A>
 template <typename ValueAdapter>
-auto Storage<T, N, A>::Insert(const_iterator pos, ValueAdapter values,
-                              size_type insert_count) -> iterator {
-  StorageView storage_view = MakeStorageView();
+auto Storage<T, N, A>::Insert(ConstIterator<A> pos, ValueAdapter values,
+                              SizeType<A> insert_count) -> Iterator<A> {
+  StorageView<A> storage_view = MakeStorageView();
 
-  size_type insert_index =
-      std::distance(const_iterator(storage_view.data), pos);
-  size_type insert_end_index = insert_index + insert_count;
-  size_type new_size = storage_view.size + insert_count;
+  auto insert_index = static_cast<SizeType<A>>(
+      std::distance(ConstIterator<A>(storage_view.data), pos));
+  SizeType<A> insert_end_index = insert_index + insert_count;
+  SizeType<A> new_size = storage_view.size + insert_count;
 
   if (new_size > storage_view.capacity) {
-    AllocationTransaction allocation_tx(GetAllocPtr());
-    ConstructionTransaction construction_tx(GetAllocPtr());
-    ConstructionTransaction move_construciton_tx(GetAllocPtr());
+    AllocationTransaction<A> allocation_tx(GetAllocator());
+    ConstructionTransaction<A> construction_tx(GetAllocator());
+    ConstructionTransaction<A> move_construction_tx(GetAllocator());
 
-    IteratorValueAdapter<MoveIterator> move_values(
-        MoveIterator(storage_view.data));
+    IteratorValueAdapter<A, MoveIterator<A>> move_values(
+        MoveIterator<A>(storage_view.data));
 
-    size_type new_capacity = ComputeCapacity(storage_view.capacity, new_size);
-    pointer new_data = allocation_tx.Allocate(new_capacity);
+    SizeType<A> requested_capacity =
+        ComputeCapacity(storage_view.capacity, new_size);
+    Pointer<A> new_data = allocation_tx.Allocate(requested_capacity);
 
-    construction_tx.Construct(new_data + insert_index, &values, insert_count);
+    construction_tx.Construct(new_data + insert_index, values, insert_count);
 
-    move_construciton_tx.Construct(new_data, &move_values, insert_index);
+    move_construction_tx.Construct(new_data, move_values, insert_index);
 
-    inlined_vector_internal::ConstructElements(
-        GetAllocPtr(), new_data + insert_end_index, &move_values,
-        storage_view.size - insert_index);
+    ConstructElements<A>(GetAllocator(), new_data + insert_end_index,
+                         move_values, storage_view.size - insert_index);
 
-    inlined_vector_internal::DestroyElements(GetAllocPtr(), storage_view.data,
-                                             storage_view.size);
+    DestroyAdapter<A>::DestroyElements(GetAllocator(), storage_view.data,
+                                       storage_view.size);
 
-    construction_tx.Commit();
-    move_construciton_tx.Commit();
+    std::move(construction_tx).Commit();
+    std::move(move_construction_tx).Commit();
     DeallocateIfAllocated();
-    AcquireAllocatedData(&allocation_tx);
+    SetAllocation(std::move(allocation_tx).Release());
 
     SetAllocatedSize(new_size);
-    return iterator(new_data + insert_index);
+    return Iterator<A>(new_data + insert_index);
   } else {
-    size_type move_construction_destination_index =
+    SizeType<A> move_construction_destination_index =
         (std::max)(insert_end_index, storage_view.size);
 
-    ConstructionTransaction move_construction_tx(GetAllocPtr());
+    ConstructionTransaction<A> move_construction_tx(GetAllocator());
 
-    IteratorValueAdapter<MoveIterator> move_construction_values(
-        MoveIterator(storage_view.data +
-                     (move_construction_destination_index - insert_count)));
-    absl::Span<value_type> move_construction = {
+    IteratorValueAdapter<A, MoveIterator<A>> move_construction_values(
+        MoveIterator<A>(storage_view.data +
+                        (move_construction_destination_index - insert_count)));
+    absl::Span<ValueType<A>> move_construction = {
         storage_view.data + move_construction_destination_index,
         new_size - move_construction_destination_index};
 
-    pointer move_assignment_values = storage_view.data + insert_index;
-    absl::Span<value_type> move_assignment = {
+    Pointer<A> move_assignment_values = storage_view.data + insert_index;
+    absl::Span<ValueType<A>> move_assignment = {
         storage_view.data + insert_end_index,
         move_construction_destination_index - insert_end_index};
 
-    absl::Span<value_type> insert_assignment = {move_assignment_values,
-                                                move_construction.size()};
+    absl::Span<ValueType<A>> insert_assignment = {move_assignment_values,
+                                                  move_construction.size()};
 
-    absl::Span<value_type> insert_construction = {
+    absl::Span<ValueType<A>> insert_construction = {
         insert_assignment.data() + insert_assignment.size(),
         insert_count - insert_assignment.size()};
 
     move_construction_tx.Construct(move_construction.data(),
-                                   &move_construction_values,
+                                   move_construction_values,
                                    move_construction.size());
 
-    for (pointer destination = move_assignment.data() + move_assignment.size(),
-                 last_destination = move_assignment.data(),
-                 source = move_assignment_values + move_assignment.size();
-         ;) {
-      --destination;
-      --source;
-      if (destination < last_destination) break;
-      *destination = std::move(*source);
-    }
+    std::move_backward(move_assignment_values,
+                       move_assignment_values + move_assignment.size(),
+                       move_assignment.data() + move_assignment.size());
 
-    inlined_vector_internal::AssignElements(insert_assignment.data(), &values,
-                                            insert_assignment.size());
+    AssignElements<A>(insert_assignment.data(), values,
+                      insert_assignment.size());
 
-    inlined_vector_internal::ConstructElements(
-        GetAllocPtr(), insert_construction.data(), &values,
-        insert_construction.size());
+    ConstructElements<A>(GetAllocator(), insert_construction.data(), values,
+                         insert_construction.size());
 
-    move_construction_tx.Commit();
+    std::move(move_construction_tx).Commit();
 
     AddSize(insert_count);
-    return iterator(storage_view.data + insert_index);
+    return Iterator<A>(storage_view.data + insert_index);
   }
 }
 
 template <typename T, size_t N, typename A>
 template <typename... Args>
-auto Storage<T, N, A>::EmplaceBack(Args&&... args) -> reference {
-  StorageView storage_view = MakeStorageView();
-
-  AllocationTransaction allocation_tx(GetAllocPtr());
-
-  IteratorValueAdapter<MoveIterator> move_values(
-      MoveIterator(storage_view.data));
-
-  pointer construct_data;
-  if (storage_view.size == storage_view.capacity) {
-    size_type new_capacity = NextCapacity(storage_view.capacity);
-    pointer new_data = allocation_tx.Allocate(new_capacity);
-
-    construct_data = new_data;
-  } else {
-    construct_data = storage_view.data;
+auto Storage<T, N, A>::EmplaceBack(Args&&... args) -> Reference<A> {
+  StorageView<A> storage_view = MakeStorageView();
+  const SizeType<A> n = storage_view.size;
+  if (ABSL_PREDICT_TRUE(n != storage_view.capacity)) {
+    // Fast path; new element fits.
+    Pointer<A> last_ptr = storage_view.data + n;
+    AllocatorTraits<A>::construct(GetAllocator(), last_ptr,
+                                  std::forward<Args>(args)...);
+    AddSize(1);
+    return *last_ptr;
   }
+  // TODO(b/173712035): Annotate with musttail attribute to prevent regression.
+  return EmplaceBackSlow(std::forward<Args>(args)...);
+}
 
-  AllocatorTraits::construct(*GetAllocPtr(), construct_data + storage_view.size,
-                             std::forward<Args>(args)...);
-
-  if (allocation_tx.DidAllocate()) {
-    ABSL_INTERNAL_TRY {
-      inlined_vector_internal::ConstructElements(
-          GetAllocPtr(), allocation_tx.GetData(), &move_values,
-          storage_view.size);
-    }
-    ABSL_INTERNAL_CATCH_ANY {
-      AllocatorTraits::destroy(*GetAllocPtr(),
-                               construct_data + storage_view.size);
-      ABSL_INTERNAL_RETHROW;
-    }
-
-    inlined_vector_internal::DestroyElements(GetAllocPtr(), storage_view.data,
-                                             storage_view.size);
-
-    DeallocateIfAllocated();
-    AcquireAllocatedData(&allocation_tx);
-    SetIsAllocated();
+template <typename T, size_t N, typename A>
+template <typename... Args>
+auto Storage<T, N, A>::EmplaceBackSlow(Args&&... args) -> Reference<A> {
+  StorageView<A> storage_view = MakeStorageView();
+  AllocationTransaction<A> allocation_tx(GetAllocator());
+  IteratorValueAdapter<A, MoveIterator<A>> move_values(
+      MoveIterator<A>(storage_view.data));
+  SizeType<A> requested_capacity = NextCapacity(storage_view.capacity);
+  Pointer<A> construct_data = allocation_tx.Allocate(requested_capacity);
+  Pointer<A> last_ptr = construct_data + storage_view.size;
+
+  // Construct new element.
+  AllocatorTraits<A>::construct(GetAllocator(), last_ptr,
+                                std::forward<Args>(args)...);
+  // Move elements from old backing store to new backing store.
+  ABSL_INTERNAL_TRY {
+    ConstructElements<A>(GetAllocator(), allocation_tx.GetData(), move_values,
+                         storage_view.size);
   }
+  ABSL_INTERNAL_CATCH_ANY {
+    AllocatorTraits<A>::destroy(GetAllocator(), last_ptr);
+    ABSL_INTERNAL_RETHROW;
+  }
+  // Destroy elements in old backing store.
+  DestroyAdapter<A>::DestroyElements(GetAllocator(), storage_view.data,
+                                     storage_view.size);
 
+  DeallocateIfAllocated();
+  SetAllocation(std::move(allocation_tx).Release());
+  SetIsAllocated();
   AddSize(1);
-  return *(construct_data + storage_view.size);
+  return *last_ptr;
 }
 
 template <typename T, size_t N, typename A>
-auto Storage<T, N, A>::Erase(const_iterator from, const_iterator to)
-    -> iterator {
-  assert(from != to);
-
-  StorageView storage_view = MakeStorageView();
-
-  size_type erase_size = std::distance(from, to);
-  size_type erase_index =
-      std::distance(const_iterator(storage_view.data), from);
-  size_type erase_end_index = erase_index + erase_size;
-
-  IteratorValueAdapter<MoveIterator> move_values(
-      MoveIterator(storage_view.data + erase_end_index));
-
-  inlined_vector_internal::AssignElements(storage_view.data + erase_index,
-                                          &move_values,
-                                          storage_view.size - erase_end_index);
+auto Storage<T, N, A>::Erase(ConstIterator<A> from,
+                             ConstIterator<A> to) -> Iterator<A> {
+  StorageView<A> storage_view = MakeStorageView();
+
+  auto erase_size = static_cast<SizeType<A>>(std::distance(from, to));
+  auto erase_index = static_cast<SizeType<A>>(
+      std::distance(ConstIterator<A>(storage_view.data), from));
+  SizeType<A> erase_end_index = erase_index + erase_size;
+
+  // Fast path: if the value type is trivially relocatable and we know
+  // the allocator doesn't do anything fancy, then we know it is legal for us to
+  // simply destroy the elements in the "erasure window" (which cannot throw)
+  // and then memcpy downward to close the window.
+  if (absl::is_trivially_relocatable<ValueType<A>>::value &&
+      std::is_nothrow_destructible_v<ValueType<A>> &&
+      std::is_same_v<A, std::allocator<ValueType<A>>>) {
+    DestroyAdapter<A>::DestroyElements(
+        GetAllocator(), storage_view.data + erase_index, erase_size);
+    std::memmove(
+        reinterpret_cast<char*>(storage_view.data + erase_index),
+        reinterpret_cast<const char*>(storage_view.data + erase_end_index),
+        (storage_view.size - erase_end_index) * sizeof(ValueType<A>));
+  } else {
+    IteratorValueAdapter<A, MoveIterator<A>> move_values(
+        MoveIterator<A>(storage_view.data + erase_end_index));
 
-  inlined_vector_internal::DestroyElements(
-      GetAllocPtr(), storage_view.data + (storage_view.size - erase_size),
-      erase_size);
+    AssignElements<A>(storage_view.data + erase_index, move_values,
+                      storage_view.size - erase_end_index);
 
+    DestroyAdapter<A>::DestroyElements(
+        GetAllocator(), storage_view.data + (storage_view.size - erase_size),
+        erase_size);
+  }
   SubtractSize(erase_size);
-  return iterator(storage_view.data + erase_index);
+  return Iterator<A>(storage_view.data + erase_index);
 }
 
 template <typename T, size_t N, typename A>
-auto Storage<T, N, A>::Reserve(size_type requested_capacity) -> void {
-  StorageView storage_view = MakeStorageView();
+auto Storage<T, N, A>::Reserve(SizeType<A> requested_capacity) -> void {
+  StorageView<A> storage_view = MakeStorageView();
 
   if (ABSL_PREDICT_FALSE(requested_capacity <= storage_view.capacity)) return;
 
-  AllocationTransaction allocation_tx(GetAllocPtr());
+  AllocationTransaction<A> allocation_tx(GetAllocator());
 
-  IteratorValueAdapter<MoveIterator> move_values(
-      MoveIterator(storage_view.data));
+  IteratorValueAdapter<A, MoveIterator<A>> move_values(
+      MoveIterator<A>(storage_view.data));
 
-  size_type new_capacity =
+  SizeType<A> new_requested_capacity =
       ComputeCapacity(storage_view.capacity, requested_capacity);
-  pointer new_data = allocation_tx.Allocate(new_capacity);
+  Pointer<A> new_data = allocation_tx.Allocate(new_requested_capacity);
 
-  inlined_vector_internal::ConstructElements(GetAllocPtr(), new_data,
-                                             &move_values, storage_view.size);
+  ConstructElements<A>(GetAllocator(), new_data, move_values,
+                       storage_view.size);
 
-  inlined_vector_internal::DestroyElements(GetAllocPtr(), storage_view.data,
-                                           storage_view.size);
+  DestroyAdapter<A>::DestroyElements(GetAllocator(), storage_view.data,
+                                     storage_view.size);
 
   DeallocateIfAllocated();
-  AcquireAllocatedData(&allocation_tx);
+  SetAllocation(std::move(allocation_tx).Release());
   SetIsAllocated();
 }
 
 template <typename T, size_t N, typename A>
 auto Storage<T, N, A>::ShrinkToFit() -> void {
   // May only be called on allocated instances!
-  assert(GetIsAllocated());
+  ABSL_ASSERT(GetIsAllocated());
 
-  StorageView storage_view{GetAllocatedData(), GetSize(),
-                           GetAllocatedCapacity()};
+  StorageView<A> storage_view{GetAllocatedData(), GetSize(),
+                              GetAllocatedCapacity()};
 
   if (ABSL_PREDICT_FALSE(storage_view.size == storage_view.capacity)) return;
 
-  AllocationTransaction allocation_tx(GetAllocPtr());
+  AllocationTransaction<A> allocation_tx(GetAllocator());
 
-  IteratorValueAdapter<MoveIterator> move_values(
-      MoveIterator(storage_view.data));
+  IteratorValueAdapter<A, MoveIterator<A>> move_values(
+      MoveIterator<A>(storage_view.data));
 
-  pointer construct_data;
+  Pointer<A> construct_data;
   if (storage_view.size > GetInlinedCapacity()) {
-    size_type new_capacity = storage_view.size;
-    pointer new_data = allocation_tx.Allocate(new_capacity);
-
-    construct_data = new_data;
+    SizeType<A> requested_capacity = storage_view.size;
+    construct_data = allocation_tx.Allocate(requested_capacity);
+    if (allocation_tx.GetCapacity() >= storage_view.capacity) {
+      // Already using the smallest available heap allocation.
+      return;
+    }
   } else {
     construct_data = GetInlinedData();
   }
 
   ABSL_INTERNAL_TRY {
-    inlined_vector_internal::ConstructElements(GetAllocPtr(), construct_data,
-                                               &move_values, storage_view.size);
+    ConstructElements<A>(GetAllocator(), construct_data, move_values,
+                         storage_view.size);
   }
   ABSL_INTERNAL_CATCH_ANY {
-    SetAllocatedData(storage_view.data, storage_view.capacity);
+    SetAllocation({storage_view.data, storage_view.capacity});
     ABSL_INTERNAL_RETHROW;
   }
 
-  inlined_vector_internal::DestroyElements(GetAllocPtr(), storage_view.data,
-                                           storage_view.size);
+  DestroyAdapter<A>::DestroyElements(GetAllocator(), storage_view.data,
+                                     storage_view.size);
 
-  AllocatorTraits::deallocate(*GetAllocPtr(), storage_view.data,
-                              storage_view.capacity);
+  MallocAdapter<A>::Deallocate(GetAllocator(), storage_view.data,
+                               storage_view.capacity);
 
   if (allocation_tx.DidAllocate()) {
-    AcquireAllocatedData(&allocation_tx);
+    SetAllocation(std::move(allocation_tx).Release());
   } else {
     UnsetIsAllocated();
   }
@@ -827,67 +978,128 @@ auto Storage<T, N, A>::ShrinkToFit() -> void {
 template <typename T, size_t N, typename A>
 auto Storage<T, N, A>::Swap(Storage* other_storage_ptr) -> void {
   using std::swap;
-  assert(this != other_storage_ptr);
+  ABSL_ASSERT(this != other_storage_ptr);
 
   if (GetIsAllocated() && other_storage_ptr->GetIsAllocated()) {
     swap(data_.allocated, other_storage_ptr->data_.allocated);
   } else if (!GetIsAllocated() && !other_storage_ptr->GetIsAllocated()) {
-    Storage* small_ptr = this;
-    Storage* large_ptr = other_storage_ptr;
-    if (small_ptr->GetSize() > large_ptr->GetSize()) swap(small_ptr, large_ptr);
-
-    for (size_type i = 0; i < small_ptr->GetSize(); ++i) {
-      swap(small_ptr->GetInlinedData()[i], large_ptr->GetInlinedData()[i]);
-    }
-
-    IteratorValueAdapter<MoveIterator> move_values(
-        MoveIterator(large_ptr->GetInlinedData() + small_ptr->GetSize()));
-
-    inlined_vector_internal::ConstructElements(
-        large_ptr->GetAllocPtr(),
-        small_ptr->GetInlinedData() + small_ptr->GetSize(), &move_values,
-        large_ptr->GetSize() - small_ptr->GetSize());
-
-    inlined_vector_internal::DestroyElements(
-        large_ptr->GetAllocPtr(),
-        large_ptr->GetInlinedData() + small_ptr->GetSize(),
-        large_ptr->GetSize() - small_ptr->GetSize());
+    SwapInlinedElements(SwapInlinedElementsPolicy{}, other_storage_ptr);
   } else {
     Storage* allocated_ptr = this;
     Storage* inlined_ptr = other_storage_ptr;
     if (!allocated_ptr->GetIsAllocated()) swap(allocated_ptr, inlined_ptr);
 
-    StorageView allocated_storage_view{allocated_ptr->GetAllocatedData(),
-                                       allocated_ptr->GetSize(),
-                                       allocated_ptr->GetAllocatedCapacity()};
+    StorageView<A> allocated_storage_view{
+        allocated_ptr->GetAllocatedData(), allocated_ptr->GetSize(),
+        allocated_ptr->GetAllocatedCapacity()};
 
-    IteratorValueAdapter<MoveIterator> move_values(
-        MoveIterator(inlined_ptr->GetInlinedData()));
+    IteratorValueAdapter<A, MoveIterator<A>> move_values(
+        MoveIterator<A>(inlined_ptr->GetInlinedData()));
 
     ABSL_INTERNAL_TRY {
-      inlined_vector_internal::ConstructElements(
-          inlined_ptr->GetAllocPtr(), allocated_ptr->GetInlinedData(),
-          &move_values, inlined_ptr->GetSize());
+      ConstructElements<A>(inlined_ptr->GetAllocator(),
+                           allocated_ptr->GetInlinedData(), move_values,
+                           inlined_ptr->GetSize());
     }
     ABSL_INTERNAL_CATCH_ANY {
-      allocated_ptr->SetAllocatedData(allocated_storage_view.data,
-                                      allocated_storage_view.capacity);
+      allocated_ptr->SetAllocation(Allocation<A>{
+          allocated_storage_view.data, allocated_storage_view.capacity});
       ABSL_INTERNAL_RETHROW;
     }
 
-    inlined_vector_internal::DestroyElements(inlined_ptr->GetAllocPtr(),
-                                             inlined_ptr->GetInlinedData(),
-                                             inlined_ptr->GetSize());
+    DestroyAdapter<A>::DestroyElements(inlined_ptr->GetAllocator(),
+                                       inlined_ptr->GetInlinedData(),
+                                       inlined_ptr->GetSize());
 
-    inlined_ptr->SetAllocatedData(allocated_storage_view.data,
-                                  allocated_storage_view.capacity);
+    inlined_ptr->SetAllocation(Allocation<A>{allocated_storage_view.data,
+                                             allocated_storage_view.capacity});
   }
 
   swap(GetSizeAndIsAllocated(), other_storage_ptr->GetSizeAndIsAllocated());
-  swap(*GetAllocPtr(), *other_storage_ptr->GetAllocPtr());
+  swap(GetAllocator(), other_storage_ptr->GetAllocator());
+}
+
+template <typename T, size_t N, typename A>
+void Storage<T, N, A>::SwapN(ElementwiseSwapPolicy, Storage* other,
+                             SizeType<A> n) {
+  std::swap_ranges(GetInlinedData(), GetInlinedData() + n,
+                   other->GetInlinedData());
 }
 
+template <typename T, size_t N, typename A>
+void Storage<T, N, A>::SwapN(ElementwiseConstructPolicy, Storage* other,
+                             SizeType<A> n) {
+  Pointer<A> a = GetInlinedData();
+  Pointer<A> b = other->GetInlinedData();
+  // see note on allocators in `SwapInlinedElements`.
+  A& allocator_a = GetAllocator();
+  A& allocator_b = other->GetAllocator();
+  for (SizeType<A> i = 0; i < n; ++i, ++a, ++b) {
+    ValueType<A> tmp(std::move(*a));
+
+    AllocatorTraits<A>::destroy(allocator_a, a);
+    ABSL_INTERNAL_TRY {
+      AllocatorTraits<A>::construct(allocator_b, a, std::move(*b));
+    }
+    ABSL_INTERNAL_CATCH_ANY {
+      AllocatorTraits<A>::construct(allocator_a, a, std::move(tmp));
+      ABSL_INTERNAL_RETHROW;
+    }
+
+    AllocatorTraits<A>::destroy(allocator_b, b);
+    ABSL_INTERNAL_TRY {
+      AllocatorTraits<A>::construct(allocator_a, b, std::move(tmp));
+    }
+    ABSL_INTERNAL_CATCH_ANY {
+      AllocatorTraits<A>::construct(allocator_b, b, std::move(*a));
+      ABSL_INTERNAL_RETHROW;
+    }
+  }
+}
+
+template <typename T, size_t N, typename A>
+void Storage<T, N, A>::SwapInlinedElements(MemcpyPolicy, Storage* other) {
+  Data tmp = data_;
+  data_ = other->data_;
+  other->data_ = tmp;
+}
+
+template <typename T, size_t N, typename A>
+template <typename NotMemcpyPolicy>
+void Storage<T, N, A>::SwapInlinedElements(NotMemcpyPolicy policy,
+                                           Storage* other) {
+  // Note: `destroy` needs to use pre-swap allocator while `construct` -
+  // post-swap allocator. Allocators will be swapped later on outside of
+  // `SwapInlinedElements`.
+  Storage* small_ptr = this;
+  Storage* large_ptr = other;
+  if (small_ptr->GetSize() > large_ptr->GetSize()) {
+    std::swap(small_ptr, large_ptr);
+  }
+
+  auto small_size = small_ptr->GetSize();
+  auto diff = large_ptr->GetSize() - small_size;
+  SwapN(policy, other, small_size);
+
+  IteratorValueAdapter<A, MoveIterator<A>> move_values(
+      MoveIterator<A>(large_ptr->GetInlinedData() + small_size));
+
+  ConstructElements<A>(large_ptr->GetAllocator(),
+                       small_ptr->GetInlinedData() + small_size, move_values,
+                       diff);
+
+  DestroyAdapter<A>::DestroyElements(large_ptr->GetAllocator(),
+                                     large_ptr->GetInlinedData() + small_size,
+                                     diff);
+}
+
+// End ignore "array-bounds"
+#if !defined(__clang__) && defined(__GNUC__)
+#pragma GCC diagnostic pop
+#endif
+
 }  // namespace inlined_vector_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
-#endif  // ABSL_CONTAINER_INTERNAL_INLINED_VECTOR_INTERNAL_H_
+#endif  // ABSL_CONTAINER_INTERNAL_INLINED_VECTOR_H_
diff --git a/absl/container/internal/layout.h b/absl/container/internal/layout.h
index bbdde50796e..0f9084ceb86 100644
--- a/absl/container/internal/layout.h
+++ b/absl/container/internal/layout.h
@@ -55,7 +55,7 @@
 // `Partial()` comes in handy when the array sizes are embedded into the
 // allocation.
 //
-//   // size_t[1] containing N, size_t[1] containing M, double[N], int[M].
+//   // size_t[0] containing N, size_t[1] containing M, double[N], int[M].
 //   using L = Layout<size_t, size_t, double, int>;
 //
 //   unsigned char* Allocate(size_t n, size_t m) {
@@ -81,9 +81,30 @@
 //   }
 //
 // The layout we used above combines fixed-size with dynamically-sized fields.
-// This is quite common. Layout is optimized for this use case and generates
-// optimal code. All computations that can be performed at compile time are
-// indeed performed at compile time.
+// This is quite common. Layout is optimized for this use case and attempts to
+// generate optimal code. To help the compiler do that in more cases, you can
+// specify the fixed sizes using `WithStaticSizes`. This ensures that all
+// computations that can be performed at compile time are indeed performed at
+// compile time. Note that sometimes the `template` keyword is needed. E.g.:
+//
+//   using SL = L::template WithStaticSizes<1, 1>;
+//
+//   void Use(unsigned char* p) {
+//     // First, extract N and M.
+//     // Using `prefix` we can access the first three arrays but not more.
+//     //
+//     // More details: The first element always has offset 0. `SL`
+//     // has offsets for the second and third array based on sizes of
+//     // the first and second array, specified via `WithStaticSizes`.
+//     constexpr auto prefix = SL::Partial();
+//     size_t n = *prefix.Pointer<0>(p);
+//     size_t m = *prefix.Pointer<1>(p);
+//
+//     // Now we can get a pointer to the final payload.
+//     const SL layout(n, m);
+//     double* a = layout.Pointer<double>(p);
+//     int* b = layout.Pointer<int>(p);
+//   }
 //
 // Efficiency tip: The order of fields matters. In `Layout<T1, ..., TN>` try to
 // ensure that `alignof(T1) >= ... >= alignof(TN)`. This way you'll have no
@@ -107,7 +128,7 @@
 //     CompactString(const char* s = "") {
 //       const size_t size = strlen(s);
 //       // size_t[1] followed by char[size + 1].
-//       const L layout(1, size + 1);
+//       const L layout(size + 1);
 //       p_.reset(new unsigned char[layout.AllocSize()]);
 //       // If running under ASAN, mark the padding bytes, if any, to catch
 //       // memory errors.
@@ -125,14 +146,13 @@
 //
 //     const char* c_str() const {
 //       // Equivalent to reinterpret_cast<char*>(p.get() + sizeof(size_t)).
-//       // The argument in Partial(1) specifies that we have size_t[1] in front
-//       // of the characters.
-//       return L::Partial(1).Pointer<char>(p_.get());
+//       return L::Partial().Pointer<char>(p_.get());
 //     }
 //
 //    private:
-//     // Our heap allocation contains a size_t followed by an array of chars.
-//     using L = Layout<size_t, char>;
+//     // Our heap allocation contains a single size_t followed by an array of
+//     // chars.
+//     using L = Layout<size_t, char>::WithStaticSizes<1>;
 //     std::unique_ptr<unsigned char[]> p_;
 //   };
 //
@@ -146,11 +166,12 @@
 //
 // The interface exported by this file consists of:
 // - class `Layout<>` and its public members.
-// - The public members of class `internal_layout::LayoutImpl<>`. That class
-//   isn't intended to be used directly, and its name and template parameter
-//   list are internal implementation details, but the class itself provides
-//   most of the functionality in this file. See comments on its members for
-//   detailed documentation.
+// - The public members of classes `internal_layout::LayoutWithStaticSizes<>`
+//   and `internal_layout::LayoutImpl<>`. Those classes aren't intended to be
+//   used directly, and their name and template parameter list are internal
+//   implementation details, but the classes themselves provide most of the
+//   functionality in this file. See comments on their members for detailed
+//   documentation.
 //
 // `Layout<T1,... Tn>::Partial(count1,..., countm)` (where `m` <= `n`) returns a
 // `LayoutImpl<>` object. `Layout<T1,..., Tn> layout(count1,..., countn)`
@@ -163,31 +184,27 @@
 #include <assert.h>
 #include <stddef.h>
 #include <stdint.h>
-#include <ostream>
+
+#include <array>
 #include <string>
 #include <tuple>
 #include <type_traits>
 #include <typeinfo>
 #include <utility>
 
-#ifdef ADDRESS_SANITIZER
-#include <sanitizer/asan_interface.h>
-#endif
-
+#include "absl/base/config.h"
+#include "absl/debugging/internal/demangle.h"
 #include "absl/meta/type_traits.h"
 #include "absl/strings/str_cat.h"
 #include "absl/types/span.h"
 #include "absl/utility/utility.h"
 
-#if defined(__GXX_RTTI)
-#define ABSL_INTERNAL_HAS_CXA_DEMANGLE
-#endif
-
-#ifdef ABSL_INTERNAL_HAS_CXA_DEMANGLE
-#include <cxxabi.h>
+#ifdef ABSL_HAVE_ADDRESS_SANITIZER
+#include <sanitizer/asan_interface.h>
 #endif
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
 // A type wrapper that instructs `Layout` to use the specific alignment for the
@@ -213,9 +230,6 @@ struct NotAligned<const Aligned<T, N>> {
 template <size_t>
 using IntToSize = size_t;
 
-template <class>
-using TypeToSize = size_t;
-
 template <class T>
 struct Type : NotAligned<T> {
   using type = T;
@@ -247,11 +261,10 @@ struct AlignOf<Aligned<T, N>> {
 
 // Does `Ts...` contain `T`?
 template <class T, class... Ts>
-using Contains = absl::disjunction<std::is_same<T, Ts>...>;
+using Contains = std::disjunction<std::is_same<T, Ts>...>;
 
 template <class From, class To>
-using CopyConst =
-    typename std::conditional<std::is_const<From>::value, const To, To>::type;
+using CopyConst = std::conditional_t<std::is_const_v<From>, const To, To>;
 
 // Note: We're not qualifying this with absl:: because it doesn't compile under
 // MSVC.
@@ -291,36 +304,26 @@ constexpr size_t Max(size_t a, size_t b, Ts... rest) {
 template <class T>
 std::string TypeName() {
   std::string out;
-  int status = 0;
-  char* demangled = nullptr;
-#ifdef ABSL_INTERNAL_HAS_CXA_DEMANGLE
-  demangled = abi::__cxa_demangle(typeid(T).name(), nullptr, nullptr, &status);
-#endif
-  if (status == 0 && demangled != nullptr) {  // Demangling succeeded.
-    absl::StrAppend(&out, "<", demangled, ">");
-    free(demangled);
-  } else {
-#if defined(__GXX_RTTI) || defined(_CPPRTTI)
-    absl::StrAppend(&out, "<", typeid(T).name(), ">");
+#ifdef ABSL_INTERNAL_HAS_RTTI
+  absl::StrAppend(&out, "<",
+                  absl::debugging_internal::DemangleString(typeid(T).name()),
+                  ">");
 #endif
-  }
   return out;
 }
 
 }  // namespace adl_barrier
 
-template <bool C>
-using EnableIf = typename std::enable_if<C, int>::type;
-
 // Can `T` be a template argument of `Layout`?
 template <class T>
-using IsLegalElementType = std::integral_constant<
-    bool, !std::is_reference<T>::value && !std::is_volatile<T>::value &&
-              !std::is_reference<typename Type<T>::type>::value &&
-              !std::is_volatile<typename Type<T>::type>::value &&
-              adl_barrier::IsPow2(AlignOf<T>::value)>;
-
-template <class Elements, class SizeSeq, class OffsetSeq>
+using IsLegalElementType =
+    std::bool_constant<!std::is_reference_v<T> && !std::is_volatile_v<T> &&
+                       !std::is_reference_v<typename Type<T>::type> &&
+                       !std::is_volatile_v<typename Type<T>::type> &&
+                       adl_barrier::IsPow2(AlignOf<T>::value)>;
+
+template <class Elements, class StaticSizeSeq, class RuntimeSizeSeq,
+          class SizeSeq, class OffsetSeq>
 class LayoutImpl;
 
 // Public base class of `Layout` and the result type of `Layout::Partial()`.
@@ -328,31 +331,49 @@ class LayoutImpl;
 // `Elements...` contains all template arguments of `Layout` that created this
 // instance.
 //
-// `SizeSeq...` is `[0, NumSizes)` where `NumSizes` is the number of arguments
-// passed to `Layout::Partial()` or `Layout::Layout()`.
+// `StaticSizeSeq...` is an index_sequence containing the sizes specified at
+// compile-time.
+//
+// `RuntimeSizeSeq...` is `[0, NumRuntimeSizes)`, where `NumRuntimeSizes` is the
+// number of arguments passed to `Layout::Partial()` or `Layout::Layout()`.
+//
+// `SizeSeq...` is `[0, NumSizes)` where `NumSizes` is `NumRuntimeSizes` plus
+// the number of sizes in `StaticSizeSeq`.
 //
 // `OffsetSeq...` is `[0, NumOffsets)` where `NumOffsets` is
 // `Min(sizeof...(Elements), NumSizes + 1)` (the number of arrays for which we
 // can compute offsets).
-template <class... Elements, size_t... SizeSeq, size_t... OffsetSeq>
-class LayoutImpl<std::tuple<Elements...>, absl::index_sequence<SizeSeq...>,
-                 absl::index_sequence<OffsetSeq...>> {
+template <class... Elements, size_t... StaticSizeSeq, size_t... RuntimeSizeSeq,
+          size_t... SizeSeq, size_t... OffsetSeq>
+class LayoutImpl<std::tuple<Elements...>, std::index_sequence<StaticSizeSeq...>,
+                 std::index_sequence<RuntimeSizeSeq...>,
+                 std::index_sequence<SizeSeq...>,
+                 std::index_sequence<OffsetSeq...>> {
  private:
   static_assert(sizeof...(Elements) > 0, "At least one field is required");
-  static_assert(absl::conjunction<IsLegalElementType<Elements>...>::value,
+  static_assert(std::conjunction_v<IsLegalElementType<Elements>...>,
                 "Invalid element type (see IsLegalElementType)");
+  static_assert(sizeof...(StaticSizeSeq) <= sizeof...(Elements),
+                "Too many static sizes specified");
 
   enum {
     NumTypes = sizeof...(Elements),
+    NumStaticSizes = sizeof...(StaticSizeSeq),
+    NumRuntimeSizes = sizeof...(RuntimeSizeSeq),
     NumSizes = sizeof...(SizeSeq),
     NumOffsets = sizeof...(OffsetSeq),
   };
 
   // These are guaranteed by `Layout`.
+  static_assert(NumStaticSizes + NumRuntimeSizes == NumSizes, "Internal error");
+  static_assert(NumSizes <= NumTypes, "Internal error");
   static_assert(NumOffsets == adl_barrier::Min(NumTypes, NumSizes + 1),
                 "Internal error");
   static_assert(NumTypes > 0, "Internal error");
 
+  static constexpr std::array<size_t, sizeof...(StaticSizeSeq)> kStaticSizes = {
+      StaticSizeSeq...};
+
   // Returns the index of `T` in `Elements...`. Results in a compilation error
   // if `Elements...` doesn't contain exactly one instance of `T`.
   template <class T>
@@ -365,7 +386,7 @@ class LayoutImpl<std::tuple<Elements...>, absl::index_sequence<SizeSeq...>,
 
   template <size_t N>
   using ElementAlignment =
-      AlignOf<typename std::tuple_element<N, std::tuple<Elements...>>::type>;
+      AlignOf<std::tuple_element_t<N, std::tuple<Elements...>>>;
 
  public:
   // Element types of all arrays packed in a tuple.
@@ -373,9 +394,9 @@ class LayoutImpl<std::tuple<Elements...>, absl::index_sequence<SizeSeq...>,
 
   // Element type of the Nth array.
   template <size_t N>
-  using ElementType = typename std::tuple_element<N, ElementTypes>::type;
+  using ElementType = std::tuple_element_t<N, ElementTypes>;
 
-  constexpr explicit LayoutImpl(IntToSize<SizeSeq>... sizes)
+  constexpr explicit LayoutImpl(IntToSize<RuntimeSizeSeq>... sizes)
       : size_{sizes...} {}
 
   // Alignment of the layout, equal to the strictest alignment of all elements.
@@ -392,17 +413,16 @@ class LayoutImpl<std::tuple<Elements...>, absl::index_sequence<SizeSeq...>,
   //   assert(x.Offset<1>() == 16);  // The doubles starts from 16.
   //
   // Requires: `N <= NumSizes && N < sizeof...(Ts)`.
-  template <size_t N, EnableIf<N == 0> = 0>
-  constexpr size_t Offset() const {
-    return 0;
-  }
-
-  template <size_t N, EnableIf<N != 0> = 0>
+  template <size_t N>
   constexpr size_t Offset() const {
-    static_assert(N < NumOffsets, "Index out of bounds");
-    return adl_barrier::Align(
-        Offset<N - 1>() + SizeOf<ElementType<N - 1>>() * size_[N - 1],
-        ElementAlignment<N>::value);
+    if constexpr (N == 0) {
+      return 0;
+    } else {
+      static_assert(N < NumOffsets, "Index out of bounds");
+      return adl_barrier::Align(
+          Offset<N - 1>() + SizeOf<ElementType<N - 1>>::value * Size<N - 1>(),
+          ElementAlignment<N>::value);
+    }
   }
 
   // Offset in bytes of the array with the specified element type. There must
@@ -423,8 +443,7 @@ class LayoutImpl<std::tuple<Elements...>, absl::index_sequence<SizeSeq...>,
     return {{Offset<OffsetSeq>()...}};
   }
 
-  // The number of elements in the Nth array. This is the Nth argument of
-  // `Layout::Partial()` or `Layout::Layout()` (zero-based).
+  // The number of elements in the Nth array (zero-based).
   //
   //   // int[3], 4 bytes of padding, double[4].
   //   Layout<int, double> x(3, 4);
@@ -434,8 +453,12 @@ class LayoutImpl<std::tuple<Elements...>, absl::index_sequence<SizeSeq...>,
   // Requires: `N < NumSizes`.
   template <size_t N>
   constexpr size_t Size() const {
-    static_assert(N < NumSizes, "Index out of bounds");
-    return size_[N];
+    if constexpr (N < NumStaticSizes) {
+      return kStaticSizes[N];
+    } else {
+      static_assert(N < NumSizes, "Index out of bounds");
+      return size_[N - NumStaticSizes];
+    }
   }
 
   // The number of elements in the array with the specified element type.
@@ -470,7 +493,7 @@ class LayoutImpl<std::tuple<Elements...>, absl::index_sequence<SizeSeq...>,
   // Requires: `p` is aligned to `Alignment()`.
   template <size_t N, class Char>
   CopyConst<Char, ElementType<N>>* Pointer(Char* p) const {
-    using C = typename std::remove_const<Char>::type;
+    using C = std::remove_const_t<Char>;
     static_assert(
         std::is_same<C, char>() || std::is_same<C, unsigned char>() ||
             std::is_same<C, signed char>(),
@@ -512,13 +535,8 @@ class LayoutImpl<std::tuple<Elements...>, absl::index_sequence<SizeSeq...>,
   //   std::tie(ints, doubles) = x.Pointers(p);
   //
   // Requires: `p` is aligned to `Alignment()`.
-  //
-  // Note: We're not using ElementType alias here because it does not compile
-  // under MSVC.
   template <class Char>
-  std::tuple<CopyConst<
-      Char, typename std::tuple_element<OffsetSeq, ElementTypes>::type>*...>
-  Pointers(Char* p) const {
+  auto Pointers(Char* p) const {
     return std::tuple<CopyConst<Char, ElementType<OffsetSeq>>*...>(
         Pointer<OffsetSeq>(p)...);
   }
@@ -571,15 +589,10 @@ class LayoutImpl<std::tuple<Elements...>, absl::index_sequence<SizeSeq...>,
   //
   // Requires: `p` is aligned to `Alignment()`.
   //
-  // Note: We're not using ElementType alias here because it does not compile
-  // under MSVC.
+  // Note: We mark the parameter as maybe_unused because GCC detects it is not
+  // used when `SizeSeq` is empty [-Werror=unused-but-set-parameter].
   template <class Char>
-  std::tuple<SliceType<CopyConst<
-      Char, typename std::tuple_element<SizeSeq, ElementTypes>::type>>...>
-  Slices(Char* p) const {
-    // Workaround for https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63875 (fixed
-    // in 6.1).
-    (void)p;
+  auto Slices([[maybe_unused]] Char* p) const {
     return std::tuple<SliceType<CopyConst<Char, ElementType<SizeSeq>>>...>(
         Slice<SizeSeq>(p)...);
   }
@@ -594,7 +607,7 @@ class LayoutImpl<std::tuple<Elements...>, absl::index_sequence<SizeSeq...>,
   constexpr size_t AllocSize() const {
     static_assert(NumTypes == NumSizes, "You must specify sizes of all fields");
     return Offset<NumTypes - 1>() +
-           SizeOf<ElementType<NumTypes - 1>>() * size_[NumTypes - 1];
+           SizeOf<ElementType<NumTypes - 1>>::value * Size<NumTypes - 1>();
   }
 
   // If built with --config=asan, poisons padding bytes (if any) in the
@@ -604,24 +617,23 @@ class LayoutImpl<std::tuple<Elements...>, absl::index_sequence<SizeSeq...>,
   // `Char` must be `[const] [signed|unsigned] char`.
   //
   // Requires: `p` is aligned to `Alignment()`.
-  template <class Char, size_t N = NumOffsets - 1, EnableIf<N == 0> = 0>
-  void PoisonPadding(const Char* p) const {
-    Pointer<0>(p);  // verify the requirements on `Char` and `p`
-  }
-
-  template <class Char, size_t N = NumOffsets - 1, EnableIf<N != 0> = 0>
+  template <class Char, size_t N = NumOffsets - 1>
   void PoisonPadding(const Char* p) const {
-    static_assert(N < NumOffsets, "Index out of bounds");
-    (void)p;
-#ifdef ADDRESS_SANITIZER
+    if constexpr (N == 0) {
+      Pointer<0>(p);  // verify the requirements on `Char` and `p`
+    } else {
+      static_assert(N < NumOffsets, "Index out of bounds");
+      (void)p;
+#ifdef ABSL_HAVE_ADDRESS_SANITIZER
     PoisonPadding<Char, N - 1>(p);
     // The `if` is an optimization. It doesn't affect the observable behaviour.
     if (ElementAlignment<N - 1>::value % ElementAlignment<N>::value) {
       size_t start =
-          Offset<N - 1>() + SizeOf<ElementType<N - 1>>() * size_[N - 1];
+          Offset<N - 1>() + SizeOf<ElementType<N - 1>>::value * Size<N - 1>();
       ASAN_POISON_MEMORY_REGION(p + start, Offset<N>() - start);
     }
 #endif
+    }
   }
 
   // Human-readable description of the memory layout. Useful for debugging.
@@ -642,52 +654,61 @@ class LayoutImpl<std::tuple<Elements...>, absl::index_sequence<SizeSeq...>,
   // produce "unsigned*" where another produces "unsigned int *".
   std::string DebugString() const {
     const auto offsets = Offsets();
-    const size_t sizes[] = {SizeOf<ElementType<OffsetSeq>>()...};
+    const size_t sizes[] = {SizeOf<ElementType<OffsetSeq>>::value...};
     const std::string types[] = {
         adl_barrier::TypeName<ElementType<OffsetSeq>>()...};
     std::string res = absl::StrCat("@0", types[0], "(", sizes[0], ")");
     for (size_t i = 0; i != NumOffsets - 1; ++i) {
-      absl::StrAppend(&res, "[", size_[i], "]; @", offsets[i + 1], types[i + 1],
-                      "(", sizes[i + 1], ")");
+      absl::StrAppend(&res, "[", DebugSize(i), "]; @", offsets[i + 1],
+                      types[i + 1], "(", sizes[i + 1], ")");
     }
     // NumSizes is a constant that may be zero. Some compilers cannot see that
     // inside the if statement "size_[NumSizes - 1]" must be valid.
     int last = static_cast<int>(NumSizes) - 1;
     if (NumTypes == NumSizes && last >= 0) {
-      absl::StrAppend(&res, "[", size_[last], "]");
+      absl::StrAppend(&res, "[", DebugSize(static_cast<size_t>(last)), "]");
     }
     return res;
   }
 
  private:
+  size_t DebugSize(size_t n) const {
+    if (n < NumStaticSizes) {
+      return kStaticSizes[n];
+    } else {
+      return size_[n - NumStaticSizes];
+    }
+  }
+
   // Arguments of `Layout::Partial()` or `Layout::Layout()`.
-  size_t size_[NumSizes > 0 ? NumSizes : 1];
+  size_t size_[NumRuntimeSizes > 0 ? NumRuntimeSizes : 1];
 };
 
-template <size_t NumSizes, class... Ts>
+template <class StaticSizeSeq, size_t NumRuntimeSizes, class... Ts>
 using LayoutType = LayoutImpl<
-    std::tuple<Ts...>, absl::make_index_sequence<NumSizes>,
-    absl::make_index_sequence<adl_barrier::Min(sizeof...(Ts), NumSizes + 1)>>;
-
-}  // namespace internal_layout
+    std::tuple<Ts...>, StaticSizeSeq, std::make_index_sequence<NumRuntimeSizes>,
+    std::make_index_sequence<NumRuntimeSizes + StaticSizeSeq::size()>,
+    std::make_index_sequence<adl_barrier::Min(
+        sizeof...(Ts), NumRuntimeSizes + StaticSizeSeq::size() + 1)>>;
+
+template <class StaticSizeSeq, class... Ts>
+class LayoutWithStaticSizes
+    : public LayoutType<StaticSizeSeq,
+                        sizeof...(Ts) - adl_barrier::Min(sizeof...(Ts),
+                                                         StaticSizeSeq::size()),
+                        Ts...> {
+ private:
+  using Super =
+      LayoutType<StaticSizeSeq,
+                 sizeof...(Ts) -
+                     adl_barrier::Min(sizeof...(Ts), StaticSizeSeq::size()),
+                 Ts...>;
 
-// Descriptor of arrays of various types and sizes laid out in memory one after
-// another. See the top of the file for documentation.
-//
-// Check out the public API of internal_layout::LayoutImpl above. The type is
-// internal to the library but its methods are public, and they are inherited
-// by `Layout`.
-template <class... Ts>
-class Layout : public internal_layout::LayoutType<sizeof...(Ts), Ts...> {
  public:
-  static_assert(sizeof...(Ts) > 0, "At least one field is required");
-  static_assert(
-      absl::conjunction<internal_layout::IsLegalElementType<Ts>...>::value,
-      "Invalid element type (see IsLegalElementType)");
-
   // The result type of `Partial()` with `NumSizes` arguments.
   template <size_t NumSizes>
-  using PartialType = internal_layout::LayoutType<NumSizes, Ts...>;
+  using PartialType =
+      internal_layout::LayoutType<StaticSizeSeq, NumSizes, Ts...>;
 
   // `Layout` knows the element types of the arrays we want to lay out in
   // memory but not the number of elements in each array.
@@ -713,14 +734,17 @@ class Layout : public internal_layout::LayoutType<sizeof...(Ts), Ts...> {
   // Note: The sizes of the arrays must be specified in number of elements,
   // not in bytes.
   //
-  // Requires: `sizeof...(Sizes) <= sizeof...(Ts)`.
+  // Requires: `sizeof...(Sizes) + NumStaticSizes <= sizeof...(Ts)`.
   // Requires: all arguments are convertible to `size_t`.
   template <class... Sizes>
   static constexpr PartialType<sizeof...(Sizes)> Partial(Sizes&&... sizes) {
-    static_assert(sizeof...(Sizes) <= sizeof...(Ts), "");
-    return PartialType<sizeof...(Sizes)>(absl::forward<Sizes>(sizes)...);
+    static_assert(sizeof...(Sizes) + StaticSizeSeq::size() <= sizeof...(Ts));
+    return PartialType<sizeof...(Sizes)>(
+        static_cast<size_t>(std::forward<Sizes>(sizes))...);
   }
 
+  // Inherit LayoutType's constructor.
+  //
   // Creates a layout with the sizes of all arrays specified. If you know
   // only the sizes of the first N arrays (where N can be zero), you can use
   // `Partial()` defined above. The constructor is essentially equivalent to
@@ -729,11 +753,74 @@ class Layout : public internal_layout::LayoutType<sizeof...(Ts), Ts...> {
   //
   // Note: The sizes of the arrays must be specified in number of elements,
   // not in bytes.
-  constexpr explicit Layout(internal_layout::TypeToSize<Ts>... sizes)
-      : internal_layout::LayoutType<sizeof...(Ts), Ts...>(sizes...) {}
+  //
+  // Implementation note: we do this via a `using` declaration instead of
+  // defining our own explicit constructor because the signature of LayoutType's
+  // constructor depends on RuntimeSizeSeq, which we don't have access to here.
+  // If we defined our own constructor here, it would have to use a parameter
+  // pack and then cast the arguments to size_t when calling the superclass
+  // constructor, similar to what Partial() does. But that would suffer from the
+  // same problem that Partial() has, which is that the parameter types are
+  // inferred from the arguments, which may be signed types, which must then be
+  // cast to size_t. This can lead to negative values being silently (i.e. with
+  // no compiler warnings) cast to an unsigned type. Having a constructor with
+  // size_t parameters helps the compiler generate better warnings about
+  // potential bad casts, while avoiding false warnings when positive literal
+  // arguments are used. If an argument is a positive literal integer (e.g.
+  // `1`), the compiler will understand that it can be safely converted to
+  // size_t, and hence not generate a warning. But if a negative literal (e.g.
+  // `-1`) or a variable with signed type is used, then it can generate a
+  // warning about a potentially unsafe implicit cast. It would be great if we
+  // could do this for Partial() too, but unfortunately as of C++23 there seems
+  // to be no way to define a function with a variable number of parameters of a
+  // certain type, a.k.a. homogeneous function parameter packs. So we're forced
+  // to choose between explicitly casting the arguments to size_t, which
+  // suppresses all warnings, even potentially valid ones, or implicitly casting
+  // them to size_t, which generates bogus warnings whenever literal arguments
+  // are used, even if they're positive.
+  using Super::Super;
+};
+
+}  // namespace internal_layout
+
+// Descriptor of arrays of various types and sizes laid out in memory one after
+// another. See the top of the file for documentation.
+//
+// Check out the public API of internal_layout::LayoutWithStaticSizes and
+// internal_layout::LayoutImpl above. Those types are internal to the library
+// but their methods are public, and they are inherited by `Layout`.
+template <class... Ts>
+class Layout
+    : public internal_layout::LayoutWithStaticSizes<std::make_index_sequence<0>,
+                                                    Ts...> {
+ private:
+  using Super =
+      internal_layout::LayoutWithStaticSizes<std::make_index_sequence<0>,
+                                             Ts...>;
+
+ public:
+  // If you know the sizes of some or all of the arrays at compile time, you can
+  // use `WithStaticSizes` or `WithStaticSizeSequence` to create a `Layout` type
+  // with those sizes baked in. This can help the compiler generate optimal code
+  // for calculating array offsets and AllocSize().
+  //
+  // Like `Partial()`, the N sizes you specify are for the first N arrays, and
+  // they specify the number of elements in each array, not the number of bytes.
+  template <class StaticSizeSeq>
+  using WithStaticSizeSequence =
+      internal_layout::LayoutWithStaticSizes<StaticSizeSeq, Ts...>;
+
+  template <size_t... StaticSizes>
+  using WithStaticSizes =
+      WithStaticSizeSequence<std::index_sequence<StaticSizes...>>;
+
+  // Inherit LayoutWithStaticSizes's constructor, which requires you to specify
+  // all the array sizes.
+  using Super::Super;
 };
 
 }  // namespace container_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_INTERNAL_LAYOUT_H_
diff --git a/absl/container/internal/layout_benchmark.cc b/absl/container/internal/layout_benchmark.cc
new file mode 100644
index 00000000000..d6f2669760b
--- /dev/null
+++ b/absl/container/internal/layout_benchmark.cc
@@ -0,0 +1,295 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// Every benchmark should have the same performance as the corresponding
+// headroom benchmark.
+
+#include <cstddef>
+#include <cstdint>
+
+#include "absl/base/internal/raw_logging.h"
+#include "absl/container/internal/layout.h"
+#include "benchmark/benchmark.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace container_internal {
+namespace {
+
+using ::benchmark::DoNotOptimize;
+
+using Int128 = int64_t[2];
+
+constexpr size_t MyAlign(size_t n, size_t m) { return (n + m - 1) & ~(m - 1); }
+
+// This benchmark provides the upper bound on performance for BM_OffsetConstant.
+template <size_t Offset, class... Ts>
+void BM_OffsetConstantHeadroom(benchmark::State& state) {
+  for (auto _ : state) {
+    DoNotOptimize(Offset);
+  }
+}
+
+template <size_t Offset, class... Ts>
+void BM_OffsetConstantStatic(benchmark::State& state) {
+  using L = typename Layout<Ts...>::template WithStaticSizes<3, 5, 7>;
+  ABSL_RAW_CHECK(L::Partial().template Offset<3>() == Offset, "Invalid offset");
+  for (auto _ : state) {
+    DoNotOptimize(L::Partial().template Offset<3>());
+  }
+}
+
+template <size_t Offset, class... Ts>
+void BM_OffsetConstant(benchmark::State& state) {
+  using L = Layout<Ts...>;
+  ABSL_RAW_CHECK(L::Partial(3, 5, 7).template Offset<3>() == Offset,
+                 "Invalid offset");
+  for (auto _ : state) {
+    DoNotOptimize(L::Partial(3, 5, 7).template Offset<3>());
+  }
+}
+
+template <size_t Offset, class... Ts>
+void BM_OffsetConstantIndirect(benchmark::State& state) {
+  using L = Layout<Ts...>;
+  auto p = L::Partial(3, 5, 7);
+  ABSL_RAW_CHECK(p.template Offset<3>() == Offset, "Invalid offset");
+  for (auto _ : state) {
+    DoNotOptimize(p);
+    DoNotOptimize(p.template Offset<3>());
+  }
+}
+
+template <class... Ts>
+size_t PartialOffset(size_t k);
+
+template <>
+size_t PartialOffset<int8_t, int16_t, int32_t, Int128>(size_t k) {
+  constexpr size_t o = MyAlign(MyAlign(3 * 1, 2) + 5 * 2, 4);
+  return MyAlign(o + k * 4, 8);
+}
+
+template <>
+size_t PartialOffset<Int128, int32_t, int16_t, int8_t>(size_t k) {
+  // No alignment is necessary.
+  return 3 * 16 + 5 * 4 + k * 2;
+}
+
+// This benchmark provides the upper bound on performance for BM_OffsetVariable.
+template <size_t Offset, class... Ts>
+void BM_OffsetPartialHeadroom(benchmark::State& state) {
+  size_t k = 7;
+  ABSL_RAW_CHECK(PartialOffset<Ts...>(k) == Offset, "Invalid offset");
+  for (auto _ : state) {
+    DoNotOptimize(k);
+    DoNotOptimize(PartialOffset<Ts...>(k));
+  }
+}
+
+template <size_t Offset, class... Ts>
+void BM_OffsetPartialStatic(benchmark::State& state) {
+  using L = typename Layout<Ts...>::template WithStaticSizes<3, 5>;
+  size_t k = 7;
+  ABSL_RAW_CHECK(L::Partial(k).template Offset<3>() == Offset,
+                 "Invalid offset");
+  for (auto _ : state) {
+    DoNotOptimize(k);
+    DoNotOptimize(L::Partial(k).template Offset<3>());
+  }
+}
+
+template <size_t Offset, class... Ts>
+void BM_OffsetPartial(benchmark::State& state) {
+  using L = Layout<Ts...>;
+  size_t k = 7;
+  ABSL_RAW_CHECK(L::Partial(3, 5, k).template Offset<3>() == Offset,
+                 "Invalid offset");
+  for (auto _ : state) {
+    DoNotOptimize(k);
+    DoNotOptimize(L::Partial(3, 5, k).template Offset<3>());
+  }
+}
+
+template <class... Ts>
+size_t VariableOffset(size_t n, size_t m, size_t k);
+
+template <>
+size_t VariableOffset<int8_t, int16_t, int32_t, Int128>(size_t n, size_t m,
+                                                        size_t k) {
+  return MyAlign(MyAlign(MyAlign(n * 1, 2) + m * 2, 4) + k * 4, 8);
+}
+
+template <>
+size_t VariableOffset<Int128, int32_t, int16_t, int8_t>(size_t n, size_t m,
+                                                        size_t k) {
+  // No alignment is necessary.
+  return n * 16 + m * 4 + k * 2;
+}
+
+// This benchmark provides the upper bound on performance for BM_OffsetVariable.
+template <size_t Offset, class... Ts>
+void BM_OffsetVariableHeadroom(benchmark::State& state) {
+  size_t n = 3;
+  size_t m = 5;
+  size_t k = 7;
+  ABSL_RAW_CHECK(VariableOffset<Ts...>(n, m, k) == Offset, "Invalid offset");
+  for (auto _ : state) {
+    DoNotOptimize(n);
+    DoNotOptimize(m);
+    DoNotOptimize(k);
+    DoNotOptimize(VariableOffset<Ts...>(n, m, k));
+  }
+}
+
+template <size_t Offset, class... Ts>
+void BM_OffsetVariable(benchmark::State& state) {
+  using L = Layout<Ts...>;
+  size_t n = 3;
+  size_t m = 5;
+  size_t k = 7;
+  ABSL_RAW_CHECK(L::Partial(n, m, k).template Offset<3>() == Offset,
+                 "Invalid offset");
+  for (auto _ : state) {
+    DoNotOptimize(n);
+    DoNotOptimize(m);
+    DoNotOptimize(k);
+    DoNotOptimize(L::Partial(n, m, k).template Offset<3>());
+  }
+}
+
+template <class... Ts>
+size_t AllocSize(size_t x);
+
+template <>
+size_t AllocSize<int8_t, int16_t, int32_t, Int128>(size_t x) {
+  constexpr size_t o =
+      Layout<int8_t, int16_t, int32_t, Int128>::Partial(3, 5, 7)
+          .template Offset<Int128>();
+  return o + sizeof(Int128) * x;
+}
+
+template <>
+size_t AllocSize<Int128, int32_t, int16_t, int8_t>(size_t x) {
+  constexpr size_t o =
+      Layout<Int128, int32_t, int16_t, int8_t>::Partial(3, 5, 7)
+          .template Offset<int8_t>();
+  return o + sizeof(int8_t) * x;
+}
+
+// This benchmark provides the upper bound on performance for BM_AllocSize
+template <size_t Size, class... Ts>
+void BM_AllocSizeHeadroom(benchmark::State& state) {
+  size_t x = 9;
+  ABSL_RAW_CHECK(AllocSize<Ts...>(x) == Size, "Invalid size");
+  for (auto _ : state) {
+    DoNotOptimize(x);
+    DoNotOptimize(AllocSize<Ts...>(x));
+  }
+}
+
+template <size_t Size, class... Ts>
+void BM_AllocSizeStatic(benchmark::State& state) {
+  using L = typename Layout<Ts...>::template WithStaticSizes<3, 5, 7>;
+  size_t x = 9;
+  ABSL_RAW_CHECK(L(x).AllocSize() == Size, "Invalid offset");
+  for (auto _ : state) {
+    DoNotOptimize(x);
+    DoNotOptimize(L(x).AllocSize());
+  }
+}
+
+template <size_t Size, class... Ts>
+void BM_AllocSize(benchmark::State& state) {
+  using L = Layout<Ts...>;
+  size_t n = 3;
+  size_t m = 5;
+  size_t k = 7;
+  size_t x = 9;
+  ABSL_RAW_CHECK(L(n, m, k, x).AllocSize() == Size, "Invalid offset");
+  for (auto _ : state) {
+    DoNotOptimize(n);
+    DoNotOptimize(m);
+    DoNotOptimize(k);
+    DoNotOptimize(x);
+    DoNotOptimize(L(n, m, k, x).AllocSize());
+  }
+}
+
+template <size_t Size, class... Ts>
+void BM_AllocSizeIndirect(benchmark::State& state) {
+  using L = Layout<Ts...>;
+  auto l = L(3, 5, 7, 9);
+  ABSL_RAW_CHECK(l.AllocSize() == Size, "Invalid offset");
+  for (auto _ : state) {
+    DoNotOptimize(l);
+    DoNotOptimize(l.AllocSize());
+  }
+}
+
+// Run all benchmarks in two modes:
+//
+//   Layout with padding: int8_t[3], int16_t[5], int32_t[7], Int128[?].
+//   Layout without padding: Int128[3], int32_t[5], int16_t[7], int8_t[?].
+
+#define OFFSET_BENCHMARK(NAME, OFFSET, T1, T2, T3, T4) \
+  auto& NAME##_##OFFSET##_##T1##_##T2##_##T3##_##T4 =  \
+      NAME<OFFSET, T1, T2, T3, T4>;                    \
+  BENCHMARK(NAME##_##OFFSET##_##T1##_##T2##_##T3##_##T4)
+
+OFFSET_BENCHMARK(BM_OffsetConstantHeadroom, 48, int8_t, int16_t, int32_t,
+                 Int128);
+OFFSET_BENCHMARK(BM_OffsetConstantStatic, 48, int8_t, int16_t, int32_t, Int128);
+OFFSET_BENCHMARK(BM_OffsetConstant, 48, int8_t, int16_t, int32_t, Int128);
+OFFSET_BENCHMARK(BM_OffsetConstantIndirect, 48, int8_t, int16_t, int32_t,
+                 Int128);
+
+OFFSET_BENCHMARK(BM_OffsetConstantHeadroom, 82, Int128, int32_t, int16_t,
+                 int8_t);
+OFFSET_BENCHMARK(BM_OffsetConstantStatic, 82, Int128, int32_t, int16_t, int8_t);
+OFFSET_BENCHMARK(BM_OffsetConstant, 82, Int128, int32_t, int16_t, int8_t);
+OFFSET_BENCHMARK(BM_OffsetConstantIndirect, 82, Int128, int32_t, int16_t,
+                 int8_t);
+
+OFFSET_BENCHMARK(BM_OffsetPartialHeadroom, 48, int8_t, int16_t, int32_t,
+                 Int128);
+OFFSET_BENCHMARK(BM_OffsetPartialStatic, 48, int8_t, int16_t, int32_t, Int128);
+OFFSET_BENCHMARK(BM_OffsetPartial, 48, int8_t, int16_t, int32_t, Int128);
+
+OFFSET_BENCHMARK(BM_OffsetPartialHeadroom, 82, Int128, int32_t, int16_t,
+                 int8_t);
+OFFSET_BENCHMARK(BM_OffsetPartialStatic, 82, Int128, int32_t, int16_t, int8_t);
+OFFSET_BENCHMARK(BM_OffsetPartial, 82, Int128, int32_t, int16_t, int8_t);
+
+OFFSET_BENCHMARK(BM_OffsetVariableHeadroom, 48, int8_t, int16_t, int32_t,
+                 Int128);
+OFFSET_BENCHMARK(BM_OffsetVariable, 48, int8_t, int16_t, int32_t, Int128);
+
+OFFSET_BENCHMARK(BM_OffsetVariableHeadroom, 82, Int128, int32_t, int16_t,
+                 int8_t);
+OFFSET_BENCHMARK(BM_OffsetVariable, 82, Int128, int32_t, int16_t, int8_t);
+
+OFFSET_BENCHMARK(BM_AllocSizeHeadroom, 192, int8_t, int16_t, int32_t, Int128);
+OFFSET_BENCHMARK(BM_AllocSizeStatic, 192, int8_t, int16_t, int32_t, Int128);
+OFFSET_BENCHMARK(BM_AllocSize, 192, int8_t, int16_t, int32_t, Int128);
+OFFSET_BENCHMARK(BM_AllocSizeIndirect, 192, int8_t, int16_t, int32_t, Int128);
+
+OFFSET_BENCHMARK(BM_AllocSizeHeadroom, 91, Int128, int32_t, int16_t, int8_t);
+OFFSET_BENCHMARK(BM_AllocSizeStatic, 91, Int128, int32_t, int16_t, int8_t);
+OFFSET_BENCHMARK(BM_AllocSize, 91, Int128, int32_t, int16_t, int8_t);
+OFFSET_BENCHMARK(BM_AllocSizeIndirect, 91, Int128, int32_t, int16_t, int8_t);
+
+}  // namespace
+}  // namespace container_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/container/internal/layout_test.cc b/absl/container/internal/layout_test.cc
index 33b72bd372e..12039e9892a 100644
--- a/absl/container/internal/layout_test.cc
+++ b/absl/container/internal/layout_test.cc
@@ -17,17 +17,26 @@
 // We need ::max_align_t because some libstdc++ versions don't provide
 // std::max_align_t
 #include <stddef.h>
+
 #include <cstdint>
+#include <cstring>
+#include <initializer_list>
 #include <memory>
-#include <sstream>
+#include <ostream>
+#include <string>
+#include <tuple>
 #include <type_traits>
+#include <utility>
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
-#include "absl/base/internal/raw_logging.h"
+#include "absl/base/config.h"
+#include "absl/log/check.h"
 #include "absl/types/span.h"
+#include "absl/utility/utility.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 namespace {
 
@@ -35,13 +44,13 @@ using ::absl::Span;
 using ::testing::ElementsAre;
 
 size_t Distance(const void* from, const void* to) {
-  ABSL_RAW_CHECK(from <= to, "Distance must be non-negative");
+  CHECK_LE(from, to) << "Distance must be non-negative";
   return static_cast<const char*>(to) - static_cast<const char*>(from);
 }
 
 template <class Expected, class Actual>
 Expected Type(Actual val) {
-  static_assert(std::is_same<Expected, Actual>(), "");
+  static_assert(std::is_same<Expected, Actual>());
   return val;
 }
 
@@ -60,26 +69,24 @@ struct alignas(8) Int128 {
 // int64_t is *not* 8-byte aligned on all platforms!
 struct alignas(8) Int64 {
   int64_t a;
-  friend bool operator==(Int64 lhs, Int64 rhs) {
-    return lhs.a == rhs.a;
-  }
+  friend bool operator==(Int64 lhs, Int64 rhs) { return lhs.a == rhs.a; }
 };
 
 // Properties of types that this test relies on.
-static_assert(sizeof(int8_t) == 1, "");
-static_assert(alignof(int8_t) == 1, "");
-static_assert(sizeof(int16_t) == 2, "");
-static_assert(alignof(int16_t) == 2, "");
-static_assert(sizeof(int32_t) == 4, "");
-static_assert(alignof(int32_t) == 4, "");
-static_assert(sizeof(Int64) == 8, "");
-static_assert(alignof(Int64) == 8, "");
-static_assert(sizeof(Int128) == 16, "");
-static_assert(alignof(Int128) == 8, "");
+static_assert(sizeof(int8_t) == 1);
+static_assert(alignof(int8_t) == 1);
+static_assert(sizeof(int16_t) == 2);
+static_assert(alignof(int16_t) == 2);
+static_assert(sizeof(int32_t) == 4);
+static_assert(alignof(int32_t) == 4);
+static_assert(sizeof(Int64) == 8);
+static_assert(alignof(Int64) == 8);
+static_assert(sizeof(Int128) == 16);
+static_assert(alignof(Int128) == 8);
 
 template <class Expected, class Actual>
 void SameType() {
-  static_assert(std::is_same<Expected, Actual>(), "");
+  static_assert(std::is_same<Expected, Actual>());
 }
 
 TEST(Layout, ElementType) {
@@ -125,8 +132,10 @@ TEST(Layout, ElementTypes) {
   {
     using L = Layout<int32_t, int32_t>;
     SameType<std::tuple<int32_t, int32_t>, L::ElementTypes>();
-    SameType<std::tuple<int32_t, int32_t>, decltype(L::Partial())::ElementTypes>();
-    SameType<std::tuple<int32_t, int32_t>, decltype(L::Partial(0))::ElementTypes>();
+    SameType<std::tuple<int32_t, int32_t>,
+             decltype(L::Partial())::ElementTypes>();
+    SameType<std::tuple<int32_t, int32_t>,
+             decltype(L::Partial(0))::ElementTypes>();
   }
   {
     using L = Layout<int8_t, int32_t, Int128>;
@@ -261,6 +270,35 @@ TEST(Layout, Offsets) {
   }
 }
 
+TEST(Layout, StaticOffsets) {
+  using L = Layout<int8_t, int32_t, Int128>;
+  {
+    using SL = L::WithStaticSizes<>;
+    EXPECT_THAT(SL::Partial().Offsets(), ElementsAre(0));
+    EXPECT_THAT(SL::Partial(5).Offsets(), ElementsAre(0, 8));
+    EXPECT_THAT(SL::Partial(5, 3, 1).Offsets(), ElementsAre(0, 8, 24));
+    EXPECT_THAT(SL(5, 3, 1).Offsets(), ElementsAre(0, 8, 24));
+  }
+  {
+    using SL = L::WithStaticSizes<5>;
+    EXPECT_THAT(SL::Partial().Offsets(), ElementsAre(0, 8));
+    EXPECT_THAT(SL::Partial(3).Offsets(), ElementsAre(0, 8, 24));
+    EXPECT_THAT(SL::Partial(3, 1).Offsets(), ElementsAre(0, 8, 24));
+    EXPECT_THAT(SL(3, 1).Offsets(), ElementsAre(0, 8, 24));
+  }
+  {
+    using SL = L::WithStaticSizes<5, 3>;
+    EXPECT_THAT(SL::Partial().Offsets(), ElementsAre(0, 8, 24));
+    EXPECT_THAT(SL::Partial(1).Offsets(), ElementsAre(0, 8, 24));
+    EXPECT_THAT(SL(1).Offsets(), ElementsAre(0, 8, 24));
+  }
+  {
+    using SL = L::WithStaticSizes<5, 3, 1>;
+    EXPECT_THAT(SL::Partial().Offsets(), ElementsAre(0, 8, 24));
+    EXPECT_THAT(SL().Offsets(), ElementsAre(0, 8, 24));
+  }
+}
+
 TEST(Layout, AllocSize) {
   {
     using L = Layout<int32_t>;
@@ -285,6 +323,30 @@ TEST(Layout, AllocSize) {
   }
 }
 
+TEST(Layout, StaticAllocSize) {
+  using L = Layout<int8_t, int32_t, Int128>;
+  {
+    using SL = L::WithStaticSizes<>;
+    EXPECT_EQ(136, SL::Partial(3, 5, 7).AllocSize());
+    EXPECT_EQ(136, SL(3, 5, 7).AllocSize());
+  }
+  {
+    using SL = L::WithStaticSizes<3>;
+    EXPECT_EQ(136, SL::Partial(5, 7).AllocSize());
+    EXPECT_EQ(136, SL(5, 7).AllocSize());
+  }
+  {
+    using SL = L::WithStaticSizes<3, 5>;
+    EXPECT_EQ(136, SL::Partial(7).AllocSize());
+    EXPECT_EQ(136, SL(7).AllocSize());
+  }
+  {
+    using SL = L::WithStaticSizes<3, 5, 7>;
+    EXPECT_EQ(136, SL::Partial().AllocSize());
+    EXPECT_EQ(136, SL().AllocSize());
+  }
+}
+
 TEST(Layout, SizeByIndex) {
   {
     using L = Layout<int32_t>;
@@ -360,23 +422,47 @@ TEST(Layout, Sizes) {
   }
 }
 
+TEST(Layout, StaticSize) {
+  using L = Layout<int8_t, int32_t, Int128>;
+  {
+    using SL = L::WithStaticSizes<>;
+    EXPECT_THAT(SL::Partial().Sizes(), ElementsAre());
+    EXPECT_THAT(SL::Partial(3).Size<0>(), 3);
+    EXPECT_THAT(SL::Partial(3).Size<int8_t>(), 3);
+    EXPECT_THAT(SL::Partial(3).Sizes(), ElementsAre(3));
+    EXPECT_THAT(SL::Partial(3, 5, 7).Size<0>(), 3);
+    EXPECT_THAT(SL::Partial(3, 5, 7).Size<int8_t>(), 3);
+    EXPECT_THAT(SL::Partial(3, 5, 7).Size<2>(), 7);
+    EXPECT_THAT(SL::Partial(3, 5, 7).Size<Int128>(), 7);
+    EXPECT_THAT(SL::Partial(3, 5, 7).Sizes(), ElementsAre(3, 5, 7));
+    EXPECT_THAT(SL(3, 5, 7).Size<0>(), 3);
+    EXPECT_THAT(SL(3, 5, 7).Size<int8_t>(), 3);
+    EXPECT_THAT(SL(3, 5, 7).Size<2>(), 7);
+    EXPECT_THAT(SL(3, 5, 7).Size<Int128>(), 7);
+    EXPECT_THAT(SL(3, 5, 7).Sizes(), ElementsAre(3, 5, 7));
+  }
+}
+
 TEST(Layout, PointerByIndex) {
-  alignas(max_align_t) const unsigned char p[100] = {};
+  alignas(max_align_t) const unsigned char p[100] = {0};
   {
     using L = Layout<int32_t>;
     EXPECT_EQ(0, Distance(p, Type<const int32_t*>(L::Partial().Pointer<0>(p))));
-    EXPECT_EQ(0, Distance(p, Type<const int32_t*>(L::Partial(3).Pointer<0>(p))));
+    EXPECT_EQ(0,
+              Distance(p, Type<const int32_t*>(L::Partial(3).Pointer<0>(p))));
     EXPECT_EQ(0, Distance(p, Type<const int32_t*>(L(3).Pointer<0>(p))));
   }
   {
     using L = Layout<int32_t, int32_t>;
     EXPECT_EQ(0, Distance(p, Type<const int32_t*>(L::Partial().Pointer<0>(p))));
-    EXPECT_EQ(0, Distance(p, Type<const int32_t*>(L::Partial(3).Pointer<0>(p))));
-    EXPECT_EQ(12, Distance(p, Type<const int32_t*>(L::Partial(3).Pointer<1>(p))));
     EXPECT_EQ(0,
-              Distance(p, Type<const int32_t*>(L::Partial(3, 5).Pointer<0>(p))));
+              Distance(p, Type<const int32_t*>(L::Partial(3).Pointer<0>(p))));
     EXPECT_EQ(12,
-              Distance(p, Type<const int32_t*>(L::Partial(3, 5).Pointer<1>(p))));
+              Distance(p, Type<const int32_t*>(L::Partial(3).Pointer<1>(p))));
+    EXPECT_EQ(
+        0, Distance(p, Type<const int32_t*>(L::Partial(3, 5).Pointer<0>(p))));
+    EXPECT_EQ(
+        12, Distance(p, Type<const int32_t*>(L::Partial(3, 5).Pointer<1>(p))));
     EXPECT_EQ(0, Distance(p, Type<const int32_t*>(L(3, 5).Pointer<0>(p))));
     EXPECT_EQ(12, Distance(p, Type<const int32_t*>(L(3, 5).Pointer<1>(p))));
   }
@@ -384,39 +470,44 @@ TEST(Layout, PointerByIndex) {
     using L = Layout<int8_t, int32_t, Int128>;
     EXPECT_EQ(0, Distance(p, Type<const int8_t*>(L::Partial().Pointer<0>(p))));
     EXPECT_EQ(0, Distance(p, Type<const int8_t*>(L::Partial(0).Pointer<0>(p))));
-    EXPECT_EQ(0, Distance(p, Type<const int32_t*>(L::Partial(0).Pointer<1>(p))));
+    EXPECT_EQ(0,
+              Distance(p, Type<const int32_t*>(L::Partial(0).Pointer<1>(p))));
     EXPECT_EQ(0, Distance(p, Type<const int8_t*>(L::Partial(1).Pointer<0>(p))));
-    EXPECT_EQ(4, Distance(p, Type<const int32_t*>(L::Partial(1).Pointer<1>(p))));
+    EXPECT_EQ(4,
+              Distance(p, Type<const int32_t*>(L::Partial(1).Pointer<1>(p))));
     EXPECT_EQ(0, Distance(p, Type<const int8_t*>(L::Partial(5).Pointer<0>(p))));
-    EXPECT_EQ(8, Distance(p, Type<const int32_t*>(L::Partial(5).Pointer<1>(p))));
+    EXPECT_EQ(8,
+              Distance(p, Type<const int32_t*>(L::Partial(5).Pointer<1>(p))));
     EXPECT_EQ(0,
               Distance(p, Type<const int8_t*>(L::Partial(0, 0).Pointer<0>(p))));
-    EXPECT_EQ(0,
-              Distance(p, Type<const int32_t*>(L::Partial(0, 0).Pointer<1>(p))));
+    EXPECT_EQ(
+        0, Distance(p, Type<const int32_t*>(L::Partial(0, 0).Pointer<1>(p))));
     EXPECT_EQ(0,
               Distance(p, Type<const Int128*>(L::Partial(0, 0).Pointer<2>(p))));
     EXPECT_EQ(0,
               Distance(p, Type<const int8_t*>(L::Partial(1, 0).Pointer<0>(p))));
-    EXPECT_EQ(4,
-              Distance(p, Type<const int32_t*>(L::Partial(1, 0).Pointer<1>(p))));
+    EXPECT_EQ(
+        4, Distance(p, Type<const int32_t*>(L::Partial(1, 0).Pointer<1>(p))));
     EXPECT_EQ(8,
               Distance(p, Type<const Int128*>(L::Partial(1, 0).Pointer<2>(p))));
     EXPECT_EQ(0,
               Distance(p, Type<const int8_t*>(L::Partial(5, 3).Pointer<0>(p))));
-    EXPECT_EQ(8,
-              Distance(p, Type<const int32_t*>(L::Partial(5, 3).Pointer<1>(p))));
+    EXPECT_EQ(
+        8, Distance(p, Type<const int32_t*>(L::Partial(5, 3).Pointer<1>(p))));
     EXPECT_EQ(24,
               Distance(p, Type<const Int128*>(L::Partial(5, 3).Pointer<2>(p))));
     EXPECT_EQ(
         0, Distance(p, Type<const int8_t*>(L::Partial(0, 0, 0).Pointer<0>(p))));
     EXPECT_EQ(
-        0, Distance(p, Type<const int32_t*>(L::Partial(0, 0, 0).Pointer<1>(p))));
+        0,
+        Distance(p, Type<const int32_t*>(L::Partial(0, 0, 0).Pointer<1>(p))));
     EXPECT_EQ(
         0, Distance(p, Type<const Int128*>(L::Partial(0, 0, 0).Pointer<2>(p))));
     EXPECT_EQ(
         0, Distance(p, Type<const int8_t*>(L::Partial(1, 0, 0).Pointer<0>(p))));
     EXPECT_EQ(
-        4, Distance(p, Type<const int32_t*>(L::Partial(1, 0, 0).Pointer<1>(p))));
+        4,
+        Distance(p, Type<const int32_t*>(L::Partial(1, 0, 0).Pointer<1>(p))));
     EXPECT_EQ(
         8, Distance(p, Type<const Int128*>(L::Partial(1, 0, 0).Pointer<2>(p))));
     EXPECT_EQ(
@@ -425,7 +516,8 @@ TEST(Layout, PointerByIndex) {
         24,
         Distance(p, Type<const Int128*>(L::Partial(5, 3, 1).Pointer<2>(p))));
     EXPECT_EQ(
-        8, Distance(p, Type<const int32_t*>(L::Partial(5, 3, 1).Pointer<1>(p))));
+        8,
+        Distance(p, Type<const int32_t*>(L::Partial(5, 3, 1).Pointer<1>(p))));
     EXPECT_EQ(0, Distance(p, Type<const int8_t*>(L(5, 3, 1).Pointer<0>(p))));
     EXPECT_EQ(24, Distance(p, Type<const Int128*>(L(5, 3, 1).Pointer<2>(p))));
     EXPECT_EQ(8, Distance(p, Type<const int32_t*>(L(5, 3, 1).Pointer<1>(p))));
@@ -433,83 +525,86 @@ TEST(Layout, PointerByIndex) {
 }
 
 TEST(Layout, PointerByType) {
-  alignas(max_align_t) const unsigned char p[100] = {};
+  alignas(max_align_t) const unsigned char p[100] = {0};
   {
     using L = Layout<int32_t>;
-    EXPECT_EQ(0,
-              Distance(p, Type<const int32_t*>(L::Partial().Pointer<int32_t>(p))));
-    EXPECT_EQ(0,
-              Distance(p, Type<const int32_t*>(L::Partial(3).Pointer<int32_t>(p))));
+    EXPECT_EQ(
+        0, Distance(p, Type<const int32_t*>(L::Partial().Pointer<int32_t>(p))));
+    EXPECT_EQ(
+        0,
+        Distance(p, Type<const int32_t*>(L::Partial(3).Pointer<int32_t>(p))));
     EXPECT_EQ(0, Distance(p, Type<const int32_t*>(L(3).Pointer<int32_t>(p))));
   }
   {
     using L = Layout<int8_t, int32_t, Int128>;
-    EXPECT_EQ(0, Distance(p, Type<const int8_t*>(L::Partial().Pointer<int8_t>(p))));
-    EXPECT_EQ(0,
-              Distance(p, Type<const int8_t*>(L::Partial(0).Pointer<int8_t>(p))));
-    EXPECT_EQ(0,
-              Distance(p, Type<const int32_t*>(L::Partial(0).Pointer<int32_t>(p))));
-    EXPECT_EQ(0,
-              Distance(p, Type<const int8_t*>(L::Partial(1).Pointer<int8_t>(p))));
-    EXPECT_EQ(4,
-              Distance(p, Type<const int32_t*>(L::Partial(1).Pointer<int32_t>(p))));
-    EXPECT_EQ(0,
-              Distance(p, Type<const int8_t*>(L::Partial(5).Pointer<int8_t>(p))));
-    EXPECT_EQ(8,
-              Distance(p, Type<const int32_t*>(L::Partial(5).Pointer<int32_t>(p))));
     EXPECT_EQ(
-        0, Distance(p, Type<const int8_t*>(L::Partial(0, 0).Pointer<int8_t>(p))));
+        0, Distance(p, Type<const int8_t*>(L::Partial().Pointer<int8_t>(p))));
     EXPECT_EQ(
-        0, Distance(p, Type<const int32_t*>(L::Partial(0, 0).Pointer<int32_t>(p))));
+        0, Distance(p, Type<const int8_t*>(L::Partial(0).Pointer<int8_t>(p))));
     EXPECT_EQ(
         0,
-        Distance(p, Type<const Int128*>(L::Partial(0, 0).Pointer<Int128>(p))));
+        Distance(p, Type<const int32_t*>(L::Partial(0).Pointer<int32_t>(p))));
     EXPECT_EQ(
-        0, Distance(p, Type<const int8_t*>(L::Partial(1, 0).Pointer<int8_t>(p))));
+        0, Distance(p, Type<const int8_t*>(L::Partial(1).Pointer<int8_t>(p))));
     EXPECT_EQ(
-        4, Distance(p, Type<const int32_t*>(L::Partial(1, 0).Pointer<int32_t>(p))));
-    EXPECT_EQ(
-        8,
-        Distance(p, Type<const Int128*>(L::Partial(1, 0).Pointer<Int128>(p))));
+        4,
+        Distance(p, Type<const int32_t*>(L::Partial(1).Pointer<int32_t>(p))));
     EXPECT_EQ(
-        0, Distance(p, Type<const int8_t*>(L::Partial(5, 3).Pointer<int8_t>(p))));
+        0, Distance(p, Type<const int8_t*>(L::Partial(5).Pointer<int8_t>(p))));
     EXPECT_EQ(
-        8, Distance(p, Type<const int32_t*>(L::Partial(5, 3).Pointer<int32_t>(p))));
+        8,
+        Distance(p, Type<const int32_t*>(L::Partial(5).Pointer<int32_t>(p))));
     EXPECT_EQ(
-        24,
-        Distance(p, Type<const Int128*>(L::Partial(5, 3).Pointer<Int128>(p))));
+        0,
+        Distance(p, Type<const int8_t*>(L::Partial(0, 0).Pointer<int8_t>(p))));
+    EXPECT_EQ(0, Distance(p, Type<const int32_t*>(
+                                 L::Partial(0, 0).Pointer<int32_t>(p))));
     EXPECT_EQ(
         0,
-        Distance(p, Type<const int8_t*>(L::Partial(0, 0, 0).Pointer<int8_t>(p))));
+        Distance(p, Type<const Int128*>(L::Partial(0, 0).Pointer<Int128>(p))));
     EXPECT_EQ(
         0,
-        Distance(p, Type<const int32_t*>(L::Partial(0, 0, 0).Pointer<int32_t>(p))));
-    EXPECT_EQ(0, Distance(p, Type<const Int128*>(
-                                 L::Partial(0, 0, 0).Pointer<Int128>(p))));
+        Distance(p, Type<const int8_t*>(L::Partial(1, 0).Pointer<int8_t>(p))));
+    EXPECT_EQ(4, Distance(p, Type<const int32_t*>(
+                                 L::Partial(1, 0).Pointer<int32_t>(p))));
+    EXPECT_EQ(
+        8,
+        Distance(p, Type<const Int128*>(L::Partial(1, 0).Pointer<Int128>(p))));
     EXPECT_EQ(
         0,
-        Distance(p, Type<const int8_t*>(L::Partial(1, 0, 0).Pointer<int8_t>(p))));
+        Distance(p, Type<const int8_t*>(L::Partial(5, 3).Pointer<int8_t>(p))));
+    EXPECT_EQ(8, Distance(p, Type<const int32_t*>(
+                                 L::Partial(5, 3).Pointer<int32_t>(p))));
     EXPECT_EQ(
-        4,
-        Distance(p, Type<const int32_t*>(L::Partial(1, 0, 0).Pointer<int32_t>(p))));
+        24,
+        Distance(p, Type<const Int128*>(L::Partial(5, 3).Pointer<Int128>(p))));
+    EXPECT_EQ(0, Distance(p, Type<const int8_t*>(
+                                 L::Partial(0, 0, 0).Pointer<int8_t>(p))));
+    EXPECT_EQ(0, Distance(p, Type<const int32_t*>(
+                                 L::Partial(0, 0, 0).Pointer<int32_t>(p))));
+    EXPECT_EQ(0, Distance(p, Type<const Int128*>(
+                                 L::Partial(0, 0, 0).Pointer<Int128>(p))));
+    EXPECT_EQ(0, Distance(p, Type<const int8_t*>(
+                                 L::Partial(1, 0, 0).Pointer<int8_t>(p))));
+    EXPECT_EQ(4, Distance(p, Type<const int32_t*>(
+                                 L::Partial(1, 0, 0).Pointer<int32_t>(p))));
     EXPECT_EQ(8, Distance(p, Type<const Int128*>(
                                  L::Partial(1, 0, 0).Pointer<Int128>(p))));
-    EXPECT_EQ(
-        0,
-        Distance(p, Type<const int8_t*>(L::Partial(5, 3, 1).Pointer<int8_t>(p))));
+    EXPECT_EQ(0, Distance(p, Type<const int8_t*>(
+                                 L::Partial(5, 3, 1).Pointer<int8_t>(p))));
     EXPECT_EQ(24, Distance(p, Type<const Int128*>(
                                   L::Partial(5, 3, 1).Pointer<Int128>(p))));
-    EXPECT_EQ(
-        8,
-        Distance(p, Type<const int32_t*>(L::Partial(5, 3, 1).Pointer<int32_t>(p))));
+    EXPECT_EQ(8, Distance(p, Type<const int32_t*>(
+                                 L::Partial(5, 3, 1).Pointer<int32_t>(p))));
     EXPECT_EQ(24,
               Distance(p, Type<const Int128*>(L(5, 3, 1).Pointer<Int128>(p))));
-    EXPECT_EQ(8, Distance(p, Type<const int32_t*>(L(5, 3, 1).Pointer<int32_t>(p))));
+    EXPECT_EQ(
+        8, Distance(p, Type<const int32_t*>(L(5, 3, 1).Pointer<int32_t>(p))));
   }
 }
 
 TEST(Layout, MutablePointerByIndex) {
-  alignas(max_align_t) unsigned char p[100];
+  alignas(max_align_t) unsigned char p[100] = {0};
   {
     using L = Layout<int32_t>;
     EXPECT_EQ(0, Distance(p, Type<int32_t*>(L::Partial().Pointer<0>(p))));
@@ -545,15 +640,18 @@ TEST(Layout, MutablePointerByIndex) {
     EXPECT_EQ(8, Distance(p, Type<int32_t*>(L::Partial(5, 3).Pointer<1>(p))));
     EXPECT_EQ(24, Distance(p, Type<Int128*>(L::Partial(5, 3).Pointer<2>(p))));
     EXPECT_EQ(0, Distance(p, Type<int8_t*>(L::Partial(0, 0, 0).Pointer<0>(p))));
-    EXPECT_EQ(0, Distance(p, Type<int32_t*>(L::Partial(0, 0, 0).Pointer<1>(p))));
+    EXPECT_EQ(0,
+              Distance(p, Type<int32_t*>(L::Partial(0, 0, 0).Pointer<1>(p))));
     EXPECT_EQ(0, Distance(p, Type<Int128*>(L::Partial(0, 0, 0).Pointer<2>(p))));
     EXPECT_EQ(0, Distance(p, Type<int8_t*>(L::Partial(1, 0, 0).Pointer<0>(p))));
-    EXPECT_EQ(4, Distance(p, Type<int32_t*>(L::Partial(1, 0, 0).Pointer<1>(p))));
+    EXPECT_EQ(4,
+              Distance(p, Type<int32_t*>(L::Partial(1, 0, 0).Pointer<1>(p))));
     EXPECT_EQ(8, Distance(p, Type<Int128*>(L::Partial(1, 0, 0).Pointer<2>(p))));
     EXPECT_EQ(0, Distance(p, Type<int8_t*>(L::Partial(5, 3, 1).Pointer<0>(p))));
     EXPECT_EQ(24,
               Distance(p, Type<Int128*>(L::Partial(5, 3, 1).Pointer<2>(p))));
-    EXPECT_EQ(8, Distance(p, Type<int32_t*>(L::Partial(5, 3, 1).Pointer<1>(p))));
+    EXPECT_EQ(8,
+              Distance(p, Type<int32_t*>(L::Partial(5, 3, 1).Pointer<1>(p))));
     EXPECT_EQ(0, Distance(p, Type<int8_t*>(L(5, 3, 1).Pointer<0>(p))));
     EXPECT_EQ(24, Distance(p, Type<Int128*>(L(5, 3, 1).Pointer<2>(p))));
     EXPECT_EQ(8, Distance(p, Type<int32_t*>(L(5, 3, 1).Pointer<1>(p))));
@@ -561,52 +659,65 @@ TEST(Layout, MutablePointerByIndex) {
 }
 
 TEST(Layout, MutablePointerByType) {
-  alignas(max_align_t) unsigned char p[100];
+  alignas(max_align_t) unsigned char p[100] = {0};
   {
     using L = Layout<int32_t>;
     EXPECT_EQ(0, Distance(p, Type<int32_t*>(L::Partial().Pointer<int32_t>(p))));
-    EXPECT_EQ(0, Distance(p, Type<int32_t*>(L::Partial(3).Pointer<int32_t>(p))));
+    EXPECT_EQ(0,
+              Distance(p, Type<int32_t*>(L::Partial(3).Pointer<int32_t>(p))));
     EXPECT_EQ(0, Distance(p, Type<int32_t*>(L(3).Pointer<int32_t>(p))));
   }
   {
     using L = Layout<int8_t, int32_t, Int128>;
     EXPECT_EQ(0, Distance(p, Type<int8_t*>(L::Partial().Pointer<int8_t>(p))));
     EXPECT_EQ(0, Distance(p, Type<int8_t*>(L::Partial(0).Pointer<int8_t>(p))));
-    EXPECT_EQ(0, Distance(p, Type<int32_t*>(L::Partial(0).Pointer<int32_t>(p))));
+    EXPECT_EQ(0,
+              Distance(p, Type<int32_t*>(L::Partial(0).Pointer<int32_t>(p))));
     EXPECT_EQ(0, Distance(p, Type<int8_t*>(L::Partial(1).Pointer<int8_t>(p))));
-    EXPECT_EQ(4, Distance(p, Type<int32_t*>(L::Partial(1).Pointer<int32_t>(p))));
+    EXPECT_EQ(4,
+              Distance(p, Type<int32_t*>(L::Partial(1).Pointer<int32_t>(p))));
     EXPECT_EQ(0, Distance(p, Type<int8_t*>(L::Partial(5).Pointer<int8_t>(p))));
-    EXPECT_EQ(8, Distance(p, Type<int32_t*>(L::Partial(5).Pointer<int32_t>(p))));
-    EXPECT_EQ(0, Distance(p, Type<int8_t*>(L::Partial(0, 0).Pointer<int8_t>(p))));
-    EXPECT_EQ(0, Distance(p, Type<int32_t*>(L::Partial(0, 0).Pointer<int32_t>(p))));
+    EXPECT_EQ(8,
+              Distance(p, Type<int32_t*>(L::Partial(5).Pointer<int32_t>(p))));
+    EXPECT_EQ(0,
+              Distance(p, Type<int8_t*>(L::Partial(0, 0).Pointer<int8_t>(p))));
+    EXPECT_EQ(
+        0, Distance(p, Type<int32_t*>(L::Partial(0, 0).Pointer<int32_t>(p))));
     EXPECT_EQ(0,
               Distance(p, Type<Int128*>(L::Partial(0, 0).Pointer<Int128>(p))));
-    EXPECT_EQ(0, Distance(p, Type<int8_t*>(L::Partial(1, 0).Pointer<int8_t>(p))));
-    EXPECT_EQ(4, Distance(p, Type<int32_t*>(L::Partial(1, 0).Pointer<int32_t>(p))));
+    EXPECT_EQ(0,
+              Distance(p, Type<int8_t*>(L::Partial(1, 0).Pointer<int8_t>(p))));
+    EXPECT_EQ(
+        4, Distance(p, Type<int32_t*>(L::Partial(1, 0).Pointer<int32_t>(p))));
     EXPECT_EQ(8,
               Distance(p, Type<Int128*>(L::Partial(1, 0).Pointer<Int128>(p))));
-    EXPECT_EQ(0, Distance(p, Type<int8_t*>(L::Partial(5, 3).Pointer<int8_t>(p))));
-    EXPECT_EQ(8, Distance(p, Type<int32_t*>(L::Partial(5, 3).Pointer<int32_t>(p))));
+    EXPECT_EQ(0,
+              Distance(p, Type<int8_t*>(L::Partial(5, 3).Pointer<int8_t>(p))));
+    EXPECT_EQ(
+        8, Distance(p, Type<int32_t*>(L::Partial(5, 3).Pointer<int32_t>(p))));
     EXPECT_EQ(24,
               Distance(p, Type<Int128*>(L::Partial(5, 3).Pointer<Int128>(p))));
-    EXPECT_EQ(0,
-              Distance(p, Type<int8_t*>(L::Partial(0, 0, 0).Pointer<int8_t>(p))));
-    EXPECT_EQ(0,
-              Distance(p, Type<int32_t*>(L::Partial(0, 0, 0).Pointer<int32_t>(p))));
+    EXPECT_EQ(
+        0, Distance(p, Type<int8_t*>(L::Partial(0, 0, 0).Pointer<int8_t>(p))));
+    EXPECT_EQ(
+        0,
+        Distance(p, Type<int32_t*>(L::Partial(0, 0, 0).Pointer<int32_t>(p))));
     EXPECT_EQ(
         0, Distance(p, Type<Int128*>(L::Partial(0, 0, 0).Pointer<Int128>(p))));
-    EXPECT_EQ(0,
-              Distance(p, Type<int8_t*>(L::Partial(1, 0, 0).Pointer<int8_t>(p))));
-    EXPECT_EQ(4,
-              Distance(p, Type<int32_t*>(L::Partial(1, 0, 0).Pointer<int32_t>(p))));
+    EXPECT_EQ(
+        0, Distance(p, Type<int8_t*>(L::Partial(1, 0, 0).Pointer<int8_t>(p))));
+    EXPECT_EQ(
+        4,
+        Distance(p, Type<int32_t*>(L::Partial(1, 0, 0).Pointer<int32_t>(p))));
     EXPECT_EQ(
         8, Distance(p, Type<Int128*>(L::Partial(1, 0, 0).Pointer<Int128>(p))));
-    EXPECT_EQ(0,
-              Distance(p, Type<int8_t*>(L::Partial(5, 3, 1).Pointer<int8_t>(p))));
+    EXPECT_EQ(
+        0, Distance(p, Type<int8_t*>(L::Partial(5, 3, 1).Pointer<int8_t>(p))));
     EXPECT_EQ(
         24, Distance(p, Type<Int128*>(L::Partial(5, 3, 1).Pointer<Int128>(p))));
-    EXPECT_EQ(8,
-              Distance(p, Type<int32_t*>(L::Partial(5, 3, 1).Pointer<int32_t>(p))));
+    EXPECT_EQ(
+        8,
+        Distance(p, Type<int32_t*>(L::Partial(5, 3, 1).Pointer<int32_t>(p))));
     EXPECT_EQ(0, Distance(p, Type<int8_t*>(L(5, 3, 1).Pointer<int8_t>(p))));
     EXPECT_EQ(24, Distance(p, Type<Int128*>(L(5, 3, 1).Pointer<Int128>(p))));
     EXPECT_EQ(8, Distance(p, Type<int32_t*>(L(5, 3, 1).Pointer<int32_t>(p))));
@@ -614,7 +725,7 @@ TEST(Layout, MutablePointerByType) {
 }
 
 TEST(Layout, Pointers) {
-  alignas(max_align_t) const unsigned char p[100] = {};
+  alignas(max_align_t) const unsigned char p[100] = {0};
   using L = Layout<int8_t, int8_t, Int128>;
   {
     const auto x = L::Partial();
@@ -650,7 +761,7 @@ TEST(Layout, Pointers) {
 }
 
 TEST(Layout, MutablePointers) {
-  alignas(max_align_t) unsigned char p[100];
+  alignas(max_align_t) unsigned char p[100] = {0};
   using L = Layout<int8_t, int8_t, Int128>;
   {
     const auto x = L::Partial();
@@ -682,8 +793,63 @@ TEST(Layout, MutablePointers) {
   }
 }
 
+TEST(Layout, StaticPointers) {
+  alignas(max_align_t) const unsigned char p[100] = {0};
+  using L = Layout<int8_t, int8_t, Int128>;
+  {
+    const auto x = L::WithStaticSizes<>::Partial();
+    EXPECT_EQ(std::make_tuple(x.Pointer<0>(p)),
+              Type<std::tuple<const int8_t*>>(x.Pointers(p)));
+  }
+  {
+    const auto x = L::WithStaticSizes<>::Partial(1);
+    EXPECT_EQ(std::make_tuple(x.Pointer<0>(p), x.Pointer<1>(p)),
+              (Type<std::tuple<const int8_t*, const int8_t*>>(x.Pointers(p))));
+  }
+  {
+    const auto x = L::WithStaticSizes<1>::Partial();
+    EXPECT_EQ(std::make_tuple(x.Pointer<0>(p), x.Pointer<1>(p)),
+              (Type<std::tuple<const int8_t*, const int8_t*>>(x.Pointers(p))));
+  }
+  {
+    const auto x = L::WithStaticSizes<>::Partial(1, 2, 3);
+    EXPECT_EQ(
+        std::make_tuple(x.Pointer<0>(p), x.Pointer<1>(p), x.Pointer<2>(p)),
+        (Type<std::tuple<const int8_t*, const int8_t*, const Int128*>>(
+            x.Pointers(p))));
+  }
+  {
+    const auto x = L::WithStaticSizes<1>::Partial(2, 3);
+    EXPECT_EQ(
+        std::make_tuple(x.Pointer<0>(p), x.Pointer<1>(p), x.Pointer<2>(p)),
+        (Type<std::tuple<const int8_t*, const int8_t*, const Int128*>>(
+            x.Pointers(p))));
+  }
+  {
+    const auto x = L::WithStaticSizes<1, 2>::Partial(3);
+    EXPECT_EQ(
+        std::make_tuple(x.Pointer<0>(p), x.Pointer<1>(p), x.Pointer<2>(p)),
+        (Type<std::tuple<const int8_t*, const int8_t*, const Int128*>>(
+            x.Pointers(p))));
+  }
+  {
+    const auto x = L::WithStaticSizes<1, 2, 3>::Partial();
+    EXPECT_EQ(
+        std::make_tuple(x.Pointer<0>(p), x.Pointer<1>(p), x.Pointer<2>(p)),
+        (Type<std::tuple<const int8_t*, const int8_t*, const Int128*>>(
+            x.Pointers(p))));
+  }
+  {
+    const L::WithStaticSizes<1, 2, 3> x;
+    EXPECT_EQ(
+        std::make_tuple(x.Pointer<0>(p), x.Pointer<1>(p), x.Pointer<2>(p)),
+        (Type<std::tuple<const int8_t*, const int8_t*, const Int128*>>(
+            x.Pointers(p))));
+  }
+}
+
 TEST(Layout, SliceByIndexSize) {
-  alignas(max_align_t) const unsigned char p[100] = {};
+  alignas(max_align_t) const unsigned char p[100] = {0};
   {
     using L = Layout<int32_t>;
     EXPECT_EQ(0, L::Partial(0).Slice<0>(p).size());
@@ -711,7 +877,7 @@ TEST(Layout, SliceByIndexSize) {
 }
 
 TEST(Layout, SliceByTypeSize) {
-  alignas(max_align_t) const unsigned char p[100] = {};
+  alignas(max_align_t) const unsigned char p[100] = {0};
   {
     using L = Layout<int32_t>;
     EXPECT_EQ(0, L::Partial(0).Slice<int32_t>(p).size());
@@ -731,9 +897,8 @@ TEST(Layout, SliceByTypeSize) {
     EXPECT_EQ(7, L(3, 5, 7).Slice<Int128>(p).size());
   }
 }
-
 TEST(Layout, MutableSliceByIndexSize) {
-  alignas(max_align_t) unsigned char p[100];
+  alignas(max_align_t) unsigned char p[100] = {0};
   {
     using L = Layout<int32_t>;
     EXPECT_EQ(0, L::Partial(0).Slice<0>(p).size());
@@ -761,7 +926,7 @@ TEST(Layout, MutableSliceByIndexSize) {
 }
 
 TEST(Layout, MutableSliceByTypeSize) {
-  alignas(max_align_t) unsigned char p[100];
+  alignas(max_align_t) unsigned char p[100] = {0};
   {
     using L = Layout<int32_t>;
     EXPECT_EQ(0, L::Partial(0).Slice<int32_t>(p).size());
@@ -782,72 +947,110 @@ TEST(Layout, MutableSliceByTypeSize) {
   }
 }
 
+TEST(Layout, StaticSliceSize) {
+  alignas(max_align_t) const unsigned char cp[100] = {0};
+  alignas(max_align_t) unsigned char p[100] = {0};
+  using L = Layout<int8_t, int32_t, Int128>;
+  using SL = L::WithStaticSizes<3, 5>;
+
+  EXPECT_EQ(3, SL::Partial().Slice<0>(cp).size());
+  EXPECT_EQ(3, SL::Partial().Slice<int8_t>(cp).size());
+  EXPECT_EQ(3, SL::Partial(7).Slice<0>(cp).size());
+  EXPECT_EQ(3, SL::Partial(7).Slice<int8_t>(cp).size());
+
+  EXPECT_EQ(5, SL::Partial().Slice<1>(cp).size());
+  EXPECT_EQ(5, SL::Partial().Slice<int32_t>(cp).size());
+  EXPECT_EQ(5, SL::Partial(7).Slice<1>(cp).size());
+  EXPECT_EQ(5, SL::Partial(7).Slice<int32_t>(cp).size());
+
+  EXPECT_EQ(7, SL::Partial(7).Slice<2>(cp).size());
+  EXPECT_EQ(7, SL::Partial(7).Slice<Int128>(cp).size());
+
+  EXPECT_EQ(3, SL::Partial().Slice<0>(p).size());
+  EXPECT_EQ(3, SL::Partial().Slice<int8_t>(p).size());
+  EXPECT_EQ(3, SL::Partial(7).Slice<0>(p).size());
+  EXPECT_EQ(3, SL::Partial(7).Slice<int8_t>(p).size());
+
+  EXPECT_EQ(5, SL::Partial().Slice<1>(p).size());
+  EXPECT_EQ(5, SL::Partial().Slice<int32_t>(p).size());
+  EXPECT_EQ(5, SL::Partial(7).Slice<1>(p).size());
+  EXPECT_EQ(5, SL::Partial(7).Slice<int32_t>(p).size());
+
+  EXPECT_EQ(7, SL::Partial(7).Slice<2>(p).size());
+  EXPECT_EQ(7, SL::Partial(7).Slice<Int128>(p).size());
+}
+
 TEST(Layout, SliceByIndexData) {
-  alignas(max_align_t) const unsigned char p[100] = {};
+  alignas(max_align_t) const unsigned char p[100] = {0};
   {
     using L = Layout<int32_t>;
     EXPECT_EQ(
-        0,
-        Distance(p, Type<Span<const int32_t>>(L::Partial(0).Slice<0>(p)).data()));
+        0, Distance(
+               p, Type<Span<const int32_t>>(L::Partial(0).Slice<0>(p)).data()));
     EXPECT_EQ(
-        0,
-        Distance(p, Type<Span<const int32_t>>(L::Partial(3).Slice<0>(p)).data()));
-    EXPECT_EQ(0, Distance(p, Type<Span<const int32_t>>(L(3).Slice<0>(p)).data()));
+        0, Distance(
+               p, Type<Span<const int32_t>>(L::Partial(3).Slice<0>(p)).data()));
+    EXPECT_EQ(0,
+              Distance(p, Type<Span<const int32_t>>(L(3).Slice<0>(p)).data()));
   }
   {
     using L = Layout<int32_t, int32_t>;
     EXPECT_EQ(
-        0,
-        Distance(p, Type<Span<const int32_t>>(L::Partial(3).Slice<0>(p)).data()));
+        0, Distance(
+               p, Type<Span<const int32_t>>(L::Partial(3).Slice<0>(p)).data()));
     EXPECT_EQ(
         0,
-        Distance(p,
-                 Type<Span<const int32_t>>(L::Partial(3, 5).Slice<0>(p)).data()));
+        Distance(
+            p, Type<Span<const int32_t>>(L::Partial(3, 5).Slice<0>(p)).data()));
     EXPECT_EQ(
         12,
-        Distance(p,
-                 Type<Span<const int32_t>>(L::Partial(3, 5).Slice<1>(p)).data()));
-    EXPECT_EQ(0,
-              Distance(p, Type<Span<const int32_t>>(L(3, 5).Slice<0>(p)).data()));
-    EXPECT_EQ(12,
-              Distance(p, Type<Span<const int32_t>>(L(3, 5).Slice<1>(p)).data()));
+        Distance(
+            p, Type<Span<const int32_t>>(L::Partial(3, 5).Slice<1>(p)).data()));
+    EXPECT_EQ(
+        0, Distance(p, Type<Span<const int32_t>>(L(3, 5).Slice<0>(p)).data()));
+    EXPECT_EQ(
+        12, Distance(p, Type<Span<const int32_t>>(L(3, 5).Slice<1>(p)).data()));
   }
   {
     using L = Layout<int8_t, int32_t, Int128>;
     EXPECT_EQ(
-        0,
-        Distance(p, Type<Span<const int8_t>>(L::Partial(0).Slice<0>(p)).data()));
-    EXPECT_EQ(
-        0,
-        Distance(p, Type<Span<const int8_t>>(L::Partial(1).Slice<0>(p)).data()));
+        0, Distance(
+               p, Type<Span<const int8_t>>(L::Partial(0).Slice<0>(p)).data()));
     EXPECT_EQ(
-        0,
-        Distance(p, Type<Span<const int8_t>>(L::Partial(5).Slice<0>(p)).data()));
+        0, Distance(
+               p, Type<Span<const int8_t>>(L::Partial(1).Slice<0>(p)).data()));
     EXPECT_EQ(
         0, Distance(
-               p, Type<Span<const int8_t>>(L::Partial(0, 0).Slice<0>(p)).data()));
+               p, Type<Span<const int8_t>>(L::Partial(5).Slice<0>(p)).data()));
     EXPECT_EQ(
         0,
-        Distance(p,
-                 Type<Span<const int32_t>>(L::Partial(0, 0).Slice<1>(p)).data()));
+        Distance(
+            p, Type<Span<const int8_t>>(L::Partial(0, 0).Slice<0>(p)).data()));
     EXPECT_EQ(
-        0, Distance(
-               p, Type<Span<const int8_t>>(L::Partial(1, 0).Slice<0>(p)).data()));
+        0,
+        Distance(
+            p, Type<Span<const int32_t>>(L::Partial(0, 0).Slice<1>(p)).data()));
+    EXPECT_EQ(
+        0,
+        Distance(
+            p, Type<Span<const int8_t>>(L::Partial(1, 0).Slice<0>(p)).data()));
     EXPECT_EQ(
         4,
-        Distance(p,
-                 Type<Span<const int32_t>>(L::Partial(1, 0).Slice<1>(p)).data()));
+        Distance(
+            p, Type<Span<const int32_t>>(L::Partial(1, 0).Slice<1>(p)).data()));
     EXPECT_EQ(
-        0, Distance(
-               p, Type<Span<const int8_t>>(L::Partial(5, 3).Slice<0>(p)).data()));
+        0,
+        Distance(
+            p, Type<Span<const int8_t>>(L::Partial(5, 3).Slice<0>(p)).data()));
     EXPECT_EQ(
         8,
-        Distance(p,
-                 Type<Span<const int32_t>>(L::Partial(5, 3).Slice<1>(p)).data()));
+        Distance(
+            p, Type<Span<const int32_t>>(L::Partial(5, 3).Slice<1>(p)).data()));
     EXPECT_EQ(
         0,
         Distance(
-            p, Type<Span<const int8_t>>(L::Partial(0, 0, 0).Slice<0>(p)).data()));
+            p,
+            Type<Span<const int8_t>>(L::Partial(0, 0, 0).Slice<0>(p)).data()));
     EXPECT_EQ(
         0,
         Distance(
@@ -861,7 +1064,8 @@ TEST(Layout, SliceByIndexData) {
     EXPECT_EQ(
         0,
         Distance(
-            p, Type<Span<const int8_t>>(L::Partial(1, 0, 0).Slice<0>(p)).data()));
+            p,
+            Type<Span<const int8_t>>(L::Partial(1, 0, 0).Slice<0>(p)).data()));
     EXPECT_EQ(
         4,
         Distance(
@@ -875,7 +1079,8 @@ TEST(Layout, SliceByIndexData) {
     EXPECT_EQ(
         0,
         Distance(
-            p, Type<Span<const int8_t>>(L::Partial(5, 3, 1).Slice<0>(p)).data()));
+            p,
+            Type<Span<const int8_t>>(L::Partial(5, 3, 1).Slice<0>(p)).data()));
     EXPECT_EQ(
         24,
         Distance(
@@ -887,133 +1092,132 @@ TEST(Layout, SliceByIndexData) {
             p,
             Type<Span<const int32_t>>(L::Partial(5, 3, 1).Slice<1>(p)).data()));
     EXPECT_EQ(
-        0, Distance(p, Type<Span<const int8_t>>(L(5, 3, 1).Slice<0>(p)).data()));
+        0,
+        Distance(p, Type<Span<const int8_t>>(L(5, 3, 1).Slice<0>(p)).data()));
     EXPECT_EQ(
         24,
         Distance(p, Type<Span<const Int128>>(L(5, 3, 1).Slice<2>(p)).data()));
     EXPECT_EQ(
-        8, Distance(p, Type<Span<const int32_t>>(L(5, 3, 1).Slice<1>(p)).data()));
+        8,
+        Distance(p, Type<Span<const int32_t>>(L(5, 3, 1).Slice<1>(p)).data()));
   }
 }
 
 TEST(Layout, SliceByTypeData) {
-  alignas(max_align_t) const unsigned char p[100] = {};
+  alignas(max_align_t) const unsigned char p[100] = {0};
   {
     using L = Layout<int32_t>;
     EXPECT_EQ(
         0,
         Distance(
-            p, Type<Span<const int32_t>>(L::Partial(0).Slice<int32_t>(p)).data()));
+            p,
+            Type<Span<const int32_t>>(L::Partial(0).Slice<int32_t>(p)).data()));
     EXPECT_EQ(
         0,
         Distance(
-            p, Type<Span<const int32_t>>(L::Partial(3).Slice<int32_t>(p)).data()));
+            p,
+            Type<Span<const int32_t>>(L::Partial(3).Slice<int32_t>(p)).data()));
     EXPECT_EQ(
-        0, Distance(p, Type<Span<const int32_t>>(L(3).Slice<int32_t>(p)).data()));
+        0,
+        Distance(p, Type<Span<const int32_t>>(L(3).Slice<int32_t>(p)).data()));
   }
   {
     using L = Layout<int8_t, int32_t, Int128>;
-    EXPECT_EQ(
-        0, Distance(
-               p, Type<Span<const int8_t>>(L::Partial(0).Slice<int8_t>(p)).data()));
-    EXPECT_EQ(
-        0, Distance(
-               p, Type<Span<const int8_t>>(L::Partial(1).Slice<int8_t>(p)).data()));
-    EXPECT_EQ(
-        0, Distance(
-               p, Type<Span<const int8_t>>(L::Partial(5).Slice<int8_t>(p)).data()));
-    EXPECT_EQ(
-        0,
-        Distance(
-            p, Type<Span<const int8_t>>(L::Partial(0, 0).Slice<int8_t>(p)).data()));
     EXPECT_EQ(
         0,
         Distance(
             p,
-            Type<Span<const int32_t>>(L::Partial(0, 0).Slice<int32_t>(p)).data()));
+            Type<Span<const int8_t>>(L::Partial(0).Slice<int8_t>(p)).data()));
     EXPECT_EQ(
         0,
-        Distance(
-            p, Type<Span<const int8_t>>(L::Partial(1, 0).Slice<int8_t>(p)).data()));
-    EXPECT_EQ(
-        4,
         Distance(
             p,
-            Type<Span<const int32_t>>(L::Partial(1, 0).Slice<int32_t>(p)).data()));
+            Type<Span<const int8_t>>(L::Partial(1).Slice<int8_t>(p)).data()));
     EXPECT_EQ(
         0,
-        Distance(
-            p, Type<Span<const int8_t>>(L::Partial(5, 3).Slice<int8_t>(p)).data()));
-    EXPECT_EQ(
-        8,
         Distance(
             p,
-            Type<Span<const int32_t>>(L::Partial(5, 3).Slice<int32_t>(p)).data()));
+            Type<Span<const int8_t>>(L::Partial(5).Slice<int8_t>(p)).data()));
     EXPECT_EQ(
         0,
-        Distance(
-            p,
-            Type<Span<const int8_t>>(L::Partial(0, 0, 0).Slice<int8_t>(p)).data()));
+        Distance(p, Type<Span<const int8_t>>(L::Partial(0, 0).Slice<int8_t>(p))
+                        .data()));
+    EXPECT_EQ(0, Distance(p, Type<Span<const int32_t>>(
+                                 L::Partial(0, 0).Slice<int32_t>(p))
+                                 .data()));
     EXPECT_EQ(
         0,
-        Distance(p, Type<Span<const int32_t>>(L::Partial(0, 0, 0).Slice<int32_t>(p))
+        Distance(p, Type<Span<const int8_t>>(L::Partial(1, 0).Slice<int8_t>(p))
                         .data()));
-    EXPECT_EQ(0, Distance(p, Type<Span<const Int128>>(
-                                 L::Partial(0, 0, 0).Slice<Int128>(p))
+    EXPECT_EQ(4, Distance(p, Type<Span<const int32_t>>(
+                                 L::Partial(1, 0).Slice<int32_t>(p))
                                  .data()));
     EXPECT_EQ(
         0,
-        Distance(
-            p,
-            Type<Span<const int8_t>>(L::Partial(1, 0, 0).Slice<int8_t>(p)).data()));
-    EXPECT_EQ(
-        4,
-        Distance(p, Type<Span<const int32_t>>(L::Partial(1, 0, 0).Slice<int32_t>(p))
+        Distance(p, Type<Span<const int8_t>>(L::Partial(5, 3).Slice<int8_t>(p))
                         .data()));
+    EXPECT_EQ(8, Distance(p, Type<Span<const int32_t>>(
+                                 L::Partial(5, 3).Slice<int32_t>(p))
+                                 .data()));
+    EXPECT_EQ(0, Distance(p, Type<Span<const int8_t>>(
+                                 L::Partial(0, 0, 0).Slice<int8_t>(p))
+                                 .data()));
+    EXPECT_EQ(0, Distance(p, Type<Span<const int32_t>>(
+                                 L::Partial(0, 0, 0).Slice<int32_t>(p))
+                                 .data()));
+    EXPECT_EQ(0, Distance(p, Type<Span<const Int128>>(
+                                 L::Partial(0, 0, 0).Slice<Int128>(p))
+                                 .data()));
+    EXPECT_EQ(0, Distance(p, Type<Span<const int8_t>>(
+                                 L::Partial(1, 0, 0).Slice<int8_t>(p))
+                                 .data()));
+    EXPECT_EQ(4, Distance(p, Type<Span<const int32_t>>(
+                                 L::Partial(1, 0, 0).Slice<int32_t>(p))
+                                 .data()));
     EXPECT_EQ(8, Distance(p, Type<Span<const Int128>>(
                                  L::Partial(1, 0, 0).Slice<Int128>(p))
                                  .data()));
-    EXPECT_EQ(
-        0,
-        Distance(
-            p,
-            Type<Span<const int8_t>>(L::Partial(5, 3, 1).Slice<int8_t>(p)).data()));
+    EXPECT_EQ(0, Distance(p, Type<Span<const int8_t>>(
+                                 L::Partial(5, 3, 1).Slice<int8_t>(p))
+                                 .data()));
     EXPECT_EQ(24, Distance(p, Type<Span<const Int128>>(
                                   L::Partial(5, 3, 1).Slice<Int128>(p))
                                   .data()));
-    EXPECT_EQ(
-        8,
-        Distance(p, Type<Span<const int32_t>>(L::Partial(5, 3, 1).Slice<int32_t>(p))
-                        .data()));
+    EXPECT_EQ(8, Distance(p, Type<Span<const int32_t>>(
+                                 L::Partial(5, 3, 1).Slice<int32_t>(p))
+                                 .data()));
     EXPECT_EQ(
         0,
-        Distance(p, Type<Span<const int8_t>>(L(5, 3, 1).Slice<int8_t>(p)).data()));
+        Distance(p,
+                 Type<Span<const int8_t>>(L(5, 3, 1).Slice<int8_t>(p)).data()));
     EXPECT_EQ(
         24,
         Distance(p,
                  Type<Span<const Int128>>(L(5, 3, 1).Slice<Int128>(p)).data()));
     EXPECT_EQ(
-        8, Distance(
-               p, Type<Span<const int32_t>>(L(5, 3, 1).Slice<int32_t>(p)).data()));
+        8,
+        Distance(
+            p, Type<Span<const int32_t>>(L(5, 3, 1).Slice<int32_t>(p)).data()));
   }
 }
 
 TEST(Layout, MutableSliceByIndexData) {
-  alignas(max_align_t) unsigned char p[100];
+  alignas(max_align_t) unsigned char p[100] = {0};
   {
     using L = Layout<int32_t>;
-    EXPECT_EQ(0,
-              Distance(p, Type<Span<int32_t>>(L::Partial(0).Slice<0>(p)).data()));
-    EXPECT_EQ(0,
-              Distance(p, Type<Span<int32_t>>(L::Partial(3).Slice<0>(p)).data()));
+    EXPECT_EQ(
+        0, Distance(p, Type<Span<int32_t>>(L::Partial(0).Slice<0>(p)).data()));
+    EXPECT_EQ(
+        0, Distance(p, Type<Span<int32_t>>(L::Partial(3).Slice<0>(p)).data()));
     EXPECT_EQ(0, Distance(p, Type<Span<int32_t>>(L(3).Slice<0>(p)).data()));
   }
   {
     using L = Layout<int32_t, int32_t>;
-    EXPECT_EQ(0,
-              Distance(p, Type<Span<int32_t>>(L::Partial(3).Slice<0>(p)).data()));
     EXPECT_EQ(
-        0, Distance(p, Type<Span<int32_t>>(L::Partial(3, 5).Slice<0>(p)).data()));
+        0, Distance(p, Type<Span<int32_t>>(L::Partial(3).Slice<0>(p)).data()));
+    EXPECT_EQ(
+        0,
+        Distance(p, Type<Span<int32_t>>(L::Partial(3, 5).Slice<0>(p)).data()));
     EXPECT_EQ(
         12,
         Distance(p, Type<Span<int32_t>>(L::Partial(3, 5).Slice<1>(p)).data()));
@@ -1022,123 +1226,149 @@ TEST(Layout, MutableSliceByIndexData) {
   }
   {
     using L = Layout<int8_t, int32_t, Int128>;
-    EXPECT_EQ(0,
-              Distance(p, Type<Span<int8_t>>(L::Partial(0).Slice<0>(p)).data()));
-    EXPECT_EQ(0,
-              Distance(p, Type<Span<int8_t>>(L::Partial(1).Slice<0>(p)).data()));
-    EXPECT_EQ(0,
-              Distance(p, Type<Span<int8_t>>(L::Partial(5).Slice<0>(p)).data()));
-    EXPECT_EQ(
-        0, Distance(p, Type<Span<int8_t>>(L::Partial(0, 0).Slice<0>(p)).data()));
     EXPECT_EQ(
-        0, Distance(p, Type<Span<int32_t>>(L::Partial(0, 0).Slice<1>(p)).data()));
+        0, Distance(p, Type<Span<int8_t>>(L::Partial(0).Slice<0>(p)).data()));
     EXPECT_EQ(
-        0, Distance(p, Type<Span<int8_t>>(L::Partial(1, 0).Slice<0>(p)).data()));
+        0, Distance(p, Type<Span<int8_t>>(L::Partial(1).Slice<0>(p)).data()));
     EXPECT_EQ(
-        4, Distance(p, Type<Span<int32_t>>(L::Partial(1, 0).Slice<1>(p)).data()));
+        0, Distance(p, Type<Span<int8_t>>(L::Partial(5).Slice<0>(p)).data()));
     EXPECT_EQ(
-        0, Distance(p, Type<Span<int8_t>>(L::Partial(5, 3).Slice<0>(p)).data()));
+        0,
+        Distance(p, Type<Span<int8_t>>(L::Partial(0, 0).Slice<0>(p)).data()));
     EXPECT_EQ(
-        8, Distance(p, Type<Span<int32_t>>(L::Partial(5, 3).Slice<1>(p)).data()));
+        0,
+        Distance(p, Type<Span<int32_t>>(L::Partial(0, 0).Slice<1>(p)).data()));
     EXPECT_EQ(
         0,
-        Distance(p, Type<Span<int8_t>>(L::Partial(0, 0, 0).Slice<0>(p)).data()));
+        Distance(p, Type<Span<int8_t>>(L::Partial(1, 0).Slice<0>(p)).data()));
+    EXPECT_EQ(
+        4,
+        Distance(p, Type<Span<int32_t>>(L::Partial(1, 0).Slice<1>(p)).data()));
     EXPECT_EQ(
         0,
-        Distance(p, Type<Span<int32_t>>(L::Partial(0, 0, 0).Slice<1>(p)).data()));
+        Distance(p, Type<Span<int8_t>>(L::Partial(5, 3).Slice<0>(p)).data()));
+    EXPECT_EQ(
+        8,
+        Distance(p, Type<Span<int32_t>>(L::Partial(5, 3).Slice<1>(p)).data()));
+    EXPECT_EQ(
+        0, Distance(
+               p, Type<Span<int8_t>>(L::Partial(0, 0, 0).Slice<0>(p)).data()));
+    EXPECT_EQ(
+        0, Distance(
+               p, Type<Span<int32_t>>(L::Partial(0, 0, 0).Slice<1>(p)).data()));
     EXPECT_EQ(
         0, Distance(
                p, Type<Span<Int128>>(L::Partial(0, 0, 0).Slice<2>(p)).data()));
     EXPECT_EQ(
-        0,
-        Distance(p, Type<Span<int8_t>>(L::Partial(1, 0, 0).Slice<0>(p)).data()));
+        0, Distance(
+               p, Type<Span<int8_t>>(L::Partial(1, 0, 0).Slice<0>(p)).data()));
     EXPECT_EQ(
-        4,
-        Distance(p, Type<Span<int32_t>>(L::Partial(1, 0, 0).Slice<1>(p)).data()));
+        4, Distance(
+               p, Type<Span<int32_t>>(L::Partial(1, 0, 0).Slice<1>(p)).data()));
     EXPECT_EQ(
         8, Distance(
                p, Type<Span<Int128>>(L::Partial(1, 0, 0).Slice<2>(p)).data()));
     EXPECT_EQ(
-        0,
-        Distance(p, Type<Span<int8_t>>(L::Partial(5, 3, 1).Slice<0>(p)).data()));
+        0, Distance(
+               p, Type<Span<int8_t>>(L::Partial(5, 3, 1).Slice<0>(p)).data()));
     EXPECT_EQ(
         24, Distance(
                 p, Type<Span<Int128>>(L::Partial(5, 3, 1).Slice<2>(p)).data()));
     EXPECT_EQ(
-        8,
-        Distance(p, Type<Span<int32_t>>(L::Partial(5, 3, 1).Slice<1>(p)).data()));
-    EXPECT_EQ(0, Distance(p, Type<Span<int8_t>>(L(5, 3, 1).Slice<0>(p)).data()));
+        8, Distance(
+               p, Type<Span<int32_t>>(L::Partial(5, 3, 1).Slice<1>(p)).data()));
+    EXPECT_EQ(0,
+              Distance(p, Type<Span<int8_t>>(L(5, 3, 1).Slice<0>(p)).data()));
     EXPECT_EQ(24,
               Distance(p, Type<Span<Int128>>(L(5, 3, 1).Slice<2>(p)).data()));
-    EXPECT_EQ(8, Distance(p, Type<Span<int32_t>>(L(5, 3, 1).Slice<1>(p)).data()));
+    EXPECT_EQ(8,
+              Distance(p, Type<Span<int32_t>>(L(5, 3, 1).Slice<1>(p)).data()));
   }
 }
 
 TEST(Layout, MutableSliceByTypeData) {
-  alignas(max_align_t) unsigned char p[100];
+  alignas(max_align_t) unsigned char p[100] = {0};
   {
     using L = Layout<int32_t>;
     EXPECT_EQ(
-        0,
-        Distance(p, Type<Span<int32_t>>(L::Partial(0).Slice<int32_t>(p)).data()));
+        0, Distance(
+               p, Type<Span<int32_t>>(L::Partial(0).Slice<int32_t>(p)).data()));
     EXPECT_EQ(
-        0,
-        Distance(p, Type<Span<int32_t>>(L::Partial(3).Slice<int32_t>(p)).data()));
-    EXPECT_EQ(0, Distance(p, Type<Span<int32_t>>(L(3).Slice<int32_t>(p)).data()));
+        0, Distance(
+               p, Type<Span<int32_t>>(L::Partial(3).Slice<int32_t>(p)).data()));
+    EXPECT_EQ(0,
+              Distance(p, Type<Span<int32_t>>(L(3).Slice<int32_t>(p)).data()));
   }
   {
     using L = Layout<int8_t, int32_t, Int128>;
     EXPECT_EQ(
-        0, Distance(p, Type<Span<int8_t>>(L::Partial(0).Slice<int8_t>(p)).data()));
+        0,
+        Distance(p, Type<Span<int8_t>>(L::Partial(0).Slice<int8_t>(p)).data()));
     EXPECT_EQ(
-        0, Distance(p, Type<Span<int8_t>>(L::Partial(1).Slice<int8_t>(p)).data()));
+        0,
+        Distance(p, Type<Span<int8_t>>(L::Partial(1).Slice<int8_t>(p)).data()));
     EXPECT_EQ(
-        0, Distance(p, Type<Span<int8_t>>(L::Partial(5).Slice<int8_t>(p)).data()));
+        0,
+        Distance(p, Type<Span<int8_t>>(L::Partial(5).Slice<int8_t>(p)).data()));
     EXPECT_EQ(
         0,
-        Distance(p, Type<Span<int8_t>>(L::Partial(0, 0).Slice<int8_t>(p)).data()));
+        Distance(p,
+                 Type<Span<int8_t>>(L::Partial(0, 0).Slice<int8_t>(p)).data()));
     EXPECT_EQ(
-        0, Distance(
-               p, Type<Span<int32_t>>(L::Partial(0, 0).Slice<int32_t>(p)).data()));
+        0,
+        Distance(
+            p, Type<Span<int32_t>>(L::Partial(0, 0).Slice<int32_t>(p)).data()));
     EXPECT_EQ(
         0,
-        Distance(p, Type<Span<int8_t>>(L::Partial(1, 0).Slice<int8_t>(p)).data()));
+        Distance(p,
+                 Type<Span<int8_t>>(L::Partial(1, 0).Slice<int8_t>(p)).data()));
     EXPECT_EQ(
-        4, Distance(
-               p, Type<Span<int32_t>>(L::Partial(1, 0).Slice<int32_t>(p)).data()));
+        4,
+        Distance(
+            p, Type<Span<int32_t>>(L::Partial(1, 0).Slice<int32_t>(p)).data()));
     EXPECT_EQ(
         0,
-        Distance(p, Type<Span<int8_t>>(L::Partial(5, 3).Slice<int8_t>(p)).data()));
+        Distance(p,
+                 Type<Span<int8_t>>(L::Partial(5, 3).Slice<int8_t>(p)).data()));
     EXPECT_EQ(
-        8, Distance(
-               p, Type<Span<int32_t>>(L::Partial(5, 3).Slice<int32_t>(p)).data()));
+        8,
+        Distance(
+            p, Type<Span<int32_t>>(L::Partial(5, 3).Slice<int32_t>(p)).data()));
     EXPECT_EQ(
-        0, Distance(
-               p, Type<Span<int8_t>>(L::Partial(0, 0, 0).Slice<int8_t>(p)).data()));
+        0,
+        Distance(
+            p,
+            Type<Span<int8_t>>(L::Partial(0, 0, 0).Slice<int8_t>(p)).data()));
     EXPECT_EQ(
         0,
         Distance(
-            p, Type<Span<int32_t>>(L::Partial(0, 0, 0).Slice<int32_t>(p)).data()));
+            p,
+            Type<Span<int32_t>>(L::Partial(0, 0, 0).Slice<int32_t>(p)).data()));
     EXPECT_EQ(
         0,
         Distance(
             p,
             Type<Span<Int128>>(L::Partial(0, 0, 0).Slice<Int128>(p)).data()));
     EXPECT_EQ(
-        0, Distance(
-               p, Type<Span<int8_t>>(L::Partial(1, 0, 0).Slice<int8_t>(p)).data()));
+        0,
+        Distance(
+            p,
+            Type<Span<int8_t>>(L::Partial(1, 0, 0).Slice<int8_t>(p)).data()));
     EXPECT_EQ(
         4,
         Distance(
-            p, Type<Span<int32_t>>(L::Partial(1, 0, 0).Slice<int32_t>(p)).data()));
+            p,
+            Type<Span<int32_t>>(L::Partial(1, 0, 0).Slice<int32_t>(p)).data()));
     EXPECT_EQ(
         8,
         Distance(
             p,
             Type<Span<Int128>>(L::Partial(1, 0, 0).Slice<Int128>(p)).data()));
     EXPECT_EQ(
-        0, Distance(
-               p, Type<Span<int8_t>>(L::Partial(5, 3, 1).Slice<int8_t>(p)).data()));
+        0,
+        Distance(
+            p,
+            Type<Span<int8_t>>(L::Partial(5, 3, 1).Slice<int8_t>(p)).data()));
     EXPECT_EQ(
         24,
         Distance(
@@ -1147,17 +1377,52 @@ TEST(Layout, MutableSliceByTypeData) {
     EXPECT_EQ(
         8,
         Distance(
-            p, Type<Span<int32_t>>(L::Partial(5, 3, 1).Slice<int32_t>(p)).data()));
-    EXPECT_EQ(0,
-              Distance(p, Type<Span<int8_t>>(L(5, 3, 1).Slice<int8_t>(p)).data()));
+            p,
+            Type<Span<int32_t>>(L::Partial(5, 3, 1).Slice<int32_t>(p)).data()));
+    EXPECT_EQ(
+        0, Distance(p, Type<Span<int8_t>>(L(5, 3, 1).Slice<int8_t>(p)).data()));
     EXPECT_EQ(
         24,
         Distance(p, Type<Span<Int128>>(L(5, 3, 1).Slice<Int128>(p)).data()));
     EXPECT_EQ(
-        8, Distance(p, Type<Span<int32_t>>(L(5, 3, 1).Slice<int32_t>(p)).data()));
+        8,
+        Distance(p, Type<Span<int32_t>>(L(5, 3, 1).Slice<int32_t>(p)).data()));
   }
 }
 
+TEST(Layout, StaticSliceData) {
+  alignas(max_align_t) const unsigned char cp[100] = {0};
+  alignas(max_align_t) unsigned char p[100] = {0};
+  using L = Layout<int8_t, int32_t, Int128>;
+  using SL = L::WithStaticSizes<3, 5>;
+
+  EXPECT_EQ(0, Distance(cp, SL::Partial().Slice<0>(cp).data()));
+  EXPECT_EQ(0, Distance(cp, SL::Partial().Slice<int8_t>(cp).data()));
+  EXPECT_EQ(0, Distance(cp, SL::Partial(7).Slice<0>(cp).data()));
+  EXPECT_EQ(0, Distance(cp, SL::Partial(7).Slice<int8_t>(cp).data()));
+
+  EXPECT_EQ(4, Distance(cp, SL::Partial().Slice<1>(cp).data()));
+  EXPECT_EQ(4, Distance(cp, SL::Partial().Slice<int32_t>(cp).data()));
+  EXPECT_EQ(4, Distance(cp, SL::Partial(7).Slice<1>(cp).data()));
+  EXPECT_EQ(4, Distance(cp, SL::Partial(7).Slice<int32_t>(cp).data()));
+
+  EXPECT_EQ(24, Distance(cp, SL::Partial(7).Slice<2>(cp).data()));
+  EXPECT_EQ(24, Distance(cp, SL::Partial(7).Slice<Int128>(cp).data()));
+
+  EXPECT_EQ(0, Distance(p, SL::Partial().Slice<0>(p).data()));
+  EXPECT_EQ(0, Distance(p, SL::Partial().Slice<int8_t>(p).data()));
+  EXPECT_EQ(0, Distance(p, SL::Partial(7).Slice<0>(p).data()));
+  EXPECT_EQ(0, Distance(p, SL::Partial(7).Slice<int8_t>(p).data()));
+
+  EXPECT_EQ(4, Distance(p, SL::Partial().Slice<1>(p).data()));
+  EXPECT_EQ(4, Distance(p, SL::Partial().Slice<int32_t>(p).data()));
+  EXPECT_EQ(4, Distance(p, SL::Partial(7).Slice<1>(p).data()));
+  EXPECT_EQ(4, Distance(p, SL::Partial(7).Slice<int32_t>(p).data()));
+
+  EXPECT_EQ(24, Distance(p, SL::Partial(7).Slice<2>(p).data()));
+  EXPECT_EQ(24, Distance(p, SL::Partial(7).Slice<Int128>(p).data()));
+}
+
 MATCHER_P(IsSameSlice, slice, "") {
   return arg.size() == slice.size() && arg.data() == slice.data();
 }
@@ -1170,9 +1435,9 @@ class TupleMatcher {
   template <typename Tuple>
   bool MatchAndExplain(const Tuple& p,
                        testing::MatchResultListener* /* listener */) const {
-    static_assert(std::tuple_size<Tuple>::value == sizeof...(M), "");
+    static_assert(std::tuple_size_v<Tuple> == sizeof...(M));
     return MatchAndExplainImpl(
-        p, absl::make_index_sequence<std::tuple_size<Tuple>::value>{});
+        p, std::make_index_sequence<std::tuple_size_v<Tuple>>{});
   }
 
   // For the matcher concept. Left empty as we don't really need the diagnostics
@@ -1182,11 +1447,10 @@ class TupleMatcher {
 
  private:
   template <typename Tuple, size_t... Is>
-  bool MatchAndExplainImpl(const Tuple& p, absl::index_sequence<Is...>) const {
+  bool MatchAndExplainImpl(const Tuple& p, std::index_sequence<Is...>) const {
     // Using std::min as a simple variadic "and".
     return std::min(
-        {true, testing::SafeMatcherCast<
-                   const typename std::tuple_element<Is, Tuple>::type&>(
+        {true, testing::SafeMatcherCast<const std::tuple_element_t<Is, Tuple>&>(
                    std::get<Is>(matchers_))
                    .Matches(std::get<Is>(p))...});
   }
@@ -1201,7 +1465,7 @@ testing::PolymorphicMatcher<TupleMatcher<M...>> Tuple(M... matchers) {
 }
 
 TEST(Layout, Slices) {
-  alignas(max_align_t) const unsigned char p[100] = {};
+  alignas(max_align_t) const unsigned char p[100] = {0};
   using L = Layout<int8_t, int8_t, Int128>;
   {
     const auto x = L::Partial();
@@ -1235,7 +1499,7 @@ TEST(Layout, Slices) {
 }
 
 TEST(Layout, MutableSlices) {
-  alignas(max_align_t) unsigned char p[100] = {};
+  alignas(max_align_t) unsigned char p[100] = {0};
   using L = Layout<int8_t, int8_t, Int128>;
   {
     const auto x = L::Partial();
@@ -1253,17 +1517,54 @@ TEST(Layout, MutableSlices) {
   }
   {
     const auto x = L::Partial(1, 2, 3);
-    EXPECT_THAT(
-        (Type<std::tuple<Span<int8_t>, Span<int8_t>, Span<Int128>>>(x.Slices(p))),
-        Tuple(IsSameSlice(x.Slice<0>(p)), IsSameSlice(x.Slice<1>(p)),
-              IsSameSlice(x.Slice<2>(p))));
+    EXPECT_THAT((Type<std::tuple<Span<int8_t>, Span<int8_t>, Span<Int128>>>(
+                    x.Slices(p))),
+                Tuple(IsSameSlice(x.Slice<0>(p)), IsSameSlice(x.Slice<1>(p)),
+                      IsSameSlice(x.Slice<2>(p))));
   }
   {
     const L x(1, 2, 3);
+    EXPECT_THAT((Type<std::tuple<Span<int8_t>, Span<int8_t>, Span<Int128>>>(
+                    x.Slices(p))),
+                Tuple(IsSameSlice(x.Slice<0>(p)), IsSameSlice(x.Slice<1>(p)),
+                      IsSameSlice(x.Slice<2>(p))));
+  }
+}
+
+TEST(Layout, StaticSlices) {
+  alignas(max_align_t) const unsigned char cp[100] = {0};
+  alignas(max_align_t) unsigned char p[100] = {0};
+  using SL = Layout<int8_t, int8_t, Int128>::WithStaticSizes<1, 2>;
+  {
+    const auto x = SL::Partial();
     EXPECT_THAT(
-        (Type<std::tuple<Span<int8_t>, Span<int8_t>, Span<Int128>>>(x.Slices(p))),
-        Tuple(IsSameSlice(x.Slice<0>(p)), IsSameSlice(x.Slice<1>(p)),
-              IsSameSlice(x.Slice<2>(p))));
+        (Type<std::tuple<Span<const int8_t>, Span<const int8_t>>>(
+            x.Slices(cp))),
+        Tuple(IsSameSlice(x.Slice<0>(cp)), IsSameSlice(x.Slice<1>(cp))));
+    EXPECT_THAT((Type<std::tuple<Span<int8_t>, Span<int8_t>>>(x.Slices(p))),
+                Tuple(IsSameSlice(x.Slice<0>(p)), IsSameSlice(x.Slice<1>(p))));
+  }
+  {
+    const auto x = SL::Partial(3);
+    EXPECT_THAT((Type<std::tuple<Span<const int8_t>, Span<const int8_t>,
+                                 Span<const Int128>>>(x.Slices(cp))),
+                Tuple(IsSameSlice(x.Slice<0>(cp)), IsSameSlice(x.Slice<1>(cp)),
+                      IsSameSlice(x.Slice<2>(cp))));
+    EXPECT_THAT((Type<std::tuple<Span<int8_t>, Span<int8_t>, Span<Int128>>>(
+                    x.Slices(p))),
+                Tuple(IsSameSlice(x.Slice<0>(p)), IsSameSlice(x.Slice<1>(p)),
+                      IsSameSlice(x.Slice<2>(p))));
+  }
+  {
+    const SL x(3);
+    EXPECT_THAT((Type<std::tuple<Span<const int8_t>, Span<const int8_t>,
+                                 Span<const Int128>>>(x.Slices(cp))),
+                Tuple(IsSameSlice(x.Slice<0>(cp)), IsSameSlice(x.Slice<1>(cp)),
+                      IsSameSlice(x.Slice<2>(cp))));
+    EXPECT_THAT((Type<std::tuple<Span<int8_t>, Span<int8_t>, Span<Int128>>>(
+                    x.Slices(p))),
+                Tuple(IsSameSlice(x.Slice<0>(p)), IsSameSlice(x.Slice<1>(p)),
+                      IsSameSlice(x.Slice<2>(p))));
   }
 }
 
@@ -1283,29 +1584,72 @@ TEST(Layout, CustomAlignment) {
 TEST(Layout, OverAligned) {
   constexpr size_t M = alignof(max_align_t);
   constexpr Layout<unsigned char, Aligned<unsigned char, 2 * M>> x(1, 3);
+#ifdef __GNUC__
+  // Using __attribute__ ((aligned ())) instead of alignas to bypass a gcc bug:
+  // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89357
+  __attribute__((aligned(2 * M))) unsigned char p[x.AllocSize()];
+#else
   alignas(2 * M) unsigned char p[x.AllocSize()];
+#endif
   EXPECT_EQ(2 * M + 3, x.AllocSize());
   EXPECT_THAT(x.Pointers(p), Tuple(p + 0, p + 2 * M));
 }
 
 TEST(Layout, Alignment) {
-  static_assert(Layout<int8_t>::Alignment() == 1, "");
-  static_assert(Layout<int32_t>::Alignment() == 4, "");
-  static_assert(Layout<Int64>::Alignment() == 8, "");
-  static_assert(Layout<Aligned<int8_t, 64>>::Alignment() == 64, "");
-  static_assert(Layout<int8_t, int32_t, Int64>::Alignment() == 8, "");
-  static_assert(Layout<int8_t, Int64, int32_t>::Alignment() == 8, "");
-  static_assert(Layout<int32_t, int8_t, Int64>::Alignment() == 8, "");
-  static_assert(Layout<int32_t, Int64, int8_t>::Alignment() == 8, "");
-  static_assert(Layout<Int64, int8_t, int32_t>::Alignment() == 8, "");
-  static_assert(Layout<Int64, int32_t, int8_t>::Alignment() == 8, "");
+  static_assert(Layout<int8_t>::Alignment() == 1);
+  static_assert(Layout<int32_t>::Alignment() == 4);
+  static_assert(Layout<Int64>::Alignment() == 8);
+  static_assert(Layout<Aligned<int8_t, 64>>::Alignment() == 64);
+  static_assert(Layout<int8_t, int32_t, Int64>::Alignment() == 8);
+  static_assert(Layout<int8_t, Int64, int32_t>::Alignment() == 8);
+  static_assert(Layout<int32_t, int8_t, Int64>::Alignment() == 8);
+  static_assert(Layout<int32_t, Int64, int8_t>::Alignment() == 8);
+  static_assert(Layout<Int64, int8_t, int32_t>::Alignment() == 8);
+  static_assert(Layout<Int64, int32_t, int8_t>::Alignment() == 8);
+  static_assert(Layout<Int64, int32_t, int8_t>::Alignment() == 8);
+  static_assert(Layout<Aligned<int8_t, 64>>::WithStaticSizes<>::Alignment() ==
+                64);
+  static_assert(Layout<Aligned<int8_t, 64>>::WithStaticSizes<2>::Alignment() ==
+                64);
+}
+
+TEST(Layout, StaticAlignment) {
+  static_assert(Layout<int8_t>::WithStaticSizes<>::Alignment() == 1);
+  static_assert(Layout<int8_t>::WithStaticSizes<0>::Alignment() == 1);
+  static_assert(Layout<int8_t>::WithStaticSizes<7>::Alignment() == 1);
+  static_assert(Layout<int32_t>::WithStaticSizes<>::Alignment() == 4);
+  static_assert(Layout<int32_t>::WithStaticSizes<0>::Alignment() == 4);
+  static_assert(Layout<int32_t>::WithStaticSizes<3>::Alignment() == 4);
+  static_assert(Layout<Aligned<int8_t, 64>>::WithStaticSizes<>::Alignment() ==
+                64);
+  static_assert(Layout<Aligned<int8_t, 64>>::WithStaticSizes<0>::Alignment() ==
+                64);
+  static_assert(Layout<Aligned<int8_t, 64>>::WithStaticSizes<2>::Alignment() ==
+                64);
+  static_assert(
+      Layout<int32_t, Int64, int8_t>::WithStaticSizes<>::Alignment() == 8);
+  static_assert(
+      Layout<int32_t, Int64, int8_t>::WithStaticSizes<0, 0, 0>::Alignment() ==
+      8);
+  static_assert(
+      Layout<int32_t, Int64, int8_t>::WithStaticSizes<1, 1, 1>::Alignment() ==
+      8);
 }
 
 TEST(Layout, ConstexprPartial) {
   constexpr size_t M = alignof(max_align_t);
   constexpr Layout<unsigned char, Aligned<unsigned char, 2 * M>> x(1, 3);
-  static_assert(x.Partial(1).template Offset<1>() == 2 * M, "");
+  static_assert(x.Partial(1).template Offset<1>() == 2 * M);
+}
+
+TEST(Layout, StaticConstexpr) {
+  constexpr size_t M = alignof(max_align_t);
+  using L = Layout<unsigned char, Aligned<unsigned char, 2 * M>>;
+  using SL = L::WithStaticSizes<1, 3>;
+  constexpr SL x;
+  static_assert(x.Offset<1>() == 2 * M);
 }
+
 // [from, to)
 struct Region {
   size_t from;
@@ -1313,7 +1657,7 @@ struct Region {
 };
 
 void ExpectRegionPoisoned(const unsigned char* p, size_t n, bool poisoned) {
-#ifdef ADDRESS_SANITIZER
+#ifdef ABSL_HAVE_ADDRESS_SANITIZER
   for (size_t i = 0; i != n; ++i) {
     EXPECT_EQ(poisoned, __asan_address_is_poisoned(p + i));
   }
@@ -1380,6 +1724,41 @@ TEST(Layout, PoisonPadding) {
   }
 }
 
+TEST(Layout, StaticPoisonPadding) {
+  using L = Layout<int8_t, Int64, int32_t, Int128>;
+  using SL = L::WithStaticSizes<1, 2>;
+
+  constexpr size_t n = L::Partial(1, 2, 3, 4).AllocSize();
+  {
+    constexpr auto x = SL::Partial();
+    alignas(max_align_t) const unsigned char c[n] = {};
+    x.PoisonPadding(c);
+    EXPECT_EQ(x.Slices(c), x.Slices(c));
+    ExpectPoisoned(c, {{1, 8}});
+  }
+  {
+    constexpr auto x = SL::Partial(3);
+    alignas(max_align_t) const unsigned char c[n] = {};
+    x.PoisonPadding(c);
+    EXPECT_EQ(x.Slices(c), x.Slices(c));
+    ExpectPoisoned(c, {{1, 8}, {36, 40}});
+  }
+  {
+    constexpr auto x = SL::Partial(3, 4);
+    alignas(max_align_t) const unsigned char c[n] = {};
+    x.PoisonPadding(c);
+    EXPECT_EQ(x.Slices(c), x.Slices(c));
+    ExpectPoisoned(c, {{1, 8}, {36, 40}});
+  }
+  {
+    constexpr SL x(3, 4);
+    alignas(max_align_t) const unsigned char c[n] = {};
+    x.PoisonPadding(c);
+    EXPECT_EQ(x.Slices(c), x.Slices(c));
+    ExpectPoisoned(c, {{1, 8}, {36, 40}});
+  }
+}
+
 TEST(Layout, DebugString) {
   {
     constexpr auto x = Layout<int8_t, int32_t, int8_t, Int128>::Partial();
@@ -1395,7 +1774,8 @@ TEST(Layout, DebugString) {
               x.DebugString());
   }
   {
-    constexpr auto x = Layout<int8_t, int32_t, int8_t, Int128>::Partial(1, 2, 3);
+    constexpr auto x =
+        Layout<int8_t, int32_t, int8_t, Int128>::Partial(1, 2, 3);
     EXPECT_EQ(
         "@0<signed char>(1)[1]; @4<int>(4)[2]; @12<signed char>(1)[3]; "
         "@16" +
@@ -1403,7 +1783,8 @@ TEST(Layout, DebugString) {
         x.DebugString());
   }
   {
-    constexpr auto x = Layout<int8_t, int32_t, int8_t, Int128>::Partial(1, 2, 3, 4);
+    constexpr auto x =
+        Layout<int8_t, int32_t, int8_t, Int128>::Partial(1, 2, 3, 4);
     EXPECT_EQ(
         "@0<signed char>(1)[1]; @4<int>(4)[2]; @12<signed char>(1)[3]; "
         "@16" +
@@ -1420,6 +1801,62 @@ TEST(Layout, DebugString) {
   }
 }
 
+TEST(Layout, StaticDebugString) {
+  {
+    constexpr auto x =
+        Layout<int8_t, int32_t, int8_t, Int128>::WithStaticSizes<>::Partial();
+    EXPECT_EQ("@0<signed char>(1)", x.DebugString());
+  }
+  {
+    constexpr auto x =
+        Layout<int8_t, int32_t, int8_t, Int128>::WithStaticSizes<>::Partial(1);
+    EXPECT_EQ("@0<signed char>(1)[1]; @4<int>(4)", x.DebugString());
+  }
+  {
+    constexpr auto x =
+        Layout<int8_t, int32_t, int8_t, Int128>::WithStaticSizes<1>::Partial();
+    EXPECT_EQ("@0<signed char>(1)[1]; @4<int>(4)", x.DebugString());
+  }
+  {
+    constexpr auto x =
+        Layout<int8_t, int32_t, int8_t, Int128>::WithStaticSizes<>::Partial(1,
+                                                                            2);
+    EXPECT_EQ("@0<signed char>(1)[1]; @4<int>(4)[2]; @12<signed char>(1)",
+              x.DebugString());
+  }
+  {
+    constexpr auto x =
+        Layout<int8_t, int32_t, int8_t, Int128>::WithStaticSizes<1>::Partial(2);
+    EXPECT_EQ("@0<signed char>(1)[1]; @4<int>(4)[2]; @12<signed char>(1)",
+              x.DebugString());
+  }
+  {
+    constexpr auto x = Layout<int8_t, int32_t, int8_t,
+                              Int128>::WithStaticSizes<1, 2>::Partial();
+    EXPECT_EQ("@0<signed char>(1)[1]; @4<int>(4)[2]; @12<signed char>(1)",
+              x.DebugString());
+  }
+  {
+    constexpr auto x = Layout<int8_t, int32_t, int8_t,
+                              Int128>::WithStaticSizes<1, 2, 3, 4>::Partial();
+    EXPECT_EQ(
+        "@0<signed char>(1)[1]; @4<int>(4)[2]; @12<signed char>(1)[3]; "
+        "@16" +
+            Int128::Name() + "(16)[4]",
+        x.DebugString());
+  }
+  {
+    constexpr Layout<int8_t, int32_t, int8_t, Int128>::WithStaticSizes<1, 2, 3,
+                                                                       4>
+        x;
+    EXPECT_EQ(
+        "@0<signed char>(1)[1]; @4<int>(4)[2]; @12<signed char>(1)[3]; "
+        "@16" +
+            Int128::Name() + "(16)[4]",
+        x.DebugString());
+  }
+}
+
 TEST(Layout, CharTypes) {
   constexpr Layout<int32_t> x(1);
   alignas(max_align_t) char c[x.AllocSize()] = {};
@@ -1558,8 +1995,38 @@ TEST(CompactString, Works) {
   EXPECT_STREQ("hello", s.c_str());
 }
 
+// Same as the previous CompactString example, except we set the first array
+// size to 1 statically, since we know it is always 1. This allows us to compute
+// the offset of the character array at compile time.
+class StaticCompactString {
+ public:
+  StaticCompactString(const char* s = "") {  // NOLINT
+    const size_t size = strlen(s);
+    const SL layout(size + 1);
+    p_.reset(new unsigned char[layout.AllocSize()]);
+    layout.PoisonPadding(p_.get());
+    *layout.Pointer<size_t>(p_.get()) = size;
+    memcpy(layout.Pointer<char>(p_.get()), s, size + 1);
+  }
+
+  size_t size() const { return *SL::Partial().Pointer<size_t>(p_.get()); }
+
+  const char* c_str() const { return SL::Partial().Pointer<char>(p_.get()); }
+
+ private:
+  using SL = Layout<size_t, char>::WithStaticSizes<1>;
+  std::unique_ptr<unsigned char[]> p_;
+};
+
+TEST(StaticCompactString, Works) {
+  StaticCompactString s = "hello";
+  EXPECT_EQ(5, s.size());
+  EXPECT_STREQ("hello", s.c_str());
+}
+
 }  // namespace example
 
 }  // namespace
 }  // namespace container_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/container/internal/node_hash_policy.h b/absl/container/internal/node_slot_policy.h
similarity index 80%
rename from absl/container/internal/node_hash_policy.h
rename to absl/container/internal/node_slot_policy.h
index 19b4fc098fb..4a7dca27c4b 100644
--- a/absl/container/internal/node_hash_policy.h
+++ b/absl/container/internal/node_slot_policy.h
@@ -30,8 +30,8 @@
 // It may also optionally define `value()` and `apply()`. For documentation on
 // these, see hash_policy_traits.h.
 
-#ifndef ABSL_CONTAINER_INTERNAL_NODE_HASH_POLICY_H_
-#define ABSL_CONTAINER_INTERNAL_NODE_HASH_POLICY_H_
+#ifndef ABSL_CONTAINER_INTERNAL_NODE_SLOT_POLICY_H_
+#define ABSL_CONTAINER_INTERNAL_NODE_SLOT_POLICY_H_
 
 #include <cassert>
 #include <cstddef>
@@ -39,15 +39,17 @@
 #include <type_traits>
 #include <utility>
 
+#include "absl/base/config.h"
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
 template <class Reference, class Policy>
-struct node_hash_policy {
-  static_assert(std::is_lvalue_reference<Reference>::value, "");
+struct node_slot_policy {
+  static_assert(std::is_lvalue_reference_v<Reference>);
 
-  using slot_type = typename std::remove_cv<
-      typename std::remove_reference<Reference>::type>::type*;
+  using slot_type = std::remove_cv_t<std::remove_reference_t<Reference>>*;
 
   template <class Alloc, class... Args>
   static void construct(Alloc* alloc, slot_type* slot, Args&&... args) {
@@ -59,9 +61,12 @@ struct node_hash_policy {
     Policy::delete_element(alloc, *slot);
   }
 
+  // Returns true_type to indicate that transfer can use memcpy.
   template <class Alloc>
-  static void transfer(Alloc*, slot_type* new_slot, slot_type* old_slot) {
+  static std::true_type transfer(Alloc*, slot_type* new_slot,
+                                 slot_type* old_slot) {
     *new_slot = *old_slot;
+    return {};
   }
 
   static size_t space_used(const slot_type* slot) {
@@ -83,6 +88,7 @@ struct node_hash_policy {
 };
 
 }  // namespace container_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
-#endif  // ABSL_CONTAINER_INTERNAL_NODE_HASH_POLICY_H_
+#endif  // ABSL_CONTAINER_INTERNAL_NODE_SLOT_POLICY_H_
diff --git a/absl/container/internal/node_hash_policy_test.cc b/absl/container/internal/node_slot_policy_test.cc
similarity index 87%
rename from absl/container/internal/node_hash_policy_test.cc
rename to absl/container/internal/node_slot_policy_test.cc
index f1d3ec3014a..d4ea919d2c8 100644
--- a/absl/container/internal/node_hash_policy_test.cc
+++ b/absl/container/internal/node_slot_policy_test.cc
@@ -12,21 +12,23 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-#include "absl/container/internal/node_hash_policy.h"
+#include "absl/container/internal/node_slot_policy.h"
 
 #include <memory>
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
+#include "absl/base/config.h"
 #include "absl/container/internal/hash_policy_traits.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 namespace {
 
 using ::testing::Pointee;
 
-struct Policy : node_hash_policy<int&, Policy> {
+struct Policy : node_slot_policy<int&, Policy> {
   using key_type = int;
   using init_type = int;
 
@@ -60,8 +62,10 @@ TEST_F(NodeTest, transfer) {
   int* b = &s;
   NodePolicy::transfer(&alloc, &a, &b);
   EXPECT_EQ(&s, a);
+  EXPECT_TRUE(NodePolicy::transfer_uses_memcpy());
 }
 
 }  // namespace
 }  // namespace container_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/container/internal/raw_hash_map.h b/absl/container/internal/raw_hash_map.h
index 7dad120ad12..8301b50d084 100644
--- a/absl/container/internal/raw_hash_map.h
+++ b/absl/container/internal/raw_hash_map.h
@@ -19,15 +19,32 @@
 #include <type_traits>
 #include <utility>
 
-#include "absl/base/internal/throw_delegate.h"
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/throw_delegate.h"
+#include "absl/container/internal/common_policy_traits.h"
 #include "absl/container/internal/container_memory.h"
 #include "absl/container/internal/raw_hash_set.h"  // IWYU pragma: export
+#include "absl/meta/type_traits.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
-template <class Policy, class Hash, class Eq, class Alloc>
-class raw_hash_map : public raw_hash_set<Policy, Hash, Eq, Alloc> {
+template <class Policy, class... Params>
+class raw_hash_map;
+
+template <typename Policy, typename Hash, typename Eq, typename Alloc>
+struct InstantiateRawHashMap {
+  using type = typename ApplyWithoutDefaultSuffix<
+      raw_hash_map,
+      TypeList<int, typename Policy::DefaultHash, typename Policy::DefaultEq,
+               typename Policy::DefaultAlloc>,
+      TypeList<Policy, Hash, Eq, Alloc>>::type;
+};
+
+template <class Policy, class... Params>
+class raw_hash_map : public raw_hash_set<Policy, Params...> {
   // P is Policy. It's passed as a template argument to support maps that have
   // incomplete types as values, as in unordered_map<K, IncompleteType>.
   // MappedReference<> may be a non-reference type.
@@ -40,19 +57,49 @@ class raw_hash_map : public raw_hash_set<Policy, Hash, Eq, Alloc> {
   using MappedConstReference = decltype(P::value(
       std::addressof(std::declval<typename raw_hash_map::const_reference>())));
 
-  using KeyArgImpl =
-      KeyArg<IsTransparent<Eq>::value && IsTransparent<Hash>::value>;
+  using Hash = typename raw_hash_map::raw_hash_set::hasher;
+  using Eq = typename raw_hash_map::raw_hash_set::key_equal;
+  using Alloc = typename raw_hash_map::raw_hash_set::allocator_type;
+
+  template <class K>
+  using key_arg =
+      typename KeyArg<IsTransparent<Eq>::value && IsTransparent<Hash>::value>::
+          template type<K, typename Policy::key_type>;
+
+  // NOTE: The mess here is to shorten the code for the (very repetitive)
+  // function overloads, and to allow the lifetime-bound overloads to dispatch
+  // to the non-lifetime-bound overloads, to ensure there is a single source of
+  // truth for each overload set.
+  //
+  // Enabled if an assignment from the given type would require the
+  // source object to remain alive for the life of the element.
+  //
+  // TODO(b/402804213): Remove these traits and simplify the overloads whenever
+  // we have a better mechanism available to handle lifetime analysis.
+  template <class K, bool Value, typename = void>
+  using LifetimeBoundK = HasValue<
+      Value, std::conditional_t<policy_trait_element_is_owner<Policy>::value,
+                                std::false_type,
+                                type_traits_internal::IsLifetimeBoundAssignment<
+                                    typename Policy::key_type, K>>>;
+  template <class V, bool Value, typename = void>
+  using LifetimeBoundV =
+      HasValue<Value, type_traits_internal::IsLifetimeBoundAssignment<
+                          typename Policy::mapped_type, V>>;
+  template <class K, bool KValue, class V, bool VValue, typename... Dummy>
+  using LifetimeBoundKV =
+      std::conjunction<LifetimeBoundK<K, KValue, std::void_t<Dummy...>>,
+                       LifetimeBoundV<V, VValue>>;
 
  public:
   using key_type = typename Policy::key_type;
   using mapped_type = typename Policy::mapped_type;
-  template <class K>
-  using key_arg = typename KeyArgImpl::template type<K, key_type>;
 
-  static_assert(!std::is_reference<key_type>::value, "");
-  // TODO(alkis): remove this assertion and verify that reference mapped_type is
-  // supported.
-  static_assert(!std::is_reference<mapped_type>::value, "");
+  static_assert(!std::is_reference_v<key_type>);
+
+  // TODO(b/187807849): Evaluate whether to support reference mapped_type and
+  // remove this assertion if/when it is supported.
+  static_assert(!std::is_reference_v<mapped_type>);
 
   using iterator = typename raw_hash_map::raw_hash_set::iterator;
   using const_iterator = typename raw_hash_map::raw_hash_set::const_iterator;
@@ -67,129 +114,267 @@ class raw_hash_map : public raw_hash_set<Policy, Hash, Eq, Alloc> {
   //   union { int n : 1; };
   //   flat_hash_map<int, int> m;
   //   m.insert_or_assign(n, n);
-  template <class K = key_type, class V = mapped_type, K* = nullptr,
-            V* = nullptr>
-  std::pair<iterator, bool> insert_or_assign(key_arg<K>&& k, V&& v) {
-    return insert_or_assign_impl(std::forward<K>(k), std::forward<V>(v));
-  }
+  //
+  // TODO(b/402804213): Remove these macros whenever we have a better mechanism
+  // available to handle lifetime analysis.
+#define ABSL_INTERNAL_X(Func, Callee, KQual, VQual, KValue, VValue, Tail, ...) \
+  template <                                                                   \
+      typename K = key_type, class V = mapped_type,                            \
+      ABSL_INTERNAL_IF_##KValue##_NOR_##VValue(                                \
+          int = (EnableIf<LifetimeBoundKV<K, KValue, V, VValue,                \
+                                          IfRRef<int KQual>::AddPtr<K>,        \
+                                          IfRRef<int VQual>::AddPtr<V>>>()),   \
+          ABSL_INTERNAL_SINGLE_ARG(                                            \
+              int&...,                                                         \
+              decltype(EnableIf<LifetimeBoundKV<K, KValue, V, VValue>>()) =    \
+                  0))>                                                         \
+  decltype(auto) Func(                                                         \
+      __VA_ARGS__ key_arg<K> KQual k ABSL_INTERNAL_IF_##KValue(                \
+          ABSL_INTERNAL_ATTRIBUTE_CAPTURED_BY_THIS),                           \
+      V VQual v ABSL_INTERNAL_IF_##VValue(                                     \
+          ABSL_INTERNAL_ATTRIBUTE_CAPTURED_BY_THIS))                           \
+      ABSL_ATTRIBUTE_LIFETIME_BOUND {                                          \
+    return ABSL_INTERNAL_IF_##KValue##_OR_##VValue(                            \
+        (this->template Func<K, V, 0>), Callee)(                               \
+        std::forward<decltype(k)>(k), std::forward<decltype(v)>(v)) Tail;      \
+  }                                                                            \
+  static_assert(true, "This is to force a semicolon.")
 
-  template <class K = key_type, class V = mapped_type, K* = nullptr>
-  std::pair<iterator, bool> insert_or_assign(key_arg<K>&& k, const V& v) {
-    return insert_or_assign_impl(std::forward<K>(k), v);
-  }
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_impl, const&, const&,
+                  false, false, ABSL_INTERNAL_SINGLE_ARG());
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_impl, const&, const&,
+                  false, true, ABSL_INTERNAL_SINGLE_ARG());
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_impl, const&, const&, true,
+                  false, ABSL_INTERNAL_SINGLE_ARG());
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_impl, const&, const&, true,
+                  true, ABSL_INTERNAL_SINGLE_ARG());
 
-  template <class K = key_type, class V = mapped_type, V* = nullptr>
-  std::pair<iterator, bool> insert_or_assign(const key_arg<K>& k, V&& v) {
-    return insert_or_assign_impl(k, std::forward<V>(v));
-  }
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_impl, const&, &&, false,
+                  false, ABSL_INTERNAL_SINGLE_ARG());
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_impl, const&, &&, false,
+                  true, ABSL_INTERNAL_SINGLE_ARG());
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_impl, const&, &&, true,
+                  false, ABSL_INTERNAL_SINGLE_ARG());
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_impl, const&, &&, true,
+                  true, ABSL_INTERNAL_SINGLE_ARG());
 
-  template <class K = key_type, class V = mapped_type>
-  std::pair<iterator, bool> insert_or_assign(const key_arg<K>& k, const V& v) {
-    return insert_or_assign_impl(k, v);
-  }
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_impl, &&, const&, false,
+                  false, ABSL_INTERNAL_SINGLE_ARG());
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_impl, &&, const&, false,
+                  true, ABSL_INTERNAL_SINGLE_ARG());
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_impl, &&, const&, true,
+                  false, ABSL_INTERNAL_SINGLE_ARG());
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_impl, &&, const&, true,
+                  true, ABSL_INTERNAL_SINGLE_ARG());
 
-  template <class K = key_type, class V = mapped_type, K* = nullptr,
-            V* = nullptr>
-  iterator insert_or_assign(const_iterator, key_arg<K>&& k, V&& v) {
-    return insert_or_assign(std::forward<K>(k), std::forward<V>(v)).first;
-  }
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_impl, &&, &&, false, false,
+                  ABSL_INTERNAL_SINGLE_ARG());
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_impl, &&, &&, false, true,
+                  ABSL_INTERNAL_SINGLE_ARG());
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_impl, &&, &&, true, false,
+                  ABSL_INTERNAL_SINGLE_ARG());
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_impl, &&, &&, true, true,
+                  ABSL_INTERNAL_SINGLE_ARG());
 
-  template <class K = key_type, class V = mapped_type, K* = nullptr>
-  iterator insert_or_assign(const_iterator, key_arg<K>&& k, const V& v) {
-    return insert_or_assign(std::forward<K>(k), v).first;
-  }
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_impl, const&, const&,
+                  false, false, .first, const_iterator ABSL_INTERNAL_COMMA);
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_impl, const&, const&,
+                  false, true, .first, const_iterator ABSL_INTERNAL_COMMA);
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_impl, const&, const&, true,
+                  false, .first, const_iterator ABSL_INTERNAL_COMMA);
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_impl, const&, const&, true,
+                  true, .first, const_iterator ABSL_INTERNAL_COMMA);
 
-  template <class K = key_type, class V = mapped_type, V* = nullptr>
-  iterator insert_or_assign(const_iterator, const key_arg<K>& k, V&& v) {
-    return insert_or_assign(k, std::forward<V>(v)).first;
-  }
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_impl, const&, &&, false,
+                  false, .first, const_iterator ABSL_INTERNAL_COMMA);
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_impl, const&, &&, false,
+                  true, .first, const_iterator ABSL_INTERNAL_COMMA);
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_impl, const&, &&, true,
+                  false, .first, const_iterator ABSL_INTERNAL_COMMA);
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_impl, const&, &&, true,
+                  true, .first, const_iterator ABSL_INTERNAL_COMMA);
 
-  template <class K = key_type, class V = mapped_type>
-  iterator insert_or_assign(const_iterator, const key_arg<K>& k, const V& v) {
-    return insert_or_assign(k, v).first;
-  }
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_impl, &&, const&, false,
+                  false, .first, const_iterator ABSL_INTERNAL_COMMA);
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_impl, &&, const&, false,
+                  true, .first, const_iterator ABSL_INTERNAL_COMMA);
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_impl, &&, const&, true,
+                  false, .first, const_iterator ABSL_INTERNAL_COMMA);
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_impl, &&, const&, true,
+                  true, .first, const_iterator ABSL_INTERNAL_COMMA);
+
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_impl, &&, &&, false, false,
+                  .first, const_iterator ABSL_INTERNAL_COMMA);
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_impl, &&, &&, false, true,
+                  .first, const_iterator ABSL_INTERNAL_COMMA);
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_impl, &&, &&, true, false,
+                  .first, const_iterator ABSL_INTERNAL_COMMA);
+  ABSL_INTERNAL_X(insert_or_assign, insert_or_assign_impl, &&, &&, true, true,
+                  .first, const_iterator ABSL_INTERNAL_COMMA);
+#undef ABSL_INTERNAL_X
 
   // All `try_emplace()` overloads make the same guarantees regarding rvalue
   // arguments as `std::unordered_map::try_emplace()`, namely that these
   // functions will not move from rvalue arguments if insertions do not happen.
-  template <class K = key_type, class... Args,
-            typename std::enable_if<
-                !std::is_convertible<K, const_iterator>::value, int>::type = 0,
-            K* = nullptr>
-  std::pair<iterator, bool> try_emplace(key_arg<K>&& k, Args&&... args) {
-    return try_emplace_impl(std::forward<K>(k), std::forward<Args>(args)...);
+  template <
+      class K = key_type, int = EnableIf<LifetimeBoundK<K, false, K*>>(),
+      class... Args,
+      std::enable_if_t<!std::is_convertible_v<K, const_iterator>, int> = 0>
+  std::pair<iterator, bool> try_emplace(key_arg<K>&& k, Args&&... args)
+      ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return try_emplace_impl(std::forward<key_arg<K>>(k),
+                            std::forward<Args>(args)...);
   }
 
-  template <class K = key_type, class... Args,
-            typename std::enable_if<
-                !std::is_convertible<K, const_iterator>::value, int>::type = 0>
-  std::pair<iterator, bool> try_emplace(const key_arg<K>& k, Args&&... args) {
+  template <
+      class K = key_type, class... Args,
+      EnableIf<LifetimeBoundK<K, true, K*>> = 0,
+      std::enable_if_t<!std::is_convertible_v<K, const_iterator>, int> = 0>
+  std::pair<iterator, bool> try_emplace(
+      key_arg<K>&& k ABSL_INTERNAL_ATTRIBUTE_CAPTURED_BY_THIS,
+      Args&&... args) ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return this->template try_emplace<K, 0>(std::forward<key_arg<K>>(k),
+                                            std::forward<Args>(args)...);
+  }
+
+  template <
+      class K = key_type, int = EnableIf<LifetimeBoundK<K, false>>(),
+      class... Args,
+      std::enable_if_t<!std::is_convertible_v<K, const_iterator>, int> = 0>
+  std::pair<iterator, bool> try_emplace(const key_arg<K>& k, Args&&... args)
+      ABSL_ATTRIBUTE_LIFETIME_BOUND {
     return try_emplace_impl(k, std::forward<Args>(args)...);
   }
+  template <
+      class K = key_type, class... Args, EnableIf<LifetimeBoundK<K, true>> = 0,
+      std::enable_if_t<!std::is_convertible_v<K, const_iterator>, int> = 0>
+  std::pair<iterator, bool> try_emplace(
+      const key_arg<K>& k ABSL_INTERNAL_ATTRIBUTE_CAPTURED_BY_THIS,
+      Args&&... args) ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return this->template try_emplace<K, 0>(k, std::forward<Args>(args)...);
+  }
 
-  template <class K = key_type, class... Args, K* = nullptr>
-  iterator try_emplace(const_iterator, key_arg<K>&& k, Args&&... args) {
-    return try_emplace(std::forward<K>(k), std::forward<Args>(args)...).first;
+  template <class K = key_type, int = EnableIf<LifetimeBoundK<K, false, K*>>(),
+            class... Args>
+  iterator try_emplace(const_iterator, key_arg<K>&& k,
+                       Args&&... args) ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return try_emplace(std::forward<key_arg<K>>(k), std::forward<Args>(args)...)
+        .first;
+  }
+  template <class K = key_type, class... Args,
+            EnableIf<LifetimeBoundK<K, true, K*>> = 0>
+  iterator try_emplace(const_iterator hint,
+                       key_arg<K>&& k ABSL_INTERNAL_ATTRIBUTE_CAPTURED_BY_THIS,
+                       Args&&... args) ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return this->template try_emplace<K, 0>(hint, std::forward<key_arg<K>>(k),
+                                            std::forward<Args>(args)...);
   }
 
-  template <class K = key_type, class... Args>
-  iterator try_emplace(const_iterator, const key_arg<K>& k, Args&&... args) {
+  template <class K = key_type, int = EnableIf<LifetimeBoundK<K, false>>(),
+            class... Args>
+  iterator try_emplace(const_iterator, const key_arg<K>& k,
+                       Args&&... args) ABSL_ATTRIBUTE_LIFETIME_BOUND {
     return try_emplace(k, std::forward<Args>(args)...).first;
   }
+  template <class K = key_type, class... Args,
+            EnableIf<LifetimeBoundK<K, true>> = 0>
+  iterator try_emplace(const_iterator hint,
+                       const key_arg<K>& k
+                           ABSL_INTERNAL_ATTRIBUTE_CAPTURED_BY_THIS,
+                       Args&&... args) ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return this->template try_emplace<K, 0>(hint, k,
+                                            std::forward<Args>(args)...);
+  }
 
   template <class K = key_type, class P = Policy>
-  MappedReference<P> at(const key_arg<K>& key) {
+  MappedReference<P> at(const key_arg<K>& key) ABSL_ATTRIBUTE_LIFETIME_BOUND {
     auto it = this->find(key);
     if (it == this->end()) {
-      base_internal::ThrowStdOutOfRange(
-          "absl::container_internal::raw_hash_map<>::at");
+      ThrowStdOutOfRange("absl::container_internal::raw_hash_map<>::at");
     }
     return Policy::value(&*it);
   }
 
   template <class K = key_type, class P = Policy>
-  MappedConstReference<P> at(const key_arg<K>& key) const {
+  MappedConstReference<P> at(const key_arg<K>& key) const
+      ABSL_ATTRIBUTE_LIFETIME_BOUND {
     auto it = this->find(key);
     if (it == this->end()) {
-      base_internal::ThrowStdOutOfRange(
-          "absl::container_internal::raw_hash_map<>::at");
+      ThrowStdOutOfRange("absl::container_internal::raw_hash_map<>::at");
     }
     return Policy::value(&*it);
   }
 
-  template <class K = key_type, class P = Policy, K* = nullptr>
-  MappedReference<P> operator[](key_arg<K>&& key) {
-    return Policy::value(&*try_emplace(std::forward<K>(key)).first);
+  template <class K = key_type, class P = Policy,
+            int = EnableIf<LifetimeBoundK<K, false, K*>>()>
+  MappedReference<P> operator[](key_arg<K>&& key)
+      ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    // It is safe to use unchecked_deref here because try_emplace
+    // will always return an iterator pointing to a valid item in the table,
+    // since it inserts if nothing is found for the given key.
+    return Policy::value(&this->unchecked_deref(
+        try_emplace(std::forward<key_arg<K>>(key)).first));
+  }
+  template <class K = key_type, class P = Policy, int&...,
+            EnableIf<LifetimeBoundK<K, true, K*>> = 0>
+  MappedReference<P> operator[](
+      key_arg<K>&& key ABSL_INTERNAL_ATTRIBUTE_CAPTURED_BY_THIS)
+      ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return this->template operator[]<K, P, 0>(std::forward<key_arg<K>>(key));
   }
 
-  template <class K = key_type, class P = Policy>
-  MappedReference<P> operator[](const key_arg<K>& key) {
-    return Policy::value(&*try_emplace(key).first);
+  template <class K = key_type, class P = Policy,
+            int = EnableIf<LifetimeBoundK<K, false>>()>
+  MappedReference<P> operator[](const key_arg<K>& key)
+      ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    // It is safe to use unchecked_deref here because try_emplace
+    // will always return an iterator pointing to a valid item in the table,
+    // since it inserts if nothing is found for the given key.
+    return Policy::value(&this->unchecked_deref(try_emplace(key).first));
+  }
+  template <class K = key_type, class P = Policy, int&...,
+            EnableIf<LifetimeBoundK<K, true>> = 0>
+  MappedReference<P> operator[](
+      const key_arg<K>& key ABSL_INTERNAL_ATTRIBUTE_CAPTURED_BY_THIS)
+      ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return this->template operator[]<K, P, 0>(key);
   }
 
  private:
+  static_assert(
+      std::is_same_v<
+          typename InstantiateRawHashMap<Policy, Hash, Eq, Alloc>::type,
+          raw_hash_map>,
+      "Redundant template parameters were passed. Use InstantiateRawHashMap<> "
+      "instead");
+
   template <class K, class V>
-  std::pair<iterator, bool> insert_or_assign_impl(K&& k, V&& v) {
+  std::pair<iterator, bool> insert_or_assign_impl(K&& k, V&& v)
+      ABSL_ATTRIBUTE_LIFETIME_BOUND {
     auto res = this->find_or_prepare_insert(k);
-    if (res.second)
+    if (res.second) {
       this->emplace_at(res.first, std::forward<K>(k), std::forward<V>(v));
-    else
-      Policy::value(&*this->iterator_at(res.first)) = std::forward<V>(v);
-    return {this->iterator_at(res.first), res.second};
+    } else {
+      Policy::value(&Policy::element(res.first)) = std::forward<V>(v);
+    }
+    return {this->non_iterable_iterator_at_slot(res.first), res.second};
   }
 
   template <class K = key_type, class... Args>
-  std::pair<iterator, bool> try_emplace_impl(K&& k, Args&&... args) {
+  std::pair<iterator, bool> try_emplace_impl(K&& k, Args&&... args)
+      ABSL_ATTRIBUTE_LIFETIME_BOUND {
     auto res = this->find_or_prepare_insert(k);
-    if (res.second)
+    if (res.second) {
       this->emplace_at(res.first, std::piecewise_construct,
                        std::forward_as_tuple(std::forward<K>(k)),
                        std::forward_as_tuple(std::forward<Args>(args)...));
-    return {this->iterator_at(res.first), res.second};
+    }
+    return {this->non_iterable_iterator_at_slot(res.first), res.second};
   }
 };
 
 }  // namespace container_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_INTERNAL_RAW_HASH_MAP_H_
diff --git a/absl/container/internal/raw_hash_set.cc b/absl/container/internal/raw_hash_set.cc
index ac2d10a7666..3f9263c4d3d 100644
--- a/absl/container/internal/raw_hash_set.cc
+++ b/absl/container/internal/raw_hash_set.cc
@@ -14,33 +14,2487 @@
 
 #include "absl/container/internal/raw_hash_set.h"
 
+#include <algorithm>
 #include <atomic>
+#include <cassert>
 #include <cstddef>
+#include <cstdint>
+#include <cstring>
+#include <memory>
+#include <tuple>
+#include <utility>
 
+#include "absl/base/attributes.h"
 #include "absl/base/config.h"
+#include "absl/base/dynamic_annotations.h"
+#include "absl/base/internal/endian.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/base/optimization.h"
+#include "absl/container/internal/container_memory.h"
+#include "absl/container/internal/hashtable_control_bytes.h"
+#include "absl/container/internal/hashtablez_sampler.h"
+#include "absl/container/internal/raw_hash_set_resize_impl.h"
+#include "absl/functional/function_ref.h"
+#include "absl/hash/hash.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
-constexpr size_t Group::kWidth;
+// Represents a control byte corresponding to a full slot with arbitrary hash.
+constexpr ctrl_t ZeroCtrlT() { return static_cast<ctrl_t>(0); }
+
+// A single byte for default-constructed iterators. We leave it uninitialized
+// because reading this memory is a bug.
+ABSL_DLL char kDefaultIterSlot;
+
+// We need one full byte followed by a sentinel byte for iterator::operator++.
+ABSL_CONST_INIT ABSL_DLL const ctrl_t kSooControl[2] = {ZeroCtrlT(),
+                                                        ctrl_t::kSentinel};
+// We need one full byte followed by a sentinel byte for iterator::operator++.
+ABSL_CONST_INIT ABSL_DLL const ctrl_t kInsertIteratorControl[2] = {
+    ZeroCtrlT(), ctrl_t::kSentinel};
+
+namespace {
+
+#ifdef ABSL_SWISSTABLE_ASSERT
+#error ABSL_SWISSTABLE_ASSERT cannot be directly set
+#else
+// We use this macro for assertions that users may see when the table is in an
+// invalid state that sanitizers may help diagnose.
+#define ABSL_SWISSTABLE_ASSERT(CONDITION) \
+  assert((CONDITION) && "Try enabling sanitizers.")
+#endif
+
+void ValidateMaxSize([[maybe_unused]] size_t size,
+                     [[maybe_unused]] size_t key_size,
+                     [[maybe_unused]] size_t slot_size) {
+  ABSL_SWISSTABLE_ASSERT(size <= MaxValidSize(key_size, slot_size));
+}
+void ValidateMaxCapacity(size_t capacity, size_t key_size, size_t slot_size) {
+  if (capacity <= 1) return;
+  ValidateMaxSize(CapacityToGrowth(PreviousCapacity(capacity)), key_size,
+                  slot_size);
+}
 
 // Returns "random" seed.
 inline size_t RandomSeed() {
-#if ABSL_HAVE_THREAD_LOCAL
+  constexpr size_t kIncrement = 0xad53;
+#ifdef ABSL_HAVE_THREAD_LOCAL
   static thread_local size_t counter = 0;
-  size_t value = ++counter;
+  counter += kIncrement;
+  size_t value = counter;
 #else   // ABSL_HAVE_THREAD_LOCAL
   static std::atomic<size_t> counter(0);
-  size_t value = counter.fetch_add(1, std::memory_order_relaxed);
+  size_t value = counter.fetch_add(kIncrement, std::memory_order_relaxed);
 #endif  // ABSL_HAVE_THREAD_LOCAL
   return value ^ static_cast<size_t>(reinterpret_cast<uintptr_t>(&counter));
 }
 
-bool ShouldInsertBackwards(size_t hash, ctrl_t* ctrl) {
-  // To avoid problems with weak hashes and single bit tests, we use % 13.
-  // TODO(kfm,sbenza): revisit after we do unconditional mixing
-  return (H1(hash, ctrl) ^ RandomSeed()) % 13 > 6;
+bool ShouldRehashForBugDetection(size_t capacity) {
+  // Note: we can't use the abseil-random library because abseil-random
+  // depends on swisstable. We want to return true with probability
+  // `min(1, RehashProbabilityConstant() / capacity())`. In order to do this,
+  // we probe based on a random hash and see if the offset is less than
+  // RehashProbabilityConstant().
+  return probe(ProbeCapacity{capacity}, absl::HashOf(RandomSeed()))
+             .offset() < RehashProbabilityConstant();
+}
+
+// Find a non-deterministic hash for single group table.
+// Last two bits are used to find a position for a newly inserted element after
+// resize.
+// This function basically using H2 last bits to save on shift operation.
+size_t SingleGroupTableH1(size_t hash, PerTableSeed seed) {
+  return hash ^ seed.seed();
+}
+
+// Returns the offset of the new element after resize from capacity 1 to 3.
+size_t Resize1To3NewOffset(size_t hash, PerTableSeed seed) {
+  // After resize from capacity 1 to 3, we always have exactly the slot with
+  // index 1 occupied, so we need to insert either at index 0 or index 2.
+  static_assert(SooSlotIndex() == 1);
+  return SingleGroupTableH1(hash, seed) & 2;
+}
+
+// Returns the address of the ith slot in slots where each slot occupies
+// slot_size.
+inline void* SlotAddress(void* slot_array, size_t slot, size_t slot_size) {
+  return static_cast<void*>(static_cast<char*>(slot_array) +
+                            (slot * slot_size));
+}
+
+// Returns the address of the slot `i` iterations after `slot` assuming each
+// slot has the specified size.
+inline void* NextSlot(void* slot, size_t slot_size, size_t i = 1) {
+  return reinterpret_cast<void*>(reinterpret_cast<uintptr_t>(slot) +
+                                 slot_size * i);
+}
+
+// Returns the address of the slot just before `slot` assuming each slot has the
+// specified size.
+inline void* PrevSlot(void* slot, size_t slot_size) {
+  return reinterpret_cast<void*>(reinterpret_cast<uintptr_t>(slot) - slot_size);
+}
+
+}  // namespace
+
+// Must be defined out-of-line to avoid MSVC error C2482 on some platforms,
+// which is caused by non-constexpr initialization.
+uint8_t NextHashTableSeed() {
+  return static_cast<uint8_t>(RandomSeed());
+}
+
+GenerationType* EmptyGeneration() {
+  if (SwisstableGenerationsEnabled()) {
+    constexpr size_t kNumEmptyGenerations = 1024;
+    static constexpr GenerationType kEmptyGenerations[kNumEmptyGenerations]{};
+    return const_cast<GenerationType*>(
+        &kEmptyGenerations[RandomSeed() % kNumEmptyGenerations]);
+  }
+  return nullptr;
+}
+
+bool CommonFieldsGenerationInfoEnabled::
+    should_rehash_for_bug_detection_on_insert(size_t capacity) const {
+  if (reserved_growth_ == kReservedGrowthJustRanOut) return true;
+  if (reserved_growth_ > 0) return false;
+  return ShouldRehashForBugDetection(capacity);
+}
+
+bool CommonFieldsGenerationInfoEnabled::should_rehash_for_bug_detection_on_move(
+    size_t capacity) const {
+  return ShouldRehashForBugDetection(capacity);
+}
+
+namespace {
+
+// Probes an array of control bits using a probe sequence,
+// and returns the mask corresponding to the first group with a deleted or empty
+// slot.
+inline Group::NonIterableBitMaskType probe_till_first_non_full_group(
+    const ctrl_t* ctrl, probe_seq<Group::kWidth>& seq,
+    [[maybe_unused]] size_t capacity) {
+  while (true) {
+    GroupFullEmptyOrDeleted g{ctrl + seq.offset()};
+    auto mask = g.MaskEmptyOrDeleted();
+    if (mask) {
+      return mask;
+    }
+    seq.next();
+    ABSL_SWISSTABLE_ASSERT(seq.index() <= capacity && "full table!");
+  }
+}
+
+FindInfo find_first_non_full_from_h1(const ctrl_t* ctrl, size_t h1,
+                                     HashtableCapacity capacity) {
+  const size_t cap = capacity.capacity();
+  auto seq = probe_h1(ProbeCapacity{cap}, h1);
+  if (IsEmptyOrDeleted(ctrl[seq.offset()])) {
+    return {seq.offset(), /*probe_length=*/0};
+  }
+  auto mask = probe_till_first_non_full_group(ctrl, seq, cap);
+  return {seq.offset(mask.LowestBitSet()), seq.index()};
+}
+
+// Probes an array of control bits using a probe sequence derived from `hash`,
+// and returns the offset corresponding to the first deleted or empty slot.
+//
+// Behavior when the entire table is full is undefined.
+//
+// NOTE: this function must work with tables having both empty and deleted
+// slots in the same group. Such tables appear during `erase()`.
+FindInfo find_first_non_full(const CommonFields& common, size_t hash) {
+  return find_first_non_full_from_h1(common.control(), H1(hash),
+                                     common.capacity_impl());
+}
+
+// Same as `find_first_non_full`, but returns the mask corresponding to the
+// first group with a deleted or empty slot.
+std::pair<FindInfo, Group::NonIterableBitMaskType> find_first_non_full_group(
+    const CommonFields& common, size_t hash) {
+  auto seq = probe(common, hash);
+  auto mask =
+      probe_till_first_non_full_group(common.control(), seq, common.capacity());
+  return {{seq.offset(), seq.index()}, mask};
+}
+
+// Whether a table fits in half a group. A half-group table fits entirely into a
+// probing group, i.e., has a capacity < `Group::kWidth`.
+//
+// In half-group mode we are able to use the whole capacity. The extra control
+// bytes give us at least one "empty" control byte to stop the iteration.
+// This is important to make 1 a valid capacity.
+//
+// In half-group mode only the first `capacity` control bytes after the sentinel
+// are valid. The rest contain dummy ctrl_t::kEmpty values that do not
+// represent a real slot.
+constexpr bool is_half_group(size_t capacity) {
+  return capacity < Group::kWidth - 1;
+}
+
+template <class Fn>
+void IterateOverFullSlotsImpl(const CommonFields& c, size_t slot_size, Fn cb) {
+  const size_t cap = c.capacity();
+  ABSL_ASSUME(cap > kMaxSmallCapacity);
+  const ctrl_t* ctrl = c.control();
+  void* slot = c.slot_array(cap);
+  if (is_half_group(cap)) {
+    // Mirrored/cloned control bytes in half-group table are also located in the
+    // first group (starting from position 0). We are taking group from position
+    // `capacity` in order to avoid duplicates.
+
+    // Half-group tables capacity fits into portable group, where
+    // GroupPortableImpl::MaskFull is more efficient for the
+    // capacity <= GroupPortableImpl::kWidth.
+    ABSL_SWISSTABLE_ASSERT(cap <= GroupPortableImpl::kWidth &&
+                           "unexpectedly large half-group capacity");
+    static_assert(Group::kWidth >= GroupPortableImpl::kWidth,
+                  "unexpected group width");
+    // Group starts from kSentinel slot, so indices in the mask will
+    // be increased by 1.
+    const auto mask = GroupPortableImpl(ctrl + cap).MaskFull();
+    --ctrl;
+    slot = PrevSlot(slot, slot_size);
+    for (uint32_t i : mask) {
+      cb(ctrl + i, SlotAddress(slot, i, slot_size));
+    }
+    return;
+  }
+  size_t remaining = c.size();
+  ABSL_ATTRIBUTE_UNUSED const size_t original_size_for_assert = remaining;
+  while (remaining != 0) {
+    for (uint32_t i : GroupFullEmptyOrDeleted(ctrl).MaskFull()) {
+      ABSL_SWISSTABLE_ASSERT(IsFull(ctrl[i]) &&
+                             "hash table was modified unexpectedly");
+      cb(ctrl + i, SlotAddress(slot, i, slot_size));
+      --remaining;
+    }
+    ctrl += Group::kWidth;
+    slot = NextSlot(slot, slot_size, Group::kWidth);
+    ABSL_SWISSTABLE_ASSERT(
+        (remaining == 0 || *(ctrl - 1) != ctrl_t::kSentinel) &&
+        "hash table was modified unexpectedly");
+  }
+  // NOTE: erasure of the current element is allowed in callback for
+  // absl::erase_if specialization. So we use `>=`.
+  ABSL_SWISSTABLE_ASSERT(original_size_for_assert >= c.size() &&
+                         "hash table was modified unexpectedly");
+}
+
+// NOTE: we don't use structure with bit fields for GrowthInfo because for
+// correctness we rely on the lower bound being the most significant byte.
+
+// Returns the increment that needs to be added to the packed full growth info
+// in order to increase lower bound by lower_bound_increment and increase
+// overflow growth left by overflow_increment.
+constexpr uint64_t GetPackedIncrement(uint64_t lower_bound_increment,
+                                      uint64_t overflow_increment) {
+  return (lower_bound_increment << GrowthInfoAccessor::kLowerBoundShift) +
+         overflow_increment;
+}
+
+// Returns the increment that needs to be added to the packed full growth info
+// in order to increase lower bound by overflow_to_lower_bound_size and
+// decrease overflow growth left by overflow_to_lower_bound_size.
+constexpr uint64_t GetRebalanceIncrement(
+    uint64_t overflow_to_lower_bound_size) {
+  return GetPackedIncrement(overflow_to_lower_bound_size,
+                            0u - overflow_to_lower_bound_size);
+}
+
+// Returns the number of elements left to grow in the full growth info.
+constexpr uint64_t GetOverflowGrowthLeftFromPacked(
+    uint64_t packed_full_growth_info) {
+  constexpr uint64_t kFullGrowthMask =
+      (uint64_t{1} << GrowthInfoAccessor::kLowerBoundShift) - 1;
+  return packed_full_growth_info & kFullGrowthMask;
+}
+
+// Returns the GrowthInfoLowerBound object containing the information
+// about minimum growth left.
+constexpr GrowthInfoLowerBound GetGrowthInfoLowerBoundFromPacked(
+    uint64_t packed_full_growth_info) {
+  return GrowthInfoLowerBound(packed_full_growth_info >>
+                              GrowthInfoAccessor::kLowerBoundShift);
+}
+
+// Returns the number of elements left to grow in the lower bound.
+constexpr uint64_t GetGrowthLeftLowerBoundFromPacked(
+    uint64_t packed_full_growth_info) {
+  return GetGrowthInfoLowerBoundFromPacked(packed_full_growth_info)
+      .GetGrowthLeft();
+}
+
+// Returns the total number of elements left to grow in the full growth info.
+// Assumes that the table has capacity > kMaxGrowthLeftLowerBound.
+uint64_t GetGrowthLeftTotalBigCapacity(void* full_growth_info) {
+  uint64_t packed_full_growth_left = little_endian::Load64(full_growth_info);
+  return GetOverflowGrowthLeftFromPacked(packed_full_growth_left) +
+         GetGrowthLeftLowerBoundFromPacked(packed_full_growth_left);
+}
+
+}  // namespace
+
+void CommonFields::AssertNotDebugCapacityImpl() const {
+  const HashtableCapacity cap = maybe_invalid_capacity();
+  if (ABSL_PREDICT_TRUE(cap.IsValid())) {
+    return;
+  }
+  assert(!cap.IsReentrance() &&
+         "Reentrant container access during element construction/destruction "
+         "is not allowed.");
+  if (cap.IsDestroyed()) {
+    ABSL_RAW_LOG(FATAL, "Use of destroyed hash table.");
+  }
+  if (SwisstableGenerationsEnabled() && ABSL_PREDICT_FALSE(cap.IsMovedFrom())) {
+    if (cap.IsSelfMovedFrom()) {
+      // If this log triggers, then a hash table was move-assigned to itself
+      // and then used again later without being reinitialized.
+      ABSL_RAW_LOG(FATAL, "Use of self-move-assigned hash table.");
+    }
+    ABSL_RAW_LOG(FATAL, "Use of moved-from hash table.");
+  }
+}
+
+void GrowthInfoAccessor::InitGrowthLeftNoDeleted(size_t growth_left,
+                                                 size_t capacity) {
+  if (capacity <= GrowthInfoLowerBound::kMaxGrowthLeftLowerBound) {
+    *growth_info_lower_bound_ = static_cast<uint8_t>(growth_left);
+  } else {
+    uint64_t lower_bound =
+        (std::min)(uint64_t{growth_left},
+                   GrowthInfoLowerBound::kMaxGrowthLeftLowerBound);
+    little_endian::Store64(
+        full_growth_info_ptr(),
+        GetPackedIncrement(lower_bound, growth_left - lower_bound));
+  }
+}
+
+GrowthInfoLowerBound GrowthInfoAccessor::RebalanceGrowthLeftLowerBound(
+    size_t capacity) {
+  auto growth_left_lower_bound = GetGrowthInfoLowerBound();
+  if (capacity <= GrowthInfoLowerBound::kMaxGrowthLeftLowerBound ||
+      // For tables with deleted slots, we often call rebalance even if
+      // we have growth left in the lower bound.
+      growth_left_lower_bound.HasDeletedAndGrowthLeft()) {
+    return growth_left_lower_bound;
+  } else {
+    return RebalanceGrowthLeftLowerBoundLargeCapacity();
+  }
+}
+
+size_t GrowthInfoAccessor::GetGrowthLeftTotalSlow(size_t capacity) const {
+  if (capacity <= GrowthInfoLowerBound::kMaxGrowthLeftLowerBound) {
+    return GetGrowthLeftLowerBound();
+  } else {
+    return static_cast<size_t>(
+        GetGrowthLeftTotalBigCapacity(full_growth_info_ptr()));
+  }
+}
+
+ABSL_ATTRIBUTE_NOINLINE GrowthInfoLowerBound
+GrowthInfoAccessor::RebalanceGrowthLeftLowerBoundLargeCapacity() {
+  void* full_growth_info = full_growth_info_ptr();
+  uint64_t packed_full_growth_info = little_endian::Load64(full_growth_info);
+  uint64_t overflow_growth_left =
+      GetOverflowGrowthLeftFromPacked(packed_full_growth_info);
+  uint64_t lower_bound_growth_left =
+      GetGrowthLeftLowerBoundFromPacked(packed_full_growth_info);
+  uint64_t overflow_to_lower_bound_size =
+      (std::min)(overflow_growth_left,
+                 GrowthInfoLowerBound::kMaxGrowthLeftLowerBound -
+                     lower_bound_growth_left);
+  packed_full_growth_info +=
+      GetRebalanceIncrement(overflow_to_lower_bound_size);
+  little_endian::Store64(full_growth_info, packed_full_growth_info);
+  auto result = GetGrowthInfoLowerBoundFromPacked(packed_full_growth_info);
+  ABSL_SWISSTABLE_ASSERT(result.HasNoDeleted() ==
+                         GetGrowthInfoLowerBound().HasNoDeleted());
+  ABSL_SWISSTABLE_ASSERT(
+      (result.GetGrowthLeft() > 0 ||
+       GetGrowthLeftTotalBigCapacity(full_growth_info_ptr()) == 0) &&
+      "rebalance may return 0 only if we have absolutely no growth left");
+  return result;
+}
+
+void GrowthInfoAccessor::OverwriteFullAsEmpty() {
+  if (GetGrowthLeftLowerBound() <
+      GrowthInfoLowerBound::kMaxGrowthLeftLowerBound) {
+    ++(*growth_info_lower_bound_);
+  } else {
+    constexpr uint64_t kIncrement = GetPackedIncrement(
+        /*lower_bound_increment=*/0, /*overflow_increment=*/1);
+    void* const full_growth_info = full_growth_info_ptr();
+    little_endian::Store64(
+        full_growth_info, little_endian::Load64(full_growth_info) + kIncrement);
+  }
+}
+
+void ConvertDeletedToEmptyAndFullToDeleted(ctrl_t* ctrl, size_t capacity) {
+  ABSL_SWISSTABLE_ASSERT(ctrl[capacity] == ctrl_t::kSentinel);
+  ABSL_SWISSTABLE_ASSERT(IsValidCapacity(capacity));
+  for (ctrl_t* pos = ctrl; pos < ctrl + capacity; pos += Group::kWidth) {
+    Group{pos}.ConvertSpecialToEmptyAndFullToDeleted(pos);
+  }
+  // Copy the cloned ctrl bytes.
+  std::memcpy(ctrl + capacity + 1, ctrl, NumClonedBytes());
+  ctrl[capacity] = ctrl_t::kSentinel;
+}
+
+void IterateOverFullSlots(const CommonFields& c, size_t slot_size,
+                          absl::FunctionRef<void(const ctrl_t*, void*)> cb) {
+  IterateOverFullSlotsImpl(c, slot_size, cb);
+}
+
+HashtablezInfoHandle CommonFields::infoz_ptr() const {
+  // growth_info is stored before control bytes.
+  ABSL_SWISSTABLE_ASSERT(has_infoz());
+  HashtablezInfoHandle res;
+  void* src = reinterpret_cast<char*>(control()) -
+              MetadataBeforeControlSize(/*has_infoz=*/true, capacity());
+  std::memcpy(&res, src, sizeof(HashtablezInfoHandle));
+  return res;
+}
+
+void CommonFields::set_infoz(HashtablezInfoHandle infoz) {
+  ABSL_SWISSTABLE_ASSERT(has_infoz());
+  void* dst = reinterpret_cast<char*>(control()) -
+              MetadataBeforeControlSize(/*has_infoz=*/true, capacity());
+  std::memcpy(dst, &infoz, sizeof(HashtablezInfoHandle));
+}
+
+namespace {
+
+void ResetGrowthLeft(GrowthInfoAccessor growth_info, size_t capacity,
+                     size_t occupied_elements) {
+  growth_info.InitGrowthLeftNoDeleted(
+      CapacityToGrowth(capacity) - occupied_elements, capacity);
+}
+
+// Finds guaranteed to exists empty slot from the given position.
+// NOTE: this function is almost never triggered inside of the
+// DropDeletesWithoutResize, so we keep it simple.
+// The table is rather sparse, so empty slot will be found very quickly.
+size_t FindEmptySlot(size_t start, size_t end, const ctrl_t* ctrl) {
+  for (size_t i = start; i < end; ++i) {
+    if (IsEmpty(ctrl[i])) {
+      return i;
+    }
+  }
+  ABSL_UNREACHABLE();
+}
+
+// Finds guaranteed to exist full slot starting from the given position.
+// NOTE: this function is only triggered for rehash(0), when we need to
+// go back to SOO state, so we keep it simple.
+size_t FindFirstFullSlot(size_t start, size_t end, const ctrl_t* ctrl) {
+  for (size_t i = start; i < end; ++i) {
+    if (IsFull(ctrl[i])) {
+      return i;
+    }
+  }
+  ABSL_UNREACHABLE();
+}
+
+void PrepareInsertCommon(CommonFields& common) {
+  common.increment_size();
+  common.maybe_increment_generation_on_insert();
+}
+
+// Sets sanitizer poisoning for slot corresponding to control byte being set.
+inline void DoSanitizeOnSetCtrl(const CommonFields& c, size_t i, ctrl_t h,
+                                size_t slot_size) {
+  const size_t cap = c.capacity();
+  ABSL_ASSUME(cap > kMaxSmallCapacity);
+  ABSL_SWISSTABLE_ASSERT(i < cap);
+  auto* slot_i = static_cast<const char*>(c.slot_array(cap)) + i * slot_size;
+  if (IsFull(h)) {
+    SanitizerUnpoisonMemoryRegion(slot_i, slot_size);
+  } else {
+    SanitizerPoisonMemoryRegion(slot_i, slot_size);
+  }
+}
+
+// Sets `ctrl[i]` to `h`.
+//
+// Unlike setting it directly, this function will perform bounds checks and
+// mirror the value to the cloned tail if necessary.
+inline void SetCtrlNoSanitizeImpl(const CommonFields& c, size_t i, ctrl_t h) {
+  ABSL_SWISSTABLE_ASSERT(i < c.capacity());
+  ctrl_t* ctrl = c.control();
+  const size_t cap = c.capacity();
+  ctrl[i] = h;
+  ctrl[((i - NumClonedBytes()) & cap) + (NumClonedBytes() & cap)] = h;
+}
+
+inline void SetCtrl(const CommonFields& c, size_t i, ctrl_t h,
+                    size_t slot_size) {
+  ABSL_SWISSTABLE_ASSERT(!c.is_small());
+  DoSanitizeOnSetCtrl(c, i, h, slot_size);
+  SetCtrlNoSanitizeImpl(c, i, h);
+}
+// Overload for setting to an occupied `h2_t` rather than a special `ctrl_t`.
+inline void SetCtrl(const CommonFields& c, size_t i, h2_t h, size_t slot_size) {
+  SetCtrl(c, i, static_cast<ctrl_t>(h), slot_size);
+}
+
+// Sets `ctrl[i]` to `ctrl_t::kSentinel`.
+//
+// Unlike setting it directly, this function will perform bounds checks and
+// mirror the value to the cloned tail if necessary.
+inline void BlockCtrl(const CommonFields& c, size_t i) {
+  ABSL_SWISSTABLE_ASSERT(!c.is_small());
+  SetCtrlNoSanitizeImpl(c, i, ctrl_t::kSentinel);
+}
+
+// Like SetCtrl, but in a single group table, we can save some operations when
+// setting the cloned control byte.
+inline void SetCtrlInSingleGroupTable(const CommonFields& c, size_t i, ctrl_t h,
+                                      size_t slot_size) {
+  const size_t cap = c.capacity();
+  ABSL_SWISSTABLE_ASSERT(!c.is_small());
+  ABSL_SWISSTABLE_ASSERT(is_single_group(cap));
+  DoSanitizeOnSetCtrl(c, i, h, slot_size);
+  ctrl_t* ctrl = c.control();
+  ctrl[i] = h;
+  ctrl[i + cap + 1] = h;
+}
+// Overload for setting to an occupied `h2_t` rather than a special `ctrl_t`.
+inline void SetCtrlInSingleGroupTable(const CommonFields& c, size_t i, h2_t h,
+                                      size_t slot_size) {
+  SetCtrlInSingleGroupTable(c, i, static_cast<ctrl_t>(h), slot_size);
+}
+
+// Like SetCtrl, but in a table with capacity >= Group::kWidth - 1,
+// we can save some operations when setting the cloned control byte.
+inline void SetCtrlInLargeTable(const CommonFields& c, size_t i, ctrl_t h,
+                                size_t slot_size) {
+  ABSL_SWISSTABLE_ASSERT(c.capacity() >= Group::kWidth - 1);
+  DoSanitizeOnSetCtrl(c, i, h, slot_size);
+  ctrl_t* ctrl = c.control();
+  ctrl[i] = h;
+  ctrl[((i - NumClonedBytes()) & c.capacity()) + NumClonedBytes()] = h;
+}
+// Overload for setting to an occupied `h2_t` rather than a special `ctrl_t`.
+inline void SetCtrlInLargeTable(const CommonFields& c, size_t i, h2_t h,
+                                size_t slot_size) {
+  SetCtrlInLargeTable(c, i, static_cast<ctrl_t>(h), slot_size);
+}
+
+void BlockControlBytes(CommonFields& common, size_t blocked_element_count) {
+  const size_t capacity = common.capacity();
+  while (blocked_element_count > 0) {
+    BlockCtrl(common, capacity - blocked_element_count);
+    --blocked_element_count;
+  }
+}
+
+void* DropDeletesWithoutResizeAndPrepareInsert(
+    CommonFields& common, const PolicyFunctions& __restrict policy,
+    size_t new_hash) {
+  void* set = &common;
+  const size_t capacity = common.capacity();
+  ABSL_SWISSTABLE_ASSERT(IsValidCapacity(capacity));
+  ABSL_SWISSTABLE_ASSERT(!is_single_group(capacity));
+  ABSL_ASSUME(capacity > kMaxSmallCapacity);
+
+  ctrl_t* ctrl = common.control();
+  void* slot_array = common.slot_array(capacity);
+  // Algorithm:
+  // - mark all DELETED slots as EMPTY
+  // - mark all FULL slots as DELETED
+  // - for each slot marked as DELETED
+  //     hash = Hash(element)
+  //     target = find_first_non_full(hash)
+  //     if target is in the same group
+  //       mark slot as FULL
+  //     else if target is EMPTY
+  //       transfer element to target
+  //       mark slot as EMPTY
+  //       mark target as FULL
+  //     else if target is DELETED
+  //       swap current element with target element
+  //       mark target as FULL
+  //       repeat procedure for current slot with moved from element (target)
+  const size_t blocked_element_count = common.blocked_element_count();
+  ConvertDeletedToEmptyAndFullToDeleted(ctrl, capacity);
+  BlockControlBytes(common, blocked_element_count);
+  const void* hash_fn = policy.hash_fn(common);
+  auto hasher = policy.hash_slot;
+  auto transfer_n = policy.transfer_n;
+  const size_t slot_size = policy.slot_size;
+
+  size_t total_probe_length = 0;
+  void* slot_ptr = SlotAddress(slot_array, 0, slot_size);
+
+  // The index of an empty slot that can be used as temporary memory for
+  // the swap operation.
+  constexpr size_t kUnknownId = ~size_t{};
+  size_t tmp_space_id = kUnknownId;
+
+  for (size_t i = 0; i != capacity;
+       ++i, slot_ptr = NextSlot(slot_ptr, slot_size)) {
+    ABSL_SWISSTABLE_ASSERT(slot_ptr == SlotAddress(slot_array, i, slot_size));
+    if (IsEmpty(ctrl[i])) {
+      tmp_space_id = i;
+      continue;
+    }
+    if (!IsDeleted(ctrl[i])) continue;
+    const size_t hash = (*hasher)(hash_fn, slot_ptr, common.seed().seed());
+    const FindInfo target = find_first_non_full(common, hash);
+    const size_t new_i = target.offset;
+    total_probe_length += target.probe_length;
+
+    // Verify if the old and new i fall within the same group wrt the hash.
+    // If they do, we don't need to move the object as it falls already in the
+    // best probe we can.
+    const size_t probe_offset = probe(common, hash).offset();
+    const h2_t h2 = H2(hash);
+    const auto probe_index = [probe_offset, capacity](size_t pos) {
+      return ((pos - probe_offset) & capacity) / Group::kWidth;
+    };
+
+    // Element doesn't move.
+    if (ABSL_PREDICT_TRUE(probe_index(new_i) == probe_index(i))) {
+      SetCtrlInLargeTable(common, i, h2, slot_size);
+      continue;
+    }
+
+    void* new_slot_ptr = SlotAddress(slot_array, new_i, slot_size);
+    if (IsEmpty(ctrl[new_i])) {
+      // Transfer element to the empty spot.
+      // SetCtrl poisons/unpoisons the slots so we have to call it at the
+      // right time.
+      SetCtrlInLargeTable(common, new_i, h2, slot_size);
+      (*transfer_n)(set, new_slot_ptr, slot_ptr, 1);
+      SetCtrlInLargeTable(common, i, ctrl_t::kEmpty, slot_size);
+      // Initialize or change empty space id.
+      tmp_space_id = i;
+    } else {
+      ABSL_SWISSTABLE_ASSERT(IsDeleted(ctrl[new_i]));
+      SetCtrlInLargeTable(common, new_i, h2, slot_size);
+      // Until we are done rehashing, DELETED marks previously FULL slots.
+
+      if (tmp_space_id == kUnknownId) {
+        tmp_space_id = FindEmptySlot(i + 1, capacity, ctrl);
+      }
+      void* tmp_space = SlotAddress(slot_array, tmp_space_id, slot_size);
+      SanitizerUnpoisonMemoryRegion(tmp_space, slot_size);
+
+      // Swap i and new_i elements.
+      (*transfer_n)(set, tmp_space, new_slot_ptr, 1);
+      (*transfer_n)(set, new_slot_ptr, slot_ptr, 1);
+      (*transfer_n)(set, slot_ptr, tmp_space, 1);
+
+      SanitizerPoisonMemoryRegion(tmp_space, slot_size);
+
+      // repeat the processing of the ith slot
+      --i;
+      slot_ptr = PrevSlot(slot_ptr, slot_size);
+    }
+  }
+  // Prepare insert for the new element.
+  PrepareInsertCommon(common);
+  ResetGrowthLeft(common.growth_info(), capacity,
+                  common.size() + blocked_element_count);
+  FindInfo find_info = find_first_non_full(common, new_hash);
+  SetCtrlInLargeTable(common, find_info.offset, H2(new_hash), slot_size);
+  common.infoz().RecordInsertMiss(new_hash, find_info.probe_length);
+  common.infoz().RecordRehash(total_probe_length);
+  return SlotAddress(slot_array, find_info.offset, slot_size);
+}
+
+bool WasNeverFull(CommonFields& c, size_t index) {
+  if (is_single_group(c.capacity())) {
+    return true;
+  }
+  const size_t index_before = (index - Group::kWidth) & c.capacity();
+  const auto empty_after = Group(c.control() + index).MaskEmpty();
+  const auto empty_before = Group(c.control() + index_before).MaskEmpty();
+
+  // We count how many consecutive non empties we have to the right and to the
+  // left of `it`. If the sum is >= kWidth then there is at least one probe
+  // window that might have seen a full group.
+  return empty_before && empty_after &&
+         static_cast<size_t>(empty_after.TrailingZeros()) +
+                 empty_before.LeadingZeros() <
+             Group::kWidth;
+}
+
+// Updates the control bytes to indicate a completely empty table such that all
+// control bytes are kEmpty except for the kSentinel bytes.
+// If the table has blocked elements, last `blocked_element_count` are set to
+// kSentinel.
+void ResetCtrl(CommonFields& common, size_t slot_size,
+               size_t blocked_element_count) {
+  ABSL_SWISSTABLE_ASSERT(IsCapacityValidForBlockedElements(common.capacity()) ||
+                         blocked_element_count == 0);
+  const size_t capacity = common.capacity();
+  ctrl_t* ctrl = common.control();
+  static constexpr size_t kTwoGroupCapacity = 2 * Group::kWidth - 1;
+  if (ABSL_PREDICT_TRUE(capacity <= kTwoGroupCapacity)) {
+    if (IsSmallCapacity(capacity)) return;
+    std::memset(ctrl, static_cast<int8_t>(ctrl_t::kEmpty), Group::kWidth);
+    std::memset(ctrl + capacity, static_cast<int8_t>(ctrl_t::kEmpty),
+                Group::kWidth);
+    if (capacity == kTwoGroupCapacity) {
+      std::memset(ctrl + Group::kWidth, static_cast<int8_t>(ctrl_t::kEmpty),
+                  Group::kWidth);
+    }
+  } else {
+    std::memset(ctrl, static_cast<int8_t>(ctrl_t::kEmpty),
+                capacity + 1 + NumClonedBytes());
+  }
+  ctrl[capacity] = ctrl_t::kSentinel;
+  SanitizerPoisonMemoryRegion(common.slot_array(capacity),
+                              slot_size * (capacity - blocked_element_count));
+  BlockControlBytes(common, blocked_element_count);
+}
+
+// Initializes control bytes for growing from capacity 1 to 3.
+// `orig_h2` is placed in the position `SooSlotIndex()`.
+// `new_h2` is placed in the position `new_offset`.
+ABSL_ATTRIBUTE_ALWAYS_INLINE inline void InitializeThreeElementsControlBytes(
+    h2_t orig_h2, h2_t new_h2, size_t new_offset, ctrl_t* new_ctrl) {
+  static constexpr size_t kNewCapacity = NextCapacity(SooCapacity());
+  static_assert(kNewCapacity == 3);
+  static_assert(is_single_group(kNewCapacity));
+  static_assert(SooSlotIndex() == 1);
+  ABSL_SWISSTABLE_ASSERT(new_offset == 0 || new_offset == 2);
+
+  static constexpr uint64_t kEmptyXorSentinel =
+      static_cast<uint8_t>(ctrl_t::kEmpty) ^
+      static_cast<uint8_t>(ctrl_t::kSentinel);
+  static constexpr uint64_t kEmpty64 = static_cast<uint8_t>(ctrl_t::kEmpty);
+  static constexpr size_t kMirroredSooSlotIndex =
+      SooSlotIndex() + kNewCapacity + 1;
+  // The first 8 bytes, where SOO slot original and mirrored positions are
+  // replaced with 0.
+  // Result will look like: E0ESE0EE
+  static constexpr uint64_t kFirstCtrlBytesWithZeroes =
+      k8EmptyBytes ^ (kEmpty64 << (8 * SooSlotIndex())) ^
+      (kEmptyXorSentinel << (8 * kNewCapacity)) ^
+      (kEmpty64 << (8 * kMirroredSooSlotIndex));
+
+  const uint64_t soo_h2 = static_cast<uint64_t>(orig_h2);
+  const uint64_t new_h2_xor_empty =
+      static_cast<uint64_t>(new_h2 ^ static_cast<uint8_t>(ctrl_t::kEmpty));
+  // Fill the original and mirrored bytes for SOO slot.
+  // Result will look like:
+  // EHESEHEE
+  // Where H = soo_h2, E = kEmpty, S = kSentinel.
+  uint64_t first_ctrl_bytes =
+      ((soo_h2 << (8 * SooSlotIndex())) | kFirstCtrlBytesWithZeroes) |
+      (soo_h2 << (8 * kMirroredSooSlotIndex));
+  // Replace original and mirrored empty bytes for the new position.
+  // Result for new_offset 0 will look like:
+  // NHESNHEE
+  // Where H = soo_h2, N = H2(new_hash), E = kEmpty, S = kSentinel.
+  // Result for new_offset 2 will look like:
+  // EHNSEHNE
+  first_ctrl_bytes ^= (new_h2_xor_empty << (8 * new_offset));
+  size_t new_mirrored_offset = new_offset + kNewCapacity + 1;
+  first_ctrl_bytes ^= (new_h2_xor_empty << (8 * new_mirrored_offset));
+
+  // Fill last bytes with kEmpty.
+  std::memset(new_ctrl + kNewCapacity, static_cast<int8_t>(ctrl_t::kEmpty),
+              Group::kWidth);
+  // Overwrite the first 8 bytes with first_ctrl_bytes.
+  absl::little_endian::Store64(new_ctrl, first_ctrl_bytes);
+
+  // Example for group size 16:
+  // new_ctrl after 1st memset =      ???EEEEEEEEEEEEEEEE
+  // new_offset 0:
+  // new_ctrl after 2nd store  =      NHESNHEEEEEEEEEEEEE
+  // new_offset 2:
+  // new_ctrl after 2nd store  =      EHNSEHNEEEEEEEEEEEE
+
+  // Example for group size 8:
+  // new_ctrl after 1st memset =      ???EEEEEEEE
+  // new_offset 0:
+  // new_ctrl after 2nd store  =      NHESNHEEEEE
+  // new_offset 2:
+  // new_ctrl after 2nd store  =      EHNSEHNEEEE
+}
+
+// ClearBackingArrayNoReuse clears the backing array and sets the common
+// fields to the default values for empty non-allocated tables.
+// REQUIRES: c.capacity > policy.soo_capacity.
+void ClearBackingArrayNoReuse(CommonFields& c,
+                              const PolicyFunctions& __restrict policy,
+                              void* alloc) {
+  ABSL_SWISSTABLE_ASSERT(c.capacity() > policy.soo_capacity());
+  // We need to record infoz before calling dealloc, which will unregister
+  // infoz.
+  c.infoz().RecordClearedReservation();
+  c.infoz().RecordStorageChanged(0, policy.soo_capacity());
+  c.infoz().Unregister();
+  (*policy.dealloc)(alloc, c.capacity(), c.control(), policy.slot_size,
+                    policy.slot_align, c.has_infoz(),
+                    c.blocked_element_count());
+  c = policy.soo_enabled ? CommonFields{soo_tag_t{}}
+                         : CommonFields{non_soo_tag_t{}};
+}
+
+template <bool kSooEnabled>
+void* SingleSlotAddress(CommonFields& c) {
+  return kSooEnabled ? c.soo_data() : c.slot_array(/*capacity=*/1);
+}
+
+template <bool kSooEnabled>
+void DecrementSmallSize(CommonFields& c) {
+  if constexpr (kSooEnabled) {
+    c.set_empty_soo();
+  } else {
+    c.decrement_size();
+  }
+}
+
+}  // namespace
+
+void EraseMetaOnlySmall(CommonFields& c, bool soo_enabled, size_t slot_size) {
+  ABSL_SWISSTABLE_ASSERT(c.is_small());
+  if (soo_enabled) {
+    c.set_empty_soo();
+    return;
+  }
+  c.decrement_size();
+  c.infoz().RecordErase();
+  SanitizerPoisonMemoryRegion(SingleSlotAddress</*kSooEnabled=*/false>(c),
+                              slot_size);
+}
+
+void EraseMetaOnlyLarge(CommonFields& c, size_t index, size_t slot_size) {
+  ABSL_SWISSTABLE_ASSERT(!c.is_small());
+  ABSL_SWISSTABLE_ASSERT(IsFull(c.control()[index]) &&
+                         "erasing a dangling iterator");
+  c.decrement_size();
+  c.infoz().RecordErase();
+
+  if (WasNeverFull(c, index)) {
+    SetCtrl(c, index, ctrl_t::kEmpty, slot_size);
+    c.growth_info().OverwriteFullAsEmpty();
+    return;
+  }
+
+  c.growth_info().OverwriteFullAsDeleted();
+  SetCtrlInLargeTable(c, index, ctrl_t::kDeleted, slot_size);
+}
+
+void ClearBackingArray(CommonFields& c,
+                       const PolicyFunctions& __restrict policy, void* alloc,
+                       bool reuse) {
+  ABSL_SWISSTABLE_ASSERT(c.capacity() > kMaxSmallCapacity);
+  if (reuse) {
+    const size_t blocked_element_count = c.blocked_element_count();
+    c.set_size_to_zero();
+    ABSL_SWISSTABLE_ASSERT(c.capacity() > policy.soo_capacity());
+    ResetCtrl(c, policy.slot_size, blocked_element_count);
+    ResetGrowthLeft(c.growth_info(), c.capacity(), blocked_element_count);
+    ABSL_SWISSTABLE_ASSERT(c.blocked_element_count() == blocked_element_count);
+    c.infoz().RecordStorageChanged(0, c.capacity());
+  } else {
+    ClearBackingArrayNoReuse(c, policy, alloc);
+  }
+}
+
+void DestroySlots(CommonFields& c, size_t slot_size,
+                  DestroySlotFn destroy_slot) {
+  ABSL_SWISSTABLE_ASSERT(!c.is_small());
+  ABSL_SWISSTABLE_ASSERT(destroy_slot != nullptr);
+  auto destroy_slot_wrapper = [&](const ctrl_t*, void* slot) {
+    destroy_slot(&c, slot);
+  };
+  if constexpr (SwisstableGenerationsOrDebugEnabled()) {
+    CommonFields common_copy(non_soo_tag_t{}, c);
+    c.set_capacity(HashtableCapacity::CreateDestroyed());
+    IterateOverFullSlotsImpl(common_copy, slot_size, destroy_slot_wrapper);
+    c.set_capacity(common_copy.capacity());
+  } else {
+    IterateOverFullSlotsImpl(c, slot_size, destroy_slot_wrapper);
+  }
+}
+
+void DeallocBackingArray(CommonFields& c, size_t slot_size, size_t slot_align,
+                         DeallocBackingArrayFn dealloc, void* alloc) {
+  const size_t cap = c.capacity();
+  c.infoz().Unregister();
+  dealloc(alloc, cap, c.control(), slot_size, slot_align, c.has_infoz(),
+          c.blocked_element_count());
+}
+
+template <bool kSooEnabled>
+void Clear(CommonFields& c, const PolicyFunctions& __restrict policy,
+           DestroySlotFn destroy_slot, void* alloc) {
+  if (SwisstableGenerationsEnabled() &&
+      c.maybe_invalid_capacity().IsMovedFrom()) {
+    c.set_capacity(policy.soo_capacity());
+  }
+  c.AssertNotDebugCapacity();
+  const size_t cap = c.capacity();
+  if constexpr (kSooEnabled) {
+    ABSL_ASSUME(cap > 0);
+  }
+  if (c.is_small()) {
+    if (!c.empty()) {
+      if (destroy_slot != nullptr) {
+        destroy_slot(&c, SingleSlotAddress<kSooEnabled>(c));
+      }
+      DecrementSmallSize<kSooEnabled>(c);
+      c.infoz().RecordStorageChanged(0, cap);
+    }
+  } else {
+    if (destroy_slot != nullptr) {
+      DestroySlots(c, policy.slot_size, destroy_slot);
+    }
+    // Iterating over this container is O(bucket_count()). When bucket_count()
+    // is much greater than size(), iteration becomes prohibitively expensive.
+    // For clear() it is more important to reuse the allocated array when the
+    // container is small because allocation takes comparatively long time
+    // compared to destruction of the elements of the container. So we pick the
+    // largest bucket_count() threshold for which iteration is still fast and
+    // past that we simply deallocate the array.
+    ClearBackingArray(c, policy, alloc, /*reuse=*/cap < 128);
+  }
+  c.set_reserved_growth(0);
+  c.set_reservation_size(0);
+}
+
+void DestructSoo(CommonFields& c, size_t slot_size, size_t slot_align,
+                 DestroySlotFn destroy_slot, DeallocBackingArrayFn dealloc,
+                 void* alloc) {
+  ABSL_SWISSTABLE_ASSERT(!c.is_small() || !c.empty());
+  if (c.is_small()) {
+    ABSL_SWISSTABLE_ASSERT(destroy_slot != nullptr);
+    destroy_slot(&c, c.soo_data());
+    return;
+  }
+  if (destroy_slot != nullptr) {
+    DestroySlots(c, slot_size, destroy_slot);
+  }
+  DeallocBackingArray(c, slot_size, slot_align, dealloc, alloc);
+}
+
+void DestructNonSoo(CommonFields& c, size_t slot_size, size_t slot_align,
+                    DestroySlotFn destroy_slot, DeallocBackingArrayFn dealloc,
+                    void* alloc) {
+  ABSL_SWISSTABLE_ASSERT(c.capacity() > 0);
+  if (destroy_slot != nullptr) {
+    if (c.is_small()) {
+      if (!c.empty()) {
+        static_assert(kMaxSmallCapacity == 1);
+        destroy_slot(&c, c.slot_array(/*capacity=*/1));
+      }
+    } else {
+      DestroySlots(c, slot_size, destroy_slot);
+    }
+  }
+  DeallocBackingArray(c, slot_size, slot_align, dealloc, alloc);
+}
+
+namespace {
+
+// Iterates over full slots in old table, finds new positions for them and
+// transfers the slots.
+// This function is used for reserving or rehashing non-empty tables.
+// This use case is rare so the function is type erased.
+// Returns the total probe length.
+size_t FindNewPositionsAndTransferSlots(
+    CommonFields& common, const PolicyFunctions& __restrict policy,
+    ctrl_t* old_ctrl, void* old_slots, size_t old_capacity) {
+  void* new_slots = common.slot_array(common.capacity());
+  const void* hash_fn = policy.hash_fn(common);
+  const size_t slot_size = policy.slot_size;
+  const size_t seed = common.seed().seed();
+
+  const auto insert_slot = [&](void* slot) {
+    size_t hash = policy.hash_slot(hash_fn, slot, seed);
+    FindInfo target;
+    if (common.is_small()) {
+      target = FindInfo{0, 0};
+    } else {
+      target = find_first_non_full(common, hash);
+      SetCtrl(common, target.offset, H2(hash), slot_size);
+    }
+    policy.transfer_n(&common, SlotAddress(new_slots, target.offset, slot_size),
+                      slot, 1);
+    return target.probe_length;
+  };
+  if (IsSmallCapacity(old_capacity)) {
+    if (common.size() == 1) insert_slot(old_slots);
+    return 0;
+  }
+  size_t total_probe_length = 0;
+  for (size_t i = 0; i < old_capacity; ++i) {
+    if (IsFull(old_ctrl[i])) {
+      total_probe_length += insert_slot(old_slots);
+    }
+    old_slots = NextSlot(old_slots, slot_size);
+  }
+  return total_probe_length;
+}
+
+void ReportGrowthToInfozImpl(CommonFields& common, HashtablezInfoHandle infoz,
+                             size_t hash, size_t total_probe_length,
+                             size_t distance_from_desired) {
+  ABSL_SWISSTABLE_ASSERT(infoz.IsSampled());
+  infoz.RecordStorageChanged(common.size() - 1, common.capacity());
+  infoz.RecordRehash(total_probe_length);
+  infoz.RecordInsertMiss(hash, distance_from_desired);
+  common.set_has_infoz();
+  // TODO(b/413062340): we could potentially store infoz in place of the
+  // control pointer for the capacity 1 case.
+  common.set_infoz(infoz);
+}
+
+// Specialization to avoid passing two 0s from hot function.
+ABSL_ATTRIBUTE_NOINLINE void ReportSingleGroupTableGrowthToInfoz(
+    CommonFields& common, HashtablezInfoHandle infoz, size_t hash) {
+  ReportGrowthToInfozImpl(common, infoz, hash, /*total_probe_length=*/0,
+                          /*distance_from_desired=*/0);
 }
 
+ABSL_ATTRIBUTE_NOINLINE void ReportGrowthToInfoz(CommonFields& common,
+                                                 HashtablezInfoHandle infoz,
+                                                 size_t hash,
+                                                 size_t total_probe_length,
+                                                 size_t distance_from_desired) {
+  ReportGrowthToInfozImpl(common, infoz, hash, total_probe_length,
+                          distance_from_desired);
+}
+
+ABSL_ATTRIBUTE_NOINLINE void ReportResizeToInfoz(CommonFields& common,
+                                                 HashtablezInfoHandle infoz,
+                                                 size_t total_probe_length) {
+  ABSL_SWISSTABLE_ASSERT(infoz.IsSampled());
+  infoz.RecordStorageChanged(common.size(), common.capacity());
+  infoz.RecordRehash(total_probe_length);
+  common.set_has_infoz();
+  common.set_infoz(infoz);
+}
+
+struct BackingArrayPtrs {
+  ctrl_t* ctrl;
+  void* slots;
+};
+
+BackingArrayPtrs AllocBackingArray(CommonFields& common,
+                                   const PolicyFunctions& __restrict policy,
+                                   size_t new_capacity, bool has_infoz,
+                                   void* alloc, size_t blocked_element_count) {
+  RawHashSetLayout layout(new_capacity, policy.slot_size, policy.slot_align,
+                          has_infoz, blocked_element_count);
+  // Perform a direct call in the common case to allow for profile-guided
+  // heap optimization (PGHO) to understand which allocation function is used.
+  constexpr size_t kDefaultAlignment = BackingArrayAlignment(alignof(size_t));
+  char* mem = static_cast<char*>(
+      ABSL_PREDICT_TRUE(
+          policy.alloc ==
+          (&AllocateBackingArray<kDefaultAlignment, std::allocator<char>>))
+          ? AllocateBackingArray<kDefaultAlignment, std::allocator<char>>(
+                alloc, layout.alloc_size())
+          : policy.alloc(alloc, layout.alloc_size()));
+  const GenerationType old_generation = common.generation();
+  common.set_generation_ptr(
+      reinterpret_cast<GenerationType*>(mem + layout.generation_offset()));
+  common.set_generation(NextGeneration(old_generation));
+
+  return {reinterpret_cast<ctrl_t*>(mem + layout.control_offset()),
+          mem + layout.slot_offset()};
+}
+
+void ResizeEmptyNonAllocatedTableImpl(CommonFields& common,
+                                      const PolicyFunctions& __restrict policy,
+                                      size_t new_capacity,
+                                      size_t blocked_element_count,
+                                      bool force_infoz) {
+  ABSL_SWISSTABLE_ASSERT(IsValidCapacity(new_capacity));
+  ABSL_SWISSTABLE_ASSERT(new_capacity > policy.soo_capacity());
+  ABSL_SWISSTABLE_ASSERT(!force_infoz || policy.soo_enabled);
+  ABSL_SWISSTABLE_ASSERT(common.capacity() == policy.soo_capacity());
+  ABSL_SWISSTABLE_ASSERT(common.empty());
+  const size_t slot_size = policy.slot_size;
+  HashtablezInfoHandle infoz;
+  const bool should_sample =
+      policy.is_hashtablez_eligible && (force_infoz || ShouldSampleNextTable());
+  if (ABSL_PREDICT_FALSE(should_sample)) {
+    infoz = ForcedTrySample(slot_size, policy.key_size, policy.value_size,
+                            policy.soo_capacity());
+  }
+  const bool has_infoz = infoz.IsSampled();
+  void* alloc = policy.get_char_alloc(common);
+
+  common.set_capacity(new_capacity);
+  common.init_blocked_element_count(blocked_element_count);
+  const auto [new_ctrl, new_slots] = AllocBackingArray(
+      common, policy, new_capacity, has_infoz, alloc, blocked_element_count);
+  common.set_control(new_ctrl);
+  common.generate_new_seed(has_infoz);
+
+  ResetCtrl(common, slot_size, blocked_element_count);
+  if (GrowthInfoSizeForCapacity(new_capacity) > 0) {
+    ResetGrowthLeft(GetGrowthInfoFromControl(new_ctrl), new_capacity,
+                    blocked_element_count);
+  }
+
+  if (ABSL_PREDICT_FALSE(has_infoz)) {
+    ReportResizeToInfoz(common, infoz, 0);
+  }
+}
+
+// If the table was SOO, initializes new control bytes and transfers slot.
+// After transferring the slot, sets control and slots in CommonFields.
+// It is rare to resize an SOO table with one element to a large size.
+// Requires: `c` contains SOO data.
+void InsertOldSooSlotAndInitializeControlBytes(
+    CommonFields& c, const PolicyFunctions& __restrict policy, ctrl_t* new_ctrl,
+    void* new_slots, bool has_infoz) {
+  ABSL_SWISSTABLE_ASSERT(c.size() == policy.soo_capacity());
+  ABSL_SWISSTABLE_ASSERT(policy.soo_enabled);
+  const size_t new_capacity = c.capacity();
+
+  c.generate_new_seed(has_infoz);
+
+  const size_t soo_slot_hash =
+      policy.hash_slot(policy.hash_fn(c), c.soo_data(), c.seed().seed());
+  size_t offset = probe(ProbeCapacity{new_capacity}, soo_slot_hash).offset();
+  offset = offset == new_capacity ? 0 : offset;
+  SanitizerPoisonMemoryRegion(new_slots, policy.slot_size * new_capacity);
+  void* target_slot = SlotAddress(new_slots, offset, policy.slot_size);
+  SanitizerUnpoisonMemoryRegion(target_slot, policy.slot_size);
+  policy.transfer_n(&c, target_slot, c.soo_data(), 1);
+  c.set_control(new_ctrl);
+  ResetCtrl(c, policy.slot_size, /*blocked_element_count=*/0);
+  SetCtrl(c, offset, H2(soo_slot_hash), policy.slot_size);
+}
+
+enum class ResizeFullSooTableSamplingMode {
+  kNoSampling,
+  // Force sampling. If the table was still not sampled, do not resize.
+  kForceSampleNoResizeIfUnsampled,
+};
+
+void AssertSoo([[maybe_unused]] CommonFields& common,
+               [[maybe_unused]] const PolicyFunctions& __restrict policy) {
+  ABSL_SWISSTABLE_ASSERT(policy.soo_enabled);
+  ABSL_SWISSTABLE_ASSERT(common.capacity() == policy.soo_capacity());
+}
+void AssertFullSoo([[maybe_unused]] CommonFields& common,
+                   [[maybe_unused]] const PolicyFunctions& __restrict policy) {
+  AssertSoo(common, policy);
+  ABSL_SWISSTABLE_ASSERT(common.size() == policy.soo_capacity());
+}
+
+void ResizeFullSooTable(CommonFields& common,
+                        const PolicyFunctions& __restrict policy,
+                        size_t new_capacity,
+                        ResizeFullSooTableSamplingMode sampling_mode) {
+  AssertFullSoo(common, policy);
+  const size_t slot_size = policy.slot_size;
+  void* alloc = policy.get_char_alloc(common);
+  constexpr size_t kTableSize = 1;
+
+  HashtablezInfoHandle infoz;
+  bool has_infoz = false;
+  if (sampling_mode ==
+      ResizeFullSooTableSamplingMode::kForceSampleNoResizeIfUnsampled) {
+    if (ABSL_PREDICT_FALSE(policy.is_hashtablez_eligible)) {
+      infoz = ForcedTrySample(slot_size, policy.key_size, policy.value_size,
+                              policy.soo_capacity());
+    }
+
+    if (!infoz.IsSampled()) return;
+    has_infoz = true;
+  }
+
+  common.set_capacity(new_capacity);
+
+  // We do not set control and slots in CommonFields yet to avoid overriding
+  // SOO data.
+  const auto [new_ctrl, new_slots] =
+      AllocBackingArray(common, policy, new_capacity, has_infoz, alloc,
+                        /*blocked_element_count=*/0);
+
+  InsertOldSooSlotAndInitializeControlBytes(common, policy, new_ctrl, new_slots,
+                                            has_infoz);
+  ResetGrowthLeft(common.growth_info(), new_capacity, kTableSize);
+  if (has_infoz) {
+    common.set_has_infoz();
+    common.set_infoz(infoz);
+    infoz.RecordStorageChanged(kTableSize, new_capacity);
+  }
+}
+
+void GrowIntoSingleGroupShuffleControlBytes(ctrl_t* __restrict old_ctrl,
+                                            size_t old_capacity,
+                                            size_t old_blocked_element_count,
+                                            ctrl_t* __restrict new_ctrl,
+                                            size_t new_capacity) {
+  ABSL_SWISSTABLE_ASSERT(is_single_group(new_capacity));
+  constexpr size_t kHalfWidth = Group::kWidth / 2;
+  ABSL_ASSUME(old_capacity < kHalfWidth);
+  ABSL_ASSUME(old_capacity > 0);
+  static_assert(Group::kWidth == 8 || Group::kWidth == 16,
+                "Group size is not supported.");
+
+  // NOTE: operations are done with compile time known size = 8.
+  // Compiler optimizes that into single ASM operation.
+
+  // Load the bytes from old_capacity. This contains
+  // - the sentinel byte
+  // - all the old control bytes
+  // - the rest is filled with kEmpty bytes
+  // Example:
+  // old_ctrl =     012S012EEEEEEEEE...
+  // copied_bytes = S012EEEE
+  // Example with blocked elements:
+  // old_ctrl =     01SS01SEEEEEEEEE...
+  // copied_bytes = S01SEEEE
+  uint64_t copied_bytes = absl::little_endian::Load64(old_ctrl + old_capacity);
+
+  // We change the sentinel byte to kEmpty before storing to both the start of
+  // the new_ctrl, and past the end of the new_ctrl later for the new cloned
+  // bytes. Note that this is faster than setting the sentinel byte to kEmpty
+  // after the copy directly in new_ctrl because we are limited on store
+  // bandwidth.
+  static constexpr uint64_t kEmptyXorSentinel =
+      static_cast<uint8_t>(ctrl_t::kEmpty) ^
+      static_cast<uint8_t>(ctrl_t::kSentinel);
+
+  // Replace the first byte kSentinel with kEmpty.
+  // Resulting bytes will be shifted by one byte old control blocks.
+  // Example:
+  // old_ctrl = 012S012EEEEEEEEE...
+  // before =   S012EEEE
+  // after  =   E012EEEE
+  copied_bytes ^= kEmptyXorSentinel;
+
+  if (ABSL_PREDICT_FALSE(old_blocked_element_count > 0)) {
+    // Replacing blocked sentinel elements with kEmpty.
+    static constexpr uint64_t kAllBytesEmptyXorSentinel =
+        kEmptyXorSentinel * uint64_t{0x0101010101010101};
+    uint64_t blocked_mask = kAllBytesEmptyXorSentinel;
+    // Keep old_blocked_element_count bytes in the mask.
+    blocked_mask >>= 64 - old_blocked_element_count * 8;
+    // Shift the mask to the start of the blocked elements bytes.
+    blocked_mask <<= (old_capacity - old_blocked_element_count + 1) * 8;
+    // Example with blocked elements:
+    // old_ctrl = 0SSS0SSEEEEEEEEE...
+    // before =   E0SSEEEE
+    // after  =   E0EEEEEE
+    copied_bytes ^= blocked_mask;
+  }
+
+  if (Group::kWidth == 8) {
+    // With group size 8, we can grow with two write operations.
+    ABSL_SWISSTABLE_ASSERT(old_capacity < 8 &&
+                           "old_capacity is too large for group size 8");
+    absl::little_endian::Store64(new_ctrl, copied_bytes);
+
+    static constexpr uint64_t kSentinal64 =
+        static_cast<uint8_t>(ctrl_t::kSentinel);
+
+    // Prepend kSentinel byte to the beginning of copied_bytes.
+    // We have maximum 3 non-empty bytes at the beginning of copied_bytes for
+    // group size 8.
+    // Example:
+    // old_ctrl = 012S012EEEE
+    // before =   E012EEEE
+    // after  =   SE012EEE
+    copied_bytes = (copied_bytes << 8) ^ kSentinal64;
+    absl::little_endian::Store64(new_ctrl + new_capacity, copied_bytes);
+    // Example for capacity 3:
+    // old_ctrl = 012S012EEEE
+    // After the first store:
+    //           >!
+    // new_ctrl = E012EEEE???????
+    // After the second store:
+    //                  >!
+    // new_ctrl = E012EEESE012EEE
+    return;
+  }
+
+  ABSL_SWISSTABLE_ASSERT(Group::kWidth == 16);  // NOLINT(misc-static-assert)
+
+  // Fill the second half of the main control bytes with kEmpty.
+  // For small capacity that may write into mirrored control bytes.
+  // It is fine as we will overwrite all the bytes later.
+  std::memset(new_ctrl + kHalfWidth, static_cast<int8_t>(ctrl_t::kEmpty),
+              kHalfWidth);
+  // Fill the second half of the mirrored control bytes with kEmpty.
+  std::memset(new_ctrl + new_capacity + kHalfWidth,
+              static_cast<int8_t>(ctrl_t::kEmpty), kHalfWidth);
+  // Copy the first half of the non-mirrored control bytes.
+  absl::little_endian::Store64(new_ctrl, copied_bytes);
+  new_ctrl[new_capacity] = ctrl_t::kSentinel;
+  // Copy the first half of the mirrored control bytes.
+  absl::little_endian::Store64(new_ctrl + new_capacity + 1, copied_bytes);
+
+  // Example for growth capacity 1->3:
+  // old_ctrl =                  0S0EEEEEEEEEEEEEE
+  // new_ctrl at the end =       E0ESE0EEEEEEEEEEEEE
+  //                                    >!
+  // new_ctrl after 1st memset = ????????EEEEEEEE???
+  //                                       >!
+  // new_ctrl after 2nd memset = ????????EEEEEEEEEEE
+  //                            >!
+  // new_ctrl after 1st store =  E0EEEEEEEEEEEEEEEEE
+  // new_ctrl after kSentinel =  E0ESEEEEEEEEEEEEEEE
+  //                                >!
+  // new_ctrl after 2nd store =  E0ESE0EEEEEEEEEEEEE
+
+  // Example for growth capacity 3->7:
+  // old_ctrl =                  012S012EEEEEEEEEEEE
+  // new_ctrl at the end =       E012EEESE012EEEEEEEEEEE
+  //                                    >!
+  // new_ctrl after 1st memset = ????????EEEEEEEE???????
+  //                                           >!
+  // new_ctrl after 2nd memset = ????????EEEEEEEEEEEEEEE
+  //                            >!
+  // new_ctrl after 1st store =  E012EEEEEEEEEEEEEEEEEEE
+  // new_ctrl after kSentinel =  E012EEESEEEEEEEEEEEEEEE
+  //                                >!
+  // new_ctrl after 2nd store =  E012EEESE012EEEEEEEEEEE
+
+  // Example for growth capacity 7->15:
+  // old_ctrl =                  0123456S0123456EEEEEEEE
+  // new_ctrl at the end =       E0123456EEEEEEESE0123456EEEEEEE
+  //                                    >!
+  // new_ctrl after 1st memset = ????????EEEEEEEE???????????????
+  //                                                   >!
+  // new_ctrl after 2nd memset = ????????EEEEEEEE???????EEEEEEEE
+  //                            >!
+  // new_ctrl after 1st store =  E0123456EEEEEEEE???????EEEEEEEE
+  // new_ctrl after kSentinel =  E0123456EEEEEEES???????EEEEEEEE
+  //                                            >!
+  // new_ctrl after 2nd store =  E0123456EEEEEEESE0123456EEEEEEE
+}
+
+// Size of the buffer we allocate on stack for storing probed elements in
+// GrowToNextCapacity algorithm.
+constexpr size_t kProbedElementsBufferSize = 512;
+
+// Decodes information about probed elements from contiguous memory.
+// Finds new position for each element and transfers it to the new slots.
+// Returns the total probe length.
+template <typename ProbedItem>
+ABSL_ATTRIBUTE_NOINLINE size_t DecodeAndInsertImpl(
+    CommonFields& c, const PolicyFunctions& __restrict policy,
+    const ProbedItem* start, const ProbedItem* end, void* old_slots) {
+  const HashtableCapacity new_capacity = c.capacity_impl();
+
+  void* new_slots = c.slot_array(new_capacity.capacity());
+  ctrl_t* new_ctrl = c.control();
+  size_t total_probe_length = 0;
+
+  const size_t slot_size = policy.slot_size;
+  auto transfer_n = policy.transfer_n;
+
+  for (; start < end; ++start) {
+    const FindInfo target = find_first_non_full_from_h1(
+        new_ctrl, static_cast<size_t>(start->h1), new_capacity);
+    total_probe_length += target.probe_length;
+    const size_t old_index = static_cast<size_t>(start->source_offset);
+    const size_t new_i = target.offset;
+    ABSL_SWISSTABLE_ASSERT(old_index < new_capacity.capacity() / 2);
+    ABSL_SWISSTABLE_ASSERT(new_i < new_capacity.capacity());
+    ABSL_SWISSTABLE_ASSERT(IsEmpty(new_ctrl[new_i]));
+    void* src_slot = SlotAddress(old_slots, old_index, slot_size);
+    void* dst_slot = SlotAddress(new_slots, new_i, slot_size);
+    SanitizerUnpoisonMemoryRegion(dst_slot, slot_size);
+    transfer_n(&c, dst_slot, src_slot, 1);
+    SetCtrlInLargeTable(c, new_i, static_cast<h2_t>(start->h2), slot_size);
+  }
+  return total_probe_length;
+}
+
+// Sentinel value for the start of marked elements.
+// Signals that there are no marked elements.
+constexpr size_t kNoMarkedElementsSentinel = ~size_t{};
+
+// Process probed elements that did not fit into available buffers.
+// We marked them in control bytes as kMarkedForSlowTransfer.
+// Hash recomputation and full probing is done here.
+// This use case should be extremely rare.
+ABSL_ATTRIBUTE_NOINLINE size_t ProcessProbedMarkedElements(
+    CommonFields& c, const PolicyFunctions& __restrict policy, ctrl_t* old_ctrl,
+    void* old_slots, size_t start) {
+  size_t old_capacity = PreviousCapacity(c.capacity());
+  const size_t slot_size = policy.slot_size;
+  void* new_slots = c.slot_array(c.capacity());
+  size_t total_probe_length = 0;
+  const void* hash_fn = policy.hash_fn(c);
+  auto hash_slot = policy.hash_slot;
+  auto transfer_n = policy.transfer_n;
+  const size_t seed = c.seed().seed();
+  for (size_t old_index = start; old_index < old_capacity; ++old_index) {
+    if (old_ctrl[old_index] != ctrl_t::kMarkedForSlowTransfer) {
+      continue;
+    }
+    void* src_slot = SlotAddress(old_slots, old_index, slot_size);
+    const size_t hash = hash_slot(hash_fn, src_slot, seed);
+    const FindInfo target = find_first_non_full(c, hash);
+    total_probe_length += target.probe_length;
+    const size_t new_i = target.offset;
+    void* dst_slot = SlotAddress(new_slots, new_i, slot_size);
+    SetCtrlInLargeTable(c, new_i, H2(hash), slot_size);
+    transfer_n(&c, dst_slot, src_slot, 1);
+  }
+  return total_probe_length;
+}
+
+// The largest old capacity for which it is guaranteed that all probed elements
+// fit in ProbedItemEncoder's local buffer.
+// For such tables, `encode_probed_element` is trivial.
+constexpr size_t kMaxLocalBufferOldCapacity =
+    kProbedElementsBufferSize / sizeof(ProbedItem4Bytes) - 1;
+static_assert(IsValidCapacity(kMaxLocalBufferOldCapacity));
+constexpr size_t kMaxLocalBufferNewCapacity =
+    NextCapacity(kMaxLocalBufferOldCapacity);
+static_assert(kMaxLocalBufferNewCapacity <= ProbedItem4Bytes::kMaxNewCapacity);
+static_assert(NextCapacity(kMaxLocalBufferNewCapacity) <=
+              ProbedItem4Bytes::kMaxNewCapacity);
+
+// Initializes mirrored control bytes after
+// transfer_unprobed_elements_to_next_capacity.
+void InitializeMirroredControlBytes(ctrl_t* new_ctrl, size_t new_capacity) {
+  std::memcpy(new_ctrl + new_capacity,
+              // We own GrowthInfo just before control bytes. So it is ok
+              // to read one byte from it.
+              new_ctrl - 1, Group::kWidth);
+  new_ctrl[new_capacity] = ctrl_t::kSentinel;
+}
+
+// Encodes probed elements into available memory.
+// At first, a local (on stack) buffer is used. The size of the buffer is
+// kProbedElementsBufferSize bytes.
+// When the local buffer is full, we switch to `control_` buffer. We are allowed
+// to overwrite `control_` buffer till the `source_offset` byte. In case we have
+// no space in `control_` buffer, we fallback to a naive algorithm for all the
+// rest of the probed elements. We mark elements as kSentinel in control bytes
+// and later process them fully. See ProcessMarkedElements for details. It
+// should be extremely rare.
+template <typename ProbedItemType,
+          // If true, we only use the local buffer and never switch to the
+          // control buffer.
+          bool kGuaranteedFitToBuffer = false>
+class ProbedItemEncoder {
+ public:
+  using ProbedItem = ProbedItemType;
+  explicit ProbedItemEncoder(ctrl_t* control) : control_(control) {}
+
+  // Encode item into the best available location.
+  void EncodeItem(ProbedItem item) {
+    if (ABSL_PREDICT_FALSE(!kGuaranteedFitToBuffer && pos_ >= end_)) {
+      return ProcessEncodeWithOverflow(item);
+    }
+    ABSL_SWISSTABLE_ASSERT(pos_ < end_);
+    *pos_ = item;
+    ++pos_;
+  }
+
+  // Decodes information about probed elements from all available sources.
+  // Finds new position for each element and transfers it to the new slots.
+  // Returns the total probe length.
+  size_t DecodeAndInsertToTable(CommonFields& common,
+                                const PolicyFunctions& __restrict policy,
+                                void* old_slots) const {
+    if (pos_ == buffer_) {
+      return 0;
+    }
+    if constexpr (kGuaranteedFitToBuffer) {
+      return DecodeAndInsertImpl(common, policy, buffer_, pos_, old_slots);
+    }
+    size_t total_probe_length = DecodeAndInsertImpl(
+        common, policy, buffer_,
+        local_buffer_full_ ? buffer_ + kBufferSize : pos_, old_slots);
+    if (!local_buffer_full_) {
+      return total_probe_length;
+    }
+    total_probe_length +=
+        DecodeAndInsertToTableOverflow(common, policy, old_slots);
+    return total_probe_length;
+  }
+
+ private:
+  static ProbedItem* AlignToNextItem(void* ptr) {
+    return reinterpret_cast<ProbedItem*>(AlignUpTo(
+        reinterpret_cast<uintptr_t>(ptr), alignof(ProbedItem)));
+  }
+
+  ProbedItem* OverflowBufferStart() const {
+    ABSL_SWISSTABLE_ASSERT(!kGuaranteedFitToBuffer &&
+                           "OverflowBufferStart should not be called when "
+                           "kGuaranteedFitToBuffer is true.");
+    // We reuse GrowthInfo memory as well.
+    return AlignToNextItem(
+        control_ - MetadataBeforeControlSize(/*has_infoz=*/false,
+                                 NextCapacity(kMaxLocalBufferOldCapacity)));
+  }
+
+  // Encodes item when previously allocated buffer is full.
+  // At first that happens when local buffer is full.
+  // We switch from the local buffer to the control buffer.
+  // Every time this function is called, the available buffer is extended till
+  // `item.source_offset` byte in the control buffer.
+  // After the buffer is extended, this function wouldn't be called till the
+  // buffer is exhausted.
+  //
+  // If there's no space in the control buffer, we fallback to naive algorithm
+  // and mark probed elements as kMarkedForSlowTransfer in the control buffer.
+  // In this case, we will call this function for every subsequent probed
+  // element.
+  ABSL_ATTRIBUTE_NOINLINE void ProcessEncodeWithOverflow(ProbedItem item) {
+    if (!local_buffer_full_) {
+      local_buffer_full_ = true;
+      pos_ = OverflowBufferStart();
+    }
+    const size_t source_offset = static_cast<size_t>(item.source_offset);
+    // We are in fallback mode so we can't reuse control buffer anymore.
+    // Probed elements are marked as kMarkedForSlowTransfer in the control
+    // buffer.
+    if (ABSL_PREDICT_FALSE(marked_elements_starting_position_ !=
+                           kNoMarkedElementsSentinel)) {
+      control_[source_offset] = ctrl_t::kMarkedForSlowTransfer;
+      return;
+    }
+    // Refresh the end pointer to the new available position.
+    // Invariant: if pos < end, then we have at least sizeof(ProbedItem) bytes
+    // to write.
+    end_ = control_ + source_offset + 1 - sizeof(ProbedItem);
+    if (ABSL_PREDICT_TRUE(pos_ < end_)) {
+      *pos_ = item;
+      ++pos_;
+      return;
+    }
+    control_[source_offset] = ctrl_t::kMarkedForSlowTransfer;
+    marked_elements_starting_position_ = source_offset;
+    // Now we will always fall down to `ProcessEncodeWithOverflow`.
+    ABSL_SWISSTABLE_ASSERT(pos_ >= end_);
+  }
+
+  // Decodes information about probed elements from control buffer and processes
+  // marked elements.
+  // Finds new position for each element and transfers it to the new slots.
+  // Returns the total probe length.
+  ABSL_ATTRIBUTE_NOINLINE size_t DecodeAndInsertToTableOverflow(
+      CommonFields& common, const PolicyFunctions& __restrict policy,
+      void* old_slots) const {
+    ABSL_SWISSTABLE_ASSERT(local_buffer_full_ &&
+                           "must not be called when local buffer is not full");
+    size_t total_probe_length = DecodeAndInsertImpl(
+        common, policy, OverflowBufferStart(), pos_, old_slots);
+    if (ABSL_PREDICT_TRUE(marked_elements_starting_position_ ==
+                          kNoMarkedElementsSentinel)) {
+      return total_probe_length;
+    }
+    total_probe_length +=
+        ProcessProbedMarkedElements(common, policy, control_, old_slots,
+                                    marked_elements_starting_position_);
+    return total_probe_length;
+  }
+
+  static constexpr size_t kBufferSize =
+      kProbedElementsBufferSize / sizeof(ProbedItem);
+  ProbedItem buffer_[kBufferSize];
+  // If local_buffer_full_ is false, then pos_/end_ are in the local buffer,
+  // otherwise, they're in the overflow buffer.
+  ProbedItem* pos_ = buffer_;
+  const void* end_ = buffer_ + kBufferSize;
+  ctrl_t* const control_;
+  size_t marked_elements_starting_position_ = kNoMarkedElementsSentinel;
+  bool local_buffer_full_ = false;
+};
+
+// Grows to next capacity with specified encoder type.
+// Encoder is used to store probed elements that are processed later.
+// Different encoder is used depending on the capacity of the table.
+// Returns total probe length.
+template <typename Encoder>
+size_t GrowToNextCapacity(CommonFields& common,
+                          const PolicyFunctions& __restrict policy,
+                          ctrl_t* old_ctrl, void* old_slots) {
+  using ProbedItem = typename Encoder::ProbedItem;
+  ABSL_SWISSTABLE_ASSERT(common.capacity() <= ProbedItem::kMaxNewCapacity);
+  Encoder encoder(old_ctrl);
+  policy.transfer_unprobed_elements_to_next_capacity(
+      common, old_ctrl, old_slots, &encoder,
+      [](void* probed_storage, h2_t h2, size_t source_offset, size_t h1) {
+        auto encoder_ptr = static_cast<Encoder*>(probed_storage);
+        encoder_ptr->EncodeItem(ProbedItem(h2, source_offset, h1));
+      });
+  InitializeMirroredControlBytes(common.control(), common.capacity());
+  return encoder.DecodeAndInsertToTable(common, policy, old_slots);
+}
+
+// Grows to next capacity for relatively small tables so that even if all
+// elements are probed, we don't need to overflow the local buffer.
+// Returns total probe length.
+size_t GrowToNextCapacityThatFitsInLocalBuffer(
+    CommonFields& common, const PolicyFunctions& __restrict policy,
+    ctrl_t* old_ctrl, void* old_slots) {
+  ABSL_SWISSTABLE_ASSERT(common.capacity() <= kMaxLocalBufferNewCapacity);
+  return GrowToNextCapacity<
+      ProbedItemEncoder<ProbedItem4Bytes, /*kGuaranteedFitToBuffer=*/true>>(
+      common, policy, old_ctrl, old_slots);
+}
+
+// Grows to next capacity with different encodings. Returns total probe length.
+// These functions are useful to simplify profile analysis.
+size_t GrowToNextCapacity4BytesEncoder(CommonFields& common,
+                                       const PolicyFunctions& __restrict policy,
+                                       ctrl_t* old_ctrl, void* old_slots) {
+  return GrowToNextCapacity<ProbedItemEncoder<ProbedItem4Bytes>>(
+      common, policy, old_ctrl, old_slots);
+}
+size_t GrowToNextCapacity8BytesEncoder(CommonFields& common,
+                                       const PolicyFunctions& __restrict policy,
+                                       ctrl_t* old_ctrl, void* old_slots) {
+  return GrowToNextCapacity<ProbedItemEncoder<ProbedItem8Bytes>>(
+      common, policy, old_ctrl, old_slots);
+}
+size_t GrowToNextCapacity16BytesEncoder(
+    CommonFields& common, const PolicyFunctions& __restrict policy,
+    ctrl_t* old_ctrl, void* old_slots) {
+  return GrowToNextCapacity<ProbedItemEncoder<ProbedItem16Bytes>>(
+      common, policy, old_ctrl, old_slots);
+}
+
+// Grows to next capacity for tables with relatively large capacity so that we
+// can't guarantee that all probed elements fit in the local buffer. Returns
+// total probe length.
+size_t GrowToNextCapacityOverflowLocalBuffer(
+    CommonFields& common, const PolicyFunctions& __restrict policy,
+    ctrl_t* old_ctrl, void* old_slots) {
+  const size_t new_capacity = common.capacity();
+  if (ABSL_PREDICT_TRUE(new_capacity <= ProbedItem4Bytes::kMaxNewCapacity)) {
+    return GrowToNextCapacity4BytesEncoder(common, policy, old_ctrl, old_slots);
+  }
+  if (ABSL_PREDICT_TRUE(new_capacity <= ProbedItem8Bytes::kMaxNewCapacity)) {
+    return GrowToNextCapacity8BytesEncoder(common, policy, old_ctrl, old_slots);
+  }
+  // 16 bytes encoding supports the maximum swisstable capacity.
+  return GrowToNextCapacity16BytesEncoder(common, policy, old_ctrl, old_slots);
+}
+
+// Dispatches to the appropriate `GrowToNextCapacity*` function based on the
+// capacity of the table. Returns total probe length.
+ABSL_ATTRIBUTE_NOINLINE
+size_t GrowToNextCapacityDispatch(CommonFields& common,
+                                  const PolicyFunctions& __restrict policy,
+                                  ctrl_t* old_ctrl, void* old_slots) {
+  const size_t new_capacity = common.capacity();
+  if (ABSL_PREDICT_TRUE(new_capacity <= kMaxLocalBufferNewCapacity)) {
+    return GrowToNextCapacityThatFitsInLocalBuffer(common, policy, old_ctrl,
+                                                   old_slots);
+  } else {
+    return GrowToNextCapacityOverflowLocalBuffer(common, policy, old_ctrl,
+                                                 old_slots);
+  }
+}
+
+void IncrementSmallSizeNonSoo(CommonFields& common,
+                              const PolicyFunctions& __restrict policy) {
+  ABSL_SWISSTABLE_ASSERT(common.is_small());
+  common.increment_size();
+  SanitizerUnpoisonMemoryRegion(
+      SingleSlotAddress</*kSooEnabled=*/false>(common), policy.slot_size);
+}
+
+void IncrementSmallSize(CommonFields& common,
+                        const PolicyFunctions& __restrict policy) {
+  ABSL_SWISSTABLE_ASSERT(common.is_small());
+  if (policy.soo_enabled) {
+    common.set_full_soo();
+  } else {
+    IncrementSmallSizeNonSoo(common, policy);
+  }
+}
+
+void* Grow1To3AndPrepareInsert(CommonFields& common,
+                               const PolicyFunctions& __restrict policy,
+                               absl::FunctionRef<size_t(size_t)> get_hash) {
+  // TODO(b/413062340): Refactor to reuse more code with
+  // GrowSooTableToNextCapacityAndPrepareInsert.
+  ABSL_SWISSTABLE_ASSERT(common.capacity() == 1);
+  ABSL_SWISSTABLE_ASSERT(!common.empty());
+  ABSL_SWISSTABLE_ASSERT(!policy.soo_enabled);
+  // 1-element tables can't have any blocked elements.
+  ABSL_SWISSTABLE_ASSERT(common.blocked_element_count() == 0);
+  constexpr size_t kOldCapacity = 1;
+  constexpr size_t kNewCapacity = NextCapacity(kOldCapacity);
+  void* old_slots = common.slot_array(kOldCapacity);
+
+  const size_t slot_size = policy.slot_size;
+  const size_t slot_align = policy.slot_align;
+  void* alloc = policy.get_char_alloc(common);
+  HashtablezInfoHandle infoz = common.infoz();
+  const bool has_infoz = infoz.IsSampled();
+  common.set_capacity(kNewCapacity);
+
+  const auto [new_ctrl, new_slots] =
+      AllocBackingArray(common, policy, kNewCapacity, has_infoz, alloc,
+                        /*blocked_element_count=*/0);
+  common.set_control(new_ctrl);
+  SanitizerPoisonMemoryRegion(new_slots, kNewCapacity * slot_size);
+
+  if (ABSL_PREDICT_TRUE(!has_infoz)) {
+    // When we're sampled, we already have a seed.
+    common.generate_new_seed(/*has_infoz=*/false);
+  }
+  const size_t new_hash = get_hash(common.seed().seed());
+  h2_t new_h2 = H2(new_hash);
+  size_t orig_hash =
+      policy.hash_slot(policy.hash_fn(common), old_slots, common.seed().seed());
+  size_t offset = Resize1To3NewOffset(new_hash, common.seed());
+  InitializeThreeElementsControlBytes(H2(orig_hash), new_h2, offset, new_ctrl);
+
+  void* old_element_target = NextSlot(new_slots, slot_size);
+  SanitizerUnpoisonMemoryRegion(old_element_target, slot_size);
+  policy.transfer_n(&common, old_element_target, old_slots, 1);
+
+  void* new_element_target_slot = SlotAddress(new_slots, offset, slot_size);
+  SanitizerUnpoisonMemoryRegion(new_element_target_slot, slot_size);
+
+  policy.dealloc(alloc, kOldCapacity,
+                 // old_slots == old_ctrl in case of capacity == 1.
+                 static_cast<ctrl_t*>(old_slots),
+                 slot_size, slot_align, has_infoz,
+                 /*blocked_element_count=*/0);
+  PrepareInsertCommon(common);
+  ABSL_SWISSTABLE_ASSERT(common.size() == 2);
+  GetGrowthInfoFromControl(new_ctrl).InitGrowthLeftNoDeleted(kNewCapacity - 2,
+                                                             kNewCapacity);
+
+  if (ABSL_PREDICT_FALSE(has_infoz)) {
+    ReportSingleGroupTableGrowthToInfoz(common, infoz, new_hash);
+  }
+  return new_element_target_slot;
+}
+
+// Grows to next capacity and prepares insert for the given new_hash.
+// Returns the offset of the new element.
+void* GrowToNextCapacityAndPrepareInsert(
+    CommonFields& common, const PolicyFunctions& __restrict policy,
+    size_t new_hash) {
+  const size_t old_capacity = common.capacity();
+  ABSL_SWISSTABLE_ASSERT(
+      common.growth_info().GetGrowthLeftTotalSlow(old_capacity) == 0);
+  ABSL_SWISSTABLE_ASSERT(old_capacity > policy.soo_capacity());
+  ABSL_SWISSTABLE_ASSERT(!IsSmallCapacity(old_capacity));
+  ABSL_ASSUME(old_capacity > kMaxSmallCapacity);
+
+  const size_t new_capacity = NextCapacity(old_capacity);
+  ctrl_t* old_ctrl = common.control();
+  void* old_slots = common.slot_array(old_capacity);
+  size_t old_blocked_element_count = common.blocked_element_count();
+
+  HashtablezInfoHandle infoz = common.infoz();
+  const bool has_infoz = infoz.IsSampled();
+  common.set_capacity(new_capacity);
+  common.set_blocked_element_count_to_zero();
+  const size_t slot_size = policy.slot_size;
+  const size_t slot_align = policy.slot_align;
+  void* alloc = policy.get_char_alloc(common);
+
+  const auto [new_ctrl, new_slots] =
+      AllocBackingArray(common, policy, new_capacity, has_infoz, alloc,
+                        /*blocked_element_count=*/0);
+  common.set_control(new_ctrl);
+  SanitizerPoisonMemoryRegion(new_slots, new_capacity * slot_size);
+
+  h2_t new_h2 = H2(new_hash);
+  size_t total_probe_length = 0;
+  FindInfo find_info;
+  if (ABSL_PREDICT_TRUE(is_single_group(new_capacity))) {
+    size_t offset;
+    const size_t old_size = common.size();
+    GrowIntoSingleGroupShuffleControlBytes(old_ctrl, old_capacity,
+                                           old_blocked_element_count, new_ctrl,
+                                           new_capacity);
+    // We put the new element either at the beginning or at the end of the
+    // table with approximately equal probability.
+    offset =
+        SingleGroupTableH1(new_hash, common.seed()) & 1 ? 0 : new_capacity - 1;
+
+    ABSL_SWISSTABLE_ASSERT(IsEmpty(new_ctrl[offset]));
+    SetCtrlInSingleGroupTable(common, offset, new_h2, policy.slot_size);
+    find_info = FindInfo{offset, 0};
+    // Single group tables have all slots full on resize. So we can transfer
+    // all slots without checking the control bytes.
+    ABSL_SWISSTABLE_ASSERT(common.size() + old_blocked_element_count ==
+                           old_capacity);
+    void* target = NextSlot(new_slots, slot_size);
+    SanitizerUnpoisonMemoryRegion(target, old_size * slot_size);
+    policy.transfer_n(&common, target, old_slots, old_size);
+  } else {
+    total_probe_length =
+        GrowToNextCapacityDispatch(common, policy, old_ctrl, old_slots);
+    find_info = find_first_non_full(common, new_hash);
+    SetCtrlInLargeTable(common, find_info.offset, new_h2, policy.slot_size);
+  }
+  ABSL_SWISSTABLE_ASSERT(old_capacity > policy.soo_capacity());
+  (*policy.dealloc)(alloc, old_capacity, old_ctrl, slot_size, slot_align,
+                    has_infoz, old_blocked_element_count);
+  PrepareInsertCommon(common);
+  ResetGrowthLeft(GetGrowthInfoFromControl(new_ctrl), new_capacity,
+                  common.size());
+
+  if (ABSL_PREDICT_FALSE(has_infoz)) {
+    ReportGrowthToInfoz(common, infoz, new_hash, total_probe_length,
+                        find_info.probe_length);
+  }
+  return SlotAddress(new_slots, find_info.offset, policy.slot_size);
+}
+
+}  // namespace
+
+void* PrepareInsertSmallNonSoo(CommonFields& common,
+                               const PolicyFunctions& __restrict policy,
+                               absl::FunctionRef<size_t(size_t)> get_hash) {
+  ABSL_SWISSTABLE_ASSERT(common.is_small());
+  ABSL_SWISSTABLE_ASSERT(!policy.soo_enabled);
+  if (common.capacity() == 1) {
+    if (common.empty()) {
+      IncrementSmallSizeNonSoo(common, policy);
+      if (common.has_infoz()) {
+        common.infoz().RecordInsertMiss(get_hash(common.seed().seed()),
+                                        /*distance_from_desired=*/0);
+      }
+      return common.slot_array(/*capacity=*/1);
+    } else {
+      return Grow1To3AndPrepareInsert(common, policy, get_hash);
+    }
+  }
+
+  // Growing from 0 to 1 capacity.
+  ABSL_SWISSTABLE_ASSERT(common.capacity() == 0);
+  constexpr size_t kNewCapacity = 1;
+
+  common.set_capacity(kNewCapacity);
+  HashtablezInfoHandle infoz;
+  const bool should_sample =
+      policy.is_hashtablez_eligible && ShouldSampleNextTable();
+  if (ABSL_PREDICT_FALSE(should_sample)) {
+    infoz = ForcedTrySample(policy.slot_size, policy.key_size,
+                            policy.value_size, policy.soo_capacity());
+  }
+  const bool has_infoz = infoz.IsSampled();
+  void* alloc = policy.get_char_alloc(common);
+
+  const auto [new_ctrl, new_slots] =
+      AllocBackingArray(common, policy, kNewCapacity, has_infoz, alloc,
+                        /*blocked_element_count=*/0);
+  common.set_control(new_ctrl);
+
+  static_assert(NextCapacity(0) == 1);
+  PrepareInsertCommon(common);
+
+  if (ABSL_PREDICT_FALSE(has_infoz)) {
+    common.generate_new_seed(/*has_infoz=*/true);
+    ReportSingleGroupTableGrowthToInfoz(common, infoz,
+                                        get_hash(common.seed().seed()));
+  }
+  return new_slots;
+}
+
+namespace {
+
+// Called whenever the table needs to vacate empty slots either by removing
+// tombstones via rehash or growth to next capacity.
+ABSL_ATTRIBUTE_NOINLINE
+void* RehashOrGrowToNextCapacityAndPrepareInsert(
+    CommonFields& common, const PolicyFunctions& __restrict policy,
+    size_t new_hash) {
+  ABSL_SWISSTABLE_ASSERT(
+      !common.growth_info().GetGrowthInfoLowerBound().HasNoDeleted());
+  const size_t cap = common.capacity();
+  ABSL_ASSUME(cap > 0);
+  // Do these calculations in 64-bit to avoid overflow.
+  if (common.size() * uint64_t{32} <=
+      (cap - kMaxBlockedElementsForLargeTables) * uint64_t{25}) {
+    // Squash DELETED without growing if there is enough capacity.
+    //
+    // Rehash in place if the current size is <= 25/32 of capacity.
+    // Rationale for such a high factor: 1) DropDeletesWithoutResize() is
+    // faster than resize, and 2) it takes quite a bit of work to add
+    // tombstones.  In the worst case, seems to take approximately 4
+    // insert/erase pairs to create a single tombstone and so if we are
+    // rehashing because of tombstones, we can afford to rehash-in-place as
+    // long as we are reclaiming at least 1/8 the capacity without doing more
+    // than 2X the work.  (Where "work" is defined to be size() for rehashing
+    // or rehashing in place, and 1 for an insert or erase.)  But rehashing in
+    // place is faster per operation than inserting or even doubling the size
+    // of the table, so we actually afford to reclaim even less space from a
+    // resize-in-place.  The decision is to rehash in place if we can reclaim
+    // at about 1/8th of the usable capacity (specifically 3/28 of the
+    // capacity) which means that the total cost of rehashing will be a small
+    // fraction of the total work.
+    //
+    // Here is output of an experiment using the BM_CacheInSteadyState
+    // benchmark running the old case (where we rehash-in-place only if we can
+    // reclaim at least 7/16*capacity) vs. this code (which rehashes in place
+    // if we can recover 3/32*capacity).
+    //
+    // Note that although in the worst-case number of rehashes jumped up from
+    // 15 to 190, but the number of operations per second is almost the same.
+    //
+    // Abridged output of running BM_CacheInSteadyState benchmark from
+    // raw_hash_set_benchmark.   N is the number of insert/erase operations.
+    //
+    //      | OLD (recover >= 7/16        | NEW (recover >= 3/32)
+    // size |    N/s LoadFactor NRehashes |    N/s LoadFactor NRehashes
+    //  448 | 145284       0.44        18 | 140118       0.44        19
+    //  493 | 152546       0.24        11 | 151417       0.48        28
+    //  538 | 151439       0.26        11 | 151152       0.53        38
+    //  583 | 151765       0.28        11 | 150572       0.57        50
+    //  628 | 150241       0.31        11 | 150853       0.61        66
+    //  672 | 149602       0.33        12 | 150110       0.66        90
+    //  717 | 149998       0.35        12 | 149531       0.70       129
+    //  762 | 149836       0.37        13 | 148559       0.74       190
+    //  807 | 149736       0.39        14 | 151107       0.39        14
+    //  852 | 150204       0.42        15 | 151019       0.42        15
+    return DropDeletesWithoutResizeAndPrepareInsert(common, policy, new_hash);
+  } else {
+    // Otherwise grow the container.
+    return GrowToNextCapacityAndPrepareInsert(common, policy, new_hash);
+  }
+}
+
+// Slow path for PrepareInsertLarge that is called when the table has deleted
+// slots or need to be resized or rehashed.
+ABSL_ATTRIBUTE_NOINLINE
+void* PrepareInsertLargeSlow(CommonFields& common,
+                             const PolicyFunctions& __restrict policy,
+                             size_t hash) {
+  GrowthInfoAccessor growth_info = common.growth_info();
+  const size_t cap = common.capacity();
+  ABSL_ASSUME(cap > kMaxSmallCapacity);
+  GrowthInfoLowerBound growth_info_lower_bound =
+      growth_info.RebalanceGrowthLeftLowerBound(cap);
+  if (ABSL_PREDICT_TRUE(
+          growth_info_lower_bound.HasNoGrowthLeftAndNoDeleted())) {
+    // Table without deleted slots (>95% cases) that needs to be resized.
+    return GrowToNextCapacityAndPrepareInsert(common, policy, hash);
+  }
+  if (ABSL_PREDICT_FALSE(
+          growth_info_lower_bound.HasNoGrowthLeftAndHaveDeleted())) {
+    // Table with deleted slots that needs to be rehashed or resized.
+    return RehashOrGrowToNextCapacityAndPrepareInsert(common, policy, hash);
+  }
+  // Covers two cases:
+  // 1. Table with deleted slots that has space for the inserting element.
+  // 2. Table without deleted slots that has space and GrowthInfoView was
+  //    rebalanced.
+  FindInfo target = find_first_non_full(common, hash);
+  PrepareInsertCommon(common);
+  growth_info.OverwriteControlAsFull(common.control()[target.offset]);
+  SetCtrlInLargeTable(common, target.offset, H2(hash), policy.slot_size);
+  common.infoz().RecordInsertMiss(hash, target.probe_length);
+  return SlotAddress(common.slot_array(cap), target.offset, policy.slot_size);
+}
+
+// Resizes empty non-allocated SOO table to NextCapacity(SooCapacity()),
+// forces the table to be sampled and prepares the insert.
+// SOO tables need to switch from SOO to heap in order to store the infoz.
+// Requires:
+//   1. `c.capacity() == SooCapacity()`.
+//   2. `c.empty()`.
+ABSL_ATTRIBUTE_NOINLINE void*
+GrowEmptySooTableToNextCapacityForceSamplingAndPrepareInsert(
+    CommonFields& common, const PolicyFunctions& __restrict policy,
+    absl::FunctionRef<size_t(size_t)> get_hash) {
+  const size_t kNewCapacity = NextCapacity(SooCapacity());
+  ResizeEmptyNonAllocatedTableImpl(common, policy, kNewCapacity,
+                                   /*blocked_element_count=*/0,
+                                   /*force_infoz=*/true);
+  PrepareInsertCommon(common);
+  common.growth_info().OverwriteEmptyAsFull();
+  const size_t new_hash = get_hash(common.seed().seed());
+  SetCtrlInSingleGroupTable(common, SooSlotIndex(), H2(new_hash),
+                            policy.slot_size);
+  common.infoz().RecordInsertMiss(new_hash, /*distance_from_desired=*/0);
+  return SlotAddress(common.slot_array(kNewCapacity), SooSlotIndex(),
+                     policy.slot_size);
+}
+
+// Returns the number of elements to block for the given capacity and reserved
+// size.
+size_t BlockedElementCountForReservedTable(size_t capacity,
+                                           size_t reserved_size) {
+  if (!IsCapacityValidForBlockedElements(capacity)) {
+    return 0;
+  }
+  const size_t blocked_elements = CapacityToGrowth(capacity) - reserved_size;
+  if (is_single_group(capacity)) {
+    // Single group tables never probes, so we can block all the slots.
+    return blocked_elements;
+  }
+  return (std::min)(blocked_elements, kMaxBlockedElementsForLargeTables);
+}
+
+// Resizes empty non-allocated table to the capacity to fit new_size elements.
+// Requires:
+//   1. `c.capacity() == policy.soo_capacity()`.
+//   2. `c.empty()`.
+//   3. `new_size > policy.soo_capacity()`.
+// The table will be attempted to be sampled.
+void ReserveEmptyNonAllocatedTableToFitNewSize(
+    CommonFields& common, const PolicyFunctions& __restrict policy,
+    size_t new_size) {
+  ValidateMaxSize(new_size, policy.key_size, policy.slot_size);
+  ABSL_ASSUME(new_size > 0);
+  const size_t new_capacity = SizeToCapacity(new_size);
+  ResizeEmptyNonAllocatedTableImpl(
+      common, policy, new_capacity,
+      BlockedElementCountForReservedTable(new_capacity, new_size),
+      /*force_infoz=*/false);
+  // This is after resize, to ensure that we have completed the allocation
+  // and have potentially sampled the hashtable.
+  common.infoz().RecordReservation(new_size);
+}
+
+// Type erased version of raw_hash_set::reserve for tables that have an
+// allocated backing array.
+//
+// Requires:
+//   1. `c.capacity() > policy.soo_capacity()` OR `!c.empty()`.
+// Reserving already allocated tables is considered to be a rare case.
+ABSL_ATTRIBUTE_NOINLINE void ReserveAllocatedTable(
+    CommonFields& common, const PolicyFunctions& __restrict policy,
+    size_t new_size) {
+  const size_t cap = common.capacity();
+  ValidateMaxSize(new_size, policy.key_size, policy.slot_size);
+  ABSL_ASSUME(new_size > 0);
+  const size_t new_capacity = SizeToCapacity(new_size);
+  if (cap == policy.soo_capacity()) {
+    ABSL_SWISSTABLE_ASSERT(!common.empty());
+    ResizeFullSooTable(common, policy, new_capacity,
+                       ResizeFullSooTableSamplingMode::kNoSampling);
+  } else {
+    ABSL_SWISSTABLE_ASSERT(cap > policy.soo_capacity());
+    // TODO(b/382423690): consider using GrowToNextCapacity, when applicable.
+    ResizeAllocatedTableWithSeedChange(common, policy, new_capacity);
+  }
+  common.infoz().RecordReservation(new_size);
+}
+
+// As `ResizeFullSooTableToNextCapacity`, except that we also force the SOO
+// table to be sampled. SOO tables need to switch from SOO to heap in order to
+// store the infoz. No-op if sampling is disabled or not possible.
+void GrowFullSooTableToNextCapacityForceSampling(
+    CommonFields& common, const PolicyFunctions& __restrict policy) {
+  AssertFullSoo(common, policy);
+  ResizeFullSooTable(
+      common, policy, NextCapacity(SooCapacity()),
+      ResizeFullSooTableSamplingMode::kForceSampleNoResizeIfUnsampled);
+}
+
+}  // namespace
+
+void* GetRefForEmptyClass(CommonFields& common) {
+  // Empty base optimization typically make the empty base class address to be
+  // the same as the first address of the derived class object.
+  // But we generally assume that for empty classes we can return any valid
+  // pointer.
+  return &common;
+}
+
+void ResizeAllocatedTableWithSeedChange(
+    CommonFields& common, const PolicyFunctions& __restrict policy,
+    size_t new_capacity) {
+  ABSL_SWISSTABLE_ASSERT(IsValidCapacity(new_capacity));
+  ABSL_SWISSTABLE_ASSERT(new_capacity > policy.soo_capacity());
+
+  const size_t old_capacity = common.capacity();
+  ctrl_t* const old_ctrl = common.control();
+  void* const old_slots = common.slot_array(old_capacity);
+  const size_t old_blocked_element_count = common.blocked_element_count();
+
+  const size_t slot_size = policy.slot_size;
+  const size_t slot_align = policy.slot_align;
+  HashtablezInfoHandle infoz = common.infoz();
+  const bool has_infoz = infoz.IsSampled();
+  void* alloc = policy.get_char_alloc(common);
+
+  common.set_capacity(new_capacity);
+  common.set_blocked_element_count_to_zero();
+  const auto [new_ctrl, new_slots] =
+      AllocBackingArray(common, policy, new_capacity, has_infoz, alloc,
+                        /*blocked_element_count=*/0);
+  common.set_control(new_ctrl);
+  common.generate_new_seed(has_infoz);
+
+  size_t total_probe_length = 0;
+  ResetCtrl(common, slot_size, /*blocked_element_count=*/0);
+  ABSL_SWISSTABLE_ASSERT(old_capacity > 0);
+  total_probe_length = FindNewPositionsAndTransferSlots(
+      common, policy, old_ctrl, old_slots, old_capacity);
+  (*policy.dealloc)(alloc, old_capacity, old_ctrl, slot_size, slot_align,
+                    has_infoz, old_blocked_element_count);
+  if (GrowthInfoSizeForCapacity(new_capacity) > 0) {
+    ResetGrowthLeft(GetGrowthInfoFromControl(new_ctrl), new_capacity,
+                    common.size());
+  }
+
+  if (ABSL_PREDICT_FALSE(has_infoz)) {
+    ReportResizeToInfoz(common, infoz, total_probe_length);
+  }
+}
+
+// Resizes a full SOO table to the NextCapacity(SooCapacity()).
+template <size_t SooSlotMemcpySize, bool TransferUsesMemcpy>
+void* GrowSooTableToNextCapacityAndPrepareInsert(
+    CommonFields& common, const PolicyFunctions& __restrict policy,
+    absl::FunctionRef<size_t(size_t)> get_hash, bool force_sampling) {
+  AssertSoo(common, policy);
+  if (ABSL_PREDICT_FALSE(force_sampling)) {
+    // The table is empty, it is only used for forced sampling of SOO tables.
+    return GrowEmptySooTableToNextCapacityForceSamplingAndPrepareInsert(
+        common, policy, get_hash);
+  }
+  ABSL_SWISSTABLE_ASSERT(common.size() == policy.soo_capacity());
+  static constexpr size_t kNewCapacity = NextCapacity(SooCapacity());
+  const size_t slot_size = policy.slot_size;
+  void* alloc = policy.get_char_alloc(common);
+  common.set_capacity(kNewCapacity);
+
+  // Since the table is not empty, it will not be sampled.
+  // The decision to sample was already made during the first insertion.
+  //
+  // We do not set control and slots in CommonFields yet to avoid overriding
+  // SOO data.
+  const auto [new_ctrl, new_slots] = AllocBackingArray(
+      common, policy, kNewCapacity, /*has_infoz=*/false, alloc,
+      /*blocked_element_count=*/0);
+
+  PrepareInsertCommon(common);
+  ABSL_SWISSTABLE_ASSERT(common.size() == 2);
+  GetGrowthInfoFromControl(new_ctrl).InitGrowthLeftNoDeleted(kNewCapacity - 2,
+                                                             kNewCapacity);
+  common.generate_new_seed(/*has_infoz=*/false);
+  const h2_t soo_slot_h2 = H2(policy.hash_slot(
+      policy.hash_fn(common), common.soo_data(), common.seed().seed()));
+  const size_t new_hash = get_hash(common.seed().seed());
+
+  const size_t offset = Resize1To3NewOffset(new_hash, common.seed());
+  InitializeThreeElementsControlBytes(soo_slot_h2, H2(new_hash), offset,
+                                      new_ctrl);
+
+  SanitizerPoisonMemoryRegion(new_slots, slot_size * kNewCapacity);
+  void* target_slot = SlotAddress(new_slots, SooSlotIndex(), slot_size);
+  SanitizerUnpoisonMemoryRegion(target_slot, slot_size);
+  if constexpr (TransferUsesMemcpy) {
+    // Target slot is placed at index 1, but capacity is at
+    // minimum 3. So we are allowed to copy at least twice as much
+    // memory.
+    static_assert(SooSlotIndex() == 1);
+    static_assert(SooSlotMemcpySize > 0);
+    static_assert(SooSlotMemcpySize <= MaxSooSlotSize());
+    ABSL_SWISSTABLE_ASSERT(SooSlotMemcpySize <= 2 * slot_size);
+    ABSL_SWISSTABLE_ASSERT(SooSlotMemcpySize >= slot_size);
+    void* next_slot = SlotAddress(target_slot, 1, slot_size);
+    SanitizerUnpoisonMemoryRegion(next_slot, SooSlotMemcpySize - slot_size);
+    std::memcpy(target_slot, common.soo_data(), SooSlotMemcpySize);
+    SanitizerPoisonMemoryRegion(next_slot, SooSlotMemcpySize - slot_size);
+  } else {
+    static_assert(SooSlotMemcpySize == 0);
+    policy.transfer_n(&common, target_slot, common.soo_data(), 1);
+  }
+  common.set_control(new_ctrl);
+
+  // Full SOO table couldn't be sampled. If SOO table is sampled, it would
+  // have been resized to the next capacity.
+  ABSL_SWISSTABLE_ASSERT(!common.infoz().IsSampled());
+  void* new_slot = SlotAddress(new_slots, offset, slot_size);
+  SanitizerUnpoisonMemoryRegion(new_slot, slot_size);
+  return new_slot;
+}
+
+void Rehash(CommonFields& common, const PolicyFunctions& __restrict policy,
+            size_t n) {
+  const size_t cap = common.capacity();
+
+  auto clear_backing_array = [&]() {
+    ClearBackingArrayNoReuse(common, policy, policy.get_char_alloc(common));
+  };
+
+  const size_t slot_size = policy.slot_size;
+
+  if (n == 0) {
+    if (cap <= policy.soo_capacity()) return;
+    if (common.empty()) {
+      clear_backing_array();
+      return;
+    }
+    if (common.size() <= policy.soo_capacity()) {
+      // When the table is already sampled, we keep it sampled.
+      if (common.infoz().IsSampled()) {
+        static constexpr size_t kInitialSampledCapacity =
+            NextCapacity(SooCapacity());
+        if (cap > kInitialSampledCapacity) {
+          ResizeAllocatedTableWithSeedChange(common, policy,
+                                             kInitialSampledCapacity);
+        }
+        // This asserts that we didn't lose sampling coverage in `resize`.
+        ABSL_SWISSTABLE_ASSERT(common.infoz().IsSampled());
+        return;
+      }
+      ABSL_SWISSTABLE_ASSERT(slot_size <= sizeof(HeapOrSoo));
+      ABSL_SWISSTABLE_ASSERT(policy.slot_align <= alignof(HeapOrSoo));
+      HeapOrSoo tmp_slot;
+      size_t begin_offset = FindFirstFullSlot(0, cap, common.control());
+      policy.transfer_n(
+          &common, &tmp_slot,
+          SlotAddress(common.slot_array(cap), begin_offset, slot_size), 1);
+      clear_backing_array();
+      policy.transfer_n(&common, common.soo_data(), &tmp_slot, 1);
+      common.set_full_soo();
+      return;
+    }
+  }
+
+  // bitor is a faster way of doing `max` here. We will round up to the next
+  // power-of-2-minus-1, so bitor is good enough.
+  const size_t new_capacity =
+      NormalizeCapacity(n | SizeToCapacity(common.size()));
+  ValidateMaxCapacity(new_capacity, policy.key_size, policy.slot_size);
+  // n == 0 unconditionally rehashes as per the standard.
+  if (n == 0 || new_capacity > cap) {
+    if (cap == policy.soo_capacity()) {
+      if (common.empty()) {
+        ResizeEmptyNonAllocatedTableImpl(common, policy, new_capacity,
+                                         /*blocked_element_count=*/0,
+                                         /*force_infoz=*/false);
+      } else {
+        ResizeFullSooTable(common, policy, new_capacity,
+                           ResizeFullSooTableSamplingMode::kNoSampling);
+      }
+    } else {
+      ResizeAllocatedTableWithSeedChange(common, policy, new_capacity);
+    }
+    // This is after resize, to ensure that we have completed the allocation
+    // and have potentially sampled the hashtable.
+    common.infoz().RecordReservation(n);
+  }
+}
+
+void Copy(CommonFields& common, const PolicyFunctions& __restrict policy,
+          const CommonFields& other,
+          absl::FunctionRef<void(void*, const void*)> copy_fn) {
+  const size_t size = other.size();
+  ABSL_SWISSTABLE_ASSERT(size > 0);
+  const size_t soo_capacity = policy.soo_capacity();
+  const size_t slot_size = policy.slot_size;
+  const bool soo_enabled = policy.soo_enabled;
+  if (size == 1) {
+    if (!soo_enabled) {
+      ReserveEmptyNonAllocatedTableToFitNewSize(common, policy, 1);
+      common.infoz().RecordStorageChanged(1, 1);
+    }
+    IncrementSmallSize(common, policy);
+    const size_t other_capacity = other.capacity();
+    const void* other_slot =
+        other_capacity <= soo_capacity ? other.soo_data()
+        : IsSmallCapacity(other_capacity)
+            ? other.slot_array(other_capacity)
+            : SlotAddress(other.slot_array(other_capacity),
+                          FindFirstFullSlot(0, other_capacity, other.control()),
+                          slot_size);
+    copy_fn(soo_enabled ? common.soo_data()
+                        : SingleSlotAddress</*kSooEnabled=*/false>(common),
+            other_slot);
+
+    if (soo_enabled && policy.is_hashtablez_eligible &&
+        ShouldSampleNextTable()) {
+      GrowFullSooTableToNextCapacityForceSampling(common, policy);
+    }
+    return;
+  }
+
+  ReserveTableToFitNewSize(common, policy, size);
+  const size_t blocked_element_count = common.blocked_element_count();
+  auto infoz = common.infoz();
+  ABSL_SWISSTABLE_ASSERT(other.capacity() > soo_capacity);
+  const size_t cap = common.capacity();
+  ABSL_SWISSTABLE_ASSERT(cap > soo_capacity);
+  ABSL_ASSUME(cap > kMaxSmallCapacity);
+  size_t offset = cap;
+  const void* hash_fn = policy.hash_fn(common);
+  auto hasher = policy.hash_slot;
+  const size_t seed = common.seed().seed();
+  void* target_slot_array = common.slot_array(cap);
+  IterateOverFullSlotsImpl(
+      other, slot_size, [&](const ctrl_t*, void* that_slot) {
+        // The table is guaranteed to be empty, so we can do faster than
+        // a full `insert`.
+        const size_t hash = (*hasher)(hash_fn, that_slot, seed);
+        FindInfo target = find_first_non_full(common, hash);
+        infoz.RecordInsertMiss(hash, target.probe_length);
+        offset = target.offset;
+        SetCtrl(common, offset, H2(hash), slot_size);
+        copy_fn(SlotAddress(target_slot_array, offset, slot_size), that_slot);
+        common.maybe_increment_generation_on_insert();
+      });
+  common.increment_size(size);
+  ResetGrowthLeft(common.growth_info(), cap, size + blocked_element_count);
+}
+
+void ReserveTableToFitNewSize(CommonFields& common,
+                              const PolicyFunctions& __restrict policy,
+                              size_t new_size) {
+  new_size =
+      std::min(new_size, MaxValidSize(policy.key_size, policy.slot_size));
+  common.reset_reserved_growth(new_size);
+  common.set_reservation_size(new_size);
+  ABSL_SWISSTABLE_ASSERT(new_size > policy.soo_capacity());
+  const size_t cap = common.capacity();
+  if (ABSL_PREDICT_TRUE(common.empty() && cap <= policy.soo_capacity())) {
+    return ReserveEmptyNonAllocatedTableToFitNewSize(common, policy, new_size);
+  }
+
+  ABSL_SWISSTABLE_ASSERT(!common.empty() || cap > policy.soo_capacity());
+  ABSL_SWISSTABLE_ASSERT(cap > 0);
+  const size_t max_size_before_growth =
+      IsSmallCapacity(cap)
+          ? cap
+          : common.size() + common.growth_info().GetGrowthLeftTotalSlow(cap);
+  if (new_size <= max_size_before_growth) {
+    return;
+  }
+  ReserveAllocatedTable(common, policy, new_size);
+}
+
+namespace {
+void* PrepareInsertLargeImpl(CommonFields& common,
+                             const PolicyFunctions& __restrict policy,
+                             size_t hash,
+                             Group::NonIterableBitMaskType mask_empty,
+                             FindInfo target_group) {
+  ABSL_SWISSTABLE_ASSERT(!common.is_small());
+  GrowthInfoAccessor growth_info = common.growth_info();
+  // When there are no deleted slots in the table
+  // and growth_left is positive, we can insert at the first
+  // empty slot in the probe sequence (target).
+  if (ABSL_PREDICT_FALSE(
+          !growth_info.GetGrowthInfoLowerBound().HasNoDeletedAndGrowthLeft())) {
+    return PrepareInsertLargeSlow(common, policy, hash);
+  }
+  PrepareInsertCommon(common);
+  growth_info.OverwriteEmptyAsFull();
+  const size_t cap = common.capacity();
+  ABSL_ASSUME(cap > kMaxSmallCapacity);
+  target_group.offset += mask_empty.LowestBitSet();
+  target_group.offset &= cap;
+  SetCtrl(common, target_group.offset, H2(hash), policy.slot_size);
+  common.infoz().RecordInsertMiss(hash, target_group.probe_length);
+  return SlotAddress(common.slot_array(cap), target_group.offset,
+                     policy.slot_size);
+}
+}  // namespace
+
+void* PrepareInsertLarge(CommonFields& common,
+                         const PolicyFunctions& __restrict policy, size_t hash,
+                         Group::NonIterableBitMaskType mask_empty,
+                         FindInfo target_group) {
+  // NOLINTNEXTLINE(misc-static-assert)
+  ABSL_SWISSTABLE_ASSERT(!SwisstableGenerationsEnabled());
+  return PrepareInsertLargeImpl(common, policy, hash, mask_empty, target_group);
+}
+
+void* PrepareInsertLargeGenerationsEnabled(
+    CommonFields& common, const PolicyFunctions& __restrict policy, size_t hash,
+    Group::NonIterableBitMaskType mask_empty, FindInfo target_group,
+    absl::FunctionRef<size_t(size_t)> recompute_hash) {
+  // NOLINTNEXTLINE(misc-static-assert)
+  ABSL_SWISSTABLE_ASSERT(SwisstableGenerationsEnabled());
+  const size_t cap = common.capacity();
+  const size_t growth_left = common.growth_info().GetGrowthLeftTotalSlow(cap);
+  // As an optimization, we avoid calling ShouldRehashForBugDetection if we
+  // will end up rehashing anyways.
+  if (growth_left > 0 && common.should_rehash_for_bug_detection_on_insert()) {
+    // Move to a different heap allocation in order to detect bugs.
+    ResizeAllocatedTableWithSeedChange(common, policy, cap);
+    hash = recompute_hash(common.seed().seed());
+    std::tie(target_group, mask_empty) =
+        find_first_non_full_group(common, hash);
+  }
+  return PrepareInsertLargeImpl(common, policy, hash, mask_empty, target_group);
+}
+
+namespace {
+// Returns true if the following is true
+// 1. OptimalMemcpySizeForSooSlotTransfer(left) >
+//    OptimalMemcpySizeForSooSlotTransfer(left - 1)
+// 2. OptimalMemcpySizeForSooSlotTransfer(left) are equal for all i in [left,
+// right].
+// This function is used to verify that we have all the possible template
+// instantiations for GrowFullSooTableToNextCapacity.
+// With this verification the problem may be detected at compile time instead of
+// link time.
+constexpr bool VerifyOptimalMemcpySizeForSooSlotTransferRange(size_t left,
+                                                              size_t right) {
+  size_t optimal_size_for_range = OptimalMemcpySizeForSooSlotTransfer(left);
+  if (optimal_size_for_range <= OptimalMemcpySizeForSooSlotTransfer(left - 1)) {
+    return false;
+  }
+  for (size_t i = left + 1; i <= right; ++i) {
+    if (OptimalMemcpySizeForSooSlotTransfer(i) != optimal_size_for_range) {
+      return false;
+    }
+  }
+  return true;
+}
+}  // namespace
+
+// Extern template instantiation for inline function.
+template size_t TryFindNewIndexWithoutProbing(size_t h1, size_t old_index,
+                                              size_t old_capacity,
+                                              ctrl_t* new_ctrl,
+                                              size_t new_capacity);
+
+// We need to instantiate ALL possible template combinations because we define
+// the function in the cc file.
+template void* GrowSooTableToNextCapacityAndPrepareInsert<0, false>(
+    CommonFields&, const PolicyFunctions&, absl::FunctionRef<size_t(size_t)>,
+    bool);
+template void* GrowSooTableToNextCapacityAndPrepareInsert<
+    OptimalMemcpySizeForSooSlotTransfer(1), true>(
+    CommonFields&, const PolicyFunctions&, absl::FunctionRef<size_t(size_t)>,
+    bool);
+
+static_assert(VerifyOptimalMemcpySizeForSooSlotTransferRange(2, 3));
+template void* GrowSooTableToNextCapacityAndPrepareInsert<
+    OptimalMemcpySizeForSooSlotTransfer(3), true>(
+    CommonFields&, const PolicyFunctions&, absl::FunctionRef<size_t(size_t)>,
+    bool);
+
+#if UINTPTR_MAX == UINT32_MAX
+static_assert(MaxSooSlotSize() == 4);
+static_assert(VerifyOptimalMemcpySizeForSooSlotTransferRange(2, 4));
+#else
+static_assert(VerifyOptimalMemcpySizeForSooSlotTransferRange(4, 8));
+template void* GrowSooTableToNextCapacityAndPrepareInsert<
+    OptimalMemcpySizeForSooSlotTransfer(8), true>(
+    CommonFields&, const PolicyFunctions&, absl::FunctionRef<size_t(size_t)>,
+    bool);
+static_assert(MaxSooSlotSize() == 8);
+#endif
+
+template void* AllocateBackingArray<BackingArrayAlignment(alignof(size_t)),
+                                    std::allocator<char>>(void* alloc,
+                                                          size_t n);
+template void DeallocateBackingArray<BackingArrayAlignment(alignof(size_t)),
+                                     std::allocator<char>>(
+    void* alloc, size_t capacity, ctrl_t* ctrl, size_t slot_size,
+    size_t slot_align, bool had_infoz, size_t blocked_element_count);
+
+template void Clear<true>(CommonFields& c, const PolicyFunctions& policy,
+                          DestroySlotFn destroy_slot, void* alloc);
+template void Clear<false>(CommonFields& c, const PolicyFunctions& policy,
+                           DestroySlotFn destroy_slot, void* alloc);
+
 }  // namespace container_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/container/internal/raw_hash_set.h b/absl/container/internal/raw_hash_set.h
index 2e6f4dd3e66..ad416cd79b5 100644
--- a/absl/container/internal/raw_hash_set.h
+++ b/absl/container/internal/raw_hash_set.h
@@ -13,6 +13,7 @@
 // limitations under the License.
 //
 // An open-addressing
+// [https://en.wikipedia.org/wiki/Open_addressing]
 // hashtable with quadratic probing.
 //
 // This is a low level hashtable on top of which different interfaces can be
@@ -41,60 +42,155 @@
 // When heterogeneous lookup is disabled, only the explicit `key_type` overloads
 // exist.
 //
-// find() also supports passing the hash explicitly:
-//
-//   iterator find(const key_type& key, size_t hash);
-//   template <class U>
-//   iterator find(const U& key, size_t hash);
-//
 // In addition the pointer to element and iterator stability guarantees are
 // weaker: all iterators and pointers are invalidated after a new element is
 // inserted.
 //
 // IMPLEMENTATION DETAILS
 //
-// The table stores elements inline in a slot array. In addition to the slot
-// array the table maintains some control state per slot. The extra state is one
-// byte per slot and stores empty or deleted marks, or alternatively 7 bits from
-// the hash of an occupied slot. The table is split into logical groups of
-// slots, like so:
+// # Table Layout
+//
+// A raw_hash_set's backing array consists of control bytes followed by slots
+// that may or may not contain objects.
+//
+// The layout of the backing array, for `capacity` slots, is thus, as a
+// pseudo-struct:
+//
+//   struct BackingArray {
+//     // Sampling handler. This field isn't present when the sampling is
+//     // disabled or this allocation hasn't been selected for sampling.
+//     HashtablezInfoHandle infoz_;  // optional
+//     // Additional number that can be added to growth_left_lower_bound.
+//     // Only stored for tables with large capacities.
+//     uint8_t growth_left_overflow[7];  // optional
+//     // The minimum number of elements we can insert before growing the
+//     // capacity.
+//     uint8_t growth_left_lower_bound;
+//     // Control bytes for the "real" slots.
+//     ctrl_t ctrl[capacity];
+//     // Always `ctrl_t::kSentinel`. This is used by iterators to find when to
+//     // stop and serves no other purpose.
+//     ctrl_t sentinel;
+//     // A copy of the first `kWidth - 1` elements of `ctrl`. This is used so
+//     // that if a probe sequence picks a value near the end of `ctrl`,
+//     // `Group` will have valid control bytes to look at.
+//     ctrl_t clones[kWidth - 1];
+//     // The actual slot data.
+//     slot_type slots[capacity];
+//   };
+//
+// The length of this array is computed by `RawHashSetLayout::alloc_size` below.
+//
+// Control bytes (`ctrl_t`) are bytes (collected into groups of a
+// platform-specific size) that define the state of the corresponding slot in
+// the slot array. Group manipulation is tightly optimized to be as efficient
+// as possible: SSE and friends on x86, clever bit operations on other arches.
 //
 //      Group 1         Group 2        Group 3
 // +---------------+---------------+---------------+
 // | | | | | | | | | | | | | | | | | | | | | | | | |
 // +---------------+---------------+---------------+
 //
-// On lookup the hash is split into two parts:
-// - H2: 7 bits (those stored in the control bytes)
-// - H1: the rest of the bits
-// The groups are probed using H1. For each group the slots are matched to H2 in
-// parallel. Because H2 is 7 bits (128 states) and the number of slots per group
-// is low (8 or 16) in almost all cases a match in H2 is also a lookup hit.
+// Each control byte is either a special value for empty slots, deleted slots
+// (sometimes called *tombstones*), and a special end-of-table marker used by
+// iterators, or, if occupied, seven bits (H2) from the hash of the value in the
+// corresponding slot.
+//
+// Storing control bytes in a separate array also has beneficial cache effects,
+// since more logical slots will fit into a cache line.
+//
+// # Small Object Optimization (SOO)
+//
+// When the size/alignment of the value_type and the capacity of the table are
+// small, we enable small object optimization and store the values inline in
+// the raw_hash_set object. This optimization allows us to avoid
+// allocation/deallocation as well as cache/dTLB misses.
+//
+// # Hashing
+//
+// We compute two separate hashes, `H1` and `H2`, from the hash of an object.
+// `H1(hash(x))` is an index into `slots`, and essentially the starting point
+// for the probe sequence. `H2(hash(x))` is a 7-bit value used to filter out
+// objects that cannot possibly be the one we are looking for.
+//
+// # Table operations.
+//
+// The key operations are `insert`, `find`, and `erase`.
 //
-// On insert, once the right group is found (as in lookup), its slots are
-// filled in order.
+// Since `insert` and `erase` are implemented in terms of `find`, we describe
+// `find` first. To `find` a value `x`, we compute `hash(x)`. From
+// `H1(hash(x))` and the capacity, we construct a `probe_seq` that visits every
+// group of slots in some interesting order.
 //
-// On erase a slot is cleared. In case the group did not have any empty slots
-// before the erase, the erased slot is marked as deleted.
+// We now walk through these indices. At each index, we select the entire group
+// starting with that index and extract potential candidates: occupied slots
+// with a control byte equal to `H2(hash(x))`. If we find an empty slot in the
+// group, we stop and return an error. Each candidate slot `y` is compared with
+// `x`; if `x == y`, we are done and return `&y`; otherwise we continue to the
+// next probe index. Tombstones effectively behave like full slots that never
+// match the value we're looking for.
 //
-// Groups without empty slots (but maybe with deleted slots) extend the probe
-// sequence. The probing algorithm is quadratic. Given N the number of groups,
-// the probing function for the i'th probe is:
+// The `H2` bits ensure when we compare a slot to an object with `==`, we are
+// likely to have actually found the object.  That is, the chance is low that
+// `==` is called and returns `false`.  Thus, when we search for an object, we
+// are unlikely to call `==` many times.  This likelyhood can be analyzed as
+// follows (assuming that H2 is a random enough hash function).
 //
-//   P(0) = H1 % N
+// Let's assume that there are `k` "wrong" objects that must be examined in a
+// probe sequence.  For example, when doing a `find` on an object that is in the
+// table, `k` is the number of objects between the start of the probe sequence
+// and the final found object (not including the final found object).  The
+// expected number of objects with an H2 match is then `k/128`.  Measurements
+// and analysis indicate that even at high load factors, `k` is less than 32,
+// meaning that the number of "false positive" comparisons we must perform is
+// less than 1/8 per `find`.
+
+// `insert` is implemented in terms of `unchecked_insert`, which inserts a
+// value presumed to not be in the table (violating this requirement will cause
+// the table to behave erratically). Given `x` and its hash `hash(x)`, to insert
+// it, we construct a `probe_seq` once again, and use it to find the first
+// group with an unoccupied (empty *or* deleted) slot. We place `x` into the
+// first such slot in the group and mark it as full with `x`'s H2.
 //
-//   P(i) = (P(i - 1) + i) % N
+// To `insert`, we compose `unchecked_insert` with `find`. We compute `h(x)` and
+// perform a `find` to see if it's already present; if it is, we're done. If
+// it's not, we may decide the table is getting overcrowded (i.e. the load
+// factor is greater than 7/8 for big tables; tables smaller than one probing
+// group use a max load factor of 1); in this case, we allocate a bigger array,
+// `unchecked_insert` each element of the table into the new array (we know that
+// no insertion here will insert an already-present value), and discard the old
+// backing array. At this point, we may `unchecked_insert` the value `x`.
 //
-// This probing function guarantees that after N probes, all the groups of the
-// table will be probed exactly once.
+// Below, `unchecked_insert` is partly implemented by `prepare_insert`, which
+// presents a viable, initialized slot pointee to the caller.
+//
+// `erase` is implemented in terms of `erase_at`, which takes an index to a
+// slot. Given an offset, we simply create a tombstone and destroy its contents.
+// If we can prove that the slot would not appear in a probe sequence, we can
+// make the slot as empty, instead. We can prove this by observing that if a
+// group has any empty slots, it has never been full (assuming we never create
+// an empty slot in a group with no empties, which this heuristic guarantees we
+// never do) and find would stop at this group anyways (since it does not probe
+// beyond groups with empties).
+//
+// `erase` is `erase_at` composed with `find`: if we
+// have a value `x`, we can perform a `find`, and then `erase_at` the resulting
+// slot.
+//
+// To iterate, we simply traverse the array, skipping empty and deleted slots
+// and stopping when we hit a `kSentinel`.
 
 #ifndef ABSL_CONTAINER_INTERNAL_RAW_HASH_SET_H_
 #define ABSL_CONTAINER_INTERNAL_RAW_HASH_SET_H_
 
 #include <algorithm>
+#include <cassert>
 #include <cmath>
+#include <cstddef>
 #include <cstdint>
 #include <cstring>
+#include <functional>
+#include <initializer_list>
 #include <iterator>
 #include <limits>
 #include <memory>
@@ -102,48 +198,111 @@
 #include <type_traits>
 #include <utility>
 
-#include "absl/base/internal/bits.h"
+#include "absl/base/attributes.h"
+#include "absl/base/casts.h"
+#include "absl/base/config.h"
 #include "absl/base/internal/endian.h"
+#include "absl/base/internal/iterator_traits.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/base/macros.h"
+#include "absl/base/optimization.h"
+#include "absl/base/options.h"
 #include "absl/base/port.h"
-#include "absl/container/internal/common.h"
+#include "absl/base/prefetch.h"
+#include "absl/container/internal/common.h"  // IWYU pragma: export // for node_handle
+#include "absl/container/internal/common_policy_traits.h"
 #include "absl/container/internal/compressed_tuple.h"
 #include "absl/container/internal/container_memory.h"
+#include "absl/container/internal/hash_function_defaults.h"
 #include "absl/container/internal/hash_policy_traits.h"
+#include "absl/container/internal/hashtable_control_bytes.h"
 #include "absl/container/internal/hashtable_debug_hooks.h"
 #include "absl/container/internal/hashtablez_sampler.h"
-#include "absl/container/internal/have_sse.h"
-#include "absl/container/internal/layout.h"
+#include "absl/functional/function_ref.h"
+#include "absl/hash/hash.h"
+#include "absl/hash/internal/weakly_mixed_integer.h"
 #include "absl/memory/memory.h"
 #include "absl/meta/type_traits.h"
+#include "absl/numeric/bits.h"
 #include "absl/utility/utility.h"
 
+#if ABSL_INTERNAL_CPLUSPLUS_LANG >= 202002L
+#include <ranges>  // NOLINT(build/c++20)
+#endif
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
-template <size_t Width>
-class probe_seq {
- public:
-  probe_seq(size_t hash, size_t mask) {
-    assert(((mask + 1) & mask) == 0 && "not a mask");
-    mask_ = mask;
-    offset_ = hash & mask_;
-  }
-  size_t offset() const { return offset_; }
-  size_t offset(size_t i) const { return (offset_ + i) & mask_; }
+#ifdef ABSL_SWISSTABLE_ENABLE_GENERATIONS
+#error ABSL_SWISSTABLE_ENABLE_GENERATIONS cannot be directly set
+#elif (defined(ABSL_HAVE_ADDRESS_SANITIZER) ||   \
+       defined(ABSL_HAVE_HWADDRESS_SANITIZER) || \
+       defined(ABSL_HAVE_MEMORY_SANITIZER)) &&   \
+    !defined(NDEBUG_SANITIZER)  // If defined, performance is important.
+// When compiled in sanitizer mode, we add generation integers to the backing
+// array and iterators. In the backing array, we store the generation between
+// the control bytes and the slots. When iterators are dereferenced, we assert
+// that the container has not been mutated in a way that could cause iterator
+// invalidation since the iterator was initialized.
+#define ABSL_SWISSTABLE_ENABLE_GENERATIONS
+#endif
 
-  void next() {
-    index_ += Width;
-    offset_ += index_;
-    offset_ &= mask_;
-  }
-  // 0-based probe index. The i-th probe in the probe sequence.
-  size_t index() const { return index_; }
+#ifdef ABSL_SWISSTABLE_ASSERT
+#error ABSL_SWISSTABLE_ASSERT cannot be directly set
+#else
+// We use this macro for assertions that users may see when the table is in an
+// invalid state that sanitizers may help diagnose.
+#define ABSL_SWISSTABLE_ASSERT(CONDITION) \
+  assert((CONDITION) && "Try enabling sanitizers.")
+#endif
 
- private:
-  size_t mask_;
-  size_t offset_;
-  size_t index_ = 0;
-};
+// We use uint8_t so we don't need to worry about padding.
+using GenerationType = uint8_t;
+
+// A sentinel value for empty generations. Using 0 makes it easy to constexpr
+// initialize an array of this value.
+constexpr GenerationType SentinelEmptyGeneration() { return 0; }
+
+constexpr GenerationType NextGeneration(GenerationType generation) {
+  return ++generation == SentinelEmptyGeneration() ? ++generation : generation;
+}
+
+#ifdef ABSL_SWISSTABLE_ENABLE_GENERATIONS
+constexpr bool SwisstableGenerationsEnabled() { return true; }
+constexpr size_t NumGenerationBytes() { return sizeof(GenerationType); }
+#else
+constexpr bool SwisstableGenerationsEnabled() { return false; }
+constexpr size_t NumGenerationBytes() { return 0; }
+#endif
+
+constexpr bool SwisstableGenerationsOrDebugEnabled() {
+#ifndef NDEBUG
+  return true;
+#endif
+  return SwisstableGenerationsEnabled();
+}
+
+template <typename AllocType>
+void SwapAlloc(AllocType& lhs, AllocType& rhs,
+               std::true_type /* propagate_on_container_swap */) {
+  using std::swap;
+  swap(lhs, rhs);
+}
+template <typename AllocType>
+void SwapAlloc([[maybe_unused]] AllocType& lhs, [[maybe_unused]] AllocType& rhs,
+               std::false_type /* propagate_on_container_swap */) {
+  assert(lhs == rhs &&
+         "It's UB to call swap with unequal non-propagating allocators.");
+}
+
+template <typename AllocType>
+void CopyAlloc(AllocType& lhs, AllocType& rhs,
+               std::true_type /* propagate_alloc */) {
+  lhs = rhs;
+}
+template <typename AllocType>
+void CopyAlloc(AllocType&, AllocType&, std::false_type /* propagate_alloc */) {}
 
 template <class ContainerKey, class Hash, class Eq>
 struct RequireUsableKey {
@@ -160,370 +319,1934 @@ struct IsDecomposable : std::false_type {};
 
 template <class Policy, class Hash, class Eq, class... Ts>
 struct IsDecomposable<
-    absl::void_t<decltype(
-        Policy::apply(RequireUsableKey<typename Policy::key_type, Hash, Eq>(),
-                      std::declval<Ts>()...))>,
+    std::void_t<decltype(Policy::apply(
+        RequireUsableKey<typename Policy::key_type, Hash, Eq>(),
+        std::declval<Ts>()...))>,
     Policy, Hash, Eq, Ts...> : std::true_type {};
 
-// TODO(alkis): Switch to std::is_nothrow_swappable when gcc/clang supports it.
-template <class T>
-constexpr bool IsNoThrowSwappable() {
-  using std::swap;
-  return noexcept(swap(std::declval<T&>(), std::declval<T&>()));
+ABSL_DLL extern char kDefaultIterSlot;
+
+// Returns a pointer to a control byte that can be used by default-constructed
+// iterators. We don't expect this pointer to be dereferenced.
+inline void* DefaultIterSlot() { return &kDefaultIterSlot; }
+
+// For use in SOO iterators.
+// TODO(b/289225379): we could potentially get rid of this by adding an is_soo
+// bit in iterators. This would add branches but reduce cache misses.
+ABSL_DLL extern const ctrl_t kSooControl[2];
+
+// Returns a pointer to a full byte followed by a sentinel byte.
+inline ctrl_t* SooControl() {
+  // Const must be cast away here; no uses of this function will actually write
+  // to it because it is only used for SOO iterators.
+  return const_cast<ctrl_t*>(kSooControl);
+}
+// Whether ctrl is from the SooControl array.
+inline bool IsSooControl(const ctrl_t* ctrl) { return ctrl == SooControl(); }
+
+// For use in iterators returned by `insert` and similar.
+ABSL_DLL extern const ctrl_t kInsertIteratorControl[2];
+
+// Returns a pointer to a full byte followed by a sentinel byte.
+inline ctrl_t* InsertIteratorControl() {
+  // Const must be cast away here; no uses of this function will actually write
+  // to it because it is only used for iterators returned by `insert` and
+  // similar.
+  return const_cast<ctrl_t*>(kInsertIteratorControl);
+}
+// Whether ctrl is special value for iterators returned by `insert` and similar.
+inline bool IsInsertIteratorControl(const ctrl_t* ctrl) {
+  return ctrl == InsertIteratorControl();
 }
 
-template <typename T>
-int TrailingZeros(T x) {
-  return sizeof(T) == 8 ? base_internal::CountTrailingZerosNonZero64(
-                              static_cast<uint64_t>(x))
-                        : base_internal::CountTrailingZerosNonZero32(
-                              static_cast<uint32_t>(x));
+// Returns a pointer to a generation to use for an empty hashtable.
+GenerationType* EmptyGeneration();
+
+// Returns whether `generation` is a generation for an empty hashtable that
+// could be returned by EmptyGeneration().
+inline bool IsEmptyGeneration(const GenerationType* generation) {
+  return *generation == SentinelEmptyGeneration();
 }
 
-template <typename T>
-int LeadingZeros(T x) {
-  return sizeof(T) == 8
-             ? base_internal::CountLeadingZeros64(static_cast<uint64_t>(x))
-             : base_internal::CountLeadingZeros32(static_cast<uint32_t>(x));
+// We only allow a maximum of 1 SOO element, which makes the implementation
+// much simpler. Complications with multiple SOO elements include:
+// - Satisfying the guarantee that erasing one element doesn't invalidate
+//   iterators to other elements means we would probably need actual SOO
+//   control bytes.
+// - In order to prevent user code from depending on iteration order for small
+//   tables, we would need to randomize the iteration order somehow.
+constexpr size_t SooCapacity() { return 1; }
+// Maximum capacity of a table where we don't need to hash any keys.
+inline constexpr size_t kMaxSmallCapacity = 1;
+// Sentinel type to indicate SOO CommonFields construction.
+struct soo_tag_t {};
+// Sentinel type to indicate SOO CommonFields construction with full size.
+struct full_soo_tag_t {};
+// Sentinel type to indicate non-SOO CommonFields construction.
+struct non_soo_tag_t {};
+// Sentinel value to indicate an uninitialized value explicitly.
+struct uninitialized_tag_t {};
+// Sentinel value to indicate creation of an empty table without a seed.
+struct no_seed_empty_tag_t {};
+
+// Returns whether `n` is a valid capacity (i.e., number of slots).
+//
+// A valid capacity is a non-zero integer `2^m - 1`.
+constexpr bool IsValidCapacity(size_t n) { return ((n + 1) & n) == 0 && n > 0; }
+
+// Whether a table is small enough that we don't need to hash any keys.
+constexpr bool IsSmallCapacity(size_t capacity) {
+  return capacity <= kMaxSmallCapacity;
+}
+
+// Whether a table fits entirely into a probing group.
+// Arbitrary order of elements in such tables is correct.
+constexpr bool is_single_group(size_t capacity) {
+  return capacity <= Group::kWidth;
+}
+
+// Whether `cap` is a valid capacity for a table that can store blocked
+// elements.
+constexpr bool IsCapacityValidForBlockedElements(size_t cap) {
+  return !IsSmallCapacity(cap);
+}
+
+// Converts `n` into the next valid capacity, per `IsValidCapacity`.
+constexpr size_t NormalizeCapacity(size_t n) {
+  return n ? ~size_t{} >> countl_zero(n) : 1;
 }
 
-// An abstraction over a bitmask. It provides an easy way to iterate through the
-// indexes of the set bits of a bitmask.  When Shift=0 (platforms with SSE),
-// this is a true bitmask.  On non-SSE, platforms the arithematic used to
-// emulate the SSE behavior works in bytes (Shift=3) and leaves each bytes as
-// either 0x00 or 0x80.
+// Returns the next valid capacity after `n`.
+constexpr size_t NextCapacity(size_t n) {
+  ABSL_SWISSTABLE_ASSERT(IsValidCapacity(n) || n == 0);
+  return n * 2 + 1;
+}
+
+// Returns the previous valid capacity before `n`.
+constexpr size_t PreviousCapacity(size_t n) {
+  ABSL_SWISSTABLE_ASSERT(IsValidCapacity(n));
+  return n / 2;
+}
+
+// General notes on capacity/growth methods below:
+// - We use 7/8th as maximum load factor. For 16-wide groups, that gives an
+//   average of two empty slots per group.
+// - For (capacity+1) < Group::kWidth, growth == capacity. In this case, we
+//   never need to probe (the whole table fits in one group) so we don't need a
+//   load factor less than 1.
+// - For tables with capacity <= kMaxCapacityForLoadFactorOne, we leave one
+//   empty slot.
+// - For capacity > kMaxCapacityForLoadFactorOne, growth is 7/8*capacity.
+constexpr inline size_t kMaxCapacityForLoadFactorOne = Group::kWidth * 4 - 1;
+
+// Given `capacity`, applies the load factor; i.e., it returns the maximum
+// number of values we should put into the table before a resizing rehash.
+constexpr size_t CapacityToGrowth(size_t capacity) {
+  ABSL_SWISSTABLE_ASSERT(IsValidCapacity(capacity));
+  // `capacity*7/8`
+  if (capacity <= kMaxCapacityForLoadFactorOne) {
+    // For small capacities we leave at most one empty slot.
+    return capacity - (capacity >= Group::kWidth - 1);
+  }
+  return capacity - capacity / 8;
+}
+
+// Given `size`, "unapplies" the load factor to find how large the capacity
+// should be to stay within the load factor.
 //
-// For example:
-//   for (int i : BitMask<uint32_t, 16>(0x5)) -> yields 0, 2
-//   for (int i : BitMask<uint64_t, 8, 3>(0x0000000080800000)) -> yields 2, 3
-template <class T, int SignificantBits, int Shift = 0>
-class BitMask {
-  static_assert(std::is_unsigned<T>::value, "");
-  static_assert(Shift == 0 || Shift == 3, "");
+// For size == 0, returns 0.
+// For other values, returns the same as `NormalizeCapacity(size*8/7)`.
+constexpr size_t SizeToCapacity(size_t size) {
+  if (size == 0) {
+    return 0;
+  }
+  // The minimum possible capacity is NormalizeCapacity(size).
+  // Shifting right `~size_t{}` by `leading_zeros` yields
+  // NormalizeCapacity(size).
+  int leading_zeros = absl::countl_zero(
+      size +
+      // Tables larger than half a group require at least one empty slot.
+      (size >= Group::kWidth / 2));
+  if (size < kMaxCapacityForLoadFactorOne) {
+    return (~size_t{}) >> leading_zeros;
+  }
+  constexpr size_t kLast3Bits = size_t{7} << (sizeof(size_t) * 8 - 3);
+  // max_size_for_next_capacity = max_load_factor * next_capacity
+  //                            = (7/8) * (~size_t{} >> leading_zeros)
+  //                            = (7/8*~size_t{}) >> leading_zeros
+  //                            = kLast3Bits >> leading_zeros
+  size_t max_size_for_next_capacity = kLast3Bits >> leading_zeros;
+  // Decrease shift if size is too big for the minimum capacity.
+  leading_zeros -= static_cast<int>(size > max_size_for_next_capacity);
+  return (~size_t{}) >> leading_zeros;
+}
+
+// The mode we store capacity in the table.
+enum HashtableCapacityStorageMode {
+  // Capacity stored as size_t as a full number.
+  kCapacityByValue,
+  // Capacity stored as uint8_t as log2, i.e. capacity = 2^capacity_ - 1.
+  kCapacityByLog,
+};
+
+// The number of slots in the backing array. This is always 2^N-1 for an
+// integer N.
+// NOTE: this class exists to simplify experiments with different ways to store
+// capacity within size.
+// NOTE: we tried experimenting with compressing the capacity and storing it
+// together with size_: (a) using 6 bits to store the corresponding power (N in
+// 2^N-1), and (b) storing 2^N as the most significant bit of size_ and storing
+// size in the low bits. Both of these experiments were regressions, presumably
+// because we need capacity to do find operations.
+template <HashtableCapacityStorageMode StorageMode>
+class HashtableCapacityImpl {
+  using IntType =
+      std::conditional_t<StorageMode == kCapacityByValue, size_t, uint8_t>;
 
  public:
-  // These are useful for unit tests (gunit).
-  using value_type = int;
-  using iterator = BitMask;
-  using const_iterator = BitMask;
-
-  explicit BitMask(T mask) : mask_(mask) {}
-  BitMask& operator++() {
-    mask_ &= (mask_ - 1);
-    return *this;
+  static constexpr HashtableCapacityImpl CreateDestroyed() {
+    return HashtableCapacityImpl(kDestroyed);
+  }
+  static constexpr HashtableCapacityImpl CreateReentrance() {
+    return HashtableCapacityImpl(kReentrance);
   }
-  explicit operator bool() const { return mask_ != 0; }
-  int operator*() const { return LowestBitSet(); }
-  int LowestBitSet() const {
-    return container_internal::TrailingZeros(mask_) >> Shift;
+  static constexpr HashtableCapacityImpl CreateMovedFrom() {
+    return HashtableCapacityImpl(kMovedFrom);
   }
-  int HighestBitSet() const {
-    return (sizeof(T) * CHAR_BIT - container_internal::LeadingZeros(mask_) -
-            1) >>
-           Shift;
+  static constexpr HashtableCapacityImpl CreateSelfMovedFrom() {
+    return HashtableCapacityImpl(kSelfMovedFrom);
   }
 
-  BitMask begin() const { return *this; }
-  BitMask end() const { return BitMask(0); }
+  explicit HashtableCapacityImpl(uninitialized_tag_t) {}
+  explicit constexpr HashtableCapacityImpl(size_t capacity)
+      : capacity_data_(static_cast<IntType>(
+            StorageMode == kCapacityByValue ? capacity
+                                            : TrailingZeros(capacity + 1))) {
+    ABSL_SWISSTABLE_ASSERT(capacity == 0 || IsValidCapacity(capacity));
+  }
+
+  // Creates capacity from the value that was returned by `ToRawData()`.
+  // This is needed to use bitfield for capacity.
+  // At least on Windows combination uint8_t and uint64_t bitfield in one struct
+  // is not optimized by compiler.
+  static HashtableCapacityImpl FromRawData(uint64_t capacity) {
+    auto cap = HashtableCapacityImpl(uninitialized_tag_t{});
+    cap.capacity_data_ = static_cast<IntType>(capacity);
+    return cap;
+  }
+  IntType ToRawData() const { return capacity_data_; }
 
-  int TrailingZeros() const {
-    return container_internal::TrailingZeros(mask_) >> Shift;
+  constexpr bool IsValid() const {
+    return capacity_data_ <= kAboveMaxValidCapacity;
   }
 
-  int LeadingZeros() const {
-    constexpr int total_significant_bits = SignificantBits << Shift;
-    constexpr int extra_bits = sizeof(T) * 8 - total_significant_bits;
-    return container_internal::LeadingZeros(mask_ << extra_bits) >> Shift;
+  constexpr bool IsDestroyed() const { return capacity_data_ == kDestroyed; }
+  constexpr bool IsReentrance() const { return capacity_data_ == kReentrance; }
+  // Returns true if the table is moved-from including self moved-from.
+  constexpr bool IsMovedFrom() const { return capacity_data_ >= kMovedFrom; }
+  constexpr bool IsSelfMovedFrom() const {
+    return capacity_data_ == kSelfMovedFrom;
+  }
+
+  constexpr size_t capacity() const {
+    ABSL_SWISSTABLE_ASSERT(IsValid());
+    return StorageMode == kCapacityByValue ? capacity_data_
+                                           : (size_t{1} << capacity_data_) - 1;
+  }
+
+  constexpr bool is_small() const {
+    // Small tables have capacity 0 or 1. This expression is valid for both
+    // capacity storage modes.
+    // Comparing capacity_data_ directly leads to a better generated code.
+    // One byte comparison is used before computing the capacity in order to
+    // detect small tables faster for critical path.
+    static_assert(kMaxSmallCapacity == 1);
+    return capacity_data_ <= 1;
   }
 
  private:
-  friend bool operator==(const BitMask& a, const BitMask& b) {
-    return a.mask_ == b.mask_;
+  // We use these sentinel capacity values in debug mode to indicate different
+  // classes of bugs.
+  enum InvalidCapacity : IntType {
+    kAboveMaxValidCapacity = StorageMode == kCapacityByValue
+                                 ? (std::numeric_limits<IntType>::max)() - 100
+                                 : 64 - 10,
+    kReentrance,
+    kDestroyed,
+
+    // These two must be last because we use `>= kMovedFrom` to mean moved-from.
+    kMovedFrom,
+    kSelfMovedFrom,
+  };
+
+  explicit constexpr HashtableCapacityImpl(InvalidCapacity capacity)
+      : capacity_data_(capacity) {
+    ABSL_SWISSTABLE_ASSERT(capacity_data_ > kAboveMaxValidCapacity);
   }
-  friend bool operator!=(const BitMask& a, const BitMask& b) {
-    return a.mask_ != b.mask_;
+
+  // Capacity is stored as a value or as a log2 depending on `StorageMode`.
+  IntType capacity_data_;
+};
+
+template <HashtableCapacityStorageMode StorageMode>
+class HashtableInlineDataImpl;
+
+// Returns next per-table seed.
+uint8_t NextHashTableSeed();
+
+// Per table hash salt. This gets mixed into H1 to randomize iteration order
+// per-table.
+// The seed is needed to ensure non-determinism of iteration order.
+class PerTableSeed {
+ public:
+  // Returns the seed for the table.
+  size_t seed() const { return seed_; }
+
+ private:
+  template <HashtableCapacityStorageMode StorageModeOfData>
+  friend class HashtableInlineDataImpl;
+
+  explicit PerTableSeed(uint64_t seed)
+      : seed_(static_cast<uint16_t>(seed)) {}
+
+  const uint16_t seed_;
+};
+
+// Represents blocked elements info: log2_period and tail_blocked.
+// Every `2**log2_period` is a blocked slot. The first blocked slot is at
+// index `2**log2_period-1`. E.g. if log2_period is 2, then every 4th slot
+// is blocked: 0, 1, 2, X, 4, 5, 6, X, ...
+//
+// tail_blocked is the number of blocked slots at the end in addition.
+// E.g., log2_period = 2 and tail_blocked = 3, then there are 6 blocked for
+// capacity = 15.
+// slots: 0, 1, 2, X, 4, 5, 6, X, 8, 9, 10, X, X, X, X, S. (S = sentinel)
+class BlockedInfo {
+ public:
+  constexpr BlockedInfo(uint8_t log2_period, uint8_t tail_blocked)
+      : log2_period_(log2_period), tail_blocked_(tail_blocked) {
+    ABSL_ASSUME(log2_period < 64);
+  }
+
+  // Returns the log2 of the period for blocked elements.
+  // Every `2**K` element is blocked starting from index `2**K - 1`.
+  constexpr uint8_t log2_period() const { return log2_period_; }
+  // Returns the number of blocked elements at the end of the table.
+  constexpr uint8_t tail_blocked() const { return tail_blocked_; }
+
+  // Returns the number of blocked elements before the given index.
+  // Doesn't account for tail_blocked because there are no useful indices in
+  // the blocked tail.
+  constexpr size_t blocked_before(size_t index) const {
+    return index >> log2_period();
   }
 
-  T mask_;
+  // Returns the number of blocked elements in the table.
+  constexpr size_t total_blocked_count(size_t capacity) const {
+    return blocked_before(capacity) + tail_blocked();
+  }
+
+ private:
+  uint8_t log2_period_;
+  uint8_t tail_blocked_;
 };
 
-using ctrl_t = signed char;
-using h2_t = uint8_t;
+// Capacity, size and also has additionally
+// 1) one bit that stores whether we have infoz.
+// 2) kBlockedElementsBitCount bits that stores number of blocked elements in
+//    the table.
+// 3) kSeedBitCount bits for the seed. (For SOO tables, the lowest bit of the
+//    seed is repurposed to track if sampling has been tried).
+template <HashtableCapacityStorageMode StorageMode>
+class HashtableInlineDataImpl {
+  // The number of bits in the seed. It is big enough to ensure
+  // non-determinism of iteration order. We store the seed inside a uint64_t
+  // together with size and other metadata. When absl::Hash is inlined, it can
+  // have lower latency knowing that the high bits of the seed are zero.
+  static constexpr size_t kSeedBitCount = 5;
+
+ public:
+  static constexpr HashtableCapacityStorageMode kStorageMode = StorageMode;
+  using HashtableCapacity = HashtableCapacityImpl<StorageMode>;
+  static constexpr size_t kBlockedElementBitCount = 3;
+  static constexpr size_t kMaxBlockedElementCount =
+      (uint64_t{1} << kBlockedElementBitCount) - 1;
+  static constexpr size_t kCapacityBitCount =
+      StorageMode == kCapacityByValue ? sizeof(HashtableCapacity) * 8 : 6;
+  static constexpr size_t kCapacityBitStoredInDataCount =
+      StorageMode == kCapacityByValue ? 0 : kCapacityBitCount;
+  static constexpr size_t kSizeBitCount =
+      64 - (kBlockedElementBitCount + kSeedBitCount +
+            /*has_infoz*/ 1 + kCapacityBitStoredInDataCount);
+
+  explicit HashtableInlineDataImpl(uninitialized_tag_t) {}
+  explicit HashtableInlineDataImpl(HashtableCapacity capacity,
+                                   no_seed_empty_tag_t)
+      : capacity_internal_(capacity.ToRawData()), data_(0) {}
+  HashtableInlineDataImpl(HashtableCapacity capacity, full_soo_tag_t,
+                          bool has_tried_sampling)
+      : capacity_internal_(capacity.ToRawData()),
+        data_(kSizeOneNoMetadata |
+              (has_tried_sampling ? kSooHasTriedSamplingMask : 0)) {}
+
+  HashtableCapacity capacity() const {
+    return HashtableCapacity::FromRawData(capacity_internal_);
+  }
+  bool is_small() const { return capacity().is_small(); }
+
+  void set_capacity(HashtableCapacity c) { capacity_internal_ = c.ToRawData(); }
+  void set_capacity(size_t c) { set_capacity(HashtableCapacity(c)); }
+
+  // Returns actual size of the table.
+  size_t size() const { return static_cast<size_t>(data_ >> kSizeShift); }
+  void increment_size() { data_ += kSizeOneNoMetadata; }
+  void increment_size(size_t size) {
+    data_ += static_cast<uint64_t>(size) << kSizeShift;
+  }
+  void decrement_size() { data_ -= kSizeOneNoMetadata; }
+  // Returns true if the table is empty.
+  bool empty() const { return data_ < kSizeOneNoMetadata; }
+
+  // Returns true if an empty SOO table has already queried should_sample_soo().
+  bool soo_has_tried_sampling() const {
+    return (data_ & kSooHasTriedSamplingMask) != 0;
+  }
 
-// The values here are selected for maximum performance. See the static asserts
-// below for details.
-enum Ctrl : ctrl_t {
-  kEmpty = -128,   // 0b10000000
-  kDeleted = -2,   // 0b11111110
-  kSentinel = -1,  // 0b11111111
+  // Records that an empty SOO table has tried sampling.
+  void set_soo_has_tried_sampling() { data_ |= kSooHasTriedSamplingMask; }
+
+  // Sets the size, but keeps all the metadata bits.
+  void set_size(size_t size) {
+    data_ =
+        (data_ & kMetadataMask) | (static_cast<uint64_t>(size) << kSizeShift);
+  }
+
+  PerTableSeed seed() const {
+    return PerTableSeed(ToPublicSeed(data_ & kSeedMask));
+  }
+
+  void generate_new_seed() { set_seed(NextHashTableSeed()); }
+
+  // We need to use a constant seed when the table is sampled so that sampled
+  // hashes use the same seed and can e.g. identify stuck bits accurately.
+  void set_sampled_seed() { set_seed(kSampledSeed); }
+
+  bool is_sampled_seed() const {
+    return seed().seed() == ToPublicSeed(kSampledSeed);
+  }
+
+  // Returns true if the table has infoz.
+  bool has_infoz() const {
+    return ABSL_PREDICT_FALSE((data_ & kHasInfozMask) != 0);
+  }
+
+  // Sets the has_infoz bit.
+  void set_has_infoz() { data_ |= kHasInfozMask; }
+
+  // Returns the number of blocked elements in the table.
+  size_t blocked_element_count() const {
+    return (data_ & kBlockedElementMask) >> kBlockedElementsShift;
+  }
+  // Initializes the number of blocked elements in the table.
+  // Requires:
+  //   1. `blocked_element_count() == 0`.
+  //   2. `count <= kMaxBlockedElementCount`.
+  void init_blocked_element_count(uint64_t count) {
+    ABSL_SWISSTABLE_ASSERT(blocked_element_count() == 0);
+    ABSL_SWISSTABLE_ASSERT(count <= kMaxBlockedElementCount);
+    data_ |= count << kBlockedElementsShift;
+  }
+  void set_blocked_element_count_to_zero() { data_ &= ~kBlockedElementMask; }
+
+  void set_no_seed_for_testing() { data_ &= ~kSeedMask; }
+
+ private:
+  // Bit layout of `data_` and `capacity_internal_` from MSB to LSB:
+  // (55/49 bits)   : size
+  // (3 bits)       : blocked_element_count
+  // (1 bit)        : has_infoz
+  // (5 bits)       : seed
+  // (6 bits)       : capacity (only for kCapacityByLog)
+  // We don't split these components of `data_` into separate bit field elements
+  // because we get worse generated code that way.
+
+  static constexpr size_t kDataBitCount =
+      kSeedBitCount + 1 + kSizeBitCount + kBlockedElementBitCount;
+  static constexpr size_t kSizeShift = kDataBitCount - kSizeBitCount;
+  static constexpr uint64_t kSizeOneNoMetadata = uint64_t{1} << kSizeShift;
+  static constexpr uint64_t kMetadataMask = kSizeOneNoMetadata - 1;
+  static constexpr uint64_t kSeedMask = (uint64_t{1} << kSeedBitCount) - 1;
+  // The next bit after the seed.
+  static constexpr uint64_t kHasInfozMask = kSeedMask + 1;
+  static constexpr uint64_t kBlockedElementsShift = kSeedBitCount + 1;
+  static constexpr uint64_t kBlockedElementMask = kMaxBlockedElementCount
+                                                  << kBlockedElementsShift;
+  // For SOO tables, the seed is unused, and bit 0 is repurposed to track
+  // whether the table has already queried should_sample_soo().
+  static constexpr uint64_t kSooHasTriedSamplingMask = 1;
+
+  // We need to use a constant seed when the table is sampled so that sampled
+  // hashes use the same seed and can e.g. identify stuck bits accurately.
+  static constexpr uint8_t kSampledSeed = (1 << kSeedBitCount) - 1;
+
+  static constexpr uint64_t ToPublicSeed(uint64_t seed) {
+    // In kCapacityByLog mode, we shift public seed to the left to keep bits of
+    // the seed in the original place. It allows us to use single instruction to
+    // access the seed (e.g., `andl $0x7c0, %r8d`).
+    return seed << kCapacityBitStoredInDataCount;
+  }
+
+  void set_seed(uint8_t seed) {
+    data_ = (data_ & ~kSeedMask) | (seed & kSeedMask);
+  }
+
+  uint64_t capacity_internal_ : kCapacityBitCount;
+  uint64_t data_ : kDataBitCount;
 };
+
 static_assert(
-    kEmpty & kDeleted & kSentinel & 0x80,
-    "Special markers need to have the MSB to make checking for them efficient");
-static_assert(kEmpty < kSentinel && kDeleted < kSentinel,
-              "kEmpty and kDeleted must be smaller than kSentinel to make the "
-              "SIMD test of IsEmptyOrDeleted() efficient");
-static_assert(kSentinel == -1,
-              "kSentinel must be -1 to elide loading it from memory into SIMD "
-              "registers (pcmpeqd xmm, xmm)");
-static_assert(kEmpty == -128,
-              "kEmpty must be -128 to make the SIMD check for its "
-              "existence efficient (psignb xmm, xmm)");
-static_assert(~kEmpty & ~kDeleted & kSentinel & 0x7F,
-              "kEmpty and kDeleted must share an unset bit that is not shared "
-              "by kSentinel to make the scalar test for MatchEmptyOrDeleted() "
-              "efficient");
-static_assert(kDeleted == -2,
-              "kDeleted must be -2 to make the implementation of "
-              "ConvertSpecialToEmptyAndFullToDeleted efficient");
-
-// A single block of empty control bytes for tables without any slots allocated.
-// This enables removing a branch in the hot path of find().
-inline ctrl_t* EmptyGroup() {
-  alignas(16) static constexpr ctrl_t empty_group[] = {
-      kSentinel, kEmpty, kEmpty, kEmpty, kEmpty, kEmpty, kEmpty, kEmpty,
-      kEmpty,    kEmpty, kEmpty, kEmpty, kEmpty, kEmpty, kEmpty, kEmpty};
-  return const_cast<ctrl_t*>(empty_group);
-}
+    sizeof(HashtableInlineDataImpl<kCapacityByValue>::HashtableCapacity) ==
+    sizeof(size_t));
+// NOTE: some platforms have this size to be equal to 12 for two reasons:
+// 1) alignof(uint64_t) == 4.
+// 2) sizeof(size_t) == sizeof(HashtableCapacityImpl<kCapacityByValue>) == 4.
+static_assert(sizeof(HashtableInlineDataImpl<kCapacityByValue>) <= 16);
+static_assert(
+    sizeof(HashtableInlineDataImpl<kCapacityByLog>::HashtableCapacity) == 1);
+static_assert(sizeof(HashtableInlineDataImpl<kCapacityByLog>) == 8);
+
+#ifndef ABSL_SWISSTABLE_INTERNAL_ENABLE_CAPACITY_BY_VALUE
+using HashtableInlineData = HashtableInlineDataImpl<kCapacityByLog>;
+#else
+using HashtableInlineData = HashtableInlineDataImpl<kCapacityByValue>;
+#endif  // ABSL_SWISSTABLE_INTERNAL_ENABLE_CAPACITY_BY_VALUE
+using HashtableCapacity = HashtableInlineData::HashtableCapacity;
+
+// For large tables, we limit the number of blocked elements to maintain O(1)
+// average case lookup complexity.
+constexpr size_t kMaxBlockedElementsForLargeTables = 5;
+static_assert(kMaxBlockedElementsForLargeTables <=
+              HashtableInlineData::kMaxBlockedElementCount);
+
+// H1 is just the low bits of the hash.
+inline size_t H1(size_t hash) { return hash; }
+
+// Extracts the H2 portion of a hash: the 7 most significant bits.
+//
+// These are used as an occupied control byte.
+inline h2_t H2(size_t hash) { return hash >> (sizeof(size_t) * 8 - 7); }
+
+// When there is an insertion with no reserved growth, we rehash with
+// probability `min(1, RehashProbabilityConstant() / capacity())`. Using a
+// constant divided by capacity ensures that inserting N elements is still O(N)
+// in the average case. Using the constant 16 means that we expect to rehash ~8
+// times more often than when generations are disabled. We are adding expected
+// rehash_probability * #insertions/capacity_growth = 16/capacity * ((7/8 -
+// 7/16) * capacity)/capacity_growth = ~7 extra rehashes per capacity growth.
+inline size_t RehashProbabilityConstant() { return 16; }
+
+class CommonFieldsGenerationInfoEnabled {
+  // A sentinel value for reserved_growth_ indicating that we just ran out of
+  // reserved growth on the last insertion. When reserve is called and then
+  // insertions take place, reserved_growth_'s state machine is N, ..., 1,
+  // kReservedGrowthJustRanOut, 0.
+  static constexpr size_t kReservedGrowthJustRanOut =
+      (std::numeric_limits<size_t>::max)();
+
+ public:
+  CommonFieldsGenerationInfoEnabled() = default;
+  CommonFieldsGenerationInfoEnabled(CommonFieldsGenerationInfoEnabled&& that)
+      : reserved_growth_(that.reserved_growth_),
+        reservation_size_(that.reservation_size_),
+        generation_(that.generation_) {
+    that.reserved_growth_ = 0;
+    that.reservation_size_ = 0;
+    that.generation_ = EmptyGeneration();
+  }
+  CommonFieldsGenerationInfoEnabled& operator=(
+      CommonFieldsGenerationInfoEnabled&&) = default;
+
+  // Whether we should rehash on insert in order to detect bugs of using invalid
+  // references. We rehash on the first insertion after reserved_growth_ reaches
+  // 0 after a call to reserve. We also do a rehash with low probability
+  // whenever reserved_growth_ is zero.
+  bool should_rehash_for_bug_detection_on_insert(size_t capacity) const;
+  // Similar to above, except that we don't depend on reserved_growth_.
+  bool should_rehash_for_bug_detection_on_move(size_t capacity) const;
+  void maybe_increment_generation_on_insert() {
+    if (reserved_growth_ == kReservedGrowthJustRanOut) reserved_growth_ = 0;
+
+    if (reserved_growth_ > 0) {
+      if (--reserved_growth_ == 0) reserved_growth_ = kReservedGrowthJustRanOut;
+    } else {
+      increment_generation();
+    }
+  }
+  void increment_generation() { *generation_ = NextGeneration(*generation_); }
+  void reset_reserved_growth(size_t reservation, size_t size) {
+    reserved_growth_ = reservation - size;
+  }
+  size_t reserved_growth() const { return reserved_growth_; }
+  void set_reserved_growth(size_t r) { reserved_growth_ = r; }
+  size_t reservation_size() const { return reservation_size_; }
+  void set_reservation_size(size_t r) { reservation_size_ = r; }
+  GenerationType generation() const { return *generation_; }
+  void set_generation(GenerationType g) { *generation_ = g; }
+  GenerationType* generation_ptr() const { return generation_; }
+  void set_generation_ptr(GenerationType* g) { generation_ = g; }
+
+ private:
+  // The number of insertions remaining that are guaranteed to not rehash due to
+  // a prior call to reserve. Note: we store reserved growth in addition to
+  // reservation size because calls to erase() decrease size_ but don't decrease
+  // reserved growth.
+  size_t reserved_growth_ = 0;
+  // The maximum argument to reserve() since the container was cleared. We need
+  // to keep track of this, in addition to reserved growth, because we reset
+  // reserved growth to this when erase(begin(), end()) is called.
+  size_t reservation_size_ = 0;
+  // Pointer to the generation counter, which is used to validate iterators and
+  // is stored in the backing array between the control bytes and the slots.
+  // Note that we can't store the generation inside the container itself and
+  // keep a pointer to the container in the iterators because iterators must
+  // remain valid when the container is moved.
+  // Note: we could derive this pointer from the control pointer, but it makes
+  // the code more complicated, and there's a benefit in having the sizes of
+  // raw_hash_set in sanitizer mode and non-sanitizer mode a bit more different,
+  // which is that tests are less likely to rely on the size remaining the same.
+  GenerationType* generation_ = EmptyGeneration();
+};
+
+class CommonFieldsGenerationInfoDisabled {
+ public:
+  CommonFieldsGenerationInfoDisabled() = default;
+  CommonFieldsGenerationInfoDisabled(CommonFieldsGenerationInfoDisabled&&) =
+      default;
+  CommonFieldsGenerationInfoDisabled& operator=(
+      CommonFieldsGenerationInfoDisabled&&) = default;
+
+  bool should_rehash_for_bug_detection_on_insert(size_t) const { return false; }
+  bool should_rehash_for_bug_detection_on_move(size_t) const { return false; }
+  void maybe_increment_generation_on_insert() {}
+  void increment_generation() {}
+  void reset_reserved_growth(size_t, size_t) {}
+  size_t reserved_growth() const { return 0; }
+  void set_reserved_growth(size_t) {}
+  size_t reservation_size() const { return 0; }
+  void set_reservation_size(size_t) {}
+  GenerationType generation() const { return 0; }
+  void set_generation(GenerationType) {}
+  GenerationType* generation_ptr() const { return nullptr; }
+  void set_generation_ptr(GenerationType*) {}
+};
+
+class HashSetIteratorGenerationInfoEnabled {
+ public:
+  HashSetIteratorGenerationInfoEnabled() = default;
+  explicit HashSetIteratorGenerationInfoEnabled(
+      const GenerationType* generation_ptr)
+      : generation_ptr_(generation_ptr), generation_(*generation_ptr) {}
+
+  GenerationType generation() const { return generation_; }
+  void reset_generation() { generation_ = *generation_ptr_; }
+  const GenerationType* generation_ptr() const { return generation_ptr_; }
+  void set_generation_ptr(const GenerationType* ptr) { generation_ptr_ = ptr; }
 
-// Mixes a randomly generated per-process seed with `hash` and `ctrl` to
-// randomize insertion order within groups.
-bool ShouldInsertBackwards(size_t hash, ctrl_t* ctrl);
+ private:
+  const GenerationType* generation_ptr_ = EmptyGeneration();
+  GenerationType generation_ = *generation_ptr_;
+};
+
+class HashSetIteratorGenerationInfoDisabled {
+ public:
+  HashSetIteratorGenerationInfoDisabled() = default;
+  explicit HashSetIteratorGenerationInfoDisabled(const GenerationType*) {}
 
-// Returns a hash seed.
+  GenerationType generation() const { return 0; }
+  void reset_generation() {}
+  const GenerationType* generation_ptr() const { return nullptr; }
+  void set_generation_ptr(const GenerationType*) {}
+};
+
+#ifdef ABSL_SWISSTABLE_ENABLE_GENERATIONS
+using CommonFieldsGenerationInfo = CommonFieldsGenerationInfoEnabled;
+using HashSetIteratorGenerationInfo = HashSetIteratorGenerationInfoEnabled;
+#else
+using CommonFieldsGenerationInfo = CommonFieldsGenerationInfoDisabled;
+using HashSetIteratorGenerationInfo = HashSetIteratorGenerationInfoDisabled;
+#endif
+
+// Stored the information regarding number of slots we can still fill
+// without needing to rehash.
+//
+// We want to ensure sufficient number of empty slots in the table in order
+// to keep probe sequences relatively short. Empty slot in the probe group
+// is required to stop probing.
+//
+// Tombstones (kDeleted slots) are not included in the growth capacity,
+// because we'd like to rehash when the table is filled with tombstones and/or
+// full slots.
+//
+// GrowthInfo also stores a bit that encodes whether table may have any
+// deleted slots.
+// Most of the tables (>95%) have no deleted slots, so some functions can
+// be more efficient with this information.
+//
+// Callers can also force a rehash via the standard `rehash(0)`,
+// which will recompute this value as a side-effect.
+//
+// See also `CapacityToGrowth()`.
 //
-// The seed consists of the ctrl_ pointer, which adds enough entropy to ensure
-// non-determinism of iteration order in most cases.
-inline size_t HashSeed(const ctrl_t* ctrl) {
-  // The low bits of the pointer have little or no entropy because of
-  // alignment. We shift the pointer to try to use higher entropy bits. A
-  // good number seems to be 12 bits, because that aligns with page size.
-  return reinterpret_cast<uintptr_t>(ctrl) >> 12;
+// GrowthInfo is stored as 1 or 8 bytes at the beginning of the backing array.
+// For capacity <= kMaxGrowthLeftLowerBound we store single byte, otherwise we
+// store 8 bytes. Byte before the first control byte for all tables is always
+// used to store GrowthInfoLowerBound. That helps to avoid any branching in the
+// hottest code accessing GrowthInfo. GrowthInfoLowerBound has 7 bits to store
+// the growth left and 1 bit to store whether the table has any deleted slots.
+// For capacity > kMaxGrowthLeftLowerBound we use another 7 bytes to store the
+// full GrowthInfo. GrowthInfo for capacity > kMaxGrowthLeftLowerBound is stored
+// as uint64_t in little endian encoding. Most significant 8 bits (last byte in
+// little endian encoding) contains GrowthInfoLowerBound.
+class GrowthInfoAccessor;
+
+// One byte encoding of lower bound GrowthInfo.
+// It encodes number of growth left from 0 to kMaxGrowthLeftLowerBound and
+// whether the table has any deleted slots.
+class GrowthInfoLowerBound {
+ public:
+  static constexpr uint8_t kGrowthLeftMask = 0x7Fu;
+  static constexpr uint8_t kDeletedBit = 0x80u;
+  static constexpr uint64_t kMaxGrowthLeftLowerBound = 127;
+  static_assert(kMaxGrowthLeftLowerBound == kGrowthLeftMask);
+
+  explicit constexpr GrowthInfoLowerBound(uint8_t growth_left)
+      : growth_left_(growth_left) {}
+
+  // Returns true if table satisfies two properties:
+  // 1. Guaranteed to have no kDeleted slots.
+  // 2. There is a place for at least one element to grow.
+  constexpr bool HasNoDeletedAndGrowthLeft() const {
+    return static_cast<int8_t>(growth_left_) > 0;
+  }
+
+  // Returns true if table satisfies two properties:
+  // 1. May have kDeleted slots (kDeletedBit == 1).
+  // 2. There is a place for at least one element to grow.
+  constexpr bool HasDeletedAndGrowthLeft() const {
+    return growth_left_ > kDeletedBit;
+  }
+
+  // Returns true if the table satisfies two properties:
+  // 1. Guaranteed to have no kDeleted slots.
+  // 2. There is no growth left.
+  constexpr bool HasNoGrowthLeftAndNoDeleted() const {
+    return growth_left_ == 0;
+  }
+
+  // Returns true if GetGrowthLeft() == 0 and HasNoDeleted() is false.
+  // It is slightly more efficient.
+  constexpr bool HasNoGrowthLeftAndHaveDeleted() const {
+    return growth_left_ == kDeletedBit;
+  }
+
+  // Returns true if table guaranteed to have no kDeleted slots.
+  constexpr bool HasNoDeleted() const {
+    return (growth_left_ & kDeletedBit) == 0;
+  }
+
+  // Returns the minimum number of elements left to grow.
+  // Use GrowthInfoView::GetGrowthLeftTotal() to get the total number of
+  // elements left to grow. For tables with capacity <=
+  // kMaxGrowthLeftLowerBound, this is the same as GetGrowthLeftTotal().
+  constexpr uint8_t GetGrowthLeft() const {
+    return growth_left_ & kGrowthLeftMask;
+  }
+
+ private:
+  uint8_t growth_left_;
+};
+
+// GrowthInfo is stored in the backing array, and this class provides a simple
+// interface to access and modify it.
+class GrowthInfoAccessor {
+ public:
+  // GrowthInfoLowerBound is stored in the most significant 8 bits of the
+  // full growth info.
+  static constexpr uint64_t kLowerBoundShift = 64 - 8;
+
+  explicit GrowthInfoAccessor(void* control)
+      : growth_info_lower_bound_(reinterpret_cast<uint8_t*>(control) - 1 -
+                                 NumGenerationBytes()) {}
+
+  // Initializes the GrowthInfo assuming we can grow `growth_left` elements
+  // and there are no kDeleted slots in the table.
+  void InitGrowthLeftNoDeleted(size_t growth_left, size_t capacity);
+
+  // Returns a GrowthInfoLowerBound object containing the information
+  // about minimum growth left.
+  // It guarantees that GetGrowthLeft() will be > 0 if GetGrowthLeftTotal() > 0.
+  // It may optionally borrow some growth left from the full_growth_info.
+  GrowthInfoLowerBound RebalanceGrowthLeftLowerBound(size_t capacity);
+
+  // Overwrites single full slot with an empty slot.
+  void OverwriteFullAsEmpty();
+
+  // Overwrites single empty slot with a full slot.
+  // Must be called when GetGrowthLeftLowerBound() > 0.
+  void OverwriteEmptyAsFull() {
+    ABSL_SWISSTABLE_ASSERT(GetGrowthLeftLowerBound() > 0);
+    --(*growth_info_lower_bound_);
+  }
+
+  // Overwrites specified control element with full slot.
+  // Must be called when GetGrowthLeftLowerBound() >= IsEmpty(ctrl).
+  void OverwriteControlAsFull(ctrl_t ctrl) {
+    ABSL_SWISSTABLE_ASSERT(GetGrowthLeftLowerBound() >=
+                           static_cast<size_t>(IsEmpty(ctrl)));
+    *growth_info_lower_bound_ -= static_cast<size_t>(IsEmpty(ctrl));
+  }
+
+  // Overwrites single full slot with a deleted slot.
+  void OverwriteFullAsDeleted() {
+    *growth_info_lower_bound_ |= GrowthInfoLowerBound::kDeletedBit;
+  }
+
+  // Returns a GrowthInfoLowerBound object containing the information
+  // about minimum growth left.
+  GrowthInfoLowerBound GetGrowthInfoLowerBound() const {
+    return GrowthInfoLowerBound(*growth_info_lower_bound_);
+  }
+
+  // Returns the minimum number of elements left to grow.
+  size_t GetGrowthLeftLowerBound() const {
+    return GetGrowthInfoLowerBound().GetGrowthLeft();
+  }
+
+  // The number of slots we can still fill without needing to rehash.
+  // Hot code paths should try to work with
+  // growth_info().GetGrowthLeftLowerBound() instead.
+  size_t GetGrowthLeftTotalSlow(size_t capacity) const;
+
+ private:
+  void* full_growth_info_ptr() const { return growth_info_lower_bound_ - 7; }
+
+  GrowthInfoLowerBound RebalanceGrowthLeftLowerBoundLargeCapacity();
+
+  // Pointer to the GrowthInfoLowerBound data.
+  // For large capacities, 7 bytes before this pointer is used to store
+  // the full growth info.
+  // NOTE: using a pointer here can result in the compiler being forced to
+  // assume aliasing can happen. So in hot code paths, we try to work with
+  // GrowthInfoLowerBound directly
+  uint8_t* growth_info_lower_bound_;
+};
+
+// Returns the number of "cloned control bytes".
+//
+// This is the number of control bytes that are present both at the beginning
+// of the control byte array and at the end, such that we can create a
+// `Group::kWidth`-width probe window starting from any control byte.
+constexpr size_t NumClonedBytes() { return Group::kWidth - 1; }
+
+// Returns the number of control bytes including cloned.
+constexpr size_t NumControlBytes(size_t capacity) {
+  return IsSmallCapacity(capacity) ? 0 : capacity + 1 + NumClonedBytes();
 }
 
-inline size_t H1(size_t hash, const ctrl_t* ctrl) {
-  return (hash >> 7) ^ HashSeed(ctrl);
+// Returns the size in bytes table with given capacity use to store GrowthInfo.
+// Returns 0 for small tables that doesn't store GrowthInfo.
+constexpr size_t GrowthInfoSizeForCapacity(size_t capacity) {
+  if (IsSmallCapacity(capacity)) {
+    return 0;
+  }
+  return capacity <= GrowthInfoLowerBound::kMaxGrowthLeftLowerBound
+             ? sizeof(uint8_t)
+             : sizeof(uint64_t);
 }
-inline ctrl_t H2(size_t hash) { return hash & 0x7F; }
-
-inline bool IsEmpty(ctrl_t c) { return c == kEmpty; }
-inline bool IsFull(ctrl_t c) { return c >= 0; }
-inline bool IsDeleted(ctrl_t c) { return c == kDeleted; }
-inline bool IsEmptyOrDeleted(ctrl_t c) { return c < kSentinel; }
-
-#if SWISSTABLE_HAVE_SSE2
-
-// https://github.com/abseil/abseil-cpp/issues/209
-// https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87853
-// _mm_cmpgt_epi8 is broken under GCC with -funsigned-char
-// Work around this by using the portable implementation of Group
-// when using -funsigned-char under GCC.
-inline __m128i _mm_cmpgt_epi8_fixed(__m128i a, __m128i b) {
-#if defined(__GNUC__) && !defined(__clang__)
-  if (std::is_unsigned<char>::value) {
-    const __m128i mask = _mm_set1_epi8(0x80);
-    const __m128i diff = _mm_subs_epi8(b, a);
-    return _mm_cmpeq_epi8(_mm_and_si128(diff, mask), mask);
+
+// Computes the size of the metadata before the control bytes. infoz,
+// growth_info and generation are stored at the beginning of the backing array.
+constexpr size_t MetadataBeforeControlSize(bool has_infoz, size_t capacity) {
+  if (ABSL_PREDICT_FALSE(has_infoz)) {
+    // We always allocate 8 bytes of growth info for sampled tables to allow
+    // branchless access to infoz pointer.
+    return sizeof(HashtablezInfoHandle) + sizeof(uint64_t) +
+           NumGenerationBytes();
   }
-#endif
-  return _mm_cmpgt_epi8(a, b);
+  return GrowthInfoSizeForCapacity(capacity) + NumGenerationBytes();
 }
 
-struct GroupSse2Impl {
-  static constexpr size_t kWidth = 16;  // the number of slots per group
+// Returns the offset of the next item after `offset` that is aligned to `align`
+// bytes. `align` must be a power of two.
+constexpr size_t AlignUpTo(size_t offset, size_t align) {
+  return (offset + align - 1) & (~align + 1);
+}
 
-  explicit GroupSse2Impl(const ctrl_t* pos) {
-    ctrl = _mm_loadu_si128(reinterpret_cast<const __m128i*>(pos));
+// Helper class for computing offsets and allocation size of hash set fields.
+class RawHashSetLayout {
+ public:
+  explicit RawHashSetLayout(size_t capacity, size_t slot_size,
+                            size_t slot_align, bool has_infoz,
+                            size_t blocked_element_count)
+      : control_offset_(MetadataBeforeControlSize(has_infoz, capacity)),
+        generation_offset_(control_offset_ - NumGenerationBytes()),
+        slot_offset_(control_offset_ + NumControlBytes(capacity)) {
+    ABSL_SWISSTABLE_ASSERT(IsValidCapacity(capacity));
+    size_t aligned_slot_offset = AlignUpTo(slot_offset_, slot_align);
+    size_t slot_array_padding = aligned_slot_offset - slot_offset_;
+    slot_offset_ = aligned_slot_offset;
+    ABSL_SWISSTABLE_ASSERT(
+        slot_size <=
+        ((std::numeric_limits<size_t>::max)() - slot_offset_) / capacity);
+    control_offset_ += slot_array_padding;
+    generation_offset_ += slot_array_padding;
+    ABSL_SWISSTABLE_ASSERT(!IsSmallCapacity(capacity) ||
+                           control_offset_ == slot_offset_);
+    alloc_size_ = slot_offset_ + (capacity - blocked_element_count) * slot_size;
   }
 
-  // Returns a bitmask representing the positions of slots that match hash.
-  BitMask<uint32_t, kWidth> Match(h2_t hash) const {
-    auto match = _mm_set1_epi8(hash);
-    return BitMask<uint32_t, kWidth>(
-        _mm_movemask_epi8(_mm_cmpeq_epi8(match, ctrl)));
+  // Returns precomputed offset from the start of the backing allocation of
+  // control.
+  size_t control_offset() const { return control_offset_; }
+
+  // Given the capacity of a table, computes the offset (from the start of the
+  // backing allocation) of the generation counter (if it exists).
+  size_t generation_offset() const { return generation_offset_; }
+
+  // Given the capacity of a table, computes the offset (from the start of the
+  // backing allocation) at which the slots begin.
+  size_t slot_offset() const { return slot_offset_; }
+
+  // Given the capacity of a table, computes the total size of the backing
+  // array.
+  size_t alloc_size() const { return alloc_size_; }
+
+ private:
+  size_t control_offset_;
+  size_t generation_offset_;
+  size_t slot_offset_;
+  size_t alloc_size_;
+};
+
+struct HashtableFreeFunctionsAccess;
+
+// This allows us to work around an uninitialized memory warning when
+// constructing begin() iterators in empty hashtables.
+template <typename T>
+union MaybeInitializedPtr {
+  T* get() const { ABSL_SWISSTABLE_IGNORE_UNINITIALIZED_RETURN(p); }
+  void set(T* ptr) { p = ptr; }
+
+  T* p;
+};
+
+struct HeapPtrs {
+  // The control bytes (and, also, a pointer near to the base of the backing
+  // array).
+  //
+  // This contains `capacity + 1 + NumClonedBytes()` entries.
+  //
+  // Note that growth_info is stored immediately before this pointer.
+  // May be uninitialized for small tables.
+  MaybeInitializedPtr<ctrl_t> control;
+};
+
+// Returns the maximum size of the SOO slot.
+constexpr size_t MaxSooSlotSize() { return sizeof(HeapPtrs); }
+
+// Manages the backing array pointers or the SOO slot. When raw_hash_set::is_soo
+// is true, the SOO slot is stored in `soo_data`. Otherwise, we use `heap`.
+union HeapOrSoo {
+  MaybeInitializedPtr<ctrl_t>& control() {
+    ABSL_SWISSTABLE_IGNORE_UNINITIALIZED_RETURN(heap.control);
+  }
+  MaybeInitializedPtr<ctrl_t> control() const {
+    ABSL_SWISSTABLE_IGNORE_UNINITIALIZED_RETURN(heap.control);
+  }
+  void* get_soo_data() {
+    ABSL_SWISSTABLE_IGNORE_UNINITIALIZED_RETURN(soo_data);
+  }
+  const void* get_soo_data() const {
+    ABSL_SWISSTABLE_IGNORE_UNINITIALIZED_RETURN(soo_data);
   }
 
-  // Returns a bitmask representing the positions of empty slots.
-  BitMask<uint32_t, kWidth> MatchEmpty() const {
-#if SWISSTABLE_HAVE_SSSE3
-    // This only works because kEmpty is -128.
-    return BitMask<uint32_t, kWidth>(
-        _mm_movemask_epi8(_mm_sign_epi8(ctrl, ctrl)));
-#else
-    return Match(static_cast<h2_t>(kEmpty));
-#endif
+  HeapPtrs heap;
+  unsigned char soo_data[MaxSooSlotSize()];
+};
+
+// Returns a reference to the GrowthInfo object stored immediately before
+// `control`.
+inline GrowthInfoAccessor GetGrowthInfoFromControl(ctrl_t* control) {
+  return GrowthInfoAccessor(control);
+}
+
+// CommonFields hold the fields in raw_hash_set that do not depend
+// on template parameters. This allows us to conveniently pass all
+// of this state to helper functions as a single argument.
+class CommonFields : public CommonFieldsGenerationInfo {
+ public:
+  explicit CommonFields(soo_tag_t)
+      : inline_data_(HashtableCapacity(SooCapacity()), no_seed_empty_tag_t{}) {}
+  explicit CommonFields(full_soo_tag_t, bool has_tried_sampling)
+      : inline_data_(HashtableCapacity(SooCapacity()), full_soo_tag_t{},
+                     has_tried_sampling) {}
+  explicit CommonFields(non_soo_tag_t)
+      : inline_data_(HashtableCapacity(0), no_seed_empty_tag_t{}) {}
+  // For use in swapping.
+  explicit CommonFields(uninitialized_tag_t)
+      : inline_data_(uninitialized_tag_t{}) {}
+
+  // Not copyable
+  CommonFields(const CommonFields&) = delete;
+  CommonFields& operator=(const CommonFields&) = delete;
+
+  // Copy with guarantee that it is not SOO.
+  CommonFields(non_soo_tag_t, const CommonFields& that)
+      : inline_data_(that.inline_data_), heap_or_soo_(that.heap_or_soo_) {}
+
+  // Movable
+  CommonFields(CommonFields&& that) = default;
+  CommonFields& operator=(CommonFields&&) = default;
+
+  template <bool kSooEnabled>
+  static CommonFields CreateDefault() {
+    return kSooEnabled ? CommonFields{soo_tag_t{}}
+                       : CommonFields{non_soo_tag_t{}};
   }
 
-  // Returns a bitmask representing the positions of empty or deleted slots.
-  BitMask<uint32_t, kWidth> MatchEmptyOrDeleted() const {
-    auto special = _mm_set1_epi8(kSentinel);
-    return BitMask<uint32_t, kWidth>(
-        _mm_movemask_epi8(_mm_cmpgt_epi8_fixed(special, ctrl)));
+  // The inline data for SOO is written on top of control_/slots_.
+  const void* soo_data() const { return heap_or_soo_.get_soo_data(); }
+  void* soo_data() { return heap_or_soo_.get_soo_data(); }
+
+  ctrl_t* control() const {
+    ABSL_SWISSTABLE_ASSERT(capacity() > 0);
+    // Assume that the control bytes don't alias `this`.
+    ctrl_t* ctrl = heap_or_soo_.control().get();
+    [[maybe_unused]] size_t num_control_bytes = NumControlBytes(capacity());
+    ABSL_ASSUME(reinterpret_cast<uintptr_t>(ctrl + num_control_bytes) <=
+                    reinterpret_cast<uintptr_t>(this) ||
+                reinterpret_cast<uintptr_t>(this + 1) <=
+                    reinterpret_cast<uintptr_t>(ctrl));
+    ABSL_SWISSTABLE_IGNORE_UNINITIALIZED_RETURN(ctrl);
   }
 
-  // Returns the number of trailing empty or deleted elements in the group.
-  uint32_t CountLeadingEmptyOrDeleted() const {
-    auto special = _mm_set1_epi8(kSentinel);
-    return TrailingZeros(
-        _mm_movemask_epi8(_mm_cmpgt_epi8_fixed(special, ctrl)) + 1);
+  void set_control(ctrl_t* c) { heap_or_soo_.control().set(c); }
+
+  // Note: we can't use slots() because Qt defines "slots" as a macro.
+  // Returns pointer to the slots of a table with explicit capacity that must be
+  // equal to the actual capacity of the table.
+  // Capacity is often known at compile time or already in register with some
+  // ABSL_ASSUME conditions. We require passing it explicitly to eliminate
+  // branches inside of NumControlBytes in majority of cases.
+  void* slot_array(size_t capacity) const {
+    ABSL_SWISSTABLE_ASSERT(capacity == this->capacity());
+    ctrl_t* ctrl = control();
+    return ctrl + NumControlBytes(capacity);
   }
 
-  void ConvertSpecialToEmptyAndFullToDeleted(ctrl_t* dst) const {
-    auto msbs = _mm_set1_epi8(static_cast<char>(-128));
-    auto x126 = _mm_set1_epi8(126);
-#if SWISSTABLE_HAVE_SSSE3
-    auto res = _mm_or_si128(_mm_shuffle_epi8(x126, ctrl), msbs);
-#else
-    auto zero = _mm_setzero_si128();
-    auto special_mask = _mm_cmpgt_epi8_fixed(zero, ctrl);
-    auto res = _mm_or_si128(msbs, _mm_andnot_si128(special_mask, x126));
-#endif
-    _mm_storeu_si128(reinterpret_cast<__m128i*>(dst), res);
+  // The number of filled slots.
+  size_t size() const { return inline_data_.size(); }
+  // Sets the size to zero, but keeps hashinfoz bit and seed.
+  void set_size_to_zero() { inline_data_.set_size(0); }
+  void set_empty_soo() {
+    AssertInSooMode();
+    inline_data_.set_size(0);
+  }
+  void set_full_soo() {
+    AssertInSooMode();
+    inline_data_.set_size(1);
+  }
+  void increment_size() {
+    ABSL_SWISSTABLE_ASSERT(size() < capacity());
+    inline_data_.increment_size();
+  }
+  void increment_size(size_t n) {
+    ABSL_SWISSTABLE_ASSERT(size() + n <= capacity());
+    inline_data_.increment_size(n);
+  }
+  void decrement_size() {
+    ABSL_SWISSTABLE_ASSERT(!empty());
+    inline_data_.decrement_size();
+  }
+  bool empty() const { return inline_data_.empty(); }
+  void set_soo_has_tried_sampling() {
+    inline_data_.set_soo_has_tried_sampling();
+  }
+  bool soo_has_tried_sampling() const {
+    return inline_data_.soo_has_tried_sampling();
   }
 
-  __m128i ctrl;
-};
-#endif  // SWISSTABLE_HAVE_SSE2
+  // The seed used for the hash function.
+  PerTableSeed seed() const { return inline_data_.seed(); }
+  // Generates a new seed the hash function.
+  // The table will be invalidated if `!empty()` because hash is being changed.
+  // In such cases, we will need to rehash the table.
+  void generate_new_seed(bool has_infoz) {
+    // Note: we can't use has_infoz() here because we set has_infoz later than
+    // we generate the seed.
+    if (ABSL_PREDICT_FALSE(has_infoz)) {
+      inline_data_.set_sampled_seed();
+      return;
+    }
+    inline_data_.generate_new_seed();
+  }
+  void set_no_seed_for_testing() { inline_data_.set_no_seed_for_testing(); }
 
-struct GroupPortableImpl {
-  static constexpr size_t kWidth = 8;
+  HashtableCapacity capacity_impl() const {
+    HashtableCapacity cap = inline_data_.capacity();
+    ABSL_SWISSTABLE_ASSERT(cap.IsValid());
+    return cap;
+  }
+  size_t capacity() const { return capacity_impl().capacity(); }
+  // We have a separate alias for callsites in which the capacity may be
+  // invalid.
+  HashtableCapacity maybe_invalid_capacity() const {
+    return inline_data_.capacity();
+  }
+  void set_capacity(HashtableCapacity c) { inline_data_.set_capacity(c); }
+  void set_capacity(size_t c) {
+    set_capacity(HashtableCapacity(c));
+  }
+  bool is_small() const { return inline_data_.is_small(); }
+
+  GrowthInfoAccessor growth_info() const {
+    ABSL_SWISSTABLE_ASSERT(GrowthInfoSizeForCapacity(capacity()) > 0);
+    return GetGrowthInfoFromControl(control());
+  }
+
+  bool has_infoz() const { return inline_data_.has_infoz(); }
+  void set_has_infoz() {
+    ABSL_SWISSTABLE_ASSERT(inline_data_.is_sampled_seed());
+    inline_data_.set_has_infoz();
+  }
 
-  explicit GroupPortableImpl(const ctrl_t* pos)
-      : ctrl(little_endian::Load64(pos)) {}
+  HashtablezInfoHandle infoz_ptr() const;
 
-  BitMask<uint64_t, kWidth, 3> Match(h2_t hash) const {
-    // For the technique, see:
-    // http://graphics.stanford.edu/~seander/bithacks.html##ValueInWord
-    // (Determine if a word has a byte equal to n).
-    //
-    // Caveat: there are false positives but:
-    // - they only occur if there is a real match
-    // - they never occur on kEmpty, kDeleted, kSentinel
-    // - they will be handled gracefully by subsequent checks in code
-    //
-    // Example:
-    //   v = 0x1716151413121110
-    //   hash = 0x12
-    //   retval = (v - lsbs) & ~v & msbs = 0x0000000080800000
-    constexpr uint64_t msbs = 0x8080808080808080ULL;
-    constexpr uint64_t lsbs = 0x0101010101010101ULL;
-    auto x = ctrl ^ (lsbs * hash);
-    return BitMask<uint64_t, kWidth, 3>((x - lsbs) & ~x & msbs);
+  HashtablezInfoHandle infoz() {
+    return has_infoz() ? infoz_ptr() : HashtablezInfoHandle();
   }
+  void set_infoz(HashtablezInfoHandle infoz);
 
-  BitMask<uint64_t, kWidth, 3> MatchEmpty() const {
-    constexpr uint64_t msbs = 0x8080808080808080ULL;
-    return BitMask<uint64_t, kWidth, 3>((ctrl & (~ctrl << 6)) & msbs);
+  bool should_rehash_for_bug_detection_on_insert() const {
+    if constexpr (!SwisstableGenerationsEnabled()) {
+      return false;
+    }
+    return CommonFieldsGenerationInfo::
+        should_rehash_for_bug_detection_on_insert(capacity());
+  }
+  bool should_rehash_for_bug_detection_on_move() const {
+    return CommonFieldsGenerationInfo::should_rehash_for_bug_detection_on_move(
+        capacity());
+  }
+  void reset_reserved_growth(size_t reservation) {
+    CommonFieldsGenerationInfo::reset_reserved_growth(reservation, size());
   }
 
-  BitMask<uint64_t, kWidth, 3> MatchEmptyOrDeleted() const {
-    constexpr uint64_t msbs = 0x8080808080808080ULL;
-    return BitMask<uint64_t, kWidth, 3>((ctrl & (~ctrl << 7)) & msbs);
+  // Returns the number of blocked elements in the table.
+  // Blocked elements are located at the end of the table and do not have
+  // corresponding slots.
+  // Control bytes are set to kSentinel for blocked elements.
+  size_t blocked_element_count() const {
+    return inline_data_.blocked_element_count();
+  }
+  // Initializes the number of blocked elements in the table.
+  // Requires:
+  //   1. `blocked_element_count() == 0`.
+  //   2. `count <= kMaxBlockedElementCount`.
+  void init_blocked_element_count(size_t count) {
+    inline_data_.init_blocked_element_count(count);
+  }
+  void set_blocked_element_count_to_zero() {
+    inline_data_.set_blocked_element_count_to_zero();
   }
 
-  uint32_t CountLeadingEmptyOrDeleted() const {
-    constexpr uint64_t gaps = 0x00FEFEFEFEFEFEFEULL;
-    return (TrailingZeros(((~ctrl & (ctrl >> 7)) | gaps) + 1) + 7) >> 3;
+  // The size of the backing array allocation.
+  size_t alloc_size(size_t slot_size, size_t slot_align) const {
+    return RawHashSetLayout(capacity(), slot_size, slot_align, has_infoz(),
+                            blocked_element_count())
+        .alloc_size();
   }
 
-  void ConvertSpecialToEmptyAndFullToDeleted(ctrl_t* dst) const {
-    constexpr uint64_t msbs = 0x8080808080808080ULL;
-    constexpr uint64_t lsbs = 0x0101010101010101ULL;
-    auto x = ctrl & msbs;
-    auto res = (~x + (x >> 7)) & ~lsbs;
-    little_endian::Store64(dst, res);
+  // Move fields other than heap_or_soo_.
+  void move_non_heap_or_soo_fields(CommonFields& that) {
+    static_cast<CommonFieldsGenerationInfo&>(*this) =
+        std::move(static_cast<CommonFieldsGenerationInfo&>(that));
+    inline_data_ = that.inline_data_;
   }
 
-  uint64_t ctrl;
-};
+  // Returns the number of control bytes set to kDeleted. For testing only.
+  size_t TombstonesCount() const {
+    return static_cast<size_t>(
+        std::count(control(), control() + capacity(), ctrl_t::kDeleted));
+  }
 
-#if SWISSTABLE_HAVE_SSE2
-using Group = GroupSse2Impl;
-#else
-using Group = GroupPortableImpl;
+  // Helper to enable sanitizer mode validation to protect against reentrant
+  // calls during element constructor/destructor.
+  template <typename F>
+  void RunWithReentrancyGuard(F f) {
+#ifdef NDEBUG
+    f();
+    return;
 #endif
+    const HashtableCapacity cap = maybe_invalid_capacity();
+    set_capacity(HashtableCapacity::CreateReentrance());
+    f();
+    set_capacity(cap);
+  }
 
-template <class Policy, class Hash, class Eq, class Alloc>
-class raw_hash_set;
+  // Asserts that the capacity is not a sentinel invalid value.
+  void AssertNotDebugCapacity() const {
+    if (!SwisstableGenerationsOrDebugEnabled()) {
+      return;
+    }
+    AssertNotDebugCapacityImpl();
+  }
 
-inline bool IsValidCapacity(size_t n) { return ((n + 1) & n) == 0 && n > 0; }
+ private:
+  // We store the has_infoz bit in the lowest bit of size_.
+  static constexpr size_t HasInfozShift() { return 1; }
+  static constexpr size_t HasInfozMask() {
+    return (size_t{1} << HasInfozShift()) - 1;
+  }
+
+  // We can't assert that SOO is enabled because we don't have SooEnabled(), but
+  // we assert what we can.
+  void AssertInSooMode() const {
+    ABSL_SWISSTABLE_ASSERT(capacity() == SooCapacity());
+    ABSL_SWISSTABLE_ASSERT(!has_infoz());
+  }
+
+  void AssertNotDebugCapacityImpl() const;
+
+  HashtableInlineData inline_data_;
+
+  // Either the heap pointer or the SOO slot.
+  HeapOrSoo heap_or_soo_;
+};
 
+template <class Policy, class... Params>
+class raw_hash_set;
+
+// Applies the following mapping to every byte in the control array:
+//   * kDeleted -> kEmpty
+//   * kEmpty -> kEmpty
+//   * _ -> kDeleted
 // PRECONDITION:
 //   IsValidCapacity(capacity)
-//   ctrl[capacity] == kSentinel
-//   ctrl[i] != kSentinel for all i < capacity
-// Applies mapping for every byte in ctrl:
-//   DELETED -> EMPTY
-//   EMPTY -> EMPTY
-//   FULL -> DELETED
-inline void ConvertDeletedToEmptyAndFullToDeleted(
-    ctrl_t* ctrl, size_t capacity) {
-  assert(ctrl[capacity] == kSentinel);
-  assert(IsValidCapacity(capacity));
-  for (ctrl_t* pos = ctrl; pos != ctrl + capacity + 1; pos += Group::kWidth) {
-    Group{pos}.ConvertSpecialToEmptyAndFullToDeleted(pos);
-  }
-  // Copy the cloned ctrl bytes.
-  std::memcpy(ctrl + capacity + 1, ctrl, Group::kWidth);
-  ctrl[capacity] = kSentinel;
+//   ctrl[capacity] == ctrl_t::kSentinel
+//   ctrl[i] != ctrl_t::kSentinel for all i < capacity
+void ConvertDeletedToEmptyAndFullToDeleted(ctrl_t* ctrl, size_t capacity);
+
+template <class InputIter>
+size_t SelectReservationSizeForIterRange(InputIter first, InputIter last,
+                                         size_t reservation_size) {
+  if (reservation_size != 0) {
+    return reservation_size;
+  }
+  if (base_internal::IsAtLeastIterator<std::random_access_iterator_tag,
+                                       InputIter>()) {
+    return static_cast<size_t>(std::distance(first, last));
+  }
+  return 0;
 }
 
-// Rounds up the capacity to the next power of 2 minus 1, with a minimum of 1.
-inline size_t NormalizeCapacity(size_t n) {
-  return n ? ~size_t{} >> LeadingZeros(n) : 1;
+constexpr bool SwisstableDebugEnabled() {
+#if defined(ABSL_SWISSTABLE_ENABLE_GENERATIONS) || \
+    ABSL_OPTION_HARDENED == 1 || !defined(NDEBUG)
+  return true;
+#else
+  return false;
+#endif
 }
 
-// We use 7/8th as maximum load factor.
-// For 16-wide groups, that gives an average of two empty slots per group.
-inline size_t CapacityToGrowth(size_t capacity) {
-  assert(IsValidCapacity(capacity));
-  // `capacity*7/8`
-  if (Group::kWidth == 8 && capacity == 7) {
-    // x-x/8 does not work when x==7.
-    return 6;
+// Dereferences `ptr`. The function is named in order to provide a helpful error
+// message when users see crashing stack traces. Note that this function is not
+// guaranteed to crash when `ptr` is invalid if sanitizer mode is not enabled.
+template <typename T>
+T CrashIfIteratorIsInvalid(const T* ptr) {
+  // If the following line(s) crash, then it's likely that `ptr` is from a
+  // backing array that has been deallocated. If you see a crash here, it likely
+  // means that you are comparing an invalid iterator from a table that has
+  // rehashed, moved, or been destroyed. In such cases, it is often helpful to
+  // reproduce the issue with --config=asan and (assuming there's a crash here)
+  // examine the corresponding deallocation stack trace.
+  T ret = *ptr;
+  // Force a read with inline asm to make sure that a crash happens here, rather
+  // than later when the value is used.
+#ifdef __clang__
+  asm("" : "+r"(ret));
+#endif
+  return ret;
+}
+
+// Note: we take control pointers by reference in a few Assert* functions below
+// so that it's not UB if they're uninitialized as long as we don't read them
+// (when slot is null).
+
+inline void AssertIsFull(const ctrl_t* const& ctrl, const void* slot,
+                         GenerationType generation,
+                         const GenerationType* generation_ptr,
+                         const char* operation) {
+  if (!SwisstableDebugEnabled()) return;
+  // `SwisstableDebugEnabled()` is also true for release builds with hardening
+  // enabled. To minimize their impact in those builds:
+  // - use `ABSL_PREDICT_FALSE()` to provide a compiler hint for code layout
+  // - use `ABSL_RAW_LOG()` with a format string to reduce code size and improve
+  //   the chances that the hot paths will be inlined.
+  if (ABSL_PREDICT_FALSE(slot == nullptr)) {
+    ABSL_RAW_LOG(FATAL, "%s called on end() iterator.", operation);
   }
-  return capacity - capacity / 8;
+  if (ABSL_PREDICT_FALSE(slot == DefaultIterSlot())) {
+    ABSL_RAW_LOG(FATAL, "%s called on default-constructed iterator.",
+                 operation);
+  }
+  if (SwisstableGenerationsEnabled()) {
+    if (ABSL_PREDICT_FALSE(generation !=
+                           CrashIfIteratorIsInvalid(generation_ptr))) {
+      ABSL_RAW_LOG(FATAL,
+                   "%s called on invalid iterator. The table could have "
+                   "rehashed or moved since this iterator was initialized.",
+                   operation);
+    }
+    if (ABSL_PREDICT_FALSE(!IsFull(CrashIfIteratorIsInvalid(ctrl)))) {
+      ABSL_RAW_LOG(
+          FATAL,
+          "%s called on invalid iterator. The element was likely erased.",
+          operation);
+    }
+  } else {
+    if (ABSL_PREDICT_FALSE(!IsFull(CrashIfIteratorIsInvalid(ctrl)))) {
+      ABSL_RAW_LOG(
+          FATAL,
+          "%s called on invalid iterator. The element might have been erased "
+          "or the table might have rehashed. Consider running with "
+          "--config=asan to diagnose rehashing issues.",
+          operation);
+    }
+  }
+}
+
+// Note that for comparisons, null/end iterators are valid.
+inline void AssertIsValidForComparison(const ctrl_t* const& ctrl,
+                                       const void* slot,
+                                       GenerationType generation,
+                                       const GenerationType* generation_ptr) {
+  if (!SwisstableDebugEnabled()) return;
+  const bool ctrl_is_valid_for_comparison =
+      slot == nullptr || slot == DefaultIterSlot() ||
+      IsFull(CrashIfIteratorIsInvalid(ctrl));
+  if (SwisstableGenerationsEnabled()) {
+    if (ABSL_PREDICT_FALSE(generation !=
+                           CrashIfIteratorIsInvalid(generation_ptr))) {
+      // Note: in the case of a rehash, we would expect to see a sanitizer crash
+      // in CrashIfIteratorIsInvalid so this assertion will only catch moved
+      // table cases, unless we're using a custom allocator that does not
+      // deallocate the old backing array (e.g. an arena allocator).
+      ABSL_RAW_LOG(
+          FATAL,
+          "Invalid iterator comparison. The table was likely moved (or "
+          "possibly rehashed) since this iterator was initialized.");
+    }
+    if (ABSL_PREDICT_FALSE(!ctrl_is_valid_for_comparison)) {
+      ABSL_RAW_LOG(
+          FATAL, "Invalid iterator comparison. The element was likely erased.");
+    }
+  } else {
+    ABSL_HARDENING_ASSERT_SLOW(
+        ctrl_is_valid_for_comparison &&
+        "Invalid iterator comparison. The element might have been erased or "
+        "the table might have rehashed. Consider running with --config=asan to "
+        "diagnose rehashing issues.");
+  }
+}
+
+// If the two iterators come from the same container, then their pointers will
+// interleave such that ctrl_a <= ctrl_b < slot_a <= slot_b or vice/versa.
+inline bool AreItersFromSameContainer(const ctrl_t* const& ctrl_a,
+                                      const ctrl_t* const& ctrl_b,
+                                      const void* slot_a, const void* slot_b) {
+  // If either slot is null, then we can't tell.
+  if (slot_a == nullptr || slot_b == nullptr) return true;
+  // If either slot is iterator returned by insert, then we can't tell.
+  if (IsInsertIteratorControl(ctrl_a) || IsInsertIteratorControl(ctrl_b)) {
+    return true;
+  }
+  const bool a_is_soo = IsSooControl(ctrl_a);
+  if (a_is_soo != IsSooControl(ctrl_b)) return false;
+  if (a_is_soo) return slot_a == slot_b;
+
+  const void* low_ctrl = ctrl_a;
+  const void* hi_ctrl = ctrl_b;
+  if (ctrl_a > ctrl_b) {
+    std::swap(low_ctrl, hi_ctrl);
+    std::swap(slot_a, slot_b);
+  }
+  return hi_ctrl < slot_a && slot_a <= slot_b;
+}
+
+// Asserts that two iterators come from the same container.
+// Note: we take slots by reference so that it's not UB if they're uninitialized
+// as long as we don't read them (when ctrl is null).
+inline void AssertSameContainer(const ctrl_t* const& ctrl_a,
+                                const ctrl_t* const& ctrl_b, const void* slot_a,
+                                const void* slot_b,
+                                const GenerationType* generation_ptr_a,
+                                const GenerationType* generation_ptr_b) {
+  if (!SwisstableDebugEnabled()) return;
+  // `SwisstableDebugEnabled()` is also true for release builds with hardening
+  // enabled. To minimize their impact in those builds:
+  // - use `ABSL_PREDICT_FALSE()` to provide a compiler hint for code layout
+  // - use `ABSL_RAW_LOG()` with a format string to reduce code size and improve
+  //   the chances that the hot paths will be inlined.
+
+  // fail_if(is_invalid, message) crashes when is_invalid is true and provides
+  // an error message based on `message`.
+  const auto fail_if = [](bool is_invalid, const char* message) {
+    if (ABSL_PREDICT_FALSE(is_invalid)) {
+      ABSL_RAW_LOG(FATAL, "Invalid iterator comparison. %s", message);
+    }
+  };
+
+  const bool a_is_default = slot_a == DefaultIterSlot();
+  const bool b_is_default = slot_b == DefaultIterSlot();
+  if (a_is_default && b_is_default) return;
+  fail_if(a_is_default != b_is_default,
+          "Comparing default-constructed hashtable iterator with a "
+          "non-default-constructed hashtable iterator.");
+
+  if (SwisstableGenerationsEnabled()) {
+    if (ABSL_PREDICT_TRUE(generation_ptr_a == generation_ptr_b)) return;
+    const bool a_is_empty = IsEmptyGeneration(generation_ptr_a);
+    const bool b_is_empty = IsEmptyGeneration(generation_ptr_b);
+    fail_if(a_is_empty != b_is_empty,
+            "Comparing an iterator from an empty hashtable with an iterator "
+            "from a non-empty hashtable.");
+    fail_if(a_is_empty && b_is_empty,
+            "Comparing iterators from different empty hashtables.");
+
+    const bool a_is_end = slot_a == nullptr;
+    const bool b_is_end = slot_b == nullptr;
+    fail_if(a_is_end || b_is_end,
+            "Comparing iterator with an end() iterator from a different "
+            "hashtable.");
+    fail_if(true, "Comparing non-end() iterators from different hashtables.");
+  } else {
+    ABSL_HARDENING_ASSERT_SLOW(
+        AreItersFromSameContainer(ctrl_a, ctrl_b, slot_a, slot_b) &&
+        "Invalid iterator comparison. The iterators may be from different "
+        "containers or the container might have rehashed or moved. Consider "
+        "running with --config=asan to diagnose issues.");
+  }
+}
+
+struct FindInfo {
+  size_t offset;
+  size_t probe_length;
+};
+
+struct ProbeCapacity {
+  size_t capacity;
+};
+
+// The state for a probe sequence.
+//
+// Currently, the sequence is a triangular progression of the form
+//
+//   p(i) := Width * (i^2 + i)/2 + hash (mod mask + 1)
+//
+// The use of `Width` ensures that each probe step does not overlap groups;
+// the sequence effectively outputs the addresses of *groups* (although not
+// necessarily aligned to any boundary). The `Group` machinery allows us
+// to check an entire group with minimal branching.
+//
+// Wrapping around at `mask + 1` is important, but not for the obvious reason.
+// As described above, the first few entries of the control byte array
+// are mirrored at the end of the array, which `Group` will find and use
+// for selecting candidates. However, when those candidates' slots are
+// actually inspected, there are no corresponding slots for the cloned bytes,
+// so we need to make sure we've treated those offsets as "wrapping around".
+//
+// It turns out that this probe sequence visits every group exactly once if the
+// number of groups is a power of two, since (i^2+i)/2 is a bijection in
+// Z/(2^m). See https://en.wikipedia.org/wiki/Quadratic_probing
+template <size_t Width>
+class probe_seq {
+ public:
+  // Creates a new probe sequence using `hash` as the initial value of the
+  // sequence and `capacity` as the mask to apply to each value in the
+  // progression.
+  probe_seq(ProbeCapacity capacity, size_t hash)
+      : capacity_(capacity.capacity), offset_(hash & capacity_) {}
+
+  // The offset within the table, i.e., the value `p(i)` above.
+  size_t offset() const { return offset_; }
+  size_t offset(size_t i) const { return (offset_ + i) & capacity_; }
+
+  void next() {
+    index_ += Width;
+    offset_ += index_;
+    offset_ &= capacity_;
+  }
+  // 0-based probe index, a multiple of `Width`.
+  size_t index() const { return index_; }
+
+ private:
+  size_t capacity_;
+  size_t offset_;
+  size_t index_ = 0;
+};
+
+// Begins a probing operation on `common.control`, using `hash`.
+inline probe_seq<Group::kWidth> probe_h1(ProbeCapacity capacity, size_t h1) {
+  return probe_seq<Group::kWidth>(capacity, h1);
+}
+inline probe_seq<Group::kWidth> probe(ProbeCapacity capacity, size_t hash) {
+  return probe_h1(capacity, H1(hash));
+}
+inline probe_seq<Group::kWidth> probe(const CommonFields& common, size_t hash) {
+  return probe(ProbeCapacity{common.capacity()}, hash);
+}
+
+constexpr size_t kProbedElementIndexSentinel = ~size_t{};
+
+// Implementation detail of transfer_unprobed_elements_to_next_capacity_fn.
+// Tries to find the new index for an element whose hash corresponds to
+// `h1` for growth to the next capacity.
+// Returns kProbedElementIndexSentinel if full probing is required.
+//
+// If element is located in the first probing group in the table before growth,
+// returns one of two positions: `old_index` or `old_index + old_capacity + 1`.
+//
+// Otherwise, we will try to insert it into the first probe group of the new
+// table. We only attempt to do so if the first probe group is already
+// initialized.
+template <typename = void>
+inline size_t TryFindNewIndexWithoutProbing(size_t h1, size_t old_index,
+                                            size_t old_capacity,
+                                            ctrl_t* new_ctrl,
+                                            size_t new_capacity) {
+  size_t index_diff = old_index - h1;
+  // The first probe group starts with h1 & capacity.
+  // All following groups start at (h1 + Group::kWidth * K) & capacity.
+  // We can find an index within the floating group as index_diff modulo
+  // Group::kWidth.
+  // Both old and new capacity are larger than Group::kWidth so we can avoid
+  // computing `& capacity`.
+  size_t in_floating_group_index = index_diff & (Group::kWidth - 1);
+  // By subtracting we will get the difference between the first probe group
+  // and the probe group corresponding to old_index.
+  index_diff -= in_floating_group_index;
+  if (ABSL_PREDICT_TRUE((index_diff & old_capacity) == 0)) {
+    size_t new_index = (h1 + in_floating_group_index) & new_capacity;
+    ABSL_ASSUME(new_index != kProbedElementIndexSentinel);
+    return new_index;
+  }
+  ABSL_SWISSTABLE_ASSERT(((old_index - h1) & old_capacity) >= Group::kWidth);
+  // Try to insert element into the first probe group.
+  // new_ctrl is not yet fully initialized so we can't use regular search via
+  // find_first_non_full.
+
+  // We can search in the first probe group only if it is located in already
+  // initialized part of the table.
+  if (ABSL_PREDICT_FALSE((h1 & old_capacity) >= old_index)) {
+    return kProbedElementIndexSentinel;
+  }
+  size_t offset = h1 & new_capacity;
+  Group new_g(new_ctrl + offset);
+  if (auto mask = new_g.MaskNonFull(); ABSL_PREDICT_TRUE(mask)) {
+    size_t result = offset + mask.LowestBitSet();
+    ABSL_ASSUME(result != kProbedElementIndexSentinel);
+    return result;
+  }
+  return kProbedElementIndexSentinel;
+}
+
+// Extern template for inline function keeps possibility of inlining.
+// When compiler decided to not inline, no symbols will be added to the
+// corresponding translation unit.
+extern template size_t TryFindNewIndexWithoutProbing(size_t h1,
+                                                     size_t old_index,
+                                                     size_t old_capacity,
+                                                     ctrl_t* new_ctrl,
+                                                     size_t new_capacity);
+
+// The HashtablezInfoHandle is stored before the control bytes.
+// NOTE: The growth_info is also stored before the backing array, but it doesn't
+// have alignment requirements. For small tables it is 1 byte, for larger tables
+// it is 8 bytes, but we use unaligned load.
+constexpr size_t BackingArrayAlignment(size_t align_of_slot) {
+  return (std::max)(align_of_slot, alignof(HashtablezInfoHandle));
+}
+
+// Iterates over all full slots and calls `cb(const ctrl_t*, void*)`.
+// No insertion to the table is allowed during `cb` call.
+// Erasure is allowed only for the element passed to the callback.
+// The table must not be in SOO mode.
+void IterateOverFullSlots(const CommonFields& c, size_t slot_size,
+                          absl::FunctionRef<void(const ctrl_t*, void*)> cb);
+
+template <typename CharAlloc>
+constexpr bool ShouldSampleHashtablezInfoForAlloc() {
+  // Folks with custom allocators often make unwarranted assumptions about the
+  // behavior of their classes vis-a-vis trivial destructability and what
+  // calls they will or won't make.  Avoid sampling for people with custom
+  // allocators to get us out of this mess.  This is not a hard guarantee but
+  // a workaround while we plan the exact guarantee we want to provide.
+  return std::is_same_v<CharAlloc, std::allocator<char>>;
+}
+
+// Allocates `n` bytes for a backing array.
+template <size_t AlignOfBackingArray, typename Alloc>
+void* AllocateBackingArray(void* alloc, size_t n) {
+  return Allocate<AlignOfBackingArray>(static_cast<Alloc*>(alloc), n);
+}
+
+template <size_t AlignOfBackingArray, typename Alloc>
+void DeallocateBackingArray(void* alloc, size_t capacity, ctrl_t* ctrl,
+                            size_t slot_size, size_t slot_align, bool had_infoz,
+                            size_t blocked_element_count) {
+  RawHashSetLayout layout(capacity, slot_size, slot_align, had_infoz,
+                          blocked_element_count);
+  void* backing_array = ctrl - layout.control_offset();
+  // Unpoison before returning the memory to the allocator.
+  SanitizerUnpoisonMemoryRegion(backing_array, layout.alloc_size());
+  Deallocate<AlignOfBackingArray>(static_cast<Alloc*>(alloc), backing_array,
+                                  layout.alloc_size());
+}
+
+using DeallocBackingArrayFn =
+    decltype(&DeallocateBackingArray<8, std::allocator<char>>);
+
+// PolicyFunctions bundles together some information for a particular
+// raw_hash_set<T, ...> instantiation. This information is passed to
+// type-erased functions that want to do small amounts of type-specific
+// work.
+struct PolicyFunctions {
+  uint32_t key_size;
+  uint32_t value_size;
+  uint32_t slot_size;
+  uint16_t slot_align;
+  bool soo_enabled;
+  bool is_hashtablez_eligible;
+
+  // Returns the pointer to the hash function stored in the set.
+  void* (*hash_fn)(CommonFields& common);
+
+  // Returns the hash of the pointed-to slot.
+  HashSlotFn hash_slot;
+
+  // Transfers the contents of `count` slots from src_slot to dst_slot.
+  // We use ability to transfer several slots in single group table growth.
+  void (*transfer_n)(void* set, void* dst_slot, void* src_slot, size_t count);
+
+  // Returns the pointer to the CharAlloc stored in the set.
+  void* (*get_char_alloc)(CommonFields& common);
+
+  // Allocates n bytes for the backing store for common.
+  void* (*alloc)(void* alloc, size_t n);
+
+  // Deallocates the backing store from common.
+  DeallocBackingArrayFn dealloc;
+
+  // Implementation detail of GrowToNextCapacity.
+  // Iterates over all full slots and transfers unprobed elements.
+  // Initializes the new control bytes except mirrored bytes and kSentinel.
+  // Caller must finish the initialization.
+  // All slots corresponding to the full control bytes are transferred.
+  // Probed elements are reported by `encode_probed_element` callback.
+  // encode_probed_element may overwrite old_ctrl buffer till source_offset.
+  // Different encoding is used depending on the capacity of the table.
+  // See ProbedItem*Bytes classes for details.
+  void (*transfer_unprobed_elements_to_next_capacity)(
+      CommonFields& common, const ctrl_t* old_ctrl, void* old_slots,
+      // TODO(b/382423690): Try to use absl::FunctionRef here.
+      void* probed_storage,
+      void (*encode_probed_element)(void* probed_storage, h2_t h2,
+                                    size_t source_offset, size_t h1));
+
+  uint8_t soo_capacity() const {
+    return static_cast<uint8_t>(soo_enabled ? SooCapacity() : 0);
+  }
+};
+
+// The following functions are used for calculating the max valid size of the
+// table. This is important for security to avoid overflowing size_t when
+// calculating the allocation size of the backing array
+// (https://nvd.nist.gov/vuln/detail/CVE-2025-0838). We also limit the max valid
+// size based on the size of the key_type, and this is an optimization because
+// we ABSL_ASSUME that the size is less than MaxValidSize, which can enable
+// other optimizations for tables with small keys.
+
+template <size_t kSizeOfSizeT = sizeof(size_t)>
+constexpr size_t MaxSizeAtMaxValidCapacity(size_t slot_size) {
+  using SizeT = std::conditional_t<kSizeOfSizeT == 4, uint32_t, uint64_t>;
+  // We shift right by 2 for a safe margin against overflow.
+  constexpr SizeT kMaxValidCapacity = ~SizeT{} >> 2;
+  return CapacityToGrowth(kMaxValidCapacity) / slot_size;
+}
+
+constexpr size_t MaxStorableSize() {
+  return static_cast<size_t>(uint64_t{1}
+                             << HashtableInlineData::kSizeBitCount) -
+         1;
+}
+
+// There are no more than 2^sizeof(key_type) unique key_types (and hashtable
+// keys must be unique) so we can't have a hashtable with more than
+// 2^sizeof(key_type) elements.
+template <size_t kSizeOfSizeT = sizeof(size_t)>
+constexpr size_t MaxValidSizeForKeySize(size_t key_size) {
+  if (key_size < kSizeOfSizeT) return size_t{1} << 8 * key_size;
+  return (std::numeric_limits<size_t>::max)();
+}
+
+template <size_t kSizeOfSizeT = sizeof(size_t)>
+constexpr size_t MaxValidSizeForSlotSize(size_t slot_size) {
+  if constexpr (kSizeOfSizeT == 8) {
+    // For small slot sizes we are limited by HashtableStackData::kSizeBitCount.
+    if (slot_size < size_t{1} << (64 - HashtableInlineData::kSizeBitCount)) {
+      return MaxStorableSize();
+    }
+  }
+  return MaxSizeAtMaxValidCapacity<kSizeOfSizeT>(slot_size);
 }
-// From desired "growth" to a lowerbound of the necessary capacity.
-// Might not be a valid one and required NormalizeCapacity().
-inline size_t GrowthToLowerboundCapacity(size_t growth) {
-  // `growth*8/7`
-  if (Group::kWidth == 8 && growth == 7) {
-    // x+(x-1)/7 does not work when x==7.
-    return 8;
-  }
-  return growth + static_cast<size_t>((static_cast<int64_t>(growth) - 1) / 7);
+
+// Returns the maximum valid size for a table, given the key size and slot size.
+// Template parameter is only used to enable testing.
+template <size_t kSizeOfSizeT = sizeof(size_t)>
+constexpr size_t MaxValidSize(size_t key_size, size_t slot_size) {
+  return (std::min)(MaxValidSizeForKeySize<kSizeOfSizeT>(key_size),
+                    MaxValidSizeForSlotSize<kSizeOfSizeT>(slot_size));
+}
+
+// Returns the index of the SOO slot when growing from SOO to non-SOO in a
+// single group. See also InitializeSmallControlBytesAfterSoo(). It's important
+// to use index 1 so that when resizing from capacity 1 to 3, we can still have
+// random iteration order between the first two inserted elements.
+// I.e. it allows inserting the second element at either index 0 or 2.
+constexpr size_t SooSlotIndex() { return 1; }
+
+// Maximum capacity for the algorithm for small table after SOO.
+// Note that typical size after SOO is 3, but we allow up to 7.
+// Allowing till 16 would require additional store that can be avoided.
+constexpr size_t MaxSmallAfterSooCapacity() { return 7; }
+
+// Type erased version of raw_hash_set::reserve. Requires:
+//   1. `new_size > policy.soo_capacity`.
+//   2. `new_size <= kMaxValidSize`.
+void ReserveTableToFitNewSize(CommonFields& common,
+                              const PolicyFunctions& policy, size_t new_size);
+
+// Type erased version of raw_hash_set::rehash.
+// Requires: `n <= MaxValidCapacity()`.
+void Rehash(CommonFields& common, const PolicyFunctions& policy, size_t n);
+
+// Type erased version of copy constructor.
+void Copy(CommonFields& common, const PolicyFunctions& policy,
+          const CommonFields& other,
+          absl::FunctionRef<void(void*, const void*)> copy_fn);
+
+// Returns the optimal size for memcpy when transferring SOO slot.
+// Otherwise, returns the optimal size for memcpy SOO slot transfer
+// to SooSlotIndex().
+// At the destination we are allowed to copy upto twice more bytes,
+// because there is at least one more slot after SooSlotIndex().
+// The result must not exceed MaxSooSlotSize().
+// Some of the cases are merged to minimize the number of function
+// instantiations.
+constexpr size_t OptimalMemcpySizeForSooSlotTransfer(
+    size_t slot_size, size_t max_soo_slot_size = MaxSooSlotSize()) {
+  static_assert(MaxSooSlotSize() >= 4, "unexpectedly small SOO slot size");
+  static_assert(MaxSooSlotSize() <= 8, "unexpectedly large SOO slot size");
+  if (slot_size == 1) {
+    return 1;
+  }
+  if (slot_size <= 3) {
+    return 4;
+  }
+  if (slot_size == max_soo_slot_size) {
+    return max_soo_slot_size;
+  }
+  // We are merging 4 and 8 into one case because we expect them to be the
+  // hottest cases. Copying 8 bytes is as fast on common architectures.
+  return 8;
 }
 
+// Resizes SOO table to the NextCapacity(SooCapacity()) and prepares insert for
+// the given new_hash. Returns the new slot.
+// All possible template combinations are defined in cc file to improve
+// compilation time.
+template <size_t SooSlotMemcpySize, bool TransferUsesMemcpy>
+void* GrowSooTableToNextCapacityAndPrepareInsert(
+    CommonFields& common, const PolicyFunctions& policy,
+    absl::FunctionRef<size_t(size_t)> get_hash, bool force_sampling);
+
+// PrepareInsert for small tables (is_small()==true).
+// Returns the new slot.
+// Hash is only computed if the table is sampled or grew to large size
+// (is_small()==false).
+void* PrepareInsertSmallNonSoo(CommonFields& common,
+                               const PolicyFunctions& policy,
+                               absl::FunctionRef<size_t(size_t)> get_hash);
+
+// Resizes table with allocated slots and change the table seed.
+// Tables with SOO enabled must have capacity > policy.soo_capacity.
+// No sampling will be performed since table is already allocated.
+void ResizeAllocatedTableWithSeedChange(CommonFields& common,
+                                        const PolicyFunctions& policy,
+                                        size_t new_capacity);
+
+// ClearBackingArray clears the backing array, either modifying it in place,
+// or creating a new one based on the value of "reuse".
+// REQUIRES: c.capacity > MaxSmallCapacity().
+void ClearBackingArray(CommonFields& c, const PolicyFunctions& policy,
+                       void* alloc, bool reuse);
+
+using DestroySlotFn = void (*)(void* set, void* slot);
+
+// Destroys all full slots in the backing array.
+// REQUIRES: !is_small(c.capacity()).
+// REQUIRES: destroy_slot != nullptr.
+void DestroySlots(CommonFields& c, size_t slot_size,
+                  DestroySlotFn destroy_slot);
+
+// Deallocates the backing array and unregister infoz if necessary.
+// REQUIRES: c.capacity > raw_hash_set::DefaultCapacity().
+void DeallocBackingArray(CommonFields& c, size_t slot_size, size_t slot_align,
+                         DeallocBackingArrayFn dealloc, void* alloc);
+
+// Type erased version of raw_hash_set::clear.
+template <bool kSooEnabled>
+void Clear(CommonFields& c, const PolicyFunctions& policy,
+           DestroySlotFn destroy_slot, void* alloc);
+
+// NOTE: Destruct* functions couldn't use PolicyFunctions in order to support
+// incomplete types.
+// TODO(b/515666499): try to use PolicyFunctions since it makes code simpler and
+// binary size smaller.
+
+// Destructs all elements and deallocates the backing array for SOO tables.
+// REQUIRES: !c.is_small || !c.empty()
+// REQUIRES: !c.is_small || destroy_slot != nullptr
+void DestructSoo(CommonFields& c, size_t slot_size, size_t slot_align,
+                 DestroySlotFn destroy_slot, DeallocBackingArrayFn dealloc,
+                 void* alloc);
+
+// Destructs all elements and deallocates the backing array for non-SOO tables.
+// REQUIRES: c.capacity > 0.
+void DestructNonSoo(CommonFields& c, size_t slot_size, size_t slot_align,
+                    DestroySlotFn destroy_slot, DeallocBackingArrayFn dealloc,
+                    void* alloc);
+
+// Type-erased versions of raw_hash_set::erase_meta_only_{small,large}.
+void EraseMetaOnlySmall(CommonFields& c, bool soo_enabled, size_t slot_size);
+void EraseMetaOnlyLarge(CommonFields& c, size_t index, size_t slot_size);
+
+// For trivially relocatable types we use memcpy directly. This allows us to
+// share the same function body for raw_hash_set instantiations that have the
+// same slot size as long as they are relocatable.
+// Separate function for relocating single slot cause significant binary bloat.
+template <size_t SizeOfSlot>
+ABSL_ATTRIBUTE_NOINLINE void TransferNRelocatable(void*, void* dst, void* src,
+                                                  size_t count) {
+  // TODO(b/382423690): Experiment with making specialization for power of 2 and
+  // non power of 2. This would require passing the size of the slot.
+  memcpy(dst, src, SizeOfSlot * count);
+}
+
+// Returns a pointer to `common`. This is used to implement type erased
+// raw_hash_set::get_hash_ref_fn and raw_hash_set::get_alloc_ref_fn for the
+// empty class cases.
+void* GetRefForEmptyClass(CommonFields& common);
+
+// Given the hash of a value not currently in the table and the first group with
+// an empty slot in the probe sequence, finds a viable slot to insert it at.
+//
+// In case there's no space left, the table can be resized or rehashed
+// (for tables with deleted slots, see FindInsertPositionWithGrowthOrRehash).
+//
+// In the case of absence of deleted slots and positive growth_left, the element
+// can be inserted in one of the empty slots in the provided `target_group`.
+//
+// When the table has deleted slots (according to GrowthInfo), the target
+// position will be searched one more time using `find_first_non_full`.
+//
+// REQUIRES: `!common.is_small()`.
+// REQUIRES: At least one non-full slot available.
+// REQUIRES: `mask_empty` is a mask containing empty slots for the
+//           `target_group`.
+// REQUIRES: `target_group` is a starting position for the group that has
+//            at least one empty slot.
+void* PrepareInsertLarge(CommonFields& common, const PolicyFunctions& policy,
+                         size_t hash, Group::NonIterableBitMaskType mask_empty,
+                         FindInfo target_group);
+
+// Same as above, but with generations enabled, we may end up changing the seed,
+// which means we need to be able to recompute the hash.
+void* PrepareInsertLargeGenerationsEnabled(
+    CommonFields& common, const PolicyFunctions& policy, size_t hash,
+    Group::NonIterableBitMaskType mask_empty, FindInfo target_group,
+    absl::FunctionRef<size_t(size_t)> recompute_hash);
+
+template <typename Policy, typename Hash, typename Eq, typename Alloc>
+struct InstantiateRawHashSet {
+  using type = typename ApplyWithoutDefaultSuffix<
+      raw_hash_set,
+      TypeList<void, typename Policy::DefaultHash, typename Policy::DefaultEq,
+               typename Policy::DefaultAlloc>,
+      TypeList<Policy, Hash, Eq, Alloc>>::type;
+};
+
+// A SwissTable.
+//
 // Policy: a policy defines how to perform different operations on
 // the slots of the hashtable (see hash_policy_traits.h for the full interface
 // of policy).
 //
+// Params...: a variadic list of parameters that allows us to omit default
+//            types. This reduces the mangled name of the class and the size of
+//            debug strings like __PRETTY_FUNCTION__. Default types do not give
+//            any new information.
+//
 // Hash: a (possibly polymorphic) functor that hashes keys of the hashtable. The
 // functor should accept a key and return size_t as hash. For best performance
 // it is important that the hash function provides high entropy across all bits
 // of the hash.
+// This is the first element in `Params...` if it exists, or Policy::DefaultHash
+// otherwise.
 //
 // Eq: a (possibly polymorphic) functor that compares two keys for equality. It
 // should accept two (of possibly different type) keys and return a bool: true
 // if they are equal, false if they are not. If two keys compare equal, then
 // their hash values as defined by Hash MUST be equal.
+// This is the second element in `Params...` if it exists, or Policy::DefaultEq
+// otherwise.
 //
-// Allocator: an Allocator [https://devdocs.io/cpp/concept/allocator] with which
+// Allocator: an Allocator
+// [https://en.cppreference.com/w/cpp/named_req/Allocator] with which
 // the storage of the hashtable will be allocated and the elements will be
 // constructed and destroyed.
-template <class Policy, class Hash, class Eq, class Alloc>
+// This is the third element in `Params...` if it exists, or
+// Policy::DefaultAlloc otherwise.
+template <class Policy, class... Params>
 class raw_hash_set {
   using PolicyTraits = hash_policy_traits<Policy>;
+  using Hash = GetFromListOr<typename Policy::DefaultHash, 0, Params...>;
+  using Eq = GetFromListOr<typename Policy::DefaultEq, 1, Params...>;
+  using Alloc = GetFromListOr<typename Policy::DefaultAlloc, 2, Params...>;
   using KeyArgImpl =
       KeyArg<IsTransparent<Eq>::value && IsTransparent<Hash>::value>;
 
+  static_assert(
+      std::is_same_v<
+          typename InstantiateRawHashSet<Policy, Hash, Eq, Alloc>::type,
+          raw_hash_set>,
+      "Redundant template parameters were passed. Use InstantiateRawHashSet<> "
+      "instead");
+
  public:
   using init_type = typename PolicyTraits::init_type;
   using key_type = typename PolicyTraits::key_type;
-  // TODO(sbenza): Hide slot_type as it is an implementation detail. Needs user
-  // code fixes!
-  using slot_type = typename PolicyTraits::slot_type;
   using allocator_type = Alloc;
   using size_type = size_t;
   using difference_type = ptrdiff_t;
@@ -533,11 +2256,12 @@ class raw_hash_set {
   using value_type = typename PolicyTraits::value_type;
   using reference = value_type&;
   using const_reference = const value_type&;
-  using pointer = typename absl::allocator_traits<
+  using pointer = typename std::allocator_traits<
       allocator_type>::template rebind_traits<value_type>::pointer;
-  using const_pointer = typename absl::allocator_traits<
+  using const_pointer = typename std::allocator_traits<
       allocator_type>::template rebind_traits<value_type>::const_pointer;
 
+ private:
   // Alias used for heterogeneous lookup functions.
   // `key_arg<K>` evaluates to `K` when the functors are transparent and to
   // `key_type` otherwise. It permits template argument deduction on `K` for the
@@ -545,85 +2269,152 @@ class raw_hash_set {
   template <class K>
   using key_arg = typename KeyArgImpl::template type<K, key_type>;
 
- private:
+  using slot_type = typename PolicyTraits::slot_type;
+
+  constexpr static bool kIsDefaultHash =
+      std::is_same_v<hasher, absl::Hash<key_type>> ||
+      std::is_same_v<hasher, absl::container_internal::StringHash>;
+  // For non-default hashers it is required to have low bits entropy because
+  // (a) in such cases, the seed is xor'ed with the hash value rather than being
+  // used as a seed for the hash function, (b) the seed has low bits that are
+  // all 0s, and (c) we require random iteration order for small tables.
+  // In ToPublicSeed we shift the seed by kCapacityBitStoredInDataCount as
+  // performance optimization for default hashers. For non-default hashers, we
+  // shift it back.
+  constexpr static size_t kSeedShift =
+      kIsDefaultHash ? 0 : HashtableInlineData::kCapacityBitStoredInDataCount;
+
+  // TODO(b/289225379): we could add extra SOO space inside raw_hash_set
+  // after CommonFields to allow inlining larger slot_types (e.g. std::string),
+  // but it's a bit complicated if we want to support incomplete mapped_type in
+  // flat_hash_map. We could potentially do this for flat_hash_set and for an
+  // allowlist of `mapped_type`s of flat_hash_map that includes e.g. arithmetic
+  // types, strings, cords, and pairs/tuples of allowlisted types.
+  constexpr static bool SooEnabled() {
+    return PolicyTraits::soo_enabled() &&
+           sizeof(slot_type) <= sizeof(HeapOrSoo) &&
+           alignof(slot_type) <= alignof(HeapOrSoo);
+  }
+
+  constexpr static size_t DefaultCapacity() {
+    return SooEnabled() ? SooCapacity() : 0;
+  }
+  constexpr static size_t MaxValidSize() {
+    return container_internal::MaxValidSize(sizeof(key_type),
+                                            sizeof(slot_type));
+  }
+  constexpr static size_t MaxValidCapacity() {
+    return SizeToCapacity(MaxValidSize());
+  }
+
+  // Whether `size` fits in the SOO capacity of this table.
+  bool fits_in_soo(size_t size) const {
+    return SooEnabled() && size <= SooCapacity();
+  }
+  // Whether this table is in SOO mode or non-SOO mode.
+  bool is_soo() const {
+    HashtableCapacity cap = maybe_invalid_capacity();
+    return cap.IsValid() && fits_in_soo(cap.capacity());
+  }
+  bool is_full_soo() const { return is_soo() && !empty(); }
+
+  bool is_small() const { return common().is_small(); }
+
   // Give an early error when key_type is not hashable/eq.
   auto KeyTypeCanBeHashed(const Hash& h, const key_type& k) -> decltype(h(k));
   auto KeyTypeCanBeEq(const Eq& eq, const key_type& k) -> decltype(eq(k, k));
 
-  using Layout = absl::container_internal::Layout<ctrl_t, slot_type>;
-
-  static Layout MakeLayout(size_t capacity) {
-    assert(IsValidCapacity(capacity));
-    return Layout(capacity + Group::kWidth + 1, capacity);
-  }
+  // Try to be helpful when the hasher returns an unreasonable type.
+  using key_hash_result =
+      absl::remove_cvref_t<decltype(std::declval<const Hash&>()(
+          std::declval<const key_type&>()))>;
+  static_assert(sizeof(key_hash_result) >= sizeof(size_t),
+                "`Hash::operator()` should return a `size_t`");
 
-  using AllocTraits = absl::allocator_traits<allocator_type>;
-  using SlotAlloc = typename absl::allocator_traits<
+  using AllocTraits = std::allocator_traits<allocator_type>;
+  using SlotAlloc = typename std::allocator_traits<
       allocator_type>::template rebind_alloc<slot_type>;
-  using SlotAllocTraits = typename absl::allocator_traits<
+  // People are often sloppy with the exact type of their allocator (sometimes
+  // it has an extra const or is missing the pair, but rebinds made it work
+  // anyway).
+  using CharAlloc =
+      typename std::allocator_traits<Alloc>::template rebind_alloc<char>;
+  using SlotAllocTraits = typename std::allocator_traits<
       allocator_type>::template rebind_traits<slot_type>;
 
-  static_assert(std::is_lvalue_reference<reference>::value,
+  static_assert(std::is_lvalue_reference_v<reference>,
                 "Policy::element() must return a reference");
 
-  template <typename T>
-  struct SameAsElementReference
-      : std::is_same<typename std::remove_cv<
-                         typename std::remove_reference<reference>::type>::type,
-                     typename std::remove_cv<
-                         typename std::remove_reference<T>::type>::type> {};
-
   // An enabler for insert(T&&): T must be convertible to init_type or be the
   // same as [cv] value_type [ref].
-  // Note: we separate SameAsElementReference into its own type to avoid using
-  // reference unless we need to. MSVC doesn't seem to like it in some
-  // cases.
   template <class T>
-  using RequiresInsertable = typename std::enable_if<
-      absl::disjunction<std::is_convertible<T, init_type>,
-                        SameAsElementReference<T>>::value,
-      int>::type;
+  using Insertable = std::disjunction<
+      std::is_same<absl::remove_cvref_t<reference>, absl::remove_cvref_t<T>>,
+      std::is_convertible<T, init_type>>;
+  template <class T>
+  using IsNotBitField = std::is_pointer<T*>;
 
   // RequiresNotInit is a workaround for gcc prior to 7.1.
   // See https://godbolt.org/g/Y4xsUh.
   template <class T>
-  using RequiresNotInit =
-      typename std::enable_if<!std::is_same<T, init_type>::value, int>::type;
+  using RequiresNotInit = std::enable_if_t<!std::is_same_v<T, init_type>, int>;
 
   template <class... Ts>
   using IsDecomposable = IsDecomposable<void, PolicyTraits, Hash, Eq, Ts...>;
 
+  template <class T>
+  using IsDecomposableAndInsertable =
+      IsDecomposable<std::enable_if_t<Insertable<T>::value, T>>;
+
+  // Evaluates to true if an assignment from the given type would require the
+  // source object to remain alive for the life of the element.
+  template <class U>
+  using IsLifetimeBoundAssignmentFrom = std::conditional_t<
+      policy_trait_element_is_owner<Policy>::value, std::false_type,
+      type_traits_internal::IsLifetimeBoundAssignment<init_type, U>>;
+
  public:
-  static_assert(std::is_same<pointer, value_type*>::value,
+  static_assert(std::is_same_v<pointer, value_type*>,
                 "Allocators with custom pointer types are not supported");
-  static_assert(std::is_same<const_pointer, const value_type*>::value,
+  static_assert(std::is_same_v<const_pointer, const value_type*>,
                 "Allocators with custom pointer types are not supported");
 
-  class iterator {
+  class iterator : private HashSetIteratorGenerationInfo {
     friend class raw_hash_set;
+    friend struct HashtableFreeFunctionsAccess;
 
    public:
     using iterator_category = std::forward_iterator_tag;
     using value_type = typename raw_hash_set::value_type;
     using reference =
-        absl::conditional_t<PolicyTraits::constant_iterators::value,
+        std::conditional_t<PolicyTraits::constant_iterators::value,
                             const value_type&, value_type&>;
-    using pointer = absl::remove_reference_t<reference>*;
+    using pointer = std::remove_reference_t<reference>*;
     using difference_type = typename raw_hash_set::difference_type;
 
-    iterator() {}
+    // We use DefaultIterSlot() for default-constructed iterators so that
+    // they can be distinguished from end iterators, which have nullptr slot_.
+    iterator() : slot_(static_cast<slot_type*>(DefaultIterSlot())) {}
 
     // PRECONDITION: not an end() iterator.
-    reference operator*() const { return PolicyTraits::element(slot_); }
+    reference operator*() const {
+      assert_is_full("operator*()");
+      return unchecked_deref();
+    }
 
     // PRECONDITION: not an end() iterator.
-    pointer operator->() const { return &operator*(); }
+    pointer operator->() const {
+      assert_is_full("operator->");
+      return &operator*();
+    }
 
     // PRECONDITION: not an end() iterator.
     iterator& operator++() {
+      assert_is_full("operator++");
       ++ctrl_;
       ++slot_;
       skip_empty_or_deleted();
+      if (ABSL_PREDICT_FALSE(*ctrl_ == ctrl_t::kSentinel)) slot_ = nullptr;
       return *this;
     }
     // PRECONDITION: not an end() iterator.
@@ -634,38 +2425,71 @@ class raw_hash_set {
     }
 
     friend bool operator==(const iterator& a, const iterator& b) {
-      return a.ctrl_ == b.ctrl_;
+      AssertIsValidForComparison(a.ctrl_, a.slot_, a.generation(),
+                                 a.generation_ptr());
+      AssertIsValidForComparison(b.ctrl_, b.slot_, b.generation(),
+                                 b.generation_ptr());
+      AssertSameContainer(a.ctrl_, b.ctrl_, a.slot_, b.slot_,
+                          a.generation_ptr(), b.generation_ptr());
+      return a.unchecked_equals(b);
     }
     friend bool operator!=(const iterator& a, const iterator& b) {
       return !(a == b);
     }
 
-   private:
-    iterator(ctrl_t* ctrl) : ctrl_(ctrl) {}  // for end()
-    iterator(ctrl_t* ctrl, slot_type* slot) : ctrl_(ctrl), slot_(slot) {}
+   private:
+    iterator(ctrl_t* ctrl, slot_type* slot,
+             const GenerationType* generation_ptr)
+        : HashSetIteratorGenerationInfo(generation_ptr),
+          ctrl_(ctrl),
+          slot_(slot) {
+      // This assumption helps the compiler know that any non-end iterator is
+      // not equal to any end iterator.
+      ABSL_ASSUME(slot != nullptr);
+    }
+    // For end() iterators.
+    explicit iterator(const GenerationType* generation_ptr)
+        : HashSetIteratorGenerationInfo(generation_ptr), slot_(nullptr) {}
+
+    void assert_is_full(const char* operation) const {
+      AssertIsFull(ctrl_, slot_, generation(), generation_ptr(), operation);
+    }
 
+    // Fixes up `ctrl_` to point to a full or sentinel by advancing `ctrl_` and
+    // `slot_` until they reach one.
     void skip_empty_or_deleted() {
       while (IsEmptyOrDeleted(*ctrl_)) {
-        // ctrl is not necessarily aligned to Group::kWidth. It is also likely
-        // to read past the space for ctrl bytes and into slots. This is ok
-        // because ctrl has sizeof() == 1 and slot has sizeof() >= 1 so there
-        // is no way to read outside the combined slot array.
-        uint32_t shift = Group{ctrl_}.CountLeadingEmptyOrDeleted();
-        ctrl_ += shift;
-        slot_ += shift;
+        ++ctrl_;
+        ++slot_;
       }
     }
 
-    ctrl_t* ctrl_ = nullptr;
-    // To avoid uninitialized member warnigs, put slot_ in an anonymous union.
+    // An equality check which skips ABSL Hardening iterator invalidation
+    // checks.
+    // Should be used when the lifetimes of the iterators are well-enough
+    // understood to prove that they cannot be invalid.
+    bool unchecked_equals(const iterator& b) const { return slot_ == b.slot(); }
+
+    // Dereferences the iterator without ABSL Hardening iterator invalidation
+    // checks.
+    reference unchecked_deref() const { return PolicyTraits::element(slot_); }
+
+    ctrl_t* control() const { return ctrl_; }
+    slot_type* slot() const { return slot_; }
+
+    // To avoid uninitialized member warnings, put ctrl_ in an anonymous union.
     // The member is not initialized on singleton and end iterators.
     union {
-      slot_type* slot_;
+      ctrl_t* ctrl_;
     };
+    slot_type* slot_;
   };
 
   class const_iterator {
     friend class raw_hash_set;
+    template <class Container, typename Enabler>
+    friend struct absl::container_internal::hashtable_debug_internal::
+        HashtableDebugAccess;
 
    public:
     using iterator_category = typename iterator::iterator_category;
@@ -674,9 +2498,9 @@ class raw_hash_set {
     using pointer = typename raw_hash_set::const_pointer;
     using difference_type = typename raw_hash_set::difference_type;
 
-    const_iterator() {}
+    const_iterator() = default;
     // Implicit construction from iterator.
-    const_iterator(iterator i) : inner_(std::move(i)) {}
+    const_iterator(iterator i) : inner_(std::move(i)) {}  // NOLINT
 
     reference operator*() const { return *inner_; }
     pointer operator->() const { return inner_.operator->(); }
@@ -695,8 +2519,15 @@ class raw_hash_set {
     }
 
    private:
-    const_iterator(const ctrl_t* ctrl, const slot_type* slot)
-        : inner_(const_cast<ctrl_t*>(ctrl), const_cast<slot_type*>(slot)) {}
+    const_iterator(const ctrl_t* ctrl, const slot_type* slot,
+                   const GenerationType* gen)
+        : inner_(const_cast<ctrl_t*>(ctrl), const_cast<slot_type*>(slot), gen) {
+    }
+    bool unchecked_equals(const const_iterator& b) const {
+      return inner_.unchecked_equals(b.inner_);
+    }
+    ctrl_t* control() const { return inner_.control(); }
+    slot_type* slot() const { return inner_.slot(); }
 
     iterator inner_;
   };
@@ -704,49 +2535,76 @@ class raw_hash_set {
   using node_type = node_handle<Policy, hash_policy_traits<Policy>, Alloc>;
   using insert_return_type = InsertReturnType<iterator, node_type>;
 
+  // Note: can't use `= default` due to non-default noexcept (causes
+  // problems for some compilers). NOLINTNEXTLINE
   raw_hash_set() noexcept(
-      std::is_nothrow_default_constructible<hasher>::value&&
-          std::is_nothrow_default_constructible<key_equal>::value&&
-              std::is_nothrow_default_constructible<allocator_type>::value) {}
+      std::is_nothrow_default_constructible_v<hasher> &&
+      std::is_nothrow_default_constructible_v<key_equal> &&
+      std::is_nothrow_default_constructible_v<allocator_type>) {}
 
-  explicit raw_hash_set(size_t bucket_count, const hasher& hash = hasher(),
+  explicit raw_hash_set(size_t reservation_size, const hasher& hash = hasher(),
                         const key_equal& eq = key_equal(),
                         const allocator_type& alloc = allocator_type())
-      : ctrl_(EmptyGroup()), settings_(0, hash, eq, alloc) {
-    if (bucket_count) {
-      capacity_ = NormalizeCapacity(bucket_count);
-      reset_growth_left();
-      initialize_slots();
+      : settings_(CommonFields::CreateDefault<SooEnabled()>(), hash, eq,
+                  alloc) {
+    if (reservation_size > DefaultCapacity()) {
+      ReserveTableToFitNewSize(common(), GetPolicyFunctions(),
+                               reservation_size);
     }
   }
 
-  raw_hash_set(size_t bucket_count, const hasher& hash,
+  raw_hash_set(size_t reservation_size, const hasher& hash,
                const allocator_type& alloc)
-      : raw_hash_set(bucket_count, hash, key_equal(), alloc) {}
+      : raw_hash_set(reservation_size, hash, key_equal(), alloc) {}
 
-  raw_hash_set(size_t bucket_count, const allocator_type& alloc)
-      : raw_hash_set(bucket_count, hasher(), key_equal(), alloc) {}
+  raw_hash_set(size_t reservation_size, const allocator_type& alloc)
+      : raw_hash_set(reservation_size, hasher(), key_equal(), alloc) {}
 
   explicit raw_hash_set(const allocator_type& alloc)
       : raw_hash_set(0, hasher(), key_equal(), alloc) {}
 
   template <class InputIter>
-  raw_hash_set(InputIter first, InputIter last, size_t bucket_count = 0,
+  raw_hash_set(InputIter first, InputIter last, size_t reservation_size = 0,
                const hasher& hash = hasher(), const key_equal& eq = key_equal(),
                const allocator_type& alloc = allocator_type())
-      : raw_hash_set(bucket_count, hash, eq, alloc) {
+      : raw_hash_set(
+            SelectReservationSizeForIterRange(first, last, reservation_size),
+            hash, eq, alloc) {
     insert(first, last);
   }
 
   template <class InputIter>
-  raw_hash_set(InputIter first, InputIter last, size_t bucket_count,
+  raw_hash_set(InputIter first, InputIter last, size_t reservation_size,
                const hasher& hash, const allocator_type& alloc)
-      : raw_hash_set(first, last, bucket_count, hash, key_equal(), alloc) {}
+      : raw_hash_set(first, last, reservation_size, hash, key_equal(), alloc) {}
 
   template <class InputIter>
-  raw_hash_set(InputIter first, InputIter last, size_t bucket_count,
+  raw_hash_set(InputIter first, InputIter last, size_t reservation_size,
+               const allocator_type& alloc)
+      : raw_hash_set(first, last, reservation_size, hasher(), key_equal(),
+                     alloc) {}
+
+#if defined(__cpp_lib_containers_ranges) && \
+    __cpp_lib_containers_ranges >= 202202L
+  template <typename R>
+  raw_hash_set(std::from_range_t, R&& rg, size_type reservation_size = 0,
+               const hasher& hash = hasher(), const key_equal& eq = key_equal(),
+               const allocator_type& alloc = allocator_type())
+      : raw_hash_set(std::begin(rg), std::end(rg), reservation_size, hash, eq,
+                     alloc) {}
+
+  template <typename R>
+  raw_hash_set(std::from_range_t, R&& rg, size_type reservation_size,
                const allocator_type& alloc)
-      : raw_hash_set(first, last, bucket_count, hasher(), key_equal(), alloc) {}
+      : raw_hash_set(std::from_range, std::forward<R>(rg), reservation_size,
+                     hasher(), key_equal(), alloc) {}
+
+  template <typename R>
+  raw_hash_set(std::from_range_t, R&& rg, size_type reservation_size,
+               const hasher& hash, const allocator_type& alloc)
+      : raw_hash_set(std::from_range, std::forward<R>(rg), reservation_size,
+                     hash, key_equal(), alloc) {}
+#endif
 
   template <class InputIter>
   raw_hash_set(InputIter first, InputIter last, const allocator_type& alloc)
@@ -773,36 +2631,43 @@ class raw_hash_set {
   //   absl::flat_hash_set<int> a, b{a};
   //
   // RequiresNotInit<T> is a workaround for gcc prior to 7.1.
-  template <class T, RequiresNotInit<T> = 0, RequiresInsertable<T> = 0>
-  raw_hash_set(std::initializer_list<T> init, size_t bucket_count = 0,
+  template <class T, RequiresNotInit<T> = 0,
+            std::enable_if_t<Insertable<T>::value, int> = 0>
+  raw_hash_set(std::initializer_list<T> init, size_t reservation_size = 0,
                const hasher& hash = hasher(), const key_equal& eq = key_equal(),
                const allocator_type& alloc = allocator_type())
-      : raw_hash_set(init.begin(), init.end(), bucket_count, hash, eq, alloc) {}
+      : raw_hash_set(init.begin(), init.end(), reservation_size, hash, eq,
+                     alloc) {}
 
-  raw_hash_set(std::initializer_list<init_type> init, size_t bucket_count = 0,
-               const hasher& hash = hasher(), const key_equal& eq = key_equal(),
+  raw_hash_set(std::initializer_list<init_type> init,
+               size_t reservation_size = 0, const hasher& hash = hasher(),
+               const key_equal& eq = key_equal(),
                const allocator_type& alloc = allocator_type())
-      : raw_hash_set(init.begin(), init.end(), bucket_count, hash, eq, alloc) {}
+      : raw_hash_set(init.begin(), init.end(), reservation_size, hash, eq,
+                     alloc) {}
 
-  template <class T, RequiresNotInit<T> = 0, RequiresInsertable<T> = 0>
-  raw_hash_set(std::initializer_list<T> init, size_t bucket_count,
+  template <class T, RequiresNotInit<T> = 0,
+            std::enable_if_t<Insertable<T>::value, int> = 0>
+  raw_hash_set(std::initializer_list<T> init, size_t reservation_size,
                const hasher& hash, const allocator_type& alloc)
-      : raw_hash_set(init, bucket_count, hash, key_equal(), alloc) {}
+      : raw_hash_set(init, reservation_size, hash, key_equal(), alloc) {}
 
-  raw_hash_set(std::initializer_list<init_type> init, size_t bucket_count,
+  raw_hash_set(std::initializer_list<init_type> init, size_t reservation_size,
                const hasher& hash, const allocator_type& alloc)
-      : raw_hash_set(init, bucket_count, hash, key_equal(), alloc) {}
+      : raw_hash_set(init, reservation_size, hash, key_equal(), alloc) {}
 
-  template <class T, RequiresNotInit<T> = 0, RequiresInsertable<T> = 0>
-  raw_hash_set(std::initializer_list<T> init, size_t bucket_count,
+  template <class T, RequiresNotInit<T> = 0,
+            std::enable_if_t<Insertable<T>::value, int> = 0>
+  raw_hash_set(std::initializer_list<T> init, size_t reservation_size,
                const allocator_type& alloc)
-      : raw_hash_set(init, bucket_count, hasher(), key_equal(), alloc) {}
+      : raw_hash_set(init, reservation_size, hasher(), key_equal(), alloc) {}
 
-  raw_hash_set(std::initializer_list<init_type> init, size_t bucket_count,
+  raw_hash_set(std::initializer_list<init_type> init, size_t reservation_size,
                const allocator_type& alloc)
-      : raw_hash_set(init, bucket_count, hasher(), key_equal(), alloc) {}
+      : raw_hash_set(init, reservation_size, hasher(), key_equal(), alloc) {}
 
-  template <class T, RequiresNotInit<T> = 0, RequiresInsertable<T> = 0>
+  template <class T, RequiresNotInit<T> = 0,
+            std::enable_if_t<Insertable<T>::value, int> = 0>
   raw_hash_set(std::initializer_list<T> init, const allocator_type& alloc)
       : raw_hash_set(init, 0, hasher(), key_equal(), alloc) {}
 
@@ -812,125 +2677,137 @@ class raw_hash_set {
 
   raw_hash_set(const raw_hash_set& that)
       : raw_hash_set(that, AllocTraits::select_on_container_copy_construction(
-                               that.alloc_ref())) {}
+                               allocator_type(that.char_alloc_ref()))) {}
 
   raw_hash_set(const raw_hash_set& that, const allocator_type& a)
       : raw_hash_set(0, that.hash_ref(), that.eq_ref(), a) {
-    reserve(that.size());
-    // Because the table is guaranteed to be empty, we can do something faster
-    // than a full `insert`.
-    for (const auto& v : that) {
-      const size_t hash = PolicyTraits::apply(HashElement{hash_ref()}, v);
-      auto target = find_first_non_full(hash);
-      set_ctrl(target.offset, H2(hash));
-      emplace_at(target.offset, v);
-      infoz_.RecordInsert(hash, target.probe_length);
-    }
-    size_ = that.size();
-    growth_left() -= that.size();
-  }
-
-  raw_hash_set(raw_hash_set&& that) noexcept(
-      std::is_nothrow_copy_constructible<hasher>::value&&
-          std::is_nothrow_copy_constructible<key_equal>::value&&
-              std::is_nothrow_copy_constructible<allocator_type>::value)
-      : ctrl_(absl::exchange(that.ctrl_, EmptyGroup())),
-        slots_(absl::exchange(that.slots_, nullptr)),
-        size_(absl::exchange(that.size_, 0)),
-        capacity_(absl::exchange(that.capacity_, 0)),
-        infoz_(absl::exchange(that.infoz_, HashtablezInfoHandle())),
-        // Hash, equality and allocator are copied instead of moved because
-        // `that` must be left valid. If Hash is std::function<Key>, moving it
-        // would create a nullptr functor that cannot be called.
-        settings_(that.settings_) {
-    // growth_left was copied above, reset the one from `that`.
-    that.growth_left() = 0;
+    that.AssertNotDebugCapacity();
+    if (that.empty()) return;
+    Copy(common(), GetPolicyFunctions(), that.common(),
+         [this](void* dst, const void* src) {
+           // TODO(b/413598253): type erase for trivially copyable types via
+           // PolicyTraits.
+           construct(to_slot(dst),
+                     PolicyTraits::element(
+                         static_cast<slot_type*>(const_cast<void*>(src))));
+         });
+  }
+
+  ABSL_ATTRIBUTE_NOINLINE raw_hash_set(raw_hash_set&& that) noexcept(
+      std::is_nothrow_copy_constructible_v<hasher> &&
+      std::is_nothrow_copy_constructible_v<key_equal> &&
+      std::is_nothrow_copy_constructible_v<allocator_type>)
+      :  // Hash, equality and allocator are copied instead of moved because
+         // `that` must be left valid. If Hash is std::function<Key>, moving it
+         // would create a nullptr functor that cannot be called.
+         // Note: we avoid using exchange for better generated code.
+        settings_(PolicyTraits::transfer_uses_memcpy() || !that.is_full_soo()
+                      ? std::move(that.common())
+                      : CommonFields{full_soo_tag_t{},
+                                     that.common().soo_has_tried_sampling()},
+                  that.hash_ref(), that.eq_ref(), that.char_alloc_ref()) {
+    if (!PolicyTraits::transfer_uses_memcpy() && that.is_full_soo()) {
+      transfer(soo_slot(), that.soo_slot());
+    }
+    that.common() = CommonFields::CreateDefault<SooEnabled()>();
+    annotate_for_bug_detection_on_move(that);
   }
 
   raw_hash_set(raw_hash_set&& that, const allocator_type& a)
-      : ctrl_(EmptyGroup()),
-        slots_(nullptr),
-        size_(0),
-        capacity_(0),
-        settings_(0, that.hash_ref(), that.eq_ref(), a) {
-    if (a == that.alloc_ref()) {
-      std::swap(ctrl_, that.ctrl_);
-      std::swap(slots_, that.slots_);
-      std::swap(size_, that.size_);
-      std::swap(capacity_, that.capacity_);
-      std::swap(growth_left(), that.growth_left());
-      std::swap(infoz_, that.infoz_);
+      : settings_(CommonFields::CreateDefault<SooEnabled()>(), that.hash_ref(),
+                  that.eq_ref(), a) {
+    if (CharAlloc(a) == that.char_alloc_ref()) {
+      swap_common(that);
+      annotate_for_bug_detection_on_move(that);
     } else {
-      reserve(that.size());
-      // Note: this will copy elements of dense_set and unordered_set instead of
-      // moving them. This can be fixed if it ever becomes an issue.
-      for (auto& elem : that) insert(std::move(elem));
+      move_elements_allocs_unequal(std::move(that));
     }
   }
 
   raw_hash_set& operator=(const raw_hash_set& that) {
-    raw_hash_set tmp(that,
-                     AllocTraits::propagate_on_container_copy_assignment::value
-                         ? that.alloc_ref()
-                         : alloc_ref());
-    swap(tmp);
-    return *this;
+    that.AssertNotDebugCapacity();
+    if (ABSL_PREDICT_FALSE(this == &that)) return *this;
+    constexpr bool propagate_alloc =
+        AllocTraits::propagate_on_container_copy_assignment::value;
+    // TODO(ezb): maybe avoid allocating a new backing array if this->capacity()
+    // is an exact match for that.size(). If this->capacity() is too big, then
+    // it would make iteration very slow to reuse the allocation. Maybe we can
+    // do the same heuristic as clear() and reuse if it's small enough.
+    allocator_type alloc(propagate_alloc ? that.char_alloc_ref()
+                                         : char_alloc_ref());
+    raw_hash_set tmp(that, alloc);
+    // NOLINTNEXTLINE: not returning *this for performance.
+    return assign_impl<propagate_alloc>(std::move(tmp));
   }
 
   raw_hash_set& operator=(raw_hash_set&& that) noexcept(
-      absl::allocator_traits<allocator_type>::is_always_equal::value&&
-          std::is_nothrow_move_assignable<hasher>::value&&
-              std::is_nothrow_move_assignable<key_equal>::value) {
+      AllocTraits::is_always_equal::value &&
+      std::is_nothrow_move_assignable_v<hasher> &&
+      std::is_nothrow_move_assignable_v<key_equal>) {
     // TODO(sbenza): We should only use the operations from the noexcept clause
     // to make sure we actually adhere to that contract.
+    // NOLINTNEXTLINE: not returning *this for performance.
     return move_assign(
         std::move(that),
         typename AllocTraits::propagate_on_container_move_assignment());
   }
 
-  ~raw_hash_set() { destroy_slots(); }
+  ~raw_hash_set() {
+    destructor_impl();
+    if constexpr (SwisstableGenerationsOrDebugEnabled()) {
+      common().set_capacity(HashtableCapacity::CreateDestroyed());
+    }
+  }
 
-  iterator begin() {
-    auto it = iterator_at(0);
+  iterator begin() ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    if (ABSL_PREDICT_FALSE(empty())) return end();
+    if (is_small()) return single_iterator();
+    iterator it = {control(), slot_array(capacity()),
+                   common().generation_ptr()};
     it.skip_empty_or_deleted();
+    ABSL_SWISSTABLE_ASSERT(IsFull(*it.control()));
     return it;
   }
-  iterator end() { return {ctrl_ + capacity_}; }
+  iterator end() ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    AssertNotDebugCapacity();
+    return iterator(common().generation_ptr());
+  }
 
-  const_iterator begin() const {
+  const_iterator begin() const ABSL_ATTRIBUTE_LIFETIME_BOUND {
     return const_cast<raw_hash_set*>(this)->begin();
   }
-  const_iterator end() const { return const_cast<raw_hash_set*>(this)->end(); }
-  const_iterator cbegin() const { return begin(); }
-  const_iterator cend() const { return end(); }
+  const_iterator end() const ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return const_cast<raw_hash_set*>(this)->end();
+  }
+  const_iterator cbegin() const ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return begin();
+  }
+  const_iterator cend() const ABSL_ATTRIBUTE_LIFETIME_BOUND { return end(); }
 
   bool empty() const { return !size(); }
-  size_t size() const { return size_; }
-  size_t capacity() const { return capacity_; }
-  size_t max_size() const { return (std::numeric_limits<size_t>::max)(); }
+  size_t size() const {
+    AssertNotDebugCapacity();
+    const size_t size = common().size();
+    [[maybe_unused]] const size_t kMaxValidSize = MaxValidSize();
+    ABSL_ASSUME(size <= kMaxValidSize);
+    return size;
+  }
+  size_t capacity() const {
+    const size_t cap = common().capacity();
+    // Compiler complains when using functions in ASSUME so use local variables.
+    [[maybe_unused]] const bool kIsValid = IsValidCapacity(cap);
+    [[maybe_unused]] const size_t kDefaultCapacity = DefaultCapacity();
+    [[maybe_unused]] const size_t kMaxValidCapacity = MaxValidCapacity();
+    ABSL_ASSUME(kIsValid || cap == 0);
+    ABSL_ASSUME(cap >= kDefaultCapacity);
+    ABSL_ASSUME(cap <= kMaxValidCapacity);
+    return cap;
+  }
+  size_t max_size() const { return MaxValidSize(); }
 
   ABSL_ATTRIBUTE_REINITIALIZES void clear() {
-    // Iterating over this container is O(bucket_count()). When bucket_count()
-    // is much greater than size(), iteration becomes prohibitively expensive.
-    // For clear() it is more important to reuse the allocated array when the
-    // container is small because allocation takes comparatively long time
-    // compared to destruction of the elements of the container. So we pick the
-    // largest bucket_count() threshold for which iteration is still fast and
-    // past that we simply deallocate the array.
-    if (capacity_ > 127) {
-      destroy_slots();
-    } else if (capacity_) {
-      for (size_t i = 0; i != capacity_; ++i) {
-        if (IsFull(ctrl_[i])) {
-          PolicyTraits::destroy(&alloc_ref(), slots_ + i);
-        }
-      }
-      size_ = 0;
-      reset_ctrl();
-      reset_growth_left();
-    }
-    assert(empty());
-    infoz_.RecordStorageChanged(0, capacity_);
+    Clear<SooEnabled()>(common(), GetPolicyFunctions(), get_destroy_slot_fn(),
+                        &char_alloc_ref());
   }
 
   // This overload kicks in when the argument is an rvalue of insertable and
@@ -938,13 +2815,26 @@ class raw_hash_set {
   //
   //   flat_hash_map<std::string, int> m;
   //   m.insert(std::make_pair("abc", 42));
-  template <class T, RequiresInsertable<T> = 0,
-            typename std::enable_if<IsDecomposable<T>::value, int>::type = 0,
-            T* = nullptr>
-  std::pair<iterator, bool> insert(T&& value) {
+  template <class T,
+            int = std::enable_if_t<IsDecomposableAndInsertable<T>::value &&
+                                       IsNotBitField<T>::value &&
+                                       !IsLifetimeBoundAssignmentFrom<T>::value,
+                                   int>()>
+  std::pair<iterator, bool> insert(T&& value) ABSL_ATTRIBUTE_LIFETIME_BOUND {
     return emplace(std::forward<T>(value));
   }
 
+  template <class T, int&...,
+            std::enable_if_t<IsDecomposableAndInsertable<T>::value &&
+                                 IsNotBitField<T>::value &&
+                                 IsLifetimeBoundAssignmentFrom<T>::value,
+                             int> = 0>
+  std::pair<iterator, bool> insert(
+      T&& value ABSL_INTERNAL_ATTRIBUTE_CAPTURED_BY_THIS)
+      ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return this->template insert<T, 0>(std::forward<T>(value));
+  }
+
   // This overload kicks in when the argument is a bitfield or an lvalue of
   // insertable and decomposable type.
   //
@@ -956,61 +2846,94 @@ class raw_hash_set {
   //   const char* p = "hello";
   //   s.insert(p);
   //
-  // TODO(romanp): Once we stop supporting gcc 5.1 and below, replace
-  // RequiresInsertable<T> with RequiresInsertable<const T&>.
-  // We are hitting this bug: https://godbolt.org/g/1Vht4f.
-  template <
-      class T, RequiresInsertable<T> = 0,
-      typename std::enable_if<IsDecomposable<const T&>::value, int>::type = 0>
-  std::pair<iterator, bool> insert(const T& value) {
+  template <class T, int = std::enable_if_t<
+                         IsDecomposableAndInsertable<const T&>::value &&
+                             !IsLifetimeBoundAssignmentFrom<const T&>::value,
+                         int>()>
+  std::pair<iterator, bool> insert(const T& value)
+      ABSL_ATTRIBUTE_LIFETIME_BOUND {
     return emplace(value);
   }
+  template <class T, int&...,
+            std::enable_if_t<IsDecomposableAndInsertable<const T&>::value &&
+                                 IsLifetimeBoundAssignmentFrom<const T&>::value,
+                             int> = 0>
+  std::pair<iterator, bool> insert(
+      const T& value ABSL_INTERNAL_ATTRIBUTE_CAPTURED_BY_THIS)
+      ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return this->template insert<T, 0>(value);
+  }
 
   // This overload kicks in when the argument is an rvalue of init_type. Its
   // purpose is to handle brace-init-list arguments.
   //
   //   flat_hash_map<std::string, int> s;
   //   s.insert({"abc", 42});
-  std::pair<iterator, bool> insert(init_type&& value) {
+  std::pair<iterator, bool> insert(init_type&& value)
+      ABSL_ATTRIBUTE_LIFETIME_BOUND
+#if ABSL_INTERNAL_CPLUSPLUS_LANG >= 202002L
+    requires(!IsLifetimeBoundAssignmentFrom<init_type>::value)
+#endif
+  {
+    return emplace(std::move(value));
+  }
+#if ABSL_INTERNAL_CPLUSPLUS_LANG >= 202002L
+  std::pair<iterator, bool> insert(
+      init_type&& value ABSL_INTERNAL_ATTRIBUTE_CAPTURED_BY_THIS)
+      ABSL_ATTRIBUTE_LIFETIME_BOUND
+    requires(IsLifetimeBoundAssignmentFrom<init_type>::value)
+  {
     return emplace(std::move(value));
   }
+#endif
 
-  template <class T, RequiresInsertable<T> = 0,
-            typename std::enable_if<IsDecomposable<T>::value, int>::type = 0,
-            T* = nullptr>
-  iterator insert(const_iterator, T&& value) {
+  template <class T,
+            int = std::enable_if_t<IsDecomposableAndInsertable<T>::value &&
+                                       IsNotBitField<T>::value &&
+                                       !IsLifetimeBoundAssignmentFrom<T>::value,
+                                   int>()>
+  iterator insert(const_iterator, T&& value) ABSL_ATTRIBUTE_LIFETIME_BOUND {
     return insert(std::forward<T>(value)).first;
   }
+  template <class T, int&...,
+            std::enable_if_t<IsDecomposableAndInsertable<T>::value &&
+                                 IsNotBitField<T>::value &&
+                                 IsLifetimeBoundAssignmentFrom<T>::value,
+                             int> = 0>
+  iterator insert(const_iterator hint,
+                  T&& value ABSL_INTERNAL_ATTRIBUTE_CAPTURED_BY_THIS)
+      ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return this->template insert<T, 0>(hint, std::forward<T>(value));
+  }
 
-  // TODO(romanp): Once we stop supporting gcc 5.1 and below, replace
-  // RequiresInsertable<T> with RequiresInsertable<const T&>.
-  // We are hitting this bug: https://godbolt.org/g/1Vht4f.
-  template <
-      class T, RequiresInsertable<T> = 0,
-      typename std::enable_if<IsDecomposable<const T&>::value, int>::type = 0>
-  iterator insert(const_iterator, const T& value) {
+  template <class T, std::enable_if_t<
+                         IsDecomposableAndInsertable<const T&>::value, int> = 0>
+  iterator insert(const_iterator,
+                  const T& value) ABSL_ATTRIBUTE_LIFETIME_BOUND {
     return insert(value).first;
   }
 
-  iterator insert(const_iterator, init_type&& value) {
+  iterator insert(const_iterator,
+                  init_type&& value) ABSL_ATTRIBUTE_LIFETIME_BOUND {
     return insert(std::move(value)).first;
   }
 
   template <class InputIt>
   void insert(InputIt first, InputIt last) {
-    for (; first != last; ++first) insert(*first);
+    insert_range(first, last);
   }
 
-  template <class T, RequiresNotInit<T> = 0, RequiresInsertable<const T&> = 0>
+  template <class T, RequiresNotInit<T> = 0,
+            std::enable_if_t<Insertable<const T&>::value, int> = 0>
   void insert(std::initializer_list<T> ilist) {
-    insert(ilist.begin(), ilist.end());
+    insert_range(ilist.begin(), ilist.end());
   }
 
   void insert(std::initializer_list<init_type> ilist) {
-    insert(ilist.begin(), ilist.end());
+    insert_range(ilist.begin(), ilist.end());
   }
 
-  insert_return_type insert(node_type&& node) {
+  insert_return_type insert(node_type&& node) ABSL_ATTRIBUTE_LIFETIME_BOUND {
     if (!node) return {end(), false, node_type()};
     const auto& elem = PolicyTraits::element(CommonAccess::GetSlot(node));
     auto res = PolicyTraits::apply(
@@ -1024,8 +2947,11 @@ class raw_hash_set {
     }
   }
 
-  iterator insert(const_iterator, node_type&& node) {
-    return insert(std::move(node)).first;
+  iterator insert(const_iterator,
+                  node_type&& node) ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    auto res = insert(std::move(node));
+    node = std::move(res.node);
+    return res.position;
   }
 
   // This overload kicks in if we can deduce the key from args. This enables us
@@ -1037,9 +2963,10 @@ class raw_hash_set {
   //   flat_hash_map<std::string, std::string> m = {{"abc", "def"}};
   //   // Creates no std::string copies and makes no heap allocations.
   //   m.emplace("abc", "xyz");
-  template <class... Args, typename std::enable_if<
-                               IsDecomposable<Args...>::value, int>::type = 0>
-  std::pair<iterator, bool> emplace(Args&&... args) {
+  template <class... Args,
+            std::enable_if_t<IsDecomposable<Args...>::value, int> = 0>
+  std::pair<iterator, bool> emplace(Args&&... args)
+      ABSL_ATTRIBUTE_LIFETIME_BOUND {
     return PolicyTraits::apply(EmplaceDecomposable{*this},
                                std::forward<Args>(args)...);
   }
@@ -1047,30 +2974,37 @@ class raw_hash_set {
   // This overload kicks in if we cannot deduce the key from args. It constructs
   // value_type unconditionally and then either moves it into the table or
   // destroys.
-  template <class... Args, typename std::enable_if<
-                               !IsDecomposable<Args...>::value, int>::type = 0>
-  std::pair<iterator, bool> emplace(Args&&... args) {
-    typename std::aligned_storage<sizeof(slot_type), alignof(slot_type)>::type
-        raw;
-    slot_type* slot = reinterpret_cast<slot_type*>(&raw);
-
-    PolicyTraits::construct(&alloc_ref(), slot, std::forward<Args>(args)...);
+  template <class... Args,
+            std::enable_if_t<!IsDecomposable<Args...>::value, int> = 0>
+  std::pair<iterator, bool> emplace(Args&&... args)
+      ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    alignas(slot_type) unsigned char raw[sizeof(slot_type)];
+    slot_type* slot = to_slot(&raw);
+
+    construct(slot, std::forward<Args>(args)...);
     const auto& elem = PolicyTraits::element(slot);
     return PolicyTraits::apply(InsertSlot<true>{*this, std::move(*slot)}, elem);
   }
 
   template <class... Args>
-  iterator emplace_hint(const_iterator, Args&&... args) {
+  iterator emplace_hint(const_iterator,
+                        Args&&... args) ABSL_ATTRIBUTE_LIFETIME_BOUND {
     return emplace(std::forward<Args>(args)...).first;
   }
 
   // Extension API: support for lazy emplace.
   //
   // Looks up key in the table. If found, returns the iterator to the element.
-  // Otherwise calls f with one argument of type raw_hash_set::constructor. f
-  // MUST call raw_hash_set::constructor with arguments as if a
-  // raw_hash_set::value_type is constructed, otherwise the behavior is
-  // undefined.
+  // Otherwise calls `f` with one argument of type `raw_hash_set::constructor`,
+  // and returns an iterator to the new element.
+  //
+  // `f` must abide by several restrictions:
+  //  - it MUST call `raw_hash_set::constructor` with arguments as if a
+  //    `raw_hash_set::value_type` is constructed,
+  //  - it MUST NOT access the container before the call to
+  //    `raw_hash_set::constructor`, and
+  //  - it MUST NOT erase the lazily emplaced element.
+  // Doing any of these is undefined behavior.
   //
   // For example:
   //
@@ -1092,7 +3026,7 @@ class raw_hash_set {
    public:
     template <class... Args>
     void operator()(Args&&... args) const {
-      assert(*slot_);
+      ABSL_SWISSTABLE_ASSERT(*slot_);
       PolicyTraits::construct(alloc_, *slot_, std::forward<Args>(args)...);
       *slot_ = nullptr;
     }
@@ -1105,14 +3039,16 @@ class raw_hash_set {
   };
 
   template <class K = key_type, class F>
-  iterator lazy_emplace(const key_arg<K>& key, F&& f) {
+  iterator lazy_emplace(const key_arg<K>& key,
+                        F&& f) ABSL_ATTRIBUTE_LIFETIME_BOUND {
     auto res = find_or_prepare_insert(key);
     if (res.second) {
-      slot_type* slot = slots_ + res.first;
-      std::forward<F>(f)(constructor(&alloc_ref(), &slot));
-      assert(!slot);
+      slot_type* slot = res.first;
+      allocator_type alloc(char_alloc_ref());
+      std::forward<F>(f)(constructor(&alloc, &slot));
+      ABSL_SWISSTABLE_ASSERT(!slot);
     }
-    return iterator_at(res.first);
+    return non_iterable_iterator_at_slot(res.first);
   }
 
   // Extension API: support for heterogeneous keys.
@@ -1132,30 +3068,44 @@ class raw_hash_set {
     return 1;
   }
 
-  // Erases the element pointed to by `it`.  Unlike `std::unordered_set::erase`,
-  // this method returns void to reduce algorithmic complexity to O(1).  The
-  // iterator is invalidated, so any increment should be done before calling
-  // erase.  In order to erase while iterating across a map, use the following
-  // idiom (which also works for standard containers):
-  //
-  // for (auto it = m.begin(), end = m.end(); it != end;) {
-  //   // `erase()` will invalidate `it`, so advance `it` first.
-  //   auto copy_it = it++;
-  //   if (<pred>) {
-  //     m.erase(copy_it);
-  //   }
-  // }
+  // Erases the element pointed to by `it`. Unlike `std::unordered_set::erase`,
+  // this method returns void to reduce algorithmic complexity to O(1). The
+  // iterator is invalidated so any increment should be done before calling
+  // erase (e.g. `erase(it++)`).
   void erase(const_iterator cit) { erase(cit.inner_); }
 
   // This overload is necessary because otherwise erase<K>(const K&) would be
   // a better match if non-const iterator is passed as an argument.
   void erase(iterator it) {
-    assert(it != end());
-    PolicyTraits::destroy(&alloc_ref(), it.slot_);
+    ABSL_SWISSTABLE_ASSERT(capacity() > 0);
+    AssertNotDebugCapacity();
+    it.assert_is_full("erase()");
+    destroy(it.slot());
     erase_meta_only(it);
   }
 
-  iterator erase(const_iterator first, const_iterator last) {
+  // TODO(b/515666499): Type erase entire function or begin/end case.
+  iterator erase(const_iterator first,
+                 const_iterator last) ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    AssertNotDebugCapacity();
+    // We check for empty and for is_small because clear_backing_array requires
+    // that capacity() > MaxSmallCapacity() as a precondition.
+    if (empty()) return end();
+    if (first == last) return last.inner_;
+    if (is_small()) {
+      destroy(single_slot());
+      erase_meta_only_small();
+      return end();
+    }
+    if (first == begin() && last == end()) {
+      // TODO(ezb): we access control bytes in destroy_slots so it could make
+      // sense to combine destroy_slots and clear_backing_array to avoid cache
+      // misses when the table is large. Note that we also do this in clear().
+      destroy_slots();
+      clear_backing_array(/*reuse=*/true);
+      common().set_reserved_growth(common().reservation_size());
+      return end();
+    }
     while (first != last) {
       erase(first++);
     }
@@ -1164,76 +3114,81 @@ class raw_hash_set {
 
   // Moves elements from `src` into `this`.
   // If the element already exists in `this`, it is left unmodified in `src`.
-  template <typename H, typename E>
-  void merge(raw_hash_set<Policy, H, E, Alloc>& src) {  // NOLINT
+  template <
+      typename... Params2,
+      typename = std::enable_if_t<std::is_same_v<
+          Alloc, typename raw_hash_set<Policy, Params2...>::allocator_type>>>
+  void merge(raw_hash_set<Policy, Params2...>& src) {  // NOLINT
+    AssertNotDebugCapacity();
+    src.AssertNotDebugCapacity();
     assert(this != &src);
-    for (auto it = src.begin(), e = src.end(); it != e; ++it) {
-      if (PolicyTraits::apply(InsertSlot<false>{*this, std::move(*it.slot_)},
-                              PolicyTraits::element(it.slot_))
-              .second) {
-        src.erase_meta_only(it);
-      }
+    // Returns whether insertion took place.
+    const auto insert_slot = [this](slot_type* src_slot) {
+      return PolicyTraits::apply(InsertSlot<false>{*this, std::move(*src_slot)},
+                                 PolicyTraits::element(src_slot))
+          .second;
+    };
+
+    if (src.is_small()) {
+      if (src.empty()) return;
+      if (insert_slot(src.single_slot()))
+        src.erase_meta_only_small();
+      return;
+    }
+    for (auto it = src.begin(), e = src.end(); it != e;) {
+      auto next = std::next(it);
+      if (insert_slot(it.slot())) src.erase_meta_only_large(it);
+      it = next;
     }
   }
 
-  template <typename H, typename E>
-  void merge(raw_hash_set<Policy, H, E, Alloc>&& src) {
+  template <
+      typename... Params2,
+      typename = std::enable_if_t<std::is_same_v<
+          Alloc, typename raw_hash_set<Policy, Params2...>::allocator_type>>>
+  void merge(raw_hash_set<Policy, Params2...>&& src) {  // NOLINT
     merge(src);
   }
 
   node_type extract(const_iterator position) {
-    auto node =
-        CommonAccess::Transfer<node_type>(alloc_ref(), position.inner_.slot_);
+    AssertNotDebugCapacity();
+    position.inner_.assert_is_full("extract()");
+    allocator_type alloc(char_alloc_ref());
+    auto node = CommonAccess::Transfer<node_type>(alloc, position.slot());
     erase_meta_only(position);
     return node;
   }
 
-  template <
-      class K = key_type,
-      typename std::enable_if<!std::is_same<K, iterator>::value, int>::type = 0>
+  template <class K = key_type,
+            std::enable_if_t<!std::is_same_v<K, iterator>, int> = 0>
   node_type extract(const key_arg<K>& key) {
     auto it = find(key);
     return it == end() ? node_type() : extract(const_iterator{it});
   }
 
   void swap(raw_hash_set& that) noexcept(
-      IsNoThrowSwappable<hasher>() && IsNoThrowSwappable<key_equal>() &&
-      (!AllocTraits::propagate_on_container_swap::value ||
-       IsNoThrowSwappable<allocator_type>())) {
+      AllocTraits::is_always_equal::value &&
+      std::is_nothrow_swappable_v<hasher> &&
+      std::is_nothrow_swappable_v<key_equal>) {
+    AssertNotDebugCapacity();
+    that.AssertNotDebugCapacity();
     using std::swap;
-    swap(ctrl_, that.ctrl_);
-    swap(slots_, that.slots_);
-    swap(size_, that.size_);
-    swap(capacity_, that.capacity_);
-    swap(growth_left(), that.growth_left());
+    swap_common(that);
     swap(hash_ref(), that.hash_ref());
     swap(eq_ref(), that.eq_ref());
-    swap(infoz_, that.infoz_);
-    if (AllocTraits::propagate_on_container_swap::value) {
-      swap(alloc_ref(), that.alloc_ref());
-    } else {
-      // If the allocators do not compare equal it is officially undefined
-      // behavior. We choose to do nothing.
-    }
+    SwapAlloc(char_alloc_ref(), that.char_alloc_ref(),
+              typename AllocTraits::propagate_on_container_swap{});
   }
 
   void rehash(size_t n) {
-    if (n == 0 && capacity_ == 0) return;
-    if (n == 0 && size_ == 0) {
-      destroy_slots();
-      infoz_.RecordStorageChanged(0, 0);
-      return;
-    }
-    // bitor is a faster way of doing `max` here. We will round up to the next
-    // power-of-2-minus-1, so bitor is good enough.
-    auto m = NormalizeCapacity(n | GrowthToLowerboundCapacity(size()));
-    // n == 0 unconditionally rehashes as per the standard.
-    if (n == 0 || m > capacity_) {
-      resize(m);
-    }
+    Rehash(common(), GetPolicyFunctions(), (std::min)(n, MaxValidCapacity()));
   }
 
-  void reserve(size_t n) { rehash(GrowthToLowerboundCapacity(n)); }
+  void reserve(size_t n) {
+    if (ABSL_PREDICT_TRUE(n > DefaultCapacity())) {
+      ReserveTableToFitNewSize(common(), GetPolicyFunctions(), n);
+    }
+  }
 
   // Extension API: support for heterogeneous keys.
   //
@@ -1255,73 +3210,73 @@ class raw_hash_set {
   // NOTE: This is a very low level operation and should not be used without
   // specific benchmarks indicating its importance.
   template <class K = key_type>
-  void prefetch(const key_arg<K>& key) const {
-    (void)key;
-#if defined(__GNUC__)
-    auto seq = probe(hash_ref()(key));
-    __builtin_prefetch(static_cast<const void*>(ctrl_ + seq.offset()));
-    __builtin_prefetch(static_cast<const void*>(slots_ + seq.offset()));
-#endif  // __GNUC__
+  void prefetch([[maybe_unused]] const key_arg<K>& key) const {
+    if (capacity() == DefaultCapacity()) return;
+    // Avoid probing if we won't be able to prefetch the addresses received.
+#ifdef ABSL_HAVE_PREFETCH
+    prefetch_heap_block();
+    if (is_small()) return;
+    auto seq = probe(common(), hash_of(key));
+    PrefetchToLocalCache(control() + seq.offset());
+    PrefetchToLocalCache(slot_array(capacity()) + seq.offset());
+#endif  // ABSL_HAVE_PREFETCH
   }
 
-  // The API of find() has two extensions.
-  //
-  // 1. The hash can be passed by the user. It must be equal to the hash of the
-  // key.
-  //
-  // 2. The type of the key argument doesn't have to be key_type. This is so
-  // called heterogeneous key support.
   template <class K = key_type>
-  iterator find(const key_arg<K>& key, size_t hash) {
-    auto seq = probe(hash);
-    while (true) {
-      Group g{ctrl_ + seq.offset()};
-      for (int i : g.Match(H2(hash))) {
-        if (ABSL_PREDICT_TRUE(PolicyTraits::apply(
-                EqualElement<K>{key, eq_ref()},
-                PolicyTraits::element(slots_ + seq.offset(i)))))
-          return iterator_at(seq.offset(i));
-      }
-      if (ABSL_PREDICT_TRUE(g.MatchEmpty())) return end();
-      seq.next();
-    }
+  ABSL_DEPRECATE_AND_INLINE()
+  iterator find(const key_arg<K>& key,
+                size_t) ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return find(key);
   }
+  // The API of find() has one extension: the type of the key argument doesn't
+  // have to be key_type. This is so called heterogeneous key support.
   template <class K = key_type>
-  iterator find(const key_arg<K>& key) {
-    return find(key, hash_ref()(key));
+  iterator find(const key_arg<K>& key) ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    AssertOnFind(key);
+    if (is_small()) return find_small(key);
+    prefetch_heap_block();
+    return find_large(key);
   }
 
   template <class K = key_type>
-  const_iterator find(const key_arg<K>& key, size_t hash) const {
-    return const_cast<raw_hash_set*>(this)->find(key, hash);
+  ABSL_DEPRECATE_AND_INLINE()
+  const_iterator find(const key_arg<K>& key,
+                      size_t) const ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return find(key);
   }
   template <class K = key_type>
-  const_iterator find(const key_arg<K>& key) const {
-    return find(key, hash_ref()(key));
+  const_iterator find(const key_arg<K>& key) const
+      ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return const_cast<raw_hash_set*>(this)->find(key);
   }
 
   template <class K = key_type>
   bool contains(const key_arg<K>& key) const {
-    return find(key) != end();
+    // Here neither the iterator returned by `find()` nor `end()` can be invalid
+    // outside of potential thread-safety issues.
+    // `find()`'s return value is constructed, used, and then destructed
+    // all in this context.
+    return !find(key).unchecked_equals(end());
   }
 
   template <class K = key_type>
-  std::pair<iterator, iterator> equal_range(const key_arg<K>& key) {
+  std::pair<iterator, iterator> equal_range(const key_arg<K>& key)
+      ABSL_ATTRIBUTE_LIFETIME_BOUND {
     auto it = find(key);
     if (it != end()) return {it, std::next(it)};
     return {it, it};
   }
   template <class K = key_type>
   std::pair<const_iterator, const_iterator> equal_range(
-      const key_arg<K>& key) const {
+      const key_arg<K>& key) const ABSL_ATTRIBUTE_LIFETIME_BOUND {
     auto it = find(key);
     if (it != end()) return {it, std::next(it)};
     return {it, it};
   }
 
-  size_t bucket_count() const { return capacity_; }
+  size_t bucket_count() const { return capacity(); }
   float load_factor() const {
-    return capacity_ ? static_cast<double>(size()) / capacity_ : 0.0;
+    return capacity() ? static_cast<double>(size()) / capacity() : 0.0;
   }
   float max_load_factor() const { return 1.0f; }
   void max_load_factor(float) {
@@ -1330,15 +3285,28 @@ class raw_hash_set {
 
   hasher hash_function() const { return hash_ref(); }
   key_equal key_eq() const { return eq_ref(); }
-  allocator_type get_allocator() const { return alloc_ref(); }
+  allocator_type get_allocator() const {
+    return allocator_type(char_alloc_ref());
+  }
 
   friend bool operator==(const raw_hash_set& a, const raw_hash_set& b) {
     if (a.size() != b.size()) return false;
     const raw_hash_set* outer = &a;
     const raw_hash_set* inner = &b;
     if (outer->capacity() > inner->capacity()) std::swap(outer, inner);
-    for (const value_type& elem : *outer)
-      if (!inner->has_element(elem)) return false;
+    for (const value_type& elem : *outer) {
+      auto it = PolicyTraits::apply(FindElement{*inner}, elem);
+      if (it == inner->end()) return false;
+      // Note: we used key_equal to check for key equality in FindElement, but
+      // we may need to do an additional comparison using
+      // value_type::operator==. E.g. the keys could be equal and the
+      // mapped_types could be unequal in a map or even in a set, key_equal
+      // could ignore some fields that aren't ignored by operator==.
+      static constexpr bool kKeyEqIsValueEq =
+          std::is_same_v<key_type, value_type> &&
+          std::is_same_v<key_equal, hash_default_eq<key_type>>;
+      if (!kKeyEqIsValueEq && !(*it == elem)) return false;
+    }
     return true;
   }
 
@@ -1346,6 +3314,13 @@ class raw_hash_set {
     return !(a == b);
   }
 
+  template <typename H>
+  friend std::enable_if_t<H::template is_hashable<value_type>::value, H>
+  AbslHashValue(H h, const raw_hash_set& s) {
+    return H::combine(H::combine_unordered(std::move(h), s.begin(), s.end()),
+                      hash_internal::WeaklyMixedInteger{s.size()});
+  }
+
   friend void swap(raw_hash_set& a,
                    raw_hash_set& b) noexcept(noexcept(a.swap(b))) {
     a.swap(b);
@@ -1356,6 +3331,8 @@ class raw_hash_set {
   friend struct absl::container_internal::hashtable_debug_internal::
       HashtableDebugAccess;
 
+  friend struct absl::container_internal::HashtableFreeFunctionsAccess;
+
   struct FindElement {
     template <class K, class... Args>
     const_iterator operator()(const K& key, Args&&...) const {
@@ -1364,24 +3341,6 @@ class raw_hash_set {
     const raw_hash_set& s;
   };
 
-  struct HashElement {
-    template <class K, class... Args>
-    size_t operator()(const K& key, Args&&...) const {
-      return h(key);
-    }
-    const hasher& h;
-  };
-
-  template <class K1>
-  struct EqualElement {
-    template <class K2, class... Args>
-    bool operator()(const K2& lhs, Args&&...) const {
-      return eq(lhs, rhs);
-    }
-    const K1& rhs;
-    const key_equal& eq;
-  };
-
   struct EmplaceDecomposable {
     template <class K, class... Args>
     std::pair<iterator, bool> operator()(const K& key, Args&&... args) const {
@@ -1389,7 +3348,7 @@ class raw_hash_set {
       if (res.second) {
         s.emplace_at(res.first, std::forward<Args>(args)...);
       }
-      return {s.iterator_at(res.first), res.second};
+      return {s.non_iterable_iterator_at_slot(res.first), res.second};
     }
     raw_hash_set& s;
   };
@@ -1400,448 +3359,851 @@ class raw_hash_set {
     std::pair<iterator, bool> operator()(const K& key, Args&&...) && {
       auto res = s.find_or_prepare_insert(key);
       if (res.second) {
-        PolicyTraits::transfer(&s.alloc_ref(), s.slots_ + res.first, &slot);
+        s.transfer(res.first, &slot);
       } else if (do_destroy) {
-        PolicyTraits::destroy(&s.alloc_ref(), &slot);
+        s.destroy(&slot);
       }
-      return {s.iterator_at(res.first), res.second};
+      return {s.non_iterable_iterator_at_slot(res.first), res.second};
     }
     raw_hash_set& s;
     // Constructed slot. Either moved into place or destroyed.
     slot_type&& slot;
   };
 
-  // "erases" the object from the container, except that it doesn't actually
-  // destroy the object. It only updates all the metadata of the class.
-  // This can be used in conjunction with Policy::transfer to move the object to
-  // another place.
-  void erase_meta_only(const_iterator it) {
-    assert(IsFull(*it.inner_.ctrl_) && "erasing a dangling iterator");
-    --size_;
-    const size_t index = it.inner_.ctrl_ - ctrl_;
-    const size_t index_before = (index - Group::kWidth) & capacity_;
-    const auto empty_after = Group(it.inner_.ctrl_).MatchEmpty();
-    const auto empty_before = Group(ctrl_ + index_before).MatchEmpty();
-
-    // We count how many consecutive non empties we have to the right and to the
-    // left of `it`. If the sum is >= kWidth then there is at least one probe
-    // window that might have seen a full group.
-    bool was_never_full =
-        empty_before && empty_after &&
-        static_cast<size_t>(empty_after.TrailingZeros() +
-                            empty_before.LeadingZeros()) < Group::kWidth;
-
-    set_ctrl(index, was_never_full ? kEmpty : kDeleted);
-    growth_left() += was_never_full;
-    infoz_.RecordErase();
-  }
-
-  void initialize_slots() {
-    assert(capacity_);
-    // Folks with custom allocators often make unwarranted assumptions about the
-    // behavior of their classes vis-a-vis trivial destructability and what
-    // calls they will or wont make.  Avoid sampling for people with custom
-    // allocators to get us out of this mess.  This is not a hard guarantee but
-    // a workaround while we plan the exact guarantee we want to provide.
-    //
-    // People are often sloppy with the exact type of their allocator (sometimes
-    // it has an extra const or is missing the pair, but rebinds made it work
-    // anyway).  To avoid the ambiguity, we work off SlotAlloc which we have
-    // bound more carefully.
-    if (std::is_same<SlotAlloc, std::allocator<slot_type>>::value &&
-        slots_ == nullptr) {
-      infoz_ = Sample();
-    }
-
-    auto layout = MakeLayout(capacity_);
-    char* mem = static_cast<char*>(
-        Allocate<Layout::Alignment()>(&alloc_ref(), layout.AllocSize()));
-    ctrl_ = reinterpret_cast<ctrl_t*>(layout.template Pointer<0>(mem));
-    slots_ = layout.template Pointer<1>(mem);
-    reset_ctrl();
-    reset_growth_left();
-    infoz_.RecordStorageChanged(size_, capacity_);
+  template <typename... Args>
+  void construct(slot_type* slot, Args&&... args) {
+    common().RunWithReentrancyGuard([&] {
+      allocator_type alloc(char_alloc_ref());
+      PolicyTraits::construct(&alloc, slot, std::forward<Args>(args)...);
+    });
+  }
+  void destroy(slot_type* slot) {
+    common().RunWithReentrancyGuard([&] {
+      allocator_type alloc(char_alloc_ref());
+      PolicyTraits::destroy(&alloc, slot);
+    });
+  }
+  void transfer(slot_type* to, slot_type* from) {
+    common().RunWithReentrancyGuard([&] {
+      allocator_type alloc(char_alloc_ref());
+      PolicyTraits::transfer(&alloc, to, from);
+    });
   }
 
-  void destroy_slots() {
-    if (!capacity_) return;
-    for (size_t i = 0; i != capacity_; ++i) {
-      if (IsFull(ctrl_[i])) {
-        PolicyTraits::destroy(&alloc_ref(), slots_ + i);
+  // TODO(b/289225379): consider having a helper class that has the impls for
+  // SOO functionality.
+  template <class K = key_type>
+  ABSL_ATTRIBUTE_ALWAYS_INLINE iterator find_small(const key_arg<K>& key) {
+    ABSL_SWISSTABLE_ASSERT(is_small());
+    return empty() || !equal_to(key, single_slot()) ? end() : single_iterator();
+  }
+
+  template <class K = key_type>
+  iterator find_large(const key_arg<K>& key) {
+    ABSL_SWISSTABLE_ASSERT(!is_small());
+    const size_t cap = common().capacity();
+    ABSL_ASSUME(cap > kMaxSmallCapacity);
+    const size_t hash = hash_of(key);
+    auto seq = probe(ProbeCapacity{cap}, hash);
+    const h2_t h2 = H2(hash);
+    ctrl_t* ctrl = control();
+    slot_type* slot_array = to_slot(common().slot_array(cap));
+    while (true) {
+#ifndef ABSL_HAVE_MEMORY_SANITIZER
+      absl::PrefetchToLocalCache(slot_array + seq.offset());
+#endif
+      Group g{ctrl + seq.offset()};
+      for (uint32_t i : g.Match(h2)) {
+        const size_t offset = seq.offset(i);
+        if (ABSL_PREDICT_TRUE(equal_to(key, slot_array + offset)))
+          return iterator_at_ptr(ctrl + offset, slot_array + offset);
       }
+      if (ABSL_PREDICT_TRUE(g.MaskEmpty())) return end();
+      seq.next();
+      ABSL_SWISSTABLE_ASSERT(seq.index() <= cap && "full table!");
     }
-    auto layout = MakeLayout(capacity_);
-    // Unpoison before returning the memory to the allocator.
-    SanitizerUnpoisonMemoryRegion(slots_, sizeof(slot_type) * capacity_);
-    Deallocate<Layout::Alignment()>(&alloc_ref(), ctrl_, layout.AllocSize());
-    ctrl_ = EmptyGroup();
-    slots_ = nullptr;
-    size_ = 0;
-    capacity_ = 0;
-    growth_left() = 0;
-  }
-
-  void resize(size_t new_capacity) {
-    assert(IsValidCapacity(new_capacity));
-    auto* old_ctrl = ctrl_;
-    auto* old_slots = slots_;
-    const size_t old_capacity = capacity_;
-    capacity_ = new_capacity;
-    initialize_slots();
-
-    size_t total_probe_length = 0;
-    for (size_t i = 0; i != old_capacity; ++i) {
-      if (IsFull(old_ctrl[i])) {
-        size_t hash = PolicyTraits::apply(HashElement{hash_ref()},
-                                          PolicyTraits::element(old_slots + i));
-        auto target = find_first_non_full(hash);
-        size_t new_i = target.offset;
-        total_probe_length += target.probe_length;
-        set_ctrl(new_i, H2(hash));
-        PolicyTraits::transfer(&alloc_ref(), slots_ + new_i, old_slots + i);
-      }
+  }
+
+  // Returns true if the table needs to be sampled. This keeps track of whether
+  // sampling has already been evaluated and ensures that it can only return
+  // true on its first evaluation. All subsequent calls will return false.
+  //
+  // This should be called on insertion into an empty SOO table and in copy
+  // construction when the size can fit in SOO capacity.
+  bool should_sample_soo() {
+    ABSL_SWISSTABLE_ASSERT(is_soo());
+    if constexpr (!ShouldSampleHashtablezInfoForAlloc<CharAlloc>()) {
+      return false;
     }
-    if (old_capacity) {
-      SanitizerUnpoisonMemoryRegion(old_slots,
-                                    sizeof(slot_type) * old_capacity);
-      auto layout = MakeLayout(old_capacity);
-      Deallocate<Layout::Alignment()>(&alloc_ref(), old_ctrl,
-                                      layout.AllocSize());
-    }
-    infoz_.RecordRehash(total_probe_length);
-  }
-
-  void drop_deletes_without_resize() ABSL_ATTRIBUTE_NOINLINE {
-    assert(IsValidCapacity(capacity_));
-    assert(!is_small());
-    // Algorithm:
-    // - mark all DELETED slots as EMPTY
-    // - mark all FULL slots as DELETED
-    // - for each slot marked as DELETED
-    //     hash = Hash(element)
-    //     target = find_first_non_full(hash)
-    //     if target is in the same group
-    //       mark slot as FULL
-    //     else if target is EMPTY
-    //       transfer element to target
-    //       mark slot as EMPTY
-    //       mark target as FULL
-    //     else if target is DELETED
-    //       swap current element with target element
-    //       mark target as FULL
-    //       repeat procedure for current slot with moved from element (target)
-    ConvertDeletedToEmptyAndFullToDeleted(ctrl_, capacity_);
-    typename std::aligned_storage<sizeof(slot_type), alignof(slot_type)>::type
-        raw;
-    size_t total_probe_length = 0;
-    slot_type* slot = reinterpret_cast<slot_type*>(&raw);
-    for (size_t i = 0; i != capacity_; ++i) {
-      if (!IsDeleted(ctrl_[i])) continue;
-      size_t hash = PolicyTraits::apply(HashElement{hash_ref()},
-                                        PolicyTraits::element(slots_ + i));
-      auto target = find_first_non_full(hash);
-      size_t new_i = target.offset;
-      total_probe_length += target.probe_length;
-
-      // Verify if the old and new i fall within the same group wrt the hash.
-      // If they do, we don't need to move the object as it falls already in the
-      // best probe we can.
-      const auto probe_index = [&](size_t pos) {
-        return ((pos - probe(hash).offset()) & capacity_) / Group::kWidth;
-      };
-
-      // Element doesn't move.
-      if (ABSL_PREDICT_TRUE(probe_index(new_i) == probe_index(i))) {
-        set_ctrl(i, H2(hash));
-        continue;
-      }
-      if (IsEmpty(ctrl_[new_i])) {
-        // Transfer element to the empty spot.
-        // set_ctrl poisons/unpoisons the slots so we have to call it at the
-        // right time.
-        set_ctrl(new_i, H2(hash));
-        PolicyTraits::transfer(&alloc_ref(), slots_ + new_i, slots_ + i);
-        set_ctrl(i, kEmpty);
-      } else {
-        assert(IsDeleted(ctrl_[new_i]));
-        set_ctrl(new_i, H2(hash));
-        // Until we are done rehashing, DELETED marks previously FULL slots.
-        // Swap i and new_i elements.
-        PolicyTraits::transfer(&alloc_ref(), slot, slots_ + i);
-        PolicyTraits::transfer(&alloc_ref(), slots_ + i, slots_ + new_i);
-        PolicyTraits::transfer(&alloc_ref(), slots_ + new_i, slot);
-        --i;  // repeat
+    if (common().soo_has_tried_sampling()) {
+      // Already evaluated sampling on this SOO table; do not re-evaluate
+      // sampling each time it transitions from empty to full SOO state.
+      return false;
+    }
+    // TODO: b/396049910 -- consider managing this flag on the 1->0 size
+    // transition of SOO tables rather than the 0->1 transition.
+    common().set_soo_has_tried_sampling();
+    return ABSL_PREDICT_FALSE(ShouldSampleNextTable());
+  }
+
+  void clear_backing_array(bool reuse) {
+    ABSL_SWISSTABLE_ASSERT(capacity() > kMaxSmallCapacity);
+    ClearBackingArray(common(), GetPolicyFunctions(), &char_alloc_ref(), reuse);
+  }
+
+  void destroy_slots() {
+    ABSL_SWISSTABLE_ASSERT(!is_small());
+    if (PolicyTraits::template destroy_is_trivial<Alloc>()) return;
+    DestroySlots(common(), sizeof(slot_type), get_destroy_slot_fn());
+  }
+
+  void dealloc() {
+    ABSL_SWISSTABLE_ASSERT(capacity() > DefaultCapacity());
+    DeallocBackingArray(common(), sizeof(slot_type), alignof(slot_type),
+                        get_dealloc_backing_array_fn(), &char_alloc_ref());
+  }
+
+  void destructor_impl() {
+    if (SwisstableGenerationsEnabled() &&
+        maybe_invalid_capacity().IsMovedFrom()) {
+      return;
+    }
+    if constexpr (SooEnabled()) {
+      if (is_small() &&
+          (PolicyTraits::template destroy_is_trivial<Alloc>() || empty())) {
+        return;
       }
+      DestructSoo(common(), sizeof(slot_type), alignof(slot_type),
+                  get_destroy_slot_fn(), get_dealloc_backing_array_fn(),
+                  &char_alloc_ref());
+    } else {
+      if (capacity() == 0) return;
+      DestructNonSoo(common(), sizeof(slot_type), alignof(slot_type),
+                     get_destroy_slot_fn(), get_dealloc_backing_array_fn(),
+                     &char_alloc_ref());
+    }
+  }
+
+  // Erases, but does not destroy, the value pointed to by `it`.
+  //
+  // This merely updates the pertinent control byte. This can be used in
+  // conjunction with Policy::transfer to move the object to another place.
+  void erase_meta_only(const_iterator it) {
+    if (is_small()) {
+      erase_meta_only_small();
+      return;
     }
-    reset_growth_left();
-    infoz_.RecordRehash(total_probe_length);
+    erase_meta_only_large(it);
+  }
+  void erase_meta_only_small() {
+    EraseMetaOnlySmall(common(), SooEnabled(), sizeof(slot_type));
+  }
+  void erase_meta_only_large(const_iterator it) {
+    EraseMetaOnlyLarge(common(),
+                       // `it` can be non-iterable iterator, so we can't use
+                       // it.control().
+                       static_cast<size_t>(it.slot() - slot_array(capacity())),
+                       sizeof(slot_type));
+  }
+
+  template <class K>
+  ABSL_ATTRIBUTE_ALWAYS_INLINE bool equal_to(const K& key,
+                                             slot_type* slot) const {
+    return PolicyTraits::apply(EqualElement<K, key_equal>{key, eq_ref()},
+                               PolicyTraits::element(slot));
+  }
+  template <class K>
+  ABSL_ATTRIBUTE_ALWAYS_INLINE size_t hash_of(const K& key) const {
+    return HashElement<hasher, kIsDefaultHash, kSeedShift>{
+        hash_ref(), common().seed().seed()}(key);
+  }
+  ABSL_ATTRIBUTE_ALWAYS_INLINE size_t hash_of(slot_type* slot) const {
+    return PolicyTraits::apply(
+        HashElement<hasher, kIsDefaultHash, kSeedShift>{hash_ref(),
+                                                        common().seed().seed()},
+        PolicyTraits::element(slot));
   }
 
-  void rehash_and_grow_if_necessary() {
-    if (capacity_ == 0) {
-      resize(1);
-    } else if (size() <= CapacityToGrowth(capacity()) / 2) {
-      // Squash DELETED without growing if there is enough capacity.
-      drop_deletes_without_resize();
+  // Casting directly from e.g. char* to slot_type* can cause compilation errors
+  // on objective-C. This function converts to void* first, avoiding the issue.
+  static ABSL_ATTRIBUTE_ALWAYS_INLINE slot_type* to_slot(void* buf) {
+    return static_cast<slot_type*>(buf);
+  }
+
+  // Requires that lhs does not have a full SOO slot.
+  static void move_common(bool rhs_is_full_soo, CharAlloc& rhs_alloc,
+                          CommonFields& lhs, CommonFields&& rhs) {
+    if (PolicyTraits::transfer_uses_memcpy() || !rhs_is_full_soo) {
+      lhs = std::move(rhs);
     } else {
-      // Otherwise grow the container.
-      resize(capacity_ * 2 + 1);
+      lhs.move_non_heap_or_soo_fields(rhs);
+      rhs.RunWithReentrancyGuard([&] {
+        lhs.RunWithReentrancyGuard([&] {
+          PolicyTraits::transfer(&rhs_alloc, to_slot(lhs.soo_data()),
+                                 to_slot(rhs.soo_data()));
+        });
+      });
     }
   }
 
-  bool has_element(const value_type& elem) const {
-    size_t hash = PolicyTraits::apply(HashElement{hash_ref()}, elem);
-    auto seq = probe(hash);
-    while (true) {
-      Group g{ctrl_ + seq.offset()};
-      for (int i : g.Match(H2(hash))) {
-        if (ABSL_PREDICT_TRUE(PolicyTraits::element(slots_ + seq.offset(i)) ==
-                              elem))
-          return true;
-      }
-      if (ABSL_PREDICT_TRUE(g.MatchEmpty())) return false;
-      seq.next();
-      assert(seq.index() < capacity_ && "full table!");
+  // Swaps common fields making sure to avoid memcpy'ing a full SOO slot if we
+  // aren't allowed to do so.
+  void swap_common(raw_hash_set& that) {
+    using std::swap;
+    if (PolicyTraits::transfer_uses_memcpy()) {
+      swap(common(), that.common());
+      return;
     }
-    return false;
+    CommonFields tmp = CommonFields(uninitialized_tag_t{});
+    const bool that_is_full_soo = that.is_full_soo();
+    move_common(that_is_full_soo, that.char_alloc_ref(), tmp,
+                std::move(that.common()));
+    move_common(is_full_soo(), char_alloc_ref(), that.common(),
+                std::move(common()));
+    move_common(that_is_full_soo, that.char_alloc_ref(), common(),
+                std::move(tmp));
   }
 
-  // Probes the raw_hash_set with the probe sequence for hash and returns the
-  // pointer to the first empty or deleted slot.
-  // NOTE: this function must work with tables having both kEmpty and kDelete
-  // in one group. Such tables appears during drop_deletes_without_resize.
-  //
-  // This function is very useful when insertions happen and:
-  // - the input is already a set
-  // - there are enough slots
-  // - the element with the hash is not in the table
-  struct FindInfo {
-    size_t offset;
-    size_t probe_length;
-  };
-  FindInfo find_first_non_full(size_t hash) {
-    auto seq = probe(hash);
-    while (true) {
-      Group g{ctrl_ + seq.offset()};
-      auto mask = g.MatchEmptyOrDeleted();
-      if (mask) {
-#if !defined(NDEBUG)
-        // We want to add entropy even when ASLR is not enabled.
-        // In debug build we will randomly insert in either the front or back of
-        // the group.
-        // TODO(kfm,sbenza): revisit after we do unconditional mixing
-        if (!is_small() && ShouldInsertBackwards(hash, ctrl_)) {
-          return {seq.offset(mask.HighestBitSet()), seq.index()};
-        }
-#endif
-        return {seq.offset(mask.LowestBitSet()), seq.index()};
-      }
-      assert(seq.index() < capacity_ && "full table!");
-      seq.next();
+  void annotate_for_bug_detection_on_move([[maybe_unused]] raw_hash_set& that) {
+    // We only enable moved-from validation when generations are enabled (rather
+    // than using NDEBUG) to avoid issues in which NDEBUG is enabled in some
+    // translation units but not in others.
+    if (SwisstableGenerationsEnabled()) {
+      that.common().set_capacity(this == &that
+                                     ? HashtableCapacity::CreateSelfMovedFrom()
+                                     : HashtableCapacity::CreateMovedFrom());
+    }
+    if (!SwisstableGenerationsEnabled() ||
+        !maybe_invalid_capacity().IsValid() ||
+        capacity() == DefaultCapacity()) {
+      return;
+    }
+    common().increment_generation();
+    if (!empty() && common().should_rehash_for_bug_detection_on_move()) {
+      ResizeAllocatedTableWithSeedChange(common(), GetPolicyFunctions(),
+                                         capacity());
     }
   }
 
-  // TODO(alkis): Optimize this assuming *this and that don't overlap.
-  raw_hash_set& move_assign(raw_hash_set&& that, std::true_type) {
-    raw_hash_set tmp(std::move(that));
-    swap(tmp);
+  template <bool propagate_alloc>
+  raw_hash_set& assign_impl(raw_hash_set&& that) {
+    // We don't bother checking for this/that aliasing. We just need to avoid
+    // breaking the invariants in that case.
+    destructor_impl();
+    move_common(that.is_full_soo(), that.char_alloc_ref(), common(),
+                std::move(that.common()));
+    hash_ref() = that.hash_ref();
+    eq_ref() = that.eq_ref();
+    CopyAlloc(char_alloc_ref(), that.char_alloc_ref(),
+              std::bool_constant<propagate_alloc>());
+    that.common() = CommonFields::CreateDefault<SooEnabled()>();
+    annotate_for_bug_detection_on_move(that);
     return *this;
   }
-  raw_hash_set& move_assign(raw_hash_set&& that, std::false_type) {
-    raw_hash_set tmp(std::move(that), alloc_ref());
-    swap(tmp);
+
+  raw_hash_set& move_elements_allocs_unequal(raw_hash_set&& that) {
+    const size_t size = that.size();
+    if (size == 0) return *this;
+    reserve(size);
+    for (iterator it = that.begin(); it != that.end(); ++it) {
+      insert(std::move(PolicyTraits::element(it.slot())));
+      that.destroy(it.slot());
+    }
+    if (!that.is_soo()) that.dealloc();
+    that.common() = CommonFields::CreateDefault<SooEnabled()>();
+    annotate_for_bug_detection_on_move(that);
     return *this;
   }
 
- protected:
+  raw_hash_set& move_assign(raw_hash_set&& that,
+                            std::true_type /*propagate_alloc*/) {
+    return assign_impl<true>(std::move(that));
+  }
+  raw_hash_set& move_assign(raw_hash_set&& that,
+                            std::false_type /*propagate_alloc*/) {
+    if (char_alloc_ref() == that.char_alloc_ref()) {
+      return assign_impl<false>(std::move(that));
+    }
+    // Aliasing can't happen here because allocs would compare equal above.
+    assert(this != &that);
+    destructor_impl();
+    // We can't take over that's memory so we need to move each element.
+    // While moving elements, this should have that's hash/eq so copy hash/eq
+    // before moving elements.
+    hash_ref() = that.hash_ref();
+    eq_ref() = that.eq_ref();
+    return move_elements_allocs_unequal(std::move(that));
+  }
+
+  template <class K>
+  ABSL_ATTRIBUTE_ALWAYS_INLINE std::pair<slot_type*, bool>
+  find_or_prepare_insert_soo(const K& key) {
+    ABSL_SWISSTABLE_ASSERT(is_soo());
+    bool force_sampling;
+    slot_type* slot = single_slot();
+    if (empty()) {
+      if (!should_sample_soo()) {
+        common().set_full_soo();
+        return {slot, true};
+      }
+      force_sampling = true;
+    } else if (equal_to(key, slot)) {
+      return {slot, false};
+    } else {
+      force_sampling = false;
+    }
+    ABSL_SWISSTABLE_ASSERT(capacity() == 1);
+    constexpr bool kUseMemcpy =
+        PolicyTraits::transfer_uses_memcpy() && SooEnabled();
+    slot = to_slot(
+        GrowSooTableToNextCapacityAndPrepareInsert<
+            kUseMemcpy ? OptimalMemcpySizeForSooSlotTransfer(sizeof(slot_type))
+                       : 0,
+            kUseMemcpy>(
+            common(), GetPolicyFunctions(),
+            HashKey<hasher, K, kIsDefaultHash, kSeedShift>{hash_ref(), key},
+            force_sampling));
+    return {slot, true};
+  }
+
+  template <class K>
+  ABSL_ATTRIBUTE_ALWAYS_INLINE std::pair<slot_type*, bool>
+  find_or_prepare_insert_small(const K& key) {
+    ABSL_SWISSTABLE_ASSERT(is_small());
+    if constexpr (SooEnabled()) {
+      return find_or_prepare_insert_soo(key);
+    }
+    if (!empty()) {
+      if (equal_to(key, single_slot())) {
+        return {single_slot(), false};
+      }
+    }
+    return {
+        to_slot(PrepareInsertSmallNonSoo(
+            common(), GetPolicyFunctions(),
+            HashKey<hasher, K, kIsDefaultHash, kSeedShift>{hash_ref(), key})),
+        true};
+  }
+
   template <class K>
-  std::pair<size_t, bool> find_or_prepare_insert(const K& key) {
-    auto hash = hash_ref()(key);
-    auto seq = probe(hash);
+  std::pair<slot_type*, bool> find_or_prepare_insert_large(const K& key) {
+    ABSL_SWISSTABLE_ASSERT(!is_soo());
+    prefetch_heap_block();
+    const size_t cap = capacity();
+    ABSL_ASSUME(cap > kMaxSmallCapacity);
+    const size_t hash = hash_of(key);
+    auto seq = probe(ProbeCapacity{cap}, hash);
+    const h2_t h2 = H2(hash);
+    const ctrl_t* ctrl = control();
+    slot_type* slot_array = to_slot(common().slot_array(cap));
     while (true) {
-      Group g{ctrl_ + seq.offset()};
-      for (int i : g.Match(H2(hash))) {
-        if (ABSL_PREDICT_TRUE(PolicyTraits::apply(
-                EqualElement<K>{key, eq_ref()},
-                PolicyTraits::element(slots_ + seq.offset(i)))))
-          return {seq.offset(i), false};
+#ifndef ABSL_HAVE_MEMORY_SANITIZER
+      absl::PrefetchToLocalCache(slot_array + seq.offset());
+#endif
+      Group g{ctrl + seq.offset()};
+      for (uint32_t i : g.Match(h2)) {
+        slot_type* slot = slot_array + seq.offset(i);
+        if (ABSL_PREDICT_TRUE(equal_to(key, slot))) {
+          return {slot, false};
+        }
+      }
+      auto mask_empty = g.MaskEmpty();
+      if (ABSL_PREDICT_TRUE(mask_empty)) {
+        size_t target_group_offset = seq.offset();
+        void* slot = SwisstableGenerationsEnabled()
+                         ? PrepareInsertLargeGenerationsEnabled(
+                               common(), GetPolicyFunctions(), hash, mask_empty,
+                               FindInfo{target_group_offset, seq.index()},
+                               HashKey<hasher, K, kIsDefaultHash, kSeedShift>{
+                                   hash_ref(), key})
+                         : PrepareInsertLarge(
+                               common(), GetPolicyFunctions(), hash, mask_empty,
+                               FindInfo{target_group_offset, seq.index()});
+        return {to_slot(slot), true};
       }
-      if (ABSL_PREDICT_TRUE(g.MatchEmpty())) break;
       seq.next();
+      ABSL_SWISSTABLE_ASSERT(seq.index() <= capacity() && "full table!");
     }
-    return {prepare_insert(hash), true};
   }
 
-  size_t prepare_insert(size_t hash) ABSL_ATTRIBUTE_NOINLINE {
-    auto target = find_first_non_full(hash);
-    if (ABSL_PREDICT_FALSE(growth_left() == 0 &&
-                           !IsDeleted(ctrl_[target.offset]))) {
-      rehash_and_grow_if_necessary();
-      target = find_first_non_full(hash);
+  template <class InputIt>
+  void insert_range(InputIt first, InputIt last) {
+    for (; first != last; ++first) emplace(*first);
+  }
+
+ protected:
+  // Asserts for correctness that we run on find/find_or_prepare_insert.
+  template <class K>
+  void AssertOnFind([[maybe_unused]] const K& key) {
+    AssertHashEqConsistent(key);
+    AssertNotDebugCapacity();
+  }
+
+  // Asserts that the capacity is not a sentinel invalid value.
+  void AssertNotDebugCapacity() const { common().AssertNotDebugCapacity(); }
+
+  // Asserts that hash and equal functors provided by the user are consistent,
+  // meaning that `eq(k1, k2)` implies `hash(k1)==hash(k2)`.
+  template <class K>
+  void AssertHashEqConsistent(const K& key) {
+#ifdef NDEBUG
+    return;
+#endif
+    // If the hash/eq functors are known to be consistent, then skip validation.
+    if (std::is_same_v<hasher, absl::container_internal::StringHash> &&
+        std::is_same_v<key_equal, absl::container_internal::StringEq>) {
+      return;
+    }
+    if (std::is_scalar_v<key_type> &&
+        std::is_same_v<hasher, absl::Hash<key_type>> &&
+        std::is_same_v<key_equal, std::equal_to<key_type>>) {
+      return;
+    }
+    if (empty()) return;
+
+    const size_t hash_of_arg = hash_of(key);
+    const auto assert_consistent = [&](const ctrl_t*, void* slot) {
+      const bool is_key_equal = equal_to(key, to_slot(slot));
+      if (!is_key_equal) return;
+
+      [[maybe_unused]] const bool is_hash_equal =
+          hash_of_arg == hash_of(to_slot(slot));
+      assert((!is_key_equal || is_hash_equal) &&
+             "eq(k1, k2) must imply that hash(k1) == hash(k2). "
+             "hash/eq functors are inconsistent.");
+    };
+
+    if (is_small()) {
+      assert_consistent(/*unused*/ nullptr, single_slot());
+      return;
     }
-    ++size_;
-    growth_left() -= IsEmpty(ctrl_[target.offset]);
-    set_ctrl(target.offset, H2(hash));
-    infoz_.RecordInsert(hash, target.probe_length);
-    return target.offset;
+    // We only do validation for small tables so that it's constant time.
+    if (capacity() > 16) return;
+    IterateOverFullSlots(common(), sizeof(slot_type), assert_consistent);
+  }
+
+  // Attempts to find `key` in the table; if it isn't found, returns an iterator
+  // where the value can be inserted into, with the control byte already set to
+  // `key`'s H2. Returns a bool indicating whether an insertion can take place.
+  template <class K>
+  std::pair<slot_type*, bool> find_or_prepare_insert(const K& key) {
+    AssertOnFind(key);
+    return is_small() ? find_or_prepare_insert_small(key)
+                      : find_or_prepare_insert_large(key);
   }
 
   // Constructs the value in the space pointed by the iterator. This only works
   // after an unsuccessful find_or_prepare_insert() and before any other
   // modifications happen in the raw_hash_set.
   //
-  // PRECONDITION: i is an index returned from find_or_prepare_insert(k), where
-  // k is the key decomposed from `forward<Args>(args)...`, and the bool
-  // returned by find_or_prepare_insert(k) was true.
+  // PRECONDITION: iter was returned from find_or_prepare_insert(k), where k is
+  // the key decomposed from `forward<Args>(args)...`, and the bool returned by
+  // find_or_prepare_insert(k) was true.
   // POSTCONDITION: *m.iterator_at(i) == value_type(forward<Args>(args)...).
   template <class... Args>
-  void emplace_at(size_t i, Args&&... args) {
-    PolicyTraits::construct(&alloc_ref(), slots_ + i,
-                            std::forward<Args>(args)...);
-
-    assert(PolicyTraits::apply(FindElement{*this}, *iterator_at(i)) ==
-               iterator_at(i) &&
+  void emplace_at(slot_type* slot, Args&&... args) {
+    construct(slot, std::forward<Args>(args)...);
+
+    // When is_small, find calls find_small and if size is 0, then it will
+    // return an end iterator. This can happen in the raw_hash_set copy ctor.
+    assert((is_small() ||
+            PolicyTraits::apply(FindElement{*this}, PolicyTraits::element(slot))
+                    .slot() == slot) &&
            "constructed value does not match the lookup key");
   }
 
-  iterator iterator_at(size_t i) { return {ctrl_ + i, slots_ + i}; }
-  const_iterator iterator_at(size_t i) const { return {ctrl_ + i, slots_ + i}; }
+  // Special iterator that can be returned by insert/emplace functions.
+  // It is non-iterable, meaning that std::next(it) always points to end().
+  iterator non_iterable_iterator_at_slot(slot_type* slot)
+      ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return {InsertIteratorControl(), slot, common().generation_ptr()};
+  }
+  iterator iterator_at(size_t i) ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return {control() + i, slot_array() + i, common().generation_ptr()};
+  }
+  const_iterator iterator_at(size_t i) const ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return const_cast<raw_hash_set*>(this)->iterator_at(i);
+  }
+  iterator iterator_at_ptr(ctrl_t* ctrl, void* slot)
+      ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return {ctrl, to_slot(slot), common().generation_ptr()};
+  }
+
+  reference unchecked_deref(iterator it) { return it.unchecked_deref(); }
 
  private:
   friend struct RawHashSetTestOnlyAccess;
 
-  probe_seq<Group::kWidth> probe(size_t hash) const {
-    return probe_seq<Group::kWidth>(H1(hash, ctrl_), capacity_);
+  GrowthInfoAccessor growth_info() const { return common().growth_info(); }
+
+  // Prefetch the heap-allocated memory region to resolve potential TLB and
+  // cache misses. This is intended to overlap with execution of calculating the
+  // hash for a key.
+  void prefetch_heap_block() const {
+    ABSL_SWISSTABLE_ASSERT(!is_soo());
+#if ABSL_HAVE_BUILTIN(__builtin_prefetch) || defined(__GNUC__)
+    __builtin_prefetch(control(), 0, 1);
+#endif
+  }
+
+  CommonFields& common() { return settings_.template get<0>(); }
+  const CommonFields& common() const { return settings_.template get<0>(); }
+
+  // For use when the capacity is potentially invalid we return
+  // HashtableCapacity directly.
+  HashtableCapacity maybe_invalid_capacity() const {
+    return common().maybe_invalid_capacity();
+  }
+  ctrl_t* control() const {
+    ABSL_SWISSTABLE_ASSERT(!is_soo());
+    return common().control();
+  }
+  slot_type* slot_array(size_t capacity) const {
+    ABSL_SWISSTABLE_ASSERT(!is_soo());
+    return static_cast<slot_type*>(common().slot_array(capacity));
+  }
+  slot_type* soo_slot() {
+    ABSL_SWISSTABLE_ASSERT(is_soo());
+    ABSL_SWISSTABLE_IGNORE_UNINITIALIZED_RETURN(
+        static_cast<slot_type*>(common().soo_data()));
+  }
+  const slot_type* soo_slot() const {
+    ABSL_SWISSTABLE_IGNORE_UNINITIALIZED_RETURN(
+        const_cast<raw_hash_set*>(this)->soo_slot());
+  }
+  slot_type* single_slot() {
+    ABSL_SWISSTABLE_ASSERT(is_small());
+    return SooEnabled()
+               ? soo_slot()
+               : to_slot(common().slot_array(/*capacity=*/1));
+  }
+  const slot_type* single_slot() const {
+    return const_cast<raw_hash_set*>(this)->single_slot();
+  }
+  void decrement_small_size() {
+    ABSL_SWISSTABLE_ASSERT(is_small());
+    SooEnabled() ? common().set_empty_soo() : common().decrement_size();
+    if (!SooEnabled()) {
+      SanitizerPoisonObject(single_slot());
+    }
+  }
+  iterator single_iterator() {
+    return {SooControl(), single_slot(), common().generation_ptr()};
+  }
+  const_iterator single_iterator() const {
+    return const_cast<raw_hash_set*>(this)->single_iterator();
+  }
+  HashtablezInfoHandle infoz() {
+    ABSL_SWISSTABLE_ASSERT(!is_soo());
+    return common().infoz();
   }
 
-  // Reset all ctrl bytes back to kEmpty, except the sentinel.
-  void reset_ctrl() {
-    std::memset(ctrl_, kEmpty, capacity_ + Group::kWidth);
-    ctrl_[capacity_] = kSentinel;
-    SanitizerPoisonMemoryRegion(slots_, sizeof(slot_type) * capacity_);
+  hasher& hash_ref() { return settings_.template get<1>(); }
+  const hasher& hash_ref() const { return settings_.template get<1>(); }
+  key_equal& eq_ref() { return settings_.template get<2>(); }
+  const key_equal& eq_ref() const { return settings_.template get<2>(); }
+  CharAlloc& char_alloc_ref() { return settings_.template get<3>(); }
+  const CharAlloc& char_alloc_ref() const {
+    return settings_.template get<3>();
   }
 
-  void reset_growth_left() {
-    growth_left() = CapacityToGrowth(capacity()) - size_;
+  static void* get_char_alloc_ref_fn(CommonFields& common) {
+    auto* h = reinterpret_cast<raw_hash_set*>(&common);
+    return &h->char_alloc_ref();
+  }
+  static void* get_hash_ref_fn(CommonFields& common) {
+    auto* h = reinterpret_cast<raw_hash_set*>(&common);
+    // TODO(b/397453582): Remove support for const hasher.
+    return const_cast<std::remove_const_t<hasher>*>(&h->hash_ref());
+  }
+  static void transfer_n_slots_fn(void* set, void* dst, void* src,
+                                  size_t count) {
+    auto* src_slot = to_slot(src);
+    auto* dst_slot = to_slot(dst);
+
+    auto* h = static_cast<raw_hash_set*>(set);
+    for (; count > 0; --count, ++src_slot, ++dst_slot) {
+      h->transfer(dst_slot, src_slot);
+    }
   }
 
-  // Sets the control byte, and if `i < Group::kWidth`, set the cloned byte at
-  // the end too.
-  void set_ctrl(size_t i, ctrl_t h) {
-    assert(i < capacity_);
+  static void destroy_slot_fn_impl(void* set, void* slot) {
+    auto* h = static_cast<raw_hash_set*>(set);
+    h->destroy(to_slot(slot));
+  }
+  static constexpr DestroySlotFn get_destroy_slot_fn() {
+    return PolicyTraits::template destroy_is_trivial<Alloc>()
+               ? nullptr
+               : &raw_hash_set::destroy_slot_fn_impl;
+  }
 
-    if (IsFull(h)) {
-      SanitizerUnpoisonObject(slots_ + i);
-    } else {
-      SanitizerPoisonObject(slots_ + i);
+  // TODO(b/382423690): Try to type erase entire function or at least type erase
+  // by GetKey + Hash for memcpyable types.
+  // TODO(b/382423690): Try to type erase for big slots: sizeof(slot_type) > 16.
+  static void transfer_unprobed_elements_to_next_capacity_fn(
+      CommonFields& common, const ctrl_t* old_ctrl, void* old_slots,
+      void* probed_storage,
+      void (*encode_probed_element)(void* probed_storage, h2_t h2,
+                                    size_t source_offset, size_t h1)) {
+    const size_t new_capacity = common.capacity();
+    ABSL_ASSUME(new_capacity > kMaxSmallCapacity);
+    const size_t old_capacity = PreviousCapacity(new_capacity);
+    ABSL_ASSUME(old_capacity + 1 >= Group::kWidth);
+    ABSL_ASSUME((old_capacity + 1) % Group::kWidth == 0);
+
+    auto* set = reinterpret_cast<raw_hash_set*>(&common);
+    slot_type* old_slots_ptr = to_slot(old_slots);
+    ctrl_t* new_ctrl = common.control();
+    slot_type* new_slots = set->slot_array(new_capacity);
+
+    for (size_t group_index = 0; group_index < old_capacity;
+         group_index += Group::kWidth) {
+      GroupFullEmptyOrDeleted old_g(old_ctrl + group_index);
+      std::memset(new_ctrl + group_index, static_cast<int8_t>(ctrl_t::kEmpty),
+                  Group::kWidth);
+      std::memset(new_ctrl + group_index + old_capacity + 1,
+                  static_cast<int8_t>(ctrl_t::kEmpty), Group::kWidth);
+      // TODO(b/382423690): try to type erase everything outside of the loop.
+      // We will share a lot of code in expense of one function call per group.
+      for (auto in_fixed_group_index : old_g.MaskFull()) {
+        size_t old_index = group_index + in_fixed_group_index;
+        slot_type* old_slot = old_slots_ptr + old_index;
+        // TODO(b/382423690): try to avoid entire hash calculation since we need
+        // only one new bit of h1.
+        size_t hash = set->hash_of(old_slot);
+        size_t h1 = H1(hash);
+        h2_t h2 = H2(hash);
+        size_t new_index = TryFindNewIndexWithoutProbing(
+            h1, old_index, old_capacity, new_ctrl, new_capacity);
+        // Note that encode_probed_element is allowed to use old_ctrl buffer
+        // till and included the old_index.
+        if (ABSL_PREDICT_FALSE(new_index == kProbedElementIndexSentinel)) {
+          encode_probed_element(probed_storage, h2, old_index, h1);
+          continue;
+        }
+        ABSL_SWISSTABLE_ASSERT((new_index & old_capacity) <= old_index);
+        ABSL_SWISSTABLE_ASSERT(IsEmpty(new_ctrl[new_index]));
+        new_ctrl[new_index] = static_cast<ctrl_t>(h2);
+        auto* new_slot = new_slots + new_index;
+        SanitizerUnpoisonMemoryRegion(new_slot, sizeof(slot_type));
+        set->transfer(new_slot, old_slot);
+        SanitizerPoisonMemoryRegion(old_slot, sizeof(slot_type));
+      }
     }
+  }
 
-    ctrl_[i] = h;
-    ctrl_[((i - Group::kWidth) & capacity_) + 1 +
-          ((Group::kWidth - 1) & capacity_)] = h;
+  static constexpr DeallocBackingArrayFn get_dealloc_backing_array_fn() {
+    return &DeallocateBackingArray<BackingArrayAlignment(alignof(slot_type)),
+                                   CharAlloc>;
   }
 
-  size_t& growth_left() { return settings_.template get<0>(); }
+  static const PolicyFunctions& GetPolicyFunctions() {
+    static_assert(sizeof(slot_type) <= (std::numeric_limits<uint32_t>::max)(),
+                  "Slot size is too large. Use std::unique_ptr for value type "
+                  "or use absl::node_hash_{map,set}.");
+    static_assert(alignof(slot_type) <=
+                  size_t{(std::numeric_limits<uint16_t>::max)()});
+    static_assert(sizeof(key_type) <=
+                  size_t{(std::numeric_limits<uint32_t>::max)()});
+    static_assert(sizeof(value_type) <=
+                  size_t{(std::numeric_limits<uint32_t>::max)()});
+    static constexpr size_t kBackingArrayAlignment =
+        BackingArrayAlignment(alignof(slot_type));
+    static constexpr PolicyFunctions value = {
+        static_cast<uint32_t>(sizeof(key_type)),
+        static_cast<uint32_t>(sizeof(value_type)),
+        static_cast<uint32_t>(sizeof(slot_type)),
+        static_cast<uint16_t>(alignof(slot_type)), SooEnabled(),
+        ShouldSampleHashtablezInfoForAlloc<CharAlloc>(),
+        // TODO(b/328722020): try to type erase
+        // for standard layout and alignof(Hash) <= alignof(CommonFields).
+        std::is_empty_v<hasher> ? &GetRefForEmptyClass
+                                : &raw_hash_set::get_hash_ref_fn,
+        PolicyTraits::template get_hash_slot_fn<hasher, kIsDefaultHash,
+                                                kSeedShift>(),
+        PolicyTraits::transfer_uses_memcpy()
+            ? TransferNRelocatable<sizeof(slot_type)>
+            : &raw_hash_set::transfer_n_slots_fn,
+        std::is_empty_v<Alloc> ? &GetRefForEmptyClass
+                               : &raw_hash_set::get_char_alloc_ref_fn,
+        &AllocateBackingArray<kBackingArrayAlignment, CharAlloc>,
+        get_dealloc_backing_array_fn(),
+        &raw_hash_set::transfer_unprobed_elements_to_next_capacity_fn};
+    return value;
+  }
 
-  // The representation of the object has two modes:
-  //  - small: For capacities < kWidth-1
-  //  - large: For the rest.
-  //
-  // Differences:
-  //  - In small mode we are able to use the whole capacity. The extra control
-  //  bytes give us at least one "empty" control byte to stop the iteration.
-  //  This is important to make 1 a valid capacity.
-  //
-  //  - In small mode only the first `capacity()` control bytes after the
-  //  sentinel are valid. The rest contain dummy kEmpty values that do not
-  //  represent a real slot. This is important to take into account on
-  //  find_first_non_full(), where we never try ShouldInsertBackwards() for
-  //  small tables.
-  bool is_small() const { return capacity_ < Group::kWidth - 1; }
+  // Bundle together CommonFields plus other objects which might be empty.
+  // CompressedTuple will ensure that sizeof is not affected by any of the empty
+  // fields that occur after CommonFields.
+  absl::container_internal::CompressedTuple<CommonFields, hasher, key_equal,
+                                            CharAlloc>
+      settings_{CommonFields::CreateDefault<SooEnabled()>(), hasher{},
+                key_equal{}, CharAlloc{}};
+};
 
-  hasher& hash_ref() { return settings_.template get<1>(); }
-  const hasher& hash_ref() const { return settings_.template get<1>(); }
-  key_equal& eq_ref() { return settings_.template get<2>(); }
-  const key_equal& eq_ref() const { return settings_.template get<2>(); }
-  allocator_type& alloc_ref() { return settings_.template get<3>(); }
-  const allocator_type& alloc_ref() const {
-    return settings_.template get<3>();
+// Friend access for free functions in raw_hash_set.h.
+struct HashtableFreeFunctionsAccess {
+  template <class Predicate, typename Set>
+  static typename Set::size_type EraseIf(Predicate& pred, Set* c) {
+    if (c->empty()) {
+      return 0;
+    }
+    if (c->is_small()) {
+      auto it = c->single_iterator();
+      if (!pred(*it)) {
+        ABSL_SWISSTABLE_ASSERT(c->size() == 1 &&
+                               "hash table was modified unexpectedly");
+        return 0;
+      }
+      c->destroy(it.slot());
+      c->erase_meta_only_small();
+      return 1;
+    }
+    [[maybe_unused]] const size_t original_size_for_assert = c->size();
+    size_t num_deleted = 0;
+    using SlotType = typename Set::slot_type;
+    IterateOverFullSlots(
+        c->common(), sizeof(SlotType),
+        [&](const ctrl_t* ctrl, void* slot_void) {
+          auto* slot = static_cast<SlotType*>(slot_void);
+          if (pred(Set::PolicyTraits::element(slot))) {
+            c->destroy(slot);
+            EraseMetaOnlyLarge(c->common(),
+                               static_cast<size_t>(ctrl - c->control()),
+                               sizeof(*slot));
+            ++num_deleted;
+          }
+        });
+    // NOTE: IterateOverFullSlots allow removal of the current element, so we
+    // verify the size additionally here.
+    ABSL_SWISSTABLE_ASSERT(original_size_for_assert - num_deleted ==
+                               c->size() &&
+                           "hash table was modified unexpectedly");
+    return num_deleted;
   }
 
-  // TODO(alkis): Investigate removing some of these fields:
-  // - ctrl/slots can be derived from each other
-  // - size can be moved into the slot array
-  ctrl_t* ctrl_ = EmptyGroup();    // [(capacity + 1) * ctrl_t]
-  slot_type* slots_ = nullptr;     // [capacity * slot_type]
-  size_t size_ = 0;                // number of full slots
-  size_t capacity_ = 0;            // total number of slots
-  HashtablezInfoHandle infoz_;
-  absl::container_internal::CompressedTuple<size_t /* growth_left */, hasher,
-                                            key_equal, allocator_type>
-      settings_{0, hasher{}, key_equal{}, allocator_type{}};
+  template <class Callback, typename Set>
+  static void ForEach(Callback& cb, Set* c) {
+    if (c->empty()) {
+      return;
+    }
+    if (c->is_small()) {
+      cb(*c->single_iterator());
+      return;
+    }
+    using SlotType = typename Set::slot_type;
+    using ElementTypeWithConstness = decltype(*c->begin());
+    IterateOverFullSlots(
+        c->common(), sizeof(SlotType), [&cb](const ctrl_t*, void* slot) {
+          ElementTypeWithConstness& element =
+              Set::PolicyTraits::element(static_cast<SlotType*>(slot));
+          cb(element);
+        });
+  }
 };
 
+// Erases all elements that satisfy the predicate `pred` from the container `c`.
+template <typename P, typename... Params, typename Predicate>
+typename raw_hash_set<P, Params...>::size_type EraseIf(
+    Predicate& pred, raw_hash_set<P, Params...>* c) {
+  return HashtableFreeFunctionsAccess::EraseIf(pred, c);
+}
+
+// Calls `cb` for all elements in the container `c`.
+template <typename P, typename... Params, typename Callback>
+void ForEach(Callback& cb, raw_hash_set<P, Params...>* c) {
+  return HashtableFreeFunctionsAccess::ForEach(cb, c);
+}
+template <typename P, typename... Params, typename Callback>
+void ForEach(Callback& cb, const raw_hash_set<P, Params...>* c) {
+  return HashtableFreeFunctionsAccess::ForEach(cb, c);
+}
+
 namespace hashtable_debug_internal {
 template <typename Set>
-struct HashtableDebugAccess<Set, absl::void_t<typename Set::raw_hash_set>> {
+struct HashtableDebugAccess<Set, std::void_t<typename Set::raw_hash_set>> {
   using Traits = typename Set::PolicyTraits;
   using Slot = typename Traits::slot_type;
 
+  constexpr static bool kIsDefaultHash = Set::kIsDefaultHash;
+
   static size_t GetNumProbes(const Set& set,
                              const typename Set::key_type& key) {
+    if (set.is_small()) return 0;
     size_t num_probes = 0;
-    size_t hash = set.hash_ref()(key);
-    auto seq = set.probe(hash);
+    const size_t hash = set.hash_of(key);
+    auto seq = probe(set.common(), hash);
+    const h2_t h2 = H2(hash);
+    const ctrl_t* ctrl = set.control();
     while (true) {
-      container_internal::Group g{set.ctrl_ + seq.offset()};
-      for (int i : g.Match(container_internal::H2(hash))) {
-        if (Traits::apply(
-                typename Set::template EqualElement<typename Set::key_type>{
-                    key, set.eq_ref()},
-                Traits::element(set.slots_ + seq.offset(i))))
+      container_internal::Group g{ctrl + seq.offset()};
+      for (uint32_t i : g.Match(h2)) {
+        if (set.equal_to(key, set.slot_array(set.capacity()) + seq.offset(i)))
           return num_probes;
         ++num_probes;
       }
-      if (g.MatchEmpty()) return num_probes;
+      if (g.MaskEmpty()) return num_probes;
       seq.next();
       ++num_probes;
     }
   }
 
   static size_t AllocatedByteSize(const Set& c) {
-    size_t capacity = c.capacity_;
+    size_t capacity = c.capacity();
     if (capacity == 0) return 0;
-    auto layout = Set::MakeLayout(capacity);
-    size_t m = layout.AllocSize();
+    size_t m =
+        c.is_soo() ? 0 : c.common().alloc_size(sizeof(Slot), alignof(Slot));
 
     size_t per_slot = Traits::space_used(static_cast<const Slot*>(nullptr));
     if (per_slot != ~size_t{}) {
       m += per_slot * c.size();
     } else {
-      for (size_t i = 0; i != capacity; ++i) {
-        if (container_internal::IsFull(c.ctrl_[i])) {
-          m += Traits::space_used(c.slots_ + i);
-        }
+      for (auto it = c.begin(); it != c.end(); ++it) {
+        m += Traits::space_used(it.slot());
       }
     }
     return m;
   }
-
-  static size_t LowerBoundAllocatedByteSize(size_t size) {
-    size_t capacity = GrowthToLowerboundCapacity(size);
-    if (capacity == 0) return 0;
-    auto layout = Set::MakeLayout(NormalizeCapacity(capacity));
-    size_t m = layout.AllocSize();
-    size_t per_slot = Traits::space_used(static_cast<const Slot*>(nullptr));
-    if (per_slot != ~size_t{}) {
-      m += per_slot * size;
-    }
-    return m;
-  }
 };
 
 }  // namespace hashtable_debug_internal
+
+// Extern template instantiations reduce binary size and linker input size.
+// Function definition is in raw_hash_set.cc.
+extern template void* GrowSooTableToNextCapacityAndPrepareInsert<0, false>(
+    CommonFields&, const PolicyFunctions&, absl::FunctionRef<size_t(size_t)>,
+    bool);
+extern template void* GrowSooTableToNextCapacityAndPrepareInsert<1, true>(
+    CommonFields&, const PolicyFunctions&, absl::FunctionRef<size_t(size_t)>,
+    bool);
+extern template void* GrowSooTableToNextCapacityAndPrepareInsert<4, true>(
+    CommonFields&, const PolicyFunctions&, absl::FunctionRef<size_t(size_t)>,
+    bool);
+#if UINTPTR_MAX == UINT64_MAX
+extern template void* GrowSooTableToNextCapacityAndPrepareInsert<8, true>(
+    CommonFields&, const PolicyFunctions&, absl::FunctionRef<size_t(size_t)>,
+    bool);
+#endif
+
+extern template void* AllocateBackingArray<
+    BackingArrayAlignment(alignof(size_t)), std::allocator<char>>(void* alloc,
+                                                                  size_t n);
+extern template void DeallocateBackingArray<
+    BackingArrayAlignment(alignof(size_t)), std::allocator<char>>(
+    void* alloc, size_t capacity, ctrl_t* ctrl, size_t slot_size,
+    size_t slot_align, bool had_infoz, size_t blocked_element_count);
+
+extern template void Clear<true>(CommonFields& c, const PolicyFunctions& policy,
+                                 DestroySlotFn destroy_slot, void* alloc);
+extern template void Clear<false>(CommonFields& c,
+                                  const PolicyFunctions& policy,
+                                  DestroySlotFn destroy_slot, void* alloc);
+
 }  // namespace container_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
+#undef ABSL_SWISSTABLE_ENABLE_GENERATIONS
+#undef ABSL_SWISSTABLE_ASSERT
+
 #endif  // ABSL_CONTAINER_INTERNAL_RAW_HASH_SET_H_
diff --git a/absl/container/internal/raw_hash_set_allocator_test.cc b/absl/container/internal/raw_hash_set_allocator_test.cc
index a5eff0b3d27..eaee003409b 100644
--- a/absl/container/internal/raw_hash_set_allocator_test.cc
+++ b/absl/container/internal/raw_hash_set_allocator_test.cc
@@ -12,16 +12,28 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+#include <cstddef>
+#include <cstdint>
+#include <functional>
 #include <limits>
-#include <scoped_allocator>
+#include <memory>
+#include <ostream>
+#include <set>
+#include <type_traits>
+#include <utility>
 
+#include "gmock/gmock.h"
 #include "gtest/gtest.h"
+#include "absl/base/config.h"
+#include "absl/container/internal/container_memory.h"
 #include "absl/container/internal/raw_hash_set.h"
 #include "absl/container/internal/tracked.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 namespace {
+using ::testing::AnyOf;
 
 enum AllocSpec {
   kPropagateOnCopy = 1,
@@ -58,13 +70,13 @@ class CheckedAlloc {
   };
 
   using propagate_on_container_copy_assignment =
-      std::integral_constant<bool, (Spec & kPropagateOnCopy) != 0>;
+      std::bool_constant<(Spec & kPropagateOnCopy) != 0>;
 
   using propagate_on_container_move_assignment =
-      std::integral_constant<bool, (Spec & kPropagateOnMove) != 0>;
+      std::bool_constant<(Spec & kPropagateOnMove) != 0>;
 
   using propagate_on_container_swap =
-      std::integral_constant<bool, (Spec & kPropagateOnSwap) != 0>;
+      std::bool_constant<(Spec & kPropagateOnSwap) != 0>;
 
   CheckedAlloc select_on_container_copy_construction() const {
     if (Spec & kPropagateOnCopy) return *this;
@@ -122,7 +134,7 @@ class CheckedAlloc {
 };
 
 struct Identity {
-  int32_t operator()(int32_t v) const { return v; }
+  size_t operator()(int32_t v) const { return static_cast<size_t>(v); }
 };
 
 struct Policy {
@@ -130,6 +142,10 @@ struct Policy {
   using init_type = Tracked<int32_t>;
   using key_type = int32_t;
 
+  using DefaultHash = void;
+  using DefaultEq = void;
+  using DefaultAlloc = void;
+
   template <class allocator_type, class... Args>
   static void construct(allocator_type* alloc, slot_type* slot,
                         Args&&... args) {
@@ -167,6 +183,11 @@ struct Policy {
   }
 
   static slot_type& element(slot_type* slot) { return *slot; }
+
+  template <class Hash, bool kIsDefault, size_t kSeedShift>
+  static constexpr HashSlotFn get_hash_slot_fn() {
+    return nullptr;
+  }
 };
 
 template <int Spec>
@@ -275,34 +296,38 @@ TEST_F(NoPropagateOnCopy, CopyConstructorWithDifferentAlloc) {
 }
 
 TEST_F(PropagateOnAll, MoveConstructor) {
-  auto it = t1.insert(0).first;
+  t1.insert(0);
   Table u(std::move(t1));
-  EXPECT_EQ(1, a1.num_allocs());
-  EXPECT_EQ(0, it->num_moves());
+  auto it = u.begin();
+  EXPECT_THAT(a1.num_allocs(), AnyOf(1, 2));
+  EXPECT_THAT(it->num_moves(), AnyOf(0, 1));
   EXPECT_EQ(0, it->num_copies());
 }
 
 TEST_F(NoPropagateOnMove, MoveConstructor) {
-  auto it = t1.insert(0).first;
+  t1.insert(0);
   Table u(std::move(t1));
-  EXPECT_EQ(1, a1.num_allocs());
-  EXPECT_EQ(0, it->num_moves());
+  auto it = u.begin();
+  EXPECT_THAT(a1.num_allocs(), AnyOf(1, 2));
+  EXPECT_THAT(it->num_moves(), AnyOf(0, 1));
   EXPECT_EQ(0, it->num_copies());
 }
 
 TEST_F(PropagateOnAll, MoveConstructorWithSameAlloc) {
-  auto it = t1.insert(0).first;
+  t1.insert(0);
   Table u(std::move(t1), a1);
-  EXPECT_EQ(1, a1.num_allocs());
-  EXPECT_EQ(0, it->num_moves());
+  auto it = u.begin();
+  EXPECT_THAT(a1.num_allocs(), AnyOf(1, 2));
+  EXPECT_THAT(it->num_moves(), AnyOf(0, 1));
   EXPECT_EQ(0, it->num_copies());
 }
 
 TEST_F(NoPropagateOnMove, MoveConstructorWithSameAlloc) {
-  auto it = t1.insert(0).first;
+  t1.insert(0);
   Table u(std::move(t1), a1);
-  EXPECT_EQ(1, a1.num_allocs());
-  EXPECT_EQ(0, it->num_moves());
+  auto it = u.begin();
+  EXPECT_THAT(a1.num_allocs(), AnyOf(1, 2));
+  EXPECT_THAT(it->num_moves(), AnyOf(0, 1));
   EXPECT_EQ(0, it->num_copies());
 }
 
@@ -312,8 +337,8 @@ TEST_F(PropagateOnAll, MoveConstructorWithDifferentAlloc) {
   it = u.find(0);
   EXPECT_EQ(a2, u.get_allocator());
   EXPECT_EQ(1, a1.num_allocs());
-  EXPECT_EQ(1, a2.num_allocs());
-  EXPECT_EQ(1, it->num_moves());
+  EXPECT_THAT(a2.num_allocs(), AnyOf(1, 2));
+  EXPECT_THAT(it->num_moves(), AnyOf(1, 2));
   EXPECT_EQ(0, it->num_copies());
 }
 
@@ -323,8 +348,8 @@ TEST_F(NoPropagateOnMove, MoveConstructorWithDifferentAlloc) {
   it = u.find(0);
   EXPECT_EQ(a2, u.get_allocator());
   EXPECT_EQ(1, a1.num_allocs());
-  EXPECT_EQ(1, a2.num_allocs());
-  EXPECT_EQ(1, it->num_moves());
+  EXPECT_THAT(a2.num_allocs(), AnyOf(1, 2));
+  EXPECT_THAT(it->num_moves(), AnyOf(1, 2));
   EXPECT_EQ(0, it->num_copies());
 }
 
@@ -332,8 +357,8 @@ TEST_F(PropagateOnAll, CopyAssignmentWithSameAlloc) {
   auto it = t1.insert(0).first;
   Table u(0, a1);
   u = t1;
-  EXPECT_EQ(2, a1.num_allocs());
-  EXPECT_EQ(0, it->num_moves());
+  EXPECT_THAT(a1.num_allocs(), AnyOf(2, 3));
+  EXPECT_THAT(it->num_moves(), AnyOf(0, 1));
   EXPECT_EQ(1, it->num_copies());
 }
 
@@ -341,8 +366,8 @@ TEST_F(NoPropagateOnCopy, CopyAssignmentWithSameAlloc) {
   auto it = t1.insert(0).first;
   Table u(0, a1);
   u = t1;
-  EXPECT_EQ(2, a1.num_allocs());
-  EXPECT_EQ(0, it->num_moves());
+  EXPECT_THAT(a1.num_allocs(), AnyOf(2, 3));
+  EXPECT_THAT(it->num_moves(), AnyOf(0, 1));
   EXPECT_EQ(1, it->num_copies());
 }
 
@@ -351,9 +376,9 @@ TEST_F(PropagateOnAll, CopyAssignmentWithDifferentAlloc) {
   Table u(0, a2);
   u = t1;
   EXPECT_EQ(a1, u.get_allocator());
-  EXPECT_EQ(2, a1.num_allocs());
+  EXPECT_THAT(a1.num_allocs(), AnyOf(2, 3));
   EXPECT_EQ(0, a2.num_allocs());
-  EXPECT_EQ(0, it->num_moves());
+  EXPECT_THAT(it->num_moves(), AnyOf(0, 1));
   EXPECT_EQ(1, it->num_copies());
 }
 
@@ -363,66 +388,138 @@ TEST_F(NoPropagateOnCopy, CopyAssignmentWithDifferentAlloc) {
   u = t1;
   EXPECT_EQ(a2, u.get_allocator());
   EXPECT_EQ(1, a1.num_allocs());
-  EXPECT_EQ(1, a2.num_allocs());
-  EXPECT_EQ(0, it->num_moves());
+  EXPECT_THAT(a2.num_allocs(), AnyOf(1, 2));
+  EXPECT_THAT(it->num_moves(), AnyOf(0, 1));
   EXPECT_EQ(1, it->num_copies());
 }
 
 TEST_F(PropagateOnAll, MoveAssignmentWithSameAlloc) {
-  auto it = t1.insert(0).first;
+  t1.insert(0);
   Table u(0, a1);
   u = std::move(t1);
+  auto it = u.begin();
   EXPECT_EQ(a1, u.get_allocator());
-  EXPECT_EQ(1, a1.num_allocs());
-  EXPECT_EQ(0, it->num_moves());
+  EXPECT_THAT(a1.num_allocs(), AnyOf(1, 2));
+  EXPECT_THAT(it->num_moves(), AnyOf(0, 1));
   EXPECT_EQ(0, it->num_copies());
 }
 
 TEST_F(NoPropagateOnMove, MoveAssignmentWithSameAlloc) {
-  auto it = t1.insert(0).first;
+  t1.insert(0);
   Table u(0, a1);
   u = std::move(t1);
+  auto it = u.begin();
   EXPECT_EQ(a1, u.get_allocator());
-  EXPECT_EQ(1, a1.num_allocs());
-  EXPECT_EQ(0, it->num_moves());
+  EXPECT_THAT(a1.num_allocs(), AnyOf(1, 2));
+  EXPECT_THAT(it->num_moves(), AnyOf(0, 1));
   EXPECT_EQ(0, it->num_copies());
 }
 
 TEST_F(PropagateOnAll, MoveAssignmentWithDifferentAlloc) {
-  auto it = t1.insert(0).first;
+  t1.insert(0);
   Table u(0, a2);
   u = std::move(t1);
+  auto it = u.begin();
   EXPECT_EQ(a1, u.get_allocator());
-  EXPECT_EQ(1, a1.num_allocs());
+  EXPECT_THAT(a1.num_allocs(), AnyOf(1, 2));
   EXPECT_EQ(0, a2.num_allocs());
-  EXPECT_EQ(0, it->num_moves());
+  EXPECT_THAT(it->num_moves(), AnyOf(0, 1));
   EXPECT_EQ(0, it->num_copies());
 }
 
 TEST_F(NoPropagateOnMove, MoveAssignmentWithDifferentAlloc) {
-  auto it = t1.insert(0).first;
+  t1.insert(0);
   Table u(0, a2);
   u = std::move(t1);
-  it = u.find(0);
+  auto it = u.find(0);
   EXPECT_EQ(a2, u.get_allocator());
   EXPECT_EQ(1, a1.num_allocs());
-  EXPECT_EQ(1, a2.num_allocs());
-  EXPECT_EQ(1, it->num_moves());
+  EXPECT_THAT(a2.num_allocs(), AnyOf(1, 2));
+  EXPECT_THAT(it->num_moves(), AnyOf(1, 2));
   EXPECT_EQ(0, it->num_copies());
 }
 
 TEST_F(PropagateOnAll, Swap) {
-  auto it = t1.insert(0).first;
+  t1.insert(0);
   Table u(0, a2);
   u.swap(t1);
   EXPECT_EQ(a1, u.get_allocator());
   EXPECT_EQ(a2, t1.get_allocator());
   EXPECT_EQ(1, a1.num_allocs());
   EXPECT_EQ(0, a2.num_allocs());
+  auto it = u.begin();
   EXPECT_EQ(0, it->num_moves());
   EXPECT_EQ(0, it->num_copies());
 }
 
+// This allocator is similar to std::pmr::polymorphic_allocator.
+// Note the disabled assignment.
+template <class T>
+class PAlloc {
+  template <class>
+  friend class PAlloc;
+
+ public:
+  // types
+  using value_type = T;
+
+  PAlloc() noexcept = default;
+  explicit PAlloc(size_t id) noexcept : id_(id) {}
+  PAlloc(const PAlloc&) noexcept = default;
+  PAlloc& operator=(const PAlloc&) noexcept = delete;
+
+  template <class U>
+  PAlloc(const PAlloc<U>& that) noexcept : id_(that.id_) {}  // NOLINT
+
+  template <class U>
+  struct rebind {
+    using other = PAlloc<U>;
+  };
+
+  constexpr PAlloc select_on_container_copy_construction() const { return {}; }
+
+  // public member functions
+  T* allocate(size_t) { return new T; }
+  void deallocate(T* p, size_t) { delete p; }
+
+  friend bool operator==(const PAlloc& a, const PAlloc& b) {
+    return a.id_ == b.id_;
+  }
+  friend bool operator!=(const PAlloc& a, const PAlloc& b) { return !(a == b); }
+
+ private:
+  size_t id_ = std::numeric_limits<size_t>::max();
+};
+
+TEST(NoPropagateDeletedAssignment, CopyConstruct) {
+  using PA = PAlloc<char>;
+  using Table = raw_hash_set<Policy, Identity, std::equal_to<int32_t>, PA>;
+
+  Table t1(PA{1}), t2(t1);
+  EXPECT_EQ(t1.get_allocator(), PA(1));
+  EXPECT_EQ(t2.get_allocator(), PA());
+}
+
+TEST(NoPropagateDeletedAssignment, CopyAssignment) {
+  using PA = PAlloc<char>;
+  using Table = raw_hash_set<Policy, Identity, std::equal_to<int32_t>, PA>;
+
+  Table t1(PA{1}), t2(PA{2});
+  t1 = t2;
+  EXPECT_EQ(t1.get_allocator(), PA(1));
+  EXPECT_EQ(t2.get_allocator(), PA(2));
+}
+
+TEST(NoPropagateDeletedAssignment, MoveAssignment) {
+  using PA = PAlloc<char>;
+  using Table = raw_hash_set<Policy, Identity, std::equal_to<int32_t>, PA>;
+
+  Table t1(PA{1}), t2(PA{2});
+  t1 = std::move(t2);
+  EXPECT_EQ(t1.get_allocator(), PA(1));
+}
+
 }  // namespace
 }  // namespace container_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/container/internal/raw_hash_set_benchmark.cc b/absl/container/internal/raw_hash_set_benchmark.cc
new file mode 100644
index 00000000000..4c0a78a2b67
--- /dev/null
+++ b/absl/container/internal/raw_hash_set_benchmark.cc
@@ -0,0 +1,721 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <algorithm>
+#include <array>
+#include <cmath>
+#include <cstddef>
+#include <cstdint>
+#include <limits>
+#include <numeric>
+#include <random>
+#include <string>
+#include <tuple>
+#include <type_traits>
+#include <utility>
+#include <vector>
+
+#include "absl/base/internal/raw_logging.h"
+#include "absl/container/internal/container_memory.h"
+#include "absl/container/internal/hash_function_defaults.h"
+#include "absl/container/internal/raw_hash_set.h"
+#include "absl/random/random.h"
+#include "absl/strings/str_format.h"
+#include "benchmark/benchmark.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace container_internal {
+
+struct RawHashSetTestOnlyAccess {
+  template <typename C>
+  static auto GetSlots(const C& c) -> decltype(c.slots_) {
+    return c.slots_;
+  }
+};
+
+namespace {
+
+struct IntPolicy {
+  using slot_type = int64_t;
+  using key_type = int64_t;
+  using init_type = int64_t;
+
+  using DefaultHash = void;
+  using DefaultEq = void;
+  using DefaultAlloc = void;
+
+  static void construct(void*, int64_t* slot, int64_t v) { *slot = v; }
+  static void destroy(void*, int64_t*) {}
+  static void transfer(void*, int64_t* new_slot, int64_t* old_slot) {
+    *new_slot = *old_slot;
+  }
+
+  static int64_t& element(slot_type* slot) { return *slot; }
+
+  template <class F>
+  static auto apply(F&& f, int64_t x) -> decltype(std::forward<F>(f)(x, x)) {
+    return std::forward<F>(f)(x, x);
+  }
+
+  template <class Hash, bool kIsDefault, size_t kSeedShift>
+  static constexpr HashSlotFn get_hash_slot_fn() {
+    return nullptr;
+  }
+};
+
+class StringPolicy {
+  template <class F, class K, class V,
+            class = std::enable_if_t<
+                std::is_convertible_v<const K&, absl::string_view>>>
+  decltype(std::declval<F>()(
+      std::declval<const absl::string_view&>(), std::piecewise_construct,
+      std::declval<std::tuple<K>>(),
+      std::declval<V>())) static apply_impl(F&& f,
+                                            std::pair<std::tuple<K>, V> p) {
+    const absl::string_view& key = std::get<0>(p.first);
+    return std::forward<F>(f)(key, std::piecewise_construct, std::move(p.first),
+                              std::move(p.second));
+  }
+
+ public:
+  struct slot_type {
+    struct ctor {};
+
+    template <class... Ts>
+    slot_type(ctor, Ts&&... ts) : pair(std::forward<Ts>(ts)...) {}
+
+    std::pair<std::string, std::string> pair;
+  };
+
+  using key_type = std::string;
+  using init_type = std::pair<std::string, std::string>;
+
+  using DefaultHash = void;
+  using DefaultEq = void;
+  using DefaultAlloc = void;
+
+  template <class allocator_type, class... Args>
+  static void construct(allocator_type* alloc, slot_type* slot, Args... args) {
+    std::allocator_traits<allocator_type>::construct(
+        *alloc, slot, typename slot_type::ctor(), std::forward<Args>(args)...);
+  }
+
+  template <class allocator_type>
+  static void destroy(allocator_type* alloc, slot_type* slot) {
+    std::allocator_traits<allocator_type>::destroy(*alloc, slot);
+  }
+
+  template <class allocator_type>
+  static void transfer(allocator_type* alloc, slot_type* new_slot,
+                       slot_type* old_slot) {
+    construct(alloc, new_slot, std::move(old_slot->pair));
+    destroy(alloc, old_slot);
+  }
+
+  static std::pair<std::string, std::string>& element(slot_type* slot) {
+    return slot->pair;
+  }
+
+  template <class F, class... Args>
+  static auto apply(F&& f, Args&&... args)
+      -> decltype(apply_impl(std::forward<F>(f),
+                             PairArgs(std::forward<Args>(args)...))) {
+    return apply_impl(std::forward<F>(f),
+                      PairArgs(std::forward<Args>(args)...));
+  }
+
+  template <class Hash, bool kIsDefault, size_t kSeedShift>
+  static constexpr HashSlotFn get_hash_slot_fn() {
+    return nullptr;
+  }
+};
+
+struct StringHash : container_internal::hash_default_hash<absl::string_view> {
+  using is_transparent = void;
+};
+struct StringEq : std::equal_to<absl::string_view> {
+  using is_transparent = void;
+};
+
+struct StringTable
+    : raw_hash_set<StringPolicy, StringHash, StringEq, std::allocator<int>> {
+  using Base = typename StringTable::raw_hash_set;
+  StringTable() {}
+  using Base::Base;
+};
+
+struct IntTable
+    : raw_hash_set<IntPolicy, container_internal::hash_default_hash<int64_t>,
+                   std::equal_to<int64_t>, std::allocator<int64_t>> {
+  using Base = typename IntTable::raw_hash_set;
+  IntTable() {}
+  using Base::Base;
+};
+
+struct string_generator {
+  template <class RNG>
+  std::string operator()(RNG& rng) const {
+    std::string res;
+    res.resize(size);
+    std::uniform_int_distribution<uint32_t> printable_ascii(0x20, 0x7E);
+    std::generate(res.begin(), res.end(), [&] { return printable_ascii(rng); });
+    return res;
+  }
+
+  size_t size;
+};
+
+// Model a cache in steady state.
+//
+// On a table of size N, keep deleting the LRU entry and add a random one.
+void BM_CacheInSteadyState(benchmark::State& state) {
+  absl::InsecureBitGen rng;
+  string_generator gen{12};
+  StringTable t;
+  std::deque<std::string> keys;
+  while (t.size() < state.range(0)) {
+    auto x = t.emplace(gen(rng), gen(rng));
+    if (x.second) keys.push_back(x.first->first);
+  }
+  ABSL_RAW_CHECK(state.range(0) >= 10, "");
+  while (state.KeepRunning()) {
+    // Some cache hits.
+    std::deque<std::string>::const_iterator it;
+    for (int i = 0; i != 90; ++i) {
+      if (i % 10 == 0) it = keys.end();
+      ::benchmark::DoNotOptimize(t.find(*--it));
+    }
+    // Some cache misses.
+    for (int i = 0; i != 10; ++i) ::benchmark::DoNotOptimize(t.find(gen(rng)));
+    ABSL_RAW_CHECK(t.erase(keys.front()), keys.front().c_str());
+    keys.pop_front();
+    while (true) {
+      auto x = t.emplace(gen(rng), gen(rng));
+      if (x.second) {
+        keys.push_back(x.first->first);
+        break;
+      }
+    }
+  }
+  state.SetItemsProcessed(state.iterations());
+  state.SetLabel(absl::StrFormat("load_factor=%.2f", t.load_factor()));
+}
+
+void CacheInSteadyStateArgs(::benchmark::Benchmark* bm) {
+  // The default.
+  const float max_load_factor = 0.875;
+  // When the cache is at the steady state, the probe sequence will equal
+  // capacity if there is no reclamation of deleted slots. Pick a number large
+  // enough to make the benchmark slow for that case.
+  const size_t capacity = 1 << 10;
+
+  // Check N data points to cover load factors in [0.4, 0.8).
+  const size_t kNumPoints = 10;
+  for (size_t i = 0; i != kNumPoints; ++i)
+    bm->Arg(std::ceil(
+        capacity * (max_load_factor + i * max_load_factor / kNumPoints) / 2));
+}
+BENCHMARK(BM_CacheInSteadyState)->Apply(CacheInSteadyStateArgs);
+
+void BM_EraseEmplace(benchmark::State& state) {
+  IntTable t;
+  int64_t size = state.range(0);
+  for (int64_t i = 0; i < size; ++i) {
+    t.emplace(i);
+  }
+  while (state.KeepRunningBatch(size)) {
+    for (int64_t i = 0; i < size; ++i) {
+      benchmark::DoNotOptimize(t);
+      t.erase(i);
+      t.emplace(i);
+    }
+  }
+}
+BENCHMARK(BM_EraseEmplace)->Arg(1)->Arg(2)->Arg(4)->Arg(8)->Arg(16)->Arg(100);
+
+void BM_EraseEmplaceString(benchmark::State& state) {
+  StringTable t;
+  int64_t size = state.range(0);
+  for (int64_t i = 0; i < size; ++i) {
+    std::string s = std::to_string(i);
+    t.emplace(s, s);
+  }
+  while (state.KeepRunningBatch(size)) {
+    for (int64_t i = 0; i < size; ++i) {
+      benchmark::DoNotOptimize(t);
+      std::string s = std::to_string(i);
+      t.erase(s);
+      t.emplace(s, s);
+    }
+  }
+}
+BENCHMARK(BM_EraseEmplaceString)
+    ->Arg(1)
+    ->Arg(2)
+    ->Arg(4)
+    ->Arg(8)
+    ->Arg(16)
+    ->Arg(100);
+
+void BM_EndComparison(benchmark::State& state) {
+  StringTable t = {{"a", "a"}, {"b", "b"}};
+  auto it = t.begin();
+  for (auto i : state) {
+    benchmark::DoNotOptimize(t);
+    benchmark::DoNotOptimize(it);
+    benchmark::DoNotOptimize(it != t.end());
+  }
+}
+BENCHMARK(BM_EndComparison);
+
+void BM_Iteration(benchmark::State& state) {
+  absl::InsecureBitGen rng;
+  string_generator gen{12};
+  StringTable t;
+
+  size_t capacity = state.range(0);
+  size_t size = state.range(1);
+  t.reserve(capacity);
+
+  while (t.size() < size) {
+    t.emplace(gen(rng), gen(rng));
+  }
+
+  for (auto i : state) {
+    benchmark::DoNotOptimize(t);
+    for (auto it = t.begin(); it != t.end(); ++it) {
+      benchmark::DoNotOptimize(*it);
+    }
+  }
+}
+
+BENCHMARK(BM_Iteration)
+    ->ArgPair(1, 1)
+    ->ArgPair(2, 2)
+    ->ArgPair(4, 4)
+    ->ArgPair(7, 7)
+    ->ArgPair(10, 10)
+    ->ArgPair(15, 15)
+    ->ArgPair(16, 16)
+    ->ArgPair(54, 54)
+    ->ArgPair(100, 100)
+    ->ArgPair(400, 400)
+    // empty
+    ->ArgPair(0, 0)
+    ->ArgPair(10, 0)
+    ->ArgPair(100, 0)
+    ->ArgPair(1000, 0)
+    ->ArgPair(10000, 0)
+    // sparse
+    ->ArgPair(100, 1)
+    ->ArgPair(1000, 10);
+
+void BM_CopyCtorSparseInt(benchmark::State& state) {
+  absl::InsecureBitGen rng;
+  IntTable t;
+  std::uniform_int_distribution<uint64_t> dist(0, ~uint64_t{});
+
+  size_t size = state.range(0);
+  t.reserve(size * 10);
+  while (t.size() < size) {
+    t.emplace(dist(rng));
+  }
+
+  for (auto i : state) {
+    IntTable t2 = t;
+    benchmark::DoNotOptimize(t2);
+  }
+}
+BENCHMARK(BM_CopyCtorSparseInt)->Range(1, 4096);
+
+void BM_CopyCtorInt(benchmark::State& state) {
+  absl::InsecureBitGen rng;
+  IntTable t;
+  std::uniform_int_distribution<uint64_t> dist(0, ~uint64_t{});
+
+  size_t size = state.range(0);
+  while (t.size() < size) {
+    t.emplace(dist(rng));
+  }
+
+  for (auto i : state) {
+    IntTable t2 = t;
+    benchmark::DoNotOptimize(t2);
+  }
+}
+BENCHMARK(BM_CopyCtorInt)->Range(0, 4096);
+
+void BM_CopyCtorString(benchmark::State& state) {
+  absl::InsecureBitGen rng;
+  StringTable t;
+  std::uniform_int_distribution<uint64_t> dist(0, ~uint64_t{});
+
+  size_t size = state.range(0);
+  while (t.size() < size) {
+    t.emplace(std::to_string(dist(rng)), std::to_string(dist(rng)));
+  }
+
+  for (auto i : state) {
+    StringTable t2 = t;
+    benchmark::DoNotOptimize(t2);
+  }
+}
+BENCHMARK(BM_CopyCtorString)->Range(0, 4096);
+
+void BM_CopyAssign(benchmark::State& state) {
+  absl::InsecureBitGen rng;
+  IntTable t;
+  std::uniform_int_distribution<uint64_t> dist(0, ~uint64_t{});
+  while (t.size() < state.range(0)) {
+    t.emplace(dist(rng));
+  }
+
+  IntTable t2;
+  for (auto _ : state) {
+    t2 = t;
+    benchmark::DoNotOptimize(t2);
+  }
+}
+BENCHMARK(BM_CopyAssign)->Range(128, 4096);
+
+void BM_RangeCtor(benchmark::State& state) {
+  absl::InsecureBitGen rng;
+  std::uniform_int_distribution<uint64_t> dist(0, ~uint64_t{});
+  std::vector<int> values;
+  const size_t desired_size = state.range(0);
+  while (values.size() < desired_size) {
+    values.emplace_back(dist(rng));
+  }
+
+  for (auto unused : state) {
+    IntTable t{values.begin(), values.end()};
+    benchmark::DoNotOptimize(t);
+  }
+}
+BENCHMARK(BM_RangeCtor)->Range(128, 65536);
+
+void BM_NoOpReserveIntTable(benchmark::State& state) {
+  IntTable t;
+  t.reserve(100000);
+  for (auto _ : state) {
+    benchmark::DoNotOptimize(t);
+    t.reserve(100000);
+  }
+}
+BENCHMARK(BM_NoOpReserveIntTable);
+
+void BM_NoOpReserveStringTable(benchmark::State& state) {
+  StringTable t;
+  t.reserve(100000);
+  for (auto _ : state) {
+    benchmark::DoNotOptimize(t);
+    t.reserve(100000);
+  }
+}
+BENCHMARK(BM_NoOpReserveStringTable);
+
+void BM_ReserveIntTable(benchmark::State& state) {
+  constexpr size_t kBatchSize = 1024;
+  size_t reserve_size = static_cast<size_t>(state.range(0));
+
+  std::vector<IntTable> tables;
+  while (state.KeepRunningBatch(kBatchSize)) {
+    state.PauseTiming();
+    tables.clear();
+    tables.resize(kBatchSize);
+    state.ResumeTiming();
+    for (auto& t : tables) {
+      benchmark::DoNotOptimize(t);
+      t.reserve(reserve_size);
+      benchmark::DoNotOptimize(t);
+    }
+  }
+}
+BENCHMARK(BM_ReserveIntTable)
+    ->Arg(1)
+    ->Arg(2)
+    ->Arg(4)
+    ->Arg(8)
+    ->Arg(16)
+    ->Arg(32)
+    ->Arg(64)
+    ->Arg(128)
+    ->Arg(256)
+    ->Arg(512);
+
+void BM_ReserveStringTable(benchmark::State& state) {
+  constexpr size_t kBatchSize = 1024;
+  size_t reserve_size = static_cast<size_t>(state.range(0));
+
+  std::vector<StringTable> tables;
+  while (state.KeepRunningBatch(kBatchSize)) {
+    state.PauseTiming();
+    tables.clear();
+    tables.resize(kBatchSize);
+    state.ResumeTiming();
+    for (auto& t : tables) {
+      benchmark::DoNotOptimize(t);
+      t.reserve(reserve_size);
+      benchmark::DoNotOptimize(t);
+    }
+  }
+}
+BENCHMARK(BM_ReserveStringTable)
+    ->Arg(1)
+    ->Arg(2)
+    ->Arg(4)
+    ->Arg(8)
+    ->Arg(16)
+    ->Arg(32)
+    ->Arg(64)
+    ->Arg(128)
+    ->Arg(256)
+    ->Arg(512);
+
+// Like std::iota, except that ctrl_t doesn't support operator++.
+template <typename CtrlIter>
+void Iota(CtrlIter begin, CtrlIter end, int value) {
+  for (; begin != end; ++begin, ++value) {
+    *begin = static_cast<ctrl_t>(value);
+  }
+}
+
+void BM_Group_Match(benchmark::State& state) {
+  std::array<ctrl_t, Group::kWidth> group;
+  Iota(group.begin(), group.end(), -4);
+  Group g{group.data()};
+  h2_t h = 1;
+  for (auto _ : state) {
+    ::benchmark::DoNotOptimize(h);
+    ::benchmark::DoNotOptimize(g);
+    ::benchmark::DoNotOptimize(g.Match(h));
+  }
+}
+BENCHMARK(BM_Group_Match);
+
+void BM_GroupPortable_Match(benchmark::State& state) {
+  std::array<ctrl_t, GroupPortableImpl::kWidth> group;
+  Iota(group.begin(), group.end(), -4);
+  GroupPortableImpl g{group.data()};
+  h2_t h = 1;
+  for (auto _ : state) {
+    ::benchmark::DoNotOptimize(h);
+    ::benchmark::DoNotOptimize(g);
+    ::benchmark::DoNotOptimize(g.Match(h));
+  }
+}
+BENCHMARK(BM_GroupPortable_Match);
+
+void BM_Group_MaskEmpty(benchmark::State& state) {
+  std::array<ctrl_t, Group::kWidth> group;
+  Iota(group.begin(), group.end(), -4);
+  Group g{group.data()};
+  for (auto _ : state) {
+    ::benchmark::DoNotOptimize(g);
+    ::benchmark::DoNotOptimize(g.MaskEmpty());
+  }
+}
+BENCHMARK(BM_Group_MaskEmpty);
+
+void BM_Group_MaskEmptyOrDeleted(benchmark::State& state) {
+  std::array<ctrl_t, Group::kWidth> group;
+  Iota(group.begin(), group.end(), -4);
+  Group g{group.data()};
+  for (auto _ : state) {
+    ::benchmark::DoNotOptimize(g);
+    ::benchmark::DoNotOptimize(g.MaskEmptyOrDeleted());
+  }
+}
+BENCHMARK(BM_Group_MaskEmptyOrDeleted);
+
+void BM_Group_MaskNonFull(benchmark::State& state) {
+  std::array<ctrl_t, Group::kWidth> group;
+  Iota(group.begin(), group.end(), -4);
+  Group g{group.data()};
+  for (auto _ : state) {
+    ::benchmark::DoNotOptimize(g);
+    ::benchmark::DoNotOptimize(g.MaskNonFull());
+  }
+}
+BENCHMARK(BM_Group_MaskNonFull);
+
+void BM_Group_MatchFirstEmptyOrDeleted(benchmark::State& state) {
+  std::array<ctrl_t, Group::kWidth> group;
+  Iota(group.begin(), group.end(), -2);
+  Group g{group.data()};
+  for (auto _ : state) {
+    ::benchmark::DoNotOptimize(g);
+    ::benchmark::DoNotOptimize(g.MaskEmptyOrDeleted().LowestBitSet());
+  }
+}
+BENCHMARK(BM_Group_MatchFirstEmptyOrDeleted);
+
+void BM_Group_MatchFirstNonFull(benchmark::State& state) {
+  std::array<ctrl_t, Group::kWidth> group;
+  Iota(group.begin(), group.end(), -2);
+  Group g{group.data()};
+  for (auto _ : state) {
+    ::benchmark::DoNotOptimize(g);
+    ::benchmark::DoNotOptimize(g.MaskNonFull().LowestBitSet());
+  }
+}
+BENCHMARK(BM_Group_MatchFirstNonFull);
+
+void BM_DropDeletes(benchmark::State& state) {
+  constexpr size_t capacity = (1 << 20) - 1;
+  std::vector<ctrl_t> ctrl(capacity + 1 + Group::kWidth);
+  ctrl[capacity] = ctrl_t::kSentinel;
+  std::vector<ctrl_t> pattern = {ctrl_t::kEmpty,   static_cast<ctrl_t>(2),
+                                 ctrl_t::kDeleted, static_cast<ctrl_t>(2),
+                                 ctrl_t::kEmpty,   static_cast<ctrl_t>(1),
+                                 ctrl_t::kDeleted};
+  for (size_t i = 0; i != capacity; ++i) {
+    ctrl[i] = pattern[i % pattern.size()];
+  }
+  while (state.KeepRunning()) {
+    state.PauseTiming();
+    std::vector<ctrl_t> ctrl_copy = ctrl;
+    state.ResumeTiming();
+    ConvertDeletedToEmptyAndFullToDeleted(ctrl_copy.data(), capacity);
+    ::benchmark::DoNotOptimize(ctrl_copy[capacity]);
+  }
+}
+BENCHMARK(BM_DropDeletes);
+
+void BM_Resize(benchmark::State& state) {
+  // For now just measure a small cheap hash table since we
+  // are mostly interested in the overhead of type-erasure
+  // in resize().
+  constexpr int kElements = 64;
+  const int kCapacity = kElements * 2;
+
+  IntTable table;
+  for (int i = 0; i < kElements; i++) {
+    table.insert(i);
+  }
+  for (auto unused : state) {
+    table.rehash(0);
+    table.rehash(kCapacity);
+  }
+}
+BENCHMARK(BM_Resize);
+
+void BM_EraseIf(benchmark::State& state) {
+  int64_t num_elements = state.range(0);
+  size_t num_erased = static_cast<size_t>(state.range(1));
+
+  constexpr size_t kRepetitions = 64;
+
+  absl::BitGen rng;
+
+  std::vector<std::vector<int64_t>> keys(kRepetitions);
+  std::vector<IntTable> tables;
+  std::vector<int64_t> threshold;
+  for (auto& k : keys) {
+    tables.push_back(IntTable());
+    auto& table = tables.back();
+    for (int64_t i = 0; i < num_elements; i++) {
+      // We use random keys to reduce noise.
+      k.push_back(
+          absl::Uniform<int64_t>(rng, 0, std::numeric_limits<int64_t>::max()));
+      if (!table.insert(k.back()).second) {
+        k.pop_back();
+        --i;  // duplicated value, retrying
+      }
+    }
+    std::sort(k.begin(), k.end());
+    threshold.push_back(static_cast<int64_t>(num_erased) < num_elements
+                            ? k[num_erased]
+                            : std::numeric_limits<int64_t>::max());
+  }
+
+  while (state.KeepRunningBatch(static_cast<int64_t>(kRepetitions) *
+                                std::max(num_elements, int64_t{1}))) {
+    benchmark::DoNotOptimize(tables);
+    for (size_t t_id = 0; t_id < kRepetitions; t_id++) {
+      auto& table = tables[t_id];
+      benchmark::DoNotOptimize(num_erased);
+      auto pred = [t = threshold[t_id]](int64_t key) { return key < t; };
+      benchmark::DoNotOptimize(pred);
+      benchmark::DoNotOptimize(table);
+      absl::container_internal::EraseIf(pred, &table);
+    }
+    state.PauseTiming();
+    for (size_t t_id = 0; t_id < kRepetitions; t_id++) {
+      auto& k = keys[t_id];
+      auto& table = tables[t_id];
+      for (size_t i = 0; i < num_erased; i++) {
+        table.insert(k[i]);
+      }
+    }
+    state.ResumeTiming();
+  }
+}
+
+BENCHMARK(BM_EraseIf)
+    ->ArgNames({"num_elements", "num_erased"})
+    ->ArgPair(10, 0)
+    ->ArgPair(1000, 0)
+    ->ArgPair(10, 5)
+    ->ArgPair(1000, 500)
+    ->ArgPair(10, 10)
+    ->ArgPair(1000, 1000);
+
+}  // namespace
+}  // namespace container_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+// These methods are here to make it easy to examine the assembly for targeted
+// parts of the API.
+auto CodegenAbslRawHashSetInt64Find(absl::container_internal::IntTable* table,
+                                    int64_t key) -> decltype(table->find(key)) {
+  return table->find(key);
+}
+
+bool CodegenAbslRawHashSetInt64FindNeEnd(
+    absl::container_internal::IntTable* table, int64_t key) {
+  return table->find(key) != table->end();
+}
+
+// This is useful because the find isn't inlined but the iterator comparison is.
+bool CodegenAbslRawHashSetStringFindNeEnd(
+    absl::container_internal::StringTable* table, const std::string& key) {
+  return table->find(key) != table->end();
+}
+
+auto CodegenAbslRawHashSetInt64Insert(absl::container_internal::IntTable* table,
+                                      int64_t key)
+    -> decltype(table->insert(key)) {
+  return table->insert(key);
+}
+
+bool CodegenAbslRawHashSetInt64Contains(
+    absl::container_internal::IntTable* table, int64_t key) {
+  return table->contains(key);
+}
+
+void CodegenAbslRawHashSetInt64Iterate(
+    absl::container_internal::IntTable* table) {
+  for (auto x : *table) benchmark::DoNotOptimize(x);
+}
+
+int odr =
+    (::benchmark::DoNotOptimize(std::make_tuple(
+         &CodegenAbslRawHashSetInt64Find, &CodegenAbslRawHashSetInt64FindNeEnd,
+         &CodegenAbslRawHashSetStringFindNeEnd,
+         &CodegenAbslRawHashSetInt64Insert, &CodegenAbslRawHashSetInt64Contains,
+         &CodegenAbslRawHashSetInt64Iterate)),
+     1);
diff --git a/absl/container/internal/raw_hash_set_probe_benchmark.cc b/absl/container/internal/raw_hash_set_probe_benchmark.cc
new file mode 100644
index 00000000000..fee9531aca3
--- /dev/null
+++ b/absl/container/internal/raw_hash_set_probe_benchmark.cc
@@ -0,0 +1,583 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// Generates probe length statistics for many combinations of key types and key
+// distributions, all using the default hash function for swisstable.
+
+#include <algorithm>
+#include <cstddef>
+#include <cstdint>
+#include <limits>
+#include <memory>
+#include <optional>
+#include <regex>  // NOLINT
+#include <string>
+#include <utility>
+#include <vector>
+
+#include "absl/base/no_destructor.h"
+#include "absl/container/flat_hash_map.h"
+#include "absl/container/internal/hash_function_defaults.h"
+#include "absl/container/internal/hashtable_debug.h"
+#include "absl/container/internal/raw_hash_set.h"
+#include "absl/random/distributions.h"
+#include "absl/random/random.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/str_format.h"
+#include "absl/strings/string_view.h"
+#include "absl/strings/strip.h"
+
+namespace {
+
+enum class OutputStyle { kRegular, kBenchmark };
+
+// The --benchmark command line flag.
+// This is populated from main().
+// When run in "benchmark" mode, we have different output. This allows
+// A/B comparisons with tools like `benchy`.
+absl::string_view benchmarks;
+
+OutputStyle output() {
+  return !benchmarks.empty() ? OutputStyle::kBenchmark : OutputStyle::kRegular;
+}
+
+template <class T>
+struct Policy {
+  using slot_type = T;
+  using key_type = T;
+  using init_type = T;
+
+  using DefaultHash = void;
+  using DefaultEq = void;
+  using DefaultAlloc = void;
+
+  template <class allocator_type, class Arg>
+  static void construct(allocator_type* alloc, slot_type* slot,
+                        const Arg& arg) {
+    std::allocator_traits<allocator_type>::construct(*alloc, slot, arg);
+  }
+
+  template <class allocator_type>
+  static void destroy(allocator_type* alloc, slot_type* slot) {
+    std::allocator_traits<allocator_type>::destroy(*alloc, slot);
+  }
+
+  static slot_type& element(slot_type* slot) { return *slot; }
+
+  template <class F, class... Args>
+  static auto apply(F&& f, const slot_type& arg)
+      -> decltype(std::forward<F>(f)(arg, arg)) {
+    return std::forward<F>(f)(arg, arg);
+  }
+
+  template <class Hash, bool kIsDefault, size_t kSeedShift>
+  static constexpr auto get_hash_slot_fn() {
+    return nullptr;
+  }
+};
+
+absl::BitGen& GlobalBitGen() {
+  static absl::NoDestructor<absl::BitGen> value;
+  return *value;
+}
+
+// Keeps a pool of allocations and randomly gives one out.
+// This introduces more randomization to the addresses given to swisstable and
+// should help smooth out this factor from probe length calculation.
+template <class T>
+class RandomizedAllocator {
+ public:
+  using value_type = T;
+
+  RandomizedAllocator() = default;
+  template <typename U>
+  RandomizedAllocator(RandomizedAllocator<U>) {}  // NOLINT
+
+  static T* allocate(size_t n) {
+    auto& pointers = GetPointers(n);
+    // Fill the pool
+    while (pointers.size() < kRandomPool) {
+      pointers.push_back(std::allocator<T>{}.allocate(n));
+    }
+
+    // Choose a random one.
+    size_t i = absl::Uniform<size_t>(GlobalBitGen(), 0, pointers.size());
+    T* result = pointers[i];
+    pointers[i] = pointers.back();
+    pointers.pop_back();
+    return result;
+  }
+
+  static void deallocate(T* p, size_t n) {
+    // Just put it back on the pool. No need to release the memory.
+    GetPointers(n).push_back(p);
+  }
+
+ private:
+  // We keep at least kRandomPool allocations for each size.
+  static constexpr size_t kRandomPool = 20;
+
+  static std::vector<T*>& GetPointers(size_t n) {
+    static absl::NoDestructor<absl::flat_hash_map<size_t, std::vector<T*>>> m;
+    return (*m)[n];
+  }
+};
+
+template <class T>
+struct DefaultHash {
+  using type = absl::container_internal::hash_default_hash<T>;
+};
+
+template <class T>
+using DefaultHashT = typename DefaultHash<T>::type;
+
+template <class T>
+struct Table : absl::container_internal::raw_hash_set<
+                   Policy<T>, DefaultHashT<T>,
+                   absl::container_internal::hash_default_eq<T>,
+                   RandomizedAllocator<T>> {};
+
+struct LoadSizes {
+  size_t min_load;
+  size_t max_load;
+};
+
+LoadSizes GetMinMaxLoadSizes() {
+  static const auto sizes = [] {
+    Table<int> t;
+
+    // First, fill enough to have a good distribution.
+    constexpr size_t kMinSize = 10000;
+    while (t.size() < kMinSize) t.insert(t.size());
+
+    const auto reach_min_load_factor = [&] {
+      const double lf = t.load_factor();
+      while (lf <= t.load_factor()) t.insert(t.size());
+    };
+
+    // Then, insert until we reach min load factor.
+    reach_min_load_factor();
+    const size_t min_load_size = t.size();
+
+    // Keep going until we hit min load factor again, then go back one.
+    t.insert(t.size());
+    reach_min_load_factor();
+
+    return LoadSizes{min_load_size, t.size() - 1};
+  }();
+  return sizes;
+}
+
+struct Ratios {
+  double min_load;
+  double avg_load;
+  double max_load;
+};
+
+// See absl/container/internal/hashtable_debug.h for details on
+// probe length calculation.
+template <class ElemFn>
+Ratios CollectMeanProbeLengths() {
+  const auto min_max_sizes = GetMinMaxLoadSizes();
+
+  ElemFn elem;
+  using Key = decltype(elem());
+  Table<Key> t;
+
+  Ratios result;
+  while (t.size() < min_max_sizes.min_load) t.insert(elem());
+  result.min_load =
+      absl::container_internal::GetHashtableDebugProbeSummary(t).mean;
+
+  while (t.size() < (min_max_sizes.min_load + min_max_sizes.max_load) / 2)
+    t.insert(elem());
+  result.avg_load =
+      absl::container_internal::GetHashtableDebugProbeSummary(t).mean;
+
+  while (t.size() < min_max_sizes.max_load) t.insert(elem());
+  result.max_load =
+      absl::container_internal::GetHashtableDebugProbeSummary(t).mean;
+
+  return result;
+}
+
+template <int Align>
+uintptr_t PointerForAlignment() {
+  alignas(Align) static constexpr uintptr_t kInitPointer = 0;
+  return reinterpret_cast<uintptr_t>(&kInitPointer);
+}
+
+// This incomplete type is used for testing hash of pointers of different
+// alignments.
+// NOTE: We are generating invalid pointer values on the fly with
+// reinterpret_cast. There are not "safely derived" pointers so using them is
+// technically UB. It is unlikely to be a problem, though.
+template <int Align>
+struct Ptr;
+
+template <int Align>
+Ptr<Align>* MakePtr(uintptr_t v) {
+  if (sizeof(v) == 8) {
+    constexpr int kCopyBits = 16;
+    // Ensure high bits are all the same.
+    v = static_cast<uintptr_t>(static_cast<intptr_t>(v << kCopyBits) >>
+                               kCopyBits);
+  }
+  return reinterpret_cast<Ptr<Align>*>(v);
+}
+
+enum class StringSize { kSmall, kMedium, kLarge, kExtraLarge };
+constexpr char kStringFormat[] = "%s/name-%07d-of-9999999.txt";
+
+template <StringSize size>
+struct String {
+  std::string value;
+  static std::string Make(uint32_t v) {
+    switch (size) {
+      case StringSize::kSmall:
+        return absl::StrCat(v);
+      case StringSize::kMedium:  // < 32 bytes
+        return absl::StrFormat(kStringFormat, "/path", v);
+      case StringSize::kLarge:  // 33-64 bytes
+        return absl::StrFormat(kStringFormat, "/path/to/file", v);
+      case StringSize::kExtraLarge:  // > 64 bytes
+        return absl::StrFormat(kStringFormat,
+                               "/path/to/a/very/long/file/name/so/that/total/"
+                               "length/is/larger/than/64/bytes",
+                               v);
+      default:
+        return "";
+    }
+  }
+};
+
+template <class T>
+struct Sequential {
+  T operator()() const { return current++; }
+  mutable T current{};
+};
+
+template <int Align>
+struct Sequential<Ptr<Align>*> {
+  Ptr<Align>* operator()() const {
+    auto* result = MakePtr<Align>(current);
+    current += Align;
+    return result;
+  }
+  mutable uintptr_t current = PointerForAlignment<Align>();
+};
+
+template <StringSize size>
+struct Sequential<String<size>> {
+  std::string operator()() const { return String<size>::Make(current++); }
+  mutable uint32_t current = 0;
+};
+
+template <class T, class U>
+struct Sequential<std::pair<T, U>> {
+  mutable Sequential<T> tseq;
+  mutable Sequential<U> useq;
+
+  using RealT = decltype(tseq());
+  using RealU = decltype(useq());
+
+  mutable std::vector<RealT> ts;
+  mutable std::vector<RealU> us;
+  mutable size_t ti = 0, ui = 0;
+
+  std::pair<RealT, RealU> operator()() const {
+    std::pair<RealT, RealU> value{get_t(), get_u()};
+    if (ti == 0) {
+      ti = ui + 1;
+      ui = 0;
+    } else {
+      --ti;
+      ++ui;
+    }
+    return value;
+  }
+
+  RealT get_t() const {
+    while (ti >= ts.size()) ts.push_back(tseq());
+    return ts[ti];
+  }
+
+  RealU get_u() const {
+    while (ui >= us.size()) us.push_back(useq());
+    return us[ui];
+  }
+};
+
+template <class T, int percent_skip>
+struct AlmostSequential {
+  mutable Sequential<T> current;
+
+  auto operator()() const -> decltype(current()) {
+    while (absl::Uniform(GlobalBitGen(), 0.0, 1.0) <= percent_skip / 100.)
+      current();
+    return current();
+  }
+};
+
+struct Uniform {
+  template <typename T>
+  T operator()(T) const {
+    return absl::Uniform<T>(absl::IntervalClosed, GlobalBitGen(), T{0}, ~T{0});
+  }
+};
+
+struct Gaussian {
+  template <typename T>
+  T operator()(T) const {
+    double d;
+    do {
+      d = absl::Gaussian<double>(GlobalBitGen(), 1e6, 1e4);
+    } while (d <= 0 || d > std::numeric_limits<T>::max() / 2);
+    return static_cast<T>(d);
+  }
+};
+
+struct Zipf {
+  template <typename T>
+  T operator()(T) const {
+    return absl::Zipf<T>(GlobalBitGen(), std::numeric_limits<T>::max(), 1.6);
+  }
+};
+
+template <class T, class Dist>
+struct Random {
+  T operator()() const { return Dist{}(T{}); }
+};
+
+template <class Dist, int Align>
+struct Random<Ptr<Align>*, Dist> {
+  Ptr<Align>* operator()() const {
+    return MakePtr<Align>(Random<uintptr_t, Dist>{}() * Align);
+  }
+};
+
+template <class Dist, StringSize size>
+struct Random<String<size>, Dist> {
+  std::string operator()() const {
+    return String<size>::Make(Random<uint32_t, Dist>{}());
+  }
+};
+
+template <class T, class U, class Dist>
+struct Random<std::pair<T, U>, Dist> {
+  auto operator()() const
+      -> decltype(std::make_pair(Random<T, Dist>{}(), Random<U, Dist>{}())) {
+    return std::make_pair(Random<T, Dist>{}(), Random<U, Dist>{}());
+  }
+};
+
+template <typename>
+std::string Name();
+
+std::string Name(uint32_t*) { return "u32"; }
+std::string Name(uint64_t*) { return "u64"; }
+
+template <int Align>
+std::string Name(Ptr<Align>**) {
+  return absl::StrCat("Ptr", Align);
+}
+
+template <StringSize size>
+std::string Name(String<size>*) {
+  switch (size) {
+    case StringSize::kSmall:
+      return "StrS";
+    case StringSize::kMedium:
+      return "StrM";
+    case StringSize::kLarge:
+      return "StrL";
+    case StringSize::kExtraLarge:
+      return "StrXL";
+    default:
+      return "";
+  }
+}
+
+template <class T, class U>
+std::string Name(std::pair<T, U>*) {
+  if (output() == OutputStyle::kBenchmark)
+    return absl::StrCat("P_", Name<T>(), "_", Name<U>());
+  return absl::StrCat("P<", Name<T>(), ",", Name<U>(), ">");
+}
+
+template <class T>
+std::string Name(Sequential<T>*) {
+  return "Sequential";
+}
+
+template <class T, int P>
+std::string Name(AlmostSequential<T, P>*) {
+  return absl::StrCat("AlmostSeq_", P);
+}
+
+template <class T>
+std::string Name(Random<T, Uniform>*) {
+  return "UnifRand";
+}
+
+template <class T>
+std::string Name(Random<T, Gaussian>*) {
+  return "GausRand";
+}
+
+template <class T>
+std::string Name(Random<T, Zipf>*) {
+  return "ZipfRand";
+}
+
+template <typename T>
+std::string Name() {
+  return Name(static_cast<T*>(nullptr));
+}
+
+constexpr int kNameWidth = 15;
+constexpr int kDistWidth = 16;
+
+bool CanRunBenchmark(absl::string_view name) {
+  static const absl::NoDestructor<std::optional<std::regex>> filter([] {
+    return benchmarks.empty() || benchmarks == "all"
+               ? std::nullopt
+               : std::make_optional(std::regex(std::string(benchmarks)));
+  }());
+  return !filter->has_value() || std::regex_search(std::string(name), **filter);
+}
+
+struct Result {
+  std::string name;
+  std::string dist_name;
+  Ratios ratios;
+};
+
+template <typename T, typename Dist>
+void RunForTypeAndDistribution(std::vector<Result>& results) {
+  std::string name = absl::StrCat(Name<T>(), "/", Name<Dist>());
+  // We have to check against all three names (min/avg/max) before we run it.
+  // If any of them is enabled, we run it.
+  if (!CanRunBenchmark(absl::StrCat(name, "/min")) &&
+      !CanRunBenchmark(absl::StrCat(name, "/avg")) &&
+      !CanRunBenchmark(absl::StrCat(name, "/max"))) {
+    return;
+  }
+  results.push_back({Name<T>(), Name<Dist>(), CollectMeanProbeLengths<Dist>()});
+}
+
+template <class T>
+void RunForType(std::vector<Result>& results) {
+  RunForTypeAndDistribution<T, Sequential<T>>(results);
+  RunForTypeAndDistribution<T, AlmostSequential<T, 20>>(results);
+  RunForTypeAndDistribution<T, AlmostSequential<T, 50>>(results);
+  RunForTypeAndDistribution<T, Random<T, Uniform>>(results);
+#ifdef NDEBUG
+  // Disable these in non-opt mode because they take too long.
+  RunForTypeAndDistribution<T, Random<T, Gaussian>>(results);
+  RunForTypeAndDistribution<T, Random<T, Zipf>>(results);
+#endif  // NDEBUG
+}
+
+}  // namespace
+
+int main(int argc, char** argv) {
+  // Parse the benchmark flags. Ignore all of them except the regex pattern.
+  for (int i = 1; i < argc; ++i) {
+    absl::string_view arg = argv[i];
+    const auto next = [&] { return argv[std::min(i + 1, argc - 1)]; };
+
+    if (absl::ConsumePrefix(&arg, "--benchmark_filter")) {
+      if (arg == "") {
+        // --benchmark_filter X
+        benchmarks = next();
+      } else if (absl::ConsumePrefix(&arg, "=")) {
+        // --benchmark_filter=X
+        benchmarks = arg;
+      }
+    }
+
+    // Any --benchmark flag turns on the mode.
+    if (absl::ConsumePrefix(&arg, "--benchmark")) {
+      if (benchmarks.empty()) benchmarks="all";
+    }
+  }
+
+  std::vector<Result> results;
+  RunForType<uint64_t>(results);
+  RunForType<Ptr<8>*>(results);
+  RunForType<Ptr<16>*>(results);
+  RunForType<Ptr<32>*>(results);
+  RunForType<Ptr<64>*>(results);
+  RunForType<std::pair<uint32_t, uint32_t>>(results);
+  RunForType<String<StringSize::kSmall>>(results);
+  RunForType<String<StringSize::kMedium>>(results);
+  RunForType<String<StringSize::kLarge>>(results);
+  RunForType<String<StringSize::kExtraLarge>>(results);
+  RunForType<std::pair<uint64_t, String<StringSize::kSmall>>>(results);
+  RunForType<std::pair<String<StringSize::kSmall>, uint64_t>>(results);
+  RunForType<std::pair<uint64_t, String<StringSize::kMedium>>>(results);
+  RunForType<std::pair<String<StringSize::kMedium>, uint64_t>>(results);
+  RunForType<std::pair<uint64_t, String<StringSize::kLarge>>>(results);
+  RunForType<std::pair<String<StringSize::kLarge>, uint64_t>>(results);
+  RunForType<std::pair<uint64_t, String<StringSize::kExtraLarge>>>(results);
+  RunForType<std::pair<String<StringSize::kExtraLarge>, uint64_t>>(results);
+
+  switch (output()) {
+    case OutputStyle::kRegular:
+      absl::PrintF("%-*s%-*s       Min       Avg       Max\n%s\n", kNameWidth,
+                   "Type", kDistWidth, "Distribution",
+                   std::string(kNameWidth + kDistWidth + 10 * 3, '-'));
+      for (const auto& result : results) {
+        absl::PrintF("%-*s%-*s  %8.4f  %8.4f  %8.4f\n", kNameWidth, result.name,
+                     kDistWidth, result.dist_name, result.ratios.min_load,
+                     result.ratios.avg_load, result.ratios.max_load);
+      }
+      break;
+    case OutputStyle::kBenchmark: {
+      absl::PrintF("{\n");
+      absl::PrintF("  \"benchmarks\": [\n");
+      absl::string_view comma;
+      for (const auto& result : results) {
+        auto print = [&](absl::string_view stat, double Ratios::*val) {
+          std::string name =
+              absl::StrCat(result.name, "/", result.dist_name, "/", stat);
+          // Check the regex again. We might had have enabled only one of the
+          // stats for the benchmark.
+          if (!CanRunBenchmark(name)) return;
+          // Report at least 1, because benchy drops results with zero.
+          double reported_value = std::max(1e9 * result.ratios.*val, 1.0);
+          absl::PrintF("    %s{\n", comma);
+          absl::PrintF("      \"cpu_time\": %f,\n", reported_value);
+          absl::PrintF("      \"real_time\": %f,\n", reported_value);
+          absl::PrintF("      \"iterations\": 1,\n");
+          absl::PrintF("      \"name\": \"%s\",\n", name);
+          absl::PrintF("      \"time_unit\": \"ns\"\n");
+          absl::PrintF("    }\n");
+          comma = ",";
+        };
+        print("min", &Ratios::min_load);
+        print("avg", &Ratios::avg_load);
+        print("max", &Ratios::max_load);
+      }
+      absl::PrintF("  ],\n");
+      absl::PrintF("  \"context\": {\n");
+      absl::PrintF("  }\n");
+      absl::PrintF("}\n");
+      break;
+    }
+  }
+
+  return 0;
+}
diff --git a/absl/container/internal/raw_hash_set_resize_impl.h b/absl/container/internal/raw_hash_set_resize_impl.h
new file mode 100644
index 00000000000..ed48d96bc67
--- /dev/null
+++ b/absl/container/internal/raw_hash_set_resize_impl.h
@@ -0,0 +1,79 @@
+// Copyright 2025 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// This is a private implementation detail of resize algorithm of
+// raw_hash_set. It is exposed in a separate file for testing purposes.
+
+#ifndef ABSL_CONTAINER_INTERNAL_RAW_HASH_SET_RESIZE_IMPL_H_
+#define ABSL_CONTAINER_INTERNAL_RAW_HASH_SET_RESIZE_IMPL_H_
+
+#include <cstddef>
+#include <cstdint>
+
+#include "absl/base/config.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace container_internal {
+
+// Encoding for probed elements used for smaller tables.
+// Data is encoded into single integer.
+// Storage format for 4 bytes:
+// - 7 bits for h2
+// - 12 bits for source_offset
+// - 13 bits for h1
+// Storage format for 8 bytes:
+// - 7 bits for h2
+// - 28 bits for source_offset
+// - 29 bits for h1
+// Storage format for 16 bytes:
+// - 7 bits for h2
+// - 57 bits for source_offset
+// - 58 bits for h1
+template <typename IntType, size_t kTotalBits>
+struct ProbedItemImpl {
+  static constexpr IntType kH2Bits = 7;
+
+  static constexpr IntType kMaxOldBits = (kTotalBits - kH2Bits) / 2;
+  static constexpr IntType kMaxOldCapacity = (IntType{1} << kMaxOldBits) - 1;
+
+  // We always have one bit more for h1.
+  static constexpr IntType kMaxNewBits = kMaxOldBits + 1;
+  static constexpr IntType kMaxNewCapacity = (IntType{1} << kMaxNewBits) - 1;
+
+  static_assert(kMaxNewBits + kMaxOldBits + kH2Bits == kTotalBits);
+
+  ProbedItemImpl() = default;
+  ProbedItemImpl(uint8_t h2_arg, size_t source_offset_arg, size_t h1_arg)
+      : h2(h2_arg),
+        source_offset(static_cast<IntType>(source_offset_arg)),
+        h1(static_cast<IntType>(h1_arg)) {}
+
+  IntType h2 : kH2Bits;
+  IntType source_offset : kMaxOldBits;
+  IntType h1 : kMaxNewBits;
+};
+
+using ProbedItem4Bytes = ProbedItemImpl<uint32_t, 32>;
+static_assert(sizeof(ProbedItem4Bytes) == 4);
+using ProbedItem8Bytes = ProbedItemImpl<uint64_t, 64>;
+static_assert(sizeof(ProbedItem8Bytes) == 8);
+using ProbedItem16Bytes = ProbedItemImpl<uint64_t, 7 + 57 + 58>;
+static_assert(sizeof(ProbedItem16Bytes) == 16);
+
+}  // namespace container_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_CONTAINER_INTERNAL_RAW_HASH_SET_RESIZE_IMPL_H_
diff --git a/absl/container/internal/raw_hash_set_resize_impl_test.cc b/absl/container/internal/raw_hash_set_resize_impl_test.cc
new file mode 100644
index 00000000000..5020fca1805
--- /dev/null
+++ b/absl/container/internal/raw_hash_set_resize_impl_test.cc
@@ -0,0 +1,66 @@
+// Copyright 2025 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/container/internal/raw_hash_set_resize_impl.h"
+
+#include <cstddef>
+#include <cstdint>
+#include <limits>
+
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace container_internal {
+namespace {
+
+constexpr size_t kSmallSourceOffset = 17;
+constexpr size_t kSmallH1 = 25;
+
+template <class Encoder>
+class EncoderTest : public testing::Test {};
+
+using ProbedItemTypes =
+    ::testing::Types<ProbedItem4Bytes, ProbedItem8Bytes, ProbedItem16Bytes>;
+TYPED_TEST_SUITE(EncoderTest, ProbedItemTypes);
+
+TYPED_TEST(EncoderTest, EncodeDecodeSmall) {
+  using ProbedItem = TypeParam;
+  for (uint8_t h2 = 0; h2 < 128; ++h2) {
+    ProbedItem item(h2, kSmallSourceOffset, kSmallH1);
+    EXPECT_EQ(item.h2, h2);
+    EXPECT_EQ(item.source_offset, kSmallSourceOffset);
+    EXPECT_EQ(item.h1 & ProbedItem::kMaxNewCapacity, kSmallH1);
+  }
+}
+
+TYPED_TEST(EncoderTest, EncodeDecodeMax) {
+  using ProbedItem = TypeParam;
+  for (uint8_t h2 = 0; h2 < 128; ++h2) {
+    size_t source_offset = static_cast<size_t>(std::min<uint64_t>(
+        ProbedItem::kMaxOldCapacity, (std::numeric_limits<size_t>::max)()));
+    size_t h1 = static_cast<size_t>(std::min<uint64_t>(
+        ProbedItem::kMaxNewCapacity, (std::numeric_limits<size_t>::max)()));
+    ProbedItem item(h2, source_offset, h1);
+    EXPECT_EQ(item.h2, h2);
+    EXPECT_EQ(item.source_offset, source_offset);
+    EXPECT_EQ(item.h1 & ProbedItem::kMaxNewCapacity, h1);
+  }
+}
+
+}  // namespace
+}  // namespace container_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/container/internal/raw_hash_set_test.cc b/absl/container/internal/raw_hash_set_test.cc
index ed4ca8c88d3..ec5ec30495f 100644
--- a/absl/container/internal/raw_hash_set_test.cc
+++ b/absl/container/internal/raw_hash_set_test.cc
@@ -14,45 +14,606 @@
 
 #include "absl/container/internal/raw_hash_set.h"
 
+#include <algorithm>
+#include <array>
+#include <atomic>
+#include <bitset>
 #include <cmath>
+#include <cstddef>
 #include <cstdint>
+#include <cstring>
 #include <deque>
 #include <functional>
+#include <iostream>
+#include <iterator>
+#include <limits>
+#include <list>
+#include <map>
 #include <memory>
 #include <numeric>
+#include <optional>
+#include <ostream>
 #include <random>
+#include <set>
 #include <string>
+#include <tuple>
+#include <type_traits>
+#include <unordered_map>
+#include <unordered_set>
+#include <utility>
+#include <vector>
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
 #include "absl/base/attributes.h"
+#include "absl/base/config.h"
 #include "absl/base/internal/cycleclock.h"
-#include "absl/base/internal/raw_logging.h"
+#include "absl/base/prefetch.h"
+#include "absl/container/flat_hash_map.h"
+#include "absl/container/flat_hash_set.h"
 #include "absl/container/internal/container_memory.h"
 #include "absl/container/internal/hash_function_defaults.h"
 #include "absl/container/internal/hash_policy_testing.h"
+#include "absl/container/internal/hashtable_control_bytes.h"
 #include "absl/container/internal/hashtable_debug.h"
+#include "absl/container/internal/hashtablez_sampler.h"
+#include "absl/container/internal/raw_hash_set_resize_impl.h"
+#include "absl/container/internal/test_allocator.h"
+#include "absl/container/internal/test_instance_tracker.h"
+#include "absl/container/node_hash_set.h"
+#include "absl/functional/function_ref.h"
+#include "absl/hash/hash.h"
+#include "absl/log/check.h"
+#include "absl/log/log.h"
+#include "absl/memory/memory.h"
+#include "absl/meta/type_traits.h"
+#include "absl/numeric/int128.h"
+#include "absl/random/random.h"
+#include "absl/strings/str_cat.h"
 #include "absl/strings/string_view.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
 struct RawHashSetTestOnlyAccess {
   template <typename C>
-  static auto GetSlots(const C& c) -> decltype(c.slots_) {
-    return c.slots_;
+  static auto GetCommon(C&& c) -> decltype(std::forward<C>(c).common()) {
+    return std::forward<C>(c).common();
+  }
+  template <typename C>
+  static auto GetSlots(const C& c) -> decltype(c.slot_array(c.capacity())) {
+    return c.slot_array(c.capacity());
+  }
+  template <typename C>
+  static size_t CountTombstones(const C& c) {
+    return c.common().TombstonesCount();
   }
 };
 
 namespace {
 
-using ::testing::DoubleNear;
 using ::testing::ElementsAre;
+using ::testing::Eq;
 using ::testing::Ge;
+using ::testing::HasSubstr;
 using ::testing::Lt;
-using ::testing::Optional;
 using ::testing::Pair;
 using ::testing::UnorderedElementsAre;
+using ::testing::UnorderedElementsAreArray;
+
+// Enables sampling with 1 percent sampling rate and
+// resets the rate counter for the current thread.
+void SetSamplingRateTo1Percent() {
+  SetHashtablezEnabled(true);
+  SetHashtablezSampleParameter(100);  // Sample ~1% of tables.
+  // Reset rate counter for the current thread.
+  TestOnlyRefreshSamplingStateForCurrentThread();
+}
+
+// Disables sampling and resets the rate counter for the current thread.
+void DisableSampling() {
+  SetHashtablezEnabled(false);
+  SetHashtablezSampleParameter(1 << 16);
+  // Reset rate counter for the current thread.
+  TestOnlyRefreshSamplingStateForCurrentThread();
+}
+
+// Convenience function to static cast to ctrl_t.
+ctrl_t CtrlT(int i) { return static_cast<ctrl_t>(i); }
+
+TEST(RawHashSetLayout, SmallCapacity) {
+  {
+    SCOPED_TRACE("capacity=1 no alignment after generation");
+    constexpr size_t kSlotSize = 1;
+    RawHashSetLayout layout(1, kSlotSize, /*slot_align=*/1,
+                            /*has_infoz=*/false, /*blocked_element_count=*/0);
+    EXPECT_EQ(layout.control_offset(), NumGenerationBytes());
+    EXPECT_EQ(layout.slot_offset(), NumGenerationBytes());
+    EXPECT_EQ(layout.alloc_size(), NumGenerationBytes() + kSlotSize);
+  }
+  {
+    SCOPED_TRACE("capacity=1 with alignment after generation");
+    constexpr size_t kSlotSize = 8;
+    constexpr size_t kAlignment = 4;
+    RawHashSetLayout layout(1, kSlotSize, kAlignment,
+                            /*has_infoz=*/false, /*blocked_element_count=*/0);
+    EXPECT_EQ(layout.control_offset(),
+              NumGenerationBytes() == 0 ? 0 : kAlignment);
+    EXPECT_EQ(layout.slot_offset(), NumGenerationBytes() == 0 ? 0 : kAlignment);
+    EXPECT_EQ(layout.alloc_size(), layout.slot_offset() + kSlotSize);
+  }
+}
+
+void VerifyMiddleSizeTableLayout(size_t capacity, size_t slot_size,
+                                 size_t slot_align, bool has_infoz,
+                                 size_t blocked_element_count,
+                                 size_t padding = 0) {
+  SCOPED_TRACE(testing::Message()
+               << "capacity: " << capacity << " slot_size: " << slot_size
+               << " slot_align: " << slot_align << " has_infoz: " << has_infoz
+               << " blocked_element_count: " << blocked_element_count
+               << " padding: " << padding);
+  ASSERT_GT(capacity, 1);
+  ASSERT_LE(capacity, GrowthInfoLowerBound::kMaxGrowthLeftLowerBound);
+  RawHashSetLayout layout(capacity, slot_size, slot_align, has_infoz,
+                          blocked_element_count);
+  EXPECT_EQ(layout.control_offset(),
+            /*growth*/ 1 + padding + NumGenerationBytes());
+  size_t expected_slot_offset =
+      layout.control_offset() + NumControlBytes(capacity);
+  EXPECT_LT(padding, slot_align);
+  EXPECT_EQ(expected_slot_offset % slot_align, 0);
+  EXPECT_EQ(layout.slot_offset(), expected_slot_offset);
+  size_t allocated_values = capacity - blocked_element_count;
+  EXPECT_EQ(layout.alloc_size(),
+            expected_slot_offset + allocated_values * slot_size);
+}
+
+TEST(RawHashSetLayout, MiddleSize) {
+  VerifyMiddleSizeTableLayout(/*capacity=*/3, /*slot_size=*/4,
+                              /*slot_align=*/4, /*has_infoz=*/false,
+                              /*blocked_element_count=*/1,
+                              /*padding=*/NumGenerationBytes() == 0 ? 0 : 3);
+  VerifyMiddleSizeTableLayout(/*capacity=*/7, /*slot_size=*/4,
+                              /*slot_align=*/4, /*has_infoz=*/false,
+                              /*blocked_element_count=*/1,
+                              /*padding=*/NumGenerationBytes() == 0 ? 0 : 3);
+  VerifyMiddleSizeTableLayout(/*capacity=*/127, /*slot_size=*/8,
+                              /*slot_align=*/8, /*has_infoz=*/false,
+                              /*blocked_element_count=*/3,
+                              /*padding=*/NumGenerationBytes() == 0 ? 0 : 7);
+}
+
+#if defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
+TEST(RawHashSetLayout, SmallWithInfoZ) {
+  {
+    SCOPED_TRACE("capacity=1 no alignment after generation");
+    RawHashSetLayout layout(1, /*slot_size=*/1, /*slot_align=*/1,
+                            /*has_infoz=*/true, /*blocked_element_count=*/0);
+    EXPECT_EQ(layout.control_offset(),
+              // growth_info is always 8 bytes for sampled tables.
+              8 + sizeof(HashtablezInfoHandle) + NumGenerationBytes());
+    EXPECT_EQ(layout.slot_offset(), layout.control_offset());
+    EXPECT_EQ(layout.alloc_size(), layout.slot_offset() + 1);
+  }
+  {
+    constexpr size_t kSlotSize = 8;
+    constexpr size_t kAlignment = 8;
+    constexpr size_t kCapacity = 3;
+    RawHashSetLayout layout(kCapacity, /*slot_size=*/kSlotSize,
+                            /*slot_align=*/kAlignment,
+                            /*has_infoz=*/true, /*blocked_element_count=*/0);
+    size_t padding = NumGenerationBytes() == 0 ? 1 : 0;
+    padding += sizeof(HashtablezInfoHandle) == 4 ? 0 : 4;
+    EXPECT_EQ(layout.control_offset(),
+              // growth_info is always 8 bytes for sampled tables.
+              /*growth*/ 8 + sizeof(HashtablezInfoHandle) + padding +
+                  NumGenerationBytes());
+    size_t expected_slot_offset =
+        layout.control_offset() + NumControlBytes(kCapacity);
+    EXPECT_EQ(expected_slot_offset % kAlignment, 0);
+    EXPECT_EQ(layout.slot_offset(), expected_slot_offset);
+    EXPECT_EQ(layout.alloc_size(),
+              expected_slot_offset + kCapacity * kSlotSize);
+  }
+}
+#endif  // defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
+
+void VerifyLargeTableLayout(size_t capacity, size_t slot_size,
+                            size_t slot_align, bool has_infoz,
+                            size_t blocked_element_count) {
+  SCOPED_TRACE(testing::Message()
+               << "capacity: " << capacity << " slot_size: " << slot_size
+               << " slot_align: " << slot_align << " has_infoz: " << has_infoz
+               << " blocked_element_count: " << blocked_element_count);
+  ASSERT_GT(capacity, GrowthInfoLowerBound::kMaxGrowthLeftLowerBound);
+  RawHashSetLayout layout(capacity, slot_size, slot_align, has_infoz,
+                          blocked_element_count);
+  size_t padding = NumGenerationBytes() == 0 ? 1 : 0;
+  EXPECT_EQ(layout.control_offset(),
+            padding + (has_infoz ? 8 + sizeof(HashtablezInfoHandle) : 8) +
+                NumGenerationBytes());
+  size_t expected_slot_offset =
+      layout.control_offset() + NumControlBytes(capacity);
+  EXPECT_EQ(expected_slot_offset % slot_align, 0);
+  EXPECT_EQ(layout.slot_offset(), expected_slot_offset);
+  EXPECT_EQ(
+      layout.alloc_size(),
+      expected_slot_offset + (capacity - blocked_element_count) * slot_size);
+}
+
+TEST(RawHashSetLayout, Large) {
+  VerifyLargeTableLayout(/*capacity=*/255, /*slot_size=*/8, /*slot_align=*/8,
+                         /*has_infoz=*/false, /*blocked_element_count=*/5);
+#if defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
+  VerifyLargeTableLayout(/*capacity=*/1023, /*slot_size=*/8, /*slot_align=*/8,
+                         /*has_infoz=*/true, /*blocked_element_count=*/2);
+#endif  // defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
+}
+
+TEST(BlockedInfoTest, ConstructFromComponents) {
+  constexpr BlockedInfo info(10, 2);
+  EXPECT_EQ(info.log2_period(), 10);
+  EXPECT_EQ(info.tail_blocked(), 2);
+
+  constexpr BlockedInfo info_zero(0, 0);
+  EXPECT_EQ(info_zero.log2_period(), 0);
+  EXPECT_EQ(info_zero.tail_blocked(), 0);
+
+  constexpr BlockedInfo info_max(63, 3);
+  EXPECT_EQ(info_max.log2_period(), 63);
+  EXPECT_EQ(info_max.tail_blocked(), 3);
+}
+
+TEST(BlockedInfoTest, BlockedBefore) {
+  constexpr BlockedInfo info3(3, 0);
+  EXPECT_EQ(info3.blocked_before(0), 0);
+  EXPECT_EQ(info3.blocked_before(7), 0);
+  EXPECT_EQ(info3.blocked_before(8), 1);
+  EXPECT_EQ(info3.blocked_before(15), 1);
+  EXPECT_EQ(info3.blocked_before(16), 2);
+  EXPECT_EQ(info3.blocked_before(24), 3);
+  EXPECT_EQ(info3.blocked_before(100), 12);
+
+  constexpr BlockedInfo info0(0, 0);
+  EXPECT_EQ(info0.blocked_before(0), 0);
+  EXPECT_EQ(info0.blocked_before(5), 5);
+  EXPECT_EQ(info0.blocked_before(10), 10);
+
+  constexpr BlockedInfo info4(4, 1);
+  EXPECT_EQ(info4.blocked_before(0), 0);
+  EXPECT_EQ(info4.blocked_before(15), 0);
+  EXPECT_EQ(info4.blocked_before(16), 1);
+  EXPECT_EQ(info4.blocked_before(31), 1);
+  EXPECT_EQ(info4.blocked_before(32), 2);
+}
+
+TEST(BlockedInfoTest, TotalBlockedCount) {
+  constexpr BlockedInfo info(3, 2);
+  EXPECT_EQ(info.total_blocked_count(0), 2);
+  EXPECT_EQ(info.total_blocked_count(7), 2);
+  EXPECT_EQ(info.total_blocked_count(8), 3);
+  EXPECT_EQ(info.total_blocked_count(15), 3);
+  EXPECT_EQ(info.total_blocked_count(31), 5);
+
+  constexpr BlockedInfo info_zero(0, 0);
+  EXPECT_EQ(info_zero.total_blocked_count(0), 0);
+  EXPECT_EQ(info_zero.total_blocked_count(15), 15);
+
+  constexpr BlockedInfo info_tail(5, 3);
+  EXPECT_EQ(info_tail.total_blocked_count(31), 3);
+  EXPECT_EQ(info_tail.total_blocked_count(63), 4);
+}
+
+class GrowthInfoAllocator {
+ public:
+  explicit GrowthInfoAllocator(size_t capacity) {
+    if (capacity <= GrowthInfoLowerBound::kMaxGrowthLeftLowerBound) {
+      SanitizerPoisonMemoryRegion(control_.data(), 7);
+    }
+    SanitizerPoisonMemoryRegion(control_.data() + kControlStart, 1);
+    if constexpr (NumGenerationBytes() > 0) {
+      SanitizerPoisonMemoryRegion(
+          control_.data() + kControlStart + NumGenerationBytes(),
+          NumGenerationBytes());
+    }
+  }
+
+  GrowthInfoAccessor* operator->() { return &growth_info_; }
+
+ private:
+  static constexpr size_t kControlStart = 8 + NumGenerationBytes();
+  // We allocate on heap since ASAN fails to detect access to poisoned memory
+  // on stack.
+  std::vector<ctrl_t> control_ = std::vector<ctrl_t>(
+      9 + NumGenerationBytes(), /*garbage*/ ctrl_t::kSentinel);
+  GrowthInfoAccessor growth_info_ =
+      GrowthInfoAccessor(control_.data() + kControlStart);
+};
+
+TEST(GrowthInfoViewTest, GetGrowthLeft) {
+  constexpr size_t kCapacity = 7;
+  GrowthInfoAllocator growth_info(kCapacity);
+  growth_info->InitGrowthLeftNoDeleted(/*growth_left=*/5, kCapacity);
+  EXPECT_EQ(growth_info->GetGrowthLeftLowerBound(), 5);
+  EXPECT_EQ(growth_info->GetGrowthLeftTotalSlow(kCapacity), 5);
+  growth_info->OverwriteFullAsDeleted();
+  EXPECT_EQ(growth_info->GetGrowthLeftLowerBound(), 5);
+  EXPECT_EQ(growth_info->GetGrowthLeftTotalSlow(kCapacity), 5);
+}
+
+TEST(GrowthInfoViewTest, HasNoDeleted) {
+  constexpr size_t kCapacity = 7;
+  GrowthInfoAllocator growth_info(kCapacity);
+  growth_info->InitGrowthLeftNoDeleted(/*growth_left=*/5, kCapacity);
+  EXPECT_TRUE(growth_info->GetGrowthInfoLowerBound().HasNoDeleted());
+  growth_info->OverwriteFullAsDeleted();
+  EXPECT_FALSE(growth_info->GetGrowthInfoLowerBound().HasNoDeleted());
+  // After reinitialization we have no deleted slots.
+  growth_info->InitGrowthLeftNoDeleted(/*growth_left=*/5, kCapacity);
+  EXPECT_TRUE(growth_info->GetGrowthInfoLowerBound().HasNoDeleted());
+}
+
+TEST(GrowthInfoViewTest, HasNoDeletedAndGrowthLeft) {
+  constexpr size_t kCapacity = 7;
+  GrowthInfoAllocator growth_info(kCapacity);
+  growth_info->InitGrowthLeftNoDeleted(/*growth_left=*/5, kCapacity);
+  EXPECT_TRUE(
+      growth_info->GetGrowthInfoLowerBound().HasNoDeletedAndGrowthLeft());
+  growth_info->OverwriteFullAsDeleted();
+  EXPECT_FALSE(
+      growth_info->GetGrowthInfoLowerBound().HasNoDeletedAndGrowthLeft());
+  growth_info->InitGrowthLeftNoDeleted(/*growth_left=*/0, kCapacity);
+  EXPECT_FALSE(
+      growth_info->GetGrowthInfoLowerBound().HasNoDeletedAndGrowthLeft());
+  growth_info->OverwriteFullAsDeleted();
+  EXPECT_FALSE(
+      growth_info->GetGrowthInfoLowerBound().HasNoDeletedAndGrowthLeft());
+  // After reinitialization we have no deleted slots.
+  growth_info->InitGrowthLeftNoDeleted(/*growth_left=*/5, kCapacity);
+  EXPECT_TRUE(
+      growth_info->GetGrowthInfoLowerBound().HasNoDeletedAndGrowthLeft());
+}
+
+TEST(GrowthInfoViewTest, HasNoGrowthLeftAndNoDeleted) {
+  constexpr size_t kCapacity = 7;
+  GrowthInfoAllocator growth_info(kCapacity);
+  growth_info->InitGrowthLeftNoDeleted(/*growth_left=*/1, kCapacity);
+  EXPECT_FALSE(
+      growth_info->GetGrowthInfoLowerBound().HasNoGrowthLeftAndNoDeleted());
+  growth_info->OverwriteEmptyAsFull();
+  EXPECT_TRUE(
+      growth_info->GetGrowthInfoLowerBound().HasNoGrowthLeftAndNoDeleted());
+  growth_info->OverwriteFullAsDeleted();
+  EXPECT_FALSE(
+      growth_info->GetGrowthInfoLowerBound().HasNoGrowthLeftAndNoDeleted());
+  growth_info->OverwriteFullAsEmpty();
+  EXPECT_FALSE(
+      growth_info->GetGrowthInfoLowerBound().HasNoGrowthLeftAndNoDeleted());
+  growth_info->InitGrowthLeftNoDeleted(/*growth_left=*/0, kCapacity);
+  EXPECT_TRUE(
+      growth_info->GetGrowthInfoLowerBound().HasNoGrowthLeftAndNoDeleted());
+  growth_info->OverwriteFullAsEmpty();
+  EXPECT_FALSE(
+      growth_info->GetGrowthInfoLowerBound().HasNoGrowthLeftAndNoDeleted());
+}
+
+TEST(GrowthInfoViewTest, OverwriteFullAsEmpty) {
+  constexpr size_t kCapacity = 7;
+  GrowthInfoAllocator growth_info(kCapacity);
+  growth_info->InitGrowthLeftNoDeleted(/*growth_left=*/5, kCapacity);
+  growth_info->OverwriteFullAsEmpty();
+  EXPECT_EQ(growth_info->GetGrowthLeftLowerBound(), 6);
+  growth_info->OverwriteFullAsDeleted();
+  EXPECT_EQ(growth_info->GetGrowthLeftLowerBound(), 6);
+  growth_info->OverwriteFullAsEmpty();
+  EXPECT_EQ(growth_info->GetGrowthLeftLowerBound(), 7);
+  EXPECT_FALSE(growth_info->GetGrowthInfoLowerBound().HasNoDeleted());
+}
+
+TEST(GrowthInfoViewTest, OverwriteEmptyAsFull) {
+  constexpr size_t kCapacity = 7;
+  GrowthInfoAllocator growth_info(kCapacity);
+  growth_info->InitGrowthLeftNoDeleted(/*growth_left=*/5, kCapacity);
+  growth_info->OverwriteEmptyAsFull();
+  EXPECT_EQ(growth_info->GetGrowthLeftLowerBound(), 4);
+  growth_info->OverwriteFullAsDeleted();
+  EXPECT_EQ(growth_info->GetGrowthLeftLowerBound(), 4);
+  growth_info->OverwriteEmptyAsFull();
+  EXPECT_EQ(growth_info->GetGrowthLeftLowerBound(), 3);
+  EXPECT_FALSE(growth_info->GetGrowthInfoLowerBound().HasNoDeleted());
+}
+
+TEST(GrowthInfoViewTest, OverwriteControlAsFull) {
+  constexpr size_t kCapacity = 7;
+  GrowthInfoAllocator growth_info(kCapacity);
+  growth_info->InitGrowthLeftNoDeleted(/*growth_left=*/5, kCapacity);
+  growth_info->OverwriteControlAsFull(ctrl_t::kEmpty);
+  EXPECT_EQ(growth_info->GetGrowthLeftLowerBound(), 4);
+  growth_info->OverwriteControlAsFull(ctrl_t::kDeleted);
+  EXPECT_EQ(growth_info->GetGrowthLeftLowerBound(), 4);
+  growth_info->OverwriteFullAsDeleted();
+  growth_info->OverwriteControlAsFull(ctrl_t::kDeleted);
+  // We do not count number of deleted, so the bit sticks till the next rehash.
+  EXPECT_FALSE(
+      growth_info->GetGrowthInfoLowerBound().HasNoDeletedAndGrowthLeft());
+  EXPECT_FALSE(growth_info->GetGrowthInfoLowerBound().HasNoDeleted());
+}
+
+TEST(GrowthInfoViewTest, HasNoGrowthLeftAndHaveDeleted) {
+  constexpr size_t kCapacity = 7;
+  GrowthInfoAllocator growth_info(kCapacity);
+  growth_info->InitGrowthLeftNoDeleted(/*growth_left=*/1, kCapacity);
+  growth_info->OverwriteFullAsDeleted();
+  EXPECT_EQ(growth_info->GetGrowthLeftLowerBound(), 1);
+  EXPECT_FALSE(
+      growth_info->GetGrowthInfoLowerBound().HasNoGrowthLeftAndHaveDeleted());
+  growth_info->OverwriteControlAsFull(ctrl_t::kDeleted);
+  EXPECT_EQ(growth_info->GetGrowthLeftLowerBound(), 1);
+  EXPECT_FALSE(
+      growth_info->GetGrowthInfoLowerBound().HasNoGrowthLeftAndHaveDeleted());
+  growth_info->OverwriteFullAsEmpty();
+  EXPECT_EQ(growth_info->GetGrowthLeftLowerBound(), 2);
+  EXPECT_FALSE(
+      growth_info->GetGrowthInfoLowerBound().HasNoGrowthLeftAndHaveDeleted());
+  growth_info->OverwriteEmptyAsFull();
+  EXPECT_EQ(growth_info->GetGrowthLeftLowerBound(), 1);
+  EXPECT_FALSE(
+      growth_info->GetGrowthInfoLowerBound().HasNoGrowthLeftAndHaveDeleted());
+  growth_info->OverwriteEmptyAsFull();
+  EXPECT_EQ(growth_info->GetGrowthLeftLowerBound(), 0);
+  EXPECT_TRUE(
+      growth_info->GetGrowthInfoLowerBound().HasNoGrowthLeftAndHaveDeleted());
+}
+
+TEST(GrowthInfoViewTest, HasNoGrowthLeftAndHaveDeletedReturnFalseIfNoDeleted) {
+  constexpr size_t kCapacity = 7;
+  GrowthInfoAllocator growth_info(kCapacity);
+  growth_info->InitGrowthLeftNoDeleted(/*growth_left=*/0, kCapacity);
+  EXPECT_FALSE(
+      growth_info->GetGrowthInfoLowerBound().HasNoGrowthLeftAndHaveDeleted());
+}
+
+TEST(GrowthInfoViewTest, HasDeletedAndGrowthLeft) {
+  constexpr size_t kCapacity = 7;
+  GrowthInfoAllocator growth_info(kCapacity);
+  growth_info->InitGrowthLeftNoDeleted(/*growth_left=*/1, kCapacity);
+  EXPECT_FALSE(
+      growth_info->GetGrowthInfoLowerBound().HasDeletedAndGrowthLeft());
+  growth_info->OverwriteFullAsDeleted();
+  EXPECT_TRUE(growth_info->GetGrowthInfoLowerBound().HasDeletedAndGrowthLeft());
+  growth_info->OverwriteEmptyAsFull();
+  EXPECT_FALSE(
+      growth_info->GetGrowthInfoLowerBound().HasDeletedAndGrowthLeft());
+}
+
+TEST(GrowthInfoViewTest, BigCapacityGrowthOverflow) {
+  constexpr size_t kCapacity = 256;
+  for (bool has_deleted : {true, false}) {
+    SCOPED_TRACE(testing::Message() << "has_deleted: " << has_deleted);
+    GrowthInfoAllocator growth_info(kCapacity);
+    growth_info->InitGrowthLeftNoDeleted(
+        /*growth_left=*/GrowthInfoLowerBound::kMaxGrowthLeftLowerBound - 1,
+        kCapacity);
+    if (has_deleted) {
+      growth_info->OverwriteFullAsDeleted();
+    }
+    EXPECT_EQ(growth_info->GetGrowthLeftLowerBound(),
+              GrowthInfoLowerBound::kMaxGrowthLeftLowerBound - 1);
+    EXPECT_EQ(growth_info->GetGrowthLeftTotalSlow(kCapacity),
+              GrowthInfoLowerBound::kMaxGrowthLeftLowerBound - 1);
+    EXPECT_EQ(growth_info->GetGrowthInfoLowerBound().HasNoDeleted(),
+              !has_deleted);
+    growth_info->OverwriteFullAsEmpty();
+    EXPECT_EQ(growth_info->GetGrowthLeftLowerBound(),
+              GrowthInfoLowerBound::kMaxGrowthLeftLowerBound);
+    EXPECT_EQ(growth_info->GetGrowthLeftTotalSlow(kCapacity),
+              GrowthInfoLowerBound::kMaxGrowthLeftLowerBound);
+    EXPECT_EQ(growth_info->GetGrowthInfoLowerBound().HasNoDeleted(),
+              !has_deleted);
+    for (size_t i = 1; i <= 10; ++i) {
+      growth_info->OverwriteFullAsEmpty();
+      // LowerBound stayed the same, but total increased.
+      ASSERT_EQ(growth_info->GetGrowthLeftLowerBound(),
+                GrowthInfoLowerBound::kMaxGrowthLeftLowerBound);
+      ASSERT_EQ(growth_info->GetGrowthLeftTotalSlow(kCapacity),
+                GrowthInfoLowerBound::kMaxGrowthLeftLowerBound + i);
+      ASSERT_EQ(growth_info->GetGrowthInfoLowerBound().HasNoDeleted(),
+                !has_deleted);
+    }
+  }
+}
+
+TEST(GrowthInfoViewTest, RebalanceOnInsert) {
+  constexpr size_t kCapacity = 512;
+  constexpr size_t kOrigGrowthLeft = 260;
+  for (bool has_deleted : {false, true}) {
+    SCOPED_TRACE(testing::Message() << "has_deleted: " << has_deleted);
+    GrowthInfoAllocator growth_info(kCapacity);
+    growth_info->InitGrowthLeftNoDeleted(/*growth_left=*/kOrigGrowthLeft,
+                                         kCapacity);
+    if (has_deleted) {
+      growth_info->OverwriteFullAsDeleted();
+    }
+
+    auto has_no_growth = [has_deleted](const GrowthInfoLowerBound& info) {
+      return has_deleted ? info.HasNoGrowthLeftAndHaveDeleted()
+                         : info.HasNoGrowthLeftAndNoDeleted();
+    };
+
+    auto check_no_rebalance_if_growth_left_with_deleted =
+        [&](GrowthInfoAllocator& info) {
+          if (!has_deleted) return;
+          const size_t growth_left = info->GetGrowthLeftLowerBound();
+          if (growth_left > 0) {
+            auto lower_bound = info->RebalanceGrowthLeftLowerBound(kCapacity);
+            EXPECT_EQ(lower_bound.GetGrowthLeft(), growth_left);
+          }
+        };
+
+    EXPECT_EQ(growth_info->GetGrowthLeftLowerBound(),
+              GrowthInfoLowerBound::kMaxGrowthLeftLowerBound);
+    EXPECT_EQ(growth_info->GetGrowthLeftTotalSlow(kCapacity), kOrigGrowthLeft);
+    EXPECT_EQ(growth_info->GetGrowthInfoLowerBound().HasNoDeleted(),
+              !has_deleted);
+    for (size_t i = 0; i < GrowthInfoLowerBound::kMaxGrowthLeftLowerBound;
+         ++i) {
+      growth_info->OverwriteEmptyAsFull();
+      check_no_rebalance_if_growth_left_with_deleted(growth_info);
+    }
+    EXPECT_TRUE(has_no_growth(growth_info->GetGrowthInfoLowerBound()));
+    {
+      auto lower_bound = growth_info->RebalanceGrowthLeftLowerBound(kCapacity);
+      EXPECT_EQ(lower_bound.GetGrowthLeft(), 127);
+      EXPECT_EQ(lower_bound.HasNoDeleted(), !has_deleted);
+    }
+
+    EXPECT_EQ(growth_info->GetGrowthLeftLowerBound(), 127);
+    EXPECT_EQ(growth_info->GetGrowthLeftTotalSlow(kCapacity),
+              kOrigGrowthLeft - 127);
+    for (size_t i = 0; i < 127; ++i) {
+      growth_info->OverwriteEmptyAsFull();
+      check_no_rebalance_if_growth_left_with_deleted(growth_info);
+    }
+    EXPECT_TRUE(has_no_growth(growth_info->GetGrowthInfoLowerBound()));
+    constexpr size_t kSmallGrowthLeft = kOrigGrowthLeft - 127 * 2;
+    {
+      auto lower_bound = growth_info->RebalanceGrowthLeftLowerBound(kCapacity);
+      EXPECT_EQ(lower_bound.GetGrowthLeft(), kSmallGrowthLeft);
+      EXPECT_EQ(lower_bound.HasNoDeleted(), !has_deleted);
+    }
+    EXPECT_FALSE(has_no_growth(growth_info->GetGrowthInfoLowerBound()));
+
+    EXPECT_EQ(growth_info->GetGrowthLeftLowerBound(), kSmallGrowthLeft);
+    EXPECT_EQ(growth_info->GetGrowthLeftTotalSlow(kCapacity), kSmallGrowthLeft);
+    for (size_t i = 0; i < kSmallGrowthLeft; ++i) {
+      growth_info->OverwriteEmptyAsFull();
+      check_no_rebalance_if_growth_left_with_deleted(growth_info);
+    }
+    EXPECT_TRUE(has_no_growth(growth_info->GetGrowthInfoLowerBound()));
+    {
+      auto lower_bound = growth_info->RebalanceGrowthLeftLowerBound(kCapacity);
+      EXPECT_EQ(lower_bound.GetGrowthLeft(), 0);
+      EXPECT_EQ(lower_bound.HasNoDeleted(), !has_deleted);
+    }
+    EXPECT_TRUE(has_no_growth(growth_info->GetGrowthInfoLowerBound()));
+    EXPECT_EQ(growth_info->GetGrowthLeftLowerBound(), 0);
+    EXPECT_EQ(growth_info->GetGrowthLeftTotalSlow(kCapacity), 0);
+  }
+}
+
+TEST(Util, OptimalMemcpySizeForSooSlotTransfer) {
+  EXPECT_EQ(1, OptimalMemcpySizeForSooSlotTransfer(1));
+  ASSERT_EQ(4, OptimalMemcpySizeForSooSlotTransfer(2));
+  ASSERT_EQ(4, OptimalMemcpySizeForSooSlotTransfer(3));
+  ASSERT_EQ(4, OptimalMemcpySizeForSooSlotTransfer(4, /*max_soo_slot_size=*/4));
+  if constexpr (MaxSooSlotSize() > 4) {
+    for (size_t slot_size = 4; slot_size <= 8; ++slot_size) {
+      ASSERT_EQ(8, OptimalMemcpySizeForSooSlotTransfer(slot_size));
+    }
+  }
+}
 
 TEST(Util, NormalizeCapacity) {
   EXPECT_EQ(1, NormalizeCapacity(0));
@@ -67,17 +628,77 @@ TEST(Util, NormalizeCapacity) {
   EXPECT_EQ(15 * 2 + 1, NormalizeCapacity(15 + 2));
 }
 
+TEST(Util, SizeToCapacitySmallValues) {
+  EXPECT_EQ(SizeToCapacity(0), 0);
+  EXPECT_EQ(SizeToCapacity(1), 1);
+  EXPECT_EQ(SizeToCapacity(2), 3);
+  EXPECT_EQ(SizeToCapacity(3), 3);
+  EXPECT_EQ(SizeToCapacity(4), 7);
+  EXPECT_EQ(SizeToCapacity(5), 7);
+  EXPECT_EQ(SizeToCapacity(6), 7);
+  EXPECT_EQ(SizeToCapacity(14), 15);
+  EXPECT_EQ(SizeToCapacity(15), 31);
+  EXPECT_EQ(SizeToCapacity(28), 31);
+  EXPECT_EQ(SizeToCapacity(29), 31);
+  EXPECT_EQ(SizeToCapacity(30), 31);
+  EXPECT_EQ(SizeToCapacity(31), 63);
+  EXPECT_EQ(SizeToCapacity(56), 63);
+  if (Group::kWidth == 16) {
+    EXPECT_EQ(SizeToCapacity(7), 7);
+    EXPECT_EQ(SizeToCapacity(57), 63);
+    EXPECT_EQ(SizeToCapacity(60), 63);
+    EXPECT_EQ(SizeToCapacity(61), 63);
+    EXPECT_EQ(SizeToCapacity(62), 63);
+  } else {
+    EXPECT_EQ(SizeToCapacity(7), 15);
+    EXPECT_EQ(SizeToCapacity(57), 127);
+  }
+}
+
+TEST(Util, CapacityToGrowthSmallValues) {
+  EXPECT_EQ(CapacityToGrowth(1), 1);
+  EXPECT_EQ(CapacityToGrowth(3), 3);
+  EXPECT_EQ(CapacityToGrowth(15), 14);
+  EXPECT_EQ(CapacityToGrowth(31), 30);
+  if (Group::kWidth == 16) {
+    EXPECT_EQ(CapacityToGrowth(7), 7);
+    EXPECT_EQ(CapacityToGrowth(31), 30);
+    EXPECT_EQ(CapacityToGrowth(63), 62);
+  } else {
+    EXPECT_EQ(CapacityToGrowth(7), 6);
+    EXPECT_EQ(CapacityToGrowth(63), 56);
+  }
+  EXPECT_EQ(CapacityToGrowth(127), 112);
+}
+
+TEST(Table, ReserveGroupWidthCapacity) {
+  absl::flat_hash_set<int> set;
+  set.reserve(Group::kWidth * 2 - 2);
+  EXPECT_EQ(set.capacity(), Group::kWidth * 2 - 1);
+  set.reserve(Group::kWidth * 2 - 1);
+  EXPECT_EQ(set.capacity(), Group::kWidth * 4 - 1);
+  set.reserve(Group::kWidth * 4 - 2);
+  EXPECT_EQ(set.capacity(), Group::kWidth * 4 - 1);
+}
+
 TEST(Util, GrowthAndCapacity) {
   // Verify that GrowthToCapacity gives the minimum capacity that has enough
   // growth.
-  for (size_t growth = 0; growth < 10000; ++growth) {
+  for (size_t growth = 1; growth < 10000; ++growth) {
     SCOPED_TRACE(growth);
-    size_t capacity = NormalizeCapacity(GrowthToLowerboundCapacity(growth));
-    // The capacity is large enough for `growth`
-    EXPECT_THAT(CapacityToGrowth(capacity), Ge(growth));
+    size_t capacity = SizeToCapacity(growth);
+    ASSERT_TRUE(IsValidCapacity(capacity));
+    // The capacity is large enough for `growth`.
+    ASSERT_THAT(CapacityToGrowth(capacity), Ge(growth));
+    // For (capacity+1) < kWidth, growth should equal capacity.
+    if (capacity + 1 < Group::kWidth) {
+      ASSERT_THAT(CapacityToGrowth(capacity), Eq(capacity));
+    } else {
+      ASSERT_THAT(CapacityToGrowth(capacity), Lt(capacity));
+    }
     if (growth != 0 && capacity > 1) {
       // There is no smaller capacity that works.
-      EXPECT_THAT(CapacityToGrowth(capacity / 2), Lt(growth));
+      ASSERT_THAT(CapacityToGrowth(capacity / 2), Lt(growth)) << capacity;
     }
   }
 
@@ -85,14 +706,15 @@ TEST(Util, GrowthAndCapacity) {
        capacity = 2 * capacity + 1) {
     SCOPED_TRACE(capacity);
     size_t growth = CapacityToGrowth(capacity);
-    EXPECT_THAT(growth, Lt(capacity));
-    EXPECT_LE(GrowthToLowerboundCapacity(growth), capacity);
-    EXPECT_EQ(NormalizeCapacity(GrowthToLowerboundCapacity(growth)), capacity);
+    ASSERT_THAT(growth, Lt(capacity));
+    ASSERT_EQ(SizeToCapacity(growth), capacity);
+    ASSERT_EQ(NormalizeCapacity(SizeToCapacity(growth)), capacity);
   }
 }
 
 TEST(Util, probe_seq) {
-  probe_seq<16> seq(0, 127);
+  size_t capacity = 127;
+  probe_seq<16> seq(ProbeCapacity{capacity}, /*hash=*/0);
   auto gen = [&]() {
     size_t res = seq.offset();
     seq.next();
@@ -101,176 +723,395 @@ TEST(Util, probe_seq) {
   std::vector<size_t> offsets(8);
   std::generate_n(offsets.begin(), 8, gen);
   EXPECT_THAT(offsets, ElementsAre(0, 16, 48, 96, 32, 112, 80, 64));
-  seq = probe_seq<16>(128, 127);
+  seq = probe_seq<16>(ProbeCapacity{capacity}, /*hash=*/128);
   std::generate_n(offsets.begin(), 8, gen);
   EXPECT_THAT(offsets, ElementsAre(0, 16, 48, 96, 32, 112, 80, 64));
 }
 
-TEST(BitMask, Smoke) {
-  EXPECT_FALSE((BitMask<uint8_t, 8>(0)));
-  EXPECT_TRUE((BitMask<uint8_t, 8>(5)));
+template <typename T>
+class HashtableDataTest : public ::testing::Test {};
+
+using StorageModes = ::testing::Types<
+    std::integral_constant<HashtableCapacityStorageMode,
+                           HashtableCapacityStorageMode::kCapacityByValue>,
+    std::integral_constant<HashtableCapacityStorageMode,
+                           HashtableCapacityStorageMode::kCapacityByLog>>;
+
+TYPED_TEST_SUITE(HashtableDataTest, StorageModes);
+
+TYPED_TEST(HashtableDataTest, HashtableCapacity) {
+  constexpr HashtableCapacityStorageMode kMode = TypeParam::value;
+  using Capacity = HashtableCapacityImpl<kMode>;
+
+  Capacity cap0(0);
+  EXPECT_TRUE(cap0.IsValid());
+  EXPECT_EQ(cap0.capacity(), 0);
+  EXPECT_FALSE(cap0.IsDestroyed());
+  EXPECT_FALSE(cap0.IsReentrance());
+  EXPECT_FALSE(cap0.IsMovedFrom());
+  EXPECT_FALSE(cap0.IsSelfMovedFrom());
+
+  for (size_t i = 0, cap = 0; i < 20; ++i, cap = NextCapacity(cap)) {
+    Capacity capacity(cap);
+    ASSERT_TRUE(capacity.IsValid());
+    ASSERT_EQ(capacity.capacity(), cap);
+  }
+
+  auto destroyed = Capacity::CreateDestroyed();
+  EXPECT_FALSE(destroyed.IsValid());
+  EXPECT_TRUE(destroyed.IsDestroyed());
 
-  EXPECT_THAT((BitMask<uint8_t, 8>(0)), ElementsAre());
-  EXPECT_THAT((BitMask<uint8_t, 8>(0x1)), ElementsAre(0));
-  EXPECT_THAT((BitMask<uint8_t, 8>(0x2)), ElementsAre(1));
-  EXPECT_THAT((BitMask<uint8_t, 8>(0x3)), ElementsAre(0, 1));
-  EXPECT_THAT((BitMask<uint8_t, 8>(0x4)), ElementsAre(2));
-  EXPECT_THAT((BitMask<uint8_t, 8>(0x5)), ElementsAre(0, 2));
-  EXPECT_THAT((BitMask<uint8_t, 8>(0x55)), ElementsAre(0, 2, 4, 6));
-  EXPECT_THAT((BitMask<uint8_t, 8>(0xAA)), ElementsAre(1, 3, 5, 7));
-}
+  auto reentrance = Capacity::CreateReentrance();
+  EXPECT_FALSE(reentrance.IsValid());
+  EXPECT_TRUE(reentrance.IsReentrance());
 
-TEST(BitMask, WithShift) {
-  // See the non-SSE version of Group for details on what this math is for.
-  uint64_t ctrl = 0x1716151413121110;
-  uint64_t hash = 0x12;
-  constexpr uint64_t msbs = 0x8080808080808080ULL;
-  constexpr uint64_t lsbs = 0x0101010101010101ULL;
-  auto x = ctrl ^ (lsbs * hash);
-  uint64_t mask = (x - lsbs) & ~x & msbs;
-  EXPECT_EQ(0x0000000080800000, mask);
+  auto moved_from = Capacity::CreateMovedFrom();
+  EXPECT_FALSE(moved_from.IsValid());
+  EXPECT_TRUE(moved_from.IsMovedFrom());
+  EXPECT_FALSE(moved_from.IsSelfMovedFrom());
 
-  BitMask<uint64_t, 8, 3> b(mask);
-  EXPECT_EQ(*b, 2);
+  auto self_moved_from = Capacity::CreateSelfMovedFrom();
+  EXPECT_FALSE(self_moved_from.IsValid());
+  EXPECT_TRUE(self_moved_from.IsSelfMovedFrom());
+  EXPECT_TRUE(self_moved_from.IsMovedFrom());
 }
 
-TEST(BitMask, LeadingTrailing) {
-  EXPECT_EQ((BitMask<uint32_t, 16>(0x00001a40).LeadingZeros()), 3);
-  EXPECT_EQ((BitMask<uint32_t, 16>(0x00001a40).TrailingZeros()), 6);
+TYPED_TEST(HashtableDataTest, RawData) {
+  constexpr HashtableCapacityStorageMode kMode = TypeParam::value;
+  using Capacity = HashtableCapacityImpl<kMode>;
+
+  for (size_t i = 0, cap = 0; i < 20; ++i, cap = NextCapacity(cap)) {
+    Capacity orig_capacity(cap);
+    Capacity capacity = Capacity::FromRawData(orig_capacity.ToRawData());
+    ASSERT_TRUE(capacity.IsValid());
+    ASSERT_EQ(capacity.capacity(), cap);
+  }
+  auto orig_reentrance = Capacity::CreateReentrance();
+  Capacity reentrance = Capacity::FromRawData(orig_reentrance.ToRawData());
+  EXPECT_TRUE(reentrance.IsReentrance());
+}
 
-  EXPECT_EQ((BitMask<uint32_t, 16>(0x00000001).LeadingZeros()), 15);
-  EXPECT_EQ((BitMask<uint32_t, 16>(0x00000001).TrailingZeros()), 0);
+TYPED_TEST(HashtableDataTest, HashtableInlineDataCapacity) {
+  constexpr HashtableCapacityStorageMode kMode = TypeParam::value;
+  using InlineData = HashtableInlineDataImpl<kMode>;
+  using Capacity = HashtableCapacityImpl<kMode>;
 
-  EXPECT_EQ((BitMask<uint32_t, 16>(0x00008000).LeadingZeros()), 0);
-  EXPECT_EQ((BitMask<uint32_t, 16>(0x00008000).TrailingZeros()), 15);
+  InlineData data(Capacity(0), no_seed_empty_tag_t{});
+  EXPECT_EQ(data.capacity().capacity(), 0);
+  EXPECT_EQ(data.size(), 0);
+  EXPECT_TRUE(data.empty());
 
-  EXPECT_EQ((BitMask<uint64_t, 8, 3>(0x0000008080808000).LeadingZeros()), 3);
-  EXPECT_EQ((BitMask<uint64_t, 8, 3>(0x0000008080808000).TrailingZeros()), 1);
+  for (size_t i = 0, cap = 0; i < 20; ++i, cap = NextCapacity(cap)) {
+    data.set_capacity(cap);
+    ASSERT_EQ(data.capacity().capacity(), cap);
+  }
 
-  EXPECT_EQ((BitMask<uint64_t, 8, 3>(0x0000000000000080).LeadingZeros()), 7);
-  EXPECT_EQ((BitMask<uint64_t, 8, 3>(0x0000000000000080).TrailingZeros()), 0);
+  // Test overload from `Capacity` object.
+  for (size_t i = 0, cap = 0; i < 20; ++i, cap = NextCapacity(cap)) {
+    data.set_capacity(Capacity(cap));
+    ASSERT_EQ(data.capacity().capacity(), cap);
+  }
 
-  EXPECT_EQ((BitMask<uint64_t, 8, 3>(0x8000000000000000).LeadingZeros()), 0);
-  EXPECT_EQ((BitMask<uint64_t, 8, 3>(0x8000000000000000).TrailingZeros()), 7);
+  auto reentrance = Capacity::CreateReentrance();
+  data.set_capacity(reentrance);
+  EXPECT_TRUE(data.capacity().IsReentrance());
 }
 
-TEST(Group, EmptyGroup) {
-  for (h2_t h = 0; h != 128; ++h) EXPECT_FALSE(Group{EmptyGroup()}.Match(h));
+TYPED_TEST(HashtableDataTest, HashtableInlineDataSize) {
+  constexpr HashtableCapacityStorageMode kMode = TypeParam::value;
+  using InlineData = HashtableInlineDataImpl<kMode>;
+  using Capacity = HashtableCapacityImpl<kMode>;
+
+  InlineData data(Capacity(0), no_seed_empty_tag_t{});
+  EXPECT_EQ(data.size(), 0);
+  data.increment_size();
+  EXPECT_EQ(data.size(), 1);
+  EXPECT_FALSE(data.empty());
+
+  data.increment_size(5);
+  EXPECT_EQ(data.size(), 6);
+
+  data.decrement_size();
+  EXPECT_EQ(data.size(), 5);
+
+  constexpr size_t kHugeIncrement =
+      (size_t(1) << (sizeof(size_t) == 4 ? 31 : 42));
+  data.increment_size(kHugeIncrement);
+  EXPECT_EQ(data.size(), kHugeIncrement + 5);
+
+  EXPECT_FALSE(data.has_infoz());
+  data.set_has_infoz();
+  data.set_size(10);
+  EXPECT_EQ(data.size(), 10);
+  EXPECT_TRUE(data.has_infoz());
 }
 
-TEST(Group, Match) {
-  if (Group::kWidth == 16) {
-    ctrl_t group[] = {kEmpty, 1, kDeleted, 3, kEmpty, 5, kSentinel, 7,
-                      7,      5, 3,        1, 1,      1, 1,         1};
-    EXPECT_THAT(Group{group}.Match(0), ElementsAre());
-    EXPECT_THAT(Group{group}.Match(1), ElementsAre(1, 11, 12, 13, 14, 15));
-    EXPECT_THAT(Group{group}.Match(3), ElementsAre(3, 10));
-    EXPECT_THAT(Group{group}.Match(5), ElementsAre(5, 9));
-    EXPECT_THAT(Group{group}.Match(7), ElementsAre(7, 8));
-  } else if (Group::kWidth == 8) {
-    ctrl_t group[] = {kEmpty, 1, 2, kDeleted, 2, 1, kSentinel, 1};
-    EXPECT_THAT(Group{group}.Match(0), ElementsAre());
-    EXPECT_THAT(Group{group}.Match(1), ElementsAre(1, 5, 7));
-    EXPECT_THAT(Group{group}.Match(2), ElementsAre(2, 4));
-  } else {
-    FAIL() << "No test coverage for Group::kWidth==" << Group::kWidth;
+TYPED_TEST(HashtableDataTest, BlockedElementCount) {
+  constexpr HashtableCapacityStorageMode kMode = TypeParam::value;
+  using InlineData = HashtableInlineDataImpl<kMode>;
+  using Capacity = HashtableCapacityImpl<kMode>;
+
+  {
+    InlineData data(Capacity(0), no_seed_empty_tag_t{});
+    EXPECT_EQ(data.blocked_element_count(), 0);
   }
-}
 
-TEST(Group, MatchEmpty) {
-  if (Group::kWidth == 16) {
-    ctrl_t group[] = {kEmpty, 1, kDeleted, 3, kEmpty, 5, kSentinel, 7,
-                      7,      5, 3,        1, 1,      1, 1,         1};
-    EXPECT_THAT(Group{group}.MatchEmpty(), ElementsAre(0, 4));
-  } else if (Group::kWidth == 8) {
-    ctrl_t group[] = {kEmpty, 1, 2, kDeleted, 2, 1, kSentinel, 1};
-    EXPECT_THAT(Group{group}.MatchEmpty(), ElementsAre(0));
-  } else {
-    FAIL() << "No test coverage for Group::kWidth==" << Group::kWidth;
+  for (size_t i = 0; i <= InlineData::kMaxBlockedElementCount; ++i) {
+    InlineData data(Capacity(0), no_seed_empty_tag_t{});
+    data.init_blocked_element_count(i);
+    EXPECT_EQ(data.blocked_element_count(), i);
+    data.set_blocked_element_count_to_zero();
+    EXPECT_EQ(data.blocked_element_count(), 0);
   }
 }
 
-TEST(Group, MatchEmptyOrDeleted) {
-  if (Group::kWidth == 16) {
-    ctrl_t group[] = {kEmpty, 1, kDeleted, 3, kEmpty, 5, kSentinel, 7,
-                      7,      5, 3,        1, 1,      1, 1,         1};
-    EXPECT_THAT(Group{group}.MatchEmptyOrDeleted(), ElementsAre(0, 2, 4));
-  } else if (Group::kWidth == 8) {
-    ctrl_t group[] = {kEmpty, 1, 2, kDeleted, 2, 1, kSentinel, 1};
-    EXPECT_THAT(Group{group}.MatchEmptyOrDeleted(), ElementsAre(0, 3));
-  } else {
-    FAIL() << "No test coverage for Group::kWidth==" << Group::kWidth;
+TYPED_TEST(HashtableDataTest, MaxStorableSize) {
+  constexpr HashtableCapacityStorageMode kMode = TypeParam::value;
+  using InlineData = HashtableInlineDataImpl<kMode>;
+  using Capacity = HashtableCapacityImpl<kMode>;
+
+  InlineData data(Capacity(0), no_seed_empty_tag_t{});
+  constexpr uint64_t kMaxSize =
+      sizeof(size_t) == 4 ? ~uint32_t{}
+                          : (uint64_t{1} << InlineData::kSizeBitCount) - 1;
+  data.init_blocked_element_count(3);
+  data.increment_size(kMaxSize);
+  EXPECT_EQ(data.size(), kMaxSize);
+  // We didn't overwrite other fields.
+  EXPECT_FALSE(data.has_infoz());
+  EXPECT_EQ(data.blocked_element_count(), 3);
+}
+
+TYPED_TEST(HashtableDataTest, HashtableInlineDataMetadata) {
+  constexpr HashtableCapacityStorageMode kMode = TypeParam::value;
+  using InlineData = HashtableInlineDataImpl<kMode>;
+  using Capacity = HashtableCapacityImpl<kMode>;
+
+  InlineData data(Capacity(0), no_seed_empty_tag_t{});
+
+  // SOO sampling (test this first before seed messes with bit 0)
+  EXPECT_FALSE(data.soo_has_tried_sampling());
+  data.set_soo_has_tried_sampling();
+  EXPECT_TRUE(data.soo_has_tried_sampling());
+
+  data.set_no_seed_for_testing();
+  EXPECT_EQ(data.seed().seed(), 0);
+
+  data.set_sampled_seed();
+  EXPECT_TRUE(data.is_sampled_seed());
+
+  // Infoz
+  EXPECT_FALSE(data.has_infoz());
+  data.set_has_infoz();
+  EXPECT_TRUE(data.has_infoz());
+}
+
+TYPED_TEST(HashtableDataTest, HashtableInlineDataFullSooConstructor) {
+  constexpr HashtableCapacityStorageMode kMode = TypeParam::value;
+  using InlineData = HashtableInlineDataImpl<kMode>;
+  using Capacity = HashtableCapacityImpl<kMode>;
+
+  {
+    InlineData data_soo(Capacity(1), full_soo_tag_t{},
+                        /*has_tried_sampling=*/true);
+    EXPECT_EQ(data_soo.capacity().capacity(), 1);
+    EXPECT_EQ(data_soo.size(), 1);
+    EXPECT_TRUE(data_soo.soo_has_tried_sampling());
+  }
+
+  {
+    InlineData data_soo(Capacity(1), full_soo_tag_t{},
+                        /*has_tried_sampling=*/false);
+    EXPECT_EQ(data_soo.capacity().capacity(), 1);
+    EXPECT_EQ(data_soo.size(), 1);
+    EXPECT_FALSE(data_soo.soo_has_tried_sampling());
   }
 }
 
+TYPED_TEST(HashtableDataTest, GenerateNewSeedDoesntChangeSize) {
+  constexpr HashtableCapacityStorageMode kMode = TypeParam::value;
+  using InlineData = HashtableInlineDataImpl<kMode>;
+  using Capacity = HashtableCapacityImpl<kMode>;
+  size_t size = 1;
+  do {
+    InlineData inline_data(Capacity(15), no_seed_empty_tag_t{});
+    inline_data.increment_size(size);
+    EXPECT_EQ(inline_data.size(), size);
+    inline_data.generate_new_seed();
+    EXPECT_EQ(inline_data.size(), size);
+    size = size * 2 + 1;
+  } while (size < std::min(MaxStorableSize(),
+                           MaxSizeAtMaxValidCapacity(/*slot_size=*/1)));
+}
+
 TEST(Batch, DropDeletes) {
   constexpr size_t kCapacity = 63;
   constexpr size_t kGroupWidth = container_internal::Group::kWidth;
   std::vector<ctrl_t> ctrl(kCapacity + 1 + kGroupWidth);
-  ctrl[kCapacity] = kSentinel;
-  std::vector<ctrl_t> pattern = {kEmpty, 2, kDeleted, 2, kEmpty, 1, kDeleted};
+  ctrl[kCapacity] = ctrl_t::kSentinel;
+  std::vector<ctrl_t> pattern = {
+      ctrl_t::kEmpty, CtrlT(2), ctrl_t::kDeleted, CtrlT(2),
+      ctrl_t::kEmpty, CtrlT(1), ctrl_t::kDeleted};
   for (size_t i = 0; i != kCapacity; ++i) {
     ctrl[i] = pattern[i % pattern.size()];
     if (i < kGroupWidth - 1)
       ctrl[i + kCapacity + 1] = pattern[i % pattern.size()];
   }
   ConvertDeletedToEmptyAndFullToDeleted(ctrl.data(), kCapacity);
-  ASSERT_EQ(ctrl[kCapacity], kSentinel);
-  for (size_t i = 0; i < kCapacity + 1 + kGroupWidth; ++i) {
+  ASSERT_EQ(ctrl[kCapacity], ctrl_t::kSentinel);
+  for (size_t i = 0; i < kCapacity + kGroupWidth; ++i) {
     ctrl_t expected = pattern[i % (kCapacity + 1) % pattern.size()];
-    if (i == kCapacity) expected = kSentinel;
-    if (expected == kDeleted) expected = kEmpty;
-    if (IsFull(expected)) expected = kDeleted;
+    if (i == kCapacity) expected = ctrl_t::kSentinel;
+    if (expected == ctrl_t::kDeleted) expected = ctrl_t::kEmpty;
+    if (IsFull(expected)) expected = ctrl_t::kDeleted;
     EXPECT_EQ(ctrl[i], expected)
-        << i << " " << int{pattern[i % pattern.size()]};
+        << i << " " << static_cast<int>(pattern[i % pattern.size()]);
   }
 }
 
-TEST(Group, CountLeadingEmptyOrDeleted) {
-  const std::vector<ctrl_t> empty_examples = {kEmpty, kDeleted};
-  const std::vector<ctrl_t> full_examples = {0, 1, 2, 3, 5, 9, 127, kSentinel};
+template <class T, bool kTransferable = false, bool kSoo = false,
+          bool kDestroyTrivial = false>
+struct ValuePolicy {
+  using slot_type = T;
+  using key_type = T;
+  using init_type = T;
+
+  using DefaultHash = hash_default_hash<T>;
+  using DefaultEq = std::equal_to<T>;
+  using DefaultAlloc = std::allocator<T>;
+
+  template <class Allocator, class... Args>
+  static void construct(Allocator* alloc, slot_type* slot, Args&&... args) {
+    std::allocator_traits<Allocator>::construct(*alloc, slot,
+                                                std::forward<Args>(args)...);
+  }
 
-  for (ctrl_t empty : empty_examples) {
-    std::vector<ctrl_t> e(Group::kWidth, empty);
-    EXPECT_EQ(Group::kWidth, Group{e.data()}.CountLeadingEmptyOrDeleted());
-    for (ctrl_t full : full_examples) {
-      for (size_t i = 0; i != Group::kWidth; ++i) {
-        std::vector<ctrl_t> f(Group::kWidth, empty);
-        f[i] = full;
-        EXPECT_EQ(i, Group{f.data()}.CountLeadingEmptyOrDeleted());
-      }
-      std::vector<ctrl_t> f(Group::kWidth, empty);
-      f[Group::kWidth * 2 / 3] = full;
-      f[Group::kWidth / 2] = full;
-      EXPECT_EQ(
-          Group::kWidth / 2, Group{f.data()}.CountLeadingEmptyOrDeleted());
+  template <class Allocator>
+  static auto destroy(Allocator* alloc, slot_type* slot) {
+    std::allocator_traits<Allocator>::destroy(*alloc, slot);
+    if constexpr (kDestroyTrivial) {
+      return std::true_type{};
+    } else {
+      return std::false_type{};
     }
   }
-}
 
-struct IntPolicy {
-  using slot_type = int64_t;
-  using key_type = int64_t;
-  using init_type = int64_t;
+  template <class Allocator>
+  static std::bool_constant<kTransferable> transfer(Allocator* alloc,
+                                                    slot_type* new_slot,
+                                                    slot_type* old_slot) {
+    construct(alloc, new_slot, std::move(*old_slot));
+    destroy(alloc, old_slot);
+    return {};
+  }
+
+  static T& element(slot_type* slot) { return *slot; }
+
+  template <class F, class... Args>
+  static decltype(absl::container_internal::DecomposeValue(
+      std::declval<F>(), std::declval<Args>()...))
+  apply(F&& f, Args&&... args) {
+    return absl::container_internal::DecomposeValue(
+        std::forward<F>(f), std::forward<Args>(args)...);
+  }
 
-  static void construct(void*, int64_t* slot, int64_t v) { *slot = v; }
-  static void destroy(void*, int64_t*) {}
-  static void transfer(void*, int64_t* new_slot, int64_t* old_slot) {
-    *new_slot = *old_slot;
+  template <class Hash, bool kIsDefault, size_t kSeedShift>
+  static constexpr HashSlotFn get_hash_slot_fn() {
+    return nullptr;
   }
 
-  static int64_t& element(slot_type* slot) { return *slot; }
+  static constexpr bool soo_enabled() { return kSoo; }
+};
 
-  template <class F>
-  static auto apply(F&& f, int64_t x) -> decltype(std::forward<F>(f)(x, x)) {
-    return std::forward<F>(f)(x, x);
+using IntPolicy = ValuePolicy<int64_t>;
+using Uint8Policy = ValuePolicy<uint8_t>;
+
+// For testing SOO.
+template <int N>
+class SizedValue {
+ public:
+  SizedValue(int64_t v) {  // NOLINT
+    vals_[0] = v;
+  }
+  SizedValue() : SizedValue(0) {}
+  SizedValue(const SizedValue&) = default;
+  SizedValue& operator=(const SizedValue&) = default;
+
+  int64_t operator*() const {
+    // Suppress erroneous uninitialized memory errors on GCC.
+#if !defined(__clang__) && defined(__GNUC__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
+#endif
+    return vals_[0];
+#if !defined(__clang__) && defined(__GNUC__)
+#pragma GCC diagnostic pop
+#endif
+  }
+  explicit operator int() const { return **this; }
+  explicit operator int64_t() const { return **this; }
+
+  template <typename H>
+  friend H AbslHashValue(H h, SizedValue sv) {
+    return H::combine(std::move(h), *sv);
+  }
+  bool operator==(const SizedValue& rhs) const { return **this == *rhs; }
+
+ private:
+  static_assert(N % sizeof(int64_t) == 0);
+  static_assert(N >= sizeof(int64_t));
+  int64_t vals_[N / sizeof(int64_t)];
+};
+template <int N, bool kSoo>
+using SizedValuePolicy =
+    ValuePolicy<SizedValue<N>, /*kTransferable=*/true, kSoo>;
+
+// Value is aligned as type T and contains N copies of it.
+template <typename T, int N>
+class AlignedValue {
+ public:
+  AlignedValue(int64_t v) {  // NOLINT
+    for (int i = 0; i < N; ++i) {
+      vals_[i] = v;
+      if (sizeof(T) < sizeof(int64_t)) {
+        v >>= (8 * sizeof(T));
+      } else {
+        v = 0;
+      }
+    }
+  }
+  AlignedValue() : AlignedValue(0) {}
+  AlignedValue(const AlignedValue&) = default;
+  AlignedValue& operator=(const AlignedValue&) = default;
+
+  int64_t operator*() const {
+    if (sizeof(T) == sizeof(int64_t)) {
+      return vals_[0];
+    }
+    int64_t result = 0;
+    for (int i = N - 1; i >= 0; --i) {
+      result <<= (8 * sizeof(T));
+      result += vals_[i];
+    }
+    return result;
+  }
+  explicit operator int() const { return **this; }
+  explicit operator int64_t() const { return **this; }
+
+  template <typename H>
+  friend H AbslHashValue(H h, AlignedValue sv) {
+    return H::combine(std::move(h), *sv);
   }
+  bool operator==(const AlignedValue& rhs) const { return **this == *rhs; }
+
+ private:
+  T vals_[N];
 };
 
 class StringPolicy {
   template <class F, class K, class V,
-            class = typename std::enable_if<
-                std::is_convertible<const K&, absl::string_view>::value>::type>
+            class = std::enable_if_t<
+                std::is_convertible_v<const K&, absl::string_view>>>
   decltype(std::declval<F>()(
       std::declval<const absl::string_view&>(), std::piecewise_construct,
       std::declval<std::tuple<K>>(),
@@ -286,7 +1127,7 @@ class StringPolicy {
     struct ctor {};
 
     template <class... Ts>
-    slot_type(ctor, Ts&&... ts) : pair(std::forward<Ts>(ts)...) {}
+    explicit slot_type(ctor, Ts&&... ts) : pair(std::forward<Ts>(ts)...) {}
 
     std::pair<std::string, std::string> pair;
   };
@@ -294,6 +1135,10 @@ class StringPolicy {
   using key_type = std::string;
   using init_type = std::pair<std::string, std::string>;
 
+  using DefaultHash = void;
+  using DefaultEq = void;
+  using DefaultAlloc = void;
+
   template <class allocator_type, class... Args>
   static void construct(allocator_type* alloc, slot_type* slot, Args... args) {
     std::allocator_traits<allocator_type>::construct(
@@ -323,6 +1168,11 @@ class StringPolicy {
     return apply_impl(std::forward<F>(f),
                       PairArgs(std::forward<Args>(args)...));
   }
+
+  template <class Hash, bool kIsDefault, size_t kSeedShift>
+  static constexpr HashSlotFn get_hash_slot_fn() {
+    return nullptr;
+  }
 };
 
 struct StringHash : absl::Hash<absl::string_view> {
@@ -335,36 +1185,127 @@ struct StringEq : std::equal_to<absl::string_view> {
 struct StringTable
     : raw_hash_set<StringPolicy, StringHash, StringEq, std::allocator<int>> {
   using Base = typename StringTable::raw_hash_set;
-  StringTable() {}
+  StringTable() = default;
   using Base::Base;
 };
 
-struct IntTable
-    : raw_hash_set<IntPolicy, container_internal::hash_default_hash<int64_t>,
-                   std::equal_to<int64_t>, std::allocator<int64_t>> {
-  using Base = typename IntTable::raw_hash_set;
+template <typename T, bool kTransferable = false, bool kSoo = false,
+          bool kDestroyTrivial = false, class Alloc = std::allocator<T>>
+struct ValueTable : InstantiateRawHashSet<
+                        ValuePolicy<T, kTransferable, kSoo, kDestroyTrivial>,
+                        hash_default_hash<T>, std::equal_to<T>, Alloc>::type {
+  using Base = typename ValueTable::raw_hash_set;
   using Base::Base;
 };
 
+using IntTable = ValueTable<int64_t>;
+using Uint8Table = ValueTable<uint8_t>;
+
+using TransferableIntTable = ValueTable<int64_t, /*kTransferable=*/true>;
+
 template <typename T>
 struct CustomAlloc : std::allocator<T> {
-  CustomAlloc() {}
+  CustomAlloc() = default;
 
   template <typename U>
-  CustomAlloc(const CustomAlloc<U>& other) {}
+  explicit CustomAlloc(const CustomAlloc<U>& /*other*/) {}
 
-  template<class U> struct rebind {
+  template <class U>
+  struct rebind {
     using other = CustomAlloc<U>;
   };
 };
 
 struct CustomAllocIntTable
-    : raw_hash_set<IntPolicy, container_internal::hash_default_hash<int64_t>,
+    : raw_hash_set<IntPolicy, hash_default_hash<int64_t>,
                    std::equal_to<int64_t>, CustomAlloc<int64_t>> {
   using Base = typename CustomAllocIntTable::raw_hash_set;
   using Base::Base;
 };
 
+template <typename T>
+struct ChangingSizeAndTrackingTypeAlloc : std::allocator<T> {
+  ChangingSizeAndTrackingTypeAlloc() = default;
+
+  template <typename U>
+  explicit ChangingSizeAndTrackingTypeAlloc(
+      const ChangingSizeAndTrackingTypeAlloc<U>& other) {
+    EXPECT_EQ(other.type_id,
+              ChangingSizeAndTrackingTypeAlloc<U>::ComputeTypeId());
+  }
+
+  template <class U>
+  struct rebind {
+    using other = ChangingSizeAndTrackingTypeAlloc<U>;
+  };
+
+  T* allocate(size_t n) {
+    EXPECT_EQ(type_id, ComputeTypeId());
+    return std::allocator<T>::allocate(n);
+  }
+
+  void deallocate(T* p, std::size_t n) {
+    EXPECT_EQ(type_id, ComputeTypeId());
+    return std::allocator<T>::deallocate(p, n);
+  }
+
+  static size_t ComputeTypeId() { return absl::HashOf(typeid(T).name()); }
+
+  // We add extra data to make the allocator size being changed.
+  // This also make type_id positioned differently, so that assertions in the
+  // allocator can catch bugs more likely.
+  char data_before[sizeof(T) * 3] = {0};
+  size_t type_id = ComputeTypeId();
+  char data_after[sizeof(T) * 5] = {0};
+};
+
+struct ChangingSizeAllocIntTable
+    : raw_hash_set<IntPolicy, hash_default_hash<int64_t>,
+                   std::equal_to<int64_t>,
+                   ChangingSizeAndTrackingTypeAlloc<int64_t>> {
+  using Base = typename ChangingSizeAllocIntTable::raw_hash_set;
+  using Base::Base;
+};
+
+struct MinimumAlignmentUint8Table
+    : raw_hash_set<Uint8Policy, hash_default_hash<uint8_t>,
+                   std::equal_to<uint8_t>, MinimumAlignmentAlloc<uint8_t>> {
+  using Base = typename MinimumAlignmentUint8Table::raw_hash_set;
+  using Base::Base;
+};
+
+// Allows for freezing the allocator to expect no further allocations.
+template <typename T>
+struct FreezableAlloc : std::allocator<T> {
+  explicit FreezableAlloc(bool* f) : frozen(f) {}
+
+  template <typename U>
+  explicit FreezableAlloc(const FreezableAlloc<U>& other)
+      : frozen(other.frozen) {}
+
+  template <class U>
+  struct rebind {
+    using other = FreezableAlloc<U>;
+  };
+
+  T* allocate(size_t n) {
+    EXPECT_FALSE(*frozen);
+    return std::allocator<T>::allocate(n);
+  }
+
+  bool* frozen;
+};
+
+template <int N>
+struct FreezableSizedValueSooTable
+    : raw_hash_set<SizedValuePolicy<N, /*kSoo=*/true>,
+                   container_internal::hash_default_hash<SizedValue<N>>,
+                   std::equal_to<SizedValue<N>>,
+                   FreezableAlloc<SizedValue<N>>> {
+  using Base = typename FreezableSizedValueSooTable::raw_hash_set;
+  using Base::Base;
+};
+
 struct BadFastHash {
   template <class T>
   size_t operator()(const T&) const {
@@ -372,57 +1313,126 @@ struct BadFastHash {
   }
 };
 
-struct BadTable : raw_hash_set<IntPolicy, BadFastHash, std::equal_to<int>,
+struct BadHashFreezableIntTable
+    : raw_hash_set<IntPolicy, BadFastHash, std::equal_to<int64_t>,
+                   FreezableAlloc<int64_t>> {
+  using Base = typename BadHashFreezableIntTable::raw_hash_set;
+  using Base::Base;
+};
+
+struct BadTable : raw_hash_set<IntPolicy, BadFastHash, std::equal_to<int64_t>,
                                std::allocator<int>> {
   using Base = typename BadTable::raw_hash_set;
-  BadTable() {}
+  BadTable() = default;
   using Base::Base;
 };
 
+constexpr size_t kNonSooSize = 2 * sizeof(HeapOrSoo);
+using NonSooIntTableSlotType = SizedValue<kNonSooSize>;
+static_assert(sizeof(NonSooIntTableSlotType) > MaxSooSlotSize(), "too small");
+using NonSooIntTable = ValueTable<NonSooIntTableSlotType>;
+using NonSooIntTableTrivialDestroy =
+    ValueTable<NonSooIntTableSlotType, /*kTransferable=*/false, /*kSoo=*/false,
+               /*kDestroyTrivial=*/true>;
+using SooInt32Table =
+    ValueTable<int32_t, /*kTransferable=*/true, /*kSoo=*/true>;
+using SooIntTable = ValueTable<int64_t, /*kTransferable=*/true, /*kSoo=*/true>;
+using SooIntTableTrivialDestroy =
+    ValueTable<int64_t, /*kTransferable=*/true, /*kSoo=*/true,
+               /*kDestroyTrivial=*/true>;
+using NonMemcpyableSooIntTable =
+    ValueTable<int64_t, /*kTransferable=*/false, /*kSoo=*/true>;
+using MemcpyableSooIntCustomAllocTable =
+    ValueTable<int64_t, /*kTransferable=*/true, /*kSoo=*/true,
+               /*kDestroyTrivial=*/false,
+               ChangingSizeAndTrackingTypeAlloc<int64_t>>;
+using NonMemcpyableSooIntCustomAllocTable =
+    ValueTable<int64_t, /*kTransferable=*/false, /*kSoo=*/true,
+               /*kDestroyTrivial=*/false,
+               ChangingSizeAndTrackingTypeAlloc<int64_t>>;
+
 TEST(Table, EmptyFunctorOptimization) {
-  static_assert(std::is_empty<std::equal_to<absl::string_view>>::value, "");
-  static_assert(std::is_empty<std::allocator<int>>::value, "");
+  static_assert(std::is_empty_v<std::equal_to<absl::string_view>>);
+  static_assert(std::is_empty_v<std::allocator<int>>);
 
-  struct MockTable {
-    void* ctrl;
-    void* slots;
-    size_t size;
+  struct MockTableByValue {
     size_t capacity;
-    size_t growth_left;
-    void* infoz;
+    uint64_t size;
+    void* ctrl;
+  };
+  struct MockTableByLog {
+    uint64_t size;
+    void* ctrl;
   };
+  using MockTable =
+      std::conditional_t<HashtableInlineData::kStorageMode == kCapacityByValue,
+                         MockTableByValue, MockTableByLog>;
   struct StatelessHash {
     size_t operator()(absl::string_view) const { return 0; }
   };
   struct StatefulHash : StatelessHash {
-    size_t dummy;
+    uint64_t dummy;
   };
 
+  struct GenerationData {
+    size_t reserved_growth;
+    size_t reservation_size;
+    GenerationType* generation;
+  };
+
+// Ignore unreachable-code warning. Compiler thinks one branch of each ternary
+// conditional is unreachable.
+#if defined(__clang__)
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wunreachable-code"
+#endif
+  constexpr size_t mock_size = sizeof(MockTable);
+  constexpr size_t generation_size =
+      SwisstableGenerationsEnabled() ? sizeof(GenerationData) : 0;
+#if defined(__clang__)
+#pragma clang diagnostic pop
+#endif
+
   EXPECT_EQ(
-      sizeof(MockTable),
+      mock_size + generation_size,
       sizeof(
           raw_hash_set<StringPolicy, StatelessHash,
                        std::equal_to<absl::string_view>, std::allocator<int>>));
 
   EXPECT_EQ(
-      sizeof(MockTable) + sizeof(StatefulHash),
+      mock_size + sizeof(StatefulHash) + generation_size,
       sizeof(
           raw_hash_set<StringPolicy, StatefulHash,
                        std::equal_to<absl::string_view>, std::allocator<int>>));
 }
 
-TEST(Table, Empty) {
-  IntTable t;
-  EXPECT_EQ(0, t.size());
-  EXPECT_TRUE(t.empty());
-}
-
-#ifdef __GNUC__
-template <class T>
-ABSL_ATTRIBUTE_ALWAYS_INLINE inline void DoNotOptimize(const T& v) {
-  asm volatile("" : : "r,m"(v) : "memory");
+TEST(InstantiateRawHashSetTest, VerifyTypes) {
+  using P = ValuePolicy<int>;
+  using DA = typename P::DefaultHash;
+  using DB = typename P::DefaultEq;
+  using DC = typename P::DefaultAlloc;
+
+  struct A {};
+  struct B {};
+  struct C {};
+
+  EXPECT_TRUE((std::is_same_v<InstantiateRawHashSet<P, A, B, C>::type,
+                              raw_hash_set<P, A, B, C>>));
+  EXPECT_TRUE((std::is_same_v<InstantiateRawHashSet<P, A, B, DC>::type,
+                              raw_hash_set<P, A, B>>));
+  EXPECT_TRUE((std::is_same_v<InstantiateRawHashSet<P, A, DB, C>::type,
+                              raw_hash_set<P, A, DB, C>>));
+  EXPECT_TRUE((std::is_same_v<InstantiateRawHashSet<P, A, DB, DC>::type,
+                              raw_hash_set<P, A>>));
+  EXPECT_TRUE((std::is_same_v<InstantiateRawHashSet<P, DA, B, C>::type,
+                              raw_hash_set<P, DA, B, C>>));
+  EXPECT_TRUE((std::is_same_v<InstantiateRawHashSet<P, DA, B, DC>::type,
+                              raw_hash_set<P, DA, B>>));
+  EXPECT_TRUE((std::is_same_v<InstantiateRawHashSet<P, DA, DB, C>::type,
+                              raw_hash_set<P, DA, DB, C>>));
+  EXPECT_TRUE((std::is_same_v<InstantiateRawHashSet<P, DA, DB, DC>::type,
+                              raw_hash_set<P>>));
 }
-#endif
 
 TEST(Table, Prefetch) {
   IntTable t;
@@ -431,77 +1441,20 @@ TEST(Table, Prefetch) {
   t.prefetch(1);
   t.prefetch(2);
 
-  // Do not run in debug mode, when prefetch is not implemented, or when
-  // sanitizers are enabled, or on WebAssembly.
-#if defined(NDEBUG) && defined(__GNUC__) && defined(__x86_64__) &&          \
-    !defined(ADDRESS_SANITIZER) && !defined(MEMORY_SANITIZER) &&            \
-    !defined(THREAD_SANITIZER) && !defined(UNDEFINED_BEHAVIOR_SANITIZER) && \
-    !defined(__EMSCRIPTEN__)
-  const auto now = [] { return absl::base_internal::CycleClock::Now(); };
-
-  // Make size enough to not fit in L2 cache (16.7 Mb)
-  static constexpr int size = 1 << 22;
+  static constexpr int size = 10;
   for (int i = 0; i < size; ++i) t.insert(i);
-
-  int64_t no_prefetch = 0, prefetch = 0;
-  for (int iter = 0; iter < 10; ++iter) {
-    int64_t time = now();
-    for (int i = 0; i < size; ++i) {
-      DoNotOptimize(t.find(i));
-    }
-    no_prefetch += now() - time;
-
-    time = now();
-    for (int i = 0; i < size; ++i) {
-      t.prefetch(i + 20);
-      DoNotOptimize(t.find(i));
-    }
-    prefetch += now() - time;
+  for (int i = 0; i < size; ++i) {
+    t.prefetch(i);
+    ASSERT_TRUE(t.find(i) != t.end()) << i;
   }
-
-  // no_prefetch is at least 30% slower.
-  EXPECT_GE(1.0 * no_prefetch / prefetch, 1.3);
-#endif
-}
-
-TEST(Table, LookupEmpty) {
-  IntTable t;
-  auto it = t.find(0);
-  EXPECT_TRUE(it == t.end());
 }
 
-TEST(Table, Insert1) {
-  IntTable t;
-  EXPECT_TRUE(t.find(0) == t.end());
-  auto res = t.emplace(0);
+TEST(Table, InsertCollision) {
+  BadTable t;
+  EXPECT_TRUE(t.find(1) == t.end());
+  auto res = t.emplace(1);
   EXPECT_TRUE(res.second);
-  EXPECT_THAT(*res.first, 0);
-  EXPECT_EQ(1, t.size());
-  EXPECT_THAT(*t.find(0), 0);
-}
-
-TEST(Table, Insert2) {
-  IntTable t;
-  EXPECT_TRUE(t.find(0) == t.end());
-  auto res = t.emplace(0);
-  EXPECT_TRUE(res.second);
-  EXPECT_THAT(*res.first, 0);
-  EXPECT_EQ(1, t.size());
-  EXPECT_TRUE(t.find(1) == t.end());
-  res = t.emplace(1);
-  EXPECT_TRUE(res.second);
-  EXPECT_THAT(*res.first, 1);
-  EXPECT_EQ(2, t.size());
-  EXPECT_THAT(*t.find(0), 0);
-  EXPECT_THAT(*t.find(1), 1);
-}
-
-TEST(Table, InsertCollision) {
-  BadTable t;
-  EXPECT_TRUE(t.find(1) == t.end());
-  auto res = t.emplace(1);
-  EXPECT_TRUE(res.second);
-  EXPECT_THAT(*res.first, 1);
+  EXPECT_THAT(*res.first, 1);
   EXPECT_EQ(1, t.size());
 
   EXPECT_TRUE(t.find(2) == t.end());
@@ -543,178 +1496,426 @@ TEST(Table, InsertCollisionAndFindAfterDelete) {
   EXPECT_TRUE(t.empty());
 }
 
-TEST(Table, LazyEmplace) {
-  StringTable t;
-  bool called = false;
-  auto it = t.lazy_emplace("abc", [&](const StringTable::constructor& f) {
-    called = true;
-    f("abc", "ABC");
-  });
-  EXPECT_TRUE(called);
-  EXPECT_THAT(*it, Pair("abc", "ABC"));
-  called = false;
-  it = t.lazy_emplace("abc", [&](const StringTable::constructor& f) {
-    called = true;
-    f("abc", "DEF");
-  });
-  EXPECT_FALSE(called);
-  EXPECT_THAT(*it, Pair("abc", "ABC"));
+TEST(Table, ReservedTableWithTombstonesDestructWell) {
+  constexpr int64_t kCoef = 17;
+  for (size_t capacity = Group::kWidth * 2 - 1; capacity < 256;
+       capacity = NextCapacity(capacity)) {
+    int64_t reserve_size =
+        static_cast<int64_t>(CapacityToGrowth(capacity) - capacity / 16);
+    IntTable t;
+    t.reserve(static_cast<size_t>(reserve_size));
+    for (int64_t i = 0; i < reserve_size; ++i) {
+      ASSERT_TRUE(t.insert(i * kCoef).second);
+    }
+    ASSERT_EQ(t.size(), reserve_size);
+    ASSERT_EQ(t.capacity(), capacity);
+    // Erase and insert values until we get a tombstone.
+    for (int64_t i = reserve_size; i < static_cast<int64_t>(capacity) * 1000;
+         ++i) {
+      ASSERT_EQ(t.erase((i - reserve_size) * kCoef), 1);
+      if (RawHashSetTestOnlyAccess::CountTombstones(t) > 0) {
+        break;
+      }
+      ASSERT_TRUE(t.insert(i * kCoef).second);
+    }
+    ASSERT_GT(RawHashSetTestOnlyAccess::CountTombstones(t), 0);
+  }
 }
 
-TEST(Table, ContainsEmpty) {
-  IntTable t;
+struct BadTwoValuesHash {
+  explicit BadTwoValuesHash(size_t other_value) : other_value(other_value) {}
+  size_t operator()(int64_t x) const { return x >= 0 ? 0 : other_value; }
+  size_t other_value;
+};
 
-  EXPECT_FALSE(t.contains(0));
+struct BadTwoValuesHashTable
+    : raw_hash_set<IntPolicy, BadTwoValuesHash, std::equal_to<int64_t>,
+                   std::allocator<int>> {
+  using Base = typename BadTwoValuesHashTable::raw_hash_set;
+  BadTwoValuesHashTable() = default;
+  using Base::Base;
+};
+
+TEST(Table, ReservedTableRehashWithoutGrowthWorksWell) {
+  if (SwisstableGenerationsEnabled()) {
+    GTEST_SKIP() << "Generations enabled, so rehash happening earlier.";
+  }
+  constexpr int64_t kCoef = 17;
+  int retries = 0;
+  for (size_t capacity = 31; capacity < 256;
+       capacity = NextCapacity(capacity)) {
+    SCOPED_TRACE(absl::StrCat("capacity: ", capacity));
+    // Number of elements we reserve in order to force a rehash without
+    // growth. RehashOrGrowToNextCapacityAndPrepareInsert grow if number of full
+    // slots is greater than 25/32 of capacity. We reserve slightly less than
+    // 25/32 of capacity to have extra space for tombstones.
+    int64_t reserve_size =
+        static_cast<int64_t>((capacity - 5) * 25 / 32 - 2);
+
+    BadTwoValuesHashTable t(
+        0,
+        // Negative number goes to the end of the table.
+        // Positive numbers hash is 0, so the first Group::kWidth * 2 elements
+        // will be placed at the beginning of the table.
+        BadTwoValuesHash(static_cast<size_t>(reserve_size)));
+    // Remove seed to make table layout deterministic.
+    t.reserve(static_cast<size_t>(reserve_size));
+    RawHashSetTestOnlyAccess::GetCommon(t).set_no_seed_for_testing();
+    for (int64_t i = 1; i <= reserve_size; ++i) {
+      ASSERT_TRUE(t.insert(i * kCoef).second);
+    }
+    ASSERT_EQ(t.size(), reserve_size);
+    ASSERT_EQ(t.capacity(), capacity);
+    bool rehashed = false;
+    int64_t last_erased = 0;
+    // We erase and insert until we get a lot of tombstones to force a rehash
+    // without growth. It happens relatively quickly because of the
+    // intentionally bad hash function that place numbers to the same slot
+    // depending on the sign.
+    for (int64_t i = 1; i <= reserve_size; ++i) {
+      SCOPED_TRACE(absl::StrCat("i: ", i));
+      ASSERT_EQ(t.erase(i * kCoef), 1);
+      size_t tombstones_before = RawHashSetTestOnlyAccess::CountTombstones(t);
+      ASSERT_TRUE(t.insert(-i * kCoef).second);
+      size_t tombstones_after = RawHashSetTestOnlyAccess::CountTombstones(t);
+      if (tombstones_before > 1 && tombstones_after == 0) {
+        ASSERT_EQ(t.capacity(), capacity) << "capacity must be preserved";
+        rehashed = true;
+        last_erased = i;
+        break;
+      }
+    }
+    if (!rehashed) {
+      // In debug mode rehashing may happen earlier with some probability.
+      // See ShouldRehashForBugDetection for details.
+      capacity = PreviousCapacity(capacity);
+      ++retries;
+      ASSERT_LT(retries, 50) << "Too many retries";
+      continue;
+    }
+    // Verify that all elements are still in the table after rehash.
+    for (int64_t i = 1; i <= reserve_size; ++i) {
+      if (i <= last_erased) {
+        ASSERT_TRUE(t.contains(-i * kCoef)) << i;
+      } else {
+        ASSERT_TRUE(t.contains(i * kCoef)) << i;
+      }
+    }
+  }
 }
 
-TEST(Table, Contains1) {
-  IntTable t;
+// This test verifies that we don't rehash in place when we insert an element
+// above the growth left threshold. Otherwise we may end up with zero empty
+// slots. That would cause hard to debug infinite loop in `find`.
+// This test do the following:
+// 1. Reserve a table with `kReserveSize` elements.
+// 2. Insert `Group::kWidth` elements to fill the first group (due to bad hash
+//    function all elements are inserted into the same group).
+// 3. Erase one element to create tombstone.
+// 4. Insert the same element back. But GrowthInfo still assumes that we may
+//    have a tombstone in the table.
+// 5. Insert one more element, which should cause a rehash and growth.
+TEST(Table,
+     ReservedTableResizeNotRehashInplaceIfInsertingElementAboveGrowthLeft) {
+  if (SwisstableGenerationsEnabled()) {
+    GTEST_SKIP() << "Generations enabled, so rehash happens earlier.\n"
+                 << "Note that reservation doesn't prevent rehashing since we "
+                    "are erasing one element.";
+  }
+  constexpr int64_t kCoef = 17;
+  constexpr size_t kCapacity = 31;
+  constexpr size_t kReserveSize =
+      CapacityToGrowth(kCapacity) - kMaxBlockedElementsForLargeTables;
+
+  BadTwoValuesHashTable t(0,
+                          // Negative number goes to the end of the table.
+                          BadTwoValuesHash(kReserveSize + 2));
+  t.reserve(kReserveSize);
+  // Remove seed to make table layout deterministic.
+  RawHashSetTestOnlyAccess::GetCommon(t).set_no_seed_for_testing();
+  for (int64_t i = 0; i < static_cast<int64_t>(Group::kWidth); ++i) {
+    ASSERT_TRUE(t.insert(i * kCoef).second);
+  }
+  EXPECT_EQ(t.erase(kCoef), 1);
+  EXPECT_EQ(RawHashSetTestOnlyAccess::CountTombstones(t), 1);
+  EXPECT_TRUE(t.insert(kCoef).second);
+  EXPECT_EQ(RawHashSetTestOnlyAccess::CountTombstones(t), 0);
+  // We want to test codepath deciding whether to rehash in place or not.
+  // For this we need to potentially have tombstone.
+  EXPECT_FALSE(RawHashSetTestOnlyAccess::GetCommon(t)
+                   .growth_info()
+                   .GetGrowthInfoLowerBound()
+                   .HasNoDeleted());
+  for (int64_t i = static_cast<int64_t>(Group::kWidth);
+       i < static_cast<int64_t>(kReserveSize); ++i) {
+    ASSERT_TRUE(t.insert(i * kCoef).second);
+  }
+  EXPECT_EQ(t.size(), kReserveSize);
+  EXPECT_EQ(t.capacity(), kCapacity);
+  EXPECT_TRUE(t.insert(-57).second);
+  EXPECT_EQ(t.size(), kReserveSize + 1);
+  EXPECT_EQ(RawHashSetTestOnlyAccess::CountTombstones(t), 0);
+  EXPECT_EQ(t.capacity(), NextCapacity(kCapacity));
+  for (int64_t i = 0; i < static_cast<int64_t>(kReserveSize); ++i) {
+    ASSERT_TRUE(t.contains(i * kCoef));
+  }
+  EXPECT_TRUE(t.contains(-57));
+}
 
-  EXPECT_TRUE(t.insert(0).second);
-  EXPECT_TRUE(t.contains(0));
-  EXPECT_FALSE(t.contains(1));
+template <class TableType>
+class ExtendedSooTest : public testing::Test {};
 
-  EXPECT_EQ(1, t.erase(0));
-  EXPECT_FALSE(t.contains(0));
-}
+using ExtendedSooTableTypes =
+    ::testing::Types<SooIntTable, SooIntTableTrivialDestroy, NonSooIntTable,
+                     NonSooIntTableTrivialDestroy, NonMemcpyableSooIntTable,
+                     MemcpyableSooIntCustomAllocTable,
+                     NonMemcpyableSooIntCustomAllocTable>;
+TYPED_TEST_SUITE(ExtendedSooTest, ExtendedSooTableTypes);
 
-TEST(Table, Contains2) {
-  IntTable t;
+template <class TableType>
+class SooTest : public testing::Test {};
 
-  EXPECT_TRUE(t.insert(0).second);
-  EXPECT_TRUE(t.contains(0));
-  EXPECT_FALSE(t.contains(1));
+using SooTableTypes =
+    ::testing::Types<SooIntTable, NonSooIntTable>;
+TYPED_TEST_SUITE(SooTest, SooTableTypes);
 
-  t.clear();
-  EXPECT_FALSE(t.contains(0));
+TYPED_TEST(SooTest, Empty) {
+  TypeParam t;
+  EXPECT_EQ(0, t.size());
+  EXPECT_TRUE(t.empty());
 }
 
-int decompose_constructed;
-struct DecomposeType {
-  DecomposeType(int i) : i(i) {  // NOLINT
-    ++decompose_constructed;
-  }
-
-  explicit DecomposeType(const char* d) : DecomposeType(*d) {}
+TYPED_TEST(SooTest, LookupEmpty) {
+  TypeParam t;
+  auto it = t.find(0);
+  EXPECT_TRUE(it == t.end());
+}
 
-  int i;
-};
+TYPED_TEST(SooTest, Insert1) {
+  TypeParam t;
+  EXPECT_TRUE(t.find(0) == t.end());
+  auto res = t.emplace(0);
+  EXPECT_TRUE(res.second);
+  EXPECT_THAT(*res.first, 0);
+  EXPECT_EQ(1, t.size());
+  EXPECT_THAT(*t.find(0), 0);
+}
 
-struct DecomposeHash {
-  using is_transparent = void;
-  size_t operator()(DecomposeType a) const { return a.i; }
-  size_t operator()(int a) const { return a; }
-  size_t operator()(const char* a) const { return *a; }
-};
+TYPED_TEST(SooTest, Insert2) {
+  TypeParam t;
+  EXPECT_TRUE(t.find(0) == t.end());
+  auto res = t.emplace(0);
+  EXPECT_TRUE(res.second);
+  EXPECT_THAT(*res.first, 0);
+  EXPECT_EQ(1, t.size());
+  EXPECT_TRUE(t.find(1) == t.end());
+  res = t.emplace(1);
+  EXPECT_TRUE(res.second);
+  EXPECT_THAT(*res.first, 1);
+  EXPECT_EQ(2, t.size());
+  EXPECT_THAT(*t.find(0), 0);
+  EXPECT_THAT(*t.find(1), 1);
+}
 
-struct DecomposeEq {
-  using is_transparent = void;
-  bool operator()(DecomposeType a, DecomposeType b) const { return a.i == b.i; }
-  bool operator()(DecomposeType a, int b) const { return a.i == b; }
-  bool operator()(DecomposeType a, const char* b) const { return a.i == *b; }
-};
+TYPED_TEST(SooTest, EraseInSmallTables) {
+  for (int64_t size = 0; size < 64; ++size) {
+    TypeParam t;
+    for (int64_t i = 0; i < size; ++i) {
+      t.insert(i);
+    }
+    for (int64_t i = 0; i < size; ++i) {
+      t.erase(i);
+      EXPECT_EQ(t.size(), size - i - 1);
+      for (int64_t j = i + 1; j < size; ++j) {
+        EXPECT_THAT(*t.find(j), j);
+      }
+    }
+    EXPECT_TRUE(t.empty());
+  }
+}
 
-struct DecomposePolicy {
-  using slot_type = DecomposeType;
-  using key_type = DecomposeType;
-  using init_type = DecomposeType;
+TYPED_TEST(ExtendedSooTest, InsertWithinCapacity) {
+  using TableType = TypeParam;
+  using ReserveFn = std::function<size_t(size_t, TableType&)>;
+  ReserveFn reserve_and_insert = [](size_t size, TableType& t) {
+    t.reserve(size);
+    size_t cap = t.capacity();
+    return cap;
+  };
+  ReserveFn construct_and_insert = [](size_t size, TableType& t) {
+    t = TableType(size);
+    size_t cap = t.capacity();
+    return cap;
+  };
+  ReserveFn construct_inisitializer_list_with_reservation =
+      [](size_t size, TableType& t) {
+        t = TableType({}, size);
+        return t.capacity();
+      };
+  ReserveFn construct_range_iter_with_reservation = [](size_t size,
+                                                       TableType& t) {
+    std::vector<int> v;
+    t = TableType(v.begin(), v.end(), size);
+    return t.capacity();
+  };
 
-  template <typename T>
-  static void construct(void*, DecomposeType* slot, T&& v) {
-    *slot = DecomposeType(std::forward<T>(v));
+  std::vector<ReserveFn> reserve_and_insert_fns = {
+      reserve_and_insert, construct_and_insert,
+      construct_inisitializer_list_with_reservation,
+      construct_range_iter_with_reservation};
+  for (size_t fn_i = 0; fn_i < reserve_and_insert_fns.size(); ++fn_i) {
+    ReserveFn reserve_and_insert_fn = reserve_and_insert_fns[fn_i];
+    for (int size : {3, 5, 7, 11, 27, 36}) {
+      SCOPED_TRACE(absl::StrCat("fn_i: ", fn_i, ", size: ", size));
+      TableType t;
+      const size_t original_capacity =
+          reserve_and_insert_fn(static_cast<size_t>(size), t);
+      const auto addr = [&](int i) {
+        return reinterpret_cast<uintptr_t>(&*t.find(i));
+      };
+      // Inserting an element does not change capacity.
+      t.insert(0);
+      ASSERT_THAT(t.capacity(), original_capacity);
+      const uintptr_t original_addr_0 = addr(0);
+      // Inserting lots of duplicate elements does not rehash.
+      for (int i = 0; i < size * 5; ++i) {
+        t.insert(i % size);
+      }
+      ASSERT_THAT(t.capacity(), original_capacity);
+      ASSERT_THAT(addr(0), original_addr_0);
+    }
   }
-  static void destroy(void*, DecomposeType*) {}
-  static DecomposeType& element(slot_type* slot) { return *slot; }
+}
 
-  template <class F, class T>
-  static auto apply(F&& f, const T& x) -> decltype(std::forward<F>(f)(x, x)) {
-    return std::forward<F>(f)(x, x);
+TYPED_TEST(ExtendedSooTest, ClearDifferentSizes) {
+  for (size_t size = 0; size < 32; ++size) {
+    for (bool reserve : {false, true}) {
+      for (bool clear_via_erase : {false, true}) {
+        SCOPED_TRACE(absl::StrCat("size: ", size, ", reserve: ", reserve,
+                                  ", clear_via_erase: ", clear_via_erase));
+        TypeParam t;
+        if (reserve) {
+          t.reserve(size);
+        }
+        for (size_t i = 0; i < size; ++i) {
+          ASSERT_TRUE(t.insert(static_cast<int>(i)).second) << i;
+        }
+        if (clear_via_erase) {
+          t.erase(t.begin(), t.end());
+        } else {
+          t.clear();
+        }
+        ASSERT_EQ(t.size(), 0);
+        for (size_t i = 0; i < size; ++i) {
+          ASSERT_TRUE(t.insert(static_cast<int>(i)).second) << i;
+        }
+      }
+    }
   }
-};
-
-template <typename Hash, typename Eq>
-void TestDecompose(bool construct_three) {
-  DecomposeType elem{0};
-  const int one = 1;
-  const char* three_p = "3";
-  const auto& three = three_p;
-
-  raw_hash_set<DecomposePolicy, Hash, Eq, std::allocator<int>> set1;
-
-  decompose_constructed = 0;
-  int expected_constructed = 0;
-  EXPECT_EQ(expected_constructed, decompose_constructed);
-  set1.insert(elem);
-  EXPECT_EQ(expected_constructed, decompose_constructed);
-  set1.insert(1);
-  EXPECT_EQ(++expected_constructed, decompose_constructed);
-  set1.emplace("3");
-  EXPECT_EQ(++expected_constructed, decompose_constructed);
-  EXPECT_EQ(expected_constructed, decompose_constructed);
+}
 
-  {  // insert(T&&)
-    set1.insert(1);
-    EXPECT_EQ(expected_constructed, decompose_constructed);
+TYPED_TEST(ExtendedSooTest, ReserveTwice) {
+  for (int reserve_size = 0; reserve_size < 32; ++reserve_size) {
+    for (int reserve_size2 = reserve_size; reserve_size2 < 32;
+         ++reserve_size2) {
+      SCOPED_TRACE(absl::StrCat("reserve_size: ", reserve_size,
+                                ", reserve_size2: ", reserve_size2));
+      TypeParam t;
+      t.reserve(static_cast<size_t>(reserve_size));
+      {  // Insert first batch of elements.
+        size_t cap = t.capacity();
+        for (int i = 1; i <= reserve_size; ++i) {
+          ASSERT_TRUE(t.insert(i).second) << i;
+        }
+        ASSERT_EQ(t.capacity(), cap);
+      }
+      t.reserve(static_cast<size_t>(reserve_size2));
+      {  // Insert second batch of elements.
+        size_t cap = t.capacity();
+        for (int i = reserve_size + 1; i <= reserve_size2; ++i) {
+          ASSERT_TRUE(t.insert(i).second) << i;
+        }
+        ASSERT_EQ(t.capacity(), cap);
+      }
+      for (int i = 1; i <= reserve_size2; ++i) {
+        ASSERT_TRUE(t.contains(i)) << i;
+        // Testing missing value to verify that we correctly have empty slots.
+        ASSERT_FALSE(t.contains(-i)) << i;
+      }
+    }
   }
+}
 
-  {  // insert(const T&)
-    set1.insert(one);
-    EXPECT_EQ(expected_constructed, decompose_constructed);
+TYPED_TEST(ExtendedSooTest, GrowAfterReserve) {
+  for (int reserve_size = 1; reserve_size <= 150; ++reserve_size) {
+    TypeParam s;
+    s.reserve(static_cast<size_t>(reserve_size));
+    int size = reserve_size + 1;
+    for (int i = 1; i <= size; ++i) {
+      ASSERT_TRUE(s.insert(i).second) << i;
+      // Testing missing value to verify that we correctly have empty slots.
+      ASSERT_FALSE(s.contains(-i)) << i;
+    }
+    EXPECT_EQ(s.size(), size);
+    for (int i = 1; i <= size; ++i) {
+      ASSERT_TRUE(s.contains(i)) << i;
+      ASSERT_FALSE(s.contains(-i)) << i;
+    }
   }
+}
 
-  {  // insert(hint, T&&)
-    set1.insert(set1.begin(), 1);
-    EXPECT_EQ(expected_constructed, decompose_constructed);
+TYPED_TEST(ExtendedSooTest, ClearAfterReserve) {
+  for (size_t reserve_size :
+       std::vector<size_t>{1, 3, 4, 6, 7, 8, 13, 14, 15, 128, 150}) {
+    TypeParam s;
+    s.reserve(reserve_size);
+    for (size_t i = 0; i < reserve_size; ++i) {
+      ASSERT_TRUE(s.insert(static_cast<int>(i)).second);
+    }
+    EXPECT_EQ(s.size(), reserve_size);
+    s.clear();
+    EXPECT_EQ(s.size(), 0);
+    for (size_t i = 0; i < reserve_size; ++i) {
+      ASSERT_FALSE(s.contains(static_cast<int>(i))) << i;
+    }
+    for (size_t i = 0; i < reserve_size; ++i) {
+      ASSERT_TRUE(s.insert(static_cast<int>(i)).second) << i;
+    }
+    for (size_t i = 0; i < reserve_size; ++i) {
+      ASSERT_TRUE(s.contains(static_cast<int>(i))) << i;
+    }
   }
+}
 
-  {  // insert(hint, const T&)
-    set1.insert(set1.begin(), one);
-    EXPECT_EQ(expected_constructed, decompose_constructed);
-  }
+TYPED_TEST(SooTest, ContainsEmpty) {
+  TypeParam t;
 
-  {  // emplace(...)
-    set1.emplace(1);
-    EXPECT_EQ(expected_constructed, decompose_constructed);
-    set1.emplace("3");
-    expected_constructed += construct_three;
-    EXPECT_EQ(expected_constructed, decompose_constructed);
-    set1.emplace(one);
-    EXPECT_EQ(expected_constructed, decompose_constructed);
-    set1.emplace(three);
-    expected_constructed += construct_three;
-    EXPECT_EQ(expected_constructed, decompose_constructed);
-  }
+  EXPECT_FALSE(t.contains(0));
+}
 
-  {  // emplace_hint(...)
-    set1.emplace_hint(set1.begin(), 1);
-    EXPECT_EQ(expected_constructed, decompose_constructed);
-    set1.emplace_hint(set1.begin(), "3");
-    expected_constructed += construct_three;
-    EXPECT_EQ(expected_constructed, decompose_constructed);
-    set1.emplace_hint(set1.begin(), one);
-    EXPECT_EQ(expected_constructed, decompose_constructed);
-    set1.emplace_hint(set1.begin(), three);
-    expected_constructed += construct_three;
-    EXPECT_EQ(expected_constructed, decompose_constructed);
-  }
+TYPED_TEST(SooTest, Contains1) {
+  TypeParam t;
+
+  EXPECT_TRUE(t.insert(0).second);
+  EXPECT_TRUE(t.contains(0));
+  EXPECT_FALSE(t.contains(1));
+
+  EXPECT_EQ(1, t.erase(0));
+  EXPECT_FALSE(t.contains(0));
 }
 
-TEST(Table, Decompose) {
-  TestDecompose<DecomposeHash, DecomposeEq>(false);
+TYPED_TEST(SooTest, Contains2) {
+  TypeParam t;
 
-  struct TransparentHashIntOverload {
-    size_t operator()(DecomposeType a) const { return a.i; }
-    size_t operator()(int a) const { return a; }
-  };
-  struct TransparentEqIntOverload {
-    bool operator()(DecomposeType a, DecomposeType b) const {
-      return a.i == b.i;
-    }
-    bool operator()(DecomposeType a, int b) const { return a.i == b; }
-  };
-  TestDecompose<TransparentHashIntOverload, DecomposeEq>(true);
-  TestDecompose<TransparentHashIntOverload, TransparentEqIntOverload>(true);
-  TestDecompose<DecomposeHash, TransparentEqIntOverload>(true);
+  EXPECT_TRUE(t.insert(0).second);
+  EXPECT_TRUE(t.contains(0));
+  EXPECT_FALSE(t.contains(1));
+
+  t.clear();
+  EXPECT_FALSE(t.contains(0));
+
+  EXPECT_TRUE(t.insert(0).second);
+  EXPECT_TRUE(t.contains(0));
 }
 
 // Returns the largest m such that a table with m elements has the same number
@@ -728,76 +1929,23 @@ size_t MaxDensitySize(size_t n) {
   return t.size() - 1;
 }
 
-struct Modulo1000Hash {
-  size_t operator()(int x) const { return x % 1000; }
-};
-
-struct Modulo1000HashTable
-    : public raw_hash_set<IntPolicy, Modulo1000Hash, std::equal_to<int>,
-                          std::allocator<int>> {};
-
-// Test that rehash with no resize happen in case of many deleted slots.
-TEST(Table, RehashWithNoResize) {
-  Modulo1000HashTable t;
-  // Adding the same length (and the same hash) strings
-  // to have at least kMinFullGroups groups
-  // with Group::kWidth collisions. Then fill up to MaxDensitySize;
-  const size_t kMinFullGroups = 7;
-  std::vector<int> keys;
-  for (size_t i = 0; i < MaxDensitySize(Group::kWidth * kMinFullGroups); ++i) {
-    int k = i * 1000;
-    t.emplace(k);
-    keys.push_back(k);
-  }
-  const size_t capacity = t.capacity();
-
-  // Remove elements from all groups except the first and the last one.
-  // All elements removed from full groups will be marked as kDeleted.
-  const size_t erase_begin = Group::kWidth / 2;
-  const size_t erase_end = (t.size() / Group::kWidth - 1) * Group::kWidth;
-  for (size_t i = erase_begin; i < erase_end; ++i) {
-    EXPECT_EQ(1, t.erase(keys[i])) << i;
-  }
-  keys.erase(keys.begin() + erase_begin, keys.begin() + erase_end);
-
-  auto last_key = keys.back();
-  size_t last_key_num_probes = GetHashtableDebugNumProbes(t, last_key);
-
-  // Make sure that we have to make a lot of probes for last key.
-  ASSERT_GT(last_key_num_probes, kMinFullGroups);
-
-  int x = 1;
-  // Insert and erase one element, before inplace rehash happen.
-  while (last_key_num_probes == GetHashtableDebugNumProbes(t, last_key)) {
-    t.emplace(x);
-    ASSERT_EQ(capacity, t.capacity());
-    // All elements should be there.
-    ASSERT_TRUE(t.find(x) != t.end()) << x;
-    for (const auto& k : keys) {
-      ASSERT_TRUE(t.find(k) != t.end()) << k;
-    }
-    t.erase(x);
-    ++x;
-  }
-}
-
-TEST(Table, InsertEraseStressTest) {
-  IntTable t;
-  const size_t kMinElementCount = 250;
-  std::deque<int> keys;
-  size_t i = 0;
-  for (; i < MaxDensitySize(kMinElementCount); ++i) {
-    t.emplace(i);
-    keys.push_back(i);
-  }
-  const size_t kNumIterations = 1000000;
-  for (; i < kNumIterations; ++i) {
-    ASSERT_EQ(1, t.erase(keys.front()));
-    keys.pop_front();
-    t.emplace(i);
-    keys.push_back(i);
-  }
-}
+TYPED_TEST(ExtendedSooTest, InsertEraseStressTest) {
+  TypeParam t;
+  const size_t kMinElementCount = 50;
+  std::deque<int> keys;
+  size_t i = 0;
+  for (; i < MaxDensitySize(kMinElementCount); ++i) {
+    t.emplace(static_cast<int64_t>(i));
+    keys.push_back(i);
+  }
+  const size_t kNumIterations = 20000;
+  for (; i < kNumIterations; ++i) {
+    ASSERT_EQ(1, t.erase(keys.front()));
+    keys.pop_front();
+    t.emplace(static_cast<int64_t>(i));
+    keys.push_back(i);
+  }
+}
 
 TEST(Table, InsertOverloads) {
   StringTable t;
@@ -810,28 +1958,36 @@ TEST(Table, InsertOverloads) {
                                       Pair("DEF", "!!!")));
 }
 
-TEST(Table, LargeTable) {
-  IntTable t;
-  for (int64_t i = 0; i != 100000; ++i) t.emplace(i << 40);
-  for (int64_t i = 0; i != 100000; ++i) ASSERT_EQ(i << 40, *t.find(i << 40));
+TYPED_TEST(SooTest, LargeTable) {
+  TypeParam t;
+  for (int64_t i = 0; i != 10000; ++i) {
+    t.emplace(i << 40);
+    ASSERT_EQ(t.size(), i + 1);
+  }
+  for (int64_t i = 0; i != 10000; ++i)
+    ASSERT_EQ(i << 40, static_cast<int64_t>(*t.find(i << 40)));
 }
 
-// Timeout if copy is quadratic as it was in Rust.
-TEST(Table, EnsureNonQuadraticAsInRust) {
+// Timeout if copy is quadratic as it was in Rust. See b/34756399.
+TYPED_TEST(SooTest, EnsureNonQuadraticAsInRust) {
   static const size_t kLargeSize = 1 << 15;
 
-  IntTable t;
+  TypeParam t;
   for (size_t i = 0; i != kLargeSize; ++i) {
     t.insert(i);
   }
 
   // If this is quadratic, the test will timeout.
-  IntTable t2;
+  TypeParam t2;
   for (const auto& entry : t) t2.insert(entry);
 }
 
-TEST(Table, ClearBug) {
-  IntTable t;
+TYPED_TEST(SooTest, ClearBug) {
+  if (SwisstableGenerationsEnabled()) {
+    GTEST_SKIP() << "Generations being enabled causes extra rehashes.";
+  }
+
+  TypeParam t;
   constexpr size_t capacity = container_internal::Group::kWidth - 1;
   constexpr size_t max_size = capacity / 2 + 1;
   for (size_t i = 0; i < max_size; ++i) {
@@ -849,12 +2005,12 @@ TEST(Table, ClearBug) {
   // We are checking that original and second are close enough to each other
   // that they are probably still in the same group.  This is not strictly
   // guaranteed.
-  EXPECT_LT(std::abs(original - second),
-            capacity * sizeof(IntTable::value_type));
+  EXPECT_LT(static_cast<size_t>(std::abs(original - second)),
+            capacity * sizeof(typename TypeParam::value_type));
 }
 
-TEST(Table, Erase) {
-  IntTable t;
+TYPED_TEST(SooTest, Erase) {
+  TypeParam t;
   EXPECT_TRUE(t.find(0) == t.end());
   auto res = t.emplace(0);
   EXPECT_TRUE(res.second);
@@ -864,10 +2020,10 @@ TEST(Table, Erase) {
   EXPECT_TRUE(t.find(0) == t.end());
 }
 
-TEST(Table, EraseMaintainsValidIterator) {
-  IntTable t;
+TYPED_TEST(SooTest, EraseMaintainsValidIterator) {
+  TypeParam t;
   const int kNumElements = 100;
-  for (int i = 0; i < kNumElements; i ++) {
+  for (int i = 0; i < kNumElements; i++) {
     EXPECT_TRUE(t.emplace(i).second);
   }
   EXPECT_EQ(t.size(), kNumElements);
@@ -883,1032 +2039,3235 @@ TEST(Table, EraseMaintainsValidIterator) {
   EXPECT_EQ(num_erase_calls, kNumElements);
 }
 
-// Collect N bad keys by following algorithm:
-// 1. Create an empty table and reserve it to 2 * N.
-// 2. Insert N random elements.
-// 3. Take first Group::kWidth - 1 to bad_keys array.
-// 4. Clear the table without resize.
-// 5. Go to point 2 while N keys not collected
-std::vector<int64_t> CollectBadMergeKeys(size_t N) {
-  static constexpr int kGroupSize = Group::kWidth - 1;
-
-  auto topk_range = [](size_t b, size_t e, IntTable* t) -> std::vector<int64_t> {
-    for (size_t i = b; i != e; ++i) {
-      t->emplace(i);
-    }
-    std::vector<int64_t> res;
-    res.reserve(kGroupSize);
-    auto it = t->begin();
-    for (size_t i = b; i != e && i != b + kGroupSize; ++i, ++it) {
-      res.push_back(*it);
-    }
-    return res;
-  };
+TYPED_TEST(SooTest, EraseBeginEnd) {
+  TypeParam t;
+  for (int i = 0; i < 10; ++i) t.insert(i);
+  EXPECT_EQ(t.size(), 10);
+  t.erase(t.begin(), t.end());
+  EXPECT_EQ(t.size(), 0);
+}
 
-  std::vector<int64_t> bad_keys;
-  bad_keys.reserve(N);
-  IntTable t;
-  t.reserve(N * 2);
+TYPED_TEST(SooTest, Clear) {
+  TypeParam t;
+  EXPECT_TRUE(t.find(0) == t.end());
+  t.clear();
+  EXPECT_TRUE(t.find(0) == t.end());
+  auto res = t.emplace(0);
+  EXPECT_TRUE(res.second);
+  EXPECT_EQ(1, t.size());
+  t.clear();
+  EXPECT_EQ(0, t.size());
+  EXPECT_TRUE(t.find(0) == t.end());
+}
 
-  for (size_t b = 0; bad_keys.size() < N; b += N) {
-    auto keys = topk_range(b, b + N, &t);
-    bad_keys.insert(bad_keys.end(), keys.begin(), keys.end());
-    t.erase(t.begin(), t.end());
-    EXPECT_TRUE(t.empty());
-  }
-  return bad_keys;
+TYPED_TEST(SooTest, Swap) {
+  TypeParam t;
+  EXPECT_TRUE(t.find(0) == t.end());
+  auto res = t.emplace(0);
+  EXPECT_TRUE(res.second);
+  EXPECT_EQ(1, t.size());
+  TypeParam u;
+  t.swap(u);
+  EXPECT_EQ(0, t.size());
+  EXPECT_EQ(1, u.size());
+  EXPECT_TRUE(t.find(0) == t.end());
+  EXPECT_THAT(*u.find(0), 0);
 }
 
-struct ProbeStats {
-  // Number of elements with specific probe length over all tested tables.
-  std::vector<size_t> all_probes_histogram;
-  // Ratios total_probe_length/size for every tested table.
-  std::vector<double> single_table_ratios;
+TYPED_TEST(SooTest, Rehash) {
+  TypeParam t;
+  EXPECT_TRUE(t.find(0) == t.end());
+  t.emplace(0);
+  t.emplace(1);
+  EXPECT_EQ(2, t.size());
+  t.rehash(128);
+  EXPECT_EQ(2, t.size());
+  EXPECT_THAT(*t.find(0), 0);
+  EXPECT_THAT(*t.find(1), 1);
+}
 
-  friend ProbeStats operator+(const ProbeStats& a, const ProbeStats& b) {
-    ProbeStats res = a;
-    res.all_probes_histogram.resize(std::max(res.all_probes_histogram.size(),
-                                             b.all_probes_histogram.size()));
-    std::transform(b.all_probes_histogram.begin(), b.all_probes_histogram.end(),
-                   res.all_probes_histogram.begin(),
-                   res.all_probes_histogram.begin(), std::plus<size_t>());
-    res.single_table_ratios.insert(res.single_table_ratios.end(),
-                                   b.single_table_ratios.begin(),
-                                   b.single_table_ratios.end());
-    return res;
-  }
+TYPED_TEST(SooTest, RehashDoesNotRehashWhenNotNecessary) {
+  TypeParam t;
+  t.emplace(0);
+  t.emplace(1);
+  auto* p = &*t.find(0);
+  t.rehash(1);
+  EXPECT_EQ(p, &*t.find(0));
+}
 
-  // Average ratio total_probe_length/size over tables.
-  double AvgRatio() const {
-    return std::accumulate(single_table_ratios.begin(),
-                           single_table_ratios.end(), 0.0) /
-           single_table_ratios.size();
-  }
+TYPED_TEST(SooTest, RehashZeroForcesRehash) {
+  TypeParam t;
+  t.emplace(0);
+  t.emplace(1);
+  auto* p = &*t.find(0);
+  t.rehash(0);
+  EXPECT_NE(p, &*t.find(0));
+}
 
-  // Maximum ratio total_probe_length/size over tables.
-  double MaxRatio() const {
-    return *std::max_element(single_table_ratios.begin(),
-                             single_table_ratios.end());
+TYPED_TEST(SooTest, CopyConstruct) {
+  TypeParam t;
+  t.emplace(0);
+  EXPECT_EQ(1, t.size());
+  {
+    TypeParam u(t);
+    EXPECT_EQ(1, u.size());
+    EXPECT_THAT(*u.find(0), 0);
   }
-
-  // Percentile ratio total_probe_length/size over tables.
-  double PercentileRatio(double Percentile = 0.95) const {
-    auto r = single_table_ratios;
-    auto mid = r.begin() + static_cast<size_t>(r.size() * Percentile);
-    if (mid != r.end()) {
-      std::nth_element(r.begin(), mid, r.end());
-      return *mid;
-    } else {
-      return MaxRatio();
-    }
+  {
+    TypeParam u{t};
+    EXPECT_EQ(1, u.size());
+    EXPECT_THAT(*u.find(0), 0);
   }
-
-  // Maximum probe length over all elements and all tables.
-  size_t MaxProbe() const { return all_probes_histogram.size(); }
-
-  // Fraction of elements with specified probe length.
-  std::vector<double> ProbeNormalizedHistogram() const {
-    double total_elements = std::accumulate(all_probes_histogram.begin(),
-                                            all_probes_histogram.end(), 0ull);
-    std::vector<double> res;
-    for (size_t p : all_probes_histogram) {
-      res.push_back(p / total_elements);
-    }
-    return res;
+  {
+    TypeParam u = t;
+    EXPECT_EQ(1, u.size());
+    EXPECT_THAT(*u.find(0), 0);
   }
+}
 
-  size_t PercentileProbe(double Percentile = 0.99) const {
-    size_t idx = 0;
-    for (double p : ProbeNormalizedHistogram()) {
-      if (Percentile > p) {
-        Percentile -= p;
-        ++idx;
-      } else {
-        return idx;
+TYPED_TEST(SooTest, CopyAssignment) {
+  std::vector<size_t> sizes = {0, 1, 7, 25};
+  for (size_t source_size : sizes) {
+    for (size_t target_size : sizes) {
+      SCOPED_TRACE(absl::StrCat("source_size: ", source_size,
+                                " target_size: ", target_size));
+      TypeParam source;
+      std::vector<int> source_elements;
+      for (size_t i = 0; i < source_size; ++i) {
+        source.emplace(static_cast<int>(i) * 2);
+        source_elements.push_back(static_cast<int>(i) * 2);
+      }
+      TypeParam target;
+      for (size_t i = 0; i < target_size; ++i) {
+        target.emplace(static_cast<int>(i) * 3);
       }
+      target = source;
+      ASSERT_EQ(target.size(), source_size);
+      ASSERT_THAT(target, UnorderedElementsAreArray(source_elements));
     }
-    return idx;
   }
+}
 
-  friend std::ostream& operator<<(std::ostream& out, const ProbeStats& s) {
-    out << "{AvgRatio:" << s.AvgRatio() << ", MaxRatio:" << s.MaxRatio()
-        << ", PercentileRatio:" << s.PercentileRatio()
-        << ", MaxProbe:" << s.MaxProbe() << ", Probes=[";
-    for (double p : s.ProbeNormalizedHistogram()) {
-      out << p << ",";
+TYPED_TEST(SooTest, CopyConstructWithSampling) {
+  SetSamplingRateTo1Percent();
+  for (int i = 0; i < 10000; ++i) {
+    TypeParam t;
+    t.emplace(0);
+    EXPECT_EQ(1, t.size());
+    {
+      TypeParam u(t);
+      EXPECT_EQ(1, u.size());
+      EXPECT_THAT(*u.find(0), 0);
     }
-    out << "]}";
-
-    return out;
   }
-};
+}
 
-struct ExpectedStats {
-  double avg_ratio;
-  double max_ratio;
-  std::vector<std::pair<double, double>> pecentile_ratios;
-  std::vector<std::pair<double, double>> pecentile_probes;
+TYPED_TEST(SooTest, CopyDifferentSizes) {
+  TypeParam t;
 
-  friend std::ostream& operator<<(std::ostream& out, const ExpectedStats& s) {
-    out << "{AvgRatio:" << s.avg_ratio << ", MaxRatio:" << s.max_ratio
-        << ", PercentileRatios: [";
-    for (auto el : s.pecentile_ratios) {
-      out << el.first << ":" << el.second << ", ";
-    }
-    out << "], PercentileProbes: [";
-    for (auto el : s.pecentile_probes) {
-      out << el.first << ":" << el.second << ", ";
+  for (int i = 0; i < 100; ++i) {
+    t.emplace(i);
+    TypeParam c = t;
+    for (int j = 0; j <= i; ++j) {
+      ASSERT_TRUE(c.find(j) != c.end()) << "i=" << i << " j=" << j;
     }
-    out << "]}";
-
-    return out;
+    // Testing find miss to verify that table is not full.
+    ASSERT_TRUE(c.find(-1) == c.end());
   }
-};
+}
 
-void VerifyStats(size_t size, const ExpectedStats& exp,
-                 const ProbeStats& stats) {
-  EXPECT_LT(stats.AvgRatio(), exp.avg_ratio) << size << " " << stats;
-  EXPECT_LT(stats.MaxRatio(), exp.max_ratio) << size << " " << stats;
-  for (auto pr : exp.pecentile_ratios) {
-    EXPECT_LE(stats.PercentileRatio(pr.first), pr.second)
-        << size << " " << pr.first << " " << stats;
+TYPED_TEST(ExtendedSooTest, CopyDifferentSizesWithReserve) {
+  for (size_t size = 0; size < 153; ++size) {
+    SCOPED_TRACE(absl::StrCat("size: ", size));
+    TypeParam t;
+    t.reserve(size);
+    for (size_t i = 0; i < size; ++i) {
+      ASSERT_TRUE(t.insert(static_cast<int>(i)).second) << i;
+    }
+    auto t2 = t;
+    ASSERT_EQ(t2.size(), size);
+    for (size_t i = 0; i < size; ++i) {
+      ASSERT_TRUE(t2.contains(static_cast<int>(i))) << i;
+    }
+    ASSERT_TRUE(t2.insert(static_cast<int>(size)).second);
+    ASSERT_EQ(t2.size(), size + 1);
+    ASSERT_TRUE(t2.contains(static_cast<int>(size)));
   }
+}
 
-  for (auto pr : exp.pecentile_probes) {
-    EXPECT_LE(stats.PercentileProbe(pr.first), pr.second)
-        << size << " " << pr.first << " " << stats;
+TYPED_TEST(SooTest, CopyDifferentCapacities) {
+  for (int cap = 1; cap < 100; cap = cap * 2 + 1) {
+    TypeParam t;
+    t.reserve(static_cast<size_t>(cap));
+    for (int i = 0; i <= cap; ++i) {
+      t.emplace(i);
+      if (i != cap && i % 5 != 0) {
+        continue;
+      }
+      TypeParam c = t;
+      for (int j = 0; j <= i; ++j) {
+        ASSERT_TRUE(c.find(j) != c.end())
+            << "cap=" << cap << " i=" << i << " j=" << j;
+      }
+      // Testing find miss to verify that table is not full.
+      ASSERT_TRUE(c.find(-1) == c.end());
+    }
   }
 }
 
-using ProbeStatsPerSize = std::map<size_t, ProbeStats>;
+// Invalid iterator use can trigger crashes or invalidated iterator assertions.
+testing::Matcher<const std::string&> InvalidIteratorMatcher() {
+  return AnyOf(HasSubstr("invalidated iterator"), HasSubstr("Invalid iterator"),
+               HasSubstr("invalid iterator"),
+               HasSubstr("CrashIfIteratorIsInvalid"));
+}
 
-// Collect total ProbeStats on num_iters iterations of the following algorithm:
-// 1. Create new table and reserve it to keys.size() * 2
-// 2. Insert all keys xored with seed
-// 3. Collect ProbeStats from final table.
-ProbeStats CollectProbeStatsOnKeysXoredWithSeed(const std::vector<int64_t>& keys,
-                                                size_t num_iters) {
-  const size_t reserve_size = keys.size() * 2;
-
-  ProbeStats stats;
-
-  int64_t seed = 0x71b1a19b907d6e33;
-  while (num_iters--) {
-    seed = static_cast<int64_t>(static_cast<uint64_t>(seed) * 17 + 13);
-    IntTable t1;
-    t1.reserve(reserve_size);
-    for (const auto& key : keys) {
-      t1.emplace(key ^ seed);
-    }
-
-    auto probe_histogram = GetHashtableDebugNumProbesHistogram(t1);
-    stats.all_probes_histogram.resize(
-        std::max(stats.all_probes_histogram.size(), probe_histogram.size()));
-    std::transform(probe_histogram.begin(), probe_histogram.end(),
-                   stats.all_probes_histogram.begin(),
-                   stats.all_probes_histogram.begin(), std::plus<size_t>());
+TYPED_TEST(SooTest, NumDeletedRegression) {
+  TypeParam t;
+  t.emplace(0);
+  t.erase(t.find(0));
+  // construct over a deleted slot.
+  t.emplace(0);
+  t.clear();
+}
 
-    size_t total_probe_seq_length = 0;
-    for (size_t i = 0; i < probe_histogram.size(); ++i) {
-      total_probe_seq_length += i * probe_histogram[i];
-    }
-    stats.single_table_ratios.push_back(total_probe_seq_length * 1.0 /
-                                        keys.size());
-    t1.erase(t1.begin(), t1.end());
+TYPED_TEST(SooTest, FindFullDeletedRegression) {
+  TypeParam t;
+  for (int i = 0; i < 1000; ++i) {
+    t.emplace(i);
+    t.erase(t.find(i));
   }
-  return stats;
+  EXPECT_EQ(0, t.size());
 }
 
-ExpectedStats XorSeedExpectedStats() {
-  constexpr bool kRandomizesInserts =
-#ifdef NDEBUG
-      false;
-#else   // NDEBUG
-      true;
-#endif  // NDEBUG
+TYPED_TEST(SooTest, ReplacingDeletedSlotDoesNotRehash) {
+  // We need to disable hashtablez to avoid issues related to SOO and sampling.
+  DisableSampling();
 
-  // The effective load factor is larger in non-opt mode because we insert
-  // elements out of order.
-  switch (container_internal::Group::kWidth) {
-    case 8:
-      if (kRandomizesInserts) {
-  return {0.05,
-          1.0,
-          {{0.95, 0.5}},
-          {{0.95, 0}, {0.99, 2}, {0.999, 4}, {0.9999, 10}}};
-      } else {
-  return {0.05,
-          2.0,
-          {{0.95, 0.1}},
-          {{0.95, 0}, {0.99, 2}, {0.999, 4}, {0.9999, 10}}};
-      }
-    case 16:
-      if (kRandomizesInserts) {
-        return {0.1,
-                1.0,
-                {{0.95, 0.1}},
-                {{0.95, 0}, {0.99, 1}, {0.999, 8}, {0.9999, 15}}};
-      } else {
-        return {0.05,
-                1.0,
-                {{0.95, 0.05}},
-                {{0.95, 0}, {0.99, 1}, {0.999, 4}, {0.9999, 10}}};
-      }
+  size_t n;
+  {
+    // Compute n such that n is the maximum number of elements before rehash.
+    TypeParam t;
+    t.emplace(0);
+    size_t c = t.bucket_count();
+    for (n = 1; c == t.bucket_count(); ++n) t.emplace(n);
+    --n;
   }
-  ABSL_RAW_LOG(FATAL, "%s", "Unknown Group width");
-  return {};
+  TypeParam t;
+  t.rehash(n);
+  const size_t c = t.bucket_count();
+  for (size_t i = 0; i != n; ++i) t.emplace(i);
+  EXPECT_EQ(c, t.bucket_count()) << "rehashing threshold = " << n;
+  t.erase(0);
+  t.emplace(0);
+  EXPECT_EQ(c, t.bucket_count()) << "rehashing threshold = " << n;
 }
 
-TEST(Table, DISABLED_EnsureNonQuadraticTopNXorSeedByProbeSeqLength) {
-  ProbeStatsPerSize stats;
-  std::vector<size_t> sizes = {Group::kWidth << 5, Group::kWidth << 10};
-  for (size_t size : sizes) {
-    stats[size] =
-        CollectProbeStatsOnKeysXoredWithSeed(CollectBadMergeKeys(size), 200);
-  }
-  auto expected = XorSeedExpectedStats();
-  for (size_t size : sizes) {
-    auto& stat = stats[size];
-    VerifyStats(size, expected, stat);
-  }
+TYPED_TEST(SooTest, HintInsert) {
+  TypeParam t = {1, 2, 3};
+  auto node = t.extract(1);
+  EXPECT_THAT(t, UnorderedElementsAre(2, 3));
+  auto it = t.insert(t.begin(), std::move(node));
+  EXPECT_THAT(t, UnorderedElementsAre(1, 2, 3));
+  EXPECT_EQ(*it, 1);
+  EXPECT_FALSE(node);  // NOLINT(bugprone-use-after-move)
+
+  node = t.extract(2);
+  EXPECT_THAT(t, UnorderedElementsAre(1, 3));
+  // reinsert 2 to make the next insert fail.
+  t.insert(2);
+  EXPECT_THAT(t, UnorderedElementsAre(1, 2, 3));
+  it = t.insert(t.begin(), std::move(node));
+  EXPECT_EQ(*it, 2);
+  // The node was not emptied by the insert call.
+  EXPECT_TRUE(node);  // NOLINT(bugprone-use-after-move)
 }
 
-// Collect total ProbeStats on num_iters iterations of the following algorithm:
-// 1. Create new table
-// 2. Select 10% of keys and insert 10 elements key * 17 + j * 13
-// 3. Collect ProbeStats from final table
-ProbeStats CollectProbeStatsOnLinearlyTransformedKeys(
-    const std::vector<int64_t>& keys, size_t num_iters) {
-  ProbeStats stats;
+TYPED_TEST(SooTest, RehashZeroForSmallTable) {
+  TypeParam t{0};
+  EXPECT_EQ(t.capacity(), 1);
+  t.rehash(0);
+  EXPECT_EQ(t.capacity(), 1);
+  EXPECT_TRUE(t.contains(0));
+  t.insert(1);
+  EXPECT_EQ(t.capacity(), NextCapacity(1));
+  EXPECT_TRUE(t.contains(0));
+  EXPECT_TRUE(t.contains(1));
+}
 
-  std::random_device rd;
-  std::mt19937 rng(rd());
-  auto linear_transform = [](size_t x, size_t y) { return x * 17 + y * 13; };
-  std::uniform_int_distribution<size_t> dist(0, keys.size()-1);
-  while (num_iters--) {
-    IntTable t1;
-    size_t num_keys = keys.size() / 10;
-    size_t start = dist(rng);
-    for (size_t i = 0; i != num_keys; ++i) {
-      for (size_t j = 0; j != 10; ++j) {
-        t1.emplace(linear_transform(keys[(i + start) % keys.size()], j));
-      }
+TYPED_TEST(SooTest, RangeConstructorReservation) {
+  constexpr int kMaxSize = 25;
+  std::vector<int> v;
+  for (int size = 1; size <= kMaxSize; ++size) {
+    v.push_back(size);
+    TypeParam t(v.begin(), v.end());
+    EXPECT_THAT(t, UnorderedElementsAreArray(v));
+    size_t capacity = t.capacity();
+    t.insert(size + 1);
+    auto expected_array = v;
+    expected_array.push_back(size + 1);
+    EXPECT_THAT(t, UnorderedElementsAreArray(expected_array));
+    // Single group tables are making exact reservation.
+    if (static_cast<size_t>(size) <= CapacityToGrowth(Group::kWidth - 1)) {
+      EXPECT_GT(t.capacity(), capacity);
     }
+  }
+  v.clear();
+  v.push_back(0);
+  TypeParam t(v.begin(), v.end(), /*reservation_size=*/10);
+  EXPECT_GT(t.capacity(), 7);
+  EXPECT_THAT(t, UnorderedElementsAreArray(v));
+}
 
-    auto probe_histogram = GetHashtableDebugNumProbesHistogram(t1);
-    stats.all_probes_histogram.resize(
-        std::max(stats.all_probes_histogram.size(), probe_histogram.size()));
-    std::transform(probe_histogram.begin(), probe_histogram.end(),
-                   stats.all_probes_histogram.begin(),
-                   stats.all_probes_histogram.begin(), std::plus<size_t>());
+template <typename T>
+T MakeSimpleTable(size_t size, bool do_reserve) {
+  T t;
+  if (do_reserve) t.reserve(size);
+  while (t.size() < size) t.insert(t.size());
+  return t;
+}
 
-    size_t total_probe_seq_length = 0;
-    for (size_t i = 0; i < probe_histogram.size(); ++i) {
-      total_probe_seq_length += i * probe_histogram[i];
-    }
-    stats.single_table_ratios.push_back(total_probe_seq_length * 1.0 /
-                                        t1.size());
-    t1.erase(t1.begin(), t1.end());
+template <typename T>
+std::vector<int> OrderOfIteration(const T& t) {
+  std::vector<int> res;
+  for (auto i : t) res.push_back(static_cast<int>(i));
+  return res;
+}
+
+// Generate irrelevant seeds to avoid being stuck in the same last bit
+// in seed.
+void GenerateIrrelevantSeeds(int cnt) {
+  for (int i = cnt % 17; i > 0; --i) {
+    NextHashTableSeed();
   }
-  return stats;
 }
 
-ExpectedStats LinearTransformExpectedStats() {
-  constexpr bool kRandomizesInserts =
-#ifdef NDEBUG
-      false;
-#else   // NDEBUG
-      true;
-#endif  // NDEBUG
+template <class TableType>
+class IterationOrderTest : public testing::Test {};
 
-  // The effective load factor is larger in non-opt mode because we insert
-  // elements out of order.
-  switch (container_internal::Group::kWidth) {
-    case 8:
-      if (kRandomizesInserts) {
-        return {0.1,
-                0.5,
-                {{0.95, 0.3}},
-                {{0.95, 0}, {0.99, 1}, {0.999, 8}, {0.9999, 15}}};
-      } else {
-        return {0.15,
-                0.5,
-                {{0.95, 0.3}},
-                {{0.95, 0}, {0.99, 3}, {0.999, 15}, {0.9999, 25}}};
+struct CustomHashIntTable
+    : raw_hash_set<IntPolicy, std::hash<int64_t>> {
+  using Base = typename CustomHashIntTable::raw_hash_set;
+  using Base::Base;
+};
+
+using IterationOrderTypes =
+    ::testing::Types<SooIntTable, NonSooIntTable, CustomHashIntTable>;
+TYPED_TEST_SUITE(IterationOrderTest, IterationOrderTypes);
+
+// These IterationOrderChanges tests depend on non-deterministic behavior.
+// We are injecting non-determinism to the table.
+// We have to retry enough times to make sure that the seed changes in bits that
+// matter for the iteration order.
+TYPED_TEST(IterationOrderTest, IterationOrderChangesByInstance) {
+  DisableSampling();  // We do not want test to pass only because of sampling.
+  for (bool do_reserve : {false, true}) {
+    for (size_t size : {2u, 6u, 12u, 20u}) {
+      SCOPED_TRACE(absl::StrCat("size: ", size, " do_reserve: ", do_reserve));
+      const auto reference_table = MakeSimpleTable<TypeParam>(size, do_reserve);
+      const auto reference = OrderOfIteration(reference_table);
+
+      bool found_difference = false;
+      for (int i = 0; !found_difference && i < 500; ++i) {
+        auto new_table = MakeSimpleTable<TypeParam>(size, do_reserve);
+        found_difference = OrderOfIteration(new_table) != reference;
+        GenerateIrrelevantSeeds(i);
       }
-    case 16:
-      if (kRandomizesInserts) {
-        return {0.1,
-                0.4,
-                {{0.95, 0.3}},
-                {{0.95, 0}, {0.99, 1}, {0.999, 8}, {0.9999, 15}}};
-      } else {
-        return {0.05,
-                0.2,
-                {{0.95, 0.1}},
-                {{0.95, 0}, {0.99, 1}, {0.999, 6}, {0.9999, 10}}};
+      if (!found_difference) {
+        FAIL() << "Iteration order remained the same across many attempts.";
       }
+    }
   }
-  ABSL_RAW_LOG(FATAL, "%s", "Unknown Group width");
-  return {};
 }
 
-TEST(Table, DISABLED_EnsureNonQuadraticTopNLinearTransformByProbeSeqLength) {
-  ProbeStatsPerSize stats;
-  std::vector<size_t> sizes = {Group::kWidth << 5, Group::kWidth << 10};
-  for (size_t size : sizes) {
-    stats[size] = CollectProbeStatsOnLinearlyTransformedKeys(
-        CollectBadMergeKeys(size), 300);
-  }
-  auto expected = LinearTransformExpectedStats();
-  for (size_t size : sizes) {
-    auto& stat = stats[size];
-    VerifyStats(size, expected, stat);
+TYPED_TEST(IterationOrderTest, IterationOrderChangesOnRehash) {
+  DisableSampling();  // We do not want test to pass only because of sampling.
+
+  // We test different sizes with many small numbers, because small table
+  // resize has a different codepath.
+  // Note: iteration order for size() <= 1 is always the same.
+  for (bool do_reserve : {false, true}) {
+    for (size_t size : {2u, 3u, 6u, 7u, 12u, 15u, 20u, 50u}) {
+      for (size_t rehash_size : {
+               size_t{0},  // Force rehash is guaranteed.
+               size * 10   // Rehash to the larger capacity is guaranteed.
+           }) {
+        SCOPED_TRACE(absl::StrCat("size: ", size, " rehash_size: ", rehash_size,
+                                  " do_reserve: ", do_reserve));
+        bool ok = false;
+        auto t = MakeSimpleTable<TypeParam>(size, do_reserve);
+        const size_t original_capacity = t.capacity();
+        auto reference = OrderOfIteration(t);
+        for (int i = 0; i < 500; ++i) {
+          if (i > 0 && rehash_size != 0) {
+            // Rehash back to original size.
+            t.rehash(0);
+            ASSERT_EQ(t.capacity(), original_capacity);
+            reference = OrderOfIteration(t);
+          }
+          // Force rehash.
+          t.rehash(rehash_size);
+          auto trial = OrderOfIteration(t);
+          if (trial != reference) {
+            // We are done.
+            ok = true;
+            break;
+          }
+          GenerateIrrelevantSeeds(i);
+        }
+        EXPECT_TRUE(ok)
+            << "Iteration order remained the same across many attempts " << size
+            << "->" << rehash_size << ".";
+      }
+    }
   }
 }
 
-TEST(Table, EraseCollision) {
-  BadTable t;
+// Verify that pointers are invalidated as soon as a second element is inserted.
+// This prevents dependency on pointer stability on small tables.
+TYPED_TEST(SooTest, UnstablePointers) {
+  // We need to disable hashtablez to avoid issues related to SOO and sampling.
+  DisableSampling();
 
-  // 1 2 3
-  t.emplace(1);
-  t.emplace(2);
-  t.emplace(3);
-  EXPECT_THAT(*t.find(1), 1);
-  EXPECT_THAT(*t.find(2), 2);
-  EXPECT_THAT(*t.find(3), 3);
-  EXPECT_EQ(3, t.size());
+  TypeParam table;
 
-  // 1 DELETED 3
-  t.erase(t.find(2));
-  EXPECT_THAT(*t.find(1), 1);
-  EXPECT_TRUE(t.find(2) == t.end());
-  EXPECT_THAT(*t.find(3), 3);
-  EXPECT_EQ(2, t.size());
+  const auto addr = [&](int i) {
+    return reinterpret_cast<uintptr_t>(&*table.find(i));
+  };
 
-  // DELETED DELETED 3
-  t.erase(t.find(1));
-  EXPECT_TRUE(t.find(1) == t.end());
-  EXPECT_TRUE(t.find(2) == t.end());
-  EXPECT_THAT(*t.find(3), 3);
-  EXPECT_EQ(1, t.size());
+  table.insert(0);
+  const uintptr_t old_ptr = addr(0);
 
-  // DELETED DELETED DELETED
-  t.erase(t.find(3));
-  EXPECT_TRUE(t.find(1) == t.end());
-  EXPECT_TRUE(t.find(2) == t.end());
-  EXPECT_TRUE(t.find(3) == t.end());
-  EXPECT_EQ(0, t.size());
+  // This causes a rehash.
+  table.insert(1);
+
+  EXPECT_NE(old_ptr, addr(0));
 }
 
-TEST(Table, EraseInsertProbing) {
-  BadTable t(100);
+TYPED_TEST(SooTest, IteratorInvalidAssertsEqualityOperator) {
+  if (!IsAssertEnabled() && !SwisstableGenerationsEnabled())
+    GTEST_SKIP() << "Assertions not enabled.";
+
+  TypeParam t;
+  t.insert(1);
+  t.insert(2);
+  t.insert(3);
+  auto iter1 = t.begin();
+  auto iter2 = std::next(iter1);
+  ASSERT_NE(iter1, t.end());
+  ASSERT_NE(iter2, t.end());
+  t.erase(iter1);
+  // Extra simple "regexp" as regexp support is highly varied across platforms.
+  const char* const kErasedDeathMessage =
+      SwisstableGenerationsEnabled()
+          ? "Invalid iterator comparison.*was likely erased"
+          : "Invalid iterator comparison.*might have been erased.*config=asan";
+  EXPECT_DEATH_IF_SUPPORTED(void(iter1 == iter2), kErasedDeathMessage);
+  EXPECT_DEATH_IF_SUPPORTED(void(iter2 != iter1), kErasedDeathMessage);
+  t.erase(iter2);
+  EXPECT_DEATH_IF_SUPPORTED(void(iter1 == iter2), kErasedDeathMessage);
+
+  TypeParam t1, t2;
+  t1.insert(0);
+  t2.insert(0);
+  iter1 = t1.begin();
+  iter2 = t2.begin();
+  const char* const kContainerDiffDeathMessage =
+      SwisstableGenerationsEnabled()
+          ? "Invalid iterator comparison.*iterators from different.* hashtables"
+          : "Invalid iterator comparison.*may be from different "
+            ".*containers.*config=asan";
+  EXPECT_DEATH_IF_SUPPORTED(void(iter1 == iter2), kContainerDiffDeathMessage);
+  EXPECT_DEATH_IF_SUPPORTED(void(iter2 == iter1), kContainerDiffDeathMessage);
+}
 
-  // 1 2 3 4
-  t.emplace(1);
-  t.emplace(2);
-  t.emplace(3);
-  t.emplace(4);
+TYPED_TEST(SooTest, IteratorInvalidAssertsEqualityOperatorRehash) {
+  if (!IsAssertEnabled() && !SwisstableGenerationsEnabled())
+    GTEST_SKIP() << "Assertions not enabled.";
+#ifdef ABSL_HAVE_THREAD_SANITIZER
+  GTEST_SKIP() << "ThreadSanitizer test runs fail on use-after-free even in "
+                  "EXPECT_DEATH.";
+#endif
 
-  // 1 DELETED 3 DELETED
-  t.erase(t.find(2));
-  t.erase(t.find(4));
+  TypeParam t;
+  t.insert(0);
+  auto iter = t.begin();
 
-  // 1 10 3 11 12
-  t.emplace(10);
-  t.emplace(11);
-  t.emplace(12);
+  // Trigger a rehash in t.
+  for (int i = 0; i < 10; ++i) t.insert(i);
 
-  EXPECT_EQ(5, t.size());
-  EXPECT_THAT(t, UnorderedElementsAre(1, 10, 3, 11, 12));
+  EXPECT_DEATH_IF_SUPPORTED(void(iter == t.begin()), InvalidIteratorMatcher());
 }
 
-TEST(Table, Clear) {
-  IntTable t;
-  EXPECT_TRUE(t.find(0) == t.end());
-  t.clear();
-  EXPECT_TRUE(t.find(0) == t.end());
-  auto res = t.emplace(0);
-  EXPECT_TRUE(res.second);
-  EXPECT_EQ(1, t.size());
-  t.clear();
-  EXPECT_EQ(0, t.size());
-  EXPECT_TRUE(t.find(0) == t.end());
+TYPED_TEST(SooTest, IteratorInvalidAssertsEqualityOperatorMovedFrom) {
+  if (!SwisstableGenerationsEnabled())
+    GTEST_SKIP() << "Generations not enabled.";
+
+  TypeParam t;
+  for (int i = 0; i < 10; ++i) t.insert(i);
+  auto iter = t.begin();
+
+  TypeParam t2 = std::move(t);
+
+  EXPECT_DEATH_IF_SUPPORTED(void(iter == t2.begin()), InvalidIteratorMatcher());
 }
 
-TEST(Table, Swap) {
-  IntTable t;
-  EXPECT_TRUE(t.find(0) == t.end());
-  auto res = t.emplace(0);
-  EXPECT_TRUE(res.second);
+
+TYPED_TEST(SooTest, ReservedGrowthUpdatesWhenTableDoesntGrow) {
+  TypeParam t;
+  for (int i = 0; i < 8; ++i) t.insert(i);
+  // Want to insert twice without invalidating iterators so reserve.
+  const size_t cap = t.capacity();
+  t.reserve(t.size() + 2);
+  // We want to be testing the case in which the reserve doesn't grow the table.
+  ASSERT_EQ(cap, t.capacity());
+  auto it = t.find(0);
+  t.insert(100);
+  t.insert(200);
+  // `it` shouldn't have been invalidated.
+  EXPECT_EQ(*it, 0);
+}
+
+TYPED_TEST(SooTest, EraseIfAll) {
+  auto pred = [](const auto&) { return true; };
+  for (int size = 0; size < 100; ++size) {
+    TypeParam t;
+    for (int i = 0; i < size; ++i) t.insert(i);
+    absl::container_internal::EraseIf(pred, &t);
+    ASSERT_EQ(t.size(), 0);
+  }
+}
+
+TYPED_TEST(SooTest, EraseIfNone) {
+  auto pred = [](const auto&) { return false; };
+  TypeParam t;
+  for (size_t size = 0; size < 100; ++size) {
+    absl::container_internal::EraseIf(pred, &t);
+    ASSERT_EQ(t.size(), size);
+    t.insert(size);
+  }
+}
+
+TYPED_TEST(SooTest, EraseIfPartial) {
+  for (int mod : {0, 1}) {
+    auto pred = [&](const auto& x) {
+      return static_cast<int64_t>(x) % 2 == mod;
+    };
+    for (int size = 0; size < 100; ++size) {
+      SCOPED_TRACE(absl::StrCat(mod, " ", size));
+      TypeParam t;
+      std::vector<int64_t> expected;
+      for (int i = 0; i < size; ++i) {
+        t.insert(i);
+        if (i % 2 != mod) {
+          expected.push_back(i);
+        }
+      }
+      absl::container_internal::EraseIf(pred, &t);
+      ASSERT_THAT(t, testing::UnorderedElementsAreArray(expected));
+    }
+  }
+}
+
+TYPED_TEST(SooTest, ForEach) {
+  TypeParam t;
+  std::vector<int64_t> expected;
+  for (int size = 0; size < 100; ++size) {
+    SCOPED_TRACE(size);
+    {
+      SCOPED_TRACE("mutable iteration");
+      std::vector<int64_t> actual;
+      auto f = [&](auto& x) { actual.push_back(static_cast<int64_t>(x)); };
+      absl::container_internal::ForEach(f, &t);
+      ASSERT_THAT(actual, testing::UnorderedElementsAreArray(expected));
+    }
+    {
+      SCOPED_TRACE("const iteration");
+      std::vector<int64_t> actual;
+      auto f = [&](auto& x) {
+        static_assert(std::is_const_v<std::remove_reference_t<decltype(x)>>,
+                      "no mutable values should be passed to const ForEach");
+        actual.push_back(static_cast<int64_t>(x));
+      };
+      const auto& ct = t;
+      absl::container_internal::ForEach(f, &ct);
+      ASSERT_THAT(actual, testing::UnorderedElementsAreArray(expected));
+    }
+    t.insert(size);
+    expected.push_back(size);
+  }
+}
+
+TEST(Table, ForEachMutate) {
+  StringTable t;
+  using ValueType = StringTable::value_type;
+  std::vector<ValueType> expected;
+  for (int size = 0; size < 100; ++size) {
+    SCOPED_TRACE(size);
+    std::vector<ValueType> actual;
+    auto f = [&](ValueType& x) {
+      actual.push_back(x);
+      x.second += 'a';
+    };
+    absl::container_internal::ForEach(f, &t);
+    ASSERT_THAT(actual, testing::UnorderedElementsAreArray(expected));
+    for (ValueType& v : expected) {
+      v.second += 'a';
+    }
+    ASSERT_THAT(t, testing::UnorderedElementsAreArray(expected));
+    t.emplace(std::to_string(size), std::to_string(size));
+    expected.emplace_back(std::to_string(size), std::to_string(size));
+  }
+}
+
+TYPED_TEST(SooTest, EraseIfReentryDeath) {
+  if (!IsAssertEnabled()) GTEST_SKIP() << "Assertions not enabled.";
+
+  auto erase_if_with_removal_reentrance = [](size_t reserve_size) {
+    TypeParam t;
+    t.reserve(reserve_size);
+    int64_t first_value = -1;
+    t.insert(1024);
+    t.insert(5078);
+    auto pred = [&](const auto& x) {
+      if (first_value == -1) {
+        first_value = static_cast<int64_t>(x);
+        return false;
+      }
+      // We erase on second call to `pred` to reduce the chance that assertion
+      // will happen in IterateOverFullSlots.
+      t.erase(first_value);
+      return true;
+    };
+    absl::container_internal::EraseIf(pred, &t);
+  };
+  // Removal will likely happen in a different group.
+  EXPECT_DEATH_IF_SUPPORTED(erase_if_with_removal_reentrance(1024 * 16),
+                            "hash table was modified unexpectedly");
+  // Removal will happen in the same group.
+  EXPECT_DEATH_IF_SUPPORTED(
+      erase_if_with_removal_reentrance(CapacityToGrowth(Group::kWidth - 1)),
+      "hash table was modified unexpectedly");
+}
+
+// This test is useful to test soo branch.
+TYPED_TEST(SooTest, EraseIfReentrySingleElementDeath) {
+  if (!IsAssertEnabled()) GTEST_SKIP() << "Assertions not enabled.";
+
+  auto erase_if_with_removal_reentrance = []() {
+    TypeParam t;
+    t.insert(1024);
+    auto pred = [&](const auto& x) {
+      // We erase ourselves in order to confuse the erase_if.
+      t.erase(static_cast<int64_t>(x));
+      return false;
+    };
+    absl::container_internal::EraseIf(pred, &t);
+  };
+  EXPECT_DEATH_IF_SUPPORTED(erase_if_with_removal_reentrance(),
+                            "hash table was modified unexpectedly");
+}
+
+template <class TableType>
+class SmallTableResizeTest : public testing::Test {};
+
+using SmallTableTypes = ::testing::Types<
+    IntTable, TransferableIntTable, SooIntTable,
+    // int8
+    ValueTable<int8_t, /*kTransferable=*/true, /*kSoo=*/true>,
+    ValueTable<int8_t, /*kTransferable=*/false, /*kSoo=*/true>,
+    // int16
+    ValueTable<int16_t, /*kTransferable=*/true, /*kSoo=*/true>,
+    // int128
+    ValueTable<SizedValue<16>, /*kTransferable=*/true, /*kSoo=*/true>,
+    // Special tables.
+    MinimumAlignmentUint8Table, CustomAllocIntTable, ChangingSizeAllocIntTable,
+    BadTable,
+    // alignment 1, size 2.
+    ValueTable<AlignedValue<uint8_t, 2>, /*kTransferable=*/true, /*kSoo=*/true>,
+    // alignment 1, size 7.
+    ValueTable<AlignedValue<uint8_t, 7>, /*kTransferable=*/true, /*kSoo=*/true>,
+    ValueTable<AlignedValue<uint8_t, 7>, /*kTransferable=*/false,
+               /*kSoo=*/true>,
+    // alignment 2, size 6.
+    ValueTable<AlignedValue<uint16_t, 3>, /*kTransferable=*/true,
+               /*kSoo=*/true>>;
+TYPED_TEST_SUITE(SmallTableResizeTest, SmallTableTypes);
+
+TYPED_TEST(SmallTableResizeTest, InsertIntoSmallTable) {
+  TypeParam t;
+  for (int i = 0; i < 32; ++i) {
+    t.insert(i);
+    ASSERT_EQ(t.size(), i + 1);
+    for (int j = 0; j < i + 1; ++j) {
+      ASSERT_TRUE(t.find(j) != t.end());
+      EXPECT_EQ(*t.find(j), j);
+    }
+  }
+}
+
+TYPED_TEST(SmallTableResizeTest, ResizeGrowSmallTables) {
+  for (int source_size = 0; source_size < 32; ++source_size) {
+    for (int target_size = source_size; target_size < 32; ++target_size) {
+      for (bool rehash : {false, true}) {
+        SCOPED_TRACE(absl::StrCat("source_size: ", source_size,
+                                  ", target_size: ", target_size,
+                                  ", rehash: ", rehash));
+        TypeParam t;
+        for (int i = 0; i < source_size; ++i) {
+          t.insert(i);
+        }
+        if (rehash) {
+          t.rehash(static_cast<size_t>(target_size));
+        } else {
+          t.reserve(static_cast<size_t>(target_size));
+        }
+        for (int i = 0; i < source_size; ++i) {
+          ASSERT_TRUE(t.contains(i));
+          EXPECT_EQ(*t.find(static_cast<int>(i)), static_cast<int>(i));
+        }
+      }
+    }
+  }
+}
+
+TYPED_TEST(SmallTableResizeTest, ResizeReduceSmallTables) {
+  DisableSampling();
+  for (size_t source_size = 0; source_size < 32; ++source_size) {
+    for (size_t target_size = 0; target_size <= source_size; ++target_size) {
+      TypeParam t;
+      size_t inserted_count = std::min<size_t>(source_size, 5);
+      for (size_t i = 0; i < inserted_count; ++i) {
+        t.insert(static_cast<int>(i));
+      }
+      const size_t minimum_capacity = t.capacity();
+      t.reserve(source_size);
+      t.rehash(target_size);
+      if (target_size == 0) {
+        EXPECT_EQ(t.capacity(), minimum_capacity)
+            << "rehash(0) must resize to the minimum capacity";
+      }
+      for (size_t i = 0; i < inserted_count; ++i) {
+        ASSERT_TRUE(t.find(static_cast<int>(i)) != t.end());
+        EXPECT_EQ(*t.find(static_cast<int>(i)), static_cast<int>(i));
+      }
+    }
+  }
+}
+
+TEST(Table, LazyEmplace) {
+  StringTable t;
+  bool called = false;
+  auto it = t.lazy_emplace("abc", [&](const StringTable::constructor& f) {
+    called = true;
+    f("abc", "ABC");
+  });
+  EXPECT_TRUE(called);
+  EXPECT_THAT(*it, Pair("abc", "ABC"));
+  called = false;
+  it = t.lazy_emplace("abc", [&](const StringTable::constructor& f) {
+    called = true;
+    f("abc", "DEF");
+  });
+  EXPECT_FALSE(called);
+  EXPECT_THAT(*it, Pair("abc", "ABC"));
+}
+
+int decompose_constructed;
+int decompose_copy_constructed;
+int decompose_copy_assigned;
+int decompose_move_constructed;
+int decompose_move_assigned;
+struct DecomposeType {
+  DecomposeType(int i = 0) : i(i) {  // NOLINT
+    ++decompose_constructed;
+  }
+
+  explicit DecomposeType(const char* d) : DecomposeType(*d) {}
+
+  DecomposeType(const DecomposeType& other) : i(other.i) {
+    ++decompose_copy_constructed;
+  }
+  DecomposeType& operator=(const DecomposeType& other) {
+    ++decompose_copy_assigned;
+    i = other.i;
+    return *this;
+  }
+  DecomposeType(DecomposeType&& other) : i(other.i) {
+    ++decompose_move_constructed;
+  }
+  DecomposeType& operator=(DecomposeType&& other) {
+    ++decompose_move_assigned;
+    i = other.i;
+    return *this;
+  }
+
+  int i;
+};
+
+struct DecomposeHash {
+  using is_transparent = void;
+  size_t operator()(const DecomposeType& a) const { return a.i; }
+  size_t operator()(int a) const { return a; }
+  size_t operator()(const char* a) const { return *a; }
+};
+
+struct DecomposeEq {
+  using is_transparent = void;
+  bool operator()(const DecomposeType& a, const DecomposeType& b) const {
+    return a.i == b.i;
+  }
+  bool operator()(const DecomposeType& a, int b) const { return a.i == b; }
+  bool operator()(const DecomposeType& a, const char* b) const {
+    return a.i == *b;
+  }
+};
+
+struct DecomposePolicy {
+  using slot_type = DecomposeType;
+  using key_type = DecomposeType;
+  using init_type = DecomposeType;
+
+  using DefaultHash = void;
+  using DefaultEq = void;
+  using DefaultAlloc = void;
+
+  template <typename T>
+  static void construct(void*, DecomposeType* slot, T&& v) {
+    ::new (slot) DecomposeType(std::forward<T>(v));
+  }
+  static void destroy(void*, DecomposeType* slot) { slot->~DecomposeType(); }
+  static DecomposeType& element(slot_type* slot) { return *slot; }
+
+  template <class F, class T>
+  static auto apply(F&& f, const T& x) -> decltype(std::forward<F>(f)(x, x)) {
+    return std::forward<F>(f)(x, x);
+  }
+
+  template <class Hash, bool kIsDefault, size_t kSeedShift>
+  static constexpr HashSlotFn get_hash_slot_fn() {
+    return nullptr;
+  }
+};
+
+template <typename Hash, typename Eq>
+void TestDecompose(bool construct_three) {
+  DecomposeType elem{0};
+  const int one = 1;
+  const char* three_p = "3";
+  const auto& three = three_p;
+  const int elem_vector_count = 256;
+  std::vector<DecomposeType> elem_vector(elem_vector_count, DecomposeType{0});
+  std::iota(elem_vector.begin(), elem_vector.end(), 0);
+
+  using DecomposeSet =
+      raw_hash_set<DecomposePolicy, Hash, Eq, std::allocator<int>>;
+  DecomposeSet set1;
+
+  decompose_constructed = 0;
+  int expected_constructed = 0;
+  EXPECT_EQ(expected_constructed, decompose_constructed);
+  set1.insert(elem);
+  EXPECT_EQ(expected_constructed, decompose_constructed);
+  set1.insert(1);
+  EXPECT_EQ(++expected_constructed, decompose_constructed);
+  set1.emplace("3");
+  EXPECT_EQ(++expected_constructed, decompose_constructed);
+  EXPECT_EQ(expected_constructed, decompose_constructed);
+
+  {  // insert(T&&)
+    set1.insert(1);
+    EXPECT_EQ(expected_constructed, decompose_constructed);
+  }
+
+  {  // insert(const T&)
+    set1.insert(one);
+    EXPECT_EQ(expected_constructed, decompose_constructed);
+  }
+
+  {  // insert(hint, T&&)
+    set1.insert(set1.begin(), 1);
+    EXPECT_EQ(expected_constructed, decompose_constructed);
+  }
+
+  {  // insert(hint, const T&)
+    set1.insert(set1.begin(), one);
+    EXPECT_EQ(expected_constructed, decompose_constructed);
+  }
+
+  {  // emplace(...)
+    set1.emplace(1);
+    EXPECT_EQ(expected_constructed, decompose_constructed);
+    set1.emplace("3");
+    expected_constructed += construct_three;
+    EXPECT_EQ(expected_constructed, decompose_constructed);
+    set1.emplace(one);
+    EXPECT_EQ(expected_constructed, decompose_constructed);
+    set1.emplace(three);
+    expected_constructed += construct_three;
+    EXPECT_EQ(expected_constructed, decompose_constructed);
+  }
+
+  {  // emplace_hint(...)
+    set1.emplace_hint(set1.begin(), 1);
+    EXPECT_EQ(expected_constructed, decompose_constructed);
+    set1.emplace_hint(set1.begin(), "3");
+    expected_constructed += construct_three;
+    EXPECT_EQ(expected_constructed, decompose_constructed);
+    set1.emplace_hint(set1.begin(), one);
+    EXPECT_EQ(expected_constructed, decompose_constructed);
+    set1.emplace_hint(set1.begin(), three);
+    expected_constructed += construct_three;
+    EXPECT_EQ(expected_constructed, decompose_constructed);
+  }
+
+  decompose_copy_constructed = 0;
+  decompose_copy_assigned = 0;
+  decompose_move_constructed = 0;
+  decompose_move_assigned = 0;
+  int expected_copy_constructed = 0;
+  int expected_move_constructed = 0;
+  {  // raw_hash_set(first, last) with random-access iterators
+    DecomposeSet set2(elem_vector.begin(), elem_vector.end());
+    // Expect exactly one copy-constructor call for each element if no
+    // rehashing is done.
+    expected_copy_constructed += elem_vector_count;
+    EXPECT_EQ(expected_copy_constructed, decompose_copy_constructed);
+    EXPECT_EQ(expected_move_constructed, decompose_move_constructed);
+    EXPECT_EQ(0, decompose_move_assigned);
+    EXPECT_EQ(0, decompose_copy_assigned);
+  }
+
+  {  // raw_hash_set(first, last) with forward iterators
+    std::list<DecomposeType> elem_list(elem_vector.begin(), elem_vector.end());
+    expected_copy_constructed = decompose_copy_constructed;
+    DecomposeSet set2(elem_list.begin(), elem_list.end());
+    // Expect exactly N elements copied into set, expect at most 2*N elements
+    // moving internally for all resizing needed (for a growth factor of 2).
+    expected_copy_constructed += elem_vector_count;
+    EXPECT_EQ(expected_copy_constructed, decompose_copy_constructed);
+    expected_move_constructed += elem_vector_count;
+    EXPECT_LT(expected_move_constructed, decompose_move_constructed);
+    expected_move_constructed += elem_vector_count;
+    EXPECT_GE(expected_move_constructed, decompose_move_constructed);
+    EXPECT_EQ(0, decompose_move_assigned);
+    EXPECT_EQ(0, decompose_copy_assigned);
+    expected_copy_constructed = decompose_copy_constructed;
+    expected_move_constructed = decompose_move_constructed;
+  }
+
+  {  // insert(first, last)
+    DecomposeSet set2;
+    set2.insert(elem_vector.begin(), elem_vector.end());
+    // Expect exactly N elements copied into set, expect at most 2*N elements
+    // moving internally for all resizing needed (for a growth factor of 2).
+    const int expected_new_elements = elem_vector_count;
+    const int expected_max_element_moves = 2 * elem_vector_count;
+    expected_copy_constructed += expected_new_elements;
+    EXPECT_EQ(expected_copy_constructed, decompose_copy_constructed);
+    expected_move_constructed += expected_max_element_moves;
+    EXPECT_GE(expected_move_constructed, decompose_move_constructed);
+    EXPECT_EQ(0, decompose_move_assigned);
+    EXPECT_EQ(0, decompose_copy_assigned);
+    expected_copy_constructed = decompose_copy_constructed;
+    expected_move_constructed = decompose_move_constructed;
+  }
+}
+
+TEST(Table, Decompose) {
+  if (SwisstableGenerationsEnabled()) {
+    GTEST_SKIP() << "Generations being enabled causes extra rehashes.";
+  }
+
+  TestDecompose<DecomposeHash, DecomposeEq>(false);
+
+  struct TransparentHashIntOverload {
+    size_t operator()(const DecomposeType& a) const { return a.i; }
+    size_t operator()(int a) const { return a; }
+  };
+  struct TransparentEqIntOverload {
+    bool operator()(const DecomposeType& a, const DecomposeType& b) const {
+      return a.i == b.i;
+    }
+    bool operator()(const DecomposeType& a, int b) const { return a.i == b; }
+  };
+  TestDecompose<TransparentHashIntOverload, DecomposeEq>(true);
+  TestDecompose<TransparentHashIntOverload, TransparentEqIntOverload>(true);
+  TestDecompose<DecomposeHash, TransparentEqIntOverload>(true);
+}
+
+struct Modulo1000Hash {
+  size_t operator()(int64_t x) const { return static_cast<size_t>(x) % 1000; }
+};
+
+struct Modulo1000HashTable
+    : public raw_hash_set<IntPolicy, Modulo1000Hash, std::equal_to<int64_t>,
+                          std::allocator<int>> {};
+
+// Test that rehash with no resize happen in case of many deleted slots.
+TEST(Table, RehashWithNoResize) {
+  if (SwisstableGenerationsEnabled()) {
+    GTEST_SKIP() << "Generations being enabled causes extra rehashes.";
+  }
+
+  Modulo1000HashTable t;
+  // Adding the same length (and the same hash) strings
+  // to have at least kMinFullGroups groups
+  // with Group::kWidth collisions. Then fill up to MaxDensitySize;
+  const size_t kMinFullGroups = 7;
+  std::vector<int> keys;
+  for (size_t i = 0; i < MaxDensitySize(Group::kWidth * kMinFullGroups); ++i) {
+    int k = i * 1000;
+    t.emplace(k);
+    keys.push_back(k);
+  }
+  const size_t capacity = t.capacity();
+
+  // Remove elements from all groups except the first and the last one.
+  // All elements removed from full groups will be marked as ctrl_t::kDeleted.
+  const size_t erase_begin = Group::kWidth / 2;
+  const size_t erase_end = (t.size() / Group::kWidth - 1) * Group::kWidth;
+  for (size_t i = erase_begin; i < erase_end; ++i) {
+    EXPECT_EQ(1, t.erase(keys[i])) << i;
+  }
+  keys.erase(keys.begin() + erase_begin, keys.begin() + erase_end);
+
+  auto last_key = keys.back();
+  size_t last_key_num_probes = GetHashtableDebugNumProbes(t, last_key);
+
+  // Make sure that we have to make a lot of probes for last key.
+  ASSERT_GT(last_key_num_probes, kMinFullGroups);
+
+  int x = 1;
+  // Insert and erase one element, before inplace rehash happen.
+  while (last_key_num_probes == GetHashtableDebugNumProbes(t, last_key)) {
+    t.emplace(x);
+    ASSERT_EQ(capacity, t.capacity());
+    // All elements should be there.
+    ASSERT_TRUE(t.find(x) != t.end()) << x;
+    for (const auto& k : keys) {
+      ASSERT_TRUE(t.find(k) != t.end()) << k;
+    }
+    t.erase(x);
+    ++x;
+  }
+}
+
+// Collect N bad keys by following algorithm:
+// 1. Create an empty table and reserve it to 2 * N.
+// 2. Insert N random elements.
+// 3. Take first Group::kWidth - 1 to bad_keys array.
+// 4. Clear the table without resize.
+// 5. Go to point 2 while N keys not collected
+std::vector<int64_t> CollectBadMergeKeys(size_t N) {
+  static constexpr int kGroupSize = Group::kWidth - 1;
+
+  auto topk_range = [](size_t b, size_t e,
+                       IntTable* t) -> std::vector<int64_t> {
+    for (size_t i = b; i != e; ++i) {
+      t->emplace(i);
+    }
+    std::vector<int64_t> res;
+    res.reserve(kGroupSize);
+    auto it = t->begin();
+    for (size_t i = b; i != e && i != b + kGroupSize; ++i, ++it) {
+      res.push_back(*it);
+    }
+    return res;
+  };
+
+  std::vector<int64_t> bad_keys;
+  bad_keys.reserve(N);
+  IntTable t;
+  t.reserve(N * 2);
+
+  for (size_t b = 0; bad_keys.size() < N; b += N) {
+    auto keys = topk_range(b, b + N, &t);
+    bad_keys.insert(bad_keys.end(), keys.begin(), keys.end());
+    t.erase(t.begin(), t.end());
+    EXPECT_TRUE(t.empty());
+  }
+  return bad_keys;
+}
+
+struct ProbeStats {
+  // Number of elements with specific probe length over all tested tables.
+  std::vector<size_t> all_probes_histogram;
+  // Ratios total_probe_length/size for every tested table.
+  std::vector<double> single_table_ratios;
+
+  // Average ratio total_probe_length/size over tables.
+  double AvgRatio() const {
+    return std::accumulate(single_table_ratios.begin(),
+                           single_table_ratios.end(), 0.0) /
+           single_table_ratios.size();
+  }
+
+  // Maximum ratio total_probe_length/size over tables.
+  double MaxRatio() const {
+    return *std::max_element(single_table_ratios.begin(),
+                             single_table_ratios.end());
+  }
+
+  // Percentile ratio total_probe_length/size over tables.
+  double PercentileRatio(double Percentile = 0.95) const {
+    auto r = single_table_ratios;
+    auto mid = r.begin() + static_cast<size_t>(r.size() * Percentile);
+    if (mid != r.end()) {
+      std::nth_element(r.begin(), mid, r.end());
+      return *mid;
+    } else {
+      return MaxRatio();
+    }
+  }
+
+  // Maximum probe length over all elements and all tables.
+  size_t MaxProbe() const { return all_probes_histogram.size(); }
+
+  // Fraction of elements with specified probe length.
+  std::vector<double> ProbeNormalizedHistogram() const {
+    double total_elements = std::accumulate(all_probes_histogram.begin(),
+                                            all_probes_histogram.end(), 0ull);
+    std::vector<double> res;
+    for (size_t p : all_probes_histogram) {
+      res.push_back(p / total_elements);
+    }
+    return res;
+  }
+
+  size_t PercentileProbe(double Percentile = 0.99) const {
+    size_t idx = 0;
+    for (double p : ProbeNormalizedHistogram()) {
+      if (Percentile > p) {
+        Percentile -= p;
+        ++idx;
+      } else {
+        return idx;
+      }
+    }
+    return idx;
+  }
+
+  friend std::ostream& operator<<(std::ostream& out, const ProbeStats& s) {
+    out << "{AvgRatio:" << s.AvgRatio() << ", MaxRatio:" << s.MaxRatio()
+        << ", PercentileRatio:" << s.PercentileRatio()
+        << ", MaxProbe:" << s.MaxProbe() << ", Probes=[";
+    for (double p : s.ProbeNormalizedHistogram()) {
+      out << p << ",";
+    }
+    out << "]}";
+
+    return out;
+  }
+};
+
+struct ExpectedStats {
+  double avg_ratio;
+  double max_ratio;
+  std::vector<std::pair<double, double>> pecentile_ratios;
+  std::vector<std::pair<double, double>> pecentile_probes;
+
+  friend std::ostream& operator<<(std::ostream& out, const ExpectedStats& s) {
+    out << "{AvgRatio:" << s.avg_ratio << ", MaxRatio:" << s.max_ratio
+        << ", PercentileRatios: [";
+    for (auto el : s.pecentile_ratios) {
+      out << el.first << ":" << el.second << ", ";
+    }
+    out << "], PercentileProbes: [";
+    for (auto el : s.pecentile_probes) {
+      out << el.first << ":" << el.second << ", ";
+    }
+    out << "]}";
+
+    return out;
+  }
+};
+
+void VerifyStats(size_t size, const ExpectedStats& exp,
+                 const ProbeStats& stats) {
+  EXPECT_LT(stats.AvgRatio(), exp.avg_ratio) << size << " " << stats;
+  EXPECT_LT(stats.MaxRatio(), exp.max_ratio) << size << " " << stats;
+  for (auto pr : exp.pecentile_ratios) {
+    EXPECT_LE(stats.PercentileRatio(pr.first), pr.second)
+        << size << " " << pr.first << " " << stats;
+  }
+
+  for (auto pr : exp.pecentile_probes) {
+    EXPECT_LE(stats.PercentileProbe(pr.first), pr.second)
+        << size << " " << pr.first << " " << stats;
+  }
+}
+
+using ProbeStatsPerSize = std::map<size_t, ProbeStats>;
+
+// Collect total ProbeStats on num_iters iterations of the following algorithm:
+// 1. Create new table and reserve it to keys.size() * 2
+// 2. Insert all keys xored with seed
+// 3. Collect ProbeStats from final table.
+ProbeStats CollectProbeStatsOnKeysXoredWithSeed(
+    const std::vector<int64_t>& keys, size_t num_iters) {
+  const size_t reserve_size = keys.size() * 2;
+
+  ProbeStats stats;
+
+  int64_t seed = 0x71b1a19b907d6e33;
+  while (num_iters--) {
+    seed = static_cast<int64_t>(static_cast<uint64_t>(seed) * 17 + 13);
+    IntTable t1;
+    t1.reserve(reserve_size);
+    for (const auto& key : keys) {
+      t1.emplace(key ^ seed);
+    }
+
+    auto probe_histogram = GetHashtableDebugNumProbesHistogram(t1);
+    stats.all_probes_histogram.resize(
+        std::max(stats.all_probes_histogram.size(), probe_histogram.size()));
+    std::transform(probe_histogram.begin(), probe_histogram.end(),
+                   stats.all_probes_histogram.begin(),
+                   stats.all_probes_histogram.begin(), std::plus<size_t>());
+
+    size_t total_probe_seq_length = 0;
+    for (size_t i = 0; i < probe_histogram.size(); ++i) {
+      total_probe_seq_length += i * probe_histogram[i];
+    }
+    stats.single_table_ratios.push_back(total_probe_seq_length * 1.0 /
+                                        keys.size());
+    t1.erase(t1.begin(), t1.end());
+  }
+  return stats;
+}
+
+ExpectedStats XorSeedExpectedStats() {
+  constexpr bool kRandomizesInserts =
+#ifdef NDEBUG
+      false;
+#else   // NDEBUG
+      true;
+#endif  // NDEBUG
+
+  // The effective load factor is larger in non-opt mode because we insert
+  // elements out of order.
+  switch (container_internal::Group::kWidth) {
+    case 8:
+      if (kRandomizesInserts) {
+        return {0.05,
+                1.0,
+                {{0.95, 0.5}},
+                {{0.95, 0}, {0.99, 2}, {0.999, 4}, {0.9999, 10}}};
+      } else {
+        return {0.05,
+                2.0,
+                {{0.95, 0.1}},
+                {{0.95, 0}, {0.99, 2}, {0.999, 4}, {0.9999, 10}}};
+      }
+    case 16:
+      if (kRandomizesInserts) {
+        return {0.1,
+                2.0,
+                {{0.95, 0.1}},
+                {{0.95, 0}, {0.99, 1}, {0.999, 8}, {0.9999, 15}}};
+      } else {
+        return {0.05,
+                1.0,
+                {{0.95, 0.05}},
+                {{0.95, 0}, {0.99, 1}, {0.999, 4}, {0.9999, 10}}};
+      }
+  }
+  LOG(FATAL) << "Unknown Group width";
+  return {};
+}
+
+// TODO(b/80415403): Figure out why this test is so flaky, esp. on MSVC
+TEST(Table, DISABLED_EnsureNonQuadraticTopNXorSeedByProbeSeqLength) {
+  ProbeStatsPerSize stats;
+  std::vector<size_t> sizes = {Group::kWidth << 5, Group::kWidth << 10};
+  for (size_t size : sizes) {
+    stats[size] =
+        CollectProbeStatsOnKeysXoredWithSeed(CollectBadMergeKeys(size), 200);
+  }
+  auto expected = XorSeedExpectedStats();
+  for (size_t size : sizes) {
+    auto& stat = stats[size];
+    VerifyStats(size, expected, stat);
+    LOG(INFO) << size << " " << stat;
+  }
+}
+
+// Collect total ProbeStats on num_iters iterations of the following algorithm:
+// 1. Create new table
+// 2. Select 10% of keys and insert 10 elements key * 17 + j * 13
+// 3. Collect ProbeStats from final table
+ProbeStats CollectProbeStatsOnLinearlyTransformedKeys(
+    const std::vector<int64_t>& keys, size_t num_iters) {
+  ProbeStats stats;
+
+  absl::InsecureBitGen rng;
+  auto linear_transform = [](size_t x, size_t y) { return x * 17 + y * 13; };
+  std::uniform_int_distribution<size_t> dist(0, keys.size() - 1);
+  while (num_iters--) {
+    IntTable t1;
+    size_t num_keys = keys.size() / 10;
+    size_t start = dist(rng);
+    for (size_t i = 0; i != num_keys; ++i) {
+      for (size_t j = 0; j != 10; ++j) {
+        t1.emplace(linear_transform(keys[(i + start) % keys.size()], j));
+      }
+    }
+
+    auto probe_histogram = GetHashtableDebugNumProbesHistogram(t1);
+    stats.all_probes_histogram.resize(
+        std::max(stats.all_probes_histogram.size(), probe_histogram.size()));
+    std::transform(probe_histogram.begin(), probe_histogram.end(),
+                   stats.all_probes_histogram.begin(),
+                   stats.all_probes_histogram.begin(), std::plus<size_t>());
+
+    size_t total_probe_seq_length = 0;
+    for (size_t i = 0; i < probe_histogram.size(); ++i) {
+      total_probe_seq_length += i * probe_histogram[i];
+    }
+    stats.single_table_ratios.push_back(total_probe_seq_length * 1.0 /
+                                        t1.size());
+    t1.erase(t1.begin(), t1.end());
+  }
+  return stats;
+}
+
+ExpectedStats LinearTransformExpectedStats() {
+  constexpr bool kRandomizesInserts =
+#ifdef NDEBUG
+      false;
+#else   // NDEBUG
+      true;
+#endif  // NDEBUG
+
+  // The effective load factor is larger in non-opt mode because we insert
+  // elements out of order.
+  switch (container_internal::Group::kWidth) {
+    case 8:
+      if (kRandomizesInserts) {
+        return {0.1,
+                0.5,
+                {{0.95, 0.3}},
+                {{0.95, 0}, {0.99, 1}, {0.999, 8}, {0.9999, 15}}};
+      } else {
+        return {0.4,
+                0.6,
+                {{0.95, 0.5}},
+                {{0.95, 1}, {0.99, 14}, {0.999, 23}, {0.9999, 26}}};
+      }
+    case 16:
+      if (kRandomizesInserts) {
+        return {0.1,
+                0.4,
+                {{0.95, 0.3}},
+                {{0.95, 1}, {0.99, 2}, {0.999, 9}, {0.9999, 15}}};
+      } else {
+        return {0.05,
+                0.2,
+                {{0.95, 0.1}},
+                {{0.95, 0}, {0.99, 1}, {0.999, 6}, {0.9999, 10}}};
+      }
+  }
+  LOG(FATAL) << "Unknown Group width";
+  return {};
+}
+
+// TODO(b/80415403): Figure out why this test is so flaky.
+TEST(Table, DISABLED_EnsureNonQuadraticTopNLinearTransformByProbeSeqLength) {
+  ProbeStatsPerSize stats;
+  std::vector<size_t> sizes = {Group::kWidth << 5, Group::kWidth << 10};
+  for (size_t size : sizes) {
+    stats[size] = CollectProbeStatsOnLinearlyTransformedKeys(
+        CollectBadMergeKeys(size), 300);
+  }
+  auto expected = LinearTransformExpectedStats();
+  for (size_t size : sizes) {
+    auto& stat = stats[size];
+    VerifyStats(size, expected, stat);
+    LOG(INFO) << size << " " << stat;
+  }
+}
+
+TEST(Table, EraseCollision) {
+  BadTable t;
+
+  // 1 2 3
+  t.emplace(1);
+  t.emplace(2);
+  t.emplace(3);
+  EXPECT_THAT(*t.find(1), 1);
+  EXPECT_THAT(*t.find(2), 2);
+  EXPECT_THAT(*t.find(3), 3);
+  EXPECT_EQ(3, t.size());
+
+  // 1 DELETED 3
+  t.erase(t.find(2));
+  EXPECT_THAT(*t.find(1), 1);
+  EXPECT_TRUE(t.find(2) == t.end());
+  EXPECT_THAT(*t.find(3), 3);
+  EXPECT_EQ(2, t.size());
+
+  // DELETED DELETED 3
+  t.erase(t.find(1));
+  EXPECT_TRUE(t.find(1) == t.end());
+  EXPECT_TRUE(t.find(2) == t.end());
+  EXPECT_THAT(*t.find(3), 3);
   EXPECT_EQ(1, t.size());
-  IntTable u;
-  t.swap(u);
+
+  // DELETED DELETED DELETED
+  t.erase(t.find(3));
+  EXPECT_TRUE(t.find(1) == t.end());
+  EXPECT_TRUE(t.find(2) == t.end());
+  EXPECT_TRUE(t.find(3) == t.end());
   EXPECT_EQ(0, t.size());
+}
+
+TEST(Table, EraseInsertProbing) {
+  BadTable t(100);
+
+  // 1 2 3 4
+  t.emplace(1);
+  t.emplace(2);
+  t.emplace(3);
+  t.emplace(4);
+
+  // 1 DELETED 3 DELETED
+  t.erase(t.find(2));
+  t.erase(t.find(4));
+
+  // 1 10 3 11 12
+  t.emplace(10);
+  t.emplace(11);
+  t.emplace(12);
+
+  EXPECT_EQ(5, t.size());
+  EXPECT_THAT(t, UnorderedElementsAre(1, 10, 3, 11, 12));
+}
+
+TEST(Table, GrowthInfoDeletedBit) {
+  BadTable t;
+  int64_t init_count = static_cast<int64_t>(
+      CapacityToGrowth(NormalizeCapacity(Group::kWidth + 1)));
+  for (int64_t i = 0; i < init_count; ++i) {
+    t.insert(i);
+  }
+  EXPECT_TRUE(RawHashSetTestOnlyAccess::GetCommon(t)
+                  .growth_info()
+                  .GetGrowthInfoLowerBound()
+                  .HasNoDeleted());
+  t.erase(0);
+  EXPECT_EQ(RawHashSetTestOnlyAccess::CountTombstones(t), 1);
+  EXPECT_FALSE(RawHashSetTestOnlyAccess::GetCommon(t)
+                   .growth_info()
+                   .GetGrowthInfoLowerBound()
+                   .HasNoDeleted());
+  t.rehash(0);
+  EXPECT_EQ(RawHashSetTestOnlyAccess::CountTombstones(t), 0);
+  EXPECT_TRUE(RawHashSetTestOnlyAccess::GetCommon(t)
+                  .growth_info()
+                  .GetGrowthInfoLowerBound()
+                  .HasNoDeleted());
+}
+
+// Following two tests use non-SOO table because they test for 0 capacity.
+TEST(Table, RehashZeroDoesNotAllocateOnEmptyTable) {
+  NonSooIntTable t;
+  t.rehash(0);
+  EXPECT_EQ(0, t.bucket_count());
+}
+
+TEST(Table, RehashZeroDeallocatesEmptyTable) {
+  NonSooIntTable t;
+  t.emplace(0);
+  t.clear();
+  EXPECT_NE(0, t.bucket_count());
+  t.rehash(0);
+  EXPECT_EQ(0, t.bucket_count());
+}
+
+TEST(Table, ConstructFromInitList) {
+  using P = std::pair<std::string, std::string>;
+  struct Q {
+    operator P() const { return {}; }  // NOLINT
+  };
+  StringTable t = {P(), Q(), {}, {{}, {}}};
+}
+
+TEST(Table, CopyConstructWithAlloc) {
+  StringTable t;
+  t.emplace("a", "b");
+  EXPECT_EQ(1, t.size());
+  StringTable u(t, Alloc<std::pair<std::string, std::string>>());
   EXPECT_EQ(1, u.size());
-  EXPECT_TRUE(t.find(0) == t.end());
-  EXPECT_THAT(*u.find(0), 0);
+  EXPECT_THAT(*u.find("a"), Pair("a", "b"));
+}
+
+struct ExplicitAllocIntTable
+    : raw_hash_set<IntPolicy, hash_default_hash<int64_t>,
+                   std::equal_to<int64_t>, Alloc<int64_t>> {
+  ExplicitAllocIntTable() = default;
+};
+
+TEST(Table, AllocWithExplicitCtor) {
+  ExplicitAllocIntTable t;
+  EXPECT_EQ(0, t.size());
+}
+
+TEST(Table, MoveConstruct) {
+  {
+    StringTable t;
+    t.emplace("a", "b");
+    EXPECT_EQ(1, t.size());
+
+    StringTable u(std::move(t));
+    EXPECT_EQ(1, u.size());
+    EXPECT_THAT(*u.find("a"), Pair("a", "b"));
+  }
+  {
+    StringTable t;
+    t.emplace("a", "b");
+    EXPECT_EQ(1, t.size());
+
+    StringTable u{std::move(t)};
+    EXPECT_EQ(1, u.size());
+    EXPECT_THAT(*u.find("a"), Pair("a", "b"));
+  }
+  {
+    StringTable t;
+    t.emplace("a", "b");
+    EXPECT_EQ(1, t.size());
+
+    StringTable u = std::move(t);
+    EXPECT_EQ(1, u.size());
+    EXPECT_THAT(*u.find("a"), Pair("a", "b"));
+  }
+}
+
+TEST(Table, MoveConstructWithAlloc) {
+  StringTable t;
+  t.emplace("a", "b");
+  EXPECT_EQ(1, t.size());
+  StringTable u(std::move(t), Alloc<std::pair<std::string, std::string>>());
+  EXPECT_EQ(1, u.size());
+  EXPECT_THAT(*u.find("a"), Pair("a", "b"));
+}
+
+TEST(Table, CopyAssign) {
+  StringTable t;
+  t.emplace("a", "b");
+  EXPECT_EQ(1, t.size());
+  StringTable u;
+  u = t;
+  EXPECT_EQ(1, u.size());
+  EXPECT_THAT(*u.find("a"), Pair("a", "b"));
+}
+
+TEST(Table, CopySelfAssign) {
+  StringTable t;
+  t.emplace("a", "b");
+  EXPECT_EQ(1, t.size());
+  t = *&t;
+  EXPECT_EQ(1, t.size());
+  EXPECT_THAT(*t.find("a"), Pair("a", "b"));
+}
+
+TEST(Table, MoveAssign) {
+  StringTable t;
+  t.emplace("a", "b");
+  EXPECT_EQ(1, t.size());
+  StringTable u;
+  u = std::move(t);
+  EXPECT_EQ(1, u.size());
+  EXPECT_THAT(*u.find("a"), Pair("a", "b"));
+}
+
+TEST(Table, MoveSelfAssign) {
+  StringTable t;
+  t.emplace("a", "b");
+  EXPECT_EQ(1, t.size());
+  t = std::move(*&t);
+  if (SwisstableGenerationsEnabled()) {
+    // NOLINTNEXTLINE(bugprone-use-after-move)
+    EXPECT_DEATH_IF_SUPPORTED(t.contains("a"), "self-move-assigned");
+  }
+  // As long as we don't crash, it's fine.
+}
+
+TEST(Table, Equality) {
+  StringTable t;
+  std::vector<std::pair<std::string, std::string>> v = {{"a", "b"},
+                                                        {"aa", "bb"}};
+  t.insert(std::begin(v), std::end(v));
+  StringTable u = t;
+  EXPECT_EQ(u, t);
+}
+
+TEST(Table, Equality2) {
+  StringTable t;
+  std::vector<std::pair<std::string, std::string>> v1 = {{"a", "b"},
+                                                         {"aa", "bb"}};
+  t.insert(std::begin(v1), std::end(v1));
+  StringTable u;
+  std::vector<std::pair<std::string, std::string>> v2 = {{"a", "a"},
+                                                         {"aa", "aa"}};
+  u.insert(std::begin(v2), std::end(v2));
+  EXPECT_NE(u, t);
+}
+
+TEST(Table, Equality3) {
+  StringTable t;
+  std::vector<std::pair<std::string, std::string>> v1 = {{"b", "b"},
+                                                         {"bb", "bb"}};
+  t.insert(std::begin(v1), std::end(v1));
+  StringTable u;
+  std::vector<std::pair<std::string, std::string>> v2 = {{"a", "a"},
+                                                         {"aa", "aa"}};
+  u.insert(std::begin(v2), std::end(v2));
+  EXPECT_NE(u, t);
+}
+TEST(Table, NoThrowMoveConstruct) {
+  ASSERT_TRUE(
+      std::is_nothrow_copy_constructible_v<absl::Hash<absl::string_view>>);
+  ASSERT_TRUE(
+      std::is_nothrow_copy_constructible_v<std::equal_to<absl::string_view>>);
+  ASSERT_TRUE(std::is_nothrow_copy_constructible_v<std::allocator<int>>);
+  EXPECT_TRUE(std::is_nothrow_move_constructible_v<StringTable>);
+}
+
+TEST(Table, NoThrowMoveAssign) {
+  ASSERT_TRUE(std::is_nothrow_move_assignable_v<absl::Hash<absl::string_view>>);
+  ASSERT_TRUE(
+      std::is_nothrow_move_assignable_v<std::equal_to<absl::string_view>>);
+  ASSERT_TRUE(std::is_nothrow_move_assignable_v<std::allocator<int>>);
+  ASSERT_TRUE(
+      std::allocator_traits<std::allocator<int>>::is_always_equal::value);
+  EXPECT_TRUE(std::is_nothrow_move_assignable_v<StringTable>);
+}
+
+TEST(Table, NoThrowSwappable) {
+  ASSERT_TRUE(std::is_nothrow_swappable<absl::Hash<absl::string_view>>());
+  ASSERT_TRUE(std::is_nothrow_swappable<std::equal_to<absl::string_view>>());
+  ASSERT_TRUE(std::is_nothrow_swappable<std::allocator<int>>());
+  EXPECT_TRUE(std::is_nothrow_swappable<StringTable>());
+}
+
+TEST(Table, HeterogeneousLookup) {
+  struct Hash {
+    size_t operator()(int64_t i) const { return i; }
+    size_t operator()(double i) const {
+      ADD_FAILURE();
+      return i;
+    }
+  };
+  struct Eq {
+    bool operator()(int64_t a, int64_t b) const { return a == b; }
+    bool operator()(double a, int64_t b) const {
+      ADD_FAILURE();
+      return a == b;
+    }
+    bool operator()(int64_t a, double b) const {
+      ADD_FAILURE();
+      return a == b;
+    }
+    bool operator()(double a, double b) const {
+      ADD_FAILURE();
+      return a == b;
+    }
+  };
+
+  struct THash {
+    using is_transparent = void;
+    size_t operator()(int64_t i) const { return i; }
+    size_t operator()(double i) const { return i; }
+  };
+  struct TEq {
+    using is_transparent = void;
+    bool operator()(int64_t a, int64_t b) const { return a == b; }
+    bool operator()(double a, int64_t b) const { return a == b; }
+    bool operator()(int64_t a, double b) const { return a == b; }
+    bool operator()(double a, double b) const { return a == b; }
+  };
+
+  raw_hash_set<IntPolicy, Hash, Eq, Alloc<int64_t>> s{0, 1, 2};
+  // It will convert to int64_t before the query.
+  EXPECT_EQ(1, *s.find(double{1.1}));
+
+  raw_hash_set<IntPolicy, THash, TEq, Alloc<int64_t>> ts{0, 1, 2};
+  // It will try to use the double, and fail to find the object.
+  EXPECT_TRUE(ts.find(1.1) == ts.end());
 }
 
-TEST(Table, Rehash) {
-  IntTable t;
-  EXPECT_TRUE(t.find(0) == t.end());
-  t.emplace(0);
-  t.emplace(1);
-  EXPECT_EQ(2, t.size());
-  t.rehash(128);
-  EXPECT_EQ(2, t.size());
-  EXPECT_THAT(*t.find(0), 0);
-  EXPECT_THAT(*t.find(1), 1);
+template <class Table>
+using CallFind = decltype(std::declval<Table&>().find(17));
+
+template <class Table>
+using CallErase = decltype(std::declval<Table&>().erase(17));
+
+template <class Table>
+using CallExtract = decltype(std::declval<Table&>().extract(17));
+
+template <class Table>
+using CallPrefetch = decltype(std::declval<Table&>().prefetch(17));
+
+template <class Table>
+using CallCount = decltype(std::declval<Table&>().count(17));
+
+template <template <typename> class C, class Table, class = void>
+struct VerifyResultOf : std::false_type {};
+
+template <template <typename> class C, class Table>
+struct VerifyResultOf<C, Table, std::void_t<C<Table>>> : std::true_type {};
+
+TEST(Table, HeterogeneousLookupOverloads) {
+  using NonTransparentTable =
+      raw_hash_set<StringPolicy, absl::Hash<absl::string_view>,
+                   std::equal_to<absl::string_view>, std::allocator<int>>;
+
+  EXPECT_FALSE((VerifyResultOf<CallFind, NonTransparentTable>()));
+  EXPECT_FALSE((VerifyResultOf<CallErase, NonTransparentTable>()));
+  EXPECT_FALSE((VerifyResultOf<CallExtract, NonTransparentTable>()));
+  EXPECT_FALSE((VerifyResultOf<CallPrefetch, NonTransparentTable>()));
+  EXPECT_FALSE((VerifyResultOf<CallCount, NonTransparentTable>()));
+
+  using TransparentTable =
+      raw_hash_set<StringPolicy, hash_default_hash<absl::string_view>,
+                   hash_default_eq<absl::string_view>, std::allocator<int>>;
+
+  EXPECT_TRUE((VerifyResultOf<CallFind, TransparentTable>()));
+  EXPECT_TRUE((VerifyResultOf<CallErase, TransparentTable>()));
+  EXPECT_TRUE((VerifyResultOf<CallExtract, TransparentTable>()));
+  EXPECT_TRUE((VerifyResultOf<CallPrefetch, TransparentTable>()));
+  EXPECT_TRUE((VerifyResultOf<CallCount, TransparentTable>()));
 }
 
-TEST(Table, RehashDoesNotRehashWhenNotNecessary) {
-  IntTable t;
-  t.emplace(0);
-  t.emplace(1);
-  auto* p = &*t.find(0);
-  t.rehash(1);
-  EXPECT_EQ(p, &*t.find(0));
+TEST(Iterator, IsDefaultConstructible) {
+  StringTable::iterator i;
+  EXPECT_TRUE(i == StringTable::iterator());
 }
 
-TEST(Table, RehashZeroDoesNotAllocateOnEmptyTable) {
-  IntTable t;
-  t.rehash(0);
-  EXPECT_EQ(0, t.bucket_count());
+TEST(ConstIterator, IsDefaultConstructible) {
+  StringTable::const_iterator i;
+  EXPECT_TRUE(i == StringTable::const_iterator());
 }
 
-TEST(Table, RehashZeroDeallocatesEmptyTable) {
-  IntTable t;
-  t.emplace(0);
-  t.clear();
-  EXPECT_NE(0, t.bucket_count());
-  t.rehash(0);
-  EXPECT_EQ(0, t.bucket_count());
+TEST(Iterator, ConvertsToConstIterator) {
+  StringTable::iterator i;
+  EXPECT_TRUE(i == StringTable::const_iterator());
 }
 
-TEST(Table, RehashZeroForcesRehash) {
+TEST(Iterator, Iterates) {
   IntTable t;
-  t.emplace(0);
-  t.emplace(1);
-  auto* p = &*t.find(0);
-  t.rehash(0);
-  EXPECT_NE(p, &*t.find(0));
+  for (size_t i = 3; i != 6; ++i) EXPECT_TRUE(t.emplace(i).second);
+  EXPECT_THAT(t, UnorderedElementsAre(3, 4, 5));
 }
 
-TEST(Table, ConstructFromInitList) {
-  using P = std::pair<std::string, std::string>;
-  struct Q {
-    operator P() const { return {}; }
+TEST(Table, Merge) {
+  StringTable t1, t2;
+  t1.emplace("0", "-0");
+  t1.emplace("1", "-1");
+  t2.emplace("0", "~0");
+  t2.emplace("2", "~2");
+
+  EXPECT_THAT(t1, UnorderedElementsAre(Pair("0", "-0"), Pair("1", "-1")));
+  EXPECT_THAT(t2, UnorderedElementsAre(Pair("0", "~0"), Pair("2", "~2")));
+
+  t1.merge(t2);
+  EXPECT_THAT(t1, UnorderedElementsAre(Pair("0", "-0"), Pair("1", "-1"),
+                                       Pair("2", "~2")));
+  EXPECT_THAT(t2, UnorderedElementsAre(Pair("0", "~0")));
+}
+
+TEST(Table, MergeSmall) {
+  StringTable t1, t2;
+  t1.emplace("1", "1");
+  t2.emplace("2", "2");
+
+  EXPECT_THAT(t1, UnorderedElementsAre(Pair("1", "1")));
+  EXPECT_THAT(t2, UnorderedElementsAre(Pair("2", "2")));
+
+  t2.merge(t1);
+  EXPECT_EQ(t1.size(), 0);
+  EXPECT_THAT(t2, UnorderedElementsAre(Pair("1", "1"), Pair("2", "2")));
+}
+
+TEST(Table, IteratorEmplaceConstructibleRequirement) {
+  struct Value {
+    explicit Value(absl::string_view view) : value(view) {}
+    std::string value;
+
+    bool operator==(const Value& other) const { return value == other.value; }
   };
-  StringTable t = {P(), Q(), {}, {{}, {}}};
+  struct H {
+    size_t operator()(const Value& v) const {
+      return absl::Hash<std::string>{}(v.value);
+    }
+  };
+
+  struct Table
+      : InstantiateRawHashSet<ValuePolicy<Value>, H, std::equal_to<Value>,
+                              std::allocator<Value>>::type {
+    using Base = typename Table::raw_hash_set;
+    using Base::Base;
+  };
+
+  std::string input[3]{"A", "B", "C"};
+
+  Table t(std::begin(input), std::end(input));
+  EXPECT_THAT(t, UnorderedElementsAre(Value{"A"}, Value{"B"}, Value{"C"}));
+
+  input[0] = "D";
+  input[1] = "E";
+  input[2] = "F";
+  t.insert(std::begin(input), std::end(input));
+  EXPECT_THAT(t, UnorderedElementsAre(Value{"A"}, Value{"B"}, Value{"C"},
+                                      Value{"D"}, Value{"E"}, Value{"F"}));
 }
 
-TEST(Table, CopyConstruct) {
-  IntTable t;
-  t.emplace(0);
-  EXPECT_EQ(1, t.size());
-  {
-    IntTable u(t);
-    EXPECT_EQ(1, u.size());
-    EXPECT_THAT(*u.find(0), 0);
+TEST(Nodes, EmptyNodeType) {
+  using node_type = StringTable::node_type;
+  node_type n;
+  EXPECT_FALSE(n);
+  EXPECT_TRUE(n.empty());
+
+  EXPECT_TRUE(
+      (std::is_same_v<node_type::allocator_type, StringTable::allocator_type>));
+}
+
+TEST(Nodes, ExtractInsert) {
+  constexpr char k0[] = "Very long string zero.";
+  constexpr char k1[] = "Very long string one.";
+  constexpr char k2[] = "Very long string two.";
+  StringTable t = {{k0, ""}, {k1, ""}, {k2, ""}};
+  EXPECT_THAT(t,
+              UnorderedElementsAre(Pair(k0, ""), Pair(k1, ""), Pair(k2, "")));
+
+  auto node = t.extract(k0);
+  EXPECT_THAT(t, UnorderedElementsAre(Pair(k1, ""), Pair(k2, "")));
+  EXPECT_TRUE(node);
+  EXPECT_FALSE(node.empty());
+
+  StringTable t2;
+  StringTable::insert_return_type res = t2.insert(std::move(node));
+  EXPECT_TRUE(res.inserted);
+  EXPECT_THAT(*res.position, Pair(k0, ""));
+  EXPECT_FALSE(res.node);
+  EXPECT_THAT(t2, UnorderedElementsAre(Pair(k0, "")));
+
+  // Not there.
+  EXPECT_THAT(t, UnorderedElementsAre(Pair(k1, ""), Pair(k2, "")));
+  node = t.extract("Not there!");
+  EXPECT_THAT(t, UnorderedElementsAre(Pair(k1, ""), Pair(k2, "")));
+  EXPECT_FALSE(node);
+
+  // Inserting nothing.
+  res = t2.insert(std::move(node));
+  EXPECT_FALSE(res.inserted);
+  EXPECT_EQ(res.position, t2.end());
+  EXPECT_FALSE(res.node);
+  EXPECT_THAT(t2, UnorderedElementsAre(Pair(k0, "")));
+
+  t.emplace(k0, "1");
+  node = t.extract(k0);
+
+  // Insert duplicate.
+  res = t2.insert(std::move(node));
+  EXPECT_FALSE(res.inserted);
+  EXPECT_THAT(*res.position, Pair(k0, ""));
+  EXPECT_TRUE(res.node);
+  EXPECT_FALSE(node);  // NOLINT(bugprone-use-after-move)
+}
+
+TEST(Nodes, ExtractInsertSmall) {
+  constexpr char k0[] = "Very long string zero.";
+  StringTable t = {{k0, ""}};
+  EXPECT_THAT(t, UnorderedElementsAre(Pair(k0, "")));
+
+  auto node = t.extract(k0);
+  EXPECT_EQ(t.size(), 0);
+  EXPECT_TRUE(node);
+  EXPECT_FALSE(node.empty());
+
+  StringTable t2;
+  StringTable::insert_return_type res = t2.insert(std::move(node));
+  EXPECT_TRUE(res.inserted);
+  EXPECT_THAT(*res.position, Pair(k0, ""));
+  EXPECT_FALSE(res.node);
+  EXPECT_THAT(t2, UnorderedElementsAre(Pair(k0, "")));
+}
+
+TEST(TableDeathTest, InvalidIteratorAsserts) {
+  if (!IsAssertEnabled() && !SwisstableGenerationsEnabled())
+    GTEST_SKIP() << "Assertions not enabled.";
+
+  NonSooIntTable t;
+  // Extra simple "regexp" as regexp support is highly varied across platforms.
+  EXPECT_DEATH_IF_SUPPORTED(++t.end(), "operator.* called on end.. iterator.");
+  typename NonSooIntTable::iterator iter;
+  EXPECT_DEATH_IF_SUPPORTED(
+      ++iter, "operator.* called on default-constructed iterator.");
+  t.insert(0);
+  t.insert(1);
+  iter = t.begin();
+  t.erase(iter);
+  const char* const kErasedDeathMessage =
+      SwisstableGenerationsEnabled()
+          ? "operator.* called on invalid iterator.*was likely erased"
+          : "operator.* called on invalid iterator.*might have been "
+            "erased.*config=asan";
+  EXPECT_DEATH_IF_SUPPORTED(++iter, kErasedDeathMessage);
+}
+
+TEST(TableDeathTest, InvalidIteratorAssertsSoo) {
+  if (!IsAssertEnabled() && !SwisstableGenerationsEnabled())
+    GTEST_SKIP() << "Assertions not enabled.";
+
+  SooIntTable t;
+  // Extra simple "regexp" as regexp support is highly varied across platforms.
+  EXPECT_DEATH_IF_SUPPORTED(t.erase(t.end()),
+                            "erase.* called on end.. iterator.");
+  typename SooIntTable::iterator iter;
+  EXPECT_DEATH_IF_SUPPORTED(
+      ++iter, "operator.* called on default-constructed iterator.");
+
+  // We can't detect the erased iterator case as invalid in SOO mode because
+  // the control is static constant.
+}
+
+#if defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
+template <typename T>
+class RawHashSamplerTest : public testing::Test {};
+
+using RawHashSamplerTestTypes = ::testing::Types<
+    // 32 bits to make sure that table is Soo for 32 bits platform as well.
+    // 64 bits table is not SOO due to alignment.
+    SooInt32Table, NonSooIntTable>;
+TYPED_TEST_SUITE(RawHashSamplerTest, RawHashSamplerTestTypes);
+
+TYPED_TEST(RawHashSamplerTest, Sample) {
+  constexpr bool soo_enabled = std::is_same_v<SooInt32Table, TypeParam>;
+  // Enable the feature even if the prod default is off.
+  SetSamplingRateTo1Percent();
+
+  ASSERT_EQ(TypeParam().capacity(), soo_enabled ? SooCapacity() : 0);
+
+  auto& sampler = GlobalHashtablezSampler();
+  size_t start_size = 0;
+
+  // Reserve these utility tables, so that if they sampled, they'll be
+  // preexisting.
+  absl::flat_hash_set<const HashtablezInfo*> preexisting_info(10);
+  absl::flat_hash_map<size_t, int> observed_checksums(10);
+  absl::flat_hash_map<ssize_t, int> reservations(10);
+
+  start_size += sampler.Iterate([&](const HashtablezInfo& info) {
+    preexisting_info.insert(&info);
+    ++start_size;
+  });
+
+  std::vector<TypeParam> tables;
+  for (int i = 0; i < 1000000; ++i) {
+    tables.emplace_back();
+
+    const bool do_reserve = (i % 10 > 5);
+    const bool do_rehash = !do_reserve && (i % 10 > 0);
+
+    if (do_reserve) {
+      // Don't reserve on all tables.
+      tables.back().reserve(10 * (i % 10));
+    }
+
+    tables.back().insert(1);
+    tables.back().insert(i % 5);
+
+    if (do_rehash) {
+      // Rehash some other tables.
+      tables.back().rehash(10 * (i % 10));
+    }
   }
-  {
-    IntTable u{t};
-    EXPECT_EQ(1, u.size());
-    EXPECT_THAT(*u.find(0), 0);
+  size_t end_size = 0;
+  end_size += sampler.Iterate([&](const HashtablezInfo& info) {
+    ++end_size;
+    if (preexisting_info.contains(&info)) return;
+    reservations[info.max_reserve.load(std::memory_order_relaxed)]++;
+    EXPECT_EQ(info.inline_element_size, sizeof(typename TypeParam::value_type));
+    EXPECT_EQ(info.key_size, sizeof(typename TypeParam::key_type));
+    EXPECT_EQ(info.value_size, sizeof(typename TypeParam::value_type));
+
+    if (soo_enabled) {
+      EXPECT_EQ(info.soo_capacity, SooCapacity());
+    } else {
+      EXPECT_EQ(info.soo_capacity, 0);
+    }
+  });
+
+  // Expect that we sampled at the requested sampling rate of ~1%.
+  EXPECT_NEAR((end_size - start_size) / static_cast<double>(tables.size()),
+              0.01, 0.005);
+
+  ASSERT_EQ(reservations.size(), 10);
+  for (const auto& [reservation, count] : reservations) {
+    EXPECT_GE(reservation, 0);
+    EXPECT_LT(reservation, 100);
+
+    EXPECT_NEAR((100 * count) / static_cast<double>(tables.size()), 0.1, 0.05)
+        << reservation;
   }
-  {
-    IntTable u = t;
-    EXPECT_EQ(1, u.size());
-    EXPECT_THAT(*u.find(0), 0);
+}
+
+template <typename IntTableType>
+std::vector<const HashtablezInfo*> SampleTableMutation(
+    absl::FunctionRef<void(IntTableType&)> mutate_table) {
+  // Enable the feature even if the prod default is off.
+  SetSamplingRateTo1Percent();
+
+  auto& sampler = GlobalHashtablezSampler();
+  int64_t start_size = 0;
+  // Reserve the table, so that if it sampled, it'll be preexisting.
+  absl::flat_hash_set<const HashtablezInfo*> preexisting_info(10);
+  start_size += sampler.Iterate([&](const HashtablezInfo& info) {
+    preexisting_info.insert(&info);
+    ++start_size;
+  });
+
+  std::vector<IntTableType> tables;
+  for (int i = 0; i < 1000000; ++i) {
+    tables.emplace_back();
+    mutate_table(tables.back());
   }
-}
+  int64_t end_size = 0;
+  std::vector<const HashtablezInfo*> infos;
+  end_size += sampler.Iterate([&](const HashtablezInfo& info) {
+    ++end_size;
+    if (preexisting_info.contains(&info)) return;
+    infos.push_back(&info);
+  });
 
-TEST(Table, CopyConstructWithAlloc) {
-  StringTable t;
-  t.emplace("a", "b");
-  EXPECT_EQ(1, t.size());
-  StringTable u(t, Alloc<std::pair<std::string, std::string>>());
-  EXPECT_EQ(1, u.size());
-  EXPECT_THAT(*u.find("a"), Pair("a", "b"));
+  // Expect that we sampled at the requested sampling rate of ~1%.
+  EXPECT_NEAR((end_size - start_size) / static_cast<double>(tables.size()),
+              0.01, 0.005);
+  return infos;
 }
 
-struct ExplicitAllocIntTable
-    : raw_hash_set<IntPolicy, container_internal::hash_default_hash<int64_t>,
-                   std::equal_to<int64_t>, Alloc<int64_t>> {
-  ExplicitAllocIntTable() {}
-};
-
-TEST(Table, AllocWithExplicitCtor) {
-  ExplicitAllocIntTable t;
-  EXPECT_EQ(0, t.size());
+std::vector<const HashtablezInfo*> SampleSooMutation(
+    absl::FunctionRef<void(SooInt32Table&)> mutate_table) {
+  return SampleTableMutation<SooInt32Table>(mutate_table);
 }
 
-TEST(Table, MoveConstruct) {
-  {
-    StringTable t;
-    t.emplace("a", "b");
-    EXPECT_EQ(1, t.size());
+std::vector<const HashtablezInfo*> SampleNonSooMutation(
+    absl::FunctionRef<void(NonSooIntTable&)> mutate_table) {
+  return SampleTableMutation<NonSooIntTable>(mutate_table);
+}
 
-    StringTable u(std::move(t));
-    EXPECT_EQ(1, u.size());
-    EXPECT_THAT(*u.find("a"), Pair("a", "b"));
+TEST(RawHashSamplerTest, SooTableInsertToEmpty) {
+  if (SooInt32Table().capacity() != SooCapacity()) {
+    CHECK_LT(sizeof(void*), 8) << "missing SOO coverage";
+    GTEST_SKIP() << "not SOO on this platform";
   }
-  {
-    StringTable t;
-    t.emplace("a", "b");
-    EXPECT_EQ(1, t.size());
-
-    StringTable u{std::move(t)};
-    EXPECT_EQ(1, u.size());
-    EXPECT_THAT(*u.find("a"), Pair("a", "b"));
+  std::vector<const HashtablezInfo*> infos =
+      SampleSooMutation([](SooInt32Table& t) { t.insert(1); });
+
+  for (const HashtablezInfo* info : infos) {
+    ASSERT_EQ(info->inline_element_size,
+              sizeof(typename SooInt32Table::value_type));
+    ASSERT_EQ(info->soo_capacity, SooCapacity());
+    ASSERT_EQ(info->capacity, NextCapacity(SooCapacity()));
+    ASSERT_EQ(info->size, 1);
+    ASSERT_EQ(info->max_reserve, 0);
+    ASSERT_EQ(info->num_erases, 0);
+    ASSERT_EQ(info->max_probe_length, 0);
+    ASSERT_EQ(info->total_probe_length, 0);
   }
-  {
-    StringTable t;
-    t.emplace("a", "b");
-    EXPECT_EQ(1, t.size());
+}
 
-    StringTable u = std::move(t);
-    EXPECT_EQ(1, u.size());
-    EXPECT_THAT(*u.find("a"), Pair("a", "b"));
+// Verifies that repeated insertions and erasures on an SOO table do not cause
+// the sampling decision to be evaluated multiple times, preventing
+// oversampling.
+TEST(RawHashSamplerTest, SooTableRepeatedInsertEraseDoesNotOversample) {
+  if (SooInt32Table().capacity() != SooCapacity()) {
+    CHECK_LT(sizeof(void*), 8) << "missing SOO coverage";
+    GTEST_SKIP() << "not SOO on this platform";
   }
+  std::vector<const HashtablezInfo*> infos =
+      SampleSooMutation([](SooInt32Table& t) {
+        for (int i = 0; i < 10; ++i) {
+          t.insert(1);
+          t.erase(1);
+        }
+      });
+
+  // SampleSooMutation checks EXPECT_NEAR(sampled/total, 1%, 0.5%).
+  // If the sampling logic is incorrectly evaluated on every 0->1 element
+  // transition, the chance of being sampled per table approaches 10% (1 -
+  // 0.99^10), which is enough to cause this test to fail. By passing, this test
+  // verifies that the sampling decision is evaluated exactly once per SOO table
+  // instance.
 }
 
-TEST(Table, MoveConstructWithAlloc) {
-  StringTable t;
-  t.emplace("a", "b");
-  EXPECT_EQ(1, t.size());
-  StringTable u(std::move(t), Alloc<std::pair<std::string, std::string>>());
-  EXPECT_EQ(1, u.size());
-  EXPECT_THAT(*u.find("a"), Pair("a", "b"));
+TEST(RawHashSamplerTest, NonSooTableRepeatedInsertEraseCountSizeRight) {
+  ASSERT_EQ(NonSooIntTable().capacity(), 0);
+  std::vector<const HashtablezInfo*> infos =
+      SampleNonSooMutation([](NonSooIntTable& t) {
+        for (int i = 0; i < 10; ++i) {
+          t.insert(1);
+          t.erase(1);
+        }
+      });
+  for (const HashtablezInfo* info : infos) {
+    EXPECT_EQ(info->soo_capacity, 0);
+    ASSERT_EQ(info->capacity, 1);
+    ASSERT_EQ(info->size, 0);
+  }
 }
 
-TEST(Table, CopyAssign) {
-  StringTable t;
-  t.emplace("a", "b");
-  EXPECT_EQ(1, t.size());
-  StringTable u;
-  u = t;
-  EXPECT_EQ(1, u.size());
-  EXPECT_THAT(*u.find("a"), Pair("a", "b"));
+TEST(RawHashSamplerTest, NonSooTableRepeatedInsertClearCountSizeRight) {
+  ASSERT_EQ(NonSooIntTable().capacity(), 0);
+  std::vector<const HashtablezInfo*> infos =
+      SampleNonSooMutation([](NonSooIntTable& t) {
+        for (int i = 0; i < 10; ++i) {
+          t.insert(1);
+          t.clear();
+        }
+      });
+  for (const HashtablezInfo* info : infos) {
+    EXPECT_EQ(info->soo_capacity, 0);
+    ASSERT_EQ(info->capacity, 1);
+    ASSERT_EQ(info->size, 0);
+  }
 }
 
-TEST(Table, CopySelfAssign) {
-  StringTable t;
-  t.emplace("a", "b");
-  EXPECT_EQ(1, t.size());
-  t = *&t;
-  EXPECT_EQ(1, t.size());
-  EXPECT_THAT(*t.find("a"), Pair("a", "b"));
+// Verifies that copy-constructing or copy-assigning an SOO table does not
+// incorrectly trigger new sampling evaluations.
+TEST(RawHashSamplerTest, SooTableCopyDoesNotOversample) {
+  if (SooInt32Table().capacity() != SooCapacity()) {
+    CHECK_LT(sizeof(void*), 8) << "missing SOO coverage";
+    GTEST_SKIP() << "not SOO on this platform";
+  }
+  std::vector<const HashtablezInfo*> infos =
+      SampleSooMutation([](SooInt32Table& t) {
+        t.insert(1);
+        t.erase(1);
+        SooInt32Table t_copy(t);
+        for (int i = 0; i < 10; ++i) {
+          t_copy.insert(1);
+          t_copy.erase(1);
+        }
+        t = std::move(t_copy);
+      });
 }
 
-TEST(Table, MoveAssign) {
-  StringTable t;
-  t.emplace("a", "b");
-  EXPECT_EQ(1, t.size());
-  StringTable u;
-  u = std::move(t);
-  EXPECT_EQ(1, u.size());
-  EXPECT_THAT(*u.find("a"), Pair("a", "b"));
+// Verifies that move-constructing or move-assigning an SOO table correctly
+// transfers the sampling state and does not trigger oversampling.
+TEST(RawHashSamplerTest, SooTableMoveDoesNotOversample) {
+  if (SooInt32Table().capacity() != SooCapacity()) {
+    CHECK_LT(sizeof(void*), 8) << "missing SOO coverage";
+    GTEST_SKIP() << "not SOO on this platform";
+  }
+  std::vector<const HashtablezInfo*> infos =
+      SampleSooMutation([](SooInt32Table& t) {
+        t.insert(1);
+        t.erase(1);
+        SooInt32Table t_moved(std::move(t));
+        for (int i = 0; i < 10; ++i) {
+          t_moved.insert(1);
+          t_moved.erase(1);
+        }
+        t = std::move(t_moved);
+      });
 }
 
-TEST(Table, Equality) {
-  StringTable t;
-  std::vector<std::pair<std::string, std::string>> v = {{"a", "b"},
-                                                        {"aa", "bb"}};
-  t.insert(std::begin(v), std::end(v));
-  StringTable u = t;
-  EXPECT_EQ(u, t);
+TEST(RawHashSamplerTest, SooTableReserveToEmpty) {
+  if (SooInt32Table().capacity() != SooCapacity()) {
+    CHECK_LT(sizeof(void*), 8) << "missing SOO coverage";
+    GTEST_SKIP() << "not SOO on this platform";
+  }
+  std::vector<const HashtablezInfo*> infos =
+      SampleSooMutation([](SooInt32Table& t) { t.reserve(100); });
+
+  for (const HashtablezInfo* info : infos) {
+    ASSERT_EQ(info->inline_element_size,
+              sizeof(typename SooInt32Table::value_type));
+    ASSERT_EQ(info->soo_capacity, SooCapacity());
+    ASSERT_GE(info->capacity, 100);
+    ASSERT_EQ(info->size, 0);
+    ASSERT_EQ(info->max_reserve, 100);
+    ASSERT_EQ(info->num_erases, 0);
+    ASSERT_EQ(info->max_probe_length, 0);
+    ASSERT_EQ(info->total_probe_length, 0);
+  }
 }
 
-TEST(Table, Equality2) {
-  StringTable t;
-  std::vector<std::pair<std::string, std::string>> v1 = {{"a", "b"},
-                                                         {"aa", "bb"}};
-  t.insert(std::begin(v1), std::end(v1));
-  StringTable u;
-  std::vector<std::pair<std::string, std::string>> v2 = {{"a", "a"},
-                                                         {"aa", "aa"}};
-  u.insert(std::begin(v2), std::end(v2));
-  EXPECT_NE(u, t);
+// This tests that reserve on a full SOO table doesn't incorrectly result in new
+// (over-)sampling.
+TEST(RawHashSamplerTest, SooTableReserveToFullSoo) {
+  if (SooInt32Table().capacity() != SooCapacity()) {
+    CHECK_LT(sizeof(void*), 8) << "missing SOO coverage";
+    GTEST_SKIP() << "not SOO on this platform";
+  }
+  std::vector<const HashtablezInfo*> infos =
+      SampleSooMutation([](SooInt32Table& t) {
+        t.insert(1);
+        t.reserve(100);
+      });
+
+  for (const HashtablezInfo* info : infos) {
+    ASSERT_EQ(info->inline_element_size,
+              sizeof(typename SooInt32Table::value_type));
+    ASSERT_EQ(info->soo_capacity, SooCapacity());
+    ASSERT_GE(info->capacity, 100);
+    ASSERT_EQ(info->size, 1);
+    ASSERT_EQ(info->max_reserve, 100);
+    ASSERT_EQ(info->num_erases, 0);
+    ASSERT_EQ(info->max_probe_length, 0);
+    ASSERT_EQ(info->total_probe_length, 0);
+  }
 }
 
-TEST(Table, Equality3) {
-  StringTable t;
-  std::vector<std::pair<std::string, std::string>> v1 = {{"b", "b"},
-                                                         {"bb", "bb"}};
-  t.insert(std::begin(v1), std::end(v1));
-  StringTable u;
-  std::vector<std::pair<std::string, std::string>> v2 = {{"a", "a"},
-                                                         {"aa", "aa"}};
-  u.insert(std::begin(v2), std::end(v2));
-  EXPECT_NE(u, t);
-}
+TEST(RawHashSamplerTest, SooTableSampleOnCopy) {
+  if (SooInt32Table().capacity() != SooCapacity()) {
+    CHECK_LT(sizeof(void*), 8) << "missing SOO coverage";
+    GTEST_SKIP() << "not SOO on this platform";
+  }
 
-TEST(Table, NumDeletedRegression) {
-  IntTable t;
-  t.emplace(0);
-  t.erase(t.find(0));
-  // construct over a deleted slot.
-  t.emplace(0);
-  t.clear();
-}
+  SooInt32Table t_orig;
+  t_orig.insert(1);
 
-TEST(Table, FindFullDeletedRegression) {
-  IntTable t;
-  for (int i = 0; i < 1000; ++i) {
-    t.emplace(i);
-    t.erase(t.find(i));
+  std::vector<const HashtablezInfo*> infos =
+      SampleSooMutation([&t_orig](SooInt32Table& t) { t = t_orig; });
+
+  for (const HashtablezInfo* info : infos) {
+    ASSERT_EQ(info->inline_element_size,
+              sizeof(typename SooInt32Table::value_type));
+    ASSERT_EQ(info->soo_capacity, SooCapacity());
+    ASSERT_EQ(info->capacity, NextCapacity(SooCapacity()));
+    ASSERT_EQ(info->size, 1);
   }
-  EXPECT_EQ(0, t.size());
 }
 
-TEST(Table, ReplacingDeletedSlotDoesNotRehash) {
-  size_t n;
-  {
-    // Compute n such that n is the maximum number of elements before rehash.
-    IntTable t;
-    t.emplace(0);
-    size_t c = t.bucket_count();
-    for (n = 1; c == t.bucket_count(); ++n) t.emplace(n);
-    --n;
+TEST(RawHashSamplerTest, NonSooTableSampleOnCopy) {
+  NonSooIntTable t_orig;
+  t_orig.insert(1);
+
+  std::vector<const HashtablezInfo*> infos =
+      SampleNonSooMutation([&t_orig](NonSooIntTable& t) { t = t_orig; });
+
+  for (const HashtablezInfo* info : infos) {
+    ASSERT_EQ(info->inline_element_size,
+              sizeof(typename NonSooIntTable::value_type));
+    ASSERT_EQ(info->soo_capacity, 0);
+    ASSERT_EQ(info->capacity, 1);
+    ASSERT_EQ(info->size, 1);
   }
-  IntTable t;
-  t.rehash(n);
-  const size_t c = t.bucket_count();
-  for (size_t i = 0; i != n; ++i) t.emplace(i);
-  EXPECT_EQ(c, t.bucket_count()) << "rehashing threshold = " << n;
-  t.erase(0);
-  t.emplace(0);
-  EXPECT_EQ(c, t.bucket_count()) << "rehashing threshold = " << n;
 }
 
-TEST(Table, NoThrowMoveConstruct) {
-  ASSERT_TRUE(
-      std::is_nothrow_copy_constructible<absl::Hash<absl::string_view>>::value);
-  ASSERT_TRUE(std::is_nothrow_copy_constructible<
-              std::equal_to<absl::string_view>>::value);
-  ASSERT_TRUE(std::is_nothrow_copy_constructible<std::allocator<int>>::value);
-  EXPECT_TRUE(std::is_nothrow_move_constructible<StringTable>::value);
+// This tests that rehash(0) on a sampled table with size that fits in SOO
+// doesn't incorrectly result in losing sampling.
+TEST(RawHashSamplerTest, SooTableRehashShrinkWhenSizeFitsInSoo) {
+  if (SooInt32Table().capacity() != SooCapacity()) {
+    CHECK_LT(sizeof(void*), 8) << "missing SOO coverage";
+    GTEST_SKIP() << "not SOO on this platform";
+  }
+  std::vector<const HashtablezInfo*> infos =
+      SampleSooMutation([](SooInt32Table& t) {
+        t.reserve(100);
+        t.insert(1);
+        EXPECT_GE(t.capacity(), 100);
+        t.rehash(0);
+      });
+
+  for (const HashtablezInfo* info : infos) {
+    ASSERT_EQ(info->inline_element_size,
+              sizeof(typename SooInt32Table::value_type));
+    ASSERT_EQ(info->soo_capacity, SooCapacity());
+    ASSERT_EQ(info->capacity, NextCapacity(SooCapacity()));
+    ASSERT_EQ(info->size, 1);
+    ASSERT_EQ(info->max_reserve, 100);
+    ASSERT_EQ(info->num_erases, 0);
+    ASSERT_EQ(info->max_probe_length, 0);
+    ASSERT_EQ(info->total_probe_length, 0);
+  }
 }
 
-TEST(Table, NoThrowMoveAssign) {
-  ASSERT_TRUE(
-      std::is_nothrow_move_assignable<absl::Hash<absl::string_view>>::value);
-  ASSERT_TRUE(
-      std::is_nothrow_move_assignable<std::equal_to<absl::string_view>>::value);
-  ASSERT_TRUE(std::is_nothrow_move_assignable<std::allocator<int>>::value);
-  ASSERT_TRUE(
-      absl::allocator_traits<std::allocator<int>>::is_always_equal::value);
-  EXPECT_TRUE(std::is_nothrow_move_assignable<StringTable>::value);
+// Verifies that a moved-from table does not retain the sampled state of the
+// original table, allowing it to be used without incorrectly updating global
+// sampling stats.
+TEST(RawHashSamplerTest, MovedFromTableIsNotSampled) {
+  if (SooInt32Table().capacity() != SooCapacity()) {
+    CHECK_LT(sizeof(void*), 8) << "missing SOO coverage";
+    GTEST_SKIP() << "not SOO on this platform";
+  }
+
+  SetSamplingRateTo1Percent();
+  auto& sampler = GlobalHashtablezSampler();
+  size_t start_size = 0;
+  absl::flat_hash_set<const HashtablezInfo*> preexisting_info;
+  sampler.Iterate([&](const HashtablezInfo& info) {
+    preexisting_info.insert(&info);
+    ++start_size;
+  });
+
+  SooInt32Table t1;
+  // Loop until t1 is sampled
+  while (true) {
+    t1 = SooInt32Table();
+    t1.insert(1);
+    size_t new_size = 0;
+    sampler.Iterate([&](const HashtablezInfo&) { ++new_size; });
+    if (new_size > start_size) break;
+  }
+
+  // Move the table
+  SooInt32Table t2 = std::move(t1);
+
+  // Disable sampling to ensure any new sampling is a bug.
+  SetHashtablezEnabled(false);
+
+  // t2 is now the sampled table. t1 is moved-from.
+  // We want to verify that t2 is still sampled, and that t1 isn't sampled
+  // anymore, even if we insert a new entry into it.
+  t1.clear();  // Must clear before using a moved-from table.
+  t1.insert(2);
+  t2.insert(2);
+
+  // Verify no new sample was generated, and t2's sample size is now 2.
+  size_t final_size = 0;
+  const HashtablezInfo* latest_info = nullptr;
+  size_t dropped = sampler.Iterate([&](const HashtablezInfo& info) {
+    ++final_size;
+    if (!preexisting_info.contains(&info)) {
+      latest_info = &info;
+    }
+  });
+  EXPECT_EQ(0, dropped);
+  EXPECT_EQ(final_size, start_size + 1);
+  ASSERT_NE(latest_info, nullptr);
+  EXPECT_EQ(latest_info->size.load(std::memory_order_relaxed), 2);
 }
 
-TEST(Table, NoThrowSwappable) {
-  ASSERT_TRUE(
-      container_internal::IsNoThrowSwappable<absl::Hash<absl::string_view>>());
-  ASSERT_TRUE(container_internal::IsNoThrowSwappable<
-              std::equal_to<absl::string_view>>());
-  ASSERT_TRUE(container_internal::IsNoThrowSwappable<std::allocator<int>>());
-  EXPECT_TRUE(container_internal::IsNoThrowSwappable<StringTable>());
+#endif  // ABSL_INTERNAL_HASHTABLEZ_SAMPLE
+
+TEST(RawHashSamplerTest, DoNotSampleCustomAllocators) {
+  // Enable the feature even if the prod default is off.
+  SetSamplingRateTo1Percent();
+
+  auto& sampler = GlobalHashtablezSampler();
+  int64_t start_size = 0;
+  start_size += sampler.Iterate([&](const HashtablezInfo&) { ++start_size; });
+
+  std::vector<CustomAllocIntTable> tables;
+  for (int i = 0; i < 100000; ++i) {
+    tables.emplace_back();
+    tables.back().insert(1);
+    tables.push_back(tables.back());  // Copies the table.
+  }
+  int64_t end_size = 0;
+  end_size += sampler.Iterate([&](const HashtablezInfo&) { ++end_size; });
+
+  EXPECT_NEAR((end_size - start_size) / static_cast<double>(tables.size()),
+              0.00, 0.001);
 }
 
-TEST(Table, HeterogeneousLookup) {
-  struct Hash {
-    size_t operator()(int64_t i) const { return i; }
-    size_t operator()(double i) const {
-      ADD_FAILURE();
-      return i;
-    }
-  };
-  struct Eq {
-    bool operator()(int64_t a, int64_t b) const { return a == b; }
-    bool operator()(double a, int64_t b) const {
-      ADD_FAILURE();
-      return a == b;
-    }
-    bool operator()(int64_t a, double b) const {
-      ADD_FAILURE();
-      return a == b;
-    }
-    bool operator()(double a, double b) const {
-      ADD_FAILURE();
-      return a == b;
+#ifdef ABSL_HAVE_ADDRESS_SANITIZER
+template <class TableType>
+class SanitizerTest : public testing::Test {};
+
+using SanitizerTableTypes = ::testing::Types<IntTable, TransferableIntTable>;
+TYPED_TEST_SUITE(SanitizerTest, SanitizerTableTypes);
+
+TYPED_TEST(SanitizerTest, PoisoningUnused) {
+  TypeParam t;
+  for (size_t reserve_size = 2; reserve_size < 1024;
+       reserve_size = reserve_size * 3 / 2) {
+    t.reserve(reserve_size);
+    // Insert something to force an allocation.
+    int64_t& v = *t.insert(0).first;
+
+    // Make sure there is something to test.
+    ASSERT_GT(t.capacity(), 1);
+
+    int64_t* slots = RawHashSetTestOnlyAccess::GetSlots(t);
+    for (size_t i = 0; i < t.capacity(); ++i) {
+      EXPECT_EQ(slots + i != &v, __asan_address_is_poisoned(slots + i)) << i;
     }
-  };
+  }
+}
 
-  struct THash {
-    using is_transparent = void;
-    size_t operator()(int64_t i) const { return i; }
-    size_t operator()(double i) const { return i; }
-  };
-  struct TEq {
-    using is_transparent = void;
-    bool operator()(int64_t a, int64_t b) const { return a == b; }
-    bool operator()(double a, int64_t b) const { return a == b; }
-    bool operator()(int64_t a, double b) const { return a == b; }
-    bool operator()(double a, double b) const { return a == b; }
-  };
+TYPED_TEST(SanitizerTest, PoisoningUnusedOnGrowth) {
+  TypeParam t;
+  for (int64_t i = 0; i < 100; ++i) {
+    t.insert(i);
 
-  raw_hash_set<IntPolicy, Hash, Eq, Alloc<int64_t>> s{0, 1, 2};
-  // It will convert to int64_t before the query.
-  EXPECT_EQ(1, *s.find(double{1.1}));
+    int64_t* slots = RawHashSetTestOnlyAccess::GetSlots(t);
+    int poisoned = 0;
+    for (size_t i = 0; i < t.capacity(); ++i) {
+      poisoned += static_cast<int>(__asan_address_is_poisoned(slots + i));
+    }
+    ASSERT_EQ(poisoned, t.capacity() - t.size());
+  }
+}
 
-  raw_hash_set<IntPolicy, THash, TEq, Alloc<int64_t>> ts{0, 1, 2};
-  // It will try to use the double, and fail to find the object.
-  EXPECT_TRUE(ts.find(1.1) == ts.end());
+// TODO(b/289225379): poison inline space when empty SOO.
+TEST(Sanitizer, PoisoningOnErase) {
+  NonSooIntTable t;
+  auto& v = *t.insert(0).first;
+
+  EXPECT_FALSE(__asan_address_is_poisoned(&v));
+  t.erase(0);
+  EXPECT_TRUE(__asan_address_is_poisoned(&v));
 }
+#endif  // ABSL_HAVE_ADDRESS_SANITIZER
 
-template <class Table>
-using CallFind = decltype(std::declval<Table&>().find(17));
+template <typename T>
+class AlignOneTest : public ::testing::Test {};
+using AlignOneTestTypes =
+    ::testing::Types<Uint8Table, MinimumAlignmentUint8Table>;
+TYPED_TEST_SUITE(AlignOneTest, AlignOneTestTypes);
+
+TYPED_TEST(AlignOneTest, AlignOne) {
+  // We previously had a bug in which we were copying a control byte over the
+  // first slot when alignof(value_type) is 1. We test repeated
+  // insertions/erases and verify that the behavior is correct.
+  TypeParam t;
+  std::bitset<256> verifier;
+
+  // Do repeated insertions/erases from the table.
+  for (int64_t i = 0; i < 10000; ++i) {
+    SCOPED_TRACE(i);
+    const uint8_t u = (i * -i) & 0xFF;
+    auto it = t.find(u);
+    if (it == t.end()) {
+      ASSERT_FALSE(verifier.test(u));
+      t.insert(u);
+      verifier.set(u);
+    } else {
+      ASSERT_TRUE(verifier.test(u));
+      t.erase(it);
+      verifier.reset(u);
+    }
+  }
 
-template <class Table>
-using CallErase = decltype(std::declval<Table&>().erase(17));
+  EXPECT_EQ(t.size(), verifier.count());
+  for (uint8_t u : t) {
+    ASSERT_TRUE(verifier.test(u));
+  }
+}
 
-template <class Table>
-using CallExtract = decltype(std::declval<Table&>().extract(17));
+TEST(Iterator, InvalidUseCrashesWithSanitizers) {
+  if (!SwisstableGenerationsEnabled()) GTEST_SKIP() << "Generations disabled.";
 
-template <class Table>
-using CallPrefetch = decltype(std::declval<Table&>().prefetch(17));
+  NonSooIntTable t;
+  // Start with 1 element so that `it` is never an end iterator.
+  t.insert(-1);
+  for (int i = 0; i < 10; ++i) {
+    auto it = t.begin();
+    t.insert(i);
+    EXPECT_DEATH_IF_SUPPORTED(*it, InvalidIteratorMatcher());
+    EXPECT_DEATH_IF_SUPPORTED(void(it == t.begin()), InvalidIteratorMatcher());
+  }
+}
 
-template <class Table>
-using CallCount = decltype(std::declval<Table&>().count(17));
+TEST(Iterator, InvalidUseWithReserveCrashesWithSanitizers) {
+  if (!SwisstableGenerationsEnabled()) GTEST_SKIP() << "Generations disabled.";
+
+  IntTable t;
+  t.reserve(10);
+  t.insert(0);
+  auto it = t.begin();
+  // Reserved growth can't rehash.
+  for (int i = 1; i < 10; ++i) {
+    t.insert(i);
+    EXPECT_EQ(*it, 0);
+  }
+  // ptr will become invalidated on rehash.
+  const int64_t* ptr = &*it;
+  (void)ptr;
 
-template <template <typename> class C, class Table, class = void>
-struct VerifyResultOf : std::false_type {};
+  // erase decreases size but does not decrease reserved growth so the next
+  // insertion still invalidates iterators.
+  t.erase(0);
+  // The first insert after reserved growth is 0 is guaranteed to rehash when
+  // generations are enabled.
+  t.insert(10);
+  EXPECT_DEATH_IF_SUPPORTED(*it, InvalidIteratorMatcher());
+  EXPECT_DEATH_IF_SUPPORTED(void(it == t.begin()), InvalidIteratorMatcher());
+#ifdef ABSL_HAVE_ADDRESS_SANITIZER
+  EXPECT_DEATH_IF_SUPPORTED(std::cout << *ptr, "heap-use-after-free");
+#endif
+}
 
-template <template <typename> class C, class Table>
-struct VerifyResultOf<C, Table, absl::void_t<C<Table>>> : std::true_type {};
+TEST(Iterator, InvalidUseWithMoveCrashesWithSanitizers) {
+  if (!SwisstableGenerationsEnabled()) GTEST_SKIP() << "Generations disabled.";
+
+  NonSooIntTable t1, t2;
+  t1.insert(1);
+  auto it = t1.begin();
+  // ptr will become invalidated on rehash.
+  const auto* ptr = &*it;
+  (void)ptr;
+
+  t2 = std::move(t1);
+  EXPECT_DEATH_IF_SUPPORTED(*it, InvalidIteratorMatcher());
+  EXPECT_DEATH_IF_SUPPORTED(void(it == t2.begin()), InvalidIteratorMatcher());
+#ifdef ABSL_HAVE_ADDRESS_SANITIZER
+  EXPECT_DEATH_IF_SUPPORTED(std::cout << **ptr, "heap-use-after-free");
+#endif
+}
 
-TEST(Table, HeterogeneousLookupOverloads) {
-  using NonTransparentTable =
-      raw_hash_set<StringPolicy, absl::Hash<absl::string_view>,
-                   std::equal_to<absl::string_view>, std::allocator<int>>;
+template <class TableType>
+class InstanceTrackerTest : public testing::Test {};
 
-  EXPECT_FALSE((VerifyResultOf<CallFind, NonTransparentTable>()));
-  EXPECT_FALSE((VerifyResultOf<CallErase, NonTransparentTable>()));
-  EXPECT_FALSE((VerifyResultOf<CallExtract, NonTransparentTable>()));
-  EXPECT_FALSE((VerifyResultOf<CallPrefetch, NonTransparentTable>()));
-  EXPECT_FALSE((VerifyResultOf<CallCount, NonTransparentTable>()));
+using ::absl::test_internal::CopyableMovableInstance;
+using ::absl::test_internal::InstanceTracker;
 
-  using TransparentTable = raw_hash_set<
-      StringPolicy,
-      absl::container_internal::hash_default_hash<absl::string_view>,
-      absl::container_internal::hash_default_eq<absl::string_view>,
-      std::allocator<int>>;
+struct InstanceTrackerHash {
+  size_t operator()(const CopyableMovableInstance& t) const {
+    return absl::HashOf(t.value());
+  }
+};
 
-  EXPECT_TRUE((VerifyResultOf<CallFind, TransparentTable>()));
-  EXPECT_TRUE((VerifyResultOf<CallErase, TransparentTable>()));
-  EXPECT_TRUE((VerifyResultOf<CallExtract, TransparentTable>()));
-  EXPECT_TRUE((VerifyResultOf<CallPrefetch, TransparentTable>()));
-  EXPECT_TRUE((VerifyResultOf<CallCount, TransparentTable>()));
-}
+using InstanceTrackerTableTypes = ::testing::Types<
+    absl::node_hash_set<CopyableMovableInstance, InstanceTrackerHash>,
+    absl::flat_hash_set<CopyableMovableInstance, InstanceTrackerHash>>;
+TYPED_TEST_SUITE(InstanceTrackerTest, InstanceTrackerTableTypes);
 
-// TODO(alkis): Expand iterator tests.
-TEST(Iterator, IsDefaultConstructible) {
-  StringTable::iterator i;
-  EXPECT_TRUE(i == StringTable::iterator());
+TYPED_TEST(InstanceTrackerTest, EraseIfAll) {
+  using Table = TypeParam;
+  InstanceTracker tracker;
+  for (int size = 0; size < 100; ++size) {
+    Table t;
+    for (int i = 0; i < size; ++i) {
+      t.emplace(i);
+    }
+    absl::erase_if(t, [](const auto&) { return true; });
+    ASSERT_EQ(t.size(), 0);
+  }
+  EXPECT_EQ(tracker.live_instances(), 0);
 }
 
-TEST(ConstIterator, IsDefaultConstructible) {
-  StringTable::const_iterator i;
-  EXPECT_TRUE(i == StringTable::const_iterator());
+TYPED_TEST(InstanceTrackerTest, EraseIfNone) {
+  using Table = TypeParam;
+  InstanceTracker tracker;
+  {
+    Table t;
+    for (size_t size = 0; size < 100; ++size) {
+      absl::erase_if(t, [](const auto&) { return false; });
+      ASSERT_EQ(t.size(), size);
+      t.emplace(size);
+    }
+  }
+  EXPECT_EQ(tracker.live_instances(), 0);
 }
 
-TEST(Iterator, ConvertsToConstIterator) {
-  StringTable::iterator i;
-  EXPECT_TRUE(i == StringTable::const_iterator());
+TYPED_TEST(InstanceTrackerTest, EraseIfPartial) {
+  using Table = TypeParam;
+  InstanceTracker tracker;
+  for (int mod : {0, 1}) {
+    for (int size = 0; size < 100; ++size) {
+      SCOPED_TRACE(absl::StrCat(mod, " ", size));
+      Table t;
+      std::vector<CopyableMovableInstance> expected;
+      for (int i = 0; i < size; ++i) {
+        t.emplace(i);
+        if (i % 2 != mod) {
+          expected.emplace_back(i);
+        }
+      }
+      absl::erase_if(t, [mod](const auto& x) { return x.value() % 2 == mod; });
+      ASSERT_THAT(t, testing::UnorderedElementsAreArray(expected));
+    }
+  }
+  EXPECT_EQ(tracker.live_instances(), 0);
 }
 
-TEST(Iterator, Iterates) {
-  IntTable t;
-  for (size_t i = 3; i != 6; ++i) EXPECT_TRUE(t.emplace(i).second);
-  EXPECT_THAT(t, UnorderedElementsAre(3, 4, 5));
+TEST(Table, EraseBeginEndResetsReservedGrowth) {
+  bool frozen = false;
+  BadHashFreezableIntTable t{FreezableAlloc<int64_t>(&frozen)};
+  t.reserve(100);
+  const size_t cap = t.capacity();
+  frozen = true;  // no further allocs allowed
+
+  for (int i = 0; i < 10; ++i) {
+    // Create a long run (hash function returns constant).
+    for (int j = 0; j < 100; ++j) t.insert(j);
+    // Erase elements from the middle of the long run, which creates
+    // tombstones.
+    for (int j = 30; j < 60; ++j) t.erase(j);
+    EXPECT_EQ(t.size(), 70);
+    EXPECT_EQ(t.capacity(), cap);
+    ASSERT_EQ(RawHashSetTestOnlyAccess::CountTombstones(t), 30);
+
+    t.erase(t.begin(), t.end());
+
+    EXPECT_EQ(t.size(), 0);
+    EXPECT_EQ(t.capacity(), cap);
+    ASSERT_EQ(RawHashSetTestOnlyAccess::CountTombstones(t), 0);
+  }
 }
 
-TEST(Table, Merge) {
-  StringTable t1, t2;
-  t1.emplace("0", "-0");
-  t1.emplace("1", "-1");
-  t2.emplace("0", "~0");
-  t2.emplace("2", "~2");
+TEST(Table, GenerationInfoResetsOnClear) {
+  if (!SwisstableGenerationsEnabled()) GTEST_SKIP() << "Generations disabled.";
 
-  EXPECT_THAT(t1, UnorderedElementsAre(Pair("0", "-0"), Pair("1", "-1")));
-  EXPECT_THAT(t2, UnorderedElementsAre(Pair("0", "~0"), Pair("2", "~2")));
+  NonSooIntTable t;
+  for (int i = 0; i < 1000; ++i) t.insert(i);
+  t.reserve(t.size() + 100);
 
-  t1.merge(t2);
-  EXPECT_THAT(t1, UnorderedElementsAre(Pair("0", "-0"), Pair("1", "-1"),
-                                       Pair("2", "~2")));
-  EXPECT_THAT(t2, UnorderedElementsAre(Pair("0", "~0")));
+  t.clear();
+
+  t.insert(0);
+  auto it = t.begin();
+  t.insert(1);
+  EXPECT_DEATH_IF_SUPPORTED(*it, InvalidIteratorMatcher());
 }
 
-TEST(Nodes, EmptyNodeType) {
-  using node_type = StringTable::node_type;
-  node_type n;
-  EXPECT_FALSE(n);
-  EXPECT_TRUE(n.empty());
+TEST(Table, InvalidReferenceUseCrashesWithSanitizers) {
+  if (!SwisstableGenerationsEnabled()) GTEST_SKIP() << "Generations disabled.";
+#ifdef ABSL_HAVE_MEMORY_SANITIZER
+  GTEST_SKIP() << "MSan fails to detect some of these rehashes.";
+#endif
 
-  EXPECT_TRUE((std::is_same<node_type::allocator_type,
-                            StringTable::allocator_type>::value));
+  NonSooIntTable t;
+  t.insert(0);
+  // Rehashing is guaranteed on every insertion while capacity is less than
+  // RehashProbabilityConstant().
+  int i = 0;
+  while (t.capacity() <= RehashProbabilityConstant()) {
+    // ptr will become invalidated on rehash.
+    const auto* ptr = &*t.begin();
+    t.insert(++i);
+    EXPECT_DEATH_IF_SUPPORTED(std::cout << **ptr, "use-after-free") << i;
+  }
 }
 
-TEST(Nodes, ExtractInsert) {
-  constexpr char k0[] = "Very long std::string zero.";
-  constexpr char k1[] = "Very long std::string one.";
-  constexpr char k2[] = "Very long std::string two.";
-  StringTable t = {{k0, ""}, {k1, ""}, {k2, ""}};
-  EXPECT_THAT(t,
-              UnorderedElementsAre(Pair(k0, ""), Pair(k1, ""), Pair(k2, "")));
+TEST(Iterator, InvalidComparisonDifferentTables) {
+  if (!SwisstableGenerationsEnabled()) GTEST_SKIP() << "Generations disabled.";
+
+  NonSooIntTable t1, t2;
+  NonSooIntTable::iterator default_constructed_iter;
+  // We randomly use one of N empty generations for generations from empty
+  // hashtables. In general, we won't always detect when iterators from
+  // different empty hashtables are compared, but in this test case, we
+  // should deterministically detect the error due to our randomness yielding
+  // consecutive random generations.
+  EXPECT_DEATH_IF_SUPPORTED(void(t1.end() == t2.end()),
+                            "Invalid iterator comparison.*empty hashtables");
+  EXPECT_DEATH_IF_SUPPORTED(void(t1.end() == default_constructed_iter),
+                            "Invalid iterator comparison.*default-constructed");
+  t1.insert(0);
+  t1.insert(1);
+  EXPECT_DEATH_IF_SUPPORTED(void(t1.begin() == t2.end()),
+                            "Invalid iterator comparison.*empty hashtable");
+  EXPECT_DEATH_IF_SUPPORTED(void(t1.begin() == default_constructed_iter),
+                            "Invalid iterator comparison.*default-constructed");
+  t2.insert(0);
+  t2.insert(1);
+  EXPECT_DEATH_IF_SUPPORTED(void(t1.begin() == t2.end()),
+                            "Invalid iterator comparison.*end.. iterator");
+  EXPECT_DEATH_IF_SUPPORTED(void(t1.begin() == t2.begin()),
+                            "Invalid iterator comparison.*non-end");
+}
 
-  auto node = t.extract(k0);
-  EXPECT_THAT(t, UnorderedElementsAre(Pair(k1, ""), Pair(k2, "")));
-  EXPECT_TRUE(node);
-  EXPECT_FALSE(node.empty());
+template <typename Alloc>
+using RawHashSetAlloc = raw_hash_set<IntPolicy, hash_default_hash<int64_t>,
+                                     std::equal_to<int64_t>, Alloc>;
 
-  StringTable t2;
-  StringTable::insert_return_type res = t2.insert(std::move(node));
-  EXPECT_TRUE(res.inserted);
-  EXPECT_THAT(*res.position, Pair(k0, ""));
-  EXPECT_FALSE(res.node);
-  EXPECT_THAT(t2, UnorderedElementsAre(Pair(k0, "")));
+TEST(Table, AllocatorPropagation) { TestAllocPropagation<RawHashSetAlloc>(); }
 
-  // Not there.
-  EXPECT_THAT(t, UnorderedElementsAre(Pair(k1, ""), Pair(k2, "")));
-  node = t.extract("Not there!");
-  EXPECT_THAT(t, UnorderedElementsAre(Pair(k1, ""), Pair(k2, "")));
-  EXPECT_FALSE(node);
+struct CountedHash {
+  size_t operator()(int64_t value) const {
+    ++count;
+    return static_cast<size_t>(value);
+  }
+  mutable int count = 0;
+};
 
-  // Inserting nothing.
-  res = t2.insert(std::move(node));
-  EXPECT_FALSE(res.inserted);
-  EXPECT_EQ(res.position, t2.end());
-  EXPECT_FALSE(res.node);
-  EXPECT_THAT(t2, UnorderedElementsAre(Pair(k0, "")));
+struct CountedHashIntTable
+    : raw_hash_set<IntPolicy, CountedHash, std::equal_to<int>,
+                   std::allocator<int>> {
+  using Base = typename CountedHashIntTable::raw_hash_set;
+  using Base::Base;
+};
 
-  t.emplace(k0, "1");
-  node = t.extract(k0);
+TEST(Table, CountedHash) {
+  // Verify that raw_hash_set does not compute redundant hashes.
+#ifdef NDEBUG
+  constexpr bool kExpectMinimumHashes = true;
+#else
+  constexpr bool kExpectMinimumHashes = false;
+#endif
+  if (!kExpectMinimumHashes) {
+    GTEST_SKIP() << "Only run under NDEBUG: `assert` statements may cause "
+                    "redundant hashing.";
+  }
+  // When the table is sampled, we need to hash on the first insertion.
+  DisableSampling();
 
-  // Insert duplicate.
-  res = t2.insert(std::move(node));
-  EXPECT_FALSE(res.inserted);
-  EXPECT_THAT(*res.position, Pair(k0, ""));
-  EXPECT_TRUE(res.node);
-  EXPECT_FALSE(node);
+  using Table = CountedHashIntTable;
+  auto HashCount = [](const Table& t) { return t.hash_function().count; };
+  {
+    Table t;
+    t.find(0);
+    EXPECT_EQ(HashCount(t), 0);
+  }
+  {
+    Table t;
+    t.insert(1);
+    t.find(1);
+    EXPECT_EQ(HashCount(t), 0);
+    t.erase(1);
+    EXPECT_EQ(HashCount(t), 0);
+    t.insert(1);
+    t.insert(2);
+    EXPECT_EQ(HashCount(t), 2);
+  }
+  {
+    Table t;
+    t.insert(3);
+    EXPECT_EQ(HashCount(t), 0);
+    auto node = t.extract(3);
+    EXPECT_EQ(HashCount(t), 0);
+    t.insert(std::move(node));
+    EXPECT_EQ(HashCount(t), 0);
+  }
+  {
+    Table t;
+    t.emplace(5);
+    EXPECT_EQ(HashCount(t), 0);
+  }
+  {
+    Table src;
+    src.insert(7);
+    Table dst;
+    dst.merge(src);
+    EXPECT_EQ(HashCount(dst), 0);
+  }
 }
 
-IntTable MakeSimpleTable(size_t size) {
-  IntTable t;
-  while (t.size() < size) t.insert(t.size());
-  return t;
+// IterateOverFullSlots doesn't support SOO.
+TEST(Table, IterateOverFullSlotsEmpty) {
+  NonSooIntTable t;
+  using SlotType = NonSooIntTableSlotType;
+  auto fail_if_any = [](const ctrl_t*, void* i) {
+    FAIL() << "expected no slots " << **static_cast<SlotType*>(i);
+  };
+  for (size_t i = 2; i < 256; ++i) {
+    t.reserve(i);
+    container_internal::IterateOverFullSlots(
+        RawHashSetTestOnlyAccess::GetCommon(t), sizeof(SlotType), fail_if_any);
+  }
 }
 
-std::vector<int> OrderOfIteration(const IntTable& t) {
-  return {t.begin(), t.end()};
+TEST(Table, IterateOverFullSlotsFull) {
+  NonSooIntTable t;
+  using SlotType = NonSooIntTableSlotType;
+
+  std::vector<int64_t> expected_slots;
+  t.insert(0);
+  expected_slots.push_back(0);
+  for (int64_t idx = 1; idx < 128; ++idx) {
+    t.insert(idx);
+    expected_slots.push_back(idx);
+
+    std::vector<int64_t> slots;
+    container_internal::IterateOverFullSlots(
+        RawHashSetTestOnlyAccess::GetCommon(t), sizeof(SlotType),
+        [&t, &slots](const ctrl_t* ctrl, void* slot) {
+          SlotType* i = static_cast<SlotType*>(slot);
+          ptrdiff_t ctrl_offset =
+              ctrl - RawHashSetTestOnlyAccess::GetCommon(t).control();
+          ptrdiff_t slot_offset = i - RawHashSetTestOnlyAccess::GetSlots(t);
+          ASSERT_EQ(ctrl_offset, slot_offset);
+          slots.push_back(**i);
+        });
+    EXPECT_THAT(slots, testing::UnorderedElementsAreArray(expected_slots));
+  }
 }
 
-// These IterationOrderChanges tests depend on non-deterministic behavior.
-// We are injecting non-determinism from the pointer of the table, but do so in
-// a way that only the page matters. We have to retry enough times to make sure
-// we are touching different memory pages to cause the ordering to change.
-// We also need to keep the old tables around to avoid getting the same memory
-// blocks over and over.
-TEST(Table, IterationOrderChangesByInstance) {
-  for (size_t size : {2, 6, 12, 20}) {
-    const auto reference_table = MakeSimpleTable(size);
-    const auto reference = OrderOfIteration(reference_table);
+TEST(Table, IterateOverFullSlotsDeathOnRemoval) {
+  if (!IsAssertEnabled()) GTEST_SKIP() << "Assertions not enabled.";
+
+  auto iterate_with_reentrant_removal = [](int64_t size,
+                                           int64_t reserve_size = -1) {
+    if (reserve_size == -1) reserve_size = size;
+    for (int64_t idx = 0; idx < size; ++idx) {
+      NonSooIntTable t;
+      using SlotType = NonSooIntTableSlotType;
+      t.reserve(static_cast<size_t>(reserve_size));
+      for (int val = 0; val <= idx; ++val) {
+        t.insert(val);
+      }
 
-    std::vector<IntTable> tables;
-    bool found_difference = false;
-    for (int i = 0; !found_difference && i < 5000; ++i) {
-      tables.push_back(MakeSimpleTable(size));
-      found_difference = OrderOfIteration(tables.back()) != reference;
+      container_internal::IterateOverFullSlots(
+          RawHashSetTestOnlyAccess::GetCommon(t), sizeof(SlotType),
+          [&t](const ctrl_t*, void* slot) {
+            int64_t value = **static_cast<SlotType*>(slot);
+            // Erase the other element from 2*k and 2*k+1 pair.
+            t.erase(value ^ 1);
+          });
     }
-    if (!found_difference) {
-      FAIL()
-          << "Iteration order remained the same across many attempts with size "
-          << size;
-    }
-  }
+  };
+
+  EXPECT_DEATH_IF_SUPPORTED(iterate_with_reentrant_removal(128),
+                            "hash table was modified unexpectedly");
+  // Removal will likely happen in a different group.
+  EXPECT_DEATH_IF_SUPPORTED(iterate_with_reentrant_removal(14, 1024 * 16),
+                            "hash table was modified unexpectedly");
+  // Removal will happen in the same group.
+  EXPECT_DEATH_IF_SUPPORTED(iterate_with_reentrant_removal(static_cast<int64_t>(
+                                CapacityToGrowth(Group::kWidth - 1))),
+                            "hash table was modified unexpectedly");
 }
 
-TEST(Table, IterationOrderChangesOnRehash) {
-  std::vector<IntTable> garbage;
-  for (int i = 0; i < 5000; ++i) {
-    auto t = MakeSimpleTable(20);
-    const auto reference = OrderOfIteration(t);
-    // Force rehash to the same size.
-    t.rehash(0);
-    auto trial = OrderOfIteration(t);
-    if (trial != reference) {
-      // We are done.
-      return;
+TEST(Table, IterateOverFullSlotsDeathOnInsert) {
+  if (!IsAssertEnabled()) GTEST_SKIP() << "Assertions not enabled.";
+
+  auto iterate_with_reentrant_insert = [](int64_t reserve_size,
+                                          int64_t size_divisor = 2) {
+    int64_t size = reserve_size / size_divisor;
+    for (int64_t idx = 1; idx <= size; ++idx) {
+      NonSooIntTable t;
+      using SlotType = NonSooIntTableSlotType;
+      t.reserve(static_cast<size_t>(reserve_size));
+      for (int val = 1; val <= idx; ++val) {
+        t.insert(val);
+      }
+
+      container_internal::IterateOverFullSlots(
+          RawHashSetTestOnlyAccess::GetCommon(t), sizeof(SlotType),
+          [&t](const ctrl_t*, void* slot) {
+            int64_t value = **static_cast<SlotType*>(slot);
+            t.insert(-value);
+          });
     }
-    garbage.push_back(std::move(t));
+  };
+
+  EXPECT_DEATH_IF_SUPPORTED(iterate_with_reentrant_insert(128),
+                            "hash table was modified unexpectedly");
+  // Insert will likely happen in a different group.
+  EXPECT_DEATH_IF_SUPPORTED(iterate_with_reentrant_insert(1024 * 16, 1024 * 2),
+                            "hash table was modified unexpectedly");
+  // Insert will happen in the same group.
+  EXPECT_DEATH_IF_SUPPORTED(iterate_with_reentrant_insert(static_cast<int64_t>(
+                                CapacityToGrowth(Group::kWidth - 1))),
+                            "hash table was modified unexpectedly");
+}
+
+TEST(SooTable, Basic) {
+  bool frozen = true;
+  FreezableSizedValueSooTable<8> t{
+      FreezableAlloc<FreezableSizedValueSooTable<8>::value_type>(&frozen)};
+  if (t.capacity() != SooCapacity()) {
+    CHECK_LT(sizeof(void*), 8) << "missing SOO coverage";
+    GTEST_SKIP() << "not SOO on this platform";
   }
-  FAIL() << "Iteration order remained the same across many attempts.";
+
+  t.insert(0);
+  EXPECT_EQ(t.capacity(), 1);
+  auto it = t.find(0);
+  EXPECT_EQ(it, t.begin());
+  ASSERT_NE(it, t.end());
+  EXPECT_EQ(*it, 0);
+  EXPECT_EQ(++it, t.end());
+  EXPECT_EQ(t.find(1), t.end());
+  EXPECT_EQ(t.size(), 1);
+
+  t.erase(0);
+  EXPECT_EQ(t.size(), 0);
+  t.insert(1);
+  it = t.find(1);
+  EXPECT_EQ(it, t.begin());
+  ASSERT_NE(it, t.end());
+  EXPECT_EQ(*it, 1);
+
+  t.clear();
+  EXPECT_EQ(t.size(), 0);
 }
 
-// Verify that pointers are invalidated as soon as a second element is inserted.
-// This prevents dependency on pointer stability on small tables.
-TEST(Table, UnstablePointers) {
-  IntTable table;
+TEST(Table, RehashToSooUnsampled) {
+  SooIntTable t;
+  if (t.capacity() != SooCapacity()) {
+    CHECK_LT(sizeof(void*), 8) << "missing SOO coverage";
+    GTEST_SKIP() << "not SOO on this platform";
+  }
 
-  const auto addr = [&](int i) {
-    return reinterpret_cast<uintptr_t>(&*table.find(i));
-  };
+  // We disable hashtablez sampling for this test to ensure that the table isn't
+  // sampled. When the table is sampled, it won't rehash down to SOO.
+  DisableSampling();
 
-  table.insert(0);
-  const uintptr_t old_ptr = addr(0);
+  t.reserve(100);
+  t.insert(0);
+  EXPECT_EQ(*t.begin(), 0);
 
-  // This causes a rehash.
-  table.insert(1);
+  t.rehash(0);  // Rehash back down to SOO table.
 
-  EXPECT_NE(old_ptr, addr(0));
+  EXPECT_EQ(t.capacity(), SooCapacity());
+  EXPECT_EQ(t.size(), 1);
+  EXPECT_EQ(*t.begin(), 0);
+  EXPECT_EQ(t.find(0), t.begin());
+  EXPECT_EQ(t.find(1), t.end());
 }
 
-// Confirm that we assert if we try to erase() end().
-TEST(TableDeathTest, EraseOfEndAsserts) {
-  // Use an assert with side-effects to figure out if they are actually enabled.
-  bool assert_enabled = false;
-  assert([&]() {
-    assert_enabled = true;
-    return true;
-  }());
-  if (!assert_enabled) return;
+TEST(Table, ReserveToNonSoo) {
+  for (size_t reserve_capacity : {2u, 8u, 100000u}) {
+    SooIntTable t;
+    t.insert(0);
 
-  IntTable t;
-  // Extra simple "regexp" as regexp support is highly varied across platforms.
-  constexpr char kDeathMsg[] = "it != end";
-  EXPECT_DEATH_IF_SUPPORTED(t.erase(t.end()), kDeathMsg);
+    t.reserve(reserve_capacity);
+
+    EXPECT_EQ(t.find(0), t.begin());
+    EXPECT_EQ(t.size(), 1);
+    EXPECT_EQ(*t.begin(), 0);
+    EXPECT_EQ(t.find(1), t.end());
+  }
 }
 
-TEST(RawHashSamplerTest, Sample) {
-  // Enable the feature even if the prod default is off.
-  SetHashtablezEnabled(true);
-  SetHashtablezSampleParameter(100);
+struct InconsistentHashEqType {
+  InconsistentHashEqType(int v1, int v2) : v1(v1), v2(v2) {}
+  template <typename H>
+  friend H AbslHashValue(H h, InconsistentHashEqType t) {
+    return H::combine(std::move(h), t.v1);
+  }
+  bool operator==(InconsistentHashEqType t) const { return v2 == t.v2; }
+  int v1, v2;
+};
 
-  auto& sampler = HashtablezSampler::Global();
-  size_t start_size = 0;
-  start_size += sampler.Iterate([&](const HashtablezInfo&) { ++start_size; });
+TEST(Iterator, InconsistentHashEqFunctorsValidation) {
+  if (!IsAssertEnabled()) GTEST_SKIP() << "Assertions not enabled.";
+
+  ValueTable<InconsistentHashEqType> t;
+  for (int i = 0; i < 10; ++i) t.insert({i, i});
+  // We need to find/insert multiple times to guarantee that we get the
+  // assertion because it's possible for the hash to collide with the inserted
+  // element that has v2==0. In those cases, the new element won't be inserted.
+  auto find_conflicting_elems = [&] {
+    for (int i = 100; i < 20000; ++i) {
+      EXPECT_EQ(t.find({i, 0}), t.end());
+    }
+  };
+  EXPECT_DEATH_IF_SUPPORTED(find_conflicting_elems(),
+                            "hash/eq functors are inconsistent.");
+  auto insert_conflicting_elems = [&] {
+    for (int i = 100; i < 20000; ++i) {
+      EXPECT_EQ(t.insert({i, 0}).second, false);
+    }
+  };
+  EXPECT_DEATH_IF_SUPPORTED(insert_conflicting_elems(),
+                            "hash/eq functors are inconsistent.");
+}
 
-  std::vector<IntTable> tables;
-  for (int i = 0; i < 1000000; ++i) {
-    tables.emplace_back();
-    tables.back().insert(1);
+struct ConstructCaller {
+  explicit ConstructCaller(int v) : val(v) {}
+  ConstructCaller(int v, absl::FunctionRef<void()> func) : val(v) { func(); }
+  template <typename H>
+  friend H AbslHashValue(H h, const ConstructCaller& d) {
+    return H::combine(std::move(h), d.val);
   }
-  size_t end_size = 0;
-  end_size += sampler.Iterate([&](const HashtablezInfo&) { ++end_size; });
+  bool operator==(const ConstructCaller& c) const { return val == c.val; }
 
-  EXPECT_NEAR((end_size - start_size) / static_cast<double>(tables.size()),
-              0.01, 0.005);
-}
+  int val;
+};
 
-TEST(RawHashSamplerTest, DoNotSampleCustomAllocators) {
-  // Enable the feature even if the prod default is off.
-  SetHashtablezEnabled(true);
-  SetHashtablezSampleParameter(100);
+struct DestroyCaller {
+  explicit DestroyCaller(int v) : val(v) {}
+  DestroyCaller(int v, absl::FunctionRef<void()> func)
+      : val(v), destroy_func(func) {}
+  DestroyCaller(DestroyCaller&& that)
+      : val(that.val), destroy_func(std::move(that.destroy_func)) {
+    that.Deactivate();
+  }
+  ~DestroyCaller() {
+    if (destroy_func) (*destroy_func)();
+  }
+  void Deactivate() { destroy_func = std::nullopt; }
 
-  auto& sampler = HashtablezSampler::Global();
-  size_t start_size = 0;
-  start_size += sampler.Iterate([&](const HashtablezInfo&) { ++start_size; });
+  template <typename H>
+  friend H AbslHashValue(H h, const DestroyCaller& d) {
+    return H::combine(std::move(h), d.val);
+  }
+  bool operator==(const DestroyCaller& d) const { return val == d.val; }
 
-  std::vector<CustomAllocIntTable> tables;
-  for (int i = 0; i < 1000000; ++i) {
-    tables.emplace_back();
-    tables.back().insert(1);
+  int val;
+  std::optional<absl::FunctionRef<void()>> destroy_func;
+};
+
+TEST(Table, ReentrantCallsFail) {
+#ifdef NDEBUG
+  GTEST_SKIP() << "Reentrant checks only enabled in debug mode.";
+#else
+  {
+    ValueTable<ConstructCaller> t;
+    t.insert(ConstructCaller{0});
+    auto erase_begin = [&] { t.erase(t.begin()); };
+    EXPECT_DEATH_IF_SUPPORTED(t.emplace(1, erase_begin), "");
   }
-  size_t end_size = 0;
-  end_size += sampler.Iterate([&](const HashtablezInfo&) { ++end_size; });
+  {
+    ValueTable<DestroyCaller> t;
+    t.insert(DestroyCaller{0});
+    auto find_0 = [&] { t.find(DestroyCaller{0}); };
+    t.insert(DestroyCaller{1, find_0});
+    for (int i = 10; i < 20; ++i) t.insert(DestroyCaller{i});
+    EXPECT_DEATH_IF_SUPPORTED(t.clear(), "");
+    for (auto& elem : t) elem.Deactivate();
+  }
+  {
+    ValueTable<DestroyCaller> t;
+    t.insert(DestroyCaller{0});
+    auto insert_1 = [&] { t.insert(DestroyCaller{1}); };
+    t.insert(DestroyCaller{1, insert_1});
+    for (int i = 10; i < 20; ++i) t.insert(DestroyCaller{i});
+    EXPECT_DEATH_IF_SUPPORTED(t.clear(), "");
+    for (auto& elem : t) elem.Deactivate();
+  }
+#endif
+}
 
-  EXPECT_NEAR((end_size - start_size) / static_cast<double>(tables.size()),
-              0.00, 0.001);
+TEST(Table, DestroyedCallsFail) {
+  if (!SwisstableGenerationsOrDebugEnabled()) {
+    GTEST_SKIP() << "Validation not enabled.";
+  }
+#if !defined(__clang__) && defined(__GNUC__)
+  GTEST_SKIP() << "Flaky on GCC.";
+#elif defined(ABSL_HAVE_THREAD_SANITIZER)
+  GTEST_SKIP() << "Fails on TSan.";
+  // Note: we use else rather than endif here to avoid unreachable code errors.
+#else
+  std::optional<IntTable> t;
+  t.emplace({1});
+  IntTable* t_ptr = &*t;
+  EXPECT_TRUE(t_ptr->contains(1));
+  t.reset();
+  EXPECT_DEATH_IF_SUPPORTED(
+      t_ptr->contains(1),
+      AnyOf(HasSubstr("destroyed hash table"), HasSubstr("use-after-free"),
+            HasSubstr("use-of-uninitialized-value")));
+
+#endif  // ABSL_HAVE_THREAD_SANITIZER
 }
 
-#ifdef ADDRESS_SANITIZER
-TEST(Sanitizer, PoisoningUnused) {
-  IntTable t;
-  t.reserve(5);
-  // Insert something to force an allocation.
-  int64_t& v1 = *t.insert(0).first;
+TEST(Table, DestroyedCallsFailDuringDestruction) {
+  if (!SwisstableGenerationsOrDebugEnabled()) {
+    GTEST_SKIP() << "Validation not enabled.";
+  }
+#if !defined(__clang__) && defined(__GNUC__)
+  GTEST_SKIP() << "Flaky on GCC.";
+#endif
+#if defined(ABSL_HAVE_ADDRESS_SANITIZER) && defined(NDEBUG)
+  // TODO(b/487002780): see if we can re-enable this test in opt-ASan mode.
+  GTEST_SKIP() << "Fails to die in opt ASAN.";
+#endif
+  // When EXPECT_DEATH_IF_SUPPORTED is not executed, the code after it is not
+  // executed as well.
+  // We need to destruct the table correctly in such a case.
+  // Must be defined before the table for correct destruction order.
+  bool do_lookup = false;
+
+  using Table = absl::flat_hash_map<int, std::shared_ptr<int>>;
+  std::optional<Table> t = Table();
+  Table* t_ptr = &*t;
+  auto destroy = [&](int* ptr) {
+    if (do_lookup) {
+      ASSERT_TRUE(t_ptr->contains(*ptr));
+    }
+    delete ptr;
+  };
+  t->insert({0, std::shared_ptr<int>(new int(0), destroy)});
+  auto destroy_with_lookup = [&] {
+    do_lookup = true;
+    t.reset();
+  };
+  EXPECT_DEATH_IF_SUPPORTED(destroy_with_lookup(),
+                            AnyOf(HasSubstr("destroyed hash table"),
+                                  HasSubstr("Reentrant container access")));
+}
 
-  // Make sure there is something to test.
-  ASSERT_GT(t.capacity(), 1);
+TEST(Table, MovedFromCallsFail) {
+  if (!SwisstableGenerationsEnabled()) {
+    GTEST_SKIP() << "Moved-from checks only enabled in sanitizer mode.";
+    return;
+  }
 
-  int64_t* slots = RawHashSetTestOnlyAccess::GetSlots(t);
-  for (size_t i = 0; i < t.capacity(); ++i) {
-    EXPECT_EQ(slots + i != &v1, __asan_address_is_poisoned(slots + i));
+  {
+    ABSL_ATTRIBUTE_UNUSED IntTable t1, t2, t3;
+    t1.insert(1);
+    t2 = std::move(t1);
+    // NOLINTNEXTLINE(bugprone-use-after-move)
+    EXPECT_DEATH_IF_SUPPORTED(t1.contains(1), "moved-from");
+    // NOLINTNEXTLINE(bugprone-use-after-move)
+    EXPECT_DEATH_IF_SUPPORTED(t1.swap(t3), "moved-from");
+    // NOLINTNEXTLINE(bugprone-use-after-move)
+    EXPECT_DEATH_IF_SUPPORTED(t1.merge(t3), "moved-from");
+    // NOLINTNEXTLINE(bugprone-use-after-move)
+    EXPECT_DEATH_IF_SUPPORTED(IntTable{t1}, "moved-from");
+    // NOLINTNEXTLINE(bugprone-use-after-move)
+    EXPECT_DEATH_IF_SUPPORTED(t1.begin(), "moved-from");
+    // NOLINTNEXTLINE(bugprone-use-after-move)
+    EXPECT_DEATH_IF_SUPPORTED(t1.end(), "moved-from");
+    // NOLINTNEXTLINE(bugprone-use-after-move)
+    EXPECT_DEATH_IF_SUPPORTED(t1.size(), "moved-from");
+  }
+  {
+    ABSL_ATTRIBUTE_UNUSED IntTable t1;
+    t1.insert(1);
+    ABSL_ATTRIBUTE_UNUSED IntTable t2(std::move(t1));
+    // NOLINTNEXTLINE(bugprone-use-after-move)
+    EXPECT_DEATH_IF_SUPPORTED(t1.contains(1), "moved-from");
+    t1.clear();  // Clearing a moved-from table is allowed.
+  }
+  {
+    // Test that using a table (t3) that was moved-to from a moved-from table
+    // (t1) fails.
+    ABSL_ATTRIBUTE_UNUSED IntTable t1, t2, t3;
+    t1.insert(1);
+    t2 = std::move(t1);
+    // NOLINTNEXTLINE(bugprone-use-after-move)
+    t3 = std::move(t1);
+    EXPECT_DEATH_IF_SUPPORTED(t3.contains(1), "moved-from");
   }
 }
 
-TEST(Sanitizer, PoisoningOnErase) {
+TEST(Table, MaxValidSize) {
   IntTable t;
-  int64_t& v = *t.insert(0).first;
+  EXPECT_EQ(
+      MaxValidSize(sizeof(IntTable::key_type), sizeof(IntTable::value_type)),
+      t.max_size());
+  if constexpr (sizeof(size_t) == 8) {
+    for (size_t i = 0; i < 35; ++i) {
+      SCOPED_TRACE(i);
+      size_t slot_size = size_t{1} << i;
+      size_t key_size = slot_size;
+      size_t max_size = MaxValidSize(key_size, slot_size);
+      ASSERT_LT(max_size, uint64_t{1} << 60);
+      if (key_size <= 4) {
+        ASSERT_EQ(max_size, uint64_t{1} << 8 * key_size);
+      } else if (i <= 21) {
+        ASSERT_GE(max_size, uint64_t{1} << 40);
+      }
+      ASSERT_LE(max_size, uint64_t{1} << HashtableInlineData::kSizeBitCount);
+      ASSERT_LT(absl::uint128(max_size) * slot_size, uint64_t{1} << 63);
+    }
+  }
+  EXPECT_LT(MaxValidSize</*kSizeOfSizeT=*/4>(1, 1), 1 << 30);
+  EXPECT_LT(MaxValidSize</*kSizeOfSizeT=*/4>(2, 2), 1 << 29);
+  for (size_t i = 0; i < 29; ++i) {
+    SCOPED_TRACE(i);
+    size_t slot_size = size_t{1} << i;
+    size_t key_size = slot_size;
+    size_t max_size = MaxValidSize</*kSizeOfSizeT=*/4>(key_size, slot_size);
+    ASSERT_LT(max_size, 1 << 30);
+    size_t max_capacity = SizeToCapacity(max_size);
+    ASSERT_LT(uint64_t{max_capacity} * slot_size, size_t{1} << 31);
+    if (key_size < 4) {
+      ASSERT_EQ(max_size, uint64_t{1} << 8 * key_size);
+    } else {
+      ASSERT_GT(max_capacity, (1 << 29) / slot_size);
+    }
+    ASSERT_LT(max_capacity * slot_size, size_t{1} << 31);
+  }
+}
 
-  EXPECT_FALSE(__asan_address_is_poisoned(&v));
-  t.erase(0);
-  EXPECT_TRUE(__asan_address_is_poisoned(&v));
+// Tests that reserving enough space for more than the max number of unique keys
+// doesn't crash and we end up with kMaxValidCapacity.
+TEST(Table, MaxSizeOverflowUniqueKeys) {
+  absl::flat_hash_set<uint8_t> t8;
+  t8.reserve(1 << 9);
+  EXPECT_EQ(t8.capacity(), SizeToCapacity(t8.max_size()));
+  absl::flat_hash_set<uint16_t> t16;
+  t16.reserve(1 << 17);
+  EXPECT_EQ(t16.capacity(), SizeToCapacity(t16.max_size()));
+}
+
+// TODO(b/397453582): Remove support for const hasher and remove this test.
+TEST(Table, ConstLambdaHash) {
+  int64_t multiplier = 17;
+  // Make sure that code compiles and work OK with non-empty hasher with const
+  // qualifier.
+  const auto hash = [multiplier](SizedValue<64> value) -> size_t {
+    return static_cast<size_t>(static_cast<int64_t>(value) * multiplier);
+  };
+  static_assert(!std::is_empty_v<decltype(hash)>);
+  absl::flat_hash_set<SizedValue<64>, decltype(hash)> t(0, hash);
+  t.insert(1);
+  EXPECT_EQ(t.size(), 1);
+  EXPECT_EQ(t.find(1), t.begin());
+  EXPECT_EQ(t.find(2), t.end());
+  t.insert(2);
+  EXPECT_EQ(t.size(), 2);
+  EXPECT_NE(t.find(1), t.end());
+  EXPECT_NE(t.find(2), t.end());
+  EXPECT_EQ(t.find(3), t.end());
+}
+
+struct ZeroHash {
+  template <typename T>
+  size_t operator()(T) const {
+    return 0;
+  }
+};
+
+// This test is imitating growth of a very big table and triggers all buffer
+// overflows.
+// We try to insert all elements into the first probe group.
+// So the resize codepath in test does the following:
+// 1. Insert 16 elements into the first probe group. No other elements will be
+//    inserted into the first probe group.
+// 2. There will be enough elements to fill up the local buffer even for
+//    encoding with 4 bytes.
+// 3. After local buffer is full, we will fill up the control buffer till
+//    some point.
+// 4. Then a few times we will extend control buffer end.
+// 5. Finally we will catch up and go to overflow codepath.
+TEST(Table, GrowExtremelyLargeTable) {
+  // ProbedItem8Bytes causes OOMs on some platforms so we use ProbedItem4Bytes.
+  constexpr size_t kTargetCapacity =
+#if defined(__wasm__) || defined(__asmjs__) || defined(__i386__) || \
+    defined(_MSC_VER) || defined(ABSL_HAVE_THREAD_SANITIZER) ||     \
+    defined(ABSL_HAVE_MEMORY_SANITIZER) ||                          \
+    (!defined(__clang__) && defined(__GNUC__))
+      NextCapacity(ProbedItem4Bytes::kMaxNewCapacity);
+#else
+      NextCapacity(ProbedItem8Bytes::kMaxNewCapacity);
+#endif
+
+  absl::flat_hash_set<uint32_t, ZeroHash> t(63);
+  CommonFields& common = RawHashSetTestOnlyAccess::GetCommon(t);
+  // Set 0 seed so that H1 is always 0.
+  common.set_no_seed_for_testing();
+  ASSERT_EQ(H1(t.hash_function()(75)), 0);
+  uint8_t inserted_till = 210;
+  for (uint8_t i = 0; i < inserted_till; ++i) {
+    t.insert(i);
+  }
+  for (uint8_t i = 0; i < inserted_till; ++i) {
+    ASSERT_TRUE(t.contains(i));
+  }
+
+  for (size_t cap = t.capacity(); cap < kTargetCapacity;
+       cap = NextCapacity(cap)) {
+    ASSERT_EQ(t.capacity(), cap);
+    // Block upto 100 elements to test that kMarkedForSlowTransfer elements do
+    // not conflict with blocked elements.
+    for (size_t i = cap - 1,
+                growth_left = common.growth_info().GetGrowthLeftTotalSlow(cap),
+                blocked = 0;
+         i > cap / 2; --i) {
+      if (common.control()[i] == ctrl_t::kEmpty && growth_left > 1) {
+        growth_left--;
+        blocked++;
+        common.control()[i] = ctrl_t::kSentinel;
+        if (blocked > 100) break;
+      }
+    }
+    // Update growth info to force resize on the next insert. This way we avoid
+    // having to insert many elements.
+    common.growth_info().InitGrowthLeftNoDeleted(/*growth_left=*/0, cap);
+    t.insert(inserted_till++);
+    ASSERT_EQ(t.capacity(), NextCapacity(cap));
+    for (uint8_t i = 0; i < inserted_till; ++i) {
+      ASSERT_TRUE(t.contains(i));
+    }
+  }
+  EXPECT_EQ(t.capacity(), kTargetCapacity);
 }
-#endif  // ADDRESS_SANITIZER
 
 }  // namespace
 }  // namespace container_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/container/internal/test_allocator.h b/absl/container/internal/test_allocator.h
new file mode 100644
index 00000000000..8e365a3c5df
--- /dev/null
+++ b/absl/container/internal/test_allocator.h
@@ -0,0 +1,387 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_CONTAINER_INTERNAL_TEST_ALLOCATOR_H_
+#define ABSL_CONTAINER_INTERNAL_TEST_ALLOCATOR_H_
+
+#include <cassert>
+#include <cstddef>
+#include <cstdint>
+#include <memory>
+#include <type_traits>
+
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace container_internal {
+
+// This is a stateful allocator, but the state lives outside of the
+// allocator (in whatever test is using the allocator). This is odd
+// but helps in tests where the allocator is propagated into nested
+// containers - that chain of allocators uses the same state and is
+// thus easier to query for aggregate allocation information.
+template <typename T>
+class CountingAllocator {
+ public:
+  using Allocator = std::allocator<T>;
+  using AllocatorTraits = std::allocator_traits<Allocator>;
+  using value_type = typename AllocatorTraits::value_type;
+  using pointer = typename AllocatorTraits::pointer;
+  using const_pointer = typename AllocatorTraits::const_pointer;
+  using size_type = typename AllocatorTraits::size_type;
+  using difference_type = typename AllocatorTraits::difference_type;
+
+  CountingAllocator() = default;
+  explicit CountingAllocator(int64_t* bytes_used) : bytes_used_(bytes_used) {}
+  CountingAllocator(int64_t* bytes_used, int64_t* instance_count)
+      : bytes_used_(bytes_used), instance_count_(instance_count) {}
+
+  template <typename U>
+  CountingAllocator(const CountingAllocator<U>& x)
+      : bytes_used_(x.bytes_used_), instance_count_(x.instance_count_) {}
+
+  pointer allocate(
+      size_type n,
+      typename AllocatorTraits::const_void_pointer hint = nullptr) {
+    Allocator allocator;
+    pointer ptr = AllocatorTraits::allocate(allocator, n, hint);
+    if (bytes_used_ != nullptr) {
+      *bytes_used_ += n * sizeof(T);
+    }
+    return ptr;
+  }
+
+  void deallocate(pointer p, size_type n) {
+    Allocator allocator;
+    AllocatorTraits::deallocate(allocator, p, n);
+    if (bytes_used_ != nullptr) {
+      *bytes_used_ -= n * sizeof(T);
+    }
+  }
+
+  template <typename U, typename... Args>
+  void construct(U* p, Args&&... args) {
+    Allocator allocator;
+    AllocatorTraits::construct(allocator, p, std::forward<Args>(args)...);
+    if (instance_count_ != nullptr) {
+      *instance_count_ += 1;
+    }
+  }
+
+  template <typename U>
+  void destroy(U* p) {
+    Allocator allocator;
+    // Ignore GCC warning bug.
+#if ABSL_INTERNAL_HAVE_MIN_GNUC_VERSION(12, 0)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wuse-after-free"
+#endif
+    AllocatorTraits::destroy(allocator, p);
+#if ABSL_INTERNAL_HAVE_MIN_GNUC_VERSION(12, 0)
+#pragma GCC diagnostic pop
+#endif
+    if (instance_count_ != nullptr) {
+      *instance_count_ -= 1;
+    }
+  }
+
+  template <typename U>
+  class rebind {
+   public:
+    using other = CountingAllocator<U>;
+  };
+
+  friend bool operator==(const CountingAllocator& a,
+                         const CountingAllocator& b) {
+    return a.bytes_used_ == b.bytes_used_ &&
+           a.instance_count_ == b.instance_count_;
+  }
+
+  friend bool operator!=(const CountingAllocator& a,
+                         const CountingAllocator& b) {
+    return !(a == b);
+  }
+
+  int64_t* bytes_used_ = nullptr;
+  int64_t* instance_count_ = nullptr;
+};
+
+template <typename T>
+struct CopyAssignPropagatingCountingAlloc : public CountingAllocator<T> {
+  using propagate_on_container_copy_assignment = std::true_type;
+
+  using Base = CountingAllocator<T>;
+  using Base::Base;
+
+  template <typename U>
+  explicit CopyAssignPropagatingCountingAlloc(
+      const CopyAssignPropagatingCountingAlloc<U>& other)
+      : Base(other.bytes_used_, other.instance_count_) {}
+
+  template <typename U>
+  struct rebind {
+    using other = CopyAssignPropagatingCountingAlloc<U>;
+  };
+};
+
+template <typename T>
+struct MoveAssignPropagatingCountingAlloc : public CountingAllocator<T> {
+  using propagate_on_container_move_assignment = std::true_type;
+
+  using Base = CountingAllocator<T>;
+  using Base::Base;
+
+  template <typename U>
+  explicit MoveAssignPropagatingCountingAlloc(
+      const MoveAssignPropagatingCountingAlloc<U>& other)
+      : Base(other.bytes_used_, other.instance_count_) {}
+
+  template <typename U>
+  struct rebind {
+    using other = MoveAssignPropagatingCountingAlloc<U>;
+  };
+};
+
+template <typename T>
+struct SwapPropagatingCountingAlloc : public CountingAllocator<T> {
+  using propagate_on_container_swap = std::true_type;
+
+  using Base = CountingAllocator<T>;
+  using Base::Base;
+
+  template <typename U>
+  explicit SwapPropagatingCountingAlloc(
+      const SwapPropagatingCountingAlloc<U>& other)
+      : Base(other.bytes_used_, other.instance_count_) {}
+
+  template <typename U>
+  struct rebind {
+    using other = SwapPropagatingCountingAlloc<U>;
+  };
+};
+
+// Tries to allocate memory at the minimum alignment even when the default
+// allocator uses a higher alignment.
+template <typename T>
+struct MinimumAlignmentAlloc : std::allocator<T> {
+  MinimumAlignmentAlloc() = default;
+
+  template <typename U>
+  explicit MinimumAlignmentAlloc(const MinimumAlignmentAlloc<U>& /*other*/) {}
+
+  template <class U>
+  struct rebind {
+    using other = MinimumAlignmentAlloc<U>;
+  };
+
+  T* allocate(size_t n) {
+    T* ptr = std::allocator<T>::allocate(n + 1);
+    char* cptr = reinterpret_cast<char*>(ptr);
+    cptr += alignof(T);
+    return reinterpret_cast<T*>(cptr);
+  }
+
+  void deallocate(T* ptr, size_t n) {
+    char* cptr = reinterpret_cast<char*>(ptr);
+    cptr -= alignof(T);
+    std::allocator<T>::deallocate(reinterpret_cast<T*>(cptr), n + 1);
+  }
+};
+
+inline bool IsAssertEnabled() {
+  // Use an assert with side-effects to figure out if they are actually enabled.
+  bool assert_enabled = false;
+  assert([&]() {  // NOLINT
+    assert_enabled = true;
+    return true;
+  }());
+  return assert_enabled;
+}
+
+template <template <class Alloc> class Container>
+void TestCopyAssignAllocPropagation() {
+  int64_t bytes1 = 0, instances1 = 0, bytes2 = 0, instances2 = 0;
+  CopyAssignPropagatingCountingAlloc<int> allocator1(&bytes1, &instances1);
+  CopyAssignPropagatingCountingAlloc<int> allocator2(&bytes2, &instances2);
+
+  // Test propagating allocator_type.
+  {
+    Container<CopyAssignPropagatingCountingAlloc<int>> c1(allocator1);
+    Container<CopyAssignPropagatingCountingAlloc<int>> c2(allocator2);
+
+    for (int i = 0; i < 100; ++i) c1.insert(i);
+
+    EXPECT_NE(c2.get_allocator(), allocator1);
+    EXPECT_EQ(instances1, 100);
+    EXPECT_EQ(instances2, 0);
+
+    c2 = c1;
+
+    EXPECT_EQ(c2.get_allocator(), allocator1);
+    EXPECT_EQ(instances1, 200);
+    EXPECT_EQ(instances2, 0);
+  }
+  // Test non-propagating allocator_type with different allocators.
+  {
+    Container<CountingAllocator<int>> c1(allocator1), c2(allocator2);
+
+    for (int i = 0; i < 100; ++i) c1.insert(i);
+
+    EXPECT_EQ(c2.get_allocator(), allocator2);
+    EXPECT_EQ(instances1, 100);
+    EXPECT_EQ(instances2, 0);
+
+    c2 = c1;
+
+    EXPECT_EQ(c2.get_allocator(), allocator2);
+    EXPECT_EQ(instances1, 100);
+    EXPECT_EQ(instances2, 100);
+  }
+  EXPECT_EQ(bytes1, 0);
+  EXPECT_EQ(instances1, 0);
+  EXPECT_EQ(bytes2, 0);
+  EXPECT_EQ(instances2, 0);
+}
+
+template <template <class Alloc> class Container>
+void TestMoveAssignAllocPropagation() {
+  int64_t bytes1 = 0, instances1 = 0, bytes2 = 0, instances2 = 0;
+  MoveAssignPropagatingCountingAlloc<int> allocator1(&bytes1, &instances1);
+  MoveAssignPropagatingCountingAlloc<int> allocator2(&bytes2, &instances2);
+
+  // Test propagating allocator_type.
+  {
+    Container<MoveAssignPropagatingCountingAlloc<int>> c1(allocator1);
+    Container<MoveAssignPropagatingCountingAlloc<int>> c2(allocator2);
+
+    for (int i = 0; i < 100; ++i) c1.insert(i);
+
+    EXPECT_NE(c2.get_allocator(), allocator1);
+    EXPECT_EQ(instances1, 100);
+    EXPECT_EQ(instances2, 0);
+
+    c2 = std::move(c1);
+
+    EXPECT_EQ(c2.get_allocator(), allocator1);
+    EXPECT_EQ(instances1, 100);
+    EXPECT_EQ(instances2, 0);
+  }
+  // Test non-propagating allocator_type with equal allocators.
+  {
+    Container<CountingAllocator<int>> c1(allocator1), c2(allocator1);
+
+    for (int i = 0; i < 100; ++i) c1.insert(i);
+
+    EXPECT_EQ(c2.get_allocator(), allocator1);
+    EXPECT_EQ(instances1, 100);
+    EXPECT_EQ(instances2, 0);
+
+    c2 = std::move(c1);
+
+    EXPECT_EQ(c2.get_allocator(), allocator1);
+    EXPECT_EQ(instances1, 100);
+    EXPECT_EQ(instances2, 0);
+  }
+  // Test non-propagating allocator_type with different allocators.
+  {
+    Container<CountingAllocator<int>> c1(allocator1), c2(allocator2);
+
+    for (int i = 0; i < 100; ++i) c1.insert(i);
+
+    EXPECT_NE(c2.get_allocator(), allocator1);
+    EXPECT_EQ(instances1, 100);
+    EXPECT_EQ(instances2, 0);
+
+    c2 = std::move(c1);
+
+    EXPECT_EQ(c2.get_allocator(), allocator2);
+    EXPECT_LE(instances1, 100);  // The values in c1 may or may not have been
+                                 // destroyed at this point.
+    EXPECT_EQ(instances2, 100);
+  }
+  EXPECT_EQ(bytes1, 0);
+  EXPECT_EQ(instances1, 0);
+  EXPECT_EQ(bytes2, 0);
+  EXPECT_EQ(instances2, 0);
+}
+
+template <template <class Alloc> class Container>
+void TestSwapAllocPropagation() {
+  int64_t bytes1 = 0, instances1 = 0, bytes2 = 0, instances2 = 0;
+  SwapPropagatingCountingAlloc<int> allocator1(&bytes1, &instances1);
+  SwapPropagatingCountingAlloc<int> allocator2(&bytes2, &instances2);
+
+  // Test propagating allocator_type.
+  {
+    Container<SwapPropagatingCountingAlloc<int>> c1(allocator1), c2(allocator2);
+
+    for (int i = 0; i < 100; ++i) c1.insert(i);
+
+    EXPECT_NE(c2.get_allocator(), allocator1);
+    EXPECT_EQ(instances1, 100);
+    EXPECT_EQ(instances2, 0);
+
+    c2.swap(c1);
+
+    EXPECT_EQ(c2.get_allocator(), allocator1);
+    EXPECT_EQ(instances1, 100);
+    EXPECT_EQ(instances2, 0);
+  }
+  // Test non-propagating allocator_type with equal allocators.
+  {
+    Container<CountingAllocator<int>> c1(allocator1), c2(allocator1);
+
+    for (int i = 0; i < 100; ++i) c1.insert(i);
+
+    EXPECT_EQ(c2.get_allocator(), allocator1);
+    EXPECT_EQ(instances1, 100);
+    EXPECT_EQ(instances2, 0);
+
+    c2.swap(c1);
+
+    EXPECT_EQ(c2.get_allocator(), allocator1);
+    EXPECT_EQ(instances1, 100);
+    EXPECT_EQ(instances2, 0);
+  }
+  // Test non-propagating allocator_type with different allocators.
+  {
+    Container<CountingAllocator<int>> c1(allocator1), c2(allocator2);
+
+    for (int i = 0; i < 100; ++i) c1.insert(i);
+
+    EXPECT_NE(c1.get_allocator(), c2.get_allocator());
+    if (IsAssertEnabled()) {
+      EXPECT_DEATH_IF_SUPPORTED(c2.swap(c1), "");
+    }
+  }
+  EXPECT_EQ(bytes1, 0);
+  EXPECT_EQ(instances1, 0);
+  EXPECT_EQ(bytes2, 0);
+  EXPECT_EQ(instances2, 0);
+}
+
+template <template <class Alloc> class Container>
+void TestAllocPropagation() {
+  TestCopyAssignAllocPropagation<Container>();
+  TestMoveAssignAllocPropagation<Container>();
+  TestSwapAllocPropagation<Container>();
+}
+
+}  // namespace container_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_CONTAINER_INTERNAL_TEST_ALLOCATOR_H_
diff --git a/absl/container/internal/test_instance_tracker.cc b/absl/container/internal/test_instance_tracker.cc
index 5a66cb4d14d..f9947f0475d 100644
--- a/absl/container/internal/test_instance_tracker.cc
+++ b/absl/container/internal/test_instance_tracker.cc
@@ -15,6 +15,7 @@
 #include "absl/container/internal/test_instance_tracker.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace test_internal {
 int BaseCountedInstance::num_instances_ = 0;
 int BaseCountedInstance::num_live_instances_ = 0;
@@ -24,4 +25,5 @@ int BaseCountedInstance::num_swaps_ = 0;
 int BaseCountedInstance::num_comparisons_ = 0;
 
 }  // namespace test_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/container/internal/test_instance_tracker.h b/absl/container/internal/test_instance_tracker.h
index c4731dbe3d1..5ff6fd714e2 100644
--- a/absl/container/internal/test_instance_tracker.h
+++ b/absl/container/internal/test_instance_tracker.h
@@ -21,6 +21,7 @@
 #include "absl/types/compare.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace test_internal {
 
 // A type that counts number of occurrences of the type, the live occurrences of
@@ -267,6 +268,7 @@ class MovableOnlyInstance : public BaseCountedInstance {
 };
 
 }  // namespace test_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_INTERNAL_TEST_INSTANCE_TRACKER_H_
diff --git a/absl/container/internal/tracked.h b/absl/container/internal/tracked.h
index 75173ab00aa..6962eb86e9c 100644
--- a/absl/container/internal/tracked.h
+++ b/absl/container/internal/tracked.h
@@ -16,10 +16,14 @@
 #define ABSL_CONTAINER_INTERNAL_TRACKED_H_
 
 #include <stddef.h>
+
 #include <memory>
 #include <utility>
 
+#include "absl/base/config.h"
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
 // A class that tracks its copies and moves so that it can be queried in tests.
@@ -46,12 +50,14 @@ class Tracked {
     num_moves_ = that.num_moves_;
     num_copies_ = that.num_copies_;
     ++(*num_copies_);
+    return *this;
   }
   Tracked& operator=(Tracked&& that) {
     val_ = std::move(that.val_);
     num_moves_ = std::move(that.num_moves_);
     num_copies_ = std::move(that.num_copies_);
     ++(*num_moves_);
+    return *this;
   }
 
   const T& val() const { return val_; }
@@ -63,8 +69,8 @@ class Tracked {
     return !(a == b);
   }
 
-  size_t num_copies() { return *num_copies_; }
-  size_t num_moves() { return *num_moves_; }
+  size_t num_copies() const { return *num_copies_; }
+  size_t num_moves() const { return *num_moves_; }
 
  private:
   T val_;
@@ -73,6 +79,7 @@ class Tracked {
 };
 
 }  // namespace container_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_INTERNAL_TRACKED_H_
diff --git a/absl/container/internal/unordered_map_constructor_test.h b/absl/container/internal/unordered_map_constructor_test.h
index 68817e4e4cc..1076aea7a6c 100644
--- a/absl/container/internal/unordered_map_constructor_test.h
+++ b/absl/container/internal/unordered_map_constructor_test.h
@@ -16,14 +16,17 @@
 #define ABSL_CONTAINER_INTERNAL_UNORDERED_MAP_CONSTRUCTOR_TEST_H_
 
 #include <algorithm>
+#include <unordered_map>
 #include <vector>
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
+#include "absl/base/config.h"
 #include "absl/container/internal/hash_generator_testing.h"
 #include "absl/container/internal/hash_policy_testing.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
 template <class UnordMap>
@@ -83,28 +86,8 @@ TYPED_TEST_P(ConstructorTest, BucketCountHashEqualAlloc) {
   EXPECT_GE(m.bucket_count(), 123);
 }
 
-template <typename T>
-struct is_std_unordered_map : std::false_type {};
-
-template <typename... T>
-struct is_std_unordered_map<std::unordered_map<T...>> : std::true_type {};
-
-#if defined(UNORDERED_MAP_CXX14) || defined(UNORDERED_MAP_CXX17)
-using has_cxx14_std_apis = std::true_type;
-#else
-using has_cxx14_std_apis = std::false_type;
-#endif
-
-template <typename T>
-using expect_cxx14_apis =
-    absl::disjunction<absl::negation<is_std_unordered_map<T>>,
-                      has_cxx14_std_apis>;
-
 template <typename TypeParam>
-void BucketCountAllocTest(std::false_type) {}
-
-template <typename TypeParam>
-void BucketCountAllocTest(std::true_type) {
+void BucketCountAllocTest() {
   using A = typename TypeParam::allocator_type;
   A alloc(0);
   TypeParam m(123, alloc);
@@ -115,14 +98,11 @@ void BucketCountAllocTest(std::true_type) {
 }
 
 TYPED_TEST_P(ConstructorTest, BucketCountAlloc) {
-  BucketCountAllocTest<TypeParam>(expect_cxx14_apis<TypeParam>());
+  BucketCountAllocTest<TypeParam>();
 }
 
 template <typename TypeParam>
-void BucketCountHashAllocTest(std::false_type) {}
-
-template <typename TypeParam>
-void BucketCountHashAllocTest(std::true_type) {
+void BucketCountHashAllocTest() {
   using H = typename TypeParam::hasher;
   using A = typename TypeParam::allocator_type;
   H hasher;
@@ -136,25 +116,11 @@ void BucketCountHashAllocTest(std::true_type) {
 }
 
 TYPED_TEST_P(ConstructorTest, BucketCountHashAlloc) {
-  BucketCountHashAllocTest<TypeParam>(expect_cxx14_apis<TypeParam>());
+  BucketCountHashAllocTest<TypeParam>();
 }
 
-#if ABSL_UNORDERED_SUPPORTS_ALLOC_CTORS
-using has_alloc_std_constructors = std::true_type;
-#else
-using has_alloc_std_constructors = std::false_type;
-#endif
-
-template <typename T>
-using expect_alloc_constructors =
-    absl::disjunction<absl::negation<is_std_unordered_map<T>>,
-                      has_alloc_std_constructors>;
-
 template <typename TypeParam>
-void AllocTest(std::false_type) {}
-
-template <typename TypeParam>
-void AllocTest(std::true_type) {
+void AllocTest() {
   using A = typename TypeParam::allocator_type;
   A alloc(0);
   TypeParam m(alloc);
@@ -163,12 +129,10 @@ void AllocTest(std::true_type) {
   EXPECT_THAT(m, ::testing::UnorderedElementsAre());
 }
 
-TYPED_TEST_P(ConstructorTest, Alloc) {
-  AllocTest<TypeParam>(expect_alloc_constructors<TypeParam>());
-}
+TYPED_TEST_P(ConstructorTest, Alloc) { AllocTest<TypeParam>(); }
 
 TYPED_TEST_P(ConstructorTest, InputIteratorBucketHashEqualAlloc) {
-  using T = hash_internal::GeneratedType<TypeParam>;
+  using T = GeneratedType<TypeParam>;
   using H = typename TypeParam::hasher;
   using E = typename TypeParam::key_equal;
   using A = typename TypeParam::allocator_type;
@@ -176,8 +140,7 @@ TYPED_TEST_P(ConstructorTest, InputIteratorBucketHashEqualAlloc) {
   E equal;
   A alloc(0);
   std::vector<T> values;
-  std::generate_n(std::back_inserter(values), 10,
-                  hash_internal::Generator<T>());
+  std::generate_n(std::back_inserter(values), 10, UniqueGenerator<T>());
   TypeParam m(values.begin(), values.end(), 123, hasher, equal, alloc);
   EXPECT_EQ(m.hash_function(), hasher);
   EXPECT_EQ(m.key_eq(), equal);
@@ -187,16 +150,12 @@ TYPED_TEST_P(ConstructorTest, InputIteratorBucketHashEqualAlloc) {
 }
 
 template <typename TypeParam>
-void InputIteratorBucketAllocTest(std::false_type) {}
-
-template <typename TypeParam>
-void InputIteratorBucketAllocTest(std::true_type) {
-  using T = hash_internal::GeneratedType<TypeParam>;
+void InputIteratorBucketAllocTest() {
+  using T = GeneratedType<TypeParam>;
   using A = typename TypeParam::allocator_type;
   A alloc(0);
   std::vector<T> values;
-  std::generate_n(std::back_inserter(values), 10,
-                  hash_internal::Generator<T>());
+  std::generate_n(std::back_inserter(values), 10, UniqueGenerator<T>());
   TypeParam m(values.begin(), values.end(), 123, alloc);
   EXPECT_EQ(m.get_allocator(), alloc);
   EXPECT_THAT(items(m), ::testing::UnorderedElementsAreArray(values));
@@ -204,22 +163,18 @@ void InputIteratorBucketAllocTest(std::true_type) {
 }
 
 TYPED_TEST_P(ConstructorTest, InputIteratorBucketAlloc) {
-  InputIteratorBucketAllocTest<TypeParam>(expect_cxx14_apis<TypeParam>());
+  InputIteratorBucketAllocTest<TypeParam>();
 }
 
 template <typename TypeParam>
-void InputIteratorBucketHashAllocTest(std::false_type) {}
-
-template <typename TypeParam>
-void InputIteratorBucketHashAllocTest(std::true_type) {
-  using T = hash_internal::GeneratedType<TypeParam>;
+void InputIteratorBucketHashAllocTest() {
+  using T = GeneratedType<TypeParam>;
   using H = typename TypeParam::hasher;
   using A = typename TypeParam::allocator_type;
   H hasher;
   A alloc(0);
   std::vector<T> values;
-  std::generate_n(std::back_inserter(values), 10,
-                  hash_internal::Generator<T>());
+  std::generate_n(std::back_inserter(values), 10, UniqueGenerator<T>());
   TypeParam m(values.begin(), values.end(), 123, hasher, alloc);
   EXPECT_EQ(m.hash_function(), hasher);
   EXPECT_EQ(m.get_allocator(), alloc);
@@ -228,19 +183,20 @@ void InputIteratorBucketHashAllocTest(std::true_type) {
 }
 
 TYPED_TEST_P(ConstructorTest, InputIteratorBucketHashAlloc) {
-  InputIteratorBucketHashAllocTest<TypeParam>(expect_cxx14_apis<TypeParam>());
+  InputIteratorBucketHashAllocTest<TypeParam>();
 }
 
 TYPED_TEST_P(ConstructorTest, CopyConstructor) {
-  using T = hash_internal::GeneratedType<TypeParam>;
+  using T = GeneratedType<TypeParam>;
   using H = typename TypeParam::hasher;
   using E = typename TypeParam::key_equal;
   using A = typename TypeParam::allocator_type;
   H hasher;
   E equal;
   A alloc(0);
+  UniqueGenerator<T> gen;
   TypeParam m(123, hasher, equal, alloc);
-  for (size_t i = 0; i != 10; ++i) m.insert(hash_internal::Generator<T>()());
+  for (size_t i = 0; i != 10; ++i) m.insert(gen());
   TypeParam n(m);
   EXPECT_EQ(m.hash_function(), n.hash_function());
   EXPECT_EQ(m.key_eq(), n.key_eq());
@@ -249,19 +205,17 @@ TYPED_TEST_P(ConstructorTest, CopyConstructor) {
 }
 
 template <typename TypeParam>
-void CopyConstructorAllocTest(std::false_type) {}
-
-template <typename TypeParam>
-void CopyConstructorAllocTest(std::true_type) {
-  using T = hash_internal::GeneratedType<TypeParam>;
+void CopyConstructorAllocTest() {
+  using T = GeneratedType<TypeParam>;
   using H = typename TypeParam::hasher;
   using E = typename TypeParam::key_equal;
   using A = typename TypeParam::allocator_type;
   H hasher;
   E equal;
   A alloc(0);
+  UniqueGenerator<T> gen;
   TypeParam m(123, hasher, equal, alloc);
-  for (size_t i = 0; i != 10; ++i) m.insert(hash_internal::Generator<T>()());
+  for (size_t i = 0; i != 10; ++i) m.insert(gen());
   TypeParam n(m, A(11));
   EXPECT_EQ(m.hash_function(), n.hash_function());
   EXPECT_EQ(m.key_eq(), n.key_eq());
@@ -270,21 +224,22 @@ void CopyConstructorAllocTest(std::true_type) {
 }
 
 TYPED_TEST_P(ConstructorTest, CopyConstructorAlloc) {
-  CopyConstructorAllocTest<TypeParam>(expect_alloc_constructors<TypeParam>());
+  CopyConstructorAllocTest<TypeParam>();
 }
 
 // TODO(alkis): Test non-propagating allocators on copy constructors.
 
 TYPED_TEST_P(ConstructorTest, MoveConstructor) {
-  using T = hash_internal::GeneratedType<TypeParam>;
+  using T = GeneratedType<TypeParam>;
   using H = typename TypeParam::hasher;
   using E = typename TypeParam::key_equal;
   using A = typename TypeParam::allocator_type;
   H hasher;
   E equal;
   A alloc(0);
+  UniqueGenerator<T> gen;
   TypeParam m(123, hasher, equal, alloc);
-  for (size_t i = 0; i != 10; ++i) m.insert(hash_internal::Generator<T>()());
+  for (size_t i = 0; i != 10; ++i) m.insert(gen());
   TypeParam t(m);
   TypeParam n(std::move(t));
   EXPECT_EQ(m.hash_function(), n.hash_function());
@@ -294,19 +249,17 @@ TYPED_TEST_P(ConstructorTest, MoveConstructor) {
 }
 
 template <typename TypeParam>
-void MoveConstructorAllocTest(std::false_type) {}
-
-template <typename TypeParam>
-void MoveConstructorAllocTest(std::true_type) {
-  using T = hash_internal::GeneratedType<TypeParam>;
+void MoveConstructorAllocTest() {
+  using T = GeneratedType<TypeParam>;
   using H = typename TypeParam::hasher;
   using E = typename TypeParam::key_equal;
   using A = typename TypeParam::allocator_type;
   H hasher;
   E equal;
   A alloc(0);
+  UniqueGenerator<T> gen;
   TypeParam m(123, hasher, equal, alloc);
-  for (size_t i = 0; i != 10; ++i) m.insert(hash_internal::Generator<T>()());
+  for (size_t i = 0; i != 10; ++i) m.insert(gen());
   TypeParam t(m);
   TypeParam n(std::move(t), A(1));
   EXPECT_EQ(m.hash_function(), n.hash_function());
@@ -316,14 +269,14 @@ void MoveConstructorAllocTest(std::true_type) {
 }
 
 TYPED_TEST_P(ConstructorTest, MoveConstructorAlloc) {
-  MoveConstructorAllocTest<TypeParam>(expect_alloc_constructors<TypeParam>());
+  MoveConstructorAllocTest<TypeParam>();
 }
 
 // TODO(alkis): Test non-propagating allocators on move constructors.
 
 TYPED_TEST_P(ConstructorTest, InitializerListBucketHashEqualAlloc) {
-  using T = hash_internal::GeneratedType<TypeParam>;
-  hash_internal::Generator<T> gen;
+  using T = GeneratedType<TypeParam>;
+  UniqueGenerator<T> gen;
   std::initializer_list<T> values = {gen(), gen(), gen(), gen(), gen()};
   using H = typename TypeParam::hasher;
   using E = typename TypeParam::key_equal;
@@ -340,13 +293,10 @@ TYPED_TEST_P(ConstructorTest, InitializerListBucketHashEqualAlloc) {
 }
 
 template <typename TypeParam>
-void InitializerListBucketAllocTest(std::false_type) {}
-
-template <typename TypeParam>
-void InitializerListBucketAllocTest(std::true_type) {
-  using T = hash_internal::GeneratedType<TypeParam>;
+void InitializerListBucketAllocTest() {
+  using T = GeneratedType<TypeParam>;
   using A = typename TypeParam::allocator_type;
-  hash_internal::Generator<T> gen;
+  UniqueGenerator<T> gen;
   std::initializer_list<T> values = {gen(), gen(), gen(), gen(), gen()};
   A alloc(0);
   TypeParam m(values, 123, alloc);
@@ -356,20 +306,17 @@ void InitializerListBucketAllocTest(std::true_type) {
 }
 
 TYPED_TEST_P(ConstructorTest, InitializerListBucketAlloc) {
-  InitializerListBucketAllocTest<TypeParam>(expect_cxx14_apis<TypeParam>());
+  InitializerListBucketAllocTest<TypeParam>();
 }
 
 template <typename TypeParam>
-void InitializerListBucketHashAllocTest(std::false_type) {}
-
-template <typename TypeParam>
-void InitializerListBucketHashAllocTest(std::true_type) {
-  using T = hash_internal::GeneratedType<TypeParam>;
+void InitializerListBucketHashAllocTest() {
+  using T = GeneratedType<TypeParam>;
   using H = typename TypeParam::hasher;
   using A = typename TypeParam::allocator_type;
   H hasher;
   A alloc(0);
-  hash_internal::Generator<T> gen;
+  UniqueGenerator<T> gen;
   std::initializer_list<T> values = {gen(), gen(), gen(), gen(), gen()};
   TypeParam m(values, 123, hasher, alloc);
   EXPECT_EQ(m.hash_function(), hasher);
@@ -379,18 +326,18 @@ void InitializerListBucketHashAllocTest(std::true_type) {
 }
 
 TYPED_TEST_P(ConstructorTest, InitializerListBucketHashAlloc) {
-  InitializerListBucketHashAllocTest<TypeParam>(expect_cxx14_apis<TypeParam>());
+  InitializerListBucketHashAllocTest<TypeParam>();
 }
 
 TYPED_TEST_P(ConstructorTest, Assignment) {
-  using T = hash_internal::GeneratedType<TypeParam>;
+  using T = GeneratedType<TypeParam>;
   using H = typename TypeParam::hasher;
   using E = typename TypeParam::key_equal;
   using A = typename TypeParam::allocator_type;
   H hasher;
   E equal;
   A alloc(0);
-  hash_internal::Generator<T> gen;
+  UniqueGenerator<T> gen;
   TypeParam m({gen(), gen(), gen()}, 123, hasher, equal, alloc);
   TypeParam n;
   n = m;
@@ -403,14 +350,14 @@ TYPED_TEST_P(ConstructorTest, Assignment) {
 // (it depends on traits).
 
 TYPED_TEST_P(ConstructorTest, MoveAssignment) {
-  using T = hash_internal::GeneratedType<TypeParam>;
+  using T = GeneratedType<TypeParam>;
   using H = typename TypeParam::hasher;
   using E = typename TypeParam::key_equal;
   using A = typename TypeParam::allocator_type;
   H hasher;
   E equal;
   A alloc(0);
-  hash_internal::Generator<T> gen;
+  UniqueGenerator<T> gen;
   TypeParam m({gen(), gen(), gen()}, 123, hasher, equal, alloc);
   TypeParam t(m);
   TypeParam n;
@@ -421,8 +368,8 @@ TYPED_TEST_P(ConstructorTest, MoveAssignment) {
 }
 
 TYPED_TEST_P(ConstructorTest, AssignmentFromInitializerList) {
-  using T = hash_internal::GeneratedType<TypeParam>;
-  hash_internal::Generator<T> gen;
+  using T = GeneratedType<TypeParam>;
+  UniqueGenerator<T> gen;
   std::initializer_list<T> values = {gen(), gen(), gen(), gen(), gen()};
   TypeParam m;
   m = values;
@@ -430,8 +377,8 @@ TYPED_TEST_P(ConstructorTest, AssignmentFromInitializerList) {
 }
 
 TYPED_TEST_P(ConstructorTest, AssignmentOverwritesExisting) {
-  using T = hash_internal::GeneratedType<TypeParam>;
-  hash_internal::Generator<T> gen;
+  using T = GeneratedType<TypeParam>;
+  UniqueGenerator<T> gen;
   TypeParam m({gen(), gen(), gen()});
   TypeParam n({gen()});
   n = m;
@@ -439,8 +386,8 @@ TYPED_TEST_P(ConstructorTest, AssignmentOverwritesExisting) {
 }
 
 TYPED_TEST_P(ConstructorTest, MoveAssignmentOverwritesExisting) {
-  using T = hash_internal::GeneratedType<TypeParam>;
-  hash_internal::Generator<T> gen;
+  using T = GeneratedType<TypeParam>;
+  UniqueGenerator<T> gen;
   TypeParam m({gen(), gen(), gen()});
   TypeParam t(m);
   TypeParam n({gen()});
@@ -449,8 +396,8 @@ TYPED_TEST_P(ConstructorTest, MoveAssignmentOverwritesExisting) {
 }
 
 TYPED_TEST_P(ConstructorTest, AssignmentFromInitializerListOverwritesExisting) {
-  using T = hash_internal::GeneratedType<TypeParam>;
-  hash_internal::Generator<T> gen;
+  using T = GeneratedType<TypeParam>;
+  UniqueGenerator<T> gen;
   std::initializer_list<T> values = {gen(), gen(), gen(), gen(), gen()};
   TypeParam m;
   m = values;
@@ -458,8 +405,8 @@ TYPED_TEST_P(ConstructorTest, AssignmentFromInitializerListOverwritesExisting) {
 }
 
 TYPED_TEST_P(ConstructorTest, AssignmentOnSelf) {
-  using T = hash_internal::GeneratedType<TypeParam>;
-  hash_internal::Generator<T> gen;
+  using T = GeneratedType<TypeParam>;
+  UniqueGenerator<T> gen;
   std::initializer_list<T> values = {gen(), gen(), gen(), gen(), gen()};
   TypeParam m(values);
   m = *&m;  // Avoid -Wself-assign
@@ -470,7 +417,7 @@ TYPED_TEST_P(ConstructorTest, AssignmentOnSelf) {
 // containers in unspecified state (and in practice in causes memory-leak
 // according to heap-checker!).
 
-REGISTER_TYPED_TEST_CASE_P(
+REGISTER_TYPED_TEST_SUITE_P(
     ConstructorTest, NoArgs, BucketCount, BucketCountHash, BucketCountHashEqual,
     BucketCountHashEqualAlloc, BucketCountAlloc, BucketCountHashAlloc, Alloc,
     InputIteratorBucketHashEqualAlloc, InputIteratorBucketAlloc,
@@ -482,6 +429,7 @@ REGISTER_TYPED_TEST_CASE_P(
     AssignmentFromInitializerListOverwritesExisting, AssignmentOnSelf);
 
 }  // namespace container_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_INTERNAL_UNORDERED_MAP_CONSTRUCTOR_TEST_H_
diff --git a/absl/container/internal/unordered_map_lookup_test.h b/absl/container/internal/unordered_map_lookup_test.h
index ebd3612bdf3..ba037c0563f 100644
--- a/absl/container/internal/unordered_map_lookup_test.h
+++ b/absl/container/internal/unordered_map_lookup_test.h
@@ -21,6 +21,7 @@
 #include "absl/container/internal/hash_policy_testing.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
 template <class UnordMap>
@@ -29,10 +30,9 @@ class LookupTest : public ::testing::Test {};
 TYPED_TEST_SUITE_P(LookupTest);
 
 TYPED_TEST_P(LookupTest, At) {
-  using T = hash_internal::GeneratedType<TypeParam>;
+  using T = GeneratedType<TypeParam>;
   std::vector<T> values;
-  std::generate_n(std::back_inserter(values), 10,
-                  hash_internal::Generator<T>());
+  std::generate_n(std::back_inserter(values), 10, Generator<T>());
   TypeParam m(values.begin(), values.end());
   for (const auto& p : values) {
     const auto& val = m.at(p.first);
@@ -41,11 +41,10 @@ TYPED_TEST_P(LookupTest, At) {
 }
 
 TYPED_TEST_P(LookupTest, OperatorBracket) {
-  using T = hash_internal::GeneratedType<TypeParam>;
+  using T = GeneratedType<TypeParam>;
   using V = typename TypeParam::mapped_type;
   std::vector<T> values;
-  std::generate_n(std::back_inserter(values), 10,
-                  hash_internal::Generator<T>());
+  std::generate_n(std::back_inserter(values), 10, Generator<T>());
   TypeParam m;
   for (const auto& p : values) {
     auto& val = m[p.first];
@@ -57,10 +56,9 @@ TYPED_TEST_P(LookupTest, OperatorBracket) {
 }
 
 TYPED_TEST_P(LookupTest, Count) {
-  using T = hash_internal::GeneratedType<TypeParam>;
+  using T = GeneratedType<TypeParam>;
   std::vector<T> values;
-  std::generate_n(std::back_inserter(values), 10,
-                  hash_internal::Generator<T>());
+  std::generate_n(std::back_inserter(values), 10, Generator<T>());
   TypeParam m;
   for (const auto& p : values)
     EXPECT_EQ(0, m.count(p.first)) << ::testing::PrintToString(p.first);
@@ -71,10 +69,9 @@ TYPED_TEST_P(LookupTest, Count) {
 
 TYPED_TEST_P(LookupTest, Find) {
   using std::get;
-  using T = hash_internal::GeneratedType<TypeParam>;
+  using T = GeneratedType<TypeParam>;
   std::vector<T> values;
-  std::generate_n(std::back_inserter(values), 10,
-                  hash_internal::Generator<T>());
+  std::generate_n(std::back_inserter(values), 10, Generator<T>());
   TypeParam m;
   for (const auto& p : values)
     EXPECT_TRUE(m.end() == m.find(p.first))
@@ -89,10 +86,9 @@ TYPED_TEST_P(LookupTest, Find) {
 
 TYPED_TEST_P(LookupTest, EqualRange) {
   using std::get;
-  using T = hash_internal::GeneratedType<TypeParam>;
+  using T = GeneratedType<TypeParam>;
   std::vector<T> values;
-  std::generate_n(std::back_inserter(values), 10,
-                  hash_internal::Generator<T>());
+  std::generate_n(std::back_inserter(values), 10, Generator<T>());
   TypeParam m;
   for (const auto& p : values) {
     auto r = m.equal_range(p.first);
@@ -106,10 +102,11 @@ TYPED_TEST_P(LookupTest, EqualRange) {
   }
 }
 
-REGISTER_TYPED_TEST_CASE_P(LookupTest, At, OperatorBracket, Count, Find,
-                           EqualRange);
+REGISTER_TYPED_TEST_SUITE_P(LookupTest, At, OperatorBracket, Count, Find,
+                            EqualRange);
 
 }  // namespace container_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_INTERNAL_UNORDERED_MAP_LOOKUP_TEST_H_
diff --git a/absl/container/internal/unordered_map_members_test.h b/absl/container/internal/unordered_map_members_test.h
index 1bf31ab4095..e9f4979512a 100644
--- a/absl/container/internal/unordered_map_members_test.h
+++ b/absl/container/internal/unordered_map_members_test.h
@@ -21,6 +21,7 @@
 #include "absl/meta/type_traits.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
 template <class UnordMap>
@@ -35,10 +36,10 @@ TYPED_TEST_P(MembersTest, Typedefs) {
   EXPECT_TRUE((std::is_same<std::pair<const typename TypeParam::key_type,
                                       typename TypeParam::mapped_type>,
                             typename TypeParam::value_type>()));
-  EXPECT_TRUE((absl::conjunction<
-               absl::negation<std::is_signed<typename TypeParam::size_type>>,
+  EXPECT_TRUE((std::conjunction<
+               std::negation<std::is_signed<typename TypeParam::size_type>>,
                std::is_integral<typename TypeParam::size_type>>()));
-  EXPECT_TRUE((absl::conjunction<
+  EXPECT_TRUE((std::conjunction<
                std::is_signed<typename TypeParam::difference_type>,
                std::is_integral<typename TypeParam::difference_type>>()));
   EXPECT_TRUE((std::is_convertible<
@@ -80,6 +81,7 @@ TYPED_TEST_P(MembersTest, BeginEnd) {
 REGISTER_TYPED_TEST_SUITE_P(MembersTest, Typedefs, SimpleFunctions, BeginEnd);
 
 }  // namespace container_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_INTERNAL_UNORDERED_MAP_MEMBERS_TEST_H_
diff --git a/absl/container/internal/unordered_map_modifiers_test.h b/absl/container/internal/unordered_map_modifiers_test.h
index f6aff5422b4..bb41e07ba76 100644
--- a/absl/container/internal/unordered_map_modifiers_test.h
+++ b/absl/container/internal/unordered_map_modifiers_test.h
@@ -16,6 +16,7 @@
 #define ABSL_CONTAINER_INTERNAL_UNORDERED_MAP_MODIFIERS_TEST_H_
 
 #include <memory>
+#include <type_traits>
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
@@ -23,6 +24,7 @@
 #include "absl/container/internal/hash_policy_testing.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
 template <class UnordMap>
@@ -31,10 +33,9 @@ class ModifiersTest : public ::testing::Test {};
 TYPED_TEST_SUITE_P(ModifiersTest);
 
 TYPED_TEST_P(ModifiersTest, Clear) {
-  using T = hash_internal::GeneratedType<TypeParam>;
+  using T = GeneratedType<TypeParam>;
   std::vector<T> values;
-  std::generate_n(std::back_inserter(values), 10,
-                  hash_internal::Generator<T>());
+  std::generate_n(std::back_inserter(values), 10, Generator<T>());
   TypeParam m(values.begin(), values.end());
   ASSERT_THAT(items(m), ::testing::UnorderedElementsAreArray(values));
   m.clear();
@@ -43,148 +44,170 @@ TYPED_TEST_P(ModifiersTest, Clear) {
 }
 
 TYPED_TEST_P(ModifiersTest, Insert) {
-  using T = hash_internal::GeneratedType<TypeParam>;
+  using T = GeneratedType<TypeParam>;
   using V = typename TypeParam::mapped_type;
-  T val = hash_internal::Generator<T>()();
+  T val = Generator<T>()();
   TypeParam m;
   auto p = m.insert(val);
   EXPECT_TRUE(p.second);
   EXPECT_EQ(val, *p.first);
-  T val2 = {val.first, hash_internal::Generator<V>()()};
+  T val2 = {val.first, Generator<V>()()};
   p = m.insert(val2);
   EXPECT_FALSE(p.second);
   EXPECT_EQ(val, *p.first);
 }
 
 TYPED_TEST_P(ModifiersTest, InsertHint) {
-  using T = hash_internal::GeneratedType<TypeParam>;
+  using T = GeneratedType<TypeParam>;
   using V = typename TypeParam::mapped_type;
-  T val = hash_internal::Generator<T>()();
+  T val = Generator<T>()();
   TypeParam m;
   auto it = m.insert(m.end(), val);
   EXPECT_TRUE(it != m.end());
   EXPECT_EQ(val, *it);
-  T val2 = {val.first, hash_internal::Generator<V>()()};
+  T val2 = {val.first, Generator<V>()()};
   it = m.insert(it, val2);
   EXPECT_TRUE(it != m.end());
   EXPECT_EQ(val, *it);
 }
 
 TYPED_TEST_P(ModifiersTest, InsertRange) {
-  using T = hash_internal::GeneratedType<TypeParam>;
+  using T = GeneratedType<TypeParam>;
   std::vector<T> values;
-  std::generate_n(std::back_inserter(values), 10,
-                  hash_internal::Generator<T>());
+  std::generate_n(std::back_inserter(values), 10, Generator<T>());
   TypeParam m;
   m.insert(values.begin(), values.end());
   ASSERT_THAT(items(m), ::testing::UnorderedElementsAreArray(values));
 }
 
+TYPED_TEST_P(ModifiersTest, InsertWithinCapacity) {
+  using T = GeneratedType<TypeParam>;
+  using V = typename TypeParam::mapped_type;
+  T val = Generator<T>()();
+  TypeParam m;
+  m.reserve(10);
+  const size_t original_capacity = m.bucket_count();
+  m.insert(val);
+  EXPECT_EQ(m.bucket_count(), original_capacity);
+  T val2 = {val.first, Generator<V>()()};
+  m.insert(val2);
+  EXPECT_EQ(m.bucket_count(), original_capacity);
+}
+
+TYPED_TEST_P(ModifiersTest, InsertRangeWithinCapacity) {
+#if !defined(__GLIBCXX__)
+  using T = GeneratedType<TypeParam>;
+  std::vector<T> base_values;
+  std::generate_n(std::back_inserter(base_values), 10, Generator<T>());
+  std::vector<T> values;
+  while (values.size() != 100) {
+    std::copy_n(base_values.begin(), 10, std::back_inserter(values));
+  }
+  TypeParam m;
+  m.reserve(10);
+  const size_t original_capacity = m.bucket_count();
+  m.insert(values.begin(), values.end());
+  EXPECT_EQ(m.bucket_count(), original_capacity);
+#endif
+}
+
 TYPED_TEST_P(ModifiersTest, InsertOrAssign) {
-#ifdef UNORDERED_MAP_CXX17
   using std::get;
   using K = typename TypeParam::key_type;
   using V = typename TypeParam::mapped_type;
-  K k = hash_internal::Generator<K>()();
-  V val = hash_internal::Generator<V>()();
+  K k = Generator<K>()();
+  V val = Generator<V>()();
   TypeParam m;
   auto p = m.insert_or_assign(k, val);
   EXPECT_TRUE(p.second);
   EXPECT_EQ(k, get<0>(*p.first));
   EXPECT_EQ(val, get<1>(*p.first));
-  V val2 = hash_internal::Generator<V>()();
+  V val2 = Generator<V>()();
   p = m.insert_or_assign(k, val2);
   EXPECT_FALSE(p.second);
   EXPECT_EQ(k, get<0>(*p.first));
   EXPECT_EQ(val2, get<1>(*p.first));
-#endif
 }
 
 TYPED_TEST_P(ModifiersTest, InsertOrAssignHint) {
-#ifdef UNORDERED_MAP_CXX17
   using std::get;
   using K = typename TypeParam::key_type;
   using V = typename TypeParam::mapped_type;
-  K k = hash_internal::Generator<K>()();
-  V val = hash_internal::Generator<V>()();
+  K k = Generator<K>()();
+  V val = Generator<V>()();
   TypeParam m;
   auto it = m.insert_or_assign(m.end(), k, val);
   EXPECT_TRUE(it != m.end());
   EXPECT_EQ(k, get<0>(*it));
   EXPECT_EQ(val, get<1>(*it));
-  V val2 = hash_internal::Generator<V>()();
+  V val2 = Generator<V>()();
   it = m.insert_or_assign(it, k, val2);
   EXPECT_EQ(k, get<0>(*it));
   EXPECT_EQ(val2, get<1>(*it));
-#endif
 }
 
 TYPED_TEST_P(ModifiersTest, Emplace) {
-  using T = hash_internal::GeneratedType<TypeParam>;
+  using T = GeneratedType<TypeParam>;
   using V = typename TypeParam::mapped_type;
-  T val = hash_internal::Generator<T>()();
+  T val = Generator<T>()();
   TypeParam m;
   // TODO(alkis): We need a way to run emplace in a more meaningful way. Perhaps
   // with test traits/policy.
   auto p = m.emplace(val);
   EXPECT_TRUE(p.second);
   EXPECT_EQ(val, *p.first);
-  T val2 = {val.first, hash_internal::Generator<V>()()};
+  T val2 = {val.first, Generator<V>()()};
   p = m.emplace(val2);
   EXPECT_FALSE(p.second);
   EXPECT_EQ(val, *p.first);
 }
 
 TYPED_TEST_P(ModifiersTest, EmplaceHint) {
-  using T = hash_internal::GeneratedType<TypeParam>;
+  using T = GeneratedType<TypeParam>;
   using V = typename TypeParam::mapped_type;
-  T val = hash_internal::Generator<T>()();
+  T val = Generator<T>()();
   TypeParam m;
   // TODO(alkis): We need a way to run emplace in a more meaningful way. Perhaps
   // with test traits/policy.
   auto it = m.emplace_hint(m.end(), val);
   EXPECT_EQ(val, *it);
-  T val2 = {val.first, hash_internal::Generator<V>()()};
+  T val2 = {val.first, Generator<V>()()};
   it = m.emplace_hint(it, val2);
   EXPECT_EQ(val, *it);
 }
 
 TYPED_TEST_P(ModifiersTest, TryEmplace) {
-#ifdef UNORDERED_MAP_CXX17
-  using T = hash_internal::GeneratedType<TypeParam>;
+  using T = GeneratedType<TypeParam>;
   using V = typename TypeParam::mapped_type;
-  T val = hash_internal::Generator<T>()();
+  T val = Generator<T>()();
   TypeParam m;
   // TODO(alkis): We need a way to run emplace in a more meaningful way. Perhaps
   // with test traits/policy.
   auto p = m.try_emplace(val.first, val.second);
   EXPECT_TRUE(p.second);
   EXPECT_EQ(val, *p.first);
-  T val2 = {val.first, hash_internal::Generator<V>()()};
+  T val2 = {val.first, Generator<V>()()};
   p = m.try_emplace(val2.first, val2.second);
   EXPECT_FALSE(p.second);
   EXPECT_EQ(val, *p.first);
-#endif
 }
 
 TYPED_TEST_P(ModifiersTest, TryEmplaceHint) {
-#ifdef UNORDERED_MAP_CXX17
-  using T = hash_internal::GeneratedType<TypeParam>;
+  using T = GeneratedType<TypeParam>;
   using V = typename TypeParam::mapped_type;
-  T val = hash_internal::Generator<T>()();
+  T val = Generator<T>()();
   TypeParam m;
   // TODO(alkis): We need a way to run emplace in a more meaningful way. Perhaps
   // with test traits/policy.
   auto it = m.try_emplace(m.end(), val.first, val.second);
   EXPECT_EQ(val, *it);
-  T val2 = {val.first, hash_internal::Generator<V>()()};
+  T val2 = {val.first, Generator<V>()()};
   it = m.try_emplace(it, val2.first, val2.second);
   EXPECT_EQ(val, *it);
-#endif
 }
 
 template <class V>
-using IfNotVoid = typename std::enable_if<!std::is_void<V>::value, V>::type;
+using IfNotVoid = std::enable_if_t<!std::is_void_v<V>, V>;
 
 // In openmap we chose not to return the iterator from erase because that's
 // more expensive. As such we adapt erase to return an iterator here.
@@ -203,11 +226,10 @@ struct EraseFirst {
 };
 
 TYPED_TEST_P(ModifiersTest, Erase) {
-  using T = hash_internal::GeneratedType<TypeParam>;
+  using T = GeneratedType<TypeParam>;
   using std::get;
   std::vector<T> values;
-  std::generate_n(std::back_inserter(values), 10,
-                  hash_internal::Generator<T>());
+  std::generate_n(std::back_inserter(values), 10, Generator<T>());
   TypeParam m(values.begin(), values.end());
   ASSERT_THAT(items(m), ::testing::UnorderedElementsAreArray(values));
   auto& first = *m.begin();
@@ -222,10 +244,9 @@ TYPED_TEST_P(ModifiersTest, Erase) {
 }
 
 TYPED_TEST_P(ModifiersTest, EraseRange) {
-  using T = hash_internal::GeneratedType<TypeParam>;
+  using T = GeneratedType<TypeParam>;
   std::vector<T> values;
-  std::generate_n(std::back_inserter(values), 10,
-                  hash_internal::Generator<T>());
+  std::generate_n(std::back_inserter(values), 10, Generator<T>());
   TypeParam m(values.begin(), values.end());
   ASSERT_THAT(items(m), ::testing::UnorderedElementsAreArray(values));
   auto it = m.erase(m.begin(), m.end());
@@ -234,10 +255,9 @@ TYPED_TEST_P(ModifiersTest, EraseRange) {
 }
 
 TYPED_TEST_P(ModifiersTest, EraseKey) {
-  using T = hash_internal::GeneratedType<TypeParam>;
+  using T = GeneratedType<TypeParam>;
   std::vector<T> values;
-  std::generate_n(std::back_inserter(values), 10,
-                  hash_internal::Generator<T>());
+  std::generate_n(std::back_inserter(values), 10, Generator<T>());
   TypeParam m(values.begin(), values.end());
   ASSERT_THAT(items(m), ::testing::UnorderedElementsAreArray(values));
   EXPECT_EQ(1, m.erase(values[0].first));
@@ -247,11 +267,11 @@ TYPED_TEST_P(ModifiersTest, EraseKey) {
 }
 
 TYPED_TEST_P(ModifiersTest, Swap) {
-  using T = hash_internal::GeneratedType<TypeParam>;
+  using T = GeneratedType<TypeParam>;
   std::vector<T> v1;
   std::vector<T> v2;
-  std::generate_n(std::back_inserter(v1), 5, hash_internal::Generator<T>());
-  std::generate_n(std::back_inserter(v2), 5, hash_internal::Generator<T>());
+  std::generate_n(std::back_inserter(v1), 5, Generator<T>());
+  std::generate_n(std::back_inserter(v2), 5, Generator<T>());
   TypeParam m1(v1.begin(), v1.end());
   TypeParam m2(v2.begin(), v2.end());
   EXPECT_THAT(items(m1), ::testing::UnorderedElementsAreArray(v1));
@@ -264,10 +284,12 @@ TYPED_TEST_P(ModifiersTest, Swap) {
 // TODO(alkis): Write tests for extract.
 // TODO(alkis): Write tests for merge.
 
-REGISTER_TYPED_TEST_CASE_P(ModifiersTest, Clear, Insert, InsertHint,
-                           InsertRange, InsertOrAssign, InsertOrAssignHint,
-                           Emplace, EmplaceHint, TryEmplace, TryEmplaceHint,
-                           Erase, EraseRange, EraseKey, Swap);
+REGISTER_TYPED_TEST_SUITE_P(ModifiersTest, Clear, Insert, InsertHint,
+                            InsertRange, InsertWithinCapacity,
+                            InsertRangeWithinCapacity, InsertOrAssign,
+                            InsertOrAssignHint, Emplace, EmplaceHint,
+                            TryEmplace, TryEmplaceHint, Erase, EraseRange,
+                            EraseKey, Swap);
 
 template <typename Type>
 struct is_unique_ptr : std::false_type {};
@@ -285,30 +307,31 @@ class UniquePtrModifiersTest : public ::testing::Test {
   }
 };
 
+GTEST_ALLOW_UNINSTANTIATED_PARAMETERIZED_TEST(UniquePtrModifiersTest);
+
 TYPED_TEST_SUITE_P(UniquePtrModifiersTest);
 
 // Test that we do not move from rvalue arguments if an insertion does not
 // happen.
 TYPED_TEST_P(UniquePtrModifiersTest, TryEmplace) {
-#ifdef UNORDERED_MAP_CXX17
-  using T = hash_internal::GeneratedType<TypeParam>;
+  using T = GeneratedType<TypeParam>;
   using V = typename TypeParam::mapped_type;
-  T val = hash_internal::Generator<T>()();
+  T val = Generator<T>()();
   TypeParam m;
   auto p = m.try_emplace(val.first, std::move(val.second));
   EXPECT_TRUE(p.second);
   // A moved from std::unique_ptr is guaranteed to be nullptr.
   EXPECT_EQ(val.second, nullptr);
-  T val2 = {val.first, hash_internal::Generator<V>()()};
+  T val2 = {val.first, Generator<V>()()};
   p = m.try_emplace(val2.first, std::move(val2.second));
   EXPECT_FALSE(p.second);
   EXPECT_NE(val2.second, nullptr);
-#endif
 }
 
 REGISTER_TYPED_TEST_SUITE_P(UniquePtrModifiersTest, TryEmplace);
 
 }  // namespace container_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_INTERNAL_UNORDERED_MAP_MODIFIERS_TEST_H_
diff --git a/absl/container/internal/unordered_map_test.cc b/absl/container/internal/unordered_map_test.cc
index 114b342def7..9cbf512f32b 100644
--- a/absl/container/internal/unordered_map_test.cc
+++ b/absl/container/internal/unordered_map_test.cc
@@ -21,6 +21,7 @@
 #include "absl/container/internal/unordered_map_modifiers_test.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 namespace {
 
@@ -45,4 +46,5 @@ INSTANTIATE_TYPED_TEST_SUITE_P(UnorderedMap, UniquePtrModifiersTest,
 
 }  // namespace
 }  // namespace container_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/container/internal/unordered_set_constructor_test.h b/absl/container/internal/unordered_set_constructor_test.h
index f484468393e..7038a0c8b2c 100644
--- a/absl/container/internal/unordered_set_constructor_test.h
+++ b/absl/container/internal/unordered_set_constructor_test.h
@@ -21,11 +21,13 @@
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
+#include "absl/base/config.h"
 #include "absl/container/internal/hash_generator_testing.h"
 #include "absl/container/internal/hash_policy_testing.h"
 #include "absl/meta/type_traits.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
 template <class UnordMap>
@@ -93,28 +95,8 @@ TYPED_TEST_P(ConstructorTest, BucketCountHashEqualAlloc) {
   EXPECT_GE(cm.bucket_count(), 123);
 }
 
-template <typename T>
-struct is_std_unordered_set : std::false_type {};
-
-template <typename... T>
-struct is_std_unordered_set<std::unordered_set<T...>> : std::true_type {};
-
-#if defined(UNORDERED_SET_CXX14) || defined(UNORDERED_SET_CXX17)
-using has_cxx14_std_apis = std::true_type;
-#else
-using has_cxx14_std_apis = std::false_type;
-#endif
-
-template <typename T>
-using expect_cxx14_apis =
-    absl::disjunction<absl::negation<is_std_unordered_set<T>>,
-                      has_cxx14_std_apis>;
-
 template <typename TypeParam>
-void BucketCountAllocTest(std::false_type) {}
-
-template <typename TypeParam>
-void BucketCountAllocTest(std::true_type) {
+void BucketCountAllocTest() {
   using A = typename TypeParam::allocator_type;
   A alloc(0);
   TypeParam m(123, alloc);
@@ -125,14 +107,11 @@ void BucketCountAllocTest(std::true_type) {
 }
 
 TYPED_TEST_P(ConstructorTest, BucketCountAlloc) {
-  BucketCountAllocTest<TypeParam>(expect_cxx14_apis<TypeParam>());
+  BucketCountAllocTest<TypeParam>();
 }
 
 template <typename TypeParam>
-void BucketCountHashAllocTest(std::false_type) {}
-
-template <typename TypeParam>
-void BucketCountHashAllocTest(std::true_type) {
+void BucketCountHashAllocTest() {
   using H = typename TypeParam::hasher;
   using A = typename TypeParam::allocator_type;
   H hasher;
@@ -146,25 +125,11 @@ void BucketCountHashAllocTest(std::true_type) {
 }
 
 TYPED_TEST_P(ConstructorTest, BucketCountHashAlloc) {
-  BucketCountHashAllocTest<TypeParam>(expect_cxx14_apis<TypeParam>());
+  BucketCountHashAllocTest<TypeParam>();
 }
 
-#if ABSL_UNORDERED_SUPPORTS_ALLOC_CTORS
-using has_alloc_std_constructors = std::true_type;
-#else
-using has_alloc_std_constructors = std::false_type;
-#endif
-
-template <typename T>
-using expect_alloc_constructors =
-    absl::disjunction<absl::negation<is_std_unordered_set<T>>,
-                      has_alloc_std_constructors>;
-
 template <typename TypeParam>
-void AllocTest(std::false_type) {}
-
-template <typename TypeParam>
-void AllocTest(std::true_type) {
+void AllocTest() {
   using A = typename TypeParam::allocator_type;
   A alloc(0);
   TypeParam m(alloc);
@@ -173,12 +138,10 @@ void AllocTest(std::true_type) {
   EXPECT_THAT(keys(m), ::testing::UnorderedElementsAre());
 }
 
-TYPED_TEST_P(ConstructorTest, Alloc) {
-  AllocTest<TypeParam>(expect_alloc_constructors<TypeParam>());
-}
+TYPED_TEST_P(ConstructorTest, Alloc) { AllocTest<TypeParam>(); }
 
 TYPED_TEST_P(ConstructorTest, InputIteratorBucketHashEqualAlloc) {
-  using T = hash_internal::GeneratedType<TypeParam>;
+  using T = GeneratedType<TypeParam>;
   using H = typename TypeParam::hasher;
   using E = typename TypeParam::key_equal;
   using A = typename TypeParam::allocator_type;
@@ -186,8 +149,7 @@ TYPED_TEST_P(ConstructorTest, InputIteratorBucketHashEqualAlloc) {
   E equal;
   A alloc(0);
   std::vector<T> values;
-  for (size_t i = 0; i != 10; ++i)
-    values.push_back(hash_internal::Generator<T>()());
+  for (size_t i = 0; i != 10; ++i) values.push_back(Generator<T>()());
   TypeParam m(values.begin(), values.end(), 123, hasher, equal, alloc);
   EXPECT_EQ(m.hash_function(), hasher);
   EXPECT_EQ(m.key_eq(), equal);
@@ -197,16 +159,12 @@ TYPED_TEST_P(ConstructorTest, InputIteratorBucketHashEqualAlloc) {
 }
 
 template <typename TypeParam>
-void InputIteratorBucketAllocTest(std::false_type) {}
-
-template <typename TypeParam>
-void InputIteratorBucketAllocTest(std::true_type) {
-  using T = hash_internal::GeneratedType<TypeParam>;
+void InputIteratorBucketAllocTest() {
+  using T = GeneratedType<TypeParam>;
   using A = typename TypeParam::allocator_type;
   A alloc(0);
   std::vector<T> values;
-  for (size_t i = 0; i != 10; ++i)
-    values.push_back(hash_internal::Generator<T>()());
+  for (size_t i = 0; i != 10; ++i) values.push_back(Generator<T>()());
   TypeParam m(values.begin(), values.end(), 123, alloc);
   EXPECT_EQ(m.get_allocator(), alloc);
   EXPECT_THAT(keys(m), ::testing::UnorderedElementsAreArray(values));
@@ -214,22 +172,18 @@ void InputIteratorBucketAllocTest(std::true_type) {
 }
 
 TYPED_TEST_P(ConstructorTest, InputIteratorBucketAlloc) {
-  InputIteratorBucketAllocTest<TypeParam>(expect_cxx14_apis<TypeParam>());
+  InputIteratorBucketAllocTest<TypeParam>();
 }
 
 template <typename TypeParam>
-void InputIteratorBucketHashAllocTest(std::false_type) {}
-
-template <typename TypeParam>
-void InputIteratorBucketHashAllocTest(std::true_type) {
-  using T = hash_internal::GeneratedType<TypeParam>;
+void InputIteratorBucketHashAllocTest() {
+  using T = GeneratedType<TypeParam>;
   using H = typename TypeParam::hasher;
   using A = typename TypeParam::allocator_type;
   H hasher;
   A alloc(0);
   std::vector<T> values;
-  for (size_t i = 0; i != 10; ++i)
-    values.push_back(hash_internal::Generator<T>()());
+  for (size_t i = 0; i != 10; ++i) values.push_back(Generator<T>()());
   TypeParam m(values.begin(), values.end(), 123, hasher, alloc);
   EXPECT_EQ(m.hash_function(), hasher);
   EXPECT_EQ(m.get_allocator(), alloc);
@@ -238,11 +192,11 @@ void InputIteratorBucketHashAllocTest(std::true_type) {
 }
 
 TYPED_TEST_P(ConstructorTest, InputIteratorBucketHashAlloc) {
-  InputIteratorBucketHashAllocTest<TypeParam>(expect_cxx14_apis<TypeParam>());
+  InputIteratorBucketHashAllocTest<TypeParam>();
 }
 
 TYPED_TEST_P(ConstructorTest, CopyConstructor) {
-  using T = hash_internal::GeneratedType<TypeParam>;
+  using T = GeneratedType<TypeParam>;
   using H = typename TypeParam::hasher;
   using E = typename TypeParam::key_equal;
   using A = typename TypeParam::allocator_type;
@@ -250,7 +204,7 @@ TYPED_TEST_P(ConstructorTest, CopyConstructor) {
   E equal;
   A alloc(0);
   TypeParam m(123, hasher, equal, alloc);
-  for (size_t i = 0; i != 10; ++i) m.insert(hash_internal::Generator<T>()());
+  for (size_t i = 0; i != 10; ++i) m.insert(Generator<T>()());
   TypeParam n(m);
   EXPECT_EQ(m.hash_function(), n.hash_function());
   EXPECT_EQ(m.key_eq(), n.key_eq());
@@ -260,11 +214,8 @@ TYPED_TEST_P(ConstructorTest, CopyConstructor) {
 }
 
 template <typename TypeParam>
-void CopyConstructorAllocTest(std::false_type) {}
-
-template <typename TypeParam>
-void CopyConstructorAllocTest(std::true_type) {
-  using T = hash_internal::GeneratedType<TypeParam>;
+void CopyConstructorAllocTest() {
+  using T = GeneratedType<TypeParam>;
   using H = typename TypeParam::hasher;
   using E = typename TypeParam::key_equal;
   using A = typename TypeParam::allocator_type;
@@ -272,7 +223,7 @@ void CopyConstructorAllocTest(std::true_type) {
   E equal;
   A alloc(0);
   TypeParam m(123, hasher, equal, alloc);
-  for (size_t i = 0; i != 10; ++i) m.insert(hash_internal::Generator<T>()());
+  for (size_t i = 0; i != 10; ++i) m.insert(Generator<T>()());
   TypeParam n(m, A(11));
   EXPECT_EQ(m.hash_function(), n.hash_function());
   EXPECT_EQ(m.key_eq(), n.key_eq());
@@ -281,13 +232,13 @@ void CopyConstructorAllocTest(std::true_type) {
 }
 
 TYPED_TEST_P(ConstructorTest, CopyConstructorAlloc) {
-  CopyConstructorAllocTest<TypeParam>(expect_alloc_constructors<TypeParam>());
+  CopyConstructorAllocTest<TypeParam>();
 }
 
 // TODO(alkis): Test non-propagating allocators on copy constructors.
 
 TYPED_TEST_P(ConstructorTest, MoveConstructor) {
-  using T = hash_internal::GeneratedType<TypeParam>;
+  using T = GeneratedType<TypeParam>;
   using H = typename TypeParam::hasher;
   using E = typename TypeParam::key_equal;
   using A = typename TypeParam::allocator_type;
@@ -295,7 +246,7 @@ TYPED_TEST_P(ConstructorTest, MoveConstructor) {
   E equal;
   A alloc(0);
   TypeParam m(123, hasher, equal, alloc);
-  for (size_t i = 0; i != 10; ++i) m.insert(hash_internal::Generator<T>()());
+  for (size_t i = 0; i != 10; ++i) m.insert(Generator<T>()());
   TypeParam t(m);
   TypeParam n(std::move(t));
   EXPECT_EQ(m.hash_function(), n.hash_function());
@@ -305,11 +256,8 @@ TYPED_TEST_P(ConstructorTest, MoveConstructor) {
 }
 
 template <typename TypeParam>
-void MoveConstructorAllocTest(std::false_type) {}
-
-template <typename TypeParam>
-void MoveConstructorAllocTest(std::true_type) {
-  using T = hash_internal::GeneratedType<TypeParam>;
+void MoveConstructorAllocTest() {
+  using T = GeneratedType<TypeParam>;
   using H = typename TypeParam::hasher;
   using E = typename TypeParam::key_equal;
   using A = typename TypeParam::allocator_type;
@@ -317,7 +265,7 @@ void MoveConstructorAllocTest(std::true_type) {
   E equal;
   A alloc(0);
   TypeParam m(123, hasher, equal, alloc);
-  for (size_t i = 0; i != 10; ++i) m.insert(hash_internal::Generator<T>()());
+  for (size_t i = 0; i != 10; ++i) m.insert(Generator<T>()());
   TypeParam t(m);
   TypeParam n(std::move(t), A(1));
   EXPECT_EQ(m.hash_function(), n.hash_function());
@@ -327,14 +275,14 @@ void MoveConstructorAllocTest(std::true_type) {
 }
 
 TYPED_TEST_P(ConstructorTest, MoveConstructorAlloc) {
-  MoveConstructorAllocTest<TypeParam>(expect_alloc_constructors<TypeParam>());
+  MoveConstructorAllocTest<TypeParam>();
 }
 
 // TODO(alkis): Test non-propagating allocators on move constructors.
 
 TYPED_TEST_P(ConstructorTest, InitializerListBucketHashEqualAlloc) {
-  using T = hash_internal::GeneratedType<TypeParam>;
-  hash_internal::Generator<T> gen;
+  using T = GeneratedType<TypeParam>;
+  Generator<T> gen;
   std::initializer_list<T> values = {gen(), gen(), gen(), gen(), gen()};
   using H = typename TypeParam::hasher;
   using E = typename TypeParam::key_equal;
@@ -351,13 +299,10 @@ TYPED_TEST_P(ConstructorTest, InitializerListBucketHashEqualAlloc) {
 }
 
 template <typename TypeParam>
-void InitializerListBucketAllocTest(std::false_type) {}
-
-template <typename TypeParam>
-void InitializerListBucketAllocTest(std::true_type) {
-  using T = hash_internal::GeneratedType<TypeParam>;
+void InitializerListBucketAllocTest() {
+  using T = GeneratedType<TypeParam>;
   using A = typename TypeParam::allocator_type;
-  hash_internal::Generator<T> gen;
+  Generator<T> gen;
   std::initializer_list<T> values = {gen(), gen(), gen(), gen(), gen()};
   A alloc(0);
   TypeParam m(values, 123, alloc);
@@ -367,20 +312,17 @@ void InitializerListBucketAllocTest(std::true_type) {
 }
 
 TYPED_TEST_P(ConstructorTest, InitializerListBucketAlloc) {
-  InitializerListBucketAllocTest<TypeParam>(expect_cxx14_apis<TypeParam>());
+  InitializerListBucketAllocTest<TypeParam>();
 }
 
 template <typename TypeParam>
-void InitializerListBucketHashAllocTest(std::false_type) {}
-
-template <typename TypeParam>
-void InitializerListBucketHashAllocTest(std::true_type) {
-  using T = hash_internal::GeneratedType<TypeParam>;
+void InitializerListBucketHashAllocTest() {
+  using T = GeneratedType<TypeParam>;
   using H = typename TypeParam::hasher;
   using A = typename TypeParam::allocator_type;
   H hasher;
   A alloc(0);
-  hash_internal::Generator<T> gen;
+  Generator<T> gen;
   std::initializer_list<T> values = {gen(), gen(), gen(), gen(), gen()};
   TypeParam m(values, 123, hasher, alloc);
   EXPECT_EQ(m.hash_function(), hasher);
@@ -390,18 +332,18 @@ void InitializerListBucketHashAllocTest(std::true_type) {
 }
 
 TYPED_TEST_P(ConstructorTest, InitializerListBucketHashAlloc) {
-  InitializerListBucketHashAllocTest<TypeParam>(expect_cxx14_apis<TypeParam>());
+  InitializerListBucketHashAllocTest<TypeParam>();
 }
 
 TYPED_TEST_P(ConstructorTest, CopyAssignment) {
-  using T = hash_internal::GeneratedType<TypeParam>;
+  using T = GeneratedType<TypeParam>;
   using H = typename TypeParam::hasher;
   using E = typename TypeParam::key_equal;
   using A = typename TypeParam::allocator_type;
   H hasher;
   E equal;
   A alloc(0);
-  hash_internal::Generator<T> gen;
+  Generator<T> gen;
   TypeParam m({gen(), gen(), gen()}, 123, hasher, equal, alloc);
   TypeParam n;
   n = m;
@@ -414,14 +356,14 @@ TYPED_TEST_P(ConstructorTest, CopyAssignment) {
 // (it depends on traits).
 
 TYPED_TEST_P(ConstructorTest, MoveAssignment) {
-  using T = hash_internal::GeneratedType<TypeParam>;
+  using T = GeneratedType<TypeParam>;
   using H = typename TypeParam::hasher;
   using E = typename TypeParam::key_equal;
   using A = typename TypeParam::allocator_type;
   H hasher;
   E equal;
   A alloc(0);
-  hash_internal::Generator<T> gen;
+  Generator<T> gen;
   TypeParam m({gen(), gen(), gen()}, 123, hasher, equal, alloc);
   TypeParam t(m);
   TypeParam n;
@@ -432,8 +374,8 @@ TYPED_TEST_P(ConstructorTest, MoveAssignment) {
 }
 
 TYPED_TEST_P(ConstructorTest, AssignmentFromInitializerList) {
-  using T = hash_internal::GeneratedType<TypeParam>;
-  hash_internal::Generator<T> gen;
+  using T = GeneratedType<TypeParam>;
+  Generator<T> gen;
   std::initializer_list<T> values = {gen(), gen(), gen(), gen(), gen()};
   TypeParam m;
   m = values;
@@ -441,8 +383,8 @@ TYPED_TEST_P(ConstructorTest, AssignmentFromInitializerList) {
 }
 
 TYPED_TEST_P(ConstructorTest, AssignmentOverwritesExisting) {
-  using T = hash_internal::GeneratedType<TypeParam>;
-  hash_internal::Generator<T> gen;
+  using T = GeneratedType<TypeParam>;
+  Generator<T> gen;
   TypeParam m({gen(), gen(), gen()});
   TypeParam n({gen()});
   n = m;
@@ -450,8 +392,8 @@ TYPED_TEST_P(ConstructorTest, AssignmentOverwritesExisting) {
 }
 
 TYPED_TEST_P(ConstructorTest, MoveAssignmentOverwritesExisting) {
-  using T = hash_internal::GeneratedType<TypeParam>;
-  hash_internal::Generator<T> gen;
+  using T = GeneratedType<TypeParam>;
+  Generator<T> gen;
   TypeParam m({gen(), gen(), gen()});
   TypeParam t(m);
   TypeParam n({gen()});
@@ -460,8 +402,8 @@ TYPED_TEST_P(ConstructorTest, MoveAssignmentOverwritesExisting) {
 }
 
 TYPED_TEST_P(ConstructorTest, AssignmentFromInitializerListOverwritesExisting) {
-  using T = hash_internal::GeneratedType<TypeParam>;
-  hash_internal::Generator<T> gen;
+  using T = GeneratedType<TypeParam>;
+  Generator<T> gen;
   std::initializer_list<T> values = {gen(), gen(), gen(), gen(), gen()};
   TypeParam m;
   m = values;
@@ -469,15 +411,15 @@ TYPED_TEST_P(ConstructorTest, AssignmentFromInitializerListOverwritesExisting) {
 }
 
 TYPED_TEST_P(ConstructorTest, AssignmentOnSelf) {
-  using T = hash_internal::GeneratedType<TypeParam>;
-  hash_internal::Generator<T> gen;
+  using T = GeneratedType<TypeParam>;
+  Generator<T> gen;
   std::initializer_list<T> values = {gen(), gen(), gen(), gen(), gen()};
   TypeParam m(values);
   m = *&m;  // Avoid -Wself-assign.
   EXPECT_THAT(keys(m), ::testing::UnorderedElementsAreArray(values));
 }
 
-REGISTER_TYPED_TEST_CASE_P(
+REGISTER_TYPED_TEST_SUITE_P(
     ConstructorTest, NoArgs, BucketCount, BucketCountHash, BucketCountHashEqual,
     BucketCountHashEqualAlloc, BucketCountAlloc, BucketCountHashAlloc, Alloc,
     InputIteratorBucketHashEqualAlloc, InputIteratorBucketAlloc,
@@ -489,6 +431,7 @@ REGISTER_TYPED_TEST_CASE_P(
     AssignmentFromInitializerListOverwritesExisting, AssignmentOnSelf);
 
 }  // namespace container_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_INTERNAL_UNORDERED_SET_CONSTRUCTOR_TEST_H_
diff --git a/absl/container/internal/unordered_set_lookup_test.h b/absl/container/internal/unordered_set_lookup_test.h
index 05b32b5d906..932c320daf8 100644
--- a/absl/container/internal/unordered_set_lookup_test.h
+++ b/absl/container/internal/unordered_set_lookup_test.h
@@ -21,6 +21,7 @@
 #include "absl/container/internal/hash_policy_testing.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
 template <class UnordSet>
@@ -29,10 +30,9 @@ class LookupTest : public ::testing::Test {};
 TYPED_TEST_SUITE_P(LookupTest);
 
 TYPED_TEST_P(LookupTest, Count) {
-  using T = hash_internal::GeneratedType<TypeParam>;
+  using T = GeneratedType<TypeParam>;
   std::vector<T> values;
-  std::generate_n(std::back_inserter(values), 10,
-                  hash_internal::Generator<T>());
+  std::generate_n(std::back_inserter(values), 10, Generator<T>());
   TypeParam m;
   for (const auto& v : values)
     EXPECT_EQ(0, m.count(v)) << ::testing::PrintToString(v);
@@ -42,32 +42,28 @@ TYPED_TEST_P(LookupTest, Count) {
 }
 
 TYPED_TEST_P(LookupTest, Find) {
-  using T = hash_internal::GeneratedType<TypeParam>;
+  using T = GeneratedType<TypeParam>;
   std::vector<T> values;
-  std::generate_n(std::back_inserter(values), 10,
-                  hash_internal::Generator<T>());
+  std::generate_n(std::back_inserter(values), 10, Generator<T>());
   TypeParam m;
   for (const auto& v : values)
     EXPECT_TRUE(m.end() == m.find(v)) << ::testing::PrintToString(v);
   m.insert(values.begin(), values.end());
   for (const auto& v : values) {
     typename TypeParam::iterator it = m.find(v);
-    static_assert(std::is_same<const typename TypeParam::value_type&,
-                               decltype(*it)>::value,
-                  "");
+    static_assert(
+        std::is_same_v<const typename TypeParam::value_type&, decltype(*it)>);
     static_assert(std::is_same<const typename TypeParam::value_type*,
-                               decltype(it.operator->())>::value,
-                  "");
+                               decltype(it.operator->())>::value);
     EXPECT_TRUE(m.end() != it) << ::testing::PrintToString(v);
     EXPECT_EQ(v, *it) << ::testing::PrintToString(v);
   }
 }
 
 TYPED_TEST_P(LookupTest, EqualRange) {
-  using T = hash_internal::GeneratedType<TypeParam>;
+  using T = GeneratedType<TypeParam>;
   std::vector<T> values;
-  std::generate_n(std::back_inserter(values), 10,
-                  hash_internal::Generator<T>());
+  std::generate_n(std::back_inserter(values), 10, Generator<T>());
   TypeParam m;
   for (const auto& v : values) {
     auto r = m.equal_range(v);
@@ -81,9 +77,10 @@ TYPED_TEST_P(LookupTest, EqualRange) {
   }
 }
 
-REGISTER_TYPED_TEST_CASE_P(LookupTest, Count, Find, EqualRange);
+REGISTER_TYPED_TEST_SUITE_P(LookupTest, Count, Find, EqualRange);
 
 }  // namespace container_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_INTERNAL_UNORDERED_SET_LOOKUP_TEST_H_
diff --git a/absl/container/internal/unordered_set_members_test.h b/absl/container/internal/unordered_set_members_test.h
index b96c945a11b..b416fef4d78 100644
--- a/absl/container/internal/unordered_set_members_test.h
+++ b/absl/container/internal/unordered_set_members_test.h
@@ -21,6 +21,7 @@
 #include "absl/meta/type_traits.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
 template <class UnordSet>
@@ -34,10 +35,10 @@ void UseType() {}
 TYPED_TEST_P(MembersTest, Typedefs) {
   EXPECT_TRUE((std::is_same<typename TypeParam::key_type,
                             typename TypeParam::value_type>()));
-  EXPECT_TRUE((absl::conjunction<
-               absl::negation<std::is_signed<typename TypeParam::size_type>>,
+  EXPECT_TRUE((std::conjunction<
+               std::negation<std::is_signed<typename TypeParam::size_type>>,
                std::is_integral<typename TypeParam::size_type>>()));
-  EXPECT_TRUE((absl::conjunction<
+  EXPECT_TRUE((std::conjunction<
                std::is_signed<typename TypeParam::difference_type>,
                std::is_integral<typename TypeParam::difference_type>>()));
   EXPECT_TRUE((std::is_convertible<
@@ -79,6 +80,7 @@ TYPED_TEST_P(MembersTest, BeginEnd) {
 REGISTER_TYPED_TEST_SUITE_P(MembersTest, Typedefs, SimpleFunctions, BeginEnd);
 
 }  // namespace container_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_INTERNAL_UNORDERED_SET_MEMBERS_TEST_H_
diff --git a/absl/container/internal/unordered_set_modifiers_test.h b/absl/container/internal/unordered_set_modifiers_test.h
index 79a8d422be1..1d686afb21d 100644
--- a/absl/container/internal/unordered_set_modifiers_test.h
+++ b/absl/container/internal/unordered_set_modifiers_test.h
@@ -15,12 +15,15 @@
 #ifndef ABSL_CONTAINER_INTERNAL_UNORDERED_SET_MODIFIERS_TEST_H_
 #define ABSL_CONTAINER_INTERNAL_UNORDERED_SET_MODIFIERS_TEST_H_
 
+#include <type_traits>
+
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
 #include "absl/container/internal/hash_generator_testing.h"
 #include "absl/container/internal/hash_policy_testing.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
 template <class UnordSet>
@@ -29,10 +32,9 @@ class ModifiersTest : public ::testing::Test {};
 TYPED_TEST_SUITE_P(ModifiersTest);
 
 TYPED_TEST_P(ModifiersTest, Clear) {
-  using T = hash_internal::GeneratedType<TypeParam>;
+  using T = GeneratedType<TypeParam>;
   std::vector<T> values;
-  std::generate_n(std::back_inserter(values), 10,
-                  hash_internal::Generator<T>());
+  std::generate_n(std::back_inserter(values), 10, Generator<T>());
   TypeParam m(values.begin(), values.end());
   ASSERT_THAT(keys(m), ::testing::UnorderedElementsAreArray(values));
   m.clear();
@@ -41,8 +43,8 @@ TYPED_TEST_P(ModifiersTest, Clear) {
 }
 
 TYPED_TEST_P(ModifiersTest, Insert) {
-  using T = hash_internal::GeneratedType<TypeParam>;
-  T val = hash_internal::Generator<T>()();
+  using T = GeneratedType<TypeParam>;
+  T val = Generator<T>()();
   TypeParam m;
   auto p = m.insert(val);
   EXPECT_TRUE(p.second);
@@ -52,8 +54,8 @@ TYPED_TEST_P(ModifiersTest, Insert) {
 }
 
 TYPED_TEST_P(ModifiersTest, InsertHint) {
-  using T = hash_internal::GeneratedType<TypeParam>;
-  T val = hash_internal::Generator<T>()();
+  using T = GeneratedType<TypeParam>;
+  T val = Generator<T>()();
   TypeParam m;
   auto it = m.insert(m.end(), val);
   EXPECT_TRUE(it != m.end());
@@ -64,18 +66,46 @@ TYPED_TEST_P(ModifiersTest, InsertHint) {
 }
 
 TYPED_TEST_P(ModifiersTest, InsertRange) {
-  using T = hash_internal::GeneratedType<TypeParam>;
+  using T = GeneratedType<TypeParam>;
   std::vector<T> values;
-  std::generate_n(std::back_inserter(values), 10,
-                  hash_internal::Generator<T>());
+  std::generate_n(std::back_inserter(values), 10, Generator<T>());
   TypeParam m;
   m.insert(values.begin(), values.end());
   ASSERT_THAT(keys(m), ::testing::UnorderedElementsAreArray(values));
 }
 
+TYPED_TEST_P(ModifiersTest, InsertWithinCapacity) {
+  using T = GeneratedType<TypeParam>;
+  T val = Generator<T>()();
+  TypeParam m;
+  m.reserve(10);
+  const size_t original_capacity = m.bucket_count();
+  m.insert(val);
+  EXPECT_EQ(m.bucket_count(), original_capacity);
+  m.insert(val);
+  EXPECT_EQ(m.bucket_count(), original_capacity);
+}
+
+TYPED_TEST_P(ModifiersTest, InsertRangeWithinCapacity) {
+#if !defined(__GLIBCXX__)
+  using T = GeneratedType<TypeParam>;
+  std::vector<T> base_values;
+  std::generate_n(std::back_inserter(base_values), 10, Generator<T>());
+  std::vector<T> values;
+  while (values.size() != 100) {
+    values.insert(values.end(), base_values.begin(), base_values.end());
+  }
+  TypeParam m;
+  m.reserve(10);
+  const size_t original_capacity = m.bucket_count();
+  m.insert(values.begin(), values.end());
+  EXPECT_EQ(m.bucket_count(), original_capacity);
+#endif
+}
+
 TYPED_TEST_P(ModifiersTest, Emplace) {
-  using T = hash_internal::GeneratedType<TypeParam>;
-  T val = hash_internal::Generator<T>()();
+  using T = GeneratedType<TypeParam>;
+  T val = Generator<T>()();
   TypeParam m;
   // TODO(alkis): We need a way to run emplace in a more meaningful way. Perhaps
   // with test traits/policy.
@@ -88,8 +118,8 @@ TYPED_TEST_P(ModifiersTest, Emplace) {
 }
 
 TYPED_TEST_P(ModifiersTest, EmplaceHint) {
-  using T = hash_internal::GeneratedType<TypeParam>;
-  T val = hash_internal::Generator<T>()();
+  using T = GeneratedType<TypeParam>;
+  T val = Generator<T>()();
   TypeParam m;
   // TODO(alkis): We need a way to run emplace in a more meaningful way. Perhaps
   // with test traits/policy.
@@ -100,7 +130,7 @@ TYPED_TEST_P(ModifiersTest, EmplaceHint) {
 }
 
 template <class V>
-using IfNotVoid = typename std::enable_if<!std::is_void<V>::value, V>::type;
+using IfNotVoid = std::enable_if_t<!std::is_void_v<V>, V>;
 
 // In openmap we chose not to return the iterator from erase because that's
 // more expensive. As such we adapt erase to return an iterator here.
@@ -119,10 +149,9 @@ struct EraseFirst {
 };
 
 TYPED_TEST_P(ModifiersTest, Erase) {
-  using T = hash_internal::GeneratedType<TypeParam>;
+  using T = GeneratedType<TypeParam>;
   std::vector<T> values;
-  std::generate_n(std::back_inserter(values), 10,
-                  hash_internal::Generator<T>());
+  std::generate_n(std::back_inserter(values), 10, Generator<T>());
   TypeParam m(values.begin(), values.end());
   ASSERT_THAT(keys(m), ::testing::UnorderedElementsAreArray(values));
   std::vector<T> values2;
@@ -136,10 +165,9 @@ TYPED_TEST_P(ModifiersTest, Erase) {
 }
 
 TYPED_TEST_P(ModifiersTest, EraseRange) {
-  using T = hash_internal::GeneratedType<TypeParam>;
+  using T = GeneratedType<TypeParam>;
   std::vector<T> values;
-  std::generate_n(std::back_inserter(values), 10,
-                  hash_internal::Generator<T>());
+  std::generate_n(std::back_inserter(values), 10, Generator<T>());
   TypeParam m(values.begin(), values.end());
   ASSERT_THAT(keys(m), ::testing::UnorderedElementsAreArray(values));
   auto it = m.erase(m.begin(), m.end());
@@ -148,10 +176,9 @@ TYPED_TEST_P(ModifiersTest, EraseRange) {
 }
 
 TYPED_TEST_P(ModifiersTest, EraseKey) {
-  using T = hash_internal::GeneratedType<TypeParam>;
+  using T = GeneratedType<TypeParam>;
   std::vector<T> values;
-  std::generate_n(std::back_inserter(values), 10,
-                  hash_internal::Generator<T>());
+  std::generate_n(std::back_inserter(values), 10, Generator<T>());
   TypeParam m(values.begin(), values.end());
   ASSERT_THAT(keys(m), ::testing::UnorderedElementsAreArray(values));
   EXPECT_EQ(1, m.erase(values[0]));
@@ -161,11 +188,11 @@ TYPED_TEST_P(ModifiersTest, EraseKey) {
 }
 
 TYPED_TEST_P(ModifiersTest, Swap) {
-  using T = hash_internal::GeneratedType<TypeParam>;
+  using T = GeneratedType<TypeParam>;
   std::vector<T> v1;
   std::vector<T> v2;
-  std::generate_n(std::back_inserter(v1), 5, hash_internal::Generator<T>());
-  std::generate_n(std::back_inserter(v2), 5, hash_internal::Generator<T>());
+  std::generate_n(std::back_inserter(v1), 5, Generator<T>());
+  std::generate_n(std::back_inserter(v2), 5, Generator<T>());
   TypeParam m1(v1.begin(), v1.end());
   TypeParam m2(v2.begin(), v2.end());
   EXPECT_THAT(keys(m1), ::testing::UnorderedElementsAreArray(v1));
@@ -178,11 +205,13 @@ TYPED_TEST_P(ModifiersTest, Swap) {
 // TODO(alkis): Write tests for extract.
 // TODO(alkis): Write tests for merge.
 
-REGISTER_TYPED_TEST_CASE_P(ModifiersTest, Clear, Insert, InsertHint,
-                           InsertRange, Emplace, EmplaceHint, Erase, EraseRange,
-                           EraseKey, Swap);
+REGISTER_TYPED_TEST_SUITE_P(ModifiersTest, Clear, Insert, InsertHint,
+                            InsertRange, InsertWithinCapacity,
+                            InsertRangeWithinCapacity, Emplace, EmplaceHint,
+                            Erase, EraseRange, EraseKey, Swap);
 
 }  // namespace container_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_INTERNAL_UNORDERED_SET_MODIFIERS_TEST_H_
diff --git a/absl/container/internal/unordered_set_test.cc b/absl/container/internal/unordered_set_test.cc
index 6478fac17c7..a134b53984b 100644
--- a/absl/container/internal/unordered_set_test.cc
+++ b/absl/container/internal/unordered_set_test.cc
@@ -20,6 +20,7 @@
 #include "absl/container/internal/unordered_set_modifiers_test.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 namespace {
 
@@ -36,4 +37,5 @@ INSTANTIATE_TYPED_TEST_SUITE_P(UnorderedSet, ModifiersTest, SetTypes);
 
 }  // namespace
 }  // namespace container_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/container/linked_hash_map.h b/absl/container/linked_hash_map.h
new file mode 100644
index 00000000000..6d0e320cafc
--- /dev/null
+++ b/absl/container/linked_hash_map.h
@@ -0,0 +1,673 @@
+// Copyright 2025 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: linked_hash_map.h
+// -----------------------------------------------------------------------------
+//
+// This is a simple insertion-ordered map. It provides O(1) amortized
+// insertions and lookups, as well as iteration over the map in the insertion
+// order.
+//
+// This class is thread-compatible.
+// This class is NOT exception-safe.
+//
+// Iterators point into the list and should be stable in the face of
+// mutations, except for an iterator pointing to an element that was just
+// deleted.
+//
+// This class supports heterogeneous lookups.
+
+#ifndef ABSL_CONTAINER_LINKED_HASH_MAP_H_
+#define ABSL_CONTAINER_LINKED_HASH_MAP_H_
+
+#include <cassert>
+#include <cstddef>
+#include <initializer_list>
+#include <list>
+#include <memory>
+#include <string>
+#include <tuple>
+#include <type_traits>
+#include <utility>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/optimization.h"
+#include "absl/base/throw_delegate.h"
+#include "absl/container/flat_hash_set.h"
+#include "absl/container/internal/common.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+template <typename Key, typename Value,
+          typename KeyHash = typename absl::flat_hash_set<Key>::hasher,
+          typename KeyEq =
+              typename absl::flat_hash_set<Key, KeyHash>::key_equal,
+          typename Alloc = std::allocator<std::pair<const Key, Value>>>
+class linked_hash_map {
+  using KeyArgImpl = absl::container_internal::KeyArg<
+      absl::container_internal::IsTransparent<KeyEq>::value &&
+      absl::container_internal::IsTransparent<KeyHash>::value>;
+
+ public:
+  using key_type = Key;
+  using mapped_type = Value;
+  using hasher = KeyHash;
+  using key_equal = KeyEq;
+  using value_type = std::pair<const key_type, mapped_type>;
+  using allocator_type = Alloc;
+  using difference_type = ptrdiff_t;
+
+ private:
+  template <class K>
+  using key_arg = typename KeyArgImpl::template type<K, key_type>;
+
+  using ListType = std::list<value_type, Alloc>;
+
+  template <class Fn>
+  class Wrapped {
+    template <typename K>
+    static const K& ToKey(const K& k) {
+      return k;
+    }
+    static const key_type& ToKey(typename ListType::const_iterator it) {
+      return it->first;
+    }
+    static const key_type& ToKey(typename ListType::iterator it) {
+      return it->first;
+    }
+
+    Fn fn_;
+
+    friend linked_hash_map;
+
+   public:
+    using is_transparent = void;
+
+    Wrapped() = default;
+    explicit Wrapped(Fn fn) : fn_(std::move(fn)) {}
+
+    template <class... Args>
+    auto operator()(Args&&... args) const
+        -> decltype(this->fn_(ToKey(args)...)) {
+      return fn_(ToKey(args)...);
+    }
+  };
+  using SetType =
+      absl::flat_hash_set<typename ListType::iterator, Wrapped<hasher>,
+                          Wrapped<key_equal>, Alloc>;
+
+  class NodeHandle {
+   public:
+    using key_type = linked_hash_map::key_type;
+    using mapped_type = linked_hash_map::mapped_type;
+    using allocator_type = linked_hash_map::allocator_type;
+
+    constexpr NodeHandle() noexcept = default;
+    NodeHandle(NodeHandle&& nh) noexcept = default;
+    ~NodeHandle() = default;
+    NodeHandle& operator=(NodeHandle&& node) noexcept = default;
+    bool empty() const noexcept { return list_.empty(); }
+    explicit operator bool() const noexcept { return !empty(); }
+    allocator_type get_allocator() const { return list_.get_allocator(); }
+    const key_type& key() const { return list_.front().first; }
+    mapped_type& mapped() { return list_.front().second; }
+    void swap(NodeHandle& nh) noexcept { list_.swap(nh.list_); }
+
+   private:
+    friend linked_hash_map;
+
+    explicit NodeHandle(ListType list) : list_(std::move(list)) {}
+    ListType list_;
+  };
+
+  template <class Iterator, class NodeType>
+  struct InsertReturnType {
+    Iterator position;
+    bool inserted;
+    NodeType node;
+  };
+
+ public:
+  using iterator = typename ListType::iterator;
+  using const_iterator = typename ListType::const_iterator;
+  using reverse_iterator = typename ListType::reverse_iterator;
+  using const_reverse_iterator = typename ListType::const_reverse_iterator;
+  using reference = typename ListType::reference;
+  using const_reference = typename ListType::const_reference;
+  using size_type = typename ListType::size_type;
+  using pointer = typename std::allocator_traits<allocator_type>::pointer;
+  using const_pointer =
+      typename std::allocator_traits<allocator_type>::const_pointer;
+  using node_type = NodeHandle;
+  using insert_return_type = InsertReturnType<iterator, node_type>;
+
+  linked_hash_map() {}
+
+  explicit linked_hash_map(size_t reservation_size,
+                           const hasher& hash = hasher(),
+                           const key_equal& eq = key_equal(),
+                           const allocator_type& alloc = allocator_type())
+      : set_(reservation_size, Wrapped<hasher>(hash), Wrapped<key_equal>(eq),
+             alloc),
+        list_(alloc) {}
+
+  linked_hash_map(size_t reservation_size, const hasher& hash,
+                  const allocator_type& alloc)
+      : linked_hash_map(reservation_size, hash, key_equal(), alloc) {}
+
+  linked_hash_map(size_t reservation_size, const allocator_type& alloc)
+      : linked_hash_map(reservation_size, hasher(), key_equal(), alloc) {}
+
+  explicit linked_hash_map(const allocator_type& alloc)
+      : linked_hash_map(0, hasher(), key_equal(), alloc) {}
+
+  template <class InputIt>
+  linked_hash_map(InputIt first, InputIt last, size_t reservation_size = 0,
+                  const hasher& hash = hasher(),
+                  const key_equal& eq = key_equal(),
+                  const allocator_type& alloc = allocator_type())
+      : linked_hash_map(reservation_size, hash, eq, alloc) {
+    insert(first, last);
+  }
+
+  template <class InputIt>
+  linked_hash_map(InputIt first, InputIt last, size_t reservation_size,
+                  const hasher& hash, const allocator_type& alloc)
+      : linked_hash_map(first, last, reservation_size, hash, key_equal(),
+                        alloc) {}
+
+  template <class InputIt>
+  linked_hash_map(InputIt first, InputIt last, size_t reservation_size,
+                  const allocator_type& alloc)
+      : linked_hash_map(first, last, reservation_size, hasher(), key_equal(),
+                        alloc) {}
+
+  template <class InputIt>
+  linked_hash_map(InputIt first, InputIt last, const allocator_type& alloc)
+      : linked_hash_map(first, last, /*reservation_size=*/0, hasher(),
+                        key_equal(), alloc) {}
+
+  linked_hash_map(std::initializer_list<value_type> init,
+                  size_t reservation_size = 0, const hasher& hash = hasher(),
+                  const key_equal& eq = key_equal(),
+                  const allocator_type& alloc = allocator_type())
+      : linked_hash_map(init.begin(), init.end(), reservation_size, hash, eq,
+                        alloc) {}
+
+  linked_hash_map(std::initializer_list<value_type> init,
+                  size_t reservation_size, const hasher& hash,
+                  const allocator_type& alloc)
+      : linked_hash_map(init, reservation_size, hash, key_equal(), alloc) {}
+
+  linked_hash_map(std::initializer_list<value_type> init,
+                  size_t reservation_size, const allocator_type& alloc)
+      : linked_hash_map(init, reservation_size, hasher(), key_equal(), alloc) {}
+
+  linked_hash_map(std::initializer_list<value_type> init,
+                  const allocator_type& alloc)
+      : linked_hash_map(init, /*reservation_size=*/0, hasher(), key_equal(),
+                        alloc) {}
+
+  linked_hash_map(const linked_hash_map& other)
+      : linked_hash_map(0, other.hash_function(), other.key_eq(),
+                        other.get_allocator()) {
+    reserve(other.size());
+    CopyFrom(other);
+  }
+
+  linked_hash_map(const linked_hash_map& other, const allocator_type& alloc)
+      : linked_hash_map(0, other.hash_function(), other.key_eq(), alloc) {
+    reserve(other.size());
+    CopyFrom(other);
+  }
+
+  linked_hash_map(linked_hash_map&& other) noexcept
+      : set_(std::move(other.set_)), list_(std::move(other.list_)) {
+    // Since the list and set must agree for other to end up "valid",
+    // explicitly clear them.
+    other.set_.clear();
+    other.list_.clear();
+  }
+
+  linked_hash_map(linked_hash_map&& other, const allocator_type& alloc)
+      : linked_hash_map(0, other.hash_function(), other.key_eq(), alloc) {
+    if (get_allocator() == other.get_allocator()) {
+      *this = std::move(other);
+    } else {
+      CopyFrom(std::move(other));
+    }
+  }
+
+  linked_hash_map& operator=(const linked_hash_map& other) {
+    if (this != &other) {
+      // Make a new set, with other's hash/eq/alloc.
+      set_ = SetType(0, other.set_.hash_function(), other.set_.key_eq(),
+                     other.get_allocator());
+      set_.reserve(other.size());
+      // Copy the list, with other's allocator.
+      list_ = ListType(other.get_allocator());
+      CopyFrom(other);
+    }
+    return *this;
+  }
+
+  linked_hash_map& operator=(linked_hash_map&& other) noexcept {
+    if (this != &other) {
+      // underlying containers will handle progagate_on_container_move details
+      set_ = std::move(other.set_);
+      list_ = std::move(other.list_);
+      other.set_.clear();
+      other.list_.clear();
+    }
+    return *this;
+  }
+
+  linked_hash_map& operator=(std::initializer_list<value_type> values) {
+    clear();
+    reserve(values.size());
+    insert(values.begin(), values.end());
+    return *this;
+  }
+
+  // Derive size_ from set_, as list::size might be O(N).
+  size_type size() const { return set_.size(); }
+  size_type max_size() const noexcept { return ~size_type{}; }
+  bool empty() const { return set_.empty(); }
+
+  // Iteration is list-like, in insertion order.
+  // These are all forwarded.
+  iterator begin() { return list_.begin(); }
+  iterator end() { return list_.end(); }
+  const_iterator begin() const { return list_.begin(); }
+  const_iterator end() const { return list_.end(); }
+  const_iterator cbegin() const { return list_.cbegin(); }
+  const_iterator cend() const { return list_.cend(); }
+  reverse_iterator rbegin() { return list_.rbegin(); }
+  reverse_iterator rend() { return list_.rend(); }
+  const_reverse_iterator rbegin() const { return list_.rbegin(); }
+  const_reverse_iterator rend() const { return list_.rend(); }
+  const_reverse_iterator crbegin() const { return list_.crbegin(); }
+  const_reverse_iterator crend() const { return list_.crend(); }
+  reference front() { return list_.front(); }
+  reference back() { return list_.back(); }
+  const_reference front() const { return list_.front(); }
+  const_reference back() const { return list_.back(); }
+
+  void pop_front() { erase(begin()); }
+  void pop_back() { erase(std::prev(end())); }
+
+  ABSL_ATTRIBUTE_REINITIALIZES void clear() {
+    set_.clear();
+    list_.clear();
+  }
+
+  void reserve(size_t n) { set_.reserve(n); }
+  size_t capacity() const { return set_.capacity(); }
+  size_t bucket_count() const { return set_.bucket_count(); }
+  float load_factor() const { return set_.load_factor(); }
+
+  hasher hash_function() const { return set_.hash_function().fn_; }
+  key_equal key_eq() const { return set_.key_eq().fn_; }
+  allocator_type get_allocator() const { return list_.get_allocator(); }
+
+  template <class K = key_type>
+  size_type erase(const key_arg<K>& key) {
+    auto found = set_.find(key);
+    if (found == set_.end()) return 0;
+    auto list_it = *found;
+    // Erase set entry first since it refers to the list element.
+    set_.erase(found);
+    list_.erase(list_it);
+    return 1;
+  }
+
+  iterator erase(const_iterator position) {
+    auto found = set_.find(position);
+    assert(*found == position);
+    set_.erase(found);
+    return list_.erase(position);
+  }
+
+  iterator erase(iterator position) {
+    return erase(static_cast<const_iterator>(position));
+  }
+
+  iterator erase(iterator first, iterator last) {
+    while (first != last) first = erase(first);
+    return first;
+  }
+
+  iterator erase(const_iterator first, const_iterator last) {
+    while (first != last) first = erase(first);
+    if (first == end()) return end();
+    return *set_.find(first);
+  }
+
+  template <class K = key_type>
+  iterator find(const key_arg<K>& key) {
+    auto found = set_.find(key);
+    if (found == set_.end()) return end();
+    return *found;
+  }
+
+  template <class K = key_type>
+  const_iterator find(const key_arg<K>& key) const {
+    auto found = set_.find(key);
+    if (found == set_.end()) return end();
+    return *found;
+  }
+
+  template <class K = key_type>
+  size_type count(const key_arg<K>& key) const {
+    return contains(key) ? 1 : 0;
+  }
+  template <class K = key_type>
+  bool contains(const key_arg<K>& key) const {
+    return set_.contains(key);
+  }
+
+  template <class K = key_type>
+  mapped_type& at(const key_arg<K>& key) {
+    auto it = find(key);
+    if (ABSL_PREDICT_FALSE(it == end())) {
+      ThrowStdOutOfRange("absl::linked_hash_map::at");
+    }
+    return it->second;
+  }
+
+  template <class K = key_type>
+  const mapped_type& at(const key_arg<K>& key) const {
+    return const_cast<linked_hash_map*>(this)->at(key);
+  }
+
+  template <class K = key_type>
+  std::pair<iterator, iterator> equal_range(const key_arg<K>& key) {
+    auto iter = set_.find(key);
+    if (iter == set_.end()) return {end(), end()};
+    return {*iter, std::next(*iter)};
+  }
+
+  template <class K = key_type>
+  std::pair<const_iterator, const_iterator> equal_range(
+      const key_arg<K>& key) const {
+    auto iter = set_.find(key);
+    if (iter == set_.end()) return {end(), end()};
+    return {*iter, std::next(*iter)};
+  }
+
+  template <class K = key_type>
+  mapped_type& operator[](const key_arg<K>& key) {
+    return LazyEmplaceInternal(key).first->second;
+  }
+
+  template <class K = key_type, K* = nullptr>
+  mapped_type& operator[](key_arg<K>&& key) {
+    return LazyEmplaceInternal(std::forward<key_arg<K>>(key)).first->second;
+  }
+
+  std::pair<iterator, bool> insert(const value_type& v) {
+    return InsertInternal(v);
+  }
+  std::pair<iterator, bool> insert(value_type&& v) {
+    return InsertInternal(std::move(v));
+  }
+
+  iterator insert(const_iterator, const value_type& v) {
+    return insert(v).first;
+  }
+  iterator insert(const_iterator, value_type&& v) {
+    return insert(std::move(v)).first;
+  }
+
+  void insert(std::initializer_list<value_type> ilist) {
+    insert(ilist.begin(), ilist.end());
+  }
+
+  template <class InputIt>
+  void insert(InputIt first, InputIt last) {
+    for (; first != last; ++first) insert(*first);
+  }
+
+  insert_return_type insert(node_type&& node) {
+    if (node.empty()) return {end(), false, node_type()};
+    if (auto [set_itr, inserted] = set_.emplace(node.list_.begin()); inserted) {
+      list_.splice(list_.end(), node.list_);
+      return {*set_itr, true, node_type()};
+    } else {
+      return {*set_itr, false, std::move(node)};
+    }
+  }
+
+  iterator insert(const_iterator, node_type&& node) {
+    return insert(std::move(node)).first;
+  }
+
+  // The last two template parameters ensure that both arguments are rvalues
+  // (lvalue arguments are handled by the overloads below). This is necessary
+  // for supporting bitfield arguments.
+  //
+  //   union { int n : 1; };
+  //   linked_hash_map<int, int> m;
+  //   m.insert_or_assign(n, n);
+  template <class K = key_type, class V = mapped_type, K* = nullptr,
+            V* = nullptr>
+  std::pair<iterator, bool> insert_or_assign(key_arg<K>&& k, V&& v) {
+    return InsertOrAssignInternal(std::forward<key_arg<K>>(k),
+                                  std::forward<V>(v));
+  }
+
+  template <class K = key_type, class V = mapped_type, K* = nullptr>
+  std::pair<iterator, bool> insert_or_assign(key_arg<K>&& k, const V& v) {
+    return InsertOrAssignInternal(std::forward<key_arg<K>>(k), v);
+  }
+
+  template <class K = key_type, class V = mapped_type, V* = nullptr>
+  std::pair<iterator, bool> insert_or_assign(const key_arg<K>& k, V&& v) {
+    return InsertOrAssignInternal(k, std::forward<V>(v));
+  }
+
+  template <class K = key_type, class V = mapped_type>
+  std::pair<iterator, bool> insert_or_assign(const key_arg<K>& k, const V& v) {
+    return InsertOrAssignInternal(k, v);
+  }
+
+  template <class K = key_type, class V = mapped_type, K* = nullptr,
+            V* = nullptr>
+  iterator insert_or_assign(const_iterator, key_arg<K>&& k, V&& v) {
+    return insert_or_assign(std::forward<key_arg<K>>(k), std::forward<V>(v))
+        .first;
+  }
+
+  template <class K = key_type, class V = mapped_type, K* = nullptr>
+  iterator insert_or_assign(const_iterator, key_arg<K>&& k, const V& v) {
+    return insert_or_assign(std::forward<key_arg<K>>(k), v).first;
+  }
+
+  template <class K = key_type, class V = mapped_type, V* = nullptr>
+  iterator insert_or_assign(const_iterator, const key_arg<K>& k, V&& v) {
+    return insert_or_assign(k, std::forward<V>(v)).first;
+  }
+
+  template <class K = key_type, class V = mapped_type>
+  iterator insert_or_assign(const_iterator, const key_arg<K>& k, const V& v) {
+    return insert_or_assign(k, v).first;
+  }
+
+  template <typename... Args>
+  std::pair<iterator, bool> emplace(Args&&... args) {
+    ListType node_donor(get_allocator());
+    auto list_iter =
+        node_donor.emplace(node_donor.end(), std::forward<Args>(args)...);
+    auto ins = set_.insert(list_iter);
+    if (!ins.second) return {*ins.first, false};
+    list_.splice(list_.end(), node_donor, list_iter);
+    return {list_iter, true};
+  }
+
+  template <class K = key_type, class... Args, K* = nullptr>
+  iterator try_emplace(const_iterator, key_arg<K>&& k, Args&&... args) {
+    return try_emplace(std::forward<key_arg<K>>(k), std::forward<Args>(args)...)
+        .first;
+  }
+
+  template <typename... Args>
+  iterator emplace_hint(const_iterator, Args&&... args) {
+    return emplace(std::forward<Args>(args)...).first;
+  }
+
+  template <class K = key_type, typename... Args, K* = nullptr>
+  std::pair<iterator, bool> try_emplace(key_arg<K>&& key, Args&&... args) {
+    return LazyEmplaceInternal(std::forward<key_arg<K>>(key),
+                               std::forward<Args>(args)...);
+  }
+
+  template <typename H, typename E>
+  void merge(linked_hash_map<Key, Value, H, E, Alloc>& src) {
+    auto itr = src.list_.begin();
+    while (itr != src.list_.end()) {
+      if (contains(itr->first)) {
+        ++itr;
+      } else {
+        insert(src.extract(itr++));
+      }
+    }
+  }
+
+  template <typename H, typename E>
+  void merge(linked_hash_map<Key, Value, H, E, Alloc>&& src) {
+    merge(src);
+  }
+
+  node_type extract(const_iterator position) {
+    set_.erase(position->first);
+    ListType extracted_node_list(get_allocator());
+    extracted_node_list.splice(extracted_node_list.end(), list_, position);
+    return node_type(std::move(extracted_node_list));
+  }
+
+  template <class K = key_type,
+            std::enable_if_t<!std::is_same_v<K, iterator>, int> = 0>
+  node_type extract(const key_arg<K>& key) {
+    auto node = set_.extract(key);
+    if (node.empty()) return node_type();
+    ListType extracted_node_list(get_allocator());
+    extracted_node_list.splice(extracted_node_list.end(), list_, node.value());
+    return node_type(std::move(extracted_node_list));
+  }
+
+  template <typename H, typename E>
+  void splice(const_iterator, linked_hash_map<Key, Value, H, E, Alloc>& list,
+              const_iterator it) {
+    if (&list == this) {
+      list_.splice(list_.end(), list.list_, it);
+    } else {
+      insert(list.extract(it));
+    }
+  }
+
+  template <class K = key_type, typename... Args>
+  std::pair<iterator, bool> try_emplace(const key_arg<K>& key, Args&&... args) {
+    return LazyEmplaceInternal(key, std::forward<Args>(args)...);
+  }
+
+  template <class K = key_type, typename... Args>
+  iterator try_emplace(const_iterator, const key_arg<K>& key, Args&&... args) {
+    return LazyEmplaceInternal(key, std::forward<Args>(args)...).first;
+  }
+
+  void swap(linked_hash_map& other) noexcept {
+    using std::swap;
+    swap(set_, other.set_);
+    swap(list_, other.list_);
+  }
+
+  friend bool operator==(const linked_hash_map& a, const linked_hash_map& b) {
+    if (a.size() != b.size()) return false;
+    const linked_hash_map* outer = &a;
+    const linked_hash_map* inner = &b;
+    if (outer->capacity() > inner->capacity()) std::swap(outer, inner);
+    for (const value_type& elem : *outer) {
+      auto it = inner->find(elem.first);
+      if (it == inner->end()) return false;
+      if (it->second != elem.second) return false;
+    }
+
+    return true;
+  }
+
+  friend bool operator!=(const linked_hash_map& a, const linked_hash_map& b) {
+    return !(a == b);
+  }
+
+  void rehash(size_t n) { set_.rehash(n); }
+
+ private:
+  template <typename Other>
+  void CopyFrom(Other&& other) {
+    for (auto& elem : other.list_) {
+      set_.insert(list_.insert(list_.end(), std::move(elem)));
+    }
+    assert(set_.size() == list_.size());
+  }
+
+  template <typename U>
+  std::pair<iterator, bool> InsertInternal(U&& pair) {  // NOLINT(build/c++11)
+    bool constructed = false;
+    auto set_iter = set_.lazy_emplace(pair.first, [&](const auto& ctor) {
+      constructed = true;
+      ctor(list_.emplace(list_.end(), std::forward<U>(pair)));
+    });
+    return {*set_iter, constructed};
+  }
+
+  template <class K, class V>
+  std::pair<iterator, bool> InsertOrAssignInternal(K&& k, V&& v) {
+    auto [it, inserted] =
+        LazyEmplaceInternal(std::forward<K>(k), std::forward<V>(v));
+    if (!inserted) {
+      // NOLINTNEXTLINE(bugprone-use-after-move)
+      it->second = std::forward<V>(v);
+    }
+    return {it, inserted};
+  }
+
+  template <typename K, typename... Args>
+  std::pair<iterator, bool> LazyEmplaceInternal(K&& key, Args&&... args) {
+    bool constructed = false;
+    auto set_iter = set_.lazy_emplace(
+        key, [this, &constructed, &key, &args...](const auto& ctor) {
+          auto list_iter =
+              list_.emplace(list_.end(), std::piecewise_construct,
+                            std::forward_as_tuple(std::forward<K>(key)),
+                            std::forward_as_tuple(std::forward<Args>(args)...));
+          constructed = true;
+          ctor(list_iter);
+        });
+    return {*set_iter, constructed};
+  }
+
+  // The set component, used for speedy lookups.
+  SetType set_;
+
+  // The list component, used for maintaining insertion order.
+  ListType list_;
+};
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_CONTAINER_LINKED_HASH_MAP_H_
diff --git a/absl/container/linked_hash_map_benchmark.cc b/absl/container/linked_hash_map_benchmark.cc
new file mode 100644
index 00000000000..5a0db38a7bc
--- /dev/null
+++ b/absl/container/linked_hash_map_benchmark.cc
@@ -0,0 +1,140 @@
+// Copyright 2025 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <algorithm>
+#include <cstddef>
+#include <string>
+#include <vector>
+
+#include "absl/container/linked_hash_map.h"
+#include "absl/functional/function_ref.h"
+#include "absl/strings/str_format.h"
+#include "absl/strings/string_view.h"
+#include "benchmark/benchmark.h"
+
+namespace {
+
+struct BenchmarkData {
+  std::vector<std::string> sample;
+  size_t str_bytes = 0;
+  explicit BenchmarkData(int size,
+                         absl::FunctionRef<std::string(int)> factory) {
+    sample.reserve(size);
+    for (int i = 0; i < size; ++i) {
+      sample.push_back(factory(i));
+      str_bytes += sample.back().size();
+    }
+  }
+};
+
+void BenchmarkTryEmplaceStrings(benchmark::State& state,
+                                absl::FunctionRef<std::string(int)> factory) {
+  BenchmarkData data(state.range(0), factory);
+
+  // Make a batch around 1Mi bytes.
+  const size_t batch_size =
+      std::max(size_t{1}, size_t{1000000} / data.str_bytes);
+  std::vector<absl::linked_hash_map<std::string, int>> sets(batch_size);
+
+  while (state.KeepRunningBatch(batch_size)) {
+    state.PauseTiming();
+    for (auto& set : sets) set.clear();
+    state.ResumeTiming();
+    for (auto& set : sets) {
+      for (const auto& str : data.sample) {
+        benchmark::DoNotOptimize(set.try_emplace(str));
+      }
+    }
+  }
+
+  state.SetItemsProcessed(state.iterations() * state.range(0));
+  state.SetBytesProcessed(state.iterations() * data.str_bytes);
+}
+
+void BenchmarkInsertOrAssignStrings(
+    benchmark::State& state, absl::FunctionRef<std::string(int)> factory) {
+  BenchmarkData data(state.range(0), factory);
+
+  // Make a batch around 1Mi bytes.
+  const size_t batch_size = std::max(size_t{1}, 1000000 / data.str_bytes);
+  std::vector<absl::linked_hash_map<std::string, int>> sets(batch_size);
+
+  while (state.KeepRunningBatch(batch_size)) {
+    state.PauseTiming();
+    for (auto& set : sets) set.clear();
+    state.ResumeTiming();
+    for (auto& set : sets) {
+      for (const auto& str : data.sample) {
+        benchmark::DoNotOptimize(set.insert_or_assign(str, 0));
+      }
+    }
+  }
+
+  state.SetItemsProcessed(state.iterations() * state.range(0));
+  state.SetBytesProcessed(state.iterations() * data.str_bytes);
+}
+
+constexpr absl::string_view kFormatShort = "%10d";
+constexpr absl::string_view kFormatLong =
+    "a longer string that exceeds the SSO %10d";
+
+void BM_TryEmplaceShortStrings_Hit(benchmark::State& state) {
+  BenchmarkTryEmplaceStrings(
+      state, [](int i) { return absl::StrFormat(kFormatShort, i); });
+}
+BENCHMARK(BM_TryEmplaceShortStrings_Hit)->Range(1, 1 << 16);
+
+void BM_TryEmplaceLongStrings_Hit(benchmark::State& state) {
+  BenchmarkTryEmplaceStrings(
+      state, [](int i) { return absl::StrFormat(kFormatLong, i); });
+}
+BENCHMARK(BM_TryEmplaceLongStrings_Hit)->Range(1, 1 << 16);
+
+void BM_TryEmplaceShortStrings_Miss(benchmark::State& state) {
+  BenchmarkTryEmplaceStrings(
+      state, [](int i) { return absl::StrFormat(kFormatShort, i % 20); });
+}
+BENCHMARK(BM_TryEmplaceShortStrings_Miss)->Range(1, 1 << 16);
+
+void BM_TryEmplaceLongStrings_Miss(benchmark::State& state) {
+  BenchmarkTryEmplaceStrings(
+      state, [](int i) { return absl::StrFormat(kFormatLong, i % 20); });
+}
+BENCHMARK(BM_TryEmplaceLongStrings_Miss)->Range(1, 1 << 16);
+
+void BM_InsertOrAssignShortStrings_Hit(benchmark::State& state) {
+  BenchmarkInsertOrAssignStrings(
+      state, [](int i) { return absl::StrFormat(kFormatShort, i); });
+}
+BENCHMARK(BM_InsertOrAssignShortStrings_Hit)->Range(1, 1 << 16);
+
+void BM_InsertOrAssignLongStrings_Hit(benchmark::State& state) {
+  BenchmarkInsertOrAssignStrings(
+      state, [](int i) { return absl::StrFormat(kFormatLong, i); });
+}
+BENCHMARK(BM_InsertOrAssignLongStrings_Hit)->Range(1, 1 << 16);
+
+void BM_InsertOrAssignShortStrings_Miss(benchmark::State& state) {
+  BenchmarkInsertOrAssignStrings(
+      state, [](int i) { return absl::StrFormat(kFormatShort, i % 20); });
+}
+BENCHMARK(BM_InsertOrAssignShortStrings_Miss)->Range(1, 1 << 16);
+
+void BM_InsertOrAssignLongStrings_Miss(benchmark::State& state) {
+  BenchmarkInsertOrAssignStrings(
+      state, [](int i) { return absl::StrFormat(kFormatLong, i % 20); });
+}
+BENCHMARK(BM_InsertOrAssignLongStrings_Miss)->Range(1, 1 << 16);
+
+}  // namespace
diff --git a/absl/container/linked_hash_map_test.cc b/absl/container/linked_hash_map_test.cc
new file mode 100644
index 00000000000..8b95c123eee
--- /dev/null
+++ b/absl/container/linked_hash_map_test.cc
@@ -0,0 +1,1015 @@
+// Copyright 2025 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/container/linked_hash_map.h"
+
+#include <algorithm>
+#include <cstddef>
+#include <cstdint>
+#include <functional>
+#include <memory>
+#include <string>
+#include <tuple>
+#include <type_traits>
+#include <utility>
+#include <vector>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/exception_testing.h"
+#include "absl/container/internal/hash_generator_testing.h"
+#include "absl/container/internal/hash_policy_testing.h"
+#include "absl/container/internal/heterogeneous_lookup_testing.h"
+#include "absl/container/internal/test_allocator.h"
+#include "absl/container/internal/test_instance_tracker.h"
+#include "absl/container/internal/unordered_map_constructor_test.h"
+#include "absl/container/internal/unordered_map_lookup_test.h"
+#include "absl/container/internal/unordered_map_members_test.h"
+#include "absl/container/internal/unordered_map_modifiers_test.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace container_internal {
+namespace {
+
+using ::testing::ElementsAre;
+using ::testing::Pair;
+using ::testing::Pointee;
+
+template <class K, class V>
+using Map = linked_hash_map<K, V, StatefulTestingHash, StatefulTestingEqual,
+                            Alloc<std::pair<const K, V>>>;
+
+static_assert(!std::is_standard_layout<NonStandardLayout>());
+
+using MapTypes =
+    ::testing::Types<Map<int, int>, Map<std::string, int>,
+                     Map<Enum, std::string>, Map<EnumClass, int>,
+                     Map<int, NonStandardLayout>, Map<NonStandardLayout, int>>;
+
+INSTANTIATE_TYPED_TEST_SUITE_P(LinkedHashMap, ConstructorTest, MapTypes);
+INSTANTIATE_TYPED_TEST_SUITE_P(LinkedHashMap, LookupTest, MapTypes);
+INSTANTIATE_TYPED_TEST_SUITE_P(LinkedHashMap, MembersTest, MapTypes);
+INSTANTIATE_TYPED_TEST_SUITE_P(LinkedHashMap, ModifiersTest, MapTypes);
+
+// Tests that the range constructor works.
+TEST(LinkedHashMapTest, RangeConstruct) {
+  const std::pair<int, int> items[] = {{1, 2}, {2, 3}, {3, 4}};
+  EXPECT_THAT((linked_hash_map<int, int>(std::begin(items), std::end(items))),
+              ElementsAre(Pair(1, 2), Pair(2, 3), Pair(3, 4)));
+}
+
+// Tests that copying works.
+TEST(LinkedHashMapTest, Copy) {
+  linked_hash_map<int, int> m{{2, 12}, {3, 13}};
+  auto copy = m;
+
+  EXPECT_TRUE(copy.contains(2));
+  auto found = copy.find(2);
+  ASSERT_TRUE(found != copy.end());
+  for (auto iter = copy.begin(); iter != copy.end(); ++iter) {
+    if (iter == found) return;
+  }
+  FAIL() << "Copied map's find method returned an invalid iterator.";
+}
+
+// Tests that assignment works.
+TEST(LinkedHashMapTest, Assign) {
+  linked_hash_map<int, int> m{{2, 12}, {3, 13}};
+  linked_hash_map<int, int> n{{4, 14}};
+
+  n = m;
+  EXPECT_TRUE(n.contains(2));
+  auto found = n.find(2);
+  ASSERT_TRUE(found != n.end());
+  for (auto iter = n.begin(); iter != n.end(); ++iter) {
+    if (iter == found) return;
+  }
+  FAIL() << "Assigned map's find method returned an invalid iterator.";
+}
+
+// Tests that self-assignment works.
+TEST(LinkedHashMapTest, SelfAssign) {
+  linked_hash_map<int, int> a{{1, 1}, {2, 2}, {3, 3}};
+  auto& a_ref = a;
+  a = a_ref;
+  EXPECT_THAT(a, ElementsAre(Pair(1, 1), Pair(2, 2), Pair(3, 3)));
+}
+
+// Tests that move constructor works.
+TEST(LinkedHashMapTest, Move) {
+  // Use unique_ptr as an example of a non-copyable type.
+  linked_hash_map<int, std::unique_ptr<int>> m;
+  m[2] = std::make_unique<int>(12);
+  m[3] = std::make_unique<int>(13);
+  linked_hash_map<int, std::unique_ptr<int>> n = std::move(m);
+  EXPECT_THAT(n, ElementsAre(Pair(2, Pointee(12)), Pair(3, Pointee(13))));
+}
+
+// Tests that self-moving works.
+TEST(LinkedHashMapTest, SelfMove) {
+  linked_hash_map<int, int> a{{1, 1}, {2, 2}, {3, 3}};
+  auto& a_ref = a;
+  a = std::move(a_ref);
+  EXPECT_THAT(a, ElementsAre(Pair(1, 1), Pair(2, 2), Pair(3, 3)));
+}
+
+TEST(LinkedHashMapTest, CanInsertMoveOnly) {
+  linked_hash_map<int, std::unique_ptr<int>> m;
+  struct Data {
+    int k, v;
+  };
+  const Data data[] = {{1, 123}, {3, 345}, {2, 234}, {4, 456}};
+  for (const auto& kv : data)
+    m.insert({kv.k, std::make_unique<int>(int{kv.v})});
+  EXPECT_TRUE(m.contains(2));
+  auto found = m.find(2);
+  ASSERT_TRUE(found != m.end());
+  EXPECT_EQ(234, *found->second);
+}
+
+TEST(LinkedHashMapTest, CanEmplaceMoveOnly) {
+  linked_hash_map<int, std::unique_ptr<int>> m;
+  struct Data {
+    int k, v;
+  };
+  const Data data[] = {{1, 123}, {3, 345}, {2, 234}, {4, 456}};
+  for (const auto& kv : data) {
+    m.emplace(std::piecewise_construct, std::make_tuple(kv.k),
+              std::make_tuple(new int{kv.v}));
+  }
+  EXPECT_TRUE(m.contains(2));
+  auto found = m.find(2);
+  ASSERT_TRUE(found != m.end());
+  EXPECT_EQ(234, *found->second);
+}
+
+struct NoCopy {
+  explicit NoCopy(int x) : x(x) {}
+  NoCopy(const NoCopy&) = delete;
+  NoCopy& operator=(const NoCopy&) = delete;
+  NoCopy(NoCopy&&) = delete;
+  NoCopy& operator=(NoCopy&&) = delete;
+  int x;
+};
+
+TEST(LinkedHashMapTest, CanEmplaceNoMoveNoCopy) {
+  linked_hash_map<int, NoCopy> m;
+  struct Data {
+    int k, v;
+  };
+  const Data data[] = {{1, 123}, {3, 345}, {2, 234}, {4, 456}};
+  for (const auto& kv : data) {
+    m.emplace(std::piecewise_construct, std::make_tuple(kv.k),
+              std::make_tuple(kv.v));
+  }
+  EXPECT_TRUE(m.contains(2));
+  auto found = m.find(2);
+  ASSERT_TRUE(found != m.end());
+  EXPECT_EQ(234, found->second.x);
+}
+
+TEST(LinkedHashMapTest, ConstKeys) {
+  linked_hash_map<int, int> m;
+  m.insert(std::make_pair(1, 2));
+  // Test that keys are const in iteration.
+  std::pair<const int, int>& p = *m.begin();
+  EXPECT_EQ(1, p.first);
+}
+
+// Tests that iteration from begin() to end() works
+TEST(LinkedHashMapTest, Iteration) {
+  linked_hash_map<int, int> m;
+  EXPECT_TRUE(m.begin() == m.end());
+
+  m.insert(std::make_pair(2, 12));
+  m.insert(std::make_pair(1, 11));
+  m.insert(std::make_pair(3, 13));
+
+  linked_hash_map<int, int>::iterator i = m.begin();
+  ASSERT_TRUE(m.begin() == i);
+  ASSERT_TRUE(m.end() != i);
+  EXPECT_EQ(2, i->first);
+  EXPECT_EQ(12, i->second);
+
+  ++i;
+  ASSERT_TRUE(m.end() != i);
+  EXPECT_EQ(1, i->first);
+  EXPECT_EQ(11, i->second);
+
+  ++i;
+  ASSERT_TRUE(m.end() != i);
+  EXPECT_EQ(3, i->first);
+  EXPECT_EQ(13, i->second);
+
+  ++i;  // Should be the end of the line.
+  ASSERT_TRUE(m.end() == i);
+}
+
+// Tests that reverse iteration from rbegin() to rend() works
+TEST(LinkedHashMapTest, ReverseIteration) {
+  linked_hash_map<int, int> m;
+  EXPECT_TRUE(m.rbegin() == m.rend());
+
+  m.insert(std::make_pair(2, 12));
+  m.insert(std::make_pair(1, 11));
+  m.insert(std::make_pair(3, 13));
+
+  linked_hash_map<int, int>::reverse_iterator i = m.rbegin();
+  ASSERT_TRUE(m.rbegin() == i);
+  ASSERT_TRUE(m.rend() != i);
+  EXPECT_EQ(3, i->first);
+  EXPECT_EQ(13, i->second);
+
+  ++i;
+  ASSERT_TRUE(m.rend() != i);
+  EXPECT_EQ(1, i->first);
+  EXPECT_EQ(11, i->second);
+
+  ++i;
+  ASSERT_TRUE(m.rend() != i);
+  EXPECT_EQ(2, i->first);
+  EXPECT_EQ(12, i->second);
+
+  ++i;  // Should be the end of the line.
+  ASSERT_TRUE(m.rend() == i);
+}
+
+// Tests that clear() works
+TEST(LinkedHashMapTest, Clear) {
+  linked_hash_map<int, int> m;
+  m.insert(std::make_pair(2, 12));
+  m.insert(std::make_pair(1, 11));
+  m.insert(std::make_pair(3, 13));
+
+  ASSERT_EQ(3, m.size());
+
+  m.clear();
+
+  EXPECT_EQ(0, m.size());
+
+  m.clear();  // Make sure we can call it on an empty map.
+
+  EXPECT_EQ(0, m.size());
+}
+
+// Tests that size() works.
+TEST(LinkedHashMapTest, Size) {
+  linked_hash_map<int, int> m;
+  EXPECT_EQ(0, m.size());
+  m.insert(std::make_pair(2, 12));
+  EXPECT_EQ(1, m.size());
+  m.insert(std::make_pair(1, 11));
+  EXPECT_EQ(2, m.size());
+  m.insert(std::make_pair(3, 13));
+  EXPECT_EQ(3, m.size());
+  m.clear();
+  EXPECT_EQ(0, m.size());
+}
+
+// Tests empty()
+TEST(LinkedHashMapTest, Empty) {
+  linked_hash_map<int, int> m;
+  ASSERT_TRUE(m.empty());
+  m.insert(std::make_pair(2, 12));
+  ASSERT_FALSE(m.empty());
+  m.clear();
+  ASSERT_TRUE(m.empty());
+}
+
+TEST(LinkedHashMapTest, Erase) {
+  linked_hash_map<int, int> m;
+  ASSERT_EQ(0, m.size());
+  EXPECT_EQ(0, m.erase(2));  // Nothing to erase yet
+
+  m.insert(std::make_pair(2, 12));
+  ASSERT_EQ(1, m.size());
+  EXPECT_EQ(1, m.erase(2));
+  EXPECT_EQ(0, m.size());
+
+  EXPECT_EQ(0, m.erase(2));  // Make sure nothing bad happens if we repeat.
+  EXPECT_EQ(0, m.size());
+}
+
+TEST(LinkedHashMapTest, Erase2) {
+  linked_hash_map<int, int> m;
+  ASSERT_EQ(0, m.size());
+  EXPECT_EQ(0, m.erase(2));  // Nothing to erase yet
+
+  m.insert(std::make_pair(2, 12));
+  m.insert(std::make_pair(1, 11));
+  m.insert(std::make_pair(3, 13));
+  m.insert(std::make_pair(4, 14));
+  ASSERT_EQ(4, m.size());
+
+  // Erase middle two
+  EXPECT_EQ(1, m.erase(1));
+  EXPECT_EQ(1, m.erase(3));
+
+  EXPECT_EQ(2, m.size());
+
+  // Make sure we can still iterate over everything that's left.
+  linked_hash_map<int, int>::iterator it = m.begin();
+  ASSERT_TRUE(it != m.end());
+  EXPECT_EQ(12, it->second);
+  ++it;
+  ASSERT_TRUE(it != m.end());
+  EXPECT_EQ(14, it->second);
+  ++it;
+  ASSERT_TRUE(it == m.end());
+
+  EXPECT_EQ(0, m.erase(1));  // Make sure nothing bad happens if we repeat.
+  ASSERT_EQ(2, m.size());
+
+  EXPECT_EQ(1, m.erase(2));
+  EXPECT_EQ(1, m.erase(4));
+  ASSERT_EQ(0, m.size());
+
+  EXPECT_EQ(0, m.erase(1));  // Make sure nothing bad happens if we repeat.
+  ASSERT_EQ(0, m.size());
+}
+
+// Test that erase(iter,iter) and erase(iter) compile and work.
+TEST(LinkedHashMapTest, Erase3) {
+  linked_hash_map<int, int> m;
+
+  m.insert(std::make_pair(1, 11));
+  m.insert(std::make_pair(2, 12));
+  m.insert(std::make_pair(3, 13));
+  m.insert(std::make_pair(4, 14));
+
+  // Erase middle two
+  linked_hash_map<int, int>::iterator it2 = m.find(2);
+  linked_hash_map<int, int>::iterator it4 = m.find(4);
+  EXPECT_EQ(m.erase(it2, it4), m.find(4));
+  EXPECT_EQ(2, m.size());
+
+  // Make sure we can still iterate over everything that's left.
+  linked_hash_map<int, int>::iterator it = m.begin();
+  ASSERT_TRUE(it != m.end());
+  EXPECT_EQ(11, it->second);
+  ++it;
+  ASSERT_TRUE(it != m.end());
+  EXPECT_EQ(14, it->second);
+  ++it;
+  ASSERT_TRUE(it == m.end());
+
+  // Erase first one using an iterator.
+  EXPECT_EQ(m.erase(m.begin()), m.find(4));
+
+  // Only the last element should be left.
+  it = m.begin();
+  ASSERT_TRUE(it != m.end());
+  EXPECT_EQ(14, it->second);
+  ++it;
+  ASSERT_TRUE(it == m.end());
+}
+
+// Test all types of insertion
+TEST(LinkedHashMapTest, Insertion) {
+  linked_hash_map<int, int> m;
+  ASSERT_EQ(0, m.size());
+  std::pair<linked_hash_map<int, int>::iterator, bool> result;
+
+  result = m.insert(std::make_pair(2, 12));
+  ASSERT_EQ(1, m.size());
+  EXPECT_TRUE(result.second);
+  EXPECT_EQ(2, result.first->first);
+  EXPECT_EQ(12, result.first->second);
+
+  result = m.insert(std::make_pair(1, 11));
+  ASSERT_EQ(2, m.size());
+  EXPECT_TRUE(result.second);
+  EXPECT_EQ(1, result.first->first);
+  EXPECT_EQ(11, result.first->second);
+
+  result = m.insert(std::make_pair(3, 13));
+  linked_hash_map<int, int>::iterator result_iterator = result.first;
+  ASSERT_EQ(3, m.size());
+  EXPECT_TRUE(result.second);
+  EXPECT_EQ(3, result.first->first);
+  EXPECT_EQ(13, result.first->second);
+
+  result = m.insert(std::make_pair(3, 13));
+  EXPECT_EQ(3, m.size());
+  EXPECT_FALSE(result.second) << "No insertion should have occurred.";
+  EXPECT_TRUE(result_iterator == result.first)
+      << "Duplicate insertion should have given us the original iterator.";
+
+  std::vector<std::pair<int, int>> v = {{3, 13}, {4, 14}, {5, 15}};
+  m.insert(v.begin(), v.end());
+  // Expect 4 and 5 inserted, 3 not inserted.
+  EXPECT_EQ(5, m.size());
+  EXPECT_EQ(14, m.at(4));
+  EXPECT_EQ(15, m.at(5));
+}
+
+static std::pair<const int, int> Pair(int i, int j) { return {i, j}; }
+
+// Test front accessors.
+TEST(LinkedHashMapTest, Front) {
+  linked_hash_map<int, int> m;
+
+  m.insert(std::make_pair(2, 12));
+  m.insert(std::make_pair(1, 11));
+  m.insert(std::make_pair(3, 13));
+
+  EXPECT_EQ(3, m.size());
+  EXPECT_EQ(Pair(2, 12), m.front());
+  m.pop_front();
+  EXPECT_EQ(2, m.size());
+  EXPECT_EQ(Pair(1, 11), m.front());
+  m.pop_front();
+  EXPECT_EQ(1, m.size());
+  EXPECT_EQ(Pair(3, 13), m.front());
+  m.pop_front();
+  EXPECT_TRUE(m.empty());
+}
+
+// Test back accessors.
+TEST(LinkedHashMapTest, Back) {
+  linked_hash_map<int, int> m;
+
+  m.insert(std::make_pair(2, 12));
+  m.insert(std::make_pair(1, 11));
+  m.insert(std::make_pair(3, 13));
+
+  EXPECT_EQ(3, m.size());
+  EXPECT_EQ(Pair(3, 13), m.back());
+  m.pop_back();
+  EXPECT_EQ(2, m.size());
+  EXPECT_EQ(Pair(1, 11), m.back());
+  m.pop_back();
+  EXPECT_EQ(1, m.size());
+  EXPECT_EQ(Pair(2, 12), m.back());
+  m.pop_back();
+  EXPECT_TRUE(m.empty());
+}
+
+TEST(LinkedHashMapTest, Find) {
+  linked_hash_map<int, int> m;
+
+  EXPECT_TRUE(m.end() == m.find(1))
+      << "We shouldn't find anything in an empty map.";
+
+  m.insert(std::make_pair(2, 12));
+  EXPECT_TRUE(m.end() == m.find(1))
+      << "We shouldn't find an element that doesn't exist in the map.";
+
+  std::pair<linked_hash_map<int, int>::iterator, bool> result =
+      m.insert(std::make_pair(1, 11));
+  ASSERT_TRUE(result.second);
+  ASSERT_TRUE(m.end() != result.first);
+  EXPECT_TRUE(result.first == m.find(1))
+      << "We should have found an element we know exists in the map.";
+  EXPECT_EQ(11, result.first->second);
+
+  // Check that a follow-up insertion doesn't affect our original
+  m.insert(std::make_pair(3, 13));
+  linked_hash_map<int, int>::iterator it = m.find(1);
+  ASSERT_TRUE(m.end() != it);
+  EXPECT_EQ(11, it->second);
+
+  m.clear();
+  EXPECT_TRUE(m.end() == m.find(1))
+      << "We shouldn't find anything in a map that we've cleared.";
+}
+
+TEST(LinkedHashMapTest, Contains) {
+  linked_hash_map<int, int> m;
+
+  EXPECT_FALSE(m.contains(1)) << "An empty map shouldn't contain anything.";
+
+  m.insert(std::make_pair(2, 12));
+  EXPECT_FALSE(m.contains(1))
+      << "The map shouldn't contain an element that doesn't exist.";
+
+  m.insert(std::make_pair(1, 11));
+  EXPECT_TRUE(m.contains(1))
+      << "The map should contain an element that we know exists.";
+
+  m.clear();
+  EXPECT_FALSE(m.contains(1))
+      << "A map that we've cleared shouldn't contain anything.";
+}
+
+TEST(LinkedHashMapTest, At) {
+  linked_hash_map<int, int> m = {{1, 2}};
+  EXPECT_EQ(2, m.at(1));
+  ABSL_BASE_INTERNAL_EXPECT_FAIL(m.at(3), std::out_of_range,
+                                 "linked_hash_map::at");
+
+  const linked_hash_map<int, int> cm = {{1, 2}};
+  EXPECT_EQ(2, cm.at(1));
+  ABSL_BASE_INTERNAL_EXPECT_FAIL(cm.at(3), std::out_of_range,
+                                 "linked_hash_map::at");
+}
+
+TEST(LinkedHashMapTest, Swap) {
+  linked_hash_map<int, int> m1;
+  linked_hash_map<int, int> m2;
+  m1.insert(std::make_pair(1, 1));
+  m1.insert(std::make_pair(2, 2));
+  m2.insert(std::make_pair(3, 3));
+  ASSERT_EQ(2, m1.size());
+  ASSERT_EQ(1, m2.size());
+  m1.swap(m2);
+  ASSERT_EQ(1, m1.size());
+  ASSERT_EQ(2, m2.size());
+}
+
+TEST(LinkedHashMapTest, SelfSwap) {
+  linked_hash_map<int, int> a{{1, 1}, {2, 2}, {3, 3}};
+  using std::swap;
+  swap(a, a);
+  EXPECT_THAT(a, ElementsAre(Pair(1, 1), Pair(2, 2), Pair(3, 3)));
+}
+
+TEST(LinkedHashMapTest, InitializerList) {
+  linked_hash_map<int, int> m{{1, 2}, {3, 4}};
+  ASSERT_EQ(2, m.size());
+  EXPECT_TRUE(m.contains(1));
+  linked_hash_map<int, int>::iterator it = m.find(1);
+  ASSERT_TRUE(m.end() != it);
+  EXPECT_EQ(2, it->second);
+  EXPECT_TRUE(m.contains(3));
+  it = m.find(3);
+  ASSERT_TRUE(m.end() != it);
+  EXPECT_EQ(4, it->second);
+}
+
+TEST(LinkedHashMapTest, CustomHashAndEquality) {
+  struct CustomIntHash {
+    size_t operator()(int x) const { return x; }
+  };
+  struct CustomIntEq {
+    bool operator()(int x, int y) const { return x == y; }
+  };
+  linked_hash_map<int, int, CustomIntHash, CustomIntEq> m;
+  m.insert(std::make_pair(1, 1));
+  EXPECT_TRUE(m.contains(1));
+  EXPECT_EQ(1, m[1]);
+}
+
+TEST(LinkedHashMapTest, EqualRange) {
+  linked_hash_map<int, int> m{{3, 11}, {1, 13}};
+  const auto& const_m = m;
+
+  EXPECT_THAT(m.equal_range(2), testing::Pair(m.end(), m.end()));
+  EXPECT_THAT(const_m.equal_range(2),
+              testing::Pair(const_m.end(), const_m.end()));
+
+  EXPECT_THAT(m.equal_range(1), testing::Pair(m.find(1), ++m.find(1)));
+  EXPECT_THAT(const_m.equal_range(1),
+              testing::Pair(const_m.find(1), ++const_m.find(1)));
+}
+
+TEST(LinkedHashMapTest, ReserveWorks) {
+  linked_hash_map<int, int> m;
+  EXPECT_EQ(0, m.size());
+  EXPECT_EQ(0.0, m.load_factor());
+  m.reserve(10);
+  EXPECT_LE(10, m.capacity());
+  EXPECT_EQ(0, m.size());
+  EXPECT_EQ(0.0, m.load_factor());
+  m.emplace(1, 1);
+  m.emplace(2, 2);
+  EXPECT_LE(10, m.capacity());
+  EXPECT_EQ(2, m.size());
+  EXPECT_LT(0.0, m.load_factor());
+}
+
+// We require key types to have hash and equals.
+class CopyableMovableType
+    : public absl::test_internal::CopyableMovableInstance {
+ public:
+  using CopyableMovableInstance::CopyableMovableInstance;
+
+  bool operator==(const CopyableMovableType& o) const {
+    return value() == o.value();
+  }
+
+  template <typename H>
+  friend H AbslHashValue(H h, const CopyableMovableType& c) {
+    return H::combine(std::move(h), c.value());
+  }
+};
+
+TEST(LinkedHashMapTest, RValueOperatorAt) {
+  absl::test_internal::InstanceTracker tracker;
+
+  linked_hash_map<CopyableMovableType, int> map;
+  map[CopyableMovableType(1)] = 1;
+  EXPECT_EQ(tracker.copies(), 0);
+  CopyableMovableType c(2);
+  map[std::move(c)] = 2;
+  EXPECT_EQ(tracker.copies(), 0);
+  EXPECT_THAT(map, ElementsAre(Pair(CopyableMovableType(1), 1),
+                               Pair(CopyableMovableType(2), 2)));
+}
+
+TEST(LinkedHashMapTest, HeterogeneousTests) {
+  absl::test_internal::InstanceTracker tracker;
+
+  using MapType = linked_hash_map<ExpensiveType, int, HeterogeneousHash,
+                                  HeterogeneousEqual>;
+  MapType map;
+  ExpensiveType one(1);
+  map[one] = 1;
+  // Two instances: 'one' var and an instance in the map.
+  EXPECT_EQ(2, tracker.instances());
+  EXPECT_EQ(1, tracker.copies());
+  EXPECT_EQ(0, tracker.moves());
+
+  tracker.ResetCopiesMovesSwaps();
+  map[one] = 5;
+  // No construction since key==1 exists.
+  EXPECT_EQ(2, tracker.instances());
+  EXPECT_EQ(0, tracker.copies());
+  EXPECT_EQ(0, tracker.moves());
+
+  tracker.ResetCopiesMovesSwaps();
+  map[CheapType(1)] = 10;
+  // No construction since key==1 exists.
+  EXPECT_EQ(2, tracker.instances());
+  EXPECT_EQ(0, tracker.copies());
+  EXPECT_EQ(0, tracker.moves());
+
+  tracker.ResetCopiesMovesSwaps();
+  map[CheapType(2)] = 20;
+  // Construction since key==2 doesn't exist in the map.
+  EXPECT_EQ(3, tracker.instances());
+  EXPECT_EQ(0, tracker.copies());
+  EXPECT_EQ(0, tracker.moves());
+  EXPECT_THAT(map, ElementsAre(Pair(HasExpensiveValue(1), 10),
+                               Pair(HasExpensiveValue(2), 20)));
+
+  // find
+  auto itr = map.find(CheapType(1));
+  tracker.ResetCopiesMovesSwaps();
+  ASSERT_NE(itr, map.end());
+  EXPECT_EQ(10, itr->second);
+  // contains
+  EXPECT_TRUE(map.contains(CheapType(2)));
+  // count
+  EXPECT_EQ(1, map.count(CheapType(2)));
+  // equal_range
+  auto eq_itr_pair = map.equal_range(CheapType(2));
+  ASSERT_NE(eq_itr_pair.first, map.end());
+  EXPECT_EQ(20, eq_itr_pair.first->second);
+  // No construction for find, contains, count or equal_range.
+  EXPECT_EQ(3, tracker.instances());
+  EXPECT_EQ(0, tracker.copies());
+  EXPECT_EQ(0, tracker.moves());
+
+  // insert
+  auto three = MapType::value_type(CheapType(3), 30);
+  tracker.ResetCopiesMovesSwaps();
+  map.insert(three);
+  // Two instances: 'three' var and an instance in the map.
+  EXPECT_EQ(5, tracker.instances());
+  EXPECT_EQ(1, tracker.copies());
+  EXPECT_EQ(0, tracker.moves());
+
+  tracker.ResetCopiesMovesSwaps();
+  map.insert(three);
+  // No additional construction since key==3 exists.
+  EXPECT_EQ(5, tracker.instances());
+  EXPECT_EQ(0, tracker.copies());
+  EXPECT_EQ(0, tracker.moves());
+  EXPECT_THAT(map, ElementsAre(Pair(HasExpensiveValue(1), 10),
+                               Pair(HasExpensiveValue(2), 20),
+                               Pair(HasExpensiveValue(3), 30)));
+
+  // Test std::move() using operator[] and insert().
+  ExpensiveType four(4);
+  tracker.ResetCopiesMovesSwaps();
+  map[std::move(four)] = 40;
+  // Two constructions (regular and move).
+  EXPECT_EQ(7, tracker.instances());
+  EXPECT_EQ(0, tracker.copies());
+  EXPECT_EQ(1, tracker.moves());
+
+  auto five = MapType::value_type(CheapType(5), 50);
+  tracker.ResetCopiesMovesSwaps();
+  map.insert(std::move(five));
+  // Two constructions (regular and move).
+  EXPECT_EQ(9, tracker.instances());
+  EXPECT_EQ(1, tracker.copies());
+  EXPECT_EQ(0, tracker.moves());
+  EXPECT_THAT(map, ElementsAre(Pair(HasExpensiveValue(1), 10),
+                               Pair(HasExpensiveValue(2), 20),
+                               Pair(HasExpensiveValue(3), 30),
+                               Pair(HasExpensiveValue(4), 40),
+                               Pair(HasExpensiveValue(5), 50)));
+
+  // Insert using std::pair.
+  tracker.ResetCopiesMovesSwaps();
+  map.insert(std::make_pair(ExpensiveType(6), 60));
+  EXPECT_EQ(10, tracker.instances());
+  EXPECT_EQ(1, tracker.copies());
+  EXPECT_EQ(2, tracker.moves());
+
+  // Heterogeneous erase.
+  map.erase(CheapType(1));
+  // No construction and instance reduced by one.
+  tracker.ResetCopiesMovesSwaps();
+  EXPECT_EQ(9, tracker.instances());
+  EXPECT_EQ(0, tracker.copies());
+  EXPECT_EQ(0, tracker.moves());
+  EXPECT_THAT(map, ElementsAre(Pair(HasExpensiveValue(2), 20),
+                               Pair(HasExpensiveValue(3), 30),
+                               Pair(HasExpensiveValue(4), 40),
+                               Pair(HasExpensiveValue(5), 50),
+                               Pair(HasExpensiveValue(6), 60)));
+}
+
+TEST(LinkedHashMapTest, HeterogeneousStringViewLookup) {
+  linked_hash_map<std::string, int> map;
+  map["foo"] = 1;
+  map["bar"] = 2;
+  map["blah"] = 3;
+
+  {
+    absl::string_view lookup("foo");
+    auto itr = map.find(lookup);
+    ASSERT_NE(itr, map.end());
+    EXPECT_EQ(1, itr->second);
+  }
+
+  // Not found.
+  {
+    absl::string_view lookup("foobar");
+    EXPECT_EQ(map.end(), map.find(lookup));
+  }
+
+  {
+    absl::string_view lookup("blah");
+    auto itr = map.find(lookup);
+    ASSERT_NE(itr, map.end());
+    EXPECT_EQ(3, itr->second);
+  }
+}
+
+TEST(LinkedHashMapTest, UniversalReferenceWorks) {
+  linked_hash_map<std::string, int> map;
+  std::string s = "very very very very very long string";
+  map[s] = 1;
+  EXPECT_EQ(s, "very very very very very long string");
+}
+
+TEST(LinkedHashMap, ExtractInsert) {
+  linked_hash_map<int, int> m = {{1, 7}, {2, 9}};
+  auto node = m.extract(1);
+  EXPECT_TRUE(node);
+  EXPECT_EQ(node.key(), 1);
+  EXPECT_EQ(node.mapped(), 7);
+  EXPECT_THAT(m, ElementsAre(Pair(2, 9)));
+  EXPECT_FALSE(m.contains(1));
+  EXPECT_TRUE(m.contains(2));
+
+  node.mapped() = 17;
+  m.insert(std::move(node));
+  EXPECT_FALSE(node);
+  EXPECT_THAT(m, ElementsAre(Pair(2, 9), Pair(1, 17)));
+  EXPECT_TRUE(m.contains(2));
+  EXPECT_TRUE(m.contains(1));
+
+  node = m.extract(m.find(1));
+  EXPECT_TRUE(node);
+  EXPECT_EQ(node.key(), 1);
+  EXPECT_EQ(node.mapped(), 17);
+  EXPECT_THAT(m, ElementsAre(Pair(2, 9)));
+}
+
+// Verify that emplacing and extracting nodes results in the same stateful
+// allocator being used for splicing purposes, rather than another instance
+// (say, a default-constructed one), which could otherwise silently corrupt
+// memory.
+TEST(LinkedHashMap, ExtractAndEmplaceUseSameStatefulAllocator) {
+  using Alloc =
+      absl::container_internal::CountingAllocator<std::pair<const int, int>>;
+  int64_t bytes_used = 0;
+  Alloc alloc(&bytes_used);
+  linked_hash_map<int, int, linked_hash_map<int, int>::hasher, std::equal_to<>,
+                  Alloc>
+      map(alloc);
+
+  map.emplace(1, 10);
+  EXPECT_GT(bytes_used, 0) << "emplace() failed to use the same allocator";
+
+  auto node = map.extract(map.begin());
+  EXPECT_EQ(node.get_allocator(), alloc)
+      << "extract(iter) failed to use the same allocator";
+
+  map.insert(std::move(node));
+  EXPECT_EQ(map.extract(1).get_allocator(), alloc)
+      << "extract(key) failed to use the same allocator";
+}
+
+TEST(LinkedHashMap, Merge) {
+  linked_hash_map<int, int> m = {{1, 7}, {3, 6}};
+  linked_hash_map<int, int> src = {{1, 10}, {2, 9}, {4, 16}};
+
+  m.merge(src);
+
+  EXPECT_THAT(m, ElementsAre(Pair(1, 7), Pair(3, 6), Pair(2, 9), Pair(4, 16)));
+  EXPECT_THAT(src, ElementsAre(Pair(1, 10)));
+  for (int i : {2, 9, 4}) {
+    EXPECT_FALSE(src.contains(i));
+  }
+}
+
+TEST(LinkedHashMap, Splice) {
+  linked_hash_map<int, int> m = {{1, 7}, {3, 6}};
+  linked_hash_map<int, int> src = {{1, 10}, {2, 9}, {4, 16}};
+  m.splice(m.end(), m, m.begin());
+
+  EXPECT_THAT(m, ElementsAre(Pair(3, 6), Pair(1, 7)));
+
+  m.splice(m.end(), src, ++src.begin());
+  EXPECT_THAT(m, ElementsAre(Pair(3, 6), Pair(1, 7), Pair(2, 9)));
+  EXPECT_THAT(src, ElementsAre(Pair(1, 10), Pair(4, 16)));
+}
+
+TEST(LinkedHashMap, EraseRange) {
+  linked_hash_map<int, int> map = {{1, 1},  {2, 4},  {3, 9},  {4, 16}, {5, 25},
+                                   {6, 36}, {7, 49}, {8, 64}, {9, 81}};
+  auto start = map.find(3);
+  auto end = map.find(8);
+  auto itr = map.erase(start, end);
+  ASSERT_NE(itr, map.end());
+  EXPECT_THAT(*itr, Pair(8, 64));
+  EXPECT_THAT(map,
+              ElementsAre(Pair(1, 1), Pair(2, 4), Pair(8, 64), Pair(9, 81)));
+  for (int i : {1, 2, 8, 9}) {
+    EXPECT_TRUE(map.contains(i));
+  }
+  for (int i : {3, 4, 5, 6, 7}) {
+    EXPECT_FALSE(map.contains(i));
+  }
+}
+
+TEST(LinkedHashMap, InsertInitializerList) {
+  linked_hash_map<int, int> m;
+  m.insert({{1, 7}, {2, 9}, {3, 29}});
+  EXPECT_THAT(m, ElementsAre(Pair(1, 7), Pair(2, 9), Pair(3, 29)));
+}
+
+TEST(LinkedHashMap, InsertOrAssign) {
+  linked_hash_map<int, int> m;
+  {
+    auto [itr, elem_inserted] = m.insert_or_assign(10, 20);
+    EXPECT_THAT(*itr, Pair(10, 20));
+    EXPECT_EQ(elem_inserted, true);
+  }
+  {
+    auto [itr, elem_inserted] = m.insert_or_assign(10, 30);
+    EXPECT_THAT(*itr, Pair(10, 30));
+    EXPECT_EQ(elem_inserted, false);
+  }
+}
+
+TEST(LinkedHashMap, TryEmplace) {
+  linked_hash_map<int, std::pair<int, std::string>> m;
+  {
+    auto [itr, elem_inserted] = m.try_emplace(10, 20, "string");
+    EXPECT_THAT(*itr, Pair(10, Pair(20, "string")));
+    EXPECT_EQ(elem_inserted, true);
+  }
+  {
+    absl::test_internal::InstanceTracker tracker;
+    std::string s = "some string";
+    auto [itr, elem_inserted] = m.try_emplace(10, 2, std::move(s));
+    EXPECT_THAT(*itr, Pair(10, Pair(20, "string")));
+    EXPECT_EQ(elem_inserted, false);
+    EXPECT_THAT(tracker.moves(), 0);
+  }
+}
+
+TEST(LinkedHashMapTest, TryEmplace) {
+  linked_hash_map<int, std::tuple<int, std::string, std::unique_ptr<float>>>
+      map;
+  auto result = map.try_emplace(3, 2, "foo", new float(1.0));
+  EXPECT_TRUE(result.second);
+  EXPECT_EQ(std::get<0>(result.first->second), 2);
+  EXPECT_EQ(std::get<1>(result.first->second), "foo");
+  EXPECT_EQ(*std::get<2>(result.first->second), 1.0);
+
+  // Ptr should not be moved.
+  auto ptr = std::make_unique<float>(3.0);
+  auto result2 = map.try_emplace(3, 22, "foo-bar", std::move(ptr));
+  EXPECT_FALSE(result2.second);
+  EXPECT_EQ(std::get<0>(result.first->second), 2);
+  EXPECT_EQ(std::get<1>(result.first->second), "foo");
+  EXPECT_EQ(*std::get<2>(result.first->second), 1.0);
+  EXPECT_NE(ptr.get(), nullptr);
+}
+
+struct CountedHash {
+  explicit CountedHash(int* count) : count(count) {}
+  size_t operator()(int value) const {
+    ++(*count);
+    return value;
+  }
+  int* count = nullptr;
+};
+
+// Makes a map too big for small object optimization.  Counts the number of
+// hashes in `count`, but leaves `count` set to 0.
+linked_hash_map<int, std::string, CountedHash> MakeNonSmallMap(int* count) {
+  const int kFirstKey = -1000;
+  linked_hash_map<int, std::string, CountedHash> m(0, CountedHash(count));
+  for (int i = kFirstKey; i < kFirstKey + 100; ++i) {
+    m[i] = "foo";
+  }
+  *count = 0;
+  return m;
+}
+
+constexpr bool BuildHasDebugModeRehashes() {
+#if !defined(NDEBUG) || defined(ABSL_HAVE_ADDRESS_SANITIZER) || \
+    defined(ABSL_HAVE_MEMORY_SANITIZER) || defined(ABSL_HAVE_THREAD_SANITIZER)
+  return true;
+#else
+  return false;
+#endif
+}
+
+TEST(LinkedHashMapTest, HashCountInOptBuilds) {
+  if (BuildHasDebugModeRehashes()) {
+    GTEST_SKIP() << "Only run under NDEBUG: `assert` statements and sanitizer "
+                    "rehashing may cause redundant hashing.";
+  }
+
+  using Map = linked_hash_map<int, std::string, CountedHash>;
+  {
+    int count = 0;
+    Map m = MakeNonSmallMap(&count);
+    m.insert({1, "foo"});
+    EXPECT_EQ(count, 1);
+    m.erase(1);
+    EXPECT_EQ(count, 2);
+  }
+  {
+    int count = 0;
+    Map m = MakeNonSmallMap(&count);
+    m[2] = "bar";
+    EXPECT_EQ(count, 1);
+  }
+  {
+    int count = 0;
+    Map m = MakeNonSmallMap(&count);
+    m.insert({3, "baz"});
+    EXPECT_EQ(count, 1);
+    auto node = m.extract(3);
+    EXPECT_EQ(count, 2);
+    m.insert(std::move(node));
+    EXPECT_EQ(count, 3);
+  }
+  {
+    int count = 0;
+    Map m = MakeNonSmallMap(&count);
+    m.insert_or_assign(4, "qux");
+    EXPECT_EQ(count, 1);
+  }
+  {
+    int count = 0;
+    Map m = MakeNonSmallMap(&count);
+    m.emplace(5, "vog");
+    EXPECT_EQ(count, 1);
+  }
+  {
+    int count = 0;
+    Map m = MakeNonSmallMap(&count);
+    m.try_emplace(6, 'x', 42);
+    EXPECT_EQ(count, 1);
+  }
+  {
+    int src_count = 0, dst_count = 0;
+    Map src = MakeNonSmallMap(&src_count);
+    Map dst = MakeNonSmallMap(&dst_count);
+    src[7] = "siete";
+    dst.merge(src);
+    EXPECT_LE(src_count, 200);
+    EXPECT_LE(dst_count, 200);
+  }
+}
+
+}  // namespace
+}  // namespace container_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/container/linked_hash_set.h b/absl/container/linked_hash_set.h
new file mode 100644
index 00000000000..7fe0adc09d6
--- /dev/null
+++ b/absl/container/linked_hash_set.h
@@ -0,0 +1,533 @@
+// Copyright 2025 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: linked_hash_set.h
+// -----------------------------------------------------------------------------
+//
+// This is a simple insertion-ordered set. It provides O(1) amortized
+// insertions and lookups, as well as iteration over the set in the insertion
+// order.
+//
+// This class is thread-compatible.
+// This class is NOT exception-safe.
+//
+// Iterators point into the list and should be stable in the face of
+// mutations, except for an iterator pointing to an element that was just
+// deleted.
+//
+// This class supports heterogeneous lookups.
+
+#ifndef ABSL_CONTAINER_LINKED_HASH_SET_H_
+#define ABSL_CONTAINER_LINKED_HASH_SET_H_
+
+#include <cassert>
+#include <cstddef>
+#include <initializer_list>
+#include <list>
+#include <memory>
+#include <type_traits>
+#include <utility>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/container/flat_hash_set.h"
+#include "absl/container/internal/common.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+template <
+    typename Key, typename KeyHash = typename absl::flat_hash_set<Key>::hasher,
+    typename KeyEq = typename absl::flat_hash_set<Key, KeyHash>::key_equal,
+    typename Alloc = std::allocator<Key>>
+class linked_hash_set {
+  using KeyArgImpl = absl::container_internal::KeyArg<
+      absl::container_internal::IsTransparent<KeyEq>::value &&
+      absl::container_internal::IsTransparent<KeyHash>::value>;
+
+ public:
+  using key_type = Key;
+  using hasher = KeyHash;
+  using key_equal = KeyEq;
+  using value_type = key_type;
+  using allocator_type = Alloc;
+  using difference_type = ptrdiff_t;
+
+ private:
+  template <class K>
+  using key_arg = typename KeyArgImpl::template type<K, key_type>;
+
+  using ListType = std::list<key_type, Alloc>;
+
+  template <class Fn>
+  class Wrapped {
+    template <typename K>
+    static const K& ToKey(const K& k) {
+      return k;
+    }
+    static const key_type& ToKey(typename ListType::const_iterator it) {
+      return *it;
+    }
+    static const key_type& ToKey(typename ListType::iterator it) { return *it; }
+
+    Fn fn_;
+
+    friend linked_hash_set;
+
+   public:
+    using is_transparent = void;
+
+    Wrapped() = default;
+    explicit Wrapped(Fn fn) : fn_(std::move(fn)) {}
+
+    template <class... Args>
+    auto operator()(Args&&... args) const
+        -> decltype(this->fn_(ToKey(args)...)) {
+      return fn_(ToKey(args)...);
+    }
+  };
+  using SetType =
+      absl::flat_hash_set<typename ListType::iterator, Wrapped<hasher>,
+                          Wrapped<key_equal>, Alloc>;
+
+  class NodeHandle {
+   public:
+    using allocator_type = linked_hash_set::allocator_type;
+    using value_type = linked_hash_set::value_type;
+
+    constexpr NodeHandle() noexcept = default;
+    NodeHandle(NodeHandle&& nh) noexcept = default;
+    ~NodeHandle() = default;
+    NodeHandle& operator=(NodeHandle&& node) noexcept = default;
+    bool empty() const noexcept { return list_.empty(); }
+    explicit operator bool() const noexcept { return !empty(); }
+    allocator_type get_allocator() const { return list_.get_allocator(); }
+    value_type& value() { return list_.front(); }
+    void swap(NodeHandle& nh) noexcept { list_.swap(nh.list_); }
+
+   private:
+    friend linked_hash_set;
+
+    explicit NodeHandle(ListType list) : list_(std::move(list)) {}
+    ListType list_;
+  };
+
+  template <class Iterator, class NodeType>
+  struct InsertReturnType {
+    Iterator position;
+    bool inserted;
+    NodeType node;
+  };
+
+ public:
+  using iterator = typename ListType::const_iterator;
+  using const_iterator = typename ListType::const_iterator;
+  using reverse_iterator = typename ListType::const_reverse_iterator;
+  using const_reverse_iterator = typename ListType::const_reverse_iterator;
+  using reference = typename ListType::reference;
+  using const_reference = typename ListType::const_reference;
+  using pointer = typename std::allocator_traits<allocator_type>::pointer;
+  using const_pointer =
+      typename std::allocator_traits<allocator_type>::const_pointer;
+  using size_type = typename ListType::size_type;
+  using node_type = NodeHandle;
+  using insert_return_type = InsertReturnType<iterator, node_type>;
+
+  linked_hash_set() {}
+
+  explicit linked_hash_set(size_t reservation_size,
+                           const hasher& hash = hasher(),
+                           const key_equal& eq = key_equal(),
+                           const allocator_type& alloc = allocator_type())
+      : set_(reservation_size, Wrapped<hasher>(hash), Wrapped<key_equal>(eq),
+             alloc),
+        list_(alloc) {}
+
+  linked_hash_set(size_t reservation_size, const hasher& hash,
+                  const allocator_type& alloc)
+      : linked_hash_set(reservation_size, hash, key_equal(), alloc) {}
+
+  linked_hash_set(size_t reservation_size, const allocator_type& alloc)
+      : linked_hash_set(reservation_size, hasher(), key_equal(), alloc) {}
+
+  explicit linked_hash_set(const allocator_type& alloc)
+      : linked_hash_set(0, hasher(), key_equal(), alloc) {}
+
+  template <class InputIt>
+  linked_hash_set(InputIt first, InputIt last, size_t reservation_size = 0,
+                  const hasher& hash = hasher(),
+                  const key_equal& eq = key_equal(),
+                  const allocator_type& alloc = allocator_type())
+      : linked_hash_set(reservation_size, hash, eq, alloc) {
+    insert(first, last);
+  }
+
+  template <class InputIter>
+  linked_hash_set(InputIter first, InputIter last, size_t reservation_size,
+                  const hasher& hash, const allocator_type& alloc)
+      : linked_hash_set(first, last, reservation_size, hash, key_equal(),
+                        alloc) {}
+
+  template <class InputIter>
+  linked_hash_set(InputIter first, InputIter last, size_t reservation_size,
+                  const allocator_type& alloc)
+      : linked_hash_set(first, last, reservation_size, hasher(), key_equal(),
+                        alloc) {}
+
+  template <class InputIt>
+  linked_hash_set(InputIt first, InputIt last, const allocator_type& alloc)
+      : linked_hash_set(first, last, /*reservation_size=*/0, hasher(),
+                        key_equal(), alloc) {}
+
+  linked_hash_set(std::initializer_list<key_type> init,
+                  size_t reservation_size = 0, const hasher& hash = hasher(),
+                  const key_equal& eq = key_equal(),
+                  const allocator_type& alloc = allocator_type())
+      : linked_hash_set(init.begin(), init.end(), reservation_size, hash, eq,
+                        alloc) {}
+
+  linked_hash_set(std::initializer_list<key_type> init, size_t reservation_size,
+                  const allocator_type& alloc)
+      : linked_hash_set(init, reservation_size, hasher(), key_equal(), alloc) {}
+
+  linked_hash_set(std::initializer_list<key_type> init, size_t reservation_size,
+                  const hasher& hash, const allocator_type& alloc)
+      : linked_hash_set(init, reservation_size, hash, key_equal(), alloc) {}
+
+  linked_hash_set(std::initializer_list<key_type> init,
+                  const allocator_type& alloc)
+      : linked_hash_set(init, /*reservation_size=*/0, hasher(), key_equal(),
+                        alloc) {}
+
+  linked_hash_set(const linked_hash_set& other)
+      : linked_hash_set(0, other.hash_function(), other.key_eq(),
+                        other.get_allocator()) {
+    reserve(other.size());
+    CopyFrom(other);
+  }
+
+  linked_hash_set(const linked_hash_set& other, const allocator_type& alloc)
+      : linked_hash_set(0, other.hash_function(), other.key_eq(), alloc) {
+    reserve(other.size());
+    CopyFrom(other);
+  }
+
+  linked_hash_set(linked_hash_set&& other) noexcept
+      : set_(std::move(other.set_)), list_(std::move(other.list_)) {
+    // Since the list and set must agree for other to end up "valid",
+    // explicitly clear them.
+    other.set_.clear();
+    other.list_.clear();
+  }
+
+  linked_hash_set(linked_hash_set&& other, const allocator_type& alloc)
+      : linked_hash_set(0, other.hash_function(), other.key_eq(), alloc) {
+    if (get_allocator() == other.get_allocator()) {
+      *this = std::move(other);
+    } else {
+      CopyFrom(std::move(other));
+    }
+  }
+
+  linked_hash_set& operator=(const linked_hash_set& other) {
+    if (this != &other) {
+      // Make a new set, with other's hash/eq/alloc.
+      set_ = SetType(0, other.set_.hash_function(),
+                     other.set_.key_eq(), other.get_allocator());
+      set_.reserve(other.size());
+      // Copy the list, with other's allocator.
+      list_ = ListType(other.get_allocator());
+      CopyFrom(other);
+    }
+    return *this;
+  }
+
+  linked_hash_set& operator=(linked_hash_set&& other) noexcept {
+    if (this != &other) {
+      set_ = std::move(other.set_);
+      list_ = std::move(other.list_);
+      other.set_.clear();
+      other.list_.clear();
+    }
+    return *this;
+  }
+
+  linked_hash_set& operator=(std::initializer_list<key_type> values) {
+    clear();
+    reserve(values.size());
+    insert(values.begin(), values.end());
+    return *this;
+  }
+
+  // Derive size from set_, as list::size might be O(N).
+  size_type size() const { return set_.size(); }
+  size_type max_size() const noexcept { return ~size_type{}; }
+  bool empty() const { return set_.empty(); }
+
+  // Iteration is list-like, in insertion order.
+  // These are all forwarded.
+  iterator begin() { return list_.begin(); }
+  iterator end() { return list_.end(); }
+  const_iterator begin() const { return list_.begin(); }
+  const_iterator end() const { return list_.end(); }
+  const_iterator cbegin() const { return list_.cbegin(); }
+  const_iterator cend() const { return list_.cend(); }
+  reverse_iterator rbegin() { return list_.rbegin(); }
+  reverse_iterator rend() { return list_.rend(); }
+  const_reverse_iterator rbegin() const { return list_.rbegin(); }
+  const_reverse_iterator rend() const { return list_.rend(); }
+  const_reverse_iterator crbegin() const { return list_.crbegin(); }
+  const_reverse_iterator crend() const { return list_.crend(); }
+  reference front() { return list_.front(); }
+  reference back() { return list_.back(); }
+  const_reference front() const { return list_.front(); }
+  const_reference back() const { return list_.back(); }
+
+  void pop_front() { erase(begin()); }
+  void pop_back() { erase(std::prev(end())); }
+
+  ABSL_ATTRIBUTE_REINITIALIZES void clear() {
+    set_.clear();
+    list_.clear();
+  }
+
+  void reserve(size_t n) { set_.reserve(n); }
+  size_t bucket_count() const { return set_.bucket_count(); }
+  size_t capacity() const { return set_.capacity(); }
+  float load_factor() const { return set_.load_factor(); }
+
+  hasher hash_function() const { return set_.hash_function().fn_; }
+  key_equal key_eq() const { return set_.key_eq().fn_; }
+  allocator_type get_allocator() const { return list_.get_allocator(); }
+
+  template <typename K = key_type>
+  size_type erase(const key_arg<K>& key) {
+    auto found = set_.find(key);
+    if (found == set_.end()) return 0;
+    auto list_it = *found;
+    // Erase set entry first since it refers to the list element.
+    set_.erase(found);
+    list_.erase(list_it);
+    return 1;
+  }
+
+  iterator erase(const_iterator position) {
+    auto found = set_.find(position);
+    assert(*found == position);
+    set_.erase(found);
+    return list_.erase(position);
+  }
+
+  iterator erase(const_iterator first, const_iterator last) {
+    while (first != last) first = erase(first);
+    return first;
+  }
+
+  template <typename K = key_type>
+  iterator find(const key_arg<K>& key) {
+    auto found = set_.find(key);
+    if (found == set_.end()) return end();
+    return *found;
+  }
+
+  template <typename K = key_type>
+  const_iterator find(const key_arg<K>& key) const {
+    auto found = set_.find(key);
+    if (found == set_.end()) return end();
+    return *found;
+  }
+
+  template <typename K = key_type>
+  size_t count(const key_arg<K>& key) const {
+    return contains(key) ? 1 : 0;
+  }
+  template <typename K = key_type>
+  bool contains(const key_arg<K>& key) const {
+    return set_.contains(key);
+  }
+
+  template <typename K = key_type>
+  std::pair<iterator, iterator> equal_range(const key_arg<K>& key) {
+    auto iter = set_.find(key);
+    if (iter == set_.end()) return {end(), end()};
+    return {*iter, std::next(*iter)};
+  }
+
+  template <typename K = key_type>
+  std::pair<const_iterator, const_iterator> equal_range(
+      const key_arg<K>& key) const {
+    auto iter = set_.find(key);
+    if (iter == set_.end()) return {end(), end()};
+    return {*iter, std::next(*iter)};
+  }
+
+  template <typename K = key_type>
+  std::pair<iterator, bool> insert(const key_arg<K>& k) {
+    return InsertInternal(list_.end(), k);
+  }
+  template <typename K = key_type, K* = nullptr>
+  std::pair<iterator, bool> insert(key_arg<K>&& k) {
+    return InsertInternal(list_.end(), std::move(k));
+  }
+
+  template <typename K = key_type,
+            std::enable_if_t<
+                !std::is_convertible_v<const key_arg<K>&, const_iterator> &&
+                    !std::is_convertible_v<const key_arg<K>&, iterator>,
+                int> = 0>
+  iterator insert(const_iterator hint, const key_arg<K>& k) {
+    return InsertInternal(hint, k).first;
+  }
+  template <
+      typename K = key_type, K* = nullptr,
+      std::enable_if_t<!std::is_convertible_v<key_arg<K>&&, const_iterator> &&
+                           !std::is_convertible_v<key_arg<K>&&, iterator>,
+                       int> = 0>
+  iterator insert(const_iterator hint, key_arg<K>&& k) {
+    return InsertInternal(hint, std::move(k)).first;
+  }
+
+  void insert(std::initializer_list<key_type> ilist) {
+    insert(ilist.begin(), ilist.end());
+  }
+
+  template <class InputIt>
+  void insert(InputIt first, InputIt last) {
+    for (; first != last; ++first) insert(*first);
+  }
+
+  insert_return_type insert(node_type&& node) {
+    if (node.empty()) return {end(), false, node_type()};
+    if (auto [set_itr, inserted] = set_.emplace(node.list_.begin()); inserted) {
+      list_.splice(list_.end(), node.list_);
+      return {*set_itr, true, node_type()};
+    } else {
+      return {*set_itr, false, std::move(node)};
+    }
+  }
+
+  iterator insert(const_iterator, node_type&& node) {
+    return insert(std::move(node)).first;
+  }
+
+  template <typename... Args>
+  std::pair<iterator, bool> emplace(Args&&... args) {
+    return EmplaceInternal(list_.end(), std::forward<Args>(args)...);
+  }
+
+  template <typename... Args>
+  iterator emplace_hint(const_iterator hint, Args&&... args) {
+    return EmplaceInternal(hint, std::forward<Args>(args)...).first;
+  }
+
+  template <typename H, typename E>
+  void merge(linked_hash_set<Key, H, E, Alloc>& src) {
+    auto itr = src.list_.begin();
+    while (itr != src.list_.end()) {
+      if (contains(*itr)) {
+        ++itr;
+      } else {
+        insert(src.extract(itr++));
+      }
+    }
+  }
+
+  template <typename H, typename E>
+  void merge(linked_hash_set<Key, H, E, Alloc>&& src) {
+    merge(src);
+  }
+
+  node_type extract(const_iterator position) {
+    set_.erase(position);
+    ListType extracted_node_list(get_allocator());
+    extracted_node_list.splice(extracted_node_list.end(), list_, position);
+    return node_type(std::move(extracted_node_list));
+  }
+
+  template <class K = key_type,
+            typename std::enable_if_t<!std::is_same_v<K, iterator>, int> = 0>
+  node_type extract(const key_arg<K>& key) {
+    auto node = set_.extract(key);
+    if (node.empty()) return node_type();
+    ListType extracted_node_list(get_allocator());
+    extracted_node_list.splice(extracted_node_list.end(), list_, node.value());
+    return node_type(std::move(extracted_node_list));
+  }
+
+  void swap(linked_hash_set& other) noexcept {
+    using std::swap;
+    swap(set_, other.set_);
+    swap(list_, other.list_);
+  }
+
+  friend bool operator==(const linked_hash_set& a, const linked_hash_set& b) {
+    if (a.size() != b.size()) return false;
+    const linked_hash_set* outer = &a;
+    const linked_hash_set* inner = &b;
+    if (outer->capacity() > inner->capacity()) std::swap(outer, inner);
+    for (const value_type& elem : *outer)
+      if (!inner->contains(elem)) return false;
+    return true;
+  }
+
+  friend bool operator!=(const linked_hash_set& a, const linked_hash_set& b) {
+    return !(a == b);
+  }
+
+  void rehash(size_t n) { set_.rehash(n); }
+
+ private:
+  template <typename Other>
+  void CopyFrom(Other&& other) {
+    for (auto& elem : other.list_) {
+      set_.insert(list_.insert(list_.end(), std::move(elem)));
+    }
+    assert(set_.size() == list_.size());
+  }
+
+  template <typename... Args>
+  std::pair<iterator, bool> EmplaceInternal(const_iterator hint,
+                                            Args&&... args) {
+    ListType node_donor(get_allocator());
+    auto list_iter =
+        node_donor.emplace(node_donor.end(), std::forward<Args>(args)...);
+    auto ins = set_.insert(list_iter);
+    if (!ins.second) return {*ins.first, false};
+    list_.splice(hint, node_donor, list_iter);
+    return {list_iter, true};
+  }
+
+  template <typename U>
+  std::pair<iterator, bool> InsertInternal(const_iterator hint,
+                                           U&& key) {  // NOLINT(build/c++11)
+    bool constructed = false;
+    auto set_iter = set_.lazy_emplace(key, [&](const auto& ctor) {
+      constructed = true;
+      ctor(list_.emplace(hint, std::forward<U>(key)));
+    });
+    return {*set_iter, constructed};
+  }
+
+  // The set component, used for speedy lookups.
+  SetType set_;
+
+  // The list component, used for maintaining insertion order.
+  ListType list_;
+};
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_CONTAINER_LINKED_HASH_SET_H_
diff --git a/absl/container/linked_hash_set_benchmark.cc b/absl/container/linked_hash_set_benchmark.cc
new file mode 100644
index 00000000000..e790e7d1ad5
--- /dev/null
+++ b/absl/container/linked_hash_set_benchmark.cc
@@ -0,0 +1,84 @@
+// Copyright 2025 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <algorithm>
+#include <cstddef>
+#include <string>
+#include <vector>
+
+#include "absl/container/linked_hash_set.h"
+#include "absl/functional/function_ref.h"
+#include "absl/strings/str_format.h"
+#include "absl/strings/string_view.h"
+#include "benchmark/benchmark.h"
+
+namespace {
+
+void BenchmarkInsertStrings(benchmark::State& state,
+                            absl::FunctionRef<std::string(int)> factory) {
+  std::vector<std::string> sample;
+  size_t str_bytes = 0;
+  for (int i = 0; i < state.range(0); ++i) {
+    sample.push_back(factory(i));
+    str_bytes += sample.back().size();
+  }
+
+  // Make a batch around 1Mi bytes.
+  const size_t batch_size = std::max(size_t{1}, size_t{1000000} / str_bytes);
+  std::vector<absl::linked_hash_set<std::string>> sets(batch_size);
+
+  while (state.KeepRunningBatch(batch_size)) {
+    state.PauseTiming();
+    for (auto& set : sets) set.clear();
+    state.ResumeTiming();
+    for (auto& set : sets) {
+      for (const auto& str : sample) {
+        benchmark::DoNotOptimize(set.insert(str));
+      }
+    }
+  }
+
+  state.SetItemsProcessed(state.iterations() * state.range(0));
+  state.SetBytesProcessed(state.iterations() * str_bytes);
+}
+
+constexpr absl::string_view kFormatShort = "%10d";
+constexpr absl::string_view kFormatLong =
+    "a longer string that exceeds the SSO %10d";
+
+void BM_InsertShortStrings_Hit(benchmark::State& state) {
+  BenchmarkInsertStrings(
+      state, [](int i) { return absl::StrFormat(kFormatShort, i); });
+}
+BENCHMARK(BM_InsertShortStrings_Hit)->Range(1, 1 << 16);
+
+void BM_InsertLongStrings_Hit(benchmark::State& state) {
+  BenchmarkInsertStrings(state,
+                         [](int i) { return absl::StrFormat(kFormatLong, i); });
+}
+BENCHMARK(BM_InsertLongStrings_Hit)->Range(1, 1 << 16);
+
+void BM_InsertShortStrings_Miss(benchmark::State& state) {
+  BenchmarkInsertStrings(
+      state, [](int i) { return absl::StrFormat(kFormatShort, i % 20); });
+}
+BENCHMARK(BM_InsertShortStrings_Miss)->Range(1, 1 << 16);
+
+void BM_InsertLongStrings_Miss(benchmark::State& state) {
+  BenchmarkInsertStrings(
+      state, [](int i) { return absl::StrFormat(kFormatLong, i % 20); });
+}
+BENCHMARK(BM_InsertLongStrings_Miss)->Range(1, 1 << 16);
+
+}  // namespace
diff --git a/absl/container/linked_hash_set_test.cc b/absl/container/linked_hash_set_test.cc
new file mode 100644
index 00000000000..340d6e3f593
--- /dev/null
+++ b/absl/container/linked_hash_set_test.cc
@@ -0,0 +1,973 @@
+// Copyright 2025 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/container/linked_hash_set.h"
+
+#include <algorithm>
+#include <cmath>
+#include <cstddef>
+#include <cstdint>
+#include <functional>
+#include <memory>
+#include <string>
+#include <utility>
+#include <vector>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+#include "absl/container/internal/hash_generator_testing.h"
+#include "absl/container/internal/hash_policy_testing.h"
+#include "absl/container/internal/heterogeneous_lookup_testing.h"
+#include "absl/container/internal/test_allocator.h"
+#include "absl/container/internal/test_instance_tracker.h"
+#include "absl/container/internal/unordered_set_constructor_test.h"
+#include "absl/container/internal/unordered_set_lookup_test.h"
+#include "absl/container/internal/unordered_set_members_test.h"
+#include "absl/container/internal/unordered_set_modifiers_test.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace container_internal {
+namespace {
+
+using ::testing::ElementsAre;
+using ::testing::ElementsAreArray;
+using ::testing::Pointee;
+
+template <class T>
+using Set =
+    linked_hash_set<T, StatefulTestingHash, StatefulTestingEqual, Alloc<T>>;
+
+using SetTypes =
+    ::testing::Types<Set<int>, Set<std::string>, Set<Enum>, Set<EnumClass>>;
+
+INSTANTIATE_TYPED_TEST_SUITE_P(LinkedHashSet, ConstructorTest, SetTypes);
+INSTANTIATE_TYPED_TEST_SUITE_P(LinkedHashSet, LookupTest, SetTypes);
+INSTANTIATE_TYPED_TEST_SUITE_P(LinkedHashSet, MembersTest, SetTypes);
+INSTANTIATE_TYPED_TEST_SUITE_P(LinkedHashSet, ModifiersTest, SetTypes);
+
+// Tests that the range constructor works.
+TEST(LinkedHashSetTest, RangeConstruct) {
+  const auto items = {1, 2, 3};
+  EXPECT_THAT(linked_hash_set<int>(items.begin(), items.end()),
+              ElementsAre(1, 2, 3));
+}
+
+// Tests that copying works.
+TEST(LinkedHashSetTest, Copy) {
+  linked_hash_set<int> m{4, 8, 15, 16, 23, 42};
+  auto copy = m;
+
+  auto found = copy.find(8);
+  ASSERT_TRUE(found != copy.end());
+  for (auto iter = copy.begin(); iter != copy.end(); ++iter) {
+    if (iter == found) return;
+  }
+  FAIL() << "Copied set's find method returned an invalid iterator.";
+}
+
+// Tests that assignment works.
+TEST(LinkedHashSetTest, Assign) {
+  linked_hash_set<int> m{2, 3};
+  linked_hash_set<int> n{4};
+
+  n = m;
+  EXPECT_TRUE(n.contains(2));
+  auto found = n.find(2);
+  ASSERT_TRUE(found != n.end());
+  for (auto iter = n.begin(); iter != n.end(); ++iter) {
+    if (iter == found) return;
+  }
+  FAIL() << "Assigned set's find method returned an invalid iterator.";
+}
+
+// Tests that self-assignment works.
+TEST(LinkedHashSetTest, SelfAssign) {
+  linked_hash_set<int> a{1, 2, 3};
+  auto& a_ref = a;
+  a = a_ref;
+
+  EXPECT_TRUE(a.contains(2));
+  auto found = a.find(2);
+  ASSERT_TRUE(found != a.end());
+  for (auto iter = a.begin(); iter != a.end(); ++iter) {
+    if (iter == found) return;
+  }
+  FAIL() << "Assigned set's find method returned an invalid iterator.";
+}
+
+// Tests that move constructor works.
+TEST(LinkedHashSetTest, Move) {
+  // Use unique_ptr as an example of a non-copyable type.
+  linked_hash_set<std::unique_ptr<int>> m;
+  m.insert(std::make_unique<int>(2));
+  m.insert(std::make_unique<int>(3));
+  linked_hash_set<std::unique_ptr<int>> n = std::move(m);
+  EXPECT_THAT(n, ElementsAre(Pointee(2), Pointee(3)));
+}
+
+// Tests that self-moving works.
+TEST(LinkedHashSetTest, SelfMove) {
+  linked_hash_set<int> a{1, 2, 3};
+  auto& a_ref = a;
+  a = std::move(a_ref);
+  EXPECT_THAT(a, ElementsAre(1, 2, 3));
+}
+
+struct IntUniquePtrHash {
+  size_t operator()(const std::unique_ptr<int>& p) const {
+    return static_cast<size_t>(*p);
+  }
+};
+
+struct IntUniquePtrEq {
+  size_t operator()(const std::unique_ptr<int>& a,
+                    const std::unique_ptr<int>& b) const {
+    return *a == *b;
+  }
+};
+
+// Pretty artificial for a set, but unique_ptr is a convenient move-only type.
+TEST(LinkedHashSetTest, CanInsertMoveOnly) {
+  linked_hash_set<std::unique_ptr<int>, IntUniquePtrHash, IntUniquePtrEq> s;
+  std::vector<int> data = {4, 8, 15, 16, 23, 42};
+  for (int x : data) s.insert(std::make_unique<int>(x));
+  EXPECT_EQ(s.size(), data.size());
+  for (const std::unique_ptr<int>& elt : s) {
+    EXPECT_TRUE(s.contains(elt));
+    EXPECT_TRUE(s.find(elt) != s.end());
+  }
+}
+
+TEST(LinkedHashSetTest, CanMoveMoveOnly) {
+  linked_hash_set<std::unique_ptr<int>, IntUniquePtrHash, IntUniquePtrEq> s;
+  std::vector<int> data = {4, 8, 15, 16, 23, 42};
+  for (int x : data) s.insert(std::make_unique<int>(x));
+  linked_hash_set<std::unique_ptr<int>, IntUniquePtrHash, IntUniquePtrEq> ss =
+      std::move(s);
+  EXPECT_EQ(ss.size(), data.size());
+}
+
+TEST(LinkedHashSetTest, CanEmplaceMoveOnly) {
+  linked_hash_set<std::unique_ptr<int>, IntUniquePtrHash, IntUniquePtrEq> s;
+  std::vector<int> data = {4, 8, 15, 16, 23, 42};
+  for (const int x : data) {
+    s.emplace(new int{x});
+  }
+  EXPECT_EQ(s.size(), data.size());
+  for (const std::unique_ptr<int>& elt : s) {
+    EXPECT_TRUE(s.contains(elt));
+    EXPECT_TRUE(s.find(elt) != s.end());
+  }
+}
+
+TEST(LinkedHashSetTest, CanInsertTransparent) {
+  linked_hash_set<std::string> s;
+  s.insert(absl::string_view("foo"));
+  s.insert(absl::string_view("bar"));
+  s.insert(absl::string_view("foo"));
+  EXPECT_THAT(s, ElementsAre("foo", "bar"));
+}
+
+// Tests that iteration from begin() to end() works
+TEST(LinkedHashSetTest, Iteration) {
+  linked_hash_set<int> m;
+  EXPECT_TRUE(m.begin() == m.end());
+
+  m.insert(2);
+  m.insert(1);
+  m.insert(3);
+
+  linked_hash_set<int>::iterator i = m.begin();
+  ASSERT_TRUE(m.begin() == i);
+  ASSERT_TRUE(m.end() != i);
+  EXPECT_EQ(2, *i);
+
+  ++i;
+  ASSERT_TRUE(m.end() != i);
+  EXPECT_EQ(1, *i);
+
+  ++i;
+  ASSERT_TRUE(m.end() != i);
+  EXPECT_EQ(3, *i);
+
+  ++i;
+  ASSERT_TRUE(m.end() == i);
+}
+
+// Tests that reverse iteration from rbegin() to rend() works
+TEST(LinkedHashSetTest, ReverseIteration) {
+  linked_hash_set<int> m;
+  EXPECT_TRUE(m.rbegin() == m.rend());
+
+  m.insert(2);
+  m.insert(1);
+  m.insert(3);
+
+  linked_hash_set<int>::reverse_iterator i = m.rbegin();
+  ASSERT_TRUE(m.rbegin() == i);
+  ASSERT_TRUE(m.rend() != i);
+  EXPECT_EQ(3, *i);
+
+  ++i;
+  ASSERT_TRUE(m.rend() != i);
+  EXPECT_EQ(1, *i);
+
+  ++i;
+  ASSERT_TRUE(m.rend() != i);
+  EXPECT_EQ(2, *i);
+
+  ++i;
+  ASSERT_TRUE(m.rend() == i);
+}
+
+// Tests that clear() works
+TEST(LinkedHashSetTest, Clear) {
+  linked_hash_set<int> m{2, 1, 3};
+  ASSERT_EQ(3, m.size());
+
+  m.clear();
+  EXPECT_EQ(0, m.size());
+  EXPECT_FALSE(m.contains(1));
+  EXPECT_TRUE(m.find(1) == m.end());
+
+  // Make sure we can call it on an empty set.
+  m.clear();
+  EXPECT_EQ(0, m.size());
+}
+
+// Tests that size() works.
+TEST(LinkedHashSetTest, Size) {
+  linked_hash_set<int> m;
+  EXPECT_EQ(0, m.size());
+  m.insert(2);
+  EXPECT_EQ(1, m.size());
+  m.insert(11);
+  EXPECT_EQ(2, m.size());
+  m.insert(0);
+  EXPECT_EQ(3, m.size());
+  m.insert(0);
+  EXPECT_EQ(3, m.size());
+  m.clear();
+  EXPECT_EQ(0, m.size());
+}
+
+// Tests empty()
+TEST(LinkedHashSetTest, Empty) {
+  linked_hash_set<int> m;
+  ASSERT_TRUE(m.empty());
+  m.insert(2);
+  ASSERT_FALSE(m.empty());
+  m.clear();
+  ASSERT_TRUE(m.empty());
+}
+
+TEST(LinkedHashSetTest, Erase) {
+  linked_hash_set<int> m;
+  ASSERT_EQ(0, m.size());
+  EXPECT_EQ(0, m.erase(2));  // Nothing to erase yet
+
+  m.insert(2);
+  ASSERT_EQ(1, m.size());
+  EXPECT_EQ(1, m.erase(2));
+  EXPECT_EQ(0, m.size());
+  EXPECT_TRUE(m.empty());
+
+  EXPECT_EQ(0, m.erase(2));  // Make sure nothing bad happens if we repeat.
+  EXPECT_EQ(0, m.size());
+  EXPECT_TRUE(m.empty());
+}
+
+TEST(LinkedHashSetTest, Erase2) {
+  linked_hash_set<int> m;
+  ASSERT_EQ(0, m.size());
+  EXPECT_EQ(0, m.erase(2));  // Nothing to erase yet
+
+  m.insert(2);
+  m.insert(1);
+  m.insert(3);
+  m.insert(4);
+  ASSERT_EQ(4, m.size());
+
+  // Erase middle two
+  EXPECT_EQ(1, m.erase(1));
+  EXPECT_EQ(1, m.erase(3));
+
+  EXPECT_EQ(2, m.size());
+
+  // Make sure we can still iterate over everything that's left.
+  linked_hash_set<int>::iterator it = m.begin();
+  ASSERT_TRUE(it != m.end());
+  EXPECT_EQ(2, *it);
+  ++it;
+  ASSERT_TRUE(it != m.end());
+  EXPECT_EQ(4, *it);
+  ++it;
+  ASSERT_TRUE(it == m.end());
+
+  EXPECT_EQ(0, m.erase(1));  // Make sure nothing bad happens if we repeat.
+  ASSERT_EQ(2, m.size());
+
+  EXPECT_EQ(1, m.erase(2));
+  EXPECT_EQ(1, m.erase(4));
+  ASSERT_EQ(0, m.size());
+  EXPECT_TRUE(m.empty());
+
+  EXPECT_EQ(0, m.erase(1));  // Make sure nothing bad happens if we repeat.
+  ASSERT_EQ(0, m.size());
+  EXPECT_TRUE(m.empty());
+}
+
+// Test that erase(iter,iter) and erase(iter) compile and work.
+TEST(LinkedHashSetTest, Erase3) {
+  linked_hash_set<int> m;
+
+  m.insert(1);
+  m.insert(2);
+  m.insert(3);
+  m.insert(4);
+
+  // Erase middle two
+  linked_hash_set<int>::iterator it2 = m.find(2);
+  linked_hash_set<int>::iterator it4 = m.find(4);
+  EXPECT_EQ(m.erase(it2, it4), m.find(4));
+  EXPECT_FALSE(m.contains(2));
+  EXPECT_TRUE(m.find(2) == m.end());
+  EXPECT_FALSE(m.contains(3));
+  EXPECT_TRUE(m.find(3) == m.end());
+  EXPECT_EQ(2, m.size());
+
+  // Make sure we can still iterate over everything that's left.
+  linked_hash_set<int>::iterator it = m.begin();
+  ASSERT_TRUE(it != m.end());
+  EXPECT_EQ(1, *it);
+  ++it;
+  ASSERT_TRUE(it != m.end());
+  EXPECT_EQ(4, *it);
+  ++it;
+  ASSERT_TRUE(it == m.end());
+
+  // Erase first one using an iterator.
+  EXPECT_EQ(m.erase(m.begin()), m.find(4));
+  EXPECT_FALSE(m.contains(1));
+  EXPECT_TRUE(m.find(1) == m.end());
+
+  // Only the last element should be left.
+  EXPECT_TRUE(m.contains(4));
+  it = m.begin();
+  ASSERT_TRUE(it != m.end());
+  EXPECT_EQ(4, *it);
+  ++it;
+  ASSERT_TRUE(it == m.end());
+}
+
+// Test all types of insertion
+TEST(LinkedHashSetTest, Insertion) {
+  linked_hash_set<int> m;
+  ASSERT_EQ(0, m.size());
+  std::pair<linked_hash_set<int>::iterator, bool> result;
+
+  result = m.insert(2);
+  ASSERT_EQ(1, m.size());
+  EXPECT_TRUE(result.second);
+  EXPECT_EQ(2, *result.first);
+  EXPECT_TRUE(m.contains(2));
+  EXPECT_TRUE(m.find(2) != m.end());
+
+  result = m.insert(1);
+  ASSERT_EQ(2, m.size());
+  EXPECT_TRUE(result.second);
+  EXPECT_EQ(1, *result.first);
+  EXPECT_TRUE(m.contains(1));
+  EXPECT_TRUE(m.find(1) != m.end());
+
+  result = m.insert(3);
+  linked_hash_set<int>::iterator result_iterator = result.first;
+  ASSERT_EQ(3, m.size());
+  EXPECT_TRUE(result.second);
+  EXPECT_EQ(3, *result.first);
+  EXPECT_TRUE(m.contains(3));
+  EXPECT_TRUE(m.find(3) != m.end());
+
+  result = m.insert(3);
+  EXPECT_EQ(3, m.size());
+  EXPECT_FALSE(result.second) << "No insertion should have occurred.";
+  EXPECT_TRUE(result_iterator == result.first)
+      << "Duplicate insertion should have given us the original iterator.";
+  EXPECT_TRUE(m.contains(3));
+  EXPECT_TRUE(m.find(3) != m.end());
+
+  std::vector<int> v = {3, 4, 5};
+  m.insert(v.begin(), v.end());
+  // Expect 4 and 5 inserted, 3 not inserted.
+  EXPECT_EQ(5, m.size());
+  EXPECT_TRUE(m.contains(4));
+  EXPECT_NE(m.find(4), m.end());
+  EXPECT_TRUE(m.contains(5));
+  EXPECT_NE(m.find(5), m.end());
+}
+
+TEST(LinkedHashSetTest, HintedInsertionMoveable) {
+  linked_hash_set<int> m = {1, 3};
+  m.insert(m.find(3), 2);
+  EXPECT_THAT(m, ElementsAre(1, 2, 3));
+}
+
+TEST(LinkedHashSetTest, HintedInsertionReference) {
+  linked_hash_set<int> m = {1, 3};
+  const int val = 2;
+  m.insert(m.find(3), val);
+  EXPECT_THAT(m, ElementsAre(1, 2, 3));
+}
+
+TEST(LinkedHashSetTest, HintedEmplaceMoveable) {
+  linked_hash_set<int> m = {1, 3};
+  m.emplace_hint(m.find(3), 2);
+  EXPECT_THAT(m, ElementsAre(1, 2, 3));
+}
+
+TEST(LinkedHashSetTest, HintedEmplaceReference) {
+  linked_hash_set<int> m = {1, 3};
+  const int val = 2;
+  m.emplace_hint(m.find(3), val);
+  EXPECT_THAT(m, ElementsAre(1, 2, 3));
+}
+
+// Test front accessors.
+TEST(LinkedHashSetTest, Front) {
+  linked_hash_set<int> m;
+
+  m.insert(222);
+  m.insert(111);
+  m.insert(333);
+
+  EXPECT_EQ(3, m.size());
+  EXPECT_EQ(222, m.front());
+  m.pop_front();
+  EXPECT_EQ(2, m.size());
+  EXPECT_EQ(111, m.front());
+  m.pop_front();
+  EXPECT_EQ(1, m.size());
+  EXPECT_EQ(333, m.front());
+  m.pop_front();
+  EXPECT_TRUE(m.empty());
+}
+
+// Test back accessors.
+TEST(LinkedHashSetTest, Back) {
+  linked_hash_set<int> m;
+
+  m.insert(222);
+  m.insert(111);
+  m.insert(333);
+
+  EXPECT_EQ(3, m.size());
+  EXPECT_EQ(333, m.back());
+  m.pop_back();
+  EXPECT_EQ(2, m.size());
+  EXPECT_EQ(111, m.back());
+  m.pop_back();
+  EXPECT_EQ(1, m.size());
+  EXPECT_EQ(222, m.back());
+  m.pop_back();
+  EXPECT_TRUE(m.empty());
+}
+
+TEST(LinkedHashSetTest, Find) {
+  linked_hash_set<int> m;
+
+  EXPECT_TRUE(m.end() == m.find(1))
+      << "We shouldn't find anything in an empty set.";
+
+  m.insert(2);
+  EXPECT_TRUE(m.end() == m.find(1))
+      << "We shouldn't find an element that doesn't exist in the set.";
+
+  std::pair<linked_hash_set<int>::iterator, bool> result = m.insert(1);
+  ASSERT_TRUE(result.second);
+  ASSERT_TRUE(m.end() != result.first);
+  EXPECT_TRUE(result.first == m.find(1))
+      << "We should have found an element we know exists in the set.";
+  EXPECT_EQ(1, *result.first);
+
+  // Check that a follow-up insertion doesn't affect our original
+  m.insert(3);
+  linked_hash_set<int>::iterator it = m.find(1);
+  ASSERT_TRUE(m.end() != it);
+  EXPECT_EQ(1, *it);
+
+  m.clear();
+  EXPECT_TRUE(m.end() == m.find(1))
+      << "We shouldn't find anything in a set that we've cleared.";
+}
+
+TEST(LinkedHashSetTest, Contains) {
+  linked_hash_set<int> m;
+
+  EXPECT_FALSE(m.contains(1)) << "The empty set shouldn't contain anything.";
+
+  m.insert(2);
+  EXPECT_FALSE(m.contains(1))
+      << "contains() should not return true for an element that doesn't exist "
+      << "in the set.";
+
+  m.insert(1);
+  EXPECT_TRUE(m.contains(1))
+      << "contains() should return true for an element we know exists in the "
+      << "set.";
+
+  m.clear();
+  EXPECT_FALSE(m.contains(1))
+      << "A set that we've cleared shouldn't contain anything.";
+}
+
+TEST(LinkedHashSetTest, Swap) {
+  linked_hash_set<int> m1;
+  linked_hash_set<int> m2;
+  m1.insert(1);
+  m1.insert(2);
+  m2.insert(3);
+  ASSERT_EQ(2, m1.size());
+  ASSERT_EQ(1, m2.size());
+  m1.swap(m2);
+  ASSERT_EQ(1, m1.size());
+  ASSERT_EQ(2, m2.size());
+}
+
+TEST(LinkedHashSetTest, SelfSwap) {
+  linked_hash_set<int> a{1, 2, 3};
+  using std::swap;
+  swap(a, a);
+  EXPECT_THAT(a, ElementsAre(1, 2, 3));
+}
+
+TEST(LinkedHashSetTest, InitializerList) {
+  linked_hash_set<int> m{1, 3};
+  ASSERT_EQ(2, m.size());
+  EXPECT_TRUE(m.contains(1));
+  linked_hash_set<int>::iterator it = m.find(1);
+  ASSERT_TRUE(m.end() != it);
+  EXPECT_EQ(1, *it);
+  it = m.find(3);
+  EXPECT_TRUE(m.contains(3));
+  ASSERT_TRUE(m.end() != it);
+  EXPECT_EQ(3, *it);
+}
+
+TEST(LinkedHashSetTest, CustomHashAndEquality) {
+  struct CustomIntHash {
+    size_t operator()(int x) const { return 0; }
+  };
+  struct CustomIntEq {
+    bool operator()(int x, int y) const { return abs(x) == abs(y); }
+  };
+  linked_hash_set<int, CustomIntHash, CustomIntEq> m;
+  m.insert(1);
+  EXPECT_EQ(1, m.size());
+  m.insert(2);
+  EXPECT_EQ(2, m.size());
+  EXPECT_FALSE(m.insert(-2).second);
+  EXPECT_EQ(2, m.size());
+  EXPECT_TRUE(m.contains(-1));
+  EXPECT_TRUE(m.find(-1) != m.end());
+}
+
+TEST(LinkedHashSetTest, EqualRange) {
+  linked_hash_set<int> m{3, 1};
+  const auto& const_m = m;
+
+  EXPECT_THAT(m.equal_range(2), testing::Pair(m.end(), m.end()));
+  EXPECT_THAT(const_m.equal_range(2),
+              testing::Pair(const_m.end(), const_m.end()));
+
+  EXPECT_THAT(m.equal_range(1), testing::Pair(m.find(1), ++m.find(1)));
+  EXPECT_THAT(const_m.equal_range(1),
+              testing::Pair(const_m.find(1), ++const_m.find(1)));
+}
+
+TEST(LinkedHashSetTest, ReserveWorks) {
+  linked_hash_set<int> m;
+  EXPECT_EQ(0, m.size());
+  EXPECT_EQ(0.0, m.load_factor());
+  m.reserve(10);
+  EXPECT_LE(10, m.capacity());
+  EXPECT_EQ(0, m.size());
+  EXPECT_EQ(0.0, m.load_factor());
+  m.insert(1);
+  m.insert(2);
+  EXPECT_LE(10, m.capacity());
+  EXPECT_EQ(2, m.size());
+  EXPECT_LT(0.0, m.load_factor());
+}
+
+TEST(LinkedHashSetTest, HeterogeneousTests) {
+  absl::test_internal::InstanceTracker tracker;
+
+  linked_hash_set<ExpensiveType, HeterogeneousHash, HeterogeneousEqual> set;
+  ExpensiveType one(1);
+  tracker.ResetCopiesMovesSwaps();
+  set.insert(one);
+  // Two instances: 'one' var and an instance in the set.
+  EXPECT_EQ(2, tracker.instances());
+  EXPECT_EQ(1, tracker.copies());
+  EXPECT_EQ(0, tracker.moves());
+
+  tracker.ResetCopiesMovesSwaps();
+  set.insert(one);
+  // No construction since key==1 exists.
+  EXPECT_EQ(2, tracker.instances());
+  EXPECT_EQ(0, tracker.copies());
+  EXPECT_EQ(0, tracker.moves());
+
+  tracker.ResetCopiesMovesSwaps();
+  set.emplace(CheapType(1));
+  // No construction since key==1 exists.
+  EXPECT_EQ(2, tracker.instances());
+  EXPECT_EQ(0, tracker.copies());
+  EXPECT_EQ(0, tracker.moves());
+
+  tracker.ResetCopiesMovesSwaps();
+  set.emplace(CheapType(2));
+  // Construction since key==2 doesn't exist in the set.
+  EXPECT_EQ(3, tracker.instances());
+  EXPECT_EQ(0, tracker.copies());
+  EXPECT_EQ(0, tracker.moves());
+  EXPECT_THAT(set, ElementsAre(HasExpensiveValue(1), HasExpensiveValue(2)));
+
+  // find
+  tracker.ResetCopiesMovesSwaps();
+  auto itr = set.find(CheapType(1));
+  ASSERT_NE(itr, set.end());
+  EXPECT_EQ(1, itr->value());
+  // contains
+  EXPECT_TRUE(set.contains(CheapType(2)));
+  // count
+  EXPECT_EQ(1, set.count(CheapType(2)));
+  // equal_range
+  auto eq_itr_pair = set.equal_range(CheapType(2));
+  ASSERT_NE(eq_itr_pair.first, set.end());
+  EXPECT_EQ(2, eq_itr_pair.first->value());
+  // No construction for find, contains, count or equal_range.
+  EXPECT_EQ(3, tracker.instances());
+  EXPECT_EQ(0, tracker.copies());
+  EXPECT_EQ(0, tracker.moves());
+
+  // emplace
+  tracker.ResetCopiesMovesSwaps();
+  set.emplace(3);
+  // Just one construction.
+  EXPECT_EQ(4, tracker.instances());
+  EXPECT_EQ(0, tracker.copies());
+  EXPECT_EQ(0, tracker.moves());
+
+  tracker.ResetCopiesMovesSwaps();
+  set.emplace(3);
+  // No additional construction since key==3 exists.
+  EXPECT_EQ(4, tracker.instances());
+  EXPECT_EQ(0, tracker.copies());
+  EXPECT_EQ(0, tracker.moves());
+  EXPECT_THAT(set, ElementsAre(HasExpensiveValue(1), HasExpensiveValue(2),
+                               HasExpensiveValue(3)));
+
+  // Test std::move() using insert().
+  ExpensiveType four(4);
+  tracker.ResetCopiesMovesSwaps();
+  set.insert(std::move(four));
+  // Two constructions (regular and move).
+  EXPECT_EQ(6, tracker.instances());
+  EXPECT_EQ(0, tracker.copies());
+  EXPECT_EQ(1, tracker.moves());
+
+  EXPECT_THAT(set, ElementsAre(HasExpensiveValue(1), HasExpensiveValue(2),
+                               HasExpensiveValue(3), HasExpensiveValue(4)));
+
+  tracker.ResetCopiesMovesSwaps();
+  set.erase(CheapType(1));
+  // No construction and instance reduced by one.
+  EXPECT_EQ(5, tracker.instances());
+  EXPECT_EQ(0, tracker.copies());
+  EXPECT_EQ(0, tracker.moves());
+  EXPECT_THAT(set, ElementsAre(HasExpensiveValue(2), HasExpensiveValue(3),
+                               HasExpensiveValue(4)));
+}
+
+TEST(LinkedHashSetTest, HeterogeneousStringViewLookup) {
+  linked_hash_set<std::string> set;
+  set.insert("foo");
+  set.insert("bar");
+  set.insert("blah");
+
+  {
+    absl::string_view lookup("foo");
+    auto itr = set.find(lookup);
+    ASSERT_NE(itr, set.end());
+    EXPECT_EQ("foo", *itr);
+  }
+
+  // Not found.
+  {
+    absl::string_view lookup("foobar");
+    EXPECT_EQ(set.end(), set.find(lookup));
+  }
+
+  {
+    absl::string_view lookup("blah");
+    auto itr = set.find(lookup);
+    ASSERT_NE(itr, set.end());
+    EXPECT_EQ("blah", *itr);
+  }
+}
+
+TEST(LinkedHashSetTest, EmplaceString) {
+  std::vector<std::string> v = {"a", "b"};
+  linked_hash_set<absl::string_view> hs(v.begin(), v.end());
+  EXPECT_THAT(hs, ElementsAreArray(v));
+}
+
+TEST(LinkedHashSetTest, BitfieldArgument) {
+  union {
+    int n : 1;
+  };
+  n = 0;
+  linked_hash_set<int> s = {n};
+  s.insert(n);
+  s.insert(s.end(), n);
+  s.insert({n});
+  s.erase(n);
+  s.count(n);
+  s.find(n);
+  s.contains(n);
+  s.equal_range(n);
+}
+
+TEST(LinkedHashSetTest, MergeExtractInsert) {
+  struct Hash {
+    size_t operator()(const std::unique_ptr<int>& p) const { return *p; }
+  };
+  struct Eq {
+    bool operator()(const std::unique_ptr<int>& a,
+                    const std::unique_ptr<int>& b) const {
+      return *a == *b;
+    }
+  };
+  linked_hash_set<std::unique_ptr<int>, Hash, Eq> set1, set2;
+  set1.insert(std::make_unique<int>(7));
+  set1.insert(std::make_unique<int>(17));
+
+  set2.insert(std::make_unique<int>(7));
+  set2.insert(std::make_unique<int>(19));
+
+  EXPECT_THAT(set1, ElementsAre(Pointee(7), Pointee(17)));
+  EXPECT_THAT(set2, ElementsAre(Pointee(7), Pointee(19)));
+
+  set1.merge(set2);
+
+  EXPECT_THAT(set1, ElementsAre(Pointee(7), Pointee(17), Pointee(19)));
+  EXPECT_THAT(set2, ElementsAre(Pointee(7)));
+
+  auto node = set1.extract(std::make_unique<int>(7));
+  EXPECT_TRUE(node);
+  EXPECT_THAT(node.value(), Pointee(7));
+  EXPECT_THAT(set1, ElementsAre(Pointee(17), Pointee(19)));
+
+  auto insert_result = set2.insert(std::move(node));
+  EXPECT_FALSE(node);
+  EXPECT_FALSE(insert_result.inserted);
+  EXPECT_TRUE(insert_result.node);
+  EXPECT_THAT(insert_result.node.value(), Pointee(7));
+  EXPECT_EQ(**insert_result.position, 7);
+  EXPECT_NE(insert_result.position->get(), insert_result.node.value().get());
+  EXPECT_THAT(set2, ElementsAre(Pointee(7)));
+
+  node = set1.extract(std::make_unique<int>(17));
+  EXPECT_TRUE(node);
+  EXPECT_THAT(node.value(), Pointee(17));
+  EXPECT_THAT(set1, ElementsAre(Pointee(19)));
+
+  node.value() = std::make_unique<int>(23);
+
+  insert_result = set2.insert(std::move(node));
+  EXPECT_FALSE(node);
+  EXPECT_TRUE(insert_result.inserted);
+  EXPECT_FALSE(insert_result.node);
+  EXPECT_EQ(**insert_result.position, 23);
+  EXPECT_THAT(set2, ElementsAre(Pointee(7), Pointee(23)));
+}
+
+TEST(LinkedHashSet, ExtractInsert) {
+  linked_hash_set<int> s = {1, 7, 2, 9};
+  auto node = s.extract(1);
+  EXPECT_TRUE(node);
+  EXPECT_EQ(node.value(), 1);
+  EXPECT_THAT(s, ElementsAre(7, 2, 9));
+  EXPECT_FALSE(s.contains(1));
+
+  node.value() = 17;
+  s.insert(std::move(node));
+  EXPECT_FALSE(node);
+  EXPECT_THAT(s, ElementsAre(7, 2, 9, 17));
+  EXPECT_TRUE(s.contains(17));
+
+  node = s.extract(s.find(9));
+  EXPECT_TRUE(node);
+  EXPECT_EQ(node.value(), 9);
+  EXPECT_THAT(s, ElementsAre(7, 2, 17));
+  EXPECT_FALSE(s.contains(9));
+}
+
+// Verify that emplacing and extracting nodes results in the same stateful
+// allocator being used for splicing purposes, rather than another instance
+// (say, a default-constructed one), which could otherwise silently corrupt
+// memory.
+TEST(LinkedHashSet, ExtractAndEmplaceUseSameStatefulAllocator) {
+  using Alloc = absl::container_internal::CountingAllocator<int>;
+  int64_t bytes_used = 0;
+  Alloc alloc(&bytes_used);
+  linked_hash_set<int, linked_hash_set<int>::hasher, std::equal_to<>, Alloc>
+      set(alloc);
+
+  set.emplace(1);
+  EXPECT_GT(bytes_used, 0) << "emplace() failed to use the same allocator";
+
+  auto node = set.extract(set.begin());
+  EXPECT_EQ(node.get_allocator(), alloc)
+      << "extract(iter) failed to use the same allocator";
+
+  set.insert(std::move(node));
+  EXPECT_EQ(set.extract(1).get_allocator(), alloc)
+      << "extract(key) failed to use the same allocator";
+}
+
+TEST(LinkedHashSet, Merge) {
+  linked_hash_set<int> m = {1, 7, 3, 6, 10};
+  linked_hash_set<int> src = {1, 2, 9, 10, 4, 16};
+
+  m.merge(src);
+
+  EXPECT_THAT(m, ElementsAre(1, 7, 3, 6, 10, 2, 9, 4, 16));
+  for (int i : {1, 7, 3, 6, 10, 2, 9, 4, 16}) {
+    EXPECT_TRUE(m.contains(i));
+  }
+  EXPECT_THAT(src, ElementsAre(1, 10));
+  for (int i : {1, 10}) {
+    EXPECT_TRUE(src.contains(i));
+  }
+  for (int i : {2, 9, 4, 16}) {
+    EXPECT_FALSE(src.contains(i));
+  }
+}
+
+TEST(LinkedHashSet, EraseRange) {
+  linked_hash_set<int> set = {1, 2, 3, 4, 5, 25, 36, 7, 8, 9, 81};
+  auto start = set.find(3);
+  auto end = set.find(8);
+  auto itr = set.erase(start, end);
+  ASSERT_NE(itr, set.end());
+  EXPECT_THAT(*itr, 8);
+  EXPECT_THAT(set, ElementsAre(1, 2, 8, 9, 81));
+  for (int i : {1, 2, 8, 9, 81}) {
+    EXPECT_TRUE(set.contains(i));
+  }
+  for (int i : {3, 4, 5, 25, 36, 7}) {
+    EXPECT_FALSE(set.contains(i));
+  }
+}
+
+TEST(LinkedHashSet, InsertInitializerList) {
+  linked_hash_set<int> set;
+  set.insert({1, 7, 2, 9, 3, 29});
+  EXPECT_THAT(set, ElementsAre(1, 7, 2, 9, 3, 29));
+  for (int i : {1, 7, 2, 9, 3, 29}) {
+    EXPECT_TRUE(set.contains(i));
+  }
+}
+
+struct CountedHash {
+  explicit CountedHash(int* count) : count(count) {}
+  size_t operator()(int value) const {
+    ++(*count);
+    return value;
+  }
+  int* count = nullptr;
+};
+
+// Makes a set too big for small object optimization.  Counts the number of
+// hashes in `count`, but leaves `count` set to 0.
+linked_hash_set<int, CountedHash> MakeNonSmallSet(int* count) {
+  const int kFirstKey = -1000;
+  linked_hash_set<int, CountedHash> s(0, CountedHash(count));
+  for (int i = kFirstKey; i < kFirstKey + 100; ++i) {
+    s.insert(i);
+  }
+  *count = 0;
+  return s;
+}
+
+constexpr bool BuildHasDebugModeRehashes() {
+#if !defined(NDEBUG) || defined(ABSL_HAVE_ADDRESS_SANITIZER) || \
+    defined(ABSL_HAVE_MEMORY_SANITIZER) || defined(ABSL_HAVE_THREAD_SANITIZER)
+  return true;
+#else
+  return false;
+#endif
+}
+
+TEST(LinkedHashSetTest, HashCountInOptBuilds) {
+  if (BuildHasDebugModeRehashes()) {
+    GTEST_SKIP() << "Only run under NDEBUG: `assert` statements and sanitizer "
+                    "rehashing may cause redundant hashing.";
+  }
+
+  using Set = linked_hash_set<int, CountedHash>;
+  {
+    int count = 0;
+    Set s = MakeNonSmallSet(&count);
+    s.insert(1);
+    EXPECT_EQ(count, 1);
+    s.erase(1);
+    EXPECT_EQ(count, 2);
+  }
+  {
+    int count = 0;
+    Set s = MakeNonSmallSet(&count);
+    s.insert(3);
+    EXPECT_EQ(count, 1);
+    auto node = s.extract(3);
+    EXPECT_EQ(count, 2);
+    s.insert(std::move(node));
+    EXPECT_EQ(count, 3);
+  }
+  {
+    int count = 0;
+    Set s = MakeNonSmallSet(&count);
+    s.emplace(5);
+    EXPECT_EQ(count, 1);
+  }
+  {
+    int src_count = 0, dst_count = 0;
+    Set src = MakeNonSmallSet(&src_count);
+    Set dst = MakeNonSmallSet(&dst_count);
+    src.insert(7);
+    dst.merge(src);
+    EXPECT_LE(src_count, 200);
+    EXPECT_LE(dst_count, 200);
+  }
+}
+
+}  // namespace
+}  // namespace container_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/container/node_hash_map.h b/absl/container/node_hash_map.h
index a718842b653..75fc0ccab6c 100644
--- a/absl/container/node_hash_map.h
+++ b/absl/container/node_hash_map.h
@@ -32,22 +32,28 @@
 // migration, because it guarantees pointer stability. Consider migrating to
 // `node_hash_map` and perhaps converting to a more efficient `flat_hash_map`
 // upon further review.
+//
+// `node_hash_map` is not exception-safe.
 
 #ifndef ABSL_CONTAINER_NODE_HASH_MAP_H_
 #define ABSL_CONTAINER_NODE_HASH_MAP_H_
 
-#include <tuple>
+#include <cstddef>
+#include <memory>
 #include <type_traits>
 #include <utility>
 
 #include "absl/algorithm/container.h"
+#include "absl/base/attributes.h"
+#include "absl/container/hash_container_defaults.h"
 #include "absl/container/internal/container_memory.h"
-#include "absl/container/internal/hash_function_defaults.h"  // IWYU pragma: export
-#include "absl/container/internal/node_hash_policy.h"
+#include "absl/container/internal/node_slot_policy.h"
 #include "absl/container/internal/raw_hash_map.h"  // IWYU pragma: export
 #include "absl/memory/memory.h"
+#include "absl/meta/type_traits.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 template <class Key, class Value>
 class NodeHashMapPolicy;
@@ -64,7 +70,7 @@ class NodeHashMapPolicy;
 //
 // * Supports heterogeneous lookup, through `find()`, `operator[]()` and
 //   `insert()`, provided that the map is provided a compatible heterogeneous
-//   hashing function and equality operator.
+//   hashing function and equality operator. See below for details.
 // * Contains a `capacity()` member function indicating the number of element
 //   slots (open, deleted, and empty) within the hash map.
 // * Returns `void` from the `erase(iterator)` overload.
@@ -76,32 +82,58 @@ class NodeHashMapPolicy;
 // absl/hash/hash.h for information on extending Abseil hashing to user-defined
 // types.
 //
+// Using `absl::node_hash_map` at interface boundaries in dynamically loaded
+// libraries (e.g. .dll, .so) is unsupported due to way `absl::Hash` values may
+// be randomized across dynamically loaded libraries.
+//
+// To achieve heterogeneous lookup for custom types either `Hash` and `Eq` type
+// parameters can be used or `T` should have public inner types
+// `absl_container_hash` and (optionally) `absl_container_eq`. In either case,
+// `typename Hash::is_transparent` and `typename Eq::is_transparent` should be
+// well-formed. Both types are basically functors:
+// * `Hash` should support `size_t operator()(U val) const` that returns a hash
+// for the given `val`.
+// * `Eq` should support `bool operator()(U lhs, V rhs) const` that returns true
+// if `lhs` is equal to `rhs`.
+//
+// In most cases `T` needs only to provide the `absl_container_hash`. In this
+// case `std::equal_to<void>` will be used instead of `eq` part.
+//
+// PERFORMANCE WARNING: Erasure & sparsity can negatively affect performance:
+//  * Iteration takes O(capacity) time, not O(size).
+//  * erase() slows down begin() and ++iterator.
+//  * Capacity only shrinks on rehash() or clear() -- not on erase().
+//
 // Example:
 //
 //   // Create a node hash map of three strings (that map to strings)
 //   absl::node_hash_map<std::string, std::string> ducks =
 //     {{"a", "huey"}, {"b", "dewey"}, {"c", "louie"}};
 //
-//  // Insert a new element into the node hash map
-//  ducks.insert({"d", "donald"}};
+//   // Insert a new element into the node hash map
+//   ducks.insert({"d", "donald"}};
 //
-//  // Force a rehash of the node hash map
-//  ducks.rehash(0);
+//   // Force a rehash of the node hash map
+//   ducks.rehash(0);
 //
-//  // Find the element with the key "b"
-//  std::string search_key = "b";
-//  auto result = ducks.find(search_key);
-//  if (result != ducks.end()) {
-//    std::cout << "Result: " << result->second << std::endl;
-//  }
-template <class Key, class Value,
-          class Hash = absl::container_internal::hash_default_hash<Key>,
-          class Eq = absl::container_internal::hash_default_eq<Key>,
-          class Alloc = std::allocator<std::pair<const Key, Value>>>
-class node_hash_map
-    : public absl::container_internal::raw_hash_map<
+//   // Find the element with the key "b"
+//   std::string search_key = "b";
+//   auto result = ducks.find(search_key);
+//   if (result != ducks.end()) {
+//     std::cout << "Result: " << result->second << std::endl;
+//   }
+template <
+    class Key, class Value,
+    class Hash =
+        typename container_internal::NodeHashMapPolicy<Key, Value>::DefaultHash,
+    class Eq =
+        typename container_internal::NodeHashMapPolicy<Key, Value>::DefaultEq,
+    class Alloc = typename container_internal::NodeHashMapPolicy<
+        Key, Value>::DefaultAlloc>
+class ABSL_ATTRIBUTE_OWNER node_hash_map
+    : public absl::container_internal::InstantiateRawHashMap<
           absl::container_internal::NodeHashMapPolicy<Key, Value>, Hash, Eq,
-          Alloc> {
+          Alloc>::type {
   using Base = typename node_hash_map::raw_hash_map;
 
  public:
@@ -126,25 +158,37 @@ class node_hash_map
   //
   // * Copy assignment operator
   //
-  //  // Hash functor and Comparator are copied as well
-  //  absl::node_hash_map<int, std::string> map4;
-  //  map4 = map3;
+  //   // Hash functor and Comparator are copied as well
+  //   absl::node_hash_map<int, std::string> map4;
+  //   map4 = map3;
   //
   // * Move constructor
   //
   //   // Move is guaranteed efficient
   //   absl::node_hash_map<int, std::string> map5(std::move(map4));
   //
+  //   // After the move, map4 is in a valid but unspecified state. The only
+  //   // operations guaranteed to be safe on a moved-from map are destruction,
+  //   // assignment, and clear(). Any other operation (e.g. size(), empty(),
+  //   // iteration) results in undefined behavior.
+  //
   // * Move assignment operator
   //
   //   // May be efficient if allocators are compatible
   //   absl::node_hash_map<int, std::string> map6;
   //   map6 = std::move(map5);
   //
+  //   // Same moved-from guarantees apply to map5 after this operation.
+  //
   // * Range constructor
   //
   //   std::vector<std::pair<int, std::string>> v = {{1, "a"}, {2, "b"}};
   //   absl::node_hash_map<int, std::string> map7(v.begin(), v.end());
+  //
+  // * from_range constructor (C++23)
+  //
+  //   std::vector<std::pair<int, std::string>> v = {{1, "a"}, {2, "b"}};
+  //   absl::node_hash_map<int, std::string> map8(std::from_range, v);
   node_hash_map() {}
   using Base::Base;
 
@@ -214,17 +258,27 @@ class node_hash_map
   //   Erases the element at `position` of the `node_hash_map`, returning
   //   `void`.
   //
-  //   NOTE: this return behavior is different than that of STL containers in
-  //   general and `std::unordered_map` in particular.
+  //   NOTE: Returning `void` in this case is different than that of STL
+  //   containers in general and `std::unordered_map` in particular (which
+  //   return an iterator to the element following the erased element). If that
+  //   iterator is needed, simply post increment the iterator:
+  //
+  //     map.erase(it++);
+  //
   //
   // iterator erase(const_iterator first, const_iterator last):
   //
   //   Erases the elements in the open interval [`first`, `last`), returning an
-  //   iterator pointing to `last`.
+  //   iterator pointing to `last`. The special case of calling
+  //   `erase(begin(), end())` resets the reserved growth such that if
+  //   `reserve(N)` has previously been called and there has been no intervening
+  //   call to `clear()`, then after calling `erase(begin(), end())`, it is safe
+  //   to assume that inserting N elements will not cause a rehash.
   //
   // size_type erase(const key_type& key):
   //
-  //   Erases the element with the matching key, if it exists.
+  //   Erases the element with the matching key, if it exists, returning the
+  //   number of elements erased (0 or 1).
   using Base::erase;
 
   // node_hash_map::insert()
@@ -345,8 +399,8 @@ class node_hash_map
   // `node_hash_map`.
   //
   //   iterator try_emplace(const_iterator hint,
-  //                        const init_type& k, Args&&... args):
-  //   iterator try_emplace(const_iterator hint, init_type&& k, Args&&... args):
+  //                        const key_type& k, Args&&... args):
+  //   iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args):
   //
   // Inserts (via copy or move) the element of the specified key into the
   // `node_hash_map` using the position of `hint` as a non-binding suggestion
@@ -373,6 +427,7 @@ class node_hash_map
   //   key value and returns a node handle owning that extracted data. If the
   //   `node_hash_map` does not contain an element with a matching key, this
   //   function returns an empty node handle.
+  //
   using Base::extract;
 
   // node_hash_map::merge()
@@ -385,14 +440,13 @@ class node_hash_map
   // node_hash_map::swap(node_hash_map& other)
   //
   // Exchanges the contents of this `node_hash_map` with those of the `other`
-  // node hash map, avoiding invocation of any move, copy, or swap operations on
-  // individual elements.
+  // node hash map.
   //
   // All iterators and references on the `node_hash_map` remain valid, excepting
   // for the past-the-end iterator, which is invalidated.
   //
   // `swap()` requires that the node hash map's hashing and key equivalence
-  // functions be Swappable, and are exchaged using unqualified calls to
+  // functions be Swappable, and are exchanged using unqualified calls to
   // non-member `swap()`. If the map's allocator has
   // `std::allocator_traits<allocator_type>::propagate_on_container_swap::value`
   // set to `true`, the allocators are also exchanged using an unqualified call
@@ -414,7 +468,9 @@ class node_hash_map
   //
   // Sets the number of slots in the `node_hash_map` to the number needed to
   // accommodate at least `count` total elements without exceeding the current
-  // maximum load factor, and may rehash the container if needed.
+  // maximum load factor, and may rehash the container if needed. After this
+  // returns, it is guaranteed that `count - size()` elements can be inserted
+  // into the `node_hash_map` without another rehash.
   using Base::reserve;
 
   // node_hash_map::at()
@@ -513,19 +569,71 @@ class node_hash_map
   //
   // Returns the function used for comparing keys equality.
   using Base::key_eq;
+};
 
-  ABSL_DEPRECATED("Call `hash_function()` instead.")
-  typename Base::hasher hash_funct() { return this->hash_function(); }
+// erase_if(node_hash_map<>, Pred)
+//
+// Erases all elements that satisfy the predicate `pred` from the container `c`.
+// Returns the number of erased elements.
+template <typename K, typename V, typename H, typename E, typename A,
+          typename Predicate>
+typename node_hash_map<K, V, H, E, A>::size_type erase_if(
+    node_hash_map<K, V, H, E, A>& c, Predicate pred) {
+  return container_internal::EraseIf(pred, &c);
+}
+
+// swap(node_hash_map<>, node_hash_map<>)
+//
+// Swaps the contents of two `node_hash_map` containers.
+//
+// NOTE: we need to define this function template in order for
+// `flat_hash_set::swap` to be called instead of `std::swap`. Even though we
+// have `swap(raw_hash_set&, raw_hash_set&)` defined, that function requires a
+// derived-to-base conversion, whereas `std::swap` is a function template so
+// `std::swap` will be preferred by compiler.
+template <typename K, typename V, typename H, typename E, typename A>
+void swap(node_hash_map<K, V, H, E, A>& x,
+          node_hash_map<K, V, H, E, A>& y) noexcept(noexcept(x.swap(y))) {
+  return x.swap(y);
+}
 
-  ABSL_DEPRECATED("Call `rehash()` instead.")
-  void resize(typename Base::size_type hint) { this->rehash(hint); }
-};
+namespace container_internal {
+
+// c_for_each_fast(node_hash_map<>, Function)
+//
+// Container-based version of the <algorithm> `std::for_each()` function to
+// apply a function to a container's elements.
+// There is no guarantees on the order of the function calls.
+// Erasure and/or insertion of elements in the function is not allowed.
+template <typename K, typename V, typename H, typename E, typename A,
+          typename Function>
+std::decay_t<Function> c_for_each_fast(const node_hash_map<K, V, H, E, A>& c,
+                                       Function&& f) {
+  container_internal::ForEach(f, &c);
+  return f;
+}
+template <typename K, typename V, typename H, typename E, typename A,
+          typename Function>
+std::decay_t<Function> c_for_each_fast(node_hash_map<K, V, H, E, A>& c,
+                                       Function&& f) {
+  container_internal::ForEach(f, &c);
+  return f;
+}
+template <typename K, typename V, typename H, typename E, typename A,
+          typename Function>
+std::decay_t<Function> c_for_each_fast(node_hash_map<K, V, H, E, A>&& c,
+                                       Function&& f) {
+  container_internal::ForEach(f, &c);
+  return f;
+}
+
+}  // namespace container_internal
 
 namespace container_internal {
 
 template <class Key, class Value>
 class NodeHashMapPolicy
-    : public absl::container_internal::node_hash_policy<
+    : public absl::container_internal::node_slot_policy<
           std::pair<const Key, Value>&, NodeHashMapPolicy<Key, Value>> {
   using value_type = std::pair<const Key, Value>;
 
@@ -534,25 +642,28 @@ class NodeHashMapPolicy
   using mapped_type = Value;
   using init_type = std::pair</*non const*/ key_type, mapped_type>;
 
+  using DefaultHash = DefaultHashContainerHash<Key>;
+  using DefaultEq = DefaultHashContainerEq<Key>;
+  using DefaultAlloc = std::allocator<std::pair<const Key, Value>>;
+
   template <class Allocator, class... Args>
   static value_type* new_element(Allocator* alloc, Args&&... args) {
-    using PairAlloc = typename absl::allocator_traits<
+    using PairAlloc = typename std::allocator_traits<
         Allocator>::template rebind_alloc<value_type>;
     PairAlloc pair_alloc(*alloc);
-    value_type* res =
-        absl::allocator_traits<PairAlloc>::allocate(pair_alloc, 1);
-    absl::allocator_traits<PairAlloc>::construct(pair_alloc, res,
-                                                 std::forward<Args>(args)...);
+    value_type* res = std::allocator_traits<PairAlloc>::allocate(pair_alloc, 1);
+    std::allocator_traits<PairAlloc>::construct(pair_alloc, res,
+                                                std::forward<Args>(args)...);
     return res;
   }
 
   template <class Allocator>
   static void delete_element(Allocator* alloc, value_type* pair) {
-    using PairAlloc = typename absl::allocator_traits<
+    using PairAlloc = typename std::allocator_traits<
         Allocator>::template rebind_alloc<value_type>;
     PairAlloc pair_alloc(*alloc);
-    absl::allocator_traits<PairAlloc>::destroy(pair_alloc, pair);
-    absl::allocator_traits<PairAlloc>::deallocate(pair_alloc, pair, 1);
+    std::allocator_traits<PairAlloc>::destroy(pair_alloc, pair);
+    std::allocator_traits<PairAlloc>::deallocate(pair_alloc, pair, 1);
   }
 
   template <class F, class... Args>
@@ -569,6 +680,14 @@ class NodeHashMapPolicy
 
   static Value& value(value_type* elem) { return elem->second; }
   static const Value& value(const value_type* elem) { return elem->second; }
+
+  template <class Hash, bool kIsDefault, size_t kSeedShift>
+  static constexpr HashSlotFn get_hash_slot_fn() {
+    return memory_internal::IsLayoutCompatible<Key, Value>::value
+               ? &TypeErasedDerefAndApplyToSlotFirstFn<Hash, value_type,
+                                                       kIsDefault, kSeedShift>
+               : nullptr;
+  }
 };
 }  // namespace container_internal
 
@@ -581,6 +700,7 @@ struct IsUnorderedContainer<
 
 }  // namespace container_algorithm_internal
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_NODE_HASH_MAP_H_
diff --git a/absl/container/node_hash_map_test.cc b/absl/container/node_hash_map_test.cc
index 0f2714a72ce..476b12f072d 100644
--- a/absl/container/node_hash_map_test.cc
+++ b/absl/container/node_hash_map_test.cc
@@ -14,19 +14,38 @@
 
 #include "absl/container/node_hash_map.h"
 
+#include <cstddef>
+#include <new>
+#include <string>
+#include <tuple>
+#include <type_traits>
+#include <utility>
+#include <vector>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+#include "absl/container/internal/hash_policy_testing.h"
 #include "absl/container/internal/tracked.h"
 #include "absl/container/internal/unordered_map_constructor_test.h"
 #include "absl/container/internal/unordered_map_lookup_test.h"
 #include "absl/container/internal/unordered_map_members_test.h"
 #include "absl/container/internal/unordered_map_modifiers_test.h"
 
+#if ABSL_INTERNAL_CPLUSPLUS_LANG >= 202002L
+#include <ranges>  // NOLINT(build/c++20)
+#endif
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 namespace {
 
 using ::testing::Field;
+using ::testing::IsEmpty;
 using ::testing::Pair;
 using ::testing::UnorderedElementsAre;
+using ::testing::UnorderedElementsAreArray;
 
 using MapTypes = ::testing::Types<
     absl::node_hash_map<int, int, StatefulTestingHash, StatefulTestingEqual,
@@ -177,7 +196,7 @@ TEST(NodeHashMap, MergeExtractInsert) {
 
   // NonMovableKey is neither copyable nor movable. We should still be able to
   // move nodes around.
-  static_assert(!std::is_move_constructible<NonMovableKey>::value, "");
+  static_assert(!std::is_move_constructible_v<NonMovableKey>);
   set1.merge(set2);
 
   EXPECT_THAT(set1,
@@ -215,6 +234,149 @@ TEST(NodeHashMap, MergeExtractInsert) {
   EXPECT_THAT(set2, UnorderedElementsAre(Elem(7, -70), Elem(17, 23)));
 }
 
+bool FirstIsEven(std::pair<const int, int> p) { return p.first % 2 == 0; }
+
+TEST(NodeHashMap, EraseIf) {
+  // Erase all elements.
+  {
+    node_hash_map<int, int> s = {{1, 1}, {2, 2}, {3, 3}, {4, 4}, {5, 5}};
+    EXPECT_EQ(erase_if(s, [](std::pair<const int, int>) { return true; }), 5);
+    EXPECT_THAT(s, IsEmpty());
+  }
+  // Erase no elements.
+  {
+    node_hash_map<int, int> s = {{1, 1}, {2, 2}, {3, 3}, {4, 4}, {5, 5}};
+    EXPECT_EQ(erase_if(s, [](std::pair<const int, int>) { return false; }), 0);
+    EXPECT_THAT(s, UnorderedElementsAre(Pair(1, 1), Pair(2, 2), Pair(3, 3),
+                                        Pair(4, 4), Pair(5, 5)));
+  }
+  // Erase specific elements.
+  {
+    node_hash_map<int, int> s = {{1, 1}, {2, 2}, {3, 3}, {4, 4}, {5, 5}};
+    EXPECT_EQ(erase_if(s,
+                       [](std::pair<const int, int> kvp) {
+                         return kvp.first % 2 == 1;
+                       }),
+              3);
+    EXPECT_THAT(s, UnorderedElementsAre(Pair(2, 2), Pair(4, 4)));
+  }
+  // Predicate is function reference.
+  {
+    node_hash_map<int, int> s = {{1, 1}, {2, 2}, {3, 3}, {4, 4}, {5, 5}};
+    EXPECT_EQ(erase_if(s, FirstIsEven), 2);
+    EXPECT_THAT(s, UnorderedElementsAre(Pair(1, 1), Pair(3, 3), Pair(5, 5)));
+  }
+  // Predicate is function pointer.
+  {
+    node_hash_map<int, int> s = {{1, 1}, {2, 2}, {3, 3}, {4, 4}, {5, 5}};
+    EXPECT_EQ(erase_if(s, &FirstIsEven), 2);
+    EXPECT_THAT(s, UnorderedElementsAre(Pair(1, 1), Pair(3, 3), Pair(5, 5)));
+  }
+}
+
+TEST(NodeHashMap, CForEach) {
+  node_hash_map<int, int> m;
+  std::vector<std::pair<int, int>> expected;
+  for (int i = 0; i < 100; ++i) {
+    {
+      SCOPED_TRACE("mutable object iteration");
+      std::vector<std::pair<int, int>> v;
+      absl::container_internal::c_for_each_fast(
+          m, [&v](std::pair<const int, int>& p) { v.push_back(p); });
+      EXPECT_THAT(v, UnorderedElementsAreArray(expected));
+    }
+    {
+      SCOPED_TRACE("const object iteration");
+      std::vector<std::pair<int, int>> v;
+      const node_hash_map<int, int>& cm = m;
+      absl::container_internal::c_for_each_fast(
+          cm, [&v](const std::pair<const int, int>& p) { v.push_back(p); });
+      EXPECT_THAT(v, UnorderedElementsAreArray(expected));
+    }
+    {
+      SCOPED_TRACE("const object iteration");
+      std::vector<std::pair<int, int>> v;
+      absl::container_internal::c_for_each_fast(
+          node_hash_map<int, int>(m),
+          [&v](std::pair<const int, int>& p) { v.push_back(p); });
+      EXPECT_THAT(v, UnorderedElementsAreArray(expected));
+    }
+    m[i] = i;
+    expected.emplace_back(i, i);
+  }
+}
+
+TEST(NodeHashMap, CForEachMutate) {
+  node_hash_map<int, int> s;
+  std::vector<std::pair<int, int>> expected;
+  for (int i = 0; i < 100; ++i) {
+    std::vector<std::pair<int, int>> v;
+    absl::container_internal::c_for_each_fast(
+        s, [&v](std::pair<const int, int>& p) {
+          v.push_back(p);
+          p.second++;
+        });
+    EXPECT_THAT(v, UnorderedElementsAreArray(expected));
+    for (auto& p : expected) {
+      p.second++;
+    }
+    EXPECT_THAT(s, UnorderedElementsAreArray(expected));
+    s[i] = i;
+    expected.emplace_back(i, i);
+  }
+}
+
+TEST(NodeHashMap, NodeHandleMutableKeyAccess) {
+  node_hash_map<std::string, std::string> map;
+
+  map["key1"] = "mapped";
+
+  auto nh = map.extract(map.begin());
+  nh.key().resize(3);
+  map.insert(std::move(nh));
+
+  EXPECT_THAT(map, testing::ElementsAre(Pair("key", "mapped")));
+}
+
+TEST(NodeHashMap, RecursiveTypeCompiles) {
+  struct RecursiveType {
+    node_hash_map<int, RecursiveType> m;
+  };
+  RecursiveType t;
+  t.m[0] = RecursiveType{};
+}
+
+#if defined(__cpp_lib_containers_ranges) && \
+    __cpp_lib_containers_ranges >= 202202L
+TEST(NodeHashMap, FromRange) {
+  std::vector<std::pair<int, int>> v = {{1, 2}, {3, 4}, {5, 6}};
+  absl::node_hash_map<int, int> m(std::from_range, v);
+  EXPECT_THAT(m, UnorderedElementsAre(Pair(1, 2), Pair(3, 4), Pair(5, 6)));
+}
+
+TEST(NodeHashMap, FromRangeWithAllocator) {
+  std::vector<std::pair<int, int>> v = {{1, 2}, {3, 4}, {5, 6}};
+  absl::node_hash_map<int, int,
+                      absl::container_internal::hash_default_hash<int>,
+                      absl::container_internal::hash_default_eq<int>,
+                      Alloc<std::pair<const int, int>>>
+      m(std::from_range, v, 0, Alloc<std::pair<const int, int>>());
+  EXPECT_THAT(m, UnorderedElementsAre(Pair(1, 2), Pair(3, 4), Pair(5, 6)));
+}
+
+TEST(NodeHashMap, FromRangeWithHasherAndAllocator) {
+  std::vector<std::pair<int, int>> v = {{1, 2}, {3, 4}, {5, 6}};
+  using TestingHash = absl::container_internal::StatefulTestingHash;
+  absl::node_hash_map<int, int, TestingHash,
+                      absl::container_internal::hash_default_eq<int>,
+                      Alloc<std::pair<const int, int>>>
+      m(std::from_range, v, 0, TestingHash{},
+        Alloc<std::pair<const int, int>>());
+  EXPECT_THAT(m, UnorderedElementsAre(Pair(1, 2), Pair(3, 4), Pair(5, 6)));
+}
+#endif
+
 }  // namespace
 }  // namespace container_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/container/node_hash_set.h b/absl/container/node_hash_set.h
index 0cd1fe57f96..9da52e7c64c 100644
--- a/absl/container/node_hash_set.h
+++ b/absl/container/node_hash_set.h
@@ -18,7 +18,7 @@
 //
 // An `absl::node_hash_set<T>` is an unordered associative container designed to
 // be a more efficient replacement for `std::unordered_set`. Like
-// `unordered_set`, search, insertion, and deletion of map elements can be done
+// `unordered_set`, search, insertion, and deletion of set elements can be done
 // as an `O(1)` operation. However, `node_hash_set` (and other unordered
 // associative containers known as the collection of Abseil "Swiss tables")
 // contain other optimizations that result in both memory and computation
@@ -31,19 +31,27 @@
 // `node_hash_set` should be an easy migration. Consider migrating to
 // `node_hash_set` and perhaps converting to a more efficient `flat_hash_set`
 // upon further review.
+//
+// `node_hash_set` is not exception-safe.
 
 #ifndef ABSL_CONTAINER_NODE_HASH_SET_H_
 #define ABSL_CONTAINER_NODE_HASH_SET_H_
 
+#include <cstddef>
+#include <memory>
 #include <type_traits>
 
 #include "absl/algorithm/container.h"
-#include "absl/container/internal/hash_function_defaults.h"  // IWYU pragma: export
-#include "absl/container/internal/node_hash_policy.h"
+#include "absl/base/attributes.h"
+#include "absl/container/hash_container_defaults.h"
+#include "absl/container/internal/container_memory.h"
+#include "absl/container/internal/node_slot_policy.h"
 #include "absl/container/internal/raw_hash_set.h"  // IWYU pragma: export
 #include "absl/memory/memory.h"
+#include "absl/meta/type_traits.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 template <typename T>
 struct NodeHashSetPolicy;
@@ -59,8 +67,8 @@ struct NodeHashSetPolicy;
 // following notable differences:
 //
 // * Supports heterogeneous lookup, through `find()`, `operator[]()` and
-//   `insert()`, provided that the map is provided a compatible heterogeneous
-//   hashing function and equality operator.
+//   `insert()`, provided that the set is provided a compatible heterogeneous
+//   hashing function and equality operator. See below for details.
 // * Contains a `capacity()` member function indicating the number of element
 //   slots (open, deleted, and empty) within the hash set.
 // * Returns `void` from the `erase(iterator)` overload.
@@ -72,34 +80,60 @@ struct NodeHashSetPolicy;
 // absl/hash/hash.h for information on extending Abseil hashing to user-defined
 // types.
 //
+// Using `absl::node_hash_set` at interface boundaries in dynamically loaded
+// libraries (e.g. .dll, .so) is unsupported due to way `absl::Hash` values may
+// be randomized across dynamically loaded libraries.
+//
+// To achieve heterogeneous lookup for custom types either `Hash` and `Eq` type
+// parameters can be used or `T` should have public inner types
+// `absl_container_hash` and (optionally) `absl_container_eq`. In either case,
+// `typename Hash::is_transparent` and `typename Eq::is_transparent` should be
+// well-formed. Both types are basically functors:
+// * `Hash` should support `size_t operator()(U val) const` that returns a hash
+// for the given `val`.
+// * `Eq` should support `bool operator()(U lhs, V rhs) const` that returns true
+// if `lhs` is equal to `rhs`.
+//
+// In most cases `T` needs only to provide the `absl_container_hash`. In this
+// case `std::equal_to<void>` will be used instead of `eq` part.
+//
+// PERFORMANCE WARNING: Erasure & sparsity can negatively affect performance:
+//  * Iteration takes O(capacity) time, not O(size).
+//  * erase() slows down begin() and ++iterator.
+//  * Capacity only shrinks on rehash() or clear() -- not on erase().
+//
 // Example:
 //
 //   // Create a node hash set of three strings
-//   absl::node_hash_map<std::string, std::string> ducks =
-//     {"huey", "dewey"}, "louie"};
+//   absl::node_hash_set<std::string> ducks =
+//     {"huey", "dewey", "louie"};
 //
-//  // Insert a new element into the node hash map
-//  ducks.insert("donald"};
+//   // Insert a new element into the node hash set
+//   ducks.insert("donald");
 //
-//  // Force a rehash of the node hash map
-//  ducks.rehash(0);
+//   // Force a rehash of the node hash set
+//   ducks.rehash(0);
 //
-//  // See if "dewey" is present
-//  if (ducks.contains("dewey")) {
-//    std::cout << "We found dewey!" << std::endl;
-//  }
-template <class T, class Hash = absl::container_internal::hash_default_hash<T>,
-          class Eq = absl::container_internal::hash_default_eq<T>,
-          class Alloc = std::allocator<T>>
-class node_hash_set
-    : public absl::container_internal::raw_hash_set<
-          absl::container_internal::NodeHashSetPolicy<T>, Hash, Eq, Alloc> {
+//   // See if "dewey" is present
+//   if (ducks.contains("dewey")) {
+//     std::cout << "We found dewey!" << std::endl;
+//   }
+template <
+    class T,
+    class Hash = typename container_internal::NodeHashSetPolicy<T>::DefaultHash,
+    class Eq = typename container_internal::NodeHashSetPolicy<T>::DefaultEq,
+    class Alloc =
+        typename container_internal::NodeHashSetPolicy<T>::DefaultAlloc>
+class ABSL_ATTRIBUTE_OWNER node_hash_set
+    : public absl::container_internal::InstantiateRawHashSet<
+          absl::container_internal::NodeHashSetPolicy<T>, Hash, Eq,
+          Alloc>::type {
   using Base = typename node_hash_set::raw_hash_set;
 
  public:
   // Constructors and Assignment Operators
   //
-  // A node_hash_set supports the same overload set as `std::unordered_map`
+  // A node_hash_set supports the same overload set as `std::unordered_set`
   // for construction and assignment:
   //
   // *  Default constructor
@@ -110,7 +144,7 @@ class node_hash_set
   // * Initializer List constructor
   //
   //   absl::node_hash_set<std::string> set2 =
-  //       {{"huey"}, {"dewey"}, {"louie"},};
+  //       {{"huey"}, {"dewey"}, {"louie"}};
   //
   // * Copy constructor
   //
@@ -118,25 +152,37 @@ class node_hash_set
   //
   // * Copy assignment operator
   //
-  //  // Hash functor and Comparator are copied as well
-  //  absl::node_hash_set<std::string> set4;
-  //  set4 = set3;
+  //   // Hash functor and Comparator are copied as well
+  //   absl::node_hash_set<std::string> set4;
+  //   set4 = set3;
   //
   // * Move constructor
   //
   //   // Move is guaranteed efficient
   //   absl::node_hash_set<std::string> set5(std::move(set4));
   //
+  //   // After the move, set4 is in a valid but unspecified state. The only
+  //   // operations guaranteed to be safe on a moved-from set are destruction,
+  //   // assignment, and clear(). Any other operation (e.g. size(), empty(),
+  //   // iteration) results in undefined behavior.
+  //
   // * Move assignment operator
   //
   //   // May be efficient if allocators are compatible
   //   absl::node_hash_set<std::string> set6;
   //   set6 = std::move(set5);
   //
+  //   // Same moved-from guarantees apply to set5 after this operation.
+  //
   // * Range constructor
   //
   //   std::vector<std::string> v = {"a", "b"};
   //   absl::node_hash_set<std::string> set7(v.begin(), v.end());
+  //
+  // * from_range constructor (C++23)
+  //
+  //   std::vector<std::string> v = {"a", "b"};
+  //   absl::node_hash_set<std::string> set8(std::from_range, v);
   node_hash_set() {}
   using Base::Base;
 
@@ -166,7 +212,7 @@ class node_hash_set
   // available within the `node_hash_set`.
   //
   // NOTE: this member function is particular to `absl::node_hash_set` and is
-  // not provided in the `std::unordered_map` API.
+  // not provided in the `std::unordered_set` API.
   using Base::capacity;
 
   // node_hash_set::empty()
@@ -206,17 +252,27 @@ class node_hash_set
   //   Erases the element at `position` of the `node_hash_set`, returning
   //   `void`.
   //
-  //   NOTE: this return behavior is different than that of STL containers in
-  //   general and `std::unordered_map` in particular.
+  //   NOTE: Returning `void` in this case is different than that of STL
+  //   containers in general and `std::unordered_map` in particular (which
+  //   return an iterator to the element following the erased element). If that
+  //   iterator is needed, simply post increment the iterator:
+  //
+  //     map.erase(it++);
+  //
   //
   // iterator erase(const_iterator first, const_iterator last):
   //
   //   Erases the elements in the open interval [`first`, `last`), returning an
-  //   iterator pointing to `last`.
+  //   iterator pointing to `last`. The special case of calling
+  //   `erase(begin(), end())` resets the reserved growth such that if
+  //   `reserve(N)` has previously been called and there has been no intervening
+  //   call to `clear()`, then after calling `erase(begin(), end())`, it is safe
+  //   to assume that inserting N elements will not cause a rehash.
   //
   // size_type erase(const key_type& key):
   //
-  //   Erases the element with the matching key, if it exists.
+  //   Erases the element with the matching key, if it exists, returning the
+  //   number of elements erased (0 or 1).
   using Base::erase;
 
   // node_hash_set::insert()
@@ -312,7 +368,7 @@ class node_hash_set
 
   // node_hash_set::merge()
   //
-  // Extracts elements from a given `source` flat hash map into this
+  // Extracts elements from a given `source` node hash set into this
   // `node_hash_set`. If the destination `node_hash_set` already contains an
   // element with an equivalent key, that element is not extracted.
   using Base::merge;
@@ -320,15 +376,14 @@ class node_hash_set
   // node_hash_set::swap(node_hash_set& other)
   //
   // Exchanges the contents of this `node_hash_set` with those of the `other`
-  // flat hash map, avoiding invocation of any move, copy, or swap operations on
-  // individual elements.
+  // node hash set.
   //
   // All iterators and references on the `node_hash_set` remain valid, excepting
   // for the past-the-end iterator, which is invalidated.
   //
-  // `swap()` requires that the flat hash set's hashing and key equivalence
-  // functions be Swappable, and are exchaged using unqualified calls to
-  // non-member `swap()`. If the map's allocator has
+  // `swap()` requires that the node hash set's hashing and key equivalence
+  // functions be Swappable, and are exchanged using unqualified calls to
+  // non-member `swap()`. If the set's allocator has
   // `std::allocator_traits<allocator_type>::propagate_on_container_swap::value`
   // set to `true`, the allocators are also exchanged using an unqualified call
   // to non-member `swap()`; otherwise, the allocators are not swapped.
@@ -352,7 +407,9 @@ class node_hash_set
   //
   // Sets the number of slots in the `node_hash_set` to the number needed to
   // accommodate at least `count` total elements without exceeding the current
-  // maximum load factor, and may rehash the container if needed.
+  // maximum load factor, and may rehash the container if needed. After this
+  // returns, it is guaranteed that `count - size()` elements can be inserted
+  // into the `node_hash_set` without another rehash.
   using Base::reserve;
 
   // node_hash_set::contains()
@@ -383,14 +440,14 @@ class node_hash_set
   // node_hash_set::bucket_count()
   //
   // Returns the number of "buckets" within the `node_hash_set`. Note that
-  // because a flat hash map contains all elements within its internal storage,
+  // because a node hash set contains all elements within its internal storage,
   // this value simply equals the current capacity of the `node_hash_set`.
   using Base::bucket_count;
 
   // node_hash_set::load_factor()
   //
   // Returns the current load factor of the `node_hash_set` (the average number
-  // of slots occupied with a value within the hash map).
+  // of slots occupied with a value within the hash set).
   using Base::load_factor;
 
   // node_hash_set::max_load_factor()
@@ -426,41 +483,93 @@ class node_hash_set
   //
   // Returns the function used for comparing keys equality.
   using Base::key_eq;
+};
 
-  ABSL_DEPRECATED("Call `hash_function()` instead.")
-  typename Base::hasher hash_funct() { return this->hash_function(); }
+// erase_if(node_hash_set<>, Pred)
+//
+// Erases all elements that satisfy the predicate `pred` from the container `c`.
+// Returns the number of erased elements.
+template <typename T, typename H, typename E, typename A, typename Predicate>
+typename node_hash_set<T, H, E, A>::size_type erase_if(
+    node_hash_set<T, H, E, A>& c, Predicate pred) {
+  return container_internal::EraseIf(pred, &c);
+}
+
+// swap(node_hash_set<>, node_hash_set<>)
+//
+// Swaps the contents of two `node_hash_set` containers.
+//
+// NOTE: we need to define this function template in order for
+// `flat_hash_set::swap` to be called instead of `std::swap`. Even though we
+// have `swap(raw_hash_set&, raw_hash_set&)` defined, that function requires a
+// derived-to-base conversion, whereas `std::swap` is a function template so
+// `std::swap` will be preferred by compiler.
+template <typename T, typename H, typename E, typename A>
+void swap(node_hash_set<T, H, E, A>& x,
+          node_hash_set<T, H, E, A>& y) noexcept(noexcept(x.swap(y))) {
+  return x.swap(y);
+}
 
-  ABSL_DEPRECATED("Call `rehash()` instead.")
-  void resize(typename Base::size_type hint) { this->rehash(hint); }
-};
+namespace container_internal {
+
+// c_for_each_fast(node_hash_set<>, Function)
+//
+// Container-based version of the <algorithm> `std::for_each()` function to
+// apply a function to a container's elements.
+// There is no guarantees on the order of the function calls.
+// Erasure and/or insertion of elements in the function is not allowed.
+template <typename T, typename H, typename E, typename A, typename Function>
+std::decay_t<Function> c_for_each_fast(const node_hash_set<T, H, E, A>& c,
+                                       Function&& f) {
+  container_internal::ForEach(f, &c);
+  return f;
+}
+template <typename T, typename H, typename E, typename A, typename Function>
+std::decay_t<Function> c_for_each_fast(node_hash_set<T, H, E, A>& c,
+                                       Function&& f) {
+  container_internal::ForEach(f, &c);
+  return f;
+}
+template <typename T, typename H, typename E, typename A, typename Function>
+std::decay_t<Function> c_for_each_fast(node_hash_set<T, H, E, A>&& c,
+                                       Function&& f) {
+  container_internal::ForEach(f, &c);
+  return f;
+}
+
+}  // namespace container_internal
 
 namespace container_internal {
 
 template <class T>
 struct NodeHashSetPolicy
-    : absl::container_internal::node_hash_policy<T&, NodeHashSetPolicy<T>> {
+    : absl::container_internal::node_slot_policy<T&, NodeHashSetPolicy<T>> {
   using key_type = T;
   using init_type = T;
   using constant_iterators = std::true_type;
 
+  using DefaultHash = DefaultHashContainerHash<T>;
+  using DefaultEq = DefaultHashContainerEq<T>;
+  using DefaultAlloc = std::allocator<T>;
+
   template <class Allocator, class... Args>
   static T* new_element(Allocator* alloc, Args&&... args) {
     using ValueAlloc =
-        typename absl::allocator_traits<Allocator>::template rebind_alloc<T>;
+        typename std::allocator_traits<Allocator>::template rebind_alloc<T>;
     ValueAlloc value_alloc(*alloc);
-    T* res = absl::allocator_traits<ValueAlloc>::allocate(value_alloc, 1);
-    absl::allocator_traits<ValueAlloc>::construct(value_alloc, res,
-                                                  std::forward<Args>(args)...);
+    T* res = std::allocator_traits<ValueAlloc>::allocate(value_alloc, 1);
+    std::allocator_traits<ValueAlloc>::construct(value_alloc, res,
+                                                 std::forward<Args>(args)...);
     return res;
   }
 
   template <class Allocator>
   static void delete_element(Allocator* alloc, T* elem) {
     using ValueAlloc =
-        typename absl::allocator_traits<Allocator>::template rebind_alloc<T>;
+        typename std::allocator_traits<Allocator>::template rebind_alloc<T>;
     ValueAlloc value_alloc(*alloc);
-    absl::allocator_traits<ValueAlloc>::destroy(value_alloc, elem);
-    absl::allocator_traits<ValueAlloc>::deallocate(value_alloc, elem, 1);
+    std::allocator_traits<ValueAlloc>::destroy(value_alloc, elem);
+    std::allocator_traits<ValueAlloc>::deallocate(value_alloc, elem, 1);
   }
 
   template <class F, class... Args>
@@ -472,6 +581,11 @@ struct NodeHashSetPolicy
   }
 
   static size_t element_space_used(const T*) { return sizeof(T); }
+
+  template <class Hash, bool kIsDefault, size_t kSeedShift>
+  static constexpr HashSlotFn get_hash_slot_fn() {
+    return &TypeErasedDerefAndApplyToSlotFn<Hash, T, kIsDefault, kSeedShift>;
+  }
 };
 }  // namespace container_internal
 
@@ -483,6 +597,7 @@ struct IsUnorderedContainer<absl::node_hash_set<Key, Hash, KeyEqual, Allocator>>
     : std::true_type {};
 
 }  // namespace container_algorithm_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_NODE_HASH_SET_H_
diff --git a/absl/container/node_hash_set_test.cc b/absl/container/node_hash_set_test.cc
index 0ea76e7c78e..ab40282ab41 100644
--- a/absl/container/node_hash_set_test.cc
+++ b/absl/container/node_hash_set_test.cc
@@ -14,18 +14,36 @@
 
 #include "absl/container/node_hash_set.h"
 
+#include <cstddef>
+#include <memory>
+#include <string>
+#include <utility>
+#include <vector>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+#include "absl/container/internal/hash_generator_testing.h"
+#include "absl/container/internal/hash_policy_testing.h"
 #include "absl/container/internal/unordered_set_constructor_test.h"
 #include "absl/container/internal/unordered_set_lookup_test.h"
 #include "absl/container/internal/unordered_set_members_test.h"
 #include "absl/container/internal/unordered_set_modifiers_test.h"
+#include "absl/memory/memory.h"
+
+#if ABSL_INTERNAL_CPLUSPLUS_LANG >= 202002L
+#include <ranges>  // NOLINT(build/c++20)
+#endif
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 namespace {
-using ::absl::container_internal::hash_internal::Enum;
-using ::absl::container_internal::hash_internal::EnumClass;
+
+using ::testing::IsEmpty;
 using ::testing::Pointee;
 using ::testing::UnorderedElementsAre;
+using ::testing::UnorderedElementsAreArray;
 
 using SetTypes = ::testing::Types<
     node_hash_set<int, StatefulTestingHash, StatefulTestingEqual, Alloc<int>>,
@@ -57,11 +75,11 @@ TEST(NodeHashSet, MergeExtractInsert) {
     }
   };
   absl::node_hash_set<std::unique_ptr<int>, Hash, Eq> set1, set2;
-  set1.insert(absl::make_unique<int>(7));
-  set1.insert(absl::make_unique<int>(17));
+  set1.insert(std::make_unique<int>(7));
+  set1.insert(std::make_unique<int>(17));
 
-  set2.insert(absl::make_unique<int>(7));
-  set2.insert(absl::make_unique<int>(19));
+  set2.insert(std::make_unique<int>(7));
+  set2.insert(std::make_unique<int>(19));
 
   EXPECT_THAT(set1, UnorderedElementsAre(Pointee(7), Pointee(17)));
   EXPECT_THAT(set2, UnorderedElementsAre(Pointee(7), Pointee(19)));
@@ -71,7 +89,7 @@ TEST(NodeHashSet, MergeExtractInsert) {
   EXPECT_THAT(set1, UnorderedElementsAre(Pointee(7), Pointee(17), Pointee(19)));
   EXPECT_THAT(set2, UnorderedElementsAre(Pointee(7)));
 
-  auto node = set1.extract(absl::make_unique<int>(7));
+  auto node = set1.extract(std::make_unique<int>(7));
   EXPECT_TRUE(node);
   EXPECT_THAT(node.value(), Pointee(7));
   EXPECT_THAT(set1, UnorderedElementsAre(Pointee(17), Pointee(19)));
@@ -85,12 +103,12 @@ TEST(NodeHashSet, MergeExtractInsert) {
   EXPECT_NE(insert_result.position->get(), insert_result.node.value().get());
   EXPECT_THAT(set2, UnorderedElementsAre(Pointee(7)));
 
-  node = set1.extract(absl::make_unique<int>(17));
+  node = set1.extract(std::make_unique<int>(17));
   EXPECT_TRUE(node);
   EXPECT_THAT(node.value(), Pointee(17));
   EXPECT_THAT(set1, UnorderedElementsAre(Pointee(19)));
 
-  node.value() = absl::make_unique<int>(23);
+  node.value() = std::make_unique<int>(23);
 
   insert_result = set2.insert(std::move(node));
   EXPECT_FALSE(node);
@@ -100,6 +118,103 @@ TEST(NodeHashSet, MergeExtractInsert) {
   EXPECT_THAT(set2, UnorderedElementsAre(Pointee(7), Pointee(23)));
 }
 
+bool IsEven(int k) { return k % 2 == 0; }
+
+TEST(NodeHashSet, EraseIf) {
+  // Erase all elements.
+  {
+    node_hash_set<int> s = {1, 2, 3, 4, 5};
+    EXPECT_EQ(erase_if(s, [](int) { return true; }), 5);
+    EXPECT_THAT(s, IsEmpty());
+  }
+  // Erase no elements.
+  {
+    node_hash_set<int> s = {1, 2, 3, 4, 5};
+    EXPECT_EQ(erase_if(s, [](int) { return false; }), 0);
+    EXPECT_THAT(s, UnorderedElementsAre(1, 2, 3, 4, 5));
+  }
+  // Erase specific elements.
+  {
+    node_hash_set<int> s = {1, 2, 3, 4, 5};
+    EXPECT_EQ(erase_if(s, [](int k) { return k % 2 == 1; }), 3);
+    EXPECT_THAT(s, UnorderedElementsAre(2, 4));
+  }
+  // Predicate is function reference.
+  {
+    node_hash_set<int> s = {1, 2, 3, 4, 5};
+    EXPECT_EQ(erase_if(s, IsEven), 2);
+    EXPECT_THAT(s, UnorderedElementsAre(1, 3, 5));
+  }
+  // Predicate is function pointer.
+  {
+    node_hash_set<int> s = {1, 2, 3, 4, 5};
+    EXPECT_EQ(erase_if(s, &IsEven), 2);
+    EXPECT_THAT(s, UnorderedElementsAre(1, 3, 5));
+  }
+}
+
+TEST(NodeHashSet, CForEach) {
+  using ValueType = std::pair<int, int>;
+  node_hash_set<ValueType> s;
+  std::vector<ValueType> expected;
+  for (int i = 0; i < 100; ++i) {
+    {
+      SCOPED_TRACE("mutable object iteration");
+      std::vector<ValueType> v;
+      absl::container_internal::c_for_each_fast(
+          s, [&v](const ValueType& p) { v.push_back(p); });
+      ASSERT_THAT(v, UnorderedElementsAreArray(expected));
+    }
+    {
+      SCOPED_TRACE("const object iteration");
+      std::vector<ValueType> v;
+      const node_hash_set<ValueType>& cs = s;
+      absl::container_internal::c_for_each_fast(
+          cs, [&v](const ValueType& p) { v.push_back(p); });
+      ASSERT_THAT(v, UnorderedElementsAreArray(expected));
+    }
+    {
+      SCOPED_TRACE("temporary object iteration");
+      std::vector<ValueType> v;
+      absl::container_internal::c_for_each_fast(
+          node_hash_set<ValueType>(s),
+          [&v](const ValueType& p) { v.push_back(p); });
+      ASSERT_THAT(v, UnorderedElementsAreArray(expected));
+    }
+    s.emplace(i, i);
+    expected.emplace_back(i, i);
+  }
+}
+
+#if defined(__cpp_lib_containers_ranges) && \
+    __cpp_lib_containers_ranges >= 202202L
+TEST(NodeHashSet, FromRange) {
+  std::vector<int> v = {1, 2, 3, 4, 5};
+  absl::node_hash_set<int> s(std::from_range, v);
+  EXPECT_THAT(s, UnorderedElementsAre(1, 2, 3, 4, 5));
+}
+
+TEST(NodeHashSet, FromRangeWithAllocator) {
+  std::vector<int> v = {1, 2, 3, 4, 5};
+  absl::node_hash_set<int, absl::container_internal::hash_default_hash<int>,
+                      absl::container_internal::hash_default_eq<int>,
+                      Alloc<int>>
+      s(std::from_range, v, 0, Alloc<int>());
+  EXPECT_THAT(s, UnorderedElementsAre(1, 2, 3, 4, 5));
+}
+
+TEST(NodeHashSet, FromRangeWithHasherAndAllocator) {
+  std::vector<int> v = {1, 2, 3, 4, 5};
+  using TestingHash = absl::container_internal::StatefulTestingHash;
+  absl::node_hash_set<int, TestingHash,
+                      absl::container_internal::hash_default_eq<int>,
+                      Alloc<int>>
+      s(std::from_range, v, 0, TestingHash{}, Alloc<int>());
+  EXPECT_THAT(s, UnorderedElementsAre(1, 2, 3, 4, 5));
+}
+#endif
+
 }  // namespace
 }  // namespace container_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/container/sample_element_size_test.cc b/absl/container/sample_element_size_test.cc
new file mode 100644
index 00000000000..9ce19fe4f7f
--- /dev/null
+++ b/absl/container/sample_element_size_test.cc
@@ -0,0 +1,121 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <cstddef>
+#include <unordered_set>
+#include <utility>
+#include <vector>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/container/flat_hash_map.h"
+#include "absl/container/flat_hash_set.h"
+#include "absl/container/internal/hashtablez_sampler.h"
+#include "absl/container/node_hash_map.h"
+#include "absl/container/node_hash_set.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace container_internal {
+namespace {
+
+#if defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
+// Create some tables of type `Table`, then look at all the new
+// `HashtablezInfo`s to make sure that the `inline_element_size ==
+// expected_element_size`.  The `inline_element_size` is the amount of memory
+// allocated for each slot of a hash table, that is `sizeof(slot_type)`.  Add
+// the new `HashtablezInfo`s to `preexisting_info`.  Store all the new tables
+// into `tables`.
+template <class Table>
+void TestInlineElementSize(
+    HashtablezSampler& sampler,
+    // clang-tidy gives a false positive on this declaration.  This unordered
+    // set cannot be flat_hash_set, however, since that would introduce a mutex
+    // deadlock.
+    std::unordered_set<const HashtablezInfo*>& preexisting_info,  // NOLINT
+    std::vector<Table>& tables,
+    const std::vector<typename Table::value_type>& values,
+    size_t expected_element_size) {
+  EXPECT_GT(values.size(), 0);
+  for (int i = 0; i < 10; ++i) {
+    // We create a new table and must store it somewhere so that when we store
+    // a pointer to the resulting `HashtablezInfo` into `preexisting_info`
+    // that we aren't storing a dangling pointer.
+    tables.emplace_back();
+    // We must insert elements to get a hashtablez to instantiate.
+    tables.back().insert(values.begin(), values.end());
+  }
+  size_t new_count = 0;
+  EXPECT_EQ(sampler.Iterate([&](const HashtablezInfo& info) {
+    if (preexisting_info.insert(&info).second) {
+      EXPECT_EQ(info.inline_element_size, expected_element_size);
+      ++new_count;
+    }
+  }),
+            0);
+  // Make sure we actually did get a new hashtablez.
+  EXPECT_GT(new_count, 0);
+}
+
+struct bigstruct {
+  char a[1000];
+  friend bool operator==(const bigstruct& x, const bigstruct& y) {
+    return memcmp(x.a, y.a, sizeof(x.a)) == 0;
+  }
+  template <typename H>
+  friend H AbslHashValue(H h, const bigstruct& c) {
+    return H::combine_contiguous(std::move(h), c.a, sizeof(c.a));
+  }
+};
+#endif
+
+TEST(FlatHashMap, SampleElementSize) {
+#if defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
+  // Enable sampling even if the prod default is off.
+  SetHashtablezEnabled(true);
+  SetHashtablezSampleParameter(1);
+  TestOnlyRefreshSamplingStateForCurrentThread();
+
+  auto& sampler = GlobalHashtablezSampler();
+  std::vector<flat_hash_map<int, bigstruct>> flat_map_tables;
+  std::vector<flat_hash_set<bigstruct>> flat_set_tables;
+  std::vector<node_hash_map<int, bigstruct>> node_map_tables;
+  std::vector<node_hash_set<bigstruct>> node_set_tables;
+  std::vector<bigstruct> set_values = {bigstruct{{0}}, bigstruct{{1}}};
+  std::vector<std::pair<const int, bigstruct>> map_values = {{0, bigstruct{}},
+                                                             {1, bigstruct{}}};
+
+  // clang-tidy gives a false positive on this declaration.  This unordered set
+  // cannot be a flat_hash_set, however, since that would introduce a mutex
+  // deadlock.
+  std::unordered_set<const HashtablezInfo*> preexisting_info;  // NOLINT
+  EXPECT_EQ(sampler.Iterate([&](const HashtablezInfo& info) {
+    preexisting_info.insert(&info);
+  }),
+            0);
+  TestInlineElementSize(sampler, preexisting_info, flat_map_tables, map_values,
+                        sizeof(int) + sizeof(bigstruct));
+  TestInlineElementSize(sampler, preexisting_info, node_map_tables, map_values,
+                        sizeof(void*));
+  TestInlineElementSize(sampler, preexisting_info, flat_set_tables, set_values,
+                        sizeof(bigstruct));
+  TestInlineElementSize(sampler, preexisting_info, node_set_tables, set_values,
+                        sizeof(void*));
+#endif
+}
+
+}  // namespace
+}  // namespace container_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/copts/AbseilConfigureCopts.cmake b/absl/copts/AbseilConfigureCopts.cmake
index b430873c5e0..f1d8b2667d4 100644
--- a/absl/copts/AbseilConfigureCopts.cmake
+++ b/absl/copts/AbseilConfigureCopts.cmake
@@ -1,52 +1,106 @@
 # See absl/copts/copts.py and absl/copts/generate_copts.py
 include(GENERATED_AbseilCopts)
 
-set(ABSL_LSAN_LINKOPTS "")
-set(ABSL_HAVE_LSAN OFF)
 set(ABSL_DEFAULT_LINKOPTS "")
 
-if("${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU")
+if (BUILD_SHARED_LIBS AND (MSVC OR ABSL_BUILD_MONOLITHIC_SHARED_LIBS))
+  set(ABSL_BUILD_DLL TRUE)
+  set(CMAKE_WINDOWS_EXPORT_ALL_SYMBOLS ON)
+else()
+  set(ABSL_BUILD_DLL FALSE)
+endif()
+
+if(APPLE AND CMAKE_CXX_COMPILER_ID MATCHES [[Clang]])
+  # Some CMake targets (not known at the moment of processing) could be set to
+  # compile for multiple architectures as specified by the OSX_ARCHITECTURES
+  # property, which is target-specific.  We should neither inspect nor rely on
+  # any CMake property or variable to detect an architecture, in particular:
+  #
+  #   - CMAKE_OSX_ARCHITECTURES
+  #     is just an initial value for OSX_ARCHITECTURES; set too early.
+  #
+  #   - OSX_ARCHITECTURES
+  #     is a per-target property; targets could be defined later, and their
+  #     properties could be modified any time later.
+  #
+  #   - CMAKE_SYSTEM_PROCESSOR
+  #     does not reflect multiple architectures at all.
+  #
+  # When compiling for multiple architectures, a build system can invoke a
+  # compiler either
+  #
+  #   - once: a single command line for multiple architectures (Ninja build)
+  #   - twice: two command lines per each architecture (Xcode build system)
+  #
+  # If case of Xcode, it would be possible to set an Xcode-specific attributes
+  # like XCODE_ATTRIBUTE_OTHER_CPLUSPLUSFLAGS[arch=arm64] or similar.
+  #
+  # In both cases, the viable strategy is to pass all arguments at once, allowing
+  # the compiler to dispatch arch-specific arguments to a designated backend.
+  set(ABSL_RANDOM_RANDEN_COPTS "")
+  foreach(_arch IN ITEMS "x86_64" "arm64")
+    string(TOUPPER "${_arch}" _arch_uppercase)
+    string(REPLACE "X86_64" "X64" _arch_uppercase ${_arch_uppercase})
+    foreach(_flag IN LISTS ABSL_RANDOM_HWAES_${_arch_uppercase}_FLAGS)
+      list(APPEND ABSL_RANDOM_RANDEN_COPTS "SHELL:-Xarch_${_arch} ${_flag}")
+    endforeach()
+  endforeach()
+  # If a compiler happens to deal with an argument for a currently unused
+  # architecture, it will warn about an unused command line argument.
+  option(ABSL_RANDOM_RANDEN_COPTS_WARNING OFF
+         "Warn if one of ABSL_RANDOM_RANDEN_COPTS is unused")
+  if(ABSL_RANDOM_RANDEN_COPTS AND NOT ABSL_RANDOM_RANDEN_COPTS_WARNING)
+    list(APPEND ABSL_RANDOM_RANDEN_COPTS "-Wno-unused-command-line-argument")
+  endif()
+elseif(CMAKE_SYSTEM_PROCESSOR MATCHES "x86_64|amd64|AMD64")
+  if (MSVC)
+    set(ABSL_RANDOM_RANDEN_COPTS "${ABSL_RANDOM_HWAES_MSVC_X64_FLAGS}")
+  else()
+    set(ABSL_RANDOM_RANDEN_COPTS "${ABSL_RANDOM_HWAES_X64_FLAGS}")
+  endif()
+elseif(CMAKE_SYSTEM_PROCESSOR MATCHES "arm.*|aarch64")
+  if (CMAKE_SIZEOF_VOID_P STREQUAL "8")
+    set(ABSL_RANDOM_RANDEN_COPTS "${ABSL_RANDOM_HWAES_ARM64_FLAGS}")
+  elseif(CMAKE_SIZEOF_VOID_P STREQUAL "4")
+    set(ABSL_RANDOM_RANDEN_COPTS "${ABSL_RANDOM_HWAES_ARM32_FLAGS}")
+  else()
+    message(WARNING "Value of CMAKE_SIZEOF_VOID_P (${CMAKE_SIZEOF_VOID_P}) is not supported.")
+  endif()
+else()
+  set(ABSL_RANDOM_RANDEN_COPTS "")
+endif()
+
+
+if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU" OR CMAKE_CXX_COMPILER_ID STREQUAL "QCC")
   set(ABSL_DEFAULT_COPTS "${ABSL_GCC_FLAGS}")
-  set(ABSL_TEST_COPTS "${ABSL_GCC_FLAGS};${ABSL_GCC_TEST_FLAGS}")
-  set(ABSL_RANDOM_RANDEN_COPTS "${ABSL_RANDOM_HWAES_X64_FLAGS}")
-elseif("${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang")
-  # MATCHES so we get both Clang and AppleClang
+  set(ABSL_TEST_COPTS "${ABSL_GCC_TEST_FLAGS}")
+elseif(CMAKE_CXX_COMPILER_ID MATCHES "Clang")  # MATCHES so we get both Clang and AppleClang
   if(MSVC)
     # clang-cl is half MSVC, half LLVM
     set(ABSL_DEFAULT_COPTS "${ABSL_CLANG_CL_FLAGS}")
-    set(ABSL_TEST_COPTS "${ABSL_CLANG_CL_FLAGS};${ABSL_CLANG_CL_TEST_FLAGS}")
-    set(ABSL_DEFAULT_LINKOPTS "${ABSL_MSVC_LINKOPTS}")
-    set(ABSL_RANDOM_RANDEN_COPTS "${ABSL_RANDOM_HWAES_MSVC_X64_FLAGS}")
+    set(ABSL_TEST_COPTS "${ABSL_CLANG_CL_TEST_FLAGS}")
   else()
     set(ABSL_DEFAULT_COPTS "${ABSL_LLVM_FLAGS}")
-    set(ABSL_TEST_COPTS "${ABSL_LLVM_FLAGS};${ABSL_LLVM_TEST_FLAGS}")
-    set(ABSL_RANDOM_RANDEN_COPTS "${ABSL_RANDOM_HWAES_X64_FLAGS}")
-    if("${CMAKE_CXX_COMPILER_ID}" STREQUAL "Clang")
-      # AppleClang doesn't have lsan
-      # https://developer.apple.com/documentation/code_diagnostics
-      if(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 3.5)
-        set(ABSL_LSAN_LINKOPTS "-fsanitize=leak")
-        set(ABSL_HAVE_LSAN ON)
-      endif()
-    endif()
+    set(ABSL_TEST_COPTS "${ABSL_LLVM_TEST_FLAGS}")
   endif()
-elseif("${CMAKE_CXX_COMPILER_ID}" STREQUAL "MSVC")
+elseif(CMAKE_CXX_COMPILER_ID STREQUAL "IntelLLVM")
+  # IntelLLVM is similar to Clang, with some additional flags.
+  if(MSVC)
+    # clang-cl is half MSVC, half LLVM
+    set(ABSL_DEFAULT_COPTS "${ABSL_CLANG_CL_FLAGS}")
+    set(ABSL_TEST_COPTS "${ABSL_CLANG_CL_TEST_FLAGS}")
+  else()
+    set(ABSL_DEFAULT_COPTS "${ABSL_LLVM_FLAGS}")
+    set(ABSL_TEST_COPTS "${ABSL_LLVM_TEST_FLAGS}")
+  endif()
+elseif(CMAKE_CXX_COMPILER_ID STREQUAL "MSVC")
   set(ABSL_DEFAULT_COPTS "${ABSL_MSVC_FLAGS}")
-  set(ABSL_TEST_COPTS "${ABSL_MSVC_FLAGS};${ABSL_MSVC_TEST_FLAGS}")
+  set(ABSL_TEST_COPTS "${ABSL_MSVC_TEST_FLAGS}")
   set(ABSL_DEFAULT_LINKOPTS "${ABSL_MSVC_LINKOPTS}")
-  set(ABSL_RANDOM_RANDEN_COPTS "${ABSL_RANDOM_HWAES_MSVC_X64_FLAGS}")
 else()
   message(WARNING "Unknown compiler: ${CMAKE_CXX_COMPILER}.  Building with no default flags")
   set(ABSL_DEFAULT_COPTS "")
   set(ABSL_TEST_COPTS "")
-  set(ABSL_RANDOM_RANDEN_COPTS "")
 endif()
 
-if("${CMAKE_CXX_STANDARD}" EQUAL 98)
-  message(FATAL_ERROR "Abseil requires at least C++11")
-elseif(NOT "${CMAKE_CXX_STANDARD}")
-  message(STATUS "No CMAKE_CXX_STANDARD set, assuming 11")
-  set(ABSL_CXX_STANDARD 11)
-else()
-  set(ABSL_CXX_STANDARD "${CMAKE_CXX_STANDARD}")
-endif()
+set(ABSL_CXX_STANDARD "${CMAKE_CXX_STANDARD}")
diff --git a/absl/copts/GENERATED_AbseilCopts.cmake b/absl/copts/GENERATED_AbseilCopts.cmake
index 39b79c53370..b08c34c2799 100644
--- a/absl/copts/GENERATED_AbseilCopts.cmake
+++ b/absl/copts/GENERATED_AbseilCopts.cmake
@@ -5,72 +5,113 @@
 
 list(APPEND ABSL_CLANG_CL_FLAGS
     "/W3"
-    "-Wno-c++98-compat-pedantic"
-    "-Wno-conversion"
-    "-Wno-covered-switch-default"
-    "-Wno-deprecated"
-    "-Wno-disabled-macro-expansion"
-    "-Wno-double-promotion"
-    "-Wno-comma"
-    "-Wno-extra-semi"
-    "-Wno-extra-semi-stmt"
-    "-Wno-packed"
-    "-Wno-padded"
-    "-Wno-sign-compare"
-    "-Wno-float-conversion"
-    "-Wno-float-equal"
-    "-Wno-format-nonliteral"
-    "-Wno-gcc-compat"
-    "-Wno-global-constructors"
-    "-Wno-exit-time-destructors"
-    "-Wno-nested-anon-types"
-    "-Wno-non-modular-include-in-module"
-    "-Wno-old-style-cast"
-    "-Wno-range-loop-analysis"
-    "-Wno-reserved-id-macro"
-    "-Wno-shorten-64-to-32"
-    "-Wno-switch-enum"
-    "-Wno-thread-safety-negative"
-    "-Wno-unknown-warning-option"
-    "-Wno-unreachable-code"
-    "-Wno-unused-macros"
-    "-Wno-weak-vtables"
-    "-Wno-zero-as-null-pointer-constant"
-    "-Wbitfield-enum-conversion"
-    "-Wbool-conversion"
-    "-Wconstant-conversion"
-    "-Wenum-conversion"
-    "-Wint-conversion"
-    "-Wliteral-conversion"
-    "-Wnon-literal-null-conversion"
-    "-Wnull-conversion"
-    "-Wobjc-literal-conversion"
-    "-Wno-sign-conversion"
-    "-Wstring-conversion"
     "/DNOMINMAX"
     "/DWIN32_LEAN_AND_MEAN"
     "/D_CRT_SECURE_NO_WARNINGS"
     "/D_SCL_SECURE_NO_WARNINGS"
     "/D_ENABLE_EXTENDED_ALIGNED_STORAGE"
+    "-Wmost"
+    "-Wextra"
+    "-Wc++98-compat-extra-semi"
+    "-Wcast-qual"
+    "-Wconversion"
+    "-Wdeprecated-pragma"
+    "-Wfloat-overflow-conversion"
+    "-Wfloat-zero-conversion"
+    "-Wfor-loop-analysis"
+    "-Wformat-security"
+    "-Wgnu-redeclared-enum"
+    "-Winfinite-recursion"
+    "-Winvalid-constexpr"
+    "-Wliteral-conversion"
+    "-Wmissing-declarations"
+    "-Wnullability-completeness"
+    "-Woverlength-strings"
+    "-Wpointer-arith"
+    "-Wself-assign"
+    "-Wshadow-all"
+    "-Wshorten-64-to-32"
+    "-Wsign-conversion"
+    "-Wstring-conversion"
+    "-Wtautological-overlap-compare"
+    "-Wtautological-unsigned-zero-compare"
+    "-Wthread-safety"
+    "-Wundef"
+    "-Wuninitialized"
+    "-Wunreachable-code"
+    "-Wunused-comparison"
+    "-Wunused-local-typedefs"
+    "-Wunused-result"
+    "-Wvla"
+    "-Wwrite-strings"
+    "-Wno-float-conversion"
+    "-Wno-implicit-float-conversion"
+    "-Wno-implicit-int-float-conversion"
+    "-Wno-unknown-warning-option"
+    "-Wno-unused-command-line-argument"
+    "-DNOMINMAX"
 )
 
 list(APPEND ABSL_CLANG_CL_TEST_FLAGS
-    "-Wno-c99-extensions"
+    "/W3"
+    "/DNOMINMAX"
+    "/DWIN32_LEAN_AND_MEAN"
+    "/D_CRT_SECURE_NO_WARNINGS"
+    "/D_SCL_SECURE_NO_WARNINGS"
+    "/D_ENABLE_EXTENDED_ALIGNED_STORAGE"
+    "-Wmost"
+    "-Wextra"
+    "-Wc++98-compat-extra-semi"
+    "-Wcast-qual"
+    "-Wconversion"
+    "-Wdeprecated-pragma"
+    "-Wfloat-overflow-conversion"
+    "-Wfloat-zero-conversion"
+    "-Wfor-loop-analysis"
+    "-Wformat-security"
+    "-Wgnu-redeclared-enum"
+    "-Winfinite-recursion"
+    "-Winvalid-constexpr"
+    "-Wliteral-conversion"
+    "-Wmissing-declarations"
+    "-Woverlength-strings"
+    "-Wpointer-arith"
+    "-Wself-assign"
+    "-Wshadow-all"
+    "-Wstring-conversion"
+    "-Wtautological-overlap-compare"
+    "-Wtautological-unsigned-zero-compare"
+    "-Wthread-safety"
+    "-Wundef"
+    "-Wuninitialized"
+    "-Wunreachable-code"
+    "-Wunused-comparison"
+    "-Wunused-local-typedefs"
+    "-Wunused-result"
+    "-Wvla"
+    "-Wwrite-strings"
+    "-Wno-float-conversion"
+    "-Wno-implicit-float-conversion"
+    "-Wno-implicit-int-float-conversion"
+    "-Wno-unknown-warning-option"
+    "-Wno-unused-command-line-argument"
+    "-DNOMINMAX"
     "-Wno-deprecated-declarations"
-    "-Wno-missing-noreturn"
+    "-Wno-implicit-int-conversion"
     "-Wno-missing-prototypes"
     "-Wno-missing-variable-declarations"
-    "-Wno-null-conversion"
+    "-Wno-nullability-completeness"
     "-Wno-shadow"
-    "-Wno-shift-sign-overflow"
+    "-Wno-shorten-64-to-32"
     "-Wno-sign-compare"
+    "-Wno-sign-conversion"
+    "-Wno-unreachable-code-loop-increment"
     "-Wno-unused-function"
     "-Wno-unused-member-function"
     "-Wno-unused-parameter"
     "-Wno-unused-private-field"
     "-Wno-unused-template"
     "-Wno-used-but-marked-unused"
-    "-Wno-zero-as-null-pointer-constant"
     "-Wno-gnu-zero-variadic-macro-arguments"
 )
 
@@ -79,22 +120,39 @@ list(APPEND ABSL_GCC_FLAGS
     "-Wextra"
     "-Wcast-qual"
     "-Wconversion-null"
+    "-Wformat-security"
     "-Wmissing-declarations"
+    "-Wnon-virtual-dtor"
     "-Woverlength-strings"
     "-Wpointer-arith"
+    "-Wundef"
     "-Wunused-local-typedefs"
     "-Wunused-result"
     "-Wvarargs"
     "-Wvla"
     "-Wwrite-strings"
-    "-Wno-missing-field-initializers"
-    "-Wno-sign-compare"
+    "-DNOMINMAX"
 )
 
 list(APPEND ABSL_GCC_TEST_FLAGS
-    "-Wno-conversion-null"
+    "-Wall"
+    "-Wextra"
+    "-Wcast-qual"
+    "-Wconversion-null"
+    "-Wformat-security"
+    "-Wnon-virtual-dtor"
+    "-Woverlength-strings"
+    "-Wpointer-arith"
+    "-Wundef"
+    "-Wunused-local-typedefs"
+    "-Wunused-result"
+    "-Wvarargs"
+    "-Wvla"
+    "-Wwrite-strings"
+    "-DNOMINMAX"
     "-Wno-deprecated-declarations"
     "-Wno-missing-declarations"
+    "-Wno-self-move"
     "-Wno-sign-compare"
     "-Wno-unused-function"
     "-Wno-unused-parameter"
@@ -103,87 +161,119 @@ list(APPEND ABSL_GCC_TEST_FLAGS
 
 list(APPEND ABSL_LLVM_FLAGS
     "-Wall"
+    "-Wmost"
     "-Wextra"
-    "-Weverything"
-    "-Wno-c++98-compat-pedantic"
-    "-Wno-conversion"
-    "-Wno-covered-switch-default"
-    "-Wno-deprecated"
-    "-Wno-disabled-macro-expansion"
-    "-Wno-double-promotion"
-    "-Wno-comma"
-    "-Wno-extra-semi"
-    "-Wno-extra-semi-stmt"
-    "-Wno-packed"
-    "-Wno-padded"
-    "-Wno-sign-compare"
-    "-Wno-float-conversion"
-    "-Wno-float-equal"
-    "-Wno-format-nonliteral"
-    "-Wno-gcc-compat"
-    "-Wno-global-constructors"
-    "-Wno-exit-time-destructors"
-    "-Wno-nested-anon-types"
-    "-Wno-non-modular-include-in-module"
-    "-Wno-old-style-cast"
-    "-Wno-range-loop-analysis"
-    "-Wno-reserved-id-macro"
-    "-Wno-shorten-64-to-32"
-    "-Wno-switch-enum"
-    "-Wno-thread-safety-negative"
-    "-Wno-unknown-warning-option"
-    "-Wno-unreachable-code"
-    "-Wno-unused-macros"
-    "-Wno-weak-vtables"
-    "-Wno-zero-as-null-pointer-constant"
-    "-Wbitfield-enum-conversion"
-    "-Wbool-conversion"
-    "-Wconstant-conversion"
-    "-Wenum-conversion"
-    "-Wint-conversion"
+    "-Wc++98-compat-extra-semi"
+    "-Wcast-qual"
+    "-Wconversion"
+    "-Wdeprecated-pragma"
+    "-Wfloat-overflow-conversion"
+    "-Wfloat-zero-conversion"
+    "-Wfor-loop-analysis"
+    "-Wformat-security"
+    "-Wgnu-redeclared-enum"
+    "-Winfinite-recursion"
+    "-Winvalid-constexpr"
     "-Wliteral-conversion"
-    "-Wnon-literal-null-conversion"
-    "-Wnull-conversion"
-    "-Wobjc-literal-conversion"
-    "-Wno-sign-conversion"
+    "-Wmissing-declarations"
+    "-Wnullability-completeness"
+    "-Woverlength-strings"
+    "-Wpointer-arith"
+    "-Wself-assign"
+    "-Wshadow-all"
+    "-Wshorten-64-to-32"
+    "-Wsign-conversion"
     "-Wstring-conversion"
+    "-Wtautological-overlap-compare"
+    "-Wtautological-unsigned-zero-compare"
+    "-Wthread-safety"
+    "-Wundef"
+    "-Wuninitialized"
+    "-Wunreachable-code"
+    "-Wunused-comparison"
+    "-Wunused-local-typedefs"
+    "-Wunused-result"
+    "-Wvla"
+    "-Wwrite-strings"
+    "-Wno-float-conversion"
+    "-Wno-implicit-float-conversion"
+    "-Wno-implicit-int-float-conversion"
+    "-Wno-unknown-warning-option"
+    "-Wno-unused-command-line-argument"
+    "-DNOMINMAX"
 )
 
 list(APPEND ABSL_LLVM_TEST_FLAGS
-    "-Wno-c99-extensions"
+    "-Wall"
+    "-Wmost"
+    "-Wextra"
+    "-Wc++98-compat-extra-semi"
+    "-Wcast-qual"
+    "-Wconversion"
+    "-Wdeprecated-pragma"
+    "-Wfloat-overflow-conversion"
+    "-Wfloat-zero-conversion"
+    "-Wfor-loop-analysis"
+    "-Wformat-security"
+    "-Wgnu-redeclared-enum"
+    "-Winfinite-recursion"
+    "-Winvalid-constexpr"
+    "-Wliteral-conversion"
+    "-Wmissing-declarations"
+    "-Woverlength-strings"
+    "-Wpointer-arith"
+    "-Wself-assign"
+    "-Wshadow-all"
+    "-Wstring-conversion"
+    "-Wtautological-overlap-compare"
+    "-Wtautological-unsigned-zero-compare"
+    "-Wthread-safety"
+    "-Wundef"
+    "-Wuninitialized"
+    "-Wunreachable-code"
+    "-Wunused-comparison"
+    "-Wunused-local-typedefs"
+    "-Wunused-result"
+    "-Wvla"
+    "-Wwrite-strings"
+    "-Wno-float-conversion"
+    "-Wno-implicit-float-conversion"
+    "-Wno-implicit-int-float-conversion"
+    "-Wno-unknown-warning-option"
+    "-Wno-unused-command-line-argument"
+    "-DNOMINMAX"
     "-Wno-deprecated-declarations"
-    "-Wno-missing-noreturn"
+    "-Wno-implicit-int-conversion"
     "-Wno-missing-prototypes"
     "-Wno-missing-variable-declarations"
-    "-Wno-null-conversion"
+    "-Wno-nullability-completeness"
     "-Wno-shadow"
-    "-Wno-shift-sign-overflow"
+    "-Wno-shorten-64-to-32"
     "-Wno-sign-compare"
+    "-Wno-sign-conversion"
+    "-Wno-unreachable-code-loop-increment"
     "-Wno-unused-function"
     "-Wno-unused-member-function"
     "-Wno-unused-parameter"
     "-Wno-unused-private-field"
     "-Wno-unused-template"
     "-Wno-used-but-marked-unused"
-    "-Wno-zero-as-null-pointer-constant"
     "-Wno-gnu-zero-variadic-macro-arguments"
 )
 
 list(APPEND ABSL_MSVC_FLAGS
     "/W3"
-    "/DNOMINMAX"
-    "/DWIN32_LEAN_AND_MEAN"
-    "/D_CRT_SECURE_NO_WARNINGS"
-    "/D_SCL_SECURE_NO_WARNINGS"
-    "/D_ENABLE_EXTENDED_ALIGNED_STORAGE"
     "/bigobj"
     "/wd4005"
     "/wd4068"
     "/wd4180"
-    "/wd4244"
-    "/wd4267"
     "/wd4503"
     "/wd4800"
+    "/DNOMINMAX"
+    "/DWIN32_LEAN_AND_MEAN"
+    "/D_CRT_SECURE_NO_WARNINGS"
+    "/D_SCL_SECURE_NO_WARNINGS"
+    "/D_ENABLE_EXTENDED_ALIGNED_STORAGE"
 )
 
 list(APPEND ABSL_MSVC_LINKOPTS
@@ -191,25 +281,23 @@ list(APPEND ABSL_MSVC_LINKOPTS
 )
 
 list(APPEND ABSL_MSVC_TEST_FLAGS
+    "/W3"
+    "/bigobj"
+    "/wd4005"
+    "/wd4068"
+    "/wd4180"
+    "/wd4503"
+    "/wd4800"
+    "/DNOMINMAX"
+    "/DWIN32_LEAN_AND_MEAN"
+    "/D_CRT_SECURE_NO_WARNINGS"
+    "/D_SCL_SECURE_NO_WARNINGS"
+    "/D_ENABLE_EXTENDED_ALIGNED_STORAGE"
     "/wd4018"
     "/wd4101"
+    "/wd4244"
+    "/wd4267"
     "/wd4503"
     "/wd4996"
     "/DNOMINMAX"
 )
-
-list(APPEND ABSL_RANDOM_HWAES_ARM32_FLAGS
-    "-mfpu=neon"
-)
-
-list(APPEND ABSL_RANDOM_HWAES_ARM64_FLAGS
-    "-march=armv8-a+crypto"
-)
-
-list(APPEND ABSL_RANDOM_HWAES_MSVC_X64_FLAGS
-)
-
-list(APPEND ABSL_RANDOM_HWAES_X64_FLAGS
-    "-maes"
-    "-msse4.1"
-)
diff --git a/absl/copts/GENERATED_copts.bzl b/absl/copts/GENERATED_copts.bzl
index 7d645cc3374..8c1d7a059aa 100644
--- a/absl/copts/GENERATED_copts.bzl
+++ b/absl/copts/GENERATED_copts.bzl
@@ -6,72 +6,113 @@
 
 ABSL_CLANG_CL_FLAGS = [
     "/W3",
-    "-Wno-c++98-compat-pedantic",
-    "-Wno-conversion",
-    "-Wno-covered-switch-default",
-    "-Wno-deprecated",
-    "-Wno-disabled-macro-expansion",
-    "-Wno-double-promotion",
-    "-Wno-comma",
-    "-Wno-extra-semi",
-    "-Wno-extra-semi-stmt",
-    "-Wno-packed",
-    "-Wno-padded",
-    "-Wno-sign-compare",
-    "-Wno-float-conversion",
-    "-Wno-float-equal",
-    "-Wno-format-nonliteral",
-    "-Wno-gcc-compat",
-    "-Wno-global-constructors",
-    "-Wno-exit-time-destructors",
-    "-Wno-nested-anon-types",
-    "-Wno-non-modular-include-in-module",
-    "-Wno-old-style-cast",
-    "-Wno-range-loop-analysis",
-    "-Wno-reserved-id-macro",
-    "-Wno-shorten-64-to-32",
-    "-Wno-switch-enum",
-    "-Wno-thread-safety-negative",
-    "-Wno-unknown-warning-option",
-    "-Wno-unreachable-code",
-    "-Wno-unused-macros",
-    "-Wno-weak-vtables",
-    "-Wno-zero-as-null-pointer-constant",
-    "-Wbitfield-enum-conversion",
-    "-Wbool-conversion",
-    "-Wconstant-conversion",
-    "-Wenum-conversion",
-    "-Wint-conversion",
-    "-Wliteral-conversion",
-    "-Wnon-literal-null-conversion",
-    "-Wnull-conversion",
-    "-Wobjc-literal-conversion",
-    "-Wno-sign-conversion",
-    "-Wstring-conversion",
     "/DNOMINMAX",
     "/DWIN32_LEAN_AND_MEAN",
     "/D_CRT_SECURE_NO_WARNINGS",
     "/D_SCL_SECURE_NO_WARNINGS",
     "/D_ENABLE_EXTENDED_ALIGNED_STORAGE",
+    "-Wmost",
+    "-Wextra",
+    "-Wc++98-compat-extra-semi",
+    "-Wcast-qual",
+    "-Wconversion",
+    "-Wdeprecated-pragma",
+    "-Wfloat-overflow-conversion",
+    "-Wfloat-zero-conversion",
+    "-Wfor-loop-analysis",
+    "-Wformat-security",
+    "-Wgnu-redeclared-enum",
+    "-Winfinite-recursion",
+    "-Winvalid-constexpr",
+    "-Wliteral-conversion",
+    "-Wmissing-declarations",
+    "-Wnullability-completeness",
+    "-Woverlength-strings",
+    "-Wpointer-arith",
+    "-Wself-assign",
+    "-Wshadow-all",
+    "-Wshorten-64-to-32",
+    "-Wsign-conversion",
+    "-Wstring-conversion",
+    "-Wtautological-overlap-compare",
+    "-Wtautological-unsigned-zero-compare",
+    "-Wthread-safety",
+    "-Wundef",
+    "-Wuninitialized",
+    "-Wunreachable-code",
+    "-Wunused-comparison",
+    "-Wunused-local-typedefs",
+    "-Wunused-result",
+    "-Wvla",
+    "-Wwrite-strings",
+    "-Wno-float-conversion",
+    "-Wno-implicit-float-conversion",
+    "-Wno-implicit-int-float-conversion",
+    "-Wno-unknown-warning-option",
+    "-Wno-unused-command-line-argument",
+    "-DNOMINMAX",
 ]
 
 ABSL_CLANG_CL_TEST_FLAGS = [
-    "-Wno-c99-extensions",
+    "/W3",
+    "/DNOMINMAX",
+    "/DWIN32_LEAN_AND_MEAN",
+    "/D_CRT_SECURE_NO_WARNINGS",
+    "/D_SCL_SECURE_NO_WARNINGS",
+    "/D_ENABLE_EXTENDED_ALIGNED_STORAGE",
+    "-Wmost",
+    "-Wextra",
+    "-Wc++98-compat-extra-semi",
+    "-Wcast-qual",
+    "-Wconversion",
+    "-Wdeprecated-pragma",
+    "-Wfloat-overflow-conversion",
+    "-Wfloat-zero-conversion",
+    "-Wfor-loop-analysis",
+    "-Wformat-security",
+    "-Wgnu-redeclared-enum",
+    "-Winfinite-recursion",
+    "-Winvalid-constexpr",
+    "-Wliteral-conversion",
+    "-Wmissing-declarations",
+    "-Woverlength-strings",
+    "-Wpointer-arith",
+    "-Wself-assign",
+    "-Wshadow-all",
+    "-Wstring-conversion",
+    "-Wtautological-overlap-compare",
+    "-Wtautological-unsigned-zero-compare",
+    "-Wthread-safety",
+    "-Wundef",
+    "-Wuninitialized",
+    "-Wunreachable-code",
+    "-Wunused-comparison",
+    "-Wunused-local-typedefs",
+    "-Wunused-result",
+    "-Wvla",
+    "-Wwrite-strings",
+    "-Wno-float-conversion",
+    "-Wno-implicit-float-conversion",
+    "-Wno-implicit-int-float-conversion",
+    "-Wno-unknown-warning-option",
+    "-Wno-unused-command-line-argument",
+    "-DNOMINMAX",
     "-Wno-deprecated-declarations",
-    "-Wno-missing-noreturn",
+    "-Wno-implicit-int-conversion",
     "-Wno-missing-prototypes",
     "-Wno-missing-variable-declarations",
-    "-Wno-null-conversion",
+    "-Wno-nullability-completeness",
     "-Wno-shadow",
-    "-Wno-shift-sign-overflow",
+    "-Wno-shorten-64-to-32",
     "-Wno-sign-compare",
+    "-Wno-sign-conversion",
+    "-Wno-unreachable-code-loop-increment",
     "-Wno-unused-function",
     "-Wno-unused-member-function",
     "-Wno-unused-parameter",
     "-Wno-unused-private-field",
     "-Wno-unused-template",
     "-Wno-used-but-marked-unused",
-    "-Wno-zero-as-null-pointer-constant",
     "-Wno-gnu-zero-variadic-macro-arguments",
 ]
 
@@ -80,22 +121,39 @@ ABSL_GCC_FLAGS = [
     "-Wextra",
     "-Wcast-qual",
     "-Wconversion-null",
+    "-Wformat-security",
     "-Wmissing-declarations",
+    "-Wnon-virtual-dtor",
     "-Woverlength-strings",
     "-Wpointer-arith",
+    "-Wundef",
     "-Wunused-local-typedefs",
     "-Wunused-result",
     "-Wvarargs",
     "-Wvla",
     "-Wwrite-strings",
-    "-Wno-missing-field-initializers",
-    "-Wno-sign-compare",
+    "-DNOMINMAX",
 ]
 
 ABSL_GCC_TEST_FLAGS = [
-    "-Wno-conversion-null",
+    "-Wall",
+    "-Wextra",
+    "-Wcast-qual",
+    "-Wconversion-null",
+    "-Wformat-security",
+    "-Wnon-virtual-dtor",
+    "-Woverlength-strings",
+    "-Wpointer-arith",
+    "-Wundef",
+    "-Wunused-local-typedefs",
+    "-Wunused-result",
+    "-Wvarargs",
+    "-Wvla",
+    "-Wwrite-strings",
+    "-DNOMINMAX",
     "-Wno-deprecated-declarations",
     "-Wno-missing-declarations",
+    "-Wno-self-move",
     "-Wno-sign-compare",
     "-Wno-unused-function",
     "-Wno-unused-parameter",
@@ -104,87 +162,119 @@ ABSL_GCC_TEST_FLAGS = [
 
 ABSL_LLVM_FLAGS = [
     "-Wall",
+    "-Wmost",
     "-Wextra",
-    "-Weverything",
-    "-Wno-c++98-compat-pedantic",
-    "-Wno-conversion",
-    "-Wno-covered-switch-default",
-    "-Wno-deprecated",
-    "-Wno-disabled-macro-expansion",
-    "-Wno-double-promotion",
-    "-Wno-comma",
-    "-Wno-extra-semi",
-    "-Wno-extra-semi-stmt",
-    "-Wno-packed",
-    "-Wno-padded",
-    "-Wno-sign-compare",
-    "-Wno-float-conversion",
-    "-Wno-float-equal",
-    "-Wno-format-nonliteral",
-    "-Wno-gcc-compat",
-    "-Wno-global-constructors",
-    "-Wno-exit-time-destructors",
-    "-Wno-nested-anon-types",
-    "-Wno-non-modular-include-in-module",
-    "-Wno-old-style-cast",
-    "-Wno-range-loop-analysis",
-    "-Wno-reserved-id-macro",
-    "-Wno-shorten-64-to-32",
-    "-Wno-switch-enum",
-    "-Wno-thread-safety-negative",
-    "-Wno-unknown-warning-option",
-    "-Wno-unreachable-code",
-    "-Wno-unused-macros",
-    "-Wno-weak-vtables",
-    "-Wno-zero-as-null-pointer-constant",
-    "-Wbitfield-enum-conversion",
-    "-Wbool-conversion",
-    "-Wconstant-conversion",
-    "-Wenum-conversion",
-    "-Wint-conversion",
+    "-Wc++98-compat-extra-semi",
+    "-Wcast-qual",
+    "-Wconversion",
+    "-Wdeprecated-pragma",
+    "-Wfloat-overflow-conversion",
+    "-Wfloat-zero-conversion",
+    "-Wfor-loop-analysis",
+    "-Wformat-security",
+    "-Wgnu-redeclared-enum",
+    "-Winfinite-recursion",
+    "-Winvalid-constexpr",
     "-Wliteral-conversion",
-    "-Wnon-literal-null-conversion",
-    "-Wnull-conversion",
-    "-Wobjc-literal-conversion",
-    "-Wno-sign-conversion",
+    "-Wmissing-declarations",
+    "-Wnullability-completeness",
+    "-Woverlength-strings",
+    "-Wpointer-arith",
+    "-Wself-assign",
+    "-Wshadow-all",
+    "-Wshorten-64-to-32",
+    "-Wsign-conversion",
     "-Wstring-conversion",
+    "-Wtautological-overlap-compare",
+    "-Wtautological-unsigned-zero-compare",
+    "-Wthread-safety",
+    "-Wundef",
+    "-Wuninitialized",
+    "-Wunreachable-code",
+    "-Wunused-comparison",
+    "-Wunused-local-typedefs",
+    "-Wunused-result",
+    "-Wvla",
+    "-Wwrite-strings",
+    "-Wno-float-conversion",
+    "-Wno-implicit-float-conversion",
+    "-Wno-implicit-int-float-conversion",
+    "-Wno-unknown-warning-option",
+    "-Wno-unused-command-line-argument",
+    "-DNOMINMAX",
 ]
 
 ABSL_LLVM_TEST_FLAGS = [
-    "-Wno-c99-extensions",
+    "-Wall",
+    "-Wmost",
+    "-Wextra",
+    "-Wc++98-compat-extra-semi",
+    "-Wcast-qual",
+    "-Wconversion",
+    "-Wdeprecated-pragma",
+    "-Wfloat-overflow-conversion",
+    "-Wfloat-zero-conversion",
+    "-Wfor-loop-analysis",
+    "-Wformat-security",
+    "-Wgnu-redeclared-enum",
+    "-Winfinite-recursion",
+    "-Winvalid-constexpr",
+    "-Wliteral-conversion",
+    "-Wmissing-declarations",
+    "-Woverlength-strings",
+    "-Wpointer-arith",
+    "-Wself-assign",
+    "-Wshadow-all",
+    "-Wstring-conversion",
+    "-Wtautological-overlap-compare",
+    "-Wtautological-unsigned-zero-compare",
+    "-Wthread-safety",
+    "-Wundef",
+    "-Wuninitialized",
+    "-Wunreachable-code",
+    "-Wunused-comparison",
+    "-Wunused-local-typedefs",
+    "-Wunused-result",
+    "-Wvla",
+    "-Wwrite-strings",
+    "-Wno-float-conversion",
+    "-Wno-implicit-float-conversion",
+    "-Wno-implicit-int-float-conversion",
+    "-Wno-unknown-warning-option",
+    "-Wno-unused-command-line-argument",
+    "-DNOMINMAX",
     "-Wno-deprecated-declarations",
-    "-Wno-missing-noreturn",
+    "-Wno-implicit-int-conversion",
     "-Wno-missing-prototypes",
     "-Wno-missing-variable-declarations",
-    "-Wno-null-conversion",
+    "-Wno-nullability-completeness",
     "-Wno-shadow",
-    "-Wno-shift-sign-overflow",
+    "-Wno-shorten-64-to-32",
     "-Wno-sign-compare",
+    "-Wno-sign-conversion",
+    "-Wno-unreachable-code-loop-increment",
     "-Wno-unused-function",
     "-Wno-unused-member-function",
     "-Wno-unused-parameter",
     "-Wno-unused-private-field",
     "-Wno-unused-template",
     "-Wno-used-but-marked-unused",
-    "-Wno-zero-as-null-pointer-constant",
     "-Wno-gnu-zero-variadic-macro-arguments",
 ]
 
 ABSL_MSVC_FLAGS = [
     "/W3",
-    "/DNOMINMAX",
-    "/DWIN32_LEAN_AND_MEAN",
-    "/D_CRT_SECURE_NO_WARNINGS",
-    "/D_SCL_SECURE_NO_WARNINGS",
-    "/D_ENABLE_EXTENDED_ALIGNED_STORAGE",
     "/bigobj",
     "/wd4005",
     "/wd4068",
     "/wd4180",
-    "/wd4244",
-    "/wd4267",
     "/wd4503",
     "/wd4800",
+    "/DNOMINMAX",
+    "/DWIN32_LEAN_AND_MEAN",
+    "/D_CRT_SECURE_NO_WARNINGS",
+    "/D_SCL_SECURE_NO_WARNINGS",
+    "/D_ENABLE_EXTENDED_ALIGNED_STORAGE",
 ]
 
 ABSL_MSVC_LINKOPTS = [
@@ -192,25 +282,23 @@ ABSL_MSVC_LINKOPTS = [
 ]
 
 ABSL_MSVC_TEST_FLAGS = [
+    "/W3",
+    "/bigobj",
+    "/wd4005",
+    "/wd4068",
+    "/wd4180",
+    "/wd4503",
+    "/wd4800",
+    "/DNOMINMAX",
+    "/DWIN32_LEAN_AND_MEAN",
+    "/D_CRT_SECURE_NO_WARNINGS",
+    "/D_SCL_SECURE_NO_WARNINGS",
+    "/D_ENABLE_EXTENDED_ALIGNED_STORAGE",
     "/wd4018",
     "/wd4101",
+    "/wd4244",
+    "/wd4267",
     "/wd4503",
     "/wd4996",
     "/DNOMINMAX",
 ]
-
-ABSL_RANDOM_HWAES_ARM32_FLAGS = [
-    "-mfpu=neon",
-]
-
-ABSL_RANDOM_HWAES_ARM64_FLAGS = [
-    "-march=armv8-a+crypto",
-]
-
-ABSL_RANDOM_HWAES_MSVC_X64_FLAGS = [
-]
-
-ABSL_RANDOM_HWAES_X64_FLAGS = [
-    "-maes",
-    "-msse4.1",
-]
diff --git a/absl/copts/configure_copts.bzl b/absl/copts/configure_copts.bzl
index 2829e4ec6e6..9e0be28f79d 100644
--- a/absl/copts/configure_copts.bzl
+++ b/absl/copts/configure_copts.bzl
@@ -6,6 +6,8 @@ change Abseil copts, edit absl/copts/copts.py
 
 load(
     "//absl:copts/GENERATED_copts.bzl",
+    "ABSL_CLANG_CL_FLAGS",
+    "ABSL_CLANG_CL_TEST_FLAGS",
     "ABSL_GCC_FLAGS",
     "ABSL_GCC_TEST_FLAGS",
     "ABSL_LLVM_FLAGS",
@@ -13,64 +15,25 @@ load(
     "ABSL_MSVC_FLAGS",
     "ABSL_MSVC_LINKOPTS",
     "ABSL_MSVC_TEST_FLAGS",
-    "ABSL_RANDOM_HWAES_ARM32_FLAGS",
-    "ABSL_RANDOM_HWAES_ARM64_FLAGS",
-    "ABSL_RANDOM_HWAES_MSVC_X64_FLAGS",
-    "ABSL_RANDOM_HWAES_X64_FLAGS",
 )
 
 ABSL_DEFAULT_COPTS = select({
-    "//absl:windows": ABSL_MSVC_FLAGS,
-    "//absl:llvm_compiler": ABSL_LLVM_FLAGS,
-    "//conditions:default": ABSL_GCC_FLAGS,
-})
-
-# in absence of modules (--compiler=gcc or -c opt), cc_tests leak their copts
-# to their (included header) dependencies and fail to build outside absl
-ABSL_TEST_COPTS = ABSL_DEFAULT_COPTS + select({
-    "//absl:windows": ABSL_MSVC_TEST_FLAGS,
-    "//absl:llvm_compiler": ABSL_LLVM_TEST_FLAGS,
-    "//conditions:default": ABSL_GCC_TEST_FLAGS,
+    "@rules_cc//cc/compiler:msvc-cl": ABSL_MSVC_FLAGS,
+    "@rules_cc//cc/compiler:clang-cl": ABSL_CLANG_CL_FLAGS,
+    "@rules_cc//cc/compiler:clang": ABSL_LLVM_FLAGS,
+    "@rules_cc//cc/compiler:gcc": ABSL_GCC_FLAGS,
+    "//conditions:default": [],
 })
 
-ABSL_DEFAULT_LINKOPTS = select({
-    "//absl:windows": ABSL_MSVC_LINKOPTS,
+ABSL_TEST_COPTS = select({
+    "@rules_cc//cc/compiler:msvc-cl": ABSL_MSVC_TEST_FLAGS,
+    "@rules_cc//cc/compiler:clang-cl": ABSL_CLANG_CL_TEST_FLAGS,
+    "@rules_cc//cc/compiler:clang": ABSL_LLVM_TEST_FLAGS,
+    "@rules_cc//cc/compiler:gcc": ABSL_GCC_TEST_FLAGS,
     "//conditions:default": [],
 })
 
-# ABSL_RANDOM_RANDEN_COPTS blaze copts flags which are required by each
-# environment to build an accelerated RandenHwAes library.
-ABSL_RANDOM_RANDEN_COPTS = select({
-    # APPLE
-    ":cpu_darwin_x86_64": ABSL_RANDOM_HWAES_X64_FLAGS,
-    ":cpu_darwin": ABSL_RANDOM_HWAES_X64_FLAGS,
-    ":cpu_x64_windows_msvc": ABSL_RANDOM_HWAES_MSVC_X64_FLAGS,
-    ":cpu_x64_windows": ABSL_RANDOM_HWAES_MSVC_X64_FLAGS,
-    ":cpu_haswell": ABSL_RANDOM_HWAES_X64_FLAGS,
-    ":cpu_ppc": ["-mcrypto"],
-
-    # Supported by default or unsupported.
+ABSL_DEFAULT_LINKOPTS = select({
+    "@rules_cc//cc/compiler:msvc-cl": ABSL_MSVC_LINKOPTS,
     "//conditions:default": [],
 })
-
-# absl_random_randen_copts_init:
-#  Initialize the config targets based on cpu, os, etc. used to select
-#  the required values for ABSL_RANDOM_RANDEN_COPTS
-def absl_random_randen_copts_init():
-    """Initialize the config_settings used by ABSL_RANDOM_RANDEN_COPTS."""
-
-    # CPU configs.
-    # These configs have consistent flags to enable HWAES intsructions.
-    cpu_configs = [
-        "ppc",
-        "haswell",
-        "darwin_x86_64",
-        "darwin",
-        "x64_windows_msvc",
-        "x64_windows",
-    ]
-    for cpu in cpu_configs:
-        native.config_setting(
-            name = "cpu_%s" % cpu,
-            values = {"cpu": cpu},
-        )
diff --git a/absl/copts/copts.py b/absl/copts/copts.py
index a542541505b..c1d1f4a7e3d 100644
--- a/absl/copts/copts.py
+++ b/absl/copts/copts.py
@@ -11,104 +11,130 @@
 AbseilConfigureCopts.cmake.
 """
 
-# /Wall with msvc includes unhelpful warnings such as C4711, C4710, ...
-MSVC_BIG_WARNING_FLAGS = [
-    "/W3",
-]
-
-LLVM_BIG_WARNING_FLAGS = [
+ABSL_GCC_FLAGS = [
     "-Wall",
     "-Wextra",
-    "-Weverything",
+    "-Wcast-qual",
+    "-Wconversion-null",
+    "-Wformat-security",
+    "-Wmissing-declarations",
+    "-Wnon-virtual-dtor",
+    "-Woverlength-strings",
+    "-Wpointer-arith",
+    "-Wundef",
+    "-Wunused-local-typedefs",
+    "-Wunused-result",
+    "-Wvarargs",
+    "-Wvla",  # variable-length array
+    "-Wwrite-strings",
+    # Don't define min and max macros (Build on Windows using gcc)
+    "-DNOMINMAX",
 ]
 
-# Docs on single flags is preceded by a comment.
-# Docs on groups of flags is preceded by ###.
-LLVM_DISABLE_WARNINGS_FLAGS = [
-    # Abseil does not support C++98
-    "-Wno-c++98-compat-pedantic",
-    # Turns off all implicit conversion warnings. Most are re-enabled below.
-    "-Wno-conversion",
-    "-Wno-covered-switch-default",
-    "-Wno-deprecated",
-    "-Wno-disabled-macro-expansion",
-    "-Wno-double-promotion",
-    ###
-    # Turned off as they include valid C++ code.
-    "-Wno-comma",
-    "-Wno-extra-semi",
-    "-Wno-extra-semi-stmt",
-    "-Wno-packed",
-    "-Wno-padded",
-    ###
-    # Google style does not use unsigned integers, though STL containers
-    # have unsigned types.
+ABSL_GCC_TEST_ADDITIONAL_FLAGS = [
+    "-Wno-deprecated-declarations",
+    "-Wno-missing-declarations",
+    "-Wno-self-move",
     "-Wno-sign-compare",
-    ###
-    "-Wno-float-conversion",
-    "-Wno-float-equal",
-    "-Wno-format-nonliteral",
-    # Too aggressive: warns on Clang extensions enclosed in Clang-only
-    # compilation paths.
-    "-Wno-gcc-compat",
-    ###
-    # Some internal globals are necessary. Don't do this at home.
-    "-Wno-global-constructors",
-    "-Wno-exit-time-destructors",
-    ###
-    "-Wno-nested-anon-types",
-    "-Wno-non-modular-include-in-module",
-    "-Wno-old-style-cast",
-    # Warns on preferred usage of non-POD types such as string_view
-    "-Wno-range-loop-analysis",
-    "-Wno-reserved-id-macro",
-    "-Wno-shorten-64-to-32",
-    "-Wno-switch-enum",
-    "-Wno-thread-safety-negative",
-    "-Wno-unknown-warning-option",
-    "-Wno-unreachable-code",
-    # Causes warnings on include guards
-    "-Wno-unused-macros",
-    "-Wno-weak-vtables",
-    # Causes warnings on usage of types/compare.h comparison operators.
-    "-Wno-zero-as-null-pointer-constant",
-    ###
-    # Implicit conversion warnings turned off by -Wno-conversion
-    # which are re-enabled below.
-    "-Wbitfield-enum-conversion",
-    "-Wbool-conversion",
-    "-Wconstant-conversion",
-    "-Wenum-conversion",
-    "-Wint-conversion",
+    "-Wno-unused-function",
+    "-Wno-unused-parameter",
+    "-Wno-unused-private-field",
+]
+
+# https://github.com/llvm/llvm-project/issues/102982
+# A list of LLVM base flags without -Wall. This is because clang-cl
+# translates -Wall to -Weverything on Windows, mimicking MSVCs
+# behavior. On most other platforms, -Wall is just a set of very good
+# default flags.
+ABSL_LLVM_BASE_FLAGS = [
+    "-Wmost",
+    "-Wextra",
+    "-Wc++98-compat-extra-semi",
+    "-Wcast-qual",
+    "-Wconversion",
+    "-Wdeprecated-pragma",
+    "-Wfloat-overflow-conversion",
+    "-Wfloat-zero-conversion",
+    "-Wfor-loop-analysis",
+    "-Wformat-security",
+    "-Wgnu-redeclared-enum",
+    "-Winfinite-recursion",
+    "-Winvalid-constexpr",
     "-Wliteral-conversion",
-    "-Wnon-literal-null-conversion",
-    "-Wnull-conversion",
-    "-Wobjc-literal-conversion",
-    "-Wno-sign-conversion",
+    "-Wmissing-declarations",
+    "-Wnullability-completeness",
+    "-Woverlength-strings",
+    "-Wpointer-arith",
+    "-Wself-assign",
+    "-Wshadow-all",
+    "-Wshorten-64-to-32",
+    "-Wsign-conversion",
     "-Wstring-conversion",
+    "-Wtautological-overlap-compare",
+    "-Wtautological-unsigned-zero-compare",
+    "-Wthread-safety",
+    "-Wundef",
+    "-Wuninitialized",
+    "-Wunreachable-code",
+    "-Wunused-comparison",
+    "-Wunused-local-typedefs",
+    "-Wunused-result",
+    "-Wvla",
+    "-Wwrite-strings",
+    # Warnings that are enabled by group warning flags like -Wall that we
+    # explicitly disable.
+    "-Wno-float-conversion",
+    "-Wno-implicit-float-conversion",
+    "-Wno-implicit-int-float-conversion",
+    # Disable warnings on unknown warning flags (when warning flags are
+    # unknown on older compiler versions)
+    "-Wno-unknown-warning-option",
+    "-Wno-unused-command-line-argument",
+    # Don't define min and max macros (Build on Windows using clang)
+    "-DNOMINMAX",
 ]
 
-LLVM_TEST_DISABLE_WARNINGS_FLAGS = [
-    "-Wno-c99-extensions",
+ABSL_LLVM_FLAGS = ["-Wall"] + ABSL_LLVM_BASE_FLAGS
+
+ABSL_LLVM_TEST_ADDITIONAL_FLAGS = [
     "-Wno-deprecated-declarations",
-    "-Wno-missing-noreturn",
+    "-Wno-implicit-int-conversion",
     "-Wno-missing-prototypes",
     "-Wno-missing-variable-declarations",
-    "-Wno-null-conversion",
+    "-Wno-nullability-completeness",
     "-Wno-shadow",
-    "-Wno-shift-sign-overflow",
+    "-Wno-shorten-64-to-32",
     "-Wno-sign-compare",
+    "-Wno-sign-conversion",
+    "-Wno-unreachable-code-loop-increment",
     "-Wno-unused-function",
     "-Wno-unused-member-function",
     "-Wno-unused-parameter",
     "-Wno-unused-private-field",
     "-Wno-unused-template",
     "-Wno-used-but-marked-unused",
-    "-Wno-zero-as-null-pointer-constant",
     # gtest depends on this GNU extension being offered.
     "-Wno-gnu-zero-variadic-macro-arguments",
 ]
 
+# /Wall with msvc includes unhelpful warnings such as C4711, C4710, ...
+MSVC_BIG_WARNING_FLAGS = [
+    "/W3",
+]
+
+MSVC_WARNING_FLAGS = [
+    # Increase the number of sections available in object files
+    "/bigobj",
+    "/wd4005",  # macro-redefinition
+    "/wd4068",  # unknown pragma
+    # qualifier applied to function type has no meaning; ignored
+    "/wd4180",
+    # The decorated name was longer than the compiler limit
+    "/wd4503",
+    # forcing value to bool 'true' or 'false' (performance warning)
+    "/wd4800",
+]
+
 MSVC_DEFINES = [
     "/DNOMINMAX",  # Don't define min and max macros (windows.h)
     # Don't bloat namespace with incompatible winsock versions.
@@ -120,82 +146,60 @@
     "/D_ENABLE_EXTENDED_ALIGNED_STORAGE",
 ]
 
+
+def GccStyleFilterAndCombine(default_flags, test_flags):
+  """Merges default_flags and test_flags for GCC and LLVM.
+
+  Args:
+    default_flags: A list of default compiler flags
+    test_flags: A list of flags that are only used in tests
+
+  Returns:
+    A combined list of default_flags and test_flags, but with all flags of the
+    form '-Wwarning' removed if test_flags contains a flag of the form
+    '-Wno-warning'
+  """
+  remove = set(["-W" + f[5:] for f in test_flags if f[:5] == "-Wno-"])
+  return [f for f in default_flags if f not in remove] + test_flags
+
 COPT_VARS = {
-    "ABSL_GCC_FLAGS": [
-        "-Wall",
-        "-Wextra",
-        "-Wcast-qual",
-        "-Wconversion-null",
-        "-Wmissing-declarations",
-        "-Woverlength-strings",
-        "-Wpointer-arith",
-        "-Wunused-local-typedefs",
-        "-Wunused-result",
-        "-Wvarargs",
-        "-Wvla",  # variable-length array
-        "-Wwrite-strings",
-        # gcc-4.x has spurious missing field initializer warnings.
-        # https://gcc.gnu.org/bugzilla/show_bug.cgi?id=36750
-        # Remove when gcc-4.x is no longer supported.
-        "-Wno-missing-field-initializers",
-        # Google style does not use unsigned integers, though STL containers
-        # have unsigned types.
-        "-Wno-sign-compare",
-    ],
-    "ABSL_GCC_TEST_FLAGS": [
-        "-Wno-conversion-null",
-        "-Wno-deprecated-declarations",
-        "-Wno-missing-declarations",
-        "-Wno-sign-compare",
-        "-Wno-unused-function",
-        "-Wno-unused-parameter",
-        "-Wno-unused-private-field",
-    ],
-    "ABSL_LLVM_FLAGS":
-        LLVM_BIG_WARNING_FLAGS + LLVM_DISABLE_WARNINGS_FLAGS,
-    "ABSL_LLVM_TEST_FLAGS":
-        LLVM_TEST_DISABLE_WARNINGS_FLAGS,
-    "ABSL_CLANG_CL_FLAGS":
-        (MSVC_BIG_WARNING_FLAGS + LLVM_DISABLE_WARNINGS_FLAGS + MSVC_DEFINES),
-    "ABSL_CLANG_CL_TEST_FLAGS":
-        LLVM_TEST_DISABLE_WARNINGS_FLAGS,
-    "ABSL_MSVC_FLAGS":
-        MSVC_BIG_WARNING_FLAGS + MSVC_DEFINES + [
-            # Increase the number of sections available in object files
-            "/bigobj",
-            "/wd4005",  # macro-redefinition
-            "/wd4068",  # unknown pragma
-            # qualifier applied to function type has no meaning; ignored
-            "/wd4180",
-            # conversion from 'type1' to 'type2', possible loss of data
-            "/wd4244",
-            # conversion from 'size_t' to 'type', possible loss of data
-            "/wd4267",
-            # The decorated name was longer than the compiler limit
-            "/wd4503",
-            # forcing value to bool 'true' or 'false' (performance warning)
-            "/wd4800",
-        ],
-    "ABSL_MSVC_TEST_FLAGS": [
-        "/wd4018",  # signed/unsigned mismatch
-        "/wd4101",  # unreferenced local variable
-        "/wd4503",  # decorated name length exceeded, name was truncated
-        "/wd4996",  # use of deprecated symbol
-        "/DNOMINMAX",  # disable the min() and max() macros from <windows.h>
-    ],
+    "ABSL_GCC_FLAGS": ABSL_GCC_FLAGS,
+    "ABSL_GCC_TEST_FLAGS": GccStyleFilterAndCombine(
+        ABSL_GCC_FLAGS, ABSL_GCC_TEST_ADDITIONAL_FLAGS
+    ),
+    "ABSL_LLVM_FLAGS": ABSL_LLVM_FLAGS,
+    "ABSL_LLVM_TEST_FLAGS": GccStyleFilterAndCombine(
+        ABSL_LLVM_FLAGS, ABSL_LLVM_TEST_ADDITIONAL_FLAGS
+    ),
+    "ABSL_CLANG_CL_FLAGS": (
+        MSVC_BIG_WARNING_FLAGS + MSVC_DEFINES + ABSL_LLVM_BASE_FLAGS
+    ),
+    "ABSL_CLANG_CL_TEST_FLAGS": (
+        MSVC_BIG_WARNING_FLAGS
+        + MSVC_DEFINES
+        + GccStyleFilterAndCombine(
+            ABSL_LLVM_BASE_FLAGS, ABSL_LLVM_TEST_ADDITIONAL_FLAGS
+        )
+    ),
+    "ABSL_MSVC_FLAGS": (
+        MSVC_BIG_WARNING_FLAGS + MSVC_WARNING_FLAGS + MSVC_DEFINES
+    ),
+    "ABSL_MSVC_TEST_FLAGS": (
+        MSVC_BIG_WARNING_FLAGS
+        + MSVC_WARNING_FLAGS
+        + MSVC_DEFINES
+        + [
+            "/wd4018",  # signed/unsigned mismatch
+            "/wd4101",  # unreferenced local variable
+            "/wd4244",  # shortening conversion
+            "/wd4267",  # shortening conversion
+            "/wd4503",  # decorated name length exceeded, name was truncated
+            "/wd4996",  # use of deprecated symbol
+            "/DNOMINMAX",  # disable the min() and max() macros from <windows.h>
+        ]
+    ),
     "ABSL_MSVC_LINKOPTS": [
         # Object file doesn't export any previously undefined symbols
         "-ignore:4221",
     ],
-    # "HWAES" is an abbreviation for "hardware AES" (AES - Advanced Encryption
-    # Standard). These flags are used for detecting whether or not the target
-    # architecture has hardware support for AES instructions which can be used
-    # to improve performance of some random bit generators.
-    "ABSL_RANDOM_HWAES_ARM64_FLAGS": ["-march=armv8-a+crypto"],
-    "ABSL_RANDOM_HWAES_ARM32_FLAGS": ["-mfpu=neon"],
-    "ABSL_RANDOM_HWAES_X64_FLAGS": [
-        "-maes",
-        "-msse4.1",
-    ],
-    "ABSL_RANDOM_HWAES_MSVC_X64_FLAGS": [],
 }
diff --git a/absl/copts/generate_copts.py b/absl/copts/generate_copts.py
index 0e5dc9fad29..34be2fc23ba 100755
--- a/absl/copts/generate_copts.py
+++ b/absl/copts/generate_copts.py
@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python3
 """Generate Abseil compile compile option configs.
 
 Usage: <path_to_absl>/copts/generate_copts.py
diff --git a/absl/crc/BUILD.bazel b/absl/crc/BUILD.bazel
new file mode 100644
index 00000000000..88b9ea26680
--- /dev/null
+++ b/absl/crc/BUILD.bazel
@@ -0,0 +1,212 @@
+# Copyright 2022 The Abseil Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+load("@rules_cc//cc:cc_binary.bzl", "cc_binary")
+load("@rules_cc//cc:cc_library.bzl", "cc_library")
+load("@rules_cc//cc:cc_test.bzl", "cc_test")
+load(
+    "//absl:copts/configure_copts.bzl",
+    "ABSL_DEFAULT_COPTS",
+    "ABSL_DEFAULT_LINKOPTS",
+    "ABSL_TEST_COPTS",
+)
+
+package(
+    default_visibility = ["//visibility:private"],
+    features = [
+        "header_modules",
+        "layering_check",
+        "parse_headers",
+    ],
+)
+
+licenses(["notice"])
+
+cc_library(
+    name = "crc_internal",
+    srcs = [
+        "internal/crc.cc",
+        "internal/crc_internal.h",
+        "internal/crc_x86_arm_combined.cc",
+    ],
+    hdrs = [
+        "internal/crc.h",
+        "internal/crc32_x86_arm_combined_simd.h",
+    ],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:private"],
+    deps = [
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:cpu_detect",
+        "//absl/base:endian",
+        "//absl/base:prefetch",
+        "//absl/base:raw_logging_internal",
+        "//absl/memory",
+        "//absl/numeric:bits",
+    ],
+)
+
+cc_library(
+    name = "crc32c",
+    srcs = [
+        "crc32c.cc",
+        "internal/crc32c_inline.h",
+        "internal/crc_memcpy_fallback.cc",
+        "internal/crc_memcpy_x86_arm_combined.cc",
+        "internal/crc_non_temporal_memcpy.cc",
+    ],
+    hdrs = [
+        "crc32c.h",
+        "internal/crc32c.h",
+        "internal/crc_memcpy.h",
+    ],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
+    deps = [
+        ":crc_internal",
+        ":non_temporal_memcpy",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:cpu_detect",
+        "//absl/base:endian",
+        "//absl/base:prefetch",
+        "//absl/strings",
+        "//absl/strings:str_format",
+    ],
+)
+
+cc_test(
+    name = "crc32c_test",
+    srcs = ["crc32c_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:private"],
+    deps = [
+        ":crc32c",
+        "//absl/strings",
+        "//absl/strings:str_format",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_library(
+    name = "non_temporal_arm_intrinsics",
+    hdrs = ["internal/non_temporal_arm_intrinsics.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        ":__pkg__",
+    ],
+    deps = [
+        "//absl/base:config",
+    ],
+)
+
+cc_library(
+    name = "non_temporal_memcpy",
+    hdrs = ["internal/non_temporal_memcpy.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        ":__pkg__",
+    ],
+    deps = [
+        ":non_temporal_arm_intrinsics",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+    ],
+)
+
+cc_test(
+    name = "crc_memcpy_test",
+    size = "large",
+    srcs = ["internal/crc_memcpy_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    shard_count = 3,
+    visibility = ["//visibility:private"],
+    deps = [
+        ":crc32c",
+        "//absl/memory",
+        "//absl/random",
+        "//absl/random:distributions",
+        "//absl/strings",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "non_temporal_memcpy_test",
+    srcs = ["internal/non_temporal_memcpy_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:private"],
+    deps = [
+        ":non_temporal_memcpy",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_library(
+    name = "crc_cord_state",
+    srcs = ["internal/crc_cord_state.cc"],
+    hdrs = ["internal/crc_cord_state.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//absl/strings:__pkg__"],
+    deps = [
+        ":crc32c",
+        "//absl/base:config",
+        "//absl/base:no_destructor",
+        "//absl/numeric:bits",
+    ],
+)
+
+cc_test(
+    name = "crc_cord_state_test",
+    srcs = ["internal/crc_cord_state_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:private"],
+    deps = [
+        ":crc32c",
+        ":crc_cord_state",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_binary(
+    name = "crc32c_benchmark",
+    testonly = True,
+    srcs = ["crc32c_benchmark.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    tags = [
+        "benchmark",
+    ],
+    visibility = ["//visibility:private"],
+    deps = [
+        ":crc32c",
+        "//absl/memory",
+        "//absl/strings",
+        "@google_benchmark//:benchmark_main",
+    ],
+)
diff --git a/absl/crc/CMakeLists.txt b/absl/crc/CMakeLists.txt
new file mode 100644
index 00000000000..28f0bab3c6f
--- /dev/null
+++ b/absl/crc/CMakeLists.txt
@@ -0,0 +1,161 @@
+# Copyright 2022 The Abseil Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# Internal-only target, do not depend on directly.
+absl_cc_library(
+  NAME
+    crc_internal
+  HDRS
+    "internal/crc.h"
+    "internal/crc32_x86_arm_combined_simd.h"
+  SRCS
+    "internal/crc.cc"
+    "internal/crc_internal.h"
+    "internal/crc_x86_arm_combined.cc"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::base_cpu_detect
+    absl::bits
+    absl::config
+    absl::core_headers
+    absl::endian
+    absl::memory
+    absl::prefetch
+    absl::raw_logging_internal
+)
+
+absl_cc_library(
+  NAME
+    crc32c
+  HDRS
+    "crc32c.h"
+    "internal/crc32c.h"
+    "internal/crc_memcpy.h"
+  SRCS
+    "crc32c.cc"
+    "internal/crc32c_inline.h"
+    "internal/crc_memcpy_fallback.cc"
+    "internal/crc_memcpy_x86_arm_combined.cc"
+    "internal/crc_non_temporal_memcpy.cc"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::base_cpu_detect
+    absl::crc_internal
+    absl::non_temporal_memcpy
+    absl::config
+    absl::core_headers
+    absl::endian
+    absl::prefetch
+    absl::str_format
+    absl::strings
+)
+
+absl_cc_test(
+  NAME
+    crc32c_test
+  SRCS
+    "crc32c_test.cc"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::crc32c
+    absl::strings
+    absl::str_format
+    GTest::gtest_main
+)
+
+# Internal-only target, do not depend on directly.
+absl_cc_library(
+  NAME
+    non_temporal_arm_intrinsics
+  HDRS
+    "internal/non_temporal_arm_intrinsics.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::config
+)
+
+# Internal-only target, do not depend on directly.
+absl_cc_library(
+  NAME
+    non_temporal_memcpy
+  HDRS
+    "internal/non_temporal_memcpy.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::non_temporal_arm_intrinsics
+    absl::config
+    absl::core_headers
+)
+
+absl_cc_test(
+  NAME
+    crc_memcpy_test
+  SRCS
+    "internal/crc_memcpy_test.cc"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::crc32c
+    absl::memory
+    absl::random_random
+    absl::random_distributions
+    absl::strings
+    GTest::gtest_main
+)
+
+absl_cc_test(
+  NAME
+    non_temporal_memcpy_test
+  SRCS
+    "internal/non_temporal_memcpy_test.cc"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::non_temporal_memcpy
+    GTest::gtest_main
+)
+
+absl_cc_library(
+  NAME
+    crc_cord_state
+  HDRS
+    "internal/crc_cord_state.h"
+  SRCS
+    "internal/crc_cord_state.cc"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::crc32c
+    absl::config
+    absl::strings
+    absl::no_destructor
+)
+
+absl_cc_test(
+  NAME
+    crc_cord_state_test
+  SRCS
+    "internal/crc_cord_state_test.cc"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::crc_cord_state
+    absl::crc32c
+    GTest::gtest_main
+)
diff --git a/absl/crc/crc32c.cc b/absl/crc/crc32c.cc
new file mode 100644
index 00000000000..902056373c4
--- /dev/null
+++ b/absl/crc/crc32c.cc
@@ -0,0 +1,96 @@
+// Copyright 2022 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/crc/crc32c.h"
+
+#include <cstddef>
+#include <cstdint>
+
+#include "absl/crc/internal/crc.h"
+#include "absl/crc/internal/crc32c.h"
+#include "absl/crc/internal/crc_memcpy.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+namespace {
+
+const crc_internal::CRC* CrcEngine() {
+  static const crc_internal::CRC* engine = crc_internal::CRC::Crc32c();
+  return engine;
+}
+
+constexpr uint32_t kCRC32Xor = 0xffffffffU;
+
+}  // namespace
+
+namespace crc_internal {
+
+crc32c_t UnextendCrc32cByZeroes(crc32c_t initial_crc, size_t length) {
+  uint32_t crc = static_cast<uint32_t>(initial_crc) ^ kCRC32Xor;
+  CrcEngine()->UnextendByZeroes(&crc, length);
+  return static_cast<crc32c_t>(crc ^ kCRC32Xor);
+}
+
+// Called by `absl::ExtendCrc32c()` on strings with size > 64 or when hardware
+// CRC32C support is missing.
+crc32c_t ExtendCrc32cInternal(crc32c_t initial_crc,
+                              absl::string_view buf_to_add) {
+  uint32_t crc = static_cast<uint32_t>(initial_crc) ^ kCRC32Xor;
+  CrcEngine()->Extend(&crc, buf_to_add.data(), buf_to_add.size());
+  return static_cast<crc32c_t>(crc ^ kCRC32Xor);
+}
+
+}  // namespace crc_internal
+
+crc32c_t ExtendCrc32cByZeroes(crc32c_t initial_crc, size_t length) {
+  uint32_t crc = static_cast<uint32_t>(initial_crc) ^ kCRC32Xor;
+  CrcEngine()->ExtendByZeroes(&crc, length);
+  return static_cast<crc32c_t>(crc ^ kCRC32Xor);
+}
+
+crc32c_t ConcatCrc32c(crc32c_t lhs_crc, crc32c_t rhs_crc, size_t rhs_len) {
+  uint32_t result = static_cast<uint32_t>(lhs_crc);
+  CrcEngine()->ExtendByZeroes(&result, rhs_len);
+  return crc32c_t{result ^ static_cast<uint32_t>(rhs_crc)};
+}
+
+crc32c_t RemoveCrc32cPrefix(crc32c_t crc_a, crc32c_t crc_ab, size_t length_b) {
+  return ConcatCrc32c(crc_a, crc_ab, length_b);
+}
+
+crc32c_t MemcpyCrc32c(void* dest, const void* src, size_t count,
+                      crc32c_t initial_crc) {
+  return static_cast<crc32c_t>(
+      crc_internal::Crc32CAndCopy(dest, src, count, initial_crc, false));
+}
+
+// Remove a Suffix of given size from a buffer
+//
+// Given a CRC32C of an existing buffer, `full_string_crc`; the CRC32C of a
+// suffix of that buffer to remove, `suffix_crc`; and suffix buffer's length,
+// `suffix_len` return the CRC32C of the buffer with suffix removed
+//
+// This operation has a runtime cost of O(log(`suffix_len`))
+crc32c_t RemoveCrc32cSuffix(crc32c_t full_string_crc, crc32c_t suffix_crc,
+                            size_t suffix_len) {
+  uint32_t result = static_cast<uint32_t>(full_string_crc) ^
+                    static_cast<uint32_t>(suffix_crc);
+  CrcEngine()->UnextendByZeroes(&result, suffix_len);
+  return crc32c_t{result};
+}
+
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/crc/crc32c.h b/absl/crc/crc32c.h
new file mode 100644
index 00000000000..4ce941dc440
--- /dev/null
+++ b/absl/crc/crc32c.h
@@ -0,0 +1,193 @@
+// Copyright 2022 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: crc32c.h
+// -----------------------------------------------------------------------------
+//
+// This header file defines the API for computing CRC32C values as checksums
+// for arbitrary sequences of bytes provided as a string buffer.
+//
+// The API includes the basic functions for computing such CRC32C values and
+// some utility functions for performing more efficient mathematical
+// computations using an existing checksum.
+#ifndef ABSL_CRC_CRC32C_H_
+#define ABSL_CRC_CRC32C_H_
+
+#include <cstddef>
+#include <cstdint>
+#include <ostream>
+
+#include "absl/crc/internal/crc32c_inline.h"
+#include "absl/strings/str_format.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+//-----------------------------------------------------------------------------
+// crc32c_t
+//-----------------------------------------------------------------------------
+
+// `crc32c_t` defines a strongly-typed integer for holding a CRC32C value.
+//
+// Some operators are intentionally omitted. Only equality operators are defined
+// so that `crc32c_t` can be directly compared. Methods for putting `crc32c_t`
+// directly into a set are omitted because this is bug-prone due to checksum
+// collisions. Use an explicit conversion to the `uint32_t` space for operations
+// that treat `crc32c_t` as an integer.
+class crc32c_t final {
+ public:
+  crc32c_t() = default;
+  constexpr explicit crc32c_t(uint32_t crc) : crc_(crc) {}
+
+  crc32c_t(const crc32c_t&) = default;
+  crc32c_t& operator=(const crc32c_t&) = default;
+
+  explicit operator uint32_t() const { return crc_; }
+
+  friend bool operator==(crc32c_t lhs, crc32c_t rhs) {
+    return static_cast<uint32_t>(lhs) == static_cast<uint32_t>(rhs);
+  }
+
+  friend bool operator!=(crc32c_t lhs, crc32c_t rhs) { return !(lhs == rhs); }
+
+  template <typename Sink>
+  friend void AbslStringify(Sink& sink, crc32c_t crc) {
+    absl::Format(&sink, "%08x", static_cast<uint32_t>(crc));
+  }
+
+ private:
+  uint32_t crc_;
+};
+
+
+namespace crc_internal {
+// Non-inline code path for `absl::ExtendCrc32c()`. Do not call directly.
+// Call `absl::ExtendCrc32c()` (defined below) instead.
+crc32c_t ExtendCrc32cInternal(crc32c_t initial_crc,
+                              absl::string_view buf_to_add);
+}  // namespace crc_internal
+
+// -----------------------------------------------------------------------------
+// CRC32C Computation Functions
+// -----------------------------------------------------------------------------
+
+// ExtendCrc32c()
+//
+// Computes a CRC32C value from an `initial_crc` CRC32C value including the
+// `buf_to_add` bytes of an additional buffer. Using this function is more
+// efficient than computing a CRC32C value for the combined buffer from
+// scratch.
+//
+// Note: `ExtendCrc32c` with an initial_crc of 0 is equivalent to
+// `ComputeCrc32c`.
+//
+// This operation has a runtime cost of O(`buf_to_add.size()`)
+inline crc32c_t ExtendCrc32c(crc32c_t initial_crc,
+                             absl::string_view buf_to_add) {
+  // Approximately 75% of calls have size <= 64.
+  if (buf_to_add.size() <= 64) {
+    uint32_t crc = static_cast<uint32_t>(initial_crc);
+    if (crc_internal::ExtendCrc32cInline(&crc, buf_to_add.data(),
+                                         buf_to_add.size())) {
+      return crc32c_t{crc};
+    }
+  }
+  return crc_internal::ExtendCrc32cInternal(initial_crc, buf_to_add);
+}
+
+// ComputeCrc32c()
+//
+// Returns the CRC32C value of the provided string.
+inline crc32c_t ComputeCrc32c(absl::string_view buf) {
+  return ExtendCrc32c(crc32c_t{0}, buf);
+}
+
+// ExtendCrc32cByZeroes()
+//
+// Computes a CRC32C value for a buffer with an `initial_crc` CRC32C value,
+// where `length` bytes with a value of 0 are appended to the buffer. Using this
+// function is more efficient than computing a CRC32C value for the combined
+// buffer from scratch.
+//
+// This operation has a runtime cost of O(log(`length`))
+crc32c_t ExtendCrc32cByZeroes(crc32c_t initial_crc, size_t length);
+
+// MemcpyCrc32c()
+//
+// Copies `src` to `dest` using `memcpy()` semantics, returning the CRC32C
+// value of the copied buffer.
+//
+// Using `MemcpyCrc32c()` is potentially faster than performing the `memcpy()`
+// and `ComputeCrc32c()` operations separately.
+crc32c_t MemcpyCrc32c(void* dest, const void* src, size_t count,
+                      crc32c_t initial_crc = crc32c_t{0});
+
+// -----------------------------------------------------------------------------
+// CRC32C Arithmetic Functions
+// -----------------------------------------------------------------------------
+
+// The following functions perform arithmetic on CRC32C values, which are
+// generally more efficient than recalculating any given result's CRC32C value.
+
+// ConcatCrc32c()
+//
+// Calculates the CRC32C value of two buffers with known CRC32C values
+// concatenated together.
+//
+// Given a buffer with CRC32C value `crc1` and a buffer with
+// CRC32C value `crc2` and length, `crc2_length`, returns the CRC32C value of
+// the concatenation of these two buffers.
+//
+// This operation has a runtime cost of O(log(`crc2_length`)).
+crc32c_t ConcatCrc32c(crc32c_t crc1, crc32c_t crc2, size_t crc2_length);
+
+// RemoveCrc32cPrefix()
+//
+// Calculates the CRC32C value of an existing buffer with a series of bytes
+// (the prefix) removed from the beginning of that buffer.
+//
+// Given the CRC32C value of an existing buffer, `full_string_crc`; The CRC32C
+// value of a prefix of that buffer, `prefix_crc`; and the length of the buffer
+// with the prefix removed, `remaining_string_length` , return the CRC32C
+// value of the buffer with the prefix removed.
+//
+// This operation has a runtime cost of O(log(`remaining_string_length`)).
+crc32c_t RemoveCrc32cPrefix(crc32c_t prefix_crc, crc32c_t full_string_crc,
+                            size_t remaining_string_length);
+// RemoveCrc32cSuffix()
+//
+// Calculates the CRC32C value of an existing buffer with a series of bytes
+// (the suffix) removed from the end of that buffer.
+//
+// Given a CRC32C value of an existing buffer `full_string_crc`, the CRC32C
+// value of the suffix to remove `suffix_crc`, and the length of that suffix
+// `suffix_len`, returns the CRC32C value of the buffer with suffix removed.
+//
+// This operation has a runtime cost of O(log(`suffix_len`))
+crc32c_t RemoveCrc32cSuffix(crc32c_t full_string_crc, crc32c_t suffix_crc,
+                            size_t suffix_length);
+
+// operator<<
+//
+// Streams the CRC32C value `crc` to the stream `os`.
+inline std::ostream& operator<<(std::ostream& os, crc32c_t crc) {
+  return os << absl::StreamFormat("%08x", static_cast<uint32_t>(crc));
+}
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_CRC_CRC32C_H_
diff --git a/absl/crc/crc32c_benchmark.cc b/absl/crc/crc32c_benchmark.cc
new file mode 100644
index 00000000000..fb226a20cbb
--- /dev/null
+++ b/absl/crc/crc32c_benchmark.cc
@@ -0,0 +1,199 @@
+// Copyright 2022 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <cstddef>
+#include <cstdint>
+#include <memory>
+#include <string>
+
+#include "absl/crc/crc32c.h"
+#include "absl/crc/internal/crc32c.h"
+#include "absl/memory/memory.h"
+#include "absl/strings/string_view.h"
+#include "benchmark/benchmark.h"
+
+namespace {
+
+std::string TestString(size_t len) {
+  std::string result;
+  result.reserve(len);
+  for (size_t i = 0; i < len; ++i) {
+    result.push_back(static_cast<char>(i % 256));
+  }
+  return result;
+}
+
+void BM_Calculate(benchmark::State& state) {
+  int len = state.range(0);
+  std::string data = TestString(len);
+  for (auto s : state) {
+    benchmark::DoNotOptimize(data);
+    absl::crc32c_t crc = absl::ComputeCrc32c(data);
+    benchmark::DoNotOptimize(crc);
+  }
+}
+BENCHMARK(BM_Calculate)
+    ->Arg(0)
+    ->Arg(1)
+    ->Arg(100)
+    ->Arg(2048)
+    ->Arg(10000)
+    ->Arg(500000);
+
+void BM_Extend(benchmark::State& state) {
+  int len = state.range(0);
+  std::string extension = TestString(len);
+  absl::crc32c_t base = absl::crc32c_t{0xC99465AA};  // CRC32C of "Hello World"
+  for (auto s : state) {
+    benchmark::DoNotOptimize(base);
+    benchmark::DoNotOptimize(extension);
+    absl::crc32c_t crc = absl::ExtendCrc32c(base, extension);
+    benchmark::DoNotOptimize(crc);
+  }
+}
+BENCHMARK(BM_Extend)
+    ->Arg(0)
+    ->Arg(1)
+    ->Arg(100)
+    ->Arg(2048)
+    ->Arg(10000)
+    ->Arg(500000)
+    ->Arg(100 * 1000 * 1000);
+
+// Make working set >> CPU cache size to benchmark prefetches better
+void BM_ExtendCacheMiss(benchmark::State& state) {
+  int len = state.range(0);
+  constexpr int total = 300 * 1000 * 1000;
+  std::string extension = TestString(total);
+  absl::crc32c_t base = absl::crc32c_t{0xC99465AA};  // CRC32C of "Hello World"
+  for (auto s : state) {
+    for (int i = 0; i < total; i += len * 2) {
+      benchmark::DoNotOptimize(base);
+      benchmark::DoNotOptimize(extension);
+      absl::crc32c_t crc =
+          absl::ExtendCrc32c(base, absl::string_view(&extension[i], len));
+      benchmark::DoNotOptimize(crc);
+    }
+  }
+  state.SetBytesProcessed(static_cast<int64_t>(state.iterations()) * total / 2);
+}
+BENCHMARK(BM_ExtendCacheMiss)->Arg(10)->Arg(100)->Arg(1000)->Arg(100000);
+
+void BM_ExtendByZeroes(benchmark::State& state) {
+  absl::crc32c_t base = absl::crc32c_t{0xC99465AA};  // CRC32C of "Hello World"
+  int num_zeroes = state.range(0);
+  for (auto s : state) {
+    benchmark::DoNotOptimize(base);
+    absl::crc32c_t crc = absl::ExtendCrc32cByZeroes(base, num_zeroes);
+    benchmark::DoNotOptimize(crc);
+  }
+}
+BENCHMARK(BM_ExtendByZeroes)
+    ->RangeMultiplier(10)
+    ->Range(1, 1000000)
+    ->RangeMultiplier(32)
+    ->Range(1, 1 << 20);
+
+void BM_UnextendByZeroes(benchmark::State& state) {
+  absl::crc32c_t base = absl::crc32c_t{0xdeadbeef};
+  int num_zeroes = state.range(0);
+  for (auto s : state) {
+    benchmark::DoNotOptimize(base);
+    absl::crc32c_t crc =
+        absl::crc_internal::UnextendCrc32cByZeroes(base, num_zeroes);
+    benchmark::DoNotOptimize(crc);
+  }
+}
+BENCHMARK(BM_UnextendByZeroes)
+    ->RangeMultiplier(10)
+    ->Range(1, 1000000)
+    ->RangeMultiplier(32)
+    ->Range(1, 1 << 20);
+
+void BM_Concat(benchmark::State& state) {
+  int string_b_len = state.range(0);
+  std::string string_b = TestString(string_b_len);
+
+  // CRC32C of "Hello World"
+  absl::crc32c_t crc_a = absl::crc32c_t{0xC99465AA};
+  absl::crc32c_t crc_b = absl::ComputeCrc32c(string_b);
+
+  for (auto s : state) {
+    benchmark::DoNotOptimize(crc_a);
+    benchmark::DoNotOptimize(crc_b);
+    benchmark::DoNotOptimize(string_b_len);
+    absl::crc32c_t crc_ab = absl::ConcatCrc32c(crc_a, crc_b, string_b_len);
+    benchmark::DoNotOptimize(crc_ab);
+  }
+}
+BENCHMARK(BM_Concat)
+    ->RangeMultiplier(10)
+    ->Range(1, 1000000)
+    ->RangeMultiplier(32)
+    ->Range(1, 1 << 20);
+
+void BM_Memcpy(benchmark::State& state) {
+  int string_len = state.range(0);
+
+  std::string source = TestString(string_len);
+  auto dest = std::make_unique<char[]>(string_len);
+
+  for (auto s : state) {
+    benchmark::DoNotOptimize(source);
+    absl::crc32c_t crc =
+        absl::MemcpyCrc32c(dest.get(), source.data(), source.size());
+    benchmark::DoNotOptimize(crc);
+    benchmark::DoNotOptimize(dest);
+    benchmark::DoNotOptimize(dest.get());
+    benchmark::DoNotOptimize(dest[0]);
+  }
+
+  state.SetBytesProcessed(static_cast<int64_t>(state.iterations()) *
+                          state.range(0));
+}
+BENCHMARK(BM_Memcpy)->Arg(0)->Arg(1)->Arg(100)->Arg(2048)->Arg(10000)->Arg(
+    500000);
+
+void BM_RemoveSuffix(benchmark::State& state) {
+  int full_string_len = state.range(0);
+  int suffix_len = state.range(1);
+
+  std::string full_string = TestString(full_string_len);
+  std::string suffix = full_string.substr(
+    full_string_len - suffix_len, full_string_len);
+
+  absl::crc32c_t full_string_crc = absl::ComputeCrc32c(full_string);
+  absl::crc32c_t suffix_crc = absl::ComputeCrc32c(suffix);
+
+  for (auto s : state) {
+    benchmark::DoNotOptimize(full_string_crc);
+    benchmark::DoNotOptimize(suffix_crc);
+    benchmark::DoNotOptimize(suffix_len);
+    absl::crc32c_t crc = absl::RemoveCrc32cSuffix(full_string_crc, suffix_crc,
+      suffix_len);
+    benchmark::DoNotOptimize(crc);
+  }
+}
+BENCHMARK(BM_RemoveSuffix)
+    ->ArgPair(1, 1)
+    ->ArgPair(100, 10)
+    ->ArgPair(100, 100)
+    ->ArgPair(10000, 1)
+    ->ArgPair(10000, 100)
+    ->ArgPair(10000, 10000)
+    ->ArgPair(500000, 1)
+    ->ArgPair(500000, 100)
+    ->ArgPair(500000, 10000)
+    ->ArgPair(500000, 500000);
+}  // namespace
diff --git a/absl/crc/crc32c_test.cc b/absl/crc/crc32c_test.cc
new file mode 100644
index 00000000000..eed5a6c8947
--- /dev/null
+++ b/absl/crc/crc32c_test.cc
@@ -0,0 +1,257 @@
+// Copyright 2022 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/crc/crc32c.h"
+
+#include <algorithm>
+#include <array>
+#include <cstddef>
+#include <cstdint>
+#include <cstring>
+#include <limits>
+#include <sstream>
+#include <string>
+#include <tuple>
+
+#include "gtest/gtest.h"
+#include "absl/crc/internal/crc32c.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/str_format.h"
+#include "absl/strings/string_view.h"
+
+namespace {
+
+TEST(CRC32C, RFC3720) {
+  // Test the results of the vectors from
+  // https://www.rfc-editor.org/rfc/rfc3720#appendix-B.4
+  char data[32];
+
+  // 32 bytes of ones.
+  memset(data, 0, sizeof(data));
+  EXPECT_EQ(absl::ComputeCrc32c(absl::string_view(data, sizeof(data))),
+            absl::crc32c_t{0x8a9136aa});
+
+  // 32 bytes of ones.
+  memset(data, 0xff, sizeof(data));
+  EXPECT_EQ(absl::ComputeCrc32c(absl::string_view(data, sizeof(data))),
+            absl::crc32c_t{0x62a8ab43});
+
+  // 32 incrementing bytes.
+  for (int i = 0; i < 32; ++i) data[i] = static_cast<char>(i);
+  EXPECT_EQ(absl::ComputeCrc32c(absl::string_view(data, sizeof(data))),
+            absl::crc32c_t{0x46dd794e});
+
+  // 32 decrementing bytes.
+  for (int i = 0; i < 32; ++i) data[i] = static_cast<char>(31 - i);
+  EXPECT_EQ(absl::ComputeCrc32c(absl::string_view(data, sizeof(data))),
+            absl::crc32c_t{0x113fdb5c});
+
+  // An iSCSI - SCSI Read (10) Command PDU.
+  constexpr uint8_t cmd[48] = {
+      0x01, 0xc0, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+      0x00, 0x00, 0x00, 0x00, 0x14, 0x00, 0x00, 0x00, 0x00, 0x00, 0x04, 0x00,
+      0x00, 0x00, 0x00, 0x14, 0x00, 0x00, 0x00, 0x18, 0x28, 0x00, 0x00, 0x00,
+      0x00, 0x00, 0x00, 0x00, 0x02, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+  };
+  EXPECT_EQ(absl::ComputeCrc32c(absl::string_view(
+                reinterpret_cast<const char*>(cmd), sizeof(cmd))),
+            absl::crc32c_t{0xd9963a56});
+}
+
+std::string TestString(size_t len) {
+  std::string result;
+  result.reserve(len);
+  for (size_t i = 0; i < len; ++i) {
+    result.push_back(static_cast<char>(i % 256));
+  }
+  return result;
+}
+
+TEST(CRC32C, Compute) {
+  EXPECT_EQ(absl::ComputeCrc32c(""), absl::crc32c_t{0});
+  EXPECT_EQ(absl::ComputeCrc32c("hello world"), absl::crc32c_t{0xc99465aa});
+}
+
+TEST(CRC32C, Extend) {
+  uint32_t base = 0xC99465AA;  // CRC32C of "Hello World"
+  std::string extension = "Extension String";
+
+  EXPECT_EQ(
+      absl::ExtendCrc32c(absl::crc32c_t{base}, extension),
+      absl::crc32c_t{0xD2F65090});  // CRC32C of "Hello WorldExtension String"
+}
+
+TEST(CRC32C, ExtendByZeroes) {
+  std::string base = "hello world";
+  absl::crc32c_t base_crc = absl::crc32c_t{0xc99465aa};
+
+  constexpr size_t kExtendByValues[] = {100, 10000, 100000};
+  for (const size_t extend_by : kExtendByValues) {
+    SCOPED_TRACE(extend_by);
+    absl::crc32c_t crc2 = absl::ExtendCrc32cByZeroes(base_crc, extend_by);
+    EXPECT_EQ(crc2, absl::ComputeCrc32c(base + std::string(extend_by, '\0')));
+  }
+}
+
+// Test ExtendCrc32cByZeroes() for the full range of the size_t length,
+// including every bit. This is important because ExtendCrc32cByZeroes() is
+// implemented using an array of constants, where each entry in the array is
+// used only when a particular bit in the size_t length is set. This test
+// verifies that every entry in that array is correct.
+TEST(CRC32C, ExtendByZeroesAllLengthBits) {
+  absl::crc32c_t base_crc = absl::crc32c_t{0xc99465aa};
+  const std::array<std::tuple<uint64_t, absl::crc32c_t>, 5> kTestCases = {{
+      {0, absl::crc32c_t(0xc99465aa)},
+      {std::numeric_limits<uint32_t>::max(), absl::crc32c_t(0x9b1d5aaa)},
+      {0x12345678, absl::crc32c_t(0xcf0e9553)},
+      {std::numeric_limits<uint64_t>::max(), absl::crc32c_t(0xf5bff489)},
+      {0x12345678abcdefff, absl::crc32c_t(0xaa1ffb0b)},
+  }};
+  for (const auto &test_case : kTestCases) {
+    uint64_t length = std::get<0>(test_case);
+    absl::crc32c_t expected_value = std::get<1>(test_case);
+    SCOPED_TRACE(length);
+    if (length > std::numeric_limits<size_t>::max()) {
+      // On 32-bit platforms, 64-bit lengths cannot be used or tested.
+      continue;
+    }
+    EXPECT_EQ(absl::ExtendCrc32cByZeroes(base_crc, static_cast<size_t>(length)),
+              expected_value);
+  }
+}
+
+TEST(CRC32C, UnextendByZeroes) {
+  constexpr size_t kExtendByValues[] = {2, 200, 20000, 200000, 20000000};
+  constexpr size_t kUnextendByValues[] = {0, 100, 10000, 100000, 10000000};
+
+  for (auto seed_crc : {absl::crc32c_t{0}, absl::crc32c_t{0xc99465aa}}) {
+    SCOPED_TRACE(seed_crc);
+    for (const size_t size_1 : kExtendByValues) {
+      for (const size_t size_2 : kUnextendByValues) {
+        size_t extend_size = std::max(size_1, size_2);
+        size_t unextend_size = std::min(size_1, size_2);
+        SCOPED_TRACE(extend_size);
+        SCOPED_TRACE(unextend_size);
+
+        // Extending by A zeroes an unextending by B<A zeros should be identical
+        // to extending by A-B zeroes.
+        absl::crc32c_t crc1 = seed_crc;
+        crc1 = absl::ExtendCrc32cByZeroes(crc1, extend_size);
+        crc1 = absl::crc_internal::UnextendCrc32cByZeroes(crc1, unextend_size);
+
+        absl::crc32c_t crc2 = seed_crc;
+        crc2 = absl::ExtendCrc32cByZeroes(crc2, extend_size - unextend_size);
+
+        EXPECT_EQ(crc1, crc2);
+      }
+    }
+  }
+
+  constexpr size_t kSizes[] = {0, 1, 100, 10000};
+  for (const size_t size : kSizes) {
+    SCOPED_TRACE(size);
+    std::string string_before = TestString(size);
+    std::string string_after = string_before + std::string(size, '\0');
+
+    absl::crc32c_t crc_before = absl::ComputeCrc32c(string_before);
+    absl::crc32c_t crc_after = absl::ComputeCrc32c(string_after);
+
+    EXPECT_EQ(crc_before,
+              absl::crc_internal::UnextendCrc32cByZeroes(crc_after, size));
+  }
+}
+
+TEST(CRC32C, Concat) {
+  std::string hello = "Hello, ";
+  std::string world = "world!";
+  std::string hello_world = absl::StrCat(hello, world);
+
+  absl::crc32c_t crc_a = absl::ComputeCrc32c(hello);
+  absl::crc32c_t crc_b = absl::ComputeCrc32c(world);
+  absl::crc32c_t crc_ab = absl::ComputeCrc32c(hello_world);
+
+  EXPECT_EQ(absl::ConcatCrc32c(crc_a, crc_b, world.size()), crc_ab);
+}
+
+TEST(CRC32C, Memcpy) {
+  constexpr size_t kBytesSize[] = {0, 1, 20, 500, 100000};
+  for (size_t bytes : kBytesSize) {
+    SCOPED_TRACE(bytes);
+    std::string sample_string = TestString(bytes);
+    std::string target_buffer = std::string(bytes, '\0');
+
+    absl::crc32c_t memcpy_crc =
+        absl::MemcpyCrc32c(&(target_buffer[0]), sample_string.data(), bytes);
+    absl::crc32c_t compute_crc = absl::ComputeCrc32c(sample_string);
+
+    EXPECT_EQ(memcpy_crc, compute_crc);
+    EXPECT_EQ(sample_string, target_buffer);
+  }
+}
+
+TEST(CRC32C, RemovePrefix) {
+  std::string hello = "Hello, ";
+  std::string world = "world!";
+  std::string hello_world = absl::StrCat(hello, world);
+
+  absl::crc32c_t crc_a = absl::ComputeCrc32c(hello);
+  absl::crc32c_t crc_b = absl::ComputeCrc32c(world);
+  absl::crc32c_t crc_ab = absl::ComputeCrc32c(hello_world);
+
+  EXPECT_EQ(absl::RemoveCrc32cPrefix(crc_a, crc_ab, world.size()), crc_b);
+}
+
+TEST(CRC32C, RemoveSuffix) {
+  std::string hello = "Hello, ";
+  std::string world = "world!";
+  std::string hello_world = absl::StrCat(hello, world);
+
+  absl::crc32c_t crc_a = absl::ComputeCrc32c(hello);
+  absl::crc32c_t crc_b = absl::ComputeCrc32c(world);
+  absl::crc32c_t crc_ab = absl::ComputeCrc32c(hello_world);
+
+  EXPECT_EQ(absl::RemoveCrc32cSuffix(crc_ab, crc_b, world.size()), crc_a);
+}
+
+TEST(CRC32C, InsertionOperator) {
+  {
+    std::ostringstream buf;
+    buf << absl::crc32c_t{0xc99465aa};
+    EXPECT_EQ(buf.str(), "c99465aa");
+  }
+  {
+    std::ostringstream buf;
+    buf << absl::crc32c_t{0};
+    EXPECT_EQ(buf.str(), "00000000");
+  }
+  {
+    std::ostringstream buf;
+    buf << absl::crc32c_t{17};
+    EXPECT_EQ(buf.str(), "00000011");
+  }
+}
+
+TEST(CRC32C, AbslStringify) {
+  // StrFormat
+  EXPECT_EQ(absl::StrFormat("%v", absl::crc32c_t{0xc99465aa}), "c99465aa");
+  EXPECT_EQ(absl::StrFormat("%v", absl::crc32c_t{0}), "00000000");
+  EXPECT_EQ(absl::StrFormat("%v", absl::crc32c_t{17}), "00000011");
+
+  // StrCat
+  EXPECT_EQ(absl::StrCat(absl::crc32c_t{0xc99465aa}), "c99465aa");
+  EXPECT_EQ(absl::StrCat(absl::crc32c_t{0}), "00000000");
+  EXPECT_EQ(absl::StrCat(absl::crc32c_t{17}), "00000011");
+}
+
+}  // namespace
diff --git a/absl/crc/internal/crc.cc b/absl/crc/internal/crc.cc
new file mode 100644
index 00000000000..ec6a031c512
--- /dev/null
+++ b/absl/crc/internal/crc.cc
@@ -0,0 +1,436 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Implementation of CRCs (aka Rabin Fingerprints).
+// Treats the input as a polynomial with coefficients in Z(2),
+// and finds the remainder when divided by an irreducible polynomial
+// of the appropriate length.
+// It handles all CRC sizes from 8 to 128 bits.
+// It's somewhat complicated by having separate implementations optimized for
+// CRC's <=32 bits, <= 64 bits, and <= 128 bits.
+// The input string is prefixed with a "1" bit, and has "degree" "0" bits
+// appended to it before the remainder is found.   This ensures that
+// short strings are scrambled somewhat and that strings consisting
+// of all nulls have a non-zero CRC.
+//
+// Uses the "interleaved word-by-word" method from
+// "Everything we know about CRC but afraid to forget" by Andrew Kadatch
+// and Bob Jenkins,
+// http://crcutil.googlecode.com/files/crc-doc.1.0.pdf
+//
+// The idea is to compute kStride CRCs simultaneously, allowing the
+// processor to more effectively use multiple execution units. Each of
+// the CRCs is calculated on one word of data followed by kStride - 1
+// words of zeroes; the CRC starting points are staggered by one word.
+// Assuming a stride of 4 with data words "ABCDABCDABCD", the first
+// CRC is over A000A000A, the second over 0B000B000B, and so on.
+// The CRC of the whole data is then calculated by properly aligning the
+// CRCs by appending zeroes until the data lengths agree then XORing
+// the CRCs.
+
+#include "absl/crc/internal/crc.h"
+
+#include <cstddef>
+#include <cstdint>
+#include <iterator>
+
+#include "absl/base/internal/endian.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/base/prefetch.h"
+#include "absl/crc/internal/crc_internal.h"
+#include "absl/numeric/bits.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace crc_internal {
+
+namespace {
+
+// Constants
+#if defined(__i386__) || defined(__x86_64__)
+constexpr bool kNeedAlignedLoads = false;
+#else
+constexpr bool kNeedAlignedLoads = true;
+#endif
+
+// We express the number of zeroes as a number in base ZEROES_BASE. By
+// pre-computing the zero extensions for all possible components of such an
+// expression (numbers in a form a*ZEROES_BASE**b), we can calculate the
+// resulting extension by multiplying the extensions for individual components
+// using log_{ZEROES_BASE}(num_zeroes) polynomial multiplications. The tables of
+// zero extensions contain (ZEROES_BASE - 1) * (log_{ZEROES_BASE}(64)) entries.
+constexpr int ZEROES_BASE_LG = 4;                   // log_2(ZEROES_BASE)
+constexpr int ZEROES_BASE = (1 << ZEROES_BASE_LG);  // must be a power of 2
+
+constexpr uint32_t kCrc32cPoly = 0x82f63b78;
+
+uint32_t ReverseBits(uint32_t bits) {
+  bits = (bits & 0xaaaaaaaau) >> 1 | (bits & 0x55555555u) << 1;
+  bits = (bits & 0xccccccccu) >> 2 | (bits & 0x33333333u) << 2;
+  bits = (bits & 0xf0f0f0f0u) >> 4 | (bits & 0x0f0f0f0fu) << 4;
+  return absl::gbswap_32(bits);
+}
+
+// Polynomial long multiplication mod the polynomial of degree 32.
+void PolyMultiply(uint32_t* val, uint32_t m, uint32_t poly) {
+  uint32_t l = *val;
+  uint32_t result = 0;
+  auto onebit = uint32_t{0x80000000u};
+  for (uint32_t one = onebit; one != 0; one >>= 1) {
+    if ((l & one) != 0) {
+      result ^= m;
+    }
+    if (m & 1) {
+      m = (m >> 1) ^ poly;
+    } else {
+      m >>= 1;
+    }
+  }
+  *val = result;
+}
+}  // namespace
+
+void CRCImpl::FillWordTable(uint32_t poly, uint32_t last, int word_size,
+                            Uint32By256* t) {
+  for (int j = 0; j != word_size; j++) {  // for each byte of extension....
+    t[j][0] = 0;                          // a zero has no effect
+    for (int i = 128; i != 0; i >>= 1) {  // fill in entries for powers of 2
+      if (j == 0 && i == 128) {
+        t[j][i] = last;  // top bit in last byte is given
+      } else {
+        // each successive power of two is derived from the previous
+        // one, either in this table, or the last table
+        uint32_t pred;
+        if (i == 128) {
+          pred = t[j - 1][1];
+        } else {
+          pred = t[j][i << 1];
+        }
+        // Advance the CRC by one bit (multiply by X, and take remainder
+        // through one step of polynomial long division)
+        if (pred & 1) {
+          t[j][i] = (pred >> 1) ^ poly;
+        } else {
+          t[j][i] = pred >> 1;
+        }
+      }
+    }
+    // CRCs have the property that CRC(a xor b) == CRC(a) xor CRC(b)
+    // so we can make all the tables for non-powers of two by
+    // xoring previously created entries.
+    for (int i = 2; i != 256; i <<= 1) {
+      for (int k = i + 1; k != (i << 1); k++) {
+        t[j][k] = t[j][i] ^ t[j][k - i];
+      }
+    }
+  }
+}
+
+int CRCImpl::FillZeroesTable(uint32_t poly, Uint32By256* t) {
+  uint32_t inc = 1;
+  inc <<= 31;
+
+  // Extend by one zero bit. We know degree > 1 so (inc & 1) == 0.
+  inc >>= 1;
+
+  // Now extend by 2, 4, and 8 bits, so now `inc` is extended by one zero byte.
+  for (int i = 0; i < 3; ++i) {
+    PolyMultiply(&inc, inc, poly);
+  }
+
+  int j = 0;
+  for (uint64_t inc_len = 1; inc_len != 0; inc_len <<= ZEROES_BASE_LG) {
+    // Every entry in the table adds an additional inc_len zeroes.
+    uint32_t v = inc;
+    for (int a = 1; a != ZEROES_BASE; a++) {
+      t[0][j] = v;
+      PolyMultiply(&v, inc, poly);
+      j++;
+    }
+    inc = v;
+  }
+  ABSL_RAW_CHECK(j <= 256, "");
+  return j;
+}
+
+// Internal version of the "constructor".
+CRCImpl* CRCImpl::NewInternal() {
+  // Find an accelearated implementation first.
+  CRCImpl* result = TryNewCRC32AcceleratedX86ARMCombined();
+
+  // Fall back to generic implementions if no acceleration is available.
+  if (result == nullptr) {
+    result = new CRC32();
+  }
+
+  result->InitTables();
+
+  return result;
+}
+
+//  The 32-bit implementation
+
+void CRC32::InitTables() {
+  // Compute the table for extending a CRC by one byte.
+  Uint32By256* t = new Uint32By256[4];
+  FillWordTable(kCrc32cPoly, kCrc32cPoly, 1, t);
+  for (int i = 0; i != 256; i++) {
+    this->table0_[i] = t[0][i];
+  }
+
+  // Construct a table for updating the CRC by 4 bytes data followed by
+  // 12 bytes of zeroes.
+  //
+  // Note: the data word size could be larger than the CRC size; it might
+  // be slightly faster to use a 64-bit data word, but doing so doubles the
+  // table size.
+  uint32_t last = kCrc32cPoly;
+  const size_t size = 12;
+  for (size_t i = 0; i < size; ++i) {
+    last = (last >> 8) ^ this->table0_[last & 0xff];
+  }
+  FillWordTable(kCrc32cPoly, last, 4, t);
+  for (size_t b = 0; b < 4; ++b) {
+    for (int i = 0; i < 256; ++i) {
+      this->table_[b][i] = t[b][i];
+    }
+  }
+
+  int j = FillZeroesTable(kCrc32cPoly, t);
+  ABSL_RAW_CHECK(j <= static_cast<int>(std::size(this->zeroes_)), "");
+  for (int i = 0; i < j; i++) {
+    this->zeroes_[i] = t[0][i];
+  }
+
+  delete[] t;
+
+  // Build up tables for _reversing_ the operation of doing CRC operations on
+  // zero bytes.
+
+  // In C++, extending `crc` by a single zero bit is done by the following:
+  // (A)  bool low_bit_set = (crc & 1);
+  //      crc >>= 1;
+  //      if (low_bit_set) crc ^= kCrc32cPoly;
+  //
+  // In particular note that the high bit of `crc` after this operation will be
+  // set if and only if the low bit of `crc` was set before it.  This means that
+  // no information is lost, and the operation can be reversed, as follows:
+  // (B)  bool high_bit_set = (crc & 0x80000000u);
+  //      if (high_bit_set) crc ^= kCrc32cPoly;
+  //      crc <<= 1;
+  //      if (high_bit_set) crc ^= 1;
+  //
+  // Or, equivalently:
+  // (C)  bool high_bit_set = (crc & 0x80000000u);
+  //      crc <<= 1;
+  //      if (high_bit_set) crc ^= ((kCrc32cPoly << 1) ^ 1);
+  //
+  // The last observation is, if we store our checksums in variable `rcrc`,
+  // with order of the bits reversed, the inverse operation becomes:
+  // (D)  bool low_bit_set = (rcrc & 1);
+  //      rcrc >>= 1;
+  //      if (low_bit_set) rcrc ^= ReverseBits((kCrc32cPoly << 1) ^ 1)
+  //
+  // This is the same algorithm (A) that we started with, only with a different
+  // polynomial bit pattern.  This means that by building up our tables with
+  // this alternate polynomial, we can apply the CRC algorithms to a
+  // bit-reversed CRC checksum to perform inverse zero-extension.
+
+  const uint32_t kCrc32cUnextendPoly =
+      ReverseBits(static_cast<uint32_t>((kCrc32cPoly << 1) ^ 1));
+  FillWordTable(kCrc32cUnextendPoly, kCrc32cUnextendPoly, 1, &reverse_table0_);
+
+  j = FillZeroesTable(kCrc32cUnextendPoly, &reverse_zeroes_);
+  ABSL_RAW_CHECK(j <= static_cast<int>(std::size(this->reverse_zeroes_)), "");
+}
+
+void CRC32::Extend(uint32_t* crc, const void* bytes, size_t length) const {
+  const uint8_t* p = static_cast<const uint8_t*>(bytes);
+  const uint8_t* e = p + length;
+  uint32_t l = *crc;
+
+  auto step_one_byte = [this, &p, &l]() {
+    int c = (l & 0xff) ^ *p++;
+    l = this->table0_[c] ^ (l >> 8);
+  };
+
+  if (kNeedAlignedLoads) {
+    // point x at first 4-byte aligned byte in string. this might be past the
+    // end of the string.
+    const uint8_t* x = RoundUp<4>(p);
+    if (x <= e) {
+      // Process bytes until finished or p is 4-byte aligned
+      while (p != x) {
+        step_one_byte();
+      }
+    }
+  }
+
+  const size_t kSwathSize = 16;
+  if (static_cast<size_t>(e - p) >= kSwathSize) {
+    // Load one swath of data into the operating buffers.
+    uint32_t buf0 = absl::little_endian::Load32(p) ^ l;
+    uint32_t buf1 = absl::little_endian::Load32(p + 4);
+    uint32_t buf2 = absl::little_endian::Load32(p + 8);
+    uint32_t buf3 = absl::little_endian::Load32(p + 12);
+    p += kSwathSize;
+
+    // Increment a CRC value by a "swath"; this combines the four bytes
+    // starting at `ptr` and twelve zero bytes, so that four CRCs can be
+    // built incrementally and combined at the end.
+    const auto step_swath = [this](uint32_t crc_in, const std::uint8_t* ptr) {
+      return absl::little_endian::Load32(ptr) ^
+             this->table_[3][crc_in & 0xff] ^
+             this->table_[2][(crc_in >> 8) & 0xff] ^
+             this->table_[1][(crc_in >> 16) & 0xff] ^
+             this->table_[0][crc_in >> 24];
+    };
+
+    // Run one CRC calculation step over all swaths in one 16-byte stride
+    const auto step_stride = [&]() {
+      buf0 = step_swath(buf0, p);
+      buf1 = step_swath(buf1, p + 4);
+      buf2 = step_swath(buf2, p + 8);
+      buf3 = step_swath(buf3, p + 12);
+      p += 16;
+    };
+
+    // Process kStride interleaved swaths through the data in parallel.
+    while ((e - p) > kPrefetchHorizon) {
+      PrefetchToLocalCacheNta(
+          reinterpret_cast<const void*>(p + kPrefetchHorizon));
+      // Process 64 bytes at a time
+      step_stride();
+      step_stride();
+      step_stride();
+      step_stride();
+    }
+    while (static_cast<size_t>(e - p) >= kSwathSize) {
+      step_stride();
+    }
+
+    // Now advance one word at a time as far as possible. This isn't worth
+    // doing if we have word-advance tables.
+    while (static_cast<size_t>(e - p) >= 4) {
+      buf0 = step_swath(buf0, p);
+      uint32_t tmp = buf0;
+      buf0 = buf1;
+      buf1 = buf2;
+      buf2 = buf3;
+      buf3 = tmp;
+      p += 4;
+    }
+
+    // Combine the results from the different swaths. This is just a CRC
+    // on the data values in the bufX words.
+    auto combine_one_word = [this](uint32_t crc_in, uint32_t w) {
+      w ^= crc_in;
+      for (size_t i = 0; i < 4; ++i) {
+        w = (w >> 8) ^ this->table0_[w & 0xff];
+      }
+      return w;
+    };
+
+    l = combine_one_word(0, buf0);
+    l = combine_one_word(l, buf1);
+    l = combine_one_word(l, buf2);
+    l = combine_one_word(l, buf3);
+  }
+
+  // Process the last few bytes
+  while (p != e) {
+    step_one_byte();
+  }
+
+  *crc = l;
+}
+
+void CRC32::ExtendByZeroesImpl(uint32_t* crc, size_t length,
+                               const uint32_t zeroes_table[256],
+                               const uint32_t poly_table[256]) {
+  if (length != 0) {
+    uint32_t l = *crc;
+    // For each ZEROES_BASE_LG bits in length
+    // (after the low-order bits have been removed)
+    // we lookup the appropriate polynomial in the zeroes_ array
+    // and do a polynomial long multiplication (mod the CRC polynomial)
+    // to extend the CRC by the appropriate number of bits.
+    for (int i = 0; length != 0;
+         i += ZEROES_BASE - 1, length >>= ZEROES_BASE_LG) {
+      int c = length & (ZEROES_BASE - 1);  // pick next ZEROES_BASE_LG bits
+      if (c != 0) {                        // if they are not zero,
+                                           // multiply by entry in table
+        // Build a table to aid in multiplying 2 bits at a time.
+        // It takes too long to build tables for more bits.
+        uint64_t m = zeroes_table[c + i - 1];
+        m <<= 1;
+        uint64_t m2 = m << 1;
+        uint64_t mtab[4] = {0, m, m2, m2 ^ m};
+
+        // Do the multiply one byte at a time.
+        uint64_t result = 0;
+        for (int x = 0; x < 32; x += 8) {
+          // The carry-less multiply.
+          result ^= mtab[l & 3] ^ (mtab[(l >> 2) & 3] << 2) ^
+                    (mtab[(l >> 4) & 3] << 4) ^ (mtab[(l >> 6) & 3] << 6);
+          l >>= 8;
+
+          // Reduce modulo the polynomial
+          result = (result >> 8) ^ poly_table[result & 0xff];
+        }
+        l = static_cast<uint32_t>(result);
+      }
+    }
+    *crc = l;
+  }
+}
+
+void CRC32::ExtendByZeroes(uint32_t* crc, size_t length) const {
+  return CRC32::ExtendByZeroesImpl(crc, length, zeroes_, table0_);
+}
+
+void CRC32::UnextendByZeroes(uint32_t* crc, size_t length) const {
+  // See the comment in CRC32::InitTables() for an explanation of the algorithm
+  // below.
+  *crc = ReverseBits(*crc);
+  ExtendByZeroesImpl(crc, length, reverse_zeroes_, reverse_table0_);
+  *crc = ReverseBits(*crc);
+}
+
+void CRC32::Scramble(uint32_t* crc) const {
+  // Rotate by near half the word size plus 1.  See the scramble comment in
+  // crc_internal.h for an explanation.
+  constexpr int scramble_rotate = (32 / 2) + 1;
+  *crc = absl::rotr(static_cast<uint32_t>(*crc + kScrambleLo), scramble_rotate);
+}
+
+void CRC32::Unscramble(uint32_t* crc) const {
+  constexpr int scramble_rotate = (32 / 2) + 1;
+  uint64_t rotated = absl::rotl(*crc, scramble_rotate);
+  *crc = static_cast<uint32_t>(rotated - kScrambleLo);
+}
+
+// Constructor and destructor for base class CRC.
+CRC::~CRC() {}
+CRC::CRC() {}
+
+// The "constructor" for a CRC32C with a standard polynomial.
+CRC* CRC::Crc32c() {
+  static CRC* singleton = CRCImpl::NewInternal();
+  return singleton;
+}
+
+}  // namespace crc_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/crc/internal/crc.h b/absl/crc/internal/crc.h
new file mode 100644
index 00000000000..90e05f1a89b
--- /dev/null
+++ b/absl/crc/internal/crc.h
@@ -0,0 +1,84 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_CRC_INTERNAL_CRC_H_
+#define ABSL_CRC_INTERNAL_CRC_H_
+
+#include <cstddef>
+#include <cstdint>
+
+#include "absl/base/config.h"
+
+// This class implements CRCs (aka Rabin Fingerprints).
+// Treats the input as a polynomial with coefficients in Z(2),
+// and finds the remainder when divided by an primitive polynomial
+// of the appropriate length.
+
+// A polynomial is represented by the bit pattern formed by its coefficients,
+// but with the highest order bit not stored.
+// The highest degree coefficient is stored in the lowest numbered bit
+// in the lowest addressed byte.   Thus, in what follows, the highest degree
+// coefficient that is stored is in the low order bit of "lo" or "*lo".
+
+// Hardware acceleration is used when available.
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace crc_internal {
+
+class CRC {
+ public:
+  virtual ~CRC();
+
+  // If "*crc" is the CRC of bytestring A, place the CRC of
+  // the bytestring formed from the concatenation of A and the "length"
+  // bytes at "bytes" into "*crc".
+  virtual void Extend(uint32_t* crc, const void* bytes,
+                      size_t length) const = 0;
+
+  // Equivalent to Extend(crc, bytes, length) where "bytes"
+  // points to an array of "length" zero bytes.
+  virtual void ExtendByZeroes(uint32_t* crc, size_t length) const = 0;
+
+  // Inverse operation of ExtendByZeroes.  If `crc` is the CRC value of a string
+  // ending in `length` zero bytes, this returns a CRC value of that string
+  // with those zero bytes removed.
+  virtual void UnextendByZeroes(uint32_t* crc, size_t length) const = 0;
+
+  // Apply a non-linear transformation to "*crc" so that
+  // it is safe to CRC the result with the same polynomial without
+  // any reduction of error-detection ability in the outer CRC.
+  // Unscramble() performs the inverse transformation.
+  // It is strongly recommended that CRCs be scrambled before storage or
+  // transmission, and unscrambled at the other end before further manipulation.
+  virtual void Scramble(uint32_t* crc) const = 0;
+  virtual void Unscramble(uint32_t* crc) const = 0;
+
+  // Crc32c() returns the singleton implementation of CRC for the CRC32C
+  // polynomial.  Returns a handle that MUST NOT be destroyed with delete.
+  static CRC* Crc32c();
+
+ protected:
+  CRC();  // Clients may not call constructor; use Crc32c() instead.
+
+ private:
+  CRC(const CRC&) = delete;
+  CRC& operator=(const CRC&) = delete;
+};
+
+}  // namespace crc_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_CRC_INTERNAL_CRC_H_
diff --git a/absl/crc/internal/crc32_x86_arm_combined_simd.h b/absl/crc/internal/crc32_x86_arm_combined_simd.h
new file mode 100644
index 00000000000..c50a3013c74
--- /dev/null
+++ b/absl/crc/internal/crc32_x86_arm_combined_simd.h
@@ -0,0 +1,356 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_CRC_INTERNAL_CRC32_X86_ARM_COMBINED_SIMD_H_
+#define ABSL_CRC_INTERNAL_CRC32_X86_ARM_COMBINED_SIMD_H_
+
+#include <array>
+#include <cstdint>
+
+#include "absl/base/config.h"
+
+#ifdef __SSE4_2__
+#include <immintrin.h>
+#endif
+
+// -------------------------------------------------------------------------
+// Many x86 and ARM machines have CRC acceleration hardware.
+// We can do a faster version of Extend() on such machines.
+// We define a translation layer for both x86 and ARM for the ease of use and
+// most performance gains.
+
+// This implementation requires 64-bit CRC instructions (part of SSE 4.2) and
+// PCLMULQDQ instructions. 32-bit builds with SSE 4.2 do exist, so the
+// __x86_64__ condition is necessary.
+#if defined(__x86_64__) && defined(__SSE4_2__) && defined(__PCLMUL__)
+
+#include <x86intrin.h>
+#define ABSL_CRC_INTERNAL_HAVE_X86_SIMD
+
+#elif defined(_MSC_VER) && !defined(__clang__) && defined(__AVX__) && \
+    defined(_M_AMD64)
+
+// MSVC AVX (/arch:AVX) implies SSE 4.2 and PCLMULQDQ.
+#include <intrin.h>
+#define ABSL_CRC_INTERNAL_HAVE_X86_SIMD
+
+#elif defined(__aarch64__) && defined(__LITTLE_ENDIAN__) &&                 \
+    defined(__ARM_FEATURE_CRC32) && defined(ABSL_INTERNAL_HAVE_ARM_NEON) && \
+    defined(__ARM_FEATURE_CRYPTO)
+
+#include <arm_acle.h>
+#include <arm_neon.h>
+#define ABSL_CRC_INTERNAL_HAVE_ARM_SIMD
+
+#endif
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace crc_internal {
+
+#if defined(ABSL_CRC_INTERNAL_HAVE_ARM_SIMD) || \
+    defined(ABSL_CRC_INTERNAL_HAVE_X86_SIMD)
+
+#if defined(ABSL_CRC_INTERNAL_HAVE_ARM_SIMD)
+using V128 = uint64x2_t;
+#else
+// Note: Do not use __m128i_u, it is not portable.
+// Use V128_LoadU() perform an unaligned load from __m128i*.
+using V128 = __m128i;
+#endif
+
+#if defined(__AVX__)
+using V256 = __m256i;
+#else
+// Placeholder for V256 when AVX is not available.
+using V256 = std::array<uint64_t, 4>;
+#endif
+
+// Starting with the initial value in |crc|, accumulates a CRC32 value for
+// unsigned integers of different sizes.
+uint32_t CRC32_u8(uint32_t crc, uint8_t v);
+
+uint32_t CRC32_u16(uint32_t crc, uint16_t v);
+
+uint32_t CRC32_u32(uint32_t crc, uint32_t v);
+
+uint32_t CRC32_u64(uint32_t crc, uint64_t v);
+
+// Loads 128 bits of integer data. |src| must be 16-byte aligned.
+V128 V128_Load(const V128* src);
+
+// Load 128 bits of integer data. |src| does not need to be aligned.
+V128 V128_LoadU(const V128* src);
+
+// Store 128 bits of integer data. |src| must be 16-byte aligned.
+void V128_Store(V128* dst, V128 data);
+
+// Polynomially multiplies the high 64 bits of |l| and |r|.
+V128 V128_PMulHi(const V128 l, const V128 r);
+
+// Polynomially multiplies the low 64 bits of |l| and |r|.
+V128 V128_PMulLow(const V128 l, const V128 r);
+
+// Polynomially multiplies the low 64 bits of |r| and high 64 bits of |l|.
+V128 V128_PMul01(const V128 l, const V128 r);
+
+// Polynomially multiplies the low 64 bits of |l| and high 64 bits of |r|.
+V128 V128_PMul10(const V128 l, const V128 r);
+
+// Produces a XOR operation of |l| and |r|.
+V128 V128_Xor(const V128 l, const V128 r);
+
+// Produces a 3-way XOR operation of |a|, |b|, and |c|.
+// When `kUseEor3` is true, uses the ARMv8.2-A `EOR3` (3-way Exclusive-OR)
+// instruction.
+template <bool kUseEor3 = false>
+V128 V128_Xor3(const V128 a, const V128 b, const V128 c);
+
+// Sets the lower half of a 128 bit register to the given 64-bit value and
+// zeroes the upper half.
+// dst[63:0] := |r|
+// dst[127:64] := |0|
+V128 V128_From64WithZeroFill(const uint64_t r);
+
+// Extracts a 32-bit integer from |l|, selected with |imm|.
+template <int imm>
+int V128_Extract32(const V128 l);
+
+// Extracts a 64-bit integer from |l|, selected with |imm|.
+template <int imm>
+uint64_t V128_Extract64(const V128 l);
+
+// Extracts the low 64 bits from V128.
+int64_t V128_Low64(const V128 l);
+
+// Add packed 64-bit integers in |l| and |r|.
+V128 V128_Add64(const V128 l, const V128 r);
+
+#if defined(__AVX__)
+inline V256 V256_LoadU(const V256* src);
+inline V256 V256_Broadcast128(const V128* src);
+#else
+template <typename T = V256>
+T V256_LoadU(const T* src);
+
+template <typename T = V256>
+T V256_Broadcast128(const V128* src);
+#endif
+
+#endif
+
+#if defined(ABSL_CRC_INTERNAL_HAVE_X86_SIMD)
+
+inline uint32_t CRC32_u8(uint32_t crc, uint8_t v) {
+  return _mm_crc32_u8(crc, v);
+}
+
+inline uint32_t CRC32_u16(uint32_t crc, uint16_t v) {
+  return _mm_crc32_u16(crc, v);
+}
+
+inline uint32_t CRC32_u32(uint32_t crc, uint32_t v) {
+  return _mm_crc32_u32(crc, v);
+}
+
+inline uint32_t CRC32_u64(uint32_t crc, uint64_t v) {
+  return static_cast<uint32_t>(_mm_crc32_u64(crc, v));
+}
+
+inline V128 V128_Load(const V128* src) { return _mm_load_si128(src); }
+
+inline V128 V128_LoadU(const V128* src) { return _mm_loadu_si128(src); }
+
+inline void V128_Store(V128* dst, V128 data) { _mm_store_si128(dst, data); }
+
+inline V128 V128_PMulHi(const V128 l, const V128 r) {
+  return _mm_clmulepi64_si128(l, r, 0x11);
+}
+
+inline V128 V128_PMulLow(const V128 l, const V128 r) {
+  return _mm_clmulepi64_si128(l, r, 0x00);
+}
+
+inline V128 V128_PMul01(const V128 l, const V128 r) {
+  return _mm_clmulepi64_si128(l, r, 0x01);
+}
+
+inline V128 V128_PMul10(const V128 l, const V128 r) {
+  return _mm_clmulepi64_si128(l, r, 0x10);
+}
+
+inline V128 V128_Xor(const V128 l, const V128 r) { return _mm_xor_si128(l, r); }
+
+template <bool kUseEor3>
+inline V128 V128_Xor3(const V128 a, const V128 b, const V128 c) {
+  return V128_Xor(V128_Xor(a, b), c);
+}
+
+inline V128 V128_From64WithZeroFill(const uint64_t r) {
+  return _mm_set_epi64x(static_cast<int64_t>(0), static_cast<int64_t>(r));
+}
+
+template <int imm>
+inline int V128_Extract32(const V128 l) {
+  return _mm_extract_epi32(l, imm);
+}
+
+template <int imm>
+inline uint64_t V128_Extract64(const V128 l) {
+  return static_cast<uint64_t>(_mm_extract_epi64(l, imm));
+}
+
+inline int64_t V128_Low64(const V128 l) { return _mm_cvtsi128_si64(l); }
+
+inline V128 V128_Add64(const V128 l, const V128 r) {
+  return _mm_add_epi64(l, r);
+}
+
+#elif defined(ABSL_CRC_INTERNAL_HAVE_ARM_SIMD)
+
+inline uint32_t CRC32_u8(uint32_t crc, uint8_t v) { return __crc32cb(crc, v); }
+
+inline uint32_t CRC32_u16(uint32_t crc, uint16_t v) {
+  return __crc32ch(crc, v);
+}
+
+inline uint32_t CRC32_u32(uint32_t crc, uint32_t v) {
+  return __crc32cw(crc, v);
+}
+
+inline uint32_t CRC32_u64(uint32_t crc, uint64_t v) {
+  return __crc32cd(crc, v);
+}
+
+inline V128 V128_Load(const V128* src) {
+  return vld1q_u64(reinterpret_cast<const uint64_t*>(src));
+}
+
+inline V128 V128_LoadU(const V128* src) {
+  return vld1q_u64(reinterpret_cast<const uint64_t*>(src));
+}
+
+inline void V128_Store(V128* dst, V128 data) {
+  vst1q_u64(reinterpret_cast<uint64_t*>(dst), data);
+}
+
+// Using inline assembly as clang does not generate the pmull2 instruction and
+// performance drops by 15-20%.
+// TODO(b/193678732): Investigate why there is a slight performance hit when
+// using intrinsics instead of inline assembly.
+inline V128 V128_PMulHi(const V128 l, const V128 r) {
+  uint64x2_t res;
+  __asm__ __volatile__("pmull2 %0.1q, %1.2d, %2.2d \n\t"
+                       : "=w"(res)
+                       : "w"(l), "w"(r));
+  return res;
+}
+
+// TODO(b/193678732): Investigate why the compiler decides to move the constant
+// loop multiplicands from GPR to Neon registers every loop iteration.
+inline V128 V128_PMulLow(const V128 l, const V128 r) {
+  uint64x2_t res;
+  __asm__ __volatile__("pmull %0.1q, %1.1d, %2.1d \n\t"
+                       : "=w"(res)
+                       : "w"(l), "w"(r));
+  return res;
+}
+
+inline V128 V128_PMul01(const V128 l, const V128 r) {
+  return reinterpret_cast<V128>(vmull_p64(
+      reinterpret_cast<poly64_t>(vget_high_p64(vreinterpretq_p64_u64(l))),
+      reinterpret_cast<poly64_t>(vget_low_p64(vreinterpretq_p64_u64(r)))));
+}
+
+inline V128 V128_PMul10(const V128 l, const V128 r) {
+  return reinterpret_cast<V128>(vmull_p64(
+      reinterpret_cast<poly64_t>(vget_low_p64(vreinterpretq_p64_u64(l))),
+      reinterpret_cast<poly64_t>(vget_high_p64(vreinterpretq_p64_u64(r)))));
+}
+
+inline V128 V128_Xor(const V128 l, const V128 r) { return veorq_u64(l, r); }
+
+template <bool kUseEor3>
+inline V128 V128_Xor3(const V128 a, const V128 b, const V128 c) {
+#ifndef __ARM_FEATURE_SHA3
+  if constexpr (kUseEor3) {
+    // If the binary is compiled without SHA3 support, we need inline assembly
+    // to use the EOR3 instruction. We want to only use the below inline
+    // assembly block when both the CPU supports EOR3 and the binary is built
+    // without it, since we remove the sha3 extension support after the eor3
+    // instruction.
+    uint64x2_t res;
+    __asm__ __volatile__(
+        ".arch_extension sha3 \n\t"
+        "eor3 %0.16b, %1.16b, %2.16b, %3.16b \n\t"
+        ".arch_extension nosha3 \n\t"
+        : "=w"(res)
+        : "w"(a), "w"(b), "w"(c));
+    return res;
+  }
+#endif
+  return V128_Xor(V128_Xor(a, b), c);
+}
+
+inline V128 V128_From64WithZeroFill(const uint64_t r){
+  constexpr uint64x2_t kZero = {0, 0};
+  return vsetq_lane_u64(r, kZero, 0);
+}
+
+
+template <int imm>
+inline int V128_Extract32(const V128 l) {
+  return vgetq_lane_s32(vreinterpretq_s32_u64(l), imm);
+}
+
+template <int imm>
+inline uint64_t V128_Extract64(const V128 l) {
+  return vgetq_lane_u64(l, imm);
+}
+
+inline int64_t V128_Low64(const V128 l) {
+  return vgetq_lane_s64(vreinterpretq_s64_u64(l), 0);
+}
+
+inline V128 V128_Add64(const V128 l, const V128 r) { return vaddq_u64(l, r); }
+
+#endif
+
+#if defined(__AVX__) && defined(ABSL_CRC_INTERNAL_HAVE_X86_SIMD)
+inline V256 V256_LoadU(const V256* src) { return _mm256_loadu_si256(src); }
+
+inline V256 V256_Broadcast128(const V128* src) {
+  return _mm256_castps_si256(
+      _mm256_broadcast_ps(reinterpret_cast<const __m128*>(src)));
+}
+#elif defined(ABSL_CRC_INTERNAL_HAVE_X86_SIMD) || \
+    defined(ABSL_CRC_INTERNAL_HAVE_ARM_SIMD)
+template <typename T>
+inline T V256_LoadU(const T* src) {
+  (void)src;
+  return T{};
+}
+
+template <typename T>
+inline T V256_Broadcast128(const V128* src) {
+  (void)src;
+  return T{};
+}
+#endif
+
+}  // namespace crc_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_CRC_INTERNAL_CRC32_X86_ARM_COMBINED_SIMD_H_
diff --git a/absl/crc/internal/crc32c.h b/absl/crc/internal/crc32c.h
new file mode 100644
index 00000000000..20e9b73e8a1
--- /dev/null
+++ b/absl/crc/internal/crc32c.h
@@ -0,0 +1,41 @@
+// Copyright 2022 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_CRC_INTERNAL_CRC32C_H_
+#define ABSL_CRC_INTERNAL_CRC32C_H_
+
+#include <cstddef>
+
+#include "absl/base/config.h"
+#include "absl/crc/crc32c.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace crc_internal {
+
+// Modifies a CRC32 value by removing `length` bytes with a value of 0 from
+// the end of the string.
+//
+// This is the inverse operation of ExtendCrc32cByZeroes().
+//
+// This operation has a runtime cost of O(log(`length`))
+//
+// Internal implementation detail, exposed for testing only.
+crc32c_t UnextendCrc32cByZeroes(crc32c_t initial_crc, size_t length);
+
+}  // namespace crc_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_CRC_INTERNAL_CRC32C_H_
diff --git a/absl/crc/internal/crc32c_inline.h b/absl/crc/internal/crc32c_inline.h
new file mode 100644
index 00000000000..e117c8697ce
--- /dev/null
+++ b/absl/crc/internal/crc32c_inline.h
@@ -0,0 +1,73 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_CRC_INTERNAL_CRC32C_INLINE_H_
+#define ABSL_CRC_INTERNAL_CRC32C_INLINE_H_
+
+#include <cstddef>
+#include <cstdint>
+
+#include "absl/base/config.h"
+#include "absl/base/internal/endian.h"
+#include "absl/crc/internal/crc32_x86_arm_combined_simd.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace crc_internal {
+
+// CRC32C implementation optimized for small inputs.
+// Either computes crc and return true, or if there is
+// no hardware support does nothing and returns false.
+inline bool ExtendCrc32cInline(uint32_t* crc, const char* p, size_t n) {
+#if defined(ABSL_CRC_INTERNAL_HAVE_ARM_SIMD) || \
+    defined(ABSL_CRC_INTERNAL_HAVE_X86_SIMD)
+  constexpr uint32_t kCrc32Xor = 0xffffffffU;
+  *crc ^= kCrc32Xor;
+  if (n & 1) {
+    *crc = CRC32_u8(*crc, static_cast<uint8_t>(*p));
+    n--;
+    p++;
+  }
+  if (n & 2) {
+    *crc = CRC32_u16(*crc, absl::little_endian::Load16(p));
+    n -= 2;
+    p += 2;
+  }
+  if (n & 4) {
+    *crc = CRC32_u32(*crc, absl::little_endian::Load32(p));
+    n -= 4;
+    p += 4;
+  }
+  while (n) {
+    *crc = CRC32_u64(*crc, absl::little_endian::Load64(p));
+    n -= 8;
+    p += 8;
+  }
+  *crc ^= kCrc32Xor;
+  return true;
+#else
+  // No hardware support, signal the need to fallback.
+  static_cast<void>(crc);
+  static_cast<void>(p);
+  static_cast<void>(n);
+  return false;
+#endif  // defined(ABSL_CRC_INTERNAL_HAVE_ARM_SIMD) ||
+        // defined(ABSL_CRC_INTERNAL_HAVE_X86_SIMD)
+}
+
+}  // namespace crc_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_CRC_INTERNAL_CRC32C_INLINE_H_
diff --git a/absl/crc/internal/crc_cord_state.cc b/absl/crc/internal/crc_cord_state.cc
new file mode 100644
index 00000000000..cccd0b06c11
--- /dev/null
+++ b/absl/crc/internal/crc_cord_state.cc
@@ -0,0 +1,135 @@
+// Copyright 2022 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/crc/internal/crc_cord_state.h"
+
+#include <atomic>
+#include <cassert>
+#include <cstddef>
+#include <cstdint>
+
+#include "absl/base/config.h"
+#include "absl/base/no_destructor.h"
+#include "absl/crc/crc32c.h"
+#include "absl/numeric/bits.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace crc_internal {
+
+CrcCordState::RefcountedRep* CrcCordState::RefSharedEmptyRep() {
+  static absl::NoDestructor<CrcCordState::RefcountedRep> empty;
+
+  assert(empty->count.load(std::memory_order_relaxed) >= 1);
+  assert(empty->rep.removed_prefix.length == 0);
+  assert(empty->rep.prefix_crc.empty());
+
+  Ref(empty.get());
+  return empty.get();
+}
+
+CrcCordState::CrcCordState() : refcounted_rep_(new RefcountedRep) {}
+
+CrcCordState::CrcCordState(const CrcCordState& other)
+    : refcounted_rep_(other.refcounted_rep_) {
+  Ref(refcounted_rep_);
+}
+
+CrcCordState::CrcCordState(CrcCordState&& other)
+    : refcounted_rep_(other.refcounted_rep_) {
+  // Make `other` valid for use after move.
+  other.refcounted_rep_ = RefSharedEmptyRep();
+}
+
+CrcCordState& CrcCordState::operator=(const CrcCordState& other) {
+  if (this != &other) {
+    Unref(refcounted_rep_);
+    refcounted_rep_ = other.refcounted_rep_;
+    Ref(refcounted_rep_);
+  }
+  return *this;
+}
+
+CrcCordState& CrcCordState::operator=(CrcCordState&& other) {
+  if (this != &other) {
+    Unref(refcounted_rep_);
+    refcounted_rep_ = other.refcounted_rep_;
+    // Make `other` valid for use after move.
+    other.refcounted_rep_ = RefSharedEmptyRep();
+  }
+  return *this;
+}
+
+CrcCordState::~CrcCordState() {
+  Unref(refcounted_rep_);
+}
+
+crc32c_t CrcCordState::Checksum() const {
+  if (rep().prefix_crc.empty()) {
+    return absl::crc32c_t{0};
+  }
+  if (IsNormalized()) {
+    return rep().prefix_crc.back().crc;
+  }
+  return absl::RemoveCrc32cPrefix(
+      rep().removed_prefix.crc, rep().prefix_crc.back().crc,
+      rep().prefix_crc.back().length - rep().removed_prefix.length);
+}
+
+CrcCordState::PrefixCrc CrcCordState::NormalizedPrefixCrcAtNthChunk(
+    size_t n) const {
+  assert(n < NumChunks());
+  if (IsNormalized()) {
+    return rep().prefix_crc[n];
+  }
+  size_t length = rep().prefix_crc[n].length - rep().removed_prefix.length;
+  return PrefixCrc(length,
+                   absl::RemoveCrc32cPrefix(rep().removed_prefix.crc,
+                                            rep().prefix_crc[n].crc, length));
+}
+
+void CrcCordState::Normalize() {
+  if (IsNormalized() || rep().prefix_crc.empty()) {
+    return;
+  }
+
+  Rep* r = mutable_rep();
+  for (auto& prefix_crc : r->prefix_crc) {
+    size_t remaining = prefix_crc.length - r->removed_prefix.length;
+    prefix_crc.crc = absl::RemoveCrc32cPrefix(r->removed_prefix.crc,
+                                              prefix_crc.crc, remaining);
+    prefix_crc.length = remaining;
+  }
+  r->removed_prefix = PrefixCrc();
+}
+
+void CrcCordState::Poison() {
+  Rep* rep = mutable_rep();
+  if (NumChunks() > 0) {
+    for (auto& prefix_crc : rep->prefix_crc) {
+      // This is basically CRC32::Scramble().
+      uint32_t crc = static_cast<uint32_t>(prefix_crc.crc);
+      crc += 0x2e76e41b;
+      crc = absl::rotr(crc, 17);
+      prefix_crc.crc = crc32c_t{crc};
+    }
+  } else {
+    // Add a fake corrupt chunk.
+    rep->prefix_crc.emplace_back(0, crc32c_t{1});
+  }
+}
+
+}  // namespace crc_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/crc/internal/crc_cord_state.h b/absl/crc/internal/crc_cord_state.h
new file mode 100644
index 00000000000..a68e0b327b8
--- /dev/null
+++ b/absl/crc/internal/crc_cord_state.h
@@ -0,0 +1,161 @@
+// Copyright 2022 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_CRC_INTERNAL_CRC_CORD_STATE_H_
+#define ABSL_CRC_INTERNAL_CRC_CORD_STATE_H_
+
+#include <atomic>
+#include <cassert>
+#include <cstddef>
+#include <cstdint>
+#include <deque>
+
+#include "absl/base/config.h"
+#include "absl/crc/crc32c.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace crc_internal {
+
+// CrcCordState is a copy-on-write class that holds the chunked CRC32C data
+// that allows CrcCord to perform efficient substring operations. CrcCordState
+// is used as a member variable in CrcCord. When a CrcCord is converted to a
+// Cord, the CrcCordState is shallow-copied into the root node of the Cord. If
+// the converted Cord is modified outside of CrcCord, the CrcCordState is
+// discarded from the Cord. If the Cord is converted back to a CrcCord, and the
+// Cord is still carrying the CrcCordState in its root node, the CrcCord can
+// re-use the CrcCordState, making the construction of the CrcCord cheap.
+//
+// CrcCordState does not try to encapsulate the CRC32C state (CrcCord requires
+// knowledge of how CrcCordState represents the CRC32C state). It does
+// encapsulate the copy-on-write nature of the state.
+class CrcCordState {
+ public:
+  // Constructors.
+  CrcCordState();
+  CrcCordState(const CrcCordState&);
+  CrcCordState(CrcCordState&&);
+
+  // Destructor. Atomically unreferences the data.
+  ~CrcCordState();
+
+  // Copy and move operators.
+  CrcCordState& operator=(const CrcCordState&);
+  CrcCordState& operator=(CrcCordState&&);
+
+  // A (length, crc) pair.
+  struct PrefixCrc {
+    PrefixCrc() = default;
+    PrefixCrc(size_t length_arg, absl::crc32c_t crc_arg)
+        : length(length_arg), crc(crc_arg) {}
+
+    size_t length = 0;
+
+    // TODO(absl-team): Memory stomping often zeros out memory. If this struct
+    // gets overwritten, we could end up with {0, 0}, which is the correct CRC
+    // for a string of length 0. Consider storing a scrambled value and
+    // unscrambling it before verifying it.
+    absl::crc32c_t crc = absl::crc32c_t{0};
+  };
+
+  // The representation of the chunked CRC32C data.
+  struct Rep {
+    // `removed_prefix` is the crc and length of any prefix that has been
+    // removed from the Cord (for example, by calling
+    // `CrcCord::RemovePrefix()`). To get the checksum of any prefix of the
+    // cord, this value must be subtracted from `prefix_crc`. See `Checksum()`
+    // for an example.
+    //
+    // CrcCordState is said to be "normalized" if removed_prefix.length == 0.
+    PrefixCrc removed_prefix;
+
+    // A deque of (length, crc) pairs, representing length and crc of a prefix
+    // of the Cord, before removed_prefix has been subtracted. The lengths of
+    // the prefixes are stored in increasing order. If the Cord is not empty,
+    // the last value in deque is the contains the CRC32C of the entire Cord
+    // when removed_prefix is subtracted from it.
+    std::deque<PrefixCrc> prefix_crc;
+  };
+
+  // Returns a reference to the representation of the chunked CRC32C data.
+  const Rep& rep() const { return refcounted_rep_->rep; }
+
+  // Returns a mutable reference to the representation of the chunked CRC32C
+  // data. Calling this function will copy the data if another instance also
+  // holds a reference to the data, so it is important to call rep() instead if
+  // the data may not be mutated.
+  Rep* mutable_rep() {
+    if (refcounted_rep_->count.load(std::memory_order_acquire) != 1) {
+      RefcountedRep* copy = new RefcountedRep;
+      copy->rep = refcounted_rep_->rep;
+      Unref(refcounted_rep_);
+      refcounted_rep_ = copy;
+    }
+    return &refcounted_rep_->rep;
+  }
+
+  // Returns the CRC32C of the entire Cord.
+  absl::crc32c_t Checksum() const;
+
+  // Returns true if the chunked CRC32C cached is normalized.
+  bool IsNormalized() const { return rep().removed_prefix.length == 0; }
+
+  // Normalizes the chunked CRC32C checksum cache by subtracting any removed
+  // prefix from the chunks.
+  void Normalize();
+
+  // Returns the number of cached chunks.
+  size_t NumChunks() const { return rep().prefix_crc.size(); }
+
+  // Helper that returns the (length, crc) of the `n`-th cached chunked.
+  PrefixCrc NormalizedPrefixCrcAtNthChunk(size_t n) const;
+
+  // Poisons all chunks to so that Checksum() will likely be incorrect with high
+  // probability.
+  void Poison();
+
+ private:
+  struct RefcountedRep {
+    std::atomic<int32_t> count{1};
+    Rep rep;
+  };
+
+  // Adds a reference to the shared global empty `RefcountedRep`, and returns a
+  // pointer to the `RefcountedRep`. This is an optimization to avoid unneeded
+  // allocations when the allocation is unlikely to ever be used. The returned
+  // pointer can be `Unref()`ed when it is no longer needed.  Since the returned
+  // instance will always have a reference counter greater than 1, attempts to
+  // modify it (by calling `mutable_rep()`) will create a new unshared copy.
+  static RefcountedRep* RefSharedEmptyRep();
+
+  static void Ref(RefcountedRep* r) {
+    assert(r != nullptr);
+    r->count.fetch_add(1, std::memory_order_relaxed);
+  }
+
+  static void Unref(RefcountedRep* r) {
+    assert(r != nullptr);
+    if (r->count.fetch_sub(1, std::memory_order_acq_rel) == 1) {
+      delete r;
+    }
+  }
+
+  RefcountedRep* refcounted_rep_;
+};
+
+}  // namespace crc_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_CRC_INTERNAL_CRC_CORD_STATE_H_
diff --git a/absl/crc/internal/crc_cord_state_test.cc b/absl/crc/internal/crc_cord_state_test.cc
new file mode 100644
index 00000000000..e2c8e3cda13
--- /dev/null
+++ b/absl/crc/internal/crc_cord_state_test.cc
@@ -0,0 +1,124 @@
+// Copyright 2022 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/crc/internal/crc_cord_state.h"
+
+#include <algorithm>
+#include <cstdint>
+#include <string>
+#include <utility>
+
+#include "gtest/gtest.h"
+#include "absl/crc/crc32c.h"
+
+namespace {
+
+TEST(CrcCordState, Default) {
+  absl::crc_internal::CrcCordState state;
+  EXPECT_TRUE(state.IsNormalized());
+  EXPECT_EQ(state.Checksum(), absl::crc32c_t{0});
+  state.Normalize();
+  EXPECT_EQ(state.Checksum(), absl::crc32c_t{0});
+}
+
+TEST(CrcCordState, Normalize) {
+  absl::crc_internal::CrcCordState state;
+  auto* rep = state.mutable_rep();
+  rep->prefix_crc.push_back(
+      absl::crc_internal::CrcCordState::PrefixCrc(1000, absl::crc32c_t{1000}));
+  rep->prefix_crc.push_back(
+      absl::crc_internal::CrcCordState::PrefixCrc(2000, absl::crc32c_t{2000}));
+  rep->removed_prefix =
+      absl::crc_internal::CrcCordState::PrefixCrc(500, absl::crc32c_t{500});
+
+  // The removed_prefix means state is not normalized.
+  EXPECT_FALSE(state.IsNormalized());
+
+  absl::crc32c_t crc = state.Checksum();
+  state.Normalize();
+  EXPECT_TRUE(state.IsNormalized());
+
+  // The checksum should not change as a result of calling Normalize().
+  EXPECT_EQ(state.Checksum(), crc);
+  EXPECT_EQ(rep->removed_prefix.length, 0);
+}
+
+TEST(CrcCordState, Copy) {
+  absl::crc_internal::CrcCordState state;
+  auto* rep = state.mutable_rep();
+  rep->prefix_crc.push_back(
+      absl::crc_internal::CrcCordState::PrefixCrc(1000, absl::crc32c_t{1000}));
+
+  absl::crc_internal::CrcCordState copy = state;
+
+  EXPECT_EQ(state.Checksum(), absl::crc32c_t{1000});
+  EXPECT_EQ(copy.Checksum(), absl::crc32c_t{1000});
+}
+
+TEST(CrcCordState, UnsharedSelfCopy) {
+  absl::crc_internal::CrcCordState state;
+  auto* rep = state.mutable_rep();
+  rep->prefix_crc.push_back(
+      absl::crc_internal::CrcCordState::PrefixCrc(1000, absl::crc32c_t{1000}));
+
+  const absl::crc_internal::CrcCordState& ref = state;
+  state = ref;
+
+  EXPECT_EQ(state.Checksum(), absl::crc32c_t{1000});
+}
+
+TEST(CrcCordState, Move) {
+  absl::crc_internal::CrcCordState state;
+  auto* rep = state.mutable_rep();
+  rep->prefix_crc.push_back(
+      absl::crc_internal::CrcCordState::PrefixCrc(1000, absl::crc32c_t{1000}));
+
+  absl::crc_internal::CrcCordState moved = std::move(state);
+  EXPECT_EQ(moved.Checksum(), absl::crc32c_t{1000});
+}
+
+TEST(CrcCordState, UnsharedSelfMove) {
+  absl::crc_internal::CrcCordState state;
+  auto* rep = state.mutable_rep();
+  rep->prefix_crc.push_back(
+      absl::crc_internal::CrcCordState::PrefixCrc(1000, absl::crc32c_t{1000}));
+
+  absl::crc_internal::CrcCordState& ref = state;
+  state = std::move(ref);
+
+  EXPECT_EQ(state.Checksum(), absl::crc32c_t{1000});
+}
+
+TEST(CrcCordState, PoisonDefault) {
+  absl::crc_internal::CrcCordState state;
+  state.Poison();
+  EXPECT_NE(state.Checksum(), absl::crc32c_t{0});
+}
+
+TEST(CrcCordState, PoisonData) {
+  absl::crc_internal::CrcCordState state;
+  auto* rep = state.mutable_rep();
+  rep->prefix_crc.push_back(
+      absl::crc_internal::CrcCordState::PrefixCrc(1000, absl::crc32c_t{1000}));
+  rep->prefix_crc.push_back(
+      absl::crc_internal::CrcCordState::PrefixCrc(2000, absl::crc32c_t{2000}));
+  rep->removed_prefix =
+      absl::crc_internal::CrcCordState::PrefixCrc(500, absl::crc32c_t{500});
+
+  absl::crc32c_t crc = state.Checksum();
+  state.Poison();
+  EXPECT_NE(state.Checksum(), crc);
+}
+
+}  // namespace
diff --git a/absl/crc/internal/crc_internal.h b/absl/crc/internal/crc_internal.h
new file mode 100644
index 00000000000..ac9c38d68d8
--- /dev/null
+++ b/absl/crc/internal/crc_internal.h
@@ -0,0 +1,163 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_CRC_INTERNAL_CRC_INTERNAL_H_
+#define ABSL_CRC_INTERNAL_CRC_INTERNAL_H_
+
+#include <cstddef>
+#include <cstdint>
+#include <memory>
+#include <vector>
+
+#include "absl/base/internal/raw_logging.h"
+#include "absl/base/optimization.h"
+#include "absl/crc/internal/crc.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+namespace crc_internal {
+
+// Prefetch constants used in some Extend() implementations
+constexpr int kPrefetchHorizon = ABSL_CACHELINE_SIZE * 4;  // Prefetch this far
+// Shorter prefetch distance for smaller buffers
+constexpr int kPrefetchHorizonMedium = ABSL_CACHELINE_SIZE * 1;
+static_assert(kPrefetchHorizon >= 64, "CRCPrefetchHorizon less than loop len");
+
+// We require the Scramble() function:
+//  - to be reversible (Unscramble() must exist)
+//  - to be non-linear in the polynomial's Galois field (so the CRC of a
+//    scrambled CRC is not linearly affected by the scrambled CRC, even if
+//    using the same polynomial)
+//  - not to be its own inverse.  Preferably, if X=Scramble^N(X) and N!=0, then
+//    N is large.
+//  - to be fast.
+//  - not to change once defined.
+// We introduce non-linearity in two ways:
+//     Addition of a constant.
+//         - The carries introduce non-linearity; we use bits of an irrational
+//           (phi) to make it unlikely that we introduce no carries.
+//     Rotate by a constant number of bits.
+//         - We use floor(degree/2)+1, which does not divide the degree, and
+//           splits the bits nearly evenly, which makes it less likely the
+//           halves will be the same or one will be all zeroes.
+// We do both things to improve the chances of non-linearity in the face of
+// bit patterns with low numbers of bits set, while still being fast.
+// Below is the constant that we add.  The bits are the first 128 bits of the
+// fractional part of phi, with a 1 ored into the bottom bit to maximize the
+// cycle length of repeated adds.
+constexpr uint64_t kScrambleHi = (static_cast<uint64_t>(0x4f1bbcdcU) << 32) |
+                                 static_cast<uint64_t>(0xbfa53e0aU);
+constexpr uint64_t kScrambleLo = (static_cast<uint64_t>(0xf9ce6030U) << 32) |
+                                 static_cast<uint64_t>(0x2e76e41bU);
+
+class CRCImpl : public CRC {  // Implementation of the abstract class CRC
+ public:
+  using Uint32By256 = uint32_t[256];
+
+  CRCImpl() = default;
+  ~CRCImpl() override = default;
+
+  // The internal version of CRC::New().
+  static CRCImpl* NewInternal();
+
+  // Fill in a table for updating a CRC by one word of 'word_size' bytes
+  // [last_lo, last_hi] contains the answer if the last bit in the word
+  // is set.
+  static void FillWordTable(uint32_t poly, uint32_t last, int word_size,
+                            Uint32By256* t);
+
+  // Build the table for extending by zeroes, returning the number of entries.
+  // For a in {1, 2, ..., ZEROES_BASE-1}, b in {0, 1, 2, 3, ...},
+  // entry j=a-1+(ZEROES_BASE-1)*b
+  // contains a polynomial Pi such that multiplying
+  // a CRC by Pi mod P, where P is the CRC polynomial, is equivalent to
+  // appending a*2**(ZEROES_BASE_LG*b) zero bytes to the original string.
+  static int FillZeroesTable(uint32_t poly, Uint32By256* t);
+
+  virtual void InitTables() = 0;
+
+ private:
+  CRCImpl(const CRCImpl&) = delete;
+  CRCImpl& operator=(const CRCImpl&) = delete;
+};
+
+// This is the 32-bit implementation.  It handles all sizes from 8 to 32.
+class CRC32 : public CRCImpl {
+ public:
+  CRC32() = default;
+  ~CRC32() override = default;
+
+  void Extend(uint32_t* crc, const void* bytes, size_t length) const override;
+  void ExtendByZeroes(uint32_t* crc, size_t length) const override;
+  void Scramble(uint32_t* crc) const override;
+  void Unscramble(uint32_t* crc) const override;
+  void UnextendByZeroes(uint32_t* crc, size_t length) const override;
+
+  void InitTables() override;
+
+ private:
+  // Common implementation guts for ExtendByZeroes and UnextendByZeroes().
+  //
+  // zeroes_table is a table as returned by FillZeroesTable(), containing
+  // polynomials representing CRCs of strings-of-zeros of various lengths,
+  // and which can be combined by polynomial multiplication.  poly_table is
+  // a table of CRC byte extension values.  These tables are determined by
+  // the generator polynomial.
+  //
+  // These will be set to reverse_zeroes_ and reverse_table0_ for Unextend, and
+  // CRC32::zeroes_ and CRC32::table0_ for Extend.
+  static void ExtendByZeroesImpl(uint32_t* crc, size_t length,
+                                 const uint32_t zeroes_table[256],
+                                 const uint32_t poly_table[256]);
+
+  uint32_t table0_[256];  // table of byte extensions
+  uint32_t zeroes_[256];  // table of zero extensions
+
+  // table of 4-byte extensions shifted by 12 bytes of zeroes
+  uint32_t table_[4][256];
+
+  // Reverse lookup tables, using the alternate polynomial used by
+  // UnextendByZeroes().
+  uint32_t reverse_table0_[256];  // table of reverse byte extensions
+  uint32_t reverse_zeroes_[256];  // table of reverse zero extensions
+
+  CRC32(const CRC32&) = delete;
+  CRC32& operator=(const CRC32&) = delete;
+};
+
+// Helpers
+
+// RoundUp<N>(p) returns the lowest address >= p aligned to an N-byte
+// boundary.  Requires that N is a power of 2.
+template <int alignment>
+const uint8_t* RoundUp(const uint8_t* p) {
+  static_assert((alignment & (alignment - 1)) == 0, "alignment is not 2^n");
+  constexpr uintptr_t mask = alignment - 1;
+  const uintptr_t as_uintptr = reinterpret_cast<uintptr_t>(p);
+  return reinterpret_cast<const uint8_t*>((as_uintptr + mask) & ~mask);
+}
+
+// Return a newly created CRC32AcceleratedX86ARMCombined if we can use Intel's
+// or ARM's CRC acceleration for a given polynomial.  Return nullptr otherwise.
+CRCImpl* TryNewCRC32AcceleratedX86ARMCombined();
+
+// Return all possible hardware accelerated implementations. For testing only.
+std::vector<std::unique_ptr<CRCImpl>> NewCRC32AcceleratedX86ARMCombinedAll();
+
+}  // namespace crc_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_CRC_INTERNAL_CRC_INTERNAL_H_
diff --git a/absl/crc/internal/crc_memcpy.h b/absl/crc/internal/crc_memcpy.h
new file mode 100644
index 00000000000..a0fed65afa2
--- /dev/null
+++ b/absl/crc/internal/crc_memcpy.h
@@ -0,0 +1,122 @@
+// Copyright 2022 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_CRC_INTERNAL_CRC_MEMCPY_H_
+#define ABSL_CRC_INTERNAL_CRC_MEMCPY_H_
+
+#include <cstddef>
+#include <memory>
+
+#include "absl/base/config.h"
+#include "absl/crc/crc32c.h"
+#include "absl/crc/internal/crc32_x86_arm_combined_simd.h"
+
+// Defined if the class AcceleratedCrcMemcpyEngine exists.
+// TODO(b/299127771): Consider relaxing the pclmul requirement once the other
+// intrinsics are conditionally compiled without it.
+#if defined(ABSL_CRC_INTERNAL_HAVE_X86_SIMD)
+#define ABSL_INTERNAL_HAVE_X86_64_ACCELERATED_CRC_MEMCPY_ENGINE 1
+#elif defined(ABSL_CRC_INTERNAL_HAVE_ARM_SIMD)
+#define ABSL_INTERNAL_HAVE_ARM_ACCELERATED_CRC_MEMCPY_ENGINE 1
+#endif
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace crc_internal {
+
+class CrcMemcpyEngine {
+ public:
+  virtual ~CrcMemcpyEngine() = default;
+
+  virtual crc32c_t Compute(void* __restrict dst, const void* __restrict src,
+                           std::size_t length, crc32c_t initial_crc) const = 0;
+
+ protected:
+  CrcMemcpyEngine() = default;
+};
+
+class CrcMemcpy {
+ public:
+  static crc32c_t CrcAndCopy(void* __restrict dst, const void* __restrict src,
+                             std::size_t length,
+                             crc32c_t initial_crc = crc32c_t{0},
+                             bool non_temporal = false) {
+    static const ArchSpecificEngines engines = GetArchSpecificEngines();
+    auto* engine = non_temporal ? engines.non_temporal : engines.temporal;
+    return engine->Compute(dst, src, length, initial_crc);
+  }
+
+  // For testing only: get an architecture-specific engine for tests.
+  static std::unique_ptr<CrcMemcpyEngine> GetTestEngine(int vector,
+                                                        int integer);
+
+ private:
+  struct ArchSpecificEngines {
+    CrcMemcpyEngine* temporal;
+    CrcMemcpyEngine* non_temporal;
+  };
+
+  static ArchSpecificEngines GetArchSpecificEngines();
+};
+
+// Fallback CRC-memcpy engine.
+class FallbackCrcMemcpyEngine : public CrcMemcpyEngine {
+ public:
+  FallbackCrcMemcpyEngine() = default;
+  FallbackCrcMemcpyEngine(const FallbackCrcMemcpyEngine&) = delete;
+  FallbackCrcMemcpyEngine operator=(const FallbackCrcMemcpyEngine&) = delete;
+
+  crc32c_t Compute(void* __restrict dst, const void* __restrict src,
+                   std::size_t length, crc32c_t initial_crc) const override;
+};
+
+// CRC Non-Temporal-Memcpy engine.
+class CrcNonTemporalMemcpyEngine : public CrcMemcpyEngine {
+ public:
+  CrcNonTemporalMemcpyEngine() = default;
+  CrcNonTemporalMemcpyEngine(const CrcNonTemporalMemcpyEngine&) = delete;
+  CrcNonTemporalMemcpyEngine operator=(const CrcNonTemporalMemcpyEngine&) =
+      delete;
+
+  crc32c_t Compute(void* __restrict dst, const void* __restrict src,
+                   std::size_t length, crc32c_t initial_crc) const override;
+};
+
+// CRC Non-Temporal-Memcpy AVX engine.
+class CrcNonTemporalMemcpyAVXEngine : public CrcMemcpyEngine {
+ public:
+  CrcNonTemporalMemcpyAVXEngine() = default;
+  CrcNonTemporalMemcpyAVXEngine(const CrcNonTemporalMemcpyAVXEngine&) = delete;
+  CrcNonTemporalMemcpyAVXEngine operator=(
+      const CrcNonTemporalMemcpyAVXEngine&) = delete;
+
+  crc32c_t Compute(void* __restrict dst, const void* __restrict src,
+                   std::size_t length, crc32c_t initial_crc) const override;
+};
+
+// Copy source to destination and return the CRC32C of the data copied.  If an
+// accelerated version is available, use the accelerated version, otherwise use
+// the generic fallback version.
+inline crc32c_t Crc32CAndCopy(void* __restrict dst, const void* __restrict src,
+                              std::size_t length,
+                              crc32c_t initial_crc = crc32c_t{0},
+                              bool non_temporal = false) {
+  return CrcMemcpy::CrcAndCopy(dst, src, length, initial_crc, non_temporal);
+}
+
+}  // namespace crc_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_CRC_INTERNAL_CRC_MEMCPY_H_
diff --git a/absl/crc/internal/crc_memcpy_fallback.cc b/absl/crc/internal/crc_memcpy_fallback.cc
new file mode 100644
index 00000000000..e34249f07de
--- /dev/null
+++ b/absl/crc/internal/crc_memcpy_fallback.cc
@@ -0,0 +1,78 @@
+// Copyright 2022 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <cstring>
+#include <memory>
+
+#include "absl/base/config.h"
+#include "absl/crc/crc32c.h"
+#include "absl/crc/internal/crc_memcpy.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace crc_internal {
+
+absl::crc32c_t FallbackCrcMemcpyEngine::Compute(void* __restrict dst,
+                                                const void* __restrict src,
+                                                std::size_t length,
+                                                crc32c_t initial_crc) const {
+  constexpr size_t kBlockSize = 8192;
+  absl::crc32c_t crc = initial_crc;
+
+  const char* src_bytes = reinterpret_cast<const char*>(src);
+  char* dst_bytes = reinterpret_cast<char*>(dst);
+
+  // Copy + CRC loop - run 8k chunks until we are out of full chunks.  CRC
+  // then copy was found to be slightly more efficient in our test cases.
+  std::size_t offset = 0;
+  for (; offset + kBlockSize < length; offset += kBlockSize) {
+    crc = absl::ExtendCrc32c(crc,
+                             absl::string_view(src_bytes + offset, kBlockSize));
+    memcpy(dst_bytes + offset, src_bytes + offset, kBlockSize);
+  }
+
+  // Save some work if length is 0.
+  if (offset < length) {
+    std::size_t final_copy_size = length - offset;
+    crc = absl::ExtendCrc32c(
+        crc, absl::string_view(src_bytes + offset, final_copy_size));
+    memcpy(dst_bytes + offset, src_bytes + offset, final_copy_size);
+  }
+
+  return crc;
+}
+
+// Compile the following only if we don't have
+#if !defined(ABSL_INTERNAL_HAVE_X86_64_ACCELERATED_CRC_MEMCPY_ENGINE) && \
+    !defined(ABSL_INTERNAL_HAVE_ARM_ACCELERATED_CRC_MEMCPY_ENGINE)
+
+CrcMemcpy::ArchSpecificEngines CrcMemcpy::GetArchSpecificEngines() {
+  CrcMemcpy::ArchSpecificEngines engines;
+  engines.temporal = new FallbackCrcMemcpyEngine();
+  engines.non_temporal = new FallbackCrcMemcpyEngine();
+  return engines;
+}
+
+std::unique_ptr<CrcMemcpyEngine> CrcMemcpy::GetTestEngine(int /*vector*/,
+                                                          int /*integer*/) {
+  return std::make_unique<FallbackCrcMemcpyEngine>();
+}
+
+#endif  // !ABSL_INTERNAL_HAVE_X86_64_ACCELERATED_CRC_MEMCPY_ENGINE &&
+        // !ABSL_INTERNAL_HAVE_ARM_ACCELERATED_CRC_MEMCPY_ENGINE
+
+}  // namespace crc_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/crc/internal/crc_memcpy_test.cc b/absl/crc/internal/crc_memcpy_test.cc
new file mode 100644
index 00000000000..045a1e5e189
--- /dev/null
+++ b/absl/crc/internal/crc_memcpy_test.cc
@@ -0,0 +1,177 @@
+// Copyright 2022 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/crc/internal/crc_memcpy.h"
+
+#include <cstddef>
+#include <cstdint>
+#include <cstring>
+#include <limits>
+#include <memory>
+#include <string>
+#include <utility>
+
+#include "gtest/gtest.h"
+#include "absl/crc/crc32c.h"
+#include "absl/memory/memory.h"
+#include "absl/random/distributions.h"
+#include "absl/random/random.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/string_view.h"
+
+namespace {
+
+enum CrcEngine {
+  ACCELERATED = 0,
+  NONTEMPORAL = 1,
+  FALLBACK = 2,
+};
+
+// Correctness tests:
+// - Every source/destination byte alignment 0-15, every size 0-511 bytes
+// - Arbitrarily aligned source, large size
+template <size_t max_size>
+class CrcMemcpyTest : public testing::Test {
+ protected:
+  CrcMemcpyTest() {
+    source_ = std::make_unique<char[]>(kSize);
+    destination_ = std::make_unique<char[]>(kSize);
+  }
+  static constexpr size_t kAlignment = 16;
+  static constexpr size_t kMaxCopySize = max_size;
+  static constexpr size_t kSize = kAlignment + kMaxCopySize;
+  std::unique_ptr<char[]> source_;
+  std::unique_ptr<char[]> destination_;
+
+  absl::BitGen gen_;
+};
+
+// Small test is slightly larger 4096 bytes to allow coverage of the "large"
+// copy function.  The minimum size to exercise all code paths in that function
+// would be around 256 consecutive tests (getting every possible tail value
+// and 0-2 small copy loops after the main block), so testing from 4096-4500
+// will cover all of those code paths multiple times.
+typedef CrcMemcpyTest<4500> CrcSmallTest;
+typedef CrcMemcpyTest<(1 << 24)> CrcLargeTest;
+// Parametrize the small test so that it can be done with all configurations.
+template <typename ParamsT>
+class EngineParamTestTemplate : public CrcSmallTest,
+                                public ::testing::WithParamInterface<ParamsT> {
+ protected:
+  EngineParamTestTemplate() {
+    if (GetParam().crc_engine_selector == FALLBACK) {
+      engine_ = std::make_unique<absl::crc_internal::FallbackCrcMemcpyEngine>();
+    } else if (GetParam().crc_engine_selector == NONTEMPORAL) {
+      engine_ =
+          std::make_unique<absl::crc_internal::CrcNonTemporalMemcpyEngine>();
+    } else {
+      engine_ = absl::crc_internal::CrcMemcpy::GetTestEngine(
+          GetParam().vector_lanes, GetParam().integer_lanes);
+    }
+  }
+
+  // Convenience method.
+  ParamsT GetParam() const {
+    return ::testing::WithParamInterface<ParamsT>::GetParam();
+  }
+
+  std::unique_ptr<absl::crc_internal::CrcMemcpyEngine> engine_;
+};
+struct TestParams {
+  CrcEngine crc_engine_selector = ACCELERATED;
+  int vector_lanes = 0;
+  int integer_lanes = 0;
+};
+using EngineParamTest = EngineParamTestTemplate<TestParams>;
+// SmallCorrectness is designed to exercise every possible set of code paths
+// in the memcpy code, not including the loop.
+TEST_P(EngineParamTest, SmallCorrectnessCheckSourceAlignment) {
+  constexpr size_t kTestSizes[] = {0, 100, 255, 512, 1024, 4000, kMaxCopySize};
+
+  for (size_t source_alignment = 0; source_alignment < kAlignment;
+       source_alignment++) {
+    for (auto size : kTestSizes) {
+      char* base_data = static_cast<char*>(source_.get()) + source_alignment;
+      for (size_t i = 0; i < size; i++) {
+        *(base_data + i) =
+            static_cast<char>(absl::Uniform<unsigned char>(gen_));
+      }
+      SCOPED_TRACE(absl::StrCat("engine=<", GetParam().vector_lanes, ",",
+                                GetParam().integer_lanes, ">, ", "size=", size,
+                                ", source_alignment=", source_alignment));
+      absl::crc32c_t initial_crc =
+          absl::crc32c_t{absl::Uniform<uint32_t>(gen_)};
+      absl::crc32c_t experiment_crc =
+          engine_->Compute(destination_.get(), source_.get() + source_alignment,
+                           size, initial_crc);
+      // Check the memory region to make sure it is the same
+      int mem_comparison =
+          memcmp(destination_.get(), source_.get() + source_alignment, size);
+      SCOPED_TRACE(absl::StrCat("Error in memcpy of size: ", size,
+                                " with source alignment: ", source_alignment));
+      ASSERT_EQ(mem_comparison, 0);
+      absl::crc32c_t baseline_crc = absl::ExtendCrc32c(
+          initial_crc,
+          absl::string_view(
+              static_cast<char*>(source_.get()) + source_alignment, size));
+      ASSERT_EQ(baseline_crc, experiment_crc);
+    }
+  }
+}
+
+TEST_P(EngineParamTest, SmallCorrectnessCheckDestAlignment) {
+  constexpr size_t kTestSizes[] = {0, 100, 255, 512, 1024, 4000, kMaxCopySize};
+
+  for (size_t dest_alignment = 0; dest_alignment < kAlignment;
+       dest_alignment++) {
+    for (auto size : kTestSizes) {
+      char* base_data = static_cast<char*>(source_.get());
+      for (size_t i = 0; i < size; i++) {
+        *(base_data + i) =
+            static_cast<char>(absl::Uniform<unsigned char>(gen_));
+      }
+      SCOPED_TRACE(absl::StrCat("engine=<", GetParam().vector_lanes, ",",
+                                GetParam().integer_lanes, ">, ", "size=", size,
+                                ", destination_alignment=", dest_alignment));
+      absl::crc32c_t initial_crc =
+          absl::crc32c_t{absl::Uniform<uint32_t>(gen_)};
+      absl::crc32c_t experiment_crc =
+          engine_->Compute(destination_.get() + dest_alignment, source_.get(),
+                           size, initial_crc);
+      // Check the memory region to make sure it is the same
+      int mem_comparison =
+          memcmp(destination_.get() + dest_alignment, source_.get(), size);
+      SCOPED_TRACE(absl::StrCat("Error in memcpy of size: ", size,
+                                " with dest alignment: ", dest_alignment));
+      ASSERT_EQ(mem_comparison, 0);
+      absl::crc32c_t baseline_crc = absl::ExtendCrc32c(
+          initial_crc,
+          absl::string_view(static_cast<char*>(source_.get()), size));
+      ASSERT_EQ(baseline_crc, experiment_crc);
+    }
+  }
+}
+
+INSTANTIATE_TEST_SUITE_P(EngineParamTest, EngineParamTest,
+                         ::testing::Values(
+                             // Tests for configurations that may occur in prod.
+                             TestParams{ACCELERATED, 3, 0},
+                             TestParams{ACCELERATED, 1, 2},
+                             TestParams{ACCELERATED, 1, 0},
+                             // Fallback test.
+                             TestParams{FALLBACK, 0, 0},
+                             // Non Temporal
+                             TestParams{NONTEMPORAL, 0, 0}));
+
+}  // namespace
diff --git a/absl/crc/internal/crc_memcpy_x86_arm_combined.cc b/absl/crc/internal/crc_memcpy_x86_arm_combined.cc
new file mode 100644
index 00000000000..dac0446349c
--- /dev/null
+++ b/absl/crc/internal/crc_memcpy_x86_arm_combined.cc
@@ -0,0 +1,455 @@
+// Copyright 2022 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Simultaneous memcopy and CRC-32C for x86-64 and ARM 64. Uses integer
+// registers because XMM registers do not support the CRC instruction (yet).
+// While copying, compute the running CRC of the data being copied.
+//
+// It is assumed that any CPU running this code has SSE4.2 instructions
+// available (for CRC32C).  This file will do nothing if that is not true.
+//
+// The CRC instruction has a 3-byte latency, and we are stressing the ALU ports
+// here (unlike a traditional memcopy, which has almost no ALU use), so we will
+// need to copy in such a way that the CRC unit is used efficiently. We have two
+// regimes in this code:
+//  1. For operations of size < kCrcSmallSize, do the CRC then the memcpy
+//  2. For operations of size > kCrcSmallSize:
+//      a) compute an initial CRC + copy on a small amount of data to align the
+//         destination pointer on a 16-byte boundary.
+//      b) Split the data into 3 main regions and a tail (smaller than 48 bytes)
+//      c) Do the copy and CRC of the 3 main regions, interleaving (start with
+//         full cache line copies for each region, then move to single 16 byte
+//         pieces per region).
+//      d) Combine the CRCs with CRC32C::Concat.
+//      e) Copy the tail and extend the CRC with the CRC of the tail.
+// This method is not ideal for op sizes between ~1k and ~8k because CRC::Concat
+// takes a significant amount of time.  A medium-sized approach could be added
+// using 3 CRCs over fixed-size blocks where the zero-extensions required for
+// CRC32C::Concat can be precomputed.
+
+#include <array>
+#include <cstddef>
+#include <cstdint>
+#include <cstring>
+#include <memory>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/cpu_detect.h"
+#include "absl/base/optimization.h"
+#include "absl/base/prefetch.h"
+#include "absl/crc/crc32c.h"
+#include "absl/crc/internal/crc32_x86_arm_combined_simd.h"
+#include "absl/crc/internal/crc_memcpy.h"
+#include "absl/strings/string_view.h"
+
+#if defined(ABSL_INTERNAL_HAVE_X86_64_ACCELERATED_CRC_MEMCPY_ENGINE) || \
+    defined(ABSL_INTERNAL_HAVE_ARM_ACCELERATED_CRC_MEMCPY_ENGINE)
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace crc_internal {
+
+using ::absl::base_internal::CpuType;
+using ::absl::base_internal::GetCpuType;
+
+namespace {
+
+inline crc32c_t ShortCrcCopy(char* dst, const char* src, std::size_t length,
+                             crc32c_t crc) {
+  // Small copy: just go 1 byte at a time: being nice to the branch predictor
+  // is more important here than anything else
+  uint32_t crc_uint32 = static_cast<uint32_t>(crc);
+  for (std::size_t i = 0; i < length; i++) {
+    uint8_t data = *reinterpret_cast<const uint8_t*>(src);
+    crc_uint32 = CRC32_u8(crc_uint32, data);
+    *reinterpret_cast<uint8_t*>(dst) = data;
+    ++src;
+    ++dst;
+  }
+  return crc32c_t{crc_uint32};
+}
+
+constexpr size_t kIntLoadsPerVec = sizeof(V128) / sizeof(uint64_t);
+
+// Common function for copying the tails of multiple large regions.
+// Disable ubsan for benign unaligned access. See b/254108538.
+template <size_t vec_regions, size_t int_regions>
+ABSL_ATTRIBUTE_NO_SANITIZE_UNDEFINED inline void LargeTailCopy(
+    crc32c_t* crcs, char** dst, const char** src, size_t region_size,
+    size_t copy_rounds) {
+  std::array<V128, vec_regions> data;
+  std::array<uint64_t, kIntLoadsPerVec * int_regions> int_data;
+
+  while (copy_rounds > 0) {
+    for (size_t i = 0; i < vec_regions; i++) {
+      size_t region = i;
+
+      auto* vsrc = reinterpret_cast<const V128*>(*src + region_size * region);
+      auto* vdst = reinterpret_cast<V128*>(*dst + region_size * region);
+
+      // Load the blocks, unaligned
+      data[i] = V128_LoadU(vsrc);
+
+      // Store the blocks, aligned
+      V128_Store(vdst, data[i]);
+
+      // Compute the running CRC
+      crcs[region] = crc32c_t{static_cast<uint32_t>(
+          CRC32_u64(static_cast<uint32_t>(crcs[region]),
+                    static_cast<uint64_t>(V128_Extract64<0>(data[i]))))};
+      crcs[region] = crc32c_t{static_cast<uint32_t>(
+          CRC32_u64(static_cast<uint32_t>(crcs[region]),
+                    static_cast<uint64_t>(V128_Extract64<1>(data[i]))))};
+    }
+
+    for (size_t i = 0; i < int_regions; i++) {
+      size_t region = vec_regions + i;
+
+      auto* usrc =
+          reinterpret_cast<const uint64_t*>(*src + region_size * region);
+      auto* udst = reinterpret_cast<uint64_t*>(*dst + region_size * region);
+
+      for (size_t j = 0; j < kIntLoadsPerVec; j++) {
+        size_t data_index = i * kIntLoadsPerVec + j;
+
+        int_data[data_index] = *(usrc + j);
+        crcs[region] = crc32c_t{CRC32_u64(static_cast<uint32_t>(crcs[region]),
+                                          int_data[data_index])};
+
+        *(udst + j) = int_data[data_index];
+      }
+    }
+
+    // Increment pointers
+    *src += sizeof(V128);
+    *dst += sizeof(V128);
+    --copy_rounds;
+  }
+}
+
+}  // namespace
+
+template <size_t vec_regions, size_t int_regions>
+class AcceleratedCrcMemcpyEngine : public CrcMemcpyEngine {
+ public:
+  AcceleratedCrcMemcpyEngine() = default;
+  AcceleratedCrcMemcpyEngine(const AcceleratedCrcMemcpyEngine&) = delete;
+  AcceleratedCrcMemcpyEngine operator=(const AcceleratedCrcMemcpyEngine&) =
+      delete;
+
+  crc32c_t Compute(void* __restrict dst, const void* __restrict src,
+                   std::size_t length, crc32c_t initial_crc) const override;
+};
+
+// Disable ubsan for benign unaligned access. See b/254108538.
+template <size_t vec_regions, size_t int_regions>
+ABSL_ATTRIBUTE_NO_SANITIZE_UNDEFINED crc32c_t
+AcceleratedCrcMemcpyEngine<vec_regions, int_regions>::Compute(
+    void* __restrict dst, const void* __restrict src, std::size_t length,
+    crc32c_t initial_crc) const {
+  constexpr std::size_t kRegions = vec_regions + int_regions;
+  static_assert(kRegions > 0, "Must specify at least one region.");
+  constexpr uint32_t kCrcDataXor = uint32_t{0xffffffff};
+  constexpr std::size_t kBlockSize = sizeof(V128);
+  constexpr std::size_t kCopyRoundSize = kRegions * kBlockSize;
+
+  // Number of blocks per cacheline.
+  constexpr std::size_t kBlocksPerCacheLine = ABSL_CACHELINE_SIZE / kBlockSize;
+
+  char* dst_bytes = static_cast<char*>(dst);
+  const char* src_bytes = static_cast<const char*>(src);
+
+  // Make sure that one prefetch per big block is enough to cover the whole
+  // dataset, and we don't prefetch too much.
+  static_assert(ABSL_CACHELINE_SIZE % kBlockSize == 0,
+                "Cache lines are not divided evenly into blocks, may have "
+                "unintended behavior!");
+
+  // Experimentally-determined boundary between a small and large copy.
+  // Below this number, spin-up and concatenation of CRCs takes enough time that
+  // it kills the throughput gains of using 3 regions and wide vectors.
+  constexpr size_t kCrcSmallSize = 256;
+
+  // Experimentally-determined prefetch distance.  Main loop copies will
+  // prefeth data 2 cache lines ahead.
+  constexpr std::size_t kPrefetchAhead = 2 * ABSL_CACHELINE_SIZE;
+
+  // Small-size CRC-memcpy : just do CRC + memcpy
+  if (length < kCrcSmallSize) {
+    crc32c_t crc =
+        ExtendCrc32c(initial_crc, absl::string_view(src_bytes, length));
+    memcpy(dst, src, length);
+    return crc;
+  }
+
+  // Start work on the CRC: undo the XOR from the previous calculation or set up
+  // the initial value of the CRC.
+  initial_crc = crc32c_t{static_cast<uint32_t>(initial_crc) ^ kCrcDataXor};
+
+  // Do an initial alignment copy, so we can use aligned store instructions to
+  // the destination pointer.  We align the destination pointer because the
+  // penalty for an unaligned load is small compared to the penalty of an
+  // unaligned store on modern CPUs.
+  std::size_t bytes_from_last_aligned =
+      reinterpret_cast<uintptr_t>(dst) & (kBlockSize - 1);
+  if (bytes_from_last_aligned != 0) {
+    std::size_t bytes_for_alignment = kBlockSize - bytes_from_last_aligned;
+
+    // Do the short-sized copy and CRC.
+    initial_crc =
+        ShortCrcCopy(dst_bytes, src_bytes, bytes_for_alignment, initial_crc);
+    src_bytes += bytes_for_alignment;
+    dst_bytes += bytes_for_alignment;
+    length -= bytes_for_alignment;
+  }
+
+  // We are going to do the copy and CRC in kRegions regions to make sure that
+  // we can saturate the CRC unit.  The CRCs will be combined at the end of the
+  // run.  Copying will use the SSE registers, and we will extract words from
+  // the SSE registers to add to the CRC.  Initially, we run the loop one full
+  // cache line per region at a time, in order to insert prefetches.
+
+  // Initialize CRCs for kRegions regions.
+  crc32c_t crcs[kRegions];
+  crcs[0] = initial_crc;
+  for (size_t i = 1; i < kRegions; i++) {
+    crcs[i] = crc32c_t{kCrcDataXor};
+  }
+
+  // Find the number of rounds to copy and the region size.  Also compute the
+  // tail size here.
+  size_t copy_rounds = length / kCopyRoundSize;
+
+  // Find the size of each region and the size of the tail.
+  const std::size_t region_size = copy_rounds * kBlockSize;
+  const std::size_t tail_size = length - (kRegions * region_size);
+
+  // Holding registers for data in each region.
+  std::array<V128, vec_regions> vec_data;
+  std::array<uint64_t, int_regions * kIntLoadsPerVec> int_data;
+
+  // Main loop.
+  while (copy_rounds > kBlocksPerCacheLine) {
+    // Prefetch kPrefetchAhead bytes ahead of each pointer.
+    for (size_t i = 0; i < kRegions; i++) {
+      absl::PrefetchToLocalCache(src_bytes + kPrefetchAhead + region_size * i);
+#ifdef ABSL_INTERNAL_HAVE_X86_64_ACCELERATED_CRC_MEMCPY_ENGINE
+      // TODO(b/297082454): investigate dropping prefetch on x86.
+      absl::PrefetchToLocalCache(dst_bytes + kPrefetchAhead + region_size * i);
+#endif
+    }
+
+    // Load and store data, computing CRC on the way.
+    for (size_t i = 0; i < kBlocksPerCacheLine; i++) {
+      // Copy and CRC the data for the CRC regions.
+      for (size_t j = 0; j < vec_regions; j++) {
+        // Cycle which regions get vector load/store and integer load/store, to
+        // engage prefetching logic around vector load/stores and save issue
+        // slots by using the integer registers.
+        size_t region = (j + i) % kRegions;
+
+        auto* vsrc =
+            reinterpret_cast<const V128*>(src_bytes + region_size * region);
+        auto* vdst = reinterpret_cast<V128*>(dst_bytes + region_size * region);
+
+        // Load and CRC data.
+        vec_data[j] = V128_LoadU(vsrc + i);
+        crcs[region] = crc32c_t{static_cast<uint32_t>(
+            CRC32_u64(static_cast<uint32_t>(crcs[region]),
+                      static_cast<uint64_t>(V128_Extract64<0>(vec_data[j]))))};
+        crcs[region] = crc32c_t{static_cast<uint32_t>(
+            CRC32_u64(static_cast<uint32_t>(crcs[region]),
+                      static_cast<uint64_t>(V128_Extract64<1>(vec_data[j]))))};
+
+        // Store the data.
+        V128_Store(vdst + i, vec_data[j]);
+      }
+
+      // Preload the partial CRCs for the CLMUL subregions.
+      for (size_t j = 0; j < int_regions; j++) {
+        // Cycle which regions get vector load/store and integer load/store, to
+        // engage prefetching logic around vector load/stores and save issue
+        // slots by using the integer registers.
+        size_t region = (j + vec_regions + i) % kRegions;
+
+        auto* usrc =
+            reinterpret_cast<const uint64_t*>(src_bytes + region_size * region);
+        auto* udst =
+            reinterpret_cast<uint64_t*>(dst_bytes + region_size * region);
+
+        for (size_t k = 0; k < kIntLoadsPerVec; k++) {
+          size_t data_index = j * kIntLoadsPerVec + k;
+
+          // Load and CRC the data.
+          int_data[data_index] = *(usrc + i * kIntLoadsPerVec + k);
+          crcs[region] = crc32c_t{CRC32_u64(static_cast<uint32_t>(crcs[region]),
+                                            int_data[data_index])};
+
+          // Store the data.
+          *(udst + i * kIntLoadsPerVec + k) = int_data[data_index];
+        }
+      }
+    }
+
+    // Increment pointers
+    src_bytes += kBlockSize * kBlocksPerCacheLine;
+    dst_bytes += kBlockSize * kBlocksPerCacheLine;
+    copy_rounds -= kBlocksPerCacheLine;
+  }
+
+  // Copy and CRC the tails of each region.
+  LargeTailCopy<vec_regions, int_regions>(crcs, &dst_bytes, &src_bytes,
+                                          region_size, copy_rounds);
+
+  // Move the source and destination pointers to the end of the region
+  src_bytes += region_size * (kRegions - 1);
+  dst_bytes += region_size * (kRegions - 1);
+
+  // Copy and CRC the tail through the XMM registers.
+  std::size_t tail_blocks = tail_size / kBlockSize;
+  LargeTailCopy<0, 1>(&crcs[kRegions - 1], &dst_bytes, &src_bytes, 0,
+                      tail_blocks);
+
+  // Final tail copy for under 16 bytes.
+  crcs[kRegions - 1] =
+      ShortCrcCopy(dst_bytes, src_bytes, tail_size - tail_blocks * kBlockSize,
+                   crcs[kRegions - 1]);
+
+  if (kRegions == 1) {
+    // If there is only one region, finalize and return its CRC.
+    return crc32c_t{static_cast<uint32_t>(crcs[0]) ^ kCrcDataXor};
+  }
+
+  // Finalize the first CRCs: XOR the internal CRCs by the XOR mask to undo the
+  // XOR done before doing block copy + CRCs.
+  for (size_t i = 0; i + 1 < kRegions; i++) {
+    crcs[i] = crc32c_t{static_cast<uint32_t>(crcs[i]) ^ kCrcDataXor};
+  }
+
+  // Build a CRC of the first kRegions - 1 regions.
+  crc32c_t full_crc = crcs[0];
+  for (size_t i = 1; i + 1 < kRegions; i++) {
+    full_crc = ConcatCrc32c(full_crc, crcs[i], region_size);
+  }
+
+  // Finalize and concatenate the final CRC, then return.
+  crcs[kRegions - 1] =
+      crc32c_t{static_cast<uint32_t>(crcs[kRegions - 1]) ^ kCrcDataXor};
+  return ConcatCrc32c(full_crc, crcs[kRegions - 1], region_size + tail_size);
+}
+
+CrcMemcpy::ArchSpecificEngines CrcMemcpy::GetArchSpecificEngines() {
+#ifdef UNDEFINED_BEHAVIOR_SANITIZER
+  // UBSAN does not play nicely with unaligned loads (which we use a lot).
+  // Get the underlying architecture.
+  CpuType cpu_type = GetCpuType();
+  switch (cpu_type) {
+    case CpuType::kAmdRome:
+    case CpuType::kAmdNaples:
+    case CpuType::kAmdMilan:
+    case CpuType::kAmdGenoa:
+    case CpuType::kAmdRyzenV3000:
+    case CpuType::kIntelCascadelakeXeon:
+    case CpuType::kIntelSkylakeXeon:
+    case CpuType::kIntelSkylake:
+    case CpuType::kIntelBroadwell:
+    case CpuType::kIntelHaswell:
+    case CpuType::kIntelIvybridge:
+      return {
+          /*.temporal=*/new FallbackCrcMemcpyEngine(),
+          /*.non_temporal=*/new CrcNonTemporalMemcpyAVXEngine(),
+      };
+    // INTEL_SANDYBRIDGE performs better with SSE than AVX.
+    case CpuType::kIntelSandybridge:
+      return {
+          /*.temporal=*/new FallbackCrcMemcpyEngine(),
+          /*.non_temporal=*/new CrcNonTemporalMemcpyEngine(),
+      };
+    default:
+      return {/*.temporal=*/new FallbackCrcMemcpyEngine(),
+              /*.non_temporal=*/new FallbackCrcMemcpyEngine()};
+  }
+#else
+  // Get the underlying architecture.
+  CpuType cpu_type = GetCpuType();
+  switch (cpu_type) {
+    // On Zen 2, PEXTRQ uses 2 micro-ops, including one on the vector store port
+    // which data movement from the vector registers to the integer registers
+    // (where CRC32C happens) to crowd the same units as vector stores.  As a
+    // result, using that path exclusively causes bottlenecking on this port.
+    // We can avoid this bottleneck by using the integer side of the CPU for
+    // most operations rather than the vector side.  We keep a vector region to
+    // engage some of the prefetching logic in the cache hierarchy which seems
+    // to give vector instructions special treatment.  These prefetch units see
+    // strided access to each region, and do the right thing.
+    case CpuType::kAmdRome:
+    case CpuType::kAmdNaples:
+    case CpuType::kAmdMilan:
+    case CpuType::kAmdGenoa:
+    case CpuType::kAmdRyzenV3000:
+      return {
+          /*.temporal=*/new AcceleratedCrcMemcpyEngine<1, 2>(),
+          /*.non_temporal=*/new CrcNonTemporalMemcpyAVXEngine(),
+      };
+    // PCLMULQDQ is slow and we don't have wide enough issue width to take
+    // advantage of it.  For an unknown architecture, don't risk using CLMULs.
+    case CpuType::kIntelCascadelakeXeon:
+    case CpuType::kIntelSkylakeXeon:
+    case CpuType::kIntelSkylake:
+    case CpuType::kIntelBroadwell:
+    case CpuType::kIntelHaswell:
+    case CpuType::kIntelIvybridge:
+      return {
+          /*.temporal=*/new AcceleratedCrcMemcpyEngine<3, 0>(),
+          /*.non_temporal=*/new CrcNonTemporalMemcpyAVXEngine(),
+      };
+    // INTEL_SANDYBRIDGE performs better with SSE than AVX.
+    case CpuType::kIntelSandybridge:
+    // Use SIMD memcpy on ARM cores.
+    case CpuType::kArmNeoverseN1:
+    case CpuType::kArmNeoverseN2:
+    case CpuType::kArmNeoverseV1:
+    case CpuType::kArmNeoverseV2:
+    case CpuType::kNvidiaGrace:
+      return {
+          /*.temporal=*/new AcceleratedCrcMemcpyEngine<3, 0>(),
+          /*.non_temporal=*/new CrcNonTemporalMemcpyEngine(),
+      };
+    default:
+      return {/*.temporal=*/new FallbackCrcMemcpyEngine(),
+              /*.non_temporal=*/new FallbackCrcMemcpyEngine()};
+  }
+#endif  // UNDEFINED_BEHAVIOR_SANITIZER
+}
+
+// For testing, allow the user to specify which engine they want.
+std::unique_ptr<CrcMemcpyEngine> CrcMemcpy::GetTestEngine(int vector,
+                                                          int integer) {
+  if (vector == 3 && integer == 0) {
+    return std::make_unique<AcceleratedCrcMemcpyEngine<3, 0>>();
+  } else if (vector == 1 && integer == 2) {
+    return std::make_unique<AcceleratedCrcMemcpyEngine<1, 2>>();
+  } else if (vector == 1 && integer == 0) {
+    return std::make_unique<AcceleratedCrcMemcpyEngine<1, 0>>();
+  }
+  return nullptr;
+}
+
+}  // namespace crc_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_INTERNAL_HAVE_X86_64_ACCELERATED_CRC_MEMCPY_ENGINE ||
+        // ABSL_INTERNAL_HAVE_ARM_ACCELERATED_CRC_MEMCPY_ENGINE
diff --git a/absl/crc/internal/crc_non_temporal_memcpy.cc b/absl/crc/internal/crc_non_temporal_memcpy.cc
new file mode 100644
index 00000000000..a56f1eb0e06
--- /dev/null
+++ b/absl/crc/internal/crc_non_temporal_memcpy.cc
@@ -0,0 +1,93 @@
+// Copyright 2022 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <cstddef>
+
+#include "absl/base/config.h"
+#include "absl/crc/crc32c.h"
+#include "absl/crc/internal/crc_memcpy.h"
+#include "absl/crc/internal/non_temporal_memcpy.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace crc_internal {
+
+crc32c_t CrcNonTemporalMemcpyEngine::Compute(void* __restrict dst,
+                                             const void* __restrict src,
+                                             std::size_t length,
+                                             crc32c_t initial_crc) const {
+  constexpr size_t kBlockSize = 8192;
+  crc32c_t crc = initial_crc;
+
+  const char* src_bytes = reinterpret_cast<const char*>(src);
+  char* dst_bytes = reinterpret_cast<char*>(dst);
+
+  // Copy + CRC loop - run 8k chunks until we are out of full chunks.
+  std::size_t offset = 0;
+  for (; offset + kBlockSize < length; offset += kBlockSize) {
+    crc = absl::ExtendCrc32c(crc,
+                             absl::string_view(src_bytes + offset, kBlockSize));
+    non_temporal_store_memcpy(dst_bytes + offset, src_bytes + offset,
+                              kBlockSize);
+  }
+
+  // Save some work if length is 0.
+  if (offset < length) {
+    std::size_t final_copy_size = length - offset;
+    crc = ExtendCrc32c(crc,
+                       absl::string_view(src_bytes + offset, final_copy_size));
+
+    non_temporal_store_memcpy(dst_bytes + offset, src_bytes + offset,
+                              final_copy_size);
+  }
+
+  return crc;
+}
+
+crc32c_t CrcNonTemporalMemcpyAVXEngine::Compute(void* __restrict dst,
+                                                const void* __restrict src,
+                                                std::size_t length,
+                                                crc32c_t initial_crc) const {
+  constexpr size_t kBlockSize = 8192;
+  crc32c_t crc = initial_crc;
+
+  const char* src_bytes = reinterpret_cast<const char*>(src);
+  char* dst_bytes = reinterpret_cast<char*>(dst);
+
+  // Copy + CRC loop - run 8k chunks until we are out of full chunks.
+  std::size_t offset = 0;
+  for (; offset + kBlockSize < length; offset += kBlockSize) {
+    crc = ExtendCrc32c(crc, absl::string_view(src_bytes + offset, kBlockSize));
+
+    non_temporal_store_memcpy_avx(dst_bytes + offset, src_bytes + offset,
+                                  kBlockSize);
+  }
+
+  // Save some work if length is 0.
+  if (offset < length) {
+    std::size_t final_copy_size = length - offset;
+    crc = ExtendCrc32c(crc,
+                       absl::string_view(src_bytes + offset, final_copy_size));
+
+    non_temporal_store_memcpy_avx(dst_bytes + offset, src_bytes + offset,
+                                  final_copy_size);
+  }
+
+  return crc;
+}
+
+}  // namespace crc_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/crc/internal/crc_x86_arm_combined.cc b/absl/crc/internal/crc_x86_arm_combined.cc
new file mode 100644
index 00000000000..edffd36717d
--- /dev/null
+++ b/absl/crc/internal/crc_x86_arm_combined.cc
@@ -0,0 +1,922 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Hardware accelerated CRC32 computation on Intel and ARM architecture.
+
+#include <cstddef>
+#include <cstdint>
+#include <memory>
+#include <vector>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/cpu_detect.h"
+#include "absl/base/internal/endian.h"
+#include "absl/base/prefetch.h"
+#include "absl/crc/internal/crc32_x86_arm_combined_simd.h"
+#include "absl/crc/internal/crc_internal.h"
+#include "absl/memory/memory.h"
+#include "absl/numeric/bits.h"
+
+#if defined(ABSL_CRC_INTERNAL_HAVE_ARM_SIMD) || \
+    defined(ABSL_CRC_INTERNAL_HAVE_X86_SIMD)
+#define ABSL_INTERNAL_CAN_USE_SIMD_CRC32C
+#endif
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace crc_internal {
+
+using ::absl::base_internal::CpuType;
+using ::absl::base_internal::GetCpuType;
+#if defined(__aarch64__)
+using ::absl::base_internal::SupportsArmCRC32PMULL;
+#endif
+
+#if defined(ABSL_INTERNAL_CAN_USE_SIMD_CRC32C)
+
+// Implementation details not exported outside of file
+namespace {
+
+// Some machines have CRC acceleration hardware.
+// We can do a faster version of Extend() on such machines.
+class CRC32AcceleratedX86ARMCombined : public CRC32 {
+ public:
+  CRC32AcceleratedX86ARMCombined() {}
+  ~CRC32AcceleratedX86ARMCombined() override {}
+  void ExtendByZeroes(uint32_t* crc, size_t length) const override;
+  uint32_t ComputeZeroConstant(size_t length) const;
+
+ private:
+  CRC32AcceleratedX86ARMCombined(const CRC32AcceleratedX86ARMCombined&) =
+      delete;
+  CRC32AcceleratedX86ARMCombined& operator=(
+      const CRC32AcceleratedX86ARMCombined&) = delete;
+};
+
+// Constants for switching between algorithms.
+// Chosen by comparing speed at different powers of 2.
+constexpr size_t kSmallCutoff = 256;
+constexpr size_t kMediumCutoff = 2048;
+
+#define ABSL_INTERNAL_STEP1(crc, p)                   \
+  do {                                                \
+    crc = CRC32_u8(static_cast<uint32_t>(crc), *p++); \
+  } while (0)
+#define ABSL_INTERNAL_STEP2(crc, p)                                            \
+  do {                                                                         \
+    crc =                                                                      \
+        CRC32_u16(static_cast<uint32_t>(crc), absl::little_endian::Load16(p)); \
+    p += 2;                                                                    \
+  } while (0)
+#define ABSL_INTERNAL_STEP4(crc, p)                                            \
+  do {                                                                         \
+    crc =                                                                      \
+        CRC32_u32(static_cast<uint32_t>(crc), absl::little_endian::Load32(p)); \
+    p += 4;                                                                    \
+  } while (0)
+#define ABSL_INTERNAL_STEP8(crc, p)                                            \
+  do {                                                                         \
+    crc =                                                                      \
+        CRC32_u64(static_cast<uint32_t>(crc), absl::little_endian::Load64(p)); \
+    p += 8;                                                                    \
+  } while (0)
+#define ABSL_INTERNAL_STEP8BY2(crc0, crc1, p0, p1) \
+  do {                                             \
+    ABSL_INTERNAL_STEP8(crc0, p0);                 \
+    ABSL_INTERNAL_STEP8(crc1, p1);                 \
+  } while (0)
+#define ABSL_INTERNAL_STEP8BY3(crc0, crc1, crc2, p0, p1, p2) \
+  do {                                                       \
+    ABSL_INTERNAL_STEP8(crc0, p0);                           \
+    ABSL_INTERNAL_STEP8(crc1, p1);                           \
+    ABSL_INTERNAL_STEP8(crc2, p2);                           \
+  } while (0)
+
+namespace {
+
+// Does polynomial multiplication a * b * x^33 mod G.
+//
+// One of the multiplicands needs to have an extra factor of x^-33 to cancel out
+// the extra factor of x^33. The extra factor of x^33 comes from:
+//
+// - x^1 from the carry-less multiplication, due to the
+//   "least-significant-bit-first" convention of CRC-32C.
+//
+// - x^32 from using CRC32_u64() to reduce the carry-less product to 32 bits.
+//
+// Both could be avoided, but at the cost of extra instructions. It's more
+// efficient to just drop a factor of x^33 from one of the multiplicands.
+uint32_t MultiplyWithExtraX33(uint32_t a, uint32_t b) {
+  V128 a_vec = V128_From64WithZeroFill(a);
+  V128 b_vec = V128_From64WithZeroFill(b);
+  V128 res = V128_PMulLow(a_vec, b_vec);
+
+  return CRC32_u64(0, static_cast<uint64_t>(V128_Low64(res)));
+}
+
+// The number of low-order bits that ComputeZeroConstant() drops from the
+// length, i.e. treats as zeroes
+constexpr int kNumDroppedBits = 4;
+
+// Precomputed constants for faster ExtendByZeroes(). This was generated by
+// gen_crc32c_consts.py. The entry at index i is x^(2^(i + 3 + kNumDroppedBits)
+// - 33) mod G. That is x^-33 times the polynomial by which the CRC value needs
+// to be multiplied to extend it by 2^(i + 3 + kNumDroppedBits) zero bits, or
+// equivalently 2^(i + kNumDroppedBits) zero bytes. The extra factor of x^-33
+// cancels out the extra factor of x^33 that MultiplyWithExtraX33() introduces.
+constexpr uint32_t kCRC32CPowers[] = {
+    0x493c7d27, 0xba4fc28e, 0x9e4addf8, 0x0d3b6092, 0xb9e02b86, 0xdd7e3b0c,
+    0x170076fa, 0xa51b6135, 0x82f89c77, 0x54a86326, 0x1dc403cc, 0x5ae703ab,
+    0xc5013a36, 0xac2ac6dd, 0x9b4615a9, 0x688d1c61, 0xf6af14e6, 0xb6ffe386,
+    0xb717425b, 0x478b0d30, 0x54cc62e5, 0x7b2102ee, 0x8a99adef, 0xa7568c8f,
+    0xd610d67e, 0x6b086b3f, 0xd94f3c0b, 0xbf818109, 0x780d5a4d, 0x05ec76f1,
+    0x00000001, 0x493c7d27, 0xba4fc28e, 0x9e4addf8, 0x0d3b6092, 0xb9e02b86,
+    0xdd7e3b0c, 0x170076fa, 0xa51b6135, 0x82f89c77, 0x54a86326, 0x1dc403cc,
+    0x5ae703ab, 0xc5013a36, 0xac2ac6dd, 0x9b4615a9, 0x688d1c61, 0xf6af14e6,
+    0xb6ffe386, 0xb717425b, 0x478b0d30, 0x54cc62e5, 0x7b2102ee, 0x8a99adef,
+    0xa7568c8f, 0xd610d67e, 0x6b086b3f, 0xd94f3c0b, 0xbf818109, 0x780d5a4d,
+};
+// There must be an entry for each non-dropped bit in the size_t length.
+static_assert(std::size(kCRC32CPowers) >= sizeof(size_t) * 8 - kNumDroppedBits);
+
+}  // namespace
+
+// Compute a magic constant, so that multiplying by it is the same as extending
+// crc by length zeros. The lowest kNumDroppedBits of the length are ignored and
+// treated as zeroes; the caller is assumed to handle any nonzero bits there.
+#if defined(NDEBUG) && ABSL_HAVE_CPP_ATTRIBUTE(clang::no_sanitize)
+// The array accesses in this are safe: `length >= size_t{1} <<
+// kNumDroppedBits`, so `countr_zero(length >> kNumDroppedBits) < sizeof(size_t)
+// * 8 - kNumDroppedBits`, and `length & (length - 1)` cannot introduce bits
+// `>= sizeof(size_t) * 8 - kNumDroppedBits`. The compiler cannot prove this, so
+// manually disable bounds checking.
+[[clang::no_sanitize("array-bounds")]]
+#endif
+uint32_t CRC32AcceleratedX86ARMCombined::ComputeZeroConstant(
+    size_t length) const {
+  length >>= kNumDroppedBits;
+
+  int index = absl::countr_zero(length);
+  uint32_t prev = kCRC32CPowers[index];
+  length &= length - 1;
+
+  while (length) {
+    // For each bit of length, extend by 2**n zeros.
+    index = absl::countr_zero(length);
+    prev = MultiplyWithExtraX33(prev, kCRC32CPowers[index]);
+    length &= length - 1;
+  }
+  return prev;
+}
+
+void CRC32AcceleratedX86ARMCombined::ExtendByZeroes(uint32_t* crc,
+                                                    size_t length) const {
+  uint32_t val = *crc;
+  // Don't bother with multiplication for small length.
+  if (length & 1) val = CRC32_u8(val, 0);
+  if (length & 2) val = CRC32_u16(val, 0);
+  if (length & 4) val = CRC32_u32(val, 0);
+  if (length & 8) val = CRC32_u64(val, 0);
+  static_assert(kNumDroppedBits == 4);
+  if (length >= size_t{1} << kNumDroppedBits) {
+    val = MultiplyWithExtraX33(val, ComputeZeroConstant(length));
+  }
+  *crc = val;
+}
+
+// Taken from Intel paper "Fast CRC Computation for iSCSI Polynomial Using CRC32
+// Instruction"
+// https://www.intel.com/content/dam/www/public/us/en/documents/white-papers/crc-iscsi-polynomial-crc32-instruction-paper.pdf
+// We only need every 4th value, because we unroll loop by 4.
+constexpr uint64_t kClmulConstants[] = {
+    0x09e4addf8, 0x0ba4fc28e, 0x00d3b6092, 0x09e4addf8, 0x0ab7aff2a,
+    0x102f9b8a2, 0x0b9e02b86, 0x00d3b6092, 0x1bf2e8b8a, 0x18266e456,
+    0x0d270f1a2, 0x0ab7aff2a, 0x11eef4f8e, 0x083348832, 0x0dd7e3b0c,
+    0x0b9e02b86, 0x0271d9844, 0x1b331e26a, 0x06b749fb2, 0x1bf2e8b8a,
+    0x0e6fc4e6a, 0x0ce7f39f4, 0x0d7a4825c, 0x0d270f1a2, 0x026f6a60a,
+    0x12ed0daac, 0x068bce87a, 0x11eef4f8e, 0x1329d9f7e, 0x0b3e32c28,
+    0x0170076fa, 0x0dd7e3b0c, 0x1fae1cc66, 0x010746f3c, 0x086d8e4d2,
+    0x0271d9844, 0x0b3af077a, 0x093a5f730, 0x1d88abd4a, 0x06b749fb2,
+    0x0c9c8b782, 0x0cec3662e, 0x1ddffc5d4, 0x0e6fc4e6a, 0x168763fa6,
+    0x0b0cd4768, 0x19b1afbc4, 0x0d7a4825c, 0x123888b7a, 0x00167d312,
+    0x133d7a042, 0x026f6a60a, 0x000bcf5f6, 0x19d34af3a, 0x1af900c24,
+    0x068bce87a, 0x06d390dec, 0x16cba8aca, 0x1f16a3418, 0x1329d9f7e,
+    0x19fb2a8b0, 0x02178513a, 0x1a0f717c4, 0x0170076fa,
+};
+
+enum class PclmulStreamType {
+  PCLMUL,
+  VPCLMUL,
+  NEON_PCLMUL,
+  NEON_PCLMUL_EOR3,
+};
+
+// Base class for CRC32AcceleratedX86ARMCombinedMultipleStreams containing the
+// methods and data that don't need the template arguments.
+class CRC32AcceleratedX86ARMCombinedMultipleStreamsBase
+    : public CRC32AcceleratedX86ARMCombined {
+ protected:
+  // Update partialCRC with crc of 64 byte block. Calling FinalizePclmulStream
+  // would produce a single crc checksum, but it is expensive. PCLMULQDQ has a
+  // high latency, so we run 4 128-bit partial checksums that can be reduced to
+  // a single value by FinalizePclmulStream later. Computing crc for arbitrary
+  // polynomialas with PCLMULQDQ is described in Intel paper "Fast CRC
+  // Computation for Generic Polynomials Using PCLMULQDQ Instruction"
+  // https://www.intel.com/content/dam/www/public/us/en/documents/white-papers/fast-crc-computation-generic-polynomials-pclmulqdq-paper.pdf
+  // We are applying it to CRC32C polynomial.
+#if defined(ABSL_CRC_INTERNAL_HAVE_ARM_SIMD)
+  template <bool kUseEor3 = false>
+  ABSL_ATTRIBUTE_ALWAYS_INLINE void Process64BytesNeonPclmul(
+      const uint8_t* p, V128* partialCRC) const {
+    V128 loopMultiplicands =
+        V128_Load(reinterpret_cast<const V128*>(kFoldAcross512Bits));
+
+    V128 partialCRC1 = partialCRC[0];
+    V128 partialCRC2 = partialCRC[1];
+    V128 partialCRC3 = partialCRC[2];
+    V128 partialCRC4 = partialCRC[3];
+
+    V128 tmp1 = V128_PMulHi(partialCRC1, loopMultiplicands);
+    V128 tmp2 = V128_PMulHi(partialCRC2, loopMultiplicands);
+    V128 tmp3 = V128_PMulHi(partialCRC3, loopMultiplicands);
+    V128 tmp4 = V128_PMulHi(partialCRC4, loopMultiplicands);
+    V128 data1 = V128_LoadU(reinterpret_cast<const V128*>(p + 16 * 0));
+    V128 data2 = V128_LoadU(reinterpret_cast<const V128*>(p + 16 * 1));
+    V128 data3 = V128_LoadU(reinterpret_cast<const V128*>(p + 16 * 2));
+    V128 data4 = V128_LoadU(reinterpret_cast<const V128*>(p + 16 * 3));
+    partialCRC1 = V128_PMulLow(partialCRC1, loopMultiplicands);
+    partialCRC2 = V128_PMulLow(partialCRC2, loopMultiplicands);
+    partialCRC3 = V128_PMulLow(partialCRC3, loopMultiplicands);
+    partialCRC4 = V128_PMulLow(partialCRC4, loopMultiplicands);
+    partialCRC1 = V128_Xor3<kUseEor3>(tmp1, partialCRC1, data1);
+    partialCRC2 = V128_Xor3<kUseEor3>(tmp2, partialCRC2, data2);
+    partialCRC3 = V128_Xor3<kUseEor3>(tmp3, partialCRC3, data3);
+    partialCRC4 = V128_Xor3<kUseEor3>(tmp4, partialCRC4, data4);
+    partialCRC[0] = partialCRC1;
+    partialCRC[1] = partialCRC2;
+    partialCRC[2] = partialCRC3;
+    partialCRC[3] = partialCRC4;
+  }
+
+  // Reduce partialCRC produced by Process64BytesNeonPclmul into a single value,
+  // that represents crc checksum of all the processed bytes.
+  template <bool kUseEor3 = false>
+  ABSL_ATTRIBUTE_ALWAYS_INLINE uint64_t
+  FinalizeNeonPclmulStream(V128* partialCRC) const {
+    V128 partialCRC1 = partialCRC[0];
+    V128 partialCRC2 = partialCRC[1];
+    V128 partialCRC3 = partialCRC[2];
+    V128 partialCRC4 = partialCRC[3];
+
+    // Combine 4 vectors of partial crc into a single vector.
+    V128 reductionMultiplicands =
+        V128_Load(reinterpret_cast<const V128*>(kFoldAcross256Bits));
+
+    V128 low = V128_PMulLow(reductionMultiplicands, partialCRC1);
+    V128 high = V128_PMulHi(reductionMultiplicands, partialCRC1);
+
+    partialCRC1 = V128_Xor3<kUseEor3>(low, high, partialCRC3);
+
+    low = V128_PMulLow(reductionMultiplicands, partialCRC2);
+    high = V128_PMulHi(reductionMultiplicands, partialCRC2);
+
+    partialCRC2 = V128_Xor3<kUseEor3>(low, high, partialCRC4);
+
+    reductionMultiplicands =
+        V128_Load(reinterpret_cast<const V128*>(kFoldAcross128Bits));
+
+    low = V128_PMulLow(reductionMultiplicands, partialCRC1);
+    high = V128_PMulHi(reductionMultiplicands, partialCRC1);
+    V128 fullCRC = V128_Xor3<kUseEor3>(low, high, partialCRC2);
+
+    // Reduce fullCRC into scalar value.
+    uint32_t crc = 0;
+    crc = CRC32_u64(crc, V128_Extract64<0>(fullCRC));
+    crc = CRC32_u64(crc, V128_Extract64<1>(fullCRC));
+    return crc;
+  }
+
+  ABSL_ATTRIBUTE_ALWAYS_INLINE void Process64BytesPclmul(const uint8_t*,
+                                                         V128*) const {}
+
+  ABSL_ATTRIBUTE_ALWAYS_INLINE uint64_t FinalizePclmulStream(V128*) const {
+    return 0;
+  }
+#else
+  ABSL_ATTRIBUTE_ALWAYS_INLINE void Process64BytesPclmul(
+      const uint8_t* p, V128* partialCRC) const {
+    V128 loopMultiplicands =
+        V128_Load(reinterpret_cast<const V128*>(kFoldAcross512Bits));
+
+    V128 partialCRC1 = partialCRC[0];
+    V128 partialCRC2 = partialCRC[1];
+    V128 partialCRC3 = partialCRC[2];
+    V128 partialCRC4 = partialCRC[3];
+
+    V128 tmp1 = V128_PMulHi(partialCRC1, loopMultiplicands);
+    V128 tmp2 = V128_PMulHi(partialCRC2, loopMultiplicands);
+    V128 tmp3 = V128_PMulHi(partialCRC3, loopMultiplicands);
+    V128 tmp4 = V128_PMulHi(partialCRC4, loopMultiplicands);
+    V128 data1 = V128_LoadU(reinterpret_cast<const V128*>(p + 16 * 0));
+    V128 data2 = V128_LoadU(reinterpret_cast<const V128*>(p + 16 * 1));
+    V128 data3 = V128_LoadU(reinterpret_cast<const V128*>(p + 16 * 2));
+    V128 data4 = V128_LoadU(reinterpret_cast<const V128*>(p + 16 * 3));
+    partialCRC1 = V128_PMulLow(partialCRC1, loopMultiplicands);
+    partialCRC2 = V128_PMulLow(partialCRC2, loopMultiplicands);
+    partialCRC3 = V128_PMulLow(partialCRC3, loopMultiplicands);
+    partialCRC4 = V128_PMulLow(partialCRC4, loopMultiplicands);
+    partialCRC1 = V128_Xor(tmp1, partialCRC1);
+    partialCRC2 = V128_Xor(tmp2, partialCRC2);
+    partialCRC3 = V128_Xor(tmp3, partialCRC3);
+    partialCRC4 = V128_Xor(tmp4, partialCRC4);
+    partialCRC1 = V128_Xor(partialCRC1, data1);
+    partialCRC2 = V128_Xor(partialCRC2, data2);
+    partialCRC3 = V128_Xor(partialCRC3, data3);
+    partialCRC4 = V128_Xor(partialCRC4, data4);
+    partialCRC[0] = partialCRC1;
+    partialCRC[1] = partialCRC2;
+    partialCRC[2] = partialCRC3;
+    partialCRC[3] = partialCRC4;
+  }
+
+  // Reduce partialCRC produced by Process64BytesPclmul into a single value,
+  // that represents crc checksum of all the processed bytes.
+  ABSL_ATTRIBUTE_ALWAYS_INLINE uint64_t
+  FinalizePclmulStream(V128* partialCRC) const {
+    V128 partialCRC1 = partialCRC[0];
+    V128 partialCRC2 = partialCRC[1];
+    V128 partialCRC3 = partialCRC[2];
+    V128 partialCRC4 = partialCRC[3];
+
+    // Combine 4 vectors of partial crc into a single vector.
+    V128 reductionMultiplicands =
+        V128_Load(reinterpret_cast<const V128*>(kFoldAcross256Bits));
+
+    V128 low = V128_PMulLow(reductionMultiplicands, partialCRC1);
+    V128 high = V128_PMulHi(reductionMultiplicands, partialCRC1);
+
+    partialCRC1 = V128_Xor(low, high);
+    partialCRC1 = V128_Xor(partialCRC1, partialCRC3);
+
+    low = V128_PMulLow(reductionMultiplicands, partialCRC2);
+    high = V128_PMulHi(reductionMultiplicands, partialCRC2);
+
+    partialCRC2 = V128_Xor(low, high);
+    partialCRC2 = V128_Xor(partialCRC2, partialCRC4);
+
+    reductionMultiplicands =
+        V128_Load(reinterpret_cast<const V128*>(kFoldAcross128Bits));
+
+    low = V128_PMulLow(reductionMultiplicands, partialCRC1);
+    high = V128_PMulHi(reductionMultiplicands, partialCRC1);
+    V128 fullCRC = V128_Xor(low, high);
+    fullCRC = V128_Xor(fullCRC, partialCRC2);
+
+    // Reduce fullCRC into scalar value.
+    uint32_t crc = 0;
+    crc = CRC32_u64(crc, V128_Extract64<0>(fullCRC));
+    crc = CRC32_u64(crc, V128_Extract64<1>(fullCRC));
+    return crc;
+  }
+
+  template <bool kUseEor3 = false>
+  ABSL_ATTRIBUTE_ALWAYS_INLINE void Process64BytesNeonPclmul(const uint8_t*,
+                                                             V128*) const {}
+
+  template <bool kUseEor3 = false>
+  ABSL_ATTRIBUTE_ALWAYS_INLINE uint64_t FinalizeNeonPclmulStream(V128*) const {
+    return 0;
+  }
+#endif
+
+  // Update crc with 64 bytes of data from p.
+  ABSL_ATTRIBUTE_ALWAYS_INLINE uint64_t Process64BytesCRC(const uint8_t* p,
+                                                          uint64_t crc) const {
+    for (int i = 0; i < 8; i++) {
+      crc =
+          CRC32_u64(static_cast<uint32_t>(crc), absl::little_endian::Load64(p));
+      p += 8;
+    }
+    return crc;
+  }
+
+  // Same as Process64BytesCRC, but just interleaved for 2 streams.
+  ABSL_ATTRIBUTE_ALWAYS_INLINE void Process64BytesCRC2Streams(
+      const uint8_t* p0, const uint8_t* p1, uint64_t* crc) const {
+    uint64_t crc0 = crc[0];
+    uint64_t crc1 = crc[1];
+    for (int i = 0; i < 8; i++) {
+      crc0 = CRC32_u64(static_cast<uint32_t>(crc0),
+                       absl::little_endian::Load64(p0));
+      crc1 = CRC32_u64(static_cast<uint32_t>(crc1),
+                       absl::little_endian::Load64(p1));
+      p0 += 8;
+      p1 += 8;
+    }
+    crc[0] = crc0;
+    crc[1] = crc1;
+  }
+
+  // Same as Process64BytesCRC, but just interleaved for 3 streams.
+  ABSL_ATTRIBUTE_ALWAYS_INLINE void Process64BytesCRC3Streams(
+      const uint8_t* p0, const uint8_t* p1, const uint8_t* p2,
+      uint64_t* crc) const {
+    uint64_t crc0 = crc[0];
+    uint64_t crc1 = crc[1];
+    uint64_t crc2 = crc[2];
+    for (int i = 0; i < 8; i++) {
+      crc0 = CRC32_u64(static_cast<uint32_t>(crc0),
+                       absl::little_endian::Load64(p0));
+      crc1 = CRC32_u64(static_cast<uint32_t>(crc1),
+                       absl::little_endian::Load64(p1));
+      crc2 = CRC32_u64(static_cast<uint32_t>(crc2),
+                       absl::little_endian::Load64(p2));
+      p0 += 8;
+      p1 += 8;
+      p2 += 8;
+    }
+    crc[0] = crc0;
+    crc[1] = crc1;
+    crc[2] = crc2;
+  }
+
+#if defined(ABSL_CRC_INTERNAL_HAVE_X86_SIMD) && defined(__AVX__) && \
+    (!defined(_MSC_VER) || defined(__clang__))
+  // This is only used if we have vector version of PCLMULQDQ.
+  // We don't have it on arm, and it isn't supported by default
+  // compiler targets on x86. If we want to use it, we need to either use
+  // new compiler flags for the whole function and compile it twice
+  // with new and default flags or use inline asm.
+  // The code below is the same as FinalizePclmulStream, but with
+  // PCLMUL and XOR operating on 2 values in a vector at the same time.
+  ABSL_ATTRIBUTE_ALWAYS_INLINE uint64_t
+  FinalizeVpclmulStream(V256* partialCRC) const {
+    uint64_t crc = 0;
+    uint64_t low64, high64;
+    __asm__(
+        // reduce 2 256-bit vectors into s single 256 vector
+        "vbroadcasti128 %[k256], %%ymm0 \n"
+        "vpclmulqdq $0x00, %%ymm0, %[crc0], %%ymm1 \n"
+        "vpclmulqdq $0x11, %%ymm0, %[crc0], %%ymm2 \n"
+        "vpxor %%ymm2, %%ymm1, %%ymm1 \n"
+        "vpxor %[crc1], %%ymm1, %%ymm1 \n"
+        // reduce upper and lower parts of 256-bit vector
+        "vextracti128 $1, %%ymm1, %%xmm2 \n"
+        "vpclmulqdq $0x00, %[k128], %%xmm1, %%xmm3 \n"
+        "vpclmulqdq $0x11, %[k128], %%xmm1, %%xmm1 \n"
+        "vpxor %%xmm1, %%xmm3, %%xmm3 \n"
+        "vpxor %%xmm2, %%xmm3, %%xmm3 \n"
+        // Move 2 parts of 128-bit vector into scalar register
+        // and reduce using sacalr crc instruction
+        "vmovq %%xmm3, %[low] \n"
+        "vpextrq $1, %%xmm3, %[high] \n"
+        "crc32q %[low], %[crc_out] \n"
+        "crc32q %[high], %[crc_out] \n"
+        : [crc_out] "+r"(crc), [low] "=&r"(low64), [high] "=&r"(high64)
+        : [k256] "m"(*(const __m128i*)kFoldAcross256Bits),
+          [crc0] "x"(partialCRC[0]), [crc1] "x"(partialCRC[1]),
+          [k128] "m"(*(const __m128i*)kFoldAcross128Bits)
+        : "ymm0", "ymm1", "ymm2", "ymm3");
+    return crc;
+  }
+
+  ABSL_ATTRIBUTE_ALWAYS_INLINE void Process64BytesVpclmul(
+      const uint8_t* p, V256* vpartialCRC, V256 loopMultiplicands) const {
+    __asm__ volatile(
+        "vpclmulqdq $0x11, %3, %0, %%ymm0 \n"
+        "vpclmulqdq $0x11, %3, %1, %%ymm1 \n"
+        "vpclmulqdq $0x00, %3, %0, %0 \n"
+        "vpclmulqdq $0x00, %3, %1, %1 \n"
+        "vpxor %%ymm0, %0, %0 \n"
+        "vpxor %%ymm1, %1, %1 \n"
+        "vpxor (%2), %0, %0 \n"
+        "vpxor 32(%2), %1, %1 \n"
+        : "+x"(vpartialCRC[0]), "+x"(vpartialCRC[1])
+        : "r"(p), "x"(loopMultiplicands)
+        : "ymm0", "ymm1");
+  }
+#else
+  template <typename T = V256>
+  ABSL_ATTRIBUTE_ALWAYS_INLINE void Process64BytesVpclmul(const uint8_t*, T*,
+                                                          T) const {
+    static_assert(sizeof(T) == 0, "Vector PCLMUL not supported");
+  }
+  ABSL_ATTRIBUTE_ALWAYS_INLINE uint64_t FinalizeVpclmulStream(V256*) const {
+    return 0;
+  }
+#endif  // defined(ABSL_CRC_INTERNAL_HAVE_X86_SIMD) && defined(__AVX__) &&
+        // (!defined(_MSC_VER) || defined(__clang__))
+
+  // Constants generated by './scripts/gen-crc-consts.py x86_pclmul
+  // crc32_lsb_0x82f63b78' from the Linux kernel.
+  alignas(16) static constexpr uint64_t kFoldAcross512Bits[2] = {
+      // (x^543 mod G) * x^32
+      0x00000000740eef02,
+      // (x^479 mod G) * x^32
+      0x000000009e4addf8};
+  alignas(16) static constexpr uint64_t kFoldAcross256Bits[2] = {
+      // (x^287 mod G) * x^32
+      0x000000003da6d0cb,
+      // (x^223 mod G) * x^32
+      0x00000000ba4fc28e};
+  alignas(16) static constexpr uint64_t kFoldAcross128Bits[2] = {
+      // (x^159 mod G) * x^32
+      0x00000000f20c0dfe,
+      // (x^95 mod G) * x^32
+      0x00000000493c7d27};
+
+  // Medium runs of bytes are broken into groups of kGroupsSmall blocks of same
+  // size. Each group is CRCed in parallel then combined at the end of the
+  // block.
+  static constexpr size_t kGroupsSmall = 3;
+  // For large runs we use up to kMaxStreams blocks computed with CRC
+  // instruction, and up to kMaxStreams blocks computed with PCLMULQDQ, which
+  // are combined in the end.
+  static constexpr size_t kMaxStreams = 3;
+};
+
+template <size_t num_crc_streams, size_t num_pclmul_streams,
+          PclmulStreamType pclmul_stream_type>
+class CRC32AcceleratedX86ARMCombinedMultipleStreams
+    : public CRC32AcceleratedX86ARMCombinedMultipleStreamsBase {
+  ABSL_ATTRIBUTE_HOT
+  void Extend(uint32_t* crc, const void* bytes,
+              const size_t length) const override {
+    static_assert(num_crc_streams >= 1 && num_crc_streams <= kMaxStreams,
+                  "Invalid number of crc streams");
+    static_assert(num_pclmul_streams >= 0 && num_pclmul_streams <= kMaxStreams,
+                  "Invalid number of pclmul streams");
+    const uint8_t* p = static_cast<const uint8_t*>(bytes);
+    const uint8_t* e = p + length;
+    uint32_t l = *crc;
+    uint64_t l64;
+
+    // For small blocks just run simple loop, because cost of combining multiple
+    // streams is significant.
+    if (num_crc_streams > 1 && (length < kSmallCutoff)) {
+      // fallthrough; Use the same strategy as we do for processing the
+      // remaining bytes after any other strategy.
+    } else if (length < kMediumCutoff) {
+      // For medium blocks we run 3 crc streams and combine them as described in
+      // Intel paper above. Running 4th stream doesn't help, because crc
+      // instruction has latency 3 and throughput 1.
+      l64 = l;
+      if (num_crc_streams > 1) {
+        uint64_t l641 = 0;
+        uint64_t l642 = 0;
+        const size_t blockSize = 32;
+        size_t bs = static_cast<size_t>(e - p) / kGroupsSmall / blockSize;
+        const uint8_t* p1 = p + bs * blockSize;
+        const uint8_t* p2 = p1 + bs * blockSize;
+
+        for (size_t i = 0; i + 1 < bs; ++i) {
+          ABSL_INTERNAL_STEP8BY3(l64, l641, l642, p, p1, p2);
+          ABSL_INTERNAL_STEP8BY3(l64, l641, l642, p, p1, p2);
+          ABSL_INTERNAL_STEP8BY3(l64, l641, l642, p, p1, p2);
+          ABSL_INTERNAL_STEP8BY3(l64, l641, l642, p, p1, p2);
+          PrefetchToLocalCache(
+              reinterpret_cast<const char*>(p + kPrefetchHorizonMedium));
+          PrefetchToLocalCache(
+              reinterpret_cast<const char*>(p1 + kPrefetchHorizonMedium));
+          PrefetchToLocalCache(
+              reinterpret_cast<const char*>(p2 + kPrefetchHorizonMedium));
+        }
+        // Don't run crc on last 8 bytes.
+        ABSL_INTERNAL_STEP8BY3(l64, l641, l642, p, p1, p2);
+        ABSL_INTERNAL_STEP8BY3(l64, l641, l642, p, p1, p2);
+        ABSL_INTERNAL_STEP8BY3(l64, l641, l642, p, p1, p2);
+        ABSL_INTERNAL_STEP8BY2(l64, l641, p, p1);
+
+        V128 magic = *(reinterpret_cast<const V128*>(kClmulConstants) + bs - 1);
+
+        V128 tmp = V128_From64WithZeroFill(l64);
+
+        V128 res1 = V128_PMulLow(tmp, magic);
+
+        tmp = V128_From64WithZeroFill(l641);
+
+        V128 res2 = V128_PMul10(tmp, magic);
+        V128 x = V128_Xor(res1, res2);
+        l64 = static_cast<uint64_t>(V128_Low64(x)) ^
+              absl::little_endian::Load64(p2);
+        l64 = CRC32_u64(static_cast<uint32_t>(l642), l64);
+
+        p = p2 + 8;
+      } else {
+        while ((e - p) >= 64) {
+          l64 = Process64BytesCRC(p, l64);
+          p += 64;
+        }
+      }
+      l = static_cast<uint32_t>(l64);
+    } else {
+      // There is a lot of data, we can ignore combine costs and run all
+      // requested streams (num_crc_streams + num_pclmul_streams),
+      // using prefetch. CRC and PCLMULQDQ use different cpu execution units,
+      // so on some cpus it makes sense to execute both of them for different
+      // streams.
+
+      // Point x at first 8-byte aligned byte in string.
+      const uint8_t* x = RoundUp<8>(p);
+      // Process bytes until p is 8-byte aligned, if that isn't past the end.
+      while (p != x) {
+        ABSL_INTERNAL_STEP1(l, p);
+      }
+
+      size_t bs = static_cast<size_t>(e - p) /
+                  (num_crc_streams + num_pclmul_streams) / 64;
+      const uint8_t* stream_start = p;
+      const uint8_t* crc_streams[kMaxStreams];
+      for (size_t i = 0; i < num_crc_streams; i++) {
+        crc_streams[i] = stream_start;
+        stream_start += bs * 64;
+      }
+      const uint8_t* pclmul_streams[kMaxStreams];
+      for (size_t i = 0; i < num_pclmul_streams; i++) {
+        pclmul_streams[i] = stream_start;
+        stream_start += bs * 64;
+      }
+
+      // Per stream crc sums.
+      uint64_t l64_crc[kMaxStreams] = {l};
+      uint64_t l64_pclmul[kMaxStreams] = {0};
+
+      // Peel first iteration, because PCLMULQDQ stream, needs setup.
+      if (num_crc_streams == 1) {
+        l64_crc[0] = Process64BytesCRC(crc_streams[0], l64_crc[0]);
+        crc_streams[0] += 16 * 4;
+      } else if (num_crc_streams == 2) {
+        Process64BytesCRC2Streams(crc_streams[0], crc_streams[1], l64_crc);
+        crc_streams[0] += 16 * 4;
+        crc_streams[1] += 16 * 4;
+      } else {
+        Process64BytesCRC3Streams(crc_streams[0], crc_streams[1],
+                                  crc_streams[2], l64_crc);
+        crc_streams[0] += 16 * 4;
+        crc_streams[1] += 16 * 4;
+        crc_streams[2] += 16 * 4;
+      }
+
+      // Align to 32 bytes for vpclmul implementation.
+      alignas(32) V128 partialCRC[kMaxStreams][4];
+      for (size_t i = 0; i < num_pclmul_streams; i++) {
+        InitPclmulStream(&pclmul_streams[i], partialCRC[i]);
+      }
+
+      for (size_t i = 1; i < bs; i++) {
+        // Prefetch data for next iterations.
+        for (size_t j = 0; j < num_crc_streams; j++) {
+          PrefetchToLocalCache(
+              reinterpret_cast<const char*>(crc_streams[j] + kPrefetchHorizon));
+        }
+        for (size_t j = 0; j < num_pclmul_streams; j++) {
+          PrefetchToLocalCache(reinterpret_cast<const char*>(pclmul_streams[j] +
+                                                             kPrefetchHorizon));
+        }
+
+        // We process each stream in 64 byte blocks. This can be written as
+        // for (int i = 0; i < num_pclmul_streams; i++) {
+        //   Process64BytesPclmul(pclmul_streams[i], partialCRC[i]);
+        //   pclmul_streams[i] += 16 * 4;
+        // }
+        // for (int i = 0; i < num_crc_streams; i++) {
+        //   l64_crc[i] = Process64BytesCRC(crc_streams[i], l64_crc[i]);
+        //   crc_streams[i] += 16*4;
+        // }
+        // But unrolling and interleaving PCLMULQDQ and CRC blocks manually
+        // gives ~2% performance boost.
+        if (num_crc_streams == 1) {
+          l64_crc[0] = Process64BytesCRC(crc_streams[0], l64_crc[0]);
+          crc_streams[0] += 16 * 4;
+        } else if (num_crc_streams == 2) {
+          Process64BytesCRC2Streams(crc_streams[0], crc_streams[1], l64_crc);
+          crc_streams[0] += 16 * 4;
+          crc_streams[1] += 16 * 4;
+        } else {
+          Process64BytesCRC3Streams(crc_streams[0], crc_streams[1],
+                                    crc_streams[2], l64_crc);
+          crc_streams[0] += 16 * 4;
+          crc_streams[1] += 16 * 4;
+          crc_streams[2] += 16 * 4;
+        }
+        for (size_t j = 0; j < num_pclmul_streams; j++) {
+          ProcessPclmulStream(&pclmul_streams[j], partialCRC[j]);
+        }
+      }
+
+      // PCLMULQDQ based streams require special final step;
+      // CRC based don't.
+      for (size_t i = 0; i < num_pclmul_streams; i++) {
+        l64_pclmul[i] = FinalizePclmulStream(partialCRC[i]);
+      }
+
+      // Combine all streams into single result.
+      static_assert(64 % (1 << kNumDroppedBits) == 0);
+      uint32_t magic = ComputeZeroConstant(bs * 64);
+      l64 = l64_crc[0];
+      for (size_t i = 1; i < num_crc_streams; i++) {
+        l64 = MultiplyWithExtraX33(static_cast<uint32_t>(l64), magic);
+        l64 ^= l64_crc[i];
+      }
+      for (size_t i = 0; i < num_pclmul_streams; i++) {
+        l64 = MultiplyWithExtraX33(static_cast<uint32_t>(l64), magic);
+        l64 ^= l64_pclmul[i];
+      }
+
+      // Update p.
+      if constexpr (num_pclmul_streams > 0) {
+        p = pclmul_streams[num_pclmul_streams - 1];
+      } else {
+        p = crc_streams[num_crc_streams - 1];
+      }
+      l = static_cast<uint32_t>(l64);
+    }
+
+    uint64_t remaining_bytes = static_cast<uint64_t>(e - p);
+    // Process the remaining bytes.
+    while ((e - p) >= 16) {
+      ABSL_INTERNAL_STEP8(l, p);
+      ABSL_INTERNAL_STEP8(l, p);
+    }
+
+    if (remaining_bytes & 8) {
+      ABSL_INTERNAL_STEP8(l, p);
+    }
+    if (remaining_bytes & 4) {
+      ABSL_INTERNAL_STEP4(l, p);
+    }
+    if (remaining_bytes & 2) {
+      ABSL_INTERNAL_STEP2(l, p);
+    }
+    if (remaining_bytes & 1) {
+      ABSL_INTERNAL_STEP1(l, p);
+    }
+
+    *crc = l;
+  }
+
+ private:
+  ABSL_ATTRIBUTE_ALWAYS_INLINE void InitPclmulStream(
+      const uint8_t** pclmul_stream, V128* partialCRC) const {
+    if constexpr (pclmul_stream_type == PclmulStreamType::VPCLMUL) {
+      V256* vpartialCRC = reinterpret_cast<V256*>(partialCRC);
+      vpartialCRC[0] =
+          V256_LoadU(reinterpret_cast<const V256*>(*pclmul_stream + 32 * 0));
+      vpartialCRC[1] =
+          V256_LoadU(reinterpret_cast<const V256*>(*pclmul_stream + 32 * 1));
+    } else {
+      partialCRC[0] =
+          V128_LoadU(reinterpret_cast<const V128*>(*pclmul_stream + 16 * 0));
+      partialCRC[1] =
+          V128_LoadU(reinterpret_cast<const V128*>(*pclmul_stream + 16 * 1));
+      partialCRC[2] =
+          V128_LoadU(reinterpret_cast<const V128*>(*pclmul_stream + 16 * 2));
+      partialCRC[3] =
+          V128_LoadU(reinterpret_cast<const V128*>(*pclmul_stream + 16 * 3));
+    }
+    *pclmul_stream += 16 * 4;
+  }
+
+  ABSL_ATTRIBUTE_ALWAYS_INLINE void ProcessPclmulStream(
+      const uint8_t** pclmul_stream, V128* partialCRC) const {
+    if constexpr (pclmul_stream_type == PclmulStreamType::VPCLMUL) {
+      V256 loopMultiplicands =
+          V256_Broadcast128(reinterpret_cast<const V128*>(kFoldAcross512Bits));
+      Process64BytesVpclmul(*pclmul_stream, reinterpret_cast<V256*>(partialCRC),
+                            loopMultiplicands);
+    } else if constexpr (pclmul_stream_type == PclmulStreamType::NEON_PCLMUL ||
+                         pclmul_stream_type ==
+                             PclmulStreamType::NEON_PCLMUL_EOR3) {
+      constexpr bool kUseEor3 =
+          (pclmul_stream_type == PclmulStreamType::NEON_PCLMUL_EOR3);
+      Process64BytesNeonPclmul<kUseEor3>(*pclmul_stream, partialCRC);
+    } else {
+      Process64BytesPclmul(*pclmul_stream, partialCRC);
+    }
+    *pclmul_stream += 16 * 4;
+  }
+
+  ABSL_ATTRIBUTE_ALWAYS_INLINE uint64_t
+  FinalizePclmulStream(V128* partialCRC) const {
+    if constexpr (pclmul_stream_type == PclmulStreamType::VPCLMUL) {
+      return FinalizeVpclmulStream(reinterpret_cast<V256*>(partialCRC));
+    } else if constexpr (pclmul_stream_type == PclmulStreamType::NEON_PCLMUL ||
+                         pclmul_stream_type ==
+                             PclmulStreamType::NEON_PCLMUL_EOR3) {
+      constexpr bool kUseEor3 =
+          (pclmul_stream_type == PclmulStreamType::NEON_PCLMUL_EOR3);
+      return FinalizeNeonPclmulStream<kUseEor3>(partialCRC);
+    } else {
+      return CRC32AcceleratedX86ARMCombinedMultipleStreamsBase::
+          FinalizePclmulStream(partialCRC);
+    }
+  }
+};
+
+#undef ABSL_INTERNAL_STEP8BY3
+#undef ABSL_INTERNAL_STEP8BY2
+#undef ABSL_INTERNAL_STEP8
+#undef ABSL_INTERNAL_STEP4
+#undef ABSL_INTERNAL_STEP2
+#undef ABSL_INTERNAL_STEP1
+
+}  // namespace
+
+// Intel processors with SSE4.2 have an instruction for one particular
+// 32-bit CRC polynomial:  crc32c
+CRCImpl* TryNewCRC32AcceleratedX86ARMCombined() {
+  CpuType type = GetCpuType();
+  switch (type) {
+    case CpuType::kAmdRome:
+      return new CRC32AcceleratedX86ARMCombinedMultipleStreams<
+          3, 0, PclmulStreamType::PCLMUL>();
+    case CpuType::kIntelHaswell:
+    case CpuType::kAmdNaples:
+      return new CRC32AcceleratedX86ARMCombinedMultipleStreams<
+          3, 1, PclmulStreamType::PCLMUL>();
+    case CpuType::kAmdMilan:
+    case CpuType::kAmdGenoa:
+    case CpuType::kAmdTurin:
+#if defined(ABSL_CRC_INTERNAL_HAVE_X86_SIMD) && defined(__AVX__) && \
+    (!defined(_MSC_VER) || defined(__clang__))
+      // We don't have vector pclmul on arm, but this still needs to
+      // compile.
+      return new CRC32AcceleratedX86ARMCombinedMultipleStreams<
+          3, 1, PclmulStreamType::VPCLMUL>();
+#else
+      return new CRC32AcceleratedX86ARMCombinedMultipleStreams<
+          3, 1, PclmulStreamType::PCLMUL>();
+#endif
+    // PCLMULQDQ is fast, use combined PCLMULQDQ + CRC implementation.
+    case CpuType::kIntelCascadelakeXeon:
+    case CpuType::kIntelSkylakeXeon:
+    case CpuType::kIntelBroadwell:
+    case CpuType::kIntelSkylake:
+    case CpuType::kIntelIcelake:
+    case CpuType::kIntelSapphirerapids:
+    case CpuType::kIntelEmeraldrapids:
+    case CpuType::kIntelGraniterapids:
+      return new CRC32AcceleratedX86ARMCombinedMultipleStreams<
+          3, 2, PclmulStreamType::PCLMUL>();
+    // PCLMULQDQ is slow, don't use it.
+    case CpuType::kIntelIvybridge:
+    case CpuType::kIntelSandybridge:
+    case CpuType::kIntelWestmere:
+      return new CRC32AcceleratedX86ARMCombinedMultipleStreams<
+          3, 0, PclmulStreamType::PCLMUL>();
+    case CpuType::kArmNeoverseN1:
+    case CpuType::kArmNeoverseV1:
+      return new CRC32AcceleratedX86ARMCombinedMultipleStreams<
+          1, 1, PclmulStreamType::NEON_PCLMUL>();
+    case CpuType::kArmNeoverseN2:
+    case CpuType::kArmNeoverseN3:
+    case CpuType::kNvidiaGrace:
+      return new CRC32AcceleratedX86ARMCombinedMultipleStreams<
+          1, 1, PclmulStreamType::NEON_PCLMUL_EOR3>();
+    case CpuType::kAmpereSiryn:
+      return new CRC32AcceleratedX86ARMCombinedMultipleStreams<
+          3, 2, PclmulStreamType::NEON_PCLMUL_EOR3>();
+    case CpuType::kArmNeoverseV2:
+      return new CRC32AcceleratedX86ARMCombinedMultipleStreams<
+          1, 2, PclmulStreamType::NEON_PCLMUL_EOR3>();
+#if defined(__aarch64__)
+    default:
+      // Not all ARM processors support the needed instructions, so check here
+      // before trying to use an accelerated implementation.
+      if (SupportsArmCRC32PMULL()) {
+        return new CRC32AcceleratedX86ARMCombinedMultipleStreams<
+            1, 1, PclmulStreamType::NEON_PCLMUL>();
+      } else {
+        return nullptr;
+      }
+#else
+    default:
+      // Something else, play it safe and assume slow PCLMULQDQ.
+      return new CRC32AcceleratedX86ARMCombinedMultipleStreams<
+          3, 0, PclmulStreamType::PCLMUL>();
+#endif
+  }
+}
+
+#else  // !ABSL_INTERNAL_CAN_USE_SIMD_CRC32C
+
+// no hardware acceleration available
+CRCImpl* TryNewCRC32AcceleratedX86ARMCombined() { return nullptr; }
+
+#endif
+
+}  // namespace crc_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/crc/internal/gen_crc32c_consts.py b/absl/crc/internal/gen_crc32c_consts.py
new file mode 100755
index 00000000000..f78ae302d5c
--- /dev/null
+++ b/absl/crc/internal/gen_crc32c_consts.py
@@ -0,0 +1,90 @@
+#!/usr/bin/env python3
+#
+# Copyright 2025 The Abseil Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""This script generates kCRC32CPowers[]."""
+
+
+def poly_mul(a, b):
+  """Polynomial multiplication: a * b."""
+  product = 0
+  for i in range(b.bit_length()):
+    if (b & (1 << i)) != 0:
+      product ^= a << i
+  return product
+
+
+def poly_div(a, b):
+  """Polynomial division: floor(a / b)."""
+  q = 0
+  while a.bit_length() >= b.bit_length():
+    q ^= 1 << (a.bit_length() - b.bit_length())
+    a ^= b << (a.bit_length() - b.bit_length())
+  return q
+
+
+def poly_reduce(a, b):
+  """Polynomial reduction: a mod b."""
+  return a ^ poly_mul(poly_div(a, b), b)
+
+
+def poly_exp(a, b, g):
+  """Polynomial exponentiation: a^b mod g."""
+  if b == 1:
+    return poly_reduce(a, g)
+  c = poly_exp(a, b // 2, g)
+  c = poly_mul(c, c)
+  if b % 2 != 0:
+    c = poly_mul(c, a)
+  return poly_reduce(c, g)
+
+
+def bitreflect(a, num_bits):
+  """Reflects the bits of the given integer."""
+  if a.bit_length() > num_bits:
+    raise ValueError(f'Integer has more than {num_bits} bits')
+  return sum(((a >> i) & 1) << (num_bits - 1 - i) for i in range(num_bits))
+
+
+G = 0x11EDC6F41  # The CRC-32C reducing polynomial, in the "natural" bit order
+CRC_BITS = 32  # The degree of G, i.e. the 32 in "CRC-32C"
+LSB_FIRST = True  # CRC-32C is a least-significant-bit-first CRC
+NUM_SIZE_BITS = 64  # The maximum number of bits in the length (size_t)
+NUM_DROPPED_BITS = 4  # The number of bits dropped from the length
+LOG2_BITS_PER_BYTE = 3  # log2 of the number of bits in a byte, i.e. log2(8)
+X = 2  # The polynomial 'x', in the "natural" bit order
+
+
+def print_crc32c_powers():
+  """Generates kCRC32CPowers[].
+
+  kCRC32CPowers[] is an array of length NUM_SIZE_BITS - NUM_DROPPED_BITS,
+  whose i'th entry is x^(2^(i + LOG2_BITS_PER_BYTE + NUM_DROPPED_BITS) -
+  CRC_BITS - 1) mod G. See kCRC32CPowers[] in the C++ source for more info.
+  """
+  for i in range(NUM_SIZE_BITS - NUM_DROPPED_BITS):
+    poly = poly_exp(
+        X,
+        2 ** (i + LOG2_BITS_PER_BYTE + NUM_DROPPED_BITS)
+        - CRC_BITS
+        - (1 if LSB_FIRST else 0),
+        G,
+    )
+    poly = bitreflect(poly, CRC_BITS)
+    print(f'0x{poly:0{2*CRC_BITS//8}x}, ', end='')
+
+
+if __name__ == '__main__':
+  print_crc32c_powers()
diff --git a/absl/crc/internal/non_temporal_arm_intrinsics.h b/absl/crc/internal/non_temporal_arm_intrinsics.h
new file mode 100644
index 00000000000..a0d2f087a7b
--- /dev/null
+++ b/absl/crc/internal/non_temporal_arm_intrinsics.h
@@ -0,0 +1,81 @@
+// Copyright 2022 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// SKIP_ABSL_INLINE_NAMESPACE_CHECK
+
+#ifndef ABSL_CRC_INTERNAL_NON_TEMPORAL_ARM_INTRINSICS_H_
+#define ABSL_CRC_INTERNAL_NON_TEMPORAL_ARM_INTRINSICS_H_
+
+#include "absl/base/config.h"
+
+#ifdef __aarch64__
+#include <arm_neon.h>
+
+typedef int64x2_t __m128i; /* 128-bit vector containing integers */
+#define vreinterpretq_m128i_s64(x) (x)
+#define vreinterpretq_s64_m128i(x) (x)
+
+// Guarantees that every preceding store is globally visible before any
+// subsequent store.
+// https://msdn.microsoft.com/en-us/library/5h2w73d1%28v=vs.90%29.aspx
+static inline __attribute__((always_inline)) void _mm_sfence(void) {
+  __sync_synchronize();
+}
+
+// Load 128-bits of integer data from unaligned memory into dst. This intrinsic
+// may perform better than _mm_loadu_si128 when the data crosses a cache line
+// boundary.
+//
+//   dst[127:0] := MEM[mem_addr+127:mem_addr]
+//
+// https://software.intel.com/sites/landingpage/IntrinsicsGuide/#text=_mm_lddqu_si128
+#define _mm_lddqu_si128 _mm_loadu_si128
+
+// Loads 128-bit value. :
+// https://msdn.microsoft.com/zh-cn/library/f4k12ae8(v=vs.90).aspx
+static inline __attribute__((always_inline)) __m128i _mm_loadu_si128(
+    const __m128i *p) {
+  return vreinterpretq_m128i_s64(vld1q_s64((const int64_t*)p));
+}
+
+// Stores the data in a to the address p without polluting the caches.  If the
+// cache line containing address p is already in the cache, the cache will be
+// updated.
+// https://msdn.microsoft.com/en-us/library/ba08y07y%28v=vs.90%29.aspx
+static inline __attribute__((always_inline)) void _mm_stream_si128(__m128i *p,
+                                                                   __m128i a) {
+#if ABSL_HAVE_BUILTIN(__builtin_nontemporal_store)
+  __builtin_nontemporal_store(a, p);
+#else
+  vst1q_s64((int64_t *)p, vreinterpretq_s64_m128i(a));
+#endif
+}
+
+// Sets the 16 signed 8-bit integer values.
+// https://msdn.microsoft.com/en-us/library/x0cx8zd3(v=vs.90).aspx
+static inline __attribute__((always_inline)) __m128i _mm_set_epi8(
+    signed char b15, signed char b14, signed char b13, signed char b12,
+    signed char b11, signed char b10, signed char b9, signed char b8,
+    signed char b7, signed char b6, signed char b5, signed char b4,
+    signed char b3, signed char b2, signed char b1, signed char b0) {
+  int8_t __attribute__((aligned(16)))
+  data[16] = {(int8_t)b0,  (int8_t)b1,  (int8_t)b2,  (int8_t)b3,
+              (int8_t)b4,  (int8_t)b5,  (int8_t)b6,  (int8_t)b7,
+              (int8_t)b8,  (int8_t)b9,  (int8_t)b10, (int8_t)b11,
+              (int8_t)b12, (int8_t)b13, (int8_t)b14, (int8_t)b15};
+  return (__m128i)vld1q_s8(data);
+}
+#endif  // __aarch64__
+
+#endif  // ABSL_CRC_INTERNAL_NON_TEMPORAL_ARM_INTRINSICS_H_
diff --git a/absl/crc/internal/non_temporal_memcpy.h b/absl/crc/internal/non_temporal_memcpy.h
new file mode 100644
index 00000000000..96eb17834ce
--- /dev/null
+++ b/absl/crc/internal/non_temporal_memcpy.h
@@ -0,0 +1,195 @@
+// Copyright 2022 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_CRC_INTERNAL_NON_TEMPORAL_MEMCPY_H_
+#define ABSL_CRC_INTERNAL_NON_TEMPORAL_MEMCPY_H_
+
+#include <algorithm>
+#include <cassert>
+#include <cstdint>
+#include <cstring>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/optimization.h"
+
+#ifdef _MSC_VER
+#include <intrin.h>
+#endif
+
+#if defined(__SSE__) || defined(__AVX__)
+// Pulls in both SSE and AVX intrinsics.
+#include <immintrin.h>
+#endif
+
+#ifdef __aarch64__
+#include "absl/crc/internal/non_temporal_arm_intrinsics.h"
+#endif
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace crc_internal {
+
+// This non-temporal memcpy does regular load and non-temporal store memory
+// copy. It is compatible to both 16-byte aligned and unaligned addresses. If
+// data at the destination is not immediately accessed, using non-temporal
+// memcpy can save 1 DRAM load of the destination cacheline.
+constexpr size_t kCacheLineSize = ABSL_CACHELINE_SIZE;
+
+// If the objects overlap, the behavior is undefined. Uses regular memcpy
+// instead of non-temporal memcpy if the required CPU intrinsics are unavailable
+// at compile time.
+inline void *non_temporal_store_memcpy(void *__restrict dst,
+                                       const void *__restrict src, size_t len) {
+#if defined(__SSE3__) || defined(__aarch64__) || \
+    (defined(_MSC_VER) && defined(__AVX__))
+  // This implementation requires SSE3.
+  // MSVC cannot target SSE3 directly, but when MSVC targets AVX,
+  // SSE3 support is implied.
+  uint8_t *d = reinterpret_cast<uint8_t *>(dst);
+  const uint8_t *s = reinterpret_cast<const uint8_t *>(src);
+
+  // memcpy() the misaligned header. At the end of this if block, <d> is
+  // aligned to a 64-byte cacheline boundary or <len> == 0.
+  if (reinterpret_cast<uintptr_t>(d) & (kCacheLineSize - 1)) {
+    uintptr_t bytes_before_alignment_boundary =
+        kCacheLineSize -
+        (reinterpret_cast<uintptr_t>(d) & (kCacheLineSize - 1));
+    size_t header_len = (std::min)(bytes_before_alignment_boundary, len);
+    assert(bytes_before_alignment_boundary < kCacheLineSize);
+    memcpy(d, s, header_len);
+    d += header_len;
+    s += header_len;
+    len -= header_len;
+  }
+
+  if (len >= kCacheLineSize) {
+    _mm_sfence();
+    __m128i *dst_cacheline = reinterpret_cast<__m128i *>(d);
+    const __m128i *src_cacheline = reinterpret_cast<const __m128i *>(s);
+    constexpr int kOpsPerCacheLine = kCacheLineSize / sizeof(__m128i);
+    size_t loops = len / kCacheLineSize;
+
+    while (len >= kCacheLineSize) {
+      __m128i temp1, temp2, temp3, temp4;
+      temp1 = _mm_lddqu_si128(src_cacheline + 0);
+      temp2 = _mm_lddqu_si128(src_cacheline + 1);
+      temp3 = _mm_lddqu_si128(src_cacheline + 2);
+      temp4 = _mm_lddqu_si128(src_cacheline + 3);
+      _mm_stream_si128(dst_cacheline + 0, temp1);
+      _mm_stream_si128(dst_cacheline + 1, temp2);
+      _mm_stream_si128(dst_cacheline + 2, temp3);
+      _mm_stream_si128(dst_cacheline + 3, temp4);
+      src_cacheline += kOpsPerCacheLine;
+      dst_cacheline += kOpsPerCacheLine;
+      len -= kCacheLineSize;
+    }
+    d += loops * kCacheLineSize;
+    s += loops * kCacheLineSize;
+    _mm_sfence();
+  }
+
+  // memcpy the tail.
+  if (len) {
+    memcpy(d, s, len);
+  }
+  return dst;
+#else
+  // Fallback to regular memcpy.
+  return memcpy(dst, src, len);
+#endif  // __SSE3__ || __aarch64__ || (_MSC_VER && __AVX__)
+}
+
+// We try to force non_temporal_store_memcpy_avx to use AVX instructions
+// so that we can select it at runtime when AVX is available.
+// Clang on Windows has gnu::target but does not make AVX types like __m256i
+// available when trying to force specific functions to use AVX compiles.
+#if ABSL_HAVE_CPP_ATTRIBUTE(gnu::target) && !defined(_MSC_VER) && \
+    (defined(__x86_64__) || defined(__i386__))
+#define ABSL_INTERNAL_CAN_FORCE_AVX 1
+#endif
+
+// If the objects overlap, the behavior is undefined. Uses regular memcpy
+// instead of non-temporal memcpy if the required CPU intrinsics are unavailable
+// at compile time.
+#ifdef ABSL_INTERNAL_CAN_FORCE_AVX
+[[gnu::target("avx")]]
+#endif
+inline void *non_temporal_store_memcpy_avx(void *__restrict dst,
+                                           const void *__restrict src,
+                                           size_t len) {
+  // This function requires AVX. If possible we compile it with AVX even if the
+  // translation unit isn't built with AVX support. This works because we only
+  // select this implementation at runtime if the CPU supports AVX.
+  // MSVC AVX support implies SSE3 support.
+#if ((defined(__AVX__) || defined(ABSL_INTERNAL_CAN_FORCE_AVX)) && \
+     defined(__SSE3__)) ||                                         \
+    (defined(_MSC_VER) && defined(__AVX__))
+  uint8_t *d = reinterpret_cast<uint8_t *>(dst);
+  const uint8_t *s = reinterpret_cast<const uint8_t *>(src);
+
+  // memcpy() the misaligned header. At the end of this if block, <d> is
+  // aligned to a 64-byte cacheline boundary or <len> == 0.
+  if (reinterpret_cast<uintptr_t>(d) & (kCacheLineSize - 1)) {
+    uintptr_t bytes_before_alignment_boundary =
+        kCacheLineSize -
+        (reinterpret_cast<uintptr_t>(d) & (kCacheLineSize - 1));
+    size_t header_len = (std::min)(bytes_before_alignment_boundary, len);
+    assert(bytes_before_alignment_boundary < kCacheLineSize);
+    memcpy(d, s, header_len);
+    d += header_len;
+    s += header_len;
+    len -= header_len;
+  }
+
+  if (len >= kCacheLineSize) {
+    _mm_sfence();
+    __m256i *dst_cacheline = reinterpret_cast<__m256i *>(d);
+    const __m256i *src_cacheline = reinterpret_cast<const __m256i *>(s);
+    constexpr int kOpsPerCacheLine = kCacheLineSize / sizeof(__m256i);
+    size_t loops = len / kCacheLineSize;
+
+    while (len >= kCacheLineSize) {
+      __m256i temp1, temp2;
+      temp1 = _mm256_lddqu_si256(src_cacheline + 0);
+      temp2 = _mm256_lddqu_si256(src_cacheline + 1);
+      _mm256_stream_si256(dst_cacheline + 0, temp1);
+      _mm256_stream_si256(dst_cacheline + 1, temp2);
+      src_cacheline += kOpsPerCacheLine;
+      dst_cacheline += kOpsPerCacheLine;
+      len -= kCacheLineSize;
+    }
+    d += loops * kCacheLineSize;
+    s += loops * kCacheLineSize;
+    _mm_sfence();
+  }
+
+  // memcpy the tail.
+  if (len) {
+    memcpy(d, s, len);
+  }
+  return dst;
+#else
+  // Fallback to regular memcpy so that this function compiles.
+  return memcpy(dst, src, len);
+#endif
+}
+
+#undef ABSL_INTERNAL_CAN_FORCE_AVX
+
+}  // namespace crc_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_CRC_INTERNAL_NON_TEMPORAL_MEMCPY_H_
diff --git a/absl/crc/internal/non_temporal_memcpy_test.cc b/absl/crc/internal/non_temporal_memcpy_test.cc
new file mode 100644
index 00000000000..179998ebfbe
--- /dev/null
+++ b/absl/crc/internal/non_temporal_memcpy_test.cc
@@ -0,0 +1,91 @@
+// Copyright 2022 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/crc/internal/non_temporal_memcpy.h"
+
+#include <algorithm>
+#include <cstddef>
+#include <cstdint>
+#include <iostream>
+#include <vector>
+
+#include "gtest/gtest.h"
+
+namespace {
+
+struct TestParam {
+  size_t copy_size;
+  uint32_t src_offset;
+  uint32_t dst_offset;
+};
+
+class NonTemporalMemcpyTest : public testing::TestWithParam<TestParam> {
+ protected:
+  void SetUp() override {
+    // Make buf_size multiple of 16 bytes.
+    size_t buf_size = ((std::max(GetParam().src_offset, GetParam().dst_offset) +
+                        GetParam().copy_size) +
+                       15) /
+                      16 * 16;
+    a_.resize(buf_size);
+    b_.resize(buf_size);
+    for (size_t i = 0; i < buf_size; i++) {
+      a_[i] = static_cast<uint8_t>(i % 256);
+      b_[i] = ~a_[i];
+    }
+  }
+
+  std::vector<uint8_t> a_, b_;
+};
+
+TEST_P(NonTemporalMemcpyTest, SSEEquality) {
+  uint8_t *src = a_.data() + GetParam().src_offset;
+  uint8_t *dst = b_.data() + GetParam().dst_offset;
+  absl::crc_internal::non_temporal_store_memcpy(dst, src, GetParam().copy_size);
+  for (size_t i = 0; i < GetParam().copy_size; i++) {
+    EXPECT_EQ(src[i], dst[i]);
+  }
+}
+
+#ifdef __AVX__
+TEST_P(NonTemporalMemcpyTest, AVXEquality) {
+  uint8_t* src = a_.data() + GetParam().src_offset;
+  uint8_t* dst = b_.data() + GetParam().dst_offset;
+
+  absl::crc_internal::non_temporal_store_memcpy_avx(dst, src,
+                                                    GetParam().copy_size);
+  for (size_t i = 0; i < GetParam().copy_size; i++) {
+    EXPECT_EQ(src[i], dst[i]);
+  }
+}
+#endif
+
+// 63B is smaller than one cacheline operation thus the non-temporal routine
+// will not be called.
+// 4352B is sufficient for testing 4092B data copy with room for offsets.
+constexpr TestParam params[] = {
+    {63, 0, 0},       {58, 5, 5},    {61, 2, 0},    {61, 0, 2},
+    {58, 5, 2},       {4096, 0, 0},  {4096, 0, 1},  {4096, 0, 2},
+    {4096, 0, 3},     {4096, 0, 4},  {4096, 0, 5},  {4096, 0, 6},
+    {4096, 0, 7},     {4096, 0, 8},  {4096, 0, 9},  {4096, 0, 10},
+    {4096, 0, 11},    {4096, 0, 12}, {4096, 0, 13}, {4096, 0, 14},
+    {4096, 0, 15},    {4096, 7, 7},  {4096, 3, 0},  {4096, 1, 0},
+    {4096, 9, 3},     {4096, 9, 11}, {8192, 0, 0},  {8192, 5, 2},
+    {1024768, 7, 11}, {1, 0, 0},     {1, 0, 1},     {1, 1, 0},
+    {1, 1, 1}};
+
+INSTANTIATE_TEST_SUITE_P(ParameterizedNonTemporalMemcpyTest,
+                         NonTemporalMemcpyTest, testing::ValuesIn(params));
+
+}  // namespace
diff --git a/absl/debugging/BUILD.bazel b/absl/debugging/BUILD.bazel
index b87c55a54f6..d3eb74da5ae 100644
--- a/absl/debugging/BUILD.bazel
+++ b/absl/debugging/BUILD.bazel
@@ -14,7 +14,9 @@
 # limitations under the License.
 #
 
-load("@rules_cc//cc:defs.bzl", "cc_library", "cc_test")
+load("@rules_cc//cc:cc_binary.bzl", "cc_binary")
+load("@rules_cc//cc:cc_library.bzl", "cc_library")
+load("@rules_cc//cc:cc_test.bzl", "cc_test")
 load(
     "//absl:copts/configure_copts.bzl",
     "ABSL_DEFAULT_COPTS",
@@ -23,22 +25,58 @@ load(
 )
 
 package(
-    default_visibility = ["//visibility:public"],
+    default_visibility = ["//visibility:private"],
+    features = [
+        "header_modules",
+        "layering_check",
+        "parse_headers",
+    ],
 )
 
-licenses(["notice"])  # Apache 2.0
+licenses(["notice"])
 
 cc_library(
     name = "stacktrace",
     srcs = [
+        "internal/stacktrace_aarch64-inl.inc",
+        "internal/stacktrace_arm-inl.inc",
+        "internal/stacktrace_config.h",
+        "internal/stacktrace_emscripten-inl.inc",
+        "internal/stacktrace_generic-inl.inc",
+        "internal/stacktrace_powerpc-inl.inc",
+        "internal/stacktrace_riscv-inl.inc",
+        "internal/stacktrace_unimplemented-inl.inc",
+        "internal/stacktrace_win32-inl.inc",
+        "internal/stacktrace_x86-inl.inc",
         "stacktrace.cc",
     ],
     hdrs = ["stacktrace.h"],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
     deps = [
         ":debugging_internal",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:dynamic_annotations",
+        "//absl/base:malloc_internal",
+        "//absl/base:raw_logging_internal",
+    ],
+)
+
+cc_test(
+    name = "stacktrace_test",
+    srcs = ["stacktrace_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":stacktrace",
+        "//absl/base:config",
         "//absl/base:core_headers",
+        "//absl/base:errno_saver",
+        "//absl/types:span",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -46,7 +84,9 @@ cc_library(
     name = "symbolize",
     srcs = [
         "symbolize.cc",
+        "symbolize_darwin.inc",
         "symbolize_elf.inc",
+        "symbolize_emscripten.inc",
         "symbolize_unimplemented.inc",
         "symbolize_win32.inc",
     ],
@@ -55,31 +95,56 @@ cc_library(
         "symbolize.h",
     ],
     copts = ABSL_DEFAULT_COPTS,
-    linkopts = ABSL_DEFAULT_LINKOPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS + select({
+        "@rules_cc//cc/compiler:msvc-cl": ["-DEFAULTLIB:dbghelp.lib"],
+        "@rules_cc//cc/compiler:clang-cl": ["-DEFAULTLIB:dbghelp.lib"],
+        "//absl:mingw_compiler": [
+            "-DEFAULTLIB:dbghelp.lib",
+            "-ldbghelp",
+        ],
+        "//conditions:default": [],
+    }),
+    visibility = ["//visibility:public"],
     deps = [
         ":debugging_internal",
         ":demangle_internal",
         "//absl/base",
+        "//absl/base:config",
         "//absl/base:core_headers",
         "//absl/base:dynamic_annotations",
         "//absl/base:malloc_internal",
         "//absl/base:raw_logging_internal",
+        "//absl/strings",
     ],
 )
 
 cc_test(
     name = "symbolize_test",
     srcs = ["symbolize_test.cc"],
-    copts = ABSL_TEST_COPTS,
-    linkopts = ABSL_DEFAULT_LINKOPTS,
+    copts = ABSL_TEST_COPTS + select({
+        "@rules_cc//cc/compiler:msvc-cl": ["/Z7"],
+        "@rules_cc//cc/compiler:clang-cl": ["/Z7"],
+        "//conditions:default": [],
+    }),
+    linkopts = ABSL_DEFAULT_LINKOPTS + select({
+        "@rules_cc//cc/compiler:msvc-cl": ["/DEBUG"],
+        "@rules_cc//cc/compiler:clang-cl": ["/DEBUG"],
+        "//conditions:default": [],
+    }),
     deps = [
         ":stack_consumption",
         ":symbolize",
         "//absl/base",
+        "//absl/base:config",
         "//absl/base:core_headers",
-        "//absl/base:raw_logging_internal",
+        "//absl/base:malloc_internal",
+        "//absl/cleanup",
+        "//absl/log",
+        "//absl/log:check",
         "//absl/memory",
-        "@com_google_googletest//:gtest",
+        "//absl/strings",
+        "//absl/strings:str_format",
+        "@googletest//:gtest",
     ],
 )
 
@@ -93,10 +158,14 @@ cc_library(
     ],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
-    visibility = ["//visibility:private"],
+    visibility = [
+        "//absl/log/internal:__pkg__",
+        "@do_not_use_for_gloop_visibility_only//gloop:__subpackages__",
+    ],
     deps = [
         ":stacktrace",
         ":symbolize",
+        "//absl/base:config",
         "//absl/base:core_headers",
         "//absl/base:raw_logging_internal",
     ],
@@ -108,6 +177,7 @@ cc_library(
     hdrs = ["failure_signal_handler.h"],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
     deps = [
         ":examine_stack",
         ":stacktrace",
@@ -123,16 +193,20 @@ cc_test(
     srcs = ["failure_signal_handler_test.cc"],
     copts = ABSL_TEST_COPTS,
     linkopts = select({
-        "//absl:windows": [],
+        "@rules_cc//cc/compiler:msvc-cl": [],
+        "@rules_cc//cc/compiler:clang-cl": [],
+        "@rules_cc//cc/compiler:emscripten": [],
         "//conditions:default": ["-pthread"],
     }) + ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":failure_signal_handler",
         ":stacktrace",
         ":symbolize",
+        "//absl/base:core_headers",
         "//absl/base:raw_logging_internal",
+        "//absl/log:check",
         "//absl/strings",
-        "@com_google_googletest//:gtest",
+        "@googletest//:gtest",
     ],
 )
 
@@ -145,22 +219,20 @@ cc_library(
     ],
     hdrs = [
         "internal/address_is_readable.h",
+        "internal/addresses.h",
         "internal/elf_mem_image.h",
-        "internal/stacktrace_aarch64-inl.inc",
-        "internal/stacktrace_arm-inl.inc",
-        "internal/stacktrace_config.h",
-        "internal/stacktrace_generic-inl.inc",
-        "internal/stacktrace_powerpc-inl.inc",
-        "internal/stacktrace_unimplemented-inl.inc",
-        "internal/stacktrace_win32-inl.inc",
-        "internal/stacktrace_x86-inl.inc",
         "internal/vdso_support.h",
     ],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "@do_not_use_for_gloop_visibility_only//gloop:__subpackages__",
+    ],
     deps = [
+        "//absl/base:config",
         "//absl/base:core_headers",
         "//absl/base:dynamic_annotations",
+        "//absl/base:errno_saver",
         "//absl/base:raw_logging_internal",
     ],
 )
@@ -170,9 +242,19 @@ cc_library(
     srcs = ["internal/demangle.cc"],
     hdrs = ["internal/demangle.h"],
     copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl/container:__pkg__",
+        "//absl/debugging:__pkg__",
+        "@do_not_use_for_gloop_visibility_only//gloop/base:__pkg__",
+    ],
     deps = [
+        ":demangle_rust",
         "//absl/base",
+        "//absl/base:config",
         "//absl/base:core_headers",
+        "//absl/base:nullability",
+        "//absl/numeric:bits",
     ],
 )
 
@@ -184,105 +266,161 @@ cc_test(
     deps = [
         ":demangle_internal",
         ":stack_consumption",
+        "//absl/base:config",
         "//absl/base:core_headers",
-        "//absl/base:raw_logging_internal",
+        "//absl/log",
         "//absl/memory",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
 cc_library(
-    name = "leak_check",
-    srcs = ["leak_check.cc"],
-    hdrs = ["leak_check.h"],
+    name = "bounded_utf8_length_sequence",
+    hdrs = ["internal/bounded_utf8_length_sequence.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        "//absl/base:config",
+        "//absl/numeric:bits",
+    ],
+)
+
+cc_test(
+    name = "bounded_utf8_length_sequence_test",
+    srcs = ["internal/bounded_utf8_length_sequence_test.cc"],
+    copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
-    deps = ["//absl/base:core_headers"],
+    deps = [
+        ":bounded_utf8_length_sequence",
+        "//absl/base:config",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
 )
 
-# Adding a dependency to leak_check_disable will disable
-# sanitizer leak checking (asan/lsan) in a test without
-# the need to mess around with build features.
 cc_library(
-    name = "leak_check_disable",
-    srcs = ["leak_check_disable.cc"],
+    name = "decode_rust_punycode",
+    srcs = ["internal/decode_rust_punycode.cc"],
+    hdrs = ["internal/decode_rust_punycode.h"],
+    copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
-    linkstatic = 1,
-    alwayslink = 1,
+    deps = [
+        ":bounded_utf8_length_sequence",
+        ":utf8_for_code_point",
+        "//absl/base:config",
+        "//absl/base:nullability",
+    ],
 )
 
-# These targets exists for use in tests only, explicitly configuring the
-# LEAK_SANITIZER macro. It must be linked with -fsanitize=leak for lsan.
-ABSL_LSAN_LINKOPTS = select({
-    "//absl:llvm_compiler": ["-fsanitize=leak"],
-    "//conditions:default": [],
-})
+cc_test(
+    name = "decode_rust_punycode_test",
+    srcs = ["internal/decode_rust_punycode_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":decode_rust_punycode",
+        "//absl/base:config",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
 
 cc_library(
-    name = "leak_check_api_enabled_for_testing",
-    testonly = 1,
-    srcs = ["leak_check.cc"],
-    hdrs = ["leak_check.h"],
-    copts = select({
-        "//absl:llvm_compiler": ["-DLEAK_SANITIZER"],
-        "//conditions:default": [],
-    }),
+    name = "demangle_rust",
+    srcs = ["internal/demangle_rust.cc"],
+    hdrs = ["internal/demangle_rust.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "@do_not_use_for_gloop_visibility_only//gloop/util/symbolize:__pkg__",
+    ],
+    deps = [
+        ":decode_rust_punycode",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+    ],
+)
+
+cc_test(
+    name = "demangle_rust_test",
+    srcs = ["internal/demangle_rust_test.cc"],
+    copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
-    visibility = ["//visibility:private"],
+    deps = [
+        ":demangle_rust",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
 )
 
 cc_library(
-    name = "leak_check_api_disabled_for_testing",
-    testonly = 1,
-    srcs = ["leak_check.cc"],
-    hdrs = ["leak_check.h"],
-    copts = ["-ULEAK_SANITIZER"],
+    name = "utf8_for_code_point",
+    srcs = ["internal/utf8_for_code_point.cc"],
+    hdrs = ["internal/utf8_for_code_point.h"],
+    copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
-    visibility = ["//visibility:private"],
+    deps = ["//absl/base:config"],
 )
 
 cc_test(
-    name = "leak_check_test",
-    srcs = ["leak_check_test.cc"],
-    copts = select({
-        "//absl:llvm_compiler": ["-DABSL_EXPECT_LEAK_SANITIZER"],
-        "//conditions:default": [],
-    }),
-    linkopts = ABSL_LSAN_LINKOPTS + ABSL_DEFAULT_LINKOPTS,
-    tags = ["notsan"],
+    name = "utf8_for_code_point_test",
+    srcs = ["internal/utf8_for_code_point_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
-        ":leak_check_api_enabled_for_testing",
-        "//absl/base",
-        "@com_google_googletest//:gtest_main",
+        ":utf8_for_code_point",
+        "//absl/base:config",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_library(
+    name = "leak_check",
+    srcs = ["leak_check.cc"],
+    hdrs = ["leak_check.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
+    deps = [
+        "//absl/base:config",
+        "//absl/base:core_headers",
     ],
 )
 
 cc_test(
-    name = "leak_check_no_lsan_test",
+    name = "leak_check_test",
     srcs = ["leak_check_test.cc"],
-    copts = ["-UABSL_EXPECT_LEAK_SANITIZER"],
+    copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
-    tags = ["noasan"],
+    tags = ["notsan"],
     deps = [
-        ":leak_check_api_disabled_for_testing",
-        "//absl/base",  # for raw_logging
-        "@com_google_googletest//:gtest_main",
+        ":leak_check",
+        "//absl/base:config",
+        "//absl/log",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
-# Test that leak checking is skipped when lsan is enabled but
-# ":leak_check_disable" is linked in.
-#
-# This test should fail in the absence of a dependency on ":leak_check_disable"
-cc_test(
-    name = "disabled_leak_check_test",
+# Binary that leaks memory and expects to fail on exit.  This isn't a
+# test that expected to pass on its own; it exists to be called by a
+# script that checks exit status and output.
+# TODO(absl-team): Write a test to run this with a script that
+# verifies that it correctly fails.
+cc_binary(
+    name = "leak_check_fail_test_binary",
     srcs = ["leak_check_fail_test.cc"],
-    linkopts = ABSL_LSAN_LINKOPTS + ABSL_DEFAULT_LINKOPTS,
-    tags = ["notsan"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
-        ":leak_check_api_enabled_for_testing",
-        ":leak_check_disable",
-        "//absl/base",
-        "@com_google_googletest//:gtest_main",
+        ":leak_check",
+        "//absl/log",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -293,8 +431,8 @@ cc_library(
     hdrs = ["internal/stack_consumption.h"],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
-    visibility = ["//visibility:private"],
     deps = [
+        "//absl/base:config",
         "//absl/base:core_headers",
         "//absl/base:raw_logging_internal",
     ],
@@ -305,10 +443,28 @@ cc_test(
     srcs = ["internal/stack_consumption_test.cc"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    tags = ["notsan"],
     deps = [
         ":stack_consumption",
         "//absl/base:core_headers",
-        "//absl/base:raw_logging_internal",
-        "@com_google_googletest//:gtest_main",
+        "//absl/log",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_binary(
+    name = "stacktrace_benchmark",
+    testonly = 1,
+    srcs = ["stacktrace_benchmark.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    tags = ["benchmark"],
+    deps = [
+        ":stacktrace",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/cleanup",
+        "@google_benchmark//:benchmark_main",
     ],
 )
diff --git a/absl/debugging/CMakeLists.txt b/absl/debugging/CMakeLists.txt
index c409e33e39c..fbdb550f832 100644
--- a/absl/debugging/CMakeLists.txt
+++ b/absl/debugging/CMakeLists.txt
@@ -14,21 +14,55 @@
 # limitations under the License.
 #
 
+find_library(EXECINFO_LIBRARY execinfo)
+
 absl_cc_library(
   NAME
     stacktrace
   HDRS
     "stacktrace.h"
+    "internal/stacktrace_aarch64-inl.inc"
+    "internal/stacktrace_arm-inl.inc"
+    "internal/stacktrace_config.h"
+    "internal/stacktrace_emscripten-inl.inc"
+    "internal/stacktrace_generic-inl.inc"
+    "internal/stacktrace_powerpc-inl.inc"
+    "internal/stacktrace_riscv-inl.inc"
+    "internal/stacktrace_unimplemented-inl.inc"
+    "internal/stacktrace_win32-inl.inc"
+    "internal/stacktrace_x86-inl.inc"
   SRCS
     "stacktrace.cc"
   COPTS
     ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    $<$<BOOL:${EXECINFO_LIBRARY}>:${EXECINFO_LIBRARY}>
   DEPS
     absl::debugging_internal
+    absl::config
     absl::core_headers
+    absl::dynamic_annotations
+    absl::malloc_internal
+    absl::raw_logging_internal
   PUBLIC
 )
 
+absl_cc_test(
+  NAME
+    stacktrace_test
+  SRCS
+    "stacktrace_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::stacktrace
+    absl::config
+    absl::core_headers
+    absl::errno_saver
+    absl::span
+    GTest::gmock_main
+)
+
 absl_cc_library(
   NAME
     symbolize
@@ -37,21 +71,26 @@ absl_cc_library(
     "internal/symbolize.h"
   SRCS
     "symbolize.cc"
+    "symbolize_darwin.inc"
     "symbolize_elf.inc"
+    "symbolize_emscripten.inc"
     "symbolize_unimplemented.inc"
     "symbolize_win32.inc"
   COPTS
     ${ABSL_DEFAULT_COPTS}
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
+    $<$<BOOL:${MINGW}>:-ldbghelp>
   DEPS
     absl::debugging_internal
     absl::demangle_internal
     absl::base
+    absl::config
     absl::core_headers
     absl::dynamic_annotations
     absl::malloc_internal
     absl::raw_logging_internal
+    absl::strings
   PUBLIC
 )
 
@@ -62,16 +101,26 @@ absl_cc_test(
     "symbolize_test.cc"
   COPTS
     ${ABSL_TEST_COPTS}
+    $<$<BOOL:${MSVC}>:-Z7>
+  LINKOPTS
+    $<$<BOOL:${MSVC}>:-DEBUG>
   DEPS
-    absl::stack_consumption
-    absl::symbolize
     absl::base
+    absl::check
+    absl::cleanup
+    absl::config
     absl::core_headers
+    absl::log
+    absl::malloc_internal
     absl::memory
-    absl::raw_logging_internal
-    gmock
+    absl::stack_consumption
+    absl::strings
+    absl::str_format
+    absl::symbolize
+    GTest::gmock
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     examine_stack
@@ -84,6 +133,7 @@ absl_cc_library(
   DEPS
     absl::stacktrace
     absl::symbolize
+    absl::config
     absl::core_headers
     absl::raw_logging_internal
 )
@@ -115,29 +165,24 @@ absl_cc_test(
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
+    absl::check
     absl::failure_signal_handler
     absl::stacktrace
     absl::symbolize
     absl::strings
     absl::raw_logging_internal
     Threads::Threads
-    gmock
+    GTest::gmock
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     debugging_internal
   HDRS
     "internal/address_is_readable.h"
+    "internal/addresses.h"
     "internal/elf_mem_image.h"
-    "internal/stacktrace_aarch64-inl.inc"
-    "internal/stacktrace_arm-inl.inc"
-    "internal/stacktrace_config.h"
-    "internal/stacktrace_generic-inl.inc"
-    "internal/stacktrace_powerpc-inl.inc"
-    "internal/stacktrace_unimplemented-inl.inc"
-    "internal/stacktrace_win32-inl.inc"
-    "internal/stacktrace_x86-inl.inc"
     "internal/vdso_support.h"
   SRCS
     "internal/address_is_readable.cc"
@@ -147,10 +192,13 @@ absl_cc_library(
     ${ABSL_DEFAULT_COPTS}
   DEPS
     absl::core_headers
+    absl::config
     absl::dynamic_annotations
+    absl::errno_saver
     absl::raw_logging_internal
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     demangle_internal
@@ -161,8 +209,8 @@ absl_cc_library(
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
-    absl::base
-    absl::core_headers
+    absl::config
+    absl::demangle_rust
   PUBLIC
 )
 
@@ -176,109 +224,157 @@ absl_cc_test(
   DEPS
     absl::demangle_internal
     absl::stack_consumption
+    absl::config
     absl::core_headers
+    absl::log
     absl::memory
-    absl::raw_logging_internal
-    gmock_main
+    GTest::gmock_main
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
-    leak_check
+    bounded_utf8_length_sequence
   HDRS
-    "leak_check.h"
-  SRCS
-    "leak_check.cc"
+    "internal/bounded_utf8_length_sequence.h"
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
-    absl::core_headers
-  PUBLIC
+    absl::bits
+    absl::config
 )
 
-absl_cc_library(
+absl_cc_test(
   NAME
-    leak_check_disable
+    bounded_utf8_length_sequence_test
   SRCS
-    "leak_check_disable.cc"
+    "internal/bounded_utf8_length_sequence_test.cc"
   COPTS
-    ${ABSL_DEFAULT_COPTS}
-  PUBLIC
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::bounded_utf8_length_sequence
+    absl::config
+    GTest::gmock_main
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
-    leak_check_api_enabled_for_testing
+    decode_rust_punycode
   HDRS
-    "leak_check.h"
+    "internal/decode_rust_punycode.h"
   SRCS
-    "leak_check.cc"
+    "internal/decode_rust_punycode.cc"
   COPTS
     ${ABSL_DEFAULT_COPTS}
-    $<$<BOOL:${ABSL_HAVE_LSAN}>:-DLEAK_SANITIZER>
-  TESTONLY
+  DEPS
+    absl::bounded_utf8_length_sequence
+    absl::config
+    absl::nullability
+    absl::utf8_for_code_point
+)
+
+absl_cc_test(
+  NAME
+    decode_rust_punycode_test
+  SRCS
+    "internal/decode_rust_punycode_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::decode_rust_punycode
+    absl::config
+    GTest::gmock_main
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
-    leak_check_api_disabled_for_testing
+    demangle_rust
   HDRS
-    "leak_check.h"
+    "internal/demangle_rust.h"
   SRCS
-    "leak_check.cc"
+    "internal/demangle_rust.cc"
   COPTS
     ${ABSL_DEFAULT_COPTS}
-    "-ULEAK_SANITIZER"
-  TESTONLY
+  DEPS
+    absl::config
+    absl::core_headers
+    absl::decode_rust_punycode
 )
 
 absl_cc_test(
   NAME
-    leak_check_test
+    demangle_rust_test
   SRCS
-    "leak_check_test.cc"
+    "internal/demangle_rust_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::demangle_rust
+    absl::config
+    GTest::gmock_main
+)
+
+# Internal-only target, do not depend on directly.
+absl_cc_library(
+  NAME
+    utf8_for_code_point
+  HDRS
+    "internal/utf8_for_code_point.h"
+  SRCS
+    "internal/utf8_for_code_point.cc"
   COPTS
     ${ABSL_DEFAULT_COPTS}
-    "$<$<BOOL:${ABSL_HAVE_LSAN}>:-DABSL_EXPECT_LEAK_SANITIZER>"
-  LINKOPTS
-    "${ABSL_LSAN_LINKOPTS}"
   DEPS
-    absl::leak_check_api_enabled_for_testing
-    absl::base
-    gmock_main
+    absl::config
 )
 
 absl_cc_test(
   NAME
-    leak_check_no_lsan_test
+    utf8_for_code_point_test
   SRCS
-    "leak_check_test.cc"
+    "internal/utf8_for_code_point_test.cc"
   COPTS
     ${ABSL_TEST_COPTS}
-    "-UABSL_EXPECT_LEAK_SANITIZER"
   DEPS
-    absl::leak_check_api_disabled_for_testing
-    absl::base
-    gmock_main
+    absl::utf8_for_code_point
+    absl::config
+    GTest::gmock_main
+)
+
+absl_cc_library(
+  NAME
+    leak_check
+  HDRS
+    "leak_check.h"
+  SRCS
+    "leak_check.cc"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::config
+    absl::core_headers
+  PUBLIC
 )
 
 absl_cc_test(
   NAME
-    disabled_leak_check_test
+    leak_check_test
   SRCS
-    "leak_check_fail_test.cc"
+    "leak_check_test.cc"
   COPTS
     ${ABSL_TEST_COPTS}
   LINKOPTS
-    "${ABSL_LSAN_LINKOPTS}"
+    ${ABSL_DEFAULT_LINKOPTS}
   DEPS
-    absl::leak_check_api_enabled_for_testing
-    absl::leak_check_disable
+    absl::leak_check
     absl::base
-    absl::raw_logging_internal
-    gmock_main
+    absl::log
+    GTest::gmock_main
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     stack_consumption
@@ -289,6 +385,7 @@ absl_cc_library(
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
+    absl::config
     absl::core_headers
     absl::raw_logging_internal
   TESTONLY
@@ -304,8 +401,8 @@ absl_cc_test(
   DEPS
     absl::stack_consumption
     absl::core_headers
-    absl::raw_logging_internal
-    gmock_main
+    absl::log
+    GTest::gmock_main
 )
 
 # component target
diff --git a/absl/debugging/failure_signal_handler.cc b/absl/debugging/failure_signal_handler.cc
index c6a4d962b42..e15d49de599 100644
--- a/absl/debugging/failure_signal_handler.cc
+++ b/absl/debugging/failure_signal_handler.cc
@@ -16,18 +16,6 @@
 
 #include "absl/debugging/failure_signal_handler.h"
 
-#include "absl/base/config.h"
-
-#ifdef _WIN32
-#include <windows.h>
-#else
-#include <unistd.h>
-#endif
-
-#ifdef ABSL_HAVE_MMAP
-#include <sys/mman.h>
-#endif
-
 #include <algorithm>
 #include <atomic>
 #include <cerrno>
@@ -37,16 +25,65 @@
 #include <ctime>
 
 #include "absl/base/attributes.h"
+#include "absl/base/config.h"
 #include "absl/base/internal/raw_logging.h"
 #include "absl/base/internal/sysinfo.h"
 #include "absl/debugging/internal/examine_stack.h"
 #include "absl/debugging/stacktrace.h"
 
-#ifndef _WIN32
+#ifdef _WIN32
+#include <windows.h>
+#else
+#include <pthread.h>
+#include <sched.h>
+#include <unistd.h>
+#endif
+
+#ifdef __APPLE__
+#include <TargetConditionals.h>
+#endif
+
+#ifdef ABSL_HAVE_MMAP
+#include <sys/mman.h>
+#if defined(MAP_ANON) && !defined(MAP_ANONYMOUS)
+#define MAP_ANONYMOUS MAP_ANON
+#endif
+#endif
+
+#ifdef __linux__
+#include <sys/prctl.h>
+#endif
+
+#if !defined(_WIN32) && !defined(__wasi__)
 #define ABSL_HAVE_SIGACTION
+// Apple WatchOS and TVOS don't allow sigaltstack
+// Apple macOS has sigaltstack, but using it makes backtrace() unusable.
+#if !(defined(TARGET_OS_OSX) && TARGET_OS_OSX) &&     \
+    !(defined(TARGET_OS_WATCH) && TARGET_OS_WATCH) && \
+    !(defined(TARGET_OS_TV) && TARGET_OS_TV) && !defined(__QNX__)
+#define ABSL_HAVE_SIGALTSTACK
+#endif
+#endif
+
+// ABSL_HAVE_PTHREAD_CPU_NUMBER_NP
+//
+// Checks whether pthread_cpu_number_np is available.
+#ifdef ABSL_HAVE_PTHREAD_CPU_NUMBER_NP
+#error ABSL_HAVE_PTHREAD_CPU_NUMBER_NP cannot be directly set
+#elif defined(__APPLE__) &&                                        \
+    ((defined(__ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__) &&    \
+      __ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__ >= 110000) ||  \
+     (defined(__ENVIRONMENT_IPHONE_OS_VERSION_MIN_REQUIRED__) &&   \
+      __ENVIRONMENT_IPHONE_OS_VERSION_MIN_REQUIRED__ >= 140200) || \
+     (defined(__ENVIRONMENT_WATCH_OS_VERSION_MIN_REQUIRED__) &&    \
+      __ENVIRONMENT_WATCH_OS_VERSION_MIN_REQUIRED__ >= 70100) ||   \
+     (defined(__ENVIRONMENT_TV_OS_VERSION_MIN_REQUIRED__) &&       \
+      __ENVIRONMENT_TV_OS_VERSION_MIN_REQUIRED__ >= 140200))
+#define ABSL_HAVE_PTHREAD_CPU_NUMBER_NP 1
 #endif
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 ABSL_CONST_INIT static FailureSignalHandlerOptions fsh_options;
 
@@ -64,10 +101,10 @@ struct FailureSignalData {
   struct sigaction previous_action;
   // StructSigaction is used to silence -Wmissing-field-initializers.
   using StructSigaction = struct sigaction;
-  #define FSD_PREVIOUS_INIT FailureSignalData::StructSigaction()
+#define FSD_PREVIOUS_INIT FailureSignalData::StructSigaction()
 #else
   void (*previous_handler)(int);
-  #define FSD_PREVIOUS_INIT SIG_DFL
+#define FSD_PREVIOUS_INIT SIG_DFL
 #endif
 };
 
@@ -116,17 +153,19 @@ const char* FailureSignalToString(int signo) {
 
 }  // namespace debugging_internal
 
-#ifndef _WIN32
+#ifdef ABSL_HAVE_SIGALTSTACK
 
 static bool SetupAlternateStackOnce() {
-#if defined(__wasm__) || defined (__asjms__)
-  const size_t page_mask = getpagesize() - 1;
+#if defined(__wasm__) || defined(__asmjs__)
+  const size_t page_mask = static_cast<size_t>(getpagesize()) - 1;
 #else
-  const size_t page_mask = sysconf(_SC_PAGESIZE) - 1;
+  const size_t page_mask = static_cast<size_t>(sysconf(_SC_PAGESIZE)) - 1;
 #endif
-  size_t stack_size = (std::max(SIGSTKSZ, 65536) + page_mask) & ~page_mask;
-#if defined(ADDRESS_SANITIZER) || defined(MEMORY_SANITIZER) || \
-    defined(THREAD_SANITIZER)
+  size_t stack_size =
+      (std::max(static_cast<size_t>(SIGSTKSZ), size_t{65536}) + page_mask) &
+      ~page_mask;
+#if defined(ABSL_HAVE_ADDRESS_SANITIZER) || \
+    defined(ABSL_HAVE_MEMORY_SANITIZER) || defined(ABSL_HAVE_THREAD_SANITIZER)
   // Account for sanitizer instrumentation requiring additional stack space.
   stack_size *= 5;
 #endif
@@ -138,9 +177,6 @@ static bool SetupAlternateStackOnce() {
 #ifdef ABSL_HAVE_MMAP
 #ifndef MAP_STACK
 #define MAP_STACK 0
-#endif
-#if defined(MAP_ANON) && !defined(MAP_ANONYMOUS)
-#define MAP_ANONYMOUS MAP_ANON
 #endif
   sigstk.ss_sp = mmap(nullptr, sigstk.ss_size, PROT_READ | PROT_WRITE,
                       MAP_PRIVATE | MAP_ANONYMOUS | MAP_STACK, -1, 0);
@@ -157,6 +193,20 @@ static bool SetupAlternateStackOnce() {
   if (sigaltstack(&sigstk, nullptr) != 0) {
     ABSL_RAW_LOG(FATAL, "sigaltstack() failed with errno=%d", errno);
   }
+
+#ifdef __linux__
+#if defined(PR_SET_VMA) && defined(PR_SET_VMA_ANON_NAME)
+  // Make a best-effort attempt to name the allocated region in
+  // /proc/$PID/smaps.
+  //
+  // The call to prctl() may fail if the kernel was not configured with the
+  // CONFIG_ANON_VMA_NAME kernel option.  This is OK since the call is
+  // primarily a debugging aid.
+  prctl(PR_SET_VMA, PR_SET_VMA_ANON_NAME, sigstk.ss_sp, sigstk.ss_size,
+        "absl-signalstack");
+#endif
+#endif  // __linux__
+
   return true;
 }
 
@@ -168,7 +218,7 @@ static bool SetupAlternateStackOnce() {
 // Returns the appropriate flag for sig_action.sa_flags
 // if the system supports using an alternate stack.
 static int MaybeSetupAlternateStack() {
-#ifndef _WIN32
+#ifdef ABSL_HAVE_SIGALTSTACK
   ABSL_ATTRIBUTE_UNUSED static const bool kOnce = SetupAlternateStackOnce();
   return SA_ONSTACK;
 #else
@@ -203,23 +253,24 @@ static void InstallOneFailureHandler(FailureSignalData* data,
 
 #endif
 
-static void WriteToStderr(const char* data) {
-  int old_errno = errno;
-  absl::raw_logging_internal::SafeWriteToStderr(data, strlen(data));
-  errno = old_errno;
-}
-
-static void WriteSignalMessage(int signo, void (*writerfn)(const char*)) {
-  char buf[64];
+static void WriteSignalMessage(int signo, int cpu,
+                               void (*writerfn)(const char*)) {
+  char buf[96];
+  char on_cpu[32] = {0};
+  if (cpu != -1) {
+    snprintf(on_cpu, sizeof(on_cpu), " on cpu %d", cpu);
+  }
   const char* const signal_string =
       debugging_internal::FailureSignalToString(signo);
   if (signal_string != nullptr && signal_string[0] != '\0') {
-    snprintf(buf, sizeof(buf), "*** %s received at time=%ld ***\n",
+    snprintf(buf, sizeof(buf), "*** %s received at time=%ld%s ***\n",
              signal_string,
-             static_cast<long>(time(nullptr)));  // NOLINT(runtime/int)
+             static_cast<long>(time(nullptr)),  // NOLINT(runtime/int)
+             on_cpu);
   } else {
-    snprintf(buf, sizeof(buf), "*** Signal %d received at time=%ld ***\n",
-             signo, static_cast<long>(time(nullptr)));  // NOLINT(runtime/int)
+    snprintf(buf, sizeof(buf), "*** Signal %d received at time=%ld%s ***\n",
+             signo, static_cast<long>(time(nullptr)),  // NOLINT(runtime/int)
+             on_cpu);
   }
   writerfn(buf);
 }
@@ -259,10 +310,10 @@ ABSL_ATTRIBUTE_NOINLINE static void WriteStackTrace(
 // Called by AbslFailureSignalHandler() to write the failure info. It is
 // called once with writerfn set to WriteToStderr() and then possibly
 // with writerfn set to the user provided function.
-static void WriteFailureInfo(int signo, void* ucontext,
+static void WriteFailureInfo(int signo, void* ucontext, int cpu,
                              void (*writerfn)(const char*)) {
   WriterFnStruct writerfn_struct{writerfn};
-  WriteSignalMessage(signo, writerfn);
+  WriteSignalMessage(signo, cpu, writerfn);
   WriteStackTrace(ucontext, fsh_options.symbolize_stacktrace, WriterFnWrapper,
                   &writerfn_struct);
 }
@@ -272,12 +323,13 @@ static void WriteFailureInfo(int signo, void* ucontext,
 // some platforms.
 static void PortableSleepForSeconds(int seconds) {
 #ifdef _WIN32
-  Sleep(seconds * 1000);
+  Sleep(static_cast<DWORD>(seconds * 1000));
 #else
   struct timespec sleep_time;
   sleep_time.tv_sec = seconds;
   sleep_time.tv_nsec = 0;
-  while (nanosleep(&sleep_time, &sleep_time) != 0 && errno == EINTR) {}
+  while (nanosleep(&sleep_time, &sleep_time) != 0 && errno == EINTR) {
+  }
 #endif
 }
 
@@ -287,9 +339,7 @@ static void PortableSleepForSeconds(int seconds) {
 // set amount of time. If AbslFailureSignalHandler() hangs for more than
 // the alarm timeout, ImmediateAbortSignalHandler() will abort the
 // program.
-static void ImmediateAbortSignalHandler(int) {
-  RaiseToDefaultHandler(SIGABRT);
-}
+static void ImmediateAbortSignalHandler(int) { RaiseToDefaultHandler(SIGABRT); }
 #endif
 
 // absl::base_internal::GetTID() returns pid_t on most platforms, but
@@ -297,6 +347,20 @@ static void ImmediateAbortSignalHandler(int) {
 using GetTidType = decltype(absl::base_internal::GetTID());
 ABSL_CONST_INIT static std::atomic<GetTidType> failed_tid(0);
 
+static int GetCpuNumber() {
+#ifdef ABSL_HAVE_SCHED_GETCPU
+  return sched_getcpu();
+#elif defined(ABSL_HAVE_PTHREAD_CPU_NUMBER_NP)
+  size_t cpu_num;
+  if (pthread_cpu_number_np(&cpu_num) == 0) {
+    return static_cast<int>(cpu_num);
+  }
+  return -1;
+#else
+  return -1;
+#endif
+}
+
 #ifndef ABSL_HAVE_SIGACTION
 static void AbslFailureSignalHandler(int signo) {
   void* ucontext = nullptr;
@@ -306,9 +370,9 @@ static void AbslFailureSignalHandler(int signo, siginfo_t*, void* ucontext) {
 
   const GetTidType this_tid = absl::base_internal::GetTID();
   GetTidType previous_failed_tid = 0;
-  if (!failed_tid.compare_exchange_strong(
-          previous_failed_tid, static_cast<intptr_t>(this_tid),
-          std::memory_order_acq_rel, std::memory_order_relaxed)) {
+  if (!failed_tid.compare_exchange_strong(previous_failed_tid, this_tid,
+                                          std::memory_order_acq_rel,
+                                          std::memory_order_relaxed)) {
     ABSL_RAW_LOG(
         ERROR,
         "Signal %d raised at PC=%p while already in AbslFailureSignalHandler()",
@@ -318,28 +382,43 @@ static void AbslFailureSignalHandler(int signo, siginfo_t*, void* ucontext) {
       // a bit for it to finish. If the other thread doesn't kill us,
       // we do so after sleeping.
       PortableSleepForSeconds(3);
-      RaiseToDefaultHandler(signo);
-      // The recursively raised signal may be blocked until we return.
-      return;
+    } else {
+      // Same thread re-entered: the handler itself faulted. Do NOT fall through
+      // and re-run the body (which would recurse under SA_NODEFER, resetting
+      // the alarm each time and consuming the unguarded altstack). Instead,
+      // terminate now.
     }
+
+    RaiseToDefaultHandler(signo);
+    // The recursively raised signal may be blocked until we return.
+    return;
   }
 
+  // Increase the chance that the CPU we report was the same CPU on which the
+  // signal was received by doing this as early as possible, i.e. after
+  // verifying that this is not a recursive signal handler invocation.
+  int my_cpu = GetCpuNumber();
+
 #ifdef ABSL_HAVE_ALARM
   // Set an alarm to abort the program in case this code hangs or deadlocks.
   if (fsh_options.alarm_on_failure_secs > 0) {
     alarm(0);  // Cancel any existing alarms.
     signal(SIGALRM, ImmediateAbortSignalHandler);
-    alarm(fsh_options.alarm_on_failure_secs);
+    alarm(static_cast<unsigned int>(fsh_options.alarm_on_failure_secs));
   }
 #endif
 
   // First write to stderr.
-  WriteFailureInfo(signo, ucontext, WriteToStderr);
+  WriteFailureInfo(
+      signo, ucontext, my_cpu, +[](const char* data) {
+        absl::raw_log_internal::AsyncSignalSafeWriteError(data, strlen(data));
+      });
 
   // Riskier code (because it is less likely to be async-signal-safe)
   // goes after this point.
   if (fsh_options.writerfn != nullptr) {
-    WriteFailureInfo(signo, ucontext, fsh_options.writerfn);
+    WriteFailureInfo(signo, ucontext, my_cpu, fsh_options.writerfn);
+    fsh_options.writerfn(nullptr);
   }
 
   if (fsh_options.call_previous_handler) {
@@ -356,4 +435,5 @@ void InstallFailureSignalHandler(const FailureSignalHandlerOptions& options) {
   }
 }
 
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/debugging/failure_signal_handler.h b/absl/debugging/failure_signal_handler.h
index 1beb78b9378..4117facb56e 100644
--- a/absl/debugging/failure_signal_handler.h
+++ b/absl/debugging/failure_signal_handler.h
@@ -33,7 +33,7 @@
 // }
 //
 // Any program that raises a fatal signal (such as `SIGSEGV`, `SIGILL`,
-// `SIGFPE`, `SIGABRT`, `SIGTERM`, `SIGBUG`, and `SIGTRAP`) will call the
+// `SIGFPE`, `SIGABRT`, `SIGTERM`, `SIGBUS`, and `SIGTRAP`) will call the
 // installed failure signal handler and provide debugging information to stderr.
 //
 // Note that you should *not* install the Abseil failure signal handler more
@@ -44,7 +44,10 @@
 #ifndef ABSL_DEBUGGING_FAILURE_SIGNAL_HANDLER_H_
 #define ABSL_DEBUGGING_FAILURE_SIGNAL_HANDLER_H_
 
+#include "absl/base/config.h"
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 // FailureSignalHandlerOptions
 //
@@ -59,7 +62,7 @@ struct FailureSignalHandlerOptions {
   // If true, try to run signal handlers on an alternate stack (if supported on
   // the given platform). An alternate stack is useful for program crashes due
   // to a stack overflow; by running on a alternate stack, the signal handler
-  // may run even when normal stack space has been exausted. The downside of
+  // may run even when normal stack space has been exhausted. The downside of
   // using an alternate stack is that extra memory for the alternate stack needs
   // to be pre-allocated.
   bool use_alternate_stack = true;
@@ -85,9 +88,9 @@ struct FailureSignalHandlerOptions {
   bool call_previous_handler = false;
 
   // If non-null, indicates a pointer to a callback function that will be called
-  // upon failure, with a std::string argument containing failure data. This function
+  // upon failure, with a string argument containing failure data. This function
   // may be used as a hook to write failure data to a secondary location, such
-  // as a log file. This function may also be called with null data, as a hint
+  // as a log file. This function will also be called with null data, as a hint
   // to flush any buffered data before the program may be terminated. Consider
   // flushing any buffered data in all calls to this function.
   //
@@ -112,6 +115,7 @@ namespace debugging_internal {
 const char* FailureSignalToString(int signo);
 }  // namespace debugging_internal
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_DEBUGGING_FAILURE_SIGNAL_HANDLER_H_
diff --git a/absl/debugging/failure_signal_handler_test.cc b/absl/debugging/failure_signal_handler_test.cc
index bb2cc48eb4a..7a84e388b6a 100644
--- a/absl/debugging/failure_signal_handler_test.cc
+++ b/absl/debugging/failure_signal_handler_test.cc
@@ -21,16 +21,22 @@
 #include <cstdlib>
 #include <cstring>
 #include <fstream>
+#include <string>
 
+#include "gmock/gmock.h"
 #include "gtest/gtest.h"
+#include "absl/base/attributes.h"
 #include "absl/base/internal/raw_logging.h"
 #include "absl/debugging/stacktrace.h"
 #include "absl/debugging/symbolize.h"
+#include "absl/log/check.h"
 #include "absl/strings/match.h"
 #include "absl/strings/str_cat.h"
 
 namespace {
 
+using testing::StartsWith;
+
 #if GTEST_HAS_DEATH_TEST
 
 // For the parameterized death tests. GetParam() returns the signal number.
@@ -52,7 +58,7 @@ TEST_P(FailureSignalHandlerDeathTest, AbslFailureSignal) {
               exit_regex);
 #else
   // Windows doesn't have testing::KilledBySignal().
-  EXPECT_DEATH(InstallHandlerAndRaise(signo), exit_regex);
+  EXPECT_DEATH_IF_SUPPORTED(InstallHandlerAndRaise(signo), exit_regex);
 #endif
 }
 
@@ -84,7 +90,7 @@ std::string GetTmpDir() {
 // This function runs in a fork()ed process on most systems.
 void InstallHandlerWithWriteToFileAndRaise(const char* file, int signo) {
   error_file = fopen(file, "w");
-  ABSL_RAW_CHECK(error_file != nullptr, "Failed create error_file");
+  CHECK_NE(error_file, nullptr) << "Failed create error_file";
   absl::FailureSignalHandlerOptions options;
   options.writerfn = WriteToErrorFile;
   absl::InstallFailureSignalHandler(options);
@@ -104,8 +110,8 @@ TEST_P(FailureSignalHandlerDeathTest, AbslFatalSignalsWithWriterFn) {
               testing::KilledBySignal(signo), exit_regex);
 #else
   // Windows doesn't have testing::KilledBySignal().
-  EXPECT_DEATH(InstallHandlerWithWriteToFileAndRaise(file.c_str(), signo),
-               exit_regex);
+  EXPECT_DEATH_IF_SUPPORTED(
+      InstallHandlerWithWriteToFileAndRaise(file.c_str(), signo), exit_regex);
 #endif
 
   // Open the file in this process and check its contents.
@@ -113,15 +119,21 @@ TEST_P(FailureSignalHandlerDeathTest, AbslFatalSignalsWithWriterFn) {
   ASSERT_TRUE(error_output.is_open()) << file;
   std::string error_line;
   std::getline(error_output, error_line);
-  EXPECT_TRUE(absl::StartsWith(
+  EXPECT_THAT(
       error_line,
-      absl::StrCat("*** ",
-                   absl::debugging_internal::FailureSignalToString(signo),
-                   " received at ")));
+      StartsWith(absl::StrCat(
+          "*** ", absl::debugging_internal::FailureSignalToString(signo),
+          " received at ")));
+
+  // On platforms where it is possible to get the current CPU, the
+  // CPU number is also logged. Check that it is present in output.
+#if defined(__linux__)
+  EXPECT_THAT(error_line, testing::HasSubstr(" on cpu "));
+#endif
 
   if (absl::debugging_internal::StackTraceWorksForTest()) {
     std::getline(error_output, error_line);
-    EXPECT_TRUE(absl::StartsWith(error_line, "PC: "));
+    EXPECT_THAT(error_line, StartsWith("PC: "));
   }
 }
 
diff --git a/absl/debugging/internal/address_is_readable.cc b/absl/debugging/internal/address_is_readable.cc
index 99c4c64be81..e9be2caf5a7 100644
--- a/absl/debugging/internal/address_is_readable.cc
+++ b/absl/debugging/internal/address_is_readable.cc
@@ -17,117 +17,84 @@
 
 #include "absl/debugging/internal/address_is_readable.h"
 
+#include <cerrno>
+
 #if !defined(__linux__) || defined(__ANDROID__)
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace debugging_internal {
 
 // On platforms other than Linux, just return true.
 bool AddressIsReadable(const void* /* addr */) { return true; }
 
 }  // namespace debugging_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
-#else
+#else  // __linux__ && !__ANDROID__
 
-#include <fcntl.h>
-#include <sys/syscall.h>
+#include <stdint.h>
+#include <syscall.h>
 #include <unistd.h>
-#include <atomic>
-#include <cerrno>
-#include <cstdint>
 
+#include "absl/base/internal/errno_saver.h"
 #include "absl/base/internal/raw_logging.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace debugging_internal {
 
-// Pack a pid and two file descriptors into a 64-bit word,
-// using 16, 24, and 24 bits for each respectively.
-static uint64_t Pack(uint64_t pid, uint64_t read_fd, uint64_t write_fd) {
-  ABSL_RAW_CHECK((read_fd >> 24) == 0 && (write_fd >> 24) == 0,
-                 "fd out of range");
-  return (pid << 48) | ((read_fd & 0xffffff) << 24) | (write_fd & 0xffffff);
-}
+// NOTE: be extra careful about adding any interposable function calls here
+// (such as open(), read(), etc.). These symbols may be interposed and will get
+// invoked in contexts they don't expect.
+//
+// NOTE: any new system calls here may also require sandbox reconfiguration.
+//
+bool AddressIsReadable(const void *addr) {
+  // rt_sigprocmask below checks 8 contiguous bytes. If addr resides in the
+  // last 7 bytes of a page (unaligned), rt_sigprocmask would additionally
+  // check the readability of the next page, which is not desired. Align
+  // address on 8-byte boundary to check only the current page.
+  const uintptr_t u_addr = reinterpret_cast<uintptr_t>(addr) & ~uintptr_t{7};
+  addr = reinterpret_cast<const void *>(u_addr);
 
-// Unpack x into a pid and two file descriptors, where x was created with
-// Pack().
-static void Unpack(uint64_t x, int *pid, int *read_fd, int *write_fd) {
-  *pid = x >> 48;
-  *read_fd = (x >> 24) & 0xffffff;
-  *write_fd = x & 0xffffff;
-}
+  // rt_sigprocmask below will succeed for this input.
+  if (addr == nullptr) return false;
 
-// Return whether the byte at *addr is readable, without faulting.
-// Save and restores errno.   Returns true on systems where
-// unimplemented.
-// This is a namespace-scoped variable for correct zero-initialization.
-static std::atomic<uint64_t> pid_and_fds;  // initially 0, an invalid pid.
-bool AddressIsReadable(const void *addr) {
-  int save_errno = errno;
-  // We test whether a byte is readable by using write().  Normally, this would
-  // be done via a cached file descriptor to /dev/null, but linux fails to
-  // check whether the byte is readable when the destination is /dev/null, so
-  // we use a cached pipe.  We store the pid of the process that created the
-  // pipe to handle the case where a process forks, and the child closes all
-  // the file descriptors and then calls this routine.  This is not perfect:
-  // the child could use the routine, then close all file descriptors and then
-  // use this routine again.  But the likely use of this routine is when
-  // crashing, to test the validity of pages when dumping the stack.  Beware
-  // that we may leak file descriptors, but we're unlikely to leak many.
-  int bytes_written;
-  int current_pid = getpid() & 0xffff;   // we use only the low order 16 bits
-  do {  // until we do not get EBADF trying to use file descriptors
-    int pid;
-    int read_fd;
-    int write_fd;
-    uint64_t local_pid_and_fds = pid_and_fds.load(std::memory_order_relaxed);
-    Unpack(local_pid_and_fds, &pid, &read_fd, &write_fd);
-    while (current_pid != pid) {
-      int p[2];
-      // new pipe
-      if (pipe(p) != 0) {
-        ABSL_RAW_LOG(FATAL, "Failed to create pipe, errno=%d", errno);
-      }
-      fcntl(p[0], F_SETFD, FD_CLOEXEC);
-      fcntl(p[1], F_SETFD, FD_CLOEXEC);
-      uint64_t new_pid_and_fds = Pack(current_pid, p[0], p[1]);
-      if (pid_and_fds.compare_exchange_strong(
-              local_pid_and_fds, new_pid_and_fds, std::memory_order_relaxed,
-              std::memory_order_relaxed)) {
-        local_pid_and_fds = new_pid_and_fds;  // fds exposed to other threads
-      } else {  // fds not exposed to other threads; we can close them.
-        close(p[0]);
-        close(p[1]);
-        local_pid_and_fds = pid_and_fds.load(std::memory_order_relaxed);
-      }
-      Unpack(local_pid_and_fds, &pid, &read_fd, &write_fd);
-    }
-    errno = 0;
-    // Use syscall(SYS_write, ...) instead of write() to prevent ASAN
-    // and other checkers from complaining about accesses to arbitrary
-    // memory.
-    do {
-      bytes_written = syscall(SYS_write, write_fd, addr, 1);
-    } while (bytes_written == -1 && errno == EINTR);
-    if (bytes_written == 1) {   // remove the byte from the pipe
-      char c;
-      while (read(read_fd, &c, 1) == -1 && errno == EINTR) {
-      }
-    }
-    if (errno == EBADF) {  // Descriptors invalid.
-      // If pid_and_fds contains the problematic file descriptors we just used,
-      // this call will forget them, and the loop will try again.
-      pid_and_fds.compare_exchange_strong(local_pid_and_fds, 0,
-                                          std::memory_order_relaxed,
-                                          std::memory_order_relaxed);
-    }
-  } while (errno == EBADF);
-  errno = save_errno;
-  return bytes_written == 1;
+  absl::base_internal::ErrnoSaver errno_saver;
+
+  // Here we probe with some syscall which
+  // - accepts an 8-byte region of user memory as input
+  // - tests for EFAULT before other validation
+  // - has no problematic side-effects
+  //
+  // rt_sigprocmask(2) works for this.  It copies sizeof(kernel_sigset_t)==8
+  // bytes from the address into the kernel memory before any validation.
+  //
+  // The call can never succeed, since the `how` parameter is not one of
+  // SIG_BLOCK, SIG_UNBLOCK, SIG_SETMASK.
+  //
+  // This strategy depends on Linux implementation details,
+  // so we rely on the test to alert us if it stops working.
+  //
+  // Some discarded past approaches:
+  // - msync() doesn't reject PROT_NONE regions
+  // - write() on /dev/null doesn't return EFAULT
+  // - write() on a pipe requires creating it and draining the writes
+  // - connect() works but is problematic for sandboxes and needs a valid
+  //   file descriptor
+  //
+  // This can never succeed (invalid first argument to sigprocmask).
+  ABSL_RAW_CHECK(syscall(SYS_rt_sigprocmask, ~0, addr, nullptr,
+                         /*sizeof(kernel_sigset_t)*/ 8) == -1,
+                 "unexpected success");
+  ABSL_RAW_CHECK(errno == EFAULT || errno == EINVAL, "unexpected errno");
+  return errno != EFAULT;
 }
 
 }  // namespace debugging_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
-#endif
+#endif  // __linux__ && !__ANDROID__
diff --git a/absl/debugging/internal/address_is_readable.h b/absl/debugging/internal/address_is_readable.h
index ca8003e6292..4bbaf4d69bd 100644
--- a/absl/debugging/internal/address_is_readable.h
+++ b/absl/debugging/internal/address_is_readable.h
@@ -15,7 +15,10 @@
 #ifndef ABSL_DEBUGGING_INTERNAL_ADDRESS_IS_READABLE_H_
 #define ABSL_DEBUGGING_INTERNAL_ADDRESS_IS_READABLE_H_
 
+#include "absl/base/config.h"
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace debugging_internal {
 
 // Return whether the byte at *addr is readable, without faulting.
@@ -23,6 +26,7 @@ namespace debugging_internal {
 bool AddressIsReadable(const void *addr);
 
 }  // namespace debugging_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_DEBUGGING_INTERNAL_ADDRESS_IS_READABLE_H_
diff --git a/absl/debugging/internal/addresses.h b/absl/debugging/internal/addresses.h
new file mode 100644
index 00000000000..504fd6f5d1d
--- /dev/null
+++ b/absl/debugging/internal/addresses.h
@@ -0,0 +1,57 @@
+// Copyright 2025 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_DEBUGGING_INTERNAL_ADDRESSES_H_
+#define ABSL_DEBUGGING_INTERNAL_ADDRESSES_H_
+
+#include <stdint.h>
+
+#include "absl/base/config.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace debugging_internal {
+
+// Removes any metadata (tag bits) from the given pointer, converting it into a
+// user-readable address.
+inline uintptr_t StripPointerMetadata(uintptr_t ptr) {
+#if defined(__aarch64__)
+  // When PAC-RET (-mbranch-protection=pac-ret) is enabled, return addresses
+  // stored on the stack will be signed, which means that pointer bits outside
+  // of the virtual address range are potentially set. Since the stacktrace code
+  // is expected to return normal code pointers, this function clears those
+  // bits.
+  register uintptr_t x30 __asm__("x30") = ptr;
+  // The normal instruction for clearing PAC bits is XPACI, but for
+  // compatibility with ARM platforms that do not support pointer
+  // authentication, we use the hint space instruction XPACLRI instead. Hint
+  // space instructions behave as NOPs on unsupported platforms.
+#define ABSL_XPACLRI_HINT "hint #0x7;"
+  asm(ABSL_XPACLRI_HINT : "+r"(x30));  // asm("xpaclri" : "+r"(x30));
+#undef ABSL_XPACLRI_HINT
+  return x30;
+#else
+  return ptr;
+#endif
+}
+
+inline uintptr_t StripPointerMetadata(void* ptr) {
+  return StripPointerMetadata(reinterpret_cast<uintptr_t>(ptr));
+}
+
+}  // namespace debugging_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_DEBUGGING_INTERNAL_ADDRESSES_H_
diff --git a/absl/debugging/internal/bounded_utf8_length_sequence.h b/absl/debugging/internal/bounded_utf8_length_sequence.h
new file mode 100644
index 00000000000..188e06c4afb
--- /dev/null
+++ b/absl/debugging/internal/bounded_utf8_length_sequence.h
@@ -0,0 +1,126 @@
+// Copyright 2024 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_DEBUGGING_INTERNAL_BOUNDED_UTF8_LENGTH_SEQUENCE_H_
+#define ABSL_DEBUGGING_INTERNAL_BOUNDED_UTF8_LENGTH_SEQUENCE_H_
+
+#include <cstdint>
+
+#include "absl/base/config.h"
+#include "absl/numeric/bits.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace debugging_internal {
+
+// A sequence of up to max_elements integers between 1 and 4 inclusive, whose
+// insertion operation computes the sum of all the elements before the insertion
+// point.  This is useful in decoding Punycode, where one needs to know where in
+// a UTF-8 byte stream the n-th code point begins.
+//
+// BoundedUtf8LengthSequence is async-signal-safe and suitable for use in
+// symbolizing stack traces in a signal handler, provided max_elements is not
+// improvidently large.  For inputs of lengths accepted by the Rust demangler,
+// up to a couple hundred code points, InsertAndReturnSumOfPredecessors should
+// run in a few dozen clock cycles, on par with the other arithmetic required
+// for Punycode decoding.
+template <uint32_t max_elements>
+class BoundedUtf8LengthSequence {
+ public:
+  // Constructs an empty sequence.
+  BoundedUtf8LengthSequence() = default;
+
+  // Inserts `utf_length` at position `index`, shifting any existing elements at
+  // or beyond `index` one position to the right.  If the sequence is already
+  // full, the rightmost element is discarded.
+  //
+  // Returns the sum of the elements at positions 0 to `index - 1` inclusive.
+  // If `index` is greater than the number of elements already inserted, the
+  // excess positions in the range count 1 apiece.
+  //
+  // REQUIRES: index < max_elements and 1 <= utf8_length <= 4.
+  uint32_t InsertAndReturnSumOfPredecessors(
+      uint32_t index, uint32_t utf8_length) {
+    // The caller shouldn't pass out-of-bounds inputs, but if it does happen,
+    // clamp the values and try to continue.  If we're being called from a
+    // signal handler, the last thing we want to do is crash.  Emitting
+    // malformed UTF-8 is a lesser evil.
+    if (index >= max_elements) index = max_elements - 1;
+    if (utf8_length == 0 || utf8_length > 4) utf8_length = 1;
+
+    const uint32_t word_index = index/32;
+    const uint32_t bit_index = 2 * (index % 32);
+    const uint64_t ones_bit = uint64_t{1} << bit_index;
+
+    // Compute the sum of predecessors.
+    //   - Each value from 1 to 4 is represented by a bit field with value from
+    //     0 to 3, so the desired sum is index plus the sum of the
+    //     representations actually stored.
+    //   - For each bit field, a set low bit should contribute 1 to the sum, and
+    //     a set high bit should contribute 2.
+    //   - Another way to say the same thing is that each set bit contributes 1,
+    //     and each set high bit contributes an additional 1.
+    //   - So the sum we want is index + popcount(everything) + popcount(bits in
+    //     odd positions).
+    const uint64_t odd_bits_mask = 0xaaaaaaaaaaaaaaaa;
+    const uint64_t lower_seminibbles_mask = ones_bit - 1;
+    const uint64_t higher_seminibbles_mask = ~lower_seminibbles_mask;
+    const uint64_t same_word_bits_below_insertion =
+        rep_[word_index] & lower_seminibbles_mask;
+    int full_popcount = absl::popcount(same_word_bits_below_insertion);
+    int odd_popcount =
+        absl::popcount(same_word_bits_below_insertion & odd_bits_mask);
+    for (uint32_t j = word_index; j > 0; --j) {
+      const uint64_t word_below_insertion = rep_[j - 1];
+      full_popcount += absl::popcount(word_below_insertion);
+      odd_popcount += absl::popcount(word_below_insertion & odd_bits_mask);
+    }
+    const uint32_t sum_of_predecessors =
+        index + static_cast<uint32_t>(full_popcount + odd_popcount);
+
+    // Now insert utf8_length's representation, shifting successors up one
+    // place.
+    for (uint32_t j = max_elements/32 - 1; j > word_index; --j) {
+      rep_[j] = (rep_[j] << 2) | (rep_[j - 1] >> 62);
+    }
+    rep_[word_index] =
+        (rep_[word_index] & lower_seminibbles_mask) |
+        (uint64_t{utf8_length - 1} << bit_index) |
+        ((rep_[word_index] & higher_seminibbles_mask) << 2);
+
+    return sum_of_predecessors;
+  }
+
+ private:
+  // If the (32 * i + j)-th element of the represented sequence has the value k
+  // (0 <= j < 32, 1 <= k <= 4), then bits 2 * j and 2 * j + 1 of rep_[i]
+  // contain the seminibble (k - 1).
+  //
+  // In particular, the zero-initialization of rep_ makes positions not holding
+  // any inserted element count as 1 in InsertAndReturnSumOfPredecessors.
+  //
+  // Example: rep_ = {0xb1, ... the rest zeroes ...} represents the sequence
+  // (2, 1, 4, 3, ... the rest 1's ...).  Constructing the sequence of Unicode
+  // code points "Àa🂻中" = {U+00C0, U+0061, U+1F0BB, U+4E2D} (among many
+  // other examples) would yield this value of rep_.
+  static_assert(max_elements > 0 && max_elements % 32 == 0,
+                "max_elements must be a positive multiple of 32");
+  uint64_t rep_[max_elements/32] = {};
+};
+
+}  // namespace debugging_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_DEBUGGING_INTERNAL_BOUNDED_UTF8_LENGTH_SEQUENCE_H_
diff --git a/absl/debugging/internal/bounded_utf8_length_sequence_test.cc b/absl/debugging/internal/bounded_utf8_length_sequence_test.cc
new file mode 100644
index 00000000000..17a24fd1e8a
--- /dev/null
+++ b/absl/debugging/internal/bounded_utf8_length_sequence_test.cc
@@ -0,0 +1,126 @@
+// Copyright 2024 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/debugging/internal/bounded_utf8_length_sequence.h"
+
+#include <cstdint>
+
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace debugging_internal {
+namespace {
+
+TEST(BoundedUtf8LengthSequenceTest, RemembersAValueOfOneCorrectly) {
+  BoundedUtf8LengthSequence<32> seq;
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(0, 1), 0);
+  EXPECT_EQ(seq.InsertAndReturnSumOfPredecessors(1, 1), 1);
+}
+
+TEST(BoundedUtf8LengthSequenceTest, RemembersAValueOfTwoCorrectly) {
+  BoundedUtf8LengthSequence<32> seq;
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(0, 2), 0);
+  EXPECT_EQ(seq.InsertAndReturnSumOfPredecessors(1, 1), 2);
+}
+
+TEST(BoundedUtf8LengthSequenceTest, RemembersAValueOfThreeCorrectly) {
+  BoundedUtf8LengthSequence<32> seq;
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(0, 3), 0);
+  EXPECT_EQ(seq.InsertAndReturnSumOfPredecessors(1, 1), 3);
+}
+
+TEST(BoundedUtf8LengthSequenceTest, RemembersAValueOfFourCorrectly) {
+  BoundedUtf8LengthSequence<32> seq;
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(0, 4), 0);
+  EXPECT_EQ(seq.InsertAndReturnSumOfPredecessors(1, 1), 4);
+}
+
+TEST(BoundedUtf8LengthSequenceTest, RemembersSeveralAppendedValues) {
+  BoundedUtf8LengthSequence<32> seq;
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(0, 1), 0);
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(1, 4), 1);
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(2, 2), 5);
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(3, 3), 7);
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(4, 1), 10);
+}
+
+TEST(BoundedUtf8LengthSequenceTest, RemembersSeveralPrependedValues) {
+  BoundedUtf8LengthSequence<32> seq;
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(0, 4), 0);
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(0, 3), 0);
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(0, 2), 0);
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(0, 1), 0);
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(4, 1), 10);
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(3, 1), 6);
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(2, 1), 3);
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(1, 1), 1);
+}
+
+TEST(BoundedUtf8LengthSequenceTest, RepeatedInsertsShiftValuesOutTheRightEnd) {
+  BoundedUtf8LengthSequence<32> seq;
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(0, 2), 0);
+  for (uint32_t i = 1; i < 31; ++i) {
+    ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(0, 1), 0)
+        << "while moving the 2 into position " << i;
+    ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(31, 1), 32)
+        << "after moving the 2 into position " << i;
+  }
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(0, 1), 0)
+      << "while moving the 2 into position 31";
+  EXPECT_EQ(seq.InsertAndReturnSumOfPredecessors(31, 1), 31)
+      << "after moving the 2 into position 31";
+}
+
+TEST(BoundedUtf8LengthSequenceTest, InsertsIntoWord1LeaveWord0Untouched) {
+  BoundedUtf8LengthSequence<64> seq;
+  for (uint32_t i = 0; i < 32; ++i) {
+    ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(i, 2), 2 * i)
+        << "at index " << i;
+  }
+  EXPECT_EQ(seq.InsertAndReturnSumOfPredecessors(32, 1), 64);
+  EXPECT_EQ(seq.InsertAndReturnSumOfPredecessors(32, 1), 64);
+}
+
+TEST(BoundedUtf8LengthSequenceTest, InsertsIntoWord0ShiftValuesIntoWord1) {
+  BoundedUtf8LengthSequence<64> seq;
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(29, 2), 29);
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(30, 3), 31);
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(31, 4), 34);
+
+  // Pushing two 1's on the front moves the 3 and 4 into the high word.
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(0, 1), 0);
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(0, 1), 0);
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(34, 1), 31 + 2 + 3 + 4);
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(32, 1), 31 + 2);
+}
+
+TEST(BoundedUtf8LengthSequenceTest, ValuesAreShiftedCorrectlyAmongThreeWords) {
+  BoundedUtf8LengthSequence<96> seq;
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(31, 3), 31);
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(63, 4), 62 + 3);
+
+  // This insertion moves both the 3 and the 4 up a word.
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(0, 1), 0);
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(65, 1), 63 + 3 + 4);
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(64, 1), 63 + 3);
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(33, 1), 32 + 3);
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(32, 1), 32);
+}
+
+}  // namespace
+}  // namespace debugging_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/debugging/internal/decode_rust_punycode.cc b/absl/debugging/internal/decode_rust_punycode.cc
new file mode 100644
index 00000000000..6652dc221a3
--- /dev/null
+++ b/absl/debugging/internal/decode_rust_punycode.cc
@@ -0,0 +1,258 @@
+// Copyright 2024 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/debugging/internal/decode_rust_punycode.h"
+
+#include <cstddef>
+#include <cstdint>
+#include <cstring>
+
+#include "absl/base/config.h"
+#include "absl/base/nullability.h"
+#include "absl/debugging/internal/bounded_utf8_length_sequence.h"
+#include "absl/debugging/internal/utf8_for_code_point.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace debugging_internal {
+
+namespace {
+
+// Decoding Punycode requires repeated random-access insertion into a stream of
+// variable-length UTF-8 code-point encodings.  We need this to be tolerably
+// fast (no N^2 slowdown for unfortunate inputs), and we can't allocate any data
+// structures on the heap (async-signal-safety).
+//
+// It is pragmatic to impose a moderately low limit on the identifier length and
+// bail out if we ever hit it.  Then BoundedUtf8LengthSequence efficiently
+// determines where to insert the next code point, and memmove efficiently makes
+// room for it.
+//
+// The chosen limit is a round number several times larger than identifiers
+// expected in practice, yet still small enough that a memmove of this many
+// UTF-8 characters is not much more expensive than the division and modulus
+// operations that Punycode decoding requires.
+constexpr uint32_t kMaxChars = 256;
+
+// Constants from RFC 3492 section 5.
+constexpr uint32_t kBase = 36, kTMin = 1, kTMax = 26, kSkew = 38, kDamp = 700;
+
+constexpr uint32_t kMaxCodePoint = 0x10ffff;
+
+// Overflow threshold in DecodeRustPunycode's inner loop; see comments there.
+constexpr uint32_t kMaxI = 1 << 30;
+
+// If punycode_begin .. punycode_end begins with a prefix matching the regular
+// expression [0-9a-zA-Z_]+_, removes that prefix, copies all but the final
+// underscore into out_begin .. out_end, sets num_ascii_chars to the number of
+// bytes copied, and returns true.  (A prefix of this sort represents the
+// nonempty subsequence of ASCII characters in the corresponding plaintext.)
+//
+// If punycode_begin .. punycode_end does not contain an underscore, sets
+// num_ascii_chars to zero and returns true.  (The encoding of a plaintext
+// without any ASCII characters does not carry such a prefix.)
+//
+// Returns false and zeroes num_ascii_chars on failure (either parse error or
+// not enough space in the output buffer).
+bool ConsumeOptionalAsciiPrefix(const char*& punycode_begin,
+                                const char* const punycode_end,
+                                char* const out_begin,
+                                char* const out_end,
+                                uint32_t& num_ascii_chars) {
+  num_ascii_chars = 0;
+
+  // Remember the last underscore if any.  Also use the same string scan to
+  // reject any ASCII bytes that do not belong in an identifier, including NUL,
+  // as well as non-ASCII bytes, which should have been delta-encoded instead.
+  int last_underscore = -1;
+  for (int i = 0; i < punycode_end - punycode_begin; ++i) {
+    const char c = punycode_begin[i];
+    if (c == '_') {
+      last_underscore = i;
+      continue;
+    }
+    // We write out the meaning of absl::ascii_isalnum rather than call that
+    // function because its documentation does not promise it will remain
+    // async-signal-safe under future development.
+    if ('a' <= c && c <= 'z') continue;
+    if ('A' <= c && c <= 'Z') continue;
+    if ('0' <= c && c <= '9') continue;
+    return false;
+  }
+
+  // If there was no underscore, that means there were no ASCII characters in
+  // the plaintext, so there is no prefix to consume.  Our work is done.
+  if (last_underscore < 0) return true;
+
+  // Otherwise there will be an underscore delimiter somewhere.  It can't be
+  // initial because then there would be no ASCII characters to its left, and no
+  // delimiter would have been added in that case.
+  if (last_underscore == 0) return false;
+
+  // Any other position is reasonable.  Make sure there's room in the buffer.
+  if (last_underscore + 1 > out_end - out_begin) return false;
+
+  // Consume and write out the ASCII characters.
+  num_ascii_chars = static_cast<uint32_t>(last_underscore);
+  std::memcpy(out_begin, punycode_begin, num_ascii_chars);
+  out_begin[num_ascii_chars] = '\0';
+  punycode_begin += num_ascii_chars + 1;
+  return true;
+}
+
+// Returns the value of `c` as a base-36 digit according to RFC 3492 section 5,
+// or -1 if `c` is not such a digit.
+int DigitValue(char c) {
+  if ('0' <= c && c <= '9') return c - '0' + 26;
+  if ('a' <= c && c <= 'z') return c - 'a';
+  if ('A' <= c && c <= 'Z') return c - 'A';
+  return -1;
+}
+
+// Consumes the next delta encoding from punycode_begin .. punycode_end,
+// updating i accordingly.  Returns true on success.  Returns false on parse
+// failure or arithmetic overflow.
+bool ScanNextDelta(const char*& punycode_begin, const char* const punycode_end,
+                   uint32_t bias, uint32_t& i) {
+  uint64_t w = 1;  // 64 bits to prevent overflow in w *= kBase - t
+
+  // "for k = base to infinity in steps of base do begin ... end" in RFC 3492
+  // section 6.2.  Each loop iteration scans one digit of the delta.
+  for (uint32_t k = kBase; punycode_begin != punycode_end; k += kBase) {
+    const int digit_value = DigitValue(*punycode_begin++);
+    if (digit_value < 0) return false;
+
+    // Compute this in 64-bit arithmetic so we can check for overflow afterward.
+    const uint64_t new_i = i + static_cast<uint64_t>(digit_value) * w;
+
+    // Valid deltas are bounded by (#chars already emitted) * kMaxCodePoint, but
+    // invalid input could encode an arbitrarily large delta.  Nip that in the
+    // bud here.
+    static_assert(
+        kMaxI >= kMaxChars * kMaxCodePoint,
+        "kMaxI is too small to prevent spurious failures on good input");
+    if (new_i > kMaxI) return false;
+
+    static_assert(
+        kMaxI < (uint64_t{1} << 32),
+        "Make kMaxI smaller or i 64 bits wide to prevent silent wraparound");
+    i = static_cast<uint32_t>(new_i);
+
+    // Compute the threshold that determines whether this is the last digit and
+    // (if not) what the next digit's place value will be.  This logic from RFC
+    // 3492 section 6.2 is explained in section 3.3.
+    uint32_t t;
+    if (k <= bias + kTMin) {
+      t = kTMin;
+    } else if (k >= bias + kTMax) {
+      t = kTMax;
+    } else {
+      t = k - bias;
+    }
+    if (static_cast<uint32_t>(digit_value) < t) return true;
+
+    // If this gets too large, the range check on new_i in the next iteration
+    // will catch it.  We know this multiplication will not overwrap because w
+    // is 64 bits wide.
+    w *= kBase - t;
+  }
+  return false;
+}
+
+}  // namespace
+
+char* absl_nullable DecodeRustPunycode(DecodeRustPunycodeOptions options) {
+  const char* punycode_begin = options.punycode_begin;
+  const char* const punycode_end = options.punycode_end;
+  char* const out_begin = options.out_begin;
+  char* const out_end = options.out_end;
+
+  // Write a NUL terminator first.  Later memcpy calls will keep bumping it
+  // along to its new right place.
+  const size_t out_size = static_cast<size_t>(out_end - out_begin);
+  if (out_size == 0) return nullptr;
+  *out_begin = '\0';
+
+  // RFC 3492 section 6.2 begins here.  We retain the names of integer variables
+  // appearing in that text.
+  uint32_t n = 128, i = 0, bias = 72, num_chars = 0;
+
+  // If there are any ASCII characters, consume them and their trailing
+  // underscore delimiter.
+  if (!ConsumeOptionalAsciiPrefix(punycode_begin, punycode_end,
+                                  out_begin, out_end, num_chars)) {
+    return nullptr;
+  }
+  uint32_t total_utf8_bytes = num_chars;
+
+  BoundedUtf8LengthSequence<kMaxChars> utf8_lengths;
+
+  // "while the input is not exhausted do begin ... end"
+  while (punycode_begin != punycode_end) {
+    if (num_chars >= kMaxChars) return nullptr;
+
+    const uint32_t old_i = i;
+
+    if (!ScanNextDelta(punycode_begin, punycode_end, bias, i)) return nullptr;
+
+    // Update bias as in RFC 3492 section 6.1.  (We have inlined adapt.)
+    uint32_t delta = i - old_i;
+    delta /= (old_i == 0 ? kDamp : 2);
+    delta += delta/(num_chars + 1);
+    bias = 0;
+    while (delta > ((kBase - kTMin) * kTMax)/2) {
+      delta /= kBase - kTMin;
+      bias += kBase;
+    }
+    bias += ((kBase - kTMin + 1) * delta)/(delta + kSkew);
+
+    // Back in section 6.2, compute the new code point and insertion index.
+    static_assert(
+        kMaxI + kMaxCodePoint < (uint64_t{1} << 32),
+        "Make kMaxI smaller or n 64 bits wide to prevent silent wraparound");
+    n += i/(num_chars + 1);
+    i %= num_chars + 1;
+
+    // To actually insert, we need to convert the code point n to UTF-8 and the
+    // character index i to an index into the byte stream emitted so far.  First
+    // prepare the UTF-8 encoding for n, rejecting surrogates, overlarge values,
+    // and anything that won't fit into the remaining output storage.
+    Utf8ForCodePoint utf8_for_code_point(n);
+    if (!utf8_for_code_point.ok()) return nullptr;
+    if (total_utf8_bytes + utf8_for_code_point.length + 1 > out_size) {
+      return nullptr;
+    }
+
+    // Now insert the new character into both our length map and the output.
+    uint32_t n_index =
+        utf8_lengths.InsertAndReturnSumOfPredecessors(
+            i, utf8_for_code_point.length);
+    std::memmove(
+        out_begin + n_index + utf8_for_code_point.length, out_begin + n_index,
+        total_utf8_bytes + 1 - n_index);
+    std::memcpy(out_begin + n_index, utf8_for_code_point.bytes,
+                utf8_for_code_point.length);
+    total_utf8_bytes += utf8_for_code_point.length;
+    ++num_chars;
+
+    // Finally, advance to the next state before continuing.
+    ++i;
+  }
+
+  return out_begin + total_utf8_bytes;
+}
+
+}  // namespace debugging_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/debugging/internal/decode_rust_punycode.h b/absl/debugging/internal/decode_rust_punycode.h
new file mode 100644
index 00000000000..44aad8adb2b
--- /dev/null
+++ b/absl/debugging/internal/decode_rust_punycode.h
@@ -0,0 +1,55 @@
+// Copyright 2024 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_DEBUGGING_INTERNAL_DECODE_RUST_PUNYCODE_H_
+#define ABSL_DEBUGGING_INTERNAL_DECODE_RUST_PUNYCODE_H_
+
+#include "absl/base/config.h"
+#include "absl/base/nullability.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace debugging_internal {
+
+struct DecodeRustPunycodeOptions {
+  const char* absl_nonnull punycode_begin;
+  const char* absl_nonnull punycode_end;
+  char* absl_nonnull out_begin;
+  char* absl_nonnull out_end;
+};
+
+// Given Rust Punycode in `punycode_begin .. punycode_end`, writes the
+// corresponding UTF-8 plaintext into `out_begin .. out_end`, followed by a NUL
+// character, and returns a pointer to that final NUL on success.  On failure
+// returns a null pointer, and the contents of `out_begin .. out_end` are
+// unspecified.
+//
+// Failure occurs in precisely these cases:
+//   - Any input byte does not match [0-9a-zA-Z_].
+//   - The first input byte is an underscore, but no other underscore appears in
+//     the input.
+//   - The delta sequence does not represent a valid sequence of code-point
+//     insertions.
+//   - The plaintext would contain more than 256 code points.
+//
+// DecodeRustPunycode is async-signal-safe with bounded runtime and a small
+// stack footprint, making it suitable for use in demangling Rust symbol names
+// from a signal handler.
+char* absl_nullable DecodeRustPunycode(DecodeRustPunycodeOptions options);
+
+}  // namespace debugging_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_DEBUGGING_INTERNAL_DECODE_RUST_PUNYCODE_H_
diff --git a/absl/debugging/internal/decode_rust_punycode_test.cc b/absl/debugging/internal/decode_rust_punycode_test.cc
new file mode 100644
index 00000000000..78d1c33283d
--- /dev/null
+++ b/absl/debugging/internal/decode_rust_punycode_test.cc
@@ -0,0 +1,606 @@
+// Copyright 2024 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/debugging/internal/decode_rust_punycode.h"
+
+#include <cstddef>
+#include <cstring>
+#include <string>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace debugging_internal {
+namespace {
+
+using ::testing::AllOf;
+using ::testing::Eq;
+using ::testing::IsNull;
+using ::testing::Pointee;
+using ::testing::ResultOf;
+using ::testing::StrEq;
+
+class DecodeRustPunycodeTest : public ::testing::Test {
+ protected:
+  void FillBufferWithNonzeroBytes() {
+    // The choice of nonzero value to fill with is arbitrary.  The point is just
+    // to fail tests if DecodeRustPunycode forgets to write the final NUL
+    // character.
+    std::memset(buffer_storage_, 0xab, sizeof(buffer_storage_));
+  }
+
+  DecodeRustPunycodeOptions WithAmpleSpace() {
+    FillBufferWithNonzeroBytes();
+
+    DecodeRustPunycodeOptions options;
+    options.punycode_begin = punycode_.data();
+    options.punycode_end = punycode_.data() + punycode_.size();
+    options.out_begin = buffer_storage_;
+    options.out_end = buffer_storage_ + sizeof(buffer_storage_);
+    return options;
+  }
+
+  DecodeRustPunycodeOptions WithJustEnoughSpace() {
+    FillBufferWithNonzeroBytes();
+
+    const size_t begin_offset = sizeof(buffer_storage_) - plaintext_.size() - 1;
+    DecodeRustPunycodeOptions options;
+    options.punycode_begin = punycode_.data();
+    options.punycode_end = punycode_.data() + punycode_.size();
+    options.out_begin = buffer_storage_ + begin_offset;
+    options.out_end = buffer_storage_ + sizeof(buffer_storage_);
+    return options;
+  }
+
+  DecodeRustPunycodeOptions WithOneByteTooFew() {
+    FillBufferWithNonzeroBytes();
+
+    const size_t begin_offset = sizeof(buffer_storage_) - plaintext_.size();
+    DecodeRustPunycodeOptions options;
+    options.punycode_begin = punycode_.data();
+    options.punycode_end = punycode_.data() + punycode_.size();
+    options.out_begin = buffer_storage_ + begin_offset;
+    options.out_end = buffer_storage_ + sizeof(buffer_storage_);
+    return options;
+  }
+
+  // Matches a correct return value of DecodeRustPunycode when `golden` is the
+  // expected plaintext output.
+  auto PointsToTheNulAfter(const std::string& golden) {
+    const size_t golden_size = golden.size();
+    return AllOf(
+        Pointee(Eq('\0')),
+        ResultOf("preceding string body",
+                 [golden_size](const char* p) { return p - golden_size; },
+                 StrEq(golden)));
+  }
+
+  std::string punycode_;
+  std::string plaintext_;
+  char buffer_storage_[1024];
+};
+
+TEST_F(DecodeRustPunycodeTest, MapsEmptyToEmpty) {
+  punycode_ = "";
+  plaintext_ = "";
+
+  ASSERT_THAT(DecodeRustPunycode(WithAmpleSpace()),
+              PointsToTheNulAfter(plaintext_));
+  ASSERT_THAT(DecodeRustPunycode(WithJustEnoughSpace()),
+              PointsToTheNulAfter(plaintext_));
+  EXPECT_THAT(DecodeRustPunycode(WithOneByteTooFew()), IsNull());
+}
+
+TEST_F(DecodeRustPunycodeTest,
+       StripsTheTrailingDelimiterFromAPureRunOfBasicChars) {
+  punycode_ = "foo_";
+  plaintext_ = "foo";
+
+  ASSERT_THAT(DecodeRustPunycode(WithAmpleSpace()),
+              PointsToTheNulAfter(plaintext_));
+  ASSERT_THAT(DecodeRustPunycode(WithJustEnoughSpace()),
+              PointsToTheNulAfter(plaintext_));
+  EXPECT_THAT(DecodeRustPunycode(WithOneByteTooFew()), IsNull());
+}
+
+TEST_F(DecodeRustPunycodeTest, TreatsTheLastUnderscoreAsTheDelimiter) {
+  punycode_ = "foo_bar_";
+  plaintext_ = "foo_bar";
+
+  ASSERT_THAT(DecodeRustPunycode(WithAmpleSpace()),
+              PointsToTheNulAfter(plaintext_));
+  ASSERT_THAT(DecodeRustPunycode(WithJustEnoughSpace()),
+              PointsToTheNulAfter(plaintext_));
+  EXPECT_THAT(DecodeRustPunycode(WithOneByteTooFew()), IsNull());
+}
+
+TEST_F(DecodeRustPunycodeTest, AcceptsALeadingUnderscoreIfNotTheDelimiter) {
+  punycode_ = "_foo_";
+  plaintext_ = "_foo";
+
+  ASSERT_THAT(DecodeRustPunycode(WithAmpleSpace()),
+              PointsToTheNulAfter(plaintext_));
+  ASSERT_THAT(DecodeRustPunycode(WithJustEnoughSpace()),
+              PointsToTheNulAfter(plaintext_));
+  EXPECT_THAT(DecodeRustPunycode(WithOneByteTooFew()), IsNull());
+}
+
+TEST_F(DecodeRustPunycodeTest, RejectsALeadingUnderscoreDelimiter) {
+  punycode_ = "_foo";
+
+  EXPECT_THAT(DecodeRustPunycode(WithAmpleSpace()), IsNull());
+}
+
+TEST_F(DecodeRustPunycodeTest, RejectsEmbeddedNul) {
+  punycode_ = std::string("foo\0bar_", 8);
+
+  EXPECT_THAT(DecodeRustPunycode(WithAmpleSpace()), IsNull());
+}
+
+TEST_F(DecodeRustPunycodeTest, RejectsAsciiCharsOtherThanIdentifierChars) {
+  punycode_ = "foo\007_";
+  EXPECT_THAT(DecodeRustPunycode(WithAmpleSpace()), IsNull());
+
+  punycode_ = "foo-_";
+  EXPECT_THAT(DecodeRustPunycode(WithAmpleSpace()), IsNull());
+
+  punycode_ = "foo;_";
+  EXPECT_THAT(DecodeRustPunycode(WithAmpleSpace()), IsNull());
+
+  punycode_ = "foo\177_";
+  EXPECT_THAT(DecodeRustPunycode(WithAmpleSpace()), IsNull());
+}
+
+TEST_F(DecodeRustPunycodeTest, RejectsRawNonAsciiChars) {
+  punycode_ = "\x80";
+  EXPECT_THAT(DecodeRustPunycode(WithAmpleSpace()), IsNull());
+
+  punycode_ = "\x80_";
+  EXPECT_THAT(DecodeRustPunycode(WithAmpleSpace()), IsNull());
+
+  punycode_ = "\xff";
+  EXPECT_THAT(DecodeRustPunycode(WithAmpleSpace()), IsNull());
+
+  punycode_ = "\xff_";
+  EXPECT_THAT(DecodeRustPunycode(WithAmpleSpace()), IsNull());
+}
+
+TEST_F(DecodeRustPunycodeTest, RecognizesU0080) {
+  // a encodes 0, so the output is the smallest non-ASCII code point standing
+  // alone.  (U+0080 PAD is not an identifier character, but DecodeRustPunycode
+  // does not check whether non-ASCII characters could belong to an identifier.)
+  punycode_ = "a";
+  plaintext_ = "\xc2\x80";
+
+  ASSERT_THAT(DecodeRustPunycode(WithAmpleSpace()),
+              PointsToTheNulAfter(plaintext_));
+  ASSERT_THAT(DecodeRustPunycode(WithJustEnoughSpace()),
+              PointsToTheNulAfter(plaintext_));
+  EXPECT_THAT(DecodeRustPunycode(WithOneByteTooFew()), IsNull());
+}
+
+TEST_F(DecodeRustPunycodeTest, OneByteDeltaSequencesMustBeA) {
+  // Because bias = 72 for the first code point, any digit but a/A is nonfinal
+  // in one of the first two bytes of a delta sequence.
+  punycode_ = "b";
+  EXPECT_THAT(DecodeRustPunycode(WithAmpleSpace()), IsNull());
+
+  punycode_ = "z";
+  EXPECT_THAT(DecodeRustPunycode(WithAmpleSpace()), IsNull());
+
+  punycode_ = "0";
+  EXPECT_THAT(DecodeRustPunycode(WithAmpleSpace()), IsNull());
+
+  punycode_ = "9";
+  EXPECT_THAT(DecodeRustPunycode(WithAmpleSpace()), IsNull());
+}
+
+TEST_F(DecodeRustPunycodeTest, AcceptsDeltaSequenceBA) {
+  punycode_ = "ba";
+  plaintext_ = "\xc2\x81";
+
+  ASSERT_THAT(DecodeRustPunycode(WithAmpleSpace()),
+              PointsToTheNulAfter(plaintext_));
+  ASSERT_THAT(DecodeRustPunycode(WithJustEnoughSpace()),
+              PointsToTheNulAfter(plaintext_));
+  EXPECT_THAT(DecodeRustPunycode(WithOneByteTooFew()), IsNull());
+}
+
+TEST_F(DecodeRustPunycodeTest, AcceptsOtherDeltaSequencesWithSecondByteA) {
+  punycode_ = "ca";
+  plaintext_ = "\xc2\x82";
+  EXPECT_THAT(DecodeRustPunycode(WithJustEnoughSpace()),
+              PointsToTheNulAfter(plaintext_));
+
+  punycode_ = "za";
+  plaintext_ = "\xc2\x99";
+  EXPECT_THAT(DecodeRustPunycode(WithJustEnoughSpace()),
+              PointsToTheNulAfter(plaintext_));
+
+  punycode_ = "0a";
+  plaintext_ = "\xc2\x9a";
+  EXPECT_THAT(DecodeRustPunycode(WithJustEnoughSpace()),
+              PointsToTheNulAfter(plaintext_));
+
+  punycode_ = "1a";
+  plaintext_ = "\xc2\x9b";
+  EXPECT_THAT(DecodeRustPunycode(WithJustEnoughSpace()),
+              PointsToTheNulAfter(plaintext_));
+
+  punycode_ = "9a";
+  plaintext_ = "£";  // Pound sign, U+00A3
+  EXPECT_THAT(DecodeRustPunycode(WithJustEnoughSpace()),
+              PointsToTheNulAfter(plaintext_));
+}
+
+TEST_F(DecodeRustPunycodeTest, RejectsDeltaWhereTheSecondAndLastDigitIsNotA) {
+  punycode_ = "bb";
+  EXPECT_THAT(DecodeRustPunycode(WithAmpleSpace()), IsNull());
+
+  punycode_ = "zz";
+  EXPECT_THAT(DecodeRustPunycode(WithAmpleSpace()), IsNull());
+
+  punycode_ = "00";
+  EXPECT_THAT(DecodeRustPunycode(WithAmpleSpace()), IsNull());
+
+  punycode_ = "99";
+  EXPECT_THAT(DecodeRustPunycode(WithAmpleSpace()), IsNull());
+}
+
+TEST_F(DecodeRustPunycodeTest, AcceptsDeltasWithSecondByteBFollowedByA) {
+  punycode_ = "bba";
+  plaintext_ = "¤";  // U+00A4
+  EXPECT_THAT(DecodeRustPunycode(WithJustEnoughSpace()),
+              PointsToTheNulAfter(plaintext_));
+
+  punycode_ = "cba";
+  plaintext_ = "¥";  // U+00A5
+  EXPECT_THAT(DecodeRustPunycode(WithJustEnoughSpace()),
+              PointsToTheNulAfter(plaintext_));
+
+  punycode_ = "zba";
+  plaintext_ = "¼";  // U+00BC
+  EXPECT_THAT(DecodeRustPunycode(WithJustEnoughSpace()),
+              PointsToTheNulAfter(plaintext_));
+
+  punycode_ = "0ba";
+  plaintext_ = "½";  // U+00BD
+  EXPECT_THAT(DecodeRustPunycode(WithJustEnoughSpace()),
+              PointsToTheNulAfter(plaintext_));
+
+  punycode_ = "1ba";
+  plaintext_ = "¾";  // U+00BE
+  EXPECT_THAT(DecodeRustPunycode(WithJustEnoughSpace()),
+              PointsToTheNulAfter(plaintext_));
+
+  punycode_ = "9ba";
+  plaintext_ = "Æ";  // U+00C6
+  EXPECT_THAT(DecodeRustPunycode(WithJustEnoughSpace()),
+              PointsToTheNulAfter(plaintext_));
+}
+
+// Tests beyond this point use characters allowed in identifiers, so you can
+// prepend _RNvC1cu<decimal length><underscore if [0-9_] follows> to a test
+// input and run it through another Rust demangler to verify that the
+// corresponding golden output is correct.
+
+TEST_F(DecodeRustPunycodeTest, AcceptsTwoByteCharAlone) {
+  punycode_ = "0ca";
+  plaintext_ = "à";
+
+  ASSERT_THAT(DecodeRustPunycode(WithAmpleSpace()),
+              PointsToTheNulAfter(plaintext_));
+  ASSERT_THAT(DecodeRustPunycode(WithJustEnoughSpace()),
+              PointsToTheNulAfter(plaintext_));
+  EXPECT_THAT(DecodeRustPunycode(WithOneByteTooFew()), IsNull());
+}
+
+TEST_F(DecodeRustPunycodeTest, AcceptsTwoByteCharBeforeBasicChars) {
+  punycode_ = "_la_mode_yya";
+  plaintext_ = "à_la_mode";
+
+  ASSERT_THAT(DecodeRustPunycode(WithAmpleSpace()),
+              PointsToTheNulAfter(plaintext_));
+  ASSERT_THAT(DecodeRustPunycode(WithJustEnoughSpace()),
+              PointsToTheNulAfter(plaintext_));
+  EXPECT_THAT(DecodeRustPunycode(WithOneByteTooFew()), IsNull());
+}
+
+TEST_F(DecodeRustPunycodeTest, AcceptsTwoByteCharAmidBasicChars) {
+  punycode_ = "verre__vin_m4a";
+  plaintext_ = "verre_à_vin";
+
+  ASSERT_THAT(DecodeRustPunycode(WithAmpleSpace()),
+              PointsToTheNulAfter(plaintext_));
+  ASSERT_THAT(DecodeRustPunycode(WithJustEnoughSpace()),
+              PointsToTheNulAfter(plaintext_));
+  EXPECT_THAT(DecodeRustPunycode(WithOneByteTooFew()), IsNull());
+}
+
+TEST_F(DecodeRustPunycodeTest, AcceptsTwoByteCharAfterBasicChars) {
+  punycode_ = "belt_3na";
+  plaintext_ = "beltà";
+
+  ASSERT_THAT(DecodeRustPunycode(WithAmpleSpace()),
+              PointsToTheNulAfter(plaintext_));
+  ASSERT_THAT(DecodeRustPunycode(WithJustEnoughSpace()),
+              PointsToTheNulAfter(plaintext_));
+  EXPECT_THAT(DecodeRustPunycode(WithOneByteTooFew()), IsNull());
+}
+
+TEST_F(DecodeRustPunycodeTest, AcceptsRepeatedTwoByteChar) {
+  punycode_ = "0caaaa";
+  plaintext_ = "àààà";
+
+  ASSERT_THAT(DecodeRustPunycode(WithAmpleSpace()),
+              PointsToTheNulAfter(plaintext_));
+  ASSERT_THAT(DecodeRustPunycode(WithJustEnoughSpace()),
+              PointsToTheNulAfter(plaintext_));
+  EXPECT_THAT(DecodeRustPunycode(WithOneByteTooFew()), IsNull());
+}
+
+TEST_F(DecodeRustPunycodeTest, AcceptsNearbyTwoByteCharsInOrder) {
+  punycode_ = "3camsuz";
+  plaintext_ = "ãéïôù";
+
+  ASSERT_THAT(DecodeRustPunycode(WithAmpleSpace()),
+              PointsToTheNulAfter(plaintext_));
+  ASSERT_THAT(DecodeRustPunycode(WithJustEnoughSpace()),
+              PointsToTheNulAfter(plaintext_));
+  EXPECT_THAT(DecodeRustPunycode(WithOneByteTooFew()), IsNull());
+}
+
+TEST_F(DecodeRustPunycodeTest, AcceptsNearbyTwoByteCharsOutOfOrder) {
+  punycode_ = "3caltsx";
+  plaintext_ = "ùéôãï";
+
+  ASSERT_THAT(DecodeRustPunycode(WithAmpleSpace()),
+              PointsToTheNulAfter(plaintext_));
+  ASSERT_THAT(DecodeRustPunycode(WithJustEnoughSpace()),
+              PointsToTheNulAfter(plaintext_));
+  EXPECT_THAT(DecodeRustPunycode(WithOneByteTooFew()), IsNull());
+}
+
+TEST_F(DecodeRustPunycodeTest, AcceptsThreeByteCharAlone) {
+  punycode_ = "fiq";
+  plaintext_ = "中";
+
+  ASSERT_THAT(DecodeRustPunycode(WithAmpleSpace()),
+              PointsToTheNulAfter(plaintext_));
+  ASSERT_THAT(DecodeRustPunycode(WithJustEnoughSpace()),
+              PointsToTheNulAfter(plaintext_));
+  EXPECT_THAT(DecodeRustPunycode(WithOneByteTooFew()), IsNull());
+}
+
+TEST_F(DecodeRustPunycodeTest, AcceptsRepeatedThreeByteChar) {
+  punycode_ = "fiqaaaa";
+  plaintext_ = "中中中中中";
+
+  ASSERT_THAT(DecodeRustPunycode(WithAmpleSpace()),
+              PointsToTheNulAfter(plaintext_));
+  ASSERT_THAT(DecodeRustPunycode(WithJustEnoughSpace()),
+              PointsToTheNulAfter(plaintext_));
+  EXPECT_THAT(DecodeRustPunycode(WithOneByteTooFew()), IsNull());
+}
+
+TEST_F(DecodeRustPunycodeTest, AcceptsThreeByteCharsInOrder) {
+  punycode_ = "fiq228c";
+  plaintext_ = "中文";
+
+  ASSERT_THAT(DecodeRustPunycode(WithAmpleSpace()),
+              PointsToTheNulAfter(plaintext_));
+  ASSERT_THAT(DecodeRustPunycode(WithJustEnoughSpace()),
+              PointsToTheNulAfter(plaintext_));
+  EXPECT_THAT(DecodeRustPunycode(WithOneByteTooFew()), IsNull());
+}
+
+TEST_F(DecodeRustPunycodeTest, AcceptsNearbyThreeByteCharsOutOfOrder) {
+  punycode_ = "fiq128c";
+  plaintext_ = "文中";
+
+  ASSERT_THAT(DecodeRustPunycode(WithAmpleSpace()),
+              PointsToTheNulAfter(plaintext_));
+  ASSERT_THAT(DecodeRustPunycode(WithJustEnoughSpace()),
+              PointsToTheNulAfter(plaintext_));
+  EXPECT_THAT(DecodeRustPunycode(WithOneByteTooFew()), IsNull());
+}
+
+TEST_F(DecodeRustPunycodeTest, AcceptsFourByteCharAlone) {
+  punycode_ = "uy7h";
+  plaintext_ = "🂻";
+
+  ASSERT_THAT(DecodeRustPunycode(WithAmpleSpace()),
+              PointsToTheNulAfter(plaintext_));
+  ASSERT_THAT(DecodeRustPunycode(WithJustEnoughSpace()),
+              PointsToTheNulAfter(plaintext_));
+  EXPECT_THAT(DecodeRustPunycode(WithOneByteTooFew()), IsNull());
+}
+
+TEST_F(DecodeRustPunycodeTest, AcceptsFourByteCharBeforeBasicChars) {
+  punycode_ = "jack__uh63d";
+  plaintext_ = "jack_🂻";
+
+  ASSERT_THAT(DecodeRustPunycode(WithAmpleSpace()),
+              PointsToTheNulAfter(plaintext_));
+  ASSERT_THAT(DecodeRustPunycode(WithJustEnoughSpace()),
+              PointsToTheNulAfter(plaintext_));
+  EXPECT_THAT(DecodeRustPunycode(WithOneByteTooFew()), IsNull());
+}
+
+TEST_F(DecodeRustPunycodeTest, AcceptsFourByteCharAmidBasicChars) {
+  punycode_ = "jack__of_hearts_ki37n";
+  plaintext_ = "jack_🂻_of_hearts";
+
+  ASSERT_THAT(DecodeRustPunycode(WithAmpleSpace()),
+              PointsToTheNulAfter(plaintext_));
+  ASSERT_THAT(DecodeRustPunycode(WithJustEnoughSpace()),
+              PointsToTheNulAfter(plaintext_));
+  EXPECT_THAT(DecodeRustPunycode(WithOneByteTooFew()), IsNull());
+}
+
+TEST_F(DecodeRustPunycodeTest, AcceptsFourByteCharAfterBasicChars) {
+  punycode_ = "_of_hearts_kz45i";
+  plaintext_ = "🂻_of_hearts";
+
+  ASSERT_THAT(DecodeRustPunycode(WithAmpleSpace()),
+              PointsToTheNulAfter(plaintext_));
+  ASSERT_THAT(DecodeRustPunycode(WithJustEnoughSpace()),
+              PointsToTheNulAfter(plaintext_));
+  EXPECT_THAT(DecodeRustPunycode(WithOneByteTooFew()), IsNull());
+}
+
+TEST_F(DecodeRustPunycodeTest, AcceptsRepeatedFourByteChar) {
+  punycode_ = "uy7haaaa";
+  plaintext_ = "🂻🂻🂻🂻🂻";
+
+  ASSERT_THAT(DecodeRustPunycode(WithAmpleSpace()),
+              PointsToTheNulAfter(plaintext_));
+  ASSERT_THAT(DecodeRustPunycode(WithJustEnoughSpace()),
+              PointsToTheNulAfter(plaintext_));
+  EXPECT_THAT(DecodeRustPunycode(WithOneByteTooFew()), IsNull());
+}
+
+TEST_F(DecodeRustPunycodeTest, AcceptsNearbyFourByteCharsInOrder) {
+  punycode_ = "8x7hcjmf";
+  plaintext_ = "🂦🂧🂪🂭🂮";
+
+  ASSERT_THAT(DecodeRustPunycode(WithAmpleSpace()),
+              PointsToTheNulAfter(plaintext_));
+  ASSERT_THAT(DecodeRustPunycode(WithJustEnoughSpace()),
+              PointsToTheNulAfter(plaintext_));
+  EXPECT_THAT(DecodeRustPunycode(WithOneByteTooFew()), IsNull());
+}
+
+TEST_F(DecodeRustPunycodeTest, AcceptsNearbyFourByteCharsOutOfOrder) {
+  punycode_ = "8x7hcild";
+  plaintext_ = "🂮🂦🂭🂪🂧";
+
+  ASSERT_THAT(DecodeRustPunycode(WithAmpleSpace()),
+              PointsToTheNulAfter(plaintext_));
+  ASSERT_THAT(DecodeRustPunycode(WithJustEnoughSpace()),
+              PointsToTheNulAfter(plaintext_));
+  EXPECT_THAT(DecodeRustPunycode(WithOneByteTooFew()), IsNull());
+}
+
+TEST_F(DecodeRustPunycodeTest, AcceptsAMixtureOfByteLengths) {
+  punycode_ = "3caltsx2079ivf8aiuy7cja3a6ak";
+  plaintext_ = "ùéôãï中文🂮🂦🂭🂪🂧";
+
+  ASSERT_THAT(DecodeRustPunycode(WithAmpleSpace()),
+              PointsToTheNulAfter(plaintext_));
+  ASSERT_THAT(DecodeRustPunycode(WithJustEnoughSpace()),
+              PointsToTheNulAfter(plaintext_));
+  EXPECT_THAT(DecodeRustPunycode(WithOneByteTooFew()), IsNull());
+}
+
+TEST_F(DecodeRustPunycodeTest, RejectsOverlargeDeltas) {
+  punycode_ = "123456789a";
+
+  EXPECT_THAT(DecodeRustPunycode(WithAmpleSpace()), IsNull());
+}
+
+// Finally, we test on a few prose and poetry snippets as a defense in depth.
+// If our artificial short test inputs did not exercise a bug that is tickled by
+// patterns typical of real human writing, maybe real human writing will catch
+// that.
+//
+// These test inputs are extracted from texts old enough to be out of copyright
+// that probe a variety of ranges of code-point space.  All are longer than 32
+// code points, so they exercise the carrying of seminibbles from one uint64_t
+// to the next higher one in BoundedUtf8LengthSequence.
+
+// The first three lines of the Old English epic _Beowulf_, mostly ASCII with a
+// few archaic two-byte letters interspersed.
+TEST_F(DecodeRustPunycodeTest, Beowulf) {
+  punycode_ = "hwt_we_gardena_in_geardagum_"
+              "eodcyninga_rym_gefrunon_"
+              "hu_a_elingas_ellen_fremedon_hxg9c70do9alau";
+  plaintext_ = "hwæt_we_gardena_in_geardagum_"
+               "þeodcyninga_þrym_gefrunon_"
+               "hu_ða_æþelingas_ellen_fremedon";
+
+  ASSERT_THAT(DecodeRustPunycode(WithAmpleSpace()),
+              PointsToTheNulAfter(plaintext_));
+  ASSERT_THAT(DecodeRustPunycode(WithJustEnoughSpace()),
+              PointsToTheNulAfter(plaintext_));
+  EXPECT_THAT(DecodeRustPunycode(WithOneByteTooFew()), IsNull());
+}
+
+// The whole of 過故人莊 by the 8th-century Chinese poet 孟浩然
+// (Meng Haoran), exercising three-byte-character processing.
+TEST_F(DecodeRustPunycodeTest, MengHaoran) {
+  punycode_ = "gmq4ss0cfvao1e2wg8mcw8b0wkl9a7tt90a8riuvbk7t8kbv9a66ogofvzlf6"
+              "3d01ybn1u28dyqi5q2cxyyxnk5d2gx1ks9ddvfm17bk6gbsd6wftrav60u4ta";
+  plaintext_ = "故人具雞黍" "邀我至田家"
+               "綠樹村邊合" "青山郭外斜"
+               "開軒面場圃" "把酒話桑麻"
+               "待到重陽日" "還來就菊花";
+
+  ASSERT_THAT(DecodeRustPunycode(WithAmpleSpace()),
+              PointsToTheNulAfter(plaintext_));
+  ASSERT_THAT(DecodeRustPunycode(WithJustEnoughSpace()),
+              PointsToTheNulAfter(plaintext_));
+  EXPECT_THAT(DecodeRustPunycode(WithOneByteTooFew()), IsNull());
+}
+
+// A poem of the 8th-century Japanese poet 山上憶良 (Yamanoue no Okura).
+// Japanese mixes two-byte and three-byte characters: a good workout for codecs.
+TEST_F(DecodeRustPunycodeTest, YamanoueNoOkura) {
+  punycode_ = "48jdaa3a6ccpepjrsmlb0q4bwcdtid8fg6c0cai9822utqeruk3om0u4f2wbp0"
+              "em23do0op23cc2ff70mb6tae8aq759gja";
+  plaintext_ = "瓜食めば"
+               "子ども思ほゆ"
+               "栗食めば"
+               "まして偲はゆ"
+               "何処より"
+               "来りしものそ"
+               "眼交に"
+               "もとな懸りて"
+               "安眠し寝さぬ";
+
+  ASSERT_THAT(DecodeRustPunycode(WithAmpleSpace()),
+              PointsToTheNulAfter(plaintext_));
+  ASSERT_THAT(DecodeRustPunycode(WithJustEnoughSpace()),
+              PointsToTheNulAfter(plaintext_));
+  EXPECT_THAT(DecodeRustPunycode(WithOneByteTooFew()), IsNull());
+}
+
+// The first two lines of the Phoenician-language inscription on the sarcophagus
+// of Eshmunazar II of Sidon, 6th century BCE.  Phoenician and many other
+// archaic scripts are allocated in the Supplemental Multilingual Plane (U+10000
+// through U+1FFFF) and thus exercise four-byte-character processing.
+TEST_F(DecodeRustPunycodeTest, EshmunazarSarcophagus) {
+  punycode_ = "wj9caaabaabbaaohcacxvhdc7bgxbccbdcjeacddcedcdlddbdbddcdbdcknfcee"
+              "ifel8del2a7inq9fhcpxikms7a4a9ac9ataaa0g";
+  plaintext_ = "𐤁𐤉𐤓𐤇𐤁𐤋𐤁𐤔𐤍𐤕𐤏𐤎𐤓"
+               "𐤅𐤀𐤓𐤁𐤏𐤗𐤖𐤖𐤖𐤖𐤋𐤌𐤋𐤊𐤉𐤌𐤋𐤊"
+               "𐤀𐤔𐤌𐤍𐤏𐤆𐤓𐤌𐤋𐤊𐤑𐤃𐤍𐤌"
+               "𐤁𐤍𐤌𐤋𐤊𐤕𐤁𐤍𐤕𐤌𐤋𐤊𐤑𐤃𐤍𐤌"
+               "𐤃𐤁𐤓𐤌𐤋𐤊𐤀𐤔𐤌𐤍𐤏𐤆𐤓𐤌𐤋𐤊"
+               "𐤑𐤃𐤍𐤌𐤋𐤀𐤌𐤓𐤍𐤂𐤆𐤋𐤕";
+
+  ASSERT_THAT(DecodeRustPunycode(WithAmpleSpace()),
+              PointsToTheNulAfter(plaintext_));
+  ASSERT_THAT(DecodeRustPunycode(WithJustEnoughSpace()),
+              PointsToTheNulAfter(plaintext_));
+  EXPECT_THAT(DecodeRustPunycode(WithOneByteTooFew()), IsNull());
+}
+
+}  // namespace
+}  // namespace debugging_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/debugging/internal/demangle.cc b/absl/debugging/internal/demangle.cc
index 52a553fd058..e97a8ffbbed 100644
--- a/absl/debugging/internal/demangle.cc
+++ b/absl/debugging/internal/demangle.cc
@@ -14,16 +14,27 @@
 
 // For reference check out:
 // https://itanium-cxx-abi.github.io/cxx-abi/abi.html#mangling
-//
-// Note that we only have partial C++11 support yet.
 
 #include "absl/debugging/internal/demangle.h"
 
+#include <algorithm>
+#include <cstddef>
 #include <cstdint>
 #include <cstdio>
+#include <cstdlib>
+#include <cstring>
 #include <limits>
+#include <string>
+
+#include "absl/base/config.h"
+#include "absl/debugging/internal/demangle_rust.h"
+
+#ifdef ABSL_INTERNAL_HAS_CXA_DEMANGLE
+#include <cxxabi.h>
+#endif
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace debugging_internal {
 
 typedef struct {
@@ -35,14 +46,16 @@ typedef struct {
 
 // List of operators from Itanium C++ ABI.
 static const AbbrevPair kOperatorList[] = {
-    // New has special syntax (not currently supported).
+    // New has special syntax.
     {"nw", "new", 0},
     {"na", "new[]", 0},
 
-    // Works except that the 'gs' prefix is not supported.
+    // Special-cased elsewhere to support the optional gs prefix.
     {"dl", "delete", 1},
     {"da", "delete[]", 1},
 
+    {"aw", "co_await", 1},
+
     {"ps", "+", 1},  // "positive"
     {"ng", "-", 1},  // "negative"
     {"ad", "&", 1},  // "address-of"
@@ -70,6 +83,7 @@ static const AbbrevPair kOperatorList[] = {
     {"rs", ">>", 2},
     {"lS", "<<=", 2},
     {"rS", ">>=", 2},
+    {"ss", "<=>", 2},
     {"eq", "==", 2},
     {"ne", "!=", 2},
     {"lt", "<", 2},
@@ -89,10 +103,13 @@ static const AbbrevPair kOperatorList[] = {
     {"qu", "?", 3},
     {"st", "sizeof", 0},  // Special syntax
     {"sz", "sizeof", 1},  // Not a real operator name, but used in expressions.
+    {"sZ", "sizeof...", 0},  // Special syntax
     {nullptr, nullptr, 0},
 };
 
 // List of builtin types from Itanium C++ ABI.
+//
+// Invariant: only one- or two-character type abbreviations here.
 static const AbbrevPair kBuiltinTypeList[] = {
     {"v", "void", 0},
     {"w", "wchar_t", 0},
@@ -115,6 +132,17 @@ static const AbbrevPair kBuiltinTypeList[] = {
     {"e", "long double", 0},
     {"g", "__float128", 0},
     {"z", "ellipsis", 0},
+
+    {"De", "decimal128", 0},      // IEEE 754r decimal floating point (128 bits)
+    {"Dd", "decimal64", 0},       // IEEE 754r decimal floating point (64 bits)
+    {"Dc", "decltype(auto)", 0},
+    {"Da", "auto", 0},
+    {"Dn", "std::nullptr_t", 0},  // i.e., decltype(nullptr)
+    {"Df", "decimal32", 0},       // IEEE 754r decimal floating point (32 bits)
+    {"Di", "char32_t", 0},
+    {"Du", "char8_t", 0},
+    {"Ds", "char16_t", 0},
+    {"Dh", "float16", 0},         // IEEE 754r half-precision float (16 bits)
     {nullptr, nullptr, 0},
 };
 
@@ -137,12 +165,12 @@ static const AbbrevPair kSubstitutionList[] = {
 // State needed for demangling.  This struct is copied in almost every stack
 // frame, so every byte counts.
 typedef struct {
-  int mangled_idx;                   // Cursor of mangled name.
-  int out_cur_idx;                   // Cursor of output std::string.
-  int prev_name_idx;                 // For constructors/destructors.
-  signed int prev_name_length : 16;  // For constructors/destructors.
-  signed int nest_level : 15;        // For nested names.
-  unsigned int append : 1;           // Append flag.
+  int mangled_idx;                     // Cursor of mangled name.
+  int out_cur_idx;                     // Cursor of output string.
+  int prev_name_idx;                   // For constructors/destructors.
+  unsigned int prev_name_length : 16;  // For constructors/destructors.
+  signed int nest_level : 15;          // For nested names.
+  unsigned int append : 1;             // Append flag.
   // Note: for some reason MSVC can't pack "bool append : 1" into the same int
   // with the above two fields, so we use an int instead.  Amusingly it can pack
   // "signed bool" as expected, but relying on that to continue to be a legal
@@ -159,15 +187,56 @@ static_assert(sizeof(ParseState) == 4 * sizeof(int),
 // Only one copy of this exists for each call to Demangle, so the size of this
 // struct is nearly inconsequential.
 typedef struct {
-  const char *mangled_begin;  // Beginning of input std::string.
-  char *out;                  // Beginning of output std::string.
+  const char *mangled_begin;  // Beginning of input string.
+  char *out;                  // Beginning of output string.
   int out_end_idx;            // One past last allowed output character.
   int recursion_depth;        // For stack exhaustion prevention.
   int steps;               // Cap how much work we'll do, regardless of depth.
   ParseState parse_state;  // Backtrackable state copied for most frames.
+
+  // Conditionally compiled support for marking the position of the first
+  // construct Demangle couldn't parse.  This preprocessor symbol is intended
+  // for use by Abseil demangler maintainers only; its behavior is not part of
+  // Abseil's public interface.
+#ifdef ABSL_INTERNAL_DEMANGLE_RECORDS_HIGH_WATER_MARK
+  int high_water_mark;  // Input position where parsing failed.
+  bool too_complex;  // True if any guard.IsTooComplex() call returned true.
+#endif
 } State;
 
 namespace {
+
+#ifdef ABSL_INTERNAL_DEMANGLE_RECORDS_HIGH_WATER_MARK
+void UpdateHighWaterMark(State *state) {
+  if (state->high_water_mark < state->parse_state.mangled_idx) {
+    state->high_water_mark = state->parse_state.mangled_idx;
+  }
+}
+
+void ReportHighWaterMark(State *state) {
+  // Write out the mangled name with the trouble point marked, provided that the
+  // output buffer is large enough and the mangled name did not hit a complexity
+  // limit (in which case the high water mark wouldn't point out an unparsable
+  // construct, only the point where a budget ran out).
+  const size_t input_length = std::strlen(state->mangled_begin);
+  if (input_length + 6 > static_cast<size_t>(state->out_end_idx) ||
+      state->too_complex) {
+    if (state->out_end_idx > 0) state->out[0] = '\0';
+    return;
+  }
+  const size_t high_water_mark = static_cast<size_t>(state->high_water_mark);
+  std::memcpy(state->out, state->mangled_begin, high_water_mark);
+  std::memcpy(state->out + high_water_mark, "--!--", 5);
+  std::memcpy(state->out + high_water_mark + 5,
+              state->mangled_begin + high_water_mark,
+              input_length - high_water_mark);
+  state->out[input_length + 5] = '\0';
+}
+#else
+void UpdateHighWaterMark(State *) {}
+void ReportHighWaterMark(State *) {}
+#endif
+
 // Prevent deep recursion / stack exhaustion.
 // Also prevent unbounded handling of complex inputs.
 class ComplexityGuard {
@@ -179,7 +248,7 @@ class ComplexityGuard {
   ~ComplexityGuard() { --state_->recursion_depth; }
 
   // 256 levels of recursion seems like a reasonable upper limit on depth.
-  // 128 is not enough to demagle synthetic tests from demangle_unittest.txt:
+  // 128 is not enough to demangle synthetic tests from demangle_unittest.txt:
   // "_ZaaZZZZ..." and "_ZaaZcvZcvZ..."
   static constexpr int kRecursionDepthLimit = 256;
 
@@ -200,8 +269,14 @@ class ComplexityGuard {
   static constexpr int kParseStepsLimit = 1 << 17;
 
   bool IsTooComplex() const {
-    return state_->recursion_depth > kRecursionDepthLimit ||
-           state_->steps > kParseStepsLimit;
+    if (state_->recursion_depth > kRecursionDepthLimit ||
+        state_->steps > kParseStepsLimit) {
+#ifdef ABSL_INTERNAL_DEMANGLE_RECORDS_HIGH_WATER_MARK
+      state_->too_complex = true;
+#endif
+      return true;
+    }
+    return false;
   }
 
  private:
@@ -221,8 +296,8 @@ static size_t StrLen(const char *str) {
 }
 
 // Returns true if "str" has at least "n" characters remaining.
-static bool AtLeastNumCharsRemaining(const char *str, int n) {
-  for (int i = 0; i < n; ++i) {
+static bool AtLeastNumCharsRemaining(const char *str, size_t n) {
+  for (size_t i = 0; i < n; ++i) {
     if (str[i] == '\0') {
       return false;
     }
@@ -239,18 +314,24 @@ static bool StrPrefix(const char *str, const char *prefix) {
   return prefix[i] == '\0';  // Consumed everything in "prefix".
 }
 
-static void InitState(State *state, const char *mangled, char *out,
-                      int out_size) {
+static void InitState(State* state,
+                      const char* mangled,
+                      char* out,
+                      size_t out_size) {
   state->mangled_begin = mangled;
   state->out = out;
-  state->out_end_idx = out_size;
+  state->out_end_idx = static_cast<int>(out_size);
   state->recursion_depth = 0;
   state->steps = 0;
+#ifdef ABSL_INTERNAL_DEMANGLE_RECORDS_HIGH_WATER_MARK
+  state->high_water_mark = 0;
+  state->too_complex = false;
+#endif
 
   state->parse_state.mangled_idx = 0;
   state->parse_state.out_cur_idx = 0;
   state->parse_state.prev_name_idx = 0;
-  state->parse_state.prev_name_length = -1;
+  state->parse_state.prev_name_length = 0;
   state->parse_state.nest_level = -1;
   state->parse_state.append = true;
 }
@@ -267,13 +348,14 @@ static bool ParseOneCharToken(State *state, const char one_char_token) {
   if (guard.IsTooComplex()) return false;
   if (RemainingInput(state)[0] == one_char_token) {
     ++state->parse_state.mangled_idx;
+    UpdateHighWaterMark(state);
     return true;
   }
   return false;
 }
 
-// Returns true and advances "mangled_cur" if we find "two_char_token"
-// at "mangled_cur" position.  It is assumed that "two_char_token" does
+// Returns true and advances "mangled_idx" if we find "two_char_token"
+// at "mangled_idx" position.  It is assumed that "two_char_token" does
 // not contain '\0'.
 static bool ParseTwoCharToken(State *state, const char *two_char_token) {
   ComplexityGuard guard(state);
@@ -281,11 +363,45 @@ static bool ParseTwoCharToken(State *state, const char *two_char_token) {
   if (RemainingInput(state)[0] == two_char_token[0] &&
       RemainingInput(state)[1] == two_char_token[1]) {
     state->parse_state.mangled_idx += 2;
+    UpdateHighWaterMark(state);
+    return true;
+  }
+  return false;
+}
+
+// Returns true and advances "mangled_idx" if we find "three_char_token"
+// at "mangled_idx" position.  It is assumed that "three_char_token" does
+// not contain '\0'.
+static bool ParseThreeCharToken(State *state, const char *three_char_token) {
+  ComplexityGuard guard(state);
+  if (guard.IsTooComplex()) return false;
+  if (RemainingInput(state)[0] == three_char_token[0] &&
+      RemainingInput(state)[1] == three_char_token[1] &&
+      RemainingInput(state)[2] == three_char_token[2]) {
+    state->parse_state.mangled_idx += 3;
+    UpdateHighWaterMark(state);
     return true;
   }
   return false;
 }
 
+// Returns true and advances "mangled_idx" if we find a copy of the
+// NUL-terminated string "long_token" at "mangled_idx" position.
+static bool ParseLongToken(State *state, const char *long_token) {
+  ComplexityGuard guard(state);
+  if (guard.IsTooComplex()) return false;
+  int i = 0;
+  for (; long_token[i] != '\0'; ++i) {
+    // Note that we cannot run off the end of the NUL-terminated input here.
+    // Inside the loop body, long_token[i] is known to be different from NUL.
+    // So if we read the NUL on the end of the input here, we return at once.
+    if (RemainingInput(state)[i] != long_token[i]) return false;
+  }
+  state->parse_state.mangled_idx += i;
+  UpdateHighWaterMark(state);
+  return true;
+}
+
 // Returns true and advances "mangled_cur" if we find any character in
 // "char_class" at "mangled_cur" position.
 static bool ParseCharClass(State *state, const char *char_class) {
@@ -298,6 +414,7 @@ static bool ParseCharClass(State *state, const char *char_class) {
   for (; *p != '\0'; ++p) {
     if (RemainingInput(state)[0] == *p) {
       ++state->parse_state.mangled_idx;
+      UpdateHighWaterMark(state);
       return true;
     }
   }
@@ -340,22 +457,35 @@ static bool ZeroOrMore(ParseFunc parse_func, State *state) {
 }
 
 // Append "str" at "out_cur_idx".  If there is an overflow, out_cur_idx is
-// set to out_end_idx+1.  The output string is ensured to
-// always terminate with '\0' as long as there is no overflow.
-static void Append(State *state, const char *const str, const int length) {
-  for (int i = 0; i < length; ++i) {
-    if (state->parse_state.out_cur_idx + 1 <
-        state->out_end_idx) {  // +1 for '\0'
-      state->out[state->parse_state.out_cur_idx++] = str[i];
-    } else {
-      // signal overflow
-      state->parse_state.out_cur_idx = state->out_end_idx + 1;
-      break;
-    }
+// set to out_end_idx+1.  The output buffer is always terminated with '\0' if it
+// has nonzero length.
+static void Append(State *state, const char *const str, const size_t length) {
+  if (length == 0) {
+    return;
   }
-  if (state->parse_state.out_cur_idx < state->out_end_idx) {
-    state->out[state->parse_state.out_cur_idx] =
-        '\0';  // Terminate it with '\0'
+
+  // Figure out how much space is remaining in the output buffer to copy into.
+  const int cap = state->out_end_idx - state->parse_state.out_cur_idx;
+
+  // If overflow was already signaled (negative value, set further below) or
+  // there is zero space to write into, we cannot do anything.
+  if (cap <= 0) {
+    return;
+  }
+
+  // Copy the number of characters requested, capped by the amount of space
+  // remaining.
+  std::char_traits<char>::copy(state->out + state->parse_state.out_cur_idx, str,
+                               (std::min)(length, static_cast<size_t>(cap)));
+
+  // Did we copy everything we needed to, with enough room to NUL-terminate?
+  if (length < static_cast<size_t>(cap)) {
+    state->parse_state.out_cur_idx += static_cast<int>(length);
+    state->out[state->parse_state.out_cur_idx] = '\0';
+  } else {
+    // No, we ran out of space. Signal overflow, and NUL-terminate for safety.
+    state->parse_state.out_cur_idx = state->out_end_idx + 1;
+    state->out[state->out_end_idx - 1] = '\0';
   }
 }
 
@@ -368,57 +498,34 @@ static bool IsAlpha(char c) {
 
 static bool IsDigit(char c) { return c >= '0' && c <= '9'; }
 
-// Returns true if "str" is a function clone suffix.  These suffixes are used
-// by GCC 4.5.x and later versions (and our locally-modified version of GCC
-// 4.4.x) to indicate functions which have been cloned during optimization.
-// We treat any sequence (.<alpha>+.<digit>+)+ as a function clone suffix.
-static bool IsFunctionCloneSuffix(const char *str) {
-  size_t i = 0;
-  while (str[i] != '\0') {
-    // Consume a single .<alpha>+.<digit>+ sequence.
-    if (str[i] != '.' || !IsAlpha(str[i + 1])) {
-      return false;
-    }
-    i += 2;
-    while (IsAlpha(str[i])) {
-      ++i;
-    }
-    if (str[i] != '.' || !IsDigit(str[i + 1])) {
-      return false;
-    }
-    i += 2;
-    while (IsDigit(str[i])) {
-      ++i;
-    }
-  }
-  return true;  // Consumed everything in "str".
-}
-
 static bool EndsWith(State *state, const char chr) {
   return state->parse_state.out_cur_idx > 0 &&
+         state->parse_state.out_cur_idx < state->out_end_idx &&
          chr == state->out[state->parse_state.out_cur_idx - 1];
 }
 
 // Append "str" with some tweaks, iff "append" state is true.
 static void MaybeAppendWithLength(State *state, const char *const str,
-                                  const int length) {
+                                  const size_t length) {
   if (state->parse_state.append && length > 0) {
     // Append a space if the output buffer ends with '<' and "str"
     // starts with '<' to avoid <<<.
     if (str[0] == '<' && EndsWith(state, '<')) {
       Append(state, " ", 1);
     }
-    // Remember the last identifier name for ctors/dtors.
-    if (IsAlpha(str[0]) || str[0] == '_') {
+    // Remember the last identifier name for ctors/dtors,
+    // but only if we haven't yet overflown the buffer.
+    if (state->parse_state.out_cur_idx < state->out_end_idx &&
+        (IsAlpha(str[0]) || str[0] == '_')) {
       state->parse_state.prev_name_idx = state->parse_state.out_cur_idx;
-      state->parse_state.prev_name_length = length;
+      state->parse_state.prev_name_length = static_cast<unsigned int>(length);
     }
     Append(state, str, length);
   }
 }
 
 // Appends a positive decimal number to the output if appending is enabled.
-static bool MaybeAppendDecimal(State *state, unsigned int val) {
+static bool MaybeAppendDecimal(State *state, int val) {
   // Max {32-64}-bit unsigned int is 20 digits.
   constexpr size_t kMaxLength = 20;
   char buf[kMaxLength];
@@ -430,12 +537,12 @@ static bool MaybeAppendDecimal(State *state, unsigned int val) {
     // one-past-the-end and manipulate one character before the pointer.
     char *p = &buf[kMaxLength];
     do {  // val=0 is the only input that should write a leading zero digit.
-      *--p = (val % 10) + '0';
+      *--p = static_cast<char>((val % 10) + '0');
       val /= 10;
     } while (p > buf && val != 0);
 
     // 'p' landed on the last character we set.  How convenient.
-    Append(state, p, kMaxLength - (p - buf));
+    Append(state, p, kMaxLength - static_cast<size_t>(p - buf));
   }
 
   return true;
@@ -445,7 +552,7 @@ static bool MaybeAppendDecimal(State *state, unsigned int val) {
 // Returns true so that it can be placed in "if" conditions.
 static bool MaybeAppend(State *state, const char *const str) {
   if (state->parse_state.append) {
-    int length = StrLen(str);
+    size_t length = StrLen(str);
     MaybeAppendWithLength(state, str, length);
   }
   return true;
@@ -500,10 +607,10 @@ static void MaybeCancelLastSeparator(State *state) {
 
 // Returns true if the identifier of the given length pointed to by
 // "mangled_cur" is anonymous namespace.
-static bool IdentifierIsAnonymousNamespace(State *state, int length) {
+static bool IdentifierIsAnonymousNamespace(State *state, size_t length) {
   // Returns true if "anon_prefix" is a proper prefix of "mangled_cur".
   static const char anon_prefix[] = "_GLOBAL__N_";
-  return (length > static_cast<int>(sizeof(anon_prefix) - 1) &&
+  return (length > (sizeof(anon_prefix) - 1) &&
           StrPrefix(RemainingInput(state), anon_prefix));
 }
 
@@ -521,31 +628,45 @@ static bool ParseUnnamedTypeName(State *state);
 static bool ParseNumber(State *state, int *number_out);
 static bool ParseFloatNumber(State *state);
 static bool ParseSeqId(State *state);
-static bool ParseIdentifier(State *state, int length);
+static bool ParseIdentifier(State *state, size_t length);
 static bool ParseOperatorName(State *state, int *arity);
+static bool ParseConversionOperatorType(State *state);
 static bool ParseSpecialName(State *state);
 static bool ParseCallOffset(State *state);
 static bool ParseNVOffset(State *state);
 static bool ParseVOffset(State *state);
+static bool ParseAbiTags(State *state);
 static bool ParseCtorDtorName(State *state);
 static bool ParseDecltype(State *state);
 static bool ParseType(State *state);
 static bool ParseCVQualifiers(State *state);
+static bool ParseExtendedQualifier(State *state);
 static bool ParseBuiltinType(State *state);
+static bool ParseVendorExtendedType(State *state);
 static bool ParseFunctionType(State *state);
 static bool ParseBareFunctionType(State *state);
+static bool ParseOverloadAttribute(State *state);
 static bool ParseClassEnumType(State *state);
 static bool ParseArrayType(State *state);
 static bool ParsePointerToMemberType(State *state);
 static bool ParseTemplateParam(State *state);
+static bool ParseTemplateParamDecl(State *state);
 static bool ParseTemplateTemplateParam(State *state);
 static bool ParseTemplateArgs(State *state);
 static bool ParseTemplateArg(State *state);
 static bool ParseBaseUnresolvedName(State *state);
 static bool ParseUnresolvedName(State *state);
+static bool ParseUnresolvedQualifierLevel(State *state);
+static bool ParseUnionSelector(State* state);
+static bool ParseFunctionParam(State* state);
+static bool ParseBracedExpression(State *state);
 static bool ParseExpression(State *state);
+static bool ParseInitializer(State *state);
 static bool ParseExprPrimary(State *state);
-static bool ParseExprCastValue(State *state);
+static bool ParseExprCastValueAndTrailingE(State *state);
+static bool ParseQRequiresClauseExpr(State *state);
+static bool ParseRequirement(State *state);
+static bool ParseTypeConstraint(State *state);
 static bool ParseLocalName(State *state);
 static bool ParseLocalNameSuffix(State *state);
 static bool ParseDiscriminator(State *state);
@@ -580,7 +701,7 @@ static bool ParseSubstitution(State *state, bool accept_std);
 //
 // Reference:
 // - Itanium C++ ABI
-//   <https://mentorembedded.github.io/cxx-abi/abi.html#mangling>
+//   <https://itanium-cxx-abi.github.io/cxx-abi/abi.html#mangling>
 
 // <mangled-name> ::= _Z <encoding>
 static bool ParseMangledName(State *state) {
@@ -590,22 +711,34 @@ static bool ParseMangledName(State *state) {
 }
 
 // <encoding> ::= <(function) name> <bare-function-type>
+//                [`Q` <requires-clause expr>]
 //            ::= <(data) name>
 //            ::= <special-name>
+//
+// NOTE: Based on http://shortn/_Hoq9qG83rx
 static bool ParseEncoding(State *state) {
   ComplexityGuard guard(state);
   if (guard.IsTooComplex()) return false;
-  // Implementing the first two productions together as <name>
-  // [<bare-function-type>] avoids exponential blowup of backtracking.
+  // Since the first two productions both start with <name>, attempt
+  // to parse it only once to avoid exponential blowup of backtracking.
   //
-  // Since Optional(...) can't fail, there's no need to copy the state for
-  // backtracking.
-  if (ParseName(state) && Optional(ParseBareFunctionType(state))) {
+  // We're careful about exponential blowup because <encoding> recursively
+  // appears in other productions downstream of its first two productions,
+  // which means that every call to `ParseName` would possibly indirectly
+  // result in two calls to `ParseName` etc.
+  if (ParseName(state)) {
+    if (!ParseBareFunctionType(state)) {
+      return true;  // <(data) name>
+    }
+
+    // Parsed: <(function) name> <bare-function-type>
+    // Pending: [`Q` <requires-clause expr>]
+    ParseQRequiresClauseExpr(state);  // restores state on failure
     return true;
   }
 
   if (ParseSpecialName(state)) {
-    return true;
+    return true;  // <special-name>
   }
   return false;
 }
@@ -691,19 +824,26 @@ static bool ParseNestedName(State *state) {
 // <prefix> ::= <prefix> <unqualified-name>
 //          ::= <template-prefix> <template-args>
 //          ::= <template-param>
+//          ::= <decltype>
 //          ::= <substitution>
 //          ::= # empty
 // <template-prefix> ::= <prefix> <(template) unqualified-name>
 //                   ::= <template-param>
 //                   ::= <substitution>
+//                   ::= <vendor-extended-type>
 static bool ParsePrefix(State *state) {
   ComplexityGuard guard(state);
   if (guard.IsTooComplex()) return false;
   bool has_something = false;
   while (true) {
     MaybeAppendSeparator(state);
-    if (ParseTemplateParam(state) ||
+    if (ParseTemplateParam(state) || ParseDecltype(state) ||
         ParseSubstitution(state, /*accept_std=*/true) ||
+        // Although the official grammar does not mention it, nested-names
+        // shaped like Nu14__some_builtinIiE6memberE occur in practice, and it
+        // is not clear what else a compiler is supposed to do when a
+        // vendor-extended type has named members.
+        ParseVendorExtendedType(state) ||
         ParseUnscopedName(state) ||
         (ParseOneCharToken(state, 'M') && ParseUnnamedTypeName(state))) {
       has_something = true;
@@ -720,17 +860,68 @@ static bool ParsePrefix(State *state) {
   return true;
 }
 
-// <unqualified-name> ::= <operator-name>
-//                    ::= <ctor-dtor-name>
-//                    ::= <source-name>
-//                    ::= <local-source-name> // GCC extension; see below.
-//                    ::= <unnamed-type-name>
+// <unqualified-name> ::= <operator-name> [<abi-tags>]
+//                    ::= <ctor-dtor-name> [<abi-tags>]
+//                    ::= <source-name> [<abi-tags>]
+//                    ::= <local-source-name> [<abi-tags>]
+//                    ::= <unnamed-type-name> [<abi-tags>]
+//                    ::= DC <source-name>+ E  # C++17 structured binding
+//                    ::= F <source-name>  # C++20 constrained friend
+//                    ::= F <operator-name>  # C++20 constrained friend
+//
+// <local-source-name> is a GCC extension; see below.
+//
+// For the F notation for constrained friends, see
+// https://github.com/itanium-cxx-abi/cxx-abi/issues/24#issuecomment-1491130332.
 static bool ParseUnqualifiedName(State *state) {
   ComplexityGuard guard(state);
   if (guard.IsTooComplex()) return false;
-  return (ParseOperatorName(state, nullptr) || ParseCtorDtorName(state) ||
-          ParseSourceName(state) || ParseLocalSourceName(state) ||
-          ParseUnnamedTypeName(state));
+  if (ParseOperatorName(state, nullptr) || ParseCtorDtorName(state) ||
+      ParseSourceName(state) || ParseLocalSourceName(state) ||
+      ParseUnnamedTypeName(state)) {
+    return ParseAbiTags(state);
+  }
+
+  // DC <source-name>+ E
+  ParseState copy = state->parse_state;
+  if (ParseTwoCharToken(state, "DC") && OneOrMore(ParseSourceName, state) &&
+      ParseOneCharToken(state, 'E')) {
+    return true;
+  }
+  state->parse_state = copy;
+
+  // F <source-name>
+  // F <operator-name>
+  if (ParseOneCharToken(state, 'F') && MaybeAppend(state, "friend ") &&
+      (ParseSourceName(state) || ParseOperatorName(state, nullptr))) {
+    return true;
+  }
+  state->parse_state = copy;
+
+  return false;
+}
+
+// <abi-tags> ::= <abi-tag> [<abi-tags>]
+// <abi-tag>  ::= B <source-name>
+static bool ParseAbiTags(State *state) {
+  ComplexityGuard guard(state);
+  if (guard.IsTooComplex()) return false;
+
+  for (;;) {
+    const ParseState copy = state->parse_state;
+    if (!ParseOneCharToken(state, 'B')) {
+      break;
+    }
+    MaybeAppend(state, "[abi:");
+
+    if (!ParseSourceName(state)) {
+      state->parse_state = copy;
+      return false;
+    }
+    MaybeAppend(state, "]");
+  }
+
+  return true;
 }
 
 // <source-name> ::= <positive length number> <identifier>
@@ -739,7 +930,8 @@ static bool ParseSourceName(State *state) {
   if (guard.IsTooComplex()) return false;
   ParseState copy = state->parse_state;
   int length = -1;
-  if (ParseNumber(state, &length) && ParseIdentifier(state, length)) {
+  if (ParseNumber(state, &length) &&
+      ParseIdentifier(state, static_cast<size_t>(length))) {
     return true;
   }
   state->parse_state = copy;
@@ -766,7 +958,11 @@ static bool ParseLocalSourceName(State *state) {
 // <unnamed-type-name> ::= Ut [<(nonnegative) number>] _
 //                     ::= <closure-type-name>
 // <closure-type-name> ::= Ul <lambda-sig> E [<(nonnegative) number>] _
-// <lambda-sig>        ::= <(parameter) type>+
+// <lambda-sig>        ::= <template-param-decl>* <(parameter) type>+
+//
+// For <template-param-decl>* in <lambda-sig> see:
+//
+// https://github.com/itanium-cxx-abi/cxx-abi/issues/31
 static bool ParseUnnamedTypeName(State *state) {
   ComplexityGuard guard(state);
   if (guard.IsTooComplex()) return false;
@@ -777,6 +973,7 @@ static bool ParseUnnamedTypeName(State *state) {
 
   // Unnamed type local to function or class.
   if (ParseTwoCharToken(state, "Ut") && Optional(ParseNumber(state, &which)) &&
+      which >= -1 &&                                   // Don't print garbage.
       which <= std::numeric_limits<int>::max() - 2 &&  // Don't overflow.
       ParseOneCharToken(state, '_')) {
     MaybeAppend(state, "{unnamed type#");
@@ -789,8 +986,10 @@ static bool ParseUnnamedTypeName(State *state) {
   // Closure type.
   which = -1;
   if (ParseTwoCharToken(state, "Ul") && DisableAppend(state) &&
+      ZeroOrMore(ParseTemplateParamDecl, state) &&
       OneOrMore(ParseType, state) && RestoreAppend(state, copy.append) &&
       ParseOneCharToken(state, 'E') && Optional(ParseNumber(state, &which)) &&
+      which >= -1 &&                                   // Don't print garbage.
       which <= std::numeric_limits<int>::max() - 2 &&  // Don't overflow.
       ParseOneCharToken(state, '_')) {
     MaybeAppend(state, "{lambda()#");
@@ -817,7 +1016,7 @@ static bool ParseNumber(State *state, int *number_out) {
   uint64_t number = 0;
   for (; *p != '\0'; ++p) {
     if (IsDigit(*p)) {
-      number = number * 10 + (*p - '0');
+      number = number * 10 + static_cast<uint64_t>(*p - '0');
     } else {
       break;
     }
@@ -829,10 +1028,12 @@ static bool ParseNumber(State *state, int *number_out) {
     number = ~number + 1;
   }
   if (p != RemainingInput(state)) {  // Conversion succeeded.
-    state->parse_state.mangled_idx += p - RemainingInput(state);
+    state->parse_state.mangled_idx +=
+        static_cast<int>(p - RemainingInput(state));
+    UpdateHighWaterMark(state);
     if (number_out != nullptr) {
       // Note: possibly truncate "number".
-      *number_out = number;
+      *number_out = static_cast<int>(number);
     }
     return true;
   }
@@ -851,7 +1052,9 @@ static bool ParseFloatNumber(State *state) {
     }
   }
   if (p != RemainingInput(state)) {  // Conversion succeeded.
-    state->parse_state.mangled_idx += p - RemainingInput(state);
+    state->parse_state.mangled_idx +=
+        static_cast<int>(p - RemainingInput(state));
+    UpdateHighWaterMark(state);
     return true;
   }
   return false;
@@ -869,17 +1072,19 @@ static bool ParseSeqId(State *state) {
     }
   }
   if (p != RemainingInput(state)) {  // Conversion succeeded.
-    state->parse_state.mangled_idx += p - RemainingInput(state);
+    state->parse_state.mangled_idx +=
+        static_cast<int>(p - RemainingInput(state));
+    UpdateHighWaterMark(state);
     return true;
   }
   return false;
 }
 
 // <identifier> ::= <unqualified source code identifier> (of given length)
-static bool ParseIdentifier(State *state, int length) {
+static bool ParseIdentifier(State *state, size_t length) {
   ComplexityGuard guard(state);
   if (guard.IsTooComplex()) return false;
-  if (length < 0 || !AtLeastNumCharsRemaining(RemainingInput(state), length)) {
+  if (!AtLeastNumCharsRemaining(RemainingInput(state), length)) {
     return false;
   }
   if (IdentifierIsAnonymousNamespace(state, length)) {
@@ -887,12 +1092,14 @@ static bool ParseIdentifier(State *state, int length) {
   } else {
     MaybeAppendWithLength(state, RemainingInput(state), length);
   }
-  state->parse_state.mangled_idx += length;
+  state->parse_state.mangled_idx += static_cast<int>(length);
+  UpdateHighWaterMark(state);
   return true;
 }
 
 // <operator-name> ::= nw, and other two letters cases
 //                 ::= cv <type>  # (cast)
+//                 ::= li <source-name>  # C++11 user-defined literal
 //                 ::= v  <digit> <source-name> # vendor extended operator
 static bool ParseOperatorName(State *state, int *arity) {
   ComplexityGuard guard(state);
@@ -903,7 +1110,7 @@ static bool ParseOperatorName(State *state, int *arity) {
   // First check with "cv" (cast) case.
   ParseState copy = state->parse_state;
   if (ParseTwoCharToken(state, "cv") && MaybeAppend(state, "operator ") &&
-      EnterNestedName(state) && ParseType(state) &&
+      EnterNestedName(state) && ParseConversionOperatorType(state) &&
       LeaveNestedName(state, copy.nest_level)) {
     if (arity != nullptr) {
       *arity = 1;
@@ -912,6 +1119,13 @@ static bool ParseOperatorName(State *state, int *arity) {
   }
   state->parse_state = copy;
 
+  // Then user-defined literals.
+  if (ParseTwoCharToken(state, "li") && MaybeAppend(state, "operator\"\" ") &&
+      ParseSourceName(state)) {
+    return true;
+  }
+  state->parse_state = copy;
+
   // Then vendor extended operators.
   if (ParseOneCharToken(state, 'v') && ParseDigit(state, arity) &&
       ParseSourceName(state)) {
@@ -939,34 +1153,119 @@ static bool ParseOperatorName(State *state, int *arity) {
       }
       MaybeAppend(state, p->real_name);
       state->parse_state.mangled_idx += 2;
+      UpdateHighWaterMark(state);
       return true;
     }
   }
   return false;
 }
 
+// <operator-name> ::= cv <type>  # (cast)
+//
+// The name of a conversion operator is the one place where cv-qualifiers, *, &,
+// and other simple type combinators are expected to appear in our stripped-down
+// demangling (elsewhere they appear in function signatures or template
+// arguments, which we omit from the output).  We make reasonable efforts to
+// render simple cases accurately.
+static bool ParseConversionOperatorType(State *state) {
+  ComplexityGuard guard(state);
+  if (guard.IsTooComplex()) return false;
+  ParseState copy = state->parse_state;
+
+  // Scan pointers, const, and other easy mangling prefixes with postfix
+  // demanglings.  Remember the range of input for later rescanning.
+  //
+  // See `ParseType` and the `switch` below for the meaning of each char.
+  const char* begin_simple_prefixes = RemainingInput(state);
+  while (ParseCharClass(state, "OPRCGrVK")) {}
+  const char* end_simple_prefixes = RemainingInput(state);
+
+  // Emit the base type first.
+  if (!ParseType(state)) {
+    state->parse_state = copy;
+    return false;
+  }
+
+  // Then rescan the easy type combinators in reverse order to emit their
+  // demanglings in the expected output order.
+  while (begin_simple_prefixes != end_simple_prefixes) {
+    switch (*--end_simple_prefixes) {
+      case 'P':
+        MaybeAppend(state, "*");
+        break;
+      case 'R':
+        MaybeAppend(state, "&");
+        break;
+      case 'O':
+        MaybeAppend(state, "&&");
+        break;
+      case 'C':
+        MaybeAppend(state, " _Complex");
+        break;
+      case 'G':
+        MaybeAppend(state, " _Imaginary");
+        break;
+      case 'r':
+        MaybeAppend(state, " restrict");
+        break;
+      case 'V':
+        MaybeAppend(state, " volatile");
+        break;
+      case 'K':
+        MaybeAppend(state, " const");
+        break;
+    }
+  }
+  return true;
+}
+
 // <special-name> ::= TV <type>
 //                ::= TT <type>
 //                ::= TI <type>
 //                ::= TS <type>
+//                ::= TW <name>  # thread-local wrapper
+//                ::= TH <name>  # thread-local initialization
 //                ::= Tc <call-offset> <call-offset> <(base) encoding>
 //                ::= GV <(object) name>
+//                ::= GR <(object) name> [<seq-id>] _
 //                ::= T <call-offset> <(base) encoding>
+//                ::= GTt <encoding>  # transaction-safe entry point
+//                ::= TA <template-arg>  # nontype template parameter object
 // G++ extensions:
 //                ::= TC <type> <(offset) number> _ <(base) type>
 //                ::= TF <type>
 //                ::= TJ <type>
-//                ::= GR <name>
+//                ::= GR <name>  # without final _, perhaps an earlier form?
 //                ::= GA <encoding>
 //                ::= Th <call-offset> <(base) encoding>
 //                ::= Tv <call-offset> <(base) encoding>
 //
-// Note: we don't care much about them since they don't appear in
-// stack traces.  The are special data.
+// Note: Most of these are special data, not functions that occur in stack
+// traces.  Exceptions are TW and TH, which denote functions supporting the
+// thread_local feature.  For these see:
+//
+// https://maskray.me/blog/2021-02-14-all-about-thread-local-storage
+//
+// For TA see https://github.com/itanium-cxx-abi/cxx-abi/issues/63.
 static bool ParseSpecialName(State *state) {
   ComplexityGuard guard(state);
   if (guard.IsTooComplex()) return false;
   ParseState copy = state->parse_state;
+
+  if (ParseTwoCharToken(state, "TW")) {
+    MaybeAppend(state, "thread-local wrapper routine for ");
+    if (ParseName(state)) return true;
+    state->parse_state = copy;
+    return false;
+  }
+
+  if (ParseTwoCharToken(state, "TH")) {
+    MaybeAppend(state, "thread-local initialization routine for ");
+    if (ParseName(state)) return true;
+    state->parse_state = copy;
+    return false;
+  }
+
   if (ParseOneCharToken(state, 'T') && ParseCharClass(state, "VTIS") &&
       ParseType(state)) {
     return true;
@@ -1005,12 +1304,30 @@ static bool ParseSpecialName(State *state) {
   }
   state->parse_state = copy;
 
-  if (ParseTwoCharToken(state, "GR") && ParseName(state)) {
+  // <special-name> ::= GR <(object) name> [<seq-id>] _  # modern standard
+  //                ::= GR <(object) name>  # also recognized
+  if (ParseTwoCharToken(state, "GR")) {
+    MaybeAppend(state, "reference temporary for ");
+    if (!ParseName(state)) {
+      state->parse_state = copy;
+      return false;
+    }
+    const bool has_seq_id = ParseSeqId(state);
+    const bool has_underscore = ParseOneCharToken(state, '_');
+    if (has_seq_id && !has_underscore) {
+      state->parse_state = copy;
+      return false;
+    }
+    return true;
+  }
+
+  if (ParseTwoCharToken(state, "GA") && ParseEncoding(state)) {
     return true;
   }
   state->parse_state = copy;
 
-  if (ParseTwoCharToken(state, "GA") && ParseEncoding(state)) {
+  if (ParseThreeCharToken(state, "GTt") &&
+      MaybeAppend(state, "transaction clone for ") && ParseEncoding(state)) {
     return true;
   }
   state->parse_state = copy;
@@ -1020,6 +1337,18 @@ static bool ParseSpecialName(State *state) {
     return true;
   }
   state->parse_state = copy;
+
+  if (ParseTwoCharToken(state, "TA")) {
+    bool append = state->parse_state.append;
+    DisableAppend(state);
+    if (ParseTemplateArg(state)) {
+      RestoreAppend(state, append);
+      MaybeAppend(state, "template parameter object");
+      return true;
+    }
+  }
+  state->parse_state = copy;
+
   return false;
 }
 
@@ -1064,20 +1393,28 @@ static bool ParseVOffset(State *state) {
   return false;
 }
 
-// <ctor-dtor-name> ::= C1 | C2 | C3
+// <ctor-dtor-name> ::= C1 | C2 | C3 | CI1 <base-class-type> | CI2
+// <base-class-type>
 //                  ::= D0 | D1 | D2
 // # GCC extensions: "unified" constructor/destructor.  See
-// # https://github.com/gcc-mirror/gcc/blob/7ad17b583c3643bd4557f29b8391ca7ef08391f5/gcc/cp/mangle.c#L1847
+// #
+// https://github.com/gcc-mirror/gcc/blob/7ad17b583c3643bd4557f29b8391ca7ef08391f5/gcc/cp/mangle.c#L1847
 //                  ::= C4 | D4
 static bool ParseCtorDtorName(State *state) {
   ComplexityGuard guard(state);
   if (guard.IsTooComplex()) return false;
   ParseState copy = state->parse_state;
-  if (ParseOneCharToken(state, 'C') && ParseCharClass(state, "1234")) {
-    const char *const prev_name = state->out + state->parse_state.prev_name_idx;
-    MaybeAppendWithLength(state, prev_name,
-                          state->parse_state.prev_name_length);
-    return true;
+  if (ParseOneCharToken(state, 'C')) {
+    if (ParseCharClass(state, "1234")) {
+      const char *const prev_name =
+          state->out + state->parse_state.prev_name_idx;
+      MaybeAppendWithLength(state, prev_name,
+                            state->parse_state.prev_name_length);
+      return true;
+    } else if (ParseOneCharToken(state, 'I') && ParseCharClass(state, "12") &&
+               ParseClassEnumType(state)) {
+      return true;
+    }
   }
   state->parse_state = copy;
 
@@ -1115,7 +1452,6 @@ static bool ParseDecltype(State *state) {
 //        ::= O <type>   # rvalue reference-to (C++0x)
 //        ::= C <type>   # complex pair (C 2000)
 //        ::= G <type>   # imaginary (C 2000)
-//        ::= U <source-name> <type>  # vendor extended type qualifier
 //        ::= <builtin-type>
 //        ::= <function-type>
 //        ::= <class-enum-type>  # note: just an alias for <name>
@@ -1126,6 +1462,9 @@ static bool ParseDecltype(State *state) {
 //        ::= <decltype>
 //        ::= <substitution>
 //        ::= Dp <type>          # pack expansion of (C++0x)
+//        ::= Dv <(elements) number> _ <type>  # GNU vector extension
+//        ::= Dv <(bytes) expression> _ <type>
+//        ::= Dk <type-constraint>  # constrained auto
 //
 static bool ParseType(State *state) {
   ComplexityGuard guard(state);
@@ -1168,18 +1507,6 @@ static bool ParseType(State *state) {
   }
   state->parse_state = copy;
 
-  // nullptr_t, i.e. decltype(nullptr).
-  if (ParseTwoCharToken(state, "Dn")) {
-    return true;
-  }
-  state->parse_state = copy;
-
-  if (ParseOneCharToken(state, 'U') && ParseSourceName(state) &&
-      ParseType(state)) {
-    return true;
-  }
-  state->parse_state = copy;
-
   if (ParseBuiltinType(state) || ParseFunctionType(state) ||
       ParseClassEnumType(state) || ParseArrayType(state) ||
       ParsePointerToMemberType(state) || ParseDecltype(state) ||
@@ -1198,65 +1525,236 @@ static bool ParseType(State *state) {
     return true;
   }
 
-  return false;
+  // GNU vector extension Dv <number> _ <type>
+  if (ParseTwoCharToken(state, "Dv") && ParseNumber(state, nullptr) &&
+      ParseOneCharToken(state, '_') && ParseType(state)) {
+    return true;
+  }
+  state->parse_state = copy;
+
+  // GNU vector extension Dv <expression> _ <type>
+  if (ParseTwoCharToken(state, "Dv") && ParseExpression(state) &&
+      ParseOneCharToken(state, '_') && ParseType(state)) {
+    return true;
+  }
+  state->parse_state = copy;
+
+  if (ParseTwoCharToken(state, "Dk") && ParseTypeConstraint(state)) {
+    return true;
+  }
+  state->parse_state = copy;
+
+  // For this notation see CXXNameMangler::mangleType in Clang's source code.
+  // The relevant logic and its comment "not clear how to mangle this!" date
+  // from 2011, so it may be with us awhile.
+  return ParseLongToken(state, "_SUBSTPACK_");
 }
 
+// <qualifiers> ::= <extended-qualifier>* <CV-qualifiers>
 // <CV-qualifiers> ::= [r] [V] [K]
+//
 // We don't allow empty <CV-qualifiers> to avoid infinite loop in
 // ParseType().
 static bool ParseCVQualifiers(State *state) {
   ComplexityGuard guard(state);
   if (guard.IsTooComplex()) return false;
   int num_cv_qualifiers = 0;
+  while (ParseExtendedQualifier(state)) ++num_cv_qualifiers;
   num_cv_qualifiers += ParseOneCharToken(state, 'r');
   num_cv_qualifiers += ParseOneCharToken(state, 'V');
   num_cv_qualifiers += ParseOneCharToken(state, 'K');
   return num_cv_qualifiers > 0;
 }
 
-// <builtin-type> ::= v, etc.
-//                ::= u <source-name>
+// <extended-qualifier> ::= U <source-name> [<template-args>]
+static bool ParseExtendedQualifier(State *state) {
+  ComplexityGuard guard(state);
+  if (guard.IsTooComplex()) return false;
+  ParseState copy = state->parse_state;
+
+  if (!ParseOneCharToken(state, 'U')) return false;
+
+  bool append = state->parse_state.append;
+  DisableAppend(state);
+  if (!ParseSourceName(state)) {
+    state->parse_state = copy;
+    return false;
+  }
+  Optional(ParseTemplateArgs(state));
+  RestoreAppend(state, append);
+  return true;
+}
+
+// <builtin-type> ::= v, etc.  # single-character builtin types
+//                ::= <vendor-extended-type>
+//                ::= Dd, etc.  # two-character builtin types
+//                ::= DB (<number> | <expression>) _  # _BitInt(N)
+//                ::= DU (<number> | <expression>) _  # unsigned _BitInt(N)
+//                ::= DF <number> _  # _FloatN (N bits)
+//                ::= DF <number> x  # _FloatNx
+//                ::= DF16b  # std::bfloat16_t
+//
+// Not supported:
+//                ::= [DS] DA <fixed-point-size>
+//                ::= [DS] DR <fixed-point-size>
+// because real implementations of N1169 fixed-point are scant.
 static bool ParseBuiltinType(State *state) {
   ComplexityGuard guard(state);
   if (guard.IsTooComplex()) return false;
-  const AbbrevPair *p;
-  for (p = kBuiltinTypeList; p->abbrev != nullptr; ++p) {
-    if (RemainingInput(state)[0] == p->abbrev[0]) {
-      MaybeAppend(state, p->real_name);
-      ++state->parse_state.mangled_idx;
+  ParseState copy = state->parse_state;
+
+  // DB (<number> | <expression>) _  # _BitInt(N)
+  // DU (<number> | <expression>) _  # unsigned _BitInt(N)
+  if (ParseTwoCharToken(state, "DB") ||
+      (ParseTwoCharToken(state, "DU") && MaybeAppend(state, "unsigned "))) {
+    bool append = state->parse_state.append;
+    DisableAppend(state);
+    int number = -1;
+    if (!ParseNumber(state, &number) && !ParseExpression(state)) {
+      state->parse_state = copy;
+      return false;
+    }
+    RestoreAppend(state, append);
+
+    if (!ParseOneCharToken(state, '_')) {
+      state->parse_state = copy;
+      return false;
+    }
+
+    MaybeAppend(state, "_BitInt(");
+    if (number >= 0) {
+      MaybeAppendDecimal(state, number);
+    } else {
+      MaybeAppend(state, "?");  // the best we can do for dependent sizes
+    }
+    MaybeAppend(state, ")");
+    return true;
+  }
+
+  // DF <number> _  # _FloatN
+  // DF <number> x  # _FloatNx
+  // DF16b  # std::bfloat16_t
+  if (ParseTwoCharToken(state, "DF")) {
+    if (ParseThreeCharToken(state, "16b")) {
+      MaybeAppend(state, "std::bfloat16_t");
+      return true;
+    }
+    int number = 0;
+    if (!ParseNumber(state, &number)) {
+      state->parse_state = copy;
+      return false;
+    }
+    MaybeAppend(state, "_Float");
+    if (number >= 0) {
+      MaybeAppendDecimal(state, number);
+    } else {
+      MaybeAppend(state, "?");  // the best we can do for an invalid width
+    }
+    if (ParseOneCharToken(state, 'x')) {
+      MaybeAppend(state, "x");
       return true;
     }
+    if (ParseOneCharToken(state, '_')) return true;
+    state->parse_state = copy;
+    return false;
+  }
+
+  for (const AbbrevPair *p = kBuiltinTypeList; p->abbrev != nullptr; ++p) {
+    // Guaranteed only 1- or 2-character strings in kBuiltinTypeList.
+    if (p->abbrev[1] == '\0') {
+      if (ParseOneCharToken(state, p->abbrev[0])) {
+        MaybeAppend(state, p->real_name);
+        return true;  // ::= v, etc.  # single-character builtin types
+      }
+    } else if (p->abbrev[2] == '\0' && ParseTwoCharToken(state, p->abbrev)) {
+      MaybeAppend(state, p->real_name);
+      return true;  // ::= Dd, etc.  # two-character builtin types
+    }
   }
 
+  return ParseVendorExtendedType(state);
+}
+
+// <vendor-extended-type> ::= u <source-name> [<template-args>]
+static bool ParseVendorExtendedType(State *state) {
+  ComplexityGuard guard(state);
+  if (guard.IsTooComplex()) return false;
+
   ParseState copy = state->parse_state;
-  if (ParseOneCharToken(state, 'u') && ParseSourceName(state)) {
+  if (ParseOneCharToken(state, 'u') && ParseSourceName(state) &&
+      Optional(ParseTemplateArgs(state))) {
     return true;
   }
   state->parse_state = copy;
   return false;
 }
 
-// <function-type> ::= F [Y] <bare-function-type> E
-static bool ParseFunctionType(State *state) {
+//  <exception-spec> ::= Do                # non-throwing
+//                                           exception-specification (e.g.,
+//                                           noexcept, throw())
+//                   ::= DO <expression> E # computed (instantiation-dependent)
+//                                           noexcept
+//                   ::= Dw <type>+ E      # dynamic exception specification
+//                                           with instantiation-dependent types
+static bool ParseExceptionSpec(State *state) {
   ComplexityGuard guard(state);
   if (guard.IsTooComplex()) return false;
+
+  if (ParseTwoCharToken(state, "Do")) return true;
+
   ParseState copy = state->parse_state;
-  if (ParseOneCharToken(state, 'F') &&
-      Optional(ParseOneCharToken(state, 'Y')) && ParseBareFunctionType(state) &&
+  if (ParseTwoCharToken(state, "DO") && ParseExpression(state) &&
+      ParseOneCharToken(state, 'E')) {
+    return true;
+  }
+  state->parse_state = copy;
+  if (ParseTwoCharToken(state, "Dw") && OneOrMore(ParseType, state) &&
       ParseOneCharToken(state, 'E')) {
     return true;
   }
   state->parse_state = copy;
+
   return false;
 }
 
-// <bare-function-type> ::= <(signature) type>+
+// <function-type> ::=
+//     [exception-spec] [Dx] F [Y] <bare-function-type> [<ref-qualifier>] E
+//
+// <ref-qualifier> ::= R | O
+static bool ParseFunctionType(State *state) {
+  ComplexityGuard guard(state);
+  if (guard.IsTooComplex()) return false;
+  ParseState copy = state->parse_state;
+  Optional(ParseExceptionSpec(state));
+  Optional(ParseTwoCharToken(state, "Dx"));
+  if (!ParseOneCharToken(state, 'F')) {
+    state->parse_state = copy;
+    return false;
+  }
+  Optional(ParseOneCharToken(state, 'Y'));
+  if (!ParseBareFunctionType(state)) {
+    state->parse_state = copy;
+    return false;
+  }
+  Optional(ParseCharClass(state, "RO"));
+  if (!ParseOneCharToken(state, 'E')) {
+    state->parse_state = copy;
+    return false;
+  }
+  return true;
+}
+
+// <bare-function-type> ::= <overload-attribute>* <(signature) type>+
+//
+// The <overload-attribute>* prefix is nonstandard; see the comment on
+// ParseOverloadAttribute.
 static bool ParseBareFunctionType(State *state) {
   ComplexityGuard guard(state);
   if (guard.IsTooComplex()) return false;
   ParseState copy = state->parse_state;
   DisableAppend(state);
-  if (OneOrMore(ParseType, state)) {
+  if (ZeroOrMore(ParseOverloadAttribute, state) &&
+      OneOrMore(ParseType, state)) {
     RestoreAppend(state, copy.append);
     MaybeAppend(state, "()");
     return true;
@@ -1265,11 +1763,43 @@ static bool ParseBareFunctionType(State *state) {
   return false;
 }
 
+// <overload-attribute> ::= Ua <name>
+//
+// The nonstandard <overload-attribute> production is sufficient to accept the
+// current implementation of __attribute__((enable_if(condition, "message")))
+// and future attributes of a similar shape.  See
+// https://clang.llvm.org/docs/AttributeReference.html#enable-if and the
+// definition of CXXNameMangler::mangleFunctionEncodingBareType in Clang's
+// source code.
+static bool ParseOverloadAttribute(State *state) {
+  ComplexityGuard guard(state);
+  if (guard.IsTooComplex()) return false;
+  ParseState copy = state->parse_state;
+  if (ParseTwoCharToken(state, "Ua") && ParseName(state)) {
+    return true;
+  }
+  state->parse_state = copy;
+  return false;
+}
+
 // <class-enum-type> ::= <name>
+//                   ::= Ts <name>  # struct Name or class Name
+//                   ::= Tu <name>  # union Name
+//                   ::= Te <name>  # enum Name
+//
+// See http://shortn/_W3YrltiEd0.
 static bool ParseClassEnumType(State *state) {
   ComplexityGuard guard(state);
   if (guard.IsTooComplex()) return false;
-  return ParseName(state);
+  ParseState copy = state->parse_state;
+  if (Optional(ParseTwoCharToken(state, "Ts") ||
+               ParseTwoCharToken(state, "Tu") ||
+               ParseTwoCharToken(state, "Te")) &&
+      ParseName(state)) {
+    return true;
+  }
+  state->parse_state = copy;
+  return false;
 }
 
 // <array-type> ::= A <(positive dimension) number> _ <(element) type>
@@ -1306,27 +1836,89 @@ static bool ParsePointerToMemberType(State *state) {
 
 // <template-param> ::= T_
 //                  ::= T <parameter-2 non-negative number> _
+//                  ::= TL <level-1> __
+//                  ::= TL <level-1> _ <parameter-2 non-negative number> _
 static bool ParseTemplateParam(State *state) {
   ComplexityGuard guard(state);
   if (guard.IsTooComplex()) return false;
   if (ParseTwoCharToken(state, "T_")) {
     MaybeAppend(state, "?");  // We don't support template substitutions.
-    return true;
+    return true;              // ::= T_
   }
 
   ParseState copy = state->parse_state;
   if (ParseOneCharToken(state, 'T') && ParseNumber(state, nullptr) &&
       ParseOneCharToken(state, '_')) {
     MaybeAppend(state, "?");  // We don't support template substitutions.
-    return true;
+    return true;              // ::= T <parameter-2 non-negative number> _
   }
   state->parse_state = copy;
-  return false;
-}
 
-// <template-template-param> ::= <template-param>
-//                           ::= <substitution>
-static bool ParseTemplateTemplateParam(State *state) {
+  if (ParseTwoCharToken(state, "TL") && ParseNumber(state, nullptr)) {
+    if (ParseTwoCharToken(state, "__")) {
+      MaybeAppend(state, "?");  // We don't support template substitutions.
+      return true;              // ::= TL <level-1> __
+    }
+
+    if (ParseOneCharToken(state, '_') && ParseNumber(state, nullptr) &&
+        ParseOneCharToken(state, '_')) {
+      MaybeAppend(state, "?");  // We don't support template substitutions.
+      return true;  // ::= TL <level-1> _ <parameter-2 non-negative number> _
+    }
+  }
+  state->parse_state = copy;
+  return false;
+}
+
+// <template-param-decl>
+//   ::= Ty                                  # template type parameter
+//   ::= Tk <concept name> [<template-args>] # constrained type parameter
+//   ::= Tn <type>                           # template non-type parameter
+//   ::= Tt <template-param-decl>* E         # template template parameter
+//   ::= Tp <template-param-decl>            # template parameter pack
+//
+// NOTE: <concept name> is just a <name>: http://shortn/_MqJVyr0fc1
+// TODO(b/324066279): Implement optional suffix for `Tt`:
+// [Q <requires-clause expr>]
+static bool ParseTemplateParamDecl(State *state) {
+  ComplexityGuard guard(state);
+  if (guard.IsTooComplex()) return false;
+  ParseState copy = state->parse_state;
+
+  if (ParseTwoCharToken(state, "Ty")) {
+    return true;
+  }
+  state->parse_state = copy;
+
+  if (ParseTwoCharToken(state, "Tk") && ParseName(state) &&
+      Optional(ParseTemplateArgs(state))) {
+    return true;
+  }
+  state->parse_state = copy;
+
+  if (ParseTwoCharToken(state, "Tn") && ParseType(state)) {
+    return true;
+  }
+  state->parse_state = copy;
+
+  if (ParseTwoCharToken(state, "Tt") &&
+      ZeroOrMore(ParseTemplateParamDecl, state) &&
+      ParseOneCharToken(state, 'E')) {
+    return true;
+  }
+  state->parse_state = copy;
+
+  if (ParseTwoCharToken(state, "Tp") && ParseTemplateParamDecl(state)) {
+    return true;
+  }
+  state->parse_state = copy;
+
+  return false;
+}
+
+// <template-template-param> ::= <template-param>
+//                           ::= <substitution>
+static bool ParseTemplateTemplateParam(State *state) {
   ComplexityGuard guard(state);
   if (guard.IsTooComplex()) return false;
   return (ParseTemplateParam(state) ||
@@ -1334,13 +1926,14 @@ static bool ParseTemplateTemplateParam(State *state) {
           ParseSubstitution(state, /*accept_std=*/false));
 }
 
-// <template-args> ::= I <template-arg>+ E
+// <template-args> ::= I <template-arg>+ [Q <requires-clause expr>] E
 static bool ParseTemplateArgs(State *state) {
   ComplexityGuard guard(state);
   if (guard.IsTooComplex()) return false;
   ParseState copy = state->parse_state;
   DisableAppend(state);
   if (ParseOneCharToken(state, 'I') && OneOrMore(ParseTemplateArg, state) &&
+      Optional(ParseQRequiresClauseExpr(state)) &&
       ParseOneCharToken(state, 'E')) {
     RestoreAppend(state, copy.append);
     MaybeAppend(state, "<>");
@@ -1350,7 +1943,8 @@ static bool ParseTemplateArgs(State *state) {
   return false;
 }
 
-// <template-arg>  ::= <type>
+// <template-arg>  ::= <template-param-decl> <template-arg>
+//                 ::= <type>
 //                 ::= <expr-primary>
 //                 ::= J <template-arg>* E        # argument pack
 //                 ::= X <expression> E
@@ -1434,7 +2028,7 @@ static bool ParseTemplateArg(State *state) {
   //     ::= L <source-name> [<template-args>] [<expr-cast-value> E]
   if (ParseLocalSourceName(state) && Optional(ParseTemplateArgs(state))) {
     copy = state->parse_state;
-    if (ParseExprCastValue(state) && ParseOneCharToken(state, 'E')) {
+    if (ParseExprCastValueAndTrailingE(state)) {
       return true;
     }
     state->parse_state = copy;
@@ -1453,6 +2047,12 @@ static bool ParseTemplateArg(State *state) {
     return true;
   }
   state->parse_state = copy;
+
+  if (ParseTemplateParamDecl(state) && ParseTemplateArg(state)) {
+    return true;
+  }
+  state->parse_state = copy;
+
   return false;
 }
 
@@ -1507,6 +2107,13 @@ static bool ParseBaseUnresolvedName(State *state) {
 //                         <base-unresolved-name>
 //                   ::= [gs] sr <unresolved-qualifier-level>+ E
 //                         <base-unresolved-name>
+//                   ::= sr St <simple-id> <simple-id>  # nonstandard
+//
+// The last case is not part of the official grammar but has been observed in
+// real-world examples that the GNU demangler (but not the LLVM demangler) is
+// able to decode; see demangle_test.cc for one such symbol name.  The shape
+// sr St <simple-id> <simple-id> was inferred by closed-box testing of the GNU
+// demangler.
 static bool ParseUnresolvedName(State *state) {
   ComplexityGuard guard(state);
   if (guard.IsTooComplex()) return false;
@@ -1526,7 +2133,7 @@ static bool ParseUnresolvedName(State *state) {
 
   if (ParseTwoCharToken(state, "sr") && ParseOneCharToken(state, 'N') &&
       ParseUnresolvedType(state) &&
-      OneOrMore(/* <unresolved-qualifier-level> ::= */ ParseSimpleId, state) &&
+      OneOrMore(ParseUnresolvedQualifierLevel, state) &&
       ParseOneCharToken(state, 'E') && ParseBaseUnresolvedName(state)) {
     return true;
   }
@@ -1534,34 +2141,160 @@ static bool ParseUnresolvedName(State *state) {
 
   if (Optional(ParseTwoCharToken(state, "gs")) &&
       ParseTwoCharToken(state, "sr") &&
-      OneOrMore(/* <unresolved-qualifier-level> ::= */ ParseSimpleId, state) &&
+      OneOrMore(ParseUnresolvedQualifierLevel, state) &&
       ParseOneCharToken(state, 'E') && ParseBaseUnresolvedName(state)) {
     return true;
   }
   state->parse_state = copy;
 
+  if (ParseTwoCharToken(state, "sr") && ParseTwoCharToken(state, "St") &&
+      ParseSimpleId(state) && ParseSimpleId(state)) {
+    return true;
+  }
+  state->parse_state = copy;
+
   return false;
 }
 
+// <unresolved-qualifier-level> ::= <simple-id>
+//                              ::= <substitution> <template-args>
+//
+// The production <substitution> <template-args> is nonstandard but is observed
+// in practice.  An upstream discussion on the best shape of <unresolved-name>
+// has not converged:
+//
+// https://github.com/itanium-cxx-abi/cxx-abi/issues/38
+static bool ParseUnresolvedQualifierLevel(State *state) {
+  ComplexityGuard guard(state);
+  if (guard.IsTooComplex()) return false;
+
+  if (ParseSimpleId(state)) return true;
+
+  ParseState copy = state->parse_state;
+  if (ParseSubstitution(state, /*accept_std=*/false) &&
+      ParseTemplateArgs(state)) {
+    return true;
+  }
+  state->parse_state = copy;
+  return false;
+}
+
+// <union-selector> ::= _ [<number>]
+//
+// https://github.com/itanium-cxx-abi/cxx-abi/issues/47
+static bool ParseUnionSelector(State *state) {
+  return ParseOneCharToken(state, '_') && Optional(ParseNumber(state, nullptr));
+}
+
+// <function-param> ::= fp <(top-level) CV-qualifiers> _
+//                  ::= fp <(top-level) CV-qualifiers> <number> _
+//                  ::= fL <number> p <(top-level) CV-qualifiers> _
+//                  ::= fL <number> p <(top-level) CV-qualifiers> <number> _
+//                  ::= fpT  # this
+static bool ParseFunctionParam(State *state) {
+  ComplexityGuard guard(state);
+  if (guard.IsTooComplex()) return false;
+
+  ParseState copy = state->parse_state;
+
+  // Function-param expression (level 0).
+  if (ParseTwoCharToken(state, "fp") && Optional(ParseCVQualifiers(state)) &&
+      Optional(ParseNumber(state, nullptr)) && ParseOneCharToken(state, '_')) {
+    return true;
+  }
+  state->parse_state = copy;
+
+  // Function-param expression (level 1+).
+  if (ParseTwoCharToken(state, "fL") && Optional(ParseNumber(state, nullptr)) &&
+      ParseOneCharToken(state, 'p') && Optional(ParseCVQualifiers(state)) &&
+      Optional(ParseNumber(state, nullptr)) && ParseOneCharToken(state, '_')) {
+    return true;
+  }
+  state->parse_state = copy;
+
+  return ParseThreeCharToken(state, "fpT");
+}
+
+// <braced-expression> ::= <expression>
+//                     ::= di <field source-name> <braced-expression>
+//                     ::= dx <index expression> <braced-expression>
+//                     ::= dX <expression> <expression> <braced-expression>
+static bool ParseBracedExpression(State *state) {
+  ComplexityGuard guard(state);
+  if (guard.IsTooComplex()) return false;
+
+  ParseState copy = state->parse_state;
+
+  if (ParseTwoCharToken(state, "di") && ParseSourceName(state) &&
+      ParseBracedExpression(state)) {
+    return true;
+  }
+  state->parse_state = copy;
+
+  if (ParseTwoCharToken(state, "dx") && ParseExpression(state) &&
+      ParseBracedExpression(state)) {
+    return true;
+  }
+  state->parse_state = copy;
+
+  if (ParseTwoCharToken(state, "dX") &&
+      ParseExpression(state) && ParseExpression(state) &&
+      ParseBracedExpression(state)) {
+    return true;
+  }
+  state->parse_state = copy;
+
+  return ParseExpression(state);
+}
+
 // <expression> ::= <1-ary operator-name> <expression>
 //              ::= <2-ary operator-name> <expression> <expression>
 //              ::= <3-ary operator-name> <expression> <expression> <expression>
+//              ::= pp_ <expression>  # ++e; pp <expression> is e++
+//              ::= mm_ <expression>  # --e; mm <expression> is e--
 //              ::= cl <expression>+ E
+//              ::= cp <simple-id> <expression>* E # Clang-specific.
+//              ::= so <type> <expression> [<number>] <union-selector>* [p] E
 //              ::= cv <type> <expression>      # type (expression)
 //              ::= cv <type> _ <expression>* E # type (expr-list)
+//              ::= tl <type> <braced-expression>* E
+//              ::= il <braced-expression>* E
+//              ::= [gs] nw <expression>* _ <type> E
+//              ::= [gs] nw <expression>* _ <type> <initializer>
+//              ::= [gs] na <expression>* _ <type> E
+//              ::= [gs] na <expression>* _ <type> <initializer>
+//              ::= [gs] dl <expression>
+//              ::= [gs] da <expression>
+//              ::= dc <type> <expression>
+//              ::= sc <type> <expression>
+//              ::= cc <type> <expression>
+//              ::= rc <type> <expression>
+//              ::= ti <type>
+//              ::= te <expression>
 //              ::= st <type>
+//              ::= at <type>
+//              ::= az <expression>
+//              ::= nx <expression>
 //              ::= <template-param>
 //              ::= <function-param>
+//              ::= sZ <template-param>
+//              ::= sZ <function-param>
+//              ::= sP <template-arg>* E
 //              ::= <expr-primary>
 //              ::= dt <expression> <unresolved-name> # expr.name
 //              ::= pt <expression> <unresolved-name> # expr->name
 //              ::= sp <expression>         # argument pack expansion
+//              ::= fl <binary operator-name> <expression>
+//              ::= fr <binary operator-name> <expression>
+//              ::= fL <binary operator-name> <expression> <expression>
+//              ::= fR <binary operator-name> <expression> <expression>
+//              ::= tw <expression>
+//              ::= tr
 //              ::= sr <type> <unqualified-name> <template-args>
 //              ::= sr <type> <unqualified-name>
-// <function-param> ::= fp <(top-level) CV-qualifiers> _
-//                  ::= fp <(top-level) CV-qualifiers> <number> _
-//                  ::= fL <number> p <(top-level) CV-qualifiers> _
-//                  ::= fL <number> p <(top-level) CV-qualifiers> <number> _
+//              ::= u <source-name> <template-arg>* E  # vendor extension
+//              ::= rq <requirement>+ E
+//              ::= rQ <bare-function-type> _ <requirement>+ E
 static bool ParseExpression(State *state) {
   ComplexityGuard guard(state);
   if (guard.IsTooComplex()) return false;
@@ -1569,25 +2302,91 @@ static bool ParseExpression(State *state) {
     return true;
   }
 
-  // Object/function call expression.
   ParseState copy = state->parse_state;
+
+  // Object/function call expression.
   if (ParseTwoCharToken(state, "cl") && OneOrMore(ParseExpression, state) &&
       ParseOneCharToken(state, 'E')) {
     return true;
   }
   state->parse_state = copy;
 
-  // Function-param expression (level 0).
-  if (ParseTwoCharToken(state, "fp") && Optional(ParseCVQualifiers(state)) &&
-      Optional(ParseNumber(state, nullptr)) && ParseOneCharToken(state, '_')) {
+  // Preincrement and predecrement.  Postincrement and postdecrement are handled
+  // by the operator-name logic later on.
+  if ((ParseThreeCharToken(state, "pp_") ||
+       ParseThreeCharToken(state, "mm_")) &&
+      ParseExpression(state)) {
     return true;
   }
   state->parse_state = copy;
 
-  // Function-param expression (level 1+).
-  if (ParseTwoCharToken(state, "fL") && Optional(ParseNumber(state, nullptr)) &&
-      ParseOneCharToken(state, 'p') && Optional(ParseCVQualifiers(state)) &&
-      Optional(ParseNumber(state, nullptr)) && ParseOneCharToken(state, '_')) {
+  // Clang-specific "cp <simple-id> <expression>* E"
+  //   https://clang.llvm.org/doxygen/ItaniumMangle_8cpp_source.html#l04338
+  if (ParseTwoCharToken(state, "cp") && ParseSimpleId(state) &&
+      ZeroOrMore(ParseExpression, state) && ParseOneCharToken(state, 'E')) {
+    return true;
+  }
+  state->parse_state = copy;
+
+  // <expression> ::= so <type> <expression> [<number>] <union-selector>* [p] E
+  //
+  // https://github.com/itanium-cxx-abi/cxx-abi/issues/47
+  if (ParseTwoCharToken(state, "so") && ParseType(state) &&
+      ParseExpression(state) && Optional(ParseNumber(state, nullptr)) &&
+      ZeroOrMore(ParseUnionSelector, state) &&
+      Optional(ParseOneCharToken(state, 'p')) &&
+      ParseOneCharToken(state, 'E')) {
+    return true;
+  }
+  state->parse_state = copy;
+
+  // <expression> ::= <function-param>
+  if (ParseFunctionParam(state)) return true;
+  state->parse_state = copy;
+
+  // <expression> ::= tl <type> <braced-expression>* E
+  if (ParseTwoCharToken(state, "tl") && ParseType(state) &&
+      ZeroOrMore(ParseBracedExpression, state) &&
+      ParseOneCharToken(state, 'E')) {
+    return true;
+  }
+  state->parse_state = copy;
+
+  // <expression> ::= il <braced-expression>* E
+  if (ParseTwoCharToken(state, "il") &&
+      ZeroOrMore(ParseBracedExpression, state) &&
+      ParseOneCharToken(state, 'E')) {
+    return true;
+  }
+  state->parse_state = copy;
+
+  // <expression> ::= [gs] nw <expression>* _ <type> E
+  //              ::= [gs] nw <expression>* _ <type> <initializer>
+  //              ::= [gs] na <expression>* _ <type> E
+  //              ::= [gs] na <expression>* _ <type> <initializer>
+  if (Optional(ParseTwoCharToken(state, "gs")) &&
+      (ParseTwoCharToken(state, "nw") || ParseTwoCharToken(state, "na")) &&
+      ZeroOrMore(ParseExpression, state) && ParseOneCharToken(state, '_') &&
+      ParseType(state) &&
+      (ParseOneCharToken(state, 'E') || ParseInitializer(state))) {
+    return true;
+  }
+  state->parse_state = copy;
+
+  // <expression> ::= [gs] dl <expression>
+  //              ::= [gs] da <expression>
+  if (Optional(ParseTwoCharToken(state, "gs")) &&
+      (ParseTwoCharToken(state, "dl") || ParseTwoCharToken(state, "da")) &&
+      ParseExpression(state)) {
+    return true;
+  }
+  state->parse_state = copy;
+
+  // dynamic_cast, static_cast, const_cast, reinterpret_cast.
+  //
+  // <expression> ::= (dc | sc | cc | rc) <type> <expression>
+  if (ParseCharClass(state, "dscr") && ParseOneCharToken(state, 'c') &&
+      ParseType(state) && ParseExpression(state)) {
     return true;
   }
   state->parse_state = copy;
@@ -1629,15 +2428,96 @@ static bool ParseExpression(State *state) {
   }
   state->parse_state = copy;
 
+  // typeid(type)
+  if (ParseTwoCharToken(state, "ti") && ParseType(state)) {
+    return true;
+  }
+  state->parse_state = copy;
+
+  // typeid(expression)
+  if (ParseTwoCharToken(state, "te") && ParseExpression(state)) {
+    return true;
+  }
+  state->parse_state = copy;
+
   // sizeof type
   if (ParseTwoCharToken(state, "st") && ParseType(state)) {
     return true;
   }
   state->parse_state = copy;
 
+  // alignof(type)
+  if (ParseTwoCharToken(state, "at") && ParseType(state)) {
+    return true;
+  }
+  state->parse_state = copy;
+
+  // alignof(expression), a GNU extension
+  if (ParseTwoCharToken(state, "az") && ParseExpression(state)) {
+    return true;
+  }
+  state->parse_state = copy;
+
+  // noexcept(expression) appearing as an expression in a dependent signature
+  if (ParseTwoCharToken(state, "nx") && ParseExpression(state)) {
+    return true;
+  }
+  state->parse_state = copy;
+
+  // sizeof...(pack)
+  //
+  // <expression> ::= sZ <template-param>
+  //              ::= sZ <function-param>
+  if (ParseTwoCharToken(state, "sZ") &&
+      (ParseFunctionParam(state) || ParseTemplateParam(state))) {
+    return true;
+  }
+  state->parse_state = copy;
+
+  // sizeof...(pack) captured from an alias template
+  //
+  // <expression> ::= sP <template-arg>* E
+  if (ParseTwoCharToken(state, "sP") && ZeroOrMore(ParseTemplateArg, state) &&
+      ParseOneCharToken(state, 'E')) {
+    return true;
+  }
+  state->parse_state = copy;
+
+  // Unary folds (... op pack) and (pack op ...).
+  //
+  // <expression> ::= fl <binary operator-name> <expression>
+  //              ::= fr <binary operator-name> <expression>
+  if ((ParseTwoCharToken(state, "fl") || ParseTwoCharToken(state, "fr")) &&
+      ParseOperatorName(state, nullptr) && ParseExpression(state)) {
+    return true;
+  }
+  state->parse_state = copy;
+
+  // Binary folds (init op ... op pack) and (pack op ... op init).
+  //
+  // <expression> ::= fL <binary operator-name> <expression> <expression>
+  //              ::= fR <binary operator-name> <expression> <expression>
+  if ((ParseTwoCharToken(state, "fL") || ParseTwoCharToken(state, "fR")) &&
+      ParseOperatorName(state, nullptr) && ParseExpression(state) &&
+      ParseExpression(state)) {
+    return true;
+  }
+  state->parse_state = copy;
+
+  // tw <expression>: throw e
+  if (ParseTwoCharToken(state, "tw") && ParseExpression(state)) {
+    return true;
+  }
+  state->parse_state = copy;
+
+  // tr: throw (rethrows an exception from the handler that caught it)
+  if (ParseTwoCharToken(state, "tr")) return true;
+
   // Object and pointer member access expressions.
+  //
+  // <expression> ::= (dt | pt) <expression> <unresolved-name>
   if ((ParseTwoCharToken(state, "dt") || ParseTwoCharToken(state, "pt")) &&
-      ParseExpression(state) && ParseType(state)) {
+      ParseExpression(state) && ParseUnresolvedName(state)) {
     return true;
   }
   state->parse_state = copy;
@@ -1657,9 +2537,61 @@ static bool ParseExpression(State *state) {
   }
   state->parse_state = copy;
 
+  // Vendor extended expressions
+  if (ParseOneCharToken(state, 'u') && ParseSourceName(state) &&
+      ZeroOrMore(ParseTemplateArg, state) && ParseOneCharToken(state, 'E')) {
+    return true;
+  }
+  state->parse_state = copy;
+
+  // <expression> ::= rq <requirement>+ E
+  //
+  // https://github.com/itanium-cxx-abi/cxx-abi/issues/24
+  if (ParseTwoCharToken(state, "rq") && OneOrMore(ParseRequirement, state) &&
+      ParseOneCharToken(state, 'E')) {
+    return true;
+  }
+  state->parse_state = copy;
+
+  // <expression> ::= rQ <bare-function-type> _ <requirement>+ E
+  //
+  // https://github.com/itanium-cxx-abi/cxx-abi/issues/24
+  if (ParseTwoCharToken(state, "rQ") && ParseBareFunctionType(state) &&
+      ParseOneCharToken(state, '_') && OneOrMore(ParseRequirement, state) &&
+      ParseOneCharToken(state, 'E')) {
+    return true;
+  }
+  state->parse_state = copy;
+
   return ParseUnresolvedName(state);
 }
 
+// <initializer> ::= pi <expression>* E
+//               ::= il <braced-expression>* E
+//
+// The il ... E form is not in the ABI spec but is seen in practice for
+// braced-init-lists in new-expressions, which are standard syntax from C++11
+// on.
+static bool ParseInitializer(State *state) {
+  ComplexityGuard guard(state);
+  if (guard.IsTooComplex()) return false;
+  ParseState copy = state->parse_state;
+
+  if (ParseTwoCharToken(state, "pi") && ZeroOrMore(ParseExpression, state) &&
+      ParseOneCharToken(state, 'E')) {
+    return true;
+  }
+  state->parse_state = copy;
+
+  if (ParseTwoCharToken(state, "il") &&
+      ZeroOrMore(ParseBracedExpression, state) &&
+      ParseOneCharToken(state, 'E')) {
+    return true;
+  }
+  state->parse_state = copy;
+  return false;
+}
+
 // <expr-primary> ::= L <type> <(value) number> E
 //                ::= L <type> <(value) float> E
 //                ::= L <mangled-name> E
@@ -1702,10 +2634,35 @@ static bool ParseExprPrimary(State *state) {
     return false;
   }
 
-  // The merged cast production.
-  if (ParseOneCharToken(state, 'L') && ParseType(state) &&
-      ParseExprCastValue(state)) {
-    return true;
+  if (ParseOneCharToken(state, 'L')) {
+    // There are two special cases in which a literal may or must contain a type
+    // without a value.  The first is that both LDnE and LDn0E are valid
+    // encodings of nullptr, used in different situations.  Recognize LDnE here,
+    // leaving LDn0E to be recognized by the general logic afterward.
+    if (ParseThreeCharToken(state, "DnE")) return true;
+
+    // The second special case is a string literal, currently mangled in C++98
+    // style as LA<length + 1>_KcE.  This is inadequate to support C++11 and
+    // later versions, and the discussion of this problem has not converged.
+    //
+    // https://github.com/itanium-cxx-abi/cxx-abi/issues/64
+    //
+    // For now the bare-type mangling is what's used in practice, so we
+    // recognize this form and only this form if an array type appears here.
+    // Someday we'll probably have to accept a new form of value mangling in
+    // LA...E constructs.  (Note also that C++20 allows a wide range of
+    // class-type objects as template arguments, so someday their values will be
+    // mangled and we'll have to recognize them here too.)
+    if (RemainingInput(state)[0] == 'A' /* an array type follows */) {
+      if (ParseType(state) && ParseOneCharToken(state, 'E')) return true;
+      state->parse_state = copy;
+      return false;
+    }
+
+    // The merged cast production.
+    if (ParseType(state) && ParseExprCastValueAndTrailingE(state)) {
+      return true;
+    }
   }
   state->parse_state = copy;
 
@@ -1719,7 +2676,7 @@ static bool ParseExprPrimary(State *state) {
 }
 
 // <number> or <float>, followed by 'E', as described above ParseExprPrimary.
-static bool ParseExprCastValue(State *state) {
+static bool ParseExprCastValueAndTrailingE(State *state) {
   ComplexityGuard guard(state);
   if (guard.IsTooComplex()) return false;
   // We have to be able to backtrack after accepting a number because we could
@@ -1731,39 +2688,150 @@ static bool ParseExprCastValue(State *state) {
   }
   state->parse_state = copy;
 
-  if (ParseFloatNumber(state) && ParseOneCharToken(state, 'E')) {
+  if (ParseFloatNumber(state)) {
+    // <float> for ordinary floating-point types
+    if (ParseOneCharToken(state, 'E')) return true;
+
+    // <float> _ <float> for complex floating-point types
+    if (ParseOneCharToken(state, '_') && ParseFloatNumber(state) &&
+        ParseOneCharToken(state, 'E')) {
+      return true;
+    }
+  }
+  state->parse_state = copy;
+
+  return false;
+}
+
+// Parses `Q <requires-clause expr>`.
+// If parsing fails, applies backtracking to `state`.
+//
+// This function covers two symbols instead of one for convenience,
+// because in LLVM's Itanium ABI mangling grammar, <requires-clause expr>
+// always appears after Q.
+//
+// Does not emit the parsed `requires` clause to simplify the implementation.
+// In other words, these two functions' mangled names will demangle identically:
+//
+// template <typename T>
+// int foo(T) requires IsIntegral<T>;
+//
+// vs.
+//
+// template <typename T>
+// int foo(T);
+static bool ParseQRequiresClauseExpr(State *state) {
+  ComplexityGuard guard(state);
+  if (guard.IsTooComplex()) return false;
+  ParseState copy = state->parse_state;
+  DisableAppend(state);
+
+  // <requires-clause expr> is just an <expression>: http://shortn/_9E1Ul0rIM8
+  if (ParseOneCharToken(state, 'Q') && ParseExpression(state)) {
+    RestoreAppend(state, copy.append);
+    return true;
+  }
+
+  // also restores append
+  state->parse_state = copy;
+  return false;
+}
+
+// <requirement> ::= X <expression> [N] [R <type-constraint>]
+// <requirement> ::= T <type>
+// <requirement> ::= Q <constraint-expression>
+//
+// <constraint-expression> ::= <expression>
+//
+// https://github.com/itanium-cxx-abi/cxx-abi/issues/24
+static bool ParseRequirement(State *state) {
+  ComplexityGuard guard(state);
+  if (guard.IsTooComplex()) return false;
+
+  ParseState copy = state->parse_state;
+
+  if (ParseOneCharToken(state, 'X') && ParseExpression(state) &&
+      Optional(ParseOneCharToken(state, 'N')) &&
+      // This logic backtracks cleanly if we eat an R but a valid type doesn't
+      // follow it.
+      (!ParseOneCharToken(state, 'R') || ParseTypeConstraint(state))) {
     return true;
   }
   state->parse_state = copy;
 
+  if (ParseOneCharToken(state, 'T') && ParseType(state)) return true;
+  state->parse_state = copy;
+
+  if (ParseOneCharToken(state, 'Q') && ParseExpression(state)) return true;
+  state->parse_state = copy;
+
   return false;
 }
 
+// <type-constraint> ::= <name>
+static bool ParseTypeConstraint(State *state) {
+  return ParseName(state);
+}
+
 // <local-name> ::= Z <(function) encoding> E <(entity) name> [<discriminator>]
 //              ::= Z <(function) encoding> E s [<discriminator>]
+//              ::= Z <(function) encoding> E d [<(parameter) number>] _ <name>
 //
 // Parsing a common prefix of these two productions together avoids an
 // exponential blowup of backtracking.  Parse like:
 //   <local-name> := Z <encoding> E <local-name-suffix>
 //   <local-name-suffix> ::= s [<discriminator>]
+//                       ::= d [<(parameter) number>] _ <name>
 //                       ::= <name> [<discriminator>]
 
 static bool ParseLocalNameSuffix(State *state) {
   ComplexityGuard guard(state);
   if (guard.IsTooComplex()) return false;
+  ParseState copy = state->parse_state;
+
+  // <local-name-suffix> ::= d [<(parameter) number>] _ <name>
+  if (ParseOneCharToken(state, 'd') &&
+      (IsDigit(RemainingInput(state)[0]) || RemainingInput(state)[0] == '_')) {
+    int number = -1;
+    Optional(ParseNumber(state, &number));
+    if (number < -1 || number > 2147483645) {
+      // Work around overflow cases.  We do not expect these outside of a fuzzer
+      // or other source of adversarial input.  If we do detect overflow here,
+      // we'll print {default arg#1}.
+      number = -1;
+    }
+    number += 2;
+
+    // The ::{default arg#1}:: infix must be rendered before the lambda itself,
+    // so print this before parsing the rest of the <local-name-suffix>.
+    MaybeAppend(state, "::{default arg#");
+    MaybeAppendDecimal(state, number);
+    MaybeAppend(state, "}::");
+    if (ParseOneCharToken(state, '_') && ParseName(state)) return true;
 
+    // On late parse failure, roll back not only the input but also the output,
+    // whose trailing NUL was overwritten.
+    state->parse_state = copy;
+    if (state->parse_state.append &&
+        state->parse_state.out_cur_idx < state->out_end_idx) {
+      state->out[state->parse_state.out_cur_idx] = '\0';
+    }
+    return false;
+  }
+  state->parse_state = copy;
+
+  // <local-name-suffix> ::= <name> [<discriminator>]
   if (MaybeAppend(state, "::") && ParseName(state) &&
       Optional(ParseDiscriminator(state))) {
     return true;
   }
-
-  // Since we're not going to overwrite the above "::" by re-parsing the
-  // <encoding> (whose trailing '\0' byte was in the byte now holding the
-  // first ':'), we have to rollback the "::" if the <name> parse failed.
-  if (state->parse_state.append) {
-    state->out[state->parse_state.out_cur_idx - 2] = '\0';
+  state->parse_state = copy;
+  if (state->parse_state.append &&
+      state->parse_state.out_cur_idx < state->out_end_idx) {
+    state->out[state->parse_state.out_cur_idx] = '\0';
   }
 
+  // <local-name-suffix> ::= s [<discriminator>]
   return ParseOneCharToken(state, 's') && Optional(ParseDiscriminator(state));
 }
 
@@ -1779,12 +2847,22 @@ static bool ParseLocalName(State *state) {
   return false;
 }
 
-// <discriminator> := _ <(non-negative) number>
+// <discriminator> := _ <digit>
+//                 := __ <number (>= 10)> _
 static bool ParseDiscriminator(State *state) {
   ComplexityGuard guard(state);
   if (guard.IsTooComplex()) return false;
   ParseState copy = state->parse_state;
-  if (ParseOneCharToken(state, '_') && ParseNumber(state, nullptr)) {
+
+  // Both forms start with _ so parse that first.
+  if (!ParseOneCharToken(state, '_')) return false;
+
+  // <digit>
+  if (ParseDigit(state, nullptr)) return true;
+
+  // _ <number> _
+  if (ParseOneCharToken(state, '_') && ParseNumber(state, nullptr) &&
+      ParseOneCharToken(state, '_')) {
     return true;
   }
   state->parse_state = copy;
@@ -1830,6 +2908,7 @@ static bool ParseSubstitution(State *state, bool accept_std) {
           MaybeAppend(state, p->real_name);
         }
         ++state->parse_state.mangled_idx;
+        UpdateHighWaterMark(state);
         return true;
       }
     }
@@ -1846,7 +2925,7 @@ static bool ParseTopLevelMangledName(State *state) {
   if (ParseMangledName(state)) {
     if (RemainingInput(state)[0] != '\0') {
       // Drop trailing function clone suffix, if any.
-      if (IsFunctionCloneSuffix(RemainingInput(state))) {
+      if (RemainingInput(state)[0] == '.') {
         return true;
       }
       // Append trailing version suffix if any.
@@ -1855,10 +2934,13 @@ static bool ParseTopLevelMangledName(State *state) {
         MaybeAppend(state, RemainingInput(state));
         return true;
       }
+      ReportHighWaterMark(state);
       return false;  // Unconsumed suffix.
     }
     return true;
   }
+
+  ReportHighWaterMark(state);
   return false;
 }
 
@@ -1867,11 +2949,33 @@ static bool Overflowed(const State *state) {
 }
 
 // The demangler entry point.
-bool Demangle(const char *mangled, char *out, int out_size) {
+bool Demangle(const char* mangled, char* out, size_t out_size) {
+  if (mangled[0] == '_' && mangled[1] == 'R') {
+    return DemangleRustSymbolEncoding(mangled, out, out_size);
+  }
+
   State state;
   InitState(&state, mangled, out, out_size);
-  return ParseTopLevelMangledName(&state) && !Overflowed(&state);
+  return ParseTopLevelMangledName(&state) && !Overflowed(&state) &&
+         state.parse_state.out_cur_idx > 0;
+}
+
+std::string DemangleString(const char* mangled) {
+  std::string out;
+  int status = 0;
+  char* demangled = nullptr;
+#ifdef ABSL_INTERNAL_HAS_CXA_DEMANGLE
+  demangled = abi::__cxa_demangle(mangled, nullptr, nullptr, &status);
+#endif
+  if (status == 0 && demangled != nullptr) {
+    out.append(demangled);
+    free(demangled);
+  } else {
+    out.append(mangled);
+  }
+  return out;
 }
 
 }  // namespace debugging_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/debugging/internal/demangle.h b/absl/debugging/internal/demangle.h
index 81bb0dfd4bd..93193a97f57 100644
--- a/absl/debugging/internal/demangle.h
+++ b/absl/debugging/internal/demangle.h
@@ -12,13 +12,25 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-// An async-signal-safe and thread-safe demangler for Itanium C++ ABI
-// (aka G++ V3 ABI).
+#ifndef ABSL_DEBUGGING_INTERNAL_DEMANGLE_H_
+#define ABSL_DEBUGGING_INTERNAL_DEMANGLE_H_
+
+#include <cstddef>
+#include <string>
+
+#include "absl/base/config.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace debugging_internal {
+
+// Demangle `mangled`.  On success, return true and write the
+// demangled symbol name to `out`.  Otherwise, return false.
+// `out` is modified even if demangling is unsuccessful.
 //
-// The demangler is implemented to be used in async signal handlers to
-// symbolize stack traces.  We cannot use libstdc++'s
-// abi::__cxa_demangle() in such signal handlers since it's not async
-// signal safe (it uses malloc() internally).
+// This function provides an alternative to libstdc++'s abi::__cxa_demangle,
+// which is not async signal safe (it uses malloc internally).  It's intended to
+// be used in async signal handlers to symbolize stack traces.
 //
 // Note that this demangler doesn't support full demangling.  More
 // specifically, it doesn't print types of function parameters and
@@ -30,38 +42,37 @@
 //
 // Example:
 //
-// | Mangled Name  | The Demangler | abi::__cxa_demangle()
-// |---------------|---------------|-----------------------
-// | _Z1fv         | f()           | f()
-// | _Z1fi         | f()           | f(int)
-// | _Z3foo3bar    | foo()         | foo(bar)
-// | _Z1fIiEvi     | f<>()         | void f<int>(int)
-// | _ZN1N1fE      | N::f          | N::f
-// | _ZN3Foo3BarEv | Foo::Bar()    | Foo::Bar()
-// | _Zrm1XS_"     | operator%()   | operator%(X, X)
-// | _ZN3FooC1Ev   | Foo::Foo()    | Foo::Foo()
-// | _Z1fSs        | f()           | f(std::basic_string<char,
-// |               |               |   std::char_traits<char>,
-// |               |               |   std::allocator<char> >)
+// | Mangled Name  | Demangle    | DemangleString
+// |---------------|-------------|-----------------------
+// | _Z1fv         | f()         | f()
+// | _Z1fi         | f()         | f(int)
+// | _Z3foo3bar    | foo()       | foo(bar)
+// | _Z1fIiEvi     | f<>()       | void f<int>(int)
+// | _ZN1N1fE      | N::f        | N::f
+// | _ZN3Foo3BarEv | Foo::Bar()  | Foo::Bar()
+// | _Zrm1XS_"     | operator%() | operator%(X, X)
+// | _ZN3FooC1Ev   | Foo::Foo()  | Foo::Foo()
+// | _Z1fSs        | f()         | f(std::basic_string<char,
+// |               |             |   std::char_traits<char>,
+// |               |             |   std::allocator<char> >)
 //
 // See the unit test for more examples.
 //
+// Demangle also recognizes Rust mangled names by delegating the parsing of
+// anything that starts with _R to DemangleRustSymbolEncoding (demangle_rust.h).
+//
 // Note: we might want to write demanglers for ABIs other than Itanium
 // C++ ABI in the future.
-//
-
-#ifndef ABSL_DEBUGGING_INTERNAL_DEMANGLE_H_
-#define ABSL_DEBUGGING_INTERNAL_DEMANGLE_H_
+bool Demangle(const char* mangled, char* out, size_t out_size);
 
-namespace absl {
-namespace debugging_internal {
-
-// Demangle `mangled`.  On success, return true and write the
-// demangled symbol name to `out`.  Otherwise, return false.
-// `out` is modified even if demangling is unsuccessful.
-bool Demangle(const char *mangled, char *out, int out_size);
+// A wrapper around `abi::__cxa_demangle()`.  On success, returns the demangled
+// name.  On failure, returns the input mangled name.
+//
+// This function is not async-signal-safe.
+std::string DemangleString(const char* mangled);
 
 }  // namespace debugging_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_DEBUGGING_INTERNAL_DEMANGLE_H_
diff --git a/absl/debugging/internal/demangle_rust.cc b/absl/debugging/internal/demangle_rust.cc
new file mode 100644
index 00000000000..6ca481bdc3c
--- /dev/null
+++ b/absl/debugging/internal/demangle_rust.cc
@@ -0,0 +1,949 @@
+// Copyright 2024 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/debugging/internal/demangle_rust.h"
+
+#include <cstddef>
+#include <cstdint>
+#include <cstring>
+#include <limits>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/debugging/internal/decode_rust_punycode.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace debugging_internal {
+
+namespace {
+
+// Same step limit as the C++ demangler in demangle.cc uses.
+constexpr int kMaxReturns = 1 << 17;
+
+bool IsDigit(char c) { return '0' <= c && c <= '9'; }
+bool IsLower(char c) { return 'a' <= c && c <= 'z'; }
+bool IsUpper(char c) { return 'A' <= c && c <= 'Z'; }
+bool IsAlpha(char c) { return IsLower(c) || IsUpper(c); }
+bool IsIdentifierChar(char c) { return IsAlpha(c) || IsDigit(c) || c == '_'; }
+bool IsLowerHexDigit(char c) { return IsDigit(c) || ('a' <= c && c <= 'f'); }
+
+const char* BasicTypeName(char c) {
+  switch (c) {
+    case 'a': return "i8";
+    case 'b': return "bool";
+    case 'c': return "char";
+    case 'd': return "f64";
+    case 'e': return "str";
+    case 'f': return "f32";
+    case 'h': return "u8";
+    case 'i': return "isize";
+    case 'j': return "usize";
+    case 'l': return "i32";
+    case 'm': return "u32";
+    case 'n': return "i128";
+    case 'o': return "u128";
+    case 'p': return "_";
+    case 's': return "i16";
+    case 't': return "u16";
+    case 'u': return "()";
+    case 'v': return "...";
+    case 'x': return "i64";
+    case 'y': return "u64";
+    case 'z': return "!";
+  }
+  return nullptr;
+}
+
+// Parser for Rust symbol mangling v0, whose grammar is defined here:
+//
+// https://doc.rust-lang.org/rustc/symbol-mangling/v0.html#symbol-grammar-summary
+class RustSymbolParser {
+ public:
+  // Prepares to demangle the given encoding, a Rust symbol name starting with
+  // _R, into the output buffer [out, out_end).  The caller is expected to
+  // continue by calling the new object's Parse function.
+  RustSymbolParser(const char* encoding, char* out, char* const out_end)
+      : encoding_(encoding), out_(out), out_end_(out_end) {
+    if (out_ != out_end_) *out_ = '\0';
+  }
+
+  // Parses the constructor's encoding argument, writing output into the range
+  // [out, out_end).  Returns true on success and false for input whose
+  // structure was not recognized or exceeded implementation limits, such as by
+  // nesting structures too deep.  In either case *this should not be used
+  // again.
+  [[nodiscard]] bool Parse() && {
+    // Recursively parses the grammar production named by callee, then resumes
+    // execution at the next statement.
+    //
+    // Recursive-descent parsing is a beautifully readable translation of a
+    // grammar, but it risks stack overflow if implemented by naive recursion on
+    // the C++ call stack.  So we simulate recursion by goto and switch instead,
+    // keeping a bounded stack of "return addresses" in the recursion_stack_
+    // member.
+    //
+    // The callee argument is a statement label.  We goto that label after
+    // saving the "return address" on recursion_stack_.  The next continue
+    // statement in the for loop below "returns" from this "call".
+    //
+    // The caller argument names the return point.  Each value of caller must
+    // appear in only one ABSL_DEMANGLER_RECURSE call and be listed in the
+    // definition of enum ReturnAddress.  The switch implements the control
+    // transfer from the end of a "called" subroutine back to the statement
+    // after the "call".
+    //
+    // Note that not all the grammar productions have to be packed into the
+    // switch, but only those which appear in a cycle in the grammar.  Anything
+    // acyclic can be written as ordinary functions and function calls, e.g.,
+    // ParseIdentifier.
+#define ABSL_DEMANGLER_RECURSE(callee, caller) \
+    do { \
+      if (recursion_depth_ == kStackSize) return false; \
+      /* The next continue will switch on this saved value ... */ \
+      recursion_stack_[recursion_depth_++] = caller; \
+      goto callee; \
+      /* ... and will land here, resuming the suspended code. */ \
+      case caller: {} \
+    } while (0)
+
+    // Parse the encoding, counting completed recursive calls to guard against
+    // excessively complex input and infinite-loop bugs.
+    int iter = 0;
+    goto whole_encoding;
+    for (; iter < kMaxReturns && recursion_depth_ > 0; ++iter) {
+      // This switch resumes the code path most recently suspended by
+      // ABSL_DEMANGLER_RECURSE.
+      switch (recursion_stack_[--recursion_depth_]) {
+        //
+        // symbol-name ->
+        // _R decimal-number? path instantiating-crate? vendor-specific-suffix?
+        whole_encoding:
+          if (!Eat('_') || !Eat('R')) return false;
+          // decimal-number? is always empty today, so proceed to path, which
+          // can't start with a decimal digit.
+          ABSL_DEMANGLER_RECURSE(path, kInstantiatingCrate);
+          if (IsAlpha(Peek())) {
+            ++silence_depth_;  // Print nothing more from here on.
+            ABSL_DEMANGLER_RECURSE(path, kVendorSpecificSuffix);
+          }
+          switch (Take()) {
+            case '.': case '$': case '\0': return true;
+          }
+          return false;  // unexpected trailing content
+
+        // path -> crate-root | inherent-impl | trait-impl | trait-definition |
+        //         nested-path | generic-args | backref
+        //
+        // Note that ABSL_DEMANGLER_RECURSE does not work inside a nested switch
+        // (which would hide the generated case label).  Thus we jump out of the
+        // inner switch with gotos before performing any fake recursion.
+        path:
+          switch (Take()) {
+            case 'C': goto crate_root;
+            case 'M': goto inherent_impl;
+            case 'X': goto trait_impl;
+            case 'Y': goto trait_definition;
+            case 'N': goto nested_path;
+            case 'I': goto generic_args;
+            case 'B': goto path_backref;
+            default: return false;
+          }
+
+        // crate-root -> C identifier (C consumed above)
+        crate_root:
+          if (!ParseIdentifier()) return false;
+          continue;
+
+        // inherent-impl -> M impl-path type (M already consumed)
+        inherent_impl:
+          if (!Emit("<")) return false;
+          ABSL_DEMANGLER_RECURSE(impl_path, kInherentImplType);
+          ABSL_DEMANGLER_RECURSE(type, kInherentImplEnding);
+          if (!Emit(">")) return false;
+          continue;
+
+        // trait-impl -> X impl-path type path (X already consumed)
+        trait_impl:
+          if (!Emit("<")) return false;
+          ABSL_DEMANGLER_RECURSE(impl_path, kTraitImplType);
+          ABSL_DEMANGLER_RECURSE(type, kTraitImplInfix);
+          if (!Emit(" as ")) return false;
+          ABSL_DEMANGLER_RECURSE(path, kTraitImplEnding);
+          if (!Emit(">")) return false;
+          continue;
+
+        // impl-path -> disambiguator? path (but never print it!)
+        impl_path:
+          ++silence_depth_;
+          {
+            int ignored_disambiguator;
+            if (!ParseDisambiguator(ignored_disambiguator)) return false;
+          }
+          ABSL_DEMANGLER_RECURSE(path, kImplPathEnding);
+          --silence_depth_;
+          continue;
+
+        // trait-definition -> Y type path (Y already consumed)
+        trait_definition:
+          if (!Emit("<")) return false;
+          ABSL_DEMANGLER_RECURSE(type, kTraitDefinitionInfix);
+          if (!Emit(" as ")) return false;
+          ABSL_DEMANGLER_RECURSE(path, kTraitDefinitionEnding);
+          if (!Emit(">")) return false;
+          continue;
+
+        // nested-path -> N namespace path identifier (N already consumed)
+        // namespace -> lower | upper
+        nested_path:
+          // Uppercase namespaces must be saved on a stack so we can print
+          // ::{closure#0} or ::{shim:vtable#0} or ::{X:name#0} as needed.
+          if (IsUpper(Peek())) {
+            if (!PushNamespace(Take())) return false;
+            ABSL_DEMANGLER_RECURSE(path, kIdentifierInUppercaseNamespace);
+            if (!Emit("::")) return false;
+            if (!ParseIdentifier(PopNamespace())) return false;
+            continue;
+          }
+
+          // Lowercase namespaces, however, are never represented in the output;
+          // they all emit just ::name.
+          if (IsLower(Take())) {
+            ABSL_DEMANGLER_RECURSE(path, kIdentifierInLowercaseNamespace);
+            if (!Emit("::")) return false;
+            if (!ParseIdentifier()) return false;
+            continue;
+          }
+
+          // Neither upper or lower
+          return false;
+
+        // type -> basic-type | array-type | slice-type | tuple-type |
+        //         ref-type | mut-ref-type | const-ptr-type | mut-ptr-type |
+        //         fn-type | dyn-trait-type | path | backref
+        //
+        // We use ifs instead of switch (Take()) because the default case jumps
+        // to path, which will need to see the first character not yet Taken
+        // from the input.  Because we do not use a nested switch here,
+        // ABSL_DEMANGLER_RECURSE works fine in the 'S' case.
+        type:
+          if (IsLower(Peek())) {
+            const char* type_name = BasicTypeName(Take());
+            if (type_name == nullptr || !Emit(type_name)) return false;
+            continue;
+          }
+          if (Eat('A')) {
+            // array-type = A type const
+            if (!Emit("[")) return false;
+            ABSL_DEMANGLER_RECURSE(type, kArraySize);
+            if (!Emit("; ")) return false;
+            ABSL_DEMANGLER_RECURSE(constant, kFinishArray);
+            if (!Emit("]")) return false;
+            continue;
+          }
+          if (Eat('S')) {
+            if (!Emit("[")) return false;
+            ABSL_DEMANGLER_RECURSE(type, kSliceEnding);
+            if (!Emit("]")) return false;
+            continue;
+          }
+          if (Eat('T')) goto tuple_type;
+          if (Eat('R')) {
+            if (!Emit("&")) return false;
+            if (!ParseOptionalLifetime()) return false;
+            goto type;
+          }
+          if (Eat('Q')) {
+            if (!Emit("&mut ")) return false;
+            if (!ParseOptionalLifetime()) return false;
+            goto type;
+          }
+          if (Eat('P')) {
+            if (!Emit("*const ")) return false;
+            goto type;
+          }
+          if (Eat('O')) {
+            if (!Emit("*mut ")) return false;
+            goto type;
+          }
+          if (Eat('F')) goto fn_type;
+          if (Eat('D')) goto dyn_trait_type;
+          if (Eat('B')) goto type_backref;
+          goto path;
+
+        // tuple-type -> T type* E (T already consumed)
+        tuple_type:
+          if (!Emit("(")) return false;
+
+          // The toolchain should call the unit type u instead of TE, but the
+          // grammar and other demanglers also recognize TE, so we do too.
+          if (Eat('E')) {
+            if (!Emit(")")) return false;
+            continue;
+          }
+
+          // A tuple with one element is rendered (type,) instead of (type).
+          ABSL_DEMANGLER_RECURSE(type, kAfterFirstTupleElement);
+          if (Eat('E')) {
+            if (!Emit(",)")) return false;
+            continue;
+          }
+
+          // A tuple with two elements is of course (x, y).
+          if (!Emit(", ")) return false;
+          ABSL_DEMANGLER_RECURSE(type, kAfterSecondTupleElement);
+          if (Eat('E')) {
+            if (!Emit(")")) return false;
+            continue;
+          }
+
+          // And (x, y, z) for three elements.
+          if (!Emit(", ")) return false;
+          ABSL_DEMANGLER_RECURSE(type, kAfterThirdTupleElement);
+          if (Eat('E')) {
+            if (!Emit(")")) return false;
+            continue;
+          }
+
+          // For longer tuples we write (x, y, z, ...), printing none of the
+          // content of the fourth and later types.  Thus we avoid exhausting
+          // output buffers and human readers' patience when some library has a
+          // long tuple as an implementation detail, without having to
+          // completely obfuscate all tuples.
+          if (!Emit(", ...)")) return false;
+          ++silence_depth_;
+          while (!Eat('E')) {
+            ABSL_DEMANGLER_RECURSE(type, kAfterSubsequentTupleElement);
+          }
+          --silence_depth_;
+          continue;
+
+        // fn-type -> F fn-sig (F already consumed)
+        // fn-sig -> binder? U? (K abi)? type* E type
+        // abi -> C | undisambiguated-identifier
+        //
+        // We follow the C++ demangler in suppressing details of function
+        // signatures.  Every function type is rendered "fn...".
+        fn_type:
+          if (!Emit("fn...")) return false;
+          ++silence_depth_;
+          if (!ParseOptionalBinder()) return false;
+          (void)Eat('U');
+          if (Eat('K')) {
+            if (!Eat('C') && !ParseUndisambiguatedIdentifier()) return false;
+          }
+          while (!Eat('E')) {
+            ABSL_DEMANGLER_RECURSE(type, kContinueParameterList);
+          }
+          ABSL_DEMANGLER_RECURSE(type, kFinishFn);
+          --silence_depth_;
+          continue;
+
+        // dyn-trait-type -> D dyn-bounds lifetime (D already consumed)
+        // dyn-bounds -> binder? dyn-trait* E
+        //
+        // The grammar strangely allows an empty trait list, even though the
+        // compiler should never output one.  We follow existing demanglers in
+        // rendering DEL_ as "dyn ".
+        //
+        // Because auto traits lengthen a type name considerably without
+        // providing much value to a search for related source code, it would be
+        // desirable to abbreviate
+        //     dyn main::Trait + std::marker::Copy + std::marker::Send
+        // to
+        //     dyn main::Trait + ...,
+        // eliding the auto traits.  But it is difficult to do so correctly, in
+        // part because there is no guarantee that the mangling will list the
+        // main trait first.  So we just print all the traits in their order of
+        // appearance in the mangled name.
+        dyn_trait_type:
+          if (!Emit("dyn ")) return false;
+          if (!ParseOptionalBinder()) return false;
+          if (!Eat('E')) {
+            ABSL_DEMANGLER_RECURSE(dyn_trait, kBeginAutoTraits);
+            while (!Eat('E')) {
+              if (!Emit(" + ")) return false;
+              ABSL_DEMANGLER_RECURSE(dyn_trait, kContinueAutoTraits);
+            }
+          }
+          if (!ParseRequiredLifetime()) return false;
+          continue;
+
+        // dyn-trait -> path dyn-trait-assoc-binding*
+        // dyn-trait-assoc-binding -> p undisambiguated-identifier type
+        //
+        // We render nonempty binding lists as <>, omitting their contents as
+        // for generic-args.
+        dyn_trait:
+          ABSL_DEMANGLER_RECURSE(path, kContinueDynTrait);
+          if (Peek() == 'p') {
+            if (!Emit("<>")) return false;
+            ++silence_depth_;
+            while (Eat('p')) {
+              if (!ParseUndisambiguatedIdentifier()) return false;
+              ABSL_DEMANGLER_RECURSE(type, kContinueAssocBinding);
+            }
+            --silence_depth_;
+          }
+          continue;
+
+        // const -> type const-data | p | backref
+        //
+        // const is a C++ keyword, so we use the label `constant` instead.
+        constant:
+          if (Eat('B')) goto const_backref;
+          if (Eat('p')) {
+            if (!Emit("_")) return false;
+            continue;
+          }
+
+          // Scan the type without printing it.
+          //
+          // The Rust language restricts the type of a const generic argument
+          // much more than the mangling grammar does.  We do not enforce this.
+          //
+          // We also do not bother printing false, true, 'A', and '\u{abcd}' for
+          // the types bool and char.  Because we do not print generic-args
+          // contents, we expect to print constants only in array sizes, and
+          // those should not be bool or char.
+          ++silence_depth_;
+          ABSL_DEMANGLER_RECURSE(type, kConstData);
+          --silence_depth_;
+
+          // const-data -> n? hex-digit* _
+          //
+          // Although the grammar doesn't say this, existing demanglers expect
+          // that zero is 0, not an empty digit sequence, and no nonzero value
+          // may have leading zero digits.  Also n0_ is accepted and printed as
+          // -0, though a toolchain will probably never write that encoding.
+          if (Eat('n') && !EmitChar('-')) return false;
+          if (!Emit("0x")) return false;
+          if (Eat('0')) {
+            if (!EmitChar('0')) return false;
+            if (!Eat('_')) return false;
+            continue;
+          }
+          while (IsLowerHexDigit(Peek())) {
+            if (!EmitChar(Take())) return false;
+          }
+          if (!Eat('_')) return false;
+          continue;
+
+        // generic-args -> I path generic-arg* E (I already consumed)
+        //
+        // We follow the C++ demangler in omitting all the arguments from the
+        // output, printing only the list opening and closing tokens.
+        generic_args:
+          ABSL_DEMANGLER_RECURSE(path, kBeginGenericArgList);
+          if (!Emit("::<>")) return false;
+          ++silence_depth_;
+          while (!Eat('E')) {
+            ABSL_DEMANGLER_RECURSE(generic_arg, kContinueGenericArgList);
+          }
+          --silence_depth_;
+          continue;
+
+        // generic-arg -> lifetime | type | K const
+        generic_arg:
+          if (Peek() == 'L') {
+            if (!ParseOptionalLifetime()) return false;
+            continue;
+          }
+          if (Eat('K')) goto constant;
+          goto type;
+
+        // backref -> B base-62-number (B already consumed)
+        //
+        // The BeginBackref call parses and range-checks the base-62-number.  We
+        // always do that much.
+        //
+        // The recursive call parses and prints what the backref points at.  We
+        // save CPU and stack by skipping this work if the output would be
+        // suppressed anyway.
+        path_backref:
+          if (!BeginBackref()) return false;
+          if (silence_depth_ == 0) {
+            ABSL_DEMANGLER_RECURSE(path, kPathBackrefEnding);
+          }
+          EndBackref();
+          continue;
+
+        // This represents the same backref production as in path_backref but
+        // parses the target as a type instead of a path.
+        type_backref:
+          if (!BeginBackref()) return false;
+          if (silence_depth_ == 0) {
+            ABSL_DEMANGLER_RECURSE(type, kTypeBackrefEnding);
+          }
+          EndBackref();
+          continue;
+
+        const_backref:
+          if (!BeginBackref()) return false;
+          if (silence_depth_ == 0) {
+            ABSL_DEMANGLER_RECURSE(constant, kConstantBackrefEnding);
+          }
+          EndBackref();
+          continue;
+      }
+    }
+
+    return false;  // hit iteration limit or a bug in our stack handling
+  }
+
+ private:
+  // Enumerates resumption points for ABSL_DEMANGLER_RECURSE calls.
+  enum ReturnAddress : uint8_t {
+    kInstantiatingCrate,
+    kVendorSpecificSuffix,
+    kIdentifierInUppercaseNamespace,
+    kIdentifierInLowercaseNamespace,
+    kInherentImplType,
+    kInherentImplEnding,
+    kTraitImplType,
+    kTraitImplInfix,
+    kTraitImplEnding,
+    kImplPathEnding,
+    kTraitDefinitionInfix,
+    kTraitDefinitionEnding,
+    kArraySize,
+    kFinishArray,
+    kSliceEnding,
+    kAfterFirstTupleElement,
+    kAfterSecondTupleElement,
+    kAfterThirdTupleElement,
+    kAfterSubsequentTupleElement,
+    kContinueParameterList,
+    kFinishFn,
+    kBeginAutoTraits,
+    kContinueAutoTraits,
+    kContinueDynTrait,
+    kContinueAssocBinding,
+    kConstData,
+    kBeginGenericArgList,
+    kContinueGenericArgList,
+    kPathBackrefEnding,
+    kTypeBackrefEnding,
+    kConstantBackrefEnding,
+  };
+
+  // Element counts for the stack arrays.  Larger stack sizes accommodate more
+  // deeply nested names at the cost of a larger footprint on the C++ call
+  // stack.
+  enum {
+    // Maximum recursive calls outstanding at one time.
+    kStackSize = 256,
+
+    // Maximum N<uppercase> nested-paths open at once.  We do not expect
+    // closures inside closures inside closures as much as functions inside
+    // modules inside other modules, so we can use a smaller array here.
+    kNamespaceStackSize = 64,
+
+    // Maximum number of nested backrefs.  We can keep this stack pretty small
+    // because we do not follow backrefs inside generic-args or other contexts
+    // that suppress printing, so deep stacking is unlikely in practice.
+    kPositionStackSize = 16,
+  };
+
+  // Returns the next input character without consuming it.
+  char Peek() const { return encoding_[pos_]; }
+
+  // Consumes and returns the next input character.
+  char Take() { return encoding_[pos_++]; }
+
+  // If the next input character is the given character, consumes it and returns
+  // true; otherwise returns false without consuming a character.
+  [[nodiscard]] bool Eat(char want) {
+    if (encoding_[pos_] != want) return false;
+    ++pos_;
+    return true;
+  }
+
+  // Provided there is enough remaining output space, appends c to the output,
+  // writing a fresh NUL terminator afterward, and returns true.  Returns false
+  // if the output buffer had less than two bytes free.
+  [[nodiscard]] bool EmitChar(char c) {
+    if (silence_depth_ > 0) return true;
+    if (out_end_ - out_ < 2) return false;
+    *out_++ = c;
+    *out_ = '\0';
+    return true;
+  }
+
+  // Provided there is enough remaining output space, appends the C string token
+  // to the output, followed by a NUL character, and returns true.  Returns
+  // false if not everything fit into the output buffer.
+  [[nodiscard]] bool Emit(const char* token) {
+    if (silence_depth_ > 0) return true;
+    const size_t token_length = std::strlen(token);
+    const size_t bytes_to_copy = token_length + 1;  // token and final NUL
+    if (static_cast<size_t>(out_end_ - out_) < bytes_to_copy) return false;
+    std::memcpy(out_, token, bytes_to_copy);
+    out_ += token_length;
+    return true;
+  }
+
+  // Provided there is enough remaining output space, appends the decimal form
+  // of disambiguator (if it's nonnegative) or "?" (if it's negative) to the
+  // output, followed by a NUL character, and returns true.  Returns false if
+  // not everything fit into the output buffer.
+  [[nodiscard]] bool EmitDisambiguator(int disambiguator) {
+    if (disambiguator < 0) return EmitChar('?');  // parsed but too large
+    if (disambiguator == 0) return EmitChar('0');
+    // Convert disambiguator to decimal text.  Three digits per byte is enough
+    // because 999 > 256.  The bound will remain correct even if future
+    // maintenance changes the type of the disambiguator variable.
+    char digits[3 * sizeof(disambiguator)] = {};
+    size_t leading_digit_index = sizeof(digits) - 1;
+    for (; disambiguator > 0; disambiguator /= 10) {
+      digits[--leading_digit_index] =
+          static_cast<char>('0' + disambiguator % 10);
+    }
+    return Emit(digits + leading_digit_index);
+  }
+
+  // Consumes an optional disambiguator (s123_) from the input.
+  //
+  // On success returns true and fills value with the encoded value if it was
+  // not too big, otherwise with -1.  If the optional disambiguator was omitted,
+  // value is 0.  On parse failure returns false and sets value to -1.
+  [[nodiscard]] bool ParseDisambiguator(int& value) {
+    value = -1;
+
+    // disambiguator = s base-62-number
+    //
+    // Disambiguators are optional.  An omitted disambiguator is zero.
+    if (!Eat('s')) {
+      value = 0;
+      return true;
+    }
+    int base_62_value = 0;
+    if (!ParseBase62Number(base_62_value)) return false;
+    value = base_62_value < 0 ? -1 : base_62_value + 1;
+    return true;
+  }
+
+  // Consumes a base-62 number like _ or 123_ from the input.
+  //
+  // On success returns true and fills value with the encoded value if it was
+  // not too big, otherwise with -1.  On parse failure returns false and sets
+  // value to -1.
+  [[nodiscard]] bool ParseBase62Number(int& value) {
+    value = -1;
+
+    // base-62-number = (digit | lower | upper)* _
+    //
+    // An empty base-62 digit sequence means 0.
+    if (Eat('_')) {
+      value = 0;
+      return true;
+    }
+
+    // A nonempty digit sequence denotes its base-62 value plus 1.
+    int encoded_number = 0;
+    bool overflowed = false;
+    for (int scanned = 0; IsAlpha(Peek()) || IsDigit(Peek()); ++scanned) {
+      // Cap the scan length: a u64 fits in 11 base-62 digits, and int overflows
+      // after ~5, so anything beyond ~16 digits is already failing to parse.
+      if (scanned >= 16) {
+        // Reject pathologically long runs so a backref cannot re-scan
+        // arbitrarily long stretches of input per iteration.
+        return false;
+      }
+
+      const char c = Take();
+      if (encoded_number >= std::numeric_limits<int>::max()/62) {
+        // If we are close to overflowing an int, keep parsing but stop updating
+        // encoded_number and remember to return -1 at the end.  The point is to
+        // avoid undefined behavior while parsing crate-root disambiguators,
+        // which are large in practice but not shown in demangling, while
+        // successfully computing closure and shim disambiguators, which are
+        // typically small and are printed out.
+        overflowed = true;
+      } else {
+        int digit;
+        if (IsDigit(c)) {
+          digit = c - '0';
+        } else if (IsLower(c)) {
+          digit = c - 'a' + 10;
+        } else {
+          digit = c - 'A' + 36;
+        }
+        encoded_number = 62 * encoded_number + digit;
+      }
+    }
+
+    if (!Eat('_')) return false;
+    if (!overflowed) value = encoded_number + 1;
+    return true;
+  }
+
+  // Consumes an identifier from the input, returning true on success.
+  //
+  // A nonzero uppercase_namespace specifies the character after the N in a
+  // nested-identifier, e.g., 'C' for a closure, allowing ParseIdentifier to
+  // write out the name with the conventional decoration for that namespace.
+  [[nodiscard]] bool ParseIdentifier(char uppercase_namespace = '\0') {
+    // identifier -> disambiguator? undisambiguated-identifier
+    int disambiguator = 0;
+    if (!ParseDisambiguator(disambiguator)) return false;
+
+    return ParseUndisambiguatedIdentifier(uppercase_namespace, disambiguator);
+  }
+
+  // Consumes from the input an identifier with no preceding disambiguator,
+  // returning true on success.
+  //
+  // When ParseIdentifier calls this, it passes the N<namespace> character and
+  // disambiguator value so that "{closure#42}" and similar forms can be
+  // rendered correctly.
+  //
+  // At other appearances of undisambiguated-identifier in the grammar, this
+  // treatment is not applicable, and the call site omits both arguments.
+  [[nodiscard]] bool ParseUndisambiguatedIdentifier(
+      char uppercase_namespace = '\0', int disambiguator = 0) {
+    // undisambiguated-identifier -> u? decimal-number _? bytes
+    const bool is_punycoded = Eat('u');
+    if (!IsDigit(Peek())) return false;
+    int num_bytes = 0;
+    if (!ParseDecimalNumber(num_bytes)) return false;
+    (void)Eat('_');  // optional separator, needed if a digit follows
+    if (is_punycoded) {
+      // A length exceeding the remaining input would make punycode_end point
+      // past the end of the buffer, forming an out-of-bounds pointer.
+      if (static_cast<size_t>(num_bytes) > std::strlen(&encoding_[pos_])) {
+        return false;
+      }
+      if (silence_depth_ == 0) {
+        DecodeRustPunycodeOptions options;
+        options.punycode_begin = &encoding_[pos_];
+        options.punycode_end = &encoding_[pos_] + num_bytes;
+        options.out_begin = out_;
+        options.out_end = out_end_;
+        out_ = DecodeRustPunycode(options);
+        if (out_ == nullptr) return false;
+        pos_ += static_cast<size_t>(num_bytes);
+      } else {
+        // Output is silenced, so like EmitChar and Emit we produce nothing and
+        // use no output space.  Still consume the identifier's encoded bytes,
+        // stopping at a premature NUL exactly as the raw-bytes branch below
+        // does, so a suppressed punycoded identifier (e.g. a punycoded
+        // fn-signature abi) cannot leak into the demangling.
+        for (int i = 0; i < num_bytes; ++i) {
+          if (Take() == '\0') return false;
+        }
+      }
+    }
+
+    // Emit the beginnings of braced forms like {shim:vtable#0}.
+    if (uppercase_namespace != '\0') {
+      switch (uppercase_namespace) {
+        case 'C':
+          if (!Emit("{closure")) return false;
+          break;
+        case 'S':
+          if (!Emit("{shim")) return false;
+          break;
+        default:
+          if (!EmitChar('{') || !EmitChar(uppercase_namespace)) return false;
+          break;
+      }
+      if (num_bytes > 0 && !Emit(":")) return false;
+    }
+
+    // Emit the name itself.
+    if (!is_punycoded) {
+      for (int i = 0; i < num_bytes; ++i) {
+        const char c = Take();
+        if (!IsIdentifierChar(c) &&
+            // The spec gives toolchains the choice of Punycode or raw UTF-8 for
+            // identifiers containing code points above 0x7f, so accept bytes
+            // with the high bit set.
+            (c & 0x80) == 0) {
+          return false;
+        }
+        if (!EmitChar(c)) return false;
+      }
+    }
+
+    // Emit the endings of braced forms, e.g., "#42}".
+    if (uppercase_namespace != '\0') {
+      if (!EmitChar('#')) return false;
+      if (!EmitDisambiguator(disambiguator)) return false;
+      if (!EmitChar('}')) return false;
+    }
+
+    return true;
+  }
+
+  // Consumes a decimal number like 0 or 123 from the input.  On success returns
+  // true and fills value with the encoded value.  If the encoded value is too
+  // large or otherwise unparsable, returns false and sets value to -1.
+  [[nodiscard]] bool ParseDecimalNumber(int& value) {
+    value = -1;
+    if (!IsDigit(Peek())) return false;
+    int encoded_number = Take() - '0';
+    if (encoded_number == 0) {
+      // Decimal numbers are never encoded with extra leading zeroes.
+      value = 0;
+      return true;
+    }
+    while (IsDigit(Peek()) &&
+           // avoid overflow
+           encoded_number < std::numeric_limits<int>::max()/10) {
+      encoded_number = 10 * encoded_number + (Take() - '0');
+    }
+    if (IsDigit(Peek())) return false;  // too big
+    value = encoded_number;
+    return true;
+  }
+
+  // Consumes a binder of higher-ranked lifetimes if one is present.  On success
+  // returns true and discards the encoded lifetime count.  On parse failure
+  // returns false.
+  [[nodiscard]] bool ParseOptionalBinder() {
+    // binder -> G base-62-number
+    if (!Eat('G')) return true;
+    int ignored_binding_count;
+    return ParseBase62Number(ignored_binding_count);
+  }
+
+  // Consumes a lifetime if one is present.
+  //
+  // On success returns true and discards the lifetime index.  We do not print
+  // or even range-check lifetimes because they are a finer detail than other
+  // things we omit from output, such as the entire contents of generic-args.
+  //
+  // On parse failure returns false.
+  [[nodiscard]] bool ParseOptionalLifetime() {
+    // lifetime -> L base-62-number
+    if (!Eat('L')) return true;
+    int ignored_de_bruijn_index;
+    return ParseBase62Number(ignored_de_bruijn_index);
+  }
+
+  // Consumes a lifetime just like ParseOptionalLifetime, but returns false if
+  // there is no lifetime here.
+  [[nodiscard]] bool ParseRequiredLifetime() {
+    if (Peek() != 'L') return false;
+    return ParseOptionalLifetime();
+  }
+
+  // Pushes ns onto the namespace stack and returns true if the stack is not
+  // full, else returns false.
+  [[nodiscard]] bool PushNamespace(char ns) {
+    if (namespace_depth_ == kNamespaceStackSize) return false;
+    namespace_stack_[namespace_depth_++] = ns;
+    return true;
+  }
+
+  // Pops the last pushed namespace.  Requires that the namespace stack is not
+  // empty (namespace_depth_ > 0).
+  char PopNamespace() { return namespace_stack_[--namespace_depth_]; }
+
+  // Pushes position onto the position stack and returns true if the stack is
+  // not full, else returns false.
+  [[nodiscard]] bool PushPosition(int position) {
+    if (position_depth_ == kPositionStackSize) return false;
+    position_stack_[position_depth_++] = position;
+    return true;
+  }
+
+  // Pops the last pushed input position.  Requires that the position stack is
+  // not empty (position_depth_ > 0).
+  int PopPosition() { return position_stack_[--position_depth_]; }
+
+  // Consumes a base-62-number denoting a backref target, pushes the current
+  // input position on the data stack, and sets the input position to the
+  // beginning of the backref target.  Returns true on success.  Returns false
+  // if parsing failed, the stack is exhausted, or the backref target position
+  // is out of range.
+  [[nodiscard]] bool BeginBackref() {
+    // backref = B base-62-number (B already consumed)
+    //
+    // Reject backrefs that don't parse, overflow int, or don't point backward.
+    // If the offset looks fine, adjust it to account for the _R prefix.
+    int offset = 0;
+    const int offset_of_this_backref =
+        pos_ - 2 /* _R */ - 1 /* B already consumed */;
+    if (!ParseBase62Number(offset) || offset < 0 ||
+        offset >= offset_of_this_backref) {
+      return false;
+    }
+    offset += 2;
+
+    // Save the old position to restore later.
+    if (!PushPosition(pos_)) return false;
+
+    // Move the input position to the backref target.
+    //
+    // Note that we do not check whether the new position points to the
+    // beginning of a construct matching the context in which the backref
+    // appeared.  We just jump to it and see whether nested parsing succeeds.
+    // We therefore accept various wrong manglings, e.g., a type backref
+    // pointing to an 'l' character inside an identifier, which happens to mean
+    // i32 when parsed as a type mangling.  This saves the complexity and RAM
+    // footprint of remembering which offsets began which kinds of
+    // substructures.  Existing demanglers take similar shortcuts.
+    pos_ = offset;
+    return true;
+  }
+
+  // Cleans up after a backref production by restoring the previous input
+  // position from the data stack.
+  void EndBackref() { pos_ = PopPosition(); }
+
+  // The leftmost recursion_depth_ elements of recursion_stack_ contain the
+  // ReturnAddresses pushed by ABSL_DEMANGLER_RECURSE calls not yet completed.
+  ReturnAddress recursion_stack_[kStackSize] = {};
+  int recursion_depth_ = 0;
+
+  // The leftmost namespace_depth_ elements of namespace_stack_ contain the
+  // uppercase namespace identifiers for open nested-paths, e.g., 'C' for a
+  // closure.
+  char namespace_stack_[kNamespaceStackSize] = {};
+  int namespace_depth_ = 0;
+
+  // The leftmost position_depth_ elements of position_stack_ contain the input
+  // positions to return to after fully printing the targets of backrefs.
+  int position_stack_[kPositionStackSize] = {};
+  int position_depth_ = 0;
+
+  // Anything parsed while silence_depth_ > 0 contributes nothing to the
+  // demangled output.  For constructs omitted from the demangling, such as
+  // impl-path and the contents of generic-args, we will increment
+  // silence_depth_ on the way in and decrement silence_depth_ on the way out.
+  int silence_depth_ = 0;
+
+  // Input: encoding_ points to a Rust mangled symbol, and encoding_[pos_] is
+  // the next input character to be scanned.
+  int pos_ = 0;
+  const char* encoding_ = nullptr;
+
+  // Output: *out_ is where the next output character should be written, and
+  // out_end_ points past the last byte of available space.
+  char* out_ = nullptr;
+  char* out_end_ = nullptr;
+};
+
+}  // namespace
+
+bool DemangleRustSymbolEncoding(const char* mangled, char* out,
+                                size_t out_size) {
+  return RustSymbolParser(mangled, out, out + out_size).Parse();
+}
+
+}  // namespace debugging_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/debugging/internal/demangle_rust.h b/absl/debugging/internal/demangle_rust.h
new file mode 100644
index 00000000000..299c17a3d3f
--- /dev/null
+++ b/absl/debugging/internal/demangle_rust.h
@@ -0,0 +1,50 @@
+// Copyright 2024 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_DEBUGGING_INTERNAL_DEMANGLE_RUST_H_
+#define ABSL_DEBUGGING_INTERNAL_DEMANGLE_RUST_H_
+
+#include <cstddef>
+
+#include "absl/base/config.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace debugging_internal {
+
+// Demangle the Rust encoding `mangled`.  On success, return true and write the
+// demangled symbol name to `out`.  Otherwise, return false, leaving unspecified
+// contents in `out`.  For example, calling DemangleRustSymbolEncoding with
+// `mangled = "_RNvC8my_crate7my_func"` will yield `my_crate::my_func` in `out`,
+// provided `out_size` is large enough for that value and its trailing NUL.
+//
+// DemangleRustSymbolEncoding is async-signal-safe and runs in bounded C++
+// call-stack space.  It is suitable for symbolizing stack traces in a signal
+// handler.
+//
+// Note that this demangler purposefully omits some details: generic argument
+// lists become `<>`, function types `fn...`, and long tuples `(t, u, v, ...)`.
+// This simplification suits crash backtracing, where the signal handler must
+// not `malloc`, and the human troubleshooter won't want the `file:line` blame
+// drowned in generic arguments.  Applications better served by a freely
+// allocating detailed demangler might prefer to use Rust's own `rustc-demangle`
+// crate.
+bool DemangleRustSymbolEncoding(const char* mangled, char* out,
+                                size_t out_size);
+
+}  // namespace debugging_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_DEBUGGING_INTERNAL_DEMANGLE_RUST_H_
diff --git a/absl/debugging/internal/demangle_rust_test.cc b/absl/debugging/internal/demangle_rust_test.cc
new file mode 100644
index 00000000000..3f0a904731f
--- /dev/null
+++ b/absl/debugging/internal/demangle_rust_test.cc
@@ -0,0 +1,612 @@
+// Copyright 2024 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/debugging/internal/demangle_rust.h"
+
+#include <cstddef>
+#include <string>
+
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace debugging_internal {
+namespace {
+
+// If DemangleRustSymbolEncoding(mangled, <buffer with room for buffer_size
+// chars>, buffer_size) returns true and seems not to have overrun its output
+// buffer, returns the string written by DemangleRustSymbolEncoding; otherwise
+// returns an error message.
+std::string ResultOfDemangling(const char* mangled, size_t buffer_size) {
+  // Fill the buffer with something other than NUL so we test whether Demangle
+  // appends trailing NUL as expected.
+  std::string buffer(buffer_size + 1, '~');
+  constexpr char kCanaryCharacter = 0x7f;  // arbitrary unlikely value
+  buffer[buffer_size] = kCanaryCharacter;
+  if (!DemangleRustSymbolEncoding(mangled, &buffer[0], buffer_size)) {
+    return "Failed parse";
+  }
+  if (buffer[buffer_size] != kCanaryCharacter) {
+    return "Buffer overrun by output: " + buffer.substr(0, buffer_size + 1)
+        + "...";
+  }
+  return buffer.data();  // Not buffer itself: this trims trailing padding.
+}
+
+// Tests that DemangleRustSymbolEncoding converts mangled into plaintext given
+// enough output buffer space but returns false and avoids overrunning a buffer
+// that is one byte too short.
+//
+// The lambda wrapping allows ASSERT_EQ to branch out the first time an
+// expectation is not satisfied, preventing redundant errors for the same bug.
+//
+// We test first with excess space so that if the algorithm just computes the
+// wrong answer, it will be clear from the error log that the bounds checks are
+// unlikely to be the code at fault.
+#define EXPECT_DEMANGLING(mangled, plaintext) \
+  do { \
+    [] { \
+      constexpr size_t plenty_of_space = sizeof(plaintext) + 128; \
+      constexpr size_t just_enough_space = sizeof(plaintext); \
+      constexpr size_t one_byte_too_few = sizeof(plaintext) - 1; \
+      const char* expected_plaintext = plaintext; \
+      const char* expected_error = "Failed parse"; \
+      ASSERT_EQ(ResultOfDemangling(mangled, plenty_of_space), \
+                expected_plaintext); \
+      ASSERT_EQ(ResultOfDemangling(mangled, just_enough_space), \
+                expected_plaintext); \
+      ASSERT_EQ(ResultOfDemangling(mangled, one_byte_too_few), \
+                expected_error); \
+    }(); \
+  } while (0)
+
+// Tests that DemangleRustSymbolEncoding rejects the given input (typically, a
+// truncation of a real Rust symbol name).
+#define EXPECT_DEMANGLING_FAILS(mangled) \
+    do { \
+      constexpr size_t plenty_of_space = 1024; \
+      const char* expected_error = "Failed parse"; \
+      EXPECT_EQ(ResultOfDemangling(mangled, plenty_of_space), expected_error); \
+    } while (0)
+
+// Piping grep -C 1 _R demangle_test.cc into your favorite c++filt
+// implementation allows you to verify that the goldens below are reasonable.
+
+TEST(DemangleRust, EmptyDemangling) {
+  EXPECT_TRUE(DemangleRustSymbolEncoding("_RC0", nullptr, 0));
+}
+
+TEST(DemangleRust, FunctionAtCrateLevel) {
+  EXPECT_DEMANGLING("_RNvC10crate_name9func_name", "crate_name::func_name");
+  EXPECT_DEMANGLING(
+      "_RNvCs09azAZ_10crate_name9func_name", "crate_name::func_name");
+}
+
+TEST(DemangleRust, TruncationsOfFunctionAtCrateLevel) {
+  EXPECT_DEMANGLING_FAILS("_R");
+  EXPECT_DEMANGLING_FAILS("_RN");
+  EXPECT_DEMANGLING_FAILS("_RNvC");
+  EXPECT_DEMANGLING_FAILS("_RNvC10");
+  EXPECT_DEMANGLING_FAILS("_RNvC10crate_nam");
+  EXPECT_DEMANGLING_FAILS("_RNvC10crate_name");
+  EXPECT_DEMANGLING_FAILS("_RNvC10crate_name9");
+  EXPECT_DEMANGLING_FAILS("_RNvC10crate_name9func_nam");
+  EXPECT_DEMANGLING_FAILS("_RNvCs");
+  EXPECT_DEMANGLING_FAILS("_RNvCs09azAZ");
+  EXPECT_DEMANGLING_FAILS("_RNvCs09azAZ_");
+}
+
+TEST(DemangleRust, VendorSuffixes) {
+  EXPECT_DEMANGLING("_RNvC10crate_name9func_name.!@#", "crate_name::func_name");
+  EXPECT_DEMANGLING("_RNvC10crate_name9func_name$!@#", "crate_name::func_name");
+}
+
+TEST(DemangleRust, UnicodeIdentifiers) {
+  EXPECT_DEMANGLING("_RNvC7ice_cap17Eyjafjallajökull",
+                    "ice_cap::Eyjafjallajökull");
+  EXPECT_DEMANGLING("_RNvC7ice_caps_u19Eyjafjallajkull_jtb",
+                    "ice_cap::Eyjafjallajökull");
+
+  // A punycode byte count larger than the remaining input is rejected instead
+  // of running the decoder past the end of the buffer.
+  EXPECT_DEMANGLING_FAILS("_RNvC7ice_caps_u2000000000Eyjafjallajkull_jtb");
+  EXPECT_DEMANGLING_FAILS("_RNvC7ice_caps_u99Eyj_a");
+}
+
+TEST(DemangleRust, FunctionInModule) {
+  EXPECT_DEMANGLING("_RNvNtCs09azAZ_10crate_name11module_name9func_name",
+                    "crate_name::module_name::func_name");
+}
+
+TEST(DemangleRust, FunctionInFunction) {
+  EXPECT_DEMANGLING(
+      "_RNvNvCs09azAZ_10crate_name15outer_func_name15inner_func_name",
+      "crate_name::outer_func_name::inner_func_name");
+}
+
+TEST(DemangleRust, ClosureInFunction) {
+  EXPECT_DEMANGLING(
+      "_RNCNvCs09azAZ_10crate_name9func_name0",
+      "crate_name::func_name::{closure#0}");
+  EXPECT_DEMANGLING(
+      "_RNCNvCs09azAZ_10crate_name9func_name0Cs123_12client_crate",
+      "crate_name::func_name::{closure#0}");
+}
+
+TEST(DemangleRust, ClosureNumbering) {
+  EXPECT_DEMANGLING(
+      "_RNCNvCs09azAZ_10crate_name9func_names_0Cs123_12client_crate",
+      "crate_name::func_name::{closure#1}");
+  EXPECT_DEMANGLING(
+      "_RNCNvCs09azAZ_10crate_name9func_names0_0Cs123_12client_crate",
+      "crate_name::func_name::{closure#2}");
+  EXPECT_DEMANGLING(
+      "_RNCNvCs09azAZ_10crate_name9func_names9_0Cs123_12client_crate",
+      "crate_name::func_name::{closure#11}");
+  EXPECT_DEMANGLING(
+      "_RNCNvCs09azAZ_10crate_name9func_namesa_0Cs123_12client_crate",
+      "crate_name::func_name::{closure#12}");
+  EXPECT_DEMANGLING(
+      "_RNCNvCs09azAZ_10crate_name9func_namesz_0Cs123_12client_crate",
+      "crate_name::func_name::{closure#37}");
+  EXPECT_DEMANGLING(
+      "_RNCNvCs09azAZ_10crate_name9func_namesA_0Cs123_12client_crate",
+      "crate_name::func_name::{closure#38}");
+  EXPECT_DEMANGLING(
+      "_RNCNvCs09azAZ_10crate_name9func_namesZ_0Cs123_12client_crate",
+      "crate_name::func_name::{closure#63}");
+  EXPECT_DEMANGLING(
+      "_RNCNvCs09azAZ_10crate_name9func_names10_0Cs123_12client_crate",
+      "crate_name::func_name::{closure#64}");
+  EXPECT_DEMANGLING(
+      "_RNCNvCs09azAZ_10crate_name9func_namesg6_0Cs123_12client_crate",
+      "crate_name::func_name::{closure#1000}");
+}
+
+TEST(DemangleRust, ClosureNumberOverflowingInt) {
+  EXPECT_DEMANGLING(
+      "_RNCNvCs09azAZ_10crate_name9func_names1234567_0Cs123_12client_crate",
+      "crate_name::func_name::{closure#?}");
+}
+
+TEST(DemangleRust, Base62NumberScanLimit) {
+  // Up to 16 base-62 digits is permitted (though int overflow yields "?").
+  EXPECT_DEMANGLING(
+      "_RNCNvCs09azAZ_10crate_name9func_names0123456789abcdef_0Cs123_12client_"
+      "crate",
+      "crate_name::func_name::{closure#?}");
+
+  // Beyond 16 base-62 digits is rejected to prevent excessive re-scanning.
+  EXPECT_DEMANGLING_FAILS(
+      "_RNCNvCs09azAZ_10crate_name9func_names0123456789abcdef0_0Cs123_12client_"
+      "crate");
+  EXPECT_DEMANGLING_FAILS("_RB0123456789abcdef0_");
+}
+
+TEST(DemangleRust, UnexpectedlyNamedClosure) {
+  EXPECT_DEMANGLING(
+      "_RNCNvCs123_10crate_name9func_name12closure_nameCs456_12client_crate",
+      "crate_name::func_name::{closure:closure_name#0}");
+  EXPECT_DEMANGLING(
+      "_RNCNvCs123_10crate_name9func_names2_12closure_nameCs456_12client_crate",
+      "crate_name::func_name::{closure:closure_name#4}");
+}
+
+TEST(DemangleRust, ItemNestedInsideClosure) {
+  EXPECT_DEMANGLING(
+      "_RNvNCNvCs123_10crate_name9func_name015inner_func_nameCs_12client_crate",
+      "crate_name::func_name::{closure#0}::inner_func_name");
+}
+
+TEST(DemangleRust, Shim) {
+  EXPECT_DEMANGLING(
+      "_RNSNvCs123_10crate_name9func_name6vtableCs456_12client_crate",
+      "crate_name::func_name::{shim:vtable#0}");
+}
+
+TEST(DemangleRust, UnknownUppercaseNamespace) {
+  EXPECT_DEMANGLING(
+      "_RNXNvCs123_10crate_name9func_name14mystery_objectCs456_12client_crate",
+      "crate_name::func_name::{X:mystery_object#0}");
+}
+
+TEST(DemangleRust, NestedUppercaseNamespaces) {
+  EXPECT_DEMANGLING(
+      "_RNCNXNYCs123_10crate_names0_1ys1_1xs2_0Cs456_12client_crate",
+      "crate_name::{Y:y#2}::{X:x#3}::{closure#4}");
+}
+
+TEST(DemangleRust, TraitDefinition) {
+  EXPECT_DEMANGLING(
+      "_RNvYNtC7crate_a9my_structNtC7crate_b8my_trait1f",
+      "<crate_a::my_struct as crate_b::my_trait>::f");
+}
+
+TEST(DemangleRust, BasicTypeNames) {
+  EXPECT_DEMANGLING("_RNvYaNtC1c1t1f", "<i8 as c::t>::f");
+  EXPECT_DEMANGLING("_RNvYbNtC1c1t1f", "<bool as c::t>::f");
+  EXPECT_DEMANGLING("_RNvYcNtC1c1t1f", "<char as c::t>::f");
+  EXPECT_DEMANGLING("_RNvYdNtC1c1t1f", "<f64 as c::t>::f");
+  EXPECT_DEMANGLING("_RNvYeNtC1c1t1f", "<str as c::t>::f");
+  EXPECT_DEMANGLING("_RNvYfNtC1c1t1f", "<f32 as c::t>::f");
+  EXPECT_DEMANGLING("_RNvYhNtC1c1t1f", "<u8 as c::t>::f");
+  EXPECT_DEMANGLING("_RNvYiNtC1c1t1f", "<isize as c::t>::f");
+  EXPECT_DEMANGLING("_RNvYjNtC1c1t1f", "<usize as c::t>::f");
+  EXPECT_DEMANGLING("_RNvYlNtC1c1t1f", "<i32 as c::t>::f");
+  EXPECT_DEMANGLING("_RNvYmNtC1c1t1f", "<u32 as c::t>::f");
+  EXPECT_DEMANGLING("_RNvYnNtC1c1t1f", "<i128 as c::t>::f");
+  EXPECT_DEMANGLING("_RNvYoNtC1c1t1f", "<u128 as c::t>::f");
+  EXPECT_DEMANGLING("_RNvYpNtC1c1t1f", "<_ as c::t>::f");
+  EXPECT_DEMANGLING("_RNvYsNtC1c1t1f", "<i16 as c::t>::f");
+  EXPECT_DEMANGLING("_RNvYtNtC1c1t1f", "<u16 as c::t>::f");
+  EXPECT_DEMANGLING("_RNvYuNtC1c1t1f", "<() as c::t>::f");
+  EXPECT_DEMANGLING("_RNvYvNtC1c1t1f", "<... as c::t>::f");
+  EXPECT_DEMANGLING("_RNvYxNtC1c1t1f", "<i64 as c::t>::f");
+  EXPECT_DEMANGLING("_RNvYyNtC1c1t1f", "<u64 as c::t>::f");
+  EXPECT_DEMANGLING("_RNvYzNtC1c1t1f", "<! as c::t>::f");
+
+  EXPECT_DEMANGLING_FAILS("_RNvYkNtC1c1t1f");
+}
+
+TEST(DemangleRust, SliceTypes) {
+  EXPECT_DEMANGLING("_RNvYSlNtC1c1t1f", "<[i32] as c::t>::f");
+  EXPECT_DEMANGLING("_RNvYSNtC1d1sNtC1c1t1f", "<[d::s] as c::t>::f");
+}
+
+TEST(DemangleRust, ImmutableReferenceTypes) {
+  EXPECT_DEMANGLING("_RNvYRlNtC1c1t1f", "<&i32 as c::t>::f");
+  EXPECT_DEMANGLING("_RNvYRNtC1d1sNtC1c1t1f", "<&d::s as c::t>::f");
+}
+
+TEST(DemangleRust, MutableReferenceTypes) {
+  EXPECT_DEMANGLING("_RNvYQlNtC1c1t1f", "<&mut i32 as c::t>::f");
+  EXPECT_DEMANGLING("_RNvYQNtC1d1sNtC1c1t1f", "<&mut d::s as c::t>::f");
+}
+
+TEST(DemangleRust, ConstantRawPointerTypes) {
+  EXPECT_DEMANGLING("_RNvYPlNtC1c1t1f", "<*const i32 as c::t>::f");
+  EXPECT_DEMANGLING("_RNvYPNtC1d1sNtC1c1t1f", "<*const d::s as c::t>::f");
+}
+
+TEST(DemangleRust, MutableRawPointerTypes) {
+  EXPECT_DEMANGLING("_RNvYOlNtC1c1t1f", "<*mut i32 as c::t>::f");
+  EXPECT_DEMANGLING("_RNvYONtC1d1sNtC1c1t1f", "<*mut d::s as c::t>::f");
+}
+
+TEST(DemangleRust, TupleLength0) {
+  EXPECT_DEMANGLING("_RNvYTENtC1c1t1f", "<() as c::t>::f");
+}
+
+TEST(DemangleRust, TupleLength1) {
+  EXPECT_DEMANGLING("_RNvYTlENtC1c1t1f", "<(i32,) as c::t>::f");
+  EXPECT_DEMANGLING("_RNvYTNtC1d1sENtC1c1t1f", "<(d::s,) as c::t>::f");
+}
+
+TEST(DemangleRust, TupleLength2) {
+  EXPECT_DEMANGLING("_RNvYTlmENtC1c1t1f", "<(i32, u32) as c::t>::f");
+  EXPECT_DEMANGLING("_RNvYTNtC1d1xNtC1e1yENtC1c1t1f",
+                    "<(d::x, e::y) as c::t>::f");
+}
+
+TEST(DemangleRust, TupleLength3) {
+  EXPECT_DEMANGLING("_RNvYTlmnENtC1c1t1f", "<(i32, u32, i128) as c::t>::f");
+  EXPECT_DEMANGLING("_RNvYTNtC1d1xNtC1e1yNtC1f1zENtC1c1t1f",
+                    "<(d::x, e::y, f::z) as c::t>::f");
+}
+
+TEST(DemangleRust, LongerTuplesAbbreviated) {
+  EXPECT_DEMANGLING("_RNvYTlmnoENtC1c1t1f",
+                    "<(i32, u32, i128, ...) as c::t>::f");
+  EXPECT_DEMANGLING("_RNvYTlmnNtC1d1xNtC1e1yENtC1c1t1f",
+                    "<(i32, u32, i128, ...) as c::t>::f");
+}
+
+TEST(DemangleRust, PathBackrefToCrate) {
+  EXPECT_DEMANGLING("_RNvYNtC8my_crate9my_structNtB4_8my_trait1f",
+                    "<my_crate::my_struct as my_crate::my_trait>::f");
+}
+
+TEST(DemangleRust, PathBackrefToNestedPath) {
+  EXPECT_DEMANGLING("_RNvYNtNtC1c1m1sNtB4_1t1f", "<c::m::s as c::m::t>::f");
+}
+
+TEST(DemangleRust, PathBackrefAsInstantiatingCrate) {
+  EXPECT_DEMANGLING("_RNCNvC8my_crate7my_func0B3_",
+                    "my_crate::my_func::{closure#0}");
+}
+
+TEST(DemangleRust, TypeBackrefsNestedInTuple) {
+  EXPECT_DEMANGLING("_RNvYTTRlB4_ERB3_ENtC1c1t1f",
+                    "<((&i32, &i32), &(&i32, &i32)) as c::t>::f");
+}
+
+TEST(DemangleRust, NoInfiniteLoopOnBackrefToTheWhole) {
+  EXPECT_DEMANGLING_FAILS("_RB_");
+  EXPECT_DEMANGLING_FAILS("_RNvB_1sNtC1c1t1f");
+}
+
+TEST(DemangleRust, NoCrashOnForwardBackref) {
+  EXPECT_DEMANGLING_FAILS("_RB0_");
+  EXPECT_DEMANGLING_FAILS("_RB1_");
+  EXPECT_DEMANGLING_FAILS("_RB2_");
+  EXPECT_DEMANGLING_FAILS("_RB3_");
+  EXPECT_DEMANGLING_FAILS("_RB4_");
+}
+
+TEST(DemangleRust, PathBackrefsDoNotRecurseDuringSilence) {
+  // B_ points at the value f (the whole mangling), so the cycle would lead to
+  // parse failure if the parser tried to parse what was pointed to.
+  EXPECT_DEMANGLING("_RNvYTlmnNtB_1sENtC1c1t1f",
+                    "<(i32, u32, i128, ...) as c::t>::f");
+}
+
+TEST(DemangleRust, TypeBackrefsDoNotRecurseDuringSilence) {
+  // B2_ points at the tuple type, likewise making a cycle that the parser
+  // avoids following.
+  EXPECT_DEMANGLING("_RNvYTlmnB2_ENtC1c1t1f",
+                    "<(i32, u32, i128, ...) as c::t>::f");
+}
+
+TEST(DemangleRust, ConstBackrefsDoNotRecurseDuringSilence) {
+  // B_ points at the whole I...E mangling, which does not parse as a const.
+  EXPECT_DEMANGLING("_RINvC1c1fAlB_E", "c::f::<>");
+}
+
+TEST(DemangleRust, ReturnFromBackrefToInputPosition256) {
+  // Show that we can resume at input positions that don't fit into a byte.
+  EXPECT_DEMANGLING("_RNvYNtC1c238very_long_type_"
+                    "ABCDEFGHIJabcdefghijABCDEFGHIJabcdefghij"
+                    "ABCDEFGHIJabcdefghijABCDEFGHIJabcdefghij"
+                    "ABCDEFGHIJabcdefghijABCDEFGHIJabcdefghij"
+                    "ABCDEFGHIJabcdefghijABCDEFGHIJabcdefghij"
+                    "ABCDEFGHIJabcdefghijABCDEFGHIJabcdefghij"
+                    "ABCDEFGHIJabcdefghijABC"
+                    "NtB4_1t1f",
+                    "<c::very_long_type_"
+                    "ABCDEFGHIJabcdefghijABCDEFGHIJabcdefghij"
+                    "ABCDEFGHIJabcdefghijABCDEFGHIJabcdefghij"
+                    "ABCDEFGHIJabcdefghijABCDEFGHIJabcdefghij"
+                    "ABCDEFGHIJabcdefghijABCDEFGHIJabcdefghij"
+                    "ABCDEFGHIJabcdefghijABCDEFGHIJabcdefghij"
+                    "ABCDEFGHIJabcdefghijABC"
+                    " as c::t>::f");
+}
+
+TEST(DemangleRust, EmptyGenericArgs) {
+  EXPECT_DEMANGLING("_RINvC1c1fE", "c::f::<>");
+}
+
+TEST(DemangleRust, OneSimpleTypeInGenericArgs) {
+  EXPECT_DEMANGLING("_RINvC1c1flE",  // c::f::<i32>
+                    "c::f::<>");
+}
+
+TEST(DemangleRust, OneTupleInGenericArgs) {
+  EXPECT_DEMANGLING("_RINvC1c1fTlmEE",  // c::f::<(i32, u32)>
+                    "c::f::<>");
+}
+
+TEST(DemangleRust, OnePathInGenericArgs) {
+  EXPECT_DEMANGLING("_RINvC1c1fNtC1d1sE",  // c::f::<d::s>
+                    "c::f::<>");
+}
+
+TEST(DemangleRust, LongerGenericArgs) {
+  EXPECT_DEMANGLING("_RINvC1c1flmRNtC1d1sE",  // c::f::<i32, u32, &d::s>
+                    "c::f::<>");
+}
+
+TEST(DemangleRust, BackrefInGenericArgs) {
+  EXPECT_DEMANGLING("_RINvC1c1fRlB7_NtB2_1sE",  // c::f::<&i32, &i32, c::s>
+                    "c::f::<>");
+}
+
+TEST(DemangleRust, NestedGenericArgs) {
+  EXPECT_DEMANGLING("_RINvC1c1fINtB2_1slEmE",  // c::f::<c::s::<i32>, u32>
+                    "c::f::<>");
+}
+
+TEST(DemangleRust, MonomorphicEntityNestedInsideGeneric) {
+  EXPECT_DEMANGLING("_RNvINvC1c1fppE1g",  // c::f::<_, _>::g
+                    "c::f::<>::g");
+}
+
+TEST(DemangleRust, ArrayTypeWithSimpleElementType) {
+  EXPECT_DEMANGLING("_RNvYAlj1f_NtC1c1t1f", "<[i32; 0x1f] as c::t>::f");
+}
+
+TEST(DemangleRust, ArrayTypeWithComplexElementType) {
+  EXPECT_DEMANGLING("_RNvYAINtC1c1slEj1f_NtB6_1t1f",
+                    "<[c::s::<>; 0x1f] as c::t>::f");
+}
+
+TEST(DemangleRust, NestedArrayType) {
+  EXPECT_DEMANGLING("_RNvYAAlj1f_j2e_NtC1c1t1f",
+                    "<[[i32; 0x1f]; 0x2e] as c::t>::f");
+}
+
+TEST(DemangleRust, BackrefArraySize) {
+  EXPECT_DEMANGLING("_RNvYAAlj1f_B5_NtC1c1t1f",
+                    "<[[i32; 0x1f]; 0x1f] as c::t>::f");
+}
+
+TEST(DemangleRust, ZeroArraySize) {
+  EXPECT_DEMANGLING("_RNvYAlj0_NtC1c1t1f", "<[i32; 0x0] as c::t>::f");
+}
+
+TEST(DemangleRust, SurprisingMinusesInArraySize) {
+  // Compilers shouldn't do this stuff, but existing demanglers accept it.
+  EXPECT_DEMANGLING("_RNvYAljn0_NtC1c1t1f", "<[i32; -0x0] as c::t>::f");
+  EXPECT_DEMANGLING("_RNvYAljn42_NtC1c1t1f", "<[i32; -0x42] as c::t>::f");
+}
+
+TEST(DemangleRust, NumberAsGenericArg) {
+  EXPECT_DEMANGLING("_RINvC1c1fKl8_E",  // c::f::<0x8>
+                    "c::f::<>");
+}
+
+TEST(DemangleRust, NumberAsFirstOfTwoGenericArgs) {
+  EXPECT_DEMANGLING("_RINvC1c1fKl8_mE",  // c::f::<0x8, u32>
+                    "c::f::<>");
+}
+
+TEST(DemangleRust, NumberAsSecondOfTwoGenericArgs) {
+  EXPECT_DEMANGLING("_RINvC1c1fmKl8_E",  // c::f::<u32, 0x8>
+                    "c::f::<>");
+}
+
+TEST(DemangleRust, NumberPlaceholder) {
+  EXPECT_DEMANGLING("_RNvINvC1c1fKpE1g",  // c::f::<_>::g
+                    "c::f::<>::g");
+}
+
+TEST(DemangleRust, InherentImplWithoutDisambiguator) {
+  EXPECT_DEMANGLING("_RNvMNtC8my_crate6my_modNtB2_9my_struct7my_func",
+                    "<my_crate::my_mod::my_struct>::my_func");
+}
+
+TEST(DemangleRust, InherentImplWithDisambiguator) {
+  EXPECT_DEMANGLING("_RNvMs_NtC8my_crate6my_modNtB4_9my_struct7my_func",
+                    "<my_crate::my_mod::my_struct>::my_func");
+}
+
+TEST(DemangleRust, TraitImplWithoutDisambiguator) {
+  EXPECT_DEMANGLING("_RNvXC8my_crateNtB2_9my_structNtB2_8my_trait7my_func",
+                    "<my_crate::my_struct as my_crate::my_trait>::my_func");
+}
+
+TEST(DemangleRust, TraitImplWithDisambiguator) {
+  EXPECT_DEMANGLING("_RNvXs_C8my_crateNtB4_9my_structNtB4_8my_trait7my_func",
+                    "<my_crate::my_struct as my_crate::my_trait>::my_func");
+}
+
+TEST(DemangleRust, TraitImplWithNonpathSelfType) {
+  EXPECT_DEMANGLING("_RNvXC8my_crateRlNtB2_8my_trait7my_func",
+                    "<&i32 as my_crate::my_trait>::my_func");
+}
+
+TEST(DemangleRust, ThunkType) {
+  EXPECT_DEMANGLING("_RNvYFEuNtC1c1t1f",  // <fn() as c::t>::f
+                    "<fn... as c::t>::f");
+}
+
+TEST(DemangleRust, NontrivialFunctionReturnType) {
+  EXPECT_DEMANGLING(
+      "_RNvYFERTlmENtC1c1t1f",  // <fn() -> &(i32, u32) as c::t>::f
+      "<fn... as c::t>::f");
+}
+
+TEST(DemangleRust, OneParameterType) {
+  EXPECT_DEMANGLING("_RNvYFlEuNtC1c1t1f",  // <fn(i32) as c::t>::f
+                    "<fn... as c::t>::f");
+}
+
+TEST(DemangleRust, TwoParameterTypes) {
+  EXPECT_DEMANGLING("_RNvYFlmEuNtC1c1t1f",  // <fn(i32, u32) as c::t>::f
+                    "<fn... as c::t>::f");
+}
+
+TEST(DemangleRust, ExternC) {
+  EXPECT_DEMANGLING("_RNvYFKCEuNtC1c1t1f",  // <extern "C" fn() as c::t>>::f
+                    "<fn... as c::t>::f");
+}
+
+TEST(DemangleRust, ExternOther) {
+  EXPECT_DEMANGLING(
+      "_RNvYFK5not_CEuNtC1c1t1f",  // <extern "not-C" fn() as c::t>::f
+      "<fn... as c::t>::f");
+}
+
+TEST(DemangleRust, ExternPunycodedAbiIsSuppressed) {
+  // The abi is part of the silenced function signature, so a punycoded abi must
+  // be suppressed like any other identifier; its decoded form must not leak
+  // into the output.
+  EXPECT_DEMANGLING(
+      // <extern "Eyjafjallajökull" fn() as c::t>::f
+      "_RNvYFKu19Eyjafjallajkull_jtbEuNtC1c1t1f", "<fn... as c::t>::f");
+}
+
+TEST(DemangleRust, Unsafe) {
+  EXPECT_DEMANGLING("_RNvYFUEuNtC1c1t1f",  // <unsafe fn() as c::t>::f
+                    "<fn... as c::t>::f");
+}
+
+TEST(DemangleRust, Binder) {
+  EXPECT_DEMANGLING(
+      // <for<'a> fn(&'a i32) -> &'a i32 as c::t>::f
+      "_RNvYFG_RL0_lEB5_NtC1c1t1f",
+      "<fn... as c::t>::f");
+}
+
+TEST(DemangleRust, AllFnSigFeaturesInOrder) {
+  EXPECT_DEMANGLING(
+      // <for<'a> unsafe extern "C" fn(&'a i32) -> &'a i32 as c::t>::f
+      "_RNvYFG_UKCRL0_lEB8_NtC1c1t1f",
+      "<fn... as c::t>::f");
+}
+
+TEST(DemangleRust, LifetimeInGenericArgs) {
+  EXPECT_DEMANGLING("_RINvC1c1fINtB2_1sL_EE",  // c::f::<c::s::<'_>>
+                    "c::f::<>");
+}
+
+TEST(DemangleRust, EmptyDynTrait) {
+  // This shouldn't happen, but the grammar allows it and existing demanglers
+  // accept it.
+  EXPECT_DEMANGLING("_RNvYDEL_NtC1c1t1f",
+                    "<dyn  as c::t>::f");
+}
+
+TEST(DemangleRust, SimpleDynTrait) {
+  EXPECT_DEMANGLING("_RNvYDNtC1c1tEL_NtC1d1u1f",
+                    "<dyn c::t as d::u>::f");
+}
+
+TEST(DemangleRust, DynTraitWithOneAssociatedType) {
+  EXPECT_DEMANGLING(
+      "_RNvYDNtC1c1tp1xlEL_NtC1d1u1f",  // <dyn c::t<x = i32> as d::u>::f
+      "<dyn c::t<> as d::u>::f");
+}
+
+TEST(DemangleRust, DynTraitWithTwoAssociatedTypes) {
+  EXPECT_DEMANGLING(
+      // <dyn c::t<x = i32, y = u32> as d::u>::f
+      "_RNvYDNtC1c1tp1xlp1ymEL_NtC1d1u1f",
+      "<dyn c::t<> as d::u>::f");
+}
+
+TEST(DemangleRust, DynTraitPlusAutoTrait) {
+  EXPECT_DEMANGLING(
+      "_RNvYDNtC1c1tNtNtC3std6marker4SendEL_NtC1d1u1f",
+      "<dyn c::t + std::marker::Send as d::u>::f");
+}
+
+TEST(DemangleRust, DynTraitPlusTwoAutoTraits) {
+  EXPECT_DEMANGLING(
+      "_RNvYDNtC1c1tNtNtC3std6marker4CopyNtBc_4SyncEL_NtC1d1u1f",
+      "<dyn c::t + std::marker::Copy + std::marker::Sync as d::u>::f");
+}
+
+TEST(DemangleRust, HigherRankedDynTrait) {
+  EXPECT_DEMANGLING(
+      // <dyn for<'a> c::t::<&'a i32> as d::u>::f
+      "_RNvYDG_INtC1c1tRL0_lEEL_NtC1d1u1f",
+      "<dyn c::t::<> as d::u>::f");
+}
+
+}  // namespace
+}  // namespace debugging_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/debugging/internal/demangle_test.cc b/absl/debugging/internal/demangle_test.cc
index a68ce3249bc..3023c5c834d 100644
--- a/absl/debugging/internal/demangle_test.cc
+++ b/absl/debugging/internal/demangle_test.cc
@@ -14,29 +14,523 @@
 
 #include "absl/debugging/internal/demangle.h"
 
+#include <array>
 #include <cstdlib>
+#include <memory>
 #include <string>
 
+#include "gmock/gmock.h"
 #include "gtest/gtest.h"
-#include "absl/base/internal/raw_logging.h"
+#include "absl/base/config.h"
 #include "absl/debugging/internal/stack_consumption.h"
+#include "absl/log/log.h"
 #include "absl/memory/memory.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace debugging_internal {
 namespace {
 
-// A wrapper function for Demangle() to make the unit test simple.
-static const char *DemangleIt(const char * const mangled) {
-  static char demangled[4096];
-  if (Demangle(mangled, demangled, sizeof(demangled))) {
-    return demangled;
-  } else {
-    return mangled;
-  }
+using ::testing::Contains;
+using ::testing::ContainsRegex;
+
+TEST(Demangle, FunctionTemplate) {
+  char tmp[100];
+
+  // template <typename T>
+  // int foo(T);
+  //
+  // foo<int>(5);
+  ASSERT_TRUE(Demangle("_Z3fooIiEiT_", tmp, sizeof(tmp)));
+  EXPECT_STREQ(tmp, "foo<>()");
+}
+
+TEST(Demangle, FunctionTemplateWithNesting) {
+  char tmp[100];
+
+  // template <typename T>
+  // int foo(T);
+  //
+  // foo<Wrapper<int>>({ .value = 5 });
+  ASSERT_TRUE(Demangle("_Z3fooI7WrapperIiEEiT_", tmp, sizeof(tmp)));
+  EXPECT_STREQ(tmp, "foo<>()");
+}
+
+TEST(Demangle, FunctionTemplateWithNonTypeParamConstraint) {
+  char tmp[100];
+
+  // template <std::integral T>
+  // int foo(T);
+  //
+  // foo<int>(5);
+  ASSERT_TRUE(Demangle("_Z3fooITkSt8integraliEiT_", tmp, sizeof(tmp)));
+  EXPECT_STREQ(tmp, "foo<>()");
+}
+
+TEST(Demangle, FunctionTemplateWithFunctionRequiresClause) {
+  char tmp[100];
+
+  // template <typename T>
+  // int foo() requires std::integral<T>;
+  //
+  // foo<int>();
+  ASSERT_TRUE(Demangle("_Z3fooIiEivQsr3stdE8integralIT_E", tmp, sizeof(tmp)));
+  EXPECT_STREQ(tmp, "foo<>()");
+}
+
+TEST(Demangle, FunctionWithTemplateParamRequiresClause) {
+  char tmp[100];
+
+  // template <typename T>
+  //     requires std::integral<T>
+  // int foo();
+  //
+  // foo<int>();
+  ASSERT_TRUE(Demangle("_Z3fooIiQsr3stdE8integralIT_EEiv", tmp, sizeof(tmp)));
+  EXPECT_STREQ(tmp, "foo<>()");
+}
+
+TEST(Demangle, FunctionWithTemplateParamAndFunctionRequiresClauses) {
+  char tmp[100];
+
+  // template <typename T>
+  //     requires std::integral<T>
+  // int foo() requires std::integral<T>;
+  //
+  // foo<int>();
+  ASSERT_TRUE(Demangle("_Z3fooIiQsr3stdE8integralIT_EEivQsr3stdE8integralIS0_E",
+                       tmp, sizeof(tmp)));
+  EXPECT_STREQ(tmp, "foo<>()");
+}
+
+TEST(Demangle, FunctionTemplateBacktracksOnMalformedRequiresClause) {
+  char tmp[100];
+
+  // template <typename T>
+  // int foo(T);
+  //
+  // foo<int>(5);
+  // Except there's an extra `Q` where the mangled requires clause would be.
+  ASSERT_FALSE(Demangle("_Z3fooIiQEiT_", tmp, sizeof(tmp)));
+}
+
+TEST(Demangle, FunctionTemplateWithAutoParam) {
+  char tmp[100];
+
+  // template <auto>
+  // void foo();
+  //
+  // foo<1>();
+  ASSERT_TRUE(Demangle("_Z3fooITnDaLi1EEvv", tmp, sizeof(tmp)));
+  EXPECT_STREQ(tmp, "foo<>()");
+}
+
+TEST(Demangle, FunctionTemplateWithNonTypeParamPack) {
+  char tmp[100];
+
+  // template <int&..., typename T>
+  // void foo(T);
+  //
+  // foo(2);
+  ASSERT_TRUE(Demangle("_Z3fooITpTnRiJEiEvT0_", tmp, sizeof(tmp)));
+  EXPECT_STREQ(tmp, "foo<>()");
+}
+
+TEST(Demangle, FunctionTemplateTemplateParamWithConstrainedArg) {
+  char tmp[100];
+
+  // template <typename T>
+  // concept True = true;
+  //
+  // template <typename T> requires True<T>
+  // struct Fooer {};
+  //
+  // template <template <typename T> typename>
+  // void foo() {}
+  //
+  // foo<Fooer>();
+  ASSERT_TRUE(Demangle("_Z3fooITtTyE5FooerEvv", tmp, sizeof(tmp)));
+  EXPECT_STREQ(tmp, "foo<>()");
+}
+
+TEST(Demangle, ConstrainedAutoInFunctionTemplate) {
+  char tmp[100];
+
+  // template <typename T> concept C = true;
+  // template <C auto N> void f() {}
+  // template void f<0>();
+  ASSERT_TRUE(Demangle("_Z1fITnDk1CLi0EEvv", tmp, sizeof(tmp)));
+  EXPECT_STREQ(tmp, "f<>()");
+}
+
+TEST(Demangle, ConstrainedFriendFunctionTemplate) {
+  char tmp[100];
+
+  // Source:
+  //
+  // namespace ns {
+  // template <class T> struct Y {
+  //   friend void y(Y) requires true {}
+  // };
+  // }  // namespace ns
+  //
+  // y(ns::Y<int>{});
+  //
+  // LLVM demangling:
+  //
+  // ns::Y<int>::friend y(ns::Y<int>) requires true
+  ASSERT_TRUE(Demangle("_ZN2ns1YIiEF1yES1_QLb1E", tmp, sizeof(tmp)));
+  EXPECT_STREQ(tmp, "ns::Y<>::friend y()");
+}
+
+TEST(Demangle, ConstrainedFriendOperatorTemplate) {
+  char tmp[100];
+
+  // ns::Y<int>::friend operator*(ns::Y<int>) requires true
+  ASSERT_TRUE(Demangle("_ZN2ns1YIiEFdeES1_QLb1E", tmp, sizeof(tmp)));
+  EXPECT_STREQ(tmp, "ns::Y<>::friend operator*()");
+}
+
+TEST(Demangle, NonTemplateBuiltinType) {
+  char tmp[100];
+
+  // void foo(__my_builtin_type t);
+  //
+  // foo({});
+  ASSERT_TRUE(Demangle("_Z3foou17__my_builtin_type", tmp, sizeof(tmp)));
+  EXPECT_STREQ(tmp, "foo()");
+}
+
+TEST(Demangle, SingleArgTemplateBuiltinType) {
+  char tmp[100];
+
+  // template <typename T>
+  // __my_builtin_type<T> foo();
+  //
+  // foo<int>();
+  ASSERT_TRUE(Demangle("_Z3fooIiEu17__my_builtin_typeIT_Ev", tmp, sizeof(tmp)));
+  EXPECT_STREQ(tmp, "foo<>()");
+}
+
+TEST(Demangle, TwoArgTemplateBuiltinType) {
+  char tmp[100];
+
+  // template <typename T, typename U>
+  // __my_builtin_type<T, U> foo();
+  //
+  // foo<int, char>();
+  ASSERT_TRUE(
+      Demangle("_Z3fooIicEu17__my_builtin_typeIT_T0_Ev", tmp, sizeof(tmp)));
+  EXPECT_STREQ(tmp, "foo<>()");
+}
+
+TEST(Demangle, TypeNestedUnderTemplatedBuiltinType) {
+  char tmp[100];
+
+  // Source:
+  //
+  // template <typename T>
+  // typename std::remove_reference_t<T>::type f(T t);
+  //
+  // struct C { using type = C; };
+  //
+  // f<const C&>(C{});
+  //
+  // These days std::remove_reference_t is implemented in terms of a vendor
+  // builtin __remove_reference_t.  A full demangling might look like:
+  //
+  // __remove_reference_t<C const&>::type f<C const&>(C const&)
+  ASSERT_TRUE(Demangle("_Z1fIRK1CENu20__remove_reference_tIT_E4typeES3_",
+                       tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
+TEST(Demangle, TemplateTemplateParamSubstitution) {
+  char tmp[100];
+
+  // template <typename T>
+  // concept True = true;
+  //
+  // template<std::integral T, T> struct Foolable {};
+  // template<template<typename T, T> typename> void foo() {}
+  //
+  // template void foo<Foolable>();
+  ASSERT_TRUE(Demangle("_Z3fooITtTyTnTL0__E8FoolableEvv", tmp, sizeof(tmp)));
+  EXPECT_STREQ(tmp, "foo<>()");
 }
 
-// Test corner cases of bounary conditions.
+TEST(Demangle, TemplateParamSubstitutionWithGenericLambda) {
+  char tmp[100];
+
+  // template <typename>
+  // struct Fooer {
+  //     template <typename>
+  //     void foo(decltype([](auto x, auto y) {})) {}
+  // };
+  //
+  // Fooer<int> f;
+  // f.foo<int>({});
+  ASSERT_TRUE(
+      Demangle("_ZN5FooerIiE3fooIiEEvNS0_UlTL0__TL0_0_E_E", tmp, sizeof(tmp)));
+  EXPECT_STREQ(tmp, "Fooer<>::foo<>()");
+}
+
+TEST(Demangle, LambdaRequiresTrue) {
+  char tmp[100];
+
+  // auto $_0::operator()<int>(int) const requires true
+  ASSERT_TRUE(Demangle("_ZNK3$_0clIiEEDaT_QLb1E", tmp, sizeof(tmp)));
+  EXPECT_STREQ(tmp, "$_0::operator()<>()");
+}
+
+TEST(Demangle, LambdaRequiresSimpleExpression) {
+  char tmp[100];
+
+  // auto $_0::operator()<int>(int) const requires 2 + 2 == 4
+  ASSERT_TRUE(Demangle("_ZNK3$_0clIiEEDaT_QeqplLi2ELi2ELi4E",
+                       tmp, sizeof(tmp)));
+  EXPECT_STREQ(tmp, "$_0::operator()<>()");
+}
+
+TEST(Demangle, LambdaRequiresRequiresExpressionContainingTrue) {
+  char tmp[100];
+
+  // auto $_0::operator()<int>(int) const requires requires { true; }
+  ASSERT_TRUE(Demangle("_ZNK3$_0clIiEEDaT_QrqXLb1EE", tmp, sizeof(tmp)));
+  EXPECT_STREQ(tmp, "$_0::operator()<>()");
+}
+
+TEST(Demangle, LambdaRequiresRequiresExpressionContainingConcept) {
+  char tmp[100];
+
+  // auto $_0::operator()<int>(int) const
+  // requires requires { std::same_as<decltype(fp), int>; }
+  ASSERT_TRUE(Demangle("_ZNK3$_0clIiEEDaT_QrqXsr3stdE7same_asIDtfp_EiEE",
+                       tmp, sizeof(tmp)));
+  EXPECT_STREQ(tmp, "$_0::operator()<>()");
+}
+
+TEST(Demangle, LambdaRequiresRequiresExpressionContainingNoexceptExpression) {
+  char tmp[100];
+
+  // auto $_0::operator()<int>(int) const
+  // requires requires { {fp + fp} noexcept; }
+  ASSERT_TRUE(Demangle("_ZNK3$_0clIiEEDaT_QrqXplfp_fp_NE", tmp, sizeof(tmp)));
+  EXPECT_STREQ(tmp, "$_0::operator()<>()");
+}
+
+TEST(Demangle, LambdaRequiresRequiresExpressionContainingReturnTypeConstraint) {
+  char tmp[100];
+
+  // auto $_0::operator()<int>(int) const
+  // requires requires { {fp + fp} -> std::same_as<decltype(fp)>; }
+  ASSERT_TRUE(Demangle("_ZNK3$_0clIiEEDaT_QrqXplfp_fp_RNSt7same_asIDtfp_EEEE",
+                       tmp, sizeof(tmp)));
+  EXPECT_STREQ(tmp, "$_0::operator()<>()");
+}
+
+TEST(Demangle, LambdaRequiresRequiresExpressionWithBothNoexceptAndReturnType) {
+  char tmp[100];
+
+  // auto $_0::operator()<int>(int) const
+  // requires requires { {fp + fp} noexcept -> std::same_as<decltype(fp)>; }
+  ASSERT_TRUE(Demangle("_ZNK3$_0clIiEEDaT_QrqXplfp_fp_NRNSt7same_asIDtfp_EEEE",
+                       tmp, sizeof(tmp)));
+  EXPECT_STREQ(tmp, "$_0::operator()<>()");
+}
+
+TEST(Demangle, LambdaRequiresRequiresExpressionContainingType) {
+  char tmp[100];
+
+  // auto $_0::operator()<S>(S) const
+  // requires requires { typename S::T; }
+  ASSERT_TRUE(Demangle("_ZNK3$_0clI1SEEDaT_QrqTNS2_1TEE", tmp, sizeof(tmp)));
+  EXPECT_STREQ(tmp, "$_0::operator()<>()");
+}
+
+TEST(Demangle, LambdaRequiresRequiresExpressionNestingAnotherRequires) {
+  char tmp[100];
+
+  // auto $_0::operator()<int>(int) const requires requires { requires true; }
+  ASSERT_TRUE(Demangle("_ZNK3$_0clIiEEDaT_QrqQLb1EE", tmp, sizeof(tmp)));
+  EXPECT_STREQ(tmp, "$_0::operator()<>()");
+}
+
+TEST(Demangle, LambdaRequiresRequiresExpressionContainingTwoRequirements) {
+  char tmp[100];
+
+  // auto $_0::operator()<int>(int) const
+  // requires requires { requires true; requires 2 + 2 == 4; }
+  ASSERT_TRUE(Demangle("_ZNK3$_0clIiEEDaT_QrqXLb1EXeqplLi2ELi2ELi4EE",
+                       tmp, sizeof(tmp)));
+  EXPECT_STREQ(tmp, "$_0::operator()<>()");
+}
+
+TEST(Demangle, RequiresExpressionWithItsOwnParameter) {
+  char tmp[100];
+
+  // S<requires (int) { fp + fp; }> f<int>(int)
+  ASSERT_TRUE(Demangle("_Z1fIiE1SIXrQT__XplfL0p_fp_EEES1_", tmp, sizeof(tmp)));
+  EXPECT_STREQ(tmp, "f<>()");
+}
+
+TEST(Demangle, LambdaWithExplicitTypeArgument) {
+  char tmp[100];
+
+  // Source:
+  //
+  // template <class T> T f(T t) {
+  //   return []<class U>(U u) { return u + u; }(t);
+  // }
+  //
+  // template int f<int>(int);
+  //
+  // Full LLVM demangling of the lambda call operator:
+  //
+  // auto int f<int>(int)::'lambda'<typename $T>(int)::
+  // operator()<int>(int) const
+  ASSERT_TRUE(Demangle("_ZZ1fIiET_S0_ENKUlTyS0_E_clIiEEDaS0_",
+                       tmp, sizeof(tmp)));
+  EXPECT_STREQ(tmp, "f<>()::{lambda()#1}::operator()<>()");
+}
+
+TEST(Demangle, LambdaWithExplicitPackArgument) {
+  char tmp[100];
+
+  // Source:
+  //
+  // template <class T> T h(T t) {
+  //   return []<class... U>(U... u) {
+  //     return ((u + u) + ... + 0);
+  //   }(t);
+  // }
+  //
+  // template int h<int>(int);
+  //
+  // Full LLVM demangling of the lambda call operator:
+  //
+  // auto int f<int>(int)::'lambda'<typename ...$T>($T...)::
+  // operator()<int>($T...) const
+  ASSERT_TRUE(Demangle("_ZZ1fIiET_S0_ENKUlTpTyDpT_E_clIJiEEEDaS2_",
+                       tmp, sizeof(tmp)));
+  EXPECT_STREQ(tmp, "f<>()::{lambda()#1}::operator()<>()");
+}
+
+TEST(Demangle, LambdaInClassMemberDefaultArgument) {
+  char tmp[100];
+
+  // Source:
+  //
+  // struct S {
+  //   static auto f(void (*g)() = [] {}) { return g; }
+  // };
+  // void (*p)() = S::f();
+  //
+  // Full LLVM demangling of the lambda call operator:
+  //
+  // S::f(void (*)())::'lambda'()::operator()() const
+  //
+  // Full GNU binutils demangling:
+  //
+  // S::f(void (*)())::{default arg#1}::{lambda()#1}::operator()() const
+  ASSERT_TRUE(Demangle("_ZZN1S1fEPFvvEEd_NKUlvE_clEv", tmp, sizeof(tmp)));
+  EXPECT_STREQ(tmp, "S::f()::{default arg#1}::{lambda()#1}::operator()()");
+
+  // The same but in the second rightmost default argument.
+  ASSERT_TRUE(Demangle("_ZZN1S1fEPFvvEEd0_NKUlvE_clEv", tmp, sizeof(tmp)));
+  EXPECT_STREQ(tmp, "S::f()::{default arg#2}::{lambda()#1}::operator()()");
+
+  // Reject negative <(parameter) number> values.
+  ASSERT_FALSE(Demangle("_ZZN1S1fEPFvvEEdn1_NKUlvE_clEv", tmp, sizeof(tmp)));
+}
+
+TEST(Demangle, AvoidSignedOverflowForUnfortunateParameterNumbers) {
+  char tmp[100];
+
+  // Here <number> + 2 fits in an int, but just barely.  (We expect no such
+  // input in practice: real functions don't have billions of arguments.)
+  ASSERT_TRUE(Demangle("_ZZN1S1fEPFvvEEd2147483645_NKUlvE_clEv",
+                       tmp, sizeof(tmp)));
+  EXPECT_STREQ(tmp,
+               "S::f()::{default arg#2147483647}::{lambda()#1}::operator()()");
+
+  // Now <number> is an int, but <number> + 2 is not.
+  ASSERT_TRUE(Demangle("_ZZN1S1fEPFvvEEd2147483646_NKUlvE_clEv",
+                       tmp, sizeof(tmp)));
+  EXPECT_STREQ(tmp, "S::f()::{default arg#1}::{lambda()#1}::operator()()");
+
+  // <number> is the largest int.
+  ASSERT_TRUE(Demangle("_ZZN1S1fEPFvvEEd2147483647_NKUlvE_clEv",
+                       tmp, sizeof(tmp)));
+  EXPECT_STREQ(tmp, "S::f()::{default arg#1}::{lambda()#1}::operator()()");
+
+  // <number> itself does not fit into an int.  ParseNumber truncates the value
+  // to int, yielding a large negative number, which we strain out.
+  ASSERT_TRUE(Demangle("_ZZN1S1fEPFvvEEd2147483648_NKUlvE_clEv",
+                       tmp, sizeof(tmp)));
+  EXPECT_STREQ(tmp, "S::f()::{default arg#1}::{lambda()#1}::operator()()");
+}
+
+TEST(Demangle, NegativeUnnamedTypeNumbers) {
+  char tmp[100];
+
+  // An omitted <number> denotes index 1 and is left as the -1 sentinel.
+  ASSERT_TRUE(Demangle("_ZUt_", tmp, sizeof(tmp)));
+  EXPECT_STREQ(tmp, "{unnamed type#1}");
+  ASSERT_TRUE(Demangle("_ZUlvE_", tmp, sizeof(tmp)));
+  EXPECT_STREQ(tmp, "{lambda()#1}");
+
+  // Reject an explicitly negative <number>.  Left unstrained, <number> + 2 is
+  // negative, and MaybeAppendDecimal emits (val % 10) + '0' per digit, which
+  // for a negative val yields characters below '0'.
+  ASSERT_FALSE(Demangle("_ZUtn3_", tmp, sizeof(tmp)));
+  ASSERT_FALSE(Demangle("_ZUlvEn3_", tmp, sizeof(tmp)));
+
+  // ParseNumber truncates to int, so an in-range-looking <number> can also
+  // arrive negative.
+  ASSERT_FALSE(Demangle("_ZUt2147483648_", tmp, sizeof(tmp)));
+  ASSERT_FALSE(Demangle("_ZUlvE2147483648_", tmp, sizeof(tmp)));
+
+  // The largest <number> whose index still fits in an int is unaffected.
+  ASSERT_TRUE(Demangle("_ZUt2147483645_", tmp, sizeof(tmp)));
+  EXPECT_STREQ(tmp, "{unnamed type#2147483647}");
+}
+
+TEST(Demangle, SubstpackNotationForTroublesomeTemplatePack) {
+  char tmp[100];
+
+  // Source:
+  //
+  // template <template <class> class, template <class> class> struct B {};
+  //
+  // template <template <class> class... T> struct A {
+  //   template <template <class> class... U> void f(B<T, U>&&...) {}
+  // };
+  //
+  // template void A<>::f<>();
+  //
+  // LLVM can't demangle its own _SUBSTPACK_ notation.
+  ASSERT_TRUE(Demangle("_ZN1AIJEE1fIJEEEvDpO1BI_SUBSTPACK_T_E",
+                       tmp, sizeof(tmp)));
+  EXPECT_STREQ(tmp, "A<>::f<>()");
+}
+
+TEST(Demangle, TemplateTemplateParamAppearingAsBackrefFollowedByTemplateArgs) {
+  char tmp[100];
+
+  // Source:
+  //
+  // template <template <class> class C> struct W {
+  //   template <class T> static decltype(C<T>::m()) f() { return {}; }
+  // };
+  //
+  // template <class T> struct S { static int m() { return 0; } };
+  // template decltype(S<int>::m()) W<S>::f<int>();
+  ASSERT_TRUE(Demangle("_ZN1WI1SE1fIiEEDTclsrS0_IT_EE1mEEv", tmp, sizeof(tmp)));
+  EXPECT_STREQ(tmp, "W<>::f<>()");
+}
+
+// Test corner cases of boundary conditions.
 TEST(Demangle, CornerCases) {
   char tmp[10];
   EXPECT_TRUE(Demangle("_Z6foobarv", tmp, sizeof(tmp)));
@@ -68,22 +562,1405 @@ TEST(Demangle, Clones) {
   EXPECT_STREQ("Foo()", tmp);
   EXPECT_TRUE(Demangle("_ZL3Foov.isra.2.constprop.18", tmp, sizeof(tmp)));
   EXPECT_STREQ("Foo()", tmp);
-  // Invalid (truncated), should not demangle.
-  EXPECT_FALSE(Demangle("_ZL3Foov.clo", tmp, sizeof(tmp)));
-  // Invalid (.clone. not followed by number), should not demangle.
-  EXPECT_FALSE(Demangle("_ZL3Foov.clone.", tmp, sizeof(tmp)));
-  // Invalid (.clone. followed by non-number), should not demangle.
-  EXPECT_FALSE(Demangle("_ZL3Foov.clone.foo", tmp, sizeof(tmp)));
-  // Invalid (.constprop. not followed by number), should not demangle.
-  EXPECT_FALSE(Demangle("_ZL3Foov.isra.2.constprop.", tmp, sizeof(tmp)));
+  // Demangle suffixes produced by -funique-internal-linkage-names.
+  EXPECT_TRUE(Demangle("_ZL3Foov.__uniq.12345", tmp, sizeof(tmp)));
+  EXPECT_STREQ("Foo()", tmp);
+  EXPECT_TRUE(Demangle("_ZL3Foov.__uniq.12345.isra.2.constprop.18", tmp,
+                       sizeof(tmp)));
+  EXPECT_STREQ("Foo()", tmp);
+  // Suffixes without the number should also demangle.
+  EXPECT_TRUE(Demangle("_ZL3Foov.clo", tmp, sizeof(tmp)));
+  EXPECT_STREQ("Foo()", tmp);
+  // Suffixes with just the number should also demangle.
+  EXPECT_TRUE(Demangle("_ZL3Foov.123", tmp, sizeof(tmp)));
+  EXPECT_STREQ("Foo()", tmp);
+  // (.clone. followed by non-number), should also demangle.
+  EXPECT_TRUE(Demangle("_ZL3Foov.clone.foo", tmp, sizeof(tmp)));
+  EXPECT_STREQ("Foo()", tmp);
+  // (.clone. followed by multiple numbers), should also demangle.
+  EXPECT_TRUE(Demangle("_ZL3Foov.clone.123.456", tmp, sizeof(tmp)));
+  EXPECT_STREQ("Foo()", tmp);
+  // (a long valid suffix), should demangle.
+  EXPECT_TRUE(Demangle("_ZL3Foov.part.9.165493.constprop.775.31805", tmp,
+                       sizeof(tmp)));
+  EXPECT_STREQ("Foo()", tmp);
+  // Other suffixes should demangle too.
+  EXPECT_TRUE(Demangle("_ZL3Foov.", tmp, sizeof(tmp)));
+  EXPECT_STREQ("Foo()", tmp);
+  EXPECT_TRUE(Demangle("_ZL3Foov.abc123", tmp, sizeof(tmp)));
+  EXPECT_STREQ("Foo()", tmp);
+  EXPECT_TRUE(Demangle("_ZL3Foov.clone.", tmp, sizeof(tmp)));
+  EXPECT_STREQ("Foo()", tmp);
+  EXPECT_TRUE(Demangle("_ZL3Foov.isra.2.constprop.", tmp, sizeof(tmp)));
+  EXPECT_STREQ("Foo()", tmp);
+}
+
+TEST(Demangle, Discriminators) {
+  char tmp[80];
+
+  // Source:
+  //
+  // using Thunk = void (*)();
+  //
+  // Thunk* f() {
+  //   static Thunk thunks[12] = {};
+  //
+  // #define THUNK(i) [backslash here]
+  //   do { struct S { static void g() {} }; thunks[i] = &S::g; } while (0)
+  //
+  //   THUNK(0);
+  //   [... repeat for 1 to 10 ...]
+  //   THUNK(11);
+  //
+  //   return thunks;
+  // }
+  //
+  // The test inputs are manglings of some of the S::g member functions.
+
+  // The first one omits the discriminator.
+  EXPECT_TRUE(Demangle("_ZZ1fvEN1S1gEv", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f()::S::g()", tmp);
+
+  // The second one encodes 0.
+  EXPECT_TRUE(Demangle("_ZZ1fvEN1S1gE_0v", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f()::S::g()", tmp);
+
+  // The eleventh one encodes 9.
+  EXPECT_TRUE(Demangle("_ZZ1fvEN1S1gE_9v", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f()::S::g()", tmp);
+
+  // The twelfth one encodes 10 with extra underscores delimiting it.
+  EXPECT_TRUE(Demangle("_ZZ1fvEN1S1gE__10_v", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f()::S::g()", tmp);
+}
+
+TEST(Demangle, SingleDigitDiscriminatorFollowedByADigit) {
+  char tmp[80];
+
+  // Don't parse 911 as a number.
+  EXPECT_TRUE(Demangle("_ZZ1fvEN1S1gE_911return_type", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f()::S::g()", tmp);
+}
+
+TEST(Demangle, LiteralOfGlobalNamespaceEnumType) {
+  char tmp[80];
+
+  // void f<(E)42>()
+  EXPECT_TRUE(Demangle("_Z1fIL1E42EEvv", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
+TEST(Demangle, NullptrLiterals) {
+  char tmp[80];
+
+  // void f<nullptr>()
+  EXPECT_TRUE(Demangle("_Z1fILDnEEvv", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+
+  // also void f<nullptr>()
+  EXPECT_TRUE(Demangle("_Z1fILDn0EEvv", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
+TEST(Demangle, StringLiterals) {
+  char tmp[80];
+
+  // void f<"<char const [42]>">()
+  EXPECT_TRUE(Demangle("_Z1fILA42_KcEEvv", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
+TEST(Demangle, ComplexFloatingPointLiterals) {
+  char tmp[80];
+
+  // Source (use g++ -fext-numeric-literals to compile):
+  //
+  // using C = double _Complex;
+  // template <class T> void f(char (&)[sizeof(C{sizeof(T)} + 4.0j)]) {}
+  // template void f<int>(char (&)[sizeof(C{sizeof(int)} + 4.0j)]);
+  //
+  // GNU demangling:
+  //
+  // void f<int>(char (&) [sizeof (double _Complex{sizeof (int)}+
+  // ((double _Complex)0000000000000000_4010000000000000))])
+  EXPECT_TRUE(Demangle(
+      "_Z1fIiEvRAszpltlCdstT_ELS0_0000000000000000_4010000000000000E_c",
+      tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
+TEST(Demangle, Float128) {
+  char tmp[80];
+
+  // S::operator _Float128() const
+  EXPECT_TRUE(Demangle("_ZNK1ScvDF128_Ev", tmp, sizeof(tmp)));
+  EXPECT_STREQ("S::operator _Float128()", tmp);
+}
+
+TEST(Demangle, Float128x) {
+  char tmp[80];
+
+  // S::operator _Float128x() const
+  EXPECT_TRUE(Demangle("_ZNK1ScvDF128xEv", tmp, sizeof(tmp)));
+  EXPECT_STREQ("S::operator _Float128x()", tmp);
+}
+
+TEST(Demangle, InvalidFloatNWidth) {
+  char tmp[80];
+
+  // A negative or overflowed _FloatN width is not printable, so render it as
+  // "?" like the sibling _BitInt path instead of emitting garbage bytes.
+  EXPECT_TRUE(Demangle("_ZNK1ScvDFn3_Ev", tmp, sizeof(tmp)));
+  EXPECT_STREQ("S::operator _Float?()", tmp);
+
+  EXPECT_TRUE(Demangle("_ZNK1ScvDF2147483648_Ev", tmp, sizeof(tmp)));
+  EXPECT_STREQ("S::operator _Float?()", tmp);
+}
+
+TEST(Demangle, Bfloat16) {
+  char tmp[80];
+
+  // S::operator std::bfloat16_t() const
+  EXPECT_TRUE(Demangle("_ZNK1ScvDF16bEv", tmp, sizeof(tmp)));
+  EXPECT_STREQ("S::operator std::bfloat16_t()", tmp);
+}
+
+TEST(Demangle, SimpleSignedBitInt) {
+  char tmp[80];
+
+  // S::operator _BitInt(256)() const
+  EXPECT_TRUE(Demangle("_ZNK1ScvDB256_Ev", tmp, sizeof(tmp)));
+  EXPECT_STREQ("S::operator _BitInt(256)()", tmp);
+}
+
+TEST(Demangle, SimpleUnsignedBitInt) {
+  char tmp[80];
+
+  // S::operator unsigned _BitInt(256)() const
+  EXPECT_TRUE(Demangle("_ZNK1ScvDU256_Ev", tmp, sizeof(tmp)));
+  EXPECT_STREQ("S::operator unsigned _BitInt(256)()", tmp);
+}
+
+TEST(Demangle, DependentBitInt) {
+  char tmp[80];
+
+  // S::operator _BitInt(256)<256>() const
+  EXPECT_TRUE(Demangle("_ZNK1ScvDBT__ILi256EEEv", tmp, sizeof(tmp)));
+  EXPECT_STREQ("S::operator _BitInt(?)<>()", tmp);
+}
+
+TEST(Demangle, ConversionToPointerType) {
+  char tmp[80];
+
+  // S::operator int*() const
+  EXPECT_TRUE(Demangle("_ZNK1ScvPiEv", tmp, sizeof(tmp)));
+  EXPECT_STREQ("S::operator int*()", tmp);
+}
+
+TEST(Demangle, ConversionToLvalueReferenceType) {
+  char tmp[80];
+
+  // S::operator int&() const
+  EXPECT_TRUE(Demangle("_ZNK1ScvRiEv", tmp, sizeof(tmp)));
+  EXPECT_STREQ("S::operator int&()", tmp);
+}
+
+TEST(Demangle, ConversionToRvalueReferenceType) {
+  char tmp[80];
+
+  // S::operator int&&() const
+  EXPECT_TRUE(Demangle("_ZNK1ScvOiEv", tmp, sizeof(tmp)));
+  EXPECT_STREQ("S::operator int&&()", tmp);
+}
+
+TEST(Demangle, ConversionToComplexFloatingPointType) {
+  char tmp[80];
+
+  // S::operator float _Complex() const
+  EXPECT_TRUE(Demangle("_ZNK1ScvCfEv", tmp, sizeof(tmp)));
+  EXPECT_STREQ("S::operator float _Complex()", tmp);
+}
+
+TEST(Demangle, ConversionToImaginaryFloatingPointType) {
+  char tmp[80];
+
+  // S::operator float _Imaginary() const
+  EXPECT_TRUE(Demangle("_ZNK1ScvGfEv", tmp, sizeof(tmp)));
+  EXPECT_STREQ("S::operator float _Imaginary()", tmp);
+}
+
+TEST(Demangle, ConversionToPointerToCvQualifiedType) {
+  char tmp[80];
+
+  // S::operator int const volatile restrict*() const
+  EXPECT_TRUE(Demangle("_ZNK1ScvPrVKiEv", tmp, sizeof(tmp)));
+  EXPECT_STREQ("S::operator int const volatile restrict*()", tmp);
+}
+
+TEST(Demangle, ConversionToLayeredPointerType) {
+  char tmp[80];
+
+  // S::operator int const* const*() const
+  EXPECT_TRUE(Demangle("_ZNK1ScvPKPKiEv", tmp, sizeof(tmp)));
+  EXPECT_STREQ("S::operator int const* const*()", tmp);
+}
+
+TEST(Demangle, ConversionToTypeWithExtendedQualifier) {
+  char tmp[80];
+
+  // S::operator int const AS128*() const
+  //
+  // Because our scan of easy type constructors stops at the extended qualifier,
+  // the demangling preserves the * but loses the const.
+  EXPECT_TRUE(Demangle("_ZNK1ScvPU5AS128KiEv", tmp, sizeof(tmp)));
+  EXPECT_STREQ("S::operator int*()", tmp);
+}
+
+TEST(Demangle, GlobalInitializers) {
+  char tmp[80];
+
+  // old form without suffix
+  EXPECT_TRUE(Demangle("_ZGR1v", tmp, sizeof(tmp)));
+  EXPECT_STREQ("reference temporary for v", tmp);
+
+  // modern form for the whole initializer
+  EXPECT_TRUE(Demangle("_ZGR1v_", tmp, sizeof(tmp)));
+  EXPECT_STREQ("reference temporary for v", tmp);
+
+  // next subobject in depth-first preorder traversal
+  EXPECT_TRUE(Demangle("_ZGR1v0_", tmp, sizeof(tmp)));
+  EXPECT_STREQ("reference temporary for v", tmp);
+
+  // subobject with a larger seq-id
+  EXPECT_TRUE(Demangle("_ZGR1v1Z_", tmp, sizeof(tmp)));
+  EXPECT_STREQ("reference temporary for v", tmp);
+}
+
+TEST(Demangle, StructuredBindings) {
+  char tmp[80];
+
+  // Source:
+  //
+  // struct S { int a, b; };
+  // const auto& [x, y] = S{1, 2};
+
+  // [x, y]
+  EXPECT_TRUE(Demangle("_ZDC1x1yE", tmp, sizeof(tmp)));
+
+  // reference temporary for [x, y]
+  EXPECT_TRUE(Demangle("_ZGRDC1x1yE_", tmp, sizeof(tmp)));
+}
+
+// Test the GNU abi_tag extension.
+TEST(Demangle, AbiTags) {
+  char tmp[80];
+
+  // Mangled name generated via:
+  // struct [[gnu::abi_tag("abc")]] A{};
+  // A a;
+  EXPECT_TRUE(Demangle("_Z1aB3abc", tmp, sizeof(tmp)));
+  EXPECT_STREQ("a[abi:abc]", tmp);
+
+  // Mangled name generated via:
+  // struct B {
+  //   B [[gnu::abi_tag("xyz")]] (){};
+  // };
+  // B b;
+  EXPECT_TRUE(Demangle("_ZN1BC2B3xyzEv", tmp, sizeof(tmp)));
+  EXPECT_STREQ("B::B[abi:xyz]()", tmp);
+
+  // Mangled name generated via:
+  // [[gnu::abi_tag("foo", "bar")]] void C() {}
+  EXPECT_TRUE(Demangle("_Z1CB3barB3foov", tmp, sizeof(tmp)));
+  EXPECT_STREQ("C[abi:bar][abi:foo]()", tmp);
+}
+
+TEST(Demangle, SimpleGnuVectorSize) {
+  char tmp[80];
+
+  // Source:
+  //
+  // #define VECTOR(size) __attribute__((vector_size(size)))
+  // void f(int x VECTOR(32)) {}
+  //
+  // The attribute's size is a number of bytes.  The compiler verifies that this
+  // value corresponds to a whole number of elements and emits the number of
+  // elements as a <number> in the mangling.  With sizeof(int) == 4, that yields
+  // 32/4 = 8.
+  //
+  // LLVM demangling:
+  //
+  // f(int vector[8])
+  EXPECT_TRUE(Demangle("_Z1fDv8_i", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f()", tmp);
+}
+
+TEST(Demangle, GnuVectorSizeIsATemplateParameter) {
+  char tmp[80];
+
+  // Source:
+  //
+  // #define VECTOR(size) __attribute__((vector_size(size)))
+  // template <int n> void f(int x VECTOR(n)) {}
+  // template void f<32>(int x VECTOR(32));
+  //
+  // LLVM demangling:
+  //
+  // void f<32>(int vector[32])
+  //
+  // Because the size was dependent on a template parameter, it was encoded
+  // using the general expression encoding.  Nothing in the mangling says how
+  // big the element type is, so the demangler is unable to show the element
+  // count 8 instead of the byte count 32.  Arguably it would have been better
+  // to make the narrow production encode the byte count, so that nondependent
+  // and dependent versions of a 32-byte vector would both come out as
+  // vector[32].
+  EXPECT_TRUE(Demangle("_Z1fILi32EEvDvT__i", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
+TEST(Demangle, GnuVectorSizeIsADependentOperatorExpression) {
+  char tmp[80];
+
+  // Source:
+  //
+  // #define VECTOR(size) __attribute__((vector_size(size)))
+  // template <int n> void f(int x VECTOR(2 * n)) {}
+  // template void f<32>(int x VECTOR(2 * 32));
+  //
+  // LLVM demangling:
+  //
+  // void f<32>(int vector[2 * 32])
+  EXPECT_TRUE(Demangle("_Z1fILi32EEvDvmlLi2ET__i", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
+TEST(Demangle, SimpleAddressSpace) {
+  char tmp[80];
+
+  // Source:
+  //
+  // void f(const int __attribute__((address_space(128)))*) {}
+  //
+  // LLVM demangling:
+  //
+  // f(int const AS128*)
+  //
+  // Itanium ABI 5.1.5.1, "Qualified types", notes that address_space is mangled
+  // nonuniformly as a legacy exception: the number is part of the source-name
+  // if nondependent but is an expression in template-args if dependent.  Thus
+  // it is a convenient test case for both forms.
+  EXPECT_TRUE(Demangle("_Z1fPU5AS128Ki", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f()", tmp);
+}
+
+TEST(Demangle, DependentAddressSpace) {
+  char tmp[80];
+
+  // Source:
+  //
+  // template <int n> void f (const int __attribute__((address_space(n)))*) {}
+  // template void f<128>(const int __attribute__((address_space(128)))*);
+  //
+  // LLVM demangling:
+  //
+  // void f<128>(int AS<128>*)
+  EXPECT_TRUE(Demangle("_Z1fILi128EEvPU2ASIT_Ei", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
+TEST(Demangle, TransactionSafeEntryPoint) {
+  char tmp[80];
+
+  EXPECT_TRUE(Demangle("_ZGTt1fv", tmp, sizeof(tmp)));
+  EXPECT_STREQ("transaction clone for f()", tmp);
+}
+
+TEST(Demangle, TransactionSafeFunctionType) {
+  char tmp[80];
+
+  // GNU demangling: f(void (*)() transaction_safe)
+  EXPECT_TRUE(Demangle("_Z1fPDxFvvE", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f()", tmp);
+}
+
+TEST(Demangle, TemplateParameterObject) {
+  char tmp[80];
+
+  // Source:
+  //
+  // struct S { int x, y; };
+  // template <S s, const S* p = &s> void f() {}
+  // template void f<S{1, 2}>();
+  //
+  // LLVM demangling:
+  //
+  // void f<S{1, 2}, &template parameter object for S{1, 2}>()
+  EXPECT_TRUE(Demangle("_Z1fIXtl1SLi1ELi2EEEXadL_ZTAXtlS0_Li1ELi2EEEEEEvv",
+                       tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+
+  // The name of the object standing alone.
+  //
+  // LLVM demangling: template parameter object for S{1, 2}
+  EXPECT_TRUE(Demangle("_ZTAXtl1SLi1ELi2EEE", tmp, sizeof(tmp)));
+  EXPECT_STREQ("template parameter object", tmp);
+}
+
+TEST(Demangle, EnableIfAttributeOnGlobalFunction) {
+  char tmp[80];
+
+  // int f(long l) __attribute__((enable_if(l >= 0, ""))) { return l; }
+  //
+  // f(long) [enable_if:fp >= 0]
+  EXPECT_TRUE(Demangle("_Z1fUa9enable_ifIXgefL0p_Li0EEEl", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f()", tmp);
+}
+
+TEST(Demangle, EnableIfAttributeOnNamespaceScopeFunction) {
+  char tmp[80];
+
+  // namespace ns {
+  // int f(long l) __attribute__((enable_if(l >= 0, ""))) { return l; }
+  // }  // namespace ns
+  //
+  // ns::f(long) [enable_if:fp >= 0]
+  EXPECT_TRUE(Demangle("_ZN2ns1fEUa9enable_ifIXgefL0p_Li0EEEl",
+              tmp, sizeof(tmp)));
+  EXPECT_STREQ("ns::f()", tmp);
+}
+
+TEST(Demangle, EnableIfAttributeOnFunctionTemplate) {
+  char tmp[80];
+
+  // template <class T>
+  // T f(T t) __attribute__((enable_if(t >= T{}, ""))) { return t; }
+  // template int f<int>(int);
+  //
+  // int f<int>(int) [enable_if:fp >= int{}]
+  EXPECT_TRUE(Demangle("_Z1fIiEUa9enable_ifIXgefL0p_tliEEET_S0_",
+              tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
+TEST(Demangle, ThisPointerInDependentSignature) {
+  char tmp[80];
+
+  // decltype(g<int>(this)) S::f<int>()
+  EXPECT_TRUE(Demangle("_ZN1S1fIiEEDTcl1gIT_EfpTEEv", tmp, sizeof(tmp)));
+  EXPECT_STREQ("S::f<>()", tmp);
+}
+
+TEST(Demangle, DependentMemberOperatorCall) {
+  char tmp[80];
+
+  // decltype(fp.operator()()) f<C>(C)
+  EXPECT_TRUE(Demangle("_Z1fI1CEDTcldtfp_onclEET_", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
+TEST(Demangle, TypeNestedUnderDecltype) {
+  char tmp[80];
+
+  // Source:
+  //
+  // template <class T> struct S { using t = int; };
+  // template <class T> decltype(S<T>{})::t f() { return {}; }
+  // void g() { f<int>(); }
+  //
+  // Full LLVM demangling of the instantiation of f:
+  //
+  // decltype(S<int>{})::t f<int>()
+  EXPECT_TRUE(Demangle("_Z1fIiENDTtl1SIT_EEE1tEv", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
+TEST(Demangle, ElaboratedTypes) {
+  char tmp[80];
+
+  // Source:
+  //
+  // template <class T> struct S { class C {}; };
+  // template <class T> void f(class S<T>::C) {}
+  // template void f<int>(class S<int>::C);
+  //
+  // LLVM demangling:
+  //
+  // void f<int>(struct S<int>::C)
+  EXPECT_TRUE(Demangle("_Z1fIiEvTsN1SIT_E1CE", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+
+  // The like for unions.
+  EXPECT_TRUE(Demangle("_Z1fIiEvTuN1SIT_E1CE", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+
+  // The like for enums.
+  EXPECT_TRUE(Demangle("_Z1fIiEvTeN1SIT_E1CE", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
+// Test subobject-address template parameters.
+TEST(Demangle, SubobjectAddresses) {
+  char tmp[80];
+
+  // void f<a.<char const at offset 123>>()
+  EXPECT_TRUE(Demangle("_Z1fIXsoKcL_Z1aE123EEEvv", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+
+  // void f<&a.<char const at offset 0>>()
+  EXPECT_TRUE(Demangle("_Z1fIXadsoKcL_Z1aEEEEvv", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+
+  // void f<&a.<char const at offset 123>>()
+  EXPECT_TRUE(Demangle("_Z1fIXadsoKcL_Z1aE123EEEvv", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+
+  // void f<&a.<char const at offset 123>>(), past the end this time
+  EXPECT_TRUE(Demangle("_Z1fIXadsoKcL_Z1aE123pEEEvv", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+
+  // void f<&a.<char const at offset 0>>() with union-selectors
+  EXPECT_TRUE(Demangle("_Z1fIXadsoKcL_Z1aE__1_234EEEvv", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+
+  // void f<&a.<char const at offset 123>>(), past the end, with union-selector
+  EXPECT_TRUE(Demangle("_Z1fIXadsoKcL_Z1aE123_456pEEEvv", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
+TEST(Demangle, Preincrement) {
+  char tmp[80];
+
+  // Source:
+  //
+  // template <class T> auto f(T t) -> decltype(T{++t}) { return t; }
+  // template auto f<int>(int t) -> decltype(int{++t});
+  //
+  // Full LLVM demangling of the instantiation of f:
+  //
+  // decltype(int{++fp}) f<int>(int)
+  EXPECT_TRUE(Demangle("_Z1fIiEDTtlT_pp_fp_EES0_", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
+TEST(Demangle, Postincrement) {
+  char tmp[80];
+
+  // Source:
+  //
+  // template <class T> auto f(T t) -> decltype(T{t++}) { return t; }
+  // template auto f<int>(int t) -> decltype(int{t++});
+  //
+  // Full LLVM demangling of the instantiation of f:
+  //
+  // decltype(int{fp++}) f<int>(int)
+  EXPECT_TRUE(Demangle("_Z1fIiEDTtlT_ppfp_EES0_", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
+TEST(Demangle, Predecrement) {
+  char tmp[80];
+
+  // Source:
+  //
+  // template <class T> auto f(T t) -> decltype(T{--t}) { return t; }
+  // template auto f<int>(int t) -> decltype(int{--t});
+  //
+  // Full LLVM demangling of the instantiation of f:
+  //
+  // decltype(int{--fp}) f<int>(int)
+  EXPECT_TRUE(Demangle("_Z1fIiEDTtlT_mm_fp_EES0_", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
+TEST(Demangle, Postdecrement) {
+  char tmp[80];
+
+  // Source:
+  //
+  // template <class T> auto f(T t) -> decltype(T{t--}) { return t; }
+  // template auto f<int>(int t) -> decltype(int{t--});
+  //
+  // Full LLVM demangling of the instantiation of f:
+  //
+  // decltype(int{fp--}) f<int>(int)
+  EXPECT_TRUE(Demangle("_Z1fIiEDTtlT_mmfp_EES0_", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
+TEST(Demangle, UnaryFoldExpressions) {
+  char tmp[80];
+
+  // Source:
+  //
+  // template <bool b> struct S {};
+  //
+  // template <class... T> auto f(T... t) -> S<((sizeof(T) == 4) || ...)> {
+  //   return {};
+  // }
+  //
+  // void g() { f(1, 2L); }
+  //
+  // Full LLVM demangling of the instantiation of f:
+  //
+  // S<((sizeof (int) == 4, sizeof (long) == 4) || ...)> f<int, long>(int, long)
+  EXPECT_TRUE(Demangle("_Z1fIJilEE1SIXfrooeqstT_Li4EEEDpS1_",
+                       tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+
+  // The like with a left fold.
+  //
+  // S<(... || (sizeof (int) == 4, sizeof (long) == 4))> f<int, long>(int, long)
+  EXPECT_TRUE(Demangle("_Z1fIJilEE1SIXflooeqstT_Li4EEEDpS1_",
+                       tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
+TEST(Demangle, BinaryFoldExpressions) {
+  char tmp[80];
+
+  // Source:
+  //
+  // template <bool b> struct S {};
+  //
+  // template <class... T> auto f(T... t)
+  //     -> S<((sizeof(T) == 4) || ... || false)> {
+  //   return {};
+  // }
+  //
+  // void g() { f(1, 2L); }
+  //
+  // Full LLVM demangling of the instantiation of f:
+  //
+  // S<((sizeof (int) == 4, sizeof (long) == 4) || ... || false)>
+  // f<int, long>(int, long)
+  EXPECT_TRUE(Demangle("_Z1fIJilEE1SIXfRooeqstT_Li4ELb0EEEDpS1_",
+                       tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+
+  // The like with a left fold.
+  //
+  // S<(false || ... || (sizeof (int) == 4, sizeof (long) == 4))>
+  // f<int, long>(int, long)
+  EXPECT_TRUE(Demangle("_Z1fIJilEE1SIXfLooLb0EeqstT_Li4EEEDpS1_",
+                       tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
+TEST(Demangle, SizeofPacks) {
+  char tmp[80];
+
+  // template <size_t i> struct S {};
+  //
+  // template <class... T> auto f(T... p) -> S<sizeof...(T)> { return {}; }
+  // template auto f<int, long>(int, long) -> S<2>;
+  //
+  // template <class... T> auto g(T... p) -> S<sizeof...(p)> { return {}; }
+  // template auto g<int, long>(int, long) -> S<2>;
+
+  // S<sizeof...(int, long)> f<int, long>(int, long)
+  EXPECT_TRUE(Demangle("_Z1fIJilEE1SIXsZT_EEDpT_", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+
+  // S<sizeof... (fp)> g<int, long>(int, long)
+  EXPECT_TRUE(Demangle("_Z1gIJilEE1SIXsZfp_EEDpT_", tmp, sizeof(tmp)));
+  EXPECT_STREQ("g<>()", tmp);
+}
+
+TEST(Demangle, SizeofPackInvolvingAnAliasTemplate) {
+  char tmp[80];
+
+  // Source:
+  //
+  // template <class... T> using A = char[sizeof...(T)];
+  // template <class... U> void f(const A<U..., int>&) {}
+  // template void f<int>(const A<int, int>&);
+  //
+  // Full LLVM demangling of the instantiation of f:
+  //
+  // void f<int>(char const (&) [sizeof... (int, int)])
+  EXPECT_TRUE(Demangle("_Z1fIJiEEvRAsPDpT_iE_Kc", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
+TEST(Demangle, UserDefinedLiteral) {
+  char tmp[80];
+
+  // Source:
+  //
+  // unsigned long long operator""_lit(unsigned long long x) { return x; }
+  //
+  // LLVM demangling:
+  //
+  // operator"" _lit(unsigned long long)
+  EXPECT_TRUE(Demangle("_Zli4_lity", tmp, sizeof(tmp)));
+  EXPECT_STREQ("operator\"\" _lit()", tmp);
+}
+
+TEST(Demangle, Spaceship) {
+  char tmp[80];
+
+  // #include <compare>
+  //
+  // struct S { auto operator<=>(const S&) const = default; };
+  // auto (S::*f) = &S::operator<=>;  // make sure S::operator<=> is emitted
+  //
+  // template <class T> auto g(T x, T y) -> decltype(x <=> y) {
+  //   return x <=> y;
+  // }
+  // template auto g<S>(S x, S y) -> decltype(x <=> y);
+
+  // S::operator<=>(S const&) const
+  EXPECT_TRUE(Demangle("_ZNK1SssERKS_", tmp, sizeof(tmp)));
+  EXPECT_STREQ("S::operator<=>()", tmp);
+
+  // decltype(fp <=> fp0) g<S>(S, S)
+  EXPECT_TRUE(Demangle("_Z1gI1SEDTssfp_fp0_ET_S2_", tmp, sizeof(tmp)));
+  EXPECT_STREQ("g<>()", tmp);
+}
+
+TEST(Demangle, CoAwait) {
+  char tmp[80];
+
+  // ns::Awaitable::operator co_await() const
+  EXPECT_TRUE(Demangle("_ZNK2ns9AwaitableawEv", tmp, sizeof(tmp)));
+  EXPECT_STREQ("ns::Awaitable::operator co_await()", tmp);
+}
+
+TEST(Demangle, VendorExtendedExpressions) {
+  char tmp[80];
+
+  // void f<__e()>()
+  EXPECT_TRUE(Demangle("_Z1fIXu3__eEEEvv", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+
+  // void f<__e(int, long)>()
+  EXPECT_TRUE(Demangle("_Z1fIXu3__eilEEEvv", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
+TEST(Demangle, DirectListInitialization) {
+  char tmp[80];
+
+  // template <class T> decltype(T{}) f() { return T{}; }
+  // template decltype(int{}) f<int>();
+  //
+  // struct XYZ { int x, y, z; };
+  // template <class T> decltype(T{1, 2, 3}) g() { return T{1, 2, 3}; }
+  // template decltype(XYZ{1, 2, 3}) g<XYZ>();
+  //
+  // template <class T> decltype(T{.x = 1, .y = 2, .z = 3}) h() {
+  //   return T{.x = 1, .y = 2, .z = 3};
+  // }
+  // template decltype(XYZ{.x = 1, .y = 2, .z = 3}) h<XYZ>();
+  //
+  // // The following two cases require full C99 designated initializers,
+  // // not part of C++ but likely available as an extension if you ask your
+  // // compiler nicely.
+  //
+  // struct A { int a[4]; };
+  // template <class T> decltype(T{.a[2] = 42}) i() { return T{.a[2] = 42}; }
+  // template decltype(A{.a[2] = 42}) i<A>();
+  //
+  // template <class T> decltype(T{.a[1 ... 3] = 42}) j() {
+  //   return T{.a[1 ... 3] = 42};
+  // }
+  // template decltype(A{.a[1 ... 3] = 42}) j<A>();
+
+  // decltype(int{}) f<int>()
+  EXPECT_TRUE(Demangle("_Z1fIiEDTtlT_EEv", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+
+  // decltype(XYZ{1, 2, 3}) g<XYZ>()
+  EXPECT_TRUE(Demangle("_Z1gI3XYZEDTtlT_Li1ELi2ELi3EEEv", tmp, sizeof(tmp)));
+  EXPECT_STREQ("g<>()", tmp);
+
+  // decltype(XYZ{.x = 1, .y = 2, .z = 3}) h<XYZ>()
+  EXPECT_TRUE(Demangle("_Z1hI3XYZEDTtlT_di1xLi1Edi1yLi2Edi1zLi3EEEv",
+                       tmp, sizeof(tmp)));
+  EXPECT_STREQ("h<>()", tmp);
+
+  // decltype(A{.a[2] = 42}) i<A>()
+  EXPECT_TRUE(Demangle("_Z1iI1AEDTtlT_di1adxLi2ELi42EEEv", tmp, sizeof(tmp)));
+  EXPECT_STREQ("i<>()", tmp);
+
+  // decltype(A{.a[1 ... 3] = 42}) j<A>()
+  EXPECT_TRUE(Demangle("_Z1jI1AEDTtlT_di1adXLi1ELi3ELi42EEEv",
+                       tmp, sizeof(tmp)));
+  EXPECT_STREQ("j<>()", tmp);
+}
+
+TEST(Demangle, SimpleInitializerLists) {
+  char tmp[80];
+
+  // Common preamble of source-code examples in this test function:
+  //
+  // #include <initializer_list>
+  //
+  // template <class T> void g(std::initializer_list<T>) {}
+
+  // Source:
+  //
+  // template <class T> auto f() -> decltype(g<T>({})) {}
+  // template auto f<int>() -> decltype(g<int>({}));
+  //
+  // Full LLVM demangling of the instantiation of f:
+  //
+  // decltype(g<int>({})) f<int>()
+  EXPECT_TRUE(Demangle("_Z1fIiEDTcl1gIT_EilEEEv", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+
+  // Source:
+  //
+  // template <class T> auto f(T x) -> decltype(g({x})) {}
+  // template auto f<int>(int x) -> decltype(g({x}));
+  //
+  // Full LLVM demangling of the instantiation of f:
+  //
+  // decltype(g({fp})) f<int>(int)
+  EXPECT_TRUE(Demangle("_Z1fIiEDTcl1gilfp_EEET_", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+
+  // Source:
+  //
+  // template <class T> auto f(T x, T y) -> decltype(g({x, y})) {}
+  // template auto f<int>(int x, int y) -> decltype(g({x, y}));
+  //
+  // Full LLVM demangling of the instantiation of f:
+  //
+  // decltype(g({fp, fp0})) f<int>(int, int)
+  EXPECT_TRUE(Demangle("_Z1fIiEDTcl1gilfp_fp0_EEET_S1_", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
+TEST(Demangle, BracedListImplicitlyConstructingAClassObject) {
+  char tmp[80];
+
+  // Source:
+  //
+  // struct S { int v; };
+  // void g(S) {}
+  // template <class T> auto f(T x) -> decltype(g({.v = x})) {}
+  // template auto f<int>(int x) -> decltype(g({.v = x}));
+  //
+  // Full LLVM demangling of the instantiation of f:
+  //
+  // decltype(g({.v = fp})) f<int>(int)
+  EXPECT_TRUE(Demangle("_Z1fIiEDTcl1gildi1vfp_EEET_", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
+TEST(Demangle, SimpleNewExpression) {
+  char tmp[80];
+
+  // Source:
+  //
+  // template <class T> decltype(T{*new T}) f() { return T{}; }
+  // template decltype(int{*new int}) f<int>();
+  //
+  // Full LLVM demangling of the instantiation of f:
+  //
+  // decltype(int{*(new int)}) f<int>()
+  EXPECT_TRUE(Demangle("_Z1fIiEDTtlT_denw_S0_EEEv", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
+TEST(Demangle, NewExpressionWithEmptyParentheses) {
+  char tmp[80];
+
+  // Source:
+  //
+  // template <class T> decltype(T{*new T()}) f() { return T{}; }
+  // template decltype(int{*new int()}) f<int>();
+  //
+  // Full LLVM demangling of the instantiation of f:
+  //
+  // decltype(int{*(new int)}) f<int>()
+  EXPECT_TRUE(Demangle("_Z1fIiEDTtlT_denw_S0_piEEEv", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
+TEST(Demangle, NewExpressionWithNonemptyParentheses) {
+  char tmp[80];
+
+  // Source:
+  //
+  // template <class T> decltype(T{*new T(42)}) f() { return T{}; }
+  // template decltype(int{*new int(42)}) f<int>();
+  //
+  // Full LLVM demangling of the instantiation of f:
+  //
+  // decltype(int{*(new int(42))}) f<int>()
+  EXPECT_TRUE(Demangle("_Z1fIiEDTtlT_denw_S0_piLi42EEEEv", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
+TEST(Demangle, PlacementNewExpression) {
+  char tmp[80];
+
+  // Source:
+  //
+  // #include <new>
+  //
+  // template <class T> auto f(T t) -> decltype(T{*new (&t) T(42)}) {
+  //   return t;
+  // }
+  // template auto f<int>(int t) -> decltype(int{*new (&t) int(42)});
+  //
+  // Full LLVM demangling of the instantiation of f:
+  //
+  // decltype(int{*(new(&fp) int(42))}) f<int>(int)
+  EXPECT_TRUE(Demangle("_Z1fIiEDTtlT_denwadfp__S0_piLi42EEEES0_",
+                       tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
+TEST(Demangle, GlobalScopeNewExpression) {
+  char tmp[80];
+
+  // Source:
+  //
+  // template <class T> decltype(T{*::new T}) f() { return T{}; }
+  // template decltype(int{*::new int}) f<int>();
+  //
+  // Full LLVM demangling of the instantiation of f:
+  //
+  // decltype(int{*(::new int)}) f<int>()
+  EXPECT_TRUE(Demangle("_Z1fIiEDTtlT_degsnw_S0_EEEv", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
+TEST(Demangle, NewExpressionWithEmptyBraces) {
+  char tmp[80];
+
+  // Source:
+  //
+  // template <class T> decltype(T{*new T{}}) f() { return T{}; }
+  // template decltype(int{*new int{}}) f<int>();
+  //
+  // GNU demangling:
+  //
+  // decltype (int{*(new int{})}) f<int>()
+  EXPECT_TRUE(Demangle("_Z1fIiEDTtlT_denw_S0_ilEEEv", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
+TEST(Demangle, NewExpressionWithNonemptyBraces) {
+  char tmp[80];
+
+  // Source:
+  //
+  // template <class T> decltype(T{*new T{42}}) f() { return T{}; }
+  // template decltype(int{*new int{42}}) f<int>();
+  //
+  // GNU demangling:
+  //
+  // decltype (int{*(new int{42})}) f<int>()
+  EXPECT_TRUE(Demangle("_Z1fIiEDTtlT_denw_S0_ilLi42EEEEv", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
+TEST(Demangle, SimpleArrayNewExpression) {
+  char tmp[80];
+
+  // Source:
+  //
+  // template <class T> decltype(T{*new T[1]}) f() { return T{}; }
+  // template decltype(int{*new int[1]}) f<int>();
+  //
+  // Full LLVM demangling of the instantiation of f:
+  //
+  // decltype(int{*(new[] int)}) f<int>()
+  EXPECT_TRUE(Demangle("_Z1fIiEDTtlT_dena_S0_EEEv", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
+TEST(Demangle, ArrayNewExpressionWithEmptyParentheses) {
+  char tmp[80];
+
+  // Source:
+  //
+  // template <class T> decltype(T{*new T[1]()}) f() { return T{}; }
+  // template decltype(int{*new int[1]()}) f<int>();
+  //
+  // Full LLVM demangling of the instantiation of f:
+  //
+  // decltype(int{*(new[] int)}) f<int>()
+  EXPECT_TRUE(Demangle("_Z1fIiEDTtlT_dena_S0_piEEEv", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
+TEST(Demangle, ArrayPlacementNewExpression) {
+  char tmp[80];
+
+  // Source:
+  //
+  // #include <new>
+  //
+  // template <class T> auto f(T t) -> decltype(T{*new (&t) T[1]}) {
+  //   return T{};
+  // }
+  // template auto f<int>(int t) -> decltype(int{*new (&t) int[1]});
+  //
+  // Full LLVM demangling of the instantiation of f:
+  //
+  // decltype(int{*(new[](&fp) int)}) f<int>(int)
+  EXPECT_TRUE(Demangle("_Z1fIiEDTtlT_denaadfp__S0_EEES0_", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
+TEST(Demangle, GlobalScopeArrayNewExpression) {
+  char tmp[80];
+
+  // Source:
+  //
+  // template <class T> decltype(T{*::new T[1]}) f() { return T{}; }
+  // template decltype(int{*::new int[1]}) f<int>();
+  //
+  // Full LLVM demangling of the instantiation of f:
+  //
+  // decltype(int{*(::new[] int)}) f<int>()
+  EXPECT_TRUE(Demangle("_Z1fIiEDTtlT_degsna_S0_EEEv", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
+TEST(Demangle, ArrayNewExpressionWithTwoElementsInBraces) {
+  char tmp[80];
+
+  // Source:
+  //
+  // template <class T> decltype(T{*new T[2]{1, 2}}) f() { return T{}; }
+  // template decltype(int{*new int[2]{1, 2}}) f<int>();
+  //
+  // GNU demangling:
+  //
+  // decltype (int{*(new int{1, 2})}) f<int>()
+  EXPECT_TRUE(Demangle("_Z1fIiEDTtlT_dena_S0_ilLi1ELi2EEEEv",
+                       tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
+TEST(Demangle, SimpleDeleteExpression) {
+  char tmp[80];
+
+  // Source:
+  //
+  // template <class T> auto f(T* p) -> decltype(delete p) {}
+  // template auto f<int>(int* p) -> decltype(delete p);
+  //
+  // LLVM demangling:
+  //
+  // decltype(delete fp) f<int>(int*)
+  EXPECT_TRUE(Demangle("_Z1fIiEDTdlfp_EPT_", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
+TEST(Demangle, GlobalScopeDeleteExpression) {
+  char tmp[80];
+
+  // Source:
+  //
+  // template <class T> auto f(T* p) -> decltype(::delete p) {}
+  // template auto f<int>(int* p) -> decltype(::delete p);
+  //
+  // LLVM demangling:
+  //
+  // decltype(::delete fp) f<int>(int*)
+  EXPECT_TRUE(Demangle("_Z1fIiEDTgsdlfp_EPT_", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
+TEST(Demangle, SimpleArrayDeleteExpression) {
+  char tmp[80];
+
+  // Source:
+  //
+  // template <class T> auto f(T* a) -> decltype(delete[] a) {}
+  // template auto f<int>(int* a) -> decltype(delete[] a);
+  //
+  // LLVM demangling:
+  //
+  // decltype(delete[] fp) f<int>(int*)
+  EXPECT_TRUE(Demangle("_Z1fIiEDTdafp_EPT_", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
+TEST(Demangle, GlobalScopeArrayDeleteExpression) {
+  char tmp[80];
+
+  // Source:
+  //
+  // template <class T> auto f(T* a) -> decltype(::delete[] a) {}
+  // template auto f<int>(int* a) -> decltype(::delete[] a);
+  //
+  // LLVM demangling:
+  //
+  // decltype(::delete[] fp) f<int>(int*)
+  EXPECT_TRUE(Demangle("_Z1fIiEDTgsdafp_EPT_", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
+TEST(Demangle, ReferenceQualifiedFunctionTypes) {
+  char tmp[80];
+
+  // void f(void (*)() const &, int)
+  EXPECT_TRUE(Demangle("_Z1fPKFvvREi", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f()", tmp);
+
+  // void f(void (*)() &&, int)
+  EXPECT_TRUE(Demangle("_Z1fPFvvOEi", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f()", tmp);
+
+  // void f(void (*)(int&) &, int)
+  EXPECT_TRUE(Demangle("_Z1fPFvRiREi", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f()", tmp);
+
+  // void f(void (*)(S&&) &&, int)
+  EXPECT_TRUE(Demangle("_Z1fPFvO1SOEi", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f()", tmp);
+}
+
+TEST(Demangle, DynamicCast) {
+  char tmp[80];
+
+  // Source:
+  //
+  // template <class T> auto f(T* p) -> decltype(dynamic_cast<const T*>(p)) {
+  //   return p;
+  // }
+  // struct S {};
+  // void g(S* p) { f(p); }
+  //
+  // Full LLVM demangling of the instantiation of f:
+  //
+  // decltype(dynamic_cast<S const*>(fp)) f<S>(S*)
+  EXPECT_TRUE(Demangle("_Z1fI1SEDTdcPKT_fp_EPS1_", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
+TEST(Demangle, StaticCast) {
+  char tmp[80];
+
+  // Source:
+  //
+  // template <class T> auto f(T* p) -> decltype(static_cast<const T*>(p)) {
+  //   return p;
+  // }
+  // void g(int* p) { f(p); }
+  //
+  // Full LLVM demangling of the instantiation of f:
+  //
+  // decltype(static_cast<int const*>(fp)) f<int>(int*)
+  EXPECT_TRUE(Demangle("_Z1fIiEDTscPKT_fp_EPS0_", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
+TEST(Demangle, ConstCast) {
+  char tmp[80];
+
+  // Source:
+  //
+  // template <class T> auto f(T* p) -> decltype(const_cast<const T*>(p)) {
+  //   return p;
+  // }
+  // void g(int* p) { f(p); }
+  //
+  // Full LLVM demangling of the instantiation of f:
+  //
+  // decltype(const_cast<int const*>(fp)) f<int>(int*)
+  EXPECT_TRUE(Demangle("_Z1fIiEDTccPKT_fp_EPS0_", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
+TEST(Demangle, ReinterpretCast) {
+  char tmp[80];
+
+  // Source:
+  //
+  // template <class T> auto f(T* p)
+  //     -> decltype(reinterpret_cast<const T*>(p)) {
+  //   return p;
+  // }
+  // void g(int* p) { f(p); }
+  //
+  // Full LLVM demangling of the instantiation of f:
+  //
+  // decltype(reinterpret_cast<int const*>(fp)) f<int>(int*)
+  EXPECT_TRUE(Demangle("_Z1fIiEDTrcPKT_fp_EPS0_", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
+TEST(Demangle, TypeidType) {
+  char tmp[80];
+
+  // Source:
+  //
+  // #include <typeinfo>
+  //
+  // template <class T> decltype(typeid(T).name()) f(T) { return nullptr; }
+  // template decltype(typeid(int).name()) f<int>(int);
+  //
+  // Full LLVM demangling of the instantiation of f:
+  //
+  // decltype(typeid (int).name()) f<int>(int)
+  EXPECT_TRUE(Demangle("_Z1fIiEDTcldttiT_4nameEES0_", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
+TEST(Demangle, TypeidExpression) {
+  char tmp[80];
+
+  // Source:
+  //
+  // #include <typeinfo>
+  //
+  // template <class T> decltype(typeid(T{}).name()) f(T) { return nullptr; }
+  // template decltype(typeid(int{}).name()) f<int>(int);
+  //
+  // Full LLVM demangling of the instantiation of f:
+  //
+  // decltype(typeid (int{}).name()) f<int>(int)
+  EXPECT_TRUE(Demangle("_Z1fIiEDTcldttetlT_E4nameEES0_", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
+TEST(Demangle, AlignofType) {
+  char tmp[80];
+
+  // Source:
+  //
+  // template <class T> T f(T (&a)[alignof(T)]) { return a[0]; }
+  // template int f<int>(int (&)[alignof(int)]);
+  //
+  // Full LLVM demangling of the instantiation of f:
+  //
+  // int f<int>(int (&) [alignof (int)])
+  EXPECT_TRUE(Demangle("_Z1fIiET_RAatS0__S0_", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
+TEST(Demangle, AlignofExpression) {
+  char tmp[80];
+
+  // Source (note that this uses a GNU extension; it is not standard C++):
+  //
+  // template <class T> T f(T (&a)[alignof(T{})]) { return a[0]; }
+  // template int f<int>(int (&)[alignof(int{})]);
+  //
+  // Full LLVM demangling of the instantiation of f:
+  //
+  // int f<int>(int (&) [alignof (int{})])
+  EXPECT_TRUE(Demangle("_Z1fIiET_RAaztlS0_E_S0_", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
+TEST(Demangle, NoexceptExpression) {
+  char tmp[80];
+
+  // Source:
+  //
+  // template <class T> void f(T (&a)[noexcept(T{})]) {}
+  // template void f<int>(int (&)[noexcept(int{})]);
+  //
+  // Full LLVM demangling of the instantiation of f:
+  //
+  // void f<int>(int (&) [noexcept (int{})])
+  EXPECT_TRUE(Demangle("_Z1fIiEvRAnxtlT_E_S0_", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
+TEST(Demangle, UnaryThrow) {
+  char tmp[80];
+
+  // Source:
+  //
+  // template <bool b> decltype(b ? throw b : 0) f() { return 0; }
+  // template decltype(false ? throw false : 0) f<false>();
+  //
+  // Full LLVM demangling of the instantiation of f:
+  //
+  // decltype(false ? throw false : 0) f<false>()
+  EXPECT_TRUE(Demangle("_Z1fILb0EEDTquT_twT_Li0EEv", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
+TEST(Demangle, NullaryThrow) {
+  char tmp[80];
+
+  // Source:
+  //
+  // template <bool b> decltype(b ? throw : 0) f() { return 0; }
+  // template decltype(false ? throw : 0) f<false>();
+  //
+  // Full LLVM demangling of the instantiation of f:
+  //
+  // decltype(false ? throw : 0) f<false>()
+  EXPECT_TRUE(Demangle("_Z1fILb0EEDTquT_trLi0EEv", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
+TEST(Demangle, ThreadLocalWrappers) {
+  char tmp[80];
+
+  EXPECT_TRUE(Demangle("_ZTWN2ns3varE", tmp, sizeof(tmp)));
+  EXPECT_STREQ("thread-local wrapper routine for ns::var", tmp);
+
+  EXPECT_TRUE(Demangle("_ZTHN2ns3varE", tmp, sizeof(tmp)));
+  EXPECT_STREQ("thread-local initialization routine for ns::var", tmp);
+}
+
+TEST(Demangle, DubiousSrStSymbols) {
+  char tmp[80];
+
+  // GNU demangling (not accepted by LLVM):
+  //
+  // S<std::u<char>::v> f<char>()
+  EXPECT_TRUE(Demangle("_Z1fIcE1SIXsrSt1uIT_E1vEEv", tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+
+  // A real case from the wild.
+  //
+  // GNU demangling (not accepted by LLVM) with line breaks and indentation
+  // added for readability:
+  //
+  // __gnu_cxx::__enable_if<std::__is_char<char>::__value, bool>::__type
+  // std::operator==<char>(
+  //     std::__cxx11::basic_string<char, std::char_traits<char>,
+  //                                std::allocator<char> > const&,
+  //     std::__cxx11::basic_string<char, std::char_traits<char>,
+  //                                std::allocator<char> > const&)
+  EXPECT_TRUE(Demangle(
+      "_ZSteqIcEN9__gnu_cxx11__enable_if"
+      "IXsrSt9__is_charIT_E7__valueEbE"
+      "6__typeE"
+      "RKNSt7__cxx1112basic_stringIS3_St11char_traitsIS3_ESaIS3_EEESE_",
+      tmp, sizeof(tmp)));
+  EXPECT_STREQ("std::operator==<>()", tmp);
+}
+
+// Test one Rust symbol to exercise Demangle's delegation path.  Rust demangling
+// itself is more thoroughly tested in demangle_rust_test.cc.
+TEST(Demangle, DelegatesToDemangleRustSymbolEncoding) {
+  char tmp[80];
+
+  EXPECT_TRUE(Demangle("_RNvC8my_crate7my_func", tmp, sizeof(tmp)));
+  EXPECT_STREQ("my_crate::my_func", tmp);
+}
+
+TEST(Demangle, DemanglingNulTerminatesOnParsingFailure) {
+  std::array buf = {'\xAA', '\xAA', '\xAA', '\xAA'};
+  EXPECT_FALSE(Demangle("_ZN1xBE", std::data(buf), std::size(buf)));
+  // Ensure string is properly NUL-terminated despite parsing failure.
+  EXPECT_THAT(buf, Contains('\0'));
 }
 
 // Tests that verify that Demangle footprint is within some limit.
 // They are not to be run under sanitizers as the sanitizers increase
 // stack consumption by about 4x.
-#if defined(ABSL_INTERNAL_HAVE_DEBUGGING_STACK_CONSUMPTION) &&   \
-    !defined(ADDRESS_SANITIZER) && !defined(MEMORY_SANITIZER) && \
-    !defined(THREAD_SANITIZER)
+#if defined(ABSL_INTERNAL_HAVE_DEBUGGING_STACK_CONSUMPTION) && \
+    !defined(ABSL_HAVE_ADDRESS_SANITIZER) &&                   \
+    !defined(ABSL_HAVE_MEMORY_SANITIZER) &&                    \
+    !defined(ABSL_HAVE_THREAD_SANITIZER)
 
 static const char *g_mangled;
 static char g_demangle_buffer[4096];
@@ -102,15 +1979,15 @@ static const char *DemangleStackConsumption(const char *mangled,
                                             int *stack_consumed) {
   g_mangled = mangled;
   *stack_consumed = GetSignalHandlerStackConsumption(DemangleSignalHandler);
-  ABSL_RAW_LOG(INFO, "Stack consumption of Demangle: %d", *stack_consumed);
+  LOG(INFO) << "Stack consumption of Demangle: " << *stack_consumed;
   return g_demangle_result;
 }
 
-// Demangle stack consumption should be within 8kB for simple mangled names
+// Demangle stack consumption should be within 9kB for simple mangled names
 // with some level of nesting. With alternate signal stack we have 64K,
 // but some signal handlers run on thread stack, and could have arbitrarily
 // little space left (so we don't want to make this number too large).
-const int kStackConsumptionUpperLimit = 8192;
+const int kStackConsumptionUpperLimit = 9670;
 
 // Returns a mangled name nested to the given depth.
 static std::string NestedMangledName(int depth) {
@@ -170,7 +2047,7 @@ TEST(Demangle, DemangleStackConsumption) {
 
 static void TestOnInput(const char* input) {
   static const int kOutSize = 1048576;
-  auto out = absl::make_unique<char[]>(kOutSize);
+  auto out = std::make_unique<char[]>(kOutSize);
   Demangle(input, out.get(), kOutSize);
 }
 
@@ -188,6 +2065,33 @@ TEST(DemangleRegression, DeeplyNestedArrayType) {
   TestOnInput(data.c_str());
 }
 
+TEST(DemangleRegression, ShortOutputBuffer) {
+  // This should not crash.
+  char buffer[1];
+  EXPECT_FALSE(
+      absl::debugging_internal::Demangle("_ZZ2wwE", buffer, sizeof(buffer)));
+}
+
+struct Base {
+  virtual ~Base() = default;
+};
+
+struct Derived : public Base {};
+
+TEST(DemangleStringTest, SupportsSymbolNameReturnedByTypeId) {
+  EXPECT_EQ(DemangleString(typeid(int).name()), "int");
+  // We want to test that `DemangleString` can demangle the symbol names
+  // returned by `typeid`, but without hard-coding the actual demangled values
+  // (because they are platform-specific).
+  EXPECT_THAT(
+      DemangleString(typeid(Base).name()),
+      ContainsRegex("absl.*debugging_internal.*anonymous namespace.*::Base"));
+  EXPECT_THAT(DemangleString(typeid(Derived).name()),
+              ContainsRegex(
+                  "absl.*debugging_internal.*anonymous namespace.*::Derived"));
+}
+
 }  // namespace
 }  // namespace debugging_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/debugging/internal/elf_mem_image.cc b/absl/debugging/internal/elf_mem_image.cc
index e7408bca8e4..2c168309839 100644
--- a/absl/debugging/internal/elf_mem_image.cc
+++ b/absl/debugging/internal/elf_mem_image.cc
@@ -20,8 +20,12 @@
 #ifdef ABSL_HAVE_ELF_MEM_IMAGE  // defined in elf_mem_image.h
 
 #include <string.h>
+
 #include <cassert>
 #include <cstddef>
+#include <cstdint>
+
+#include "absl/base/config.h"
 #include "absl/base/internal/raw_logging.h"
 
 // From binutils/include/elf/common.h (this doesn't appear to be documented
@@ -38,15 +42,16 @@
 #define VERSYM_VERSION 0x7fff
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace debugging_internal {
 
 namespace {
 
-#if __WORDSIZE == 32
+#if __SIZEOF_POINTER__ == 4
 const int kElfClass = ELFCLASS32;
 int ElfBind(const ElfW(Sym) *symbol) { return ELF32_ST_BIND(symbol->st_info); }
 int ElfType(const ElfW(Sym) *symbol) { return ELF32_ST_TYPE(symbol->st_info); }
-#elif __WORDSIZE == 64
+#elif __SIZEOF_POINTER__ == 8
 const int kElfClass = ELFCLASS64;
 int ElfBind(const ElfW(Sym) *symbol) { return ELF64_ST_BIND(symbol->st_info); }
 int ElfType(const ElfW(Sym) *symbol) { return ELF64_ST_TYPE(symbol->st_info); }
@@ -84,30 +89,22 @@ ElfMemImage::ElfMemImage(const void *base) {
   Init(base);
 }
 
-int ElfMemImage::GetNumSymbols() const {
-  if (!hash_) {
-    return 0;
-  }
-  // See http://www.caldera.com/developers/gabi/latest/ch5.dynamic.html#hash
-  return hash_[1];
-}
+uint32_t ElfMemImage::GetNumSymbols() const { return num_syms_; }
 
-const ElfW(Sym) *ElfMemImage::GetDynsym(int index) const {
+const ElfW(Sym) * ElfMemImage::GetDynsym(uint32_t index) const {
   ABSL_RAW_CHECK(index < GetNumSymbols(), "index out of range");
   return dynsym_ + index;
 }
 
-const ElfW(Versym) *ElfMemImage::GetVersym(int index) const {
+const ElfW(Versym) *ElfMemImage::GetVersym(uint32_t index) const {
   ABSL_RAW_CHECK(index < GetNumSymbols(), "index out of range");
   return versym_ + index;
 }
 
 const ElfW(Phdr) *ElfMemImage::GetPhdr(int index) const {
-  ABSL_RAW_CHECK(index < ehdr_->e_phnum, "index out of range");
-  return GetTableElement<ElfW(Phdr)>(ehdr_,
-                                     ehdr_->e_phoff,
-                                     ehdr_->e_phentsize,
-                                     index);
+  ABSL_RAW_CHECK(index >= 0 && index < ehdr_->e_phnum, "index out of range");
+  return GetTableElement<ElfW(Phdr)>(ehdr_, ehdr_->e_phoff, ehdr_->e_phentsize,
+                                     static_cast<size_t>(index));
 }
 
 const char *ElfMemImage::GetDynstr(ElfW(Word) offset) const {
@@ -154,10 +151,11 @@ void ElfMemImage::Init(const void *base) {
   dynstr_    = nullptr;
   versym_    = nullptr;
   verdef_    = nullptr;
-  hash_      = nullptr;
+  num_syms_ = 0;
   strsize_   = 0;
   verdefnum_ = 0;
-  link_base_ = ~0L;  // Sentinel: PT_LOAD .p_vaddr can't possibly be this.
+  // Sentinel: PT_LOAD .p_vaddr can't possibly be this.
+  link_base_ = ~ElfW(Addr){0};  // NOLINT(readability/braces)
   if (!base) {
     return;
   }
@@ -174,17 +172,17 @@ void ElfMemImage::Init(const void *base) {
   }
   switch (base_as_char[EI_DATA]) {
     case ELFDATA2LSB: {
-      if (__LITTLE_ENDIAN != __BYTE_ORDER) {
-        assert(false);
-        return;
-      }
+#ifndef ABSL_IS_LITTLE_ENDIAN
+      assert(false);
+      return;
+#endif
       break;
     }
     case ELFDATA2MSB: {
-      if (__BIG_ENDIAN != __BYTE_ORDER) {
-        assert(false);
-        return;
-      }
+#ifndef ABSL_IS_BIG_ENDIAN
+      assert(false);
+      return;
+#endif
       break;
     }
     default: {
@@ -216,14 +214,19 @@ void ElfMemImage::Init(const void *base) {
   }
   ptrdiff_t relocation =
       base_as_char - reinterpret_cast<const char *>(link_base_);
-  ElfW(Dyn) *dynamic_entry =
-      reinterpret_cast<ElfW(Dyn) *>(dynamic_program_header->p_vaddr +
-                                    relocation);
+  ElfW(Dyn)* dynamic_entry = reinterpret_cast<ElfW(Dyn)*>(
+      static_cast<intptr_t>(dynamic_program_header->p_vaddr) + relocation);
+  uint32_t *sysv_hash = nullptr;
+  uint32_t *gnu_hash = nullptr;
   for (; dynamic_entry->d_tag != DT_NULL; ++dynamic_entry) {
-    const ElfW(Xword) value = dynamic_entry->d_un.d_val + relocation;
+    const auto value =
+        static_cast<intptr_t>(dynamic_entry->d_un.d_val) + relocation;
     switch (dynamic_entry->d_tag) {
       case DT_HASH:
-        hash_ = reinterpret_cast<ElfW(Word) *>(value);
+        sysv_hash = reinterpret_cast<uint32_t *>(value);
+        break;
+      case DT_GNU_HASH:
+        gnu_hash = reinterpret_cast<uint32_t *>(value);
         break;
       case DT_SYMTAB:
         dynsym_ = reinterpret_cast<ElfW(Sym) *>(value);
@@ -238,23 +241,48 @@ void ElfMemImage::Init(const void *base) {
         verdef_ = reinterpret_cast<ElfW(Verdef) *>(value);
         break;
       case DT_VERDEFNUM:
-        verdefnum_ = dynamic_entry->d_un.d_val;
+        verdefnum_ = static_cast<size_t>(dynamic_entry->d_un.d_val);
         break;
       case DT_STRSZ:
-        strsize_ = dynamic_entry->d_un.d_val;
+        strsize_ = static_cast<size_t>(dynamic_entry->d_un.d_val);
         break;
       default:
         // Unrecognized entries explicitly ignored.
         break;
     }
   }
-  if (!hash_ || !dynsym_ || !dynstr_ || !versym_ ||
+  if ((!sysv_hash && !gnu_hash) || !dynsym_ || !dynstr_ || !versym_ ||
       !verdef_ || !verdefnum_ || !strsize_) {
     assert(false);  // invalid VDSO
     // Mark this image as not present. Can not recur infinitely.
     Init(nullptr);
     return;
   }
+  if (sysv_hash) {
+    num_syms_ = sysv_hash[1];
+  } else {
+    assert(gnu_hash);
+    // Compute the number of symbols for DT_GNU_HASH, which is specified by
+    // https://sourceware.org/gnu-gabi/program-loading-and-dynamic-linking.txt
+    uint32_t nbuckets = gnu_hash[0];
+    // The buckets array is located after the header (4 uint32) and the bloom
+    // filter (size_t array of gnu_hash[2] elements).
+    uint32_t *buckets = gnu_hash + 4 + sizeof(size_t) / 4 * gnu_hash[2];
+    // Find the chain of the last non-empty bucket.
+    uint32_t idx = 0;
+    for (uint32_t i = nbuckets; i > 0;) {
+      idx = buckets[--i];
+      if (idx != 0) break;
+    }
+    if (idx != 0) {
+      // Find the last element of the chain, which has an odd value.
+      // Add one to get the number of symbols.
+      uint32_t *chain = buckets + nbuckets - gnu_hash[1];
+      while (chain[idx++] % 2 == 0) {
+      }
+    }
+    num_syms_ = idx;
+  }
 }
 
 bool ElfMemImage::LookupSymbol(const char *name,
@@ -299,9 +327,9 @@ bool ElfMemImage::LookupSymbolByAddress(const void *address,
   return false;
 }
 
-ElfMemImage::SymbolIterator::SymbolIterator(const void *const image, int index)
-    : index_(index), image_(image) {
-}
+ElfMemImage::SymbolIterator::SymbolIterator(const void *const image,
+                                            uint32_t index)
+    : index_(index), image_(image) {}
 
 const ElfMemImage::SymbolInfo *ElfMemImage::SymbolIterator::operator->() const {
   return &info_;
@@ -334,7 +362,7 @@ ElfMemImage::SymbolIterator ElfMemImage::end() const {
   return SymbolIterator(this, GetNumSymbols());
 }
 
-void ElfMemImage::SymbolIterator::Update(int increment) {
+void ElfMemImage::SymbolIterator::Update(uint32_t increment) {
   const ElfMemImage *image = reinterpret_cast<const ElfMemImage *>(image_);
   ABSL_RAW_CHECK(image->IsPresent() || increment == 0, "");
   if (!image->IsPresent()) {
@@ -349,7 +377,11 @@ void ElfMemImage::SymbolIterator::Update(int increment) {
   const ElfW(Versym) *version_symbol = image->GetVersym(index_);
   ABSL_RAW_CHECK(symbol && version_symbol, "");
   const char *const symbol_name = image->GetDynstr(symbol->st_name);
+#if defined(__NetBSD__)
+  const int version_index = version_symbol->vs_vers & VERSYM_VERSION;
+#else
   const ElfW(Versym) version_index = version_symbol[0] & VERSYM_VERSION;
+#endif
   const ElfW(Verdef) *version_definition = nullptr;
   const char *version_name = "";
   if (symbol->st_shndx == SHN_UNDEF) {
@@ -375,6 +407,7 @@ void ElfMemImage::SymbolIterator::Update(int increment) {
 }
 
 }  // namespace debugging_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_HAVE_ELF_MEM_IMAGE
diff --git a/absl/debugging/internal/elf_mem_image.h b/absl/debugging/internal/elf_mem_image.h
index d84200db1e1..4d31a474cfd 100644
--- a/absl/debugging/internal/elf_mem_image.h
+++ b/absl/debugging/internal/elf_mem_image.h
@@ -22,6 +22,10 @@
 // Including this will define the __GLIBC__ macro if glibc is being
 // used.
 #include <climits>
+#include <cstddef>
+#include <cstdint>
+
+#include "absl/base/config.h"
 
 // Maybe one day we can rewrite this file not to require the elf
 // symbol extensions in glibc, but for right now we need them.
@@ -29,8 +33,10 @@
 #error ABSL_HAVE_ELF_MEM_IMAGE cannot be directly set
 #endif
 
-#if defined(__ELF__) && defined(__GLIBC__) && !defined(__native_client__) && \
-    !defined(__asmjs__) && !defined(__wasm__)
+#if defined(__ELF__) && !defined(__OpenBSD__) && !defined(__QNX__) &&    \
+    !defined(__asmjs__) && !defined(__wasm__) && !defined(__HAIKU__) &&  \
+    !defined(__sun) && !defined(__VXWORKS__) && !defined(__hexagon__) && \
+    !defined(__XTENSA__)
 #define ABSL_HAVE_ELF_MEM_IMAGE 1
 #endif
 
@@ -38,7 +44,12 @@
 
 #include <link.h>  // for ElfW
 
+#if defined(__FreeBSD__) && !defined(ElfW)
+#define ElfW(x) __ElfN(x)
+#endif
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace debugging_internal {
 
 // An in-memory ELF image (may not exist on disk).
@@ -73,10 +84,10 @@ class ElfMemImage {
     bool operator!=(const SymbolIterator &rhs) const;
     bool operator==(const SymbolIterator &rhs) const;
    private:
-    SymbolIterator(const void *const image, int index);
-    void Update(int incr);
+    SymbolIterator(const void *const image, uint32_t index);
+    void Update(uint32_t incr);
     SymbolInfo info_;
-    int index_;
+    uint32_t index_;
     const void *const image_;
   };
 
@@ -85,14 +96,14 @@ class ElfMemImage {
   void                 Init(const void *base);
   bool                 IsPresent() const { return ehdr_ != nullptr; }
   const ElfW(Phdr)*    GetPhdr(int index) const;
-  const ElfW(Sym)*     GetDynsym(int index) const;
-  const ElfW(Versym)*  GetVersym(int index) const;
+  const ElfW(Sym) * GetDynsym(uint32_t index) const;
+  const ElfW(Versym)*  GetVersym(uint32_t index) const;
   const ElfW(Verdef)*  GetVerdef(int index) const;
   const ElfW(Verdaux)* GetVerdefAux(const ElfW(Verdef) *verdef) const;
   const char*          GetDynstr(ElfW(Word) offset) const;
   const void*          GetSymAddr(const ElfW(Sym) *sym) const;
   const char*          GetVerstr(ElfW(Word) offset) const;
-  int                  GetNumSymbols() const;
+  uint32_t GetNumSymbols() const;
 
   SymbolIterator begin() const;
   SymbolIterator end() const;
@@ -115,14 +126,15 @@ class ElfMemImage {
   const ElfW(Sym) *dynsym_;
   const ElfW(Versym) *versym_;
   const ElfW(Verdef) *verdef_;
-  const ElfW(Word) *hash_;
   const char *dynstr_;
+  uint32_t num_syms_;
   size_t strsize_;
   size_t verdefnum_;
   ElfW(Addr) link_base_;     // Link-time base (p_vaddr of first PT_LOAD).
 };
 
 }  // namespace debugging_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_HAVE_ELF_MEM_IMAGE
diff --git a/absl/debugging/internal/examine_stack.cc b/absl/debugging/internal/examine_stack.cc
index 1ebc788fe3d..9869c18a732 100644
--- a/absl/debugging/internal/examine_stack.cc
+++ b/absl/debugging/internal/examine_stack.cc
@@ -16,51 +16,217 @@
 
 #include "absl/debugging/internal/examine_stack.h"
 
-#ifndef _WIN32
-#include <unistd.h>
-#endif
-
 #include <csignal>
+#include <cstdint>
 #include <cstdio>
+#include <iterator>
 
 #include "absl/base/attributes.h"
+#include "absl/base/config.h"
 #include "absl/base/internal/raw_logging.h"
 #include "absl/base/macros.h"
 #include "absl/debugging/stacktrace.h"
 #include "absl/debugging/symbolize.h"
 
+#ifndef _WIN32
+#include <unistd.h>
+#endif
+
+#ifdef ABSL_HAVE_MMAP
+#include <sys/mman.h>
+#if defined(MAP_ANON) && !defined(MAP_ANONYMOUS)
+#define MAP_ANONYMOUS MAP_ANON
+#endif
+#endif
+
+#if defined(__linux__) || defined(__APPLE__)
+#include <sys/ucontext.h>
+#endif
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace debugging_internal {
 
+namespace {
+constexpr int kDefaultDumpStackFramesLimit = 64;
+// The %p field width for printf() functions is two characters per byte,
+// and two extra for the leading "0x".
+constexpr int kPrintfPointerFieldWidth = 2 + 2 * sizeof(void*);
+
+ABSL_CONST_INIT SymbolizeUrlEmitter debug_stack_trace_hook = nullptr;
+
+// Async-signal safe mmap allocator.
+void* Allocate(size_t num_bytes) {
+#ifdef ABSL_HAVE_MMAP
+  void* p = ::mmap(nullptr, num_bytes, PROT_READ | PROT_WRITE,
+                   MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+  return p == MAP_FAILED ? nullptr : p;
+#else
+  (void)num_bytes;
+  return nullptr;
+#endif  // ABSL_HAVE_MMAP
+}
+
+void Deallocate(void* p, size_t size) {
+#ifdef ABSL_HAVE_MMAP
+  ::munmap(p, size);
+#else
+  (void)p;
+  (void)size;
+#endif  // ABSL_HAVE_MMAP
+}
+
+// Print a program counter only.
+void DumpPC(OutputWriter* writer, void* writer_arg, void* const pc,
+            const char* const prefix) {
+  char buf[100];
+  snprintf(buf, sizeof(buf), "%s@ %*p\n", prefix, kPrintfPointerFieldWidth, pc);
+  writer(buf, writer_arg);
+}
+
+// Print a program counter and the corresponding stack frame size.
+void DumpPCAndFrameSize(OutputWriter* writer, void* writer_arg, void* const pc,
+                        int framesize, const char* const prefix) {
+  char buf[100];
+  if (framesize <= 0) {
+    snprintf(buf, sizeof(buf), "%s@ %*p  (unknown)\n", prefix,
+             kPrintfPointerFieldWidth, pc);
+  } else {
+    snprintf(buf, sizeof(buf), "%s@ %*p  %9d\n", prefix,
+             kPrintfPointerFieldWidth, pc, framesize);
+  }
+  writer(buf, writer_arg);
+}
+
+// Print a program counter and the corresponding symbol.
+void DumpPCAndSymbol(OutputWriter* writer, void* writer_arg, void* const pc,
+                     const char* const prefix) {
+  char tmp[1024];
+  const char* symbol = "(unknown)";
+  // Symbolizes the previous address of pc because pc may be in the
+  // next function.  The overrun happens when the function ends with
+  // a call to a function annotated noreturn (e.g. CHECK).
+  // If symbolization of pc-1 fails, also try pc on the off-chance
+  // that we crashed on the first instruction of a function (that
+  // actually happens very often for e.g. __restore_rt).
+  const uintptr_t prev_pc = reinterpret_cast<uintptr_t>(pc) - 1;
+  if (absl::Symbolize(reinterpret_cast<const char*>(prev_pc), tmp,
+                      sizeof(tmp)) ||
+      absl::Symbolize(pc, tmp, sizeof(tmp))) {
+    symbol = tmp;
+  }
+  char buf[1024];
+  snprintf(buf, sizeof(buf), "%s@ %*p  %s\n", prefix, kPrintfPointerFieldWidth,
+           pc, symbol);
+  writer(buf, writer_arg);
+}
+
+// Print a program counter, its stack frame size, and its symbol name.
+// Note that there is a separate symbolize_pc argument. Return addresses may be
+// at the end of the function, and this allows the caller to back up from pc if
+// appropriate.
+void DumpPCAndFrameSizeAndSymbol(OutputWriter* writer, void* writer_arg,
+                                 void* const pc, void* const symbolize_pc,
+                                 int framesize, const char* const prefix) {
+  char tmp[1024];
+  const char* symbol = "(unknown)";
+  if (absl::Symbolize(symbolize_pc, tmp, sizeof(tmp))) {
+    symbol = tmp;
+  }
+  char buf[1024];
+  if (framesize <= 0) {
+    snprintf(buf, sizeof(buf), "%s@ %*p  (unknown)  %s\n", prefix,
+             kPrintfPointerFieldWidth, pc, symbol);
+  } else {
+    snprintf(buf, sizeof(buf), "%s@ %*p  %9d  %s\n", prefix,
+             kPrintfPointerFieldWidth, pc, framesize, symbol);
+  }
+  writer(buf, writer_arg);
+}
+
+}  // namespace
+
+void RegisterDebugStackTraceHook(SymbolizeUrlEmitter hook) {
+  debug_stack_trace_hook = hook;
+}
+
+SymbolizeUrlEmitter GetDebugStackTraceHook() { return debug_stack_trace_hook; }
+
 // Returns the program counter from signal context, nullptr if
 // unknown. vuc is a ucontext_t*. We use void* to avoid the use of
 // ucontext_t on non-POSIX systems.
-void* GetProgramCounter(void* vuc) {
+void* GetProgramCounter(void* const vuc) {
 #ifdef __linux__
   if (vuc != nullptr) {
     ucontext_t* context = reinterpret_cast<ucontext_t*>(vuc);
 #if defined(__aarch64__)
     return reinterpret_cast<void*>(context->uc_mcontext.pc);
+#elif defined(__alpha__)
+    return reinterpret_cast<void*>(context->uc_mcontext.sc_pc);
 #elif defined(__arm__)
     return reinterpret_cast<void*>(context->uc_mcontext.arm_pc);
+#elif defined(__hppa__)
+    return reinterpret_cast<void*>(context->uc_mcontext.sc_iaoq[0]);
 #elif defined(__i386__)
-    if (14 < ABSL_ARRAYSIZE(context->uc_mcontext.gregs))
+    if (14 < std::size(context->uc_mcontext.gregs))
       return reinterpret_cast<void*>(context->uc_mcontext.gregs[14]);
+#elif defined(__ia64__)
+    return reinterpret_cast<void*>(context->uc_mcontext.sc_ip);
+#elif defined(__m68k__)
+    return reinterpret_cast<void*>(context->uc_mcontext.gregs[16]);
 #elif defined(__mips__)
     return reinterpret_cast<void*>(context->uc_mcontext.pc);
 #elif defined(__powerpc64__)
     return reinterpret_cast<void*>(context->uc_mcontext.gp_regs[32]);
 #elif defined(__powerpc__)
-    return reinterpret_cast<void*>(context->uc_mcontext.regs->nip);
+    return reinterpret_cast<void*>(context->uc_mcontext.uc_regs->gregs[32]);
+#elif defined(__riscv)
+    return reinterpret_cast<void*>(context->uc_mcontext.__gregs[REG_PC]);
 #elif defined(__s390__) && !defined(__s390x__)
     return reinterpret_cast<void*>(context->uc_mcontext.psw.addr & 0x7fffffff);
 #elif defined(__s390__) && defined(__s390x__)
     return reinterpret_cast<void*>(context->uc_mcontext.psw.addr);
+#elif defined(__sh__)
+    return reinterpret_cast<void*>(context->uc_mcontext.pc);
+#elif defined(__sparc__) && !defined(__arch64__)
+    return reinterpret_cast<void*>(context->uc_mcontext.gregs[19]);
+#elif defined(__sparc__) && defined(__arch64__)
+    return reinterpret_cast<void*>(context->uc_mcontext.mc_gregs[19]);
 #elif defined(__x86_64__)
-    if (16 < ABSL_ARRAYSIZE(context->uc_mcontext.gregs))
+    if (16 < std::size(context->uc_mcontext.gregs))
       return reinterpret_cast<void*>(context->uc_mcontext.gregs[16]);
+#elif defined(__e2k__)
+    return reinterpret_cast<void*>(context->uc_mcontext.cr0_hi);
+#elif defined(__loongarch__)
+    return reinterpret_cast<void*>(context->uc_mcontext.__pc);
 #else
 #error "Undefined Architecture."
+#endif
+  }
+#elif defined(__APPLE__)
+  if (vuc != nullptr) {
+    ucontext_t* signal_ucontext = reinterpret_cast<ucontext_t*>(vuc);
+#if defined(__aarch64__)
+    return reinterpret_cast<void*>(
+        __darwin_arm_thread_state64_get_pc(signal_ucontext->uc_mcontext->__ss));
+#elif defined(__arm__)
+#if __DARWIN_UNIX03
+    return reinterpret_cast<void*>(signal_ucontext->uc_mcontext->__ss.__pc);
+#else
+    return reinterpret_cast<void*>(signal_ucontext->uc_mcontext->ss.pc);
+#endif
+#elif defined(__i386__)
+#if __DARWIN_UNIX03
+    return reinterpret_cast<void*>(signal_ucontext->uc_mcontext->__ss.__eip);
+#else
+    return reinterpret_cast<void*>(signal_ucontext->uc_mcontext->ss.eip);
+#endif
+#elif defined(__x86_64__)
+#if __DARWIN_UNIX03
+    return reinterpret_cast<void*>(signal_ucontext->uc_mcontext->__ss.__rip);
+#else
+    return reinterpret_cast<void*>(signal_ucontext->uc_mcontext->ss.rip);
+#endif
 #endif
   }
 #elif defined(__akaros__)
@@ -71,59 +237,17 @@ void* GetProgramCounter(void* vuc) {
   return nullptr;
 }
 
-// The %p field width for printf() functions is two characters per byte,
-// and two extra for the leading "0x".
-static constexpr int kPrintfPointerFieldWidth = 2 + 2 * sizeof(void*);
-
-// Print a program counter, its stack frame size, and its symbol name.
-// Note that there is a separate symbolize_pc argument. Return addresses may be
-// at the end of the function, and this allows the caller to back up from pc if
-// appropriate.
-static void DumpPCAndFrameSizeAndSymbol(void (*writerfn)(const char*, void*),
-                                        void* writerfn_arg, void* pc,
-                                        void* symbolize_pc, int framesize,
-                                        const char* const prefix) {
-  char tmp[1024];
-  const char* symbol = "(unknown)";
-  if (absl::Symbolize(symbolize_pc, tmp, sizeof(tmp))) {
-    symbol = tmp;
-  }
-  char buf[1024];
-  if (framesize <= 0) {
-    snprintf(buf, sizeof(buf), "%s@ %*p  (unknown)  %s\n", prefix,
-             kPrintfPointerFieldWidth, pc, symbol);
-  } else {
-    snprintf(buf, sizeof(buf), "%s@ %*p  %9d  %s\n", prefix,
-             kPrintfPointerFieldWidth, pc, framesize, symbol);
-  }
-  writerfn(buf, writerfn_arg);
-}
-
-// Print a program counter and the corresponding stack frame size.
-static void DumpPCAndFrameSize(void (*writerfn)(const char*, void*),
-                               void* writerfn_arg, void* pc, int framesize,
-                               const char* const prefix) {
-  char buf[100];
-  if (framesize <= 0) {
-    snprintf(buf, sizeof(buf), "%s@ %*p  (unknown)\n", prefix,
-             kPrintfPointerFieldWidth, pc);
-  } else {
-    snprintf(buf, sizeof(buf), "%s@ %*p  %9d\n", prefix,
-             kPrintfPointerFieldWidth, pc, framesize);
-  }
-  writerfn(buf, writerfn_arg);
-}
-
-void DumpPCAndFrameSizesAndStackTrace(
-    void* pc, void* const stack[], int frame_sizes[], int depth,
-    int min_dropped_frames, bool symbolize_stacktrace,
-    void (*writerfn)(const char*, void*), void* writerfn_arg) {
+void DumpPCAndFrameSizesAndStackTrace(void* const pc, void* const stack[],
+                                      int frame_sizes[], int depth,
+                                      int min_dropped_frames,
+                                      bool symbolize_stacktrace,
+                                      OutputWriter* writer, void* writer_arg) {
   if (pc != nullptr) {
     // We don't know the stack frame size for PC, use 0.
     if (symbolize_stacktrace) {
-      DumpPCAndFrameSizeAndSymbol(writerfn, writerfn_arg, pc, pc, 0, "PC: ");
+      DumpPCAndFrameSizeAndSymbol(writer, writer_arg, pc, pc, 0, "PC: ");
     } else {
-      DumpPCAndFrameSize(writerfn, writerfn_arg, pc, 0, "PC: ");
+      DumpPCAndFrameSize(writer, writer_arg, pc, 0, "PC: ");
     }
   }
   for (int i = 0; i < depth; i++) {
@@ -133,21 +257,65 @@ void DumpPCAndFrameSizesAndStackTrace(
       // call to a function annotated noreturn (e.g. CHECK). Note that we don't
       // do this for pc above, as the adjustment is only correct for return
       // addresses.
-      DumpPCAndFrameSizeAndSymbol(writerfn, writerfn_arg, stack[i],
+      DumpPCAndFrameSizeAndSymbol(writer, writer_arg, stack[i],
                                   reinterpret_cast<char*>(stack[i]) - 1,
                                   frame_sizes[i], "    ");
     } else {
-      DumpPCAndFrameSize(writerfn, writerfn_arg, stack[i], frame_sizes[i],
-                         "    ");
+      DumpPCAndFrameSize(writer, writer_arg, stack[i], frame_sizes[i], "    ");
     }
   }
   if (min_dropped_frames > 0) {
     char buf[100];
     snprintf(buf, sizeof(buf), "    @ ... and at least %d more frames\n",
              min_dropped_frames);
-    writerfn(buf, writerfn_arg);
+    writer(buf, writer_arg);
+  }
+}
+
+// Dump current stack trace as directed by writer.
+// Make sure this function is not inlined to avoid skipping too many top frames.
+ABSL_ATTRIBUTE_NOINLINE
+void DumpStackTrace(int min_dropped_frames, int max_num_frames,
+                    bool symbolize_stacktrace, OutputWriter* writer,
+                    void* writer_arg) {
+  // Print stack trace
+  void* stack_buf[kDefaultDumpStackFramesLimit];
+  void** stack = stack_buf;
+  int num_stack = kDefaultDumpStackFramesLimit;
+  size_t allocated_bytes = 0;
+
+  if (num_stack >= max_num_frames) {
+    // User requested fewer frames than we already have space for.
+    num_stack = max_num_frames;
+  } else {
+    const size_t needed_bytes =
+        static_cast<size_t>(max_num_frames) * sizeof(stack[0]);
+    void* p = Allocate(needed_bytes);
+    if (p != nullptr) {  // We got the space.
+      num_stack = max_num_frames;
+      stack = reinterpret_cast<void**>(p);
+      allocated_bytes = needed_bytes;
+    }
+  }
+
+  int depth = absl::GetStackTrace(stack, num_stack, min_dropped_frames + 1);
+  for (int i = 0; i < depth; i++) {
+    if (symbolize_stacktrace) {
+      DumpPCAndSymbol(writer, writer_arg, stack[static_cast<size_t>(i)],
+                      "    ");
+    } else {
+      DumpPC(writer, writer_arg, stack[static_cast<size_t>(i)], "    ");
+    }
+  }
+
+  auto hook = GetDebugStackTraceHook();
+  if (hook != nullptr) {
+    hook(stack, depth, /*crash_pc=*/nullptr, writer, writer_arg);
   }
+
+  if (allocated_bytes != 0) Deallocate(stack, allocated_bytes);
 }
 
 }  // namespace debugging_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/debugging/internal/examine_stack.h b/absl/debugging/internal/examine_stack.h
index 56c9763eb73..5fa897d78a5 100644
--- a/absl/debugging/internal/examine_stack.h
+++ b/absl/debugging/internal/examine_stack.h
@@ -17,22 +17,49 @@
 #ifndef ABSL_DEBUGGING_INTERNAL_EXAMINE_STACK_H_
 #define ABSL_DEBUGGING_INTERNAL_EXAMINE_STACK_H_
 
+#include "absl/base/config.h"
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace debugging_internal {
 
+// Type of function used for printing in stack trace dumping, etc.
+// We avoid closures to keep things simple.
+typedef void OutputWriter(const char*, void*);
+
+// RegisterDebugStackTraceHook() allows to register a single routine
+// `hook` that is called each time DumpStackTrace() is called.
+// `hook` may be called from a signal handler.
+typedef void (*SymbolizeUrlEmitter)(void* const stack[], int depth,
+                                    const void* crash_pc, OutputWriter* writer,
+                                    void* writer_arg);
+
+// Registration of SymbolizeUrlEmitter for use inside of a signal handler.
+// This is inherently unsafe and must be signal safe code.
+void RegisterDebugStackTraceHook(SymbolizeUrlEmitter hook);
+SymbolizeUrlEmitter GetDebugStackTraceHook();
+
 // Returns the program counter from signal context, or nullptr if
 // unknown. `vuc` is a ucontext_t*. We use void* to avoid the use of
 // ucontext_t on non-POSIX systems.
-void* GetProgramCounter(void* vuc);
+void* GetProgramCounter(void* const vuc);
 
-// Uses `writerfn` to dump the program counter, stack trace, and stack
+// Uses `writer` to dump the program counter, stack trace, and stack
 // frame sizes.
-void DumpPCAndFrameSizesAndStackTrace(
-    void* pc, void* const stack[], int frame_sizes[], int depth,
-    int min_dropped_frames, bool symbolize_stacktrace,
-    void (*writerfn)(const char*, void*), void* writerfn_arg);
+void DumpPCAndFrameSizesAndStackTrace(void* const pc, void* const stack[],
+                                      int frame_sizes[], int depth,
+                                      int min_dropped_frames,
+                                      bool symbolize_stacktrace,
+                                      OutputWriter* writer, void* writer_arg);
+
+// Dump current stack trace omitting the topmost `min_dropped_frames` stack
+// frames.
+void DumpStackTrace(int min_dropped_frames, int max_num_frames,
+                    bool symbolize_stacktrace, OutputWriter* writer,
+                    void* writer_arg);
 
 }  // namespace debugging_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_DEBUGGING_INTERNAL_EXAMINE_STACK_H_
diff --git a/absl/debugging/internal/stack_consumption.cc b/absl/debugging/internal/stack_consumption.cc
index 4b05f4955ee..b54a1b28c01 100644
--- a/absl/debugging/internal/stack_consumption.cc
+++ b/absl/debugging/internal/stack_consumption.cc
@@ -18,15 +18,19 @@
 #ifdef ABSL_INTERNAL_HAVE_DEBUGGING_STACK_CONSUMPTION
 
 #include <signal.h>
+#include <string.h>
 #include <sys/mman.h>
 #include <unistd.h>
 
-#include <string.h>
-
 #include "absl/base/attributes.h"
 #include "absl/base/internal/raw_logging.h"
 
+#if defined(MAP_ANON) && !defined(MAP_ANONYMOUS)
+#define MAP_ANONYMOUS MAP_ANON
+#endif
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace debugging_internal {
 namespace {
 
@@ -41,7 +45,8 @@ namespace {
 // one of them is null, the results of p<q, p>q, p<=q, and p>=q are
 // unspecified. Therefore, instead we hardcode the direction of the
 // stack on platforms we know about.
-#if defined(__i386__) || defined(__x86_64__) || defined(__ppc__)
+#if defined(__i386__) || defined(__x86_64__) || defined(__ppc__) || \
+    defined(__aarch64__) || defined(__riscv)
 constexpr bool kStackGrowsDown = true;
 #else
 #error Need to define kStackGrowsDown
@@ -115,10 +120,11 @@ int GetSignalHandlerStackConsumption(void (*signal_handler)(int)) {
   // Set up the alt-signal-stack (and save the older one).
   stack_t sigstk;
   memset(&sigstk, 0, sizeof(sigstk));
-  stack_t old_sigstk;
   sigstk.ss_sp = altstack;
   sigstk.ss_size = kAlternateStackSize;
   sigstk.ss_flags = 0;
+  stack_t old_sigstk;
+  memset(&old_sigstk, 0, sizeof(old_sigstk));
   ABSL_RAW_CHECK(sigaltstack(&sigstk, &old_sigstk) == 0,
                  "sigaltstack() failed");
 
@@ -152,6 +158,15 @@ int GetSignalHandlerStackConsumption(void (*signal_handler)(int)) {
   int signal_handler_stack_consumption = GetStackConsumption(altstack);
 
   // Now restore the old alt-signal-stack and signal handlers.
+  if (old_sigstk.ss_sp == nullptr && old_sigstk.ss_size == 0 &&
+      (old_sigstk.ss_flags & SS_DISABLE)) {
+    // https://git.musl-libc.org/cgit/musl/commit/src/signal/sigaltstack.c?id=7829f42a2c8944555439380498ab8b924d0f2070
+    // The original stack has ss_size==0 and ss_flags==SS_DISABLE, but some
+    // versions of musl have a bug that rejects ss_size==0. Work around this by
+    // setting ss_size to MINSIGSTKSZ, which should be ignored by the kernel
+    // when SS_DISABLE is set.
+    old_sigstk.ss_size = static_cast<size_t>(MINSIGSTKSZ);
+  }
   ABSL_RAW_CHECK(sigaltstack(&old_sigstk, nullptr) == 0,
                  "sigaltstack() failed");
   ABSL_RAW_CHECK(sigaction(SIGUSR1, &old_sa1, nullptr) == 0,
@@ -167,6 +182,25 @@ int GetSignalHandlerStackConsumption(void (*signal_handler)(int)) {
 }
 
 }  // namespace debugging_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#else
+
+// https://github.com/abseil/abseil-cpp/issues/1465
+// CMake builds on Apple platforms error when libraries are empty.
+// Our CMake configuration can avoid this error on header-only libraries,
+// but since this library is conditionally empty, including a single
+// variable is an easy workaround.
+#ifdef __APPLE__
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace debugging_internal {
+extern const char kAvoidEmptyStackConsumptionLibraryWarning;
+const char kAvoidEmptyStackConsumptionLibraryWarning = 0;
+}  // namespace debugging_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
+#endif  // __APPLE__
 
 #endif  // ABSL_INTERNAL_HAVE_DEBUGGING_STACK_CONSUMPTION
diff --git a/absl/debugging/internal/stack_consumption.h b/absl/debugging/internal/stack_consumption.h
index b860a3c1a17..f5ba5575d1b 100644
--- a/absl/debugging/internal/stack_consumption.h
+++ b/absl/debugging/internal/stack_consumption.h
@@ -18,15 +18,19 @@
 #ifndef ABSL_DEBUGGING_INTERNAL_STACK_CONSUMPTION_H_
 #define ABSL_DEBUGGING_INTERNAL_STACK_CONSUMPTION_H_
 
+#include "absl/base/config.h"
+
 // The code in this module is not portable.
 // Use this feature test macro to detect its availability.
 #ifdef ABSL_INTERNAL_HAVE_DEBUGGING_STACK_CONSUMPTION
 #error ABSL_INTERNAL_HAVE_DEBUGGING_STACK_CONSUMPTION cannot be set directly
-#elif !defined(__APPLE__) && !defined(_WIN32) && \
-    (defined(__i386__) || defined(__x86_64__) || defined(__ppc__))
+#elif !defined(__APPLE__) && !defined(_WIN32) && !defined(__Fuchsia__) && \
+    (defined(__i386__) || defined(__x86_64__) || defined(__ppc__) || \
+     defined(__aarch64__) || defined(__riscv))
 #define ABSL_INTERNAL_HAVE_DEBUGGING_STACK_CONSUMPTION 1
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace debugging_internal {
 
 // Returns the stack consumption in bytes for the code exercised by
@@ -38,6 +42,7 @@ namespace debugging_internal {
 int GetSignalHandlerStackConsumption(void (*signal_handler)(int));
 
 }  // namespace debugging_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_INTERNAL_HAVE_DEBUGGING_STACK_CONSUMPTION
diff --git a/absl/debugging/internal/stack_consumption_test.cc b/absl/debugging/internal/stack_consumption_test.cc
index 68bfa126e23..0255ac8f36c 100644
--- a/absl/debugging/internal/stack_consumption_test.cc
+++ b/absl/debugging/internal/stack_consumption_test.cc
@@ -20,9 +20,10 @@
 #include <string.h>
 
 #include "gtest/gtest.h"
-#include "absl/base/internal/raw_logging.h"
+#include "absl/log/log.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace debugging_internal {
 namespace {
 
@@ -32,7 +33,7 @@ static void SimpleSignalHandler(int signo) {
 
   // Never true, but prevents compiler from optimizing buf out.
   if (signo == 0) {
-    ABSL_RAW_LOG(INFO, "%p", static_cast<void*>(buf));
+    LOG(INFO) << static_cast<void*>(buf);
   }
 }
 
@@ -43,6 +44,7 @@ TEST(SignalHandlerStackConsumptionTest, MeasuresStackConsumption) {
 
 }  // namespace
 }  // namespace debugging_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_INTERNAL_HAVE_DEBUGGING_STACK_CONSUMPTION
diff --git a/absl/debugging/internal/stacktrace_aarch64-inl.inc b/absl/debugging/internal/stacktrace_aarch64-inl.inc
index 7ed6b3eb82d..79d685810e7 100644
--- a/absl/debugging/internal/stacktrace_aarch64-inl.inc
+++ b/absl/debugging/internal/stacktrace_aarch64-inl.inc
@@ -3,23 +3,31 @@
 
 // Generate stack tracer for aarch64
 
-#if defined(__linux__)
-#include <sys/mman.h>
-#include <ucontext.h>
-#include <unistd.h>
-#endif
-
 #include <atomic>
 #include <cassert>
+#include <cstddef>
 #include <cstdint>
 #include <iostream>
+#include <limits>
 
 #include "absl/base/attributes.h"
 #include "absl/debugging/internal/address_is_readable.h"
+#include "absl/debugging/internal/addresses.h"
 #include "absl/debugging/internal/vdso_support.h"  // a no-op on non-elf or non-glibc systems
 #include "absl/debugging/stacktrace.h"
 
-static const uintptr_t kUnknownFrameSize = 0;
+#if defined(__linux__)
+#include <signal.h>
+#include <sys/mman.h>
+#include <ucontext.h>
+#include <unistd.h>
+#endif
+
+static const size_t kUnknownFrameSize = 0;
+// Stack end to use when we don't know the actual stack end
+// (effectively just the end of address space).
+constexpr uintptr_t kUnknownStackEnd =
+    std::numeric_limits<size_t>::max() - sizeof(void *);
 
 #if defined(__linux__)
 // Returns the address of the VDSO __kernel_rt_sigreturn function, if present.
@@ -37,8 +45,11 @@ static const unsigned char* GetKernelRtSigreturnAddress() {
   absl::debugging_internal::VDSOSupport vdso;
   if (vdso.IsPresent()) {
     absl::debugging_internal::VDSOSupport::SymbolInfo symbol_info;
-    if (!vdso.LookupSymbol("__kernel_rt_sigreturn", "LINUX_2.6.39", STT_FUNC,
-                           &symbol_info) ||
+    auto lookup = [&](int type) {
+      return vdso.LookupSymbol("__kernel_rt_sigreturn", "LINUX_2.6.39", type,
+                               &symbol_info);
+    };
+    if ((!lookup(STT_FUNC) && !lookup(STT_NOTYPE)) ||
         symbol_info.address == nullptr) {
       // Unexpected: VDSO is present, yet the expected symbol is missing
       // or null.
@@ -62,11 +73,28 @@ static const unsigned char* GetKernelRtSigreturnAddress() {
 // Compute the size of a stack frame in [low..high).  We assume that
 // low < high.  Return size of kUnknownFrameSize.
 template<typename T>
-static inline uintptr_t ComputeStackFrameSize(const T* low,
-                                              const T* high) {
+static size_t ComputeStackFrameSize(const T* low,
+                                           const T* high) {
   const char* low_char_ptr = reinterpret_cast<const char *>(low);
   const char* high_char_ptr = reinterpret_cast<const char *>(high);
-  return low < high ? high_char_ptr - low_char_ptr : kUnknownFrameSize;
+  return low < high ? static_cast<size_t>(high_char_ptr - low_char_ptr)
+                    : kUnknownFrameSize;
+}
+
+// Saves stack info that is expensive to calculate to avoid recalculating per frame.
+struct StackInfo {
+  uintptr_t stack_low;
+  uintptr_t stack_high;
+  uintptr_t sig_stack_low;
+  uintptr_t sig_stack_high;
+};
+
+static bool InsideSignalStack(void** ptr, const StackInfo* stack_info) {
+  uintptr_t comparable_ptr = reinterpret_cast<uintptr_t>(ptr);
+  if (stack_info->sig_stack_high == kUnknownStackEnd)
+    return false;
+  return (comparable_ptr >= stack_info->sig_stack_low &&
+          comparable_ptr < stack_info->sig_stack_high);
 }
 
 // Given a pointer to a stack frame, locate and return the calling
@@ -74,9 +102,12 @@ static inline uintptr_t ComputeStackFrameSize(const T* low,
 // checks (the strictness of which is controlled by the boolean parameter
 // "STRICT_UNWINDING") to reduce the chance that a bad pointer is returned.
 template<bool STRICT_UNWINDING, bool WITH_CONTEXT>
-static void **NextStackFrame(void **old_frame_pointer, const void *uc) {
+ABSL_ATTRIBUTE_NO_SANITIZE_ADDRESS  // May read random elements from stack.
+ABSL_ATTRIBUTE_NO_SANITIZE_MEMORY   // May read random elements from stack.
+ABSL_ATTRIBUTE_NO_SANITIZE_THREAD   // May read random elements from stack.
+static void **NextStackFrame(void **old_frame_pointer, const void *uc,
+                             const StackInfo *stack_info) {
   void **new_frame_pointer = reinterpret_cast<void**>(*old_frame_pointer);
-  bool check_frame_size = true;
 
 #if defined(__linux__)
   if (WITH_CONTEXT && uc != nullptr) {
@@ -88,103 +119,186 @@ static void **NextStackFrame(void **old_frame_pointer, const void *uc) {
       void **const pre_signal_frame_pointer =
           reinterpret_cast<void **>(ucv->uc_mcontext.regs[29]);
 
-      // Check that alleged frame pointer is actually readable. This is to
-      // prevent "double fault" in case we hit the first fault due to e.g.
-      // stack corruption.
-      if (!absl::debugging_internal::AddressIsReadable(
-              pre_signal_frame_pointer))
-        return nullptr;
-
-      // Alleged frame pointer is readable, use it for further unwinding.
-      new_frame_pointer = pre_signal_frame_pointer;
-
-      // Skip frame size check if we return from a signal. We may be using a
-      // an alternate stack for signals.
-      check_frame_size = false;
+      // The most recent signal always needs special handling to find the frame
+      // pointer, but a nested signal does not.  If pre_signal_frame_pointer is
+      // earlier in the stack than the old_frame_pointer, then use it. If it is
+      // later, then we have already unwound through it and it needs no special
+      // handling.
+      if (pre_signal_frame_pointer > old_frame_pointer) {
+        new_frame_pointer = pre_signal_frame_pointer;
+      }
     }
   }
 #endif
 
-  // aarch64 ABI requires stack pointer to be 16-byte-aligned.
-  if ((reinterpret_cast<uintptr_t>(new_frame_pointer) & 15) != 0)
+  // The frame pointer should be 8-byte aligned.
+  if ((reinterpret_cast<uintptr_t>(new_frame_pointer) & 7) != 0)
     return nullptr;
 
-  // Check frame size.  In strict mode, we assume frames to be under
-  // 100,000 bytes.  In non-strict mode, we relax the limit to 1MB.
-  if (check_frame_size) {
-    const uintptr_t max_size = STRICT_UNWINDING ? 100000 : 1000000;
-    const uintptr_t frame_size =
+  uintptr_t new_fp_comparable = reinterpret_cast<uintptr_t>(new_frame_pointer);
+  // Only check the size if both frames are in the same stack.
+  const bool old_inside_signal_stack =
+      InsideSignalStack(old_frame_pointer, stack_info);
+  const bool new_inside_signal_stack =
+      InsideSignalStack(new_frame_pointer, stack_info);
+  if (new_inside_signal_stack == old_inside_signal_stack) {
+    // Check frame size.  In strict mode, we assume frames to be under
+    // 100,000 bytes.  In non-strict mode, we relax the limit to 1MB.
+    const size_t max_size = STRICT_UNWINDING ? 100000 : 1000000;
+    const size_t frame_size =
         ComputeStackFrameSize(old_frame_pointer, new_frame_pointer);
-    if (frame_size == kUnknownFrameSize || frame_size > max_size)
-      return nullptr;
+    if (frame_size == kUnknownFrameSize)
+       return nullptr;
+    // A very large frame may mean corrupt memory or an erroneous frame
+    // pointer. But also maybe just a plain-old large frame.  Assume that if the
+    // frame is within a known stack, then it is valid.
+    if (frame_size > max_size) {
+      size_t stack_low = stack_info->stack_low;
+      size_t stack_high = stack_info->stack_high;
+      if (new_inside_signal_stack) {
+        stack_low = stack_info->sig_stack_low;
+        stack_high = stack_info->sig_stack_high;
+      }
+      if (stack_high < kUnknownStackEnd &&
+          static_cast<size_t>(getpagesize()) < stack_low) {
+        // Stack bounds are known.
+        if (!(stack_low < new_fp_comparable &&
+              new_fp_comparable <= stack_high)) {
+          // new_frame_pointer is not within a known stack.
+          return nullptr;
+        }
+      } else {
+        // Stack bounds are unknown, prefer truncated stack to possible crash.
+        return nullptr;
+      }
+    }
+  }
+  // Verify that the candidate frame pointer lies inside a positively discovered
+  // and narrowed thread stack boundary. When stack boundaries are unknown (e.g.
+  // in open-source builds or outside Linux threads), stack_low defaults to
+  // getpagesize() and stack_high defaults to kUnknownStackEnd. Requiring both
+  // stack_high < kUnknownStackEnd and getpagesize() < stack_low ensures we only
+  // short-circuit AddressIsReadable() when thread stack bounds are known.
+  const bool new_inside_known_thread_stack =
+      stack_info->stack_high < kUnknownStackEnd &&
+      static_cast<size_t>(getpagesize()) < stack_info->stack_low &&
+      new_fp_comparable >= stack_info->stack_low &&
+      new_fp_comparable < stack_info->stack_high;
+  if (new_inside_signal_stack || new_inside_known_thread_stack ||
+      absl::debugging_internal::AddressIsReadable(new_frame_pointer)) {
+    return new_frame_pointer;
   }
 
-  return new_frame_pointer;
+  return nullptr;
 }
 
 template <bool IS_STACK_FRAMES, bool IS_WITH_CONTEXT>
-static int UnwindImpl(void** result, int* sizes, int max_depth, int skip_count,
-                      const void *ucp, int *min_dropped_frames) {
+// We count on the bottom frame being this one. See the comment
+// at prev_return_address
+ABSL_ATTRIBUTE_NOINLINE
+ABSL_ATTRIBUTE_NO_SANITIZE_ADDRESS  // May read random elements from stack.
+ABSL_ATTRIBUTE_NO_SANITIZE_MEMORY   // May read random elements from stack.
+ABSL_ATTRIBUTE_NO_SANITIZE_THREAD   // May read random elements from stack.
+static int UnwindImpl(void **result, uintptr_t *frames, int *sizes,
+                      int max_depth, int skip_count, const void *ucp,
+                      int *min_dropped_frames) {
 #ifdef __GNUC__
   void **frame_pointer = reinterpret_cast<void**>(__builtin_frame_address(0));
 #else
 # error reading stack point not yet supported on this platform.
 #endif
-
   skip_count++;    // Skip the frame for this function.
   int n = 0;
 
+  // Assume that the first page is not stack.
+  StackInfo stack_info;
+  stack_info.stack_low = static_cast<uintptr_t>(getpagesize());
+  stack_info.stack_high = kUnknownStackEnd;
+  stack_info.sig_stack_low = stack_info.stack_low;
+  stack_info.sig_stack_high = kUnknownStackEnd;
+
+#if defined(__linux__)
+  // Similarly for the signal stack, retrieve the information into sig_stack and
+  // then pass it down per call. Calling sigaltstack with nullptr as the first
+  // argument doesn't change the signal stack.
+  stack_t sig_stack;
+  if (sigaltstack(nullptr, &sig_stack) == 0) {
+    stack_info.sig_stack_low = reinterpret_cast<uintptr_t>(sig_stack.ss_sp);
+    stack_info.sig_stack_high = stack_info.sig_stack_low + sig_stack.ss_size;
+  }
+#endif
+
   // The frame pointer points to low address of a frame.  The first 64-bit
   // word of a frame points to the next frame up the call chain, which normally
   // is just after the high address of the current frame.  The second word of
-  // a frame contains return adress of to the caller.   To find a pc value
+  // a frame contains return address of to the caller.   To find a pc value
   // associated with the current frame, we need to go down a level in the call
   // chain.  So we remember return the address of the last frame seen.  This
   // does not work for the first stack frame, which belongs to UnwindImp() but
   // we skip the frame for UnwindImp() anyway.
   void* prev_return_address = nullptr;
+  // The nth frame size is the difference between the nth frame pointer and the
+  // the frame pointer below it in the call chain. There is no frame below the
+  // leaf frame, but this function is the leaf anyway, and we skip it.
+  void** prev_frame_pointer = nullptr;
 
-  while (frame_pointer && n < max_depth) {
-    // The absl::GetStackFrames routine is called when we are in some
-    // informational context (the failure signal handler for example).
-    // Use the non-strict unwinding rules to produce a stack trace
-    // that is as complete as possible (even if it contains a few bogus
-    // entries in some rare cases).
-    void **next_frame_pointer =
-        NextStackFrame<!IS_STACK_FRAMES, IS_WITH_CONTEXT>(frame_pointer, ucp);
-
+   while (frame_pointer && n < max_depth) {
     if (skip_count > 0) {
       skip_count--;
     } else {
-      result[n] = prev_return_address;
+      result[n] = reinterpret_cast<void *>(
+          absl::debugging_internal::StripPointerMetadata(prev_return_address));
       if (IS_STACK_FRAMES) {
-        sizes[n] = ComputeStackFrameSize(frame_pointer, next_frame_pointer);
+        if (frames != nullptr) {
+          frames[n] = absl::debugging_internal::StripPointerMetadata(
+                          prev_frame_pointer) +
+                      2 * sizeof(void *) /* go past the return address */;
+        }
+        if (sizes != nullptr) {
+          sizes[n] = static_cast<int>(
+              ComputeStackFrameSize(prev_frame_pointer, frame_pointer));
+        }
       }
       n++;
     }
     prev_return_address = frame_pointer[1];
-    frame_pointer = next_frame_pointer;
+    prev_frame_pointer = frame_pointer;
+    // The absl::GetStackFrames routine is called when we are in some
+    // informational context (the failure signal handler for example).
+    // Use the non-strict unwinding rules to produce a stack trace
+    // that is as complete as possible (even if it contains a few bogus
+    // entries in some rare cases).
+    frame_pointer = NextStackFrame<!IS_STACK_FRAMES, IS_WITH_CONTEXT>(
+        frame_pointer, ucp, &stack_info);
   }
+
   if (min_dropped_frames != nullptr) {
     // Implementation detail: we clamp the max of frames we are willing to
     // count, so as not to spend too much time in the loop below.
     const int kMaxUnwind = 200;
-    int j = 0;
-    for (; frame_pointer != nullptr && j < kMaxUnwind; j++) {
-      frame_pointer =
-          NextStackFrame<!IS_STACK_FRAMES, IS_WITH_CONTEXT>(frame_pointer, ucp);
+    int num_dropped_frames = 0;
+    for (int j = 0; frame_pointer != nullptr && j < kMaxUnwind; j++) {
+      if (skip_count > 0) {
+        skip_count--;
+      } else {
+        num_dropped_frames++;
+      }
+      frame_pointer = NextStackFrame<!IS_STACK_FRAMES, IS_WITH_CONTEXT>(
+          frame_pointer, ucp, &stack_info);
     }
-    *min_dropped_frames = j;
+    *min_dropped_frames = num_dropped_frames;
   }
   return n;
 }
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace debugging_internal {
 bool StackTraceWorksForTest() {
   return true;
 }
 }  // namespace debugging_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_DEBUGGING_INTERNAL_STACKTRACE_AARCH64_INL_H_
diff --git a/absl/debugging/internal/stacktrace_arm-inl.inc b/absl/debugging/internal/stacktrace_arm-inl.inc
index c84083379bb..3feb5218638 100644
--- a/absl/debugging/internal/stacktrace_arm-inl.inc
+++ b/absl/debugging/internal/stacktrace_arm-inl.inc
@@ -1,15 +1,25 @@
-// Copyright 2011 and onwards Google Inc.
-// All rights reserved.
+// Copyright 2017 The Abseil Authors.
 //
-// Author: Doug Kwan
-// This is inspired by Craig Silverstein's PowerPC stacktrace code.
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
 //
+// This is inspired by Craig Silverstein's PowerPC stacktrace code.
 
 #ifndef ABSL_DEBUGGING_INTERNAL_STACKTRACE_ARM_INL_H_
 #define ABSL_DEBUGGING_INTERNAL_STACKTRACE_ARM_INL_H_
 
 #include <cstdint>
 
+#include "absl/debugging/internal/addresses.h"
 #include "absl/debugging/stacktrace.h"
 
 // WARNING:
@@ -58,8 +68,9 @@ void StacktraceArmDummyFunction() { __asm__ volatile(""); }
 #endif
 
 template <bool IS_STACK_FRAMES, bool IS_WITH_CONTEXT>
-static int UnwindImpl(void** result, int* sizes, int max_depth, int skip_count,
-                      const void * /* ucp */, int *min_dropped_frames) {
+static int UnwindImpl(void **result, uintptr_t *frames, int *sizes,
+                      int max_depth, int skip_count, const void * /* ucp */,
+                      int *min_dropped_frames) {
 #ifdef __GNUC__
   void **sp = reinterpret_cast<void**>(__builtin_frame_address(0));
 #else
@@ -88,11 +99,18 @@ static int UnwindImpl(void** result, int* sizes, int max_depth, int skip_count,
       result[n] = *sp;
 
       if (IS_STACK_FRAMES) {
-        if (next_sp > sp) {
-          sizes[n] = (uintptr_t)next_sp - (uintptr_t)sp;
-        } else {
-          // A frame-size of 0 is used to indicate unknown frame size.
-          sizes[n] = 0;
+        if (frames != nullptr) {
+          frames[n] = absl::debugging_internal::StripPointerMetadata(sp) +
+                      1 * sizeof(void *) /* go past the return address */;
+        }
+        if (sizes != nullptr) {
+          if (next_sp > sp) {
+            sizes[n] = absl::debugging_internal::StripPointerMetadata(next_sp) -
+                       absl::debugging_internal::StripPointerMetadata(sp);
+          } else {
+            // A frame-size of 0 is used to indicate unknown frame size.
+            sizes[n] = 0;
+          }
         }
       }
       n++;
@@ -103,21 +121,28 @@ static int UnwindImpl(void** result, int* sizes, int max_depth, int skip_count,
     // Implementation detail: we clamp the max of frames we are willing to
     // count, so as not to spend too much time in the loop below.
     const int kMaxUnwind = 200;
-    int j = 0;
-    for (; sp != nullptr && j < kMaxUnwind; j++) {
+    int num_dropped_frames = 0;
+    for (int j = 0; sp != nullptr && j < kMaxUnwind; j++) {
+      if (skip_count > 0) {
+        skip_count--;
+      } else {
+        num_dropped_frames++;
+      }
       sp = NextStackFrame<!IS_STACK_FRAMES>(sp);
     }
-    *min_dropped_frames = j;
+    *min_dropped_frames = num_dropped_frames;
   }
   return n;
 }
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace debugging_internal {
 bool StackTraceWorksForTest() {
   return false;
 }
 }  // namespace debugging_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_DEBUGGING_INTERNAL_STACKTRACE_ARM_INL_H_
diff --git a/absl/debugging/internal/stacktrace_config.h b/absl/debugging/internal/stacktrace_config.h
index d4e8480a8e2..1ce2ad6b337 100644
--- a/absl/debugging/internal/stacktrace_config.h
+++ b/absl/debugging/internal/stacktrace_config.h
@@ -18,9 +18,14 @@
  * This header is "private" to stacktrace.cc.
  * DO NOT include it into any other files.
 */
+
+// SKIP_ABSL_INLINE_NAMESPACE_CHECK
+
 #ifndef ABSL_DEBUGGING_INTERNAL_STACKTRACE_CONFIG_H_
 #define ABSL_DEBUGGING_INTERNAL_STACKTRACE_CONFIG_H_
 
+#include "absl/base/config.h"
+
 #if defined(ABSL_STACKTRACE_INL_HEADER)
 #error ABSL_STACKTRACE_INL_HEADER cannot be directly set
 
@@ -28,43 +33,67 @@
 #define ABSL_STACKTRACE_INL_HEADER \
     "absl/debugging/internal/stacktrace_win32-inl.inc"
 
-#elif defined(__linux__) && !defined(__ANDROID__)
+#elif defined(__APPLE__)
+#ifdef ABSL_HAVE_THREAD_LOCAL
+// Thread local support required for UnwindImpl.
+#define ABSL_STACKTRACE_INL_HEADER \
+  "absl/debugging/internal/stacktrace_generic-inl.inc"
+#endif  // defined(ABSL_HAVE_THREAD_LOCAL)
 
-#if !defined(NO_FRAME_POINTER)
-# if defined(__i386__) || defined(__x86_64__)
+// Emscripten stacktraces rely on JS. Do not use them in standalone mode.
+#elif defined(__EMSCRIPTEN__) && !defined(STANDALONE_WASM)
 #define ABSL_STACKTRACE_INL_HEADER \
-    "absl/debugging/internal/stacktrace_x86-inl.inc"
-# elif defined(__ppc__) || defined(__PPC__)
+  "absl/debugging/internal/stacktrace_emscripten-inl.inc"
+
+#elif defined(__ANDROID__) && __ANDROID_API__ >= 33
+#ifdef ABSL_HAVE_THREAD_LOCAL
+// Use the generic implementation for Android 33+ (Android T+). This is the
+// first version of Android for which <execinfo.h> implements backtrace().
 #define ABSL_STACKTRACE_INL_HEADER \
-    "absl/debugging/internal/stacktrace_powerpc-inl.inc"
-# elif defined(__aarch64__)
+  "absl/debugging/internal/stacktrace_generic-inl.inc"
+#endif  // defined(ABSL_HAVE_THREAD_LOCAL)
+
+#elif defined(__linux__) && !defined(__ANDROID__)
+
+#if defined(NO_FRAME_POINTER) && \
+    (defined(__i386__) || defined(__x86_64__) || defined(__aarch64__))
+// Note: The libunwind-based implementation is not available to open-source
+// users.
 #define ABSL_STACKTRACE_INL_HEADER \
-    "absl/debugging/internal/stacktrace_aarch64-inl.inc"
-# elif defined(__arm__)
+  "absl/debugging/internal/stacktrace_libunwind-inl.inc"
+#define STACKTRACE_USES_LIBUNWIND 1
+#elif defined(NO_FRAME_POINTER)
+#if __has_include(<execinfo.h>) && defined(ABSL_HAVE_THREAD_LOCAL)
 // Note: When using glibc this may require -funwind-tables to function properly.
 #define ABSL_STACKTRACE_INL_HEADER \
   "absl/debugging/internal/stacktrace_generic-inl.inc"
-# else
+#endif  // __has_include(<execinfo.h>) && defined(ABSL_HAVE_THREAD_LOCAL)
+#elif defined(__i386__) || defined(__x86_64__)
 #define ABSL_STACKTRACE_INL_HEADER \
-   "absl/debugging/internal/stacktrace_unimplemented-inl.inc"
-# endif
-#else  // defined(NO_FRAME_POINTER)
-# if defined(__i386__) || defined(__x86_64__) || defined(__aarch64__)
+  "absl/debugging/internal/stacktrace_x86-inl.inc"
+#elif defined(__ppc__) || defined(__PPC__)
 #define ABSL_STACKTRACE_INL_HEADER \
-    "absl/debugging/internal/stacktrace_generic-inl.inc"
-# elif defined(__ppc__) || defined(__PPC__)
+  "absl/debugging/internal/stacktrace_powerpc-inl.inc"
+#elif defined(__aarch64__)
 #define ABSL_STACKTRACE_INL_HEADER \
-    "absl/debugging/internal/stacktrace_generic-inl.inc"
-# else
+  "absl/debugging/internal/stacktrace_aarch64-inl.inc"
+#elif defined(__riscv)
 #define ABSL_STACKTRACE_INL_HEADER \
-   "absl/debugging/internal/stacktrace_unimplemented-inl.inc"
-# endif
-#endif  // NO_FRAME_POINTER
-
+  "absl/debugging/internal/stacktrace_riscv-inl.inc"
 #else
+#if __has_include(<execinfo.h>) && defined(ABSL_HAVE_THREAD_LOCAL)
+// Note: When using glibc this may require -funwind-tables to function properly.
 #define ABSL_STACKTRACE_INL_HEADER \
-  "absl/debugging/internal/stacktrace_unimplemented-inl.inc"
+  "absl/debugging/internal/stacktrace_generic-inl.inc"
+#endif  // __has_include(<execinfo.h>) && defined(ABSL_HAVE_THREAD_LOCAL)
+#endif
+
+#endif  // defined(__linux__) && !defined(__ANDROID__)
 
+// Fallback to the empty implementation.
+#if !defined(ABSL_STACKTRACE_INL_HEADER)
+#define ABSL_STACKTRACE_INL_HEADER \
+  "absl/debugging/internal/stacktrace_unimplemented-inl.inc"
 #endif
 
 #endif  // ABSL_DEBUGGING_INTERNAL_STACKTRACE_CONFIG_H_
diff --git a/absl/debugging/internal/stacktrace_emscripten-inl.inc b/absl/debugging/internal/stacktrace_emscripten-inl.inc
new file mode 100644
index 00000000000..3c125e2f3c5
--- /dev/null
+++ b/absl/debugging/internal/stacktrace_emscripten-inl.inc
@@ -0,0 +1,121 @@
+// Copyright 2017 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// Portable implementation - just use glibc
+//
+// Note:  The glibc implementation may cause a call to malloc.
+// This can cause a deadlock in HeapProfiler.
+
+#ifndef ABSL_DEBUGGING_INTERNAL_STACKTRACE_EMSCRIPTEN_INL_H_
+#define ABSL_DEBUGGING_INTERNAL_STACKTRACE_EMSCRIPTEN_INL_H_
+
+#include <emscripten.h>
+#include <stdint.h>
+
+#include <atomic>
+#include <cstddef>
+#include <cstring>
+
+#include "absl/base/attributes.h"
+#include "absl/debugging/stacktrace.h"
+
+extern "C" {
+uintptr_t emscripten_stack_snapshot();
+uint32_t emscripten_stack_unwind_buffer(uintptr_t pc, void *buffer,
+                                        uint32_t depth);
+}
+
+// Sometimes, we can try to get a stack trace from within a stack
+// trace, which can cause a self-deadlock.
+// Protect against such reentrant call by failing to get a stack trace.
+//
+// We use __thread here because the code here is extremely low level -- it is
+// called while collecting stack traces from within malloc and mmap, and thus
+// can not call anything which might call malloc or mmap itself.
+static __thread int recursive = 0;
+
+// The stack trace function might be invoked very early in the program's
+// execution (e.g. from the very first malloc).
+// As such, we suppress usage of backtrace during this early stage of execution.
+static std::atomic<bool> disable_stacktraces(true);  // Disabled until healthy.
+// Waiting until static initializers run seems to be late enough.
+// This file is included into stacktrace.cc so this will only run once.
+ABSL_ATTRIBUTE_UNUSED static int stacktraces_enabler = []() {
+  disable_stacktraces.store(false, std::memory_order_relaxed);
+  return 0;
+}();
+
+template <bool IS_STACK_FRAMES, bool IS_WITH_CONTEXT>
+static int UnwindImpl(void **result, uintptr_t *frames, int *sizes,
+                      int max_depth, int skip_count, const void *ucp,
+                      int *min_dropped_frames) {
+  if (recursive || disable_stacktraces.load(std::memory_order_relaxed)) {
+    return 0;
+  }
+  ++recursive;
+
+  static_cast<void>(ucp);  // Unused.
+  constexpr int kStackLength = 64;
+  void *stack[kStackLength];
+
+  int size;
+  uintptr_t pc = emscripten_stack_snapshot();
+  size =
+      static_cast<int>(emscripten_stack_unwind_buffer(pc, stack, kStackLength));
+
+  int num_frames = size - skip_count;
+  if (num_frames < 0) num_frames = 0;
+  if (num_frames > max_depth) num_frames = max_depth;
+
+  int result_count = 0;
+  for (int i = 0; i < num_frames; i++) {
+    int stack_index = i + skip_count;
+    // Follow x86 and stop if the return address is null (end of stack).
+    if (stack[stack_index] == nullptr) {
+      break;
+    }
+    result[result_count++] = stack[stack_index];
+  }
+
+  if (IS_STACK_FRAMES) {
+    // No implementation for finding out the stack frames yet.
+    if (frames != nullptr) {
+      memset(frames, 0, sizeof(*frames) * static_cast<size_t>(result_count));
+    }
+    if (sizes != nullptr) {
+      memset(sizes, 0, sizeof(*sizes) * static_cast<size_t>(result_count));
+    }
+  }
+  if (min_dropped_frames != nullptr) {
+    if (size - skip_count - max_depth > 0) {
+      *min_dropped_frames = size - skip_count - max_depth;
+    } else {
+      *min_dropped_frames = 0;
+    }
+  }
+
+  --recursive;
+
+  return result_count;
+}
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace debugging_internal {
+bool StackTraceWorksForTest() { return true; }
+}  // namespace debugging_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_DEBUGGING_INTERNAL_STACKTRACE_EMSCRIPTEN_INL_H_
diff --git a/absl/debugging/internal/stacktrace_generic-inl.inc b/absl/debugging/internal/stacktrace_generic-inl.inc
index 81a49ef2e86..c94bf3e51dd 100644
--- a/absl/debugging/internal/stacktrace_generic-inl.inc
+++ b/absl/debugging/internal/stacktrace_generic-inl.inc
@@ -1,7 +1,16 @@
-// Copyright 2000 - 2007 Google Inc.
-// All rights reserved.
+// Copyright 2017 The Abseil Authors.
 //
-// Author: Sanjay Ghemawat
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
 //
 // Portable implementation - just use glibc
 //
@@ -47,8 +56,9 @@ ABSL_ATTRIBUTE_UNUSED static int stacktraces_enabler = []() {
 }();
 
 template <bool IS_STACK_FRAMES, bool IS_WITH_CONTEXT>
-static int UnwindImpl(void** result, int* sizes, int max_depth, int skip_count,
-                      const void *ucp, int *min_dropped_frames) {
+static int UnwindImpl(void** result, uintptr_t* frames, int* sizes,
+                      int max_depth, int skip_count, const void* ucp,
+                      int* min_dropped_frames) {
   if (recursive || disable_stacktraces.load(std::memory_order_relaxed)) {
     return 0;
   }
@@ -61,17 +71,29 @@ static int UnwindImpl(void** result, int* sizes, int max_depth, int skip_count,
 
   size = backtrace(stack, kStackLength);
   skip_count++;  // we want to skip the current frame as well
-  int result_count = size - skip_count;
-  if (result_count < 0)
-    result_count = 0;
-  if (result_count > max_depth)
-    result_count = max_depth;
-  for (int i = 0; i < result_count; i++)
-    result[i] = stack[i + skip_count];
+
+  int num_frames = size - skip_count;
+  if (num_frames < 0) num_frames = 0;
+  if (num_frames > max_depth) num_frames = max_depth;
+
+  int result_count = 0;
+  for (int i = 0; i < num_frames; i++) {
+    int stack_index = i + skip_count;
+    // Follow x86 and stop if the return address is null (end of stack).
+    if (stack[stack_index] == nullptr) {
+      break;
+    }
+    result[result_count++] = stack[stack_index];
+  }
 
   if (IS_STACK_FRAMES) {
-    // No implementation for finding out the stack frame sizes yet.
-    memset(sizes, 0, sizeof(*sizes) * result_count);
+    // No implementation for finding out the stack frames yet.
+    if (frames != nullptr) {
+      memset(frames, 0, sizeof(*frames) * static_cast<size_t>(result_count));
+    }
+    if (sizes != nullptr) {
+      memset(sizes, 0, sizeof(*sizes) * static_cast<size_t>(result_count));
+    }
   }
   if (min_dropped_frames != nullptr) {
     if (size - skip_count - max_depth > 0) {
@@ -87,11 +109,13 @@ static int UnwindImpl(void** result, int* sizes, int max_depth, int skip_count,
 }
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace debugging_internal {
 bool StackTraceWorksForTest() {
   return true;
 }
 }  // namespace debugging_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_DEBUGGING_INTERNAL_STACKTRACE_GENERIC_INL_H_
diff --git a/absl/debugging/internal/stacktrace_powerpc-inl.inc b/absl/debugging/internal/stacktrace_powerpc-inl.inc
index 3a070ee45d4..f23c29cb0a0 100644
--- a/absl/debugging/internal/stacktrace_powerpc-inl.inc
+++ b/absl/debugging/internal/stacktrace_powerpc-inl.inc
@@ -21,12 +21,8 @@
 #ifndef ABSL_DEBUGGING_INTERNAL_STACKTRACE_POWERPC_INL_H_
 #define ABSL_DEBUGGING_INTERNAL_STACKTRACE_POWERPC_INL_H_
 
-#if defined(__linux__)
-#include <asm/ptrace.h>   // for PT_NIP.
-#include <ucontext.h>     // for ucontext_t
-#endif
-
 #include <unistd.h>
+
 #include <cassert>
 #include <cstdint>
 #include <cstdio>
@@ -34,30 +30,36 @@
 #include "absl/base/attributes.h"
 #include "absl/base/optimization.h"
 #include "absl/base/port.h"
-#include "absl/debugging/stacktrace.h"
 #include "absl/debugging/internal/address_is_readable.h"
+#include "absl/debugging/internal/addresses.h"
 #include "absl/debugging/internal/vdso_support.h"  // a no-op on non-elf or non-glibc systems
+#include "absl/debugging/stacktrace.h"
+
+#if defined(__linux__)
+#include <asm/ptrace.h>   // for PT_NIP.
+#include <ucontext.h>     // for ucontext_t
+#endif
 
 // Given a stack pointer, return the saved link register value.
 // Note that this is the link register for a callee.
-static inline void *StacktracePowerPCGetLR(void **sp) {
+static inline void **StacktracePowerPCGetLRPtr(void **sp) {
   // PowerPC has 3 main ABIs, which say where in the stack the
   // Link Register is.  For DARWIN and AIX (used by apple and
   // linux ppc64), it's in sp[2].  For SYSV (used by linux ppc),
   // it's in sp[1].
 #if defined(_CALL_AIX) || defined(_CALL_DARWIN)
-  return *(sp+2);
+  return (sp + 2);
 #elif defined(_CALL_SYSV)
-  return *(sp+1);
+  return (sp + 1);
 #elif defined(__APPLE__) || defined(__FreeBSD__) || \
     (defined(__linux__) && defined(__PPC64__))
   // This check is in case the compiler doesn't define _CALL_AIX/etc.
-  return *(sp+2);
+  return (sp + 2);
 #elif defined(__linux)
   // This check is in case the compiler doesn't define _CALL_SYSV.
-  return *(sp+1);
+  return (sp + 1);
 #else
-#error Need to specify the PPC ABI for your archiecture.
+#error Need to specify the PPC ABI for your architecture.
 #endif
 }
 
@@ -68,6 +70,7 @@ static inline void *StacktracePowerPCGetLR(void **sp) {
 template<bool STRICT_UNWINDING, bool IS_WITH_CONTEXT>
 ABSL_ATTRIBUTE_NO_SANITIZE_ADDRESS  // May read random elements from stack.
 ABSL_ATTRIBUTE_NO_SANITIZE_MEMORY   // May read random elements from stack.
+ABSL_ATTRIBUTE_NO_SANITIZE_THREAD   // May read random elements from stack.
 static void **NextStackFrame(void **old_sp, const void *uc) {
   void **new_sp = (void **) *old_sp;
   enum { kStackAlignment = 16 };
@@ -90,6 +93,13 @@ static void **NextStackFrame(void **old_sp, const void *uc) {
   }
   if ((uintptr_t)new_sp % kStackAlignment != 0) return nullptr;
 
+  // Verify readability before using new_sp.
+  if (!STRICT_UNWINDING && new_sp != nullptr &&
+      !absl::debugging_internal::AddressIsReadable(
+          StacktracePowerPCGetLRPtr(new_sp))) {
+    return nullptr;
+  }
+
 #if defined(__linux__)
   enum StackTraceKernelSymbolStatus {
       kNotInitialized = 0, kAddressValid, kAddressInvalid };
@@ -125,13 +135,17 @@ static void **NextStackFrame(void **old_sp, const void *uc) {
       }
     }
 
-    if (new_sp != nullptr &&
-        kernel_symbol_status == kAddressValid &&
-        StacktracePowerPCGetLR(new_sp) == kernel_sigtramp_rt64_address) {
+    if (new_sp != nullptr && kernel_symbol_status == kAddressValid &&
+        *StacktracePowerPCGetLRPtr(new_sp) == kernel_sigtramp_rt64_address) {
       const ucontext_t* signal_context =
           reinterpret_cast<const ucontext_t*>(uc);
       void **const sp_before_signal =
-          reinterpret_cast<void**>(signal_context->uc_mcontext.gp_regs[PT_R1]);
+#if defined(__PPC64__)
+          reinterpret_cast<void **>(signal_context->uc_mcontext.gp_regs[PT_R1]);
+#else
+          reinterpret_cast<void **>(
+              signal_context->uc_mcontext.uc_regs->gregs[PT_R1]);
+#endif
       // Check that alleged sp before signal is nonnull and is reasonably
       // aligned.
       if (sp_before_signal != nullptr &&
@@ -159,8 +173,10 @@ ABSL_ATTRIBUTE_NOINLINE static void AbslStacktracePowerPCDummyFunction() {
 template <bool IS_STACK_FRAMES, bool IS_WITH_CONTEXT>
 ABSL_ATTRIBUTE_NO_SANITIZE_ADDRESS  // May read random elements from stack.
 ABSL_ATTRIBUTE_NO_SANITIZE_MEMORY   // May read random elements from stack.
-static int UnwindImpl(void** result, int* sizes, int max_depth, int skip_count,
-                      const void *ucp, int *min_dropped_frames) {
+ABSL_ATTRIBUTE_NO_SANITIZE_THREAD   // May read random elements from stack.
+static int UnwindImpl(void **result, uintptr_t *frames, int *sizes,
+                      int max_depth, int skip_count, const void *ucp,
+                      int *min_dropped_frames) {
   void **sp;
   // Apple macOS uses an old version of gnu as -- both Darwin 7.9.0 (Panther)
   // and Darwin 8.8.1 (Tiger) use as 1.38.  This means we have to use a
@@ -206,13 +222,22 @@ static int UnwindImpl(void** result, int* sizes, int max_depth, int skip_count,
     if (skip_count > 0) {
       skip_count--;
     } else {
-      result[n] = StacktracePowerPCGetLR(sp);
+      void **lr = StacktracePowerPCGetLRPtr(sp);
+      result[n] = *lr;
       if (IS_STACK_FRAMES) {
-        if (next_sp > sp) {
-          sizes[n] = (uintptr_t)next_sp - (uintptr_t)sp;
-        } else {
-          // A frame-size of 0 is used to indicate unknown frame size.
-          sizes[n] = 0;
+        if (frames != nullptr) {
+          frames[n] = absl::debugging_internal::StripPointerMetadata(lr) +
+                      1 * sizeof(void *) /* go past the return address */;
+        }
+        if (sizes != nullptr) {
+          if (next_sp > sp) {
+            sizes[n] = static_cast<int>(
+                absl::debugging_internal::StripPointerMetadata(next_sp) -
+                absl::debugging_internal::StripPointerMetadata(sp));
+          } else {
+            // A frame-size of 0 is used to indicate unknown frame size.
+            sizes[n] = 0;
+          }
         }
       }
       n++;
@@ -226,21 +251,28 @@ static int UnwindImpl(void** result, int* sizes, int max_depth, int skip_count,
     // Implementation detail: we clamp the max of frames we are willing to
     // count, so as not to spend too much time in the loop below.
     const int kMaxUnwind = 1000;
-    int j = 0;
-    for (; next_sp != nullptr && j < kMaxUnwind; j++) {
+    int num_dropped_frames = 0;
+    for (int j = 0; next_sp != nullptr && j < kMaxUnwind; j++) {
+      if (skip_count > 0) {
+        skip_count--;
+      } else {
+        num_dropped_frames++;
+      }
       next_sp = NextStackFrame<!IS_STACK_FRAMES, IS_WITH_CONTEXT>(next_sp, ucp);
     }
-    *min_dropped_frames = j;
+    *min_dropped_frames = num_dropped_frames;
   }
   return n;
 }
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace debugging_internal {
 bool StackTraceWorksForTest() {
   return true;
 }
 }  // namespace debugging_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_DEBUGGING_INTERNAL_STACKTRACE_POWERPC_INL_H_
diff --git a/absl/debugging/internal/stacktrace_riscv-inl.inc b/absl/debugging/internal/stacktrace_riscv-inl.inc
new file mode 100644
index 00000000000..e950e76c0f3
--- /dev/null
+++ b/absl/debugging/internal/stacktrace_riscv-inl.inc
@@ -0,0 +1,208 @@
+// Copyright 2021 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_DEBUGGING_INTERNAL_STACKTRACE_RISCV_INL_H_
+#define ABSL_DEBUGGING_INTERNAL_STACKTRACE_RISCV_INL_H_
+
+// Generate stack trace for riscv
+
+#include <sys/ucontext.h>
+
+#include <atomic>
+#include <cassert>
+#include <cstdint>
+#include <iostream>
+#include <limits>
+#include <utility>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/debugging/internal/addresses.h"
+#include "absl/debugging/stacktrace.h"
+
+#if defined(__linux__)
+#include <sys/mman.h>
+#include <ucontext.h>
+#include <unistd.h>
+#endif
+
+static constexpr ptrdiff_t kUnknownFrameSize = 0;
+
+// Compute the size of a stack frame in [low..high).  We assume that low < high.
+// Return size of kUnknownFrameSize.
+template <typename T>
+static inline ptrdiff_t ComputeStackFrameSize(const T *low, const T *high) {
+  const char *low_char_ptr = reinterpret_cast<const char *>(low);
+  const char *high_char_ptr = reinterpret_cast<const char *>(high);
+  return low < high ? static_cast<ptrdiff_t>(high_char_ptr - low_char_ptr)
+                    : kUnknownFrameSize;
+}
+
+// Given a pointer to a stack frame, locate and return the calling stackframe,
+// or return null if no stackframe can be found. Perform sanity checks (the
+// strictness of which is controlled by the boolean parameter
+// "STRICT_UNWINDING") to reduce the chance that a bad pointer is returned.
+template <bool STRICT_UNWINDING, bool WITH_CONTEXT>
+ABSL_ATTRIBUTE_NO_SANITIZE_ADDRESS  // May read random elements from stack.
+ABSL_ATTRIBUTE_NO_SANITIZE_MEMORY   // May read random elements from stack.
+ABSL_ATTRIBUTE_NO_SANITIZE_THREAD   // May read random elements from stack.
+static void ** NextStackFrame(void **old_frame_pointer, const void *uc,
+                              const std::pair<size_t, size_t> range) {
+  //               .
+  //               .
+  //               .
+  //   +-> +----------------+
+  //   |   | return address |
+  //   |   |   previous fp  |
+  //   |   |      ...       |
+  //   |   +----------------+ <-+
+  //   |   | return address |   |
+  //   +---|-  previous fp  |   |
+  //       |      ...       |   |
+  // $fp ->|----------------+   |
+  //       | return address |   |
+  //       |   previous fp -|---+
+  // $sp ->|      ...       |
+  //       +----------------+
+  void **new_frame_pointer = reinterpret_cast<void **>(old_frame_pointer[-2]);
+  uintptr_t frame_pointer = reinterpret_cast<uintptr_t>(new_frame_pointer);
+
+  // The RISCV ELF psABI mandates that the stack pointer is always 16-byte
+  // aligned.
+  // TODO(#1236) this doesn't hold for ILP32E which only mandates a 4-byte
+  // alignment.
+  if (frame_pointer & 15)
+    return nullptr;
+
+  // If the new frame pointer matches the signal context, avoid terminating
+  // early to deal with alternate signal stacks.
+  if (WITH_CONTEXT)
+    if (const ucontext_t *ucv = static_cast<const ucontext_t *>(uc))
+      // RISCV ELF psABI has the frame pointer at x8/fp/s0.
+      // -- RISCV psABI Table 18.2
+      if (ucv->uc_mcontext.__gregs[8] == frame_pointer)
+        return new_frame_pointer;
+
+  // Check frame size.  In strict mode, we assume frames to be under 100,000
+  // bytes.  In non-strict mode, we relax the limit to 1MB.
+  const ptrdiff_t max_size = STRICT_UNWINDING ? 100000 : 1000000;
+  const ptrdiff_t frame_size =
+      ComputeStackFrameSize(old_frame_pointer, new_frame_pointer);
+  if (frame_size == kUnknownFrameSize) {
+    if (STRICT_UNWINDING)
+      return nullptr;
+
+    // In non-strict mode permit non-contiguous stacks (e.g. alternate signal
+    // frame handling).
+    if (reinterpret_cast<uintptr_t>(new_frame_pointer) < range.first ||
+        reinterpret_cast<uintptr_t>(new_frame_pointer) > range.second)
+      return nullptr;
+  }
+
+  if (frame_size > max_size)
+    return nullptr;
+
+  return new_frame_pointer;
+}
+
+template <bool IS_STACK_FRAMES, bool IS_WITH_CONTEXT>
+ABSL_ATTRIBUTE_NO_SANITIZE_ADDRESS  // May read random elements from stack.
+ABSL_ATTRIBUTE_NO_SANITIZE_MEMORY   // May read random elements from stack.
+ABSL_ATTRIBUTE_NO_SANITIZE_THREAD   // May read random elements from stack.
+static int UnwindImpl(void **result, uintptr_t *frames, int *sizes,
+                      int max_depth, int skip_count, const void *ucp,
+                      int *min_dropped_frames) {
+  // The `frame_pointer` that is computed here points to the top of the frame.
+  // The two words preceding the address are the return address and the previous
+  // frame pointer.
+#if defined(__GNUC__)
+  void **frame_pointer = reinterpret_cast<void **>(__builtin_frame_address(0));
+#else
+#error reading stack pointer not yet supported on this platform
+#endif
+
+  std::pair<size_t, size_t> stack = {
+      // assume that the first page is not the stack.
+      static_cast<size_t>(sysconf(_SC_PAGESIZE)),
+      std::numeric_limits<size_t>::max() - sizeof(void *)
+  };
+
+  int n = 0;
+  void *return_address = nullptr;
+  while (frame_pointer && n < max_depth) {
+    return_address = frame_pointer[-1];
+    // Follow x86 and stop if the return address is null (end of stack).
+    if (return_address == nullptr) {
+      break;
+    }
+
+    // The absl::GetStackFrames routine is called when we are in some
+    // informational context (the failure signal handler for example).  Use the
+    // non-strict unwinding rules to produce a stack trace that is as complete
+    // as possible (even if it contains a few bogus entries in some rare cases).
+    void **next_frame_pointer =
+        NextStackFrame<!IS_STACK_FRAMES, IS_WITH_CONTEXT>(frame_pointer, ucp,
+                                                          stack);
+
+    if (skip_count > 0) {
+      skip_count--;
+    } else {
+      result[n] = return_address;
+      if (IS_STACK_FRAMES) {
+        // NextStackFrame() has already checked that frame size fits to int
+        if (frames != nullptr) {
+          frames[n] =
+              absl::debugging_internal::StripPointerMetadata(frame_pointer);
+        }
+        if (sizes != nullptr) {
+          sizes[n] = static_cast<int>(
+              ComputeStackFrameSize(frame_pointer, next_frame_pointer));
+        }
+      }
+      n++;
+    }
+
+    frame_pointer = next_frame_pointer;
+  }
+
+  if (min_dropped_frames != nullptr) {
+    // Implementation detail: we clamp the max of frames we are willing to
+    // count, so as not to spend too much time in the loop below.
+    const int kMaxUnwind = 200;
+    int num_dropped_frames = 0;
+    for (int j = 0; frame_pointer != nullptr && j < kMaxUnwind; j++) {
+      if (skip_count > 0) {
+        skip_count--;
+      } else {
+        num_dropped_frames++;
+      }
+      frame_pointer =
+          NextStackFrame<!IS_STACK_FRAMES, IS_WITH_CONTEXT>(frame_pointer, ucp,
+                                                            stack);
+    }
+    *min_dropped_frames = num_dropped_frames;
+  }
+
+  return n;
+}
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace debugging_internal {
+bool StackTraceWorksForTest() { return true; }
+}  // namespace debugging_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif
diff --git a/absl/debugging/internal/stacktrace_unimplemented-inl.inc b/absl/debugging/internal/stacktrace_unimplemented-inl.inc
index e256fdd4ae7..ec63940b845 100644
--- a/absl/debugging/internal/stacktrace_unimplemented-inl.inc
+++ b/absl/debugging/internal/stacktrace_unimplemented-inl.inc
@@ -2,9 +2,10 @@
 #define ABSL_DEBUGGING_INTERNAL_STACKTRACE_UNIMPLEMENTED_INL_H_
 
 template <bool IS_STACK_FRAMES, bool IS_WITH_CONTEXT>
-static int UnwindImpl(void** /* result */, int* /* sizes */,
-                      int /* max_depth */, int /* skip_count */,
-                      const void* /* ucp */, int *min_dropped_frames) {
+static int UnwindImpl(void** /* result */, uintptr_t* /* frames */,
+                      int* /* sizes */, int /* max_depth */,
+                      int /* skip_count */, const void* /* ucp */,
+                      int* min_dropped_frames) {
   if (min_dropped_frames != nullptr) {
     *min_dropped_frames = 0;
   }
@@ -12,11 +13,13 @@ static int UnwindImpl(void** /* result */, int* /* sizes */,
 }
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace debugging_internal {
 bool StackTraceWorksForTest() {
   return false;
 }
 }  // namespace debugging_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_DEBUGGING_INTERNAL_STACKTRACE_UNIMPLEMENTED_INL_H_
diff --git a/absl/debugging/internal/stacktrace_win32-inl.inc b/absl/debugging/internal/stacktrace_win32-inl.inc
index b46491f8143..f57c187253f 100644
--- a/absl/debugging/internal/stacktrace_win32-inl.inc
+++ b/absl/debugging/internal/stacktrace_win32-inl.inc
@@ -37,33 +37,29 @@
 #ifndef ABSL_DEBUGGING_INTERNAL_STACKTRACE_WIN32_INL_H_
 #define ABSL_DEBUGGING_INTERNAL_STACKTRACE_WIN32_INL_H_
 
-#include <windows.h>    // for GetProcAddress and GetModuleHandle
-#include <cassert>
-
-typedef USHORT NTAPI RtlCaptureStackBackTrace_Function(
-    IN ULONG frames_to_skip,
-    IN ULONG frames_to_capture,
-    OUT PVOID *backtrace,
-    OUT PULONG backtrace_hash);
+#include <windows.h>  // CaptureStackBackTrace
 
-// Load the function we need at static init time, where we don't have
-// to worry about someone else holding the loader's lock.
-static RtlCaptureStackBackTrace_Function* const RtlCaptureStackBackTrace_fn =
-   (RtlCaptureStackBackTrace_Function*)
-   GetProcAddress(GetModuleHandleA("ntdll.dll"), "RtlCaptureStackBackTrace");
+#include <cassert>
 
 template <bool IS_STACK_FRAMES, bool IS_WITH_CONTEXT>
-static int UnwindImpl(void** result, int* sizes, int max_depth, int skip_count,
-                      const void *ucp, int *min_dropped_frames) {
-  int n = 0;
-  if (!RtlCaptureStackBackTrace_fn) {
-    // can't find a stacktrace with no function to call
+static int UnwindImpl(void** result, uintptr_t* frames, int* sizes,
+                      int max_depth, int skip_count, const void*,
+                      int* min_dropped_frames) {
+  USHORT n = 0;
+  if (skip_count < 0 || max_depth < 0) {
+    // can't get a stacktrace with no function/invalid args
   } else {
-    n = (int)RtlCaptureStackBackTrace_fn(skip_count + 2, max_depth, result, 0);
+    n = CaptureStackBackTrace(static_cast<ULONG>(skip_count) + 2,
+                              static_cast<ULONG>(max_depth), result, 0);
   }
   if (IS_STACK_FRAMES) {
-    // No implementation for finding out the stack frame sizes yet.
-    memset(sizes, 0, sizeof(*sizes) * n);
+    // No implementation for finding out the stack frames yet.
+    if (frames != nullptr) {
+      memset(frames, 0, sizeof(*frames) * n);
+    }
+    if (sizes != nullptr) {
+      memset(sizes, 0, sizeof(*sizes) * n);
+    }
   }
   if (min_dropped_frames != nullptr) {
     // Not implemented.
@@ -73,11 +69,13 @@ static int UnwindImpl(void** result, int* sizes, int max_depth, int skip_count,
 }
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace debugging_internal {
 bool StackTraceWorksForTest() {
   return false;
 }
 }  // namespace debugging_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_DEBUGGING_INTERNAL_STACKTRACE_WIN32_INL_H_
diff --git a/absl/debugging/internal/stacktrace_x86-inl.inc b/absl/debugging/internal/stacktrace_x86-inl.inc
index 9494441e1cf..55cf29a8cf2 100644
--- a/absl/debugging/internal/stacktrace_x86-inl.inc
+++ b/absl/debugging/internal/stacktrace_x86-inl.inc
@@ -17,30 +17,32 @@
 #ifndef ABSL_DEBUGGING_INTERNAL_STACKTRACE_X86_INL_INC_
 #define ABSL_DEBUGGING_INTERNAL_STACKTRACE_X86_INL_INC_
 
-#if defined(__linux__) && (defined(__i386__) || defined(__x86_64__))
-#include <ucontext.h>  // for ucontext_t
-#endif
-
-#if !defined(_WIN32)
-#include <unistd.h>
-#endif
-
 #include <cassert>
+#include <cstddef>
 #include <cstdint>
+#include <limits>
 
+#include "absl/base/attributes.h"
 #include "absl/base/macros.h"
 #include "absl/base/port.h"
 #include "absl/debugging/internal/address_is_readable.h"
+#include "absl/debugging/internal/addresses.h"
 #include "absl/debugging/internal/vdso_support.h"  // a no-op on non-elf or non-glibc systems
 #include "absl/debugging/stacktrace.h"
 
-#include "absl/base/internal/raw_logging.h"
+#if defined(__linux__) && (defined(__i386__) || defined(__x86_64__))
+#include <ucontext.h>  // for ucontext_t
+#endif
+
+#if !defined(_WIN32)
+#include <unistd.h>
+#endif
 
 using absl::debugging_internal::AddressIsReadable;
 
 #if defined(__linux__) && defined(__i386__)
 // Count "push %reg" instructions in VDSO __kernel_vsyscall(),
-// preceeding "syscall" or "sysenter".
+// preceding "syscall" or "sysenter".
 // If __kernel_vsyscall uses frame pointer, answer 0.
 //
 // kMaxBytes tells how many instruction bytes of __kernel_vsyscall
@@ -112,6 +114,10 @@ static int CountPushInstructions(const unsigned char *const addr) {
 
 // Assume stack frames larger than 100,000 bytes are bogus.
 static const int kMaxFrameBytes = 100000;
+// Stack end to use when we don't know the actual stack end
+// (effectively just the end of address space).
+constexpr uintptr_t kUnknownStackEnd =
+    std::numeric_limits<size_t>::max() - sizeof(void *);
 
 // Returns the stack frame pointer from signal context, 0 if unknown.
 // vuc is a ucontext_t *.  We use void* to avoid the use
@@ -132,21 +138,21 @@ static uintptr_t GetFP(const void *vuc) {
     const uintptr_t bp = 0;
     const uintptr_t sp = 0;
 #endif
-    // Sanity-check that the base pointer is valid.  It should be as long as
-    // SHRINK_WRAP_FRAME_POINTER is not set, but it's possible that some code in
-    // the process is compiled with --copt=-fomit-frame-pointer or
+    // Sanity-check that the base pointer is valid. It's possible that some
+    // code in the process is compiled with --copt=-fomit-frame-pointer or
     // --copt=-momit-leaf-frame-pointer.
     //
     // TODO(bcmills): -momit-leaf-frame-pointer is currently the default
     // behavior when building with clang.  Talk to the C++ toolchain team about
     // fixing that.
-    if (bp >= sp && bp - sp <= kMaxFrameBytes) return bp;
+    if (bp >= sp && bp - sp <= kMaxFrameBytes)
+      return static_cast<uintptr_t>(bp);
 
     // If bp isn't a plausible frame pointer, return the stack pointer instead.
     // If we're lucky, it points to the start of a stack frame; otherwise, we'll
     // get one frame of garbage in the stack trace and fail the sanity check on
     // the next iteration.
-    return sp;
+    return static_cast<uintptr_t>(sp);
   }
 #endif
   return 0;
@@ -159,7 +165,9 @@ static uintptr_t GetFP(const void *vuc) {
 template <bool STRICT_UNWINDING, bool WITH_CONTEXT>
 ABSL_ATTRIBUTE_NO_SANITIZE_ADDRESS  // May read random elements from stack.
 ABSL_ATTRIBUTE_NO_SANITIZE_MEMORY   // May read random elements from stack.
-static void **NextStackFrame(void **old_fp, const void *uc) {
+ABSL_ATTRIBUTE_NO_SANITIZE_THREAD   // May read random elements from stack.
+static void **NextStackFrame(void **old_fp, const void *uc,
+                             size_t stack_low, size_t stack_high) {
   void **new_fp = (void **)*old_fp;
 
 #if defined(__linux__) && defined(__i386__)
@@ -173,6 +181,7 @@ static void **NextStackFrame(void **old_fp, const void *uc) {
     static const unsigned char *kernel_rt_sigreturn_address = nullptr;
     static const unsigned char *kernel_vsyscall_address = nullptr;
     if (num_push_instructions == -1) {
+#ifdef ABSL_HAVE_VDSO_SUPPORT
       absl::debugging_internal::VDSOSupport vdso;
       if (vdso.IsPresent()) {
         absl::debugging_internal::VDSOSupport::SymbolInfo
@@ -201,6 +210,9 @@ static void **NextStackFrame(void **old_fp, const void *uc) {
       } else {
         num_push_instructions = 0;
       }
+#else  // ABSL_HAVE_VDSO_SUPPORT
+      num_push_instructions = 0;
+#endif  // ABSL_HAVE_VDSO_SUPPORT
     }
     if (num_push_instructions != 0 && kernel_rt_sigreturn_address != nullptr &&
         old_fp[1] == kernel_rt_sigreturn_address) {
@@ -234,6 +246,7 @@ static void **NextStackFrame(void **old_fp, const void *uc) {
   }
 #endif
 
+  const size_t page_size = static_cast<size_t>(getpagesize());
   const uintptr_t old_fp_u = reinterpret_cast<uintptr_t>(old_fp);
   const uintptr_t new_fp_u = reinterpret_cast<uintptr_t>(new_fp);
 
@@ -243,17 +256,47 @@ static void **NextStackFrame(void **old_fp, const void *uc) {
   // using an alternate signal stack.
   //
   // TODO(bcmills): The GetFP call should be completely unnecessary when
-  // SHRINK_WRAP_FRAME_POINTER is set (because we should be back in the thread's
+  // ENABLE_COMBINED_UNWINDER is set (because we should be back in the thread's
   // stack by this point), but it is empirically still needed (e.g. when the
   // stack includes a call to abort).  unw_get_reg returns UNW_EBADREG for some
   // frames.  Figure out why GetValidFrameAddr and/or libunwind isn't doing what
   // it's supposed to.
   if (STRICT_UNWINDING &&
       (!WITH_CONTEXT || uc == nullptr || new_fp_u != GetFP(uc))) {
-    // With the stack growing downwards, older stack frame must be
-    // at a greater address that the current one.
-    if (new_fp_u <= old_fp_u) return nullptr;
-    if (new_fp_u - old_fp_u > kMaxFrameBytes) return nullptr;
+    // With the stack growing downwards, older stack frame should be
+    // at a greater address that the current one. However if we get multiple
+    // signals handled on altstack the new frame pointer might return to the
+    // main stack, but be different than the value from the most recent
+    // ucontext.
+    // If we get a very large frame size, it may be an indication that we
+    // guessed frame pointers incorrectly and now risk a paging fault
+    // dereferencing a wrong frame pointer. Or maybe not because large frames
+    // are possible as well. The main stack is assumed to be readable,
+    // so we assume the large frame is legit if we know the real stack bounds
+    // and are within the stack.
+    if (new_fp_u <= old_fp_u || new_fp_u - old_fp_u > kMaxFrameBytes) {
+      if (stack_high < kUnknownStackEnd && page_size < stack_low) {
+        // Stack bounds are known.
+        if (!(stack_low < new_fp_u && new_fp_u <= stack_high)) {
+          // new_fp_u is not within the known stack.
+          return nullptr;
+        }
+      } else {
+        // Stack bounds are unknown, prefer truncated stack to possible crash.
+        return nullptr;
+      }
+    }
+    if (stack_low < old_fp_u && old_fp_u <= stack_high) {
+      // Old BP was in the expected stack region...
+      if (!(stack_low < new_fp_u && new_fp_u <= stack_high)) {
+        // ... but new BP is outside of expected stack region.
+        // It is most likely bogus.
+        return nullptr;
+      }
+    } else {
+      // We may be here if we are executing in a co-routine with a
+      // separate stack. We can't do safety checks in this case.
+    }
   } else {
     if (new_fp == nullptr) return nullptr;  // skip AddressIsReadable() below
     // In the non-strict mode, allow discontiguous stack frames.
@@ -269,7 +312,12 @@ static void **NextStackFrame(void **old_fp, const void *uc) {
   if (new_fp_u >= 0xffffe000) return nullptr;
 #endif
 #if !defined(_WIN32)
-  if (!STRICT_UNWINDING) {
+  const uintptr_t old_fp_page = old_fp_u & ~(page_size - 1);
+  const uintptr_t new_fp_page = new_fp_u & ~(page_size - 1);
+  if (old_fp_page == new_fp_page && (new_fp_u & (sizeof(void*) - 1)) == 0) {
+    // We dereferenced the old_fp above, so it is safe to dereference
+    // new_fp if it's on the same page as the old_fp and is aligned.
+  } else if (!STRICT_UNWINDING) {
     // Lax sanity checks cause a crash in 32-bit tcmalloc/crash_reason_test
     // on AMD-based machines with VDSO-enabled kernels.
     // Make an extra sanity check to insure new_fp is readable.
@@ -287,29 +335,44 @@ static void **NextStackFrame(void **old_fp, const void *uc) {
 template <bool IS_STACK_FRAMES, bool IS_WITH_CONTEXT>
 ABSL_ATTRIBUTE_NO_SANITIZE_ADDRESS  // May read random elements from stack.
 ABSL_ATTRIBUTE_NO_SANITIZE_MEMORY   // May read random elements from stack.
+ABSL_ATTRIBUTE_NO_SANITIZE_THREAD   // May read random elements from stack.
 ABSL_ATTRIBUTE_NOINLINE
-static int UnwindImpl(void **result, int *sizes, int max_depth, int skip_count,
-                      const void *ucp, int *min_dropped_frames) {
+static int UnwindImpl(void **result, uintptr_t *frames, int *sizes,
+                      int max_depth, int skip_count, const void *ucp,
+                      int *min_dropped_frames) {
   int n = 0;
   void **fp = reinterpret_cast<void **>(__builtin_frame_address(0));
 
+  // Assume that the first page is not stack.
+  size_t stack_low = static_cast<size_t>(getpagesize());
+  size_t stack_high = kUnknownStackEnd;
+
   while (fp && n < max_depth) {
     if (*(fp + 1) == reinterpret_cast<void *>(0)) {
       // In 64-bit code, we often see a frame that
       // points to itself and has a return address of 0.
       break;
     }
-    void **next_fp = NextStackFrame<!IS_STACK_FRAMES, IS_WITH_CONTEXT>(fp, ucp);
+    void **next_fp = NextStackFrame<!IS_STACK_FRAMES, IS_WITH_CONTEXT>(
+        fp, ucp, stack_low, stack_high);
     if (skip_count > 0) {
       skip_count--;
     } else {
       result[n] = *(fp + 1);
       if (IS_STACK_FRAMES) {
-        if (next_fp > fp) {
-          sizes[n] = (uintptr_t)next_fp - (uintptr_t)fp;
-        } else {
-          // A frame-size of 0 is used to indicate unknown frame size.
-          sizes[n] = 0;
+        if (frames) {
+          frames[n] = absl::debugging_internal::StripPointerMetadata(fp) +
+                      2 * sizeof(void *) /* go past the return address */;
+        }
+        if (sizes) {
+          if (next_fp > fp) {
+            sizes[n] = static_cast<int>(
+                absl::debugging_internal::StripPointerMetadata(next_fp) -
+                absl::debugging_internal::StripPointerMetadata(fp));
+          } else {
+            // A frame-size of 0 is used to indicate unknown frame size.
+            sizes[n] = 0;
+          }
         }
       }
       n++;
@@ -320,21 +383,29 @@ static int UnwindImpl(void **result, int *sizes, int max_depth, int skip_count,
     // Implementation detail: we clamp the max of frames we are willing to
     // count, so as not to spend too much time in the loop below.
     const int kMaxUnwind = 1000;
-    int j = 0;
-    for (; fp != nullptr && j < kMaxUnwind; j++) {
-      fp = NextStackFrame<!IS_STACK_FRAMES, IS_WITH_CONTEXT>(fp, ucp);
+    int num_dropped_frames = 0;
+    for (int j = 0; fp != nullptr && j < kMaxUnwind; j++) {
+      if (skip_count > 0) {
+        skip_count--;
+      } else {
+        num_dropped_frames++;
+      }
+      fp = NextStackFrame<!IS_STACK_FRAMES, IS_WITH_CONTEXT>(fp, ucp, stack_low,
+                                                             stack_high);
     }
-    *min_dropped_frames = j;
+    *min_dropped_frames = num_dropped_frames;
   }
   return n;
 }
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace debugging_internal {
 bool StackTraceWorksForTest() {
   return true;
 }
 }  // namespace debugging_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_DEBUGGING_INTERNAL_STACKTRACE_X86_INL_INC_
diff --git a/absl/debugging/internal/symbolize.h b/absl/debugging/internal/symbolize.h
index 3e537893097..fd655449c3e 100644
--- a/absl/debugging/internal/symbolize.h
+++ b/absl/debugging/internal/symbolize.h
@@ -18,21 +18,27 @@
 #ifndef ABSL_DEBUGGING_INTERNAL_SYMBOLIZE_H_
 #define ABSL_DEBUGGING_INTERNAL_SYMBOLIZE_H_
 
+#ifdef __cplusplus
+
 #include <cstddef>
 #include <cstdint>
+#include <functional>
+#include <memory>
+
+#include "absl/base/config.h"
+#include "absl/strings/string_view.h"
 
 #ifdef ABSL_INTERNAL_HAVE_ELF_SYMBOLIZE
 #error ABSL_INTERNAL_HAVE_ELF_SYMBOLIZE cannot be directly set
-#elif defined(__ELF__) && defined(__GLIBC__) && !defined(__native_client__) && \
-    !defined(__asmjs__) && !defined(__wasm__)
+#elif defined(__ELF__) && defined(__GLIBC__) && !defined(__asmjs__) \
+      && !defined(__wasm__)
 #define ABSL_INTERNAL_HAVE_ELF_SYMBOLIZE 1
 
 #include <elf.h>
 #include <link.h>  // For ElfW() macro.
-#include <functional>
-#include <string>
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace debugging_internal {
 
 // Iterates over all sections, invoking callback on each with the section name
@@ -42,7 +48,7 @@ namespace debugging_internal {
 //
 // This is not async-signal-safe.
 bool ForEachSection(int fd,
-                    const std::function<bool(const std::string& name,
+                    const std::function<bool(absl::string_view name,
                                              const ElfW(Shdr) &)>& callback);
 
 // Gets the section header for the given name, if it exists. Returns true on
@@ -51,51 +57,75 @@ bool GetSectionHeaderByName(int fd, const char *name, size_t name_len,
                             ElfW(Shdr) *out);
 
 }  // namespace debugging_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_INTERNAL_HAVE_ELF_SYMBOLIZE
 
+#ifdef ABSL_INTERNAL_HAVE_DARWIN_SYMBOLIZE
+#error ABSL_INTERNAL_HAVE_DARWIN_SYMBOLIZE cannot be directly set
+#elif defined(__APPLE__)
+#define ABSL_INTERNAL_HAVE_DARWIN_SYMBOLIZE 1
+#endif
+
+#ifdef ABSL_INTERNAL_HAVE_EMSCRIPTEN_SYMBOLIZE
+#error ABSL_INTERNAL_HAVE_EMSCRIPTEN_SYMBOLIZE cannot be directly set
+#elif defined(__EMSCRIPTEN__)
+#define ABSL_INTERNAL_HAVE_EMSCRIPTEN_SYMBOLIZE 1
+#endif
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace debugging_internal {
 
-struct SymbolDecoratorArgs {
-  // The program counter we are getting symbolic name for.
-  const void *pc;
-  // 0 for main executable, load address for shared libraries.
-  ptrdiff_t relocation;
-  // Read-only file descriptor for ELF image covering "pc",
-  // or -1 if no such ELF image exists in /proc/self/maps.
-  int fd;
-  // Output buffer, size.
-  // Note: the buffer may not be empty -- default symbolizer may have already
-  // produced some output, and earlier decorators may have adorned it in
-  // some way. You are free to replace or augment the contents (within the
-  // symbol_buf_size limit).
-  char *const symbol_buf;
-  size_t symbol_buf_size;
-  // Temporary scratch space, size.
-  // Use that space in preference to allocating your own stack buffer to
-  // conserve stack.
-  char *const tmp_buf;
-  size_t tmp_buf_size;
-  // User-provided argument
-  void* arg;
-};
-using SymbolDecorator = void (*)(const SymbolDecoratorArgs *);
+class SymbolDecorator;
 
-// Installs a function-pointer as a decorator. Returns a value less than zero
-// if the system cannot install the decorator. Otherwise, returns a unique
-// identifier corresponding to the decorator. This identifier can be used to
-// uninstall the decorator - See RemoveSymbolDecorator() below.
-int InstallSymbolDecorator(SymbolDecorator decorator, void* arg);
+class SymbolDecoratorDeleter {
+ public:
+  void operator()(SymbolDecorator* ptr);
+};
 
-// Removes a previously installed function-pointer decorator. Parameter "ticket"
-// is the return-value from calling InstallSymbolDecorator().
-bool RemoveSymbolDecorator(int ticket);
+using SymbolDecoratorPtr =
+    std::unique_ptr<SymbolDecorator, SymbolDecoratorDeleter>;
+
+// Represents an object that can add additional information to a symbol
+// name.
+class SymbolDecorator {
+ public:
+  // The signature of a factory function used to register and create a symbol
+  // decorator. This function may be called from a signal handler, so it must
+  // use an async-signal-safe allocation mechanism to allocate the returned
+  // object.
+  using Factory = SymbolDecoratorPtr(int fd);
+
+  virtual ~SymbolDecorator() = default;
+
+  // Decorates symbol name with additional information.
+  //
+  // pc represents the program counter we are getting symbolic name for.
+  // relocation is difference between the link-time and the load-time address.
+  // symbol_buf and symbol_buf_size represent the output buffer and its size.
+  // Note that the buffer may not be empty -- default symbolizer may have
+  // already produced some output. You are free to replace or augment the
+  // contents (within the symbol_buf_size limit). tmp_buf and tmp_buf_size
+  // represent temporary scratch space and its size. Use that space in
+  // preference to allocating your own stack buffer to conserve stack.
+  //
+  // This function will not be called concurrently for the same object, but it
+  // may be called from a signal handler, so it must avoid any operation that is
+  // not async-signal-safe. However, it does not have to be reentrant -- if it
+  // is interrupted by a signal and the handler tries to symbolize, the request
+  // will go to a new SymbolDecorator instance.
+  virtual void Decorate(
+      const void* pc,
+      ptrdiff_t relocation,
+      char* symbol_buf, size_t symbol_buf_size,
+      char* tmp_buf, size_t tmp_buf_size) const = 0;
+};
 
-// Remove all installed decorators.  Returns true if successful, false if
-// symbolization is currently in progress.
-bool RemoveAllSymbolDecorators(void);
+// Registers a new symbol decorator factory and returns the previous one.
+SymbolDecorator::Factory* SetSymbolDecoratorFactory(
+    SymbolDecorator::Factory* factory);
 
 // Registers an address range to a file mapping.
 //
@@ -104,19 +134,29 @@ bool RemoveAllSymbolDecorators(void);
 //   filename != nullptr
 //
 // Returns true if the file was successfully registered.
-bool RegisterFileMappingHint(
-    const void* start, const void* end, uint64_t offset, const char* filename);
+bool RegisterFileMappingHint(const void* start, const void* end,
+                             uint64_t offset, const char* filename);
 
 // Looks up the file mapping registered by RegisterFileMappingHint for an
 // address range. If there is one, the file name is stored in *filename and
 // *start and *end are modified to reflect the registered mapping. Returns
 // whether any hint was found.
-bool GetFileMappingHint(const void** start,
-                        const void** end,
-                        uint64_t    *  offset,
+bool GetFileMappingHint(const void** start, const void** end, uint64_t* offset,
                         const char** filename);
 
 }  // namespace debugging_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
+#endif  // __cplusplus
+
+#include <stdbool.h>
+
+#ifdef __cplusplus
+extern "C"
+#endif  // __cplusplus
+    bool ABSL_INTERNAL_C_SYMBOL(AbslInternalGetFileMappingHint)(
+        const void** start, const void** end, uint64_t* offset,
+        const char** filename);
+
 #endif  // ABSL_DEBUGGING_INTERNAL_SYMBOLIZE_H_
diff --git a/absl/debugging/internal/utf8_for_code_point.cc b/absl/debugging/internal/utf8_for_code_point.cc
new file mode 100644
index 00000000000..658a3b513b7
--- /dev/null
+++ b/absl/debugging/internal/utf8_for_code_point.cc
@@ -0,0 +1,70 @@
+// Copyright 2024 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/debugging/internal/utf8_for_code_point.h"
+
+#include <cstdint>
+
+#include "absl/base/config.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace debugging_internal {
+namespace {
+
+// UTF-8 encoding bounds.
+constexpr uint32_t kMinSurrogate = 0xd800, kMaxSurrogate = 0xdfff;
+constexpr uint32_t kMax1ByteCodePoint = 0x7f;
+constexpr uint32_t kMax2ByteCodePoint = 0x7ff;
+constexpr uint32_t kMax3ByteCodePoint = 0xffff;
+constexpr uint32_t kMaxCodePoint = 0x10ffff;
+
+}  // namespace
+
+Utf8ForCodePoint::Utf8ForCodePoint(uint64_t code_point) {
+  if (code_point <= kMax1ByteCodePoint) {
+    length = 1;
+    bytes[0] = static_cast<char>(code_point);
+    return;
+  }
+
+  if (code_point <= kMax2ByteCodePoint) {
+    length = 2;
+    bytes[0] = static_cast<char>(0xc0 | (code_point >> 6));
+    bytes[1] = static_cast<char>(0x80 | (code_point & 0x3f));
+    return;
+  }
+
+  if (kMinSurrogate <= code_point && code_point <= kMaxSurrogate) return;
+
+  if (code_point <= kMax3ByteCodePoint) {
+    length = 3;
+    bytes[0] = static_cast<char>(0xe0 | (code_point >> 12));
+    bytes[1] = static_cast<char>(0x80 | ((code_point >> 6) & 0x3f));
+    bytes[2] = static_cast<char>(0x80 | (code_point & 0x3f));
+    return;
+  }
+
+  if (code_point > kMaxCodePoint) return;
+
+  length = 4;
+  bytes[0] = static_cast<char>(0xf0 | (code_point >> 18));
+  bytes[1] = static_cast<char>(0x80 | ((code_point >> 12) & 0x3f));
+  bytes[2] = static_cast<char>(0x80 | ((code_point >> 6) & 0x3f));
+  bytes[3] = static_cast<char>(0x80 | (code_point & 0x3f));
+}
+
+}  // namespace debugging_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/debugging/internal/utf8_for_code_point.h b/absl/debugging/internal/utf8_for_code_point.h
new file mode 100644
index 00000000000..f23cde6d953
--- /dev/null
+++ b/absl/debugging/internal/utf8_for_code_point.h
@@ -0,0 +1,47 @@
+// Copyright 2024 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_DEBUGGING_INTERNAL_UTF8_FOR_CODE_POINT_H_
+#define ABSL_DEBUGGING_INTERNAL_UTF8_FOR_CODE_POINT_H_
+
+#include <cstdint>
+
+#include "absl/base/config.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace debugging_internal {
+
+struct Utf8ForCodePoint {
+  // Converts a Unicode code point to the corresponding UTF-8 byte sequence.
+  // Async-signal-safe to support use in symbolizing stack traces from a signal
+  // handler.
+  explicit Utf8ForCodePoint(uint64_t code_point);
+
+  // Returns true if the constructor's code_point argument was valid.
+  bool ok() const { return length != 0; }
+
+  // If code_point was in range, then 1 <= length <= 4, and the UTF-8 encoding
+  // is found in bytes[0 .. (length - 1)].  If code_point was invalid, then
+  // length == 0.  In either case, the contents of bytes[length .. 3] are
+  // unspecified.
+  char bytes[4] = {};
+  uint32_t length = 0;
+};
+
+}  // namespace debugging_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_DEBUGGING_INTERNAL_UTF8_FOR_CODE_POINT_H_
diff --git a/absl/debugging/internal/utf8_for_code_point_test.cc b/absl/debugging/internal/utf8_for_code_point_test.cc
new file mode 100644
index 00000000000..dd0591ae985
--- /dev/null
+++ b/absl/debugging/internal/utf8_for_code_point_test.cc
@@ -0,0 +1,175 @@
+// Copyright 2024 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/debugging/internal/utf8_for_code_point.h"
+
+#include <cstdint>
+
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace debugging_internal {
+namespace {
+
+TEST(Utf8ForCodePointTest, RecognizesTheSmallestCodePoint) {
+  Utf8ForCodePoint utf8(uint64_t{0});
+  ASSERT_EQ(utf8.length, 1);
+  EXPECT_EQ(utf8.bytes[0], '\0');
+}
+
+TEST(Utf8ForCodePointTest, RecognizesAsciiSmallA) {
+  Utf8ForCodePoint utf8(uint64_t{'a'});
+  ASSERT_EQ(utf8.length, 1);
+  EXPECT_EQ(utf8.bytes[0], 'a');
+}
+
+TEST(Utf8ForCodePointTest, RecognizesTheLargestOneByteCodePoint) {
+  Utf8ForCodePoint utf8(uint64_t{0x7f});
+  ASSERT_EQ(utf8.length, 1);
+  EXPECT_EQ(utf8.bytes[0], '\x7f');
+}
+
+TEST(Utf8ForCodePointTest, RecognizesTheSmallestTwoByteCodePoint) {
+  Utf8ForCodePoint utf8(uint64_t{0x80});
+  ASSERT_EQ(utf8.length, 2);
+  EXPECT_EQ(utf8.bytes[0], static_cast<char>(0xc2));
+  EXPECT_EQ(utf8.bytes[1], static_cast<char>(0x80));
+}
+
+TEST(Utf8ForCodePointTest, RecognizesSmallNWithTilde) {
+  Utf8ForCodePoint utf8(uint64_t{0xf1});
+  ASSERT_EQ(utf8.length, 2);
+  const char* want = "ñ";
+  EXPECT_EQ(utf8.bytes[0], want[0]);
+  EXPECT_EQ(utf8.bytes[1], want[1]);
+}
+
+TEST(Utf8ForCodePointTest, RecognizesCapitalPi) {
+  Utf8ForCodePoint utf8(uint64_t{0x3a0});
+  ASSERT_EQ(utf8.length, 2);
+  const char* want = "Π";
+  EXPECT_EQ(utf8.bytes[0], want[0]);
+  EXPECT_EQ(utf8.bytes[1], want[1]);
+}
+
+TEST(Utf8ForCodePointTest, RecognizesTheLargestTwoByteCodePoint) {
+  Utf8ForCodePoint utf8(uint64_t{0x7ff});
+  ASSERT_EQ(utf8.length, 2);
+  EXPECT_EQ(utf8.bytes[0], static_cast<char>(0xdf));
+  EXPECT_EQ(utf8.bytes[1], static_cast<char>(0xbf));
+}
+
+TEST(Utf8ForCodePointTest, RecognizesTheSmallestThreeByteCodePoint) {
+  Utf8ForCodePoint utf8(uint64_t{0x800});
+  ASSERT_EQ(utf8.length, 3);
+  EXPECT_EQ(utf8.bytes[0], static_cast<char>(0xe0));
+  EXPECT_EQ(utf8.bytes[1], static_cast<char>(0xa0));
+  EXPECT_EQ(utf8.bytes[2], static_cast<char>(0x80));
+}
+
+TEST(Utf8ForCodePointTest, RecognizesTheChineseCharacterZhong1AsInZhong1Wen2) {
+  Utf8ForCodePoint utf8(uint64_t{0x4e2d});
+  ASSERT_EQ(utf8.length, 3);
+  const char* want = "中";
+  EXPECT_EQ(utf8.bytes[0], want[0]);
+  EXPECT_EQ(utf8.bytes[1], want[1]);
+  EXPECT_EQ(utf8.bytes[2], want[2]);
+}
+
+TEST(Utf8ForCodePointTest, RecognizesOneBeforeTheSmallestSurrogate) {
+  Utf8ForCodePoint utf8(uint64_t{0xd7ff});
+  ASSERT_EQ(utf8.length, 3);
+  EXPECT_EQ(utf8.bytes[0], static_cast<char>(0xed));
+  EXPECT_EQ(utf8.bytes[1], static_cast<char>(0x9f));
+  EXPECT_EQ(utf8.bytes[2], static_cast<char>(0xbf));
+}
+
+TEST(Utf8ForCodePointTest, RejectsTheSmallestSurrogate) {
+  Utf8ForCodePoint utf8(uint64_t{0xd800});
+  EXPECT_EQ(utf8.length, 0);
+}
+
+TEST(Utf8ForCodePointTest, RejectsTheLargestSurrogate) {
+  Utf8ForCodePoint utf8(uint64_t{0xdfff});
+  EXPECT_EQ(utf8.length, 0);
+}
+
+TEST(Utf8ForCodePointTest, RecognizesOnePastTheLargestSurrogate) {
+  Utf8ForCodePoint utf8(uint64_t{0xe000});
+  ASSERT_EQ(utf8.length, 3);
+  EXPECT_EQ(utf8.bytes[0], static_cast<char>(0xee));
+  EXPECT_EQ(utf8.bytes[1], static_cast<char>(0x80));
+  EXPECT_EQ(utf8.bytes[2], static_cast<char>(0x80));
+}
+
+TEST(Utf8ForCodePointTest, RecognizesTheLargestThreeByteCodePoint) {
+  Utf8ForCodePoint utf8(uint64_t{0xffff});
+  ASSERT_EQ(utf8.length, 3);
+  EXPECT_EQ(utf8.bytes[0], static_cast<char>(0xef));
+  EXPECT_EQ(utf8.bytes[1], static_cast<char>(0xbf));
+  EXPECT_EQ(utf8.bytes[2], static_cast<char>(0xbf));
+}
+
+TEST(Utf8ForCodePointTest, RecognizesTheSmallestFourByteCodePoint) {
+  Utf8ForCodePoint utf8(uint64_t{0x10000});
+  ASSERT_EQ(utf8.length, 4);
+  EXPECT_EQ(utf8.bytes[0], static_cast<char>(0xf0));
+  EXPECT_EQ(utf8.bytes[1], static_cast<char>(0x90));
+  EXPECT_EQ(utf8.bytes[2], static_cast<char>(0x80));
+  EXPECT_EQ(utf8.bytes[3], static_cast<char>(0x80));
+}
+
+TEST(Utf8ForCodePointTest, RecognizesTheJackOfHearts) {
+  Utf8ForCodePoint utf8(uint64_t{0x1f0bb});
+  ASSERT_EQ(utf8.length, 4);
+  const char* want = "🂻";
+  EXPECT_EQ(utf8.bytes[0], want[0]);
+  EXPECT_EQ(utf8.bytes[1], want[1]);
+  EXPECT_EQ(utf8.bytes[2], want[2]);
+  EXPECT_EQ(utf8.bytes[3], want[3]);
+}
+
+TEST(Utf8ForCodePointTest, RecognizesTheLargestFourByteCodePoint) {
+  Utf8ForCodePoint utf8(uint64_t{0x10ffff});
+  ASSERT_EQ(utf8.length, 4);
+  EXPECT_EQ(utf8.bytes[0], static_cast<char>(0xf4));
+  EXPECT_EQ(utf8.bytes[1], static_cast<char>(0x8f));
+  EXPECT_EQ(utf8.bytes[2], static_cast<char>(0xbf));
+  EXPECT_EQ(utf8.bytes[3], static_cast<char>(0xbf));
+}
+
+TEST(Utf8ForCodePointTest, RejectsTheSmallestOverlargeCodePoint) {
+  Utf8ForCodePoint utf8(uint64_t{0x110000});
+  EXPECT_EQ(utf8.length, 0);
+}
+
+TEST(Utf8ForCodePointTest, RejectsAThroughlyOverlargeCodePoint) {
+  Utf8ForCodePoint utf8(uint64_t{0xffffffff00000000});
+  EXPECT_EQ(utf8.length, 0);
+}
+
+TEST(Utf8ForCodePointTest, OkReturnsTrueForAValidCodePoint) {
+  EXPECT_TRUE(Utf8ForCodePoint(uint64_t{0}).ok());
+}
+
+TEST(Utf8ForCodePointTest, OkReturnsFalseForAnInvalidCodePoint) {
+  EXPECT_FALSE(Utf8ForCodePoint(uint64_t{0xffffffff00000000}).ok());
+}
+
+}  // namespace
+}  // namespace debugging_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/debugging/internal/vdso_support.cc b/absl/debugging/internal/vdso_support.cc
index d13ef25d3f8..9d96f3bc87e 100644
--- a/absl/debugging/internal/vdso_support.cc
+++ b/absl/debugging/internal/vdso_support.cc
@@ -18,14 +18,28 @@
 
 #include "absl/debugging/internal/vdso_support.h"
 
+#include <atomic>
+
+#include "absl/base/attributes.h"
+#include "absl/debugging/internal/elf_mem_image.h"
+
 #ifdef ABSL_HAVE_VDSO_SUPPORT     // defined in vdso_support.h
 
 #include <errno.h>
 #include <fcntl.h>
+#if __has_include(<syscall.h>)
+#include <syscall.h>
+#elif __has_include(<sys/syscall.h>)
 #include <sys/syscall.h>
+#endif
 #include <unistd.h>
 
-#if __GLIBC_PREREQ(2, 16)  // GLIBC-2.16 implements getauxval.
+#if !defined(__UCLIBC__) && defined(__GLIBC__) && \
+    (__GLIBC__ > 2 || (__GLIBC__ == 2 && __GLIBC_MINOR__ >= 16))
+#define ABSL_HAVE_GETAUXVAL
+#endif
+
+#ifdef ABSL_HAVE_GETAUXVAL
 #include <sys/auxv.h>
 #endif
 
@@ -37,14 +51,28 @@
 #define AT_SYSINFO_EHDR 33  // for crosstoolv10
 #endif
 
+#if defined(__NetBSD__)
+using Elf32_auxv_t = Aux32Info;
+using Elf64_auxv_t = Aux64Info;
+#endif
+#if defined(__FreeBSD__)
+#if defined(__ELF_WORD_SIZE) && __ELF_WORD_SIZE == 64
+using Elf64_auxv_t = Elf64_Auxinfo;
+#endif
+using Elf32_auxv_t = Elf32_Auxinfo;
+#endif
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace debugging_internal {
 
 ABSL_CONST_INIT
 std::atomic<const void *> VDSOSupport::vdso_base_(
     debugging_internal::ElfMemImage::kInvalidBase);
 
-std::atomic<VDSOSupport::GetCpuFn> VDSOSupport::getcpu_fn_(&InitAndGetCPU);
+ABSL_CONST_INIT std::atomic<VDSOSupport::GetCpuFn> VDSOSupport::getcpu_fn_(
+    &InitAndGetCPU);
+
 VDSOSupport::VDSOSupport()
     // If vdso_base_ is still set to kInvalidBase, we got here
     // before VDSOSupport::Init has been called. Call it now.
@@ -64,7 +92,7 @@ VDSOSupport::VDSOSupport()
 // the operation should be idempotent.
 const void *VDSOSupport::Init() {
   const auto kInvalidBase = debugging_internal::ElfMemImage::kInvalidBase;
-#if __GLIBC_PREREQ(2, 16)
+#ifdef ABSL_HAVE_GETAUXVAL
   if (vdso_base_.load(std::memory_order_relaxed) == kInvalidBase) {
     errno = 0;
     const void *const sysinfo_ehdr =
@@ -73,17 +101,8 @@ const void *VDSOSupport::Init() {
       vdso_base_.store(sysinfo_ehdr, std::memory_order_relaxed);
     }
   }
-#endif  // __GLIBC_PREREQ(2, 16)
+#endif  // ABSL_HAVE_GETAUXVAL
   if (vdso_base_.load(std::memory_order_relaxed) == kInvalidBase) {
-    // Valgrind zaps AT_SYSINFO_EHDR and friends from the auxv[]
-    // on stack, and so glibc works as if VDSO was not present.
-    // But going directly to kernel via /proc/self/auxv below bypasses
-    // Valgrind zapping. So we check for Valgrind separately.
-    if (RunningOnValgrind()) {
-      vdso_base_.store(nullptr, std::memory_order_relaxed);
-      getcpu_fn_.store(&GetCPUViaSyscall, std::memory_order_relaxed);
-      return nullptr;
-    }
     int fd = open("/proc/self/auxv", O_RDONLY);
     if (fd == -1) {
       // Kernel too old to have a VDSO.
@@ -94,8 +113,13 @@ const void *VDSOSupport::Init() {
     ElfW(auxv_t) aux;
     while (read(fd, &aux, sizeof(aux)) == sizeof(aux)) {
       if (aux.a_type == AT_SYSINFO_EHDR) {
+#if defined(__NetBSD__)
+        vdso_base_.store(reinterpret_cast<void *>(aux.a_v),
+                         std::memory_order_relaxed);
+#else
         vdso_base_.store(reinterpret_cast<void *>(aux.a_un.a_val),
                          std::memory_order_relaxed);
+#endif
         break;
       }
     }
@@ -167,26 +191,19 @@ long VDSOSupport::InitAndGetCPU(unsigned *cpu,  // NOLINT(runtime/int)
 // This function must be very fast, and may be called from very
 // low level (e.g. tcmalloc). Hence I avoid things like
 // GoogleOnceInit() and ::operator new.
+// The destination in VDSO is unknown to CFI and VDSO does not set MSAN
+// shadow for the return value.
+ABSL_ATTRIBUTE_NO_SANITIZE_CFI
 ABSL_ATTRIBUTE_NO_SANITIZE_MEMORY
 int GetCPU() {
   unsigned cpu;
-  int ret_code = (*VDSOSupport::getcpu_fn_)(&cpu, nullptr, nullptr);
-  return ret_code == 0 ? cpu : ret_code;
+  long ret_code =  // NOLINT(runtime/int)
+      (*VDSOSupport::getcpu_fn_)(&cpu, nullptr, nullptr);
+  return ret_code == 0 ? static_cast<int>(cpu) : static_cast<int>(ret_code);
 }
 
-// We need to make sure VDSOSupport::Init() is called before
-// InitGoogle() does any setuid or chroot calls.  If VDSOSupport
-// is used in any global constructor, this will happen, since
-// VDSOSupport's constructor calls Init.  But if not, we need to
-// ensure it here, with a global constructor of our own.  This
-// is an allowed exception to the normal rule against non-trivial
-// global constructors.
-static class VDSOInitHelper {
- public:
-  VDSOInitHelper() { VDSOSupport::Init(); }
-} vdso_init_helper;
-
 }  // namespace debugging_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_HAVE_VDSO_SUPPORT
diff --git a/absl/debugging/internal/vdso_support.h b/absl/debugging/internal/vdso_support.h
index 9895b48dd36..6562c6c2350 100644
--- a/absl/debugging/internal/vdso_support.h
+++ b/absl/debugging/internal/vdso_support.h
@@ -53,6 +53,7 @@
 #endif
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace debugging_internal {
 
 // NOTE: this class may be used from within tcmalloc, and can not
@@ -149,6 +150,7 @@ class VDSOSupport {
 int GetCPU();
 
 }  // namespace debugging_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_HAVE_ELF_MEM_IMAGE
diff --git a/absl/debugging/leak_check.cc b/absl/debugging/leak_check.cc
index ffe3d1bda9d..1370bcc4559 100644
--- a/absl/debugging/leak_check.cc
+++ b/absl/debugging/leak_check.cc
@@ -11,30 +11,39 @@
 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 // See the License for the specific language governing permissions and
 // limitations under the License.
-
+//
 // Wrappers around lsan_interface functions.
-// When lsan is not linked in, these functions are not available,
-// therefore Abseil code which depends on these functions is conditioned on the
-// definition of LEAK_SANITIZER.
+//
+// These are always-available run-time functions manipulating the LeakSanitizer,
+// even when the lsan_interface (and LeakSanitizer) is not available. When
+// LeakSanitizer is not linked in, these functions become no-op stubs.
+
 #include "absl/debugging/leak_check.h"
 
-#ifndef LEAK_SANITIZER
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
 
-namespace absl {
-bool HaveLeakSanitizer() { return false; }
-void DoIgnoreLeak(const void*) { }
-void RegisterLivePointers(const void*, size_t) { }
-void UnRegisterLivePointers(const void*, size_t) { }
-LeakCheckDisabler::LeakCheckDisabler() { }
-LeakCheckDisabler::~LeakCheckDisabler() { }
-}  // namespace absl
-
-#else
+#if defined(ABSL_HAVE_LEAK_SANITIZER)
 
 #include <sanitizer/lsan_interface.h>
 
+#if ABSL_HAVE_ATTRIBUTE_WEAK
+extern "C" ABSL_ATTRIBUTE_WEAK int __lsan_is_turned_off() { return 0; }
+#endif
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 bool HaveLeakSanitizer() { return true; }
+
+#if ABSL_HAVE_ATTRIBUTE_WEAK
+bool LeakCheckerIsActive() {
+  return __lsan_is_turned_off() == 0;
+}
+#else
+bool LeakCheckerIsActive() { return true; }
+#endif
+
+bool FindAndReportLeaks() { return __lsan_do_recoverable_leak_check() != 0; }
 void DoIgnoreLeak(const void* ptr) { __lsan_ignore_object(ptr); }
 void RegisterLivePointers(const void* ptr, size_t size) {
   __lsan_register_root_region(ptr, size);
@@ -44,6 +53,21 @@ void UnRegisterLivePointers(const void* ptr, size_t size) {
 }
 LeakCheckDisabler::LeakCheckDisabler() { __lsan_disable(); }
 LeakCheckDisabler::~LeakCheckDisabler() { __lsan_enable(); }
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#else  // defined(ABSL_HAVE_LEAK_SANITIZER)
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+bool HaveLeakSanitizer() { return false; }
+bool LeakCheckerIsActive() { return false; }
+void DoIgnoreLeak(const void*) { }
+void RegisterLivePointers(const void*, size_t) { }
+void UnRegisterLivePointers(const void*, size_t) { }
+LeakCheckDisabler::LeakCheckDisabler() = default;
+LeakCheckDisabler::~LeakCheckDisabler() = default;
+ABSL_NAMESPACE_END
 }  // namespace absl
 
-#endif  // LEAK_SANITIZER
+#endif  // defined(ABSL_HAVE_LEAK_SANITIZER)
diff --git a/absl/debugging/leak_check.h b/absl/debugging/leak_check.h
index 4d489c58284..6bd79406236 100644
--- a/absl/debugging/leak_check.h
+++ b/absl/debugging/leak_check.h
@@ -24,7 +24,24 @@
 // Note: this leak checking API is not yet supported in MSVC.
 // Leak checking is enabled by default in all ASan builds.
 //
-// See https://github.com/google/sanitizers/wiki/AddressSanitizerLeakSanitizer
+// https://clang.llvm.org/docs/LeakSanitizer.html
+// https://github.com/google/sanitizers/wiki/AddressSanitizerLeakSanitizer
+//
+// GCC and Clang both automatically enable LeakSanitizer when AddressSanitizer
+// is enabled. To use the mode, simply pass `-fsanitize=address` to both the
+// compiler and linker. An example Bazel command could be
+//
+//   $ bazel test --copt=-fsanitize=address --linkopt=-fsanitize=address ...
+//
+// GCC and Clang auto support a standalone LeakSanitizer mode (a mode which does
+// not also use AddressSanitizer). To use the mode, simply pass
+// `-fsanitize=leak` to both the compiler and linker. Since GCC does not
+// currently provide a way of detecting this mode at compile-time, GCC users
+// must also pass -DLEAK_SANITIZER to the compiler. An example Bazel command
+// could be
+//
+//   $ bazel test --copt=-DLEAK_SANITIZER --copt=-fsanitize=leak
+//     --linkopt=-fsanitize=leak ...
 //
 // -----------------------------------------------------------------------------
 #ifndef ABSL_DEBUGGING_LEAK_CHECK_H_
@@ -32,7 +49,10 @@
 
 #include <cstddef>
 
+#include "absl/base/config.h"
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 // HaveLeakSanitizer()
 //
@@ -40,6 +60,12 @@ namespace absl {
 // currently built into this target.
 bool HaveLeakSanitizer();
 
+// LeakCheckerIsActive()
+//
+// Returns true if a leak-checking sanitizer (either ASan or standalone LSan) is
+// currently built into this target and is turned on.
+bool LeakCheckerIsActive();
+
 // DoIgnoreLeak()
 //
 // Implements `IgnoreLeak()` below. This function should usually
@@ -59,7 +85,8 @@ void DoIgnoreLeak(const void* ptr);
 //
 // If the passed `ptr` does not point to an actively allocated object at the
 // time `IgnoreLeak()` is called, the call is a no-op; if it is actively
-// allocated, the object must not get deallocated later.
+// allocated, leak sanitizer will assume this object is referenced even if
+// there is no actual reference in user memory.
 //
 template <typename T>
 T* IgnoreLeak(T* ptr) {
@@ -67,6 +94,19 @@ T* IgnoreLeak(T* ptr) {
   return ptr;
 }
 
+// FindAndReportLeaks()
+//
+// If any leaks are detected, prints a leak report and returns true.  This
+// function may be called repeatedly, and does not affect end-of-process leak
+// checking.
+//
+// Example:
+// if (FindAndReportLeaks()) {
+//   ... diagnostic already printed. Exit with failure code.
+//   exit(1)
+// }
+bool FindAndReportLeaks();
+
 // LeakCheckDisabler
 //
 // This helper class indicates that any heap allocations done in the code block
@@ -104,6 +144,7 @@ void RegisterLivePointers(const void* ptr, size_t size);
 // `RegisterLivePointers()`, enabling leak checking of those pointers.
 void UnRegisterLivePointers(const void* ptr, size_t size);
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_DEBUGGING_LEAK_CHECK_H_
diff --git a/absl/debugging/leak_check_fail_test.cc b/absl/debugging/leak_check_fail_test.cc
index 2887ceab147..46e9fb6297b 100644
--- a/absl/debugging/leak_check_fail_test.cc
+++ b/absl/debugging/leak_check_fail_test.cc
@@ -13,9 +13,10 @@
 // limitations under the License.
 
 #include <memory>
+
 #include "gtest/gtest.h"
-#include "absl/base/internal/raw_logging.h"
 #include "absl/debugging/leak_check.h"
+#include "absl/log/log.h"
 
 namespace {
 
@@ -25,7 +26,7 @@ TEST(LeakCheckTest, LeakMemory) {
   // failed exit code.
 
   char* foo = strdup("lsan should complain about this leaked string");
-  ABSL_RAW_LOG(INFO, "Should detect leaked std::string %s", foo);
+  LOG(INFO) << "Should detect leaked string " << foo;
 }
 
 TEST(LeakCheckTest, LeakMemoryAfterDisablerScope) {
@@ -34,8 +35,7 @@ TEST(LeakCheckTest, LeakMemoryAfterDisablerScope) {
   // failed exit code.
   { absl::LeakCheckDisabler disabler; }
   char* foo = strdup("lsan should also complain about this leaked string");
-  ABSL_RAW_LOG(INFO, "Re-enabled leak detection.Should detect leaked std::string %s",
-               foo);
+  LOG(INFO) << "Re-enabled leak detection.Should detect leaked string " << foo;
 }
 
 }  // namespace
diff --git a/absl/debugging/leak_check_test.cc b/absl/debugging/leak_check_test.cc
index 93a7edd2d0e..6f0135ed792 100644
--- a/absl/debugging/leak_check_test.cc
+++ b/absl/debugging/leak_check_test.cc
@@ -15,28 +15,27 @@
 #include <string>
 
 #include "gtest/gtest.h"
-#include "absl/base/internal/raw_logging.h"
+#include "absl/base/config.h"
 #include "absl/debugging/leak_check.h"
+#include "absl/log/log.h"
 
 namespace {
 
-TEST(LeakCheckTest, DetectLeakSanitizer) {
-#ifdef ABSL_EXPECT_LEAK_SANITIZER
-  EXPECT_TRUE(absl::HaveLeakSanitizer());
-#else
-  EXPECT_FALSE(absl::HaveLeakSanitizer());
-#endif
-}
-
 TEST(LeakCheckTest, IgnoreLeakSuppressesLeakedMemoryErrors) {
+  if (!absl::LeakCheckerIsActive()) {
+    GTEST_SKIP() << "LeakChecker is not active";
+  }
   auto foo = absl::IgnoreLeak(new std::string("some ignored leaked string"));
-  ABSL_RAW_LOG(INFO, "Ignoring leaked std::string %s", foo->c_str());
+  LOG(INFO) << "Ignoring leaked string " << foo;
 }
 
 TEST(LeakCheckTest, LeakCheckDisablerIgnoresLeak) {
+  if (!absl::LeakCheckerIsActive()) {
+    GTEST_SKIP() << "LeakChecker is not active";
+  }
   absl::LeakCheckDisabler disabler;
-  auto foo = new std::string("some std::string leaked while checks are disabled");
-  ABSL_RAW_LOG(INFO, "Ignoring leaked std::string %s", foo->c_str());
+  auto foo = new std::string("some string leaked while checks are disabled");
+  LOG(INFO) << "Ignoring leaked string " << foo;
 }
 
 }  // namespace
diff --git a/absl/debugging/stacktrace.cc b/absl/debugging/stacktrace.cc
index 9de8782f4ca..9689337099d 100644
--- a/absl/debugging/stacktrace.cc
+++ b/absl/debugging/stacktrace.cc
@@ -36,75 +36,124 @@
 
 #include "absl/debugging/stacktrace.h"
 
+#include <stddef.h>
+#include <stdint.h>
+#include <stdlib.h>
+
+#include <algorithm>
 #include <atomic>
 
 #include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/optimization.h"
 #include "absl/base/port.h"
 #include "absl/debugging/internal/stacktrace_config.h"
 
 #if defined(ABSL_STACKTRACE_INL_HEADER)
 #include ABSL_STACKTRACE_INL_HEADER
 #else
-# error Cannot calculate stack trace: will need to write for your environment
-
-# include "absl/debugging/internal/stacktrace_aarch64-inl.inc"
-# include "absl/debugging/internal/stacktrace_arm-inl.inc"
-# include "absl/debugging/internal/stacktrace_generic-inl.inc"
-# include "absl/debugging/internal/stacktrace_powerpc-inl.inc"
-# include "absl/debugging/internal/stacktrace_unimplemented-inl.inc"
-# include "absl/debugging/internal/stacktrace_win32-inl.inc"
-# include "absl/debugging/internal/stacktrace_x86-inl.inc"
+#error Cannot calculate stack trace: will need to write for your environment
+
+#include "absl/debugging/internal/stacktrace_aarch64-inl.inc"
+#include "absl/debugging/internal/stacktrace_arm-inl.inc"
+#include "absl/debugging/internal/stacktrace_emscripten-inl.inc"
+#include "absl/debugging/internal/stacktrace_generic-inl.inc"
+#include "absl/debugging/internal/stacktrace_powerpc-inl.inc"
+#include "absl/debugging/internal/stacktrace_riscv-inl.inc"
+#include "absl/debugging/internal/stacktrace_unimplemented-inl.inc"
+#include "absl/debugging/internal/stacktrace_win32-inl.inc"
+#include "absl/debugging/internal/stacktrace_x86-inl.inc"
 #endif
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace {
 
+
 typedef int (*Unwinder)(void**, int*, int, int, const void*, int*);
 std::atomic<Unwinder> custom;
 
 template <bool IS_STACK_FRAMES, bool IS_WITH_CONTEXT>
-ABSL_ATTRIBUTE_ALWAYS_INLINE inline int Unwind(void** result, int* sizes,
-                                               int max_depth, int skip_count,
-                                               const void* uc,
-                                               int* min_dropped_frames) {
-  Unwinder f = &UnwindImpl<IS_STACK_FRAMES, IS_WITH_CONTEXT>;
-  Unwinder g = custom.load(std::memory_order_acquire);
-  if (g != nullptr) f = g;
+ABSL_ATTRIBUTE_ALWAYS_INLINE inline int Unwind(void** result, uintptr_t* frames,
+                                               int* sizes, size_t max_depth,
+                                               int skip_count, const void* uc,
+                                               int* min_dropped_frames,
+                                               bool unwind_with_fixup = true) {
+  unwind_with_fixup =
+      unwind_with_fixup && internal_stacktrace::ShouldFixUpStack();
+
+#ifdef _WIN32
+  if (unwind_with_fixup) {
+    // TODO(b/434184677): Fixups are flaky and not supported on Windows
+    unwind_with_fixup = false;
+#ifndef NDEBUG
+    abort();
+#endif
+  }
+#endif
 
+  Unwinder g = custom.load(std::memory_order_acquire);
+  size_t size;
   // Add 1 to skip count for the unwinder function itself
-  int size = (*f)(result, sizes, max_depth, skip_count + 1, uc,
-                  min_dropped_frames);
-  // To disable tail call to (*f)(...)
+  ++skip_count;
+  if (g != nullptr) {
+    size = static_cast<size_t>((*g)(result, sizes, static_cast<int>(max_depth),
+                                    skip_count, uc, min_dropped_frames));
+    // Frame pointers aren't returned by existing hooks, so clear them.
+    if (frames != nullptr) {
+      std::fill(frames, frames + size, uintptr_t());
+    }
+  } else {
+    size = static_cast<size_t>(UnwindImpl<IS_STACK_FRAMES, IS_WITH_CONTEXT>(
+        result, frames, sizes, static_cast<int>(max_depth), skip_count, uc,
+        min_dropped_frames));
+  }
+  if (unwind_with_fixup) {
+    internal_stacktrace::FixUpStack(result, frames, sizes, max_depth, size);
+  }
+
   ABSL_BLOCK_TAIL_CALL_OPTIMIZATION();
-  return size;
+  return static_cast<int>(size);
 }
 
 }  // anonymous namespace
 
 ABSL_ATTRIBUTE_NOINLINE ABSL_ATTRIBUTE_NO_TAIL_CALL int GetStackFrames(
     void** result, int* sizes, int max_depth, int skip_count) {
-  return Unwind<true, false>(result, sizes, max_depth, skip_count, nullptr,
-                             nullptr);
+  return Unwind<true, false>(result, nullptr, sizes,
+                             static_cast<size_t>(max_depth), skip_count,
+                             nullptr, nullptr);
+}
+
+ABSL_ATTRIBUTE_NOINLINE ABSL_ATTRIBUTE_NO_TAIL_CALL int
+internal_stacktrace::GetStackTraceNoFixup(void** result, int max_depth,
+                                          int skip_count) {
+  return Unwind<false, false>(result, nullptr, nullptr,
+                              static_cast<size_t>(max_depth), skip_count,
+                              nullptr, nullptr, /*unwind_with_fixup=*/false);
 }
 
 ABSL_ATTRIBUTE_NOINLINE ABSL_ATTRIBUTE_NO_TAIL_CALL int
 GetStackFramesWithContext(void** result, int* sizes, int max_depth,
                           int skip_count, const void* uc,
                           int* min_dropped_frames) {
-  return Unwind<true, true>(result, sizes, max_depth, skip_count, uc,
+  return Unwind<true, true>(result, nullptr, sizes,
+                            static_cast<size_t>(max_depth), skip_count, uc,
                             min_dropped_frames);
 }
 
 ABSL_ATTRIBUTE_NOINLINE ABSL_ATTRIBUTE_NO_TAIL_CALL int GetStackTrace(
     void** result, int max_depth, int skip_count) {
-  return Unwind<false, false>(result, nullptr, max_depth, skip_count, nullptr,
-                              nullptr);
+  return Unwind<false, false>(result, nullptr, nullptr,
+                              static_cast<size_t>(max_depth), skip_count,
+                              nullptr, nullptr);
 }
 
 ABSL_ATTRIBUTE_NOINLINE ABSL_ATTRIBUTE_NO_TAIL_CALL int
 GetStackTraceWithContext(void** result, int max_depth, int skip_count,
                          const void* uc, int* min_dropped_frames) {
-  return Unwind<false, true>(result, nullptr, max_depth, skip_count, uc,
+  return Unwind<false, true>(result, nullptr, nullptr,
+                             static_cast<size_t>(max_depth), skip_count, uc,
                              min_dropped_frames);
 }
 
@@ -115,7 +164,7 @@ void SetStackUnwinder(Unwinder w) {
 int DefaultStackUnwinder(void** pcs, int* sizes, int depth, int skip,
                          const void* uc, int* min_dropped_frames) {
   skip++;  // For this function
-  Unwinder f = nullptr;
+  decltype(&UnwindImpl<false, false>) f;
   if (sizes == nullptr) {
     if (uc == nullptr) {
       f = &UnwindImpl<false, false>;
@@ -129,10 +178,28 @@ int DefaultStackUnwinder(void** pcs, int* sizes, int depth, int skip,
       f = &UnwindImpl<true, true>;
     }
   }
-  volatile int x = 0;
-  int n = (*f)(pcs, sizes, depth, skip, uc, min_dropped_frames);
-  x = 1; (void) x;  // To disable tail call to (*f)(...)
+  int n = (*f)(pcs, nullptr, sizes, depth, skip, uc, min_dropped_frames);
+  ABSL_BLOCK_TAIL_CALL_OPTIMIZATION();
   return n;
 }
 
+ABSL_ATTRIBUTE_WEAK bool internal_stacktrace::ShouldFixUpStack() {
+  return false;
+}
+
+// Fixes up the stack trace of the current thread, in the first `depth` frames
+// of each buffer. The buffers need to be larger than `depth`, to accommodate
+// any newly inserted elements. `depth` is updated to reflect the new number of
+// elements valid across all the buffers. (It is therefore recommended that all
+// buffer sizes be equal.)
+//
+// The `frames` and `sizes` parameters denote the bounds of the stack frame
+// corresponding to each instruction pointer in the `pcs`.
+// Any elements inside these buffers may be zero or null, in which case that
+// information is assumed to be absent/unavailable.
+ABSL_ATTRIBUTE_WEAK void internal_stacktrace::FixUpStack(void**, uintptr_t*,
+                                                         int*, size_t,
+                                                         size_t&) {}
+
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/debugging/stacktrace.h b/absl/debugging/stacktrace.h
index 3fc1c03f3e6..7e0bd7b558f 100644
--- a/absl/debugging/stacktrace.h
+++ b/absl/debugging/stacktrace.h
@@ -31,15 +31,28 @@
 #ifndef ABSL_DEBUGGING_STACKTRACE_H_
 #define ABSL_DEBUGGING_STACKTRACE_H_
 
+#include <stddef.h>
+#include <stdint.h>
+
+#include "absl/base/config.h"
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+namespace internal_stacktrace {
+
+// As above, but skips fix-ups for efficiency.
+extern int GetStackTraceNoFixup(void** result, int max_depth, int skip_count);
+
+}  // namespace internal_stacktrace
 
 // GetStackFrames()
 //
 // Records program counter values for up to `max_depth` frames, skipping the
-// most recent `skip_count` stack frames, and stores their corresponding values
-// and sizes in `results` and `sizes` buffers. (Note that the frame generated
-// for the `absl::GetStackFrames()` routine itself is also skipped.)
-// routine itself.
+// most recent `skip_count` stack frames, stores their corresponding values
+// and sizes in `results` and `sizes` buffers, and returns the number of frames
+// stored. (Note that the frame generated for the `absl::GetStackFrames()`
+// routine itself is also skipped.)
 //
 // Example:
 //
@@ -54,8 +67,8 @@ namespace absl {
 // The current stack frame would consist of three function calls: `bar()`,
 // `foo()`, and then `main()`; however, since the `GetStackFrames()` call sets
 // `skip_count` to `1`, it will skip the frame for `bar()`, the most recently
-// invoked function call. It will therefore return two program counters and will
-// produce values that map to the following function calls:
+// invoked function call. It will therefore return 2 and fill `result` with
+// program counters within the following functions:
 //
 //      result[0]       foo()
 //      result[1]       main()
@@ -82,9 +95,10 @@ extern int GetStackFrames(void** result, int* sizes, int max_depth,
 //
 // Records program counter values obtained from a signal handler. Records
 // program counter values for up to `max_depth` frames, skipping the most recent
-// `skip_count` stack frames, and stores their corresponding values and sizes in
-// `results` and `sizes` buffers. (Note that the frame generated for the
-// `absl::GetStackFramesWithContext()` routine itself is also skipped.)
+// `skip_count` stack frames, stores their corresponding values and sizes in
+// `results` and `sizes` buffers, and returns the number of frames stored. (Note
+// that the frame generated for the `absl::GetStackFramesWithContext()` routine
+// itself is also skipped.)
 //
 // The `uc` parameter, if non-null, should be a pointer to a `ucontext_t` value
 // passed to a signal handler registered via the `sa_sigaction` field of a
@@ -105,8 +119,9 @@ extern int GetStackFramesWithContext(void** result, int* sizes, int max_depth,
 // GetStackTrace()
 //
 // Records program counter values for up to `max_depth` frames, skipping the
-// most recent `skip_count` stack frames, and stores their corresponding values
-// in `results`. Note that this function is similar to `absl::GetStackFrames()`
+// most recent `skip_count` stack frames, stores their corresponding values
+// in `results`, and returns the number of frames
+// stored. Note that this function is similar to `absl::GetStackFrames()`
 // except that it returns the stack trace only, and not stack frame sizes.
 //
 // Example:
@@ -131,9 +146,9 @@ extern int GetStackTrace(void** result, int max_depth, int skip_count);
 //
 // Records program counter values obtained from a signal handler. Records
 // program counter values for up to `max_depth` frames, skipping the most recent
-// `skip_count` stack frames, and stores their corresponding values in
-// `results`. (Note that the frame generated for the
-// `absl::GetStackFramesWithContext()` routine itself is also skipped.)
+// `skip_count` stack frames, stores their corresponding values in `results`,
+// and returns the number of frames stored. (Note that the frame generated for
+// the `absl::GetStackFramesWithContext()` routine itself is also skipped.)
 //
 // The `uc` parameter, if non-null, should be a pointer to a `ucontext_t` value
 // passed to a signal handler registered via the `sa_sigaction` field of a
@@ -220,6 +235,26 @@ namespace debugging_internal {
 // working.
 extern bool StackTraceWorksForTest();
 }  // namespace debugging_internal
+
+namespace internal_stacktrace {
+extern bool ShouldFixUpStack();
+
+// Fixes up the stack trace of the current thread, in the first `depth` frames
+// of each buffer. The buffers need to be larger than `depth`, to accommodate
+// any newly inserted elements. `depth` is updated to reflect the new number of
+// elements valid across all the buffers. (It is therefore recommended that all
+// buffer sizes be equal.)
+//
+// The `frames` and `sizes` parameters denote the bounds of the stack frame
+// corresponding to each instruction pointer in the `pcs`.
+// Any elements inside these buffers may be zero or null, in which case that
+// information is assumed to be absent/unavailable.
+extern void FixUpStack(void** pcs, uintptr_t* frames, int* sizes,
+                       size_t capacity, size_t& depth);
+
+}  // namespace internal_stacktrace
+
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_DEBUGGING_STACKTRACE_H_
diff --git a/absl/debugging/stacktrace_benchmark.cc b/absl/debugging/stacktrace_benchmark.cc
new file mode 100644
index 00000000000..eef9850a116
--- /dev/null
+++ b/absl/debugging/stacktrace_benchmark.cc
@@ -0,0 +1,78 @@
+// Copyright 2022 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <stddef.h>
+#include <stdint.h>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/optimization.h"
+#include "absl/cleanup/cleanup.h"
+#include "absl/debugging/stacktrace.h"
+#include "benchmark/benchmark.h"
+
+static bool g_enable_fixup = false;
+
+#if ABSL_HAVE_ATTRIBUTE_WEAK
+// Override these weak symbols if possible.
+bool absl::internal_stacktrace::ShouldFixUpStack() { return g_enable_fixup; }
+void absl::internal_stacktrace::FixUpStack(void**, uintptr_t*, int*, size_t,
+                                           size_t&) {}
+#endif
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace {
+
+static constexpr int kMaxStackDepth = 100;
+static constexpr int kCacheSize = (1 << 16);
+void* pcs[kMaxStackDepth];
+
+ABSL_ATTRIBUTE_NOINLINE void func(benchmark::State& state, int x, int depth) {
+  if (x <= 0) {
+    // Touch a significant amount of memory so that the stack is likely to be
+    // not cached in the L1 cache.
+    state.PauseTiming();
+    int* arr = new int[kCacheSize];
+    for (int i = 0; i < kCacheSize; ++i) benchmark::DoNotOptimize(arr[i] = 100);
+    delete[] arr;
+    state.ResumeTiming();
+    benchmark::DoNotOptimize(absl::GetStackTrace(pcs, depth, 0));
+    return;
+  }
+  ABSL_BLOCK_TAIL_CALL_OPTIMIZATION();
+  func(state, --x, depth);
+}
+
+template <bool EnableFixup>
+void BM_GetStackTrace(benchmark::State& state) {
+  const Cleanup restore_state(
+      [prev = g_enable_fixup]() { g_enable_fixup = prev; });
+  g_enable_fixup = EnableFixup;
+  int depth = state.range(0);
+  for (auto s : state) {
+    func(state, depth, depth);
+  }
+}
+
+#if ABSL_HAVE_ATTRIBUTE_WEAK
+auto& BM_GetStackTraceWithFixup = BM_GetStackTrace<true>;
+BENCHMARK(BM_GetStackTraceWithFixup)->DenseRange(10, kMaxStackDepth, 10);
+#endif
+
+auto& BM_GetStackTraceWithoutFixup = BM_GetStackTrace<false>;
+BENCHMARK(BM_GetStackTraceWithoutFixup)->DenseRange(10, kMaxStackDepth, 10);
+}  // namespace
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/debugging/stacktrace_test.cc b/absl/debugging/stacktrace_test.cc
new file mode 100644
index 00000000000..0bf94f70d57
--- /dev/null
+++ b/absl/debugging/stacktrace_test.cc
@@ -0,0 +1,430 @@
+// Copyright 2023 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/debugging/stacktrace.h"
+
+#include <stddef.h>
+#include <stdint.h>
+
+#include <cerrno>
+#include <csignal>
+#include <cstring>
+#include <memory>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/errno_saver.h"
+#include "absl/base/optimization.h"
+#include "absl/types/span.h"
+
+static int g_should_fixup_calls = 0;
+static int g_fixup_calls = 0;
+static bool g_enable_fixup = false;
+static uintptr_t g_last_fixup_frame_address = 0;
+
+#if ABSL_HAVE_ATTRIBUTE_WEAK
+bool absl::internal_stacktrace::ShouldFixUpStack() {
+  ++g_should_fixup_calls;
+  return g_enable_fixup;
+}
+
+void absl::internal_stacktrace::FixUpStack(void**, uintptr_t*, int*, size_t,
+                                           size_t&) {
+  const void* frame_address = nullptr;
+#if ABSL_HAVE_BUILTIN(__builtin_frame_address)
+  frame_address = __builtin_frame_address(0);
+#endif
+  g_last_fixup_frame_address = reinterpret_cast<uintptr_t>(frame_address);
+  ++g_fixup_calls;
+}
+#endif
+
+namespace {
+
+using ::testing::ContainerEq;
+using ::testing::Contains;
+using ::testing::internal::Cleanup;
+
+struct StackTrace {
+  static constexpr int kStackCount = 64;
+  int depth;
+  void* result[kStackCount];
+  int sizes[kStackCount];
+};
+
+// This test is currently only known to pass on Linux x86_64/aarch64.
+#if defined(__linux__) && (defined(__x86_64__) || defined(__aarch64__))
+ABSL_ATTRIBUTE_NOINLINE void Unwind(void* p) {
+  ABSL_ATTRIBUTE_UNUSED static void* volatile sink = p;
+  constexpr int kSize = 16;
+  void* stack[kSize];
+  int frames[kSize];
+  absl::GetStackTrace(stack, kSize, 0);
+  absl::GetStackFrames(stack, frames, kSize, 0);
+}
+
+ABSL_ATTRIBUTE_NOINLINE void HugeFrame() {
+  char buffer[1 << 20];
+  Unwind(buffer);
+  ABSL_BLOCK_TAIL_CALL_OPTIMIZATION();
+}
+
+TEST(StackTrace, HugeFrame) {
+  // Ensure that the unwinder is not confused by very large stack frames.
+  HugeFrame();
+  ABSL_BLOCK_TAIL_CALL_OPTIMIZATION();
+}
+#endif
+
+// This is a separate function to avoid inlining.
+ABSL_ATTRIBUTE_NOINLINE static void FixupNoFixupEquivalenceNoInline() {
+#if !ABSL_HAVE_ATTRIBUTE_WEAK
+  const char* kSkipReason = "Need weak symbol support";
+#elif defined(__riscv)
+  const char* kSkipReason =
+      "Skipping test on RISC-V due to pre-existing failure";
+#elif defined(_WIN32)
+  // TODO(b/434184677): Add support for fixups on Windows if needed
+  const char* kSkipReason =
+      "Skipping test on Windows due to lack of support for fixups";
+#else
+  const char* kSkipReason = nullptr;
+#endif
+
+  // This conditional is to avoid an unreachable code warning.
+  if (kSkipReason != nullptr) {
+    GTEST_SKIP() << kSkipReason;
+  }
+
+  bool can_rely_on_frame_pointers = false;
+  if (!can_rely_on_frame_pointers) {
+    GTEST_SKIP() << "Frame pointers are required, but not guaranteed in OSS";
+  }
+
+  // This test is known not to pass on MSVC (due to weak symbols)
+
+  const Cleanup restore_state([enable_fixup = g_enable_fixup,
+                               fixup_calls = g_fixup_calls,
+                               should_fixup_calls = g_should_fixup_calls]() {
+    g_enable_fixup = enable_fixup;
+    g_fixup_calls = fixup_calls;
+    g_should_fixup_calls = should_fixup_calls;
+  });
+
+  constexpr int kSkip = 1;  // Skip our own frame, whose return PCs won't match
+  constexpr auto kStackCount = 1;
+
+  StackTrace a;
+  StackTrace b;
+
+  // ==========================================================================
+
+  g_fixup_calls = 0;
+  g_should_fixup_calls = 0;
+  a.depth = absl::GetStackTrace(a.result, kStackCount, kSkip);
+  g_enable_fixup = !g_enable_fixup;
+  b.depth = absl::GetStackTrace(b.result, kStackCount, kSkip);
+  EXPECT_THAT(
+      absl::MakeSpan(a.result, static_cast<size_t>(a.depth)),
+      ContainerEq(absl::MakeSpan(b.result, static_cast<size_t>(b.depth))));
+  EXPECT_GT(g_should_fixup_calls, 0);
+  EXPECT_GE(g_should_fixup_calls, g_fixup_calls);
+
+  // ==========================================================================
+
+  g_fixup_calls = 0;
+  g_should_fixup_calls = 0;
+  a.depth = absl::GetStackFrames(a.result, a.sizes, kStackCount, kSkip);
+  g_enable_fixup = !g_enable_fixup;
+  b.depth = absl::GetStackFrames(b.result, b.sizes, kStackCount, kSkip);
+  EXPECT_THAT(
+      absl::MakeSpan(a.result, static_cast<size_t>(a.depth)),
+      ContainerEq(absl::MakeSpan(b.result, static_cast<size_t>(b.depth))));
+  EXPECT_THAT(
+      absl::MakeSpan(a.sizes, static_cast<size_t>(a.depth)),
+      ContainerEq(absl::MakeSpan(b.sizes, static_cast<size_t>(b.depth))));
+  EXPECT_GT(g_should_fixup_calls, 0);
+  EXPECT_GE(g_should_fixup_calls, g_fixup_calls);
+
+  // ==========================================================================
+
+  g_fixup_calls = 0;
+  g_should_fixup_calls = 0;
+  a.depth = absl::GetStackTraceWithContext(a.result, kStackCount, kSkip,
+                                           nullptr, nullptr);
+  g_enable_fixup = !g_enable_fixup;
+  b.depth = absl::GetStackTraceWithContext(b.result, kStackCount, kSkip,
+                                           nullptr, nullptr);
+  EXPECT_THAT(
+      absl::MakeSpan(a.result, static_cast<size_t>(a.depth)),
+      ContainerEq(absl::MakeSpan(b.result, static_cast<size_t>(b.depth))));
+  EXPECT_GT(g_should_fixup_calls, 0);
+  EXPECT_GE(g_should_fixup_calls, g_fixup_calls);
+
+  // ==========================================================================
+
+  g_fixup_calls = 0;
+  g_should_fixup_calls = 0;
+  a.depth = absl::GetStackFramesWithContext(a.result, a.sizes, kStackCount,
+                                            kSkip, nullptr, nullptr);
+  g_enable_fixup = !g_enable_fixup;
+  b.depth = absl::GetStackFramesWithContext(b.result, b.sizes, kStackCount,
+                                            kSkip, nullptr, nullptr);
+  EXPECT_THAT(
+      absl::MakeSpan(a.result, static_cast<size_t>(a.depth)),
+      ContainerEq(absl::MakeSpan(b.result, static_cast<size_t>(b.depth))));
+  EXPECT_THAT(
+      absl::MakeSpan(a.sizes, static_cast<size_t>(a.depth)),
+      ContainerEq(absl::MakeSpan(b.sizes, static_cast<size_t>(b.depth))));
+  EXPECT_GT(g_should_fixup_calls, 0);
+  EXPECT_GE(g_should_fixup_calls, g_fixup_calls);
+}
+
+TEST(StackTrace, FixupNoFixupEquivalence) { FixupNoFixupEquivalenceNoInline(); }
+
+TEST(StackTrace, FixupLowStackUsage) {
+#if !ABSL_HAVE_ATTRIBUTE_WEAK
+  const char* kSkipReason = "Skipping test on MSVC due to weak symbols";
+#elif defined(_WIN32)
+  // TODO(b/434184677): Add support for fixups on Windows if needed
+  const char* kSkipReason =
+      "Skipping test on Windows due to lack of support for fixups";
+#else
+  const char* kSkipReason = nullptr;
+#endif
+
+  // This conditional is to avoid an unreachable code warning.
+  if (kSkipReason != nullptr) {
+    GTEST_SKIP() << kSkipReason;
+  }
+
+  const Cleanup restore_state([enable_fixup = g_enable_fixup,
+                               fixup_calls = g_fixup_calls,
+                               should_fixup_calls = g_should_fixup_calls]() {
+    g_enable_fixup = enable_fixup;
+    g_fixup_calls = fixup_calls;
+    g_should_fixup_calls = should_fixup_calls;
+  });
+
+  g_enable_fixup = true;
+
+  // Request a ton of stack frames, regardless of how many are actually used.
+  // It's fine to request more frames than we have, since functions preallocate
+  // memory before discovering how high the stack really is, and we're really
+  // just trying to make sure the preallocations don't overflow the stack.
+  //
+  // Note that we loop in order to cover all sides of any branches in the
+  // implementation that switch allocation behavior (e.g., from stack to heap)
+  // and to ensure that no sides allocate too much stack space.
+  constexpr size_t kPageSize = 4096;
+  for (size_t depth = 2; depth < (1 << 20); depth += depth / 2) {
+    const auto stack = std::make_unique<void*[]>(depth);
+    const auto frames = std::make_unique<int[]>(depth);
+
+    absl::GetStackFrames(stack.get(), frames.get(), static_cast<int>(depth), 0);
+    const void* frame_address = nullptr;
+#if ABSL_HAVE_BUILTIN(__builtin_frame_address)
+    frame_address = __builtin_frame_address(0);
+#endif
+    size_t stack_usage =
+        reinterpret_cast<uintptr_t>(frame_address) - g_last_fixup_frame_address;
+    EXPECT_LT(stack_usage, kPageSize);
+  }
+}
+
+TEST(StackTrace, CustomUnwinderPerformsFixup) {
+#if !ABSL_HAVE_ATTRIBUTE_WEAK
+  const char* kSkipReason = "Need weak symbol support";
+#elif defined(_WIN32)
+  // TODO(b/434184677): Add support for fixups on Windows if needed
+  const char* kSkipReason =
+      "Skipping test on Windows due to lack of support for fixups";
+#else
+  const char* kSkipReason = nullptr;
+#endif
+
+  // This conditional is to avoid an unreachable code warning.
+  if (kSkipReason != nullptr) {
+    GTEST_SKIP() << kSkipReason;
+  }
+
+  constexpr int kSkip = 1;  // Skip our own frame, whose return PCs won't match
+  constexpr auto kStackCount = 1;
+
+  absl::SetStackUnwinder(absl::DefaultStackUnwinder);
+  const Cleanup restore_state([enable_fixup = g_enable_fixup,
+                               fixup_calls = g_fixup_calls,
+                               should_fixup_calls = g_should_fixup_calls]() {
+    absl::SetStackUnwinder(nullptr);
+    g_enable_fixup = enable_fixup;
+    g_fixup_calls = fixup_calls;
+    g_should_fixup_calls = should_fixup_calls;
+  });
+
+  StackTrace trace;
+
+  g_enable_fixup = true;
+  g_should_fixup_calls = 0;
+  g_fixup_calls = 0;
+  absl::GetStackTrace(trace.result, kSkip, kStackCount);
+  EXPECT_GT(g_should_fixup_calls, 0);
+  EXPECT_GT(g_fixup_calls, 0);
+
+  g_enable_fixup = true;
+  g_should_fixup_calls = 0;
+  g_fixup_calls = 0;
+  absl::GetStackFrames(trace.result, trace.sizes, kSkip, kStackCount);
+  EXPECT_GT(g_should_fixup_calls, 0);
+  EXPECT_GT(g_fixup_calls, 0);
+
+  g_enable_fixup = true;
+  g_should_fixup_calls = 0;
+  g_fixup_calls = 0;
+  absl::GetStackTraceWithContext(trace.result, kSkip, kStackCount, nullptr,
+                                 nullptr);
+  EXPECT_GT(g_should_fixup_calls, 0);
+  EXPECT_GT(g_fixup_calls, 0);
+
+  g_enable_fixup = true;
+  g_should_fixup_calls = 0;
+  g_fixup_calls = 0;
+  absl::GetStackFramesWithContext(trace.result, trace.sizes, kSkip, kStackCount,
+                                  nullptr, nullptr);
+  EXPECT_GT(g_should_fixup_calls, 0);
+  EXPECT_GT(g_fixup_calls, 0);
+}
+
+// This test is Linux specific.
+#if defined(__linux__)
+const void* g_return_address = nullptr;
+bool g_sigusr2_raised = false;
+
+ABSL_ATTRIBUTE_NO_SANITIZE_ADDRESS void SigUsr2Handler(int, siginfo_t*,
+                                                       void* uc) {
+  absl::base_internal::ErrnoSaver errno_saver;
+  // Many platforms don't support this by default.
+  bool support_is_expected = false;
+  constexpr int kMaxStackDepth = 64;
+  void* result[kMaxStackDepth];
+  int depth =
+      absl::GetStackTraceWithContext(result, kMaxStackDepth, 0, uc, nullptr);
+  // Verify we can unwind past the nested signal handlers.
+  if (support_is_expected) {
+    EXPECT_THAT(absl::MakeSpan(result, static_cast<size_t>(depth)),
+                Contains(g_return_address).Times(1));
+  }
+  depth = absl::GetStackTrace(result, kMaxStackDepth, 0);
+  if (support_is_expected) {
+    EXPECT_THAT(absl::MakeSpan(result, static_cast<size_t>(depth)),
+                Contains(g_return_address).Times(1));
+  }
+  g_sigusr2_raised = true;
+}
+
+ABSL_ATTRIBUTE_NO_SANITIZE_ADDRESS void SigUsr1Handler(int, siginfo_t*, void*) {
+  raise(SIGUSR2);
+  ABSL_BLOCK_TAIL_CALL_OPTIMIZATION();
+}
+
+ABSL_ATTRIBUTE_NO_SANITIZE_ADDRESS ABSL_ATTRIBUTE_NOINLINE void RaiseSignal() {
+  g_return_address = __builtin_return_address(0);
+  raise(SIGUSR1);
+  ABSL_BLOCK_TAIL_CALL_OPTIMIZATION();
+}
+
+ABSL_ATTRIBUTE_NO_SANITIZE_ADDRESS ABSL_ATTRIBUTE_NOINLINE void
+TestNestedSignal() {
+  constexpr size_t kAltstackSize = 1 << 14;
+  // Allocate altstack on regular stack to make sure it'll have a higher
+  // address than some of the regular stack frames.
+  char space[kAltstackSize];
+  stack_t altstack;
+  stack_t old_stack;
+  altstack.ss_sp = space;
+  altstack.ss_size = kAltstackSize;
+  altstack.ss_flags = 0;
+  ASSERT_EQ(sigaltstack(&altstack, &old_stack), 0) << strerror(errno);
+  struct sigaction act;
+  struct sigaction oldusr1act;
+  struct sigaction oldusr2act;
+  act.sa_sigaction = SigUsr1Handler;
+  act.sa_flags = SA_SIGINFO | SA_ONSTACK;
+  sigemptyset(&act.sa_mask);
+  ASSERT_EQ(sigaction(SIGUSR1, &act, &oldusr1act), 0) << strerror(errno);
+  act.sa_sigaction = SigUsr2Handler;
+  ASSERT_EQ(sigaction(SIGUSR2, &act, &oldusr2act), 0) << strerror(errno);
+  RaiseSignal();
+  ASSERT_EQ(sigaltstack(&old_stack, nullptr), 0) << strerror(errno);
+  ASSERT_EQ(sigaction(SIGUSR1, &oldusr1act, nullptr), 0) << strerror(errno);
+  ASSERT_EQ(sigaction(SIGUSR2, &oldusr2act, nullptr), 0) << strerror(errno);
+  ABSL_BLOCK_TAIL_CALL_OPTIMIZATION();
+}
+
+TEST(StackTrace, NestedSignal) {
+  // Verify we can unwind past the nested signal handlers.
+  TestNestedSignal();
+  EXPECT_TRUE(g_sigusr2_raised);
+}
+#endif
+
+TEST(StackTrace, NoNullptrInPopulatedRange) {
+  constexpr int kMaxDepth = 1024;
+  void* results[kMaxDepth];
+  int depth = absl::GetStackTrace(results, kMaxDepth, 0);
+  for (int i = 0; i < depth; ++i) {
+    EXPECT_NE(results[i], nullptr) << "Unexpected nullptr found at index " << i;
+  }
+}
+
+
+#if defined(__aarch64__) && defined(__linux__)
+static void CorruptedSigStackHandler(int, siginfo_t*, void*) {
+  void** fp = reinterpret_cast<void**>(__builtin_frame_address(0));
+  void* saved_fp = fp[0];
+  fp[0] = reinterpret_cast<void*>(0x7deadbeef000ULL);  // Unmapped address
+
+  void* stack[16];
+  absl::GetStackTrace(stack, 16, 0);
+
+  fp[0] = saved_fp;
+}
+#endif
+
+TEST(StackTrace, CorruptedSignalStackFrameSafety) {
+#if defined(__aarch64__) && defined(__linux__)
+  stack_t sigstk{};
+  constexpr size_t kAltstackSize = 1 << 14;
+  char altstack[kAltstackSize];
+  sigstk.ss_sp = altstack;
+  sigstk.ss_size = kAltstackSize;
+  sigstk.ss_flags = 0;
+  ASSERT_EQ(sigaltstack(&sigstk, nullptr), 0);
+
+  struct sigaction act{}, oldact{};
+  act.sa_sigaction = CorruptedSigStackHandler;
+  act.sa_flags = SA_SIGINFO | SA_ONSTACK;
+  ASSERT_EQ(sigaction(SIGUSR1, &act, &oldact), 0);
+
+  raise(SIGUSR1);
+
+  sigaction(SIGUSR1, &oldact, nullptr);
+  stack_t disable_stk{};
+  disable_stk.ss_flags = SS_DISABLE;
+  sigaltstack(&disable_stk, nullptr);
+#endif
+}
+
+}  // namespace
diff --git a/absl/debugging/symbolize.cc b/absl/debugging/symbolize.cc
index 24e3a7f0a0e..147fda9059f 100644
--- a/absl/debugging/symbolize.cc
+++ b/absl/debugging/symbolize.cc
@@ -14,15 +14,49 @@
 
 #include "absl/debugging/symbolize.h"
 
+#include "absl/base/config.h"
+#include "absl/base/internal/low_level_alloc.h"
+#include "absl/debugging/internal/symbolize.h"
+
+#ifdef _WIN32
+#include <winapifamily.h>
+#if WINAPI_FAMILY_PARTITION(WINAPI_PARTITION_DESKTOP)
+// UWP doesn't have access to win32 APIs.
+#define ABSL_INTERNAL_HAVE_SYMBOLIZE_WIN32
+#endif
+#endif
+
+// Emscripten symbolization relies on JS. Do not use them in standalone mode.
+#if defined(__EMSCRIPTEN__) && !defined(STANDALONE_WASM)
+#define ABSL_INTERNAL_HAVE_SYMBOLIZE_WASM
+#endif
+
 #if defined(ABSL_INTERNAL_HAVE_ELF_SYMBOLIZE)
 #include "absl/debugging/symbolize_elf.inc"
-#elif defined(_WIN32) && defined(_DEBUG)
-// The Windows Symbolizer only works in debug mode. Note that _DEBUG
-// is the macro that defines whether or not MS C-Runtime debug info is
-// available. Note that the PDB files containing the debug info must
-// also be available to the program at runtime for the symbolizer to
-// work.
+#elif defined(ABSL_INTERNAL_HAVE_SYMBOLIZE_WIN32)
+// The Windows Symbolizer only works if PDB files containing the debug info
+// are available to the program at runtime.
 #include "absl/debugging/symbolize_win32.inc"
+#elif defined(__APPLE__)
+#include "absl/debugging/symbolize_darwin.inc"
+#elif defined(ABSL_INTERNAL_HAVE_SYMBOLIZE_WASM)
+#include "absl/debugging/symbolize_emscripten.inc"
 #else
 #include "absl/debugging/symbolize_unimplemented.inc"
 #endif
+
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+namespace debugging_internal {
+
+void SymbolDecoratorDeleter::operator()(SymbolDecorator* ptr) {
+  ptr->~SymbolDecorator();
+  base_internal::LowLevelAlloc::Free(ptr);
+}
+
+}  // namespace debugging_internal
+
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/debugging/symbolize.h b/absl/debugging/symbolize.h
index a73dbd9eabe..43d93a86822 100644
--- a/absl/debugging/symbolize.h
+++ b/absl/debugging/symbolize.h
@@ -55,6 +55,7 @@
 #include "absl/debugging/internal/symbolize.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 // InitializeSymbolizer()
 //
@@ -70,7 +71,7 @@ namespace absl {
 //   // Now you can use the symbolizer
 // }
 void InitializeSymbolizer(const char* argv0);
-
+//
 // Symbolize()
 //
 // Symbolizes a program counter (instruction pointer value) `pc` and, on
@@ -88,10 +89,11 @@ void InitializeSymbolizer(const char* argv0);
 //     if (absl::Symbolize(pc, tmp, sizeof(tmp))) {
 //       symbol = tmp;
 //     }
-//     absl::PrintF("%*p  %s\n", pc, symbol);
+//     absl::PrintF("%p  %s\n", pc, symbol);
 //  }
 bool Symbolize(const void *pc, char *out, int out_size);
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_DEBUGGING_SYMBOLIZE_H_
diff --git a/absl/debugging/symbolize_darwin.inc b/absl/debugging/symbolize_darwin.inc
new file mode 100644
index 00000000000..d2144818142
--- /dev/null
+++ b/absl/debugging/symbolize_darwin.inc
@@ -0,0 +1,99 @@
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <cxxabi.h>
+#include <execinfo.h>
+
+#include <algorithm>
+#include <cstring>
+
+#include "absl/base/internal/raw_logging.h"
+#include "absl/debugging/internal/demangle.h"
+#include "absl/strings/numbers.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+void InitializeSymbolizer(const char*) {}
+
+namespace debugging_internal {
+namespace {
+
+static std::string GetSymbolString(absl::string_view backtrace_line) {
+  // Example Backtrace lines:
+  // 0   libimaging_shared.dylib             0x018c152a
+  // _ZNSt11_Deque_baseIN3nik7mediadb4PageESaIS2_EE17_M_initialize_mapEm + 3478
+  //
+  // or
+  // 0   libimaging_shared.dylib             0x0000000001895c39
+  // _ZN3nik4util19register_shared_ptrINS_3gpu7TextureEEEvPKvS5_ + 39
+  //
+  // or
+  // 0   mysterious_app                      0x0124000120120009 main + 17
+  auto address_pos = backtrace_line.find(" 0x");
+  if (address_pos == absl::string_view::npos) return std::string();
+  absl::string_view symbol_view = backtrace_line.substr(address_pos + 1);
+
+  auto space_pos = symbol_view.find(" ");
+  if (space_pos == absl::string_view::npos) return std::string();
+  symbol_view = symbol_view.substr(space_pos + 1);  // to mangled symbol
+
+  auto plus_pos = symbol_view.find(" + ");
+  if (plus_pos == absl::string_view::npos) return std::string();
+  symbol_view = symbol_view.substr(0, plus_pos);  // strip remainng
+
+  return std::string(symbol_view);
+}
+
+}  // namespace
+}  // namespace debugging_internal
+
+bool Symbolize(const void* pc, char* out, int out_size) {
+  if (out_size <= 0 || pc == nullptr) {
+    out = nullptr;
+    return false;
+  }
+
+  // This allocates a char* array.
+  char** frame_strings = backtrace_symbols(const_cast<void**>(&pc), 1);
+
+  if (frame_strings == nullptr) return false;
+
+  std::string symbol = debugging_internal::GetSymbolString(frame_strings[0]);
+  free(frame_strings);
+
+  char tmp_buf[1024];
+  if (debugging_internal::Demangle(symbol.c_str(), tmp_buf, sizeof(tmp_buf))) {
+    strncpy(out, tmp_buf,
+            std::min(static_cast<size_t>(out_size), std::size(tmp_buf)));
+  } else {
+    strncpy(out, symbol.c_str(), static_cast<size_t>(out_size));
+  }
+
+  if (out[out_size - 1] != '\0') {
+    // strncpy() does not '\0' terminate when it truncates.
+    static constexpr char kEllipsis[] = "...";
+    size_t ellipsis_size =
+        std::min(sizeof(kEllipsis) - 1, static_cast<size_t>(out_size) - 1);
+    memcpy(out + out_size - ellipsis_size - 1, kEllipsis, ellipsis_size);
+    out[out_size - 1] = '\0';
+  }
+
+  return true;
+}
+
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/debugging/symbolize_elf.inc b/absl/debugging/symbolize_elf.inc
index 14f0c9723a3..06c56344543 100644
--- a/absl/debugging/symbolize_elf.inc
+++ b/absl/debugging/symbolize_elf.inc
@@ -52,11 +52,13 @@
 #include <elf.h>
 #include <fcntl.h>
 #include <link.h>  // For ElfW() macro.
+#include <sys/resource.h>
 #include <sys/stat.h>
 #include <sys/types.h>
 #include <unistd.h>
 
 #include <algorithm>
+#include <array>
 #include <atomic>
 #include <cerrno>
 #include <cinttypes>
@@ -65,6 +67,8 @@
 #include <cstdio>
 #include <cstdlib>
 #include <cstring>
+#include <iterator>
+#include <memory>
 
 #include "absl/base/casts.h"
 #include "absl/base/dynamic_annotations.h"
@@ -74,13 +78,25 @@
 #include "absl/base/port.h"
 #include "absl/debugging/internal/demangle.h"
 #include "absl/debugging/internal/vdso_support.h"
+#include "absl/strings/string_view.h"
+
+#if defined(__FreeBSD__) && !defined(ElfW)
+#define ElfW(x) __ElfN(x)
+#endif
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 // Value of argv[0]. Used by MaybeInitializeObjFile().
 static char *argv0_value = nullptr;
 
 void InitializeSymbolizer(const char *argv0) {
+#ifdef ABSL_HAVE_VDSO_SUPPORT
+  // We need to make sure VDSOSupport::Init() is called before any setuid or
+  // chroot calls, so InitializeSymbolizer() should be called very early in the
+  // life of a program.
+  absl::debugging_internal::VDSOSupport::Init();
+#endif
   if (argv0_value != nullptr) {
     free(argv0_value);
     argv0_value = nullptr;
@@ -111,28 +127,27 @@ namespace {
 // Some platforms use a special .opd section to store function pointers.
 const char kOpdSectionName[] = ".opd";
 
-#if (defined(__powerpc__) && !(_CALL_ELF > 1)) || defined(__ia64)
+#if defined(__powerpc64__) && defined(_CALL_ELF)
+#if _CALL_ELF <= 1
+#define ABSL_INTERNAL_HAVE_PPC64_ELFV1_ABI 1
+#endif
+#endif
+#if defined(ABSL_INTERNAL_HAVE_PPC64_ELFV1_ABI) || defined(__ia64)
 // Use opd section for function descriptors on these platforms, the function
 // address is the first word of the descriptor.
-enum { kPlatformUsesOPDSections = 1 };
-#else  // not PPC or IA64
-enum { kPlatformUsesOPDSections = 0 };
+//
+// https://maskray.me/blog/2023-02-26-linker-notes-on-power-isa notes that
+// opd sections are used on 64-bit PowerPC with the ELFv1 ABI.
+inline constexpr bool kPlatformUsesOPDSections = true;
+#else
+inline constexpr bool kPlatformUsesOPDSections = false;
 #endif
 
 // This works for PowerPC & IA64 only.  A function descriptor consist of two
 // pointers and the first one is the function's entry.
 const size_t kFunctionDescriptorSize = sizeof(void *) * 2;
 
-const int kMaxDecorators = 10;  // Seems like a reasonable upper limit.
-
-struct InstalledSymbolDecorator {
-  SymbolDecorator fn;
-  void *arg;
-  int ticket;
-};
-
-int g_num_decorators;
-InstalledSymbolDecorator g_decorators[kMaxDecorators];
+std::atomic<SymbolDecorator::Factory*> g_decorator_factory = nullptr;
 
 struct FileMappingHint {
   const void *start;
@@ -141,24 +156,16 @@ struct FileMappingHint {
   const char *filename;
 };
 
-// Protects g_decorators.
-// We are using SpinLock and not a Mutex here, because we may be called
-// from inside Mutex::Lock itself, and it prohibits recursive calls.
-// This happens in e.g. base/stacktrace_syscall_unittest.
-// Moreover, we are using only TryLock(), if the decorator list
-// is being modified (is busy), we skip all decorators, and possibly
-// loose some info. Sorry, that's the best we could do.
-base_internal::SpinLock g_decorators_mu(base_internal::kLinkerInitialized);
-
 const int kMaxFileMappingHints = 8;
 int g_num_file_mapping_hints;
 FileMappingHint g_file_mapping_hints[kMaxFileMappingHints];
 // Protects g_file_mapping_hints.
-base_internal::SpinLock g_file_mapping_mu(base_internal::kLinkerInitialized);
+ABSL_CONST_INIT absl::base_internal::SpinLock g_file_mapping_mu(
+    absl::base_internal::SCHEDULE_KERNEL_ONLY);
 
 // Async-signal-safe function to zero a buffer.
 // memset() is not guaranteed to be async-signal-safe.
-static void SafeMemZero(void* p, size_t size) {
+static void SafeMemZero(void *p, size_t size) {
   unsigned char *c = static_cast<unsigned char *>(p);
   while (size--) {
     *c++ = 0;
@@ -166,26 +173,23 @@ static void SafeMemZero(void* p, size_t size) {
 }
 
 struct ObjFile {
-  ObjFile()
-      : filename(nullptr),
-        start_addr(nullptr),
-        end_addr(nullptr),
-        offset(0),
-        fd(-1),
-        elf_type(-1) {
-    SafeMemZero(&elf_header, sizeof(elf_header));
-  }
-
-  char *filename;
-  const void *start_addr;
-  const void *end_addr;
-  uint64_t offset;
+  char *filename = nullptr;
+  const void *start_addr = nullptr;
+  const void *end_addr = nullptr;
+  uint64_t offset = 0;
 
   // The following fields are initialized on the first access to the
   // object file.
-  int fd;
-  int elf_type;
-  ElfW(Ehdr) elf_header;
+  int fd = -1;
+  int elf_type = -1;
+  ElfW(Ehdr) elf_header = {};
+
+  // PT_LOAD program header describing executable code.
+  // Normally we expect just one, but SWIFT binaries have two.
+  // CUDA binaries have 3 (see cr/473913254 description).
+  std::array<ElfW(Phdr), 4> phdr = {};
+
+  SymbolDecoratorPtr decorator = {};
 };
 
 // Build 4-way associative cache for symbols. Within each cache line, symbols
@@ -202,29 +206,6 @@ struct SymbolCacheLine {
   uint32_t age[ASSOCIATIVITY];
 };
 
-// ---------------------------------------------------------------
-// An async-signal-safe arena for LowLevelAlloc
-static std::atomic<base_internal::LowLevelAlloc::Arena *> g_sig_safe_arena;
-
-static base_internal::LowLevelAlloc::Arena *SigSafeArena() {
-  return g_sig_safe_arena.load(std::memory_order_acquire);
-}
-
-static void InitSigSafeArena() {
-  if (SigSafeArena() == nullptr) {
-    base_internal::LowLevelAlloc::Arena *new_arena =
-        base_internal::LowLevelAlloc::NewArena(
-            base_internal::LowLevelAlloc::kAsyncSignalSafe);
-    base_internal::LowLevelAlloc::Arena *old_value = nullptr;
-    if (!g_sig_safe_arena.compare_exchange_strong(old_value, new_arena,
-                                                  std::memory_order_release,
-                                                  std::memory_order_relaxed)) {
-      // We lost a race to allocate an arena; deallocate.
-      base_internal::LowLevelAlloc::DeleteArena(new_arena);
-    }
-  }
-}
-
 // ---------------------------------------------------------------
 // An AddrMap is a vector of ObjFile, using SigSafeArena() for allocation.
 
@@ -232,21 +213,21 @@ class AddrMap {
  public:
   AddrMap() : size_(0), allocated_(0), obj_(nullptr) {}
   ~AddrMap() { base_internal::LowLevelAlloc::Free(obj_); }
-  int Size() const { return size_; }
-  ObjFile *At(int i) { return &obj_[i]; }
+  size_t Size() const { return size_; }
+  ObjFile *At(size_t i) { return &obj_[i]; }
   ObjFile *Add();
   void Clear();
 
  private:
-  int size_;       // count of valid elements (<= allocated_)
-  int allocated_;  // count of allocated elements
-  ObjFile *obj_;   // array of allocated_ elements
+  size_t size_;       // count of valid elements (<= allocated_)
+  size_t allocated_;  // count of allocated elements
+  ObjFile *obj_;      // array of allocated_ elements
   AddrMap(const AddrMap &) = delete;
   AddrMap &operator=(const AddrMap &) = delete;
 };
 
 void AddrMap::Clear() {
-  for (int i = 0; i != size_; i++) {
+  for (size_t i = 0; i != size_; i++) {
     At(i)->~ObjFile();
   }
   size_ = 0;
@@ -254,12 +235,13 @@ void AddrMap::Clear() {
 
 ObjFile *AddrMap::Add() {
   if (size_ == allocated_) {
-    int new_allocated = allocated_ * 2 + 50;
+    size_t new_allocated = allocated_ * 2 + 50;
     ObjFile *new_obj_ =
         static_cast<ObjFile *>(base_internal::LowLevelAlloc::AllocWithArena(
-            new_allocated * sizeof(*new_obj_), SigSafeArena()));
+            new_allocated * sizeof(*new_obj_), base_internal::SigSafeArena()));
     if (obj_) {
-      memcpy(new_obj_, obj_, allocated_ * sizeof(*new_obj_));
+      std::uninitialized_move_n(obj_, size_, new_obj_);
+      std::destroy_n(obj_, size_);
       base_internal::LowLevelAlloc::Free(obj_);
     }
     obj_ = new_obj_;
@@ -268,6 +250,30 @@ ObjFile *AddrMap::Add() {
   return new (&obj_[size_++]) ObjFile;
 }
 
+class CachingFile {
+ public:
+  // Setup reader for fd that uses buf[0, buf_size-1] as a cache.
+  CachingFile(int fd, char *buf, size_t buf_size)
+      : fd_(fd),
+        cache_(buf),
+        cache_size_(buf_size),
+        cache_start_(0),
+        cache_limit_(0) {}
+
+  int fd() const { return fd_; }
+  ssize_t ReadFromOffset(void *buf, size_t count, off_t offset);
+  bool ReadFromOffsetExact(void *buf, size_t count, off_t offset);
+
+ private:
+  // Bytes [cache_start_, cache_limit_-1] from fd_ are stored in
+  // a prefix of cache_[0, cache_size_-1].
+  int fd_;
+  char *cache_;
+  size_t cache_size_;
+  off_t cache_start_;
+  off_t cache_limit_;
+};
+
 // ---------------------------------------------------------------
 
 enum FindSymbolResult { SYMBOL_NOT_FOUND = 1, SYMBOL_TRUNCATED, SYMBOL_FOUND };
@@ -278,11 +284,14 @@ class Symbolizer {
   ~Symbolizer();
   const char *GetSymbol(const void *const pc);
 
+  void UpdateDecoratorsIfNeeded(SymbolDecorator::Factory* decorator_factory);
+
  private:
   char *CopyString(const char *s) {
-    int len = strlen(s);
-    char *dst = static_cast<char *>(
-        base_internal::LowLevelAlloc::AllocWithArena(len + 1, SigSafeArena()));
+    size_t len = strlen(s);
+    char *dst =
+        static_cast<char *>(base_internal::LowLevelAlloc::AllocWithArena(
+            len + 1, base_internal::SigSafeArena()));
     ABSL_RAW_CHECK(dst != nullptr, "out of memory");
     memcpy(dst, s, len + 1);
     return dst;
@@ -301,21 +310,25 @@ class Symbolizer {
   FindSymbolResult GetSymbolFromObjectFile(const ObjFile &obj,
                                            const void *const pc,
                                            const ptrdiff_t relocation,
-                                           char *out, int out_size,
-                                           char *tmp_buf, int tmp_buf_size);
+                                           char *out, size_t out_size,
+                                           char *tmp_buf, size_t tmp_buf_size);
+  const char *GetUncachedSymbol(const void *pc);
 
   enum {
     SYMBOL_BUF_SIZE = 3072,
     TMP_BUF_SIZE = 1024,
     SYMBOL_CACHE_LINES = 128,
+    FILE_CACHE_SIZE = 8192,
   };
 
   AddrMap addr_map_;
+  SymbolDecorator::Factory* decorator_factory_ = nullptr;
 
   bool ok_;
   bool addr_map_read_;
 
   char symbol_buf_[SYMBOL_BUF_SIZE];
+  char file_cache_[FILE_CACHE_SIZE];
 
   // tmp_buf_ will be used to store arrays of ElfW(Shdr) and ElfW(Sym)
   // so we ensure that tmp_buf_ is properly aligned to store either.
@@ -328,15 +341,55 @@ class Symbolizer {
   SymbolCacheLine symbol_cache_[SYMBOL_CACHE_LINES];
 };
 
+// Protect against client code closing low-valued file descriptors it doesn't
+// actually own.
+int OpenReadOnlyWithHighFD(const char *fname) {
+  static int high_fd = [] {
+    struct rlimit rlim{};
+    const int rc = getrlimit(RLIMIT_NOFILE, &rlim);
+    if (rc == 0 && rlim.rlim_cur >= 2000) {
+      const int max_fd = static_cast<int>(rlim.rlim_cur);
+
+      // This will return 2000 on reasonably-configured systems.
+      return std::min<int>(2000, max_fd - 1000);
+    }
+    ABSL_RAW_LOG(WARNING, "Unable to get high fd: rc=%d, limit=%ld",  //
+                 rc, static_cast<long>(rlim.rlim_cur));
+    return -1;
+  }();
+  constexpr int kOpenFlags = O_RDONLY | O_CLOEXEC;
+  if (high_fd >= 1000) {
+    const int fd = open(fname, kOpenFlags);
+    if (fd != -1 && fd < high_fd) {
+      // Try to relocate fd to high range.
+      static_assert(kOpenFlags & O_CLOEXEC,
+                    "F_DUPFD_CLOEXEC assumes O_CLOEXEC");
+      const int fd2 = fcntl(fd, F_DUPFD_CLOEXEC, high_fd);
+      if (fd2 != -1) {
+        // Successfully obtained high fd. Use it.
+        close(fd);
+        return fd2;
+      } else {
+        ABSL_RAW_LOG(WARNING, "Unable to dup fd=%d above %d, errno=%d", fd,
+                     high_fd, errno);
+      }
+    }
+    // Either open failed and fd==-1, or fd is already above high_fd, or fcntl
+    // failed and fd is valid (but low).
+    return fd;
+  }
+  return open(fname, kOpenFlags);
+}
+
 static std::atomic<Symbolizer *> g_cached_symbolizer;
 
 }  // namespace
 
-static int SymbolizerSize() {
+static size_t SymbolizerSize() {
 #if defined(__wasm__) || defined(__asmjs__)
-  int pagesize = getpagesize();
+  auto pagesize = static_cast<size_t>(getpagesize());
 #else
-  int pagesize = sysconf(_SC_PAGESIZE);
+  auto pagesize = static_cast<size_t>(sysconf(_SC_PAGESIZE));
 #endif
   return ((sizeof(Symbolizer) - 1) / pagesize + 1) * pagesize;
 }
@@ -344,14 +397,17 @@ static int SymbolizerSize() {
 // Return (and set null) g_cached_symbolized_state if it is not null.
 // Otherwise return a new symbolizer.
 static Symbolizer *AllocateSymbolizer() {
-  InitSigSafeArena();
+  base_internal::InitSigSafeArena();
   Symbolizer *symbolizer =
       g_cached_symbolizer.exchange(nullptr, std::memory_order_acquire);
-  if (symbolizer != nullptr) {
-    return symbolizer;
-  }
-  return new (base_internal::LowLevelAlloc::AllocWithArena(
-      SymbolizerSize(), SigSafeArena())) Symbolizer();
+  if (symbolizer == nullptr) {
+    void* ptr = base_internal::LowLevelAlloc::AllocWithArena(
+        SymbolizerSize(), base_internal::SigSafeArena());
+    symbolizer = new (ptr) Symbolizer();
+  }
+  symbolizer->UpdateDecoratorsIfNeeded(
+      g_decorator_factory.load(std::memory_order_acquire));
+  return symbolizer;
 }
 
 // Set g_cached_symbolize_state to s if it is null, otherwise
@@ -368,7 +424,7 @@ static void FreeSymbolizer(Symbolizer *s) {
 
 Symbolizer::Symbolizer() : ok_(true), addr_map_read_(false) {
   for (SymbolCacheLine &symbol_cache_line : symbol_cache_) {
-    for (size_t j = 0; j < ABSL_ARRAYSIZE(symbol_cache_line.name); ++j) {
+    for (size_t j = 0; j < std::size(symbol_cache_line.name); ++j) {
       symbol_cache_line.pc[j] = nullptr;
       symbol_cache_line.name[j] = nullptr;
       symbol_cache_line.age[j] = 0;
@@ -385,6 +441,26 @@ Symbolizer::~Symbolizer() {
   ClearAddrMap();
 }
 
+void Symbolizer::UpdateDecoratorsIfNeeded(
+    SymbolDecorator::Factory* decorator_factory) {
+  if (decorator_factory_ == decorator_factory) return;
+
+  for (SymbolCacheLine& symbol_cache_line : symbol_cache_) {
+    for (size_t j = 0; j < std::size(symbol_cache_line.name); ++j) {
+      symbol_cache_line.pc[j] = nullptr;
+      base_internal::LowLevelAlloc::Free(symbol_cache_line.name[j]);
+      symbol_cache_line.name[j] = nullptr;
+      symbol_cache_line.age[j] = 0;
+    }
+  }
+
+  decorator_factory_ = decorator_factory;
+  for (size_t i = 0, end = addr_map_.Size(); i < end; ++i) {
+    ObjFile* obj = addr_map_.At(i);
+    obj->decorator.reset();
+  }
+}
+
 // We don't use assert() since it's not guaranteed to be
 // async-signal-safe.  Instead we define a minimal assertion
 // macro. So far, we don't need pretty printing for __FILE__, etc.
@@ -408,40 +484,64 @@ static ssize_t ReadPersistent(int fd, void *buf, size_t count) {
     if (len == 0) {  // Reached EOF.
       break;
     }
-    num_bytes += len;
+    num_bytes += static_cast<size_t>(len);
   }
   SAFE_ASSERT(num_bytes <= count);
   return static_cast<ssize_t>(num_bytes);
 }
 
-// Read up to "count" bytes from "offset" in the file pointed by file
-// descriptor "fd" into the buffer starting at "buf".  On success,
-// return the number of bytes read.  Otherwise, return -1.
-static ssize_t ReadFromOffset(const int fd, void *buf, const size_t count,
-                              const off_t offset) {
-  off_t off = lseek(fd, offset, SEEK_SET);
-  if (off == (off_t)-1) {
-    ABSL_RAW_LOG(WARNING, "lseek(%d, %ju, SEEK_SET) failed: errno=%d", fd,
-                 static_cast<uintmax_t>(offset), errno);
-    return -1;
+// Read up to "count" bytes from "offset" into the buffer starting at "buf",
+// while handling short reads and EINTR.  On success, return the number of bytes
+// read.  Otherwise, return -1.
+ssize_t CachingFile::ReadFromOffset(void *buf, size_t count, off_t offset) {
+  char *dst = static_cast<char *>(buf);
+  size_t read = 0;
+  while (read < count) {
+    // Look in cache first.
+    if (offset >= cache_start_ && offset < cache_limit_) {
+      const char *hit_start = &cache_[offset - cache_start_];
+      const size_t n =
+          std::min(count - read, static_cast<size_t>(cache_limit_ - offset));
+      memcpy(dst, hit_start, n);
+      dst += n;
+      read += static_cast<size_t>(n);
+      offset += static_cast<off_t>(n);
+      continue;
+    }
+
+    cache_start_ = 0;
+    cache_limit_ = 0;
+    ssize_t n = pread(fd_, cache_, cache_size_, offset);
+    if (n < 0) {
+      if (errno == EINTR) {
+        continue;
+      }
+      ABSL_RAW_LOG(WARNING, "read failed: errno=%d", errno);
+      return -1;
+    }
+    if (n == 0) {  // Reached EOF.
+      break;
+    }
+
+    cache_start_ = offset;
+    cache_limit_ = offset + static_cast<off_t>(n);
+    // Next iteration will copy from cache into dst.
   }
-  return ReadPersistent(fd, buf, count);
+  return static_cast<ssize_t>(read);
 }
 
-// Try reading exactly "count" bytes from "offset" bytes in a file
-// pointed by "fd" into the buffer starting at "buf" while handling
-// short reads and EINTR.  On success, return true. Otherwise, return
-// false.
-static bool ReadFromOffsetExact(const int fd, void *buf, const size_t count,
-                                const off_t offset) {
-  ssize_t len = ReadFromOffset(fd, buf, count, offset);
+// Try reading exactly "count" bytes from "offset" bytes into the buffer
+// starting at "buf" while handling short reads and EINTR.  On success, return
+// true. Otherwise, return false.
+bool CachingFile::ReadFromOffsetExact(void *buf, size_t count, off_t offset) {
+  ssize_t len = ReadFromOffset(buf, count, offset);
   return len >= 0 && static_cast<size_t>(len) == count;
 }
 
 // Returns elf_header.e_type if the file pointed by fd is an ELF binary.
-static int FileGetElfType(const int fd) {
+static int FileGetElfType(CachingFile *file) {
   ElfW(Ehdr) elf_header;
-  if (!ReadFromOffsetExact(fd, &elf_header, sizeof(elf_header), 0)) {
+  if (!file->ReadFromOffsetExact(&elf_header, sizeof(elf_header), 0)) {
     return -1;
   }
   if (memcmp(elf_header.e_ident, ELFMAG, SELFMAG) != 0) {
@@ -456,30 +556,36 @@ static int FileGetElfType(const int fd) {
 // To keep stack consumption low, we would like this function to not get
 // inlined.
 static ABSL_ATTRIBUTE_NOINLINE bool GetSectionHeaderByType(
-    const int fd, ElfW(Half) sh_num, const off_t sh_offset, ElfW(Word) type,
-    ElfW(Shdr) * out, char *tmp_buf, int tmp_buf_size) {
+    CachingFile *file, ElfW(Half) sh_num, const off_t sh_offset,
+    ElfW(Word) type, ElfW(Shdr) * out, char *tmp_buf, size_t tmp_buf_size) {
   ElfW(Shdr) *buf = reinterpret_cast<ElfW(Shdr) *>(tmp_buf);
-  const int buf_entries = tmp_buf_size / sizeof(buf[0]);
-  const int buf_bytes = buf_entries * sizeof(buf[0]);
+  const size_t buf_entries = tmp_buf_size / sizeof(buf[0]);
+  const size_t buf_bytes = buf_entries * sizeof(buf[0]);
 
-  for (int i = 0; i < sh_num;) {
-    const ssize_t num_bytes_left = (sh_num - i) * sizeof(buf[0]);
-    const ssize_t num_bytes_to_read =
+  for (size_t i = 0; static_cast<int>(i) < sh_num;) {
+    const size_t num_bytes_left =
+        (static_cast<size_t>(sh_num) - i) * sizeof(buf[0]);
+    const size_t num_bytes_to_read =
         (buf_bytes > num_bytes_left) ? num_bytes_left : buf_bytes;
-    const off_t offset = sh_offset + i * sizeof(buf[0]);
-    const ssize_t len = ReadFromOffset(fd, buf, num_bytes_to_read, offset);
-    if (len % sizeof(buf[0]) != 0) {
+    const off_t offset = sh_offset + static_cast<off_t>(i * sizeof(buf[0]));
+    const ssize_t len = file->ReadFromOffset(buf, num_bytes_to_read, offset);
+    if (len <= 0) {
+      ABSL_RAW_LOG(WARNING, "Reading %zu bytes from offset %ju returned %zd.",
+                   num_bytes_to_read, static_cast<intmax_t>(offset), len);
+      return false;
+    }
+    if (static_cast<size_t>(len) % sizeof(buf[0]) != 0) {
       ABSL_RAW_LOG(
           WARNING,
-          "Reading %zd bytes from offset %ju returned %zd which is not a "
+          "Reading %zu bytes from offset %jd returned %zd which is not a "
           "multiple of %zu.",
-          num_bytes_to_read, static_cast<uintmax_t>(offset), len,
+          num_bytes_to_read, static_cast<intmax_t>(offset), len,
           sizeof(buf[0]));
       return false;
     }
-    const ssize_t num_headers_in_buf = len / sizeof(buf[0]);
+    const size_t num_headers_in_buf = static_cast<size_t>(len) / sizeof(buf[0]);
     SAFE_ASSERT(num_headers_in_buf <= buf_entries);
-    for (int j = 0; j < num_headers_in_buf; ++j) {
+    for (size_t j = 0; j < num_headers_in_buf; ++j) {
       if (buf[j].sh_type == type) {
         *out = buf[j];
         return true;
@@ -494,41 +600,52 @@ static ABSL_ATTRIBUTE_NOINLINE bool GetSectionHeaderByType(
 // but there has (as yet) been no need for anything longer either.
 const int kMaxSectionNameLen = 64;
 
+// Small cache to use for miscellaneous file reads.
+const int kSmallFileCacheSize = 100;
+
 bool ForEachSection(int fd,
-                    const std::function<bool(const std::string &name,
+                    const std::function<bool(absl::string_view name,
                                              const ElfW(Shdr) &)> &callback) {
+  char buf[kSmallFileCacheSize];
+  CachingFile file(fd, buf, sizeof(buf));
+
   ElfW(Ehdr) elf_header;
-  if (!ReadFromOffsetExact(fd, &elf_header, sizeof(elf_header), 0)) {
+  if (!file.ReadFromOffsetExact(&elf_header, sizeof(elf_header), 0)) {
+    return false;
+  }
+
+  // Technically it can be larger, but in practice this never happens.
+  if (elf_header.e_shentsize != sizeof(ElfW(Shdr))) {
     return false;
   }
 
   ElfW(Shdr) shstrtab;
-  off_t shstrtab_offset =
-      (elf_header.e_shoff + elf_header.e_shentsize * elf_header.e_shstrndx);
-  if (!ReadFromOffsetExact(fd, &shstrtab, sizeof(shstrtab), shstrtab_offset)) {
+  off_t shstrtab_offset = static_cast<off_t>(elf_header.e_shoff) +
+                          elf_header.e_shentsize * elf_header.e_shstrndx;
+  if (!file.ReadFromOffsetExact(&shstrtab, sizeof(shstrtab), shstrtab_offset)) {
     return false;
   }
 
   for (int i = 0; i < elf_header.e_shnum; ++i) {
     ElfW(Shdr) out;
     off_t section_header_offset =
-        (elf_header.e_shoff + elf_header.e_shentsize * i);
-    if (!ReadFromOffsetExact(fd, &out, sizeof(out), section_header_offset)) {
+        static_cast<off_t>(elf_header.e_shoff) + elf_header.e_shentsize * i;
+    if (!file.ReadFromOffsetExact(&out, sizeof(out), section_header_offset)) {
       return false;
     }
-    off_t name_offset = shstrtab.sh_offset + out.sh_name;
-    char header_name[kMaxSectionNameLen + 1];
+    off_t name_offset = static_cast<off_t>(shstrtab.sh_offset) + out.sh_name;
+    char header_name[kMaxSectionNameLen];
     ssize_t n_read =
-        ReadFromOffset(fd, &header_name, kMaxSectionNameLen, name_offset);
-    if (n_read == -1) {
+        file.ReadFromOffset(&header_name, kMaxSectionNameLen, name_offset);
+    if (n_read < 0) {
       return false;
     } else if (n_read > kMaxSectionNameLen) {
       // Long read?
       return false;
     }
-    header_name[n_read] = '\0';
 
-    std::string name(header_name);
+    absl::string_view name(header_name,
+                           strnlen(header_name, static_cast<size_t>(n_read)));
     if (!callback(name, out)) {
       break;
     }
@@ -549,26 +666,33 @@ bool GetSectionHeaderByName(int fd, const char *name, size_t name_len,
     return false;
   }
 
+  char buf[kSmallFileCacheSize];
+  CachingFile file(fd, buf, sizeof(buf));
   ElfW(Ehdr) elf_header;
-  if (!ReadFromOffsetExact(fd, &elf_header, sizeof(elf_header), 0)) {
+  if (!file.ReadFromOffsetExact(&elf_header, sizeof(elf_header), 0)) {
+    return false;
+  }
+
+  // Technically it can be larger, but in practice this never happens.
+  if (elf_header.e_shentsize != sizeof(ElfW(Shdr))) {
     return false;
   }
 
   ElfW(Shdr) shstrtab;
-  off_t shstrtab_offset =
-      (elf_header.e_shoff + elf_header.e_shentsize * elf_header.e_shstrndx);
-  if (!ReadFromOffsetExact(fd, &shstrtab, sizeof(shstrtab), shstrtab_offset)) {
+  off_t shstrtab_offset = static_cast<off_t>(elf_header.e_shoff) +
+                          elf_header.e_shentsize * elf_header.e_shstrndx;
+  if (!file.ReadFromOffsetExact(&shstrtab, sizeof(shstrtab), shstrtab_offset)) {
     return false;
   }
 
   for (int i = 0; i < elf_header.e_shnum; ++i) {
     off_t section_header_offset =
-        (elf_header.e_shoff + elf_header.e_shentsize * i);
-    if (!ReadFromOffsetExact(fd, out, sizeof(*out), section_header_offset)) {
+        static_cast<off_t>(elf_header.e_shoff) + elf_header.e_shentsize * i;
+    if (!file.ReadFromOffsetExact(out, sizeof(*out), section_header_offset)) {
       return false;
     }
-    off_t name_offset = shstrtab.sh_offset + out->sh_name;
-    ssize_t n_read = ReadFromOffset(fd, &header_name, name_len, name_offset);
+    off_t name_offset = static_cast<off_t>(shstrtab.sh_offset) + out->sh_name;
+    ssize_t n_read = file.ReadFromOffset(&header_name, name_len, name_offset);
     if (n_read < 0) {
       return false;
     } else if (static_cast<size_t>(n_read) != name_len) {
@@ -589,7 +713,7 @@ static bool ShouldPickFirstSymbol(const ElfW(Sym) & symbol1,
   // If one of the symbols is weak and the other is not, pick the one
   // this is not a weak symbol.
   char bind1 = ELF_ST_BIND(symbol1.st_info);
-  char bind2 = ELF_ST_BIND(symbol1.st_info);
+  char bind2 = ELF_ST_BIND(symbol2.st_info);
   if (bind1 == STB_WEAK && bind2 != STB_WEAK) return false;
   if (bind2 == STB_WEAK && bind1 != STB_WEAK) return true;
 
@@ -605,7 +729,7 @@ static bool ShouldPickFirstSymbol(const ElfW(Sym) & symbol1,
   // If one of the symbols has no type and the other is not, pick the
   // one that has a type.
   char type1 = ELF_ST_TYPE(symbol1.st_info);
-  char type2 = ELF_ST_TYPE(symbol1.st_info);
+  char type2 = ELF_ST_TYPE(symbol2.st_info);
   if (type1 != STT_NOTYPE && type2 == STT_NOTYPE) {
     return true;
   }
@@ -618,17 +742,19 @@ static bool ShouldPickFirstSymbol(const ElfW(Sym) & symbol1,
 }
 
 // Return true if an address is inside a section.
-static bool InSection(const void *address, const ElfW(Shdr) * section) {
-  const char *start = reinterpret_cast<const char *>(section->sh_addr);
+static bool InSection(const void *address, ptrdiff_t relocation,
+                      const ElfW(Shdr) * section) {
+  const char *start = reinterpret_cast<const char *>(
+      section->sh_addr + static_cast<ElfW(Addr)>(relocation));
   size_t size = static_cast<size_t>(section->sh_size);
   return start <= address && address < (start + size);
 }
 
 static const char *ComputeOffset(const char *base, ptrdiff_t offset) {
-  // Note: cast to uintptr_t to avoid undefined behavior when base evaluates to
+  // Note: cast to intptr_t to avoid undefined behavior when base evaluates to
   // zero and offset is non-zero.
-  return reinterpret_cast<const char *>(
-      reinterpret_cast<uintptr_t>(base) + offset);
+  return reinterpret_cast<const char *>(reinterpret_cast<intptr_t>(base) +
+                                        offset);
 }
 
 // Read a symbol table and look for the symbol containing the
@@ -641,42 +767,45 @@ static const char *ComputeOffset(const char *base, ptrdiff_t offset) {
 // To keep stack consumption low, we would like this function to not get
 // inlined.
 static ABSL_ATTRIBUTE_NOINLINE FindSymbolResult FindSymbol(
-    const void *const pc, const int fd, char *out, int out_size,
+    const void *const pc, CachingFile *file, char *out, size_t out_size,
     ptrdiff_t relocation, const ElfW(Shdr) * strtab, const ElfW(Shdr) * symtab,
-    const ElfW(Shdr) * opd, char *tmp_buf, int tmp_buf_size) {
+    const ElfW(Shdr) * opd, char *tmp_buf, size_t tmp_buf_size) {
   if (symtab == nullptr) {
     return SYMBOL_NOT_FOUND;
   }
 
   // Read multiple symbols at once to save read() calls.
   ElfW(Sym) *buf = reinterpret_cast<ElfW(Sym) *>(tmp_buf);
-  const int buf_entries = tmp_buf_size / sizeof(buf[0]);
+  const size_t buf_entries = tmp_buf_size / sizeof(buf[0]);
 
-  const int num_symbols = symtab->sh_size / symtab->sh_entsize;
+  const size_t num_symbols = symtab->sh_size / symtab->sh_entsize;
 
   // On platforms using an .opd section (PowerPC & IA64), a function symbol
   // has the address of a function descriptor, which contains the real
   // starting address.  However, we do not always want to use the real
   // starting address because we sometimes want to symbolize a function
   // pointer into the .opd section, e.g. FindSymbol(&foo,...).
-  const bool pc_in_opd =
-      kPlatformUsesOPDSections && opd != nullptr && InSection(pc, opd);
+  const bool pc_in_opd = kPlatformUsesOPDSections && opd != nullptr &&
+                         InSection(pc, relocation, opd);
   const bool deref_function_descriptor_pointer =
       kPlatformUsesOPDSections && opd != nullptr && !pc_in_opd;
 
   ElfW(Sym) best_match;
   SafeMemZero(&best_match, sizeof(best_match));
   bool found_match = false;
-  for (int i = 0; i < num_symbols;) {
-    off_t offset = symtab->sh_offset + i * symtab->sh_entsize;
-    const int num_remaining_symbols = num_symbols - i;
-    const int entries_in_chunk = std::min(num_remaining_symbols, buf_entries);
-    const int bytes_in_chunk = entries_in_chunk * sizeof(buf[0]);
-    const ssize_t len = ReadFromOffset(fd, buf, bytes_in_chunk, offset);
-    SAFE_ASSERT(len % sizeof(buf[0]) == 0);
-    const ssize_t num_symbols_in_buf = len / sizeof(buf[0]);
+  for (size_t i = 0; i < num_symbols;) {
+    off_t offset =
+        static_cast<off_t>(symtab->sh_offset + i * symtab->sh_entsize);
+    const size_t num_remaining_symbols = num_symbols - i;
+    const size_t entries_in_chunk =
+        std::min(num_remaining_symbols, buf_entries);
+    const size_t bytes_in_chunk = entries_in_chunk * sizeof(buf[0]);
+    const ssize_t len = file->ReadFromOffset(buf, bytes_in_chunk, offset);
+    SAFE_ASSERT(len > 0);
+    SAFE_ASSERT(static_cast<size_t>(len) % sizeof(buf[0]) == 0);
+    const size_t num_symbols_in_buf = static_cast<size_t>(len) / sizeof(buf[0]);
     SAFE_ASSERT(num_symbols_in_buf <= entries_in_chunk);
-    for (int j = 0; j < num_symbols_in_buf; ++j) {
+    for (size_t j = 0; j < num_symbols_in_buf; ++j) {
       const ElfW(Sym) &symbol = buf[j];
 
       // For a DSO, a symbol address is relocated by the loading address.
@@ -686,8 +815,18 @@ static ABSL_ATTRIBUTE_NOINLINE FindSymbolResult FindSymbol(
       const char *start_address =
           ComputeOffset(original_start_address, relocation);
 
+#ifdef __arm__
+      // ARM functions are always aligned to multiples of two bytes; the
+      // lowest-order bit in start_address is ignored by the CPU and indicates
+      // whether the function contains ARM (0) or Thumb (1) code. We don't care
+      // about what encoding is being used; we just want the real start address
+      // of the function.
+      start_address = reinterpret_cast<const char *>(
+          reinterpret_cast<uintptr_t>(start_address) & ~1u);
+#endif
+
       if (deref_function_descriptor_pointer &&
-          InSection(original_start_address, opd)) {
+          InSection(original_start_address, /*relocation=*/0, opd)) {
         // The opd section is mapped into memory.  Just dereference
         // start_address to get the first double word, which points to the
         // function entry.
@@ -696,7 +835,8 @@ static ABSL_ATTRIBUTE_NOINLINE FindSymbolResult FindSymbol(
 
       // If pc is inside the .opd section, it points to a function descriptor.
       const size_t size = pc_in_opd ? kFunctionDescriptorSize : symbol.st_size;
-      const void *const end_address = ComputeOffset(start_address, size);
+      const void *const end_address =
+          ComputeOffset(start_address, static_cast<ptrdiff_t>(size));
       if (symbol.st_value != 0 &&  // Skip null value symbols.
           symbol.st_shndx != 0 &&  // Skip undefined symbols.
 #ifdef STT_TLS
@@ -714,16 +854,18 @@ static ABSL_ATTRIBUTE_NOINLINE FindSymbolResult FindSymbol(
   }
 
   if (found_match) {
-    const size_t off = strtab->sh_offset + best_match.st_name;
-    const ssize_t n_read = ReadFromOffset(fd, out, out_size, off);
+    const off_t off =
+        static_cast<off_t>(strtab->sh_offset) + best_match.st_name;
+    const ssize_t n_read = file->ReadFromOffset(out, out_size, off);
     if (n_read <= 0) {
       // This should never happen.
       ABSL_RAW_LOG(WARNING,
-                   "Unable to read from fd %d at offset %zu: n_read = %zd", fd,
-                   off, n_read);
+                   "Unable to read from fd %d at offset %lld: n_read = %zd",
+                   file->fd(), static_cast<long long>(off), n_read);
       return SYMBOL_NOT_FOUND;
     }
-    ABSL_RAW_CHECK(n_read <= out_size, "ReadFromOffset read too much data.");
+    ABSL_RAW_CHECK(static_cast<size_t>(n_read) <= out_size,
+                   "ReadFromOffset read too much data.");
 
     // strtab->sh_offset points into .strtab-like section that contains
     // NUL-terminated strings: '\0foo\0barbaz\0...".
@@ -731,7 +873,7 @@ static ABSL_ATTRIBUTE_NOINLINE FindSymbolResult FindSymbol(
     // sh_offset+st_name points to the start of symbol name, but we don't know
     // how long the symbol is, so we try to read as much as we have space for,
     // and usually over-read (i.e. there is a NUL somewhere before n_read).
-    if (memchr(out, '\0', n_read) == nullptr) {
+    if (memchr(out, '\0', static_cast<size_t>(n_read)) == nullptr) {
       // Either out_size was too small (n_read == out_size and no NUL), or
       // we tried to read past the EOF (n_read < out_size) and .strtab is
       // corrupt (missing terminating NUL; should never happen for valid ELF).
@@ -749,7 +891,7 @@ static ABSL_ATTRIBUTE_NOINLINE FindSymbolResult FindSymbol(
 // See FindSymbol() comment for description of return value.
 FindSymbolResult Symbolizer::GetSymbolFromObjectFile(
     const ObjFile &obj, const void *const pc, const ptrdiff_t relocation,
-    char *out, int out_size, char *tmp_buf, int tmp_buf_size) {
+    char *out, size_t out_size, char *tmp_buf, size_t tmp_buf_size) {
   ElfW(Shdr) symtab;
   ElfW(Shdr) strtab;
   ElfW(Shdr) opd;
@@ -769,20 +911,24 @@ FindSymbolResult Symbolizer::GetSymbolFromObjectFile(
     }
   }
 
+  CachingFile file(obj.fd, file_cache_, sizeof(file_cache_));
+
   // Consult a regular symbol table, then fall back to the dynamic symbol table.
   for (const auto symbol_table_type : {SHT_SYMTAB, SHT_DYNSYM}) {
-    if (!GetSectionHeaderByType(obj.fd, obj.elf_header.e_shnum,
-                                obj.elf_header.e_shoff, symbol_table_type,
+    if (!GetSectionHeaderByType(&file, obj.elf_header.e_shnum,
+                                static_cast<off_t>(obj.elf_header.e_shoff),
+                                static_cast<ElfW(Word)>(symbol_table_type),
                                 &symtab, tmp_buf, tmp_buf_size)) {
       continue;
     }
-    if (!ReadFromOffsetExact(
-            obj.fd, &strtab, sizeof(strtab),
-            obj.elf_header.e_shoff + symtab.sh_link * sizeof(symtab))) {
+    if (!file.ReadFromOffsetExact(
+            &strtab, sizeof(strtab),
+            static_cast<off_t>(obj.elf_header.e_shoff +
+                               symtab.sh_link * sizeof(symtab)))) {
       continue;
     }
     const FindSymbolResult rc =
-        FindSymbol(pc, obj.fd, out, out_size, relocation, &strtab, &symtab,
+        FindSymbol(pc, &file, out, out_size, relocation, &strtab, &symtab,
                    opd_ptr, tmp_buf, tmp_buf_size);
     if (rc != SYMBOL_NOT_FOUND) {
       return rc;
@@ -803,7 +949,7 @@ class FileDescriptor {
 
   ~FileDescriptor() {
     if (fd_ >= 0) {
-      NO_INTR(close(fd_));
+      close(fd_);
     }
   }
 
@@ -820,7 +966,7 @@ class FileDescriptor {
 // and snprintf().
 class LineReader {
  public:
-  explicit LineReader(int fd, char *buf, int buf_len)
+  explicit LineReader(int fd, char *buf, size_t buf_len)
       : fd_(fd),
         buf_len_(buf_len),
         buf_(buf),
@@ -848,12 +994,12 @@ class LineReader {
       bol_ = eol_ + 1;            // Advance to the next line in the buffer.
       SAFE_ASSERT(bol_ <= eod_);  // "bol_" can point to "eod_".
       if (!HasCompleteLine()) {
-        const int incomplete_line_length = eod_ - bol_;
+        const auto incomplete_line_length = static_cast<size_t>(eod_ - bol_);
         // Move the trailing incomplete line to the beginning.
         memmove(buf_, bol_, incomplete_line_length);
         // Read text from file and append it.
         char *const append_pos = buf_ + incomplete_line_length;
-        const int capacity_left = buf_len_ - incomplete_line_length;
+        const size_t capacity_left = buf_len_ - incomplete_line_length;
         const ssize_t num_bytes =
             ReadPersistent(fd_, append_pos, capacity_left);
         if (num_bytes <= 0) {  // EOF or error.
@@ -876,7 +1022,8 @@ class LineReader {
 
  private:
   char *FindLineFeed() const {
-    return reinterpret_cast<char *>(memchr(bol_, '\n', eod_ - bol_));
+    return reinterpret_cast<char *>(
+        memchr(bol_, '\n', static_cast<size_t>(eod_ - bol_)));
   }
 
   bool BufferIsEmpty() const { return buf_ == eod_; }
@@ -886,7 +1033,7 @@ class LineReader {
   }
 
   const int fd_;
-  const int buf_len_;
+  const size_t buf_len_;
   char *const buf_;
   char *bol_;
   char *eol_;
@@ -904,7 +1051,8 @@ static const char *GetHex(const char *start, const char *end,
     int ch = *p;
     if ((ch >= '0' && ch <= '9') || (ch >= 'A' && ch <= 'F') ||
         (ch >= 'a' && ch <= 'f')) {
-      hex = (hex << 4) | (ch < 'A' ? ch - '0' : (ch & 0xF) + 9);
+      hex = (hex << 4) |
+            static_cast<uint64_t>(ch < 'A' ? ch - '0' : (ch & 0xF) + 9);
     } else {  // Encountered the first non-hex character.
       break;
     }
@@ -936,7 +1084,7 @@ static bool ShouldUseMapping(const char *const flags) {
 static ABSL_ATTRIBUTE_NOINLINE bool ReadAddrMap(
     bool (*callback)(const char *filename, const void *const start_addr,
                      const void *const end_addr, uint64_t offset, void *arg),
-    void *arg, void *tmp_buf, int tmp_buf_size) {
+    void *arg, void *tmp_buf, size_t tmp_buf_size) {
   // Use /proc/self/task/<pid>/maps instead of /proc/self/maps. The latter
   // requires kernel to stop all threads, and is significantly slower when there
   // are 1000s of threads.
@@ -944,7 +1092,7 @@ static ABSL_ATTRIBUTE_NOINLINE bool ReadAddrMap(
   snprintf(maps_path, sizeof(maps_path), "/proc/self/task/%d/maps", getpid());
 
   int maps_fd;
-  NO_INTR(maps_fd = open(maps_path, O_RDONLY));
+  NO_INTR(maps_fd = OpenReadOnlyWithHighFD(maps_path));
   FileDescriptor wrapped_maps_fd(maps_fd);
   if (wrapped_maps_fd.get() < 0) {
     ABSL_RAW_LOG(WARNING, "%s: errno=%d", maps_path, errno);
@@ -1051,10 +1199,10 @@ ObjFile *Symbolizer::FindObjFile(const void *const addr, size_t len) {
       }
     }
 
-    int lo = 0;
-    int hi = addr_map_.Size();
+    size_t lo = 0;
+    size_t hi = addr_map_.Size();
     while (lo < hi) {
-      int mid = (lo + hi) / 2;
+      size_t mid = (lo + hi) / 2;
       if (addr < addr_map_.At(mid)->end_addr) {
         hi = mid;
       } else {
@@ -1065,7 +1213,8 @@ ObjFile *Symbolizer::FindObjFile(const void *const addr, size_t len) {
       ObjFile *obj = addr_map_.At(lo);
       SAFE_ASSERT(obj->end_addr > addr);
       if (addr >= obj->start_addr &&
-          reinterpret_cast<const char *>(addr) + len <= obj->end_addr)
+          len <= static_cast<size_t>(static_cast<const char*>(obj->end_addr) -
+                                     static_cast<const char*>(addr)))
         return obj;
     }
 
@@ -1076,11 +1225,11 @@ ObjFile *Symbolizer::FindObjFile(const void *const addr, size_t len) {
 }
 
 void Symbolizer::ClearAddrMap() {
-  for (int i = 0; i != addr_map_.Size(); i++) {
+  for (size_t i = 0; i != addr_map_.Size(); i++) {
     ObjFile *o = addr_map_.At(i);
     base_internal::LowLevelAlloc::Free(o->filename);
     if (o->fd >= 0) {
-      NO_INTR(close(o->fd));
+      close(o->fd);
     }
   }
   addr_map_.Clear();
@@ -1096,7 +1245,7 @@ bool Symbolizer::RegisterObjFile(const char *filename,
 
   // Files are supposed to be added in the increasing address order.  Make
   // sure that's the case.
-  int addr_map_size = impl->addr_map_.Size();
+  size_t addr_map_size = impl->addr_map_.Size();
   if (addr_map_size != 0) {
     ObjFile *old = impl->addr_map_.At(addr_map_size - 1);
     if (old->end_addr > end_addr) {
@@ -1116,6 +1265,14 @@ bool Symbolizer::RegisterObjFile(const char *filename,
                      reinterpret_cast<uintptr_t>(old->end_addr), old->filename);
       }
       return true;
+    } else if (old->end_addr == start_addr &&
+               reinterpret_cast<uintptr_t>(old->start_addr) - old->offset ==
+                   reinterpret_cast<uintptr_t>(start_addr) - offset &&
+               strcmp(old->filename, filename) == 0) {
+      // Two contiguous map entries that span a contiguous region of the file,
+      // perhaps because some part of the file was mlock()ed. Combine them.
+      old->end_addr = end_addr;
+      return true;
     }
   }
   ObjFile *obj = impl->addr_map_.Add();
@@ -1132,12 +1289,12 @@ bool Symbolizer::RegisterObjFile(const char *filename,
 // where the input symbol is demangled in-place.
 // To keep stack consumption low, we would like this function to not
 // get inlined.
-static ABSL_ATTRIBUTE_NOINLINE void DemangleInplace(char *out, int out_size,
+static ABSL_ATTRIBUTE_NOINLINE void DemangleInplace(char *out, size_t out_size,
                                                     char *tmp_buf,
-                                                    int tmp_buf_size) {
+                                                    size_t tmp_buf_size) {
   if (Demangle(out, tmp_buf, tmp_buf_size)) {
     // Demangling succeeded. Copy to out if the space allows.
-    int len = strlen(tmp_buf);
+    size_t len = strlen(tmp_buf);
     if (len + 1 <= out_size) {  // +1 for '\0'.
       SAFE_ASSERT(len < tmp_buf_size);
       memmove(out, tmp_buf, len + 1);
@@ -1164,7 +1321,7 @@ const char *Symbolizer::FindSymbolInCache(const void *const pc) {
   if (pc == nullptr) return nullptr;
 
   SymbolCacheLine *line = GetCacheLine(pc);
-  for (size_t i = 0; i < ABSL_ARRAYSIZE(line->pc); ++i) {
+  for (size_t i = 0; i < std::size(line->pc); ++i) {
     if (line->pc[i] == pc) {
       AgeSymbols(line);
       line->age[i] = 0;
@@ -1180,8 +1337,9 @@ const char *Symbolizer::InsertSymbolInCache(const void *const pc,
 
   SymbolCacheLine *line = GetCacheLine(pc);
   uint32_t max_age = 0;
-  int oldest_index = -1;
-  for (size_t i = 0; i < ABSL_ARRAYSIZE(line->pc); ++i) {
+  size_t oldest_index = 0;
+  bool found_oldest_index = false;
+  for (size_t i = 0; i < std::size(line->pc); ++i) {
     if (line->pc[i] == nullptr) {
       AgeSymbols(line);
       line->pc[i] = pc;
@@ -1192,11 +1350,12 @@ const char *Symbolizer::InsertSymbolInCache(const void *const pc,
     if (line->age[i] >= max_age) {
       max_age = line->age[i];
       oldest_index = i;
+      found_oldest_index = true;
     }
   }
 
   AgeSymbols(line);
-  ABSL_RAW_CHECK(oldest_index >= 0, "Corrupt cache");
+  ABSL_RAW_CHECK(found_oldest_index, "Corrupt cache");
   base_internal::LowLevelAlloc::Free(line->name[oldest_index]);
   line->pc[oldest_index] = pc;
   line->name[oldest_index] = CopyString(name);
@@ -1208,7 +1367,7 @@ static void MaybeOpenFdFromSelfExe(ObjFile *obj) {
   if (memcmp(obj->start_addr, ELFMAG, SELFMAG) != 0) {
     return;
   }
-  int fd = open("/proc/self/exe", O_RDONLY);
+  int fd = OpenReadOnlyWithHighFD("/proc/self/exe");
   if (fd == -1) {
     return;
   }
@@ -1230,9 +1389,10 @@ static void MaybeOpenFdFromSelfExe(ObjFile *obj) {
   obj->fd = fd;
 }
 
-static bool MaybeInitializeObjFile(ObjFile *obj) {
+static bool MaybeInitializeObjFile(
+    ObjFile* obj, SymbolDecorator::Factory* decorator_factory) {
   if (obj->fd < 0) {
-    obj->fd = open(obj->filename, O_RDONLY);
+    obj->fd = OpenReadOnlyWithHighFD(obj->filename);
 
     if (obj->fd < 0) {
       // Getting /proc/self/exe here means that we were hinted.
@@ -1240,7 +1400,7 @@ static bool MaybeInitializeObjFile(ObjFile *obj) {
         // /proc/self/exe may be inaccessible (due to setuid, etc.), so try
         // accessing the binary via argv0.
         if (argv0_value != nullptr) {
-          obj->fd = open(argv0_value, O_RDONLY);
+          obj->fd = OpenReadOnlyWithHighFD(argv0_value);
         }
       } else {
         MaybeOpenFdFromSelfExe(obj);
@@ -1251,18 +1411,69 @@ static bool MaybeInitializeObjFile(ObjFile *obj) {
       ABSL_RAW_LOG(WARNING, "%s: open failed: errno=%d", obj->filename, errno);
       return false;
     }
-    obj->elf_type = FileGetElfType(obj->fd);
+
+    char buf[kSmallFileCacheSize];
+    CachingFile file(obj->fd, buf, sizeof(buf));
+
+    obj->elf_type = FileGetElfType(&file);
     if (obj->elf_type < 0) {
       ABSL_RAW_LOG(WARNING, "%s: wrong elf type: %d", obj->filename,
                    obj->elf_type);
       return false;
     }
 
-    if (!ReadFromOffsetExact(obj->fd, &obj->elf_header, sizeof(obj->elf_header),
-                             0)) {
+    if (!file.ReadFromOffsetExact(&obj->elf_header, sizeof(obj->elf_header),
+                                  0)) {
       ABSL_RAW_LOG(WARNING, "%s: failed to read elf header", obj->filename);
       return false;
     }
+    const int phnum = obj->elf_header.e_phnum;
+    const int phentsize = obj->elf_header.e_phentsize;
+    auto phoff = static_cast<off_t>(obj->elf_header.e_phoff);
+    size_t num_interesting_load_segments = 0;
+    for (int j = 0; j < phnum; j++) {
+      ElfW(Phdr) phdr;
+      if (!file.ReadFromOffsetExact(&phdr, sizeof(phdr), phoff)) {
+        ABSL_RAW_LOG(WARNING, "%s: failed to read program header %d",
+                     obj->filename, j);
+        return false;
+      }
+      phoff += phentsize;
+
+#ifdef ABSL_INTERNAL_HAVE_PPC64_ELFV1_ABI
+      // On the PowerPC 64-bit ELFv1 ABI, function pointers actually point to
+      // function descriptors. These descriptors are stored in an .opd section,
+      // which is mapped read-only. We thus need to look at all readable
+      // segments, not just the executable ones.
+      constexpr int interesting = PF_R;
+#else
+      constexpr int interesting = PF_X | PF_R;
+#endif
+
+      if (phdr.p_type != PT_LOAD ||
+          (phdr.p_flags & interesting) != interesting) {
+        // Not a LOAD segment, not executable code, and not a function
+        // descriptor.
+        continue;
+      }
+      if (num_interesting_load_segments < obj->phdr.size()) {
+        memcpy(&obj->phdr[num_interesting_load_segments++], &phdr,
+               sizeof(phdr));
+      } else {
+        ABSL_RAW_LOG(
+            WARNING, "%s: too many interesting LOAD segments: %zu >= %zu",
+            obj->filename, num_interesting_load_segments, obj->phdr.size());
+        break;
+      }
+    }
+    if (num_interesting_load_segments == 0) {
+      // This object has no interesting LOAD segments. That's unexpected.
+      ABSL_RAW_LOG(WARNING, "%s: no interesting LOAD segments", obj->filename);
+      return false;
+    }
+  }
+  if (decorator_factory != nullptr && obj->decorator == nullptr) {
+    obj->decorator = decorator_factory(obj->fd);
   }
   return true;
 }
@@ -1274,35 +1485,57 @@ static bool MaybeInitializeObjFile(ObjFile *obj) {
 // they are called here as well.
 // To keep stack consumption low, we would like this function to not
 // get inlined.
-const char *Symbolizer::GetSymbol(const void *const pc) {
-  const char *entry = FindSymbolInCache(pc);
-  if (entry != nullptr) {
-    return entry;
-  }
-  symbol_buf_[0] = '\0';
-
+const char *Symbolizer::GetUncachedSymbol(const void *pc) {
   ObjFile *const obj = FindObjFile(pc, 1);
   ptrdiff_t relocation = 0;
-  int fd = -1;
   if (obj != nullptr) {
-    if (MaybeInitializeObjFile(obj)) {
-      if (obj->elf_type == ET_DYN &&
-          reinterpret_cast<uint64_t>(obj->start_addr) >= obj->offset) {
+    if (MaybeInitializeObjFile(obj, decorator_factory_)) {
+      const size_t start_addr = reinterpret_cast<size_t>(obj->start_addr);
+      if (obj->elf_type == ET_DYN && start_addr >= obj->offset) {
         // This object was relocated.
         //
         // For obj->offset > 0, adjust the relocation since a mapping at offset
         // X in the file will have a start address of [true relocation]+X.
-        relocation = reinterpret_cast<ptrdiff_t>(obj->start_addr) - obj->offset;
+        relocation = static_cast<ptrdiff_t>(start_addr - obj->offset);
+
+        // Note: some binaries have multiple LOAD segments that can contain
+        // function pointers. We must find the right one.
+        ElfW(Phdr) *phdr = nullptr;
+        for (size_t j = 0; j < obj->phdr.size(); j++) {
+          ElfW(Phdr) &p = obj->phdr[j];
+          if (p.p_type != PT_LOAD) {
+            // We only expect PT_LOADs. This must be PT_NULL that we didn't
+            // write over (i.e. we exhausted all interesting PT_LOADs).
+            ABSL_RAW_CHECK(p.p_type == PT_NULL, "unexpected p_type");
+            break;
+          }
+          if (pc <
+              reinterpret_cast<void *>(start_addr + p.p_vaddr + p.p_memsz)) {
+            phdr = &p;
+            break;
+          }
+        }
+        if (phdr == nullptr) {
+          // That's unexpected. Hope for the best.
+          ABSL_RAW_LOG(
+              WARNING,
+              "%s: unable to find LOAD segment for pc: %p, start_addr: %zx",
+              obj->filename, pc, start_addr);
+        } else {
+          // Adjust relocation in case phdr.p_vaddr != 0.
+          // This happens for binaries linked with `lld --rosegment`, and for
+          // binaries linked with BFD `ld -z separate-code`.
+          relocation -= phdr->p_vaddr - phdr->p_offset;
+        }
       }
 
-      fd = obj->fd;
-    }
-    if (GetSymbolFromObjectFile(*obj, pc, relocation, symbol_buf_,
-                                sizeof(symbol_buf_), tmp_buf_,
-                                sizeof(tmp_buf_)) == SYMBOL_FOUND) {
-      // Only try to demangle the symbol name if it fit into symbol_buf_.
-      DemangleInplace(symbol_buf_, sizeof(symbol_buf_), tmp_buf_,
-                      sizeof(tmp_buf_));
+      if (GetSymbolFromObjectFile(*obj, pc, relocation, symbol_buf_,
+                                  sizeof(symbol_buf_), tmp_buf_,
+                                  sizeof(tmp_buf_)) == SYMBOL_FOUND) {
+        // Only try to demangle the symbol name if it fit into symbol_buf_.
+        DemangleInplace(symbol_buf_, sizeof(symbol_buf_), tmp_buf_,
+                        sizeof(tmp_buf_));
+      }
     }
   } else {
 #if ABSL_HAVE_VDSO_SUPPORT
@@ -1320,17 +1553,9 @@ const char *Symbolizer::GetSymbol(const void *const pc) {
 #endif
   }
 
-  if (g_decorators_mu.TryLock()) {
-    if (g_num_decorators > 0) {
-      SymbolDecoratorArgs decorator_args = {
-          pc,       relocation,       fd,     symbol_buf_, sizeof(symbol_buf_),
-          tmp_buf_, sizeof(tmp_buf_), nullptr};
-      for (int i = 0; i < g_num_decorators; ++i) {
-        decorator_args.arg = g_decorators[i].arg;
-        g_decorators[i].fn(&decorator_args);
-      }
-    }
-    g_decorators_mu.Unlock();
+  if (obj != nullptr && obj->decorator != nullptr) {
+    obj->decorator->Decorate(pc, relocation, symbol_buf_, sizeof(symbol_buf_),
+                             tmp_buf_, sizeof(tmp_buf_));
   }
   if (symbol_buf_[0] == '\0') {
     return nullptr;
@@ -1339,61 +1564,55 @@ const char *Symbolizer::GetSymbol(const void *const pc) {
   return InsertSymbolInCache(pc, symbol_buf_);
 }
 
-bool RemoveAllSymbolDecorators(void) {
-  if (!g_decorators_mu.TryLock()) {
-    // Someone else is using decorators. Get out.
-    return false;
+const char *Symbolizer::GetSymbol(const void *pc) {
+  const char *entry = FindSymbolInCache(pc);
+  if (entry != nullptr) {
+    return entry;
   }
-  g_num_decorators = 0;
-  g_decorators_mu.Unlock();
-  return true;
-}
+  symbol_buf_[0] = '\0';
 
-bool RemoveSymbolDecorator(int ticket) {
-  if (!g_decorators_mu.TryLock()) {
-    // Someone else is using decorators. Get out.
-    return false;
-  }
-  for (int i = 0; i < g_num_decorators; ++i) {
-    if (g_decorators[i].ticket == ticket) {
-      while (i < g_num_decorators - 1) {
-        g_decorators[i] = g_decorators[i + 1];
-        ++i;
-      }
-      g_num_decorators = i;
-      break;
+#ifdef __hppa__
+  {
+    // In some contexts (e.g., return addresses), PA-RISC uses the lowest two
+    // bits of the address to indicate the privilege level. Clear those bits
+    // before trying to symbolize.
+    const auto pc_bits = reinterpret_cast<uintptr_t>(pc);
+    const auto address = pc_bits & ~0x3;
+    entry = GetUncachedSymbol(reinterpret_cast<const void *>(address));
+    if (entry != nullptr) {
+      return entry;
     }
-  }
-  g_decorators_mu.Unlock();
-  return true;  // Decorator is known to be removed.
-}
 
-int InstallSymbolDecorator(SymbolDecorator decorator, void *arg) {
-  static int ticket = 0;
+    // In some contexts, PA-RISC also uses bit 1 of the address to indicate that
+    // this is a cross-DSO function pointer. Such function pointers actually
+    // point to a procedure label, a struct whose first 32-bit (pointer) element
+    // actually points to the function text. With no symbol found for this
+    // address so far, try interpreting it as a cross-DSO function pointer and
+    // see how that goes.
+    if (pc_bits & 0x2) {
+      return GetUncachedSymbol(*reinterpret_cast<const void *const *>(address));
+    }
 
-  if (!g_decorators_mu.TryLock()) {
-    // Someone else is using decorators. Get out.
-    return false;
-  }
-  int ret = ticket;
-  if (g_num_decorators >= kMaxDecorators) {
-    ret = -1;
-  } else {
-    g_decorators[g_num_decorators] = {decorator, arg, ticket++};
-    ++g_num_decorators;
+    return nullptr;
   }
-  g_decorators_mu.Unlock();
-  return ret;
+#else
+  return GetUncachedSymbol(pc);
+#endif
+}
+
+SymbolDecorator::Factory* SetSymbolDecoratorFactory(
+    SymbolDecorator::Factory* factory) {
+  return g_decorator_factory.exchange(factory, std::memory_order_acq_rel);
 }
 
-bool RegisterFileMappingHint(const void *start, const void *end, uint64_t offset,
-                             const char *filename) {
+bool RegisterFileMappingHint(const void *start, const void *end,
+                             uint64_t offset, const char *filename) {
   SAFE_ASSERT(start <= end);
   SAFE_ASSERT(filename != nullptr);
 
-  InitSigSafeArena();
+  base_internal::InitSigSafeArena();
 
-  if (!g_file_mapping_mu.TryLock()) {
+  if (!g_file_mapping_mu.try_lock()) {
     return false;
   }
 
@@ -1401,10 +1620,11 @@ bool RegisterFileMappingHint(const void *start, const void *end, uint64_t offset
   if (g_num_file_mapping_hints >= kMaxFileMappingHints) {
     ret = false;
   } else {
-    // TODO(ckennelly): Move this into a std::string copy routine.
-    int len = strlen(filename);
-    char *dst = static_cast<char *>(
-        base_internal::LowLevelAlloc::AllocWithArena(len + 1, SigSafeArena()));
+    // TODO(ckennelly): Move this into a string copy routine.
+    size_t len = strlen(filename);
+    char *dst =
+        static_cast<char *>(base_internal::LowLevelAlloc::AllocWithArena(
+            len + 1, base_internal::SigSafeArena()));
     ABSL_RAW_CHECK(dst != nullptr, "out of memory");
     memcpy(dst, filename, len + 1);
 
@@ -1415,13 +1635,13 @@ bool RegisterFileMappingHint(const void *start, const void *end, uint64_t offset
     hint.filename = dst;
   }
 
-  g_file_mapping_mu.Unlock();
+  g_file_mapping_mu.unlock();
   return ret;
 }
 
 bool GetFileMappingHint(const void **start, const void **end, uint64_t *offset,
                         const char **filename) {
-  if (!g_file_mapping_mu.TryLock()) {
+  if (!g_file_mapping_mu.try_lock()) {
     return false;
   }
   bool found = false;
@@ -1444,7 +1664,7 @@ bool GetFileMappingHint(const void **start, const void **end, uint64_t *offset,
       break;
     }
   }
-  g_file_mapping_mu.Unlock();
+  g_file_mapping_mu.unlock();
   return found;
 }
 
@@ -1452,27 +1672,38 @@ bool GetFileMappingHint(const void **start, const void **end, uint64_t *offset,
 
 bool Symbolize(const void *pc, char *out, int out_size) {
   // Symbolization is very slow under tsan.
-  ANNOTATE_IGNORE_READS_AND_WRITES_BEGIN();
+  ABSL_ANNOTATE_IGNORE_READS_AND_WRITES_BEGIN();
   SAFE_ASSERT(out_size >= 0);
   debugging_internal::Symbolizer *s = debugging_internal::AllocateSymbolizer();
   const char *name = s->GetSymbol(pc);
   bool ok = false;
   if (name != nullptr && out_size > 0) {
-    strncpy(out, name, out_size);
+    strncpy(out, name, static_cast<size_t>(out_size));
     ok = true;
-    if (out[out_size - 1] != '\0') {
+    if (out[static_cast<size_t>(out_size) - 1] != '\0') {
       // strncpy() does not '\0' terminate when it truncates.  Do so, with
       // trailing ellipsis.
       static constexpr char kEllipsis[] = "...";
-      int ellipsis_size =
-          std::min(implicit_cast<int>(strlen(kEllipsis)), out_size - 1);
-      memcpy(out + out_size - ellipsis_size - 1, kEllipsis, ellipsis_size);
-      out[out_size - 1] = '\0';
+      size_t ellipsis_size =
+          std::min(strlen(kEllipsis), static_cast<size_t>(out_size) - 1);
+      memcpy(out + static_cast<size_t>(out_size) - ellipsis_size - 1, kEllipsis,
+             ellipsis_size);
+      out[static_cast<size_t>(out_size) - 1] = '\0';
     }
   }
   debugging_internal::FreeSymbolizer(s);
-  ANNOTATE_IGNORE_READS_AND_WRITES_END();
+  ABSL_ANNOTATE_IGNORE_READS_AND_WRITES_END();
   return ok;
 }
 
+ABSL_NAMESPACE_END
 }  // namespace absl
+
+extern "C" bool ABSL_INTERNAL_C_SYMBOL(AbslInternalGetFileMappingHint)(
+    const void** start, const void** end, uint64_t* offset,
+    const char** filename) {
+  return absl::debugging_internal::GetFileMappingHint(start, end, offset,
+                                                      filename);
+}
+
+#undef ABSL_INTERNAL_HAVE_PPC64_ELFV1_ABI
diff --git a/absl/debugging/symbolize_emscripten.inc b/absl/debugging/symbolize_emscripten.inc
new file mode 100644
index 00000000000..5756325a457
--- /dev/null
+++ b/absl/debugging/symbolize_emscripten.inc
@@ -0,0 +1,63 @@
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <cxxabi.h>
+#include <emscripten.h>
+
+#include <algorithm>
+#include <cstring>
+
+#include "absl/base/internal/raw_logging.h"
+#include "absl/debugging/internal/demangle.h"
+#include "absl/strings/numbers.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/string_view.h"
+
+extern "C" {
+const char* emscripten_pc_get_function(const void* pc);
+void* emscripten_stack_snapshot();
+}
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+void InitializeSymbolizer(const char*) {}
+
+bool Symbolize(const void* pc, char* out, int out_size) {
+  if (pc == nullptr || out_size <= 0) {
+    return false;
+  }
+  // Need to capture a snapshot first.
+  emscripten_stack_snapshot();
+  const char* func_name = emscripten_pc_get_function(pc);
+  if (func_name == nullptr) {
+    return false;
+  }
+
+  strncpy(out, func_name, static_cast<size_t>(out_size));
+
+  if (out[out_size - 1] != '\0') {
+    // strncpy() does not '\0' terminate when it truncates.
+    static constexpr char kEllipsis[] = "...";
+    size_t ellipsis_size =
+        std::min(sizeof(kEllipsis) - 1, static_cast<size_t>(out_size) - 1);
+    memcpy(out + out_size - ellipsis_size - 1, kEllipsis, ellipsis_size);
+    out[out_size - 1] = '\0';
+  }
+
+  return true;
+}
+
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/debugging/symbolize_test.cc b/absl/debugging/symbolize_test.cc
index 533799a6ebc..2cb16381b6f 100644
--- a/absl/debugging/symbolize_test.cc
+++ b/absl/debugging/symbolize_test.cc
@@ -14,31 +14,69 @@
 
 #include "absl/debugging/symbolize.h"
 
-#ifndef _WIN32
-#include <fcntl.h>
-#include <sys/mman.h>
-#endif
-
+#include <cstddef>
+#include <cstdint>
 #include <cstring>
 #include <iostream>
 #include <memory>
+#include <string>
+#include <vector>
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
 #include "absl/base/attributes.h"
 #include "absl/base/casts.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/low_level_alloc.h"
 #include "absl/base/internal/per_thread_tls.h"
-#include "absl/base/internal/raw_logging.h"
 #include "absl/base/optimization.h"
+#include "absl/cleanup/cleanup.h"
 #include "absl/debugging/internal/stack_consumption.h"
+#include "absl/debugging/internal/symbolize.h"
+#include "absl/log/check.h"
+#include "absl/log/log.h"
 #include "absl/memory/memory.h"
+#include "absl/strings/str_format.h"
+#include "absl/strings/string_view.h"
+
+#ifdef __EMSCRIPTEN__
+#include <emscripten.h>
+#endif
+
+#ifndef _WIN32
+#include <fcntl.h>
+#include <sys/mman.h>
+#endif
+
+#if defined(MAP_ANON) && !defined(MAP_ANONYMOUS)
+#define MAP_ANONYMOUS MAP_ANON
+#endif
 
 using testing::Contains;
 
+#ifdef _WIN32
+#define ABSL_SYMBOLIZE_TEST_NOINLINE __declspec(noinline)
+#else
+#define ABSL_SYMBOLIZE_TEST_NOINLINE ABSL_ATTRIBUTE_NOINLINE
+#endif
+
 // Functions to symbolize. Use C linkage to avoid mangled names.
 extern "C" {
-void nonstatic_func() { ABSL_BLOCK_TAIL_CALL_OPTIMIZATION(); }
-static void static_func() { ABSL_BLOCK_TAIL_CALL_OPTIMIZATION(); }
+ABSL_SYMBOLIZE_TEST_NOINLINE void nonstatic_func() {
+  // The next line makes this a unique function to prevent the compiler from
+  // folding identical functions together.
+  volatile int x = __LINE__;
+  static_cast<void>(x);
+  ABSL_BLOCK_TAIL_CALL_OPTIMIZATION();
+}
+
+ABSL_SYMBOLIZE_TEST_NOINLINE static void static_func() {
+  // The next line makes this a unique function to prevent the compiler from
+  // folding identical functions together.
+  volatile int x = __LINE__;
+  static_cast<void>(x);
+  ABSL_BLOCK_TAIL_CALL_OPTIMIZATION();
+}
 }  // extern "C"
 
 struct Foo {
@@ -46,7 +84,11 @@ struct Foo {
 };
 
 // A C++ method that should have a mangled name.
-void ABSL_ATTRIBUTE_NOINLINE Foo::func(int) {
+ABSL_SYMBOLIZE_TEST_NOINLINE void Foo::func(int) {
+  // The next line makes this a unique function to prevent the compiler from
+  // folding identical functions together.
+  volatile int x = __LINE__;
+  static_cast<void>(x);
   ABSL_BLOCK_TAIL_CALL_OPTIMIZATION();
 }
 
@@ -56,21 +98,13 @@ int ABSL_ATTRIBUTE_SECTION_VARIABLE(.text.unlikely) unlikely_func() {
   return 0;
 }
 
-int ABSL_ATTRIBUTE_SECTION_VARIABLE(.text.hot) hot_func() {
-  return 0;
-}
+int ABSL_ATTRIBUTE_SECTION_VARIABLE(.text.hot) hot_func() { return 0; }
 
-int ABSL_ATTRIBUTE_SECTION_VARIABLE(.text.startup) startup_func() {
-  return 0;
-}
+int ABSL_ATTRIBUTE_SECTION_VARIABLE(.text.startup) startup_func() { return 0; }
 
-int ABSL_ATTRIBUTE_SECTION_VARIABLE(.text.exit) exit_func() {
-  return 0;
-}
+int ABSL_ATTRIBUTE_SECTION_VARIABLE(.text.exit) exit_func() { return 0; }
 
-int /*ABSL_ATTRIBUTE_SECTION_VARIABLE(.text)*/ regular_func() {
-  return 0;
-}
+int /*ABSL_ATTRIBUTE_SECTION_VARIABLE(.text)*/ regular_func() { return 0; }
 
 // Thread-local data may confuse the symbolizer, ensure that it does not.
 // Variable sizes and order are important.
@@ -89,7 +123,8 @@ static volatile bool volatile_bool = false;
 // Force the binary to be large enough that a THP .text remap will succeed.
 static constexpr size_t kHpageSize = 1 << 21;
 const char kHpageTextPadding[kHpageSize * 4] ABSL_ATTRIBUTE_SECTION_VARIABLE(
-    .text) = "";
+        .text) = "";
+
 #endif  // !defined(__EMSCRIPTEN__)
 
 static char try_symbolize_buffer[4096];
@@ -99,15 +134,17 @@ static char try_symbolize_buffer[4096];
 // absl::Symbolize() returns false, otherwise returns try_symbolize_buffer with
 // the result of absl::Symbolize().
 static const char *TrySymbolizeWithLimit(void *pc, int limit) {
-  ABSL_RAW_CHECK(limit <= sizeof(try_symbolize_buffer),
-                 "try_symbolize_buffer is too small");
+  CHECK_LE(limit, sizeof(try_symbolize_buffer))
+      << "try_symbolize_buffer is too small";
 
   // Use the heap to facilitate heap and buffer sanitizer tools.
-  auto heap_buffer = absl::make_unique<char[]>(sizeof(try_symbolize_buffer));
+  auto heap_buffer = std::make_unique<char[]>(sizeof(try_symbolize_buffer));
   bool found = absl::Symbolize(pc, heap_buffer.get(), limit);
   if (found) {
-    ABSL_RAW_CHECK(strnlen(heap_buffer.get(), limit) < limit,
-                   "absl::Symbolize() did not properly terminate the string");
+    CHECK_LT(static_cast<int>(
+                 strnlen(heap_buffer.get(), static_cast<size_t>(limit))),
+             limit)
+        << "absl::Symbolize() did not properly terminate the string";
     strncpy(try_symbolize_buffer, heap_buffer.get(),
             sizeof(try_symbolize_buffer) - 1);
     try_symbolize_buffer[sizeof(try_symbolize_buffer) - 1] = '\0';
@@ -121,15 +158,26 @@ static const char *TrySymbolize(void *pc) {
   return TrySymbolizeWithLimit(pc, sizeof(try_symbolize_buffer));
 }
 
-#ifdef ABSL_INTERNAL_HAVE_ELF_SYMBOLIZE
+#if defined(ABSL_INTERNAL_HAVE_ELF_SYMBOLIZE) ||    \
+    defined(ABSL_INTERNAL_HAVE_DARWIN_SYMBOLIZE) || \
+    defined(ABSL_INTERNAL_HAVE_EMSCRIPTEN_SYMBOLIZE)
+
+// Test with a return address.
+void ABSL_ATTRIBUTE_NOINLINE TestWithReturnAddress() {
+#if defined(ABSL_HAVE_ATTRIBUTE_NOINLINE)
+  void *return_address = __builtin_return_address(0);
+  const char *symbol = TrySymbolize(return_address);
+  ASSERT_NE(symbol, nullptr) << "TestWithReturnAddress failed";
+  EXPECT_STREQ(symbol, "main") << "TestWithReturnAddress failed";
+#endif
+}
 
 TEST(Symbolize, Cached) {
   // Compilers should give us pointers to them.
-  EXPECT_STREQ("nonstatic_func", TrySymbolize((void *)(&nonstatic_func)));
-
+  EXPECT_STREQ("nonstatic_func", TrySymbolize((void*)(&nonstatic_func)));
   // The name of an internal linkage symbol is not specified; allow either a
   // mangled or an unmangled name here.
-  const char *static_func_symbol = TrySymbolize((void *)(&static_func));
+  const char* static_func_symbol = TrySymbolize((void*)(&static_func));
   EXPECT_TRUE(strcmp("static_func", static_func_symbol) == 0 ||
               strcmp("static_func()", static_func_symbol) == 0);
 
@@ -139,33 +187,38 @@ TEST(Symbolize, Cached) {
 TEST(Symbolize, Truncation) {
   constexpr char kNonStaticFunc[] = "nonstatic_func";
   EXPECT_STREQ("nonstatic_func",
-               TrySymbolizeWithLimit((void *)(&nonstatic_func),
+               TrySymbolizeWithLimit((void*)(&nonstatic_func),
                                      strlen(kNonStaticFunc) + 1));
   EXPECT_STREQ("nonstatic_...",
-               TrySymbolizeWithLimit((void *)(&nonstatic_func),
+               TrySymbolizeWithLimit((void*)(&nonstatic_func),
                                      strlen(kNonStaticFunc) + 0));
   EXPECT_STREQ("nonstatic...",
-               TrySymbolizeWithLimit((void *)(&nonstatic_func),
+               TrySymbolizeWithLimit((void*)(&nonstatic_func),
                                      strlen(kNonStaticFunc) - 1));
-  EXPECT_STREQ("n...", TrySymbolizeWithLimit((void *)(&nonstatic_func), 5));
-  EXPECT_STREQ("...", TrySymbolizeWithLimit((void *)(&nonstatic_func), 4));
-  EXPECT_STREQ("..", TrySymbolizeWithLimit((void *)(&nonstatic_func), 3));
-  EXPECT_STREQ(".", TrySymbolizeWithLimit((void *)(&nonstatic_func), 2));
-  EXPECT_STREQ("", TrySymbolizeWithLimit((void *)(&nonstatic_func), 1));
-  EXPECT_EQ(nullptr, TrySymbolizeWithLimit((void *)(&nonstatic_func), 0));
+  EXPECT_STREQ("n...", TrySymbolizeWithLimit((void*)(&nonstatic_func), 5));
+  EXPECT_STREQ("...", TrySymbolizeWithLimit((void*)(&nonstatic_func), 4));
+  EXPECT_STREQ("..", TrySymbolizeWithLimit((void*)(&nonstatic_func), 3));
+  EXPECT_STREQ(".", TrySymbolizeWithLimit((void*)(&nonstatic_func), 2));
+  EXPECT_STREQ("", TrySymbolizeWithLimit((void*)(&nonstatic_func), 1));
+  EXPECT_EQ(nullptr, TrySymbolizeWithLimit((void*)(&nonstatic_func), 0));
 }
 
 TEST(Symbolize, SymbolizeWithDemangling) {
   Foo::func(100);
-  EXPECT_STREQ("Foo::func()", TrySymbolize((void *)(&Foo::func)));
+#ifdef __EMSCRIPTEN__
+  // Emscripten's online symbolizer is more precise with arguments.
+  EXPECT_STREQ("Foo::func(int)", TrySymbolize((void*)(&Foo::func)));
+#else
+  EXPECT_STREQ("Foo::func()", TrySymbolize((void*)(&Foo::func)));
+#endif
 }
 
 TEST(Symbolize, SymbolizeSplitTextSections) {
-  EXPECT_STREQ("unlikely_func()", TrySymbolize((void *)(&unlikely_func)));
-  EXPECT_STREQ("hot_func()", TrySymbolize((void *)(&hot_func)));
-  EXPECT_STREQ("startup_func()", TrySymbolize((void *)(&startup_func)));
-  EXPECT_STREQ("exit_func()", TrySymbolize((void *)(&exit_func)));
-  EXPECT_STREQ("regular_func()", TrySymbolize((void *)(&regular_func)));
+  EXPECT_STREQ("unlikely_func()", TrySymbolize((void*)(&unlikely_func)));
+  EXPECT_STREQ("hot_func()", TrySymbolize((void*)(&hot_func)));
+  EXPECT_STREQ("startup_func()", TrySymbolize((void*)(&startup_func)));
+  EXPECT_STREQ("exit_func()", TrySymbolize((void*)(&exit_func)));
+  EXPECT_STREQ("regular_func()", TrySymbolize((void*)(&regular_func)));
 }
 
 // Tests that verify that Symbolize stack footprint is within some limit.
@@ -193,10 +246,10 @@ static const char *SymbolizeStackConsumption(void *pc, int *stack_consumed) {
 }
 
 static int GetStackConsumptionUpperLimit() {
-  // Symbolize stack consumption should be within 2kB.
-  int stack_consumption_upper_limit = 2048;
-#if defined(ADDRESS_SANITIZER) || defined(MEMORY_SANITIZER) || \
-    defined(THREAD_SANITIZER)
+  // Symbolize stack consumption should be within 4kB.
+  int stack_consumption_upper_limit = 4096;
+#if defined(ABSL_HAVE_ADDRESS_SANITIZER) || \
+    defined(ABSL_HAVE_MEMORY_SANITIZER) || defined(ABSL_HAVE_THREAD_SANITIZER)
   // Account for sanitizer instrumentation requiring additional stack space.
   stack_consumption_upper_limit *= 5;
 #endif
@@ -235,14 +288,14 @@ TEST(Symbolize, SymbolizeWithDemanglingStackConsumption) {
 
 #endif  // ABSL_INTERNAL_HAVE_DEBUGGING_STACK_CONSUMPTION
 
+#if !defined(ABSL_INTERNAL_HAVE_DARWIN_SYMBOLIZE) && \
+    !defined(ABSL_INTERNAL_HAVE_EMSCRIPTEN_SYMBOLIZE)
 // Use a 64K page size for PPC.
 const size_t kPageSize = 64 << 10;
 // We place a read-only symbols into the .text section and verify that we can
 // symbolize them and other symbols after remapping them.
-const char kPadding0[kPageSize * 4] ABSL_ATTRIBUTE_SECTION_VARIABLE(.text) =
-    "";
-const char kPadding1[kPageSize * 4] ABSL_ATTRIBUTE_SECTION_VARIABLE(.text) =
-    "";
+const char kPadding0[kPageSize * 4] ABSL_ATTRIBUTE_SECTION_VARIABLE(.text) = "";
+const char kPadding1[kPageSize * 4] ABSL_ATTRIBUTE_SECTION_VARIABLE(.text) = "";
 
 static int FilterElfHeader(struct dl_phdr_info *info, size_t size, void *data) {
   for (int i = 0; i < info->dlpi_phnum; i++) {
@@ -273,8 +326,8 @@ static int FilterElfHeader(struct dl_phdr_info *info, size_t size, void *data) {
 TEST(Symbolize, SymbolizeWithMultipleMaps) {
   // Force kPadding0 and kPadding1 to be linked in.
   if (volatile_bool) {
-    ABSL_RAW_LOG(INFO, "%s", kPadding0);
-    ABSL_RAW_LOG(INFO, "%s", kPadding1);
+    LOG(INFO) << kPadding0;
+    LOG(INFO) << kPadding1;
   }
 
   // Verify we can symbolize everything.
@@ -324,44 +377,41 @@ TEST(Symbolize, SymbolizeWithMultipleMaps) {
   }
 }
 
-// Appends string(*args->arg) to args->symbol_buf.
-static void DummySymbolDecorator(
-    const absl::debugging_internal::SymbolDecoratorArgs *args) {
-  std::string *message = static_cast<std::string *>(args->arg);
-  strncat(args->symbol_buf, message->c_str(),
-          args->symbol_buf_size - strlen(args->symbol_buf) - 1);
-}
-
-TEST(Symbolize, InstallAndRemoveSymbolDecorators) {
-  int ticket_a;
-  std::string a_message("a");
-  EXPECT_GE(ticket_a = absl::debugging_internal::InstallSymbolDecorator(
-                DummySymbolDecorator, &a_message),
-            0);
-
-  int ticket_b;
-  std::string b_message("b");
-  EXPECT_GE(ticket_b = absl::debugging_internal::InstallSymbolDecorator(
-                DummySymbolDecorator, &b_message),
-            0);
+template <char C>
+class TestSymbolDecorator final
+    : public absl::debugging_internal::SymbolDecorator {
+ public:
+  static absl::debugging_internal::SymbolDecoratorPtr Factory(int /*fd*/) {
+    void* ptr = absl::base_internal::LowLevelAlloc::AllocWithArena(
+        sizeof(TestSymbolDecorator), absl::base_internal::SigSafeArena());
+    return absl::debugging_internal::SymbolDecoratorPtr(
+        new (ptr) TestSymbolDecorator());
+  }
 
-  int ticket_c;
-  std::string c_message("c");
-  EXPECT_GE(ticket_c = absl::debugging_internal::InstallSymbolDecorator(
-                DummySymbolDecorator, &c_message),
-            0);
+  void Decorate(const void* /*pc*/, ptrdiff_t /*relocation*/, char* symbol_buf,
+                size_t symbol_buf_size, char* /*tmp_buf*/,
+                size_t /*tmp_buf_size*/) const override {
+    const size_t len = strlen(symbol_buf);
+    absl::SNPrintF(symbol_buf + len, symbol_buf_size - len, " hello %c", C);
+  }
+};
 
-  char *address = reinterpret_cast<char *>(1);
-  EXPECT_STREQ("abc", TrySymbolize(address++));
+TEST(Symbolize, SetSymbolDecorator) {
+  absl::Cleanup cleanup =
+      [old_decorator = absl::debugging_internal::SetSymbolDecoratorFactory(
+           &TestSymbolDecorator<'a'>::Factory)] {
+        absl::debugging_internal::SetSymbolDecoratorFactory(old_decorator);
+      };
 
-  EXPECT_TRUE(absl::debugging_internal::RemoveSymbolDecorator(ticket_b));
+  EXPECT_STREQ("nonstatic_func hello a",
+               TrySymbolize(reinterpret_cast<void*>(&nonstatic_func)));
 
-  EXPECT_STREQ("ac", TrySymbolize(address++));
+  EXPECT_EQ(absl::debugging_internal::SetSymbolDecoratorFactory(
+                &TestSymbolDecorator<'b'>::Factory),
+            &TestSymbolDecorator<'a'>::Factory);
 
-  // Cleanup: remove all remaining decorators so other stack traces don't
-  // get mystery "ac" decoration.
-  EXPECT_TRUE(absl::debugging_internal::RemoveSymbolDecorator(ticket_a));
-  EXPECT_TRUE(absl::debugging_internal::RemoveSymbolDecorator(ticket_c));
+  EXPECT_STREQ("nonstatic_func hello b",
+               TrySymbolize(reinterpret_cast<void*>(&nonstatic_func)));
 }
 
 // Some versions of Clang with optimizations enabled seem to be able
@@ -376,8 +426,8 @@ TEST(Symbolize, ForEachSection) {
 
   std::vector<std::string> sections;
   ASSERT_TRUE(absl::debugging_internal::ForEachSection(
-      fd, [&sections](const std::string &name, const ElfW(Shdr) &) {
-        sections.push_back(name);
+      fd, [&sections](const absl::string_view name, const ElfW(Shdr) &) {
+        sections.emplace_back(name);
         return true;
       }));
 
@@ -390,15 +440,17 @@ TEST(Symbolize, ForEachSection) {
 
   close(fd);
 }
+#endif  // !ABSL_INTERNAL_HAVE_DARWIN_SYMBOLIZE &&
+        // !ABSL_INTERNAL_HAVE_EMSCRIPTEN_SYMBOLIZE
 
 // x86 specific tests.  Uses some inline assembler.
 extern "C" {
 inline void *ABSL_ATTRIBUTE_ALWAYS_INLINE inline_func() {
   void *pc = nullptr;
 #if defined(__i386__)
-  __asm__ __volatile__("call 1f;\n 1: pop %[PC]" : [ PC ] "=r"(pc));
+  __asm__ __volatile__("call 1f;\n 1: pop %[PC]" : [PC] "=r"(pc));
 #elif defined(__x86_64__)
-  __asm__ __volatile__("leaq 0(%%rip),%[PC];\n" : [ PC ] "=r"(pc));
+  __asm__ __volatile__("leaq 0(%%rip),%[PC];\n" : [PC] "=r"(pc));
 #endif
   return pc;
 }
@@ -406,9 +458,9 @@ inline void *ABSL_ATTRIBUTE_ALWAYS_INLINE inline_func() {
 void *ABSL_ATTRIBUTE_NOINLINE non_inline_func() {
   void *pc = nullptr;
 #if defined(__i386__)
-  __asm__ __volatile__("call 1f;\n 1: pop %[PC]" : [ PC ] "=r"(pc));
+  __asm__ __volatile__("call 1f;\n 1: pop %[PC]" : [PC] "=r"(pc));
 #elif defined(__x86_64__)
-  __asm__ __volatile__("leaq 0(%%rip),%[PC];\n" : [ PC ] "=r"(pc));
+  __asm__ __volatile__("leaq 0(%%rip),%[PC];\n" : [PC] "=r"(pc));
 #endif
   return pc;
 }
@@ -418,10 +470,9 @@ void ABSL_ATTRIBUTE_NOINLINE TestWithPCInsideNonInlineFunction() {
     (defined(__i386__) || defined(__x86_64__))
   void *pc = non_inline_func();
   const char *symbol = TrySymbolize(pc);
-  ABSL_RAW_CHECK(symbol != nullptr, "TestWithPCInsideNonInlineFunction failed");
-  ABSL_RAW_CHECK(strcmp(symbol, "non_inline_func") == 0,
-                 "TestWithPCInsideNonInlineFunction failed");
-  std::cout << "TestWithPCInsideNonInlineFunction passed" << std::endl;
+  ASSERT_NE(symbol, nullptr) << "TestWithPCInsideNonInlineFunction failed";
+  EXPECT_STREQ(symbol, "non_inline_func")
+      << "TestWithPCInsideNonInlineFunction failed";
 #endif
 }
 
@@ -430,33 +481,65 @@ void ABSL_ATTRIBUTE_NOINLINE TestWithPCInsideInlineFunction() {
     (defined(__i386__) || defined(__x86_64__))
   void *pc = inline_func();  // Must be inlined.
   const char *symbol = TrySymbolize(pc);
-  ABSL_RAW_CHECK(symbol != nullptr, "TestWithPCInsideInlineFunction failed");
-  ABSL_RAW_CHECK(strcmp(symbol, __FUNCTION__) == 0,
-                 "TestWithPCInsideInlineFunction failed");
-  std::cout << "TestWithPCInsideInlineFunction passed" << std::endl;
+  ASSERT_NE(symbol, nullptr) << "TestWithPCInsideInlineFunction failed";
+  EXPECT_STREQ(symbol, __FUNCTION__) << "TestWithPCInsideInlineFunction failed";
 #endif
 }
 }
 
-// Test with a return address.
-void ABSL_ATTRIBUTE_NOINLINE TestWithReturnAddress() {
+#if defined(__arm__) && ABSL_HAVE_ATTRIBUTE(target) && \
+    ((__ARM_ARCH >= 7) || !defined(__ARM_PCS_VFP))
+// Test that we correctly identify bounds of Thumb functions on ARM.
+//
+// Thumb functions have the lowest-order bit set in their addresses in the ELF
+// symbol table. This requires some extra logic to properly compute function
+// bounds. To test this logic, nudge a Thumb function right up against an ARM
+// function and try to symbolize the ARM function.
+//
+// A naive implementation will simply use the Thumb function's entry point as
+// written in the symbol table and will therefore treat the Thumb function as
+// extending one byte further in the instruction stream than it actually does.
+// When asked to symbolize the start of the ARM function, it will identify an
+// overlap between the Thumb and ARM functions, and it will return the name of
+// the Thumb function.
+//
+// A correct implementation, on the other hand, will null out the lowest-order
+// bit in the Thumb function's entry point. It will correctly compute the end of
+// the Thumb function, it will find no overlap between the Thumb and ARM
+// functions, and it will return the name of the ARM function.
+//
+// Unfortunately we cannot perform this test on armv6 or lower systems that use
+// the hard float ABI because gcc refuses to compile thumb functions on such
+// systems with a "sorry, unimplemented: Thumb-1 hard-float VFP ABI" error.
+
+__attribute__((target("thumb"))) int ArmThumbOverlapThumb(int x) {
+  return x * x * x;
+}
+
+__attribute__((target("arm"))) int ArmThumbOverlapArm(int x) {
+  return x * x * x;
+}
+
+void ABSL_ATTRIBUTE_NOINLINE TestArmThumbOverlap() {
 #if defined(ABSL_HAVE_ATTRIBUTE_NOINLINE)
-  void *return_address = __builtin_return_address(0);
-  const char *symbol = TrySymbolize(return_address);
-  ABSL_RAW_CHECK(symbol != nullptr, "TestWithReturnAddress failed");
-  ABSL_RAW_CHECK(strcmp(symbol, "main") == 0, "TestWithReturnAddress failed");
-  std::cout << "TestWithReturnAddress passed" << std::endl;
+  const char *symbol = TrySymbolize((void *)&ArmThumbOverlapArm);
+  ASSERT_NE(symbol, nullptr) << "TestArmThumbOverlap failed";
+  EXPECT_STREQ("ArmThumbOverlapArm()", symbol) << "TestArmThumbOverlap failed";
 #endif
 }
 
-#elif defined(_WIN32) && defined(_DEBUG)
+#endif  // defined(__arm__) && ABSL_HAVE_ATTRIBUTE(target) && ((__ARM_ARCH >= 7)
+        // || !defined(__ARM_PCS_VFP))
+
+#elif defined(_WIN32)
+#if !defined(ABSL_CONSUME_DLL)
 
 TEST(Symbolize, Basics) {
   EXPECT_STREQ("nonstatic_func", TrySymbolize((void *)(&nonstatic_func)));
 
   // The name of an internal linkage symbol is not specified; allow either a
   // mangled or an unmangled name here.
-  const char* static_func_symbol = TrySymbolize((void *)(&static_func));
+  const char *static_func_symbol = TrySymbolize((void *)(&static_func));
   ASSERT_TRUE(static_func_symbol != nullptr);
   EXPECT_TRUE(strstr(static_func_symbol, "static_func") != nullptr);
 
@@ -483,13 +566,13 @@ TEST(Symbolize, Truncation) {
 }
 
 TEST(Symbolize, SymbolizeWithDemangling) {
-  const char* result = TrySymbolize((void *)(&Foo::func));
+  const char *result = TrySymbolize((void *)(&Foo::func));
   ASSERT_TRUE(result != nullptr);
   EXPECT_TRUE(strstr(result, "Foo::func") != nullptr) << result;
 }
 
-#else  // Symbolizer unimplemented
-
+#endif  // !defined(ABSL_CONSUME_DLL)
+#else   // Symbolizer unimplemented
 TEST(Symbolize, Unimplemented) {
   char buf[64];
   EXPECT_FALSE(absl::Symbolize((void *)(&nonstatic_func), buf, sizeof(buf)));
@@ -503,7 +586,7 @@ int main(int argc, char **argv) {
 #if !defined(__EMSCRIPTEN__)
   // Make sure kHpageTextPadding is linked into the binary.
   if (volatile_bool) {
-    ABSL_RAW_LOG(INFO, "%s", kHpageTextPadding);
+    LOG(INFO) << kHpageTextPadding;
   }
 #endif  // !defined(__EMSCRIPTEN__)
 
@@ -516,11 +599,24 @@ int main(int argc, char **argv) {
   absl::InitializeSymbolizer(argv[0]);
   testing::InitGoogleTest(&argc, argv);
 
-#ifdef ABSL_INTERNAL_HAVE_ELF_SYMBOLIZE
+#if defined(ABSL_INTERNAL_HAVE_ELF_SYMBOLIZE) ||        \
+    defined(ABSL_INTERNAL_HAVE_EMSCRIPTEN_SYMBOLIZE) || \
+    defined(ABSL_INTERNAL_HAVE_DARWIN_SYMBOLIZE)
   TestWithPCInsideInlineFunction();
   TestWithPCInsideNonInlineFunction();
   TestWithReturnAddress();
+#if defined(__arm__) && ABSL_HAVE_ATTRIBUTE(target) && \
+    ((__ARM_ARCH >= 7) || !defined(__ARM_PCS_VFP))
+  TestArmThumbOverlap();
+#endif
 #endif
 
+#if !defined(__EMSCRIPTEN__)
+  // All of these test cases rely on symbolizing function pointers.
+  // On most platforms, function pointers directly map to PC.
+  // In WebAssembly, function pointers are indices into the function table
+  // and there is no longer a mapping from function index back into the
+  // file offset for symbolization.
   return RUN_ALL_TESTS();
+#endif  // !defined(__EMSCRIPTEN__)
 }
diff --git a/absl/debugging/symbolize_unimplemented.inc b/absl/debugging/symbolize_unimplemented.inc
index 7c580fe447e..b0faa9d9ba0 100644
--- a/absl/debugging/symbolize_unimplemented.inc
+++ b/absl/debugging/symbolize_unimplemented.inc
@@ -14,15 +14,16 @@
 
 #include <cstdint>
 
-#include "absl/base/internal/raw_logging.h"
+#include "absl/debugging/internal/symbolize.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 namespace debugging_internal {
 
-int InstallSymbolDecorator(SymbolDecorator, void*) { return -1; }
-bool RemoveSymbolDecorator(int) { return false; }
-bool RemoveAllSymbolDecorators(void) { return false; }
+SymbolDecorator::Factory* SetSymbolDecoratorFactory(SymbolDecorator::Factory*) {
+  return nullptr;
+}
 bool RegisterFileMappingHint(const void *, const void *, uint64_t, const char *) {
   return false;
 }
@@ -35,4 +36,5 @@ bool GetFileMappingHint(const void **, const void **, uint64_t *, const char **)
 void InitializeSymbolizer(const char*) {}
 bool Symbolize(const void *, char *, int) { return false; }
 
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/debugging/symbolize_win32.inc b/absl/debugging/symbolize_win32.inc
index 5a55f29d296..eaeefb98639 100644
--- a/absl/debugging/symbolize_win32.inc
+++ b/absl/debugging/symbolize_win32.inc
@@ -16,11 +16,12 @@
 // https://msdn.microsoft.com/en-us/library/windows/desktop/ms680578(v=vs.85).aspx
 
 #include <windows.h>
+#include <winternl.h>
 
-// MSVC header DbgHelp.h has a warning for an ignored typedef.
+// MSVC header dbghelp.h has a warning for an ignored typedef.
 #pragma warning(push)
 #pragma warning(disable:4091)
-#include <DbgHelp.h>
+#include <dbghelp.h>
 #pragma warning(pop)
 
 #pragma comment(lib, "dbghelp.lib")
@@ -31,10 +32,11 @@
 #include "absl/base/internal/raw_logging.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 static HANDLE process = NULL;
 
-void InitializeSymbolizer(const char *argv0) {
+void InitializeSymbolizer(const char*) {
   if (process != nullptr) {
     return;
   }
@@ -44,37 +46,54 @@ void InitializeSymbolizer(const char *argv0) {
   // symbols be loaded. This is the fastest, most efficient way to use
   // the symbol handler.
   SymSetOptions(SYMOPT_DEFERRED_LOADS | SYMOPT_UNDNAME);
-  if (!SymInitialize(process, nullptr, true)) {
-    // GetLastError() returns a Win32 DWORD, but we assign to
-    // unsigned long long to simplify the ABSL_RAW_LOG case below.  The uniform
-    // initialization guarantees this is not a narrowing conversion.
-    const unsigned long long error{GetLastError()};  // NOLINT(runtime/int)
-    ABSL_RAW_LOG(FATAL, "SymInitialize() failed: %llu", error);
+  DWORD syminitialize_error;
+  constexpr int kSymInitializeRetries = 5;
+  for (int i = 0; i < kSymInitializeRetries; ++i) {
+    if (SymInitialize(process, nullptr, true)) {
+      return;
+    }
+
+    // SymInitialize can fail sometimes with a STATUS_INFO_LENGTH_MISMATCH
+    // NTSTATUS (0xC0000004), which can happen when a module load occurs during
+    // the SymInitialize call. In this case, we should try calling SymInitialize
+    // again.
+    constexpr DWORD kStatusInfoLengthMismatch = DWORD{0xC0000004};
+    syminitialize_error = GetLastError();
+    if (syminitialize_error != kStatusInfoLengthMismatch) {
+      break;
+    }
   }
+
+  // GetLastError() returns a Win32 DWORD, but we assign to
+  // unsigned long long to simplify the ABSL_RAW_LOG case below.  The uniform
+  // initialization guarantees this is not a narrowing conversion.
+  const unsigned long long error{syminitialize_error};
+  ABSL_RAW_LOG(FATAL, "SymInitialize() failed: %llu", error);
 }
 
-bool Symbolize(const void *pc, char *out, int out_size) {
+bool Symbolize(const void* pc, char* out, int out_size) {
   if (out_size <= 0) {
     return false;
   }
-  std::aligned_storage<sizeof(SYMBOL_INFO) + MAX_SYM_NAME,
-                       alignof(SYMBOL_INFO)>::type buf;
-  SYMBOL_INFO *symbol = reinterpret_cast<SYMBOL_INFO *>(&buf);
+  alignas(SYMBOL_INFO) char buf[sizeof(SYMBOL_INFO) + MAX_SYM_NAME];
+  SYMBOL_INFO* symbol = reinterpret_cast<SYMBOL_INFO*>(buf);
   symbol->SizeOfStruct = sizeof(SYMBOL_INFO);
   symbol->MaxNameLen = MAX_SYM_NAME;
   if (!SymFromAddr(process, reinterpret_cast<DWORD64>(pc), nullptr, symbol)) {
     return false;
   }
-  strncpy(out, symbol->Name, out_size);
-  if (out[out_size - 1] != '\0') {
+  const size_t out_size_t = static_cast<size_t>(out_size);
+  strncpy(out, symbol->Name, out_size_t);
+  if (out[out_size_t - 1] != '\0') {
     // strncpy() does not '\0' terminate when it truncates.
     static constexpr char kEllipsis[] = "...";
-    int ellipsis_size =
-        std::min<int>(sizeof(kEllipsis) - 1, out_size - 1);
-    memcpy(out + out_size - ellipsis_size - 1, kEllipsis, ellipsis_size);
-    out[out_size - 1] = '\0';
+    size_t ellipsis_size =
+        std::min(sizeof(kEllipsis) - 1, out_size_t - 1);
+    memcpy(out + out_size_t - ellipsis_size - 1, kEllipsis, ellipsis_size);
+    out[out_size_t - 1] = '\0';
   }
   return true;
 }
 
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/flags/BUILD.bazel b/absl/flags/BUILD.bazel
index 2bf562f898c..4dd3b33fa32 100644
--- a/absl/flags/BUILD.bazel
+++ b/absl/flags/BUILD.bazel
@@ -14,7 +14,9 @@
 # limitations under the License.
 #
 
-load("@rules_cc//cc:defs.bzl", "cc_library", "cc_test")
+load("@rules_cc//cc:cc_binary.bzl", "cc_binary")
+load("@rules_cc//cc:cc_library.bzl", "cc_library")
+load("@rules_cc//cc:cc_test.bzl", "cc_test")
 load(
     "//absl:copts/configure_copts.bzl",
     "ABSL_DEFAULT_COPTS",
@@ -22,25 +24,31 @@ load(
     "ABSL_TEST_COPTS",
 )
 
-package(default_visibility = ["//visibility:public"])
+package(
+    default_visibility = ["//visibility:private"],
+    features = [
+        "header_modules",
+        "layering_check",
+        "parse_headers",
+    ],
+)
 
-licenses(["notice"])  # Apache 2.0
+licenses(["notice"])
 
 cc_library(
-    name = "flag_internal",
-    srcs = [
-        "internal/flag.cc",
-    ],
+    name = "path_util",
     hdrs = [
-        "internal/flag.h",
+        "internal/path_util.h",
     ],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
-    visibility = ["//visibility:private"],
+    visibility = [
+        "//absl/flags:__pkg__",
+        "@do_not_use_for_gloop_visibility_only//gloop/base:__pkg__",
+    ],
     deps = [
-        ":handle",
-        ":registry",
-        "//absl/synchronization",
+        "//absl/base:config",
+        "//absl/strings",
     ],
 )
 
@@ -56,29 +64,19 @@ cc_library(
     linkopts = ABSL_DEFAULT_LINKOPTS,
     visibility = [
         "//absl/flags:__pkg__",
+        "//absl/log:__pkg__",
+        "@do_not_use_for_gloop_visibility_only//gloop/base:__pkg__",
     ],
     deps = [
         ":path_util",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:no_destructor",
         "//absl/strings",
         "//absl/synchronization",
     ],
 )
 
-cc_library(
-    name = "path_util",
-    hdrs = [
-        "internal/path_util.h",
-    ],
-    copts = ABSL_DEFAULT_COPTS,
-    linkopts = ABSL_DEFAULT_LINKOPTS,
-    visibility = [
-        "//absl/flags:__pkg__",
-    ],
-    deps = [
-        "//absl/strings",
-    ],
-)
-
 cc_library(
     name = "config",
     srcs = [
@@ -90,10 +88,13 @@ cc_library(
     ],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
     deps = [
         ":path_util",
         ":program_name",
+        "//absl/base:config",
         "//absl/base:core_headers",
+        "//absl/base:no_destructor",
         "//absl/strings",
         "//absl/synchronization",
     ],
@@ -109,15 +110,20 @@ cc_library(
     ],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
     deps = [
+        "//absl/base:config",
         "//absl/base:core_headers",
+        "//absl/base:log_severity",
+        "//absl/numeric:int128",
         "//absl/strings",
         "//absl/strings:str_format",
+        "//absl/types:optional",
     ],
 )
 
 cc_library(
-    name = "handle",
+    name = "commandlineflag_internal",
     srcs = [
         "internal/commandlineflag.cc",
     ],
@@ -127,63 +133,134 @@ cc_library(
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     visibility = [
-        "//absl/flags:__pkg__",
+        "@do_not_use_for_gloop_visibility_only//gloop/base:__pkg__",
     ],
     deps = [
-        ":config",
-        ":marshalling",
-        "//absl/base:core_headers",
-        "//absl/base:raw_logging_internal",
+        "//absl/base:config",
+        "//absl/base:fast_type_id",
+    ],
+)
+
+cc_library(
+    name = "commandlineflag",
+    srcs = [
+        "commandlineflag.cc",
+    ],
+    hdrs = [
+        "commandlineflag.h",
+    ],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
+    deps = [
+        ":commandlineflag_internal",
+        "//absl/base:config",
+        "//absl/base:fast_type_id",
+        "//absl/base:nullability",
         "//absl/strings",
-        "//absl/synchronization",
         "//absl/types:optional",
     ],
 )
 
 cc_library(
-    name = "registry",
+    name = "private_handle_accessor",
     srcs = [
-        "internal/registry.cc",
-        "internal/type_erased.cc",
+        "internal/private_handle_accessor.cc",
     ],
     hdrs = [
-        "internal/registry.h",
-        "internal/type_erased.h",
+        "internal/private_handle_accessor.h",
     ],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     visibility = [
         "//absl/flags:__pkg__",
+        "//absl/flags/rust:__pkg__",
+        "@do_not_use_for_gloop_visibility_only//gloop/base:__pkg__",
+    ],
+    deps = [
+        ":commandlineflag",
+        ":commandlineflag_internal",
+        "//absl/base:config",
+        "//absl/strings",
+    ],
+)
+
+cc_library(
+    name = "reflection",
+    srcs = [
+        "reflection.cc",
     ],
+    hdrs = [
+        "internal/registry.h",
+        "reflection.h",
+    ],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
     deps = [
+        ":commandlineflag",
+        ":commandlineflag_internal",
         ":config",
-        ":handle",
+        ":private_handle_accessor",
+        "//absl/base:config",
         "//absl/base:core_headers",
-        "//absl/base:dynamic_annotations",
-        "//absl/base:raw_logging_internal",
+        "//absl/base:fast_type_id",
+        "//absl/base:no_destructor",
+        "//absl/container:flat_hash_map",
         "//absl/strings",
         "//absl/synchronization",
     ],
 )
 
 cc_library(
-    name = "flag",
+    name = "flag_internal",
     srcs = [
-        "flag.cc",
+        "internal/flag.cc",
     ],
+    hdrs = [
+        "internal/flag.h",
+        "internal/sequence_lock.h",
+    ],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//absl/base:__subpackages__"],
+    deps = [
+        ":commandlineflag",
+        ":commandlineflag_internal",
+        ":config",
+        ":marshalling",
+        ":reflection",
+        "//absl/base",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:dynamic_annotations",
+        "//absl/base:fast_type_id",
+        "//absl/base:no_destructor",
+        "//absl/memory",
+        "//absl/meta:type_traits",
+        "//absl/strings",
+        "//absl/synchronization",
+        "//absl/utility",
+    ],
+)
+
+cc_library(
+    name = "flag",
     hdrs = [
         "declare.h",
         "flag.h",
     ],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
     deps = [
+        ":commandlineflag",
         ":config",
         ":flag_internal",
-        ":handle",
-        ":marshalling",
-        "//absl/base",
+        ":reflection",
+        "//absl/base:config",
         "//absl/base:core_headers",
+        "//absl/base:nullability",
         "//absl/strings",
     ],
 )
@@ -200,13 +277,20 @@ cc_library(
     linkopts = ABSL_DEFAULT_LINKOPTS,
     visibility = [
         "//absl/flags:__pkg__",
+        "@do_not_use_for_gloop_visibility_only//gloop/base:__pkg__",
     ],
     deps = [
+        ":commandlineflag",
         ":config",
         ":flag",
-        ":handle",
+        ":flag_internal",
         ":path_util",
+        ":private_handle_accessor",
         ":program_name",
+        ":reflection",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:no_destructor",
         "//absl/strings",
         "//absl/synchronization",
     ],
@@ -222,8 +306,12 @@ cc_library(
     ],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
     deps = [
         ":usage_internal",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:raw_logging_internal",
         "//absl/strings",
         "//absl/synchronization",
     ],
@@ -238,13 +326,22 @@ cc_library(
     ],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
     deps = [
+        ":commandlineflag",
+        ":commandlineflag_internal",
         ":config",
         ":flag",
+        ":flag_internal",
+        ":private_handle_accessor",
         ":program_name",
-        ":registry",
+        ":reflection",
         ":usage",
         ":usage_internal",
+        "//absl/algorithm:container",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:no_destructor",
         "//absl/strings",
         "//absl/synchronization",
     ],
@@ -257,18 +354,27 @@ cc_test(
     name = "commandlineflag_test",
     size = "small",
     srcs = [
-        "internal/commandlineflag_test.cc",
+        "commandlineflag_test.cc",
     ],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    tags = [
+        "no_test:os:android",
+        "no_test:os:ios",
+        "no_test_android",
+        "no_test_ios",
+        "no_test_wasm",
+    ],
     deps = [
+        ":commandlineflag",
         ":config",
         ":flag",
-        ":handle",
-        ":registry",
+        ":private_handle_accessor",
+        ":reflection",
         "//absl/memory",
         "//absl/strings",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -282,7 +388,8 @@ cc_test(
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":config",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -295,58 +402,113 @@ cc_test(
     ],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    tags = [
+        "no_test:os:android",
+        "no_test:os:ios",
+        "no_test_android",
+        "no_test_ios",
+        "no_test_wasm",
+    ],
     deps = [
         ":config",
         ":flag",
+        ":flag_internal",
+        ":marshalling",
+        ":parse",
+        ":reflection",
+        "//absl/base:core_headers",
+        "//absl/base:malloc_internal",
+        "//absl/base:raw_logging_internal",
+        "//absl/numeric:int128",
         "//absl/strings",
-        "@com_google_googletest//:gtest_main",
+        "//absl/time",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
-cc_test(
-    name = "marshalling_test",
-    size = "small",
+cc_binary(
+    name = "flag_benchmark",
+    testonly = True,
     srcs = [
-        "marshalling_test.cc",
+        "flag_benchmark.cc",
     ],
     copts = ABSL_TEST_COPTS,
-    linkopts = ABSL_DEFAULT_LINKOPTS,
+    linkopts = select({
+        "//conditions:default": [],
+    }) + ABSL_DEFAULT_LINKOPTS,
+    tags = ["benchmark"],
+    visibility = ["//visibility:private"],
     deps = [
+        "flag_benchmark.lds",
+        ":flag",
         ":marshalling",
-        "@com_google_googletest//:gtest_main",
+        ":parse",
+        ":reflection",
+        "//absl/strings",
+        "//absl/time",
+        "@google_benchmark//:benchmark_main",
     ],
 )
 
 cc_test(
-    name = "path_util_test",
+    name = "marshalling_test",
     size = "small",
     srcs = [
-        "internal/path_util_test.cc",
+        "marshalling_test.cc",
     ],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
-        ":path_util",
-        "@com_google_googletest//:gtest_main",
+        ":marshalling",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
 cc_test(
     name = "parse_test",
     size = "small",
+    timeout = "moderate",
     srcs = [
         "parse_test.cc",
     ],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    tags = [
+        "no_test:os:android",
+        "no_test:os:ios",
+        "no_test_android",
+        "no_test_ios",
+        "no_test_wasm",
+    ],
     deps = [
+        ":config",
         ":flag",
         ":parse",
-        "//absl/base:raw_logging_internal",
+        ":reflection",
+        ":usage_internal",
         "//absl/base:scoped_set_env",
+        "//absl/log",
         "//absl/strings",
         "//absl/types:span",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "path_util_test",
+    size = "small",
+    srcs = [
+        "internal/path_util_test.cc",
+    ],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":path_util",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -358,27 +520,62 @@ cc_test(
     ],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    tags = [
+        "no_test_fuchsia_x64",
+        "no_test_wasm",
+    ],
     deps = [
         ":program_name",
         "//absl/strings",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
 cc_test(
-    name = "type_erased_test",
+    name = "reflection_test",
     size = "small",
     srcs = [
-        "internal/type_erased_test.cc",
+        "reflection_test.cc",
     ],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    tags = [
+        "no_test:os:android",
+        "no_test:os:ios",
+        "no_test_android",
+        "no_test_ios",
+        "no_test_wasm",
+    ],
     deps = [
+        ":config",
         ":flag",
-        ":registry",
+        ":reflection",
         "//absl/memory",
         "//absl/strings",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "sequence_lock_test",
+    size = "small",
+    timeout = "moderate",
+    srcs = [
+        "internal/sequence_lock_test.cc",
+    ],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    shard_count = 31,
+    tags = ["no_test_wasm"],
+    deps = [
+        ":flag_internal",
+        "//absl/base",
+        "//absl/container:fixed_array",
+        "//absl/time",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -395,7 +592,8 @@ cc_test(
         ":path_util",
         ":program_name",
         "//absl/strings",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -407,16 +605,23 @@ cc_test(
     ],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    tags = [
+        "no_test:os:android",
+        "no_test:os:ios",
+        "no_test_android",
+        "no_test_ios",
+        "no_test_wasm",
+    ],
     deps = [
         ":config",
         ":flag",
         ":parse",
         ":path_util",
         ":program_name",
+        ":reflection",
         ":usage",
         ":usage_internal",
-        "//absl/memory",
         "//absl/strings",
-        "@com_google_googletest//:gtest",
+        "@googletest//:gtest",
     ],
 )
diff --git a/absl/flags/CMakeLists.txt b/absl/flags/CMakeLists.txt
index 3a7162f9347..0c75c93dee8 100644
--- a/absl/flags/CMakeLists.txt
+++ b/absl/flags/CMakeLists.txt
@@ -17,19 +17,16 @@
 # Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
-    flags_internal
-  SRCS
-    "internal/flag.cc"
+    flags_path_util
   HDRS
-    "internal/flag.h"
+    "internal/path_util.h"
   COPTS
     ${ABSL_DEFAULT_COPTS}
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
-    absl::flags_handle
-    absl::flags_registry
-    absl::synchronization
+    absl::config
+    absl::strings
   PUBLIC
 )
 
@@ -46,27 +43,15 @@ absl_cc_library(
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
+    absl::config
+    absl::core_headers
+    absl::no_destructor
     absl::flags_path_util
     absl::strings
     absl::synchronization
   PUBLIC
 )
 
-# Internal-only target, do not depend on directly.
-absl_cc_library(
-  NAME
-    flags_path_util
-  HDRS
-    "internal/path_util.h"
-  COPTS
-    ${ABSL_DEFAULT_COPTS}
-  LINKOPTS
-    ${ABSL_DEFAULT_LINKOPTS}
-  DEPS
-    absl::strings
-  PUBLIC
-)
-
 absl_cc_library(
   NAME
     flags_config
@@ -80,9 +65,11 @@ absl_cc_library(
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
+    absl::config
     absl::flags_path_util
     absl::flags_program_name
     absl::core_headers
+    absl::no_destructor
     absl::strings
     absl::synchronization
 )
@@ -99,7 +86,11 @@ absl_cc_library(
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
+    absl::config
     absl::core_headers
+    absl::log_severity
+    absl::int128
+    absl::optional
     absl::strings
     absl::str_format
 )
@@ -107,7 +98,7 @@ absl_cc_library(
 # Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
-    flags_handle
+    flags_commandlineflag_internal
   SRCS
     "internal/commandlineflag.cc"
   HDRS
@@ -117,44 +108,105 @@ absl_cc_library(
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
-    absl::flags_config
-    absl::flags_marshalling
-    absl::core_headers
+    absl::config
+    absl::dynamic_annotations
+    absl::fast_type_id
+)
+
+absl_cc_library(
+  NAME
+    flags_commandlineflag
+  SRCS
+    "commandlineflag.cc"
+  HDRS
+    "commandlineflag.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::config
+    absl::fast_type_id
+    absl::flags_commandlineflag_internal
+    absl::nullability
     absl::optional
-    absl::raw_logging_internal
     absl::strings
-    absl::synchronization
 )
 
 # Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
-    flags_registry
+    flags_private_handle_accessor
+  SRCS
+    "internal/private_handle_accessor.cc"
+  HDRS
+    "internal/private_handle_accessor.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::config
+    absl::flags_commandlineflag
+    absl::flags_commandlineflag_internal
+    absl::strings
+)
+
+absl_cc_library(
+  NAME
+    flags_reflection
   SRCS
-    "internal/registry.cc"
-    "internal/type_erased.cc"
+    "reflection.cc"
   HDRS
+    "reflection.h"
     "internal/registry.h"
-    "internal/type_erased.h"
   COPTS
     ${ABSL_DEFAULT_COPTS}
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
+    absl::config
+    absl::fast_type_id
+    absl::flags_commandlineflag
+    absl::flags_private_handle_accessor
     absl::flags_config
-    absl::flags_handle
-    absl::core_headers
-    absl::dynamic_annotations
-    absl::raw_logging_internal
     absl::strings
     absl::synchronization
+    absl::flat_hash_map
+    absl::no_destructor
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
-    flags
+    flags_internal
   SRCS
-    "flag.cc"
+    "internal/flag.cc"
+  HDRS
+    "internal/flag.h"
+    "internal/sequence_lock.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::base
+    absl::config
+    absl::fast_type_id
+    absl::flags_commandlineflag
+    absl::flags_commandlineflag_internal
+    absl::flags_config
+    absl::flags_marshalling
+    absl::no_destructor
+    absl::synchronization
+    absl::meta
+    absl::utility
+  PUBLIC
+)
+
+absl_cc_library(
+  NAME
+    flags
   HDRS
     "declare.h"
     "flag.h"
@@ -163,12 +215,13 @@ absl_cc_library(
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
+    absl::config
+    absl::flags_commandlineflag
     absl::flags_config
-    absl::flags_handle
     absl::flags_internal
-    absl::flags_marshalling
-    absl::base
+    absl::flags_reflection
     absl::core_headers
+    absl::nullability
     absl::strings
 )
 
@@ -185,11 +238,15 @@ absl_cc_library(
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
+    absl::config
     absl::flags_config
     absl::flags
-    absl::flags_handle
+    absl::flags_commandlineflag
+    absl::flags_internal
     absl::flags_path_util
+    absl::flags_private_handle_accessor
     absl::flags_program_name
+    absl::flags_reflection
     absl::strings
     absl::synchronization
 )
@@ -206,7 +263,11 @@ absl_cc_library(
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
+    absl::config
+    absl::core_headers
     absl::flags_usage_internal
+    absl::no_destructor
+    absl::raw_logging_internal
     absl::strings
     absl::synchronization
 )
@@ -224,33 +285,42 @@ absl_cc_library(
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
+    absl::algorithm_container
+    absl::config
+    absl::core_headers
     absl::flags_config
     absl::flags
+    absl::flags_commandlineflag
+    absl::flags_commandlineflag_internal
+    absl::flags_internal
+    absl::flags_private_handle_accessor
     absl::flags_program_name
-    absl::flags_registry
+    absl::flags_reflection
     absl::flags_usage
+    absl::no_destructor
     absl::strings
     absl::synchronization
 )
 
 ############################################################################
-# Unit tests in alpahabetical order.
+# Unit tests in alphabetical order.
 
 absl_cc_test(
   NAME
     flags_commandlineflag_test
   SRCS
-    "internal/commandlineflag_test.cc"
+    "commandlineflag_test.cc"
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
     absl::flags
+    absl::flags_commandlineflag
     absl::flags_config
-    absl::flags_handle
-    absl::flags_registry
+    absl::flags_private_handle_accessor
+    absl::flags_reflection
     absl::memory
     absl::strings
-    gtest_main
+    GTest::gtest_main
 )
 
 absl_cc_test(
@@ -262,7 +332,7 @@ absl_cc_test(
     ${ABSL_TEST_COPTS}
   DEPS
     absl::flags_config
-    gtest_main
+    GTest::gtest_main
 )
 
 absl_cc_test(
@@ -274,10 +344,18 @@ absl_cc_test(
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
+    absl::core_headers
     absl::flags
     absl::flags_config
+    absl::flags_internal
+    absl::flags_marshalling
+    absl::flags_parse
+    absl::flags_reflection
+    absl::int128
+    absl::raw_logging_internal
     absl::strings
-    gtest_main
+    absl::time
+    GTest::gtest_main
 )
 
 absl_cc_test(
@@ -289,7 +367,7 @@ absl_cc_test(
     ${ABSL_TEST_COPTS}
   DEPS
     absl::flags_marshalling
-    gtest_main
+    GTest::gtest_main
 )
 
 absl_cc_test(
@@ -301,12 +379,15 @@ absl_cc_test(
     ${ABSL_TEST_COPTS}
   DEPS
     absl::flags
+    absl::flags_config
     absl::flags_parse
-    absl::raw_logging_internal
+    absl::flags_reflection
+    absl::flags_usage_internal
+    absl::log
     absl::scoped_set_env
     absl::span
     absl::strings
-    gmock_main
+    GTest::gmock_main
 )
 
 absl_cc_test(
@@ -318,7 +399,7 @@ absl_cc_test(
     ${ABSL_TEST_COPTS}
   DEPS
     absl::flags_path_util
-    gtest_main
+    GTest::gtest_main
 )
 
 absl_cc_test(
@@ -331,22 +412,38 @@ absl_cc_test(
   DEPS
     absl::flags_program_name
     absl::strings
-    gtest_main
+    GTest::gtest_main
 )
 
 absl_cc_test(
   NAME
-    flags_type_erased_test
+    flags_reflection_test
   SRCS
-    "internal/type_erased_test.cc"
+    "reflection_test.cc"
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
     absl::flags
-    absl::flags_registry
+    absl::flags_config
+    absl::flags_reflection
+    absl::flags_usage
     absl::memory
     absl::strings
-    gtest_main
+    GTest::gmock_main
+)
+
+absl_cc_test(
+  NAME
+    flags_sequence_lock_test
+  SRCS
+    "internal/sequence_lock_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::base
+    absl::flags_internal
+    absl::time
+    GTest::gmock_main
 )
 
 absl_cc_test(
@@ -361,7 +458,7 @@ absl_cc_test(
     absl::flags_path_util
     absl::flags_program_name
     absl::strings
-    gtest_main
+    GTest::gtest_main
 )
 
 absl_cc_test(
@@ -377,8 +474,8 @@ absl_cc_test(
     absl::flags_path_util
     absl::flags_program_name
     absl::flags_parse
+    absl::flags_reflection
     absl::flags_usage
-    absl::memory
     absl::strings
-    gtest
+    GTest::gmock
 )
diff --git a/absl/flags/commandlineflag.cc b/absl/flags/commandlineflag.cc
new file mode 100644
index 00000000000..4140230651a
--- /dev/null
+++ b/absl/flags/commandlineflag.cc
@@ -0,0 +1,35 @@
+//
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/flags/commandlineflag.h"
+
+#include <string>
+
+#include "absl/base/config.h"
+#include "absl/flags/internal/commandlineflag.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+absl::string_view CommandLineFlag::TypeName() const { return ""; }
+bool CommandLineFlag::IsRetired() const { return false; }
+bool CommandLineFlag::ParseFrom(absl::string_view value, std::string* error) {
+  return ParseFrom(value, flags_internal::SET_FLAGS_VALUE,
+                   flags_internal::kProgrammaticChange, *error);
+}
+
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/flags/commandlineflag.h b/absl/flags/commandlineflag.h
new file mode 100644
index 00000000000..9336d6d4a08
--- /dev/null
+++ b/absl/flags/commandlineflag.h
@@ -0,0 +1,220 @@
+//
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: commandlineflag.h
+// -----------------------------------------------------------------------------
+//
+// This header file defines the `CommandLineFlag`, which acts as a type-erased
+// handle for accessing metadata about the Abseil Flag in question.
+//
+// Because an actual Abseil flag is of an unspecified type, you should not
+// manipulate or interact directly with objects of that type. Instead, use the
+// CommandLineFlag type as an intermediary.
+#ifndef ABSL_FLAGS_COMMANDLINEFLAG_H_
+#define ABSL_FLAGS_COMMANDLINEFLAG_H_
+
+#include <memory>
+#include <optional>
+#include <string>
+
+#include "absl/base/config.h"
+#include "absl/base/fast_type_id.h"
+#include "absl/base/nullability.h"
+#include "absl/flags/internal/commandlineflag.h"
+#include "absl/strings/string_view.h"
+#include "absl/types/optional.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace flags_internal {
+class PrivateHandleAccessor;
+}  // namespace flags_internal
+
+// CommandLineFlag
+//
+// This type acts as a type-erased handle for an instance of an Abseil Flag and
+// holds reflection information pertaining to that flag. Use CommandLineFlag to
+// access a flag's name, location, help string etc.
+//
+// To obtain an absl::CommandLineFlag, invoke `absl::FindCommandLineFlag()`
+// passing it the flag name string.
+//
+// Example:
+//
+//   // Obtain reflection handle for a flag named "flagname".
+//   const absl::CommandLineFlag* my_flag_data =
+//        absl::FindCommandLineFlag("flagname");
+//
+//   // Now you can get flag info from that reflection handle.
+//   std::string flag_location = my_flag_data->Filename();
+//   ...
+
+// These are only used as constexpr global objects.
+// They do not use a virtual destructor to simplify their implementation.
+// They are not destroyed except at program exit, so leaks do not matter.
+#if defined(__GNUC__) && !defined(__clang__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wnon-virtual-dtor"
+#endif
+class CommandLineFlag {
+ public:
+  constexpr CommandLineFlag() = default;
+
+  // Not copyable/assignable.
+  CommandLineFlag(const CommandLineFlag&) = delete;
+  CommandLineFlag& operator=(const CommandLineFlag&) = delete;
+
+  // absl::CommandLineFlag::IsOfType()
+  //
+  // Return true iff flag has type T.
+  template <typename T>
+  inline bool IsOfType() const {
+    return TypeId() == FastTypeId<T>();
+  }
+
+  // absl::CommandLineFlag::TryGet()
+  //
+  // Attempts to retrieve the flag value. Returns value on success,
+  // std::nullopt otherwise.
+  template <typename T>
+  std::optional<T> TryGet() const {
+    if (IsRetired() || !IsOfType<T>()) {
+      return std::nullopt;
+    }
+
+    // Implementation notes:
+    //
+    // We are wrapping a union around the value of `T` to serve three purposes:
+    //
+    //  1. `U.value` has correct size and alignment for a value of type `T`
+    //  2. The `U.value` constructor is not invoked since U's constructor does
+    //     not do it explicitly.
+    //  3. The `U.value` destructor is invoked since U's destructor does it
+    //     explicitly. This makes `U` a kind of RAII wrapper around non default
+    //     constructible value of T, which is destructed when we leave the
+    //     scope. We do need to destroy U.value, which is constructed by
+    //     CommandLineFlag::Read even though we left it in a moved-from state
+    //     after std::move.
+    //
+    // All of this serves to avoid requiring `T` being default constructible.
+    union U {
+      T value;
+      U() {}
+      ~U() { value.~T(); }
+    };
+    U u;
+
+    Read(&u.value);
+    // allow retired flags to be "read", so we can report invalid access.
+    if (IsRetired()) {
+      return std::nullopt;
+    }
+    return std::move(u.value);
+  }
+
+  // absl::CommandLineFlag::Name()
+  //
+  // Returns name of this flag.
+  virtual absl::string_view Name() const = 0;
+
+  // absl::CommandLineFlag::Filename()
+  //
+  // Returns name of the file where this flag is defined.
+  virtual std::string Filename() const = 0;
+
+  // absl::CommandLineFlag::Help()
+  //
+  // Returns help message associated with this flag.
+  virtual std::string Help() const = 0;
+
+  // absl::CommandLineFlag::IsRetired()
+  //
+  // Returns true iff this object corresponds to retired flag.
+  virtual bool IsRetired() const;
+
+  // absl::CommandLineFlag::DefaultValue()
+  //
+  // Returns the default value for this flag.
+  virtual std::string DefaultValue() const = 0;
+
+  // absl::CommandLineFlag::CurrentValue()
+  //
+  // Returns the current value for this flag.
+  virtual std::string CurrentValue() const = 0;
+
+  // absl::CommandLineFlag::ParseFrom()
+  //
+  // Sets the value of the flag based on specified string `value`. If the flag
+  // was successfully set to new value, it returns true. Otherwise, sets `error`
+  // to indicate the error, leaves the flag unchanged, and returns false.
+  bool ParseFrom(absl::string_view value, std::string* absl_nonnull error);
+
+ protected:
+  ~CommandLineFlag() = default;
+
+ private:
+  friend class flags_internal::PrivateHandleAccessor;
+
+  // Sets the value of the flag based on specified string `value`. If the flag
+  // was successfully set to new value, it returns true. Otherwise, sets `error`
+  // to indicate the error, leaves the flag unchanged, and returns false. There
+  // are three ways to set the flag's value:
+  //  * Update the current flag value
+  //  * Update the flag's default value
+  //  * Update the current flag value if it was never set before
+  // The mode is selected based on `set_mode` parameter.
+  virtual bool ParseFrom(absl::string_view value,
+                         flags_internal::FlagSettingMode set_mode,
+                         flags_internal::ValueSource source,
+                         std::string& error) = 0;
+
+  // Returns id of the flag's value type.
+  virtual flags_internal::FlagFastTypeId TypeId() const = 0;
+
+  // Interface to save flag to some persistent state. Returns current flag state
+  // or nullptr if flag does not support saving and restoring a state.
+  virtual std::unique_ptr<flags_internal::FlagStateInterface> SaveState() = 0;
+
+  // Copy-construct a new value of the flag's type in a memory referenced by
+  // the dst based on the current flag's value.
+  virtual void Read(void* absl_nonnull dst) const = 0;
+
+  // To be deleted. Used to return true if flag's current value originated from
+  // command line.
+  virtual bool IsSpecifiedOnCommandLine() const = 0;
+
+  // Validates supplied value using validator or parseflag routine
+  virtual bool ValidateInputValue(absl::string_view value) const = 0;
+
+  // Checks that flags default value can be converted to string and back to the
+  // flag's value type.
+  virtual void CheckDefaultValueParsingRoundtrip() const = 0;
+
+  // absl::CommandLineFlag::TypeName()
+  //
+  // Returns string representation of the type of this flag
+  // (the way it is spelled in the ABSL_FLAG macro).
+  // The default implementation returns the empty string.
+  virtual absl::string_view TypeName() const;
+};
+#if defined(__GNUC__) && !defined(__clang__)
+#pragma GCC diagnostic pop
+#endif
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_FLAGS_COMMANDLINEFLAG_H_
diff --git a/absl/flags/commandlineflag_test.cc b/absl/flags/commandlineflag_test.cc
new file mode 100644
index 00000000000..b6d01b2ac58
--- /dev/null
+++ b/absl/flags/commandlineflag_test.cc
@@ -0,0 +1,236 @@
+//
+//  Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/flags/commandlineflag.h"
+
+#include <memory>
+#include <string>
+
+#include "gtest/gtest.h"
+#include "absl/flags/config.h"
+#include "absl/flags/flag.h"
+#include "absl/flags/internal/private_handle_accessor.h"
+#include "absl/flags/reflection.h"
+#include "absl/flags/usage_config.h"
+#include "absl/memory/memory.h"
+#include "absl/strings/match.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/string_view.h"
+
+ABSL_FLAG(int, int_flag, 201, "int_flag help");
+ABSL_FLAG(std::string, string_flag, "dflt",
+          absl::StrCat("string_flag", " help"));
+ABSL_RETIRED_FLAG(bool, bool_retired_flag, false, "bool_retired_flag help");
+
+// These are only used to test default values.
+ABSL_FLAG(int, int_flag2, 201, "");
+ABSL_FLAG(std::string, string_flag2, "dflt", "");
+
+namespace {
+
+namespace flags = absl::flags_internal;
+
+class CommandLineFlagTest : public testing::Test {
+ protected:
+  static void SetUpTestSuite() {
+    // Install a function to normalize filenames before this test is run.
+    absl::FlagsUsageConfig default_config;
+    default_config.normalize_filename = &CommandLineFlagTest::NormalizeFileName;
+    absl::SetFlagsUsageConfig(default_config);
+  }
+
+  void SetUp() override {
+#if ABSL_FLAGS_STRIP_NAMES
+    GTEST_SKIP() << "This test requires flag names to be present";
+#endif
+    flag_saver_ = std::make_unique<absl::FlagSaver>();
+  }
+  void TearDown() override { flag_saver_.reset(); }
+
+ private:
+  static std::string NormalizeFileName(absl::string_view fname) {
+#ifdef _WIN32
+    std::string normalized(fname);
+    std::replace(normalized.begin(), normalized.end(), '\\', '/');
+    fname = normalized;
+#endif
+    return std::string(fname);
+  }
+
+  std::unique_ptr<absl::FlagSaver> flag_saver_;
+};
+
+TEST_F(CommandLineFlagTest, TestAttributesAccessMethods) {
+  auto* flag_01 = absl::FindCommandLineFlag("int_flag");
+
+  ASSERT_TRUE(flag_01);
+  EXPECT_EQ(flag_01->Name(), "int_flag");
+  EXPECT_EQ(flag_01->Help(), "int_flag help");
+  EXPECT_TRUE(!flag_01->IsRetired());
+  EXPECT_TRUE(flag_01->IsOfType<int>());
+  EXPECT_TRUE(!flag_01->IsOfType<bool>());
+  EXPECT_TRUE(!flag_01->IsOfType<std::string>());
+  EXPECT_TRUE(absl::EndsWith(flag_01->Filename(),
+                             "absl/flags/commandlineflag_test.cc"))
+      << flag_01->Filename();
+
+  auto* flag_02 = absl::FindCommandLineFlag("string_flag");
+
+  ASSERT_TRUE(flag_02);
+  EXPECT_EQ(flag_02->Name(), "string_flag");
+  EXPECT_EQ(flag_02->Help(), "string_flag help");
+  EXPECT_TRUE(!flag_02->IsRetired());
+  EXPECT_TRUE(flag_02->IsOfType<std::string>());
+  EXPECT_TRUE(!flag_02->IsOfType<bool>());
+  EXPECT_TRUE(!flag_02->IsOfType<int>());
+  EXPECT_TRUE(absl::EndsWith(flag_02->Filename(),
+                             "absl/flags/commandlineflag_test.cc"))
+      << flag_02->Filename();
+}
+
+// --------------------------------------------------------------------
+
+TEST_F(CommandLineFlagTest, TestValueAccessMethods) {
+  absl::SetFlag(&FLAGS_int_flag2, 301);
+  auto* flag_01 = absl::FindCommandLineFlag("int_flag2");
+
+  ASSERT_TRUE(flag_01);
+  EXPECT_EQ(flag_01->CurrentValue(), "301");
+  EXPECT_EQ(flag_01->DefaultValue(), "201");
+
+  absl::SetFlag(&FLAGS_string_flag2, "new_str_value");
+  auto* flag_02 = absl::FindCommandLineFlag("string_flag2");
+
+  ASSERT_TRUE(flag_02);
+  EXPECT_EQ(flag_02->CurrentValue(), "new_str_value");
+  EXPECT_EQ(flag_02->DefaultValue(), "dflt");
+}
+
+// --------------------------------------------------------------------
+
+TEST_F(CommandLineFlagTest, TestParseFromCurrentValue) {
+  std::string err;
+
+  auto* flag_01 = absl::FindCommandLineFlag("int_flag");
+  EXPECT_FALSE(
+      flags::PrivateHandleAccessor::IsSpecifiedOnCommandLine(*flag_01));
+
+  EXPECT_TRUE(flags::PrivateHandleAccessor::ParseFrom(
+      *flag_01, "11", flags::SET_FLAGS_VALUE, flags::kProgrammaticChange, err));
+  EXPECT_EQ(absl::GetFlag(FLAGS_int_flag), 11);
+  EXPECT_FALSE(
+      flags::PrivateHandleAccessor::IsSpecifiedOnCommandLine(*flag_01));
+
+  EXPECT_TRUE(flags::PrivateHandleAccessor::ParseFrom(
+      *flag_01, "-123", flags::SET_FLAGS_VALUE, flags::kProgrammaticChange,
+      err));
+  EXPECT_EQ(absl::GetFlag(FLAGS_int_flag), -123);
+  EXPECT_FALSE(
+      flags::PrivateHandleAccessor::IsSpecifiedOnCommandLine(*flag_01));
+
+  EXPECT_TRUE(!flags::PrivateHandleAccessor::ParseFrom(
+      *flag_01, "xyz", flags::SET_FLAGS_VALUE, flags::kProgrammaticChange,
+      err));
+  EXPECT_EQ(absl::GetFlag(FLAGS_int_flag), -123);
+  EXPECT_EQ(err, "Illegal value 'xyz' specified for flag 'int_flag'");
+  EXPECT_FALSE(
+      flags::PrivateHandleAccessor::IsSpecifiedOnCommandLine(*flag_01));
+
+  EXPECT_TRUE(!flags::PrivateHandleAccessor::ParseFrom(
+      *flag_01, "A1", flags::SET_FLAGS_VALUE, flags::kProgrammaticChange, err));
+  EXPECT_EQ(absl::GetFlag(FLAGS_int_flag), -123);
+  EXPECT_EQ(err, "Illegal value 'A1' specified for flag 'int_flag'");
+  EXPECT_FALSE(
+      flags::PrivateHandleAccessor::IsSpecifiedOnCommandLine(*flag_01));
+
+  EXPECT_TRUE(flags::PrivateHandleAccessor::ParseFrom(
+      *flag_01, "0x10", flags::SET_FLAGS_VALUE, flags::kProgrammaticChange,
+      err));
+  EXPECT_EQ(absl::GetFlag(FLAGS_int_flag), 16);
+  EXPECT_FALSE(
+      flags::PrivateHandleAccessor::IsSpecifiedOnCommandLine(*flag_01));
+
+  EXPECT_TRUE(flags::PrivateHandleAccessor::ParseFrom(
+      *flag_01, "011", flags::SET_FLAGS_VALUE, flags::kCommandLine, err));
+  EXPECT_EQ(absl::GetFlag(FLAGS_int_flag), 11);
+  EXPECT_TRUE(flags::PrivateHandleAccessor::IsSpecifiedOnCommandLine(*flag_01));
+
+  EXPECT_TRUE(!flags::PrivateHandleAccessor::ParseFrom(
+      *flag_01, "", flags::SET_FLAGS_VALUE, flags::kProgrammaticChange, err));
+  EXPECT_EQ(err, "Illegal value '' specified for flag 'int_flag'");
+
+  auto* flag_02 = absl::FindCommandLineFlag("string_flag");
+  EXPECT_TRUE(flags::PrivateHandleAccessor::ParseFrom(
+      *flag_02, "xyz", flags::SET_FLAGS_VALUE, flags::kProgrammaticChange,
+      err));
+  EXPECT_EQ(absl::GetFlag(FLAGS_string_flag), "xyz");
+
+  EXPECT_TRUE(flags::PrivateHandleAccessor::ParseFrom(
+      *flag_02, "", flags::SET_FLAGS_VALUE, flags::kProgrammaticChange, err));
+  EXPECT_EQ(absl::GetFlag(FLAGS_string_flag), "");
+}
+
+// --------------------------------------------------------------------
+
+TEST_F(CommandLineFlagTest, TestParseFromDefaultValue) {
+  std::string err;
+
+  auto* flag_01 = absl::FindCommandLineFlag("int_flag");
+
+  EXPECT_TRUE(flags::PrivateHandleAccessor::ParseFrom(
+      *flag_01, "111", flags::SET_FLAGS_DEFAULT, flags::kProgrammaticChange,
+      err));
+  EXPECT_EQ(flag_01->DefaultValue(), "111");
+
+  auto* flag_02 = absl::FindCommandLineFlag("string_flag");
+
+  EXPECT_TRUE(flags::PrivateHandleAccessor::ParseFrom(
+      *flag_02, "abc", flags::SET_FLAGS_DEFAULT, flags::kProgrammaticChange,
+      err));
+  EXPECT_EQ(flag_02->DefaultValue(), "abc");
+}
+
+// --------------------------------------------------------------------
+
+TEST_F(CommandLineFlagTest, TestParseFromIfDefault) {
+  std::string err;
+
+  auto* flag_01 = absl::FindCommandLineFlag("int_flag");
+
+  EXPECT_TRUE(flags::PrivateHandleAccessor::ParseFrom(
+      *flag_01, "22", flags::SET_FLAG_IF_DEFAULT, flags::kProgrammaticChange,
+      err))
+      << err;
+  EXPECT_EQ(absl::GetFlag(FLAGS_int_flag), 22);
+
+  EXPECT_TRUE(flags::PrivateHandleAccessor::ParseFrom(
+      *flag_01, "33", flags::SET_FLAG_IF_DEFAULT, flags::kProgrammaticChange,
+      err));
+  EXPECT_EQ(absl::GetFlag(FLAGS_int_flag), 22);
+  // EXPECT_EQ(err, "ERROR: int_flag is already set to 22");
+
+  // Reset back to default value
+  EXPECT_TRUE(flags::PrivateHandleAccessor::ParseFrom(
+      *flag_01, "201", flags::SET_FLAGS_VALUE, flags::kProgrammaticChange,
+      err));
+
+  EXPECT_TRUE(flags::PrivateHandleAccessor::ParseFrom(
+      *flag_01, "33", flags::SET_FLAG_IF_DEFAULT, flags::kProgrammaticChange,
+      err));
+  EXPECT_EQ(absl::GetFlag(FLAGS_int_flag), 201);
+  // EXPECT_EQ(err, "ERROR: int_flag is already set to 201");
+}
+
+}  // namespace
diff --git a/absl/flags/config.h b/absl/flags/config.h
index a9fd97ad0f3..14c4235bb3c 100644
--- a/absl/flags/config.h
+++ b/absl/flags/config.h
@@ -45,4 +45,24 @@
 #define ABSL_FLAGS_STRIP_HELP ABSL_FLAGS_STRIP_NAMES
 #endif
 
+// These macros represent the "source of truth" for the list of supported
+// built-in types.
+#define ABSL_FLAGS_INTERNAL_BUILTIN_TYPES(A) \
+  A(bool, bool)                              \
+  A(short, short)                            \
+  A(unsigned short, unsigned_short)          \
+  A(int, int)                                \
+  A(unsigned int, unsigned_int)              \
+  A(long, long)                              \
+  A(unsigned long, unsigned_long)            \
+  A(long long, long_long)                    \
+  A(unsigned long long, unsigned_long_long)  \
+  A(double, double)                          \
+  A(float, float)
+
+#define ABSL_FLAGS_INTERNAL_SUPPORTED_TYPES(A) \
+  ABSL_FLAGS_INTERNAL_BUILTIN_TYPES(A)         \
+  A(std::string, std_string)                   \
+  A(std::vector<std::string>, std_vector_of_string)
+
 #endif  // ABSL_FLAGS_CONFIG_H_
diff --git a/absl/flags/config_test.cc b/absl/flags/config_test.cc
index 638998667e7..5c74d765f24 100644
--- a/absl/flags/config_test.cc
+++ b/absl/flags/config_test.cc
@@ -13,13 +13,12 @@
 // limitations under the License.
 
 #include "absl/flags/config.h"
+#include "gtest/gtest.h"
 
 #ifdef __APPLE__
 #include <TargetConditionals.h>
 #endif
 
-#include "gtest/gtest.h"
-
 #ifndef ABSL_FLAGS_STRIP_NAMES
 #error ABSL_FLAGS_STRIP_NAMES is not defined
 #endif
diff --git a/absl/flags/declare.h b/absl/flags/declare.h
index 0a113a2b27a..a0b12bb51e9 100644
--- a/absl/flags/declare.h
+++ b/absl/flags/declare.h
@@ -25,9 +25,10 @@
 #ifndef ABSL_FLAGS_DECLARE_H_
 #define ABSL_FLAGS_DECLARE_H_
 
-#include "absl/strings/string_view.h"
+#include "absl/base/config.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace flags_internal {
 
 // absl::Flag<T> represents a flag of type 'T' created by ABSL_FLAG.
@@ -39,14 +40,10 @@ class Flag;
 // Flag
 //
 // Forward declaration of the `absl::Flag` type for use in defining the macro.
-#if defined(_MSC_VER)
-template <typename T>
-class Flag;
-#else
 template <typename T>
 using Flag = flags_internal::Flag<T>;
-#endif
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 // ABSL_DECLARE_FLAG()
@@ -58,6 +55,23 @@ using Flag = flags_internal::Flag<T>;
 // The ABSL_DECLARE_FLAG(type, name) macro expands to:
 //
 //   extern absl::Flag<type> FLAGS_name;
-#define ABSL_DECLARE_FLAG(type, name) extern ::absl::Flag<type> FLAGS_##name
+#define ABSL_DECLARE_FLAG(type, name) ABSL_DECLARE_FLAG_INTERNAL(type, name)
+
+// Internal implementation of ABSL_DECLARE_FLAG to allow macro expansion of its
+// arguments. Clients must use ABSL_DECLARE_FLAG instead.
+//
+// The non-MSVC implementation declares the flag twice. This is to allow
+// applying attributes to the second declaration. However, this causes a
+// compile error (C4273) in MSVC if a `__declspec` is prepended to the macro.
+#if defined(_MSC_VER)
+#define ABSL_DECLARE_FLAG_INTERNAL(type, name) \
+  extern absl::Flag<type> FLAGS_##name
+#else
+#define ABSL_DECLARE_FLAG_INTERNAL(type, name)               \
+  extern absl::Flag<type> FLAGS_##name;                      \
+  namespace absl /* block flags in namespaces */ {}          \
+  /* second redeclaration is to allow applying attributes */ \
+  extern absl::Flag<type> FLAGS_##name
+#endif  // _MSC_VER
 
 #endif  // ABSL_FLAGS_DECLARE_H_
diff --git a/absl/flags/flag.cc b/absl/flags/flag.cc
deleted file mode 100644
index a02d069e9b0..00000000000
--- a/absl/flags/flag.cc
+++ /dev/null
@@ -1,60 +0,0 @@
-//
-//  Copyright 2019 The Abseil Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-#include "absl/flags/flag.h"
-
-#include <cstring>
-
-namespace absl {
-
-// We want to validate the type mismatch between type definition and
-// declaration. The lock-free implementation does not allow us to do it,
-// so in debug builds we always use the slower implementation, which always
-// validates the type.
-#ifndef NDEBUG
-#define ABSL_FLAGS_ATOMIC_GET(T) \
-  T GetFlag(const absl::Flag<T>& flag) { return flag.Get(); }
-#else
-#define ABSL_FLAGS_ATOMIC_GET(T)         \
-  T GetFlag(const absl::Flag<T>& flag) { \
-    T result;                            \
-    if (flag.AtomicGet(&result)) {       \
-      return result;                     \
-    }                                    \
-    return flag.Get();                   \
-  }
-#endif
-
-ABSL_FLAGS_INTERNAL_FOR_EACH_LOCK_FREE(ABSL_FLAGS_ATOMIC_GET)
-
-#undef ABSL_FLAGS_ATOMIC_GET
-
-// This global nutex protects on-demand construction of flag objects in MSVC
-// builds.
-#if defined(_MSC_VER)
-
-namespace flags_internal {
-
-ABSL_CONST_INIT static absl::Mutex construction_guard(absl::kConstInit);
-
-void LockGlobalConstructionGuard() { construction_guard.Lock(); }
-
-void UnlockGlobalConstructionGuard() { construction_guard.Unlock(); }
-
-}  // namespace flags_internal
-
-#endif
-
-}  // namespace absl
diff --git a/absl/flags/flag.h b/absl/flags/flag.h
index 86ad59dd9f1..4c328e38c90 100644
--- a/absl/flags/flag.h
+++ b/absl/flags/flag.h
@@ -29,15 +29,22 @@
 #ifndef ABSL_FLAGS_FLAG_H_
 #define ABSL_FLAGS_FLAG_H_
 
+#include <cstdint>
+#include <string>
+#include <type_traits>
+
 #include "absl/base/attributes.h"
-#include "absl/base/casts.h"
+#include "absl/base/config.h"
+#include "absl/base/nullability.h"
+#include "absl/base/optimization.h"
+#include "absl/flags/commandlineflag.h"
 #include "absl/flags/config.h"
-#include "absl/flags/declare.h"
-#include "absl/flags/internal/commandlineflag.h"
 #include "absl/flags/internal/flag.h"
-#include "absl/flags/marshalling.h"
+#include "absl/flags/internal/registry.h"
+#include "absl/strings/string_view.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 // Flag
 //
@@ -63,100 +70,13 @@ namespace absl {
 //      ABSL_FLAG(int, count, 0, "Count of items to process");
 //
 // No public methods of `absl::Flag<T>` are part of the Abseil Flags API.
-#if !defined(_MSC_VER)
-template <typename T>
-using Flag = flags_internal::Flag<T>;
-#else
-// MSVC debug builds do not implement constexpr correctly for classes with
-// virtual methods. To work around this we adding level of indirection, so that
-// the class `absl::Flag` contains an `internal::Flag*` (instead of being an
-// alias to that class) and dynamically allocates an instance when necessary.
-// We also forward all calls to internal::Flag methods via trampoline methods.
-// In this setup the `absl::Flag` class does not have virtual methods and thus
-// MSVC is able to initialize it at link time. To deal with multiple threads
-// accessing the flag for the first time concurrently we use an atomic boolean
-// indicating if flag object is constructed. We also employ the double-checked
-// locking pattern where the second level of protection is a global Mutex, so
-// if two threads attempt to construct the flag concurrently only one wins.
-
-namespace flags_internal {
-void LockGlobalConstructionGuard();
-void UnlockGlobalConstructionGuard();
-}  // namespace flags_internal
+//
+// For type support of Abseil Flags, see the marshalling.h header file, which
+// discusses supported standard types, optional flags, and additional Abseil
+// type support.
 
 template <typename T>
-class Flag {
- public:
-  constexpr Flag(const char* name, const flags_internal::HelpGenFunc help_gen,
-                 const char* filename,
-                 const flags_internal::FlagMarshallingOpFn marshalling_op,
-                 const flags_internal::InitialValGenFunc initial_value_gen)
-      : name_(name),
-        help_gen_(help_gen),
-        filename_(filename),
-        marshalling_op_(marshalling_op),
-        initial_value_gen_(initial_value_gen),
-        inited_(false) {}
-
-  flags_internal::Flag<T>* GetImpl() const {
-    if (!inited_.load(std::memory_order_acquire)) {
-      flags_internal::LockGlobalConstructionGuard();
-
-      if (inited_.load(std::memory_order_acquire)) {
-        return impl_;
-      }
-
-      impl_ = new flags_internal::Flag<T>(name_, help_gen_, filename_,
-                                          marshalling_op_, initial_value_gen_);
-      inited_.store(true, std::memory_order_release);
-
-      flags_internal::UnlockGlobalConstructionGuard();
-    }
-
-    return impl_;
-  }
-
-  // absl::Flag API
-  bool IsRetired() const { return GetImpl()->IsRetired(); }
-  bool IsAbseilFlag() const { return GetImpl()->IsAbseilFlag(); }
-  absl::string_view Name() const { return GetImpl()->Name(); }
-  std::string Help() const { return GetImpl()->Help(); }
-  bool IsModified() const { return GetImpl()->IsModified(); }
-  void SetModified(bool is_modified) { GetImpl()->SetModified(is_modified); }
-  bool IsSpecifiedOnCommandLine() const {
-    GetImpl()->IsSpecifiedOnCommandLine();
-  }
-  absl::string_view Typename() const { return GetImpl()->Typename(); }
-  std::string Filename() const { return GetImpl()->Filename(); }
-  std::string DefaultValue() const { return GetImpl()->DefaultValue(); }
-  std::string CurrentValue() const { return GetImpl()->CurrentValue(); }
-  bool HasValidatorFn() const { return GetImpl()->HasValidatorFn(); }
-  bool InvokeValidator(const void* value) const {
-    return GetImpl()->InvokeValidator(value);
-  }
-  template <typename T1>
-  inline bool IsOfType() const {
-    return GetImpl()->template IsOfType<T1>();
-  }
-  T Get() const { return GetImpl()->Get(); }
-  bool AtomicGet(T* v) const { return GetImpl()->AtomicGet(v); }
-  void Set(const T& v) { GetImpl()->Set(v); }
-  void SetCallback(const flags_internal::FlagCallback mutation_callback) {
-    GetImpl()->SetCallback(mutation_callback);
-  }
-  void InvokeCallback() { GetImpl()->InvokeCallback(); }
-
- private:
-  const char* name_;
-  const flags_internal::HelpGenFunc help_gen_;
-  const char* filename_;
-  const flags_internal::FlagMarshallingOpFn marshalling_op_;
-  const flags_internal::InitialValGenFunc initial_value_gen_;
-
-  mutable std::atomic<bool> inited_;
-  mutable flags_internal::Flag<T>* impl_ = nullptr;
-};
-#endif
+using Flag = flags_internal::Flag<T>;
 
 // GetFlag()
 //
@@ -175,23 +95,10 @@ class Flag {
 //   // FLAGS_firstname is a Flag of type `std::string`
 //   std::string first_name = absl::GetFlag(FLAGS_firstname);
 template <typename T>
-ABSL_MUST_USE_RESULT T GetFlag(const absl::Flag<T>& flag) {
-#define ABSL_FLAGS_INTERNAL_LOCK_FREE_VALIDATE(BIT) \
-  static_assert(                                    \
-      !std::is_same<T, BIT>::value,                 \
-      "Do not specify explicit template parameters to absl::GetFlag");
-  ABSL_FLAGS_INTERNAL_FOR_EACH_LOCK_FREE(ABSL_FLAGS_INTERNAL_LOCK_FREE_VALIDATE)
-#undef ABSL_FLAGS_INTERNAL_LOCK_FREE_VALIDATE
-
-  return flag.Get();
+[[nodiscard]] T GetFlag(const absl::Flag<T>& flag) {
+  return flags_internal::FlagImplPeer::InvokeGet<T>(flag);
 }
 
-// Overload for `GetFlag()` for types that support lock-free reads.
-#define ABSL_FLAGS_INTERNAL_LOCK_FREE_EXPORT(T) \
-  ABSL_MUST_USE_RESULT T GetFlag(const absl::Flag<T>& flag);
-ABSL_FLAGS_INTERNAL_FOR_EACH_LOCK_FREE(ABSL_FLAGS_INTERNAL_LOCK_FREE_EXPORT)
-#undef ABSL_FLAGS_INTERNAL_LOCK_FREE_EXPORT
-
 // SetFlag()
 //
 // Sets the value of an `absl::Flag` to the value `v`. Do not construct an
@@ -200,19 +107,35 @@ ABSL_FLAGS_INTERNAL_FOR_EACH_LOCK_FREE(ABSL_FLAGS_INTERNAL_LOCK_FREE_EXPORT)
 // thread-safe, but is potentially expensive. Avoid setting flags in general,
 // but especially within performance-critical code.
 template <typename T>
-void SetFlag(absl::Flag<T>* flag, const T& v) {
-  flag->Set(v);
+void SetFlag(absl::Flag<T>* absl_nonnull flag, const T& v) {
+  flags_internal::FlagImplPeer::InvokeSet(*flag, v);
 }
 
 // Overload of `SetFlag()` to allow callers to pass in a value that is
 // convertible to `T`. E.g., use this overload to pass a "const char*" when `T`
 // is `std::string`.
 template <typename T, typename V>
-void SetFlag(absl::Flag<T>* flag, const V& v) {
+void SetFlag(absl::Flag<T>* absl_nonnull flag, const V& v) {
   T value(v);
-  flag->Set(value);
+  flags_internal::FlagImplPeer::InvokeSet(*flag, value);
+}
+
+// GetFlagReflectionHandle()
+//
+// Returns the reflection handle corresponding to specified Abseil Flag
+// instance. Use this handle to access flag's reflection information, like name,
+// location, default value etc.
+//
+// Example:
+//
+//   std::string = absl::GetFlagReflectionHandle(FLAGS_count).DefaultValue();
+
+template <typename T>
+const CommandLineFlag& GetFlagReflectionHandle(const absl::Flag<T>& f) {
+  return flags_internal::FlagImplPeer::InvokeReflect(f);
 }
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 
@@ -252,6 +175,8 @@ void SetFlag(absl::Flag<T>* flag, const V& v) {
 //
 //   ABSL_FLAG(T, name, default_value, help).OnUpdate(callback);
 //
+// `callback` should be convertible to `void (*)()`.
+//
 // After any setting of the flag value, the callback will be called at least
 // once. A rapid sequence of changes may be merged together into the same
 // callback. No concurrent calls to the callback will be made for the same
@@ -266,33 +191,32 @@ void SetFlag(absl::Flag<T>* flag, const V& v) {
 // Note: ABSL_FLAG.OnUpdate() does not have a public definition. Hence, this
 // comment serves as its API documentation.
 
-
 // -----------------------------------------------------------------------------
 // Implementation details below this section
 // -----------------------------------------------------------------------------
 
 // ABSL_FLAG_IMPL macro definition conditional on ABSL_FLAGS_STRIP_NAMES
+#define ABSL_FLAG_IMPL_FLAG_PTR(flag) flag
+#define ABSL_FLAG_IMPL_HELP_ARG(name)                      \
+  absl::flags_internal::HelpArg<AbslFlagHelpGenFor##name>( \
+      FLAGS_help_storage_##name)
+#define ABSL_FLAG_IMPL_DEFAULT_ARG(Type, name) \
+  absl::flags_internal::DefaultArg<Type, AbslFlagDefaultGenFor##name>(0)
 
 #if ABSL_FLAGS_STRIP_NAMES
 #define ABSL_FLAG_IMPL_FLAGNAME(txt) ""
+#define ABSL_FLAG_IMPL_TYPENAME(txt) ""
 #define ABSL_FLAG_IMPL_FILENAME() ""
-#if !defined(_MSC_VER)
-#define ABSL_FLAG_IMPL_REGISTRAR(T, flag) \
-  absl::flags_internal::FlagRegistrar<T, false>(&flag)
-#else
-#define ABSL_FLAG_IMPL_REGISTRAR(T, flag) \
-  absl::flags_internal::FlagRegistrar<T, false>(flag.GetImpl())
-#endif
+#define ABSL_FLAG_IMPL_REGISTRAR(T, flag)                                      \
+  absl::flags_internal::FlagRegistrar<T, false>(ABSL_FLAG_IMPL_FLAG_PTR(flag), \
+                                                nullptr)
 #else
 #define ABSL_FLAG_IMPL_FLAGNAME(txt) txt
+#define ABSL_FLAG_IMPL_TYPENAME(txt) txt
 #define ABSL_FLAG_IMPL_FILENAME() __FILE__
-#if !defined(_MSC_VER)
-#define ABSL_FLAG_IMPL_REGISTRAR(T, flag) \
-  absl::flags_internal::FlagRegistrar<T, true>(&flag)
-#else
-#define ABSL_FLAG_IMPL_REGISTRAR(T, flag) \
-  absl::flags_internal::FlagRegistrar<T, true>(flag.GetImpl())
-#endif
+#define ABSL_FLAG_IMPL_REGISTRAR(T, flag)                                     \
+  absl::flags_internal::FlagRegistrar<T, true>(ABSL_FLAG_IMPL_FLAG_PTR(flag), \
+                                               __FILE__)
 #endif
 
 // ABSL_FLAG_IMPL macro definition conditional on ABSL_FLAGS_STRIP_HELP
@@ -303,32 +227,56 @@ void SetFlag(absl::Flag<T>* flag, const V& v) {
 #define ABSL_FLAG_IMPL_FLAGHELP(txt) txt
 #endif
 
-#define ABSL_FLAG_IMPL_DECLARE_HELP_WRAPPER(name, txt) \
-  static std::string AbslFlagsWrapHelp##name() {       \
-    return ABSL_FLAG_IMPL_FLAGHELP(txt);               \
-  }
-
-#define ABSL_FLAG_IMPL_DECLARE_DEF_VAL_WRAPPER(name, Type, default_value)   \
-  static void* AbslFlagsInitFlag##name() {                                  \
-    return absl::flags_internal::MakeFromDefaultValue<Type>(default_value); \
-  }
+// AbslFlagHelpGenFor##name is used to encapsulate both immediate (method Const)
+// and lazy (method NonConst) evaluation of help message expression. We choose
+// between the two via the call to HelpArg in absl::Flag instantiation below.
+// If help message expression is constexpr evaluable compiler will optimize
+// away this whole struct.
+// TODO(rogeeff): place these generated structs into local namespace and apply
+// ABSL_INTERNAL_UNIQUE_SHORT_NAME.
+// TODO(rogeeff): Apply __attribute__((nodebug)) to FLAGS_help_storage_##name
+#define ABSL_FLAG_IMPL_DECLARE_HELP_WRAPPER(name, txt)                       \
+  struct AbslFlagHelpGenFor##name {                                          \
+    /* The expression is run in the caller as part of the   */               \
+    /* default value argument. That keeps temporaries alive */               \
+    /* long enough for NonConst to work correctly.          */               \
+    static constexpr absl::string_view Value(                                \
+        absl::string_view absl_flag_help ABSL_ATTRIBUTE_LIFETIME_BOUND  =     \
+            ABSL_FLAG_IMPL_FLAGHELP(txt)) {                                  \
+      return absl_flag_help;                                                 \
+    }                                                                        \
+    static std::string NonConst() { return std::string(Value()); }           \
+  };                                                                         \
+  constexpr auto FLAGS_help_storage_##name ABSL_INTERNAL_UNIQUE_SMALL_NAME() \
+      ABSL_ATTRIBUTE_SECTION_VARIABLE(flags_help_cold) =                     \
+          absl::flags_internal::HelpStringAsArray<AbslFlagHelpGenFor##name>( \
+              0);
+
+#define ABSL_FLAG_IMPL_DECLARE_DEF_VAL_WRAPPER(name, Type, default_value)     \
+  struct AbslFlagDefaultGenFor##name {                                        \
+    Type value = absl::flags_internal::InitDefaultValue<Type>(default_value); \
+    static void Gen(void* absl_flag_default_loc) {                            \
+      new (absl_flag_default_loc) Type(AbslFlagDefaultGenFor##name{}.value);  \
+    }                                                                         \
+  };
 
 // ABSL_FLAG_IMPL
 //
 // Note: Name of registrar object is not arbitrary. It is used to "grab"
 // global name for FLAGS_no<flag_name> symbol, thus preventing the possibility
 // of defining two flags with names foo and nofoo.
-#define ABSL_FLAG_IMPL(Type, name, default_value, help)             \
-  namespace absl /* block flags in namespaces */ {}                 \
-  ABSL_FLAG_IMPL_DECLARE_DEF_VAL_WRAPPER(name, Type, default_value) \
-  ABSL_FLAG_IMPL_DECLARE_HELP_WRAPPER(name, help)                   \
-  ABSL_CONST_INIT absl::Flag<Type> FLAGS_##name(                    \
-      ABSL_FLAG_IMPL_FLAGNAME(#name), &AbslFlagsWrapHelp##name,     \
-      ABSL_FLAG_IMPL_FILENAME(),                                    \
-      &absl::flags_internal::FlagMarshallingOps<Type>,              \
-      &AbslFlagsInitFlag##name);                                    \
-  extern bool FLAGS_no##name;                                       \
-  bool FLAGS_no##name = ABSL_FLAG_IMPL_REGISTRAR(Type, FLAGS_##name)
+#define ABSL_FLAG_IMPL(Type, name, default_value, help)               \
+  extern ::absl::Flag<Type> FLAGS_##name;                             \
+  namespace absl /* block flags in namespaces */ {}                   \
+  ABSL_FLAG_IMPL_DECLARE_DEF_VAL_WRAPPER(name, Type, default_value)   \
+  ABSL_FLAG_IMPL_DECLARE_HELP_WRAPPER(name, help)                     \
+  ABSL_CONST_INIT absl::Flag<Type> FLAGS_##name{                      \
+      ABSL_FLAG_IMPL_FLAGNAME(#name), ABSL_FLAG_IMPL_TYPENAME(#Type), \
+      ABSL_FLAG_IMPL_FILENAME(), ABSL_FLAG_IMPL_HELP_ARG(name),       \
+      ABSL_FLAG_IMPL_DEFAULT_ARG(Type, name)};                        \
+  extern absl::flags_internal::FlagRegistrarEmpty FLAGS_no##name;     \
+  absl::flags_internal::FlagRegistrarEmpty FLAGS_no##name =           \
+      ABSL_FLAG_IMPL_REGISTRAR(Type, FLAGS_##name)
 
 // ABSL_RETIRED_FLAG
 //
@@ -347,13 +295,13 @@ void SetFlag(absl::Flag<T>* flag, const V& v) {
 // arguments unchanged (unless of course you actually want to retire the flag
 // type at this time as well).
 //
-// `default_value` is only used as a double check on the type. `explanation` is
-// unused.
-// TODO(rogeeff): Return an anonymous struct instead of bool, and place it into
-// the unnamed namespace.
-#define ABSL_RETIRED_FLAG(type, flagname, default_value, explanation) \
-  ABSL_ATTRIBUTE_UNUSED static const bool ignored_##flagname =        \
-      ([] { return type(default_value); },                            \
-       absl::flags_internal::RetiredFlag<type>(#flagname))
+// `default_value` and `explanation` are unused.
+// TODO(rogeeff): replace RETIRED_FLAGS with FLAGS once forward declarations of
+// retired flags are cleaned up.
+#define ABSL_RETIRED_FLAG(type, name, default_value, explanation)      \
+  static absl::flags_internal::RetiredFlag<type> RETIRED_FLAGS_##name; \
+  ABSL_ATTRIBUTE_UNUSED static const auto RETIRED_FLAGS_REG_##name =   \
+      (RETIRED_FLAGS_##name.Retire(#name),                             \
+       ::absl::flags_internal::FlagRegistrarEmpty{})
 
 #endif  // ABSL_FLAGS_FLAG_H_
diff --git a/absl/flags/flag_benchmark.cc b/absl/flags/flag_benchmark.cc
new file mode 100644
index 00000000000..bbe9cbae490
--- /dev/null
+++ b/absl/flags/flag_benchmark.cc
@@ -0,0 +1,251 @@
+//
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <stdint.h>
+
+#include <optional>
+#include <string>
+#include <vector>
+
+#include "absl/flags/flag.h"
+#include "absl/flags/marshalling.h"
+#include "absl/flags/parse.h"
+#include "absl/flags/reflection.h"
+#include "absl/strings/string_view.h"
+#include "absl/time/time.h"
+#include "benchmark/benchmark.h"
+
+namespace {
+using String = std::string;
+using VectorOfStrings = std::vector<std::string>;
+using AbslDuration = absl::Duration;
+
+// We do not want to take over marshalling for the types std::optional<int>,
+// std::optional<std::string> which we do not own. Instead we introduce unique
+// "aliases" to these types, which we do.
+using AbslOptionalInt = std::optional<int>;
+struct OptionalInt : AbslOptionalInt {
+  using AbslOptionalInt::AbslOptionalInt;
+};
+// Next two functions represent Abseil Flags marshalling for OptionalInt.
+bool AbslParseFlag(absl::string_view src, OptionalInt* flag,
+                   std::string* error) {
+  int val;
+  if (src.empty())
+    flag->reset();
+  else if (!absl::ParseFlag(src, &val, error))
+    return false;
+  *flag = val;
+  return true;
+}
+std::string AbslUnparseFlag(const OptionalInt& flag) {
+  return !flag ? "" : absl::UnparseFlag(*flag);
+}
+
+using AbslOptionalString = std::optional<std::string>;
+struct OptionalString : AbslOptionalString {
+  using AbslOptionalString::AbslOptionalString;
+};
+// Next two functions represent Abseil Flags marshalling for OptionalString.
+bool AbslParseFlag(absl::string_view src, OptionalString* flag,
+                   std::string* error) {
+  std::string val;
+  if (src.empty())
+    flag->reset();
+  else if (!absl::ParseFlag(src, &val, error))
+    return false;
+  *flag = val;
+  return true;
+}
+std::string AbslUnparseFlag(const OptionalString& flag) {
+  return !flag ? "" : absl::UnparseFlag(*flag);
+}
+
+struct UDT {
+  UDT() = default;
+  UDT(const UDT&) {}
+  UDT& operator=(const UDT&) { return *this; }
+};
+// Next two functions represent Abseil Flags marshalling for UDT.
+bool AbslParseFlag(absl::string_view, UDT*, std::string*) { return true; }
+std::string AbslUnparseFlag(const UDT&) { return ""; }
+
+}  // namespace
+
+#define BENCHMARKED_TYPES(A) \
+  A(bool)                    \
+  A(int16_t)                 \
+  A(uint16_t)                \
+  A(int32_t)                 \
+  A(uint32_t)                \
+  A(int64_t)                 \
+  A(uint64_t)                \
+  A(double)                  \
+  A(float)                   \
+  A(String)                  \
+  A(VectorOfStrings)         \
+  A(OptionalInt)             \
+  A(OptionalString)          \
+  A(AbslDuration)            \
+  A(UDT)
+
+#define REPLICATE_0(A, T, name, index) A(T, name, index)
+#define REPLICATE_1(A, T, name, index) \
+  REPLICATE_0(A, T, name, index##0) REPLICATE_0(A, T, name, index##1)
+#define REPLICATE_2(A, T, name, index) \
+  REPLICATE_1(A, T, name, index##0) REPLICATE_1(A, T, name, index##1)
+#define REPLICATE_3(A, T, name, index) \
+  REPLICATE_2(A, T, name, index##0) REPLICATE_2(A, T, name, index##1)
+#define REPLICATE_4(A, T, name, index) \
+  REPLICATE_3(A, T, name, index##0) REPLICATE_3(A, T, name, index##1)
+#define REPLICATE_5(A, T, name, index) \
+  REPLICATE_4(A, T, name, index##0) REPLICATE_4(A, T, name, index##1)
+#define REPLICATE_6(A, T, name, index) \
+  REPLICATE_5(A, T, name, index##0) REPLICATE_5(A, T, name, index##1)
+#define REPLICATE_7(A, T, name, index) \
+  REPLICATE_6(A, T, name, index##0) REPLICATE_6(A, T, name, index##1)
+#define REPLICATE_8(A, T, name, index) \
+  REPLICATE_7(A, T, name, index##0) REPLICATE_7(A, T, name, index##1)
+#define REPLICATE_9(A, T, name, index) \
+  REPLICATE_8(A, T, name, index##0) REPLICATE_8(A, T, name, index##1)
+#if defined(_MSC_VER)
+#define REPLICATE(A, T, name) \
+  REPLICATE_7(A, T, name, 0) REPLICATE_7(A, T, name, 1)
+#define SINGLE_FLAG(T) FLAGS_##T##_flag_00000000
+#else
+#define REPLICATE(A, T, name) \
+  REPLICATE_9(A, T, name, 0) REPLICATE_9(A, T, name, 1)
+#define SINGLE_FLAG(T) FLAGS_##T##_flag_0000000000
+#endif
+#define REPLICATE_ALL(A, T, name) \
+  REPLICATE_9(A, T, name, 0) REPLICATE_9(A, T, name, 1)
+
+#define COUNT(T, name, index) +1
+constexpr size_t kNumFlags = 0 REPLICATE(COUNT, _, _);
+
+#if defined(__clang__) && defined(__linux__)
+// Force the flags used for benchmarks into a separate ELF section.
+// This ensures that, even when other parts of the code might change size,
+// the layout of the flags across cachelines is kept constant. This makes
+// benchmark results more reproducible across unrelated code changes.
+#pragma clang section data = ".benchmark_flags"
+#endif
+#define DEFINE_FLAG(T, name, index) ABSL_FLAG(T, name##_##index, {}, "");
+#define FLAG_DEF(T) REPLICATE(DEFINE_FLAG, T, T##_flag)
+BENCHMARKED_TYPES(FLAG_DEF)
+#if defined(__clang__) && defined(__linux__)
+#pragma clang section data = ""
+#endif
+// Register thousands of flags to bloat up the size of the registry.
+// This mimics real life production binaries.
+#define BLOAT_FLAG(_unused1, _unused2, index) \
+  ABSL_FLAG(int, bloat_flag_##index, 0, "");
+REPLICATE_ALL(BLOAT_FLAG, _, _)
+
+namespace {
+
+#define FLAG_PTR(T, name, index) &FLAGS_##name##_##index,
+#define FLAG_PTR_ARR(T)                              \
+  static constexpr absl::Flag<T>* FlagPtrs_##T[] = { \
+      REPLICATE(FLAG_PTR, T, T##_flag)};
+BENCHMARKED_TYPES(FLAG_PTR_ARR)
+
+#define BM_SingleGetFlag(T)                                    \
+  void BM_SingleGetFlag_##T(benchmark::State& state) {         \
+    for (auto _ : state) {                                     \
+      benchmark::DoNotOptimize(absl::GetFlag(SINGLE_FLAG(T))); \
+    }                                                          \
+  }                                                            \
+  BENCHMARK(BM_SingleGetFlag_##T)->ThreadRange(1, 16);
+
+BENCHMARKED_TYPES(BM_SingleGetFlag)
+
+template <typename T>
+struct Accumulator {
+  using type = T;
+};
+template <>
+struct Accumulator<String> {
+  using type = size_t;
+};
+template <>
+struct Accumulator<VectorOfStrings> {
+  using type = size_t;
+};
+template <>
+struct Accumulator<OptionalInt> {
+  using type = bool;
+};
+template <>
+struct Accumulator<OptionalString> {
+  using type = bool;
+};
+template <>
+struct Accumulator<UDT> {
+  using type = bool;
+};
+
+template <typename T>
+void Accumulate(typename Accumulator<T>::type& a, const T& f) {
+  a += f;
+}
+void Accumulate(bool& a, bool f) { a = a || f; }
+void Accumulate(size_t& a, const std::string& f) { a += f.size(); }
+void Accumulate(size_t& a, const std::vector<std::string>& f) { a += f.size(); }
+void Accumulate(bool& a, const OptionalInt& f) { a |= f.has_value(); }
+void Accumulate(bool& a, const OptionalString& f) { a |= f.has_value(); }
+void Accumulate(bool& a, const UDT& f) {
+  a |= reinterpret_cast<int64_t>(&f) & 0x1;
+}
+
+#define BM_ManyGetFlag(T)                            \
+  void BM_ManyGetFlag_##T(benchmark::State& state) { \
+    Accumulator<T>::type res = {};                   \
+    while (state.KeepRunningBatch(kNumFlags)) {      \
+      for (auto* flag_ptr : FlagPtrs_##T) {          \
+        Accumulate(res, absl::GetFlag(*flag_ptr));   \
+      }                                              \
+    }                                                \
+    benchmark::DoNotOptimize(res);                   \
+  }                                                  \
+  BENCHMARK(BM_ManyGetFlag_##T)->ThreadRange(1, 8);
+
+BENCHMARKED_TYPES(BM_ManyGetFlag)
+
+void BM_ThreadedFindCommandLineFlag(benchmark::State& state) {
+  char dummy[] = "dummy";
+  char* argv[] = {dummy};
+  // We need to ensure that flags have been parsed. That is where the registry
+  // is finalized.
+  absl::ParseCommandLine(1, argv);
+
+  while (state.KeepRunningBatch(kNumFlags)) {
+    for (auto* flag_ptr : FlagPtrs_bool) {
+      benchmark::DoNotOptimize(absl::FindCommandLineFlag(flag_ptr->Name()));
+    }
+  }
+}
+BENCHMARK(BM_ThreadedFindCommandLineFlag)->ThreadRange(1, 16);
+
+}  // namespace
+
+#ifdef __llvm__
+// To view disassembly use: gdb ${BINARY}  -batch -ex "disassemble /s $FUNC"
+#define InvokeGetFlag(T)                                             \
+  T AbslInvokeGetFlag##T() { return absl::GetFlag(SINGLE_FLAG(T)); } \
+  int odr##T = (benchmark::DoNotOptimize(AbslInvokeGetFlag##T), 1);
+
+BENCHMARKED_TYPES(InvokeGetFlag)
+#endif  // __llvm__
diff --git a/absl/flags/flag_benchmark.lds b/absl/flags/flag_benchmark.lds
new file mode 100644
index 00000000000..af115dfc009
--- /dev/null
+++ b/absl/flags/flag_benchmark.lds
@@ -0,0 +1,13 @@
+/* This linker script forces the flags used by flags_benchmark
+ * into a separate page-aligned section. This isn't necessary for
+ * correctness but ensures that the benchmark results are more
+ * reproducible across unrelated code changes.
+ */
+SECTIONS {
+  .benchmark_flags : {
+    . = ALIGN(0x1000);
+    * (.benchmark_flags);
+  }
+}
+
+INSERT AFTER .data
diff --git a/absl/flags/flag_test.cc b/absl/flags/flag_test.cc
index a652042c8aa..1dd2f5c97be 100644
--- a/absl/flags/flag_test.cc
+++ b/absl/flags/flag_test.cc
@@ -15,15 +15,35 @@
 
 #include "absl/flags/flag.h"
 
-#include <algorithm>
+#include <stddef.h>
+#include <stdint.h>
+
+#include <atomic>
+#include <iterator>
+#include <optional>
 #include <string>
+#include <thread>  // NOLINT
+#include <vector>
 
 #include "gtest/gtest.h"
+#include "absl/base/attributes.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/base/macros.h"
+#include "absl/flags/config.h"
+#include "absl/flags/declare.h"
+#include "absl/flags/internal/flag.h"
+#include "absl/flags/marshalling.h"
+#include "absl/flags/parse.h"
+#include "absl/flags/reflection.h"
 #include "absl/flags/usage_config.h"
+#include "absl/numeric/int128.h"
 #include "absl/strings/match.h"
 #include "absl/strings/numbers.h"
 #include "absl/strings/str_cat.h"
 #include "absl/strings/str_split.h"
+#include "absl/strings/string_view.h"
+#include "absl/time/clock.h"
+#include "absl/time/time.h"
 
 ABSL_DECLARE_FLAG(int64_t, mistyped_int_flag);
 ABSL_DECLARE_FLAG(std::vector<std::string>, mistyped_string_flag);
@@ -32,37 +52,20 @@ namespace {
 
 namespace flags = absl::flags_internal;
 
-std::string TestHelpMsg() { return "help"; }
+std::string TestHelpMsg() { return "dynamic help"; }
+#if defined(_MSC_VER) && !defined(__clang__)
+std::string TestLiteralHelpMsg() { return "literal help"; }
+#endif
 template <typename T>
-void* TestMakeDflt() {
-  return new T{};
+void TestMakeDflt(void* dst) {
+  new (dst) T{};
 }
 void TestCallback() {}
 
-template <typename T>
-bool TestConstructionFor() {
-  constexpr flags::Flag<T> f1("f1", &TestHelpMsg, "file",
-                              &absl::flags_internal::FlagMarshallingOps<T>,
-                              &TestMakeDflt<T>);
-  EXPECT_EQ(f1.Name(), "f1");
-  EXPECT_EQ(f1.Help(), "help");
-  EXPECT_EQ(f1.Filename(), "file");
-
-  ABSL_CONST_INIT static flags::Flag<T> f2(
-      "f2", &TestHelpMsg, "file", &absl::flags_internal::FlagMarshallingOps<T>,
-      &TestMakeDflt<T>);
-  flags::FlagRegistrar<T, false>(&f2).OnUpdate(TestCallback);
-
-  EXPECT_EQ(f2.Name(), "f2");
-  EXPECT_EQ(f2.Help(), "help");
-  EXPECT_EQ(f2.Filename(), "file");
-
-  return true;
-}
-
 struct UDT {
   UDT() = default;
   UDT(const UDT&) = default;
+  UDT& operator=(const UDT&) = default;
 };
 bool AbslParseFlag(absl::string_view, UDT*, std::string*) { return true; }
 std::string AbslUnparseFlag(const UDT&) { return ""; }
@@ -85,21 +88,124 @@ class FlagTest : public testing::Test {
 #endif
     return std::string(fname);
   }
+  absl::FlagSaver flag_saver_;
 };
 
-TEST_F(FlagTest, TestConstruction) {
-  TestConstructionFor<bool>();
-  TestConstructionFor<int16_t>();
-  TestConstructionFor<uint16_t>();
-  TestConstructionFor<int32_t>();
-  TestConstructionFor<uint32_t>();
-  TestConstructionFor<int64_t>();
-  TestConstructionFor<uint64_t>();
-  TestConstructionFor<double>();
-  TestConstructionFor<float>();
-  TestConstructionFor<std::string>();
+struct S1 {
+  S1() = default;
+  S1(const S1&) = default;
+  int32_t f1;
+  int64_t f2;
+};
+
+struct S2 {
+  S2() = default;
+  S2(const S2&) = default;
+  int64_t f1;
+  double f2;
+};
+
+TEST_F(FlagTest, Traits) {
+  EXPECT_EQ(flags::StorageKind<int>(),
+            flags::FlagValueStorageKind::kValueAndInitBit);
+  EXPECT_EQ(flags::StorageKind<bool>(),
+            flags::FlagValueStorageKind::kValueAndInitBit);
+  EXPECT_EQ(flags::StorageKind<double>(),
+            flags::FlagValueStorageKind::kOneWordAtomic);
+  EXPECT_EQ(flags::StorageKind<int64_t>(),
+            flags::FlagValueStorageKind::kOneWordAtomic);
+
+  EXPECT_EQ(flags::StorageKind<S1>(),
+            flags::FlagValueStorageKind::kSequenceLocked);
+  EXPECT_EQ(flags::StorageKind<S2>(),
+            flags::FlagValueStorageKind::kSequenceLocked);
+// Make sure absl::Duration uses the sequence-locked code path. MSVC 2015
+// doesn't consider absl::Duration to be trivially-copyable so we just
+// restrict this to clang as it seems to be a well-behaved compiler.
+#ifdef __clang__
+  EXPECT_EQ(flags::StorageKind<absl::Duration>(),
+            flags::FlagValueStorageKind::kSequenceLocked);
+#endif
+
+  EXPECT_EQ(flags::StorageKind<std::string>(),
+            flags::FlagValueStorageKind::kHeapAllocated);
+  EXPECT_EQ(flags::StorageKind<std::vector<std::string>>(),
+            flags::FlagValueStorageKind::kHeapAllocated);
+
+  EXPECT_EQ(flags::StorageKind<absl::int128>(),
+            flags::FlagValueStorageKind::kSequenceLocked);
+  EXPECT_EQ(flags::StorageKind<absl::uint128>(),
+            flags::FlagValueStorageKind::kSequenceLocked);
+}
+
+// --------------------------------------------------------------------
+
+constexpr flags::FlagHelpArg help_arg{flags::FlagHelpMsg("literal help"),
+                                      flags::FlagHelpKind::kLiteral};
+
+using String = std::string;
+using int128 = absl::int128;
+using uint128 = absl::uint128;
+
+#define DEFINE_CONSTRUCTED_FLAG(T, dflt, dflt_kind)                        \
+  constexpr flags::FlagDefaultArg f1default##T{                            \
+      flags::FlagDefaultSrc{dflt}, flags::FlagDefaultKind::dflt_kind};     \
+  constexpr absl::Flag<T> f1##T{"f1", #T, "file", help_arg, f1default##T}; \
+  ABSL_CONST_INIT absl::Flag<T> f2##T {                                    \
+    "f2", #T, "file",                                                      \
+        {flags::FlagHelpMsg(&TestHelpMsg), flags::FlagHelpKind::kGenFunc}, \
+        flags::FlagDefaultArg {                                            \
+      flags::FlagDefaultSrc(&TestMakeDflt<T>),                             \
+          flags::FlagDefaultKind::kGenFunc                                 \
+    }                                                                      \
+  }
+
+DEFINE_CONSTRUCTED_FLAG(bool, true, kOneWord);
+DEFINE_CONSTRUCTED_FLAG(int16_t, 1, kOneWord);
+DEFINE_CONSTRUCTED_FLAG(uint16_t, 2, kOneWord);
+DEFINE_CONSTRUCTED_FLAG(int32_t, 3, kOneWord);
+DEFINE_CONSTRUCTED_FLAG(uint32_t, 4, kOneWord);
+DEFINE_CONSTRUCTED_FLAG(int64_t, 5, kOneWord);
+DEFINE_CONSTRUCTED_FLAG(uint64_t, 6, kOneWord);
+DEFINE_CONSTRUCTED_FLAG(float, 7.8, kOneWord);
+DEFINE_CONSTRUCTED_FLAG(double, 9.10, kOneWord);
+DEFINE_CONSTRUCTED_FLAG(String, &TestMakeDflt<String>, kGenFunc);
+DEFINE_CONSTRUCTED_FLAG(UDT, &TestMakeDflt<UDT>, kGenFunc);
+DEFINE_CONSTRUCTED_FLAG(int128, 13, kGenFunc);
+DEFINE_CONSTRUCTED_FLAG(uint128, 14, kGenFunc);
+
+template <typename T>
+bool TestConstructionFor(const absl::Flag<T>& f1, absl::Flag<T>& f2) {
+  EXPECT_EQ(absl::GetFlagReflectionHandle(f1).Name(), "f1");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(f1).Help(), "literal help");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(f1).Filename(), "file");
+
+  flags::FlagRegistrar<T, false>(ABSL_FLAG_IMPL_FLAG_PTR(f2), nullptr)
+      .OnUpdate(TestCallback);
+
+  EXPECT_EQ(absl::GetFlagReflectionHandle(f2).Name(), "f2");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(f2).Help(), "dynamic help");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(f2).Filename(), "file");
+
+  return true;
+}
 
-  TestConstructionFor<UDT>();
+#define TEST_CONSTRUCTED_FLAG(T) TestConstructionFor(f1##T, f2##T);
+
+TEST_F(FlagTest, TestConstruction) {
+  TEST_CONSTRUCTED_FLAG(bool);
+  TEST_CONSTRUCTED_FLAG(int16_t);
+  TEST_CONSTRUCTED_FLAG(uint16_t);
+  TEST_CONSTRUCTED_FLAG(int32_t);
+  TEST_CONSTRUCTED_FLAG(uint32_t);
+  TEST_CONSTRUCTED_FLAG(int64_t);
+  TEST_CONSTRUCTED_FLAG(uint64_t);
+  TEST_CONSTRUCTED_FLAG(float);
+  TEST_CONSTRUCTED_FLAG(double);
+  TEST_CONSTRUCTED_FLAG(String);
+  TEST_CONSTRUCTED_FLAG(UDT);
+  TEST_CONSTRUCTED_FLAG(int128);
+  TEST_CONSTRUCTED_FLAG(uint128);
 }
 
 // --------------------------------------------------------------------
@@ -117,31 +223,67 @@ ABSL_DECLARE_FLAG(uint64_t, test_flag_08);
 ABSL_DECLARE_FLAG(double, test_flag_09);
 ABSL_DECLARE_FLAG(float, test_flag_10);
 ABSL_DECLARE_FLAG(std::string, test_flag_11);
+ABSL_DECLARE_FLAG(absl::Duration, test_flag_12);
+ABSL_DECLARE_FLAG(absl::int128, test_flag_13);
+ABSL_DECLARE_FLAG(absl::uint128, test_flag_14);
 
 namespace {
 
-#if !ABSL_FLAGS_STRIP_NAMES
-
 TEST_F(FlagTest, TestFlagDeclaration) {
+#if ABSL_FLAGS_STRIP_NAMES
+  GTEST_SKIP() << "This test requires flag names to be present";
+#endif
   // test that we can access flag objects.
-  EXPECT_EQ(FLAGS_test_flag_01.Name(), "test_flag_01");
-  EXPECT_EQ(FLAGS_test_flag_02.Name(), "test_flag_02");
-  EXPECT_EQ(FLAGS_test_flag_03.Name(), "test_flag_03");
-  EXPECT_EQ(FLAGS_test_flag_04.Name(), "test_flag_04");
-  EXPECT_EQ(FLAGS_test_flag_05.Name(), "test_flag_05");
-  EXPECT_EQ(FLAGS_test_flag_06.Name(), "test_flag_06");
-  EXPECT_EQ(FLAGS_test_flag_07.Name(), "test_flag_07");
-  EXPECT_EQ(FLAGS_test_flag_08.Name(), "test_flag_08");
-  EXPECT_EQ(FLAGS_test_flag_09.Name(), "test_flag_09");
-  EXPECT_EQ(FLAGS_test_flag_10.Name(), "test_flag_10");
-  EXPECT_EQ(FLAGS_test_flag_11.Name(), "test_flag_11");
-}
-#endif  // !ABSL_FLAGS_STRIP_NAMES
-
-// --------------------------------------------------------------------
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_01).Name(),
+            "test_flag_01");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_02).Name(),
+            "test_flag_02");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_03).Name(),
+            "test_flag_03");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_04).Name(),
+            "test_flag_04");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_05).Name(),
+            "test_flag_05");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_06).Name(),
+            "test_flag_06");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_07).Name(),
+            "test_flag_07");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_08).Name(),
+            "test_flag_08");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_09).Name(),
+            "test_flag_09");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_10).Name(),
+            "test_flag_10");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_11).Name(),
+            "test_flag_11");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_12).Name(),
+            "test_flag_12");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_13).Name(),
+            "test_flag_13");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_14).Name(),
+            "test_flag_14");
+}
 
 }  // namespace
 
+#if ABSL_FLAGS_STRIP_NAMES
+// The intent of this helper struct and an expression below is to make sure that
+// in the configuration where ABSL_FLAGS_STRIP_NAMES=1 registrar construction
+// (in cases of no Tail calls like OnUpdate) is constexpr and thus can and
+// should be completely optimized away, thus avoiding the cost/overhead of
+// static initializers.
+struct VerifyConsteval {
+  friend consteval flags::FlagRegistrarEmpty operator+(
+      flags::FlagRegistrarEmpty, VerifyConsteval) {
+    return {};
+  }
+};
+
+ABSL_FLAG(int, test_registrar_const_init, 0, "") + VerifyConsteval();
+#endif
+
+// --------------------------------------------------------------------
+
 ABSL_FLAG(bool, test_flag_01, true, "test flag 01");
 ABSL_FLAG(int, test_flag_02, 1234, "test flag 02");
 ABSL_FLAG(int16_t, test_flag_03, -34, "test flag 03");
@@ -153,73 +295,207 @@ ABSL_FLAG(uint64_t, test_flag_08, 9876543, "test flag 08");
 ABSL_FLAG(double, test_flag_09, -9.876e-50, "test flag 09");
 ABSL_FLAG(float, test_flag_10, 1.234e12f, "test flag 10");
 ABSL_FLAG(std::string, test_flag_11, "", "test flag 11");
+ABSL_FLAG(absl::Duration, test_flag_12, absl::Minutes(10), "test flag 12");
+ABSL_FLAG(absl::int128, test_flag_13, absl::MakeInt128(-1, 0), "test flag 13");
+ABSL_FLAG(absl::uint128, test_flag_14, absl::MakeUint128(0, 0xFFFAAABBBCCCDDD),
+          "test flag 14");
 
 namespace {
 
-#if !ABSL_FLAGS_STRIP_NAMES
 TEST_F(FlagTest, TestFlagDefinition) {
+#if ABSL_FLAGS_STRIP_NAMES
+  GTEST_SKIP() << "This test requires flag names to be present";
+#endif
   absl::string_view expected_file_name = "absl/flags/flag_test.cc";
 
-  EXPECT_EQ(FLAGS_test_flag_01.Name(), "test_flag_01");
-  EXPECT_EQ(FLAGS_test_flag_01.Help(), "test flag 01");
-  EXPECT_TRUE(absl::EndsWith(FLAGS_test_flag_01.Filename(), expected_file_name))
-      << FLAGS_test_flag_01.Filename();
-
-  EXPECT_EQ(FLAGS_test_flag_02.Name(), "test_flag_02");
-  EXPECT_EQ(FLAGS_test_flag_02.Help(), "test flag 02");
-  EXPECT_TRUE(absl::EndsWith(FLAGS_test_flag_02.Filename(), expected_file_name))
-      << FLAGS_test_flag_02.Filename();
-
-  EXPECT_EQ(FLAGS_test_flag_03.Name(), "test_flag_03");
-  EXPECT_EQ(FLAGS_test_flag_03.Help(), "test flag 03");
-  EXPECT_TRUE(absl::EndsWith(FLAGS_test_flag_03.Filename(), expected_file_name))
-      << FLAGS_test_flag_03.Filename();
-
-  EXPECT_EQ(FLAGS_test_flag_04.Name(), "test_flag_04");
-  EXPECT_EQ(FLAGS_test_flag_04.Help(), "test flag 04");
-  EXPECT_TRUE(absl::EndsWith(FLAGS_test_flag_04.Filename(), expected_file_name))
-      << FLAGS_test_flag_04.Filename();
-
-  EXPECT_EQ(FLAGS_test_flag_05.Name(), "test_flag_05");
-  EXPECT_EQ(FLAGS_test_flag_05.Help(), "test flag 05");
-  EXPECT_TRUE(absl::EndsWith(FLAGS_test_flag_05.Filename(), expected_file_name))
-      << FLAGS_test_flag_05.Filename();
-
-  EXPECT_EQ(FLAGS_test_flag_06.Name(), "test_flag_06");
-  EXPECT_EQ(FLAGS_test_flag_06.Help(), "test flag 06");
-  EXPECT_TRUE(absl::EndsWith(FLAGS_test_flag_06.Filename(), expected_file_name))
-      << FLAGS_test_flag_06.Filename();
-
-  EXPECT_EQ(FLAGS_test_flag_07.Name(), "test_flag_07");
-  EXPECT_EQ(FLAGS_test_flag_07.Help(), "test flag 07");
-  EXPECT_TRUE(absl::EndsWith(FLAGS_test_flag_07.Filename(), expected_file_name))
-      << FLAGS_test_flag_07.Filename();
-
-  EXPECT_EQ(FLAGS_test_flag_08.Name(), "test_flag_08");
-  EXPECT_EQ(FLAGS_test_flag_08.Help(), "test flag 08");
-  EXPECT_TRUE(absl::EndsWith(FLAGS_test_flag_08.Filename(), expected_file_name))
-      << FLAGS_test_flag_08.Filename();
-
-  EXPECT_EQ(FLAGS_test_flag_09.Name(), "test_flag_09");
-  EXPECT_EQ(FLAGS_test_flag_09.Help(), "test flag 09");
-  EXPECT_TRUE(absl::EndsWith(FLAGS_test_flag_09.Filename(), expected_file_name))
-      << FLAGS_test_flag_09.Filename();
-
-  EXPECT_EQ(FLAGS_test_flag_10.Name(), "test_flag_10");
-  EXPECT_EQ(FLAGS_test_flag_10.Help(), "test flag 10");
-  EXPECT_TRUE(absl::EndsWith(FLAGS_test_flag_10.Filename(), expected_file_name))
-      << FLAGS_test_flag_10.Filename();
-
-  EXPECT_EQ(FLAGS_test_flag_11.Name(), "test_flag_11");
-  EXPECT_EQ(FLAGS_test_flag_11.Help(), "test flag 11");
-  EXPECT_TRUE(absl::EndsWith(FLAGS_test_flag_11.Filename(), expected_file_name))
-      << FLAGS_test_flag_11.Filename();
-}
-#endif  // !ABSL_FLAGS_STRIP_NAMES
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_01).Name(),
+            "test_flag_01");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_01).Help(),
+            "test flag 01");
+  EXPECT_TRUE(absl::EndsWith(
+      absl::GetFlagReflectionHandle(FLAGS_test_flag_01).Filename(),
+      expected_file_name))
+      << absl::GetFlagReflectionHandle(FLAGS_test_flag_01).Filename();
+
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_02).Name(),
+            "test_flag_02");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_02).Help(),
+            "test flag 02");
+  EXPECT_TRUE(absl::EndsWith(
+      absl::GetFlagReflectionHandle(FLAGS_test_flag_02).Filename(),
+      expected_file_name))
+      << absl::GetFlagReflectionHandle(FLAGS_test_flag_02).Filename();
+
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_03).Name(),
+            "test_flag_03");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_03).Help(),
+            "test flag 03");
+  EXPECT_TRUE(absl::EndsWith(
+      absl::GetFlagReflectionHandle(FLAGS_test_flag_03).Filename(),
+      expected_file_name))
+      << absl::GetFlagReflectionHandle(FLAGS_test_flag_03).Filename();
+
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_04).Name(),
+            "test_flag_04");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_04).Help(),
+            "test flag 04");
+  EXPECT_TRUE(absl::EndsWith(
+      absl::GetFlagReflectionHandle(FLAGS_test_flag_04).Filename(),
+      expected_file_name))
+      << absl::GetFlagReflectionHandle(FLAGS_test_flag_04).Filename();
+
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_05).Name(),
+            "test_flag_05");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_05).Help(),
+            "test flag 05");
+  EXPECT_TRUE(absl::EndsWith(
+      absl::GetFlagReflectionHandle(FLAGS_test_flag_05).Filename(),
+      expected_file_name))
+      << absl::GetFlagReflectionHandle(FLAGS_test_flag_05).Filename();
+
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_06).Name(),
+            "test_flag_06");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_06).Help(),
+            "test flag 06");
+  EXPECT_TRUE(absl::EndsWith(
+      absl::GetFlagReflectionHandle(FLAGS_test_flag_06).Filename(),
+      expected_file_name))
+      << absl::GetFlagReflectionHandle(FLAGS_test_flag_06).Filename();
+
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_07).Name(),
+            "test_flag_07");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_07).Help(),
+            "test flag 07");
+  EXPECT_TRUE(absl::EndsWith(
+      absl::GetFlagReflectionHandle(FLAGS_test_flag_07).Filename(),
+      expected_file_name))
+      << absl::GetFlagReflectionHandle(FLAGS_test_flag_07).Filename();
+
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_08).Name(),
+            "test_flag_08");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_08).Help(),
+            "test flag 08");
+  EXPECT_TRUE(absl::EndsWith(
+      absl::GetFlagReflectionHandle(FLAGS_test_flag_08).Filename(),
+      expected_file_name))
+      << absl::GetFlagReflectionHandle(FLAGS_test_flag_08).Filename();
+
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_09).Name(),
+            "test_flag_09");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_09).Help(),
+            "test flag 09");
+  EXPECT_TRUE(absl::EndsWith(
+      absl::GetFlagReflectionHandle(FLAGS_test_flag_09).Filename(),
+      expected_file_name))
+      << absl::GetFlagReflectionHandle(FLAGS_test_flag_09).Filename();
+
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_10).Name(),
+            "test_flag_10");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_10).Help(),
+            "test flag 10");
+  EXPECT_TRUE(absl::EndsWith(
+      absl::GetFlagReflectionHandle(FLAGS_test_flag_10).Filename(),
+      expected_file_name))
+      << absl::GetFlagReflectionHandle(FLAGS_test_flag_10).Filename();
+
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_11).Name(),
+            "test_flag_11");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_11).Help(),
+            "test flag 11");
+  EXPECT_TRUE(absl::EndsWith(
+      absl::GetFlagReflectionHandle(FLAGS_test_flag_11).Filename(),
+      expected_file_name))
+      << absl::GetFlagReflectionHandle(FLAGS_test_flag_11).Filename();
+
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_12).Name(),
+            "test_flag_12");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_12).Help(),
+            "test flag 12");
+  EXPECT_TRUE(absl::EndsWith(
+      absl::GetFlagReflectionHandle(FLAGS_test_flag_12).Filename(),
+      expected_file_name))
+      << absl::GetFlagReflectionHandle(FLAGS_test_flag_12).Filename();
+
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_13).Name(),
+            "test_flag_13");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_13).Help(),
+            "test flag 13");
+  EXPECT_TRUE(absl::EndsWith(
+      absl::GetFlagReflectionHandle(FLAGS_test_flag_13).Filename(),
+      expected_file_name))
+      << absl::GetFlagReflectionHandle(FLAGS_test_flag_13).Filename();
+
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_14).Name(),
+            "test_flag_14");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_14).Help(),
+            "test flag 14");
+  EXPECT_TRUE(absl::EndsWith(
+      absl::GetFlagReflectionHandle(FLAGS_test_flag_14).Filename(),
+      expected_file_name))
+      << absl::GetFlagReflectionHandle(FLAGS_test_flag_14).Filename();
+}
 
 // --------------------------------------------------------------------
 
 TEST_F(FlagTest, TestDefault) {
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_01).DefaultValue(),
+            "true");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_02).DefaultValue(),
+            "1234");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_03).DefaultValue(),
+            "-34");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_04).DefaultValue(),
+            "189");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_05).DefaultValue(),
+            "10765");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_06).DefaultValue(),
+            "40000");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_07).DefaultValue(),
+            "-1234567");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_08).DefaultValue(),
+            "9876543");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_09).DefaultValue(),
+            "-9.876e-50");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_10).DefaultValue(),
+            "1.234e+12");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_11).DefaultValue(),
+            "");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_12).DefaultValue(),
+            "10m");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_13).DefaultValue(),
+            "-18446744073709551616");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_14).DefaultValue(),
+            "1152827684197027293");
+
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_01).CurrentValue(),
+            "true");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_02).CurrentValue(),
+            "1234");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_03).CurrentValue(),
+            "-34");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_04).CurrentValue(),
+            "189");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_05).CurrentValue(),
+            "10765");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_06).CurrentValue(),
+            "40000");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_07).CurrentValue(),
+            "-1234567");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_08).CurrentValue(),
+            "9876543");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_09).CurrentValue(),
+            "-9.876e-50");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_10).CurrentValue(),
+            "1.234e+12");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_11).CurrentValue(),
+            "");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_12).CurrentValue(),
+            "10m");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_13).CurrentValue(),
+            "-18446744073709551616");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_14).CurrentValue(),
+            "1152827684197027293");
+
   EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_01), true);
   EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_02), 1234);
   EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_03), -34);
@@ -231,6 +507,73 @@ TEST_F(FlagTest, TestDefault) {
   EXPECT_NEAR(absl::GetFlag(FLAGS_test_flag_09), -9.876e-50, 1e-55);
   EXPECT_NEAR(absl::GetFlag(FLAGS_test_flag_10), 1.234e12f, 1e5f);
   EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_11), "");
+  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_12), absl::Minutes(10));
+  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_13), absl::MakeInt128(-1, 0));
+  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_14),
+            absl::MakeUint128(0, 0xFFFAAABBBCCCDDD));
+}
+
+// --------------------------------------------------------------------
+
+struct NonTriviallyCopyableAggregate {
+  NonTriviallyCopyableAggregate() = default;
+  // NOLINTNEXTLINE
+  NonTriviallyCopyableAggregate(const NonTriviallyCopyableAggregate& rhs)
+      : value(rhs.value) {}
+  // NOLINTNEXTLINE
+  NonTriviallyCopyableAggregate& operator=(
+      const NonTriviallyCopyableAggregate& rhs) {
+    value = rhs.value;
+    return *this;
+  }
+
+  int value;
+};
+bool AbslParseFlag(absl::string_view src, NonTriviallyCopyableAggregate* f,
+                   std::string* e) {
+  return absl::ParseFlag(src, &f->value, e);
+}
+std::string AbslUnparseFlag(const NonTriviallyCopyableAggregate& ntc) {
+  return absl::StrCat(ntc.value);
+}
+
+bool operator==(const NonTriviallyCopyableAggregate& ntc1,
+                const NonTriviallyCopyableAggregate& ntc2) {
+  return ntc1.value == ntc2.value;
+}
+
+}  // namespace
+
+ABSL_FLAG(bool, test_flag_eb_01, {}, "");
+ABSL_FLAG(int32_t, test_flag_eb_02, {}, "");
+ABSL_FLAG(int64_t, test_flag_eb_03, {}, "");
+ABSL_FLAG(double, test_flag_eb_04, {}, "");
+ABSL_FLAG(std::string, test_flag_eb_05, {}, "");
+ABSL_FLAG(NonTriviallyCopyableAggregate, test_flag_eb_06, {}, "");
+
+namespace {
+
+TEST_F(FlagTest, TestEmptyBracesDefault) {
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_eb_01).DefaultValue(),
+            "false");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_eb_02).DefaultValue(),
+            "0");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_eb_03).DefaultValue(),
+            "0");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_eb_04).DefaultValue(),
+            "0");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_eb_05).DefaultValue(),
+            "");
+  EXPECT_EQ(absl::GetFlagReflectionHandle(FLAGS_test_flag_eb_06).DefaultValue(),
+            "0");
+
+  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_eb_01), false);
+  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_eb_02), 0);
+  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_eb_03), 0);
+  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_eb_04), 0.0);
+  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_eb_05), "");
+  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_eb_06),
+            NonTriviallyCopyableAggregate{});
 }
 
 // --------------------------------------------------------------------
@@ -268,6 +611,93 @@ TEST_F(FlagTest, TestGetSet) {
 
   absl::SetFlag(&FLAGS_test_flag_11, "asdf");
   EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_11), "asdf");
+
+  absl::SetFlag(&FLAGS_test_flag_12, absl::Seconds(110));
+  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_12), absl::Seconds(110));
+
+  absl::SetFlag(&FLAGS_test_flag_13, absl::MakeInt128(-1, 0));
+  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_13), absl::MakeInt128(-1, 0));
+
+  absl::SetFlag(&FLAGS_test_flag_14, absl::MakeUint128(0, 0xFFFAAABBBCCCDDD));
+  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_14),
+            absl::MakeUint128(0, 0xFFFAAABBBCCCDDD));
+}
+
+// --------------------------------------------------------------------
+
+TEST_F(FlagTest, TestGetViaReflection) {
+#if ABSL_FLAGS_STRIP_NAMES
+  GTEST_SKIP() << "This test requires flag names to be present";
+#endif
+  auto* handle = absl::FindCommandLineFlag("test_flag_01");
+  EXPECT_EQ(*handle->TryGet<bool>(), true);
+  handle = absl::FindCommandLineFlag("test_flag_02");
+  EXPECT_EQ(*handle->TryGet<int>(), 1234);
+  handle = absl::FindCommandLineFlag("test_flag_03");
+  EXPECT_EQ(*handle->TryGet<int16_t>(), -34);
+  handle = absl::FindCommandLineFlag("test_flag_04");
+  EXPECT_EQ(*handle->TryGet<uint16_t>(), 189);
+  handle = absl::FindCommandLineFlag("test_flag_05");
+  EXPECT_EQ(*handle->TryGet<int32_t>(), 10765);
+  handle = absl::FindCommandLineFlag("test_flag_06");
+  EXPECT_EQ(*handle->TryGet<uint32_t>(), 40000);
+  handle = absl::FindCommandLineFlag("test_flag_07");
+  EXPECT_EQ(*handle->TryGet<int64_t>(), -1234567);
+  handle = absl::FindCommandLineFlag("test_flag_08");
+  EXPECT_EQ(*handle->TryGet<uint64_t>(), 9876543);
+  handle = absl::FindCommandLineFlag("test_flag_09");
+  EXPECT_NEAR(*handle->TryGet<double>(), -9.876e-50, 1e-55);
+  handle = absl::FindCommandLineFlag("test_flag_10");
+  EXPECT_NEAR(*handle->TryGet<float>(), 1.234e12f, 1e5f);
+  handle = absl::FindCommandLineFlag("test_flag_11");
+  EXPECT_EQ(*handle->TryGet<std::string>(), "");
+  handle = absl::FindCommandLineFlag("test_flag_12");
+  EXPECT_EQ(*handle->TryGet<absl::Duration>(), absl::Minutes(10));
+  handle = absl::FindCommandLineFlag("test_flag_13");
+  EXPECT_EQ(*handle->TryGet<absl::int128>(), absl::MakeInt128(-1, 0));
+  handle = absl::FindCommandLineFlag("test_flag_14");
+  EXPECT_EQ(*handle->TryGet<absl::uint128>(),
+            absl::MakeUint128(0, 0xFFFAAABBBCCCDDD));
+}
+
+// --------------------------------------------------------------------
+
+TEST_F(FlagTest, ConcurrentSetAndGet) {
+#if ABSL_FLAGS_STRIP_NAMES
+  GTEST_SKIP() << "This test requires flag names to be present";
+#endif
+  static constexpr int kNumThreads = 8;
+  // Two arbitrary durations. One thread will concurrently flip the flag
+  // between these two values, while the other threads read it and verify
+  // that no other value is seen.
+  static const absl::Duration kValidDurations[] = {
+      absl::Seconds(int64_t{0x6cebf47a9b68c802}) + absl::Nanoseconds(229702057),
+      absl::Seconds(int64_t{0x23fec0307e4e9d3}) + absl::Nanoseconds(44555374)};
+  absl::SetFlag(&FLAGS_test_flag_12, kValidDurations[0]);
+
+  std::atomic<bool> stop{false};
+  std::vector<std::thread> threads;
+  auto* handle = absl::FindCommandLineFlag("test_flag_12");
+  for (int i = 0; i < kNumThreads; i++) {
+    threads.emplace_back([&]() {
+      while (!stop.load(std::memory_order_relaxed)) {
+        // Try loading the flag both directly and via a reflection
+        // handle.
+        absl::Duration v = absl::GetFlag(FLAGS_test_flag_12);
+        EXPECT_TRUE(v == kValidDurations[0] || v == kValidDurations[1]);
+        v = *handle->TryGet<absl::Duration>();
+        EXPECT_TRUE(v == kValidDurations[0] || v == kValidDurations[1]);
+      }
+    });
+  }
+  absl::Time end_time = absl::Now() + absl::Seconds(1);
+  size_t i = 0;
+  while (absl::Now() < end_time) {
+    absl::SetFlag(&FLAGS_test_flag_12,
+                  kValidDurations[i++ % std::size(kValidDurations)]);
+  }
+  stop.store(true, std::memory_order_relaxed);
+  for (auto& t : threads) t.join();
 }
 
 // --------------------------------------------------------------------
@@ -276,28 +706,33 @@ int GetDflt1() { return 1; }
 
 }  // namespace
 
-ABSL_FLAG(int, test_flag_12, GetDflt1(), "test flag 12");
-ABSL_FLAG(std::string, test_flag_13, absl::StrCat("AAA", "BBB"),
-          "test flag 13");
+ABSL_FLAG(int, test_int_flag_with_non_const_default, GetDflt1(),
+          "test int flag non const default");
+ABSL_FLAG(std::string, test_string_flag_with_non_const_default,
+          absl::StrCat("AAA", "BBB"), "test string flag non const default");
 
 namespace {
 
 TEST_F(FlagTest, TestNonConstexprDefault) {
-  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_12), 1);
-  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_13), "AAABBB");
+  EXPECT_EQ(absl::GetFlag(FLAGS_test_int_flag_with_non_const_default), 1);
+  EXPECT_EQ(absl::GetFlag(FLAGS_test_string_flag_with_non_const_default),
+            "AAABBB");
 }
 
 // --------------------------------------------------------------------
 
 }  // namespace
 
-ABSL_FLAG(bool, test_flag_14, true, absl::StrCat("test ", "flag ", "14"));
+ABSL_FLAG(bool, test_flag_with_non_const_help, true,
+          absl::StrCat("test ", "flag ", "non const help"));
 
 namespace {
 
 #if !ABSL_FLAGS_STRIP_HELP
 TEST_F(FlagTest, TestNonConstexprHelp) {
-  EXPECT_EQ(FLAGS_test_flag_14.Help(), "test flag 14");
+  EXPECT_EQ(
+      absl::GetFlagReflectionHandle(FLAGS_test_flag_with_non_const_help).Help(),
+      "test flag non const help");
 }
 #endif  //! ABSL_FLAGS_STRIP_HELP
 
@@ -363,34 +798,43 @@ std::string AbslUnparseFlag(const CustomUDT& f) {
 
 }  // namespace
 
-ABSL_FLAG(CustomUDT, test_flag_15, CustomUDT(), "test flag 15");
+ABSL_FLAG(CustomUDT, test_flag_custom_udt, CustomUDT(), "test flag custom UDT");
 
 namespace {
 
 TEST_F(FlagTest, TestCustomUDT) {
-  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_15), CustomUDT(1, 1));
-  absl::SetFlag(&FLAGS_test_flag_15, CustomUDT(2, 3));
-  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_15), CustomUDT(2, 3));
+  EXPECT_EQ(flags::StorageKind<CustomUDT>(),
+            flags::FlagValueStorageKind::kOneWordAtomic);
+  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_custom_udt), CustomUDT(1, 1));
+  absl::SetFlag(&FLAGS_test_flag_custom_udt, CustomUDT(2, 3));
+  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_custom_udt), CustomUDT(2, 3));
 }
 
 // MSVC produces link error on the type mismatch.
 // Linux does not have build errors and validations work as expected.
-#if 0  // !defined(_WIN32) && GTEST_HAS_DEATH_TEST
+#if !defined(_WIN32) && GTEST_HAS_DEATH_TEST
+using FlagDeathTest = FlagTest;
+
+TEST_F(FlagDeathTest, TestTypeMismatchValidations) {
+#if ABSL_FLAGS_STRIP_NAMES
+  GTEST_SKIP() << "This test requires flag names to be present";
+#endif
+#if !defined(NDEBUG)
+  EXPECT_DEATH_IF_SUPPORTED(
+      static_cast<void>(absl::GetFlag(FLAGS_mistyped_int_flag)),
+      "Flag 'mistyped_int_flag' is defined as one type and declared "
+      "as another");
+  EXPECT_DEATH_IF_SUPPORTED(
+      static_cast<void>(absl::GetFlag(FLAGS_mistyped_string_flag)),
+      "Flag 'mistyped_string_flag' is defined as one type and "
+      "declared as another");
+#endif
 
-TEST(Flagtest, TestTypeMismatchValidations) {
-  // For builtin types, GetFlag() only does validation in debug mode.
-  EXPECT_DEBUG_DEATH(
-      absl::GetFlag(FLAGS_mistyped_int_flag),
+  EXPECT_DEATH_IF_SUPPORTED(
+      absl::SetFlag(&FLAGS_mistyped_int_flag, 1),
       "Flag 'mistyped_int_flag' is defined as one type and declared "
       "as another");
-  EXPECT_DEATH(absl::SetFlag(&FLAGS_mistyped_int_flag, 0),
-               "Flag 'mistyped_int_flag' is defined as one type and declared "
-               "as another");
-
-  EXPECT_DEATH(absl::GetFlag(FLAGS_mistyped_string_flag),
-               "Flag 'mistyped_string_flag' is defined as one type and "
-               "declared as another");
-  EXPECT_DEATH(
+  EXPECT_DEATH_IF_SUPPORTED(
       absl::SetFlag(&FLAGS_mistyped_string_flag, std::vector<std::string>{}),
       "Flag 'mistyped_string_flag' is defined as one type and declared as "
       "another");
@@ -428,16 +872,17 @@ std::string AbslUnparseFlag(const ConversionTestVal& val) {
 
 // Flag default values can be specified with a value that converts to the flag
 // value type implicitly.
-ABSL_FLAG(ConversionTestVal, test_flag_16,
-          ConversionTestVal::ViaImplicitConv::kTen, "test flag 16");
+ABSL_FLAG(ConversionTestVal, test_flag_implicit_conv,
+          ConversionTestVal::ViaImplicitConv::kTen,
+          "test flag init via implicit conversion");
 
 namespace {
 
 TEST_F(FlagTest, CanSetViaImplicitConversion) {
-  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_16).a, 10);
-  absl::SetFlag(&FLAGS_test_flag_16,
+  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_implicit_conv).a, 10);
+  absl::SetFlag(&FLAGS_test_flag_implicit_conv,
                 ConversionTestVal::ViaImplicitConv::kEleven);
-  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_16).a, 11);
+  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_implicit_conv).a, 11);
 }
 
 // --------------------------------------------------------------------
@@ -482,25 +927,456 @@ TEST_F(FlagTest, TestNonDefaultConstructibleType) {
   EXPECT_EQ(absl::GetFlag(FLAGS_ndc_flag2).value, 25);
 }
 
-// --------------------------------------------------------------------
-
 }  // namespace
 
+// --------------------------------------------------------------------
+
 ABSL_RETIRED_FLAG(bool, old_bool_flag, true, "old descr");
 ABSL_RETIRED_FLAG(int, old_int_flag, (int)std::sqrt(10), "old descr");
 ABSL_RETIRED_FLAG(std::string, old_str_flag, "", absl::StrCat("old ", "descr"));
 
 namespace {
 
+bool initialization_order_fiasco_test ABSL_ATTRIBUTE_UNUSED = [] {
+  // Iterate over all the flags during static initialization.
+  // This should not trigger ASan's initialization-order-fiasco.
+  auto* handle1 = absl::FindCommandLineFlag("flag_on_separate_file");
+  auto* handle2 = absl::FindCommandLineFlag("retired_flag_on_separate_file");
+  if (handle1 != nullptr && handle2 != nullptr) {
+    return handle1->Name() == handle2->Name();
+  }
+  return true;
+}();
+
 TEST_F(FlagTest, TestRetiredFlagRegistration) {
-  bool is_bool = false;
-  EXPECT_TRUE(flags::IsRetiredFlag("old_bool_flag", &is_bool));
-  EXPECT_TRUE(is_bool);
-  EXPECT_TRUE(flags::IsRetiredFlag("old_int_flag", &is_bool));
-  EXPECT_FALSE(is_bool);
-  EXPECT_TRUE(flags::IsRetiredFlag("old_str_flag", &is_bool));
-  EXPECT_FALSE(is_bool);
-  EXPECT_FALSE(flags::IsRetiredFlag("some_other_flag", &is_bool));
+  auto* handle = absl::FindCommandLineFlag("old_bool_flag");
+  EXPECT_TRUE(handle->IsOfType<bool>());
+  EXPECT_TRUE(handle->IsRetired());
+  handle = absl::FindCommandLineFlag("old_int_flag");
+  EXPECT_TRUE(handle->IsOfType<int>());
+  EXPECT_TRUE(handle->IsRetired());
+  handle = absl::FindCommandLineFlag("old_str_flag");
+  EXPECT_TRUE(handle->IsOfType<std::string>());
+  EXPECT_TRUE(handle->IsRetired());
+}
+
+}  // namespace
+
+// --------------------------------------------------------------------
+
+namespace {
+
+// User-defined type with small alignment, but size exceeding 16.
+struct SmallAlignUDT {
+  SmallAlignUDT() : c('A'), s(12) {}
+  char c;
+  int16_t s;
+  char bytes[14];
+};
+
+bool AbslParseFlag(absl::string_view, SmallAlignUDT*, std::string*) {
+  return true;
+}
+std::string AbslUnparseFlag(const SmallAlignUDT&) { return ""; }
+
+}  // namespace
+
+ABSL_FLAG(SmallAlignUDT, test_flag_sa_udt, {}, "help");
+
+namespace {
+
+TEST_F(FlagTest, TestSmallAlignUDT) {
+  EXPECT_EQ(flags::StorageKind<SmallAlignUDT>(),
+            flags::FlagValueStorageKind::kSequenceLocked);
+  SmallAlignUDT value = absl::GetFlag(FLAGS_test_flag_sa_udt);
+  EXPECT_EQ(value.c, 'A');
+  EXPECT_EQ(value.s, 12);
+
+  value.c = 'B';
+  value.s = 45;
+  absl::SetFlag(&FLAGS_test_flag_sa_udt, value);
+  value = absl::GetFlag(FLAGS_test_flag_sa_udt);
+  EXPECT_EQ(value.c, 'B');
+  EXPECT_EQ(value.s, 45);
+}
+}  // namespace
+
+// --------------------------------------------------------------------
+
+namespace {
+
+// User-defined not trivially copyable type.
+template <int id>
+struct NonTriviallyCopyableUDT {
+  NonTriviallyCopyableUDT() : c('A') { s_num_instance++; }
+  NonTriviallyCopyableUDT(const NonTriviallyCopyableUDT& rhs) : c(rhs.c) {
+    s_num_instance++;
+  }
+  NonTriviallyCopyableUDT& operator=(const NonTriviallyCopyableUDT& rhs) {
+    c = rhs.c;
+    return *this;
+  }
+  ~NonTriviallyCopyableUDT() { s_num_instance--; }
+
+  static uint64_t s_num_instance;
+  char c;
+};
+
+template <int id>
+uint64_t NonTriviallyCopyableUDT<id>::s_num_instance = 0;
+
+template <int id>
+bool AbslParseFlag(absl::string_view txt, NonTriviallyCopyableUDT<id>* f,
+                   std::string*) {
+  f->c = txt.empty() ? '\0' : txt[0];
+  return true;
+}
+template <int id>
+std::string AbslUnparseFlag(const NonTriviallyCopyableUDT<id>&) {
+  return "";
+}
+
+template <int id, typename F>
+void TestExpectedLeaks(
+    F&& f, uint64_t num_leaks,
+    std::optional<uint64_t> num_new_instances = std::nullopt) {
+  if (!num_new_instances.has_value()) num_new_instances = num_leaks;
+
+  auto num_leaked_before = flags::NumLeakedFlagValues();
+  auto num_instances_before = NonTriviallyCopyableUDT<id>::s_num_instance;
+  f();
+  EXPECT_EQ(num_leaked_before + num_leaks, flags::NumLeakedFlagValues());
+  EXPECT_EQ(num_instances_before + num_new_instances.value(),
+            NonTriviallyCopyableUDT<id>::s_num_instance);
+}
+}  // namespace
+
+ABSL_FLAG(NonTriviallyCopyableUDT<1>, test_flag_ntc_udt1, {}, "help");
+ABSL_FLAG(NonTriviallyCopyableUDT<2>, test_flag_ntc_udt2, {}, "help");
+ABSL_FLAG(NonTriviallyCopyableUDT<3>, test_flag_ntc_udt3, {}, "help");
+ABSL_FLAG(NonTriviallyCopyableUDT<4>, test_flag_ntc_udt4, {}, "help");
+ABSL_FLAG(NonTriviallyCopyableUDT<5>, test_flag_ntc_udt5, {}, "help");
+
+namespace {
+
+TEST_F(FlagTest, TestNonTriviallyCopyableGetSetSet) {
+  EXPECT_EQ(flags::StorageKind<NonTriviallyCopyableUDT<1>>(),
+            flags::FlagValueStorageKind::kHeapAllocated);
+
+  TestExpectedLeaks<1>(
+      [&] {
+        NonTriviallyCopyableUDT<1> value =
+            absl::GetFlag(FLAGS_test_flag_ntc_udt1);
+        EXPECT_EQ(value.c, 'A');
+      },
+      0);
+
+  TestExpectedLeaks<1>(
+      [&] {
+        NonTriviallyCopyableUDT<1> value;
+        value.c = 'B';
+        absl::SetFlag(&FLAGS_test_flag_ntc_udt1, value);
+        EXPECT_EQ(value.c, 'B');
+      },
+      1);
+
+  TestExpectedLeaks<1>(
+      [&] {
+        NonTriviallyCopyableUDT<1> value;
+        value.c = 'C';
+        absl::SetFlag(&FLAGS_test_flag_ntc_udt1, value);
+      },
+      0);
+}
+
+TEST_F(FlagTest, TestNonTriviallyCopyableParseSet) {
+  TestExpectedLeaks<2>(
+      [&] {
+        const char* in_argv[] = {"testbin", "--test_flag_ntc_udt2=A"};
+        absl::ParseCommandLine(2, const_cast<char**>(in_argv));
+      },
+      0);
+
+  TestExpectedLeaks<2>(
+      [&] {
+        NonTriviallyCopyableUDT<2> value;
+        value.c = 'B';
+        absl::SetFlag(&FLAGS_test_flag_ntc_udt2, value);
+        EXPECT_EQ(value.c, 'B');
+      },
+      0);
+}
+
+TEST_F(FlagTest, TestNonTriviallyCopyableSet) {
+  TestExpectedLeaks<3>(
+      [&] {
+        NonTriviallyCopyableUDT<3> value;
+        value.c = 'B';
+        absl::SetFlag(&FLAGS_test_flag_ntc_udt3, value);
+        EXPECT_EQ(value.c, 'B');
+      },
+      0);
+}
+
+// One new instance created during initialization and stored in the flag.
+auto premain_utd4_get =
+    (TestExpectedLeaks<4>([] { (void)absl::GetFlag(FLAGS_test_flag_ntc_udt4); },
+                          0, 1),
+     false);
+
+TEST_F(FlagTest, TestNonTriviallyCopyableGetBeforeMainParseGet) {
+  TestExpectedLeaks<4>(
+      [&] {
+        const char* in_argv[] = {"testbin", "--test_flag_ntc_udt4=C"};
+        absl::ParseCommandLine(2, const_cast<char**>(in_argv));
+      },
+      1);
+
+  TestExpectedLeaks<4>(
+      [&] {
+        NonTriviallyCopyableUDT<4> value =
+            absl::GetFlag(FLAGS_test_flag_ntc_udt4);
+        EXPECT_EQ(value.c, 'C');
+      },
+      0);
+}
+
+// One new instance created during initialization, which is reused since it was
+// never read.
+auto premain_utd5_set = (TestExpectedLeaks<5>(
+                             [] {
+                               NonTriviallyCopyableUDT<5> value;
+                               value.c = 'B';
+                               absl::SetFlag(&FLAGS_test_flag_ntc_udt5, value);
+                             },
+                             0, 1),
+                         false);
+
+TEST_F(FlagTest, TestNonTriviallyCopyableSetParseGet) {
+  TestExpectedLeaks<5>(
+      [&] {
+        const char* in_argv[] = {"testbin", "--test_flag_ntc_udt5=C"};
+        absl::ParseCommandLine(2, const_cast<char**>(in_argv));
+      },
+      0);
+
+  TestExpectedLeaks<5>(
+      [&] {
+        NonTriviallyCopyableUDT<5> value =
+            absl::GetFlag(FLAGS_test_flag_ntc_udt5);
+        EXPECT_EQ(value.c, 'C');
+      },
+      0);
+}
+
+}  // namespace
+
+// --------------------------------------------------------------------
+
+namespace {
+
+enum TestE { A = 1, B = 2, C = 3 };
+
+struct EnumWrapper {
+  EnumWrapper() : e(A) {}
+
+  TestE e;
+};
+
+bool AbslParseFlag(absl::string_view, EnumWrapper*, std::string*) {
+  return true;
+}
+std::string AbslUnparseFlag(const EnumWrapper&) { return ""; }
+
+}  // namespace
+
+ABSL_FLAG(EnumWrapper, test_enum_wrapper_flag, {}, "help");
+
+TEST_F(FlagTest, TesTypeWrappingEnum) {
+  EnumWrapper value = absl::GetFlag(FLAGS_test_enum_wrapper_flag);
+  EXPECT_EQ(value.e, A);
+
+  value.e = B;
+  absl::SetFlag(&FLAGS_test_enum_wrapper_flag, value);
+  value = absl::GetFlag(FLAGS_test_enum_wrapper_flag);
+  EXPECT_EQ(value.e, B);
+}
+
+// This is a compile test to ensure macros are expanded within ABSL_FLAG and
+// ABSL_DECLARE_FLAG.
+#define FLAG_NAME_MACRO(name) prefix_##name
+ABSL_DECLARE_FLAG(int, FLAG_NAME_MACRO(test_macro_named_flag));
+ABSL_FLAG(int, FLAG_NAME_MACRO(test_macro_named_flag), 0,
+          "Testing macro expansion within ABSL_FLAG");
+
+TEST_F(FlagTest, MacroWithinAbslFlag) {
+  EXPECT_EQ(absl::GetFlag(FLAGS_prefix_test_macro_named_flag), 0);
+  absl::SetFlag(&FLAGS_prefix_test_macro_named_flag, 1);
+  EXPECT_EQ(absl::GetFlag(FLAGS_prefix_test_macro_named_flag), 1);
+}
+
+// --------------------------------------------------------------------
+
+ABSL_FLAG(std::optional<bool>, optional_bool, std::nullopt, "help");
+ABSL_FLAG(std::optional<int>, optional_int, {}, "help");
+ABSL_FLAG(std::optional<double>, optional_double, 9.3, "help");
+ABSL_FLAG(std::optional<std::string>, optional_string, std::nullopt, "help");
+ABSL_FLAG(std::optional<absl::Duration>, optional_duration, std::nullopt,
+          "help");
+ABSL_FLAG(std::optional<std::optional<int>>, optional_optional_int,
+          std::nullopt, "help");
+#if defined(ABSL_HAVE_STD_OPTIONAL) && !defined(ABSL_USES_STD_OPTIONAL)
+ABSL_FLAG(std::optional<int64_t>, std_optional_int64, std::nullopt, "help");
+#endif
+
+namespace {
+
+TEST_F(FlagTest, TestOptionalBool) {
+  EXPECT_FALSE(absl::GetFlag(FLAGS_optional_bool).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_optional_bool), std::nullopt);
+
+  absl::SetFlag(&FLAGS_optional_bool, false);
+  EXPECT_TRUE(absl::GetFlag(FLAGS_optional_bool).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_optional_bool), false);
+
+  absl::SetFlag(&FLAGS_optional_bool, true);
+  EXPECT_TRUE(absl::GetFlag(FLAGS_optional_bool).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_optional_bool), true);
+
+  absl::SetFlag(&FLAGS_optional_bool, std::nullopt);
+  EXPECT_FALSE(absl::GetFlag(FLAGS_optional_bool).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_optional_bool), std::nullopt);
+}
+
+// --------------------------------------------------------------------
+
+TEST_F(FlagTest, TestOptionalInt) {
+  EXPECT_FALSE(absl::GetFlag(FLAGS_optional_int).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_optional_int), std::nullopt);
+
+  absl::SetFlag(&FLAGS_optional_int, 0);
+  EXPECT_TRUE(absl::GetFlag(FLAGS_optional_int).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_optional_int), 0);
+
+  absl::SetFlag(&FLAGS_optional_int, 10);
+  EXPECT_TRUE(absl::GetFlag(FLAGS_optional_int).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_optional_int), 10);
+
+  absl::SetFlag(&FLAGS_optional_int, std::nullopt);
+  EXPECT_FALSE(absl::GetFlag(FLAGS_optional_int).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_optional_int), std::nullopt);
+}
+
+// --------------------------------------------------------------------
+
+TEST_F(FlagTest, TestOptionalDouble) {
+  EXPECT_TRUE(absl::GetFlag(FLAGS_optional_double).has_value());
+  EXPECT_DOUBLE_EQ(*absl::GetFlag(FLAGS_optional_double), 9.3);
+
+  absl::SetFlag(&FLAGS_optional_double, 0.0);
+  EXPECT_TRUE(absl::GetFlag(FLAGS_optional_double).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_optional_double), 0.0);
+
+  absl::SetFlag(&FLAGS_optional_double, 1.234);
+  EXPECT_TRUE(absl::GetFlag(FLAGS_optional_double).has_value());
+  EXPECT_DOUBLE_EQ(*absl::GetFlag(FLAGS_optional_double), 1.234);
+
+  absl::SetFlag(&FLAGS_optional_double, std::nullopt);
+  EXPECT_FALSE(absl::GetFlag(FLAGS_optional_double).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_optional_double), std::nullopt);
+}
+
+// --------------------------------------------------------------------
+
+TEST_F(FlagTest, TestOptionalString) {
+  EXPECT_FALSE(absl::GetFlag(FLAGS_optional_string).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_optional_string), std::nullopt);
+
+  // Setting optional string to "" leads to undefined behavior.
+
+  absl::SetFlag(&FLAGS_optional_string, " ");
+  EXPECT_TRUE(absl::GetFlag(FLAGS_optional_string).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_optional_string), " ");
+
+  absl::SetFlag(&FLAGS_optional_string, "QWERTY");
+  EXPECT_TRUE(absl::GetFlag(FLAGS_optional_string).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_optional_string), "QWERTY");
+
+  absl::SetFlag(&FLAGS_optional_string, std::nullopt);
+  EXPECT_FALSE(absl::GetFlag(FLAGS_optional_string).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_optional_string), std::nullopt);
 }
 
+// --------------------------------------------------------------------
+
+TEST_F(FlagTest, TestOptionalDuration) {
+  EXPECT_FALSE(absl::GetFlag(FLAGS_optional_duration).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_optional_duration), std::nullopt);
+
+  absl::SetFlag(&FLAGS_optional_duration, absl::ZeroDuration());
+  EXPECT_TRUE(absl::GetFlag(FLAGS_optional_duration).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_optional_duration), absl::Seconds(0));
+
+  absl::SetFlag(&FLAGS_optional_duration, absl::Hours(3));
+  EXPECT_TRUE(absl::GetFlag(FLAGS_optional_duration).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_optional_duration), absl::Hours(3));
+
+  absl::SetFlag(&FLAGS_optional_duration, std::nullopt);
+  EXPECT_FALSE(absl::GetFlag(FLAGS_optional_duration).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_optional_duration), std::nullopt);
+}
+
+// --------------------------------------------------------------------
+
+TEST_F(FlagTest, TestOptionalOptional) {
+  EXPECT_FALSE(absl::GetFlag(FLAGS_optional_optional_int).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_optional_optional_int), std::nullopt);
+
+  std::optional<int> nullint{std::nullopt};
+
+  absl::SetFlag(&FLAGS_optional_optional_int, nullint);
+  EXPECT_TRUE(absl::GetFlag(FLAGS_optional_optional_int).has_value());
+  EXPECT_NE(absl::GetFlag(FLAGS_optional_optional_int), nullint);
+  EXPECT_EQ(absl::GetFlag(FLAGS_optional_optional_int),
+            std::optional<std::optional<int>>{nullint});
+
+  absl::SetFlag(&FLAGS_optional_optional_int, 0);
+  EXPECT_TRUE(absl::GetFlag(FLAGS_optional_optional_int).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_optional_optional_int), 0);
+
+  absl::SetFlag(&FLAGS_optional_optional_int, std::optional<int>{0});
+  EXPECT_TRUE(absl::GetFlag(FLAGS_optional_optional_int).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_optional_optional_int), 0);
+  EXPECT_EQ(absl::GetFlag(FLAGS_optional_optional_int), std::optional<int>{0});
+
+  absl::SetFlag(&FLAGS_optional_optional_int, std::nullopt);
+  EXPECT_FALSE(absl::GetFlag(FLAGS_optional_optional_int).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_optional_optional_int), std::nullopt);
+}
+
+// --------------------------------------------------------------------
+
+#if defined(ABSL_HAVE_STD_OPTIONAL) && !defined(ABSL_USES_STD_OPTIONAL)
+
+TEST_F(FlagTest, TestStdOptional) {
+  EXPECT_FALSE(absl::GetFlag(FLAGS_std_optional_int64).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_std_optional_int64), std::nullopt);
+
+  absl::SetFlag(&FLAGS_std_optional_int64, 0);
+  EXPECT_TRUE(absl::GetFlag(FLAGS_std_optional_int64).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_std_optional_int64), 0);
+
+  absl::SetFlag(&FLAGS_std_optional_int64, 0xFFFFFFFFFF16);
+  EXPECT_TRUE(absl::GetFlag(FLAGS_std_optional_int64).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_std_optional_int64), 0xFFFFFFFFFF16);
+
+  absl::SetFlag(&FLAGS_std_optional_int64, std::nullopt);
+  EXPECT_FALSE(absl::GetFlag(FLAGS_std_optional_int64).has_value());
+  EXPECT_EQ(absl::GetFlag(FLAGS_std_optional_int64), std::nullopt);
+}
+
+// --------------------------------------------------------------------
+
+#endif
+
 }  // namespace
diff --git a/absl/flags/flag_test_defs.cc b/absl/flags/flag_test_defs.cc
index 3366c580327..4e1693cdb9c 100644
--- a/absl/flags/flag_test_defs.cc
+++ b/absl/flags/flag_test_defs.cc
@@ -20,3 +20,5 @@
 
 ABSL_FLAG(int, mistyped_int_flag, 0, "");
 ABSL_FLAG(std::string, mistyped_string_flag, "", "");
+ABSL_FLAG(bool, flag_on_separate_file, false, "");
+ABSL_RETIRED_FLAG(bool, retired_flag_on_separate_file, false, "");
diff --git a/absl/flags/internal/commandlineflag.cc b/absl/flags/internal/commandlineflag.cc
index 53e2b84ecc1..3c114d1017d 100644
--- a/absl/flags/internal/commandlineflag.cc
+++ b/absl/flags/internal/commandlineflag.cc
@@ -1,5 +1,5 @@
 //
-// Copyright 2019 The Abseil Authors.
+// Copyright 2020 The Abseil Authors.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
@@ -15,380 +15,12 @@
 
 #include "absl/flags/internal/commandlineflag.h"
 
-#include <cassert>
-
-#include "absl/base/internal/raw_logging.h"
-#include "absl/base/optimization.h"
-#include "absl/flags/config.h"
-#include "absl/flags/usage_config.h"
-#include "absl/strings/str_cat.h"
-#include "absl/synchronization/mutex.h"
-
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace flags_internal {
 
-// The help message indicating that the commandline flag has been
-// 'stripped'. It will not show up when doing "-help" and its
-// variants. The flag is stripped if ABSL_FLAGS_STRIP_HELP is set to 1
-// before including absl/flags/flag.h
-
-// This is used by this file, and also in commandlineflags_reporting.cc
-const char kStrippedFlagHelp[] = "\001\002\003\004 (unknown) \004\003\002\001";
-
-namespace {
-
-// Currently we only validate flag values for user-defined flag types.
-bool ShouldValidateFlagValue(const CommandLineFlag& flag) {
-#define DONT_VALIDATE(T) \
-  if (flag.IsOfType<T>()) return false;
-  ABSL_FLAGS_INTERNAL_FOR_EACH_LOCK_FREE(DONT_VALIDATE)
-  DONT_VALIDATE(std::string)
-  DONT_VALIDATE(std::vector<std::string>)
-#undef DONT_VALIDATE
-
-  return true;
-}
-
-}  // namespace
-
-absl::Mutex* InitFlag(CommandLineFlag* flag) {
-  ABSL_CONST_INIT static absl::Mutex init_lock(absl::kConstInit);
-  absl::Mutex* mu;
-
-  {
-    absl::MutexLock lock(&init_lock);
-
-    if (flag->locks_ == nullptr) {  // Must initialize Mutexes for this flag.
-      flag->locks_ = new flags_internal::CommandLineFlagLocks;
-    }
-
-    mu = &flag->locks_->primary_mu;
-  }
-
-  {
-    absl::MutexLock lock(mu);
-
-    if (!flag->IsRetired() && flag->def_ == nullptr) {
-      // Need to initialize def and cur fields.
-      flag->def_ = (*flag->make_init_value_)();
-      flag->cur_ = Clone(flag->op_, flag->def_);
-      UpdateCopy(flag);
-      flag->inited_.store(true, std::memory_order_release);
-      flag->InvokeCallback();
-    }
-  }
-
-  flag->inited_.store(true, std::memory_order_release);
-  return mu;
-}
-
-// Ensure that the lazily initialized fields of *flag have been initialized,
-// and return &flag->locks_->primary_mu.
-absl::Mutex* CommandLineFlag::InitFlagIfNecessary() const
-    ABSL_LOCK_RETURNED(locks_->primary_mu) {
-  if (!inited_.load(std::memory_order_acquire)) {
-    return InitFlag(const_cast<CommandLineFlag*>(this));
-  }
-
-  // All fields initialized; locks_ is therefore safe to read.
-  return &locks_->primary_mu;
-}
-
-bool CommandLineFlag::IsModified() const {
-  absl::MutexLock l(InitFlagIfNecessary());
-  return modified_;
-}
-
-void CommandLineFlag::SetModified(bool is_modified) {
-  absl::MutexLock l(InitFlagIfNecessary());
-  modified_ = is_modified;
-}
-
-bool CommandLineFlag::IsSpecifiedOnCommandLine() const {
-  absl::MutexLock l(InitFlagIfNecessary());
-  return on_command_line_;
-}
-
-absl::string_view CommandLineFlag::Typename() const {
-  // We do not store/report type in Abseil Flags, so that user do not rely on in
-  // at runtime
-  if (IsAbseilFlag() || IsRetired()) return "";
-
-#define HANDLE_V1_BUILTIN_TYPE(t) \
-  if (IsOfType<t>()) {            \
-    return #t;                    \
-  }
-
-  HANDLE_V1_BUILTIN_TYPE(bool);
-  HANDLE_V1_BUILTIN_TYPE(int32_t);
-  HANDLE_V1_BUILTIN_TYPE(int64_t);
-  HANDLE_V1_BUILTIN_TYPE(uint64_t);
-  HANDLE_V1_BUILTIN_TYPE(double);
-#undef HANDLE_V1_BUILTIN_TYPE
-
-  if (IsOfType<std::string>()) {
-    return "string";
-  }
-
-  return "";
-}
-
-std::string CommandLineFlag::Filename() const {
-  return flags_internal::GetUsageConfig().normalize_filename(filename_);
-}
-
-std::string CommandLineFlag::DefaultValue() const {
-  absl::MutexLock l(InitFlagIfNecessary());
-
-  return Unparse(marshalling_op_, def_);
-}
-
-std::string CommandLineFlag::CurrentValue() const {
-  absl::MutexLock l(InitFlagIfNecessary());
-
-  return Unparse(marshalling_op_, cur_);
-}
-
-// Attempts to parse supplied `value` string using parsing routine in the `flag`
-// argument. If parsing is successful, it will try to validate that the parsed
-// value is valid for the specified 'flag'. Finally this function stores the
-// parsed value in 'dst' assuming it is a pointer to the flag's value type. In
-// case if any error is encountered in either step, the error message is stored
-// in 'err'
-bool TryParseLocked(CommandLineFlag* flag, void* dst, absl::string_view value,
-                    std::string* err)
-    ABSL_EXCLUSIVE_LOCKS_REQUIRED(flag->locks_->primary_mu) {
-  void* tentative_value = Clone(flag->op_, flag->def_);
-  std::string parse_err;
-  if (!Parse(flag->marshalling_op_, value, tentative_value, &parse_err)) {
-    auto type_name = flag->Typename();
-    absl::string_view err_sep = parse_err.empty() ? "" : "; ";
-    absl::string_view typename_sep = type_name.empty() ? "" : " ";
-    *err = absl::StrCat("Illegal value '", value, "' specified for",
-                        typename_sep, type_name, " flag '", flag->Name(), "'",
-                        err_sep, parse_err);
-    Delete(flag->op_, tentative_value);
-    return false;
-  }
-
-  if (!flag->InvokeValidator(tentative_value)) {
-    *err = absl::StrCat("Failed validation of new value '",
-                        Unparse(flag->marshalling_op_, tentative_value),
-                        "' for flag '", flag->Name(), "'");
-    Delete(flag->op_, tentative_value);
-    return false;
-  }
-
-  flag->counter_++;
-  Copy(flag->op_, tentative_value, dst);
-  Delete(flag->op_, tentative_value);
-  return true;
-}
-
-// Sets the value of the flag based on specified string `value`. If the flag
-// was successfully set to new value, it returns true. Otherwise, sets `err`
-// to indicate the error, leaves the flag unchanged, and returns false. There
-// are three ways to set the flag's value:
-//  * Update the current flag value
-//  * Update the flag's default value
-//  * Update the current flag value if it was never set before
-// The mode is selected based on 'set_mode' parameter.
-bool CommandLineFlag::SetFromString(absl::string_view value,
-                                    FlagSettingMode set_mode,
-                                    ValueSource source, std::string* err) {
-  if (IsRetired()) return false;
-
-  absl::MutexLock l(InitFlagIfNecessary());
-
-  // Direct-access flags can be modified without going through the
-  // flag API. Detect such changes and update the flag->modified_ bit.
-  if (!IsAbseilFlag()) {
-    if (!modified_ && ChangedDirectly(this, cur_, def_)) {
-      modified_ = true;
-    }
-  }
-
-  switch (set_mode) {
-    case SET_FLAGS_VALUE: {
-      // set or modify the flag's value
-      if (!TryParseLocked(this, cur_, value, err)) return false;
-      modified_ = true;
-      UpdateCopy(this);
-      InvokeCallback();
-
-      if (source == kCommandLine) {
-        on_command_line_ = true;
-      }
-      break;
-    }
-    case SET_FLAG_IF_DEFAULT: {
-      // set the flag's value, but only if it hasn't been set by someone else
-      if (!modified_) {
-        if (!TryParseLocked(this, cur_, value, err)) return false;
-        modified_ = true;
-        UpdateCopy(this);
-        InvokeCallback();
-      } else {
-        // TODO(rogeeff): review and fix this semantic. Currently we do not fail
-        // in this case if flag is modified. This is misleading since the flag's
-        // value is not updated even though we return true.
-        // *err = absl::StrCat(Name(), " is already set to ",
-        //                     CurrentValue(), "\n");
-        // return false;
-        return true;
-      }
-      break;
-    }
-    case SET_FLAGS_DEFAULT: {
-      // modify the flag's default-value
-      if (!TryParseLocked(this, def_, value, err)) return false;
-
-      if (!modified_) {
-        // Need to set both defvalue *and* current, in this case
-        Copy(op_, def_, cur_);
-        UpdateCopy(this);
-        InvokeCallback();
-      }
-      break;
-    }
-    default: {
-      // unknown set_mode
-      assert(false);
-      return false;
-    }
-  }
-
-  return true;
-}
-
-void CommandLineFlag::CheckDefaultValueParsingRoundtrip() const {
-  std::string v = DefaultValue();
-
-  absl::MutexLock lock(InitFlagIfNecessary());
-
-  void* dst = Clone(op_, def_);
-  std::string error;
-  if (!flags_internal::Parse(marshalling_op_, v, dst, &error)) {
-    ABSL_INTERNAL_LOG(
-        FATAL,
-        absl::StrCat("Flag ", Name(), " (from ", Filename(),
-                     "): std::string form of default value '", v,
-                     "' could not be parsed; error=", error));
-  }
-
-  // We do not compare dst to def since parsing/unparsing may make
-  // small changes, e.g., precision loss for floating point types.
-  Delete(op_, dst);
-}
-
-bool CommandLineFlag::ValidateDefaultValue() const {
-  absl::MutexLock lock(InitFlagIfNecessary());
-  return InvokeValidator(def_);
-}
-
-bool CommandLineFlag::ValidateInputValue(absl::string_view value) const {
-  absl::MutexLock l(InitFlagIfNecessary());  // protect default value access
-
-  void* obj = Clone(op_, def_);
-  std::string ignored_error;
-  const bool result =
-      flags_internal::Parse(marshalling_op_, value, obj, &ignored_error) &&
-      InvokeValidator(obj);
-  Delete(op_, obj);
-  return result;
-}
-
-void CommandLineFlag::Read(void* dst,
-                           const flags_internal::FlagOpFn dst_op) const {
-  absl::ReaderMutexLock l(InitFlagIfNecessary());
-
-  // `dst_op` is the unmarshaling operation corresponding to the declaration
-  // visibile at the call site. `op` is the Flag's defined unmarshalling
-  // operation. They must match for this operation to be well-defined.
-  if (ABSL_PREDICT_FALSE(dst_op != op_)) {
-    ABSL_INTERNAL_LOG(
-        ERROR,
-        absl::StrCat("Flag '", Name(),
-                     "' is defined as one type and declared as another"));
-  }
-  CopyConstruct(op_, cur_, dst);
-}
-
-void CommandLineFlag::Write(const void* src,
-                            const flags_internal::FlagOpFn src_op) {
-  absl::MutexLock l(InitFlagIfNecessary());
-
-  // `src_op` is the marshalling operation corresponding to the declaration
-  // visible at the call site. `op` is the Flag's defined marshalling operation.
-  // They must match for this operation to be well-defined.
-  if (ABSL_PREDICT_FALSE(src_op != op_)) {
-    ABSL_INTERNAL_LOG(
-        ERROR,
-        absl::StrCat("Flag '", Name(),
-                     "' is defined as one type and declared as another"));
-  }
-
-  if (ShouldValidateFlagValue(*this)) {
-    void* obj = Clone(op_, src);
-    std::string ignored_error;
-    std::string src_as_str = Unparse(marshalling_op_, src);
-    if (!Parse(marshalling_op_, src_as_str, obj, &ignored_error) ||
-        !InvokeValidator(obj)) {
-      ABSL_INTERNAL_LOG(ERROR, absl::StrCat("Attempt to set flag '", Name(),
-                                            "' to invalid value ", src_as_str));
-    }
-    Delete(op_, obj);
-  }
-
-  modified_ = true;
-  counter_++;
-  Copy(op_, src, cur_);
-
-  UpdateCopy(this);
-  InvokeCallback();
-}
-
-std::string HelpText::GetHelpText() const {
-  if (help_function_) return help_function_();
-  if (help_message_) return help_message_;
-
-  return {};
-}
-
-// Update any copy of the flag value that is stored in an atomic word.
-// In addition if flag has a mutation callback this function invokes it.
-void UpdateCopy(CommandLineFlag* flag) {
-#define STORE_ATOMIC(T)           \
-  else if (flag->IsOfType<T>()) { \
-    flag->StoreAtomic();          \
-  }
-
-  if (false) {
-  }
-  ABSL_FLAGS_INTERNAL_FOR_EACH_LOCK_FREE(STORE_ATOMIC)
-#undef STORE_ATOMIC
-}
-
-// Return true iff flag value was changed via direct-access.
-bool ChangedDirectly(CommandLineFlag* flag, const void* a, const void* b) {
-  if (!flag->IsAbseilFlag()) {
-// Need to compare values for direct-access flags.
-#define CHANGED_FOR_TYPE(T)                                                  \
-  if (flag->IsOfType<T>()) {                                                 \
-    return *reinterpret_cast<const T*>(a) != *reinterpret_cast<const T*>(b); \
-  }
-
-    CHANGED_FOR_TYPE(bool);
-    CHANGED_FOR_TYPE(int32_t);
-    CHANGED_FOR_TYPE(int64_t);
-    CHANGED_FOR_TYPE(uint64_t);
-    CHANGED_FOR_TYPE(double);
-    CHANGED_FOR_TYPE(std::string);
-#undef CHANGED_FOR_TYPE
-  }
-
-  return false;
-}
+FlagStateInterface::~FlagStateInterface() = default;
 
 }  // namespace flags_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/flags/internal/commandlineflag.h b/absl/flags/internal/commandlineflag.h
index 284286b6be1..e2c4c82fed8 100644
--- a/absl/flags/internal/commandlineflag.h
+++ b/absl/flags/internal/commandlineflag.h
@@ -16,29 +16,19 @@
 #ifndef ABSL_FLAGS_INTERNAL_COMMANDLINEFLAG_H_
 #define ABSL_FLAGS_INTERNAL_COMMANDLINEFLAG_H_
 
-#include <atomic>
-
-#include "absl/base/macros.h"
-#include "absl/flags/marshalling.h"
-#include "absl/synchronization/mutex.h"
-#include "absl/types/optional.h"
+#include "absl/base/config.h"
+#include "absl/base/fast_type_id.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace flags_internal {
 
-// Type-specific operations, eg., parsing, copying, etc. are provided
-// by function specific to that type with a signature matching FlagOpFn.
-enum FlagOp {
-  kDelete,
-  kClone,
-  kCopy,
-  kCopyConstruct,
-  kSizeof,
-  kParse,
-  kUnparse
-};
-using FlagOpFn = void* (*)(FlagOp, const void*, void*);
-using FlagMarshallingOpFn = void* (*)(FlagOp, const void*, void*, void*);
+// An alias for flag fast type id. This value identifies the flag value type
+// similarly to typeid(T), without relying on RTTI being available. In most
+// cases this id is enough to uniquely identify the flag's value type. In a few
+// cases we'll have to resort to using actual RTTI implementation if it is
+// available.
+using FlagFastTypeId = absl::FastTypeIdType;
 
 // Options that control SetCommandLineOptionWithMode.
 enum FlagSettingMode {
@@ -53,7 +43,7 @@ enum FlagSettingMode {
   SET_FLAGS_DEFAULT
 };
 
-// Options that control SetFromString: Source of a value.
+// Options that control ParseFrom: Source of a value.
 enum ValueSource {
   // Flag is being set by value specified on a command line.
   kCommandLine,
@@ -61,309 +51,18 @@ enum ValueSource {
   kProgrammaticChange,
 };
 
-// Signature for the help generation function used as an argument for the
-// absl::Flag constructor.
-using HelpGenFunc = std::string (*)();
-
-// Signature for the function generating the initial flag value based (usually
-// based on default value supplied in flag's definition)
-using InitialValGenFunc = void* (*)();
-
-extern const char kStrippedFlagHelp[];
-
-// The per-type function
-template <typename T>
-void* FlagOps(FlagOp op, const void* v1, void* v2) {
-  switch (op) {
-    case kDelete:
-      delete static_cast<const T*>(v1);
-      return nullptr;
-    case kClone:
-      return new T(*static_cast<const T*>(v1));
-    case kCopy:
-      *static_cast<T*>(v2) = *static_cast<const T*>(v1);
-      return nullptr;
-    case kCopyConstruct:
-      new (v2) T(*static_cast<const T*>(v1));
-      return nullptr;
-    case kSizeof:
-      return reinterpret_cast<void*>(sizeof(T));
-    default:
-      return nullptr;
-  }
-}
-
-template <typename T>
-void* FlagMarshallingOps(FlagOp op, const void* v1, void* v2, void* v3) {
-  switch (op) {
-    case kParse: {
-      // initialize the temporary instance of type T based on current value in
-      // destination (which is going to be flag's default value).
-      T temp(*static_cast<T*>(v2));
-      if (!absl::ParseFlag<T>(*static_cast<const absl::string_view*>(v1), &temp,
-                              static_cast<std::string*>(v3))) {
-        return nullptr;
-      }
-      *static_cast<T*>(v2) = std::move(temp);
-      return v2;
-    }
-    case kUnparse:
-      *static_cast<std::string*>(v2) =
-          absl::UnparseFlag<T>(*static_cast<const T*>(v1));
-      return nullptr;
-    default:
-      return nullptr;
-  }
-}
-
-// Functions that invoke flag-type-specific operations.
-inline void Delete(FlagOpFn op, const void* obj) {
-  op(flags_internal::kDelete, obj, nullptr);
-}
-
-inline void* Clone(FlagOpFn op, const void* obj) {
-  return op(flags_internal::kClone, obj, nullptr);
-}
-
-inline void Copy(FlagOpFn op, const void* src, void* dst) {
-  op(flags_internal::kCopy, src, dst);
-}
-
-inline void CopyConstruct(FlagOpFn op, const void* src, void* dst) {
-  op(flags_internal::kCopyConstruct, src, dst);
-}
-
-inline bool Parse(FlagMarshallingOpFn op, absl::string_view text, void* dst,
-                  std::string* error) {
-  return op(flags_internal::kParse, &text, dst, error) != nullptr;
-}
-
-inline std::string Unparse(FlagMarshallingOpFn op, const void* val) {
-  std::string result;
-  op(flags_internal::kUnparse, val, &result, nullptr);
-  return result;
-}
-
-inline size_t Sizeof(FlagOpFn op) {
-  // This sequence of casts reverses the sequence from base::internal::FlagOps()
-  return static_cast<size_t>(reinterpret_cast<intptr_t>(
-      op(flags_internal::kSizeof, nullptr, nullptr)));
-}
-
-// The following struct contains the locks in a CommandLineFlag struct.
-// They are in a separate struct that is lazily allocated to avoid problems
-// with static initialization and to avoid multiple allocations.
-struct CommandLineFlagLocks {
-  absl::Mutex primary_mu;   // protects several fields in CommandLineFlag
-  absl::Mutex callback_mu;  // used to serialize callbacks
-};
-
-// Holds either a pointer to help text or a function which produces it.  This is
-// needed for supporting both static initialization of Flags while supporting
-// the legacy registration framework.  We can't use absl::variant<const char*,
-// const char*(*)()> since anybody passing 0 or nullptr in to a CommandLineFlag
-// would find an ambiguity.
-class HelpText {
- public:
-  static constexpr HelpText FromFunctionPointer(const HelpGenFunc fn) {
-    return HelpText(fn, nullptr);
-  }
-  static constexpr HelpText FromStaticCString(const char* msg) {
-    return HelpText(nullptr, msg);
-  }
-
-  std::string GetHelpText() const;
-
-  HelpText() = delete;
-  HelpText(const HelpText&) = default;
-  HelpText(HelpText&&) = default;
-
- private:
-  explicit constexpr HelpText(const HelpGenFunc fn, const char* msg)
-      : help_function_(fn), help_message_(msg) {}
-
-  HelpGenFunc help_function_;
-  const char* help_message_;
-};
-
-// Holds all information for a flag.
-class CommandLineFlag {
+// Handle to FlagState objects. Specific flag state objects will restore state
+// of a flag produced this flag state from method CommandLineFlag::SaveState().
+class FlagStateInterface {
  public:
-  constexpr CommandLineFlag(
-      const char* name, HelpText help_text, const char* filename,
-      const flags_internal::FlagOpFn op,
-      const flags_internal::FlagMarshallingOpFn marshalling_op,
-      const flags_internal::InitialValGenFunc initial_value_gen, void* def,
-      void* cur)
-      : name_(name),
-        help_(help_text),
-        filename_(filename),
-        op_(op),
-        marshalling_op_(marshalling_op),
-        make_init_value_(initial_value_gen),
-        inited_(false),
-        modified_(false),
-        on_command_line_(false),
-        def_(def),
-        cur_(cur),
-        counter_(0),
-        locks_(nullptr) {}
-
-  // Virtual destructor
-  virtual void Destroy() const = 0;
+  virtual ~FlagStateInterface();
 
-  // Not copyable/assignable.
-  CommandLineFlag(const CommandLineFlag&) = delete;
-  CommandLineFlag& operator=(const CommandLineFlag&) = delete;
-
-  // Access methods.
-
-  // Returns true iff this object corresponds to retired flag
-  virtual bool IsRetired() const { return false; }
-  // Returns true iff this is a handle to an Abseil Flag.
-  virtual bool IsAbseilFlag() const { return true; }
-
-  absl::string_view Name() const { return name_; }
-  std::string Help() const { return help_.GetHelpText(); }
-  bool IsModified() const;
-  void SetModified(bool is_modified);
-  bool IsSpecifiedOnCommandLine() const;
-
-  absl::string_view Typename() const;
-  std::string Filename() const;
-  std::string DefaultValue() const;
-  std::string CurrentValue() const;
-
-  // Interface to store the value in atomic if one used. This is short term
-  // interface. To be reworked once cur_ is moved.
-  virtual void StoreAtomic() {}
-
-  // Interfaces to operate on validators.
-  virtual bool HasValidatorFn() const { return false; }
-  virtual bool InvokeValidator(const void* /*value*/) const { return true; }
-  // Invoke the flag validators for old flags.
-  // TODO(rogeeff): implement proper validators for Abseil Flags
-  bool ValidateDefaultValue() const;
-  bool ValidateInputValue(absl::string_view value) const;
-
-  // Return true iff flag has type T.
-  template <typename T>
-  inline bool IsOfType() const {
-    return op_ == &flags_internal::FlagOps<T>;
-  }
-
-  // Attempts to retrieve the flag value. Returns value on success,
-  // absl::nullopt otherwise.
-  template <typename T>
-  absl::optional<T> Get() const {
-    if (IsRetired() || flags_internal::FlagOps<T> != op_) return absl::nullopt;
-
-    T res;
-    Read(&res, flags_internal::FlagOps<T>);
-
-    return res;
-  }
-
-  // Interfaces to overate on callbacks.
-  virtual void InvokeCallback() {}
-
-  // Sets the value of the flag based on specified std::string `value`. If the flag
-  // was successfully set to new value, it returns true. Otherwise, sets `error`
-  // to indicate the error, leaves the flag unchanged, and returns false. There
-  // are three ways to set the flag's value:
-  //  * Update the current flag value
-  //  * Update the flag's default value
-  //  * Update the current flag value if it was never set before
-  // The mode is selected based on `set_mode` parameter.
-  bool SetFromString(absl::string_view value,
-                     flags_internal::FlagSettingMode set_mode,
-                     flags_internal::ValueSource source, std::string* error);
-
-  void CheckDefaultValueParsingRoundtrip() const;
-
-  // Constant configuration for a particular flag.
- protected:
-  ~CommandLineFlag() = default;
-
-  const char* const name_;
-  const HelpText help_;
-  const char* const filename_;
-
-  const FlagOpFn op_;                         // Type-specific handler
-  const FlagMarshallingOpFn marshalling_op_;  // Marshalling ops handler
-  const InitialValGenFunc make_init_value_;  // Makes initial value for the flag
-  std::atomic<bool> inited_;  // fields have been lazily initialized
-
-  // Mutable state (guarded by locks_->primary_mu).
-  bool modified_;         // Has flag value been modified?
-  bool on_command_line_;  // Specified on command line.
-  void* def_;             // Lazily initialized pointer to default value
-  void* cur_;             // Lazily initialized pointer to current value
-  int64_t counter_;         // Mutation counter
-
-  // Lazily initialized mutexes for this flag value.  We cannot inline a
-  // SpinLock or Mutex here because those have non-constexpr constructors and
-  // so would prevent constant initialization of this type.
-  // TODO(rogeeff): fix it once Mutex has constexpr constructor
-  struct CommandLineFlagLocks* locks_;  // locks, laziliy allocated.
-
-  // Ensure that the lazily initialized fields of *flag have been initialized,
-  // and return the lock which should be locked when flag's state is mutated.
-  absl::Mutex* InitFlagIfNecessary() const ABSL_LOCK_RETURNED(locks_->primary_mu);
-
-  // copy construct new value of flag's type in a memory referenced by dst
-  // based on current flag's value
-  void Read(void* dst, const flags_internal::FlagOpFn dst_op) const;
-  // updates flag's value to *src (locked)
-  void Write(const void* src, const flags_internal::FlagOpFn src_op);
-
-  friend class FlagRegistry;
-  friend class FlagPtrMap;
-  friend class FlagSaverImpl;
-  friend bool TryParseLocked(CommandLineFlag* flag, void* dst,
-                             absl::string_view value, std::string* err);
-  friend absl::Mutex* InitFlag(CommandLineFlag* flag);
-
-  // This is a short term, until we completely rework persistent state
-  // storage API.
-  virtual void* GetValidator() const { return nullptr; }
-  virtual bool SetValidator(void*) { return false; }
+  // Restores the flag originated this object to the saved state.
+  virtual void Restore() && = 0;
 };
 
-// Update any copy of the flag value that is stored in an atomic word.
-// In addition if flag has a mutation callback this function invokes it. While
-// callback is being invoked the primary flag's mutex is unlocked and it is
-// re-locked back after call to callback is completed. Callback invocation is
-// guarded by flag's secondary mutex instead which prevents concurrent callback
-// invocation. Note that it is possible for other thread to grab the primary
-// lock and update flag's value at any time during the callback invocation.
-// This is by design. Callback can get a value of the flag if necessary, but it
-// might be different from the value initiated the callback and it also can be
-// different by the time the callback invocation is completed.
-// Requires that *primary_lock be held in exclusive mode; it may be released
-// and reacquired by the implementation.
-void UpdateCopy(CommandLineFlag* flag);
-// Return true iff flag value was changed via direct-access.
-bool ChangedDirectly(CommandLineFlag* flag, const void* a, const void* b);
-
-// This macro is the "source of truth" for the list of supported flag types we
-// expect to perform lock free operations on. Specifically it generates code,
-// a one argument macro operating on a type, supplied as a macro argument, for
-// each type in the list.
-#define ABSL_FLAGS_INTERNAL_FOR_EACH_LOCK_FREE(A) \
-  A(bool)                                         \
-  A(short)                                        \
-  A(unsigned short)                               \
-  A(int)                                          \
-  A(unsigned int)                                 \
-  A(long)                                         \
-  A(unsigned long)                                \
-  A(long long)                                    \
-  A(unsigned long long)                           \
-  A(double)                                       \
-  A(float)
-
 }  // namespace flags_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_FLAGS_INTERNAL_COMMANDLINEFLAG_H_
diff --git a/absl/flags/internal/commandlineflag_test.cc b/absl/flags/internal/commandlineflag_test.cc
deleted file mode 100644
index 5a0c271e853..00000000000
--- a/absl/flags/internal/commandlineflag_test.cc
+++ /dev/null
@@ -1,218 +0,0 @@
-//
-//  Copyright 2019 The Abseil Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-#include "absl/flags/internal/commandlineflag.h"
-
-#include <algorithm>
-#include <string>
-
-#include "gtest/gtest.h"
-#include "absl/flags/flag.h"
-#include "absl/flags/internal/registry.h"
-#include "absl/flags/usage_config.h"
-#include "absl/memory/memory.h"
-#include "absl/strings/match.h"
-#include "absl/strings/str_cat.h"
-
-ABSL_FLAG(int, int_flag, 201, "int_flag help");
-ABSL_FLAG(std::string, string_flag, "dflt",
-          absl::StrCat("string_flag", " help"));
-ABSL_RETIRED_FLAG(bool, bool_retired_flag, false, "bool_retired_flag help");
-
-namespace {
-
-namespace flags = absl::flags_internal;
-
-class CommandLineFlagTest : public testing::Test {
- protected:
-  static void SetUpTestSuite() {
-    // Install a function to normalize filenames before this test is run.
-    absl::FlagsUsageConfig default_config;
-    default_config.normalize_filename = &CommandLineFlagTest::NormalizeFileName;
-    absl::SetFlagsUsageConfig(default_config);
-  }
-
-  void SetUp() override { flag_saver_ = absl::make_unique<flags::FlagSaver>(); }
-  void TearDown() override { flag_saver_.reset(); }
-
- private:
-  static std::string NormalizeFileName(absl::string_view fname) {
-#ifdef _WIN32
-    std::string normalized(fname);
-    std::replace(normalized.begin(), normalized.end(), '\\', '/');
-    fname = normalized;
-#endif
-    return std::string(fname);
-  }
-
-  std::unique_ptr<flags::FlagSaver> flag_saver_;
-};
-
-TEST_F(CommandLineFlagTest, TestAttributesAccessMethods) {
-  auto* flag_01 = flags::FindCommandLineFlag("int_flag");
-
-  ASSERT_TRUE(flag_01);
-  EXPECT_EQ(flag_01->Name(), "int_flag");
-  EXPECT_EQ(flag_01->Help(), "int_flag help");
-  EXPECT_EQ(flag_01->Typename(), "");
-  EXPECT_TRUE(!flag_01->IsRetired());
-  EXPECT_TRUE(flag_01->IsOfType<int>());
-  EXPECT_TRUE(
-      absl::EndsWith(flag_01->Filename(),
-                     "absl/flags/internal/commandlineflag_test.cc"))
-      << flag_01->Filename();
-
-  auto* flag_02 = flags::FindCommandLineFlag("string_flag");
-
-  ASSERT_TRUE(flag_02);
-  EXPECT_EQ(flag_02->Name(), "string_flag");
-  EXPECT_EQ(flag_02->Help(), "string_flag help");
-  EXPECT_EQ(flag_02->Typename(), "");
-  EXPECT_TRUE(!flag_02->IsRetired());
-  EXPECT_TRUE(flag_02->IsOfType<std::string>());
-  EXPECT_TRUE(
-      absl::EndsWith(flag_02->Filename(),
-                     "absl/flags/internal/commandlineflag_test.cc"))
-      << flag_02->Filename();
-
-  auto* flag_03 = flags::FindRetiredFlag("bool_retired_flag");
-
-  ASSERT_TRUE(flag_03);
-  EXPECT_EQ(flag_03->Name(), "bool_retired_flag");
-  EXPECT_EQ(flag_03->Help(), "");
-  EXPECT_EQ(flag_03->Typename(), "");
-  EXPECT_TRUE(flag_03->IsRetired());
-  EXPECT_TRUE(flag_03->IsOfType<bool>());
-  EXPECT_EQ(flag_03->Filename(), "RETIRED");
-}
-
-// --------------------------------------------------------------------
-
-TEST_F(CommandLineFlagTest, TestValueAccessMethods) {
-  absl::SetFlag(&FLAGS_int_flag, 301);
-  auto* flag_01 = flags::FindCommandLineFlag("int_flag");
-
-  ASSERT_TRUE(flag_01);
-  EXPECT_EQ(flag_01->CurrentValue(), "301");
-  EXPECT_EQ(flag_01->DefaultValue(), "201");
-
-  absl::SetFlag(&FLAGS_string_flag, "new_str_value");
-  auto* flag_02 = flags::FindCommandLineFlag("string_flag");
-
-  ASSERT_TRUE(flag_02);
-  EXPECT_EQ(flag_02->CurrentValue(), "new_str_value");
-  EXPECT_EQ(flag_02->DefaultValue(), "dflt");
-}
-
-// --------------------------------------------------------------------
-
-TEST_F(CommandLineFlagTest, TestSetFromStringCurrentValue) {
-  std::string err;
-
-  auto* flag_01 = flags::FindCommandLineFlag("int_flag");
-  EXPECT_FALSE(flag_01->IsSpecifiedOnCommandLine());
-
-  EXPECT_TRUE(flag_01->SetFromString("11", flags::SET_FLAGS_VALUE,
-                                     flags::kProgrammaticChange, &err));
-  EXPECT_EQ(absl::GetFlag(FLAGS_int_flag), 11);
-  EXPECT_FALSE(flag_01->IsSpecifiedOnCommandLine());
-
-  EXPECT_TRUE(flag_01->SetFromString("-123", flags::SET_FLAGS_VALUE,
-                                     flags::kProgrammaticChange, &err));
-  EXPECT_EQ(absl::GetFlag(FLAGS_int_flag), -123);
-  EXPECT_FALSE(flag_01->IsSpecifiedOnCommandLine());
-
-  EXPECT_TRUE(!flag_01->SetFromString("xyz", flags::SET_FLAGS_VALUE,
-                                      flags::kProgrammaticChange, &err));
-  EXPECT_EQ(absl::GetFlag(FLAGS_int_flag), -123);
-  EXPECT_EQ(err, "Illegal value 'xyz' specified for flag 'int_flag'");
-  EXPECT_FALSE(flag_01->IsSpecifiedOnCommandLine());
-
-  EXPECT_TRUE(!flag_01->SetFromString("A1", flags::SET_FLAGS_VALUE,
-                                      flags::kProgrammaticChange, &err));
-  EXPECT_EQ(absl::GetFlag(FLAGS_int_flag), -123);
-  EXPECT_EQ(err, "Illegal value 'A1' specified for flag 'int_flag'");
-  EXPECT_FALSE(flag_01->IsSpecifiedOnCommandLine());
-
-  EXPECT_TRUE(flag_01->SetFromString("0x10", flags::SET_FLAGS_VALUE,
-                                     flags::kProgrammaticChange, &err));
-  EXPECT_EQ(absl::GetFlag(FLAGS_int_flag), 16);
-  EXPECT_FALSE(flag_01->IsSpecifiedOnCommandLine());
-
-  EXPECT_TRUE(flag_01->SetFromString("011", flags::SET_FLAGS_VALUE,
-                                     flags::kCommandLine, &err));
-  EXPECT_EQ(absl::GetFlag(FLAGS_int_flag), 11);
-  EXPECT_TRUE(flag_01->IsSpecifiedOnCommandLine());
-
-  EXPECT_TRUE(!flag_01->SetFromString("", flags::SET_FLAGS_VALUE,
-                                      flags::kProgrammaticChange, &err));
-  EXPECT_EQ(err, "Illegal value '' specified for flag 'int_flag'");
-
-  auto* flag_02 = flags::FindCommandLineFlag("string_flag");
-  EXPECT_TRUE(flag_02->SetFromString("xyz", flags::SET_FLAGS_VALUE,
-                                     flags::kProgrammaticChange, &err));
-  EXPECT_EQ(absl::GetFlag(FLAGS_string_flag), "xyz");
-
-  EXPECT_TRUE(flag_02->SetFromString("", flags::SET_FLAGS_VALUE,
-                                     flags::kProgrammaticChange, &err));
-  EXPECT_EQ(absl::GetFlag(FLAGS_string_flag), "");
-}
-
-// --------------------------------------------------------------------
-
-TEST_F(CommandLineFlagTest, TestSetFromStringDefaultValue) {
-  std::string err;
-
-  auto* flag_01 = flags::FindCommandLineFlag("int_flag");
-
-  EXPECT_TRUE(flag_01->SetFromString("111", flags::SET_FLAGS_DEFAULT,
-                                     flags::kProgrammaticChange, &err));
-  EXPECT_EQ(flag_01->DefaultValue(), "111");
-
-  auto* flag_02 = flags::FindCommandLineFlag("string_flag");
-
-  EXPECT_TRUE(flag_02->SetFromString("abc", flags::SET_FLAGS_DEFAULT,
-                                     flags::kProgrammaticChange, &err));
-  EXPECT_EQ(flag_02->DefaultValue(), "abc");
-}
-
-// --------------------------------------------------------------------
-
-TEST_F(CommandLineFlagTest, TestSetFromStringIfDefault) {
-  std::string err;
-
-  auto* flag_01 = flags::FindCommandLineFlag("int_flag");
-
-  EXPECT_TRUE(flag_01->SetFromString("22", flags::SET_FLAG_IF_DEFAULT,
-                                     flags::kProgrammaticChange, &err))
-      << err;
-  EXPECT_EQ(absl::GetFlag(FLAGS_int_flag), 22);
-
-  EXPECT_TRUE(flag_01->SetFromString("33", flags::SET_FLAG_IF_DEFAULT,
-                                     flags::kProgrammaticChange, &err));
-  EXPECT_EQ(absl::GetFlag(FLAGS_int_flag), 22);
-  // EXPECT_EQ(err, "ERROR: int_flag is already set to 22");
-
-  // Reset back to default value
-  EXPECT_TRUE(flag_01->SetFromString("201", flags::SET_FLAGS_VALUE,
-                                     flags::kProgrammaticChange, &err));
-
-  EXPECT_TRUE(flag_01->SetFromString("33", flags::SET_FLAG_IF_DEFAULT,
-                                     flags::kProgrammaticChange, &err));
-  EXPECT_EQ(absl::GetFlag(FLAGS_int_flag), 201);
-  // EXPECT_EQ(err, "ERROR: int_flag is already set to 201");
-}
-
-}  // namespace
diff --git a/absl/flags/internal/flag.cc b/absl/flags/internal/flag.cc
index 0f4035819cd..0a1dff28135 100644
--- a/absl/flags/internal/flag.cc
+++ b/absl/flags/internal/flag.cc
@@ -15,37 +15,698 @@
 
 #include "absl/flags/internal/flag.h"
 
+#include <assert.h>
+#include <stddef.h>
+#include <stdint.h>
+#include <string.h>
+
+#include <array>
+#include <atomic>
+#include <cstring>
+#include <memory>
+#include <string>
+#include <typeinfo>
+#include <utility>
+#include <vector>
+
+#include "absl/base/attributes.h"
+#include "absl/base/call_once.h"
+#include "absl/base/casts.h"
+#include "absl/base/config.h"
+#include "absl/base/const_init.h"
+#include "absl/base/dynamic_annotations.h"
+#include "absl/base/fast_type_id.h"
+#include "absl/base/no_destructor.h"
+#include "absl/base/optimization.h"
+#include "absl/base/thread_annotations.h"
+#include "absl/flags/config.h"
+#include "absl/flags/internal/commandlineflag.h"
+#include "absl/flags/usage_config.h"
+#include "absl/memory/memory.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/string_view.h"
 #include "absl/synchronization/mutex.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace flags_internal {
 
-// If the flag has a mutation callback this function invokes it. While the
-// callback is being invoked the primary flag's mutex is unlocked and it is
-// re-locked back after call to callback is completed. Callback invocation is
-// guarded by flag's secondary mutex instead which prevents concurrent
-// callback invocation. Note that it is possible for other thread to grab the
-// primary lock and update flag's value at any time during the callback
-// invocation. This is by design. Callback can get a value of the flag if
-// necessary, but it might be different from the value initiated the callback
-// and it also can be different by the time the callback invocation is
-// completed. Requires that *primary_lock be held in exclusive mode; it may be
-// released and reacquired by the implementation.
-void InvokeCallback(absl::Mutex* primary_mu, absl::Mutex* callback_mu,
-                    FlagCallback cb) ABSL_EXCLUSIVE_LOCKS_REQUIRED(primary_mu) {
-  if (!cb) return;
+// The help message indicating that the commandline flag has been stripped. It
+// will not show up when doing "-help" and its variants. The flag is stripped
+// if ABSL_FLAGS_STRIP_HELP is set to 1 before including absl/flags/flag.h
+const char kStrippedFlagHelp[] = "\001\002\003\004 (unknown) \004\003\002\001";
+
+namespace {
+
+// Currently we only validate flag values for user-defined flag types.
+bool ShouldValidateFlagValue(FlagFastTypeId flag_type_id) {
+#define DONT_VALIDATE(T, _) \
+  if (flag_type_id == absl::FastTypeId<T>()) return false;
+  ABSL_FLAGS_INTERNAL_SUPPORTED_TYPES(DONT_VALIDATE)
+#undef DONT_VALIDATE
+
+  return true;
+}
+
+// RAII helper used to temporarily unlock and relock `absl::Mutex`.
+// This is used when we need to ensure that locks are released while
+// invoking user supplied callbacks and then reacquired, since callbacks may
+// need to acquire these locks themselves.
+class MutexRelock {
+ public:
+  explicit MutexRelock(absl::Mutex& mu) : mu_(mu) { mu_.unlock(); }
+  ~MutexRelock() { mu_.lock(); }
+
+  MutexRelock(const MutexRelock&) = delete;
+  MutexRelock& operator=(const MutexRelock&) = delete;
+
+ private:
+  absl::Mutex& mu_;
+};
+
+// This is a freelist of leaked flag values and guard for its access.
+// When we can't guarantee it is safe to reuse the memory for flag values,
+// we move the memory to the freelist where it lives indefinitely, so it can
+// still be safely accessed. This also prevents leak checkers from complaining
+// about the leaked memory that can no longer be accessed through any pointer.
+absl::Mutex& FreelistMutex() {
+  static absl::NoDestructor<absl::Mutex> mutex;
+  return *mutex;
+}
+ABSL_CONST_INIT std::vector<void*>* s_freelist ABSL_GUARDED_BY(FreelistMutex())
+    ABSL_PT_GUARDED_BY(FreelistMutex()) = nullptr;
+
+void AddToFreelist(void* p) {
+  absl::MutexLock l(FreelistMutex());
+  if (!s_freelist) {
+    s_freelist = new std::vector<void*>;
+  }
+  s_freelist->push_back(p);
+}
+
+}  // namespace
+
+///////////////////////////////////////////////////////////////////////////////
+
+uint64_t NumLeakedFlagValues() {
+  absl::MutexLock l(FreelistMutex());
+  return s_freelist == nullptr ? 0u : s_freelist->size();
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Persistent state of the flag data.
+
+class FlagImpl;
+
+class FlagState : public flags_internal::FlagStateInterface {
+ public:
+  template <typename V>
+  FlagState(FlagImpl& flag_impl, const V& v, bool modified,
+            bool on_command_line, int64_t counter)
+      : flag_impl_(flag_impl),
+        value_(v),
+        modified_(modified),
+        on_command_line_(on_command_line),
+        counter_(counter) {}
+
+  ~FlagState() override {
+    if (flag_impl_.ValueStorageKind() != FlagValueStorageKind::kHeapAllocated &&
+        flag_impl_.ValueStorageKind() != FlagValueStorageKind::kSequenceLocked)
+      return;
+    flags_internal::Delete(flag_impl_.op_, value_.heap_allocated);
+  }
+
+ private:
+  friend class FlagImpl;
+
+  // Restores the flag to the saved state.
+  void Restore() && override {
+    if (!std::move(flag_impl_).RestoreState(*this)) return;
+
+    ABSL_INTERNAL_LOG(INFO,
+                      absl::StrCat("Restore saved value of ", flag_impl_.Name(),
+                                   " to: ", flag_impl_.CurrentValue()));
+  }
+
+  // Flag and saved flag data.
+  FlagImpl& flag_impl_;
+  union SavedValue {
+    explicit SavedValue(void* v) : heap_allocated(v) {}
+    explicit SavedValue(int64_t v) : one_word(v) {}
+
+    void* heap_allocated;
+    int64_t one_word;
+  } value_;
+  bool modified_;
+  bool on_command_line_;
+  int64_t counter_;
+};
+
+///////////////////////////////////////////////////////////////////////////////
+// Flag implementation, which does not depend on flag value type.
+
+DynValueDeleter::DynValueDeleter(FlagOpFn op_arg) : op(op_arg) {}
+
+void DynValueDeleter::operator()(void* ptr) const {
+  if (op == nullptr) return;
+
+  Delete(op, ptr);
+}
+
+MaskedPointer::MaskedPointer(ptr_t rhs, bool is_candidate) : ptr_(rhs) {
+  if (is_candidate) {
+    ApplyMask(kUnprotectedReadCandidate);
+  }
+}
+
+bool MaskedPointer::IsUnprotectedReadCandidate() const {
+  return CheckMask(kUnprotectedReadCandidate);
+}
 
-  // When executing the callback we need the primary flag's mutex to be
-  // unlocked so that callback can retrieve the flag's value.
-  primary_mu->Unlock();
+bool MaskedPointer::HasBeenRead() const { return CheckMask(kHasBeenRead); }
 
-  {
-    absl::MutexLock lock(callback_mu);
-    cb();
+void MaskedPointer::Set(FlagOpFn op, const void* src, bool is_candidate) {
+  flags_internal::Copy(op, src, Ptr());
+  if (is_candidate) {
+    ApplyMask(kUnprotectedReadCandidate);
   }
+}
+void MaskedPointer::MarkAsRead() { ApplyMask(kHasBeenRead); }
+
+void MaskedPointer::ApplyMask(mask_t mask) {
+  ptr_ = reinterpret_cast<ptr_t>(reinterpret_cast<mask_t>(ptr_) | mask);
+}
+bool MaskedPointer::CheckMask(mask_t mask) const {
+  return (reinterpret_cast<mask_t>(ptr_) & mask) != 0;
+}
+
+void FlagImpl::Init() {
+  new (&data_guard_) absl::Mutex;
+
+  auto def_kind = static_cast<FlagDefaultKind>(def_kind_);
+
+  switch (ValueStorageKind()) {
+    case FlagValueStorageKind::kValueAndInitBit:
+    case FlagValueStorageKind::kOneWordAtomic: {
+      alignas(int64_t) std::array<char, sizeof(int64_t)> buf{};
+      if (def_kind == FlagDefaultKind::kGenFunc) {
+        (*default_value_.gen_func)(buf.data());
+      } else {
+        assert(def_kind != FlagDefaultKind::kDynamicValue);
+        std::memcpy(buf.data(), &default_value_, Sizeof(op_));
+      }
+      if (ValueStorageKind() == FlagValueStorageKind::kValueAndInitBit) {
+        // We presume here the memory layout of FlagValueAndInitBit struct.
+        uint8_t initialized = 1;
+        std::memcpy(buf.data() + Sizeof(op_), &initialized,
+                    sizeof(initialized));
+      }
+      // Type can contain valid uninitialized bits, e.g. padding.
+      ABSL_ANNOTATE_MEMORY_IS_INITIALIZED(buf.data(), buf.size());
+      OneWordValue().store(absl::bit_cast<int64_t>(buf),
+                           std::memory_order_release);
+      break;
+    }
+    case FlagValueStorageKind::kSequenceLocked: {
+      // For this storage kind the default_value_ always points to gen_func
+      // during initialization.
+      assert(def_kind == FlagDefaultKind::kGenFunc);
+      (*default_value_.gen_func)(AtomicBufferValue());
+      break;
+    }
+    case FlagValueStorageKind::kHeapAllocated:
+      // For this storage kind the default_value_ always points to gen_func
+      // during initialization.
+      assert(def_kind == FlagDefaultKind::kGenFunc);
+      // Flag value initially points to the internal buffer.
+      MaskedPointer ptr_value = PtrStorage().load(std::memory_order_acquire);
+      (*default_value_.gen_func)(ptr_value.Ptr());
+      // Default value is a candidate for an unprotected read.
+      PtrStorage().store(MaskedPointer(ptr_value.Ptr(), true),
+                         std::memory_order_release);
+      break;
+  }
+  seq_lock_.MarkInitialized();
+}
+
+absl::Mutex& FlagImpl::DataGuard() const {
+  absl::call_once(const_cast<FlagImpl*>(this)->init_control_, &FlagImpl::Init,
+                  const_cast<FlagImpl*>(this));
+
+  // data_guard_ is initialized inside Init.
+  return *reinterpret_cast<absl::Mutex*>(&data_guard_);
+}
+
+void FlagImpl::AssertValidType(FlagFastTypeId rhs_type_id,
+                               const std::type_info* (*gen_rtti)()) const {
+  FlagFastTypeId lhs_type_id = flags_internal::FastTypeId(op_);
+
+  // `rhs_type_id` is the fast type id corresponding to the declaration
+  // visible at the call site. `lhs_type_id` is the fast type id
+  // corresponding to the type specified in flag definition. They must match
+  //  for this operation to be well-defined.
+  if (ABSL_PREDICT_TRUE(lhs_type_id == rhs_type_id)) return;
+
+  const std::type_info* lhs_runtime_type_id =
+      flags_internal::RuntimeTypeId(op_);
+  const std::type_info* rhs_runtime_type_id = (*gen_rtti)();
+
+  if (lhs_runtime_type_id == rhs_runtime_type_id) return;
+
+#ifdef ABSL_INTERNAL_HAS_RTTI
+  if (*lhs_runtime_type_id == *rhs_runtime_type_id) return;
+#endif
+
+  ABSL_INTERNAL_LOG(
+      FATAL, absl::StrCat("Flag '", Name(),
+                          "' is defined as one type and declared as another"));
+}
+
+std::unique_ptr<void, DynValueDeleter> FlagImpl::MakeInitValue() const {
+  void* res = nullptr;
+  switch (DefaultKind()) {
+    case FlagDefaultKind::kDynamicValue:
+      res = flags_internal::Clone(op_, default_value_.dynamic_value);
+      break;
+    case FlagDefaultKind::kGenFunc:
+      res = flags_internal::Alloc(op_);
+      (*default_value_.gen_func)(res);
+      break;
+    default:
+      res = flags_internal::Clone(op_, &default_value_);
+      break;
+  }
+  return {res, DynValueDeleter{op_}};
+}
+
+void FlagImpl::StoreValue(const void* src, ValueSource source) {
+  switch (ValueStorageKind()) {
+    case FlagValueStorageKind::kValueAndInitBit:
+    case FlagValueStorageKind::kOneWordAtomic: {
+      // Load the current value to avoid setting 'init' bit manually.
+      int64_t one_word_val = OneWordValue().load(std::memory_order_acquire);
+      std::memcpy(&one_word_val, src, Sizeof(op_));
+      OneWordValue().store(one_word_val, std::memory_order_release);
+      seq_lock_.IncrementModificationCount();
+      break;
+    }
+    case FlagValueStorageKind::kSequenceLocked: {
+      seq_lock_.Write(AtomicBufferValue(), src, Sizeof(op_));
+      break;
+    }
+    case FlagValueStorageKind::kHeapAllocated:
+      MaskedPointer ptr_value = PtrStorage().load(std::memory_order_acquire);
+
+      if (ptr_value.IsUnprotectedReadCandidate() && ptr_value.HasBeenRead()) {
+        // If current value is a candidate for an unprotected read and if it was
+        // already read at least once, follow up reads (if any) are done without
+        // mutex protection. We can't guarantee it is safe to reuse this memory
+        // since it may have been accessed by another thread concurrently, so
+        // instead we move the memory to a freelist so it can still be safely
+        // accessed, and allocate a new one for the new value.
+        AddToFreelist(ptr_value.Ptr());
+        ptr_value = MaskedPointer(Clone(op_, src), source == kCommandLine);
+      } else {
+        // Current value either was set programmatically or was never read.
+        // We can reuse the memory since all accesses to this value (if any)
+        // were protected by mutex. That said, if a new value comes from command
+        // line it now becomes a candidate for an unprotected read.
+        ptr_value.Set(op_, src, source == kCommandLine);
+      }
+
+      PtrStorage().store(ptr_value, std::memory_order_release);
+      seq_lock_.IncrementModificationCount();
+      break;
+  }
+  modified_ = true;
+  InvokeCallback();
+}
+
+absl::string_view FlagImpl::Name() const { return name_; }
+
+absl::string_view FlagImpl::TypeName() const { return type_name_; }
+
+std::string FlagImpl::Filename() const {
+  return flags_internal::GetUsageConfig().normalize_filename(filename_);
+}
+
+std::string FlagImpl::Help() const {
+  return HelpSourceKind() == FlagHelpKind::kLiteral ? help_.literal
+                                                    : help_.gen_func();
+}
+
+FlagFastTypeId FlagImpl::TypeId() const {
+  return flags_internal::FastTypeId(op_);
+}
+
+int64_t FlagImpl::ModificationCount() const {
+  return seq_lock_.ModificationCount();
+}
+
+bool FlagImpl::IsSpecifiedOnCommandLine() const {
+  absl::MutexLock l(DataGuard());
+  return on_command_line_;
+}
+
+std::string FlagImpl::DefaultValue() const {
+  absl::MutexLock l(DataGuard());
+
+  auto obj = MakeInitValue();
+  return flags_internal::Unparse(op_, obj.get());
+}
+
+std::string FlagImpl::CurrentValue() const {
+  auto& guard = DataGuard();  // Make sure flag initialized
+  switch (ValueStorageKind()) {
+    case FlagValueStorageKind::kValueAndInitBit:
+    case FlagValueStorageKind::kOneWordAtomic: {
+      const auto one_word_val =
+          absl::bit_cast<std::array<char, sizeof(int64_t)>>(
+              OneWordValue().load(std::memory_order_acquire));
+      return flags_internal::Unparse(op_, one_word_val.data());
+    }
+    case FlagValueStorageKind::kSequenceLocked: {
+      std::unique_ptr<void, DynValueDeleter> cloned(flags_internal::Alloc(op_),
+                                                    DynValueDeleter{op_});
+      ReadSequenceLockedData(cloned.get());
+      return flags_internal::Unparse(op_, cloned.get());
+    }
+    case FlagValueStorageKind::kHeapAllocated: {
+      absl::MutexLock l(guard);
+      return flags_internal::Unparse(
+          op_, PtrStorage().load(std::memory_order_acquire).Ptr());
+    }
+  }
+
+  return "";
+}
+
+void FlagImpl::SetCallback(const FlagCallbackFunc mutation_callback) {
+  absl::MutexLock l(DataGuard());
+
+  if (callback_ == nullptr) {
+    callback_ = new FlagCallback;
+  }
+  callback_->func = mutation_callback;
+
+  InvokeCallback();
+}
+
+void FlagImpl::InvokeCallback() const {
+  if (!callback_) return;
+
+  // Make a copy of the C-style function pointer that we are about to invoke
+  // before we release the lock guarding it.
+  FlagCallbackFunc cb = callback_->func;
+
+  // If the flag has a mutation callback this function invokes it. While the
+  // callback is being invoked the primary flag's mutex is unlocked and it is
+  // re-locked back after call to callback is completed. Callback invocation is
+  // guarded by flag's secondary mutex instead which prevents concurrent
+  // callback invocation. Note that it is possible for other thread to grab the
+  // primary lock and update flag's value at any time during the callback
+  // invocation. This is by design. Callback can get a value of the flag if
+  // necessary, but it might be different from the value initiated the callback
+  // and it also can be different by the time the callback invocation is
+  // completed. Requires that *primary_lock be held in exclusive mode; it may be
+  // released and reacquired by the implementation.
+  MutexRelock relock(DataGuard());
+  absl::MutexLock lock(callback_->guard);
+  cb();
+}
+
+std::unique_ptr<FlagStateInterface> FlagImpl::SaveState() {
+  absl::MutexLock l(DataGuard());
+
+  bool modified = modified_;
+  bool on_command_line = on_command_line_;
+  switch (ValueStorageKind()) {
+    case FlagValueStorageKind::kValueAndInitBit:
+    case FlagValueStorageKind::kOneWordAtomic: {
+      return std::make_unique<FlagState>(
+          *this, OneWordValue().load(std::memory_order_acquire), modified,
+          on_command_line, ModificationCount());
+    }
+    case FlagValueStorageKind::kSequenceLocked: {
+      void* cloned = flags_internal::Alloc(op_);
+      // Read is guaranteed to be successful because we hold the lock.
+      bool success =
+          seq_lock_.TryRead(cloned, AtomicBufferValue(), Sizeof(op_));
+      assert(success);
+      static_cast<void>(success);
+      return std::make_unique<FlagState>(*this, cloned, modified,
+                                         on_command_line, ModificationCount());
+    }
+    case FlagValueStorageKind::kHeapAllocated: {
+      return std::make_unique<FlagState>(
+          *this,
+          flags_internal::Clone(
+              op_, PtrStorage().load(std::memory_order_acquire).Ptr()),
+          modified, on_command_line, ModificationCount());
+    }
+  }
+  return nullptr;
+}
+
+bool FlagImpl::RestoreState(const FlagState& flag_state) {
+  absl::MutexLock l(DataGuard());
+  if (flag_state.counter_ == ModificationCount()) {
+    return false;
+  }
+
+  switch (ValueStorageKind()) {
+    case FlagValueStorageKind::kValueAndInitBit:
+    case FlagValueStorageKind::kOneWordAtomic:
+      StoreValue(&flag_state.value_.one_word, kProgrammaticChange);
+      break;
+    case FlagValueStorageKind::kSequenceLocked:
+    case FlagValueStorageKind::kHeapAllocated:
+      StoreValue(flag_state.value_.heap_allocated, kProgrammaticChange);
+      break;
+  }
+
+  modified_ = flag_state.modified_;
+  on_command_line_ = flag_state.on_command_line_;
+
+  return true;
+}
+
+template <typename StorageT>
+StorageT* FlagImpl::OffsetValue() const {
+  char* p = reinterpret_cast<char*>(const_cast<FlagImpl*>(this));
+  // The offset is deduced via Flag value type specific op_.
+  ptrdiff_t offset = flags_internal::ValueOffset(op_);
+
+  return reinterpret_cast<StorageT*>(p + offset);
+}
+
+std::atomic<uint64_t>* FlagImpl::AtomicBufferValue() const {
+  assert(ValueStorageKind() == FlagValueStorageKind::kSequenceLocked);
+  return OffsetValue<std::atomic<uint64_t>>();
+}
+
+std::atomic<int64_t>& FlagImpl::OneWordValue() const {
+  assert(ValueStorageKind() == FlagValueStorageKind::kOneWordAtomic ||
+         ValueStorageKind() == FlagValueStorageKind::kValueAndInitBit);
+  return OffsetValue<FlagOneWordValue>()->value;
+}
+
+std::atomic<MaskedPointer>& FlagImpl::PtrStorage() const {
+  assert(ValueStorageKind() == FlagValueStorageKind::kHeapAllocated);
+  return OffsetValue<FlagMaskedPointerValue>()->value;
+}
+
+// Attempts to parse supplied `value` string using parsing routine in the `flag`
+// argument. If parsing successful, this function replaces the dst with newly
+// parsed value. In case if any error is encountered in either step, the error
+// message is stored in 'err'
+std::unique_ptr<void, DynValueDeleter> FlagImpl::TryParse(
+    absl::string_view value, std::string& err) const {
+  std::unique_ptr<void, DynValueDeleter> tentative_value = MakeInitValue();
+
+  std::string parse_err;
+  if (!flags_internal::Parse(op_, value, tentative_value.get(), &parse_err)) {
+    absl::string_view err_sep = parse_err.empty() ? "" : "; ";
+    err = absl::StrCat("Illegal value '", value, "' specified for flag '",
+                       Name(), "'", err_sep, parse_err);
+    return nullptr;
+  }
+
+  return tentative_value;
+}
+
+void FlagImpl::Read(void* dst) const {
+  auto& guard = DataGuard();  // Make sure flag initialized
+  switch (ValueStorageKind()) {
+    case FlagValueStorageKind::kValueAndInitBit:
+    case FlagValueStorageKind::kOneWordAtomic: {
+      const int64_t one_word_val =
+          OneWordValue().load(std::memory_order_acquire);
+      std::memcpy(dst, &one_word_val, Sizeof(op_));
+      break;
+    }
+    case FlagValueStorageKind::kSequenceLocked: {
+      ReadSequenceLockedData(dst);
+      break;
+    }
+    case FlagValueStorageKind::kHeapAllocated: {
+      absl::MutexLock l(guard);
+      MaskedPointer ptr_value = PtrStorage().load(std::memory_order_acquire);
+
+      flags_internal::CopyConstruct(op_, ptr_value.Ptr(), dst);
+
+      // For unprotected read candidates, mark that the value as has been read.
+      if (ptr_value.IsUnprotectedReadCandidate() && !ptr_value.HasBeenRead()) {
+        ptr_value.MarkAsRead();
+        PtrStorage().store(ptr_value, std::memory_order_release);
+      }
+      break;
+    }
+  }
+}
+
+int64_t FlagImpl::ReadOneWord() const {
+  assert(ValueStorageKind() == FlagValueStorageKind::kOneWordAtomic ||
+         ValueStorageKind() == FlagValueStorageKind::kValueAndInitBit);
+  auto& guard = DataGuard();  // Make sure flag initialized
+  (void)guard;
+  return OneWordValue().load(std::memory_order_acquire);
+}
+
+bool FlagImpl::ReadOneBool() const {
+  assert(ValueStorageKind() == FlagValueStorageKind::kValueAndInitBit);
+  auto& guard = DataGuard();  // Make sure flag initialized
+  (void)guard;
+  return absl::bit_cast<FlagValueAndInitBit<bool>>(
+             OneWordValue().load(std::memory_order_acquire))
+      .value;
+}
+
+void FlagImpl::ReadSequenceLockedData(void* dst) const {
+  size_t size = Sizeof(op_);
+  // Attempt to read using the sequence lock.
+  if (ABSL_PREDICT_TRUE(seq_lock_.TryRead(dst, AtomicBufferValue(), size))) {
+    return;
+  }
+  // We failed due to contention. Acquire the lock to prevent contention
+  // and try again.
+  absl::ReaderMutexLock l(DataGuard());
+  bool success = seq_lock_.TryRead(dst, AtomicBufferValue(), size);
+  assert(success);
+  static_cast<void>(success);
+}
+
+void FlagImpl::Write(const void* src) {
+  absl::MutexLock l(DataGuard());
+
+  if (ShouldValidateFlagValue(flags_internal::FastTypeId(op_))) {
+    std::unique_ptr<void, DynValueDeleter> obj{flags_internal::Clone(op_, src),
+                                               DynValueDeleter{op_}};
+    std::string ignored_error;
+    std::string src_as_str = flags_internal::Unparse(op_, src);
+    if (!flags_internal::Parse(op_, src_as_str, obj.get(), &ignored_error)) {
+      ABSL_INTERNAL_LOG(ERROR, absl::StrCat("Attempt to set flag '", Name(),
+                                            "' to invalid value ", src_as_str));
+    }
+  }
+
+  StoreValue(src, kProgrammaticChange);
+}
+
+// Sets the value of the flag based on specified string `value`. If the flag
+// was successfully set to new value, it returns true. Otherwise, sets `err`
+// to indicate the error, leaves the flag unchanged, and returns false. There
+// are three ways to set the flag's value:
+//  * Update the current flag value
+//  * Update the flag's default value
+//  * Update the current flag value if it was never set before
+// The mode is selected based on 'set_mode' parameter.
+bool FlagImpl::ParseFrom(absl::string_view value, FlagSettingMode set_mode,
+                         ValueSource source, std::string& err) {
+  absl::MutexLock l(DataGuard());
+
+  switch (set_mode) {
+    case SET_FLAGS_VALUE: {
+      // set or modify the flag's value
+      auto tentative_value = TryParse(value, err);
+      if (!tentative_value) return false;
+
+      StoreValue(tentative_value.get(), source);
+
+      if (source == kCommandLine) {
+        on_command_line_ = true;
+      }
+      break;
+    }
+    case SET_FLAG_IF_DEFAULT: {
+      // set the flag's value, but only if it hasn't been set by someone else
+      if (modified_) {
+        // TODO(rogeeff): review and fix this semantic. Currently we do not fail
+        // in this case if flag is modified. This is misleading since the flag's
+        // value is not updated even though we return true.
+        // *err = absl::StrCat(Name(), " is already set to ",
+        //                     CurrentValue(), "\n");
+        // return false;
+        return true;
+      }
+      auto tentative_value = TryParse(value, err);
+      if (!tentative_value) return false;
+
+      StoreValue(tentative_value.get(), source);
+      break;
+    }
+    case SET_FLAGS_DEFAULT: {
+      auto tentative_value = TryParse(value, err);
+      if (!tentative_value) return false;
+
+      if (DefaultKind() == FlagDefaultKind::kDynamicValue) {
+        void* old_value = default_value_.dynamic_value;
+        default_value_.dynamic_value = tentative_value.release();
+        tentative_value.reset(old_value);
+      } else {
+        default_value_.dynamic_value = tentative_value.release();
+        def_kind_ = static_cast<uint8_t>(FlagDefaultKind::kDynamicValue);
+      }
+
+      if (!modified_) {
+        // Need to set both default value *and* current, in this case.
+        StoreValue(default_value_.dynamic_value, source);
+        modified_ = false;
+      }
+      break;
+    }
+  }
+
+  return true;
+}
+
+void FlagImpl::CheckDefaultValueParsingRoundtrip() const {
+  std::string v = DefaultValue();
+
+  absl::MutexLock lock(DataGuard());
+
+  auto dst = MakeInitValue();
+  std::string error;
+  if (!flags_internal::Parse(op_, v, dst.get(), &error)) {
+    ABSL_INTERNAL_LOG(
+        FATAL,
+        absl::StrCat("Flag ", Name(), " (from ", Filename(),
+                     "): string form of default value '", v,
+                     "' could not be parsed; error=", error));
+  }
+
+  // We do not compare dst to def since parsing/unparsing may make
+  // small changes, e.g., precision loss for floating point types.
+}
+
+bool FlagImpl::ValidateInputValue(absl::string_view value) const {
+  absl::MutexLock l(DataGuard());
 
-  primary_mu->Lock();
+  auto obj = MakeInitValue();
+  std::string ignored_error;
+  return flags_internal::Parse(op_, value, obj.get(), &ignored_error);
 }
 
 }  // namespace flags_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/flags/internal/flag.h b/absl/flags/internal/flag.h
index 16330380f54..7855a104b45 100644
--- a/absl/flags/internal/flag.h
+++ b/absl/flags/internal/flag.h
@@ -16,58 +16,805 @@
 #ifndef ABSL_FLAGS_INTERNAL_FLAG_H_
 #define ABSL_FLAGS_INTERNAL_FLAG_H_
 
+#include <stddef.h>
+#include <stdint.h>
+
+#include <atomic>
 #include <cstring>
+#include <memory>
+#include <string>
+#include <type_traits>
+#include <typeinfo>
 
+#include "absl/base/attributes.h"
+#include "absl/base/call_once.h"
+#include "absl/base/casts.h"
+#include "absl/base/config.h"
+#include "absl/base/optimization.h"
+#include "absl/base/thread_annotations.h"
+#include "absl/flags/commandlineflag.h"
+#include "absl/flags/config.h"
 #include "absl/flags/internal/commandlineflag.h"
 #include "absl/flags/internal/registry.h"
+#include "absl/flags/internal/sequence_lock.h"
+#include "absl/flags/marshalling.h"
+#include "absl/meta/type_traits.h"
+#include "absl/strings/string_view.h"
+#include "absl/synchronization/mutex.h"
+#include "absl/utility/utility.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+///////////////////////////////////////////////////////////////////////////////
+// Forward declaration of absl::Flag<T> public API.
+namespace flags_internal {
+template <typename T>
+class Flag;
+}  // namespace flags_internal
+
+template <typename T>
+using Flag = flags_internal::Flag<T>;
+
+template <typename T>
+[[nodiscard]] T GetFlag(const absl::Flag<T>& flag);
+
+template <typename T>
+void SetFlag(absl::Flag<T>* flag, const T& v);
+
+template <typename T, typename V>
+void SetFlag(absl::Flag<T>* flag, const V& v);
+
+template <typename U>
+const CommandLineFlag& GetFlagReflectionHandle(const absl::Flag<U>& f);
+
+///////////////////////////////////////////////////////////////////////////////
+// Flag value type operations, eg., parsing, copying, etc. are provided
+// by function specific to that type with a signature matching FlagOpFn.
+
 namespace flags_internal {
 
-constexpr int64_t AtomicInit() { return 0xababababababababll; }
+enum class FlagOp {
+  kAlloc,
+  kDelete,
+  kCopy,
+  kCopyConstruct,
+  kSizeof,
+  kFastTypeId,
+  kRuntimeTypeId,
+  kParse,
+  kUnparse,
+  kValueOffset,
+};
+using FlagOpFn = void* (*)(FlagOp, const void*, void*, void*);
+
+// Forward declaration for Flag value specific operations.
+template <typename T>
+void* FlagOps(FlagOp op, const void* v1, void* v2, void* v3);
+
+// Allocate aligned memory for a flag value.
+inline void* Alloc(FlagOpFn op) {
+  return op(FlagOp::kAlloc, nullptr, nullptr, nullptr);
+}
+// Deletes memory interpreting obj as flag value type pointer.
+inline void Delete(FlagOpFn op, void* obj) {
+  op(FlagOp::kDelete, nullptr, obj, nullptr);
+}
+// Copies src to dst interpreting as flag value type pointers.
+inline void Copy(FlagOpFn op, const void* src, void* dst) {
+  op(FlagOp::kCopy, src, dst, nullptr);
+}
+// Construct a copy of flag value in a location pointed by dst
+// based on src - pointer to the flag's value.
+inline void CopyConstruct(FlagOpFn op, const void* src, void* dst) {
+  op(FlagOp::kCopyConstruct, src, dst, nullptr);
+}
+// Makes a copy of flag value pointed by obj.
+inline void* Clone(FlagOpFn op, const void* obj) {
+  void* res = flags_internal::Alloc(op);
+  flags_internal::CopyConstruct(op, obj, res);
+  return res;
+}
+// Returns true if parsing of input text is successful.
+inline bool Parse(FlagOpFn op, absl::string_view text, void* dst,
+                  std::string* error) {
+  return op(FlagOp::kParse, &text, dst, error) != nullptr;
+}
+// Returns string representing supplied value.
+inline std::string Unparse(FlagOpFn op, const void* val) {
+  std::string result;
+  op(FlagOp::kUnparse, val, &result, nullptr);
+  return result;
+}
+// Returns size of flag value type.
+inline size_t Sizeof(FlagOpFn op) {
+  // This sequence of casts reverses the sequence from
+  // `flags_internal::FlagOps()`
+  return static_cast<size_t>(reinterpret_cast<intptr_t>(
+      op(FlagOp::kSizeof, nullptr, nullptr, nullptr)));
+}
+// Returns fast type id corresponding to the value type.
+inline FlagFastTypeId FastTypeId(FlagOpFn op) {
+  return absl::bit_cast<FlagFastTypeId>(
+      op(FlagOp::kFastTypeId, nullptr, nullptr, nullptr));
+}
+// Returns fast type id corresponding to the value type.
+inline const std::type_info* RuntimeTypeId(FlagOpFn op) {
+  return reinterpret_cast<const std::type_info*>(
+      op(FlagOp::kRuntimeTypeId, nullptr, nullptr, nullptr));
+}
+// Returns offset of the field value_ from the field impl_ inside of
+// absl::Flag<T> data. Given FlagImpl pointer p you can get the
+// location of the corresponding value as:
+//      reinterpret_cast<char*>(p) + ValueOffset().
+inline ptrdiff_t ValueOffset(FlagOpFn op) {
+  // This sequence of casts reverses the sequence from
+  // `flags_internal::FlagOps()`
+  return static_cast<ptrdiff_t>(reinterpret_cast<intptr_t>(
+      op(FlagOp::kValueOffset, nullptr, nullptr, nullptr)));
+}
+
+// Returns an address of RTTI's typeid(T).
+template <typename T>
+inline const std::type_info* GenRuntimeTypeId() {
+#ifdef ABSL_INTERNAL_HAS_RTTI
+  return &typeid(T);
+#else
+  return nullptr;
+#endif
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Flag help auxiliary structs.
+
+// This is help argument for absl::Flag encapsulating the string literal pointer
+// or pointer to function generating it as well as enum discriminating two
+// cases.
+using HelpGenFunc = std::string (*)();
+
+template <size_t N>
+struct FixedCharArray {
+  char value[N];
+
+  template <size_t... I>
+  static constexpr FixedCharArray<N> FromLiteralString(
+      absl::string_view str, std::index_sequence<I...>) {
+    return (void)str, FixedCharArray<N>({{str[I]..., '\0'}});
+  }
+};
+
+template <typename Gen, size_t N = Gen::Value().size()>
+constexpr FixedCharArray<N + 1> HelpStringAsArray(int) {
+  return FixedCharArray<N + 1>::FromLiteralString(
+      Gen::Value(), std::make_index_sequence<N>{});
+}
+
+template <typename Gen>
+constexpr std::false_type HelpStringAsArray(char) {
+  return std::false_type{};
+}
+
+union FlagHelpMsg {
+  constexpr explicit FlagHelpMsg(const char* help_msg) : literal(help_msg) {}
+  constexpr explicit FlagHelpMsg(HelpGenFunc help_gen) : gen_func(help_gen) {}
+
+  const char* literal;
+  HelpGenFunc gen_func;
+};
+
+enum class FlagHelpKind : uint8_t { kLiteral = 0, kGenFunc = 1 };
+
+struct FlagHelpArg {
+  FlagHelpMsg source;
+  FlagHelpKind kind;
+};
+
+extern const char kStrippedFlagHelp[];
+
+// These two HelpArg overloads allows us to select at compile time one of two
+// way to pass Help argument to absl::Flag. We'll be passing
+// AbslFlagHelpGenFor##name as Gen and integer 0 as a single argument to prefer
+// first overload if possible. If help message is evaluatable on constexpr
+// context We'll be able to make FixedCharArray out of it and we'll choose first
+// overload. In this case the help message expression is immediately evaluated
+// and is used to construct the absl::Flag. No additional code is generated by
+// ABSL_FLAG Otherwise SFINAE kicks in and first overload is dropped from the
+// consideration, in which case the second overload will be used. The second
+// overload does not attempt to evaluate the help message expression
+// immediately and instead delays the evaluation by returning the function
+// pointer (&T::NonConst) generating the help message when necessary. This is
+// evaluatable in constexpr context, but the cost is an extra function being
+// generated in the ABSL_FLAG code.
+template <typename Gen, size_t N>
+constexpr FlagHelpArg HelpArg(const FixedCharArray<N>& value) {
+  return {FlagHelpMsg(value.value), FlagHelpKind::kLiteral};
+}
+
+template <typename Gen>
+constexpr FlagHelpArg HelpArg(std::false_type) {
+  return {FlagHelpMsg(&Gen::NonConst), FlagHelpKind::kGenFunc};
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Flag default value auxiliary structs.
+
+// Signature for the function generating the initial flag value (usually
+// based on default value supplied in flag's definition)
+using FlagDfltGenFunc = void (*)(void*);
+
+union FlagDefaultSrc {
+  constexpr explicit FlagDefaultSrc(FlagDfltGenFunc gen_func_arg)
+      : gen_func(gen_func_arg) {}
+
+#define ABSL_FLAGS_INTERNAL_DFLT_FOR_TYPE(T, name) \
+  T name##_value;                                  \
+  constexpr explicit FlagDefaultSrc(T value) : name##_value(value) {}  // NOLINT
+  ABSL_FLAGS_INTERNAL_BUILTIN_TYPES(ABSL_FLAGS_INTERNAL_DFLT_FOR_TYPE)
+#undef ABSL_FLAGS_INTERNAL_DFLT_FOR_TYPE
+
+  void* dynamic_value;
+  FlagDfltGenFunc gen_func;
+};
+
+enum class FlagDefaultKind : uint8_t {
+  kDynamicValue = 0,
+  kGenFunc = 1,
+  kOneWord = 2  // for default values UP to one word in size
+};
+
+struct FlagDefaultArg {
+  FlagDefaultSrc source;
+  FlagDefaultKind kind;
+};
+
+// This struct and corresponding overload to InitDefaultValue are used to
+// facilitate usage of {} as default value in ABSL_FLAG macro.
+// TODO(rogeeff): Fix handling types with explicit constructors.
+struct EmptyBraces {};
+
+template <typename T>
+constexpr T InitDefaultValue(T t) {
+  return t;
+}
+
+template <typename T>
+constexpr T InitDefaultValue(EmptyBraces) {
+  return T{};
+}
+
+template <typename ValueT, typename GenT,
+          std::enable_if_t<std::is_integral_v<ValueT>, int> = ((void)GenT{}, 0)>
+constexpr FlagDefaultArg DefaultArg(int) {
+  return {FlagDefaultSrc(GenT{}.value), FlagDefaultKind::kOneWord};
+}
+
+template <typename ValueT, typename GenT>
+constexpr FlagDefaultArg DefaultArg(char) {
+  return {FlagDefaultSrc(&GenT::Gen), FlagDefaultKind::kGenFunc};
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Flag storage selector traits. Each trait indicates what kind of storage kind
+// to use for the flag value.
+
+template <typename T>
+using FlagUseValueAndInitBitStorage =
+    std::bool_constant<std::is_trivially_copyable_v<T> &&
+                       std::is_default_constructible_v<T> && (sizeof(T) < 8)>;
+
+template <typename T>
+using FlagUseOneWordStorage =
+    std::bool_constant<std::is_trivially_copyable_v<T> && (sizeof(T) <= 8)>;
+
+template <class T>
+using FlagUseSequenceLockStorage =
+    std::bool_constant<std::is_trivially_copyable_v<T> && (sizeof(T) > 8)>;
+
+enum class FlagValueStorageKind : uint8_t {
+  kValueAndInitBit = 0,
+  kOneWordAtomic = 1,
+  kSequenceLocked = 2,
+  kHeapAllocated = 3,
+};
+
+// This constexpr function returns the storage kind for the given flag value
+// type.
+template <typename T>
+static constexpr FlagValueStorageKind StorageKind() {
+  return FlagUseValueAndInitBitStorage<T>::value
+             ? FlagValueStorageKind::kValueAndInitBit
+         : FlagUseOneWordStorage<T>::value
+             ? FlagValueStorageKind::kOneWordAtomic
+         : FlagUseSequenceLockStorage<T>::value
+             ? FlagValueStorageKind::kSequenceLocked
+             : FlagValueStorageKind::kHeapAllocated;
+}
+
+// This is a base class for the storage classes used by kOneWordAtomic and
+// kValueAndInitBit storage kinds. It literally just stores the one word value
+// as an atomic. By default, it is initialized to a magic value that is unlikely
+// a valid value for the flag value type.
+struct FlagOneWordValue {
+  constexpr static int64_t Uninitialized() {
+    return static_cast<int64_t>(0xababababababababll);
+  }
+
+  constexpr FlagOneWordValue() : value(Uninitialized()) {}
+  constexpr explicit FlagOneWordValue(int64_t v) : value(v) {}
+  std::atomic<int64_t> value;
+};
+
+// This class represents a memory layout used by kValueAndInitBit storage kind.
+template <typename T>
+struct alignas(8) FlagValueAndInitBit {
+  T value;
+  // Use an int instead of a bool to guarantee that a non-zero value has
+  // a bit set.
+  uint8_t init;
+};
+
+// This class implements an aligned pointer with two options stored via masks
+// in unused bits of the pointer value (due to alignment requirement).
+//  - IsUnprotectedReadCandidate - indicates that the value can be switched to
+//    unprotected read without a lock.
+//  - HasBeenRead - indicates that the value has been read at least once.
+//  - AllowsUnprotectedRead - combination of the two options above and indicates
+//    that the value can now be read without a lock.
+// Further details of these options and their use is covered in the description
+// of the FlagValue<T, FlagValueStorageKind::kHeapAllocated> specialization.
+class MaskedPointer {
+ public:
+  using mask_t = uintptr_t;
+  using ptr_t = void*;
+
+  static constexpr int RequiredAlignment() { return 4; }
+
+  constexpr MaskedPointer() : ptr_(nullptr) {}
+  constexpr explicit MaskedPointer(ptr_t rhs) : ptr_(rhs) {}
+  MaskedPointer(ptr_t rhs, bool is_candidate);
+
+  MaskedPointer(const MaskedPointer& rhs) = default;
+  MaskedPointer& operator=(const MaskedPointer& rhs) = default;
+
+  void* Ptr() const {
+    return reinterpret_cast<void*>(reinterpret_cast<mask_t>(ptr_) &
+                                   kPtrValueMask);
+  }
+  bool AllowsUnprotectedRead() const {
+    return (reinterpret_cast<mask_t>(ptr_) & kAllowsUnprotectedRead) ==
+           kAllowsUnprotectedRead;
+  }
+  bool IsUnprotectedReadCandidate() const;
+  bool HasBeenRead() const;
+
+  void Set(FlagOpFn op, const void* src, bool is_candidate);
+  void MarkAsRead();
+
+ private:
+  // Masks
+  // Indicates that the flag value either default or originated from command
+  // line.
+  static constexpr mask_t kUnprotectedReadCandidate = 0x1u;
+  // Indicates that flag has been read.
+  static constexpr mask_t kHasBeenRead = 0x2u;
+  static constexpr mask_t kAllowsUnprotectedRead =
+      kUnprotectedReadCandidate | kHasBeenRead;
+  static constexpr mask_t kPtrValueMask = ~kAllowsUnprotectedRead;
+
+  void ApplyMask(mask_t mask);
+  bool CheckMask(mask_t mask) const;
+
+  ptr_t ptr_;
+};
+
+// This class implements a type erased storage of the heap allocated flag value.
+// It is used as a base class for the storage class for kHeapAllocated storage
+// kind. The initial_buffer is expected to have an alignment of at least
+// MaskedPointer::RequiredAlignment(), so that the bits used by the
+// MaskedPointer to store masks are set to 0. This guarantees that value starts
+// in an uninitialized state.
+struct FlagMaskedPointerValue {
+  constexpr explicit FlagMaskedPointerValue(MaskedPointer::ptr_t initial_buffer)
+      : value(MaskedPointer(initial_buffer)) {}
+
+  std::atomic<MaskedPointer> value;
+};
+
+// This is the forward declaration for the template that represents a storage
+// for the flag values. This template is expected to be explicitly specialized
+// for each storage kind and it does not have a generic default
+// implementation.
+template <typename T,
+          FlagValueStorageKind Kind = flags_internal::StorageKind<T>()>
+struct FlagValue;
+
+// This specialization represents the storage of flag values types with the
+// kValueAndInitBit storage kind. It is based on the FlagOneWordValue class
+// and relies on memory layout in FlagValueAndInitBit<T> to indicate that the
+// value has been initialized or not.
+template <typename T>
+struct FlagValue<T, FlagValueStorageKind::kValueAndInitBit> : FlagOneWordValue {
+  constexpr FlagValue() : FlagOneWordValue(0) {}
+  bool Get(const SequenceLock&, T& dst) const {
+    int64_t storage = value.load(std::memory_order_acquire);
+    if (ABSL_PREDICT_FALSE(storage == 0)) {
+      // This assert is to ensure that the initialization inside FlagImpl::Init
+      // is able to set init member correctly.
+      static_assert(offsetof(FlagValueAndInitBit<T>, init) == sizeof(T),
+                    "Unexpected memory layout of FlagValueAndInitBit");
+      return false;
+    }
+    dst = absl::bit_cast<FlagValueAndInitBit<T>>(storage).value;
+    return true;
+  }
+};
+
+// This specialization represents the storage of flag values types with the
+// kOneWordAtomic storage kind. It is based on the FlagOneWordValue class
+// and relies on the magic uninitialized state of default constructed instead of
+// FlagOneWordValue to indicate that the value has been initialized or not.
+template <typename T>
+struct FlagValue<T, FlagValueStorageKind::kOneWordAtomic> : FlagOneWordValue {
+  constexpr FlagValue() : FlagOneWordValue() {}
+  bool Get(const SequenceLock&, T& dst) const {
+    int64_t one_word_val = value.load(std::memory_order_acquire);
+    if (ABSL_PREDICT_FALSE(one_word_val == FlagOneWordValue::Uninitialized())) {
+      return false;
+    }
+    std::memcpy(&dst, static_cast<const void*>(&one_word_val), sizeof(T));
+    return true;
+  }
+};
+
+// This specialization represents the storage of flag values types with the
+// kSequenceLocked storage kind. This storage is used by trivially copyable
+// types with size greater than 8 bytes. This storage relies on uninitialized
+// state of the SequenceLock to indicate that the value has been initialized or
+// not. This storage also provides lock-free read access to the underlying
+// value once it is initialized.
+template <typename T>
+struct FlagValue<T, FlagValueStorageKind::kSequenceLocked> {
+  bool Get(const SequenceLock& lock, T& dst) const {
+    return lock.TryRead(&dst, value_words, sizeof(T));
+  }
+
+  static constexpr int kNumWords =
+      flags_internal::AlignUp(sizeof(T), sizeof(uint64_t)) / sizeof(uint64_t);
+
+  alignas(T) alignas(
+      std::atomic<uint64_t>) std::atomic<uint64_t> value_words[kNumWords];
+};
+
+// This specialization represents the storage of flag values types with the
+// kHeapAllocated storage kind. This is a storage of last resort and is used
+// if none of other storage kinds are applicable.
+//
+// Generally speaking the values with this storage kind can't be accessed
+// atomically and thus can't be read without holding a lock. If we would ever
+// want to avoid the lock, we'd need to leak the old value every time new flag
+// value is being set (since we are in danger of having a race condition
+// otherwise).
+//
+// Instead of doing that, this implementation attempts to cater to some common
+// use cases by allowing at most 2 values to be leaked - default value and
+// value set from the command line.
+//
+// This specialization provides an initial buffer for the first flag value. This
+// is where the default value is going to be stored. We attempt to reuse this
+// buffer if possible, including storing the value set from the command line
+// there.
+//
+// As long as we only read this value, we can access it without a lock (in
+// practice we still use the lock for the very first read to be able set
+// "has been read" option on this flag).
+//
+// If flag is specified on the command line we store the parsed value either
+// in the internal buffer (if the default value never been read) or we leak the
+// default value and allocate the new storage for the parse value. This value is
+// also a candidate for an unprotected read. If flag is set programmatically
+// after the command line is parsed, the storage for this value is going to be
+// leaked. Note that in both scenarios we are not going to have a real leak.
+// Instead we'll store the leaked value pointers in the internal freelist to
+// avoid triggering the memory leak checker complains.
+//
+// If the flag is ever set programmatically, it stops being the candidate for an
+// unprotected read, and any follow up access to the flag value requires a lock.
+// Note that if the value if set programmatically before the command line is
+// parsed, we can switch back to enabling unprotected reads for that value.
+template <typename T>
+struct FlagValue<T, FlagValueStorageKind::kHeapAllocated>
+    : FlagMaskedPointerValue {
+  // We const initialize the value with unmasked pointer to the internal buffer,
+  // making sure it is not a candidate for unprotected read. This way we can
+  // ensure Init is done before any access to the flag value.
+  constexpr FlagValue() : FlagMaskedPointerValue(&buffer[0]) {}
+
+  bool Get(const SequenceLock&, T& dst) const {
+    MaskedPointer ptr_value = value.load(std::memory_order_acquire);
+
+    if (ABSL_PREDICT_TRUE(ptr_value.AllowsUnprotectedRead())) {
+      ::new (static_cast<void*>(&dst)) T(*static_cast<T*>(ptr_value.Ptr()));
+      return true;
+    }
+    return false;
+  }
+
+  alignas(MaskedPointer::RequiredAlignment()) alignas(
+      T) char buffer[sizeof(T)]{};
+};
+
+///////////////////////////////////////////////////////////////////////////////
+// Flag callback auxiliary structs.
 
 // Signature for the mutation callback used by watched Flags
 // The callback is noexcept.
 // TODO(rogeeff): add noexcept after C++17 support is added.
-using FlagCallback = void (*)();
+using FlagCallbackFunc = void (*)();
+
+struct FlagCallback {
+  FlagCallbackFunc func;
+  absl::Mutex guard;  // Guard for concurrent callback invocations.
+};
+
+///////////////////////////////////////////////////////////////////////////////
+// Flag implementation, which does not depend on flag value type.
+// The class encapsulates the Flag's data and access to it.
+
+struct DynValueDeleter {
+  explicit DynValueDeleter(FlagOpFn op_arg = nullptr);
+  void operator()(void* ptr) const;
+
+  FlagOpFn op;
+};
+
+class FlagState;
+
+// These are only used as constexpr global objects.
+// They do not use a virtual destructor to simplify their implementation.
+// They are not destroyed except at program exit, so leaks do not matter.
+#if defined(__GNUC__) && !defined(__clang__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wnon-virtual-dtor"
+#endif
+class FlagImpl final : public CommandLineFlag {
+ public:
+  constexpr FlagImpl(const char* name, const char* type_name,
+                     const char* filename, FlagOpFn op, FlagHelpArg help,
+                     FlagValueStorageKind value_kind,
+                     FlagDefaultArg default_arg)
+      : name_(name),
+        type_name_(type_name),
+        filename_(filename),
+        op_(op),
+        help_(help.source),
+        help_source_kind_(static_cast<uint8_t>(help.kind)),
+        value_storage_kind_(static_cast<uint8_t>(value_kind)),
+        def_kind_(static_cast<uint8_t>(default_arg.kind)),
+        modified_(false),
+        on_command_line_(false),
+        callback_(nullptr),
+        default_value_(default_arg.source),
+        data_guard_{} {}
+
+  // Constant access methods
+  int64_t ReadOneWord() const ABSL_LOCKS_EXCLUDED(DataGuard());
+  bool ReadOneBool() const ABSL_LOCKS_EXCLUDED(DataGuard());
+  void Read(void* dst) const override ABSL_LOCKS_EXCLUDED(DataGuard());
+  void Read(bool* value) const ABSL_LOCKS_EXCLUDED(DataGuard()) {
+    *value = ReadOneBool();
+  }
+  template <typename T,
+            std::enable_if_t<flags_internal::StorageKind<T>() ==
+                                  FlagValueStorageKind::kOneWordAtomic,
+                              int> = 0>
+  void Read(T* value) const ABSL_LOCKS_EXCLUDED(DataGuard()) {
+    int64_t v = ReadOneWord();
+    std::memcpy(value, static_cast<const void*>(&v), sizeof(T));
+  }
+  template <typename T,
+            std::enable_if_t<flags_internal::StorageKind<T>() ==
+                                 FlagValueStorageKind::kValueAndInitBit,
+                             int> = 0>
+  void Read(T* value) const ABSL_LOCKS_EXCLUDED(DataGuard()) {
+    *value = absl::bit_cast<FlagValueAndInitBit<T>>(ReadOneWord()).value;
+  }
+
+  // Mutating access methods
+  void Write(const void* src) ABSL_LOCKS_EXCLUDED(DataGuard());
+
+  // Interfaces to operate on callbacks.
+  void SetCallback(const FlagCallbackFunc mutation_callback)
+      ABSL_LOCKS_EXCLUDED(DataGuard());
+  void InvokeCallback() const ABSL_EXCLUSIVE_LOCKS_REQUIRED(DataGuard());
+
+  // Used in read/write operations to validate source/target has correct type.
+  // For example if flag is declared as absl::Flag<int> FLAGS_foo, a call to
+  // absl::GetFlag(FLAGS_foo) validates that the type of FLAGS_foo is indeed
+  // int. To do that we pass the assumed type id (which is deduced from type
+  // int) as an argument `type_id`, which is in turn is validated against the
+  // type id stored in flag object by flag definition statement.
+  void AssertValidType(FlagFastTypeId type_id,
+                       const std::type_info* (*gen_rtti)()) const;
+
+ private:
+  template <typename T>
+  friend class Flag;
+  friend class FlagState;
+
+  // Ensures that `data_guard_` is initialized and returns it.
+  absl::Mutex& DataGuard() const
+      ABSL_LOCK_RETURNED(reinterpret_cast<absl::Mutex*>(data_guard_));
+  // Returns heap allocated value of type T initialized with default value.
+  std::unique_ptr<void, DynValueDeleter> MakeInitValue() const
+      ABSL_EXCLUSIVE_LOCKS_REQUIRED(DataGuard());
+  // Flag initialization called via absl::call_once.
+  void Init();
+
+  // Offset value access methods. One per storage kind. These methods to not
+  // respect const correctness, so be very careful using them.
+
+  // This is a shared helper routine which encapsulates most of the magic. Since
+  // it is only used inside the three routines below, which are defined in
+  // flag.cc, we can define it in that file as well.
+  template <typename StorageT>
+  StorageT* OffsetValue() const;
+
+  // The same as above, but used for sequencelock-protected storage.
+  std::atomic<uint64_t>* AtomicBufferValue() const;
+
+  // This is an accessor for a value stored as one word atomic. Returns a
+  // mutable reference to an atomic value.
+  std::atomic<int64_t>& OneWordValue() const;
+
+  std::atomic<MaskedPointer>& PtrStorage() const;
+
+  // Attempts to parse supplied `value` string. If parsing is successful,
+  // returns new value. Otherwise returns nullptr.
+  std::unique_ptr<void, DynValueDeleter> TryParse(absl::string_view value,
+                                                  std::string& err) const
+      ABSL_EXCLUSIVE_LOCKS_REQUIRED(DataGuard());
+  // Stores the flag value based on the pointer to the source.
+  void StoreValue(const void* src, ValueSource source)
+      ABSL_EXCLUSIVE_LOCKS_REQUIRED(DataGuard());
 
-void InvokeCallback(absl::Mutex* primary_mu, absl::Mutex* callback_mu,
-                    FlagCallback cb) ABSL_EXCLUSIVE_LOCKS_REQUIRED(primary_mu);
+  // Copy the flag data, protected by `seq_lock_` into `dst`.
+  //
+  // REQUIRES: ValueStorageKind() == kSequenceLocked.
+  void ReadSequenceLockedData(void* dst) const ABSL_LOCKS_EXCLUDED(DataGuard());
+
+  FlagHelpKind HelpSourceKind() const {
+    return static_cast<FlagHelpKind>(help_source_kind_);
+  }
+  FlagValueStorageKind ValueStorageKind() const {
+    return static_cast<FlagValueStorageKind>(value_storage_kind_);
+  }
+  FlagDefaultKind DefaultKind() const
+      ABSL_EXCLUSIVE_LOCKS_REQUIRED(DataGuard()) {
+    return static_cast<FlagDefaultKind>(def_kind_);
+  }
+
+  // CommandLineFlag interface implementation
+  absl::string_view Name() const override;
+  absl::string_view TypeName() const override;
+  std::string Filename() const override;
+  std::string Help() const override;
+  FlagFastTypeId TypeId() const override;
+  bool IsSpecifiedOnCommandLine() const override
+      ABSL_LOCKS_EXCLUDED(DataGuard());
+  std::string DefaultValue() const override ABSL_LOCKS_EXCLUDED(DataGuard());
+  std::string CurrentValue() const override ABSL_LOCKS_EXCLUDED(DataGuard());
+  bool ValidateInputValue(absl::string_view value) const override
+      ABSL_LOCKS_EXCLUDED(DataGuard());
+  void CheckDefaultValueParsingRoundtrip() const override
+      ABSL_LOCKS_EXCLUDED(DataGuard());
+
+  int64_t ModificationCount() const ABSL_EXCLUSIVE_LOCKS_REQUIRED(DataGuard());
+
+  // Interfaces to save and restore flags to/from persistent state.
+  // Returns current flag state or nullptr if flag does not support
+  // saving and restoring a state.
+  std::unique_ptr<FlagStateInterface> SaveState() override
+      ABSL_LOCKS_EXCLUDED(DataGuard());
+
+  // Restores the flag state to the supplied state object. If there is
+  // nothing to restore returns false. Otherwise returns true.
+  bool RestoreState(const FlagState& flag_state)
+      ABSL_LOCKS_EXCLUDED(DataGuard());
+
+  bool ParseFrom(absl::string_view value, FlagSettingMode set_mode,
+                 ValueSource source, std::string& error) override
+      ABSL_LOCKS_EXCLUDED(DataGuard());
+
+  // Immutable flag's state.
+
+  // Flags name passed to ABSL_FLAG as second arg.
+  const char* const name_;
+
+  // Flags type passed to ABSL_FLAG as first arg.
+  const char* const type_name_;
+
+  // The file name where ABSL_FLAG resides.
+  const char* const filename_;
+  // Type-specific operations vtable.
+  const FlagOpFn op_;
+  // Help message literal or function to generate it.
+  const FlagHelpMsg help_;
+  // Indicates if help message was supplied as literal or generator func.
+  const uint8_t help_source_kind_ : 1;
+  // Kind of storage this flag is using for the flag's value.
+  const uint8_t value_storage_kind_ : 2;
+
+  uint8_t : 0;  // The bytes containing the const bitfields must not be
+                // shared with bytes containing the mutable bitfields.
+
+  // Mutable flag's state (guarded by `data_guard_`).
+
+  // def_kind_ is not guard by DataGuard() since it is accessed in Init without
+  // locks.
+  uint8_t def_kind_ : 2;
+  // Has this flag's value been modified?
+  bool modified_ : 1 ABSL_GUARDED_BY(DataGuard());
+  // Has this flag been specified on command line.
+  bool on_command_line_ : 1 ABSL_GUARDED_BY(DataGuard());
+
+  // Unique tag for absl::call_once call to initialize this flag.
+  absl::once_flag init_control_;
+
+  // Sequence lock / mutation counter.
+  flags_internal::SequenceLock seq_lock_;
+
+  // Optional flag's callback and absl::Mutex to guard the invocations.
+  FlagCallback* callback_ ABSL_GUARDED_BY(DataGuard());
+  // Either a pointer to the function generating the default value based on the
+  // value specified in ABSL_FLAG or pointer to the dynamically set default
+  // value via SetCommandLineOptionWithMode. def_kind_ is used to distinguish
+  // these two cases.
+  FlagDefaultSrc default_value_;
+
+  // This is reserved space for an absl::Mutex to guard flag data. It will be
+  // initialized in FlagImpl::Init via placement new.
+  // We can't use "absl::Mutex data_guard_", since this class is not literal.
+  // We do not want to use "absl::Mutex* data_guard_", since this would require
+  // heap allocation during initialization, which is both slows program startup
+  // and can fail. Using reserved space + placement new allows us to avoid both
+  // problems.
+  alignas(absl::Mutex) mutable unsigned char data_guard_[sizeof(absl::Mutex)];
+};
+#if defined(__GNUC__) && !defined(__clang__)
+#pragma GCC diagnostic pop
+#endif
+
+///////////////////////////////////////////////////////////////////////////////
+// The Flag object parameterized by the flag's value type. This class implements
+// flag reflection handle interface.
 
-// This is "unspecified" implementation of absl::Flag<T> type.
 template <typename T>
-class Flag final : public flags_internal::CommandLineFlag {
+class Flag {
  public:
-  constexpr Flag(const char* name, const flags_internal::HelpGenFunc help_gen,
-                 const char* filename,
-                 const flags_internal::FlagMarshallingOpFn marshalling_op,
-                 const flags_internal::InitialValGenFunc initial_value_gen)
-      : flags_internal::CommandLineFlag(
-            name, flags_internal::HelpText::FromFunctionPointer(help_gen),
-            filename, &flags_internal::FlagOps<T>, marshalling_op,
-            initial_value_gen,
-            /*def=*/nullptr,
-            /*cur=*/nullptr),
-        atomic_(flags_internal::AtomicInit()),
-        callback_(nullptr) {}
+  constexpr Flag(const char* name, const char* type_name, const char* filename,
+                 FlagHelpArg help, const FlagDefaultArg default_arg)
+      : impl_(name, type_name, filename, &FlagOps<T>, help,
+              flags_internal::StorageKind<T>(), default_arg),
+        value_() {}
+
+  // CommandLineFlag interface
+  absl::string_view Name() const { return impl_.Name(); }
+  std::string Filename() const { return impl_.Filename(); }
+  std::string Help() const { return impl_.Help(); }
+  // Do not use. To be removed.
+  bool IsSpecifiedOnCommandLine() const {
+    return impl_.IsSpecifiedOnCommandLine();
+  }
+  std::string DefaultValue() const { return impl_.DefaultValue(); }
+  std::string CurrentValue() const { return impl_.CurrentValue(); }
+
+ private:
+  template <typename, bool>
+  friend class FlagRegistrar;
+  friend class FlagImplPeer;
 
   T Get() const {
-    // Implementation notes:
-    //
-    // We are wrapping a union around the value of `T` to serve three purposes:
-    //
-    //  1. `U.value` has correct size and alignment for a value of type `T`
-    //  2. The `U.value` constructor is not invoked since U's constructor does
-    //  not
-    //     do it explicitly.
-    //  3. The `U.value` destructor is invoked since U's destructor does it
-    //     explicitly. This makes `U` a kind of RAII wrapper around non default
-    //     constructible value of T, which is destructed when we leave the
-    //     scope. We do need to destroy U.value, which is constructed by
-    //     CommandLineFlag::Read even though we left it in a moved-from state
-    //     after std::move.
-    //
-    // All of this serves to avoid requiring `T` being default constructible.
+    // See implementation notes in CommandLineFlag::Get().
     union U {
       T value;
       U() {}
@@ -75,97 +822,144 @@ class Flag final : public flags_internal::CommandLineFlag {
     };
     U u;
 
-    Read(&u.value, &flags_internal::FlagOps<T>);
-    return std::move(u.value);
-  }
+#if !defined(NDEBUG)
+    impl_.AssertValidType(absl::FastTypeId<T>(), &GenRuntimeTypeId<T>);
+#endif
 
-  bool AtomicGet(T* v) const {
-    const int64_t r = atomic_.load(std::memory_order_acquire);
-    if (r != flags_internal::AtomicInit()) {
-      std::memcpy(v, &r, sizeof(T));
-      return true;
+    if (ABSL_PREDICT_FALSE(!value_.Get(impl_.seq_lock_, u.value))) {
+      impl_.Read(&u.value);
     }
-
-    return false;
+    return std::move(u.value);
+  }
+  void Set(const T& v) {
+    impl_.AssertValidType(absl::FastTypeId<T>(), &GenRuntimeTypeId<T>);
+    impl_.Write(&v);
   }
 
-  void Set(const T& v) { Write(&v, &flags_internal::FlagOps<T>); }
+  // Access to the reflection.
+  const CommandLineFlag& Reflect() const { return impl_; }
 
-  void SetCallback(const flags_internal::FlagCallback mutation_callback) {
-    absl::MutexLock l(InitFlagIfNecessary());
+  // Flag's data
+  // The implementation depends on value_ field to be placed exactly after the
+  // impl_ field, so that impl_ can figure out the offset to the value and
+  // access it.
+  FlagImpl impl_;
+  FlagValue<T> value_;
+};
 
-    callback_ = mutation_callback;
+///////////////////////////////////////////////////////////////////////////////
+// Trampoline for friend access
 
-    InvokeCallback();
+class FlagImplPeer {
+ public:
+  template <typename T, typename FlagType>
+  static T InvokeGet(const FlagType& flag) {
+    return flag.Get();
   }
-  void InvokeCallback() override
-      ABSL_EXCLUSIVE_LOCKS_REQUIRED(locks_->primary_mu) {
-    flags_internal::InvokeCallback(&locks_->primary_mu, &locks_->callback_mu,
-                                   callback_);
+  template <typename FlagType, typename T>
+  static void InvokeSet(FlagType& flag, const T& v) {
+    flag.Set(v);
   }
-
- private:
-  void Destroy() const override {
-    // Values are heap allocated Abseil Flags.
-    if (cur_) Delete(op_, cur_);
-    if (def_) Delete(op_, def_);
-
-    delete locks_;
+  template <typename FlagType>
+  static const CommandLineFlag& InvokeReflect(const FlagType& f) {
+    return f.Reflect();
   }
+};
 
-  void StoreAtomic() override {
-    if (sizeof(T) <= sizeof(int64_t)) {
-      int64_t t = 0;
-      std::memcpy(&t, cur_, (std::min)(sizeof(T), sizeof(int64_t)));
-      atomic_.store(t, std::memory_order_release);
+///////////////////////////////////////////////////////////////////////////////
+// Implementation of Flag value specific operations routine.
+template <typename T>
+void* FlagOps(FlagOp op, const void* v1, void* v2, void* v3) {
+  struct AlignedSpace {
+    alignas(MaskedPointer::RequiredAlignment()) alignas(
+        T) unsigned char buf[sizeof(T)];
+  };
+  using Allocator = std::allocator<AlignedSpace>;
+  switch (op) {
+    case FlagOp::kAlloc: {
+      Allocator alloc;
+      return std::allocator_traits<Allocator>::allocate(alloc, 1);
+    }
+    case FlagOp::kDelete: {
+      T* p = static_cast<T*>(v2);
+      p->~T();
+      Allocator alloc;
+      std::allocator_traits<Allocator>::deallocate(
+          alloc, reinterpret_cast<AlignedSpace*>(p), 1);
+      return nullptr;
+    }
+    case FlagOp::kCopy:
+      *static_cast<T*>(v2) = *static_cast<const T*>(v1);
+      return nullptr;
+    case FlagOp::kCopyConstruct:
+      new (v2) T(*static_cast<const T*>(v1));
+      return nullptr;
+    case FlagOp::kSizeof:
+      return reinterpret_cast<void*>(static_cast<uintptr_t>(sizeof(T)));
+    case FlagOp::kFastTypeId:
+      return absl::bit_cast<void*>(absl::FastTypeId<T>());
+    case FlagOp::kRuntimeTypeId:
+      return const_cast<std::type_info*>(GenRuntimeTypeId<T>());
+    case FlagOp::kParse: {
+      // Initialize the temporary instance of type T based on current value in
+      // destination (which is going to be flag's default value).
+      T temp(*static_cast<T*>(v2));
+      if (!absl::ParseFlag<T>(*static_cast<const absl::string_view*>(v1), &temp,
+                              static_cast<std::string*>(v3))) {
+        return nullptr;
+      }
+      *static_cast<T*>(v2) = std::move(temp);
+      return v2;
+    }
+    case FlagOp::kUnparse:
+      *static_cast<std::string*>(v2) =
+          absl::UnparseFlag<T>(*static_cast<const T*>(v1));
+      return nullptr;
+    case FlagOp::kValueOffset: {
+      // Round sizeof(FlagImp) to a multiple of alignof(FlagValue<T>) to get the
+      // offset of the data.
+      size_t round_to = alignof(FlagValue<T>);
+      size_t offset = (sizeof(FlagImpl) + round_to - 1) / round_to * round_to;
+      return reinterpret_cast<void*>(offset);
     }
   }
+  return nullptr;
+}
 
-  // Flag's data
-  // For some types, a copy of the current value is kept in an atomically
-  // accessible field.
-  std::atomic<int64_t> atomic_;
-  FlagCallback callback_;  // Mutation callback
-};
-
+///////////////////////////////////////////////////////////////////////////////
 // This class facilitates Flag object registration and tail expression-based
 // flag definition, for example:
 // ABSL_FLAG(int, foo, 42, "Foo help").OnUpdate(NotifyFooWatcher);
+struct FlagRegistrarEmpty {};
 template <typename T, bool do_register>
 class FlagRegistrar {
  public:
-  explicit FlagRegistrar(Flag<T>* flag) : flag_(flag) {
-    if (do_register) flags_internal::RegisterCommandLineFlag(flag_);
+  constexpr explicit FlagRegistrar(Flag<T>& flag, const char* filename)
+      : flag_(flag) {
+    if (do_register)
+      flags_internal::RegisterCommandLineFlag(flag_.impl_, filename);
   }
 
-  FlagRegistrar& OnUpdate(flags_internal::FlagCallback cb) && {
-    flag_->SetCallback(cb);
+  FlagRegistrar OnUpdate(FlagCallbackFunc cb) && {
+    flag_.impl_.SetCallback(cb);
     return *this;
   }
 
-  // Make the registrar "die" gracefully as a bool on a line where registration
-  // happens. Registrar objects are intended to live only as temporary.
-  operator bool() const { return true; }  // NOLINT
+  // Makes the registrar die gracefully as an empty struct on a line where
+  // registration happens. Registrar objects are intended to live only as
+  // temporary.
+  constexpr operator FlagRegistrarEmpty() const { return {}; }  // NOLINT
 
  private:
-  Flag<T>* flag_;  // Flag being registered (not owned).
+  Flag<T>& flag_;  // Flag being registered (not owned).
 };
 
-// This struct and corresponding overload to MakeDefaultValue are used to
-// facilitate usage of {} as default value in ABSL_FLAG macro.
-struct EmptyBraces {};
-
-template <typename T>
-T* MakeFromDefaultValue(T t) {
-  return new T(std::move(t));
-}
-
-template <typename T>
-T* MakeFromDefaultValue(EmptyBraces) {
-  return new T;
-}
+///////////////////////////////////////////////////////////////////////////////
+// Test only API
+uint64_t NumLeakedFlagValues();
 
 }  // namespace flags_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_FLAGS_INTERNAL_FLAG_H_
diff --git a/absl/flags/internal/parse.h b/absl/flags/internal/parse.h
index fd3aca61f40..aa29dea00d5 100644
--- a/absl/flags/internal/parse.h
+++ b/absl/flags/internal/parse.h
@@ -16,10 +16,15 @@
 #ifndef ABSL_FLAGS_INTERNAL_PARSE_H_
 #define ABSL_FLAGS_INTERNAL_PARSE_H_
 
+#include <iostream>
+#include <ostream>
 #include <string>
 #include <vector>
 
+#include "absl/base/config.h"
 #include "absl/flags/declare.h"
+#include "absl/flags/internal/usage.h"
+#include "absl/strings/string_view.h"
 
 ABSL_DECLARE_FLAG(std::vector<std::string>, flagfile);
 ABSL_DECLARE_FLAG(std::vector<std::string>, fromenv);
@@ -27,9 +32,9 @@ ABSL_DECLARE_FLAG(std::vector<std::string>, tryfromenv);
 ABSL_DECLARE_FLAG(std::vector<std::string>, undefok);
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace flags_internal {
 
-enum class ArgvListAction { kRemoveParsedArgs, kKeepParsedArgs };
 enum class UsageFlagsAction { kHandleUsage, kIgnoreUsage };
 enum class OnUndefinedFlag {
   kIgnoreUndefined,
@@ -37,12 +42,35 @@ enum class OnUndefinedFlag {
   kAbortIfUndefined
 };
 
-std::vector<char*> ParseCommandLineImpl(int argc, char* argv[],
-                                        ArgvListAction arg_list_act,
-                                        UsageFlagsAction usage_flag_act,
-                                        OnUndefinedFlag on_undef_flag);
+// This is not a public interface. This interface exists to expose the ability
+// to change help output stream in case of parsing errors. This is used by
+// internal unit tests to validate expected outputs.
+// When this was written, `EXPECT_EXIT` only supported matchers on stderr,
+// but not on stdout.
+std::vector<char*> ParseCommandLineImpl(
+    int argc, char* argv[], UsageFlagsAction usage_flag_action,
+    OnUndefinedFlag undef_flag_action,
+    std::ostream& error_help_output = std::cout);
+
+// --------------------------------------------------------------------
+// Inspect original command line
+
+// Returns true if flag with specified name was either present on the original
+// command line or specified in flag file present on the original command line.
+bool WasPresentOnCommandLine(absl::string_view flag_name);
+
+// Return existing flags similar to the parameter, in order to help in case of
+// misspellings.
+std::vector<std::string> GetMisspellingHints(absl::string_view flag);
+
+// IsIndirectFlagExpansionEnabled()
+//
+// If DisableFlagfileAndEnvParsing() has been called, returns false. Otherwise,
+// returns true.
+[[nodiscard]] bool IsIndirectFlagExpansionEnabled();
 
 }  // namespace flags_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_FLAGS_INTERNAL_PARSE_H_
diff --git a/absl/flags/internal/path_util.h b/absl/flags/internal/path_util.h
index 5615c0e6c59..a6594d3347e 100644
--- a/absl/flags/internal/path_util.h
+++ b/absl/flags/internal/path_util.h
@@ -16,10 +16,11 @@
 #ifndef ABSL_FLAGS_INTERNAL_PATH_UTIL_H_
 #define ABSL_FLAGS_INTERNAL_PATH_UTIL_H_
 
-#include "absl/strings/match.h"
+#include "absl/base/config.h"
 #include "absl/strings/string_view.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace flags_internal {
 
 // A portable interface that returns the basename of the filename passed as an
@@ -55,6 +56,7 @@ inline absl::string_view Package(absl::string_view filename) {
 }
 
 }  // namespace flags_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_FLAGS_INTERNAL_PATH_UTIL_H_
diff --git a/absl/flags/internal/private_handle_accessor.cc b/absl/flags/internal/private_handle_accessor.cc
new file mode 100644
index 00000000000..610d1ffdf7f
--- /dev/null
+++ b/absl/flags/internal/private_handle_accessor.cc
@@ -0,0 +1,69 @@
+//
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/flags/internal/private_handle_accessor.h"
+
+#include <memory>
+#include <string>
+
+#include "absl/base/config.h"
+#include "absl/flags/commandlineflag.h"
+#include "absl/flags/internal/commandlineflag.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace flags_internal {
+
+FlagFastTypeId PrivateHandleAccessor::TypeId(const CommandLineFlag& flag) {
+  return flag.TypeId();
+}
+
+std::unique_ptr<FlagStateInterface> PrivateHandleAccessor::SaveState(
+    CommandLineFlag& flag) {
+  return flag.SaveState();
+}
+
+bool PrivateHandleAccessor::IsSpecifiedOnCommandLine(
+    const CommandLineFlag& flag) {
+  return flag.IsSpecifiedOnCommandLine();
+}
+
+bool PrivateHandleAccessor::ValidateInputValue(const CommandLineFlag& flag,
+                                               absl::string_view value) {
+  return flag.ValidateInputValue(value);
+}
+
+void PrivateHandleAccessor::CheckDefaultValueParsingRoundtrip(
+    const CommandLineFlag& flag) {
+  flag.CheckDefaultValueParsingRoundtrip();
+}
+
+bool PrivateHandleAccessor::ParseFrom(CommandLineFlag& flag,
+                                      absl::string_view value,
+                                      flags_internal::FlagSettingMode set_mode,
+                                      flags_internal::ValueSource source,
+                                      std::string& error) {
+  return flag.ParseFrom(value, set_mode, source, error);
+}
+
+absl::string_view PrivateHandleAccessor::TypeName(const CommandLineFlag& flag) {
+  return flag.TypeName();
+}
+
+}  // namespace flags_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
diff --git a/absl/flags/internal/private_handle_accessor.h b/absl/flags/internal/private_handle_accessor.h
new file mode 100644
index 00000000000..bf4154c5a5b
--- /dev/null
+++ b/absl/flags/internal/private_handle_accessor.h
@@ -0,0 +1,64 @@
+//
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_FLAGS_INTERNAL_PRIVATE_HANDLE_ACCESSOR_H_
+#define ABSL_FLAGS_INTERNAL_PRIVATE_HANDLE_ACCESSOR_H_
+
+#include <memory>
+#include <string>
+
+#include "absl/base/config.h"
+#include "absl/flags/commandlineflag.h"
+#include "absl/flags/internal/commandlineflag.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace flags_internal {
+
+// This class serves as a trampoline to access private methods of
+// CommandLineFlag. This class is intended for use exclusively internally inside
+// of the Abseil Flags implementation.
+class PrivateHandleAccessor {
+ public:
+  // Access to CommandLineFlag::TypeId.
+  static FlagFastTypeId TypeId(const CommandLineFlag& flag);
+
+  // Access to CommandLineFlag::SaveState.
+  static std::unique_ptr<FlagStateInterface> SaveState(CommandLineFlag& flag);
+
+  // Access to CommandLineFlag::IsSpecifiedOnCommandLine.
+  static bool IsSpecifiedOnCommandLine(const CommandLineFlag& flag);
+
+  // Access to CommandLineFlag::ValidateInputValue.
+  static bool ValidateInputValue(const CommandLineFlag& flag,
+                                 absl::string_view value);
+
+  // Access to CommandLineFlag::CheckDefaultValueParsingRoundtrip.
+  static void CheckDefaultValueParsingRoundtrip(const CommandLineFlag& flag);
+
+  static bool ParseFrom(CommandLineFlag& flag, absl::string_view value,
+                        flags_internal::FlagSettingMode set_mode,
+                        flags_internal::ValueSource source, std::string& error);
+
+  // Access to CommandLineFlag::TypeName.
+  static absl::string_view TypeName(const CommandLineFlag& flag);
+};
+
+}  // namespace flags_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_FLAGS_INTERNAL_PRIVATE_HANDLE_ACCESSOR_H_
diff --git a/absl/flags/internal/program_name.cc b/absl/flags/internal/program_name.cc
index f0811f14486..23185c626a0 100644
--- a/absl/flags/internal/program_name.cc
+++ b/absl/flags/internal/program_name.cc
@@ -17,37 +17,45 @@
 
 #include <string>
 
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/no_destructor.h"
+#include "absl/base/thread_annotations.h"
 #include "absl/flags/internal/path_util.h"
+#include "absl/strings/string_view.h"
 #include "absl/synchronization/mutex.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace flags_internal {
 
-ABSL_CONST_INIT static absl::Mutex program_name_guard(absl::kConstInit);
-ABSL_CONST_INIT static std::string* program_name
-    ABSL_GUARDED_BY(program_name_guard) = nullptr;
+static absl::Mutex& ProgramNameMutex() {
+  static absl::NoDestructor<absl::Mutex> mutex;
+  return *mutex;
+}
+ABSL_CONST_INIT static std::string* program_name ABSL_GUARDED_BY(
+    ProgramNameMutex()) ABSL_PT_GUARDED_BY(ProgramNameMutex()) = nullptr;
 
 std::string ProgramInvocationName() {
-  absl::MutexLock l(&program_name_guard);
-
+  absl::MutexLock l(ProgramNameMutex());
   return program_name ? *program_name : "UNKNOWN";
 }
 
 std::string ShortProgramInvocationName() {
-  absl::MutexLock l(&program_name_guard);
-
+  absl::MutexLock l(ProgramNameMutex());
   return program_name ? std::string(flags_internal::Basename(*program_name))
                       : "UNKNOWN";
 }
 
 void SetProgramInvocationName(absl::string_view prog_name_str) {
-  absl::MutexLock l(&program_name_guard);
-
-  if (!program_name)
+  absl::MutexLock l(ProgramNameMutex());
+  if (!program_name) {
     program_name = new std::string(prog_name_str);
-  else
+  } else {
     program_name->assign(prog_name_str.data(), prog_name_str.size());
+  }
 }
 
 }  // namespace flags_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/flags/internal/program_name.h b/absl/flags/internal/program_name.h
index 326f24bb653..b99b94fe18a 100644
--- a/absl/flags/internal/program_name.h
+++ b/absl/flags/internal/program_name.h
@@ -18,12 +18,14 @@
 
 #include <string>
 
+#include "absl/base/config.h"
 #include "absl/strings/string_view.h"
 
 // --------------------------------------------------------------------
 // Program name
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace flags_internal {
 
 // Returns program invocation name or "UNKNOWN" if `SetProgramInvocationName()`
@@ -42,6 +44,7 @@ std::string ShortProgramInvocationName();
 void SetProgramInvocationName(absl::string_view prog_name_str);
 
 }  // namespace flags_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_FLAGS_INTERNAL_PROGRAM_NAME_H_
diff --git a/absl/flags/internal/program_name_test.cc b/absl/flags/internal/program_name_test.cc
index ed69218b839..aff9f6315e4 100644
--- a/absl/flags/internal/program_name_test.cc
+++ b/absl/flags/internal/program_name_test.cc
@@ -15,14 +15,17 @@
 
 #include "absl/flags/internal/program_name.h"
 
+#include <string>
+
 #include "gtest/gtest.h"
 #include "absl/strings/match.h"
+#include "absl/strings/string_view.h"
 
 namespace {
 
 namespace flags = absl::flags_internal;
 
-TEST(FlagsPathUtilTest, TestInitialProgamName) {
+TEST(FlagsPathUtilTest, TestProgamNameInterfaces) {
   flags::SetProgramInvocationName("absl/flags/program_name_test");
   std::string program_name = flags::ProgramInvocationName();
   for (char& c : program_name)
@@ -40,9 +43,7 @@ TEST(FlagsPathUtilTest, TestInitialProgamName) {
 
   EXPECT_TRUE(absl::EndsWith(program_name, expect_name)) << program_name;
   EXPECT_EQ(flags::ShortProgramInvocationName(), expect_basename);
-}
 
-TEST(FlagsPathUtilTest, TestProgamNameInterfaces) {
   flags::SetProgramInvocationName("a/my_test");
 
   EXPECT_EQ(flags::ProgramInvocationName(), "a/my_test");
diff --git a/absl/flags/internal/registry.cc b/absl/flags/internal/registry.cc
deleted file mode 100644
index 4bea313bb1e..00000000000
--- a/absl/flags/internal/registry.cc
+++ /dev/null
@@ -1,404 +0,0 @@
-//
-// Copyright 2019 The Abseil Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-#include "absl/flags/internal/registry.h"
-
-#include "absl/base/dynamic_annotations.h"
-#include "absl/base/internal/raw_logging.h"
-#include "absl/flags/config.h"
-#include "absl/flags/usage_config.h"
-#include "absl/strings/str_cat.h"
-#include "absl/strings/string_view.h"
-#include "absl/synchronization/mutex.h"
-
-// --------------------------------------------------------------------
-// FlagRegistry implementation
-//    A FlagRegistry holds all flag objects indexed
-//    by their names so that if you know a flag's name you can access or
-//    set it.
-
-namespace absl {
-namespace flags_internal {
-
-// --------------------------------------------------------------------
-// FlagRegistry
-//    A FlagRegistry singleton object holds all flag objects indexed
-//    by their names so that if you know a flag's name (as a C
-//    string), you can access or set it.  If the function is named
-//    FooLocked(), you must own the registry lock before calling
-//    the function; otherwise, you should *not* hold the lock, and
-//    the function will acquire it itself if needed.
-// --------------------------------------------------------------------
-
-class FlagRegistry {
- public:
-  FlagRegistry() = default;
-  ~FlagRegistry() {
-    for (auto& p : flags_) {
-      p.second->Destroy();
-    }
-  }
-
-  // Store a flag in this registry.  Takes ownership of *flag.
-  void RegisterFlag(CommandLineFlag* flag);
-
-  void Lock() ABSL_EXCLUSIVE_LOCK_FUNCTION(lock_) { lock_.Lock(); }
-  void Unlock() ABSL_UNLOCK_FUNCTION(lock_) { lock_.Unlock(); }
-
-  // Returns the flag object for the specified name, or nullptr if not found.
-  // Will emit a warning if a 'retired' flag is specified.
-  CommandLineFlag* FindFlagLocked(absl::string_view name);
-
-  // Returns the retired flag object for the specified name, or nullptr if not
-  // found or not retired.  Does not emit a warning.
-  CommandLineFlag* FindRetiredFlagLocked(absl::string_view name);
-
-  static FlagRegistry* GlobalRegistry();  // returns a singleton registry
-
- private:
-  friend class FlagSaverImpl;  // reads all the flags in order to copy them
-  friend void ForEachFlagUnlocked(
-      std::function<void(CommandLineFlag*)> visitor);
-
-  // The map from name to flag, for FindFlagLocked().
-  using FlagMap = std::map<absl::string_view, CommandLineFlag*>;
-  using FlagIterator = FlagMap::iterator;
-  using FlagConstIterator = FlagMap::const_iterator;
-  FlagMap flags_;
-
-  absl::Mutex lock_;
-
-  // Disallow
-  FlagRegistry(const FlagRegistry&);
-  FlagRegistry& operator=(const FlagRegistry&);
-};
-
-FlagRegistry* FlagRegistry::GlobalRegistry() {
-  static FlagRegistry* global_registry = new FlagRegistry;
-  return global_registry;
-}
-
-namespace {
-
-class FlagRegistryLock {
- public:
-  explicit FlagRegistryLock(FlagRegistry* fr) : fr_(fr) { fr_->Lock(); }
-  ~FlagRegistryLock() { fr_->Unlock(); }
-
- private:
-  FlagRegistry* const fr_;
-};
-
-}  // namespace
-
-void FlagRegistry::RegisterFlag(CommandLineFlag* flag) {
-  FlagRegistryLock registry_lock(this);
-  std::pair<FlagIterator, bool> ins =
-      flags_.insert(FlagMap::value_type(flag->Name(), flag));
-  if (ins.second == false) {  // means the name was already in the map
-    CommandLineFlag* old_flag = ins.first->second;
-    if (flag->IsRetired() != old_flag->IsRetired()) {
-      // All registrations must agree on the 'retired' flag.
-      flags_internal::ReportUsageError(
-          absl::StrCat(
-              "Retired flag '", flag->Name(),
-              "' was defined normally in file '",
-              (flag->IsRetired() ? old_flag->Filename() : flag->Filename()),
-              "'."),
-          true);
-    } else if (flag->op_ != old_flag->op_) {
-      flags_internal::ReportUsageError(
-          absl::StrCat("Flag '", flag->Name(),
-                       "' was defined more than once but with "
-                       "differing types. Defined in files '",
-                       old_flag->Filename(), "' and '", flag->Filename(),
-                       "' with types '", old_flag->Typename(), "' and '",
-                       flag->Typename(), "', respectively."),
-          true);
-    } else if (old_flag->IsRetired()) {
-      // Retired definitions are idempotent. Just keep the old one.
-      flag->Destroy();
-      return;
-    } else if (old_flag->Filename() != flag->Filename()) {
-      flags_internal::ReportUsageError(
-          absl::StrCat("Flag '", flag->Name(),
-                       "' was defined more than once (in files '",
-                       old_flag->Filename(), "' and '", flag->Filename(),
-                       "')."),
-          true);
-    } else {
-      flags_internal::ReportUsageError(
-          absl::StrCat(
-              "Something wrong with flag '", flag->Name(), "' in file '",
-              flag->Filename(), "'. One possibility: file '", flag->Filename(),
-              "' is being linked both statically and dynamically into this "
-              "executable. e.g. some files listed as srcs to a test and also "
-              "listed as srcs of some shared lib deps of the same test."),
-          true);
-    }
-    // All cases above are fatal, except for the retired flags.
-    std::exit(1);
-  }
-}
-
-CommandLineFlag* FlagRegistry::FindFlagLocked(absl::string_view name) {
-  FlagConstIterator i = flags_.find(name);
-  if (i == flags_.end()) {
-    return nullptr;
-  }
-
-  if (i->second->IsRetired()) {
-    flags_internal::ReportUsageError(
-        absl::StrCat("Accessing retired flag '", name, "'"), false);
-  }
-
-  return i->second;
-}
-
-CommandLineFlag* FlagRegistry::FindRetiredFlagLocked(absl::string_view name) {
-  FlagConstIterator i = flags_.find(name);
-  if (i == flags_.end() || !i->second->IsRetired()) {
-    return nullptr;
-  }
-
-  return i->second;
-}
-
-// --------------------------------------------------------------------
-// FlagSaver
-// FlagSaverImpl
-//    This class stores the states of all flags at construct time,
-//    and restores all flags to that state at destruct time.
-//    Its major implementation challenge is that it never modifies
-//    pointers in the 'main' registry, so global FLAG_* vars always
-//    point to the right place.
-// --------------------------------------------------------------------
-
-class FlagSaverImpl {
- public:
-  // Constructs an empty FlagSaverImpl object.
-  FlagSaverImpl() {}
-  ~FlagSaverImpl() {
-    // reclaim memory from each of our CommandLineFlags
-    for (const SavedFlag& src : backup_registry_) {
-      Delete(src.op, src.current);
-      Delete(src.op, src.default_value);
-    }
-  }
-
-  // Saves the flag states from the flag registry into this object.
-  // It's an error to call this more than once.
-  // Must be called when the registry mutex is not held.
-  void SaveFromRegistry() {
-    assert(backup_registry_.empty());  // call only once!
-    SavedFlag saved;
-    flags_internal::ForEachFlag([&](flags_internal::CommandLineFlag* flag) {
-      if (flag->IsRetired()) return;
-
-      saved.name = flag->Name();
-      saved.op = flag->op_;
-      saved.marshalling_op = flag->marshalling_op_;
-      {
-        absl::MutexLock l(flag->InitFlagIfNecessary());
-        saved.validator = flag->GetValidator();
-        saved.modified = flag->modified_;
-        saved.on_command_line = flag->on_command_line_;
-        saved.current = Clone(saved.op, flag->cur_);
-        saved.default_value = Clone(saved.op, flag->def_);
-        saved.counter = flag->counter_;
-      }
-      backup_registry_.push_back(saved);
-    });
-  }
-
-  // Restores the saved flag states into the flag registry.  We
-  // assume no flags were added or deleted from the registry since
-  // the SaveFromRegistry; if they were, that's trouble!  Must be
-  // called when the registry mutex is not held.
-  void RestoreToRegistry() {
-    FlagRegistry* const global_registry = FlagRegistry::GlobalRegistry();
-    FlagRegistryLock frl(global_registry);
-    for (const SavedFlag& src : backup_registry_) {
-      CommandLineFlag* flag = global_registry->FindFlagLocked(src.name);
-      // If null, flag got deleted from registry.
-      if (!flag) continue;
-
-      bool restored = false;
-      {
-        // This function encapsulate the lock.
-        flag->SetValidator(src.validator);
-
-        absl::MutexLock l(flag->InitFlagIfNecessary());
-        flag->modified_ = src.modified;
-        flag->on_command_line_ = src.on_command_line;
-        if (flag->counter_ != src.counter ||
-            ChangedDirectly(flag, src.default_value, flag->def_)) {
-          restored = true;
-          Copy(src.op, src.default_value, flag->def_);
-        }
-        if (flag->counter_ != src.counter ||
-            ChangedDirectly(flag, src.current, flag->cur_)) {
-          restored = true;
-          Copy(src.op, src.current, flag->cur_);
-          UpdateCopy(flag);
-          flag->InvokeCallback();
-        }
-      }
-
-      if (restored) {
-        flag->counter_++;
-
-        // Revalidate the flag because the validator might store state based
-        // on the flag's value, which just changed due to the restore.
-        // Failing validation is ignored because it's assumed that the flag
-        // was valid previously and there's little that can be done about it
-        // here, anyway.
-        flag->ValidateInputValue(flag->CurrentValue());
-
-        ABSL_INTERNAL_LOG(
-            INFO, absl::StrCat("Restore saved value of ", flag->Name(), ": ",
-                               Unparse(src.marshalling_op, src.current)));
-      }
-    }
-  }
-
- private:
-  struct SavedFlag {
-    absl::string_view name;
-    FlagOpFn op;
-    FlagMarshallingOpFn marshalling_op;
-    int64_t counter;
-    void* validator;
-    bool modified;
-    bool on_command_line;
-    const void* current;        // nullptr after restore
-    const void* default_value;  // nullptr after restore
-  };
-
-  std::vector<SavedFlag> backup_registry_;
-
-  FlagSaverImpl(const FlagSaverImpl&);  // no copying!
-  void operator=(const FlagSaverImpl&);
-};
-
-FlagSaver::FlagSaver() : impl_(new FlagSaverImpl()) {
-  impl_->SaveFromRegistry();
-}
-
-void FlagSaver::Ignore() {
-  delete impl_;
-  impl_ = nullptr;
-}
-
-FlagSaver::~FlagSaver() {
-  if (!impl_) return;
-
-  impl_->RestoreToRegistry();
-  delete impl_;
-}
-
-// --------------------------------------------------------------------
-
-CommandLineFlag* FindCommandLineFlag(absl::string_view name) {
-  if (name.empty()) return nullptr;
-  FlagRegistry* const registry = FlagRegistry::GlobalRegistry();
-  FlagRegistryLock frl(registry);
-
-  return registry->FindFlagLocked(name);
-}
-
-CommandLineFlag* FindRetiredFlag(absl::string_view name) {
-  FlagRegistry* const registry = FlagRegistry::GlobalRegistry();
-  FlagRegistryLock frl(registry);
-
-  return registry->FindRetiredFlagLocked(name);
-}
-
-// --------------------------------------------------------------------
-
-void ForEachFlagUnlocked(std::function<void(CommandLineFlag*)> visitor) {
-  FlagRegistry* const registry = FlagRegistry::GlobalRegistry();
-  for (FlagRegistry::FlagConstIterator i = registry->flags_.begin();
-       i != registry->flags_.end(); ++i) {
-    visitor(i->second);
-  }
-}
-
-void ForEachFlag(std::function<void(CommandLineFlag*)> visitor) {
-  FlagRegistry* const registry = FlagRegistry::GlobalRegistry();
-  FlagRegistryLock frl(registry);
-  ForEachFlagUnlocked(visitor);
-}
-
-// --------------------------------------------------------------------
-
-bool RegisterCommandLineFlag(CommandLineFlag* flag) {
-  FlagRegistry::GlobalRegistry()->RegisterFlag(flag);
-  return true;
-}
-
-// --------------------------------------------------------------------
-
-namespace {
-
-class RetiredFlagObj final : public flags_internal::CommandLineFlag {
- public:
-  constexpr RetiredFlagObj(const char* name, FlagOpFn ops,
-                           FlagMarshallingOpFn marshalling_ops)
-      : flags_internal::CommandLineFlag(
-            name, flags_internal::HelpText::FromStaticCString(nullptr),
-            /*filename=*/"RETIRED", ops, marshalling_ops,
-            /*initial_value_gen=*/nullptr,
-            /*def=*/nullptr,
-            /*cur=*/nullptr) {}
-
- private:
-  bool IsRetired() const override { return true; }
-
-  void Destroy() const override {
-    // Values are heap allocated for Retired Flags.
-    if (cur_) Delete(op_, cur_);
-    if (def_) Delete(op_, def_);
-
-    if (locks_) delete locks_;
-
-    delete this;
-  }
-};
-
-}  // namespace
-
-bool Retire(const char* name, FlagOpFn ops,
-            FlagMarshallingOpFn marshalling_ops) {
-  auto* flag = new flags_internal::RetiredFlagObj(name, ops, marshalling_ops);
-  FlagRegistry::GlobalRegistry()->RegisterFlag(flag);
-  return true;
-}
-
-// --------------------------------------------------------------------
-
-bool IsRetiredFlag(absl::string_view name, bool* type_is_bool) {
-  assert(!name.empty());
-  CommandLineFlag* flag = flags_internal::FindRetiredFlag(name);
-  if (flag == nullptr) {
-    return false;
-  }
-  assert(type_is_bool);
-  *type_is_bool = flag->IsOfType<bool>();
-  return true;
-}
-
-}  // namespace flags_internal
-}  // namespace absl
diff --git a/absl/flags/internal/registry.h b/absl/flags/internal/registry.h
index eb134a9f81e..be9aaccc7ef 100644
--- a/absl/flags/internal/registry.h
+++ b/absl/flags/internal/registry.h
@@ -17,31 +17,29 @@
 #define ABSL_FLAGS_INTERNAL_REGISTRY_H_
 
 #include <functional>
-#include <map>
-#include <string>
 
-#include "absl/base/macros.h"
+#include "absl/base/config.h"
+#include "absl/base/fast_type_id.h"
+#include "absl/flags/commandlineflag.h"
 #include "absl/flags/internal/commandlineflag.h"
+#include "absl/strings/string_view.h"
 
 // --------------------------------------------------------------------
 // Global flags registry API.
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace flags_internal {
 
-CommandLineFlag* FindCommandLineFlag(absl::string_view name);
-CommandLineFlag* FindRetiredFlag(absl::string_view name);
-
-// Executes specified visitor for each non-retired flag in the registry.
-// Requires the caller hold the registry lock.
-void ForEachFlagUnlocked(std::function<void(CommandLineFlag*)> visitor);
 // Executes specified visitor for each non-retired flag in the registry. While
 // callback are executed, the registry is locked and can't be changed.
-void ForEachFlag(std::function<void(CommandLineFlag*)> visitor);
+void ForEachFlag(std::function<void(CommandLineFlag&)> visitor);
 
 //-----------------------------------------------------------------------------
 
-bool RegisterCommandLineFlag(CommandLineFlag*);
+bool RegisterCommandLineFlag(CommandLineFlag&, const char* filename);
+
+void FinalizeRegistry();
 
 //-----------------------------------------------------------------------------
 // Retired registrations:
@@ -76,47 +74,25 @@ bool RegisterCommandLineFlag(CommandLineFlag*);
 //
 
 // Retire flag with name "name" and type indicated by ops.
-bool Retire(const char* name, FlagOpFn ops,
-            FlagMarshallingOpFn marshalling_ops);
+void Retire(const char* name, FlagFastTypeId type_id, unsigned char* buf);
+
+constexpr size_t kRetiredFlagObjSize = 3 * sizeof(void*);
+constexpr size_t kRetiredFlagObjAlignment = alignof(void*);
 
 // Registered a retired flag with name 'flag_name' and type 'T'.
 template <typename T>
-inline bool RetiredFlag(const char* flag_name) {
-  return flags_internal::Retire(flag_name, flags_internal::FlagOps<T>,
-                                flags_internal::FlagMarshallingOps<T>);
-}
-
-// If the flag is retired, returns true and indicates in |*type_is_bool|
-// whether the type of the retired flag is a bool.
-// Only to be called by code that needs to explicitly ignore retired flags.
-bool IsRetiredFlag(absl::string_view name, bool* type_is_bool);
-
-//-----------------------------------------------------------------------------
-// Saves the states (value, default value, whether the user has set
-// the flag, registered validators, etc) of all flags, and restores
-// them when the FlagSaver is destroyed.
-//
-// This class is thread-safe.  However, its destructor writes to
-// exactly the set of flags that have changed value during its
-// lifetime, so concurrent _direct_ access to those flags
-// (i.e. FLAGS_foo instead of {Get,Set}CommandLineOption()) is unsafe.
-
-class FlagSaver {
+class RetiredFlag {
  public:
-  FlagSaver();
-  ~FlagSaver();
-
-  FlagSaver(const FlagSaver&) = delete;
-  void operator=(const FlagSaver&) = delete;
-
-  // Prevents saver from restoring the saved state of flags.
-  void Ignore();
+  void Retire(const char* flag_name) {
+    flags_internal::Retire(flag_name, absl::FastTypeId<T>(), buf_);
+  }
 
  private:
-  class FlagSaverImpl* impl_;  // we use pimpl here to keep API steady
+  alignas(kRetiredFlagObjAlignment) unsigned char buf_[kRetiredFlagObjSize];
 };
 
 }  // namespace flags_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_FLAGS_INTERNAL_REGISTRY_H_
diff --git a/absl/flags/internal/sequence_lock.h b/absl/flags/internal/sequence_lock.h
new file mode 100644
index 00000000000..36318ab9d36
--- /dev/null
+++ b/absl/flags/internal/sequence_lock.h
@@ -0,0 +1,187 @@
+//
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_FLAGS_INTERNAL_SEQUENCE_LOCK_H_
+#define ABSL_FLAGS_INTERNAL_SEQUENCE_LOCK_H_
+
+#include <stddef.h>
+#include <stdint.h>
+
+#include <atomic>
+#include <cassert>
+#include <cstring>
+
+#include "absl/base/optimization.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace flags_internal {
+
+// Align 'x' up to the nearest 'align' bytes.
+inline constexpr size_t AlignUp(size_t x, size_t align) {
+  return align * ((x + align - 1) / align);
+}
+
+// A SequenceLock implements lock-free reads. A sequence counter is incremented
+// before and after each write, and readers access the counter before and after
+// accessing the protected data. If the counter is verified to not change during
+// the access, and the sequence counter value was even, then the reader knows
+// that the read was race-free and valid. Otherwise, the reader must fall back
+// to a Mutex-based code path.
+//
+// This particular SequenceLock starts in an "uninitialized" state in which
+// TryRead() returns false. It must be enabled by calling MarkInitialized().
+// This serves as a marker that the associated flag value has not yet been
+// initialized and a slow path needs to be taken.
+//
+// The memory reads and writes protected by this lock must use the provided
+// `TryRead()` and `Write()` functions. These functions behave similarly to
+// `memcpy()`, with one oddity: the protected data must be an array of
+// `std::atomic<uint64>`. This is to comply with the C++ standard, which
+// considers data races on non-atomic objects to be undefined behavior. See "Can
+// Seqlocks Get Along With Programming Language Memory Models?"[1] by Hans J.
+// Boehm for more details.
+//
+// [1] https://www.hpl.hp.com/techreports/2012/HPL-2012-68.pdf
+class SequenceLock {
+ public:
+  constexpr SequenceLock() : lock_(kUninitialized) {}
+
+  // Mark that this lock is ready for use.
+  void MarkInitialized() {
+    assert(lock_.load(std::memory_order_relaxed) == kUninitialized);
+    lock_.store(0, std::memory_order_release);
+  }
+
+  // Copy "size" bytes of data from "src" to "dst", protected as a read-side
+  // critical section of the sequence lock.
+  //
+  // Unlike traditional sequence lock implementations which loop until getting a
+  // clean read, this implementation returns false in the case of concurrent
+  // calls to `Write`. In such a case, the caller should fall back to a
+  // locking-based slow path.
+  //
+  // Returns false if the sequence lock was not yet marked as initialized.
+  //
+  // NOTE: If this returns false, "dst" may be overwritten with undefined
+  // (potentially uninitialized) data.
+  bool TryRead(void* dst, const std::atomic<uint64_t>* src, size_t size) const {
+    // Acquire barrier ensures that no loads done by f() are reordered
+    // above the first load of the sequence counter.
+    int64_t seq_before = lock_.load(std::memory_order_acquire);
+    if (ABSL_PREDICT_FALSE(seq_before & 1) == 1) return false;
+    RelaxedCopyFromAtomic(dst, src, size);
+    // Another acquire fence ensures that the load of 'lock_' below is
+    // strictly ordered after the RelaxedCopyToAtomic call above.
+    std::atomic_thread_fence(std::memory_order_acquire);
+    int64_t seq_after = lock_.load(std::memory_order_relaxed);
+    return ABSL_PREDICT_TRUE(seq_before == seq_after);
+  }
+
+  // Copy "size" bytes from "src" to "dst" as a write-side critical section
+  // of the sequence lock. Any concurrent readers will be forced to retry
+  // until they get a read that does not conflict with this write.
+  //
+  // This call must be externally synchronized against other calls to Write,
+  // but may proceed concurrently with reads.
+  void Write(std::atomic<uint64_t>* dst, const void* src, size_t size) {
+    // We can use relaxed instructions to increment the counter since we
+    // are extenally synchronized. The std::atomic_thread_fence below
+    // ensures that the counter updates don't get interleaved with the
+    // copy to the data.
+    int64_t orig_seq = lock_.load(std::memory_order_relaxed);
+    assert((orig_seq & 1) == 0);  // Must be initially unlocked.
+    lock_.store(orig_seq + 1, std::memory_order_relaxed);
+
+    // We put a release fence between update to lock_ and writes to shared data.
+    // Thus all stores to shared data are effectively release operations and
+    // update to lock_ above cannot be re-ordered past any of them. Note that
+    // this barrier is not for the fetch_add above.  A release barrier for the
+    // fetch_add would be before it, not after.
+    std::atomic_thread_fence(std::memory_order_release);
+    RelaxedCopyToAtomic(dst, src, size);
+    // "Release" semantics ensure that none of the writes done by
+    // RelaxedCopyToAtomic() can be reordered after the following modification.
+    lock_.store(orig_seq + 2, std::memory_order_release);
+  }
+
+  // Return the number of times that Write() has been called.
+  //
+  // REQUIRES: This must be externally synchronized against concurrent calls to
+  // `Write()` or `IncrementModificationCount()`.
+  // REQUIRES: `MarkInitialized()` must have been previously called.
+  int64_t ModificationCount() const {
+    int64_t val = lock_.load(std::memory_order_relaxed);
+    assert(val != kUninitialized && (val & 1) == 0);
+    return val / 2;
+  }
+
+  // REQUIRES: This must be externally synchronized against concurrent calls to
+  // `Write()` or `ModificationCount()`.
+  // REQUIRES: `MarkInitialized()` must have been previously called.
+  void IncrementModificationCount() {
+    int64_t val = lock_.load(std::memory_order_relaxed);
+    assert(val != kUninitialized);
+    lock_.store(val + 2, std::memory_order_relaxed);
+  }
+
+ private:
+  // Perform the equivalent of "memcpy(dst, src, size)", but using relaxed
+  // atomics.
+  static void RelaxedCopyFromAtomic(void* dst, const std::atomic<uint64_t>* src,
+                                    size_t size) {
+    char* dst_byte = static_cast<char*>(dst);
+    while (size >= sizeof(uint64_t)) {
+      uint64_t word = src->load(std::memory_order_relaxed);
+      std::memcpy(dst_byte, &word, sizeof(word));
+      dst_byte += sizeof(word);
+      src++;
+      size -= sizeof(word);
+    }
+    if (size > 0) {
+      uint64_t word = src->load(std::memory_order_relaxed);
+      std::memcpy(dst_byte, &word, size);
+    }
+  }
+
+  // Perform the equivalent of "memcpy(dst, src, size)", but using relaxed
+  // atomics.
+  static void RelaxedCopyToAtomic(std::atomic<uint64_t>* dst, const void* src,
+                                  size_t size) {
+    const char* src_byte = static_cast<const char*>(src);
+    while (size >= sizeof(uint64_t)) {
+      uint64_t word;
+      std::memcpy(&word, src_byte, sizeof(word));
+      dst->store(word, std::memory_order_relaxed);
+      src_byte += sizeof(word);
+      dst++;
+      size -= sizeof(word);
+    }
+    if (size > 0) {
+      uint64_t word = 0;
+      std::memcpy(&word, src_byte, size);
+      dst->store(word, std::memory_order_relaxed);
+    }
+  }
+
+  static constexpr int64_t kUninitialized = -1;
+  std::atomic<int64_t> lock_;
+};
+
+}  // namespace flags_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_FLAGS_INTERNAL_SEQUENCE_LOCK_H_
diff --git a/absl/flags/internal/sequence_lock_test.cc b/absl/flags/internal/sequence_lock_test.cc
new file mode 100644
index 00000000000..a244d67d18b
--- /dev/null
+++ b/absl/flags/internal/sequence_lock_test.cc
@@ -0,0 +1,169 @@
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+#include "absl/flags/internal/sequence_lock.h"
+
+#include <algorithm>
+#include <atomic>
+#include <thread>  // NOLINT(build/c++11)
+#include <tuple>
+#include <vector>
+
+#include "gtest/gtest.h"
+#include "absl/base/internal/sysinfo.h"
+#include "absl/container/fixed_array.h"
+#include "absl/time/clock.h"
+
+namespace {
+
+namespace flags = absl::flags_internal;
+
+class ConcurrentSequenceLockTest
+    : public testing::TestWithParam<std::tuple<int, int>> {
+ public:
+  ConcurrentSequenceLockTest()
+      : buf_bytes_(std::get<0>(GetParam())),
+        num_threads_(std::get<1>(GetParam())) {}
+
+ protected:
+  const int buf_bytes_;
+  const int num_threads_;
+};
+
+TEST_P(ConcurrentSequenceLockTest, ReadAndWrite) {
+  const int buf_words =
+      flags::AlignUp(buf_bytes_, sizeof(uint64_t)) / sizeof(uint64_t);
+
+  // The buffer that will be protected by the SequenceLock.
+  absl::FixedArray<std::atomic<uint64_t>> protected_buf(buf_words);
+  for (auto& v : protected_buf) v = -1;
+
+  flags::SequenceLock seq_lock;
+  std::atomic<bool> stop{false};
+  std::atomic<int64_t> bad_reads{0};
+  std::atomic<int64_t> good_reads{0};
+  std::atomic<int64_t> unsuccessful_reads{0};
+
+  // Start a bunch of threads which read 'protected_buf' under the sequence
+  // lock. The main thread will concurrently update 'protected_buf'. The updates
+  // always consist of an array of identical integers. The reader ensures that
+  // any data it reads matches that pattern (i.e. the reads are not "torn").
+  std::vector<std::thread> threads;
+  for (int i = 0; i < num_threads_; i++) {
+    threads.emplace_back([&]() {
+      absl::FixedArray<char> local_buf(buf_bytes_);
+      while (!stop.load(std::memory_order_relaxed)) {
+        if (seq_lock.TryRead(local_buf.data(), protected_buf.data(),
+                             buf_bytes_)) {
+          bool good = true;
+          for (const auto& v : local_buf) {
+            if (v != local_buf[0]) good = false;
+          }
+          if (good) {
+            good_reads.fetch_add(1, std::memory_order_relaxed);
+          } else {
+            bad_reads.fetch_add(1, std::memory_order_relaxed);
+          }
+        } else {
+          unsuccessful_reads.fetch_add(1, std::memory_order_relaxed);
+        }
+      }
+    });
+  }
+  while (unsuccessful_reads.load(std::memory_order_relaxed) < num_threads_) {
+    absl::SleepFor(absl::Milliseconds(1));
+  }
+  seq_lock.MarkInitialized();
+
+  // Run a maximum of 5 seconds. On Windows, the scheduler behavior seems
+  // somewhat unfair and without an explicit timeout for this loop, the tests
+  // can run a long time.
+  absl::Time deadline = absl::Now() + absl::Seconds(5);
+  for (int i = 0; i < 100 && absl::Now() < deadline; i++) {
+    absl::FixedArray<char> writer_buf(buf_bytes_);
+    for (auto& v : writer_buf) v = i;
+    seq_lock.Write(protected_buf.data(), writer_buf.data(), buf_bytes_);
+    absl::SleepFor(absl::Microseconds(10));
+  }
+  stop.store(true, std::memory_order_relaxed);
+  for (auto& t : threads) t.join();
+  ASSERT_GE(good_reads, 0);
+  ASSERT_EQ(bad_reads, 0);
+}
+
+// Simple helper for generating a range of thread counts.
+// Generates [low, low*scale, low*scale^2, ...high)
+// (even if high is between low*scale^k and low*scale^(k+1)).
+std::vector<int> MultiplicativeRange(int low, int high, int scale) {
+  std::vector<int> result;
+  for (int current = low; current < high; current *= scale) {
+    result.push_back(current);
+  }
+  result.push_back(high);
+  return result;
+}
+
+#if defined(ABSL_HAVE_THREAD_SANITIZER)
+// With TSAN, a lot of threads contending for atomic access on the sequence
+// lock make this test run too slowly.
+const int kMaxThreads = std::min(absl::base_internal::NumCPUs(), 4);
+#else
+const int kMaxThreads = absl::base_internal::NumCPUs();
+#endif
+
+// Return all of the interesting buffer sizes worth testing:
+// powers of two and adjacent values.
+std::vector<int> InterestingBufferSizes() {
+  std::vector<int> ret;
+  for (int v : MultiplicativeRange(1, 128, 2)) {
+    ret.push_back(v);
+    if (v > 1) {
+      ret.push_back(v - 1);
+    }
+    ret.push_back(v + 1);
+  }
+  return ret;
+}
+
+INSTANTIATE_TEST_SUITE_P(
+    TestManyByteSizes, ConcurrentSequenceLockTest,
+    testing::Combine(
+        // Buffer size (bytes).
+        testing::ValuesIn(InterestingBufferSizes()),
+        // Number of reader threads.
+        testing::ValuesIn(MultiplicativeRange(1, kMaxThreads, 2))));
+
+// Simple single-threaded test, parameterized by the size of the buffer to be
+// protected.
+class SequenceLockTest : public testing::TestWithParam<int> {};
+
+TEST_P(SequenceLockTest, SingleThreaded) {
+  const int size = GetParam();
+  absl::FixedArray<std::atomic<uint64_t>> protected_buf(
+      flags::AlignUp(size, sizeof(uint64_t)) / sizeof(uint64_t));
+
+  flags::SequenceLock seq_lock;
+  seq_lock.MarkInitialized();
+
+  std::vector<char> src_buf(size, 'x');
+  seq_lock.Write(protected_buf.data(), src_buf.data(), size);
+
+  std::vector<char> dst_buf(size, '0');
+  ASSERT_TRUE(seq_lock.TryRead(dst_buf.data(), protected_buf.data(), size));
+  ASSERT_EQ(src_buf, dst_buf);
+}
+INSTANTIATE_TEST_SUITE_P(TestManyByteSizes, SequenceLockTest,
+                         // Buffer size (bytes).
+                         testing::Range(1, 128));
+
+}  // namespace
diff --git a/absl/flags/internal/type_erased.cc b/absl/flags/internal/type_erased.cc
deleted file mode 100644
index a1650fcf71e..00000000000
--- a/absl/flags/internal/type_erased.cc
+++ /dev/null
@@ -1,82 +0,0 @@
-//
-// Copyright 2019 The Abseil Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-#include "absl/flags/internal/type_erased.h"
-
-#include "absl/base/internal/raw_logging.h"
-#include "absl/flags/config.h"
-#include "absl/flags/usage_config.h"
-#include "absl/strings/str_cat.h"
-
-namespace absl {
-namespace flags_internal {
-
-bool GetCommandLineOption(absl::string_view name, std::string* value) {
-  if (name.empty()) return false;
-  assert(value);
-
-  CommandLineFlag* flag = flags_internal::FindCommandLineFlag(name);
-  if (flag == nullptr || flag->IsRetired()) {
-    return false;
-  }
-
-  *value = flag->CurrentValue();
-  return true;
-}
-
-bool SetCommandLineOption(absl::string_view name, absl::string_view value) {
-  return SetCommandLineOptionWithMode(name, value,
-                                      flags_internal::SET_FLAGS_VALUE);
-}
-
-bool SetCommandLineOptionWithMode(absl::string_view name,
-                                  absl::string_view value,
-                                  FlagSettingMode set_mode) {
-  CommandLineFlag* flag = flags_internal::FindCommandLineFlag(name);
-
-  if (!flag || flag->IsRetired()) return false;
-
-  std::string error;
-  if (!flag->SetFromString(value, set_mode, kProgrammaticChange, &error)) {
-    // Errors here are all of the form: the provided name was a recognized
-    // flag, but the value was invalid (bad type, or validation failed).
-    flags_internal::ReportUsageError(error, false);
-    return false;
-  }
-
-  return true;
-}
-
-// --------------------------------------------------------------------
-
-bool IsValidFlagValue(absl::string_view name, absl::string_view value) {
-  CommandLineFlag* flag = flags_internal::FindCommandLineFlag(name);
-
-  return flag != nullptr &&
-         (flag->IsRetired() || flag->ValidateInputValue(value));
-}
-
-// --------------------------------------------------------------------
-
-bool SpecifiedOnCommandLine(absl::string_view name) {
-  CommandLineFlag* flag = flags_internal::FindCommandLineFlag(name);
-  if (flag != nullptr && !flag->IsRetired()) {
-    return flag->IsSpecifiedOnCommandLine();
-  }
-  return false;
-}
-
-}  // namespace flags_internal
-}  // namespace absl
diff --git a/absl/flags/internal/type_erased.h b/absl/flags/internal/type_erased.h
deleted file mode 100644
index a955116628f..00000000000
--- a/absl/flags/internal/type_erased.h
+++ /dev/null
@@ -1,86 +0,0 @@
-//
-// Copyright 2019 The Abseil Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-#ifndef ABSL_FLAGS_INTERNAL_TYPE_ERASED_H_
-#define ABSL_FLAGS_INTERNAL_TYPE_ERASED_H_
-
-#include <string>
-
-#include "absl/flags/internal/commandlineflag.h"
-#include "absl/flags/internal/registry.h"
-
-// --------------------------------------------------------------------
-// Registry interfaces operating on type erased handles.
-
-namespace absl {
-namespace flags_internal {
-
-// If a flag named "name" exists, store its current value in *OUTPUT
-// and return true.  Else return false without changing *OUTPUT.
-// Thread-safe.
-bool GetCommandLineOption(absl::string_view name, std::string* value);
-
-// Set the value of the flag named "name" to value.  If successful,
-// returns true.  If not successful (e.g., the flag was not found or
-// the value is not a valid value), returns false.
-// Thread-safe.
-bool SetCommandLineOption(absl::string_view name, absl::string_view value);
-
-bool SetCommandLineOptionWithMode(absl::string_view name,
-                                  absl::string_view value,
-                                  FlagSettingMode set_mode);
-
-//-----------------------------------------------------------------------------
-
-// Returns true iff all of the following conditions are true:
-// (a) "name" names a registered flag
-// (b) "value" can be parsed succesfully according to the type of the flag
-// (c) parsed value passes any validator associated with the flag
-bool IsValidFlagValue(absl::string_view name, absl::string_view value);
-
-//-----------------------------------------------------------------------------
-
-// Returns true iff a flag named "name" was specified on the command line
-// (either directly, or via one of --flagfile or --fromenv or --tryfromenv).
-//
-// Any non-command-line modification of the flag does not affect the
-// result of this function.  So for example, if a flag was passed on
-// the command line but then reset via SET_FLAGS_DEFAULT, this
-// function will still return true.
-bool SpecifiedOnCommandLine(absl::string_view name);
-
-//-----------------------------------------------------------------------------
-
-// If a flag with specified "name" exists and has type T, store
-// its current value in *dst and return true.  Else return false
-// without touching *dst.  T must obey all of the requirements for
-// types passed to DEFINE_FLAG.
-template <typename T>
-inline bool GetByName(absl::string_view name, T* dst) {
-  CommandLineFlag* flag = flags_internal::FindCommandLineFlag(name);
-  if (!flag) return false;
-
-  if (auto val = flag->Get<T>()) {
-    *dst = *val;
-    return true;
-  }
-
-  return false;
-}
-
-}  // namespace flags_internal
-}  // namespace absl
-
-#endif  // ABSL_FLAGS_INTERNAL_TYPE_ERASED_H_
diff --git a/absl/flags/internal/type_erased_test.cc b/absl/flags/internal/type_erased_test.cc
deleted file mode 100644
index ac749a60754..00000000000
--- a/absl/flags/internal/type_erased_test.cc
+++ /dev/null
@@ -1,147 +0,0 @@
-//
-//  Copyright 2019 The Abseil Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-#include "absl/flags/internal/type_erased.h"
-
-#include <cmath>
-
-#include "gtest/gtest.h"
-#include "absl/flags/flag.h"
-#include "absl/memory/memory.h"
-#include "absl/strings/str_cat.h"
-
-ABSL_FLAG(int, int_flag, 1, "int_flag help");
-ABSL_FLAG(std::string, string_flag, "dflt", "string_flag help");
-ABSL_RETIRED_FLAG(bool, bool_retired_flag, false, "bool_retired_flag help");
-
-namespace {
-
-namespace flags = absl::flags_internal;
-
-class TypeErasedTest : public testing::Test {
- protected:
-  void SetUp() override { flag_saver_ = absl::make_unique<flags::FlagSaver>(); }
-  void TearDown() override { flag_saver_.reset(); }
-
- private:
-  std::unique_ptr<flags::FlagSaver> flag_saver_;
-};
-
-// --------------------------------------------------------------------
-
-TEST_F(TypeErasedTest, TestGetCommandLineOption) {
-  std::string value;
-  EXPECT_TRUE(flags::GetCommandLineOption("int_flag", &value));
-  EXPECT_EQ(value, "1");
-
-  EXPECT_TRUE(flags::GetCommandLineOption("string_flag", &value));
-  EXPECT_EQ(value, "dflt");
-
-  EXPECT_FALSE(flags::GetCommandLineOption("bool_retired_flag", &value));
-
-  EXPECT_FALSE(flags::GetCommandLineOption("unknown_flag", &value));
-}
-
-// --------------------------------------------------------------------
-
-TEST_F(TypeErasedTest, TestSetCommandLineOption) {
-  EXPECT_TRUE(flags::SetCommandLineOption("int_flag", "101"));
-  EXPECT_EQ(absl::GetFlag(FLAGS_int_flag), 101);
-
-  EXPECT_TRUE(flags::SetCommandLineOption("string_flag", "asdfgh"));
-  EXPECT_EQ(absl::GetFlag(FLAGS_string_flag), "asdfgh");
-
-  EXPECT_FALSE(flags::SetCommandLineOption("bool_retired_flag", "true"));
-
-  EXPECT_FALSE(flags::SetCommandLineOption("unknown_flag", "true"));
-}
-
-// --------------------------------------------------------------------
-
-TEST_F(TypeErasedTest, TestSetCommandLineOptionWithMode_SET_FLAGS_VALUE) {
-  EXPECT_TRUE(flags::SetCommandLineOptionWithMode("int_flag", "101",
-                                                  flags::SET_FLAGS_VALUE));
-  EXPECT_EQ(absl::GetFlag(FLAGS_int_flag), 101);
-
-  EXPECT_TRUE(flags::SetCommandLineOptionWithMode("string_flag", "asdfgh",
-                                                  flags::SET_FLAGS_VALUE));
-  EXPECT_EQ(absl::GetFlag(FLAGS_string_flag), "asdfgh");
-
-  EXPECT_FALSE(flags::SetCommandLineOptionWithMode("bool_retired_flag", "true",
-                                                   flags::SET_FLAGS_VALUE));
-
-  EXPECT_FALSE(flags::SetCommandLineOptionWithMode("unknown_flag", "true",
-                                                   flags::SET_FLAGS_VALUE));
-}
-
-// --------------------------------------------------------------------
-
-TEST_F(TypeErasedTest, TestSetCommandLineOptionWithMode_SET_FLAG_IF_DEFAULT) {
-  EXPECT_TRUE(flags::SetCommandLineOptionWithMode("int_flag", "101",
-                                                  flags::SET_FLAG_IF_DEFAULT));
-  EXPECT_EQ(absl::GetFlag(FLAGS_int_flag), 101);
-
-  // This semantic is broken. We return true instead of false. Value is not
-  // updated.
-  EXPECT_TRUE(flags::SetCommandLineOptionWithMode("int_flag", "202",
-                                                  flags::SET_FLAG_IF_DEFAULT));
-  EXPECT_EQ(absl::GetFlag(FLAGS_int_flag), 101);
-
-  EXPECT_TRUE(flags::SetCommandLineOptionWithMode("string_flag", "asdfgh",
-                                                  flags::SET_FLAG_IF_DEFAULT));
-  EXPECT_EQ(absl::GetFlag(FLAGS_string_flag), "asdfgh");
-
-  EXPECT_FALSE(flags::SetCommandLineOptionWithMode("bool_retired_flag", "true",
-                                                   flags::SET_FLAG_IF_DEFAULT));
-
-  EXPECT_FALSE(flags::SetCommandLineOptionWithMode("unknown_flag", "true",
-                                                   flags::SET_FLAG_IF_DEFAULT));
-}
-
-// --------------------------------------------------------------------
-
-TEST_F(TypeErasedTest, TestSetCommandLineOptionWithMode_SET_FLAGS_DEFAULT) {
-  EXPECT_TRUE(flags::SetCommandLineOptionWithMode("int_flag", "101",
-                                                  flags::SET_FLAGS_DEFAULT));
-
-  EXPECT_TRUE(flags::SetCommandLineOptionWithMode("string_flag", "asdfgh",
-                                                  flags::SET_FLAGS_DEFAULT));
-  EXPECT_EQ(absl::GetFlag(FLAGS_string_flag), "asdfgh");
-
-  EXPECT_FALSE(flags::SetCommandLineOptionWithMode("bool_retired_flag", "true",
-                                                   flags::SET_FLAGS_DEFAULT));
-
-  EXPECT_FALSE(flags::SetCommandLineOptionWithMode("unknown_flag", "true",
-                                                   flags::SET_FLAGS_DEFAULT));
-
-  // This should be successfull, since flag is still is not set
-  EXPECT_TRUE(flags::SetCommandLineOptionWithMode("int_flag", "202",
-                                                  flags::SET_FLAG_IF_DEFAULT));
-  EXPECT_EQ(absl::GetFlag(FLAGS_int_flag), 202);
-}
-
-// --------------------------------------------------------------------
-
-TEST_F(TypeErasedTest, TestIsValidFlagValue) {
-  EXPECT_TRUE(flags::IsValidFlagValue("int_flag", "57"));
-  EXPECT_TRUE(flags::IsValidFlagValue("int_flag", "-101"));
-  EXPECT_FALSE(flags::IsValidFlagValue("int_flag", "1.1"));
-
-  EXPECT_TRUE(flags::IsValidFlagValue("string_flag", "#%^#%^$%DGHDG$W%adsf"));
-
-  EXPECT_TRUE(flags::IsValidFlagValue("bool_retired_flag", "true"));
-}
-
-}  // namespace
diff --git a/absl/flags/internal/usage.cc b/absl/flags/internal/usage.cc
index 03048514ae0..3c44271a70c 100644
--- a/absl/flags/internal/usage.cc
+++ b/absl/flags/internal/usage.cc
@@ -15,38 +15,57 @@
 
 #include "absl/flags/internal/usage.h"
 
+#include <stdint.h>
+
+#include <algorithm>
+#include <cstdlib>
+#include <functional>
+#include <iterator>
 #include <map>
+#include <ostream>
 #include <string>
-
+#include <utility>
+#include <vector>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/no_destructor.h"
+#include "absl/base/thread_annotations.h"
+#include "absl/flags/commandlineflag.h"
 #include "absl/flags/flag.h"
+#include "absl/flags/internal/flag.h"
 #include "absl/flags/internal/path_util.h"
+#include "absl/flags/internal/private_handle_accessor.h"
 #include "absl/flags/internal/program_name.h"
+#include "absl/flags/internal/registry.h"
 #include "absl/flags/usage_config.h"
-#include "absl/strings/ascii.h"
+#include "absl/strings/match.h"
 #include "absl/strings/str_cat.h"
 #include "absl/strings/str_split.h"
 #include "absl/strings/string_view.h"
+#include "absl/strings/strip.h"
 #include "absl/synchronization/mutex.h"
 
-ABSL_FLAG(bool, help, false,
-          "show help on important flags for this binary [tip: all flags can "
-          "have two dashes]");
-ABSL_FLAG(bool, helpfull, false, "show help on all flags");
-ABSL_FLAG(bool, helpshort, false,
-          "show help on only the main module for this program");
-ABSL_FLAG(bool, helppackage, false,
-          "show help on all modules in the main package");
-ABSL_FLAG(bool, version, false, "show version and build info and exit");
-ABSL_FLAG(bool, only_check_args, false, "exit after checking all flags");
-ABSL_FLAG(std::string, helpon, "",
-          "show help on the modules named by this flag value");
-ABSL_FLAG(std::string, helpmatch, "",
-          "show help on modules whose name contains the specified substr");
+// Dummy global variables to prevent anyone else defining these.
+bool FLAGS_help = false;
+bool FLAGS_helpfull = false;
+bool FLAGS_helpshort = false;
+bool FLAGS_helppackage = false;
+bool FLAGS_version = false;
+bool FLAGS_only_check_args = false;
+bool FLAGS_helpon = false;
+bool FLAGS_helpmatch = false;
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace flags_internal {
 namespace {
 
+using PerFlagFilter = std::function<bool(const absl::CommandLineFlag&)>;
+
+// Maximum length size in a human readable format.
+constexpr size_t kHrfMaxLineLength = 80;
+
 // This class is used to emit an XML element with `tag` and `text`.
 // It adds opening and closing tags and escapes special characters in the text.
 // For example:
@@ -78,8 +97,16 @@ class XMLElement {
         case '>':
           out << "&gt;";
           break;
+        case '\n':
+        case '\v':
+        case '\f':
+        case '\t':
+          out << " ";
+          break;
         default:
-          out << c;
+          if (IsValidXmlCharacter(static_cast<unsigned char>(c))) {
+            out << c;
+          }
           break;
       }
     }
@@ -88,6 +115,7 @@ class XMLElement {
   }
 
  private:
+  static bool IsValidXmlCharacter(unsigned char c) { return c >= 0x20; }
   absl::string_view tag_;
   absl::string_view txt_;
 };
@@ -99,22 +127,25 @@ class FlagHelpPrettyPrinter {
  public:
   // Pretty printer holds on to the std::ostream& reference to direct an output
   // to that stream.
-  FlagHelpPrettyPrinter(int max_line_len, std::ostream* out)
-      : out_(*out),
+  FlagHelpPrettyPrinter(size_t max_line_len, size_t min_line_len,
+                        size_t wrapped_line_indent, std::ostream& out)
+      : out_(out),
         max_line_len_(max_line_len),
+        min_line_len_(min_line_len),
+        wrapped_line_indent_(wrapped_line_indent),
         line_len_(0),
         first_line_(true) {}
 
   void Write(absl::string_view str, bool wrap_line = false) {
-    // Empty std::string - do nothing.
+    // Empty string - do nothing.
     if (str.empty()) return;
 
     std::vector<absl::string_view> tokens;
     if (wrap_line) {
       for (auto line : absl::StrSplit(str, absl::ByAnyChar("\n\r"))) {
         if (!tokens.empty()) {
-          // Keep line separators in the input std::string.
-          tokens.push_back("\n");
+          // Keep line separators in the input string.
+          tokens.emplace_back("\n");
         }
         for (auto token :
              absl::StrSplit(line, absl::ByAnyChar(" \t"), absl::SkipEmpty())) {
@@ -128,13 +159,13 @@ class FlagHelpPrettyPrinter {
     for (auto token : tokens) {
       bool new_line = (line_len_ == 0);
 
-      // Respect line separators in the input std::string.
+      // Respect line separators in the input string.
       if (token == "\n") {
         EndLine();
         continue;
       }
 
-      // Write the token, ending the std::string first if necessary/possible.
+      // Write the token, ending the string first if necessary/possible.
       if (!new_line && (line_len_ + token.size() >= max_line_len_)) {
         EndLine();
         new_line = true;
@@ -154,13 +185,12 @@ class FlagHelpPrettyPrinter {
 
   void StartLine() {
     if (first_line_) {
-      out_ << "    ";
-      line_len_ = 4;
+      line_len_ = min_line_len_;
       first_line_ = false;
     } else {
-      out_ << "      ";
-      line_len_ = 6;
+      line_len_ = min_line_len_ + wrapped_line_indent_;
     }
+    out_ << std::string(line_len_, ' ');
   }
   void EndLine() {
     out_ << '\n';
@@ -169,38 +199,36 @@ class FlagHelpPrettyPrinter {
 
  private:
   std::ostream& out_;
-  const int max_line_len_;
-  int line_len_;
+  const size_t max_line_len_;
+  const size_t min_line_len_;
+  const size_t wrapped_line_indent_;
+  size_t line_len_;
   bool first_line_;
 };
 
-void FlagHelpHumanReadable(const flags_internal::CommandLineFlag& flag,
-                           std::ostream* out) {
-  FlagHelpPrettyPrinter printer(80, out);  // Max line length is 80.
+void FlagHelpHumanReadable(const CommandLineFlag& flag, std::ostream& out) {
+  FlagHelpPrettyPrinter printer(kHrfMaxLineLength, 4, 2, out);
 
   // Flag name.
-  printer.Write(absl::StrCat("-", flag.Name()));
+  printer.Write(absl::StrCat("--", flag.Name()));
 
   // Flag help.
   printer.Write(absl::StrCat("(", flag.Help(), ");"), /*wrap_line=*/true);
 
-  // Flag data type (for V1 flags only).
-  if (!flag.IsAbseilFlag() && !flag.IsRetired()) {
-    printer.Write(absl::StrCat("type: ", flag.Typename(), ";"));
-  }
-
   // The listed default value will be the actual default from the flag
   // definition in the originating source file, unless the value has
   // subsequently been modified using SetCommandLineOption() with mode
   // SET_FLAGS_DEFAULT.
   std::string dflt_val = flag.DefaultValue();
+  std::string curr_val = flag.CurrentValue();
+  bool is_modified = curr_val != dflt_val;
+
   if (flag.IsOfType<std::string>()) {
     dflt_val = absl::StrCat("\"", dflt_val, "\"");
   }
   printer.Write(absl::StrCat("default: ", dflt_val, ";"));
 
-  if (flag.IsModified()) {
-    std::string curr_val = flag.CurrentValue();
+  if (is_modified) {
     if (flag.IsOfType<std::string>()) {
       curr_val = absl::StrCat("\"", curr_val, "\"");
     }
@@ -215,7 +243,7 @@ void FlagHelpHumanReadable(const flags_internal::CommandLineFlag& flag,
 // If a flag's help message has been stripped (e.g. by adding '#define
 // STRIP_FLAG_HELP 1' then this flag will not be displayed by '--help'
 // and its variants.
-void FlagsHelpImpl(std::ostream& out, flags_internal::FlagKindFilter filter_cb,
+void FlagsHelpImpl(std::ostream& out, PerFlagFilter filter_cb,
                    HelpFormat format, absl::string_view program_usage_message) {
   if (format == HelpFormat::kHumanReadable) {
     out << flags_internal::ShortProgramInvocationName() << ": "
@@ -223,6 +251,9 @@ void FlagsHelpImpl(std::ostream& out, flags_internal::FlagKindFilter filter_cb,
   } else {
     // XML schema is not a part of our public API for now.
     out << "<?xml version=\"1.0\"?>\n"
+        << "<!-- This output should be used with care. We do not report type "
+           "names for flags with user defined types -->\n"
+        << "<!-- Prefer flag only_check_args for validating flag inputs -->\n"
         // The document.
         << "<AllFlags>\n"
         // The program name and usage.
@@ -231,50 +262,54 @@ void FlagsHelpImpl(std::ostream& out, flags_internal::FlagKindFilter filter_cb,
         << XMLElement("usage", program_usage_message) << '\n';
   }
 
-  // Map of package name to
+  // Ordered map of package name to
   //   map of file name to
   //     vector of flags in the file.
   // This map is used to output matching flags grouped by package and file
   // name.
   std::map<std::string,
-           std::map<std::string,
-                    std::vector<const flags_internal::CommandLineFlag*>>>
+           std::map<std::string, std::vector<const absl::CommandLineFlag*>>>
       matching_flags;
 
-  flags_internal::ForEachFlag([&](flags_internal::CommandLineFlag* flag) {
-    std::string flag_filename = flag->Filename();
-
+  flags_internal::ForEachFlag([&](absl::CommandLineFlag& flag) {
     // Ignore retired flags.
-    if (flag->IsRetired()) return;
+    if (flag.IsRetired()) return;
 
     // If the flag has been stripped, pretend that it doesn't exist.
-    if (flag->Help() == flags_internal::kStrippedFlagHelp) return;
+    if (flag.Help() == flags_internal::kStrippedFlagHelp) return;
 
     // Make sure flag satisfies the filter
-    if (!filter_cb || !filter_cb(flag_filename)) return;
+    if (!filter_cb(flag)) return;
+
+    std::string flag_filename = flag.Filename();
 
     matching_flags[std::string(flags_internal::Package(flag_filename))]
                   [flag_filename]
-                      .push_back(flag);
+                      .push_back(&flag);
   });
 
-  absl::string_view
-      package_separator;             // controls blank lines between packages.
-  absl::string_view file_separator;  // controls blank lines between files.
-  for (const auto& package : matching_flags) {
+  absl::string_view package_separator;  // controls blank lines between packages
+  absl::string_view file_separator;     // controls blank lines between files
+  for (auto& package : matching_flags) {
     if (format == HelpFormat::kHumanReadable) {
       out << package_separator;
       package_separator = "\n\n";
     }
 
     file_separator = "";
-    for (const auto& flags_in_file : package.second) {
+    for (auto& flags_in_file : package.second) {
       if (format == HelpFormat::kHumanReadable) {
         out << file_separator << "  Flags from " << flags_in_file.first
             << ":\n";
         file_separator = "\n";
       }
 
+      std::sort(std::begin(flags_in_file.second),
+                std::end(flags_in_file.second),
+                [](const CommandLineFlag* lhs, const CommandLineFlag* rhs) {
+                  return lhs->Name() < rhs->Name();
+                });
+
       for (const auto* flag : flags_in_file.second) {
         flags_internal::FlagHelp(out, *flag, format);
       }
@@ -282,32 +317,51 @@ void FlagsHelpImpl(std::ostream& out, flags_internal::FlagKindFilter filter_cb,
   }
 
   if (format == HelpFormat::kHumanReadable) {
+    FlagHelpPrettyPrinter printer(kHrfMaxLineLength, 0, 0, out);
+
     if (filter_cb && matching_flags.empty()) {
-      out << "  No modules matched: use -helpfull\n";
+      printer.Write("No flags matched.\n", true);
     }
+    printer.EndLine();
+    printer.Write(
+        "Try --helpfull to get a list of all flags or --help=substring "
+        "shows help for flags which include specified substring in either "
+        "in the name, or description or path.\n",
+        true);
   } else {
     // The end of the document.
     out << "</AllFlags>\n";
   }
 }
 
+void FlagsHelpImpl(std::ostream& out,
+                   flags_internal::FlagKindFilter filename_filter_cb,
+                   HelpFormat format, absl::string_view program_usage_message) {
+  FlagsHelpImpl(
+      out,
+      [&](const absl::CommandLineFlag& flag) {
+        return filename_filter_cb && filename_filter_cb(flag.Filename());
+      },
+      format, program_usage_message);
+}
+
 }  // namespace
 
 // --------------------------------------------------------------------
 // Produces the help message describing specific flag.
-void FlagHelp(std::ostream& out, const flags_internal::CommandLineFlag& flag,
+void FlagHelp(std::ostream& out, const CommandLineFlag& flag,
               HelpFormat format) {
   if (format == HelpFormat::kHumanReadable)
-    flags_internal::FlagHelpHumanReadable(flag, &out);
+    flags_internal::FlagHelpHumanReadable(flag, out);
 }
 
 // --------------------------------------------------------------------
-// Produces the help messages for all flags matching the filter.
+// Produces the help messages for all flags matching the filename filter.
 // If filter is empty produces help messages for all flags.
 void FlagsHelp(std::ostream& out, absl::string_view filter, HelpFormat format,
                absl::string_view program_usage_message) {
   flags_internal::FlagKindFilter filter_cb = [&](absl::string_view filename) {
-    return filter.empty() || filename.find(filter) != absl::string_view::npos;
+    return filter.empty() || absl::StrContains(filename, filter);
   };
   flags_internal::FlagsHelpImpl(out, filter_cb, format, program_usage_message);
 }
@@ -315,69 +369,190 @@ void FlagsHelp(std::ostream& out, absl::string_view filter, HelpFormat format,
 // --------------------------------------------------------------------
 // Checks all the 'usage' command line flags to see if any have been set.
 // If so, handles them appropriately.
-int HandleUsageFlags(std::ostream& out,
-                     absl::string_view program_usage_message) {
-  if (absl::GetFlag(FLAGS_helpshort)) {
-    flags_internal::FlagsHelpImpl(
-        out, flags_internal::GetUsageConfig().contains_helpshort_flags,
-        HelpFormat::kHumanReadable, program_usage_message);
-    return 1;
+HelpMode HandleUsageFlags(std::ostream& out,
+                          absl::string_view program_usage_message) {
+  switch (GetFlagsHelpMode()) {
+    case HelpMode::kNone:
+      break;
+    case HelpMode::kImportant:
+      flags_internal::FlagsHelpImpl(
+          out, flags_internal::GetUsageConfig().contains_help_flags,
+          GetFlagsHelpFormat(), program_usage_message);
+      break;
+
+    case HelpMode::kShort:
+      flags_internal::FlagsHelpImpl(
+          out, flags_internal::GetUsageConfig().contains_helpshort_flags,
+          GetFlagsHelpFormat(), program_usage_message);
+      break;
+
+    case HelpMode::kFull:
+      flags_internal::FlagsHelp(out, "", GetFlagsHelpFormat(),
+                                program_usage_message);
+      break;
+
+    case HelpMode::kPackage:
+      flags_internal::FlagsHelpImpl(
+          out, flags_internal::GetUsageConfig().contains_helppackage_flags,
+          GetFlagsHelpFormat(), program_usage_message);
+      break;
+
+    case HelpMode::kMatch: {
+      std::string substr = GetFlagsHelpMatchSubstr();
+      if (substr.empty()) {
+        // show all options
+        flags_internal::FlagsHelp(out, substr, GetFlagsHelpFormat(),
+                                  program_usage_message);
+      } else {
+        auto filter_cb = [&substr](const absl::CommandLineFlag& flag) {
+          if (absl::StrContains(flag.Name(), substr)) return true;
+          if (absl::StrContains(flag.Filename(), substr)) return true;
+          if (absl::StrContains(flag.Help(), substr)) return true;
+
+          return false;
+        };
+        flags_internal::FlagsHelpImpl(
+            out, filter_cb, HelpFormat::kHumanReadable, program_usage_message);
+      }
+      break;
+    }
+    case HelpMode::kVersion:
+      if (flags_internal::GetUsageConfig().version_string)
+        out << flags_internal::GetUsageConfig().version_string();
+      // Unlike help, we may be asking for version in a script, so return 0
+      break;
+
+    case HelpMode::kOnlyCheckArgs:
+      break;
   }
 
-  if (absl::GetFlag(FLAGS_helpfull)) {
-    // show all options
-    flags_internal::FlagsHelp(out, "", HelpFormat::kHumanReadable,
-                              program_usage_message);
-    return 1;
-  }
+  return GetFlagsHelpMode();
+}
 
-  if (!absl::GetFlag(FLAGS_helpon).empty()) {
-    flags_internal::FlagsHelp(
-        out, absl::StrCat("/", absl::GetFlag(FLAGS_helpon), "."),
-        HelpFormat::kHumanReadable, program_usage_message);
-    return 1;
-  }
+// --------------------------------------------------------------------
+// Globals representing usage reporting flags
 
-  if (!absl::GetFlag(FLAGS_helpmatch).empty()) {
-    flags_internal::FlagsHelp(out, absl::GetFlag(FLAGS_helpmatch),
-                              HelpFormat::kHumanReadable,
-                              program_usage_message);
-    return 1;
-  }
+namespace {
 
-  if (absl::GetFlag(FLAGS_help)) {
-    flags_internal::FlagsHelpImpl(
-        out, flags_internal::GetUsageConfig().contains_help_flags,
-        HelpFormat::kHumanReadable, program_usage_message);
+absl::Mutex& HelpAttributesMutex() {
+  static absl::NoDestructor<absl::Mutex> mutex;
+  return *mutex;
+}
+ABSL_CONST_INIT std::string* match_substr ABSL_GUARDED_BY(HelpAttributesMutex())
+    ABSL_PT_GUARDED_BY(HelpAttributesMutex()) = nullptr;
+ABSL_CONST_INIT HelpMode help_mode ABSL_GUARDED_BY(HelpAttributesMutex()) =
+    HelpMode::kNone;
+ABSL_CONST_INIT HelpFormat help_format ABSL_GUARDED_BY(HelpAttributesMutex()) =
+    HelpFormat::kHumanReadable;
 
-    out << "\nTry --helpfull to get a list of all flags.\n";
+}  // namespace
 
-    return 1;
-  }
+std::string GetFlagsHelpMatchSubstr() {
+  absl::MutexLock l(HelpAttributesMutex());
+  if (match_substr == nullptr) return "";
+  return *match_substr;
+}
 
-  if (absl::GetFlag(FLAGS_helppackage)) {
-    flags_internal::FlagsHelpImpl(
-        out, flags_internal::GetUsageConfig().contains_helppackage_flags,
-        HelpFormat::kHumanReadable, program_usage_message);
+void SetFlagsHelpMatchSubstr(absl::string_view substr) {
+  absl::MutexLock l(HelpAttributesMutex());
+  if (match_substr == nullptr) match_substr = new std::string;
+  match_substr->assign(substr.data(), substr.size());
+}
 
-    out << "\nTry --helpfull to get a list of all flags.\n";
+HelpMode GetFlagsHelpMode() {
+  absl::MutexLock l(HelpAttributesMutex());
+  return help_mode;
+}
+
+void SetFlagsHelpMode(HelpMode mode) {
+  absl::MutexLock l(HelpAttributesMutex());
+  help_mode = mode;
+}
+
+HelpFormat GetFlagsHelpFormat() {
+  absl::MutexLock l(HelpAttributesMutex());
+  return help_format;
+}
+
+void SetFlagsHelpFormat(HelpFormat format) {
+  absl::MutexLock l(HelpAttributesMutex());
+  help_format = format;
+}
+
+// Deduces usage flags from the input argument in a form --name=value or
+// --name. argument is already split into name and value before we call this
+// function.
+bool DeduceUsageFlags(absl::string_view name, absl::string_view value) {
+  if (absl::ConsumePrefix(&name, "help")) {
+    if (name.empty()) {
+      if (value.empty()) {
+        SetFlagsHelpMode(HelpMode::kImportant);
+      } else {
+        SetFlagsHelpMode(HelpMode::kMatch);
+        SetFlagsHelpMatchSubstr(value);
+      }
+      return true;
+    }
+
+    if (name == "match") {
+      SetFlagsHelpMode(HelpMode::kMatch);
+      SetFlagsHelpMatchSubstr(value);
+      return true;
+    }
+
+    if (name == "on") {
+      SetFlagsHelpMode(HelpMode::kMatch);
+      SetFlagsHelpMatchSubstr(absl::StrCat("/", value, "."));
+      return true;
+    }
+
+    if (name == "full") {
+      SetFlagsHelpMode(HelpMode::kFull);
+      return true;
+    }
+
+    if (name == "short") {
+      SetFlagsHelpMode(HelpMode::kShort);
+      return true;
+    }
+
+    if (name == "package") {
+      SetFlagsHelpMode(HelpMode::kPackage);
+      return true;
+    }
 
-    return 1;
+    return false;
   }
 
-  if (absl::GetFlag(FLAGS_version)) {
-    if (flags_internal::GetUsageConfig().version_string)
-      out << flags_internal::GetUsageConfig().version_string();
-    // Unlike help, we may be asking for version in a script, so return 0
-    return 0;
+  if (name == "version") {
+    SetFlagsHelpMode(HelpMode::kVersion);
+    return true;
   }
 
-  if (absl::GetFlag(FLAGS_only_check_args)) {
-    return 0;
+  if (name == "only_check_args") {
+    SetFlagsHelpMode(HelpMode::kOnlyCheckArgs);
+    return true;
   }
 
-  return -1;
+  return false;
 }
 
+// --------------------------------------------------------------------
+
+void MaybeExit(HelpMode mode) {
+  switch (mode) {
+    case flags_internal::HelpMode::kNone:
+      return;
+    case flags_internal::HelpMode::kOnlyCheckArgs:
+    case flags_internal::HelpMode::kVersion:
+      std::exit(0);
+    default:  // For all the other modes we exit with 1
+      std::exit(1);
+  }
+}
+
+// --------------------------------------------------------------------
+
 }  // namespace flags_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/flags/internal/usage.h b/absl/flags/internal/usage.h
index 76075b0829d..a96cbf38292 100644
--- a/absl/flags/internal/usage.h
+++ b/absl/flags/internal/usage.h
@@ -17,16 +17,18 @@
 #define ABSL_FLAGS_INTERNAL_USAGE_H_
 
 #include <iosfwd>
+#include <ostream>
 #include <string>
 
-#include "absl/flags/declare.h"
-#include "absl/flags/internal/commandlineflag.h"
+#include "absl/base/config.h"
+#include "absl/flags/commandlineflag.h"
 #include "absl/strings/string_view.h"
 
 // --------------------------------------------------------------------
 // Usage reporting interfaces
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace flags_internal {
 
 // The format to report the help messages in.
@@ -34,8 +36,21 @@ enum class HelpFormat {
   kHumanReadable,
 };
 
-// Outputs the help message describing specific flag.
-void FlagHelp(std::ostream& out, const flags_internal::CommandLineFlag& flag,
+// The kind of usage help requested.
+enum class HelpMode {
+  kNone,
+  kImportant,
+  kShort,
+  kFull,
+  kPackage,
+  kMatch,
+  kVersion,
+  kOnlyCheckArgs
+};
+
+// Streams the help message describing `flag` to `out`.
+// The default value for `flag` is included in the output.
+void FlagHelp(std::ostream& out, const CommandLineFlag& flag,
               HelpFormat format = HelpFormat::kHumanReadable);
 
 // Produces the help messages for all flags matching the filter. A flag matches
@@ -54,25 +69,38 @@ void FlagsHelp(std::ostream& out, absl::string_view filter,
 
 // If any of the 'usage' related command line flags (listed on the bottom of
 // this file) has been set this routine produces corresponding help message in
-// the specified output stream and returns:
-//  0 - if "version" or "only_check_flags" flags were set and handled.
-//  1 - if some other 'usage' related flag was set and handled.
-// -1 - if no usage flags were set on a commmand line.
-// Non negative return values are expected to be used as an exit code for a
-// binary.
-int HandleUsageFlags(std::ostream& out,
-                     absl::string_view program_usage_message);
+// the specified output stream and returns HelpMode that was handled. Otherwise
+// it returns HelpMode::kNone.
+HelpMode HandleUsageFlags(std::ostream& out,
+                          absl::string_view program_usage_message);
+
+// --------------------------------------------------------------------
+// Encapsulates the logic of exiting the binary depending on handled help mode.
+
+void MaybeExit(HelpMode mode);
+
+// --------------------------------------------------------------------
+// Globals representing usage reporting flags
+
+// Returns substring to filter help output (--help=substr argument)
+std::string GetFlagsHelpMatchSubstr();
+// Returns the requested help mode.
+HelpMode GetFlagsHelpMode();
+// Returns the requested help format.
+HelpFormat GetFlagsHelpFormat();
+
+// These are corresponding setters to the attributes above.
+void SetFlagsHelpMatchSubstr(absl::string_view);
+void SetFlagsHelpMode(HelpMode);
+void SetFlagsHelpFormat(HelpFormat);
+
+// Deduces usage flags from the input argument in a form --name=value or
+// --name. argument is already split into name and value before we call this
+// function.
+bool DeduceUsageFlags(absl::string_view name, absl::string_view value);
 
 }  // namespace flags_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
-ABSL_DECLARE_FLAG(bool, help);
-ABSL_DECLARE_FLAG(bool, helpfull);
-ABSL_DECLARE_FLAG(bool, helpshort);
-ABSL_DECLARE_FLAG(bool, helppackage);
-ABSL_DECLARE_FLAG(bool, version);
-ABSL_DECLARE_FLAG(bool, only_check_args);
-ABSL_DECLARE_FLAG(std::string, helpon);
-ABSL_DECLARE_FLAG(std::string, helpmatch);
-
 #endif  // ABSL_FLAGS_INTERNAL_USAGE_H_
diff --git a/absl/flags/internal/usage_test.cc b/absl/flags/internal/usage_test.cc
index d15e44847ff..9b6d730ca2c 100644
--- a/absl/flags/internal/usage_test.cc
+++ b/absl/flags/internal/usage_test.cc
@@ -15,17 +15,22 @@
 
 #include "absl/flags/internal/usage.h"
 
+#include <stdint.h>
+
 #include <sstream>
+#include <string>
 
+#include "gmock/gmock.h"
 #include "gtest/gtest.h"
+#include "absl/flags/config.h"
 #include "absl/flags/flag.h"
-#include "absl/flags/internal/path_util.h"
+#include "absl/flags/internal/parse.h"
 #include "absl/flags/internal/program_name.h"
-#include "absl/flags/parse.h"
+#include "absl/flags/reflection.h"
 #include "absl/flags/usage.h"
 #include "absl/flags/usage_config.h"
-#include "absl/memory/memory.h"
 #include "absl/strings/match.h"
+#include "absl/strings/string_view.h"
 
 ABSL_FLAG(int, usage_reporting_test_flag_01, 101,
           "usage_reporting_test_flag_01 help message");
@@ -35,15 +40,20 @@ ABSL_FLAG(double, usage_reporting_test_flag_03, 1.03,
           "usage_reporting_test_flag_03 help message");
 ABSL_FLAG(int64_t, usage_reporting_test_flag_04, 1000000000000004L,
           "usage_reporting_test_flag_04 help message");
+ABSL_FLAG(std::string, usage_reporting_test_flag_07, "\r\n\f\v\a\b\t ",
+          "usage_reporting_test_flag_07 help \r\n\f\v\a\b\t ");
 
 static const char kTestUsageMessage[] = "Custom usage message";
 
 struct UDT {
   UDT() = default;
   UDT(const UDT&) = default;
+  UDT& operator=(const UDT&) = default;
 };
-bool AbslParseFlag(absl::string_view, UDT*, std::string*) { return true; }
-std::string AbslUnparseFlag(const UDT&) { return "UDT{}"; }
+static bool AbslParseFlag(absl::string_view, UDT*, std::string*) {
+  return true;
+}
+static std::string AbslUnparseFlag(const UDT&) { return "UDT{}"; }
 
 ABSL_FLAG(UDT, usage_reporting_test_flag_05, {},
           "usage_reporting_test_flag_05 help message");
@@ -83,9 +93,19 @@ class UsageReportingTest : public testing::Test {
     default_config.normalize_filename = &NormalizeFileName;
     absl::SetFlagsUsageConfig(default_config);
   }
+  ~UsageReportingTest() override {
+    flags::SetFlagsHelpMode(flags::HelpMode::kNone);
+    flags::SetFlagsHelpMatchSubstr("");
+    flags::SetFlagsHelpFormat(flags::HelpFormat::kHumanReadable);
+  }
+  void SetUp() override {
+#if ABSL_FLAGS_STRIP_NAMES
+    GTEST_SKIP() << "This test requires flag names to be present";
+#endif
+  }
 
  private:
-  flags::FlagSaver flag_saver_;
+  absl::FlagSaver flag_saver_;
 };
 
 // --------------------------------------------------------------------
@@ -93,73 +113,79 @@ class UsageReportingTest : public testing::Test {
 using UsageReportingDeathTest = UsageReportingTest;
 
 TEST_F(UsageReportingDeathTest, TestSetProgramUsageMessage) {
+#if !defined(GTEST_HAS_ABSL) || !GTEST_HAS_ABSL
+  // Check for kTestUsageMessage set in main() below.
   EXPECT_EQ(absl::ProgramUsageMessage(), kTestUsageMessage);
-
-#ifndef _WIN32
-  // TODO(rogeeff): figure out why this does not work on Windows.
-  EXPECT_DEATH(absl::SetProgramUsageMessage("custom usage message"),
-               ".*SetProgramUsageMessage\\(\\) called twice.*");
+#else
+  // Check for part of the usage message set by GoogleTest.
+  EXPECT_THAT(absl::ProgramUsageMessage(),
+              ::testing::HasSubstr(
+                  "This program contains tests written using Google Test"));
 #endif
+
+  EXPECT_DEATH_IF_SUPPORTED(
+      absl::SetProgramUsageMessage("custom usage message"),
+      ::testing::HasSubstr("SetProgramUsageMessage() called twice"));
 }
 
 // --------------------------------------------------------------------
 
 TEST_F(UsageReportingTest, TestFlagHelpHRF_on_flag_01) {
-  const auto* flag = flags::FindCommandLineFlag("usage_reporting_test_flag_01");
+  const auto* flag = absl::FindCommandLineFlag("usage_reporting_test_flag_01");
   std::stringstream test_buf;
 
   flags::FlagHelp(test_buf, *flag, flags::HelpFormat::kHumanReadable);
   EXPECT_EQ(
       test_buf.str(),
-      R"(    -usage_reporting_test_flag_01 (usage_reporting_test_flag_01 help message);
+      R"(    --usage_reporting_test_flag_01 (usage_reporting_test_flag_01 help message);
       default: 101;
 )");
 }
 
 TEST_F(UsageReportingTest, TestFlagHelpHRF_on_flag_02) {
-  const auto* flag = flags::FindCommandLineFlag("usage_reporting_test_flag_02");
+  const auto* flag = absl::FindCommandLineFlag("usage_reporting_test_flag_02");
   std::stringstream test_buf;
 
   flags::FlagHelp(test_buf, *flag, flags::HelpFormat::kHumanReadable);
   EXPECT_EQ(
       test_buf.str(),
-      R"(    -usage_reporting_test_flag_02 (usage_reporting_test_flag_02 help message);
+      R"(    --usage_reporting_test_flag_02 (usage_reporting_test_flag_02 help message);
       default: false;
 )");
 }
 
 TEST_F(UsageReportingTest, TestFlagHelpHRF_on_flag_03) {
-  const auto* flag = flags::FindCommandLineFlag("usage_reporting_test_flag_03");
+  const auto* flag = absl::FindCommandLineFlag("usage_reporting_test_flag_03");
   std::stringstream test_buf;
 
   flags::FlagHelp(test_buf, *flag, flags::HelpFormat::kHumanReadable);
   EXPECT_EQ(
       test_buf.str(),
-      R"(    -usage_reporting_test_flag_03 (usage_reporting_test_flag_03 help message);
+      R"(    --usage_reporting_test_flag_03 (usage_reporting_test_flag_03 help message);
       default: 1.03;
 )");
 }
 
 TEST_F(UsageReportingTest, TestFlagHelpHRF_on_flag_04) {
-  const auto* flag = flags::FindCommandLineFlag("usage_reporting_test_flag_04");
+  const auto* flag = absl::FindCommandLineFlag("usage_reporting_test_flag_04");
   std::stringstream test_buf;
 
   flags::FlagHelp(test_buf, *flag, flags::HelpFormat::kHumanReadable);
   EXPECT_EQ(
       test_buf.str(),
-      R"(    -usage_reporting_test_flag_04 (usage_reporting_test_flag_04 help message);
+      R"(    --usage_reporting_test_flag_04 (usage_reporting_test_flag_04 help message);
       default: 1000000000000004;
 )");
 }
 
 TEST_F(UsageReportingTest, TestFlagHelpHRF_on_flag_05) {
-  const auto* flag = flags::FindCommandLineFlag("usage_reporting_test_flag_05");
+  const auto* flag = absl::FindCommandLineFlag("usage_reporting_test_flag_05");
   std::stringstream test_buf;
 
   flags::FlagHelp(test_buf, *flag, flags::HelpFormat::kHumanReadable);
   EXPECT_EQ(
       test_buf.str(),
-      R"(    -usage_reporting_test_flag_05 (usage_reporting_test_flag_05 help message);
+      R"(    --usage_reporting_test_flag_05 (usage_reporting_test_flag_05 help message);
       default: UDT{};
 )");
 }
@@ -171,21 +197,29 @@ TEST_F(UsageReportingTest, TestFlagsHelpHRF) {
       R"(usage_test: Custom usage message
 
   Flags from absl/flags/internal/usage_test.cc:
-    -usage_reporting_test_flag_01 (usage_reporting_test_flag_01 help message);
+    --usage_reporting_test_flag_01 (usage_reporting_test_flag_01 help message);
       default: 101;
-    -usage_reporting_test_flag_02 (usage_reporting_test_flag_02 help message);
+    --usage_reporting_test_flag_02 (usage_reporting_test_flag_02 help message);
       default: false;
-    -usage_reporting_test_flag_03 (usage_reporting_test_flag_03 help message);
+    --usage_reporting_test_flag_03 (usage_reporting_test_flag_03 help message);
       default: 1.03;
-    -usage_reporting_test_flag_04 (usage_reporting_test_flag_04 help message);
+    --usage_reporting_test_flag_04 (usage_reporting_test_flag_04 help message);
       default: 1000000000000004;
-    -usage_reporting_test_flag_05 (usage_reporting_test_flag_05 help message);
+    --usage_reporting_test_flag_05 (usage_reporting_test_flag_05 help message);
       default: UDT{};
-    -usage_reporting_test_flag_06 (usage_reporting_test_flag_06 help message.
+    --usage_reporting_test_flag_06 (usage_reporting_test_flag_06 help message.
 
       Some more help.
       Even more long long long long long long long long long long long long help
-      message.); default: "";
+      message.); default: "";)"
+
+      "\n    --usage_reporting_test_flag_07 (usage_reporting_test_flag_07 "
+      "help\n\n      \f\v\a\b ); default: \"\r\n\f\v\a\b\t \";\n"
+
+      R"(
+Try --helpfull to get a list of all flags or --help=substring shows help for
+flags which include specified substring in either in the name, or description or
+path.
 )";
 
   std::stringstream test_buf_01;
@@ -209,7 +243,11 @@ TEST_F(UsageReportingTest, TestFlagsHelpHRF) {
   EXPECT_EQ(test_buf_04.str(),
             R"(usage_test: Custom usage message
 
-  No modules matched: use -helpfull
+No flags matched.
+
+Try --helpfull to get a list of all flags or --help=substring shows help for
+flags which include specified substring in either in the name, or description or
+path.
 )");
 
   std::stringstream test_buf_05;
@@ -221,119 +259,208 @@ TEST_F(UsageReportingTest, TestFlagsHelpHRF) {
       absl::StartsWith(test_out_str, "usage_test: Custom usage message"));
   EXPECT_TRUE(absl::StrContains(
       test_out_str, "Flags from absl/flags/internal/usage_test.cc:"));
-  EXPECT_TRUE(absl::StrContains(test_out_str,
-                                "Flags from absl/flags/internal/usage.cc:"));
   EXPECT_TRUE(
       absl::StrContains(test_out_str, "-usage_reporting_test_flag_01 "));
-  EXPECT_TRUE(absl::StrContains(test_out_str, "-help (show help"))
-      << test_out_str;
 }
 
 // --------------------------------------------------------------------
 
 TEST_F(UsageReportingTest, TestNoUsageFlags) {
   std::stringstream test_buf;
-  EXPECT_EQ(flags::HandleUsageFlags(test_buf, kTestUsageMessage), -1);
+  EXPECT_EQ(flags::HandleUsageFlags(test_buf, kTestUsageMessage),
+            flags::HelpMode::kNone);
 }
 
 // --------------------------------------------------------------------
 
 TEST_F(UsageReportingTest, TestUsageFlag_helpshort) {
-  absl::SetFlag(&FLAGS_helpshort, true);
+  flags::SetFlagsHelpMode(flags::HelpMode::kShort);
 
   std::stringstream test_buf;
-  EXPECT_EQ(flags::HandleUsageFlags(test_buf, kTestUsageMessage), 1);
-  EXPECT_EQ(test_buf.str(),
-            R"(usage_test: Custom usage message
+  EXPECT_EQ(flags::HandleUsageFlags(test_buf, kTestUsageMessage),
+            flags::HelpMode::kShort);
+  EXPECT_EQ(
+      test_buf.str(),
+      R"(usage_test: Custom usage message
 
   Flags from absl/flags/internal/usage_test.cc:
-    -usage_reporting_test_flag_01 (usage_reporting_test_flag_01 help message);
+    --usage_reporting_test_flag_01 (usage_reporting_test_flag_01 help message);
       default: 101;
-    -usage_reporting_test_flag_02 (usage_reporting_test_flag_02 help message);
+    --usage_reporting_test_flag_02 (usage_reporting_test_flag_02 help message);
       default: false;
-    -usage_reporting_test_flag_03 (usage_reporting_test_flag_03 help message);
+    --usage_reporting_test_flag_03 (usage_reporting_test_flag_03 help message);
       default: 1.03;
-    -usage_reporting_test_flag_04 (usage_reporting_test_flag_04 help message);
+    --usage_reporting_test_flag_04 (usage_reporting_test_flag_04 help message);
       default: 1000000000000004;
-    -usage_reporting_test_flag_05 (usage_reporting_test_flag_05 help message);
+    --usage_reporting_test_flag_05 (usage_reporting_test_flag_05 help message);
       default: UDT{};
-    -usage_reporting_test_flag_06 (usage_reporting_test_flag_06 help message.
+    --usage_reporting_test_flag_06 (usage_reporting_test_flag_06 help message.
 
       Some more help.
       Even more long long long long long long long long long long long long help
-      message.); default: "";
+      message.); default: "";)"
+
+      "\n    --usage_reporting_test_flag_07 (usage_reporting_test_flag_07 "
+      "help\n\n      \f\v\a\b ); default: \"\r\n\f\v\a\b\t \";\n"
+
+      R"(
+Try --helpfull to get a list of all flags or --help=substring shows help for
+flags which include specified substring in either in the name, or description or
+path.
 )");
 }
 
 // --------------------------------------------------------------------
 
-TEST_F(UsageReportingTest, TestUsageFlag_help) {
-  absl::SetFlag(&FLAGS_help, true);
+TEST_F(UsageReportingTest, TestUsageFlag_help_simple) {
+  flags::SetFlagsHelpMode(flags::HelpMode::kImportant);
 
   std::stringstream test_buf;
-  EXPECT_EQ(flags::HandleUsageFlags(test_buf, kTestUsageMessage), 1);
+  EXPECT_EQ(flags::HandleUsageFlags(test_buf, kTestUsageMessage),
+            flags::HelpMode::kImportant);
+  EXPECT_EQ(
+      test_buf.str(),
+      R"(usage_test: Custom usage message
+
+  Flags from absl/flags/internal/usage_test.cc:
+    --usage_reporting_test_flag_01 (usage_reporting_test_flag_01 help message);
+      default: 101;
+    --usage_reporting_test_flag_02 (usage_reporting_test_flag_02 help message);
+      default: false;
+    --usage_reporting_test_flag_03 (usage_reporting_test_flag_03 help message);
+      default: 1.03;
+    --usage_reporting_test_flag_04 (usage_reporting_test_flag_04 help message);
+      default: 1000000000000004;
+    --usage_reporting_test_flag_05 (usage_reporting_test_flag_05 help message);
+      default: UDT{};
+    --usage_reporting_test_flag_06 (usage_reporting_test_flag_06 help message.
+
+      Some more help.
+      Even more long long long long long long long long long long long long help
+      message.); default: "";)"
+
+      "\n    --usage_reporting_test_flag_07 (usage_reporting_test_flag_07 "
+      "help\n\n      \f\v\a\b ); default: \"\r\n\f\v\a\b\t \";\n"
+
+      R"(
+Try --helpfull to get a list of all flags or --help=substring shows help for
+flags which include specified substring in either in the name, or description or
+path.
+)");
+}
+
+// --------------------------------------------------------------------
+
+TEST_F(UsageReportingTest, TestUsageFlag_help_one_flag) {
+  flags::SetFlagsHelpMode(flags::HelpMode::kMatch);
+  flags::SetFlagsHelpMatchSubstr("usage_reporting_test_flag_06");
+
+  std::stringstream test_buf;
+  EXPECT_EQ(flags::HandleUsageFlags(test_buf, kTestUsageMessage),
+            flags::HelpMode::kMatch);
   EXPECT_EQ(test_buf.str(),
             R"(usage_test: Custom usage message
 
   Flags from absl/flags/internal/usage_test.cc:
-    -usage_reporting_test_flag_01 (usage_reporting_test_flag_01 help message);
+    --usage_reporting_test_flag_06 (usage_reporting_test_flag_06 help message.
+
+      Some more help.
+      Even more long long long long long long long long long long long long help
+      message.); default: "";
+
+Try --helpfull to get a list of all flags or --help=substring shows help for
+flags which include specified substring in either in the name, or description or
+path.
+)");
+}
+
+// --------------------------------------------------------------------
+
+TEST_F(UsageReportingTest, TestUsageFlag_help_multiple_flag) {
+  flags::SetFlagsHelpMode(flags::HelpMode::kMatch);
+  flags::SetFlagsHelpMatchSubstr("test_flag");
+
+  std::stringstream test_buf;
+  EXPECT_EQ(flags::HandleUsageFlags(test_buf, kTestUsageMessage),
+            flags::HelpMode::kMatch);
+  EXPECT_EQ(
+      test_buf.str(),
+      R"(usage_test: Custom usage message
+
+  Flags from absl/flags/internal/usage_test.cc:
+    --usage_reporting_test_flag_01 (usage_reporting_test_flag_01 help message);
       default: 101;
-    -usage_reporting_test_flag_02 (usage_reporting_test_flag_02 help message);
+    --usage_reporting_test_flag_02 (usage_reporting_test_flag_02 help message);
       default: false;
-    -usage_reporting_test_flag_03 (usage_reporting_test_flag_03 help message);
+    --usage_reporting_test_flag_03 (usage_reporting_test_flag_03 help message);
       default: 1.03;
-    -usage_reporting_test_flag_04 (usage_reporting_test_flag_04 help message);
+    --usage_reporting_test_flag_04 (usage_reporting_test_flag_04 help message);
       default: 1000000000000004;
-    -usage_reporting_test_flag_05 (usage_reporting_test_flag_05 help message);
+    --usage_reporting_test_flag_05 (usage_reporting_test_flag_05 help message);
       default: UDT{};
-    -usage_reporting_test_flag_06 (usage_reporting_test_flag_06 help message.
+    --usage_reporting_test_flag_06 (usage_reporting_test_flag_06 help message.
 
       Some more help.
       Even more long long long long long long long long long long long long help
-      message.); default: "";
+      message.); default: "";)"
+
+      "\n    --usage_reporting_test_flag_07 (usage_reporting_test_flag_07 "
+      "help\n\n      \f\v\a\b ); default: \"\r\n\f\v\a\b\t \";\n"
 
-Try --helpfull to get a list of all flags.
+      R"(
+Try --helpfull to get a list of all flags or --help=substring shows help for
+flags which include specified substring in either in the name, or description or
+path.
 )");
 }
 
 // --------------------------------------------------------------------
 
 TEST_F(UsageReportingTest, TestUsageFlag_helppackage) {
-  absl::SetFlag(&FLAGS_helppackage, true);
+  flags::SetFlagsHelpMode(flags::HelpMode::kPackage);
 
   std::stringstream test_buf;
-  EXPECT_EQ(flags::HandleUsageFlags(test_buf, kTestUsageMessage), 1);
-  EXPECT_EQ(test_buf.str(),
-            R"(usage_test: Custom usage message
+  EXPECT_EQ(flags::HandleUsageFlags(test_buf, kTestUsageMessage),
+            flags::HelpMode::kPackage);
+  EXPECT_EQ(
+      test_buf.str(),
+      R"(usage_test: Custom usage message
 
   Flags from absl/flags/internal/usage_test.cc:
-    -usage_reporting_test_flag_01 (usage_reporting_test_flag_01 help message);
+    --usage_reporting_test_flag_01 (usage_reporting_test_flag_01 help message);
       default: 101;
-    -usage_reporting_test_flag_02 (usage_reporting_test_flag_02 help message);
+    --usage_reporting_test_flag_02 (usage_reporting_test_flag_02 help message);
       default: false;
-    -usage_reporting_test_flag_03 (usage_reporting_test_flag_03 help message);
+    --usage_reporting_test_flag_03 (usage_reporting_test_flag_03 help message);
       default: 1.03;
-    -usage_reporting_test_flag_04 (usage_reporting_test_flag_04 help message);
+    --usage_reporting_test_flag_04 (usage_reporting_test_flag_04 help message);
       default: 1000000000000004;
-    -usage_reporting_test_flag_05 (usage_reporting_test_flag_05 help message);
+    --usage_reporting_test_flag_05 (usage_reporting_test_flag_05 help message);
       default: UDT{};
-    -usage_reporting_test_flag_06 (usage_reporting_test_flag_06 help message.
+    --usage_reporting_test_flag_06 (usage_reporting_test_flag_06 help message.
 
       Some more help.
       Even more long long long long long long long long long long long long help
-      message.); default: "";
+      message.); default: "";)"
+
+      "\n    --usage_reporting_test_flag_07 (usage_reporting_test_flag_07 "
+      "help\n\n      \f\v\a\b ); default: \"\r\n\f\v\a\b\t \";\n"
 
-Try --helpfull to get a list of all flags.
+      R"(
+Try --helpfull to get a list of all flags or --help=substring shows help for
+flags which include specified substring in either in the name, or description or
+path.
 )");
 }
 
 // --------------------------------------------------------------------
 
 TEST_F(UsageReportingTest, TestUsageFlag_version) {
-  absl::SetFlag(&FLAGS_version, true);
+  flags::SetFlagsHelpMode(flags::HelpMode::kVersion);
 
   std::stringstream test_buf;
-  EXPECT_EQ(flags::HandleUsageFlags(test_buf, kTestUsageMessage), 0);
+  EXPECT_EQ(flags::HandleUsageFlags(test_buf, kTestUsageMessage),
+            flags::HelpMode::kVersion);
 #ifndef NDEBUG
   EXPECT_EQ(test_buf.str(), "usage_test\nDebug build (NDEBUG not #defined)\n");
 #else
@@ -344,49 +471,66 @@ TEST_F(UsageReportingTest, TestUsageFlag_version) {
 // --------------------------------------------------------------------
 
 TEST_F(UsageReportingTest, TestUsageFlag_only_check_args) {
-  absl::SetFlag(&FLAGS_only_check_args, true);
+  flags::SetFlagsHelpMode(flags::HelpMode::kOnlyCheckArgs);
 
   std::stringstream test_buf;
-  EXPECT_EQ(flags::HandleUsageFlags(test_buf, kTestUsageMessage), 0);
+  EXPECT_EQ(flags::HandleUsageFlags(test_buf, kTestUsageMessage),
+            flags::HelpMode::kOnlyCheckArgs);
   EXPECT_EQ(test_buf.str(), "");
 }
 
 // --------------------------------------------------------------------
 
 TEST_F(UsageReportingTest, TestUsageFlag_helpon) {
-  absl::SetFlag(&FLAGS_helpon, "bla-bla");
+  flags::SetFlagsHelpMode(flags::HelpMode::kMatch);
+  flags::SetFlagsHelpMatchSubstr("/bla-bla.");
 
   std::stringstream test_buf_01;
-  EXPECT_EQ(flags::HandleUsageFlags(test_buf_01, kTestUsageMessage), 1);
+  EXPECT_EQ(flags::HandleUsageFlags(test_buf_01, kTestUsageMessage),
+            flags::HelpMode::kMatch);
   EXPECT_EQ(test_buf_01.str(),
             R"(usage_test: Custom usage message
 
-  No modules matched: use -helpfull
+No flags matched.
+
+Try --helpfull to get a list of all flags or --help=substring shows help for
+flags which include specified substring in either in the name, or description or
+path.
 )");
 
-  absl::SetFlag(&FLAGS_helpon, "usage_test");
+  flags::SetFlagsHelpMatchSubstr("/usage_test.");
 
   std::stringstream test_buf_02;
-  EXPECT_EQ(flags::HandleUsageFlags(test_buf_02, kTestUsageMessage), 1);
-  EXPECT_EQ(test_buf_02.str(),
-            R"(usage_test: Custom usage message
+  EXPECT_EQ(flags::HandleUsageFlags(test_buf_02, kTestUsageMessage),
+            flags::HelpMode::kMatch);
+  EXPECT_EQ(
+      test_buf_02.str(),
+      R"(usage_test: Custom usage message
 
   Flags from absl/flags/internal/usage_test.cc:
-    -usage_reporting_test_flag_01 (usage_reporting_test_flag_01 help message);
+    --usage_reporting_test_flag_01 (usage_reporting_test_flag_01 help message);
       default: 101;
-    -usage_reporting_test_flag_02 (usage_reporting_test_flag_02 help message);
+    --usage_reporting_test_flag_02 (usage_reporting_test_flag_02 help message);
       default: false;
-    -usage_reporting_test_flag_03 (usage_reporting_test_flag_03 help message);
+    --usage_reporting_test_flag_03 (usage_reporting_test_flag_03 help message);
       default: 1.03;
-    -usage_reporting_test_flag_04 (usage_reporting_test_flag_04 help message);
+    --usage_reporting_test_flag_04 (usage_reporting_test_flag_04 help message);
       default: 1000000000000004;
-    -usage_reporting_test_flag_05 (usage_reporting_test_flag_05 help message);
+    --usage_reporting_test_flag_05 (usage_reporting_test_flag_05 help message);
       default: UDT{};
-    -usage_reporting_test_flag_06 (usage_reporting_test_flag_06 help message.
+    --usage_reporting_test_flag_06 (usage_reporting_test_flag_06 help message.
 
       Some more help.
       Even more long long long long long long long long long long long long help
-      message.); default: "";
+      message.); default: "";)"
+
+      "\n    --usage_reporting_test_flag_07 (usage_reporting_test_flag_07 "
+      "help\n\n      \f\v\a\b ); default: \"\r\n\f\v\a\b\t \";\n"
+
+      R"(
+Try --helpfull to get a list of all flags or --help=substring shows help for
+flags which include specified substring in either in the name, or description or
+path.
 )");
 }
 
@@ -397,8 +541,10 @@ TEST_F(UsageReportingTest, TestUsageFlag_helpon) {
 int main(int argc, char* argv[]) {
   (void)absl::GetFlag(FLAGS_undefok);  // Force linking of parse.cc
   flags::SetProgramInvocationName("usage_test");
+#if !defined(GTEST_HAS_ABSL) || !GTEST_HAS_ABSL
+  // GoogleTest calls absl::SetProgramUsageMessage() already.
   absl::SetProgramUsageMessage(kTestUsageMessage);
+#endif
   ::testing::InitGoogleTest(&argc, argv);
-
   return RUN_ALL_TESTS();
 }
diff --git a/absl/flags/marshalling.cc b/absl/flags/marshalling.cc
index f4ebe0e3433..99d1eddad5d 100644
--- a/absl/flags/marshalling.cc
+++ b/absl/flags/marshalling.cc
@@ -15,40 +15,37 @@
 
 #include "absl/flags/marshalling.h"
 
+#include <stddef.h>
+
+#include <cmath>
 #include <limits>
+#include <sstream>
+#include <string>
 #include <type_traits>
+#include <vector>
 
+#include "absl/base/config.h"
+#include "absl/base/log_severity.h"
 #include "absl/base/macros.h"
+#include "absl/numeric/int128.h"
+#include "absl/strings/ascii.h"
 #include "absl/strings/match.h"
 #include "absl/strings/numbers.h"
 #include "absl/strings/str_cat.h"
 #include "absl/strings/str_format.h"
 #include "absl/strings/str_join.h"
 #include "absl/strings/str_split.h"
+#include "absl/strings/string_view.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace flags_internal {
 
 // --------------------------------------------------------------------
 // AbslParseFlag specializations for boolean type.
 
 bool AbslParseFlag(absl::string_view text, bool* dst, std::string*) {
-  const char* kTrue[] = {"1", "t", "true", "y", "yes"};
-  const char* kFalse[] = {"0", "f", "false", "n", "no"};
-  static_assert(sizeof(kTrue) == sizeof(kFalse), "true_false_equal");
-
-  text = absl::StripAsciiWhitespace(text);
-
-  for (size_t i = 0; i < ABSL_ARRAYSIZE(kTrue); ++i) {
-    if (absl::EqualsIgnoreCase(text, kTrue[i])) {
-      *dst = true;
-      return true;
-    } else if (absl::EqualsIgnoreCase(text, kFalse[i])) {
-      *dst = false;
-      return true;
-    }
-  }
-  return false;  // didn't match a legal input
+  return SimpleAtob(absl::StripAsciiWhitespace(text), dst);
 }
 
 // --------------------------------------------------------------------
@@ -58,21 +55,24 @@ bool AbslParseFlag(absl::string_view text, bool* dst, std::string*) {
 // puts us in base 16.  But leading 0 does not put us in base 8. It
 // caused too many bugs when we had that behavior.
 static int NumericBase(absl::string_view text) {
-  const bool hex = (text.size() >= 2 && text[0] == '0' &&
-                    (text[1] == 'x' || text[1] == 'X'));
+  if (text.empty()) return 0;
+  size_t num_start = (text[0] == '-' || text[0] == '+') ? 1 : 0;
+  const bool hex = (text.size() >= num_start + 2 && text[num_start] == '0' &&
+                    (text[num_start + 1] == 'x' || text[num_start + 1] == 'X'));
   return hex ? 16 : 10;
 }
 
 template <typename IntType>
-inline bool ParseFlagImpl(absl::string_view text, IntType* dst) {
+inline bool ParseFlagImpl(absl::string_view text, IntType& dst) {
   text = absl::StripAsciiWhitespace(text);
 
-  return absl::numbers_internal::safe_strtoi_base(text, dst, NumericBase(text));
+  return absl::numbers_internal::safe_strtoi_base(text, &dst,
+                                                  NumericBase(text));
 }
 
 bool AbslParseFlag(absl::string_view text, short* dst, std::string*) {
   int val;
-  if (!ParseFlagImpl(text, &val)) return false;
+  if (!ParseFlagImpl(text, val)) return false;
   if (static_cast<short>(val) != val)  // worked, but number out of range
     return false;
   *dst = static_cast<short>(val);
@@ -81,7 +81,7 @@ bool AbslParseFlag(absl::string_view text, short* dst, std::string*) {
 
 bool AbslParseFlag(absl::string_view text, unsigned short* dst, std::string*) {
   unsigned int val;
-  if (!ParseFlagImpl(text, &val)) return false;
+  if (!ParseFlagImpl(text, val)) return false;
   if (static_cast<unsigned short>(val) !=
       val)  // worked, but number out of range
     return false;
@@ -90,28 +90,54 @@ bool AbslParseFlag(absl::string_view text, unsigned short* dst, std::string*) {
 }
 
 bool AbslParseFlag(absl::string_view text, int* dst, std::string*) {
-  return ParseFlagImpl(text, dst);
+  return ParseFlagImpl(text, *dst);
 }
 
 bool AbslParseFlag(absl::string_view text, unsigned int* dst, std::string*) {
-  return ParseFlagImpl(text, dst);
+  return ParseFlagImpl(text, *dst);
 }
 
 bool AbslParseFlag(absl::string_view text, long* dst, std::string*) {
-  return ParseFlagImpl(text, dst);
+  return ParseFlagImpl(text, *dst);
 }
 
 bool AbslParseFlag(absl::string_view text, unsigned long* dst, std::string*) {
-  return ParseFlagImpl(text, dst);
+  return ParseFlagImpl(text, *dst);
 }
 
 bool AbslParseFlag(absl::string_view text, long long* dst, std::string*) {
-  return ParseFlagImpl(text, dst);
+  return ParseFlagImpl(text, *dst);
 }
 
 bool AbslParseFlag(absl::string_view text, unsigned long long* dst,
                    std::string*) {
-  return ParseFlagImpl(text, dst);
+  return ParseFlagImpl(text, *dst);
+}
+
+bool AbslParseFlag(absl::string_view text, absl::int128* dst, std::string*) {
+  text = absl::StripAsciiWhitespace(text);
+
+  // check hex
+  int base = NumericBase(text);
+  if (!absl::numbers_internal::safe_strto128_base(text, dst, base)) {
+    return false;
+  }
+
+  return base == 16 ? absl::SimpleHexAtoi(text, dst)
+                    : absl::SimpleAtoi(text, dst);
+}
+
+bool AbslParseFlag(absl::string_view text, absl::uint128* dst, std::string*) {
+  text = absl::StripAsciiWhitespace(text);
+
+  // check hex
+  int base = NumericBase(text);
+  if (!absl::numbers_internal::safe_strtou128_base(text, dst, base)) {
+    return false;
+  }
+
+  return base == 16 ? absl::SimpleHexAtoi(text, dst)
+                    : absl::SimpleAtoi(text, dst);
 }
 
 // --------------------------------------------------------------------
@@ -160,9 +186,20 @@ std::string Unparse(long v) { return absl::StrCat(v); }
 std::string Unparse(unsigned long v) { return absl::StrCat(v); }
 std::string Unparse(long long v) { return absl::StrCat(v); }
 std::string Unparse(unsigned long long v) { return absl::StrCat(v); }
+std::string Unparse(absl::int128 v) {
+  std::stringstream ss;
+  ss << v;
+  return ss.str();
+}
+std::string Unparse(absl::uint128 v) {
+  std::stringstream ss;
+  ss << v;
+  return ss.str();
+}
+
 template <typename T>
 std::string UnparseFloatingPointVal(T v) {
-  // digits10 is guaranteed to roundtrip correctly in std::string -> value -> std::string
+  // digits10 is guaranteed to roundtrip correctly in string -> value -> string
   // conversions, but may not be enough to represent all the values correctly.
   std::string digit10_str =
       absl::StrFormat("%.*g", std::numeric_limits<T>::digits10, v);
@@ -195,6 +232,14 @@ bool AbslParseFlag(absl::string_view text, absl::LogSeverity* dst,
     *err = "no value provided";
     return false;
   }
+  if (absl::EqualsIgnoreCase(text, "dfatal")) {
+    *dst = absl::kLogDebugFatal;
+    return true;
+  }
+  if (absl::EqualsIgnoreCase(text, "klogdebugfatal")) {
+    *dst = absl::kLogDebugFatal;
+    return true;
+  }
   if (text.front() == 'k' || text.front() == 'K') text.remove_prefix(1);
   if (absl::EqualsIgnoreCase(text, "info")) {
     *dst = absl::LogSeverity::kInfo;
@@ -212,12 +257,13 @@ bool AbslParseFlag(absl::string_view text, absl::LogSeverity* dst,
     *dst = absl::LogSeverity::kFatal;
     return true;
   }
-  std::underlying_type<absl::LogSeverity>::type numeric_value;
+  std::underlying_type_t<absl::LogSeverity> numeric_value;
   if (absl::ParseFlag(text, &numeric_value, err)) {
     *dst = static_cast<absl::LogSeverity>(numeric_value);
     return true;
   }
-  *err = "only integers and absl::LogSeverity enumerators are accepted";
+  *err =
+      "only integers, absl::LogSeverity enumerators, and DFATAL are accepted";
   return false;
 }
 
@@ -226,4 +272,5 @@ std::string AbslUnparseFlag(absl::LogSeverity v) {
   return absl::UnparseFlag(static_cast<int>(v));
 }
 
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/flags/marshalling.h b/absl/flags/marshalling.h
index 6d391804370..5e04d6ac2ce 100644
--- a/absl/flags/marshalling.h
+++ b/absl/flags/marshalling.h
@@ -33,16 +33,17 @@
 // * `double`
 // * `std::string`
 // * `std::vector<std::string>`
-// * `absl::LogSeverity` (provided here due to dependency ordering)
+// * `std::optional<T>`
+// * `absl::LogSeverity` (provided natively for layering reasons)
 //
 // Note that support for integral types is implemented using overloads for
 // variable-width fundamental types (`short`, `int`, `long`, etc.). However,
 // you should prefer the fixed-width integral types (`int32_t`, `uint64_t`,
 // etc.) we've noted above within flag definitions.
-
 //
 // In addition, several Abseil libraries provide their own custom support for
-// Abseil flags.
+// Abseil flags. Documentation for these formats is provided in the type's
+// `AbslParseFlag()` definition.
 //
 // The Abseil time library provides the following support for civil time values:
 //
@@ -65,6 +66,42 @@
 // below.)
 //
 // -----------------------------------------------------------------------------
+// Optional Flags
+// -----------------------------------------------------------------------------
+//
+// The Abseil flags library supports flags of type `std::optional<T>` where
+// `T` is a type of one of the supported flags. We refer to this flag type as
+// an "optional flag." An optional flag is either "valueless", holding no value
+// of type `T` (indicating that the flag has not been set) or a value of type
+// `T`. The valueless state in C++ code is represented by a value of
+// `std::nullopt` for the optional flag.
+//
+// Using `std::nullopt` as an optional flag's default value allows you to check
+// whether such a flag was ever specified on the command line:
+//
+//   if (absl::GetFlag(FLAGS_foo).has_value()) {
+//     // flag was set on command line
+//   } else {
+//     // flag was not passed on command line
+//   }
+//
+// Using an optional flag in this manner avoids common workarounds for
+// indicating such an unset flag (such as using sentinel values to indicate this
+// state).
+//
+// An optional flag also allows a developer to pass a flag in an "unset"
+// valueless state on the command line, allowing the flag to later be set in
+// binary logic. An optional flag's valueless state is indicated by the special
+// notation of passing the value as an empty string through the syntax `--flag=`
+// or `--flag ""`.
+//
+//   $ binary_with_optional --flag_in_unset_state=
+//   $ binary_with_optional --flag_in_unset_state ""
+//
+// Note: as a result of the above syntax requirements, an optional flag cannot
+// be set to a `T` of any value which unparses to the empty string.
+//
+// -----------------------------------------------------------------------------
 // Adding Type Support for Abseil Flags
 // -----------------------------------------------------------------------------
 //
@@ -83,7 +120,7 @@
 //   // AbslParseFlag converts from a string to OutputMode.
 //   // Must be in same namespace as OutputMode.
 //
-//   // Parses an OutputMode from the command line flag value `text. Returns
+//   // Parses an OutputMode from the command line flag value `text`. Returns
 //   // `true` and sets `*mode` on success; returns `false` and sets `*error`
 //   // on failure.
 //   bool AbslParseFlag(absl::string_view text,
@@ -139,7 +176,7 @@
 //
 //   // Within the implementation, `AbslParseFlag()` will, in turn invoke
 //   // `absl::ParseFlag()` on its constituent `int` and `std::string` types
-//   // (which have built-in Abseil flag support.
+//   // (which have built-in Abseil flag support).
 //
 //   bool AbslParseFlag(absl::string_view text, MyFlagType* flag,
 //                      std::string* err) {
@@ -162,12 +199,25 @@
 #ifndef ABSL_FLAGS_MARSHALLING_H_
 #define ABSL_FLAGS_MARSHALLING_H_
 
+#include <optional>
 #include <string>
 #include <vector>
 
+#include "absl/base/config.h"
+#include "absl/numeric/int128.h"
 #include "absl/strings/string_view.h"
+#include "absl/types/optional.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+// Forward declaration to be used inside composable flag parse/unparse
+// implementations
+template <typename T>
+inline bool ParseFlag(absl::string_view input, T* dst, std::string* error);
+template <typename T>
+inline std::string UnparseFlag(const T& v);
+
 namespace flags_internal {
 
 // Overloads of `AbslParseFlag()` and `AbslUnparseFlag()` for fundamental types.
@@ -181,11 +231,27 @@ bool AbslParseFlag(absl::string_view, unsigned long*, std::string*);   // NOLINT
 bool AbslParseFlag(absl::string_view, long long*, std::string*);       // NOLINT
 bool AbslParseFlag(absl::string_view, unsigned long long*,             // NOLINT
                    std::string*);
+bool AbslParseFlag(absl::string_view, absl::int128*, std::string*);    // NOLINT
+bool AbslParseFlag(absl::string_view, absl::uint128*, std::string*);   // NOLINT
 bool AbslParseFlag(absl::string_view, float*, std::string*);
 bool AbslParseFlag(absl::string_view, double*, std::string*);
 bool AbslParseFlag(absl::string_view, std::string*, std::string*);
 bool AbslParseFlag(absl::string_view, std::vector<std::string>*, std::string*);
 
+template <typename T>
+bool AbslParseFlag(absl::string_view text, std::optional<T>* f,
+                   std::string* err) {
+  if (text.empty()) {
+    *f = std::nullopt;
+    return true;
+  }
+  T value;
+  if (!absl::ParseFlag(text, &value, err)) return false;
+
+  *f = std::move(value);
+  return true;
+}
+
 template <typename T>
 bool InvokeParseFlag(absl::string_view input, T* dst, std::string* err) {
   // Comment on next line provides a good compiler error message if T
@@ -199,6 +265,11 @@ bool InvokeParseFlag(absl::string_view input, T* dst, std::string* err) {
 std::string AbslUnparseFlag(absl::string_view v);
 std::string AbslUnparseFlag(const std::vector<std::string>&);
 
+template <typename T>
+std::string AbslUnparseFlag(const std::optional<T>& f) {
+  return f.has_value() ? absl::UnparseFlag(*f) : "";
+}
+
 template <typename T>
 std::string Unparse(const T& v) {
   // Comment on next line provides a good compiler error message if T does not
@@ -216,6 +287,8 @@ std::string Unparse(long v);                // NOLINT
 std::string Unparse(unsigned long v);       // NOLINT
 std::string Unparse(long long v);           // NOLINT
 std::string Unparse(unsigned long long v);  // NOLINT
+std::string Unparse(absl::int128 v);
+std::string Unparse(absl::uint128 v);
 std::string Unparse(float v);
 std::string Unparse(double v);
 
@@ -256,6 +329,7 @@ enum class LogSeverity : int;
 bool AbslParseFlag(absl::string_view, absl::LogSeverity*, std::string*);
 std::string AbslUnparseFlag(absl::LogSeverity);
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_FLAGS_MARSHALLING_H_
diff --git a/absl/flags/marshalling_test.cc b/absl/flags/marshalling_test.cc
index 37cd1940e34..a5faabca06e 100644
--- a/absl/flags/marshalling_test.cc
+++ b/absl/flags/marshalling_test.cc
@@ -15,7 +15,13 @@
 
 #include "absl/flags/marshalling.h"
 
+#include <stdint.h>
+
 #include <cmath>
+#include <limits>
+#include <optional>
+#include <string>
+#include <vector>
 
 #include "gtest/gtest.h"
 
@@ -132,11 +138,10 @@ TEST(MarshallingTest, TestInt16Parsing) {
   EXPECT_EQ(value, 16);
   EXPECT_TRUE(absl::ParseFlag("0X234", &value, &err));
   EXPECT_EQ(value, 564);
-  // TODO(rogeeff): fix below validations
-  EXPECT_FALSE(absl::ParseFlag("-0x7FFD", &value, &err));
-  EXPECT_NE(value, -3);
-  EXPECT_FALSE(absl::ParseFlag("+0x31", &value, &err));
-  EXPECT_NE(value, 49);
+  EXPECT_TRUE(absl::ParseFlag("-0x7FFD", &value, &err));
+  EXPECT_EQ(value, -32765);
+  EXPECT_TRUE(absl::ParseFlag("+0x31", &value, &err));
+  EXPECT_EQ(value, 49);
 
   // Whitespace handling
   EXPECT_TRUE(absl::ParseFlag("10  ", &value, &err));
@@ -189,9 +194,8 @@ TEST(MarshallingTest, TestUint16Parsing) {
   EXPECT_EQ(value, 16);
   EXPECT_TRUE(absl::ParseFlag("0X234", &value, &err));
   EXPECT_EQ(value, 564);
-  // TODO(rogeeff): fix below validations
-  EXPECT_FALSE(absl::ParseFlag("+0x31", &value, &err));
-  EXPECT_NE(value, 49);
+  EXPECT_TRUE(absl::ParseFlag("+0x31", &value, &err));
+  EXPECT_EQ(value, 49);
 
   // Whitespace handling
   EXPECT_TRUE(absl::ParseFlag("10  ", &value, &err));
@@ -249,11 +253,11 @@ TEST(MarshallingTest, TestInt32Parsing) {
   EXPECT_EQ(value, 16);
   EXPECT_TRUE(absl::ParseFlag("0X234", &value, &err));
   EXPECT_EQ(value, 564);
-  // TODO(rogeeff): fix below validations
-  EXPECT_FALSE(absl::ParseFlag("-0x7FFFFFFD", &value, &err));
-  EXPECT_NE(value, -3);
-  EXPECT_FALSE(absl::ParseFlag("+0x31", &value, &err));
-  EXPECT_NE(value, 49);
+
+  EXPECT_TRUE(absl::ParseFlag("-0x7FFFFFFD", &value, &err));
+  EXPECT_EQ(value, -2147483645);
+  EXPECT_TRUE(absl::ParseFlag("+0x31", &value, &err));
+  EXPECT_EQ(value, 49);
 
   // Whitespace handling
   EXPECT_TRUE(absl::ParseFlag("10  ", &value, &err));
@@ -306,9 +310,8 @@ TEST(MarshallingTest, TestUint32Parsing) {
   EXPECT_EQ(value, 564);
   EXPECT_TRUE(absl::ParseFlag("0xFFFFFFFD", &value, &err));
   EXPECT_EQ(value, 4294967293);
-  // TODO(rogeeff): fix below validations
-  EXPECT_FALSE(absl::ParseFlag("+0x31", &value, &err));
-  EXPECT_NE(value, 49);
+  EXPECT_TRUE(absl::ParseFlag("+0x31", &value, &err));
+  EXPECT_EQ(value, 49);
 
   // Whitespace handling
   EXPECT_TRUE(absl::ParseFlag("10  ", &value, &err));
@@ -366,11 +369,12 @@ TEST(MarshallingTest, TestInt64Parsing) {
   EXPECT_EQ(value, 16);
   EXPECT_TRUE(absl::ParseFlag("0XFFFAAABBBCCCDDD", &value, &err));
   EXPECT_EQ(value, 1152827684197027293);
-  // TODO(rogeeff): fix below validation
-  EXPECT_FALSE(absl::ParseFlag("-0x7FFFFFFFFFFFFFFE", &value, &err));
-  EXPECT_NE(value, -2);
-  EXPECT_FALSE(absl::ParseFlag("+0x31", &value, &err));
-  EXPECT_NE(value, 49);
+  EXPECT_TRUE(absl::ParseFlag("-0x7FFFFFFFFFFFFFFE", &value, &err));
+  EXPECT_EQ(value, -9223372036854775806);
+  EXPECT_TRUE(absl::ParseFlag("-0x02", &value, &err));
+  EXPECT_EQ(value, -2);
+  EXPECT_TRUE(absl::ParseFlag("+0x31", &value, &err));
+  EXPECT_EQ(value, 49);
 
   // Whitespace handling
   EXPECT_TRUE(absl::ParseFlag("10  ", &value, &err));
@@ -423,9 +427,8 @@ TEST(MarshallingTest, TestUInt64Parsing) {
   EXPECT_EQ(value, 16);
   EXPECT_TRUE(absl::ParseFlag("0XFFFF", &value, &err));
   EXPECT_EQ(value, 65535);
-  // TODO(rogeeff): fix below validation
-  EXPECT_FALSE(absl::ParseFlag("+0x31", &value, &err));
-  EXPECT_NE(value, 49);
+  EXPECT_TRUE(absl::ParseFlag("+0x31", &value, &err));
+  EXPECT_EQ(value, 49);
 
   // Whitespace handling
   EXPECT_TRUE(absl::ParseFlag("10  ", &value, &err));
@@ -450,6 +453,125 @@ TEST(MarshallingTest, TestUInt64Parsing) {
 
 // --------------------------------------------------------------------
 
+TEST(MarshallingTest, TestInt128Parsing) {
+  std::string err;
+  absl::int128 value;
+
+  // Decimal values.
+  EXPECT_TRUE(absl::ParseFlag("0", &value, &err));
+  EXPECT_EQ(value, 0);
+  EXPECT_TRUE(absl::ParseFlag("1", &value, &err));
+  EXPECT_EQ(value, 1);
+  EXPECT_TRUE(absl::ParseFlag("-1", &value, &err));
+  EXPECT_EQ(value, -1);
+  EXPECT_TRUE(absl::ParseFlag("123", &value, &err));
+  EXPECT_EQ(value, 123);
+  EXPECT_TRUE(absl::ParseFlag("-98765", &value, &err));
+  EXPECT_EQ(value, -98765);
+  EXPECT_TRUE(absl::ParseFlag("+3", &value, &err));
+  EXPECT_EQ(value, 3);
+
+  // Leading zero values.
+  EXPECT_TRUE(absl::ParseFlag("01", &value, &err));
+  EXPECT_EQ(value, 1);
+  EXPECT_TRUE(absl::ParseFlag("001", &value, &err));
+  EXPECT_EQ(value, 1);
+  EXPECT_TRUE(absl::ParseFlag("0000100", &value, &err));
+  EXPECT_EQ(value, 100);
+
+  // Hex values.
+  EXPECT_TRUE(absl::ParseFlag("0x10", &value, &err));
+  EXPECT_EQ(value, 16);
+  EXPECT_TRUE(absl::ParseFlag("0xFFFAAABBBCCCDDD", &value, &err));
+  EXPECT_EQ(value, 1152827684197027293);
+  EXPECT_TRUE(absl::ParseFlag("0xFFF0FFFFFFFFFFFFFFF", &value, &err));
+  EXPECT_EQ(value, absl::MakeInt128(0x000000000000fff, 0xFFFFFFFFFFFFFFF));
+
+  EXPECT_TRUE(absl::ParseFlag("-0x10000000000000000", &value, &err));
+  EXPECT_EQ(value, absl::MakeInt128(-1, 0));
+  EXPECT_TRUE(absl::ParseFlag("+0x31", &value, &err));
+  EXPECT_EQ(value, 49);
+
+  // Whitespace handling
+  EXPECT_TRUE(absl::ParseFlag("16  ", &value, &err));
+  EXPECT_EQ(value, 16);
+  EXPECT_TRUE(absl::ParseFlag("  16", &value, &err));
+  EXPECT_EQ(value, 16);
+  EXPECT_TRUE(absl::ParseFlag("  0100  ", &value, &err));
+  EXPECT_EQ(value, 100);
+  EXPECT_TRUE(absl::ParseFlag(" 0x7B    ", &value, &err));
+  EXPECT_EQ(value, 123);
+
+  // Invalid values.
+  EXPECT_FALSE(absl::ParseFlag("", &value, &err));
+  EXPECT_FALSE(absl::ParseFlag(" ", &value, &err));
+  EXPECT_FALSE(absl::ParseFlag("  ", &value, &err));
+  EXPECT_FALSE(absl::ParseFlag("--1", &value, &err));
+  EXPECT_FALSE(absl::ParseFlag("\n", &value, &err));
+  EXPECT_FALSE(absl::ParseFlag("\t", &value, &err));
+  EXPECT_FALSE(absl::ParseFlag("2U", &value, &err));
+  EXPECT_FALSE(absl::ParseFlag("FFF", &value, &err));
+}
+
+// --------------------------------------------------------------------
+
+TEST(MarshallingTest, TestUint128Parsing) {
+  std::string err;
+  absl::uint128 value;
+
+  // Decimal values.
+  EXPECT_TRUE(absl::ParseFlag("0", &value, &err));
+  EXPECT_EQ(value, 0);
+  EXPECT_TRUE(absl::ParseFlag("1", &value, &err));
+  EXPECT_EQ(value, 1);
+  EXPECT_TRUE(absl::ParseFlag("123", &value, &err));
+  EXPECT_EQ(value, 123);
+  EXPECT_TRUE(absl::ParseFlag("+3", &value, &err));
+  EXPECT_EQ(value, 3);
+
+  // Leading zero values.
+  EXPECT_TRUE(absl::ParseFlag("01", &value, &err));
+  EXPECT_EQ(value, 1);
+  EXPECT_TRUE(absl::ParseFlag("001", &value, &err));
+  EXPECT_EQ(value, 1);
+  EXPECT_TRUE(absl::ParseFlag("0000100", &value, &err));
+  EXPECT_EQ(value, 100);
+
+  // Hex values.
+  EXPECT_TRUE(absl::ParseFlag("0x10", &value, &err));
+  EXPECT_EQ(value, 16);
+  EXPECT_TRUE(absl::ParseFlag("0xFFFAAABBBCCCDDD", &value, &err));
+  EXPECT_EQ(value, 1152827684197027293);
+  EXPECT_TRUE(absl::ParseFlag("0xFFF0FFFFFFFFFFFFFFF", &value, &err));
+  EXPECT_EQ(value, absl::MakeInt128(0x000000000000fff, 0xFFFFFFFFFFFFFFF));
+  EXPECT_TRUE(absl::ParseFlag("+0x31", &value, &err));
+  EXPECT_EQ(value, 49);
+
+  // Whitespace handling
+  EXPECT_TRUE(absl::ParseFlag("16  ", &value, &err));
+  EXPECT_EQ(value, 16);
+  EXPECT_TRUE(absl::ParseFlag("  16", &value, &err));
+  EXPECT_EQ(value, 16);
+  EXPECT_TRUE(absl::ParseFlag("  0100  ", &value, &err));
+  EXPECT_EQ(value, 100);
+  EXPECT_TRUE(absl::ParseFlag(" 0x7B    ", &value, &err));
+  EXPECT_EQ(value, 123);
+
+  // Invalid values.
+  EXPECT_FALSE(absl::ParseFlag("", &value, &err));
+  EXPECT_FALSE(absl::ParseFlag(" ", &value, &err));
+  EXPECT_FALSE(absl::ParseFlag("  ", &value, &err));
+  EXPECT_FALSE(absl::ParseFlag("-1", &value, &err));
+  EXPECT_FALSE(absl::ParseFlag("--1", &value, &err));
+  EXPECT_FALSE(absl::ParseFlag("\n", &value, &err));
+  EXPECT_FALSE(absl::ParseFlag("\t", &value, &err));
+  EXPECT_FALSE(absl::ParseFlag("2U", &value, &err));
+  EXPECT_FALSE(absl::ParseFlag("FFF", &value, &err));
+  EXPECT_FALSE(absl::ParseFlag("-0x10000000000000000", &value, &err));
+}
+
+// --------------------------------------------------------------------
+
 TEST(MarshallingTest, TestFloatParsing) {
   std::string err;
   float value;
@@ -654,6 +776,88 @@ TEST(MarshallingTest, TestVectorOfStringParsing) {
 
 // --------------------------------------------------------------------
 
+TEST(MarshallingTest, TestOptionalBoolParsing) {
+  std::string err;
+  std::optional<bool> value;
+
+  EXPECT_TRUE(absl::ParseFlag("", &value, &err));
+  EXPECT_FALSE(value.has_value());
+
+  EXPECT_TRUE(absl::ParseFlag("true", &value, &err));
+  EXPECT_TRUE(value.has_value());
+  EXPECT_TRUE(*value);
+
+  EXPECT_TRUE(absl::ParseFlag("false", &value, &err));
+  EXPECT_TRUE(value.has_value());
+  EXPECT_FALSE(*value);
+
+  EXPECT_FALSE(absl::ParseFlag("nullopt", &value, &err));
+}
+
+// --------------------------------------------------------------------
+
+TEST(MarshallingTest, TestOptionalIntParsing) {
+  std::string err;
+  std::optional<int> value;
+
+  EXPECT_TRUE(absl::ParseFlag("", &value, &err));
+  EXPECT_FALSE(value.has_value());
+
+  EXPECT_TRUE(absl::ParseFlag("10", &value, &err));
+  EXPECT_TRUE(value.has_value());
+  EXPECT_EQ(*value, 10);
+
+  EXPECT_TRUE(absl::ParseFlag("0x1F", &value, &err));
+  EXPECT_TRUE(value.has_value());
+  EXPECT_EQ(*value, 31);
+
+  EXPECT_FALSE(absl::ParseFlag("nullopt", &value, &err));
+}
+
+// --------------------------------------------------------------------
+
+TEST(MarshallingTest, TestOptionalDoubleParsing) {
+  std::string err;
+  std::optional<double> value;
+
+  EXPECT_TRUE(absl::ParseFlag("", &value, &err));
+  EXPECT_FALSE(value.has_value());
+
+  EXPECT_TRUE(absl::ParseFlag("1.11", &value, &err));
+  EXPECT_TRUE(value.has_value());
+  EXPECT_EQ(*value, 1.11);
+
+  EXPECT_TRUE(absl::ParseFlag("-0.12", &value, &err));
+  EXPECT_TRUE(value.has_value());
+  EXPECT_EQ(*value, -0.12);
+
+  EXPECT_FALSE(absl::ParseFlag("nullopt", &value, &err));
+}
+
+// --------------------------------------------------------------------
+
+TEST(MarshallingTest, TestOptionalStringParsing) {
+  std::string err;
+  std::optional<std::string> value;
+
+  EXPECT_TRUE(absl::ParseFlag("", &value, &err));
+  EXPECT_FALSE(value.has_value());
+
+  EXPECT_TRUE(absl::ParseFlag(" ", &value, &err));
+  EXPECT_TRUE(value.has_value());
+  EXPECT_EQ(*value, " ");
+
+  EXPECT_TRUE(absl::ParseFlag("aqswde", &value, &err));
+  EXPECT_TRUE(value.has_value());
+  EXPECT_EQ(*value, "aqswde");
+
+  EXPECT_TRUE(absl::ParseFlag("nullopt", &value, &err));
+  EXPECT_TRUE(value.has_value());
+  EXPECT_EQ(*value, "nullopt");
+}
+
+// --------------------------------------------------------------------
+
 TEST(MarshallingTest, TestBoolUnparsing) {
   EXPECT_EQ(absl::UnparseFlag(true), "true");
   EXPECT_EQ(absl::UnparseFlag(false), "false");
@@ -757,6 +961,40 @@ TEST(MarshallingTest, TestUint64Unparsing) {
 
 // --------------------------------------------------------------------
 
+TEST(MarshallingTest, TestInt128Unparsing) {
+  absl::int128 value;
+
+  value = 1;
+  EXPECT_EQ(absl::UnparseFlag(value), "1");
+  value = 0;
+  EXPECT_EQ(absl::UnparseFlag(value), "0");
+  value = -1;
+  EXPECT_EQ(absl::UnparseFlag(value), "-1");
+  value = 123456789L;
+  EXPECT_EQ(absl::UnparseFlag(value), "123456789");
+  value = -987654321L;
+  EXPECT_EQ(absl::UnparseFlag(value), "-987654321");
+  value = 0x7FFFFFFFFFFFFFFF;
+  EXPECT_EQ(absl::UnparseFlag(value), "9223372036854775807");
+}
+
+// --------------------------------------------------------------------
+
+TEST(MarshallingTest, TestUint128Unparsing) {
+  absl::uint128 value;
+
+  value = 1;
+  EXPECT_EQ(absl::UnparseFlag(value), "1");
+  value = 0;
+  EXPECT_EQ(absl::UnparseFlag(value), "0");
+  value = 123456789L;
+  EXPECT_EQ(absl::UnparseFlag(value), "123456789");
+  value = absl::MakeUint128(0, 0xFFFFFFFFFFFFFFFF);
+  EXPECT_EQ(absl::UnparseFlag(value), "18446744073709551615");
+}
+
+// --------------------------------------------------------------------
+
 TEST(MarshallingTest, TestFloatUnparsing) {
   float value;
 
@@ -803,6 +1041,90 @@ TEST(MarshallingTest, TestStringUnparsing) {
 
 // --------------------------------------------------------------------
 
+TEST(MarshallingTest, TestOptionalBoolUnparsing) {
+  std::optional<bool> value;
+
+  EXPECT_EQ(absl::UnparseFlag(value), "");
+  value = true;
+  EXPECT_EQ(absl::UnparseFlag(value), "true");
+  value = false;
+  EXPECT_EQ(absl::UnparseFlag(value), "false");
+  value = std::nullopt;
+  EXPECT_EQ(absl::UnparseFlag(value), "");
+}
+
+// --------------------------------------------------------------------
+
+TEST(MarshallingTest, TestOptionalIntUnparsing) {
+  std::optional<int> value;
+
+  EXPECT_EQ(absl::UnparseFlag(value), "");
+  value = 0;
+  EXPECT_EQ(absl::UnparseFlag(value), "0");
+  value = -12;
+  EXPECT_EQ(absl::UnparseFlag(value), "-12");
+  value = std::nullopt;
+  EXPECT_EQ(absl::UnparseFlag(value), "");
+}
+
+// --------------------------------------------------------------------
+
+TEST(MarshallingTest, TestOptionalDoubleUnparsing) {
+  std::optional<double> value;
+
+  EXPECT_EQ(absl::UnparseFlag(value), "");
+  value = 1.;
+  EXPECT_EQ(absl::UnparseFlag(value), "1");
+  value = -1.23;
+  EXPECT_EQ(absl::UnparseFlag(value), "-1.23");
+  value = std::nullopt;
+  EXPECT_EQ(absl::UnparseFlag(value), "");
+}
+
+// --------------------------------------------------------------------
+
+TEST(MarshallingTest, TestOptionalStringUnparsing) {
+  std::optional<std::string> strvalue;
+  EXPECT_EQ(absl::UnparseFlag(strvalue), "");
+
+  strvalue = "asdfg";
+  EXPECT_EQ(absl::UnparseFlag(strvalue), "asdfg");
+
+  strvalue = " ";
+  EXPECT_EQ(absl::UnparseFlag(strvalue), " ");
+
+  strvalue = "";  // It is UB to set an optional string flag to ""
+  EXPECT_EQ(absl::UnparseFlag(strvalue), "");
+}
+
+// --------------------------------------------------------------------
+
+#if defined(ABSL_HAVE_STD_OPTIONAL) && !defined(ABSL_USES_STD_OPTIONAL)
+
+TEST(MarshallingTest, TestStdOptionalUnparsing) {
+  std::optional<std::string> strvalue;
+  EXPECT_EQ(absl::UnparseFlag(strvalue), "");
+
+  strvalue = "asdfg";
+  EXPECT_EQ(absl::UnparseFlag(strvalue), "asdfg");
+
+  strvalue = " ";
+  EXPECT_EQ(absl::UnparseFlag(strvalue), " ");
+
+  strvalue = "";  // It is UB to set an optional string flag to ""
+  EXPECT_EQ(absl::UnparseFlag(strvalue), "");
+
+  std::optional<int> intvalue;
+  EXPECT_EQ(absl::UnparseFlag(intvalue), "");
+
+  intvalue = 10;
+  EXPECT_EQ(absl::UnparseFlag(intvalue), "10");
+}
+
+// --------------------------------------------------------------------
+
+#endif
+
 template <typename T>
 void TestRoundtrip(T v) {
   T new_v;
diff --git a/absl/flags/parse.cc b/absl/flags/parse.cc
index 16c4d68fc27..940bf565c45 100644
--- a/absl/flags/parse.cc
+++ b/absl/flags/parse.cc
@@ -17,49 +17,106 @@
 
 #include <stdlib.h>
 
+#include <algorithm>
+#include <atomic>
+#include <cstdint>
+#include <cstdlib>
 #include <fstream>
 #include <iostream>
+#include <ostream>
+#include <string>
 #include <tuple>
-
-#ifdef _WIN32
-#include <windows.h>
-#endif
-
+#include <utility>
+#include <vector>
+
+#include "absl/algorithm/container.h"
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/no_destructor.h"
+#include "absl/base/thread_annotations.h"
+#include "absl/flags/commandlineflag.h"
+#include "absl/flags/config.h"
 #include "absl/flags/flag.h"
+#include "absl/flags/internal/commandlineflag.h"
+#include "absl/flags/internal/flag.h"
+#include "absl/flags/internal/parse.h"
+#include "absl/flags/internal/private_handle_accessor.h"
 #include "absl/flags/internal/program_name.h"
-#include "absl/flags/internal/registry.h"
 #include "absl/flags/internal/usage.h"
+#include "absl/flags/reflection.h"
 #include "absl/flags/usage.h"
 #include "absl/flags/usage_config.h"
+#include "absl/strings/ascii.h"
+#include "absl/strings/internal/damerau_levenshtein_distance.h"
 #include "absl/strings/str_cat.h"
+#include "absl/strings/str_join.h"
+#include "absl/strings/string_view.h"
 #include "absl/strings/strip.h"
 #include "absl/synchronization/mutex.h"
 
+#ifdef _WIN32
+#include <windows.h>
+#endif
+
 // --------------------------------------------------------------------
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace flags_internal {
 namespace {
 
-ABSL_CONST_INIT absl::Mutex processing_checks_guard(absl::kConstInit);
+ABSL_CONST_INIT std::atomic<bool> g_disable_indirect_flag_expansion(false);
+
+absl::Mutex& ProcessingChecksMutex() {
+  static absl::NoDestructor<absl::Mutex> mutex;
+  return *mutex;
+}
 
 ABSL_CONST_INIT bool flagfile_needs_processing
-    ABSL_GUARDED_BY(processing_checks_guard) = false;
+    ABSL_GUARDED_BY(ProcessingChecksMutex()) = false;
 ABSL_CONST_INIT bool fromenv_needs_processing
-    ABSL_GUARDED_BY(processing_checks_guard) = false;
+    ABSL_GUARDED_BY(ProcessingChecksMutex()) = false;
 ABSL_CONST_INIT bool tryfromenv_needs_processing
-    ABSL_GUARDED_BY(processing_checks_guard) = false;
+    ABSL_GUARDED_BY(ProcessingChecksMutex()) = false;
+
+absl::Mutex& SpecifiedFlagsMutex() {
+  static absl::NoDestructor<absl::Mutex> mutex;
+  return *mutex;
+}
+
+ABSL_CONST_INIT std::vector<const CommandLineFlag*>* specified_flags
+    ABSL_GUARDED_BY(SpecifiedFlagsMutex()) = nullptr;
+
+// Suggesting at most kMaxHints flags in case of misspellings.
+ABSL_CONST_INIT const size_t kMaxHints = 100;
+// Suggesting only flags which have a smaller distance than kMaxDistance.
+ABSL_CONST_INIT const size_t kMaxDistance = 3;
+
+struct SpecifiedFlagsCompare {
+  bool operator()(const CommandLineFlag* a, const CommandLineFlag* b) const {
+    return a->Name() < b->Name();
+  }
+  bool operator()(const CommandLineFlag* a, absl::string_view b) const {
+    return a->Name() < b;
+  }
+  bool operator()(absl::string_view a, const CommandLineFlag* b) const {
+    return a < b->Name();
+  }
+};
 
 }  // namespace
 }  // namespace flags_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
+// These flags influence how command line flags are parsed and are only intended
+// to be set on the command line.  Avoid reading or setting them from C++ code.
 ABSL_FLAG(std::vector<std::string>, flagfile, {},
           "comma-separated list of files to load flags from")
     .OnUpdate([]() {
       if (absl::GetFlag(FLAGS_flagfile).empty()) return;
 
-      absl::MutexLock l(&absl::flags_internal::processing_checks_guard);
+      absl::MutexLock l(absl::flags_internal::ProcessingChecksMutex());
 
       // Setting this flag twice before it is handled most likely an internal
       // error and should be reviewed by developers.
@@ -75,7 +132,7 @@ ABSL_FLAG(std::vector<std::string>, fromenv, {},
     .OnUpdate([]() {
       if (absl::GetFlag(FLAGS_fromenv).empty()) return;
 
-      absl::MutexLock l(&absl::flags_internal::processing_checks_guard);
+      absl::MutexLock l(absl::flags_internal::ProcessingChecksMutex());
 
       // Setting this flag twice before it is handled most likely an internal
       // error and should be reviewed by developers.
@@ -91,7 +148,7 @@ ABSL_FLAG(std::vector<std::string>, tryfromenv, {},
     .OnUpdate([]() {
       if (absl::GetFlag(FLAGS_tryfromenv).empty()) return;
 
-      absl::MutexLock l(&absl::flags_internal::processing_checks_guard);
+      absl::MutexLock l(absl::flags_internal::ProcessingChecksMutex());
 
       // Setting this flag twice before it is handled most likely an internal
       // error and should be reviewed by developers.
@@ -103,12 +160,15 @@ ABSL_FLAG(std::vector<std::string>, tryfromenv, {},
       absl::flags_internal::tryfromenv_needs_processing = true;
     });
 
+// Rather than reading or setting --undefok from C++ code, please consider using
+// ABSL_RETIRED_FLAG instead.
 ABSL_FLAG(std::vector<std::string>, undefok, {},
           "comma-separated list of flag names that it is okay to specify "
           "on the command line even if the program does not define a flag "
           "with that name");
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace flags_internal {
 
 namespace {
@@ -123,14 +183,14 @@ class ArgsList {
   // Returns success status: true if parsing successful, false otherwise.
   bool ReadFromFlagfile(const std::string& flag_file_name);
 
-  int Size() const { return args_.size() - next_arg_; }
-  int FrontIndex() const { return next_arg_; }
+  size_t Size() const { return args_.size() - next_arg_; }
+  size_t FrontIndex() const { return next_arg_; }
   absl::string_view Front() const { return args_[next_arg_]; }
   void PopFront() { next_arg_++; }
 
  private:
   std::vector<std::string> args_;
-  int next_arg_;
+  size_t next_arg_;
 };
 
 bool ArgsList::ReadFromFlagfile(const std::string& flag_file_name) {
@@ -145,12 +205,14 @@ bool ArgsList::ReadFromFlagfile(const std::string& flag_file_name) {
 
   // This argument represents fake argv[0], which should be present in all arg
   // lists.
-  args_.push_back("");
+  args_.emplace_back("");
 
   std::string line;
   bool success = true;
+  int line_number = 0;
 
   while (std::getline(flag_file, line)) {
+    line_number++;
     absl::string_view stripped = absl::StripLeadingAsciiWhitespace(line);
 
     if (stripped.empty() || stripped[0] == '#') {
@@ -167,13 +229,13 @@ bool ArgsList::ReadFromFlagfile(const std::string& flag_file_name) {
         break;
       }
 
-      args_.push_back(std::string(stripped));
+      args_.emplace_back(stripped);
       continue;
     }
 
     flags_internal::ReportUsageError(
-        absl::StrCat("Unexpected line in the flagfile ", flag_file_name, ": ",
-                     line),
+        absl::StrCat("Unexpected line ", line_number, " in the flagfile ",
+                     flag_file_name),
         true);
 
     success = false;
@@ -187,7 +249,7 @@ bool ArgsList::ReadFromFlagfile(const std::string& flag_file_name) {
 // Reads the environment variable with name `name` and stores results in
 // `value`. If variable is not present in environment returns false, otherwise
 // returns true.
-bool GetEnvVar(const char* var_name, std::string* var_value) {
+bool GetEnvVar(const char* var_name, std::string& var_value) {
 #ifdef _WIN32
   char buf[1024];
   auto get_res = GetEnvironmentVariableA(var_name, buf, sizeof(buf));
@@ -199,14 +261,14 @@ bool GetEnvVar(const char* var_name, std::string* var_value) {
     return false;
   }
 
-  *var_value = std::string(buf, get_res);
+  var_value = std::string(buf, get_res);
 #else
   const char* val = ::getenv(var_name);
   if (val == nullptr) {
     return false;
   }
 
-  *var_value = val;
+  var_value = val;
 #endif
 
   return true;
@@ -233,7 +295,7 @@ std::tuple<absl::string_view, absl::string_view, bool> SplitNameAndValue(
     return std::make_tuple("", "", false);
   }
 
-  auto equal_sign_pos = arg.find("=");
+  auto equal_sign_pos = arg.find('=');
 
   absl::string_view flag_name = arg.substr(0, equal_sign_pos);
 
@@ -254,11 +316,11 @@ std::tuple<absl::string_view, absl::string_view, bool> SplitNameAndValue(
 //  found flag or nullptr
 //  is negative in case of --nofoo
 std::tuple<CommandLineFlag*, bool> LocateFlag(absl::string_view flag_name) {
-  CommandLineFlag* flag = flags_internal::FindCommandLineFlag(flag_name);
+  CommandLineFlag* flag = absl::FindCommandLineFlag(flag_name);
   bool is_negative = false;
 
   if (!flag && absl::ConsumePrefix(&flag_name, "no")) {
-    flag = flags_internal::FindCommandLineFlag(flag_name);
+    flag = absl::FindCommandLineFlag(flag_name);
     is_negative = true;
   }
 
@@ -271,18 +333,17 @@ std::tuple<CommandLineFlag*, bool> LocateFlag(absl::string_view flag_name) {
 // back.
 void CheckDefaultValuesParsingRoundtrip() {
 #ifndef NDEBUG
-  flags_internal::ForEachFlag([&](CommandLineFlag* flag) {
-    if (flag->IsRetired()) return;
+  flags_internal::ForEachFlag([&](CommandLineFlag& flag) {
+    if (flag.IsRetired()) return;
 
-#define IGNORE_TYPE(T) \
-  if (flag->IsOfType<T>()) return;
+#define ABSL_FLAGS_INTERNAL_IGNORE_TYPE(T, _) \
+  if (flag.IsOfType<T>()) return;
 
-    ABSL_FLAGS_INTERNAL_FOR_EACH_LOCK_FREE(IGNORE_TYPE)
-    IGNORE_TYPE(std::string)
-    IGNORE_TYPE(std::vector<std::string>)
-#undef IGNORE_TYPE
+    ABSL_FLAGS_INTERNAL_SUPPORTED_TYPES(ABSL_FLAGS_INTERNAL_IGNORE_TYPE)
+#undef ABSL_FLAGS_INTERNAL_IGNORE_TYPE
 
-    flag->CheckDefaultValueParsingRoundtrip();
+    flags_internal::PrivateHandleAccessor::CheckDefaultValueParsingRoundtrip(
+        flag);
   });
 #endif
 }
@@ -295,13 +356,19 @@ void CheckDefaultValuesParsingRoundtrip() {
 // the first flagfile in the input list are processed before the second flagfile
 // etc.
 bool ReadFlagfiles(const std::vector<std::string>& flagfiles,
-                   std::vector<ArgsList>* input_args) {
+                   std::vector<ArgsList>& input_args) {
+  if (!flags_internal::IsIndirectFlagExpansionEnabled()) {
+    flags_internal::ReportUsageError(
+        "Skipping expansion of --flagfile as it is disabled.", false);
+    return true;
+  }
+
   bool success = true;
   for (auto it = flagfiles.rbegin(); it != flagfiles.rend(); ++it) {
     ArgsList al;
 
     if (al.ReadFromFlagfile(*it)) {
-      input_args->push_back(al);
+      input_args.push_back(al);
     } else {
       success = false;
     }
@@ -316,14 +383,21 @@ bool ReadFlagfiles(const std::vector<std::string>& flagfiles,
 // `flag_name` is a string from the input flag_names list. If successful we
 // append a single ArgList at the end of the input_args.
 bool ReadFlagsFromEnv(const std::vector<std::string>& flag_names,
-                      std::vector<ArgsList>* input_args,
+                      std::vector<ArgsList>& input_args,
                       bool fail_on_absent_in_env) {
+  if (!flags_internal::IsIndirectFlagExpansionEnabled()) {
+    flags_internal::ReportUsageError(
+        "Skipping expansion of --fromenv/--tryfromenv as it is disabled.",
+        false);
+    return true;
+  }
+
   bool success = true;
   std::vector<std::string> args;
 
   // This argument represents fake argv[0], which should be present in all arg
   // lists.
-  args.push_back("");
+  args.emplace_back("");
 
   for (const auto& flag_name : flag_names) {
     // Avoid infinite recursion.
@@ -337,7 +411,7 @@ bool ReadFlagsFromEnv(const std::vector<std::string>& flag_names,
 
     const std::string envname = absl::StrCat("FLAGS_", flag_name);
     std::string envval;
-    if (!GetEnvVar(envname.c_str(), &envval)) {
+    if (!GetEnvVar(envname.c_str(), envval)) {
       if (fail_on_absent_in_env) {
         flags_internal::ReportUsageError(
             absl::StrCat(envname, " not found in environment"), true);
@@ -352,7 +426,7 @@ bool ReadFlagsFromEnv(const std::vector<std::string>& flag_names,
   }
 
   if (success) {
-    input_args->emplace_back(args);
+    input_args.emplace_back(args);
   }
 
   return success;
@@ -362,17 +436,17 @@ bool ReadFlagsFromEnv(const std::vector<std::string>& flag_names,
 
 // Returns success status, which is true if were able to handle all generator
 // flags (flagfile, fromenv, tryfromemv) successfully.
-bool HandleGeneratorFlags(std::vector<ArgsList>* input_args,
-                          std::vector<std::string>* flagfile_value) {
+bool HandleGeneratorFlags(std::vector<ArgsList>& input_args,
+                          std::vector<std::string>& flagfile_value) {
   bool success = true;
 
-  absl::MutexLock l(&flags_internal::processing_checks_guard);
+  absl::MutexLock l(flags_internal::ProcessingChecksMutex());
 
   // flagfile could have been set either on a command line or
   // programmatically before invoking ParseCommandLine. Note that we do not
   // actually process arguments specified in the flagfile, but instead
   // create a secondary arguments list to be processed along with the rest
-  // of the comamnd line arguments. Since we always the process most recently
+  // of the command line arguments. Since we always the process most recently
   // created list of arguments first, this will result in flagfile argument
   // being processed before any other argument in the command line. If
   // FLAGS_flagfile contains more than one file name we create multiple new
@@ -388,9 +462,9 @@ bool HandleGeneratorFlags(std::vector<ArgsList>* input_args,
   if (flags_internal::flagfile_needs_processing) {
     auto flagfiles = absl::GetFlag(FLAGS_flagfile);
 
-    if (input_args->size() == 1) {
-      flagfile_value->insert(flagfile_value->end(), flagfiles.begin(),
-                             flagfiles.end());
+    if (input_args.size() == 1) {
+      flagfile_value.insert(flagfile_value.end(), flagfiles.begin(),
+                            flagfiles.end());
     }
 
     success &= ReadFlagfiles(flagfiles, input_args);
@@ -429,7 +503,7 @@ void ResetGeneratorFlags(const std::vector<std::string>& flagfile_value) {
   // going to be {"f1", "f2"}
   if (!flagfile_value.empty()) {
     absl::SetFlag(&FLAGS_flagfile, flagfile_value);
-    absl::MutexLock l(&flags_internal::processing_checks_guard);
+    absl::MutexLock l(flags_internal::ProcessingChecksMutex());
     flags_internal::flagfile_needs_processing = false;
   }
 
@@ -441,7 +515,7 @@ void ResetGeneratorFlags(const std::vector<std::string>& flagfile_value) {
     absl::SetFlag(&FLAGS_tryfromenv, {});
   }
 
-  absl::MutexLock l(&flags_internal::processing_checks_guard);
+  absl::MutexLock l(flags_internal::ProcessingChecksMutex());
   flags_internal::fromenv_needs_processing = false;
   flags_internal::tryfromenv_needs_processing = false;
 }
@@ -517,12 +591,12 @@ std::tuple<bool, absl::string_view> DeduceFlagValue(const CommandLineFlag& flag,
     curr_list->PopFront();
     value = curr_list->Front();
 
-    // Heuristic to detect the case where someone treats a std::string arg
+    // Heuristic to detect the case where someone treats a string arg
     // like a bool or just forgets to pass a value:
     // --my_string_var --foo=bar
-    // We look for a flag of std::string type, whose value begins with a
+    // We look for a flag of string type, whose value begins with a
     // dash and corresponds to known flag or standalone --.
-    if (value[0] == '-' && flag.IsOfType<std::string>()) {
+    if (!value.empty() && value[0] == '-' && flag.IsOfType<std::string>()) {
       auto maybe_flag_name = std::get<0>(SplitNameAndValue(value.substr(1)));
 
       if (maybe_flag_name.empty() ||
@@ -555,52 +629,187 @@ bool CanIgnoreUndefinedFlag(absl::string_view flag_name) {
   return false;
 }
 
+// --------------------------------------------------------------------
+
+void ReportUnrecognizedFlags(
+    const std::vector<UnrecognizedFlag>& unrecognized_flags,
+    bool report_as_fatal_error) {
+  for (const auto& unrecognized : unrecognized_flags) {
+    // Verify if flag_name has the "no" already removed
+    std::vector<std::string> misspelling_hints;
+    if (unrecognized.source == UnrecognizedFlag::kFromArgv) {
+      misspelling_hints =
+          flags_internal::GetMisspellingHints(unrecognized.flag_name);
+    }
+
+    if (misspelling_hints.empty()) {
+      flags_internal::ReportUsageError(
+          absl::StrCat("Unknown command line flag '", unrecognized.flag_name,
+                       "'"),
+          report_as_fatal_error);
+    } else {
+      flags_internal::ReportUsageError(
+          absl::StrCat("Unknown command line flag '", unrecognized.flag_name,
+                       "'. Did you mean: ",
+                       absl::StrJoin(misspelling_hints, ", "), " ?"),
+          report_as_fatal_error);
+    }
+  }
+}
+
 }  // namespace
 
 // --------------------------------------------------------------------
 
+bool WasPresentOnCommandLine(absl::string_view flag_name) {
+  absl::ReaderMutexLock l(SpecifiedFlagsMutex());
+  ABSL_INTERNAL_CHECK(specified_flags != nullptr,
+                      "ParseCommandLine is not invoked yet");
+
+  return std::binary_search(specified_flags->begin(), specified_flags->end(),
+                            flag_name, SpecifiedFlagsCompare{});
+}
+
+// --------------------------------------------------------------------
+
+struct BestHints {
+  explicit BestHints(uint8_t _max) : best_distance(_max + 1) {}
+  bool AddHint(absl::string_view hint, uint8_t distance) {
+    if (hints.size() >= kMaxHints) return false;
+    if (distance == best_distance) {
+      hints.emplace_back(hint);
+    }
+    if (distance < best_distance) {
+      best_distance = distance;
+      hints = std::vector<std::string>{std::string(hint)};
+    }
+    return true;
+  }
+
+  uint8_t best_distance;
+  std::vector<std::string> hints;
+};
+
+// Return the list of flags with the smallest Damerau-Levenshtein distance to
+// the given flag.
+std::vector<std::string> GetMisspellingHints(const absl::string_view flag) {
+  const size_t maxCutoff = std::min(flag.size() / 2 + 1, kMaxDistance);
+  auto undefok = absl::GetFlag(FLAGS_undefok);
+  BestHints best_hints(static_cast<uint8_t>(maxCutoff));
+  flags_internal::ForEachFlag([&](const CommandLineFlag& f) {
+    if (best_hints.hints.size() >= kMaxHints) return;
+    uint8_t distance = strings_internal::CappedDamerauLevenshteinDistance(
+        flag, f.Name(), best_hints.best_distance);
+    best_hints.AddHint(f.Name(), distance);
+    // For boolean flags, also calculate distance to the negated form.
+    if (f.IsOfType<bool>()) {
+      const std::string negated_flag = absl::StrCat("no", f.Name());
+      distance = strings_internal::CappedDamerauLevenshteinDistance(
+          flag, negated_flag, best_hints.best_distance);
+      best_hints.AddHint(negated_flag, distance);
+    }
+  });
+  // Finally calculate distance to flags in "undefok".
+  absl::c_for_each(undefok, [&](const absl::string_view f) {
+    if (best_hints.hints.size() >= kMaxHints) return;
+    uint8_t distance = strings_internal::CappedDamerauLevenshteinDistance(
+        flag, f, best_hints.best_distance);
+    best_hints.AddHint(absl::StrCat(f, " (undefok)"), distance);
+  });
+  return best_hints.hints;
+}
+
+// --------------------------------------------------------------------
+
 std::vector<char*> ParseCommandLineImpl(int argc, char* argv[],
-                                        ArgvListAction arg_list_act,
-                                        UsageFlagsAction usage_flag_act,
-                                        OnUndefinedFlag on_undef_flag) {
+                                        UsageFlagsAction usage_flag_action,
+                                        OnUndefinedFlag undef_flag_action,
+                                        std::ostream& error_help_output) {
+  std::vector<char*> positional_args;
+  std::vector<UnrecognizedFlag> unrecognized_flags;
+
+  auto help_mode = flags_internal::ParseAbseilFlagsOnlyImpl(
+      argc, argv, positional_args, unrecognized_flags, usage_flag_action);
+
+  if (undef_flag_action != OnUndefinedFlag::kIgnoreUndefined) {
+    flags_internal::ReportUnrecognizedFlags(
+        unrecognized_flags,
+        (undef_flag_action == OnUndefinedFlag::kAbortIfUndefined));
+
+    if (undef_flag_action == OnUndefinedFlag::kAbortIfUndefined) {
+      if (!unrecognized_flags.empty()) {
+        flags_internal::HandleUsageFlags(error_help_output,
+        ProgramUsageMessage()); std::exit(1);
+      }
+    }
+  }
+
+  flags_internal::MaybeExit(help_mode);
+
+  return positional_args;
+}
+
+// --------------------------------------------------------------------
+
+// This function handles all Abseil Flags and built-in usage flags and, if any
+// help mode was handled, it returns that help mode. The caller of this function
+// can decide to exit based on the returned help mode.
+// The caller may decide to handle unrecognized positional arguments and
+// unrecognized flags first before exiting.
+//
+// Returns:
+// * HelpMode::kFull if parsing errors were detected in recognized arguments
+// * The HelpMode that was handled in case when `usage_flag_action` is
+//   UsageFlagsAction::kHandleUsage and a usage flag was specified on the
+//   commandline
+// * Otherwise it returns HelpMode::kNone
+HelpMode ParseAbseilFlagsOnlyImpl(
+    int argc, char* argv[], std::vector<char*>& positional_args,
+    std::vector<UnrecognizedFlag>& unrecognized_flags,
+    UsageFlagsAction usage_flag_action) {
   ABSL_INTERNAL_CHECK(argc > 0, "Missing argv[0]");
 
-  // This routine does not return anything since we abort on failure.
-  CheckDefaultValuesParsingRoundtrip();
+  using flags_internal::ArgsList;
+  using flags_internal::specified_flags;
 
   std::vector<std::string> flagfile_value;
-
   std::vector<ArgsList> input_args;
-  input_args.push_back(ArgsList(argc, argv));
 
-  std::vector<char*> output_args;
-  std::vector<char*> positional_args;
-  output_args.reserve(argc);
+  // Once parsing has started we will not allow more flag registrations.
+  flags_internal::FinalizeRegistry();
+
+  // This routine does not return anything since we abort on failure.
+  flags_internal::CheckDefaultValuesParsingRoundtrip();
 
-  // This is the list of undefined flags. The element of the list is the pair
-  // consisting of boolean indicating if flag came from command line (vs from
-  // some flag file we've read) and flag name.
-  // TODO(rogeeff): Eliminate the first element in the pair after cleanup.
-  std::vector<std::pair<bool, std::string>> undefined_flag_names;
+  input_args.push_back(ArgsList(argc, argv));
 
   // Set program invocation name if it is not set before.
-  if (ProgramInvocationName() == "UNKNOWN") {
+  if (flags_internal::ProgramInvocationName() == "UNKNOWN") {
     flags_internal::SetProgramInvocationName(argv[0]);
   }
-  output_args.push_back(argv[0]);
+  positional_args.push_back(argv[0]);
+
+  absl::MutexLock l(flags_internal::SpecifiedFlagsMutex());
+  if (specified_flags == nullptr) {
+    specified_flags = new std::vector<const CommandLineFlag*>;
+  } else {
+    specified_flags->clear();
+  }
 
-  // Iterate through the list of the input arguments. First level are arguments
-  // originated from argc/argv. Following levels are arguments originated from
-  // recursive parsing of flagfile(s).
+  // Iterate through the list of the input arguments. First level are
+  // arguments originated from argc/argv. Following levels are arguments
+  // originated from recursive parsing of flagfile(s).
   bool success = true;
   while (!input_args.empty()) {
-    // 10. First we process the built-in generator flags.
-    success &= HandleGeneratorFlags(&input_args, &flagfile_value);
+    // First we process the built-in generator flags.
+    success &= flags_internal::HandleGeneratorFlags(input_args, flagfile_value);
 
-    // 30. Select top-most (most recent) arguments list. If it is empty drop it
+    // Select top-most (most recent) arguments list. If it is empty drop it
     // and re-try.
     ArgsList& curr_list = input_args.back();
 
+    // Every ArgsList starts with real or fake program name, so we can always
+    // start by skipping it.
     curr_list.PopFront();
 
     if (curr_list.Size() == 0) {
@@ -608,13 +817,13 @@ std::vector<char*> ParseCommandLineImpl(int argc, char* argv[],
       continue;
     }
 
-    // 40. Pick up the front remaining argument in the current list. If current
-    // stack of argument lists contains only one element - we are processing an
-    // argument from the original argv.
+    // Handle the next argument in the current list. If the stack of argument
+    // lists contains only one element - we are processing an argument from
+    // the original argv.
     absl::string_view arg(curr_list.Front());
     bool arg_from_argv = input_args.size() == 1;
 
-    // 50. If argument does not start with - or is just "-" - this is
+    // If argument does not start with '-' or is just "-" - this is
     // positional argument.
     if (!absl::ConsumePrefix(&arg, "-") || arg.empty()) {
       ABSL_INTERNAL_CHECK(arg_from_argv,
@@ -624,23 +833,20 @@ std::vector<char*> ParseCommandLineImpl(int argc, char* argv[],
       continue;
     }
 
-    if (arg_from_argv && (arg_list_act == ArgvListAction::kKeepParsedArgs)) {
-      output_args.push_back(argv[curr_list.FrontIndex()]);
-    }
-
-    // 60. Split the current argument on '=' to figure out the argument
-    // name and value. If flag name is empty it means we've got "--". value
-    // can be empty either if there were no '=' in argument std::string at all or
+    // Split the current argument on '=' to deduce the argument flag name and
+    // value. If flag name is empty it means we've got an "--" argument. Value
+    // can be empty either if there were no '=' in argument string at all or
     // an argument looked like "--foo=". In a latter case is_empty_value is
     // true.
     absl::string_view flag_name;
     absl::string_view value;
     bool is_empty_value = false;
 
-    std::tie(flag_name, value, is_empty_value) = SplitNameAndValue(arg);
+    std::tie(flag_name, value, is_empty_value) =
+        flags_internal::SplitNameAndValue(arg);
 
-    // 70. "--" alone means what it does for GNU: stop flags parsing. We do
-    // not support positional arguments in flagfiles, so we just drop them.
+    // Standalone "--" argument indicates that the rest of the arguments are
+    // positional. We do not support positional arguments in flagfiles.
     if (flag_name.empty()) {
       ABSL_INTERNAL_CHECK(arg_from_argv,
                           "Flagfile cannot contain positional argument");
@@ -649,103 +855,122 @@ std::vector<char*> ParseCommandLineImpl(int argc, char* argv[],
       break;
     }
 
-    // 80. Locate the flag based on flag name. Handle both --foo and --nofoo
+    // Locate the flag based on flag name. Handle both --foo and --nofoo.
     CommandLineFlag* flag = nullptr;
     bool is_negative = false;
-    std::tie(flag, is_negative) = LocateFlag(flag_name);
+    std::tie(flag, is_negative) = flags_internal::LocateFlag(flag_name);
 
     if (flag == nullptr) {
-      if (on_undef_flag != OnUndefinedFlag::kIgnoreUndefined) {
-        undefined_flag_names.emplace_back(arg_from_argv,
-                                          std::string(flag_name));
+      // Usage flags are not modeled as Abseil flags. Locate them separately.
+      if (flags_internal::DeduceUsageFlags(flag_name, value)) {
+        continue;
       }
+      unrecognized_flags.emplace_back(arg_from_argv
+                                          ? UnrecognizedFlag::kFromArgv
+                                          : UnrecognizedFlag::kFromFlagfile,
+                                      flag_name);
       continue;
     }
 
-    // 90. Deduce flag's value (from this or next argument)
-    auto curr_index = curr_list.FrontIndex();
+    // Deduce flag's value (from this or next argument).
     bool value_success = true;
-    std::tie(value_success, value) =
-        DeduceFlagValue(*flag, value, is_negative, is_empty_value, &curr_list);
+    std::tie(value_success, value) = flags_internal::DeduceFlagValue(
+        *flag, value, is_negative, is_empty_value, &curr_list);
     success &= value_success;
 
-    // If above call consumed an argument, it was a standalone value
-    if (arg_from_argv && (arg_list_act == ArgvListAction::kKeepParsedArgs) &&
-        (curr_index != curr_list.FrontIndex())) {
-      output_args.push_back(argv[curr_list.FrontIndex()]);
-    }
-
-    // 100. Set the located flag to a new new value, unless it is retired.
-    // Setting retired flag fails, but we ignoring it here.
-    if (flag->IsRetired()) continue;
-
+    // Set the located flag to a new value, unless it is retired. Setting
+    // retired flag fails, but we ignoring it here while also reporting access
+    // to retired flag.
     std::string error;
-    if (!flag->SetFromString(value, SET_FLAGS_VALUE, kCommandLine, &error)) {
+    if (!flags_internal::PrivateHandleAccessor::ParseFrom(
+            *flag, value, flags_internal::SET_FLAGS_VALUE,
+            flags_internal::kCommandLine, error)) {
+      if (flag->IsRetired()) continue;
+
       flags_internal::ReportUsageError(error, true);
       success = false;
+    } else {
+      specified_flags->push_back(flag);
     }
   }
 
-  for (const auto& flag_name : undefined_flag_names) {
-    if (CanIgnoreUndefinedFlag(flag_name.second)) continue;
+  flags_internal::ResetGeneratorFlags(flagfile_value);
 
-    flags_internal::ReportUsageError(
-        absl::StrCat("Unknown command line flag '", flag_name.second, "'"),
-        true);
+  // All the remaining arguments are positional.
+  if (!input_args.empty()) {
+    for (size_t arg_index = input_args.back().FrontIndex();
+         arg_index < static_cast<size_t>(argc); ++arg_index) {
+      positional_args.push_back(argv[arg_index]);
+    }
+  }
 
-    success = false;
+  // Trim and sort the vector.
+  specified_flags->shrink_to_fit();
+  std::sort(specified_flags->begin(), specified_flags->end(),
+            flags_internal::SpecifiedFlagsCompare{});
+
+  // Filter out unrecognized flags, which are ok to ignore.
+  std::vector<UnrecognizedFlag> filtered;
+  filtered.reserve(unrecognized_flags.size());
+  for (const auto& unrecognized : unrecognized_flags) {
+    if (flags_internal::CanIgnoreUndefinedFlag(unrecognized.flag_name))
+      continue;
+    filtered.push_back(unrecognized);
   }
 
-#if ABSL_FLAGS_STRIP_NAMES
+  std::swap(unrecognized_flags, filtered);
+
   if (!success) {
+#if ABSL_FLAGS_STRIP_NAMES
     flags_internal::ReportUsageError(
         "NOTE: command line flags are disabled in this build", true);
-  }
+#else
+    flags_internal::HandleUsageFlags(std::cerr, ProgramUsageMessage());
 #endif
-
-  if (!success) {
-    flags_internal::HandleUsageFlags(std::cout,
-                                     ProgramUsageMessage());
-    std::exit(1);
+    return HelpMode::kFull;  // We just need to make sure the exit with
+                             // code 1.
   }
 
-  if (usage_flag_act == UsageFlagsAction::kHandleUsage) {
-    int exit_code = flags_internal::HandleUsageFlags(
-        std::cout, ProgramUsageMessage());
+  return usage_flag_action == UsageFlagsAction::kHandleUsage
+             ? flags_internal::HandleUsageFlags(std::cout,
+                                                ProgramUsageMessage())
+             : HelpMode::kNone;
+}
 
-    if (exit_code != -1) {
-      std::exit(exit_code);
-    }
-  }
+bool IsIndirectFlagExpansionEnabled() {
+  return !g_disable_indirect_flag_expansion;
+}
 
-  ResetGeneratorFlags(flagfile_value);
+}  // namespace flags_internal
 
-  // Reinstate positional args which were intermixed with flags in the arguments
-  // list.
-  for (auto arg : positional_args) {
-    output_args.push_back(arg);
-  }
+void DisableFlagfileAndEnvParsing() {
+  flags_internal::g_disable_indirect_flag_expansion = true;
+}
 
-  // All the remaining arguments are positional.
-  if (!input_args.empty()) {
-    for (int arg_index = input_args.back().FrontIndex(); arg_index < argc;
-         ++arg_index) {
-      output_args.push_back(argv[arg_index]);
-    }
-  }
+void ParseAbseilFlagsOnly(int argc, char* argv[],
+                          std::vector<char*>& positional_args,
+                          std::vector<UnrecognizedFlag>& unrecognized_flags) {
+  auto help_mode = flags_internal::ParseAbseilFlagsOnlyImpl(
+      argc, argv, positional_args, unrecognized_flags,
+      flags_internal::UsageFlagsAction::kHandleUsage);
 
-  return output_args;
+  flags_internal::MaybeExit(help_mode);
 }
 
-}  // namespace flags_internal
+// --------------------------------------------------------------------
+
+void ReportUnrecognizedFlags(
+    const std::vector<UnrecognizedFlag>& unrecognized_flags) {
+  flags_internal::ReportUnrecognizedFlags(unrecognized_flags, true);
+}
 
 // --------------------------------------------------------------------
 
 std::vector<char*> ParseCommandLine(int argc, char* argv[]) {
   return flags_internal::ParseCommandLineImpl(
-      argc, argv, flags_internal::ArgvListAction::kRemoveParsedArgs,
-      flags_internal::UsageFlagsAction::kHandleUsage,
+      argc, argv, flags_internal::UsageFlagsAction::kHandleUsage,
       flags_internal::OnUndefinedFlag::kAbortIfUndefined);
 }
 
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/flags/parse.h b/absl/flags/parse.h
index dbb75101568..fdb3d1eb89d 100644
--- a/absl/flags/parse.h
+++ b/absl/flags/parse.h
@@ -26,33 +26,120 @@
 #include <string>
 #include <vector>
 
+#include "absl/base/config.h"
 #include "absl/flags/internal/parse.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+// This type represent information about an unrecognized flag in the command
+// line.
+struct UnrecognizedFlag {
+  enum Source { kFromArgv, kFromFlagfile };
+
+  explicit UnrecognizedFlag(Source s, absl::string_view f)
+      : source(s), flag_name(f) {}
+  // This field indicates where we found this flag: on the original command line
+  // or read in some flag file.
+  Source source;
+  // Name of the flag we did not recognize in --flag_name=value or --flag_name.
+  std::string flag_name;
+};
+
+inline bool operator==(const UnrecognizedFlag& lhs,
+                       const UnrecognizedFlag& rhs) {
+  return lhs.source == rhs.source && lhs.flag_name == rhs.flag_name;
+}
+
+namespace flags_internal {
+
+HelpMode ParseAbseilFlagsOnlyImpl(
+    int argc, char* argv[], std::vector<char*>& positional_args,
+    std::vector<UnrecognizedFlag>& unrecognized_flags,
+    UsageFlagsAction usage_flag_action);
+
+}  // namespace flags_internal
+
+// ParseAbseilFlagsOnly()
+//
+// Parses a list of command-line arguments, passed in the `argc` and `argv[]`
+// parameters, into a set of Abseil Flag values, returning any unparsed
+// arguments in `positional_args` and `unrecognized_flags` output parameters.
+//
+// This function classifies all the arguments (including content of the
+// flagfiles, if any) into one of the following groups:
+//
+//   * arguments specified as "--flag=value" or "--flag value" that match
+//     registered or built-in Abseil Flags. These are "Abseil Flag arguments."
+//   * arguments specified as "--flag" that are unrecognized as Abseil Flags
+//   * arguments that are not specified as "--flag" are positional arguments
+//   * arguments that follow the flag-terminating delimiter (`--`) are also
+//     treated as positional arguments regardless of their syntax.
+//
+// All of the deduced Abseil Flag arguments are then parsed into their
+// corresponding flag values. If any syntax errors are found in these arguments,
+// the binary exits with code 1.
+//
+// This function also handles Abseil Flags built-in usage flags (e.g. --help)
+// if any were present on the command line.
+//
+// All the remaining positional arguments including original program name
+// (argv[0]) are are returned in the `positional_args` output parameter.
+//
+// All unrecognized flags that are not otherwise ignored are returned in the
+// `unrecognized_flags` output parameter. Note that the special `undefok`
+// flag allows you to specify flags which can be safely ignored; `undefok`
+// specifies these flags as a comma-separated list. Any unrecognized flags
+// that appear within `undefok` will therefore be ignored and not included in
+// the `unrecognized_flag` output parameter.
+//
+void ParseAbseilFlagsOnly(int argc, char* argv[],
+                          std::vector<char*>& positional_args,
+                          std::vector<UnrecognizedFlag>& unrecognized_flags);
+
+// ReportUnrecognizedFlags()
+//
+// Reports an error to `stderr` for all non-ignored unrecognized flags in
+// the provided `unrecognized_flags` list.
+void ReportUnrecognizedFlags(
+    const std::vector<UnrecognizedFlag>& unrecognized_flags);
 
 // ParseCommandLine()
 //
-// Parses the set of command-line arguments passed in the `argc` (argument
-// count) and `argv[]` (argument vector) parameters from `main()`, assigning
-// values to any defined Abseil flags. (Any arguments passed after the
-// flag-terminating delimiter (`--`) are treated as positional arguments and
-// ignored.)
-//
-// Any command-line flags (and arguments to those flags) are parsed into Abseil
-// Flag values, if those flags are defined. Any undefined flags will either
-// return an error, or be ignored if that flag is designated using `undefok` to
-// indicate "undefined is OK."
-//
-// Any command-line positional arguments not part of any command-line flag (or
-// arguments to a flag) are returned in a vector, with the program invocation
-// name at position 0 of that vector. (Note that this includes positional
-// arguments after the flag-terminating delimiter `--`.)
-//
-// After all flags and flag arguments are parsed, this function looks for any
-// built-in usage flags (e.g. `--help`), and if any were specified, it reports
-// help messages and then exits the program.
+// First parses Abseil Flags only from the command line according to the
+// description in `ParseAbseilFlagsOnly`. In addition this function handles
+// unrecognized and usage flags.
+//
+// If any unrecognized flags are located they are reported using
+// `ReportUnrecognizedFlags`.
+//
+// If any errors detected during command line parsing, this routine reports a
+// usage message and aborts the program.
+//
+// If any built-in usage flags were specified on the command line (e.g.
+// `--help`), this function reports help messages and then gracefully exits the
+// program.
+//
+// This function returns all the remaining positional arguments collected by
+// `ParseAbseilFlagsOnly`.
 std::vector<char*> ParseCommandLine(int argc, char* argv[]);
 
+// DisableFlagfileAndEnvParsing()
+//
+// Disables the processing of flags that load values from secondary sources,
+// specifically `--flagfile`, `--fromenv`, and `--tryfromenv`. When disabled,
+// occurrences of these flags on the command line are skipped without opening
+// files or inspecting environment variables, and a warning is printed to
+// stderr. Direct command-line flags passed via argv are still parsed normally.
+//
+// This is primarily intended as a security precaution for privileged or setuid
+// processes parsing untrusted command lines prior to dropping privileges.
+//
+// Should only be called in `main()` before calling `absl::ParseCommandLine()`
+// or `absl::ParseAbseilFlagsOnly()`.
+void DisableFlagfileAndEnvParsing();
+
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_FLAGS_PARSE_H_
diff --git a/absl/flags/parse_test.cc b/absl/flags/parse_test.cc
index 447a3bc7bdc..138cf6e09d1 100644
--- a/absl/flags/parse_test.cc
+++ b/absl/flags/parse_test.cc
@@ -15,14 +15,24 @@
 
 #include "absl/flags/parse.h"
 
+#include <stdlib.h>
+
 #include <fstream>
+#include <iostream>
+#include <string>
+#include <vector>
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
-#include "absl/base/internal/raw_logging.h"
 #include "absl/base/internal/scoped_set_env.h"
+#include "absl/flags/config.h"
 #include "absl/flags/flag.h"
+#include "absl/flags/internal/parse.h"
+#include "absl/flags/internal/usage.h"
+#include "absl/flags/reflection.h"
+#include "absl/log/log.h"
 #include "absl/strings/str_cat.h"
+#include "absl/strings/string_view.h"
 #include "absl/strings/substitute.h"
 #include "absl/types/span.h"
 
@@ -30,6 +40,36 @@
 #include <windows.h>
 #endif
 
+// Define 125 similar flags to test kMaxHints for flag suggestions.
+#define FLAG_MULT(x) F3(x)
+#define TEST_FLAG_HEADER FLAG_HEADER_
+
+#define F(name) ABSL_FLAG(int, name, 0, "")
+
+#define F1(name) \
+  F(name##1);    \
+  F(name##2);    \
+  F(name##3);    \
+  F(name##4);    \
+  F(name##5)
+/**/
+#define F2(name) \
+  F1(name##1);   \
+  F1(name##2);   \
+  F1(name##3);   \
+  F1(name##4);   \
+  F1(name##5)
+/**/
+#define F3(name) \
+  F2(name##1);   \
+  F2(name##2);   \
+  F2(name##3);   \
+  F2(name##4);   \
+  F2(name##5)
+/**/
+
+FLAG_MULT(TEST_FLAG_HEADER);
+
 namespace {
 
 using absl::base_internal::ScopedSetEnv;
@@ -37,6 +77,7 @@ using absl::base_internal::ScopedSetEnv;
 struct UDT {
   UDT() = default;
   UDT(const UDT&) = default;
+  UDT& operator=(const UDT&) = default;
   UDT(int v) : value(v) {}  // NOLINT
 
   int value;
@@ -92,8 +133,11 @@ const std::string& GetTestTempDir() {
 
       auto len = GetTempPathA(MAX_PATH, temp_path_buffer);
       if (len < MAX_PATH && len != 0) {
-        std::string temp_dir_name = absl::StrCat(
-            temp_path_buffer, "\\parse_test.", GetCurrentProcessId());
+        std::string temp_dir_name = temp_path_buffer;
+        if (!absl::EndsWith(temp_dir_name, "\\")) {
+          temp_dir_name.push_back('\\');
+        }
+        absl::StrAppend(&temp_dir_name, "parse_test.", GetCurrentProcessId());
         if (CreateDirectoryA(temp_dir_name.c_str(), nullptr)) {
           *res = temp_dir_name;
         }
@@ -104,11 +148,10 @@ const std::string& GetTestTempDir() {
         *res = unique_name;
       }
 #endif
+    }
 
-      if (res->empty()) {
-        ABSL_INTERNAL_LOG(FATAL,
-                          "Failed to make temporary directory for data files");
-      }
+    if (res->empty()) {
+      LOG(FATAL) << "Failed to make temporary directory for data files";
     }
 
 #ifdef _WIN32
@@ -155,13 +198,13 @@ constexpr const char* const ff2_data[] = {
 // Builds flagfile flag in the flagfile_flag buffer and returns it. This
 // function also creates a temporary flagfile based on FlagfileData input.
 // We create a flagfile in a temporary directory with the name specified in
-// FlagfileData and populate it with lines specifed in FlagfileData. If $0 is
+// FlagfileData and populate it with lines specified in FlagfileData. If $0 is
 // referenced in any of the lines in FlagfileData they are replaced with
 // temporary directory location. This way we can test inclusion of one flagfile
 // from another flagfile.
 const char* GetFlagfileFlag(const std::vector<FlagfileData>& ffd,
-                            std::string* flagfile_flag) {
-  *flagfile_flag = "--flagfile=";
+                            std::string& flagfile_flag) {
+  flagfile_flag = "--flagfile=";
   absl::string_view separator;
   for (const auto& flagfile_data : ffd) {
     std::string flagfile_name =
@@ -172,11 +215,11 @@ const char* GetFlagfileFlag(const std::vector<FlagfileData>& ffd,
       flagfile_out << absl::Substitute(line, GetTestTempDir()) << "\n";
     }
 
-    absl::StrAppend(flagfile_flag, separator, flagfile_name);
+    absl::StrAppend(&flagfile_flag, separator, flagfile_name);
     separator = ",";
   }
 
-  return flagfile_flag->c_str();
+  return flagfile_flag.c_str();
 }
 
 }  // namespace
@@ -193,15 +236,58 @@ ABSL_RETIRED_FLAG(std::string, legacy_str, "l", "");
 namespace {
 
 namespace flags = absl::flags_internal;
+using testing::AllOf;
 using testing::ElementsAreArray;
+using testing::HasSubstr;
 
 class ParseTest : public testing::Test {
+ public:
+  ~ParseTest() override { flags::SetFlagsHelpMode(flags::HelpMode::kNone); }
+
+  void SetUp() override {
+#if ABSL_FLAGS_STRIP_NAMES
+    GTEST_SKIP() << "This test requires flag names to be present";
+#endif
+  }
+
  private:
-  flags::FlagSaver flag_saver_;
+  absl::FlagSaver flag_saver_;
 };
 
 // --------------------------------------------------------------------
 
+template <int N>
+flags::HelpMode InvokeParseAbslOnlyImpl(const char* (&in_argv)[N]) {
+  std::vector<char*> positional_args;
+  std::vector<absl::UnrecognizedFlag> unrecognized_flags;
+
+  return flags::ParseAbseilFlagsOnlyImpl(N, const_cast<char**>(in_argv),
+                                         positional_args, unrecognized_flags,
+                                         flags::UsageFlagsAction::kHandleUsage);
+}
+
+// --------------------------------------------------------------------
+
+template <int N>
+void InvokeParseAbslOnly(const char* (&in_argv)[N]) {
+  std::vector<char*> positional_args;
+  std::vector<absl::UnrecognizedFlag> unrecognized_flags;
+
+  absl::ParseAbseilFlagsOnly(2, const_cast<char**>(in_argv), positional_args,
+                             unrecognized_flags);
+}
+
+// --------------------------------------------------------------------
+
+template <int N>
+std::vector<char*> InvokeParseCommandLineImpl(const char* (&in_argv)[N]) {
+  return flags::ParseCommandLineImpl(
+      N, const_cast<char**>(in_argv), flags::UsageFlagsAction::kHandleUsage,
+      flags::OnUndefinedFlag::kAbortIfUndefined, std::cerr);
+}
+
+// --------------------------------------------------------------------
+
 template <int N>
 std::vector<char*> InvokeParse(const char* (&in_argv)[N]) {
   return absl::ParseCommandLine(N, const_cast<char**>(in_argv));
@@ -470,21 +556,22 @@ TEST_F(ParseDeathTest, TestUndefinedArg) {
       "testbin",
       "--undefined_flag",
   };
-  EXPECT_DEATH(InvokeParse(in_args1),
-               "Unknown command line flag 'undefined_flag'");
+  EXPECT_DEATH_IF_SUPPORTED(InvokeParse(in_args1),
+                            "Unknown command line flag 'undefined_flag'");
 
   const char* in_args2[] = {
       "testbin",
       "--noprefixed_flag",
   };
-  EXPECT_DEATH(InvokeParse(in_args2),
-               "Unknown command line flag 'noprefixed_flag'");
+  EXPECT_DEATH_IF_SUPPORTED(InvokeParse(in_args2),
+                            "Unknown command line flag 'noprefixed_flag'");
 
   const char* in_args3[] = {
       "testbin",
       "--Int_flag=1",
   };
-  EXPECT_DEATH(InvokeParse(in_args3), "Unknown command line flag 'Int_flag'");
+  EXPECT_DEATH_IF_SUPPORTED(InvokeParse(in_args3),
+                            "Unknown command line flag 'Int_flag'");
 }
 
 // --------------------------------------------------------------------
@@ -494,7 +581,7 @@ TEST_F(ParseDeathTest, TestInvalidBoolFlagFormat) {
       "testbin",
       "--bool_flag=",
   };
-  EXPECT_DEATH(
+  EXPECT_DEATH_IF_SUPPORTED(
       InvokeParse(in_args1),
       "Missing the value after assignment for the boolean flag 'bool_flag'");
 
@@ -502,7 +589,7 @@ TEST_F(ParseDeathTest, TestInvalidBoolFlagFormat) {
       "testbin",
       "--nobool_flag=true",
   };
-  EXPECT_DEATH(InvokeParse(in_args2),
+  EXPECT_DEATH_IF_SUPPORTED(InvokeParse(in_args2),
                "Negative form with assignment is not valid for the boolean "
                "flag 'bool_flag'");
 }
@@ -514,14 +601,14 @@ TEST_F(ParseDeathTest, TestInvalidNonBoolFlagFormat) {
       "testbin",
       "--nostring_flag",
   };
-  EXPECT_DEATH(InvokeParse(in_args1),
+  EXPECT_DEATH_IF_SUPPORTED(InvokeParse(in_args1),
                "Negative form is not valid for the flag 'string_flag'");
 
   const char* in_args2[] = {
       "testbin",
       "--int_flag",
   };
-  EXPECT_DEATH(InvokeParse(in_args2),
+  EXPECT_DEATH_IF_SUPPORTED(InvokeParse(in_args2),
                "Missing the value for the flag 'int_flag'");
 }
 
@@ -532,7 +619,7 @@ TEST_F(ParseDeathTest, TestInvalidUDTFlagFormat) {
       "testbin",
       "--udt_flag=1",
   };
-  EXPECT_DEATH(InvokeParse(in_args1),
+  EXPECT_DEATH_IF_SUPPORTED(InvokeParse(in_args1),
                "Illegal value '1' specified for flag 'udt_flag'; Use values A, "
                "AAA instead");
 
@@ -541,13 +628,56 @@ TEST_F(ParseDeathTest, TestInvalidUDTFlagFormat) {
       "--udt_flag",
       "AA",
   };
-  EXPECT_DEATH(InvokeParse(in_args2),
+  EXPECT_DEATH_IF_SUPPORTED(InvokeParse(in_args2),
                "Illegal value 'AA' specified for flag 'udt_flag'; Use values "
                "A, AAA instead");
 }
 
 // --------------------------------------------------------------------
 
+TEST_F(ParseDeathTest, TestFlagSuggestions) {
+  const char* in_args1[] = {
+      "testbin",
+      "--legacy_boo",
+  };
+  EXPECT_DEATH_IF_SUPPORTED(
+      InvokeParse(in_args1),
+      "Unknown command line flag 'legacy_boo'. Did you mean: legacy_bool ?");
+
+  const char* in_args2[] = {"testbin", "--foo", "--undefok=foo1"};
+  EXPECT_DEATH_IF_SUPPORTED(
+      InvokeParse(in_args2),
+      "Unknown command line flag 'foo'. Did you mean: foo1 \\(undefok\\)?");
+
+  const char* in_args3[] = {
+      "testbin",
+      "--nolegacy_ino",
+  };
+  EXPECT_DEATH_IF_SUPPORTED(InvokeParse(in_args3),
+                            "Unknown command line flag 'nolegacy_ino'. Did "
+                            "you mean: nolegacy_bool, legacy_int ?");
+}
+
+// --------------------------------------------------------------------
+
+TEST_F(ParseTest, GetHints) {
+  EXPECT_THAT(absl::flags_internal::GetMisspellingHints("legacy_boo"),
+              testing::ContainerEq(std::vector<std::string>{"legacy_bool"}));
+  EXPECT_THAT(absl::flags_internal::GetMisspellingHints("nolegacy_itn"),
+              testing::ContainerEq(std::vector<std::string>{"legacy_int"}));
+  EXPECT_THAT(absl::flags_internal::GetMisspellingHints("nolegacy_int1"),
+              testing::ContainerEq(std::vector<std::string>{"legacy_int"}));
+  EXPECT_THAT(absl::flags_internal::GetMisspellingHints("nolegacy_int"),
+              testing::ContainerEq(std::vector<std::string>{"legacy_int"}));
+  EXPECT_THAT(absl::flags_internal::GetMisspellingHints("nolegacy_ino"),
+              testing::ContainerEq(
+                  std::vector<std::string>{"nolegacy_bool", "legacy_int"}));
+  EXPECT_THAT(
+      absl::flags_internal::GetMisspellingHints("FLAG_HEADER_000").size(), 100);
+}
+
+// --------------------------------------------------------------------
+
 TEST_F(ParseTest, TestLegacyFlags) {
   const char* in_args1[] = {
       "testbin",
@@ -576,14 +706,14 @@ TEST_F(ParseTest, TestSimpleValidFlagfile) {
   const char* in_args1[] = {
       "testbin",
       GetFlagfileFlag({{"parse_test.ff1", absl::MakeConstSpan(ff1_data)}},
-                      &flagfile_flag),
+                      flagfile_flag),
   };
   TestParse(in_args1, -1, 0.1, "q2w2  ", true);
 
   const char* in_args2[] = {
       "testbin",
       GetFlagfileFlag({{"parse_test.ff2", absl::MakeConstSpan(ff2_data)}},
-                      &flagfile_flag),
+                      flagfile_flag),
   };
   TestParse(in_args2, 100, 0.1, "q2w2  ", false);
 }
@@ -597,7 +727,7 @@ TEST_F(ParseTest, TestValidMultiFlagfile) {
       "testbin",
       GetFlagfileFlag({{"parse_test.ff2", absl::MakeConstSpan(ff2_data)},
                        {"parse_test.ff1", absl::MakeConstSpan(ff1_data)}},
-                      &flagfile_flag),
+                      flagfile_flag),
   };
   TestParse(in_args1, -1, 0.1, "q2w2  ", true);
 }
@@ -610,7 +740,7 @@ TEST_F(ParseTest, TestFlagfileMixedWithRegularFlags) {
   const char* in_args1[] = {
       "testbin", "--int_flag=3",
       GetFlagfileFlag({{"parse_test.ff1", absl::MakeConstSpan(ff1_data)}},
-                      &flagfile_flag),
+                      flagfile_flag),
       "-double_flag=0.2"};
   TestParse(in_args1, -1, 0.2, "q2w2  ", true);
 }
@@ -625,10 +755,14 @@ TEST_F(ParseTest, TestFlagfileInFlagfile) {
       "--flagfile=$0/parse_test.ff2",
   };
 
+  GetFlagfileFlag({{"parse_test.ff2", absl::MakeConstSpan(ff2_data)},
+                   {"parse_test.ff1", absl::MakeConstSpan(ff1_data)}},
+                      flagfile_flag);
+
   const char* in_args1[] = {
       "testbin",
       GetFlagfileFlag({{"parse_test.ff3", absl::MakeConstSpan(ff3_data)}},
-                      &flagfile_flag),
+                      flagfile_flag),
   };
   TestParse(in_args1, 100, 0.1, "q2w2  ", false);
 }
@@ -645,9 +779,9 @@ TEST_F(ParseDeathTest, TestInvalidFlagfiles) {
   const char* in_args1[] = {
       "testbin",
       GetFlagfileFlag({{"parse_test.ff4",
-                        absl::MakeConstSpan(ff4_data)}}, &flagfile_flag),
+                        absl::MakeConstSpan(ff4_data)}}, flagfile_flag),
   };
-  EXPECT_DEATH(InvokeParse(in_args1),
+  EXPECT_DEATH_IF_SUPPORTED(InvokeParse(in_args1),
                "Unknown command line flag 'unknown_flag'");
 
   constexpr const char* const ff5_data[] = {
@@ -657,9 +791,9 @@ TEST_F(ParseDeathTest, TestInvalidFlagfiles) {
   const char* in_args2[] = {
       "testbin",
       GetFlagfileFlag({{"parse_test.ff5",
-                        absl::MakeConstSpan(ff5_data)}}, &flagfile_flag),
+                        absl::MakeConstSpan(ff5_data)}}, flagfile_flag),
   };
-  EXPECT_DEATH(InvokeParse(in_args2),
+  EXPECT_DEATH_IF_SUPPORTED(InvokeParse(in_args2),
                "Unknown command line flag 'int_flag 10'");
 
   constexpr const char* const ff6_data[] = {
@@ -669,16 +803,17 @@ TEST_F(ParseDeathTest, TestInvalidFlagfiles) {
   const char* in_args3[] = {
       "testbin",
       GetFlagfileFlag({{"parse_test.ff6", absl::MakeConstSpan(ff6_data)}},
-                      &flagfile_flag),
+                      flagfile_flag),
   };
-  EXPECT_DEATH(InvokeParse(in_args3),
+  EXPECT_DEATH_IF_SUPPORTED(InvokeParse(in_args3),
                "Flagfile can't contain position arguments or --");
 
   const char* in_args4[] = {
       "testbin",
       "--flagfile=invalid_flag_file",
   };
-  EXPECT_DEATH(InvokeParse(in_args4), "Can't open flagfile invalid_flag_file");
+  EXPECT_DEATH_IF_SUPPORTED(InvokeParse(in_args4),
+                            "Can't open flagfile invalid_flag_file");
 
   constexpr const char* const ff7_data[] = {
       "--int_flag=10",
@@ -689,10 +824,10 @@ TEST_F(ParseDeathTest, TestInvalidFlagfiles) {
   const char* in_args5[] = {
       "testbin",
       GetFlagfileFlag({{"parse_test.ff7", absl::MakeConstSpan(ff7_data)}},
-                      &flagfile_flag),
+                      flagfile_flag),
   };
-  EXPECT_DEATH(InvokeParse(in_args5),
-               "Unexpected line in the flagfile .*: \\*bin\\*");
+  EXPECT_DEATH_IF_SUPPORTED(InvokeParse(in_args5),
+               "Unexpected line 2 in the flagfile .*");
 }
 
 // --------------------------------------------------------------------
@@ -713,7 +848,7 @@ TEST_F(ParseTest, TestReadingRequiredFlagsFromEnv) {
 TEST_F(ParseDeathTest, TestReadingUnsetRequiredFlagsFromEnv) {
   const char* in_args1[] = {"testbin", "--fromenv=int_flag"};
 
-  EXPECT_DEATH(InvokeParse(in_args1),
+  EXPECT_DEATH_IF_SUPPORTED(InvokeParse(in_args1),
                "FLAGS_int_flag not found in environment");
 }
 
@@ -724,7 +859,8 @@ TEST_F(ParseDeathTest, TestRecursiveFlagsFromEnv) {
 
   ScopedSetEnv set_tryfromenv("FLAGS_tryfromenv", "int_flag");
 
-  EXPECT_DEATH(InvokeParse(in_args1), "Infinite recursion on flag tryfromenv");
+  EXPECT_DEATH_IF_SUPPORTED(InvokeParse(in_args1),
+                            "Infinite recursion on flag tryfromenv");
 }
 
 // --------------------------------------------------------------------
@@ -757,102 +893,263 @@ TEST_F(ParseTest, TestReadingFlagsFromEnvMoxedWithRegularFlags) {
 
 // --------------------------------------------------------------------
 
-TEST_F(ParseTest, TestKeepParsedArgs) {
+TEST_F(ParseDeathTest, TestSimpleHelpFlagHandling) {
   const char* in_args1[] = {
-      "testbin",        "arg1", "--bool_flag",
-      "--int_flag=211", "arg2", "--double_flag=1.1",
-      "--string_flag",  "asd",  "--",
-      "arg3",           "arg4",
+      "testbin",
+      "--help",
   };
 
-  auto out_args1 = InvokeParse(in_args1);
+  EXPECT_EQ(InvokeParseAbslOnlyImpl(in_args1), flags::HelpMode::kImportant);
+  EXPECT_EXIT(InvokeParse(in_args1), testing::ExitedWithCode(1), "");
 
-  EXPECT_THAT(
-      out_args1,
-      ElementsAreArray({absl::string_view("testbin"), absl::string_view("arg1"),
-                        absl::string_view("arg2"), absl::string_view("arg3"),
-                        absl::string_view("arg4")}));
+  const char* in_args2[] = {
+      "testbin",
+      "--help",
+      "--int_flag=3",
+  };
 
-  auto out_args2 = flags::ParseCommandLineImpl(
-      11, const_cast<char**>(in_args1), flags::ArgvListAction::kKeepParsedArgs,
-      flags::UsageFlagsAction::kHandleUsage,
-      flags::OnUndefinedFlag::kAbortIfUndefined);
+  EXPECT_EQ(InvokeParseAbslOnlyImpl(in_args2), flags::HelpMode::kImportant);
+  EXPECT_EQ(absl::GetFlag(FLAGS_int_flag), 3);
 
-  EXPECT_THAT(
-      out_args2,
-      ElementsAreArray({absl::string_view("testbin"),
-                        absl::string_view("--bool_flag"),
-                        absl::string_view("--int_flag=211"),
-                        absl::string_view("--double_flag=1.1"),
-                        absl::string_view("--string_flag"),
-                        absl::string_view("asd"), absl::string_view("--"),
-                        absl::string_view("arg1"), absl::string_view("arg2"),
-                        absl::string_view("arg3"), absl::string_view("arg4")}));
+  const char* in_args3[] = {"testbin", "--help", "some_positional_arg"};
+
+  EXPECT_EQ(InvokeParseAbslOnlyImpl(in_args3), flags::HelpMode::kImportant);
 }
 
 // --------------------------------------------------------------------
 
-TEST_F(ParseTest, TestIgnoreUndefinedFlags) {
+TEST_F(ParseTest, TestSubstringHelpFlagHandling) {
   const char* in_args1[] = {
       "testbin",
-      "arg1",
-      "--undef_flag=aa",
-      "--int_flag=21",
+      "--help=abcd",
   };
 
-  auto out_args1 = flags::ParseCommandLineImpl(
-      4, const_cast<char**>(in_args1), flags::ArgvListAction::kRemoveParsedArgs,
-      flags::UsageFlagsAction::kHandleUsage,
-      flags::OnUndefinedFlag::kIgnoreUndefined);
+  EXPECT_EQ(InvokeParseAbslOnlyImpl(in_args1), flags::HelpMode::kMatch);
+  EXPECT_EQ(flags::GetFlagsHelpMatchSubstr(), "abcd");
+}
+
+// --------------------------------------------------------------------
 
-  EXPECT_THAT(out_args1, ElementsAreArray({absl::string_view("testbin"),
-                                           absl::string_view("arg1")}));
+TEST_F(ParseDeathTest, TestVersionHandling) {
+  const char* in_args1[] = {
+      "testbin",
+      "--version",
+  };
 
-  EXPECT_EQ(absl::GetFlag(FLAGS_int_flag), 21);
+  EXPECT_EQ(InvokeParseAbslOnlyImpl(in_args1), flags::HelpMode::kVersion);
+}
 
-  const char* in_args2[] = {
+// --------------------------------------------------------------------
+
+TEST_F(ParseTest, TestCheckArgsHandling) {
+  const char* in_args1[] = {"testbin", "--only_check_args", "--int_flag=211"};
+
+  EXPECT_EQ(InvokeParseAbslOnlyImpl(in_args1), flags::HelpMode::kOnlyCheckArgs);
+  EXPECT_EXIT(InvokeParseAbslOnly(in_args1), testing::ExitedWithCode(0), "");
+  EXPECT_EXIT(InvokeParse(in_args1), testing::ExitedWithCode(0), "");
+
+  const char* in_args2[] = {"testbin", "--only_check_args", "--unknown_flag=a"};
+
+  EXPECT_EQ(InvokeParseAbslOnlyImpl(in_args2), flags::HelpMode::kOnlyCheckArgs);
+  EXPECT_EXIT(InvokeParseAbslOnly(in_args2), testing::ExitedWithCode(0), "");
+  EXPECT_EXIT(InvokeParse(in_args2), testing::ExitedWithCode(1), "");
+}
+
+// --------------------------------------------------------------------
+
+TEST_F(ParseTest, WasPresentOnCommandLine) {
+  const char* in_args1[] = {
+      "testbin",        "arg1", "--bool_flag",
+      "--int_flag=211", "arg2", "--double_flag=1.1",
+      "--string_flag",  "asd",  "--",
+      "--some_flag",    "arg4",
+  };
+
+  InvokeParse(in_args1);
+
+  EXPECT_TRUE(flags::WasPresentOnCommandLine("bool_flag"));
+  EXPECT_TRUE(flags::WasPresentOnCommandLine("int_flag"));
+  EXPECT_TRUE(flags::WasPresentOnCommandLine("double_flag"));
+  EXPECT_TRUE(flags::WasPresentOnCommandLine("string_flag"));
+  EXPECT_FALSE(flags::WasPresentOnCommandLine("some_flag"));
+  EXPECT_FALSE(flags::WasPresentOnCommandLine("another_flag"));
+}
+
+// --------------------------------------------------------------------
+
+TEST_F(ParseTest, ParseAbseilFlagsOnlySuccess) {
+  const char* in_args[] = {
       "testbin",
       "arg1",
-      "--undef_flag=aa",
-      "--string_flag=AA",
+      "--bool_flag",
+      "--int_flag=211",
+      "arg2",
+      "--double_flag=1.1",
+      "--undef_flag1",
+      "--undef_flag2=123",
+      "--string_flag",
+      "asd",
+      "--",
+      "--some_flag",
+      "arg4",
   };
 
-  auto out_args2 = flags::ParseCommandLineImpl(
-      4, const_cast<char**>(in_args2), flags::ArgvListAction::kKeepParsedArgs,
-      flags::UsageFlagsAction::kHandleUsage,
-      flags::OnUndefinedFlag::kIgnoreUndefined);
+  std::vector<char*> positional_args;
+  std::vector<absl::UnrecognizedFlag> unrecognized_flags;
+
+  absl::ParseAbseilFlagsOnly(13, const_cast<char**>(in_args), positional_args,
+                             unrecognized_flags);
+  EXPECT_THAT(positional_args,
+              ElementsAreArray(
+                  {absl::string_view("testbin"), absl::string_view("arg1"),
+                   absl::string_view("arg2"), absl::string_view("--some_flag"),
+                   absl::string_view("arg4")}));
+  EXPECT_THAT(unrecognized_flags,
+              ElementsAreArray(
+                  {absl::UnrecognizedFlag(absl::UnrecognizedFlag::kFromArgv,
+                                          "undef_flag1"),
+                   absl::UnrecognizedFlag(absl::UnrecognizedFlag::kFromArgv,
+                                          "undef_flag2")}));
+}
 
-  EXPECT_THAT(
-      out_args2,
-      ElementsAreArray(
-          {absl::string_view("testbin"), absl::string_view("--undef_flag=aa"),
-           absl::string_view("--string_flag=AA"), absl::string_view("arg1")}));
+// --------------------------------------------------------------------
 
-  EXPECT_EQ(absl::GetFlag(FLAGS_string_flag), "AA");
+TEST_F(ParseDeathTest, ParseAbseilFlagsOnlyFailure) {
+  const char* in_args[] = {
+      "testbin",
+      "--int_flag=21.1",
+  };
+
+  EXPECT_DEATH_IF_SUPPORTED(
+      InvokeParseAbslOnly(in_args),
+      "Illegal value '21.1' specified for flag 'int_flag'");
 }
 
 // --------------------------------------------------------------------
 
-TEST_F(ParseDeathTest, TestHelpFlagHandling) {
-  const char* in_args1[] = {
+TEST_F(ParseTest, UndefOkFlagsAreIgnored) {
+  const char* in_args[] = {
+      "testbin",           "--undef_flag1",
+      "--undef_flag2=123", "--undefok=undef_flag2",
+      "--undef_flag3",     "value",
+  };
+
+  std::vector<char*> positional_args;
+  std::vector<absl::UnrecognizedFlag> unrecognized_flags;
+
+  absl::ParseAbseilFlagsOnly(6, const_cast<char**>(in_args), positional_args,
+                             unrecognized_flags);
+  EXPECT_THAT(positional_args, ElementsAreArray({absl::string_view("testbin"),
+                                                 absl::string_view("value")}));
+  EXPECT_THAT(unrecognized_flags,
+              ElementsAreArray(
+                  {absl::UnrecognizedFlag(absl::UnrecognizedFlag::kFromArgv,
+                                          "undef_flag1"),
+                   absl::UnrecognizedFlag(absl::UnrecognizedFlag::kFromArgv,
+                                          "undef_flag3")}));
+}
+
+// --------------------------------------------------------------------
+
+TEST_F(ParseTest, AllUndefOkFlagsAreIgnored) {
+  const char* in_args[] = {
       "testbin",
-      "--help",
+      "--undef_flag1",
+      "--undef_flag2=123",
+      "--undefok=undef_flag2,undef_flag1,undef_flag3",
+      "--undef_flag3",
+      "value",
+      "--",
+      "--undef_flag4",
   };
 
-  EXPECT_EXIT(InvokeParse(in_args1), testing::ExitedWithCode(1), "");
+  std::vector<char*> positional_args;
+  std::vector<absl::UnrecognizedFlag> unrecognized_flags;
 
-  const char* in_args2[] = {
+  absl::ParseAbseilFlagsOnly(8, const_cast<char**>(in_args), positional_args,
+                             unrecognized_flags);
+  EXPECT_THAT(positional_args,
+              ElementsAreArray({absl::string_view("testbin"),
+                                absl::string_view("value"),
+                                absl::string_view("--undef_flag4")}));
+  EXPECT_THAT(unrecognized_flags, testing::IsEmpty());
+}
+
+// --------------------------------------------------------------------
+
+TEST_F(ParseDeathTest, ExitOnUnrecognizedFlagPrintsHelp) {
+  const char* in_args[] = {
       "testbin",
-      "--help",
-      "--int_flag=3",
+      "--undef_flag1",
+      "--help=int_flag",
   };
 
-  auto out_args2 = flags::ParseCommandLineImpl(
-      3, const_cast<char**>(in_args2), flags::ArgvListAction::kRemoveParsedArgs,
-      flags::UsageFlagsAction::kIgnoreUsage,
-      flags::OnUndefinedFlag::kAbortIfUndefined);
+  EXPECT_EXIT(InvokeParseCommandLineImpl(in_args), testing::ExitedWithCode(1),
+              AllOf(HasSubstr("Unknown command line flag 'undef_flag1'"),
+                    HasSubstr("Try --helpfull to get a list of all flags")));
+}
 
-  EXPECT_EQ(absl::GetFlag(FLAGS_int_flag), 3);
+// --------------------------------------------------------------------
+
+TEST_F(ParseDeathTest, DisableFlagfileAndEnvParsing) {
+  EXPECT_EXIT(
+      ([]() {
+        absl::DisableFlagfileAndEnvParsing();
+        std::string flagfile_flag;
+        const char* args[] = {
+            "testbin",
+            "--string_flag=abc",
+            GetFlagfileFlag({{"parse_test.ff_disabled", {"--int_flag=123"}}},
+                            flagfile_flag),
+        };
+        InvokeParse(args);
+        exit(absl::GetFlag(FLAGS_int_flag) == 123 ||
+                     absl::GetFlag(FLAGS_string_flag) != "abc"
+                 ? 1
+                 : 0);
+      })(),
+      testing::ExitedWithCode(0),
+      "Skipping expansion of --flagfile as it is disabled");
+
+  EXPECT_EXIT(([]() {
+                absl::DisableFlagfileAndEnvParsing();
+                ScopedSetEnv set_int_flag("FLAGS_int_flag", "123");
+                const char* args[] = {"testbin", "--string_flag=abc",
+                                      "--fromenv=int_flag"};
+                InvokeParse(args);
+                exit(absl::GetFlag(FLAGS_int_flag) == 123 ||
+                             absl::GetFlag(FLAGS_string_flag) != "abc"
+                         ? 1
+                         : 0);
+              })(),
+              testing::ExitedWithCode(0),
+              "Skipping expansion of --fromenv/--tryfromenv as it is disabled");
+
+  EXPECT_EXIT(([]() {
+                absl::DisableFlagfileAndEnvParsing();
+                ScopedSetEnv set_int_flag("FLAGS_int_flag", "123");
+                const char* args[] = {"testbin", "--string_flag=abc",
+                                      "--tryfromenv=int_flag"};
+                InvokeParse(args);
+                exit(absl::GetFlag(FLAGS_int_flag) == 123 ||
+                             absl::GetFlag(FLAGS_string_flag) != "abc"
+                         ? 1
+                         : 0);
+              })(),
+              testing::ExitedWithCode(0),
+              "Skipping expansion of --fromenv/--tryfromenv as it is disabled");
+}
+
+TEST_F(ParseDeathTest, IsIndirectFlagExpansionEnabledWorks) {
+  EXPECT_EXIT(
+      ([]() {
+        if (!absl::flags_internal::IsIndirectFlagExpansionEnabled()) {
+          exit(1);
+        }
+        absl::DisableFlagfileAndEnvParsing();
+        exit(absl::flags_internal::IsIndirectFlagExpansionEnabled() ? 2 : 0);
+      })(),
+      testing::ExitedWithCode(0), "");
 }
 
 }  // namespace
diff --git a/absl/flags/reflection.cc b/absl/flags/reflection.cc
new file mode 100644
index 00000000000..e92ff961b45
--- /dev/null
+++ b/absl/flags/reflection.cc
@@ -0,0 +1,368 @@
+//
+//  Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/flags/reflection.h"
+
+#include <assert.h>
+
+#include <atomic>
+#include <string>
+#include <utility>
+
+#include "absl/base/config.h"
+#include "absl/base/no_destructor.h"
+#include "absl/base/thread_annotations.h"
+#include "absl/container/flat_hash_map.h"
+#include "absl/flags/commandlineflag.h"
+#include "absl/flags/internal/private_handle_accessor.h"
+#include "absl/flags/internal/registry.h"
+#include "absl/flags/usage_config.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/string_view.h"
+#include "absl/synchronization/mutex.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace flags_internal {
+
+// --------------------------------------------------------------------
+// FlagRegistry
+//    A FlagRegistry singleton object holds all flag objects indexed by their
+//    names so that if you know a flag's name, you can access or set it. If the
+//    function is named FooLocked(), you must own the registry lock before
+//    calling the function; otherwise, you should *not* hold the lock, and the
+//    function will acquire it itself if needed.
+// --------------------------------------------------------------------
+
+class FlagRegistry {
+ public:
+  FlagRegistry() = default;
+  ~FlagRegistry() = default;
+
+  // Store a flag in this registry. Takes ownership of *flag.
+  void RegisterFlag(CommandLineFlag& flag, const char* filename);
+
+  void lock() ABSL_EXCLUSIVE_LOCK_FUNCTION(lock_) { lock_.lock(); }
+  inline void Lock() ABSL_EXCLUSIVE_LOCK_FUNCTION(lock_) { lock(); }
+
+  void unlock() ABSL_UNLOCK_FUNCTION(lock_) { lock_.unlock(); }
+  inline void Unlock() ABSL_UNLOCK_FUNCTION(lock_) { unlock(); }
+
+  // Returns the flag object for the specified name, or nullptr if not found.
+  // Will emit a warning if a 'retired' flag is specified.
+  CommandLineFlag* FindFlag(absl::string_view name);
+
+  static FlagRegistry& GlobalRegistry();  // returns a singleton registry
+
+ private:
+  friend class flags_internal::FlagSaverImpl;  // reads all the flags in order
+                                               // to copy them
+  friend void ForEachFlag(std::function<void(CommandLineFlag&)> visitor);
+  friend void FinalizeRegistry();
+
+  // The map from name to flag, for FindFlag().
+  using FlagMap = absl::flat_hash_map<absl::string_view, CommandLineFlag*>;
+  using FlagIterator = FlagMap::iterator;
+  using FlagConstIterator = FlagMap::const_iterator;
+  FlagMap flags_;
+  std::vector<CommandLineFlag*> flat_flags_;
+  std::atomic<bool> finalized_flags_{false};
+
+  absl::Mutex lock_;
+
+  // Disallow
+  FlagRegistry(const FlagRegistry&);
+  FlagRegistry& operator=(const FlagRegistry&);
+};
+
+namespace {
+
+class FlagRegistryLock {
+ public:
+  explicit FlagRegistryLock(FlagRegistry& fr) : fr_(fr) { fr_.lock(); }
+  ~FlagRegistryLock() { fr_.unlock(); }
+
+ private:
+  FlagRegistry& fr_;
+};
+
+}  // namespace
+
+CommandLineFlag* FlagRegistry::FindFlag(absl::string_view name) {
+  if (finalized_flags_.load(std::memory_order_acquire)) {
+    // We could save some gcus here if we make `Name()` be non-virtual.
+    // We could move the `const char*` name to the base class.
+    auto it = std::partition_point(
+        flat_flags_.begin(), flat_flags_.end(),
+        [=](CommandLineFlag* f) { return f->Name() < name; });
+    if (it != flat_flags_.end() && (*it)->Name() == name) return *it;
+  }
+
+  FlagRegistryLock frl(*this);
+  auto it = flags_.find(name);
+  return it != flags_.end() ? it->second : nullptr;
+}
+
+void FlagRegistry::RegisterFlag(CommandLineFlag& flag, const char* filename) {
+  if (filename != nullptr &&
+      flag.Filename() != GetUsageConfig().normalize_filename(filename)) {
+    flags_internal::ReportUsageError(
+        absl::StrCat(
+            "Inconsistency between flag object and registration for flag '",
+            flag.Name(),
+            "', likely due to duplicate flags or an ODR violation. Relevant "
+            "files: ",
+            flag.Filename(), " and ", filename),
+        true);
+    std::exit(1);
+  }
+
+  FlagRegistryLock registry_lock(*this);
+
+  std::pair<FlagIterator, bool> ins =
+      flags_.insert(FlagMap::value_type(flag.Name(), &flag));
+  if (ins.second == false) {  // means the name was already in the map
+    CommandLineFlag& old_flag = *ins.first->second;
+    if (flag.IsRetired() != old_flag.IsRetired()) {
+      // All registrations must agree on the 'retired' flag.
+      flags_internal::ReportUsageError(
+          absl::StrCat(
+              "Retired flag '", flag.Name(), "' was defined normally in file '",
+              (flag.IsRetired() ? old_flag.Filename() : flag.Filename()), "'."),
+          true);
+    } else if (flags_internal::PrivateHandleAccessor::TypeId(flag) !=
+               flags_internal::PrivateHandleAccessor::TypeId(old_flag)) {
+      flags_internal::ReportUsageError(
+          absl::StrCat("Flag '", flag.Name(),
+                       "' was defined more than once but with "
+                       "differing types. Defined in files '",
+                       old_flag.Filename(), "' and '", flag.Filename(), "'."),
+          true);
+    } else if (old_flag.IsRetired()) {
+      return;
+    } else if (old_flag.Filename() != flag.Filename()) {
+      flags_internal::ReportUsageError(
+          absl::StrCat("Flag '", flag.Name(),
+                       "' was defined more than once (in files '",
+                       old_flag.Filename(), "' and '", flag.Filename(), "')."),
+          true);
+    } else {
+      flags_internal::ReportUsageError(
+          absl::StrCat(
+              "Something is wrong with flag '", flag.Name(), "' in file '",
+              flag.Filename(), "'. One possibility: file '", flag.Filename(),
+              "' is being linked both statically and dynamically into this "
+              "executable. e.g. some files listed as srcs to a test and also "
+              "listed as srcs of some shared lib deps of the same test."),
+          true);
+    }
+    // All cases above are fatal, except for the retired flags.
+    std::exit(1);
+  }
+}
+
+FlagRegistry& FlagRegistry::GlobalRegistry() {
+  static absl::NoDestructor<FlagRegistry> global_registry;
+  return *global_registry;
+}
+
+// --------------------------------------------------------------------
+
+void ForEachFlag(std::function<void(CommandLineFlag&)> visitor) {
+  FlagRegistry& registry = FlagRegistry::GlobalRegistry();
+
+  if (registry.finalized_flags_.load(std::memory_order_acquire)) {
+    for (const auto& i : registry.flat_flags_) visitor(*i);
+  }
+
+  FlagRegistryLock frl(registry);
+  for (const auto& i : registry.flags_) visitor(*i.second);
+}
+
+// --------------------------------------------------------------------
+
+bool RegisterCommandLineFlag(CommandLineFlag& flag, const char* filename) {
+  FlagRegistry::GlobalRegistry().RegisterFlag(flag, filename);
+  return true;
+}
+
+void FinalizeRegistry() {
+  auto& registry = FlagRegistry::GlobalRegistry();
+  FlagRegistryLock frl(registry);
+  if (registry.finalized_flags_.load(std::memory_order_relaxed)) {
+    // Was already finalized. Ignore the second time.
+    return;
+  }
+  registry.flat_flags_.reserve(registry.flags_.size());
+  for (const auto& f : registry.flags_) {
+    registry.flat_flags_.push_back(f.second);
+  }
+  std::sort(std::begin(registry.flat_flags_), std::end(registry.flat_flags_),
+            [](const CommandLineFlag* lhs, const CommandLineFlag* rhs) {
+              return lhs->Name() < rhs->Name();
+            });
+  registry.flags_.clear();
+  registry.finalized_flags_.store(true, std::memory_order_release);
+}
+
+// --------------------------------------------------------------------
+
+namespace {
+
+// These are only used as constexpr global objects.
+// They do not use a virtual destructor to simplify their implementation.
+// They are not destroyed except at program exit, so leaks do not matter.
+#if defined(__GNUC__) && !defined(__clang__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wnon-virtual-dtor"
+#endif
+class RetiredFlagObj final : public CommandLineFlag {
+ public:
+  constexpr RetiredFlagObj(const char* name, FlagFastTypeId type_id)
+      : name_(name), type_id_(type_id) {}
+
+ private:
+  absl::string_view Name() const override { return name_; }
+  std::string Filename() const override {
+    OnAccess();
+    return "RETIRED";
+  }
+  FlagFastTypeId TypeId() const override { return type_id_; }
+  std::string Help() const override {
+    OnAccess();
+    return "";
+  }
+  bool IsRetired() const override { return true; }
+  bool IsSpecifiedOnCommandLine() const override {
+    OnAccess();
+    return false;
+  }
+  std::string DefaultValue() const override {
+    OnAccess();
+    return "";
+  }
+  std::string CurrentValue() const override {
+    OnAccess();
+    return "";
+  }
+
+  // Any input is valid
+  bool ValidateInputValue(absl::string_view) const override {
+    OnAccess();
+    return true;
+  }
+
+  std::unique_ptr<flags_internal::FlagStateInterface> SaveState() override {
+    return nullptr;
+  }
+
+  bool ParseFrom(absl::string_view, flags_internal::FlagSettingMode,
+                 flags_internal::ValueSource, std::string&) override {
+    OnAccess();
+    return false;
+  }
+
+  void CheckDefaultValueParsingRoundtrip() const override { OnAccess(); }
+
+  void Read(void*) const override { OnAccess(); }
+
+  void OnAccess() const {
+    flags_internal::ReportUsageError(
+        absl::StrCat("Accessing retired flag '", name_, "'"), false);
+  }
+
+  // Data members
+  const char* const name_;
+  const FlagFastTypeId type_id_;
+};
+#if defined(__GNUC__) && !defined(__clang__)
+#pragma GCC diagnostic pop
+#endif
+
+}  // namespace
+
+void Retire(const char* name, FlagFastTypeId type_id, unsigned char* buf) {
+  static_assert(sizeof(RetiredFlagObj) == kRetiredFlagObjSize);
+  static_assert(alignof(RetiredFlagObj) == kRetiredFlagObjAlignment);
+  auto* flag = ::new (buf) flags_internal::RetiredFlagObj(name, type_id);
+  FlagRegistry::GlobalRegistry().RegisterFlag(*flag, nullptr);
+}
+
+// --------------------------------------------------------------------
+
+class FlagSaverImpl {
+ public:
+  FlagSaverImpl() = default;
+  FlagSaverImpl(const FlagSaverImpl&) = delete;
+  void operator=(const FlagSaverImpl&) = delete;
+
+  // Saves the flag states from the flag registry into this object.
+  // It's an error to call this more than once.
+  void SaveFromRegistry() {
+    assert(backup_registry_.empty());  // call only once!
+    flags_internal::ForEachFlag([&](CommandLineFlag& flag) {
+      if (auto flag_state =
+              flags_internal::PrivateHandleAccessor::SaveState(flag)) {
+        backup_registry_.emplace_back(std::move(flag_state));
+      }
+    });
+  }
+
+  // Restores the saved flag states into the flag registry.
+  void RestoreToRegistry() && {
+    for (const auto& flag_state : backup_registry_) {
+      std::move(*flag_state).Restore();
+    }
+  }
+
+ private:
+  std::vector<std::unique_ptr<flags_internal::FlagStateInterface>>
+      backup_registry_;
+};
+
+}  // namespace flags_internal
+
+FlagSaver::FlagSaver() : impl_(new flags_internal::FlagSaverImpl) {
+  impl_->SaveFromRegistry();
+}
+
+FlagSaver::~FlagSaver() {
+  if (!impl_) return;
+
+  std::move(*impl_).RestoreToRegistry();
+  delete impl_;
+}
+
+// --------------------------------------------------------------------
+
+CommandLineFlag* FindCommandLineFlag(absl::string_view name) {
+  if (name.empty()) return nullptr;
+  flags_internal::FlagRegistry& registry =
+      flags_internal::FlagRegistry::GlobalRegistry();
+  return registry.FindFlag(name);
+}
+
+// --------------------------------------------------------------------
+
+absl::flat_hash_map<absl::string_view, absl::CommandLineFlag*> GetAllFlags() {
+  absl::flat_hash_map<absl::string_view, absl::CommandLineFlag*> res;
+  flags_internal::ForEachFlag([&](CommandLineFlag& flag) {
+    if (!flag.IsRetired()) res.insert({flag.Name(), &flag});
+  });
+  return res;
+}
+
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/flags/reflection.h b/absl/flags/reflection.h
new file mode 100644
index 00000000000..e6baf5de4b0
--- /dev/null
+++ b/absl/flags/reflection.h
@@ -0,0 +1,90 @@
+//
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: reflection.h
+// -----------------------------------------------------------------------------
+//
+// This file defines the routines to access and operate on an Abseil Flag's
+// reflection handle.
+
+#ifndef ABSL_FLAGS_REFLECTION_H_
+#define ABSL_FLAGS_REFLECTION_H_
+
+#include <string>
+
+#include "absl/base/config.h"
+#include "absl/container/flat_hash_map.h"
+#include "absl/flags/commandlineflag.h"
+#include "absl/flags/internal/commandlineflag.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace flags_internal {
+class FlagSaverImpl;
+}  // namespace flags_internal
+
+// FindCommandLineFlag()
+//
+// Returns the reflection handle of an Abseil flag of the specified name, or
+// `nullptr` if not found. This function will emit a warning if the name of a
+// 'retired' flag is specified.
+absl::CommandLineFlag* FindCommandLineFlag(absl::string_view name);
+
+// Returns current state of the Flags registry in a form of mapping from flag
+// name to a flag reflection handle.
+absl::flat_hash_map<absl::string_view, absl::CommandLineFlag*> GetAllFlags();
+
+//------------------------------------------------------------------------------
+// FlagSaver
+//------------------------------------------------------------------------------
+//
+// A FlagSaver object stores the state of flags in the scope where the FlagSaver
+// is defined, allowing modification of those flags within that scope and
+// automatic restoration of the flags to their previous state upon leaving the
+// scope.
+//
+// A FlagSaver can be used within tests to temporarily change the test
+// environment and restore the test case to its previous state.
+//
+// Example:
+//
+//   void MyFunc() {
+//    absl::FlagSaver fs;
+//    ...
+//    absl::SetFlag(&FLAGS_myFlag, otherValue);
+//    ...
+//  } // scope of FlagSaver left, flags return to previous state
+//
+// This class is thread-safe.
+
+class FlagSaver {
+ public:
+  FlagSaver();
+  ~FlagSaver();
+
+  FlagSaver(const FlagSaver&) = delete;
+  void operator=(const FlagSaver&) = delete;
+
+ private:
+  flags_internal::FlagSaverImpl* impl_;
+};
+
+//-----------------------------------------------------------------------------
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_FLAGS_REFLECTION_H_
diff --git a/absl/flags/reflection_test.cc b/absl/flags/reflection_test.cc
new file mode 100644
index 00000000000..996a458fa79
--- /dev/null
+++ b/absl/flags/reflection_test.cc
@@ -0,0 +1,268 @@
+//
+//  Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/flags/reflection.h"
+
+#include <memory>
+#include <string>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/flags/config.h"
+#include "absl/flags/flag.h"
+#include "absl/memory/memory.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/str_split.h"
+
+ABSL_FLAG(int, int_flag, 1, "int_flag help");
+ABSL_FLAG(std::string, string_flag, "dflt", "string_flag help");
+ABSL_RETIRED_FLAG(bool, bool_retired_flag, false, "bool_retired_flag help");
+
+namespace {
+
+class ReflectionTest : public testing::Test {
+ protected:
+  void SetUp() override {
+#if ABSL_FLAGS_STRIP_NAMES
+    GTEST_SKIP() << "This test requires flag names to be present";
+#endif
+    flag_saver_ = std::make_unique<absl::FlagSaver>();
+  }
+  void TearDown() override { flag_saver_.reset(); }
+
+ private:
+  std::unique_ptr<absl::FlagSaver> flag_saver_;
+};
+
+// --------------------------------------------------------------------
+
+TEST_F(ReflectionTest, TestFindCommandLineFlag) {
+  auto* handle = absl::FindCommandLineFlag("some_flag");
+  EXPECT_EQ(handle, nullptr);
+
+  handle = absl::FindCommandLineFlag("int_flag");
+  EXPECT_NE(handle, nullptr);
+
+  handle = absl::FindCommandLineFlag("string_flag");
+  EXPECT_NE(handle, nullptr);
+
+  handle = absl::FindCommandLineFlag("bool_retired_flag");
+  EXPECT_NE(handle, nullptr);
+}
+
+// --------------------------------------------------------------------
+
+TEST_F(ReflectionTest, TestGetAllFlags) {
+  auto all_flags = absl::GetAllFlags();
+  EXPECT_NE(all_flags.find("int_flag"), all_flags.end());
+  EXPECT_EQ(all_flags.find("bool_retired_flag"), all_flags.end());
+  EXPECT_EQ(all_flags.find("some_undefined_flag"), all_flags.end());
+
+  std::vector<absl::string_view> flag_names_first_attempt;
+  auto all_flags_1 = absl::GetAllFlags();
+  for (auto f : all_flags_1) {
+    flag_names_first_attempt.push_back(f.first);
+  }
+
+  std::vector<absl::string_view> flag_names_second_attempt;
+  auto all_flags_2 = absl::GetAllFlags();
+  for (auto f : all_flags_2) {
+    flag_names_second_attempt.push_back(f.first);
+  }
+
+  EXPECT_THAT(flag_names_first_attempt,
+              ::testing::UnorderedElementsAreArray(flag_names_second_attempt));
+}
+
+// --------------------------------------------------------------------
+
+struct CustomUDT {
+  CustomUDT() : a(1), b(1) {}
+  CustomUDT(int a_, int b_) : a(a_), b(b_) {}
+
+  friend bool operator==(const CustomUDT& f1, const CustomUDT& f2) {
+    return f1.a == f2.a && f1.b == f2.b;
+  }
+
+  int a;
+  int b;
+};
+bool AbslParseFlag(absl::string_view in, CustomUDT* f, std::string*) {
+  std::vector<absl::string_view> parts =
+      absl::StrSplit(in, ':', absl::SkipWhitespace());
+
+  if (parts.size() != 2) return false;
+
+  if (!absl::SimpleAtoi(parts[0], &f->a)) return false;
+
+  if (!absl::SimpleAtoi(parts[1], &f->b)) return false;
+
+  return true;
+}
+std::string AbslUnparseFlag(const CustomUDT& f) {
+  return absl::StrCat(f.a, ":", f.b);
+}
+
+}  // namespace
+
+// --------------------------------------------------------------------
+
+ABSL_FLAG(bool, test_flag_01, true, "");
+ABSL_FLAG(int, test_flag_02, 1234, "");
+ABSL_FLAG(int16_t, test_flag_03, -34, "");
+ABSL_FLAG(uint16_t, test_flag_04, 189, "");
+ABSL_FLAG(int32_t, test_flag_05, 10765, "");
+ABSL_FLAG(uint32_t, test_flag_06, 40000, "");
+ABSL_FLAG(int64_t, test_flag_07, -1234567, "");
+ABSL_FLAG(uint64_t, test_flag_08, 9876543, "");
+ABSL_FLAG(double, test_flag_09, -9.876e-50, "");
+ABSL_FLAG(float, test_flag_10, 1.234e12f, "");
+ABSL_FLAG(std::string, test_flag_11, "", "");
+ABSL_FLAG(absl::Duration, test_flag_12, absl::Minutes(10), "");
+static int counter = 0;
+ABSL_FLAG(int, test_flag_13, 200, "").OnUpdate([]() { counter++; });
+ABSL_FLAG(CustomUDT, test_flag_14, {}, "");
+
+namespace {
+
+TEST_F(ReflectionTest, TestFlagSaverInScope) {
+  {
+    absl::FlagSaver s;
+    counter = 0;
+    absl::SetFlag(&FLAGS_test_flag_01, false);
+    absl::SetFlag(&FLAGS_test_flag_02, -1021);
+    absl::SetFlag(&FLAGS_test_flag_03, 6009);
+    absl::SetFlag(&FLAGS_test_flag_04, 44);
+    absl::SetFlag(&FLAGS_test_flag_05, +800);
+    absl::SetFlag(&FLAGS_test_flag_06, -40978756);
+    absl::SetFlag(&FLAGS_test_flag_07, 23405);
+    absl::SetFlag(&FLAGS_test_flag_08, 975310);
+    absl::SetFlag(&FLAGS_test_flag_09, 1.00001);
+    absl::SetFlag(&FLAGS_test_flag_10, -3.54f);
+    absl::SetFlag(&FLAGS_test_flag_11, "asdf");
+    absl::SetFlag(&FLAGS_test_flag_12, absl::Hours(20));
+    absl::SetFlag(&FLAGS_test_flag_13, 4);
+    absl::SetFlag(&FLAGS_test_flag_14, CustomUDT{-1, -2});
+  }
+
+  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_01), true);
+  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_02), 1234);
+  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_03), -34);
+  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_04), 189);
+  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_05), 10765);
+  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_06), 40000);
+  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_07), -1234567);
+  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_08), 9876543);
+  EXPECT_NEAR(absl::GetFlag(FLAGS_test_flag_09), -9.876e-50, 1e-55);
+  EXPECT_NEAR(absl::GetFlag(FLAGS_test_flag_10), 1.234e12f, 1e5f);
+  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_11), "");
+  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_12), absl::Minutes(10));
+  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_13), 200);
+  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_14), CustomUDT{});
+  EXPECT_EQ(counter, 2);
+}
+
+// --------------------------------------------------------------------
+
+TEST_F(ReflectionTest, TestFlagSaverVsUpdateViaReflection) {
+  {
+    absl::FlagSaver s;
+    counter = 0;
+    std::string error;
+    EXPECT_TRUE(
+        absl::FindCommandLineFlag("test_flag_01")->ParseFrom("false", &error))
+        << error;
+    EXPECT_TRUE(
+        absl::FindCommandLineFlag("test_flag_02")->ParseFrom("-4536", &error))
+        << error;
+    EXPECT_TRUE(
+        absl::FindCommandLineFlag("test_flag_03")->ParseFrom("111", &error))
+        << error;
+    EXPECT_TRUE(
+        absl::FindCommandLineFlag("test_flag_04")->ParseFrom("909", &error))
+        << error;
+    EXPECT_TRUE(
+        absl::FindCommandLineFlag("test_flag_05")->ParseFrom("-2004", &error))
+        << error;
+    EXPECT_TRUE(
+        absl::FindCommandLineFlag("test_flag_06")->ParseFrom("1000023", &error))
+        << error;
+    EXPECT_TRUE(
+        absl::FindCommandLineFlag("test_flag_07")->ParseFrom("69305", &error))
+        << error;
+    EXPECT_TRUE(absl::FindCommandLineFlag("test_flag_08")
+                    ->ParseFrom("1000000001", &error))
+        << error;
+    EXPECT_TRUE(
+        absl::FindCommandLineFlag("test_flag_09")->ParseFrom("2.09021", &error))
+        << error;
+    EXPECT_TRUE(
+        absl::FindCommandLineFlag("test_flag_10")->ParseFrom("-33.1", &error))
+        << error;
+    EXPECT_TRUE(
+        absl::FindCommandLineFlag("test_flag_11")->ParseFrom("ADD_FOO", &error))
+        << error;
+    EXPECT_TRUE(absl::FindCommandLineFlag("test_flag_12")
+                    ->ParseFrom("3h11m16s", &error))
+        << error;
+    EXPECT_TRUE(
+        absl::FindCommandLineFlag("test_flag_13")->ParseFrom("0", &error))
+        << error;
+    EXPECT_TRUE(
+        absl::FindCommandLineFlag("test_flag_14")->ParseFrom("10:1", &error))
+        << error;
+  }
+
+  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_01), true);
+  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_02), 1234);
+  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_03), -34);
+  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_04), 189);
+  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_05), 10765);
+  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_06), 40000);
+  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_07), -1234567);
+  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_08), 9876543);
+  EXPECT_NEAR(absl::GetFlag(FLAGS_test_flag_09), -9.876e-50, 1e-55);
+  EXPECT_NEAR(absl::GetFlag(FLAGS_test_flag_10), 1.234e12f, 1e5f);
+  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_11), "");
+  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_12), absl::Minutes(10));
+  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_13), 200);
+  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_14), CustomUDT{});
+  EXPECT_EQ(counter, 2);
+}
+
+// --------------------------------------------------------------------
+
+TEST_F(ReflectionTest, TestMultipleFlagSaversInEnclosedScopes) {
+  {
+    absl::FlagSaver s;
+    absl::SetFlag(&FLAGS_test_flag_08, 10);
+    EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_08), 10);
+    {
+      absl::FlagSaver s;
+      absl::SetFlag(&FLAGS_test_flag_08, 20);
+      EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_08), 20);
+      {
+        absl::FlagSaver s;
+        absl::SetFlag(&FLAGS_test_flag_08, -200);
+        EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_08), -200);
+      }
+      EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_08), 20);
+    }
+    EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_08), 10);
+  }
+  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_08), 9876543);
+}
+
+}  // namespace
diff --git a/absl/flags/usage.cc b/absl/flags/usage.cc
index dff7a3dab39..e42b4540228 100644
--- a/absl/flags/usage.cc
+++ b/absl/flags/usage.cc
@@ -14,12 +14,21 @@
 // limitations under the License.
 #include "absl/flags/usage.h"
 
+#include <stdlib.h>
+
 #include <string>
 
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/const_init.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/base/thread_annotations.h"
 #include "absl/flags/internal/usage.h"
+#include "absl/strings/string_view.h"
 #include "absl/synchronization/mutex.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace flags_internal {
 namespace {
 ABSL_CONST_INIT absl::Mutex usage_message_guard(absl::kConstInit);
@@ -31,7 +40,7 @@ ABSL_CONST_INIT std::string* program_usage_message
 // --------------------------------------------------------------------
 // Sets the "usage" message to be used by help reporting routines.
 void SetProgramUsageMessage(absl::string_view new_usage_message) {
-  absl::MutexLock l(&flags_internal::usage_message_guard);
+  absl::MutexLock l(flags_internal::usage_message_guard);
 
   if (flags_internal::program_usage_message != nullptr) {
     ABSL_INTERNAL_LOG(FATAL, "SetProgramUsageMessage() called twice.");
@@ -46,11 +55,12 @@ void SetProgramUsageMessage(absl::string_view new_usage_message) {
 // Note: We able to return string_view here only because calling
 // SetProgramUsageMessage twice is prohibited.
 absl::string_view ProgramUsageMessage() {
-  absl::MutexLock l(&flags_internal::usage_message_guard);
+  absl::MutexLock l(flags_internal::usage_message_guard);
 
   return flags_internal::program_usage_message != nullptr
              ? absl::string_view(*flags_internal::program_usage_message)
              : "Warning: SetProgramUsageMessage() never called";
 }
 
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/flags/usage.h b/absl/flags/usage.h
index 3a12107137a..ad12ab7ad90 100644
--- a/absl/flags/usage.h
+++ b/absl/flags/usage.h
@@ -16,12 +16,14 @@
 #ifndef ABSL_FLAGS_USAGE_H_
 #define ABSL_FLAGS_USAGE_H_
 
+#include "absl/base/config.h"
 #include "absl/strings/string_view.h"
 
 // --------------------------------------------------------------------
 // Usage reporting interfaces
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 // Sets the "usage" message to be used by help reporting routines.
 // For example:
@@ -35,6 +37,7 @@ void SetProgramUsageMessage(absl::string_view new_usage_message);
 // Returns the usage message set by SetProgramUsageMessage().
 absl::string_view ProgramUsageMessage();
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_FLAGS_USAGE_H_
diff --git a/absl/flags/usage_config.cc b/absl/flags/usage_config.cc
index f97bf300dde..bbc020d1f44 100644
--- a/absl/flags/usage_config.cc
+++ b/absl/flags/usage_config.cc
@@ -15,13 +15,19 @@
 
 #include "absl/flags/usage_config.h"
 
+#include <functional>
 #include <iostream>
-#include <memory>
+#include <string>
 
 #include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/const_init.h"
+#include "absl/base/no_destructor.h"
+#include "absl/base/thread_annotations.h"
 #include "absl/flags/internal/path_util.h"
 #include "absl/flags/internal/program_name.h"
-#include "absl/strings/str_cat.h"
+#include "absl/strings/match.h"
+#include "absl/strings/string_view.h"
 #include "absl/strings/strip.h"
 #include "absl/synchronization/mutex.h"
 
@@ -29,11 +35,13 @@ extern "C" {
 
 // Additional report of fatal usage error message before we std::exit. Error is
 // fatal if is_fatal argument to ReportUsageError is true.
-ABSL_ATTRIBUTE_WEAK void AbslInternalReportFatalUsageError(absl::string_view) {}
+ABSL_ATTRIBUTE_WEAK void ABSL_INTERNAL_C_SYMBOL(
+    AbslInternalReportFatalUsageError)(absl::string_view) {}
 
 }  // extern "C"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace flags_internal {
 
 namespace {
@@ -45,10 +53,15 @@ namespace {
 bool ContainsHelpshortFlags(absl::string_view filename) {
   // By default we only want flags in binary's main. We expect the main
   // routine to reside in <program>.cc or <program>-main.cc or
-  // <program>_main.cc, where the <program> is the name of the binary.
+  // <program>_main.cc, where the <program> is the name of the binary
+  // (without .exe on Windows).
   auto suffix = flags_internal::Basename(filename);
-  if (!absl::ConsumePrefix(&suffix,
-                           flags_internal::ShortProgramInvocationName()))
+  auto program_name = flags_internal::ShortProgramInvocationName();
+  absl::string_view program_name_ref = program_name;
+#if defined(_WIN32)
+  absl::ConsumeSuffix(&program_name_ref, ".exe");
+#endif
+  if (!absl::ConsumePrefix(&suffix, program_name_ref))
     return false;
   return absl::StartsWith(suffix, ".") || absl::StartsWith(suffix, "-main.") ||
          absl::StartsWith(suffix, "_main.");
@@ -92,14 +105,18 @@ std::string NormalizeFilename(absl::string_view filename) {
 
 // --------------------------------------------------------------------
 
-ABSL_CONST_INIT absl::Mutex custom_usage_config_guard(absl::kConstInit);
+absl::Mutex& CustomUsageConfigMutex() {
+  static absl::NoDestructor<absl::Mutex> mutex;
+  return *mutex;
+}
 ABSL_CONST_INIT FlagsUsageConfig* custom_usage_config
-    ABSL_GUARDED_BY(custom_usage_config_guard) = nullptr;
+    ABSL_GUARDED_BY(CustomUsageConfigMutex())
+        ABSL_PT_GUARDED_BY(CustomUsageConfigMutex()) = nullptr;
 
 }  // namespace
 
 FlagsUsageConfig GetUsageConfig() {
-  absl::MutexLock l(&custom_usage_config_guard);
+  absl::MutexLock l(CustomUsageConfigMutex());
 
   if (custom_usage_config) return *custom_usage_config;
 
@@ -117,14 +134,14 @@ void ReportUsageError(absl::string_view msg, bool is_fatal) {
   std::cerr << "ERROR: " << msg << std::endl;
 
   if (is_fatal) {
-    AbslInternalReportFatalUsageError(msg);
+    ABSL_INTERNAL_C_SYMBOL(AbslInternalReportFatalUsageError)(msg);
   }
 }
 
 }  // namespace flags_internal
 
 void SetFlagsUsageConfig(FlagsUsageConfig usage_config) {
-  absl::MutexLock l(&flags_internal::custom_usage_config_guard);
+  absl::MutexLock l(flags_internal::CustomUsageConfigMutex());
 
   if (!usage_config.contains_helpshort_flags)
     usage_config.contains_helpshort_flags =
@@ -149,4 +166,5 @@ void SetFlagsUsageConfig(FlagsUsageConfig usage_config) {
     flags_internal::custom_usage_config = new FlagsUsageConfig(usage_config);
 }
 
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/flags/usage_config.h b/absl/flags/usage_config.h
index bfd0eedbc4a..ded70300f09 100644
--- a/absl/flags/usage_config.h
+++ b/absl/flags/usage_config.h
@@ -27,6 +27,7 @@
 #include <functional>
 #include <string>
 
+#include "absl/base/config.h"
 #include "absl/strings/string_view.h"
 
 // -----------------------------------------------------------------------------
@@ -54,6 +55,7 @@
 //     Shows help on modules whose name contains the specified substring
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 namespace flags_internal {
 using FlagKindFilter = std::function<bool (absl::string_view)>;
@@ -88,7 +90,7 @@ struct FlagsUsageConfig {
   // program output.
   flags_internal::FlagKindFilter contains_helppackage_flags;
 
-  // Generates std::string containing program version. This is the std::string reported
+  // Generates string containing program version. This is the string reported
   // when user specifies --version in a command line.
   std::function<std::string()> version_string;
 
@@ -118,13 +120,15 @@ FlagsUsageConfig GetUsageConfig();
 void ReportUsageError(absl::string_view msg, bool is_fatal);
 
 }  // namespace flags_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 extern "C" {
 
 // Additional report of fatal usage error message before we std::exit. Error is
 // fatal if is_fatal argument to ReportUsageError is true.
-void AbslInternalReportFatalUsageError(absl::string_view);
+void ABSL_INTERNAL_C_SYMBOL(AbslInternalReportFatalUsageError)(
+    absl::string_view);
 
 }  // extern "C"
 
diff --git a/absl/flags/usage_config_test.cc b/absl/flags/usage_config_test.cc
index 3bde13af8f8..e57a8832f64 100644
--- a/absl/flags/usage_config_test.cc
+++ b/absl/flags/usage_config_test.cc
@@ -15,10 +15,13 @@
 
 #include "absl/flags/usage_config.h"
 
+#include <string>
+
 #include "gtest/gtest.h"
 #include "absl/flags/internal/path_util.h"
 #include "absl/flags/internal/program_name.h"
 #include "absl/strings/match.h"
+#include "absl/strings/string_view.h"
 
 namespace {
 
@@ -81,7 +84,11 @@ TEST_F(FlagsUsageConfigTest, TestGetSetFlagsUsageConfig) {
 // --------------------------------------------------------------------
 
 TEST_F(FlagsUsageConfigTest, TestContainsHelpshortFlags) {
+#if defined(_WIN32)
+  flags::SetProgramInvocationName("usage_config_test.exe");
+#else
   flags::SetProgramInvocationName("usage_config_test");
+#endif
 
   auto config = flags::GetUsageConfig();
   EXPECT_TRUE(config.contains_helpshort_flags("adir/cd/usage_config_test.cc"));
diff --git a/absl/functional/BUILD.bazel b/absl/functional/BUILD.bazel
new file mode 100644
index 00000000000..c41b211c889
--- /dev/null
+++ b/absl/functional/BUILD.bazel
@@ -0,0 +1,207 @@
+#
+# Copyright 2019 The Abseil Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+load("@rules_cc//cc:cc_binary.bzl", "cc_binary")
+load("@rules_cc//cc:cc_library.bzl", "cc_library")
+load("@rules_cc//cc:cc_test.bzl", "cc_test")
+load(
+    "//absl:copts/configure_copts.bzl",
+    "ABSL_DEFAULT_COPTS",
+    "ABSL_DEFAULT_LINKOPTS",
+    "ABSL_TEST_COPTS",
+)
+
+package(
+    default_visibility = ["//visibility:private"],
+    features = [
+        "header_modules",
+        "layering_check",
+        "parse_headers",
+    ],
+)
+
+licenses(["notice"])
+
+cc_library(
+    name = "any_invocable",
+    srcs = ["internal/any_invocable.h"],
+    hdrs = ["any_invocable.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
+    deps = [
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:nullability",
+        "//absl/meta:type_traits",
+        "//absl/utility",
+    ],
+)
+
+cc_test(
+    name = "any_invocable_test",
+    srcs = [
+        "any_invocable_test.h",
+        "any_invocable_test_inst1.cc",
+        "any_invocable_test_inst2.cc",
+        "any_invocable_test_inst3.cc",
+        "any_invocable_test_inst4.cc",
+        "internal/any_invocable.h",
+    ],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":any_invocable",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:nullability",
+        "//absl/meta:type_traits",
+        "//absl/utility",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_library(
+    name = "bind_back",
+    srcs = ["internal/back_binder.h"],
+    hdrs = ["bind_back.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
+    deps = [
+        "//absl/base:config",
+        "//absl/container:compressed_tuple",
+        "//absl/utility",
+    ],
+)
+
+cc_test(
+    name = "bind_back_test",
+    srcs = ["bind_back_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":bind_back",
+        "//absl/memory",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_library(
+    name = "bind_front",
+    srcs = ["internal/front_binder.h"],
+    hdrs = ["bind_front.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
+    deps = [
+        "//absl/container:compressed_tuple",
+        "//absl/meta:type_traits",
+        "//absl/utility",
+    ],
+)
+
+cc_test(
+    name = "bind_front_test",
+    srcs = ["bind_front_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":bind_front",
+        "//absl/memory",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_library(
+    name = "function_ref",
+    srcs = ["internal/function_ref.h"],
+    hdrs = ["function_ref.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
+    deps = [
+        ":any_invocable",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/meta:type_traits",
+        "//absl/utility",
+    ],
+)
+
+cc_test(
+    name = "function_ref_test",
+    size = "small",
+    srcs = ["function_ref_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    deps = [
+        ":any_invocable",
+        ":function_ref",
+        "//absl/base:config",
+        "//absl/container:test_instance_tracker",
+        "//absl/memory",
+        "//absl/utility",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_library(
+    name = "overload",
+    hdrs = ["overload.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
+    deps = [
+        "//absl/base:config",
+        "//absl/meta:type_traits",
+    ],
+)
+
+cc_test(
+    name = "overload_test",
+    size = "small",
+    srcs = ["overload_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    deps = [
+        ":overload",
+        "//absl/base:config",
+        "//absl/strings",
+        "//absl/strings:string_view",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_binary(
+    name = "function_type_benchmark",
+    testonly = True,
+    srcs = [
+        "function_type_benchmark.cc",
+    ],
+    copts = ABSL_TEST_COPTS,
+    tags = ["benchmark"],
+    visibility = ["//visibility:private"],
+    deps = [
+        ":any_invocable",
+        ":function_ref",
+        "//absl/base:core_headers",
+        "@google_benchmark//:benchmark_main",
+    ],
+)
diff --git a/absl/functional/CMakeLists.txt b/absl/functional/CMakeLists.txt
new file mode 100644
index 00000000000..77801749c0d
--- /dev/null
+++ b/absl/functional/CMakeLists.txt
@@ -0,0 +1,171 @@
+#
+# Copyright 2019 The Abseil Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+absl_cc_library(
+  NAME
+    any_invocable
+  SRCS
+    "internal/any_invocable.h"
+  HDRS
+    "any_invocable.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::base
+    absl::config
+    absl::core_headers
+    absl::type_traits
+    absl::utility
+  PUBLIC
+)
+
+absl_cc_test(
+  NAME
+    any_invocable_test
+  SRCS
+    "any_invocable_test.h"
+    "any_invocable_test_inst1.cc"
+    "any_invocable_test_inst2.cc"
+    "any_invocable_test_inst3.cc"
+    "any_invocable_test_inst4.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::any_invocable
+    absl::base
+    absl::config
+    absl::core_headers
+    absl::type_traits
+    absl::utility
+    GTest::gmock_main
+)
+
+absl_cc_library(
+  NAME
+    bind_back
+  SRCS
+    "internal/back_binder.h"
+  HDRS
+    "bind_back.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::compressed_tuple
+    absl::config
+    absl::utility
+  PUBLIC
+)
+
+absl_cc_test(
+  NAME
+    bind_back_test
+  SRCS
+    "bind_back_test.cc"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::bind_back
+    absl::memory
+    GTest::gmock_main
+)
+
+absl_cc_library(
+  NAME
+    bind_front
+  SRCS
+    "internal/front_binder.h"
+  HDRS
+    "bind_front.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::compressed_tuple
+  PUBLIC
+)
+
+absl_cc_test(
+  NAME
+    bind_front_test
+  SRCS
+    "bind_front_test.cc"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::bind_front
+    absl::memory
+    GTest::gmock_main
+)
+
+absl_cc_library(
+  NAME
+    function_ref
+  SRCS
+    "internal/function_ref.h"
+  HDRS
+    "function_ref.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::config
+    absl::core_headers
+    absl::any_invocable
+    absl::meta
+    absl::utility
+  PUBLIC
+)
+
+absl_cc_test(
+  NAME
+    function_ref_test
+  SRCS
+    "function_ref_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::config
+    absl::function_ref
+    absl::memory
+    absl::test_instance_tracker
+    absl::utility
+    GTest::gmock_main
+)
+
+absl_cc_library(
+  NAME
+    overload
+  HDRS
+    "overload.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::meta
+  PUBLIC
+)
+
+absl_cc_test(
+  NAME
+    overload_test
+  SRCS
+    "overload_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::config
+    absl::overload
+    absl::string_view
+    absl::strings
+    GTest::gmock_main
+)
diff --git a/absl/functional/any_invocable.h b/absl/functional/any_invocable.h
new file mode 100644
index 00000000000..867885e5280
--- /dev/null
+++ b/absl/functional/any_invocable.h
@@ -0,0 +1,335 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: any_invocable.h
+// -----------------------------------------------------------------------------
+//
+// This header file defines an `absl::AnyInvocable` type that assumes ownership
+// and wraps an object of an invocable type. (Invocable types adhere to the
+// concept specified in https://en.cppreference.com/w/cpp/concepts/invocable.)
+//
+// In general, prefer `absl::AnyInvocable` when you need a type-erased
+// function parameter that needs to take ownership of the type.
+//
+// NOTE: `absl::AnyInvocable` is similar to the C++23 `std::move_only_function`
+// abstraction, but has a slightly different API and is not designed to be a
+// drop-in replacement or backfill of that type.
+//
+// Credits to Matt Calabrese (https://github.com/mattcalabrese) for the original
+// implementation.
+
+#ifndef ABSL_FUNCTIONAL_ANY_INVOCABLE_H_
+#define ABSL_FUNCTIONAL_ANY_INVOCABLE_H_
+
+#include <cstddef>
+#include <functional>
+#include <initializer_list>
+#include <type_traits>
+#include <utility>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/nullability.h"
+#include "absl/functional/internal/any_invocable.h"
+#include "absl/meta/type_traits.h"
+#include "absl/utility/utility.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+// absl::AnyInvocable
+//
+// `absl::AnyInvocable` is a functional wrapper type, like `std::function`, that
+// assumes ownership of an invocable object. Unlike `std::function`, an
+// `absl::AnyInvocable` is more type-safe and provides the following additional
+// benefits:
+//
+// * Properly adheres to const correctness of the underlying type
+// * Is move-only so avoids concurrency problems with copied invocables and
+//   unnecessary copies in general.
+// * Supports reference qualifiers allowing it to perform unique actions (noted
+//   below).
+//
+// `absl::AnyInvocable` is a template, and an `absl::AnyInvocable` instantiation
+// may wrap any invocable object with a compatible function signature, e.g.
+// having arguments and return types convertible to types matching the
+// `absl::AnyInvocable` signature, and also matching any stated reference
+// qualifiers, as long as that type is moveable. It therefore provides broad
+// type erasure for functional objects.
+//
+// An `absl::AnyInvocable` is typically used as a type-erased function parameter
+// for accepting various functional objects:
+//
+// // Define a function taking an AnyInvocable parameter.
+// void my_func(absl::AnyInvocable<int()> f) {
+//   ...
+// };
+//
+// // That function can accept any invocable type:
+//
+// // Accept a function reference. We don't need to move a reference.
+// int func1() { return 0; };
+// my_func(func1);
+//
+// // Accept a lambda. We use std::move here because otherwise my_func would
+// // copy the lambda.
+// auto lambda = []() { return 0; };
+// my_func(std::move(lambda));
+//
+// // Accept a function pointer. We don't need to move a function pointer.
+// func2 = &func1;
+// my_func(func2);
+//
+// // Accept an std::function by moving it. Note that the lambda is copyable
+// // (satisfying std::function requirements) and moveable (satisfying
+// // absl::AnyInvocable requirements).
+// std::function<int()> func6 = []() { return 0; };
+// my_func(std::move(func6));
+//
+// `AnyInvocable` also properly respects `const` qualifiers, reference
+// qualifiers, and the `noexcept` specification  as part of the user-specified
+// function type (e.g. `AnyInvocable<void() const && noexcept>`). These
+// qualifiers will be applied to the `AnyInvocable` object's `operator()`, and
+// the underlying invocable must be compatible with those qualifiers.
+//
+// Comparison of const and non-const function types:
+//
+//   // Store a closure inside of `func` with the function type `int()`.
+//   // Note that we have made `func` itself `const`.
+//   const AnyInvocable<int()> func = [](){ return 0; };
+//
+//   func();  // Compile-error: the passed type `int()` isn't `const`.
+//
+//   // Store a closure inside of `const_func` with the function type
+//   // `int() const`.
+//   // Note that we have also made `const_func` itself `const`.
+//   const AnyInvocable<int() const> const_func = [](){ return 0; };
+//
+//   const_func();  // Fine: `int() const` is `const`.
+//
+// In the above example, the call `func()` would have compiled if
+// `std::function` were used even though the types are not const compatible.
+// This is a bug, and using `absl::AnyInvocable` properly detects that bug.
+//
+// In addition to affecting the signature of `operator()`, the `const` and
+// reference qualifiers of the function type also appropriately constrain which
+// kinds of invocable objects you are allowed to place into the `AnyInvocable`
+// instance. If you specify a function type that is const-qualified, then
+// anything that you attempt to put into the `AnyInvocable` must be callable on
+// a `const` instance of that type.
+//
+// Constraint example:
+//
+//   // Fine because the lambda is callable when `const`.
+//   AnyInvocable<int() const> func = [=](){ return 0; };
+//
+//   // This is a compile-error because the lambda isn't callable when `const`.
+//   AnyInvocable<int() const> error = [=]() mutable { return 0; };
+//
+// An `&&` qualifier can be used to express that an `absl::AnyInvocable`
+// instance should be invoked at most once:
+//
+//   // Invokes `continuation` with the logical result of an operation when
+//   // that operation completes (common in asynchronous code).
+//   void CallOnCompletion(AnyInvocable<void(int)&&> continuation) {
+//     int result_of_foo = foo();
+//
+//     // `std::move` is required because the `operator()` of `continuation` is
+//     // rvalue-reference qualified.
+//     std::move(continuation)(result_of_foo);
+//   }
+//
+// Attempting to call `absl::AnyInvocable` multiple times in such a case
+// results in undefined behavior.
+//
+// Invoking an empty `absl::AnyInvocable` results in undefined behavior:
+//
+//   // Create an empty instance using the default constructor.
+//   AnyInvocable<void()> empty;
+//   empty();  // WARNING: Undefined behavior!
+template <class Sig>
+class ABSL_NULLABILITY_COMPATIBLE ABSL_ATTRIBUTE_OWNER AnyInvocable
+    : private internal_any_invocable::Impl<Sig> {
+ private:
+  static_assert(
+      std::is_function_v<Sig>,
+      "The template argument of AnyInvocable must be a function type.");
+
+  using Impl = internal_any_invocable::Impl<Sig>;
+
+ public:
+  // The return type of Sig
+  using result_type = typename Impl::result_type;
+  using absl_internal_is_view = std::false_type;
+
+  // Constructors
+
+  // Constructs the `AnyInvocable` in an empty state.
+  // Invoking it results in undefined behavior.
+  AnyInvocable() noexcept = default;
+  AnyInvocable(std::nullptr_t) noexcept {}  // NOLINT
+
+  // Constructs the `AnyInvocable` from an existing `AnyInvocable` by a move.
+  // Note that `f` is not guaranteed to be empty after move-construction,
+  // although it may be.
+  AnyInvocable(AnyInvocable&& /*f*/) noexcept = default;
+
+  // Constructs an `AnyInvocable` from an invocable object.
+  //
+  // Upon construction, `*this` is only empty if `f` is a function pointer or
+  // member pointer type and is null, or if `f` is an `AnyInvocable` that is
+  // empty.
+  template <class F, typename = std::enable_if_t<
+                         internal_any_invocable::CanConvert<Sig, F>::value>>
+  AnyInvocable(F&& f)  // NOLINT
+      : Impl(internal_any_invocable::ConversionConstruct(),
+             std::forward<F>(f)) {}
+
+  // Constructs an `AnyInvocable` that holds an invocable object of type `T`,
+  // which is constructed in-place from the given arguments.
+  //
+  // Example:
+  //
+  //   AnyInvocable<int(int)> func(
+  //       std::in_place_type<PossiblyImmovableType>, arg1, arg2);
+  //
+  template <class T, class... Args,
+            typename = std::enable_if_t<
+                internal_any_invocable::CanEmplace<Sig, T, Args...>::value>>
+  explicit AnyInvocable(std::in_place_type_t<T>, Args&&... args)
+      : Impl(std::in_place_type<std::decay_t<T>>, std::forward<Args>(args)...) {
+    static_assert(std::is_same_v<T, std::decay_t<T>>,
+                  "The explicit template argument of in_place_type is required "
+                  "to be an unqualified object type.");
+  }
+
+  // Overload of the above constructor to support list-initialization.
+  template <class T, class U, class... Args,
+            typename = std::enable_if_t<internal_any_invocable::CanEmplace<
+                Sig, T, std::initializer_list<U>&, Args...>::value>>
+  explicit AnyInvocable(std::in_place_type_t<T>, std::initializer_list<U> ilist,
+                        Args&&... args)
+      : Impl(std::in_place_type<std::decay_t<T>>, ilist,
+             std::forward<Args>(args)...) {
+    static_assert(std::is_same_v<T, std::decay_t<T>>,
+                  "The explicit template argument of in_place_type is required "
+                  "to be an unqualified object type.");
+  }
+
+  // Assignment Operators
+
+  // Assigns an `AnyInvocable` through move-assignment.
+  // Note that `f` is not guaranteed to be empty after move-assignment
+  // although it may be.
+  AnyInvocable& operator=(AnyInvocable&& /*f*/) noexcept = default;
+
+  // Assigns an `AnyInvocable` from a nullptr, clearing the `AnyInvocable`. If
+  // not empty, destroys the target, putting `*this` into an empty state.
+  AnyInvocable& operator=(std::nullptr_t) noexcept {
+    this->Clear();
+    return *this;
+  }
+
+  // Assigns an `AnyInvocable` from an existing `AnyInvocable` instance.
+  //
+  // Upon assignment, `*this` is only empty if `f` is a function pointer or
+  // member pointer type and is null, or if `f` is an `AnyInvocable` that is
+  // empty.
+  template <class F, typename = std::enable_if_t<
+                         internal_any_invocable::CanAssign<Sig, F>::value>>
+  AnyInvocable& operator=(F&& f) {
+    *this = AnyInvocable(std::forward<F>(f));
+    return *this;
+  }
+
+  // Assigns an `AnyInvocable` from a reference to an invocable object.
+  // Upon assignment, stores a reference to the invocable object in the
+  // `AnyInvocable` instance.
+  template <
+      class F,
+      typename = std::enable_if_t<
+          internal_any_invocable::CanAssignReferenceWrapper<Sig, F>::value>>
+  AnyInvocable& operator=(std::reference_wrapper<F> f) noexcept {
+    *this = AnyInvocable(f);
+    return *this;
+  }
+
+  // Destructor
+
+  // If not empty, destroys the target.
+  ~AnyInvocable() = default;
+
+  // absl::AnyInvocable::swap()
+  //
+  // Exchanges the targets of `*this` and `other`.
+  void swap(AnyInvocable& other) noexcept { std::swap(*this, other); }
+
+  // absl::AnyInvocable::operator bool()
+  //
+  // Returns `true` if `*this` is not empty.
+  //
+  // WARNING: An `AnyInvocable` that wraps an empty `std::function` is not
+  // itself empty. This behavior is consistent with the standard equivalent
+  // `std::move_only_function`. In the following example, `a()` will actually
+  // invoke `f()`, leading to an `std::bad_function_call` exception:
+  //   std::function<void()> f;  // empty
+  //   absl::AnyInvocable<void()> a = f;  // not empty
+  //
+  // Invoking an empty `AnyInvocable` results in undefined behavior.
+  explicit operator bool() const noexcept { return this->HasValue(); }
+
+  // Invokes the target object of `*this`. `*this` must not be empty.
+  //
+  // Note: The signature of this function call operator is the same as the
+  //       template parameter `Sig`.
+  using Impl::operator();
+
+  // Equality operators
+
+  // Returns `true` if `f` is empty.
+  friend bool operator==(const AnyInvocable& f, std::nullptr_t) noexcept {
+    return !f.HasValue();
+  }
+
+  // Returns `true` if `f` is empty.
+  friend bool operator==(std::nullptr_t, const AnyInvocable& f) noexcept {
+    return !f.HasValue();
+  }
+
+  // Returns `false` if `f` is empty.
+  friend bool operator!=(const AnyInvocable& f, std::nullptr_t) noexcept {
+    return f.HasValue();
+  }
+
+  // Returns `false` if `f` is empty.
+  friend bool operator!=(std::nullptr_t, const AnyInvocable& f) noexcept {
+    return f.HasValue();
+  }
+
+  // swap()
+  //
+  // Exchanges the targets of `f1` and `f2`.
+  friend void swap(AnyInvocable& f1, AnyInvocable& f2) noexcept { f1.swap(f2); }
+
+ private:
+  // Friending other instantiations is necessary for conversions.
+  template <bool /*SigIsNoexcept*/, class /*ReturnType*/, class... /*P*/>
+  friend class internal_any_invocable::CoreImpl;
+};
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_FUNCTIONAL_ANY_INVOCABLE_H_
diff --git a/absl/functional/any_invocable_test.h b/absl/functional/any_invocable_test.h
new file mode 100644
index 00000000000..d667fa427ec
--- /dev/null
+++ b/absl/functional/any_invocable_test.h
@@ -0,0 +1,1597 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// To prevent compiler memory exhaustion (OOM / Killed signal terminates
+// cc1plus) during parallel builds with GCC, the test suite instantiations have
+// been split into four separate compilation units: any_invocable_test_inst1.cc,
+// any_invocable_test_inst2.cc, any_invocable_test_inst3.cc, and
+// any_invocable_test_inst4.cc. The test definitions remain here in this
+// header.
+
+// SKIP_ABSL_INLINE_NAMESPACE_CHECK
+
+#ifndef ABSL_FUNCTIONAL_ANY_INVOCABLE_TEST_H_
+#define ABSL_FUNCTIONAL_ANY_INVOCABLE_TEST_H_
+
+#include "absl/functional/any_invocable.h"
+
+#include <cstddef>
+#include <cstdlib>
+#include <functional>
+#include <initializer_list>
+#include <iterator>
+#include <memory>
+#include <numeric>
+#include <type_traits>
+
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+#include "absl/base/nullability.h"
+#include "absl/meta/type_traits.h"
+#include "absl/utility/utility.h"
+
+static_assert(absl::internal_any_invocable::kStorageSize >= sizeof(void*),
+              "These tests assume that the small object storage is at least "
+              "the size of a pointer.");
+
+namespace absl_any_invocable_test {
+
+// A dummy type we use when passing qualifiers to metafunctions
+struct _ {};
+
+template <class T>
+struct Wrapper {
+  template <class U, class = std::enable_if_t<std::is_convertible_v<U, T>>>
+  Wrapper(U&&);  // NOLINT
+};
+
+// This will cause a recursive trait instantiation if the SFINAE checks are
+// not ordered correctly for constructibility.
+static_assert(std::is_constructible_v<Wrapper<absl::AnyInvocable<void()>>,
+                                      Wrapper<absl::AnyInvocable<void()>>>);
+
+// A metafunction that takes the cv and l-value reference qualifiers that were
+// associated with a function type (here passed via qualifiers of an object
+// type), and .
+template <class Qualifiers, class This>
+struct QualifiersForThisImpl {
+  static_assert(std::is_object_v<This>);
+  using type =
+      std::conditional_t<std::is_const_v<Qualifiers>, const This, This>&;
+};
+
+template <class Qualifiers, class This>
+struct QualifiersForThisImpl<Qualifiers&, This>
+    : QualifiersForThisImpl<Qualifiers, This> {};
+
+template <class Qualifiers, class This>
+struct QualifiersForThisImpl<Qualifiers&&, This> {
+  static_assert(std::is_object_v<This>);
+  using type =
+      std::conditional_t<std::is_const_v<Qualifiers>, const This, This>&&;
+};
+
+template <class Qualifiers, class This>
+using QualifiersForThis =
+    typename QualifiersForThisImpl<Qualifiers, This>::type;
+
+// A metafunction that takes the cv and l-value reference qualifier of T and
+// applies them to U's function type qualifiers.
+template <class T, class Fun>
+struct GiveQualifiersToFunImpl;
+
+template <class T, class R, class... P>
+struct GiveQualifiersToFunImpl<T, R(P...)> {
+  using type = std::conditional_t<std::is_const_v<T>, R(P...) const, R(P...)>;
+};
+
+template <class T, class R, class... P>
+struct GiveQualifiersToFunImpl<T&, R(P...)> {
+  using type =
+      std::conditional_t<std::is_const_v<T>, R(P...) const&, R(P...) &>;
+};
+
+template <class T, class R, class... P>
+struct GiveQualifiersToFunImpl<T&&, R(P...)> {
+  using type =
+      std::conditional_t<std::is_const_v<T>, R(P...) const&&, R(P...) &&>;
+};
+
+template <class T, class R, class... P>
+struct GiveQualifiersToFunImpl<T, R(P...) noexcept> {
+  using type = std::conditional_t<std::is_const_v<T>, R(P...) const noexcept,
+                                  R(P...) noexcept>;
+};
+
+template <class T, class R, class... P>
+struct GiveQualifiersToFunImpl<T&, R(P...) noexcept> {
+  using type = std::conditional_t<std::is_const_v<T>, R(P...) const & noexcept,
+                                  R(P...) & noexcept>;
+};
+
+template <class T, class R, class... P>
+struct GiveQualifiersToFunImpl<T&&, R(P...) noexcept> {
+  using type = std::conditional_t<std::is_const_v<T>, R(P...) const && noexcept,
+                                  R(P...) && noexcept>;
+};
+
+template <class T, class Fun>
+using GiveQualifiersToFun = typename GiveQualifiersToFunImpl<T, Fun>::type;
+
+// This is used in template parameters to decide whether or not to use a type
+// that fits in the small object optimization storage.
+enum class ObjSize { small, large };
+
+// A base type that is used with classes as a means to insert an
+// appropriately-sized dummy datamember when Size is ObjSize::large so that the
+// user's class type is guaranteed to not fit in small object storage.
+template <ObjSize Size>
+struct TypeErasedPadding;
+
+template <>
+struct TypeErasedPadding<ObjSize::small> {};
+
+template <>
+struct TypeErasedPadding<ObjSize::large> {
+  char dummy_data[absl::internal_any_invocable::kStorageSize + 1] = {};
+};
+
+struct Int {
+  Int(int v) noexcept : value(v) {}  // NOLINT
+#ifndef _MSC_VER
+  Int(Int&&) noexcept {
+    // NOTE: Prior to C++17, this not being called requires optimizations to
+    //       take place when performing the top-level invocation. In practice,
+    //       most supported compilers perform this optimization prior to C++17.
+    std::abort();
+  }
+#else
+  Int(Int&& v) noexcept = default;
+#endif
+  operator int() && noexcept { return value; }  // NOLINT
+
+  int MemberFunctionAdd(int const& b, int c) noexcept {  // NOLINT
+    return value + b + c;
+  }
+
+  int value;
+};
+
+enum class Movable { no, yes, nothrow, trivial };
+
+enum class NothrowCall { no, yes };
+
+enum class Destructible { nothrow, trivial };
+
+enum class ObjAlign : std::size_t {
+  normal = absl::internal_any_invocable::kAlignment,
+  large = absl::internal_any_invocable::kAlignment * 2,
+};
+
+// A function-object template that has knobs for each property that can affect
+// how the object is stored in AnyInvocable.
+template <Movable Movability, Destructible Destructibility, class Qual,
+          NothrowCall CallExceptionSpec, ObjSize Size, ObjAlign Alignment>
+struct add;
+
+#define ABSL_INTERNALS_ADD(qual)                                              \
+  template <NothrowCall CallExceptionSpec, ObjSize Size, ObjAlign Alignment>  \
+  struct alignas(static_cast<std::size_t>(Alignment))                         \
+      add<Movable::trivial, Destructible::trivial, _ qual, CallExceptionSpec, \
+          Size, Alignment> : TypeErasedPadding<Size> {                        \
+    explicit add(int state_init) : state(state_init) {}                       \
+    explicit add(std::initializer_list<int> state_init, int tail)             \
+        : state(std::accumulate(std::begin(state_init), std::end(state_init), \
+                                0) +                                          \
+                tail) {}                                                      \
+    add(add&& other) = default; /*NOLINT*/                                    \
+    Int operator()(int a, int b, int c) qual                                  \
+        noexcept(CallExceptionSpec == NothrowCall::yes) {                     \
+      return state + a + b + c;                                               \
+    }                                                                         \
+    int state;                                                                \
+  };                                                                          \
+                                                                              \
+  template <NothrowCall CallExceptionSpec, ObjSize Size, ObjAlign Alignment>  \
+  struct alignas(static_cast<std::size_t>(Alignment))                         \
+      add<Movable::trivial, Destructible::nothrow, _ qual, CallExceptionSpec, \
+          Size, Alignment> : TypeErasedPadding<Size> {                        \
+    explicit add(int state_init) : state(state_init) {}                       \
+    explicit add(std::initializer_list<int> state_init, int tail)             \
+        : state(std::accumulate(std::begin(state_init), std::end(state_init), \
+                                0) +                                          \
+                tail) {}                                                      \
+    ~add() noexcept {}                                                        \
+    add(add&& other) = default; /*NOLINT*/                                    \
+    Int operator()(int a, int b, int c) qual                                  \
+        noexcept(CallExceptionSpec == NothrowCall::yes) {                     \
+      return state + a + b + c;                                               \
+    }                                                                         \
+    int state;                                                                \
+  }
+
+// Explicitly specify an empty argument.
+// MSVC (at least up to _MSC_VER 1931, if not beyond) warns that
+// ABSL_INTERNALS_ADD() is an undefined zero-arg overload.
+#define ABSL_INTERNALS_NOARG
+ABSL_INTERNALS_ADD(ABSL_INTERNALS_NOARG);
+#undef ABSL_INTERNALS_NOARG
+
+ABSL_INTERNALS_ADD(const);
+ABSL_INTERNALS_ADD(&);
+ABSL_INTERNALS_ADD(const&);
+ABSL_INTERNALS_ADD(&&);       // NOLINT
+ABSL_INTERNALS_ADD(const&&);  // NOLINT
+
+#undef ABSL_INTERNALS_ADD
+
+template <Destructible Destructibility, class Qual,
+          NothrowCall CallExceptionSpec, ObjSize Size, ObjAlign Alignment>
+struct add<Movable::no, Destructibility, Qual, CallExceptionSpec, Size,
+           Alignment> : private add<Movable::trivial, Destructibility, Qual,
+                                    CallExceptionSpec, Size, Alignment> {
+  using Base = add<Movable::trivial, Destructibility, Qual, CallExceptionSpec,
+                   Size, Alignment>;
+
+  explicit add(int state_init) : Base(state_init) {}
+
+  explicit add(std::initializer_list<int> state_init, int tail)
+      : Base(state_init, tail) {}
+
+  add(add&&) = delete;
+
+  using Base::operator();
+  using Base::state;
+};
+
+template <Destructible Destructibility, class Qual,
+          NothrowCall CallExceptionSpec, ObjSize Size, ObjAlign Alignment>
+struct add<Movable::yes, Destructibility, Qual, CallExceptionSpec, Size,
+           Alignment> : private add<Movable::trivial, Destructibility, Qual,
+                                    CallExceptionSpec, Size, Alignment> {
+  using Base = add<Movable::trivial, Destructibility, Qual, CallExceptionSpec,
+                   Size, Alignment>;
+
+  explicit add(int state_init) : Base(state_init) {}
+
+  explicit add(std::initializer_list<int> state_init, int tail)
+      : Base(state_init, tail) {}
+
+  add(add&& other) noexcept(false) : Base(other.state) {}  // NOLINT
+
+  using Base::operator();
+  using Base::state;
+};
+
+template <Destructible Destructibility, class Qual,
+          NothrowCall CallExceptionSpec, ObjSize Size, ObjAlign Alignment>
+struct add<Movable::nothrow, Destructibility, Qual, CallExceptionSpec, Size,
+           Alignment> : private add<Movable::trivial, Destructibility, Qual,
+                                    CallExceptionSpec, Size, Alignment> {
+  using Base = add<Movable::trivial, Destructibility, Qual, CallExceptionSpec,
+                   Size, Alignment>;
+
+  explicit add(int state_init) : Base(state_init) {}
+
+  explicit add(std::initializer_list<int> state_init, int tail)
+      : Base(state_init, tail) {}
+
+  add(add&& other) noexcept : Base(other.state) {}
+
+  using Base::operator();
+  using Base::state;
+};
+
+// Actual non-member functions rather than function objects
+inline Int add_function(Int&& a, int b, int c) noexcept {
+  return a.value + b + c;
+}
+
+inline Int mult_function(Int&& a, int b, int c) noexcept {
+  return a.value * b * c;
+}
+
+inline Int square_function(Int const&& a) noexcept { return a.value * a.value; }
+
+template <class Sig>
+using AnyInvocable = absl::AnyInvocable<Sig>;
+
+// Instantiations of this template contains all of the compile-time parameters
+// for a given instantiation of the AnyInvocable test suite.
+template <Movable Movability, Destructible Destructibility, class Qual,
+          NothrowCall CallExceptionSpec, ObjSize Size, ObjAlign Alignment>
+struct TestParams {
+  static constexpr Movable kMovability = Movability;
+  static constexpr Destructible kDestructibility = Destructibility;
+  using Qualifiers = Qual;
+  static constexpr NothrowCall kCallExceptionSpec = CallExceptionSpec;
+  static constexpr bool kIsNoexcept = kCallExceptionSpec == NothrowCall::yes;
+  static constexpr bool kIsRvalueQualified = std::is_rvalue_reference_v<Qual>;
+  static constexpr ObjSize kSize = Size;
+  static constexpr ObjAlign kAlignment = Alignment;
+
+  // These types are used when testing with member object pointer Invocables
+  using UnqualifiedUnaryFunType = int(Int const&&) noexcept(CallExceptionSpec ==
+                                                            NothrowCall::yes);
+  using UnaryFunType = GiveQualifiersToFun<Qualifiers, UnqualifiedUnaryFunType>;
+  using MemObjPtrType = int(Int::*);
+  using UnaryAnyInvType = AnyInvocable<UnaryFunType>;
+  using UnaryThisParamType = QualifiersForThis<Qualifiers, UnaryAnyInvType>;
+
+  template <class T>
+  static UnaryThisParamType ToUnaryThisParam(T&& fun) {
+    return static_cast<UnaryThisParamType>(fun);
+  }
+
+  // This function type intentionally uses 3 "kinds" of parameter types.
+  //     - A user-defined type
+  //     - A reference type
+  //     - A scalar type
+  //
+  // These were chosen because internal forwarding takes place on parameters
+  // differently depending based on type properties (scalars are forwarded by
+  // value).
+  using ResultType = Int;
+  using AnyInvocableFunTypeNotNoexcept = Int(Int, const int&, int);
+  using UnqualifiedFunType =
+      std::conditional_t<kIsNoexcept, Int(Int, const int&, int) noexcept,
+                         Int(Int, const int&, int)>;
+  using FunType = GiveQualifiersToFun<Qualifiers, UnqualifiedFunType>;
+  using MemFunPtrType =
+      std::conditional_t<kIsNoexcept, Int (Int::*)(const int&, int) noexcept,
+                         Int (Int::*)(const int&, int)>;
+  using AnyInvType = AnyInvocable<FunType>;
+  using AddType = add<kMovability, kDestructibility, Qualifiers,
+                      kCallExceptionSpec, kSize, kAlignment>;
+  using ThisParamType = QualifiersForThis<Qualifiers, AnyInvType>;
+
+  template <class T>
+  static ThisParamType ToThisParam(T&& fun) {
+    return static_cast<ThisParamType>(fun);
+  }
+
+  // These typedefs are used when testing void return type covariance.
+  using UnqualifiedVoidFunType =
+      std::conditional_t<kIsNoexcept, void(Int, const int&, int) noexcept,
+                         void(Int, const int&, int)>;
+  using VoidFunType = GiveQualifiersToFun<Qualifiers, UnqualifiedVoidFunType>;
+  using VoidAnyInvType = AnyInvocable<VoidFunType>;
+  using VoidThisParamType = QualifiersForThis<Qualifiers, VoidAnyInvType>;
+
+  template <class T>
+  static VoidThisParamType ToVoidThisParam(T&& fun) {
+    return static_cast<VoidThisParamType>(fun);
+  }
+
+  using CompatibleAnyInvocableFunType =
+      std::conditional_t<std::is_rvalue_reference_v<Qual>,
+                         GiveQualifiersToFun<const _&&, UnqualifiedFunType>,
+                         GiveQualifiersToFun<const _&, UnqualifiedFunType>>;
+
+  using CompatibleAnyInvType = AnyInvocable<CompatibleAnyInvocableFunType>;
+
+  using IncompatibleInvocable =
+      std::conditional_t<std::is_rvalue_reference_v<Qual>,
+                         GiveQualifiersToFun<_&, UnqualifiedFunType>(_::*),
+                         GiveQualifiersToFun<_&&, UnqualifiedFunType>(_::*)>;
+};
+
+// Given a member-pointer type, this metafunction yields the target type of the
+// pointer, not including the class-type. It is used to verify that the function
+// call operator of AnyInvocable has the proper signature, corresponding to the
+// function type that the user provided.
+template <class MemberPtrType>
+struct MemberTypeOfImpl;
+
+template <class Class, class T>
+struct MemberTypeOfImpl<T(Class::*)> {
+  using type = T;
+};
+
+template <class MemberPtrType>
+using MemberTypeOf = typename MemberTypeOfImpl<MemberPtrType>::type;
+
+template <class T, class = void>
+struct IsMemberSwappableImpl : std::false_type {
+  static constexpr bool kIsNothrow = false;
+};
+
+template <class T>
+struct IsMemberSwappableImpl<
+    T, std::void_t<decltype(std::declval<T&>().swap(std::declval<T&>()))>>
+    : std::true_type {
+  static constexpr bool kIsNothrow =
+      noexcept(std::declval<T&>().swap(std::declval<T&>()));
+};
+
+template <class T>
+using IsMemberSwappable = IsMemberSwappableImpl<T>;
+
+template <class T>
+using IsNothrowMemberSwappable =
+    std::bool_constant<IsMemberSwappableImpl<T>::kIsNothrow>;
+
+template <class T>
+class AnyInvTestBasic : public ::testing::Test {};
+
+TYPED_TEST_SUITE_P(AnyInvTestBasic);
+
+TYPED_TEST_P(AnyInvTestBasic, DefaultConstruction) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+
+  AnyInvType fun;
+
+  EXPECT_FALSE(static_cast<bool>(fun));
+
+  EXPECT_TRUE(std::is_nothrow_default_constructible_v<AnyInvType>);
+}
+
+TYPED_TEST_P(AnyInvTestBasic, ConstructionNullptr) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+
+  AnyInvType fun = nullptr;
+
+  EXPECT_FALSE(static_cast<bool>(fun));
+
+  EXPECT_TRUE((std::is_nothrow_constructible_v<AnyInvType, std::nullptr_t>));
+}
+
+TYPED_TEST_P(AnyInvTestBasic, ConstructionNullFunctionPtr) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+  using UnqualifiedFunType = typename TypeParam::UnqualifiedFunType;
+
+  UnqualifiedFunType* const null_fun_ptr = nullptr;
+  AnyInvType fun = null_fun_ptr;
+
+  EXPECT_FALSE(static_cast<bool>(fun));
+}
+
+TYPED_TEST_P(AnyInvTestBasic, ConstructionNullMemberFunctionPtr) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+  using MemFunPtrType = typename TypeParam::MemFunPtrType;
+
+  const MemFunPtrType null_mem_fun_ptr = nullptr;
+  AnyInvType fun = null_mem_fun_ptr;
+
+  EXPECT_FALSE(static_cast<bool>(fun));
+}
+
+TYPED_TEST_P(AnyInvTestBasic, ConstructionNullMemberObjectPtr) {
+  using UnaryAnyInvType = typename TypeParam::UnaryAnyInvType;
+  using MemObjPtrType = typename TypeParam::MemObjPtrType;
+
+  const MemObjPtrType null_mem_obj_ptr = nullptr;
+  UnaryAnyInvType fun = null_mem_obj_ptr;
+
+  EXPECT_FALSE(static_cast<bool>(fun));
+}
+
+TYPED_TEST_P(AnyInvTestBasic, ConstructionMemberFunctionPtr) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+
+  AnyInvType fun = &Int::MemberFunctionAdd;
+
+  EXPECT_TRUE(static_cast<bool>(fun));
+  EXPECT_EQ(24, TypeParam::ToThisParam(fun)(7, 8, 9).value);
+}
+
+TYPED_TEST_P(AnyInvTestBasic, ConstructionMemberObjectPtr) {
+  using UnaryAnyInvType = typename TypeParam::UnaryAnyInvType;
+
+  UnaryAnyInvType fun = &Int::value;
+
+  EXPECT_TRUE(static_cast<bool>(fun));
+  EXPECT_EQ(13, TypeParam::ToUnaryThisParam(fun)(13));
+}
+
+TYPED_TEST_P(AnyInvTestBasic, ConstructionFunctionReferenceDecay) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+
+  AnyInvType fun = add_function;
+
+  EXPECT_TRUE(static_cast<bool>(fun));
+  EXPECT_EQ(24, TypeParam::ToThisParam(fun)(7, 8, 9).value);
+}
+
+TYPED_TEST_P(AnyInvTestBasic, ConstructionCompatibleAnyInvocableEmpty) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+  using CompatibleAnyInvType = typename TypeParam::CompatibleAnyInvType;
+
+  CompatibleAnyInvType other;
+  AnyInvType fun = std::move(other);
+
+  EXPECT_FALSE(static_cast<bool>(other));  // NOLINT
+  EXPECT_EQ(other, nullptr);               // NOLINT
+  EXPECT_EQ(nullptr, other);               // NOLINT
+
+  EXPECT_FALSE(static_cast<bool>(fun));
+}
+
+TYPED_TEST_P(AnyInvTestBasic, ConstructionCompatibleAnyInvocableNonempty) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+  using CompatibleAnyInvType = typename TypeParam::CompatibleAnyInvType;
+
+  CompatibleAnyInvType other = &add_function;
+  AnyInvType fun = std::move(other);
+
+  EXPECT_FALSE(static_cast<bool>(other));  // NOLINT
+  EXPECT_EQ(other, nullptr);               // NOLINT
+  EXPECT_EQ(nullptr, other);               // NOLINT
+
+  EXPECT_TRUE(static_cast<bool>(fun));
+  EXPECT_EQ(24, TypeParam::ToThisParam(fun)(7, 8, 9).value);
+}
+
+TYPED_TEST_P(AnyInvTestBasic, ConversionToBool) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+
+  {
+    AnyInvType fun;
+
+    // This tests contextually-convertible-to-bool.
+    EXPECT_FALSE(fun ? true : false);  // NOLINT
+
+    // Make sure that the conversion is not implicit.
+    EXPECT_TRUE((std::is_nothrow_constructible_v<bool, const AnyInvType&>));
+    EXPECT_FALSE((std::is_convertible_v<const AnyInvType&, bool>));
+  }
+
+  {
+    AnyInvType fun = &add_function;
+
+    // This tests contextually-convertible-to-bool.
+    EXPECT_TRUE(fun ? true : false);  // NOLINT
+  }
+}
+
+TYPED_TEST_P(AnyInvTestBasic, Invocation) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+
+  using FunType = typename TypeParam::FunType;
+  using AnyInvCallType = MemberTypeOf<decltype(&AnyInvType::operator())>;
+
+  // Make sure the function call operator of AnyInvocable always has the
+  // type that was specified via the template argument.
+  EXPECT_TRUE((std::is_same_v<AnyInvCallType, FunType>));
+
+  AnyInvType fun = &add_function;
+
+  EXPECT_EQ(24, TypeParam::ToThisParam(fun)(7, 8, 9).value);
+}
+
+TYPED_TEST_P(AnyInvTestBasic, InPlaceConstruction) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+  using AddType = typename TypeParam::AddType;
+
+  AnyInvType fun(std::in_place_type<AddType>, 5);
+
+  EXPECT_TRUE(static_cast<bool>(fun));
+  EXPECT_EQ(29, TypeParam::ToThisParam(fun)(7, 8, 9).value);
+}
+
+TYPED_TEST_P(AnyInvTestBasic, InPlaceConstructionInitializerList) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+  using AddType = typename TypeParam::AddType;
+
+  AnyInvType fun(std::in_place_type<AddType>, {1, 2, 3, 4}, 5);
+
+  EXPECT_TRUE(static_cast<bool>(fun));
+  EXPECT_EQ(39, TypeParam::ToThisParam(fun)(7, 8, 9).value);
+}
+
+TYPED_TEST_P(AnyInvTestBasic, InPlaceNullFunPtrConstruction) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+  using UnqualifiedFunType = typename TypeParam::UnqualifiedFunType;
+
+  AnyInvType fun(std::in_place_type<UnqualifiedFunType*>, nullptr);
+
+  // In-place construction does not lead to empty.
+  EXPECT_TRUE(static_cast<bool>(fun));
+}
+
+TYPED_TEST_P(AnyInvTestBasic, InPlaceNullFunPtrConstructionValueInit) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+  using UnqualifiedFunType = typename TypeParam::UnqualifiedFunType;
+
+  AnyInvType fun(std::in_place_type<UnqualifiedFunType*>);
+
+  // In-place construction does not lead to empty.
+  EXPECT_TRUE(static_cast<bool>(fun));
+}
+
+TYPED_TEST_P(AnyInvTestBasic, InPlaceNullMemFunPtrConstruction) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+  using MemFunPtrType = typename TypeParam::MemFunPtrType;
+
+  AnyInvType fun(std::in_place_type<MemFunPtrType>, nullptr);
+
+  // In-place construction does not lead to empty.
+  EXPECT_TRUE(static_cast<bool>(fun));
+}
+
+TYPED_TEST_P(AnyInvTestBasic, InPlaceNullMemFunPtrConstructionValueInit) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+  using MemFunPtrType = typename TypeParam::MemFunPtrType;
+
+  AnyInvType fun(std::in_place_type<MemFunPtrType>);
+
+  // In-place construction does not lead to empty.
+  EXPECT_TRUE(static_cast<bool>(fun));
+}
+
+TYPED_TEST_P(AnyInvTestBasic, InPlaceNullMemObjPtrConstruction) {
+  using UnaryAnyInvType = typename TypeParam::UnaryAnyInvType;
+  using MemObjPtrType = typename TypeParam::MemObjPtrType;
+
+  UnaryAnyInvType fun(std::in_place_type<MemObjPtrType>, nullptr);
+
+  // In-place construction does not lead to empty.
+  EXPECT_TRUE(static_cast<bool>(fun));
+}
+
+TYPED_TEST_P(AnyInvTestBasic, InPlaceNullMemObjPtrConstructionValueInit) {
+  using UnaryAnyInvType = typename TypeParam::UnaryAnyInvType;
+  using MemObjPtrType = typename TypeParam::MemObjPtrType;
+
+  UnaryAnyInvType fun(std::in_place_type<MemObjPtrType>);
+
+  // In-place construction does not lead to empty.
+  EXPECT_TRUE(static_cast<bool>(fun));
+}
+
+TYPED_TEST_P(AnyInvTestBasic, InPlaceVoidCovarianceConstruction) {
+  using VoidAnyInvType = typename TypeParam::VoidAnyInvType;
+  using AddType = typename TypeParam::AddType;
+
+  VoidAnyInvType fun(std::in_place_type<AddType>, 5);
+
+  EXPECT_TRUE(static_cast<bool>(fun));
+}
+
+TYPED_TEST_P(AnyInvTestBasic, MoveConstructionFromEmpty) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+
+  absl_nullable AnyInvType source_fun;
+  absl_nullable AnyInvType fun(std::move(source_fun));
+
+  EXPECT_FALSE(static_cast<bool>(fun));
+
+  EXPECT_TRUE(std::is_nothrow_move_constructible_v<AnyInvType>);
+}
+
+TYPED_TEST_P(AnyInvTestBasic, MoveConstructionFromNonEmpty) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+  using AddType = typename TypeParam::AddType;
+
+  AnyInvType source_fun(std::in_place_type<AddType>, 5);
+  AnyInvType fun(std::move(source_fun));
+
+  EXPECT_TRUE(static_cast<bool>(fun));
+  EXPECT_EQ(29, TypeParam::ToThisParam(fun)(7, 8, 9).value);
+
+  EXPECT_TRUE(std::is_nothrow_move_constructible_v<AnyInvType>);
+}
+
+TYPED_TEST_P(AnyInvTestBasic, ComparisonWithNullptrEmpty) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+
+  AnyInvType fun;
+
+  EXPECT_TRUE(fun == nullptr);
+  EXPECT_TRUE(nullptr == fun);
+
+  EXPECT_FALSE(fun != nullptr);
+  EXPECT_FALSE(nullptr != fun);
+}
+
+TYPED_TEST_P(AnyInvTestBasic, ComparisonWithNullptrNonempty) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+  using AddType = typename TypeParam::AddType;
+
+  AnyInvType fun(std::in_place_type<AddType>, 5);
+
+  EXPECT_FALSE(fun == nullptr);
+  EXPECT_FALSE(nullptr == fun);
+
+  EXPECT_TRUE(fun != nullptr);
+  EXPECT_TRUE(nullptr != fun);
+}
+
+TYPED_TEST_P(AnyInvTestBasic, ResultType) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+  using ExpectedResultType = typename TypeParam::ResultType;
+
+  EXPECT_TRUE(
+      (std::is_same_v<typename AnyInvType::result_type, ExpectedResultType>));
+}
+
+template <class T>
+class AnyInvTestCombinatoric : public ::testing::Test {};
+
+TYPED_TEST_SUITE_P(AnyInvTestCombinatoric);
+
+TYPED_TEST_P(AnyInvTestCombinatoric, MoveAssignEmptyEmptyLhsRhs) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+
+  AnyInvType source_fun;
+  AnyInvType fun;
+
+  fun = std::move(source_fun);
+
+  EXPECT_FALSE(static_cast<bool>(fun));
+}
+
+TYPED_TEST_P(AnyInvTestCombinatoric, MoveAssignEmptyLhsNonemptyRhs) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+  using AddType = typename TypeParam::AddType;
+
+  AnyInvType source_fun(std::in_place_type<AddType>, 5);
+  AnyInvType fun;
+
+  fun = std::move(source_fun);
+
+  EXPECT_TRUE(static_cast<bool>(fun));
+  EXPECT_EQ(29, TypeParam::ToThisParam(fun)(7, 8, 9).value);
+}
+
+TYPED_TEST_P(AnyInvTestCombinatoric, MoveAssignNonemptyEmptyLhsRhs) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+  using AddType = typename TypeParam::AddType;
+
+  AnyInvType source_fun;
+  AnyInvType fun(std::in_place_type<AddType>, 5);
+
+  fun = std::move(source_fun);
+
+  EXPECT_FALSE(static_cast<bool>(fun));
+}
+
+TYPED_TEST_P(AnyInvTestCombinatoric, MoveAssignNonemptyLhsNonemptyRhs) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+  using AddType = typename TypeParam::AddType;
+
+  AnyInvType source_fun(std::in_place_type<AddType>, 5);
+  AnyInvType fun(std::in_place_type<AddType>, 20);
+
+  fun = std::move(source_fun);
+
+  EXPECT_TRUE(static_cast<bool>(fun));
+  EXPECT_EQ(29, TypeParam::ToThisParam(fun)(7, 8, 9).value);
+}
+
+TYPED_TEST_P(AnyInvTestCombinatoric, SelfMoveAssignEmpty) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+
+  AnyInvType source_fun;
+  source_fun = std::move(source_fun);
+
+  // This space intentionally left blank.
+}
+
+TYPED_TEST_P(AnyInvTestCombinatoric, SelfMoveAssignNonempty) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+  using AddType = typename TypeParam::AddType;
+
+  AnyInvType source_fun(std::in_place_type<AddType>, 5);
+  source_fun = std::move(source_fun);
+
+  // This space intentionally left blank.
+}
+
+TYPED_TEST_P(AnyInvTestCombinatoric, AssignNullptrEmptyLhs) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+
+  AnyInvType fun;
+  fun = nullptr;
+
+  EXPECT_FALSE(static_cast<bool>(fun));
+}
+
+TYPED_TEST_P(AnyInvTestCombinatoric, AssignNullFunctionPtrEmptyLhs) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+  using UnqualifiedFunType = typename TypeParam::UnqualifiedFunType;
+
+  UnqualifiedFunType* const null_fun_ptr = nullptr;
+  AnyInvType fun;
+  fun = null_fun_ptr;
+
+  EXPECT_FALSE(static_cast<bool>(fun));
+}
+
+TYPED_TEST_P(AnyInvTestCombinatoric, AssignNullMemberFunctionPtrEmptyLhs) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+  using MemFunPtrType = typename TypeParam::MemFunPtrType;
+
+  const MemFunPtrType null_mem_fun_ptr = nullptr;
+  AnyInvType fun;
+  fun = null_mem_fun_ptr;
+
+  EXPECT_FALSE(static_cast<bool>(fun));
+}
+
+TYPED_TEST_P(AnyInvTestCombinatoric, AssignNullMemberObjectPtrEmptyLhs) {
+  using UnaryAnyInvType = typename TypeParam::UnaryAnyInvType;
+  using MemObjPtrType = typename TypeParam::MemObjPtrType;
+
+  const MemObjPtrType null_mem_obj_ptr = nullptr;
+  UnaryAnyInvType fun;
+  fun = null_mem_obj_ptr;
+
+  EXPECT_FALSE(static_cast<bool>(fun));
+}
+
+TYPED_TEST_P(AnyInvTestCombinatoric, AssignMemberFunctionPtrEmptyLhs) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+
+  AnyInvType fun;
+  fun = &Int::MemberFunctionAdd;
+
+  EXPECT_TRUE(static_cast<bool>(fun));
+  EXPECT_EQ(24, TypeParam::ToThisParam(fun)(7, 8, 9).value);
+}
+
+TYPED_TEST_P(AnyInvTestCombinatoric, AssignMemberObjectPtrEmptyLhs) {
+  using UnaryAnyInvType = typename TypeParam::UnaryAnyInvType;
+
+  UnaryAnyInvType fun;
+  fun = &Int::value;
+
+  EXPECT_TRUE(static_cast<bool>(fun));
+  EXPECT_EQ(13, TypeParam::ToUnaryThisParam(fun)(13));
+}
+
+TYPED_TEST_P(AnyInvTestCombinatoric, AssignFunctionReferenceDecayEmptyLhs) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+
+  AnyInvType fun;
+  fun = add_function;
+
+  EXPECT_TRUE(static_cast<bool>(fun));
+  EXPECT_EQ(24, TypeParam::ToThisParam(fun)(7, 8, 9).value);
+}
+
+TYPED_TEST_P(AnyInvTestCombinatoric,
+             AssignCompatibleAnyInvocableEmptyLhsEmptyRhs) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+  using CompatibleAnyInvType = typename TypeParam::CompatibleAnyInvType;
+
+  CompatibleAnyInvType other;
+  AnyInvType fun;
+  fun = std::move(other);
+
+  EXPECT_FALSE(static_cast<bool>(other));  // NOLINT
+  EXPECT_EQ(other, nullptr);               // NOLINT
+  EXPECT_EQ(nullptr, other);               // NOLINT
+
+  EXPECT_FALSE(static_cast<bool>(fun));
+}
+
+TYPED_TEST_P(AnyInvTestCombinatoric,
+             AssignCompatibleAnyInvocableEmptyLhsNonemptyRhs) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+  using CompatibleAnyInvType = typename TypeParam::CompatibleAnyInvType;
+
+  CompatibleAnyInvType other = &add_function;
+  AnyInvType fun;
+  fun = std::move(other);
+
+  EXPECT_FALSE(static_cast<bool>(other));  // NOLINT
+
+  EXPECT_TRUE(static_cast<bool>(fun));
+  EXPECT_EQ(24, TypeParam::ToThisParam(fun)(7, 8, 9).value);
+}
+
+TYPED_TEST_P(AnyInvTestCombinatoric, AssignNullptrNonemptyLhs) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+
+  AnyInvType fun = &mult_function;
+  fun = nullptr;
+
+  EXPECT_FALSE(static_cast<bool>(fun));
+}
+
+TYPED_TEST_P(AnyInvTestCombinatoric, AssignNullFunctionPtrNonemptyLhs) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+  using UnqualifiedFunType = typename TypeParam::UnqualifiedFunType;
+
+  UnqualifiedFunType* const null_fun_ptr = nullptr;
+  AnyInvType fun = &mult_function;
+  fun = null_fun_ptr;
+
+  EXPECT_FALSE(static_cast<bool>(fun));
+}
+
+TYPED_TEST_P(AnyInvTestCombinatoric, AssignNullMemberFunctionPtrNonemptyLhs) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+  using MemFunPtrType = typename TypeParam::MemFunPtrType;
+
+  const MemFunPtrType null_mem_fun_ptr = nullptr;
+  AnyInvType fun = &mult_function;
+  fun = null_mem_fun_ptr;
+
+  EXPECT_FALSE(static_cast<bool>(fun));
+}
+
+TYPED_TEST_P(AnyInvTestCombinatoric, AssignNullMemberObjectPtrNonemptyLhs) {
+  using UnaryAnyInvType = typename TypeParam::UnaryAnyInvType;
+  using MemObjPtrType = typename TypeParam::MemObjPtrType;
+
+  const MemObjPtrType null_mem_obj_ptr = nullptr;
+  UnaryAnyInvType fun = &square_function;
+  fun = null_mem_obj_ptr;
+
+  EXPECT_FALSE(static_cast<bool>(fun));
+}
+
+TYPED_TEST_P(AnyInvTestCombinatoric, AssignMemberFunctionPtrNonemptyLhs) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+
+  AnyInvType fun = &mult_function;
+  fun = &Int::MemberFunctionAdd;
+
+  EXPECT_TRUE(static_cast<bool>(fun));
+  EXPECT_EQ(24, TypeParam::ToThisParam(fun)(7, 8, 9).value);
+}
+
+TYPED_TEST_P(AnyInvTestCombinatoric, AssignMemberObjectPtrNonemptyLhs) {
+  using UnaryAnyInvType = typename TypeParam::UnaryAnyInvType;
+
+  UnaryAnyInvType fun = &square_function;
+  fun = &Int::value;
+
+  EXPECT_TRUE(static_cast<bool>(fun));
+  EXPECT_EQ(13, TypeParam::ToUnaryThisParam(fun)(13));
+}
+
+TYPED_TEST_P(AnyInvTestCombinatoric, AssignFunctionReferenceDecayNonemptyLhs) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+
+  AnyInvType fun = &mult_function;
+  fun = add_function;
+
+  EXPECT_TRUE(static_cast<bool>(fun));
+  EXPECT_EQ(24, TypeParam::ToThisParam(fun)(7, 8, 9).value);
+}
+
+TYPED_TEST_P(AnyInvTestCombinatoric,
+             AssignCompatibleAnyInvocableNonemptyLhsEmptyRhs) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+  using CompatibleAnyInvType = typename TypeParam::CompatibleAnyInvType;
+
+  CompatibleAnyInvType other;
+  AnyInvType fun = &mult_function;
+  fun = std::move(other);
+
+  EXPECT_FALSE(static_cast<bool>(other));  // NOLINT
+  EXPECT_EQ(other, nullptr);               // NOLINT
+  EXPECT_EQ(nullptr, other);               // NOLINT
+
+  EXPECT_FALSE(static_cast<bool>(fun));
+}
+
+TYPED_TEST_P(AnyInvTestCombinatoric,
+             AssignCompatibleAnyInvocableNonemptyLhsNonemptyRhs) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+  using CompatibleAnyInvType = typename TypeParam::CompatibleAnyInvType;
+
+  CompatibleAnyInvType other = &add_function;
+  AnyInvType fun = &mult_function;
+  fun = std::move(other);
+
+  EXPECT_FALSE(static_cast<bool>(other));  // NOLINT
+
+  EXPECT_TRUE(static_cast<bool>(fun));
+  EXPECT_EQ(24, TypeParam::ToThisParam(fun)(7, 8, 9).value);
+}
+
+TYPED_TEST_P(AnyInvTestCombinatoric, SwapEmptyLhsEmptyRhs) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+
+  // Swap idiom
+  {
+    AnyInvType fun;
+    AnyInvType other;
+
+    using std::swap;
+    swap(fun, other);
+
+    EXPECT_FALSE(static_cast<bool>(fun));
+    EXPECT_FALSE(static_cast<bool>(other));
+
+    EXPECT_TRUE(
+        std::is_nothrow_swappable_v<AnyInvType>);
+  }
+
+  // Member swap
+  {
+    AnyInvType fun;
+    AnyInvType other;
+
+    fun.swap(other);
+
+    EXPECT_FALSE(static_cast<bool>(fun));
+    EXPECT_FALSE(static_cast<bool>(other));
+
+    EXPECT_TRUE(IsNothrowMemberSwappable<AnyInvType>::value);
+  }
+}
+
+TYPED_TEST_P(AnyInvTestCombinatoric, SwapEmptyLhsNonemptyRhs) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+  using AddType = typename TypeParam::AddType;
+
+  // Swap idiom
+  {
+    AnyInvType fun;
+    AnyInvType other(std::in_place_type<AddType>, 5);
+
+    using std::swap;
+    swap(fun, other);
+
+    EXPECT_TRUE(static_cast<bool>(fun));
+    EXPECT_FALSE(static_cast<bool>(other));
+
+    EXPECT_EQ(29, TypeParam::ToThisParam(fun)(7, 8, 9).value);
+
+    EXPECT_TRUE(
+        std::is_nothrow_swappable_v<AnyInvType>);
+  }
+
+  // Member swap
+  {
+    AnyInvType fun;
+    AnyInvType other(std::in_place_type<AddType>, 5);
+
+    fun.swap(other);
+
+    EXPECT_TRUE(static_cast<bool>(fun));
+    EXPECT_FALSE(static_cast<bool>(other));
+
+    EXPECT_EQ(29, TypeParam::ToThisParam(fun)(7, 8, 9).value);
+
+    EXPECT_TRUE(IsNothrowMemberSwappable<AnyInvType>::value);
+  }
+}
+
+TYPED_TEST_P(AnyInvTestCombinatoric, SwapNonemptyLhsEmptyRhs) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+  using AddType = typename TypeParam::AddType;
+
+  // Swap idiom
+  {
+    AnyInvType fun(std::in_place_type<AddType>, 5);
+    AnyInvType other;
+
+    using std::swap;
+    swap(fun, other);
+
+    EXPECT_FALSE(static_cast<bool>(fun));
+    EXPECT_TRUE(static_cast<bool>(other));
+
+    EXPECT_EQ(29, TypeParam::ToThisParam(other)(7, 8, 9).value);
+
+    EXPECT_TRUE(
+        std::is_nothrow_swappable_v<AnyInvType>);
+  }
+
+  // Member swap
+  {
+    AnyInvType fun(std::in_place_type<AddType>, 5);
+    AnyInvType other;
+
+    fun.swap(other);
+
+    EXPECT_FALSE(static_cast<bool>(fun));
+    EXPECT_TRUE(static_cast<bool>(other));
+
+    EXPECT_EQ(29, TypeParam::ToThisParam(other)(7, 8, 9).value);
+
+    EXPECT_TRUE(IsNothrowMemberSwappable<AnyInvType>::value);
+  }
+}
+
+TYPED_TEST_P(AnyInvTestCombinatoric, SwapNonemptyLhsNonemptyRhs) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+  using AddType = typename TypeParam::AddType;
+
+  // Swap idiom
+  {
+    AnyInvType fun(std::in_place_type<AddType>, 5);
+    AnyInvType other(std::in_place_type<AddType>, 6);
+
+    using std::swap;
+    swap(fun, other);
+
+    EXPECT_TRUE(static_cast<bool>(fun));
+    EXPECT_TRUE(static_cast<bool>(other));
+
+    EXPECT_EQ(30, TypeParam::ToThisParam(fun)(7, 8, 9).value);
+    EXPECT_EQ(29, TypeParam::ToThisParam(other)(7, 8, 9).value);
+
+    EXPECT_TRUE(
+        std::is_nothrow_swappable_v<AnyInvType>);
+  }
+
+  // Member swap
+  {
+    AnyInvType fun(std::in_place_type<AddType>, 5);
+    AnyInvType other(std::in_place_type<AddType>, 6);
+
+    fun.swap(other);
+
+    EXPECT_TRUE(static_cast<bool>(fun));
+    EXPECT_TRUE(static_cast<bool>(other));
+
+    EXPECT_EQ(30, TypeParam::ToThisParam(fun)(7, 8, 9).value);
+    EXPECT_EQ(29, TypeParam::ToThisParam(other)(7, 8, 9).value);
+
+    EXPECT_TRUE(IsNothrowMemberSwappable<AnyInvType>::value);
+  }
+}
+
+template <class T>
+class AnyInvTestMovable : public ::testing::Test {};
+
+TYPED_TEST_SUITE_P(AnyInvTestMovable);
+
+TYPED_TEST_P(AnyInvTestMovable, ConversionConstructionUserDefinedType) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+  using AddType = typename TypeParam::AddType;
+
+  AnyInvType fun(AddType(5));
+
+  EXPECT_TRUE(static_cast<bool>(fun));
+  EXPECT_EQ(29, TypeParam::ToThisParam(fun)(7, 8, 9).value);
+}
+
+TYPED_TEST_P(AnyInvTestMovable, ConversionConstructionVoidCovariance) {
+  using VoidAnyInvType = typename TypeParam::VoidAnyInvType;
+  using AddType = typename TypeParam::AddType;
+
+  VoidAnyInvType fun(AddType(5));
+
+  EXPECT_TRUE(static_cast<bool>(fun));
+}
+
+TYPED_TEST_P(AnyInvTestMovable, ConversionAssignUserDefinedTypeEmptyLhs) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+  using AddType = typename TypeParam::AddType;
+
+  AnyInvType fun;
+  fun = AddType(5);
+
+  EXPECT_TRUE(static_cast<bool>(fun));
+  EXPECT_EQ(29, TypeParam::ToThisParam(fun)(7, 8, 9).value);
+}
+
+TYPED_TEST_P(AnyInvTestMovable, ConversionAssignUserDefinedTypeNonemptyLhs) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+  using AddType = typename TypeParam::AddType;
+
+  AnyInvType fun = &add_function;
+  fun = AddType(5);
+
+  EXPECT_TRUE(static_cast<bool>(fun));
+  EXPECT_EQ(29, TypeParam::ToThisParam(fun)(7, 8, 9).value);
+}
+
+TYPED_TEST_P(AnyInvTestMovable, ConversionAssignVoidCovariance) {
+  using VoidAnyInvType = typename TypeParam::VoidAnyInvType;
+  using AddType = typename TypeParam::AddType;
+
+  VoidAnyInvType fun;
+  fun = AddType(5);
+
+  EXPECT_TRUE(static_cast<bool>(fun));
+}
+
+template <class T>
+class AnyInvTestNoexceptFalse : public ::testing::Test {};
+
+TYPED_TEST_SUITE_P(AnyInvTestNoexceptFalse);
+
+TYPED_TEST_P(AnyInvTestNoexceptFalse, ConversionConstructionConstraints) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+
+  EXPECT_TRUE(
+      (std::is_constructible_v<
+          AnyInvType, typename TypeParam::AnyInvocableFunTypeNotNoexcept*>));
+  EXPECT_FALSE(
+      (std::is_constructible_v<AnyInvType,
+                               typename TypeParam::IncompatibleInvocable>));
+}
+
+TYPED_TEST_P(AnyInvTestNoexceptFalse, ConversionAssignConstraints) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+
+  EXPECT_TRUE(
+      (std::is_assignable_v<
+          AnyInvType&, typename TypeParam::AnyInvocableFunTypeNotNoexcept*>));
+  EXPECT_FALSE(
+      (std::is_assignable_v<AnyInvType&,
+                            typename TypeParam::IncompatibleInvocable>));
+}
+
+template <class T>
+class AnyInvTestNoexceptTrue : public ::testing::Test {};
+
+TYPED_TEST_SUITE_P(AnyInvTestNoexceptTrue);
+
+TYPED_TEST_P(AnyInvTestNoexceptTrue, ConversionConstructionConstraints) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+
+  EXPECT_FALSE(
+      (std::is_constructible_v<
+          AnyInvType, typename TypeParam::AnyInvocableFunTypeNotNoexcept*>));
+  EXPECT_FALSE(
+      (std::is_constructible_v<AnyInvType,
+                               typename TypeParam::IncompatibleInvocable>));
+}
+
+TYPED_TEST_P(AnyInvTestNoexceptTrue, ConversionAssignConstraints) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+
+  EXPECT_FALSE(
+      (std::is_assignable_v<
+          AnyInvType&, typename TypeParam::AnyInvocableFunTypeNotNoexcept*>));
+  EXPECT_FALSE(
+      (std::is_assignable_v<AnyInvType&,
+                            typename TypeParam::IncompatibleInvocable>));
+}
+
+template <class T>
+class AnyInvTestNonRvalue : public ::testing::Test {};
+
+TYPED_TEST_SUITE_P(AnyInvTestNonRvalue);
+
+TYPED_TEST_P(AnyInvTestNonRvalue, ConversionConstructionReferenceWrapper) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+  using AddType = typename TypeParam::AddType;
+
+  AddType add(4);
+  AnyInvType fun = std::ref(add);
+  add.state = 5;
+
+  EXPECT_TRUE(static_cast<bool>(fun));
+  EXPECT_EQ(29, TypeParam::ToThisParam(fun)(7, 8, 9).value);
+
+  EXPECT_TRUE(static_cast<bool>(fun));
+  EXPECT_EQ(38, TypeParam::ToThisParam(fun)(10, 11, 12).value);
+}
+
+TYPED_TEST_P(AnyInvTestNonRvalue, NonMoveableResultType) {
+  // Define a result type that cannot be copy- or move-constructed.
+  struct Result {
+    int x;
+
+    explicit Result(const int x_in) : x(x_in) {}
+    Result(Result&&) = delete;
+  };
+
+  static_assert(!std::is_move_constructible_v<Result>);
+  static_assert(!std::is_copy_constructible_v<Result>);
+
+  // Assumption check: it should nevertheless be possible to use functors that
+  // return a Result struct according to the language rules.
+  const auto return_17 = []() noexcept { return Result(17); };
+  EXPECT_EQ(17, return_17().x);
+
+  // Just like plain functors, it should work fine to use an AnyInvocable that
+  // returns the non-moveable type.
+  using UnqualifiedFun =
+      std::conditional_t<TypeParam::kIsNoexcept, Result() noexcept, Result()>;
+
+  using Fun =
+      GiveQualifiersToFun<typename TypeParam::Qualifiers, UnqualifiedFun>;
+
+  AnyInvocable<Fun> any_inv(return_17);
+  EXPECT_EQ(17, any_inv().x);
+}
+
+TYPED_TEST_P(AnyInvTestNonRvalue, ConversionAssignReferenceWrapperEmptyLhs) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+  using AddType = typename TypeParam::AddType;
+
+  AddType add(4);
+  AnyInvType fun;
+  fun = std::ref(add);
+  add.state = 5;
+  EXPECT_TRUE((std::is_nothrow_assignable_v<AnyInvType&,
+                                            std::reference_wrapper<AddType>>));
+
+  EXPECT_TRUE(static_cast<bool>(fun));
+  EXPECT_EQ(29, TypeParam::ToThisParam(fun)(7, 8, 9).value);
+
+  EXPECT_TRUE(static_cast<bool>(fun));
+  EXPECT_EQ(38, TypeParam::ToThisParam(fun)(10, 11, 12).value);
+}
+
+TYPED_TEST_P(AnyInvTestNonRvalue, ConversionAssignReferenceWrapperNonemptyLhs) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+  using AddType = typename TypeParam::AddType;
+
+  AddType add(4);
+  AnyInvType fun = &mult_function;
+  fun = std::ref(add);
+  add.state = 5;
+  EXPECT_TRUE((std::is_nothrow_assignable_v<AnyInvType&,
+                                            std::reference_wrapper<AddType>>));
+
+  EXPECT_TRUE(static_cast<bool>(fun));
+  EXPECT_EQ(29, TypeParam::ToThisParam(fun)(7, 8, 9).value);
+
+  EXPECT_TRUE(static_cast<bool>(fun));
+  EXPECT_EQ(38, TypeParam::ToThisParam(fun)(10, 11, 12).value);
+}
+
+template <class T>
+class AnyInvTestRvalue : public ::testing::Test {};
+
+TYPED_TEST_SUITE_P(AnyInvTestRvalue);
+
+TYPED_TEST_P(AnyInvTestRvalue, ConversionConstructionReferenceWrapper) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+  using AddType = typename TypeParam::AddType;
+
+  EXPECT_FALSE(
+      (std::is_convertible_v<std::reference_wrapper<AddType>, AnyInvType>));
+}
+
+TYPED_TEST_P(AnyInvTestRvalue, NonMoveableResultType) {
+  // Define a result type that cannot be copy- or move-constructed.
+  struct Result {
+    int x;
+
+    explicit Result(const int x_in) : x(x_in) {}
+    Result(Result&&) = delete;
+  };
+
+  static_assert(!std::is_move_constructible_v<Result>);
+  static_assert(!std::is_copy_constructible_v<Result>);
+
+  // Assumption check: it should nevertheless be possible to use functors that
+  // return a Result struct according to the language rules.
+  const auto return_17 = []() noexcept { return Result(17); };
+  EXPECT_EQ(17, return_17().x);
+
+  // Just like plain functors, it should work fine to use an AnyInvocable that
+  // returns the non-moveable type.
+  using UnqualifiedFun =
+      std::conditional_t<TypeParam::kIsNoexcept, Result() noexcept, Result()>;
+
+  using Fun =
+      GiveQualifiersToFun<typename TypeParam::Qualifiers, UnqualifiedFun>;
+
+  EXPECT_EQ(17, AnyInvocable<Fun>(return_17)().x);
+}
+
+TYPED_TEST_P(AnyInvTestRvalue, ConversionAssignReferenceWrapper) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+  using AddType = typename TypeParam::AddType;
+
+  EXPECT_FALSE(
+      (std::is_assignable_v<AnyInvType&, std::reference_wrapper<AddType>>));
+}
+
+TYPED_TEST_P(AnyInvTestRvalue, NonConstCrashesOnSecondCall) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+  using AddType = typename TypeParam::AddType;
+
+  AnyInvType fun(std::in_place_type<AddType>, 5);
+
+  EXPECT_TRUE(static_cast<bool>(fun));
+  std::move(fun)(7, 8, 9);
+
+  // Ensure we're still valid
+  EXPECT_TRUE(static_cast<bool>(fun));  // NOLINT(bugprone-use-after-move)
+
+#if !defined(NDEBUG)
+  EXPECT_DEATH_IF_SUPPORTED(std::move(fun)(7, 8, 9), "");
+#endif
+}
+
+// Ensure that any qualifiers (in particular &&-qualifiers) do not affect
+// when the destructor is actually run.
+TYPED_TEST_P(AnyInvTestRvalue, QualifierIndependentObjectLifetime) {
+  using AnyInvType = typename TypeParam::AnyInvType;
+
+  auto refs = std::make_shared<std::nullptr_t>();
+  {
+    AnyInvType fun([refs](auto&&...) noexcept { return 0; });
+    EXPECT_GT(refs.use_count(), 1);
+
+    std::move(fun)(7, 8, 9);
+
+    // Ensure destructor hasn't run even if rref-qualified
+    EXPECT_GT(refs.use_count(), 1);
+  }
+  EXPECT_EQ(refs.use_count(), 1);
+}
+
+// NOTE: This test suite originally attempted to enumerate all possible
+// combinations of type properties but the build-time started getting too large.
+// Instead, it is now assumed that certain parameters are orthogonal and so
+// some combinations are elided.
+
+// A metafunction to form a TypeList of all cv and non-rvalue ref combinations,
+// coupled with all of the other explicitly specified parameters.
+template <Movable Mov, Destructible Dest, NothrowCall CallExceptionSpec,
+          ObjSize Size, ObjAlign Align>
+using NonRvalueQualifiedTestParams = ::testing::Types<               //
+    TestParams<Mov, Dest, _, CallExceptionSpec, Size, Align>,        //
+    TestParams<Mov, Dest, const _, CallExceptionSpec, Size, Align>,  //
+    TestParams<Mov, Dest, _&, CallExceptionSpec, Size, Align>,       //
+    TestParams<Mov, Dest, const _&, CallExceptionSpec, Size, Align>>;
+
+// A metafunction to form a TypeList of const and non-const rvalue ref
+// qualifiers, coupled with all of the other explicitly specified parameters.
+template <Movable Mov, Destructible Dest, NothrowCall CallExceptionSpec,
+          ObjSize Size, ObjAlign Align>
+using RvalueQualifiedTestParams = ::testing::Types<
+    TestParams<Mov, Dest, _&&, CallExceptionSpec, Size, Align>,       //
+    TestParams<Mov, Dest, const _&&, CallExceptionSpec, Size, Align>  //
+    >;
+
+// All qualifier combinations and a noexcept function type
+using TestParameterListNonRvalueQualifiersNothrowCall =
+    NonRvalueQualifiedTestParams<Movable::trivial, Destructible::trivial,
+                                 NothrowCall::yes, ObjSize::small,
+                                 ObjAlign::normal>;
+using TestParameterListRvalueQualifiersNothrowCall =
+    RvalueQualifiedTestParams<Movable::trivial, Destructible::trivial,
+                              NothrowCall::yes, ObjSize::small,
+                              ObjAlign::normal>;
+
+// All qualifier combinations and a non-noexcept function type
+using TestParameterListNonRvalueQualifiersCallMayThrow =
+    NonRvalueQualifiedTestParams<Movable::trivial, Destructible::trivial,
+                                 NothrowCall::no, ObjSize::small,
+                                 ObjAlign::normal>;
+using TestParameterListRvalueQualifiersCallMayThrow =
+    RvalueQualifiedTestParams<Movable::trivial, Destructible::trivial,
+                              NothrowCall::no, ObjSize::small,
+                              ObjAlign::normal>;
+
+// Lists of various cases that should lead to remote storage
+using TestParameterListRemoteMovable = ::testing::Types<
+    // "Normal" aligned types that are large and have trivial destructors
+    TestParams<Movable::trivial, Destructible::trivial, _, NothrowCall::no,
+               ObjSize::large, ObjAlign::normal>,  //
+    TestParams<Movable::nothrow, Destructible::trivial, _, NothrowCall::no,
+               ObjSize::large, ObjAlign::normal>,  //
+    TestParams<Movable::yes, Destructible::trivial, _, NothrowCall::no,
+               ObjSize::small, ObjAlign::normal>,  //
+    TestParams<Movable::yes, Destructible::trivial, _, NothrowCall::no,
+               ObjSize::large, ObjAlign::normal>,  //
+
+    // Same as above but with non-trivial destructors
+    TestParams<Movable::trivial, Destructible::nothrow, _, NothrowCall::no,
+               ObjSize::large, ObjAlign::normal>,  //
+    TestParams<Movable::nothrow, Destructible::nothrow, _, NothrowCall::no,
+               ObjSize::large, ObjAlign::normal>,  //
+    TestParams<Movable::yes, Destructible::nothrow, _, NothrowCall::no,
+               ObjSize::small, ObjAlign::normal>,  //
+    TestParams<Movable::yes, Destructible::nothrow, _, NothrowCall::no,
+               ObjSize::large, ObjAlign::normal>  //
+
+    // Types that must use remote storage because of a large alignment.
+    ,
+    TestParams<Movable::trivial, Destructible::trivial, _, NothrowCall::no,
+               ObjSize::small, ObjAlign::large>,  //
+    TestParams<Movable::nothrow, Destructible::trivial, _, NothrowCall::no,
+               ObjSize::small, ObjAlign::large>,  //
+    TestParams<Movable::trivial, Destructible::nothrow, _, NothrowCall::no,
+               ObjSize::small, ObjAlign::large>,  //
+    TestParams<Movable::nothrow, Destructible::nothrow, _, NothrowCall::no,
+               ObjSize::small, ObjAlign::large>  //
+    >;
+using TestParameterListRemoteNonMovable = ::testing::Types<
+    // "Normal" aligned types that are large and have trivial destructors
+    TestParams<Movable::no, Destructible::trivial, _, NothrowCall::no,
+               ObjSize::small, ObjAlign::normal>,  //
+    TestParams<Movable::no, Destructible::trivial, _, NothrowCall::no,
+               ObjSize::large, ObjAlign::normal>,  //
+    // Same as above but with non-trivial destructors
+    TestParams<Movable::no, Destructible::nothrow, _, NothrowCall::no,
+               ObjSize::small, ObjAlign::normal>,  //
+    TestParams<Movable::no, Destructible::nothrow, _, NothrowCall::no,
+               ObjSize::large, ObjAlign::normal>  //
+    >;
+
+// Parameters that lead to local storage
+using TestParameterListLocal = ::testing::Types<
+    // Types that meet the requirements and have trivial destructors
+    TestParams<Movable::trivial, Destructible::trivial, _, NothrowCall::no,
+               ObjSize::small, ObjAlign::normal>,  //
+    TestParams<Movable::nothrow, Destructible::trivial, _, NothrowCall::no,
+               ObjSize::small, ObjAlign::normal>,  //
+
+    // Same as above but with non-trivial destructors
+    TestParams<Movable::trivial, Destructible::trivial, _, NothrowCall::no,
+               ObjSize::small, ObjAlign::normal>,  //
+    TestParams<Movable::nothrow, Destructible::trivial, _, NothrowCall::no,
+               ObjSize::small, ObjAlign::normal>  //
+    >;
+
+// All of the tests that are run for every possible combination of types.
+REGISTER_TYPED_TEST_SUITE_P(
+    AnyInvTestBasic, DefaultConstruction, ConstructionNullptr,
+    ConstructionNullFunctionPtr, ConstructionNullMemberFunctionPtr,
+    ConstructionNullMemberObjectPtr, ConstructionMemberFunctionPtr,
+    ConstructionMemberObjectPtr, ConstructionFunctionReferenceDecay,
+    ConstructionCompatibleAnyInvocableEmpty,
+    ConstructionCompatibleAnyInvocableNonempty, InPlaceConstruction,
+    ConversionToBool, Invocation, InPlaceConstructionInitializerList,
+    InPlaceNullFunPtrConstruction, InPlaceNullFunPtrConstructionValueInit,
+    InPlaceNullMemFunPtrConstruction, InPlaceNullMemFunPtrConstructionValueInit,
+    InPlaceNullMemObjPtrConstruction, InPlaceNullMemObjPtrConstructionValueInit,
+    InPlaceVoidCovarianceConstruction, MoveConstructionFromEmpty,
+    MoveConstructionFromNonEmpty, ComparisonWithNullptrEmpty,
+    ComparisonWithNullptrNonempty, ResultType);
+
+// Tests for functions that take two operands.
+REGISTER_TYPED_TEST_SUITE_P(
+    AnyInvTestCombinatoric, MoveAssignEmptyEmptyLhsRhs,
+    MoveAssignEmptyLhsNonemptyRhs, MoveAssignNonemptyEmptyLhsRhs,
+    MoveAssignNonemptyLhsNonemptyRhs, SelfMoveAssignEmpty,
+    SelfMoveAssignNonempty, AssignNullptrEmptyLhs,
+    AssignNullFunctionPtrEmptyLhs, AssignNullMemberFunctionPtrEmptyLhs,
+    AssignNullMemberObjectPtrEmptyLhs, AssignMemberFunctionPtrEmptyLhs,
+    AssignMemberObjectPtrEmptyLhs, AssignFunctionReferenceDecayEmptyLhs,
+    AssignCompatibleAnyInvocableEmptyLhsEmptyRhs,
+    AssignCompatibleAnyInvocableEmptyLhsNonemptyRhs, AssignNullptrNonemptyLhs,
+    AssignNullFunctionPtrNonemptyLhs, AssignNullMemberFunctionPtrNonemptyLhs,
+    AssignNullMemberObjectPtrNonemptyLhs, AssignMemberFunctionPtrNonemptyLhs,
+    AssignMemberObjectPtrNonemptyLhs, AssignFunctionReferenceDecayNonemptyLhs,
+    AssignCompatibleAnyInvocableNonemptyLhsEmptyRhs,
+    AssignCompatibleAnyInvocableNonemptyLhsNonemptyRhs, SwapEmptyLhsEmptyRhs,
+    SwapEmptyLhsNonemptyRhs, SwapNonemptyLhsEmptyRhs,
+    SwapNonemptyLhsNonemptyRhs);
+
+REGISTER_TYPED_TEST_SUITE_P(AnyInvTestMovable,
+                            ConversionConstructionUserDefinedType,
+                            ConversionConstructionVoidCovariance,
+                            ConversionAssignUserDefinedTypeEmptyLhs,
+                            ConversionAssignUserDefinedTypeNonemptyLhs,
+                            ConversionAssignVoidCovariance);
+
+REGISTER_TYPED_TEST_SUITE_P(AnyInvTestNoexceptFalse,
+                            ConversionConstructionConstraints,
+                            ConversionAssignConstraints);
+
+REGISTER_TYPED_TEST_SUITE_P(AnyInvTestNoexceptTrue,
+                            ConversionConstructionConstraints,
+                            ConversionAssignConstraints);
+
+REGISTER_TYPED_TEST_SUITE_P(AnyInvTestNonRvalue,
+                            ConversionConstructionReferenceWrapper,
+                            NonMoveableResultType,
+                            ConversionAssignReferenceWrapperEmptyLhs,
+                            ConversionAssignReferenceWrapperNonemptyLhs);
+
+REGISTER_TYPED_TEST_SUITE_P(AnyInvTestRvalue,
+                            ConversionConstructionReferenceWrapper,
+                            NonMoveableResultType,
+                            ConversionAssignReferenceWrapper,
+                            NonConstCrashesOnSecondCall,
+                            QualifierIndependentObjectLifetime);
+
+// Minimal SFINAE testing for platforms where we can't run the tests, but we can
+// build binaries for.
+static_assert(std::is_convertible_v<void (*)(), absl::AnyInvocable<void() &&>>);
+static_assert(!std::is_convertible_v<void*, absl::AnyInvocable<void() &&>>);
+
+}  // namespace absl_any_invocable_test
+
+#endif  // ABSL_FUNCTIONAL_ANY_INVOCABLE_TEST_H_
diff --git a/absl/functional/any_invocable_test_inst1.cc b/absl/functional/any_invocable_test_inst1.cc
new file mode 100644
index 00000000000..68a5955fa48
--- /dev/null
+++ b/absl/functional/any_invocable_test_inst1.cc
@@ -0,0 +1,60 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// To prevent compiler memory exhaustion (OOM / Killed signal terminates
+// cc1plus) during parallel builds with GCC, the test suite instantiations have
+// been split into multiple compilation units.
+
+// SKIP_ABSL_INLINE_NAMESPACE_CHECK
+
+#include "absl/functional/any_invocable_test.h"
+
+namespace absl_any_invocable_test {
+
+INSTANTIATE_TYPED_TEST_SUITE_P(
+    NonRvalueCallMayThrow, AnyInvTestBasic,
+    TestParameterListNonRvalueQualifiersCallMayThrow);
+
+INSTANTIATE_TYPED_TEST_SUITE_P(RvalueCallMayThrow, AnyInvTestBasic,
+                               TestParameterListRvalueQualifiersCallMayThrow);
+
+INSTANTIATE_TYPED_TEST_SUITE_P(
+    NonRvalueCallMayThrow, AnyInvTestCombinatoric,
+    TestParameterListNonRvalueQualifiersCallMayThrow);
+
+INSTANTIATE_TYPED_TEST_SUITE_P(RvalueCallMayThrow, AnyInvTestCombinatoric,
+                               TestParameterListRvalueQualifiersCallMayThrow);
+
+INSTANTIATE_TYPED_TEST_SUITE_P(
+    NonRvalueCallMayThrow, AnyInvTestMovable,
+    TestParameterListNonRvalueQualifiersCallMayThrow);
+
+INSTANTIATE_TYPED_TEST_SUITE_P(RvalueCallMayThrow, AnyInvTestMovable,
+                               TestParameterListRvalueQualifiersCallMayThrow);
+
+INSTANTIATE_TYPED_TEST_SUITE_P(
+    NonRvalueCallMayThrow, AnyInvTestNoexceptFalse,
+    TestParameterListNonRvalueQualifiersCallMayThrow);
+
+INSTANTIATE_TYPED_TEST_SUITE_P(RvalueCallMayThrow, AnyInvTestNoexceptFalse,
+                               TestParameterListRvalueQualifiersCallMayThrow);
+
+INSTANTIATE_TYPED_TEST_SUITE_P(
+    NonRvalueCallMayThrow, AnyInvTestNonRvalue,
+    TestParameterListNonRvalueQualifiersCallMayThrow);
+
+INSTANTIATE_TYPED_TEST_SUITE_P(RvalueCallMayThrow, AnyInvTestRvalue,
+                               TestParameterListRvalueQualifiersCallMayThrow);
+
+}  // namespace absl_any_invocable_test
diff --git a/absl/functional/any_invocable_test_inst2.cc b/absl/functional/any_invocable_test_inst2.cc
new file mode 100644
index 00000000000..5b5f22a25d3
--- /dev/null
+++ b/absl/functional/any_invocable_test_inst2.cc
@@ -0,0 +1,40 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// To prevent compiler memory exhaustion (OOM / Killed signal terminates
+// cc1plus) during parallel builds with GCC, the test suite instantiations have
+// been split into multiple compilation units.
+
+// SKIP_ABSL_INLINE_NAMESPACE_CHECK
+
+#include "absl/functional/any_invocable_test.h"
+
+namespace absl_any_invocable_test {
+
+INSTANTIATE_TYPED_TEST_SUITE_P(RemoteMovable, AnyInvTestBasic,
+                               TestParameterListRemoteMovable);
+
+INSTANTIATE_TYPED_TEST_SUITE_P(RemoteMovable, AnyInvTestCombinatoric,
+                               TestParameterListRemoteMovable);
+
+INSTANTIATE_TYPED_TEST_SUITE_P(RemoteMovable, AnyInvTestMovable,
+                               TestParameterListRemoteMovable);
+
+INSTANTIATE_TYPED_TEST_SUITE_P(RemoteMovable, AnyInvTestNoexceptFalse,
+                               TestParameterListRemoteMovable);
+
+INSTANTIATE_TYPED_TEST_SUITE_P(RemoteMovable, AnyInvTestNonRvalue,
+                               TestParameterListRemoteMovable);
+
+}  // namespace absl_any_invocable_test
diff --git a/absl/functional/any_invocable_test_inst3.cc b/absl/functional/any_invocable_test_inst3.cc
new file mode 100644
index 00000000000..ba23e6391dc
--- /dev/null
+++ b/absl/functional/any_invocable_test_inst3.cc
@@ -0,0 +1,55 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// To prevent compiler memory exhaustion (OOM / Killed signal terminates
+// cc1plus) during parallel builds with GCC, the test suite instantiations have
+// been split into multiple compilation units.
+
+// SKIP_ABSL_INLINE_NAMESPACE_CHECK
+
+#include "absl/functional/any_invocable_test.h"
+
+namespace absl_any_invocable_test {
+
+INSTANTIATE_TYPED_TEST_SUITE_P(NonRvalueCallNothrow, AnyInvTestBasic,
+                               TestParameterListNonRvalueQualifiersNothrowCall);
+
+INSTANTIATE_TYPED_TEST_SUITE_P(CallNothrowRvalue, AnyInvTestBasic,
+                               TestParameterListRvalueQualifiersNothrowCall);
+
+INSTANTIATE_TYPED_TEST_SUITE_P(NonRvalueCallNothrow, AnyInvTestCombinatoric,
+                               TestParameterListNonRvalueQualifiersNothrowCall);
+
+INSTANTIATE_TYPED_TEST_SUITE_P(RvalueCallNothrow, AnyInvTestCombinatoric,
+                               TestParameterListRvalueQualifiersNothrowCall);
+
+INSTANTIATE_TYPED_TEST_SUITE_P(NonRvalueCallNothrow, AnyInvTestMovable,
+                               TestParameterListNonRvalueQualifiersNothrowCall);
+
+INSTANTIATE_TYPED_TEST_SUITE_P(RvalueCallNothrow, AnyInvTestMovable,
+                               TestParameterListRvalueQualifiersNothrowCall);
+
+INSTANTIATE_TYPED_TEST_SUITE_P(NonRvalueCallNothrow, AnyInvTestNoexceptTrue,
+                               TestParameterListNonRvalueQualifiersNothrowCall);
+
+INSTANTIATE_TYPED_TEST_SUITE_P(RvalueCallNothrow, AnyInvTestNoexceptTrue,
+                               TestParameterListRvalueQualifiersNothrowCall);
+
+INSTANTIATE_TYPED_TEST_SUITE_P(NonRvalueCallNothrow, AnyInvTestNonRvalue,
+                               TestParameterListNonRvalueQualifiersNothrowCall);
+
+INSTANTIATE_TYPED_TEST_SUITE_P(CallNothrowRvalue, AnyInvTestRvalue,
+                               TestParameterListRvalueQualifiersNothrowCall);
+
+}  // namespace absl_any_invocable_test
diff --git a/absl/functional/any_invocable_test_inst4.cc b/absl/functional/any_invocable_test_inst4.cc
new file mode 100644
index 00000000000..0549a1e1291
--- /dev/null
+++ b/absl/functional/any_invocable_test_inst4.cc
@@ -0,0 +1,51 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// To prevent compiler memory exhaustion (OOM / Killed signal terminates
+// cc1plus) during parallel builds with GCC, the test suite instantiations have
+// been split into multiple compilation units.
+
+// SKIP_ABSL_INLINE_NAMESPACE_CHECK
+
+#include "absl/functional/any_invocable_test.h"
+
+namespace absl_any_invocable_test {
+
+INSTANTIATE_TYPED_TEST_SUITE_P(RemoteNonMovable, AnyInvTestBasic,
+                               TestParameterListRemoteNonMovable);
+
+INSTANTIATE_TYPED_TEST_SUITE_P(Local, AnyInvTestBasic, TestParameterListLocal);
+
+INSTANTIATE_TYPED_TEST_SUITE_P(RemoteNonMovable, AnyInvTestCombinatoric,
+                               TestParameterListRemoteNonMovable);
+
+INSTANTIATE_TYPED_TEST_SUITE_P(Local, AnyInvTestCombinatoric,
+                               TestParameterListLocal);
+
+INSTANTIATE_TYPED_TEST_SUITE_P(Local, AnyInvTestMovable,
+                               TestParameterListLocal);
+
+INSTANTIATE_TYPED_TEST_SUITE_P(RemoteNonMovable, AnyInvTestNoexceptFalse,
+                               TestParameterListRemoteNonMovable);
+
+INSTANTIATE_TYPED_TEST_SUITE_P(Local, AnyInvTestNoexceptFalse,
+                               TestParameterListLocal);
+
+INSTANTIATE_TYPED_TEST_SUITE_P(RemoteNonMovable, AnyInvTestNonRvalue,
+                               TestParameterListRemoteNonMovable);
+
+INSTANTIATE_TYPED_TEST_SUITE_P(Local, AnyInvTestNonRvalue,
+                               TestParameterListLocal);
+
+}  // namespace absl_any_invocable_test
diff --git a/absl/functional/bind_back.h b/absl/functional/bind_back.h
new file mode 100644
index 00000000000..8d9ff7cd3c8
--- /dev/null
+++ b/absl/functional/bind_back.h
@@ -0,0 +1,77 @@
+// Copyright 2026 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: bind_back.h
+// -----------------------------------------------------------------------------
+//
+// `absl::bind_back()` returns a functor by binding a number of arguments to
+// the back of a provided (usually more generic) functor. Unlike `std::bind`,
+// it does not require the use of argument placeholders. The simpler syntax of
+// `absl::bind_back()` allows you to avoid known misuses with `std::bind()`.
+//
+// `absl::bind_back()` is meant as a drop-in replacement for C++23's
+// `std::bind_back()`, which similarly resolves these issues with
+// `std::bind()`. Both `bind_back()` alternatives, unlike `std::bind()`, allow
+// partial function application. (See
+// https://en.wikipedia.org/wiki/Partial_application).
+
+#ifndef ABSL_FUNCTIONAL_BIND_BACK_H_
+#define ABSL_FUNCTIONAL_BIND_BACK_H_
+
+#if __has_include(<version>)
+#include <version>
+#endif
+
+#if defined(__cpp_lib_bind_back) && __cpp_lib_bind_back >= 202202L
+#include <functional>  // For std::bind_back.
+#endif
+
+#include <utility>
+
+#include "absl/base/config.h"
+#include "absl/functional/internal/back_binder.h"
+#include "absl/utility/utility.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+// bind_back()
+//
+// Binds the last N arguments of an invocable object and stores them by value.
+//
+// Like `std::bind()`, `absl::bind_back()` is implicitly convertible to
+// `std::function`.  In particular, it may be used as a simpler replacement for
+// `std::bind()` in most cases, as it does not require placeholders to be
+// specified. More importantly, it provides more reliable correctness guarantees
+// than `std::bind()`; while `std::bind()` will silently ignore passing more
+// parameters than expected, for example, `absl::bind_back()` will report such
+// mis-uses as errors. In C++23, `absl::bind_back` is replaced by
+// `std::bind_back`.
+//
+#if defined(__cpp_lib_bind_back) && __cpp_lib_bind_back >= 202202L
+using std::bind_back;
+#else
+template <class F, class... BoundArgs>
+constexpr functional_internal::bind_back_t<F, BoundArgs...> bind_back(
+    F&& func, BoundArgs&&... args) {
+  return functional_internal::bind_back_t<F, BoundArgs...>(
+      std::in_place, std::forward<F>(func), std::forward<BoundArgs>(args)...);
+}
+#endif
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_FUNCTIONAL_BIND_BACK_H_
diff --git a/absl/functional/bind_back_test.cc b/absl/functional/bind_back_test.cc
new file mode 100644
index 00000000000..77accc4dbbf
--- /dev/null
+++ b/absl/functional/bind_back_test.cc
@@ -0,0 +1,237 @@
+// Copyright 2026 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/functional/bind_back.h"
+
+#include <stddef.h>
+
+#include <functional>
+#include <memory>
+#include <string>
+#include <utility>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/memory/memory.h"
+
+namespace {
+
+char CharAt(const char* s, size_t index) { return s[index]; }
+
+TEST(BindTest, Basics) {
+  EXPECT_EQ('C', absl::bind_back(CharAt)("ABC", 2));
+  EXPECT_EQ('C', absl::bind_back(CharAt, 2)("ABC"));
+  EXPECT_EQ('C', absl::bind_back(CharAt, "ABC", 2)());
+}
+
+TEST(BindTest, Lambda) {
+  auto lambda = [](int x, int y, int z) { return x + y + z; };
+  EXPECT_EQ(6, absl::bind_back(lambda)(1, 2, 3));
+  EXPECT_EQ(6, absl::bind_back(lambda, 3)(1, 2));
+  EXPECT_EQ(6, absl::bind_back(lambda, 2, 3)(1));
+  EXPECT_EQ(6, absl::bind_back(lambda, 1, 2, 3)());
+}
+
+struct Functor {
+  std::string operator()() & { return "&"; }
+  std::string operator()() const& { return "const&"; }
+  std::string operator()() && { return "&&"; }
+  std::string operator()() const&& { return "const&&"; }
+};
+
+TEST(BindTest, PerfectForwardingOfBoundArgs) {
+  auto f = absl::bind_back(Functor());
+  const auto& cf = f;
+  EXPECT_EQ("&", f());
+  EXPECT_EQ("const&", cf());
+  EXPECT_EQ("&&", std::move(f)());
+  EXPECT_EQ("const&&", std::move(cf)());
+}
+
+struct ArgDescribe {
+  std::string operator()(int&) const { return "&"; }             // NOLINT
+  std::string operator()(const int&) const { return "const&"; }  // NOLINT
+  std::string operator()(int&&) const { return "&&"; }
+  std::string operator()(const int&&) const { return "const&&"; }
+};
+
+TEST(BindTest, PerfectForwardingOfFreeArgs) {
+  ArgDescribe f;
+  int i;
+  EXPECT_EQ("&", absl::bind_back(f)(static_cast<int&>(i)));
+  EXPECT_EQ("const&", absl::bind_back(f)(static_cast<const int&>(i)));
+  EXPECT_EQ("&&", absl::bind_back(f)(static_cast<int&&>(i)));
+  EXPECT_EQ("const&&", absl::bind_back(f)(static_cast<const int&&>(i)));
+}
+
+struct NonCopyableFunctor {
+  NonCopyableFunctor() = default;
+  NonCopyableFunctor(const NonCopyableFunctor&) = delete;
+  NonCopyableFunctor& operator=(const NonCopyableFunctor&) = delete;
+  const NonCopyableFunctor* operator()() const { return this; }
+};
+
+TEST(BindTest, RefToFunctor) {
+  // It won't copy/move the functor and use the original object.
+  NonCopyableFunctor ncf;
+  auto bound_ncf = absl::bind_back(std::ref(ncf));
+  auto bound_ncf_copy = bound_ncf;
+  EXPECT_EQ(&ncf, bound_ncf_copy());
+}
+
+struct Struct {
+  std::string value;
+};
+
+TEST(BindTest, StoreByCopy) {
+  Struct s = {"hello"};
+  auto f = absl::bind_back(&Struct::value, s);
+  auto g = f;
+  EXPECT_EQ("hello", f());
+  EXPECT_EQ("hello", g());
+  EXPECT_NE(&s.value, &f());
+  EXPECT_NE(&s.value, &g());
+  EXPECT_NE(&g(), &f());
+}
+
+struct NonCopyable {
+  explicit NonCopyable(const std::string& s) : value(s) {}
+  NonCopyable(const NonCopyable&) = delete;
+  NonCopyable& operator=(const NonCopyable&) = delete;
+
+  std::string value;
+};
+
+const std::string& GetNonCopyableValue(const NonCopyable& n) { return n.value; }
+
+TEST(BindTest, StoreByRef) {
+  NonCopyable s("hello");
+  auto f = absl::bind_back(&GetNonCopyableValue, std::ref(s));
+  EXPECT_EQ("hello", f());
+  EXPECT_EQ(&s.value, &f());
+  auto g = std::move(f);  // NOLINT
+  EXPECT_EQ("hello", g());
+  EXPECT_EQ(&s.value, &g());
+  s.value = "goodbye";
+  EXPECT_EQ("goodbye", g());
+}
+
+TEST(BindTest, StoreByCRef) {
+  NonCopyable s("hello");
+  auto f = absl::bind_back(&GetNonCopyableValue, std::cref(s));
+  EXPECT_EQ("hello", f());
+  EXPECT_EQ(&s.value, &f());
+  auto g = std::move(f);  // NOLINT
+  EXPECT_EQ("hello", g());
+  EXPECT_EQ(&s.value, &g());
+  s.value = "goodbye";
+  EXPECT_EQ("goodbye", g());
+}
+
+const std::string& GetNonCopyableValueByWrapper(
+    std::reference_wrapper<NonCopyable> n) {
+  return n.get().value;
+}
+
+TEST(BindTest, StoreByRefInvokeByWrapper) {
+  NonCopyable s("hello");
+  auto f = absl::bind_back(GetNonCopyableValueByWrapper, std::ref(s));
+  EXPECT_EQ("hello", f());
+  EXPECT_EQ(&s.value, &f());
+  auto g = std::move(f);
+  EXPECT_EQ("hello", g());
+  EXPECT_EQ(&s.value, &g());
+  s.value = "goodbye";
+  EXPECT_EQ("goodbye", g());
+}
+
+TEST(BindTest, StoreByPointer) {
+  NonCopyable s("hello");
+  auto f = absl::bind_back(&NonCopyable::value, &s);
+  EXPECT_EQ("hello", f());
+  EXPECT_EQ(&s.value, &f());
+  auto g = std::move(f);
+  EXPECT_EQ("hello", g());
+  EXPECT_EQ(&s.value, &g());
+}
+
+struct MyStruct {
+  int x;
+  int Add(int y) const { return x + y; }
+};
+
+TEST(BindTest, MemberFunctionFreeInstance) {
+  MyStruct s{10};
+  auto f = absl::bind_back(&MyStruct::Add, 5);
+  EXPECT_EQ(f(s), 15);
+}
+
+int Sink(std::unique_ptr<int> p) { return *p; }
+
+std::unique_ptr<int> Factory(int n) { return std::make_unique<int>(n); }
+
+TEST(BindTest, NonCopyableArg) {
+  EXPECT_EQ(42, absl::bind_back(Sink)(std::make_unique<int>(42)));
+  EXPECT_EQ(42, absl::bind_back(Sink, std::make_unique<int>(42))());
+}
+
+TEST(BindTest, NonCopyableResult) {
+  EXPECT_THAT(absl::bind_back(Factory)(42), ::testing::Pointee(42));
+  EXPECT_THAT(absl::bind_back(Factory, 42)(), ::testing::Pointee(42));
+}
+
+// is_copy_constructible<FalseCopyable<unique_ptr<T>> is true but an attempt to
+// instantiate the copy constructor leads to a compile error. This is similar
+// to how standard containers behave.
+template <class T>
+struct FalseCopyable {
+  FalseCopyable() = default;
+  FalseCopyable(const FalseCopyable& other) : m(other.m) {}
+  FalseCopyable(FalseCopyable&& other) : m(std::move(other.m)) {}
+  T m;
+};
+
+int GetMember(FalseCopyable<std::unique_ptr<int>> x) { return *x.m; }
+
+TEST(BindTest, WrappedMoveOnly) {
+  FalseCopyable<std::unique_ptr<int>> x;
+  x.m = std::make_unique<int>(42);
+  auto f = absl::bind_back(&GetMember, std::move(x));
+  EXPECT_EQ(42, std::move(f)());
+}
+
+int Plus(int a, int b) { return a + b; }
+
+TEST(BindTest, ConstExpr) {
+  constexpr auto f = absl::bind_back(CharAt);
+  EXPECT_EQ(f("ABC", 1), 'B');
+  static constexpr int five = 5;
+  constexpr auto plus5 = absl::bind_back(Plus, five);
+  EXPECT_EQ(plus5(1), 6);
+
+  static constexpr char data[] = "DEF";
+  constexpr auto g = absl::bind_back(CharAt, 1);
+  EXPECT_EQ(g(data), 'E');
+}
+
+struct ManglingCall {
+  int operator()(int, double, std::string) const { return 0; }
+};
+
+TEST(BindTest, Mangling) {
+  // We just want to generate a particular instantiation to see its mangling.
+  absl::bind_back(ManglingCall{}, 3.3, "A")(1);
+}
+
+}  // namespace
diff --git a/absl/functional/bind_front.h b/absl/functional/bind_front.h
new file mode 100644
index 00000000000..4dcee393f02
--- /dev/null
+++ b/absl/functional/bind_front.h
@@ -0,0 +1,198 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: bind_front.h
+// -----------------------------------------------------------------------------
+//
+// `absl::bind_front()` returns a functor by binding a number of arguments to
+// the front of a provided (usually more generic) functor. Unlike `std::bind`,
+// it does not require the use of argument placeholders. The simpler syntax of
+// `absl::bind_front()` allows you to avoid known misuses with `std::bind()`.
+//
+// `absl::bind_front()` is meant as a drop-in replacement for C++20's upcoming
+// `std::bind_front()`, which similarly resolves these issues with
+// `std::bind()`. Both `bind_front()` alternatives, unlike `std::bind()`, allow
+// partial function application. (See
+// https://en.wikipedia.org/wiki/Partial_application).
+
+#ifndef ABSL_FUNCTIONAL_BIND_FRONT_H_
+#define ABSL_FUNCTIONAL_BIND_FRONT_H_
+
+#if __has_include(<version>)
+#include <version>
+#endif
+
+#if defined(__cpp_lib_bind_front) && __cpp_lib_bind_front >= 201907L
+#include <functional>  // For std::bind_front.
+#endif  // defined(__cpp_lib_bind_front) && __cpp_lib_bind_front >= 201907L
+
+#include <utility>
+
+#include "absl/functional/internal/front_binder.h"
+#include "absl/utility/utility.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+// bind_front()
+//
+// Binds the first N arguments of an invocable object and stores them by value.
+//
+// Like `std::bind()`, `absl::bind_front()` is implicitly convertible to
+// `std::function`.  In particular, it may be used as a simpler replacement for
+// `std::bind()` in most cases, as it does not require placeholders to be
+// specified. More importantly, it provides more reliable correctness guarantees
+// than `std::bind()`; while `std::bind()` will silently ignore passing more
+// parameters than expected, for example, `absl::bind_front()` will report such
+// mis-uses as errors. In C++20, `absl::bind_front` is replaced by
+// `std::bind_front`.
+//
+// absl::bind_front(a...) can be seen as storing the results of
+// std::make_tuple(a...).
+//
+// Example: Binding a free function.
+//
+//   int Minus(int a, int b) { return a - b; }
+//
+//   assert(absl::bind_front(Minus)(3, 2) == 3 - 2);
+//   assert(absl::bind_front(Minus, 3)(2) == 3 - 2);
+//   assert(absl::bind_front(Minus, 3, 2)() == 3 - 2);
+//
+// Example: Binding a member function.
+//
+//   struct Math {
+//     int Double(int a) const { return 2 * a; }
+//   };
+//
+//   Math math;
+//
+//   assert(absl::bind_front(&Math::Double)(&math, 3) == 2 * 3);
+//   // Stores a pointer to math inside the functor.
+//   assert(absl::bind_front(&Math::Double, &math)(3) == 2 * 3);
+//   // Stores a copy of math inside the functor.
+//   assert(absl::bind_front(&Math::Double, math)(3) == 2 * 3);
+//   // Stores std::unique_ptr<Math> inside the functor.
+//   assert(absl::bind_front(&Math::Double,
+//                           std::unique_ptr<Math>(new Math))(3) == 2 * 3);
+//
+// Example: Using `absl::bind_front()`, instead of `std::bind()`, with
+//          `std::function`.
+//
+//   class FileReader {
+//    public:
+//     void ReadFileAsync(const std::string& filename, std::string* content,
+//                        const std::function<void()>& done) {
+//       // Calls Executor::Schedule(std::function<void()>).
+//       Executor::DefaultExecutor()->Schedule(
+//           absl::bind_front(&FileReader::BlockingRead, this,
+//                            filename, content, done));
+//     }
+//
+//    private:
+//     void BlockingRead(const std::string& filename, std::string* content,
+//                       const std::function<void()>& done) {
+//       CHECK_OK(file::GetContents(filename, content, {}));
+//       done();
+//     }
+//   };
+//
+// `absl::bind_front()` stores bound arguments explicitly using the type passed
+// rather than implicitly based on the type accepted by its functor.
+//
+// Example: Binding arguments explicitly.
+//
+//   void LogStringView(absl::string_view sv) {
+//     LOG(INFO) << sv;
+//   }
+//
+//   Executor* e = Executor::DefaultExecutor();
+//   std::string s = "hello";
+//   absl::string_view sv = s;
+//
+//   // absl::bind_front(LogStringView, arg) makes a copy of arg and stores it.
+//   e->Schedule(absl::bind_front(LogStringView, sv)); // ERROR: dangling
+//                                                     // string_view.
+//
+//   e->Schedule(absl::bind_front(LogStringView, s));  // OK: stores a copy of
+//                                                     // s.
+//
+// To store some of the arguments passed to `absl::bind_front()` by reference,
+//  use std::ref()` and `std::cref()`.
+//
+// Example: Storing some of the bound arguments by reference.
+//
+//   class Service {
+//    public:
+//     void Serve(const Request& req, std::function<void()>* done) {
+//       // The request protocol buffer won't be deleted until done is called.
+//       // It's safe to store a reference to it inside the functor.
+//       Executor::DefaultExecutor()->Schedule(
+//           absl::bind_front(&Service::BlockingServe, this, std::cref(req),
+//           done));
+//     }
+//
+//    private:
+//     void BlockingServe(const Request& req, std::function<void()>* done);
+//   };
+//
+// Example: Storing bound arguments by reference.
+//
+//   void Print(const std::string& a, const std::string& b) {
+//     std::cerr << a << b;
+//   }
+//
+//   std::string hi = "Hello, ";
+//   std::vector<std::string> names = {"Chuk", "Gek"};
+//   // Doesn't copy hi.
+//   for_each(names.begin(), names.end(),
+//            absl::bind_front(Print, std::ref(hi)));
+//
+//   // DO NOT DO THIS: the functor may outlive "hi", resulting in
+//   // dangling references.
+//   foo->DoInFuture(absl::bind_front(Print, std::ref(hi), "Guest"));  // BAD!
+//   auto f = absl::bind_front(Print, std::ref(hi), "Guest"); // BAD!
+//
+// Example: Storing reference-like types.
+//
+//   void Print(absl::string_view a, const std::string& b) {
+//     std::cerr << a << b;
+//   }
+//
+//   std::string hi = "Hello, ";
+//   // Copies "hi".
+//   absl::bind_front(Print, hi)("Chuk");
+//
+//   // Compile error: std::reference_wrapper<const string> is not implicitly
+//   // convertible to string_view.
+//   // absl::bind_front(Print, std::cref(hi))("Chuk");
+//
+//   // Doesn't copy "hi".
+//   absl::bind_front(Print, absl::string_view(hi))("Chuk");
+//
+#if defined(__cpp_lib_bind_front) && __cpp_lib_bind_front >= 201907L
+using std::bind_front;
+#else   // defined(__cpp_lib_bind_front) && __cpp_lib_bind_front >= 201907L
+template <class F, class... BoundArgs>
+constexpr functional_internal::bind_front_t<F, BoundArgs...> bind_front(
+    F&& func, BoundArgs&&... args) {
+  return functional_internal::bind_front_t<F, BoundArgs...>(
+      std::in_place, std::forward<F>(func), std::forward<BoundArgs>(args)...);
+}
+#endif  // defined(__cpp_lib_bind_front) && __cpp_lib_bind_front >= 201907L
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_FUNCTIONAL_BIND_FRONT_H_
diff --git a/absl/functional/bind_front_test.cc b/absl/functional/bind_front_test.cc
new file mode 100644
index 00000000000..a759dbe775c
--- /dev/null
+++ b/absl/functional/bind_front_test.cc
@@ -0,0 +1,227 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/functional/bind_front.h"
+
+#include <stddef.h>
+
+#include <functional>
+#include <memory>
+#include <string>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/memory/memory.h"
+
+namespace {
+
+char CharAt(const char* s, size_t index) { return s[index]; }
+
+TEST(BindTest, Basics) {
+  EXPECT_EQ('C', absl::bind_front(CharAt)("ABC", 2));
+  EXPECT_EQ('C', absl::bind_front(CharAt, "ABC")(2));
+  EXPECT_EQ('C', absl::bind_front(CharAt, "ABC", 2)());
+}
+
+TEST(BindTest, Lambda) {
+  auto lambda = [](int x, int y, int z) { return x + y + z; };
+  EXPECT_EQ(6, absl::bind_front(lambda)(1, 2, 3));
+  EXPECT_EQ(6, absl::bind_front(lambda, 1)(2, 3));
+  EXPECT_EQ(6, absl::bind_front(lambda, 1, 2)(3));
+  EXPECT_EQ(6, absl::bind_front(lambda, 1, 2, 3)());
+}
+
+struct Functor {
+  std::string operator()() & { return "&"; }
+  std::string operator()() const& { return "const&"; }
+  std::string operator()() && { return "&&"; }
+  std::string operator()() const&& { return "const&&"; }
+};
+
+TEST(BindTest, PerfectForwardingOfBoundArgs) {
+  auto f = absl::bind_front(Functor());
+  const auto& cf = f;
+  EXPECT_EQ("&", f());
+  EXPECT_EQ("const&", cf());
+  EXPECT_EQ("&&", std::move(f)());
+  EXPECT_EQ("const&&", std::move(cf)());
+}
+
+struct ArgDescribe {
+  std::string operator()(int&) const { return "&"; }             // NOLINT
+  std::string operator()(const int&) const { return "const&"; }  // NOLINT
+  std::string operator()(int&&) const { return "&&"; }
+  std::string operator()(const int&&) const { return "const&&"; }
+};
+
+TEST(BindTest, PerfectForwardingOfFreeArgs) {
+  ArgDescribe f;
+  int i;
+  EXPECT_EQ("&", absl::bind_front(f)(static_cast<int&>(i)));
+  EXPECT_EQ("const&", absl::bind_front(f)(static_cast<const int&>(i)));
+  EXPECT_EQ("&&", absl::bind_front(f)(static_cast<int&&>(i)));
+  EXPECT_EQ("const&&", absl::bind_front(f)(static_cast<const int&&>(i)));
+}
+
+struct NonCopyableFunctor {
+  NonCopyableFunctor() = default;
+  NonCopyableFunctor(const NonCopyableFunctor&) = delete;
+  NonCopyableFunctor& operator=(const NonCopyableFunctor&) = delete;
+  const NonCopyableFunctor* operator()() const { return this; }
+};
+
+TEST(BindTest, RefToFunctor) {
+  // It won't copy/move the functor and use the original object.
+  NonCopyableFunctor ncf;
+  auto bound_ncf = absl::bind_front(std::ref(ncf));
+  auto bound_ncf_copy = bound_ncf;
+  EXPECT_EQ(&ncf, bound_ncf_copy());
+}
+
+struct Struct {
+  std::string value;
+};
+
+TEST(BindTest, StoreByCopy) {
+  Struct s = {"hello"};
+  auto f = absl::bind_front(&Struct::value, s);
+  auto g = f;
+  EXPECT_EQ("hello", f());
+  EXPECT_EQ("hello", g());
+  EXPECT_NE(&s.value, &f());
+  EXPECT_NE(&s.value, &g());
+  EXPECT_NE(&g(), &f());
+}
+
+struct NonCopyable {
+  explicit NonCopyable(const std::string& s) : value(s) {}
+  NonCopyable(const NonCopyable&) = delete;
+  NonCopyable& operator=(const NonCopyable&) = delete;
+
+  std::string value;
+};
+
+const std::string& GetNonCopyableValue(const NonCopyable& n) { return n.value; }
+
+TEST(BindTest, StoreByRef) {
+  NonCopyable s("hello");
+  auto f = absl::bind_front(&GetNonCopyableValue, std::ref(s));
+  EXPECT_EQ("hello", f());
+  EXPECT_EQ(&s.value, &f());
+  auto g = std::move(f);  // NOLINT
+  EXPECT_EQ("hello", g());
+  EXPECT_EQ(&s.value, &g());
+  s.value = "goodbye";
+  EXPECT_EQ("goodbye", g());
+}
+
+TEST(BindTest, StoreByCRef) {
+  NonCopyable s("hello");
+  auto f = absl::bind_front(&GetNonCopyableValue, std::cref(s));
+  EXPECT_EQ("hello", f());
+  EXPECT_EQ(&s.value, &f());
+  auto g = std::move(f);  // NOLINT
+  EXPECT_EQ("hello", g());
+  EXPECT_EQ(&s.value, &g());
+  s.value = "goodbye";
+  EXPECT_EQ("goodbye", g());
+}
+
+const std::string& GetNonCopyableValueByWrapper(
+    std::reference_wrapper<NonCopyable> n) {
+  return n.get().value;
+}
+
+TEST(BindTest, StoreByRefInvokeByWrapper) {
+  NonCopyable s("hello");
+  auto f = absl::bind_front(GetNonCopyableValueByWrapper, std::ref(s));
+  EXPECT_EQ("hello", f());
+  EXPECT_EQ(&s.value, &f());
+  auto g = std::move(f);
+  EXPECT_EQ("hello", g());
+  EXPECT_EQ(&s.value, &g());
+  s.value = "goodbye";
+  EXPECT_EQ("goodbye", g());
+}
+
+TEST(BindTest, StoreByPointer) {
+  NonCopyable s("hello");
+  auto f = absl::bind_front(&NonCopyable::value, &s);
+  EXPECT_EQ("hello", f());
+  EXPECT_EQ(&s.value, &f());
+  auto g = std::move(f);
+  EXPECT_EQ("hello", g());
+  EXPECT_EQ(&s.value, &g());
+}
+
+int Sink(std::unique_ptr<int> p) {
+  return *p;
+}
+
+std::unique_ptr<int> Factory(int n) { return std::make_unique<int>(n); }
+
+TEST(BindTest, NonCopyableArg) {
+  EXPECT_EQ(42, absl::bind_front(Sink)(std::make_unique<int>(42)));
+  EXPECT_EQ(42, absl::bind_front(Sink, std::make_unique<int>(42))());
+}
+
+TEST(BindTest, NonCopyableResult) {
+  EXPECT_THAT(absl::bind_front(Factory)(42), ::testing::Pointee(42));
+  EXPECT_THAT(absl::bind_front(Factory, 42)(), ::testing::Pointee(42));
+}
+
+// is_copy_constructible<FalseCopyable<unique_ptr<T>> is true but an attempt to
+// instantiate the copy constructor leads to a compile error. This is similar
+// to how standard containers behave.
+template <class T>
+struct FalseCopyable {
+  FalseCopyable() {}
+  FalseCopyable(const FalseCopyable& other) : m(other.m) {}
+  FalseCopyable(FalseCopyable&& other) : m(std::move(other.m)) {}
+  T m;
+};
+
+int GetMember(FalseCopyable<std::unique_ptr<int>> x) { return *x.m; }
+
+TEST(BindTest, WrappedMoveOnly) {
+  FalseCopyable<std::unique_ptr<int>> x;
+  x.m = std::make_unique<int>(42);
+  auto f = absl::bind_front(&GetMember, std::move(x));
+  EXPECT_EQ(42, std::move(f)());
+}
+
+int Plus(int a, int b) { return a + b; }
+
+TEST(BindTest, ConstExpr) {
+  constexpr auto f = absl::bind_front(CharAt);
+  EXPECT_EQ(f("ABC", 1), 'B');
+  static constexpr int five = 5;
+  constexpr auto plus5 = absl::bind_front(Plus, five);
+  EXPECT_EQ(plus5(1), 6);
+
+  static constexpr char data[] = "DEF";
+  constexpr auto g = absl::bind_front(CharAt, data);
+  EXPECT_EQ(g(1), 'E');
+}
+
+struct ManglingCall {
+  int operator()(int, double, std::string) const { return 0; }
+};
+
+TEST(BindTest, Mangling) {
+  // We just want to generate a particular instantiation to see its mangling.
+  absl::bind_front(ManglingCall{}, 1, 3.3)("A");
+}
+
+}  // namespace
diff --git a/absl/functional/function_ref.h b/absl/functional/function_ref.h
new file mode 100644
index 00000000000..f6b2f225e40
--- /dev/null
+++ b/absl/functional/function_ref.h
@@ -0,0 +1,249 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: function_ref.h
+// -----------------------------------------------------------------------------
+//
+// This header file defines the `absl::FunctionRef` type for holding a
+// non-owning reference to an object of any invocable type. This function
+// reference is typically most useful as a type-erased argument type for
+// accepting function types that neither take ownership nor copy the type; using
+// the reference type in this case avoids a copy and an allocation. Best
+// practices of other non-owning reference-like objects (such as
+// `absl::string_view`) apply here.
+//
+//  An `absl::FunctionRef` is similar in usage to a `std::function` but has the
+//  following differences:
+//
+//  * It doesn't own the underlying object.
+//  * It doesn't have a null or empty state.
+//  * It never performs deep copies or allocations.
+//  * It's much faster and cheaper to construct.
+//  * It's trivially copyable and destructable.
+//
+// Generally, `absl::FunctionRef` should not be used as a return value, data
+// member, or to initialize a `std::function`. Such usages will often lead to
+// problematic lifetime issues. Once you convert something to an
+// `absl::FunctionRef` you cannot make a deep copy later.
+//
+// This class is suitable for use wherever a "const std::function<>&"
+// would be used without making a copy. ForEach functions and other versions of
+// the visitor pattern are a good example of when this class should be used.
+//
+// This class is trivial to copy and should be passed by value.
+#ifndef ABSL_FUNCTIONAL_FUNCTION_REF_H_
+#define ABSL_FUNCTIONAL_FUNCTION_REF_H_
+
+#include <cassert>
+#include <type_traits>
+#include <utility>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/functional/internal/function_ref.h"
+#include "absl/meta/type_traits.h"
+#include "absl/utility/utility.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+// FunctionRef
+//
+// Dummy class declaration to allow the partial specialization based on function
+// types below.
+template <typename T>
+class FunctionRef;
+
+// FunctionRef
+//
+// An `absl::FunctionRef` is a lightweight wrapper to any invocable object with
+// a compatible signature. Generally, an `absl::FunctionRef` should only be used
+// as an argument type and should be preferred as an argument over a const
+// reference to a `std::function`. `absl::FunctionRef` itself does not allocate,
+// although the wrapped invocable may.
+//
+// Example:
+//
+//   // The following function takes a function callback by const reference
+//   bool Visitor(const std::function<void(my_proto&,
+//                                         absl::string_view)>& callback);
+//
+//   // Assuming that the function is not stored or otherwise copied, it can be
+//   // replaced by an `absl::FunctionRef`:
+//   bool Visitor(absl::FunctionRef<void(my_proto&, absl::string_view)>
+//                  callback);
+template <typename R, typename... Args>
+class ABSL_ATTRIBUTE_VIEW FunctionRef<R(Args...)> {
+ protected:
+  // Used to disable constructors for objects that are not compatible with the
+  // signature of this FunctionRef.
+  template <typename F, typename... U>
+  using EnableIfCompatible =
+      std::enable_if_t<std::is_invocable_r_v<R, F, U..., Args...>>;
+
+  // Internal constructor to supersede the copying constructor
+  template <typename F>
+  // NOLINTNEXTLINE(google-explicit-constructor)
+  FunctionRef(std::in_place_t, F&& f ABSL_ATTRIBUTE_LIFETIME_BOUND) noexcept
+      : invoker_(&absl::functional_internal::InvokeObject<F&, R, Args...>) {
+    absl::functional_internal::AssertNonNull(f);
+    ptr_.obj = &f;
+  }
+
+ public:
+  // Constructs a FunctionRef from any invocable type.
+  template <typename F,
+            typename = EnableIfCompatible<std::enable_if_t<
+                !std::is_same_v<FunctionRef, absl::remove_cvref_t<F>>, F&>>>
+  // NOLINTNEXTLINE(google-explicit-constructor)
+  FunctionRef(F&& f ABSL_ATTRIBUTE_LIFETIME_BOUND) noexcept
+      : FunctionRef(std::in_place, std::forward<F>(f)) {}
+
+  // Overload for function pointers. This eliminates a level of indirection that
+  // would happen if the above overload was used (it lets us store the pointer
+  // instead of a pointer to a pointer).
+  //
+  // This overload is also used for references to functions, since references to
+  // functions can decay to function pointers implicitly.
+  template <typename F, typename = EnableIfCompatible<F*>,
+            absl::functional_internal::EnableIf<std::is_function_v<F>> = 0>
+  // NOLINTNEXTLINE(google-explicit-constructor)
+  FunctionRef(F* f ABSL_ATTRIBUTE_LIFETIME_BOUND) noexcept
+      : invoker_(&absl::functional_internal::InvokeFunction<F*, R, Args...>) {
+    assert(f != nullptr);
+    ptr_.fun = reinterpret_cast<decltype(ptr_.fun)>(f);
+  }
+
+#if ABSL_INTERNAL_CPLUSPLUS_LANG >= 202002L
+  // Similar to the other overloads, but passes the address of a known callable
+  // `F` at compile time. This allows calling arbitrary functions while avoiding
+  // an indirection.
+  // Needs C++20 as `nontype_t` needs C++20 for `auto` template parameters.
+  template <auto F, typename = EnableIfCompatible<decltype(F)>>
+  FunctionRef(nontype_t<F>) noexcept  // NOLINT(google-explicit-constructor)
+      : invoker_(&absl::functional_internal::InvokeFunction<decltype(F), F, R,
+                                                            Args...>) {}
+
+  template <
+      auto F, typename Obj,
+      typename = EnableIfCompatible<decltype(F), std::remove_reference_t<Obj>&>,
+      absl::functional_internal::EnableIf<!std::is_rvalue_reference_v<Obj&&>> =
+          0>
+  // NOLINTNEXTLINE(google-explicit-constructor)
+  FunctionRef(nontype_t<F>, Obj&& obj ABSL_ATTRIBUTE_LIFETIME_BOUND) noexcept
+      : invoker_(&absl::functional_internal::InvokeObject<Obj&, decltype(F), F,
+                                                          R, Args...>) {
+    ptr_.obj = std::addressof(obj);
+  }
+
+  template <auto F, typename Obj,
+            typename = EnableIfCompatible<decltype(F), Obj*>>
+  // NOLINTNEXTLINE(google-explicit-constructor)
+  FunctionRef(nontype_t<F>, Obj* obj ABSL_ATTRIBUTE_LIFETIME_BOUND) noexcept
+      : invoker_(&absl::functional_internal::InvokePtr<Obj, decltype(F), F, R,
+                                                       Args...>) {
+    ptr_.obj = obj;
+  }
+#endif
+
+  using absl_internal_is_view = std::true_type;
+
+  // Call the underlying object.
+  R operator()(Args... args) const {
+    return invoker_(ptr_, std::forward<Args>(args)...);
+  }
+
+ private:
+  absl::functional_internal::VoidPtr ptr_;
+  absl::functional_internal::Invoker<R, Args...> invoker_;
+};
+
+// Allow const qualified function signatures. Since FunctionRef requires
+// constness anyway we can just make this a no-op.
+template <typename R, typename... Args>
+class ABSL_ATTRIBUTE_VIEW
+    FunctionRef<R(Args...) const> : private FunctionRef<R(Args...)> {
+  using Base = FunctionRef<R(Args...)>;
+
+  template <typename F, typename... U>
+  using EnableIfCompatible =
+      typename Base::template EnableIfCompatible<F, U...>;
+
+ public:
+  template <
+      typename F,
+      typename = EnableIfCompatible<std::enable_if_t<
+          !std::is_same_v<FunctionRef, absl::remove_cvref_t<F>>, const F&>>>
+  // NOLINTNEXTLINE(google-explicit-constructor)
+  FunctionRef(const F& f ABSL_ATTRIBUTE_LIFETIME_BOUND) noexcept
+      : Base(std::in_place_t(), f) {}
+
+  template <typename F, typename = EnableIfCompatible<F*>,
+            absl::functional_internal::EnableIf<std::is_function_v<F>> = 0>
+  // NOLINTNEXTLINE(google-explicit-constructor)
+  FunctionRef(F* f ABSL_ATTRIBUTE_LIFETIME_BOUND) noexcept : Base(f) {}
+
+#if ABSL_INTERNAL_CPLUSPLUS_LANG >= 202002L
+  template <auto F, typename = EnableIfCompatible<decltype(F)>>
+  // NOLINTNEXTLINE(google-explicit-constructor)
+  FunctionRef(nontype_t<F> arg) noexcept : Base(arg) {}
+
+  template <auto F, typename Obj,
+            typename = EnableIfCompatible<decltype(F),
+                                          const std::remove_reference_t<Obj>&>,
+            absl::functional_internal::EnableIf<
+                !std::is_rvalue_reference_v<Obj&&>> = 0>
+  // NOLINTNEXTLINE(google-explicit-constructor)
+  FunctionRef(nontype_t<F> arg,
+              Obj&& obj ABSL_ATTRIBUTE_LIFETIME_BOUND) noexcept
+      : Base(arg, std::forward<Obj>(obj)) {}
+
+  template <auto F, typename Obj,
+            typename = EnableIfCompatible<decltype(F), const Obj*>>
+  // NOLINTNEXTLINE(google-explicit-constructor)
+  FunctionRef(nontype_t<F> arg,
+              const Obj* obj ABSL_ATTRIBUTE_LIFETIME_BOUND) noexcept
+      : Base(arg, obj) {}
+#endif
+
+  using absl_internal_is_view = std::true_type;
+
+  using Base::operator();
+};
+
+template <class F>
+FunctionRef(F*) -> FunctionRef<F>;
+
+#if ABSL_INTERNAL_CPLUSPLUS_LANG >= 202002L
+template <auto Func>
+FunctionRef(nontype_t<Func>)
+    -> FunctionRef<std::remove_pointer_t<decltype(Func)>>;
+
+template <class M, class T, M T::* Func, class U>
+FunctionRef(nontype_t<Func>, U&&)
+    -> FunctionRef<std::enable_if_t<std::is_member_pointer_v<M T::*>, M>>;
+
+template <class M, class T, M T::* Func, class U>
+FunctionRef(nontype_t<Func>, U&&) -> FunctionRef<std::enable_if_t<
+    std::is_object_v<M>, std::invoke_result_t<decltype(Func), U&>()>>;
+
+template <class R, class T, class... Args, R (*Func)(T, Args...), class U>
+FunctionRef(nontype_t<Func>, U&&) -> FunctionRef<R(Args...)>;
+#endif
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_FUNCTIONAL_FUNCTION_REF_H_
diff --git a/absl/functional/function_ref_test.cc b/absl/functional/function_ref_test.cc
new file mode 100644
index 00000000000..69513f42148
--- /dev/null
+++ b/absl/functional/function_ref_test.cc
@@ -0,0 +1,413 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/functional/function_ref.h"
+
+#include <functional>
+#include <memory>
+#include <type_traits>
+#include <utility>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/container/internal/test_instance_tracker.h"
+#include "absl/functional/any_invocable.h"
+#include "absl/memory/memory.h"
+#include "absl/utility/utility.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace {
+
+struct Class {
+  int Func() { return 42; }
+  int CFunc() const { return 43; }
+};
+
+int Function() { return 1337; }
+
+template <typename T>
+T Dereference(const T* v) {
+  return *v;
+}
+
+template <typename T>
+T Copy(const T& v) {
+  return v;
+}
+
+TEST(FunctionRefTest, Function1) {
+  FunctionRef<int()> ref(&Function);
+  EXPECT_EQ(1337, ref());
+}
+
+TEST(FunctionRefTest, Function2) {
+  FunctionRef<int()> ref(Function);
+  EXPECT_EQ(1337, ref());
+}
+
+TEST(FunctionRefTest, ConstFunction) {
+  FunctionRef<int() const> ref(Function);
+  EXPECT_EQ(1337, ref());
+}
+
+TEST(FunctionRefTest, CopyAssignment) {
+  FunctionRef<int()> a = +[]() -> int {
+    ADD_FAILURE() << "Unexpectedly called";
+    return 0;
+  };
+  FunctionRef<int()> b = +[] { return 1337; };
+  a = b;
+  EXPECT_EQ(1337, a());
+}
+
+int NoExceptFunction() noexcept { return 1337; }
+
+// TODO(jdennett): Add a test for noexcept member functions.
+TEST(FunctionRefTest, NoExceptFunction) {
+  FunctionRef<int()> ref(NoExceptFunction);
+  EXPECT_EQ(1337, ref());
+}
+
+TEST(FunctionRefTest, ForwardsArgs) {
+  auto l = [](std::unique_ptr<int> i) { return *i; };
+  FunctionRef<int(std::unique_ptr<int>)> ref(l);
+  EXPECT_EQ(42, ref(std::make_unique<int>(42)));
+}
+
+TEST(FunctionRef, ReturnMoveOnly) {
+  auto l = [] { return std::make_unique<int>(29); };
+  FunctionRef<std::unique_ptr<int>()> ref(l);
+  EXPECT_EQ(29, *ref());
+}
+
+TEST(FunctionRef, ManyArgs) {
+  auto l = [](int a, int b, int c) { return a + b + c; };
+  FunctionRef<int(int, int, int)> ref(l);
+  EXPECT_EQ(6, ref(1, 2, 3));
+}
+
+TEST(FunctionRef, VoidResultFromNonVoidFunctor) {
+  bool ran = false;
+  auto l = [&]() -> int {
+    ran = true;
+    return 2;
+  };
+  FunctionRef<void()> ref(l);
+  ref();
+  EXPECT_TRUE(ran);
+}
+
+TEST(FunctionRef, CastFromDerived) {
+  struct Base {};
+  struct Derived : public Base {};
+
+  Derived d;
+  auto l1 = [&](Base* b) { EXPECT_EQ(&d, b); };
+  FunctionRef<void(Derived*)> ref1(l1);
+  ref1(&d);
+
+  auto l2 = [&]() -> Derived* { return &d; };
+  FunctionRef<Base*()> ref2(l2);
+  EXPECT_EQ(&d, ref2());
+}
+
+TEST(FunctionRef, VoidResultFromNonVoidFuncton) {
+  FunctionRef<void()> ref(Function);
+  ref();
+}
+
+TEST(FunctionRef, MemberPtr) {
+  struct S {
+    int i;
+  };
+
+  S s{1100111};
+  auto mem_ptr = &S::i;
+  FunctionRef<int(const S& s)> ref(mem_ptr);
+  EXPECT_EQ(1100111, ref(s));
+}
+
+TEST(FunctionRef, MemberFun) {
+  struct S {
+    int i;
+    int get_i() const { return i; }
+  };
+
+  S s{22};
+  auto mem_fun_ptr = &S::get_i;
+  FunctionRef<int(const S& s)> ref(mem_fun_ptr);
+  EXPECT_EQ(22, ref(s));
+}
+
+TEST(FunctionRef, MemberFunRefqualified) {
+  struct S {
+    int i;
+    int get_i() && { return i; }
+  };
+  auto mem_fun_ptr = &S::get_i;
+  S s{22};
+  FunctionRef<int(S && s)> ref(mem_fun_ptr);
+  EXPECT_EQ(22, ref(std::move(s)));
+}
+
+#if !defined(_WIN32) && defined(GTEST_HAS_DEATH_TEST)
+
+TEST(FunctionRef, MemberFunRefqualifiedNull) {
+  struct S {
+    int i;
+    int get_i() && { return i; }
+  };
+  auto mem_fun_ptr = &S::get_i;
+  mem_fun_ptr = nullptr;
+  EXPECT_DEBUG_DEATH({ FunctionRef<int(S && s)> ref(mem_fun_ptr); }, "");
+}
+
+TEST(FunctionRef, NullMemberPtrAssertFails) {
+  struct S {
+    int i;
+  };
+  using MemberPtr = int S::*;
+  MemberPtr mem_ptr = nullptr;
+  EXPECT_DEBUG_DEATH({ FunctionRef<int(const S& s)> ref(mem_ptr); }, "");
+}
+
+TEST(FunctionRef, NullStdFunctionAssertPasses) {
+  std::function<void()> function = []() {};
+  FunctionRef<void()> ref(function);
+}
+
+TEST(FunctionRef, NullStdFunctionAssertFails) {
+  std::function<void()> function = nullptr;
+  EXPECT_DEBUG_DEATH({ FunctionRef<void()> ref(function); }, "");
+}
+
+TEST(FunctionRef, NullAnyInvocableAssertPasses) {
+  AnyInvocable<void() const> invocable = []() {};
+  FunctionRef<void()> ref(invocable);
+}
+TEST(FunctionRef, NullAnyInvocableAssertFails) {
+  AnyInvocable<void() const> invocable = nullptr;
+  EXPECT_DEBUG_DEATH({ FunctionRef<void()> ref(invocable); }, "");
+}
+
+#endif  // GTEST_HAS_DEATH_TEST
+
+TEST(FunctionRef, CopiesAndMovesPerPassByValue) {
+  absl::test_internal::InstanceTracker tracker;
+  absl::test_internal::CopyableMovableInstance instance(0);
+  auto l = [](absl::test_internal::CopyableMovableInstance) {};
+  FunctionRef<void(absl::test_internal::CopyableMovableInstance)> ref(l);
+  ref(instance);
+  EXPECT_EQ(tracker.copies(), 1);
+  EXPECT_EQ(tracker.moves(), 1);
+}
+
+TEST(FunctionRef, CopiesAndMovesPerPassByRef) {
+  absl::test_internal::InstanceTracker tracker;
+  absl::test_internal::CopyableMovableInstance instance(0);
+  auto l = [](const absl::test_internal::CopyableMovableInstance&) {};
+  FunctionRef<void(const absl::test_internal::CopyableMovableInstance&)> ref(l);
+  ref(instance);
+  EXPECT_EQ(tracker.copies(), 0);
+  EXPECT_EQ(tracker.moves(), 0);
+}
+
+TEST(FunctionRef, CopiesAndMovesPerPassByValueCallByMove) {
+  absl::test_internal::InstanceTracker tracker;
+  absl::test_internal::CopyableMovableInstance instance(0);
+  auto l = [](absl::test_internal::CopyableMovableInstance) {};
+  FunctionRef<void(absl::test_internal::CopyableMovableInstance)> ref(l);
+  ref(std::move(instance));
+  EXPECT_EQ(tracker.copies(), 0);
+  EXPECT_EQ(tracker.moves(), 2);
+}
+
+TEST(FunctionRef, CopiesAndMovesPerPassByValueToRef) {
+  absl::test_internal::InstanceTracker tracker;
+  absl::test_internal::CopyableMovableInstance instance(0);
+  auto l = [](const absl::test_internal::CopyableMovableInstance&) {};
+  FunctionRef<void(absl::test_internal::CopyableMovableInstance)> ref(l);
+  ref(std::move(instance));
+  EXPECT_EQ(tracker.copies(), 0);
+  EXPECT_EQ(tracker.moves(), 1);
+}
+
+TEST(FunctionRef, PassByValueTypes) {
+  using absl::functional_internal::Invoker;
+  using absl::functional_internal::VoidPtr;
+  using absl::test_internal::CopyableMovableInstance;
+  struct Trivial {
+    void* p[2];
+  };
+  struct LargeTrivial {
+    void* p[3];
+  };
+
+  static_assert(std::is_same_v<Invoker<void, int>, void (*)(VoidPtr, int)>,
+                "Scalar types should be passed by value");
+  static_assert(
+      std::is_same_v<Invoker<void, Trivial>, void (*)(VoidPtr, Trivial)>,
+      "Small trivial types should be passed by value");
+  static_assert(std::is_same_v<Invoker<void, LargeTrivial>,
+                               void (*)(VoidPtr, LargeTrivial&&)>,
+                "Large trivial types should be passed by rvalue reference");
+  static_assert(
+      std::is_same_v<Invoker<void, CopyableMovableInstance>,
+                     void (*)(VoidPtr, CopyableMovableInstance&&)>,
+      "Types with copy/move ctor should be passed by rvalue reference");
+
+  // References are passed as references.
+  static_assert(std::is_same_v<Invoker<void, int&>, void (*)(VoidPtr, int&)>,
+                "Reference types should be preserved");
+  static_assert(std::is_same_v<Invoker<void, CopyableMovableInstance&>,
+                               void (*)(VoidPtr, CopyableMovableInstance&)>,
+                "Reference types should be preserved");
+  static_assert(std::is_same_v<Invoker<void, CopyableMovableInstance&&>,
+                               void (*)(VoidPtr, CopyableMovableInstance&&)>,
+                "Reference types should be preserved");
+
+  // Make sure the address of an object received by reference is the same as the
+  // address of the object passed by the caller.
+  {
+    LargeTrivial obj;
+    auto test = [&obj](LargeTrivial& input) { ASSERT_EQ(&input, &obj); };
+    absl::FunctionRef<void(LargeTrivial&)> ref(test);
+    ref(obj);
+  }
+
+  {
+    Trivial obj;
+    auto test = [&obj](Trivial& input) { ASSERT_EQ(&input, &obj); };
+    absl::FunctionRef<void(Trivial&)> ref(test);
+    ref(obj);
+  }
+}
+
+TEST(FunctionRef, ReferenceToIncompleteType) {
+  struct IncompleteType;
+  auto test = [](IncompleteType&) {};
+  absl::FunctionRef<void(IncompleteType&)> ref(test);
+
+  struct IncompleteType {};
+  IncompleteType obj;
+  ref(obj);
+}
+
+TEST(FunctionRefTest, CorrectConstQualifiers) {
+  struct S {
+    int operator()() { return 42; }
+    int operator()() const { return 1337; }
+  };
+  S s;
+  EXPECT_EQ(42, FunctionRef<int()>(s)());
+  EXPECT_EQ(1337, FunctionRef<int() const>(s)());
+  EXPECT_EQ(1337, FunctionRef<int()>(std::as_const(s))());
+  EXPECT_EQ(1337, FunctionRef<int() const>(std::as_const(s))());
+}
+
+TEST(FunctionRefTest, Lambdas) {
+  // Stateless lambdas implicitly convert to function pointers, so their
+  // mutability is irrelevant.
+  EXPECT_TRUE(FunctionRef<bool()>([]() /*const*/ { return true; })());
+  EXPECT_TRUE(FunctionRef<bool()>([]() mutable { return true; })());
+  EXPECT_TRUE(FunctionRef<bool() const>([]() /*const*/ { return true; })());
+#if defined(__clang__) || (ABSL_INTERNAL_CPLUSPLUS_LANG >= 202002L && \
+                           defined(_MSC_VER) && !defined(__EDG__))
+  // MSVC has problems compiling the following code pre-C++20:
+  //   const auto f = []() mutable {};
+  //   f();
+  // EDG's MSVC-compatible mode (which Visual C++ uses for Intellisense)
+  // exhibits the bug in C++20 as well. So we don't support them.
+  EXPECT_TRUE(FunctionRef<bool() const>([]() mutable { return true; })());
+#endif
+
+  // Stateful lambdas are not implicitly convertible to function pointers, so
+  // a const stateful lambda is not mutably callable.
+  EXPECT_TRUE(FunctionRef<bool()>([v = true]() /*const*/ { return v; })());
+  EXPECT_TRUE(FunctionRef<bool()>([v = true]() mutable { return v; })());
+  EXPECT_TRUE(
+      FunctionRef<bool() const>([v = true]() /*const*/ { return v; })());
+  const auto func = [v = true]() mutable { return v; };
+  static_assert(
+      !std::is_convertible_v<decltype(func), FunctionRef<bool() const>>);
+}
+
+#if ABSL_INTERNAL_CPLUSPLUS_LANG >= 202002L
+static_assert(std::is_same_v<decltype(FunctionRef(nontype<&Class::Func>,
+                                                  std::declval<Class&>())),
+                             FunctionRef<int()>>);
+static_assert(std::is_same_v<decltype(FunctionRef(nontype<&Class::CFunc>,
+                                                  std::declval<Class&>())),
+                             FunctionRef<int() const>>);
+
+static_assert(std::is_same_v<decltype(FunctionRef(nontype<&Class::Func>,
+                                                  std::declval<Class*>())),
+                             FunctionRef<int()>>);
+static_assert(std::is_same_v<decltype(FunctionRef(nontype<&Class::CFunc>,
+                                                  std::declval<Class*>())),
+                             FunctionRef<int() const>>);
+
+TEST(FunctionRefTest, NonTypeParameterWithTemporaries) {
+  static_assert(!std::is_constructible_v<FunctionRef<int()>,
+                                         nontype_t<&Class::Func>, Class&&>);
+  static_assert(
+      !std::is_constructible_v<FunctionRef<int()>, nontype_t<&Class::Func>,
+                               const Class&&>);
+  static_assert(!std::is_constructible_v<FunctionRef<int() const>,
+                                         nontype_t<&Class::CFunc>, Class&&>);
+  static_assert(
+      !std::is_constructible_v<FunctionRef<int() const>,
+                               nontype_t<&Class::CFunc>, const Class&&>);
+}
+
+TEST(FunctionRefTest, NonTypeParameterWithDeductionGuides) {
+  EXPECT_EQ(1337, FunctionRef(nontype<&Function>)());
+  EXPECT_EQ(42, FunctionRef(nontype<&Copy<int>>,
+                            std::integral_constant<int, 42>::value)());
+  EXPECT_EQ(42, FunctionRef(nontype<&Dereference<int>>,
+                            &std::integral_constant<int, 42>::value)());
+
+  Class c;
+  EXPECT_EQ(42, FunctionRef<int()>(nontype<&Class::Func>, c)());
+  EXPECT_EQ(43, FunctionRef<int() const>(nontype<&Class::CFunc>, c)());
+
+  EXPECT_EQ(42, FunctionRef<int()>(nontype<&Class::Func>, &c)());
+  EXPECT_EQ(43, FunctionRef<int() const>(nontype<&Class::CFunc>, &c)());
+}
+#endif
+
+TEST(FunctionRefTest, OptionalArguments) {
+  struct S {
+    int operator()(int = 0) const { return 1337; }
+  };
+  S s;
+  EXPECT_EQ(1337, FunctionRef<int()>(s)());
+}
+
+TEST(FunctionRefTest, NonConstToConstConversion) {
+  // The const-qualified version might inherit from the non-const version.
+  // We want to make sure that this doesn't introduce a bug when an instance of
+  // the base (non-const) class is forwarded through the derived (const) class.
+  // This has the potential to trigger the copy constructor, thus incorrectly
+  // producing a copy rather than another indirection.
+  absl::FunctionRef<int()> a = +[]() { return 1; };
+  absl::FunctionRef<int() const> b = a;
+  a = +[]() { return 2; };
+  EXPECT_EQ(b(), 2);
+}
+
+}  // namespace
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/functional/function_type_benchmark.cc b/absl/functional/function_type_benchmark.cc
new file mode 100644
index 00000000000..513233b46b0
--- /dev/null
+++ b/absl/functional/function_type_benchmark.cc
@@ -0,0 +1,176 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <functional>
+#include <memory>
+#include <string>
+
+#include "absl/base/attributes.h"
+#include "absl/functional/any_invocable.h"
+#include "absl/functional/function_ref.h"
+#include "benchmark/benchmark.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace {
+
+int dummy = 0;
+
+void FreeFunction() { benchmark::DoNotOptimize(dummy); }
+
+struct TrivialFunctor {
+  void operator()() const { benchmark::DoNotOptimize(dummy); }
+};
+
+struct LargeFunctor {
+  void operator()() const { benchmark::DoNotOptimize(this); }
+  std::string a, b, c;
+};
+
+template <typename Function, typename... Args>
+void ABSL_ATTRIBUTE_NOINLINE CallFunction(Function f, Args&&... args) {
+  f(std::forward<Args>(args)...);
+}
+
+template <typename Function, typename Callable, typename... Args>
+void ConstructAndCallFunctionBenchmark(benchmark::State& state,
+                                       const Callable& c, Args&&... args) {
+  for (auto _ : state) {
+    CallFunction<Function>(c, std::forward<Args>(args)...);
+  }
+}
+
+void BM_TrivialStdFunction(benchmark::State& state) {
+  ConstructAndCallFunctionBenchmark<std::function<void()>>(state,
+                                                           TrivialFunctor{});
+}
+BENCHMARK(BM_TrivialStdFunction);
+
+void BM_TrivialFunctionRef(benchmark::State& state) {
+  ConstructAndCallFunctionBenchmark<FunctionRef<void()>>(state,
+                                                         TrivialFunctor{});
+}
+BENCHMARK(BM_TrivialFunctionRef);
+
+void BM_TrivialAnyInvocable(benchmark::State& state) {
+  ConstructAndCallFunctionBenchmark<AnyInvocable<void()>>(state,
+                                                          TrivialFunctor{});
+}
+BENCHMARK(BM_TrivialAnyInvocable);
+
+void BM_LargeStdFunction(benchmark::State& state) {
+  ConstructAndCallFunctionBenchmark<std::function<void()>>(state,
+                                                           LargeFunctor{});
+}
+BENCHMARK(BM_LargeStdFunction);
+
+void BM_LargeFunctionRef(benchmark::State& state) {
+  ConstructAndCallFunctionBenchmark<FunctionRef<void()>>(state, LargeFunctor{});
+}
+BENCHMARK(BM_LargeFunctionRef);
+
+
+void BM_LargeAnyInvocable(benchmark::State& state) {
+  ConstructAndCallFunctionBenchmark<AnyInvocable<void()>>(state,
+                                                          LargeFunctor{});
+}
+BENCHMARK(BM_LargeAnyInvocable);
+
+void BM_FunPtrStdFunction(benchmark::State& state) {
+  ConstructAndCallFunctionBenchmark<std::function<void()>>(state, FreeFunction);
+}
+BENCHMARK(BM_FunPtrStdFunction);
+
+void BM_FunPtrFunctionRef(benchmark::State& state) {
+  ConstructAndCallFunctionBenchmark<FunctionRef<void()>>(state, FreeFunction);
+}
+BENCHMARK(BM_FunPtrFunctionRef);
+
+void BM_FunPtrAnyInvocable(benchmark::State& state) {
+  ConstructAndCallFunctionBenchmark<AnyInvocable<void()>>(state, FreeFunction);
+}
+BENCHMARK(BM_FunPtrAnyInvocable);
+
+// Doesn't include construction or copy overhead in the loop.
+template <typename Function, typename Callable, typename... Args>
+void CallFunctionBenchmark(benchmark::State& state, const Callable& c,
+                           Args... args) {
+  Function f = c;
+  for (auto _ : state) {
+    benchmark::DoNotOptimize(&f);
+    f(args...);
+  }
+}
+
+struct FunctorWithTrivialArgs {
+  void operator()(int a, int b, int c) const {
+    benchmark::DoNotOptimize(a);
+    benchmark::DoNotOptimize(b);
+    benchmark::DoNotOptimize(c);
+  }
+};
+
+void BM_TrivialArgsStdFunction(benchmark::State& state) {
+  CallFunctionBenchmark<std::function<void(int, int, int)>>(
+      state, FunctorWithTrivialArgs{}, 1, 2, 3);
+}
+BENCHMARK(BM_TrivialArgsStdFunction);
+
+void BM_TrivialArgsFunctionRef(benchmark::State& state) {
+  CallFunctionBenchmark<FunctionRef<void(int, int, int)>>(
+      state, FunctorWithTrivialArgs{}, 1, 2, 3);
+}
+BENCHMARK(BM_TrivialArgsFunctionRef);
+
+void BM_TrivialArgsAnyInvocable(benchmark::State& state) {
+  CallFunctionBenchmark<AnyInvocable<void(int, int, int)>>(
+      state, FunctorWithTrivialArgs{}, 1, 2, 3);
+}
+BENCHMARK(BM_TrivialArgsAnyInvocable);
+
+struct FunctorWithNonTrivialArgs {
+  void operator()(std::string a, std::string b, std::string c) const {
+    benchmark::DoNotOptimize(&a);
+    benchmark::DoNotOptimize(&b);
+    benchmark::DoNotOptimize(&c);
+  }
+};
+
+void BM_NonTrivialArgsStdFunction(benchmark::State& state) {
+  std::string a, b, c;
+  CallFunctionBenchmark<
+      std::function<void(std::string, std::string, std::string)>>(
+      state, FunctorWithNonTrivialArgs{}, a, b, c);
+}
+BENCHMARK(BM_NonTrivialArgsStdFunction);
+
+void BM_NonTrivialArgsFunctionRef(benchmark::State& state) {
+  std::string a, b, c;
+  CallFunctionBenchmark<
+      FunctionRef<void(std::string, std::string, std::string)>>(
+      state, FunctorWithNonTrivialArgs{}, a, b, c);
+}
+BENCHMARK(BM_NonTrivialArgsFunctionRef);
+
+void BM_NonTrivialArgsAnyInvocable(benchmark::State& state) {
+  std::string a, b, c;
+  CallFunctionBenchmark<
+      AnyInvocable<void(std::string, std::string, std::string)>>(
+      state, FunctorWithNonTrivialArgs{}, a, b, c);
+}
+BENCHMARK(BM_NonTrivialArgsAnyInvocable);
+
+}  // namespace
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/functional/internal/any_invocable.h b/absl/functional/internal/any_invocable.h
new file mode 100644
index 00000000000..1ff8bb6fa58
--- /dev/null
+++ b/absl/functional/internal/any_invocable.h
@@ -0,0 +1,785 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// Implementation details for `absl::AnyInvocable`
+
+#ifndef ABSL_FUNCTIONAL_INTERNAL_ANY_INVOCABLE_H_
+#define ABSL_FUNCTIONAL_INTERNAL_ANY_INVOCABLE_H_
+
+////////////////////////////////////////////////////////////////////////////////
+//                                                                            //
+// This implementation chooses between local storage and remote storage for   //
+// the contained target object based on the target object's size, alignment   //
+// requirements, and whether or not it has a nothrow move constructor.        //
+// Additional optimizations are performed when the object is a trivially      //
+// copyable type [basic.types].                                               //
+//                                                                            //
+// There are three datamembers per `AnyInvocable` instance                    //
+//                                                                            //
+// 1) A union containing either                                               //
+//        - A pointer to the target object referred to via a void*, or        //
+//        - the target object, emplaced into a raw char buffer                //
+//                                                                            //
+// 2) A function pointer to a "manager" function operation that takes a       //
+//    discriminator and logically branches to either perform a move operation //
+//    or destroy operation based on that discriminator.                       //
+//                                                                            //
+// 3) A function pointer to an "invoker" function operation that invokes the  //
+//    target object, directly returning the result.                           //
+//                                                                            //
+// When in the logically empty state, the manager function is an empty        //
+// function and the invoker function is one that would be undefined behavior  //
+// to call.                                                                   //
+//                                                                            //
+// An additional optimization is performed when converting from one           //
+// AnyInvocable to another where only the noexcept specification and/or the   //
+// cv/ref qualifiers of the function type differ. In these cases, the         //
+// conversion works by "moving the guts", similar to if they were the same    //
+// exact type, as opposed to having to perform an additional layer of         //
+// wrapping through remote storage.                                           //
+//                                                                            //
+////////////////////////////////////////////////////////////////////////////////
+
+// IWYU pragma: private, include "absl/functional/any_invocable.h"
+
+#include <cassert>
+#include <cstddef>
+#include <cstring>
+#include <exception>
+#include <functional>
+#include <memory>
+#include <new>
+#include <type_traits>
+#include <utility>
+
+#include "absl/base/config.h"
+#include "absl/base/macros.h"
+#include "absl/base/nullability.h"
+#include "absl/base/optimization.h"
+#include "absl/meta/type_traits.h"
+#include "absl/utility/utility.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+// Defined in functional/any_invocable.h
+template <class Sig>
+class ABSL_NULLABILITY_COMPATIBLE AnyInvocable;
+
+namespace internal_any_invocable {
+
+// Constants relating to the small-object-storage for AnyInvocable
+enum StorageProperty : std::size_t {
+  kAlignment = alignof(std::max_align_t),  // The alignment of the storage
+  kStorageSize = sizeof(void*) * 2         // The size of the storage
+};
+
+////////////////////////////////////////////////////////////////////////////////
+//
+// A metafunction for checking if a type is an AnyInvocable instantiation.
+// This is used during conversion operations.
+template <class T>
+struct IsAnyInvocable : std::false_type {};
+
+template <class Sig>
+struct IsAnyInvocable<AnyInvocable<Sig>> : std::true_type {};
+//
+////////////////////////////////////////////////////////////////////////////////
+
+// A metafunction that tells us whether or not a target function type should be
+// stored locally in the small object optimization storage
+template <class T>
+constexpr bool IsStoredLocally() {
+  if constexpr (sizeof(T) <= kStorageSize && alignof(T) <= kAlignment &&
+                kAlignment % alignof(T) == 0) {
+    return std::is_nothrow_move_constructible_v<T>;
+  }
+  return false;
+}
+
+// An implementation of std::remove_cvref_t of C++20.
+template <class T>
+using RemoveCVRef = std::remove_cv_t<std::remove_reference_t<T>>;
+
+// An implementation of std::invoke_r of C++23.
+template <class ReturnType, class F, class... P>
+ReturnType InvokeR(F&& f, P&&... args) {
+  if constexpr (std::is_void_v<ReturnType>) {
+    std::invoke(std::forward<F>(f), std::forward<P>(args)...);
+  } else {
+    return std::invoke(std::forward<F>(f), std::forward<P>(args)...);
+  }
+}
+
+//
+////////////////////////////////////////////////////////////////////////////////
+
+////////////////////////////////////////////////////////////////////////////////
+///
+// A metafunction that takes a "T" corresponding to a parameter type of the
+// user's specified function type, and yields the parameter type to use for the
+// type-erased invoker. In order to prevent observable moves, this must be
+// either a reference or, if the type is trivial, the original parameter type
+// itself. Since the parameter type may be incomplete at the point that this
+// metafunction is used, we can only do this optimization for scalar types
+// rather than for any trivial type.
+template <typename T>
+T ForwardImpl(std::true_type);
+
+template <typename T>
+T&& ForwardImpl(std::false_type);
+
+// NOTE: We deliberately use an intermediate struct instead of a direct alias,
+// as a workaround for b/206991861 on MSVC versions < 1924.
+template <class T>
+struct ForwardedParameter {
+  using type =
+      decltype((ForwardImpl<T>)(std::bool_constant<std::is_scalar_v<T>>()));
+};
+
+template <class T>
+using ForwardedParameterType = typename ForwardedParameter<T>::type;
+//
+////////////////////////////////////////////////////////////////////////////////
+
+// A discriminator when calling the "manager" function that describes operation
+// type-erased operation should be invoked.
+//
+// "dispose" specifies that the manager should perform a destroy.
+//
+// "relocate_from_to" specifies that the manager should perform a move.
+//
+// "relocate_from_to_and_query_rust" is identical to "relocate_from_to" for C++
+// managers, but instructs Rust managers to perform a special operation that
+// can be detected by the caller.
+enum class FunctionToCall : unsigned char {
+  dispose,
+  relocate_from_to,
+  relocate_from_to_and_query_rust,
+};
+
+// The portion of `AnyInvocable` state that contains either a pointer to the
+// target object or the object itself in local storage
+union TypeErasedState {
+  struct {
+    // A pointer to the type-erased object when remotely stored
+    void* target;
+    // The size of the object for `RemoteManagerTrivial`
+    std::size_t size;
+  } remote;
+
+  // Local-storage for the type-erased object when small and trivial enough
+  alignas(kAlignment) unsigned char storage[kStorageSize];
+};
+
+// A typed accessor for the object in `TypeErasedState` storage
+template <class T>
+T& ObjectInLocalStorage(TypeErasedState* const state) {
+  // We launder here because the storage may be reused with the same type.
+  return *std::launder(reinterpret_cast<T*>(&state->storage));
+}
+
+// The type for functions issuing lifetime-related operations: move and dispose
+// A pointer to such a function is contained in each `AnyInvocable` instance.
+// NOTE: When specifying `FunctionToCall::`dispose, the same state must be
+// passed as both "from" and "to".
+using ManagerType = void(FunctionToCall /*operation*/,
+                         TypeErasedState* /*from*/,
+                         TypeErasedState* /*to*/) noexcept(true);
+
+// The type for functions issuing the actual invocation of the object
+// A pointer to such a function is contained in each AnyInvocable instance.
+template <bool SigIsNoexcept, class ReturnType, class... P>
+using InvokerType = ReturnType(
+    TypeErasedState*, ForwardedParameterType<P>...) noexcept(SigIsNoexcept);
+
+// The manager that is used when AnyInvocable is empty
+inline void EmptyManager(FunctionToCall /*operation*/,
+                         TypeErasedState* /*from*/,
+                         TypeErasedState* /*to*/) noexcept {}
+
+// The manager that is used when a target function is in local storage and is
+// a trivially copyable type.
+inline void LocalManagerTrivial(FunctionToCall /*operation*/,
+                                TypeErasedState* const from,
+                                TypeErasedState* const to) noexcept {
+  // This single statement without branching handles both possible operations.
+  //
+  // For FunctionToCall::dispose, "from" and "to" point to the same state, and
+  // so this assignment logically would do nothing.
+  //
+  // Note: Correctness here relies on http://wg21.link/p0593, which has only
+  // become standard in C++20, though implementations do not break it in
+  // practice for earlier versions of C++.
+  //
+  // The correct way to do this without that paper is to first placement-new a
+  // default-constructed T in "to->storage" prior to the memmove, but doing so
+  // requires a different function to be created for each T that is stored
+  // locally, which can cause unnecessary bloat and be less cache friendly.
+  *to = *from;
+
+  // Note: Because the type is trivially copyable, the destructor does not need
+  // to be called ("trivially copyable" requires a trivial destructor).
+}
+
+// The manager that is used when a target function is in local storage and is
+// not a trivially copyable type.
+template <class T>
+void LocalManagerNontrivial(FunctionToCall operation,
+                            TypeErasedState* const from,
+                            TypeErasedState* const to) noexcept {
+  static_assert(IsStoredLocally<T>(),
+                "Local storage must only be used for supported types.");
+  static_assert(!std::is_trivially_copyable_v<T>,
+                "Locally stored types must be trivially copyable.");
+
+  T& from_object = (ObjectInLocalStorage<T>)(from);
+
+  switch (operation) {
+    case FunctionToCall::relocate_from_to:
+    case FunctionToCall::relocate_from_to_and_query_rust:
+      // NOTE: Requires that the left-hand operand is already empty.
+      ::new (static_cast<void*>(&to->storage)) T(std::move(from_object));
+      [[fallthrough]];
+    case FunctionToCall::dispose:
+      from_object.~T();  // Must not throw. // NOLINT
+      return;
+  }
+  ABSL_UNREACHABLE();
+}
+
+// The invoker that is used when a target function is in local storage
+// Note: QualTRef here is the target function type along with cv and reference
+// qualifiers that must be used when calling the function.
+template <bool SigIsNoexcept, class ReturnType, class QualTRef, class... P>
+ReturnType LocalInvoker(
+    TypeErasedState* const state,
+    ForwardedParameterType<P>... args) noexcept(SigIsNoexcept) {
+  using RawT = RemoveCVRef<QualTRef>;
+  static_assert(
+      IsStoredLocally<RawT>(),
+      "Target object must be in local storage in order to be invoked from it.");
+
+  auto& f = (ObjectInLocalStorage<RawT>)(state);
+  return (InvokeR<ReturnType>)(static_cast<QualTRef>(f),
+                               static_cast<ForwardedParameterType<P>>(args)...);
+}
+
+// The manager that is used when a target function is in remote storage and it
+// has a trivial destructor
+inline void RemoteManagerTrivial(FunctionToCall operation,
+                                 TypeErasedState* const from,
+                                 TypeErasedState* const to) noexcept {
+  switch (operation) {
+    case FunctionToCall::relocate_from_to:
+    case FunctionToCall::relocate_from_to_and_query_rust:
+      // NOTE: Requires that the left-hand operand is already empty.
+      to->remote = from->remote;
+      return;
+    case FunctionToCall::dispose:
+#if defined(__cpp_sized_deallocation)
+      ::operator delete(from->remote.target, from->remote.size);
+#else   // __cpp_sized_deallocation
+      ::operator delete(from->remote.target);
+#endif  // __cpp_sized_deallocation
+      return;
+  }
+  ABSL_UNREACHABLE();
+}
+
+// The manager that is used when a target function is in remote storage and the
+// destructor of the type is not trivial
+template <class T>
+void RemoteManagerNontrivial(FunctionToCall operation,
+                             TypeErasedState* const from,
+                             TypeErasedState* const to) noexcept {
+  static_assert(!IsStoredLocally<T>(),
+                "Remote storage must only be used for types that do not "
+                "qualify for local storage.");
+
+  switch (operation) {
+    case FunctionToCall::relocate_from_to:
+    case FunctionToCall::relocate_from_to_and_query_rust:
+      // NOTE: Requires that the left-hand operand is already empty.
+      to->remote.target = from->remote.target;
+      return;
+    case FunctionToCall::dispose:
+      ::delete static_cast<T*>(from->remote.target);  // Must not throw.
+      return;
+  }
+  ABSL_UNREACHABLE();
+}
+
+// The invoker that is used when a target function is in remote storage
+template <bool SigIsNoexcept, class ReturnType, class QualTRef, class... P>
+ReturnType RemoteInvoker(
+    TypeErasedState* const state,
+    ForwardedParameterType<P>... args) noexcept(SigIsNoexcept) {
+  using RawT = RemoveCVRef<QualTRef>;
+  static_assert(!IsStoredLocally<RawT>(),
+                "Target object must be in remote storage in order to be "
+                "invoked from it.");
+
+  auto& f = *static_cast<RawT*>(state->remote.target);
+  return (InvokeR<ReturnType>)(static_cast<QualTRef>(f),
+                               static_cast<ForwardedParameterType<P>>(args)...);
+}
+
+////////////////////////////////////////////////////////////////////////////////
+//
+// A metafunction that checks if a type T is an instantiation of
+// std::in_place_type_t (needed for constructor constraints of AnyInvocable).
+template <class T>
+struct IsInPlaceType : std::false_type {};
+
+template <class T>
+struct IsInPlaceType<std::in_place_type_t<T>> : std::true_type {};
+//
+////////////////////////////////////////////////////////////////////////////////
+
+// A constructor name-tag used with CoreImpl (below) to request the
+// conversion-constructor. QualDecayedTRef is the decayed-type of the object to
+// wrap, along with the cv and reference qualifiers that must be applied when
+// performing an invocation of the wrapped object.
+template <class QualDecayedTRef>
+struct TypedConversionConstruct {};
+
+// A helper base class for all core operations of AnyInvocable. Most notably,
+// this class creates the function call operator and constraint-checkers so that
+// the top-level class does not have to be a series of partial specializations.
+//
+// Note: This definition exists (as opposed to being a declaration) so that if
+// the user of the top-level template accidentally passes a template argument
+// that is not a function type, they will get a static_assert in AnyInvocable's
+// class body rather than an error stating that Impl is not defined.
+template <class Sig>
+class Impl {};  // Note: This is partially-specialized later.
+
+// A std::unique_ptr deleter that deletes memory allocated via ::operator new.
+#if defined(__cpp_sized_deallocation)
+class TrivialDeleter {
+ public:
+  explicit TrivialDeleter(std::size_t size) : size_(size) {}
+
+  void operator()(void* target) const {
+    ::operator delete(target, size_);
+  }
+
+ private:
+  std::size_t size_;
+};
+#else   // __cpp_sized_deallocation
+class TrivialDeleter {
+ public:
+  explicit TrivialDeleter(std::size_t) {}
+
+  void operator()(void* target) const { ::operator delete(target); }
+};
+#endif  // __cpp_sized_deallocation
+
+template <bool SigIsNoexcept, class ReturnType, class... P>
+class CoreImpl;
+
+constexpr bool IsCompatibleConversion(void*, void*) { return false; }
+template <bool NoExceptSrc, bool NoExceptDest, class... T>
+constexpr bool IsCompatibleConversion(CoreImpl<NoExceptSrc, T...>*,
+                                      CoreImpl<NoExceptDest, T...>*) {
+  return !NoExceptDest || NoExceptSrc;
+}
+
+// A helper base class for all core operations of AnyInvocable that do not
+// depend on the cv/ref qualifiers of the function type.
+template <bool SigIsNoexcept, class ReturnType, class... P>
+class CoreImpl {
+ public:
+  using result_type = ReturnType;
+
+  CoreImpl() noexcept : manager_(EmptyManager), invoker_(nullptr) {}
+
+  // Note: QualDecayedTRef here includes the cv-ref qualifiers associated with
+  // the invocation of the Invocable. The unqualified type is the target object
+  // type to be stored.
+  template <class QualDecayedTRef, class F>
+  explicit CoreImpl(TypedConversionConstruct<QualDecayedTRef>, F&& f) {
+    using DecayedT = RemoveCVRef<QualDecayedTRef>;
+
+    if constexpr (std::is_pointer_v<DecayedT> ||
+                  std::is_member_pointer_v<DecayedT>) {
+      // This condition handles types that decay into pointers. This includes
+      // function references, which cannot be null. GCC warns against comparing
+      // their decayed form with nullptr (https://godbolt.org/z/9r9TMTcPK).
+      // We could work around this warning with constexpr programming, using
+      // std::is_function_v<std::remove_reference_t<F>>, but we choose to ignore
+      // it instead of writing more code.
+#if !defined(__clang__) && defined(__GNUC__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wpragmas"
+#pragma GCC diagnostic ignored "-Waddress"
+#pragma GCC diagnostic ignored "-Wnonnull-compare"
+#endif
+      if (static_cast<DecayedT>(f) == nullptr) {
+#if !defined(__clang__) && defined(__GNUC__)
+#pragma GCC diagnostic pop
+#endif
+        manager_ = EmptyManager;
+        invoker_ = nullptr;
+      } else {
+        InitializeStorage<QualDecayedTRef>(std::forward<F>(f));
+      }
+    } else if constexpr (IsCompatibleAnyInvocable<DecayedT>::value) {
+      // In this case we can "steal the guts" of the other AnyInvocable.
+      f.manager_(FunctionToCall::relocate_from_to, &f.state_, &state_);
+      manager_ = f.manager_;
+      invoker_ = f.invoker_;
+
+      f.manager_ = EmptyManager;
+      f.invoker_ = nullptr;
+    } else if constexpr (IsAnyInvocable<DecayedT>::value) {
+      if (f.HasValue()) {
+        InitializeStorage<QualDecayedTRef>(std::forward<F>(f));
+      } else {
+        manager_ = EmptyManager;
+        invoker_ = nullptr;
+      }
+    } else {
+      InitializeStorage<QualDecayedTRef>(std::forward<F>(f));
+    }
+  }
+
+  // Note: QualTRef here includes the cv-ref qualifiers associated with the
+  // invocation of the Invocable. The unqualified type is the target object
+  // type to be stored.
+  template <class QualTRef, class... Args>
+  explicit CoreImpl(std::in_place_type_t<QualTRef>, Args&&... args) {
+    InitializeStorage<QualTRef>(std::forward<Args>(args)...);
+  }
+
+  CoreImpl(CoreImpl&& other) noexcept {
+    other.manager_(FunctionToCall::relocate_from_to, &other.state_, &state_);
+    manager_ = other.manager_;
+    invoker_ = other.invoker_;
+    other.manager_ = EmptyManager;
+    other.invoker_ = nullptr;
+  }
+
+  CoreImpl& operator=(CoreImpl&& other) noexcept {
+    // Put the left-hand operand in an empty state.
+    //
+    // Note: A full reset that leaves us with an object that has its invariants
+    // intact is necessary in order to handle self-move. This is required by
+    // types that are used with certain operations of the standard library, such
+    // as the default definition of std::swap when both operands target the same
+    // object.
+    Clear();
+
+    // Perform the actual move/destroy operation on the target function.
+    other.manager_(FunctionToCall::relocate_from_to, &other.state_, &state_);
+    manager_ = other.manager_;
+    invoker_ = other.invoker_;
+    other.manager_ = EmptyManager;
+    other.invoker_ = nullptr;
+
+    return *this;
+  }
+
+  ~CoreImpl() { manager_(FunctionToCall::dispose, &state_, &state_); }
+
+  // Check whether or not the AnyInvocable is in the empty state.
+  bool HasValue() const { return invoker_ != nullptr; }
+
+  // Effects: Puts the object into its empty state.
+  void Clear() {
+    manager_(FunctionToCall::dispose, &state_, &state_);
+    manager_ = EmptyManager;
+    invoker_ = nullptr;
+  }
+
+  // Use local (inline) storage for applicable target object types.
+  template <class QualTRef, class... Args>
+  void InitializeStorage(Args&&... args) {
+    using RawT = RemoveCVRef<QualTRef>;
+    if constexpr (IsStoredLocally<RawT>()) {
+      ::new (static_cast<void*>(&state_.storage))
+          RawT(std::forward<Args>(args)...);
+      invoker_ = LocalInvoker<SigIsNoexcept, ReturnType, QualTRef, P...>;
+      // We can simplify our manager if we know the type is trivially copyable.
+      if constexpr (std::is_trivially_copyable_v<RawT>) {
+        manager_ = LocalManagerTrivial;
+      } else {
+        manager_ = LocalManagerNontrivial<RawT>;
+      }
+    } else {
+      InitializeRemoteManager<RawT>(std::forward<Args>(args)...);
+      // This is set after everything else in case an exception is thrown in an
+      // earlier step of the initialization.
+      invoker_ = RemoteInvoker<SigIsNoexcept, ReturnType, QualTRef, P...>;
+    }
+  }
+
+  template <class T, class... Args>
+  void InitializeRemoteManager(Args&&... args) {
+    if constexpr (std::is_trivially_destructible_v<T> &&
+                  alignof(T) <= ABSL_INTERNAL_DEFAULT_NEW_ALIGNMENT) {
+      // unique_ptr is used for exception-safety in case construction throws.
+      std::unique_ptr<void, TrivialDeleter> uninitialized_target(
+          ::operator new(sizeof(T)), TrivialDeleter(sizeof(T)));
+      ::new (uninitialized_target.get()) T(std::forward<Args>(args)...);
+      state_.remote.target = uninitialized_target.release();
+      state_.remote.size = sizeof(T);
+      manager_ = RemoteManagerTrivial;
+    } else {
+      state_.remote.target = ::new T(std::forward<Args>(args)...);
+      manager_ = RemoteManagerNontrivial<T>;
+    }
+  }
+
+  //////////////////////////////////////////////////////////////////////////////
+  //
+  // Type trait to determine if the template argument is an AnyInvocable whose
+  // function type is compatible enough with ours such that we can
+  // "move the guts" out of it when moving, rather than having to place a new
+  // object into remote storage.
+
+  template <typename Other>
+  struct IsCompatibleAnyInvocable {
+    static constexpr bool value = false;
+  };
+
+  template <typename Sig>
+  struct IsCompatibleAnyInvocable<AnyInvocable<Sig>> {
+    static constexpr bool value =
+        (IsCompatibleConversion)(static_cast<
+                                     typename AnyInvocable<Sig>::CoreImpl*>(
+                                     nullptr),
+                                 static_cast<CoreImpl*>(nullptr));
+  };
+
+  //
+  //////////////////////////////////////////////////////////////////////////////
+
+  TypeErasedState state_;
+  ManagerType* manager_;
+  InvokerType<SigIsNoexcept, ReturnType, P...>* invoker_;
+};
+
+// A constructor name-tag used with Impl to request the
+// conversion-constructor
+struct ConversionConstruct {};
+
+////////////////////////////////////////////////////////////////////////////////
+//
+// A metafunction that is normally an identity metafunction except that when
+// given a std::reference_wrapper<T>, it yields T&. This is necessary because
+// currently std::reference_wrapper's operator() is not conditionally noexcept,
+// so when checking if such an Invocable is nothrow-invocable, we must pull out
+// the underlying type.
+template <class T>
+struct UnwrapStdReferenceWrapperImpl {
+  using type = T;
+};
+
+template <class T>
+struct UnwrapStdReferenceWrapperImpl<std::reference_wrapper<T>> {
+  using type = T&;
+};
+
+template <class T>
+using UnwrapStdReferenceWrapper =
+    typename UnwrapStdReferenceWrapperImpl<T>::type;
+//
+////////////////////////////////////////////////////////////////////////////////
+
+// An alias that always yields std::true_type (used with constraints) where
+// substitution failures happen when forming the template arguments.
+//
+// NOTE: We avoid std::void_t here to avoid a bug in GCC < 11:
+// https://godbolt.org/z/sxbfGMdcb
+template <class... T>
+using TrueAlias = std::bool_constant<sizeof(std::common_type<T...>*) != 0>;
+
+/*SFINAE constraints for the conversion-constructor.*/
+template <class Sig, class F,
+          class = std::enable_if_t<
+              !std::is_same_v<RemoveCVRef<F>, AnyInvocable<Sig>>>>
+using CanConvert =
+    TrueAlias<std::enable_if_t<!IsInPlaceType<RemoveCVRef<F>>::value>,
+              std::enable_if_t<Impl<Sig>::template CallIsValid<F>::value>,
+              std::enable_if_t<
+                  Impl<Sig>::template CallIsNoexceptIfSigIsNoexcept<F>::value>,
+              std::enable_if_t<std::is_constructible_v<std::decay_t<F>, F>>>;
+
+/*SFINAE constraints for the std::in_place constructors.*/
+template <class Sig, class F, class... Args>
+using CanEmplace = TrueAlias<
+    std::enable_if_t<Impl<Sig>::template CallIsValid<F>::value>,
+    std::enable_if_t<
+        Impl<Sig>::template CallIsNoexceptIfSigIsNoexcept<F>::value>,
+    std::enable_if_t<std::is_constructible_v<std::decay_t<F>, Args...>>>;
+
+/*SFINAE constraints for the conversion-assign operator.*/
+template <class Sig, class F,
+          class = std::enable_if_t<
+              !std::is_same_v<RemoveCVRef<F>, AnyInvocable<Sig>>>>
+using CanAssign =
+    TrueAlias<std::enable_if_t<Impl<Sig>::template CallIsValid<F>::value>,
+              std::enable_if_t<
+                  Impl<Sig>::template CallIsNoexceptIfSigIsNoexcept<F>::value>,
+              std::enable_if_t<std::is_constructible_v<std::decay_t<F>, F>>>;
+
+/*SFINAE constraints for the reference-wrapper conversion-assign operator.*/
+template <class Sig, class F>
+using CanAssignReferenceWrapper = TrueAlias<
+    std::enable_if_t<
+        Impl<Sig>::template CallIsValid<std::reference_wrapper<F>>::value>,
+    std::enable_if_t<Impl<Sig>::template CallIsNoexceptIfSigIsNoexcept<
+        std::reference_wrapper<F>>::value>>;
+
+// The constraint for checking whether or not a call meets the noexcept
+// callability requirements. We use a preprocessor macro because specifying it
+// this way as opposed to a disjunction/branch can improve the user-side error
+// messages and avoids an instantiation of std::is_nothrow_invocable_r in the
+// cases where the user did not specify a noexcept function type.
+//
+// The disjunction below is because we can't rely on std::is_nothrow_invocable_r
+// to give the right result when ReturnType is non-moveable in toolchains that
+// don't treat non-moveable result types correctly. For example this was the
+// case in libc++ before commit c3a24882 (2022-05).
+#define ABSL_INTERNAL_ANY_INVOCABLE_NOEXCEPT_CONSTRAINT_true(inv_quals)     \
+  std::enable_if_t<std::disjunction_v<                                      \
+      std::is_nothrow_invocable_r<                                          \
+          ReturnType, UnwrapStdReferenceWrapper<std::decay_t<F>> inv_quals, \
+          P...>,                                                            \
+      std::conjunction<                                                     \
+          std::is_nothrow_invocable<                                        \
+              UnwrapStdReferenceWrapper<std::decay_t<F>> inv_quals, P...>,  \
+          std::is_same<                                                     \
+              ReturnType,                                                   \
+              std::invoke_result_t<                                         \
+                  UnwrapStdReferenceWrapper<std::decay_t<F>> inv_quals,     \
+                  P...>>>>>
+
+#define ABSL_INTERNAL_ANY_INVOCABLE_NOEXCEPT_CONSTRAINT_false(inv_quals)
+//
+////////////////////////////////////////////////////////////////////////////////
+
+// A macro to generate partial specializations of Impl with the different
+// combinations of supported cv/reference qualifiers and noexcept specifier.
+//
+// Here, `cv` are the cv-qualifiers if any, `ref` is the ref-qualifier if any,
+// inv_quals is the reference type to be used when invoking the target, and
+// noex is "true" if the function type is noexcept, or false if it is not.
+//
+// The CallIsValid condition is more complicated than simply using
+// std::is_invocable_r because we can't rely on it to give the right result
+// when ReturnType is non-moveable in toolchains that don't treat non-moveable
+// result types correctly. For example this was the case in libc++ before commit
+// c3a24882 (2022-05).
+#define ABSL_INTERNAL_ANY_INVOCABLE_IMPL_(cv, ref, inv_quals, noex)            \
+  template <class ReturnType, class... P>                                      \
+  class Impl<ReturnType(P...) cv ref noexcept(noex)>                           \
+      : public CoreImpl<noex, ReturnType, P...> {                              \
+   public:                                                                     \
+    /*The base class, which contains the datamembers and core operations*/     \
+    using Core = CoreImpl<noex, ReturnType, P...>;                             \
+                                                                               \
+    /*SFINAE constraint to check if F is invocable with the proper signature*/ \
+    template <class F>                                                         \
+    using CallIsValid = TrueAlias<std::enable_if_t<std::disjunction<           \
+        std::is_invocable_r<ReturnType, std::decay_t<F> inv_quals, P...>,      \
+        std::is_same<ReturnType,                                               \
+                     std::invoke_result_t<std::decay_t<F> inv_quals, P...>>>:: \
+                                                       value>>;                \
+                                                                               \
+    /*SFINAE constraint to check if F is nothrow-invocable when necessary*/    \
+    template <class F>                                                         \
+    using CallIsNoexceptIfSigIsNoexcept =                                      \
+        TrueAlias<ABSL_INTERNAL_ANY_INVOCABLE_NOEXCEPT_CONSTRAINT_##noex(      \
+            inv_quals)>;                                                       \
+                                                                               \
+    /*Put the AnyInvocable into an empty state.*/                              \
+    Impl() = default;                                                          \
+                                                                               \
+    /*The implementation of a conversion-constructor from "f*/                 \
+    /*This forwards to Core, attaching inv_quals so that the base class*/      \
+    /*knows how to properly type-erase the invocation.*/                       \
+    template <class F>                                                         \
+    explicit Impl(ConversionConstruct, F&& f)                                  \
+        : Core(TypedConversionConstruct<std::decay_t<F> inv_quals>(),          \
+               std::forward<F>(f)) {}                                          \
+                                                                               \
+    /*Forward along the in-place construction parameters.*/                    \
+    template <class T, class... Args>                                          \
+    explicit Impl(std::in_place_type_t<T>, Args&&... args)                     \
+        : Core(std::in_place_type<std::decay_t<T> inv_quals>,                  \
+               std::forward<Args>(args)...) {}                                 \
+                                                                               \
+    /*Raises a fatal error when the AnyInvocable is invoked after a move*/     \
+    static ReturnType InvokedAfterMove(                                        \
+        TypeErasedState*, ForwardedParameterType<P>...) noexcept(noex) {       \
+      ABSL_HARDENING_ASSERT(false && "AnyInvocable use-after-move");           \
+      std::terminate();                                                        \
+    }                                                                          \
+                                                                               \
+    InvokerType<noex, ReturnType, P...>* ExtractInvoker() cv {                 \
+      using QualifiedTestType = int cv ref;                                    \
+      auto* invoker = this->invoker_;                                          \
+      if (!std::is_const_v<QualifiedTestType> &&                               \
+          std::is_rvalue_reference_v<QualifiedTestType>) {                     \
+        ABSL_ASSERT([this]() {                                                 \
+          /* We checked that this isn't const above, so const_cast is safe */  \
+          const_cast<Impl*>(this)->invoker_ = InvokedAfterMove;                \
+          return this->HasValue();                                             \
+        }());                                                                  \
+      }                                                                        \
+      return invoker;                                                          \
+    }                                                                          \
+                                                                               \
+    /*The actual invocation operation with the proper signature*/              \
+    ReturnType operator()(P... args) cv ref noexcept(noex) {                   \
+      assert(this->invoker_ != nullptr);                                       \
+      return this->ExtractInvoker()(                                           \
+          const_cast<TypeErasedState*>(&this->state_),                         \
+          static_cast<ForwardedParameterType<P>>(args)...);                    \
+    }                                                                          \
+  }
+
+// A convenience macro that defines specializations for the noexcept(true) and
+// noexcept(false) forms, given the other properties.
+#define ABSL_INTERNAL_ANY_INVOCABLE_IMPL(cv, ref, inv_quals)    \
+  ABSL_INTERNAL_ANY_INVOCABLE_IMPL_(cv, ref, inv_quals, false); \
+  ABSL_INTERNAL_ANY_INVOCABLE_IMPL_(cv, ref, inv_quals, true)
+
+// Non-ref-qualified partial specializations
+ABSL_INTERNAL_ANY_INVOCABLE_IMPL(, , &);
+ABSL_INTERNAL_ANY_INVOCABLE_IMPL(const, , const&);
+
+// Lvalue-ref-qualified partial specializations
+ABSL_INTERNAL_ANY_INVOCABLE_IMPL(, &, &);
+ABSL_INTERNAL_ANY_INVOCABLE_IMPL(const, &, const&);
+
+// Rvalue-ref-qualified partial specializations
+ABSL_INTERNAL_ANY_INVOCABLE_IMPL(, &&, &&);
+ABSL_INTERNAL_ANY_INVOCABLE_IMPL(const, &&, const&&);
+
+// Undef the detail-only macros.
+#undef ABSL_INTERNAL_ANY_INVOCABLE_IMPL
+#undef ABSL_INTERNAL_ANY_INVOCABLE_IMPL_
+#undef ABSL_INTERNAL_ANY_INVOCABLE_NOEXCEPT_CONSTRAINT_false
+#undef ABSL_INTERNAL_ANY_INVOCABLE_NOEXCEPT_CONSTRAINT_true
+
+}  // namespace internal_any_invocable
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_FUNCTIONAL_INTERNAL_ANY_INVOCABLE_H_
diff --git a/absl/functional/internal/back_binder.h b/absl/functional/internal/back_binder.h
new file mode 100644
index 00000000000..71cfc7a2491
--- /dev/null
+++ b/absl/functional/internal/back_binder.h
@@ -0,0 +1,95 @@
+// Copyright 2026 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Implementation details for `absl::bind_back()`.
+
+#ifndef ABSL_FUNCTIONAL_INTERNAL_BACK_BINDER_H_
+#define ABSL_FUNCTIONAL_INTERNAL_BACK_BINDER_H_
+
+#include <cstddef>
+#include <type_traits>
+#include <utility>
+
+#include "absl/base/config.h"
+#include "absl/container/internal/compressed_tuple.h"
+#include "absl/utility/utility.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace functional_internal {
+
+// Invoke the method, expanding the tuple of bound arguments.
+template <class R, class Tuple, size_t... Idx, class... Args>
+constexpr R ApplyBack(Tuple&& bound, std::index_sequence<Idx...>,
+                      Args&&... free) {
+  return std::invoke(std::forward<Tuple>(bound).template get<0>(),
+                     std::forward<Args>(free)...,
+                     std::forward<Tuple>(bound).template get<Idx + 1>()...);
+}
+
+template <class F, class... BoundArgs>
+class BackBinder {
+  using BoundArgsT = absl::container_internal::CompressedTuple<F, BoundArgs...>;
+  using Idx = std::make_index_sequence<sizeof...(BoundArgs)>;
+
+  BoundArgsT bound_args_;
+
+ public:
+  template <class... Ts>
+  constexpr explicit BackBinder(std::in_place_t, Ts&&... ts)
+      : bound_args_(std::forward<Ts>(ts)...) {}
+
+  template <class... FreeArgs,
+            class R = std::invoke_result_t<F&, FreeArgs&&..., BoundArgs&...>>
+  constexpr R operator()(FreeArgs&&... free_args) & {
+    return functional_internal::ApplyBack<R>(
+        bound_args_, Idx(), std::forward<FreeArgs>(free_args)...);
+  }
+
+  template <class... FreeArgs,
+            class R = std::invoke_result_t<const F&, FreeArgs&&...,
+                                           const BoundArgs&...>>
+  constexpr R operator()(FreeArgs&&... free_args) const& {
+    return functional_internal::ApplyBack<R>(
+        bound_args_, Idx(), std::forward<FreeArgs>(free_args)...);
+  }
+
+  template <class... FreeArgs,
+            class R = std::invoke_result_t<F&&, FreeArgs&&..., BoundArgs&&...>>
+  constexpr R operator()(FreeArgs&&... free_args) && {
+    // This overload is called when *this is an rvalue. If some of the bound
+    // arguments are stored by value or rvalue reference, we move them.
+    return functional_internal::ApplyBack<R>(
+        std::move(bound_args_), Idx(), std::forward<FreeArgs>(free_args)...);
+  }
+
+  template <class... FreeArgs,
+            class R = std::invoke_result_t<const F&&, FreeArgs&&...,
+                                           const BoundArgs&&...>>
+  constexpr R operator()(FreeArgs&&... free_args) const&& {
+    // This overload is called when *this is an rvalue. If some of the bound
+    // arguments are stored by value or rvalue reference, we move them.
+    return functional_internal::ApplyBack<R>(
+        std::move(bound_args_), Idx(), std::forward<FreeArgs>(free_args)...);
+  }
+};
+
+template <class F, class... BoundArgs>
+using bind_back_t = BackBinder<std::decay_t<F>, std::decay_t<BoundArgs>...>;
+
+}  // namespace functional_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_FUNCTIONAL_INTERNAL_BACK_BINDER_H_
diff --git a/absl/functional/internal/front_binder.h b/absl/functional/internal/front_binder.h
new file mode 100644
index 00000000000..d0c3a5a0506
--- /dev/null
+++ b/absl/functional/internal/front_binder.h
@@ -0,0 +1,93 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Implementation details for `absl::bind_front()`.
+
+#ifndef ABSL_FUNCTIONAL_INTERNAL_FRONT_BINDER_H_
+#define ABSL_FUNCTIONAL_INTERNAL_FRONT_BINDER_H_
+
+#include <cstddef>
+#include <type_traits>
+#include <utility>
+
+#include "absl/container/internal/compressed_tuple.h"
+#include "absl/meta/type_traits.h"
+#include "absl/utility/utility.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace functional_internal {
+
+// Invoke the method, expanding the tuple of bound arguments.
+template <class R, class Tuple, size_t... Idx, class... Args>
+R Apply(Tuple&& bound, std::index_sequence<Idx...>, Args&&... free) {
+  return std::invoke(std::forward<Tuple>(bound).template get<Idx>()...,
+                     std::forward<Args>(free)...);
+}
+
+template <class F, class... BoundArgs>
+class FrontBinder {
+  using BoundArgsT = absl::container_internal::CompressedTuple<F, BoundArgs...>;
+  using Idx = std::make_index_sequence<sizeof...(BoundArgs) + 1>;
+
+  BoundArgsT bound_args_;
+
+ public:
+  template <class... Ts>
+  constexpr explicit FrontBinder(std::in_place_t, Ts&&... ts)
+      : bound_args_(std::forward<Ts>(ts)...) {}
+
+  template <class... FreeArgs,
+            class R = std::invoke_result_t<F&, BoundArgs&..., FreeArgs&&...>>
+  R operator()(FreeArgs&&... free_args) & {
+    return functional_internal::Apply<R>(bound_args_, Idx(),
+                                         std::forward<FreeArgs>(free_args)...);
+  }
+
+  template <class... FreeArgs,
+            class R = std::invoke_result_t<const F&, const BoundArgs&...,
+                                           FreeArgs&&...>>
+  R operator()(FreeArgs&&... free_args) const& {
+    return functional_internal::Apply<R>(bound_args_, Idx(),
+                                         std::forward<FreeArgs>(free_args)...);
+  }
+
+  template <class... FreeArgs,
+            class R = std::invoke_result_t<F&&, BoundArgs&&..., FreeArgs&&...>>
+  R operator()(FreeArgs&&... free_args) && {
+    // This overload is called when *this is an rvalue. If some of the bound
+    // arguments are stored by value or rvalue reference, we move them.
+    return functional_internal::Apply<R>(std::move(bound_args_), Idx(),
+                                         std::forward<FreeArgs>(free_args)...);
+  }
+
+  template <class... FreeArgs,
+            class R = std::invoke_result_t<const F&&, const BoundArgs&&...,
+                                           FreeArgs&&...>>
+  R operator()(FreeArgs&&... free_args) const&& {
+    // This overload is called when *this is an rvalue. If some of the bound
+    // arguments are stored by value or rvalue reference, we move them.
+    return functional_internal::Apply<R>(std::move(bound_args_), Idx(),
+                                         std::forward<FreeArgs>(free_args)...);
+  }
+};
+
+template <class F, class... BoundArgs>
+using bind_front_t = FrontBinder<std::decay_t<F>, std::decay_t<BoundArgs>...>;
+
+}  // namespace functional_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_FUNCTIONAL_INTERNAL_FRONT_BINDER_H_
diff --git a/absl/functional/internal/function_ref.h b/absl/functional/internal/function_ref.h
new file mode 100644
index 00000000000..1e905fa2571
--- /dev/null
+++ b/absl/functional/internal/function_ref.h
@@ -0,0 +1,149 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_FUNCTIONAL_INTERNAL_FUNCTION_REF_H_
+#define ABSL_FUNCTIONAL_INTERNAL_FUNCTION_REF_H_
+
+#include <cassert>
+#include <functional>
+#include <type_traits>
+
+#include "absl/functional/any_invocable.h"
+#include "absl/meta/type_traits.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace functional_internal {
+
+// Like a void* that can handle function pointers as well. The standard does not
+// allow function pointers to round-trip through void*, but void(*)() is fine.
+//
+// Note: It's important that this class remains trivial and is the same size as
+// a pointer, since this allows the compiler to perform tail-call optimizations
+// when the underlying function is a callable object with a matching signature.
+union VoidPtr {
+  const void* obj;
+  void (*fun)();
+};
+
+// Chooses the best type for passing T as an argument.
+// Attempt to be close to SystemV AMD64 ABI. Objects with trivial copy ctor are
+// passed by value.
+template <typename T, bool IsLValueReference = std::is_lvalue_reference_v<T>>
+struct PassByValue : std::false_type {};
+
+template <typename T>
+struct PassByValue<T, /*IsLValueReference=*/false>
+    : std::bool_constant<
+          std::is_trivially_copy_constructible_v<T> &&
+          std::is_trivially_copy_assignable_v<std::remove_cv_t<T>> &&
+          std::is_trivially_destructible_v<T> &&
+          sizeof(T) <= 2 * sizeof(void*)> {};
+
+template <typename T>
+struct ForwardT : std::conditional<PassByValue<T>::value, T, T&&> {};
+
+// An Invoker takes a pointer to the type-erased invokable object, followed by
+// the arguments that the invokable object expects.
+//
+// Note: The order of arguments here is an optimization, since member functions
+// have an implicit "this" pointer as their first argument, putting VoidPtr
+// first allows the compiler to perform tail-call optimization in many cases.
+template <typename R, typename... Args>
+using Invoker = R (*)(VoidPtr, typename ForwardT<Args>::type...);
+
+//
+// InvokeObject and InvokeFunction provide static "Invoke" functions that can be
+// used as Invokers for objects or functions respectively.
+//
+// static_cast<R> handles the case the return type is void.
+template <typename Obj, typename R, typename... Args>
+R InvokeObject(VoidPtr ptr, typename ForwardT<Args>::type... args) {
+  using T = std::remove_reference_t<Obj>;
+  return static_cast<R>(std::invoke(
+      std::forward<Obj>(*const_cast<T*>(static_cast<const T*>(ptr.obj))),
+      std::forward<typename ForwardT<Args>::type>(args)...));
+}
+
+template <typename Obj, typename Fun, Fun F, typename R, typename... Args>
+R InvokeObject(VoidPtr ptr, typename ForwardT<Args>::type... args) {
+  using T = std::remove_reference_t<Obj>;
+  Obj&& obj =
+      std::forward<Obj>(*const_cast<T*>(static_cast<const T*>(ptr.obj)));
+  // Avoid std::invoke() since the callee is a known function at compile time
+  if constexpr (std::is_member_function_pointer_v<Fun>) {
+    return static_cast<R>((std::forward<Obj>(obj).*F)(
+        std::forward<typename ForwardT<Args>::type>(args)...));
+  } else {
+    return static_cast<R>(
+        F(std::forward<Obj>(obj),
+          std::forward<typename ForwardT<Args>::type>(args)...));
+  }
+}
+
+template <typename T, typename Fun, Fun F, typename R, typename... Args>
+R InvokePtr(VoidPtr ptr, typename ForwardT<Args>::type... args) {
+  T* obj = const_cast<T*>(static_cast<const T*>(ptr.obj));
+  // Avoid std::invoke() since the callee is a known function at compile time
+  if constexpr (std::is_member_function_pointer_v<Fun>) {
+    return static_cast<R>(
+        (obj->*F)(std::forward<typename ForwardT<Args>::type>(args)...));
+  } else {
+    return static_cast<R>(
+        F(obj, std::forward<typename ForwardT<Args>::type>(args)...));
+  }
+}
+
+template <typename Fun, typename R, typename... Args>
+R InvokeFunction(VoidPtr ptr, typename ForwardT<Args>::type... args) {
+  auto f = reinterpret_cast<Fun>(ptr.fun);
+  return static_cast<R>(
+      std::invoke(f, std::forward<typename ForwardT<Args>::type>(args)...));
+}
+
+template <typename Fun, Fun F, typename R, typename... Args>
+R InvokeFunction(VoidPtr, typename ForwardT<Args>::type... args) {
+  return static_cast<R>(
+      F(std::forward<typename ForwardT<Args>::type>(args)...));
+}
+
+template <typename Sig>
+void AssertNonNull(const std::function<Sig>& f) {
+  assert(f != nullptr);
+  (void)f;
+}
+
+template <typename Sig>
+void AssertNonNull(const AnyInvocable<Sig>& f) {
+  assert(f != nullptr);
+  (void)f;
+}
+
+template <typename F>
+void AssertNonNull(const F&) {}
+
+template <typename F, typename C>
+void AssertNonNull(F C::* f) {
+  assert(f != nullptr);
+  (void)f;
+}
+
+template <bool C>
+using EnableIf = typename ::std::enable_if_t<C, int>;
+
+}  // namespace functional_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_FUNCTIONAL_INTERNAL_FUNCTION_REF_H_
diff --git a/absl/functional/overload.h b/absl/functional/overload.h
new file mode 100644
index 00000000000..35eec96292d
--- /dev/null
+++ b/absl/functional/overload.h
@@ -0,0 +1,72 @@
+// Copyright 2023 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: overload.h
+// -----------------------------------------------------------------------------
+//
+// `absl::Overload` is a functor that provides overloads based on the functors
+// with which it is created. This can, for example, be used to locally define an
+// anonymous visitor type for `std::visit` inside a function using lambdas.
+//
+// Before using this function, consider whether named function overloads would
+// be a better design.
+//
+// Example:
+//
+//     std::variant<std::string, int32_t, int64_t> v(int32_t{1});
+//     const size_t result =
+//         std::visit(absl::Overload{
+//                        [](const std::string& s) { return s.size(); },
+//                        [](const auto& s) { return sizeof(s); },
+//                    },
+//                    v);
+//     assert(result == 4);
+//
+
+#ifndef ABSL_FUNCTIONAL_OVERLOAD_H_
+#define ABSL_FUNCTIONAL_OVERLOAD_H_
+
+#include "absl/base/config.h"
+#include "absl/meta/type_traits.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+template <typename... T>
+struct Overload final : T... {
+  using T::operator()...;
+
+  // For historical reasons we want to support use that looks like a function
+  // call:
+  //
+  //     absl::Overload(lambda_1, lambda_2)
+  //
+  // This works automatically in C++20 because we have support for parenthesized
+  // aggregate initialization. Before then we must provide a constructor that
+  // makes this work.
+  //
+  constexpr explicit Overload(T... ts) : T(std::move(ts))... {}
+};
+
+// Before C++20, which added support for CTAD for aggregate types, we must also
+// teach the compiler how to deduce the template arguments for Overload.
+//
+template <typename... T>
+Overload(T...) -> Overload<T...>;
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_FUNCTIONAL_OVERLOAD_H_
diff --git a/absl/functional/overload_test.cc b/absl/functional/overload_test.cc
new file mode 100644
index 00000000000..bb510203d7a
--- /dev/null
+++ b/absl/functional/overload_test.cc
@@ -0,0 +1,207 @@
+// Copyright 2023 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/functional/overload.h"
+
+#include <cstdint>
+#include <string>
+#include <type_traits>
+#include <variant>
+
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/string_view.h"
+
+namespace {
+
+TEST(OverloadTest, DispatchConsidersTypeWithAutoFallback) {
+  auto overloaded = absl::Overload{
+      [](int v) { return absl::StrCat("int ", v); },
+      [](double v) { return absl::StrCat("double ", v); },
+      [](const char* v) { return absl::StrCat("const char* ", v); },
+      [](auto v) { return absl::StrCat("auto ", v); },
+  };
+
+  EXPECT_EQ("int 1", overloaded(1));
+  EXPECT_EQ("double 2.5", overloaded(2.5));
+  EXPECT_EQ("const char* hello", overloaded("hello"));
+  EXPECT_EQ("auto 1.5", overloaded(1.5f));
+}
+
+TEST(OverloadTest, DispatchConsidersNumberOfArguments) {
+  auto overloaded = absl::Overload{
+      [](int a) { return a + 1; },
+      [](int a, int b) { return a * b; },
+      []() -> absl::string_view { return "none"; },
+  };
+
+  EXPECT_EQ(3, overloaded(2));
+  EXPECT_EQ(21, overloaded(3, 7));
+  EXPECT_EQ("none", overloaded());
+}
+
+TEST(OverloadTest, SupportsConstantEvaluation) {
+  auto overloaded = absl::Overload{
+      [](int a) { return a + 1; },
+      [](int a, int b) { return a * b; },
+      []() -> absl::string_view { return "none"; },
+  };
+
+  static_assert(overloaded() == "none");
+  static_assert(overloaded(2) == 3);
+  static_assert(overloaded(3, 7) == 21);
+}
+
+TEST(OverloadTest, PropogatesDefaults) {
+  auto overloaded = absl::Overload{
+      [](int a, int b = 5) { return a * b; },
+      [](double c) { return c; },
+  };
+
+  EXPECT_EQ(21, overloaded(3, 7));
+  EXPECT_EQ(35, overloaded(7));
+  EXPECT_EQ(2.5, overloaded(2.5));
+}
+
+TEST(OverloadTest, AmbiguousWithDefaultsNotInvocable) {
+  auto overloaded = absl::Overload{
+      [](int a, int b = 5) { return a * b; },
+      [](int c) { return c; },
+  };
+
+  static_assert(!std::is_invocable_v<decltype(overloaded), int>);
+  static_assert(std::is_invocable_v<decltype(overloaded), int, int>);
+}
+
+TEST(OverloadTest, AmbiguousDuplicatesNotInvocable) {
+  auto overloaded = absl::Overload{
+      [](int a) { return a; },
+      [](int c) { return c; },
+  };
+
+  static_assert(!std::is_invocable_v<decltype(overloaded), int>);
+}
+
+TEST(OverloadTest, AmbiguousConversionNotInvocable) {
+  auto overloaded = absl::Overload{
+      [](uint16_t a) { return a; },
+      [](uint64_t c) { return c; },
+  };
+
+  static_assert(!std::is_invocable_v<decltype(overloaded), int>);
+}
+
+TEST(OverloadTest, AmbiguousConversionWithAutoNotInvocable) {
+  auto overloaded = absl::Overload{
+      [](auto a) { return a; },
+      [](auto c) { return c; },
+  };
+
+  static_assert(!std::is_invocable_v<decltype(overloaded), int>);
+}
+
+#if ABSL_INTERNAL_CPLUSPLUS_LANG >= 202002L
+
+TEST(OverloadTest, AmbiguousConversionWithAutoAndTemplateNotInvocable) {
+  auto overloaded = absl::Overload{
+      [](auto a) { return a; },
+      []<class T>(T c) { return c; },
+  };
+
+  static_assert(!std::is_invocable_v<decltype(overloaded), int>);
+}
+
+TEST(OverloadTest, DispatchConsidersTypeWithTemplateFallback) {
+  auto overloaded = absl::Overload{
+      [](int a) { return a; },
+      []<class T>(T c) { return c * 2; },
+  };
+
+  EXPECT_EQ(7, overloaded(7));
+  EXPECT_EQ(14.0, overloaded(7.0));
+}
+
+#endif  // ABSL_INTERNAL_CPLUSPLUS_LANG >= 202002L
+
+TEST(OverloadTest, DispatchConsidersSfinae) {
+  auto overloaded = absl::Overload{
+      [](auto a) -> decltype(a + 1) { return a + 1; },
+  };
+
+  static_assert(std::is_invocable_v<decltype(overloaded), int>);
+  static_assert(!std::is_invocable_v<decltype(overloaded), std::string>);
+}
+
+TEST(OverloadTest, VariantVisitDispatchesCorrectly) {
+  std::variant<int, double, std::string> v(1);
+  auto overloaded = absl::Overload{
+      [](int) -> absl::string_view { return "int"; },
+      [](double) -> absl::string_view { return "double"; },
+      [](const std::string&) -> absl::string_view { return "string"; },
+  };
+
+  EXPECT_EQ("int", std::visit(overloaded, v));
+  v = 1.1;
+  EXPECT_EQ("double", std::visit(overloaded, v));
+  v = "hello";
+  EXPECT_EQ("string", std::visit(overloaded, v));
+}
+
+TEST(OverloadTest, VariantVisitWithAutoFallbackDispatchesCorrectly) {
+  std::variant<std::string, int32_t, int64_t> v(int32_t{1});
+  auto overloaded = absl::Overload{
+      [](const std::string& s) { return s.size(); },
+      [](const auto& s) { return sizeof(s); },
+  };
+
+  EXPECT_EQ(4, std::visit(overloaded, v));
+  v = int64_t{1};
+  EXPECT_EQ(8, std::visit(overloaded, v));
+  v = std::string("hello");
+  EXPECT_EQ(5, std::visit(overloaded, v));
+}
+
+// This API used to be exported as a function, so it should also work fine to
+// use parantheses when initializing it.
+TEST(OverloadTest, UseWithParentheses) {
+  const auto overloaded =
+      absl::Overload([](const std::string& s) { return s.size(); },
+                     [](const auto& s) { return sizeof(s); });
+
+  std::variant<std::string, int32_t, int64_t> v(int32_t{1});
+  EXPECT_EQ(4, std::visit(overloaded, v));
+
+  v = int64_t{1};
+  EXPECT_EQ(8, std::visit(overloaded, v));
+
+  v = std::string("hello");
+  EXPECT_EQ(5, std::visit(overloaded, v));
+}
+
+TEST(OverloadTest, HasConstexprConstructor) {
+  constexpr auto overloaded = absl::Overload{
+      [](int v) { return absl::StrCat("int ", v); },
+      [](double v) { return absl::StrCat("double ", v); },
+      [](const char* v) { return absl::StrCat("const char* ", v); },
+      [](auto v) { return absl::StrCat("auto ", v); },
+  };
+
+  EXPECT_EQ("int 1", overloaded(1));
+  EXPECT_EQ("double 2.5", overloaded(2.5));
+  EXPECT_EQ("const char* hello", overloaded("hello"));
+  EXPECT_EQ("auto 1.5", overloaded(1.5f));
+}
+
+}  // namespace
diff --git a/absl/hash/BUILD.bazel b/absl/hash/BUILD.bazel
index c51248a5cb9..41d7d686b0c 100644
--- a/absl/hash/BUILD.bazel
+++ b/absl/hash/BUILD.bazel
@@ -14,7 +14,9 @@
 # limitations under the License.
 #
 
-load("@rules_cc//cc:defs.bzl", "cc_library", "cc_test")
+load("@rules_cc//cc:cc_binary.bzl", "cc_binary")
+load("@rules_cc//cc:cc_library.bzl", "cc_library")
+load("@rules_cc//cc:cc_test.bzl", "cc_test")
 load(
     "//absl:copts/configure_copts.bzl",
     "ABSL_DEFAULT_COPTS",
@@ -22,9 +24,16 @@ load(
     "ABSL_TEST_COPTS",
 )
 
-package(default_visibility = ["//visibility:public"])
+package(
+    default_visibility = ["//visibility:private"],
+    features = [
+        "header_modules",
+        "layering_check",
+        "parse_headers",
+    ],
+)
 
-licenses(["notice"])  # Apache 2.0
+licenses(["notice"])
 
 cc_library(
     name = "hash",
@@ -35,12 +44,18 @@ cc_library(
     hdrs = ["hash.h"],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
     deps = [
         ":city",
+        ":weakly_mixed_integer",
+        "//absl/base:config",
         "//absl/base:core_headers",
         "//absl/base:endian",
+        "//absl/base:prefetch",
         "//absl/container:fixed_array",
+        "//absl/functional:function_ref",
         "//absl/meta:type_traits",
+        "//absl/numeric:bits",
         "//absl/numeric:int128",
         "//absl/strings",
         "//absl/types:optional",
@@ -51,44 +66,98 @@ cc_library(
 
 cc_library(
     name = "hash_testing",
-    testonly = 1,
+    testonly = True,
     hdrs = ["hash_testing.h"],
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
     deps = [
         ":spy_hash_state",
         "//absl/meta:type_traits",
         "//absl/strings",
         "//absl/types:variant",
-        "@com_google_googletest//:gtest",
+        "@googletest//:gtest",
     ],
 )
 
 cc_test(
     name = "hash_test",
-    srcs = ["hash_test.cc"],
+    size = "large",
+    srcs = [
+        "hash_test.cc",
+        "internal/hash_test.h",
+    ],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":hash",
         ":hash_testing",
-        "//absl/base:core_headers",
+        ":spy_hash_state",
+        "//absl/base:config",
+        "//absl/container:flat_hash_map",
         "//absl/container:flat_hash_set",
-        "//absl/hash:spy_hash_state",
+        "//absl/memory",
         "//absl/meta:type_traits",
-        "//absl/numeric:int128",
-        "@com_google_googletest//:gtest_main",
+        "//absl/numeric:bits",
+        "//absl/strings:cord",
+        "//absl/strings:cord_test_helpers",
+        "//absl/strings:string_view",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "hash_instantiated_test",
+    srcs = [
+        "hash_instantiated_test.cc",
+        "internal/hash_test.h",
+    ],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":hash",
+        ":hash_testing",
+        "//absl/base:config",
+        "//absl/container:btree",
+        "//absl/container:flat_hash_map",
+        "//absl/container:flat_hash_set",
+        "//absl/container:node_hash_map",
+        "//absl/container:node_hash_set",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_binary(
+    name = "hash_benchmark",
+    testonly = True,
+    srcs = ["hash_benchmark.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    tags = ["benchmark"],
+    visibility = ["//visibility:private"],
+    deps = [
+        ":hash",
+        "//absl/base:core_headers",
+        "//absl/container:flat_hash_set",
+        "//absl/random",
+        "//absl/strings",
+        "//absl/strings:cord",
+        "//absl/strings:cord_test_helpers",
+        "@google_benchmark//:benchmark_main",
     ],
 )
 
 cc_library(
     name = "spy_hash_state",
-    testonly = 1,
+    testonly = True,
     hdrs = ["internal/spy_hash_state.h"],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     visibility = ["//visibility:private"],
     deps = [
         ":hash",
+        ":weakly_mixed_integer",
         "//absl/strings",
         "//absl/strings:str_format",
     ],
@@ -102,10 +171,14 @@ cc_library(
     ],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl/hash:__pkg__",
+    ],
     deps = [
         "//absl/base:config",
         "//absl/base:core_headers",
         "//absl/base:endian",
+        "//absl/numeric:bits",
     ],
 )
 
@@ -116,6 +189,36 @@ cc_test(
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":city",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_library(
+    name = "weakly_mixed_integer",
+    hdrs = ["internal/weakly_mixed_integer.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl/container:__pkg__",
+        "//absl/strings:__pkg__",
+        "//absl/types:__pkg__",
+    ],
+    deps = [
+        "//absl/base:config",
+    ],
+)
+
+cc_test(
+    name = "low_level_hash_test",
+    srcs = ["internal/low_level_hash_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:private"],
+    deps = [
+        ":hash",
+        "//absl/strings",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
diff --git a/absl/hash/CMakeLists.txt b/absl/hash/CMakeLists.txt
index febc551fced..c593a5bc3c6 100644
--- a/absl/hash/CMakeLists.txt
+++ b/absl/hash/CMakeLists.txt
@@ -25,16 +25,20 @@ absl_cc_library(
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
+    absl::bits
+    absl::city
+    absl::config
     absl::core_headers
     absl::endian
     absl::fixed_array
+    absl::function_ref
     absl::meta
     absl::int128
     absl::strings
     absl::optional
     absl::variant
     absl::utility
-    absl::city
+    absl::weakly_mixed_integer
   PUBLIC
 )
 
@@ -50,8 +54,9 @@ absl_cc_library(
     absl::meta
     absl::strings
     absl::variant
-    gmock
+    GTest::gmock
   TESTONLY
+  PUBLIC
 )
 
 absl_cc_test(
@@ -59,19 +64,48 @@ absl_cc_test(
     hash_test
   SRCS
     "hash_test.cc"
+    "internal/hash_test.h"
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
+    absl::bits
+    absl::cord_test_helpers
+    absl::flat_hash_map
+    absl::flat_hash_set
     absl::hash
     absl::hash_testing
-    absl::core_headers
-    absl::flat_hash_set
-    absl::spy_hash_state
+    absl::memory
     absl::meta
-    absl::int128
-    gmock_main
+    absl::spy_hash_state
+    absl::string_view
+    GTest::gmock_main
+)
+
+absl_cc_test(
+  NAME
+    hash_instantiated_test
+  SRCS
+    "hash_instantiated_test.cc"
+    "internal/hash_test.h"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::hash
+    absl::hash_testing
+    absl::config
+    absl::btree
+    absl::flat_hash_map
+    absl::flat_hash_set
+    absl::node_hash_map
+    absl::node_hash_set
+    GTest::gmock_main
 )
 
+# Internal-only target, do not depend on directly.
+#
+# Note: Even though external code should not depend on this target
+# directly, it must be marked PUBLIC since it is a dependency of
+# hash_testing.
 absl_cc_library(
   NAME
     spy_hash_state
@@ -83,9 +117,12 @@ absl_cc_library(
     absl::hash
     absl::strings
     absl::str_format
+    absl::weakly_mixed_integer
   TESTONLY
+  PUBLIC
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     city
@@ -98,6 +135,7 @@ absl_cc_library(
   DEPS
     absl::config
     absl::core_headers
+    absl::bits
     absl::endian
 )
 
@@ -110,6 +148,30 @@ absl_cc_test(
     ${ABSL_TEST_COPTS}
   DEPS
     absl::city
-    gmock_main
+    GTest::gmock_main
 )
 
+# Internal-only target, do not depend on directly.
+absl_cc_library(
+  NAME
+    weakly_mixed_integer
+  HDRS
+    "internal/weakly_mixed_integer.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::config
+)
+
+absl_cc_test(
+  NAME
+    low_level_hash_test
+  SRCS
+    "internal/low_level_hash_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::hash
+    absl::strings
+    GTest::gmock_main
+)
diff --git a/absl/hash/hash.h b/absl/hash/hash.h
index 297dc9cb22a..7a767713c9b 100644
--- a/absl/hash/hash.h
+++ b/absl/hash/hash.h
@@ -26,9 +26,9 @@
 //     support Abseil hashing without requiring you to define a hashing
 //     algorithm.
 //   * `HashState`, a type-erased class which implements the manipulation of the
-//     hash state (H) itself, contains member functions `combine()` and
-//     `combine_contiguous()`, which you can use to contribute to an existing
-//     hash state when hashing your types.
+//     hash state (H) itself; contains member functions `combine()`,
+//     `combine_contiguous()`, and `combine_unordered()`; and which you can use
+//     to contribute to an existing hash state when hashing your types.
 //
 // Unlike `std::hash` or other hashing frameworks, the Abseil hashing framework
 // provides most of its utility by abstracting away the hash algorithm (and its
@@ -37,8 +37,16 @@
 // types. Hashing of that combined state is separately done by `absl::Hash`.
 //
 // One should assume that a hash algorithm is chosen randomly at the start of
-// each process.  E.g., absl::Hash<int>()(9) in one process and
-// absl::Hash<int>()(9) in another process are likely to differ.
+// each process.  E.g., `absl::Hash<int>{}(9)` in one process and
+// `absl::Hash<int>{}(9)` in another process are likely to differ.
+//
+// `absl::Hash` may also produce different values from different dynamically
+// loaded libraries. For this reason, `absl::Hash` values must never cross
+// boundaries in dynamically loaded libraries (including when used in types like
+// hash containers.)
+//
+// `absl::Hash` is intended to strongly mix input bits with a target of passing
+// an [Avalanche Test](https://en.wikipedia.org/wiki/Avalanche_effect).
 //
 // Example:
 //
@@ -70,9 +78,20 @@
 #ifndef ABSL_HASH_HASH_H_
 #define ABSL_HASH_HASH_H_
 
+#include <cstddef>
+#include <cstdint>
+#include <tuple>
+#include <type_traits>
+#include <utility>
+
+#include "absl/base/config.h"
+#include "absl/functional/function_ref.h"
 #include "absl/hash/internal/hash.h"
+#include "absl/hash/internal/weakly_mixed_integer.h"
+#include "absl/meta/type_traits.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 // -----------------------------------------------------------------------------
 // `absl::Hash`
@@ -84,7 +103,6 @@ namespace absl {
 //  * T is an arithmetic or pointer type
 //  * T defines an overload for `AbslHashValue(H, const T&)` for an arbitrary
 //    hash state `H`.
-//  - T defines a specialization of `HASH_NAMESPACE::hash<T>`
 //  - T defines a specialization of `std::hash<T>`
 //
 // `absl::Hash` intrinsically supports the following types:
@@ -97,18 +115,35 @@ namespace absl {
 //   * std::tuple<Ts...>, if all the Ts... are hashable
 //   * std::unique_ptr and std::shared_ptr
 //   * All string-like types including:
-//     * std::string
-//     * std::string_view (as well as any instance of std::basic_string that
-//       uses char and std::char_traits)
+//     * absl::Cord
+//     * std::string (as well as any instance of std::basic_string that
+//       uses one of {char, wchar_t, char8_t, char16_t, char32_t} and its
+//       associated std::char_traits)
+//     * std::string_view (as well as any instance of std::basic_string_view
+//       that uses one of {char, wchar_t, char8_t, char16_t, char32_t} and its
+//       associated std::char_traits)
 //  * All the standard sequence containers (provided the elements are hashable)
-//  * All the standard ordered associative containers (provided the elements are
+//  * All the standard associative containers (provided the elements are
 //    hashable)
 //  * absl types such as the following:
 //    * absl::string_view
-//    * absl::InlinedVector
-//    * absl::FixedArray
 //    * absl::uint128
 //    * absl::Time, absl::Duration, and absl::TimeZone
+//  * absl containers (provided the elements are hashable) such as the
+//    following:
+//    * absl::flat_hash_set, absl::node_hash_set, absl::btree_set
+//    * absl::flat_hash_map, absl::node_hash_map, absl::btree_map
+//    * absl::btree_multiset, absl::btree_multimap
+//    * absl::InlinedVector
+//    * absl::FixedArray
+//
+// When absl::Hash is used to hash an unordered container with a custom hash
+// functor, the elements are hashed using default absl::Hash semantics, not
+// the custom hash functor.  This is consistent with the behavior of
+// operator==() on unordered containers, which compares elements pairwise with
+// operator==() rather than the custom equality functor.  It is usually a
+// mistake to use either operator==() or absl::Hash on unordered collections
+// that use functors incompatible with operator==() equality.
 //
 // Note: the list above is not meant to be exhaustive. Additional type support
 // may be added, in which case the above list will be updated.
@@ -123,8 +158,6 @@ namespace absl {
 //   * Natively supported types out of the box (see above)
 //   * Types for which an `AbslHashValue()` overload is provided (such as
 //     user-defined types). See "Adding Type Support to `absl::Hash`" below.
-//   * Types which define a `HASH_NAMESPACE::hash<T>` specialization (aka
-//     `__gnu_cxx::hash<T>` for gcc/Clang or `stdext::hash<T>` for MSVC)
 //   * Types which define a `std::hash<T>` specialization
 //
 // The fallback to legacy hash functions exists mainly for backwards
@@ -149,7 +182,8 @@ namespace absl {
 //   that are otherwise difficult to extend using `AbslHashValue()`. (See the
 //   `HashState` class below.)
 //
-// The "hash state" concept contains two member functions for mixing hash state:
+// The "hash state" concept contains three member functions for mixing hash
+// state:
 //
 // * `H::combine(state, values...)`
 //
@@ -183,6 +217,15 @@ namespace absl {
 //    (it may perform internal optimizations). If you need this guarantee, use a
 //    loop instead.
 //
+// * `H::combine_unordered(state, begin, end)`
+//
+//    Combines a set of elements denoted by an iterator pair into a hash
+//    state, returning the updated state.  Note that the existing hash
+//    state is move-only and must be passed by value.
+//
+//    Unlike the other two methods, the hashing is order-independent.
+//    This can be used to hash unordered collections.
+//
 // -----------------------------------------------------------------------------
 // Adding Type Support to `absl::Hash`
 // -----------------------------------------------------------------------------
@@ -212,6 +255,26 @@ namespace absl {
 template <typename T>
 using Hash = absl::hash_internal::Hash<T>;
 
+// HashOf
+//
+// absl::HashOf() is a helper that generates a hash from the values of its
+// arguments.  It dispatches to absl::Hash directly, as follows:
+//  * HashOf(t) == absl::Hash<T>{}(t)
+//  * HashOf(a, b, c) == HashOf(std::make_tuple(a, b, c))
+//
+// HashOf(a1, a2, ...) == HashOf(b1, b2, ...) is guaranteed when
+//  * The argument lists have pairwise identical C++ types
+//  * a1 == b1 && a2 == b2 && ...
+//
+// The requirement that the arguments match in both type and value is critical.
+// It means that `a == b` does not necessarily imply `HashOf(a) == HashOf(b)` if
+// `a` and `b` have different types. For example, `HashOf(2) != HashOf(2.0)`.
+template <int&... ExplicitArgumentBarrier, typename... Types>
+size_t HashOf(const Types&... values) {
+  auto tuple = std::tie(values...);
+  return absl::Hash<decltype(tuple)>{}(tuple);
+}
+
 // HashState
 //
 // A type erased version of the hash state concept, for use in user-defined
@@ -219,8 +282,9 @@ using Hash = absl::hash_internal::Hash<T>;
 // classes, virtual functions, etc.). The type erasure adds overhead so it
 // should be avoided unless necessary.
 //
-// Note: This wrapper will only erase calls to:
+// Note: This wrapper will only erase calls to
 //     combine_contiguous(H, const unsigned char*, size_t)
+//     RunCombineUnordered(H, CombinerF)
 //
 // All other calls will be handled internally and will not invoke overloads
 // provided by the wrapped class.
@@ -261,8 +325,11 @@ class HashState : public hash_internal::HashStateBase<HashState> {
   // Create a new `HashState` instance that wraps `state`. All calls to
   // `combine()` and `combine_contiguous()` on the new instance will be
   // redirected to the original `state` object. The `state` object must outlive
-  // the `HashState` instance.
-  template <typename T>
+  // the `HashState` instance. `T` must be a subclass of `HashStateBase<T>` -
+  // users should not define their own HashState types.
+  template <typename T,
+            std::enable_if_t<
+                std::is_base_of_v<hash_internal::HashStateBase<T>, T>, int> = 0>
   static HashState Create(T* state) {
     HashState s;
     s.Init(state);
@@ -289,11 +356,20 @@ class HashState : public hash_internal::HashStateBase<HashState> {
     hash_state.combine_contiguous_(hash_state.state_, first, size);
     return hash_state;
   }
+
+  static HashState combine_weakly_mixed_integer(
+      HashState hash_state, hash_internal::WeaklyMixedInteger value) {
+    hash_state.combine_weakly_mixed_integer_(hash_state.state_, value);
+    return hash_state;
+  }
   using HashState::HashStateBase::combine_contiguous;
 
  private:
   HashState() = default;
 
+  friend class HashState::HashStateBase;
+  friend struct hash_internal::CombineRaw;
+
   template <typename T>
   static void CombineContiguousImpl(void* p, const unsigned char* first,
                                     size_t size) {
@@ -301,22 +377,89 @@ class HashState : public hash_internal::HashStateBase<HashState> {
     state = T::combine_contiguous(std::move(state), first, size);
   }
 
+  template <typename T>
+  static void CombineWeaklyMixedIntegerImpl(
+      void* p, hash_internal::WeaklyMixedInteger value) {
+    T& state = *static_cast<T*>(p);
+    state = T::combine_weakly_mixed_integer(std::move(state), value);
+  }
+
+  static HashState combine_raw(HashState hash_state, uint64_t value) {
+    hash_state.combine_raw_(hash_state.state_, value);
+    return hash_state;
+  }
+
+  template <typename T>
+  static void CombineRawImpl(void* p, uint64_t value) {
+    T& state = *static_cast<T*>(p);
+    state = hash_internal::CombineRaw()(std::move(state), value);
+  }
+
   template <typename T>
   void Init(T* state) {
     state_ = state;
+    combine_weakly_mixed_integer_ = &CombineWeaklyMixedIntegerImpl<T>;
     combine_contiguous_ = &CombineContiguousImpl<T>;
+    combine_raw_ = &CombineRawImpl<T>;
+    run_combine_unordered_ = &RunCombineUnorderedImpl<T>;
+  }
+
+  template <typename HS>
+  struct CombineUnorderedInvoker {
+    template <typename T, typename ConsumerT>
+    void operator()(T inner_state, ConsumerT inner_cb) {
+      f(HashState::Create(&inner_state),
+        [&](HashState& inner_erased) { inner_cb(inner_erased.Real<T>()); });
+    }
+
+    absl::FunctionRef<void(HS, absl::FunctionRef<void(HS&)>)> f;
+  };
+
+  template <typename T>
+  static HashState RunCombineUnorderedImpl(
+      HashState state,
+      absl::FunctionRef<void(HashState, absl::FunctionRef<void(HashState&)>)>
+          f) {
+    // Note that this implementation assumes that inner_state and outer_state
+    // are the same type.  This isn't true in the SpyHash case, but SpyHash
+    // types are move-convertible to each other, so this still works.
+    T& real_state = state.Real<T>();
+    real_state = T::RunCombineUnordered(
+        std::move(real_state), CombineUnorderedInvoker<HashState>{f});
+    return state;
+  }
+
+  template <typename CombinerT>
+  static HashState RunCombineUnordered(HashState state, CombinerT combiner) {
+    auto* run = state.run_combine_unordered_;
+    return run(std::move(state), std::ref(combiner));
   }
 
   // Do not erase an already erased state.
   void Init(HashState* state) {
     state_ = state->state_;
+    combine_weakly_mixed_integer_ = state->combine_weakly_mixed_integer_;
     combine_contiguous_ = state->combine_contiguous_;
+    combine_raw_ = state->combine_raw_;
+    run_combine_unordered_ = state->run_combine_unordered_;
+  }
+
+  template <typename T>
+  T& Real() {
+    return *static_cast<T*>(state_);
   }
 
   void* state_;
+  void (*combine_weakly_mixed_integer_)(
+      void*, absl::hash_internal::WeaklyMixedInteger);
   void (*combine_contiguous_)(void*, const unsigned char*, size_t);
+  void (*combine_raw_)(void*, uint64_t);
+  HashState (*run_combine_unordered_)(
+      HashState state,
+      absl::FunctionRef<void(HashState, absl::FunctionRef<void(HashState&)>)>);
 };
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_HASH_HASH_H_
diff --git a/absl/hash/hash_benchmark.cc b/absl/hash/hash_benchmark.cc
new file mode 100644
index 00000000000..978b5bcfb63
--- /dev/null
+++ b/absl/hash/hash_benchmark.cc
@@ -0,0 +1,413 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <algorithm>
+#include <bitset>
+#include <cassert>
+#include <cstddef>
+#include <cstdint>
+#include <cstring>
+#include <string>
+#include <tuple>
+#include <type_traits>
+#include <typeindex>
+#include <utility>
+#include <vector>
+
+#include "absl/base/attributes.h"
+#include "absl/container/flat_hash_set.h"
+#include "absl/hash/hash.h"
+#include "absl/random/random.h"
+#include "absl/strings/cord.h"
+#include "absl/strings/cord_test_helpers.h"
+#include "absl/strings/string_view.h"
+#include "benchmark/benchmark.h"
+
+namespace {
+
+using absl::Hash;
+
+template <template <typename> class H, typename T>
+void RunBenchmark(benchmark::State& state, T value) {
+  H<T> h;
+  for (auto _ : state) {
+    benchmark::DoNotOptimize(value);
+    benchmark::DoNotOptimize(h(value));
+  }
+}
+
+}  // namespace
+
+template <typename T>
+using AbslHash = absl::Hash<T>;
+
+class TypeErasedInterface {
+ public:
+  virtual ~TypeErasedInterface() = default;
+
+  template <typename H>
+  friend H AbslHashValue(H state, const TypeErasedInterface& wrapper) {
+    state = H::combine(std::move(state), std::type_index(typeid(wrapper)));
+    wrapper.HashValue(absl::HashState::Create(&state));
+    return state;
+  }
+
+ private:
+  virtual void HashValue(absl::HashState state) const = 0;
+};
+
+template <typename T>
+struct TypeErasedAbslHash {
+  class Wrapper : public TypeErasedInterface {
+   public:
+    explicit Wrapper(const T& value) : value_(value) {}
+
+   private:
+    void HashValue(absl::HashState state) const override {
+      absl::HashState::combine(std::move(state), value_);
+    }
+
+    const T& value_;
+  };
+
+  size_t operator()(const T& value) {
+    return absl::Hash<Wrapper>{}(Wrapper(value));
+  }
+};
+
+absl::Cord FlatCord(size_t size) {
+  absl::Cord result(std::string(size, 'a'));
+  result.Flatten();
+  return result;
+}
+
+absl::Cord FragmentedCord(size_t size) {
+  const size_t orig_size = size;
+  std::vector<std::string> chunks;
+  size_t chunk_size = std::max<size_t>(1, size / 10);
+  while (size > chunk_size) {
+    chunks.push_back(std::string(chunk_size, 'a'));
+    size -= chunk_size;
+  }
+  if (size > 0) {
+    chunks.push_back(std::string(size, 'a'));
+  }
+  absl::Cord result = absl::MakeFragmentedCord(chunks);
+  (void) orig_size;
+  assert(result.size() == orig_size);
+  return result;
+}
+
+template <typename T>
+std::vector<T> Vector(size_t count) {
+  std::vector<T> result;
+  for (size_t v = 0; v < count; ++v) {
+    result.push_back(v);
+  }
+  return result;
+}
+
+// Bogus type that replicates an unorderd_set's bit mixing, but with
+// vector-speed iteration. This is intended to measure the overhead of unordered
+// hashing without counting the speed of unordered_set iteration.
+template <typename T>
+struct FastUnorderedSet {
+  explicit FastUnorderedSet(size_t count) {
+    for (size_t v = 0; v < count; ++v) {
+      values.push_back(v);
+    }
+  }
+  std::vector<T> values;
+
+  template <typename H>
+  friend H AbslHashValue(H h, const FastUnorderedSet& fus) {
+    return H::combine(H::combine_unordered(std::move(h), fus.values.begin(),
+                                           fus.values.end()),
+                      fus.values.size());
+  }
+};
+
+template <typename T>
+absl::flat_hash_set<T> FlatHashSet(size_t count) {
+  absl::flat_hash_set<T> result;
+  for (size_t v = 0; v < count; ++v) {
+    result.insert(v);
+  }
+  return result;
+}
+
+template <typename T>
+struct LongCombine {
+  T a[200]{};
+  template <typename H>
+  friend H AbslHashValue(H state, const LongCombine& v) {
+    // This is testing a single call to `combine` with a lot of arguments to
+    // test the performance of the folding logic.
+    return H::combine(
+        std::move(state),  //
+        v.a[0], v.a[1], v.a[2], v.a[3], v.a[4], v.a[5], v.a[6], v.a[7], v.a[8],
+        v.a[9], v.a[10], v.a[11], v.a[12], v.a[13], v.a[14], v.a[15], v.a[16],
+        v.a[17], v.a[18], v.a[19], v.a[20], v.a[21], v.a[22], v.a[23], v.a[24],
+        v.a[25], v.a[26], v.a[27], v.a[28], v.a[29], v.a[30], v.a[31], v.a[32],
+        v.a[33], v.a[34], v.a[35], v.a[36], v.a[37], v.a[38], v.a[39], v.a[40],
+        v.a[41], v.a[42], v.a[43], v.a[44], v.a[45], v.a[46], v.a[47], v.a[48],
+        v.a[49], v.a[50], v.a[51], v.a[52], v.a[53], v.a[54], v.a[55], v.a[56],
+        v.a[57], v.a[58], v.a[59], v.a[60], v.a[61], v.a[62], v.a[63], v.a[64],
+        v.a[65], v.a[66], v.a[67], v.a[68], v.a[69], v.a[70], v.a[71], v.a[72],
+        v.a[73], v.a[74], v.a[75], v.a[76], v.a[77], v.a[78], v.a[79], v.a[80],
+        v.a[81], v.a[82], v.a[83], v.a[84], v.a[85], v.a[86], v.a[87], v.a[88],
+        v.a[89], v.a[90], v.a[91], v.a[92], v.a[93], v.a[94], v.a[95], v.a[96],
+        v.a[97], v.a[98], v.a[99], v.a[100], v.a[101], v.a[102], v.a[103],
+        v.a[104], v.a[105], v.a[106], v.a[107], v.a[108], v.a[109], v.a[110],
+        v.a[111], v.a[112], v.a[113], v.a[114], v.a[115], v.a[116], v.a[117],
+        v.a[118], v.a[119], v.a[120], v.a[121], v.a[122], v.a[123], v.a[124],
+        v.a[125], v.a[126], v.a[127], v.a[128], v.a[129], v.a[130], v.a[131],
+        v.a[132], v.a[133], v.a[134], v.a[135], v.a[136], v.a[137], v.a[138],
+        v.a[139], v.a[140], v.a[141], v.a[142], v.a[143], v.a[144], v.a[145],
+        v.a[146], v.a[147], v.a[148], v.a[149], v.a[150], v.a[151], v.a[152],
+        v.a[153], v.a[154], v.a[155], v.a[156], v.a[157], v.a[158], v.a[159],
+        v.a[160], v.a[161], v.a[162], v.a[163], v.a[164], v.a[165], v.a[166],
+        v.a[167], v.a[168], v.a[169], v.a[170], v.a[171], v.a[172], v.a[173],
+        v.a[174], v.a[175], v.a[176], v.a[177], v.a[178], v.a[179], v.a[180],
+        v.a[181], v.a[182], v.a[183], v.a[184], v.a[185], v.a[186], v.a[187],
+        v.a[188], v.a[189], v.a[190], v.a[191], v.a[192], v.a[193], v.a[194],
+        v.a[195], v.a[196], v.a[197], v.a[198], v.a[199]);
+  }
+};
+
+template <typename T>
+auto MakeLongTuple() {
+  auto t1 = std::tuple<T>();
+  auto t2 = std::tuple_cat(t1, t1);
+  auto t3 = std::tuple_cat(t2, t2);
+  auto t4 = std::tuple_cat(t3, t3);
+  auto t5 = std::tuple_cat(t4, t4);
+  auto t6 = std::tuple_cat(t5, t5);
+  // Ideally this would be much larger, but some configurations can't handle
+  // making tuples with that many elements. They break inside std::tuple itself.
+  static_assert(std::tuple_size_v<decltype(t6)> == 32);
+  return t6;
+}
+
+// Generates a benchmark and a codegen method for the provided types.  The
+// codegen method provides a well known entrypoint for dumping assembly.
+#define MAKE_BENCHMARK(hash, name, ...)                          \
+  namespace {                                                    \
+  void BM_##hash##_##name(benchmark::State& state) {             \
+    RunBenchmark<hash>(state, __VA_ARGS__);                      \
+  }                                                              \
+  BENCHMARK(BM_##hash##_##name);                                 \
+  }                                                              \
+  size_t Codegen##hash##name(const decltype(__VA_ARGS__)& arg);  \
+  size_t Codegen##hash##name(const decltype(__VA_ARGS__)& arg) { \
+    return hash<decltype(__VA_ARGS__)>{}(arg);                   \
+  }                                                              \
+  bool absl_hash_test_odr_use##hash##name =                      \
+      (benchmark::DoNotOptimize(&Codegen##hash##name), false)
+
+MAKE_BENCHMARK(AbslHash, Int32, int32_t{});
+MAKE_BENCHMARK(AbslHash, Int64, int64_t{});
+MAKE_BENCHMARK(AbslHash, Double, 1.2);
+MAKE_BENCHMARK(AbslHash, DoubleZero, 0.0);
+MAKE_BENCHMARK(AbslHash, PairInt32Int32, std::pair<int32_t, int32_t>{});
+MAKE_BENCHMARK(AbslHash, PairInt64Int64, std::pair<int64_t, int64_t>{});
+MAKE_BENCHMARK(AbslHash, TupleInt32BoolInt64,
+               std::tuple<int32_t, bool, int64_t>{});
+MAKE_BENCHMARK(AbslHash, String_0, std::string());
+MAKE_BENCHMARK(AbslHash, String_1, std::string(1, 'a'));
+MAKE_BENCHMARK(AbslHash, String_2, std::string(2, 'a'));
+MAKE_BENCHMARK(AbslHash, String_4, std::string(4, 'a'));
+MAKE_BENCHMARK(AbslHash, String_8, std::string(8, 'a'));
+MAKE_BENCHMARK(AbslHash, String_10, std::string(10, 'a'));
+MAKE_BENCHMARK(AbslHash, String_30, std::string(30, 'a'));
+MAKE_BENCHMARK(AbslHash, String_90, std::string(90, 'a'));
+MAKE_BENCHMARK(AbslHash, String_200, std::string(200, 'a'));
+MAKE_BENCHMARK(AbslHash, String_5000, std::string(5000, 'a'));
+MAKE_BENCHMARK(AbslHash, Cord_Flat_0, absl::Cord());
+MAKE_BENCHMARK(AbslHash, Cord_Flat_10, FlatCord(10));
+MAKE_BENCHMARK(AbslHash, Cord_Flat_30, FlatCord(30));
+MAKE_BENCHMARK(AbslHash, Cord_Flat_90, FlatCord(90));
+MAKE_BENCHMARK(AbslHash, Cord_Flat_200, FlatCord(200));
+MAKE_BENCHMARK(AbslHash, Cord_Flat_5000, FlatCord(5000));
+MAKE_BENCHMARK(AbslHash, Cord_Fragmented_200, FragmentedCord(200));
+MAKE_BENCHMARK(AbslHash, Cord_Fragmented_5000, FragmentedCord(5000));
+MAKE_BENCHMARK(AbslHash, VectorInt64_10, Vector<int64_t>(10));
+MAKE_BENCHMARK(AbslHash, VectorInt64_100, Vector<int64_t>(100));
+MAKE_BENCHMARK(AbslHash, VectorInt64_1000, Vector<int64_t>(1000));
+MAKE_BENCHMARK(AbslHash, VectorDouble_10, Vector<double>(10));
+MAKE_BENCHMARK(AbslHash, VectorDouble_100, Vector<double>(100));
+MAKE_BENCHMARK(AbslHash, VectorDouble_1000, Vector<double>(1000));
+MAKE_BENCHMARK(AbslHash, VectorBool_10, Vector<bool>(10));
+MAKE_BENCHMARK(AbslHash, VectorBool_100, Vector<bool>(100));
+MAKE_BENCHMARK(AbslHash, VectorBool_1000, Vector<bool>(1000));
+MAKE_BENCHMARK(AbslHash, Bitset_10, std::bitset<10>());
+MAKE_BENCHMARK(AbslHash, Bitset_100, std::bitset<100>());
+MAKE_BENCHMARK(AbslHash, Bitset_1000, std::bitset<1000>());
+MAKE_BENCHMARK(AbslHash, FlatHashSetInt64_10, FlatHashSet<int64_t>(10));
+MAKE_BENCHMARK(AbslHash, FlatHashSetInt64_100, FlatHashSet<int64_t>(100));
+MAKE_BENCHMARK(AbslHash, FlatHashSetInt64_1000, FlatHashSet<int64_t>(1000));
+MAKE_BENCHMARK(AbslHash, FlatHashSetDouble_10, FlatHashSet<double>(10));
+MAKE_BENCHMARK(AbslHash, FlatHashSetDouble_100, FlatHashSet<double>(100));
+MAKE_BENCHMARK(AbslHash, FlatHashSetDouble_1000, FlatHashSet<double>(1000));
+MAKE_BENCHMARK(AbslHash, FastUnorderedSetInt64_1000,
+               FastUnorderedSet<int64_t>(1000));
+MAKE_BENCHMARK(AbslHash, FastUnorderedSetDouble_1000,
+               FastUnorderedSet<double>(1000));
+MAKE_BENCHMARK(AbslHash, PairStringString_0,
+               std::make_pair(std::string(), std::string()));
+MAKE_BENCHMARK(AbslHash, PairStringString_10,
+               std::make_pair(std::string(10, 'a'), std::string(10, 'b')));
+MAKE_BENCHMARK(AbslHash, PairStringString_30,
+               std::make_pair(std::string(30, 'a'), std::string(30, 'b')));
+MAKE_BENCHMARK(AbslHash, PairStringString_90,
+               std::make_pair(std::string(90, 'a'), std::string(90, 'b')));
+MAKE_BENCHMARK(AbslHash, PairStringString_200,
+               std::make_pair(std::string(200, 'a'), std::string(200, 'b')));
+MAKE_BENCHMARK(AbslHash, PairStringString_5000,
+               std::make_pair(std::string(5000, 'a'), std::string(5000, 'b')));
+MAKE_BENCHMARK(AbslHash, LongTupleInt32, MakeLongTuple<int>());
+MAKE_BENCHMARK(AbslHash, LongTupleString, MakeLongTuple<std::string>());
+MAKE_BENCHMARK(AbslHash, LongCombineInt32, LongCombine<int>());
+MAKE_BENCHMARK(AbslHash, LongCombineString, LongCombine<std::string>());
+
+MAKE_BENCHMARK(TypeErasedAbslHash, Int32, int32_t{});
+MAKE_BENCHMARK(TypeErasedAbslHash, Int64, int64_t{});
+MAKE_BENCHMARK(TypeErasedAbslHash, PairInt32Int32,
+               std::pair<int32_t, int32_t>{});
+MAKE_BENCHMARK(TypeErasedAbslHash, PairInt64Int64,
+               std::pair<int64_t, int64_t>{});
+MAKE_BENCHMARK(TypeErasedAbslHash, TupleInt32BoolInt64,
+               std::tuple<int32_t, bool, int64_t>{});
+MAKE_BENCHMARK(TypeErasedAbslHash, String_0, std::string());
+MAKE_BENCHMARK(TypeErasedAbslHash, String_10, std::string(10, 'a'));
+MAKE_BENCHMARK(TypeErasedAbslHash, String_30, std::string(30, 'a'));
+MAKE_BENCHMARK(TypeErasedAbslHash, String_90, std::string(90, 'a'));
+MAKE_BENCHMARK(TypeErasedAbslHash, String_200, std::string(200, 'a'));
+MAKE_BENCHMARK(TypeErasedAbslHash, String_5000, std::string(5000, 'a'));
+MAKE_BENCHMARK(TypeErasedAbslHash, VectorDouble_10,
+               std::vector<double>(10, 1.1));
+MAKE_BENCHMARK(TypeErasedAbslHash, VectorDouble_100,
+               std::vector<double>(100, 1.1));
+MAKE_BENCHMARK(TypeErasedAbslHash, VectorDouble_1000,
+               std::vector<double>(1000, 1.1));
+MAKE_BENCHMARK(TypeErasedAbslHash, VectorBool_10, std::vector<bool>(10, true));
+MAKE_BENCHMARK(TypeErasedAbslHash, VectorBool_100,
+               std::vector<bool>(100, true));
+MAKE_BENCHMARK(TypeErasedAbslHash, VectorBool_1000,
+               std::vector<bool>(1000, true));
+MAKE_BENCHMARK(TypeErasedAbslHash, Bitset_10, std::bitset<10>());
+MAKE_BENCHMARK(TypeErasedAbslHash, Bitset_100, std::bitset<100>());
+MAKE_BENCHMARK(TypeErasedAbslHash, Bitset_1000, std::bitset<1000>());
+MAKE_BENCHMARK(TypeErasedAbslHash, FlatHashSetInt64_10,
+               FlatHashSet<int64_t>(10));
+MAKE_BENCHMARK(TypeErasedAbslHash, FlatHashSetInt64_100,
+               FlatHashSet<int64_t>(100));
+MAKE_BENCHMARK(TypeErasedAbslHash, FlatHashSetInt64_1000,
+               FlatHashSet<int64_t>(1000));
+MAKE_BENCHMARK(TypeErasedAbslHash, FlatHashSetDouble_10,
+               FlatHashSet<double>(10));
+MAKE_BENCHMARK(TypeErasedAbslHash, FlatHashSetDouble_100,
+               FlatHashSet<double>(100));
+MAKE_BENCHMARK(TypeErasedAbslHash, FlatHashSetDouble_1000,
+               FlatHashSet<double>(1000));
+MAKE_BENCHMARK(TypeErasedAbslHash, FastUnorderedSetInt64_1000,
+               FastUnorderedSet<int64_t>(1000));
+MAKE_BENCHMARK(TypeErasedAbslHash, FastUnorderedSetDouble_1000,
+               FastUnorderedSet<double>(1000));
+
+// The latency benchmark attempts to model the speed of the hash function in
+// production. When a hash function is used for hashtable lookups it is rarely
+// used to hash N items in a tight loop nor on constant sized strings. Instead,
+// after hashing there is a potential equality test plus a (usually) large
+// amount of user code. To simulate this effectively we introduce a data
+// dependency between elements we hash by using the hash of the Nth element as
+// the selector of the N+1th element to hash. This isolates the hash function
+// code much like in production. As a bonus we use the hash to generate strings
+// of size [1,N] (instead of fixed N) to disable perfect branch predictions in
+// hash function implementations.
+namespace {
+// 16kb fits in L1 cache of most CPUs we care about. Keeping memory latency low
+// will allow us to attribute most time to CPU which means more accurate
+// measurements.
+static constexpr size_t kEntropySize = 16 << 10;
+static char entropy[kEntropySize + 1024];
+ABSL_ATTRIBUTE_UNUSED static const bool kInitialized = [] {
+  absl::BitGen gen;
+  static_assert(sizeof(entropy) % sizeof(uint64_t) == 0);
+  for (int i = 0; i != sizeof(entropy); i += sizeof(uint64_t)) {
+    auto rand = absl::Uniform<uint64_t>(gen);
+    memcpy(&entropy[i], &rand, sizeof(uint64_t));
+  }
+  return true;
+}();
+}  // namespace
+
+struct PodPairInt64 {
+  int64_t a;
+  int64_t b;
+
+  template <typename H>
+  friend H AbslHashValue(H h, const PodPairInt64& p) {
+    return H::combine(std::move(h), p.a, p.b);
+  }
+};
+
+template <class T>
+struct PodRand {
+  static_assert(std::is_pod_v<T>);
+  static_assert(kEntropySize + sizeof(T) < sizeof(entropy));
+
+  T Get(size_t i) const {
+    T v;
+    memcpy(&v, &entropy[i % kEntropySize], sizeof(T));
+    return v;
+  }
+};
+
+template <size_t N>
+struct StringRand {
+  static_assert(kEntropySize + N < sizeof(entropy));
+
+  absl::string_view Get(size_t i) const {
+    // This has a small bias towards small numbers. Because max N is ~200 this
+    // is very small and prefer to be very fast instead of absolutely accurate.
+    // Also we pass N = 2^K+1 so that mod reduces to a bitand.
+    size_t s = (i % (N - 1)) + 1;
+    return {&entropy[i % kEntropySize], s};
+  }
+};
+
+#define MAKE_LATENCY_BENCHMARK(hash, name, ...)              \
+  namespace {                                                \
+  void BM_latency_##hash##_##name(benchmark::State& state) { \
+    __VA_ARGS__ r;                                           \
+    hash<decltype(r.Get(0))> h;                              \
+    size_t i = 871401241;                                    \
+    for (auto _ : state) {                                   \
+      benchmark::DoNotOptimize(i = h(r.Get(i)));             \
+    }                                                        \
+  }                                                          \
+  BENCHMARK(BM_latency_##hash##_##name);                     \
+  }  // namespace
+
+MAKE_LATENCY_BENCHMARK(AbslHash, Int32, PodRand<int32_t>)
+MAKE_LATENCY_BENCHMARK(AbslHash, Int64, PodRand<int64_t>)
+MAKE_LATENCY_BENCHMARK(AbslHash, PairInt64, PodRand<PodPairInt64>)
+MAKE_LATENCY_BENCHMARK(AbslHash, String3, StringRand<3>)
+MAKE_LATENCY_BENCHMARK(AbslHash, String5, StringRand<5>)
+MAKE_LATENCY_BENCHMARK(AbslHash, String9, StringRand<9>)
+MAKE_LATENCY_BENCHMARK(AbslHash, String17, StringRand<17>)
+MAKE_LATENCY_BENCHMARK(AbslHash, String33, StringRand<33>)
+MAKE_LATENCY_BENCHMARK(AbslHash, String65, StringRand<65>)
+MAKE_LATENCY_BENCHMARK(AbslHash, String257, StringRand<257>)
diff --git a/absl/hash/hash_instantiated_test.cc b/absl/hash/hash_instantiated_test.cc
new file mode 100644
index 00000000000..a7b78260e7e
--- /dev/null
+++ b/absl/hash/hash_instantiated_test.cc
@@ -0,0 +1,223 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// This file contains a few select absl::Hash tests that, due to their reliance
+// on INSTANTIATE_TYPED_TEST_SUITE_P, require a large amount of memory to
+// compile. Put new tests in hash_test.cc, not this file.
+
+#include "absl/hash/hash.h"
+
+#include <stddef.h>
+
+#include <algorithm>
+#include <deque>
+#include <forward_list>
+#include <initializer_list>
+#include <list>
+#include <map>
+#include <set>
+#include <string>
+#include <type_traits>
+#include <unordered_map>
+#include <unordered_set>
+#include <utility>
+#include <vector>
+
+#include "gtest/gtest.h"
+#include "absl/container/btree_map.h"
+#include "absl/container/btree_set.h"
+#include "absl/container/flat_hash_map.h"
+#include "absl/container/flat_hash_set.h"
+#include "absl/container/node_hash_map.h"
+#include "absl/container/node_hash_set.h"
+#include "absl/hash/hash_testing.h"
+#include "absl/hash/internal/hash_test.h"
+
+namespace {
+
+using ::absl::hash_test_internal::is_hashable;
+using ::absl::hash_test_internal::TypeErasedContainer;
+
+// Dummy type with unordered equality and hashing semantics.  This preserves
+// input order internally, and is used below to ensure we get test coverage
+// for equal sequences with different iteration orders.
+template <typename T>
+class UnorderedSequence {
+ public:
+  UnorderedSequence() = default;
+  template <typename TT>
+  UnorderedSequence(std::initializer_list<TT> l)
+      : values_(l.begin(), l.end()) {}
+  template <typename ForwardIterator,
+            std::enable_if_t<!std::is_integral_v<ForwardIterator>, bool> = true>
+  UnorderedSequence(ForwardIterator begin, ForwardIterator end)
+      : values_(begin, end) {}
+  // one-argument constructor of value type T, to appease older toolchains that
+  // get confused by one-element initializer lists in some contexts
+  explicit UnorderedSequence(const T& v) : values_(&v, &v + 1) {}
+
+  using value_type = T;
+
+  size_t size() const { return values_.size(); }
+  typename std::vector<T>::const_iterator begin() const {
+    return values_.begin();
+  }
+  typename std::vector<T>::const_iterator end() const { return values_.end(); }
+
+  friend bool operator==(const UnorderedSequence& lhs,
+                         const UnorderedSequence& rhs) {
+    return lhs.size() == rhs.size() &&
+           std::is_permutation(lhs.begin(), lhs.end(), rhs.begin());
+  }
+  friend bool operator!=(const UnorderedSequence& lhs,
+                         const UnorderedSequence& rhs) {
+    return !(lhs == rhs);
+  }
+  template <typename H>
+  friend H AbslHashValue(H h, const UnorderedSequence& u) {
+    return H::combine(H::combine_unordered(std::move(h), u.begin(), u.end()),
+                      u.size());
+  }
+
+ private:
+  std::vector<T> values_;
+};
+
+template <typename T>
+class HashValueSequenceTest : public testing::Test {};
+TYPED_TEST_SUITE_P(HashValueSequenceTest);
+
+TYPED_TEST_P(HashValueSequenceTest, BasicUsage) {
+  EXPECT_TRUE((is_hashable<TypeParam>::value));
+
+  using IntType = typename TypeParam::value_type;
+  auto a = static_cast<IntType>(0);
+  auto b = static_cast<IntType>(23);
+  auto c = static_cast<IntType>(42);
+
+  std::vector<TypeParam> exemplars = {
+      TypeParam(),        TypeParam(),        TypeParam{a, b, c},
+      TypeParam{a, c, b}, TypeParam{c, a, b}, TypeParam{a},
+      TypeParam{a, a},    TypeParam{a, a, a}, TypeParam{a, a, b},
+      TypeParam{a, b, a}, TypeParam{b, a, a}, TypeParam{a, b},
+      TypeParam{b, c}};
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(exemplars));
+}
+
+REGISTER_TYPED_TEST_SUITE_P(HashValueSequenceTest, BasicUsage);
+using IntSequenceTypes = testing::Types<
+    std::deque<int>, std::forward_list<int>, std::list<int>, std::vector<int>,
+    std::vector<bool>, TypeErasedContainer<std::vector<int>>, std::set<int>,
+    std::multiset<int>, UnorderedSequence<int>,
+    TypeErasedContainer<UnorderedSequence<int>>, std::unordered_set<int>,
+    std::unordered_multiset<int>, absl::flat_hash_set<int>,
+    absl::node_hash_set<int>, absl::btree_set<int>>;
+INSTANTIATE_TYPED_TEST_SUITE_P(My, HashValueSequenceTest, IntSequenceTypes);
+
+template <typename T>
+class HashValueNestedSequenceTest : public testing::Test {};
+TYPED_TEST_SUITE_P(HashValueNestedSequenceTest);
+
+TYPED_TEST_P(HashValueNestedSequenceTest, BasicUsage) {
+  using T = TypeParam;
+  using V = typename T::value_type;
+  std::vector<T> exemplars = {
+      // empty case
+      T{},
+      // sets of empty sets
+      T{V{}}, T{V{}, V{}}, T{V{}, V{}, V{}},
+      // multisets of different values
+      T{V{1}}, T{V{1, 1}, V{1, 1}}, T{V{1, 1, 1}, V{1, 1, 1}, V{1, 1, 1}},
+      // various orderings of same nested sets
+      T{V{}, V{1, 2}}, T{V{}, V{2, 1}}, T{V{1, 2}, V{}}, T{V{2, 1}, V{}},
+      // various orderings of various nested sets, case 2
+      T{V{1, 2}, V{3, 4}}, T{V{1, 2}, V{4, 3}}, T{V{1, 3}, V{2, 4}},
+      T{V{1, 3}, V{4, 2}}, T{V{1, 4}, V{2, 3}}, T{V{1, 4}, V{3, 2}},
+      T{V{2, 3}, V{1, 4}}, T{V{2, 3}, V{4, 1}}, T{V{2, 4}, V{1, 3}},
+      T{V{2, 4}, V{3, 1}}, T{V{3, 4}, V{1, 2}}, T{V{3, 4}, V{2, 1}}};
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(exemplars));
+}
+
+REGISTER_TYPED_TEST_SUITE_P(HashValueNestedSequenceTest, BasicUsage);
+template <typename T>
+using TypeErasedSet = TypeErasedContainer<UnorderedSequence<T>>;
+
+using NestedIntSequenceTypes = testing::Types<
+    std::vector<std::vector<int>>, std::vector<UnorderedSequence<int>>,
+    std::vector<TypeErasedSet<int>>, UnorderedSequence<std::vector<int>>,
+    UnorderedSequence<UnorderedSequence<int>>,
+    UnorderedSequence<TypeErasedSet<int>>, TypeErasedSet<std::vector<int>>,
+    TypeErasedSet<UnorderedSequence<int>>, TypeErasedSet<TypeErasedSet<int>>>;
+INSTANTIATE_TYPED_TEST_SUITE_P(My, HashValueNestedSequenceTest,
+                               NestedIntSequenceTypes);
+
+template <typename T>
+class HashValueAssociativeMapTest : public testing::Test {};
+TYPED_TEST_SUITE_P(HashValueAssociativeMapTest);
+
+TYPED_TEST_P(HashValueAssociativeMapTest, BasicUsage) {
+  using M = TypeParam;
+  using V = typename M::value_type;
+  std::vector<M> exemplars{M{},
+                           M{V{0, "foo"}},
+                           M{V{1, "foo"}},
+                           M{V{0, "bar"}},
+                           M{V{1, "bar"}},
+                           M{V{0, "foo"}, V{42, "bar"}},
+                           M{V{42, "bar"}, V{0, "foo"}},
+                           M{V{1, "foo"}, V{42, "bar"}},
+                           M{V{1, "foo"}, V{43, "bar"}},
+                           M{V{1, "foo"}, V{43, "baz"}}};
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(exemplars));
+}
+
+REGISTER_TYPED_TEST_SUITE_P(HashValueAssociativeMapTest, BasicUsage);
+using AssociativeMapTypes = testing::Types<
+    std::map<int, std::string>, std::unordered_map<int, std::string>,
+    absl::flat_hash_map<int, std::string>,
+    absl::node_hash_map<int, std::string>, absl::btree_map<int, std::string>,
+    UnorderedSequence<std::pair<const int, std::string>>>;
+INSTANTIATE_TYPED_TEST_SUITE_P(My, HashValueAssociativeMapTest,
+                               AssociativeMapTypes);
+
+template <typename T>
+class HashValueAssociativeMultimapTest : public testing::Test {};
+TYPED_TEST_SUITE_P(HashValueAssociativeMultimapTest);
+
+TYPED_TEST_P(HashValueAssociativeMultimapTest, BasicUsage) {
+  using MM = TypeParam;
+  using V = typename MM::value_type;
+  std::vector<MM> exemplars{MM{},
+                            MM{V{0, "foo"}},
+                            MM{V{1, "foo"}},
+                            MM{V{0, "bar"}},
+                            MM{V{1, "bar"}},
+                            MM{V{0, "foo"}, V{0, "bar"}},
+                            MM{V{0, "bar"}, V{0, "foo"}},
+                            MM{V{0, "foo"}, V{42, "bar"}},
+                            MM{V{1, "foo"}, V{42, "bar"}},
+                            MM{V{1, "foo"}, V{1, "foo"}, V{43, "bar"}},
+                            MM{V{1, "foo"}, V{43, "bar"}, V{1, "foo"}},
+                            MM{V{1, "foo"}, V{43, "baz"}}};
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(exemplars));
+}
+
+REGISTER_TYPED_TEST_SUITE_P(HashValueAssociativeMultimapTest, BasicUsage);
+using AssociativeMultimapTypes =
+    testing::Types<std::multimap<int, std::string>,
+                   std::unordered_multimap<int, std::string>>;
+INSTANTIATE_TYPED_TEST_SUITE_P(My, HashValueAssociativeMultimapTest,
+                               AssociativeMultimapTypes);
+
+}  // namespace
diff --git a/absl/hash/hash_test.cc b/absl/hash/hash_test.cc
index 9a667ba703b..4198102effc 100644
--- a/absl/hash/hash_test.cc
+++ b/absl/hash/hash_test.cc
@@ -14,39 +14,61 @@
 
 #include "absl/hash/hash.h"
 
+#include <algorithm>
 #include <array>
 #include <bitset>
+#include <cstddef>
+#include <cstdint>
+#include <cstdlib>
 #include <cstring>
-#include <deque>
-#include <forward_list>
 #include <functional>
-#include <iterator>
+#include <initializer_list>
+#include <ios>
 #include <limits>
-#include <list>
-#include <map>
 #include <memory>
-#include <numeric>
-#include <random>
+#include <optional>
+#include <ostream>
 #include <set>
 #include <string>
+#include <string_view>
 #include <tuple>
 #include <type_traits>
 #include <unordered_map>
 #include <utility>
+#include <variant>
 #include <vector>
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
+#include "absl/base/config.h"
+#include "absl/container/flat_hash_map.h"
 #include "absl/container/flat_hash_set.h"
 #include "absl/hash/hash_testing.h"
+#include "absl/hash/internal/hash_test.h"
 #include "absl/hash/internal/spy_hash_state.h"
+#include "absl/memory/memory.h"
 #include "absl/meta/type_traits.h"
-#include "absl/numeric/int128.h"
+#include "absl/numeric/bits.h"
+#include "absl/strings/cord.h"
+#include "absl/strings/cord_test_helpers.h"
+#include "absl/strings/string_view.h"
+
+#ifdef ABSL_INTERNAL_STD_FILESYSTEM_PATH_HASH_AVAILABLE
+#include <filesystem>  // NOLINT
+#endif
 
 namespace {
 
-using absl::Hash;
-using absl::hash_internal::SpyHashState;
+using ::absl::Hash;
+using ::absl::container_internal::hashtable_debug_internal::
+    HashtableDebugAccess;
+using ::absl::hash_internal::SpyHashState;
+using ::absl::hash_test_internal::is_hashable;
+using ::absl::hash_test_internal::TypeErasedContainer;
+using ::absl::hash_test_internal::TypeErasedValue;
+
+template <typename T>
+using TypeErasedVector = TypeErasedContainer<std::vector<T>>;
 
 template <typename T>
 class HashValueIntTest : public testing::Test {
@@ -58,11 +80,6 @@ SpyHashState SpyHash(const T& value) {
   return SpyHashState::combine(SpyHashState(), value);
 }
 
-// Helper trait to verify if T is hashable. We use absl::Hash's poison status to
-// detect it.
-template <typename T>
-using is_hashable = std::is_default_constructible<absl::Hash<T>>;
-
 TYPED_TEST_P(HashValueIntTest, BasicUsage) {
   EXPECT_TRUE((is_hashable<TypeParam>::value));
 
@@ -80,10 +97,10 @@ TYPED_TEST_P(HashValueIntTest, FastPath) {
             absl::Hash<std::tuple<TypeParam>>{}(std::tuple<TypeParam>(n)));
 }
 
-REGISTER_TYPED_TEST_CASE_P(HashValueIntTest, BasicUsage, FastPath);
-using IntTypes = testing::Types<unsigned char, char, int, int32_t, int64_t, uint32_t,
-                                uint64_t, size_t>;
-INSTANTIATE_TYPED_TEST_CASE_P(My, HashValueIntTest, IntTypes);
+REGISTER_TYPED_TEST_SUITE_P(HashValueIntTest, BasicUsage, FastPath);
+using IntTypes = testing::Types<unsigned char, char, int, int32_t, int64_t,
+                                uint32_t, uint64_t, size_t>;
+INSTANTIATE_TYPED_TEST_SUITE_P(My, HashValueIntTest, IntTypes);
 
 enum LegacyEnum { kValue1, kValue2, kValue3 };
 
@@ -102,6 +119,15 @@ TEST(HashValueTest, EnumAndBool) {
       std::make_tuple(true, false)));
 }
 
+TEST(HashValueTest, HashConsistentAcrossIntTypes){
+  std::vector<size_t> hashes = {
+      absl::Hash<int8_t>{}(1),  absl::Hash<uint8_t>{}(1),
+      absl::Hash<int16_t>{}(1), absl::Hash<uint16_t>{}(1),
+      absl::Hash<int32_t>{}(1), absl::Hash<uint32_t>{}(1),
+      absl::Hash<int64_t>{}(1), absl::Hash<uint64_t>{}(1)};
+  EXPECT_THAT(hashes, testing::Each(absl::Hash<int>{}(1)));
+}
+
 TEST(HashValueTest, FloatingPoint) {
   EXPECT_TRUE((is_hashable<float>::value));
   EXPECT_TRUE((is_hashable<double>::value));
@@ -126,6 +152,8 @@ TEST(HashValueTest, FloatingPoint) {
 
 TEST(HashValueTest, Pointer) {
   EXPECT_TRUE((is_hashable<int*>::value));
+  EXPECT_TRUE((is_hashable<int(*)(char, float)>::value));
+  EXPECT_TRUE((is_hashable<void(*)(int, int, ...)>::value));
 
   int i;
   int* ptr = &i;
@@ -136,14 +164,17 @@ TEST(HashValueTest, Pointer) {
 }
 
 TEST(HashValueTest, PointerAlignment) {
-  // We want to make sure that pointer alignment will not cause bits to be
-  // stuck.
+  // We want to make sure that pointer alignment will not cause too many bits to
+  // be stuck.
 
   constexpr size_t kTotalSize = 1 << 20;
   std::unique_ptr<char[]> data(new char[kTotalSize]);
   constexpr size_t kLog2NumValues = 5;
   constexpr size_t kNumValues = 1 << kLog2NumValues;
 
+  int64_t test_count = 0;
+  int64_t total_stuck_bit_count = 0;
+
   for (size_t align = 1; align < kTotalSize / kNumValues;
        align < 8 ? align += 1 : align < 1024 ? align += 8 : align += 32) {
     SCOPED_TRACE(align);
@@ -161,8 +192,96 @@ TEST(HashValueTest, PointerAlignment) {
     // Limit the scope to the bits we would be using for Swisstable.
     constexpr size_t kMask = (1 << (kLog2NumValues + 7)) - 1;
     size_t stuck_bits = (~bits_or | bits_and) & kMask;
-    EXPECT_EQ(stuck_bits, 0) << "0x" << std::hex << stuck_bits;
+    int stuck_bit_count = absl::popcount(stuck_bits);
+    size_t max_stuck_bits = 8;
+    EXPECT_LE(stuck_bit_count, max_stuck_bits)
+        << "0x" << std::hex << stuck_bits;
+
+    total_stuck_bit_count += stuck_bit_count;
+    ++test_count;
   }
+  // Test that average across alignments are at most 0.2 stuck bits.
+  // As of 2025-05-30 test is also passing with 0.07 stuck bits.
+  EXPECT_LE(total_stuck_bit_count, 0.2 * test_count);
+}
+
+TEST(HashValueTest, PointerToMember) {
+  struct Bass {
+    void q() {}
+  };
+
+  struct A : Bass {
+    virtual ~A() = default;
+    virtual void vfa() {}
+
+    static auto pq() -> void (A::*)() { return &A::q; }
+  };
+
+  struct B : Bass {
+    virtual ~B() = default;
+    virtual void vfb() {}
+
+    static auto pq() -> void (B::*)() { return &B::q; }
+  };
+
+  struct Foo : A, B {
+    void f1() {}
+    void f2() const {}
+
+    int g1() & { return 0; }
+    int g2() const & { return 0; }
+    int g3() && { return 0; }
+    int g4() const && { return 0; }
+
+    int h1() & { return 0; }
+    int h2() const & { return 0; }
+    int h3() && { return 0; }
+    int h4() const && { return 0; }
+
+    int a;
+    int b;
+
+    const int c = 11;
+    const int d = 22;
+  };
+
+  EXPECT_TRUE((is_hashable<float Foo::*>::value));
+  EXPECT_TRUE((is_hashable<double (Foo::*)(int, int)&&>::value));
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(
+      std::make_tuple(&Foo::a, &Foo::b, static_cast<int Foo::*>(nullptr))));
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(
+      std::make_tuple(&Foo::c, &Foo::d, static_cast<const int Foo::*>(nullptr),
+                      &Foo::a, &Foo::b)));
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(std::make_tuple(
+      &Foo::f1, static_cast<void (Foo::*)()>(nullptr))));
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(std::make_tuple(
+      &Foo::f2, static_cast<void (Foo::*)() const>(nullptr))));
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(std::make_tuple(
+      &Foo::g1, &Foo::h1, static_cast<int (Foo::*)() &>(nullptr))));
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(std::make_tuple(
+      &Foo::g2, &Foo::h2, static_cast<int (Foo::*)() const &>(nullptr))));
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(std::make_tuple(
+      &Foo::g3, &Foo::h3, static_cast<int (Foo::*)() &&>(nullptr))));
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(std::make_tuple(
+      &Foo::g4, &Foo::h4, static_cast<int (Foo::*)() const &&>(nullptr))));
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(
+      std::make_tuple(static_cast<void (Foo::*)()>(&Foo::vfa),
+                      static_cast<void (Foo::*)()>(&Foo::vfb),
+                      static_cast<void (Foo::*)()>(nullptr))));
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(
+      std::make_tuple(static_cast<void (Foo::*)()>(Foo::A::pq()),
+                      static_cast<void (Foo::*)()>(Foo::B::pq()),
+                      static_cast<void (Foo::*)()>(nullptr))));
 }
 
 TEST(HashValueTest, PairAndTuple) {
@@ -205,7 +324,7 @@ TEST(HashValueTest, CombineContiguousWorks) {
 
 struct DummyDeleter {
   template <typename T>
-  void operator() (T* ptr) {}
+  void operator() (T*) {}
 };
 
 struct SmartPointerEq {
@@ -247,7 +366,7 @@ TEST(HashValueTest, SmartPointers) {
   EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(
       std::forward_as_tuple(&i, nullptr,                    //
                             unique1, unique2, unique_null,  //
-                            absl::make_unique<int>(),       //
+                            std::make_unique<int>(),        //
                             shared1, shared2, shared_null,  //
                             std::make_shared<int>()),
       SmartPointerEq{}));
@@ -269,37 +388,212 @@ struct WrapInTuple {
   }
 };
 
+absl::Cord FlatCord(absl::string_view sv) {
+  absl::Cord c(sv);
+  c.Flatten();
+  return c;
+}
+
+absl::Cord FragmentedCord(absl::string_view sv) {
+  if (sv.size() < 2) {
+    return absl::Cord(sv);
+  }
+  size_t halfway = sv.size() / 2;
+  std::vector<absl::string_view> parts = {sv.substr(0, halfway),
+                                          sv.substr(halfway)};
+  return absl::MakeFragmentedCord(parts);
+}
+
+#ifdef ABSL_HAVE_INTRINSIC_INT128
+TEST(HashValueTest, TestIntrinsicInt128) {
+  EXPECT_TRUE((is_hashable<__int128_t>::value));
+  EXPECT_TRUE((is_hashable<__uint128_t>::value));
+
+  absl::flat_hash_map<size_t, int> hash_to_index;
+  std::vector<__uint128_t> values;
+  for (int i = 0; i < 128; ++i) {
+    // Some arbitrary pattern to check if changing each bit changes the hash.
+    static constexpr __uint128_t kPattern =
+        __uint128_t{0x0123456789abcdef} |
+        (__uint128_t{0x0123456789abcdef} << 64);
+    const __uint128_t value = kPattern ^ (__uint128_t{1} << i);
+    const __int128_t as_signed = static_cast<__int128_t>(value);
+
+    values.push_back(value);
+    auto [it, inserted] =
+        hash_to_index.insert({absl::Hash<__uint128_t>{}(value), i});
+    ASSERT_TRUE(inserted) << "Duplicate hash: " << i << " vs " << it->second;
+
+    // Verify that the fast-path for MixingHashState does not break the hash.
+    EXPECT_EQ(absl::HashOf(value), absl::Hash<__uint128_t>{}(value));
+    EXPECT_EQ(absl::HashOf(as_signed), absl::Hash<__int128_t>{}(as_signed));
+  }
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(values));
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(
+      std::vector<__int128_t>(values.begin(), values.end())));
+}
+#endif  // ABSL_HAVE_INTRINSIC_INT128
+
 TEST(HashValueTest, Strings) {
   EXPECT_TRUE((is_hashable<std::string>::value));
 
   const std::string small = "foo";
   const std::string dup = "foofoo";
-  const std::string large = "large";
-  const std::string huge = std::string(5000, 'a');
+  const std::string large = std::string(2048, 'x');  // multiple of chunk size
+  const std::string huge = std::string(5000, 'a');   // not a multiple
 
   EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(std::make_tuple(
-      std::string(), absl::string_view(),
-      std::string(""), absl::string_view(""),
-      std::string(small), absl::string_view(small),
-      std::string(dup), absl::string_view(dup),
-      std::string(large), absl::string_view(large),
-      std::string(huge), absl::string_view(huge))));
+      std::string(), absl::string_view(), absl::Cord(), std::string(""),
+      absl::string_view(""), absl::Cord(""), std::string(small),
+      absl::string_view(small), absl::Cord(small), FragmentedCord(small),
+      std::string(dup), absl::string_view(dup), absl::Cord(dup),
+      std::string(large), absl::string_view(large), absl::Cord(large),
+      std::string(huge), absl::string_view(huge), FlatCord(huge),
+      FragmentedCord(huge))));
 
   // Also check that nested types maintain the same hash.
   const WrapInTuple t{};
   EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(std::make_tuple(
-      t(std::string()), t(absl::string_view()),
-      t(std::string("")), t(absl::string_view("")),
-      t(std::string(small)), t(absl::string_view(small)),
-      t(std::string(dup)), t(absl::string_view(dup)),
-      t(std::string(large)), t(absl::string_view(large)),
-      t(std::string(huge)), t(absl::string_view(huge)))));
-
-  // Make sure that hashing a `const char*` does not use its std::string-value.
+      t(std::string()), t(absl::string_view()), t(absl::Cord()),
+      t(std::string("")), t(absl::string_view("")), t(absl::Cord("")),
+      t(std::string(small)), t(absl::string_view(small)), t(absl::Cord(small)),
+      t(FragmentedCord(small)), t(std::string(dup)), t(absl::string_view(dup)),
+      t(absl::Cord(dup)), t(std::string(large)), t(absl::string_view(large)),
+      t(absl::Cord(large)), t(std::string(huge)), t(absl::string_view(huge)),
+      t(FlatCord(huge)), t(FragmentedCord(huge)))));
+
+  // Make sure that hashing a `const char*` does not use its string-value.
   EXPECT_NE(SpyHash(static_cast<const char*>("ABC")),
             SpyHash(absl::string_view("ABC")));
 }
 
+TEST(HashValueTest, StringsVector) {
+  using Vec = std::vector<std::string>;
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(std::make_tuple(
+      Vec{"abc", "def"}, Vec{"abcde", "f"},
+      Vec{"abcdefghijklmnopqrstuvwxyz", "ABCDEFGHIJKLMNOPQRSTUVWXYZ"},
+      Vec{"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXY", "Z"})));
+}
+
+TEST(HashValueTest, WString) {
+  EXPECT_TRUE((is_hashable<std::wstring>::value));
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(std::make_tuple(
+      std::wstring(), std::wstring(L"ABC"), std::wstring(L"ABC"),
+      std::wstring(L"Some other different string"),
+      std::wstring(L"Iñtërnâtiônàlizætiøn"))));
+}
+
+#ifdef __cpp_char8_t
+TEST(HashValueTest, U8String) {
+  EXPECT_TRUE((is_hashable<std::u8string>::value));
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(std::make_tuple(
+      std::u8string(), std::u8string(u8"ABC"), std::u8string(u8"ABC"),
+      std::u8string(u8"Some other different string"),
+      std::u8string(u8"Iñtërnâtiônàlizætiøn"))));
+}
+#endif
+
+TEST(HashValueTest, U16String) {
+  EXPECT_TRUE((is_hashable<std::u16string>::value));
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(std::make_tuple(
+      std::u16string(), std::u16string(u"ABC"), std::u16string(u"ABC"),
+      std::u16string(u"Some other different string"),
+      std::u16string(u"Iñtërnâtiônàlizætiøn"))));
+}
+
+TEST(HashValueTest, U32String) {
+  EXPECT_TRUE((is_hashable<std::u32string>::value));
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(std::make_tuple(
+      std::u32string(), std::u32string(U"ABC"), std::u32string(U"ABC"),
+      std::u32string(U"Some other different string"),
+      std::u32string(U"Iñtërnâtiônàlizætiøn"))));
+}
+
+TEST(HashValueTest, WStringView) {
+  EXPECT_TRUE((is_hashable<std::wstring_view>::value));
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(std::make_tuple(
+      std::wstring_view(), std::wstring_view(L"ABC"), std::wstring_view(L"ABC"),
+      std::wstring_view(L"Some other different string_view"),
+      std::wstring_view(L"Iñtërnâtiônàlizætiøn"))));
+}
+
+#ifdef __cpp_char8_t
+TEST(HashValueTest, U8StringView) {
+  EXPECT_TRUE((is_hashable<std::u8string_view>::value));
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(
+      std::make_tuple(std::u8string_view(), std::u8string_view(u8"ABC"),
+                      std::u8string_view(u8"ABC"),
+                      std::u8string_view(u8"Some other different string_view"),
+                      std::u8string_view(u8"Iñtërnâtiônàlizætiøn"))));
+}
+#endif
+
+TEST(HashValueTest, U16StringView) {
+  EXPECT_TRUE((is_hashable<std::u16string_view>::value));
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(
+      std::make_tuple(std::u16string_view(), std::u16string_view(u"ABC"),
+                      std::u16string_view(u"ABC"),
+                      std::u16string_view(u"Some other different string_view"),
+                      std::u16string_view(u"Iñtërnâtiônàlizætiøn"))));
+}
+
+TEST(HashValueTest, U32StringView) {
+  EXPECT_TRUE((is_hashable<std::u32string_view>::value));
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(
+      std::make_tuple(std::u32string_view(), std::u32string_view(U"ABC"),
+                      std::u32string_view(U"ABC"),
+                      std::u32string_view(U"Some other different string_view"),
+                      std::u32string_view(U"Iñtërnâtiônàlizætiøn"))));
+}
+
+TEST(HashValueTest, StdFilesystemPath) {
+#ifndef ABSL_INTERNAL_STD_FILESYSTEM_PATH_HASH_AVAILABLE
+  GTEST_SKIP() << "std::filesystem::path is unavailable on this platform";
+#else
+  EXPECT_TRUE((is_hashable<std::filesystem::path>::value));
+
+  // clang-format off
+  const auto kTestCases = std::make_tuple(
+      std::filesystem::path(),
+      std::filesystem::path("/"),
+#ifndef __GLIBCXX__
+      // libstdc++ has a known issue normalizing "//".
+      // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106452
+      std::filesystem::path("//"),
+#endif
+      std::filesystem::path("/a/b"),
+      std::filesystem::path("/a//b"),
+      std::filesystem::path("a/b"),
+      std::filesystem::path("a/b/"),
+      std::filesystem::path("a//b"),
+      std::filesystem::path("a//b/"),
+      std::filesystem::path("c:/"),
+      std::filesystem::path("c:\\"),
+      std::filesystem::path("c:\\/"),
+      std::filesystem::path("c:\\//"),
+      std::filesystem::path("c://"),
+      std::filesystem::path("c://\\"),
+      std::filesystem::path("c:/a"),
+      std::filesystem::path("c:\\a"),
+      std::filesystem::path("/e/p"),
+      std::filesystem::path("/s/../e/p"),
+      std::filesystem::path("e/p"),
+      std::filesystem::path("s/../e/p"));
+  // clang-format on
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(kTestCases));
+#endif
+}
+
 TEST(HashValueTest, StdArray) {
   EXPECT_TRUE((is_hashable<std::array<int, 3>>::value));
 
@@ -333,33 +627,8 @@ TEST(HashValueTest, StdBitset) {
        std::bitset<kNumBits>(bit_strings[5].c_str())}));
 }  // namespace
 
-template <typename T>
-class HashValueSequenceTest : public testing::Test {
-};
-TYPED_TEST_SUITE_P(HashValueSequenceTest);
-
-TYPED_TEST_P(HashValueSequenceTest, BasicUsage) {
-  EXPECT_TRUE((is_hashable<TypeParam>::value));
-
-  using ValueType = typename TypeParam::value_type;
-  auto a = static_cast<ValueType>(0);
-  auto b = static_cast<ValueType>(23);
-  auto c = static_cast<ValueType>(42);
-
-  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(
-      std::make_tuple(TypeParam(), TypeParam{}, TypeParam{a, b, c},
-                      TypeParam{a, b}, TypeParam{b, c})));
-}
-
-REGISTER_TYPED_TEST_CASE_P(HashValueSequenceTest, BasicUsage);
-using IntSequenceTypes =
-    testing::Types<std::deque<int>, std::forward_list<int>, std::list<int>,
-                   std::vector<int>, std::vector<bool>, std::set<int>,
-                   std::multiset<int>>;
-INSTANTIATE_TYPED_TEST_CASE_P(My, HashValueSequenceTest, IntSequenceTypes);
-
 // Private type that only supports AbslHashValue to make sure our chosen hash
-// implentation is recursive within absl::Hash.
+// implementation is recursive within absl::Hash.
 // It uses std::abs() on the value to provide different bitwise representations
 // of the same logical value.
 struct Private {
@@ -378,6 +647,125 @@ struct Private {
   }
 };
 
+// Test helper for combine_piecewise_buffer.  It holds a string_view to the
+// buffer-to-be-hashed.  Its AbslHashValue specialization will split up its
+// contents at the character offsets requested.
+class PiecewiseHashTester {
+ public:
+  // Create a hash view of a buffer to be hashed contiguously.
+  explicit PiecewiseHashTester(absl::string_view buf)
+      : buf_(buf), piecewise_(false), split_locations_() {}
+
+  // Create a hash view of a buffer to be hashed piecewise, with breaks at the
+  // given locations.
+  PiecewiseHashTester(absl::string_view buf, std::set<size_t> split_locations)
+      : buf_(buf),
+        piecewise_(true),
+        split_locations_(std::move(split_locations)) {}
+
+  template <typename H>
+  friend H AbslHashValue(H h, const PiecewiseHashTester& p) {
+    if (!p.piecewise_) {
+      return H::combine_contiguous(std::move(h), p.buf_.data(), p.buf_.size());
+    }
+    absl::hash_internal::PiecewiseCombiner combiner;
+    if (p.split_locations_.empty()) {
+      h = combiner.add_buffer(std::move(h), p.buf_.data(), p.buf_.size());
+      return combiner.finalize(std::move(h));
+    }
+    size_t begin = 0;
+    for (size_t next : p.split_locations_) {
+      absl::string_view chunk = p.buf_.substr(begin, next - begin);
+      h = combiner.add_buffer(std::move(h), chunk.data(), chunk.size());
+      begin = next;
+    }
+    absl::string_view last_chunk = p.buf_.substr(begin);
+    if (!last_chunk.empty()) {
+      h = combiner.add_buffer(std::move(h), last_chunk.data(),
+                              last_chunk.size());
+    }
+    return combiner.finalize(std::move(h));
+  }
+
+ private:
+  absl::string_view buf_;
+  bool piecewise_;
+  std::set<size_t> split_locations_;
+};
+
+// Dummy object that hashes as two distinct contiguous buffers, "foo" followed
+// by "bar"
+struct DummyFooBar {
+  template <typename H>
+  friend H AbslHashValue(H h, const DummyFooBar&) {
+    const char* foo = "foo";
+    const char* bar = "bar";
+    h = H::combine_contiguous(std::move(h), foo, 3);
+    h = H::combine_contiguous(std::move(h), bar, 3);
+    return h;
+  }
+};
+
+TEST(HashValueTest, CombinePiecewiseBuffer) {
+  absl::Hash<PiecewiseHashTester> hash;
+
+  // Check that hashing an empty buffer through the piecewise API works.
+  EXPECT_EQ(hash(PiecewiseHashTester("")), hash(PiecewiseHashTester("", {})));
+
+  // Similarly, small buffers should give consistent results
+  EXPECT_EQ(hash(PiecewiseHashTester("foobar")),
+            hash(PiecewiseHashTester("foobar", {})));
+  EXPECT_EQ(hash(PiecewiseHashTester("foobar")),
+            hash(PiecewiseHashTester("foobar", {3})));
+
+  // But hashing "foobar" in pieces gives a different answer than hashing "foo"
+  // contiguously, then "bar" contiguously.
+  EXPECT_NE(hash(PiecewiseHashTester("foobar", {3})),
+            absl::Hash<DummyFooBar>()(DummyFooBar{}));
+
+  // Test hashing a large buffer incrementally, broken up in several different
+  // ways.  Arrange for breaks on and near the stride boundaries to look for
+  // off-by-one errors in the implementation.
+  //
+  // This test is run on a buffer that is a multiple of the stride size, and one
+  // that isn't.
+  const size_t kChunkSize = absl::hash_internal::PiecewiseChunkSize();
+  for (size_t big_buffer_size :
+       {2 * kChunkSize + kChunkSize / 2, 3 * kChunkSize}) {
+    SCOPED_TRACE(big_buffer_size);
+    std::string big_buffer;
+    for (size_t i = 0; i < big_buffer_size; ++i) {
+      // Arbitrary string
+      big_buffer.push_back(32 + (i * (i / 3)) % 64);
+    }
+    auto big_buffer_hash = hash(PiecewiseHashTester(big_buffer));
+
+    const int possible_breaks = 9;
+    size_t breaks[possible_breaks] = {1,
+                                      kChunkSize / 2,
+                                      kChunkSize - 1,
+                                      kChunkSize,
+                                      kChunkSize + 1,
+                                      kChunkSize + kChunkSize / 2,
+                                      2 * kChunkSize - 1,
+                                      2 * kChunkSize,
+                                      2 * kChunkSize + 1};
+    for (unsigned test_mask = 0; test_mask < (1u << possible_breaks);
+         ++test_mask) {
+      SCOPED_TRACE(test_mask);
+      std::set<size_t> break_locations;
+      for (int j = 0; j < possible_breaks; ++j) {
+        if (test_mask & (1u << j)) {
+          break_locations.insert(breaks[j]);
+        }
+      }
+      ASSERT_EQ(
+          hash(PiecewiseHashTester(big_buffer, std::move(break_locations))),
+          big_buffer_hash);
+    }
+  }
+}
+
 TEST(HashValueTest, PrivateSanity) {
   // Sanity check that Private is working as the tests below expect it to work.
   EXPECT_TRUE(is_hashable<Private>::value);
@@ -386,81 +774,81 @@ TEST(HashValueTest, PrivateSanity) {
 }
 
 TEST(HashValueTest, Optional) {
-  EXPECT_TRUE(is_hashable<absl::optional<Private>>::value);
+  EXPECT_TRUE(is_hashable<std::optional<Private>>::value);
 
-  using O = absl::optional<Private>;
+  using O = std::optional<Private>;
   EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(
       std::make_tuple(O{}, O{{1}}, O{{-1}}, O{{10}})));
 }
 
 TEST(HashValueTest, Variant) {
-  using V = absl::variant<Private, std::string>;
+  using V = std::variant<Private, std::string>;
   EXPECT_TRUE(is_hashable<V>::value);
 
   EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(std::make_tuple(
       V(Private{1}), V(Private{-1}), V(Private{2}), V("ABC"), V("BCD"))));
 
-#if ABSL_META_INTERNAL_STD_HASH_SFINAE_FRIENDLY_
   struct S {};
-  EXPECT_FALSE(is_hashable<absl::variant<S>>::value);
-#endif
+  EXPECT_FALSE(is_hashable<std::variant<S>>::value);
 }
 
-TEST(HashValueTest, Maps) {
-  EXPECT_TRUE((is_hashable<std::map<int, std::string>>::value));
+TEST(HashValueTest, ReferenceWrapper) {
+  EXPECT_TRUE(is_hashable<std::reference_wrapper<Private>>::value);
 
-  using M = std::map<int, std::string>;
+  Private p1{1}, p10{10};
   EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(std::make_tuple(
-      M{}, M{{0, "foo"}}, M{{1, "foo"}}, M{{0, "bar"}}, M{{1, "bar"}},
-      M{{0, "foo"}, {42, "bar"}}, M{{1, "foo"}, {42, "bar"}},
-      M{{1, "foo"}, {43, "bar"}}, M{{1, "foo"}, {43, "baz"}})));
+      p1, p10, std::ref(p1), std::ref(p10), std::cref(p1), std::cref(p10))));
 
-  using MM = std::multimap<int, std::string>;
+  EXPECT_TRUE(is_hashable<std::reference_wrapper<int>>::value);
+  int one = 1, ten = 10;
   EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(std::make_tuple(
-      MM{}, MM{{0, "foo"}}, MM{{1, "foo"}}, MM{{0, "bar"}}, MM{{1, "bar"}},
-      MM{{0, "foo"}, {0, "bar"}}, MM{{0, "bar"}, {0, "foo"}},
-      MM{{0, "foo"}, {42, "bar"}}, MM{{1, "foo"}, {42, "bar"}},
-      MM{{1, "foo"}, {1, "foo"}, {43, "bar"}}, MM{{1, "foo"}, {43, "baz"}})));
+      one, ten, std::ref(one), std::ref(ten), std::cref(one), std::cref(ten))));
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(
+      std::make_tuple(std::tuple<std::reference_wrapper<int>>(std::ref(one)),
+                      std::tuple<std::reference_wrapper<int>>(std::ref(ten)),
+                      std::tuple<int>(one), std::tuple<int>(ten))));
 }
 
 template <typename T, typename = void>
 struct IsHashCallable : std::false_type {};
 
 template <typename T>
-struct IsHashCallable<T, absl::void_t<decltype(std::declval<absl::Hash<T>>()(
+struct IsHashCallable<T, std::void_t<decltype(std::declval<absl::Hash<T>>()(
                             std::declval<const T&>()))>> : std::true_type {};
 
 template <typename T, typename = void>
 struct IsAggregateInitializable : std::false_type {};
 
 template <typename T>
-struct IsAggregateInitializable<T, absl::void_t<decltype(T{})>>
+struct IsAggregateInitializable<T, std::void_t<decltype(T{})>>
     : std::true_type {};
 
 TEST(IsHashableTest, ValidHash) {
   EXPECT_TRUE((is_hashable<int>::value));
-  EXPECT_TRUE(std::is_default_constructible<absl::Hash<int>>::value);
-  EXPECT_TRUE(std::is_copy_constructible<absl::Hash<int>>::value);
-  EXPECT_TRUE(std::is_move_constructible<absl::Hash<int>>::value);
-  EXPECT_TRUE(absl::is_copy_assignable<absl::Hash<int>>::value);
-  EXPECT_TRUE(absl::is_move_assignable<absl::Hash<int>>::value);
+  EXPECT_TRUE(std::is_default_constructible_v<absl::Hash<int>>);
+  EXPECT_TRUE(std::is_copy_constructible_v<absl::Hash<int>>);
+  EXPECT_TRUE(std::is_move_constructible_v<absl::Hash<int>>);
+  EXPECT_TRUE(std::is_copy_assignable_v<absl::Hash<int>>);
+  EXPECT_TRUE(std::is_move_assignable_v<absl::Hash<int>>);
   EXPECT_TRUE(IsHashCallable<int>::value);
   EXPECT_TRUE(IsAggregateInitializable<absl::Hash<int>>::value);
 }
 
-#if ABSL_META_INTERNAL_STD_HASH_SFINAE_FRIENDLY_
 TEST(IsHashableTest, PoisonHash) {
   struct X {};
   EXPECT_FALSE((is_hashable<X>::value));
-  EXPECT_FALSE(std::is_default_constructible<absl::Hash<X>>::value);
-  EXPECT_FALSE(std::is_copy_constructible<absl::Hash<X>>::value);
-  EXPECT_FALSE(std::is_move_constructible<absl::Hash<X>>::value);
-  EXPECT_FALSE(absl::is_copy_assignable<absl::Hash<X>>::value);
-  EXPECT_FALSE(absl::is_move_assignable<absl::Hash<X>>::value);
+  EXPECT_FALSE(std::is_default_constructible_v<absl::Hash<X>>);
+  EXPECT_FALSE(std::is_copy_constructible_v<absl::Hash<X>>);
+  EXPECT_FALSE(std::is_move_constructible_v<absl::Hash<X>>);
+  EXPECT_FALSE(std::is_copy_assignable_v<absl::Hash<X>>);
+  EXPECT_FALSE(std::is_move_assignable_v<absl::Hash<X>>);
   EXPECT_FALSE(IsHashCallable<X>::value);
+#if !defined(__GNUC__) || defined(__clang__)
+  // TODO(b/144368551): As of GCC 8.4 this does not compile.
   EXPECT_FALSE(IsAggregateInitializable<absl::Hash<X>>::value);
+#endif
 }
-#endif  // ABSL_META_INTERNAL_STD_HASH_SFINAE_FRIENDLY_
 
 // Hashable types
 //
@@ -529,14 +917,14 @@ struct CustomHashType {
 
 template <InvokeTag allowed, InvokeTag... tags>
 struct EnableIfContained
-    : std::enable_if<absl::disjunction<
-          std::integral_constant<bool, allowed == tags>...>::value> {};
+    : std::enable_if<
+          std::disjunction_v<std::bool_constant<allowed == tags>...>> {};
 
 template <
     typename H, InvokeTag... Tags,
     typename = typename EnableIfContained<InvokeTag::kHashValue, Tags...>::type>
 H AbslHashValue(H state, CustomHashType<Tags...> t) {
-  static_assert(MinTag<Tags...>::value == InvokeTag::kHashValue, "");
+  static_assert(MinTag<Tags...>::value == InvokeTag::kHashValue);
   return H::combine(std::move(state),
                     t.value + static_cast<int>(InvokeTag::kHashValue));
 }
@@ -544,6 +932,7 @@ H AbslHashValue(H state, CustomHashType<Tags...> t) {
 }  // namespace
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace hash_internal {
 template <InvokeTag... Tags>
 struct is_uniquely_represented<
@@ -551,6 +940,7 @@ struct is_uniquely_represented<
     typename EnableIfContained<InvokeTag::kUniquelyRepresented, Tags...>::type>
     : std::true_type {};
 }  // namespace hash_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #if ABSL_HASH_INTERNAL_SUPPORT_LEGACY_HASH_
@@ -560,7 +950,7 @@ struct hash<CustomHashType<Tags...>> {
   template <InvokeTag... TagsIn, typename = typename EnableIfContained<
                                      InvokeTag::kLegacyHash, TagsIn...>::type>
   size_t operator()(CustomHashType<TagsIn...> t) const {
-    static_assert(MinTag<Tags...>::value == InvokeTag::kLegacyHash, "");
+    static_assert(MinTag<Tags...>::value == InvokeTag::kLegacyHash);
     return t.value + static_cast<int>(InvokeTag::kLegacyHash);
   }
 };
@@ -573,7 +963,7 @@ struct hash<CustomHashType<Tags...>> {
   template <InvokeTag... TagsIn, typename = typename EnableIfContained<
                                      InvokeTag::kStdHash, TagsIn...>::type>
   size_t operator()(CustomHashType<TagsIn...> t) const {
-    static_assert(MinTag<Tags...>::value == InvokeTag::kStdHash, "");
+    static_assert(MinTag<Tags...>::value == InvokeTag::kStdHash);
     return t.value + static_cast<int>(InvokeTag::kStdHash);
   }
 };
@@ -594,13 +984,11 @@ void TestCustomHashType(InvokeTagConstant<InvokeTag::kNone>, T...) {
 }
 
 void TestCustomHashType(InvokeTagConstant<InvokeTag::kNone>) {
-#if ABSL_META_INTERNAL_STD_HASH_SFINAE_FRIENDLY_
   // is_hashable is false if we don't support any of the hooks.
   using type = CustomHashType<>;
   EXPECT_FALSE(is_hashable<type>());
   EXPECT_FALSE(is_hashable<const type>());
   EXPECT_FALSE(is_hashable<const type&>());
-#endif  // ABSL_META_INTERNAL_STD_HASH_SFINAE_FRIENDLY_
 }
 
 template <InvokeTag Tag, typename... T>
@@ -637,10 +1025,10 @@ TYPED_TEST_P(HashIntTest, BasicUsage) {
             Hash<CombineVariadic<TypeParam>>()({}));
 }
 
-REGISTER_TYPED_TEST_CASE_P(HashIntTest, BasicUsage);
-using IntTypes = testing::Types<unsigned char, char, int, int32_t, int64_t, uint32_t,
-                                uint64_t, size_t>;
-INSTANTIATE_TYPED_TEST_CASE_P(My, HashIntTest, IntTypes);
+REGISTER_TYPED_TEST_SUITE_P(HashIntTest, BasicUsage);
+using IntTypes = testing::Types<unsigned char, char, int, int32_t, int64_t,
+                                uint32_t, uint64_t, size_t>;
+INSTANTIATE_TYPED_TEST_SUITE_P(My, HashIntTest, IntTypes);
 
 struct StructWithPadding {
   char c;
@@ -654,10 +1042,10 @@ struct StructWithPadding {
 
 static_assert(sizeof(StructWithPadding) > sizeof(char) + sizeof(int),
               "StructWithPadding doesn't have padding");
-static_assert(std::is_standard_layout<StructWithPadding>::value, "");
+static_assert(std::is_standard_layout_v<StructWithPadding>);
 
 // This check has to be disabled because libstdc++ doesn't support it.
-// static_assert(std::is_trivially_constructible<StructWithPadding>::value, "");
+// static_assert(std::is_trivially_constructible_v<StructWithPadding>);
 
 template <typename T>
 struct ArraySlice {
@@ -684,10 +1072,10 @@ TEST(HashTest, HashNonUniquelyRepresentedType) {
   unsigned char buffer2[kNumStructs * sizeof(StructWithPadding)];
   std::memset(buffer2, 255, sizeof(buffer2));
   auto* s2 = reinterpret_cast<StructWithPadding*>(buffer2);
-  for (int i = 0; i < kNumStructs; ++i) {
+  for (size_t i = 0; i < kNumStructs; ++i) {
     SCOPED_TRACE(i);
-    s1[i].c = s2[i].c = '0' + i;
-    s1[i].i = s2[i].i = i;
+    s1[i].c = s2[i].c = static_cast<char>('0' + i);
+    s1[i].i = s2[i].i = static_cast<int>(i);
     ASSERT_FALSE(memcmp(buffer1 + i * sizeof(StructWithPadding),
                         buffer2 + i * sizeof(StructWithPadding),
                         sizeof(StructWithPadding)) == 0)
@@ -747,29 +1135,23 @@ TEST(HashTest, SmallValueOn64ByteBoundary) {
   Hash<IntAndString>()(IntAndString{0, std::string(63, '0')});
 }
 
-struct TypeErased {
-  size_t n;
-
-  template <typename H>
-  friend H AbslHashValue(H hash_state, const TypeErased& v) {
-    v.HashValue(absl::HashState::Create(&hash_state));
-    return hash_state;
-  }
-
-  void HashValue(absl::HashState state) const {
-    absl::HashState::combine(std::move(state), n);
-  }
-};
-
 TEST(HashTest, TypeErased) {
-  EXPECT_TRUE((is_hashable<TypeErased>::value));
-  EXPECT_TRUE((is_hashable<std::pair<TypeErased, int>>::value));
+  EXPECT_TRUE((is_hashable<TypeErasedValue<size_t>>::value));
+  EXPECT_TRUE((is_hashable<std::pair<TypeErasedValue<size_t>, int>>::value));
 
-  EXPECT_EQ(SpyHash(TypeErased{7}), SpyHash(size_t{7}));
-  EXPECT_NE(SpyHash(TypeErased{7}), SpyHash(size_t{13}));
+  EXPECT_EQ(SpyHash(TypeErasedValue<size_t>(7)), SpyHash(size_t{7}));
+  EXPECT_NE(SpyHash(TypeErasedValue<size_t>(7)), SpyHash(size_t{13}));
 
-  EXPECT_EQ(SpyHash(std::make_pair(TypeErased{7}, 17)),
+  EXPECT_EQ(SpyHash(std::make_pair(TypeErasedValue<size_t>(7), 17)),
             SpyHash(std::make_pair(size_t{7}, 17)));
+
+  absl::flat_hash_set<absl::flat_hash_set<int>> ss = {{1, 2}, {3, 4}};
+  TypeErasedContainer<absl::flat_hash_set<absl::flat_hash_set<int>>> es = {
+      absl::flat_hash_set<int>{1, 2}, {3, 4}};
+  absl::flat_hash_set<TypeErasedContainer<absl::flat_hash_set<int>>> se = {
+      {1, 2}, {3, 4}};
+  EXPECT_EQ(SpyHash(ss), SpyHash(es));
+  EXPECT_EQ(SpyHash(ss), SpyHash(se));
 }
 
 struct ValueWithBoolConversion {
@@ -781,7 +1163,9 @@ struct ValueWithBoolConversion {
 namespace std {
 template <>
 struct hash<ValueWithBoolConversion> {
-  size_t operator()(ValueWithBoolConversion v) { return v.i; }
+  size_t operator()(ValueWithBoolConversion v) {
+    return static_cast<size_t>(v.i);
+  }
 };
 }  // namespace std
 
@@ -792,4 +1176,176 @@ TEST(HashTest, DoesNotUseImplicitConversionsToBool) {
             absl::Hash<ValueWithBoolConversion>()(ValueWithBoolConversion{1}));
 }
 
+TEST(HashOf, MatchesHashForSingleArgument) {
+  std::string s = "forty two";
+  double d = 42.0;
+  std::tuple<int, int> t{4, 2};
+  int i = 42;
+  int neg_i = -42;
+  int16_t i16 = 42;
+  int16_t neg_i16 = -42;
+  int8_t i8 = 42;
+  int8_t neg_i8 = -42;
+
+  EXPECT_EQ(absl::HashOf(s), absl::Hash<std::string>{}(s));
+  EXPECT_EQ(absl::HashOf(d), absl::Hash<double>{}(d));
+  EXPECT_EQ(absl::HashOf(t), (absl::Hash<std::tuple<int, int>>{}(t)));
+  EXPECT_EQ(absl::HashOf(i), absl::Hash<int>{}(i));
+  EXPECT_EQ(absl::HashOf(neg_i), absl::Hash<int>{}(neg_i));
+  EXPECT_EQ(absl::HashOf(i16), absl::Hash<int16_t>{}(i16));
+  EXPECT_EQ(absl::HashOf(neg_i16), absl::Hash<int16_t>{}(neg_i16));
+  EXPECT_EQ(absl::HashOf(i8), absl::Hash<int8_t>{}(i8));
+  EXPECT_EQ(absl::HashOf(neg_i8), absl::Hash<int8_t>{}(neg_i8));
+}
+
+TEST(HashOf, MatchesHashOfTupleForMultipleArguments) {
+  std::string hello = "hello";
+  std::string world = "world";
+
+  EXPECT_EQ(absl::HashOf(), absl::HashOf(std::make_tuple()));
+  EXPECT_EQ(absl::HashOf(hello), absl::HashOf(std::make_tuple(hello)));
+  EXPECT_EQ(absl::HashOf(hello, world),
+            absl::HashOf(std::make_tuple(hello, world)));
+}
+
+template <typename T>
+std::true_type HashOfExplicitParameter(decltype(absl::HashOf<T>(0))) {
+  return {};
+}
+template <typename T>
+std::false_type HashOfExplicitParameter(size_t) {
+  return {};
+}
+
+TEST(HashOf, CantPassExplicitTemplateParameters) {
+  EXPECT_FALSE(HashOfExplicitParameter<int>(0));
+}
+
+struct TypeErasedHashStateUser {
+  int a;
+  std::string b;
+
+  template <typename H>
+  friend H AbslHashValue(H state, const TypeErasedHashStateUser& value) {
+    absl::HashState type_erased_state = absl::HashState::Create(&state);
+    absl::HashState::combine(std::move(type_erased_state), value.a, value.b);
+    return state;
+  }
+};
+
+TEST(HashOf, MatchesTypeErasedHashState) {
+  std::string s = "s";
+  EXPECT_EQ(absl::HashOf(1, s), absl::Hash<TypeErasedHashStateUser>{}(
+                                    TypeErasedHashStateUser{1, s}));
+}
+
+struct AutoReturnTypeUser {
+  int a;
+  std::string b;
+
+  template <typename H>
+  friend auto AbslHashValue(H state, const AutoReturnTypeUser& value) {
+    return H::combine(std::move(state), value.a, value.b);
+  }
+};
+
+TEST(HashOf, AutoReturnTypeUser) {
+  std::string s = "s";
+  EXPECT_EQ(absl::HashOf(1, s),
+            absl::Hash<AutoReturnTypeUser>{}(AutoReturnTypeUser{1, s}));
+}
+
+TEST(HashOf, DoubleSignCollision) {
+  // These values differ only in their most significant bit.
+  EXPECT_NE(absl::HashOf(-1.0), absl::HashOf(1.0));
+}
+
+// Test for collisions in short strings if PrecombineLengthMix is low quality.
+TEST(PrecombineLengthMix, ShortStringCollision) {
+#if defined(__wasm__)
+  GTEST_SKIP() << "Fails flakily on wasm due to no ASLR and 32-bit size_t.";
+#endif
+#if defined(__ANDROID__) && defined(__arm__)
+  GTEST_SKIP() << "Fails on 32-bit Android due to layout changes.";
+#endif
+  std::string s1 = "00";
+  std::string s2 = "000";
+  constexpr char kMinChar = 0;
+  constexpr char kMaxChar = 32;
+  for (s1[0] = kMinChar; s1[0] < kMaxChar; ++s1[0]) {
+    for (s1[1] = kMinChar; s1[1] < kMaxChar; ++s1[1]) {
+      for (s2[0] = kMinChar; s2[0] < kMaxChar; ++s2[0]) {
+        for (s2[1] = kMinChar; s2[1] < kMaxChar; ++s2[1]) {
+          for (s2[2] = kMinChar; s2[2] < kMaxChar; ++s2[2]) {
+            ASSERT_NE(absl::HashOf(s1), absl::HashOf(s2))
+                << "s1[0]: " << static_cast<int>(s1[0])
+                << "; s1[1]: " << static_cast<int>(s1[1])
+                << "; s2[0]: " << static_cast<int>(s2[0])
+                << "; s2[1]: " << static_cast<int>(s2[1])
+                << "; s2[2]: " << static_cast<int>(s2[2]);
+          }
+        }
+      }
+    }
+  }
+}
+
+// Test that we don't cause excessive collisions on the hash table for
+// doubles in the range [-1024, 1024]. See cl/773069881 for more information.
+TEST(SwisstableCollisions, DoubleRange) {
+  absl::flat_hash_set<double> set;
+  for (double t = -1024.0; t < 1024.0; t += 1.0) {
+    set.insert(t);
+    ASSERT_LT(HashtableDebugAccess<decltype(set)>::GetNumProbes(set, t), 64)
+        << t;
+  }
+}
+
+// Test that for each pair of adjacent bytes in a string, if there's only
+// entropy in those two bytes, then we don't have excessive collisions.
+TEST(SwisstableCollisions, LowEntropyStrings) {
+  constexpr char kMinChar = 0;
+  constexpr char kMaxChar = 64;
+  // These sizes cover the different hashing cases.
+  for (size_t size : {8u, 16u, 32u, 64u, 128u}) {
+    for (size_t b = 0; b < size - 1; ++b) {
+      // Pre-reserve table capacity so the test measures hash distribution
+      // quality under standard load factors, avoiding probe length spikes
+      // caused by near-maximum load factors right before incremental resizing.
+      absl::flat_hash_set<std::string> set(
+          size_t{kMaxChar - kMinChar} * size_t{kMaxChar - kMinChar});
+      std::string s(size, '\0');
+      for (char c1 = kMinChar; c1 < kMaxChar; ++c1) {
+        for (char c2 = kMinChar; c2 < kMaxChar; ++c2) {
+          s[b] = c1;
+          s[b + 1] = c2;
+          set.insert(s);
+          ASSERT_LT(HashtableDebugAccess<decltype(set)>::GetNumProbes(set, s),
+                    64)
+              << "size: " << size << "; bit: " << b;
+        }
+      }
+    }
+  }
+}
+
+// Test that we don't have excessive collisions when keys are consecutive
+// integers rotated by N bits.
+TEST(SwisstableCollisions, LowEntropyInts) {
+  constexpr int kSizeTBits = sizeof(size_t) * 8;
+  for (int bit = 0; bit < kSizeTBits; ++bit) {
+    // Pre-reserve table capacity so the test measures hash distribution
+    // quality under standard load factors, avoiding probe length spikes
+    // caused by near-maximum load factors right before incremental resizing.
+    const size_t kNumElements = 128 * 1024;
+    absl::flat_hash_set<size_t> set(kNumElements);
+    for (size_t i = 0; i < kNumElements; ++i) {
+      size_t v = absl::rotl(i, bit);
+      set.insert(v);
+      ASSERT_LT(HashtableDebugAccess<decltype(set)>::GetNumProbes(set, v), 48)
+          << bit << " " << i;
+    }
+  }
+}
+
 }  // namespace
diff --git a/absl/hash/hash_testing.h b/absl/hash/hash_testing.h
index c45bc154982..d932f636623 100644
--- a/absl/hash/hash_testing.h
+++ b/absl/hash/hash_testing.h
@@ -15,9 +15,13 @@
 #ifndef ABSL_HASH_HASH_TESTING_H_
 #define ABSL_HASH_HASH_TESTING_H_
 
+#include <cstddef>
 #include <initializer_list>
+#include <string>
 #include <tuple>
 #include <type_traits>
+#include <utility>
+#include <variant>
 #include <vector>
 
 #include "gmock/gmock.h"
@@ -28,6 +32,7 @@
 #include "absl/types/variant.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 // Run the absl::Hash algorithm over all the elements passed in and verify that
 // their hash expansion is congruent with their `==` operator.
@@ -127,7 +132,7 @@ namespace absl {
 //   }
 //   friend bool operator==(Bad4 x, Bad4 y) {
 //    // Compare two ranges for equality. C++14 code can instead use std::equal.
-//     return absl::equal(x.p, x.p + x.size, y.p, y.p + y.size);
+//     return std::equal(x.p, x.p + x.size, y.p, y.p + y.size);
 //   }
 // };
 //
@@ -140,21 +145,20 @@ namespace absl {
 // }
 //
 template <int&... ExplicitBarrier, typename Container>
-ABSL_MUST_USE_RESULT testing::AssertionResult
-VerifyTypeImplementsAbslHashCorrectly(const Container& values);
+testing::AssertionResult VerifyTypeImplementsAbslHashCorrectly(
+    const Container& values);
 
 template <int&... ExplicitBarrier, typename Container, typename Eq>
-ABSL_MUST_USE_RESULT testing::AssertionResult
-VerifyTypeImplementsAbslHashCorrectly(const Container& values, Eq equals);
+testing::AssertionResult VerifyTypeImplementsAbslHashCorrectly(
+    const Container& values, Eq equals);
 
 template <int&..., typename T>
-ABSL_MUST_USE_RESULT testing::AssertionResult
-VerifyTypeImplementsAbslHashCorrectly(std::initializer_list<T> values);
+testing::AssertionResult VerifyTypeImplementsAbslHashCorrectly(
+    std::initializer_list<T> values);
 
 template <int&..., typename T, typename Eq>
-ABSL_MUST_USE_RESULT testing::AssertionResult
-VerifyTypeImplementsAbslHashCorrectly(std::initializer_list<T> values,
-                                      Eq equals);
+testing::AssertionResult VerifyTypeImplementsAbslHashCorrectly(
+    std::initializer_list<T> values, Eq equals);
 
 namespace hash_internal {
 
@@ -183,17 +187,17 @@ struct ExpandVisitor {
 };
 
 template <typename Container, typename Eq>
-ABSL_MUST_USE_RESULT testing::AssertionResult
-VerifyTypeImplementsAbslHashCorrectly(const Container& values, Eq equals) {
+testing::AssertionResult VerifyTypeImplementsAbslHashCorrectly(
+    const Container& values, Eq equals) {
   using V = typename Container::value_type;
 
   struct Info {
     const V& value;
     size_t index;
     std::string ToString() const {
-      return absl::visit(PrintVisitor{index}, value);
+      return std::visit(PrintVisitor{index}, value);
     }
-    SpyHashState expand() const { return absl::visit(ExpandVisitor{}, value); }
+    SpyHashState expand() const { return std::visit(ExpandVisitor{}, value); }
   };
 
   using EqClass = std::vector<Info>;
@@ -204,7 +208,7 @@ VerifyTypeImplementsAbslHashCorrectly(const Container& values, Eq equals) {
   for (const auto& value : values) {
     EqClass* c = nullptr;
     for (auto& eqclass : classes) {
-      if (absl::visit(EqVisitor<Eq>{equals}, value, eqclass[0].value)) {
+      if (std::visit(EqVisitor<Eq>{equals}, value, eqclass[0].value)) {
         c = &eqclass;
         break;
       }
@@ -243,7 +247,8 @@ VerifyTypeImplementsAbslHashCorrectly(const Container& values, Eq equals) {
       if (v.expand() != expected) {
         return testing::AssertionFailure()
                << "Values " << c[0].ToString() << " and " << v.ToString()
-               << " evaluate as equal but have an unequal hash expansion.";
+               << " evaluate as equal but have unequal hash expansions ("
+               << expected << " vs. " << v.expand() << ").";
       }
     }
 
@@ -255,17 +260,18 @@ VerifyTypeImplementsAbslHashCorrectly(const Container& values, Eq equals) {
         case SpyHashState::CompareResult::kEqual:
           return testing::AssertionFailure()
                  << "Values " << c[0].ToString() << " and " << c2[0].ToString()
-                 << " evaluate as unequal but have an equal hash expansion.";
+                 << " evaluate as unequal but have an equal hash expansion:"
+                 << c2_hash << ".";
         case SpyHashState::CompareResult::kBSuffixA:
           return testing::AssertionFailure()
-                 << "Hash expansion of " << c2[0].ToString()
+                 << "Hash expansion of " << c2[0].ToString() << ";" << c2_hash
                  << " is a suffix of the hash expansion of " << c[0].ToString()
-                 << ".";
+                 << ";" << expected << ".";
         case SpyHashState::CompareResult::kASuffixB:
           return testing::AssertionFailure()
-                 << "Hash expansion of " << c[0].ToString()
-                 << " is a suffix of the hash expansion of " << c2[0].ToString()
-                 << ".";
+                 << "Hash expansion of " << c[0].ToString() << ";"
+                 << expected << " is a suffix of the hash expansion of "
+                 << c2[0].ToString() << ";" << c2_hash << ".";
         case SpyHashState::CompareResult::kUnequal:
           break;
       }
@@ -276,7 +282,7 @@ VerifyTypeImplementsAbslHashCorrectly(const Container& values, Eq equals) {
 
 template <typename... T>
 struct TypeSet {
-  template <typename U, bool = disjunction<std::is_same<T, U>...>::value>
+  template <typename U, bool = std::disjunction_v<std::is_same<T, U>...>>
   struct Insert {
     using type = TypeSet<U, T...>;
   };
@@ -296,11 +302,11 @@ struct MakeTypeSet<T, Ts...> : MakeTypeSet<Ts...>::template Insert<T>::type {};
 
 template <typename... T>
 using VariantForTypes = typename MakeTypeSet<
-    const typename std::decay<T>::type*...>::template apply<absl::variant>;
+    const std::decay_t<T>*...>::template apply<std::variant>;
 
 template <typename Container>
 struct ContainerAsVector {
-  using V = absl::variant<const typename Container::value_type*>;
+  using V = std::variant<const typename Container::value_type*>;
   using Out = std::vector<V>;
 
   static Out Do(const Container& values) {
@@ -316,12 +322,12 @@ struct ContainerAsVector<std::tuple<T...>> {
   using Out = std::vector<V>;
 
   template <size_t... I>
-  static Out DoImpl(const std::tuple<T...>& tuple, absl::index_sequence<I...>) {
+  static Out DoImpl(const std::tuple<T...>& tuple, std::index_sequence<I...>) {
     return Out{&std::get<I>(tuple)...};
   }
 
   static Out Do(const std::tuple<T...>& values) {
-    return DoImpl(values, absl::index_sequence_for<T...>());
+    return DoImpl(values, std::index_sequence_for<T...>());
   }
 };
 
@@ -340,37 +346,37 @@ struct DefaultEquals {
 }  // namespace hash_internal
 
 template <int&..., typename Container>
-ABSL_MUST_USE_RESULT testing::AssertionResult
-VerifyTypeImplementsAbslHashCorrectly(const Container& values) {
+testing::AssertionResult VerifyTypeImplementsAbslHashCorrectly(
+    const Container& values) {
   return hash_internal::VerifyTypeImplementsAbslHashCorrectly(
       hash_internal::ContainerAsVector<Container>::Do(values),
       hash_internal::DefaultEquals{});
 }
 
 template <int&..., typename Container, typename Eq>
-ABSL_MUST_USE_RESULT testing::AssertionResult
-VerifyTypeImplementsAbslHashCorrectly(const Container& values, Eq equals) {
+testing::AssertionResult VerifyTypeImplementsAbslHashCorrectly(
+    const Container& values, Eq equals) {
   return hash_internal::VerifyTypeImplementsAbslHashCorrectly(
       hash_internal::ContainerAsVector<Container>::Do(values), equals);
 }
 
 template <int&..., typename T>
-ABSL_MUST_USE_RESULT testing::AssertionResult
-VerifyTypeImplementsAbslHashCorrectly(std::initializer_list<T> values) {
+testing::AssertionResult VerifyTypeImplementsAbslHashCorrectly(
+    std::initializer_list<T> values) {
   return hash_internal::VerifyTypeImplementsAbslHashCorrectly(
       hash_internal::ContainerAsVector<std::initializer_list<T>>::Do(values),
       hash_internal::DefaultEquals{});
 }
 
 template <int&..., typename T, typename Eq>
-ABSL_MUST_USE_RESULT testing::AssertionResult
-VerifyTypeImplementsAbslHashCorrectly(std::initializer_list<T> values,
-                                      Eq equals) {
+testing::AssertionResult VerifyTypeImplementsAbslHashCorrectly(
+    std::initializer_list<T> values, Eq equals) {
   return hash_internal::VerifyTypeImplementsAbslHashCorrectly(
       hash_internal::ContainerAsVector<std::initializer_list<T>>::Do(values),
       equals);
 }
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_HASH_HASH_TESTING_H_
diff --git a/absl/hash/internal/city.cc b/absl/hash/internal/city.cc
index dc7650a79e0..bc2999ae295 100644
--- a/absl/hash/internal/city.cc
+++ b/absl/hash/internal/city.cc
@@ -21,15 +21,19 @@
 
 #include "absl/hash/internal/city.h"
 
-#include <string.h>  // for memcpy and memset
 #include <algorithm>
+#include <cstddef>
+#include <cstdint>
+#include <utility>
 
 #include "absl/base/config.h"
 #include "absl/base/internal/endian.h"
 #include "absl/base/internal/unaligned_access.h"
 #include "absl/base/optimization.h"
+#include "absl/numeric/bits.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace hash_internal {
 
 #ifdef ABSL_IS_BIG_ENDIAN
@@ -67,11 +71,6 @@ static uint32_t fmix(uint32_t h) {
   return h;
 }
 
-static uint32_t Rotate32(uint32_t val, int shift) {
-  // Avoid shifting by 32: doing so yields an undefined result.
-  return shift == 0 ? val : ((val >> shift) | (val << (32 - shift)));
-}
-
 #undef PERMUTE3
 #define PERMUTE3(a, b, c) \
   do {                    \
@@ -82,10 +81,10 @@ static uint32_t Rotate32(uint32_t val, int shift) {
 static uint32_t Mur(uint32_t a, uint32_t h) {
   // Helper from Murmur3 for combining two 32-bit values.
   a *= c1;
-  a = Rotate32(a, 17);
+  a = absl::rotr(a, 17);
   a *= c2;
   h ^= a;
-  h = Rotate32(h, 19);
+  h = absl::rotr(h, 19);
   return h * 5 + 0xe6546b64;
 }
 
@@ -96,7 +95,7 @@ static uint32_t Hash32Len13to24(const char *s, size_t len) {
   uint32_t d = Fetch32(s + (len >> 1));
   uint32_t e = Fetch32(s);
   uint32_t f = Fetch32(s + len - 4);
-  uint32_t h = len;
+  uint32_t h = static_cast<uint32_t>(len);
 
   return fmix(Mur(f, Mur(e, Mur(d, Mur(c, Mur(b, Mur(a, h)))))));
 }
@@ -105,15 +104,15 @@ static uint32_t Hash32Len0to4(const char *s, size_t len) {
   uint32_t b = 0;
   uint32_t c = 9;
   for (size_t i = 0; i < len; i++) {
-    signed char v = s[i];
-    b = b * c1 + v;
+    signed char v = static_cast<signed char>(s[i]);
+    b = b * c1 + static_cast<uint32_t>(v);
     c ^= b;
   }
-  return fmix(Mur(b, Mur(len, c)));
+  return fmix(Mur(b, Mur(static_cast<uint32_t>(len), c)));
 }
 
 static uint32_t Hash32Len5to12(const char *s, size_t len) {
-  uint32_t a = len, b = len * 5, c = 9, d = b;
+  uint32_t a = static_cast<uint32_t>(len), b = a * 5, c = 9, d = b;
   a += Fetch32(s);
   b += Fetch32(s + len - 4);
   c += Fetch32(s + ((len >> 1) & 4));
@@ -128,46 +127,46 @@ uint32_t CityHash32(const char *s, size_t len) {
   }
 
   // len > 24
-  uint32_t h = len, g = c1 * len, f = g;
+  uint32_t h = static_cast<uint32_t>(len), g = c1 * h, f = g;
 
-  uint32_t a0 = Rotate32(Fetch32(s + len - 4) * c1, 17) * c2;
-  uint32_t a1 = Rotate32(Fetch32(s + len - 8) * c1, 17) * c2;
-  uint32_t a2 = Rotate32(Fetch32(s + len - 16) * c1, 17) * c2;
-  uint32_t a3 = Rotate32(Fetch32(s + len - 12) * c1, 17) * c2;
-  uint32_t a4 = Rotate32(Fetch32(s + len - 20) * c1, 17) * c2;
+  uint32_t a0 = absl::rotr(Fetch32(s + len - 4) * c1, 17) * c2;
+  uint32_t a1 = absl::rotr(Fetch32(s + len - 8) * c1, 17) * c2;
+  uint32_t a2 = absl::rotr(Fetch32(s + len - 16) * c1, 17) * c2;
+  uint32_t a3 = absl::rotr(Fetch32(s + len - 12) * c1, 17) * c2;
+  uint32_t a4 = absl::rotr(Fetch32(s + len - 20) * c1, 17) * c2;
   h ^= a0;
-  h = Rotate32(h, 19);
+  h = absl::rotr(h, 19);
   h = h * 5 + 0xe6546b64;
   h ^= a2;
-  h = Rotate32(h, 19);
+  h = absl::rotr(h, 19);
   h = h * 5 + 0xe6546b64;
   g ^= a1;
-  g = Rotate32(g, 19);
+  g = absl::rotr(g, 19);
   g = g * 5 + 0xe6546b64;
   g ^= a3;
-  g = Rotate32(g, 19);
+  g = absl::rotr(g, 19);
   g = g * 5 + 0xe6546b64;
   f += a4;
-  f = Rotate32(f, 19);
+  f = absl::rotr(f, 19);
   f = f * 5 + 0xe6546b64;
   size_t iters = (len - 1) / 20;
   do {
-    uint32_t b0 = Rotate32(Fetch32(s) * c1, 17) * c2;
+    uint32_t b0 = absl::rotr(Fetch32(s) * c1, 17) * c2;
     uint32_t b1 = Fetch32(s + 4);
-    uint32_t b2 = Rotate32(Fetch32(s + 8) * c1, 17) * c2;
-    uint32_t b3 = Rotate32(Fetch32(s + 12) * c1, 17) * c2;
+    uint32_t b2 = absl::rotr(Fetch32(s + 8) * c1, 17) * c2;
+    uint32_t b3 = absl::rotr(Fetch32(s + 12) * c1, 17) * c2;
     uint32_t b4 = Fetch32(s + 16);
     h ^= b0;
-    h = Rotate32(h, 18);
+    h = absl::rotr(h, 18);
     h = h * 5 + 0xe6546b64;
     f += b1;
-    f = Rotate32(f, 19);
+    f = absl::rotr(f, 19);
     f = f * c1;
     g += b2;
-    g = Rotate32(g, 18);
+    g = absl::rotr(g, 18);
     g = g * 5 + 0xe6546b64;
     h ^= b3 + b1;
-    h = Rotate32(h, 19);
+    h = absl::rotr(h, 19);
     h = h * 5 + 0xe6546b64;
     g ^= b4;
     g = absl::gbswap_32(g) * 5;
@@ -177,32 +176,21 @@ uint32_t CityHash32(const char *s, size_t len) {
     PERMUTE3(f, h, g);
     s += 20;
   } while (--iters != 0);
-  g = Rotate32(g, 11) * c1;
-  g = Rotate32(g, 17) * c1;
-  f = Rotate32(f, 11) * c1;
-  f = Rotate32(f, 17) * c1;
-  h = Rotate32(h + g, 19);
+  g = absl::rotr(g, 11) * c1;
+  g = absl::rotr(g, 17) * c1;
+  f = absl::rotr(f, 11) * c1;
+  f = absl::rotr(f, 17) * c1;
+  h = absl::rotr(h + g, 19);
   h = h * 5 + 0xe6546b64;
-  h = Rotate32(h, 17) * c1;
-  h = Rotate32(h + f, 19);
+  h = absl::rotr(h, 17) * c1;
+  h = absl::rotr(h + f, 19);
   h = h * 5 + 0xe6546b64;
-  h = Rotate32(h, 17) * c1;
+  h = absl::rotr(h, 17) * c1;
   return h;
 }
 
-// Bitwise right rotate.  Normally this will compile to a single
-// instruction, especially if the shift is a manifest constant.
-static uint64_t Rotate(uint64_t val, int shift) {
-  // Avoid shifting by 64: doing so yields an undefined result.
-  return shift == 0 ? val : ((val >> shift) | (val << (64 - shift)));
-}
-
 static uint64_t ShiftMix(uint64_t val) { return val ^ (val >> 47); }
 
-static uint64_t HashLen16(uint64_t u, uint64_t v) {
-  return Hash128to64(uint128(u, v));
-}
-
 static uint64_t HashLen16(uint64_t u, uint64_t v, uint64_t mul) {
   // Murmur-inspired hashing.
   uint64_t a = (u ^ v) * mul;
@@ -213,13 +201,18 @@ static uint64_t HashLen16(uint64_t u, uint64_t v, uint64_t mul) {
   return b;
 }
 
+static uint64_t HashLen16(uint64_t u, uint64_t v) {
+  const uint64_t kMul = 0x9ddfea08eb382d69ULL;
+  return HashLen16(u, v, kMul);
+}
+
 static uint64_t HashLen0to16(const char *s, size_t len) {
   if (len >= 8) {
     uint64_t mul = k2 + len * 2;
     uint64_t a = Fetch64(s) + k2;
     uint64_t b = Fetch64(s + len - 8);
-    uint64_t c = Rotate(b, 37) * mul + a;
-    uint64_t d = (Rotate(a, 25) + b) * mul;
+    uint64_t c = absl::rotr(b, 37) * mul + a;
+    uint64_t d = (absl::rotr(a, 25) + b) * mul;
     return HashLen16(c, d, mul);
   }
   if (len >= 4) {
@@ -228,11 +221,11 @@ static uint64_t HashLen0to16(const char *s, size_t len) {
     return HashLen16(len + (a << 3), Fetch32(s + len - 4), mul);
   }
   if (len > 0) {
-    uint8_t a = s[0];
-    uint8_t b = s[len >> 1];
-    uint8_t c = s[len - 1];
+    uint8_t a = static_cast<uint8_t>(s[0]);
+    uint8_t b = static_cast<uint8_t>(s[len >> 1]);
+    uint8_t c = static_cast<uint8_t>(s[len - 1]);
     uint32_t y = static_cast<uint32_t>(a) + (static_cast<uint32_t>(b) << 8);
-    uint32_t z = len + (static_cast<uint32_t>(c) << 2);
+    uint32_t z = static_cast<uint32_t>(len) + (static_cast<uint32_t>(c) << 2);
     return ShiftMix(y * k2 ^ z * k0) * k2;
   }
   return k2;
@@ -246,27 +239,27 @@ static uint64_t HashLen17to32(const char *s, size_t len) {
   uint64_t b = Fetch64(s + 8);
   uint64_t c = Fetch64(s + len - 8) * mul;
   uint64_t d = Fetch64(s + len - 16) * k2;
-  return HashLen16(Rotate(a + b, 43) + Rotate(c, 30) + d,
-                   a + Rotate(b + k2, 18) + c, mul);
+  return HashLen16(absl::rotr(a + b, 43) + absl::rotr(c, 30) + d,
+                   a + absl::rotr(b + k2, 18) + c, mul);
 }
 
 // Return a 16-byte hash for 48 bytes.  Quick and dirty.
 // Callers do best to use "random-looking" values for a and b.
-static std::pair<uint64_t, uint64_t> WeakHashLen32WithSeeds(uint64_t w, uint64_t x,
-                                                        uint64_t y, uint64_t z,
-                                                        uint64_t a, uint64_t b) {
+static std::pair<uint64_t, uint64_t> WeakHashLen32WithSeeds(
+    uint64_t w, uint64_t x, uint64_t y, uint64_t z, uint64_t a, uint64_t b) {
   a += w;
-  b = Rotate(b + a + z, 21);
+  b = absl::rotr(b + a + z, 21);
   uint64_t c = a;
   a += x;
   a += y;
-  b += Rotate(a, 44);
+  b += absl::rotr(a, 44);
   return std::make_pair(a + z, b + c);
 }
 
 // Return a 16-byte hash for s[0] ... s[31], a, and b.  Quick and dirty.
-static std::pair<uint64_t, uint64_t> WeakHashLen32WithSeeds(const char *s, uint64_t a,
-                                                        uint64_t b) {
+static std::pair<uint64_t, uint64_t> WeakHashLen32WithSeeds(const char *s,
+                                                            uint64_t a,
+                                                            uint64_t b) {
   return WeakHashLen32WithSeeds(Fetch64(s), Fetch64(s + 8), Fetch64(s + 16),
                                 Fetch64(s + 24), a, b);
 }
@@ -282,10 +275,10 @@ static uint64_t HashLen33to64(const char *s, size_t len) {
   uint64_t f = Fetch64(s + 24) * 9;
   uint64_t g = Fetch64(s + len - 8);
   uint64_t h = Fetch64(s + len - 16) * mul;
-  uint64_t u = Rotate(a + g, 43) + (Rotate(b, 30) + c) * 9;
+  uint64_t u = absl::rotr(a + g, 43) + (absl::rotr(b, 30) + c) * 9;
   uint64_t v = ((a + g) ^ d) + f + 1;
   uint64_t w = absl::gbswap_64((u + v) * mul) + h;
-  uint64_t x = Rotate(e + f, 42) + c;
+  uint64_t x = absl::rotr(e + f, 42) + c;
   uint64_t y = (absl::gbswap_64((v + w) * mul) + g) * mul;
   uint64_t z = e + f + c;
   a = absl::gbswap_64((x + z) * mul + y) + b;
@@ -309,18 +302,20 @@ uint64_t CityHash64(const char *s, size_t len) {
   uint64_t x = Fetch64(s + len - 40);
   uint64_t y = Fetch64(s + len - 16) + Fetch64(s + len - 56);
   uint64_t z = HashLen16(Fetch64(s + len - 48) + len, Fetch64(s + len - 24));
-  std::pair<uint64_t, uint64_t> v = WeakHashLen32WithSeeds(s + len - 64, len, z);
-  std::pair<uint64_t, uint64_t> w = WeakHashLen32WithSeeds(s + len - 32, y + k1, x);
+  std::pair<uint64_t, uint64_t> v =
+      WeakHashLen32WithSeeds(s + len - 64, len, z);
+  std::pair<uint64_t, uint64_t> w =
+      WeakHashLen32WithSeeds(s + len - 32, y + k1, x);
   x = x * k1 + Fetch64(s);
 
   // Decrease len to the nearest multiple of 64, and operate on 64-byte chunks.
   len = (len - 1) & ~static_cast<size_t>(63);
   do {
-    x = Rotate(x + y + v.first + Fetch64(s + 8), 37) * k1;
-    y = Rotate(y + v.second + Fetch64(s + 48), 42) * k1;
+    x = absl::rotr(x + y + v.first + Fetch64(s + 8), 37) * k1;
+    y = absl::rotr(y + v.second + Fetch64(s + 48), 42) * k1;
     x ^= w.second;
     y += v.first + Fetch64(s + 40);
-    z = Rotate(z + w.first, 33) * k1;
+    z = absl::rotr(z + w.first, 33) * k1;
     v = WeakHashLen32WithSeeds(s, v.second * k1, x + w.first);
     w = WeakHashLen32WithSeeds(s + 32, z + w.second, y + Fetch64(s + 16));
     std::swap(z, x);
@@ -336,9 +331,10 @@ uint64_t CityHash64WithSeed(const char *s, size_t len, uint64_t seed) {
 }
 
 uint64_t CityHash64WithSeeds(const char *s, size_t len, uint64_t seed0,
-                           uint64_t seed1) {
+                             uint64_t seed1) {
   return HashLen16(CityHash64(s, len) - seed0, seed1);
 }
 
 }  // namespace hash_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/hash/internal/city.h b/absl/hash/internal/city.h
index b43d3407dcb..393da0b95d9 100644
--- a/absl/hash/internal/city.h
+++ b/absl/hash/internal/city.h
@@ -47,16 +47,15 @@
 
 #include <stdint.h>
 #include <stdlib.h>  // for size_t.
+
 #include <utility>
 
+#include "absl/base/config.h"
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace hash_internal {
 
-typedef std::pair<uint64_t, uint64_t> uint128;
-
-inline uint64_t Uint128Low64(const uint128 &x) { return x.first; }
-inline uint64_t Uint128High64(const uint128 &x) { return x.second; }
-
 // Hash function for a byte array.
 uint64_t CityHash64(const char *s, size_t len);
 
@@ -67,25 +66,13 @@ uint64_t CityHash64WithSeed(const char *s, size_t len, uint64_t seed);
 // Hash function for a byte array.  For convenience, two seeds are also
 // hashed into the result.
 uint64_t CityHash64WithSeeds(const char *s, size_t len, uint64_t seed0,
-                           uint64_t seed1);
+                             uint64_t seed1);
 
 // Hash function for a byte array.  Most useful in 32-bit binaries.
 uint32_t CityHash32(const char *s, size_t len);
 
-// Hash 128 input bits down to 64 bits of output.
-// This is intended to be a reasonably good hash function.
-inline uint64_t Hash128to64(const uint128 &x) {
-  // Murmur-inspired hashing.
-  const uint64_t kMul = 0x9ddfea08eb382d69ULL;
-  uint64_t a = (Uint128Low64(x) ^ Uint128High64(x)) * kMul;
-  a ^= (a >> 47);
-  uint64_t b = (Uint128High64(x) ^ a) * kMul;
-  b ^= (b >> 47);
-  b *= kMul;
-  return b;
-}
-
 }  // namespace hash_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_HASH_INTERNAL_CITY_H_
diff --git a/absl/hash/internal/city_test.cc b/absl/hash/internal/city_test.cc
index 71b4eccecff..822b39974d6 100644
--- a/absl/hash/internal/city_test.cc
+++ b/absl/hash/internal/city_test.cc
@@ -15,12 +15,17 @@
 #include "absl/hash/internal/city.h"
 
 #include <string.h>
+
+#include <cstdint>
 #include <cstdio>
 #include <iostream>
+
 #include "gtest/gtest.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace hash_internal {
+namespace {
 
 static const uint64_t k0 = 0xc3a5c85c97cb3127ULL;
 static const uint64_t kSeed0 = 1234567;
@@ -589,5 +594,7 @@ TEST(CityHashTest, Unchanging) {
   TestUnchanging(testdata[i], 0, kDataSize);
 }
 
+}  // namespace
 }  // namespace hash_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/hash/internal/hash.cc b/absl/hash/internal/hash.cc
index 4ab7a9f8bfd..43cb561b954 100644
--- a/absl/hash/internal/hash.cc
+++ b/absl/hash/internal/hash.cc
@@ -14,10 +14,417 @@
 
 #include "absl/hash/internal/hash.h"
 
+#include <cassert>
+#include <cstddef>
+#include <cstdint>
+#include <type_traits>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/unaligned_access.h"
+#include "absl/base/optimization.h"
+#include "absl/base/prefetch.h"
+#include "absl/hash/internal/city.h"
+
+#ifdef ABSL_AES_INTERNAL_HAVE_X86_SIMD
+#error ABSL_AES_INTERNAL_HAVE_X86_SIMD cannot be directly set
+#elif defined(__SSE4_2__) && defined(__AES__)
+#define ABSL_AES_INTERNAL_HAVE_X86_SIMD
+#endif
+
+
+#ifdef ABSL_AES_INTERNAL_HAVE_X86_SIMD
+#include <smmintrin.h>
+#include <wmmintrin.h>
+#include <xmmintrin.h>
+#endif  // ABSL_AES_INTERNAL_HAVE_X86_SIMD
+
+#ifdef ABSL_AES_INTERNAL_HAVE_ARM_SIMD
+#error ABSL_AES_INTERNAL_HAVE_ARM_SIMD cannot be directly set
+#elif defined(ABSL_INTERNAL_HAVE_ARM_NEON) && defined(__ARM_FEATURE_CRYPTO)
+#include <arm_neon.h>
+#define ABSL_AES_INTERNAL_HAVE_ARM_SIMD
+#endif  // ABSL_INTERNAL_HAVE_ARM_NEON
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace hash_internal {
 
-ABSL_CONST_INIT const void* const CityHashState::kSeed = &kSeed;
+namespace {
+
+void PrefetchFutureDataToLocalCache(const uint8_t* ptr) {
+  PrefetchToLocalCache(ptr + 5 * ABSL_CACHELINE_SIZE);
+}
+
+#if defined(ABSL_AES_INTERNAL_HAVE_X86_SIMD) || \
+    defined(ABSL_AES_INTERNAL_HAVE_ARM_SIMD)
+
+#if defined(ABSL_AES_INTERNAL_HAVE_X86_SIMD)
+using Vector128 = __m128i;
+
+inline Vector128 Load128(const uint8_t* ptr) {
+  return _mm_loadu_si128(reinterpret_cast<const Vector128*>(ptr));
+}
+
+inline Vector128 Set128(uint64_t a, uint64_t b) {
+  return _mm_set_epi64x(static_cast<int64_t>(a), static_cast<int64_t>(b));
+}
+
+inline Vector128 Add128(Vector128 a, Vector128 b) {
+  return _mm_add_epi64(a, b);
+}
+
+inline Vector128 Sub128(Vector128 a, Vector128 b) {
+  return _mm_sub_epi64(a, b);
+}
+
+// Bits of the second argument to Encrypt128/Decrypt128 are XORed with the
+// first argument after encryption/decryption.
+
+inline Vector128 Encrypt128(Vector128 data, Vector128 key) {
+  return _mm_aesenc_si128(data, key);
+}
+
+inline Vector128 Decrypt128(Vector128 data, Vector128 key) {
+  return _mm_aesdec_si128(data, key);
+}
+
+// We use each value as the first argument to shuffle all the bits around. We do
+// not add any salt to the state or loaded data, instead we vary instructions
+// used to mix bits Encrypt128/Decrypt128 and Add128/Sub128. On x86,
+// Add128/Sub128 are combined to one instruction with data loading like
+// `vpaddq xmm1, xmm0, xmmword ptr [rdi]`.
+
+inline Vector128 MixA(Vector128 a, Vector128 state) {
+  return Decrypt128(Add128(state, a), state);
+}
+
+inline Vector128 MixB(Vector128 b, Vector128 state) {
+  return Decrypt128(Sub128(state, b), state);
+}
+
+inline Vector128 MixC(Vector128 c, Vector128 state) {
+  return Encrypt128(Add128(state, c), state);
+}
+
+inline Vector128 MixD(Vector128 d, Vector128 state) {
+  return Encrypt128(Sub128(state, d), state);
+}
+
+inline uint64_t ExtractLow64(Vector128 v) {
+  return static_cast<uint64_t>(_mm_cvtsi128_si64(v));
+}
+
+inline uint64_t ExtractHigh64(Vector128 v) {
+  return static_cast<uint64_t>(_mm_extract_epi64(v, 1));
+}
+
+inline uint64_t Mix4x16Vectors(Vector128 a, Vector128 b, Vector128 c,
+                               Vector128 d) {
+  Vector128 res128 =
+      Add128(Encrypt128(Add128(a, c), d), Decrypt128(Sub128(b, d), a));
+  uint64_t x64 = ExtractLow64(res128);
+  uint64_t y64 = ExtractHigh64(res128);
+  return x64 ^ y64;
+}
+
+#else  // ABSL_AES_INTERNAL_HAVE_ARM_SIMD
+
+using Vector128 = uint8x16_t;
+
+inline Vector128 Load128(const uint8_t* ptr) { return vld1q_u8(ptr); }
+
+inline Vector128 Set128(uint64_t a, uint64_t b) {
+  return vreinterpretq_u8_u64(vsetq_lane_u64(a, vdupq_n_u64(b), 1));
+}
+
+inline Vector128 Add128(Vector128 a, Vector128 b) {
+  return vreinterpretq_u8_u64(
+      vaddq_u64(vreinterpretq_u64_u8(a), vreinterpretq_u64_u8(b)));
+}
+
+// Bits of the second argument to Decrypt128/Encrypt128 are XORed with the
+// state argument BEFORE encryption (in x86 version they are XORed after).
+
+inline Vector128 Encrypt128(Vector128 data, Vector128 key) {
+  return vaesmcq_u8(vaeseq_u8(data, key));
+}
+
+inline Vector128 Decrypt128(Vector128 data, Vector128 key) {
+  return vaesimcq_u8(vaesdq_u8(data, key));
+}
+
+// We use decryption for a, b and encryption for c, d. That helps us to avoid
+// collisions for trivial byte rotations. Mix4x16Vectors later uses
+// encrypted/decrypted pairs differently to ensure that the order of blocks is
+// important for the hash value.
+// We also avoid using Add128/Sub128 instructions because state is being mixed
+// before encryption/decryption. On ARM, there is no fusion of load and add/sub
+// instructions so it is more expensive to use them.
+
+inline Vector128 MixA(Vector128 a, Vector128 state) {
+  return Decrypt128(a, state);
+}
+
+inline Vector128 MixB(Vector128 b, Vector128 state) {
+  return Decrypt128(b, state);
+}
+
+inline Vector128 MixC(Vector128 c, Vector128 state) {
+  return Encrypt128(c, state);
+}
+
+inline Vector128 MixD(Vector128 d, Vector128 state) {
+  return Encrypt128(d, state);
+}
+
+inline uint64_t ExtractLow64(Vector128 v) {
+  return vgetq_lane_u64(vreinterpretq_u64_u8(v), 0);
+}
+
+inline uint64_t ExtractHigh64(Vector128 v) {
+  return vgetq_lane_u64(vreinterpretq_u64_u8(v), 1);
+}
+
+uint64_t Mix4x16Vectors(Vector128 a, Vector128 b, Vector128 c, Vector128 d) {
+  Vector128 res128 = Add128(Encrypt128(a, c), Decrypt128(b, d));
+  uint64_t x64 = ExtractLow64(res128);
+  uint64_t y64 = ExtractHigh64(res128);
+  return x64 ^ y64;
+}
+
+#endif  // ABSL_AES_INTERNAL_HAVE_X86_SIMD
+
+uint64_t LowLevelHash33To64(uint64_t seed, const uint8_t* ptr, size_t len) {
+  assert(len > 32);
+  assert(len <= 64);
+  Vector128 state = Set128(seed, len);
+  Vector128 a = Load128(ptr);
+  Vector128 b = Load128(ptr + 16);
+  auto* last32_ptr = ptr + len - 32;
+  Vector128 c = Load128(last32_ptr);
+  Vector128 d = Load128(last32_ptr + 16);
+
+  Vector128 na = MixA(a, state);
+  Vector128 nb = MixB(b, state);
+  Vector128 nc = MixC(c, state);
+  Vector128 nd = MixD(d, state);
+
+  // We perform another round of encryption to mix bits between two halves of
+  // the input.
+  return Mix4x16Vectors(na, nb, nc, nd);
+}
+
+[[maybe_unused]] ABSL_ATTRIBUTE_NOINLINE uint64_t
+LowLevelHashLenGt64(uint64_t seed, const void* data, size_t len) {
+  assert(len > 64);
+  const uint8_t* ptr = static_cast<const uint8_t*>(data);
+  const uint8_t* last_32_ptr = ptr + len - 32;
+
+  // If we have more than 64 bytes, we're going to handle chunks of 64
+  // bytes at a time. We're going to build up four separate hash states
+  // which we will then hash together. This avoids short dependency chains.
+  Vector128 state0 = Set128(seed, len);
+  Vector128 state1 = state0;
+  Vector128 state2 = state1;
+  Vector128 state3 = state2;
+
+  // Mixing two 128-bit vectors at a time with corresponding states.
+  // All variables are mixed slightly differently to avoid hash collision
+  // due to trivial byte rotation.
+  // We combine state and data with _mm_add_epi64/_mm_sub_epi64 before applying
+  // AES encryption to make hash function dependent on the order of the blocks.
+  // See comments in LowLevelHash33To64 for more considerations.
+  auto mix_ab = [&state0,
+                 &state1](const uint8_t* p) ABSL_ATTRIBUTE_ALWAYS_INLINE {
+    Vector128 a = Load128(p);
+    Vector128 b = Load128(p + 16);
+    state0 = MixA(a, state0);
+    state1 = MixB(b, state1);
+  };
+  auto mix_cd = [&state2,
+                 &state3](const uint8_t* p) ABSL_ATTRIBUTE_ALWAYS_INLINE {
+    Vector128 c = Load128(p);
+    Vector128 d = Load128(p + 16);
+    state2 = MixC(c, state2);
+    state3 = MixD(d, state3);
+  };
+
+  do {
+    PrefetchFutureDataToLocalCache(ptr);
+    mix_ab(ptr);
+    mix_cd(ptr + 32);
+
+    ptr += 64;
+    len -= 64;
+  } while (len > 64);
+
+  // We now have a data `ptr` with at most 64 bytes.
+  if (len > 32) {
+    mix_ab(ptr);
+  }
+  mix_cd(last_32_ptr);
+
+  return Mix4x16Vectors(state0, state1, state2, state3);
+}
+#else
+uint64_t Mix32Bytes(const uint8_t* ptr, uint64_t current_state) {
+  uint64_t a = absl::base_internal::UnalignedLoad64(ptr);
+  uint64_t b = absl::base_internal::UnalignedLoad64(ptr + 8);
+  uint64_t c = absl::base_internal::UnalignedLoad64(ptr + 16);
+  uint64_t d = absl::base_internal::UnalignedLoad64(ptr + 24);
+
+  uint64_t cs0 = Mix(a ^ kStaticRandomData[1], b ^ current_state);
+  uint64_t cs1 = Mix(c ^ kStaticRandomData[2], d ^ current_state);
+  return cs0 ^ cs1;
+}
+
+uint64_t LowLevelHash33To64(uint64_t seed, const uint8_t* ptr, size_t len) {
+  assert(len > 32);
+  assert(len <= 64);
+  uint64_t current_state = seed ^ kStaticRandomData[0] ^ len;
+  const uint8_t* last_32_ptr = ptr + len - 32;
+  return Mix32Bytes(last_32_ptr, Mix32Bytes(ptr, current_state));
+}
+
+[[maybe_unused]] ABSL_ATTRIBUTE_NOINLINE uint64_t
+LowLevelHashLenGt64(uint64_t seed, const void* data, size_t len) {
+  assert(len > 64);
+  const uint8_t* ptr = static_cast<const uint8_t*>(data);
+  uint64_t current_state = seed ^ kStaticRandomData[0] ^ len;
+  const uint8_t* last_32_ptr = ptr + len - 32;
+  // If we have more than 64 bytes, we're going to handle chunks of 64
+  // bytes at a time. We're going to build up four separate hash states
+  // which we will then hash together. This avoids short dependency chains.
+  uint64_t duplicated_state0 = current_state;
+  uint64_t duplicated_state1 = current_state;
+  uint64_t duplicated_state2 = current_state;
+
+  do {
+    PrefetchFutureDataToLocalCache(ptr);
+
+    uint64_t a = absl::base_internal::UnalignedLoad64(ptr);
+    uint64_t b = absl::base_internal::UnalignedLoad64(ptr + 8);
+    uint64_t c = absl::base_internal::UnalignedLoad64(ptr + 16);
+    uint64_t d = absl::base_internal::UnalignedLoad64(ptr + 24);
+    uint64_t e = absl::base_internal::UnalignedLoad64(ptr + 32);
+    uint64_t f = absl::base_internal::UnalignedLoad64(ptr + 40);
+    uint64_t g = absl::base_internal::UnalignedLoad64(ptr + 48);
+    uint64_t h = absl::base_internal::UnalignedLoad64(ptr + 56);
+
+    current_state = Mix(a ^ kStaticRandomData[1], b ^ current_state);
+    duplicated_state0 = Mix(c ^ kStaticRandomData[2], d ^ duplicated_state0);
+
+    duplicated_state1 = Mix(e ^ kStaticRandomData[3], f ^ duplicated_state1);
+    duplicated_state2 = Mix(g ^ kStaticRandomData[4], h ^ duplicated_state2);
+
+    ptr += 64;
+    len -= 64;
+  } while (len > 64);
+
+  current_state = (current_state ^ duplicated_state0) ^
+                  (duplicated_state1 + duplicated_state2);
+  // We now have a data `ptr` with at most 64 bytes and the current state
+  // of the hashing state machine stored in current_state.
+  if (len > 32) {
+    current_state = Mix32Bytes(ptr, current_state);
+  }
+
+  // We now have a data `ptr` with at most 32 bytes and the current state
+  // of the hashing state machine stored in current_state. But we can
+  // safely read from `ptr + len - 32`.
+  return Mix32Bytes(last_32_ptr, current_state);
+}
+#endif  // ABSL_AES_INTERNAL_HAVE_X86_SIMD
+
+[[maybe_unused]] uint64_t LowLevelHashLenGt32(uint64_t seed, const void* data,
+                                              size_t len) {
+  assert(len > 32);
+  if (ABSL_PREDICT_FALSE(len > 64)) {
+    return LowLevelHashLenGt64(seed, data, len);
+  }
+  return LowLevelHash33To64(seed, static_cast<const uint8_t*>(data), len);
+}
+
+ABSL_ATTRIBUTE_ALWAYS_INLINE inline uint64_t HashBlockOn32Bit(
+    uint64_t state, const unsigned char* data, size_t len) {
+  // TODO(b/417141985): expose and use CityHash32WithSeed.
+  // Note: we can't use PrecombineLengthMix here because len can be up to 1024.
+  return CombineRawImpl(
+      state + len,
+      hash_internal::CityHash32(reinterpret_cast<const char*>(data), len));
+}
+
+ABSL_ATTRIBUTE_NOINLINE uint64_t
+SplitAndCombineOn32Bit(uint64_t state, const unsigned char* first, size_t len) {
+  while (len >= PiecewiseChunkSize()) {
+    state = HashBlockOn32Bit(state, first, PiecewiseChunkSize());
+    len -= PiecewiseChunkSize();
+    first += PiecewiseChunkSize();
+  }
+  // Do not call CombineContiguousImpl for empty range since it is modifying
+  // state.
+  if (len == 0) {
+    return state;
+  }
+  // Handle the remainder.
+  return CombineContiguousImpl(state, first, len,
+                               std::integral_constant<int, 4>{});
+}
+
+ABSL_ATTRIBUTE_ALWAYS_INLINE inline uint64_t HashBlockOn64Bit(
+    uint64_t state, const unsigned char* data, size_t len) {
+#ifdef ABSL_HAVE_INTRINSIC_INT128
+  return LowLevelHashLenGt32(state, data, len);
+#else
+  return hash_internal::CityHash64WithSeed(reinterpret_cast<const char*>(data),
+                                           len, state);
+#endif
+}
+
+ABSL_ATTRIBUTE_NOINLINE uint64_t
+SplitAndCombineOn64Bit(uint64_t state, const unsigned char* first, size_t len) {
+  while (len >= PiecewiseChunkSize()) {
+    state = HashBlockOn64Bit(state, first, PiecewiseChunkSize());
+    len -= PiecewiseChunkSize();
+    first += PiecewiseChunkSize();
+  }
+  // Do not call CombineContiguousImpl for empty range since it is modifying
+  // state.
+  if (len == 0) {
+    return state;
+  }
+  // Handle the remainder.
+  return CombineContiguousImpl(state, first, len,
+                               std::integral_constant<int, 8>{});
+}
+
+}  // namespace
+
+uint64_t CombineLargeContiguousImplOn32BitLengthGt8(uint64_t state,
+                                                    const unsigned char* first,
+                                                    size_t len) {
+  assert(len > 8);
+  assert(sizeof(size_t) == 4);  // NOLINT(misc-static-assert)
+  if (ABSL_PREDICT_TRUE(len <= PiecewiseChunkSize())) {
+    return HashBlockOn32Bit(state, first, len);
+  }
+  return SplitAndCombineOn32Bit(state, first, len);
+}
+
+uint64_t CombineLargeContiguousImplOn64BitLengthGt32(uint64_t state,
+                                                     const unsigned char* first,
+                                                     size_t len) {
+  assert(len > 32);
+  assert(sizeof(size_t) == 8);  // NOLINT(misc-static-assert)
+  if (ABSL_PREDICT_TRUE(len <= PiecewiseChunkSize())) {
+    return HashBlockOn64Bit(state, first, len);
+  }
+  return SplitAndCombineOn64Bit(state, first, len);
+}
+
+ABSL_CONST_INIT const void* const MixingHashState::kSeed = &kSeed;
 
 }  // namespace hash_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/hash/internal/hash.h b/absl/hash/internal/hash.h
index 4ff4a126f0c..a7dc74c60ff 100644
--- a/absl/hash/internal/hash.h
+++ b/absl/hash/internal/hash.h
@@ -19,9 +19,34 @@
 #ifndef ABSL_HASH_INTERNAL_HASH_H_
 #define ABSL_HASH_INTERNAL_HASH_H_
 
+// We include config.h here to make sure that ABSL_INTERNAL_CPLUSPLUS_LANG is
+// defined.
+#include "absl/base/config.h"
+
+// GCC15 warns that <ciso646> is deprecated in C++17 and suggests using
+// <version> instead, even though <version> is not available in C++17 mode prior
+// to GCC9.
+#if __has_include(<version>)
+#define ABSL_INTERNAL_VERSION_HEADER_AVAILABLE 1
+#endif
+
+// For feature testing and determining which headers can be included.
+#if ABSL_INTERNAL_CPLUSPLUS_LANG >= 202002L || \
+    defined(ABSL_INTERNAL_VERSION_HEADER_AVAILABLE)
+#include <version>
+#else
+#include <ciso646>
+#endif
+
+#undef ABSL_INTERNAL_VERSION_HEADER_AVAILABLE
+
 #include <algorithm>
 #include <array>
+#include <bitset>
+#include <cassert>
 #include <cmath>
+#include <cstddef>
+#include <cstdint>
 #include <cstring>
 #include <deque>
 #include <forward_list>
@@ -31,45 +56,211 @@
 #include <list>
 #include <map>
 #include <memory>
+#include <optional>
 #include <set>
 #include <string>
+#include <string_view>
 #include <tuple>
 #include <type_traits>
+#include <unordered_map>
+#include <unordered_set>
 #include <utility>
+#include <variant>
 #include <vector>
 
+#include "absl/base/attributes.h"
 #include "absl/base/internal/endian.h"
+#include "absl/base/internal/unaligned_access.h"
+#include "absl/base/optimization.h"
+#include "absl/base/options.h"
 #include "absl/base/port.h"
 #include "absl/container/fixed_array.h"
+#include "absl/hash/internal/city.h"
+#include "absl/hash/internal/weakly_mixed_integer.h"
 #include "absl/meta/type_traits.h"
+#include "absl/numeric/bits.h"
 #include "absl/numeric/int128.h"
 #include "absl/strings/string_view.h"
 #include "absl/types/optional.h"
 #include "absl/types/variant.h"
 #include "absl/utility/utility.h"
-#include "absl/hash/internal/city.h"
+
+#ifdef __APPLE__
+#include <Availability.h>
+#include <TargetConditionals.h>
+#endif
+
+#if defined(__cpp_lib_filesystem) && __cpp_lib_filesystem >= 201703L && \
+    !defined(__XTENSA__)
+#include <filesystem>  // NOLINT
+#endif
+
+// We are allowed to use a non-portable hardware-accelerated implementation in
+// headers if ABSL_OPTION_INLINE_HW_ACCEL_STRATEGY != 0
+#if ABSL_OPTION_INLINE_HW_ACCEL_STRATEGY != 0
+
+// 32-bit builds with SSE 4.2 do not have _mm_crc32_u64, so the
+// __x86_64__ condition is necessary.
+#if defined(__SSE4_2__) && defined(__x86_64__)
+
+#include <x86intrin.h>
+#define ABSL_HASH_INTERNAL_HAS_CRC32
+#define ABSL_HASH_INTERNAL_CRC32_U64 _mm_crc32_u64
+#define ABSL_HASH_INTERNAL_CRC32_U32 _mm_crc32_u32
+#define ABSL_HASH_INTERNAL_CRC32_U8 _mm_crc32_u8
+
+// 32-bit builds with AVX do not have _mm_crc32_u64, so the _M_X64 condition is
+// necessary.
+#elif defined(_MSC_VER) && !defined(__clang__) && defined(__AVX__) && \
+    defined(_M_X64)
+
+// MSVC AVX (/arch:AVX) implies SSE 4.2.
+#include <intrin.h>
+#define ABSL_HASH_INTERNAL_HAS_CRC32
+#define ABSL_HASH_INTERNAL_CRC32_U64 _mm_crc32_u64
+#define ABSL_HASH_INTERNAL_CRC32_U32 _mm_crc32_u32
+#define ABSL_HASH_INTERNAL_CRC32_U8 _mm_crc32_u8
+
+#elif defined(__ARM_FEATURE_CRC32)
+
+#include <arm_acle.h>
+#define ABSL_HASH_INTERNAL_HAS_CRC32
+// Casting to uint32_t to be consistent with x86 intrinsic (_mm_crc32_u64
+// accepts crc as 64 bit integer).
+#define ABSL_HASH_INTERNAL_CRC32_U64(crc, data) \
+  __crc32cd(static_cast<uint32_t>(crc), data)
+#define ABSL_HASH_INTERNAL_CRC32_U32 __crc32cw
+#define ABSL_HASH_INTERNAL_CRC32_U8 __crc32cb
+
+#endif  // Platform tests
+
+#endif  // ABSL_OPTION_INLINE_HW_ACCEL_STRATEGY != 0
+
+
+#if ABSL_OPTION_INLINE_HW_ACCEL_STRATEGY == 1
+#ifndef ABSL_HASH_INTERNAL_HAS_CRC32
+#error "Hardware acceleration is required by ABSL_OPTION_INLINE_HW_ACCEL_STRATEGY but not supported on this platform; see absl/base/options.h"
+#endif
+#endif
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+class HashState;
+
 namespace hash_internal {
 
-// HashStateBase
+// Internal detail: Large buffers are hashed in smaller chunks.  This function
+// returns the size of these chunks.
+constexpr size_t PiecewiseChunkSize() { return 1024; }
+
+// PiecewiseCombiner is an internal-only helper class for hashing a piecewise
+// buffer of `char` or `unsigned char` as though it were contiguous.  This class
+// provides two methods:
+//
+//   H add_buffer(state, data, size)
+//   H finalize(state)
 //
-// A hash state object represents an intermediate state in the computation
-// of an unspecified hash algorithm. `HashStateBase` provides a CRTP style
-// base class for hash state implementations. Developers adding type support
-// for `absl::Hash` should not rely on any parts of the state object other than
-// the following member functions:
+// `add_buffer` can be called zero or more times, followed by a single call to
+// `finalize`.  This will produce the same hash expansion as concatenating each
+// buffer piece into a single contiguous buffer, and passing this to
+// `H::combine_contiguous`.
+//
+//  Example usage:
+//    PiecewiseCombiner combiner;
+//    for (const auto& piece : pieces) {
+//      state = combiner.add_buffer(std::move(state), piece.data, piece.size);
+//    }
+//    return combiner.finalize(std::move(state));
+class PiecewiseCombiner {
+ public:
+  PiecewiseCombiner() = default;
+  PiecewiseCombiner(const PiecewiseCombiner&) = delete;
+  PiecewiseCombiner& operator=(const PiecewiseCombiner&) = delete;
+
+  // Appends the given range of bytes to the sequence to be hashed, which may
+  // modify the provided hash state.
+  template <typename H>
+  H add_buffer(H state, const unsigned char* data, size_t size);
+  template <typename H>
+  H add_buffer(H state, const char* data, size_t size) {
+    return add_buffer(std::move(state),
+                      reinterpret_cast<const unsigned char*>(data), size);
+  }
+
+  // Finishes combining the hash sequence, which may may modify the provided
+  // hash state.
+  //
+  // Once finalize() is called, add_buffer() may no longer be called. The
+  // resulting hash state will be the same as if the pieces passed to
+  // add_buffer() were concatenated into a single flat buffer, and then provided
+  // to H::combine_contiguous().
+  template <typename H>
+  H finalize(H state);
+
+ private:
+  unsigned char buf_[PiecewiseChunkSize()];
+  size_t position_ = 0;
+  bool added_something_ = false;
+};
+
+// Trait class which returns true if T is hashable by the absl::Hash framework.
+// Used for the AbslHashValue implementations for composite types below.
+template <typename T>
+struct is_hashable;
+
+// HashStateBase is an internal implementation detail that contains common
+// implementation details for all of the "hash state objects" objects generated
+// by Abseil.  This is not a public API; users should not create classes that
+// inherit from this.
+//
+// A hash state object is the template argument `H` passed to `AbslHashValue`.
+// It represents an intermediate state in the computation of an unspecified hash
+// algorithm. `HashStateBase` provides a CRTP style base class for hash state
+// implementations. Developers adding type support for `absl::Hash` should not
+// rely on any parts of the state object other than the following member
+// functions:
 //
 //   * HashStateBase::combine()
 //   * HashStateBase::combine_contiguous()
+//   * HashStateBase::combine_unordered()
 //
-// A derived hash state class of type `H` must provide a static member function
+// A derived hash state class of type `H` must provide a public member function
 // with a signature similar to the following:
 //
 //    `static H combine_contiguous(H state, const unsigned char*, size_t)`.
 //
-// `HashStateBase` will provide a complete implementations for a hash state
-// object in terms of this method.
+// It must also provide a private template method named RunCombineUnordered.
+//
+// A "consumer" is a 1-arg functor returning void.  Its argument is a reference
+// to an inner hash state object, and it may be called multiple times.  When
+// called, the functor consumes the entropy from the provided state object,
+// and resets that object to its empty state.
+//
+// A "combiner" is a stateless 2-arg functor returning void.  Its arguments are
+// an inner hash state object and an ElementStateConsumer functor.  A combiner
+// uses the provided inner hash state object to hash each element of the
+// container, passing the inner hash state object to the consumer after hashing
+// each element.
+//
+// Given these definitions, a derived hash state class of type H
+// must provide a private template method with a signature similar to the
+// following:
+//
+//    `template <typename CombinerT>`
+//    `static H RunCombineUnordered(H outer_state, CombinerT combiner)`
+//
+// This function is responsible for constructing the inner state object and
+// providing a consumer to the combiner.  It uses side effects of the consumer
+// and combiner to mix the state of each element in an order-independent manner,
+// and uses this to return an updated value of `outer_state`.
+//
+// This inside-out approach generates efficient object code in the normal case,
+// but allows us to use stack storage to implement the absl::HashState type
+// erasure mechanism (avoiding heap allocations while hashing).
+//
+// `HashStateBase` will provide a complete implementation for a hash state
+// object in terms of these two methods.
 //
 // Example:
 //
@@ -78,12 +269,14 @@ namespace hash_internal {
 //       static H combine_contiguous(H state, const unsigned char*, size_t);
 //       using MyHashState::HashStateBase::combine;
 //       using MyHashState::HashStateBase::combine_contiguous;
+//       using MyHashState::HashStateBase::combine_unordered;
+//     private:
+//       template <typename CombinerT>
+//       static H RunCombineUnordered(H state, CombinerT combiner);
 //   };
 template <typename H>
 class HashStateBase {
  public:
-  // HashStateBase::combine()
-  //
   // Combines an arbitrary number of values into a hash state, returning the
   // updated state.
   //
@@ -103,8 +296,6 @@ class HashStateBase {
   static H combine(H state, const T& value, const Ts&... values);
   static H combine(H state) { return state; }
 
-  // HashStateBase::combine_contiguous()
-  //
   // Combines a contiguous array of `size` elements into a hash state, returning
   // the updated state.
   //
@@ -117,10 +308,33 @@ class HashStateBase {
   // for-loop instead.
   template <typename T>
   static H combine_contiguous(H state, const T* data, size_t size);
+
+  template <typename I>
+  static H combine_unordered(H state, I begin, I end);
+
+  using AbslInternalPiecewiseCombiner = PiecewiseCombiner;
+
+  template <typename T>
+  using is_hashable = absl::hash_internal::is_hashable<T>;
+
+ private:
+  // Common implementation of the iteration step of a "combiner", as described
+  // above.
+  template <typename I>
+  struct CombineUnorderedCallback {
+    I begin;
+    I end;
+
+    template <typename InnerH, typename ElementStateConsumer>
+    void operator()(InnerH inner_state, ElementStateConsumer cb) {
+      for (; begin != end; ++begin) {
+        inner_state = H::combine(std::move(inner_state), *begin);
+        cb(inner_state);
+      }
+    }
+  };
 };
 
-// is_uniquely_represented
-//
 // `is_uniquely_represented<T>` is a trait class that indicates whether `T`
 // is uniquely represented.
 //
@@ -155,8 +369,6 @@ class HashStateBase {
 template <typename T, typename Enable = void>
 struct is_uniquely_represented : std::false_type {};
 
-// is_uniquely_represented<unsigned char>
-//
 // unsigned char is a synonym for "byte", so it is guaranteed to be
 // uniquely represented.
 template <>
@@ -167,26 +379,71 @@ struct is_uniquely_represented<unsigned char> : std::true_type {};
 // Integral types other than bool should be uniquely represented on any
 // platform that this will plausibly be ported to.
 template <typename Integral>
-struct is_uniquely_represented<
-    Integral, typename std::enable_if<std::is_integral<Integral>::value>::type>
+struct is_uniquely_represented<Integral,
+                               std::enable_if_t<std::is_integral_v<Integral>>>
     : std::true_type {};
 
-// is_uniquely_represented<bool>
-//
-//
 template <>
 struct is_uniquely_represented<bool> : std::false_type {};
 
-// hash_bytes()
-//
+#ifdef ABSL_HAVE_INTRINSIC_INT128
+// Specialize the trait for GNU extension types.
+template <>
+struct is_uniquely_represented<__int128> : std::true_type {};
+template <>
+struct is_uniquely_represented<unsigned __int128> : std::true_type {};
+#endif  // ABSL_HAVE_INTRINSIC_INT128
+
+template <typename T>
+struct FitsIn64Bits : std::bool_constant<sizeof(T) <= 8> {};
+
+struct CombineRaw {
+  template <typename H>
+  H operator()(H state, uint64_t value) const {
+    return H::combine_raw(std::move(state), value);
+  }
+};
+
+// For use in `raw_hash_set` to pass a seed to the hash function.
+struct HashWithSeed {
+  template <typename Hasher, typename T>
+  size_t hash(const Hasher& hasher, const T& value, size_t seed) const {
+    // NOLINTNEXTLINE(clang-diagnostic-sign-conversion)
+    return hasher.hash_with_seed(value, seed);
+  }
+};
+
 // Convenience function that combines `hash_state` with the byte representation
 // of `value`.
-template <typename H, typename T>
+template <typename H, typename T,
+          std::enable_if_t<FitsIn64Bits<T>::value, int> = 0>
+H hash_bytes(H hash_state, const T& value) {
+  const unsigned char* start = reinterpret_cast<const unsigned char*>(&value);
+  uint64_t v;
+  if constexpr (sizeof(T) == 1) {
+    v = *start;
+  } else if constexpr (sizeof(T) == 2) {
+    v = absl::base_internal::UnalignedLoad16(start);
+  } else if constexpr (sizeof(T) == 4) {
+    v = absl::base_internal::UnalignedLoad32(start);
+  } else {
+    static_assert(sizeof(T) == 8);
+    v = absl::base_internal::UnalignedLoad64(start);
+  }
+  return CombineRaw()(std::move(hash_state), v);
+}
+template <typename H, typename T,
+          std::enable_if_t<!FitsIn64Bits<T>::value, int> = 0>
 H hash_bytes(H hash_state, const T& value) {
   const unsigned char* start = reinterpret_cast<const unsigned char*>(&value);
   return H::combine_contiguous(std::move(hash_state), start, sizeof(value));
 }
 
+template <typename H>
+H hash_weakly_mixed_integer(H hash_state, WeaklyMixedInteger value) {
+  return H::combine_weakly_mixed_integer(std::move(hash_state), value);
+}
+
 // -----------------------------------------------------------------------------
 // AbslHashValue for Basic Types
 // -----------------------------------------------------------------------------
@@ -201,29 +458,29 @@ H hash_bytes(H hash_state, const T& value) {
 // We use SFINAE to ensure that this overload only accepts bool, not types that
 // are convertible to bool.
 template <typename H, typename B>
-typename std::enable_if<std::is_same<B, bool>::value, H>::type AbslHashValue(
-    H hash_state, B value) {
+std::enable_if_t<std::is_same_v<B, bool>, H> AbslHashValue(H hash_state,
+                                                           B value) {
+  // We use ~size_t{} instead of 1 so that all bits are different between
+  // true/false instead of only 1.
   return H::combine(std::move(hash_state),
-                    static_cast<unsigned char>(value ? 1 : 0));
+                    static_cast<size_t>(value ? ~size_t{} : 0));
 }
 
 // AbslHashValue() for hashing enum values
 template <typename H, typename Enum>
-typename std::enable_if<std::is_enum<Enum>::value, H>::type AbslHashValue(
-    H hash_state, Enum e) {
+std::enable_if_t<std::is_enum_v<Enum>, H> AbslHashValue(H hash_state, Enum e) {
   // In practice, we could almost certainly just invoke hash_bytes directly,
   // but it's possible that a sanitizer might one day want to
   // store data in the unused bits of an enum. To avoid that risk, we
   // convert to the underlying type before hashing. Hopefully this will get
   // optimized away; if not, we can reopen discussion with c-toolchain-team.
   return H::combine(std::move(hash_state),
-                    static_cast<typename std::underlying_type<Enum>::type>(e));
+                    static_cast<std::underlying_type_t<Enum>>(e));
 }
 // AbslHashValue() for hashing floating-point values
 template <typename H, typename Float>
-typename std::enable_if<std::is_same<Float, float>::value ||
-                            std::is_same<Float, double>::value,
-                        H>::type
+std::enable_if_t<std::is_same_v<Float, float> || std::is_same_v<Float, double>,
+                 H>
 AbslHashValue(H hash_state, Float value) {
   return hash_internal::hash_bytes(std::move(hash_state),
                                    value == 0 ? 0 : value);
@@ -234,8 +491,8 @@ AbslHashValue(H hash_state, Float value) {
 // of it. This means we can't use hash_bytes on a long double and have to
 // convert it to something else first.
 template <typename H, typename LongDouble>
-typename std::enable_if<std::is_same<LongDouble, long double>::value, H>::type
-AbslHashValue(H hash_state, LongDouble value) {
+std::enable_if_t<std::is_same_v<LongDouble, long double>, H> AbslHashValue(
+    H hash_state, LongDouble value) {
   const int category = std::fpclassify(value);
   switch (category) {
     case FP_INFINITE:
@@ -264,15 +521,28 @@ AbslHashValue(H hash_state, LongDouble value) {
   return H::combine(std::move(hash_state), category);
 }
 
+// Without this overload, an array decays to a pointer and we hash that, which
+// is not likely to be what the caller intended.
+template <typename H, typename T, size_t N>
+H AbslHashValue(H hash_state, T (&)[N]) {
+  static_assert(
+      sizeof(T) == -1,
+      "Hashing C arrays is not allowed. For string literals, wrap the literal "
+      "in absl::string_view(). To hash the array contents, use "
+      "absl::MakeSpan() or make the array an std::array. To hash the array "
+      "address, use &array[0].");
+  return hash_state;
+}
+
 // AbslHashValue() for hashing pointers
 template <typename H, typename T>
-H AbslHashValue(H hash_state, T* ptr) {
+std::enable_if_t<std::is_pointer_v<T>, H> AbslHashValue(H hash_state, T ptr) {
   auto v = reinterpret_cast<uintptr_t>(ptr);
   // Due to alignment, pointers tend to have low bits as zero, and the next few
   // bits follow a pattern since they are also multiples of some base value.
-  // Mixing the pointer twice helps prevent stuck low bits for certain alignment
-  // values.
-  return H::combine(std::move(hash_state), v, v);
+  // The PointerAlignment test verifies that our mixing is good enough to handle
+  // these cases.
+  return H::combine(std::move(hash_state), v);
 }
 
 // AbslHashValue() for hashing nullptr_t
@@ -281,31 +551,52 @@ H AbslHashValue(H hash_state, std::nullptr_t) {
   return H::combine(std::move(hash_state), static_cast<void*>(nullptr));
 }
 
+// AbslHashValue() for hashing pointers-to-member
+template <typename H, typename T, typename C>
+H AbslHashValue(H hash_state, T C::*ptr) {
+  auto salient_ptm_size = [](std::size_t n) -> std::size_t {
+#if defined(_MSC_VER)
+    // Pointers-to-member-function on MSVC consist of one pointer plus 0, 1, 2,
+    // or 3 ints. In 64-bit mode, they are 8-byte aligned and thus can contain
+    // padding (namely when they have 1 or 3 ints). The value below is a lower
+    // bound on the number of salient, non-padding bytes that we use for
+    // hashing.
+    if constexpr (alignof(T C::*) == alignof(int)) {
+      // No padding when all subobjects have the same size as the total
+      // alignment. This happens in 32-bit mode.
+      return n;
+    } else {
+      // Padding for 1 int (size 16) or 3 ints (size 24).
+      // With 2 ints, the size is 16 with no padding, which we pessimize.
+      return n == 24 ? 20 : n == 16 ? 12 : n;
+    }
+#else
+  // On other platforms, we assume that pointers-to-members do not have
+  // padding.
+    static_assert(std::has_unique_object_representations_v<T C::*>);
+    return n;
+#endif
+  };
+  return H::combine_contiguous(std::move(hash_state),
+                               reinterpret_cast<unsigned char*>(&ptr),
+                               salient_ptm_size(sizeof ptr));
+}
+
 // -----------------------------------------------------------------------------
 // AbslHashValue for Composite Types
 // -----------------------------------------------------------------------------
 
-// is_hashable()
-//
-// Trait class which returns true if T is hashable by the absl::Hash framework.
-// Used for the AbslHashValue implementations for composite types below.
-template <typename T>
-struct is_hashable;
-
 // AbslHashValue() for hashing pairs
 template <typename H, typename T1, typename T2>
-typename std::enable_if<is_hashable<T1>::value && is_hashable<T2>::value,
-                        H>::type
+std::enable_if_t<is_hashable<T1>::value && is_hashable<T2>::value, H>
 AbslHashValue(H hash_state, const std::pair<T1, T2>& p) {
   return H::combine(std::move(hash_state), p.first, p.second);
 }
 
-// hash_tuple()
-//
 // Helper function for hashing a tuple. The third argument should
 // be an index_sequence running from 0 to tuple_size<Tuple> - 1.
 template <typename H, typename Tuple, size_t... Is>
-H hash_tuple(H hash_state, const Tuple& t, absl::index_sequence<Is...>) {
+H hash_tuple(H hash_state, const Tuple& t, std::index_sequence<Is...>) {
   return H::combine(std::move(hash_state), std::get<Is>(t)...);
 }
 
@@ -315,12 +606,12 @@ template <typename H, typename... Ts>
 // This SFINAE gets MSVC confused under some conditions. Let's just disable it
 // for now.
 H
-#else  // _MSC_VER
-typename std::enable_if<absl::conjunction<is_hashable<Ts>...>::value, H>::type
+#else   // _MSC_VER
+std::enable_if_t<std::conjunction_v<is_hashable<Ts>...>, H>
 #endif  // _MSC_VER
 AbslHashValue(H hash_state, const std::tuple<Ts...>& t) {
   return hash_internal::hash_tuple(std::move(hash_state), t,
-                                   absl::make_index_sequence<sizeof...(Ts)>());
+                                   std::make_index_sequence<sizeof...(Ts)>());
 }
 
 // -----------------------------------------------------------------------------
@@ -348,28 +639,98 @@ H AbslHashValue(H hash_state, const std::shared_ptr<T>& ptr) {
 // All the string-like types supported here provide the same hash expansion for
 // the same character sequence. These types are:
 //
-//  - `std::string` (and std::basic_string<char, std::char_traits<char>, A> for
-//      any allocator A)
-//  - `absl::string_view` and `std::string_view`
+//  - `absl::Cord`
+//  - `std::string` (and std::basic_string<T, std::char_traits<T>, A> for
+//      any allocator A and any T in {char, wchar_t, char8_t, char16_t,
+//      char32_t})
+//  - `absl::string_view`, `std::string_view`, `std::wstring_view`,
+//    `std::u8string_view`, `std::u16string_view`, and `std::u32_string_view`.
 //
-// For simplicity, we currently support only `char` strings. This support may
-// be broadened, if necessary, but with some caution - this overload would
-// misbehave in cases where the traits' `eq()` member isn't equivalent to `==`
-// on the underlying character type.
+// For simplicity, we currently support only strings built on `char`, `wchar_t`,
+// `char8_t`, `char16_t`, or `char32_t`. This support may be broadened, if
+// necessary, but with some caution - this overload would misbehave in cases
+// where the traits' `eq()` member isn't equivalent to `==` on the underlying
+// character type.
 template <typename H>
 H AbslHashValue(H hash_state, absl::string_view str) {
-  return H::combine(
-      H::combine_contiguous(std::move(hash_state), str.data(), str.size()),
-      str.size());
+  return H::combine_contiguous(std::move(hash_state), str.data(), str.size());
+}
+
+// Support std::wstring, std::u8string, std::u16string and std::u32string.
+template <typename Char, typename Alloc, typename H,
+          typename = std::enable_if_t<std::is_same_v<Char, wchar_t> ||
+#ifdef __cpp_char8_t
+                                      std::is_same_v<Char, char8_t> ||
+#endif
+                                      std::is_same_v<Char, char16_t> ||
+                                      std::is_same_v<Char, char32_t>>>
+H AbslHashValue(
+    H hash_state,
+    const std::basic_string<Char, std::char_traits<Char>, Alloc>& str) {
+  return H::combine_contiguous(std::move(hash_state), str.data(), str.size());
+}
+
+// Support std::wstring_view, std::u8string_view, std::u16string_view and
+// std::u32string_view.
+template <typename Char, typename H,
+          typename = std::enable_if_t<std::is_same_v<Char, wchar_t> ||
+#ifdef __cpp_char8_t
+                                      std::is_same_v<Char, char8_t> ||
+#endif
+                                      std::is_same_v<Char, char16_t> ||
+                                      std::is_same_v<Char, char32_t>>>
+H AbslHashValue(H hash_state, std::basic_string_view<Char> str) {
+  return H::combine_contiguous(std::move(hash_state), str.data(), str.size());
+}
+
+#if defined(__cpp_lib_filesystem) && __cpp_lib_filesystem >= 201703L && \
+    (!defined(__ENVIRONMENT_IPHONE_OS_VERSION_MIN_REQUIRED__) ||        \
+     __ENVIRONMENT_IPHONE_OS_VERSION_MIN_REQUIRED__ >= 130000) &&       \
+    (!defined(__ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__) ||         \
+     __ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__ >= 101500) &&        \
+    (!defined(__XTENSA__))
+
+#define ABSL_INTERNAL_STD_FILESYSTEM_PATH_HASH_AVAILABLE 1
+
+// Support std::filesystem::path. The SFINAE is required because some string
+// types are implicitly convertible to std::filesystem::path.
+template <typename Path, typename H,
+          typename = std::enable_if_t<
+              std::is_same_v<Path, std::filesystem::path>>>
+H AbslHashValue(H hash_state, const Path& path) {
+  // Avoid deferring to std::filesystem::hash_value, as that makes it easy to
+  // generate offline collisions, bypassing per-table and per-process hash
+  // seeding. Instead, we hash it ourselves.
+  size_t count = 0;
+
+  for (const Path& component : path) {
+    std::basic_string_view<typename Path::value_type> part = component.native();
+
+    // If this is a directory separator, pretend it is the preferred directory
+    // separator (rather than the alternate separator) to ensure that equal
+    // paths produce equal hashes.
+    // Analogous to LLVM commit aa427b1aae445ed46d9f60c5e2eaac61bdf76be3.
+    if (!part.empty() &&
+        (*part.begin() == '/' || *part.begin() == Path::preferred_separator)) {
+      part = std::basic_string_view<typename Path::value_type>(
+          &Path::preferred_separator, 1);
+    }
+
+    hash_state = H::combine(std::move(hash_state), part);
+    ++count;
+  }
+  return H::combine(std::move(hash_state), count);
 }
 
+#endif  // ABSL_INTERNAL_STD_FILESYSTEM_PATH_HASH_AVAILABLE
+
 // -----------------------------------------------------------------------------
 // AbslHashValue for Sequence Containers
 // -----------------------------------------------------------------------------
 
 // AbslHashValue for hashing std::array
 template <typename H, typename T, size_t N>
-typename std::enable_if<is_hashable<T>::value, H>::type AbslHashValue(
+std::enable_if_t<is_hashable<T>::value, H> AbslHashValue(
     H hash_state, const std::array<T, N>& array) {
   return H::combine_contiguous(std::move(hash_state), array.data(),
                                array.size());
@@ -377,49 +738,100 @@ typename std::enable_if<is_hashable<T>::value, H>::type AbslHashValue(
 
 // AbslHashValue for hashing std::deque
 template <typename H, typename T, typename Allocator>
-typename std::enable_if<is_hashable<T>::value, H>::type AbslHashValue(
+std::enable_if_t<is_hashable<T>::value, H> AbslHashValue(
     H hash_state, const std::deque<T, Allocator>& deque) {
   // TODO(gromer): investigate a more efficient implementation taking
   // advantage of the chunk structure.
   for (const auto& t : deque) {
     hash_state = H::combine(std::move(hash_state), t);
   }
-  return H::combine(std::move(hash_state), deque.size());
+  return H::combine(std::move(hash_state), WeaklyMixedInteger{deque.size()});
 }
 
 // AbslHashValue for hashing std::forward_list
 template <typename H, typename T, typename Allocator>
-typename std::enable_if<is_hashable<T>::value, H>::type AbslHashValue(
+std::enable_if_t<is_hashable<T>::value, H> AbslHashValue(
     H hash_state, const std::forward_list<T, Allocator>& list) {
   size_t size = 0;
   for (const T& t : list) {
     hash_state = H::combine(std::move(hash_state), t);
     ++size;
   }
-  return H::combine(std::move(hash_state), size);
+  return H::combine(std::move(hash_state), WeaklyMixedInteger{size});
 }
 
 // AbslHashValue for hashing std::list
 template <typename H, typename T, typename Allocator>
-typename std::enable_if<is_hashable<T>::value, H>::type AbslHashValue(
+std::enable_if_t<is_hashable<T>::value, H> AbslHashValue(
     H hash_state, const std::list<T, Allocator>& list) {
   for (const auto& t : list) {
     hash_state = H::combine(std::move(hash_state), t);
   }
-  return H::combine(std::move(hash_state), list.size());
+  return H::combine(std::move(hash_state), WeaklyMixedInteger{list.size()});
 }
 
 // AbslHashValue for hashing std::vector
 //
-// Do not use this for vector<bool>. It does not have a .data(), and a fallback
-// for std::hash<> is most likely faster.
+// Do not use this for vector<bool> on platforms that have a working
+// implementation of std::hash. It does not have a .data(), and a fallback for
+// std::hash<> is most likely faster.
 template <typename H, typename T, typename Allocator>
-typename std::enable_if<is_hashable<T>::value && !std::is_same<T, bool>::value,
-                        H>::type
+std::enable_if_t<is_hashable<T>::value && !std::is_same_v<T, bool>, H>
 AbslHashValue(H hash_state, const std::vector<T, Allocator>& vector) {
-  return H::combine(H::combine_contiguous(std::move(hash_state), vector.data(),
-                                          vector.size()),
-                    vector.size());
+  return H::combine_contiguous(std::move(hash_state), vector.data(),
+                               vector.size());
+}
+
+// AbslHashValue special cases for hashing std::vector<bool>
+//
+// To achieve high performance without depending on private standard library
+// internals, we pack bits 64 at a time into uint64_t words using a fixed
+// 64-step inner loop that allows compilers to unroll bit shifts cleanly.
+//
+// This is slower than std::hash<std::vector<bool>> which can access private
+// storage directly, but more than fast enough for the very rare case of hashing
+// std::vector<bool>. In the event that higher performance is needed, a custom
+// key type is likely faster than building std::vector<bool> and hashing it,
+// otherwise users can just use std::hash as the hasher.
+template <typename H, typename T, typename Allocator>
+std::enable_if_t<is_hashable<T>::value && std::is_same_v<T, bool>, H>
+AbslHashValue(H hash_state, const std::vector<T, Allocator>& vector) {
+  typename H::AbslInternalPiecewiseCombiner combiner;
+  const size_t size = vector.size();
+  size_t i = 0;
+  // Pack full 64-bit words. Fixed inner loop count enables compiler unrolling.
+  while (i + 64 <= size) {
+    uint64_t word = 0;
+    for (size_t j = 0; j < 64; ++j) {
+      word |= static_cast<uint64_t>(vector[i + j]) << j;
+    }
+    if constexpr (absl::endian::native == absl::endian::big) {
+      word = absl::byteswap(word);
+    }
+    hash_state = combiner.add_buffer(
+        std::move(hash_state), reinterpret_cast<const unsigned char*>(&word),
+        sizeof(word));
+    i += 64;
+  }
+  // Pack remaining bits (< 64) into the final word.
+  if (i < size) {
+    uint64_t word = 0;
+    const size_t rem = size - i;
+    for (size_t j = 0; j < rem; ++j) {
+      word |= static_cast<uint64_t>(vector[i + j]) << j;
+    }
+    if constexpr (absl::endian::native == absl::endian::big) {
+      word = absl::byteswap(word);
+    }
+    hash_state = combiner.add_buffer(
+        std::move(hash_state), reinterpret_cast<const unsigned char*>(&word),
+        (rem + 7) / 8);
+  }
+  // Mix in vector.size() to distinguish vectors with trailing false/zero bits
+  // (e.g. {true} vs {true, false}) that would otherwise produce identical bit
+  // buffers.
+  return H::combine(combiner.finalize(std::move(hash_state)),
+                    WeaklyMixedInteger{size});
 }
 
 // -----------------------------------------------------------------------------
@@ -429,61 +841,112 @@ AbslHashValue(H hash_state, const std::vector<T, Allocator>& vector) {
 // AbslHashValue for hashing std::map
 template <typename H, typename Key, typename T, typename Compare,
           typename Allocator>
-typename std::enable_if<is_hashable<Key>::value && is_hashable<T>::value,
-                        H>::type
+std::enable_if_t<is_hashable<Key>::value && is_hashable<T>::value, H>
 AbslHashValue(H hash_state, const std::map<Key, T, Compare, Allocator>& map) {
   for (const auto& t : map) {
     hash_state = H::combine(std::move(hash_state), t);
   }
-  return H::combine(std::move(hash_state), map.size());
+  return H::combine(std::move(hash_state), WeaklyMixedInteger{map.size()});
 }
 
 // AbslHashValue for hashing std::multimap
 template <typename H, typename Key, typename T, typename Compare,
           typename Allocator>
-typename std::enable_if<is_hashable<Key>::value && is_hashable<T>::value,
-                        H>::type
+std::enable_if_t<is_hashable<Key>::value && is_hashable<T>::value, H>
 AbslHashValue(H hash_state,
               const std::multimap<Key, T, Compare, Allocator>& map) {
   for (const auto& t : map) {
     hash_state = H::combine(std::move(hash_state), t);
   }
-  return H::combine(std::move(hash_state), map.size());
+  return H::combine(std::move(hash_state), WeaklyMixedInteger{map.size()});
 }
 
 // AbslHashValue for hashing std::set
 template <typename H, typename Key, typename Compare, typename Allocator>
-typename std::enable_if<is_hashable<Key>::value, H>::type AbslHashValue(
+std::enable_if_t<is_hashable<Key>::value, H> AbslHashValue(
     H hash_state, const std::set<Key, Compare, Allocator>& set) {
   for (const auto& t : set) {
     hash_state = H::combine(std::move(hash_state), t);
   }
-  return H::combine(std::move(hash_state), set.size());
+  return H::combine(std::move(hash_state), WeaklyMixedInteger{set.size()});
 }
 
 // AbslHashValue for hashing std::multiset
 template <typename H, typename Key, typename Compare, typename Allocator>
-typename std::enable_if<is_hashable<Key>::value, H>::type AbslHashValue(
+std::enable_if_t<is_hashable<Key>::value, H> AbslHashValue(
     H hash_state, const std::multiset<Key, Compare, Allocator>& set) {
   for (const auto& t : set) {
     hash_state = H::combine(std::move(hash_state), t);
   }
-  return H::combine(std::move(hash_state), set.size());
+  return H::combine(std::move(hash_state), WeaklyMixedInteger{set.size()});
+}
+
+// -----------------------------------------------------------------------------
+// AbslHashValue for Unordered Associative Containers
+// -----------------------------------------------------------------------------
+
+// AbslHashValue for hashing std::unordered_set
+template <typename H, typename Key, typename Hash, typename KeyEqual,
+          typename Alloc>
+std::enable_if_t<is_hashable<Key>::value, H> AbslHashValue(
+    H hash_state, const std::unordered_set<Key, Hash, KeyEqual, Alloc>& s) {
+  return H::combine(
+      H::combine_unordered(std::move(hash_state), s.begin(), s.end()),
+      WeaklyMixedInteger{s.size()});
+}
+
+// AbslHashValue for hashing std::unordered_multiset
+template <typename H, typename Key, typename Hash, typename KeyEqual,
+          typename Alloc>
+std::enable_if_t<is_hashable<Key>::value, H> AbslHashValue(
+    H hash_state,
+    const std::unordered_multiset<Key, Hash, KeyEqual, Alloc>& s) {
+  return H::combine(
+      H::combine_unordered(std::move(hash_state), s.begin(), s.end()),
+      WeaklyMixedInteger{s.size()});
+}
+
+// AbslHashValue for hashing std::unordered_set
+template <typename H, typename Key, typename T, typename Hash,
+          typename KeyEqual, typename Alloc>
+std::enable_if_t<is_hashable<Key>::value && is_hashable<T>::value, H>
+AbslHashValue(H hash_state,
+              const std::unordered_map<Key, T, Hash, KeyEqual, Alloc>& s) {
+  return H::combine(
+      H::combine_unordered(std::move(hash_state), s.begin(), s.end()),
+      WeaklyMixedInteger{s.size()});
+}
+
+// AbslHashValue for hashing std::unordered_multiset
+template <typename H, typename Key, typename T, typename Hash,
+          typename KeyEqual, typename Alloc>
+std::enable_if_t<is_hashable<Key>::value && is_hashable<T>::value, H>
+AbslHashValue(H hash_state,
+              const std::unordered_multimap<Key, T, Hash, KeyEqual, Alloc>& s) {
+  return H::combine(
+      H::combine_unordered(std::move(hash_state), s.begin(), s.end()),
+      WeaklyMixedInteger{s.size()});
 }
 
 // -----------------------------------------------------------------------------
 // AbslHashValue for Wrapper Types
 // -----------------------------------------------------------------------------
 
-// AbslHashValue for hashing absl::optional
+// AbslHashValue for hashing std::reference_wrapper
 template <typename H, typename T>
-typename std::enable_if<is_hashable<T>::value, H>::type AbslHashValue(
-    H hash_state, const absl::optional<T>& opt) {
+std::enable_if_t<is_hashable<T>::value, H> AbslHashValue(
+    H hash_state, std::reference_wrapper<T> opt) {
+  return H::combine(std::move(hash_state), opt.get());
+}
+
+// AbslHashValue for hashing std::optional
+template <typename H, typename T>
+std::enable_if_t<is_hashable<T>::value, H> AbslHashValue(
+    H hash_state, const std::optional<T>& opt) {
   if (opt) hash_state = H::combine(std::move(hash_state), *opt);
   return H::combine(std::move(hash_state), opt.has_value());
 }
 
-// VariantVisitor
 template <typename H>
 struct VariantVisitor {
   H&& hash_state;
@@ -493,12 +956,12 @@ struct VariantVisitor {
   }
 };
 
-// AbslHashValue for hashing absl::variant
+// AbslHashValue for hashing std::variant
 template <typename H, typename... T>
-typename std::enable_if<conjunction<is_hashable<T>...>::value, H>::type
-AbslHashValue(H hash_state, const absl::variant<T...>& v) {
+std::enable_if_t<std::conjunction_v<is_hashable<T>...>, H> AbslHashValue(
+    H hash_state, const std::variant<T...>& v) {
   if (!v.valueless_by_exception()) {
-    hash_state = absl::visit(VariantVisitor<H>{std::move(hash_state)}, v);
+    hash_state = std::visit(VariantVisitor<H>{std::move(hash_state)}, v);
   }
   return H::combine(std::move(hash_state), v.index());
 }
@@ -507,43 +970,368 @@ AbslHashValue(H hash_state, const absl::variant<T...>& v) {
 // AbslHashValue for Other Types
 // -----------------------------------------------------------------------------
 
-// AbslHashValue for hashing std::bitset is not defined, for the same reason as
-// for vector<bool> (see std::vector above): It does not expose the raw bytes,
-// and a fallback to std::hash<> is most likely faster.
+// AbslHashValue for hashing std::bitset
+//
+// To achieve high performance without depending on private standard library
+// internals, we pack bits 64 at a time into uint64_t words using a fixed
+// 64-step inner loop that allows compilers to unroll bit shifts cleanly.
+//
+// This is slower than std::hash<std::bitset> which can access private storage
+// directly, but more than fast enough for the very rare case of hashing
+// std::bitset. In the event that higher-performance is needed, users can just
+// use std::hash as the hasher.
+template <typename H, size_t N>
+H AbslHashValue(H hash_state, const std::bitset<N>& set) {
+  typename H::AbslInternalPiecewiseCombiner combiner;
+  size_t i = 0;
+  // Pack full 64-bit words. Fixed inner loop count enables compiler unrolling.
+  while (i + 64 <= N) {
+    uint64_t word = 0;
+    for (size_t j = 0; j < 64; ++j) {
+      word |= static_cast<uint64_t>(set[i + j]) << j;
+    }
+    if constexpr (absl::endian::native == absl::endian::big) {
+      word = absl::byteswap(word);
+    }
+    hash_state = combiner.add_buffer(
+        std::move(hash_state), reinterpret_cast<const unsigned char*>(&word),
+        sizeof(word));
+    i += 64;
+  }
+  // Pack remaining bits (< 64) into the final word.
+  if (i < N) {
+    uint64_t word = 0;
+    const size_t rem = N - i;
+    for (size_t j = 0; j < rem; ++j) {
+      word |= static_cast<uint64_t>(set[i + j]) << j;
+    }
+    if constexpr (absl::endian::native == absl::endian::big) {
+      word = absl::byteswap(word);
+    }
+    hash_state = combiner.add_buffer(
+        std::move(hash_state), reinterpret_cast<const unsigned char*>(&word),
+        (rem + 7) / 8);
+  }
+  return H::combine(combiner.finalize(std::move(hash_state)), N);
+}
 
 // -----------------------------------------------------------------------------
 
-// hash_range_or_bytes()
-//
 // Mixes all values in the range [data, data+size) into the hash state.
 // This overload accepts only uniquely-represented types, and hashes them by
 // hashing the entire range of bytes.
 template <typename H, typename T>
-typename std::enable_if<is_uniquely_represented<T>::value, H>::type
-hash_range_or_bytes(H hash_state, const T* data, size_t size) {
+std::enable_if_t<is_uniquely_represented<T>::value, H> hash_range_or_bytes(
+    H hash_state, const T* data, size_t size) {
   const auto* bytes = reinterpret_cast<const unsigned char*>(data);
   return H::combine_contiguous(std::move(hash_state), bytes, sizeof(T) * size);
 }
 
-// hash_range_or_bytes()
 template <typename H, typename T>
-typename std::enable_if<!is_uniquely_represented<T>::value, H>::type
-hash_range_or_bytes(H hash_state, const T* data, size_t size) {
+std::enable_if_t<!is_uniquely_represented<T>::value, H> hash_range_or_bytes(
+    H hash_state, const T* data, size_t size) {
   for (const auto end = data + size; data < end; ++data) {
     hash_state = H::combine(std::move(hash_state), *data);
   }
-  return hash_state;
+  return H::combine(std::move(hash_state),
+                    hash_internal::WeaklyMixedInteger{size});
+}
+
+inline constexpr uint64_t kMul = uint64_t{0x79d5f9e0de1e8cf5};
+
+// Random data taken from the hexadecimal digits of Pi's fractional component.
+// https://en.wikipedia.org/wiki/Nothing-up-my-sleeve_number
+ABSL_CACHELINE_ALIGNED inline constexpr uint64_t kStaticRandomData[] = {
+    0x243f'6a88'85a3'08d3, 0x1319'8a2e'0370'7344, 0xa409'3822'299f'31d0,
+    0x082e'fa98'ec4e'6c89, 0x4528'21e6'38d0'1377,
+};
+
+// Extremely weak mixture of length that is mixed into the state before
+// combining the data. It is used only for small strings. This also ensures that
+// we have high entropy in all bits of the state.
+inline uint64_t PrecombineLengthMix(uint64_t state, size_t len) {
+  ABSL_ASSUME(len + sizeof(uint64_t) <= sizeof(kStaticRandomData));
+  uint64_t data = absl::base_internal::UnalignedLoad64(
+      reinterpret_cast<const unsigned char*>(&kStaticRandomData[0]) + len);
+  return state ^ data;
+}
+
+ABSL_ATTRIBUTE_ALWAYS_INLINE inline uint64_t Mix(uint64_t lhs, uint64_t rhs) {
+  // Though the 128-bit product needs multiple instructions on non-x86-64
+  // platforms, it is still a good balance between speed and hash quality.
+  absl::uint128 m = lhs;
+  m *= rhs;
+  return Uint128High64(m) ^ Uint128Low64(m);
+}
+
+// Suppress erroneous array bounds errors on GCC.
+#if defined(__GNUC__) && !defined(__clang__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Warray-bounds"
+#endif
+inline uint32_t Read4(const unsigned char* p) {
+  return absl::base_internal::UnalignedLoad32(p);
+}
+inline uint64_t Read8(const unsigned char* p) {
+  return absl::base_internal::UnalignedLoad64(p);
+}
+#if defined(__GNUC__) && !defined(__clang__)
+#pragma GCC diagnostic pop
+#endif
+
+// Reads 9 to 16 bytes from p.
+// The first 8 bytes are in .first, and the rest of the bytes are in .second
+// along with duplicated bytes from .first if len<16.
+inline std::pair<uint64_t, uint64_t> Read9To16(const unsigned char* p,
+                                               size_t len) {
+  return {Read8(p), Read8(p + len - 8)};
+}
+
+// Reads 4 to 8 bytes from p.
+// Bytes are permuted and some input bytes may be duplicated in output.
+inline uint64_t Read4To8(const unsigned char* p, size_t len) {
+  // If `len < 8`, we duplicate bytes. We always put low memory at the end.
+  // E.g., on little endian platforms:
+  // `ABCD` will be read as `ABCDABCD`.
+  // `ABCDE` will be read as `BCDEABCD`.
+  // `ABCDEF` will be read as `CDEFABCD`.
+  // `ABCDEFG` will be read as `DEFGABCD`.
+  // `ABCDEFGH` will be read as `EFGHABCD`.
+  // We also do not care about endianness. On big-endian platforms, bytes will
+  // be permuted differently. We always shift low memory by 32, because that
+  // can be pipelined earlier. Reading high memory requires computing
+  // `p + len - 4`.
+  uint64_t most_significant =
+      static_cast<uint64_t>(absl::base_internal::UnalignedLoad32(p)) << 32;
+  uint64_t least_significant =
+      absl::base_internal::UnalignedLoad32(p + len - 4);
+  return most_significant | least_significant;
+}
+
+// Reads 1 to 3 bytes from p. Some input bytes may be duplicated in output.
+inline uint32_t Read1To3(const unsigned char* p, size_t len) {
+  // The trick used by this implementation is to avoid branches.
+  // We always read three bytes by duplicating.
+  // E.g.,
+  // `A` is read as `AAA`.
+  // `AB` is read as `ABB`.
+  // `ABC` is read as `ABC`.
+  // We always shift `p[0]` so that it can be pipelined better.
+  // Other bytes require extra computation to find indices.
+  uint32_t mem0 = (static_cast<uint32_t>(p[0]) << 16) | p[len - 1];
+  uint32_t mem1 = static_cast<uint32_t>(p[len / 2]) << 8;
+  return mem0 | mem1;
+}
+
+#ifdef ABSL_HASH_INTERNAL_HAS_CRC32
+
+ABSL_ATTRIBUTE_ALWAYS_INLINE inline uint64_t CombineRawImpl(uint64_t state,
+                                                            uint64_t value) {
+  // We use a union to access the high and low 32 bits of the state.
+  union {
+    uint64_t u64;
+    struct {
+#ifdef ABSL_IS_LITTLE_ENDIAN
+      uint32_t low, high;
+#else  // big endian
+      uint32_t high, low;
+#endif
+    } u32s;
+  } s;
+  s.u64 = state;
+  // The general idea here is to do two CRC32 operations in parallel using the
+  // low and high 32 bits of state as CRC states. Note that: (1) when absl::Hash
+  // is inlined into swisstable lookups, we know that the seed's high bits are
+  // zero so s.u32s.high is available immediately. (2) We chose to multiply
+  // value by 3 for the low CRC because (a) multiplication by 3 can be done in 1
+  // cycle on x86/ARM and (b) multiplication has carry bits so it's nonlinear in
+  // GF(2) and therefore ensures that the two CRCs are independent (unlike bit
+  // rotation, XOR, etc). (3) We also tried using addition instead of
+  // multiplication by 3, but (a) code size is larger and (b) if the input keys
+  // all have 0s in the bits where the addition constant has 1s, then the
+  // addition is equivalent to XOR and linear in GF(2). (4) The union makes it
+  // easy for the compiler to understand that the high and low CRC states are
+  // independent from each other so that when CombineRawImpl is repeated (e.g.
+  // for std::pair<size_t, size_t>), the CRC chains can run in parallel. We
+  // originally tried using bswaps rather than shifting by 32 bits (to get from
+  // high to low bits) because bswap is one byte smaller in code size, but the
+  // compiler couldn't understand that the CRC chains were independent.
+  s.u32s.high =
+      static_cast<uint32_t>(ABSL_HASH_INTERNAL_CRC32_U64(s.u32s.high, value));
+  s.u32s.low = static_cast<uint32_t>(
+      ABSL_HASH_INTERNAL_CRC32_U64(s.u32s.low, 3 * value));
+  return s.u64;
+}
+#else   // ABSL_HASH_INTERNAL_HAS_CRC32
+ABSL_ATTRIBUTE_ALWAYS_INLINE inline uint64_t CombineRawImpl(uint64_t state,
+                                                            uint64_t value) {
+  return Mix(state ^ value, kMul);
+}
+#endif  // ABSL_HASH_INTERNAL_HAS_CRC32
+
+// Slow dispatch path for calls to CombineContiguousImpl with a size argument
+// larger than inlined size. Has the same effect as calling
+// CombineContiguousImpl() repeatedly with the chunk stride size.
+uint64_t CombineLargeContiguousImplOn32BitLengthGt8(uint64_t state,
+                                                    const unsigned char* first,
+                                                    size_t len);
+uint64_t CombineLargeContiguousImplOn64BitLengthGt32(uint64_t state,
+                                                     const unsigned char* first,
+                                                     size_t len);
+
+ABSL_ATTRIBUTE_ALWAYS_INLINE inline uint64_t CombineSmallContiguousImpl(
+    uint64_t state, const unsigned char* first, size_t len) {
+  ABSL_ASSUME(len <= 8);
+  uint64_t v;
+  if (len >= 4) {
+    v = Read4To8(first, len);
+  } else if (len > 0) {
+    v = Read1To3(first, len);
+  } else {
+    // Empty string must modify the state.
+    v = 0x57;
+  }
+  return CombineRawImpl(state, v);
 }
 
-#if defined(ABSL_INTERNAL_LEGACY_HASH_NAMESPACE) && \
-    ABSL_META_INTERNAL_STD_HASH_SFINAE_FRIENDLY_
+ABSL_ATTRIBUTE_ALWAYS_INLINE inline uint64_t CombineContiguousImpl9to16(
+    uint64_t state, const unsigned char* first, size_t len) {
+  ABSL_ASSUME(len >= 9);
+  ABSL_ASSUME(len <= 16);
+  // Note: any time one half of the mix function becomes zero it will fail to
+  // incorporate any bits from the other half. However, there is exactly 1 in
+  // 2^64 values for each side that achieve this, and only when the size is
+  // exactly 16 -- for smaller sizes there is an overlapping byte that makes
+  // this impossible unless the seed is *also* incredibly unlucky.
+  auto p = Read9To16(first, len);
+  return Mix(state ^ p.first, kMul ^ p.second);
+}
+
+ABSL_ATTRIBUTE_ALWAYS_INLINE inline uint64_t CombineContiguousImpl17to32(
+    uint64_t state, const unsigned char* first, size_t len) {
+  ABSL_ASSUME(len >= 17);
+  ABSL_ASSUME(len <= 32);
+  // Do two mixes of overlapping 16-byte ranges in parallel to minimize
+  // latency.
+  const uint64_t m0 =
+      Mix(Read8(first) ^ kStaticRandomData[1], Read8(first + 8) ^ state);
+
+  const unsigned char* tail_16b_ptr = first + (len - 16);
+  const uint64_t m1 = Mix(Read8(tail_16b_ptr) ^ kStaticRandomData[3],
+                          Read8(tail_16b_ptr + 8) ^ state);
+  return m0 ^ m1;
+}
+
+// Implementation of the base case for combine_contiguous where we actually
+// mix the bytes into the state.
+// Dispatch to different implementations of combine_contiguous depending
+// on the value of `sizeof(size_t)`.
+inline uint64_t CombineContiguousImpl(
+    uint64_t state, const unsigned char* first, size_t len,
+    std::integral_constant<int, 4> /* sizeof_size_t */) {
+  // For large values we use CityHash, for small ones we use custom low latency
+  // hash.
+  if (len <= 8) {
+    return CombineSmallContiguousImpl(PrecombineLengthMix(state, len), first,
+                                      len);
+  }
+  return CombineLargeContiguousImplOn32BitLengthGt8(state, first, len);
+}
+
+#ifdef ABSL_HASH_INTERNAL_HAS_CRC32
+inline uint64_t CombineContiguousImpl(
+    uint64_t state, const unsigned char* first, size_t len,
+    std::integral_constant<int, 8> /* sizeof_size_t */) {
+  if (ABSL_PREDICT_FALSE(len > 32)) {
+    return CombineLargeContiguousImplOn64BitLengthGt32(state, first, len);
+  }
+  // `mul` is the salt that is used for final mixing. It is important to fill
+  // high 32 bits because CRC wipes out high 32 bits.
+  // `rotr` is important to mix `len` into high 32 bits.
+  uint64_t mul = absl::rotr(kMul, static_cast<int>(len));
+  // Only low 32 bits of each uint64_t are used in CRC32 so we use gbswap_64 to
+  // move high 32 bits to low 32 bits. It has slightly smaller binary size than
+  // `>> 32`. `state + 8 * len` is a single instruction on both x86 and ARM, so
+  // we use it to better mix length. Although only the low 32 bits of the pair
+  // elements are used, we use pair<uint64_t, uint64_t> for better generated
+  // code.
+  std::pair<uint64_t, uint64_t> crcs = {state + 8 * len,
+                                        absl::gbswap_64(state)};
+
+  // All CRC operations here directly read bytes from the memory.
+  // Single fused instructions are used, like `crc32 rcx, qword ptr [rsi]`.
+  // On x86, llvm-mca reports latency `R + 2` for such fused instructions, while
+  // `R + 3` for two separate `mov` + `crc` instructions. `R` is the latency of
+  // reading the memory. Fused instructions also reduce register pressure
+  // allowing surrounding code to be more efficient when this code is inlined.
+  if (len > 8) {
+    crcs = {ABSL_HASH_INTERNAL_CRC32_U64(crcs.first, Read8(first)),
+            ABSL_HASH_INTERNAL_CRC32_U64(crcs.second, Read8(first + len - 8))};
+    if (len > 16) {
+      // We compute the second round of dependent CRC32 operations.
+      crcs = {ABSL_HASH_INTERNAL_CRC32_U64(crcs.first, Read8(first + len - 16)),
+              ABSL_HASH_INTERNAL_CRC32_U64(crcs.second, Read8(first + 8))};
+    }
+  } else {
+    if (len >= 4) {
+      // We use CRC for 4 bytes to benefit from the fused instruction and better
+      // hash quality.
+      // Using `xor` or `add` may reduce latency for this case, but would
+      // require more registers, more instructions and will have worse hash
+      // quality.
+      crcs = {ABSL_HASH_INTERNAL_CRC32_U32(static_cast<uint32_t>(crcs.first),
+                                           Read4(first)),
+              ABSL_HASH_INTERNAL_CRC32_U32(static_cast<uint32_t>(crcs.second),
+                                           Read4(first + len - 4))};
+    } else if (len >= 1) {
+      // We mix three bytes all into different output registers.
+      // This way, we do not need shifting of these bytes (so they don't overlap
+      // with each other).
+      crcs = {ABSL_HASH_INTERNAL_CRC32_U8(static_cast<uint32_t>(crcs.first),
+                                          first[0]),
+              ABSL_HASH_INTERNAL_CRC32_U8(static_cast<uint32_t>(crcs.second),
+                                          first[len - 1])};
+      // Middle byte is mixed weaker. It is a new byte only for len == 3.
+      // Mixing is independent from CRC operations so it is scheduled ASAP.
+      mul += first[len / 2];
+    }
+  }
+  // `mul` is mixed into both sides of `Mix` to guarantee non-zero values for
+  // both multiplicands. Using Mix instead of just multiplication here improves
+  // hash quality, especially for short strings.
+  return Mix(mul - crcs.first, crcs.second - mul);
+}
+#else
+inline uint64_t CombineContiguousImpl(
+    uint64_t state, const unsigned char* first, size_t len,
+    std::integral_constant<int, 8> /* sizeof_size_t */) {
+  // For large values we use LowLevelHash or CityHash depending on the platform,
+  // for small ones we use custom low latency hash.
+  if (len <= 8) {
+    return CombineSmallContiguousImpl(PrecombineLengthMix(state, len), first,
+                                      len);
+  }
+  if (len <= 16) {
+    return CombineContiguousImpl9to16(PrecombineLengthMix(state, len), first,
+                                      len);
+  }
+  if (len <= 32) {
+    return CombineContiguousImpl17to32(PrecombineLengthMix(state, len), first,
+                                       len);
+  }
+  // We must not mix length into the state here because calling
+  // CombineContiguousImpl twice with PiecewiseChunkSize() must be equivalent
+  // to calling CombineLargeContiguousImpl once with 2 * PiecewiseChunkSize().
+  return CombineLargeContiguousImplOn64BitLengthGt32(state, first, len);
+}
+#endif  // ABSL_HASH_INTERNAL_HAS_CRC32
+
+#if defined(ABSL_INTERNAL_LEGACY_HASH_NAMESPACE)
 #define ABSL_HASH_INTERNAL_SUPPORT_LEGACY_HASH_ 1
 #else
 #define ABSL_HASH_INTERNAL_SUPPORT_LEGACY_HASH_ 0
 #endif
 
-// HashSelect
-//
 // Type trait to select the appropriate hash implementation to use.
 // HashSelect::type<T> will give the proper hash implementation, to be invoked
 // as:
@@ -553,25 +1341,34 @@ hash_range_or_bytes(H hash_state, const T* data, size_t size) {
 // `false`.
 struct HashSelect {
  private:
+  struct WeaklyMixedIntegerProbe {
+    template <typename H>
+    static H Invoke(H state, WeaklyMixedInteger value) {
+      return hash_internal::hash_weakly_mixed_integer(std::move(state), value);
+    }
+  };
+
   struct State : HashStateBase<State> {
     static State combine_contiguous(State hash_state, const unsigned char*,
                                     size_t);
     using State::HashStateBase::combine_contiguous;
+    static State combine_raw(State state, uint64_t value);
+    static State combine_weakly_mixed_integer(State hash_state,
+                                              WeaklyMixedInteger value);
   };
 
   struct UniquelyRepresentedProbe {
     template <typename H, typename T>
     static auto Invoke(H state, const T& value)
-        -> absl::enable_if_t<is_uniquely_represented<T>::value, H> {
+        -> std::enable_if_t<is_uniquely_represented<T>::value, H> {
       return hash_internal::hash_bytes(std::move(state), value);
     }
   };
 
   struct HashValueProbe {
     template <typename H, typename T>
-    static auto Invoke(H state, const T& value) -> absl::enable_if_t<
-        std::is_same<H,
-                     decltype(AbslHashValue(std::move(state), value))>::value,
+    static auto Invoke(H state, const T& value) -> std::enable_if_t<
+        std::is_same_v<H, decltype(AbslHashValue(std::move(state), value))>,
         H> {
       return AbslHashValue(std::move(state), value);
     }
@@ -580,10 +1377,10 @@ struct HashSelect {
   struct LegacyHashProbe {
 #if ABSL_HASH_INTERNAL_SUPPORT_LEGACY_HASH_
     template <typename H, typename T>
-    static auto Invoke(H state, const T& value) -> absl::enable_if_t<
-        std::is_convertible<
+    static auto Invoke(H state, const T& value) -> std::enable_if_t<
+        std::is_convertible_v<
             decltype(ABSL_INTERNAL_LEGACY_HASH_NAMESPACE::hash<T>()(value)),
-            size_t>::value,
+            size_t>,
         H> {
       return hash_internal::hash_bytes(
           std::move(state),
@@ -595,7 +1392,7 @@ struct HashSelect {
   struct StdHashProbe {
     template <typename H, typename T>
     static auto Invoke(H state, const T& value)
-        -> absl::enable_if_t<type_traits_internal::IsHashable<T>::value, H> {
+        -> std::enable_if_t<type_traits_internal::IsHashable<T>::value, H> {
       return hash_internal::hash_bytes(std::move(state), std::hash<T>{}(value));
     }
   };
@@ -617,7 +1414,8 @@ struct HashSelect {
   // Probe each implementation in order.
   // disjunction provides short circuiting wrt instantiation.
   template <typename T>
-  using Apply = absl::disjunction<         //
+  using Apply = std::disjunction<         //
+      Probe<WeaklyMixedIntegerProbe, T>,   //
       Probe<UniquelyRepresentedProbe, T>,  //
       Probe<HashValueProbe, T>,            //
       Probe<LegacyHashProbe, T>,           //
@@ -626,126 +1424,113 @@ struct HashSelect {
 };
 
 template <typename T>
-struct is_hashable
-    : std::integral_constant<bool, HashSelect::template Apply<T>::value> {};
-
-// CityHashState
-class CityHashState : public HashStateBase<CityHashState> {
-  // absl::uint128 is not an alias or a thin wrapper around the intrinsic.
-  // We use the intrinsic when available to improve performance.
-#ifdef ABSL_HAVE_INTRINSIC_INT128
-  using uint128 = __uint128_t;
-#else   // ABSL_HAVE_INTRINSIC_INT128
-  using uint128 = absl::uint128;
-#endif  // ABSL_HAVE_INTRINSIC_INT128
-
-  static constexpr uint64_t kMul =
-      sizeof(size_t) == 4 ? uint64_t{0xcc9e2d51}
-                          : uint64_t{0x9ddfea08eb382d69};
+struct is_hashable : std::bool_constant<HashSelect::template Apply<T>::value> {
+};
 
+class ABSL_DLL MixingHashState : public HashStateBase<MixingHashState> {
   template <typename T>
   using IntegralFastPath =
-      conjunction<std::is_integral<T>, is_uniquely_represented<T>>;
+      std::conjunction<std::is_integral<T>, is_uniquely_represented<T>,
+                       FitsIn64Bits<T>>;
 
  public:
   // Move only
-  CityHashState(CityHashState&&) = default;
-  CityHashState& operator=(CityHashState&&) = default;
+  MixingHashState(MixingHashState&&) = default;
+  MixingHashState& operator=(MixingHashState&&) = default;
 
-  // CityHashState::combine_contiguous()
-  //
   // Fundamental base case for hash recursion: mixes the given range of bytes
   // into the hash state.
-  static CityHashState combine_contiguous(CityHashState hash_state,
-                                          const unsigned char* first,
-                                          size_t size) {
-    return CityHashState(
+  static MixingHashState combine_contiguous(MixingHashState hash_state,
+                                            const unsigned char* first,
+                                            size_t size) {
+    return MixingHashState(
         CombineContiguousImpl(hash_state.state_, first, size,
                               std::integral_constant<int, sizeof(size_t)>{}));
   }
-  using CityHashState::HashStateBase::combine_contiguous;
+  using MixingHashState::HashStateBase::combine_contiguous;
+
+  template <typename T>
+  static size_t hash(const T& value) {
+    return hash_with_seed(value, Seed());
+  }
 
-  // CityHashState::hash()
-  //
   // For performance reasons in non-opt mode, we specialize this for
   // integral types.
   // Otherwise we would be instantiating and calling dozens of functions for
   // something that is just one multiplication and a couple xor's.
   // The result should be the same as running the whole algorithm, but faster.
-  template <typename T, absl::enable_if_t<IntegralFastPath<T>::value, int> = 0>
-  static size_t hash(T value) {
-    return static_cast<size_t>(Mix(Seed(), static_cast<uint64_t>(value)));
+  template <typename T, std::enable_if_t<IntegralFastPath<T>::value, int> = 0>
+  static size_t hash_with_seed(T value, size_t seed) {
+    return static_cast<size_t>(
+        CombineRawImpl(seed, static_cast<std::make_unsigned_t<T>>(value)));
   }
 
-  // Overload of CityHashState::hash()
-  template <typename T, absl::enable_if_t<!IntegralFastPath<T>::value, int> = 0>
-  static size_t hash(const T& value) {
-    return static_cast<size_t>(combine(CityHashState{}, value).state_);
+  template <typename T, std::enable_if_t<!IntegralFastPath<T>::value, int> = 0>
+  static size_t hash_with_seed(const T& value, size_t seed) {
+    return static_cast<size_t>(combine(MixingHashState{seed}, value).state_);
   }
 
  private:
+  friend class MixingHashState::HashStateBase;
+  template <typename H>
+  friend H absl::hash_internal::hash_weakly_mixed_integer(H,
+                                                          WeaklyMixedInteger);
+  // Allow the HashState type-erasure implementation to invoke
+  // RunCombinedUnordered() directly.
+  friend class absl::HashState;
+  friend struct CombineRaw;
+
+  // For use in Seed().
+  static const void* const kSeed;
+
   // Invoked only once for a given argument; that plus the fact that this is
   // move-only ensures that there is only one non-moved-from object.
-  CityHashState() : state_(Seed()) {}
+  MixingHashState() : state_(Seed()) {}
 
   // Workaround for MSVC bug.
   // We make the type copyable to fix the calling convention, even though we
   // never actually copy it. Keep it private to not affect the public API of the
   // type.
-  CityHashState(const CityHashState&) = default;
-
-  explicit CityHashState(uint64_t state) : state_(state) {}
-
-  // Implementation of the base case for combine_contiguous where we actually
-  // mix the bytes into the state.
-  // Dispatch to different implementations of the combine_contiguous depending
-  // on the value of `sizeof(size_t)`.
-  static uint64_t CombineContiguousImpl(uint64_t state,
-                                        const unsigned char* first, size_t len,
-                                        std::integral_constant<int, 4>
-                                        /* sizeof_size_t */);
-  static uint64_t CombineContiguousImpl(uint64_t state,
-                                        const unsigned char* first, size_t len,
-                                        std::integral_constant<int, 8>
-                                        /* sizeof_size_t*/);
-
-  // Reads 9 to 16 bytes from p.
-  // The first 8 bytes are in .first, the rest (zero padded) bytes are in
-  // .second.
-  static std::pair<uint64_t, uint64_t> Read9To16(const unsigned char* p,
-                                                 size_t len) {
-    uint64_t high = little_endian::Load64(p + len - 8);
-    return {little_endian::Load64(p), high >> (128 - len * 8)};
-  }
-
-  // Reads 4 to 8 bytes from p. Zero pads to fill uint64_t.
-  static uint64_t Read4To8(const unsigned char* p, size_t len) {
-    return (static_cast<uint64_t>(little_endian::Load32(p + len - 4))
-            << (len - 4) * 8) |
-           little_endian::Load32(p);
-  }
-
-  // Reads 1 to 3 bytes from p. Zero pads to fill uint32_t.
-  static uint32_t Read1To3(const unsigned char* p, size_t len) {
-    return static_cast<uint32_t>((p[0]) |                         //
-                                 (p[len / 2] << (len / 2 * 8)) |  //
-                                 (p[len - 1] << ((len - 1) * 8)));
-  }
-
-  ABSL_ATTRIBUTE_ALWAYS_INLINE static uint64_t Mix(uint64_t state, uint64_t v) {
-    using MultType =
-        absl::conditional_t<sizeof(size_t) == 4, uint64_t, uint128>;
-    // We do the addition in 64-bit space to make sure the 128-bit
-    // multiplication is fast. If we were to do it as MultType the compiler has
-    // to assume that the high word is non-zero and needs to perform 2
-    // multiplications instead of one.
-    MultType m = state + v;
-    m *= kMul;
-    return static_cast<uint64_t>(m ^ (m >> (sizeof(m) * 8 / 2)));
-  }
-
-  // Seed()
-  //
+  MixingHashState(const MixingHashState&) = default;
+
+  explicit MixingHashState(uint64_t state) : state_(state) {}
+
+  // Combines a raw value from e.g. integrals/floats/pointers/etc. This allows
+  // us to be consistent with IntegralFastPath when combining raw types, but
+  // optimize Read1To3 and Read4To8 differently for the string case.
+  static MixingHashState combine_raw(MixingHashState hash_state,
+                                     uint64_t value) {
+    return MixingHashState(CombineRawImpl(hash_state.state_, value));
+  }
+
+  static MixingHashState combine_weakly_mixed_integer(
+      MixingHashState hash_state, WeaklyMixedInteger value) {
+    // Some transformation for the value is needed to make an empty
+    // string/container change the mixing hash state.
+    // We use constant smaller than 8 bits to make compiler use
+    // `add` with an immediate operand with 1 byte value.
+    return MixingHashState{hash_state.state_ + (0x57 + value.value)};
+  }
+
+  template <typename CombinerT>
+  static MixingHashState RunCombineUnordered(MixingHashState state,
+                                             CombinerT combiner) {
+    uint64_t unordered_state = 0;
+    combiner(MixingHashState{}, [&](MixingHashState& inner_state) {
+      // Add the hash state of the element to the running total, but mix the
+      // carry bit back into the low bit.  This in intended to avoid losing
+      // entropy to overflow, especially when unordered_multisets contain
+      // multiple copies of the same value.
+      auto element_state = inner_state.state_;
+      unordered_state += element_state;
+      if (unordered_state < element_state) {
+        ++unordered_state;
+      }
+      inner_state = MixingHashState{};
+    });
+    return MixingHashState::combine(std::move(state), unordered_state);
+  }
+
   // A non-deterministic seed.
   //
   // The current purpose of this seed is to generate non-deterministic results
@@ -760,63 +1545,23 @@ class CityHashState : public HashStateBase<CityHashState> {
   //
   // On other platforms this is still going to be non-deterministic but most
   // probably per-build and not per-process.
-  ABSL_ATTRIBUTE_ALWAYS_INLINE static uint64_t Seed() {
-    return static_cast<uint64_t>(reinterpret_cast<uintptr_t>(kSeed));
+  ABSL_ATTRIBUTE_ALWAYS_INLINE static size_t Seed() {
+#if (!defined(__clang__) || __clang_major__ > 11) && \
+    (!defined(__apple_build_version__) ||            \
+     __apple_build_version__ >= 19558921)  // Xcode 12
+    return static_cast<size_t>(reinterpret_cast<uintptr_t>(&kSeed));
+#else
+    // Workaround the absence of
+    // https://github.com/llvm/llvm-project/commit/bc15bf66dcca76cc06fe71fca35b74dc4d521021.
+    return static_cast<size_t>(reinterpret_cast<uintptr_t>(kSeed));
+#endif
   }
-  static const void* const kSeed;
 
   uint64_t state_;
 };
 
-// CityHashState::CombineContiguousImpl()
-inline uint64_t CityHashState::CombineContiguousImpl(
-    uint64_t state, const unsigned char* first, size_t len,
-    std::integral_constant<int, 4> /* sizeof_size_t */) {
-  // For large values we use CityHash, for small ones we just use a
-  // multiplicative hash.
-  uint64_t v;
-  if (len > 8) {
-    v = absl::hash_internal::CityHash32(reinterpret_cast<const char*>(first), len);
-  } else if (len >= 4) {
-    v = Read4To8(first, len);
-  } else if (len > 0) {
-    v = Read1To3(first, len);
-  } else {
-    // Empty ranges have no effect.
-    return state;
-  }
-  return Mix(state, v);
-}
-
-// Overload of CityHashState::CombineContiguousImpl()
-inline uint64_t CityHashState::CombineContiguousImpl(
-    uint64_t state, const unsigned char* first, size_t len,
-    std::integral_constant<int, 8> /* sizeof_size_t */) {
-  // For large values we use CityHash, for small ones we just use a
-  // multiplicative hash.
-  uint64_t v;
-  if (len > 16) {
-    v = absl::hash_internal::CityHash64(reinterpret_cast<const char*>(first), len);
-  } else if (len > 8) {
-    auto p = Read9To16(first, len);
-    state = Mix(state, p.first);
-    v = p.second;
-  } else if (len >= 4) {
-    v = Read4To8(first, len);
-  } else if (len > 0) {
-    v = Read1To3(first, len);
-  } else {
-    // Empty ranges have no effect.
-    return state;
-  }
-  return Mix(state, v);
-}
-
-
 struct AggregateBarrier {};
 
-// HashImpl
-
 // Add a private base class to make sure this type is not an aggregate.
 // Aggregates can be aggregate initialized even if the default constructor is
 // deleted.
@@ -828,12 +1573,21 @@ struct PoisonedHash : private AggregateBarrier {
 
 template <typename T>
 struct HashImpl {
-  size_t operator()(const T& value) const { return CityHashState::hash(value); }
+  size_t operator()(const T& value) const {
+    return MixingHashState::hash(value);
+  }
+
+ private:
+  friend struct HashWithSeed;
+
+  size_t hash_with_seed(const T& value, size_t seed) const {
+    return MixingHashState::hash_with_seed(value, seed);
+  }
 };
 
 template <typename T>
 struct Hash
-    : absl::conditional_t<is_hashable<T>::value, HashImpl<T>, PoisonedHash> {};
+    : std::conditional_t<is_hashable<T>::value, HashImpl<T>, PoisonedHash> {};
 
 template <typename H>
 template <typename T, typename... Ts>
@@ -843,13 +1597,69 @@ H HashStateBase<H>::combine(H state, const T& value, const Ts&... values) {
                     values...);
 }
 
-// HashStateBase::combine_contiguous()
 template <typename H>
 template <typename T>
 H HashStateBase<H>::combine_contiguous(H state, const T* data, size_t size) {
   return hash_internal::hash_range_or_bytes(std::move(state), data, size);
 }
+
+template <typename H>
+template <typename I>
+H HashStateBase<H>::combine_unordered(H state, I begin, I end) {
+  return H::RunCombineUnordered(std::move(state),
+                                CombineUnorderedCallback<I>{begin, end});
+}
+
+template <typename H>
+H PiecewiseCombiner::add_buffer(H state, const unsigned char* data,
+                                size_t size) {
+  if (position_ + size < PiecewiseChunkSize()) {
+    // This partial chunk does not fill our existing buffer
+    memcpy(buf_ + position_, data, size);
+    position_ += size;
+    return state;
+  }
+  added_something_ = true;
+  // If the buffer is partially filled we need to complete the buffer
+  // and hash it.
+  if (position_ != 0) {
+    const size_t bytes_needed = PiecewiseChunkSize() - position_;
+    memcpy(buf_ + position_, data, bytes_needed);
+    state = H::combine_contiguous(std::move(state), buf_, PiecewiseChunkSize());
+    data += bytes_needed;
+    size -= bytes_needed;
+  }
+
+  // Hash whatever chunks we can without copying
+  while (size >= PiecewiseChunkSize()) {
+    state = H::combine_contiguous(std::move(state), data, PiecewiseChunkSize());
+    data += PiecewiseChunkSize();
+    size -= PiecewiseChunkSize();
+  }
+  // Fill the buffer with the remainder
+  memcpy(buf_, data, size);
+  position_ = size;
+  return state;
+}
+
+template <typename H>
+H PiecewiseCombiner::finalize(H state) {
+  // Do not call combine_contiguous with empty remainder since it is modifying
+  // state.
+  if (added_something_ && position_ == 0) {
+    return state;
+  }
+  // We still call combine_contiguous for the entirely empty buffer.
+  return H::combine_contiguous(std::move(state), buf_, position_);
+}
+
 }  // namespace hash_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
+#undef ABSL_HASH_INTERNAL_HAS_CRC32
+#undef ABSL_HASH_INTERNAL_CRC32_U64
+#undef ABSL_HASH_INTERNAL_CRC32_U32
+#undef ABSL_HASH_INTERNAL_CRC32_U8
+
 #endif  // ABSL_HASH_INTERNAL_HASH_H_
diff --git a/absl/hash/internal/hash_test.h b/absl/hash/internal/hash_test.h
new file mode 100644
index 00000000000..962a9967e6d
--- /dev/null
+++ b/absl/hash/internal/hash_test.h
@@ -0,0 +1,88 @@
+// Copyright 2023 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Common code shared between absl/hash/hash_test.cc and
+// absl/hash/hash_instantiated_test.cc.
+
+#ifndef ABSL_HASH_INTERNAL_HASH_TEST_H_
+#define ABSL_HASH_INTERNAL_HASH_TEST_H_
+
+#include <initializer_list>
+#include <type_traits>
+#include <utility>
+
+#include "absl/base/config.h"
+#include "absl/hash/hash.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace hash_test_internal {
+
+// Utility wrapper of T for the purposes of testing the `AbslHash` type erasure
+// mechanism.  `TypeErasedValue<T>` can be constructed with a `T`, and can
+// be compared and hashed.  However, all hashing goes through the hashing
+// type-erasure framework.
+template <typename T>
+class TypeErasedValue {
+ public:
+  TypeErasedValue() = default;
+  TypeErasedValue(const TypeErasedValue&) = default;
+  TypeErasedValue(TypeErasedValue&&) = default;
+  explicit TypeErasedValue(const T& n) : n_(n) {}
+
+  template <typename H>
+  friend H AbslHashValue(H hash_state, const TypeErasedValue& v) {
+    v.HashValue(absl::HashState::Create(&hash_state));
+    return hash_state;
+  }
+
+  void HashValue(absl::HashState state) const {
+    absl::HashState::combine(std::move(state), n_);
+  }
+
+  bool operator==(const TypeErasedValue& rhs) const { return n_ == rhs.n_; }
+  bool operator!=(const TypeErasedValue& rhs) const { return !(*this == rhs); }
+
+ private:
+  T n_;
+};
+
+// A TypeErasedValue refinement, for containers.  It exposes the wrapped
+// `value_type` and is constructible from an initializer list.
+template <typename T>
+class TypeErasedContainer : public TypeErasedValue<T> {
+ public:
+  using value_type = typename T::value_type;
+  TypeErasedContainer() = default;
+  TypeErasedContainer(const TypeErasedContainer&) = default;
+  TypeErasedContainer(TypeErasedContainer&&) = default;
+  explicit TypeErasedContainer(const T& n) : TypeErasedValue<T>(n) {}
+  TypeErasedContainer(std::initializer_list<value_type> init_list)
+      : TypeErasedContainer(T(init_list.begin(), init_list.end())) {}
+  // one-argument constructor of value type T, to appease older toolchains that
+  // get confused by one-element initializer lists in some contexts
+  explicit TypeErasedContainer(const value_type& v)
+      : TypeErasedContainer(T(&v, &v + 1)) {}
+};
+
+// Helper trait to verify if T is hashable. We use absl::Hash's poison status to
+// detect it.
+template <typename T>
+using is_hashable = std::is_default_constructible<absl::Hash<T>>;
+
+}  // namespace hash_test_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_HASH_INTERNAL_HASH_TEST_H_
diff --git a/absl/hash/internal/low_level_hash_test.cc b/absl/hash/internal/low_level_hash_test.cc
new file mode 100644
index 00000000000..f1e2d15630e
--- /dev/null
+++ b/absl/hash/internal/low_level_hash_test.cc
@@ -0,0 +1,509 @@
+// Copyright 2020 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <cstddef>
+#include <cstdint>
+#include <string>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/hash/hash.h"
+#include "absl/strings/escaping.h"
+#include "absl/strings/string_view.h"
+
+#define UPDATE_GOLDEN 0
+
+namespace {
+
+TEST(LowLevelHashTest, VerifyGolden) {
+  constexpr size_t kNumGoldenOutputs = 95;
+  static struct {
+    absl::string_view base64_data;
+    uint64_t seed;
+  } cases[kNumGoldenOutputs] = {
+      {"VprUGNH+5NnNRaORxgH/ySrZFQFDL+4VAodhfBNinmn8cg==",
+       uint64_t{0x531858a40bfa7ea1}},
+      {"gc1xZaY+q0nPcUvOOnWnT3bqfmT/geth/f7Dm2e/DemMfk4=",
+       uint64_t{0x86689478a7a7e8fa}},
+      {"Mr35fIxqx1ukPAL0su1yFuzzAU3wABCLZ8+ZUFsXn47UmAph",
+       uint64_t{0x4ec948b8e7f27288}},
+      {"A9G8pw2+m7+rDtWYAdbl8tb2fT7FFo4hLi2vAsa5Y8mKH3CX3g==",
+       uint64_t{0xce46c7213c10032}},
+      {"DFaJGishGwEHDdj9ixbCoaTjz9KS0phLNWHVVdFsM93CvPft3hM=",
+       uint64_t{0xf63e96ee6f32a8b6}},
+      {"7+Ugx+Kr3aRNgYgcUxru62YkTDt5Hqis+2po81hGBkcrJg4N0uuy",
+       uint64_t{0x1cfe85e65fc5225}},
+      {"H2w6O8BUKqu6Tvj2xxaecxEI2wRgIgqnTTG1WwOgDSINR13Nm4d4Vg==",
+       uint64_t{0x45c474f1cee1d2e8}},
+      {"1XBMnIbqD5jy65xTDaf6WtiwtdtQwv1dCVoqpeKj+7cTR1SaMWMyI04=",
+       uint64_t{0x6e024e14015f329c}},
+      {"znZbdXG2TSFrKHEuJc83gPncYpzXGbAebUpP0XxzH0rpe8BaMQ17nDbt",
+       uint64_t{0x760c40502103ae1c}},
+      {"ylu8Atu13j1StlcC1MRMJJXIl7USgDDS22HgVv0WQ8hx/8pNtaiKB17hCQ==",
+       uint64_t{0x17fd05c3c560c320}},
+      {"M6ZVVzsd7vAvbiACSYHioH/440dp4xG2mLlBnxgiqEvI/aIEGpD0Sf4VS0g=",
+       uint64_t{0x8b34200a6f8e90d9}},
+      {"li3oFSXLXI+ubUVGJ4blP6mNinGKLHWkvGruun85AhVn6iuMtocbZPVhqxzn",
+       uint64_t{0x6be89e50818bdf69}},
+      {"kFuQHuUCqBF3Tc3hO4dgdIp223ShaCoog48d5Do5zMqUXOh5XpGK1t5XtxnfGA==",
+       uint64_t{0xfb389773315b47d8}},
+      {"jWmOad0v0QhXVJd1OdGuBZtDYYS8wBVHlvOeTQx9ZZnm8wLEItPMeihj72E0nWY=",
+       uint64_t{0x4f2512a23f61efee}},
+      {"z+DHU52HaOQdW4JrZwDQAebEA6rm13Zg/9lPYA3txt3NjTBqFZlOMvTRnVzRbl23",
+       uint64_t{0x59ccd92fc16c6fda}},
+      {"MmBiGDfYeTayyJa/tVycg+rN7f9mPDFaDc+23j0TlW9094er0ADigsl4QX7V3gG/qw==",
+       uint64_t{0x25c5a7f5bd330919}},
+      {"774RK+9rOL4iFvs1q2qpo/JVc/I39buvNjqEFDtDvyoB0FXxPI2vXqOrk08VPfIHkmU=",
+       uint64_t{0x51df4174d34c97d7}},
+      {"+slatXiQ7/2lK0BkVUI1qzNxOOLP3I1iK6OfHaoxgqT63FpzbElwEXSwdsryq3UlHK0I",
+       uint64_t{0x80ce6d76f89cb57}},
+      {"64mVTbQ47dHjHlOHGS/hjJwr/"
+       "K2frCNpn87exOqMzNUVYiPKmhCbfS7vBUce5tO6Ec9osQ==",
+       uint64_t{0x20961c911965f684}},
+      {"fIsaG1r530SFrBqaDj1kqE0AJnvvK8MNEZbII2Yw1OK77v0V59xabIh0B5axaz/"
+       "+a2V5WpA=",
+       uint64_t{0x4e5b926ec83868e7}},
+      {"PGih0zDEOWCYGxuHGDFu9Ivbff/"
+       "iE7BNUq65tycTR2R76TerrXALRosnzaNYO5fjFhTi+CiS",
+       uint64_t{0x3927b30b922eecef}},
+      {"RnpA/"
+       "zJnEnnLjmICORByRVb9bCOgxF44p3VMiW10G7PvW7IhwsWajlP9kIwNA9FjAD2GoQHk2Q="
+       "=",
+       uint64_t{0xbd0291284a49b61c}},
+      {"qFklMceaTHqJpy2qavJE+EVBiNFOi6OxjOA3LeIcBop1K7w8xQi3TrDk+"
+       "BrWPRIbfprszSaPfrI=",
+       uint64_t{0x73a77c575bcc956}},
+      {"cLbfUtLl3EcQmITWoTskUR8da/VafRDYF/ylPYwk7/"
+       "zazk6ssyrzxMN3mmSyvrXR2yDGNZ3WDrTT",
+       uint64_t{0x766a0e2ade6d09a6}},
+      {"s/"
+       "Jf1+"
+       "FbsbCpXWPTUSeWyMH6e4CvTFvPE5Fs6Z8hvFITGyr0dtukHzkI84oviVLxhM1xMxrMAy1db"
+       "w==",
+       uint64_t{0x2599f4f905115869}},
+      {"FvyQ00+j7nmYZVQ8hI1Edxd0AWplhTfWuFGiu34AK5X8u2hLX1bE97sZM0CmeLe+"
+       "7LgoUT1fJ/axybE=",
+       uint64_t{0xd8256e5444d21e53}},
+      {"L8ncxMaYLBH3g9buPu8hfpWZNlOF7nvWLNv9IozH07uQsIBWSKxoPy8+"
+       "LW4tTuzC6CIWbRGRRD1sQV/4",
+       uint64_t{0xf664a91333fb8dfd}},
+      {"CDK0meI07yrgV2kQlZZ+"
+       "wuVqhc2NmzqeLH7bmcA6kchsRWFPeVF5Wqjjaj556ABeUoUr3yBmfU3kWOakkg==",
+       uint64_t{0x9625b859be372cd1}},
+      {"d23/vc5ONh/"
+       "HkMiq+gYk4gaCNYyuFKwUkvn46t+dfVcKfBTYykr4kdvAPNXGYLjM4u1YkAEFpJP+"
+       "nX7eOvs=",
+       uint64_t{0x7b99940782e29898}},
+      {"NUR3SRxBkxTSbtQORJpu/GdR6b/h6sSGfsMj/KFd99ahbh+9r7LSgSGmkGVB/"
+       "mGoT0pnMTQst7Lv2q6QN6Vm",
+       uint64_t{0x4fe12fa5383b51a8}},
+      {"2BOFlcI3Z0RYDtS9T9Ie9yJoXlOdigpPeeT+CRujb/"
+       "O39Ih5LPC9hP6RQk1kYESGyaLZZi3jtabHs7DiVx/VDg==",
+       uint64_t{0xe2ccb09ac0f5b4b6}},
+      {"FF2HQE1FxEvWBpg6Z9zAMH+Zlqx8S1JD/"
+       "wIlViL6ZDZY63alMDrxB0GJQahmAtjlm26RGLnjW7jmgQ4Ie3I+014=",
+       uint64_t{0x7d0a37adbd7b753b}},
+      {"tHmO7mqVL/PX11nZrz50Hc+M17Poj5lpnqHkEN+4bpMx/"
+       "YGbkrGOaYjoQjgmt1X2QyypK7xClFrjeWrCMdlVYtbW",
+       uint64_t{0xd3ae96ef9f7185f2}},
+      {"/WiHi9IQcxRImsudkA/KOTqGe8/"
+       "gXkhKIHkjddv5S9hi02M049dIK3EUyAEjkjpdGLUs+BN0QzPtZqjIYPOgwsYE9g==",
+       uint64_t{0x4fb88ea63f79a0d8}},
+      {"qds+1ExSnU11L4fTSDz/QE90g4Jh6ioqSh3KDOTOAo2pQGL1k/"
+       "9CCC7J23YF27dUTzrWsCQA2m4epXoCc3yPHb3xElA=",
+       uint64_t{0xed564e259bb5ebe9}},
+      {"8FVYHx40lSQPTHheh08Oq0/"
+       "pGm2OlG8BEf8ezvAxHuGGdgCkqpXIueJBF2mQJhTfDy5NncO8ntS7vaKs7sCNdDaNGOEi",
+       uint64_t{0x3e3256b60c428000}},
+      {"4ZoEIrJtstiCkeew3oRzmyJHVt/pAs2pj0HgHFrBPztbQ10NsQ/"
+       "lM6DM439QVxpznnBSiHMgMQJhER+70l72LqFTO1JiIQ==",
+       uint64_t{0xfb05bad59ec8705}},
+      {"hQPtaYI+wJyxXgwD5n8jGIKFKaFA/"
+       "P83KqCKZfPthnjwdOFysqEOYwAaZuaaiv4cDyi9TyS8hk5cEbNP/jrI7q6pYGBLbsM=",
+       uint64_t{0xafdc251dbf97b5f8}},
+      {"S4gpMSKzMD7CWPsSfLeYyhSpfWOntyuVZdX1xSBjiGvsspwOZcxNKCRIOqAA0moUfOh3I5+"
+       "juQV4rsqYElMD/gWfDGpsWZKQ",
+       uint64_t{0x10ec9c92ddb5dcbc}},
+      {"oswxop+"
+       "bthuDLT4j0PcoSKby4LhF47ZKg8K17xxHf74UsGCzTBbOz0MM8hQEGlyqDT1iUiAYnaPaUp"
+       "L2mRK0rcIUYA4qLt5uOw==",
+       uint64_t{0x9a767d5822c7dac4}},
+      {"0II/"
+       "697p+"
+       "BtLSjxj5989OXI004TogEb94VUnDzOVSgMXie72cuYRvTFNIBgtXlKfkiUjeqVpd4a+"
+       "n5bxNOD1TGrjQtzKU5r7obo=",
+       uint64_t{0xee46254080d6e2db}},
+      {"E84YZW2qipAlMPmctrg7TKlwLZ68l4L+c0xRDUfyyFrA4MAti0q9sHq3TDFviH0Y+"
+       "Kq3tEE5srWFA8LM9oomtmvm5PYxoaarWPLc",
+       uint64_t{0xbbb669588d8bf398}},
+      {"x3pa4HIElyZG0Nj7Vdy9IdJIR4izLmypXw5PCmZB5y68QQ4uRaVVi3UthsoJROvbjDJkP2D"
+       "Q6L/eN8pFeLFzNPKBYzcmuMOb5Ull7w==",
+       uint64_t{0xdc2afaa529beef44}},
+      {"jVDKGYIuWOP/"
+       "QKLdd2wi8B2VJA8Wh0c8PwrXJVM8FOGM3voPDVPyDJOU6QsBDPseoR8uuKd19OZ/"
+       "zAvSCB+zlf6upAsBlheUKgCfKww=",
+       uint64_t{0xf1f67391d45013a8}},
+      {"mkquunhmYe1aR2wmUz4vcvLEcKBoe6H+kjUok9VUn2+eTSkWs4oDDtJvNCWtY5efJwg/"
+       "j4PgjRYWtqnrCkhaqJaEvkkOwVfgMIwF3e+d",
+       uint64_t{0x16fce2b8c65a3429}},
+      {"fRelvKYonTQ+s+rnnvQw+JzGfFoPixtna0vzcSjiDqX5s2Kg2//"
+       "UGrK+AVCyMUhO98WoB1DDbrsOYSw2QzrcPe0+3ck9sePvb+Q/IRaHbw==",
+       uint64_t{0xf4b096699f49fe67}},
+      {"DUwXFJzagljo44QeJ7/"
+       "6ZKw4QXV18lhkYT2jglMr8WB3CHUU4vdsytvw6AKv42ZcG6fRkZkq9fpnmXy6xG0aO3WPT1"
+       "eHuyFirAlkW+zKtwg=",
+       uint64_t{0xca584c4bc8198682}},
+      {"cYmZCrOOBBongNTr7e4nYn52uQUy2mfe48s50JXx2AZ6cRAt/"
+       "xRHJ5QbEoEJOeOHsJyM4nbzwFm++SlT6gFZZHJpkXJ92JkR86uS/eV1hJUR",
+       uint64_t{0xed269fc3818b6aad}},
+      {"EXeHBDfhwzAKFhsMcH9+2RHwV+mJaN01+9oacF6vgm8mCXRd6jeN9U2oAb0of5c5cO4i+"
+       "Vb/LlHZSMI490SnHU0bejhSCC2gsC5d2K30ER3iNA==",
+       uint64_t{0x33f253cbb8fe66a8}},
+      {"FzkzRYoNjkxFhZDso94IHRZaJUP61nFYrh5MwDwv9FNoJ5jyNCY/"
+       "eazPZk+tbmzDyJIGw2h3GxaWZ9bSlsol/vK98SbkMKCQ/wbfrXRLcDzdd/8=",
+       uint64_t{0xd0b76b2c1523d99c}},
+      {"Re4aXISCMlYY/XsX7zkIFR04ta03u4zkL9dVbLXMa/q6hlY/CImVIIYRN3VKP4pnd0AUr/"
+       "ugkyt36JcstAInb4h9rpAGQ7GMVOgBniiMBZ/MGU7H",
+       uint64_t{0xfd28f0811a2a237f}},
+      {"ueLyMcqJXX+MhO4UApylCN9WlTQ+"
+       "ltJmItgG7vFUtqs2qNwBMjmAvr5u0sAKd8jpzV0dDPTwchbIeAW5zbtkA2NABJV6hFM48ib"
+       "4/J3A5mseA3cS8w==",
+       uint64_t{0x6261fb136482e84}},
+      {"6Si7Yi11L+jZMkwaN+GUuzXMrlvEqviEkGOilNq0h8TdQyYKuFXzkYc/"
+       "q74gP3pVCyiwz9KpVGMM9vfnq36riMHRknkmhQutxLZs5fbmOgEO69HglCU=",
+       uint64_t{0x458efc750bca7c3a}},
+      {"Q6AbOofGuTJOegPh9Clm/"
+       "9crtUMQqylKrTc1fhfJo1tqvpXxhU4k08kntL1RG7woRnFrVh2UoMrL1kjin+s9CanT+"
+       "y4hHwLqRranl9FjvxfVKm3yvg68",
+       uint64_t{0xa7e69ff84e5e7c27}},
+      {"ieQEbIPvqY2YfIjHnqfJiO1/MIVRk0RoaG/WWi3kFrfIGiNLCczYoklgaecHMm/"
+       "1sZ96AjO+a5stQfZbJQwS7Sc1ODABEdJKcTsxeW2hbh9A6CFzpowP1A==",
+       uint64_t{0x3c59bfd0c29efe9e}},
+      {"zQUv8hFB3zh2GGl3KTvCmnfzE+"
+       "SUgQPVaSVIELFX5H9cE3FuVFGmymkPQZJLAyzC90Cmi8GqYCvPqTuAAB//"
+       "XTJxy4bCcVArgZG9zJXpjowpNBfr3ngWrSE=",
+       uint64_t{0x10befacc6afd298d}},
+      {"US4hcC1+op5JKGC7eIs8CUgInjKWKlvKQkapulxW262E/"
+       "B2ye79QxOexf188u2mFwwe3WTISJHRZzS61IwljqAWAWoBAqkUnW8SHmIDwHUP31J0p5sGd"
+       "P47L",
+       uint64_t{0x41d5320b0a38efa7}},
+      {"9bHUWFna2LNaGF6fQLlkx1Hkt24nrkLE2CmFdWgTQV3FFbUe747SSqYw6ebpTa07MWSpWRP"
+       "sHesVo2B9tqHbe7eQmqYebPDFnNqrhSdZwFm9arLQVs+7a3Ic6A==",
+       uint64_t{0x58db1c7450fe17f3}},
+      {"Kb3DpHRUPhtyqgs3RuXjzA08jGb59hjKTOeFt1qhoINfYyfTt2buKhD6YVffRCPsgK9SeqZ"
+       "qRPJSyaqsa0ovyq1WnWW8jI/NhvAkZTVHUrX2pC+cD3OPYT05Dag=",
+       uint64_t{0x6098c055a335b7a6}},
+      {"gzxyMJIPlU+bJBwhFUCHSofZ/"
+       "319LxqMoqnt3+L6h2U2+ZXJCSsYpE80xmR0Ta77Jq54o92SMH87HV8dGOaCTuAYF+"
+       "lDL42SY1P316Cl0sZTS2ow3ZqwGbcPNs/1",
+       uint64_t{0x1bbacec67845a801}},
+      {"uR7V0TW+FGVMpsifnaBAQ3IGlr1wx5sKd7TChuqRe6OvUXTlD4hKWy8S+"
+       "8yyOw8lQabism19vOQxfmocEOW/"
+       "vzY0pEa87qHrAZy4s9fH2Bltu8vaOIe+agYohhYORQ==",
+       uint64_t{0xc419cfc7442190}},
+      {"1UR5eoo2aCwhacjZHaCh9bkOsITp6QunUxHQ2SfeHv0imHetzt/"
+       "Z70mhyWZBalv6eAx+YfWKCUib2SHDtz/"
+       "A2dc3hqUWX5VfAV7FQsghPUAtu6IiRatq4YSLpDvKZBQ=",
+       uint64_t{0xc95e510d94ba270c}},
+      {"opubR7H63BH7OtY+Avd7QyQ25UZ8kLBdFDsBTwZlY6gA/"
+       "u+x+"
+       "czC9AaZMgmQrUy15DH7YMGsvdXnviTtI4eVI4aF1H9Rl3NXMKZgwFOsdTfdcZeeHVRzBBKX"
+       "8jUfh1il",
+       uint64_t{0xff1ae05c98089c3f}},
+      {"DC0kXcSXtfQ9FbSRwirIn5tgPri0sbzHSa78aDZVDUKCMaBGyFU6BmrulywYX8yzvwprdLs"
+       "oOwTWN2wMjHlPDqrvVHNEjnmufRDblW+nSS+xtKNs3N5xsxXdv6JXDrAB/Q==",
+       uint64_t{0x90c02b8dceced493}},
+      {"BXRBk+3wEP3Lpm1y75wjoz+PgB0AMzLe8tQ1AYU2/"
+       "oqrQB2YMC6W+9QDbcOfkGbeH+b7IBkt/"
+       "gwCMw2HaQsRFEsurXtcQ3YwRuPz5XNaw5NAvrNa67Fm7eRzdE1+hWLKtA8=",
+       uint64_t{0x9f8a76697ab1aa36}},
+      {"RRBSvEGYnzR9E45Aps/+WSnpCo/X7gJLO4DRnUqFrJCV/kzWlusLE/"
+       "6ZU6RoUf2ROwcgEvUiXTGjLs7ts3t9SXnJHxC1KiOzxHdYLMhVvgNd3hVSAXODpKFSkVXND"
+       "55G2L1W",
+       uint64_t{0x6ba1bf3d811a531d}},
+      {"jeh6Qazxmdi57pa9S3XSnnZFIRrnc6s8QLrah5OX3SB/V2ErSPoEAumavzQPkdKF1/"
+       "SfvmdL+qgF1C+Yawy562QaFqwVGq7+tW0yxP8FStb56ZRgNI4IOmI30s1Ei7iops9Uuw==",
+       uint64_t{0x6a418974109c67b4}},
+      {"6QO5nnDrY2/"
+       "wrUXpltlKy2dSBcmK15fOY092CR7KxAjNfaY+"
+       "aAmtWbbzQk3MjBg03x39afSUN1fkrWACdyQKRaGxgwq6MGNxI6W+8DLWJBHzIXrntrE/"
+       "ml6fnNXEpxplWJ1vEs4=",
+       uint64_t{0x8472f1c2b3d230a3}},
+      {"0oPxeEHhqhcFuwonNfLd5jF3RNATGZS6NPoS0WklnzyokbTqcl4BeBkMn07+fDQv83j/"
+       "BpGUwcWO05f3+DYzocfnizpFjLJemFGsls3gxcBYxcbqWYev51tG3lN9EvRE+X9+Pwww",
+       uint64_t{0x5e06068f884e73a7}},
+      {"naSBSjtOKgAOg8XVbR5cHAW3Y+QL4Pb/JO9/"
+       "oy6L08wvVRZqo0BrssMwhzBP401Um7A4ppAupbQeJFdMrysY34AuSSNvtNUy5VxjNECwiNt"
+       "gwYHw7yakDUv8WvonctmnoSPKENegQg==",
+       uint64_t{0x55290b1a8f170f59}},
+      {"vPyl8DxVeRe1OpilKb9KNwpGkQRtA94UpAHetNh+"
+       "95V7nIW38v7PpzhnTWIml5kw3So1Si0TXtIUPIbsu32BNhoH7QwFvLM+"
+       "JACgSpc5e3RjsL6Qwxxi11npwxRmRUqATDeMUfRAjxg=",
+       uint64_t{0x5501cfd83dfe706a}},
+      {"QC9i2GjdTMuNC1xQJ74ngKfrlA4w3o58FhvNCltdIpuMhHP1YsDA78scQPLbZ3OCUgeQguY"
+       "f/vw6zAaVKSgwtaykqg5ka/4vhz4hYqWU5ficdXqClHl+zkWEY26slCNYOM5nnDlly8Cj",
+       uint64_t{0xe43ed13d13a66990}},
+      {"7CNIgQhAHX27nxI0HeB5oUTnTdgKpRDYDKwRcXfSFGP1XeT9nQF6WKCMjL1tBV6x7KuJ91G"
+       "Zz11F4c+8s+MfqEAEpd4FHzamrMNjGcjCyrVtU6y+7HscMVzr7Q/"
+       "ODLcPEFztFnwjvCjmHw==",
+       uint64_t{0xdf43bc375cf5283f}},
+      {"Qa/hC2RPXhANSospe+gUaPfjdK/yhQvfm4cCV6/pdvCYWPv8p1kMtKOX3h5/"
+       "8oZ31fsmx4Axphu5qXJokuhZKkBUJueuMpxRyXpwSWz2wELx5glxF7CM0Fn+"
+       "OevnkhUn5jsPlG2r5jYlVn8=",
+       uint64_t{0x8112b806d288d7b5}},
+      {"kUw/0z4l3a89jTwN5jpG0SHY5km/"
+       "IVhTjgM5xCiPRLncg40aqWrJ5vcF891AOq5hEpSq0bUCJUMFXgct7kvnys905HjerV7Vs1G"
+       "y84tgVJ70/2+pAZTsB/PzNOE/G6sOj4+GbTzkQu819OLB",
+       uint64_t{0xd52a18abb001cb46}},
+      {"VDdfSDbO8Tdj3T5W0XM3EI7iHh5xpIutiM6dvcJ/fhe23V/srFEkDy5iZf/"
+       "VnA9kfi2C79ENnFnbOReeuZW1b3MUXB9lgC6U4pOTuC+"
+       "jHK3Qnpyiqzj7h3ISJSuo2pob7vY6VHZo6Fn7exEqHg==",
+       uint64_t{0xe12b76a2433a1236}},
+      {"Ldfvy3ORdquM/R2fIkhH/ONi69mcP1AEJ6n/"
+       "oropwecAsLJzQSgezSY8bEiEs0VnFTBBsW+RtZY6tDj03fnb3amNUOq1b7jbqyQkL9hpl+"
+       "2Z2J8IaVSeownWl+bQcsR5/xRktIMckC5AtF4YHfU=",
+       uint64_t{0x175bf7319cf1fa00}},
+      {"BrbNpb42+"
+       "VzZAjJw6QLirXzhweCVRfwlczzZ0VX2xluskwBqyfnGovz5EuX79JJ31VNXa5hTkAyQat3l"
+       "YKRADTdAdwE5PqM1N7YaMqqsqoAAAeuYVXuk5eWCykYmClNdSspegwgCuT+403JigBzi",
+       uint64_t{0xd63d57b3f67525ae}},
+      {"gB3NGHJJvVcuPyF0ZSvHwnWSIfmaI7La24VMPQVoIIWF7Z74NltPZZpx2f+cocESM+"
+       "ILzQW9p+BC8x5IWz7N4Str2WLGKMdgmaBfNkEhSHQDU0IJEOnpUt0HmjhFaBlx0/"
+       "LTmhua+rQ6Wup8ezLwfg==",
+       uint64_t{0x933faea858832b73}},
+      {"hTKHlRxx6Pl4gjG+6ksvvj0CWFicUg3WrPdSJypDpq91LUWRni2KF6+"
+       "81ZoHBFhEBrCdogKqeK+hy9bLDnx7g6rAFUjtn1+cWzQ2YjiOpz4+"
+       "ROBB7lnwjyTGWzJD1rXtlso1g2qVH8XJVigC5M9AIxM=",
+       uint64_t{0x53d061e5f8e7c04f}},
+      {"IWQBelSQnhrr0F3BhUpXUIDauhX6f95Qp+A0diFXiUK7irwPG1oqBiqHyK/SH/"
+       "9S+"
+       "rln9DlFROAmeFdH0OCJi2tFm4afxYzJTFR4HnR4cG4x12JqHaZLQx6iiu6CE3rtWBVz99oA"
+       "wCZUOEXIsLU24o2Y",
+       uint64_t{0xdb4124556dd515e0}},
+      {"TKo+l+"
+       "1dOXdLvIrFqeLaHdm0HZnbcdEgOoLVcGRiCbAMR0j5pIFw8D36tefckAS1RCFOH5IgP8yiF"
+       "T0Gd0a2hI3+"
+       "fTKA7iK96NekxWeoeqzJyctc6QsoiyBlkZerRxs5RplrxoeNg29kKDTM0K94mnhD9g==",
+       uint64_t{0x4fb31a0dd681ee71}},
+      {"YU4e7G6EfQYvxCFoCrrT0EFgVLHFfOWRTJQJ5gxM3G2b+"
+       "1kJf9YPrpsxF6Xr6nYtS8reEEbDoZJYqnlk9lXSkVArm88Cqn6d25VCx3+"
+       "49MqC0trIlXtb7SXUUhwpJK16T0hJUfPH7s5cMZXc6YmmbFuBNPE=",
+       uint64_t{0x27cc72eefa138e4c}},
+      {"/I/"
+       "eImMwPo1U6wekNFD1Jxjk9XQVi1D+"
+       "FPdqcHifYXQuP5aScNQfxMAmaPR2XhuOQhADV5tTVbBKwCDCX4E3jcDNHzCiPvViZF1W27t"
+       "xaf2BbFQdwKrNCmrtzcluBFYu0XZfc7RU1RmxK/RtnF1qHsq/O4pp",
+       uint64_t{0x44bc2dfba4bd3ced}},
+      {"CJTT9WGcY2XykTdo8KodRIA29qsqY0iHzWZRjKHb9alwyJ7RZAE3V5Juv4MY3MeYEr1EPCC"
+       "MxO7yFXqT8XA8YTjaMp3bafRt17Pw8JC4iKJ1zN+WWKOESrj+"
+       "3aluGQqn8z1EzqY4PH7rLG575PYeWsP98BugdA==",
+       uint64_t{0x242da1e3a439bed8}},
+      {"ZlhyQwLhXQyIUEnMH/"
+       "AEW27vh9xrbNKJxpWGtrEmKhd+nFqAfbeNBQjW0SfG1YI0xQkQMHXjuTt4P/"
+       "EpZRtA47ibZDVS8TtaxwyBjuIDwqcN09eCtpC+Ls+"
+       "vWDTLmBeDM3u4hmzz4DQAYsLiZYSJcldg9Q3wszw=",
+       uint64_t{0xdc559c746e35c139}},
+      {"v2KU8y0sCrBghmnm8lzGJlwo6D6ObccAxCf10heoDtYLosk4ztTpLlpSFEyu23MLA1tJkcg"
+       "Rko04h19QMG0mOw/"
+       "wc93EXAweriBqXfvdaP85sZABwiKO+6rtS9pacRVpYYhHJeVTQ5NzrvBvi1huxAr+"
+       "xswhVMfL",
+       uint64_t{0xd0b0350275b9989}},
+      {"QhKlnIS6BuVCTQsnoE67E/"
+       "yrgogE8EwO7xLaEGei26m0gEU4OksefJgppDh3X0x0Cs78Dr9IHK5b977CmZlrTRmwhlP8p"
+       "M+UzXPNRNIZuN3ntOum/QhUWP8SGpirheXENWsXMQ/"
+       "nxtxakyEtrNkKk471Oov9juP8oQ==",
+       uint64_t{0xb04489e41d17730c}},
+      {"/ZRMgnoRt+Uo6fUPr9FqQvKX7syhgVqWu+"
+       "WUSsiQ68UlN0efSP6Eced5gJZL6tg9gcYJIkhjuQNITU0Q3TjVAnAcobgbJikCn6qZ6pRxK"
+       "BY4MTiAlfGD3T7R7hwJwx554MAy++Zb/YUFlnCaCJiwQMnowF7aQzwYFCo=",
+       uint64_t{0x2217285eb4572156}},
+      {"NB7tU5fNE8nI+SXGfipc7sRkhnSkUF1krjeo6k+8FITaAtdyz+"
+       "o7mONgXmGLulBPH9bEwyYhKNVY0L+njNQrZ9YC2aXsFD3PdZsxAFaBT3VXEzh+"
+       "NGBTjDASNL3mXyS8Yv1iThGfHoY7T4aR0NYGJ+k+pR6f+KrPC96M",
+       uint64_t{0x12c2e8e68aede73b}},
+      {"8T6wrqCtEO6/rwxF6lvMeyuigVOLwPipX/FULvwyu+1wa5sQGav/"
+       "2FsLHUVn6cGSi0LlFwLewGHPFJDLR0u4t7ZUyM//"
+       "x6da0sWgOa5hzDqjsVGmjxEHXiaXKW3i4iSZNuxoNbMQkIbVML+"
+       "DkYu9ND0O2swg4itGeVSzXA==",
+       uint64_t{0x4d612125bdc4fd00}},
+      {"Ntf1bMRdondtMv1CYr3G80iDJ4WSAlKy5H34XdGruQiCrnRGDBa+"
+       "eUi7vKp4gp3BBcVGl8eYSasVQQjn7MLvb3BjtXx6c/"
+       "bCL7JtpzQKaDnPr9GWRxpBXVxKREgMM7d8lm35EODv0w+"
+       "hQLfVSh8OGs7fsBb68nNWPLeeSOo=",
+       uint64_t{0x81826b553954464e}},
+      {"VsSAw72Ro6xks02kaiLuiTEIWBC5bgqr4WDnmP8vglXzAhixk7td926rm9jNimL+"
+       "kroPSygZ9gl63aF5DCPOACXmsbmhDrAQuUzoh9ZKhWgElLQsrqo1KIjWoZT5b5QfVUXY9lS"
+       "IBg3U75SqORoTPq7HalxxoIT5diWOcJQi",
+       uint64_t{0xc2e5d345dc0ddd2d}},
+      {"j+loZ+C87+"
+       "bJxNVebg94gU0mSLeDulcHs84tQT7BZM2rzDSLiCNxUedHr1ZWJ9ejTiBa0dqy2I2ABc++"
+       "xzOLcv+//YfibtjKtYggC6/3rv0XCc7xu6d/"
+       "O6xO+XOBhOWAQ+IHJVHf7wZnDxIXB8AUHsnjEISKj7823biqXjyP3g==",
+       uint64_t{0x3da6830a9e32631e}},
+      {"f3LlpcPElMkspNtDq5xXyWU62erEaKn7RWKlo540gR6mZsNpK1czV/"
+       "sOmqaq8XAQLEn68LKj6/"
+       "cFkJukxRzCa4OF1a7cCAXYFp9+wZDu0bw4y63qbpjhdCl8GO6Z2lkcXy7KOzbPE01ukg7+"
+       "gN+7uKpoohgAhIwpAKQXmX5xtd0=",
+       uint64_t{0xc9ae5c8759b4877a}},
+  };
+
+#if defined(ABSL_IS_BIG_ENDIAN) || !defined(ABSL_HAVE_INTRINSIC_INT128) || \
+    UINTPTR_MAX != UINT64_MAX
+  constexpr uint64_t kGolden[kNumGoldenOutputs] = {};
+  // This conditional is to avoid an unreachable code warning.
+  bool skip = true;
+  if (skip) {
+    GTEST_SKIP()
+        << "We only maintain golden data for little endian 64 bit systems with "
+           "128 bit intristics.";
+  }
+#elif (defined(__SSE4_2__) && defined(__AES__))
+  constexpr uint64_t kGolden[kNumGoldenOutputs] = {
+      0xd6bdb2c9ba5e55f2, 0xffd3e23d4115a8ae, 0x2c3218ef486127de,
+      0x554fa7f3a262b886, 0x06304cbf82e312d3, 0x490b3fb5af80622c,
+      0x7398a90b8cc59c5d, 0x65fb3168b98030ab, 0xd4564363c53617bb,
+      0x0545c26351925fe7, 0xc30700723b634bf4, 0xfb23a140a76dbe94,
+      0x2fa1467fe218a47c, 0x92e05ec3a7b966eb, 0x6112b56e5624dd50,
+      0x8760801365f9d722, 0x41f7187b61db0e5e, 0x7fe9188a1f5f50ad,
+      0x25800bd4c2002ef1, 0x91fecd33a78ef0aa, 0x93986ad71e983613,
+      0xe4c78173c7ea537b, 0x0bbdc2bcabdb50b1, 0xd9aa134df2d87623,
+      0x6c4907c9477a9409, 0xc3e418a5dbda52e5, 0x4d24f3e9d0dda93a,
+      0xcdb565a363dbe45f, 0xa95f228c8ee57478, 0x6b8f00bab5130227,
+      0x2d05a0f44818b67a, 0xd6bf7d990b5f44cb, 0xa3608bdb4712861a,
+      0xf20c33e5e355330b, 0xbc86e1b13130180d, 0x0848221b397b839a,
+      0x17cc0acf44a7e210, 0xc18c6dc584fe0f62, 0x896c7858a59f991d,
+      0xeab1e6d7d2856ed7, 0x7e4b2d99c23edc51, 0x9aeeeb7fa46e7cf0,
+      0x161b9f2e3611790f, 0x5f82aae18d971b36, 0x8d0dd9965881e162,
+      0x56700ea26285895a, 0xcd919c86c29a053e, 0x3e5d589282d9a722,
+      0x92caee9f48a66604, 0x7e1a2fd9b06f14b0, 0xce1d5293f95b0178,
+      0x8101361290e70a11, 0x570e3e9c9eafc1c6, 0x77b6241926a7a568,
+      0x313e5cb34f346699, 0xab8ebeab0514b82b, 0x6e0a43763a310408,
+      0x761b76ec22b2e440, 0x4238c84a9ec00528, 0xb9ea1f6d4d5552af,
+      0xd21f8f110b9dc060, 0xb3d3842b69ac3689, 0xd0a88aa1dcf59869,
+      0xf3f69f637b123403, 0xf5f34b1068cac7da, 0xe69a08d604774abf,
+      0x57648d3a73332437, 0x9762947f5013d00d, 0x35c5d734a0015922,
+      0xbee2fe5a104ce209, 0xedb060efa6efca34, 0x5ccf0f4786d97bc2,
+      0x1ef8ed72e80d7bef, 0x58522deb49c5e30f, 0xde97cd2a6f8bd13b,
+      0x3fae37c6f9855d09, 0xea99ae786feca261, 0x8c6d1d46670b0943,
+      0x84658b2a232c7bfb, 0x7058b7a7968de394, 0x0d44fba68e25aa8f,
+      0xc7f687020f8eb00b, 0xbf9671e1196153d6, 0x1009be891b7f83e7,
+      0x4f9457fb4aa12865, 0x30a49d9563643b32, 0x0302e2c5b46d5a3a,
+      0x77553f42fb0bfbf7, 0x26b95e89f0077110, 0x76ce68ebe01191ba,
+      0x724110fb509e4376, 0xebe74b016b5cfb88, 0x3b0fe11dcf175fc9,
+      0x20b737b9c0490538, 0x0db21c429b45fd17,
+  };
+#elif (defined(ABSL_INTERNAL_HAVE_ARM_NEON) && defined(__ARM_FEATURE_CRYPTO))
+  constexpr uint64_t kGolden[kNumGoldenOutputs] = {
+      0xe3de56d839559570, 0x77afbb8906ccf19a, 0xe66e02ad8e92c12f,
+      0x36cc6f2fe751bc64, 0xf4d28aea3ed69ade, 0xf246fd6a4ca33ab3,
+      0xcae9edaf829a6575, 0x11fa7c88bf4cc73a, 0x98c47bfbae3d8c02,
+      0x27545ea34451a56f, 0x14b4c1a27d26d2b2, 0x7613ca9857e7cbe0,
+      0x02efc8f12e71bb88, 0x9c1a83672eee7d5b, 0x2b89aa9cfde6d9b3,
+      0xabeb2cf68ab463dc, 0xfb3db51d62d71d28, 0x3177cc3e0927e344,
+      0xb5798fc1d348beb6, 0x212c471510859095, 0x191df651c270548d,
+      0x1b15d6b8e8cc3105, 0xdc51a9e369fbab13, 0xb674fd0372fab5aa,
+      0x048aec21666fc8d5, 0x8b15383bc4a7e244, 0xf8945c253a43469f,
+      0x513f31e6bf240064, 0xc37c6225063c266f, 0x2a6747f8952ec44c,
+      0xa7a0f11d3607f268, 0x9b58d5c889166fb3, 0xb75ca76a67212a39,
+      0x9a71a476bf4933f3, 0x3f049a71b475edf7, 0x7a49c8907a278e5e,
+      0xf9a78757d9355ac9, 0xbe1bd9d70ee46398, 0x600e24a76a359a39,
+      0x235905ade793a5f0, 0x940c36e0df8ec13a, 0x9300c551cb93a286,
+      0x5e265b90e4828aba, 0x42dee1626c647735, 0x8cd32910c1f17a8c,
+      0x642f44f3f8290fb0, 0xb73ee167c5f8655e, 0x9d05deadb2a47fe4,
+      0x03b359dff5351708, 0x19a6427c0ee9c907, 0x0692dc026847e0d9,
+      0xe4023ba608b25f1e, 0xf438ad7eea71dfc1, 0x205120c9355d4ac8,
+      0xd4cf2cb1f412c846, 0x82d7d98b87ecf53a, 0x2cc779469cc3d690,
+      0xf93545883141ce8e, 0x0f794c869b82a28a, 0x014e9fccfe7f7d9e,
+      0xb6953652e49e4e0a, 0x68035eabb056e9b6, 0xf1b3a6847e610274,
+      0x59dc584137c0cb55, 0xafb36238ad797ec6, 0xb9f331777a030104,
+      0x2899a97f98140d3b, 0x0ae342d8d4f80ad0, 0xcfb25544c873fbd1,
+      0xcb2498ca84b01a4c, 0x1a50e4a0d8db8406, 0x0afbf05b7c8c146e,
+      0x1b45ee2ee670d71b, 0xef9204d9fcc31075, 0x2a5750d2ef558495,
+      0x8e151888d0ce024e, 0x26882404dd58bcc9, 0x6c830f947bf2195f,
+      0x2ff0c1fa64bf8cfb, 0x8f108f869e11d224, 0x7ce757787a04fd76,
+      0xee55da944ebbbd85, 0x43563645bb00dcd0, 0x643848bf73336681,
+      0x050f54d0478ffd52, 0xc5c1bff1bc3c008c, 0x48fd0f2729c9402d,
+      0x7685b990e3e264af, 0x940dccac3264ed16, 0x688f94ee88c9ba90,
+      0xca112c7825f3944b, 0x584cb5ddba130be2, 0xb7ced01f7140dff6,
+      0xc10dcbb4e77168e6, 0xb5ea4360351ebaef,
+  };
+#else
+  constexpr uint64_t kGolden[kNumGoldenOutputs] = {
+      0x669da02f8d009e0f, 0xceb19bf2255445cd, 0x0e746992d6d43a7c,
+      0x41ed623b9dcc5fde, 0x187a5a30d7c72edc, 0x949ae2a9c1eb925a,
+      0x7e9c76a7b7c35e68, 0x4f96bf15b8309ff6, 0x26c0c1fde233732e,
+      0xb0453f72aa151615, 0xf24b621a9ce9fece, 0x99ed798408687b5f,
+      0x3b13ec1221423b66, 0xc67cf148a28afe59, 0x22f7e0173f92e3fa,
+      0x14186c5fda6683a0, 0x97d608caa2603b2c, 0xfde3b0bbba24ffa9,
+      0xb7068eb48c472c77, 0x9e34d72866b9fda0, 0xbbb99c884cdef88e,
+      0x81d3e01f472a8a1a, 0xf84f506b3b60366d, 0xfe3f42f01300db37,
+      0xe385712a51c1f836, 0x41dfd5e394245c79, 0x60855dbedadb900a,
+      0xbdb4c0aa38567476, 0x9748802e8eec02cc, 0x5ced256d257f88de,
+      0x55acccdf9a80f155, 0xa64b55b071afbbea, 0xa205bfe6c724ce4d,
+      0x69dd26ca8ac21744, 0xef80e2ff2f6a9bc0, 0xde266c0baa202c20,
+      0xfa3463080ac74c50, 0x379d968a40125c2b, 0x4cbbd0a7b3c7d648,
+      0xc92afd93f4c665d2, 0x6e28f5adb7ae38dc, 0x7c689c9c237be35e,
+      0xaea41b29bd9d0f73, 0x832cef631d77e59f, 0x70cac8e87bc37dd3,
+      0x8e8c98bbde68e764, 0xd6117aeb3ddedded, 0xd796ab808e766240,
+      0x8953d0ea1a7d9814, 0xa212eba4281b391c, 0x21a555a8939ce597,
+      0x809d31660f6d81a8, 0x2356524b20ab400f, 0x5bc611e1e49d0478,
+      0xba9c065e2f385ce2, 0xb0a0fd12f4e83899, 0x14d076a35b1ff2ca,
+      0x8acd0bb8cf9a93c0, 0xe62e8ec094039ee4, 0x38a536a7072bdc61,
+      0xca256297602524f8, 0xfc62ebfb3530caeb, 0x8d8b0c05520569f6,
+      0xbbaca65cf154c59d, 0x3739b5ada7e338d3, 0xdb9ea31f47365340,
+      0x410b5c9c1da56755, 0x7e0abc03dbd10283, 0x136f87be70ed442e,
+      0x6b727d4feddbe1e9, 0x074ebb21183b01df, 0x3fe92185b1985484,
+      0xc5d8efd3c68305ca, 0xd9bada21b17e272e, 0x64d73133e1360f83,
+      0xeb8563aa993e21f9, 0xe5e8da50cceab28f, 0x7a6f92eb3223d2f3,
+      0xbdaf98370ea9b31b, 0x1682a84457f077bc, 0x4abd2d33b6e3be37,
+      0xb35bc81a7c9d4c04, 0x3e5bde3fb7cfe63d, 0xff3abe6e2ffec974,
+      0xb8116dd26cf6feec, 0x7a77a6e4ed0cf081, 0xb71eec2d5a184316,
+      0x6fa932f77b4da817, 0x795f79b33909b2c4, 0x1b8755ef6b5eb34e,
+      0x2255b72d7d6b2d79, 0xf2bdafafa90bd50a, 0x442a578f02cb1fc8,
+      0xc25aefe55ecf83db, 0x3114c056f9c5a676,
+  };
+#endif
+
+  auto hash_fn = [](absl::string_view s, uint64_t state) {
+    return absl::hash_internal::CombineLargeContiguousImplOn64BitLengthGt32(
+        state, reinterpret_cast<const unsigned char*>(s.data()), s.size());
+  };
+
+#if UPDATE_GOLDEN
+  (void)kGolden;  // Silence warning.
+  for (size_t i = 0; i < kNumGoldenOutputs; ++i) {
+    std::string str;
+    ASSERT_TRUE(absl::Base64Unescape(cases[i].base64_data, &str));
+    ASSERT_GT(str.size(), 32);
+    uint64_t h = hash_fn(str, cases[i].seed);
+    printf("0x%016" PRIx64 ", ", h);
+    if (i % 3 == 2) {
+      printf("\n");
+    }
+  }
+  printf("\n\n\n");
+  EXPECT_FALSE(true);
+#else
+  for (size_t i = 0; i < kNumGoldenOutputs; ++i) {
+    SCOPED_TRACE(::testing::Message()
+                 << "i = " << i << "; input = " << cases[i].base64_data);
+    std::string str;
+    ASSERT_TRUE(absl::Base64Unescape(cases[i].base64_data, &str));
+    ASSERT_GT(str.size(), 32);
+    EXPECT_EQ(hash_fn(str, cases[i].seed), kGolden[i]);
+  }
+#endif
+}
+
+}  // namespace
diff --git a/absl/hash/internal/print_hash_of.cc b/absl/hash/internal/print_hash_of.cc
index c392125a69f..883cf9935ee 100644
--- a/absl/hash/internal/print_hash_of.cc
+++ b/absl/hash/internal/print_hash_of.cc
@@ -12,6 +12,9 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+// SKIP_ABSL_INLINE_NAMESPACE_CHECK
+
+#include <cstdio>
 #include <cstdlib>
 
 #include "absl/hash/hash.h"
diff --git a/absl/hash/internal/spy_hash_state.h b/absl/hash/internal/spy_hash_state.h
index c4cc8d07337..543e7197284 100644
--- a/absl/hash/internal/spy_hash_state.h
+++ b/absl/hash/internal/spy_hash_state.h
@@ -15,16 +15,25 @@
 #ifndef ABSL_HASH_INTERNAL_SPY_HASH_STATE_H_
 #define ABSL_HASH_INTERNAL_SPY_HASH_STATE_H_
 
+#include <algorithm>
+#include <cstddef>
+#include <cstdint>
+#include <memory>
+#include <optional>
 #include <ostream>
 #include <string>
+#include <type_traits>
+#include <utility>
 #include <vector>
 
 #include "absl/hash/hash.h"
+#include "absl/hash/internal/weakly_mixed_integer.h"
 #include "absl/strings/match.h"
 #include "absl/strings/str_format.h"
 #include "absl/strings/str_join.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace hash_internal {
 
 // SpyHashState is an implementation of the HashState API that simply
@@ -39,8 +48,8 @@ namespace hash_internal {
 template <typename T>
 class SpyHashStateImpl : public HashStateBase<SpyHashStateImpl<T>> {
  public:
-  SpyHashStateImpl() : error_(std::make_shared<absl::optional<std::string>>()) {
-    static_assert(std::is_void<T>::value, "");
+  SpyHashStateImpl() : error_(std::make_shared<std::optional<std::string>>()) {
+    static_assert(std::is_void_v<T>);
   }
 
   // Move-only
@@ -146,14 +155,54 @@ class SpyHashStateImpl : public HashStateBase<SpyHashStateImpl<T>> {
   static SpyHashStateImpl combine_contiguous(SpyHashStateImpl hash_state,
                                              const unsigned char* begin,
                                              size_t size) {
-    hash_state.hash_representation_.emplace_back(
-        reinterpret_cast<const char*>(begin), size);
+    if (size == 0) {
+      return SpyHashStateImpl::combine_raw(std::move(hash_state), 0);
+    }
+    const size_t large_chunk_stride = PiecewiseChunkSize();
+    // Combining a large contiguous buffer must have the same effect as
+    // doing it piecewise by the stride length, followed by the (possibly
+    // empty) remainder.
+    while (size > large_chunk_stride) {
+      hash_state = SpyHashStateImpl::combine_contiguous(
+          std::move(hash_state), begin, large_chunk_stride);
+      begin += large_chunk_stride;
+      size -= large_chunk_stride;
+    }
+
+    if (size > 0) {
+      hash_state.hash_representation_.emplace_back(
+          reinterpret_cast<const char*>(begin), size);
+      hash_state = SpyHashStateImpl::combine_raw(std::move(hash_state), size);
+    }
     return hash_state;
   }
 
+  static SpyHashStateImpl combine_weakly_mixed_integer(
+      SpyHashStateImpl hash_state, WeaklyMixedInteger value) {
+    return combine(std::move(hash_state), value.value);
+  }
+
   using SpyHashStateImpl::HashStateBase::combine_contiguous;
 
-  absl::optional<std::string> error() const {
+  template <typename CombinerT>
+  static SpyHashStateImpl RunCombineUnordered(SpyHashStateImpl state,
+                                              CombinerT combiner) {
+    UnorderedCombinerCallback cb;
+
+    combiner(SpyHashStateImpl<void>{}, std::ref(cb));
+
+    std::sort(cb.element_hash_representations.begin(),
+              cb.element_hash_representations.end());
+    state.hash_representation_.insert(state.hash_representation_.end(),
+                                      cb.element_hash_representations.begin(),
+                                      cb.element_hash_representations.end());
+    if (cb.error && cb.error->has_value()) {
+      state.error_ = std::move(cb.error);
+    }
+    return state;
+  }
+
+  std::optional<std::string> error() const {
     if (moved_from_) {
       return "Returned a moved-from instance of the hash state object.";
     }
@@ -163,6 +212,30 @@ class SpyHashStateImpl : public HashStateBase<SpyHashStateImpl<T>> {
  private:
   template <typename U>
   friend class SpyHashStateImpl;
+  friend struct CombineRaw;
+
+  struct UnorderedCombinerCallback {
+    std::vector<std::string> element_hash_representations;
+    std::shared_ptr<std::optional<std::string>> error;
+
+    // The inner spy can have a different type.
+    template <typename U>
+    void operator()(SpyHashStateImpl<U>& inner) {
+      element_hash_representations.push_back(
+          absl::StrJoin(inner.hash_representation_, ""));
+      if (inner.error_->has_value()) {
+        error = std::move(inner.error_);
+      }
+      inner = SpyHashStateImpl<void>{};
+    }
+  };
+
+  // Combines raw data from e.g. integrals/floats/pointers/etc.
+  static SpyHashStateImpl combine_raw(SpyHashStateImpl state, uint64_t value) {
+    state.hash_representation_.emplace_back(
+        reinterpret_cast<const char*>(&value), 8);
+    return state;
+  }
 
   // This is true if SpyHashStateImpl<T> has been passed to a call of
   // AbslHashValue with the wrong type. This detects that the user called
@@ -173,7 +246,7 @@ class SpyHashStateImpl : public HashStateBase<SpyHashStateImpl<T>> {
   // This is a shared_ptr because we want all instances of the particular
   // SpyHashState run to share the field. This way we can set the error for
   // use-after-move and all the copies will see it.
-  std::shared_ptr<absl::optional<std::string>> error_;
+  std::shared_ptr<std::optional<std::string>> error_;
   bool moved_from_ = false;
 };
 
@@ -198,7 +271,7 @@ bool RunOnStartup<f>::run = (f(), true);
 template <
     typename T, typename U,
     // Only trigger for when (T != U),
-    typename = absl::enable_if_t<!std::is_same<T, U>::value>,
+    typename = std::enable_if_t<!std::is_same_v<T, U>>,
     // This statement works in two ways:
     //  - First, it instantiates RunOnStartup and forces the initialization of
     //    `run`, which set the global variable.
@@ -211,6 +284,7 @@ void AbslHashValue(SpyHashStateImpl<T>, const U&);
 using SpyHashState = SpyHashStateImpl<void>;
 
 }  // namespace hash_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_HASH_INTERNAL_SPY_HASH_STATE_H_
diff --git a/absl/hash/internal/weakly_mixed_integer.h b/absl/hash/internal/weakly_mixed_integer.h
new file mode 100644
index 00000000000..55754366c03
--- /dev/null
+++ b/absl/hash/internal/weakly_mixed_integer.h
@@ -0,0 +1,38 @@
+// Copyright 2025 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_HASH_INTERNAL_WEAKLY_MIXED_INTEGER_H_
+#define ABSL_HASH_INTERNAL_WEAKLY_MIXED_INTEGER_H_
+
+#include <cstddef>
+
+#include "absl/base/config.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace hash_internal {
+
+// Contains an integer that will be mixed into a hash state more weakly than
+// regular integers. It is useful for cases in which an integer is a part of a
+// larger object and needs to be mixed as a supplement. E.g., absl::string_view
+// and absl::Span are mixing their size wrapped with WeaklyMixedInteger.
+struct WeaklyMixedInteger {
+  size_t value;
+};
+
+}  // namespace hash_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_HASH_INTERNAL_WEAKLY_MIXED_INTEGER_H_
diff --git a/absl/log/BUILD.bazel b/absl/log/BUILD.bazel
new file mode 100644
index 00000000000..188186622ad
--- /dev/null
+++ b/absl/log/BUILD.bazel
@@ -0,0 +1,709 @@
+#
+# Copyright 2022 The Abseil Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+load("@rules_cc//cc:cc_library.bzl", "cc_library")
+load("@rules_cc//cc:cc_test.bzl", "cc_test")
+load(
+    "//absl:copts/configure_copts.bzl",
+    "ABSL_DEFAULT_COPTS",
+    "ABSL_DEFAULT_LINKOPTS",
+    "ABSL_TEST_COPTS",
+)
+
+package(
+    default_visibility = ["//visibility:private"],
+    features = [
+        "header_modules",
+        "layering_check",
+        "parse_headers",
+    ],
+)
+
+licenses(["notice"])
+
+# Public targets
+
+cc_library(
+    name = "absl_check",
+    hdrs = ["absl_check.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
+    deps = [
+        "//absl/log/internal:check_impl",
+    ],
+)
+
+cc_library(
+    name = "absl_log",
+    hdrs = ["absl_log.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
+    deps = [
+        "//absl/log/internal:log_impl",
+    ],
+)
+
+cc_library(
+    name = "check",
+    hdrs = ["check.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
+    deps = [
+        "//absl/log/internal:check_impl",
+        "//absl/log/internal:check_op",
+        "//absl/log/internal:conditions",
+        "//absl/log/internal:log_message",
+        "//absl/log/internal:strip",
+    ],
+)
+
+cc_library(
+    name = "die_if_null",
+    srcs = ["die_if_null.cc"],
+    hdrs = ["die_if_null.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
+    deps = [
+        ":log",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:nullability",
+        "//absl/base:nullability_traits_internal",
+        "//absl/strings",
+    ],
+)
+
+cc_library(
+    name = "flags",
+    srcs = ["flags.cc"],
+    hdrs = ["flags.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
+    deps = [
+        ":globals",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:log_severity",
+        "//absl/flags:flag",
+        "//absl/flags:marshalling",
+        "//absl/log/internal:config",
+        "//absl/log/internal:flags",
+        "//absl/log/internal:vlog_config",
+        "//absl/strings",
+    ],
+    # Binaries which do not access these flags from C++ still want this library linked in.
+    alwayslink = True,
+)
+
+cc_library(
+    name = "globals",
+    srcs = ["globals.cc"],
+    hdrs = ["globals.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
+    deps = [
+        "//absl/base:atomic_hook",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:log_severity",
+        "//absl/base:raw_logging_internal",
+        "//absl/hash",
+        "//absl/log/internal:vlog_config",
+        "//absl/strings",
+    ],
+)
+
+cc_library(
+    name = "initialize",
+    srcs = ["initialize.cc"],
+    hdrs = ["initialize.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
+    deps = [
+        ":globals",
+        "//absl/base:config",
+        "//absl/log/internal:globals",
+        "//absl/time",
+    ],
+)
+
+cc_library(
+    name = "log",
+    hdrs = ["log.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
+    deps = [
+        ":vlog_is_on",
+        "//absl/log/internal:log_impl",
+    ],
+)
+
+cc_library(
+    name = "log_entry",
+    srcs = ["log_entry.cc"],
+    hdrs = ["log_entry.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
+    deps = [
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:log_severity",
+        "//absl/log/internal:config",
+        "//absl/log/internal:proto",
+        "//absl/strings",
+        "//absl/time",
+        "//absl/types:span",
+    ],
+)
+
+cc_library(
+    name = "log_sink",
+    srcs = ["log_sink.cc"],
+    hdrs = ["log_sink.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
+    deps = [
+        ":log_entry",
+        "//absl/base:config",
+    ],
+)
+
+cc_library(
+    name = "log_sink_registry",
+    hdrs = ["log_sink_registry.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
+    deps = [
+        ":log_sink",
+        "//absl/base:config",
+        "//absl/base:nullability",
+        "//absl/log/internal:log_sink_set",
+    ],
+)
+
+cc_library(
+    name = "log_streamer",
+    hdrs = ["log_streamer.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
+    deps = [
+        ":absl_log",
+        "//absl/base:config",
+        "//absl/base:log_severity",
+        "//absl/strings",
+        "//absl/strings:internal",
+        "//absl/types:optional",
+        "//absl/types:source_location",
+        "//absl/utility",
+    ],
+)
+
+cc_library(
+    name = "scoped_mock_log",
+    testonly = True,
+    srcs = ["scoped_mock_log.cc"],
+    hdrs = ["scoped_mock_log.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
+    deps = [
+        ":log_entry",
+        ":log_sink",
+        ":log_sink_registry",
+        "//absl/base:config",
+        "//absl/base:log_severity",
+        "//absl/base:raw_logging_internal",
+        "//absl/strings",
+        "@googletest//:gtest",
+    ],
+)
+
+cc_library(
+    name = "structured",
+    hdrs = ["structured.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
+    deps = [
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/log/internal:structured",
+        "//absl/strings",
+    ],
+)
+
+cc_library(
+    name = "absl_vlog_is_on",
+    hdrs = ["absl_vlog_is_on.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
+    deps = [
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/log/internal:vlog_config",
+        "//absl/strings",
+    ],
+)
+
+cc_library(
+    name = "vlog_is_on",
+    hdrs = ["vlog_is_on.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
+    deps = [
+        ":absl_vlog_is_on",
+    ],
+)
+
+# TODO(b/200695798): run this in TAP projects with -DABSL_MAX_VLOG_VERBOSITY={-100,100}
+cc_test(
+    name = "vlog_is_on_test",
+    size = "small",
+    srcs = [
+        "vlog_is_on_test.cc",
+    ],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":flags",
+        ":globals",
+        ":log",
+        ":scoped_mock_log",
+        ":vlog_is_on",
+        "//absl/base:log_severity",
+        "//absl/flags:flag",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+# Test targets
+
+cc_test(
+    name = "absl_check_test",
+    size = "small",
+    timeout = "moderate",
+    srcs = ["absl_check_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    tags = [
+        "no_test:os:ios",
+        "no_test_ios",
+        "no_test_wasm",
+    ],
+    deps = [
+        ":absl_check",
+        ":check_test_impl",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "absl_log_basic_test",
+    size = "small",
+    timeout = "moderate",
+    srcs = ["absl_log_basic_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":absl_log",
+        ":log_basic_test_impl",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "check_test",
+    size = "small",
+    timeout = "moderate",
+    srcs = ["check_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    tags = [
+        "no_test:os:ios",
+        "no_test_ios",
+        "no_test_wasm",
+    ],
+    deps = [
+        ":check",
+        ":check_test_impl",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_library(
+    name = "check_test_impl",
+    testonly = True,
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    tags = [
+        "no_test:os:ios",
+        "no_test_ios",
+        "no_test_wasm",
+    ],
+    textual_hdrs = ["check_test_impl.inc"],
+    visibility = ["//visibility:private"],
+    deps = [
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/log/internal:test_helpers",
+        "//absl/status",
+        "//absl/strings",
+        "//absl/strings:string_view",
+        "@googletest//:gtest",
+    ],
+)
+
+cc_test(
+    name = "die_if_null_test",
+    size = "small",
+    srcs = ["die_if_null_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":die_if_null",
+        "//absl/base:core_headers",
+        "//absl/log/internal:test_helpers",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "flags_test",
+    size = "small",
+    srcs = ["flags_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":flags",
+        ":globals",
+        ":log",
+        ":scoped_mock_log",
+        "//absl/base:core_headers",
+        "//absl/base:log_severity",
+        "//absl/flags:flag",
+        "//absl/flags:reflection",
+        "//absl/log/internal:flags",
+        "//absl/log/internal:test_helpers",
+        "//absl/log/internal:test_matchers",
+        "//absl/strings",
+        "//absl/types:source_location",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "globals_test",
+    size = "small",
+    srcs = ["globals_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":globals",
+        ":log",
+        ":scoped_mock_log",
+        "//absl/base:core_headers",
+        "//absl/base:log_severity",
+        "//absl/log/internal:globals",
+        "//absl/log/internal:test_helpers",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "log_basic_test",
+    size = "small",
+    timeout = "moderate",
+    srcs = ["log_basic_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":log",
+        ":log_basic_test_impl",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_library(
+    name = "log_basic_test_impl",
+    testonly = True,
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    textual_hdrs = ["log_basic_test_impl.inc"],
+    visibility = ["//visibility:private"],
+    deps = [
+        ":globals",
+        ":log_entry",
+        ":scoped_mock_log",
+        "//absl/base",
+        "//absl/base:log_severity",
+        "//absl/log/internal:globals",
+        "//absl/log/internal:test_actions",
+        "//absl/log/internal:test_helpers",
+        "//absl/log/internal:test_matchers",
+        "@googletest//:gtest",
+    ],
+)
+
+cc_test(
+    name = "log_entry_test",
+    size = "small",
+    srcs = ["log_entry_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":log_entry",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:log_severity",
+        "//absl/log/internal:append_truncated",
+        "//absl/log/internal:format",
+        "//absl/log/internal:test_helpers",
+        "//absl/strings",
+        "//absl/time",
+        "//absl/types:span",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "log_format_test",
+    size = "small",
+    srcs = ["log_format_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":check",
+        ":log",
+        ":log_entry",
+        ":scoped_mock_log",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/log/internal:test_matchers",
+        "//absl/strings",
+        "//absl/strings:str_format",
+        "//absl/types:source_location",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "log_macro_hygiene_test",
+    size = "small",
+    srcs = ["log_macro_hygiene_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":log",
+        ":scoped_mock_log",
+        "//absl/base:core_headers",
+        "//absl/base:log_severity",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "log_sink_test",
+    size = "medium",
+    srcs = ["log_sink_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    tags = [
+        "no_test:os:ios",
+        "no_test_ios",
+        "no_test_wasm",
+    ],
+    deps = [
+        ":log",
+        ":log_entry",
+        ":log_sink",
+        ":log_sink_registry",
+        ":scoped_mock_log",
+        "//absl/base:core_headers",
+        "//absl/base:log_severity",
+        "//absl/log/internal:test_actions",
+        "//absl/log/internal:test_helpers",
+        "//absl/log/internal:test_matchers",
+        "//absl/strings",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "log_streamer_test",
+    size = "medium",
+    srcs = ["log_streamer_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":log",
+        ":log_streamer",
+        ":scoped_mock_log",
+        "//absl/base",
+        "//absl/base:core_headers",
+        "//absl/base:log_severity",
+        "//absl/log/internal:test_actions",
+        "//absl/log/internal:test_helpers",
+        "//absl/log/internal:test_matchers",
+        "//absl/strings",
+        "//absl/types:source_location",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "log_modifier_methods_test",
+    size = "small",
+    srcs = ["log_modifier_methods_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":log",
+        ":log_entry",
+        ":log_sink",
+        ":scoped_mock_log",
+        "//absl/base:log_severity",
+        "//absl/log/internal:test_actions",
+        "//absl/log/internal:test_helpers",
+        "//absl/log/internal:test_matchers",
+        "//absl/strings",
+        "//absl/time",
+        "//absl/types:source_location",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "scoped_mock_log_test",
+    size = "small",
+    srcs = ["scoped_mock_log_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    linkstatic = 1,
+    tags = [
+        "no_test:os:ios",
+        "no_test_ios",
+        "no_test_wasm",
+    ],
+    deps = [
+        ":globals",
+        ":log",
+        ":log_entry",
+        ":scoped_mock_log",
+        "//absl/base:core_headers",
+        "//absl/base:log_severity",
+        "//absl/log/internal:test_helpers",
+        "//absl/log/internal:test_matchers",
+        "//absl/memory",
+        "//absl/strings",
+        "//absl/synchronization",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "stripping_test",
+    size = "small",
+    srcs = ["stripping_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    # This test requires all code live in the binary (instead of shared libraries)
+    # because we test for the existence of specific literals in the binary.
+    linkstatic = 1,
+    deps = [
+        ":check",
+        ":log",
+        "//absl/base:log_severity",
+        "//absl/base:strerror",
+        "//absl/flags:program_name",
+        "//absl/log/internal:test_helpers",
+        "//absl/status",
+        "//absl/strings",
+        "//absl/strings:str_format",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "structured_test",
+    size = "small",
+    srcs = ["structured_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":log",
+        ":scoped_mock_log",
+        ":structured",
+        "//absl/base:core_headers",
+        "//absl/log/internal:test_helpers",
+        "//absl/log/internal:test_matchers",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "log_benchmark",
+    size = "small",
+    srcs = ["log_benchmark.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    tags = ["benchmark"],
+    deps = [
+        ":check",
+        ":flags",
+        ":globals",
+        ":log",
+        ":log_entry",
+        ":log_sink",
+        ":log_sink_registry",
+        ":vlog_is_on",
+        "//absl/base:core_headers",
+        "//absl/base:log_severity",
+        "//absl/flags:flag",
+        "//absl/log/internal:flags",
+        "@google_benchmark//:benchmark_main",
+    ],
+)
diff --git a/absl/log/CMakeLists.txt b/absl/log/CMakeLists.txt
new file mode 100644
index 00000000000..e0a307b3aff
--- /dev/null
+++ b/absl/log/CMakeLists.txt
@@ -0,0 +1,1241 @@
+# Copyright 2022 The Abseil Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+# Internal targets
+absl_cc_library(
+  NAME
+    log_internal_check_impl
+  SRCS
+  HDRS
+    "internal/check_impl.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::core_headers
+    absl::log_internal_check_op
+    absl::log_internal_conditions
+    absl::log_internal_strip
+)
+
+absl_cc_library(
+  NAME
+    log_internal_check_op
+  SRCS
+    "internal/check_op.cc"
+  HDRS
+    "internal/check_op.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::base
+    absl::config
+    absl::core_headers
+    absl::has_ostream_operator
+    absl::leak_check
+    absl::log_internal_nullguard
+    absl::log_internal_nullstream
+    absl::log_internal_strip
+    absl::nullability
+    absl::strings
+)
+
+absl_cc_library(
+  NAME
+    log_internal_conditions
+  SRCS
+    "internal/conditions.cc"
+  HDRS
+    "internal/conditions.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::base
+    absl::config
+    absl::core_headers
+    absl::log_internal_voidify
+)
+
+absl_cc_library(
+  NAME
+    log_internal_config
+  SRCS
+  HDRS
+    "internal/config.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::config
+    absl::core_headers
+)
+
+absl_cc_library(
+  NAME
+    log_internal_flags
+  SRCS
+  HDRS
+    "internal/flags.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::flags
+)
+
+absl_cc_library(
+  NAME
+    log_internal_format
+  SRCS
+    "internal/log_format.cc"
+  HDRS
+    "internal/log_format.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::config
+    absl::core_headers
+    absl::log_internal_append_truncated
+    absl::log_internal_config
+    absl::log_internal_globals
+    absl::log_severity
+    absl::strings
+    absl::str_format
+    absl::time
+    absl::span
+)
+
+absl_cc_library(
+  NAME
+    log_internal_globals
+  SRCS
+    "internal/globals.cc"
+  HDRS
+    "internal/globals.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::config
+    absl::core_headers
+    absl::log_severity
+    absl::raw_logging_internal
+    absl::strings
+    absl::time
+)
+
+absl_cc_library(
+  NAME
+    log_internal_log_impl
+  SRCS
+  HDRS
+    "internal/log_impl.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::log_internal_conditions
+    absl::log_internal_message
+    absl::log_internal_strip
+    absl::absl_vlog_is_on
+)
+
+absl_cc_library(
+  NAME
+    log_internal_proto
+  SRCS
+    "internal/proto.cc"
+  HDRS
+    "internal/proto.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::base
+    absl::config
+    absl::core_headers
+    absl::strings
+    absl::span
+)
+
+absl_cc_library(
+  NAME
+    log_internal_message
+  SRCS
+    "internal/log_message.cc"
+  HDRS
+    "internal/log_message.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::base
+    absl::config
+    absl::core_headers
+    absl::errno_saver
+    absl::examine_stack
+    absl::inlined_vector
+    absl::log_internal_append_truncated
+    absl::log_internal_format
+    absl::log_internal_globals
+    absl::log_internal_proto
+    absl::log_internal_log_sink_set
+    absl::log_internal_nullguard
+    absl::log_internal_structured_proto
+    absl::log_globals
+    absl::log_entry
+    absl::log_severity
+    absl::log_sink
+    absl::log_sink_registry
+    absl::memory
+    absl::nullability
+    absl::raw_logging_internal
+    absl::source_location
+    absl::span
+    absl::strerror
+    absl::strings
+    absl::strings_internal
+    absl::time
+)
+
+absl_cc_library(
+  NAME
+    log_internal_log_sink_set
+  SRCS
+    "internal/log_sink_set.cc"
+  HDRS
+    "internal/log_sink_set.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+    $<$<BOOL:${ANDROID}>:-llog>
+  DEPS
+    absl::base
+    absl::cleanup
+    absl::config
+    absl::core_headers
+    absl::log_internal_config
+    absl::log_internal_globals
+    absl::log_globals
+    absl::log_entry
+    absl::log_severity
+    absl::log_sink
+    absl::no_destructor
+    absl::raw_logging_internal
+    absl::synchronization
+    absl::span
+    absl::strings
+)
+
+absl_cc_library(
+  NAME
+    log_internal_nullguard
+  SRCS
+    "internal/nullguard.cc"
+  HDRS
+    "internal/nullguard.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::config
+    absl::core_headers
+)
+
+absl_cc_library(
+  NAME
+    log_internal_nullstream
+  SRCS
+  HDRS
+    "internal/nullstream.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::config
+    absl::core_headers
+    absl::log_severity
+    absl::strings
+)
+
+absl_cc_library(
+  NAME
+    log_internal_strip
+  SRCS
+  HDRS
+    "internal/strip.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::core_headers
+    absl::log_internal_message
+    absl::log_internal_nullstream
+    absl::log_severity
+)
+
+absl_cc_library(
+  NAME
+    log_internal_test_actions
+  SRCS
+    "internal/test_actions.cc"
+  HDRS
+    "internal/test_actions.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::config
+    absl::core_headers
+    absl::log_entry
+    absl::log_severity
+    absl::strings
+    absl::time
+  TESTONLY
+)
+
+absl_cc_library(
+  NAME
+    log_internal_test_helpers
+  SRCS
+    "internal/test_helpers.cc"
+  HDRS
+    "internal/test_helpers.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::config
+    absl::log_globals
+    absl::log_initialize
+    absl::log_internal_globals
+    absl::log_severity
+    GTest::gtest
+  TESTONLY
+)
+
+absl_cc_library(
+  NAME
+    log_internal_test_matchers
+  SRCS
+    "internal/test_matchers.cc"
+  HDRS
+    "internal/test_matchers.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::config
+    absl::core_headers
+    absl::log_entry
+    absl::log_internal_test_helpers
+    absl::log_severity
+    absl::strings
+    absl::time
+    GTest::gtest
+    GTest::gmock
+  TESTONLY
+)
+
+absl_cc_library(
+  NAME
+    log_internal_voidify
+  SRCS
+  HDRS
+    "internal/voidify.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::config
+    absl::core_headers
+)
+
+absl_cc_library(
+  NAME
+    log_internal_append_truncated
+  SRCS
+  HDRS
+    "internal/append_truncated.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::config
+    absl::strings
+    absl::strings_internal
+    absl::span
+)
+
+# Public targets
+absl_cc_library(
+  NAME
+    absl_check
+  SRCS
+  HDRS
+    "absl_check.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::log_internal_check_impl
+  PUBLIC
+)
+
+absl_cc_library(
+  NAME
+    absl_log
+  SRCS
+  HDRS
+    "absl_log.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::log_internal_log_impl
+  PUBLIC
+)
+
+absl_cc_library(
+  NAME
+    check
+  SRCS
+  HDRS
+    "check.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::log_internal_check_impl
+    absl::core_headers
+    absl::log_internal_check_op
+    absl::log_internal_conditions
+    absl::log_internal_message
+    absl::log_internal_strip
+  PUBLIC
+)
+
+absl_cc_library(
+  NAME
+    die_if_null
+  SRCS
+    "die_if_null.cc"
+  HDRS
+    "die_if_null.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::config
+    absl::core_headers
+    absl::log
+    absl::nullability
+    absl::nullability_traits_internal
+    absl::strings
+  PUBLIC
+)
+
+# Warning: Many linkers will strip the contents of this library because its
+# symbols are only used in a global constructor. A workaround is for clients
+# to link this using $<LINK_LIBRARY:WHOLE_ARCHIVE,absl::log_flags> instead of
+# the plain absl::log_flags.
+# TODO(b/320467376): Implement the equivalent of Bazel's alwayslink=True.
+absl_cc_library(
+  NAME
+    log_flags
+  SRCS
+    "flags.cc"
+  HDRS
+    "flags.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::config
+    absl::core_headers
+    absl::log_globals
+    absl::log_severity
+    absl::log_internal_config
+    absl::log_internal_flags
+    absl::flags
+    absl::flags_marshalling
+    absl::strings
+    absl::vlog_config_internal
+  PUBLIC
+)
+
+absl_cc_library(
+  NAME
+    log_globals
+  SRCS
+    "globals.cc"
+  HDRS
+    "globals.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::atomic_hook
+    absl::config
+    absl::core_headers
+    absl::hash
+    absl::log_severity
+    absl::raw_logging_internal
+    absl::strings
+    absl::vlog_config_internal
+)
+
+absl_cc_library(
+  NAME
+    log_initialize
+  SRCS
+    "initialize.cc"
+  HDRS
+    "initialize.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::config
+    absl::log_globals
+    absl::log_internal_globals
+    absl::time
+  PUBLIC
+)
+
+absl_cc_library(
+  NAME
+    log
+  SRCS
+  HDRS
+    "log.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::log_internal_log_impl
+    absl::vlog_is_on
+  PUBLIC
+)
+
+absl_cc_library(
+  NAME
+    log_entry
+  SRCS
+    "log_entry.cc"
+  HDRS
+    "log_entry.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::config
+    absl::core_headers
+    absl::log_internal_config
+    absl::log_internal_proto
+    absl::log_severity
+    absl::span
+    absl::strings
+    absl::time
+  PUBLIC
+)
+
+absl_cc_library(
+  NAME
+    log_sink
+  SRCS
+    "log_sink.cc"
+  HDRS
+    "log_sink.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::config
+    absl::log_entry
+  PUBLIC
+)
+
+absl_cc_library(
+  NAME
+    log_sink_registry
+  SRCS
+  HDRS
+    "log_sink_registry.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::config
+    absl::log_sink
+    absl::log_internal_log_sink_set
+    absl::nullability
+  PUBLIC
+)
+
+absl_cc_library(
+  NAME
+    log_streamer
+  SRCS
+  HDRS
+    "log_streamer.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::config
+    absl::absl_log
+    absl::log_severity
+    absl::optional
+    absl::source_location
+    absl::strings
+    absl::strings_internal
+    absl::utility
+  PUBLIC
+)
+
+absl_cc_library(
+  NAME
+    scoped_mock_log
+  SRCS
+    "scoped_mock_log.cc"
+  HDRS
+    "scoped_mock_log.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::config
+    absl::log_entry
+    absl::log_severity
+    absl::log_sink
+    absl::log_sink_registry
+    absl::raw_logging_internal
+    absl::strings
+    GTest::gmock
+    GTest::gtest
+  PUBLIC
+  TESTONLY
+)
+
+absl_cc_library(
+  NAME
+    log_internal_structured
+  HDRS
+    "internal/structured.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::any_invocable
+    absl::config
+    absl::core_headers
+    absl::log_internal_message
+    absl::log_internal_structured_proto
+    absl::strings
+)
+
+absl_cc_library(
+  NAME
+    log_internal_structured_proto
+  SRCS
+    "internal/structured_proto.cc"
+  HDRS
+    "internal/structured_proto.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::log_internal_proto
+    absl::config
+    absl::span
+    absl::strings
+    absl::variant
+  PUBLIC
+)
+
+absl_cc_test(
+  NAME
+    log_internal_structured_proto_test
+  SRCS
+    "internal/structured_proto_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::config
+    absl::log_internal_structured_proto
+    absl::span
+    absl::string_view
+    absl::utility
+    GTest::gmock_main
+)
+
+absl_cc_library(
+  NAME
+    log_structured
+  HDRS
+    "structured.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::config
+    absl::core_headers
+    absl::log_internal_structured
+    absl::strings
+  PUBLIC
+)
+
+absl_cc_library(
+  NAME
+    vlog_config_internal
+  SRCS
+    "internal/vlog_config.cc"
+  HDRS
+    "internal/vlog_config.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::base
+    absl::config
+    absl::core_headers
+    absl::log_internal_fnmatch
+    absl::memory
+    absl::no_destructor
+    absl::nullability
+    absl::strings
+    absl::synchronization
+)
+
+absl_cc_library(
+  NAME
+    absl_vlog_is_on
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  HDRS
+    "absl_vlog_is_on.h"
+  DEPS
+    absl::vlog_config_internal
+    absl::config
+    absl::core_headers
+    absl::strings
+)
+
+absl_cc_library(
+  NAME
+    vlog_is_on
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  HDRS
+    "vlog_is_on.h"
+  DEPS
+    absl::absl_vlog_is_on
+)
+
+absl_cc_test(
+  NAME
+    vlog_is_on_test
+  SRCS
+    "vlog_is_on_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::log
+    absl::log_flags
+    absl::log_globals
+    absl::scoped_mock_log
+    absl::vlog_is_on
+    absl::log_severity
+    absl::flags
+    GTest::gmock_main
+)
+
+absl_cc_library(
+  NAME
+    log_internal_fnmatch
+  SRCS
+    "internal/fnmatch.cc"
+  HDRS
+    "internal/fnmatch.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::config
+    absl::strings
+)
+
+# Test targets
+
+absl_cc_test(
+  NAME
+    absl_check_test
+  SRCS
+    "absl_check_test.cc"
+    "check_test_impl.inc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::absl_check
+    absl::config
+    absl::core_headers
+    absl::log_internal_test_helpers
+    absl::status
+    absl::strings
+    absl::string_view
+    GTest::gmock_main
+)
+
+absl_cc_test(
+  NAME
+    absl_log_basic_test
+  SRCS
+    "log_basic_test.cc"
+    "log_basic_test_impl.inc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::base
+    absl::absl_log
+    absl::log_entry
+    absl::log_globals
+    absl::log_severity
+    absl::log_internal_globals
+    absl::log_internal_test_actions
+    absl::log_internal_test_helpers
+    absl::log_internal_test_matchers
+    absl::scoped_mock_log
+    GTest::gmock_main
+)
+
+absl_cc_test(
+  NAME
+    check_test
+  SRCS
+    "check_test.cc"
+    "check_test_impl.inc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::check
+    absl::config
+    absl::core_headers
+    absl::log_internal_test_helpers
+    absl::status
+    absl::strings
+    absl::string_view
+    GTest::gmock_main
+)
+
+absl_cc_test(
+  NAME
+    die_if_null_test
+  SRCS
+    "die_if_null_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::core_headers
+    absl::die_if_null
+    absl::log_internal_test_helpers
+    GTest::gtest_main
+)
+
+absl_cc_test(
+  NAME
+    log_basic_test
+  SRCS
+    "log_basic_test.cc"
+    "log_basic_test_impl.inc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::base
+    absl::log
+    absl::log_entry
+    absl::log_globals
+    absl::log_severity
+    absl::log_internal_test_actions
+    absl::log_internal_test_helpers
+    absl::log_internal_test_matchers
+    absl::scoped_mock_log
+    GTest::gmock_main
+)
+
+absl_cc_test(
+  NAME
+    log_entry_test
+  SRCS
+    "log_entry_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::config
+    absl::core_headers
+    absl::log_entry
+    absl::log_internal_append_truncated
+    absl::log_internal_format
+    absl::log_internal_globals
+    absl::log_internal_test_helpers
+    absl::log_severity
+    absl::span
+    absl::strings
+    absl::time
+    GTest::gmock_main
+)
+
+absl_cc_test(
+  NAME
+    log_flags_test
+  SRCS
+    "flags_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::core_headers
+    absl::log
+    absl::log_flags
+    absl::log_globals
+    absl::log_internal_flags
+    absl::log_internal_test_helpers
+    absl::log_internal_test_matchers
+    absl::log_severity
+    absl::flags
+    absl::flags_reflection
+    absl::scoped_mock_log
+    absl::strings
+    GTest::gmock_main
+)
+
+absl_cc_test(
+  NAME
+    log_globals_test
+  SRCS
+    "globals_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::core_headers
+    absl::log
+    absl::log_globals
+    absl::log_internal_globals
+    absl::log_internal_test_helpers
+    absl::log_severity
+    absl::scoped_mock_log
+    GTest::gmock_main
+)
+
+absl_cc_test(
+  NAME
+    log_format_test
+  SRCS
+    "log_format_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::check
+    absl::config
+    absl::log
+    absl::log_internal_test_matchers
+    absl::scoped_mock_log
+    absl::source_location
+    absl::str_format
+    absl::strings
+    GTest::gmock_main
+)
+
+absl_cc_test(
+  NAME
+    log_macro_hygiene_test
+  SRCS
+    "log_macro_hygiene_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::core_headers
+    absl::log
+    absl::log_severity
+    absl::scoped_mock_log
+    GTest::gmock_main
+)
+
+absl_cc_test(
+  NAME
+    log_sink_test
+  SRCS
+    "log_sink_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::core_headers
+    absl::log
+    absl::log_internal_test_actions
+    absl::log_internal_test_helpers
+    absl::log_internal_test_matchers
+    absl::log_sink
+    absl::log_sink_registry
+    absl::log_severity
+    absl::scoped_mock_log
+    absl::strings
+    GTest::gmock_main
+)
+
+absl_cc_test(
+  NAME
+    log_streamer_test
+  SRCS
+    "log_streamer_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::base
+    absl::core_headers
+    absl::log
+    absl::log_internal_test_actions
+    absl::log_internal_test_helpers
+    absl::log_internal_test_matchers
+    absl::log_streamer
+    absl::log_severity
+    absl::scoped_mock_log
+    absl::source_location
+    absl::strings
+    GTest::gmock_main
+)
+
+absl_cc_test(
+  NAME
+    log_modifier_methods_test
+  SRCS
+    "log_modifier_methods_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::log
+    absl::log_internal_test_actions
+    absl::log_internal_test_helpers
+    absl::log_internal_test_matchers
+    absl::log_sink
+    absl::scoped_mock_log
+    absl::source_location
+    absl::strings
+    absl::time
+    GTest::gmock_main
+)
+
+absl_cc_test(
+  NAME
+    scoped_mock_log_test
+  SRCS
+    "scoped_mock_log_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::core_headers
+    absl::log
+    absl::log_globals
+    absl::log_internal_globals
+    absl::log_internal_test_helpers
+    absl::log_internal_test_matchers
+    absl::log_severity
+    absl::memory
+    absl::scoped_mock_log
+    absl::strings
+    absl::synchronization
+    GTest::gmock
+    GTest::gtest_main
+)
+
+absl_cc_test(
+  NAME
+    log_internal_stderr_log_sink_test
+  SRCS
+    "internal/stderr_log_sink_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::core_headers
+    absl::log
+    absl::log_globals
+    absl::log_internal_test_helpers
+    absl::log_severity
+    GTest::gmock_main
+)
+
+absl_cc_test(
+  NAME
+    log_stripping_test
+  SRCS
+    "stripping_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::check
+    absl::flags_program_name
+    absl::log
+    absl::log_internal_test_helpers
+    absl::log_severity
+    absl::status
+    absl::strerror
+    absl::strings
+    absl::str_format
+    GTest::gmock_main
+)
+
+absl_cc_test(
+  NAME
+    log_structured_test
+  SRCS
+    "structured_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::core_headers
+    absl::log
+    absl::log_internal_test_helpers
+    absl::log_internal_test_matchers
+    absl::log_structured
+    absl::scoped_mock_log
+    GTest::gmock_main
+)
+
+absl_cc_test(
+  NAME
+    internal_fnmatch_test
+  SRCS
+    "internal/fnmatch_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::log_internal_fnmatch
+    GTest::gmock_main
+)
+
+absl_cc_library(
+  NAME
+    log_internal_container
+  HDRS
+    "internal/container.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::config
+    absl::requires_internal
+    absl::strings
+)
+
+absl_cc_test(
+  NAME
+    internal_container_test
+  SRCS
+    "internal/container_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::config
+    absl::log_internal_container
+    absl::span
+    absl::strings
+    absl::str_format
+    GTest::gmock_main
+)
diff --git a/absl/log/absl_check.h b/absl/log/absl_check.h
new file mode 100644
index 00000000000..1bb43bd3442
--- /dev/null
+++ b/absl/log/absl_check.h
@@ -0,0 +1,117 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: log/absl_check.h
+// -----------------------------------------------------------------------------
+//
+// This header declares a family of `ABSL_CHECK` macros as alternative spellings
+// for `CHECK` macros in `check.h`.
+//
+// Except for those whose names begin with `ABSL_DCHECK`, these macros are not
+// controlled by `NDEBUG` (cf. `assert`), so the check will be executed
+// regardless of compilation mode. `ABSL_CHECK` and friends are thus useful for
+// confirming invariants in situations where continuing to run would be worse
+// than terminating, e.g., due to risk of data corruption or security
+// compromise.  It is also more robust and portable to deliberately terminate
+// at a particular place with a useful message and backtrace than to assume some
+// ultimately unspecified and unreliable crashing behavior (such as a
+// "segmentation fault").
+//
+// For full documentation of each macro, see comments in `check.h`, which has an
+// identical set of macros without the ABSL_* prefix.
+
+#ifndef ABSL_LOG_ABSL_CHECK_H_
+#define ABSL_LOG_ABSL_CHECK_H_
+
+#include "absl/log/internal/check_impl.h"
+
+#define ABSL_CHECK(condition) \
+  ABSL_LOG_INTERNAL_CHECK_IMPL((condition), #condition)
+#define ABSL_QCHECK(condition) \
+  ABSL_LOG_INTERNAL_QCHECK_IMPL((condition), #condition)
+#define ABSL_PCHECK(condition) \
+  ABSL_LOG_INTERNAL_PCHECK_IMPL((condition), #condition)
+#define ABSL_DCHECK(condition) \
+  ABSL_LOG_INTERNAL_DCHECK_IMPL((condition), #condition)
+
+#define ABSL_CHECK_EQ(val1, val2) \
+  ABSL_LOG_INTERNAL_CHECK_EQ_IMPL((val1), #val1, (val2), #val2)
+#define ABSL_CHECK_NE(val1, val2) \
+  ABSL_LOG_INTERNAL_CHECK_NE_IMPL((val1), #val1, (val2), #val2)
+#define ABSL_CHECK_LE(val1, val2) \
+  ABSL_LOG_INTERNAL_CHECK_LE_IMPL((val1), #val1, (val2), #val2)
+#define ABSL_CHECK_LT(val1, val2) \
+  ABSL_LOG_INTERNAL_CHECK_LT_IMPL((val1), #val1, (val2), #val2)
+#define ABSL_CHECK_GE(val1, val2) \
+  ABSL_LOG_INTERNAL_CHECK_GE_IMPL((val1), #val1, (val2), #val2)
+#define ABSL_CHECK_GT(val1, val2) \
+  ABSL_LOG_INTERNAL_CHECK_GT_IMPL((val1), #val1, (val2), #val2)
+#define ABSL_QCHECK_EQ(val1, val2) \
+  ABSL_LOG_INTERNAL_QCHECK_EQ_IMPL((val1), #val1, (val2), #val2)
+#define ABSL_QCHECK_NE(val1, val2) \
+  ABSL_LOG_INTERNAL_QCHECK_NE_IMPL((val1), #val1, (val2), #val2)
+#define ABSL_QCHECK_LE(val1, val2) \
+  ABSL_LOG_INTERNAL_QCHECK_LE_IMPL((val1), #val1, (val2), #val2)
+#define ABSL_QCHECK_LT(val1, val2) \
+  ABSL_LOG_INTERNAL_QCHECK_LT_IMPL((val1), #val1, (val2), #val2)
+#define ABSL_QCHECK_GE(val1, val2) \
+  ABSL_LOG_INTERNAL_QCHECK_GE_IMPL((val1), #val1, (val2), #val2)
+#define ABSL_QCHECK_GT(val1, val2) \
+  ABSL_LOG_INTERNAL_QCHECK_GT_IMPL((val1), #val1, (val2), #val2)
+#define ABSL_DCHECK_EQ(val1, val2) \
+  ABSL_LOG_INTERNAL_DCHECK_EQ_IMPL((val1), #val1, (val2), #val2)
+#define ABSL_DCHECK_NE(val1, val2) \
+  ABSL_LOG_INTERNAL_DCHECK_NE_IMPL((val1), #val1, (val2), #val2)
+#define ABSL_DCHECK_LE(val1, val2) \
+  ABSL_LOG_INTERNAL_DCHECK_LE_IMPL((val1), #val1, (val2), #val2)
+#define ABSL_DCHECK_LT(val1, val2) \
+  ABSL_LOG_INTERNAL_DCHECK_LT_IMPL((val1), #val1, (val2), #val2)
+#define ABSL_DCHECK_GE(val1, val2) \
+  ABSL_LOG_INTERNAL_DCHECK_GE_IMPL((val1), #val1, (val2), #val2)
+#define ABSL_DCHECK_GT(val1, val2) \
+  ABSL_LOG_INTERNAL_DCHECK_GT_IMPL((val1), #val1, (val2), #val2)
+
+#define ABSL_CHECK_OK(status) ABSL_LOG_INTERNAL_CHECK_OK_IMPL((status), #status)
+#define ABSL_QCHECK_OK(status) \
+  ABSL_LOG_INTERNAL_QCHECK_OK_IMPL((status), #status)
+#define ABSL_DCHECK_OK(status) \
+  ABSL_LOG_INTERNAL_DCHECK_OK_IMPL((status), #status)
+
+#define ABSL_CHECK_STREQ(s1, s2) \
+  ABSL_LOG_INTERNAL_CHECK_STREQ_IMPL((s1), #s1, (s2), #s2)
+#define ABSL_CHECK_STRNE(s1, s2) \
+  ABSL_LOG_INTERNAL_CHECK_STRNE_IMPL((s1), #s1, (s2), #s2)
+#define ABSL_CHECK_STRCASEEQ(s1, s2) \
+  ABSL_LOG_INTERNAL_CHECK_STRCASEEQ_IMPL((s1), #s1, (s2), #s2)
+#define ABSL_CHECK_STRCASENE(s1, s2) \
+  ABSL_LOG_INTERNAL_CHECK_STRCASENE_IMPL((s1), #s1, (s2), #s2)
+#define ABSL_QCHECK_STREQ(s1, s2) \
+  ABSL_LOG_INTERNAL_QCHECK_STREQ_IMPL((s1), #s1, (s2), #s2)
+#define ABSL_QCHECK_STRNE(s1, s2) \
+  ABSL_LOG_INTERNAL_QCHECK_STRNE_IMPL((s1), #s1, (s2), #s2)
+#define ABSL_QCHECK_STRCASEEQ(s1, s2) \
+  ABSL_LOG_INTERNAL_QCHECK_STRCASEEQ_IMPL((s1), #s1, (s2), #s2)
+#define ABSL_QCHECK_STRCASENE(s1, s2) \
+  ABSL_LOG_INTERNAL_QCHECK_STRCASENE_IMPL((s1), #s1, (s2), #s2)
+#define ABSL_DCHECK_STREQ(s1, s2) \
+  ABSL_LOG_INTERNAL_DCHECK_STREQ_IMPL((s1), #s1, (s2), #s2)
+#define ABSL_DCHECK_STRNE(s1, s2) \
+  ABSL_LOG_INTERNAL_DCHECK_STRNE_IMPL((s1), #s1, (s2), #s2)
+#define ABSL_DCHECK_STRCASEEQ(s1, s2) \
+  ABSL_LOG_INTERNAL_DCHECK_STRCASEEQ_IMPL((s1), #s1, (s2), #s2)
+#define ABSL_DCHECK_STRCASENE(s1, s2) \
+  ABSL_LOG_INTERNAL_DCHECK_STRCASENE_IMPL((s1), #s1, (s2), #s2)
+
+#endif  // ABSL_LOG_ABSL_CHECK_H_
diff --git a/absl/log/absl_check_test.cc b/absl/log/absl_check_test.cc
new file mode 100644
index 00000000000..d84940fa129
--- /dev/null
+++ b/absl/log/absl_check_test.cc
@@ -0,0 +1,58 @@
+//
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/log/absl_check.h"
+
+#define ABSL_TEST_CHECK ABSL_CHECK
+#define ABSL_TEST_CHECK_OK ABSL_CHECK_OK
+#define ABSL_TEST_CHECK_EQ ABSL_CHECK_EQ
+#define ABSL_TEST_CHECK_NE ABSL_CHECK_NE
+#define ABSL_TEST_CHECK_GE ABSL_CHECK_GE
+#define ABSL_TEST_CHECK_LE ABSL_CHECK_LE
+#define ABSL_TEST_CHECK_GT ABSL_CHECK_GT
+#define ABSL_TEST_CHECK_LT ABSL_CHECK_LT
+#define ABSL_TEST_CHECK_STREQ ABSL_CHECK_STREQ
+#define ABSL_TEST_CHECK_STRNE ABSL_CHECK_STRNE
+#define ABSL_TEST_CHECK_STRCASEEQ ABSL_CHECK_STRCASEEQ
+#define ABSL_TEST_CHECK_STRCASENE ABSL_CHECK_STRCASENE
+
+#define ABSL_TEST_DCHECK ABSL_DCHECK
+#define ABSL_TEST_DCHECK_OK ABSL_DCHECK_OK
+#define ABSL_TEST_DCHECK_EQ ABSL_DCHECK_EQ
+#define ABSL_TEST_DCHECK_NE ABSL_DCHECK_NE
+#define ABSL_TEST_DCHECK_GE ABSL_DCHECK_GE
+#define ABSL_TEST_DCHECK_LE ABSL_DCHECK_LE
+#define ABSL_TEST_DCHECK_GT ABSL_DCHECK_GT
+#define ABSL_TEST_DCHECK_LT ABSL_DCHECK_LT
+#define ABSL_TEST_DCHECK_STREQ ABSL_DCHECK_STREQ
+#define ABSL_TEST_DCHECK_STRNE ABSL_DCHECK_STRNE
+#define ABSL_TEST_DCHECK_STRCASEEQ ABSL_DCHECK_STRCASEEQ
+#define ABSL_TEST_DCHECK_STRCASENE ABSL_DCHECK_STRCASENE
+
+#define ABSL_TEST_QCHECK ABSL_QCHECK
+#define ABSL_TEST_QCHECK_OK ABSL_QCHECK_OK
+#define ABSL_TEST_QCHECK_EQ ABSL_QCHECK_EQ
+#define ABSL_TEST_QCHECK_NE ABSL_QCHECK_NE
+#define ABSL_TEST_QCHECK_GE ABSL_QCHECK_GE
+#define ABSL_TEST_QCHECK_LE ABSL_QCHECK_LE
+#define ABSL_TEST_QCHECK_GT ABSL_QCHECK_GT
+#define ABSL_TEST_QCHECK_LT ABSL_QCHECK_LT
+#define ABSL_TEST_QCHECK_STREQ ABSL_QCHECK_STREQ
+#define ABSL_TEST_QCHECK_STRNE ABSL_QCHECK_STRNE
+#define ABSL_TEST_QCHECK_STRCASEEQ ABSL_QCHECK_STRCASEEQ
+#define ABSL_TEST_QCHECK_STRCASENE ABSL_QCHECK_STRCASENE
+
+#include "gtest/gtest.h"
+#include "absl/log/check_test_impl.inc"
diff --git a/absl/log/absl_log.h b/absl/log/absl_log.h
new file mode 100644
index 00000000000..0fd9ae3fc30
--- /dev/null
+++ b/absl/log/absl_log.h
@@ -0,0 +1,115 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: log/absl_log.h
+// -----------------------------------------------------------------------------
+//
+// This header declares a family of `ABSL_LOG` macros as alternative spellings
+// for macros in `log.h`.
+//
+// Basic invocation looks like this:
+//
+//   ABSL_LOG(INFO) << "Found " << num_cookies << " cookies";
+//
+// Most `ABSL_LOG` macros take a severity level argument. The severity levels
+// are `INFO`, `WARNING`, `ERROR`, and `FATAL`.
+//
+// For full documentation, see comments in `log.h`, which includes full
+// reference documentation on use of the equivalent `LOG` macro and has an
+// identical set of macros without the ABSL_* prefix.
+
+#ifndef ABSL_LOG_ABSL_LOG_H_
+#define ABSL_LOG_ABSL_LOG_H_
+
+#include "absl/log/internal/log_impl.h"
+
+#define ABSL_LOG(severity) ABSL_LOG_INTERNAL_LOG_IMPL(_##severity)
+#define ABSL_PLOG(severity) ABSL_LOG_INTERNAL_PLOG_IMPL(_##severity)
+#define ABSL_DLOG(severity) ABSL_LOG_INTERNAL_DLOG_IMPL(_##severity)
+
+#define ABSL_VLOG(verbose_level) ABSL_LOG_INTERNAL_VLOG_IMPL(verbose_level)
+#define ABSL_DVLOG(verbose_level) ABSL_LOG_INTERNAL_DVLOG_IMPL(verbose_level)
+
+#define ABSL_LOG_IF(severity, condition) \
+  ABSL_LOG_INTERNAL_LOG_IF_IMPL(_##severity, condition)
+#define ABSL_PLOG_IF(severity, condition) \
+  ABSL_LOG_INTERNAL_PLOG_IF_IMPL(_##severity, condition)
+#define ABSL_DLOG_IF(severity, condition) \
+  ABSL_LOG_INTERNAL_DLOG_IF_IMPL(_##severity, condition)
+
+#define ABSL_LOG_EVERY_N(severity, n) \
+  ABSL_LOG_INTERNAL_LOG_EVERY_N_IMPL(_##severity, n)
+#define ABSL_LOG_FIRST_N(severity, n) \
+  ABSL_LOG_INTERNAL_LOG_FIRST_N_IMPL(_##severity, n)
+#define ABSL_LOG_EVERY_POW_2(severity) \
+  ABSL_LOG_INTERNAL_LOG_EVERY_POW_2_IMPL(_##severity)
+#define ABSL_LOG_EVERY_N_SEC(severity, n_seconds) \
+  ABSL_LOG_INTERNAL_LOG_EVERY_N_SEC_IMPL(_##severity, n_seconds)
+
+#define ABSL_PLOG_EVERY_N(severity, n) \
+  ABSL_LOG_INTERNAL_PLOG_EVERY_N_IMPL(_##severity, n)
+#define ABSL_PLOG_FIRST_N(severity, n) \
+  ABSL_LOG_INTERNAL_PLOG_FIRST_N_IMPL(_##severity, n)
+#define ABSL_PLOG_EVERY_POW_2(severity) \
+  ABSL_LOG_INTERNAL_PLOG_EVERY_POW_2_IMPL(_##severity)
+#define ABSL_PLOG_EVERY_N_SEC(severity, n_seconds) \
+  ABSL_LOG_INTERNAL_PLOG_EVERY_N_SEC_IMPL(_##severity, n_seconds)
+
+#define ABSL_DLOG_EVERY_N(severity, n) \
+  ABSL_LOG_INTERNAL_DLOG_EVERY_N_IMPL(_##severity, n)
+#define ABSL_DLOG_FIRST_N(severity, n) \
+  ABSL_LOG_INTERNAL_DLOG_FIRST_N_IMPL(_##severity, n)
+#define ABSL_DLOG_EVERY_POW_2(severity) \
+  ABSL_LOG_INTERNAL_DLOG_EVERY_POW_2_IMPL(_##severity)
+#define ABSL_DLOG_EVERY_N_SEC(severity, n_seconds) \
+  ABSL_LOG_INTERNAL_DLOG_EVERY_N_SEC_IMPL(_##severity, n_seconds)
+
+#define ABSL_VLOG_EVERY_N(verbose_level, n) \
+  ABSL_LOG_INTERNAL_VLOG_EVERY_N_IMPL(verbose_level, n)
+#define ABSL_VLOG_FIRST_N(verbose_level, n) \
+  ABSL_LOG_INTERNAL_VLOG_FIRST_N_IMPL(verbose_level, n)
+#define ABSL_VLOG_EVERY_POW_2(verbose_level, n) \
+  ABSL_LOG_INTERNAL_VLOG_EVERY_POW_2_IMPL(verbose_level, n)
+#define ABSL_VLOG_EVERY_N_SEC(verbose_level, n) \
+  ABSL_LOG_INTERNAL_VLOG_EVERY_N_SEC_IMPL(verbose_level, n)
+
+#define ABSL_LOG_IF_EVERY_N(severity, condition, n) \
+  ABSL_LOG_INTERNAL_LOG_IF_EVERY_N_IMPL(_##severity, condition, n)
+#define ABSL_LOG_IF_FIRST_N(severity, condition, n) \
+  ABSL_LOG_INTERNAL_LOG_IF_FIRST_N_IMPL(_##severity, condition, n)
+#define ABSL_LOG_IF_EVERY_POW_2(severity, condition) \
+  ABSL_LOG_INTERNAL_LOG_IF_EVERY_POW_2_IMPL(_##severity, condition)
+#define ABSL_LOG_IF_EVERY_N_SEC(severity, condition, n_seconds) \
+  ABSL_LOG_INTERNAL_LOG_IF_EVERY_N_SEC_IMPL(_##severity, condition, n_seconds)
+
+#define ABSL_PLOG_IF_EVERY_N(severity, condition, n) \
+  ABSL_LOG_INTERNAL_PLOG_IF_EVERY_N_IMPL(_##severity, condition, n)
+#define ABSL_PLOG_IF_FIRST_N(severity, condition, n) \
+  ABSL_LOG_INTERNAL_PLOG_IF_FIRST_N_IMPL(_##severity, condition, n)
+#define ABSL_PLOG_IF_EVERY_POW_2(severity, condition) \
+  ABSL_LOG_INTERNAL_PLOG_IF_EVERY_POW_2_IMPL(_##severity, condition)
+#define ABSL_PLOG_IF_EVERY_N_SEC(severity, condition, n_seconds) \
+  ABSL_LOG_INTERNAL_PLOG_IF_EVERY_N_SEC_IMPL(_##severity, condition, n_seconds)
+
+#define ABSL_DLOG_IF_EVERY_N(severity, condition, n) \
+  ABSL_LOG_INTERNAL_DLOG_IF_EVERY_N_IMPL(_##severity, condition, n)
+#define ABSL_DLOG_IF_FIRST_N(severity, condition, n) \
+  ABSL_LOG_INTERNAL_DLOG_IF_FIRST_N_IMPL(_##severity, condition, n)
+#define ABSL_DLOG_IF_EVERY_POW_2(severity, condition) \
+  ABSL_LOG_INTERNAL_DLOG_IF_EVERY_POW_2_IMPL(_##severity, condition)
+#define ABSL_DLOG_IF_EVERY_N_SEC(severity, condition, n_seconds) \
+  ABSL_LOG_INTERNAL_DLOG_IF_EVERY_N_SEC_IMPL(_##severity, condition, n_seconds)
+
+#endif  // ABSL_LOG_ABSL_LOG_H_
diff --git a/absl/debugging/leak_check_disable.cc b/absl/log/absl_log_basic_test.cc
similarity index 67%
rename from absl/debugging/leak_check_disable.cc
rename to absl/log/absl_log_basic_test.cc
index 924d6e3d543..7378f5a8023 100644
--- a/absl/debugging/leak_check_disable.cc
+++ b/absl/log/absl_log_basic_test.cc
@@ -1,4 +1,5 @@
-// Copyright 2017 The Abseil Authors.
+//
+// Copyright 2022 The Abseil Authors.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
@@ -12,9 +13,10 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-// Disable LeakSanitizer when this file is linked in.
-// This function overrides __lsan_is_turned_off from sanitizer/lsan_interface.h
-extern "C" int __lsan_is_turned_off();
-extern "C" int __lsan_is_turned_off() {
-  return 1;
-}
+#include "absl/log/absl_log.h"
+
+#define ABSL_TEST_LOG ABSL_LOG
+#define ABSL_TEST_DLOG ABSL_DLOG
+
+#include "gtest/gtest.h"
+#include "absl/log/log_basic_test_impl.inc"
diff --git a/absl/log/absl_vlog_is_on.h b/absl/log/absl_vlog_is_on.h
new file mode 100644
index 00000000000..1a6792cbd1a
--- /dev/null
+++ b/absl/log/absl_vlog_is_on.h
@@ -0,0 +1,93 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: log/absl_vlog_is_on.h
+// -----------------------------------------------------------------------------
+//
+// This header defines the `ABSL_VLOG_IS_ON()` macro that controls the
+// variable-verbosity conditional logging.
+//
+// It's used by `VLOG` in log.h, or it can also be used directly like this:
+//
+//   if (ABSL_VLOG_IS_ON(2)) {
+//     foo_server.RecomputeStatisticsExpensive();
+//     LOG(INFO) << foo_server.LastStatisticsAsString();
+//   }
+//
+// Each source file has an effective verbosity level that's a non-negative
+// integer computed from the `--vmodule` and `--v` flags.
+// `ABSL_VLOG_IS_ON(n)` is true, and `VLOG(n)` logs, if that effective verbosity
+// level is greater than or equal to `n`.
+//
+// `--vmodule` takes a comma-delimited list of key=value pairs.  Each key is a
+// pattern matched against filenames, and the values give the effective severity
+// level applied to matching files.  '?' and '*' characters in patterns are
+// interpreted as single-character and zero-or-more-character wildcards.
+// Patterns including a slash character are matched against full pathnames,
+// while those without are matched against basenames only.  One suffix (i.e. the
+// last . and everything after it) is stripped from each filename prior to
+// matching, as is the special suffix "-inl".
+//
+// Example: --vmodule=module_a=1,module_b=2
+//
+// Files are matched against globs in `--vmodule` in order, and the first match
+// determines the verbosity level.
+//
+// Files which do not match any pattern in `--vmodule` use the value of `--v` as
+// their effective verbosity level.  The default is 0.
+//
+// SetVLogLevel helper function is provided to do limited dynamic control over
+// V-logging by appending to `--vmodule`. Because these go at the beginning of
+// the list, they take priority over any globs previously added.
+//
+// Resetting --vmodule will override all previous modifications to `--vmodule`,
+// including via SetVLogLevel.
+
+#ifndef ABSL_LOG_ABSL_VLOG_IS_ON_H_
+#define ABSL_LOG_ABSL_VLOG_IS_ON_H_
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/log/internal/vlog_config.h"  // IWYU pragma: export
+#include "absl/strings/string_view.h"
+
+// This is expanded at the callsite to allow the compiler to optimize
+// always-false cases out of the build.
+// An ABSL_MAX_VLOG_VERBOSITY of 2 means that VLOG(3) and above should never
+// log.
+#ifdef ABSL_MAX_VLOG_VERBOSITY
+#define ABSL_LOG_INTERNAL_MAX_LOG_VERBOSITY_CHECK(x) \
+  ((x) <= ABSL_MAX_VLOG_VERBOSITY)&&
+#else
+#define ABSL_LOG_INTERNAL_MAX_LOG_VERBOSITY_CHECK(x)
+#endif
+
+// Each ABSL_VLOG_IS_ON call site gets its own VLogSite that registers with the
+// global linked list of sites to asynchronously update its verbosity level on
+// changes to --v or --vmodule. The verbosity can also be set by manually
+// calling SetVLogLevel.
+//
+// ABSL_VLOG_IS_ON is not async signal safe, but it is guaranteed not to
+// allocate new memory.
+#define ABSL_VLOG_IS_ON(verbose_level)                                     \
+  (ABSL_LOG_INTERNAL_MAX_LOG_VERBOSITY_CHECK(verbose_level)[]()            \
+       ->::absl::log_internal::VLogSite *                                  \
+   {                                                                       \
+     ABSL_CONST_INIT static ::absl::log_internal::VLogSite site(__FILE__); \
+     return &site;                                                         \
+   }()                                                                     \
+       ->IsEnabled(verbose_level))
+
+#endif  // ABSL_LOG_ABSL_VLOG_IS_ON_H_
diff --git a/absl/log/check.h b/absl/log/check.h
new file mode 100644
index 00000000000..10674c92994
--- /dev/null
+++ b/absl/log/check.h
@@ -0,0 +1,210 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: log/check.h
+// -----------------------------------------------------------------------------
+//
+// This header declares a family of `CHECK` macros.
+//
+// `CHECK` macros terminate the program with a fatal error if the specified
+// condition is not true.
+//
+// Except for those whose names begin with `DCHECK`, these macros are not
+// controlled by `NDEBUG` (cf. `assert`), so the check will be executed
+// regardless of compilation mode. `CHECK` and friends are thus useful for
+// confirming invariants in situations where continuing to run would be worse
+// than terminating, e.g., due to risk of data corruption or security
+// compromise.  It is also more robust and portable to deliberately terminate
+// at a particular place with a useful message and backtrace than to assume some
+// ultimately unspecified and unreliable crashing behavior (such as a
+// "segmentation fault").
+
+#ifndef ABSL_LOG_CHECK_H_
+#define ABSL_LOG_CHECK_H_
+
+#include "absl/log/internal/check_impl.h"
+#include "absl/log/internal/check_op.h"     // IWYU pragma: export
+#include "absl/log/internal/conditions.h"   // IWYU pragma: export
+#include "absl/log/internal/log_message.h"  // IWYU pragma: export
+#include "absl/log/internal/strip.h"        // IWYU pragma: export
+
+// CHECK()
+//
+// `CHECK` enforces that the `condition` is true. If the condition is false,
+// the program is terminated with a fatal error.
+//
+// The message may include additional information such as stack traces, when
+// available.
+//
+// Example:
+//
+//   CHECK(!cheese.empty()) << "Out of Cheese";
+//
+// Might produce a message like:
+//
+//   Check failed: !cheese.empty() Out of Cheese
+#define CHECK(condition) ABSL_LOG_INTERNAL_CHECK_IMPL((condition), #condition)
+
+// QCHECK()
+//
+// `QCHECK` behaves like `CHECK` but does not print a full stack trace and does
+// not run registered error handlers (as `QFATAL`).  It is useful when the
+// problem is definitely unrelated to program flow, e.g. when validating user
+// input.
+#define QCHECK(condition) ABSL_LOG_INTERNAL_QCHECK_IMPL((condition), #condition)
+
+// PCHECK()
+//
+// `PCHECK` behaves like `CHECK` but appends a description of the current state
+// of `errno` to the failure message.
+//
+// Example:
+//
+//   int fd = open("/var/empty/missing", O_RDONLY);
+//   PCHECK(fd != -1) << "posix is difficult";
+//
+// Might produce a message like:
+//
+//   Check failed: fd != -1 posix is difficult: No such file or directory [2]
+#define PCHECK(condition) ABSL_LOG_INTERNAL_PCHECK_IMPL((condition), #condition)
+
+// DCHECK()
+//
+// `DCHECK` behaves like `CHECK` in debug mode and does nothing otherwise (as
+// `DLOG`).  Unlike with `CHECK` (but as with `assert`), it is not safe to rely
+// on evaluation of `condition`: when `NDEBUG` is enabled, DCHECK does not
+// evaluate the condition.
+#define DCHECK(condition) ABSL_LOG_INTERNAL_DCHECK_IMPL((condition), #condition)
+
+// `CHECK_EQ` and friends are syntactic sugar for `CHECK(x == y)` that
+// automatically output the expression being tested and the evaluated values on
+// either side.
+//
+// Example:
+//
+//   int x = 3, y = 5;
+//   CHECK_EQ(2 * x, y) << "oops!";
+//
+// Might produce a message like:
+//
+//   Check failed: 2 * x == y (6 vs. 5) oops!
+//
+// The values must implement the appropriate comparison operator as well as
+// either `operator<<(std::ostream&, ...)` or `AbslStringify`.  Care is taken to
+// ensure that each argument is evaluated exactly once, and that anything which
+// is legal to pass as a function argument is legal here.  In particular, the
+// arguments may be temporary expressions which will end up being destroyed at
+// the end of the statement,
+//
+// Example:
+//
+//   CHECK_EQ(std::string("abc")[1], 'b');
+//
+// WARNING: Passing `NULL` as an argument to `CHECK_EQ` and similar macros does
+// not compile.  Use `nullptr` instead.
+#define CHECK_EQ(val1, val2) \
+  ABSL_LOG_INTERNAL_CHECK_EQ_IMPL((val1), #val1, (val2), #val2)
+#define CHECK_NE(val1, val2) \
+  ABSL_LOG_INTERNAL_CHECK_NE_IMPL((val1), #val1, (val2), #val2)
+#define CHECK_LE(val1, val2) \
+  ABSL_LOG_INTERNAL_CHECK_LE_IMPL((val1), #val1, (val2), #val2)
+#define CHECK_LT(val1, val2) \
+  ABSL_LOG_INTERNAL_CHECK_LT_IMPL((val1), #val1, (val2), #val2)
+#define CHECK_GE(val1, val2) \
+  ABSL_LOG_INTERNAL_CHECK_GE_IMPL((val1), #val1, (val2), #val2)
+#define CHECK_GT(val1, val2) \
+  ABSL_LOG_INTERNAL_CHECK_GT_IMPL((val1), #val1, (val2), #val2)
+#define QCHECK_EQ(val1, val2) \
+  ABSL_LOG_INTERNAL_QCHECK_EQ_IMPL((val1), #val1, (val2), #val2)
+#define QCHECK_NE(val1, val2) \
+  ABSL_LOG_INTERNAL_QCHECK_NE_IMPL((val1), #val1, (val2), #val2)
+#define QCHECK_LE(val1, val2) \
+  ABSL_LOG_INTERNAL_QCHECK_LE_IMPL((val1), #val1, (val2), #val2)
+#define QCHECK_LT(val1, val2) \
+  ABSL_LOG_INTERNAL_QCHECK_LT_IMPL((val1), #val1, (val2), #val2)
+#define QCHECK_GE(val1, val2) \
+  ABSL_LOG_INTERNAL_QCHECK_GE_IMPL((val1), #val1, (val2), #val2)
+#define QCHECK_GT(val1, val2) \
+  ABSL_LOG_INTERNAL_QCHECK_GT_IMPL((val1), #val1, (val2), #val2)
+#define DCHECK_EQ(val1, val2) \
+  ABSL_LOG_INTERNAL_DCHECK_EQ_IMPL((val1), #val1, (val2), #val2)
+#define DCHECK_NE(val1, val2) \
+  ABSL_LOG_INTERNAL_DCHECK_NE_IMPL((val1), #val1, (val2), #val2)
+#define DCHECK_LE(val1, val2) \
+  ABSL_LOG_INTERNAL_DCHECK_LE_IMPL((val1), #val1, (val2), #val2)
+#define DCHECK_LT(val1, val2) \
+  ABSL_LOG_INTERNAL_DCHECK_LT_IMPL((val1), #val1, (val2), #val2)
+#define DCHECK_GE(val1, val2) \
+  ABSL_LOG_INTERNAL_DCHECK_GE_IMPL((val1), #val1, (val2), #val2)
+#define DCHECK_GT(val1, val2) \
+  ABSL_LOG_INTERNAL_DCHECK_GT_IMPL((val1), #val1, (val2), #val2)
+
+// `CHECK_OK` and friends validate that the provided `absl::Status` or
+// `absl::StatusOr<T>` is OK.  If it isn't, they print a failure message that
+// includes the actual status and terminate the program.
+//
+// As with all `DCHECK` variants, `DCHECK_OK` has no effect (not even
+// evaluating its argument) if `NDEBUG` is enabled.
+//
+// Example:
+//
+//   CHECK_OK(FunctionReturnsStatus(x, y, z)) << "oops!";
+//
+// Might produce a message like:
+//
+//   Check failed: FunctionReturnsStatus(x, y, z) is OK (ABORTED: timeout) oops!
+#define CHECK_OK(status) ABSL_LOG_INTERNAL_CHECK_OK_IMPL((status), #status)
+#define QCHECK_OK(status) ABSL_LOG_INTERNAL_QCHECK_OK_IMPL((status), #status)
+#define DCHECK_OK(status) ABSL_LOG_INTERNAL_DCHECK_OK_IMPL((status), #status)
+
+// `CHECK_STREQ` and friends provide `CHECK_EQ` functionality for C strings,
+// i.e., null-terminated char arrays.  The `CASE` versions are case-insensitive.
+//
+// Example:
+//
+//   CHECK_STREQ(argv[0], "./skynet");
+//
+// Note that both arguments may be temporary strings which are destroyed by the
+// compiler at the end of the current full expression.
+//
+// Example:
+//
+//   CHECK_STREQ(Foo().c_str(), Bar().c_str());
+#define CHECK_STREQ(s1, s2) \
+  ABSL_LOG_INTERNAL_CHECK_STREQ_IMPL((s1), #s1, (s2), #s2)
+#define CHECK_STRNE(s1, s2) \
+  ABSL_LOG_INTERNAL_CHECK_STRNE_IMPL((s1), #s1, (s2), #s2)
+#define CHECK_STRCASEEQ(s1, s2) \
+  ABSL_LOG_INTERNAL_CHECK_STRCASEEQ_IMPL((s1), #s1, (s2), #s2)
+#define CHECK_STRCASENE(s1, s2) \
+  ABSL_LOG_INTERNAL_CHECK_STRCASENE_IMPL((s1), #s1, (s2), #s2)
+#define QCHECK_STREQ(s1, s2) \
+  ABSL_LOG_INTERNAL_QCHECK_STREQ_IMPL((s1), #s1, (s2), #s2)
+#define QCHECK_STRNE(s1, s2) \
+  ABSL_LOG_INTERNAL_QCHECK_STRNE_IMPL((s1), #s1, (s2), #s2)
+#define QCHECK_STRCASEEQ(s1, s2) \
+  ABSL_LOG_INTERNAL_QCHECK_STRCASEEQ_IMPL((s1), #s1, (s2), #s2)
+#define QCHECK_STRCASENE(s1, s2) \
+  ABSL_LOG_INTERNAL_QCHECK_STRCASENE_IMPL((s1), #s1, (s2), #s2)
+#define DCHECK_STREQ(s1, s2) \
+  ABSL_LOG_INTERNAL_DCHECK_STREQ_IMPL((s1), #s1, (s2), #s2)
+#define DCHECK_STRNE(s1, s2) \
+  ABSL_LOG_INTERNAL_DCHECK_STRNE_IMPL((s1), #s1, (s2), #s2)
+#define DCHECK_STRCASEEQ(s1, s2) \
+  ABSL_LOG_INTERNAL_DCHECK_STRCASEEQ_IMPL((s1), #s1, (s2), #s2)
+#define DCHECK_STRCASENE(s1, s2) \
+  ABSL_LOG_INTERNAL_DCHECK_STRCASENE_IMPL((s1), #s1, (s2), #s2)
+
+#endif  // ABSL_LOG_CHECK_H_
diff --git a/absl/log/check_test.cc b/absl/log/check_test.cc
new file mode 100644
index 00000000000..ef415bd4227
--- /dev/null
+++ b/absl/log/check_test.cc
@@ -0,0 +1,58 @@
+//
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/log/check.h"
+
+#define ABSL_TEST_CHECK CHECK
+#define ABSL_TEST_CHECK_OK CHECK_OK
+#define ABSL_TEST_CHECK_EQ CHECK_EQ
+#define ABSL_TEST_CHECK_NE CHECK_NE
+#define ABSL_TEST_CHECK_GE CHECK_GE
+#define ABSL_TEST_CHECK_LE CHECK_LE
+#define ABSL_TEST_CHECK_GT CHECK_GT
+#define ABSL_TEST_CHECK_LT CHECK_LT
+#define ABSL_TEST_CHECK_STREQ CHECK_STREQ
+#define ABSL_TEST_CHECK_STRNE CHECK_STRNE
+#define ABSL_TEST_CHECK_STRCASEEQ CHECK_STRCASEEQ
+#define ABSL_TEST_CHECK_STRCASENE CHECK_STRCASENE
+
+#define ABSL_TEST_DCHECK DCHECK
+#define ABSL_TEST_DCHECK_OK DCHECK_OK
+#define ABSL_TEST_DCHECK_EQ DCHECK_EQ
+#define ABSL_TEST_DCHECK_NE DCHECK_NE
+#define ABSL_TEST_DCHECK_GE DCHECK_GE
+#define ABSL_TEST_DCHECK_LE DCHECK_LE
+#define ABSL_TEST_DCHECK_GT DCHECK_GT
+#define ABSL_TEST_DCHECK_LT DCHECK_LT
+#define ABSL_TEST_DCHECK_STREQ DCHECK_STREQ
+#define ABSL_TEST_DCHECK_STRNE DCHECK_STRNE
+#define ABSL_TEST_DCHECK_STRCASEEQ DCHECK_STRCASEEQ
+#define ABSL_TEST_DCHECK_STRCASENE DCHECK_STRCASENE
+
+#define ABSL_TEST_QCHECK QCHECK
+#define ABSL_TEST_QCHECK_OK QCHECK_OK
+#define ABSL_TEST_QCHECK_EQ QCHECK_EQ
+#define ABSL_TEST_QCHECK_NE QCHECK_NE
+#define ABSL_TEST_QCHECK_GE QCHECK_GE
+#define ABSL_TEST_QCHECK_LE QCHECK_LE
+#define ABSL_TEST_QCHECK_GT QCHECK_GT
+#define ABSL_TEST_QCHECK_LT QCHECK_LT
+#define ABSL_TEST_QCHECK_STREQ QCHECK_STREQ
+#define ABSL_TEST_QCHECK_STRNE QCHECK_STRNE
+#define ABSL_TEST_QCHECK_STRCASEEQ QCHECK_STRCASEEQ
+#define ABSL_TEST_QCHECK_STRCASENE QCHECK_STRCASENE
+
+#include "gtest/gtest.h"
+#include "absl/log/check_test_impl.inc"
diff --git a/absl/log/check_test_impl.inc b/absl/log/check_test_impl.inc
new file mode 100644
index 00000000000..c78bbd412a7
--- /dev/null
+++ b/absl/log/check_test_impl.inc
@@ -0,0 +1,1033 @@
+//
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// SKIP_ABSL_INLINE_NAMESPACE_CHECK
+
+#ifndef ABSL_LOG_CHECK_TEST_IMPL_H_
+#define ABSL_LOG_CHECK_TEST_IMPL_H_
+
+// Verify that both sets of macros behave identically by parameterizing the
+// entire test file.
+#ifndef ABSL_TEST_CHECK
+#error ABSL_TEST_CHECK must be defined for these tests to work.
+#endif
+
+#include <cstdint>
+#include <limits>
+#include <ostream>
+#include <string>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/log/internal/test_helpers.h"
+#include "absl/status/status.h"
+#include "absl/strings/string_view.h"
+#include "absl/strings/substitute.h"
+
+// NOLINTBEGIN(misc-definitions-in-headers)
+
+namespace absl_log_internal {
+
+using ::testing::AllOf;
+using ::testing::AnyOf;
+using ::testing::ContainsRegex;
+using ::testing::HasSubstr;
+using ::testing::Not;
+
+auto* test_env ABSL_ATTRIBUTE_UNUSED = ::testing::AddGlobalTestEnvironment(
+    new absl::log_internal::LogTestEnvironment);
+
+#if GTEST_HAS_DEATH_TEST
+
+TEST(CHECKDeathTest, TestBasicValues) {
+  ABSL_TEST_CHECK(true);
+
+  EXPECT_DEATH(ABSL_TEST_CHECK(false), "Check failed: false");
+
+  int i = 2;
+  ABSL_TEST_CHECK(i != 3);  // NOLINT
+}
+
+#endif  // GTEST_HAS_DEATH_TEST
+
+TEST(CHECKTest, TestLogicExpressions) {
+  int i = 5;
+  ABSL_TEST_CHECK(i > 0 && i < 10);
+  ABSL_TEST_CHECK(i < 0 || i > 3);
+}
+
+ABSL_CONST_INIT const auto global_var_check = [](int i) {
+  ABSL_TEST_CHECK(i > 0);  // NOLINT
+  return i + 1;
+}(3);
+
+ABSL_CONST_INIT const auto global_var = [](int i) {
+  ABSL_TEST_CHECK_GE(i, 0);  // NOLINT
+  return i + 1;
+}(global_var_check);
+
+TEST(CHECKTest, TestPlacementsInCompoundStatements) {
+  // check placement inside if/else clauses
+  if (true) ABSL_TEST_CHECK(true);
+
+  if (false)
+    ;  // NOLINT
+  else
+    ABSL_TEST_CHECK(true);
+
+  switch (0)
+  case 0:
+    ABSL_TEST_CHECK(true);  // NOLINT
+
+  constexpr auto var = [](int i) {
+    ABSL_TEST_CHECK(i > 0);  // NOLINT
+    return i + 1;
+  }(global_var);
+  (void)var;
+}
+
+TEST(CHECKTest, TestBoolConvertible) {
+  struct Tester {
+  } tester;
+  ABSL_TEST_CHECK([&]() { return &tester; }());
+}
+
+#if GTEST_HAS_DEATH_TEST
+
+TEST(CHECKDeathTest, TestChecksWithSideEffects) {
+  int var = 0;
+  ABSL_TEST_CHECK([&var]() {
+    ++var;
+    return true;
+  }());
+  EXPECT_EQ(var, 1);
+
+  EXPECT_DEATH(ABSL_TEST_CHECK([&var]() {
+                 ++var;
+                 return false;
+               }()) << var,
+               "Check failed: .* 2");
+}
+
+#endif  // GTEST_HAS_DEATH_TEST
+
+template <int a, int b>
+constexpr int sum() {
+  return a + b;
+}
+#define MACRO_ONE 1
+#define TEMPLATE_SUM(a, b) sum<a, b>()
+#define CONCAT(a, b) a b
+#define IDENTITY(x) x
+
+TEST(CHECKTest, TestPassingMacroExpansion) {
+  ABSL_TEST_CHECK(IDENTITY(true));
+  ABSL_TEST_CHECK_EQ(TEMPLATE_SUM(MACRO_ONE, 2), 3);
+  ABSL_TEST_CHECK_STREQ(CONCAT("x", "y"), "xy");
+}
+
+#if GTEST_HAS_DEATH_TEST
+
+TEST(CHECKTest, TestMacroExpansionInMessage) {
+  auto MessageGen = []() { ABSL_TEST_CHECK(IDENTITY(false)); };
+  EXPECT_DEATH(MessageGen(), HasSubstr("IDENTITY(false)"));
+}
+
+TEST(CHECKTest, TestNestedMacroExpansionInMessage) {
+  EXPECT_DEATH(ABSL_TEST_CHECK(IDENTITY(false)), HasSubstr("IDENTITY(false)"));
+}
+
+TEST(CHECKTest, TestMacroExpansionCompare) {
+  EXPECT_DEATH(ABSL_TEST_CHECK_EQ(IDENTITY(false), IDENTITY(true)),
+               HasSubstr("IDENTITY(false) == IDENTITY(true)"));
+  EXPECT_DEATH(ABSL_TEST_CHECK_GT(IDENTITY(1), IDENTITY(2)),
+               HasSubstr("IDENTITY(1) > IDENTITY(2)"));
+}
+
+TEST(CHECKTest, TestMacroExpansionStrCompare) {
+  EXPECT_DEATH(ABSL_TEST_CHECK_STREQ(IDENTITY("x"), IDENTITY("y")),
+               HasSubstr("IDENTITY(\"x\") == IDENTITY(\"y\")"));
+  EXPECT_DEATH(ABSL_TEST_CHECK_STRCASENE(IDENTITY("a"), IDENTITY("A")),
+               HasSubstr("IDENTITY(\"a\") != IDENTITY(\"A\")"));
+}
+
+TEST(CHECKTest, TestMacroExpansionStatus) {
+  EXPECT_DEATH(
+      ABSL_TEST_CHECK_OK(IDENTITY(absl::FailedPreconditionError("message"))),
+      HasSubstr("IDENTITY(absl::FailedPreconditionError(\"message\"))"));
+}
+
+TEST(CHECKTest, TestMacroExpansionComma) {
+  EXPECT_DEATH(ABSL_TEST_CHECK(TEMPLATE_SUM(MACRO_ONE, 2) == 4),
+               HasSubstr("TEMPLATE_SUM(MACRO_ONE, 2) == 4"));
+}
+
+TEST(CHECKTest, TestMacroExpansionCommaCompare) {
+  EXPECT_DEATH(
+      ABSL_TEST_CHECK_EQ(TEMPLATE_SUM(2, MACRO_ONE), TEMPLATE_SUM(3, 2)),
+      HasSubstr("TEMPLATE_SUM(2, MACRO_ONE) == TEMPLATE_SUM(3, 2)"));
+  EXPECT_DEATH(
+      ABSL_TEST_CHECK_GT(TEMPLATE_SUM(2, MACRO_ONE), TEMPLATE_SUM(3, 2)),
+      HasSubstr("TEMPLATE_SUM(2, MACRO_ONE) > TEMPLATE_SUM(3, 2)"));
+}
+
+TEST(CHECKTest, TestMacroExpansionCommaStrCompare) {
+  EXPECT_DEATH(ABSL_TEST_CHECK_STREQ(CONCAT("x", "y"), "z"),
+               HasSubstr("CONCAT(\"x\", \"y\") == \"z\""));
+  EXPECT_DEATH(ABSL_TEST_CHECK_STRNE(CONCAT("x", "y"), "xy"),
+               HasSubstr("CONCAT(\"x\", \"y\") != \"xy\""));
+}
+
+#endif  // GTEST_HAS_DEATH_TEST
+
+#undef TEMPLATE_SUM
+#undef CONCAT
+#undef MACRO
+#undef ONE
+
+#if GTEST_HAS_DEATH_TEST
+
+TEST(CHECKDeachTest, TestOrderOfInvocationsBetweenCheckAndMessage) {
+  int counter = 0;
+
+  auto GetStr = [&counter]() -> std::string {
+    return counter++ == 0 ? "" : "non-empty";
+  };
+
+  EXPECT_DEATH(ABSL_TEST_CHECK(!GetStr().empty()) << GetStr(),
+               HasSubstr("non-empty"));
+}
+
+TEST(CHECKTest, TestSecondaryFailure) {
+  auto FailingRoutine = []() {
+    ABSL_TEST_CHECK(false) << "Secondary";
+    return false;
+  };
+  EXPECT_DEATH(ABSL_TEST_CHECK(FailingRoutine()) << "Primary",
+               AllOf(HasSubstr("Secondary"), Not(HasSubstr("Primary"))));
+}
+
+TEST(CHECKTest, TestSecondaryFailureInMessage) {
+  auto MessageGen = []() {
+    ABSL_TEST_CHECK(false) << "Secondary";
+    return "Primary";
+  };
+  EXPECT_DEATH(ABSL_TEST_CHECK(false) << MessageGen(),
+               AllOf(HasSubstr("Secondary"), Not(HasSubstr("Primary"))));
+}
+
+#endif  // GTEST_HAS_DEATH_TEST
+
+TEST(CHECKTest, TestBinaryChecksWithPrimitives) {
+  ABSL_TEST_CHECK_EQ(1, 1);
+  ABSL_TEST_CHECK_NE(1, 2);
+  ABSL_TEST_CHECK_GE(1, 1);
+  ABSL_TEST_CHECK_GE(2, 1);
+  ABSL_TEST_CHECK_LE(1, 1);
+  ABSL_TEST_CHECK_LE(1, 2);
+  ABSL_TEST_CHECK_GT(2, 1);
+  ABSL_TEST_CHECK_LT(1, 2);
+}
+
+TEST(CHECKTest, TestBinaryChecksWithStringComparison) {
+  const std::string a = "a";
+  ABSL_TEST_CHECK_EQ(a, "a");
+  ABSL_TEST_CHECK_NE(a, "b");
+  ABSL_TEST_CHECK_GE(a, a);
+  ABSL_TEST_CHECK_GE("b", a);
+  ABSL_TEST_CHECK_LE(a, "a");
+  ABSL_TEST_CHECK_LE(a, "b");
+  ABSL_TEST_CHECK_GT("b", a);
+  ABSL_TEST_CHECK_LT(a, "b");
+}
+
+TEST(CHECKDeathTest, CheckWithCharStarAndStringPrintsTheCharStar) {
+  std::string str = "B";
+
+  // When the comparison happens as strings, then we print the CharT* as a
+  // string.
+  EXPECT_DEATH(ABSL_TEST_CHECK_EQ("A", str),
+               R"re(Check failed: \"A\" == str \(A vs. B\))re");
+}
+
+#if defined(GTEST_USES_SIMPLE_RE) && GTEST_USES_SIMPLE_RE
+#define POINTER_VALUE_RE R"re(\w*)re"
+#else
+#define POINTER_VALUE_RE R"re((0x)*[0-9a-fA-F]*)re"
+#endif
+
+#define POINTER_VS_POINTER_RE(lhs, rhs) "\\(" lhs " vs. " rhs "\\)"
+
+template <typename CharT>
+void TestCharStarComparison() {
+  // When the comparison happens as pointers, we only print the pointer and not
+  // interpret it as a C-String because it might not be.
+  // Leave the CharTs uninitialized to trigger ASan/MSan failures if we actually
+  // read the pointers.
+  CharT* p1 = new CharT;
+  CharT* p2 = new CharT;
+  EXPECT_DEATH(ABSL_TEST_CHECK_EQ(p1, p2),
+               R"re(Check failed: p1 == p2 )re" POINTER_VS_POINTER_RE(
+                   POINTER_VALUE_RE, POINTER_VALUE_RE));
+  CharT as_array[10];
+  EXPECT_DEATH(ABSL_TEST_CHECK_EQ(p1, as_array),
+               R"re(Check failed: p1 == as_array )re" POINTER_VS_POINTER_RE(
+                   POINTER_VALUE_RE, POINTER_VALUE_RE));
+
+  const void* as_void = as_array;
+  EXPECT_DEATH(ABSL_TEST_CHECK_EQ(as_void, p2),
+               R"re(Check failed: as_void == p2 )re" POINTER_VS_POINTER_RE(
+                   POINTER_VALUE_RE, POINTER_VALUE_RE));
+
+  delete p1;
+  delete p2;
+}
+
+TEST(CHECKDeathTest, CheckWithCharStarStringification) {
+  TestCharStarComparison<char>();
+  TestCharStarComparison<signed char>();
+  TestCharStarComparison<unsigned char>();
+  TestCharStarComparison<wchar_t>();
+#if defined(__cpp_char8_t)
+  TestCharStarComparison<char8_t>();
+#endif
+  TestCharStarComparison<char16_t>();
+  TestCharStarComparison<char32_t>();
+}
+
+// For testing using CHECK*() on anonymous enums.
+enum { CASE_A, CASE_B };
+
+TEST(CHECKTest, TestBinaryChecksWithEnumValues) {
+  // Tests using CHECK*() on anonymous enums.
+  ABSL_TEST_CHECK_EQ(CASE_A, CASE_A);
+  ABSL_TEST_CHECK_NE(CASE_A, CASE_B);
+  ABSL_TEST_CHECK_GE(CASE_A, CASE_A);
+  ABSL_TEST_CHECK_GE(CASE_B, CASE_A);
+  ABSL_TEST_CHECK_LE(CASE_A, CASE_A);
+  ABSL_TEST_CHECK_LE(CASE_A, CASE_B);
+  ABSL_TEST_CHECK_GT(CASE_B, CASE_A);
+  ABSL_TEST_CHECK_LT(CASE_A, CASE_B);
+}
+
+TEST(CHECKTest, TestBinaryChecksWithNullptr) {
+  const void* p_null = nullptr;
+  const void* p_not_null = &p_null;
+  ABSL_TEST_CHECK_EQ(p_null, nullptr);
+  ABSL_TEST_CHECK_EQ(nullptr, p_null);
+  ABSL_TEST_CHECK_NE(p_not_null, nullptr);
+  ABSL_TEST_CHECK_NE(nullptr, p_not_null);
+}
+
+struct ExampleTypeThatHasNoStreamOperator {
+  bool x;
+
+  bool operator==(const ExampleTypeThatHasNoStreamOperator& other) const {
+    return x == other.x;
+  }
+  bool operator==(const bool& other) const { return x == other; }
+};
+
+TEST(CHECKDeathTest, TestBinaryChecksWithUnprintable) {
+  ExampleTypeThatHasNoStreamOperator a{true};
+  ExampleTypeThatHasNoStreamOperator b{false};
+  ABSL_TEST_CHECK_EQ(a, a);
+  EXPECT_DEATH(ABSL_TEST_CHECK_EQ(a, b), "Check failed: a == b");
+  ABSL_TEST_CHECK_EQ(a, true);
+  EXPECT_DEATH(ABSL_TEST_CHECK_EQ(a, false),
+               "Check failed: a == false \\(UNPRINTABLE vs. 0\\)");
+}
+
+#if GTEST_HAS_DEATH_TEST
+
+// Test logging of various char-typed values by failing CHECK*().
+TEST(CHECKDeathTest, TestComparingCharsValues) {
+  {
+    char a = ';';
+    char b = 'b';
+    EXPECT_DEATH(ABSL_TEST_CHECK_EQ(a, b),
+                 "Check failed: a == b \\(';' vs. 'b'\\)");
+    b = 1;
+    EXPECT_DEATH(ABSL_TEST_CHECK_EQ(a, b),
+                 "Check failed: a == b \\(';' vs. char value 1\\)");
+  }
+  {
+    signed char a = ';';
+    signed char b = 'b';
+    EXPECT_DEATH(ABSL_TEST_CHECK_EQ(a, b),
+                 "Check failed: a == b \\(';' vs. 'b'\\)");
+    b = -128;
+    EXPECT_DEATH(ABSL_TEST_CHECK_EQ(a, b),
+                 "Check failed: a == b \\(';' vs. signed char value -128\\)");
+  }
+  {
+    unsigned char a = ';';
+    unsigned char b = 'b';
+    EXPECT_DEATH(ABSL_TEST_CHECK_EQ(a, b),
+                 "Check failed: a == b \\(';' vs. 'b'\\)");
+    b = 128;
+    EXPECT_DEATH(ABSL_TEST_CHECK_EQ(a, b),
+                 "Check failed: a == b \\(';' vs. unsigned char value 128\\)");
+  }
+}
+
+TEST(CHECKDeathTest, TestNullValuesAreReportedCleanly) {
+  const char* a = nullptr;
+  const char* b = nullptr;
+  EXPECT_DEATH(ABSL_TEST_CHECK_NE(a, b),
+               "Check failed: a != b \\(\\(null\\) vs. \\(null\\)\\)");
+
+  a = "xx";
+  EXPECT_DEATH(ABSL_TEST_CHECK_EQ(a, b),
+               R"re(Check failed: a == b )re" POINTER_VS_POINTER_RE(
+                   POINTER_VALUE_RE, "\\(null\\)"));
+  EXPECT_DEATH(ABSL_TEST_CHECK_EQ(b, a),
+               R"re(Check failed: b == a )re" POINTER_VS_POINTER_RE(
+                   "\\(null\\)", POINTER_VALUE_RE));
+
+  std::nullptr_t n{};
+  EXPECT_DEATH(ABSL_TEST_CHECK_NE(n, nullptr),
+               "Check failed: n != nullptr \\(\\(null\\) vs. \\(null\\)\\)");
+}
+
+#endif  // GTEST_HAS_DEATH_TEST
+
+TEST(CHECKTest, TestSTREQ) {
+  ABSL_TEST_CHECK_STREQ("this", "this");
+  ABSL_TEST_CHECK_STREQ(nullptr, nullptr);
+  ABSL_TEST_CHECK_STRCASEEQ("this", "tHiS");
+  ABSL_TEST_CHECK_STRCASEEQ(nullptr, nullptr);
+  ABSL_TEST_CHECK_STRNE("this", "tHiS");
+  ABSL_TEST_CHECK_STRNE("this", nullptr);
+  ABSL_TEST_CHECK_STRCASENE("this", "that");
+  ABSL_TEST_CHECK_STRCASENE(nullptr, "that");
+  ABSL_TEST_CHECK_STREQ((std::string("a") + "b").c_str(), "ab");
+  ABSL_TEST_CHECK_STREQ(std::string("test").c_str(),
+                        (std::string("te") + std::string("st")).c_str());
+}
+
+TEST(CHECKTest, TestComparisonPlacementsInCompoundStatements) {
+  // check placement inside if/else clauses
+  if (true) ABSL_TEST_CHECK_EQ(1, 1);
+  if (true) ABSL_TEST_CHECK_STREQ("c", "c");
+
+  if (false)
+    ;  // NOLINT
+  else
+    ABSL_TEST_CHECK_LE(0, 1);
+
+  if (false)
+    ;  // NOLINT
+  else
+    ABSL_TEST_CHECK_STRNE("a", "b");
+
+  switch (0)
+  case 0:
+    ABSL_TEST_CHECK_NE(1, 0);
+
+  switch (0)
+  case 0:
+    ABSL_TEST_CHECK_STRCASEEQ("A", "a");
+
+  constexpr auto var = [](int i) {
+    ABSL_TEST_CHECK_GT(i, 0);
+    return i + 1;
+  }(global_var);
+  (void)var;
+
+  // CHECK_STR... checks are not supported in constexpr routines.
+  // constexpr auto var2 = [](int i) {
+  //  ABSL_TEST_CHECK_STRNE("c", "d");
+  //  return i + 1;
+  // }(global_var);
+
+#if defined(__GNUC__)
+  int var3 = (({ ABSL_TEST_CHECK_LE(1, 2); }), global_var < 10) ? 1 : 0;
+  (void)var3;
+
+  int var4 = (({ ABSL_TEST_CHECK_STREQ("a", "a"); }), global_var < 10) ? 1 : 0;
+  (void)var4;
+#endif  // __GNUC__
+}
+
+TEST(CHECKTest, TestDCHECK) {
+#ifdef NDEBUG
+  ABSL_TEST_DCHECK(1 == 2) << " DCHECK's shouldn't be compiled in normal mode";
+#endif
+  ABSL_TEST_DCHECK(1 == 1);  // NOLINT(readability/check)
+  ABSL_TEST_DCHECK_EQ(1, 1);
+  ABSL_TEST_DCHECK_NE(1, 2);
+  ABSL_TEST_DCHECK_GE(1, 1);
+  ABSL_TEST_DCHECK_GE(2, 1);
+  ABSL_TEST_DCHECK_LE(1, 1);
+  ABSL_TEST_DCHECK_LE(1, 2);
+  ABSL_TEST_DCHECK_GT(2, 1);
+  ABSL_TEST_DCHECK_LT(1, 2);
+
+  // Test DCHECK on std::nullptr_t
+  const void* p_null = nullptr;
+  const void* p_not_null = &p_null;
+  ABSL_TEST_DCHECK_EQ(p_null, nullptr);
+  ABSL_TEST_DCHECK_EQ(nullptr, p_null);
+  ABSL_TEST_DCHECK_NE(p_not_null, nullptr);
+  ABSL_TEST_DCHECK_NE(nullptr, p_not_null);
+}
+
+TEST(CHECKTest, TestQCHECK) {
+  // The tests that QCHECK does the same as CHECK
+  ABSL_TEST_QCHECK(1 == 1);  // NOLINT(readability/check)
+  ABSL_TEST_QCHECK_EQ(1, 1);
+  ABSL_TEST_QCHECK_NE(1, 2);
+  ABSL_TEST_QCHECK_GE(1, 1);
+  ABSL_TEST_QCHECK_GE(2, 1);
+  ABSL_TEST_QCHECK_LE(1, 1);
+  ABSL_TEST_QCHECK_LE(1, 2);
+  ABSL_TEST_QCHECK_GT(2, 1);
+  ABSL_TEST_QCHECK_LT(1, 2);
+
+  // Tests using QCHECK*() on anonymous enums.
+  ABSL_TEST_QCHECK_EQ(CASE_A, CASE_A);
+  ABSL_TEST_QCHECK_NE(CASE_A, CASE_B);
+  ABSL_TEST_QCHECK_GE(CASE_A, CASE_A);
+  ABSL_TEST_QCHECK_GE(CASE_B, CASE_A);
+  ABSL_TEST_QCHECK_LE(CASE_A, CASE_A);
+  ABSL_TEST_QCHECK_LE(CASE_A, CASE_B);
+  ABSL_TEST_QCHECK_GT(CASE_B, CASE_A);
+  ABSL_TEST_QCHECK_LT(CASE_A, CASE_B);
+}
+
+TEST(CHECKTest, TestQCHECKPlacementsInCompoundStatements) {
+  // check placement inside if/else clauses
+  if (true) ABSL_TEST_QCHECK(true);
+
+  if (false)
+    ;  // NOLINT
+  else
+    ABSL_TEST_QCHECK(true);
+
+  if (false)
+    ;  // NOLINT
+  else
+    ABSL_TEST_QCHECK(true);
+
+  switch (0)
+  case 0:
+    ABSL_TEST_QCHECK(true);
+
+  constexpr auto var = [](int i) {
+    ABSL_TEST_QCHECK(i > 0);  // NOLINT
+    return i + 1;
+  }(global_var);
+  (void)var;
+
+#if defined(__GNUC__)
+  int var2 = (({ ABSL_TEST_CHECK_LE(1, 2); }), global_var < 10) ? 1 : 0;
+  (void)var2;
+#endif  // __GNUC__
+}
+
+class ComparableType {
+ public:
+  explicit ComparableType(int v) : v_(v) {}
+
+  void MethodWithCheck(int i) {
+    ABSL_TEST_CHECK_EQ(*this, i);
+    ABSL_TEST_CHECK_EQ(i, *this);
+  }
+
+  int Get() const { return v_; }
+
+ private:
+  friend bool operator==(const ComparableType& lhs, const ComparableType& rhs) {
+    return lhs.v_ == rhs.v_;
+  }
+  friend bool operator!=(const ComparableType& lhs, const ComparableType& rhs) {
+    return lhs.v_ != rhs.v_;
+  }
+  friend bool operator<(const ComparableType& lhs, const ComparableType& rhs) {
+    return lhs.v_ < rhs.v_;
+  }
+  friend bool operator<=(const ComparableType& lhs, const ComparableType& rhs) {
+    return lhs.v_ <= rhs.v_;
+  }
+  friend bool operator>(const ComparableType& lhs, const ComparableType& rhs) {
+    return lhs.v_ > rhs.v_;
+  }
+  friend bool operator>=(const ComparableType& lhs, const ComparableType& rhs) {
+    return lhs.v_ >= rhs.v_;
+  }
+  friend bool operator==(const ComparableType& lhs, int rhs) {
+    return lhs.v_ == rhs;
+  }
+  friend bool operator==(int lhs, const ComparableType& rhs) {
+    return lhs == rhs.v_;
+  }
+
+  friend std::ostream& operator<<(std::ostream& out, const ComparableType& v) {
+    return out << "ComparableType{" << v.Get() << "}";
+  }
+
+  int v_;
+};
+
+TEST(CHECKTest, TestUserDefinedCompOp) {
+  ABSL_TEST_CHECK_EQ(ComparableType{0}, ComparableType{0});
+  ABSL_TEST_CHECK_NE(ComparableType{1}, ComparableType{2});
+  ABSL_TEST_CHECK_LT(ComparableType{1}, ComparableType{2});
+  ABSL_TEST_CHECK_LE(ComparableType{1}, ComparableType{2});
+  ABSL_TEST_CHECK_GT(ComparableType{2}, ComparableType{1});
+  ABSL_TEST_CHECK_GE(ComparableType{2}, ComparableType{2});
+}
+
+TEST(CHECKTest, TestCheckInMethod) {
+  ComparableType v{1};
+  v.MethodWithCheck(1);
+}
+
+TEST(CHECKDeathTest, TestUserDefinedStreaming) {
+  ComparableType v1{1};
+  ComparableType v2{2};
+
+  EXPECT_DEATH(
+      ABSL_TEST_CHECK_EQ(v1, v2),
+      HasSubstr(
+          "Check failed: v1 == v2 (ComparableType{1} vs. ComparableType{2})"));
+}
+
+// A type that can be printed using AbslStringify.
+struct StringifiableType {
+  int x = 0;
+  explicit StringifiableType(int x) : x(x) {}
+  friend bool operator==(const StringifiableType& lhs,
+                         const StringifiableType& rhs) {
+    return lhs.x == rhs.x;
+  }
+  friend bool operator!=(const StringifiableType& lhs,
+                         const StringifiableType& rhs) {
+    return lhs.x != rhs.x;
+  }
+  friend bool operator<(const StringifiableType& lhs,
+                        const StringifiableType& rhs) {
+    return lhs.x < rhs.x;
+  }
+  friend bool operator>(const StringifiableType& lhs,
+                        const StringifiableType& rhs) {
+    return lhs.x > rhs.x;
+  }
+  friend bool operator<=(const StringifiableType& lhs,
+                         const StringifiableType& rhs) {
+    return lhs.x <= rhs.x;
+  }
+  friend bool operator>=(const StringifiableType& lhs,
+                         const StringifiableType& rhs) {
+    return lhs.x >= rhs.x;
+  }
+  template <typename Sink>
+  friend void AbslStringify(Sink& sink, const StringifiableType& obj) {
+    absl::Format(&sink, "StringifiableType{%d}", obj.x);
+  }
+
+  // Make sure no unintended copy happens.
+  StringifiableType(const StringifiableType&) = delete;
+};
+
+TEST(CHECKTest, TestUserDefinedAbslStringify) {
+  const StringifiableType v1(1);
+  const StringifiableType v2(2);
+
+  ABSL_TEST_CHECK_EQ(v1, v1);
+  ABSL_TEST_CHECK_NE(v1, v2);
+  ABSL_TEST_CHECK_LT(v1, v2);
+  ABSL_TEST_CHECK_LE(v1, v2);
+  ABSL_TEST_CHECK_GT(v2, v1);
+  ABSL_TEST_CHECK_GE(v2, v1);
+}
+
+TEST(CHECKDeathTest, TestUserDefinedAbslStringify) {
+  const StringifiableType v1(1);
+  const StringifiableType v2(2);
+
+  // Returns a matcher for the expected check failure message when comparing two
+  // values.
+  auto expected_output = [](int lhs, absl::string_view condition, int rhs) {
+    return HasSubstr(
+        absl::Substitute("Check failed: v$0 $1 v$2 (StringifiableType{$0} vs. "
+                         "StringifiableType{$2})",
+                         lhs, condition, rhs));
+  };
+  // Test comparisons where the check fails.
+  EXPECT_DEATH(ABSL_TEST_CHECK_EQ(v1, v2), expected_output(1, "==", 2));
+  EXPECT_DEATH(ABSL_TEST_CHECK_NE(v1, v1), expected_output(1, "!=", 1));
+  EXPECT_DEATH(ABSL_TEST_CHECK_LT(v2, v1), expected_output(2, "<", 1));
+  EXPECT_DEATH(ABSL_TEST_CHECK_LE(v2, v1), expected_output(2, "<=", 1));
+  EXPECT_DEATH(ABSL_TEST_CHECK_GT(v1, v2), expected_output(1, ">", 2));
+  EXPECT_DEATH(ABSL_TEST_CHECK_GE(v1, v2), expected_output(1, ">=", 2));
+}
+
+// A type that can be printed using both AbslStringify and operator<<.
+struct StringifiableStreamableType {
+  int x = 0;
+  explicit StringifiableStreamableType(int x) : x(x) {}
+
+  friend bool operator==(const StringifiableStreamableType& lhs,
+                         const StringifiableStreamableType& rhs) {
+    return lhs.x == rhs.x;
+  }
+  friend bool operator!=(const StringifiableStreamableType& lhs,
+                         const StringifiableStreamableType& rhs) {
+    return lhs.x != rhs.x;
+  }
+  template <typename Sink>
+  friend void AbslStringify(Sink& sink,
+                            const StringifiableStreamableType& obj) {
+    absl::Format(&sink, "Strigified{%d}", obj.x);
+  }
+  friend std::ostream& operator<<(std::ostream& out,
+                                  const StringifiableStreamableType& obj) {
+    return out << "Streamed{" << obj.x << "}";
+  }
+
+  // Avoid unintentional copy.
+  StringifiableStreamableType(const StringifiableStreamableType&) = delete;
+};
+
+TEST(CHECKDeathTest, TestStreamingPreferredOverAbslStringify) {
+  StringifiableStreamableType v1(1);
+  StringifiableStreamableType v2(2);
+
+  EXPECT_DEATH(
+      ABSL_TEST_CHECK_EQ(v1, v2),
+      HasSubstr("Check failed: v1 == v2 (Streamed{1} vs. Streamed{2})"));
+}
+
+// A type whose pointer can be passed to AbslStringify.
+struct PointerIsStringifiable {};
+template <typename Sink>
+void AbslStringify(Sink& sink, const PointerIsStringifiable* var) {
+  sink.Append("PointerIsStringifiable");
+}
+
+// Verifies that a pointer is printed as a number despite having AbslStringify
+// defined. Users may implement AbslStringify that dereferences the pointer, and
+// doing so as part of DCHECK would not be good.
+TEST(CHECKDeathTest, TestPointerPrintedAsNumberDespiteAbslStringify) {
+  const auto* p = reinterpret_cast<const PointerIsStringifiable*>(0x1234);
+
+  EXPECT_DEATH(ABSL_TEST_CHECK_EQ(p, nullptr),
+               ContainsRegex(
+                   "Check failed: p == nullptr \\(0+x?1234 vs. \\(null\\)\\)"));
+}
+
+// An uncopyable object with operator<<.
+struct Uncopyable {
+  int x;
+  explicit Uncopyable(int x) : x(x) {}
+  Uncopyable(const Uncopyable&) = delete;
+  friend bool operator==(const Uncopyable& lhs, const Uncopyable& rhs) {
+    return lhs.x == rhs.x;
+  }
+  friend bool operator!=(const Uncopyable& lhs, const Uncopyable& rhs) {
+    return lhs.x != rhs.x;
+  }
+  friend std::ostream& operator<<(std::ostream& os, const Uncopyable& obj) {
+    return os << "Uncopyable{" << obj.x << "}";
+  }
+};
+
+// Test that an uncopyable object can be used.
+// Will catch us if implementation has an unintended copy.
+TEST(CHECKDeathTest, TestUncopyable) {
+  const Uncopyable v1(1);
+  const Uncopyable v2(2);
+
+  EXPECT_DEATH(
+      ABSL_TEST_CHECK_EQ(v1, v2),
+      HasSubstr("Check failed: v1 == v2 (Uncopyable{1} vs. Uncopyable{2})"));
+}
+
+enum class ScopedEnum { kValue1 = 1, kValue2 = 2 };
+
+TEST(CHECKTest, TestScopedEnumComparisonChecks) {
+  ABSL_TEST_CHECK_EQ(ScopedEnum::kValue1, ScopedEnum::kValue1);
+  ABSL_TEST_CHECK_NE(ScopedEnum::kValue1, ScopedEnum::kValue2);
+  ABSL_TEST_CHECK_LT(ScopedEnum::kValue1, ScopedEnum::kValue2);
+  ABSL_TEST_CHECK_LE(ScopedEnum::kValue1, ScopedEnum::kValue2);
+  ABSL_TEST_CHECK_GT(ScopedEnum::kValue2, ScopedEnum::kValue1);
+  ABSL_TEST_CHECK_GE(ScopedEnum::kValue2, ScopedEnum::kValue2);
+  ABSL_TEST_DCHECK_EQ(ScopedEnum::kValue1, ScopedEnum::kValue1);
+  ABSL_TEST_DCHECK_NE(ScopedEnum::kValue1, ScopedEnum::kValue2);
+  ABSL_TEST_DCHECK_LT(ScopedEnum::kValue1, ScopedEnum::kValue2);
+  ABSL_TEST_DCHECK_LE(ScopedEnum::kValue1, ScopedEnum::kValue2);
+  ABSL_TEST_DCHECK_GT(ScopedEnum::kValue2, ScopedEnum::kValue1);
+  ABSL_TEST_DCHECK_GE(ScopedEnum::kValue2, ScopedEnum::kValue2);
+
+  // Check that overloads work correctly with references as well.
+  const ScopedEnum x = ScopedEnum::kValue1;
+  const ScopedEnum& x_ref = x;
+  ABSL_TEST_CHECK_EQ(x, x_ref);
+  ABSL_TEST_CHECK_EQ(x_ref, x_ref);
+}
+
+#if GTEST_HAS_DEATH_TEST
+TEST(CHECKDeathTest, TestScopedEnumCheckFailureMessagePrintsIntegerValues) {
+  const auto e1 = ScopedEnum::kValue1;
+  const auto e2 = ScopedEnum::kValue2;
+  EXPECT_DEATH(ABSL_TEST_CHECK_EQ(e1, e2),
+               ContainsRegex(R"re(Check failed:.*\(1 vs. 2\))re"));
+  EXPECT_DEATH(ABSL_TEST_CHECK_NE(e1, e1),
+               ContainsRegex(R"re(Check failed:.*\(1 vs. 1\))re"));
+  EXPECT_DEATH(ABSL_TEST_CHECK_GT(e1, e1),
+               ContainsRegex(R"re(Check failed:.*\(1 vs. 1\))re"));
+  EXPECT_DEATH(ABSL_TEST_CHECK_GE(e1, e2),
+               ContainsRegex(R"re(Check failed:.*\(1 vs. 2\))re"));
+  EXPECT_DEATH(ABSL_TEST_CHECK_LT(e2, e2),
+               ContainsRegex(R"re(Check failed:.*\(2 vs. 2\))re"));
+  EXPECT_DEATH(ABSL_TEST_CHECK_LE(e2, e1),
+               ContainsRegex(R"re(Check failed:.*\(2 vs. 1\))re"));
+
+  const auto& e1_ref = e1;
+  EXPECT_DEATH(ABSL_TEST_CHECK_NE(e1_ref, e1),
+               ContainsRegex(R"re(Check failed:.*\(1 vs. 1\))re"));
+  EXPECT_DEATH(ABSL_TEST_CHECK_NE(e1_ref, e1_ref),
+               ContainsRegex(R"re(Check failed:.*\(1 vs. 1\))re"));
+  EXPECT_DEATH(ABSL_TEST_CHECK_EQ(e2, e1_ref),
+               ContainsRegex(R"re(Check failed:.*\(2 vs. 1\))re"));
+
+#ifndef NDEBUG
+  EXPECT_DEATH(ABSL_TEST_DCHECK_EQ(e2, e1),
+               ContainsRegex(R"re(Check failed:.*\(2 vs. 1\))re"));
+#else
+  // DHECK_EQ is not evaluated in non-debug mode.
+  ABSL_TEST_DCHECK_EQ(e2, e1);
+#endif  // NDEBUG
+}
+#endif  // GTEST_HAS_DEATH_TEST
+
+enum class ScopedInt8Enum : int8_t {
+  kValue1 = 1,
+  kValue2 = 66  // Printable ascii value 'B'.
+};
+
+TEST(CHECKDeathTest, TestScopedInt8EnumCheckFailureMessagePrintsCharValues) {
+  const auto e1 = ScopedInt8Enum::kValue1;
+  const auto e2 = ScopedInt8Enum::kValue2;
+  EXPECT_DEATH(
+      ABSL_TEST_CHECK_EQ(e1, e2),
+      ContainsRegex(R"re(Check failed:.*\(signed char value 1 vs. 'B'\))re"));
+  EXPECT_DEATH(
+      ABSL_TEST_CHECK_NE(e1, e1),
+      ContainsRegex(
+          R"re(Check failed:.*\(signed char value 1 vs. signed char value 1\))re"));
+  EXPECT_DEATH(
+      ABSL_TEST_CHECK_GT(e1, e1),
+      ContainsRegex(
+          R"re(Check failed:.*\(signed char value 1 vs. signed char value 1\))re"));
+  EXPECT_DEATH(
+      ABSL_TEST_CHECK_GE(e1, e2),
+      ContainsRegex(R"re(Check failed:.*\(signed char value 1 vs. 'B'\))re"));
+  EXPECT_DEATH(ABSL_TEST_CHECK_LT(e2, e2),
+               ContainsRegex(R"re(Check failed:.*\('B' vs. 'B'\))re"));
+  EXPECT_DEATH(
+      ABSL_TEST_CHECK_LE(e2, e1),
+      ContainsRegex(R"re(Check failed:.*\('B' vs. signed char value 1\))re"));
+}
+
+enum class ScopedUnsignedEnum : uint16_t {
+  kValue1 = std::numeric_limits<uint16_t>::min(),
+  kValue2 = std::numeric_limits<uint16_t>::max()
+};
+
+TEST(CHECKDeathTest,
+     TestScopedUnsignedEnumCheckFailureMessagePrintsCorrectValues) {
+  const auto e1 = ScopedUnsignedEnum::kValue1;
+  const auto e2 = ScopedUnsignedEnum::kValue2;
+  EXPECT_DEATH(ABSL_TEST_CHECK_EQ(e1, e2),
+               ContainsRegex(R"re(Check failed:.*\(0 vs. 65535\))re"));
+  EXPECT_DEATH(ABSL_TEST_CHECK_NE(e1, e1),
+               ContainsRegex(R"re(Check failed:.*\(0 vs. 0\))re"));
+  EXPECT_DEATH(ABSL_TEST_CHECK_GT(e1, e1),
+               ContainsRegex(R"re(Check failed:.*\(0 vs. 0\))re"));
+  EXPECT_DEATH(ABSL_TEST_CHECK_GE(e1, e2),
+               ContainsRegex(R"re(Check failed:.*\(0 vs. 65535\))re"));
+  EXPECT_DEATH(ABSL_TEST_CHECK_LT(e1, e1),
+               ContainsRegex(R"re(Check failed:.*\(0 vs. 0\))re"));
+  EXPECT_DEATH(ABSL_TEST_CHECK_LE(e2, e1),
+               ContainsRegex(R"re(Check failed:.*\(65535 vs. 0\))re"));
+}
+
+enum class ScopedInt64Enum : int64_t {
+  kMin = std::numeric_limits<int64_t>::min(),
+  kMax = std::numeric_limits<int64_t>::max(),
+};
+
+// Tests that int64-backed enums are printed correctly even for very large and
+// very small values.
+TEST(CHECKDeathTest, TestScopedInt64EnumCheckFailureMessage) {
+  const auto min = ScopedInt64Enum::kMin;
+  const auto max = ScopedInt64Enum::kMax;
+  EXPECT_DEATH(
+      ABSL_TEST_CHECK_EQ(max, min),
+      ContainsRegex(
+          "Check failed:.*9223372036854775807 vs. -9223372036854775808"));
+  EXPECT_DEATH(
+      ABSL_TEST_CHECK_NE(max, max),
+      ContainsRegex(
+          "Check failed:.*9223372036854775807 vs. 9223372036854775807"));
+  EXPECT_DEATH(
+      ABSL_TEST_CHECK_GT(min, min),
+      ContainsRegex(
+          "Check failed:.*-9223372036854775808 vs. -9223372036854775808"));
+  EXPECT_DEATH(
+      ABSL_TEST_CHECK_GE(min, max),
+      ContainsRegex(
+          R"(Check failed:.*-9223372036854775808 vs. 9223372036854775807)"));
+  EXPECT_DEATH(
+      ABSL_TEST_CHECK_LT(max, max),
+      ContainsRegex(
+          R"(Check failed:.*9223372036854775807 vs. 9223372036854775807)"));
+  EXPECT_DEATH(
+      ABSL_TEST_CHECK_LE(max, min),
+      ContainsRegex(
+          R"(Check failed:.*9223372036854775807 vs. -9223372036854775808)"));
+}
+
+enum class ScopedBoolEnum : bool {
+  kFalse,
+  kTrue,
+};
+
+TEST(CHECKDeathTest, TestScopedBoolEnumCheckFailureMessagePrintsCorrectValues) {
+  const auto t = ScopedBoolEnum::kTrue;
+  const auto f = ScopedBoolEnum::kFalse;
+  EXPECT_DEATH(ABSL_TEST_CHECK_EQ(t, f),
+               ContainsRegex(R"re(Check failed:.*\(1 vs. 0\))re"));
+  EXPECT_DEATH(ABSL_TEST_CHECK_NE(f, f),
+               ContainsRegex(R"re(Check failed:.*\(0 vs. 0\))re"));
+  EXPECT_DEATH(ABSL_TEST_CHECK_GT(f, f),
+               ContainsRegex(R"re(Check failed:.*\(0 vs. 0\))re"));
+  EXPECT_DEATH(ABSL_TEST_CHECK_GE(f, t),
+               ContainsRegex(R"re(Check failed:.*\(0 vs. 1\))re"));
+  EXPECT_DEATH(ABSL_TEST_CHECK_LT(t, t),
+               ContainsRegex(R"re(Check failed:.*\(1 vs. 1\))re"));
+  EXPECT_DEATH(ABSL_TEST_CHECK_LE(t, f),
+               ContainsRegex(R"re(Check failed:.*\(1 vs. 0\))re"));
+}
+
+enum class ScopedEnumWithAbslStringify {
+  kValue1 = 1,
+  kValue2 = 2,
+  kValue3 = 3
+};
+
+template <typename Sink>
+void AbslStringify(Sink& sink, ScopedEnumWithAbslStringify v) {
+  switch (v) {
+    case ScopedEnumWithAbslStringify::kValue1:
+      sink.Append("AbslStringify: kValue1");
+      break;
+    case ScopedEnumWithAbslStringify::kValue2:
+      sink.Append("AbslStringify: kValue2");
+      break;
+    case ScopedEnumWithAbslStringify::kValue3:
+      sink.Append("AbslStringify: kValue3");
+      break;
+  }
+}
+
+#if GTEST_HAS_DEATH_TEST
+TEST(CHECKDeathTest, TestScopedEnumUsesAbslStringify) {
+  EXPECT_DEATH(ABSL_TEST_CHECK_EQ(ScopedEnumWithAbslStringify::kValue1,
+                                  ScopedEnumWithAbslStringify::kValue2),
+               ContainsRegex("Check failed:.*AbslStringify: kValue1 vs. "
+                             "AbslStringify: kValue2"));
+}
+#endif  // GTEST_HAS_DEATH_TEST
+
+enum class ScopedEnumWithOutputOperator {
+  kValue1 = 1,
+  kValue2 = 2,
+};
+
+std::ostream& operator<<(std::ostream& os, ScopedEnumWithOutputOperator v) {
+  switch (v) {
+    case ScopedEnumWithOutputOperator::kValue1:
+      os << "OutputOperator: kValue1";
+      break;
+    case ScopedEnumWithOutputOperator::kValue2:
+      os << "OutputOperator: kValue2";
+      break;
+  }
+  return os;
+}
+
+#if GTEST_HAS_DEATH_TEST
+TEST(CHECKDeathTest, TestOutputOperatorIsUsedForScopedEnum) {
+  EXPECT_DEATH(ABSL_TEST_CHECK_EQ(ScopedEnumWithOutputOperator::kValue1,
+                                  ScopedEnumWithOutputOperator::kValue2),
+               ContainsRegex("Check failed:.*OutputOperator: kValue1 vs. "
+                             "OutputOperator: kValue2"));
+}
+#endif  // GTEST_HAS_DEATH_TEST
+
+enum class ScopedEnumWithAbslStringifyAndOutputOperator {
+  kValue1 = 1,
+  kValue2 = 2,
+};
+
+template <typename Sink>
+void AbslStringify(Sink& sink, ScopedEnumWithAbslStringifyAndOutputOperator v) {
+  switch (v) {
+    case ScopedEnumWithAbslStringifyAndOutputOperator::kValue1:
+      sink.Append("AbslStringify: kValue1");
+      break;
+    case ScopedEnumWithAbslStringifyAndOutputOperator::kValue2:
+      sink.Append("AbslStringify: kValue2");
+      break;
+  }
+}
+
+std::ostream& operator<<(std::ostream& os,
+                         ScopedEnumWithAbslStringifyAndOutputOperator v) {
+  switch (v) {
+    case ScopedEnumWithAbslStringifyAndOutputOperator::kValue1:
+      os << "OutputOperator: kValue1";
+      break;
+    case ScopedEnumWithAbslStringifyAndOutputOperator::kValue2:
+      os << "OutputOperator: kValue2";
+      break;
+  }
+  return os;
+}
+
+#if GTEST_HAS_DEATH_TEST
+
+// Test that, if operator<< and AbslStringify are both defined for a scoped
+// enum, streaming takes precedence over AbslStringify.
+TEST(CHECKDeathTest, TestScopedEnumPrefersOutputOperatorOverAbslStringify) {
+  EXPECT_DEATH(
+      ABSL_TEST_CHECK_EQ(ScopedEnumWithAbslStringifyAndOutputOperator::kValue1,
+                         ScopedEnumWithAbslStringifyAndOutputOperator::kValue2),
+      ContainsRegex("Check failed:.*OutputOperator: kValue1 vs. "
+                    "OutputOperator: kValue2"));
+}
+#endif  // GTEST_HAS_DEATH_TEST
+
+}  // namespace absl_log_internal
+
+// NOLINTEND(misc-definitions-in-headers)
+
+#endif  // ABSL_LOG_CHECK_TEST_IMPL_H_
diff --git a/absl/log/die_if_null.cc b/absl/log/die_if_null.cc
new file mode 100644
index 00000000000..0d0b78ed438
--- /dev/null
+++ b/absl/log/die_if_null.cc
@@ -0,0 +1,34 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/log/die_if_null.h"
+
+#include "absl/base/config.h"
+#include "absl/base/nullability.h"
+#include "absl/log/log.h"
+#include "absl/strings/str_cat.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace log_internal {
+
+void DieBecauseNull(const char* absl_nonnull file, int line,
+                    const char* absl_nonnull exprtext) {
+  LOG(FATAL).AtLocation(file, line)
+      << absl::StrCat("Check failed: '", exprtext, "' Must be non-null");
+}
+
+}  // namespace log_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/log/die_if_null.h b/absl/log/die_if_null.h
new file mode 100644
index 00000000000..ac7dbe6885b
--- /dev/null
+++ b/absl/log/die_if_null.h
@@ -0,0 +1,97 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: log/die_if_null.h
+// -----------------------------------------------------------------------------
+//
+// This header declares macro `ABSL_DIE_IF_NULL`.
+
+#ifndef ABSL_LOG_DIE_IF_NULL_H_
+#define ABSL_LOG_DIE_IF_NULL_H_
+
+#include <stdint.h>
+
+#include <type_traits>
+#include <utility>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/nullability_traits.h"
+#include "absl/base/nullability.h"
+#include "absl/base/optimization.h"
+
+// ABSL_DIE_IF_NULL()
+//
+// `ABSL_DIE_IF_NULL` behaves as `CHECK_NE` against `nullptr` but *also*
+// "returns" its argument.  It is useful in initializers where statements (like
+// `CHECK_NE`) can't be used.  Outside initializers, prefer `CHECK` or
+// `CHECK_NE`. `ABSL_DIE_IF_NULL` works for both raw pointers and (compatible)
+// smart pointers including `std::unique_ptr` and `std::shared_ptr`; more
+// generally, it works for any type that can be compared to nullptr_t.  For
+// types that aren't raw pointers, `ABSL_DIE_IF_NULL` returns a reference to
+// its argument, preserving the value category. Example:
+//
+//   Foo() : bar_(ABSL_DIE_IF_NULL(MethodReturningUniquePtr())) {}
+//
+// Use `CHECK(ptr)` or `CHECK(ptr != nullptr)` if the returned pointer is
+// unused.
+#define ABSL_DIE_IF_NULL(val) \
+  ::absl::log_internal::DieIfNull(__FILE__, __LINE__, #val, (val))
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace log_internal {
+
+// Crashes the process after logging `exprtext` annotated at the `file` and
+// `line` location. Called when `ABSL_DIE_IF_NULL` fails. Calling this function
+// generates less code than its implementation would if inlined, for a slight
+// code size reduction each time `ABSL_DIE_IF_NULL` is called.
+[[noreturn]] ABSL_ATTRIBUTE_NOINLINE void DieBecauseNull(
+    const char* absl_nonnull file, int line, const char* absl_nonnull exprtext);
+
+// Helper for `ABSL_DIE_IF_NULL`.
+
+// Since we use `remove_reference_t` before `AddNonnullIfCompatible`, we need
+// to explicitly have overloads for both lvalue reference and rvalue reference
+// arguments and returns.
+template <typename T>
+[[nodiscard]] typename absl::base_internal::AddNonnullIfCompatible<
+    std::remove_reference_t<T>>::type&
+DieIfNull(const char* absl_nonnull file, int line,
+          const char* absl_nonnull exprtext, T& t) {
+  if (ABSL_PREDICT_FALSE(t == nullptr)) {
+    // Call a non-inline helper function for a small code size improvement.
+    DieBecauseNull(file, line, exprtext);
+  }
+  return t;
+}
+
+template <typename T>
+[[nodiscard]] typename absl::base_internal::AddNonnullIfCompatible<
+    std::remove_reference_t<T>>::type&&
+DieIfNull(const char* absl_nonnull file, int line,
+          const char* absl_nonnull exprtext, T&& t) {
+  if (ABSL_PREDICT_FALSE(t == nullptr)) {
+    // Call a non-inline helper function for a small code size improvement.
+    DieBecauseNull(file, line, exprtext);
+  }
+  return std::forward<T>(t);
+}
+
+}  // namespace log_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_LOG_DIE_IF_NULL_H_
diff --git a/absl/log/die_if_null_test.cc b/absl/log/die_if_null_test.cc
new file mode 100644
index 00000000000..b0aab781c36
--- /dev/null
+++ b/absl/log/die_if_null_test.cc
@@ -0,0 +1,107 @@
+//
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/log/die_if_null.h"
+
+#include <stdint.h>
+
+#include <memory>
+#include <utility>
+
+#include "gtest/gtest.h"
+#include "absl/base/attributes.h"
+#include "absl/log/internal/test_helpers.h"
+
+namespace {
+
+auto* test_env ABSL_ATTRIBUTE_UNUSED = ::testing::AddGlobalTestEnvironment(
+    new absl::log_internal::LogTestEnvironment);
+
+// TODO(b/69907837): Revisit these tests with the goal of making them less
+// convoluted.
+TEST(AbslDieIfNull, Simple) {
+  int64_t t;
+  void* ptr = static_cast<void*>(&t);
+  void* ref = ABSL_DIE_IF_NULL(ptr);
+  ASSERT_EQ(ptr, ref);
+
+  char* t_as_char;
+  t_as_char = ABSL_DIE_IF_NULL(reinterpret_cast<char*>(&t));
+  (void)t_as_char;
+
+  unsigned char* t_as_uchar;
+  t_as_uchar = ABSL_DIE_IF_NULL(reinterpret_cast<unsigned char*>(&t));
+  (void)t_as_uchar;
+
+  int* t_as_int;
+  t_as_int = ABSL_DIE_IF_NULL(reinterpret_cast<int*>(&t));
+  (void)t_as_int;
+
+  int64_t* t_as_int64_t;
+  t_as_int64_t = ABSL_DIE_IF_NULL(reinterpret_cast<int64_t*>(&t));
+  (void)t_as_int64_t;
+
+  std::unique_ptr<int64_t> sptr(new int64_t);
+  EXPECT_EQ(sptr.get(), ABSL_DIE_IF_NULL(sptr).get());
+  ABSL_DIE_IF_NULL(sptr).reset();
+
+  int64_t* int_ptr = new int64_t();
+  EXPECT_EQ(int_ptr, ABSL_DIE_IF_NULL(std::unique_ptr<int64_t>(int_ptr)).get());
+}
+
+#if GTEST_HAS_DEATH_TEST
+TEST(DeathCheckAbslDieIfNull, Simple) {
+  void* ptr;
+  ASSERT_DEATH({ ptr = ABSL_DIE_IF_NULL(nullptr); }, "");
+  (void)ptr;
+
+  std::unique_ptr<int64_t> sptr;
+  ASSERT_DEATH(ptr = ABSL_DIE_IF_NULL(sptr).get(), "");
+}
+#endif
+
+// Ensures that ABSL_DIE_IF_NULL works with C++11's std::unique_ptr and
+// std::shared_ptr.
+TEST(AbslDieIfNull, DoesNotCompareSmartPointerToNULL) {
+  std::unique_ptr<int> up(new int);
+  EXPECT_EQ(&up, &ABSL_DIE_IF_NULL(up));
+  ABSL_DIE_IF_NULL(up).reset();
+
+  std::shared_ptr<int> sp(new int);
+  EXPECT_EQ(&sp, &ABSL_DIE_IF_NULL(sp));
+  ABSL_DIE_IF_NULL(sp).reset();
+}
+
+// Verifies that ABSL_DIE_IF_NULL returns an rvalue reference if its argument is
+// an rvalue reference.
+TEST(AbslDieIfNull, PreservesRValues) {
+  int64_t* ptr = new int64_t();
+  auto uptr = ABSL_DIE_IF_NULL(std::unique_ptr<int64_t>(ptr));
+  EXPECT_EQ(ptr, uptr.get());
+}
+
+// Verifies that ABSL_DIE_IF_NULL returns an lvalue if its argument is an
+// lvalue.
+TEST(AbslDieIfNull, PreservesLValues) {
+  int64_t array[2] = {0};
+  int64_t* a = array + 0;
+  int64_t* b = array + 1;
+  using std::swap;
+  swap(ABSL_DIE_IF_NULL(a), ABSL_DIE_IF_NULL(b));
+  EXPECT_EQ(array + 1, a);
+  EXPECT_EQ(array + 0, b);
+}
+
+}  // namespace
diff --git a/absl/log/flags.cc b/absl/log/flags.cc
new file mode 100644
index 00000000000..287b3e96c70
--- /dev/null
+++ b/absl/log/flags.cc
@@ -0,0 +1,143 @@
+//
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/log/internal/flags.h"
+
+#include <stddef.h>
+
+#include <algorithm>
+#include <cstdlib>
+#include <string>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/log_severity.h"
+#include "absl/flags/flag.h"
+#include "absl/flags/marshalling.h"
+#include "absl/log/globals.h"
+#include "absl/log/internal/config.h"
+#include "absl/log/internal/vlog_config.h"
+#include "absl/strings/numbers.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace log_internal {
+namespace {
+
+void SyncLoggingFlags() {
+  absl::SetFlag(&FLAGS_minloglevel, static_cast<int>(absl::MinLogLevel()));
+  absl::SetFlag(&FLAGS_log_prefix, absl::ShouldPrependLogPrefix());
+}
+
+bool RegisterSyncLoggingFlags() {
+  log_internal::SetLoggingGlobalsListener(&SyncLoggingFlags);
+  return true;
+}
+
+ABSL_ATTRIBUTE_UNUSED const bool unused = RegisterSyncLoggingFlags();
+
+template <typename T>
+T GetFromEnv(const char* varname, T dflt) {
+  const char* val = ::getenv(varname);
+  if (val != nullptr) {
+    std::string err;
+    ABSL_INTERNAL_CHECK(absl::ParseFlag(val, &dflt, &err), err.c_str());
+  }
+  return dflt;
+}
+
+constexpr absl::LogSeverityAtLeast StderrThresholdDefault() {
+  return absl::LogSeverityAtLeast::kError;
+}
+
+}  // namespace
+}  // namespace log_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+ABSL_FLAG(int, stderrthreshold,
+          static_cast<int>(absl::log_internal::StderrThresholdDefault()),
+          "Log messages at or above this threshold level are copied to stderr.")
+    .OnUpdate([] {
+      absl::log_internal::RawSetStderrThreshold(
+          static_cast<absl::LogSeverityAtLeast>(
+              absl::GetFlag(FLAGS_stderrthreshold)));
+    });
+
+ABSL_FLAG(int, minloglevel, static_cast<int>(absl::LogSeverityAtLeast::kInfo),
+          "Messages logged at a lower level than this don't actually "
+          "get logged anywhere")
+    .OnUpdate([] {
+      absl::log_internal::RawSetMinLogLevel(
+          static_cast<absl::LogSeverityAtLeast>(
+              absl::GetFlag(FLAGS_minloglevel)));
+    });
+
+ABSL_FLAG(std::string, log_backtrace_at, "",
+          "Emit a backtrace when logging at file:linenum.")
+    .OnUpdate([] {
+      const std::string log_backtrace_at =
+          absl::GetFlag(FLAGS_log_backtrace_at);
+      if (log_backtrace_at.empty()) {
+        absl::ClearLogBacktraceLocation();
+        return;
+      }
+
+      const size_t last_colon = log_backtrace_at.rfind(':');
+      if (last_colon == log_backtrace_at.npos) {
+        absl::ClearLogBacktraceLocation();
+        return;
+      }
+
+      const absl::string_view file =
+          absl::string_view(log_backtrace_at).substr(0, last_colon);
+      int line;
+      if (!absl::SimpleAtoi(
+              absl::string_view(log_backtrace_at).substr(last_colon + 1),
+              &line)) {
+        absl::ClearLogBacktraceLocation();
+        return;
+      }
+      absl::SetLogBacktraceLocation(file, line);
+    });
+
+ABSL_FLAG(bool, log_prefix, true,
+          "Prepend the log prefix to the start of each log line")
+    .OnUpdate([] {
+      absl::log_internal::RawEnableLogPrefix(absl::GetFlag(FLAGS_log_prefix));
+    });
+
+ABSL_FLAG(int, v, 0,
+          "Show all VLOG(m) messages for m <= this. Overridable by --vmodule.")
+    .OnUpdate([] {
+      absl::log_internal::UpdateGlobalVLogLevel(absl::GetFlag(FLAGS_v));
+    });
+
+ABSL_FLAG(
+    std::string, vmodule, "",
+    "per-module log verbosity level."
+    " Argument is a comma-separated list of <module name>=<log level>."
+    " <module name> is a glob pattern, matched against the filename base"
+    " (that is, name ignoring .cc/.h./-inl.h)."
+    " A pattern without slashes matches just the file name portion, otherwise"
+    " the whole file path below the workspace root"
+    " (still without .cc/.h./-inl.h) is matched."
+    " ? and * in the glob pattern match any single or sequence of characters"
+    " respectively including slashes."
+    " <log level> overrides any value given by --v.")
+    .OnUpdate([] {
+      absl::log_internal::UpdateVModule(absl::GetFlag(FLAGS_vmodule));
+    });
diff --git a/absl/log/flags.h b/absl/log/flags.h
new file mode 100644
index 00000000000..146cfdd6ca0
--- /dev/null
+++ b/absl/log/flags.h
@@ -0,0 +1,43 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: log/flags.h
+// -----------------------------------------------------------------------------
+//
+
+#ifndef ABSL_LOG_FLAGS_H_
+#define ABSL_LOG_FLAGS_H_
+
+// The Abseil Logging library supports the following command line flags to
+// configure logging behavior at runtime:
+//
+// --stderrthreshold=<value>
+// Log messages at or above this threshold level are copied to stderr.
+//
+// --minloglevel=<value>
+// Messages logged at a lower level than this are discarded and don't actually
+// get logged anywhere.
+//
+// --log_backtrace_at=<file:linenum>
+// Emit a backtrace (stack trace) when logging at file:linenum.
+//
+// To use these commandline flags, the //absl/log:flags library must be
+// explicitly linked, and absl::ParseCommandLine() must be called before the
+// call to absl::InitializeLog().
+//
+// To configure the Log library programmatically, use the interfaces defined in
+// absl/log/globals.h.
+
+#endif  // ABSL_LOG_FLAGS_H_
diff --git a/absl/log/flags_test.cc b/absl/log/flags_test.cc
new file mode 100644
index 00000000000..f5a2b5189c7
--- /dev/null
+++ b/absl/log/flags_test.cc
@@ -0,0 +1,195 @@
+//
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/log/internal/flags.h"
+
+#include <string>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/attributes.h"
+#include "absl/base/log_severity.h"
+#include "absl/flags/flag.h"
+#include "absl/flags/reflection.h"
+#include "absl/log/globals.h"
+#include "absl/log/internal/test_helpers.h"
+#include "absl/log/internal/test_matchers.h"
+#include "absl/log/log.h"
+#include "absl/log/scoped_mock_log.h"
+#include "absl/strings/str_cat.h"
+
+namespace {
+using ::absl::log_internal::TextMessage;
+
+using ::testing::HasSubstr;
+using ::testing::Not;
+
+auto* test_env ABSL_ATTRIBUTE_UNUSED = ::testing::AddGlobalTestEnvironment(
+    new absl::log_internal::LogTestEnvironment);
+
+constexpr static absl::LogSeverityAtLeast DefaultStderrThreshold() {
+  return absl::LogSeverityAtLeast::kError;
+}
+
+class LogFlagsTest : public ::testing::Test {
+ protected:
+  absl::FlagSaver flag_saver_;
+};
+
+// This test is disabled because it adds order dependency to the test suite.
+// This order dependency is currently not fixable due to the way the
+// stderrthreshold global value is out of sync with the stderrthreshold flag.
+TEST_F(LogFlagsTest, DISABLED_StderrKnobsDefault) {
+  EXPECT_EQ(absl::StderrThreshold(), DefaultStderrThreshold());
+}
+
+TEST_F(LogFlagsTest, SetStderrThreshold) {
+  absl::SetFlag(&FLAGS_stderrthreshold,
+                static_cast<int>(absl::LogSeverityAtLeast::kInfo));
+
+  EXPECT_EQ(absl::StderrThreshold(), absl::LogSeverityAtLeast::kInfo);
+
+  absl::SetFlag(&FLAGS_stderrthreshold,
+                static_cast<int>(absl::LogSeverityAtLeast::kError));
+
+  EXPECT_EQ(absl::StderrThreshold(), absl::LogSeverityAtLeast::kError);
+}
+
+TEST_F(LogFlagsTest, SetMinLogLevel) {
+  absl::SetFlag(&FLAGS_minloglevel,
+                static_cast<int>(absl::LogSeverityAtLeast::kError));
+
+  EXPECT_EQ(absl::MinLogLevel(), absl::LogSeverityAtLeast::kError);
+
+  absl::log_internal::ScopedMinLogLevel scoped_min_log_level(
+      absl::LogSeverityAtLeast::kWarning);
+
+  EXPECT_EQ(absl::GetFlag(FLAGS_minloglevel),
+            static_cast<int>(absl::LogSeverityAtLeast::kWarning));
+}
+
+TEST_F(LogFlagsTest, PrependLogPrefix) {
+  absl::SetFlag(&FLAGS_log_prefix, false);
+
+  EXPECT_EQ(absl::ShouldPrependLogPrefix(), false);
+
+  absl::EnableLogPrefix(true);
+
+  EXPECT_EQ(absl::GetFlag(FLAGS_log_prefix), true);
+}
+
+TEST_F(LogFlagsTest, EmptyBacktraceAtFlag) {
+  absl::SetMinLogLevel(absl::LogSeverityAtLeast::kInfo);
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  EXPECT_CALL(test_sink, Send(TextMessage(Not(HasSubstr("(stacktrace:")))));
+
+  test_sink.StartCapturingLogs();
+  absl::SetFlag(&FLAGS_log_backtrace_at, "");
+  LOG(INFO) << "hello world";
+}
+
+TEST_F(LogFlagsTest, BacktraceAtNonsense) {
+  absl::SetMinLogLevel(absl::LogSeverityAtLeast::kInfo);
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  EXPECT_CALL(test_sink, Send(TextMessage(Not(HasSubstr("(stacktrace:")))));
+
+  test_sink.StartCapturingLogs();
+  absl::SetFlag(&FLAGS_log_backtrace_at, "gibberish");
+  LOG(INFO) << "hello world";
+}
+
+TEST_F(LogFlagsTest, BacktraceAtWrongFile) {
+  absl::SetMinLogLevel(absl::LogSeverityAtLeast::kInfo);
+  const int log_line = __LINE__ + 1;
+  auto do_log = [] { LOG(INFO) << "hello world"; };
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  EXPECT_CALL(test_sink, Send(TextMessage(Not(HasSubstr("(stacktrace:")))));
+
+  test_sink.StartCapturingLogs();
+  absl::SetFlag(&FLAGS_log_backtrace_at,
+                absl::StrCat("some_other_file.cc:", log_line));
+  do_log();
+}
+
+TEST_F(LogFlagsTest, BacktraceAtWrongLine) {
+  absl::SetMinLogLevel(absl::LogSeverityAtLeast::kInfo);
+  const int log_line = __LINE__ + 1;
+  auto do_log = [] { LOG(INFO) << "hello world"; };
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  EXPECT_CALL(test_sink, Send(TextMessage(Not(HasSubstr("(stacktrace:")))));
+
+  test_sink.StartCapturingLogs();
+  absl::SetFlag(&FLAGS_log_backtrace_at,
+                absl::StrCat("flags_test.cc:", log_line + 1));
+  do_log();
+}
+
+TEST_F(LogFlagsTest, BacktraceAtWholeFilename) {
+  absl::SetMinLogLevel(absl::LogSeverityAtLeast::kInfo);
+  const int log_line = __LINE__ + 1;
+  auto do_log = [] { LOG(INFO) << "hello world"; };
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  EXPECT_CALL(test_sink, Send(TextMessage(Not(HasSubstr("(stacktrace:")))));
+
+  test_sink.StartCapturingLogs();
+  absl::SetFlag(&FLAGS_log_backtrace_at, absl::StrCat(__FILE__, ":", log_line));
+  do_log();
+}
+
+TEST_F(LogFlagsTest, BacktraceAtNonmatchingSuffix) {
+  absl::SetMinLogLevel(absl::LogSeverityAtLeast::kInfo);
+  const int log_line = __LINE__ + 1;
+  auto do_log = [] { LOG(INFO) << "hello world"; };
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  EXPECT_CALL(test_sink, Send(TextMessage(Not(HasSubstr("(stacktrace:")))));
+
+  test_sink.StartCapturingLogs();
+  absl::SetFlag(&FLAGS_log_backtrace_at,
+                absl::StrCat("flags_test.cc:", log_line, "gibberish"));
+  do_log();
+}
+
+TEST_F(LogFlagsTest, LogsBacktrace) {
+  absl::SetMinLogLevel(absl::LogSeverityAtLeast::kInfo);
+  const int log_line = __LINE__ + 1;
+  auto do_log = [] { LOG(INFO) << "hello world"; };
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  testing::InSequence seq;
+  EXPECT_CALL(test_sink, Send(TextMessage(HasSubstr("(stacktrace:"))));
+  EXPECT_CALL(test_sink, Send(TextMessage(Not(HasSubstr("(stacktrace:")))));
+
+  test_sink.StartCapturingLogs();
+  absl::SetFlag(&FLAGS_log_backtrace_at,
+                absl::StrCat("flags_test.cc:", log_line));
+  do_log();
+  absl::SetFlag(&FLAGS_log_backtrace_at, "");
+  do_log();
+}
+
+}  // namespace
diff --git a/absl/log/globals.cc b/absl/log/globals.cc
new file mode 100644
index 00000000000..cc85438f0ba
--- /dev/null
+++ b/absl/log/globals.cc
@@ -0,0 +1,178 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/log/globals.h"
+
+#include <atomic>
+#include <cstddef>
+#include <cstdint>
+#include <cstdlib>
+#include <cstring>
+#include <string>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/atomic_hook.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/base/log_severity.h"
+#include "absl/hash/hash.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace {
+
+// These atomics represent logging library configuration.
+// Integer types are used instead of absl::LogSeverity to ensure that a
+// lock-free std::atomic is used when possible.
+ABSL_CONST_INIT std::atomic<int> min_log_level{
+    static_cast<int>(absl::LogSeverityAtLeast::kInfo)};
+ABSL_CONST_INIT std::atomic<int> stderrthreshold{
+    static_cast<int>(absl::LogSeverityAtLeast::kError)};
+// We evaluate this value as a hash comparison to avoid having to
+// hold a mutex or make a copy (to access the value of a string-typed flag) in
+// very hot codepath.
+ABSL_CONST_INIT std::atomic<size_t> log_backtrace_at_hash{0};
+ABSL_CONST_INIT std::atomic<bool> prepend_log_prefix{true};
+
+constexpr char kDefaultAndroidTag[] = "native";
+ABSL_CONST_INIT std::atomic<const char*> android_log_tag{kDefaultAndroidTag};
+
+ABSL_INTERNAL_ATOMIC_HOOK_ATTRIBUTES
+absl::base_internal::AtomicHook<log_internal::LoggingGlobalsListener>
+    logging_globals_listener;
+
+size_t HashSiteForLogBacktraceAt(absl::string_view file, int line) {
+  return absl::HashOf(file, line);
+}
+
+void TriggerLoggingGlobalsListener() {
+  auto* listener = logging_globals_listener.Load();
+  if (listener != nullptr) listener();
+}
+
+}  // namespace
+
+namespace log_internal {
+
+void RawSetMinLogLevel(absl::LogSeverityAtLeast severity) {
+  min_log_level.store(static_cast<int>(severity), std::memory_order_release);
+}
+
+void RawSetStderrThreshold(absl::LogSeverityAtLeast severity) {
+  stderrthreshold.store(static_cast<int>(severity), std::memory_order_release);
+}
+
+void RawEnableLogPrefix(bool on_off) {
+  prepend_log_prefix.store(on_off, std::memory_order_release);
+}
+
+void SetLoggingGlobalsListener(LoggingGlobalsListener l) {
+  logging_globals_listener.Store(l);
+}
+
+}  // namespace log_internal
+
+absl::LogSeverityAtLeast MinLogLevel() {
+  return static_cast<absl::LogSeverityAtLeast>(
+      min_log_level.load(std::memory_order_acquire));
+}
+
+void SetMinLogLevel(absl::LogSeverityAtLeast severity) {
+  log_internal::RawSetMinLogLevel(severity);
+  TriggerLoggingGlobalsListener();
+}
+
+namespace log_internal {
+
+ScopedMinLogLevel::ScopedMinLogLevel(absl::LogSeverityAtLeast severity)
+    : saved_severity_(absl::MinLogLevel()) {
+  absl::SetMinLogLevel(severity);
+}
+ScopedMinLogLevel::~ScopedMinLogLevel() {
+  absl::SetMinLogLevel(saved_severity_);
+}
+
+}  // namespace log_internal
+
+absl::LogSeverityAtLeast StderrThreshold() {
+  return static_cast<absl::LogSeverityAtLeast>(
+      stderrthreshold.load(std::memory_order_acquire));
+}
+
+void SetStderrThreshold(absl::LogSeverityAtLeast severity) {
+  log_internal::RawSetStderrThreshold(severity);
+  TriggerLoggingGlobalsListener();
+}
+
+ScopedStderrThreshold::ScopedStderrThreshold(absl::LogSeverityAtLeast severity)
+    : saved_severity_(absl::StderrThreshold()) {
+  absl::SetStderrThreshold(severity);
+}
+
+ScopedStderrThreshold::~ScopedStderrThreshold() {
+  absl::SetStderrThreshold(saved_severity_);
+}
+
+namespace log_internal {
+
+const char* GetAndroidNativeTag() {
+  return android_log_tag.load(std::memory_order_acquire);
+}
+
+}  // namespace log_internal
+
+void SetAndroidNativeTag(const char* tag) {
+  ABSL_CONST_INIT static std::atomic<const std::string*> user_log_tag(nullptr);
+  ABSL_INTERNAL_CHECK(tag, "tag must be non-null.");
+
+  const std::string* tag_str = new std::string(tag);
+  ABSL_INTERNAL_CHECK(
+      android_log_tag.exchange(tag_str->c_str(), std::memory_order_acq_rel) ==
+          kDefaultAndroidTag,
+      "SetAndroidNativeTag() must only be called once per process!");
+  user_log_tag.store(tag_str, std::memory_order_relaxed);
+}
+
+namespace log_internal {
+
+bool ShouldLogBacktraceAt(absl::string_view file, int line) {
+  const size_t flag_hash =
+      log_backtrace_at_hash.load(std::memory_order_relaxed);
+
+  return flag_hash != 0 && flag_hash == HashSiteForLogBacktraceAt(file, line);
+}
+
+}  // namespace log_internal
+
+void SetLogBacktraceLocation(absl::string_view file, int line) {
+  log_backtrace_at_hash.store(HashSiteForLogBacktraceAt(file, line),
+                              std::memory_order_relaxed);
+}
+
+void ClearLogBacktraceLocation() {
+  log_backtrace_at_hash.store(0, std::memory_order_relaxed);
+}
+
+bool ShouldPrependLogPrefix() {
+  return prepend_log_prefix.load(std::memory_order_acquire);
+}
+
+void EnableLogPrefix(bool on_off) {
+  log_internal::RawEnableLogPrefix(on_off);
+  TriggerLoggingGlobalsListener();
+}
+
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/log/globals.h b/absl/log/globals.h
new file mode 100644
index 00000000000..9718967ae73
--- /dev/null
+++ b/absl/log/globals.h
@@ -0,0 +1,230 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: log/globals.h
+// -----------------------------------------------------------------------------
+//
+// This header declares global logging library configuration knobs.
+
+#ifndef ABSL_LOG_GLOBALS_H_
+#define ABSL_LOG_GLOBALS_H_
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/log_severity.h"
+#include "absl/log/internal/vlog_config.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+//------------------------------------------------------------------------------
+//  Minimum Log Level
+//------------------------------------------------------------------------------
+//
+// Messages logged at or above this severity are directed to all registered log
+// sinks or skipped otherwise. This parameter can also be modified using
+// command line flag --minloglevel.
+// See absl/base/log_severity.h for descriptions of severity levels.
+
+// MinLogLevel()
+//
+// Returns the value of the Minimum Log Level parameter.
+// This function is async-signal-safe.
+[[nodiscard]] absl::LogSeverityAtLeast MinLogLevel();
+
+// SetMinLogLevel()
+//
+// Updates the value of Minimum Log Level parameter.
+// This function is async-signal-safe.
+void SetMinLogLevel(absl::LogSeverityAtLeast severity);
+
+namespace log_internal {
+
+// ScopedMinLogLevel
+//
+// RAII type used to temporarily update the Min Log Level parameter.
+class ScopedMinLogLevel final {
+ public:
+  explicit ScopedMinLogLevel(absl::LogSeverityAtLeast severity);
+  ScopedMinLogLevel(const ScopedMinLogLevel&) = delete;
+  ScopedMinLogLevel& operator=(const ScopedMinLogLevel&) = delete;
+  ~ScopedMinLogLevel();
+
+ private:
+  absl::LogSeverityAtLeast saved_severity_;
+};
+
+}  // namespace log_internal
+
+//------------------------------------------------------------------------------
+// Stderr Threshold
+//------------------------------------------------------------------------------
+//
+// Messages logged at or above this level are directed to stderr in
+// addition to other registered log sinks. This parameter can also be modified
+// using command line flag --stderrthreshold.
+// See absl/base/log_severity.h for descriptions of severity levels.
+
+// StderrThreshold()
+//
+// Returns the value of the Stderr Threshold parameter.
+// This function is async-signal-safe.
+[[nodiscard]] absl::LogSeverityAtLeast StderrThreshold();
+
+// SetStderrThreshold()
+//
+// Updates the Stderr Threshold parameter.
+// This function is async-signal-safe.
+void SetStderrThreshold(absl::LogSeverityAtLeast severity);
+inline void SetStderrThreshold(absl::LogSeverity severity) {
+  absl::SetStderrThreshold(static_cast<absl::LogSeverityAtLeast>(severity));
+}
+
+// ScopedStderrThreshold
+//
+// RAII type used to temporarily update the Stderr Threshold parameter.
+class ScopedStderrThreshold final {
+ public:
+  explicit ScopedStderrThreshold(absl::LogSeverityAtLeast severity);
+  ScopedStderrThreshold(const ScopedStderrThreshold&) = delete;
+  ScopedStderrThreshold& operator=(const ScopedStderrThreshold&) = delete;
+  ~ScopedStderrThreshold();
+
+ private:
+  absl::LogSeverityAtLeast saved_severity_;
+};
+
+//------------------------------------------------------------------------------
+// Log Backtrace At
+//------------------------------------------------------------------------------
+//
+// Users can request an existing `LOG` statement, specified by file and line
+// number, to also include a backtrace when logged.
+
+// ShouldLogBacktraceAt()
+//
+// Returns true if we should log a backtrace at the specified location.
+namespace log_internal {
+[[nodiscard]] bool ShouldLogBacktraceAt(absl::string_view file, int line);
+}  // namespace log_internal
+
+// SetLogBacktraceLocation()
+//
+// Sets the location the backtrace should be logged at.  If the specified
+// location isn't a `LOG` statement, the effect will be the same as
+// `ClearLogBacktraceLocation` (but less efficient).
+void SetLogBacktraceLocation(absl::string_view file, int line);
+
+// ClearLogBacktraceLocation()
+//
+// Clears the set location so that backtraces will no longer be logged at it.
+void ClearLogBacktraceLocation();
+
+//------------------------------------------------------------------------------
+// Prepend Log Prefix
+//------------------------------------------------------------------------------
+//
+// This option tells the logging library that every logged message
+// should include the prefix (severity, date, time, PID, etc.)
+//
+// ShouldPrependLogPrefix()
+//
+// Returns the value of the Prepend Log Prefix option.
+// This function is async-signal-safe.
+[[nodiscard]] bool ShouldPrependLogPrefix();
+
+// EnableLogPrefix()
+//
+// Updates the value of the Prepend Log Prefix option.
+// This function is async-signal-safe.
+void EnableLogPrefix(bool on_off);
+
+//------------------------------------------------------------------------------
+// `VLOG` Configuration
+//------------------------------------------------------------------------------
+//
+// These methods set the `(ABSL_)VLOG(_IS_ON)` threshold.  They allow
+// programmatic control of the thresholds set by the --v and --vmodule flags.
+//
+// Only `VLOG`s with a severity level LESS THAN OR EQUAL TO the threshold will
+// be evaluated.
+//
+// For example, if the threshold is 2, then:
+//
+//   VLOG(2) << "This message will be logged.";
+//   VLOG(3) << "This message will NOT be logged.";
+//
+// The default threshold is 0. Since `VLOG` levels must not be negative, a
+// negative threshold value will turn off all VLOGs.
+
+// SetGlobalVLogLevel()
+//
+// Sets the global `VLOG` level to threshold. Returns the previous global
+// threshold.
+inline int SetGlobalVLogLevel(int threshold) {
+  return absl::log_internal::UpdateGlobalVLogLevel(threshold);
+}
+
+// SetVLogLevel()
+//
+// Sets the `VLOG` threshold for all files that match `module_pattern`,
+// overwriting any prior value. Files that don't match aren't affected.
+// Returns the threshold that previously applied to `module_pattern`.
+inline int SetVLogLevel(absl::string_view module_pattern, int threshold) {
+  return absl::log_internal::PrependVModule(module_pattern, threshold);
+}
+
+//------------------------------------------------------------------------------
+// Configure Android Native Log Tag
+//------------------------------------------------------------------------------
+//
+// The logging library forwards to the Android system log API when built for
+// Android.  That API takes a string "tag" value in addition to a message and
+// severity level.  The tag is used to identify the source of messages and to
+// filter them.  This library uses the tag "native" by default.
+
+// SetAndroidNativeTag()
+//
+// Stores a copy of the string pointed to by `tag` and uses it as the Android
+// logging tag thereafter. `tag` must not be null.
+// This function must not be called more than once!
+void SetAndroidNativeTag(const char* tag);
+
+namespace log_internal {
+// GetAndroidNativeTag()
+//
+// Returns the configured Android logging tag.
+const char* GetAndroidNativeTag();
+}  // namespace log_internal
+
+namespace log_internal {
+
+using LoggingGlobalsListener = void (*)();
+void SetLoggingGlobalsListener(LoggingGlobalsListener l);
+
+// Internal implementation for the setter routines. These are used
+// to break circular dependencies between flags and globals. Each "Raw"
+// routine corresponds to the non-"Raw" counterpart and used to set the
+// configuration parameter directly without calling back to the listener.
+void RawSetMinLogLevel(absl::LogSeverityAtLeast severity);
+void RawSetStderrThreshold(absl::LogSeverityAtLeast severity);
+void RawEnableLogPrefix(bool on_off);
+
+}  // namespace log_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_LOG_GLOBALS_H_
diff --git a/absl/log/globals_test.cc b/absl/log/globals_test.cc
new file mode 100644
index 00000000000..78430d460f9
--- /dev/null
+++ b/absl/log/globals_test.cc
@@ -0,0 +1,156 @@
+//
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/log/globals.h"
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/attributes.h"
+#include "absl/base/log_severity.h"
+#include "absl/log/internal/globals.h"
+#include "absl/log/internal/test_helpers.h"
+#include "absl/log/log.h"
+#include "absl/log/scoped_mock_log.h"
+
+namespace {
+using ::testing::_;
+using ::testing::StrEq;
+
+auto* test_env ABSL_ATTRIBUTE_UNUSED = ::testing::AddGlobalTestEnvironment(
+    new absl::log_internal::LogTestEnvironment);
+
+constexpr static absl::LogSeverityAtLeast DefaultMinLogLevel() {
+  return absl::LogSeverityAtLeast::kInfo;
+}
+constexpr static absl::LogSeverityAtLeast DefaultStderrThreshold() {
+  return absl::LogSeverityAtLeast::kError;
+}
+
+TEST(TestGlobals, MinLogLevel) {
+  EXPECT_EQ(absl::MinLogLevel(), DefaultMinLogLevel());
+  absl::SetMinLogLevel(absl::LogSeverityAtLeast::kError);
+  EXPECT_EQ(absl::MinLogLevel(), absl::LogSeverityAtLeast::kError);
+  absl::SetMinLogLevel(DefaultMinLogLevel());
+}
+
+TEST(TestGlobals, ScopedMinLogLevel) {
+  EXPECT_EQ(absl::MinLogLevel(), DefaultMinLogLevel());
+  {
+    absl::log_internal::ScopedMinLogLevel scoped_stderr_threshold(
+        absl::LogSeverityAtLeast::kError);
+    EXPECT_EQ(absl::MinLogLevel(), absl::LogSeverityAtLeast::kError);
+  }
+  EXPECT_EQ(absl::MinLogLevel(), DefaultMinLogLevel());
+}
+
+TEST(TestGlobals, StderrThreshold) {
+  EXPECT_EQ(absl::StderrThreshold(), DefaultStderrThreshold());
+  absl::SetStderrThreshold(absl::LogSeverityAtLeast::kError);
+  EXPECT_EQ(absl::StderrThreshold(), absl::LogSeverityAtLeast::kError);
+  absl::SetStderrThreshold(DefaultStderrThreshold());
+}
+
+TEST(TestGlobals, ScopedStderrThreshold) {
+  EXPECT_EQ(absl::StderrThreshold(), DefaultStderrThreshold());
+  {
+    absl::ScopedStderrThreshold scoped_stderr_threshold(
+        absl::LogSeverityAtLeast::kError);
+    EXPECT_EQ(absl::StderrThreshold(), absl::LogSeverityAtLeast::kError);
+  }
+  EXPECT_EQ(absl::StderrThreshold(), DefaultStderrThreshold());
+}
+
+TEST(TestGlobals, LogBacktraceAt) {
+  EXPECT_FALSE(absl::log_internal::ShouldLogBacktraceAt("some_file.cc", 111));
+  absl::SetLogBacktraceLocation("some_file.cc", 111);
+  EXPECT_TRUE(absl::log_internal::ShouldLogBacktraceAt("some_file.cc", 111));
+  EXPECT_FALSE(
+      absl::log_internal::ShouldLogBacktraceAt("another_file.cc", 222));
+}
+
+TEST(TestGlobals, LogPrefix) {
+  EXPECT_TRUE(absl::ShouldPrependLogPrefix());
+  absl::EnableLogPrefix(false);
+  EXPECT_FALSE(absl::ShouldPrependLogPrefix());
+  absl::EnableLogPrefix(true);
+  EXPECT_TRUE(absl::ShouldPrependLogPrefix());
+}
+
+TEST(TestGlobals, SetGlobalVLogLevel) {
+  EXPECT_EQ(absl::SetGlobalVLogLevel(42), 0);
+  EXPECT_EQ(absl::SetGlobalVLogLevel(1337), 42);
+  // Restore the value since it affects the default unset module value for
+  // `SetVLogLevel()`.
+  EXPECT_EQ(absl::SetGlobalVLogLevel(0), 1337);
+}
+
+TEST(TestGlobals, SetVLogLevel) {
+  EXPECT_EQ(absl::SetVLogLevel("setvloglevel", 42), 0);
+  EXPECT_EQ(absl::SetVLogLevel("setvloglevel", 1337), 42);
+  EXPECT_EQ(absl::SetVLogLevel("othersetvloglevel", 50), 0);
+
+  EXPECT_EQ(absl::SetVLogLevel("*pattern*", 1), 0);
+  EXPECT_EQ(absl::SetVLogLevel("*less_generic_pattern*", 2), 1);
+  // "pattern_match" matches the pattern "*pattern*". Therefore, the previous
+  // level must be 1.
+  EXPECT_EQ(absl::SetVLogLevel("pattern_match", 3), 1);
+  // "less_generic_pattern_match" matches the pattern "*pattern*". Therefore,
+  // the previous level must be 2.
+  EXPECT_EQ(absl::SetVLogLevel("less_generic_pattern_match", 4), 2);
+}
+
+TEST(TestGlobals, AndroidLogTag) {
+  // Verify invalid tags result in a check failure.
+  EXPECT_DEATH_IF_SUPPORTED(absl::SetAndroidNativeTag(nullptr), ".*");
+
+  // Verify valid tags applied.
+  EXPECT_THAT(absl::log_internal::GetAndroidNativeTag(), StrEq("native"));
+  absl::SetAndroidNativeTag("test_tag");
+  EXPECT_THAT(absl::log_internal::GetAndroidNativeTag(), StrEq("test_tag"));
+
+  // Verify that additional calls (more than 1) result in a check failure.
+  EXPECT_DEATH_IF_SUPPORTED(absl::SetAndroidNativeTag("test_tag_fail"), ".*");
+}
+
+TEST(TestExitOnDFatal, OffTest) {
+  // Turn off...
+  absl::log_internal::SetExitOnDFatal(false);
+  EXPECT_FALSE(absl::log_internal::ExitOnDFatal());
+
+  // We don't die.
+  {
+    absl::ScopedMockLog log(absl::MockLogDefault::kDisallowUnexpected);
+
+    // LOG(DFATAL) has severity FATAL if debugging, but is
+    // downgraded to ERROR if not debugging.
+    EXPECT_CALL(log, Log(absl::kLogDebugFatal, _, "This should not be fatal"));
+
+    log.StartCapturingLogs();
+    LOG(DFATAL) << "This should not be fatal";
+  }
+}
+
+#if GTEST_HAS_DEATH_TEST
+TEST(TestDeathWhileExitOnDFatal, OnTest) {
+  absl::log_internal::SetExitOnDFatal(true);
+  EXPECT_TRUE(absl::log_internal::ExitOnDFatal());
+
+  // Death comes on little cats' feet.
+  EXPECT_DEBUG_DEATH({ LOG(DFATAL) << "This should be fatal in debug mode"; },
+                     "This should be fatal in debug mode");
+}
+#endif
+
+}  // namespace
diff --git a/absl/log/initialize.cc b/absl/log/initialize.cc
new file mode 100644
index 00000000000..ef5d31469a9
--- /dev/null
+++ b/absl/log/initialize.cc
@@ -0,0 +1,38 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/log/initialize.h"
+
+#include "absl/base/config.h"
+#include "absl/log/internal/globals.h"
+#include "absl/time/time.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+namespace {
+void InitializeLogImpl(absl::TimeZone time_zone) {
+  // This comes first since it is used by RAW_LOG.
+  absl::log_internal::SetTimeZone(time_zone);
+
+  // Note that initialization is complete, so logs can now be sent to their
+  // proper destinations rather than stderr.
+  log_internal::SetInitialized();
+}
+}  // namespace
+
+void InitializeLog() { InitializeLogImpl(absl::LocalTimeZone()); }
+
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/log/initialize.h b/absl/log/initialize.h
new file mode 100644
index 00000000000..f600eb606c7
--- /dev/null
+++ b/absl/log/initialize.h
@@ -0,0 +1,45 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: log/initialize.h
+// -----------------------------------------------------------------------------
+//
+// This header declares the Abseil Log initialization routine InitializeLog().
+
+#ifndef ABSL_LOG_INITIALIZE_H_
+#define ABSL_LOG_INITIALIZE_H_
+
+#include "absl/base/config.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+// InitializeLog()
+//
+// Initializes the Abseil logging library.
+//
+// Before this function is called, all log messages are directed only to stderr.
+// After initialization is finished, log messages are directed to all registered
+// `LogSink`s.
+//
+// It is an error to call this function twice.
+//
+// There is no corresponding function to shut down the logging library.
+void InitializeLog();
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_LOG_INITIALIZE_H_
diff --git a/absl/log/internal/BUILD.bazel b/absl/log/internal/BUILD.bazel
new file mode 100644
index 00000000000..d9cb7b20f70
--- /dev/null
+++ b/absl/log/internal/BUILD.bazel
@@ -0,0 +1,591 @@
+#
+# Copyright 2022 The Abseil Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+load("@rules_cc//cc:cc_binary.bzl", "cc_binary")
+load("@rules_cc//cc:cc_library.bzl", "cc_library")
+load("@rules_cc//cc:cc_test.bzl", "cc_test")
+load(
+    "//absl:copts/configure_copts.bzl",
+    "ABSL_DEFAULT_COPTS",
+    "ABSL_DEFAULT_LINKOPTS",
+    "ABSL_TEST_COPTS",
+)
+
+package(
+    default_visibility = [
+        ":internal_users",
+        "@do_not_use_for_gloop_visibility_only//gloop/base:__subpackages__",
+    ],
+    features = [
+        "header_modules",
+        "layering_check",
+        "parse_headers",
+    ],
+)
+
+licenses(["notice"])
+
+package_group(
+    name = "internal_users",
+    includes = [
+    ],
+    packages = [
+        "//absl/log",
+    ],
+)
+
+package_group(
+    name = "structured_proto_users",
+    packages = [
+        "//absl/log/...",
+    ],
+)
+
+cc_library(
+    name = "check_impl",
+    hdrs = ["check_impl.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":check_op",
+        ":conditions",
+        ":strip",
+        "//absl/base:core_headers",
+    ],
+)
+
+cc_library(
+    name = "check_op",
+    srcs = ["check_op.cc"],
+    hdrs = ["check_op.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//absl/log:__pkg__"],
+    deps = [
+        ":nullguard",
+        ":nullstream",
+        ":strip",
+        "//absl/base",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:nullability",
+        "//absl/debugging:leak_check",
+        "//absl/strings",
+        "//absl/strings:has_ostream_operator",
+    ],
+)
+
+cc_library(
+    name = "conditions",
+    srcs = ["conditions.cc"],
+    hdrs = ["conditions.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":voidify",
+        "//absl/base",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+    ],
+)
+
+cc_library(
+    name = "config",
+    hdrs = ["config.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl/log:__pkg__",
+        "@do_not_use_for_gloop_visibility_only//gloop/base:__subpackages__",
+    ],
+    deps = ["//absl/base:config"],
+)
+
+cc_library(
+    name = "flags",
+    hdrs = ["flags.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        ":internal_users",
+        "//absl/status:__pkg__",
+        "@do_not_use_for_gloop_visibility_only//gloop/base:__subpackages__",
+    ],
+    deps = [
+        "//absl/flags:flag",
+    ],
+)
+
+cc_library(
+    name = "format",
+    srcs = ["log_format.cc"],
+    hdrs = ["log_format.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":append_truncated",
+        ":config",
+        ":globals",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:log_severity",
+        "//absl/strings",
+        "//absl/strings:str_format",
+        "//absl/time",
+        "//absl/types:span",
+    ],
+)
+
+cc_library(
+    name = "globals",
+    srcs = ["globals.cc"],
+    hdrs = ["globals.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl/log:__pkg__",
+        "@do_not_use_for_gloop_visibility_only//gloop/base:__subpackages__",
+    ],
+    deps = [
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:log_severity",
+        "//absl/base:raw_logging_internal",
+        "//absl/strings",
+        "//absl/time",
+    ],
+)
+
+cc_library(
+    name = "log_impl",
+    hdrs = ["log_impl.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":conditions",
+        ":log_message",
+        ":strip",
+        "//absl/log:absl_vlog_is_on",
+    ],
+)
+
+cc_library(
+    name = "log_message",
+    srcs = ["log_message.cc"],
+    hdrs = ["log_message.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl/log:__pkg__",
+        "@do_not_use_for_gloop_visibility_only//gloop/base:__subpackages__",
+    ],
+    deps = [
+        ":append_truncated",
+        ":format",
+        ":globals",
+        ":log_sink_set",
+        ":nullguard",
+        ":proto",
+        ":structured_proto",
+        "//absl/base",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:errno_saver",
+        "//absl/base:log_severity",
+        "//absl/base:nullability",
+        "//absl/base:raw_logging_internal",
+        "//absl/base:strerror",
+        "//absl/container:inlined_vector",
+        "//absl/debugging:examine_stack",
+        "//absl/log:globals",
+        "//absl/log:log_entry",
+        "//absl/log:log_sink",
+        "//absl/log:log_sink_registry",
+        "//absl/strings",
+        "//absl/strings:internal",
+        "//absl/time",
+        "//absl/types:source_location",
+        "//absl/types:span",
+    ],
+)
+
+cc_library(
+    name = "append_truncated",
+    hdrs = ["append_truncated.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        "//absl/base:config",
+        "//absl/strings",
+        "//absl/strings:internal",
+        "//absl/types:span",
+    ],
+)
+
+cc_library(
+    name = "log_sink_set",
+    srcs = ["log_sink_set.cc"],
+    hdrs = ["log_sink_set.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS + select({
+        "//conditions:default": [],
+        "@platforms//os:android": ["-llog"],
+    }),
+    deps = [
+        ":config",
+        ":globals",
+        "//absl/base",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:log_severity",
+        "//absl/base:no_destructor",
+        "//absl/base:raw_logging_internal",
+        "//absl/cleanup",
+        "//absl/log:globals",
+        "//absl/log:log_entry",
+        "//absl/log:log_sink",
+        "//absl/strings",
+        "//absl/synchronization",
+        "//absl/types:span",
+    ],
+)
+
+cc_library(
+    name = "nullguard",
+    srcs = ["nullguard.cc"],
+    hdrs = ["nullguard.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        "//absl/base:config",
+        "//absl/base:core_headers",
+    ],
+)
+
+cc_library(
+    name = "nullstream",
+    hdrs = ["nullstream.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:log_severity",
+        "//absl/strings",
+    ],
+)
+
+cc_library(
+    name = "strip",
+    hdrs = ["strip.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":log_message",
+        ":nullstream",
+        "//absl/base:core_headers",
+        "//absl/base:log_severity",
+    ],
+)
+
+cc_library(
+    name = "structured",
+    hdrs = ["structured.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        ":internal_users",
+        ":structured_proto_users",
+        "@do_not_use_for_gloop_visibility_only//gloop/base:__subpackages__",
+    ],
+    deps = [
+        ":log_message",
+        ":structured_proto",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/functional:any_invocable",
+        "//absl/strings",
+    ],
+)
+
+cc_library(
+    name = "structured_proto",
+    srcs = ["structured_proto.cc"],
+    hdrs = ["structured_proto.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        ":structured_proto_users",
+    ],
+    deps = [
+        ":proto",
+        "//absl/base:config",
+        "//absl/strings",
+        "//absl/types:span",
+        "//absl/types:variant",
+    ],
+)
+
+cc_test(
+    name = "structured_proto_test",
+    srcs = ["structured_proto_test.cc"],
+    deps = [
+        ":structured_proto",
+        "//absl/base:config",
+        "//absl/strings:string_view",
+        "//absl/types:span",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_library(
+    name = "test_actions",
+    testonly = True,
+    srcs = ["test_actions.cc"],
+    hdrs = ["test_actions.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:log_severity",
+        "//absl/log:log_entry",
+        "//absl/strings",
+        "//absl/time",
+    ] + select({
+        "@rules_cc//cc/compiler:msvc-cl": [],
+        "//conditions:default": [
+        ],
+    }),
+)
+
+cc_library(
+    name = "test_helpers",
+    testonly = True,
+    srcs = ["test_helpers.cc"],
+    hdrs = ["test_helpers.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        ":internal_users",
+        ":structured_proto_users",
+        "@do_not_use_for_gloop_visibility_only//gloop/base:__subpackages__",
+    ],
+    deps = [
+        ":globals",
+        "//absl/base:config",
+        "//absl/base:log_severity",
+        "//absl/log:globals",
+        "//absl/log:initialize",
+        "@googletest//:gtest",
+    ],
+)
+
+cc_library(
+    name = "test_matchers",
+    testonly = True,
+    srcs = ["test_matchers.cc"],
+    hdrs = ["test_matchers.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        ":internal_users",
+        ":structured_proto_users",
+        "@do_not_use_for_gloop_visibility_only//gloop/base:__subpackages__",
+    ],
+    deps = [
+        ":test_helpers",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:log_severity",
+        "//absl/log:log_entry",
+        "//absl/strings",
+        "//absl/time",
+        "@googletest//:gtest",
+    ] + select({
+        "@rules_cc//cc/compiler:msvc-cl": [],
+        "//conditions:default": [
+        ],
+    }),
+)
+
+cc_library(
+    name = "voidify",
+    hdrs = ["voidify.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        "//absl/base:config",
+        "//absl/base:core_headers",
+    ],
+)
+
+cc_library(
+    name = "proto",
+    srcs = ["proto.cc"],
+    hdrs = ["proto.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        "//absl/base",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/strings",
+        "//absl/types:span",
+    ],
+)
+
+cc_library(
+    name = "fnmatch",
+    srcs = ["fnmatch.cc"],
+    hdrs = ["fnmatch.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        "//absl/base:config",
+        "//absl/strings",
+    ],
+)
+
+cc_library(
+    name = "vlog_config",
+    srcs = ["vlog_config.cc"],
+    hdrs = ["vlog_config.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl/log:__subpackages__",
+        "//absl/status:__pkg__",
+        "@do_not_use_for_gloop_visibility_only//gloop/util/status:__subpackages__",
+    ],
+    deps = [
+        ":fnmatch",
+        "//absl/base",
+        "//absl/base:base_internal",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:no_destructor",
+        "//absl/base:nullability",
+        "//absl/strings",
+        "//absl/synchronization",
+    ],
+)
+
+cc_binary(
+    name = "vlog_config_benchmark",
+    testonly = True,
+    srcs = ["vlog_config_benchmark.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    tags = [
+        "benchmark",
+    ],
+    visibility = ["//visibility:private"],
+    deps = [
+        ":vlog_config",
+        "//absl/base:config",
+        "//absl/container:layout",
+        "//absl/random:distributions",
+        "//absl/strings",
+        "@google_benchmark//:benchmark_main",
+    ],
+)
+
+# Test targets
+cc_test(
+    name = "stderr_log_sink_test",
+    size = "small",
+    timeout = "moderate",
+    srcs = ["stderr_log_sink_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    tags = [
+        "no_test:os:android",
+        "no_test:os:ios",
+        "no_test_android",
+        "no_test_darwin_arm64",
+        "no_test_darwin_x86_64",
+        "no_test_fuchsia_x64",
+        "no_test_ios",
+        "no_test_wasm",
+    ],
+    deps = [
+        ":test_helpers",
+        "//absl/base:core_headers",
+        "//absl/base:log_severity",
+        "//absl/log",
+        "//absl/log:globals",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "fnmatch_test",
+    srcs = ["fnmatch_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":fnmatch",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "fnmatch_benchmark",
+    srcs = ["fnmatch_benchmark.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    tags = ["benchmark"],
+    deps = [
+        ":fnmatch",
+        "@google_benchmark//:benchmark_main",
+    ],
+)
+
+cc_library(
+    name = "container",
+    hdrs = ["container.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl:__subpackages__",
+        "@do_not_use_for_gloop_visibility_only//gloop/util/gtl:__subpackages__",
+    ],
+    deps = [
+        "//absl/base:config",
+        "//absl/meta:requires",
+        "//absl/strings",
+    ],
+)
+
+cc_test(
+    name = "container_test",
+    srcs = ["container_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":container",
+        "//absl/base:config",
+        "//absl/strings",
+        "//absl/strings:str_format",
+        "//absl/types:span",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
diff --git a/absl/log/internal/append_truncated.h b/absl/log/internal/append_truncated.h
new file mode 100644
index 00000000000..d420a8b5c6a
--- /dev/null
+++ b/absl/log/internal/append_truncated.h
@@ -0,0 +1,75 @@
+// Copyright 2022 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_LOG_INTERNAL_APPEND_TRUNCATED_H_
+#define ABSL_LOG_INTERNAL_APPEND_TRUNCATED_H_
+
+#include <cstddef>
+#include <cstring>
+#include <string_view>
+
+#include "absl/base/config.h"
+#include "absl/strings/internal/utf8.h"
+#include "absl/strings/string_view.h"
+#include "absl/types/span.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace log_internal {
+// Copies into `dst` as many bytes of `src` as will fit, then truncates the
+// copied bytes from the front of `dst` and returns the number of bytes written.
+inline size_t AppendTruncated(absl::string_view src, absl::Span<char> &dst) {
+  if (src.size() > dst.size()) src = src.substr(0, dst.size());
+  memcpy(dst.data(), src.data(), src.size());
+  dst.remove_prefix(src.size());
+  return src.size();
+}
+// Likewise, but it also takes a wide character string and transforms it into a
+// UTF-8 encoded byte string regardless of the current locale.
+// - On platforms where `wchar_t` is 2 bytes (e.g., Windows), the input is
+//   treated as UTF-16.
+// - On platforms where `wchar_t` is 4 bytes (e.g., Linux, macOS), the input
+//   is treated as UTF-32.
+inline size_t AppendTruncated(std::wstring_view src, absl::Span<char> &dst) {
+  absl::strings_internal::ShiftState state;
+  size_t total_bytes_written = 0;
+  for (const wchar_t wc : src) {
+    // If the destination buffer might not be large enough to write the next
+    // character, stop.
+    if (dst.size() < absl::strings_internal::kMaxEncodedUTF8Size) break;
+    size_t bytes_written =
+        absl::strings_internal::WideToUtf8(wc, dst.data(), state);
+    if (bytes_written == static_cast<size_t>(-1)) {
+      // Invalid character. Encode REPLACEMENT CHARACTER (U+FFFD) instead.
+      constexpr wchar_t kReplacementCharacter = L'\uFFFD';
+      bytes_written = absl::strings_internal::WideToUtf8(kReplacementCharacter,
+                                                         dst.data(), state);
+    }
+    dst.remove_prefix(bytes_written);
+    total_bytes_written += bytes_written;
+  }
+  return total_bytes_written;
+}
+// Likewise, but `n` copies of `c`.
+inline size_t AppendTruncated(char c, size_t n, absl::Span<char> &dst) {
+  if (n > dst.size()) n = dst.size();
+  memset(dst.data(), c, n);
+  dst.remove_prefix(n);
+  return n;
+}
+}  // namespace log_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_LOG_INTERNAL_APPEND_TRUNCATED_H_
diff --git a/absl/log/internal/check_impl.h b/absl/log/internal/check_impl.h
new file mode 100644
index 00000000000..dc2e214de74
--- /dev/null
+++ b/absl/log/internal/check_impl.h
@@ -0,0 +1,151 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// SKIP_ABSL_INLINE_NAMESPACE_CHECK
+
+#ifndef ABSL_LOG_INTERNAL_CHECK_IMPL_H_
+#define ABSL_LOG_INTERNAL_CHECK_IMPL_H_
+
+#include "absl/base/optimization.h"
+#include "absl/log/internal/check_op.h"
+#include "absl/log/internal/conditions.h"
+#include "absl/log/internal/strip.h"
+
+// CHECK
+#define ABSL_LOG_INTERNAL_CHECK_IMPL(condition, condition_text)       \
+  ABSL_LOG_INTERNAL_CONDITION_FATAL(STATELESS,                        \
+                                    ABSL_PREDICT_FALSE(!(condition))) \
+  ABSL_LOG_INTERNAL_CHECK(condition_text).InternalStream()
+
+#define ABSL_LOG_INTERNAL_QCHECK_IMPL(condition, condition_text)       \
+  ABSL_LOG_INTERNAL_CONDITION_QFATAL(STATELESS,                        \
+                                     ABSL_PREDICT_FALSE(!(condition))) \
+  ABSL_LOG_INTERNAL_QCHECK(condition_text).InternalStream()
+
+#define ABSL_LOG_INTERNAL_PCHECK_IMPL(condition, condition_text) \
+  ABSL_LOG_INTERNAL_CHECK_IMPL(condition, condition_text).WithPerror()
+
+#ifndef NDEBUG
+#define ABSL_LOG_INTERNAL_DCHECK_IMPL(condition, condition_text) \
+  ABSL_LOG_INTERNAL_CHECK_IMPL(condition, condition_text)
+#else
+#define ABSL_LOG_INTERNAL_DCHECK_IMPL(condition, condition_text) \
+  ABSL_LOG_INTERNAL_CHECK_IMPL(true || (condition), "true")
+#endif
+
+// CHECK_EQ
+#define ABSL_LOG_INTERNAL_CHECK_EQ_IMPL(val1, val1_text, val2, val2_text) \
+  ABSL_LOG_INTERNAL_CHECK_OP(Check_EQ, ==, val1, val1_text, val2, val2_text)
+#define ABSL_LOG_INTERNAL_CHECK_NE_IMPL(val1, val1_text, val2, val2_text) \
+  ABSL_LOG_INTERNAL_CHECK_OP(Check_NE, !=, val1, val1_text, val2, val2_text)
+#define ABSL_LOG_INTERNAL_CHECK_LE_IMPL(val1, val1_text, val2, val2_text) \
+  ABSL_LOG_INTERNAL_CHECK_OP(Check_LE, <=, val1, val1_text, val2, val2_text)
+#define ABSL_LOG_INTERNAL_CHECK_LT_IMPL(val1, val1_text, val2, val2_text) \
+  ABSL_LOG_INTERNAL_CHECK_OP(Check_LT, <, val1, val1_text, val2, val2_text)
+#define ABSL_LOG_INTERNAL_CHECK_GE_IMPL(val1, val1_text, val2, val2_text) \
+  ABSL_LOG_INTERNAL_CHECK_OP(Check_GE, >=, val1, val1_text, val2, val2_text)
+#define ABSL_LOG_INTERNAL_CHECK_GT_IMPL(val1, val1_text, val2, val2_text) \
+  ABSL_LOG_INTERNAL_CHECK_OP(Check_GT, >, val1, val1_text, val2, val2_text)
+#define ABSL_LOG_INTERNAL_QCHECK_EQ_IMPL(val1, val1_text, val2, val2_text) \
+  ABSL_LOG_INTERNAL_QCHECK_OP(Check_EQ, ==, val1, val1_text, val2, val2_text)
+#define ABSL_LOG_INTERNAL_QCHECK_NE_IMPL(val1, val1_text, val2, val2_text) \
+  ABSL_LOG_INTERNAL_QCHECK_OP(Check_NE, !=, val1, val1_text, val2, val2_text)
+#define ABSL_LOG_INTERNAL_QCHECK_LE_IMPL(val1, val1_text, val2, val2_text) \
+  ABSL_LOG_INTERNAL_QCHECK_OP(Check_LE, <=, val1, val1_text, val2, val2_text)
+#define ABSL_LOG_INTERNAL_QCHECK_LT_IMPL(val1, val1_text, val2, val2_text) \
+  ABSL_LOG_INTERNAL_QCHECK_OP(Check_LT, <, val1, val1_text, val2, val2_text)
+#define ABSL_LOG_INTERNAL_QCHECK_GE_IMPL(val1, val1_text, val2, val2_text) \
+  ABSL_LOG_INTERNAL_QCHECK_OP(Check_GE, >=, val1, val1_text, val2, val2_text)
+#define ABSL_LOG_INTERNAL_QCHECK_GT_IMPL(val1, val1_text, val2, val2_text) \
+  ABSL_LOG_INTERNAL_QCHECK_OP(Check_GT, >, val1, val1_text, val2, val2_text)
+#ifndef NDEBUG
+#define ABSL_LOG_INTERNAL_DCHECK_EQ_IMPL(val1, val1_text, val2, val2_text) \
+  ABSL_LOG_INTERNAL_CHECK_EQ_IMPL(val1, val1_text, val2, val2_text)
+#define ABSL_LOG_INTERNAL_DCHECK_NE_IMPL(val1, val1_text, val2, val2_text) \
+  ABSL_LOG_INTERNAL_CHECK_NE_IMPL(val1, val1_text, val2, val2_text)
+#define ABSL_LOG_INTERNAL_DCHECK_LE_IMPL(val1, val1_text, val2, val2_text) \
+  ABSL_LOG_INTERNAL_CHECK_LE_IMPL(val1, val1_text, val2, val2_text)
+#define ABSL_LOG_INTERNAL_DCHECK_LT_IMPL(val1, val1_text, val2, val2_text) \
+  ABSL_LOG_INTERNAL_CHECK_LT_IMPL(val1, val1_text, val2, val2_text)
+#define ABSL_LOG_INTERNAL_DCHECK_GE_IMPL(val1, val1_text, val2, val2_text) \
+  ABSL_LOG_INTERNAL_CHECK_GE_IMPL(val1, val1_text, val2, val2_text)
+#define ABSL_LOG_INTERNAL_DCHECK_GT_IMPL(val1, val1_text, val2, val2_text) \
+  ABSL_LOG_INTERNAL_CHECK_GT_IMPL(val1, val1_text, val2, val2_text)
+#else  // ndef NDEBUG
+#define ABSL_LOG_INTERNAL_DCHECK_EQ_IMPL(val1, val1_text, val2, val2_text) \
+  ABSL_LOG_INTERNAL_DCHECK_NOP(val1, val2)
+#define ABSL_LOG_INTERNAL_DCHECK_NE_IMPL(val1, val1_text, val2, val2_text) \
+  ABSL_LOG_INTERNAL_DCHECK_NOP(val1, val2)
+#define ABSL_LOG_INTERNAL_DCHECK_LE_IMPL(val1, val1_text, val2, val2_text) \
+  ABSL_LOG_INTERNAL_DCHECK_NOP(val1, val2)
+#define ABSL_LOG_INTERNAL_DCHECK_LT_IMPL(val1, val1_text, val2, val2_text) \
+  ABSL_LOG_INTERNAL_DCHECK_NOP(val1, val2)
+#define ABSL_LOG_INTERNAL_DCHECK_GE_IMPL(val1, val1_text, val2, val2_text) \
+  ABSL_LOG_INTERNAL_DCHECK_NOP(val1, val2)
+#define ABSL_LOG_INTERNAL_DCHECK_GT_IMPL(val1, val1_text, val2, val2_text) \
+  ABSL_LOG_INTERNAL_DCHECK_NOP(val1, val2)
+#endif  // def NDEBUG
+
+// CHECK_OK
+#define ABSL_LOG_INTERNAL_CHECK_OK_IMPL(status, status_text) \
+  ABSL_LOG_INTERNAL_CHECK_OK(status, status_text)
+#define ABSL_LOG_INTERNAL_QCHECK_OK_IMPL(status, status_text) \
+  ABSL_LOG_INTERNAL_QCHECK_OK(status, status_text)
+#ifndef NDEBUG
+#define ABSL_LOG_INTERNAL_DCHECK_OK_IMPL(status, status_text) \
+  ABSL_LOG_INTERNAL_CHECK_OK(status, status_text)
+#else
+#define ABSL_LOG_INTERNAL_DCHECK_OK_IMPL(status, status_text) \
+  ABSL_LOG_INTERNAL_DCHECK_NOP(status, nullptr)
+#endif
+
+// CHECK_STREQ
+#define ABSL_LOG_INTERNAL_CHECK_STREQ_IMPL(s1, s1_text, s2, s2_text) \
+  ABSL_LOG_INTERNAL_CHECK_STROP(strcmp, ==, true, s1, s1_text, s2, s2_text)
+#define ABSL_LOG_INTERNAL_CHECK_STRNE_IMPL(s1, s1_text, s2, s2_text) \
+  ABSL_LOG_INTERNAL_CHECK_STROP(strcmp, !=, false, s1, s1_text, s2, s2_text)
+#define ABSL_LOG_INTERNAL_CHECK_STRCASEEQ_IMPL(s1, s1_text, s2, s2_text) \
+  ABSL_LOG_INTERNAL_CHECK_STROP(strcasecmp, ==, true, s1, s1_text, s2, s2_text)
+#define ABSL_LOG_INTERNAL_CHECK_STRCASENE_IMPL(s1, s1_text, s2, s2_text) \
+  ABSL_LOG_INTERNAL_CHECK_STROP(strcasecmp, !=, false, s1, s1_text, s2, s2_text)
+#define ABSL_LOG_INTERNAL_QCHECK_STREQ_IMPL(s1, s1_text, s2, s2_text) \
+  ABSL_LOG_INTERNAL_QCHECK_STROP(strcmp, ==, true, s1, s1_text, s2, s2_text)
+#define ABSL_LOG_INTERNAL_QCHECK_STRNE_IMPL(s1, s1_text, s2, s2_text) \
+  ABSL_LOG_INTERNAL_QCHECK_STROP(strcmp, !=, false, s1, s1_text, s2, s2_text)
+#define ABSL_LOG_INTERNAL_QCHECK_STRCASEEQ_IMPL(s1, s1_text, s2, s2_text) \
+  ABSL_LOG_INTERNAL_QCHECK_STROP(strcasecmp, ==, true, s1, s1_text, s2, s2_text)
+#define ABSL_LOG_INTERNAL_QCHECK_STRCASENE_IMPL(s1, s1_text, s2, s2_text) \
+  ABSL_LOG_INTERNAL_QCHECK_STROP(strcasecmp, !=, false, s1, s1_text, s2,  \
+                                 s2_text)
+#ifndef NDEBUG
+#define ABSL_LOG_INTERNAL_DCHECK_STREQ_IMPL(s1, s1_text, s2, s2_text) \
+  ABSL_LOG_INTERNAL_CHECK_STREQ_IMPL(s1, s1_text, s2, s2_text)
+#define ABSL_LOG_INTERNAL_DCHECK_STRCASEEQ_IMPL(s1, s1_text, s2, s2_text) \
+  ABSL_LOG_INTERNAL_CHECK_STRCASEEQ_IMPL(s1, s1_text, s2, s2_text)
+#define ABSL_LOG_INTERNAL_DCHECK_STRNE_IMPL(s1, s1_text, s2, s2_text) \
+  ABSL_LOG_INTERNAL_CHECK_STRNE_IMPL(s1, s1_text, s2, s2_text)
+#define ABSL_LOG_INTERNAL_DCHECK_STRCASENE_IMPL(s1, s1_text, s2, s2_text) \
+  ABSL_LOG_INTERNAL_CHECK_STRCASENE_IMPL(s1, s1_text, s2, s2_text)
+#else  // ndef NDEBUG
+#define ABSL_LOG_INTERNAL_DCHECK_STREQ_IMPL(s1, s1_text, s2, s2_text) \
+  ABSL_LOG_INTERNAL_DCHECK_NOP(s1, s2)
+#define ABSL_LOG_INTERNAL_DCHECK_STRCASEEQ_IMPL(s1, s1_text, s2, s2_text) \
+  ABSL_LOG_INTERNAL_DCHECK_NOP(s1, s2)
+#define ABSL_LOG_INTERNAL_DCHECK_STRNE_IMPL(s1, s1_text, s2, s2_text) \
+  ABSL_LOG_INTERNAL_DCHECK_NOP(s1, s2)
+#define ABSL_LOG_INTERNAL_DCHECK_STRCASENE_IMPL(s1, s1_text, s2, s2_text) \
+  ABSL_LOG_INTERNAL_DCHECK_NOP(s1, s2)
+#endif  // def NDEBUG
+
+#endif  // ABSL_LOG_INTERNAL_CHECK_IMPL_H_
diff --git a/absl/log/internal/check_op.cc b/absl/log/internal/check_op.cc
new file mode 100644
index 00000000000..510c5764130
--- /dev/null
+++ b/absl/log/internal/check_op.cc
@@ -0,0 +1,148 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/log/internal/check_op.h"
+
+#include <cstdint>
+#include <cstring>
+#include <ostream>
+#include <string>
+#include <utility>
+
+#include "absl/base/config.h"
+#include "absl/base/nullability.h"
+#include "absl/debugging/leak_check.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/string_view.h"
+
+#ifdef _MSC_VER
+#define strcasecmp _stricmp
+#else
+#include <strings.h>  // for strcasecmp, but msvc does not have this header
+#endif
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace log_internal {
+
+#define ABSL_LOG_INTERNAL_DEFINE_MAKE_CHECK_OP_STRING(x) \
+  template const char* absl_nonnull MakeCheckOpString(   \
+      x, x, const char* absl_nonnull)
+ABSL_LOG_INTERNAL_DEFINE_MAKE_CHECK_OP_STRING(bool);
+ABSL_LOG_INTERNAL_DEFINE_MAKE_CHECK_OP_STRING(int64_t);
+ABSL_LOG_INTERNAL_DEFINE_MAKE_CHECK_OP_STRING(uint64_t);
+ABSL_LOG_INTERNAL_DEFINE_MAKE_CHECK_OP_STRING(float);
+ABSL_LOG_INTERNAL_DEFINE_MAKE_CHECK_OP_STRING(double);
+ABSL_LOG_INTERNAL_DEFINE_MAKE_CHECK_OP_STRING(char);
+ABSL_LOG_INTERNAL_DEFINE_MAKE_CHECK_OP_STRING(unsigned char);
+ABSL_LOG_INTERNAL_DEFINE_MAKE_CHECK_OP_STRING(const std::string&);
+ABSL_LOG_INTERNAL_DEFINE_MAKE_CHECK_OP_STRING(const absl::string_view&);
+ABSL_LOG_INTERNAL_DEFINE_MAKE_CHECK_OP_STRING(const char*);
+ABSL_LOG_INTERNAL_DEFINE_MAKE_CHECK_OP_STRING(const signed char*);
+ABSL_LOG_INTERNAL_DEFINE_MAKE_CHECK_OP_STRING(const unsigned char*);
+ABSL_LOG_INTERNAL_DEFINE_MAKE_CHECK_OP_STRING(const void*);
+#undef ABSL_LOG_INTERNAL_DEFINE_MAKE_CHECK_OP_STRING
+
+CheckOpMessageBuilder::CheckOpMessageBuilder(
+    const char* absl_nonnull exprtext) {
+  stream_ << exprtext << " (";
+}
+
+std::ostream& CheckOpMessageBuilder::ForVar2() {
+  stream_ << " vs. ";
+  return stream_;
+}
+
+const char* absl_nonnull CheckOpMessageBuilder::NewString() {
+  stream_ << ")";
+  // There's no need to free this string since the process is crashing.
+  return absl::IgnoreLeak(new std::string(std::move(stream_).str()))->c_str();
+}
+
+void MakeCheckOpValueString(std::ostream& os, const char v) {
+  if (v >= 32 && v <= 126) {
+    os << "'" << v << "'";
+  } else {
+    os << "char value " << int{v};
+  }
+}
+
+void MakeCheckOpValueString(std::ostream& os, const signed char v) {
+  if (v >= 32 && v <= 126) {
+    os << "'" << v << "'";
+  } else {
+    os << "signed char value " << int{v};
+  }
+}
+
+void MakeCheckOpValueString(std::ostream& os, const unsigned char v) {
+  if (v >= 32 && v <= 126) {
+    os << "'" << v << "'";
+  } else {
+    os << "unsigned char value " << int{v};
+  }
+}
+
+void MakeCheckOpValueString(std::ostream& os, const void* p) {
+  if (p == nullptr) {
+    os << "(null)";
+  } else {
+    os << p;
+  }
+}
+
+std::ostream& operator<<(std::ostream& os, UnprintableWrapper) {
+  return os << "UNPRINTABLE";
+}
+
+// Helper functions for string comparisons.
+#define DEFINE_CHECK_STROP_IMPL(name, func, expected)                          \
+  const char* absl_nullable Check##func##expected##Impl(                       \
+      const char* absl_nullable s1, const char* absl_nullable s2,              \
+      const char* absl_nonnull exprtext) {                                     \
+    bool equal = s1 == s2 || (s1 && s2 && !func(s1, s2));                      \
+    if (equal == expected) {                                                   \
+      return nullptr;                                                          \
+    } else {                                                                   \
+      /* There's no need to free this string since the process is crashing. */ \
+      return absl::IgnoreLeak(new std::string(absl::StrCat(exprtext, " (", s1, \
+                                                           " vs. ", s2, ")"))) \
+          ->c_str();                                                           \
+    }                                                                          \
+  }
+DEFINE_CHECK_STROP_IMPL(CHECK_STREQ, strcmp, true)
+DEFINE_CHECK_STROP_IMPL(CHECK_STRNE, strcmp, false)
+DEFINE_CHECK_STROP_IMPL(CHECK_STRCASEEQ, strcasecmp, true)
+DEFINE_CHECK_STROP_IMPL(CHECK_STRCASENE, strcasecmp, false)
+#undef DEFINE_CHECK_STROP_IMPL
+
+namespace detect_specialization {
+
+StringifySink::StringifySink(std::ostream& os) : os_(os) {}
+
+void StringifySink::Append(absl::string_view text) { os_ << text; }
+
+void StringifySink::Append(size_t length, char ch) {
+  for (size_t i = 0; i < length; ++i) os_.put(ch);
+}
+
+void AbslFormatFlush(StringifySink* sink, absl::string_view text) {
+  sink->Append(text);
+}
+
+}  // namespace detect_specialization
+
+}  // namespace log_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/log/internal/check_op.h b/absl/log/internal/check_op.h
new file mode 100644
index 00000000000..66050f0a69e
--- /dev/null
+++ b/absl/log/internal/check_op.h
@@ -0,0 +1,556 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: log/internal/check_op.h
+// -----------------------------------------------------------------------------
+//
+// This file declares helpers routines and macros used to implement `CHECK`
+// macros.
+
+#ifndef ABSL_LOG_INTERNAL_CHECK_OP_H_
+#define ABSL_LOG_INTERNAL_CHECK_OP_H_
+
+#include <stdint.h>
+
+#include <cstddef>
+#include <ostream>
+#include <sstream>
+#include <string>
+#include <type_traits>
+#include <utility>
+
+#include "absl/base/attributes.h"
+#include "absl/base/casts.h"
+#include "absl/base/config.h"
+#include "absl/base/nullability.h"
+#include "absl/base/optimization.h"
+#include "absl/log/internal/nullguard.h"
+#include "absl/log/internal/nullstream.h"
+#include "absl/log/internal/strip.h"
+#include "absl/strings/has_absl_stringify.h"
+#include "absl/strings/has_ostream_operator.h"
+#include "absl/strings/string_view.h"
+
+// `ABSL_LOG_INTERNAL_STRIP_STRING_LITERAL` wraps string literals that
+// should be stripped when `ABSL_MIN_LOG_LEVEL` exceeds `kFatal`.
+#ifdef ABSL_MIN_LOG_LEVEL
+#define ABSL_LOG_INTERNAL_STRIP_STRING_LITERAL(literal)                \
+  (::absl::LogSeverity::kFatal >=                                      \
+           static_cast<::absl::LogSeverityAtLeast>(ABSL_MIN_LOG_LEVEL) \
+       ? (literal)                                                     \
+       : "")
+#else
+#define ABSL_LOG_INTERNAL_STRIP_STRING_LITERAL(literal) (literal)
+#endif
+
+#ifdef NDEBUG
+// `NDEBUG` is defined, so `DCHECK_EQ(x, y)` and so on do nothing.  However, we
+// still want the compiler to parse `x` and `y`, because we don't want to lose
+// potentially useful errors and warnings.
+#define ABSL_LOG_INTERNAL_DCHECK_NOP(x, y)   \
+  while (false && ((void)(x), (void)(y), 0)) \
+  ::absl::log_internal::NullStream().InternalStream()
+#endif
+
+#define ABSL_LOG_INTERNAL_CHECK_OP(name, op, val1, val1_text, val2, val2_text) \
+  while (const char* absl_nullable absl_log_internal_check_op_result           \
+         [[maybe_unused]] = ::absl::log_internal::name##Impl(                  \
+             ::absl::log_internal::GetReferenceableValue(val1),                \
+             ::absl::log_internal::GetReferenceableValue(val2),                \
+             ABSL_LOG_INTERNAL_STRIP_STRING_LITERAL(val1_text " " #op          \
+                                                              " " val2_text))) \
+    ABSL_LOG_INTERNAL_CONDITION_FATAL(STATELESS, true)                         \
+  ABSL_LOG_INTERNAL_CHECK(::absl::implicit_cast<const char* absl_nonnull>(     \
+                              absl_log_internal_check_op_result))              \
+      .InternalStream()
+#define ABSL_LOG_INTERNAL_QCHECK_OP(name, op, val1, val1_text, val2,        \
+                                    val2_text)                              \
+  while (const char* absl_nullable absl_log_internal_qcheck_op_result =     \
+             ::absl::log_internal::name##Impl(                              \
+                 ::absl::log_internal::GetReferenceableValue(val1),         \
+                 ::absl::log_internal::GetReferenceableValue(val2),         \
+                 ABSL_LOG_INTERNAL_STRIP_STRING_LITERAL(                    \
+                     val1_text " " #op " " val2_text)))                     \
+    ABSL_LOG_INTERNAL_CONDITION_QFATAL(STATELESS, true)                     \
+  ABSL_LOG_INTERNAL_QCHECK(::absl::implicit_cast<const char* absl_nonnull>( \
+                               absl_log_internal_qcheck_op_result))         \
+      .InternalStream()
+#define ABSL_LOG_INTERNAL_CHECK_STROP(func, op, expected, s1, s1_text, s2,     \
+                                      s2_text)                                 \
+  while (const char* absl_nullable absl_log_internal_check_strop_result =      \
+             ::absl::log_internal::Check##func##expected##Impl(                \
+                 (s1), (s2),                                                   \
+                 ABSL_LOG_INTERNAL_STRIP_STRING_LITERAL(s1_text " " #op        \
+                                                                " " s2_text))) \
+    ABSL_LOG_INTERNAL_CONDITION_FATAL(STATELESS, true)                         \
+  ABSL_LOG_INTERNAL_CHECK(::absl::implicit_cast<const char* absl_nonnull>(     \
+                              absl_log_internal_check_strop_result))           \
+      .InternalStream()
+#define ABSL_LOG_INTERNAL_QCHECK_STROP(func, op, expected, s1, s1_text, s2,    \
+                                       s2_text)                                \
+  while (const char* absl_nullable absl_log_internal_qcheck_strop_result =     \
+             ::absl::log_internal::Check##func##expected##Impl(                \
+                 (s1), (s2),                                                   \
+                 ABSL_LOG_INTERNAL_STRIP_STRING_LITERAL(s1_text " " #op        \
+                                                                " " s2_text))) \
+    ABSL_LOG_INTERNAL_CONDITION_QFATAL(STATELESS, true)                        \
+  ABSL_LOG_INTERNAL_QCHECK(::absl::implicit_cast<const char* absl_nonnull>(    \
+                               absl_log_internal_qcheck_strop_result))         \
+      .InternalStream()
+
+// This one is tricky:
+// * We must evaluate `val` exactly once, yet we need to do two things with it:
+//   evaluate `.ok()` and (sometimes) `.ToString()`.
+// * `val` might be an `absl::Status` or some `absl::StatusOr<T>`.
+// * `val` might be e.g. `ATemporary().GetStatus()`, which may return a
+//   reference to a member of `ATemporary` that is only valid until the end of
+//   the full expression.
+// * We don't want this file to depend on `absl::Status` `#include`s or linkage,
+//   nor do we want to move the definition to status and introduce a dependency
+//   in the other direction.  We can be assured that callers must already have a
+//   `Status` and the necessary `#include`s and linkage.
+// * Callsites should be small and fast (at least when `val.ok()`): one branch,
+//   minimal stack footprint.
+//   * In particular, the string concat stuff should be out-of-line and emitted
+//     in only one TU to save linker input size
+// * We want the `val.ok()` check inline so static analyzers and optimizers can
+//   see it.
+// * As usual, no braces so we can stream into the expansion with `operator<<`.
+// * Also as usual, it must expand to a single (partial) statement with no
+//   ambiguous-else problems.
+// * When stripped by `ABSL_MIN_LOG_LEVEL`, we must discard the `<expr> is OK`
+//   string literal and abort without doing any streaming.  We don't need to
+//   strip the call to stringify the non-ok `Status` as long as we don't log it;
+//   dropping the `Status`'s message text is out of scope.
+#define ABSL_LOG_INTERNAL_CHECK_OK(val, val_text)                         \
+  for (::std::pair<const ::absl::Status* absl_nonnull,                    \
+                   const char* absl_nonnull>                              \
+           absl_log_internal_check_ok_goo;                                \
+       absl_log_internal_check_ok_goo.first =                             \
+           ::absl::log_internal::AsStatus(val),                           \
+       absl_log_internal_check_ok_goo.second =                            \
+           ABSL_PREDICT_TRUE(absl_log_internal_check_ok_goo.first->ok())  \
+               ? "" /* Don't use nullptr, to keep the annotation happy */ \
+               : ::absl::status_internal::MakeCheckFailString(            \
+                     absl_log_internal_check_ok_goo.first,                \
+                     ABSL_LOG_INTERNAL_STRIP_STRING_LITERAL(val_text      \
+                                                            " is OK")),   \
+       !ABSL_PREDICT_TRUE(absl_log_internal_check_ok_goo.first->ok());)   \
+    ABSL_LOG_INTERNAL_CONDITION_FATAL(STATELESS, true)                    \
+  ABSL_LOG_INTERNAL_CHECK(absl_log_internal_check_ok_goo.second)          \
+      .InternalStream()
+#define ABSL_LOG_INTERNAL_QCHECK_OK(val, val_text)                        \
+  for (::std::pair<const ::absl::Status* absl_nonnull,                    \
+                   const char* absl_nonnull>                              \
+           absl_log_internal_qcheck_ok_goo;                               \
+       absl_log_internal_qcheck_ok_goo.first =                            \
+           ::absl::log_internal::AsStatus(val),                           \
+       absl_log_internal_qcheck_ok_goo.second =                           \
+           ABSL_PREDICT_TRUE(absl_log_internal_qcheck_ok_goo.first->ok()) \
+               ? "" /* Don't use nullptr, to keep the annotation happy */ \
+               : ::absl::status_internal::MakeCheckFailString(            \
+                     absl_log_internal_qcheck_ok_goo.first,               \
+                     ABSL_LOG_INTERNAL_STRIP_STRING_LITERAL(val_text      \
+                                                            " is OK")),   \
+       !ABSL_PREDICT_TRUE(absl_log_internal_qcheck_ok_goo.first->ok());)  \
+    ABSL_LOG_INTERNAL_CONDITION_QFATAL(STATELESS, true)                   \
+  ABSL_LOG_INTERNAL_QCHECK(absl_log_internal_qcheck_ok_goo.second)        \
+      .InternalStream()
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+class Status;
+template <typename T>
+class StatusOr;
+
+namespace status_internal {
+ABSL_ATTRIBUTE_PURE_FUNCTION const char* absl_nonnull MakeCheckFailString(
+    const absl::Status* absl_nonnull status, const char* absl_nonnull prefix);
+}  // namespace status_internal
+
+namespace log_internal {
+
+// Convert a Status or a StatusOr to its underlying status value.
+//
+// (This implementation does not require a dep on absl::Status to work.)
+inline const absl::Status* absl_nonnull AsStatus(const absl::Status& s) {
+  return &s;
+}
+template <typename T>
+const absl::Status* absl_nonnull AsStatus(const absl::StatusOr<T>& s) {
+  return &s.status();
+}
+
+// A helper class for formatting `expr (V1 vs. V2)` in a `CHECK_XX` statement.
+// See `MakeCheckOpString` for sample usage.
+class CheckOpMessageBuilder final {
+ public:
+  // Inserts `exprtext` and ` (` to the stream.
+  explicit CheckOpMessageBuilder(const char* absl_nonnull exprtext);
+  ~CheckOpMessageBuilder() = default;
+  // For inserting the first variable.
+  std::ostream& ForVar1() { return stream_; }
+  // For inserting the second variable (adds an intermediate ` vs. `).
+  std::ostream& ForVar2();
+  // Get the result (inserts the closing `)`).
+  const char* absl_nonnull NewString();
+
+ private:
+  std::ostringstream stream_;
+};
+
+// This formats a value for a failing `CHECK_XX` statement.  Ordinarily, it uses
+// the definition for `operator<<`, with a few special cases below.
+template <typename T>
+inline void MakeCheckOpValueString(std::ostream& os, const T& v) {
+  os << log_internal::NullGuard<T>::Guard(v);
+}
+
+// Overloads for char types provide readable values for unprintable characters.
+void MakeCheckOpValueString(std::ostream& os, char v);
+void MakeCheckOpValueString(std::ostream& os, signed char v);
+void MakeCheckOpValueString(std::ostream& os, unsigned char v);
+void MakeCheckOpValueString(std::ostream& os, const void* absl_nullable p);
+
+// A wrapper for types that have no operator<<.
+struct UnprintableWrapper {
+  template <typename T>
+  explicit UnprintableWrapper(const T&) {}
+
+  friend std::ostream& operator<<(std::ostream& os, UnprintableWrapper);
+};
+
+namespace detect_specialization {
+
+// MakeCheckOpString is being specialized for every T and U pair that is being
+// passed to the CHECK_op macros. However, there is a lot of redundancy in these
+// specializations that creates unnecessary library and binary bloat.
+// The number of instantiations tends to be O(n^2) because we have two
+// independent inputs. This technique works by reducing `n`.
+//
+// Most user-defined types being passed to CHECK_op end up being printed as a
+// builtin type. For example, enums tend to be implicitly converted to its
+// underlying type when calling operator<<, and pointers are printed with the
+// `const void*` overload.
+// To reduce the number of instantiations we coerce these values before calling
+// MakeCheckOpString instead of inside it.
+//
+// To detect if this coercion is needed, we duplicate all the relevant
+// operator<< overloads as specified in the standard, just in a different
+// namespace. If the call to `stream << value` becomes ambiguous, it means that
+// one of these overloads is the one selected by overload resolution. We then
+// do overload resolution again just with our overload set to see which one gets
+// selected. That tells us which type to coerce to.
+// If the augmented call was not ambiguous, it means that none of these were
+// selected and we can't coerce the input.
+//
+// As a secondary step to reduce code duplication, we promote integral types to
+// their 64-bit variant. This does not change the printed value, but reduces the
+// number of instantiations even further. Promoting an integer is very cheap at
+// the call site.
+int64_t operator<<(std::ostream&, short value);           // NOLINT
+int64_t operator<<(std::ostream&, unsigned short value);  // NOLINT
+int64_t operator<<(std::ostream&, int value);
+int64_t operator<<(std::ostream&, unsigned int value);
+int64_t operator<<(std::ostream&, long value);                 // NOLINT
+uint64_t operator<<(std::ostream&, unsigned long value);       // NOLINT
+int64_t operator<<(std::ostream&, long long value);            // NOLINT
+uint64_t operator<<(std::ostream&, unsigned long long value);  // NOLINT
+float operator<<(std::ostream&, float value);
+double operator<<(std::ostream&, double value);
+long double operator<<(std::ostream&, long double value);
+bool operator<<(std::ostream&, bool value);
+const void* absl_nullable operator<<(std::ostream&,
+                                     const void* absl_nullable value);
+const void* absl_nullable operator<<(std::ostream&, std::nullptr_t);
+
+// These `char` overloads are specified like this in the standard, so we have to
+// write them exactly the same to ensure the call is ambiguous.
+// If we wrote it in a different way (eg taking std::ostream instead of the
+// template) then one call might have a higher rank than the other and it would
+// not be ambiguous.
+template <typename Traits>
+char operator<<(std::basic_ostream<char, Traits>&, char);
+template <typename Traits>
+signed char operator<<(std::basic_ostream<char, Traits>&, signed char);
+template <typename Traits>
+unsigned char operator<<(std::basic_ostream<char, Traits>&, unsigned char);
+template <typename Traits>
+const char* absl_nonnull operator<<(std::basic_ostream<char, Traits>&,
+                                    const char* absl_nonnull);
+template <typename Traits>
+const signed char* absl_nonnull operator<<(std::basic_ostream<char, Traits>&,
+                                           const signed char* absl_nonnull);
+template <typename Traits>
+const unsigned char* absl_nonnull operator<<(std::basic_ostream<char, Traits>&,
+                                             const unsigned char* absl_nonnull);
+
+// This overload triggers when the call is not ambiguous.
+// It means that T is being printed with some overload not on this list.
+// We keep the value as `const T&`.
+template <typename T, typename = decltype(std::declval<std::ostream&>()
+                                          << std::declval<const T&>())>
+const T& Detect(int);
+
+// This overload triggers when the call is ambiguous.
+// It means that T is either one from this list or printed as one from this
+// list. Eg an unscoped enum that decays to `int` for printing.
+// We ask the overload set to give us the type we want to convert it to.
+template <typename T>
+decltype(detect_specialization::operator<<(
+    std::declval<std::ostream&>(), std::declval<const T&>())) Detect(char);
+
+// A sink for AbslStringify which redirects everything to a std::ostream.
+class StringifySink {
+ public:
+  explicit StringifySink(std::ostream& os ABSL_ATTRIBUTE_LIFETIME_BOUND);
+
+  void Append(absl::string_view text);
+  void Append(size_t length, char ch);
+  friend void AbslFormatFlush(StringifySink* absl_nonnull sink,
+                              absl::string_view text);
+
+ private:
+  std::ostream& os_;
+};
+
+// Wraps a type implementing AbslStringify, and implements operator<<.
+template <typename T>
+class StringifyToStreamWrapper {
+ public:
+  explicit StringifyToStreamWrapper(const T& v ABSL_ATTRIBUTE_LIFETIME_BOUND)
+      : v_(v) {}
+
+  friend std::ostream& operator<<(std::ostream& os,
+                                  const StringifyToStreamWrapper& wrapper) {
+    StringifySink sink(os);
+    AbslStringify(sink, wrapper.v_);
+    return os;
+  }
+
+ private:
+  const T& v_;
+};
+
+// This overload triggers when T implements AbslStringify.
+// StringifyToStreamWrapper is used to allow MakeCheckOpString to use
+// operator<<.
+template <typename T>
+std::enable_if_t<HasAbslStringify<T>::value,
+                 StringifyToStreamWrapper<T>>
+Detect(...);  // Ellipsis has lowest preference when int passed.
+
+// This overload triggers when T is neither possible to print nor an enum.
+template <typename T>
+std::enable_if_t<std::negation_v<std::disjunction<
+                     std::is_convertible<T, int>, std::is_enum<T>,
+                     std::is_pointer<T>, std::is_same<T, std::nullptr_t>,
+                     HasOstreamOperator<T>, HasAbslStringify<T>>>,
+                 UnprintableWrapper>
+Detect(...);
+
+// Equivalent to the updated std::underlying_type from C++20, which is no
+// longer undefined behavior for non-enum types.
+template <typename T, typename EnableT = void>
+struct UnderlyingType {};
+
+template <typename T>
+struct UnderlyingType<T, std::enable_if_t<std::is_enum_v<T>>> {
+  using type = std::underlying_type_t<T>;
+};
+template <typename T>
+using UnderlyingTypeT = typename UnderlyingType<T>::type;
+
+// This overload triggers when T is a scoped enum that has not defined an output
+// stream operator (operator<<) or AbslStringify. It causes the enum value to be
+// converted to a type that can be streamed. For consistency with other enums, a
+// scoped enum backed by a bool or char is converted to its underlying type, and
+// one backed by another integer is converted to (u)int64_t.
+template <typename T>
+std::enable_if_t<
+    std::conjunction_v<std::is_enum<T>,
+                       std::negation<std::is_convertible<T, int>>,
+                       std::negation<HasOstreamOperator<T>>,
+                       std::negation<HasAbslStringify<T>>>,
+    std::conditional_t<std::is_same_v<UnderlyingTypeT<T>, bool> ||
+                           std::is_same_v<UnderlyingTypeT<T>, char> ||
+                           std::is_same_v<UnderlyingTypeT<T>, signed char> ||
+                           std::is_same_v<UnderlyingTypeT<T>, unsigned char>,
+                       UnderlyingTypeT<T>,
+                       std::conditional_t<std::is_signed_v<UnderlyingTypeT<T>>,
+                                          int64_t, uint64_t>>>
+Detect(...);
+
+template <typename T>
+using Detected = decltype(Detect<T>(0));
+}  // namespace detect_specialization
+
+// If the comparison will happen as pointers, decay `char*` arguments to `void*`
+// when printing them. There is no evidence that they are a NULL terminated
+// C-String so printing them as such could lead to UB, and more importantly we
+// compared pointers so showing the pointers is a better result.
+template <typename T>
+constexpr bool IsCharStarOrVoidStar() {
+  if constexpr (std::is_reference_v<T>) {
+    return IsCharStarOrVoidStar<std::remove_reference_t<T>>();
+  } else if constexpr (std::is_array_v<T>) {
+    return IsCharStarOrVoidStar<std::decay_t<T>>();
+  } else {
+    using U = std::remove_const_t<std::remove_pointer_t<T>>;
+    return std::is_pointer_v<T> &&
+        (std::is_same_v<char, U> || std::is_same_v<unsigned char, U> ||
+         std::is_same_v<signed char, U> || std::is_void_v<U>);
+  }
+}
+
+template <typename T1, typename T2,
+          typename U1 = detect_specialization::Detected<T1>,
+          typename U2 = detect_specialization::Detected<T2>>
+using CheckOpStreamType =
+    std::conditional_t<IsCharStarOrVoidStar<U1>() && IsCharStarOrVoidStar<U2>(),
+                       const void*, U1>;
+
+// Build the error message string.  Specify no inlining for code size.
+template <typename T1, typename T2>
+ABSL_ATTRIBUTE_RETURNS_NONNULL const char* absl_nonnull MakeCheckOpString(
+    T1 v1, T2 v2, const char* absl_nonnull exprtext) ABSL_ATTRIBUTE_NOINLINE;
+
+template <typename T1, typename T2>
+const char* absl_nonnull MakeCheckOpString(T1 v1, T2 v2,
+                                           const char* absl_nonnull exprtext) {
+  if constexpr (std::is_same_v<CheckOpStreamType<T1, T2>, UnprintableWrapper> &&
+                std::is_same_v<CheckOpStreamType<T2, T1>, UnprintableWrapper>) {
+    // No sense printing " (UNPRINTABLE vs. UNPRINTABLE)"
+    return exprtext;
+  } else {
+    CheckOpMessageBuilder comb(exprtext);
+    MakeCheckOpValueString(comb.ForVar1(), v1);
+    MakeCheckOpValueString(comb.ForVar2(), v2);
+    return comb.NewString();
+  }
+}
+
+// Add a few commonly used instantiations as extern to reduce size of objects
+// files.
+#define ABSL_LOG_INTERNAL_DEFINE_MAKE_CHECK_OP_STRING_EXTERN(x) \
+  extern template const char* absl_nonnull MakeCheckOpString(   \
+      x, x, const char* absl_nonnull)
+ABSL_LOG_INTERNAL_DEFINE_MAKE_CHECK_OP_STRING_EXTERN(bool);
+ABSL_LOG_INTERNAL_DEFINE_MAKE_CHECK_OP_STRING_EXTERN(int64_t);
+ABSL_LOG_INTERNAL_DEFINE_MAKE_CHECK_OP_STRING_EXTERN(uint64_t);
+ABSL_LOG_INTERNAL_DEFINE_MAKE_CHECK_OP_STRING_EXTERN(float);
+ABSL_LOG_INTERNAL_DEFINE_MAKE_CHECK_OP_STRING_EXTERN(double);
+ABSL_LOG_INTERNAL_DEFINE_MAKE_CHECK_OP_STRING_EXTERN(char);
+ABSL_LOG_INTERNAL_DEFINE_MAKE_CHECK_OP_STRING_EXTERN(unsigned char);
+ABSL_LOG_INTERNAL_DEFINE_MAKE_CHECK_OP_STRING_EXTERN(const std::string&);
+ABSL_LOG_INTERNAL_DEFINE_MAKE_CHECK_OP_STRING_EXTERN(const absl::string_view&);
+ABSL_LOG_INTERNAL_DEFINE_MAKE_CHECK_OP_STRING_EXTERN(const char* absl_nonnull);
+ABSL_LOG_INTERNAL_DEFINE_MAKE_CHECK_OP_STRING_EXTERN(
+    const signed char* absl_nonnull);
+ABSL_LOG_INTERNAL_DEFINE_MAKE_CHECK_OP_STRING_EXTERN(
+    const unsigned char* absl_nonnull);
+ABSL_LOG_INTERNAL_DEFINE_MAKE_CHECK_OP_STRING_EXTERN(const void* absl_nonnull);
+#undef ABSL_LOG_INTERNAL_DEFINE_MAKE_CHECK_OP_STRING_EXTERN
+
+// `ABSL_LOG_INTERNAL_CHECK_OP_IMPL_RESULT` skips formatting the Check_OP result
+// string iff `ABSL_MIN_LOG_LEVEL` exceeds `kFatal`, instead returning an empty
+// string.
+#ifdef ABSL_MIN_LOG_LEVEL
+#define ABSL_LOG_INTERNAL_CHECK_OP_IMPL_RESULT(U1, U2, v1, v2, exprtext) \
+  ((::absl::LogSeverity::kFatal >=                                       \
+    static_cast<::absl::LogSeverityAtLeast>(ABSL_MIN_LOG_LEVEL))         \
+       ? MakeCheckOpString<U1, U2>(v1, v2, exprtext)                     \
+       : "")
+#else
+#define ABSL_LOG_INTERNAL_CHECK_OP_IMPL_RESULT(U1, U2, v1, v2, exprtext) \
+  MakeCheckOpString<U1, U2>(v1, v2, exprtext)
+#endif
+
+// Helper functions for `ABSL_LOG_INTERNAL_CHECK_OP` macro family.  The
+// `(int, int)` override works around the issue that the compiler will not
+// instantiate the template version of the function on values of unnamed enum
+// type.
+#define ABSL_LOG_INTERNAL_CHECK_OP_IMPL(name, op)                          \
+  template <typename T1, typename T2>                                      \
+  inline constexpr const char* absl_nullable name##Impl(                   \
+      const T1& v1, const T2& v2, const char* absl_nonnull exprtext) {     \
+    using U1 = CheckOpStreamType<T1, T2>;                                  \
+    using U2 = CheckOpStreamType<T2, T1>;                                  \
+    return ABSL_PREDICT_TRUE(v1 op v2)                                     \
+               ? nullptr                                                   \
+               : ABSL_LOG_INTERNAL_CHECK_OP_IMPL_RESULT(U1, U2, U1(v1),    \
+                                                        U2(v2), exprtext); \
+  }                                                                        \
+  inline constexpr const char* absl_nullable name##Impl(                   \
+      int v1, int v2, const char* absl_nonnull exprtext) {                 \
+    return name##Impl<int, int>(v1, v2, exprtext);                         \
+  }
+
+ABSL_LOG_INTERNAL_CHECK_OP_IMPL(Check_EQ, ==)
+ABSL_LOG_INTERNAL_CHECK_OP_IMPL(Check_NE, !=)
+ABSL_LOG_INTERNAL_CHECK_OP_IMPL(Check_LE, <=)
+ABSL_LOG_INTERNAL_CHECK_OP_IMPL(Check_LT, <)
+ABSL_LOG_INTERNAL_CHECK_OP_IMPL(Check_GE, >=)
+ABSL_LOG_INTERNAL_CHECK_OP_IMPL(Check_GT, >)
+#undef ABSL_LOG_INTERNAL_CHECK_OP_IMPL_RESULT
+#undef ABSL_LOG_INTERNAL_CHECK_OP_IMPL
+
+const char* absl_nullable CheckstrcmptrueImpl(
+    const char* absl_nullable s1, const char* absl_nullable s2,
+    const char* absl_nonnull exprtext);
+const char* absl_nullable CheckstrcmpfalseImpl(
+    const char* absl_nullable s1, const char* absl_nullable s2,
+    const char* absl_nonnull exprtext);
+const char* absl_nullable CheckstrcasecmptrueImpl(
+    const char* absl_nullable s1, const char* absl_nullable s2,
+    const char* absl_nonnull exprtext);
+const char* absl_nullable CheckstrcasecmpfalseImpl(
+    const char* absl_nullable s1, const char* absl_nullable s2,
+    const char* absl_nonnull exprtext);
+
+// `CHECK_EQ` and friends want to pass their arguments by reference, however
+// this winds up exposing lots of cases where people have defined and
+// initialized static const data members but never declared them (i.e. in a .cc
+// file), meaning they are not referenceable.  This function avoids that problem
+// for integers (the most common cases) by overloading for every primitive
+// integer type, even the ones we discourage, and returning them by value.
+// NOLINTBEGIN(runtime/int)
+// NOLINTBEGIN(google-runtime-int)
+template <typename T>
+constexpr const T& GetReferenceableValue(const T& t) {
+  return t;
+}
+constexpr char GetReferenceableValue(char t) { return t; }
+constexpr unsigned char GetReferenceableValue(unsigned char t) { return t; }
+constexpr signed char GetReferenceableValue(signed char t) { return t; }
+constexpr short GetReferenceableValue(short t) { return t; }
+constexpr unsigned short GetReferenceableValue(unsigned short t) { return t; }
+constexpr int GetReferenceableValue(int t) { return t; }
+constexpr unsigned int GetReferenceableValue(unsigned int t) { return t; }
+constexpr long GetReferenceableValue(long t) { return t; }
+constexpr unsigned long GetReferenceableValue(unsigned long t) { return t; }
+constexpr long long GetReferenceableValue(long long t) { return t; }
+constexpr unsigned long long GetReferenceableValue(unsigned long long t) {
+  return t;
+}
+// NOLINTEND(google-runtime-int)
+// NOLINTEND(runtime/int)
+
+}  // namespace log_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_LOG_INTERNAL_CHECK_OP_H_
diff --git a/absl/log/internal/conditions.cc b/absl/log/internal/conditions.cc
new file mode 100644
index 00000000000..a418c88428e
--- /dev/null
+++ b/absl/log/internal/conditions.cc
@@ -0,0 +1,85 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/log/internal/conditions.h"
+
+#include <atomic>
+#include <cstdint>
+
+#include "absl/base/config.h"
+#include "absl/base/internal/cycleclock.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace log_internal {
+namespace {
+
+// The following code behaves like AtomicStatsCounter::LossyAdd() for
+// speed since it is fine to lose occasional updates.
+// Returns old value of *counter.
+uint32_t LossyIncrement(std::atomic<uint32_t>* counter) {
+  const uint32_t value = counter->load(std::memory_order_relaxed);
+  counter->store(value + 1, std::memory_order_relaxed);
+  return value;
+}
+
+}  // namespace
+
+bool LogEveryNState::ShouldLog(int n) {
+  return n > 0 && (LossyIncrement(&counter_) % static_cast<uint32_t>(n)) == 0;
+}
+
+bool LogFirstNState::ShouldLog(int n) {
+  const uint32_t counter_value = counter_.load(std::memory_order_relaxed);
+  if (static_cast<int64_t>(counter_value) < n) {
+    counter_.store(counter_value + 1, std::memory_order_relaxed);
+    return true;
+  }
+  return false;
+}
+
+bool LogEveryPow2State::ShouldLog() {
+  const uint32_t new_value = LossyIncrement(&counter_) + 1;
+  return (new_value & (new_value - 1)) == 0;
+}
+
+bool LogEveryNSecState::ShouldLog(double seconds) {
+  using absl::base_internal::CycleClock;
+  LossyIncrement(&counter_);
+  const int64_t now_cycles = CycleClock::Now();
+  int64_t next_cycles = next_log_time_cycles_.load(std::memory_order_relaxed);
+#if defined(__myriad2__)
+  // myriad2 does not have 8-byte compare and exchange.  Use a racy version that
+  // is "good enough" but will over-log in the face of concurrent logging.
+  if (now_cycles > next_cycles) {
+    next_log_time_cycles_.store(
+        static_cast<int64_t>(now_cycles + seconds * CycleClock::Frequency()),
+        std::memory_order_relaxed);
+    return true;
+  }
+  return false;
+#else
+  do {
+    if (now_cycles <= next_cycles) return false;
+  } while (!next_log_time_cycles_.compare_exchange_weak(
+      next_cycles,
+      static_cast<int64_t>(now_cycles + seconds * CycleClock::Frequency()),
+      std::memory_order_relaxed, std::memory_order_relaxed));
+  return true;
+#endif
+}
+
+}  // namespace log_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/log/internal/conditions.h b/absl/log/internal/conditions.h
new file mode 100644
index 00000000000..8be029d58db
--- /dev/null
+++ b/absl/log/internal/conditions.h
@@ -0,0 +1,248 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: log/internal/conditions.h
+// -----------------------------------------------------------------------------
+//
+// This file contains implementation of conditional log statements, like LOG_IF
+// including all the ABSL_LOG_INTERNAL_..._CONDITION_... macros and
+// various condition classes like LogEveryNState.
+
+#ifndef ABSL_LOG_INTERNAL_CONDITIONS_H_
+#define ABSL_LOG_INTERNAL_CONDITIONS_H_
+
+#include <stdlib.h>
+
+#include <atomic>
+#include <cstdint>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/log/internal/voidify.h"
+
+#if defined(_WIN32) || defined(__hexagon__)
+#include <cstdlib>
+#else
+#include <unistd.h>
+#endif
+
+// `ABSL_LOG_INTERNAL_CONDITION` prefixes another macro that expands to a
+// temporary `LogMessage` instantiation followed by zero or more streamed
+// expressions.  This definition is tricky to read correctly.  It evaluates to
+// either
+//
+//   (void)0;
+//
+// or
+//
+//   ::absl::log_internal::Voidify() &&
+//       ::absl::log_internal::LogMessage(...) << "the user's message";
+//
+// If the condition is evaluable at compile time, as is often the case, it
+// compiles away to just one side or the other.
+//
+// Although this is not used anywhere a statement (e.g. `if`) could not go,
+// the ternary expression does a better job avoiding spurious diagnostics
+// (dangling else, missing switch case) and preserving noreturn semantics (e.g.
+// on `LOG(FATAL)`) without requiring braces.
+//
+// The `switch` ensures that this expansion is the beginning of a statement (as
+// opposed to an expression) and prevents shenanigans like
+// `AFunction(LOG(INFO))` and `decltype(LOG(INFO))`.  The apparently-redundant
+// `default` case makes the condition more amenable to Clang dataflow analysis.
+#define ABSL_LOG_INTERNAL_STATELESS_CONDITION(condition) \
+  switch (0)                                             \
+  case 0:                                                \
+  default:                                               \
+    !(condition) ? (void)0 : ::absl::log_internal::Voidify() &&
+
+// `ABSL_LOG_INTERNAL_STATEFUL_CONDITION` applies a condition like
+// `ABSL_LOG_INTERNAL_STATELESS_CONDITION` but adds to that a series of variable
+// declarations, including a local static object which stores the state needed
+// to implement the stateful macros like `LOG_EVERY_N`.
+//
+// `for`-loops are used to declare scoped variables without braces (to permit
+// streaming into the macro's expansion) and without the dangling-`else`
+// problems/diagnostics that come with `if`.
+//
+// Two more variables are declared in separate `for`-loops:
+//
+// * `COUNTER` implements a streamable token whose value when streamed is the
+//   number of times execution has passed through the macro.
+// * A boolean flag is used to prevent any of the `for`-loops from ever actually
+//   looping.
+#define ABSL_LOG_INTERNAL_STATEFUL_CONDITION(condition)             \
+  for (bool absl_log_internal_stateful_condition_do_log(condition); \
+       absl_log_internal_stateful_condition_do_log;                 \
+       absl_log_internal_stateful_condition_do_log = false)         \
+  ABSL_LOG_INTERNAL_STATEFUL_CONDITION_IMPL
+#define ABSL_LOG_INTERNAL_STATEFUL_CONDITION_IMPL(kind, ...)              \
+  for (static ::absl::log_internal::Log##kind##State                      \
+           absl_log_internal_stateful_condition_state;                    \
+       absl_log_internal_stateful_condition_do_log &&                     \
+       absl_log_internal_stateful_condition_state.ShouldLog(__VA_ARGS__); \
+       absl_log_internal_stateful_condition_do_log = false)               \
+    for (const uint32_t COUNTER ABSL_ATTRIBUTE_UNUSED =                   \
+             absl_log_internal_stateful_condition_state.counter();        \
+         absl_log_internal_stateful_condition_do_log;                     \
+         absl_log_internal_stateful_condition_do_log = false)             \
+  ::absl::log_internal::Voidify() &&
+
+// `ABSL_LOG_INTERNAL_CONDITION_*` serve to combine any conditions from the
+// macro (e.g. `LOG_IF` or `VLOG`) with inherent conditions (e.g.
+// `ABSL_MIN_LOG_LEVEL`) into a single boolean expression.  We could chain
+// ternary operators instead, however some versions of Clang sometimes issue
+// spurious diagnostics after such expressions due to a control flow analysis
+// bug.
+#ifdef ABSL_MIN_LOG_LEVEL
+#define ABSL_LOG_INTERNAL_CONDITION_INFO(type, condition) \
+  ABSL_LOG_INTERNAL_##type##_CONDITION(                   \
+      (condition) &&                                      \
+      ::absl::LogSeverity::kInfo >=                       \
+          static_cast<::absl::LogSeverityAtLeast>(ABSL_MIN_LOG_LEVEL))
+#define ABSL_LOG_INTERNAL_CONDITION_WARNING(type, condition) \
+  ABSL_LOG_INTERNAL_##type##_CONDITION(                      \
+      (condition) &&                                         \
+      ::absl::LogSeverity::kWarning >=                       \
+          static_cast<::absl::LogSeverityAtLeast>(ABSL_MIN_LOG_LEVEL))
+#define ABSL_LOG_INTERNAL_CONDITION_ERROR(type, condition) \
+  ABSL_LOG_INTERNAL_##type##_CONDITION(                    \
+      (condition) &&                                       \
+      ::absl::LogSeverity::kError >=                       \
+          static_cast<::absl::LogSeverityAtLeast>(ABSL_MIN_LOG_LEVEL))
+#define ABSL_LOG_INTERNAL_CONDITION_DO_NOT_SUBMIT(type, condition) \
+  ABSL_LOG_INTERNAL_CONDITION_ERROR(type, condition)
+// NOTE: Use ternary operators instead of short-circuiting to mitigate
+// https://bugs.llvm.org/show_bug.cgi?id=51928.
+#define ABSL_LOG_INTERNAL_CONDITION_FATAL(type, condition)                 \
+  ABSL_LOG_INTERNAL_##type##_CONDITION(                                    \
+      ((condition) ? (::absl::LogSeverity::kFatal >=                       \
+                              static_cast<::absl::LogSeverityAtLeast>(     \
+                                  ABSL_MIN_LOG_LEVEL)                      \
+                          ? true                                           \
+                          : (::absl::log_internal::AbortQuietly(), false)) \
+                   : false))
+// NOTE: Use ternary operators instead of short-circuiting to mitigate
+// https://bugs.llvm.org/show_bug.cgi?id=51928.
+#define ABSL_LOG_INTERNAL_CONDITION_QFATAL(type, condition)               \
+  ABSL_LOG_INTERNAL_##type##_CONDITION(                                   \
+      ((condition) ? (::absl::LogSeverity::kFatal >=                      \
+                              static_cast<::absl::LogSeverityAtLeast>(    \
+                                  ABSL_MIN_LOG_LEVEL)                     \
+                          ? true                                          \
+                          : (::absl::log_internal::ExitQuietly(), false)) \
+                   : false))
+#define ABSL_LOG_INTERNAL_CONDITION_DFATAL(type, condition)                    \
+  ABSL_LOG_INTERNAL_##type##_CONDITION(                                        \
+      (ABSL_ASSUME(absl::kLogDebugFatal == absl::LogSeverity::kError ||        \
+                   absl::kLogDebugFatal == absl::LogSeverity::kFatal),         \
+       (condition) &&                                                          \
+           (::absl::kLogDebugFatal >=                                          \
+                static_cast<::absl::LogSeverityAtLeast>(ABSL_MIN_LOG_LEVEL) || \
+            (::absl::kLogDebugFatal == ::absl::LogSeverity::kFatal &&          \
+             (::absl::log_internal::AbortQuietly(), false)))))
+
+#define ABSL_LOG_INTERNAL_CONDITION_LEVEL(severity)                            \
+  for (int absl_log_internal_severity_loop = 1;                                \
+       absl_log_internal_severity_loop; absl_log_internal_severity_loop = 0)   \
+    for (const absl::LogSeverity absl_log_internal_severity =                  \
+             ::absl::NormalizeLogSeverity(severity);                           \
+         absl_log_internal_severity_loop; absl_log_internal_severity_loop = 0) \
+  ABSL_LOG_INTERNAL_CONDITION_LEVEL_IMPL
+#define ABSL_LOG_INTERNAL_CONDITION_LEVEL_IMPL(type, condition)            \
+  ABSL_LOG_INTERNAL_##type##_CONDITION(                                    \
+      ((condition) &&                                                      \
+       (absl_log_internal_severity >=                                      \
+            static_cast<::absl::LogSeverityAtLeast>(ABSL_MIN_LOG_LEVEL) || \
+        (absl_log_internal_severity == ::absl::LogSeverity::kFatal &&      \
+         (::absl::log_internal::AbortQuietly(), false)))))
+#else  // ndef ABSL_MIN_LOG_LEVEL
+#define ABSL_LOG_INTERNAL_CONDITION_INFO(type, condition) \
+  ABSL_LOG_INTERNAL_##type##_CONDITION(condition)
+#define ABSL_LOG_INTERNAL_CONDITION_WARNING(type, condition) \
+  ABSL_LOG_INTERNAL_##type##_CONDITION(condition)
+#define ABSL_LOG_INTERNAL_CONDITION_ERROR(type, condition) \
+  ABSL_LOG_INTERNAL_##type##_CONDITION(condition)
+#define ABSL_LOG_INTERNAL_CONDITION_DO_NOT_SUBMIT(type, condition) \
+  ABSL_LOG_INTERNAL_CONDITION_ERROR(type, condition)
+#define ABSL_LOG_INTERNAL_CONDITION_FATAL(type, condition) \
+  ABSL_LOG_INTERNAL_##type##_CONDITION(condition)
+#define ABSL_LOG_INTERNAL_CONDITION_QFATAL(type, condition) \
+  ABSL_LOG_INTERNAL_##type##_CONDITION(condition)
+#define ABSL_LOG_INTERNAL_CONDITION_DFATAL(type, condition) \
+  ABSL_LOG_INTERNAL_##type##_CONDITION(condition)
+#define ABSL_LOG_INTERNAL_CONDITION_LEVEL(severity)                            \
+  for (int absl_log_internal_severity_loop = 1;                                \
+       absl_log_internal_severity_loop; absl_log_internal_severity_loop = 0)   \
+    for (const absl::LogSeverity absl_log_internal_severity =                  \
+             ::absl::NormalizeLogSeverity(severity);                           \
+         absl_log_internal_severity_loop; absl_log_internal_severity_loop = 0) \
+  ABSL_LOG_INTERNAL_CONDITION_LEVEL_IMPL
+#define ABSL_LOG_INTERNAL_CONDITION_LEVEL_IMPL(type, condition) \
+  ABSL_LOG_INTERNAL_##type##_CONDITION(condition)
+#endif  // ndef ABSL_MIN_LOG_LEVEL
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace log_internal {
+
+// Stateful condition class name should be "Log" + name + "State".
+class LogEveryNState final {
+ public:
+  bool ShouldLog(int n);
+  uint32_t counter() { return counter_.load(std::memory_order_relaxed); }
+
+ private:
+  std::atomic<uint32_t> counter_{0};
+};
+
+class LogFirstNState final {
+ public:
+  bool ShouldLog(int n);
+  uint32_t counter() { return counter_.load(std::memory_order_relaxed); }
+
+ private:
+  std::atomic<uint32_t> counter_{0};
+};
+
+class LogEveryPow2State final {
+ public:
+  bool ShouldLog();
+  uint32_t counter() { return counter_.load(std::memory_order_relaxed); }
+
+ private:
+  std::atomic<uint32_t> counter_{0};
+};
+
+class LogEveryNSecState final {
+ public:
+  bool ShouldLog(double seconds);
+  uint32_t counter() { return counter_.load(std::memory_order_relaxed); }
+
+ private:
+  std::atomic<uint32_t> counter_{0};
+  // Cycle count according to CycleClock that we should next log at.
+  std::atomic<int64_t> next_log_time_cycles_{0};
+};
+
+// Helper routines to abort the application quietly
+
+[[noreturn]] inline void AbortQuietly() { abort(); }
+[[noreturn]] inline void ExitQuietly() { _exit(1); }
+}  // namespace log_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_LOG_INTERNAL_CONDITIONS_H_
diff --git a/absl/types/bad_optional_access.cc b/absl/log/internal/config.h
similarity index 52%
rename from absl/types/bad_optional_access.cc
rename to absl/log/internal/config.h
index a791c7c21da..379e9ab974c 100644
--- a/absl/types/bad_optional_access.cc
+++ b/absl/log/internal/config.h
@@ -1,4 +1,4 @@
-// Copyright 2017 The Abseil Authors.
+// Copyright 2022 The Abseil Authors.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
@@ -11,36 +11,35 @@
 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 // See the License for the specific language governing permissions and
 // limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: log/internal/config.h
+// -----------------------------------------------------------------------------
+//
 
-#include "absl/types/bad_optional_access.h"
-
-#ifndef ABSL_HAVE_STD_OPTIONAL
-
-#include <cstdlib>
+#ifndef ABSL_LOG_INTERNAL_CONFIG_H_
+#define ABSL_LOG_INTERNAL_CONFIG_H_
 
 #include "absl/base/config.h"
-#include "absl/base/internal/raw_logging.h"
-
-namespace absl {
-
-bad_optional_access::~bad_optional_access() = default;
 
-const char* bad_optional_access::what() const noexcept {
-  return "optional has no value";
-}
+#ifdef _WIN32
+#include <cstdint>
+#else
+#include <sys/types.h>
+#endif
 
-namespace optional_internal {
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace log_internal {
 
-void throw_bad_optional_access() {
-#ifdef ABSL_HAVE_EXCEPTIONS
-  throw bad_optional_access();
+#ifdef _WIN32
+using Tid = uint32_t;
 #else
-  ABSL_RAW_LOG(FATAL, "Bad optional access");
-  abort();
+using Tid = pid_t;
 #endif
-}
 
-}  // namespace optional_internal
+}  // namespace log_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
-#endif  // ABSL_HAVE_STD_OPTIONAL
+#endif  // ABSL_LOG_INTERNAL_CONFIG_H_
diff --git a/absl/log/internal/container.h b/absl/log/internal/container.h
new file mode 100644
index 00000000000..ea2445f4cb5
--- /dev/null
+++ b/absl/log/internal/container.h
@@ -0,0 +1,312 @@
+// Copyright 2025 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// The typical use looks like this:
+//
+//   LOG(INFO) << LogContainer(container);
+//
+// By default, LogContainer() uses the LogShortUpTo100 policy: comma-space
+// separation, no newlines, and with limit of 100 items.
+//
+// Policies can be specified:
+//
+//   LOG(INFO) << LogContainer(container, LogMultiline());
+//
+// The above example will print the container using newlines between elements,
+// enclosed in [] braces.
+//
+// See below for further details on policies.
+
+#ifndef ABSL_LOG_INTERNAL_CONTAINER_H_
+#define ABSL_LOG_INTERNAL_CONTAINER_H_
+
+#include <cstdint>
+#include <limits>
+#include <ostream>
+#include <sstream>
+#include <string>
+#include <type_traits>
+
+#include "absl/base/config.h"
+#include "absl/meta/internal/requires.h"
+#include "absl/strings/str_cat.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace log_internal {
+
+// Several policy classes below determine how LogRangeToStream will
+// format a range of items.  A Policy class should have these methods:
+//
+// Called to print an individual container element.
+//   void Log(ostream &out, const ElementT &element) const;
+//
+// Called before printing the set of elements:
+//   void LogOpening(ostream &out) const;
+//
+// Called after printing the set of elements:
+//   void LogClosing(ostream &out) const;
+//
+// Called before printing the first element:
+//   void LogFirstSeparator(ostream &out) const;
+//
+// Called before printing the remaining elements:
+//   void LogSeparator(ostream &out) const;
+//
+// Returns the maximum number of elements to print:
+//   int64 MaxElements() const;
+//
+// Called to print an indication that MaximumElements() was reached:
+//   void LogEllipsis(ostream &out) const;
+
+namespace internal {
+
+struct LogBase {
+  template <typename ElementT>
+  void Log(std::ostream &out, const ElementT &element) const {  // NOLINT
+    // Fallback to `AbslStringify` if the type does not have `operator<<`.
+    if constexpr (meta_internal::Requires<std::ostream, ElementT>(
+                      [](auto&& x, auto&& y) -> decltype(x << y) {})) {
+      out << element;
+    } else {
+      out << absl::StrCat(element);
+    }
+  }
+  void LogEllipsis(std::ostream &out) const {  // NOLINT
+    out << "...";
+  }
+};
+
+struct LogShortBase : public LogBase {
+  void LogOpening(std::ostream &out) const { out << "["; }        // NOLINT
+  void LogClosing(std::ostream &out) const { out << "]"; }        // NOLINT
+  void LogFirstSeparator(std::ostream &out) const { out << ""; }  // NOLINT
+  void LogSeparator(std::ostream &out) const { out << ", "; }     // NOLINT
+};
+
+struct LogMultilineBase : public LogBase {
+  void LogOpening(std::ostream &out) const { out << "["; }          // NOLINT
+  void LogClosing(std::ostream &out) const { out << "\n]"; }        // NOLINT
+  void LogFirstSeparator(std::ostream &out) const { out << "\n"; }  // NOLINT
+  void LogSeparator(std::ostream &out) const { out << "\n"; }       // NOLINT
+};
+
+struct LogLegacyBase : public LogBase {
+  void LogOpening(std::ostream &out) const { out << ""; }         // NOLINT
+  void LogClosing(std::ostream &out) const { out << ""; }         // NOLINT
+  void LogFirstSeparator(std::ostream &out) const { out << ""; }  // NOLINT
+  void LogSeparator(std::ostream &out) const { out << " "; }      // NOLINT
+};
+
+}  // namespace internal
+
+// LogShort uses [] braces and separates items with comma-spaces.  For
+// example "[1, 2, 3]".
+struct LogShort : public internal::LogShortBase {
+  int64_t MaxElements() const { return (std::numeric_limits<int64_t>::max)(); }
+};
+
+// LogShortUpToN(max_elements) formats the same as LogShort but prints no more
+// than the max_elements elements.
+class LogShortUpToN : public internal::LogShortBase {
+ public:
+  explicit LogShortUpToN(int64_t max_elements) : max_elements_(max_elements) {}
+  int64_t MaxElements() const { return max_elements_; }
+
+ private:
+  int64_t max_elements_;
+};
+
+// LogShortUpTo100 formats the same as LogShort but prints no more
+// than 100 elements.
+struct LogShortUpTo100 : public LogShortUpToN {
+  LogShortUpTo100() : LogShortUpToN(100) {}
+};
+
+// LogMultiline uses [] braces and separates items with
+// newlines.  For example "[
+// 1
+// 2
+// 3
+// ]".
+struct LogMultiline : public internal::LogMultilineBase {
+  int64_t MaxElements() const { return (std::numeric_limits<int64_t>::max)(); }
+};
+
+// LogMultilineUpToN(max_elements) formats the same as LogMultiline but
+// prints no more than max_elements elements.
+class LogMultilineUpToN : public internal::LogMultilineBase {
+ public:
+  explicit LogMultilineUpToN(int64_t max_elements)
+      : max_elements_(max_elements) {}
+  int64_t MaxElements() const { return max_elements_; }
+
+ private:
+  int64_t max_elements_;
+};
+
+// LogMultilineUpTo100 formats the same as LogMultiline but
+// prints no more than 100 elements.
+struct LogMultilineUpTo100 : public LogMultilineUpToN {
+  LogMultilineUpTo100() : LogMultilineUpToN(100) {}
+};
+
+// The legacy behavior of LogSequence() does not use braces and
+// separates items with spaces.  For example "1 2 3".
+struct LogLegacyUpTo100 : public internal::LogLegacyBase {
+  int64_t MaxElements() const { return 100; }
+};
+struct LogLegacy : public internal::LogLegacyBase {
+  int64_t MaxElements() const { return (std::numeric_limits<int64_t>::max)(); }
+};
+
+// The default policy for new code.
+typedef LogShortUpTo100 LogDefault;
+
+// LogRangeToStream should be used to define operator<< for
+// STL and STL-like containers.  For example, see stl_logging.h.
+template <typename IteratorT, typename PolicyT>
+inline void LogRangeToStream(std::ostream &out,  // NOLINT
+                             IteratorT begin, IteratorT end,
+                             const PolicyT &policy) {
+  policy.LogOpening(out);
+  for (int64_t i = 0; begin != end && i < policy.MaxElements(); ++i, ++begin) {
+    if (i == 0) {
+      policy.LogFirstSeparator(out);
+    } else {
+      policy.LogSeparator(out);
+    }
+    policy.Log(out, *begin);
+  }
+  if (begin != end) {
+    policy.LogSeparator(out);
+    policy.LogEllipsis(out);
+  }
+  policy.LogClosing(out);
+}
+
+namespace detail {
+
+// RangeLogger is a helper class for LogRange and LogContainer; do not use it
+// directly.  This object captures iterators into the argument of the LogRange
+// and LogContainer functions, so its lifetime should be confined to a single
+// logging statement.  Objects of this type should not be assigned to local
+// variables.
+template <typename IteratorT, typename PolicyT>
+class RangeLogger {
+ public:
+  RangeLogger(const IteratorT &begin, const IteratorT &end,
+              const PolicyT &policy)
+      : begin_(begin), end_(end), policy_(policy) {}
+
+  friend std::ostream &operator<<(std::ostream &out, const RangeLogger &range) {
+    LogRangeToStream<IteratorT, PolicyT>(out, range.begin_, range.end_,
+                                         range.policy_);
+    return out;
+  }
+
+  // operator<< above is generally recommended. However, some situations may
+  // require a string, so a convenience str() method is provided as well.
+  std::string str() const {
+    std::stringstream ss;
+    ss << *this;
+    return ss.str();
+  }
+
+ private:
+  IteratorT begin_;
+  IteratorT end_;
+  PolicyT policy_;
+};
+
+template <typename E>
+class EnumLogger {
+ public:
+  explicit EnumLogger(E e) : e_(e) {}
+
+  friend std::ostream &operator<<(std::ostream &out, const EnumLogger &v) {
+    using I = std::underlying_type_t<E>;
+    return out << static_cast<I>(v.e_);
+  }
+
+ private:
+  E e_;
+};
+
+}  // namespace detail
+
+// Log a range using "policy".  For example:
+//
+//   LOG(INFO) << LogRange(start_pos, end_pos, LogMultiline());
+//
+// The above example will print the range using newlines between
+// elements, enclosed in [] braces.
+template <typename IteratorT, typename PolicyT>
+detail::RangeLogger<IteratorT, PolicyT> LogRange(const IteratorT &begin,
+                                                 const IteratorT &end,
+                                                 const PolicyT &policy) {
+  return detail::RangeLogger<IteratorT, PolicyT>(begin, end, policy);
+}
+
+// Log a range.  For example:
+//
+//   LOG(INFO) << LogRange(start_pos, end_pos);
+//
+// By default, Range() uses the LogShortUpTo100 policy: comma-space
+// separation, no newlines, and with limit of 100 items.
+template <typename IteratorT>
+detail::RangeLogger<IteratorT, LogDefault> LogRange(const IteratorT &begin,
+                                                    const IteratorT &end) {
+  return LogRange(begin, end, LogDefault());
+}
+
+// Log a container using "policy".  For example:
+//
+//   LOG(INFO) << LogContainer(container, LogMultiline());
+//
+// The above example will print the container using newlines between
+// elements, enclosed in [] braces.
+template <typename ContainerT, typename PolicyT>
+auto LogContainer(const ContainerT& container, const PolicyT& policy)
+    -> decltype(LogRange(container.begin(), container.end(), policy)) {
+  return LogRange(container.begin(), container.end(), policy);
+}
+
+// Log a container.  For example:
+//
+//   LOG(INFO) << LogContainer(container);
+//
+// By default, Container() uses the LogShortUpTo100 policy: comma-space
+// separation, no newlines, and with limit of 100 items.
+template <typename ContainerT>
+auto LogContainer(const ContainerT& container)
+    -> decltype(LogContainer(container, LogDefault())) {
+  return LogContainer(container, LogDefault());
+}
+
+// Log a (possibly scoped) enum.  For example:
+//
+//   enum class Color { kRed, kGreen, kBlue };
+//   LOG(INFO) << LogEnum(kRed);
+template <typename E>
+detail::EnumLogger<E> LogEnum(E e) {
+  static_assert(std::is_enum_v<E>, "must be an enum");
+  return detail::EnumLogger<E>(e);
+}
+
+}  // namespace log_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_LOG_INTERNAL_CONTAINER_H_
diff --git a/absl/log/internal/container_test.cc b/absl/log/internal/container_test.cc
new file mode 100644
index 00000000000..92eef748e35
--- /dev/null
+++ b/absl/log/internal/container_test.cc
@@ -0,0 +1,252 @@
+// Copyright 2025 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/log/internal/container.h"
+
+#include <cstdint>
+#include <memory>
+#include <ostream>
+#include <set>
+#include <sstream>
+#include <string>
+#include <vector>
+
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+#include "absl/strings/str_format.h"
+#include "absl/strings/str_join.h"
+#include "absl/types/span.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace log_internal {
+namespace {
+
+class ContainerLoggingTest : public ::testing::Test {
+ protected:
+  ContainerLoggingTest() : stream_(new std::stringstream) {}
+  std::ostream& stream() { return *stream_; }
+  std::string logged() {
+    std::string r = stream_->str();
+    stream_ = std::make_unique<std::stringstream>();
+    return r;
+  }
+
+ private:
+  std::unique_ptr<std::stringstream> stream_;
+};
+
+TEST_F(ContainerLoggingTest, ShortRange) {
+  std::vector<std::string> words = {"hi", "hello"};
+  LogRangeToStream(stream(), words.begin(), words.end(), LogMultiline());
+  EXPECT_EQ("[\nhi\nhello\n]", logged());
+}
+
+TEST_F(ContainerLoggingTest, LegacyRange) {
+  std::vector<int> lengths = {1, 2};
+  LogRangeToStream(stream(), lengths.begin(), lengths.end(),
+                   LogLegacyUpTo100());
+  EXPECT_EQ("1 2", logged());
+}
+
+TEST_F(ContainerLoggingTest, ToString) {
+  std::vector<int> lengths = {1, 2, 3, 4, 5};
+  EXPECT_EQ(LogContainer(lengths).str(), "[1, 2, 3, 4, 5]");
+}
+
+class UserDefFriend {
+ public:
+  explicit UserDefFriend(int i) : i_(i) {}
+
+ private:
+  friend std::ostream& operator<<(std::ostream& str, const UserDefFriend& i) {
+    return str << i.i_;
+  }
+  int i_;
+};
+
+TEST_F(ContainerLoggingTest, RangeOfUserDefined) {
+  std::vector<UserDefFriend> ints = {UserDefFriend(1), UserDefFriend(2),
+                                     UserDefFriend(3)};
+  LogRangeToStream(stream(), ints.begin(), ints.begin() + 1, LogDefault());
+  LogRangeToStream(stream(), ints.begin() + 1, ints.begin() + 2,
+                   LogMultiline());
+  LogRangeToStream(stream(), ints.begin() + 2, ints.begin() + 3, LogDefault());
+  LogRangeToStream(stream(), ints.begin(), ints.begin(), LogMultiline());
+
+  EXPECT_EQ("[1][\n2\n][3][\n]", logged());
+}
+
+TEST_F(ContainerLoggingTest, FullContainer) {
+  std::vector<int> ints;
+  std::vector<int> ints100;
+  std::vector<int> ints123;
+  int64_t max_elements = 123;
+  std::string expected1;
+  std::string expected2;
+  std::string expected3;
+  std::string expected4;
+  std::string expected5;
+  std::string expected6;
+  std::string expected7;
+  std::string expected8;
+  std::string expected9;
+  for (int i = 0; i < 1000; ++i) {
+    ints.push_back(i);
+    if (i < 100) {
+      ints100.push_back(i);
+    }
+    if (i < max_elements) {
+      ints123.push_back(i);
+    }
+  }
+  expected1 = "[\n" + absl::StrJoin(ints, "\n") + "\n]";
+  expected2 = "[" + absl::StrJoin(ints, ", ") + "]";
+  expected3 = "[\n" + absl::StrJoin(ints100, "\n") + "\n...\n]";
+  expected4 = "[" + absl::StrJoin(ints100, ", ") + ", ...]";
+  expected5 = absl::StrJoin(ints100, " ") + " ...";
+  expected6 = "[\n" + absl::StrJoin(ints, "\n") + "\n]";
+  expected7 = "[\n" + absl::StrJoin(ints123, "\n") + "\n...\n]";
+  expected8 = "[" + absl::StrJoin(ints, ", ") + "]";
+  expected9 = "[" + absl::StrJoin(ints123, ", ") + ", ...]";
+
+  LogRangeToStream(stream(), ints.begin(), ints.end(), LogMultiline());
+  EXPECT_EQ(expected1, logged());
+  LogRangeToStream(stream(), ints.begin(), ints.end(), LogShort());
+  EXPECT_EQ(expected2, logged());
+  LogRangeToStream(stream(), ints.begin(), ints.end(), LogMultilineUpTo100());
+  EXPECT_EQ(expected3, logged());
+  LogRangeToStream(stream(), ints.begin(), ints.end(), LogShortUpTo100());
+  EXPECT_EQ(expected4, logged());
+  LogRangeToStream(stream(), ints.begin(), ints.end(), LogLegacyUpTo100());
+  EXPECT_EQ(expected5, logged());
+
+  LogRangeToStream(stream(), ints.begin(), ints.end(),
+                   LogMultilineUpToN(ints.size()));
+  EXPECT_EQ(expected6, logged());
+  LogRangeToStream(stream(), ints.begin(), ints.end(),
+                   LogMultilineUpToN(max_elements));
+  EXPECT_EQ(expected7, logged());
+  LogRangeToStream(stream(), ints.begin(), ints.end(),
+                   LogShortUpToN(ints.size()));
+  EXPECT_EQ(expected8, logged());
+  LogRangeToStream(stream(), ints.begin(), ints.end(),
+                   LogShortUpToN(max_elements));
+  EXPECT_EQ(expected9, logged());
+}
+
+TEST_F(ContainerLoggingTest, LogContainer) {
+  std::set<int> ints = {1, 2, 3};
+  stream() << LogContainer(ints, LogMultiline());
+  EXPECT_EQ("[\n1\n2\n3\n]", logged());
+
+  stream() << LogContainer(ints);
+  EXPECT_EQ("[1, 2, 3]", logged());
+
+  stream() << LogContainer(std::vector<int>(ints.begin(), ints.end()),
+                           LogLegacyUpTo100());
+  EXPECT_EQ("1 2 3", logged());
+}
+
+TEST_F(ContainerLoggingTest, LogMutableSpan) {
+  std::vector<int> ints = {1, 2, 3};
+  absl::Span<int> int_span(ints);
+  stream() << LogContainer(int_span);
+  EXPECT_EQ("[1, 2, 3]", logged());
+}
+
+TEST_F(ContainerLoggingTest, LogRange) {
+  std::set<int> ints = {1, 2, 3};
+  stream() << LogRange(ints.begin(), ints.end(), LogMultiline());
+  EXPECT_EQ("[\n1\n2\n3\n]", logged());
+
+  stream() << LogRange(ints.begin(), ints.end());
+  EXPECT_EQ("[1, 2, 3]", logged());
+}
+
+// Some class with a custom Stringify
+class C {
+ public:
+  explicit C(int x) : x_(x) {}
+
+ private:
+  // This is intentionally made private for the purposes of the test;
+  //` AbslStringify` isn't meant to be called directly, and instead invoked
+  // via `StrCat` and friends.
+  template <typename Sink>
+  friend void AbslStringify(Sink& sink, const C& p) {
+    absl::Format(&sink, "C(%d)", p.x_);
+  }
+
+  int x_;
+};
+
+TEST_F(ContainerLoggingTest, LogContainerWithCustomStringify) {
+  std::vector<C> c = {C(1), C(2), C(3)};
+  stream() << LogContainer(c);
+  EXPECT_EQ("[C(1), C(2), C(3)]", logged());
+}
+
+class LogEnumTest : public ContainerLoggingTest {
+ protected:
+  enum Unscoped { kUnscoped0, kUnscoped1, kUnscoped2 };
+
+  enum StreamableUnscoped {
+    kStreamableUnscoped0,
+    kStreamableUnscoped1,
+    kStreamableUnscoped2
+  };
+
+  enum class Scoped { k0, k1, k2 };
+
+  enum class StreamableScoped { k0, k1, k2 };
+
+  friend std::ostream& operator<<(std::ostream& os, StreamableUnscoped v) {
+    return os << LogEnum(v);
+  }
+
+  friend std::ostream& operator<<(std::ostream& os, StreamableScoped v) {
+    return os << LogEnum(v);
+  }
+};
+
+TEST_F(LogEnumTest, Unscoped) {
+  stream() << LogEnum(kUnscoped0) << "," << LogEnum(kUnscoped1) << ","
+           << LogEnum(kUnscoped2);
+  EXPECT_EQ("0,1,2", logged());
+}
+
+TEST_F(LogEnumTest, StreamableUnscoped) {
+  stream() << kStreamableUnscoped0 << "," << kStreamableUnscoped1 << ","
+           << kStreamableUnscoped2;
+  EXPECT_EQ("0,1,2", logged());
+}
+
+TEST_F(LogEnumTest, Scoped) {
+  stream() << LogEnum(Scoped::k0) << "," << LogEnum(Scoped::k1) << ","
+           << LogEnum(Scoped::k2);
+  EXPECT_EQ("0,1,2", logged());
+}
+
+TEST_F(LogEnumTest, StreamableScoped) {
+  // Test using LogEnum to implement an operator<<.
+  stream() << StreamableScoped::k0 << "," << StreamableScoped::k1 << ","
+           << StreamableScoped::k2;
+  EXPECT_EQ("0,1,2", logged());
+}
+
+}  // namespace
+}  // namespace log_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/log/internal/flags.h b/absl/log/internal/flags.h
new file mode 100644
index 00000000000..c4539785ad3
--- /dev/null
+++ b/absl/log/internal/flags.h
@@ -0,0 +1,59 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: log/log_flags.h
+// -----------------------------------------------------------------------------
+//
+// This header declares set of flags which can be used to configure Abseil
+// Logging library behaviour at runtime.
+
+#ifndef ABSL_LOG_INTERNAL_FLAGS_H_
+#define ABSL_LOG_INTERNAL_FLAGS_H_
+
+#include <string>
+
+#include "absl/flags/declare.h"
+
+// !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
+// These flags should not be used in C++ code to access logging library
+// configuration knobs. Use interfaces defined in absl/log/globals.h
+// instead. It is still ok to use these flags on a command line.
+// !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
+
+// Log messages at this severity or above are sent to stderr in *addition* to
+// `LogSink`s.  Defaults to `ERROR`.  See log_severity.h for numeric values of
+// severity levels.
+ABSL_DECLARE_FLAG(int, stderrthreshold);
+
+// Log messages at this severity or above are logged; others are discarded.
+// Defaults to `INFO`, i.e. log all severities.  See log_severity.h for numeric
+// values of severity levels.
+ABSL_DECLARE_FLAG(int, minloglevel);
+
+// If specified in the form file:linenum, any messages logged from a matching
+// location will also include a backtrace.
+ABSL_DECLARE_FLAG(std::string, log_backtrace_at);
+
+// If true, the log prefix (severity, date, time, PID, etc.) is prepended to
+// each message logged. Defaults to true.
+ABSL_DECLARE_FLAG(bool, log_prefix);
+
+// Global log verbosity level. Default is 0.
+ABSL_DECLARE_FLAG(int, v);
+
+// Per-module log verbosity level. By default is empty and is unused.
+ABSL_DECLARE_FLAG(std::string, vmodule);
+
+#endif  // ABSL_LOG_INTERNAL_FLAGS_H_
diff --git a/absl/log/internal/fnmatch.cc b/absl/log/internal/fnmatch.cc
new file mode 100644
index 00000000000..26e1e57fe43
--- /dev/null
+++ b/absl/log/internal/fnmatch.cc
@@ -0,0 +1,73 @@
+// Copyright 2023 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/log/internal/fnmatch.h"
+
+#include <cstddef>
+
+#include "absl/base/config.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace log_internal {
+bool FNMatch(absl::string_view pattern, absl::string_view str) {
+  bool in_wildcard_match = false;
+  while (true) {
+    if (pattern.empty()) {
+      // `pattern` is exhausted; succeed if all of `str` was consumed matching
+      // it.
+      return in_wildcard_match || str.empty();
+    }
+    if (str.empty()) {
+      // `str` is exhausted; succeed if `pattern` is empty or all '*'s.
+      return pattern.find_first_not_of('*') == pattern.npos;
+    }
+    switch (pattern.front()) {
+      case '*':
+        pattern.remove_prefix(1);
+        in_wildcard_match = true;
+        break;
+      case '?':
+        pattern.remove_prefix(1);
+        str.remove_prefix(1);
+        break;
+      default:
+        if (in_wildcard_match) {
+          absl::string_view fixed_portion = pattern;
+          const size_t end = fixed_portion.find_first_of("*?");
+          if (end != fixed_portion.npos) {
+            fixed_portion = fixed_portion.substr(0, end);
+          }
+          const size_t match = str.find(fixed_portion);
+          if (match == str.npos) {
+            return false;
+          }
+          pattern.remove_prefix(fixed_portion.size());
+          str.remove_prefix(match + fixed_portion.size());
+          in_wildcard_match = false;
+        } else {
+          if (pattern.front() != str.front()) {
+            return false;
+          }
+          pattern.remove_prefix(1);
+          str.remove_prefix(1);
+        }
+        break;
+    }
+  }
+}
+}  // namespace log_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/log/internal/fnmatch.h b/absl/log/internal/fnmatch.h
new file mode 100644
index 00000000000..4ea147cc6f7
--- /dev/null
+++ b/absl/log/internal/fnmatch.h
@@ -0,0 +1,35 @@
+// Copyright 2023 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_LOG_INTERNAL_FNMATCH_H_
+#define ABSL_LOG_INTERNAL_FNMATCH_H_
+
+#include "absl/base/config.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace log_internal {
+// Like POSIX `fnmatch`, but:
+// * accepts `string_view`
+// * does not allocate any dynamic memory
+// * only supports * and ? wildcards and not bracket expressions [...]
+// * wildcards may match /
+// * no backslash-escaping
+bool FNMatch(absl::string_view pattern, absl::string_view str);
+}  // namespace log_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_LOG_INTERNAL_FNMATCH_H_
diff --git a/absl/log/internal/fnmatch_benchmark.cc b/absl/log/internal/fnmatch_benchmark.cc
new file mode 100644
index 00000000000..f062ba20d45
--- /dev/null
+++ b/absl/log/internal/fnmatch_benchmark.cc
@@ -0,0 +1,29 @@
+// Copyright 2023 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/log/internal/fnmatch.h"
+#include "benchmark/benchmark.h"
+
+namespace {
+void BM_FNMatch(benchmark::State& state) {
+  while (state.KeepRunning()) {
+    bool ret =
+        absl::log_internal::FNMatch("*?*asdf*?*we???asdf**asdf*we",
+                                    "QWERFASVWERASDFWEDFASDasdfQWERGFWASDERREWF"
+                                    "weHOOasdf@#$%TW#ZSERasdfQW#REGTZSERERwe");
+    benchmark::DoNotOptimize(ret);
+  }
+}
+BENCHMARK(BM_FNMatch);
+}  // namespace
diff --git a/absl/log/internal/fnmatch_test.cc b/absl/log/internal/fnmatch_test.cc
new file mode 100644
index 00000000000..614c7d35edc
--- /dev/null
+++ b/absl/log/internal/fnmatch_test.cc
@@ -0,0 +1,60 @@
+// Copyright 2023 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/log/internal/fnmatch.h"
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+
+namespace {
+using ::testing::IsFalse;
+using ::testing::IsTrue;
+
+TEST(FNMatchTest, Works) {
+  using absl::log_internal::FNMatch;
+  EXPECT_THAT(FNMatch("foo", "foo"), IsTrue());
+  EXPECT_THAT(FNMatch("foo", "bar"), IsFalse());
+  EXPECT_THAT(FNMatch("foo", "fo"), IsFalse());
+  EXPECT_THAT(FNMatch("foo", "foo2"), IsFalse());
+  EXPECT_THAT(FNMatch("foo/*", "foo/1/2/3/4"), IsTrue());
+  EXPECT_THAT(FNMatch("bar/foo.ext", "bar/foo.ext"), IsTrue());
+  EXPECT_THAT(FNMatch("*ba*r/fo*o.ext*", "bar/foo.ext"), IsTrue());
+  EXPECT_THAT(FNMatch("bar/foo.ext", "bar/baz.ext"), IsFalse());
+  EXPECT_THAT(FNMatch("bar/foo.ext", "bar/foo"), IsFalse());
+  EXPECT_THAT(FNMatch("bar/foo.ext", "bar/foo.ext.zip"), IsFalse());
+  EXPECT_THAT(FNMatch("ba?/*.ext", "bar/foo.ext"), IsTrue());
+  EXPECT_THAT(FNMatch("ba?/*.ext", "baZ/FOO.ext"), IsTrue());
+  EXPECT_THAT(FNMatch("ba?/*.ext", "barr/foo.ext"), IsFalse());
+  EXPECT_THAT(FNMatch("ba?/*.ext", "bar/foo.ext2"), IsFalse());
+  EXPECT_THAT(FNMatch("ba?/*", "bar/foo.ext2"), IsTrue());
+  EXPECT_THAT(FNMatch("ba?/*", "bar/"), IsTrue());
+  EXPECT_THAT(FNMatch("ba?/?", "bar/"), IsFalse());
+  EXPECT_THAT(FNMatch("ba?/*", "bar"), IsFalse());
+  EXPECT_THAT(FNMatch("?x", "zx"), IsTrue());
+  EXPECT_THAT(FNMatch("*b", "aab"), IsTrue());
+  EXPECT_THAT(FNMatch("a*b", "aXb"), IsTrue());
+  EXPECT_THAT(FNMatch("", ""), IsTrue());
+  EXPECT_THAT(FNMatch("", "a"), IsFalse());
+  EXPECT_THAT(FNMatch("ab*", "ab"), IsTrue());
+  EXPECT_THAT(FNMatch("ab**", "ab"), IsTrue());
+  EXPECT_THAT(FNMatch("ab*?", "ab"), IsFalse());
+  EXPECT_THAT(FNMatch("*", "bbb"), IsTrue());
+  EXPECT_THAT(FNMatch("*", ""), IsTrue());
+  EXPECT_THAT(FNMatch("?", ""), IsFalse());
+  EXPECT_THAT(FNMatch("***", "**p"), IsTrue());
+  EXPECT_THAT(FNMatch("**", "*"), IsTrue());
+  EXPECT_THAT(FNMatch("*?", "*"), IsTrue());
+}
+
+}  // namespace
diff --git a/absl/log/internal/globals.cc b/absl/log/internal/globals.cc
new file mode 100644
index 00000000000..658ebfd2bff
--- /dev/null
+++ b/absl/log/internal/globals.cc
@@ -0,0 +1,145 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/log/internal/globals.h"
+
+#include <atomic>
+#include <cstdio>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/base/log_severity.h"
+#include "absl/strings/string_view.h"
+#include "absl/strings/strip.h"  // IWYU pragma: keep
+#include "absl/time/time.h"
+
+#if defined(__EMSCRIPTEN__)
+#include <emscripten/console.h>
+#endif
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace log_internal {
+
+namespace {
+// Keeps track of whether Logging initialization is finalized.
+// Log messages generated before that will go to stderr.
+ABSL_CONST_INIT std::atomic<bool> logging_initialized(false);
+
+// The TimeZone used for logging. This may only be set once.
+ABSL_CONST_INIT std::atomic<absl::TimeZone*> timezone_ptr{nullptr};
+
+// If true, the logging library will symbolize stack in fatal messages
+ABSL_CONST_INIT std::atomic<bool> symbolize_stack_trace(true);
+
+// Specifies maximum number of stack frames to report in fatal messages.
+ABSL_CONST_INIT std::atomic<int> max_frames_in_stack_trace(64);
+
+ABSL_CONST_INIT std::atomic<bool> exit_on_dfatal(true);
+ABSL_CONST_INIT std::atomic<bool> suppress_sigabort_trace(false);
+}  // namespace
+
+bool IsInitialized() {
+  return logging_initialized.load(std::memory_order_acquire);
+}
+
+void SetInitialized() {
+  logging_initialized.store(true, std::memory_order_release);
+}
+
+void WriteToStderr(absl::string_view message, absl::LogSeverity severity) {
+  if (message.empty()) return;
+#if defined(__EMSCRIPTEN__)
+  // In WebAssembly, bypass filesystem emulation via fwrite.
+  // Skip a trailing newline character as emscripten_errn adds one itself.
+  const auto message_minus_newline = absl::StripSuffix(message, "\n");
+  // emscripten_errn was introduced in 3.1.41 but broken in standalone mode
+  // until 3.1.43.
+#if ABSL_INTERNAL_EMSCRIPTEN_VERSION >= 3001043
+  emscripten_errn(message_minus_newline.data(), message_minus_newline.size());
+#else
+  std::string null_terminated_message(message_minus_newline);
+  _emscripten_err(null_terminated_message.c_str());
+#endif
+#else
+  // Avoid using std::cerr from this module since we may get called during
+  // exit code, and cerr may be partially or fully destroyed by then.
+  std::fwrite(message.data(), message.size(), 1, stderr);
+#endif
+
+#if defined(_WIN64) || defined(_WIN32) || defined(_WIN16)
+  // C99 requires stderr to not be fully-buffered by default (7.19.3.7), but
+  // MS CRT buffers it anyway, so we must `fflush` to ensure the string hits
+  // the console/file before the program dies (and takes the libc buffers
+  // with it).
+  // https://docs.microsoft.com/en-us/cpp/c-runtime-library/stream-i-o
+  if (severity >= absl::LogSeverity::kWarning) {
+    std::fflush(stderr);
+  }
+#else
+  // Avoid unused parameter warning in this branch.
+  (void)severity;
+#endif
+}
+
+void SetTimeZone(absl::TimeZone tz) {
+  absl::TimeZone* expected = nullptr;
+  absl::TimeZone* new_tz = new absl::TimeZone(tz);
+  // timezone_ptr can only be set once, otherwise new_tz is leaked.
+  if (!timezone_ptr.compare_exchange_strong(expected, new_tz,
+                                            std::memory_order_release,
+                                            std::memory_order_relaxed)) {
+    ABSL_RAW_LOG(FATAL,
+                 "absl::log_internal::SetTimeZone() has already been called");
+  }
+}
+
+const absl::TimeZone* TimeZone() {
+  return timezone_ptr.load(std::memory_order_acquire);
+}
+
+bool ShouldSymbolizeLogStackTrace() {
+  return symbolize_stack_trace.load(std::memory_order_acquire);
+}
+
+void EnableSymbolizeLogStackTrace(bool on_off) {
+  symbolize_stack_trace.store(on_off, std::memory_order_release);
+}
+
+int MaxFramesInLogStackTrace() {
+  return max_frames_in_stack_trace.load(std::memory_order_acquire);
+}
+
+void SetMaxFramesInLogStackTrace(int max_num_frames) {
+  max_frames_in_stack_trace.store(max_num_frames, std::memory_order_release);
+}
+
+bool ExitOnDFatal() { return exit_on_dfatal.load(std::memory_order_acquire); }
+
+void SetExitOnDFatal(bool on_off) {
+  exit_on_dfatal.store(on_off, std::memory_order_release);
+}
+
+bool SuppressSigabortTrace() {
+  return suppress_sigabort_trace.load(std::memory_order_acquire);
+}
+
+bool SetSuppressSigabortTrace(bool on_off) {
+  return suppress_sigabort_trace.exchange(on_off);
+}
+
+}  // namespace log_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/log/internal/globals.h b/absl/log/internal/globals.h
new file mode 100644
index 00000000000..27bc0d09847
--- /dev/null
+++ b/absl/log/internal/globals.h
@@ -0,0 +1,101 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: log/internal/globals.h
+// -----------------------------------------------------------------------------
+//
+// This header file contains various global objects and static helper routines
+// use in logging implementation.
+
+#ifndef ABSL_LOG_INTERNAL_GLOBALS_H_
+#define ABSL_LOG_INTERNAL_GLOBALS_H_
+
+#include "absl/base/config.h"
+#include "absl/base/log_severity.h"
+#include "absl/strings/string_view.h"
+#include "absl/time/time.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace log_internal {
+
+// IsInitialized returns true if the logging library is initialized.
+// This function is async-signal-safe
+bool IsInitialized();
+
+// SetLoggingInitialized is called once after logging initialization is done.
+void SetInitialized();
+
+// Unconditionally write a `message` to stderr. If `severity` exceeds kInfo
+// we also flush the stderr stream.
+void WriteToStderr(absl::string_view message, absl::LogSeverity severity);
+
+// Set the TimeZone used for human-friendly times (for example, the log message
+// prefix) printed by the logging library. This may only be called once.
+void SetTimeZone(absl::TimeZone tz);
+
+// Returns the TimeZone used for human-friendly times (for example, the log
+// message prefix) printed by the logging library Returns nullptr prior to
+// initialization.
+const absl::TimeZone* TimeZone();
+
+// Returns true if stack traces emitted by the logging library should be
+// symbolized. This function is async-signal-safe.
+bool ShouldSymbolizeLogStackTrace();
+
+// Enables or disables symbolization of stack traces emitted by the
+// logging library. This function is async-signal-safe.
+void EnableSymbolizeLogStackTrace(bool on_off);
+
+// Returns the maximum number of frames that appear in stack traces
+// emitted by the logging library. This function is async-signal-safe.
+int MaxFramesInLogStackTrace();
+
+// Sets the maximum number of frames that appear in stack traces emitted by
+// the logging library. This function is async-signal-safe.
+void SetMaxFramesInLogStackTrace(int max_num_frames);
+
+// Determines whether we exit the program for a LOG(DFATAL) message in
+// debug mode.  It does this by skipping the call to Fail/FailQuietly.
+// This is intended for testing only.
+//
+// This can have some effects on LOG(FATAL) as well. Failure messages
+// are always allocated (rather than sharing a buffer), the crash
+// reason is not recorded, the "gwq" status message is not updated,
+// and the stack trace is not recorded.  The LOG(FATAL) *will* still
+// exit the program. Since this function is used only in testing,
+// these differences are acceptable.
+//
+// Additionally, LOG(LEVEL(FATAL)) is indistinguishable from LOG(DFATAL) and
+// will not terminate the program if SetExitOnDFatal(false) has been called.
+bool ExitOnDFatal();
+
+// SetExitOnDFatal() sets the ExitOnDFatal() status
+void SetExitOnDFatal(bool on_off);
+
+// Determines if the logging library should suppress logging of stacktraces in
+// the `SIGABRT` handler, typically because we just logged a stacktrace as part
+// of `LOG(FATAL)` and are about to send ourselves a `SIGABRT` to end the
+// program.
+bool SuppressSigabortTrace();
+
+// Sets the SuppressSigabortTrace() status and returns the previous state.
+bool SetSuppressSigabortTrace(bool on_off);
+
+}  // namespace log_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_LOG_INTERNAL_GLOBALS_H_
diff --git a/absl/log/internal/log_format.cc b/absl/log/internal/log_format.cc
new file mode 100644
index 00000000000..ac33021edf9
--- /dev/null
+++ b/absl/log/internal/log_format.cc
@@ -0,0 +1,201 @@
+//
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/log/internal/log_format.h"
+
+#include <string.h>
+
+#include <cstddef>
+#include <cstdint>
+#include <limits>
+#include <string>
+#include <type_traits>
+
+#include "absl/base/config.h"
+#include "absl/base/log_severity.h"
+#include "absl/base/optimization.h"
+#include "absl/log/internal/append_truncated.h"
+#include "absl/log/internal/config.h"
+#include "absl/log/internal/globals.h"
+#include "absl/strings/numbers.h"
+#include "absl/strings/str_format.h"
+#include "absl/strings/string_view.h"
+#include "absl/time/civil_time.h"
+#include "absl/time/time.h"
+#include "absl/types/span.h"
+
+#ifdef _MSC_VER
+#include <winsock2.h>  // For timeval
+#else
+#include <sys/time.h>
+#endif
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace log_internal {
+namespace {
+
+// The `if constexpr` avoids compiler warnings about tautological comparisons
+// when log_internal::Tid is unsigned.
+template <typename T>
+inline void PutLeadingWhitespace(T tid, char*& p) {
+  if constexpr (std::is_signed_v<T>) {
+    if (tid >= 0 && tid < 10) *p++ = ' ';
+    if (tid > -10 && tid < 100) *p++ = ' ';
+    if (tid > -100 && tid < 1000) *p++ = ' ';
+    if (tid > -1000 && tid < 10000) *p++ = ' ';
+    if (tid > -10000 && tid < 100000) *p++ = ' ';
+    if (tid > -100000 && tid < 1000000) *p++ = ' ';
+  } else {
+    if (tid < 10) *p++ = ' ';
+    if (tid < 100) *p++ = ' ';
+    if (tid < 1000) *p++ = ' ';
+    if (tid < 10000) *p++ = ' ';
+    if (tid < 100000) *p++ = ' ';
+    if (tid < 1000000) *p++ = ' ';
+  }
+}
+
+// The fields before the filename are all fixed-width except for the thread ID,
+// which is of bounded width.
+size_t FormatBoundedFields(absl::LogSeverity severity, absl::Time timestamp,
+                           log_internal::Tid tid, absl::Span<char>& buf) {
+  constexpr size_t kBoundedFieldsMaxLen =
+      sizeof("SMMDD HH:MM:SS.NNNNNN  ") +
+      (1 + std::numeric_limits<log_internal::Tid>::digits10 + 1) - sizeof("");
+  if (ABSL_PREDICT_FALSE(buf.size() < kBoundedFieldsMaxLen)) {
+    // We don't bother trying to truncate these fields if the buffer is too
+    // short (or almost too short) because it would require doing a lot more
+    // length checking (slow) and it should never happen.  A 15kB buffer should
+    // be enough for anyone.  Instead we mark `buf` full without writing
+    // anything.
+    buf.remove_suffix(buf.size());
+    return 0;
+  }
+
+  // We can't call absl::LocalTime(), localtime_r(), or anything else here that
+  // isn't async-signal-safe. We can only use the time zone if it has already
+  // been loaded.
+  const absl::TimeZone* tz = absl::log_internal::TimeZone();
+  if (ABSL_PREDICT_FALSE(tz == nullptr)) {
+    // If a time zone hasn't been set yet because we are logging before the
+    // logging library has been initialized, we fallback to a simpler, slower
+    // method. Just report the raw Unix time in seconds. We cram this into the
+    // normal time format for the benefit of parsers.
+    auto tv = absl::ToTimeval(timestamp);
+    int snprintf_result = absl::SNPrintF(
+        buf.data(), buf.size(), "%c0000 00:00:%02d.%06d %7d ",
+        absl::LogSeverityName(severity)[0], static_cast<int>(tv.tv_sec),
+        static_cast<int>(tv.tv_usec), static_cast<int>(tid));
+    if (snprintf_result >= 0) {
+      buf.remove_prefix(static_cast<size_t>(snprintf_result));
+      return static_cast<size_t>(snprintf_result);
+    }
+    return 0;
+  }
+
+  char* p = buf.data();
+  *p++ = absl::LogSeverityName(severity)[0];
+  const absl::TimeZone::CivilInfo ci = tz->At(timestamp);
+  absl::numbers_internal::PutTwoDigits(static_cast<uint32_t>(ci.cs.month()), p);
+  p += 2;
+  absl::numbers_internal::PutTwoDigits(static_cast<uint32_t>(ci.cs.day()), p);
+  p += 2;
+  *p++ = ' ';
+  absl::numbers_internal::PutTwoDigits(static_cast<uint32_t>(ci.cs.hour()), p);
+  p += 2;
+  *p++ = ':';
+  absl::numbers_internal::PutTwoDigits(static_cast<uint32_t>(ci.cs.minute()),
+                                       p);
+  p += 2;
+  *p++ = ':';
+  absl::numbers_internal::PutTwoDigits(static_cast<uint32_t>(ci.cs.second()),
+                                       p);
+  p += 2;
+  *p++ = '.';
+  const int64_t usecs = absl::ToInt64Microseconds(ci.subsecond);
+  absl::numbers_internal::PutTwoDigits(static_cast<uint32_t>(usecs / 10000), p);
+  p += 2;
+  absl::numbers_internal::PutTwoDigits(static_cast<uint32_t>(usecs / 100 % 100),
+                                       p);
+  p += 2;
+  absl::numbers_internal::PutTwoDigits(static_cast<uint32_t>(usecs % 100), p);
+  p += 2;
+  *p++ = ' ';
+  PutLeadingWhitespace(tid, p);
+  p = absl::numbers_internal::FastIntToBuffer(tid, p);
+  *p++ = ' ';
+  const size_t bytes_formatted = static_cast<size_t>(p - buf.data());
+  buf.remove_prefix(bytes_formatted);
+  return bytes_formatted;
+}
+
+size_t FormatLineNumber(int line, absl::Span<char>& buf) {
+  constexpr size_t kLineFieldMaxLen =
+      sizeof(":] ") + (1 + std::numeric_limits<int>::digits10 + 1) - sizeof("");
+  if (ABSL_PREDICT_FALSE(buf.size() < kLineFieldMaxLen)) {
+    // As above, we don't bother trying to truncate this if the buffer is too
+    // short and it should never happen.
+    buf.remove_suffix(buf.size());
+    return 0;
+  }
+  char* p = buf.data();
+  *p++ = ':';
+  p = absl::numbers_internal::FastIntToBuffer(line, p);
+  *p++ = ']';
+  *p++ = ' ';
+  const size_t bytes_formatted = static_cast<size_t>(p - buf.data());
+  buf.remove_prefix(bytes_formatted);
+  return bytes_formatted;
+}
+
+}  // namespace
+
+std::string FormatLogMessage(absl::LogSeverity severity,
+                             absl::CivilSecond civil_second,
+                             absl::Duration subsecond, log_internal::Tid tid,
+                             absl::string_view basename, int line,
+                             PrefixFormat format, absl::string_view message) {
+  return absl::StrFormat(
+      "%c%02d%02d %02d:%02d:%02d.%06d %7d %s:%d] %s%s",
+      absl::LogSeverityName(severity)[0], civil_second.month(),
+      civil_second.day(), civil_second.hour(), civil_second.minute(),
+      civil_second.second(), absl::ToInt64Microseconds(subsecond), tid,
+      basename, line, format == PrefixFormat::kRaw ? "RAW: " : "", message);
+}
+
+// This method is fairly hot, and the library always passes a huge `buf`, so we
+// save some bounds-checking cycles by not trying to do precise truncation.
+// Truncating at a field boundary is probably a better UX anyway.
+//
+// The prefix is written in three parts, each of which does a single
+// bounds-check and truncation:
+// 1. severity, timestamp, and thread ID
+// 2. filename
+// 3. line number and bracket
+size_t FormatLogPrefix(absl::LogSeverity severity, absl::Time timestamp,
+                       log_internal::Tid tid, absl::string_view basename,
+                       int line, PrefixFormat format, absl::Span<char>& buf) {
+  auto prefix_size = FormatBoundedFields(severity, timestamp, tid, buf);
+  prefix_size += log_internal::AppendTruncated(basename, buf);
+  prefix_size += FormatLineNumber(line, buf);
+  if (format == PrefixFormat::kRaw)
+    prefix_size += log_internal::AppendTruncated("RAW: ", buf);
+  return prefix_size;
+}
+
+}  // namespace log_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/log/internal/log_format.h b/absl/log/internal/log_format.h
new file mode 100644
index 00000000000..95a45edf610
--- /dev/null
+++ b/absl/log/internal/log_format.h
@@ -0,0 +1,78 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: log/internal/log_format.h
+// -----------------------------------------------------------------------------
+//
+// This file declares routines implementing formatting of log message and log
+// prefix.
+
+#ifndef ABSL_LOG_INTERNAL_LOG_FORMAT_H_
+#define ABSL_LOG_INTERNAL_LOG_FORMAT_H_
+
+#include <stddef.h>
+
+#include <string>
+
+#include "absl/base/config.h"
+#include "absl/base/log_severity.h"
+#include "absl/log/internal/config.h"
+#include "absl/strings/string_view.h"
+#include "absl/time/civil_time.h"
+#include "absl/time/time.h"
+#include "absl/types/span.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace log_internal {
+
+enum class PrefixFormat {
+  kNotRaw,
+  kRaw,
+};
+
+// Formats log message based on provided data.
+std::string FormatLogMessage(absl::LogSeverity severity,
+                             absl::CivilSecond civil_second,
+                             absl::Duration subsecond, log_internal::Tid tid,
+                             absl::string_view basename, int line,
+                             PrefixFormat format, absl::string_view message);
+
+// Formats various entry metadata into a text string meant for use as a
+// prefix on a log message string.  Writes into `buf`, advances `buf` to point
+// at the remainder of the buffer (i.e. past any written bytes), and returns the
+// number of bytes written.
+//
+// In addition to calling `buf->remove_prefix()` (or the equivalent), this
+// function may also do `buf->remove_suffix(buf->size())` in cases where no more
+// bytes (i.e. no message data) should be written into the buffer.  For example,
+// if the prefix ought to be:
+//   I0926 09:00:00.000000 1234567 foo.cc:123]
+// `buf` is too small, the function might fill the whole buffer:
+//   I0926 09:00:00.000000 1234
+// (note the apparrently incorrect thread ID), or it might write less:
+//   I0926 09:00:00.000000
+// In this case, it might also empty `buf` prior to returning to prevent
+// message data from being written into the space where a reader would expect to
+// see a thread ID.
+size_t FormatLogPrefix(absl::LogSeverity severity, absl::Time timestamp,
+                       log_internal::Tid tid, absl::string_view basename,
+                       int line, PrefixFormat format, absl::Span<char>& buf);
+
+}  // namespace log_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_LOG_INTERNAL_LOG_FORMAT_H_
diff --git a/absl/log/internal/log_impl.h b/absl/log/internal/log_impl.h
new file mode 100644
index 00000000000..2fe73de10b8
--- /dev/null
+++ b/absl/log/internal/log_impl.h
@@ -0,0 +1,280 @@
+// Copyright 2022 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// SKIP_ABSL_INLINE_NAMESPACE_CHECK
+
+#ifndef ABSL_LOG_INTERNAL_LOG_IMPL_H_
+#define ABSL_LOG_INTERNAL_LOG_IMPL_H_
+
+#include "absl/log/absl_vlog_is_on.h"
+#include "absl/log/internal/conditions.h"
+#include "absl/log/internal/log_message.h"
+#include "absl/log/internal/strip.h"
+
+// ABSL_LOG()
+#define ABSL_LOG_INTERNAL_LOG_IMPL(severity)             \
+  ABSL_LOG_INTERNAL_CONDITION##severity(STATELESS, true) \
+      ABSL_LOG_INTERNAL_LOG##severity.InternalStream()
+
+// ABSL_PLOG()
+#define ABSL_LOG_INTERNAL_PLOG_IMPL(severity)            \
+  ABSL_LOG_INTERNAL_CONDITION##severity(STATELESS, true) \
+      ABSL_LOG_INTERNAL_LOG##severity.InternalStream()   \
+          .WithPerror()
+
+// ABSL_DLOG()
+#ifndef NDEBUG
+#define ABSL_LOG_INTERNAL_DLOG_IMPL(severity)            \
+  ABSL_LOG_INTERNAL_CONDITION##severity(STATELESS, true) \
+      ABSL_LOG_INTERNAL_DLOG##severity.InternalStream()
+#else
+#define ABSL_LOG_INTERNAL_DLOG_IMPL(severity)             \
+  ABSL_LOG_INTERNAL_CONDITION##severity(STATELESS, false) \
+      ABSL_LOG_INTERNAL_DLOG##severity.InternalStream()
+#endif
+
+// The `switch` ensures that this expansion is the beginning of a statement (as
+// opposed to an expression). The use of both `case 0` and `default` is to
+// suppress a compiler warning.
+#define ABSL_LOG_INTERNAL_VLOG_IMPL(verbose_level)                     \
+  switch (const int absl_log_internal_verbose_level = (verbose_level)) \
+  case 0:                                                              \
+  default:                                                             \
+    ABSL_LOG_INTERNAL_LOG_IF_IMPL(                                     \
+        _INFO, ABSL_VLOG_IS_ON(absl_log_internal_verbose_level))       \
+        .WithVerbosity(absl_log_internal_verbose_level)
+
+#ifndef NDEBUG
+#define ABSL_LOG_INTERNAL_DVLOG_IMPL(verbose_level)                    \
+  switch (const int absl_log_internal_verbose_level = (verbose_level)) \
+  case 0:                                                              \
+  default:                                                             \
+    ABSL_LOG_INTERNAL_DLOG_IF_IMPL(                                    \
+        _INFO, ABSL_VLOG_IS_ON(absl_log_internal_verbose_level))       \
+        .WithVerbosity(absl_log_internal_verbose_level)
+#else
+#define ABSL_LOG_INTERNAL_DVLOG_IMPL(verbose_level)                       \
+  switch (const int absl_log_internal_verbose_level = (verbose_level))    \
+  case 0:                                                                 \
+  default:                                                                \
+    ABSL_LOG_INTERNAL_DLOG_IF_IMPL(                                       \
+        _INFO, false && ABSL_VLOG_IS_ON(absl_log_internal_verbose_level)) \
+        .WithVerbosity(absl_log_internal_verbose_level)
+#endif
+
+#define ABSL_LOG_INTERNAL_LOG_IF_IMPL(severity, condition)    \
+  ABSL_LOG_INTERNAL_CONDITION##severity(STATELESS, condition) \
+      ABSL_LOG_INTERNAL_LOG##severity.InternalStream()
+#define ABSL_LOG_INTERNAL_PLOG_IF_IMPL(severity, condition)   \
+  ABSL_LOG_INTERNAL_CONDITION##severity(STATELESS, condition) \
+      ABSL_LOG_INTERNAL_LOG##severity.InternalStream()        \
+          .WithPerror()
+
+#ifndef NDEBUG
+#define ABSL_LOG_INTERNAL_DLOG_IF_IMPL(severity, condition)   \
+  ABSL_LOG_INTERNAL_CONDITION##severity(STATELESS, condition) \
+      ABSL_LOG_INTERNAL_DLOG##severity.InternalStream()
+#else
+#define ABSL_LOG_INTERNAL_DLOG_IF_IMPL(severity, condition)              \
+  ABSL_LOG_INTERNAL_CONDITION##severity(STATELESS, false && (condition)) \
+      ABSL_LOG_INTERNAL_DLOG##severity.InternalStream()
+#endif
+
+// ABSL_LOG_EVERY_N
+#define ABSL_LOG_INTERNAL_LOG_EVERY_N_IMPL(severity, n)            \
+  ABSL_LOG_INTERNAL_CONDITION##severity(STATEFUL, true)(EveryN, n) \
+      ABSL_LOG_INTERNAL_LOG##severity.InternalStream()
+
+// ABSL_LOG_FIRST_N
+#define ABSL_LOG_INTERNAL_LOG_FIRST_N_IMPL(severity, n)            \
+  ABSL_LOG_INTERNAL_CONDITION##severity(STATEFUL, true)(FirstN, n) \
+      ABSL_LOG_INTERNAL_LOG##severity.InternalStream()
+
+// ABSL_LOG_EVERY_POW_2
+#define ABSL_LOG_INTERNAL_LOG_EVERY_POW_2_IMPL(severity)           \
+  ABSL_LOG_INTERNAL_CONDITION##severity(STATEFUL, true)(EveryPow2) \
+      ABSL_LOG_INTERNAL_LOG##severity.InternalStream()
+
+// ABSL_LOG_EVERY_N_SEC
+#define ABSL_LOG_INTERNAL_LOG_EVERY_N_SEC_IMPL(severity, n_seconds)           \
+  ABSL_LOG_INTERNAL_CONDITION##severity(STATEFUL, true)(EveryNSec, n_seconds) \
+      ABSL_LOG_INTERNAL_LOG##severity.InternalStream()
+
+#define ABSL_LOG_INTERNAL_PLOG_EVERY_N_IMPL(severity, n)           \
+  ABSL_LOG_INTERNAL_CONDITION##severity(STATEFUL, true)(EveryN, n) \
+      ABSL_LOG_INTERNAL_LOG##severity.InternalStream()             \
+          .WithPerror()
+
+#define ABSL_LOG_INTERNAL_PLOG_FIRST_N_IMPL(severity, n)           \
+  ABSL_LOG_INTERNAL_CONDITION##severity(STATEFUL, true)(FirstN, n) \
+      ABSL_LOG_INTERNAL_LOG##severity.InternalStream()             \
+          .WithPerror()
+
+#define ABSL_LOG_INTERNAL_PLOG_EVERY_POW_2_IMPL(severity)          \
+  ABSL_LOG_INTERNAL_CONDITION##severity(STATEFUL, true)(EveryPow2) \
+      ABSL_LOG_INTERNAL_LOG##severity.InternalStream()             \
+          .WithPerror()
+
+#define ABSL_LOG_INTERNAL_PLOG_EVERY_N_SEC_IMPL(severity, n_seconds)          \
+  ABSL_LOG_INTERNAL_CONDITION##severity(STATEFUL, true)(EveryNSec, n_seconds) \
+      ABSL_LOG_INTERNAL_LOG##severity.InternalStream()                        \
+          .WithPerror()
+
+#ifndef NDEBUG
+#define ABSL_LOG_INTERNAL_DLOG_EVERY_N_IMPL(severity, n) \
+  ABSL_LOG_INTERNAL_CONDITION_INFO(STATEFUL, true)       \
+  (EveryN, n) ABSL_LOG_INTERNAL_DLOG##severity.InternalStream()
+
+#define ABSL_LOG_INTERNAL_DLOG_FIRST_N_IMPL(severity, n) \
+  ABSL_LOG_INTERNAL_CONDITION_INFO(STATEFUL, true)       \
+  (FirstN, n) ABSL_LOG_INTERNAL_DLOG##severity.InternalStream()
+
+#define ABSL_LOG_INTERNAL_DLOG_EVERY_POW_2_IMPL(severity) \
+  ABSL_LOG_INTERNAL_CONDITION_INFO(STATEFUL, true)        \
+  (EveryPow2) ABSL_LOG_INTERNAL_DLOG##severity.InternalStream()
+
+#define ABSL_LOG_INTERNAL_DLOG_EVERY_N_SEC_IMPL(severity, n_seconds) \
+  ABSL_LOG_INTERNAL_CONDITION_INFO(STATEFUL, true)                   \
+  (EveryNSec, n_seconds) ABSL_LOG_INTERNAL_DLOG##severity.InternalStream()
+
+#else  // def NDEBUG
+#define ABSL_LOG_INTERNAL_DLOG_EVERY_N_IMPL(severity, n) \
+  ABSL_LOG_INTERNAL_CONDITION_INFO(STATEFUL, false)      \
+  (EveryN, n) ABSL_LOG_INTERNAL_DLOG##severity.InternalStream()
+
+#define ABSL_LOG_INTERNAL_DLOG_FIRST_N_IMPL(severity, n) \
+  ABSL_LOG_INTERNAL_CONDITION_INFO(STATEFUL, false)      \
+  (FirstN, n) ABSL_LOG_INTERNAL_DLOG##severity.InternalStream()
+
+#define ABSL_LOG_INTERNAL_DLOG_EVERY_POW_2_IMPL(severity) \
+  ABSL_LOG_INTERNAL_CONDITION_INFO(STATEFUL, false)       \
+  (EveryPow2) ABSL_LOG_INTERNAL_DLOG##severity.InternalStream()
+
+#define ABSL_LOG_INTERNAL_DLOG_EVERY_N_SEC_IMPL(severity, n_seconds) \
+  ABSL_LOG_INTERNAL_CONDITION_INFO(STATEFUL, false)                  \
+  (EveryNSec, n_seconds) ABSL_LOG_INTERNAL_DLOG##severity.InternalStream()
+#endif  // def NDEBUG
+
+#define ABSL_LOG_INTERNAL_VLOG_EVERY_N_IMPL(verbose_level, n)            \
+  switch (const int absl_log_internal_verbose_level = (verbose_level))   \
+  case 0:                                                                \
+  default:                                                               \
+    ABSL_LOG_INTERNAL_CONDITION_INFO(                                    \
+        STATEFUL, ABSL_VLOG_IS_ON(absl_log_internal_verbose_level))      \
+  (EveryN, n) ABSL_LOG_INTERNAL_LOG_INFO.InternalStream().WithVerbosity( \
+      absl_log_internal_verbose_level)
+
+#define ABSL_LOG_INTERNAL_VLOG_FIRST_N_IMPL(verbose_level, n)            \
+  switch (const int absl_log_internal_verbose_level = (verbose_level))   \
+  case 0:                                                                \
+  default:                                                               \
+    ABSL_LOG_INTERNAL_CONDITION_INFO(                                    \
+        STATEFUL, ABSL_VLOG_IS_ON(absl_log_internal_verbose_level))      \
+  (FirstN, n) ABSL_LOG_INTERNAL_LOG_INFO.InternalStream().WithVerbosity( \
+      absl_log_internal_verbose_level)
+
+#define ABSL_LOG_INTERNAL_VLOG_EVERY_POW_2_IMPL(verbose_level)           \
+  switch (const int absl_log_internal_verbose_level = (verbose_level))   \
+  case 0:                                                                \
+  default:                                                               \
+    ABSL_LOG_INTERNAL_CONDITION_INFO(                                    \
+        STATEFUL, ABSL_VLOG_IS_ON(absl_log_internal_verbose_level))      \
+  (EveryPow2) ABSL_LOG_INTERNAL_LOG_INFO.InternalStream().WithVerbosity( \
+      absl_log_internal_verbose_level)
+
+#define ABSL_LOG_INTERNAL_VLOG_EVERY_N_SEC_IMPL(verbose_level, n_seconds) \
+  switch (const int absl_log_internal_verbose_level = (verbose_level))    \
+  case 0:                                                                 \
+  default:                                                                \
+    ABSL_LOG_INTERNAL_CONDITION_INFO(                                     \
+        STATEFUL, ABSL_VLOG_IS_ON(absl_log_internal_verbose_level))       \
+  (EveryNSec, n_seconds) ABSL_LOG_INTERNAL_LOG_INFO.InternalStream()      \
+      .WithVerbosity(absl_log_internal_verbose_level)
+
+#define ABSL_LOG_INTERNAL_LOG_IF_EVERY_N_IMPL(severity, condition, n)   \
+  ABSL_LOG_INTERNAL_CONDITION##severity(STATEFUL, condition)(EveryN, n) \
+      ABSL_LOG_INTERNAL_LOG##severity.InternalStream()
+
+#define ABSL_LOG_INTERNAL_LOG_IF_FIRST_N_IMPL(severity, condition, n)   \
+  ABSL_LOG_INTERNAL_CONDITION##severity(STATEFUL, condition)(FirstN, n) \
+      ABSL_LOG_INTERNAL_LOG##severity.InternalStream()
+
+#define ABSL_LOG_INTERNAL_LOG_IF_EVERY_POW_2_IMPL(severity, condition)  \
+  ABSL_LOG_INTERNAL_CONDITION##severity(STATEFUL, condition)(EveryPow2) \
+      ABSL_LOG_INTERNAL_LOG##severity.InternalStream()
+
+#define ABSL_LOG_INTERNAL_LOG_IF_EVERY_N_SEC_IMPL(severity, condition, \
+                                                  n_seconds)           \
+  ABSL_LOG_INTERNAL_CONDITION##severity(STATEFUL, condition)(          \
+      EveryNSec, n_seconds) ABSL_LOG_INTERNAL_LOG##severity.InternalStream()
+
+#define ABSL_LOG_INTERNAL_PLOG_IF_EVERY_N_IMPL(severity, condition, n)  \
+  ABSL_LOG_INTERNAL_CONDITION##severity(STATEFUL, condition)(EveryN, n) \
+      ABSL_LOG_INTERNAL_LOG##severity.InternalStream()                  \
+          .WithPerror()
+
+#define ABSL_LOG_INTERNAL_PLOG_IF_FIRST_N_IMPL(severity, condition, n)  \
+  ABSL_LOG_INTERNAL_CONDITION##severity(STATEFUL, condition)(FirstN, n) \
+      ABSL_LOG_INTERNAL_LOG##severity.InternalStream()                  \
+          .WithPerror()
+
+#define ABSL_LOG_INTERNAL_PLOG_IF_EVERY_POW_2_IMPL(severity, condition) \
+  ABSL_LOG_INTERNAL_CONDITION##severity(STATEFUL, condition)(EveryPow2) \
+      ABSL_LOG_INTERNAL_LOG##severity.InternalStream()                  \
+          .WithPerror()
+
+#define ABSL_LOG_INTERNAL_PLOG_IF_EVERY_N_SEC_IMPL(severity, condition,      \
+                                                   n_seconds)                \
+  ABSL_LOG_INTERNAL_CONDITION##severity(STATEFUL, condition)(                \
+      EveryNSec, n_seconds) ABSL_LOG_INTERNAL_LOG##severity.InternalStream() \
+      .WithPerror()
+
+#ifndef NDEBUG
+#define ABSL_LOG_INTERNAL_DLOG_IF_EVERY_N_IMPL(severity, condition, n)  \
+  ABSL_LOG_INTERNAL_CONDITION##severity(STATEFUL, condition)(EveryN, n) \
+      ABSL_LOG_INTERNAL_DLOG##severity.InternalStream()
+
+#define ABSL_LOG_INTERNAL_DLOG_IF_FIRST_N_IMPL(severity, condition, n)  \
+  ABSL_LOG_INTERNAL_CONDITION##severity(STATEFUL, condition)(FirstN, n) \
+      ABSL_LOG_INTERNAL_DLOG##severity.InternalStream()
+
+#define ABSL_LOG_INTERNAL_DLOG_IF_EVERY_POW_2_IMPL(severity, condition) \
+  ABSL_LOG_INTERNAL_CONDITION##severity(STATEFUL, condition)(EveryPow2) \
+      ABSL_LOG_INTERNAL_DLOG##severity.InternalStream()
+
+#define ABSL_LOG_INTERNAL_DLOG_IF_EVERY_N_SEC_IMPL(severity, condition, \
+                                                   n_seconds)           \
+  ABSL_LOG_INTERNAL_CONDITION##severity(STATEFUL, condition)(           \
+      EveryNSec, n_seconds) ABSL_LOG_INTERNAL_DLOG##severity.InternalStream()
+
+#else  // def NDEBUG
+#define ABSL_LOG_INTERNAL_DLOG_IF_EVERY_N_IMPL(severity, condition, n)   \
+  ABSL_LOG_INTERNAL_CONDITION##severity(STATEFUL, false && (condition))( \
+      EveryN, n) ABSL_LOG_INTERNAL_DLOG##severity.InternalStream()
+
+#define ABSL_LOG_INTERNAL_DLOG_IF_FIRST_N_IMPL(severity, condition, n)   \
+  ABSL_LOG_INTERNAL_CONDITION##severity(STATEFUL, false && (condition))( \
+      FirstN, n) ABSL_LOG_INTERNAL_DLOG##severity.InternalStream()
+
+#define ABSL_LOG_INTERNAL_DLOG_IF_EVERY_POW_2_IMPL(severity, condition)  \
+  ABSL_LOG_INTERNAL_CONDITION##severity(STATEFUL, false && (condition))( \
+      EveryPow2) ABSL_LOG_INTERNAL_DLOG##severity.InternalStream()
+
+#define ABSL_LOG_INTERNAL_DLOG_IF_EVERY_N_SEC_IMPL(severity, condition,  \
+                                                   n_seconds)            \
+  ABSL_LOG_INTERNAL_CONDITION##severity(STATEFUL, false && (condition))( \
+      EveryNSec, n_seconds) ABSL_LOG_INTERNAL_DLOG##severity.InternalStream()
+#endif  // def NDEBUG
+
+#endif  // ABSL_LOG_INTERNAL_LOG_IMPL_H_
diff --git a/absl/log/internal/log_message.cc b/absl/log/internal/log_message.cc
new file mode 100644
index 00000000000..8ac81dfed0d
--- /dev/null
+++ b/absl/log/internal/log_message.cc
@@ -0,0 +1,786 @@
+//
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/log/internal/log_message.h"
+
+#include <stddef.h>
+#include <stdint.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include <algorithm>
+#include <array>
+#include <atomic>
+#include <ios>
+#include <memory>
+#include <ostream>
+#include <string>
+#include <string_view>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/base/internal/strerror.h"
+#include "absl/base/internal/sysinfo.h"
+#include "absl/base/log_severity.h"
+#include "absl/base/nullability.h"
+#include "absl/container/inlined_vector.h"
+#include "absl/debugging/internal/examine_stack.h"
+#include "absl/log/globals.h"
+#include "absl/log/internal/append_truncated.h"
+#include "absl/log/internal/globals.h"
+#include "absl/log/internal/log_format.h"
+#include "absl/log/internal/log_sink_set.h"
+#include "absl/log/internal/nullguard.h"
+#include "absl/log/internal/proto.h"
+#include "absl/log/internal/structured_proto.h"
+#include "absl/log/log_entry.h"
+#include "absl/log/log_sink.h"
+#include "absl/log/log_sink_registry.h"
+#include "absl/strings/internal/utf8.h"
+#include "absl/strings/string_view.h"
+#include "absl/time/clock.h"
+#include "absl/time/time.h"
+#include "absl/types/span.h"
+
+#ifndef _WIN32
+#include <unistd.h>
+#endif
+
+extern "C" ABSL_ATTRIBUTE_WEAK void ABSL_INTERNAL_C_SYMBOL(
+    AbslInternalOnFatalLogMessage)(const absl::LogEntry&) {
+  // Default - Do nothing
+}
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace log_internal {
+
+namespace {
+// message `logging.proto.Event`
+enum EventTag : uint8_t {
+  kFileName = 2,
+  kFileLine = 3,
+  kTimeNsecs = 4,
+  kSeverity = 5,
+  kThreadId = 6,
+  kValue = 7,
+  kSequenceNumber = 9,
+  kThreadName = 10,
+};
+
+// message `logging.proto.Value`
+enum ValueTag : uint8_t {
+  kString = 1,
+  kStringLiteral = 6,
+};
+
+// Decodes a `logging.proto.Value` from `buf` and writes a string representation
+// into `dst`.  The string representation will be truncated if `dst` is not
+// large enough to hold it.  Returns false if `dst` has size zero or one (i.e.
+// sufficient only for a nul-terminator) and no decoded data could be written.
+// This function may or may not write a nul-terminator into `dst`, and it may or
+// may not truncate the data it writes in order to do make space for that nul
+// terminator.  In any case, `dst` will be advanced to point at the byte where
+// subsequent writes should begin.
+bool PrintValue(absl::Span<char>& dst, absl::Span<const char> buf) {
+  if (dst.size() <= 1) return false;
+  ProtoField field;
+  while (field.DecodeFrom(&buf)) {
+    switch (field.tag()) {
+      case ValueTag::kString:
+      case ValueTag::kStringLiteral:
+        if (field.type() == WireType::kLengthDelimited)
+          if (log_internal::AppendTruncated(field.string_value(), dst) <
+              field.string_value().size())
+            return false;
+    }
+  }
+  return true;
+}
+
+// See `logging.proto.Severity`
+int32_t ProtoSeverity(absl::LogSeverity severity, int verbose_level) {
+  switch (severity) {
+    case absl::LogSeverity::kInfo:
+      if (verbose_level == absl::LogEntry::kNoVerbosityLevel) return 800;
+      return 600 - verbose_level;
+    case absl::LogSeverity::kWarning:
+      return 900;
+    case absl::LogSeverity::kError:
+      return 950;
+    case absl::LogSeverity::kFatal:
+      return 1100;
+    default:
+      return 800;
+  }
+}
+
+absl::string_view Basename(absl::string_view filepath) {
+#ifdef _WIN32
+  size_t path = filepath.find_last_of("/\\");
+#else
+  size_t path = filepath.find_last_of('/');
+#endif
+  if (path != filepath.npos) filepath.remove_prefix(path + 1);
+  return filepath;
+}
+
+void WriteToString(const char* data, void* str) {
+  reinterpret_cast<std::string*>(str)->append(data);
+}
+void WriteToStream(const char* data, void* os) {
+  auto* cast_os = static_cast<std::ostream*>(os);
+  *cast_os << data;
+}
+}  // namespace
+
+struct LogMessage::LogMessageData final {
+  LogMessageData(absl::string_view file, int line,
+                 absl::LogSeverity severity, absl::Time timestamp);
+  LogMessageData(const LogMessageData&) = delete;
+  LogMessageData& operator=(const LogMessageData&) = delete;
+
+  // `LogEntry` sent to `LogSink`s; contains metadata.
+  absl::LogEntry entry;
+
+  // true => this was first fatal msg
+  bool first_fatal;
+  // true => all failures should be quiet
+  bool fail_quietly;
+  // true => PLOG was requested
+  bool is_perror;
+
+  // Extra `LogSink`s to log to, in addition to `global_sinks`.
+  absl::InlinedVector<absl::LogSink* absl_nonnull, 16> extra_sinks;
+  // If true, log to `extra_sinks` but not to `global_sinks` or hardcoded
+  // non-sink targets (e.g. stderr, log files).
+  bool extra_sinks_only;
+
+  std::ostream manipulated;  // ostream with IO manipulators applied
+
+  // A `logging.proto.Event` proto message is built into `encoded_buf`.
+  std::array<char, kLogMessageBufferSize> encoded_buf;
+  // `encoded_remaining()` is the suffix of `encoded_buf` that has not been
+  // filled yet.  If a datum to be encoded does not fit into
+  // `encoded_remaining()` and cannot be truncated to fit, the size of
+  // `encoded_remaining()` will be zeroed to prevent encoding of any further
+  // data.  Note that in this case its `data()` pointer will not point past the
+  // end of `encoded_buf`.
+  // The first use of `encoded_remaining()` is our chance to record metadata
+  // after any modifications (e.g. by `AtLocation()`) but before any data have
+  // been recorded.  We want to record metadata before data so that data are
+  // preferentially truncated if we run out of buffer.
+  absl::Span<char>& encoded_remaining() {
+    if (encoded_remaining_actual_do_not_use_directly.data() == nullptr) {
+      encoded_remaining_actual_do_not_use_directly =
+          absl::MakeSpan(encoded_buf);
+      InitializeEncodingAndFormat();
+    }
+    return encoded_remaining_actual_do_not_use_directly;
+  }
+  absl::Span<char> encoded_remaining_actual_do_not_use_directly;
+
+  // A formatted string message is built in `string_buf`.
+  std::array<char, kLogMessageBufferSize> string_buf;
+
+  void InitializeEncodingAndFormat();
+  void FinalizeEncodingAndFormat();
+};
+
+LogMessage::LogMessageData::LogMessageData(absl::string_view file,
+                                           int line, absl::LogSeverity severity,
+                                           absl::Time timestamp)
+    : extra_sinks_only(false), manipulated(nullptr) {
+  // Legacy defaults for LOG's ostream:
+  manipulated.setf(std::ios_base::showbase | std::ios_base::boolalpha);
+  entry.full_filename_ = file;
+  entry.base_filename_ = Basename(file);
+  entry.line_ = line;
+  entry.prefix_ = absl::ShouldPrependLogPrefix();
+  entry.severity_ = absl::NormalizeLogSeverity(severity);
+  entry.verbose_level_ = absl::LogEntry::kNoVerbosityLevel;
+  entry.timestamp_ = timestamp;
+  entry.tid_ = absl::base_internal::GetCachedTID();
+}
+
+void LogMessage::LogMessageData::InitializeEncodingAndFormat() {
+  EncodeStringTruncate(EventTag::kFileName, entry.source_filename(),
+                       &encoded_remaining());
+  EncodeVarint(EventTag::kFileLine, entry.source_line(), &encoded_remaining());
+  EncodeVarint(EventTag::kTimeNsecs, absl::ToUnixNanos(entry.timestamp()),
+               &encoded_remaining());
+  EncodeVarint(EventTag::kSeverity,
+               ProtoSeverity(entry.log_severity(), entry.verbosity()),
+               &encoded_remaining());
+  EncodeVarint(EventTag::kThreadId, static_cast<uint64_t>(entry.tid()),
+               &encoded_remaining());
+}
+
+void LogMessage::LogMessageData::FinalizeEncodingAndFormat() {
+  // Note that `encoded_remaining()` may have zero size without pointing past
+  // the end of `encoded_buf`, so the difference between `data()` pointers is
+  // used to compute the size of `encoded_data`.
+  absl::Span<const char> encoded_data(
+      encoded_buf.data(),
+      static_cast<size_t>(encoded_remaining().data() - encoded_buf.data()));
+  // `string_remaining` is the suffix of `string_buf` that has not been filled
+  // yet.
+  absl::Span<char> string_remaining(string_buf);
+  // We may need to write a newline and nul-terminator at the end of the decoded
+  // string data.  Rather than worry about whether those should overwrite the
+  // end of the string (if the buffer is full) or be appended, we avoid writing
+  // into the last two bytes so we always have space to append.
+  string_remaining.remove_suffix(2);
+  entry.prefix_len_ =
+      entry.prefix() ? log_internal::FormatLogPrefix(
+                           entry.log_severity(), entry.timestamp(), entry.tid(),
+                           entry.source_basename(), entry.source_line(),
+                           log_internal::ThreadIsLoggingToLogSink()
+                               ? PrefixFormat::kRaw
+                               : PrefixFormat::kNotRaw,
+                           string_remaining)
+                     : 0;
+  // Decode data from `encoded_buf` until we run out of data or we run out of
+  // `string_remaining`.
+  ProtoField field;
+  while (field.DecodeFrom(&encoded_data)) {
+    switch (field.tag()) {
+      case EventTag::kValue:
+        if (field.type() != WireType::kLengthDelimited) continue;
+        if (PrintValue(string_remaining, field.bytes_value())) continue;
+        break;
+    }
+  }
+  auto chars_written =
+      static_cast<size_t>(string_remaining.data() - string_buf.data());
+    string_buf[chars_written++] = '\n';
+  string_buf[chars_written++] = '\0';
+  entry.text_message_with_prefix_and_newline_and_nul_ =
+      absl::MakeSpan(string_buf).subspan(0, chars_written);
+}
+
+LogMessage::LogMessage(const char* absl_nonnull file, int line,
+                       absl::LogSeverity severity)
+  : LogMessage(absl::string_view(file), line, severity) {}
+LogMessage::LogMessage(absl::string_view file, int line,
+                       absl::LogSeverity severity)
+    : data_(
+          std::make_unique<LogMessageData>(file, line, severity, absl::Now())) {
+  data_->first_fatal = false;
+  data_->is_perror = false;
+  data_->fail_quietly = false;
+
+  // This logs a backtrace even if the location is subsequently changed using
+  // AtLocation.  This quirk, and the behavior when AtLocation is called twice,
+  // are fixable but probably not worth fixing.
+  LogBacktraceIfNeeded();
+}
+
+LogMessage::LogMessage(const char* absl_nonnull file, int line, InfoTag)
+    : LogMessage(file, line, absl::LogSeverity::kInfo) {}
+LogMessage::LogMessage(const char* absl_nonnull file, int line, WarningTag)
+    : LogMessage(file, line, absl::LogSeverity::kWarning) {}
+LogMessage::LogMessage(const char* absl_nonnull file, int line, ErrorTag)
+    : LogMessage(file, line, absl::LogSeverity::kError) {}
+
+// This cannot go in the header since LogMessageData is defined in this file.
+LogMessage::~LogMessage() = default;
+
+LogMessage& LogMessage::AtLocation(absl::string_view file, int line) {
+  data_->entry.full_filename_ = file;
+  data_->entry.base_filename_ = Basename(file);
+  data_->entry.line_ = line;
+  LogBacktraceIfNeeded();
+  return *this;
+}
+
+LogMessage& LogMessage::NoPrefix() {
+  data_->entry.prefix_ = false;
+  return *this;
+}
+
+LogMessage& LogMessage::WithVerbosity(int verbose_level) {
+  if (verbose_level == absl::LogEntry::kNoVerbosityLevel) {
+    data_->entry.verbose_level_ = absl::LogEntry::kNoVerbosityLevel;
+  } else {
+    data_->entry.verbose_level_ = std::max(0, verbose_level);
+  }
+  return *this;
+}
+
+LogMessage& LogMessage::WithTimestamp(absl::Time timestamp) {
+  data_->entry.timestamp_ = timestamp;
+  return *this;
+}
+
+LogMessage& LogMessage::WithThreadID(absl::LogEntry::tid_t tid) {
+  data_->entry.tid_ = tid;
+  return *this;
+}
+
+LogMessage& LogMessage::WithMetadataFrom(const absl::LogEntry& entry) {
+  data_->entry.full_filename_ = entry.full_filename_;
+  data_->entry.base_filename_ = entry.base_filename_;
+  data_->entry.line_ = entry.line_;
+  data_->entry.prefix_ = entry.prefix_;
+  data_->entry.severity_ = entry.severity_;
+  data_->entry.verbose_level_ = entry.verbose_level_;
+  data_->entry.timestamp_ = entry.timestamp_;
+  data_->entry.tid_ = entry.tid_;
+  return *this;
+}
+
+LogMessage& LogMessage::WithPerror() {
+  data_->is_perror = true;
+  return *this;
+}
+
+LogMessage& LogMessage::ToSinkAlso(absl::LogSink* absl_nonnull sink) {
+  ABSL_INTERNAL_CHECK(sink, "null LogSink*");
+  data_->extra_sinks.push_back(sink);
+  return *this;
+}
+
+LogMessage& LogMessage::ToSinkOnly(absl::LogSink* absl_nonnull sink) {
+  ABSL_INTERNAL_CHECK(sink, "null LogSink*");
+  data_->extra_sinks.clear();
+  data_->extra_sinks.push_back(sink);
+  data_->extra_sinks_only = true;
+  return *this;
+}
+
+#ifdef __ELF__
+extern "C" void __gcov_dump() ABSL_ATTRIBUTE_WEAK;
+extern "C" void __gcov_flush() ABSL_ATTRIBUTE_WEAK;
+#endif
+
+void LogMessage::FailWithoutStackTrace() {
+  // Now suppress repeated trace logging:
+  log_internal::SetSuppressSigabortTrace(true);
+#if defined _DEBUG && defined COMPILER_MSVC
+  // When debugging on windows, avoid the obnoxious dialog.
+  __debugbreak();
+#endif
+
+#ifdef __ELF__
+  // For b/8737634, flush coverage if we are in coverage mode.
+  if (&__gcov_dump != nullptr) {
+    __gcov_dump();
+  } else if (&__gcov_flush != nullptr) {
+    __gcov_flush();
+  }
+#endif
+
+  abort();
+}
+
+void LogMessage::FailQuietly() {
+  // _exit. Calling abort() would trigger all sorts of death signal handlers
+  // and a detailed stack trace. Calling exit() would trigger the onexit
+  // handlers, including the heap-leak checker, which is guaranteed to fail in
+  // this case: we probably just new'ed the std::string that we logged.
+  // Anyway, if you're calling Fail or FailQuietly, you're trying to bail out
+  // of the program quickly, and it doesn't make much sense for FailQuietly to
+  // offer different guarantees about exit behavior than Fail does. (And as a
+  // consequence for QCHECK and CHECK to offer different exit behaviors)
+  _exit(1);
+}
+
+LogMessage& LogMessage::operator<<(const std::string& v) {
+  CopyToEncodedBuffer<StringType::kNotLiteral>(v);
+  return *this;
+}
+
+LogMessage& LogMessage::operator<<(absl::string_view v) {
+  CopyToEncodedBuffer<StringType::kNotLiteral>(v);
+  return *this;
+}
+
+LogMessage& LogMessage::operator<<(const std::wstring& v) {
+  CopyToEncodedBuffer<StringType::kNotLiteral>(v);
+  return *this;
+}
+
+LogMessage& LogMessage::operator<<(std::wstring_view v) {
+  CopyToEncodedBuffer<StringType::kNotLiteral>(v);
+  return *this;
+}
+
+template <>
+LogMessage& LogMessage::operator<< <const wchar_t*>(
+    const wchar_t* absl_nullable const& v) {
+  if (v == nullptr) {
+    CopyToEncodedBuffer<StringType::kNotLiteral>(
+        absl::string_view(kCharNull.data(), kCharNull.size() - 1));
+  } else {
+    CopyToEncodedBuffer<StringType::kNotLiteral>(v);
+  }
+  return *this;
+}
+
+LogMessage& LogMessage::operator<<(wchar_t v) {
+  CopyToEncodedBuffer<StringType::kNotLiteral>(std::wstring_view(&v, 1));
+  return *this;
+}
+
+LogMessage& LogMessage::operator<<(std::ostream& (*m)(std::ostream& os)) {
+  OstreamView view(*data_);
+  data_->manipulated << m;
+  return *this;
+}
+LogMessage& LogMessage::operator<<(std::ios_base& (*m)(std::ios_base& os)) {
+  OstreamView view(*data_);
+  data_->manipulated << m;
+  return *this;
+}
+// NOLINTBEGIN(runtime/int)
+// NOLINTBEGIN(google-runtime-int)
+template LogMessage& LogMessage::operator<<(const char& v);
+template LogMessage& LogMessage::operator<<(const signed char& v);
+template LogMessage& LogMessage::operator<<(const unsigned char& v);
+template LogMessage& LogMessage::operator<<(const short& v);
+template LogMessage& LogMessage::operator<<(const unsigned short& v);
+template LogMessage& LogMessage::operator<<(const int& v);
+template LogMessage& LogMessage::operator<<(const unsigned int& v);
+template LogMessage& LogMessage::operator<<(const long& v);
+template LogMessage& LogMessage::operator<<(const unsigned long& v);
+template LogMessage& LogMessage::operator<<(const long long& v);
+template LogMessage& LogMessage::operator<<(const unsigned long long& v);
+template LogMessage& LogMessage::operator<<(void* const& v);
+template LogMessage& LogMessage::operator<<(const void* const& v);
+template LogMessage& LogMessage::operator<<(const float& v);
+template LogMessage& LogMessage::operator<<(const double& v);
+template LogMessage& LogMessage::operator<<(const bool& v);
+// NOLINTEND(google-runtime-int)
+// NOLINTEND(runtime/int)
+
+void LogMessage::Flush() {
+  if (data_->entry.log_severity() < absl::MinLogLevel()) return;
+
+  if (data_->is_perror) {
+    InternalStream() << ": " << absl::base_internal::StrError(errno_saver_())
+                     << " [" << errno_saver_() << "]";
+  }
+
+  // Have we already seen a fatal message?
+  ABSL_CONST_INIT static std::atomic<bool> seen_fatal(false);
+  if (data_->entry.log_severity() == absl::LogSeverity::kFatal &&
+      absl::log_internal::ExitOnDFatal()) {
+    // Exactly one LOG(FATAL) message is responsible for aborting the process,
+    // even if multiple threads LOG(FATAL) concurrently.
+    bool expected_seen_fatal = false;
+    if (seen_fatal.compare_exchange_strong(expected_seen_fatal, true,
+                                           std::memory_order_relaxed)) {
+      data_->first_fatal = true;
+    }
+  }
+
+  data_->FinalizeEncodingAndFormat();
+  data_->entry.encoding_ =
+      absl::string_view(data_->encoded_buf.data(),
+                        static_cast<size_t>(data_->encoded_remaining().data() -
+                                            data_->encoded_buf.data()));
+  SendToLog();
+}
+
+void LogMessage::SetFailQuietly() { data_->fail_quietly = true; }
+
+LogMessage::OstreamView::OstreamView(LogMessageData& message_data)
+    : data_(message_data), encoded_remaining_copy_(data_.encoded_remaining()) {
+  // This constructor sets the `streambuf` up so that streaming into an attached
+  // ostream encodes string data in-place.  To do that, we write appropriate
+  // headers into the buffer using a copy of the buffer view so that we can
+  // decide not to keep them later if nothing is ever streamed in.  We don't
+  // know how much data we'll get, but we can use the size of the remaining
+  // buffer as an upper bound and fill in the right size once we know it.
+  message_start_ =
+      EncodeMessageStart(EventTag::kValue, encoded_remaining_copy_.size(),
+                         &encoded_remaining_copy_);
+  string_start_ =
+      EncodeMessageStart(ValueTag::kString, encoded_remaining_copy_.size(),
+                         &encoded_remaining_copy_);
+  setp(encoded_remaining_copy_.data(),
+       encoded_remaining_copy_.data() + encoded_remaining_copy_.size());
+  data_.manipulated.rdbuf(this);
+}
+
+LogMessage::OstreamView::~OstreamView() {
+  data_.manipulated.rdbuf(nullptr);
+  if (!string_start_.data()) {
+    // The second field header didn't fit.  Whether the first one did or not, we
+    // shouldn't commit `encoded_remaining_copy_`, and we also need to zero the
+    // size of `data_->encoded_remaining()` so that no more data are encoded.
+    data_.encoded_remaining().remove_suffix(data_.encoded_remaining().size());
+    return;
+  }
+  const absl::Span<const char> contents(pbase(),
+                                        static_cast<size_t>(pptr() - pbase()));
+  if (contents.empty()) return;
+  encoded_remaining_copy_.remove_prefix(contents.size());
+  EncodeMessageLength(string_start_, &encoded_remaining_copy_);
+  EncodeMessageLength(message_start_, &encoded_remaining_copy_);
+  data_.encoded_remaining() = encoded_remaining_copy_;
+}
+
+std::ostream& LogMessage::OstreamView::stream() { return data_.manipulated; }
+
+bool LogMessage::IsFatal() const {
+  return data_->entry.log_severity() == absl::LogSeverity::kFatal &&
+         absl::log_internal::ExitOnDFatal();
+}
+
+void LogMessage::PrepareToDie() {
+  // If we log a FATAL message, flush all the log destinations, then toss
+  // a signal for others to catch. We leave the logs in a state that
+  // someone else can use them (as long as they flush afterwards)
+  if (data_->first_fatal) {
+    // Notify observers about the upcoming fatal error.
+    ABSL_INTERNAL_C_SYMBOL(AbslInternalOnFatalLogMessage)(data_->entry);
+  }
+
+  if (!data_->fail_quietly) {
+    // Log the message first before we start collecting stack trace.
+    log_internal::LogToSinks(data_->entry, absl::MakeSpan(data_->extra_sinks),
+                             data_->extra_sinks_only);
+
+    // `DumpStackTrace` generates an empty string under MSVC.
+    // Adding the constant prefix here simplifies testing.
+    data_->entry.stacktrace_ = "*** Check failure stack trace: ***\n";
+    debugging_internal::DumpStackTrace(
+        0, log_internal::MaxFramesInLogStackTrace(),
+        log_internal::ShouldSymbolizeLogStackTrace(), WriteToString,
+        &data_->entry.stacktrace_);
+  }
+}
+
+void LogMessage::Die() {
+  absl::FlushLogSinks();
+
+  if (data_->fail_quietly) {
+    FailQuietly();
+  } else {
+    FailWithoutStackTrace();
+  }
+}
+
+void LogMessage::SendToLog() {
+  if (IsFatal()) PrepareToDie();
+  // Also log to all registered sinks, even if OnlyLogToStderr() is set.
+  log_internal::LogToSinks(data_->entry, absl::MakeSpan(data_->extra_sinks),
+                           data_->extra_sinks_only);
+  if (IsFatal()) Die();
+}
+
+void LogMessage::LogBacktraceIfNeeded() {
+  if (!absl::log_internal::IsInitialized()) return;
+
+  if (!absl::log_internal::ShouldLogBacktraceAt(data_->entry.source_basename(),
+                                                data_->entry.source_line()))
+    return;
+  OstreamView view(*data_);
+  view.stream() << " (stacktrace:\n";
+  debugging_internal::DumpStackTrace(
+      1, log_internal::MaxFramesInLogStackTrace(),
+      log_internal::ShouldSymbolizeLogStackTrace(), WriteToStream,
+      &view.stream());
+  view.stream() << ") ";
+}
+
+// Encodes into `data_->encoded_remaining()` a partial `logging.proto.Event`
+// containing the specified string data using a `Value` field appropriate to
+// `str_type`.  Truncates `str` if necessary, but emits nothing and marks the
+// buffer full if  even the field headers do not fit.
+template <LogMessage::StringType str_type>
+void LogMessage::CopyToEncodedBuffer(absl::string_view str) {
+  auto encoded_remaining_copy = data_->encoded_remaining();
+  constexpr uint8_t tag_value = str_type == StringType::kLiteral
+                                    ? ValueTag::kStringLiteral
+                                    : ValueTag::kString;
+  auto start = EncodeMessageStart(
+      EventTag::kValue,
+      BufferSizeFor(tag_value, WireType::kLengthDelimited) + str.size(),
+      &encoded_remaining_copy);
+  // If the `logging.proto.Event.value` field header did not fit,
+  // `EncodeMessageStart` will have zeroed `encoded_remaining_copy`'s size and
+  // `EncodeStringTruncate` will fail too.
+  if (EncodeStringTruncate(tag_value, str, &encoded_remaining_copy)) {
+    // The string may have been truncated, but the field header fit.
+    EncodeMessageLength(start, &encoded_remaining_copy);
+    data_->encoded_remaining() = encoded_remaining_copy;
+  } else {
+    // The field header(s) did not fit; zero `encoded_remaining()` so we don't
+    // write anything else later.
+    data_->encoded_remaining().remove_suffix(data_->encoded_remaining().size());
+  }
+}
+template void LogMessage::CopyToEncodedBuffer<LogMessage::StringType::kLiteral>(
+    absl::string_view str);
+template void LogMessage::CopyToEncodedBuffer<
+    LogMessage::StringType::kNotLiteral>(absl::string_view str);
+template <LogMessage::StringType str_type>
+void LogMessage::CopyToEncodedBuffer(char ch, size_t num) {
+  auto encoded_remaining_copy = data_->encoded_remaining();
+  constexpr uint8_t tag_value = str_type == StringType::kLiteral
+                                    ? ValueTag::kStringLiteral
+                                    : ValueTag::kString;
+  auto value_start = EncodeMessageStart(
+      EventTag::kValue,
+      BufferSizeFor(tag_value, WireType::kLengthDelimited) + num,
+      &encoded_remaining_copy);
+  auto str_start = EncodeMessageStart(tag_value, num, &encoded_remaining_copy);
+  if (str_start.data()) {
+    // The field headers fit.
+    log_internal::AppendTruncated(ch, num, encoded_remaining_copy);
+    EncodeMessageLength(str_start, &encoded_remaining_copy);
+    EncodeMessageLength(value_start, &encoded_remaining_copy);
+    data_->encoded_remaining() = encoded_remaining_copy;
+  } else {
+    // The field header(s) did not fit; zero `encoded_remaining()` so we don't
+    // write anything else later.
+    data_->encoded_remaining().remove_suffix(data_->encoded_remaining().size());
+  }
+}
+template void LogMessage::CopyToEncodedBuffer<LogMessage::StringType::kLiteral>(
+    char ch, size_t num);
+template void LogMessage::CopyToEncodedBuffer<
+    LogMessage::StringType::kNotLiteral>(char ch, size_t num);
+
+template <LogMessage::StringType str_type>
+void LogMessage::CopyToEncodedBuffer(std::wstring_view str) {
+  auto encoded_remaining_copy = data_->encoded_remaining();
+  constexpr uint8_t tag_value = str_type == StringType::kLiteral
+                                    ? ValueTag::kStringLiteral
+                                    : ValueTag::kString;
+  size_t max_str_byte_length =
+      absl::strings_internal::kMaxEncodedUTF8Size * str.length();
+  auto value_start =
+      EncodeMessageStart(EventTag::kValue,
+                         BufferSizeFor(tag_value, WireType::kLengthDelimited) +
+                             max_str_byte_length,
+                         &encoded_remaining_copy);
+  auto str_start = EncodeMessageStart(tag_value, max_str_byte_length,
+                                      &encoded_remaining_copy);
+  if (str_start.data()) {
+    log_internal::AppendTruncated(str, encoded_remaining_copy);
+    EncodeMessageLength(str_start, &encoded_remaining_copy);
+    EncodeMessageLength(value_start, &encoded_remaining_copy);
+    data_->encoded_remaining() = encoded_remaining_copy;
+  } else {
+    // The field header(s) did not fit; zero `encoded_remaining()` so we don't
+    // write anything else later.
+    data_->encoded_remaining().remove_suffix(data_->encoded_remaining().size());
+  }
+}
+template void LogMessage::CopyToEncodedBuffer<LogMessage::StringType::kLiteral>(
+    std::wstring_view str);
+template void LogMessage::CopyToEncodedBuffer<
+    LogMessage::StringType::kNotLiteral>(std::wstring_view str);
+
+template void LogMessage::CopyToEncodedBufferWithStructuredProtoField<
+    LogMessage::StringType::kLiteral>(StructuredProtoField field,
+                                      absl::string_view str);
+template void LogMessage::CopyToEncodedBufferWithStructuredProtoField<
+    LogMessage::StringType::kNotLiteral>(StructuredProtoField field,
+                                         absl::string_view str);
+
+template <LogMessage::StringType str_type>
+void LogMessage::CopyToEncodedBufferWithStructuredProtoField(
+    StructuredProtoField field, absl::string_view str) {
+  auto encoded_remaining_copy = data_->encoded_remaining();
+  size_t encoded_field_size = BufferSizeForStructuredProtoField(field);
+  constexpr uint8_t tag_value = str_type == StringType::kLiteral
+                                    ? ValueTag::kStringLiteral
+                                    : ValueTag::kString;
+  auto start = EncodeMessageStart(
+      EventTag::kValue,
+      encoded_field_size +
+          BufferSizeFor(tag_value, WireType::kLengthDelimited) + str.size(),
+      &encoded_remaining_copy);
+
+  // Write the encoded proto field.
+  if (!EncodeStructuredProtoField(field, encoded_remaining_copy)) {
+    // The header / field will not fit; zero `encoded_remaining()` so we
+    // don't write anything else later.
+    data_->encoded_remaining().remove_suffix(data_->encoded_remaining().size());
+    return;
+  }
+
+  // Write the string, truncating if necessary.
+  if (!EncodeStringTruncate(tag_value, str, &encoded_remaining_copy)) {
+    // The length of the string itself did not fit; zero `encoded_remaining()`
+    // so the value is not encoded at all.
+    data_->encoded_remaining().remove_suffix(data_->encoded_remaining().size());
+    return;
+  }
+
+  EncodeMessageLength(start, &encoded_remaining_copy);
+  data_->encoded_remaining() = encoded_remaining_copy;
+}
+
+// We intentionally don't return from these destructors. Disable MSVC's warning
+// about the destructor never returning as we do so intentionally here.
+#if defined(_MSC_VER) && !defined(__clang__)
+#pragma warning(push)
+#pragma warning(disable : 4722)
+#endif
+
+LogMessageFatal::LogMessageFatal(const char* absl_nonnull file, int line)
+    : LogMessage(file, line, absl::LogSeverity::kFatal) {}
+
+LogMessageFatal::LogMessageFatal(const char* absl_nonnull file, int line,
+                                 const char* absl_nonnull failure_msg)
+    : LogMessage(file, line, absl::LogSeverity::kFatal) {
+  *this << "Check failed: " << failure_msg << " ";
+}
+
+LogMessageFatal::~LogMessageFatal() { FailWithoutStackTrace(); }
+
+LogMessageDebugFatal::LogMessageDebugFatal(const char* absl_nonnull file,
+                                           int line)
+    : LogMessage(file, line, absl::LogSeverity::kFatal) {}
+
+LogMessageDebugFatal::~LogMessageDebugFatal() { FailWithoutStackTrace(); }
+
+LogMessageQuietlyDebugFatal::LogMessageQuietlyDebugFatal(
+    const char* absl_nonnull file, int line)
+    : LogMessage(file, line, absl::LogSeverity::kFatal) {
+  SetFailQuietly();
+}
+
+LogMessageQuietlyDebugFatal::~LogMessageQuietlyDebugFatal() { FailQuietly(); }
+
+LogMessageQuietlyFatal::LogMessageQuietlyFatal(const char* absl_nonnull file,
+                                               int line)
+    : LogMessage(file, line, absl::LogSeverity::kFatal) {
+  SetFailQuietly();
+}
+
+LogMessageQuietlyFatal::LogMessageQuietlyFatal(
+    const char* absl_nonnull file, int line,
+    const char* absl_nonnull failure_msg)
+    : LogMessageQuietlyFatal(file, line) {
+  *this << "Check failed: " << failure_msg << " ";
+}
+
+LogMessageQuietlyFatal::~LogMessageQuietlyFatal() { FailQuietly(); }
+#if defined(_MSC_VER) && !defined(__clang__)
+#pragma warning(pop)
+#endif
+
+}  // namespace log_internal
+
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/log/internal/log_message.h b/absl/log/internal/log_message.h
new file mode 100644
index 00000000000..b9b533ea5b1
--- /dev/null
+++ b/absl/log/internal/log_message.h
@@ -0,0 +1,469 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: log/internal/log_message.h
+// -----------------------------------------------------------------------------
+//
+// This file declares `class absl::log_internal::LogMessage`. This class more or
+// less represents a particular log message. LOG/CHECK macros create a temporary
+// instance of `LogMessage` and then stream values to it.  At the end of the
+// LOG/CHECK statement, the LogMessage is voidified by operator&&, and `Flush()`
+// directs the message to the registered log sinks.  Heap-allocation of
+// `LogMessage` is unsupported.  Construction outside of a `LOG` macro is
+// unsupported.
+
+#ifndef ABSL_LOG_INTERNAL_LOG_MESSAGE_H_
+#define ABSL_LOG_INTERNAL_LOG_MESSAGE_H_
+
+#include <wchar.h>
+
+#include <cstddef>
+#include <ios>
+#include <memory>
+#include <ostream>
+#include <streambuf>
+#include <string>
+#include <string_view>
+#include <type_traits>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/errno_saver.h"
+#include "absl/base/log_severity.h"
+#include "absl/base/nullability.h"
+#include "absl/log/internal/nullguard.h"
+#include "absl/log/internal/structured_proto.h"
+#include "absl/log/log_entry.h"
+#include "absl/log/log_sink.h"
+#include "absl/strings/has_absl_stringify.h"
+#include "absl/strings/string_view.h"
+#include "absl/time/time.h"
+#include "absl/types/source_location.h"
+#include "absl/types/span.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace log_internal {
+constexpr int kLogMessageBufferSize = 15000;
+
+enum class StructuredStringType;
+
+class LogMessage {
+ public:
+  struct InfoTag {};
+  struct WarningTag {};
+  struct ErrorTag {};
+
+  // Used for `LOG`.  Taking `const char *` instead of `string_view` keeps
+  // callsites a little bit smaller at the cost of doing `strlen` at runtime.
+  LogMessage(const char* absl_nonnull file, int line,
+             absl::LogSeverity severity) ABSL_ATTRIBUTE_COLD;
+  // Used for FFI integrations that don't have a NUL-terminated string.
+  LogMessage(absl::string_view file, int line,
+             absl::LogSeverity severity) ABSL_ATTRIBUTE_COLD;
+  // These constructors are slightly smaller/faster to call; the severity is
+  // curried into the function pointer.
+  LogMessage(const char* absl_nonnull file, int line,
+             InfoTag) ABSL_ATTRIBUTE_COLD ABSL_ATTRIBUTE_NOINLINE;
+  LogMessage(const char* absl_nonnull file, int line,
+             WarningTag) ABSL_ATTRIBUTE_COLD ABSL_ATTRIBUTE_NOINLINE;
+  LogMessage(const char* absl_nonnull file, int line,
+             ErrorTag) ABSL_ATTRIBUTE_COLD ABSL_ATTRIBUTE_NOINLINE;
+  LogMessage(const LogMessage&) = delete;
+  LogMessage& operator=(const LogMessage&) = delete;
+  ~LogMessage() ABSL_ATTRIBUTE_COLD;
+
+  // Overrides the location inferred from the callsite.  The string pointed to
+  // by `file` must be valid until the end of the statement.
+  LogMessage& AtLocation(absl::string_view file, int line);
+  // `loc` doesn't default to `absl::SourceLocation::current()` here since the
+  // callsite is already the default location for `LOG` statements.
+  LogMessage& AtLocation(absl::SourceLocation loc) {
+    return AtLocation(loc.file_name(), static_cast<int>(loc.line()));
+  }
+  // Omits the prefix from this line.  The prefix includes metadata about the
+  // logged data such as source code location and timestamp.
+  LogMessage& NoPrefix();
+  // Sets the verbosity field of the logged message as if it was logged by
+  // `VLOG(verbose_level)`.  Unlike `VLOG`, this method does not affect
+  // evaluation of the statement when the specified `verbose_level` has been
+  // disabled.  The only effect is on `absl::LogSink` implementations which
+  // make use of the `absl::LogSink::verbosity()` value.  The value
+  // `absl::LogEntry::kNoVerbosityLevel` can be specified to mark the message
+  // not verbose.
+  LogMessage& WithVerbosity(int verbose_level);
+  // Uses the specified timestamp instead of one collected in the constructor.
+  LogMessage& WithTimestamp(absl::Time timestamp);
+  // Uses the specified thread ID instead of one collected in the constructor.
+  LogMessage& WithThreadID(absl::LogEntry::tid_t tid);
+  // Copies all metadata (but no data) from the specified `absl::LogEntry`.
+  LogMessage& WithMetadataFrom(const absl::LogEntry& entry);
+  // Appends to the logged message a colon, a space, a textual description of
+  // the current value of `errno` (as by strerror(3)), and the numerical value
+  // of `errno`.
+  LogMessage& WithPerror();
+  // Sends this message to `*sink` in addition to whatever other sinks it would
+  // otherwise have been sent to.
+  LogMessage& ToSinkAlso(absl::LogSink* absl_nonnull sink);
+  // Sends this message to `*sink` and no others.
+  LogMessage& ToSinkOnly(absl::LogSink* absl_nonnull sink);
+
+  // Don't call this method from outside this library.
+  LogMessage& InternalStream() { return *this; }
+
+  // By-value overloads for small, common types let us overlook common failures
+  // to define globals and static data members (i.e. in a .cc file).
+  // NOLINTBEGIN(runtime/int)
+  // NOLINTBEGIN(google-runtime-int)
+  // clang-format off:  The CUDA toolchain cannot handle these <<<'s
+  LogMessage& operator<<(char v) { return operator<< <char>(v); }
+  LogMessage& operator<<(signed char v) { return operator<< <signed char>(v); }
+  LogMessage& operator<<(unsigned char v) {
+    return operator<< <unsigned char>(v);
+  }
+  LogMessage& operator<<(signed short v) {
+    return operator<< <signed short>(v);
+  }
+  LogMessage& operator<<(signed int v) { return operator<< <signed int>(v); }
+  LogMessage& operator<<(signed long v) {
+    return operator<< <signed long>(v);
+  }
+  LogMessage& operator<<(signed long long v) {
+    return operator<< <signed long long>(v);
+  }
+  LogMessage& operator<<(unsigned short v) {
+    return operator<< <unsigned short>(v);
+  }
+  LogMessage& operator<<(unsigned int v) {
+    return operator<< <unsigned int>(v);
+  }
+  LogMessage& operator<<(unsigned long v) {
+    return operator<< <unsigned long>(v);
+  }
+  LogMessage& operator<<(unsigned long long v) {
+    return operator<< <unsigned long long>(v);
+  }
+  LogMessage& operator<<(void* absl_nullable  v) {
+    return operator<< <void*>(v);
+  }
+  LogMessage& operator<<(const void* absl_nullable  v) {
+    return operator<< <const void*>(v);
+  }
+  LogMessage& operator<<(float v) { return operator<< <float>(v); }
+  LogMessage& operator<<(double v) { return operator<< <double>(v); }
+  LogMessage& operator<<(bool v) { return operator<< <bool>(v); }
+  // clang-format on
+  // NOLINTEND(google-runtime-int)
+  // NOLINTEND(runtime/int)
+
+  // These overloads are more efficient since no `ostream` is involved.
+  LogMessage& operator<<(const std::string& v);
+  LogMessage& operator<<(absl::string_view v);
+
+  // Wide string overloads (since std::ostream does not provide them).
+  LogMessage& operator<<(const std::wstring& v);
+  LogMessage& operator<<(std::wstring_view v);
+  // `const wchar_t*` is handled by `operator<< <const wchar_t*>`.
+  LogMessage& operator<<(wchar_t* absl_nullable v);
+  LogMessage& operator<<(wchar_t v);
+
+  // Overload for absl::SourceLocation or the std::source_location alias.
+  LogMessage& operator<<(const absl::SourceLocation& loc) {
+    OstreamView view(*data_);
+    view.stream() << loc.file_name() << ':' << loc.line();
+    return *this;
+  }
+
+  // Handle stream manipulators e.g. std::endl.
+  LogMessage& operator<<(std::ostream& (*absl_nonnull m)(std::ostream& os));
+  LogMessage& operator<<(std::ios_base& (*absl_nonnull m)(std::ios_base& os));
+
+  // Literal strings.  This allows us to record C string literals as literals in
+  // the logging.proto.Value.
+  //
+  // Allow this overload to be inlined to prevent generating instantiations of
+  // this template for every value of `SIZE` encountered in each source code
+  // file. That significantly increases linker input sizes. Inlining is cheap
+  // because the argument to this overload is almost always a string literal so
+  // the call to `strlen` can be replaced at compile time. The overloads for
+  // `char[]`/`wchar_t[]` below should not be inlined. The compiler typically
+  // does not have the string at compile time and cannot replace the call to
+  // `strlen` so inlining it increases the binary size. See the discussion on
+  // cl/107527369.
+  template <int SIZE>
+  LogMessage& operator<<(const char (&buf)[SIZE]);
+  template <int SIZE>
+  LogMessage& operator<<(const wchar_t (&buf)[SIZE]);
+
+  // This prevents non-const `char[]` arrays from looking like literals.
+  template <int SIZE>
+  LogMessage& operator<<(char (&buf)[SIZE]) ABSL_ATTRIBUTE_NOINLINE;
+  // `wchar_t[SIZE]` is handled by `operator<< <const wchar_t*>`.
+
+  // Types that support `AbslStringify()` are serialized that way.
+  // Types that don't support `AbslStringify()` but do support streaming into a
+  // `std::ostream&` are serialized that way.
+  template <typename T>
+  LogMessage& operator<<(const T& v) ABSL_ATTRIBUTE_NOINLINE;
+
+  // Dispatches the completed `absl::LogEntry` to applicable `absl::LogSink`s.
+  void Flush();
+
+  // Note: We explicitly do not support `operator<<` for non-const references
+  // because it breaks logging of non-integer bitfield types (i.e., enums).
+
+ protected:
+  // Call `abort()` or similar to perform `LOG(FATAL)` crash.  It is assumed
+  // that the caller has already generated and written the trace as appropriate.
+  [[noreturn]] static void FailWithoutStackTrace();
+
+  // Similar to `FailWithoutStackTrace()`, but without `abort()`.  Terminates
+  // the process with an error exit code.
+  [[noreturn]] static void FailQuietly();
+
+  // After this is called, failures are done as quiet as possible for this log
+  // message.
+  void SetFailQuietly();
+
+ private:
+  struct LogMessageData;  // Opaque type containing message state
+  friend class AsLiteralImpl;
+  friend class StringifySink;
+  template <StructuredStringType str_type>
+  friend class AsStructuredStringTypeImpl;
+  template <typename T>
+  friend class AsStructuredValueImpl;
+
+  // This streambuf writes directly into the structured logging buffer so that
+  // arbitrary types can be encoded as string data (using
+  // `operator<<(std::ostream &, ...)` without any extra allocation or copying.
+  // Space is reserved before the data to store the length field, which is
+  // filled in by `~OstreamView`.
+  class OstreamView final : public std::streambuf {
+   public:
+    explicit OstreamView(LogMessageData& message_data);
+    ~OstreamView() override;
+    OstreamView(const OstreamView&) = delete;
+    OstreamView& operator=(const OstreamView&) = delete;
+    std::ostream& stream();
+
+   private:
+    LogMessageData& data_;
+    absl::Span<char> encoded_remaining_copy_;
+    absl::Span<char> message_start_;
+    absl::Span<char> string_start_;
+  };
+
+  enum class StringType {
+    kLiteral,
+    kNotLiteral,
+  };
+  template <StringType str_type>
+  void CopyToEncodedBuffer(absl::string_view str) ABSL_ATTRIBUTE_NOINLINE;
+  template <StringType str_type>
+  void CopyToEncodedBuffer(char ch, size_t num) ABSL_ATTRIBUTE_NOINLINE;
+  template <StringType str_type>
+  void CopyToEncodedBuffer(std::wstring_view str) ABSL_ATTRIBUTE_NOINLINE;
+
+  // Copies `field` to the encoded buffer, then appends `str` after it
+  // (truncating `str` if necessary to fit).
+  template <StringType str_type>
+  void CopyToEncodedBufferWithStructuredProtoField(StructuredProtoField field,
+                                                   absl::string_view str)
+      ABSL_ATTRIBUTE_NOINLINE;
+
+  // Returns `true` if the message is fatal or enabled debug-fatal.
+  bool IsFatal() const;
+
+  // Records some tombstone-type data in anticipation of `Die`.
+  void PrepareToDie();
+  void Die();
+
+  void SendToLog();
+
+  // Checks `FLAGS_log_backtrace_at` and appends a backtrace if appropriate.
+  void LogBacktraceIfNeeded();
+
+  // This should be the first data member so that its initializer captures errno
+  // before any other initializers alter it (e.g. with calls to new) and so that
+  // no other destructors run afterward an alter it (e.g. with calls to delete).
+  absl::base_internal::ErrnoSaver errno_saver_;
+
+  // We keep the data in a separate struct so that each instance of `LogMessage`
+  // uses less stack space.
+  absl_nonnull std::unique_ptr<LogMessageData> data_;
+};
+
+// Explicitly specializes the generic operator<< for `const wchar_t*`
+// arguments.
+//
+// This method is used instead of a non-template `const wchar_t*` overload,
+// as the latter was found to take precedence over the array template
+// (`operator<<(const wchar_t(&)[SIZE])`) when handling string literals.
+// This specialization ensures the array template now correctly processes
+// literals.
+template <>
+LogMessage& LogMessage::operator<< <const wchar_t*>(
+    const wchar_t* absl_nullable const& v);
+
+inline LogMessage& LogMessage::operator<<(wchar_t* absl_nullable v) {
+  return operator<<(const_cast<const wchar_t*>(v));
+}
+
+// Helper class so that `AbslStringify()` can modify the LogMessage.
+class StringifySink final {
+ public:
+  explicit StringifySink(LogMessage& message) : message_(message) {}
+
+  void Append(size_t count, char ch) {
+    message_.CopyToEncodedBuffer<LogMessage::StringType::kNotLiteral>(ch,
+                                                                      count);
+  }
+
+  void Append(absl::string_view v) {
+    message_.CopyToEncodedBuffer<LogMessage::StringType::kNotLiteral>(v);
+  }
+
+  // For types that implement `AbslStringify` using `absl::Format()`.
+  friend void AbslFormatFlush(StringifySink* absl_nonnull sink,
+                              absl::string_view v) {
+    sink->Append(v);
+  }
+
+ private:
+  LogMessage& message_;
+};
+
+// Note: the following is declared `ABSL_ATTRIBUTE_NOINLINE`
+template <typename T>
+LogMessage& LogMessage::operator<<(const T& v) {
+  if constexpr (absl::HasAbslStringify<T>::value) {
+    StringifySink sink(*this);
+    // Replace with public API.
+    AbslStringify(sink, v);
+  } else {
+    OstreamView view(*data_);
+    view.stream() << log_internal::NullGuard<T>().Guard(v);
+  }
+  return *this;
+}
+
+template <int SIZE>
+LogMessage& LogMessage::operator<<(const char (&buf)[SIZE]) {
+  CopyToEncodedBuffer<StringType::kLiteral>(buf);
+  return *this;
+}
+
+template <int SIZE>
+LogMessage& LogMessage::operator<<(const wchar_t (&buf)[SIZE]) {
+  CopyToEncodedBuffer<StringType::kLiteral>(buf);
+  return *this;
+}
+
+// Note: the following is declared `ABSL_ATTRIBUTE_NOINLINE`
+template <int SIZE>
+LogMessage& LogMessage::operator<<(char (&buf)[SIZE]) {
+  CopyToEncodedBuffer<StringType::kNotLiteral>(buf);
+  return *this;
+}
+// We instantiate these specializations in the library's TU to save space in
+// other TUs.  Since the template is marked `ABSL_ATTRIBUTE_NOINLINE` we will be
+// emitting a function call either way.
+// NOLINTBEGIN(runtime/int)
+// NOLINTBEGIN(google-runtime-int)
+extern template LogMessage& LogMessage::operator<<(const char& v);
+extern template LogMessage& LogMessage::operator<<(const signed char& v);
+extern template LogMessage& LogMessage::operator<<(const unsigned char& v);
+extern template LogMessage& LogMessage::operator<<(const short& v);
+extern template LogMessage& LogMessage::operator<<(const unsigned short& v);
+extern template LogMessage& LogMessage::operator<<(const int& v);
+extern template LogMessage& LogMessage::operator<<(const unsigned int& v);
+extern template LogMessage& LogMessage::operator<<(const long& v);
+extern template LogMessage& LogMessage::operator<<(const unsigned long& v);
+extern template LogMessage& LogMessage::operator<<(const long long& v);
+extern template LogMessage& LogMessage::operator<<(const unsigned long long& v);
+extern template LogMessage& LogMessage::operator<<(
+    void* absl_nullable const& v);
+extern template LogMessage& LogMessage::operator<<(
+    const void* absl_nullable const& v);
+extern template LogMessage& LogMessage::operator<<(const float& v);
+extern template LogMessage& LogMessage::operator<<(const double& v);
+extern template LogMessage& LogMessage::operator<<(const bool& v);
+// NOLINTEND(google-runtime-int)
+// NOLINTEND(runtime/int)
+
+extern template void LogMessage::CopyToEncodedBuffer<
+    LogMessage::StringType::kLiteral>(absl::string_view str);
+extern template void LogMessage::CopyToEncodedBuffer<
+    LogMessage::StringType::kNotLiteral>(absl::string_view str);
+extern template void
+LogMessage::CopyToEncodedBuffer<LogMessage::StringType::kLiteral>(char ch,
+                                                                  size_t num);
+extern template void LogMessage::CopyToEncodedBuffer<
+    LogMessage::StringType::kNotLiteral>(char ch, size_t num);
+extern template void LogMessage::CopyToEncodedBuffer<
+    LogMessage::StringType::kLiteral>(std::wstring_view str);
+extern template void LogMessage::CopyToEncodedBuffer<
+    LogMessage::StringType::kNotLiteral>(std::wstring_view str);
+
+// `LogMessageFatal` ensures the process will exit in failure after logging this
+// message.
+class LogMessageFatal final : public LogMessage {
+ public:
+  LogMessageFatal(const char* absl_nonnull file, int line) ABSL_ATTRIBUTE_COLD;
+  LogMessageFatal(const char* absl_nonnull file, int line,
+                  const char* absl_nonnull failure_msg) ABSL_ATTRIBUTE_COLD;
+  [[noreturn]] ~LogMessageFatal();
+};
+
+// `LogMessageDebugFatal` ensures the process will exit in failure after logging
+// this message. It matches LogMessageFatal but is not [[noreturn]] as it's used
+// for DLOG(FATAL) variants.
+class LogMessageDebugFatal final : public LogMessage {
+ public:
+  LogMessageDebugFatal(const char* absl_nonnull file,
+                       int line) ABSL_ATTRIBUTE_COLD;
+  ~LogMessageDebugFatal();
+};
+
+class LogMessageQuietlyDebugFatal final : public LogMessage {
+ public:
+  // DLOG(QFATAL) calls this instead of LogMessageQuietlyFatal to make sure the
+  // destructor is not [[noreturn]] even if this is always FATAL as this is only
+  // invoked when DLOG() is enabled.
+  LogMessageQuietlyDebugFatal(const char* absl_nonnull file,
+                              int line) ABSL_ATTRIBUTE_COLD;
+  ~LogMessageQuietlyDebugFatal();
+};
+
+// Used for LOG(QFATAL) to make sure it's properly understood as [[noreturn]].
+class LogMessageQuietlyFatal final : public LogMessage {
+ public:
+  LogMessageQuietlyFatal(const char* absl_nonnull file,
+                         int line) ABSL_ATTRIBUTE_COLD;
+  LogMessageQuietlyFatal(const char* absl_nonnull file, int line,
+                         const char* absl_nonnull failure_msg)
+      ABSL_ATTRIBUTE_COLD;
+  [[noreturn]] ~LogMessageQuietlyFatal();
+};
+
+}  // namespace log_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+extern "C" ABSL_ATTRIBUTE_WEAK void ABSL_INTERNAL_C_SYMBOL(
+    AbslInternalOnFatalLogMessage)(const absl::LogEntry&);
+
+#endif  // ABSL_LOG_INTERNAL_LOG_MESSAGE_H_
diff --git a/absl/log/internal/log_sink_set.cc b/absl/log/internal/log_sink_set.cc
new file mode 100644
index 00000000000..8f0eb2279ad
--- /dev/null
+++ b/absl/log/internal/log_sink_set.cc
@@ -0,0 +1,296 @@
+//
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/log/internal/log_sink_set.h"
+
+#include <algorithm>
+#include <vector>
+
+#include "absl/base/attributes.h"
+#include "absl/base/call_once.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/base/log_severity.h"
+#include "absl/base/no_destructor.h"
+#include "absl/base/thread_annotations.h"
+#include "absl/cleanup/cleanup.h"
+#include "absl/log/globals.h"
+#include "absl/log/internal/config.h"
+#include "absl/log/internal/globals.h"
+#include "absl/log/log_entry.h"
+#include "absl/log/log_sink.h"
+#include "absl/strings/string_view.h"
+#include "absl/synchronization/mutex.h"
+#include "absl/types/span.h"
+
+#ifndef ABSL_HAVE_THREAD_LOCAL
+#include <pthread.h>
+#endif
+
+#ifdef __ANDROID__
+#include <android/log.h>
+#endif
+
+#ifdef _WIN32
+#include <windows.h>
+#endif
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace log_internal {
+namespace {
+
+// Returns a mutable reference to a thread-local variable that should be true if
+// a globally-registered `LogSink`'s `Send()` is currently being invoked on this
+// thread.
+bool& ThreadIsLoggingStatus() {
+#ifdef ABSL_HAVE_THREAD_LOCAL
+  ABSL_CONST_INIT thread_local bool thread_is_logging = false;
+  return thread_is_logging;
+#else
+  ABSL_CONST_INIT static pthread_key_t thread_is_logging_key;
+  static const bool unused = [] {
+    if (pthread_key_create(&thread_is_logging_key, [](void* data) {
+          delete reinterpret_cast<bool*>(data);
+        })) {
+      perror("pthread_key_create failed!");
+      abort();
+    }
+    return true;
+  }();
+  (void)unused;  // Fixes -wunused-variable warning
+  bool* thread_is_logging_ptr =
+      reinterpret_cast<bool*>(pthread_getspecific(thread_is_logging_key));
+
+  if (ABSL_PREDICT_FALSE(!thread_is_logging_ptr)) {
+    thread_is_logging_ptr = new bool{false};
+    if (pthread_setspecific(thread_is_logging_key, thread_is_logging_ptr)) {
+      perror("pthread_setspecific failed");
+      abort();
+    }
+  }
+  return *thread_is_logging_ptr;
+#endif
+}
+
+class StderrLogSink final : public LogSink {
+ public:
+  ~StderrLogSink() override = default;
+
+  void Send(const absl::LogEntry& entry) override {
+    if (entry.log_severity() < absl::StderrThreshold() &&
+        absl::log_internal::IsInitialized()) {
+      return;
+    }
+
+    ABSL_CONST_INIT static absl::once_flag warn_if_not_initialized;
+    absl::call_once(warn_if_not_initialized, []() {
+      if (absl::log_internal::IsInitialized()) return;
+      const char w[] =
+          "WARNING: All log messages before absl::InitializeLog() is called"
+          " are written to STDERR\n";
+      absl::log_internal::WriteToStderr(w, absl::LogSeverity::kWarning);
+    });
+
+    if (!entry.stacktrace().empty()) {
+      absl::log_internal::WriteToStderr(entry.stacktrace(),
+                                        entry.log_severity());
+    } else {
+      // TODO(b/226937039): do this outside else condition once we avoid
+      // ReprintFatalMessage
+      absl::log_internal::WriteToStderr(
+          entry.text_message_with_prefix_and_newline(), entry.log_severity());
+    }
+  }
+};
+
+#if defined(__ANDROID__)
+class AndroidLogSink final : public LogSink {
+ public:
+  ~AndroidLogSink() override = default;
+
+  void Send(const absl::LogEntry& entry) override {
+    const int level = AndroidLogLevel(entry);
+    const char* const tag = GetAndroidNativeTag();
+    __android_log_write(level, tag,
+                        entry.text_message_with_prefix_and_newline_c_str());
+    if (entry.log_severity() == absl::LogSeverity::kFatal)
+      __android_log_write(ANDROID_LOG_FATAL, tag, "terminating.\n");
+  }
+
+ private:
+  static int AndroidLogLevel(const absl::LogEntry& entry) {
+    switch (entry.log_severity()) {
+      case absl::LogSeverity::kFatal:
+        return ANDROID_LOG_FATAL;
+      case absl::LogSeverity::kError:
+        return ANDROID_LOG_ERROR;
+      case absl::LogSeverity::kWarning:
+        return ANDROID_LOG_WARN;
+      default:
+        if (entry.verbosity() >= 2) return ANDROID_LOG_VERBOSE;
+        if (entry.verbosity() == 1) return ANDROID_LOG_DEBUG;
+        return ANDROID_LOG_INFO;
+    }
+  }
+};
+#endif  // !defined(__ANDROID__)
+
+#if defined(_WIN32)
+class WindowsDebuggerLogSink final : public LogSink {
+ public:
+  ~WindowsDebuggerLogSink() override = default;
+
+  void Send(const absl::LogEntry& entry) override {
+    if (entry.log_severity() < absl::StderrThreshold() &&
+        absl::log_internal::IsInitialized()) {
+      return;
+    }
+    ::OutputDebugStringA(entry.text_message_with_prefix_and_newline_c_str());
+  }
+};
+#endif  // !defined(_WIN32)
+
+class GlobalLogSinkSet final {
+ public:
+  GlobalLogSinkSet() {
+#if defined(__myriad2__) || defined(__Fuchsia__)
+    // myriad2 and Fuchsia do not log to stderr by default.
+#else
+    static absl::NoDestructor<StderrLogSink> stderr_log_sink;
+    AddLogSink(stderr_log_sink.get());
+#endif
+#ifdef __ANDROID__
+    static absl::NoDestructor<AndroidLogSink> android_log_sink;
+    AddLogSink(android_log_sink.get());
+#endif
+#if defined(_WIN32)
+    static absl::NoDestructor<WindowsDebuggerLogSink> debugger_log_sink;
+    AddLogSink(debugger_log_sink.get());
+#endif  // !defined(_WIN32)
+  }
+
+  void LogToSinks(const absl::LogEntry& entry,
+                  absl::Span<absl::LogSink*> extra_sinks, bool extra_sinks_only)
+      ABSL_LOCKS_EXCLUDED(guard_) {
+    SendToSinks(entry, extra_sinks);
+
+    if (!extra_sinks_only) {
+      if (ThreadIsLoggingToLogSink()) {
+        absl::log_internal::WriteToStderr(
+            entry.text_message_with_prefix_and_newline(), entry.log_severity());
+      } else {
+        absl::ReaderMutexLock global_sinks_lock(guard_);
+        ThreadIsLoggingStatus() = true;
+        // Ensure the "thread is logging" status is reverted upon leaving the
+        // scope even in case of exceptions.
+        auto status_cleanup =
+            absl::MakeCleanup([] { ThreadIsLoggingStatus() = false; });
+        SendToSinks(entry, absl::MakeSpan(sinks_));
+      }
+    }
+  }
+
+  void AddLogSink(absl::LogSink* sink) ABSL_LOCKS_EXCLUDED(guard_) {
+    {
+      absl::WriterMutexLock global_sinks_lock(guard_);
+      auto pos = std::find(sinks_.begin(), sinks_.end(), sink);
+      if (pos == sinks_.end()) {
+        sinks_.push_back(sink);
+        return;
+      }
+    }
+    ABSL_INTERNAL_LOG(FATAL, "Duplicate log sinks are not supported");
+  }
+
+  void RemoveLogSink(absl::LogSink* sink) ABSL_LOCKS_EXCLUDED(guard_) {
+    {
+      absl::WriterMutexLock global_sinks_lock(guard_);
+      auto pos = std::find(sinks_.begin(), sinks_.end(), sink);
+      if (pos != sinks_.end()) {
+        sinks_.erase(pos);
+        return;
+      }
+    }
+    ABSL_INTERNAL_LOG(FATAL, "Mismatched log sink being removed");
+  }
+
+  void FlushLogSinks() ABSL_LOCKS_EXCLUDED(guard_) {
+    if (ThreadIsLoggingToLogSink()) {
+      // The thread_local condition demonstrates that we're already holding the
+      // lock in order to iterate over `sinks_` for dispatch.  The thread-safety
+      // annotations don't know this, so we use `ABSL_NO_THREAD_SAFETY_ANALYSIS`
+      guard_.AssertReaderHeld();
+      FlushLogSinksLocked();
+    } else {
+      absl::ReaderMutexLock global_sinks_lock(guard_);
+      // In case if LogSink::Flush overload decides to log
+      ThreadIsLoggingStatus() = true;
+      // Ensure the "thread is logging" status is reverted upon leaving the
+      // scope even in case of exceptions.
+      auto status_cleanup =
+          absl::MakeCleanup([] { ThreadIsLoggingStatus() = false; });
+      FlushLogSinksLocked();
+    }
+  }
+
+ private:
+  void FlushLogSinksLocked() ABSL_SHARED_LOCKS_REQUIRED(guard_) {
+    for (absl::LogSink* sink : sinks_) {
+      sink->Flush();
+    }
+  }
+
+  // Helper routine for LogToSinks.
+  static void SendToSinks(const absl::LogEntry& entry,
+                          absl::Span<absl::LogSink*> sinks) {
+    for (absl::LogSink* sink : sinks) {
+      sink->Send(entry);
+    }
+  }
+
+  using LogSinksSet = std::vector<absl::LogSink*>;
+  absl::Mutex guard_;
+  LogSinksSet sinks_ ABSL_GUARDED_BY(guard_);
+};
+
+// Returns reference to the global LogSinks set.
+GlobalLogSinkSet& GlobalSinks() {
+  static absl::NoDestructor<GlobalLogSinkSet> global_sinks;
+  return *global_sinks;
+}
+
+}  // namespace
+
+bool ThreadIsLoggingToLogSink() { return ThreadIsLoggingStatus(); }
+
+void LogToSinks(const absl::LogEntry& entry,
+                absl::Span<absl::LogSink*> extra_sinks, bool extra_sinks_only) {
+  log_internal::GlobalSinks().LogToSinks(entry, extra_sinks, extra_sinks_only);
+}
+
+void AddLogSink(absl::LogSink* sink) {
+  log_internal::GlobalSinks().AddLogSink(sink);
+}
+
+void RemoveLogSink(absl::LogSink* sink) {
+  log_internal::GlobalSinks().RemoveLogSink(sink);
+}
+
+void FlushLogSinks() { log_internal::GlobalSinks().FlushLogSinks(); }
+
+}  // namespace log_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/log/internal/log_sink_set.h b/absl/log/internal/log_sink_set.h
new file mode 100644
index 00000000000..88ab073bbd0
--- /dev/null
+++ b/absl/log/internal/log_sink_set.h
@@ -0,0 +1,54 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: log/internal/log_sink_set.h
+// -----------------------------------------------------------------------------
+
+#ifndef ABSL_LOG_INTERNAL_LOG_SINK_SET_H_
+#define ABSL_LOG_INTERNAL_LOG_SINK_SET_H_
+
+#include "absl/base/config.h"
+#include "absl/log/log_entry.h"
+#include "absl/log/log_sink.h"
+#include "absl/types/span.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace log_internal {
+
+// Returns true if a globally-registered `LogSink`'s `Send()` is currently
+// being invoked on this thread.
+bool ThreadIsLoggingToLogSink();
+
+// This function may log to two sets of sinks:
+//
+// * If `extra_sinks_only` is true, it will dispatch only to `extra_sinks`.
+//   `LogMessage::ToSinkAlso` and `LogMessage::ToSinkOnly` are used to attach
+//    extra sinks to the entry.
+// * Otherwise it will also log to the global sinks set. This set is managed
+//   by `absl::AddLogSink` and `absl::RemoveLogSink`.
+void LogToSinks(const absl::LogEntry& entry,
+                absl::Span<absl::LogSink*> extra_sinks, bool extra_sinks_only);
+
+// Implementation for operations with log sink set.
+void AddLogSink(absl::LogSink* sink);
+void RemoveLogSink(absl::LogSink* sink);
+void FlushLogSinks();
+
+}  // namespace log_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_LOG_INTERNAL_LOG_SINK_SET_H_
diff --git a/absl/log/internal/nullguard.cc b/absl/log/internal/nullguard.cc
new file mode 100644
index 00000000000..3296c0142ea
--- /dev/null
+++ b/absl/log/internal/nullguard.cc
@@ -0,0 +1,35 @@
+// Copyright 2023 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/log/internal/nullguard.h"
+
+#include <array>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace log_internal {
+
+ABSL_CONST_INIT ABSL_DLL const std::array<char, 7> kCharNull{
+    {'(', 'n', 'u', 'l', 'l', ')', '\0'}};
+ABSL_CONST_INIT ABSL_DLL const std::array<signed char, 7> kSignedCharNull{
+    {'(', 'n', 'u', 'l', 'l', ')', '\0'}};
+ABSL_CONST_INIT ABSL_DLL const std::array<unsigned char, 7> kUnsignedCharNull{
+    {'(', 'n', 'u', 'l', 'l', ')', '\0'}};
+
+}  // namespace log_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/log/internal/nullguard.h b/absl/log/internal/nullguard.h
new file mode 100644
index 00000000000..623943c512b
--- /dev/null
+++ b/absl/log/internal/nullguard.h
@@ -0,0 +1,88 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: log/internal/nullguard.h
+// -----------------------------------------------------------------------------
+//
+// NullGuard exists such that NullGuard<T>::Guard(v) returns v, unless passed a
+// nullptr_t, or a null char* or const char*, in which case it returns "(null)".
+// This allows streaming NullGuard<T>::Guard(v) to an output stream without
+// hitting undefined behavior for null values.
+
+#ifndef ABSL_LOG_INTERNAL_NULLGUARD_H_
+#define ABSL_LOG_INTERNAL_NULLGUARD_H_
+
+#include <array>
+#include <cstddef>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace log_internal {
+
+ABSL_CONST_INIT ABSL_DLL extern const std::array<char, 7> kCharNull;
+ABSL_CONST_INIT ABSL_DLL extern const std::array<signed char, 7>
+    kSignedCharNull;
+ABSL_CONST_INIT ABSL_DLL extern const std::array<unsigned char, 7>
+    kUnsignedCharNull;
+
+template <typename T>
+struct NullGuard final {
+  static const T& Guard(const T& v) { return v; }
+};
+template <>
+struct NullGuard<char*> final {
+  static const char* Guard(const char* v) { return v ? v : kCharNull.data(); }
+};
+template <>
+struct NullGuard<const char*> final {
+  static const char* Guard(const char* v) { return v ? v : kCharNull.data(); }
+};
+template <>
+struct NullGuard<signed char*> final {
+  static const signed char* Guard(const signed char* v) {
+    return v ? v : kSignedCharNull.data();
+  }
+};
+template <>
+struct NullGuard<const signed char*> final {
+  static const signed char* Guard(const signed char* v) {
+    return v ? v : kSignedCharNull.data();
+  }
+};
+template <>
+struct NullGuard<unsigned char*> final {
+  static const unsigned char* Guard(const unsigned char* v) {
+    return v ? v : kUnsignedCharNull.data();
+  }
+};
+template <>
+struct NullGuard<const unsigned char*> final {
+  static const unsigned char* Guard(const unsigned char* v) {
+    return v ? v : kUnsignedCharNull.data();
+  }
+};
+template <>
+struct NullGuard<std::nullptr_t> final {
+  static const char* Guard(const std::nullptr_t&) { return kCharNull.data(); }
+};
+
+}  // namespace log_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_LOG_INTERNAL_NULLGUARD_H_
diff --git a/absl/log/internal/nullstream.h b/absl/log/internal/nullstream.h
new file mode 100644
index 00000000000..f582a05185a
--- /dev/null
+++ b/absl/log/internal/nullstream.h
@@ -0,0 +1,129 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: log/internal/nullstream.h
+// -----------------------------------------------------------------------------
+//
+// Classes `NullStream`, `NullStreamMaybeFatal ` and `NullStreamFatal`
+// implement a subset of the `LogMessage` API and are used instead when logging
+// of messages has been disabled.
+
+#ifndef ABSL_LOG_INTERNAL_NULLSTREAM_H_
+#define ABSL_LOG_INTERNAL_NULLSTREAM_H_
+
+#include <ios>
+#include <ostream>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/log_severity.h"
+#include "absl/strings/string_view.h"
+
+#ifdef _WIN32
+#include <cstdlib>
+#else
+#include <unistd.h>
+#endif
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace log_internal {
+
+// A `NullStream` implements the API of `LogMessage` (a few methods and
+// `operator<<`) but does nothing.  All methods are defined inline so the
+// compiler can eliminate the whole instance and discard anything that's
+// streamed in.
+class NullStream {
+ public:
+  NullStream& AtLocation(absl::string_view, int) { return *this; }
+  template <typename SourceLocationType>
+  NullStream& AtLocation(SourceLocationType) {
+    return *this;
+  }
+  NullStream& NoPrefix() { return *this; }
+  NullStream& WithVerbosity(int) { return *this; }
+  template <typename TimeType>
+  NullStream& WithTimestamp(TimeType) {
+    return *this;
+  }
+  template <typename Tid>
+  NullStream& WithThreadID(Tid) {
+    return *this;
+  }
+  template <typename LogEntryType>
+  NullStream& WithMetadataFrom(const LogEntryType&) {
+    return *this;
+  }
+  NullStream& WithPerror() { return *this; }
+  template <typename LogSinkType>
+  NullStream& ToSinkAlso(LogSinkType*) {
+    return *this;
+  }
+  template <typename LogSinkType>
+  NullStream& ToSinkOnly(LogSinkType*) {
+    return *this;
+  }
+  template <typename LogSinkType>
+  NullStream& OutputToSink(LogSinkType*, bool) {
+    return *this;
+  }
+  NullStream& InternalStream() { return *this; }
+  void Flush() {}
+};
+template <typename T>
+inline NullStream& operator<<(NullStream& str, const T&) {
+  return str;
+}
+inline NullStream& operator<<(NullStream& str,
+                              std::ostream& (*)(std::ostream& os)) {
+  return str;
+}
+inline NullStream& operator<<(NullStream& str,
+                              std::ios_base& (*)(std::ios_base& os)) {
+  return str;
+}
+
+// `NullStreamMaybeFatal` implements the process termination semantics of
+// `LogMessage`, which is used for `DFATAL` severity and expression-defined
+// severity e.g. `LOG(LEVEL(HowBadIsIt()))`.  Like `LogMessage`, it terminates
+// the process when destroyed if the passed-in severity equals `FATAL`.
+class NullStreamMaybeFatal final : public NullStream {
+ public:
+  explicit NullStreamMaybeFatal(absl::LogSeverity severity)
+      : fatal_(severity == absl::LogSeverity::kFatal) {}
+  ~NullStreamMaybeFatal() {
+    if (fatal_) {
+      _exit(1);
+    }
+  }
+
+ private:
+  bool fatal_;
+};
+
+// `NullStreamFatal` implements the process termination semantics of
+// `LogMessageFatal`, which means it always terminates the process.  `DFATAL`
+// and expression-defined severity use `NullStreamMaybeFatal` above.
+class NullStreamFatal final : public NullStream {
+ public:
+  NullStreamFatal() = default;
+  [[noreturn]] ~NullStreamFatal() { _exit(1); }
+};
+
+}  // namespace log_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_LOG_INTERNAL_NULLSTREAM_H_
diff --git a/absl/log/internal/proto.cc b/absl/log/internal/proto.cc
new file mode 100644
index 00000000000..d0c57d69900
--- /dev/null
+++ b/absl/log/internal/proto.cc
@@ -0,0 +1,230 @@
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// All of the data that passes through this code is trusted because it flows
+// through a closed loop within the absl::LogMessage object. It is not a robust
+// protocol buffer encoder or decoder.
+//
+// Encoding: When `LOG(INFO) << "foo"` is called, the library uses the Encode*
+// functions to build a protocol buffer in a private, fixed-size internal buffer
+// (`LogMessageData::encoded_buf`).
+//
+// Decoding: During the same logging call, `LogMessage::Flush()` calls
+// `FinalizeEncodingAndFormat()`, which uses `ProtoField::DecodeFrom` to parse
+// that same internal buffer to generate the human-readable string for
+// text-based log sinks.
+
+#include "absl/log/internal/proto.h"
+
+#include <algorithm>
+#include <cassert>
+#include <cstddef>
+#include <cstdint>
+#include <cstring>
+
+#include "absl/base/config.h"
+#include "absl/types/span.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace log_internal {
+namespace {
+void EncodeRawVarint(uint64_t value, size_t size, absl::Span<char> *buf) {
+  for (size_t s = 0; s < size; s++) {
+    (*buf)[s] = static_cast<char>((value & 0x7f) | (s + 1 == size ? 0 : 0x80));
+    value >>= 7;
+  }
+  buf->remove_prefix(size);
+}
+}  // namespace
+
+bool EncodeVarint(uint64_t tag, uint64_t value, absl::Span<char> *buf) {
+  const uint64_t tag_type = MakeTagType(tag, WireType::kVarint);
+  const size_t tag_type_size = VarintSize(tag_type);
+  const size_t value_size = VarintSize(value);
+  if (tag_type_size + value_size > buf->size()) {
+    buf->remove_suffix(buf->size());
+    return false;
+  }
+  EncodeRawVarint(tag_type, tag_type_size, buf);
+  EncodeRawVarint(value, value_size, buf);
+  return true;
+}
+
+bool Encode64Bit(uint64_t tag, uint64_t value, absl::Span<char> *buf) {
+  const uint64_t tag_type = MakeTagType(tag, WireType::k64Bit);
+  const size_t tag_type_size = VarintSize(tag_type);
+  if (tag_type_size + sizeof(value) > buf->size()) {
+    buf->remove_suffix(buf->size());
+    return false;
+  }
+  EncodeRawVarint(tag_type, tag_type_size, buf);
+  for (size_t s = 0; s < sizeof(value); s++) {
+    (*buf)[s] = static_cast<char>(value & 0xff);
+    value >>= 8;
+  }
+  buf->remove_prefix(sizeof(value));
+  return true;
+}
+
+bool Encode32Bit(uint64_t tag, uint32_t value, absl::Span<char> *buf) {
+  const uint64_t tag_type = MakeTagType(tag, WireType::k32Bit);
+  const size_t tag_type_size = VarintSize(tag_type);
+  if (tag_type_size + sizeof(value) > buf->size()) {
+    buf->remove_suffix(buf->size());
+    return false;
+  }
+  EncodeRawVarint(tag_type, tag_type_size, buf);
+  for (size_t s = 0; s < sizeof(value); s++) {
+    (*buf)[s] = static_cast<char>(value & 0xff);
+    value >>= 8;
+  }
+  buf->remove_prefix(sizeof(value));
+  return true;
+}
+
+bool EncodeBytes(uint64_t tag, absl::Span<const char> value,
+                 absl::Span<char> *buf) {
+  const uint64_t tag_type = MakeTagType(tag, WireType::kLengthDelimited);
+  const size_t tag_type_size = VarintSize(tag_type);
+  uint64_t length = value.size();
+  const size_t length_size = VarintSize(length);
+  if (tag_type_size + length_size + value.size() > buf->size()) {
+    buf->remove_suffix(buf->size());
+    return false;
+  }
+  EncodeRawVarint(tag_type, tag_type_size, buf);
+  EncodeRawVarint(length, length_size, buf);
+  memcpy(buf->data(), value.data(), value.size());
+  buf->remove_prefix(value.size());
+  return true;
+}
+
+bool EncodeBytesTruncate(uint64_t tag, absl::Span<const char> value,
+                         absl::Span<char> *buf) {
+  const uint64_t tag_type = MakeTagType(tag, WireType::kLengthDelimited);
+  const size_t tag_type_size = VarintSize(tag_type);
+  uint64_t length = value.size();
+  const size_t length_size =
+      VarintSize(std::min<uint64_t>(length, buf->size()));
+  if (tag_type_size + length_size <= buf->size() &&
+      tag_type_size + length_size + value.size() > buf->size()) {
+    value.remove_suffix(tag_type_size + length_size + value.size() -
+                        buf->size());
+    length = value.size();
+  }
+  if (tag_type_size + length_size + value.size() > buf->size()) {
+    buf->remove_suffix(buf->size());
+    return false;
+  }
+  EncodeRawVarint(tag_type, tag_type_size, buf);
+  EncodeRawVarint(length, length_size, buf);
+  memcpy(buf->data(), value.data(), value.size());
+  buf->remove_prefix(value.size());
+  return true;
+}
+
+[[nodiscard]] absl::Span<char> EncodeMessageStart(uint64_t tag,
+                                                  uint64_t max_size,
+                                                  absl::Span<char> *buf) {
+  const uint64_t tag_type = MakeTagType(tag, WireType::kLengthDelimited);
+  const size_t tag_type_size = VarintSize(tag_type);
+  max_size = std::min<uint64_t>(max_size, buf->size());
+  const size_t length_size = VarintSize(max_size);
+  if (tag_type_size + length_size > buf->size()) {
+    buf->remove_suffix(buf->size());
+    return absl::Span<char>();
+  }
+  EncodeRawVarint(tag_type, tag_type_size, buf);
+  const absl::Span<char> ret = buf->subspan(0, length_size);
+  EncodeRawVarint(0, length_size, buf);
+  return ret;
+}
+
+void EncodeMessageLength(absl::Span<char> msg, const absl::Span<char> *buf) {
+  if (!msg.data()) return;
+  assert(buf->data() >= msg.data());
+  if (buf->data() < msg.data()) return;
+  EncodeRawVarint(
+      static_cast<uint64_t>(buf->data() - (msg.data() + msg.size())),
+      msg.size(), &msg);
+}
+
+namespace {
+uint64_t DecodeVarint(absl::Span<const char> *buf) {
+  uint64_t value = 0;
+  size_t s = 0;
+  while (s < buf->size()) {
+    value |= static_cast<uint64_t>(static_cast<unsigned char>((*buf)[s]) & 0x7f)
+             << 7 * s;
+    if (!((*buf)[s++] & 0x80)) break;
+  }
+  buf->remove_prefix(s);
+  return value;
+}
+
+uint64_t Decode64Bit(absl::Span<const char> *buf) {
+  uint64_t value = 0;
+  size_t s = 0;
+  while (s < buf->size()) {
+    value |= static_cast<uint64_t>(static_cast<unsigned char>((*buf)[s]))
+             << 8 * s;
+    if (++s == sizeof(value)) break;
+  }
+  buf->remove_prefix(s);
+  return value;
+}
+
+uint32_t Decode32Bit(absl::Span<const char> *buf) {
+  uint32_t value = 0;
+  size_t s = 0;
+  while (s < buf->size()) {
+    value |= static_cast<uint32_t>(static_cast<unsigned char>((*buf)[s]))
+             << 8 * s;
+    if (++s == sizeof(value)) break;
+  }
+  buf->remove_prefix(s);
+  return value;
+}
+}  // namespace
+
+bool ProtoField::DecodeFrom(absl::Span<const char> *data) {
+  if (data->empty()) return false;
+  const uint64_t tag_type = DecodeVarint(data);
+  tag_ = tag_type >> 3;
+  type_ = static_cast<WireType>(tag_type & 0x07);
+  switch (type_) {
+    case WireType::kVarint:
+      value_ = DecodeVarint(data);
+      break;
+    case WireType::k64Bit:
+      value_ = Decode64Bit(data);
+      break;
+    case WireType::kLengthDelimited: {
+      value_ = DecodeVarint(data);
+      data_ = data->subspan(
+          0, static_cast<size_t>(std::min<uint64_t>(value_, data->size())));
+      data->remove_prefix(data_.size());
+      break;
+    }
+    case WireType::k32Bit:
+      value_ = Decode32Bit(data);
+      break;
+  }
+  return true;
+}
+
+}  // namespace log_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/log/internal/proto.h b/absl/log/internal/proto.h
new file mode 100644
index 00000000000..23f79543231
--- /dev/null
+++ b/absl/log/internal/proto.h
@@ -0,0 +1,298 @@
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// -----------------------------------------------------------------------------
+// File: internal/proto.h
+// -----------------------------------------------------------------------------
+//
+// Declares functions for serializing and deserializing data to and from memory
+// buffers in protocol buffer wire format.  This library takes no steps to
+// ensure that the encoded data matches with any message specification.
+
+#ifndef ABSL_LOG_INTERNAL_PROTO_H_
+#define ABSL_LOG_INTERNAL_PROTO_H_
+
+#include <cstddef>
+#include <cstdint>
+#include <limits>
+
+#include "absl/base/attributes.h"
+#include "absl/base/casts.h"
+#include "absl/base/config.h"
+#include "absl/strings/string_view.h"
+#include "absl/types/span.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace log_internal {
+
+// absl::Span<char> represents a view into the available space in a mutable
+// buffer during encoding.  Encoding functions shrink the span as they go so
+// that the same view can be passed to a series of Encode functions.  If the
+// data do not fit, nothing is encoded, the view is set to size zero (so that
+// all subsequent encode calls fail), and false is returned.  Otherwise true is
+// returned.
+
+// In particular, attempting to encode a series of data into an insufficient
+// buffer has consistent and efficient behavior without any caller-side error
+// checking.  Individual values will be encoded in their entirety or not at all
+// (unless one of the `Truncate` functions is used).  Once a value is omitted
+// because it does not fit, no subsequent values will be encoded to preserve
+// ordering; the decoded sequence will be a prefix of the original sequence.
+
+// There are two ways to encode a message-typed field:
+//
+// * Construct its contents in a separate buffer and use `EncodeBytes` to copy
+//   it into the primary buffer with type, tag, and length.
+// * Use `EncodeMessageStart` to write type and tag fields and reserve space for
+//   the length field, then encode the contents directly into the buffer, then
+//   use `EncodeMessageLength` to write the actual length into the reserved
+//   bytes.  This works fine if the actual length takes fewer bytes to encode
+//   than were reserved, although you don't get your extra bytes back.
+//   This approach will always produce a valid encoding, but your protocol may
+//   require that the whole message field by omitted if the buffer is too small
+//   to contain all desired subfields.  In this case, operate on a copy of the
+//   buffer view and assign back only if everything fit, i.e. if the last
+//   `Encode` call returned true.
+
+// Encodes the specified integer as a varint field and returns true if it fits.
+// Used for int32_t, int64_t, uint32_t, uint64_t, bool, and enum field types.
+// Consumes up to kMaxVarintSize * 2 bytes (20).
+bool EncodeVarint(uint64_t tag, uint64_t value, absl::Span<char> *buf);
+inline bool EncodeVarint(uint64_t tag, int64_t value, absl::Span<char> *buf) {
+  return EncodeVarint(tag, static_cast<uint64_t>(value), buf);
+}
+inline bool EncodeVarint(uint64_t tag, uint32_t value, absl::Span<char> *buf) {
+  return EncodeVarint(tag, static_cast<uint64_t>(value), buf);
+}
+inline bool EncodeVarint(uint64_t tag, int32_t value, absl::Span<char> *buf) {
+  return EncodeVarint(tag, static_cast<uint64_t>(value), buf);
+}
+
+// Encodes the specified integer as a varint field using ZigZag encoding and
+// returns true if it fits.
+// Used for sint32 and sint64 field types.
+// Consumes up to kMaxVarintSize * 2 bytes (20).
+inline bool EncodeVarintZigZag(uint64_t tag, int64_t value,
+                               absl::Span<char> *buf) {
+  if (value < 0)
+    return EncodeVarint(tag, 2 * static_cast<uint64_t>(-(value + 1)) + 1, buf);
+  return EncodeVarint(tag, 2 * static_cast<uint64_t>(value), buf);
+}
+
+// Encodes the specified integer as a 64-bit field and returns true if it fits.
+// Used for fixed64 and sfixed64 field types.
+// Consumes up to kMaxVarintSize + 8 bytes (18).
+bool Encode64Bit(uint64_t tag, uint64_t value, absl::Span<char> *buf);
+inline bool Encode64Bit(uint64_t tag, int64_t value, absl::Span<char> *buf) {
+  return Encode64Bit(tag, static_cast<uint64_t>(value), buf);
+}
+inline bool Encode64Bit(uint64_t tag, uint32_t value, absl::Span<char> *buf) {
+  return Encode64Bit(tag, static_cast<uint64_t>(value), buf);
+}
+inline bool Encode64Bit(uint64_t tag, int32_t value, absl::Span<char> *buf) {
+  return Encode64Bit(tag, static_cast<uint64_t>(value), buf);
+}
+
+// Encodes the specified double as a 64-bit field and returns true if it fits.
+// Used for double field type.
+// Consumes up to kMaxVarintSize + 8 bytes (18).
+inline bool EncodeDouble(uint64_t tag, double value, absl::Span<char> *buf) {
+  return Encode64Bit(tag, absl::bit_cast<uint64_t>(value), buf);
+}
+
+// Encodes the specified integer as a 32-bit field and returns true if it fits.
+// Used for fixed32 and sfixed32 field types.
+// Consumes up to kMaxVarintSize + 4 bytes (14).
+bool Encode32Bit(uint64_t tag, uint32_t value, absl::Span<char> *buf);
+inline bool Encode32Bit(uint64_t tag, int32_t value, absl::Span<char> *buf) {
+  return Encode32Bit(tag, static_cast<uint32_t>(value), buf);
+}
+
+// Encodes the specified float as a 32-bit field and returns true if it fits.
+// Used for float field type.
+// Consumes up to kMaxVarintSize + 4 bytes (14).
+inline bool EncodeFloat(uint64_t tag, float value, absl::Span<char> *buf) {
+  return Encode32Bit(tag, absl::bit_cast<uint32_t>(value), buf);
+}
+
+// Encodes the specified bytes as a length-delimited field and returns true if
+// they fit.
+// Used for string, bytes, message, and packed-repeated field type.
+// Consumes up to kMaxVarintSize * 2 + value.size() bytes (20 + value.size()).
+bool EncodeBytes(uint64_t tag, absl::Span<const char> value,
+                 absl::Span<char> *buf);
+
+// Encodes as many of the specified bytes as will fit as a length-delimited
+// field and returns true as long as the field header (`tag_type` and `length`)
+// fits.
+// Used for string, bytes, message, and packed-repeated field type.
+// Consumes up to kMaxVarintSize * 2 + value.size() bytes (20 + value.size()).
+bool EncodeBytesTruncate(uint64_t tag, absl::Span<const char> value,
+                         absl::Span<char> *buf);
+
+// Encodes the specified string as a length-delimited field and returns true if
+// it fits.
+// Used for string, bytes, message, and packed-repeated field type.
+// Consumes up to kMaxVarintSize * 2 + value.size() bytes (20 + value.size()).
+inline bool EncodeString(uint64_t tag, absl::string_view value,
+                         absl::Span<char> *buf) {
+  return EncodeBytes(tag, value, buf);
+}
+
+// Encodes as much of the specified string as will fit as a length-delimited
+// field and returns true as long as the field header (`tag_type` and `length`)
+// fits.
+// Used for string, bytes, message, and packed-repeated field type.
+// Consumes up to kMaxVarintSize * 2 + value.size() bytes (20 + value.size()).
+inline bool EncodeStringTruncate(uint64_t tag, absl::string_view value,
+                                 absl::Span<char> *buf) {
+  return EncodeBytesTruncate(tag, value, buf);
+}
+
+// Encodes the header for a length-delimited field containing up to `max_size`
+// bytes or the number remaining in the buffer, whichever is less.  If the
+// header fits, a non-nullptr `Span` is returned; this must be passed to
+// `EncodeMessageLength` after all contents are encoded to finalize the length
+// field.  If the header does not fit, a nullptr `Span` is returned which is
+// safe to pass to `EncodeMessageLength` but need not be.
+// Used for string, bytes, message, and packed-repeated field type.
+// Consumes up to kMaxVarintSize * 2 bytes (20).
+[[nodiscard]] absl::Span<char> EncodeMessageStart(uint64_t tag,
+                                                  uint64_t max_size,
+                                                  absl::Span<char> *buf);
+
+// Finalizes the length field in `msg` so that it encompasses all data encoded
+// since the call to `EncodeMessageStart` which returned `msg`.  Does nothing if
+// `msg` is a `nullptr` `Span`.
+void EncodeMessageLength(absl::Span<char> msg, const absl::Span<char> *buf);
+
+enum class WireType : uint64_t {
+  kVarint = 0,
+  k64Bit = 1,
+  kLengthDelimited = 2,
+  k32Bit = 5,
+};
+
+constexpr size_t VarintSize(uint64_t value) {
+  return value < 128 ? 1 : 1 + VarintSize(value >> 7);
+}
+constexpr size_t MinVarintSize() {
+  return VarintSize((std::numeric_limits<uint64_t>::min)());
+}
+constexpr size_t MaxVarintSize() {
+  return VarintSize((std::numeric_limits<uint64_t>::max)());
+}
+
+constexpr uint64_t MaxVarintForSize(size_t size) {
+  return size >= 10 ? (std::numeric_limits<uint64_t>::max)()
+                    : (static_cast<uint64_t>(1) << size * 7) - 1;
+}
+constexpr uint64_t MakeTagType(uint64_t tag, WireType type) {
+  return tag << 3 | static_cast<uint64_t>(type);
+}
+
+// `BufferSizeFor` returns a number of bytes guaranteed to be sufficient to
+// store encoded fields as `(tag, WireType)`, regardless of data values.  This
+// only makes sense for `WireType::kLengthDelimited` if you add in the length of
+// the contents yourself, e.g. for string and bytes fields by adding the lengths
+// of any encoded strings to the return value or for submessage fields by
+// enumerating the fields you may encode into their contents.
+constexpr size_t BufferSizeFor(uint64_t tag, WireType type) {
+  size_t buffer_size = VarintSize(MakeTagType(tag, type));
+  switch (type) {
+    case WireType::kVarint:
+      buffer_size += MaxVarintSize();
+      break;
+    case WireType::k64Bit:
+      buffer_size += size_t{8};
+      break;
+    case WireType::kLengthDelimited:
+      buffer_size += MaxVarintSize();
+      break;
+    case WireType::k32Bit:
+      buffer_size += size_t{4};
+      break;
+  }
+  return buffer_size;
+}
+
+// absl::Span<const char> represents a view into the un-processed space in a
+// buffer during decoding.  Decoding functions shrink the span as they go so
+// that the same view can be decoded iteratively until all data are processed.
+// In general, if the buffer is exhausted but additional bytes are expected by
+// the decoder, it will return values as if the additional bytes were zeros.
+// Length-delimited fields are an exception - if the encoded length field
+// indicates more data bytes than are available in the buffer, the `bytes_value`
+// and `string_value` accessors will return truncated views.
+
+class ProtoField final {
+ public:
+  // Consumes bytes from `data` and returns true if there were any bytes to
+  // decode.
+  bool DecodeFrom(absl::Span<const char> *data);
+  uint64_t tag() const { return tag_; }
+  WireType type() const { return type_; }
+
+  // These value accessors will return nonsense if the data were not encoded in
+  // the corresponding wiretype from the corresponding C++ (or other language)
+  // type.
+
+  double double_value() const { return absl::bit_cast<double>(value_); }
+  float float_value() const {
+    return absl::bit_cast<float>(static_cast<uint32_t>(value_));
+  }
+  int32_t int32_value() const { return static_cast<int32_t>(value_); }
+  int64_t int64_value() const { return static_cast<int64_t>(value_); }
+  int32_t sint32_value() const {
+    if (value_ % 2) return static_cast<int32_t>(0 - ((value_ - 1) / 2) - 1);
+    return static_cast<int32_t>(value_ / 2);
+  }
+  int64_t sint64_value() const {
+    if (value_ % 2) return 0 - ((value_ - 1) / 2) - 1;
+    return value_ / 2;
+  }
+  uint32_t uint32_value() const { return static_cast<uint32_t>(value_); }
+  uint64_t uint64_value() const { return value_; }
+  bool bool_value() const { return value_ != 0; }
+  // To decode an enum, call int32_value() and cast to the appropriate type.
+  // Note that the official C++ proto compiler treats enum fields with values
+  // that do not correspond to a defined enumerator as unknown fields.
+
+  // To decode fields within a submessage field, call
+  // `DecodeNextField(field.BytesValue())`.
+  absl::Span<const char> bytes_value() const { return data_; }
+  absl::string_view string_value() const {
+    const auto data = bytes_value();
+    return absl::string_view(data.data(), data.size());
+  }
+  // Returns the encoded length of a length-delimited field.  This equals
+  // `bytes_value().size()` except when the latter has been truncated due to
+  // buffer underrun.
+  uint64_t encoded_length() const { return value_; }
+
+ private:
+  uint64_t tag_;
+  WireType type_;
+  // For `kTypeVarint`, `kType64Bit`, and `kType32Bit`, holds the decoded value.
+  // For `kTypeLengthDelimited`, holds the decoded length.
+  uint64_t value_;
+  absl::Span<const char> data_;
+};
+
+}  // namespace log_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_LOG_INTERNAL_PROTO_H_
diff --git a/absl/log/internal/stderr_log_sink_test.cc b/absl/log/internal/stderr_log_sink_test.cc
new file mode 100644
index 00000000000..763690d1b32
--- /dev/null
+++ b/absl/log/internal/stderr_log_sink_test.cc
@@ -0,0 +1,105 @@
+//
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <stdlib.h>
+
+#include <string>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/attributes.h"
+#include "absl/base/log_severity.h"
+#include "absl/log/globals.h"
+#include "absl/log/internal/test_helpers.h"
+#include "absl/log/log.h"
+
+namespace {
+using ::testing::AllOf;
+using ::testing::HasSubstr;
+
+auto* test_env ABSL_ATTRIBUTE_UNUSED = ::testing::AddGlobalTestEnvironment(
+    new absl::log_internal::LogTestEnvironment);
+
+MATCHER_P2(HasSubstrTimes, substr, expected_count, "") {
+  int count = 0;
+  std::string::size_type pos = 0;
+  std::string needle(substr);
+  while ((pos = arg.find(needle, pos)) != std::string::npos) {
+    ++count;
+    pos += needle.size();
+  }
+
+  return count == expected_count;
+}
+
+TEST(StderrLogSinkDeathTest, InfoMessagesInStderr) {
+  EXPECT_DEATH_IF_SUPPORTED(
+      {
+        absl::SetStderrThreshold(absl::LogSeverityAtLeast::kInfo);
+        LOG(INFO) << "INFO message";
+        exit(1);
+      },
+      "INFO message");
+}
+
+TEST(StderrLogSinkDeathTest, WarningMessagesInStderr) {
+  EXPECT_DEATH_IF_SUPPORTED(
+      {
+        absl::SetStderrThreshold(absl::LogSeverityAtLeast::kInfo);
+        LOG(WARNING) << "WARNING message";
+        exit(1);
+      },
+      "WARNING message");
+}
+
+TEST(StderrLogSinkDeathTest, ErrorMessagesInStderr) {
+  EXPECT_DEATH_IF_SUPPORTED(
+      {
+        absl::SetStderrThreshold(absl::LogSeverityAtLeast::kInfo);
+        LOG(ERROR) << "ERROR message";
+        exit(1);
+      },
+      "ERROR message");
+}
+
+TEST(StderrLogSinkDeathTest, FatalMessagesInStderr) {
+  char message[] = "FATAL message";
+  char stacktrace[] = "*** Check failure stack trace: ***";
+
+  int expected_count = 1;
+
+  EXPECT_DEATH_IF_SUPPORTED(
+      {
+        absl::SetStderrThreshold(absl::LogSeverityAtLeast::kInfo);
+        LOG(FATAL) << message;
+      },
+      AllOf(HasSubstrTimes(message, expected_count), HasSubstr(stacktrace)));
+}
+
+TEST(StderrLogSinkDeathTest, SecondaryFatalMessagesInStderr) {
+  auto MessageGen = []() -> std::string {
+    LOG(FATAL) << "Internal failure";
+    return "External failure";
+  };
+
+  EXPECT_DEATH_IF_SUPPORTED(
+      {
+        absl::SetStderrThreshold(absl::LogSeverityAtLeast::kInfo);
+        LOG(FATAL) << MessageGen();
+      },
+      "Internal failure");
+}
+
+}  // namespace
diff --git a/absl/log/internal/strip.h b/absl/log/internal/strip.h
new file mode 100644
index 00000000000..6fd857bdc57
--- /dev/null
+++ b/absl/log/internal/strip.h
@@ -0,0 +1,99 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: log/internal/strip.h
+// -----------------------------------------------------------------------------
+
+// SKIP_ABSL_INLINE_NAMESPACE_CHECK
+
+#ifndef ABSL_LOG_INTERNAL_STRIP_H_
+#define ABSL_LOG_INTERNAL_STRIP_H_
+
+#include "absl/base/attributes.h"  // IWYU pragma: keep
+#include "absl/base/log_severity.h"
+#include "absl/log/internal/log_message.h"
+#include "absl/log/internal/nullstream.h"
+
+// `ABSL_LOG_INTERNAL_LOG_*` evaluates to a temporary `LogMessage` object or
+// to a related object with a compatible API but different behavior.  This set
+// of defines comes in three flavors: vanilla, plus two variants that strip some
+// logging in subtly different ways for subtly different reasons (see below).
+#if defined(STRIP_LOG) && STRIP_LOG
+
+#define ABSL_LOG_INTERNAL_LOG_INFO ::absl::log_internal::NullStream()
+#define ABSL_LOG_INTERNAL_LOG_WARNING ::absl::log_internal::NullStream()
+#define ABSL_LOG_INTERNAL_LOG_ERROR ::absl::log_internal::NullStream()
+#define ABSL_LOG_INTERNAL_LOG_FATAL ::absl::log_internal::NullStreamFatal()
+#define ABSL_LOG_INTERNAL_LOG_QFATAL ::absl::log_internal::NullStreamFatal()
+#define ABSL_LOG_INTERNAL_LOG_DFATAL \
+  ::absl::log_internal::NullStreamMaybeFatal(::absl::kLogDebugFatal)
+#define ABSL_LOG_INTERNAL_LOG_LEVEL(severity) \
+  ::absl::log_internal::NullStreamMaybeFatal(absl_log_internal_severity)
+
+// Fatal `DLOG`s expand a little differently to avoid being `[[noreturn]]`.
+#define ABSL_LOG_INTERNAL_DLOG_FATAL \
+  ::absl::log_internal::NullStreamMaybeFatal(::absl::LogSeverity::kFatal)
+#define ABSL_LOG_INTERNAL_DLOG_QFATAL \
+  ::absl::log_internal::NullStreamMaybeFatal(::absl::LogSeverity::kFatal)
+
+#define ABSL_LOG_INTERNAL_CHECK(failure_message) ABSL_LOG_INTERNAL_LOG_FATAL
+#define ABSL_LOG_INTERNAL_QCHECK(failure_message) ABSL_LOG_INTERNAL_LOG_QFATAL
+
+#else  // !defined(STRIP_LOG) || !STRIP_LOG
+
+#define ABSL_LOG_INTERNAL_LOG_INFO  \
+  ::absl::log_internal::LogMessage( \
+      __FILE__, __LINE__, ::absl::log_internal::LogMessage::InfoTag{})
+#define ABSL_LOG_INTERNAL_LOG_WARNING \
+  ::absl::log_internal::LogMessage(   \
+      __FILE__, __LINE__, ::absl::log_internal::LogMessage::WarningTag{})
+#define ABSL_LOG_INTERNAL_LOG_ERROR \
+  ::absl::log_internal::LogMessage( \
+      __FILE__, __LINE__, ::absl::log_internal::LogMessage::ErrorTag{})
+#define ABSL_LOG_INTERNAL_LOG_FATAL \
+  ::absl::log_internal::LogMessageFatal(__FILE__, __LINE__)
+#define ABSL_LOG_INTERNAL_LOG_QFATAL \
+  ::absl::log_internal::LogMessageQuietlyFatal(__FILE__, __LINE__)
+#define ABSL_LOG_INTERNAL_LOG_DFATAL \
+  ::absl::log_internal::LogMessage(__FILE__, __LINE__, ::absl::kLogDebugFatal)
+#define ABSL_LOG_INTERNAL_LOG_LEVEL(severity)          \
+  ::absl::log_internal::LogMessage(__FILE__, __LINE__, \
+                                   absl_log_internal_severity)
+
+// Fatal `DLOG`s expand a little differently to avoid being `[[noreturn]]`.
+#define ABSL_LOG_INTERNAL_DLOG_FATAL \
+  ::absl::log_internal::LogMessageDebugFatal(__FILE__, __LINE__)
+#define ABSL_LOG_INTERNAL_DLOG_QFATAL \
+  ::absl::log_internal::LogMessageQuietlyDebugFatal(__FILE__, __LINE__)
+
+// These special cases dispatch to special-case constructors that allow us to
+// avoid an extra function call and shrink non-LTO binaries by a percent or so.
+#define ABSL_LOG_INTERNAL_CHECK(failure_message) \
+  ::absl::log_internal::LogMessageFatal(__FILE__, __LINE__, failure_message)
+#define ABSL_LOG_INTERNAL_QCHECK(failure_message)                  \
+  ::absl::log_internal::LogMessageQuietlyFatal(__FILE__, __LINE__, \
+                                               failure_message)
+#endif  // !defined(STRIP_LOG) || !STRIP_LOG
+
+// This part of a non-fatal `DLOG`s expands the same as `LOG`.
+#define ABSL_LOG_INTERNAL_DLOG_INFO ABSL_LOG_INTERNAL_LOG_INFO
+#define ABSL_LOG_INTERNAL_DLOG_WARNING ABSL_LOG_INTERNAL_LOG_WARNING
+#define ABSL_LOG_INTERNAL_DLOG_ERROR ABSL_LOG_INTERNAL_LOG_ERROR
+#define ABSL_LOG_INTERNAL_DLOG_DFATAL ABSL_LOG_INTERNAL_LOG_DFATAL
+#define ABSL_LOG_INTERNAL_DLOG_LEVEL ABSL_LOG_INTERNAL_LOG_LEVEL
+
+#define ABSL_LOG_INTERNAL_LOG_DO_NOT_SUBMIT ABSL_LOG_INTERNAL_LOG_ERROR
+
+#endif  // ABSL_LOG_INTERNAL_STRIP_H_
diff --git a/absl/log/internal/structured.h b/absl/log/internal/structured.h
new file mode 100644
index 00000000000..70b50e2e83f
--- /dev/null
+++ b/absl/log/internal/structured.h
@@ -0,0 +1,160 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: log/internal/structured.h
+// -----------------------------------------------------------------------------
+
+#ifndef ABSL_LOG_INTERNAL_STRUCTURED_H_
+#define ABSL_LOG_INTERNAL_STRUCTURED_H_
+
+#include <ostream>
+#include <string>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/functional/any_invocable.h"
+#include "absl/log/internal/log_message.h"
+#include "absl/log/internal/structured_proto.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace log_internal {
+
+class [[nodiscard]] AsLiteralImpl final {
+ public:
+  explicit AsLiteralImpl(absl::string_view str ABSL_ATTRIBUTE_LIFETIME_BOUND)
+      : str_(str) {}
+  AsLiteralImpl(const AsLiteralImpl&) = default;
+  AsLiteralImpl& operator=(const AsLiteralImpl&) = default;
+
+ private:
+  absl::string_view str_;
+
+  friend std::ostream& operator<<(std::ostream& os,
+                                  AsLiteralImpl&& as_literal) {
+    return os << as_literal.str_;
+  }
+  void AddToMessage(log_internal::LogMessage& m) {
+    m.CopyToEncodedBuffer<log_internal::LogMessage::StringType::kLiteral>(str_);
+  }
+  friend log_internal::LogMessage& operator<<(log_internal::LogMessage& m,
+                                              AsLiteralImpl as_literal) {
+    as_literal.AddToMessage(m);
+    return m;
+  }
+};
+
+enum class StructuredStringType {
+  kLiteral,
+  kNotLiteral,
+};
+
+// Structured log data for a string and associated structured proto field,
+// both of which must outlive this object.
+template <StructuredStringType str_type>
+class [[nodiscard]] AsStructuredStringTypeImpl final {
+ public:
+  constexpr AsStructuredStringTypeImpl(
+      absl::string_view str ABSL_ATTRIBUTE_LIFETIME_BOUND,
+      StructuredProtoField field ABSL_ATTRIBUTE_LIFETIME_BOUND)
+      : str_(str), field_(field) {}
+
+ private:
+  absl::string_view str_;
+  StructuredProtoField field_;
+
+  friend std::ostream& operator<<(std::ostream& os,
+                                  const AsStructuredStringTypeImpl& impl) {
+    return os << impl.str_;
+  }
+  void AddToMessage(LogMessage& m) const {
+    if (str_type == StructuredStringType::kLiteral) {
+      return m.CopyToEncodedBufferWithStructuredProtoField<
+          log_internal::LogMessage::StringType::kLiteral>(field_, str_);
+    } else {
+      return m.CopyToEncodedBufferWithStructuredProtoField<
+          log_internal::LogMessage::StringType::kNotLiteral>(field_, str_);
+    }
+  }
+  friend LogMessage& operator<<(LogMessage& m,
+                                const AsStructuredStringTypeImpl& impl) {
+    impl.AddToMessage(m);
+    return m;
+  }
+};
+
+using AsStructuredLiteralImpl =
+    AsStructuredStringTypeImpl<StructuredStringType::kLiteral>;
+using AsStructuredNotLiteralImpl =
+    AsStructuredStringTypeImpl<StructuredStringType::kNotLiteral>;
+
+// Structured log data for a stringifyable type T and associated structured
+// proto field, both of which must outlive this object.
+template <typename T>
+class [[nodiscard]] AsStructuredValueImpl final {
+ public:
+  using ValueFormatter = absl::AnyInvocable<std::string(T) const>;
+
+  constexpr AsStructuredValueImpl(
+      T value ABSL_ATTRIBUTE_LIFETIME_BOUND,
+      StructuredProtoField field ABSL_ATTRIBUTE_LIFETIME_BOUND,
+      ValueFormatter value_formatter =
+          [](T value) { return absl::StrCat(value); })
+      : value_(value),
+        field_(field),
+        value_formatter_(std::move(value_formatter)) {}
+
+ private:
+  T value_;
+  StructuredProtoField field_;
+  ValueFormatter value_formatter_;
+
+  friend std::ostream& operator<<(std::ostream& os,
+                                  const AsStructuredValueImpl& impl) {
+    return os << impl.value_formatter_(impl.value_);
+  }
+  void AddToMessage(LogMessage& m) const {
+    m.CopyToEncodedBufferWithStructuredProtoField<
+        log_internal::LogMessage::StringType::kNotLiteral>(
+        field_, value_formatter_(value_));
+  }
+  friend LogMessage& operator<<(LogMessage& m,
+                                const AsStructuredValueImpl& impl) {
+    impl.AddToMessage(m);
+    return m;
+  }
+};
+
+// Template deduction guide so `AsStructuredValueImpl(42, data)` works
+// without specifying the template type.
+template <typename T>
+AsStructuredValueImpl(T value, StructuredProtoField field)
+    -> AsStructuredValueImpl<T>;
+
+// Template deduction guide so `AsStructuredValueImpl(42, data, formatter)`
+// works without specifying the template type.
+template <typename T>
+AsStructuredValueImpl(
+    T value, StructuredProtoField field,
+    typename AsStructuredValueImpl<T>::ValueFormatter value_formatter)
+    -> AsStructuredValueImpl<T>;
+
+}  // namespace log_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_LOG_INTERNAL_STRUCTURED_H_
diff --git a/absl/log/internal/structured_proto.cc b/absl/log/internal/structured_proto.cc
new file mode 100644
index 00000000000..eed2e88f849
--- /dev/null
+++ b/absl/log/internal/structured_proto.cc
@@ -0,0 +1,115 @@
+//
+// Copyright 2024 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/log/internal/structured_proto.h"
+
+#include <cstdint>
+#include <variant>
+
+#include "absl/base/config.h"
+#include "absl/log/internal/proto.h"
+#include "absl/types/span.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace log_internal {
+
+namespace {
+
+// Handles protobuf-encoding a type contained inside
+// `StructuredProtoField::Varint`.
+struct VarintEncoderVisitor final {
+  template <typename T>
+  bool operator()(T value) const {
+    return EncodeVarint(field_number, value, &buf);
+  }
+
+  uint64_t field_number;
+  absl::Span<char>& buf;
+};
+
+// Handles protobuf-encoding a type contained inside
+// `StructuredProtoField::I64`.
+struct I64EncoderVisitor final {
+  bool operator()(uint64_t value) const {
+    return Encode64Bit(field_number, value, &buf);
+  }
+
+  bool operator()(int64_t value) const {
+    return Encode64Bit(field_number, value, &buf);
+  }
+
+  bool operator()(double value) const {
+    return EncodeDouble(field_number, value, &buf);
+  }
+
+  uint64_t field_number;
+  absl::Span<char>& buf;
+};
+
+// Handles protobuf-encoding a type contained inside
+// `StructuredProtoField::I32`.
+struct I32EncoderVisitor final {
+  bool operator()(uint32_t value) const {
+    return Encode32Bit(field_number, value, &buf);
+  }
+
+  bool operator()(int32_t value) const {
+    return Encode32Bit(field_number, value, &buf);
+  }
+
+  bool operator()(float value) const {
+    return EncodeFloat(field_number, value, &buf);
+  }
+
+  uint64_t field_number;
+  absl::Span<char>& buf;
+};
+
+// Handles protobuf-encoding a type contained inside `StructuredProtoField`.
+struct EncoderVisitor final {
+  bool operator()(StructuredProtoField::Varint varint) {
+    return std::visit(VarintEncoderVisitor{field_number, buf}, varint);
+  }
+
+  bool operator()(StructuredProtoField::I64 i64) {
+    return std::visit(I64EncoderVisitor{field_number, buf}, i64);
+  }
+
+  bool operator()(StructuredProtoField::LengthDelimited length_delimited) {
+    // No need for a visitor, since `StructuredProtoField::LengthDelimited` is
+    // just `absl::Span<const char>`.
+    return EncodeBytes(field_number, length_delimited, &buf);
+  }
+
+  bool operator()(StructuredProtoField::I32 i32) {
+    return std::visit(I32EncoderVisitor{field_number, buf}, i32);
+  }
+
+  uint64_t field_number;
+  absl::Span<char>& buf;
+};
+
+}  // namespace
+
+bool EncodeStructuredProtoField(StructuredProtoField field,
+                                absl::Span<char>& buf) {
+  return std::visit(EncoderVisitor{field.field_number, buf}, field.value);
+}
+
+}  // namespace log_internal
+
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/log/internal/structured_proto.h b/absl/log/internal/structured_proto.h
new file mode 100644
index 00000000000..04ec499fc8e
--- /dev/null
+++ b/absl/log/internal/structured_proto.h
@@ -0,0 +1,108 @@
+// Copyright 2024 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: log/internal/structured_proto.h
+// -----------------------------------------------------------------------------
+
+#ifndef ABSL_LOG_INTERNAL_STRUCTURED_PROTO_H_
+#define ABSL_LOG_INTERNAL_STRUCTURED_PROTO_H_
+
+#include <cstddef>
+#include <cstdint>
+#include <variant>
+
+#include "absl/base/config.h"
+#include "absl/log/internal/proto.h"
+#include "absl/types/span.h"
+#include "absl/types/variant.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace log_internal {
+
+// Sum type holding a single valid protobuf field suitable for encoding.
+struct StructuredProtoField final {
+  // Numeric type encoded with varint encoding:
+  // https://protobuf.dev/programming-guides/encoding/#varints
+  using Varint = std::variant<uint64_t, int64_t, uint32_t, int32_t, bool>;
+
+  // Fixed-length 64-bit integer encoding:
+  // https://protobuf.dev/programming-guides/encoding/#non-varints
+  using I64 = std::variant<uint64_t, int64_t, double>;
+
+  // Length-delimited record type (string, sub-message):
+  // https://protobuf.dev/programming-guides/encoding/#length-types
+  using LengthDelimited = absl::Span<const char>;
+
+  // Fixed-length 32-bit integer encoding:
+  // https://protobuf.dev/programming-guides/encoding/#non-varints
+  using I32 = std::variant<uint32_t, int32_t, float>;
+
+  // Valid record type:
+  // https://protobuf.dev/programming-guides/encoding/#structure
+  using Value = std::variant<Varint, I64, LengthDelimited, I32>;
+
+  // Field number for the protobuf value.
+  uint64_t field_number;
+
+  // Value to encode.
+  Value value;
+};
+
+// Estimates the number of bytes needed to encode `field` using
+// protobuf encoding.
+//
+// The returned value might be larger than the actual number of bytes needed.
+inline size_t BufferSizeForStructuredProtoField(StructuredProtoField field) {
+  // Visitor to estimate the number of bytes of one of the types contained
+  // inside `StructuredProtoField`.
+  struct BufferSizeVisitor final {
+    size_t operator()(StructuredProtoField::Varint /*unused*/) {
+      return BufferSizeFor(field_number, WireType::kVarint);
+    }
+
+    size_t operator()(StructuredProtoField::I64 /*unused*/) {
+      return BufferSizeFor(field_number, WireType::k64Bit);
+    }
+
+    size_t operator()(StructuredProtoField::LengthDelimited length_delimited) {
+      return BufferSizeFor(field_number, WireType::kLengthDelimited) +
+             length_delimited.size();
+    }
+
+    size_t operator()(StructuredProtoField::I32 /*unused*/) {
+      return BufferSizeFor(field_number, WireType::k32Bit);
+    }
+
+    uint64_t field_number;
+  };
+
+  return std::visit(BufferSizeVisitor{field.field_number}, field.value);
+}
+
+// Encodes `field` into `buf` using protobuf encoding.
+//
+// On success, returns `true` and advances `buf` to the end of
+// the bytes consumed.
+//
+// On failure (if `buf` was too small), returns `false`.
+bool EncodeStructuredProtoField(StructuredProtoField field,
+                                absl::Span<char>& buf);
+
+}  // namespace log_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_LOG_INTERNAL_STRUCTURED_PROTO_H_
diff --git a/absl/log/internal/structured_proto_test.cc b/absl/log/internal/structured_proto_test.cc
new file mode 100644
index 00000000000..80a3e7b6b5b
--- /dev/null
+++ b/absl/log/internal/structured_proto_test.cc
@@ -0,0 +1,119 @@
+//
+// Copyright 2024 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/log/internal/structured_proto.h"
+
+#include <cstddef>
+#include <cstdint>
+#include <string>
+#include <utility>
+#include <vector>
+
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+#include "absl/strings/string_view.h"
+#include "absl/types/span.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace log_internal {
+namespace {
+
+using ::testing::TestWithParam;
+
+struct StructuredProtoTestCase {
+  std::string test_name;
+  StructuredProtoField field;
+  std::vector<char> expected_encoded_field;
+};
+
+using StructuredProtoTest = TestWithParam<StructuredProtoTestCase>;
+
+TEST_P(StructuredProtoTest, Encoding) {
+  const StructuredProtoTestCase& test_case = GetParam();
+
+  // Greater than or equal to since BufferSizeForStructuredProtoField() is just
+  // an estimate of the data size and not an exact measurement.
+  ASSERT_GE(BufferSizeForStructuredProtoField(test_case.field),
+            test_case.expected_encoded_field.size());
+
+  std::vector<char> buf;
+  buf.resize(1024);
+
+  absl::Span<char> buf_span(buf);
+  EXPECT_TRUE(EncodeStructuredProtoField(test_case.field, buf_span));
+  size_t encoded_field_size = buf.size() - buf_span.size();
+
+  ASSERT_EQ(encoded_field_size, test_case.expected_encoded_field.size());
+  buf.resize(encoded_field_size);
+  EXPECT_EQ(buf, test_case.expected_encoded_field);
+}
+
+INSTANTIATE_TEST_SUITE_P(
+    StructuredProtoTestSuiteInstantiation,
+    StructuredProtoTest,  // This is the name of your parameterized test
+    testing::ValuesIn<StructuredProtoTestCase>({
+        {
+            "Varint",
+            {
+                42,
+                StructuredProtoField::Value{
+                    std::in_place_type<StructuredProtoField::Varint>,
+                    int32_t{23},
+                },
+            },
+            {'\xD0', '\x02', '\x17'},
+        },
+        {
+            "I64",
+            {
+                42,
+                StructuredProtoField::Value{
+                    std::in_place_type<StructuredProtoField::I64>,
+                    int64_t{23},
+                },
+            },
+            {'\xD1', '\x02', '\x17', '\x00', '\x00', '\x00', '\x00', '\x00',
+             '\x00', '\x00'},
+        },
+        {
+            "LengthDelimited",
+            {
+                42,
+                // Use a string_view so the terminating NUL is excluded.
+                absl::string_view("Hello"),
+            },
+            {'\xD2', '\x02', '\x05', 'H', 'e', 'l', 'l', 'o'},
+        },
+        {
+            "I32",
+            {
+                42,
+                StructuredProtoField::Value{
+                    std::in_place_type<StructuredProtoField::I32>,
+                    int32_t{23},
+                },
+            },
+            {'\xD5', '\x02', '\x17', '\x00', '\x00', '\x00'},
+        },
+    }),
+    [](const testing::TestParamInfo<StructuredProtoTest::ParamType>& info) {
+      return info.param.test_name;
+    });
+
+}  // namespace
+}  // namespace log_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/log/internal/test_actions.cc b/absl/log/internal/test_actions.cc
new file mode 100644
index 00000000000..5d27f14aabf
--- /dev/null
+++ b/absl/log/internal/test_actions.cc
@@ -0,0 +1,77 @@
+//
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/log/internal/test_actions.h"
+
+#include <cassert>
+#include <iostream>
+#include <string>
+#include <type_traits>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/log_severity.h"
+#include "absl/log/log_entry.h"
+#include "absl/strings/escaping.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/string_view.h"
+#include "absl/time/time.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace log_internal {
+
+void WriteToStderrWithFilename::operator()(const absl::LogEntry& entry) const {
+  std::cerr << message << " (file: " << entry.source_filename() << ")\n";
+}
+
+void WriteEntryToStderr::operator()(const absl::LogEntry& entry) const {
+  if (!message.empty()) std::cerr << message << "\n";
+
+  const std::string source_filename = absl::CHexEscape(entry.source_filename());
+  const std::string source_basename = absl::CHexEscape(entry.source_basename());
+  const std::string text_message = absl::CHexEscape(entry.text_message());
+  const std::string encoded_message = absl::CHexEscape(entry.encoded_message());
+  std::string encoded_message_str;
+  std::cerr << "LogEntry{\n"                                               //
+            << "  source_filename: \"" << source_filename << "\"\n"        //
+            << "  source_basename: \"" << source_basename << "\"\n"        //
+            << "  source_line: " << entry.source_line() << "\n"            //
+            << "  prefix: " << (entry.prefix() ? "true\n" : "false\n")     //
+            << "  log_severity: " << entry.log_severity() << "\n"          //
+            << "  timestamp: " << entry.timestamp() << "\n"                //
+            << "  text_message: \"" << text_message << "\"\n"              //
+            << "  verbosity: " << entry.verbosity() << "\n"                //
+            << "  encoded_message (raw): \"" << encoded_message << "\"\n"  //
+            << encoded_message_str                                         //
+            << "}\n";
+}
+
+void WriteEntryToStderr::operator()(absl::LogSeverity severity,
+                                    absl::string_view filename,
+                                    absl::string_view log_message) const {
+  if (!message.empty()) std::cerr << message << "\n";
+  const std::string source_filename = absl::CHexEscape(filename);
+  const std::string text_message = absl::CHexEscape(log_message);
+  std::cerr << "LogEntry{\n"                                         //
+            << "  source_filename: \"" << source_filename << "\"\n"  //
+            << "  log_severity: " << severity << "\n"                //
+            << "  text_message: \"" << text_message << "\"\n"        //
+            << "}\n";
+}
+
+}  // namespace log_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/log/internal/test_actions.h b/absl/log/internal/test_actions.h
new file mode 100644
index 00000000000..649a050521b
--- /dev/null
+++ b/absl/log/internal/test_actions.h
@@ -0,0 +1,90 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: log/internal/test_actions.h
+// -----------------------------------------------------------------------------
+//
+// This file declares Googletest's actions used in the Abseil Logging library
+// unit tests.
+
+#ifndef ABSL_LOG_INTERNAL_TEST_ACTIONS_H_
+#define ABSL_LOG_INTERNAL_TEST_ACTIONS_H_
+
+#include <iostream>
+#include <ostream>
+#include <string>
+
+#include "absl/base/config.h"
+#include "absl/base/log_severity.h"
+#include "absl/log/log_entry.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace log_internal {
+
+// These actions are used by the child process in a death test.
+//
+// Expectations set in the child cannot cause test failure in the parent
+// directly.  Instead, the child can use these actions with
+// `EXPECT_CALL`/`WillOnce` and `ON_CALL`/`WillByDefault` (for unexpected calls)
+// to write messages to stderr that the parent can match against.
+struct WriteToStderr final {
+  explicit WriteToStderr(absl::string_view m) : message(m) {}
+  std::string message;
+
+  template <typename... Args>
+  void operator()(const Args&...) const {
+    std::cerr << message << std::endl;
+  }
+};
+
+struct WriteToStderrWithFilename final {
+  explicit WriteToStderrWithFilename(absl::string_view m) : message(m) {}
+
+  std::string message;
+
+  void operator()(const absl::LogEntry& entry) const;
+};
+
+struct WriteEntryToStderr final {
+  explicit WriteEntryToStderr(absl::string_view m) : message(m) {}
+
+  std::string message = "";
+
+  void operator()(const absl::LogEntry& entry) const;
+  void operator()(absl::LogSeverity, absl::string_view,
+                  absl::string_view) const;
+};
+
+// See the documentation for `DeathTestValidateExpectations` above.
+// `DeathTestExpectedLogging` should be used once in a given death test, and the
+// applicable severity level is the one that should be passed to
+// `DeathTestValidateExpectations`.
+inline WriteEntryToStderr DeathTestExpectedLogging() {
+  return WriteEntryToStderr{"Mock received expected entry:"};
+}
+
+// `DeathTestUnexpectedLogging` should be used zero or more times to mark
+// messages that should not hit the logs as the process dies.
+inline WriteEntryToStderr DeathTestUnexpectedLogging() {
+  return WriteEntryToStderr{"Mock received unexpected entry:"};
+}
+
+}  // namespace log_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_LOG_INTERNAL_TEST_ACTIONS_H_
diff --git a/absl/log/internal/test_helpers.cc b/absl/log/internal/test_helpers.cc
new file mode 100644
index 00000000000..4a79664b277
--- /dev/null
+++ b/absl/log/internal/test_helpers.cc
@@ -0,0 +1,88 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+#include "absl/log/internal/test_helpers.h"
+
+#include <csignal>
+
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+#include "absl/base/log_severity.h"
+#include "absl/log/globals.h"
+#include "absl/log/initialize.h"
+#include "absl/log/internal/globals.h"
+
+#ifdef __Fuchsia__
+#include <zircon/syscalls.h>
+#endif
+
+#if defined(ABSL_HAVE_ALARM)
+#include <signal.h>
+#endif
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace log_internal {
+
+// Returns false if the specified severity level is disabled by
+// `ABSL_MIN_LOG_LEVEL` or `absl::MinLogLevel()`.
+bool LoggingEnabledAt(absl::LogSeverity severity) {
+  return severity >= kAbslMinLogLevel && severity >= absl::MinLogLevel();
+}
+
+// -----------------------------------------------------------------------------
+// Googletest Death Test Predicates
+// -----------------------------------------------------------------------------
+
+#if GTEST_HAS_DEATH_TEST
+
+bool DiedOfFatal(int exit_status) {
+#if defined(_WIN32)
+  // Depending on NDEBUG and (configuration?) MSVC's abort either results
+  // in error code 3 (SIGABRT) or error code 0x80000003 (breakpoint
+  // triggered).
+  return ::testing::ExitedWithCode(3)(exit_status & 0x7fffffff);
+#elif defined(__Fuchsia__)
+  // The Fuchsia death test implementation kill()'s the process when it detects
+  // an exception, so it should exit with the corresponding code. See
+  // FuchsiaDeathTest::Wait().
+  return ::testing::ExitedWithCode(ZX_TASK_RETCODE_SYSCALL_KILL)(exit_status);
+#elif defined(__ANDROID__) && defined(__aarch64__)
+  // These are all run under a qemu config that eats died-due-to-signal exit
+  // statuses.
+  return true;
+#else
+  return ::testing::KilledBySignal(SIGABRT)(exit_status);
+#endif
+}
+
+bool DiedOfQFatal(int exit_status) {
+  return ::testing::ExitedWithCode(1)(exit_status);
+}
+
+#endif
+
+// -----------------------------------------------------------------------------
+// Helper for Log initialization in test
+// -----------------------------------------------------------------------------
+
+void LogTestEnvironment::SetUp() {
+  if (!absl::log_internal::IsInitialized()) {
+    absl::InitializeLog();
+  }
+}
+
+}  // namespace log_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/log/internal/test_helpers.h b/absl/log/internal/test_helpers.h
new file mode 100644
index 00000000000..714bc7bddcb
--- /dev/null
+++ b/absl/log/internal/test_helpers.h
@@ -0,0 +1,71 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: log/internal/test_helpers.h
+// -----------------------------------------------------------------------------
+//
+// This file declares testing helpers for the logging library.
+
+#ifndef ABSL_LOG_INTERNAL_TEST_HELPERS_H_
+#define ABSL_LOG_INTERNAL_TEST_HELPERS_H_
+
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+#include "absl/base/log_severity.h"
+#include "absl/log/globals.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace log_internal {
+
+// `ABSL_MIN_LOG_LEVEL` can't be used directly since it is not always defined.
+constexpr auto kAbslMinLogLevel =
+#ifdef ABSL_MIN_LOG_LEVEL
+    static_cast<absl::LogSeverityAtLeast>(ABSL_MIN_LOG_LEVEL);
+#else
+    absl::LogSeverityAtLeast::kInfo;
+#endif
+
+// Returns false if the specified severity level is disabled by
+// `ABSL_MIN_LOG_LEVEL` or `absl::MinLogLevel()`.
+bool LoggingEnabledAt(absl::LogSeverity severity);
+
+// -----------------------------------------------------------------------------
+// Googletest Death Test Predicates
+// -----------------------------------------------------------------------------
+
+#if GTEST_HAS_DEATH_TEST
+
+bool DiedOfFatal(int exit_status);
+bool DiedOfQFatal(int exit_status);
+
+#endif
+
+// -----------------------------------------------------------------------------
+// Helper for Log initialization in test
+// -----------------------------------------------------------------------------
+
+class LogTestEnvironment : public ::testing::Environment {
+ public:
+  ~LogTestEnvironment() override = default;
+
+  void SetUp() override;
+};
+
+}  // namespace log_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_LOG_INTERNAL_TEST_HELPERS_H_
diff --git a/absl/log/internal/test_matchers.cc b/absl/log/internal/test_matchers.cc
new file mode 100644
index 00000000000..736bc7b5172
--- /dev/null
+++ b/absl/log/internal/test_matchers.cc
@@ -0,0 +1,216 @@
+//
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/log/internal/test_matchers.h"
+
+#include <ostream>
+#include <sstream>
+#include <string>
+#include <type_traits>
+#include <utility>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/log_severity.h"
+#include "absl/log/internal/test_helpers.h"
+#include "absl/log/log_entry.h"
+#include "absl/strings/string_view.h"
+#include "absl/time/clock.h"
+#include "absl/time/time.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace log_internal {
+namespace {
+using ::testing::AllOf;
+using ::testing::Ge;
+using ::testing::HasSubstr;
+using ::testing::MakeMatcher;
+using ::testing::Matcher;
+using ::testing::MatcherInterface;
+using ::testing::MatchResultListener;
+using ::testing::Not;
+using ::testing::Property;
+using ::testing::ResultOf;
+using ::testing::Truly;
+
+class AsStringImpl final
+    : public MatcherInterface<absl::string_view> {
+ public:
+  explicit AsStringImpl(
+      const Matcher<const std::string&>& str_matcher)
+      : str_matcher_(str_matcher) {}
+  bool MatchAndExplain(
+      absl::string_view actual,
+      MatchResultListener* listener) const override {
+    return str_matcher_.MatchAndExplain(std::string(actual), listener);
+  }
+  void DescribeTo(std::ostream* os) const override {
+    return str_matcher_.DescribeTo(os);
+  }
+
+  void DescribeNegationTo(std::ostream* os) const override {
+    return str_matcher_.DescribeNegationTo(os);
+  }
+
+ private:
+  const Matcher<const std::string&> str_matcher_;
+};
+
+class MatchesOstreamImpl final
+    : public MatcherInterface<absl::string_view> {
+ public:
+  explicit MatchesOstreamImpl(std::string expected)
+      : expected_(std::move(expected)) {}
+  bool MatchAndExplain(absl::string_view actual,
+                       MatchResultListener*) const override {
+    return actual == expected_;
+  }
+  void DescribeTo(std::ostream* os) const override {
+    *os << "matches the contents of the ostringstream, which are \""
+        << expected_ << "\"";
+  }
+
+  void DescribeNegationTo(std::ostream* os) const override {
+    *os << "does not match the contents of the ostringstream, which are \""
+        << expected_ << "\"";
+  }
+
+ private:
+  const std::string expected_;
+};
+}  // namespace
+
+Matcher<absl::string_view> AsString(
+    const Matcher<const std::string&>& str_matcher) {
+  return MakeMatcher(new AsStringImpl(str_matcher));
+}
+
+Matcher<const absl::LogEntry&> SourceFilename(
+    const Matcher<absl::string_view>& source_filename) {
+  return Property("source_filename", &absl::LogEntry::source_filename,
+                  source_filename);
+}
+
+Matcher<const absl::LogEntry&> SourceBasename(
+    const Matcher<absl::string_view>& source_basename) {
+  return Property("source_basename", &absl::LogEntry::source_basename,
+                  source_basename);
+}
+
+Matcher<const absl::LogEntry&> SourceLine(
+    const Matcher<int>& source_line) {
+  return Property("source_line", &absl::LogEntry::source_line, source_line);
+}
+
+Matcher<const absl::LogEntry&> Prefix(
+    const Matcher<bool>& prefix) {
+  return Property("prefix", &absl::LogEntry::prefix, prefix);
+}
+
+Matcher<const absl::LogEntry&> LogSeverity(
+    const Matcher<absl::LogSeverity>& log_severity) {
+  return Property("log_severity", &absl::LogEntry::log_severity, log_severity);
+}
+
+Matcher<const absl::LogEntry&> Timestamp(
+    const Matcher<absl::Time>& timestamp) {
+  return Property("timestamp", &absl::LogEntry::timestamp, timestamp);
+}
+
+Matcher<absl::Time> InMatchWindow() {
+  return AllOf(Ge(absl::Now()),
+               Truly([](absl::Time arg) { return arg <= absl::Now(); }));
+}
+
+Matcher<const absl::LogEntry&> ThreadID(
+    const Matcher<absl::LogEntry::tid_t>& tid) {
+  return Property("tid", &absl::LogEntry::tid, tid);
+}
+
+Matcher<const absl::LogEntry&> TextMessageWithPrefixAndNewline(
+    const Matcher<absl::string_view>&
+        text_message_with_prefix_and_newline) {
+  return Property("text_message_with_prefix_and_newline",
+                  &absl::LogEntry::text_message_with_prefix_and_newline,
+                  text_message_with_prefix_and_newline);
+}
+
+Matcher<const absl::LogEntry&> TextMessageWithPrefix(
+    const Matcher<absl::string_view>& text_message_with_prefix) {
+  return Property("text_message_with_prefix",
+                  &absl::LogEntry::text_message_with_prefix,
+                  text_message_with_prefix);
+}
+
+Matcher<const absl::LogEntry&> TextMessage(
+    const Matcher<absl::string_view>& text_message) {
+  return Property("text_message", &absl::LogEntry::text_message, text_message);
+}
+
+Matcher<const absl::LogEntry&> TextPrefix(
+    const Matcher<absl::string_view>& text_prefix) {
+  return ResultOf(
+      [](const absl::LogEntry& entry) {
+        absl::string_view msg = entry.text_message_with_prefix();
+        msg.remove_suffix(entry.text_message().size());
+        return msg;
+      },
+      text_prefix);
+}
+Matcher<const absl::LogEntry&> RawEncodedMessage(
+    const Matcher<absl::string_view>& raw_encoded_message) {
+  return Property("encoded_message", &absl::LogEntry::encoded_message,
+                  raw_encoded_message);
+}
+
+Matcher<const absl::LogEntry&> Verbosity(
+    const Matcher<int>& verbosity) {
+  return Property("verbosity", &absl::LogEntry::verbosity, verbosity);
+}
+
+Matcher<const absl::LogEntry&> Stacktrace(
+    const Matcher<absl::string_view>& stacktrace) {
+  return Property("stacktrace", &absl::LogEntry::stacktrace, stacktrace);
+}
+
+Matcher<absl::string_view> MatchesOstream(
+    const std::ostringstream& stream) {
+  return MakeMatcher(new MatchesOstreamImpl(stream.str()));
+}
+
+// We need to validate what is and isn't logged as the process dies due to
+// `FATAL`, `QFATAL`, `CHECK`, etc., but assertions inside a death test
+// subprocess don't directly affect the pass/fail status of the parent process.
+// Instead, we use the mock actions `DeathTestExpectedLogging` and
+// `DeathTestUnexpectedLogging` to write specific phrases to `stderr` that we
+// can validate in the parent process using this matcher.
+Matcher<const std::string&> DeathTestValidateExpectations() {
+  if (log_internal::LoggingEnabledAt(absl::LogSeverity::kFatal)) {
+    return Matcher<const std::string&>(
+        AllOf(HasSubstr("Mock received expected entry"),
+              Not(HasSubstr("Mock received unexpected entry"))));
+  }
+  // If `FATAL` logging is disabled, neither message should have been written.
+  return Matcher<const std::string&>(
+      AllOf(Not(HasSubstr("Mock received expected entry")),
+            Not(HasSubstr("Mock received unexpected entry"))));
+}
+
+}  // namespace log_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/log/internal/test_matchers.h b/absl/log/internal/test_matchers.h
new file mode 100644
index 00000000000..906eda24088
--- /dev/null
+++ b/absl/log/internal/test_matchers.h
@@ -0,0 +1,94 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: log/internal/test_matchers.h
+// -----------------------------------------------------------------------------
+//
+// This file declares Googletest's matchers used in the Abseil Logging library
+// unit tests.
+
+#ifndef ABSL_LOG_INTERNAL_TEST_MATCHERS_H_
+#define ABSL_LOG_INTERNAL_TEST_MATCHERS_H_
+
+#include <iosfwd>
+#include <sstream>
+#include <string>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+#include "absl/base/log_severity.h"
+#include "absl/log/internal/test_helpers.h"
+#include "absl/log/log_entry.h"
+#include "absl/strings/string_view.h"
+#include "absl/time/time.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace log_internal {
+// In some configurations, Googletest's string matchers (e.g.
+// `::testing::EndsWith`) need help to match `absl::string_view`.
+::testing::Matcher<absl::string_view> AsString(
+    const ::testing::Matcher<const std::string&>& str_matcher);
+
+// These matchers correspond to the components of `absl::LogEntry`.
+::testing::Matcher<const absl::LogEntry&> SourceFilename(
+    const ::testing::Matcher<absl::string_view>& source_filename);
+::testing::Matcher<const absl::LogEntry&> SourceBasename(
+    const ::testing::Matcher<absl::string_view>& source_basename);
+// Be careful with this one; multi-line statements using `__LINE__` evaluate
+// differently on different platforms.  In particular, the MSVC implementation
+// of `EXPECT_DEATH` returns the line number of the macro expansion to all lines
+// within the code block that's expected to die.
+::testing::Matcher<const absl::LogEntry&> SourceLine(
+    const ::testing::Matcher<int>& source_line);
+::testing::Matcher<const absl::LogEntry&> Prefix(
+    const ::testing::Matcher<bool>& prefix);
+::testing::Matcher<const absl::LogEntry&> LogSeverity(
+    const ::testing::Matcher<absl::LogSeverity>& log_severity);
+::testing::Matcher<const absl::LogEntry&> Timestamp(
+    const ::testing::Matcher<absl::Time>& timestamp);
+// Matches if the `LogEntry`'s timestamp falls after the instantiation of this
+// matcher and before its execution, as is normal when used with EXPECT_CALL.
+::testing::Matcher<absl::Time> InMatchWindow();
+::testing::Matcher<const absl::LogEntry&> ThreadID(
+    const ::testing::Matcher<absl::LogEntry::tid_t>&);
+::testing::Matcher<const absl::LogEntry&> TextMessageWithPrefixAndNewline(
+    const ::testing::Matcher<absl::string_view>&
+        text_message_with_prefix_and_newline);
+::testing::Matcher<const absl::LogEntry&> TextMessageWithPrefix(
+    const ::testing::Matcher<absl::string_view>& text_message_with_prefix);
+::testing::Matcher<const absl::LogEntry&> TextMessage(
+    const ::testing::Matcher<absl::string_view>& text_message);
+::testing::Matcher<const absl::LogEntry&> TextPrefix(
+    const ::testing::Matcher<absl::string_view>& text_prefix);
+::testing::Matcher<const absl::LogEntry&> Verbosity(
+    const ::testing::Matcher<int>& verbosity);
+::testing::Matcher<const absl::LogEntry&> Stacktrace(
+    const ::testing::Matcher<absl::string_view>& stacktrace);
+// Behaves as `Eq(stream.str())`, but produces better failure messages.
+::testing::Matcher<absl::string_view> MatchesOstream(
+    const std::ostringstream& stream);
+::testing::Matcher<const std::string&> DeathTestValidateExpectations();
+
+::testing::Matcher<const absl::LogEntry&> RawEncodedMessage(
+    const ::testing::Matcher<absl::string_view>& raw_encoded_message);
+#define ENCODED_MESSAGE(message_matcher) ::testing::_
+
+}  // namespace log_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_LOG_INTERNAL_TEST_MATCHERS_H_
diff --git a/absl/log/internal/vlog_config.cc b/absl/log/internal/vlog_config.cc
new file mode 100644
index 00000000000..79eb7df3dce
--- /dev/null
+++ b/absl/log/internal/vlog_config.cc
@@ -0,0 +1,345 @@
+// Copyright 2022 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/log/internal/vlog_config.h"
+
+#include <stddef.h>
+
+#include <algorithm>
+#include <atomic>
+#include <functional>
+#include <optional>
+#include <string>
+#include <utility>
+#include <vector>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/scheduling_mode.h"
+#include "absl/base/internal/spinlock.h"
+#include "absl/base/no_destructor.h"
+#include "absl/base/optimization.h"
+#include "absl/base/thread_annotations.h"
+#include "absl/log/internal/fnmatch.h"
+#include "absl/strings/numbers.h"
+#include "absl/strings/str_split.h"
+#include "absl/strings/string_view.h"
+#include "absl/strings/strip.h"
+#include "absl/synchronization/mutex.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace log_internal {
+
+namespace {
+
+#ifdef _WIN32
+constexpr char kPathSeparators[] = "/\\";
+#else
+constexpr char kPathSeparators[] = "/";
+#endif
+
+bool ModuleIsPath(absl::string_view module_pattern) {
+  return module_pattern.find_first_of(kPathSeparators) != module_pattern.npos;
+}
+
+absl::string_view Basename(absl::string_view file) {
+  auto sep = file.find_last_of(kPathSeparators);
+  if (sep != file.npos) {
+    file.remove_prefix(sep + 1);
+  }
+  return file;
+}
+
+}  // namespace
+
+bool VLogSite::SlowIsEnabled(int stale_v, int level) {
+  if (ABSL_PREDICT_TRUE(stale_v != kUninitialized)) {
+    // Because of the prerequisites to this function, we know that stale_v is
+    // either uninitialized or >= level. If it's not uninitialized, that means
+    // it must be >= level, thus we should log.
+    return true;
+  }
+  stale_v = log_internal::RegisterAndInitialize(this);
+  return ABSL_PREDICT_FALSE(stale_v >= level);
+}
+
+bool VLogSite::SlowIsEnabled0(int stale_v) { return SlowIsEnabled(stale_v, 0); }
+bool VLogSite::SlowIsEnabled1(int stale_v) { return SlowIsEnabled(stale_v, 1); }
+bool VLogSite::SlowIsEnabled2(int stale_v) { return SlowIsEnabled(stale_v, 2); }
+bool VLogSite::SlowIsEnabled3(int stale_v) { return SlowIsEnabled(stale_v, 3); }
+bool VLogSite::SlowIsEnabled4(int stale_v) { return SlowIsEnabled(stale_v, 4); }
+bool VLogSite::SlowIsEnabled5(int stale_v) { return SlowIsEnabled(stale_v, 5); }
+
+namespace {
+struct VModuleInfo final {
+  std::string module_pattern;
+  bool module_is_path;  // i.e. it contains a path separator.
+  int vlog_level;
+
+  // Allocates memory.
+  VModuleInfo(absl::string_view module_pattern_arg, bool module_is_path_arg,
+              int vlog_level_arg)
+      : module_pattern(std::string(module_pattern_arg)),
+        module_is_path(module_is_path_arg),
+        vlog_level(vlog_level_arg) {}
+};
+
+// `mutex` guards all of the data structures that aren't lock-free.
+// To avoid problems with the heap checker which calls into `VLOG`, `mutex` must
+// be a `SpinLock` that prevents fiber scheduling instead of a `Mutex`.
+ABSL_CONST_INIT absl::base_internal::SpinLock mutex(
+    absl::base_internal::SCHEDULE_KERNEL_ONLY);
+
+// `GetUpdateSitesMutex()` serializes updates to all of the sites (i.e. those in
+// `site_list_head`) themselves.
+absl::Mutex& GetUpdateSitesMutex() {
+  // Chromium requires no global destructors, so we can't use the
+  // absl::kConstInit idiom since absl::Mutex as a non-trivial destructor.
+  static absl::NoDestructor<absl::Mutex> update_sites_mutex ABSL_ACQUIRED_AFTER(
+      mutex);
+  return *update_sites_mutex;
+}
+
+ABSL_CONST_INIT int global_v ABSL_GUARDED_BY(mutex) = 0;
+// `site_list_head` is the head of a singly-linked list.  Traversal, insertion,
+// and reads are atomic, so no locks are required, but updates to existing
+// elements are guarded by `GetUpdateSitesMutex()`.
+ABSL_CONST_INIT std::atomic<VLogSite*> site_list_head{nullptr};
+ABSL_CONST_INIT std::vector<VModuleInfo>* vmodule_info ABSL_GUARDED_BY(mutex)
+    ABSL_PT_GUARDED_BY(mutex){nullptr};
+
+// Only used for lisp.
+ABSL_CONST_INIT std::vector<std::function<void()>>* update_callbacks
+    ABSL_GUARDED_BY(GetUpdateSitesMutex())
+        ABSL_PT_GUARDED_BY(GetUpdateSitesMutex()){nullptr};
+
+// Allocates memory.
+std::vector<VModuleInfo>& get_vmodule_info()
+    ABSL_EXCLUSIVE_LOCKS_REQUIRED(mutex) {
+  if (!vmodule_info) vmodule_info = new std::vector<VModuleInfo>;
+  return *vmodule_info;
+}
+
+// Does not allocate or take locks.
+int VLogLevel(absl::string_view file, const std::vector<VModuleInfo>* infos,
+              int current_global_v) {
+  // `infos` is null during a call to `VLOG` prior to setting `vmodule` (e.g. by
+  // parsing flags).  We can't allocate in `VLOG`, so we treat null as empty
+  // here and press on.
+  if (!infos || infos->empty()) return current_global_v;
+
+  absl::string_view stem = file;
+  absl::string_view stem_basename = Basename(stem);
+  {
+    const size_t sep = stem_basename.find('.');
+    if (sep != stem_basename.npos) {
+      stem.remove_suffix(stem_basename.size() - sep);
+      stem_basename.remove_suffix(stem_basename.size() - sep);
+    }
+    if (absl::ConsumeSuffix(&stem_basename, "-inl")) {
+      stem.remove_suffix(absl::string_view("-inl").size());
+    }
+  }
+  for (const auto& info : *infos) {
+    if (info.module_is_path) {
+      // If there are any slashes in the pattern, try to match the full
+      // name.
+      if (FNMatch(info.module_pattern, stem)) {
+        return info.vlog_level;
+      }
+    } else if (FNMatch(info.module_pattern, stem_basename)) {
+      return info.vlog_level;
+    }
+  }
+
+  return current_global_v;
+}
+
+// Allocates memory.
+int AppendVModuleLocked(absl::string_view module_pattern, int log_level)
+    ABSL_EXCLUSIVE_LOCKS_REQUIRED(mutex) {
+  for (const auto& info : get_vmodule_info()) {
+    if (FNMatch(info.module_pattern, module_pattern)) {
+      // This is a memory optimization to avoid storing patterns that will never
+      // match due to exit early semantics. Primarily optimized for our own unit
+      // tests.
+      return info.vlog_level;
+    }
+  }
+  bool module_is_path = ModuleIsPath(module_pattern);
+  get_vmodule_info().emplace_back(std::string(module_pattern), module_is_path,
+                                  log_level);
+  return global_v;
+}
+
+// Allocates memory.
+int PrependVModuleLocked(absl::string_view module_pattern, int log_level)
+    ABSL_EXCLUSIVE_LOCKS_REQUIRED(mutex) {
+  std::optional<int> old_log_level;
+  for (const auto& info : get_vmodule_info()) {
+    if (FNMatch(info.module_pattern, module_pattern)) {
+      old_log_level = info.vlog_level;
+      break;
+    }
+  }
+  bool module_is_path = ModuleIsPath(module_pattern);
+  auto iter = get_vmodule_info().emplace(get_vmodule_info().cbegin(),
+                                         std::string(module_pattern),
+                                         module_is_path, log_level);
+
+  // This is a memory optimization to avoid storing patterns that will never
+  // match due to exit early semantics. Primarily optimized for our own unit
+  // tests.
+  get_vmodule_info().erase(
+      std::remove_if(++iter, get_vmodule_info().end(),
+                     [module_pattern](const VModuleInfo& info) {
+                       // Remove the previous pattern if it is less generic than
+                       // the new one. For example, if the new pattern
+                       // `module_pattern` is "foo*" and the previous pattern
+                       // `info.module_pattern` is "foo", we should remove the
+                       // previous pattern. Because the new pattern "foo*" will
+                       // match all the files that the previous pattern "foo"
+                       // matches.
+                       return FNMatch(module_pattern, info.module_pattern);
+                     }),
+      get_vmodule_info().cend());
+  return old_log_level.value_or(global_v);
+}
+}  // namespace
+
+int VLogLevel(absl::string_view file) ABSL_LOCKS_EXCLUDED(mutex) {
+  absl::base_internal::SpinLockHolder l(mutex);
+  return VLogLevel(file, vmodule_info, global_v);
+}
+
+int RegisterAndInitialize(VLogSite* v) ABSL_LOCKS_EXCLUDED(mutex) {
+  // std::memory_order_seq_cst is overkill in this function, but given that this
+  // path is intended to be slow, it's not worth the brain power to relax that.
+  VLogSite* h = site_list_head.load(std::memory_order_seq_cst);
+
+  VLogSite* old = nullptr;
+  if (v->next_.compare_exchange_strong(old, h, std::memory_order_seq_cst,
+                                       std::memory_order_seq_cst)) {
+    // Multiple threads may attempt to register this site concurrently.
+    // By successfully setting `v->next` this thread commits to being *the*
+    // thread that installs `v` in the list.
+    while (!site_list_head.compare_exchange_weak(
+        h, v, std::memory_order_seq_cst, std::memory_order_seq_cst)) {
+      v->next_.store(h, std::memory_order_seq_cst);
+    }
+  }
+
+  int old_v = VLogSite::kUninitialized;
+  int new_v = VLogLevel(v->file_);
+  // No loop, if someone else set this, we should respect their evaluation of
+  // `VLogLevel`. This may mean we return a stale `v`, but `v` itself will
+  // always arrive at the freshest value.  Otherwise, we could be writing a
+  // stale value and clobbering the fresher one.
+  if (v->v_.compare_exchange_strong(old_v, new_v, std::memory_order_seq_cst,
+                                    std::memory_order_seq_cst)) {
+    return new_v;
+  }
+  return old_v;
+}
+
+void UpdateVLogSites() ABSL_UNLOCK_FUNCTION(mutex)
+    ABSL_LOCKS_EXCLUDED(GetUpdateSitesMutex()) {
+  std::vector<VModuleInfo> infos = get_vmodule_info();
+  int current_global_v = global_v;
+  // We need to grab `GetUpdateSitesMutex()` before we release `mutex` to ensure
+  // that updates are not interleaved (resulting in an inconsistent final state)
+  // and to ensure that the final state in the sites matches the final state of
+  // `vmodule_info`. We unlock `mutex` to ensure that uninitialized sites don't
+  // have to wait on all updates in order to acquire `mutex` and initialize
+  // themselves.
+  absl::MutexLock ul(GetUpdateSitesMutex());
+  mutex.unlock();
+  VLogSite* n = site_list_head.load(std::memory_order_seq_cst);
+  // Because sites are added to the list in the order they are executed, there
+  // tend to be clusters of entries with the same file.
+  const char* last_file = nullptr;
+  int last_file_level = 0;
+  while (n != nullptr) {
+    if (n->file_ != last_file) {
+      last_file = n->file_;
+      last_file_level = VLogLevel(n->file_, &infos, current_global_v);
+    }
+    n->v_.store(last_file_level, std::memory_order_seq_cst);
+    n = n->next_.load(std::memory_order_seq_cst);
+  }
+  if (update_callbacks) {
+    for (auto& cb : *update_callbacks) {
+      cb();
+    }
+  }
+}
+
+void UpdateVModule(absl::string_view vmodule)
+    ABSL_LOCKS_EXCLUDED(mutex, GetUpdateSitesMutex()) {
+  std::vector<std::pair<absl::string_view, int>> glob_levels;
+  for (absl::string_view glob_level : absl::StrSplit(vmodule, ',')) {
+    const size_t eq = glob_level.rfind('=');
+    if (eq == glob_level.npos) continue;
+    const absl::string_view glob = glob_level.substr(0, eq);
+    int level;
+    if (!absl::SimpleAtoi(glob_level.substr(eq + 1), &level)) continue;
+    glob_levels.emplace_back(glob, level);
+  }
+  mutex.lock();  // unlocked by UpdateVLogSites().
+  get_vmodule_info().clear();
+  for (const auto& it : glob_levels) {
+    const absl::string_view glob = it.first;
+    const int level = it.second;
+    AppendVModuleLocked(glob, level);
+  }
+  UpdateVLogSites();
+}
+
+int UpdateGlobalVLogLevel(int v)
+    ABSL_LOCKS_EXCLUDED(mutex, GetUpdateSitesMutex()) {
+  mutex.lock();  // unlocked by UpdateVLogSites().
+  const int old_global_v = global_v;
+  if (v == global_v) {
+    mutex.unlock();
+    return old_global_v;
+  }
+  global_v = v;
+  UpdateVLogSites();
+  return old_global_v;
+}
+
+int PrependVModule(absl::string_view module_pattern, int log_level)
+    ABSL_LOCKS_EXCLUDED(mutex, GetUpdateSitesMutex()) {
+  mutex.lock();  // unlocked by UpdateVLogSites().
+  int old_v = PrependVModuleLocked(module_pattern, log_level);
+  UpdateVLogSites();
+  return old_v;
+}
+
+void OnVLogVerbosityUpdate(std::function<void()> cb)
+    ABSL_LOCKS_EXCLUDED(GetUpdateSitesMutex()) {
+  absl::MutexLock ul(GetUpdateSitesMutex());
+  if (!update_callbacks)
+    update_callbacks = new std::vector<std::function<void()>>;
+  update_callbacks->push_back(std::move(cb));
+}
+
+VLogSite* SetVModuleListHeadForTestOnly(VLogSite* v) {
+  return site_list_head.exchange(v, std::memory_order_seq_cst);
+}
+
+}  // namespace log_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/log/internal/vlog_config.h b/absl/log/internal/vlog_config.h
new file mode 100644
index 00000000000..f65eb5d9fd5
--- /dev/null
+++ b/absl/log/internal/vlog_config.h
@@ -0,0 +1,164 @@
+// Copyright 2022 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// vlog_config.h
+// -----------------------------------------------------------------------------
+//
+// This header file defines `VLogSite`, a public primitive that represents
+// a callsite for the `VLOG` family of macros and related libraries.
+// It also declares and defines multiple internal utilities used to implement
+// `VLOG`, such as `VLogSiteManager`.
+
+#ifndef ABSL_LOG_INTERNAL_VLOG_CONFIG_H_
+#define ABSL_LOG_INTERNAL_VLOG_CONFIG_H_
+
+// IWYU pragma: private, include "absl/log/log.h"
+
+#include <atomic>
+#include <cstdint>
+#include <functional>
+#include <limits>
+#include <type_traits>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/nullability.h"
+#include "absl/base/optimization.h"
+#include "absl/base/thread_annotations.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace log_internal {
+
+class SyntheticBinary;
+class VLogSite;
+
+int RegisterAndInitialize(VLogSite* absl_nonnull v);
+void UpdateVLogSites();
+
+// Represents a unique callsite for a `VLOG()` or `VLOG_IS_ON()` call.
+//
+// Libraries that provide `VLOG`-like functionality should use this to
+// efficiently handle --vmodule.
+//
+// VLogSite objects must not be destroyed until the program exits. Doing so will
+// probably yield nasty segfaults in VLogSiteManager::UpdateLogSites(). The
+// recommendation is to make all such objects function-local statics.
+class VLogSite final {
+ public:
+  // `f` must not be destroyed until the program exits.
+  explicit constexpr VLogSite(const char* absl_nonnull f)
+      : file_(f), v_(kUninitialized), next_(nullptr) {}
+  VLogSite(const VLogSite&) = delete;
+  VLogSite& operator=(const VLogSite&) = delete;
+
+  // Inlining the function yields a ~3x performance improvement at the cost of a
+  // 1.5x code size increase at the call site.
+  // Takes locks but does not allocate memory.
+  ABSL_ATTRIBUTE_ALWAYS_INLINE
+  bool IsEnabled(int level) {
+    int stale_v = v_.load(std::memory_order_relaxed);
+    if (ABSL_PREDICT_TRUE(level > stale_v)) {
+      return false;
+    }
+
+    // We put everything other than the fast path, i.e. vlogging is initialized
+    // but not on, behind an out-of-line function to reduce code size.
+    // "level" is almost always a call-site constant, so we can save a bit
+    // of code space by special-casing for a few common levels.
+#if ABSL_HAVE_BUILTIN(__builtin_constant_p) || defined(__GNUC__)
+    if (__builtin_constant_p(level)) {
+      if (level == 0) return SlowIsEnabled0(stale_v);
+      if (level == 1) return SlowIsEnabled1(stale_v);
+      if (level == 2) return SlowIsEnabled2(stale_v);
+      if (level == 3) return SlowIsEnabled3(stale_v);
+      if (level == 4) return SlowIsEnabled4(stale_v);
+      if (level == 5) return SlowIsEnabled5(stale_v);
+    }
+#endif
+    return SlowIsEnabled(stale_v, level);
+  }
+
+ private:
+  friend int log_internal::RegisterAndInitialize(VLogSite* absl_nonnull v);
+  friend void log_internal::UpdateVLogSites();
+  friend class log_internal::SyntheticBinary;
+  static constexpr int kUninitialized = (std::numeric_limits<int>::max)();
+
+  // SlowIsEnabled performs slower checks to determine whether a log site is
+  // enabled. Because it is expected to be called somewhat rarely
+  // (comparatively), it is not inlined to save on code size.
+  //
+  // Prerequisites to calling SlowIsEnabled:
+  //   1) stale_v is uninitialized OR
+  //   2) stale_v is initialized and >= level (meaning we must log).
+  // Takes locks but does not allocate memory.
+  ABSL_ATTRIBUTE_NOINLINE
+  bool SlowIsEnabled(int stale_v, int level);
+  ABSL_ATTRIBUTE_NOINLINE bool SlowIsEnabled0(int stale_v);
+  ABSL_ATTRIBUTE_NOINLINE bool SlowIsEnabled1(int stale_v);
+  ABSL_ATTRIBUTE_NOINLINE bool SlowIsEnabled2(int stale_v);
+  ABSL_ATTRIBUTE_NOINLINE bool SlowIsEnabled3(int stale_v);
+  ABSL_ATTRIBUTE_NOINLINE bool SlowIsEnabled4(int stale_v);
+  ABSL_ATTRIBUTE_NOINLINE bool SlowIsEnabled5(int stale_v);
+
+  // This object is too size-sensitive to use absl::string_view.
+  const char* absl_nonnull const file_;
+  std::atomic<int> v_;
+  std::atomic<VLogSite*> next_;
+};
+static_assert(std::is_trivially_destructible_v<VLogSite>,
+              "VLogSite must be trivially destructible");
+
+// Returns the current verbose log level of `file`.
+// Does not allocate memory.
+int VLogLevel(absl::string_view file);
+
+// Registers a site `v` to get updated as `vmodule` and `v` change.  Also
+// initializes the site based on their current values, and returns that result.
+// Does not allocate memory.
+int RegisterAndInitialize(VLogSite* absl_nonnull v);
+
+// Allocates memory.
+void UpdateVLogSites();
+
+// Completely overwrites the saved value of `vmodule`.
+// Allocates memory.
+void UpdateVModule(absl::string_view vmodule);
+
+// Updates the global verbosity level to `v` and returns the prior value.
+// Allocates memory.
+int UpdateGlobalVLogLevel(int v);
+
+// Atomically prepends `module_pattern=log_level` to the start of vmodule.
+// Returns the prior value for `module_pattern` if there was an exact match and
+// `global_v` otherwise.
+// Allocates memory.
+int PrependVModule(absl::string_view module_pattern, int log_level);
+
+// Registers `on_update` to be called whenever `v` or `vmodule` change.
+// Allocates memory.
+void OnVLogVerbosityUpdate(std::function<void()> cb);
+
+// Does not allocate memory.
+VLogSite* absl_nullable SetVModuleListHeadForTestOnly(
+    VLogSite* absl_nullable v);
+
+}  // namespace log_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_LOG_INTERNAL_VLOG_CONFIG_H_
diff --git a/absl/log/internal/vlog_config_benchmark.cc b/absl/log/internal/vlog_config_benchmark.cc
new file mode 100644
index 00000000000..1916e2c11d8
--- /dev/null
+++ b/absl/log/internal/vlog_config_benchmark.cc
@@ -0,0 +1,186 @@
+// Copyright 2022 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <algorithm>
+#include <atomic>
+#include <cstddef>
+#include <cstring>
+#include <memory>
+#include <new>
+#include <random>
+#include <string>
+#include <type_traits>
+#include <utility>
+#include <vector>
+
+#include "absl/base/config.h"
+#include "absl/container/internal/layout.h"
+#include "absl/log/internal/vlog_config.h"
+#include "absl/random/distributions.h"
+#include "absl/strings/str_cat.h"
+#include "benchmark/benchmark.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace log_internal {
+// Performance of `UpdateLogSites` depends upon the number and organization of
+// `VLogSite`s in the program.  We can synthesize some on the heap to mimic
+// their layout and linkage in static data.
+class SyntheticBinary {
+ public:
+  explicit SyntheticBinary(const size_t num_tus,
+                           const size_t max_extra_static_data_bytes_per_tu,
+                           const size_t max_sites_per_tu,
+                           const int num_shuffles) {
+    per_tu_data_.reserve(num_tus);
+    auto sites = std::make_unique<VLogSite*[]>(num_tus * max_sites_per_tu);
+    for (size_t i = 0; i < num_tus; i++) {
+      const std::string filename =
+          absl::StrCat("directory-", i / 100, "/subdirectory-", i % 100 / 10,
+                       "/file-", i % 10, ".cc");
+      container_internal::Layout<char, VLogSite, char> layout(
+          filename.size() + 1,
+          absl::LogUniform<size_t>(bitgen_, 1, max_sites_per_tu),
+          absl::LogUniform<size_t>(bitgen_, 0,
+                                   max_extra_static_data_bytes_per_tu));
+      auto buf = std::make_unique<char[]>(layout.AllocSize());
+      layout.PoisonPadding(buf.get());
+      memcpy(layout.Pointer<0>(buf.get()), filename.c_str(),
+             filename.size() + 1);
+      for (VLogSite &site : layout.Slice<1>(buf.get())) {
+        sites[num_sites_++] =
+            new (&site) VLogSite(layout.Pointer<0>(buf.get()));
+        // The last one is a dangling pointer but will be fixed below.
+        site.next_.store(&site + 1, std::memory_order_seq_cst);
+      }
+      num_extra_static_data_bytes_ += layout.Size<2>();
+      per_tu_data_.push_back(PerTU{layout, std::move(buf)});
+    }
+    // Now link the files together back-to-front into a circular list.
+    for (size_t i = 0; i < num_tus; i++) {
+      auto &tu = per_tu_data_[i];
+      auto &next_tu = per_tu_data_[(i + 1) % num_tus];
+      tu.layout.Slice<1>(tu.buf.get())
+          .back()
+          .next_.store(next_tu.layout.Pointer<1>(next_tu.buf.get()),
+                       std::memory_order_seq_cst);
+    }
+    // Now do some shufflin'.
+    auto new_sites = std::make_unique<VLogSite*[]>(num_sites_);
+    for (int shuffle_num = 0; shuffle_num < num_shuffles; shuffle_num++) {
+      // Each shuffle cuts the ring into three pieces and rearranges them.
+      const size_t cut_a = absl::Uniform(bitgen_, size_t{0}, num_sites_);
+      const size_t cut_b = absl::Uniform(bitgen_, size_t{0}, num_sites_);
+      const size_t cut_c = absl::Uniform(bitgen_, size_t{0}, num_sites_);
+      if (cut_a == cut_b || cut_b == cut_c || cut_a == cut_c) continue;
+      // The same cuts, sorted:
+      const size_t cut_1 = std::min({cut_a, cut_b, cut_c});
+      const size_t cut_3 = std::max({cut_a, cut_b, cut_c});
+      const size_t cut_2 = cut_a ^ cut_b ^ cut_c ^ cut_1 ^ cut_3;
+      VLogSite *const tmp = sites[cut_1]->next_.load(std::memory_order_seq_cst);
+      sites[cut_1]->next_.store(
+          sites[cut_2]->next_.load(std::memory_order_seq_cst),
+          std::memory_order_seq_cst);
+      sites[cut_2]->next_.store(
+          sites[cut_3]->next_.load(std::memory_order_seq_cst),
+          std::memory_order_seq_cst);
+      sites[cut_3]->next_.store(tmp, std::memory_order_seq_cst);
+      memcpy(&new_sites[0], &sites[0], sizeof(VLogSite *) * (cut_1 + 1));
+      memcpy(&new_sites[cut_1 + 1], &sites[cut_2 + 1],
+             sizeof(VLogSite *) * (cut_3 - cut_2));
+      memcpy(&new_sites[cut_1 + 1 + cut_3 - cut_2], &sites[cut_1 + 1],
+             sizeof(VLogSite *) * (cut_2 - cut_1));
+      memcpy(&new_sites[cut_3 + 1], &sites[cut_3 + 1],
+             sizeof(VLogSite *) * (num_sites_ - cut_3 - 1));
+      sites.swap(new_sites);
+    }
+    const char *last_file = nullptr;
+    for (size_t i = 0; i < num_sites_; i++) {
+      if (sites[i]->file_ == last_file) continue;
+      last_file = sites[i]->file_;
+      num_new_files_++;
+    }
+    absl::log_internal::SetVModuleListHeadForTestOnly(sites[0]);
+    sites[num_tus - 1]->next_.store(nullptr, std::memory_order_seq_cst);
+  }
+  ~SyntheticBinary() {
+    static_assert(std::is_trivially_destructible_v<VLogSite>);
+    absl::log_internal::SetVModuleListHeadForTestOnly(nullptr);
+  }
+
+  size_t num_sites() const { return num_sites_; }
+  size_t num_new_files() const { return num_new_files_; }
+  size_t num_extra_static_data_bytes() const {
+    return num_extra_static_data_bytes_;
+  }
+
+ private:
+  struct PerTU {
+    container_internal::Layout<char, VLogSite, char> layout;
+    std::unique_ptr<char[]> buf;
+  };
+
+  std::mt19937 bitgen_;
+  std::vector<PerTU> per_tu_data_;
+  size_t num_sites_ = 0;
+  size_t num_new_files_ = 0;
+  size_t num_extra_static_data_bytes_ = 0;
+};
+
+namespace {
+void BM_UpdateVModuleEmpty(benchmark::State& state) {
+  SyntheticBinary bin(static_cast<size_t>(state.range(0)), 10 * 1024 * 1024,
+                      256, state.range(1));
+  for (auto s : state) {
+    absl::log_internal::UpdateVModule("");
+  }
+  state.SetItemsProcessed(static_cast<int>(bin.num_new_files()));
+}
+BENCHMARK(BM_UpdateVModuleEmpty)
+    ->ArgPair(100, 200)
+    ->ArgPair(1000, 2000)
+    ->ArgPair(10000, 20000);
+
+void BM_UpdateVModuleShort(benchmark::State& state) {
+  SyntheticBinary bin(static_cast<size_t>(state.range(0)), 10 * 1024 * 1024,
+                      256, state.range(1));
+  for (auto s : state) {
+    absl::log_internal::UpdateVModule("directory2/*=4");
+  }
+  state.SetItemsProcessed(static_cast<int>(bin.num_new_files()));
+}
+BENCHMARK(BM_UpdateVModuleShort)
+    ->ArgPair(100, 200)
+    ->ArgPair(1000, 2000)
+    ->ArgPair(10000, 20000);
+
+void BM_UpdateVModuleLong(benchmark::State& state) {
+  SyntheticBinary bin(static_cast<size_t>(state.range(0)), 10 * 1024 * 1024,
+                      256, state.range(1));
+  for (auto s : state) {
+    absl::log_internal::UpdateVModule(
+        "d?r?c?o?y2/*=4,d?*r?*c?**o?*y1/*=2,d?*rc?**o?*y3/*=2,,"
+        "d?*r?*c?**o?*1/*=1,d?*r?**o?*y1/*=2,d?*r???***y1/*=7,"
+        "d?*r?**o?*y1/aaa=6");
+  }
+  state.SetItemsProcessed(static_cast<int>(bin.num_new_files()));
+}
+BENCHMARK(BM_UpdateVModuleLong)
+    ->ArgPair(100, 200)
+    ->ArgPair(1000, 2000)
+    ->ArgPair(10000, 20000);
+}  // namespace
+}  // namespace log_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/log/internal/voidify.h b/absl/log/internal/voidify.h
new file mode 100644
index 00000000000..f42859eba14
--- /dev/null
+++ b/absl/log/internal/voidify.h
@@ -0,0 +1,50 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: log/internal/voidify.h
+// -----------------------------------------------------------------------------
+//
+// This class does the dispatching of the completed `absl::LogEntry` to
+// applicable `absl::LogSink`s, and is used to explicitly ignore values in the
+// conditional logging macros. This avoids compiler warnings like "value
+// computed is not used" and "statement has no effect".
+
+#ifndef ABSL_LOG_INTERNAL_VOIDIFY_H_
+#define ABSL_LOG_INTERNAL_VOIDIFY_H_
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace log_internal {
+
+class Voidify final {
+ public:
+  // This has to be an operator with a precedence lower than << but higher than
+  // ?:
+  template <typename T>
+  ABSL_ATTRIBUTE_COLD void operator&&(T&& message) const&& {
+    // The dispatching of the completed `absl::LogEntry` to applicable
+    // `absl::LogSink`s happens here.
+    message.Flush();
+  }
+};
+
+}  // namespace log_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_LOG_INTERNAL_VOIDIFY_H_
diff --git a/absl/log/log.h b/absl/log/log.h
new file mode 100644
index 00000000000..371c32ff80e
--- /dev/null
+++ b/absl/log/log.h
@@ -0,0 +1,378 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: log/log.h
+// -----------------------------------------------------------------------------
+//
+// This header declares a family of LOG macros.
+//
+// Basic invocation looks like this:
+//
+//   LOG(INFO) << "Found " << num_cookies << " cookies";
+//
+// Most `LOG` macros take a severity level argument.  The severity levels are
+// `INFO`, `WARNING`, `ERROR`, and `FATAL`.  They are defined
+// in absl/base/log_severity.h.
+// * The `FATAL` severity level terminates the program with a stack trace after
+//   logging its message.  Error handlers registered with `RunOnFailure`
+//   (process_state.h) are run, but exit handlers registered with `atexit(3)`
+//   are not.
+// * The `QFATAL` pseudo-severity level is equivalent to `FATAL` but triggers
+//   quieter termination messages, e.g. without a full stack trace, and skips
+//   running registered error handlers.
+// * The `DFATAL` pseudo-severity level is defined as `FATAL` in debug mode and
+//   as `ERROR` otherwise.
+// * The `DO_NOT_SUBMIT` pseudo-severity level is an alias for `ERROR`, and is
+//   intended for debugging statements that won't be submitted.  The name is
+//   chosen to be easy to spot in review and with tools in order to ensure that
+//   such statements aren't inadvertently checked in.
+//   The contract is that **it may not be checked in**, meaning that no
+//   in-contract uses will be affected if we decide in the future to remove it
+//   or change what it does.
+// Some preprocessor shenanigans are used to ensure that e.g. `LOG(INFO)` has
+// the same meaning even if a local symbol or preprocessor macro named `INFO` is
+// defined.  To specify a severity level using an expression instead of a
+// literal, use `LEVEL(expr)`.
+// Example:
+//
+//   LOG(LEVEL(stale ? absl::LogSeverity::kWarning : absl::LogSeverity::kInfo))
+//       << "Cookies are " << days << " days old";
+
+// `LOG` macros evaluate to an unterminated statement.  The value at the end of
+// the statement supports some chainable methods:
+//
+//   * .AtLocation(absl::string_view file, int line)
+//     .AtLocation(absl::SourceLocation loc)
+//     Overrides the location inferred from the callsite.  The string pointed to
+//     by `file` must be valid until the end of the statement.
+//   * .NoPrefix()
+//     Omits the prefix from this line.  The prefix includes metadata about the
+//     logged data such as source code location and timestamp.
+//   * .WithVerbosity(int verbose_level)
+//     Sets the verbosity field of the logged message as if it was logged by
+//     `VLOG(verbose_level)`.  Unlike `VLOG`, this method does not affect
+//     evaluation of the statement when the specified `verbose_level` has been
+//     disabled.  The only effect is on `LogSink` implementations which make use
+//     of the `absl::LogSink::verbosity()` value.  The value
+//     `absl::LogEntry::kNoVerbosityLevel` can be specified to mark the message
+//     not verbose.
+//   * .WithTimestamp(absl::Time timestamp)
+//     Uses the specified timestamp instead of one collected at the time of
+//     execution.
+//   * .WithThreadID(absl::LogEntry::tid_t tid)
+//     Uses the specified thread ID instead of one collected at the time of
+//     execution.
+//   * .WithMetadataFrom(const absl::LogEntry &entry)
+//     Copies all metadata (but no data) from the specified `absl::LogEntry`.
+//     This can be used to change the severity of a message, but it has some
+//     limitations:
+//     * `ABSL_MIN_LOG_LEVEL` is evaluated against the severity passed into
+//       `LOG` (or the implicit `FATAL` level of `CHECK`).
+//     * `LOG(FATAL)` and `CHECK` terminate the process unconditionally, even if
+//       the severity is changed later.
+//     `.WithMetadataFrom(entry)` should almost always be used in combination
+//     with `LOG(LEVEL(entry.log_severity()))`.
+//   * .WithPerror()
+//     Appends to the logged message a colon, a space, a textual description of
+//     the current value of `errno` (as by `strerror(3)`), and the numerical
+//     value of `errno`.
+//   * .ToSinkAlso(absl::LogSink* sink)
+//     Sends this message to `*sink` in addition to whatever other sinks it
+//     would otherwise have been sent to.  `sink` must not be null.
+//   * .ToSinkOnly(absl::LogSink* sink)
+//     Sends this message to `*sink` and no others.  `sink` must not be null.
+//
+// No interfaces in this header are async-signal-safe; their use in signal
+// handlers is unsupported and may deadlock your program or eat your lunch.
+//
+// Many logging statements are inherently conditional.  For example,
+// `LOG_IF(INFO, !foo)` does nothing if `foo` is true.  Even seemingly
+// unconditional statements like `LOG(INFO)` might be disabled at
+// compile-time to minimize binary size or for security reasons.
+//
+// * Except for the condition in a `CHECK` or `QCHECK` statement, programs must
+//   not rely on evaluation of expressions anywhere in logging statements for
+//   correctness.  For example, this is ok:
+//
+//     CHECK((fp = fopen("config.ini", "r")) != nullptr);
+//
+//   But this is probably not ok:
+//
+//     LOG(INFO) << "Server status: " << StartServerAndReturnStatusString();
+//
+//   The example below is bad too; the `i++` in the `LOG_IF` condition might
+//   not be evaluated, resulting in an infinite loop:
+//
+//     for (int i = 0; i < 1000000;)
+//       LOG_IF(INFO, i++ % 1000 == 0) << "Still working...";
+//
+// * Except where otherwise noted, conditions which cause a statement not to log
+//   also cause expressions not to be evaluated.  Programs may rely on this for
+//   performance reasons, e.g. by streaming the result of an expensive function
+//   call into a `DLOG` or `LOG_EVERY_N` statement.
+// * Care has been taken to ensure that expressions are parsed by the compiler
+//   even if they are never evaluated.  This means that syntax errors will be
+//   caught and variables will be considered used for the purposes of
+//   unused-variable diagnostics.  For example, this statement won't compile
+//   even if `INFO`-level logging has been compiled out:
+//
+//     int number_of_cakes = 40;
+//     LOG(INFO) << "Number of cakes: " << number_of_cake;  // Note the typo!
+//
+//   Similarly, this won't produce unused-variable compiler diagnostics even
+//   if `INFO`-level logging is compiled out:
+//
+//     {
+//       char fox_line1[] = "Hatee-hatee-hatee-ho!";
+//       LOG_IF(ERROR, false) << "The fox says " << fox_line1;
+//       char fox_line2[] = "A-oo-oo-oo-ooo!";
+//       LOG(INFO) << "The fox also says " << fox_line2;
+//     }
+//
+//   This error-checking is not perfect; for example, symbols that have been
+//   declared but not defined may not produce link errors if used in logging
+//   statements that compile away.
+//
+// Expressions streamed into these macros are formatted using `operator<<` just
+// as they would be if streamed into a `std::ostream`, however it should be
+// noted that their actual type is unspecified.
+//
+// To implement a custom formatting operator for a type you own, there are two
+// options: `AbslStringify()` or `std::ostream& operator<<(std::ostream&, ...)`.
+// It is recommended that users make their types loggable through
+// `AbslStringify()` as it is a universal stringification extension that also
+// enables `absl::StrFormat` and `absl::StrCat` support. If both
+// `AbslStringify()` and `std::ostream& operator<<(std::ostream&, ...)` are
+// defined, `AbslStringify()` will be used.
+//
+// To use the `AbslStringify()` API, define a friend function template in your
+// type's namespace with the following signature:
+//
+//   template <typename Sink>
+//   void AbslStringify(Sink& sink, const UserDefinedType& value);
+//
+// `Sink` has the same interface as `absl::FormatSink`, but without
+// `PutPaddedString()`.
+//
+// Example:
+//
+//   struct Point {
+//     template <typename Sink>
+//     friend void AbslStringify(Sink& sink, const Point& p) {
+//       absl::Format(&sink, "(%v, %v)", p.x, p.y);
+//     }
+//
+//     int x;
+//     int y;
+//   };
+//
+// To use `std::ostream& operator<<(std::ostream&, ...)`, define
+// `std::ostream& operator<<(std::ostream&, ...)` in your type's namespace (for
+// ADL) just as you would to stream it to `std::cout`.
+//
+// Currently `AbslStringify()` ignores output manipulators but this is not
+// guaranteed behavior and may be subject to change in the future. If you would
+// like guaranteed behavior regarding output manipulators, please use
+// `std::ostream& operator<<(std::ostream&, ...)` to make custom types loggable
+// instead.
+//
+// Those macros that support streaming honor output manipulators and `fmtflag`
+// changes that output data (e.g. `std::ends`) or control formatting of data
+// (e.g. `std::hex` and `std::fixed`), however flushing such a stream is
+// ignored.  The message produced by a log statement is sent to registered
+// `absl::LogSink` instances at the end of the statement; those sinks are
+// responsible for their own flushing (e.g. to disk) semantics.
+//
+// Flag settings are not carried over from one `LOG` statement to the next; this
+// is a bit different than e.g. `std::cout`:
+//
+//   LOG(INFO) << std::hex << 0xdeadbeef;  // logs "0xdeadbeef"
+//   LOG(INFO) << 0xdeadbeef;              // logs "3735928559"
+
+// SKIP_ABSL_INLINE_NAMESPACE_CHECK
+
+#ifndef ABSL_LOG_LOG_H_
+#define ABSL_LOG_LOG_H_
+
+#include "absl/log/internal/log_impl.h"
+
+// LOG()
+//
+// `LOG` takes a single argument which is a severity level.  Data streamed in
+// comprise the logged message.
+// Example:
+//
+//   LOG(INFO) << "Found " << num_cookies << " cookies";
+#define LOG(severity) ABSL_LOG_INTERNAL_LOG_IMPL(_##severity)
+
+// PLOG()
+//
+// `PLOG` behaves like `LOG` except that a description of the current state of
+// `errno` is appended to the streamed message.
+#define PLOG(severity) ABSL_LOG_INTERNAL_PLOG_IMPL(_##severity)
+
+// DLOG()
+//
+// `DLOG` behaves like `LOG` in debug mode (i.e. `#ifndef NDEBUG`).  Otherwise
+// it compiles away and does nothing.  Note that `DLOG(FATAL)` does not
+// terminate the program if `NDEBUG` is defined.
+#define DLOG(severity) ABSL_LOG_INTERNAL_DLOG_IMPL(_##severity)
+
+// `VLOG` uses numeric levels to provide verbose logging that can configured at
+// runtime, including at a per-module level.  `VLOG` statements are logged at
+// `INFO` severity if they are logged at all; the numeric levels are on a
+// different scale than the proper severity levels.  Positive levels are
+// disabled by default.  Negative levels should not be used.
+// Example:
+//
+//   VLOG(1) << "I print when you run the program with --v=1 or higher";
+//   VLOG(2) << "I print when you run the program with --v=2 or higher";
+//
+// See vlog_is_on.h for further documentation, including the usage of the
+// --vmodule flag to log at different levels in different source files.
+//
+// `VLOG` does not produce any output when verbose logging is not enabled.
+// However, simply testing whether verbose logging is enabled can be expensive.
+// If you don't intend to enable verbose logging in non-debug builds, consider
+// using `DVLOG` instead.
+#define VLOG(verbose_level) ABSL_LOG_INTERNAL_VLOG_IMPL(verbose_level)
+
+// `DVLOG` behaves like `VLOG` in debug mode (i.e. `#ifndef NDEBUG`).
+// Otherwise, it compiles away and does nothing.
+#define DVLOG(verbose_level) ABSL_LOG_INTERNAL_DVLOG_IMPL(verbose_level)
+
+// `LOG_IF` and friends add a second argument which specifies a condition.  If
+// the condition is false, nothing is logged.
+// Example:
+//
+//   LOG_IF(INFO, num_cookies > 10) << "Got lots of cookies";
+//
+// There is no `VLOG_IF` because the order of evaluation of the arguments is
+// ambiguous and the alternate spelling with an `if`-statement is trivial.
+#define LOG_IF(severity, condition) \
+  ABSL_LOG_INTERNAL_LOG_IF_IMPL(_##severity, condition)
+#define PLOG_IF(severity, condition) \
+  ABSL_LOG_INTERNAL_PLOG_IF_IMPL(_##severity, condition)
+#define DLOG_IF(severity, condition) \
+  ABSL_LOG_INTERNAL_DLOG_IF_IMPL(_##severity, condition)
+
+// LOG_EVERY_N
+// LOG_FIRST_N
+// LOG_EVERY_POW_2
+// LOG_EVERY_N_SEC
+//
+// These "stateful" macros log conditionally based on a hidden counter or timer.
+// When the condition is false and no logging is done, streamed operands aren't
+// evaluated either.  Each instance has its own state (i.e. counter, timer)
+// that's independent of other instances of the macros.  The macros in this
+// family are thread-safe in the sense that they are meant to be called
+// concurrently and will not invoke undefined behavior, however their
+// implementation prioritizes efficiency over exactness and may occasionally log
+// more or less often than specified.
+//
+//   * `LOG_EVERY_N` logs the first time and once every `n` times thereafter.
+//   * `LOG_FIRST_N` logs the first `n` times and then stops.
+//   * `LOG_EVERY_POW_2` logs the first, second, fourth, eighth, etc. times.
+//   * `LOG_EVERY_N_SEC` logs the first time and no more than once every `n`
+//     seconds thereafter.  `n` is passed as a floating point value.
+//
+// The `LOG_IF`... variations with an extra condition evaluate the specified
+// condition first and short-circuit if it is false.  For example, an evaluation
+// of `LOG_IF_FIRST_N` does not count against the first `n` if the specified
+// condition is false.  Stateful `VLOG`... variations likewise short-circuit
+// if `VLOG` is disabled.
+//
+// An approximate count of the number of times a particular instance's stateful
+// condition has been evaluated (i.e. excluding those where a specified `LOG_IF`
+// condition was false) can be included in the logged message by streaming the
+// symbol `COUNTER`.
+//
+// The `n` parameter need not be a constant.  Conditional logging following a
+// change to `n` isn't fully specified, but it should converge on the new value
+// within roughly `max(old_n, new_n)` evaluations or seconds.
+//
+// Examples:
+//
+//   LOG_EVERY_N(WARNING, 1000) << "Got a packet with a bad CRC (" << COUNTER
+//                              << " total)";
+//
+//   LOG_EVERY_N_SEC(INFO, 2.5) << "Got " << COUNTER << " cookies so far";
+//
+//   LOG_IF_EVERY_N(INFO, (size > 1024), 10) << "Got the " << COUNTER
+//                                           << "th big cookie";
+#define LOG_EVERY_N(severity, n) \
+  ABSL_LOG_INTERNAL_LOG_EVERY_N_IMPL(_##severity, n)
+#define LOG_FIRST_N(severity, n) \
+  ABSL_LOG_INTERNAL_LOG_FIRST_N_IMPL(_##severity, n)
+#define LOG_EVERY_POW_2(severity) \
+  ABSL_LOG_INTERNAL_LOG_EVERY_POW_2_IMPL(_##severity)
+#define LOG_EVERY_N_SEC(severity, n_seconds) \
+  ABSL_LOG_INTERNAL_LOG_EVERY_N_SEC_IMPL(_##severity, n_seconds)
+
+#define PLOG_EVERY_N(severity, n) \
+  ABSL_LOG_INTERNAL_PLOG_EVERY_N_IMPL(_##severity, n)
+#define PLOG_FIRST_N(severity, n) \
+  ABSL_LOG_INTERNAL_PLOG_FIRST_N_IMPL(_##severity, n)
+#define PLOG_EVERY_POW_2(severity) \
+  ABSL_LOG_INTERNAL_PLOG_EVERY_POW_2_IMPL(_##severity)
+#define PLOG_EVERY_N_SEC(severity, n_seconds) \
+  ABSL_LOG_INTERNAL_PLOG_EVERY_N_SEC_IMPL(_##severity, n_seconds)
+
+#define DLOG_EVERY_N(severity, n) \
+  ABSL_LOG_INTERNAL_DLOG_EVERY_N_IMPL(_##severity, n)
+#define DLOG_FIRST_N(severity, n) \
+  ABSL_LOG_INTERNAL_DLOG_FIRST_N_IMPL(_##severity, n)
+#define DLOG_EVERY_POW_2(severity) \
+  ABSL_LOG_INTERNAL_DLOG_EVERY_POW_2_IMPL(_##severity)
+#define DLOG_EVERY_N_SEC(severity, n_seconds) \
+  ABSL_LOG_INTERNAL_DLOG_EVERY_N_SEC_IMPL(_##severity, n_seconds)
+
+#define VLOG_EVERY_N(severity, n) \
+  ABSL_LOG_INTERNAL_VLOG_EVERY_N_IMPL(severity, n)
+#define VLOG_FIRST_N(severity, n) \
+  ABSL_LOG_INTERNAL_VLOG_FIRST_N_IMPL(severity, n)
+#define VLOG_EVERY_POW_2(severity) \
+  ABSL_LOG_INTERNAL_VLOG_EVERY_POW_2_IMPL(severity)
+#define VLOG_EVERY_N_SEC(severity, n_seconds) \
+  ABSL_LOG_INTERNAL_VLOG_EVERY_N_SEC_IMPL(severity, n_seconds)
+
+#define LOG_IF_EVERY_N(severity, condition, n) \
+  ABSL_LOG_INTERNAL_LOG_IF_EVERY_N_IMPL(_##severity, condition, n)
+#define LOG_IF_FIRST_N(severity, condition, n) \
+  ABSL_LOG_INTERNAL_LOG_IF_FIRST_N_IMPL(_##severity, condition, n)
+#define LOG_IF_EVERY_POW_2(severity, condition) \
+  ABSL_LOG_INTERNAL_LOG_IF_EVERY_POW_2_IMPL(_##severity, condition)
+#define LOG_IF_EVERY_N_SEC(severity, condition, n_seconds) \
+  ABSL_LOG_INTERNAL_LOG_IF_EVERY_N_SEC_IMPL(_##severity, condition, n_seconds)
+
+#define PLOG_IF_EVERY_N(severity, condition, n) \
+  ABSL_LOG_INTERNAL_PLOG_IF_EVERY_N_IMPL(_##severity, condition, n)
+#define PLOG_IF_FIRST_N(severity, condition, n) \
+  ABSL_LOG_INTERNAL_PLOG_IF_FIRST_N_IMPL(_##severity, condition, n)
+#define PLOG_IF_EVERY_POW_2(severity, condition) \
+  ABSL_LOG_INTERNAL_PLOG_IF_EVERY_POW_2_IMPL(_##severity, condition)
+#define PLOG_IF_EVERY_N_SEC(severity, condition, n_seconds) \
+  ABSL_LOG_INTERNAL_PLOG_IF_EVERY_N_SEC_IMPL(_##severity, condition, n_seconds)
+
+#define DLOG_IF_EVERY_N(severity, condition, n) \
+  ABSL_LOG_INTERNAL_DLOG_IF_EVERY_N_IMPL(_##severity, condition, n)
+#define DLOG_IF_FIRST_N(severity, condition, n) \
+  ABSL_LOG_INTERNAL_DLOG_IF_FIRST_N_IMPL(_##severity, condition, n)
+#define DLOG_IF_EVERY_POW_2(severity, condition) \
+  ABSL_LOG_INTERNAL_DLOG_IF_EVERY_POW_2_IMPL(_##severity, condition)
+#define DLOG_IF_EVERY_N_SEC(severity, condition, n_seconds) \
+  ABSL_LOG_INTERNAL_DLOG_IF_EVERY_N_SEC_IMPL(_##severity, condition, n_seconds)
+
+#endif  // ABSL_LOG_LOG_H_
diff --git a/absl/base/inline_variable_test_a.cc b/absl/log/log_basic_test.cc
similarity index 62%
rename from absl/base/inline_variable_test_a.cc
rename to absl/log/log_basic_test.cc
index d0b8e7d394b..ef8967af3d4 100644
--- a/absl/base/inline_variable_test_a.cc
+++ b/absl/log/log_basic_test.cc
@@ -1,4 +1,5 @@
-// Copyright 2017 The Abseil Authors.
+//
+// Copyright 2022 The Abseil Authors.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
@@ -12,14 +13,10 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-#include "absl/base/internal/inline_variable_testing.h"
-
-namespace absl {
-namespace inline_variable_testing_internal {
-
-const Foo& get_foo_a() { return inline_variable_foo; }
+#include "absl/log/log.h"
 
-const int& get_int_a() { return inline_variable_int; }
+#define ABSL_TEST_LOG LOG
+#define ABSL_TEST_DLOG DLOG
 
-}  // namespace inline_variable_testing_internal
-}  // namespace absl
+#include "gtest/gtest.h"
+#include "absl/log/log_basic_test_impl.inc"
diff --git a/absl/log/log_basic_test_impl.inc b/absl/log/log_basic_test_impl.inc
new file mode 100644
index 00000000000..3f007dc9cb3
--- /dev/null
+++ b/absl/log/log_basic_test_impl.inc
@@ -0,0 +1,654 @@
+//
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// SKIP_ABSL_INLINE_NAMESPACE_CHECK
+
+// The testcases in this file are expected to pass or be skipped with any value
+// of ABSL_MIN_LOG_LEVEL
+
+#ifndef ABSL_LOG_LOG_BASIC_TEST_IMPL_H_
+#define ABSL_LOG_LOG_BASIC_TEST_IMPL_H_
+
+// Verify that both sets of macros behave identically by parameterizing the
+// entire test file.
+#ifndef ABSL_TEST_LOG
+#error ABSL_TEST_LOG must be defined for these tests to work.
+#endif
+
+#ifndef ABSL_TEST_DLOG
+#error ABSL_TEST_DLOG must be defined for these tests to work.
+#endif
+
+#include <cerrno>
+#include <sstream>
+#include <string>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/internal/sysinfo.h"
+#include "absl/base/log_severity.h"
+#include "absl/log/globals.h"
+#include "absl/log/internal/globals.h"
+#include "absl/log/internal/test_actions.h"
+#include "absl/log/internal/test_helpers.h"
+#include "absl/log/internal/test_matchers.h"
+#include "absl/log/log_entry.h"
+#include "absl/log/scoped_mock_log.h"
+
+namespace absl_log_internal {
+#if GTEST_HAS_DEATH_TEST
+using ::absl::log_internal::DeathTestExpectedLogging;
+using ::absl::log_internal::DeathTestUnexpectedLogging;
+using ::absl::log_internal::DeathTestValidateExpectations;
+using ::absl::log_internal::DiedOfFatal;
+using ::absl::log_internal::DiedOfQFatal;
+#endif
+using ::absl::log_internal::InMatchWindow;
+using ::absl::log_internal::LoggingEnabledAt;
+using ::absl::log_internal::LogSeverity;
+using ::absl::log_internal::Prefix;
+using ::absl::log_internal::SourceBasename;
+using ::absl::log_internal::SourceFilename;
+using ::absl::log_internal::SourceLine;
+using ::absl::log_internal::Stacktrace;
+using ::absl::log_internal::TextMessage;
+using ::absl::log_internal::ThreadID;
+using ::absl::log_internal::Timestamp;
+using ::absl::log_internal::Verbosity;
+using ::testing::AnyNumber;
+using ::testing::Eq;
+using ::testing::IsEmpty;
+using ::testing::IsTrue;
+
+class BasicLogTest : public testing::TestWithParam<absl::LogSeverityAtLeast> {};
+
+std::string ThresholdName(
+    testing::TestParamInfo<absl::LogSeverityAtLeast> severity) {
+  std::stringstream ostr;
+  ostr << severity.param;
+  return ostr.str().substr(
+      severity.param == absl::LogSeverityAtLeast::kInfinity ? 0 : 2);
+}
+
+INSTANTIATE_TEST_SUITE_P(WithParam, BasicLogTest,
+                         testing::Values(absl::LogSeverityAtLeast::kInfo,
+                                         absl::LogSeverityAtLeast::kWarning,
+                                         absl::LogSeverityAtLeast::kError,
+                                         absl::LogSeverityAtLeast::kFatal,
+                                         absl::LogSeverityAtLeast::kInfinity),
+                         ThresholdName);
+
+TEST_P(BasicLogTest, Info) {
+  absl::log_internal::ScopedMinLogLevel scoped_min_log_level(GetParam());
+
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const int log_line = __LINE__ + 1;
+  auto do_log = [] { ABSL_TEST_LOG(INFO) << "hello world"; };
+
+  if (LoggingEnabledAt(absl::LogSeverity::kInfo)) {
+    EXPECT_CALL(
+        test_sink,
+        Send(AllOf(
+            SourceFilename(Eq(__FILE__)),
+            SourceBasename(Eq("log_basic_test_impl.inc")),
+            SourceLine(Eq(log_line)), Prefix(IsTrue()),
+            LogSeverity(Eq(absl::LogSeverity::kInfo)),
+            Timestamp(InMatchWindow()),
+            ThreadID(Eq(absl::base_internal::GetTID())),
+            TextMessage(Eq("hello world")),
+            Verbosity(Eq(absl::LogEntry::kNoVerbosityLevel)),
+            ENCODED_MESSAGE(MatchesEvent(
+                Eq(__FILE__), Eq(log_line), InMatchWindow(),
+                Eq(logging::proto::INFO), Eq(absl::base_internal::GetTID()),
+                ElementsAre(ValueWithLiteral(Eq("hello world"))))),
+            Stacktrace(IsEmpty()))));
+  }
+
+  test_sink.StartCapturingLogs();
+  do_log();
+}
+
+TEST_P(BasicLogTest, Warning) {
+  absl::log_internal::ScopedMinLogLevel scoped_min_log_level(GetParam());
+
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const int log_line = __LINE__ + 1;
+  auto do_log = [] { ABSL_TEST_LOG(WARNING) << "hello world"; };
+
+  if (LoggingEnabledAt(absl::LogSeverity::kWarning)) {
+    EXPECT_CALL(
+        test_sink,
+        Send(AllOf(
+            SourceFilename(Eq(__FILE__)),
+            SourceBasename(Eq("log_basic_test_impl.inc")),
+            SourceLine(Eq(log_line)), Prefix(IsTrue()),
+            LogSeverity(Eq(absl::LogSeverity::kWarning)),
+            Timestamp(InMatchWindow()),
+            ThreadID(Eq(absl::base_internal::GetTID())),
+            TextMessage(Eq("hello world")),
+            Verbosity(Eq(absl::LogEntry::kNoVerbosityLevel)),
+            ENCODED_MESSAGE(MatchesEvent(
+                Eq(__FILE__), Eq(log_line), InMatchWindow(),
+                Eq(logging::proto::WARNING), Eq(absl::base_internal::GetTID()),
+                ElementsAre(ValueWithLiteral(Eq("hello world"))))),
+            Stacktrace(IsEmpty()))));
+  }
+
+  test_sink.StartCapturingLogs();
+  do_log();
+}
+
+TEST_P(BasicLogTest, Error) {
+  absl::log_internal::ScopedMinLogLevel scoped_min_log_level(GetParam());
+
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const int log_line = __LINE__ + 1;
+  auto do_log = [] { ABSL_TEST_LOG(ERROR) << "hello world"; };
+
+  if (LoggingEnabledAt(absl::LogSeverity::kError)) {
+    EXPECT_CALL(
+        test_sink,
+        Send(AllOf(
+            SourceFilename(Eq(__FILE__)),
+            SourceBasename(Eq("log_basic_test_impl.inc")),
+            SourceLine(Eq(log_line)), Prefix(IsTrue()),
+            LogSeverity(Eq(absl::LogSeverity::kError)),
+            Timestamp(InMatchWindow()),
+            ThreadID(Eq(absl::base_internal::GetTID())),
+            TextMessage(Eq("hello world")),
+            Verbosity(Eq(absl::LogEntry::kNoVerbosityLevel)),
+            ENCODED_MESSAGE(MatchesEvent(
+                Eq(__FILE__), Eq(log_line), InMatchWindow(),
+                Eq(logging::proto::ERROR), Eq(absl::base_internal::GetTID()),
+                ElementsAre(ValueWithLiteral(Eq("hello world"))))),
+            Stacktrace(IsEmpty()))));
+  }
+
+  test_sink.StartCapturingLogs();
+  do_log();
+}
+
+TEST_P(BasicLogTest, DoNotSubmit) {
+  absl::log_internal::ScopedMinLogLevel scoped_min_log_level(GetParam());
+
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const int log_line = __LINE__ + 1;
+  auto do_log = [] { ABSL_TEST_LOG(DO_NOT_SUBMIT) << "hello world"; };
+
+  if (LoggingEnabledAt(absl::LogSeverity::kError)) {
+    EXPECT_CALL(
+        test_sink,
+        Send(AllOf(
+            SourceFilename(Eq(__FILE__)),
+            SourceBasename(Eq("log_basic_test_impl.inc")),
+            SourceLine(Eq(log_line)), Prefix(IsTrue()),
+            LogSeverity(Eq(absl::LogSeverity::kError)),
+            Timestamp(InMatchWindow()),
+            ThreadID(Eq(absl::base_internal::GetTID())),
+            TextMessage(Eq("hello world")),
+            Verbosity(Eq(absl::LogEntry::kNoVerbosityLevel)),
+            ENCODED_MESSAGE(MatchesEvent(
+                Eq(__FILE__), Eq(log_line), InMatchWindow(),
+                Eq(logging::proto::ERROR), Eq(absl::base_internal::GetTID()),
+                ElementsAre(ValueWithLiteral(Eq("hello world"))))),
+            Stacktrace(IsEmpty()))));
+  }
+
+  test_sink.StartCapturingLogs();
+  do_log();
+}
+
+#if GTEST_HAS_DEATH_TEST
+using BasicLogDeathTest = BasicLogTest;
+
+INSTANTIATE_TEST_SUITE_P(WithParam, BasicLogDeathTest,
+                         testing::Values(absl::LogSeverityAtLeast::kInfo,
+                                         absl::LogSeverityAtLeast::kFatal,
+                                         absl::LogSeverityAtLeast::kInfinity),
+                         ThresholdName);
+
+TEST_P(BasicLogDeathTest, Fatal) {
+  absl::log_internal::ScopedMinLogLevel scoped_min_log_level(GetParam());
+
+  const int log_line = __LINE__ + 1;
+  auto do_log = [] { ABSL_TEST_LOG(FATAL) << "hello world"; };
+
+  EXPECT_EXIT(
+      {
+        absl::ScopedMockLog test_sink(
+            absl::MockLogDefault::kDisallowUnexpected);
+        EXPECT_CALL(test_sink, Send).Times(0);
+
+        EXPECT_CALL(test_sink, Send)
+            .Times(AnyNumber())
+            .WillRepeatedly(DeathTestUnexpectedLogging());
+
+        ::testing::InSequence s;
+
+        // Note the logic in DeathTestValidateExpectations() caters for the case
+        // of logging being disabled at FATAL level.
+
+        if (LoggingEnabledAt(absl::LogSeverity::kFatal)) {
+          // The first call without the stack trace.
+          EXPECT_CALL(
+              test_sink,
+              Send(AllOf(SourceFilename(Eq(__FILE__)),
+                         SourceBasename(Eq("log_basic_test_impl.inc")),
+                         SourceLine(Eq(log_line)), Prefix(IsTrue()),
+                         LogSeverity(Eq(absl::LogSeverity::kFatal)),
+                         Timestamp(InMatchWindow()),
+                         ThreadID(Eq(absl::base_internal::GetTID())),
+                         TextMessage(Eq("hello world")),
+                         Verbosity(Eq(absl::LogEntry::kNoVerbosityLevel)),
+                         ENCODED_MESSAGE(MatchesEvent(
+                             Eq(__FILE__), Eq(log_line), InMatchWindow(),
+                             Eq(logging::proto::FATAL),
+                             Eq(absl::base_internal::GetTID()),
+                             ElementsAre(ValueWithLiteral(Eq("hello world"))))),
+                         Stacktrace(IsEmpty()))))
+              .WillOnce(DeathTestExpectedLogging());
+
+          // The second call with the stack trace.
+          EXPECT_CALL(
+              test_sink,
+              Send(AllOf(SourceFilename(Eq(__FILE__)),
+                         SourceBasename(Eq("log_basic_test_impl.inc")),
+                         SourceLine(Eq(log_line)), Prefix(IsTrue()),
+                         LogSeverity(Eq(absl::LogSeverity::kFatal)),
+                         Timestamp(InMatchWindow()),
+                         ThreadID(Eq(absl::base_internal::GetTID())),
+                         TextMessage(Eq("hello world")),
+                         Verbosity(Eq(absl::LogEntry::kNoVerbosityLevel)),
+                         ENCODED_MESSAGE(MatchesEvent(
+                             Eq(__FILE__), Eq(log_line), InMatchWindow(),
+                             Eq(logging::proto::FATAL),
+                             Eq(absl::base_internal::GetTID()),
+                             ElementsAre(ValueWithLiteral(Eq("hello world"))))),
+                         Stacktrace(Not(IsEmpty())))))
+              .WillOnce(DeathTestExpectedLogging());
+        }
+
+        test_sink.StartCapturingLogs();
+        do_log();
+      },
+      DiedOfFatal, DeathTestValidateExpectations());
+}
+
+TEST_P(BasicLogDeathTest, QFatal) {
+  absl::log_internal::ScopedMinLogLevel scoped_min_log_level(GetParam());
+
+  const int log_line = __LINE__ + 1;
+  auto do_log = [] { ABSL_TEST_LOG(QFATAL) << "hello world"; };
+
+  EXPECT_EXIT(
+      {
+        absl::ScopedMockLog test_sink(
+            absl::MockLogDefault::kDisallowUnexpected);
+        EXPECT_CALL(test_sink, Send).Times(0);
+
+        EXPECT_CALL(test_sink, Send)
+            .Times(AnyNumber())
+            .WillRepeatedly(DeathTestUnexpectedLogging());
+
+        if (LoggingEnabledAt(absl::LogSeverity::kFatal)) {
+          EXPECT_CALL(
+              test_sink,
+              Send(AllOf(SourceFilename(Eq(__FILE__)),
+                         SourceBasename(Eq("log_basic_test_impl.inc")),
+                         SourceLine(Eq(log_line)), Prefix(IsTrue()),
+                         LogSeverity(Eq(absl::LogSeverity::kFatal)),
+                         Timestamp(InMatchWindow()),
+                         ThreadID(Eq(absl::base_internal::GetTID())),
+                         TextMessage(Eq("hello world")),
+                         Verbosity(Eq(absl::LogEntry::kNoVerbosityLevel)),
+                         ENCODED_MESSAGE(MatchesEvent(
+                             Eq(__FILE__), Eq(log_line), InMatchWindow(),
+                             Eq(logging::proto::FATAL),
+                             Eq(absl::base_internal::GetTID()),
+                             ElementsAre(ValueWithLiteral(Eq("hello world"))))),
+                         Stacktrace(IsEmpty()))))
+              .WillOnce(DeathTestExpectedLogging());
+        }
+
+        test_sink.StartCapturingLogs();
+        do_log();
+      },
+      DiedOfQFatal, DeathTestValidateExpectations());
+}
+#endif
+
+#ifdef NDEBUG
+TEST_P(BasicLogTest, DFatal) {
+  absl::log_internal::ScopedMinLogLevel scoped_min_log_level(GetParam());
+
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const int log_line = __LINE__ + 1;
+  auto do_log = [] { ABSL_TEST_LOG(DFATAL) << "hello world"; };
+
+  if (LoggingEnabledAt(absl::LogSeverity::kError)) {
+    EXPECT_CALL(
+        test_sink,
+        Send(AllOf(
+            SourceFilename(Eq(__FILE__)),
+            SourceBasename(Eq("log_basic_test_impl.inc")),
+            SourceLine(Eq(log_line)), Prefix(IsTrue()),
+            LogSeverity(Eq(absl::LogSeverity::kError)),
+            Timestamp(InMatchWindow()),
+            ThreadID(Eq(absl::base_internal::GetTID())),
+            TextMessage(Eq("hello world")),
+            Verbosity(Eq(absl::LogEntry::kNoVerbosityLevel)),
+            ENCODED_MESSAGE(MatchesEvent(
+                Eq(__FILE__), Eq(log_line), InMatchWindow(),
+                Eq(logging::proto::ERROR), Eq(absl::base_internal::GetTID()),
+                ElementsAre(ValueWithLiteral(Eq("hello world"))))),
+            Stacktrace(IsEmpty()))));
+  }
+
+  test_sink.StartCapturingLogs();
+  do_log();
+}
+
+#elif GTEST_HAS_DEATH_TEST
+TEST_P(BasicLogDeathTest, DFatal) {
+  // TODO(b/242568884): re-enable once bug is fixed.
+  // absl::log_internal::ScopedMinLogLevel scoped_min_log_level(GetParam());
+
+  const int log_line = __LINE__ + 1;
+  auto do_log = [] { ABSL_TEST_LOG(DFATAL) << "hello world"; };
+
+  EXPECT_EXIT(
+      {
+        absl::ScopedMockLog test_sink(
+            absl::MockLogDefault::kDisallowUnexpected);
+        EXPECT_CALL(test_sink, Send).Times(0);
+
+        EXPECT_CALL(test_sink, Send)
+            .Times(AnyNumber())
+            .WillRepeatedly(DeathTestUnexpectedLogging());
+
+        ::testing::InSequence s;
+
+        if (LoggingEnabledAt(absl::LogSeverity::kFatal)) {
+          // The first call without the stack trace.
+          EXPECT_CALL(
+              test_sink,
+              Send(AllOf(SourceFilename(Eq(__FILE__)),
+                         SourceBasename(Eq("log_basic_test_impl.inc")),
+                         SourceLine(Eq(log_line)), Prefix(IsTrue()),
+                         LogSeverity(Eq(absl::LogSeverity::kFatal)),
+                         Timestamp(InMatchWindow()),
+                         ThreadID(Eq(absl::base_internal::GetTID())),
+                         TextMessage(Eq("hello world")),
+                         Verbosity(Eq(absl::LogEntry::kNoVerbosityLevel)),
+                         ENCODED_MESSAGE(MatchesEvent(
+                             Eq(__FILE__), Eq(log_line), InMatchWindow(),
+                             Eq(logging::proto::FATAL),
+                             Eq(absl::base_internal::GetTID()),
+                             ElementsAre(ValueWithLiteral(Eq("hello world"))))),
+                         Stacktrace(IsEmpty()))))
+              .WillOnce(DeathTestExpectedLogging());
+
+          // The second call with the stack trace.
+          EXPECT_CALL(
+              test_sink,
+              Send(AllOf(SourceFilename(Eq(__FILE__)),
+                         SourceBasename(Eq("log_basic_test_impl.inc")),
+                         SourceLine(Eq(log_line)), Prefix(IsTrue()),
+                         LogSeverity(Eq(absl::LogSeverity::kFatal)),
+                         Timestamp(InMatchWindow()),
+                         ThreadID(Eq(absl::base_internal::GetTID())),
+                         TextMessage(Eq("hello world")),
+                         Verbosity(Eq(absl::LogEntry::kNoVerbosityLevel)),
+                         ENCODED_MESSAGE(MatchesEvent(
+                             Eq(__FILE__), Eq(log_line), InMatchWindow(),
+                             Eq(logging::proto::FATAL),
+                             Eq(absl::base_internal::GetTID()),
+                             ElementsAre(ValueWithLiteral(Eq("hello world"))))),
+                         Stacktrace(Not(IsEmpty())))))
+              .WillOnce(DeathTestExpectedLogging());
+        }
+
+        test_sink.StartCapturingLogs();
+        do_log();
+      },
+      DiedOfFatal, DeathTestValidateExpectations());
+}
+#endif
+
+#ifndef NDEBUG
+TEST_P(BasicLogTest, DFatalIsCancellable) {
+  // LOG(DFATAL) does not die when DFATAL death is disabled.
+  absl::log_internal::SetExitOnDFatal(false);
+  ABSL_TEST_LOG(DFATAL) << "hello world";
+  absl::log_internal::SetExitOnDFatal(true);
+}
+
+#if GTEST_HAS_DEATH_TEST
+TEST_P(BasicLogDeathTest, DLogFatalIsNotCancellable) {
+  EXPECT_EXIT(
+      {
+        absl::log_internal::SetExitOnDFatal(false);
+        ABSL_TEST_DLOG(FATAL) << "hello world";
+        absl::log_internal::SetExitOnDFatal(true);
+      },
+      DiedOfFatal, "");
+}
+#endif
+#endif
+
+TEST_P(BasicLogTest, Level) {
+  absl::log_internal::ScopedMinLogLevel scoped_min_log_level(GetParam());
+
+  for (auto severity : {absl::LogSeverity::kInfo, absl::LogSeverity::kWarning,
+                        absl::LogSeverity::kError}) {
+    absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+    EXPECT_CALL(test_sink, Send).Times(0);
+
+    const int log_line = __LINE__ + 2;
+    auto do_log = [severity] {
+      ABSL_TEST_LOG(LEVEL(severity)) << "hello world";
+    };
+
+    if (LoggingEnabledAt(severity)) {
+      EXPECT_CALL(
+          test_sink,
+          Send(AllOf(
+              SourceFilename(Eq(__FILE__)),
+              SourceBasename(Eq("log_basic_test_impl.inc")),
+              SourceLine(Eq(log_line)), Prefix(IsTrue()),
+              LogSeverity(Eq(severity)), Timestamp(InMatchWindow()),
+              ThreadID(Eq(absl::base_internal::GetTID())),
+              TextMessage(Eq("hello world")),
+              Verbosity(Eq(absl::LogEntry::kNoVerbosityLevel)),
+              ENCODED_MESSAGE(MatchesEvent(
+                  Eq(__FILE__), Eq(log_line), InMatchWindow(),
+                  Eq(severity == absl::LogSeverity::kInfo ? logging::proto::INFO
+                     : severity == absl::LogSeverity::kWarning
+                         ? logging::proto::WARNING
+                     : severity == absl::LogSeverity::kError
+                         ? logging::proto::ERROR
+                         : 0),
+                  Eq(absl::base_internal::GetTID()),
+                  ElementsAre(ValueWithLiteral(Eq("hello world"))))),
+              Stacktrace(IsEmpty()))));
+    }
+    test_sink.StartCapturingLogs();
+    do_log();
+  }
+}
+
+#if GTEST_HAS_DEATH_TEST
+TEST_P(BasicLogDeathTest, Level) {
+  // TODO(b/242568884): re-enable once bug is fixed.
+  // absl::log_internal::ScopedMinLogLevel scoped_min_log_level(GetParam());
+
+  // Ensure that `severity` is not a compile-time constant to prove that
+  // `LOG(LEVEL(severity))` works regardless:
+  auto volatile severity = absl::LogSeverity::kFatal;
+
+  const int log_line = __LINE__ + 1;
+  auto do_log = [severity] { ABSL_TEST_LOG(LEVEL(severity)) << "hello world"; };
+
+  EXPECT_EXIT(
+      {
+        absl::ScopedMockLog test_sink(
+            absl::MockLogDefault::kDisallowUnexpected);
+        EXPECT_CALL(test_sink, Send).Times(0);
+
+        EXPECT_CALL(test_sink, Send)
+            .Times(AnyNumber())
+            .WillRepeatedly(DeathTestUnexpectedLogging());
+
+        ::testing::InSequence s;
+
+        if (LoggingEnabledAt(absl::LogSeverity::kFatal)) {
+          EXPECT_CALL(
+              test_sink,
+              Send(AllOf(SourceFilename(Eq(__FILE__)),
+                         SourceBasename(Eq("log_basic_test_impl.inc")),
+                         SourceLine(Eq(log_line)), Prefix(IsTrue()),
+                         LogSeverity(Eq(absl::LogSeverity::kFatal)),
+                         Timestamp(InMatchWindow()),
+                         ThreadID(Eq(absl::base_internal::GetTID())),
+                         TextMessage(Eq("hello world")),
+                         Verbosity(Eq(absl::LogEntry::kNoVerbosityLevel)),
+                         ENCODED_MESSAGE(MatchesEvent(
+                             Eq(__FILE__), Eq(log_line), InMatchWindow(),
+                             Eq(logging::proto::FATAL),
+                             Eq(absl::base_internal::GetTID()),
+                             ElementsAre(ValueWithLiteral(Eq("hello world"))))),
+                         Stacktrace(IsEmpty()))))
+              .WillOnce(DeathTestExpectedLogging());
+
+          EXPECT_CALL(
+              test_sink,
+              Send(AllOf(SourceFilename(Eq(__FILE__)),
+                         SourceBasename(Eq("log_basic_test_impl.inc")),
+                         SourceLine(Eq(log_line)), Prefix(IsTrue()),
+                         LogSeverity(Eq(absl::LogSeverity::kFatal)),
+                         Timestamp(InMatchWindow()),
+                         ThreadID(Eq(absl::base_internal::GetTID())),
+                         TextMessage(Eq("hello world")),
+                         Verbosity(Eq(absl::LogEntry::kNoVerbosityLevel)),
+                         ENCODED_MESSAGE(MatchesEvent(
+                             Eq(__FILE__), Eq(log_line), InMatchWindow(),
+                             Eq(logging::proto::FATAL),
+                             Eq(absl::base_internal::GetTID()),
+                             ElementsAre(ValueWithLiteral(Eq("hello world"))))),
+                         Stacktrace(Not(IsEmpty())))))
+              .WillOnce(DeathTestExpectedLogging());
+        }
+
+        test_sink.StartCapturingLogs();
+        do_log();
+      },
+      DiedOfFatal, DeathTestValidateExpectations());
+}
+#endif
+
+TEST_P(BasicLogTest, LevelClampsNegativeValues) {
+  absl::log_internal::ScopedMinLogLevel scoped_min_log_level(GetParam());
+
+  if (!LoggingEnabledAt(absl::LogSeverity::kInfo)) {
+    GTEST_SKIP() << "This test cases required INFO log to be enabled";
+    return;
+  }
+
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  EXPECT_CALL(test_sink, Send(LogSeverity(Eq(absl::LogSeverity::kInfo))));
+
+  test_sink.StartCapturingLogs();
+  ABSL_TEST_LOG(LEVEL(-1)) << "hello world";
+}
+
+TEST_P(BasicLogTest, LevelClampsLargeValues) {
+  absl::log_internal::ScopedMinLogLevel scoped_min_log_level(GetParam());
+
+  if (!LoggingEnabledAt(absl::LogSeverity::kError)) {
+    GTEST_SKIP() << "This test cases required ERROR log to be enabled";
+    return;
+  }
+
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  EXPECT_CALL(test_sink, Send(LogSeverity(Eq(absl::LogSeverity::kError))));
+
+  test_sink.StartCapturingLogs();
+  ABSL_TEST_LOG(LEVEL(static_cast<int>(absl::LogSeverity::kFatal) + 1))
+      << "hello world";
+}
+
+TEST(ErrnoPreservationTest, InSeverityExpression) {
+  errno = 77;
+  int saved_errno;
+  ABSL_TEST_LOG(LEVEL((saved_errno = errno, absl::LogSeverity::kInfo)));
+  EXPECT_THAT(saved_errno, Eq(77));
+}
+
+TEST(ErrnoPreservationTest, InStreamedExpression) {
+  if (!LoggingEnabledAt(absl::LogSeverity::kInfo)) {
+    GTEST_SKIP() << "This test cases required INFO log to be enabled";
+    return;
+  }
+
+  errno = 77;
+  int saved_errno = 0;
+  ABSL_TEST_LOG(INFO) << (saved_errno = errno, "hello world");
+  EXPECT_THAT(saved_errno, Eq(77));
+}
+
+TEST(ErrnoPreservationTest, AfterStatement) {
+  errno = 77;
+  ABSL_TEST_LOG(INFO);
+  const int saved_errno = errno;
+  EXPECT_THAT(saved_errno, Eq(77));
+}
+
+// Tests that using a variable/parameter in a logging statement suppresses
+// unused-variable/parameter warnings.
+// -----------------------------------------------------------------------
+class UnusedVariableWarningCompileTest {
+  // These four don't prove anything unless `ABSL_MIN_LOG_LEVEL` is greater than
+  // `kInfo`.
+  static void LoggedVariable() {
+    const int x = 0;
+    ABSL_TEST_LOG(INFO) << x;
+  }
+  static void LoggedParameter(const int x) { ABSL_TEST_LOG(INFO) << x; }
+  static void SeverityVariable() {
+    const int x = 0;
+    ABSL_TEST_LOG(LEVEL(x)) << "hello world";
+  }
+  static void SeverityParameter(const int x) {
+    ABSL_TEST_LOG(LEVEL(x)) << "hello world";
+  }
+};
+
+}  // namespace absl_log_internal
+
+#endif  // ABSL_LOG_LOG_BASIC_TEST_IMPL_H_
diff --git a/absl/log/log_benchmark.cc b/absl/log/log_benchmark.cc
new file mode 100644
index 00000000000..60c0fd637e4
--- /dev/null
+++ b/absl/log/log_benchmark.cc
@@ -0,0 +1,164 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/base/attributes.h"
+#include "absl/base/log_severity.h"
+#include "absl/flags/flag.h"
+#include "absl/log/check.h"
+#include "absl/log/globals.h"
+#include "absl/log/internal/flags.h"
+#include "absl/log/log.h"
+#include "absl/log/log_entry.h"
+#include "absl/log/log_sink.h"
+#include "absl/log/log_sink_registry.h"
+#include "absl/log/vlog_is_on.h"
+#include "benchmark/benchmark.h"
+
+namespace {
+
+class NullLogSink : public absl::LogSink {
+ public:
+  NullLogSink() { absl::AddLogSink(this); }
+
+  ~NullLogSink() override { absl::RemoveLogSink(this); }
+
+  void Send(const absl::LogEntry&) override {}
+};
+
+constexpr int x = -1;
+
+void BM_SuccessfulBinaryCheck(benchmark::State& state) {
+  int n = 0;
+  while (state.KeepRunningBatch(8)) {
+    CHECK_GE(n, x);
+    CHECK_GE(n, x);
+    CHECK_GE(n, x);
+    CHECK_GE(n, x);
+    CHECK_GE(n, x);
+    CHECK_GE(n, x);
+    CHECK_GE(n, x);
+    CHECK_GE(n, x);
+    ++n;
+  }
+  benchmark::DoNotOptimize(n);
+}
+BENCHMARK(BM_SuccessfulBinaryCheck);
+
+static void BM_SuccessfulUnaryCheck(benchmark::State& state) {
+  int n = 0;
+  while (state.KeepRunningBatch(8)) {
+    CHECK(n >= x);
+    CHECK(n >= x);
+    CHECK(n >= x);
+    CHECK(n >= x);
+    CHECK(n >= x);
+    CHECK(n >= x);
+    CHECK(n >= x);
+    CHECK(n >= x);
+    ++n;
+  }
+  benchmark::DoNotOptimize(n);
+}
+BENCHMARK(BM_SuccessfulUnaryCheck);
+
+static void BM_DisabledLogOverhead(benchmark::State& state) {
+  absl::ScopedStderrThreshold disable_stderr_logging(
+      absl::LogSeverityAtLeast::kInfinity);
+  absl::log_internal::ScopedMinLogLevel scoped_min_log_level(
+      absl::LogSeverityAtLeast::kInfinity);
+  for (auto _ : state) {
+    LOG(INFO);
+  }
+}
+BENCHMARK(BM_DisabledLogOverhead);
+
+static void BM_EnabledLogOverhead(benchmark::State& state) {
+  absl::ScopedStderrThreshold stderr_logging(
+      absl::LogSeverityAtLeast::kInfinity);
+  absl::log_internal::ScopedMinLogLevel scoped_min_log_level(
+      absl::LogSeverityAtLeast::kInfo);
+  ABSL_ATTRIBUTE_UNUSED NullLogSink null_sink;
+  for (auto _ : state) {
+    LOG(INFO);
+  }
+}
+BENCHMARK(BM_EnabledLogOverhead);
+
+static void BM_VlogIsOnOverhead(benchmark::State& state) {
+  // It would make sense to do this only when state.thread_index == 0,
+  // but thread_index is an int on some platforms (e.g. Android) and a
+  // function returning an int on others. So we just do it on all threads.
+  // TODO(b/152609127): set only if thread_index == 0.
+  absl::SetFlag(&FLAGS_v, 0);
+
+  while (state.KeepRunningBatch(10)) {
+    benchmark::DoNotOptimize(VLOG_IS_ON(0));  // 1
+    benchmark::DoNotOptimize(VLOG_IS_ON(0));  // 2
+    benchmark::DoNotOptimize(VLOG_IS_ON(0));  // 3
+    benchmark::DoNotOptimize(VLOG_IS_ON(0));  // 4
+    benchmark::DoNotOptimize(VLOG_IS_ON(0));  // 5
+    benchmark::DoNotOptimize(VLOG_IS_ON(0));  // 6
+    benchmark::DoNotOptimize(VLOG_IS_ON(0));  // 7
+    benchmark::DoNotOptimize(VLOG_IS_ON(0));  // 8
+    benchmark::DoNotOptimize(VLOG_IS_ON(0));  // 9
+    benchmark::DoNotOptimize(VLOG_IS_ON(0));  // 10
+  }
+}
+BENCHMARK(BM_VlogIsOnOverhead)->ThreadRange(1, 64);
+
+static void BM_VlogIsNotOnOverhead(benchmark::State& state) {
+  // It would make sense to do this only when state.thread_index == 0,
+  // but thread_index is an int on some platforms (e.g. Android) and a
+  // function returning an int on others. So we just do it on all threads.
+  // TODO(b/152609127): set only if thread_index == 0.
+  absl::SetFlag(&FLAGS_v, 0);
+
+  while (state.KeepRunningBatch(10)) {
+    benchmark::DoNotOptimize(VLOG_IS_ON(1));  // 1
+    benchmark::DoNotOptimize(VLOG_IS_ON(1));  // 2
+    benchmark::DoNotOptimize(VLOG_IS_ON(1));  // 3
+    benchmark::DoNotOptimize(VLOG_IS_ON(1));  // 4
+    benchmark::DoNotOptimize(VLOG_IS_ON(1));  // 5
+    benchmark::DoNotOptimize(VLOG_IS_ON(1));  // 6
+    benchmark::DoNotOptimize(VLOG_IS_ON(1));  // 7
+    benchmark::DoNotOptimize(VLOG_IS_ON(1));  // 8
+    benchmark::DoNotOptimize(VLOG_IS_ON(1));  // 9
+    benchmark::DoNotOptimize(VLOG_IS_ON(1));  // 10
+  }
+}
+BENCHMARK(BM_VlogIsNotOnOverhead)->ThreadRange(1, 64);
+
+static void BM_LogEveryNOverhead(benchmark::State& state) {
+  absl::ScopedStderrThreshold disable_stderr_logging(
+      absl::LogSeverityAtLeast::kInfinity);
+  absl::SetMinLogLevel(absl::LogSeverityAtLeast::kInfinity);
+  ABSL_ATTRIBUTE_UNUSED NullLogSink null_sink;
+
+  while (state.KeepRunningBatch(10)) {
+    LOG_EVERY_N_SEC(INFO, 10);
+    LOG_EVERY_N_SEC(INFO, 20);
+    LOG_EVERY_N_SEC(INFO, 30);
+    LOG_EVERY_N_SEC(INFO, 40);
+    LOG_EVERY_N_SEC(INFO, 50);
+    LOG_EVERY_N_SEC(INFO, 60);
+    LOG_EVERY_N_SEC(INFO, 70);
+    LOG_EVERY_N_SEC(INFO, 80);
+    LOG_EVERY_N_SEC(INFO, 90);
+    LOG_EVERY_N_SEC(INFO, 100);
+  }
+}
+BENCHMARK(BM_LogEveryNOverhead)->ThreadRange(1, 64);
+
+}  // namespace
+
diff --git a/absl/log/log_entry.cc b/absl/log/log_entry.cc
new file mode 100644
index 00000000000..358b8f5a7ce
--- /dev/null
+++ b/absl/log/log_entry.cc
@@ -0,0 +1,263 @@
+// Copyright 2025 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/log/log_entry.h"
+
+#include <cstddef>
+#include <cstdint>
+#include <iomanip>
+#include <ios>
+#include <ostream>
+
+#include "absl/base/config.h"
+#include "absl/log/internal/proto.h"
+#include "absl/strings/escaping.h"
+#include "absl/strings/string_view.h"
+#include "absl/time/time.h"
+#include "absl/types/span.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace {
+// message `logging.proto.Event`
+enum EventTag : uint8_t {
+  kFileName = 2,
+  kFileLine = 3,
+  kTimeNsecs = 4,
+  kSeverity = 5,
+  kThreadId = 6,
+  kValue = 7,
+  kSequenceNumber = 9,
+  kThreadName = 10,
+};
+
+// message `logging.proto.Value`
+enum ValueTag : uint8_t {
+  kString = 1,
+  kStringLiteral = 6,
+};
+
+// enum `logging.proto.Severity`
+enum Severity : int {
+  FINEST = 300,
+  FINER = 400,
+  FINE = 500,
+  VERBOSE_0 = 600,
+  CONFIG = 700,
+  INFO = 800,
+  NOTICE = 850,
+  WARNING = 900,
+  ERROR = 950,
+  SEVERE = 1000,
+  FATAL = 1100,
+};
+
+void PrintEscapedRangeTo(const absl::string_view str,
+                         const absl::string_view substr, std::ostream* os) {
+  const absl::string_view head =
+      str.substr(0, static_cast<size_t>(substr.data() - str.data()));
+  const char old_fill = os->fill();
+  const auto old_flags = os->flags();
+  *os << std::right
+      << std::setw(static_cast<int>(absl::CHexEscape(head).size())) << "";
+  switch (substr.size()) {
+    case 0:
+      *os << "\\";
+      break;
+    case 1:
+      *os << "^";
+      break;
+    default:
+      *os << "[" << std::setw(static_cast<int>(absl::CHexEscape(substr).size()))
+          << std::setfill('-') << ")";
+      break;
+  }
+  os->fill(old_fill);
+  os->flags(old_flags);
+}
+}  // namespace
+void PrintTo(const LogEntry& entry, std::ostream* os) {
+  auto text_message_with_prefix_and_newline_and_nul = absl::string_view(
+      entry.text_message_with_prefix_and_newline_and_nul_.data(),
+      entry.text_message_with_prefix_and_newline_and_nul_.size());
+  *os << "LogEntry {\n"
+      << "  source_filename: \"" << absl::CHexEscape(entry.source_filename())
+      << "\"\n"
+      << "  source_basename: \"" << absl::CHexEscape(entry.source_basename())
+      << "\"\n"
+      << "  source_line: " << entry.source_line() << "\n"
+      << "  prefix: " << std::boolalpha << entry.prefix() << "\n"
+      << "  log_severity: " << entry.log_severity() << "\n"
+      << "  verbosity: " << entry.verbosity();
+  if (entry.verbosity() == absl::LogEntry::kNoVerbosityLevel) {
+    *os << " (kNoVerbosityLevel)";
+  }
+  *os << "\n"
+      << "  timestamp: " << entry.timestamp() << "\n"
+      << "  tid: " << entry.tid() << "\n"
+      << "  text_message_with_prefix_and_newline_and_nul_: \""
+      << absl::CHexEscape(text_message_with_prefix_and_newline_and_nul)
+      << "\"\n"
+      << "  text_message_with_prefix_and_newline:           ";
+  PrintEscapedRangeTo(text_message_with_prefix_and_newline_and_nul,
+                      entry.text_message_with_prefix_and_newline(), os);
+  *os << "\n"
+      << "  text_message_with_prefix:                       ";
+  PrintEscapedRangeTo(text_message_with_prefix_and_newline_and_nul,
+                      entry.text_message_with_prefix(), os);
+  *os << "\n"
+      << "  text_message_with_newline:                      ";
+  PrintEscapedRangeTo(text_message_with_prefix_and_newline_and_nul,
+                      entry.text_message_with_newline(), os);
+  *os << "\n"
+      << "  text_message:                                   ";
+  PrintEscapedRangeTo(text_message_with_prefix_and_newline_and_nul,
+                      entry.text_message(), os);
+  *os << "\n"
+      << "  text_message_with_prefix_and_newline_c_str:     ";
+  PrintEscapedRangeTo(
+      text_message_with_prefix_and_newline_and_nul,
+      // NOLINTNEXTLINE(bugprone-string-constructor)
+      absl::string_view(entry.text_message_with_prefix_and_newline_c_str(), 0),
+      os);
+  *os << "\n"
+      << "  encoded_message (raw): \""
+      << absl::CHexEscape(entry.encoded_message()) << "\"\n"
+      << "  encoded_message {\n";
+  absl::Span<const char> event = entry.encoded_message();
+  log_internal::ProtoField field;
+  while (field.DecodeFrom(&event)) {
+    switch (field.tag()) {
+      case EventTag::kFileName:
+        *os << "    file_name: \"" << absl::CHexEscape(field.string_value())
+            << "\"\n";
+        break;
+      case EventTag::kFileLine:
+        *os << "    file_line: " << field.int32_value() << "\n";
+        break;
+      case EventTag::kTimeNsecs:
+        *os << "    time_nsecs: " << field.int64_value() << " ("
+            << absl::FromUnixNanos(field.int64_value()) << ")\n";
+        break;
+      case EventTag::kSeverity:
+        *os << "    severity: " << field.int32_value();
+        switch (field.int32_value()) {
+          case Severity::FINEST:
+            *os << " (FINEST)";
+            break;
+          case Severity::FINER:
+            *os << " (FINER)";
+            break;
+          case Severity::FINE:
+            *os << " (FINE)";
+            break;
+          case Severity::VERBOSE_0:
+            *os << " (VERBOSE_0)";
+            break;
+          case Severity::CONFIG:
+            *os << " (CONFIG)";
+            break;
+          case Severity::INFO:
+            *os << " (INFO)";
+            break;
+          case Severity::NOTICE:
+            *os << " (NOTICE)";
+            break;
+          case Severity::WARNING:
+            *os << " (WARNING)";
+            break;
+          case Severity::ERROR:
+            *os << " (ERROR)";
+            break;
+          case Severity::SEVERE:
+            *os << " (SEVERE)";
+            break;
+          case Severity::FATAL:
+            *os << " (FATAL)";
+            break;
+        }
+        *os << "\n";
+        break;
+      case EventTag::kThreadId:
+        *os << "    thread_id: " << field.int64_value() << "\n";
+        break;
+      case EventTag::kValue: {
+        *os << "    value {\n";
+        auto value = field.bytes_value();
+        while (field.DecodeFrom(&value)) {
+          switch (field.tag()) {
+            case ValueTag::kString:
+              *os << "      str: \"" << absl::CHexEscape(field.string_value())
+                  << "\"\n";
+              break;
+            case ValueTag::kStringLiteral:
+              *os << "      literal: \""
+                  << absl::CHexEscape(field.string_value()) << "\"\n";
+              break;
+            default:
+              *os << "      unknown field " << field.tag();
+              switch (field.type()) {
+                case log_internal::WireType::kVarint:
+                  *os << " (VARINT): " << std::hex << std::showbase
+                      << field.uint64_value() << std::dec << "\n";
+                  break;
+                case log_internal::WireType::k64Bit:
+                  *os << " (I64): " << std::hex << std::showbase
+                      << field.uint64_value() << std::dec << "\n";
+                  break;
+                case log_internal::WireType::kLengthDelimited:
+                  *os << " (LEN): \"" << absl::CHexEscape(field.string_value())
+                      << "\"\n";
+                  break;
+                case log_internal::WireType::k32Bit:
+                  *os << " (I32): " << std::hex << std::showbase
+                      << field.uint32_value() << std::dec << "\n";
+                  break;
+              }
+              break;
+          }
+        }
+        *os << "    }\n";
+        break;
+      }
+      default:
+        *os << "    unknown field " << field.tag();
+        switch (field.type()) {
+          case log_internal::WireType::kVarint:
+            *os << " (VARINT): " << std::hex << std::showbase
+                << field.uint64_value() << std::dec << "\n";
+            break;
+          case log_internal::WireType::k64Bit:
+            *os << " (I64): " << std::hex << std::showbase
+                << field.uint64_value() << std::dec << "\n";
+            break;
+          case log_internal::WireType::kLengthDelimited:
+            *os << " (LEN): \"" << absl::CHexEscape(field.string_value())
+                << "\"\n";
+            break;
+          case log_internal::WireType::k32Bit:
+            *os << " (I32): " << std::hex << std::showbase
+                << field.uint32_value() << std::dec << "\n";
+            break;
+        }
+        break;
+    }
+  }
+  *os << "  }\n"
+      << "  stacktrace: \"" << absl::CHexEscape(entry.stacktrace()) << "\"\n"
+      << "}";
+}
+
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/log/log_entry.h b/absl/log/log_entry.h
new file mode 100644
index 00000000000..c56685629cb
--- /dev/null
+++ b/absl/log/log_entry.h
@@ -0,0 +1,223 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: log/log_entry.h
+// -----------------------------------------------------------------------------
+//
+// This header declares `class absl::LogEntry`, which represents a log record as
+// passed to `LogSink::Send`. Data returned by pointer or by reference or by
+// `absl::string_view` must be copied if they are needed after the lifetime of
+// the `absl::LogEntry`.
+
+#ifndef ABSL_LOG_LOG_ENTRY_H_
+#define ABSL_LOG_LOG_ENTRY_H_
+
+#include <cstddef>
+#include <ostream>
+#include <string>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/log_severity.h"
+#include "absl/log/internal/config.h"
+#include "absl/strings/string_view.h"
+#include "absl/time/time.h"
+#include "absl/types/span.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+namespace log_internal {
+// Test only friend.
+class LogEntryTestPeer;
+class LogMessage;
+}  // namespace log_internal
+
+// LogEntry
+//
+// Represents a single entry in a log, i.e., one `LOG` statement or failed
+// `CHECK`.
+//
+// `LogEntry` is thread-compatible.
+class LogEntry final {
+ public:
+  using tid_t = log_internal::Tid;
+
+  // For non-verbose log entries, `verbosity()` returns `kNoVerbosityLevel`.
+  static constexpr int kNoVerbosityLevel = -1;
+  static constexpr int kNoVerboseLevel = -1;  // TO BE removed
+
+  // Pass `LogEntry` by reference, and do not store it as its state does not
+  // outlive the call to `LogSink::Send()`.
+  LogEntry(const LogEntry&) = delete;
+  LogEntry& operator=(const LogEntry&) = delete;
+
+  // Source file and line where the log message occurred.  Taken from `__FILE__`
+  // and `__LINE__` unless overridden by `LOG(...).AtLocation(...)`.
+  //
+  // Take special care not to use the values returned by `source_filename()` and
+  // `source_basename()` after the lifetime of the entry.  This is always
+  // incorrect, but it will often work in practice because they usually point
+  // into a statically allocated character array obtained from `__FILE__`.
+  // Statements like `LOG(INFO).AtLocation(std::string(...), ...)` will expose
+  // the bug.  If you need the data later, you must copy them.
+  absl::string_view source_filename() const ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return full_filename_;
+  }
+  absl::string_view source_basename() const ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return base_filename_;
+  }
+  int source_line() const { return line_; }
+
+  // LogEntry::prefix()
+  //
+  // True unless the metadata prefix was suppressed once by
+  // `LOG(...).NoPrefix()` or globally by `absl::EnableLogPrefix(false)`.
+  // Implies `text_message_with_prefix() == text_message()`.
+  bool prefix() const { return prefix_; }
+
+  // LogEntry::log_severity()
+  //
+  // Returns this entry's severity.  For `LOG`, taken from the first argument;
+  // for `CHECK`, always `absl::LogSeverity::kFatal`.
+  absl::LogSeverity log_severity() const { return severity_; }
+
+  // LogEntry::verbosity()
+  //
+  // Returns this entry's verbosity, or `kNoVerbosityLevel` for a non-verbose
+  // entry. Taken from the argument to `VLOG` or from
+  // `LOG(...).WithVerbosity(...)`.
+  int verbosity() const { return verbose_level_; }
+
+  // LogEntry::timestamp()
+  //
+  // Returns the time at which this entry was written.  Captured during
+  // evaluation of `LOG`, but can be overridden by
+  // `LOG(...).WithTimestamp(...)`.
+  //
+  // Take care not to rely on timestamps increasing monotonically, or even to
+  // rely on timestamps having any particular relationship with reality (since
+  // they can be overridden).
+  absl::Time timestamp() const { return timestamp_; }
+
+  // LogEntry::tid()
+  //
+  // Returns the ID of the thread that wrote this entry.  Captured during
+  // evaluation of `LOG`, but can be overridden by `LOG(...).WithThreadID(...)`.
+  //
+  // Take care not to *rely* on reported thread IDs as they can be overridden as
+  // specified above.
+  tid_t tid() const { return tid_; }
+
+  // Text-formatted version of the log message.  An underlying buffer holds
+  // these contiguous data:
+  //
+  // * A prefix formed by formatting metadata (timestamp, filename, line number,
+  //   etc.)
+  //   The prefix may be empty - see `LogEntry::prefix()` - and may rarely be
+  //   truncated if the metadata are very long.
+  // * The streamed data
+  //   The data may be empty if nothing was streamed, or may be truncated to fit
+  //   the buffer.
+  // * A newline
+  // * A nul terminator
+  //
+  // The newline and nul terminator will be present even if the prefix and/or
+  // data are truncated.
+  //
+  // These methods give access to the most commonly useful substrings of the
+  // buffer's contents.  Other combinations can be obtained with substring
+  // arithmetic.
+  //
+  // The buffer does not outlive the entry; if you need the data later, you must
+  // copy them.
+  absl::string_view text_message_with_prefix_and_newline() const
+      ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return absl::string_view(
+        text_message_with_prefix_and_newline_and_nul_.data(),
+        text_message_with_prefix_and_newline_and_nul_.size() - 1);
+  }
+  absl::string_view text_message_with_prefix() const
+      ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return absl::string_view(
+        text_message_with_prefix_and_newline_and_nul_.data(),
+        text_message_with_prefix_and_newline_and_nul_.size() - 2);
+  }
+  absl::string_view text_message_with_newline() const
+      ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return absl::string_view(
+        text_message_with_prefix_and_newline_and_nul_.data() + prefix_len_,
+        text_message_with_prefix_and_newline_and_nul_.size() - prefix_len_ - 1);
+  }
+  absl::string_view text_message() const ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return absl::string_view(
+        text_message_with_prefix_and_newline_and_nul_.data() + prefix_len_,
+        text_message_with_prefix_and_newline_and_nul_.size() - prefix_len_ - 2);
+  }
+  const char* text_message_with_prefix_and_newline_c_str() const
+      ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return text_message_with_prefix_and_newline_and_nul_.data();
+  }
+
+  // Returns a serialized protobuf holding the operands streamed into this
+  // log message.  The message definition is not yet published.
+  //
+  // The buffer does not outlive the entry; if you need the data later, you must
+  // copy them.
+  absl::string_view encoded_message() const ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return encoding_;
+  }
+
+  // LogEntry::stacktrace()
+  //
+  // Optional stacktrace, e.g. for `FATAL` logs and failed `CHECK`s.
+  //
+  // Fatal entries are dispatched to each sink twice: first with all data and
+  // metadata but no stacktrace, and then with the stacktrace.  This is done
+  // because stacktrace collection is sometimes slow and fallible, and it's
+  // critical to log enough information to diagnose the failure even if the
+  // stacktrace collection hangs.
+  //
+  // The buffer does not outlive the entry; if you need the data later, you must
+  // copy them.
+  absl::string_view stacktrace() const ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return stacktrace_;
+  }
+
+ private:
+  LogEntry() = default;
+
+  absl::string_view full_filename_;
+  absl::string_view base_filename_;
+  int line_;
+  bool prefix_;
+  absl::LogSeverity severity_;
+  int verbose_level_;  // >=0 for `VLOG`, etc.; otherwise `kNoVerbosityLevel`.
+  absl::Time timestamp_;
+  tid_t tid_;
+  absl::Span<const char> text_message_with_prefix_and_newline_and_nul_;
+  size_t prefix_len_;
+  absl::string_view encoding_;
+  std::string stacktrace_;
+
+  friend class log_internal::LogEntryTestPeer;
+  friend class log_internal::LogMessage;
+  friend void PrintTo(const absl::LogEntry& entry, std::ostream* os);
+};
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_LOG_LOG_ENTRY_H_
diff --git a/absl/log/log_entry_test.cc b/absl/log/log_entry_test.cc
new file mode 100644
index 00000000000..3af10b65fce
--- /dev/null
+++ b/absl/log/log_entry_test.cc
@@ -0,0 +1,468 @@
+//
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/log/log_entry.h"
+
+#include <stddef.h>
+#include <stdint.h>
+
+#include <cstring>
+#include <limits>
+#include <string>
+#include <type_traits>
+#include <utility>
+#include <vector>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/log_severity.h"
+#include "absl/log/internal/append_truncated.h"
+#include "absl/log/internal/log_format.h"
+#include "absl/log/internal/test_helpers.h"
+#include "absl/strings/numbers.h"
+#include "absl/strings/str_split.h"
+#include "absl/strings/string_view.h"
+#include "absl/time/civil_time.h"
+#include "absl/time/time.h"
+#include "absl/types/span.h"
+
+namespace {
+using ::absl::log_internal::LogEntryTestPeer;
+using ::testing::Eq;
+using ::testing::IsTrue;
+using ::testing::StartsWith;
+using ::testing::StrEq;
+
+auto* test_env ABSL_ATTRIBUTE_UNUSED = ::testing::AddGlobalTestEnvironment(
+    new absl::log_internal::LogTestEnvironment);
+}  // namespace
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace log_internal {
+
+class LogEntryTestPeer {
+ public:
+  LogEntryTestPeer(absl::string_view base_filename, int line, bool prefix,
+                   absl::LogSeverity severity, absl::string_view timestamp,
+                   absl::LogEntry::tid_t tid, PrefixFormat format,
+                   absl::string_view text_message)
+      : format_{format}, buf_(15000, '\0') {
+    entry_.base_filename_ = base_filename;
+    entry_.line_ = line;
+    entry_.prefix_ = prefix;
+    entry_.severity_ = severity;
+    std::string time_err;
+    EXPECT_THAT(
+        absl::ParseTime("%Y-%m-%d%ET%H:%M:%E*S", timestamp,
+                        absl::LocalTimeZone(), &entry_.timestamp_, &time_err),
+        IsTrue())
+        << "Failed to parse time " << timestamp << ": " << time_err;
+    entry_.tid_ = tid;
+    std::pair<absl::string_view, std::string> timestamp_bits =
+        absl::StrSplit(timestamp, absl::ByChar('.'));
+    EXPECT_THAT(absl::ParseCivilTime(timestamp_bits.first, &ci_.cs), IsTrue())
+        << "Failed to parse time " << timestamp_bits.first;
+    timestamp_bits.second.resize(9, '0');
+    int64_t nanos = 0;
+    EXPECT_THAT(absl::SimpleAtoi(timestamp_bits.second, &nanos), IsTrue())
+        << "Failed to parse time " << timestamp_bits.first;
+    ci_.subsecond = absl::Nanoseconds(nanos);
+
+    absl::Span<char> view = absl::MakeSpan(buf_);
+    view.remove_suffix(2);
+    entry_.prefix_len_ =
+        entry_.prefix_
+            ? log_internal::FormatLogPrefix(
+                  entry_.log_severity(), entry_.timestamp(), entry_.tid(),
+                  entry_.source_basename(), entry_.source_line(), format_, view)
+            : 0;
+
+    EXPECT_THAT(entry_.prefix_len_,
+                Eq(static_cast<size_t>(view.data() - buf_.data())));
+    log_internal::AppendTruncated(text_message, view);
+    view = absl::Span<char>(view.data(), view.size() + 2);
+    view[0] = '\n';
+    view[1] = '\0';
+    view.remove_prefix(2);
+    buf_.resize(static_cast<size_t>(view.data() - buf_.data()));
+    entry_.text_message_with_prefix_and_newline_and_nul_ = absl::MakeSpan(buf_);
+  }
+  LogEntryTestPeer(const LogEntryTestPeer&) = delete;
+  LogEntryTestPeer& operator=(const LogEntryTestPeer&) = delete;
+
+  std::string FormatLogMessage() const {
+    return log_internal::FormatLogMessage(
+        entry_.log_severity(), ci_.cs, ci_.subsecond, entry_.tid(),
+        entry_.source_basename(), entry_.source_line(), format_,
+        entry_.text_message());
+  }
+  std::string FormatPrefixIntoSizedBuffer(size_t sz) {
+    std::string str(sz, '\0');
+    absl::Span<char> buf(&str[0], str.size());
+    const size_t prefix_size = log_internal::FormatLogPrefix(
+        entry_.log_severity(), entry_.timestamp(), entry_.tid(),
+        entry_.source_basename(), entry_.source_line(), format_, buf);
+    EXPECT_THAT(prefix_size, Eq(static_cast<size_t>(buf.data() - str.data())));
+    str.resize(prefix_size);
+    return str;
+  }
+  const absl::LogEntry& entry() const { return entry_; }
+
+ private:
+  absl::LogEntry entry_;
+  PrefixFormat format_;
+  absl::TimeZone::CivilInfo ci_;
+  std::vector<char> buf_;
+};
+
+}  // namespace log_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+namespace {
+constexpr bool kUsePrefix = true, kNoPrefix = false;
+
+TEST(LogEntryTest, Baseline) {
+  LogEntryTestPeer entry("foo.cc", 1234, kUsePrefix, absl::LogSeverity::kInfo,
+                         "2020-01-02T03:04:05.6789", 451,
+                         absl::log_internal::PrefixFormat::kNotRaw,
+                         "hello world");
+  EXPECT_THAT(entry.FormatLogMessage(),
+              Eq("I0102 03:04:05.678900     451 foo.cc:1234] hello world"));
+  EXPECT_THAT(entry.FormatPrefixIntoSizedBuffer(1000),
+              Eq("I0102 03:04:05.678900     451 foo.cc:1234] "));
+  for (size_t sz = strlen("I0102 03:04:05.678900     451 foo.cc:1234] ") + 20;
+       sz != std::numeric_limits<size_t>::max(); sz--)
+    EXPECT_THAT("I0102 03:04:05.678900     451 foo.cc:1234] ",
+                StartsWith(entry.FormatPrefixIntoSizedBuffer(sz)));
+
+  EXPECT_THAT(entry.entry().text_message_with_prefix_and_newline(),
+              Eq("I0102 03:04:05.678900     451 foo.cc:1234] hello world\n"));
+  EXPECT_THAT(
+      entry.entry().text_message_with_prefix_and_newline_c_str(),
+      StrEq("I0102 03:04:05.678900     451 foo.cc:1234] hello world\n"));
+  EXPECT_THAT(entry.entry().text_message_with_prefix(),
+              Eq("I0102 03:04:05.678900     451 foo.cc:1234] hello world"));
+  EXPECT_THAT(entry.entry().text_message(), Eq("hello world"));
+}
+
+TEST(LogEntryTest, NoPrefix) {
+  LogEntryTestPeer entry("foo.cc", 1234, kNoPrefix, absl::LogSeverity::kInfo,
+                         "2020-01-02T03:04:05.6789", 451,
+                         absl::log_internal::PrefixFormat::kNotRaw,
+                         "hello world");
+  EXPECT_THAT(entry.FormatLogMessage(),
+              Eq("I0102 03:04:05.678900     451 foo.cc:1234] hello world"));
+  // These methods are not responsible for honoring `prefix()`.
+  EXPECT_THAT(entry.FormatPrefixIntoSizedBuffer(1000),
+              Eq("I0102 03:04:05.678900     451 foo.cc:1234] "));
+  for (size_t sz = strlen("I0102 03:04:05.678900     451 foo.cc:1234] ") + 20;
+       sz != std::numeric_limits<size_t>::max(); sz--)
+    EXPECT_THAT("I0102 03:04:05.678900     451 foo.cc:1234] ",
+                StartsWith(entry.FormatPrefixIntoSizedBuffer(sz)));
+
+  EXPECT_THAT(entry.entry().text_message_with_prefix_and_newline(),
+              Eq("hello world\n"));
+  EXPECT_THAT(entry.entry().text_message_with_prefix_and_newline_c_str(),
+              StrEq("hello world\n"));
+  EXPECT_THAT(entry.entry().text_message_with_prefix(), Eq("hello world"));
+  EXPECT_THAT(entry.entry().text_message(), Eq("hello world"));
+}
+
+TEST(LogEntryTest, EmptyFields) {
+  LogEntryTestPeer entry("", 0, kUsePrefix, absl::LogSeverity::kInfo,
+                         "2020-01-02T03:04:05", 0,
+                         absl::log_internal::PrefixFormat::kNotRaw, "");
+  const std::string format_message = entry.FormatLogMessage();
+  EXPECT_THAT(format_message, Eq("I0102 03:04:05.000000       0 :0] "));
+  EXPECT_THAT(entry.FormatPrefixIntoSizedBuffer(1000), Eq(format_message));
+  for (size_t sz = format_message.size() + 20;
+       sz != std::numeric_limits<size_t>::max(); sz--)
+    EXPECT_THAT(format_message,
+                StartsWith(entry.FormatPrefixIntoSizedBuffer(sz)));
+
+  EXPECT_THAT(entry.entry().text_message_with_prefix_and_newline(),
+              Eq("I0102 03:04:05.000000       0 :0] \n"));
+  EXPECT_THAT(entry.entry().text_message_with_prefix_and_newline_c_str(),
+              StrEq("I0102 03:04:05.000000       0 :0] \n"));
+  EXPECT_THAT(entry.entry().text_message_with_prefix(),
+              Eq("I0102 03:04:05.000000       0 :0] "));
+  EXPECT_THAT(entry.entry().text_message(), Eq(""));
+}
+
+TEST(LogEntryTest, NegativeFields) {
+  // When Abseil's minimum C++ version is C++17, this conditional can be
+  // converted to a constexpr if and the static_cast below removed.
+  if (std::is_signed_v<absl::LogEntry::tid_t>) {
+    LogEntryTestPeer entry(
+        "foo.cc", -1234, kUsePrefix, absl::LogSeverity::kInfo,
+        "2020-01-02T03:04:05.6789", static_cast<absl::LogEntry::tid_t>(-451),
+        absl::log_internal::PrefixFormat::kNotRaw, "hello world");
+    EXPECT_THAT(entry.FormatLogMessage(),
+                Eq("I0102 03:04:05.678900    -451 foo.cc:-1234] hello world"));
+    EXPECT_THAT(entry.FormatPrefixIntoSizedBuffer(1000),
+                Eq("I0102 03:04:05.678900    -451 foo.cc:-1234] "));
+    for (size_t sz =
+             strlen("I0102 03:04:05.678900    -451 foo.cc:-1234] ") + 20;
+         sz != std::numeric_limits<size_t>::max(); sz--)
+      EXPECT_THAT("I0102 03:04:05.678900    -451 foo.cc:-1234] ",
+                  StartsWith(entry.FormatPrefixIntoSizedBuffer(sz)));
+
+    EXPECT_THAT(
+        entry.entry().text_message_with_prefix_and_newline(),
+        Eq("I0102 03:04:05.678900    -451 foo.cc:-1234] hello world\n"));
+    EXPECT_THAT(
+        entry.entry().text_message_with_prefix_and_newline_c_str(),
+        StrEq("I0102 03:04:05.678900    -451 foo.cc:-1234] hello world\n"));
+    EXPECT_THAT(entry.entry().text_message_with_prefix(),
+                Eq("I0102 03:04:05.678900    -451 foo.cc:-1234] hello world"));
+    EXPECT_THAT(entry.entry().text_message(), Eq("hello world"));
+  } else {
+    LogEntryTestPeer entry("foo.cc", -1234, kUsePrefix,
+                           absl::LogSeverity::kInfo, "2020-01-02T03:04:05.6789",
+                           451, absl::log_internal::PrefixFormat::kNotRaw,
+                           "hello world");
+    EXPECT_THAT(entry.FormatLogMessage(),
+                Eq("I0102 03:04:05.678900     451 foo.cc:-1234] hello world"));
+    EXPECT_THAT(entry.FormatPrefixIntoSizedBuffer(1000),
+                Eq("I0102 03:04:05.678900     451 foo.cc:-1234] "));
+    for (size_t sz =
+             strlen("I0102 03:04:05.678900     451 foo.cc:-1234] ") + 20;
+         sz != std::numeric_limits<size_t>::max(); sz--)
+      EXPECT_THAT("I0102 03:04:05.678900     451 foo.cc:-1234] ",
+                  StartsWith(entry.FormatPrefixIntoSizedBuffer(sz)));
+
+    EXPECT_THAT(
+        entry.entry().text_message_with_prefix_and_newline(),
+        Eq("I0102 03:04:05.678900     451 foo.cc:-1234] hello world\n"));
+    EXPECT_THAT(
+        entry.entry().text_message_with_prefix_and_newline_c_str(),
+        StrEq("I0102 03:04:05.678900     451 foo.cc:-1234] hello world\n"));
+    EXPECT_THAT(entry.entry().text_message_with_prefix(),
+                Eq("I0102 03:04:05.678900     451 foo.cc:-1234] hello world"));
+    EXPECT_THAT(entry.entry().text_message(), Eq("hello world"));
+  }
+}
+
+TEST(LogEntryTest, LongFields) {
+  LogEntryTestPeer entry(
+      "I am the very model of a modern Major-General / "
+      "I've information vegetable, animal, and mineral.",
+      2147483647, kUsePrefix, absl::LogSeverity::kInfo,
+      "2020-01-02T03:04:05.678967896789", 2147483647,
+      absl::log_internal::PrefixFormat::kNotRaw,
+      "I know the kings of England, and I quote the fights historical / "
+      "From Marathon to Waterloo, in order categorical.");
+  EXPECT_THAT(entry.FormatLogMessage(),
+              Eq("I0102 03:04:05.678967 2147483647 I am the very model of a "
+                 "modern Major-General / I've information vegetable, animal, "
+                 "and mineral.:2147483647] I know the kings of England, and I "
+                 "quote the fights historical / From Marathon to Waterloo, in "
+                 "order categorical."));
+  EXPECT_THAT(entry.FormatPrefixIntoSizedBuffer(1000),
+              Eq("I0102 03:04:05.678967 2147483647 I am the very model of a "
+                 "modern Major-General / I've information vegetable, animal, "
+                 "and mineral.:2147483647] "));
+  for (size_t sz =
+           strlen("I0102 03:04:05.678967 2147483647 I am the very model of a "
+                  "modern Major-General / I've information vegetable, animal, "
+                  "and mineral.:2147483647] ") +
+           20;
+       sz != std::numeric_limits<size_t>::max(); sz--)
+    EXPECT_THAT(
+        "I0102 03:04:05.678967 2147483647 I am the very model of a "
+        "modern Major-General / I've information vegetable, animal, "
+        "and mineral.:2147483647] ",
+        StartsWith(entry.FormatPrefixIntoSizedBuffer(sz)));
+
+  EXPECT_THAT(entry.entry().text_message_with_prefix_and_newline(),
+              Eq("I0102 03:04:05.678967 2147483647 I am the very model of a "
+                 "modern Major-General / I've information vegetable, animal, "
+                 "and mineral.:2147483647] I know the kings of England, and I "
+                 "quote the fights historical / From Marathon to Waterloo, in "
+                 "order categorical.\n"));
+  EXPECT_THAT(
+      entry.entry().text_message_with_prefix_and_newline_c_str(),
+      StrEq("I0102 03:04:05.678967 2147483647 I am the very model of a "
+            "modern Major-General / I've information vegetable, animal, "
+            "and mineral.:2147483647] I know the kings of England, and I "
+            "quote the fights historical / From Marathon to Waterloo, in "
+            "order categorical.\n"));
+  EXPECT_THAT(entry.entry().text_message_with_prefix(),
+              Eq("I0102 03:04:05.678967 2147483647 I am the very model of a "
+                 "modern Major-General / I've information vegetable, animal, "
+                 "and mineral.:2147483647] I know the kings of England, and I "
+                 "quote the fights historical / From Marathon to Waterloo, in "
+                 "order categorical."));
+  EXPECT_THAT(
+      entry.entry().text_message(),
+      Eq("I know the kings of England, and I quote the fights historical / "
+         "From Marathon to Waterloo, in order categorical."));
+}
+
+TEST(LogEntryTest, LongNegativeFields) {
+  // When Abseil's minimum C++ version is C++17, this conditional can be
+  // converted to a constexpr if and the static_cast below removed.
+  if (std::is_signed_v<absl::LogEntry::tid_t>) {
+    LogEntryTestPeer entry(
+        "I am the very model of a modern Major-General / "
+        "I've information vegetable, animal, and mineral.",
+        -2147483647, kUsePrefix, absl::LogSeverity::kInfo,
+        "2020-01-02T03:04:05.678967896789",
+        static_cast<absl::LogEntry::tid_t>(-2147483647),
+        absl::log_internal::PrefixFormat::kNotRaw,
+        "I know the kings of England, and I quote the fights historical / "
+        "From Marathon to Waterloo, in order categorical.");
+    EXPECT_THAT(
+        entry.FormatLogMessage(),
+        Eq("I0102 03:04:05.678967 -2147483647 I am the very model of a "
+           "modern Major-General / I've information vegetable, animal, "
+           "and mineral.:-2147483647] I know the kings of England, and I "
+           "quote the fights historical / From Marathon to Waterloo, in "
+           "order categorical."));
+    EXPECT_THAT(entry.FormatPrefixIntoSizedBuffer(1000),
+                Eq("I0102 03:04:05.678967 -2147483647 I am the very model of a "
+                   "modern Major-General / I've information vegetable, animal, "
+                   "and mineral.:-2147483647] "));
+    for (size_t sz =
+             strlen(
+                 "I0102 03:04:05.678967 -2147483647 I am the very model of a "
+                 "modern Major-General / I've information vegetable, animal, "
+                 "and mineral.:-2147483647] ") +
+             20;
+         sz != std::numeric_limits<size_t>::max(); sz--)
+      EXPECT_THAT(
+          "I0102 03:04:05.678967 -2147483647 I am the very model of a "
+          "modern Major-General / I've information vegetable, animal, "
+          "and mineral.:-2147483647] ",
+          StartsWith(entry.FormatPrefixIntoSizedBuffer(sz)));
+
+    EXPECT_THAT(
+        entry.entry().text_message_with_prefix_and_newline(),
+        Eq("I0102 03:04:05.678967 -2147483647 I am the very model of a "
+           "modern Major-General / I've information vegetable, animal, "
+           "and mineral.:-2147483647] I know the kings of England, and I "
+           "quote the fights historical / From Marathon to Waterloo, in "
+           "order categorical.\n"));
+    EXPECT_THAT(
+        entry.entry().text_message_with_prefix_and_newline_c_str(),
+        StrEq("I0102 03:04:05.678967 -2147483647 I am the very model of a "
+              "modern Major-General / I've information vegetable, animal, "
+              "and mineral.:-2147483647] I know the kings of England, and I "
+              "quote the fights historical / From Marathon to Waterloo, in "
+              "order categorical.\n"));
+    EXPECT_THAT(
+        entry.entry().text_message_with_prefix(),
+        Eq("I0102 03:04:05.678967 -2147483647 I am the very model of a "
+           "modern Major-General / I've information vegetable, animal, "
+           "and mineral.:-2147483647] I know the kings of England, and I "
+           "quote the fights historical / From Marathon to Waterloo, in "
+           "order categorical."));
+    EXPECT_THAT(
+        entry.entry().text_message(),
+        Eq("I know the kings of England, and I quote the fights historical / "
+           "From Marathon to Waterloo, in order categorical."));
+  } else {
+    LogEntryTestPeer entry(
+        "I am the very model of a modern Major-General / "
+        "I've information vegetable, animal, and mineral.",
+        -2147483647, kUsePrefix, absl::LogSeverity::kInfo,
+        "2020-01-02T03:04:05.678967896789", 2147483647,
+        absl::log_internal::PrefixFormat::kNotRaw,
+        "I know the kings of England, and I quote the fights historical / "
+        "From Marathon to Waterloo, in order categorical.");
+    EXPECT_THAT(
+        entry.FormatLogMessage(),
+        Eq("I0102 03:04:05.678967 2147483647 I am the very model of a "
+           "modern Major-General / I've information vegetable, animal, "
+           "and mineral.:-2147483647] I know the kings of England, and I "
+           "quote the fights historical / From Marathon to Waterloo, in "
+           "order categorical."));
+    EXPECT_THAT(entry.FormatPrefixIntoSizedBuffer(1000),
+                Eq("I0102 03:04:05.678967 2147483647 I am the very model of a "
+                   "modern Major-General / I've information vegetable, animal, "
+                   "and mineral.:-2147483647] "));
+    for (size_t sz =
+             strlen(
+                 "I0102 03:04:05.678967 2147483647 I am the very model of a "
+                 "modern Major-General / I've information vegetable, animal, "
+                 "and mineral.:-2147483647] ") +
+             20;
+         sz != std::numeric_limits<size_t>::max(); sz--)
+      EXPECT_THAT(
+          "I0102 03:04:05.678967 2147483647 I am the very model of a "
+          "modern Major-General / I've information vegetable, animal, "
+          "and mineral.:-2147483647] ",
+          StartsWith(entry.FormatPrefixIntoSizedBuffer(sz)));
+
+    EXPECT_THAT(
+        entry.entry().text_message_with_prefix_and_newline(),
+        Eq("I0102 03:04:05.678967 2147483647 I am the very model of a "
+           "modern Major-General / I've information vegetable, animal, "
+           "and mineral.:-2147483647] I know the kings of England, and I "
+           "quote the fights historical / From Marathon to Waterloo, in "
+           "order categorical.\n"));
+    EXPECT_THAT(
+        entry.entry().text_message_with_prefix_and_newline_c_str(),
+        StrEq("I0102 03:04:05.678967 2147483647 I am the very model of a "
+              "modern Major-General / I've information vegetable, animal, "
+              "and mineral.:-2147483647] I know the kings of England, and I "
+              "quote the fights historical / From Marathon to Waterloo, in "
+              "order categorical.\n"));
+    EXPECT_THAT(
+        entry.entry().text_message_with_prefix(),
+        Eq("I0102 03:04:05.678967 2147483647 I am the very model of a "
+           "modern Major-General / I've information vegetable, animal, "
+           "and mineral.:-2147483647] I know the kings of England, and I "
+           "quote the fights historical / From Marathon to Waterloo, in "
+           "order categorical."));
+    EXPECT_THAT(
+        entry.entry().text_message(),
+        Eq("I know the kings of England, and I quote the fights historical / "
+           "From Marathon to Waterloo, in order categorical."));
+  }
+}
+
+TEST(LogEntryTest, Raw) {
+  LogEntryTestPeer entry("foo.cc", 1234, kUsePrefix, absl::LogSeverity::kInfo,
+                         "2020-01-02T03:04:05.6789", 451,
+                         absl::log_internal::PrefixFormat::kRaw, "hello world");
+  EXPECT_THAT(
+      entry.FormatLogMessage(),
+      Eq("I0102 03:04:05.678900     451 foo.cc:1234] RAW: hello world"));
+  EXPECT_THAT(entry.FormatPrefixIntoSizedBuffer(1000),
+              Eq("I0102 03:04:05.678900     451 foo.cc:1234] RAW: "));
+  for (size_t sz =
+           strlen("I0102 03:04:05.678900     451 foo.cc:1234] RAW: ") + 20;
+       sz != std::numeric_limits<size_t>::max(); sz--)
+    EXPECT_THAT("I0102 03:04:05.678900     451 foo.cc:1234] RAW: ",
+                StartsWith(entry.FormatPrefixIntoSizedBuffer(sz)));
+
+  EXPECT_THAT(
+      entry.entry().text_message_with_prefix_and_newline(),
+      Eq("I0102 03:04:05.678900     451 foo.cc:1234] RAW: hello world\n"));
+  EXPECT_THAT(
+      entry.entry().text_message_with_prefix_and_newline_c_str(),
+      StrEq("I0102 03:04:05.678900     451 foo.cc:1234] RAW: hello world\n"));
+  EXPECT_THAT(
+      entry.entry().text_message_with_prefix(),
+      Eq("I0102 03:04:05.678900     451 foo.cc:1234] RAW: hello world"));
+  EXPECT_THAT(entry.entry().text_message(), Eq("hello world"));
+}
+
+}  // namespace
diff --git a/absl/log/log_format_test.cc b/absl/log/log_format_test.cc
new file mode 100644
index 00000000000..eb7585c158a
--- /dev/null
+++ b/absl/log/log_format_test.cc
@@ -0,0 +1,2281 @@
+//
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <math.h>
+
+#include <cstring>
+#include <iomanip>
+#include <ios>
+#include <limits>
+#include <optional>
+#include <ostream>
+#include <sstream>
+#include <string>
+#include <string_view>
+#include <type_traits>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/log/check.h"
+#include "absl/log/internal/test_matchers.h"
+#include "absl/log/log.h"
+#include "absl/log/log_entry.h"
+#include "absl/log/scoped_mock_log.h"
+#include "absl/strings/match.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/str_format.h"
+#include "absl/strings/string_view.h"
+#include "absl/types/source_location.h"
+
+#ifdef __ANDROID__
+#include <android/api-level.h>
+#endif
+
+namespace {
+using ::absl::log_internal::AsString;
+using ::absl::log_internal::MatchesOstream;
+using ::absl::log_internal::RawEncodedMessage;
+using ::absl::log_internal::TextMessage;
+using ::absl::log_internal::TextPrefix;
+using ::testing::_;
+using ::testing::AllOf;
+using ::testing::AnyOf;
+using ::testing::Each;
+using ::testing::EndsWith;
+using ::testing::Eq;
+using ::testing::Ge;
+using ::testing::IsEmpty;
+using ::testing::Le;
+using ::testing::SizeIs;
+using ::testing::Types;
+
+// Some aspects of formatting streamed data (e.g. pointer handling) are
+// implementation-defined.  Others are buggy in supported implementations.
+// These tests validate that the formatting matches that performed by a
+// `std::ostream` and also that the result is one of a list of expected formats.
+
+std::ostringstream ComparisonStream() {
+  std::ostringstream str;
+  str.setf(std::ios_base::showbase | std::ios_base::boolalpha |
+           std::ios_base::internal);
+  return str;
+}
+
+TEST(LogFormatTest, NoMessage) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const int log_line = __LINE__ + 1;
+  auto do_log = [] { LOG(INFO); };
+
+  EXPECT_CALL(test_sink,
+              Send(AllOf(TextMessage(MatchesOstream(ComparisonStream())),
+                         TextPrefix(AsString(EndsWith(absl::StrCat(
+                             " log_format_test.cc:", log_line, "] ")))),
+                         TextMessage(IsEmpty()),
+                         ENCODED_MESSAGE(HasValues(IsEmpty())))));
+
+  test_sink.StartCapturingLogs();
+  do_log();
+}
+
+template <typename T>
+class CharLogFormatTest : public testing::Test {};
+using CharTypes = Types<char, signed char, unsigned char>;
+TYPED_TEST_SUITE(CharLogFormatTest, CharTypes);
+
+TYPED_TEST(CharLogFormatTest, Printable) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const TypeParam value = 'x';
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << value;
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(
+          TextMessage(MatchesOstream(comparison_stream)), TextMessage(Eq("x")),
+          ENCODED_MESSAGE(HasValues(ElementsAre(ValueWithStr(Eq("x"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << value;
+}
+
+TYPED_TEST(CharLogFormatTest, Unprintable) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  constexpr auto value = static_cast<TypeParam>(0xeeu);
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << value;
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(
+          TextMessage(MatchesOstream(comparison_stream)),
+          TextMessage(Eq("\xee")),
+          ENCODED_MESSAGE(HasValues(ElementsAre(ValueWithStr(Eq("\xee"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << value;
+}
+
+TEST(WideCharLogFormatTest, Printable) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  EXPECT_CALL(test_sink, Send(AllOf(TextMessage(Eq("€")),
+                                    ENCODED_MESSAGE(HasValues(
+                                        ElementsAre(ValueWithStr(Eq("€"))))))));
+
+  test_sink.StartCapturingLogs();
+  const wchar_t value = L'\u20AC';
+  LOG(INFO) << value;
+}
+
+TEST(WideCharLogFormatTest, Unprintable) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  // Using NEL (Next Line) Unicode character (U+0085).
+  // It is encoded as "\xC2\x85" in UTF-8.
+  constexpr wchar_t wide_value = L'\u0085';
+  constexpr char value[] = "\xC2\x85";
+  EXPECT_CALL(test_sink, Send(AllOf(TextMessage(Eq(value)),
+                                    ENCODED_MESSAGE(HasValues(ElementsAre(
+                                        ValueWithStr(Eq(value))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << wide_value;
+}
+
+template <typename T>
+class UnsignedIntLogFormatTest : public testing::Test {};
+using UnsignedIntTypes = Types<unsigned short, unsigned int,        // NOLINT
+                               unsigned long, unsigned long long>;  // NOLINT
+TYPED_TEST_SUITE(UnsignedIntLogFormatTest, UnsignedIntTypes);
+
+TYPED_TEST(UnsignedIntLogFormatTest, Positive) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const TypeParam value = 224;
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << value;
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(
+          TextMessage(MatchesOstream(comparison_stream)),
+          TextMessage(Eq("224")),
+          ENCODED_MESSAGE(HasValues(ElementsAre(ValueWithStr(Eq("224"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << value;
+}
+
+TYPED_TEST(UnsignedIntLogFormatTest, BitfieldPositive) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const struct {
+    TypeParam bits : 6;
+  } value{42};
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << value.bits;
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(
+          TextMessage(MatchesOstream(comparison_stream)), TextMessage(Eq("42")),
+          ENCODED_MESSAGE(HasValues(ElementsAre(ValueWithStr(Eq("42"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << value.bits;
+}
+
+template <typename T>
+class SignedIntLogFormatTest : public testing::Test {};
+using SignedIntTypes =
+    Types<signed short, signed int, signed long, signed long long>;  // NOLINT
+TYPED_TEST_SUITE(SignedIntLogFormatTest, SignedIntTypes);
+
+TYPED_TEST(SignedIntLogFormatTest, Positive) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const TypeParam value = 224;
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << value;
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(
+          TextMessage(MatchesOstream(comparison_stream)),
+          TextMessage(Eq("224")),
+          ENCODED_MESSAGE(HasValues(ElementsAre(ValueWithStr(Eq("224"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << value;
+}
+
+TYPED_TEST(SignedIntLogFormatTest, Negative) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const TypeParam value = -112;
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << value;
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(
+          TextMessage(MatchesOstream(comparison_stream)),
+          TextMessage(Eq("-112")),
+          ENCODED_MESSAGE(HasValues(ElementsAre(ValueWithStr(Eq("-112"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << value;
+}
+
+TYPED_TEST(SignedIntLogFormatTest, BitfieldPositive) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const struct {
+    TypeParam bits : 6;
+  } value{21};
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << value.bits;
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(
+          TextMessage(MatchesOstream(comparison_stream)), TextMessage(Eq("21")),
+          ENCODED_MESSAGE(HasValues(ElementsAre(ValueWithStr(Eq("21"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << value.bits;
+}
+
+TYPED_TEST(SignedIntLogFormatTest, BitfieldNegative) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const struct {
+    TypeParam bits : 6;
+  } value{-21};
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << value.bits;
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(
+          TextMessage(MatchesOstream(comparison_stream)),
+          TextMessage(Eq("-21")),
+          ENCODED_MESSAGE(HasValues(ElementsAre(ValueWithStr(Eq("-21"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << value.bits;
+}
+
+TEST(SourceLocationTest, Format) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  absl::SourceLocation loc = absl::SourceLocation::current();
+  std::string expected = absl::StrCat(__FILE__, ":", __LINE__ - 1);
+
+  EXPECT_CALL(test_sink, Send(AllOf(TextMessage(Eq(expected)),
+                                    ENCODED_MESSAGE(HasValues(ElementsAre(
+                                        ValueWithStr(Eq(expected))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << loc;
+}
+
+// Ignore these test cases on GCC due to "is too small to hold all values ..."
+// warning.
+#if !defined(__GNUC__) || defined(__clang__)
+// The implementation may choose a signed or unsigned integer type to represent
+// this enum, so it may be tested by either `UnsignedEnumLogFormatTest` or
+// `SignedEnumLogFormatTest`.
+enum MyUnsignedEnum {
+  MyUnsignedEnum_ZERO = 0,
+  MyUnsignedEnum_FORTY_TWO = 42,
+  MyUnsignedEnum_TWO_HUNDRED_TWENTY_FOUR = 224,
+};
+enum MyUnsignedIntEnum : unsigned int {
+  MyUnsignedIntEnum_ZERO = 0,
+  MyUnsignedIntEnum_FORTY_TWO = 42,
+  MyUnsignedIntEnum_TWO_HUNDRED_TWENTY_FOUR = 224,
+};
+
+template <typename T>
+class UnsignedEnumLogFormatTest : public testing::Test {};
+using UnsignedEnumTypes =
+    std::conditional_t<std::is_signed_v<std::underlying_type_t<MyUnsignedEnum>>,
+                       Types<MyUnsignedIntEnum>,
+                       Types<MyUnsignedEnum, MyUnsignedIntEnum>>;
+TYPED_TEST_SUITE(UnsignedEnumLogFormatTest, UnsignedEnumTypes);
+
+TYPED_TEST(UnsignedEnumLogFormatTest, Positive) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const TypeParam value = static_cast<TypeParam>(224);
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << value;
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(
+          TextMessage(MatchesOstream(comparison_stream)),
+          TextMessage(Eq("224")),
+          ENCODED_MESSAGE(HasValues(ElementsAre(ValueWithStr(Eq("224"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << value;
+}
+
+TYPED_TEST(UnsignedEnumLogFormatTest, BitfieldPositive) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const struct {
+    TypeParam bits : 6;
+  } value{static_cast<TypeParam>(42)};
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << value.bits;
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(
+          TextMessage(MatchesOstream(comparison_stream)), TextMessage(Eq("42")),
+          ENCODED_MESSAGE(HasValues(ElementsAre(ValueWithStr(Eq("42"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << value.bits;
+}
+
+enum MySignedEnum {
+  MySignedEnum_NEGATIVE_ONE_HUNDRED_TWELVE = -112,
+  MySignedEnum_NEGATIVE_TWENTY_ONE = -21,
+  MySignedEnum_ZERO = 0,
+  MySignedEnum_TWENTY_ONE = 21,
+  MySignedEnum_TWO_HUNDRED_TWENTY_FOUR = 224,
+};
+enum MySignedIntEnum : signed int {
+  MySignedIntEnum_NEGATIVE_ONE_HUNDRED_TWELVE = -112,
+  MySignedIntEnum_NEGATIVE_TWENTY_ONE = -21,
+  MySignedIntEnum_ZERO = 0,
+  MySignedIntEnum_TWENTY_ONE = 21,
+  MySignedIntEnum_TWO_HUNDRED_TWENTY_FOUR = 224,
+};
+
+template <typename T>
+class SignedEnumLogFormatTest : public testing::Test {};
+using SignedEnumTypes =
+    std::conditional_t<std::is_signed_v<std::underlying_type_t<MyUnsignedEnum>>,
+                       Types<MyUnsignedEnum, MySignedEnum, MySignedIntEnum>,
+                       Types<MySignedEnum, MySignedIntEnum>>;
+TYPED_TEST_SUITE(SignedEnumLogFormatTest, SignedEnumTypes);
+
+TYPED_TEST(SignedEnumLogFormatTest, Positive) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const TypeParam value = static_cast<TypeParam>(224);
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << value;
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(
+          TextMessage(MatchesOstream(comparison_stream)),
+          TextMessage(Eq("224")),
+          ENCODED_MESSAGE(HasValues(ElementsAre(ValueWithStr(Eq("224"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << value;
+}
+
+TYPED_TEST(SignedEnumLogFormatTest, Negative) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const TypeParam value = static_cast<TypeParam>(-112);
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << value;
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(
+          TextMessage(MatchesOstream(comparison_stream)),
+          TextMessage(Eq("-112")),
+          ENCODED_MESSAGE(HasValues(ElementsAre(ValueWithStr(Eq("-112"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << value;
+}
+
+TYPED_TEST(SignedEnumLogFormatTest, BitfieldPositive) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const struct {
+    TypeParam bits : 6;
+  } value{static_cast<TypeParam>(21)};
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << value.bits;
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(
+          TextMessage(MatchesOstream(comparison_stream)), TextMessage(Eq("21")),
+          ENCODED_MESSAGE(HasValues(ElementsAre(ValueWithStr(Eq("21"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << value.bits;
+}
+
+TYPED_TEST(SignedEnumLogFormatTest, BitfieldNegative) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const struct {
+    TypeParam bits : 6;
+  } value{static_cast<TypeParam>(-21)};
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << value.bits;
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(
+          TextMessage(MatchesOstream(comparison_stream)),
+          TextMessage(Eq("-21")),
+          ENCODED_MESSAGE(HasValues(ElementsAre(ValueWithStr(Eq("-21"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << value.bits;
+}
+#endif
+
+TEST(FloatLogFormatTest, Positive) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const float value = 6.02e23f;
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << value;
+
+  EXPECT_CALL(test_sink,
+              Send(AllOf(TextMessage(MatchesOstream(comparison_stream)),
+                         TextMessage(Eq("6.02e+23")),
+                         ENCODED_MESSAGE(HasValues(
+                             ElementsAre(ValueWithStr(Eq("6.02e+23"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << value;
+}
+
+TEST(FloatLogFormatTest, Negative) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const float value = -6.02e23f;
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << value;
+
+  EXPECT_CALL(test_sink,
+              Send(AllOf(TextMessage(MatchesOstream(comparison_stream)),
+                         TextMessage(Eq("-6.02e+23")),
+                         ENCODED_MESSAGE(HasValues(
+                             ElementsAre(ValueWithStr(Eq("-6.02e+23"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << value;
+}
+
+TEST(FloatLogFormatTest, NegativeExponent) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const float value = 6.02e-23f;
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << value;
+
+  EXPECT_CALL(test_sink,
+              Send(AllOf(TextMessage(MatchesOstream(comparison_stream)),
+                         TextMessage(Eq("6.02e-23")),
+                         ENCODED_MESSAGE(HasValues(
+                             ElementsAre(ValueWithStr(Eq("6.02e-23"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << value;
+}
+
+TEST(DoubleLogFormatTest, Positive) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const double value = 6.02e23;
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << value;
+
+  EXPECT_CALL(test_sink,
+              Send(AllOf(TextMessage(MatchesOstream(comparison_stream)),
+                         TextMessage(Eq("6.02e+23")),
+                         ENCODED_MESSAGE(HasValues(
+                             ElementsAre(ValueWithStr(Eq("6.02e+23"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << value;
+}
+
+TEST(DoubleLogFormatTest, Negative) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const double value = -6.02e23;
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << value;
+
+  EXPECT_CALL(test_sink,
+              Send(AllOf(TextMessage(MatchesOstream(comparison_stream)),
+                         TextMessage(Eq("-6.02e+23")),
+                         ENCODED_MESSAGE(HasValues(
+                             ElementsAre(ValueWithStr(Eq("-6.02e+23"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << value;
+}
+
+TEST(DoubleLogFormatTest, NegativeExponent) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const double value = 6.02e-23;
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << value;
+
+  EXPECT_CALL(test_sink,
+              Send(AllOf(TextMessage(MatchesOstream(comparison_stream)),
+                         TextMessage(Eq("6.02e-23")),
+                         ENCODED_MESSAGE(HasValues(
+                             ElementsAre(ValueWithStr(Eq("6.02e-23"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << value;
+}
+
+template <typename T>
+class FloatingPointLogFormatTest : public testing::Test {};
+using FloatingPointTypes = Types<float, double>;
+TYPED_TEST_SUITE(FloatingPointLogFormatTest, FloatingPointTypes);
+
+TYPED_TEST(FloatingPointLogFormatTest, Zero) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const TypeParam value = 0.0;
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << value;
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(
+          TextMessage(MatchesOstream(comparison_stream)), TextMessage(Eq("0")),
+          ENCODED_MESSAGE(HasValues(ElementsAre(ValueWithStr(Eq("0"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << value;
+}
+
+TYPED_TEST(FloatingPointLogFormatTest, Integer) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const TypeParam value = 1.0;
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << value;
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(
+          TextMessage(MatchesOstream(comparison_stream)), TextMessage(Eq("1")),
+          ENCODED_MESSAGE(HasValues(ElementsAre(ValueWithStr(Eq("1"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << value;
+}
+
+TYPED_TEST(FloatingPointLogFormatTest, Infinity) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const TypeParam value = std::numeric_limits<TypeParam>::infinity();
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << value;
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(
+          TextMessage(MatchesOstream(comparison_stream)),
+          TextMessage(AnyOf(Eq("inf"), Eq("Inf"))),
+          ENCODED_MESSAGE(HasValues(ElementsAre(ValueWithStr(Eq("inf"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << value;
+}
+
+TYPED_TEST(FloatingPointLogFormatTest, NegativeInfinity) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const TypeParam value = -std::numeric_limits<TypeParam>::infinity();
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << value;
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(
+          TextMessage(MatchesOstream(comparison_stream)),
+          TextMessage(AnyOf(Eq("-inf"), Eq("-Inf"))),
+          ENCODED_MESSAGE(HasValues(ElementsAre(ValueWithStr(Eq("-inf"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << value;
+}
+
+TYPED_TEST(FloatingPointLogFormatTest, NaN) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const TypeParam value = std::numeric_limits<TypeParam>::quiet_NaN();
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << value;
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(
+          TextMessage(MatchesOstream(comparison_stream)),
+          TextMessage(AnyOf(Eq("nan"), Eq("NaN"))),
+          ENCODED_MESSAGE(HasValues(ElementsAre(ValueWithStr(Eq("nan"))))))));
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << value;
+}
+
+TYPED_TEST(FloatingPointLogFormatTest, NegativeNaN) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const TypeParam value =
+      std::copysign(std::numeric_limits<TypeParam>::quiet_NaN(), -1.0);
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << value;
+
+  // On RISC-V, don't expect that formatting -NaN produces the same string as
+  // streaming it. #ifdefing out just the relevant line breaks the MSVC build,
+  // so duplicate the entire EXPECT_CALL.
+#ifdef __riscv
+  EXPECT_CALL(test_sink,
+              Send(AllOf(TextMessage(AnyOf(Eq("-nan"), Eq("nan"), Eq("NaN"),
+                                           Eq("-nan(ind)"))),
+                         ENCODED_MESSAGE(HasValues(ElementsAre(AnyOf(
+                             ValueWithStr(Eq("-nan")), ValueWithStr(Eq("nan")),
+                             ValueWithStr(Eq("-nan(ind)")))))))));
+#else
+  EXPECT_CALL(test_sink,
+              Send(AllOf(TextMessage(MatchesOstream(comparison_stream)),
+                         TextMessage(AnyOf(Eq("-nan"), Eq("nan"), Eq("NaN"),
+                                           Eq("-nan(ind)"))),
+                         ENCODED_MESSAGE(HasValues(ElementsAre(AnyOf(
+                             ValueWithStr(Eq("-nan")), ValueWithStr(Eq("nan")),
+                             ValueWithStr(Eq("-nan(ind)")))))))));
+#endif
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << value;
+}
+
+template <typename T>
+class VoidPtrLogFormatTest : public testing::Test {};
+using VoidPtrTypes = Types<void*, const void*>;
+TYPED_TEST_SUITE(VoidPtrLogFormatTest, VoidPtrTypes);
+
+TYPED_TEST(VoidPtrLogFormatTest, Null) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const TypeParam value = nullptr;
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << value;
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(TextMessage(MatchesOstream(comparison_stream)),
+                 TextMessage(AnyOf(Eq("(nil)"), Eq("0"), Eq("0x0"),
+                                   Eq("00000000"), Eq("0000000000000000"))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << value;
+}
+
+TYPED_TEST(VoidPtrLogFormatTest, NonNull) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const TypeParam value = reinterpret_cast<TypeParam>(0xdeadbeefULL);
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << value;
+
+  EXPECT_CALL(test_sink,
+              Send(AllOf(TextMessage(MatchesOstream(comparison_stream)),
+                         TextMessage(AnyOf(Eq("0xdeadbeef"), Eq("DEADBEEF"),
+                                           Eq("00000000DEADBEEF"))),
+                         ENCODED_MESSAGE(HasValues(ElementsAre(
+                             AnyOf(ValueWithStr(Eq("0xdeadbeef")),
+                                   ValueWithStr(Eq("00000000DEADBEEF")))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << value;
+}
+
+template <typename T>
+class VolatilePtrLogFormatTest : public testing::Test {};
+using VolatilePtrTypes = Types<
+    volatile void*, const volatile void*, volatile char*, const volatile char*,
+    volatile signed char*, const volatile signed char*, volatile unsigned char*,
+    const volatile unsigned char*, volatile wchar_t*, const volatile wchar_t*>;
+TYPED_TEST_SUITE(VolatilePtrLogFormatTest, VolatilePtrTypes);
+
+TYPED_TEST(VolatilePtrLogFormatTest, Null) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const TypeParam value = nullptr;
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << value;
+
+  // https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p1147r1.html
+#if ABSL_INTERNAL_CPLUSPLUS_LANG >= 202302L
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(TextMessage(MatchesOstream(comparison_stream)),
+                 TextMessage(AnyOf(Eq("(nil)"), Eq("0"), Eq("0x0"),
+                                   Eq("00000000"), Eq("0000000000000000"))))));
+#else
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(
+          TextMessage(MatchesOstream(comparison_stream)),
+          TextMessage(Eq("false")),
+          ENCODED_MESSAGE(HasValues(ElementsAre(ValueWithStr(Eq("false"))))))));
+#endif
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << value;
+}
+
+TYPED_TEST(VolatilePtrLogFormatTest, NonNull) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const TypeParam value = reinterpret_cast<TypeParam>(0xdeadbeefLL);
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << value;
+
+  // https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p1147r1.html
+#if ABSL_INTERNAL_CPLUSPLUS_LANG >= 202302L
+  EXPECT_CALL(test_sink,
+              Send(AllOf(TextMessage(MatchesOstream(comparison_stream)),
+                         TextMessage(AnyOf(Eq("0xdeadbeef"), Eq("DEADBEEF"),
+                                           Eq("00000000DEADBEEF"))),
+                         ENCODED_MESSAGE(HasValues(ElementsAre(
+                             AnyOf(ValueWithStr(Eq("0xdeadbeef")),
+                                   ValueWithStr(Eq("00000000DEADBEEF")))))))));
+#else
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(
+          TextMessage(MatchesOstream(comparison_stream)),
+          TextMessage(Eq("true")),
+          ENCODED_MESSAGE(HasValues(ElementsAre(ValueWithStr(Eq("true"))))))));
+#endif
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << value;
+}
+
+template <typename T>
+class CharPtrLogFormatTest : public testing::Test {};
+using CharPtrTypes = Types<char, const char, signed char, const signed char,
+                           unsigned char, const unsigned char>;
+TYPED_TEST_SUITE(CharPtrLogFormatTest, CharPtrTypes);
+
+TYPED_TEST(CharPtrLogFormatTest, Null) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  // Streaming `([cv] char *)nullptr` into a `std::ostream` is UB, and some C++
+  // standard library implementations choose to crash.  We take measures to log
+  // something useful instead of crashing, even when that differs from the
+  // standard library in use (and thus the behavior of `std::ostream`).
+  TypeParam* const value = nullptr;
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(
+          // `MatchesOstream` deliberately omitted since we deliberately differ.
+          TextMessage(Eq("(null)")), ENCODED_MESSAGE(HasValues(ElementsAre(
+                                         ValueWithStr(Eq("(null)"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << value;
+}
+
+TYPED_TEST(CharPtrLogFormatTest, NonNull) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  TypeParam data[] = {'v', 'a', 'l', 'u', 'e', '\0'};
+  TypeParam* const value = data;
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << value;
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(
+          TextMessage(MatchesOstream(comparison_stream)),
+          TextMessage(Eq("value")),
+          ENCODED_MESSAGE(HasValues(ElementsAre(ValueWithStr(Eq("value"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << value;
+}
+
+template <typename T>
+class WideCharPtrLogFormatTest : public testing::Test {};
+using WideCharPtrTypes = Types<wchar_t, const wchar_t>;
+TYPED_TEST_SUITE(WideCharPtrLogFormatTest, WideCharPtrTypes);
+
+TYPED_TEST(WideCharPtrLogFormatTest, Null) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  TypeParam* const value = nullptr;
+
+  EXPECT_CALL(test_sink, Send(AllOf(TextMessage(Eq("(null)")),
+                                    ENCODED_MESSAGE(HasValues(ElementsAre(
+                                        ValueWithStr(Eq("(null)"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << value;
+}
+
+TYPED_TEST(WideCharPtrLogFormatTest, NonNull) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  TypeParam data[] = {'v', 'a', 'l', 'u', 'e', '\0'};
+  TypeParam* const value = data;
+
+  EXPECT_CALL(test_sink, Send(AllOf(TextMessage(Eq("value")),
+                                    ENCODED_MESSAGE(HasValues(ElementsAre(
+                                        ValueWithStr(Eq("value"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << value;
+}
+
+TEST(BoolLogFormatTest, True) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const bool value = true;
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << value;
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(
+          TextMessage(MatchesOstream(comparison_stream)),
+          TextMessage(Eq("true")),
+          ENCODED_MESSAGE(HasValues(ElementsAre(ValueWithStr(Eq("true"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << value;
+}
+
+TEST(BoolLogFormatTest, False) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const bool value = false;
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << value;
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(
+          TextMessage(MatchesOstream(comparison_stream)),
+          TextMessage(Eq("false")),
+          ENCODED_MESSAGE(HasValues(ElementsAre(ValueWithStr(Eq("false"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << value;
+}
+
+TEST(LogFormatTest, StringLiteral) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << "value";
+
+  EXPECT_CALL(test_sink,
+              Send(AllOf(TextMessage(MatchesOstream(comparison_stream)),
+                         TextMessage(Eq("value")),
+                         ENCODED_MESSAGE(HasValues(
+                             ElementsAre(ValueWithLiteral(Eq("value"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << "value";
+}
+
+TEST(LogFormatTest, WideStringLiteral) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  EXPECT_CALL(test_sink, Send(AllOf(TextMessage(Eq("value")),
+                                    ENCODED_MESSAGE(HasValues(ElementsAre(
+                                        ValueWithLiteral(Eq("value"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << L"value";
+}
+
+TEST(LogFormatTest, CharArray) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  char value[] = "value";
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << value;
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(
+          TextMessage(MatchesOstream(comparison_stream)),
+          TextMessage(Eq("value")),
+          ENCODED_MESSAGE(HasValues(ElementsAre(ValueWithStr(Eq("value"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << value;
+}
+
+TEST(LogFormatTest, WideCharArray) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  wchar_t value[] = L"value";
+
+  EXPECT_CALL(test_sink, Send(AllOf(TextMessage(Eq("value")),
+                                    ENCODED_MESSAGE(HasValues(ElementsAre(
+                                        ValueWithStr(Eq("value"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << value;
+}
+
+// Comprehensive test string for validating wchar_t to UTF-8 conversion.
+// See details in absl/strings/internal/utf8_test.cc.
+//
+// clang-format off
+#define ABSL_LOG_INTERNAL_WIDE_LITERAL L"Holá €1 你好 שָׁלוֹם 👍🏻🇺🇸👩‍❤️‍💋‍👨 中"
+#define ABSL_LOG_INTERNAL_UTF8_LITERAL u8"Holá €1 你好 שָׁלוֹם 👍🏻🇺🇸👩‍❤️‍💋‍👨 中"
+// clang-format on
+
+absl::string_view GetUtf8TestString() {
+  // `u8""` forces UTF-8 encoding; MSVC will default to e.g. CP1252 (and warn)
+  // without it. However, the resulting character type differs between pre-C++20
+  // (`char`) and C++20 (`char8_t`). So we reinterpret_cast to `char*` and wrap
+  // it in a `string_view`.
+  static const absl::string_view kUtf8TestString(
+      reinterpret_cast<const char*>(ABSL_LOG_INTERNAL_UTF8_LITERAL),
+      sizeof(ABSL_LOG_INTERNAL_UTF8_LITERAL) - 1);
+  return kUtf8TestString;
+}
+
+template <typename T>
+class WideStringLogFormatTest : public testing::Test {};
+using StringTypes =
+    Types<std::wstring, const std::wstring, wchar_t[], const wchar_t*>;
+TYPED_TEST_SUITE(WideStringLogFormatTest, StringTypes);
+
+TYPED_TEST(WideStringLogFormatTest, NonLiterals) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  TypeParam value = ABSL_LOG_INTERNAL_WIDE_LITERAL;
+  absl::string_view utf8_value = GetUtf8TestString();
+
+  EXPECT_CALL(test_sink, Send(AllOf(TextMessage(Eq(utf8_value)),
+                                    ENCODED_MESSAGE(HasValues(ElementsAre(
+                                        ValueWithStr(Eq(utf8_value))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << value;
+}
+
+TEST(WideStringLogFormatTest, StringView) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  std::wstring_view value = ABSL_LOG_INTERNAL_WIDE_LITERAL;
+  absl::string_view utf8_value = GetUtf8TestString();
+
+  EXPECT_CALL(test_sink, Send(AllOf(TextMessage(Eq(utf8_value)),
+                                    ENCODED_MESSAGE(HasValues(ElementsAre(
+                                        ValueWithStr(Eq(utf8_value))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << value;
+}
+
+TEST(WideStringLogFormatTest, Literal) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  absl::string_view utf8_value = GetUtf8TestString();
+
+  EXPECT_CALL(test_sink, Send(AllOf(TextMessage(Eq(utf8_value)),
+                                    ENCODED_MESSAGE(HasValues(ElementsAre(
+                                        ValueWithLiteral(Eq(utf8_value))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << ABSL_LOG_INTERNAL_WIDE_LITERAL;
+}
+
+#undef ABSL_LOG_INTERNAL_WIDE_LITERAL
+#undef ABSL_LOG_INTERNAL_UTF8_LITERAL
+
+TYPED_TEST(WideStringLogFormatTest, IsolatedLowSurrogatesAreReplaced) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  TypeParam value = L"AAA \xDC00 BBB";
+  // NOLINTNEXTLINE(readability/utf8)
+  absl::string_view utf8_value = "AAA � BBB";
+
+  EXPECT_CALL(test_sink, Send(AllOf(TextMessage(Eq(utf8_value)),
+                                    ENCODED_MESSAGE(HasValues(ElementsAre(
+                                        ValueWithStr(Eq(utf8_value))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << value;
+}
+
+TYPED_TEST(WideStringLogFormatTest,
+           DISABLED_IsolatedHighSurrogatesAreReplaced) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  TypeParam value = L"AAA \xD800 BBB";
+  // NOLINTNEXTLINE(readability/utf8)
+  absl::string_view utf8_value = "AAA � BBB";
+  // Currently, this is "AAA \xF0\x90 BBB".
+
+  EXPECT_CALL(test_sink, Send(AllOf(TextMessage(Eq(utf8_value)),
+                                    ENCODED_MESSAGE(HasValues(ElementsAre(
+                                        ValueWithStr(Eq(utf8_value))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << value;
+}
+
+TYPED_TEST(WideStringLogFormatTest,
+           DISABLED_ConsecutiveHighSurrogatesAreReplaced) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  TypeParam value = L"AAA \xD800\xD800 BBB";
+  // NOLINTNEXTLINE(readability/utf8)
+  absl::string_view utf8_value = "AAA �� BBB";
+  // Currently, this is "AAA \xF0\x90\xF0\x90 BBB".
+
+  EXPECT_CALL(test_sink, Send(AllOf(TextMessage(Eq(utf8_value)),
+                                    ENCODED_MESSAGE(HasValues(ElementsAre(
+                                        ValueWithStr(Eq(utf8_value))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << value;
+}
+
+TYPED_TEST(WideStringLogFormatTest,
+           DISABLED_HighHighLowSurrogateSequencesAreReplaced) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  TypeParam value = L"AAA \xD800\xD800\xDC00 BBB";
+  // NOLINTNEXTLINE(readability/utf8)
+  absl::string_view utf8_value = "AAA �𐀀 BBB";
+  // Currently, this is "AAA \xF0\x90𐀀 BBB".
+
+  EXPECT_CALL(test_sink, Send(AllOf(TextMessage(Eq(utf8_value)),
+                                    ENCODED_MESSAGE(HasValues(ElementsAre(
+                                        ValueWithStr(Eq(utf8_value))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << value;
+}
+
+TYPED_TEST(WideStringLogFormatTest,
+           DISABLED_TrailingHighSurrogatesAreReplaced) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  TypeParam value = L"AAA \xD800";
+  // NOLINTNEXTLINE(readability/utf8)
+  absl::string_view utf8_value = "AAA �";
+  // Currently, this is "AAA \xF0\x90".
+
+  EXPECT_CALL(test_sink, Send(AllOf(TextMessage(Eq(utf8_value)),
+                                    ENCODED_MESSAGE(HasValues(ElementsAre(
+                                        ValueWithStr(Eq(utf8_value))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << value;
+}
+
+TYPED_TEST(WideStringLogFormatTest, EmptyWideString) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  TypeParam value = L"";
+
+  EXPECT_CALL(test_sink, Send(AllOf(TextMessage(Eq("")),
+                                    ENCODED_MESSAGE(HasValues(
+                                        ElementsAre(ValueWithStr(Eq(""))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << value;
+}
+
+TEST(WideStringLogFormatTest, MixedNarrowAndWideStrings) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+
+  EXPECT_CALL(test_sink, Log(_, _, "1234"));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << "1" << L"2" << "3" << L"4";
+}
+
+class CustomClass {};
+std::ostream& operator<<(std::ostream& os, const CustomClass&) {
+  return os << "CustomClass{}";
+}
+
+TEST(LogFormatTest, Custom) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  CustomClass value;
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << value;
+
+  EXPECT_CALL(test_sink,
+              Send(AllOf(TextMessage(MatchesOstream(comparison_stream)),
+                         TextMessage(Eq("CustomClass{}")),
+                         ENCODED_MESSAGE(HasValues(ElementsAre(
+                             ValueWithStr(Eq("CustomClass{}"))))))));
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << value;
+}
+
+class CustomClassNonCopyable {
+ public:
+  CustomClassNonCopyable() = default;
+  CustomClassNonCopyable(const CustomClassNonCopyable&) = delete;
+  CustomClassNonCopyable& operator=(const CustomClassNonCopyable&) = delete;
+};
+std::ostream& operator<<(std::ostream& os, const CustomClassNonCopyable&) {
+  return os << "CustomClassNonCopyable{}";
+}
+
+TEST(LogFormatTest, CustomNonCopyable) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  CustomClassNonCopyable value;
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << value;
+
+  EXPECT_CALL(test_sink,
+              Send(AllOf(TextMessage(MatchesOstream(comparison_stream)),
+                         TextMessage(Eq("CustomClassNonCopyable{}")),
+                         ENCODED_MESSAGE(HasValues(ElementsAre(
+                             ValueWithStr(Eq("CustomClassNonCopyable{}"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << value;
+}
+
+struct Point {
+  template <typename Sink>
+  friend void AbslStringify(Sink& sink, const Point& p) {
+    absl::Format(&sink, "(%d, %d)", p.x, p.y);
+  }
+
+  int x = 10;
+  int y = 20;
+};
+
+TEST(LogFormatTest, AbslStringifyExample) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  Point p;
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(TextMessage(Eq("(10, 20)")), TextMessage(Eq(absl::StrCat(p))),
+                 ENCODED_MESSAGE(
+                     HasValues(ElementsAre(ValueWithStr(Eq("(10, 20)"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << p;
+}
+
+struct PointWithAbslStringifiyAndOstream {
+  template <typename Sink>
+  friend void AbslStringify(Sink& sink,
+                            const PointWithAbslStringifiyAndOstream& p) {
+    absl::Format(&sink, "(%d, %d)", p.x, p.y);
+  }
+
+  int x = 10;
+  int y = 20;
+};
+
+ABSL_ATTRIBUTE_UNUSED std::ostream& operator<<(
+    std::ostream& os, const PointWithAbslStringifiyAndOstream&) {
+  return os << "Default to AbslStringify()";
+}
+
+TEST(LogFormatTest, CustomWithAbslStringifyAndOstream) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  PointWithAbslStringifiyAndOstream p;
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(TextMessage(Eq("(10, 20)")), TextMessage(Eq(absl::StrCat(p))),
+                 ENCODED_MESSAGE(
+                     HasValues(ElementsAre(ValueWithStr(Eq("(10, 20)"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << p;
+}
+
+struct PointStreamsNothing {
+  template <typename Sink>
+  friend void AbslStringify(Sink&, const PointStreamsNothing&) {}
+
+  int x = 10;
+  int y = 20;
+};
+
+TEST(LogFormatTest, AbslStringifyStreamsNothing) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  PointStreamsNothing p;
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(
+          TextMessage(Eq("77")), TextMessage(Eq(absl::StrCat(p, 77))),
+          ENCODED_MESSAGE(HasValues(ElementsAre(ValueWithStr(Eq("77"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << p << 77;
+}
+
+struct PointMultipleAppend {
+  template <typename Sink>
+  friend void AbslStringify(Sink& sink, const PointMultipleAppend& p) {
+    sink.Append("(");
+    sink.Append(absl::StrCat(p.x, ", ", p.y, ")"));
+  }
+
+  int x = 10;
+  int y = 20;
+};
+
+TEST(LogFormatTest, AbslStringifyMultipleAppend) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  PointMultipleAppend p;
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(TextMessage(Eq("(10, 20)")), TextMessage(Eq(absl::StrCat(p))),
+                 ENCODED_MESSAGE(HasValues(ElementsAre(
+                     ValueWithStr(Eq("(")), ValueWithStr(Eq("10, 20)"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << p;
+}
+
+TEST(ManipulatorLogFormatTest, BoolAlphaTrue) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const bool value = true;
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << std::noboolalpha << value << " "  //
+                    << std::boolalpha << value << " "    //
+                    << std::noboolalpha << value;
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(TextMessage(MatchesOstream(comparison_stream)),
+                 TextMessage(Eq("1 true 1")),
+                 ENCODED_MESSAGE(HasValues(ElementsAre(
+                     ValueWithStr(Eq("1")), ValueWithLiteral(Eq(" ")),
+                     ValueWithStr(Eq("true")), ValueWithLiteral(Eq(" ")),
+                     ValueWithStr(Eq("1"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << std::noboolalpha << value << " "  //
+            << std::boolalpha << value << " "    //
+            << std::noboolalpha << value;
+}
+
+TEST(ManipulatorLogFormatTest, BoolAlphaFalse) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const bool value = false;
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << std::noboolalpha << value << " "  //
+                    << std::boolalpha << value << " "    //
+                    << std::noboolalpha << value;
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(TextMessage(MatchesOstream(comparison_stream)),
+                 TextMessage(Eq("0 false 0")),
+                 ENCODED_MESSAGE(HasValues(ElementsAre(
+                     ValueWithStr(Eq("0")), ValueWithLiteral(Eq(" ")),
+                     ValueWithStr(Eq("false")), ValueWithLiteral(Eq(" ")),
+                     ValueWithStr(Eq("0"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << std::noboolalpha << value << " "  //
+            << std::boolalpha << value << " "    //
+            << std::noboolalpha << value;
+}
+
+TEST(ManipulatorLogFormatTest, ShowPoint) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const double value = 77.0;
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << std::noshowpoint << value << " "  //
+                    << std::showpoint << value << " "    //
+                    << std::noshowpoint << value;
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(TextMessage(MatchesOstream(comparison_stream)),
+                 TextMessage(Eq("77 77.0000 77")),
+                 ENCODED_MESSAGE(HasValues(ElementsAre(
+                     ValueWithStr(Eq("77")), ValueWithLiteral(Eq(" ")),
+                     ValueWithStr(Eq("77.0000")), ValueWithLiteral(Eq(" ")),
+                     ValueWithStr(Eq("77"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << std::noshowpoint << value << " "  //
+            << std::showpoint << value << " "    //
+            << std::noshowpoint << value;
+}
+
+TEST(ManipulatorLogFormatTest, ShowPos) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const int value = 77;
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << std::noshowpos << value << " "  //
+                    << std::showpos << value << " "    //
+                    << std::noshowpos << value;
+
+  EXPECT_CALL(test_sink,
+              Send(AllOf(TextMessage(MatchesOstream(comparison_stream)),
+                         TextMessage(Eq("77 +77 77")),
+                         ENCODED_MESSAGE(HasValues(ElementsAre(
+                             ValueWithStr(Eq("77")), ValueWithLiteral(Eq(" ")),
+                             ValueWithStr(Eq("+77")), ValueWithLiteral(Eq(" ")),
+                             ValueWithStr(Eq("77"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << std::noshowpos << value << " "  //
+            << std::showpos << value << " "    //
+            << std::noshowpos << value;
+}
+
+TEST(ManipulatorLogFormatTest, UppercaseFloat) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const double value = 7.7e7;
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << std::nouppercase << value << " "  //
+                    << std::uppercase << value << " "    //
+                    << std::nouppercase << value;
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(TextMessage(MatchesOstream(comparison_stream)),
+                 TextMessage(Eq("7.7e+07 7.7E+07 7.7e+07")),
+                 ENCODED_MESSAGE(HasValues(ElementsAre(
+                     ValueWithStr(Eq("7.7e+07")), ValueWithLiteral(Eq(" ")),
+                     ValueWithStr(Eq("7.7E+07")), ValueWithLiteral(Eq(" ")),
+                     ValueWithStr(Eq("7.7e+07"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << std::nouppercase << value << " "  //
+            << std::uppercase << value << " "    //
+            << std::nouppercase << value;
+}
+
+TEST(ManipulatorLogFormatTest, Hex) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const int value = 0x77;
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << std::hex << value;
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(
+          TextMessage(MatchesOstream(comparison_stream)),
+          TextMessage(Eq("0x77")),
+          ENCODED_MESSAGE(HasValues(ElementsAre(ValueWithStr(Eq("0x77"))))))));
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << std::hex << value;
+}
+
+TEST(ManipulatorLogFormatTest, Oct) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const int value = 077;
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << std::oct << value;
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(
+          TextMessage(MatchesOstream(comparison_stream)),
+          TextMessage(Eq("077")),
+          ENCODED_MESSAGE(HasValues(ElementsAre(ValueWithStr(Eq("077"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << std::oct << value;
+}
+
+TEST(ManipulatorLogFormatTest, Dec) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const int value = 77;
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << std::hex << std::dec << value;
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(
+          TextMessage(MatchesOstream(comparison_stream)), TextMessage(Eq("77")),
+          ENCODED_MESSAGE(HasValues(ElementsAre(ValueWithStr(Eq("77"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << std::hex << std::dec << value;
+}
+
+TEST(ManipulatorLogFormatTest, ShowbaseHex) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const int value = 0x77;
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << std::hex                         //
+                    << std::noshowbase << value << " "  //
+                    << std::showbase << value << " "    //
+                    << std::noshowbase << value;
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(TextMessage(MatchesOstream(comparison_stream)),
+                 TextMessage(Eq("77 0x77 77")),
+                 ENCODED_MESSAGE(HasValues(ElementsAre(
+                     ValueWithStr(Eq("77")), ValueWithLiteral(Eq(" ")),
+                     ValueWithStr(Eq("0x77")), ValueWithLiteral(Eq(" ")),
+                     ValueWithStr(Eq("77"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << std::hex                         //
+            << std::noshowbase << value << " "  //
+            << std::showbase << value << " "    //
+            << std::noshowbase << value;
+}
+
+TEST(ManipulatorLogFormatTest, ShowbaseOct) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const int value = 077;
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << std::oct                         //
+                    << std::noshowbase << value << " "  //
+                    << std::showbase << value << " "    //
+                    << std::noshowbase << value;
+
+  EXPECT_CALL(test_sink,
+              Send(AllOf(TextMessage(MatchesOstream(comparison_stream)),
+                         TextMessage(Eq("77 077 77")),
+                         ENCODED_MESSAGE(HasValues(ElementsAre(
+                             ValueWithStr(Eq("77")), ValueWithLiteral(Eq(" ")),
+                             ValueWithStr(Eq("077")), ValueWithLiteral(Eq(" ")),
+                             ValueWithStr(Eq("77"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << std::oct                         //
+            << std::noshowbase << value << " "  //
+            << std::showbase << value << " "    //
+            << std::noshowbase << value;
+}
+
+TEST(ManipulatorLogFormatTest, UppercaseHex) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const int value = 0xbeef;
+  auto comparison_stream = ComparisonStream();
+  comparison_stream                        //
+      << std::hex                          //
+      << std::nouppercase << value << " "  //
+      << std::uppercase << value << " "    //
+      << std::nouppercase << value;
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(TextMessage(MatchesOstream(comparison_stream)),
+                 TextMessage(Eq("0xbeef 0XBEEF 0xbeef")),
+                 ENCODED_MESSAGE(HasValues(ElementsAre(
+                     ValueWithStr(Eq("0xbeef")), ValueWithLiteral(Eq(" ")),
+                     ValueWithStr(Eq("0XBEEF")), ValueWithLiteral(Eq(" ")),
+                     ValueWithStr(Eq("0xbeef"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << std::hex                          //
+            << std::nouppercase << value << " "  //
+            << std::uppercase << value << " "    //
+            << std::nouppercase << value;
+}
+
+TEST(ManipulatorLogFormatTest, FixedFloat) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const double value = 7.7e7;
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << std::fixed << value;
+
+  EXPECT_CALL(test_sink,
+              Send(AllOf(TextMessage(MatchesOstream(comparison_stream)),
+                         TextMessage(Eq("77000000.000000")),
+                         ENCODED_MESSAGE(HasValues(ElementsAre(
+                             ValueWithStr(Eq("77000000.000000"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << std::fixed << value;
+}
+
+TEST(ManipulatorLogFormatTest, ScientificFloat) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const double value = 7.7e7;
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << std::scientific << value;
+
+  EXPECT_CALL(test_sink,
+              Send(AllOf(TextMessage(MatchesOstream(comparison_stream)),
+                         TextMessage(Eq("7.700000e+07")),
+                         ENCODED_MESSAGE(HasValues(
+                             ElementsAre(ValueWithStr(Eq("7.700000e+07"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << std::scientific << value;
+}
+
+#if defined(__BIONIC__) && (!defined(__ANDROID_API__) || __ANDROID_API__ < 22)
+// Bionic doesn't support `%a` until API 22, so this prints 'a' even if the
+// C++ standard library implements it correctly (by forwarding to printf).
+#elif defined(__GLIBCXX__) && __cplusplus < 201402L
+// libstdc++ shipped C++11 support without `std::hexfloat`.
+#else
+TEST(ManipulatorLogFormatTest, FixedAndScientificFloat) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const double value = 7.7e7;
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << std::setiosflags(std::ios_base::scientific |
+                                        std::ios_base::fixed)
+                    << value;
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(TextMessage(MatchesOstream(comparison_stream)),
+                 TextMessage(AnyOf(Eq("0x1.25bb50p+26"), Eq("0x1.25bb5p+26"),
+                                   Eq("0x1.25bb500000000p+26"))),
+                 ENCODED_MESSAGE(HasValues(ElementsAre(
+                     AnyOf(ValueWithStr(Eq("0x1.25bb5p+26")),
+                           ValueWithStr(Eq("0x1.25bb500000000p+26")))))))));
+
+  test_sink.StartCapturingLogs();
+
+  // This combination should mean the same thing as `std::hexfloat`.
+  LOG(INFO) << std::setiosflags(std::ios_base::scientific |
+                                std::ios_base::fixed)
+            << value;
+}
+#endif
+
+#if defined(__BIONIC__) && (!defined(__ANDROID_API__) || __ANDROID_API__ < 22)
+// Bionic doesn't support `%a` until API 22, so this prints 'a' even if the C++
+// standard library supports `std::hexfloat` (by forwarding to printf).
+#elif defined(__GLIBCXX__) && __cplusplus < 201402L
+// libstdc++ shipped C++11 support without `std::hexfloat`.
+#else
+TEST(ManipulatorLogFormatTest, HexfloatFloat) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const double value = 7.7e7;
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << std::hexfloat << value;
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(TextMessage(MatchesOstream(comparison_stream)),
+                 TextMessage(AnyOf(Eq("0x1.25bb50p+26"), Eq("0x1.25bb5p+26"),
+                                   Eq("0x1.25bb500000000p+26"))),
+                 ENCODED_MESSAGE(HasValues(ElementsAre(
+                     AnyOf(ValueWithStr(Eq("0x1.25bb5p+26")),
+                           ValueWithStr(Eq("0x1.25bb500000000p+26")))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << std::hexfloat << value;
+}
+#endif
+
+TEST(ManipulatorLogFormatTest, DefaultFloatFloat) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const double value = 7.7e7;
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << std::hexfloat << std::defaultfloat << value;
+
+  EXPECT_CALL(test_sink,
+              Send(AllOf(TextMessage(MatchesOstream(comparison_stream)),
+                         TextMessage(Eq("7.7e+07")),
+                         ENCODED_MESSAGE(HasValues(
+                             ElementsAre(ValueWithStr(Eq("7.7e+07"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << std::hexfloat << std::defaultfloat << value;
+}
+
+TEST(ManipulatorLogFormatTest, Ends) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << std::ends;
+
+  EXPECT_CALL(test_sink,
+              Send(AllOf(TextMessage(MatchesOstream(comparison_stream)),
+                         TextMessage(Eq(absl::string_view("\0", 1))),
+                         ENCODED_MESSAGE(HasValues(ElementsAre(
+                             ValueWithStr(Eq(absl::string_view("\0", 1)))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << std::ends;
+}
+
+TEST(ManipulatorLogFormatTest, Endl) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << std::endl;
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(
+          TextMessage(MatchesOstream(comparison_stream)),
+          TextMessage(Eq("\n")),
+          ENCODED_MESSAGE(HasValues(ElementsAre(ValueWithStr(Eq("\n"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << std::endl;
+}
+
+TEST(ManipulatorLogFormatTest, SetIosFlags) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const int value = 0x77;
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << std::resetiosflags(std::ios_base::basefield)
+                    << std::setiosflags(std::ios_base::hex) << value << " "  //
+                    << std::resetiosflags(std::ios_base::basefield)
+                    << std::setiosflags(std::ios_base::dec) << value;
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(
+          TextMessage(MatchesOstream(comparison_stream)),
+          TextMessage(Eq("0x77 119")),
+          // `std::setiosflags` and `std::resetiosflags` aren't manipulators.
+          // We're unable to distinguish their return type(s) from arbitrary
+          // user-defined types and thus don't suppress the empty str value.
+          ENCODED_MESSAGE(HasValues(ElementsAre(ValueWithStr(Eq("0x77")),
+                                                ValueWithLiteral(Eq(" ")),
+                                                ValueWithStr(Eq("119"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << std::resetiosflags(std::ios_base::basefield)
+            << std::setiosflags(std::ios_base::hex) << value << " "  //
+            << std::resetiosflags(std::ios_base::basefield)
+            << std::setiosflags(std::ios_base::dec) << value;
+}
+
+TEST(ManipulatorLogFormatTest, SetBase) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const int value = 0x77;
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << std::setbase(16) << value << " "  //
+                    << std::setbase(0) << value;
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(TextMessage(MatchesOstream(comparison_stream)),
+                 TextMessage(Eq("0x77 119")),
+                 // `std::setbase` isn't a manipulator.  We're unable to
+                 // distinguish its return type from arbitrary user-defined
+                 // types and thus don't suppress the empty str value.
+                 ENCODED_MESSAGE(HasValues(ElementsAre(
+                     ValueWithStr(Eq("0x77")), ValueWithLiteral(Eq(" ")),
+                     ValueWithStr(Eq("119"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << std::setbase(16) << value << " "  //
+            << std::setbase(0) << value;
+}
+
+TEST(ManipulatorLogFormatTest, SetPrecision) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const double value = 6.022140857e23;
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << std::setprecision(4) << value;
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(TextMessage(MatchesOstream(comparison_stream)),
+                 TextMessage(Eq("6.022e+23")),
+                 // `std::setprecision` isn't a manipulator.  We're unable to
+                 // distinguish its return type from arbitrary user-defined
+                 // types and thus don't suppress the empty str value.
+                 ENCODED_MESSAGE(
+                     HasValues(ElementsAre(ValueWithStr(Eq("6.022e+23"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << std::setprecision(4) << value;
+}
+
+TEST(ManipulatorLogFormatTest, SetPrecisionOverflow) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const double value = 6.022140857e23;
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << std::setprecision(200) << value;
+
+  EXPECT_CALL(test_sink,
+              Send(AllOf(TextMessage(MatchesOstream(comparison_stream)),
+                         TextMessage(Eq("602214085700000015187968")),
+                         ENCODED_MESSAGE(HasValues(ElementsAre(
+                             ValueWithStr(Eq("602214085700000015187968"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << std::setprecision(200) << value;
+}
+
+TEST(ManipulatorLogFormatTest, SetW) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const int value = 77;
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << std::setw(8) << value;
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(TextMessage(MatchesOstream(comparison_stream)),
+                 TextMessage(Eq("      77")),
+                 // `std::setw` isn't a manipulator.  We're unable to
+                 // distinguish its return type from arbitrary user-defined
+                 // types and thus don't suppress the empty str value.
+                 ENCODED_MESSAGE(
+                     HasValues(ElementsAre(ValueWithStr(Eq("      77"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << std::setw(8) << value;
+}
+
+TEST(ManipulatorLogFormatTest, Left) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const int value = -77;
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << std::left << std::setw(8) << value;
+
+  EXPECT_CALL(test_sink,
+              Send(AllOf(TextMessage(MatchesOstream(comparison_stream)),
+                         TextMessage(Eq("-77     ")),
+                         ENCODED_MESSAGE(HasValues(
+                             ElementsAre(ValueWithStr(Eq("-77     "))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << std::left << std::setw(8) << value;
+}
+
+TEST(ManipulatorLogFormatTest, Right) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const int value = -77;
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << std::right << std::setw(8) << value;
+
+  EXPECT_CALL(test_sink,
+              Send(AllOf(TextMessage(MatchesOstream(comparison_stream)),
+                         TextMessage(Eq("     -77")),
+                         ENCODED_MESSAGE(HasValues(
+                             ElementsAre(ValueWithStr(Eq("     -77"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << std::right << std::setw(8) << value;
+}
+
+TEST(ManipulatorLogFormatTest, Internal) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const int value = -77;
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << std::internal << std::setw(8) << value;
+
+  EXPECT_CALL(test_sink,
+              Send(AllOf(TextMessage(MatchesOstream(comparison_stream)),
+                         TextMessage(Eq("-     77")),
+                         ENCODED_MESSAGE(HasValues(
+                             ElementsAre(ValueWithStr(Eq("-     77"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << std::internal << std::setw(8) << value;
+}
+
+TEST(ManipulatorLogFormatTest, SetFill) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const int value = 77;
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << std::setfill('0') << std::setw(8) << value;
+
+  EXPECT_CALL(test_sink,
+              Send(AllOf(TextMessage(MatchesOstream(comparison_stream)),
+                         TextMessage(Eq("00000077")),
+                         // `std::setfill` isn't a manipulator.  We're
+                         // unable to distinguish its return
+                         // type from arbitrary user-defined types and
+                         // thus don't suppress the empty str value.
+                         ENCODED_MESSAGE(HasValues(
+                             ElementsAre(ValueWithStr(Eq("00000077"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << std::setfill('0') << std::setw(8) << value;
+}
+
+class FromCustomClass {};
+std::ostream& operator<<(std::ostream& os, const FromCustomClass&) {
+  return os << "FromCustomClass{}" << std::hex;
+}
+
+TEST(ManipulatorLogFormatTest, FromCustom) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  FromCustomClass value;
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << value << " " << 0x77;
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(TextMessage(MatchesOstream(comparison_stream)),
+                 TextMessage(Eq("FromCustomClass{} 0x77")),
+                 ENCODED_MESSAGE(HasValues(ElementsAre(
+                     ValueWithStr(Eq("FromCustomClass{}")),
+                     ValueWithLiteral(Eq(" ")), ValueWithStr(Eq("0x77"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << value << " " << 0x77;
+}
+
+class StreamsNothing {};
+std::ostream& operator<<(std::ostream& os, const StreamsNothing&) { return os; }
+
+TEST(ManipulatorLogFormatTest, CustomClassStreamsNothing) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  StreamsNothing value;
+  auto comparison_stream = ComparisonStream();
+  comparison_stream << value << 77;
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(
+          TextMessage(MatchesOstream(comparison_stream)), TextMessage(Eq("77")),
+          ENCODED_MESSAGE(HasValues(ElementsAre(ValueWithStr(Eq("77"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << value << 77;
+}
+
+struct PointPercentV {
+  template <typename Sink>
+  friend void AbslStringify(Sink& sink, const PointPercentV& p) {
+    absl::Format(&sink, "(%v, %v)", p.x, p.y);
+  }
+
+  int x = 10;
+  int y = 20;
+};
+
+TEST(ManipulatorLogFormatTest, IOManipsDoNotAffectAbslStringify) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  PointPercentV p;
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(TextMessage(Eq("(10, 20)")), TextMessage(Eq(absl::StrCat(p))),
+                 ENCODED_MESSAGE(
+                     HasValues(ElementsAre(ValueWithStr(Eq("(10, 20)"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << std::hex << p;
+}
+
+TEST(StructuredLoggingOverflowTest, TruncatesStrings) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  // This message is too long and should be truncated to some unspecified size
+  // no greater than the buffer size but not too much less either.  It should be
+  // truncated rather than discarded.
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(
+          TextMessage(AllOf(
+              SizeIs(AllOf(Ge(absl::log_internal::kLogMessageBufferSize - 256),
+                           Le(absl::log_internal::kLogMessageBufferSize))),
+              Each(Eq('x')))),
+          ENCODED_MESSAGE(HasOneStrThat(AllOf(
+              SizeIs(AllOf(Ge(absl::log_internal::kLogMessageBufferSize - 256),
+                           Le(absl::log_internal::kLogMessageBufferSize))),
+              Each(Eq('x'))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << std::string(2 * absl::log_internal::kLogMessageBufferSize, 'x');
+}
+
+TEST(StructuredLoggingOverflowTest, TruncatesWideStrings) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  // This message is too long and should be truncated to some unspecified size
+  // no greater than the buffer size but not too much less either.  It should be
+  // truncated rather than discarded.
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(
+          TextMessage(AllOf(
+              SizeIs(AllOf(Ge(absl::log_internal::kLogMessageBufferSize - 256),
+                           Le(absl::log_internal::kLogMessageBufferSize))),
+              Each(Eq('x')))),
+          ENCODED_MESSAGE(HasOneStrThat(AllOf(
+              SizeIs(AllOf(Ge(absl::log_internal::kLogMessageBufferSize - 256),
+                           Le(absl::log_internal::kLogMessageBufferSize))),
+              Each(Eq('x'))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << std::wstring(2 * absl::log_internal::kLogMessageBufferSize,
+                            L'x');
+}
+
+struct StringLike {
+  absl::string_view data;
+};
+std::ostream& operator<<(std::ostream& os, StringLike str) {
+  return os << str.data;
+}
+
+TEST(StructuredLoggingOverflowTest, TruncatesInsertionOperators) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  // This message is too long and should be truncated to some unspecified size
+  // no greater than the buffer size but not too much less either.  It should be
+  // truncated rather than discarded.
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(
+          TextMessage(AllOf(
+              SizeIs(AllOf(Ge(absl::log_internal::kLogMessageBufferSize - 256),
+                           Le(absl::log_internal::kLogMessageBufferSize))),
+              Each(Eq('x')))),
+          ENCODED_MESSAGE(HasOneStrThat(AllOf(
+              SizeIs(AllOf(Ge(absl::log_internal::kLogMessageBufferSize - 256),
+                           Le(absl::log_internal::kLogMessageBufferSize))),
+              Each(Eq('x'))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << StringLike{
+      std::string(2 * absl::log_internal::kLogMessageBufferSize, 'x')};
+}
+
+// Returns the size of the largest string that will fit in a `LOG` message
+// buffer with no prefix.
+size_t MaxLogFieldLengthNoPrefix() {
+  class StringLengthExtractorSink : public absl::LogSink {
+   public:
+    void Send(const absl::LogEntry& entry) override {
+      CHECK(!size_.has_value());
+      CHECK_EQ(entry.text_message().find_first_not_of('x'),
+               absl::string_view::npos);
+      size_.emplace(entry.text_message().size());
+    }
+    size_t size() const {
+      CHECK(size_.has_value());
+      return *size_;
+    }
+
+   private:
+    std::optional<size_t> size_;
+  } extractor_sink;
+  LOG(INFO).NoPrefix().ToSinkOnly(&extractor_sink)
+      << std::string(2 * absl::log_internal::kLogMessageBufferSize, 'x');
+  return extractor_sink.size();
+}
+
+TEST(StructuredLoggingOverflowTest, TruncatesStringsCleanly) {
+  const size_t longest_fit = MaxLogFieldLengthNoPrefix();
+  // To log a second value field, we need four bytes: two tag/type bytes and two
+  // sizes.  To put any data in the field we need a fifth byte.
+  {
+    absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+    EXPECT_CALL(test_sink, Send).Times(0);
+    EXPECT_CALL(test_sink,
+                Send(AllOf(ENCODED_MESSAGE(HasOneStrThat(
+                               AllOf(SizeIs(longest_fit), Each(Eq('x'))))),
+                           RawEncodedMessage(AsString(EndsWith("x"))))));
+    test_sink.StartCapturingLogs();
+    // x fits exactly, no part of y fits.
+    LOG(INFO).NoPrefix() << std::string(longest_fit, 'x') << "y";
+  }
+  {
+    absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+    EXPECT_CALL(test_sink, Send).Times(0);
+    EXPECT_CALL(test_sink,
+                Send(AllOf(ENCODED_MESSAGE(HasOneStrThat(
+                               AllOf(SizeIs(longest_fit - 1), Each(Eq('x'))))),
+                           RawEncodedMessage(AsString(EndsWith("x"))))));
+    test_sink.StartCapturingLogs();
+    // x fits, one byte from y's header fits but shouldn't be visible.
+    LOG(INFO).NoPrefix() << std::string(longest_fit - 1, 'x') << "y";
+  }
+  {
+    absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+    EXPECT_CALL(test_sink, Send).Times(0);
+    EXPECT_CALL(test_sink,
+                Send(AllOf(ENCODED_MESSAGE(HasOneStrThat(
+                               AllOf(SizeIs(longest_fit - 2), Each(Eq('x'))))),
+                           RawEncodedMessage(AsString(EndsWith("x"))))));
+    test_sink.StartCapturingLogs();
+    // x fits, two bytes from y's header fit but shouldn't be visible.
+    LOG(INFO).NoPrefix() << std::string(longest_fit - 2, 'x') << "y";
+  }
+  {
+    absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+    EXPECT_CALL(test_sink, Send).Times(0);
+    EXPECT_CALL(test_sink,
+                Send(AllOf(ENCODED_MESSAGE(HasOneStrThat(
+                               AllOf(SizeIs(longest_fit - 3), Each(Eq('x'))))),
+                           RawEncodedMessage(AsString(EndsWith("x"))))));
+    test_sink.StartCapturingLogs();
+    // x fits, three bytes from y's header fit but shouldn't be visible.
+    LOG(INFO).NoPrefix() << std::string(longest_fit - 3, 'x') << "y";
+  }
+  {
+    absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+    EXPECT_CALL(test_sink, Send).Times(0);
+    EXPECT_CALL(test_sink,
+                Send(AllOf(ENCODED_MESSAGE(HasOneStrAndOneLiteralThat(
+                               AllOf(SizeIs(longest_fit - 4), Each(Eq('x'))),
+                               IsEmpty())),
+                           RawEncodedMessage(Not(AsString(EndsWith("x")))))));
+    test_sink.StartCapturingLogs();
+    // x fits, all four bytes from y's header fit but no data bytes do, so we
+    // encode an empty string.
+    LOG(INFO).NoPrefix() << std::string(longest_fit - 4, 'x') << "y";
+  }
+  {
+    absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+    EXPECT_CALL(test_sink, Send).Times(0);
+    EXPECT_CALL(
+        test_sink,
+        Send(AllOf(ENCODED_MESSAGE(HasOneStrAndOneLiteralThat(
+                       AllOf(SizeIs(longest_fit - 5), Each(Eq('x'))), Eq("y"))),
+                   RawEncodedMessage(AsString(EndsWith("y"))))));
+    test_sink.StartCapturingLogs();
+    // x fits, y fits exactly.
+    LOG(INFO).NoPrefix() << std::string(longest_fit - 5, 'x') << "y";
+  }
+}
+
+TEST(StructuredLoggingOverflowTest, TruncatesInsertionOperatorsCleanly) {
+  const size_t longest_fit = MaxLogFieldLengthNoPrefix();
+  // To log a second value field, we need four bytes: two tag/type bytes and two
+  // sizes.  To put any data in the field we need a fifth byte.
+  {
+    absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+    EXPECT_CALL(test_sink, Send).Times(0);
+    EXPECT_CALL(test_sink,
+                Send(AllOf(ENCODED_MESSAGE(HasOneStrThat(
+                               AllOf(SizeIs(longest_fit), Each(Eq('x'))))),
+                           RawEncodedMessage(AsString(EndsWith("x"))))));
+    test_sink.StartCapturingLogs();
+    // x fits exactly, no part of y fits.
+    LOG(INFO).NoPrefix() << std::string(longest_fit, 'x') << StringLike{"y"};
+  }
+  {
+    absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+    EXPECT_CALL(test_sink, Send).Times(0);
+    EXPECT_CALL(test_sink,
+                Send(AllOf(ENCODED_MESSAGE(HasOneStrThat(
+                               AllOf(SizeIs(longest_fit - 1), Each(Eq('x'))))),
+                           RawEncodedMessage(AsString(EndsWith("x"))))));
+    test_sink.StartCapturingLogs();
+    // x fits, one byte from y's header fits but shouldn't be visible.
+    LOG(INFO).NoPrefix() << std::string(longest_fit - 1, 'x')
+                         << StringLike{"y"};
+  }
+  {
+    absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+    EXPECT_CALL(test_sink, Send).Times(0);
+    EXPECT_CALL(test_sink,
+                Send(AllOf(ENCODED_MESSAGE(HasOneStrThat(
+                               AllOf(SizeIs(longest_fit - 2), Each(Eq('x'))))),
+                           RawEncodedMessage(AsString(EndsWith("x"))))));
+    test_sink.StartCapturingLogs();
+    // x fits, two bytes from y's header fit but shouldn't be visible.
+    LOG(INFO).NoPrefix() << std::string(longest_fit - 2, 'x')
+                         << StringLike{"y"};
+  }
+  {
+    absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+    EXPECT_CALL(test_sink, Send).Times(0);
+    EXPECT_CALL(test_sink,
+                Send(AllOf(ENCODED_MESSAGE(HasOneStrThat(
+                               AllOf(SizeIs(longest_fit - 3), Each(Eq('x'))))),
+                           RawEncodedMessage(AsString(EndsWith("x"))))));
+    test_sink.StartCapturingLogs();
+    // x fits, three bytes from y's header fit but shouldn't be visible.
+    LOG(INFO).NoPrefix() << std::string(longest_fit - 3, 'x')
+                         << StringLike{"y"};
+  }
+  {
+    absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+    EXPECT_CALL(test_sink, Send).Times(0);
+    EXPECT_CALL(test_sink,
+                Send(AllOf(ENCODED_MESSAGE(HasOneStrThat(
+                               AllOf(SizeIs(longest_fit - 4), Each(Eq('x'))))),
+                           RawEncodedMessage(AsString(EndsWith("x"))))));
+    test_sink.StartCapturingLogs();
+    // x fits, all four bytes from y's header fit but no data bytes do.  We
+    // don't encode an empty string here because every I/O manipulator hits this
+    // codepath and those shouldn't leave empty strings behind.
+    LOG(INFO).NoPrefix() << std::string(longest_fit - 4, 'x')
+                         << StringLike{"y"};
+  }
+  {
+    absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+    EXPECT_CALL(test_sink, Send).Times(0);
+    EXPECT_CALL(
+        test_sink,
+        Send(AllOf(ENCODED_MESSAGE(HasTwoStrsThat(
+                       AllOf(SizeIs(longest_fit - 5), Each(Eq('x'))), Eq("y"))),
+                   RawEncodedMessage(AsString(EndsWith("y"))))));
+    test_sink.StartCapturingLogs();
+    // x fits, y fits exactly.
+    LOG(INFO).NoPrefix() << std::string(longest_fit - 5, 'x')
+                         << StringLike{"y"};
+  }
+}
+
+}  // namespace
diff --git a/absl/log/log_macro_hygiene_test.cc b/absl/log/log_macro_hygiene_test.cc
new file mode 100644
index 00000000000..dad9389e741
--- /dev/null
+++ b/absl/log/log_macro_hygiene_test.cc
@@ -0,0 +1,187 @@
+//
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/attributes.h"
+#include "absl/base/log_severity.h"
+#include "absl/log/log.h"
+#include "absl/log/scoped_mock_log.h"
+
+namespace {
+using ::testing::_;
+using ::testing::Eq;
+
+namespace not_absl {
+
+class Dummy {
+ public:
+  Dummy() {}
+
+ private:
+  Dummy(const Dummy&) = delete;
+  Dummy& operator=(const Dummy&) = delete;
+};
+
+// This line tests that local definitions of INFO, WARNING, ERROR, and
+// etc don't shadow the global ones used by the logging macros.  If
+// they do, the LOG() calls in the tests won't compile, catching the
+// bug.
+const Dummy INFO, WARNING, ERROR, FATAL, NUM_SEVERITIES;
+
+// These makes sure that the uses of same-named types in the
+// implementation of the logging macros are fully qualified.
+class string {};
+class vector {};
+class LogMessage {};
+class LogMessageFatal {};
+class LogMessageQuietlyFatal {};
+class LogMessageVoidify {};
+class LogSink {};
+class NullStream {};
+class NullStreamFatal {};
+
+}  // namespace not_absl
+
+using namespace not_absl;  // NOLINT
+
+// Tests for LOG(LEVEL(()).
+
+TEST(LogHygieneTest, WorksForQualifiedSeverity) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+
+  ::testing::InSequence seq;
+  EXPECT_CALL(test_sink, Log(absl::LogSeverity::kInfo, _, "To INFO"));
+  EXPECT_CALL(test_sink, Log(absl::LogSeverity::kWarning, _, "To WARNING"));
+  EXPECT_CALL(test_sink, Log(absl::LogSeverity::kError, _, "To ERROR"));
+
+  test_sink.StartCapturingLogs();
+  // Note that LOG(LEVEL()) expects the severity as a run-time
+  // expression (as opposed to a compile-time constant).  Hence we
+  // test that :: is allowed before INFO, etc.
+  LOG(LEVEL(absl::LogSeverity::kInfo)) << "To INFO";
+  LOG(LEVEL(absl::LogSeverity::kWarning)) << "To WARNING";
+  LOG(LEVEL(absl::LogSeverity::kError)) << "To ERROR";
+}
+
+TEST(LogHygieneTest, WorksWithAlternativeINFOSymbol) {
+  const double INFO ABSL_ATTRIBUTE_UNUSED = 7.77;
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+
+  EXPECT_CALL(test_sink, Log(absl::LogSeverity::kInfo, _, "Hello world"));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << "Hello world";
+}
+
+TEST(LogHygieneTest, WorksWithAlternativeWARNINGSymbol) {
+  const double WARNING ABSL_ATTRIBUTE_UNUSED = 7.77;
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+
+  EXPECT_CALL(test_sink, Log(absl::LogSeverity::kWarning, _, "Hello world"));
+
+  test_sink.StartCapturingLogs();
+  LOG(WARNING) << "Hello world";
+}
+
+TEST(LogHygieneTest, WorksWithAlternativeERRORSymbol) {
+  const double ERROR ABSL_ATTRIBUTE_UNUSED = 7.77;
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+
+  EXPECT_CALL(test_sink, Log(absl::LogSeverity::kError, _, "Hello world"));
+
+  test_sink.StartCapturingLogs();
+  LOG(ERROR) << "Hello world";
+}
+
+TEST(LogHygieneTest, WorksWithAlternativeLEVELSymbol) {
+  const double LEVEL ABSL_ATTRIBUTE_UNUSED = 7.77;
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+
+  EXPECT_CALL(test_sink, Log(absl::LogSeverity::kError, _, "Hello world"));
+
+  test_sink.StartCapturingLogs();
+  LOG(LEVEL(absl::LogSeverity::kError)) << "Hello world";
+}
+
+#define INFO Bogus
+#ifdef NDEBUG
+constexpr bool IsOptimized = false;
+#else
+constexpr bool IsOptimized = true;
+#endif
+
+TEST(LogHygieneTest, WorksWithINFODefined) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+
+  EXPECT_CALL(test_sink, Log(absl::LogSeverity::kInfo, _, "Hello world"))
+      .Times(2 + (IsOptimized ? 2 : 0));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << "Hello world";
+  LOG_IF(INFO, true) << "Hello world";
+
+  DLOG(INFO) << "Hello world";
+  DLOG_IF(INFO, true) << "Hello world";
+}
+
+#undef INFO
+
+#define _INFO Bogus
+TEST(LogHygieneTest, WorksWithUnderscoreINFODefined) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+
+  EXPECT_CALL(test_sink, Log(absl::LogSeverity::kInfo, _, "Hello world"))
+      .Times(2 + (IsOptimized ? 2 : 0));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << "Hello world";
+  LOG_IF(INFO, true) << "Hello world";
+
+  DLOG(INFO) << "Hello world";
+  DLOG_IF(INFO, true) << "Hello world";
+}
+#undef _INFO
+
+TEST(LogHygieneTest, ExpressionEvaluationInLEVELSeverity) {
+  auto i = static_cast<int>(absl::LogSeverity::kInfo);
+  LOG(LEVEL(++i)) << "hello world";  // NOLINT
+  EXPECT_THAT(i, Eq(static_cast<int>(absl::LogSeverity::kInfo) + 1));
+}
+
+TEST(LogHygieneTest, ExpressionEvaluationInStreamedMessage) {
+  int i = 0;
+  LOG(INFO) << ++i;
+  EXPECT_THAT(i, 1);
+  LOG_IF(INFO, false) << ++i;
+  EXPECT_THAT(i, 1);
+}
+
+// Tests that macros are usable in unbraced switch statements.
+// -----------------------------------------------------------
+
+class UnbracedSwitchCompileTest {
+  static void Log() {
+    switch (0) {
+      case 0:
+        LOG(INFO);
+        break;
+      default:
+        break;
+    }
+  }
+};
+
+}  // namespace
diff --git a/absl/log/log_modifier_methods_test.cc b/absl/log/log_modifier_methods_test.cc
new file mode 100644
index 00000000000..42a5147fc9c
--- /dev/null
+++ b/absl/log/log_modifier_methods_test.cc
@@ -0,0 +1,264 @@
+//
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <errno.h>
+
+#include <string>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/log_severity.h"
+#include "absl/log/internal/test_actions.h"
+#include "absl/log/internal/test_helpers.h"
+#include "absl/log/internal/test_matchers.h"
+#include "absl/log/log.h"
+#include "absl/log/log_entry.h"
+#include "absl/log/log_sink.h"
+#include "absl/log/scoped_mock_log.h"
+#include "absl/strings/match.h"
+#include "absl/strings/string_view.h"
+#include "absl/time/time.h"
+#include "absl/types/source_location.h"
+
+namespace {
+#if GTEST_HAS_DEATH_TEST
+using ::absl::log_internal::DeathTestExpectedLogging;
+using ::absl::log_internal::DeathTestUnexpectedLogging;
+using ::absl::log_internal::DeathTestValidateExpectations;
+using ::absl::log_internal::DiedOfQFatal;
+#endif
+using ::absl::log_internal::LogSeverity;
+using ::absl::log_internal::Prefix;
+using ::absl::log_internal::SourceBasename;
+using ::absl::log_internal::SourceFilename;
+using ::absl::log_internal::SourceLine;
+using ::absl::log_internal::Stacktrace;
+using ::absl::log_internal::TextMessage;
+using ::absl::log_internal::TextMessageWithPrefix;
+using ::absl::log_internal::TextMessageWithPrefixAndNewline;
+using ::absl::log_internal::TextPrefix;
+using ::absl::log_internal::ThreadID;
+using ::absl::log_internal::Timestamp;
+using ::absl::log_internal::Verbosity;
+
+using ::testing::AllOf;
+using ::testing::AnyNumber;
+using ::testing::AnyOf;
+using ::testing::Eq;
+using ::testing::IsEmpty;
+using ::testing::IsFalse;
+using ::testing::Truly;
+
+TEST(TailCallsModifiesTest, AtLocationFileLine) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(
+          // The metadata should change:
+          SourceFilename(Eq("/my/very/very/very_long_source_file.cc")),
+          SourceBasename(Eq("very_long_source_file.cc")), SourceLine(Eq(777)),
+          // The logged line should change too, even though the prefix must
+          // grow to fit the new metadata.
+          TextMessageWithPrefix(Truly([](absl::string_view msg) {
+            return absl::EndsWith(msg,
+                                  " very_long_source_file.cc:777] hello world");
+          })))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO).AtLocation("/my/very/very/very_long_source_file.cc", 777)
+      << "hello world";
+}
+
+TEST(TailCallsModifiesTest, AtLocationFileLineLifetime) {
+  // The macro takes care to not use this temporary after its lifetime.
+  // The only salient expectation is "no sanitizer diagnostics".
+  LOG(INFO).AtLocation(std::string("/my/very/very/very_long_source_file.cc"),
+                       777)
+      << "hello world";
+}
+
+TEST(TailCallsModifiesTest, AtLocationSourceLocation) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  const int log_line = __LINE__ + 1;
+  constexpr absl::SourceLocation loc = absl::SourceLocation::current();
+  auto do_log = [loc] { LOG(INFO).AtLocation(loc) << "hello world"; };
+
+  EXPECT_CALL(test_sink,
+              Send(AllOf(SourceFilename(Eq(__FILE__)),
+                         SourceBasename(Eq("log_modifier_methods_test.cc")),
+                         SourceLine(Eq(log_line)))));
+
+  test_sink.StartCapturingLogs();
+  do_log();
+}
+
+TEST(TailCallsModifiesTest, NoPrefix) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  EXPECT_CALL(test_sink, Send(AllOf(Prefix(IsFalse()), TextPrefix(IsEmpty()),
+                                    TextMessageWithPrefix(Eq("hello world")))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO).NoPrefix() << "hello world";
+}
+
+TEST(TailCallsModifiesTest, NoPrefixNoMessageNoShirtNoShoesNoService) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  EXPECT_CALL(test_sink,
+              Send(AllOf(Prefix(IsFalse()), TextPrefix(IsEmpty()),
+                         TextMessageWithPrefix(IsEmpty()),
+                         TextMessageWithPrefixAndNewline(Eq("\n")))));
+  test_sink.StartCapturingLogs();
+  LOG(INFO).NoPrefix();
+}
+
+TEST(TailCallsModifiesTest, WithVerbosity) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  EXPECT_CALL(test_sink, Send(Verbosity(Eq(2))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO).WithVerbosity(2) << "hello world";
+}
+
+TEST(TailCallsModifiesTest, WithVerbosityNoVerbosity) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  EXPECT_CALL(test_sink,
+              Send(Verbosity(Eq(absl::LogEntry::kNoVerbosityLevel))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO).WithVerbosity(2).WithVerbosity(absl::LogEntry::kNoVerbosityLevel)
+      << "hello world";
+}
+
+TEST(TailCallsModifiesTest, WithTimestamp) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  EXPECT_CALL(test_sink, Send(Timestamp(Eq(absl::UnixEpoch()))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO).WithTimestamp(absl::UnixEpoch()) << "hello world";
+}
+
+TEST(TailCallsModifiesTest, WithThreadID) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  EXPECT_CALL(test_sink,
+              Send(AllOf(ThreadID(Eq(absl::LogEntry::tid_t{1234})))));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO).WithThreadID(1234) << "hello world";
+}
+
+TEST(TailCallsModifiesTest, WithMetadataFrom) {
+  class ForwardingLogSink : public absl::LogSink {
+   public:
+    void Send(const absl::LogEntry &entry) override {
+      LOG(LEVEL(entry.log_severity())).WithMetadataFrom(entry)
+          << "forwarded: " << entry.text_message();
+    }
+  } forwarding_sink;
+
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(SourceFilename(Eq("fake/file")), SourceBasename(Eq("file")),
+                 SourceLine(Eq(123)), Prefix(IsFalse()),
+                 LogSeverity(Eq(absl::LogSeverity::kWarning)),
+                 Timestamp(Eq(absl::UnixEpoch())),
+                 ThreadID(Eq(absl::LogEntry::tid_t{456})),
+                 TextMessage(Eq("forwarded: hello world")), Verbosity(Eq(7)),
+                 ENCODED_MESSAGE(MatchesEvent(
+                     Eq("fake/file"), Eq(123), Eq(absl::UnixEpoch()),
+                     Eq(logging::proto::WARNING), Eq(456),
+                     ElementsAre(ValueWithLiteral(Eq("forwarded: ")),
+                                 ValueWithStr(Eq("hello world"))))))));
+
+  test_sink.StartCapturingLogs();
+  LOG(WARNING)
+          .AtLocation("fake/file", 123)
+          .NoPrefix()
+          .WithTimestamp(absl::UnixEpoch())
+          .WithThreadID(456)
+          .WithVerbosity(7)
+          .ToSinkOnly(&forwarding_sink)
+      << "hello world";
+}
+
+TEST(TailCallsModifiesTest, WithPerror) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(
+          TextMessage(AnyOf(Eq("hello world: Bad file number [9]"),
+                            Eq("hello world: Bad file descriptor [9]"),
+                            Eq("hello world: Bad file descriptor [8]"))),
+          ENCODED_MESSAGE(HasValues(ElementsAre(
+              ValueWithLiteral(Eq("hello world")), ValueWithLiteral(Eq(": ")),
+              AnyOf(ValueWithStr(Eq("Bad file number")),
+                    ValueWithStr(Eq("Bad file descriptor"))),
+              ValueWithLiteral(Eq(" [")),
+              AnyOf(ValueWithStr(Eq("8")), ValueWithStr(Eq("9"))),
+              ValueWithLiteral(Eq("]"))))))));
+
+  test_sink.StartCapturingLogs();
+  errno = EBADF;
+  LOG(INFO).WithPerror() << "hello world";
+}
+
+#if GTEST_HAS_DEATH_TEST
+TEST(ModifierMethodDeathTest, ToSinkOnlyQFatal) {
+  EXPECT_EXIT(
+      {
+        absl::ScopedMockLog test_sink(
+            absl::MockLogDefault::kDisallowUnexpected);
+        EXPECT_CALL(test_sink, Send).Times(0);
+
+        auto do_log = [&test_sink] {
+          LOG(QFATAL).ToSinkOnly(&test_sink.UseAsLocalSink()) << "hello world";
+        };
+
+        EXPECT_CALL(test_sink, Send)
+            .Times(AnyNumber())
+            .WillRepeatedly(DeathTestUnexpectedLogging());
+
+        EXPECT_CALL(test_sink, Send(AllOf(TextMessage(Eq("hello world")),
+                                          Stacktrace(IsEmpty()))))
+            .WillOnce(DeathTestExpectedLogging());
+
+        test_sink.StartCapturingLogs();
+        do_log();
+      },
+      DiedOfQFatal, DeathTestValidateExpectations());
+}
+#endif
+
+}  // namespace
diff --git a/absl/base/inline_variable_test_b.cc b/absl/log/log_sink.cc
similarity index 60%
rename from absl/base/inline_variable_test_b.cc
rename to absl/log/log_sink.cc
index 931d56d0571..01d7ca82512 100644
--- a/absl/base/inline_variable_test_b.cc
+++ b/absl/log/log_sink.cc
@@ -1,10 +1,10 @@
-// Copyright 2017 The Abseil Authors.
+// Copyright 2022 The Abseil Authors
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
 // You may obtain a copy of the License at
 //
-//      https://www.apache.org/licenses/LICENSE-2.0
+//     https://www.apache.org/licenses/LICENSE-2.0
 //
 // Unless required by applicable law or agreed to in writing, software
 // distributed under the License is distributed on an "AS IS" BASIS,
@@ -12,14 +12,12 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-#include "absl/base/internal/inline_variable_testing.h"
+#include "absl/log/log_sink.h"
 
-namespace absl {
-namespace inline_variable_testing_internal {
-
-const Foo& get_foo_b() { return inline_variable_foo; }
+#include "absl/base/config.h"
 
-const int& get_int_b() { return inline_variable_int; }
-
-}  // namespace inline_variable_testing_internal
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+void LogSink::KeyFunction() const {}
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/log/log_sink.h b/absl/log/log_sink.h
new file mode 100644
index 00000000000..2910070d51d
--- /dev/null
+++ b/absl/log/log_sink.h
@@ -0,0 +1,71 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: log/log_sink.h
+// -----------------------------------------------------------------------------
+//
+// This header declares the interface class `absl::LogSink`.
+
+#ifndef ABSL_LOG_LOG_SINK_H_
+#define ABSL_LOG_LOG_SINK_H_
+
+#include "absl/base/config.h"
+#include "absl/log/log_entry.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+// absl::LogSink
+//
+// `absl::LogSink` is an interface which can be extended to intercept and
+// process particular messages (with `LOG.ToSinkOnly()` or
+// `LOG.ToSinkAlso()`) or all messages (if registered with
+// `absl::AddLogSink`).  Implementations must not take any locks that might be
+// held by the `LOG` caller.
+class LogSink {
+ public:
+  virtual ~LogSink() = default;
+
+  // LogSink::Send()
+  //
+  // `Send` is called synchronously during the log statement.  `Send` must be
+  // thread-safe.
+  //
+  // It is safe to use `LOG` within an implementation of `Send`.  `ToSinkOnly`
+  // and `ToSinkAlso` are safe in general but can be used to create an infinite
+  // loop if you try.
+  virtual void Send(const absl::LogEntry& entry) = 0;
+
+  // LogSink::Flush()
+  //
+  // Sinks that buffer messages should override this method to flush the buffer
+  // and return.  `Flush` must be thread-safe.
+  virtual void Flush() {}
+
+ protected:
+  LogSink() = default;
+  // Implementations may be copyable and/or movable.
+  LogSink(const LogSink&) = default;
+  LogSink& operator=(const LogSink&) = default;
+
+ private:
+  // https://lld.llvm.org/missingkeyfunction.html#missing-key-function
+  virtual void KeyFunction() const final;  // NOLINT(readability/inheritance)
+};
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_LOG_LOG_SINK_H_
diff --git a/absl/log/log_sink_registry.h b/absl/log/log_sink_registry.h
new file mode 100644
index 00000000000..a3fa9a3db35
--- /dev/null
+++ b/absl/log/log_sink_registry.h
@@ -0,0 +1,64 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: log/log_sink_registry.h
+// -----------------------------------------------------------------------------
+//
+// This header declares APIs to operate on global set of registered log sinks.
+
+#ifndef ABSL_LOG_LOG_SINK_REGISTRY_H_
+#define ABSL_LOG_LOG_SINK_REGISTRY_H_
+
+#include "absl/base/config.h"
+#include "absl/base/nullability.h"
+#include "absl/log/internal/log_sink_set.h"
+#include "absl/log/log_sink.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+// AddLogSink(), RemoveLogSink()
+//
+// Adds or removes a `absl::LogSink` as a consumer of logging data.
+//
+// These functions are thread-safe.
+//
+// It is an error to attempt to add a sink that's already registered or to
+// attempt to remove one that isn't.
+//
+// To avoid unbounded recursion, dispatch to registered `absl::LogSink`s is
+// disabled per-thread while running the `Send()` method of registered
+// `absl::LogSink`s.  Affected messages are dispatched to a special internal
+// sink instead which writes them to `stderr`.
+//
+// Do not call these inside `absl::LogSink::Send`.
+inline void AddLogSink(absl::LogSink* absl_nonnull sink) {
+  log_internal::AddLogSink(sink);
+}
+inline void RemoveLogSink(absl::LogSink* absl_nonnull sink) {
+  log_internal::RemoveLogSink(sink);
+}
+
+// FlushLogSinks()
+//
+// Calls `absl::LogSink::Flush` on all registered sinks.
+//
+// Do not call this inside `absl::LogSink::Send`.
+inline void FlushLogSinks() { log_internal::FlushLogSinks(); }
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_LOG_LOG_SINK_REGISTRY_H_
diff --git a/absl/log/log_sink_test.cc b/absl/log/log_sink_test.cc
new file mode 100644
index 00000000000..cbb5a531b60
--- /dev/null
+++ b/absl/log/log_sink_test.cc
@@ -0,0 +1,420 @@
+//
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/log/log_sink.h"
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/attributes.h"
+#include "absl/base/log_severity.h"
+#include "absl/log/internal/test_actions.h"
+#include "absl/log/internal/test_helpers.h"
+#include "absl/log/internal/test_matchers.h"
+#include "absl/log/log.h"
+#include "absl/log/log_entry.h"
+#include "absl/log/log_sink_registry.h"
+#include "absl/log/scoped_mock_log.h"
+#include "absl/strings/string_view.h"
+
+namespace {
+
+using ::absl::log_internal::DeathTestExpectedLogging;
+using ::absl::log_internal::DeathTestUnexpectedLogging;
+using ::absl::log_internal::DeathTestValidateExpectations;
+using ::absl::log_internal::DiedOfFatal;
+using ::testing::_;
+using ::testing::AnyNumber;
+using ::testing::HasSubstr;
+using ::testing::InSequence;
+
+auto* test_env ABSL_ATTRIBUTE_UNUSED = ::testing::AddGlobalTestEnvironment(
+    new absl::log_internal::LogTestEnvironment);
+
+// Tests for global log sink registration.
+// ---------------------------------------
+
+TEST(LogSinkRegistryTest, AddLogSink) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+
+  InSequence s;
+  EXPECT_CALL(test_sink, Log(_, _, "hello world")).Times(0);
+  EXPECT_CALL(test_sink, Log(absl::LogSeverity::kInfo, __FILE__, "Test : 42"));
+  EXPECT_CALL(test_sink,
+              Log(absl::LogSeverity::kWarning, __FILE__, "Danger ahead"));
+  EXPECT_CALL(test_sink,
+              Log(absl::LogSeverity::kError, __FILE__, "This is an error"));
+
+  LOG(INFO) << "hello world";
+  test_sink.StartCapturingLogs();
+
+  LOG(INFO) << "Test : " << 42;
+  LOG(WARNING) << "Danger" << ' ' << "ahead";
+  LOG(ERROR) << "This is an error";
+
+  test_sink.StopCapturingLogs();
+  LOG(INFO) << "Goodby world";
+}
+
+TEST(LogSinkRegistryTest, MultipleLogSinks) {
+  absl::ScopedMockLog test_sink1(absl::MockLogDefault::kDisallowUnexpected);
+  absl::ScopedMockLog test_sink2(absl::MockLogDefault::kDisallowUnexpected);
+
+  ::testing::InSequence seq;
+  EXPECT_CALL(test_sink1, Log(absl::LogSeverity::kInfo, _, "First")).Times(1);
+  EXPECT_CALL(test_sink2, Log(absl::LogSeverity::kInfo, _, "First")).Times(0);
+
+  EXPECT_CALL(test_sink1, Log(absl::LogSeverity::kInfo, _, "Second")).Times(1);
+  EXPECT_CALL(test_sink2, Log(absl::LogSeverity::kInfo, _, "Second")).Times(1);
+
+  EXPECT_CALL(test_sink1, Log(absl::LogSeverity::kInfo, _, "Third")).Times(0);
+  EXPECT_CALL(test_sink2, Log(absl::LogSeverity::kInfo, _, "Third")).Times(1);
+
+  LOG(INFO) << "Before first";
+
+  test_sink1.StartCapturingLogs();
+  LOG(INFO) << "First";
+
+  test_sink2.StartCapturingLogs();
+  LOG(INFO) << "Second";
+
+  test_sink1.StopCapturingLogs();
+  LOG(INFO) << "Third";
+
+  test_sink2.StopCapturingLogs();
+  LOG(INFO) << "Fourth";
+}
+
+TEST(LogSinkRegistrationDeathTest, DuplicateSinkRegistration) {
+  ASSERT_DEATH_IF_SUPPORTED(
+      {
+        absl::ScopedMockLog sink;
+        sink.StartCapturingLogs();
+        absl::AddLogSink(&sink.UseAsLocalSink());
+      },
+      HasSubstr("Duplicate log sinks"));
+}
+
+TEST(LogSinkRegistrationDeathTest, MismatchSinkRemoval) {
+  ASSERT_DEATH_IF_SUPPORTED(
+      {
+        absl::ScopedMockLog sink;
+        absl::RemoveLogSink(&sink.UseAsLocalSink());
+      },
+      HasSubstr("Mismatched log sink"));
+}
+
+// Tests for log sink semantic.
+// ---------------------------------------
+
+TEST(LogSinkTest, FlushSinks) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+
+  EXPECT_CALL(test_sink, Flush()).Times(2);
+
+  test_sink.StartCapturingLogs();
+
+  absl::FlushLogSinks();
+  absl::FlushLogSinks();
+}
+
+TEST(LogSinkDeathTest, DeathInSend) {
+  class FatalSendSink : public absl::LogSink {
+   public:
+    void Send(const absl::LogEntry&) override { LOG(FATAL) << "goodbye world"; }
+  };
+
+  FatalSendSink sink;
+  EXPECT_EXIT({ LOG(INFO).ToSinkAlso(&sink) << "hello world"; }, DiedOfFatal,
+              _);
+}
+
+// Tests for explicit log sink redirection.
+// ---------------------------------------
+
+TEST(LogSinkTest, ToSinkAlso) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  absl::ScopedMockLog another_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Log(_, _, "hello world"));
+  EXPECT_CALL(another_sink, Log(_, _, "hello world"));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO).ToSinkAlso(&another_sink.UseAsLocalSink()) << "hello world";
+}
+
+TEST(LogSinkTest, ToSinkOnly) {
+  absl::ScopedMockLog another_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(another_sink, Log(_, _, "hello world"));
+  LOG(INFO).ToSinkOnly(&another_sink.UseAsLocalSink()) << "hello world";
+}
+
+TEST(LogSinkTest, ToManySinks) {
+  absl::ScopedMockLog sink1(absl::MockLogDefault::kDisallowUnexpected);
+  absl::ScopedMockLog sink2(absl::MockLogDefault::kDisallowUnexpected);
+  absl::ScopedMockLog sink3(absl::MockLogDefault::kDisallowUnexpected);
+  absl::ScopedMockLog sink4(absl::MockLogDefault::kDisallowUnexpected);
+  absl::ScopedMockLog sink5(absl::MockLogDefault::kDisallowUnexpected);
+
+  EXPECT_CALL(sink3, Log(_, _, "hello world"));
+  EXPECT_CALL(sink4, Log(_, _, "hello world"));
+  EXPECT_CALL(sink5, Log(_, _, "hello world"));
+
+  LOG(INFO)
+          .ToSinkAlso(&sink1.UseAsLocalSink())
+          .ToSinkAlso(&sink2.UseAsLocalSink())
+          .ToSinkOnly(&sink3.UseAsLocalSink())
+          .ToSinkAlso(&sink4.UseAsLocalSink())
+          .ToSinkAlso(&sink5.UseAsLocalSink())
+      << "hello world";
+}
+
+class ReentrancyTest : public ::testing::Test {
+ protected:
+  ReentrancyTest() = default;
+  enum class LogMode : int { kNormal, kToSinkAlso, kToSinkOnly };
+
+  class ReentrantSendLogSink : public absl::LogSink {
+   public:
+    explicit ReentrantSendLogSink(absl::LogSeverity severity,
+                                  absl::LogSink* sink, LogMode mode)
+        : severity_(severity), sink_(sink), mode_(mode) {}
+    explicit ReentrantSendLogSink(absl::LogSeverity severity)
+        : ReentrantSendLogSink(severity, nullptr, LogMode::kNormal) {}
+
+    void Send(const absl::LogEntry&) override {
+      switch (mode_) {
+        case LogMode::kNormal:
+          LOG(LEVEL(severity_)) << "The log is coming from *inside the sink*.";
+          break;
+        case LogMode::kToSinkAlso:
+          LOG(LEVEL(severity_)).ToSinkAlso(sink_)
+              << "The log is coming from *inside the sink*.";
+          break;
+        case LogMode::kToSinkOnly:
+          LOG(LEVEL(severity_)).ToSinkOnly(sink_)
+              << "The log is coming from *inside the sink*.";
+          break;
+        default:
+          LOG(FATAL) << "Invalid mode " << static_cast<int>(mode_);
+      }
+    }
+
+   private:
+    absl::LogSeverity severity_;
+    absl::LogSink* sink_;
+    LogMode mode_;
+  };
+
+  static absl::string_view LogAndReturn(absl::LogSeverity severity,
+                                        absl::string_view to_log,
+                                        absl::string_view to_return) {
+    LOG(LEVEL(severity)) << to_log;
+    return to_return;
+  }
+};
+
+TEST_F(ReentrancyTest, LogFunctionThatLogs) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+
+  InSequence seq;
+  EXPECT_CALL(test_sink, Log(absl::LogSeverity::kInfo, _, "hello"));
+  EXPECT_CALL(test_sink, Log(absl::LogSeverity::kInfo, _, "world"));
+  EXPECT_CALL(test_sink, Log(absl::LogSeverity::kWarning, _, "danger"));
+  EXPECT_CALL(test_sink, Log(absl::LogSeverity::kInfo, _, "here"));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO) << LogAndReturn(absl::LogSeverity::kInfo, "hello", "world");
+  LOG(INFO) << LogAndReturn(absl::LogSeverity::kWarning, "danger", "here");
+}
+
+TEST_F(ReentrancyTest, RegisteredLogSinkThatLogsInSend) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  ReentrantSendLogSink renentrant_sink(absl::LogSeverity::kInfo);
+  EXPECT_CALL(test_sink, Log(_, _, "hello world"));
+
+  test_sink.StartCapturingLogs();
+  absl::AddLogSink(&renentrant_sink);
+  LOG(INFO) << "hello world";
+  absl::RemoveLogSink(&renentrant_sink);
+}
+
+TEST_F(ReentrancyTest, AlsoLogSinkThatLogsInSend) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  ReentrantSendLogSink reentrant_sink(absl::LogSeverity::kInfo);
+  EXPECT_CALL(test_sink, Log(_, _, "hello world"));
+  EXPECT_CALL(test_sink,
+              Log(_, _, "The log is coming from *inside the sink*."));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO).ToSinkAlso(&reentrant_sink) << "hello world";
+}
+
+TEST_F(ReentrancyTest, RegisteredAlsoLogSinkThatLogsInSend) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  ReentrantSendLogSink reentrant_sink(absl::LogSeverity::kInfo);
+  EXPECT_CALL(test_sink, Log(_, _, "hello world"));
+  // We only call into the test_log sink once with this message, since the
+  // second time log statement is run we are in "ThreadIsLogging" mode and all
+  // the log statements are redirected into stderr.
+  EXPECT_CALL(test_sink,
+              Log(_, _, "The log is coming from *inside the sink*."));
+
+  test_sink.StartCapturingLogs();
+  absl::AddLogSink(&reentrant_sink);
+  LOG(INFO).ToSinkAlso(&reentrant_sink) << "hello world";
+  absl::RemoveLogSink(&reentrant_sink);
+}
+
+TEST_F(ReentrancyTest, OnlyLogSinkThatLogsInSend) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  ReentrantSendLogSink reentrant_sink(absl::LogSeverity::kInfo);
+  EXPECT_CALL(test_sink,
+              Log(_, _, "The log is coming from *inside the sink*."));
+
+  test_sink.StartCapturingLogs();
+  LOG(INFO).ToSinkOnly(&reentrant_sink) << "hello world";
+}
+
+TEST_F(ReentrancyTest, RegisteredOnlyLogSinkThatLogsInSend) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  ReentrantSendLogSink reentrant_sink(absl::LogSeverity::kInfo);
+  EXPECT_CALL(test_sink,
+              Log(_, _, "The log is coming from *inside the sink*."));
+
+  test_sink.StartCapturingLogs();
+  absl::AddLogSink(&reentrant_sink);
+  LOG(INFO).ToSinkOnly(&reentrant_sink) << "hello world";
+  absl::RemoveLogSink(&reentrant_sink);
+}
+
+using ReentrancyDeathTest = ReentrancyTest;
+
+TEST_F(ReentrancyDeathTest, LogFunctionThatLogsFatal) {
+  EXPECT_EXIT(
+      {
+        absl::ScopedMockLog test_sink;
+
+        EXPECT_CALL(test_sink, Log)
+            .Times(AnyNumber())
+            .WillRepeatedly(DeathTestUnexpectedLogging());
+        EXPECT_CALL(test_sink, Log(_, _, "hello"))
+            .WillOnce(DeathTestExpectedLogging());
+
+        test_sink.StartCapturingLogs();
+        LOG(INFO) << LogAndReturn(absl::LogSeverity::kFatal, "hello", "world");
+      },
+      DiedOfFatal, DeathTestValidateExpectations());
+}
+
+TEST_F(ReentrancyDeathTest, RegisteredLogSinkThatLogsFatalInSend) {
+  EXPECT_EXIT(
+      {
+        absl::ScopedMockLog test_sink;
+        ReentrantSendLogSink reentrant_sink(absl::LogSeverity::kFatal);
+        EXPECT_CALL(test_sink, Log)
+            .Times(AnyNumber())
+            .WillRepeatedly(DeathTestUnexpectedLogging());
+        EXPECT_CALL(test_sink, Log(_, _, "hello world"))
+            .WillOnce(DeathTestExpectedLogging());
+
+        test_sink.StartCapturingLogs();
+        absl::AddLogSink(&reentrant_sink);
+        LOG(INFO) << "hello world";
+        // No need to call RemoveLogSink - process is dead at this point.
+      },
+      DiedOfFatal, DeathTestValidateExpectations());
+}
+
+TEST_F(ReentrancyDeathTest, AlsoLogSinkThatLogsFatalInSend) {
+  EXPECT_EXIT(
+      {
+        absl::ScopedMockLog test_sink;
+        ReentrantSendLogSink reentrant_sink(absl::LogSeverity::kFatal);
+
+        EXPECT_CALL(test_sink, Log)
+            .Times(AnyNumber())
+            .WillRepeatedly(DeathTestUnexpectedLogging());
+        EXPECT_CALL(test_sink, Log(_, _, "hello world"))
+            .WillOnce(DeathTestExpectedLogging());
+        EXPECT_CALL(test_sink,
+                    Log(_, _, "The log is coming from *inside the sink*."))
+            .WillOnce(DeathTestExpectedLogging());
+
+        test_sink.StartCapturingLogs();
+        LOG(INFO).ToSinkAlso(&reentrant_sink) << "hello world";
+      },
+      DiedOfFatal, DeathTestValidateExpectations());
+}
+
+TEST_F(ReentrancyDeathTest, RegisteredAlsoLogSinkThatLogsFatalInSend) {
+  EXPECT_EXIT(
+      {
+        absl::ScopedMockLog test_sink;
+        ReentrantSendLogSink reentrant_sink(absl::LogSeverity::kFatal);
+        EXPECT_CALL(test_sink, Log)
+            .Times(AnyNumber())
+            .WillRepeatedly(DeathTestUnexpectedLogging());
+        EXPECT_CALL(test_sink, Log(_, _, "hello world"))
+            .WillOnce(DeathTestExpectedLogging());
+        EXPECT_CALL(test_sink,
+                    Log(_, _, "The log is coming from *inside the sink*."))
+            .WillOnce(DeathTestExpectedLogging());
+
+        test_sink.StartCapturingLogs();
+        absl::AddLogSink(&reentrant_sink);
+        LOG(INFO).ToSinkAlso(&reentrant_sink) << "hello world";
+        // No need to call RemoveLogSink - process is dead at this point.
+      },
+      DiedOfFatal, DeathTestValidateExpectations());
+}
+
+TEST_F(ReentrancyDeathTest, OnlyLogSinkThatLogsFatalInSend) {
+  EXPECT_EXIT(
+      {
+        absl::ScopedMockLog test_sink;
+        ReentrantSendLogSink reentrant_sink(absl::LogSeverity::kFatal);
+        EXPECT_CALL(test_sink, Log)
+            .Times(AnyNumber())
+            .WillRepeatedly(DeathTestUnexpectedLogging());
+        EXPECT_CALL(test_sink,
+                    Log(_, _, "The log is coming from *inside the sink*."))
+            .WillOnce(DeathTestExpectedLogging());
+
+        test_sink.StartCapturingLogs();
+        LOG(INFO).ToSinkOnly(&reentrant_sink) << "hello world";
+      },
+      DiedOfFatal, DeathTestValidateExpectations());
+}
+
+TEST_F(ReentrancyDeathTest, RegisteredOnlyLogSinkThatLogsFatalInSend) {
+  EXPECT_EXIT(
+      {
+        absl::ScopedMockLog test_sink;
+        ReentrantSendLogSink reentrant_sink(absl::LogSeverity::kFatal);
+        EXPECT_CALL(test_sink, Log)
+            .Times(AnyNumber())
+            .WillRepeatedly(DeathTestUnexpectedLogging());
+        EXPECT_CALL(test_sink,
+                    Log(_, _, "The log is coming from *inside the sink*."))
+            .WillOnce(DeathTestExpectedLogging());
+
+        test_sink.StartCapturingLogs();
+        absl::AddLogSink(&reentrant_sink);
+        LOG(INFO).ToSinkOnly(&reentrant_sink) << "hello world";
+        // No need to call RemoveLogSink - process is dead at this point.
+      },
+      DiedOfFatal, DeathTestValidateExpectations());
+}
+
+}  // namespace
diff --git a/absl/log/log_streamer.h b/absl/log/log_streamer.h
new file mode 100644
index 00000000000..294012bac8c
--- /dev/null
+++ b/absl/log/log_streamer.h
@@ -0,0 +1,208 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: log/log_streamer.h
+// -----------------------------------------------------------------------------
+//
+// This header declares the class `LogStreamer` and convenience functions to
+// construct LogStreamer objects with different associated log severity levels.
+
+#ifndef ABSL_LOG_LOG_STREAMER_H_
+#define ABSL_LOG_LOG_STREAMER_H_
+
+#include <ios>
+#include <memory>
+#include <optional>
+#include <ostream>
+#include <string>
+#include <utility>
+
+#include "absl/base/config.h"
+#include "absl/base/log_severity.h"
+#include "absl/log/absl_log.h"
+#include "absl/strings/internal/ostringstream.h"
+#include "absl/strings/string_view.h"
+#include "absl/types/optional.h"
+#include "absl/types/source_location.h"
+#include "absl/utility/utility.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+// LogStreamer
+//
+// Although you can stream into `LOG(INFO)`, you can't pass it into a function
+// that takes a `std::ostream` parameter. `LogStreamer::stream()` provides a
+// `std::ostream` that buffers everything that's streamed in.  The buffer's
+// contents are logged as if by `LOG` when the `LogStreamer` is destroyed.
+// If nothing is streamed in, an empty message is logged.  If the specified
+// severity is `absl::LogSeverity::kFatal`, the program will be terminated when
+// the `LogStreamer` is destroyed regardless of whether any data were streamed
+// in.
+//
+// Factory functions corresponding to the `absl::LogSeverity` enumerators
+// are provided for convenience; if the desired severity is variable, invoke the
+// constructor directly.
+//
+// LogStreamer is movable, but not copyable.
+//
+// Examples:
+//
+//   ShaveYakAndWriteToStream(
+//       yak, absl::LogInfoStreamer(__FILE__, __LINE__).stream());
+//
+//   {
+//     // This logs a single line containing data streamed by all three function
+//     // calls.
+//     absl::LogStreamer streamer(absl::LogSeverity::kInfo, __FILE__, __LINE__);
+//     ShaveYakAndWriteToStream(yak1, streamer.stream());
+//     streamer.stream() << " ";
+//     ShaveYakAndWriteToStream(yak2, streamer.stream());
+//     streamer.stream() << " ";
+//     ShaveYakAndWriteToStreamPointer(yak3, &streamer.stream());
+//   }
+class LogStreamer final {
+ public:
+  // LogStreamer::LogStreamer()
+  //
+  // Creates a LogStreamer with a given `severity` that will log a message
+  // attributed to the given `file` and `line`.
+  explicit LogStreamer(absl::LogSeverity severity, absl::string_view file,
+                       int line)
+      : severity_(severity),
+        line_(line),
+        file_(file),
+        stream_(std::in_place, &buf_) {
+    // To match `LOG`'s defaults:
+    stream_->setf(std::ios_base::showbase | std::ios_base::boolalpha);
+  }
+  explicit LogStreamer(
+      absl::LogSeverity severity,
+      absl::SourceLocation loc = absl::SourceLocation::current())
+      : LogStreamer(severity, loc.file_name(), static_cast<int>(loc.line())) {}
+
+  // A moved-from `absl::LogStreamer` does not `LOG` when destroyed,
+  // and a program that streams into one has undefined behavior.
+  LogStreamer(LogStreamer&& that) noexcept
+      : severity_(that.severity_),
+        line_(that.line_),
+        file_(std::move(that.file_)),
+        buf_(std::move(that.buf_)),
+        stream_(std::move(that.stream_)) {
+    if (stream_.has_value()) stream_->str(&buf_);
+    that.stream_.reset();
+  }
+  LogStreamer& operator=(LogStreamer&& that) {
+    ABSL_LOG_IF(LEVEL(severity_), stream_).AtLocation(file_, line_) << buf_;
+    severity_ = that.severity_;
+    file_ = std::move(that.file_);
+    line_ = that.line_;
+    buf_ = std::move(that.buf_);
+    stream_ = std::move(that.stream_);
+    if (stream_.has_value()) stream_->str(&buf_);
+    that.stream_.reset();
+    return *this;
+  }
+
+  // LogStreamer::~LogStreamer()
+  //
+  // Logs this LogStreamer's buffered content as if by LOG.
+  ~LogStreamer() {
+    ABSL_LOG_IF(LEVEL(severity_), stream_.has_value()).AtLocation(file_, line_)
+        << buf_;
+  }
+
+  // LogStreamer::stream()
+  //
+  // Returns the `std::ostream` to use to write into this LogStreamer' internal
+  // buffer.
+  std::ostream& stream() { return *stream_; }
+
+ private:
+  absl::LogSeverity severity_;
+  int line_;
+  std::string file_;
+  std::string buf_;
+  // A disengaged `stream_` indicates a moved-from `LogStreamer` that should not
+  // `LOG` upon destruction.
+  std::optional<absl::strings_internal::OStringStream> stream_;
+};
+
+// LogInfoStreamer()
+//
+// Returns a LogStreamer that writes at level LogSeverity::kInfo.
+inline LogStreamer LogInfoStreamer(absl::string_view file, int line) {
+  return absl::LogStreamer(absl::LogSeverity::kInfo, file, line);
+}
+
+// LogWarningStreamer()
+//
+// Returns a LogStreamer that writes at level LogSeverity::kWarning.
+inline LogStreamer LogWarningStreamer(absl::string_view file, int line) {
+  return absl::LogStreamer(absl::LogSeverity::kWarning, file, line);
+}
+
+// LogErrorStreamer()
+//
+// Returns a LogStreamer that writes at level LogSeverity::kError.
+inline LogStreamer LogErrorStreamer(absl::string_view file, int line) {
+  return absl::LogStreamer(absl::LogSeverity::kError, file, line);
+}
+
+// LogFatalStreamer()
+//
+// Returns a LogStreamer that writes at level LogSeverity::kFatal.
+//
+// The program will be terminated when this `LogStreamer` is destroyed,
+// regardless of whether any data were streamed in.
+inline LogStreamer LogFatalStreamer(absl::string_view file, int line) {
+  return absl::LogStreamer(absl::LogSeverity::kFatal, file, line);
+}
+
+// LogDebugFatalStreamer()
+//
+// Returns a LogStreamer that writes at level LogSeverity::kLogDebugFatal.
+//
+// In debug mode, the program will be terminated when this `LogStreamer` is
+// destroyed, regardless of whether any data were streamed in.
+inline LogStreamer LogDebugFatalStreamer(absl::string_view file, int line) {
+  return absl::LogStreamer(absl::kLogDebugFatal, file, line);
+}
+
+inline LogStreamer LogInfoStreamer(
+    absl::SourceLocation loc = absl::SourceLocation::current()) {
+  return absl::LogStreamer(absl::LogSeverity::kInfo, loc);
+}
+inline LogStreamer LogWarningStreamer(
+    absl::SourceLocation loc = absl::SourceLocation::current()) {
+  return absl::LogStreamer(absl::LogSeverity::kWarning, loc);
+}
+inline LogStreamer LogErrorStreamer(
+    absl::SourceLocation loc = absl::SourceLocation::current()) {
+  return absl::LogStreamer(absl::LogSeverity::kError, loc);
+}
+inline LogStreamer LogFatalStreamer(
+    absl::SourceLocation loc = absl::SourceLocation::current()) {
+  return absl::LogStreamer(absl::LogSeverity::kFatal, loc);
+}
+inline LogStreamer LogDebugFatalStreamer(
+    absl::SourceLocation loc = absl::SourceLocation::current()) {
+  return absl::LogStreamer(absl::kLogDebugFatal, loc);
+}
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_LOG_LOG_STREAMER_H_
diff --git a/absl/log/log_streamer_test.cc b/absl/log/log_streamer_test.cc
new file mode 100644
index 00000000000..e45a2935280
--- /dev/null
+++ b/absl/log/log_streamer_test.cc
@@ -0,0 +1,469 @@
+//
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/log/log_streamer.h"
+
+#include <ios>
+#include <iostream>
+#include <utility>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/attributes.h"
+#include "absl/base/internal/sysinfo.h"
+#include "absl/base/log_severity.h"
+#include "absl/log/internal/test_actions.h"
+#include "absl/log/internal/test_helpers.h"
+#include "absl/log/internal/test_matchers.h"
+#include "absl/log/log.h"
+#include "absl/log/scoped_mock_log.h"
+#include "absl/strings/string_view.h"
+#include "absl/types/source_location.h"
+
+namespace {
+using ::absl::log_internal::DeathTestExpectedLogging;
+using ::absl::log_internal::DeathTestUnexpectedLogging;
+using ::absl::log_internal::DeathTestValidateExpectations;
+#if GTEST_HAS_DEATH_TEST
+using ::absl::log_internal::DiedOfFatal;
+#endif
+using ::absl::log_internal::InMatchWindow;
+using ::absl::log_internal::LogSeverity;
+using ::absl::log_internal::Prefix;
+using ::absl::log_internal::SourceFilename;
+using ::absl::log_internal::SourceLine;
+using ::absl::log_internal::Stacktrace;
+using ::absl::log_internal::TextMessage;
+using ::absl::log_internal::ThreadID;
+using ::absl::log_internal::Timestamp;
+using ::testing::_;
+using ::testing::AnyNumber;
+using ::testing::Eq;
+using ::testing::HasSubstr;
+using ::testing::IsEmpty;
+using ::testing::IsTrue;
+
+auto* test_env ABSL_ATTRIBUTE_UNUSED = ::testing::AddGlobalTestEnvironment(
+    new absl::log_internal::LogTestEnvironment);
+
+void WriteToStream(absl::string_view data, std::ostream* os) {
+  *os << "WriteToStream: " << data;
+}
+void WriteToStreamRef(absl::string_view data, std::ostream& os) {
+  os << "WriteToStreamRef: " << data;
+}
+
+TEST(LogStreamerTest, LogInfoStreamer) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  EXPECT_CALL(
+      test_sink,
+      Send(
+          AllOf(SourceFilename(Eq("path/file.cc")), SourceLine(Eq(1234)),
+                Prefix(IsTrue()), LogSeverity(Eq(absl::LogSeverity::kInfo)),
+                Timestamp(InMatchWindow()),
+                ThreadID(Eq(absl::base_internal::GetTID())),
+                TextMessage(Eq("WriteToStream: foo")),
+                ENCODED_MESSAGE(MatchesEvent(
+                    Eq("path/file.cc"), Eq(1234), InMatchWindow(),
+                    Eq(logging::proto::INFO), Eq(absl::base_internal::GetTID()),
+                    ElementsAre(ValueWithStr(Eq("WriteToStream: foo"))))),
+                Stacktrace(IsEmpty()))));
+
+  test_sink.StartCapturingLogs();
+  WriteToStream("foo", &absl::LogInfoStreamer("path/file.cc", 1234).stream());
+}
+
+TEST(LogStreamerTest, LogWarningStreamer) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(
+          SourceFilename(Eq("path/file.cc")), SourceLine(Eq(1234)),
+          Prefix(IsTrue()), LogSeverity(Eq(absl::LogSeverity::kWarning)),
+          Timestamp(InMatchWindow()),
+          ThreadID(Eq(absl::base_internal::GetTID())),
+          TextMessage(Eq("WriteToStream: foo")),
+          ENCODED_MESSAGE(MatchesEvent(
+              Eq("path/file.cc"), Eq(1234), InMatchWindow(),
+              Eq(logging::proto::WARNING), Eq(absl::base_internal::GetTID()),
+              ElementsAre(ValueWithStr(Eq("WriteToStream: foo"))))),
+          Stacktrace(IsEmpty()))));
+
+  test_sink.StartCapturingLogs();
+  WriteToStream("foo",
+                &absl::LogWarningStreamer("path/file.cc", 1234).stream());
+}
+
+TEST(LogStreamerTest, LogErrorStreamer) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(
+          SourceFilename(Eq("path/file.cc")), SourceLine(Eq(1234)),
+          Prefix(IsTrue()), LogSeverity(Eq(absl::LogSeverity::kError)),
+          Timestamp(InMatchWindow()),
+          ThreadID(Eq(absl::base_internal::GetTID())),
+          TextMessage(Eq("WriteToStream: foo")),
+          ENCODED_MESSAGE(MatchesEvent(
+              Eq("path/file.cc"), Eq(1234), InMatchWindow(),
+              Eq(logging::proto::ERROR), Eq(absl::base_internal::GetTID()),
+              ElementsAre(ValueWithStr(Eq("WriteToStream: foo"))))),
+          Stacktrace(IsEmpty()))));
+
+  test_sink.StartCapturingLogs();
+  WriteToStream("foo", &absl::LogErrorStreamer("path/file.cc", 1234).stream());
+}
+
+#if GTEST_HAS_DEATH_TEST
+TEST(LogStreamerDeathTest, LogFatalStreamer) {
+  EXPECT_EXIT(
+      {
+        absl::ScopedMockLog test_sink;
+        EXPECT_CALL(test_sink, Send).Times(0);
+
+        EXPECT_CALL(test_sink, Send)
+            .Times(AnyNumber())
+            .WillRepeatedly(DeathTestUnexpectedLogging());
+
+        EXPECT_CALL(
+            test_sink,
+            Send(AllOf(
+                SourceFilename(Eq("path/file.cc")), SourceLine(Eq(1234)),
+                Prefix(IsTrue()), LogSeverity(Eq(absl::LogSeverity::kFatal)),
+                Timestamp(InMatchWindow()),
+                ThreadID(Eq(absl::base_internal::GetTID())),
+                TextMessage(Eq("WriteToStream: foo")),
+                ENCODED_MESSAGE(MatchesEvent(
+                    Eq("path/file.cc"), Eq(1234), InMatchWindow(),
+                    Eq(logging::proto::FATAL),
+                    Eq(absl::base_internal::GetTID()),
+                    ElementsAre(ValueWithStr(Eq("WriteToStream: foo"))))))))
+            .WillOnce(DeathTestExpectedLogging());
+
+        test_sink.StartCapturingLogs();
+        WriteToStream("foo",
+                      &absl::LogFatalStreamer("path/file.cc", 1234).stream());
+      },
+      DiedOfFatal, DeathTestValidateExpectations());
+}
+#endif
+
+#ifdef NDEBUG
+TEST(LogStreamerTest, LogDebugFatalStreamer) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(
+          SourceFilename(Eq("path/file.cc")), SourceLine(Eq(1234)),
+          Prefix(IsTrue()), LogSeverity(Eq(absl::LogSeverity::kError)),
+          Timestamp(InMatchWindow()),
+          ThreadID(Eq(absl::base_internal::GetTID())),
+          TextMessage(Eq("WriteToStream: foo")),
+          ENCODED_MESSAGE(MatchesEvent(
+              Eq("path/file.cc"), Eq(1234), InMatchWindow(),
+              Eq(logging::proto::ERROR), Eq(absl::base_internal::GetTID()),
+              ElementsAre(ValueWithStr(Eq("WriteToStream: foo"))))),
+          Stacktrace(IsEmpty()))));
+
+  test_sink.StartCapturingLogs();
+  WriteToStream("foo",
+                &absl::LogDebugFatalStreamer("path/file.cc", 1234).stream());
+}
+#elif GTEST_HAS_DEATH_TEST
+TEST(LogStreamerDeathTest, LogDebugFatalStreamer) {
+  EXPECT_EXIT(
+      {
+        absl::ScopedMockLog test_sink;
+        EXPECT_CALL(test_sink, Send).Times(0);
+
+        EXPECT_CALL(test_sink, Send)
+            .Times(AnyNumber())
+            .WillRepeatedly(DeathTestUnexpectedLogging());
+
+        EXPECT_CALL(
+            test_sink,
+            Send(AllOf(
+                SourceFilename(Eq("path/file.cc")), SourceLine(Eq(1234)),
+                Prefix(IsTrue()), LogSeverity(Eq(absl::LogSeverity::kFatal)),
+                Timestamp(InMatchWindow()),
+                ThreadID(Eq(absl::base_internal::GetTID())),
+                TextMessage(Eq("WriteToStream: foo")),
+                ENCODED_MESSAGE(MatchesEvent(
+                    Eq("path/file.cc"), Eq(1234), InMatchWindow(),
+                    Eq(logging::proto::FATAL),
+                    Eq(absl::base_internal::GetTID()),
+                    ElementsAre(ValueWithStr(Eq("WriteToStream: foo"))))))))
+            .WillOnce(DeathTestExpectedLogging());
+
+        test_sink.StartCapturingLogs();
+        WriteToStream(
+            "foo", &absl::LogDebugFatalStreamer("path/file.cc", 1234).stream());
+      },
+      DiedOfFatal, DeathTestValidateExpectations());
+}
+#endif
+
+TEST(LogStreamerTest, LogStreamer) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(
+          SourceFilename(Eq("path/file.cc")), SourceLine(Eq(1234)),
+          Prefix(IsTrue()), LogSeverity(Eq(absl::LogSeverity::kError)),
+          Timestamp(InMatchWindow()),
+          ThreadID(Eq(absl::base_internal::GetTID())),
+          TextMessage(Eq("WriteToStream: foo")),
+          ENCODED_MESSAGE(MatchesEvent(
+              Eq("path/file.cc"), Eq(1234), InMatchWindow(),
+              Eq(logging::proto::ERROR), Eq(absl::base_internal::GetTID()),
+              ElementsAre(ValueWithStr(Eq("WriteToStream: foo"))))),
+          Stacktrace(IsEmpty()))));
+
+  test_sink.StartCapturingLogs();
+  WriteToStream(
+      "foo", &absl::LogStreamer(absl::LogSeverity::kError, "path/file.cc", 1234)
+                  .stream());
+}
+
+#if GTEST_HAS_DEATH_TEST
+TEST(LogStreamerDeathTest, LogStreamer) {
+  EXPECT_EXIT(
+      {
+        absl::ScopedMockLog test_sink;
+        EXPECT_CALL(test_sink, Send).Times(0);
+
+        EXPECT_CALL(test_sink, Send)
+            .Times(AnyNumber())
+            .WillRepeatedly(DeathTestUnexpectedLogging());
+
+        EXPECT_CALL(
+            test_sink,
+            Send(AllOf(
+                SourceFilename(Eq("path/file.cc")), SourceLine(Eq(1234)),
+                Prefix(IsTrue()), LogSeverity(Eq(absl::LogSeverity::kFatal)),
+                Timestamp(InMatchWindow()),
+                ThreadID(Eq(absl::base_internal::GetTID())),
+                TextMessage(Eq("WriteToStream: foo")),
+                ENCODED_MESSAGE(MatchesEvent(
+                    Eq("path/file.cc"), Eq(1234), InMatchWindow(),
+                    Eq(logging::proto::FATAL),
+                    Eq(absl::base_internal::GetTID()),
+                    ElementsAre(ValueWithStr(Eq("WriteToStream: foo"))))))))
+            .WillOnce(DeathTestExpectedLogging());
+
+        test_sink.StartCapturingLogs();
+        WriteToStream("foo", &absl::LogStreamer(absl::LogSeverity::kFatal,
+                                                "path/file.cc", 1234)
+                                  .stream());
+      },
+      DiedOfFatal, DeathTestValidateExpectations());
+}
+#endif
+
+TEST(LogStreamerTest, PassedByReference) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(SourceFilename(Eq("path/file.cc")), SourceLine(Eq(1234)),
+                 TextMessage(Eq("WriteToStreamRef: foo")),
+                 ENCODED_MESSAGE(MatchesEvent(
+                     Eq("path/file.cc"), Eq(1234), _, _, _,
+                     ElementsAre(ValueWithStr(Eq("WriteToStreamRef: foo"))))),
+                 Stacktrace(IsEmpty()))));
+
+  test_sink.StartCapturingLogs();
+  WriteToStreamRef("foo", absl::LogInfoStreamer("path/file.cc", 1234).stream());
+}
+
+TEST(LogStreamerTest, StoredAsLocal) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  auto streamer = absl::LogInfoStreamer("path/file.cc", 1234);
+  WriteToStream("foo", &streamer.stream());
+  streamer.stream() << " ";
+  WriteToStreamRef("bar", streamer.stream());
+
+  // The call should happen when `streamer` goes out of scope; if it
+  // happened before this `EXPECT_CALL` the call would be unexpected and the
+  // test would fail.
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(SourceFilename(Eq("path/file.cc")), SourceLine(Eq(1234)),
+                 TextMessage(Eq("WriteToStream: foo WriteToStreamRef: bar")),
+                 ENCODED_MESSAGE(MatchesEvent(
+                     Eq("path/file.cc"), Eq(1234), _, _, _,
+                     ElementsAre(ValueWithStr(
+                         Eq("WriteToStream: foo WriteToStreamRef: bar"))))),
+                 Stacktrace(IsEmpty()))));
+
+  test_sink.StartCapturingLogs();
+}
+
+#if GTEST_HAS_DEATH_TEST
+TEST(LogStreamerDeathTest, StoredAsLocal) {
+  EXPECT_EXIT(
+      {
+        // This is fatal when it goes out of scope, but not until then:
+        auto streamer = absl::LogFatalStreamer("path/file.cc", 1234);
+        std::cerr << "I'm still alive" << std::endl;
+        WriteToStream("foo", &streamer.stream());
+      },
+      DiedOfFatal, HasSubstr("I'm still alive"));
+}
+#endif
+
+TEST(LogStreamerTest, LogsEmptyLine) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  EXPECT_CALL(test_sink, Send(AllOf(SourceFilename(Eq("path/file.cc")),
+                                    SourceLine(Eq(1234)), TextMessage(Eq("")),
+                                    ENCODED_MESSAGE(MatchesEvent(
+                                        Eq("path/file.cc"), Eq(1234), _, _, _,
+                                        ElementsAre(ValueWithStr(Eq(""))))),
+                                    Stacktrace(IsEmpty()))));
+
+  test_sink.StartCapturingLogs();
+  absl::LogInfoStreamer("path/file.cc", 1234);
+}
+
+#if GTEST_HAS_DEATH_TEST
+TEST(LogStreamerDeathTest, LogsEmptyLine) {
+  EXPECT_EXIT(
+      {
+        absl::ScopedMockLog test_sink;
+        EXPECT_CALL(test_sink, Send)
+            .Times(AnyNumber())
+            .WillRepeatedly(DeathTestUnexpectedLogging());
+
+        EXPECT_CALL(
+            test_sink,
+            Send(AllOf(SourceFilename(Eq("path/file.cc")), TextMessage(Eq("")),
+                       ENCODED_MESSAGE(
+                           MatchesEvent(Eq("path/file.cc"), _, _, _, _,
+                                        ElementsAre(ValueWithStr(Eq(""))))))))
+            .WillOnce(DeathTestExpectedLogging());
+
+        test_sink.StartCapturingLogs();
+        // This is fatal even though it's never used:
+        auto streamer = absl::LogFatalStreamer("path/file.cc", 1234);
+      },
+      DiedOfFatal, DeathTestValidateExpectations());
+}
+#endif
+
+TEST(LogStreamerTest, MoveConstruction) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  EXPECT_CALL(
+      test_sink,
+      Send(
+          AllOf(SourceFilename(Eq("path/file.cc")), SourceLine(Eq(1234)),
+                LogSeverity(Eq(absl::LogSeverity::kInfo)),
+                TextMessage(Eq("hello 0x10 world 0x10")),
+                ENCODED_MESSAGE(MatchesEvent(
+                    Eq("path/file.cc"), Eq(1234), _, Eq(logging::proto::INFO),
+                    _, ElementsAre(ValueWithStr(Eq("hello 0x10 world 0x10"))))),
+                Stacktrace(IsEmpty()))));
+
+  test_sink.StartCapturingLogs();
+  auto streamer1 = absl::LogInfoStreamer("path/file.cc", 1234);
+  streamer1.stream() << "hello " << std::hex << 16;
+  absl::LogStreamer streamer2(std::move(streamer1));
+  streamer2.stream() << " world " << 16;
+}
+
+TEST(LogStreamerTest, MoveAssignment) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  testing::InSequence seq;
+  EXPECT_CALL(
+      test_sink,
+      Send(AllOf(
+          SourceFilename(Eq("path/file2.cc")), SourceLine(Eq(5678)),
+          LogSeverity(Eq(absl::LogSeverity::kWarning)),
+          TextMessage(Eq("something else")),
+          ENCODED_MESSAGE(MatchesEvent(
+              Eq("path/file2.cc"), Eq(5678), _, Eq(logging::proto::WARNING), _,
+              ElementsAre(ValueWithStr(Eq("something else"))))),
+          Stacktrace(IsEmpty()))));
+  EXPECT_CALL(
+      test_sink,
+      Send(
+          AllOf(SourceFilename(Eq("path/file.cc")), SourceLine(Eq(1234)),
+                LogSeverity(Eq(absl::LogSeverity::kInfo)),
+                TextMessage(Eq("hello 0x10 world 0x10")),
+                ENCODED_MESSAGE(MatchesEvent(
+                    Eq("path/file.cc"), Eq(1234), _, Eq(logging::proto::INFO),
+                    _, ElementsAre(ValueWithStr(Eq("hello 0x10 world 0x10"))))),
+                Stacktrace(IsEmpty()))));
+
+  test_sink.StartCapturingLogs();
+  auto streamer1 = absl::LogInfoStreamer("path/file.cc", 1234);
+  streamer1.stream() << "hello " << std::hex << 16;
+  auto streamer2 = absl::LogWarningStreamer("path/file2.cc", 5678);
+  streamer2.stream() << "something else";
+  streamer2 = std::move(streamer1);
+  streamer2.stream() << " world " << 16;
+}
+
+TEST(LogStreamerTest, CorrectDefaultFlags) {
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  // The `boolalpha` and `showbase` flags should be set by default, to match
+  // `LOG`.
+  EXPECT_CALL(test_sink, Send(AllOf(TextMessage(Eq("false0xdeadbeef")))))
+      .Times(2);
+
+  test_sink.StartCapturingLogs();
+  absl::LogInfoStreamer("path/file.cc", 1234).stream()
+      << false << std::hex << 0xdeadbeef;
+  LOG(INFO) << false << std::hex << 0xdeadbeef;
+}
+
+TEST(LogStreamerTest, AtSourceLocation) {
+  const int log_line = __LINE__ + 2;
+  auto do_log = [] {
+    WriteToStream("foo", &absl::LogInfoStreamer().stream());  //
+  };
+  absl::ScopedMockLog test_sink(absl::MockLogDefault::kDisallowUnexpected);
+  EXPECT_CALL(test_sink, Send).Times(0);
+
+  EXPECT_CALL(test_sink,
+              Send(AllOf(SourceFilename(
+                             Eq(absl::SourceLocation::current().file_name())),
+                         SourceLine(Eq(log_line)))));
+
+  test_sink.StartCapturingLogs();
+  do_log();
+}
+
+}  // namespace
diff --git a/absl/log/scoped_mock_log.cc b/absl/log/scoped_mock_log.cc
new file mode 100644
index 00000000000..39a0a52aab6
--- /dev/null
+++ b/absl/log/scoped_mock_log.cc
@@ -0,0 +1,86 @@
+//
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/log/scoped_mock_log.h"
+
+#include <atomic>
+#include <string>
+
+#include "gmock/gmock.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/log/log_entry.h"
+#include "absl/log/log_sink.h"
+#include "absl/log/log_sink_registry.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+ScopedMockLog::ScopedMockLog(MockLogDefault default_exp)
+    : sink_(this), is_capturing_logs_(false), is_triggered_(false) {
+  if (default_exp == MockLogDefault::kIgnoreUnexpected) {
+    // Ignore all calls to Log we did not set expectations for.
+    EXPECT_CALL(*this, Log).Times(::testing::AnyNumber());
+  } else {
+    // Disallow all calls to Log we did not set expectations for.
+    EXPECT_CALL(*this, Log).Times(0);
+  }
+  // By default Send mock forwards to Log mock.
+  EXPECT_CALL(*this, Send)
+      .Times(::testing::AnyNumber())
+      .WillRepeatedly([this](const absl::LogEntry& entry) {
+        is_triggered_.store(true, std::memory_order_relaxed);
+        Log(entry.log_severity(), std::string(entry.source_filename()),
+            std::string(entry.text_message()));
+      });
+
+  // By default We ignore all Flush calls.
+  EXPECT_CALL(*this, Flush).Times(::testing::AnyNumber());
+}
+
+ScopedMockLog::~ScopedMockLog() {
+  ABSL_RAW_CHECK(is_triggered_.load(std::memory_order_relaxed),
+                 "Did you forget to call StartCapturingLogs()?");
+
+  if (is_capturing_logs_) StopCapturingLogs();
+}
+
+void ScopedMockLog::StartCapturingLogs() {
+  ABSL_RAW_CHECK(!is_capturing_logs_,
+                 "StartCapturingLogs() can be called only when the "
+                 "absl::ScopedMockLog object is not capturing logs.");
+
+  is_capturing_logs_ = true;
+  is_triggered_.store(true, std::memory_order_relaxed);
+  absl::AddLogSink(&sink_);
+}
+
+void ScopedMockLog::StopCapturingLogs() {
+  ABSL_RAW_CHECK(is_capturing_logs_,
+                 "StopCapturingLogs() can be called only when the "
+                 "absl::ScopedMockLog object is capturing logs.");
+
+  is_capturing_logs_ = false;
+  absl::RemoveLogSink(&sink_);
+}
+
+absl::LogSink& ScopedMockLog::UseAsLocalSink() {
+  is_triggered_.store(true, std::memory_order_relaxed);
+  return sink_;
+}
+
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/log/scoped_mock_log.h b/absl/log/scoped_mock_log.h
new file mode 100644
index 00000000000..970087374e3
--- /dev/null
+++ b/absl/log/scoped_mock_log.h
@@ -0,0 +1,203 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: log/scoped_mock_log.h
+// -----------------------------------------------------------------------------
+//
+// This header declares `class absl::ScopedMockLog`, for use in testing.
+
+#ifndef ABSL_LOG_SCOPED_MOCK_LOG_H_
+#define ABSL_LOG_SCOPED_MOCK_LOG_H_
+
+#include <atomic>
+#include <string>
+
+#include "gmock/gmock.h"
+#include "absl/base/config.h"
+#include "absl/base/log_severity.h"
+#include "absl/log/log_entry.h"
+#include "absl/log/log_sink.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+// MockLogDefault
+//
+// Controls how ScopedMockLog responds to unexpected calls by default.
+enum class MockLogDefault { kIgnoreUnexpected, kDisallowUnexpected };
+
+// ScopedMockLog
+//
+// ScopedMockLog is a LogSink that intercepts LOG() messages issued by all
+// threads when active.
+//
+// Using this together with GoogleTest, it's easy to test how a piece of code
+// calls LOG(). The typical usage, noting the distinction between
+// "uninteresting" and "unexpected", looks like this:
+//
+//   using ::testing::_;
+//   using ::testing::AnyNumber;
+//   using ::testing::EndsWith;
+//   using ::testing::kDoNotCaptureLogsYet;
+//   using ::testing::Lt;
+//
+//   TEST(FooTest, LogsCorrectly) {
+//     // Simple robust setup, ignores unexpected logs.
+//     absl::ScopedMockLog log;
+//
+//     // We expect the WARNING "Something bad!" exactly twice.
+//     EXPECT_CALL(log, Log(absl::LogSeverity::kWarning, _, "Something bad!"))
+//         .Times(2);
+//
+//     // But we want no messages from foo.cc.
+//     EXPECT_CALL(log, Log(_, EndsWith("/foo.cc"), _)).Times(0);
+//
+//     log.StartCapturingLogs();  // Call this after done setting expectations.
+//     Foo();  // Exercises the code under test.
+//   }
+//
+//   TEST(BarTest, LogsExactlyCorrectly) {
+//     // Strict checking, fails for unexpected logs.
+//     absl::ScopedMockLog log(absl::MockLogDefault::kDisallowUnexpected);
+//
+//     // ... but ignore low severity messages
+//     EXPECT_CALL(log, Log(Lt(absl::LogSeverity::kWarning), _, _))
+//         .Times(AnyNumber());
+//
+//     // We expect the ERROR "Something bad!" exactly once.
+//     EXPECT_CALL(log, Log(absl::LogSeverity::kError, EndsWith("/foo.cc"),
+//                 "Something bad!"))
+//         .Times(1);
+//
+//     log.StartCapturingLogs();  // Call this after done setting expectations.
+//     Bar();  // Exercises the code under test.
+//    }
+//
+// Note that in a multi-threaded environment, all LOG() messages from a single
+// thread will be handled in sequence, but that cannot be guaranteed for
+// messages from different threads. In fact, if the same or multiple
+// expectations are matched on two threads concurrently, their actions will be
+// executed concurrently as well and may interleave.
+class ScopedMockLog final {
+ public:
+  // ScopedMockLog::ScopedMockLog()
+  //
+  // Sets up the log and adds default expectations.
+  explicit ScopedMockLog(
+      MockLogDefault default_exp = MockLogDefault::kIgnoreUnexpected);
+  ScopedMockLog(const ScopedMockLog&) = delete;
+  ScopedMockLog& operator=(const ScopedMockLog&) = delete;
+
+  // ScopedMockLog::~ScopedMockLog()
+  //
+  // Stops intercepting logs and destroys this ScopedMockLog.
+  ~ScopedMockLog();
+
+  // ScopedMockLog::StartCapturingLogs()
+  //
+  // Starts log capturing if the object isn't already doing so. Otherwise
+  // crashes.
+  //
+  // Usually this method is called in the same thread that created this
+  // ScopedMockLog. It is the user's responsibility to not call this method if
+  // another thread may be calling it or StopCapturingLogs() at the same time.
+  // It is undefined behavior to add expectations while capturing logs is
+  // enabled.
+  void StartCapturingLogs();
+
+  // ScopedMockLog::StopCapturingLogs()
+  //
+  // Stops log capturing if the object is capturing logs. Otherwise crashes.
+  //
+  // Usually this method is called in the same thread that created this object.
+  // It is the user's responsibility to not call this method if another thread
+  // may be calling it or StartCapturingLogs() at the same time.
+  //
+  // It is UB to add expectations, while capturing logs is enabled.
+  void StopCapturingLogs();
+
+  // ScopedMockLog::UseAsLocalSink()
+  //
+  // Each `ScopedMockLog` is implemented with an `absl::LogSink`; this method
+  // returns a reference to that sink (e.g. for use with
+  // `LOG(...).ToSinkOnly()`) and marks the `ScopedMockLog` as having been used
+  // even if `StartCapturingLogs` is never called.
+  absl::LogSink& UseAsLocalSink();
+
+  // Implements the mock method:
+  //
+  //   void Log(LogSeverity severity, absl::string_view file_path,
+  //            absl::string_view message);
+  //
+  // The second argument to Log() is the full path of the source file in
+  // which the LOG() was issued.
+  //
+  // This is a shorthand form, which should be used by most users. Use the
+  // `Send` mock only if you want to add expectations for other log message
+  // attributes.
+  MOCK_METHOD(void, Log,
+              (absl::LogSeverity severity, const std::string& file_path,
+               const std::string& message));
+
+  // Implements the mock method:
+  //
+  //   void Send(const absl::LogEntry& entry);
+  //
+  // This is the most generic form of mock that can be specified. Use this mock
+  // only if you want to add expectations for log message attributes different
+  // from the log message text, log message path and log message severity.
+  //
+  // If no expectations are specified for this mock, the default action is to
+  // forward the call to the `Log` mock.  Tests using `Send` are advised to call
+  //
+  //   `EXPECT_CALL(sink, Send).Times(0);`
+  //
+  // prior to specifying other expectations to suppress forwarding to `Log`.
+  // That way, unexpected calls show up as calls to `Send` with complete data
+  // and metadata for easier debugging.
+  MOCK_METHOD(void, Send, (const absl::LogEntry&));
+
+  // Implements the mock method:
+  //
+  //   void Flush();
+  //
+  // Use this mock only if you want to add expectations for log flush calls.
+  MOCK_METHOD(void, Flush, ());
+
+ private:
+  class ForwardingSink final : public absl::LogSink {
+   public:
+    explicit ForwardingSink(ScopedMockLog* sml) : sml_(sml) {}
+    ForwardingSink(const ForwardingSink&) = delete;
+    ForwardingSink& operator=(const ForwardingSink&) = delete;
+    void Send(const absl::LogEntry& entry) override { sml_->Send(entry); }
+    void Flush() override { sml_->Flush(); }
+
+   private:
+    ScopedMockLog* sml_;
+  };
+
+  ForwardingSink sink_;
+  bool is_capturing_logs_;
+  // Until C++20, the default constructor leaves the underlying value wrapped in
+  // std::atomic uninitialized, so all constructors should be sure to initialize
+  // is_triggered_.
+  std::atomic<bool> is_triggered_;
+};
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_LOG_SCOPED_MOCK_LOG_H_
diff --git a/absl/log/scoped_mock_log_test.cc b/absl/log/scoped_mock_log_test.cc
new file mode 100644
index 00000000000..6f1d8fb7fc4
--- /dev/null
+++ b/absl/log/scoped_mock_log_test.cc
@@ -0,0 +1,296 @@
+//
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/log/scoped_mock_log.h"
+
+#include <memory>
+#include <thread>  // NOLINT(build/c++11)
+
+#include "gmock/gmock.h"
+#include "gtest/gtest-spi.h"
+#include "gtest/gtest.h"
+#include "absl/base/attributes.h"
+#include "absl/base/log_severity.h"
+#include "absl/log/globals.h"
+#include "absl/log/internal/test_helpers.h"
+#include "absl/log/internal/test_matchers.h"
+#include "absl/log/log.h"
+#include "absl/log/log_entry.h"
+#include "absl/memory/memory.h"
+#include "absl/strings/match.h"
+#include "absl/strings/string_view.h"
+#include "absl/synchronization/barrier.h"
+#include "absl/synchronization/notification.h"
+
+namespace {
+
+using ::testing::_;
+using ::testing::AnyNumber;
+using ::testing::Eq;
+using ::testing::HasSubstr;
+using ::testing::InSequence;
+using ::testing::Lt;
+using ::testing::Truly;
+using absl::log_internal::SourceBasename;
+using absl::log_internal::SourceFilename;
+using absl::log_internal::SourceLine;
+using absl::log_internal::TextMessageWithPrefix;
+using absl::log_internal::ThreadID;
+
+auto* test_env ABSL_ATTRIBUTE_UNUSED = ::testing::AddGlobalTestEnvironment(
+    new absl::log_internal::LogTestEnvironment);
+
+#if GTEST_HAS_DEATH_TEST
+TEST(ScopedMockLogDeathTest,
+     StartCapturingLogsCannotBeCalledWhenAlreadyCapturing) {
+  EXPECT_DEATH(
+      {
+        absl::ScopedMockLog log;
+        log.StartCapturingLogs();
+        log.StartCapturingLogs();
+      },
+      "StartCapturingLogs");
+}
+
+TEST(ScopedMockLogDeathTest, StopCapturingLogsCannotBeCalledWhenNotCapturing) {
+  EXPECT_DEATH(
+      {
+        absl::ScopedMockLog log;
+        log.StopCapturingLogs();
+      },
+      "StopCapturingLogs");
+}
+
+TEST(ScopedMockLogDeathTest, FailsCheckIfStartCapturingLogsIsNeverCalled) {
+  EXPECT_DEATH({ absl::ScopedMockLog log; },
+               "Did you forget to call StartCapturingLogs");
+}
+#endif
+
+// Tests that ScopedMockLog intercepts LOG()s when it's alive.
+TEST(ScopedMockLogTest, LogMockCatchAndMatchStrictExpectations) {
+  absl::ScopedMockLog log;
+
+  // The following expectations must match in the order they appear.
+  InSequence s;
+  EXPECT_CALL(log,
+              Log(absl::LogSeverity::kWarning, HasSubstr(__FILE__), "Danger."));
+  EXPECT_CALL(log, Log(absl::LogSeverity::kInfo, _, "Working...")).Times(2);
+  EXPECT_CALL(log, Log(absl::LogSeverity::kError, _, "Bad!!"));
+
+  log.StartCapturingLogs();
+  LOG(WARNING) << "Danger.";
+  LOG(INFO) << "Working...";
+  LOG(INFO) << "Working...";
+  LOG(ERROR) << "Bad!!";
+}
+
+TEST(ScopedMockLogTest, LogMockCatchAndMatchSendExpectations) {
+  absl::ScopedMockLog log;
+
+  EXPECT_CALL(
+      log,
+      Send(AllOf(SourceFilename(Eq("/my/very/very/very_long_source_file.cc")),
+                 SourceBasename(Eq("very_long_source_file.cc")),
+                 SourceLine(Eq(777)), ThreadID(Eq(absl::LogEntry::tid_t{1234})),
+                 TextMessageWithPrefix(Truly([](absl::string_view msg) {
+                   return absl::EndsWith(
+                       msg, " very_long_source_file.cc:777] Info message");
+                 })))));
+
+  log.StartCapturingLogs();
+  LOG(INFO)
+          .AtLocation("/my/very/very/very_long_source_file.cc", 777)
+          .WithThreadID(1234)
+      << "Info message";
+}
+
+TEST(ScopedMockLogTest, ScopedMockLogCanBeNice) {
+  absl::ScopedMockLog log;
+
+  InSequence s;
+  EXPECT_CALL(log,
+              Log(absl::LogSeverity::kWarning, HasSubstr(__FILE__), "Danger."));
+  EXPECT_CALL(log, Log(absl::LogSeverity::kInfo, _, "Working...")).Times(2);
+  EXPECT_CALL(log, Log(absl::LogSeverity::kError, _, "Bad!!"));
+
+  log.StartCapturingLogs();
+
+  // Any number of these are OK.
+  LOG(INFO) << "Info message.";
+  // Any number of these are OK.
+  LOG(WARNING).AtLocation("SomeOtherFile.cc", 100) << "Danger ";
+
+  LOG(WARNING) << "Danger.";
+
+  // Any number of these are OK.
+  LOG(INFO) << "Info message.";
+  // Any number of these are OK.
+  LOG(WARNING).AtLocation("SomeOtherFile.cc", 100) << "Danger ";
+
+  LOG(INFO) << "Working...";
+
+  // Any number of these are OK.
+  LOG(INFO) << "Info message.";
+  // Any number of these are OK.
+  LOG(WARNING).AtLocation("SomeOtherFile.cc", 100) << "Danger ";
+
+  LOG(INFO) << "Working...";
+
+  // Any number of these are OK.
+  LOG(INFO) << "Info message.";
+  // Any number of these are OK.
+  LOG(WARNING).AtLocation("SomeOtherFile.cc", 100) << "Danger ";
+
+  LOG(ERROR) << "Bad!!";
+
+  // Any number of these are OK.
+  LOG(INFO) << "Info message.";
+  // Any number of these are OK.
+  LOG(WARNING).AtLocation("SomeOtherFile.cc", 100) << "Danger ";
+}
+
+// Tests that ScopedMockLog generates a test failure if a message is logged
+// that is not expected (here, that means ERROR or FATAL).
+TEST(ScopedMockLogTest, RejectsUnexpectedLogs) {
+  EXPECT_NONFATAL_FAILURE(
+      {
+        absl::ScopedMockLog log(absl::MockLogDefault::kDisallowUnexpected);
+        // Any INFO and WARNING messages are permitted.
+        EXPECT_CALL(log, Log(Lt(absl::LogSeverity::kError), _, _))
+            .Times(AnyNumber());
+        log.StartCapturingLogs();
+        LOG(INFO) << "Ignored";
+        LOG(WARNING) << "Ignored";
+        LOG(ERROR) << "Should not be ignored";
+      },
+      "Should not be ignored");
+}
+
+TEST(ScopedMockLogTest, CapturesLogsAfterStartCapturingLogs) {
+  absl::SetStderrThreshold(absl::LogSeverityAtLeast::kInfinity);
+  absl::ScopedMockLog log;
+
+  // The ScopedMockLog object shouldn't see these LOGs, as it hasn't
+  // started capturing LOGs yet.
+  LOG(INFO) << "Ignored info";
+  LOG(WARNING) << "Ignored warning";
+  LOG(ERROR) << "Ignored error";
+
+  EXPECT_CALL(log, Log(absl::LogSeverity::kInfo, _, "Expected info"));
+  log.StartCapturingLogs();
+
+  // Only this LOG will be seen by the ScopedMockLog.
+  LOG(INFO) << "Expected info";
+}
+
+TEST(ScopedMockLogTest, DoesNotCaptureLogsAfterStopCapturingLogs) {
+  absl::ScopedMockLog log;
+  EXPECT_CALL(log, Log(absl::LogSeverity::kInfo, _, "Expected info"));
+
+  log.StartCapturingLogs();
+
+  // This LOG should be seen by the ScopedMockLog.
+  LOG(INFO) << "Expected info";
+
+  log.StopCapturingLogs();
+
+  // The ScopedMockLog object shouldn't see these LOGs, as it has
+  // stopped capturing LOGs.
+  LOG(INFO) << "Ignored info";
+  LOG(WARNING) << "Ignored warning";
+  LOG(ERROR) << "Ignored error";
+}
+
+// Tests that all messages are intercepted regardless of issuing thread. The
+// purpose of this test is NOT to exercise thread-safety.
+TEST(ScopedMockLogTest, LogFromMultipleThreads) {
+  absl::ScopedMockLog log;
+
+  // We don't establish an order to expectations here, since the threads may
+  // execute their log statements in different order.
+  EXPECT_CALL(log, Log(absl::LogSeverity::kInfo, __FILE__, "Thread 1"));
+  EXPECT_CALL(log, Log(absl::LogSeverity::kInfo, __FILE__, "Thread 2"));
+
+  log.StartCapturingLogs();
+
+  absl::Barrier barrier(2);
+  std::thread thread1([&barrier]() {
+    barrier.Block();
+    LOG(INFO) << "Thread 1";
+  });
+  std::thread thread2([&barrier]() {
+    barrier.Block();
+    LOG(INFO) << "Thread 2";
+  });
+
+  thread1.join();
+  thread2.join();
+}
+
+// Tests that no sequence will be imposed on two LOG message expectations from
+// different threads. This test would actually deadlock if replaced to two LOG
+// statements from the same thread.
+TEST(ScopedMockLogTest, NoSequenceWithMultipleThreads) {
+  absl::ScopedMockLog log;
+
+  absl::Barrier barrier(2);
+  EXPECT_CALL(log, Log(absl::LogSeverity::kInfo, _, _))
+      .Times(2)
+      .WillRepeatedly([&barrier]() { barrier.Block(); });
+
+  log.StartCapturingLogs();
+
+  std::thread thread1([]() { LOG(INFO) << "Thread 1"; });
+  std::thread thread2([]() { LOG(INFO) << "Thread 2"; });
+
+  thread1.join();
+  thread2.join();
+}
+
+TEST(ScopedMockLogTsanTest,
+     ScopedMockLogCanBeDeletedWhenAnotherThreadIsLogging) {
+  auto log = std::make_unique<absl::ScopedMockLog>();
+  EXPECT_CALL(*log, Log(absl::LogSeverity::kInfo, __FILE__, "Thread log"))
+      .Times(AnyNumber());
+
+  log->StartCapturingLogs();
+
+  absl::Notification logging_started;
+
+  std::thread thread([&logging_started]() {
+    for (int i = 0; i < 100; ++i) {
+      if (i == 50) logging_started.Notify();
+      LOG(INFO) << "Thread log";
+    }
+  });
+
+  logging_started.WaitForNotification();
+  log.reset();
+  thread.join();
+}
+
+TEST(ScopedMockLogTest, AsLocalSink) {
+  absl::ScopedMockLog log(absl::MockLogDefault::kDisallowUnexpected);
+
+  EXPECT_CALL(log, Log(_, _, "two"));
+  EXPECT_CALL(log, Log(_, _, "three"));
+
+  LOG(INFO) << "one";
+  LOG(INFO).ToSinkOnly(&log.UseAsLocalSink()) << "two";
+  LOG(INFO).ToSinkAlso(&log.UseAsLocalSink()) << "three";
+}
+
+}  // namespace
diff --git a/absl/log/stripping_test.cc b/absl/log/stripping_test.cc
new file mode 100644
index 00000000000..b0f718cdd0e
--- /dev/null
+++ b/absl/log/stripping_test.cc
@@ -0,0 +1,505 @@
+//
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// Tests for stripping of literal strings.
+// ---------------------------------------
+//
+// When a `LOG` statement can be trivially proved at compile time to never fire,
+// e.g. due to `ABSL_MIN_LOG_LEVEL`, `NDEBUG`, or some explicit condition, data
+// streamed in can be dropped from the compiled program completely if they are
+// not used elsewhere.  This most commonly affects string literals, which users
+// often want to strip to reduce binary size and/or redact information about
+// their program's internals (e.g. in a release build).
+//
+// These tests log strings and then validate whether they appear in the compiled
+// binary.  This is done by opening the file corresponding to the running test
+// and running a simple string search on its contents.  The strings to be logged
+// and searched for must be unique, and we must take care not to emit them into
+// the binary in any other place, e.g. when searching for them.  The latter is
+// accomplished by computing them using base64; the source string appears in the
+// binary but the target string is computed at runtime.
+
+#include <stdio.h>
+
+#include <algorithm>
+#include <cerrno>
+#include <cstring>
+#include <functional>
+#include <memory>
+#include <ostream>
+#include <string>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/internal/strerror.h"
+#include "absl/base/log_severity.h"
+#include "absl/flags/internal/program_name.h"
+#include "absl/log/check.h"
+#include "absl/log/internal/test_helpers.h"
+#include "absl/log/log.h"
+#include "absl/status/status.h"
+#include "absl/strings/escaping.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/str_format.h"
+#include "absl/strings/string_view.h"
+
+#if defined(__APPLE__)
+#include <mach-o/dyld.h>
+#elif defined(_WIN32)
+#include <Windows.h>
+#include <tchar.h>
+#endif
+
+// Set a flag that controls whether we actually execute fatal statements, but
+// prevent the compiler from optimizing it out.
+static volatile bool kReallyDie = false;
+
+namespace {
+using ::testing::_;
+using ::testing::Eq;
+using ::testing::NotNull;
+
+using absl::log_internal::kAbslMinLogLevel;
+
+std::string Base64UnescapeOrDie(absl::string_view data) {
+  std::string decoded;
+  CHECK(absl::Base64Unescape(data, &decoded));
+  return decoded;
+}
+
+// -----------------------------------------------------------------------------
+// A Googletest matcher which searches the running binary for a given string
+// -----------------------------------------------------------------------------
+
+// This matcher is used to validate that literal strings streamed into
+// `LOG` statements that ought to be compiled out (e.g. `LOG_IF(INFO, false)`)
+// do not appear in the binary.
+//
+// Note that passing the string to be sought directly to `FileHasSubstr()` all
+// but forces its inclusion in the binary regardless of the logging library's
+// behavior. For example:
+//
+//   LOG_IF(INFO, false) << "you're the man now dog";
+//   // This will always pass:
+//   // EXPECT_THAT(fp, FileHasSubstr("you're the man now dog"));
+//   // So use this instead:
+//   EXPECT_THAT(fp, FileHasSubstr(
+//       Base64UnescapeOrDie("eW91J3JlIHRoZSBtYW4gbm93IGRvZw==")));
+
+class FileHasSubstrMatcher final : public ::testing::MatcherInterface<FILE*> {
+ public:
+  explicit FileHasSubstrMatcher(absl::string_view needle) : needle_(needle) {}
+
+  bool MatchAndExplain(
+      FILE* fp, ::testing::MatchResultListener* listener) const override {
+    std::string buf(
+        std::max<std::string::size_type>(needle_.size() * 2, 163840000), '\0');
+    size_t buf_start_offset = 0;  // The file offset of the byte at `buf[0]`.
+    size_t buf_data_size = 0;     // The number of bytes of `buf` which contain
+                                  // data.
+
+    ::fseek(fp, 0, SEEK_SET);
+    while (true) {
+      // Fill the buffer to capacity or EOF:
+      while (buf_data_size < buf.size()) {
+        const size_t ret = fread(&buf[buf_data_size], sizeof(char),
+                                 buf.size() - buf_data_size, fp);
+        if (ret == 0) break;
+        buf_data_size += ret;
+      }
+      if (ferror(fp)) {
+        *listener << "error reading file";
+        return false;
+      }
+      const absl::string_view haystack(&buf[0], buf_data_size);
+      const auto off = haystack.find(needle_);
+      if (off != haystack.npos) {
+        *listener << "string found at offset " << buf_start_offset + off;
+        return true;
+      }
+      if (feof(fp)) {
+        *listener << "string not found";
+        return false;
+      }
+      // Copy the end of `buf` to the beginning so we catch matches that span
+      // buffer boundaries.  `buf` and `buf_data_size` are always large enough
+      // that these ranges don't overlap.
+      memcpy(&buf[0], &buf[buf_data_size - needle_.size()], needle_.size());
+      buf_start_offset += buf_data_size - needle_.size();
+      buf_data_size = needle_.size();
+    }
+  }
+  void DescribeTo(std::ostream* os) const override {
+    *os << "contains the string \"" << needle_ << "\" (base64(\""
+        << Base64UnescapeOrDie(needle_) << "\"))";
+  }
+
+  void DescribeNegationTo(std::ostream* os) const override {
+    *os << "does not ";
+    DescribeTo(os);
+  }
+
+ private:
+  std::string needle_;
+};
+
+class StrippingTest : public ::testing::Test {
+ protected:
+  void SetUp() override {
+#ifndef NDEBUG
+    // Non-optimized builds don't necessarily eliminate dead code at all, so we
+    // don't attempt to validate stripping against such builds.
+    GTEST_SKIP() << "StrippingTests skipped since this build is not optimized";
+#elif defined(__EMSCRIPTEN__)
+    // These tests require a way to examine the running binary and look for
+    // strings; there's no portable way to do that.
+    GTEST_SKIP()
+        << "StrippingTests skipped since this platform is not optimized";
+#endif
+  }
+
+  // Opens this program's executable file.  Returns `nullptr` and writes to
+  // `stderr` on failure.
+  std::unique_ptr<FILE, std::function<void(FILE*)>> OpenTestExecutable() {
+#if defined(__linux__)
+    std::unique_ptr<FILE, std::function<void(FILE*)>> fp(
+        fopen("/proc/self/exe", "rb"), [](FILE* fp) { fclose(fp); });
+    if (!fp) {
+      const std::string err = absl::base_internal::StrError(errno);
+      absl::FPrintF(stderr, "Failed to open /proc/self/exe: %s\n", err);
+    }
+    return fp;
+#elif defined(__Fuchsia__)
+    // TODO(b/242579714): We need to restore the test coverage on this platform.
+    std::unique_ptr<FILE, std::function<void(FILE*)>> fp(
+        fopen(absl::StrCat("/pkg/bin/",
+                           absl::flags_internal::ShortProgramInvocationName())
+                  .c_str(),
+              "rb"),
+        [](FILE* fp) { fclose(fp); });
+    if (!fp) {
+      const std::string err = absl::base_internal::StrError(errno);
+      absl::FPrintF(stderr, "Failed to open /pkg/bin/<binary name>: %s\n", err);
+    }
+    return fp;
+#elif defined(__APPLE__)
+    uint32_t size = 0;
+    int ret = _NSGetExecutablePath(nullptr, &size);
+    if (ret != -1) {
+      absl::FPrintF(stderr,
+                    "Failed to get executable path: "
+                    "_NSGetExecutablePath(nullptr) returned %d\n",
+                    ret);
+      return nullptr;
+    }
+    std::string path(size, '\0');
+    ret = _NSGetExecutablePath(&path[0], &size);
+    if (ret != 0) {
+      absl::FPrintF(
+          stderr,
+          "Failed to get executable path: _NSGetExecutablePath(buffer) "
+          "returned %d\n",
+          ret);
+      return nullptr;
+    }
+    std::unique_ptr<FILE, std::function<void(FILE*)>> fp(
+        fopen(path.c_str(), "rb"), [](FILE* fp) { fclose(fp); });
+    if (!fp) {
+      const std::string err = absl::base_internal::StrError(errno);
+      absl::FPrintF(stderr, "Failed to open executable at %s: %s\n", path, err);
+    }
+    return fp;
+#elif defined(_WIN32)
+    std::basic_string<TCHAR> path(4096, _T('\0'));
+    while (true) {
+      const uint32_t ret = ::GetModuleFileName(nullptr, &path[0],
+                                               static_cast<DWORD>(path.size()));
+      if (ret == 0) {
+        absl::FPrintF(
+            stderr,
+            "Failed to get executable path: GetModuleFileName(buffer) "
+            "returned 0\n");
+        return nullptr;
+      }
+      if (ret < path.size()) break;
+      path.resize(path.size() * 2, _T('\0'));
+    }
+    std::unique_ptr<FILE, std::function<void(FILE*)>> fp(
+        _tfopen(path.c_str(), _T("rb")), [](FILE* fp) { fclose(fp); });
+    if (!fp) absl::FPrintF(stderr, "Failed to open executable\n");
+    return fp;
+#else
+    absl::FPrintF(stderr,
+                  "OpenTestExecutable() unimplemented on this platform\n");
+    return nullptr;
+#endif
+  }
+
+  ::testing::Matcher<FILE*> FileHasSubstr(absl::string_view needle) {
+    return MakeMatcher(new FileHasSubstrMatcher(needle));
+  }
+};
+
+// This tests whether out methodology for testing stripping works on this
+// platform by looking for one string that definitely ought to be there and one
+// that definitely ought not to.  If this fails, none of the `StrippingTest`s
+// are going to produce meaningful results.
+TEST_F(StrippingTest, Control) {
+  constexpr char kEncodedPositiveControl[] =
+      "U3RyaXBwaW5nVGVzdC5Qb3NpdGl2ZUNvbnRyb2w=";
+  const std::string encoded_negative_control =
+      absl::Base64Escape("StrippingTest.NegativeControl");
+
+  // Verify this mainly so we can encode other strings and know definitely they
+  // won't encode to `kEncodedPositiveControl`.
+  EXPECT_THAT(Base64UnescapeOrDie("U3RyaXBwaW5nVGVzdC5Qb3NpdGl2ZUNvbnRyb2w="),
+              Eq("StrippingTest.PositiveControl"));
+
+  auto exe = OpenTestExecutable();
+  ASSERT_THAT(exe, NotNull());
+  EXPECT_THAT(exe.get(), FileHasSubstr(kEncodedPositiveControl));
+  EXPECT_THAT(exe.get(), Not(FileHasSubstr(encoded_negative_control)));
+}
+
+TEST_F(StrippingTest, Literal) {
+  // We need to load a copy of the needle string into memory (so we can search
+  // for it) without leaving it lying around in plaintext in the executable file
+  // as would happen if we used a literal.  We might (or might not) leave it
+  // lying around later; that's what the tests are for!
+  const std::string needle = absl::Base64Escape("StrippingTest.Literal");
+  LOG(INFO) << "U3RyaXBwaW5nVGVzdC5MaXRlcmFs";
+  auto exe = OpenTestExecutable();
+  ASSERT_THAT(exe, NotNull());
+  if (absl::LogSeverity::kInfo >= kAbslMinLogLevel) {
+    EXPECT_THAT(exe.get(), FileHasSubstr(needle));
+  } else {
+    EXPECT_THAT(exe.get(), Not(FileHasSubstr(needle)));
+  }
+}
+
+TEST_F(StrippingTest, LiteralInExpression) {
+  // We need to load a copy of the needle string into memory (so we can search
+  // for it) without leaving it lying around in plaintext in the executable file
+  // as would happen if we used a literal.  We might (or might not) leave it
+  // lying around later; that's what the tests are for!
+  const std::string needle =
+      absl::Base64Escape("StrippingTest.LiteralInExpression");
+  LOG(INFO) << absl::StrCat("secret: ",
+                            "U3RyaXBwaW5nVGVzdC5MaXRlcmFsSW5FeHByZXNzaW9u");
+  std::unique_ptr<FILE, std::function<void(FILE*)>> exe = OpenTestExecutable();
+  ASSERT_THAT(exe, NotNull());
+  if (absl::LogSeverity::kInfo >= kAbslMinLogLevel) {
+    EXPECT_THAT(exe.get(), FileHasSubstr(needle));
+  } else {
+    EXPECT_THAT(exe.get(), Not(FileHasSubstr(needle)));
+  }
+}
+
+TEST_F(StrippingTest, Fatal) {
+  // We need to load a copy of the needle string into memory (so we can search
+  // for it) without leaving it lying around in plaintext in the executable file
+  // as would happen if we used a literal.  We might (or might not) leave it
+  // lying around later; that's what the tests are for!
+  const std::string needle = absl::Base64Escape("StrippingTest.Fatal");
+  // We don't care if the LOG statement is actually executed, we're just
+  // checking that it's stripped.
+  if (kReallyDie) LOG(FATAL) << "U3RyaXBwaW5nVGVzdC5GYXRhbA==";
+
+  std::unique_ptr<FILE, std::function<void(FILE*)>> exe = OpenTestExecutable();
+  ASSERT_THAT(exe, NotNull());
+  if (absl::LogSeverity::kFatal >= kAbslMinLogLevel) {
+    EXPECT_THAT(exe.get(), FileHasSubstr(needle));
+  } else {
+    EXPECT_THAT(exe.get(), Not(FileHasSubstr(needle)));
+  }
+}
+
+TEST_F(StrippingTest, DFatal) {
+  // We need to load a copy of the needle string into memory (so we can search
+  // for it) without leaving it lying around in plaintext in the executable file
+  // as would happen if we used a literal.  We might (or might not) leave it
+  // lying around later; that's what the tests are for!
+  const std::string needle = absl::Base64Escape("StrippingTest.DFatal");
+  // We don't care if the LOG statement is actually executed, we're just
+  // checking that it's stripped.
+  if (kReallyDie) LOG(DFATAL) << "U3RyaXBwaW5nVGVzdC5ERmF0YWw=";
+
+  std::unique_ptr<FILE, std::function<void(FILE*)>> exe = OpenTestExecutable();
+  ASSERT_THAT(exe, NotNull());
+  // `DFATAL` can be `ERROR` or `FATAL`, and a compile-time optimizer doesn't
+  // know which, because `absl::kLogDebugFatal` is declared `extern` and defined
+  // in another TU.  Link-time optimization might do better.  We have six cases:
+  // |         `AMLL` is-> | `<=ERROR` | `FATAL` | `>FATAL` |
+  // | ------------------- | --------- | ------- | -------- |
+  // | `DFATAL` is `ERROR` |   present |       ? | stripped |
+  // | `DFATAL` is `FATAL` |   present | present | stripped |
+
+  // These constexpr variables are used to suppress unreachable code warnings
+  // in the if-else statements below.
+
+  // "present" in the table above: `DFATAL` exceeds `ABSL_MIN_LOG_LEVEL`, so
+  // `DFATAL` statements should not be stripped (and they should be logged
+  // when executed, but that's a different testsuite).
+  constexpr bool kExpectPresent = absl::kLogDebugFatal >= kAbslMinLogLevel;
+
+  // "stripped" in the table above: even though the compiler may not know
+  // which value `DFATAL` has, it should be able to strip it since both
+  // possible values ought to be stripped.
+  constexpr bool kExpectStripped = kAbslMinLogLevel > absl::LogSeverity::kFatal;
+
+  if (kExpectPresent) {
+    EXPECT_THAT(exe.get(), FileHasSubstr(needle));
+  } else if (kExpectStripped) {
+    EXPECT_THAT(exe.get(), Not(FileHasSubstr(needle)));
+  } else {
+    // "?" in the table above; may or may not be stripped depending on whether
+    // any link-time optimization is done.  Either outcome is ok.
+  }
+}
+
+TEST_F(StrippingTest, Level) {
+  const std::string needle = absl::Base64Escape("StrippingTest.Level");
+  volatile auto severity = absl::LogSeverity::kWarning;
+  // Ensure that `severity` is not a compile-time constant to prove that
+  // stripping works regardless:
+  LOG(LEVEL(severity)) << "U3RyaXBwaW5nVGVzdC5MZXZlbA==";
+  std::unique_ptr<FILE, std::function<void(FILE*)>> exe = OpenTestExecutable();
+  ASSERT_THAT(exe, NotNull());
+  if (absl::LogSeverity::kFatal >= kAbslMinLogLevel) {
+    // This can't be stripped at compile-time because it might evaluate to a
+    // level that shouldn't be stripped.
+    EXPECT_THAT(exe.get(), FileHasSubstr(needle));
+  } else {
+#if (defined(_MSC_VER) && !defined(__clang__)) || defined(__APPLE__)
+    // Dead code elimination misses this case.
+#else
+    // All levels should be stripped, so it doesn't matter what the severity
+    // winds up being.
+    EXPECT_THAT(exe.get(), Not(FileHasSubstr(needle)));
+#endif
+  }
+}
+
+TEST_F(StrippingTest, Check) {
+  // Here we also need a variable name with enough entropy that it's unlikely to
+  // appear in the binary by chance.  `volatile` keeps the tautological
+  // comparison (and the rest of the `CHECK`) from being optimized away.
+  const std::string var_needle = absl::Base64Escape("StrippingTestCheckVar");
+  const std::string msg_needle = absl::Base64Escape("StrippingTest.Check");
+  volatile int U3RyaXBwaW5nVGVzdENoZWNrVmFy = 0xCAFE;
+  // We don't care if the CHECK is actually executed, just that stripping works.
+  // Hiding it behind `kReallyDie` works around some overly aggressive
+  // optimizations in older versions of MSVC.
+  if (kReallyDie) {
+    CHECK(U3RyaXBwaW5nVGVzdENoZWNrVmFy != U3RyaXBwaW5nVGVzdENoZWNrVmFy)
+        << "U3RyaXBwaW5nVGVzdC5DaGVjaw==";
+  }
+
+  std::unique_ptr<FILE, std::function<void(FILE*)>> exe = OpenTestExecutable();
+  ASSERT_THAT(exe, NotNull());
+  if (absl::LogSeverity::kFatal >= kAbslMinLogLevel) {
+    EXPECT_THAT(exe.get(), FileHasSubstr(var_needle));
+    EXPECT_THAT(exe.get(), FileHasSubstr(msg_needle));
+  } else {
+    EXPECT_THAT(exe.get(), Not(FileHasSubstr(var_needle)));
+    EXPECT_THAT(exe.get(), Not(FileHasSubstr(msg_needle)));
+  }
+}
+
+TEST_F(StrippingTest, CheckOp) {
+  // See `StrippingTest.Check` for some hairy implementation notes.
+  const std::string var_needle1 =
+      absl::Base64Escape("StrippingTestCheckOpVar1");
+  const std::string var_needle2 =
+      absl::Base64Escape("StrippingTestCheckOpVar2");
+  const std::string msg_needle = absl::Base64Escape("StrippingTest.CheckOp");
+  volatile int U3RyaXBwaW5nVGVzdENoZWNrT3BWYXIx = 0xFEED;
+  volatile int U3RyaXBwaW5nVGVzdENoZWNrT3BWYXIy = 0xCAFE;
+  if (kReallyDie) {
+    CHECK_EQ(U3RyaXBwaW5nVGVzdENoZWNrT3BWYXIx, U3RyaXBwaW5nVGVzdENoZWNrT3BWYXIy)
+        << "U3RyaXBwaW5nVGVzdC5DaGVja09w";
+  }
+
+  std::unique_ptr<FILE, std::function<void(FILE*)>> exe = OpenTestExecutable();
+  ASSERT_THAT(exe, NotNull());
+
+  if (absl::LogSeverity::kFatal >= kAbslMinLogLevel) {
+    EXPECT_THAT(exe.get(), FileHasSubstr(var_needle1));
+    EXPECT_THAT(exe.get(), FileHasSubstr(var_needle2));
+    EXPECT_THAT(exe.get(), FileHasSubstr(msg_needle));
+  } else {
+    EXPECT_THAT(exe.get(), Not(FileHasSubstr(var_needle1)));
+    EXPECT_THAT(exe.get(), Not(FileHasSubstr(var_needle2)));
+    EXPECT_THAT(exe.get(), Not(FileHasSubstr(msg_needle)));
+  }
+}
+
+TEST_F(StrippingTest, CheckStrOp) {
+  // See `StrippingTest.Check` for some hairy implementation notes.
+  const std::string var_needle1 =
+      absl::Base64Escape("StrippingTestCheckStrOpVar1");
+  const std::string var_needle2 =
+      absl::Base64Escape("StrippingTestCheckStrOpVar2");
+  const std::string msg_needle = absl::Base64Escape("StrippingTest.CheckStrOp");
+  const char *volatile U3RyaXBwaW5nVGVzdENoZWNrU3RyT3BWYXIx = "FEED";
+  const char *volatile U3RyaXBwaW5nVGVzdENoZWNrU3RyT3BWYXIy = "CAFE";
+  if (kReallyDie) {
+    CHECK_STREQ(U3RyaXBwaW5nVGVzdENoZWNrU3RyT3BWYXIx,
+                U3RyaXBwaW5nVGVzdENoZWNrU3RyT3BWYXIy)
+        << "U3RyaXBwaW5nVGVzdC5DaGVja1N0ck9w";
+  }
+
+  std::unique_ptr<FILE, std::function<void(FILE*)>> exe = OpenTestExecutable();
+  ASSERT_THAT(exe, NotNull());
+
+  if (absl::LogSeverity::kFatal >= kAbslMinLogLevel) {
+    EXPECT_THAT(exe.get(), FileHasSubstr(var_needle1));
+    EXPECT_THAT(exe.get(), FileHasSubstr(var_needle2));
+    EXPECT_THAT(exe.get(), FileHasSubstr(msg_needle));
+  } else {
+    EXPECT_THAT(exe.get(), Not(FileHasSubstr(var_needle1)));
+    EXPECT_THAT(exe.get(), Not(FileHasSubstr(var_needle2)));
+    EXPECT_THAT(exe.get(), Not(FileHasSubstr(msg_needle)));
+  }
+}
+
+TEST_F(StrippingTest, CheckOk) {
+  // See `StrippingTest.Check` for some hairy implementation notes.
+  const std::string var_needle = absl::Base64Escape("StrippingTestCheckOkVar1");
+  const std::string msg_needle = absl::Base64Escape("StrippingTest.CheckOk");
+  volatile bool x = false;
+  auto U3RyaXBwaW5nVGVzdENoZWNrT2tWYXIx = absl::OkStatus();
+  if (x) {
+    U3RyaXBwaW5nVGVzdENoZWNrT2tWYXIx =
+        absl::InvalidArgumentError("Stripping this is not my job!");
+  }
+  if (kReallyDie) {
+    CHECK_OK(U3RyaXBwaW5nVGVzdENoZWNrT2tWYXIx)
+        << "U3RyaXBwaW5nVGVzdC5DaGVja09r";
+  }
+
+  std::unique_ptr<FILE, std::function<void(FILE*)>> exe = OpenTestExecutable();
+  ASSERT_THAT(exe, NotNull());
+
+  if (absl::LogSeverity::kFatal >= kAbslMinLogLevel) {
+    EXPECT_THAT(exe.get(), FileHasSubstr(var_needle));
+    EXPECT_THAT(exe.get(), FileHasSubstr(msg_needle));
+  } else {
+    EXPECT_THAT(exe.get(), Not(FileHasSubstr(var_needle)));
+    EXPECT_THAT(exe.get(), Not(FileHasSubstr(msg_needle)));
+  }
+}
+
+}  // namespace
diff --git a/absl/log/structured.h b/absl/log/structured.h
new file mode 100644
index 00000000000..89d241e4f52
--- /dev/null
+++ b/absl/log/structured.h
@@ -0,0 +1,75 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: log/structured.h
+// -----------------------------------------------------------------------------
+//
+// This header declares APIs supporting structured logging, allowing log
+// statements to be more easily parsed, especially by automated processes.
+//
+// When structured logging is in use, data streamed into a `LOG` statement are
+// encoded as `Value` fields in a `logging.proto.Event` protocol buffer message.
+// The individual data are exposed programmatically to `LogSink`s and to the
+// user via some log reading tools which are able to query the structured data
+// more usefully than would be possible if each message was a single opaque
+// string.  These helpers allow user code to add additional structure to the
+// data they stream.
+
+#ifndef ABSL_LOG_STRUCTURED_H_
+#define ABSL_LOG_STRUCTURED_H_
+
+#include <ostream>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/log/internal/structured.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+// LogAsLiteral()
+//
+// Annotates its argument as a string literal so that structured logging
+// captures it as a `literal` field instead of a `str` field (the default).
+// This does not affect the text representation, only the structure.
+//
+// Streaming `LogAsLiteral(s)` into a `std::ostream` behaves just like streaming
+// `s` directly.
+//
+// Using `LogAsLiteral()` is occasionally appropriate and useful when proxying
+// data logged from another system or another language.  For example:
+//
+//   void Logger::LogString(absl::string_view str, absl::LogSeverity severity,
+//                          const char *file, int line) {
+//     LOG(LEVEL(severity)).AtLocation(file, line) << str;
+//   }
+//   void Logger::LogStringLiteral(absl::string_view str,
+//                                 absl::LogSeverity severity, const char *file,
+//                                 int line) {
+//     LOG(LEVEL(severity)).AtLocation(file, line) << absl::LogAsLiteral(str);
+//   }
+//
+// `LogAsLiteral` should only be used as a streaming operand and not, for
+// example, as a local variable initializer.
+inline log_internal::AsLiteralImpl LogAsLiteral(
+    absl::string_view s ABSL_ATTRIBUTE_LIFETIME_BOUND) {
+  return log_internal::AsLiteralImpl(s);
+}
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_LOG_STRUCTURED_H_
diff --git a/absl/log/structured_test.cc b/absl/log/structured_test.cc
new file mode 100644
index 00000000000..cde81999a68
--- /dev/null
+++ b/absl/log/structured_test.cc
@@ -0,0 +1,64 @@
+//
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/log/structured.h"
+
+#include <ios>
+#include <sstream>
+#include <string>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/attributes.h"
+#include "absl/log/internal/test_helpers.h"
+#include "absl/log/internal/test_matchers.h"
+#include "absl/log/log.h"
+#include "absl/log/scoped_mock_log.h"
+
+namespace {
+using ::absl::log_internal::MatchesOstream;
+using ::absl::log_internal::TextMessage;
+using ::testing::ElementsAre;
+using ::testing::Eq;
+
+auto *test_env ABSL_ATTRIBUTE_UNUSED = ::testing::AddGlobalTestEnvironment(
+    new absl::log_internal::LogTestEnvironment);
+
+// Abseil Logging library uses these by default, so we set them on the
+// `std::ostream` we compare against too.
+std::ios &LoggingDefaults(std::ios &str) {
+  str.setf(std::ios_base::showbase | std::ios_base::boolalpha |
+           std::ios_base::internal);
+  return str;
+}
+
+TEST(StreamingFormatTest, LogAsLiteral) {
+  std::ostringstream stream;
+  const std::string not_a_literal("hello world");
+  stream << LoggingDefaults << absl::LogAsLiteral(not_a_literal);
+
+  absl::ScopedMockLog sink;
+  EXPECT_CALL(sink, Send).Times(0);
+
+  EXPECT_CALL(sink, Send(AllOf(TextMessage(MatchesOstream(stream)),
+                               TextMessage(Eq("hello world")),
+                               ENCODED_MESSAGE(HasValues(ElementsAre(
+                                   ValueWithLiteral(Eq("hello world"))))))));
+
+  sink.StartCapturingLogs();
+  LOG(INFO) << absl::LogAsLiteral(not_a_literal);
+}
+
+}  // namespace
diff --git a/absl/log/vlog_is_on.h b/absl/log/vlog_is_on.h
new file mode 100644
index 00000000000..67c6fb37747
--- /dev/null
+++ b/absl/log/vlog_is_on.h
@@ -0,0 +1,72 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: log/vlog_is_on.h
+// -----------------------------------------------------------------------------
+//
+// This header defines the `VLOG_IS_ON()` macro that controls the
+// variable-verbosity conditional logging.
+//
+// It's used by `VLOG` in log.h, or it can also be used directly like this:
+//
+//   if (VLOG_IS_ON(2)) {
+//     foo_server.RecomputeStatisticsExpensive();
+//     LOG(INFO) << foo_server.LastStatisticsAsString();
+//   }
+//
+// Each source file has an effective verbosity level that's a non-negative
+// integer computed from the `--vmodule` and `--v` flags.
+// `VLOG_IS_ON(n)` is true, and `VLOG(n)` logs, if that effective verbosity
+// level is greater than or equal to `n`.
+//
+// `--vmodule` takes a comma-delimited list of key=value pairs.  Each key is a
+// pattern matched against filenames, and the values give the effective severity
+// level applied to matching files.  '?' and '*' characters in patterns are
+// interpreted as single-character and zero-or-more-character wildcards.
+// Patterns including a slash character are matched against full pathnames,
+// while those without are matched against basenames only.  One suffix (i.e. the
+// last . and everything after it) is stripped from each filename prior to
+// matching, as is the special suffix "-inl".
+//
+// Example: --vmodule=module_a=1,module_b=2
+//
+// Files are matched against globs in `--vmodule` in order, and the first match
+// determines the verbosity level.
+//
+// Files which do not match any pattern in `--vmodule` use the value of `--v` as
+// their effective verbosity level.  The default is 0.
+//
+// SetVLogLevel helper function is provided to do limited dynamic control over
+// V-logging by appending to `--vmodule`. Because these go at the beginning of
+// the list, they take priority over any globs previously added.
+//
+// Resetting --vmodule will override all previous modifications to `--vmodule`,
+// including via SetVLogLevel.
+
+#ifndef ABSL_LOG_VLOG_IS_ON_H_
+#define ABSL_LOG_VLOG_IS_ON_H_
+
+#include "absl/log/absl_vlog_is_on.h"  // IWYU pragma: export
+
+// Each VLOG_IS_ON call site gets its own VLogSite that registers with the
+// global linked list of sites to asynchronously update its verbosity level on
+// changes to --v or --vmodule. The verbosity can also be set by manually
+// calling SetVLogLevel.
+//
+// VLOG_IS_ON is not async signal safe, but it is guaranteed not to allocate
+// new memory.
+#define VLOG_IS_ON(verbose_level) ABSL_VLOG_IS_ON(verbose_level)
+
+#endif  // ABSL_LOG_VLOG_IS_ON_H_
diff --git a/absl/log/vlog_is_on_test.cc b/absl/log/vlog_is_on_test.cc
new file mode 100644
index 00000000000..d523781841b
--- /dev/null
+++ b/absl/log/vlog_is_on_test.cc
@@ -0,0 +1,234 @@
+// Copyright 2023 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/log/vlog_is_on.h"
+
+#include <optional>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/log_severity.h"
+#include "absl/flags/flag.h"
+#include "absl/log/flags.h"
+#include "absl/log/globals.h"
+#include "absl/log/log.h"
+#include "absl/log/scoped_mock_log.h"
+
+namespace {
+
+using ::testing::_;
+
+std::optional<int> MaxLogVerbosity() {
+#ifdef ABSL_MAX_VLOG_VERBOSITY
+  return ABSL_MAX_VLOG_VERBOSITY;
+#else
+  return std::nullopt;
+#endif
+}
+
+std::optional<int> MinLogLevel() {
+#ifdef ABSL_MIN_LOG_LEVEL
+  return static_cast<int>(ABSL_MIN_LOG_LEVEL);
+#else
+  return std::nullopt;
+#endif
+}
+
+// This fixture is used to reset the VLOG levels to their default values before
+// each test.
+class VLogIsOnTest : public ::testing::Test {
+ protected:
+  void SetUp() override { ResetVLogLevels(); }
+
+ private:
+  // Resets the VLOG levels to their default values.
+  // It is supposed to be called in the SetUp() method of the test fixture to
+  // eliminate any side effects from other tests.
+  static void ResetVLogLevels() {
+    absl::log_internal::UpdateVModule("");
+    absl::SetGlobalVLogLevel(0);
+  }
+};
+
+TEST_F(VLogIsOnTest, GlobalWorksWithoutMaxVerbosityAndMinLogLevel) {
+  if (MaxLogVerbosity().has_value() || MinLogLevel().has_value()) {
+    GTEST_SKIP();
+  }
+
+  absl::SetGlobalVLogLevel(3);
+  absl::ScopedMockLog log(absl::MockLogDefault::kDisallowUnexpected);
+
+  EXPECT_CALL(log, Log(absl::LogSeverity::kInfo, _, "important"));
+
+  log.StartCapturingLogs();
+  VLOG(3) << "important";
+  VLOG(4) << "spam";
+}
+
+TEST_F(VLogIsOnTest, FileWorksWithoutMaxVerbosityAndMinLogLevel) {
+  if (MaxLogVerbosity().has_value() || MinLogLevel().has_value()) {
+    GTEST_SKIP();
+  }
+
+  absl::SetVLogLevel("vlog_is_on_test", 3);
+  absl::ScopedMockLog log(absl::MockLogDefault::kDisallowUnexpected);
+
+  EXPECT_CALL(log, Log(absl::LogSeverity::kInfo, _, "important"));
+
+  log.StartCapturingLogs();
+  VLOG(3) << "important";
+  VLOG(4) << "spam";
+}
+
+TEST_F(VLogIsOnTest, PatternWorksWithoutMaxVerbosityAndMinLogLevel) {
+  if (MaxLogVerbosity().has_value() || MinLogLevel().has_value()) {
+    GTEST_SKIP();
+  }
+
+  absl::SetVLogLevel("vlog_is_on*", 3);
+  absl::ScopedMockLog log(absl::MockLogDefault::kDisallowUnexpected);
+
+  EXPECT_CALL(log, Log(absl::LogSeverity::kInfo, _, "important"));
+
+  log.StartCapturingLogs();
+  VLOG(3) << "important";
+  VLOG(4) << "spam";
+}
+
+TEST_F(VLogIsOnTest,
+       PatternOverridesLessGenericOneWithoutMaxVerbosityAndMinLogLevel) {
+  if (MaxLogVerbosity().has_value() || MinLogLevel().has_value()) {
+    GTEST_SKIP();
+  }
+
+  // This should disable logging in this file
+  absl::SetVLogLevel("vlog_is_on*", -1);
+  // This overrides the previous setting, because "vlog*" is more generic than
+  // "vlog_is_on*". This should enable VLOG level 3 in this file.
+  absl::SetVLogLevel("vlog*", 3);
+  absl::ScopedMockLog log(absl::MockLogDefault::kDisallowUnexpected);
+
+  EXPECT_CALL(log, Log(absl::LogSeverity::kInfo, _, "important"));
+
+  log.StartCapturingLogs();
+  VLOG(3) << "important";
+  VLOG(4) << "spam";
+}
+
+TEST_F(VLogIsOnTest,
+       PatternDoesNotOverridesMoreGenericOneWithoutMaxVerbosityAndMinLogLevel) {
+  if (MaxLogVerbosity().has_value() || MinLogLevel().has_value()) {
+    GTEST_SKIP();
+  }
+
+  // This should enable VLOG level 3 in this file.
+  absl::SetVLogLevel("vlog*", 3);
+  // This should not change the VLOG level in this file. The pattern does not
+  // match this file and it is less generic than the previous patter "vlog*".
+  // Therefore, it does not disable VLOG level 3 in this file.
+  absl::SetVLogLevel("vlog_is_on_some_other_test*", -1);
+  absl::ScopedMockLog log(absl::MockLogDefault::kDisallowUnexpected);
+
+  EXPECT_CALL(log, Log(absl::LogSeverity::kInfo, _, "important"));
+
+  log.StartCapturingLogs();
+  VLOG(3) << "important";
+  VLOG(5) << "spam";
+}
+
+TEST_F(VLogIsOnTest, GlobalDoesNotFilterBelowMaxVerbosity) {
+  if (!MaxLogVerbosity().has_value() || *MaxLogVerbosity() < 2) {
+    GTEST_SKIP();
+  }
+
+  // Set an arbitrary high value to avoid filtering VLOGs in tests by default.
+  absl::SetGlobalVLogLevel(1000);
+  absl::ScopedMockLog log(absl::MockLogDefault::kDisallowUnexpected);
+
+  EXPECT_CALL(log, Log(absl::LogSeverity::kInfo, _, "asdf"));
+
+  log.StartCapturingLogs();
+  VLOG(2) << "asdf";
+}
+
+TEST_F(VLogIsOnTest, FileDoesNotFilterBelowMaxVerbosity) {
+  if (!MaxLogVerbosity().has_value() || *MaxLogVerbosity() < 2) {
+    GTEST_SKIP();
+  }
+
+  // Set an arbitrary high value to avoid filtering VLOGs in tests by default.
+  absl::SetVLogLevel("vlog_is_on_test", 1000);
+  absl::ScopedMockLog log(absl::MockLogDefault::kDisallowUnexpected);
+
+  EXPECT_CALL(log, Log(absl::LogSeverity::kInfo, _, "asdf"));
+
+  log.StartCapturingLogs();
+  VLOG(2) << "asdf";
+}
+
+TEST_F(VLogIsOnTest, PatternDoesNotFilterBelowMaxVerbosity) {
+  if (!MaxLogVerbosity().has_value() || *MaxLogVerbosity() < 2) {
+    GTEST_SKIP();
+  }
+
+  // Set an arbitrary high value to avoid filtering VLOGs in tests by default.
+  absl::SetVLogLevel("vlog_is_on*", 1000);
+  absl::ScopedMockLog log(absl::MockLogDefault::kDisallowUnexpected);
+
+  EXPECT_CALL(log, Log(absl::LogSeverity::kInfo, _, "asdf"));
+
+  log.StartCapturingLogs();
+  VLOG(2) << "asdf";
+}
+
+TEST_F(VLogIsOnTest, GlobalFiltersAboveMaxVerbosity) {
+  if (!MaxLogVerbosity().has_value() || *MaxLogVerbosity() >= 4) {
+    GTEST_SKIP();
+  }
+
+  // Set an arbitrary high value to avoid filtering VLOGs in tests by default.
+  absl::SetGlobalVLogLevel(1000);
+  absl::ScopedMockLog log(absl::MockLogDefault::kDisallowUnexpected);
+
+  log.StartCapturingLogs();
+  VLOG(4) << "dfgh";
+}
+
+TEST_F(VLogIsOnTest, FileFiltersAboveMaxVerbosity) {
+  if (!MaxLogVerbosity().has_value() || *MaxLogVerbosity() >= 4) {
+    GTEST_SKIP();
+  }
+
+  // Set an arbitrary high value to avoid filtering VLOGs in tests by default.
+  absl::SetVLogLevel("vlog_is_on_test", 1000);
+  absl::ScopedMockLog log(absl::MockLogDefault::kDisallowUnexpected);
+
+  log.StartCapturingLogs();
+  VLOG(4) << "dfgh";
+}
+
+TEST_F(VLogIsOnTest, PatternFiltersAboveMaxVerbosity) {
+  if (!MaxLogVerbosity().has_value() || *MaxLogVerbosity() >= 4) {
+    GTEST_SKIP();
+  }
+
+  // Set an arbitrary high value to avoid filtering VLOGs in tests by default.
+  absl::SetVLogLevel("vlog_is_on*", 1000);
+  absl::ScopedMockLog log(absl::MockLogDefault::kDisallowUnexpected);
+
+  log.StartCapturingLogs();
+  VLOG(4) << "dfgh";
+}
+
+}  // namespace
diff --git a/absl/memory/BUILD.bazel b/absl/memory/BUILD.bazel
index 2ba9d7cb98a..92f93729392 100644
--- a/absl/memory/BUILD.bazel
+++ b/absl/memory/BUILD.bazel
@@ -14,7 +14,8 @@
 # limitations under the License.
 #
 
-load("@rules_cc//cc:defs.bzl", "cc_library", "cc_test")
+load("@rules_cc//cc:cc_library.bzl", "cc_library")
+load("@rules_cc//cc:cc_test.bzl", "cc_test")
 load(
     "//absl:copts/configure_copts.bzl",
     "ABSL_DEFAULT_COPTS",
@@ -22,15 +23,26 @@ load(
     "ABSL_TEST_COPTS",
 )
 
-package(default_visibility = ["//visibility:public"])
+package(
+    default_visibility = ["//visibility:private"],
+    features = [
+        "header_modules",
+        "layering_check",
+        "parse_headers",
+    ],
+)
 
-licenses(["notice"])  # Apache 2.0
+licenses(["notice"])
 
 cc_library(
     name = "memory",
     hdrs = ["memory.h"],
     copts = ABSL_DEFAULT_COPTS,
+    defines = select({
+        "//conditions:default": [],
+    }),
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
     deps = [
         "//absl/base:core_headers",
         "//absl/meta:type_traits",
@@ -45,21 +57,7 @@ cc_test(
     deps = [
         ":memory",
         "//absl/base:core_headers",
-        "@com_google_googletest//:gtest_main",
-    ],
-)
-
-cc_test(
-    name = "memory_exception_safety_test",
-    srcs = [
-        "memory_exception_safety_test.cc",
-    ],
-    copts = ABSL_TEST_COPTS,
-    linkopts = ABSL_DEFAULT_LINKOPTS,
-    deps = [
-        ":memory",
-        "//absl/base:config",
-        "//absl/base:exception_safety_testing",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
diff --git a/absl/memory/CMakeLists.txt b/absl/memory/CMakeLists.txt
index 78fb7e1b316..c5ed4b42558 100644
--- a/absl/memory/CMakeLists.txt
+++ b/absl/memory/CMakeLists.txt
@@ -37,19 +37,5 @@ absl_cc_test(
   DEPS
     absl::memory
     absl::core_headers
-    gmock_main
-)
-
-absl_cc_test(
-  NAME
-    memory_exception_safety_test
-  SRCS
-    "memory_exception_safety_test.cc"
-  COPTS
-    ${ABSL_TEST_COPTS}
-  DEPS
-    absl::memory
-    absl::config
-    absl::exception_safety_testing
-    gmock_main
+    GTest::gmock_main
 )
diff --git a/absl/memory/memory.h b/absl/memory/memory.h
index 5a4a1a1dfe2..cbc98125099 100644
--- a/absl/memory/memory.h
+++ b/absl/memory/memory.h
@@ -29,11 +29,13 @@
 #include <new>
 #include <type_traits>
 #include <utility>
+#include <version>
 
 #include "absl/base/macros.h"
 #include "absl/meta/type_traits.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 // -----------------------------------------------------------------------------
 // Function Template: WrapUnique()
@@ -57,7 +59,7 @@ namespace absl {
 //   std::unique_ptr<X> x(NewX(1, 2));
 //
 // While `absl::WrapUnique` is useful for capturing the output of a raw
-// pointer factory, prefer 'absl::make_unique<T>(args...)' over
+// pointer factory, prefer 'std::make_unique<T>(args...)' over
 // 'absl::WrapUnique(new T(args...))'.
 //
 //   auto x = WrapUnique(new X(1, 2));  // works, but nonideal.
@@ -69,14 +71,41 @@ namespace absl {
 // obtained from array-new expressions (even though that would compile!).
 template <typename T>
 std::unique_ptr<T> WrapUnique(T* ptr) {
-  static_assert(!std::is_array<T>::value, "array types are unsupported");
-  static_assert(std::is_object<T>::value, "non-object types are unsupported");
+  static_assert(!std::is_array_v<T>, "array types are unsupported");
+  static_assert(std::is_object_v<T>, "non-object types are unsupported");
   return std::unique_ptr<T>(ptr);
 }
 
+// -----------------------------------------------------------------------------
+// Function Template: make_unique<T>()
+// -----------------------------------------------------------------------------
+//
+// Creates a `std::unique_ptr<>`, while avoiding issues creating temporaries
+// during the construction process. `absl::make_unique<>` also avoids redundant
+// type declarations, by avoiding the need to explicitly use the `new` operator.
+//
+// https://en.cppreference.com/w/cpp/memory/unique_ptr/make_unique
+//
+// For more background on why `std::unique_ptr<T>(new T(a,b))` is problematic,
+// see Herb Sutter's explanation on
+// (Exception-Safe Function Calls)[https://herbsutter.com/gotw/_102/].
+// (In general, reviewers should treat `new T(a,b)` with scrutiny.)
+//
+// Historical note: Abseil once provided a C++11 compatible implementation of
+// the C++14's `std::make_unique`. Now that C++11 support has been sunsetted,
+// `absl::make_unique` simply uses the STL-provided implementation. New code
+// should use `std::make_unique`.
+using std::make_unique ABSL_REFACTOR_INLINE;
+
+#if defined(__cpp_lib_smart_ptr_for_overwrite) && \
+    __cpp_lib_smart_ptr_for_overwrite >= 202002L
+using std::make_unique_for_overwrite;
+#else
+
 namespace memory_internal {
 
-// Traits to select proper overload and return type for `absl::make_unique<>`.
+// Traits to select proper overload and return type for
+// `absl::make_unique_for_overwrite<>`.
 template <typename T>
 struct MakeUniqueResult {
   using scalar = std::unique_ptr<T>;
@@ -92,97 +121,35 @@ struct MakeUniqueResult<T[N]> {
 
 }  // namespace memory_internal
 
-// gcc 4.8 has __cplusplus at 201301 but doesn't define make_unique.  Other
-// supported compilers either just define __cplusplus as 201103 but have
-// make_unique (msvc), or have make_unique whenever __cplusplus > 201103 (clang)
-#if (__cplusplus > 201103L || defined(_MSC_VER)) && \
-    !(defined(__GNUC__) && __GNUC__ == 4 && __GNUC_MINOR__ == 8)
-using std::make_unique;
-#else
-// -----------------------------------------------------------------------------
-// Function Template: make_unique<T>()
-// -----------------------------------------------------------------------------
-//
-// Creates a `std::unique_ptr<>`, while avoiding issues creating temporaries
-// during the construction process. `absl::make_unique<>` also avoids redundant
-// type declarations, by avoiding the need to explicitly use the `new` operator.
-//
-// This implementation of `absl::make_unique<>` is designed for C++11 code and
-// will be replaced in C++14 by the equivalent `std::make_unique<>` abstraction.
-// `absl::make_unique<>` is designed to be 100% compatible with
-// `std::make_unique<>` so that the eventual migration will involve a simple
-// rename operation.
-//
-// For more background on why `std::unique_ptr<T>(new T(a,b))` is problematic,
-// see Herb Sutter's explanation on
-// (Exception-Safe Function Calls)[https://herbsutter.com/gotw/_102/].
-// (In general, reviewers should treat `new T(a,b)` with scrutiny.)
-//
-// Example usage:
-//
-//    auto p = make_unique<X>(args...);  // 'p'  is a std::unique_ptr<X>
-//    auto pa = make_unique<X[]>(5);     // 'pa' is a std::unique_ptr<X[]>
-//
-// Three overloads of `absl::make_unique` are required:
-//
-//   - For non-array T:
-//
-//       Allocates a T with `new T(std::forward<Args> args...)`,
-//       forwarding all `args` to T's constructor.
-//       Returns a `std::unique_ptr<T>` owning that object.
-//
-//   - For an array of unknown bounds T[]:
-//
-//       `absl::make_unique<>` will allocate an array T of type U[] with
-//       `new U[n]()` and return a `std::unique_ptr<U[]>` owning that array.
-//
-//       Note that 'U[n]()' is different from 'U[n]', and elements will be
-//       value-initialized. Note as well that `std::unique_ptr` will perform its
-//       own destruction of the array elements upon leaving scope, even though
-//       the array [] does not have a default destructor.
-//
-//       NOTE: an array of unknown bounds T[] may still be (and often will be)
-//       initialized to have a size, and will still use this overload. E.g:
-//
-//         auto my_array = absl::make_unique<int[]>(10);
-//
-//   - For an array of known bounds T[N]:
-//
-//       `absl::make_unique<>` is deleted (like with `std::make_unique<>`) as
-//       this overload is not useful.
-//
-//       NOTE: an array of known bounds T[N] is not considered a useful
-//       construction, and may cause undefined behavior in templates. E.g:
-//
-//         auto my_array = absl::make_unique<int[10]>();
+// These are make_unique_for_overwrite variants modeled after
+// http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p1973r1.pdf
+// Unlike std::make_unique, values are default initialized rather than value
+// initialized.
 //
-//       In those cases, of course, you can still use the overload above and
-//       simply initialize it to its desired size:
-//
-//         auto my_array = absl::make_unique<int[]>(10);
-
-// `absl::make_unique` overload for non-array types.
-template <typename T, typename... Args>
-typename memory_internal::MakeUniqueResult<T>::scalar make_unique(
-    Args&&... args) {
-  return std::unique_ptr<T>(new T(std::forward<Args>(args)...));
+// `absl::make_unique_for_overwrite` overload for non-array types.
+template <typename T>
+typename memory_internal::MakeUniqueResult<T>::scalar
+make_unique_for_overwrite() {
+  return std::unique_ptr<T>(new T);
 }
 
-// `absl::make_unique` overload for an array T[] of unknown bounds.
-// The array allocation needs to use the `new T[size]` form and cannot take
-// element constructor arguments. The `std::unique_ptr` will manage destructing
-// these array elements.
+// `absl::make_unique_for_overwrite` overload for an array T[] of unknown
+// bounds. The array allocation needs to use the `new T[size]` form and cannot
+// take element constructor arguments. The `std::unique_ptr` will manage
+// destructing these array elements.
 template <typename T>
-typename memory_internal::MakeUniqueResult<T>::array make_unique(size_t n) {
-  return std::unique_ptr<T>(new typename absl::remove_extent_t<T>[n]());
+typename memory_internal::MakeUniqueResult<T>::array make_unique_for_overwrite(
+    size_t n) {
+  return std::unique_ptr<T>(new typename std::remove_extent_t<T>[n]);
 }
 
-// `absl::make_unique` overload for an array T[N] of known bounds.
+// `absl::make_unique_for_overwrite` overload for an array T[N] of known bounds.
 // This construction will be rejected.
 template <typename T, typename... Args>
-typename memory_internal::MakeUniqueResult<T>::invalid make_unique(
-    Args&&... /* args */) = delete;
-#endif
+typename memory_internal::MakeUniqueResult<T>::invalid
+make_unique_for_overwrite(Args&&... /* args */) = delete;
+
+#endif  // __cpp_lib_smart_ptr_for_overwrite
 
 // -----------------------------------------------------------------------------
 // Function Template: RawPtr()
@@ -208,7 +175,7 @@ inline std::nullptr_t RawPtr(std::nullptr_t) { return nullptr; }
 //
 // Example:
 //
-//     auto up = absl::make_unique<int>(10);
+//     auto up = std::make_unique<int>(10);
 //     auto sp = absl::ShareUniquePtr(std::move(up));  // shared_ptr<int>
 //     CHECK_EQ(*sp, 10);
 //     CHECK(up == nullptr);
@@ -246,369 +213,45 @@ std::weak_ptr<T> WeakenPtr(const std::shared_ptr<T>& ptr) {
   return std::weak_ptr<T>(ptr);
 }
 
-namespace memory_internal {
-
-// ExtractOr<E, O, D>::type evaluates to E<O> if possible. Otherwise, D.
-template <template <typename> class Extract, typename Obj, typename Default,
-          typename>
-struct ExtractOr {
-  using type = Default;
-};
-
-template <template <typename> class Extract, typename Obj, typename Default>
-struct ExtractOr<Extract, Obj, Default, void_t<Extract<Obj>>> {
-  using type = Extract<Obj>;
-};
-
-template <template <typename> class Extract, typename Obj, typename Default>
-using ExtractOrT = typename ExtractOr<Extract, Obj, Default, void>::type;
-
-// Extractors for the features of allocators.
-template <typename T>
-using GetPointer = typename T::pointer;
-
-template <typename T>
-using GetConstPointer = typename T::const_pointer;
-
-template <typename T>
-using GetVoidPointer = typename T::void_pointer;
-
-template <typename T>
-using GetConstVoidPointer = typename T::const_void_pointer;
-
-template <typename T>
-using GetDifferenceType = typename T::difference_type;
-
-template <typename T>
-using GetSizeType = typename T::size_type;
-
-template <typename T>
-using GetPropagateOnContainerCopyAssignment =
-    typename T::propagate_on_container_copy_assignment;
-
-template <typename T>
-using GetPropagateOnContainerMoveAssignment =
-    typename T::propagate_on_container_move_assignment;
-
-template <typename T>
-using GetPropagateOnContainerSwap = typename T::propagate_on_container_swap;
-
-template <typename T>
-using GetIsAlwaysEqual = typename T::is_always_equal;
-
-template <typename T>
-struct GetFirstArg;
-
-template <template <typename...> class Class, typename T, typename... Args>
-struct GetFirstArg<Class<T, Args...>> {
-  using type = T;
-};
-
-template <typename Ptr, typename = void>
-struct ElementType {
-  using type = typename GetFirstArg<Ptr>::type;
-};
-
-template <typename T>
-struct ElementType<T, void_t<typename T::element_type>> {
-  using type = typename T::element_type;
-};
-
-template <typename T, typename U>
-struct RebindFirstArg;
-
-template <template <typename...> class Class, typename T, typename... Args,
-          typename U>
-struct RebindFirstArg<Class<T, Args...>, U> {
-  using type = Class<U, Args...>;
-};
-
-template <typename T, typename U, typename = void>
-struct RebindPtr {
-  using type = typename RebindFirstArg<T, U>::type;
-};
-
-template <typename T, typename U>
-struct RebindPtr<T, U, void_t<typename T::template rebind<U>>> {
-  using type = typename T::template rebind<U>;
-};
-
-template <typename T, typename U>
-constexpr bool HasRebindAlloc(...) {
-  return false;
-}
-
-template <typename T, typename U>
-constexpr bool HasRebindAlloc(typename T::template rebind<U>::other*) {
-  return true;
-}
-
-template <typename T, typename U, bool = HasRebindAlloc<T, U>(nullptr)>
-struct RebindAlloc {
-  using type = typename RebindFirstArg<T, U>::type;
-};
-
-template <typename T, typename U>
-struct RebindAlloc<T, U, true> {
-  using type = typename T::template rebind<U>::other;
-};
-
-}  // namespace memory_internal
-
 // -----------------------------------------------------------------------------
 // Class Template: pointer_traits
 // -----------------------------------------------------------------------------
 //
-// An implementation of C++11's std::pointer_traits.
-//
-// Provided for portability on toolchains that have a working C++11 compiler,
-// but the standard library is lacking in C++11 support. For example, some
-// version of the Android NDK.
-//
-
+// Historical note: Abseil once provided an implementation of
+// `std::pointer_traits` for platforms that had not yet provided it. Those
+// platforms are no longer supported. New code should simply use
+// `std::pointer_traits`.
 template <typename Ptr>
-struct pointer_traits {
-  using pointer = Ptr;
-
-  // element_type:
-  // Ptr::element_type if present. Otherwise T if Ptr is a template
-  // instantiation Template<T, Args...>
-  using element_type = typename memory_internal::ElementType<Ptr>::type;
-
-  // difference_type:
-  // Ptr::difference_type if present, otherwise std::ptrdiff_t
-  using difference_type =
-      memory_internal::ExtractOrT<memory_internal::GetDifferenceType, Ptr,
-                                  std::ptrdiff_t>;
-
-  // rebind:
-  // Ptr::rebind<U> if exists, otherwise Template<U, Args...> if Ptr is a
-  // template instantiation Template<T, Args...>
-  template <typename U>
-  using rebind = typename memory_internal::RebindPtr<Ptr, U>::type;
-
-  // pointer_to:
-  // Calls Ptr::pointer_to(r)
-  static pointer pointer_to(element_type& r) {  // NOLINT(runtime/references)
-    return Ptr::pointer_to(r);
-  }
-};
-
-// Specialization for T*.
-template <typename T>
-struct pointer_traits<T*> {
-  using pointer = T*;
-  using element_type = T;
-  using difference_type = std::ptrdiff_t;
-
-  template <typename U>
-  using rebind = U*;
-
-  // pointer_to:
-  // Calls std::addressof(r)
-  static pointer pointer_to(
-      element_type& r) noexcept {  // NOLINT(runtime/references)
-    return std::addressof(r);
-  }
-};
+using pointer_traits ABSL_DEPRECATE_AND_INLINE() = std::pointer_traits<Ptr>;
 
 // -----------------------------------------------------------------------------
 // Class Template: allocator_traits
 // -----------------------------------------------------------------------------
 //
-// A C++11 compatible implementation of C++17's std::allocator_traits.
-//
+// Historical note: Abseil once provided an implementation of
+// `std::allocator_traits` for platforms that had not yet provided it. Those
+// platforms are no longer supported. New code should simply use
+// `std::allocator_traits`.
 template <typename Alloc>
-struct allocator_traits {
-  using allocator_type = Alloc;
-
-  // value_type:
-  // Alloc::value_type
-  using value_type = typename Alloc::value_type;
-
-  // pointer:
-  // Alloc::pointer if present, otherwise value_type*
-  using pointer = memory_internal::ExtractOrT<memory_internal::GetPointer,
-                                              Alloc, value_type*>;
-
-  // const_pointer:
-  // Alloc::const_pointer if present, otherwise
-  // absl::pointer_traits<pointer>::rebind<const value_type>
-  using const_pointer =
-      memory_internal::ExtractOrT<memory_internal::GetConstPointer, Alloc,
-                                  typename absl::pointer_traits<pointer>::
-                                      template rebind<const value_type>>;
-
-  // void_pointer:
-  // Alloc::void_pointer if present, otherwise
-  // absl::pointer_traits<pointer>::rebind<void>
-  using void_pointer = memory_internal::ExtractOrT<
-      memory_internal::GetVoidPointer, Alloc,
-      typename absl::pointer_traits<pointer>::template rebind<void>>;
-
-  // const_void_pointer:
-  // Alloc::const_void_pointer if present, otherwise
-  // absl::pointer_traits<pointer>::rebind<const void>
-  using const_void_pointer = memory_internal::ExtractOrT<
-      memory_internal::GetConstVoidPointer, Alloc,
-      typename absl::pointer_traits<pointer>::template rebind<const void>>;
-
-  // difference_type:
-  // Alloc::difference_type if present, otherwise
-  // absl::pointer_traits<pointer>::difference_type
-  using difference_type = memory_internal::ExtractOrT<
-      memory_internal::GetDifferenceType, Alloc,
-      typename absl::pointer_traits<pointer>::difference_type>;
+using allocator_traits ABSL_DEPRECATE_AND_INLINE() =
+    std::allocator_traits<Alloc>;
 
-  // size_type:
-  // Alloc::size_type if present, otherwise
-  // std::make_unsigned<difference_type>::type
-  using size_type = memory_internal::ExtractOrT<
-      memory_internal::GetSizeType, Alloc,
-      typename std::make_unsigned<difference_type>::type>;
-
-  // propagate_on_container_copy_assignment:
-  // Alloc::propagate_on_container_copy_assignment if present, otherwise
-  // std::false_type
-  using propagate_on_container_copy_assignment = memory_internal::ExtractOrT<
-      memory_internal::GetPropagateOnContainerCopyAssignment, Alloc,
-      std::false_type>;
-
-  // propagate_on_container_move_assignment:
-  // Alloc::propagate_on_container_move_assignment if present, otherwise
-  // std::false_type
-  using propagate_on_container_move_assignment = memory_internal::ExtractOrT<
-      memory_internal::GetPropagateOnContainerMoveAssignment, Alloc,
-      std::false_type>;
-
-  // propagate_on_container_swap:
-  // Alloc::propagate_on_container_swap if present, otherwise std::false_type
-  using propagate_on_container_swap =
-      memory_internal::ExtractOrT<memory_internal::GetPropagateOnContainerSwap,
-                                  Alloc, std::false_type>;
-
-  // is_always_equal:
-  // Alloc::is_always_equal if present, otherwise std::is_empty<Alloc>::type
-  using is_always_equal =
-      memory_internal::ExtractOrT<memory_internal::GetIsAlwaysEqual, Alloc,
-                                  typename std::is_empty<Alloc>::type>;
-
-  // rebind_alloc:
-  // Alloc::rebind<T>::other if present, otherwise Alloc<T, Args> if this Alloc
-  // is Alloc<U, Args>
-  template <typename T>
-  using rebind_alloc = typename memory_internal::RebindAlloc<Alloc, T>::type;
-
-  // rebind_traits:
-  // absl::allocator_traits<rebind_alloc<T>>
-  template <typename T>
-  using rebind_traits = absl::allocator_traits<rebind_alloc<T>>;
-
-  // allocate(Alloc& a, size_type n):
-  // Calls a.allocate(n)
-  static pointer allocate(Alloc& a,  // NOLINT(runtime/references)
-                          size_type n) {
-    return a.allocate(n);
-  }
-
-  // allocate(Alloc& a, size_type n, const_void_pointer hint):
-  // Calls a.allocate(n, hint) if possible.
-  // If not possible, calls a.allocate(n)
-  static pointer allocate(Alloc& a, size_type n,  // NOLINT(runtime/references)
-                          const_void_pointer hint) {
-    return allocate_impl(0, a, n, hint);
-  }
-
-  // deallocate(Alloc& a, pointer p, size_type n):
-  // Calls a.deallocate(p, n)
-  static void deallocate(Alloc& a, pointer p,  // NOLINT(runtime/references)
-                         size_type n) {
-    a.deallocate(p, n);
-  }
-
-  // construct(Alloc& a, T* p, Args&&... args):
-  // Calls a.construct(p, std::forward<Args>(args)...) if possible.
-  // If not possible, calls
-  //   ::new (static_cast<void*>(p)) T(std::forward<Args>(args)...)
-  template <typename T, typename... Args>
-  static void construct(Alloc& a, T* p,  // NOLINT(runtime/references)
-                        Args&&... args) {
-    construct_impl(0, a, p, std::forward<Args>(args)...);
-  }
-
-  // destroy(Alloc& a, T* p):
-  // Calls a.destroy(p) if possible. If not possible, calls p->~T().
-  template <typename T>
-  static void destroy(Alloc& a, T* p) {  // NOLINT(runtime/references)
-    destroy_impl(0, a, p);
-  }
-
-  // max_size(const Alloc& a):
-  // Returns a.max_size() if possible. If not possible, returns
-  //   std::numeric_limits<size_type>::max() / sizeof(value_type)
-  static size_type max_size(const Alloc& a) { return max_size_impl(0, a); }
-
-  // select_on_container_copy_construction(const Alloc& a):
-  // Returns a.select_on_container_copy_construction() if possible.
-  // If not possible, returns a.
-  static Alloc select_on_container_copy_construction(const Alloc& a) {
-    return select_on_container_copy_construction_impl(0, a);
-  }
-
- private:
-  template <typename A>
-  static auto allocate_impl(int, A& a,  // NOLINT(runtime/references)
-                            size_type n, const_void_pointer hint)
-      -> decltype(a.allocate(n, hint)) {
-    return a.allocate(n, hint);
-  }
-  static pointer allocate_impl(char, Alloc& a,  // NOLINT(runtime/references)
-                               size_type n, const_void_pointer) {
-    return a.allocate(n);
-  }
-
-  template <typename A, typename... Args>
-  static auto construct_impl(int, A& a,  // NOLINT(runtime/references)
-                             Args&&... args)
-      -> decltype(a.construct(std::forward<Args>(args)...)) {
-    a.construct(std::forward<Args>(args)...);
-  }
-
-  template <typename T, typename... Args>
-  static void construct_impl(char, Alloc&, T* p, Args&&... args) {
-    ::new (static_cast<void*>(p)) T(std::forward<Args>(args)...);
-  }
-
-  template <typename A, typename T>
-  static auto destroy_impl(int, A& a,  // NOLINT(runtime/references)
-                           T* p) -> decltype(a.destroy(p)) {
-    a.destroy(p);
-  }
-  template <typename T>
-  static void destroy_impl(char, Alloc&, T* p) {
-    p->~T();
-  }
+namespace memory_internal {
 
-  template <typename A>
-  static auto max_size_impl(int, const A& a) -> decltype(a.max_size()) {
-    return a.max_size();
-  }
-  static size_type max_size_impl(char, const Alloc&) {
-    return (std::numeric_limits<size_type>::max)() / sizeof(value_type);
-  }
+// ExtractOr<E, O, D>::type evaluates to E<O> if possible. Otherwise, D.
+template <template <typename> class Extract, typename Obj, typename Default,
+          typename>
+struct ExtractOr {
+  using type = Default;
+};
 
-  template <typename A>
-  static auto select_on_container_copy_construction_impl(int, const A& a)
-      -> decltype(a.select_on_container_copy_construction()) {
-    return a.select_on_container_copy_construction();
-  }
-  static Alloc select_on_container_copy_construction_impl(char,
-                                                          const Alloc& a) {
-    return a;
-  }
+template <template <typename> class Extract, typename Obj, typename Default>
+struct ExtractOr<Extract, Obj, Default, std::void_t<Extract<Obj>>> {
+  using type = Extract<Obj>;
 };
 
-namespace memory_internal {
+template <template <typename> class Extract, typename Obj, typename Default>
+using ExtractOrT = typename ExtractOr<Extract, Obj, Default, void>::type;
 
 // This template alias transforms Alloc::is_nothrow into a metafunction with
 // Alloc as a parameter so it can be used with ExtractOrT<>.
@@ -687,6 +330,7 @@ void CopyRange(Allocator& alloc, Iterator destination, InputIterator first,
   }
 }
 }  // namespace memory_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_MEMORY_MEMORY_H_
diff --git a/absl/memory/memory_exception_safety_test.cc b/absl/memory/memory_exception_safety_test.cc
deleted file mode 100644
index 729507e97f3..00000000000
--- a/absl/memory/memory_exception_safety_test.cc
+++ /dev/null
@@ -1,58 +0,0 @@
-// Copyright 2018 The Abseil Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-#include "absl/memory/memory.h"
-
-#include "absl/base/config.h"
-
-#ifdef ABSL_HAVE_EXCEPTIONS
-
-#include "gtest/gtest.h"
-#include "absl/base/internal/exception_safety_testing.h"
-
-namespace absl {
-namespace {
-
-constexpr int kLength = 50;
-using Thrower = testing::ThrowingValue<testing::TypeSpec::kEverythingThrows>;
-using ThrowerStorage =
-    absl::aligned_storage_t<sizeof(Thrower), alignof(Thrower)>;
-using ThrowerList = std::array<ThrowerStorage, kLength>;
-
-TEST(MakeUnique, CheckForLeaks) {
-  constexpr int kValue = 321;
-  auto tester = testing::MakeExceptionSafetyTester()
-                    .WithInitialValue(Thrower(kValue))
-                    // Ensures make_unique does not modify the input. The real
-                    // test, though, is ConstructorTracker checking for leaks.
-                    .WithContracts(testing::strong_guarantee);
-
-  EXPECT_TRUE(tester.Test([](Thrower* thrower) {
-    static_cast<void>(absl::make_unique<Thrower>(*thrower));
-  }));
-
-  EXPECT_TRUE(tester.Test([](Thrower* thrower) {
-    static_cast<void>(absl::make_unique<Thrower>(std::move(*thrower)));
-  }));
-
-  // Test T[n] overload
-  EXPECT_TRUE(tester.Test([&](Thrower*) {
-    static_cast<void>(absl::make_unique<Thrower[]>(kLength));
-  }));
-}
-
-}  // namespace
-}  // namespace absl
-
-#endif  // ABSL_HAVE_EXCEPTIONS
diff --git a/absl/memory/memory_test.cc b/absl/memory/memory_test.cc
index c47820e54ab..3ef6830046d 100644
--- a/absl/memory/memory_test.cc
+++ b/absl/memory/memory_test.cc
@@ -17,7 +17,9 @@
 #include "absl/memory/memory.h"
 
 #include <sys/types.h>
+
 #include <cstddef>
+#include <cstring>
 #include <memory>
 #include <string>
 #include <type_traits>
@@ -36,10 +38,10 @@ using ::testing::Return;
 // been called, via the instance_count variable.
 class DestructorVerifier {
  public:
-  DestructorVerifier() { ++instance_count_;  }
+  DestructorVerifier() { ++instance_count_; }
   DestructorVerifier(const DestructorVerifier&) = delete;
   DestructorVerifier& operator=(const DestructorVerifier&) = delete;
-  ~DestructorVerifier() {  --instance_count_; }
+  ~DestructorVerifier() { --instance_count_; }
 
   // The number of instances of this class currently active.
   static int instance_count() { return instance_count_; }
@@ -62,13 +64,24 @@ TEST(WrapUniqueTest, WrapUnique) {
   }
   EXPECT_EQ(0, DestructorVerifier::instance_count());
 }
-TEST(MakeUniqueTest, Basic) {
-  std::unique_ptr<std::string> p = absl::make_unique<std::string>();
-  EXPECT_EQ("", *p);
-  p = absl::make_unique<std::string>("hi");
-  EXPECT_EQ("hi", *p);
+
+TEST(MakeUniqueForOverwriteTest, Basic) {
+  std::unique_ptr<int> p = absl::make_unique_for_overwrite<int>();
+  p = absl::make_unique_for_overwrite<int>();
 }
 
+// The initialization tests needs to suppress dead-store elimination, otherwise
+// memset is optimized away, and lifetime is assumed begin after new, triggering
+// uninitalized variable warnings. Various tricks to prevent memset from being
+// optimized away still result in uninitialized variable warnings.  Once we move
+// to a C++20 floor we can delegate to std::make_unique_for_overwrite and avoid
+// testing absl::make_unique_for_overwrite.
+// https://github.com/gcc-mirror/gcc/blob/be1da01067c898a3e3979bfb1edd05f115ab2e3e/libstdc%2B%2B-v3/testsuite/20_util/unique_ptr/creation/for_overwrite.cc#L1
+#if defined(__GNUC__) && !defined(__clang__)
+#pragma GCC push_options
+#pragma GCC optimize("O0")
+#endif
+
 // InitializationVerifier fills in a pattern when allocated so we can
 // distinguish between its default and value initialized states (without
 // accessing truly uninitialized memory).
@@ -92,112 +105,55 @@ struct InitializationVerifier {
   int b;
 };
 
-TEST(Initialization, MakeUnique) {
-  auto p = absl::make_unique<InitializationVerifier>();
-
-  EXPECT_EQ(0, p->a);
-  EXPECT_EQ(0, p->b);
-}
+TEST(Initialization, MakeUniqueForOverwrite) {
+  auto p = absl::make_unique_for_overwrite<InitializationVerifier>();
 
-TEST(Initialization, MakeUniqueArray) {
-  auto p = absl::make_unique<InitializationVerifier[]>(2);
+  int pattern;
+  memset(&pattern, InitializationVerifier::kDefaultScalar, sizeof(pattern));
 
-  EXPECT_EQ(0, p[0].a);
-  EXPECT_EQ(0, p[0].b);
-  EXPECT_EQ(0, p[1].a);
-  EXPECT_EQ(0, p[1].b);
+  EXPECT_EQ(pattern, p->a);
+  EXPECT_EQ(pattern, p->b);
 }
 
-struct MoveOnly {
-  MoveOnly() = default;
-  explicit MoveOnly(int i1) : ip1{new int{i1}} {}
-  MoveOnly(int i1, int i2) : ip1{new int{i1}}, ip2{new int{i2}} {}
-  std::unique_ptr<int> ip1;
-  std::unique_ptr<int> ip2;
-};
+TEST(Initialization, MakeUniqueForOverwriteArray) {
+  auto p = absl::make_unique_for_overwrite<InitializationVerifier[]>(2);
 
-struct AcceptMoveOnly {
-  explicit AcceptMoveOnly(MoveOnly m) : m_(std::move(m)) {}
-  MoveOnly m_;
-};
+  int pattern;
+  memset(&pattern, InitializationVerifier::kDefaultArray, sizeof(pattern));
 
-TEST(MakeUniqueTest, MoveOnlyTypeAndValue) {
-  using ExpectedType = std::unique_ptr<MoveOnly>;
-  {
-    auto p = absl::make_unique<MoveOnly>();
-    static_assert(std::is_same<decltype(p), ExpectedType>::value,
-                  "unexpected return type");
-    EXPECT_TRUE(!p->ip1);
-    EXPECT_TRUE(!p->ip2);
-  }
-  {
-    auto p = absl::make_unique<MoveOnly>(1);
-    static_assert(std::is_same<decltype(p), ExpectedType>::value,
-                  "unexpected return type");
-    EXPECT_TRUE(p->ip1 && *p->ip1 == 1);
-    EXPECT_TRUE(!p->ip2);
-  }
-  {
-    auto p = absl::make_unique<MoveOnly>(1, 2);
-    static_assert(std::is_same<decltype(p), ExpectedType>::value,
-                  "unexpected return type");
-    EXPECT_TRUE(p->ip1 && *p->ip1 == 1);
-    EXPECT_TRUE(p->ip2 && *p->ip2 == 2);
-  }
+  EXPECT_EQ(pattern, p[0].a);
+  EXPECT_EQ(pattern, p[0].b);
+  EXPECT_EQ(pattern, p[1].a);
+  EXPECT_EQ(pattern, p[1].b);
 }
 
-TEST(MakeUniqueTest, AcceptMoveOnly) {
-  auto p = absl::make_unique<AcceptMoveOnly>(MoveOnly());
-  p = std::unique_ptr<AcceptMoveOnly>(new AcceptMoveOnly(MoveOnly()));
-}
+#if defined(__GNUC__) && !defined(__clang__)
+#pragma GCC pop_options
+#endif
 
 struct ArrayWatch {
   void* operator new[](size_t n) {
     allocs().push_back(n);
     return ::operator new[](n);
   }
-  void operator delete[](void* p) {
-    return ::operator delete[](p);
-  }
+  void operator delete[](void* p) { return ::operator delete[](p); }
   static std::vector<size_t>& allocs() {
     static auto& v = *new std::vector<size_t>;
     return v;
   }
 };
 
-TEST(Make_UniqueTest, Array) {
+TEST(MakeUniqueForOverwriteTest, Array) {
   // Ensure state is clean before we start so that these tests
   // are order-agnostic.
   ArrayWatch::allocs().clear();
 
-  auto p = absl::make_unique<ArrayWatch[]>(5);
-  static_assert(std::is_same<decltype(p),
-                             std::unique_ptr<ArrayWatch[]>>::value,
+  auto p = absl::make_unique_for_overwrite<ArrayWatch[]>(5);
+  static_assert(std::is_same_v<decltype(p), std::unique_ptr<ArrayWatch[]>>,
                 "unexpected return type");
   EXPECT_THAT(ArrayWatch::allocs(), ElementsAre(5 * sizeof(ArrayWatch)));
 }
 
-TEST(Make_UniqueTest, NotAmbiguousWithStdMakeUnique) {
-  // Ensure that absl::make_unique is not ambiguous with std::make_unique.
-  // In C++14 mode, the below call to make_unique has both types as candidates.
-  struct TakesStdType {
-    explicit TakesStdType(const std::vector<int> &vec) {}
-  };
-  using absl::make_unique;
-  (void)make_unique<TakesStdType>(std::vector<int>());
-}
-
-#if 0
-// These tests shouldn't compile.
-TEST(MakeUniqueTestNC, AcceptMoveOnlyLvalue) {
-  auto m = MoveOnly();
-  auto p = absl::make_unique<AcceptMoveOnly>(m);
-}
-TEST(MakeUniqueTestNC, KnownBoundArray) {
-  auto p = absl::make_unique<ArrayWatch[5]>();
-}
-#endif
-
 TEST(RawPtrTest, RawPointer) {
   int i = 5;
   EXPECT_EQ(&i, absl::RawPtr(&i));
@@ -239,24 +195,24 @@ TEST(RawPtrTest, NullValuedSmartPointer) {
 
 TEST(RawPtrTest, Nullptr) {
   auto p = absl::RawPtr(nullptr);
-  EXPECT_TRUE((std::is_same<std::nullptr_t, decltype(p)>::value));
+  EXPECT_TRUE((std::is_same_v<std::nullptr_t, decltype(p)>));
   EXPECT_EQ(nullptr, p);
 }
 
 TEST(RawPtrTest, Null) {
   auto p = absl::RawPtr(nullptr);
-  EXPECT_TRUE((std::is_same<std::nullptr_t, decltype(p)>::value));
+  EXPECT_TRUE((std::is_same_v<std::nullptr_t, decltype(p)>));
   EXPECT_EQ(nullptr, p);
 }
 
 TEST(RawPtrTest, Zero) {
   auto p = absl::RawPtr(nullptr);
-  EXPECT_TRUE((std::is_same<std::nullptr_t, decltype(p)>::value));
+  EXPECT_TRUE((std::is_same_v<std::nullptr_t, decltype(p)>));
   EXPECT_EQ(nullptr, p);
 }
 
 TEST(ShareUniquePtrTest, Share) {
-  auto up = absl::make_unique<int>();
+  auto up = std::make_unique<int>();
   int* rp = up.get();
   auto sp = absl::ShareUniquePtr(std::move(up));
   EXPECT_EQ(sp.get(), rp);
@@ -289,336 +245,6 @@ TEST(RawPtrTest, NotAPointer) {
 }
 */
 
-template <typename T>
-struct SmartPointer {
-  using difference_type = char;
-};
-
-struct PointerWith {
-  using element_type = int32_t;
-  using difference_type = int16_t;
-  template <typename U>
-  using rebind = SmartPointer<U>;
-
-  static PointerWith pointer_to(
-      element_type& r) {  // NOLINT(runtime/references)
-    return PointerWith{&r};
-  }
-
-  element_type* ptr;
-};
-
-template <typename... Args>
-struct PointerWithout {};
-
-TEST(PointerTraits, Types) {
-  using TraitsWith = absl::pointer_traits<PointerWith>;
-  EXPECT_TRUE((std::is_same<TraitsWith::pointer, PointerWith>::value));
-  EXPECT_TRUE((std::is_same<TraitsWith::element_type, int32_t>::value));
-  EXPECT_TRUE((std::is_same<TraitsWith::difference_type, int16_t>::value));
-  EXPECT_TRUE((
-      std::is_same<TraitsWith::rebind<int64_t>, SmartPointer<int64_t>>::value));
-
-  using TraitsWithout = absl::pointer_traits<PointerWithout<double, int>>;
-  EXPECT_TRUE((std::is_same<TraitsWithout::pointer,
-                            PointerWithout<double, int>>::value));
-  EXPECT_TRUE((std::is_same<TraitsWithout::element_type, double>::value));
-  EXPECT_TRUE(
-      (std::is_same<TraitsWithout ::difference_type, std::ptrdiff_t>::value));
-  EXPECT_TRUE((std::is_same<TraitsWithout::rebind<int64_t>,
-                            PointerWithout<int64_t, int>>::value));
-
-  using TraitsRawPtr = absl::pointer_traits<char*>;
-  EXPECT_TRUE((std::is_same<TraitsRawPtr::pointer, char*>::value));
-  EXPECT_TRUE((std::is_same<TraitsRawPtr::element_type, char>::value));
-  EXPECT_TRUE(
-      (std::is_same<TraitsRawPtr::difference_type, std::ptrdiff_t>::value));
-  EXPECT_TRUE((std::is_same<TraitsRawPtr::rebind<int64_t>, int64_t*>::value));
-}
-
-TEST(PointerTraits, Functions) {
-  int i;
-  EXPECT_EQ(&i, absl::pointer_traits<PointerWith>::pointer_to(i).ptr);
-  EXPECT_EQ(&i, absl::pointer_traits<int*>::pointer_to(i));
-}
-
-TEST(AllocatorTraits, Typedefs) {
-  struct A {
-    struct value_type {};
-  };
-  EXPECT_TRUE((
-      std::is_same<A,
-                   typename absl::allocator_traits<A>::allocator_type>::value));
-  EXPECT_TRUE(
-      (std::is_same<A::value_type,
-                    typename absl::allocator_traits<A>::value_type>::value));
-
-  struct X {};
-  struct HasPointer {
-    using value_type = X;
-    using pointer = SmartPointer<X>;
-  };
-  EXPECT_TRUE((std::is_same<SmartPointer<X>, typename absl::allocator_traits<
-                                                 HasPointer>::pointer>::value));
-  EXPECT_TRUE(
-      (std::is_same<A::value_type*,
-                    typename absl::allocator_traits<A>::pointer>::value));
-
-  EXPECT_TRUE(
-      (std::is_same<
-          SmartPointer<const X>,
-          typename absl::allocator_traits<HasPointer>::const_pointer>::value));
-  EXPECT_TRUE(
-      (std::is_same<const A::value_type*,
-                    typename absl::allocator_traits<A>::const_pointer>::value));
-
-  struct HasVoidPointer {
-    using value_type = X;
-    struct void_pointer {};
-  };
-
-  EXPECT_TRUE((std::is_same<HasVoidPointer::void_pointer,
-                            typename absl::allocator_traits<
-                                HasVoidPointer>::void_pointer>::value));
-  EXPECT_TRUE(
-      (std::is_same<SmartPointer<void>, typename absl::allocator_traits<
-                                            HasPointer>::void_pointer>::value));
-
-  struct HasConstVoidPointer {
-    using value_type = X;
-    struct const_void_pointer {};
-  };
-
-  EXPECT_TRUE(
-      (std::is_same<HasConstVoidPointer::const_void_pointer,
-                    typename absl::allocator_traits<
-                        HasConstVoidPointer>::const_void_pointer>::value));
-  EXPECT_TRUE((std::is_same<SmartPointer<const void>,
-                            typename absl::allocator_traits<
-                                HasPointer>::const_void_pointer>::value));
-
-  struct HasDifferenceType {
-    using value_type = X;
-    using difference_type = int;
-  };
-  EXPECT_TRUE(
-      (std::is_same<int, typename absl::allocator_traits<
-                             HasDifferenceType>::difference_type>::value));
-  EXPECT_TRUE((std::is_same<char, typename absl::allocator_traits<
-                                      HasPointer>::difference_type>::value));
-
-  struct HasSizeType {
-    using value_type = X;
-    using size_type = unsigned int;
-  };
-  EXPECT_TRUE((std::is_same<unsigned int, typename absl::allocator_traits<
-                                              HasSizeType>::size_type>::value));
-  EXPECT_TRUE((std::is_same<unsigned char, typename absl::allocator_traits<
-                                               HasPointer>::size_type>::value));
-
-  struct HasPropagateOnCopy {
-    using value_type = X;
-    struct propagate_on_container_copy_assignment {};
-  };
-
-  EXPECT_TRUE(
-      (std::is_same<HasPropagateOnCopy::propagate_on_container_copy_assignment,
-                    typename absl::allocator_traits<HasPropagateOnCopy>::
-                        propagate_on_container_copy_assignment>::value));
-  EXPECT_TRUE(
-      (std::is_same<std::false_type,
-                    typename absl::allocator_traits<
-                        A>::propagate_on_container_copy_assignment>::value));
-
-  struct HasPropagateOnMove {
-    using value_type = X;
-    struct propagate_on_container_move_assignment {};
-  };
-
-  EXPECT_TRUE(
-      (std::is_same<HasPropagateOnMove::propagate_on_container_move_assignment,
-                    typename absl::allocator_traits<HasPropagateOnMove>::
-                        propagate_on_container_move_assignment>::value));
-  EXPECT_TRUE(
-      (std::is_same<std::false_type,
-                    typename absl::allocator_traits<
-                        A>::propagate_on_container_move_assignment>::value));
-
-  struct HasPropagateOnSwap {
-    using value_type = X;
-    struct propagate_on_container_swap {};
-  };
-
-  EXPECT_TRUE(
-      (std::is_same<HasPropagateOnSwap::propagate_on_container_swap,
-                    typename absl::allocator_traits<HasPropagateOnSwap>::
-                        propagate_on_container_swap>::value));
-  EXPECT_TRUE(
-      (std::is_same<std::false_type, typename absl::allocator_traits<A>::
-                                         propagate_on_container_swap>::value));
-
-  struct HasIsAlwaysEqual {
-    using value_type = X;
-    struct is_always_equal {};
-  };
-
-  EXPECT_TRUE((std::is_same<HasIsAlwaysEqual::is_always_equal,
-                            typename absl::allocator_traits<
-                                HasIsAlwaysEqual>::is_always_equal>::value));
-  EXPECT_TRUE((std::is_same<std::true_type, typename absl::allocator_traits<
-                                                A>::is_always_equal>::value));
-  struct NonEmpty {
-    using value_type = X;
-    int i;
-  };
-  EXPECT_TRUE(
-      (std::is_same<std::false_type,
-                    absl::allocator_traits<NonEmpty>::is_always_equal>::value));
-}
-
-template <typename T>
-struct AllocWithPrivateInheritance : private std::allocator<T> {
-  using value_type = T;
-};
-
-TEST(AllocatorTraits, RebindWithPrivateInheritance) {
-  // Regression test for some versions of gcc that do not like the sfinae we
-  // used in combination with private inheritance.
-  EXPECT_TRUE(
-      (std::is_same<AllocWithPrivateInheritance<int>,
-                    absl::allocator_traits<AllocWithPrivateInheritance<char>>::
-                        rebind_alloc<int>>::value));
-}
-
-template <typename T>
-struct Rebound {};
-
-struct AllocWithRebind {
-  using value_type = int;
-  template <typename T>
-  struct rebind {
-    using other = Rebound<T>;
-  };
-};
-
-template <typename T, typename U>
-struct AllocWithoutRebind {
-  using value_type = int;
-};
-
-TEST(AllocatorTraits, Rebind) {
-  EXPECT_TRUE(
-      (std::is_same<Rebound<int>,
-                    typename absl::allocator_traits<
-                        AllocWithRebind>::template rebind_alloc<int>>::value));
-  EXPECT_TRUE(
-      (std::is_same<absl::allocator_traits<Rebound<int>>,
-                    typename absl::allocator_traits<
-                        AllocWithRebind>::template rebind_traits<int>>::value));
-
-  EXPECT_TRUE(
-      (std::is_same<AllocWithoutRebind<double, char>,
-                    typename absl::allocator_traits<AllocWithoutRebind<
-                        int, char>>::template rebind_alloc<double>>::value));
-  EXPECT_TRUE(
-      (std::is_same<absl::allocator_traits<AllocWithoutRebind<double, char>>,
-                    typename absl::allocator_traits<AllocWithoutRebind<
-                        int, char>>::template rebind_traits<double>>::value));
-}
-
-struct TestValue {
-  TestValue() {}
-  explicit TestValue(int* trace) : trace(trace) { ++*trace; }
-  ~TestValue() {
-    if (trace) --*trace;
-  }
-  int* trace = nullptr;
-};
-
-struct MinimalMockAllocator {
-  MinimalMockAllocator() : value(0) {}
-  explicit MinimalMockAllocator(int value) : value(value) {}
-  MinimalMockAllocator(const MinimalMockAllocator& other)
-      : value(other.value) {}
-  using value_type = TestValue;
-  MOCK_METHOD1(allocate, value_type*(size_t));
-  MOCK_METHOD2(deallocate, void(value_type*, size_t));
-
-  int value;
-};
-
-TEST(AllocatorTraits, FunctionsMinimal) {
-  int trace = 0;
-  int hint;
-  TestValue x(&trace);
-  MinimalMockAllocator mock;
-  using Traits = absl::allocator_traits<MinimalMockAllocator>;
-  EXPECT_CALL(mock, allocate(7)).WillRepeatedly(Return(&x));
-  EXPECT_CALL(mock, deallocate(&x, 7));
-
-  EXPECT_EQ(&x, Traits::allocate(mock, 7));
-  Traits::allocate(mock, 7, static_cast<const void*>(&hint));
-  EXPECT_EQ(&x, Traits::allocate(mock, 7, static_cast<const void*>(&hint)));
-  Traits::deallocate(mock, &x, 7);
-
-  EXPECT_EQ(1, trace);
-  Traits::construct(mock, &x, &trace);
-  EXPECT_EQ(2, trace);
-  Traits::destroy(mock, &x);
-  EXPECT_EQ(1, trace);
-
-  EXPECT_EQ(std::numeric_limits<size_t>::max() / sizeof(TestValue),
-            Traits::max_size(mock));
-
-  EXPECT_EQ(0, mock.value);
-  EXPECT_EQ(0, Traits::select_on_container_copy_construction(mock).value);
-}
-
-struct FullMockAllocator {
-  FullMockAllocator() : value(0) {}
-  explicit FullMockAllocator(int value) : value(value) {}
-  FullMockAllocator(const FullMockAllocator& other) : value(other.value) {}
-  using value_type = TestValue;
-  MOCK_METHOD1(allocate, value_type*(size_t));
-  MOCK_METHOD2(allocate, value_type*(size_t, const void*));
-  MOCK_METHOD2(construct, void(value_type*, int*));
-  MOCK_METHOD1(destroy, void(value_type*));
-  MOCK_CONST_METHOD0(max_size, size_t());
-  MOCK_CONST_METHOD0(select_on_container_copy_construction,
-                     FullMockAllocator());
-
-  int value;
-};
-
-TEST(AllocatorTraits, FunctionsFull) {
-  int trace = 0;
-  int hint;
-  TestValue x(&trace), y;
-  FullMockAllocator mock;
-  using Traits = absl::allocator_traits<FullMockAllocator>;
-  EXPECT_CALL(mock, allocate(7)).WillRepeatedly(Return(&x));
-  EXPECT_CALL(mock, allocate(13, &hint)).WillRepeatedly(Return(&y));
-  EXPECT_CALL(mock, construct(&x, &trace));
-  EXPECT_CALL(mock, destroy(&x));
-  EXPECT_CALL(mock, max_size()).WillRepeatedly(Return(17));
-  EXPECT_CALL(mock, select_on_container_copy_construction())
-      .WillRepeatedly(Return(FullMockAllocator(23)));
-
-  EXPECT_EQ(&x, Traits::allocate(mock, 7));
-  EXPECT_EQ(&y, Traits::allocate(mock, 13, static_cast<const void*>(&hint)));
-
-  EXPECT_EQ(1, trace);
-  Traits::construct(mock, &x, &trace);
-  EXPECT_EQ(1, trace);
-  Traits::destroy(mock, &x);
-  EXPECT_EQ(1, trace);
-
-  EXPECT_EQ(17, Traits::max_size(mock));
-
-  EXPECT_EQ(0, mock.value);
-  EXPECT_EQ(23, Traits::select_on_container_copy_construction(mock).value);
-}
-
 TEST(AllocatorNoThrowTest, DefaultAllocator) {
 #if defined(ABSL_ALLOCATOR_NOTHROW) && ABSL_ALLOCATOR_NOTHROW
   EXPECT_TRUE(absl::default_allocator_is_nothrow::value);
@@ -642,8 +268,7 @@ TEST(AllocatorNoThrowTest, CustomAllocator) {
   struct CanThrowAllocator {
     using is_nothrow = std::false_type;
   };
-  struct UnspecifiedAllocator {
-  };
+  struct UnspecifiedAllocator {};
   EXPECT_TRUE(absl::allocator_is_nothrow<NoThrowAllocator>::value);
   EXPECT_FALSE(absl::allocator_is_nothrow<CanThrowAllocator>::value);
   EXPECT_FALSE(absl::allocator_is_nothrow<UnspecifiedAllocator>::value);
diff --git a/absl/meta/BUILD.bazel b/absl/meta/BUILD.bazel
index c06d2d9708c..a4b79cc4490 100644
--- a/absl/meta/BUILD.bazel
+++ b/absl/meta/BUILD.bazel
@@ -14,7 +14,8 @@
 # limitations under the License.
 #
 
-load("@rules_cc//cc:defs.bzl", "cc_library", "cc_test")
+load("@rules_cc//cc:cc_library.bzl", "cc_library")
+load("@rules_cc//cc:cc_test.bzl", "cc_test")
 load(
     "//absl:copts/configure_copts.bzl",
     "ABSL_DEFAULT_COPTS",
@@ -22,17 +23,79 @@ load(
     "ABSL_TEST_COPTS",
 )
 
-package(default_visibility = ["//visibility:public"])
+package(
+    default_visibility = ["//visibility:private"],
+    features = [
+        "header_modules",
+        "layering_check",
+        "parse_headers",
+    ],
+)
+
+licenses(["notice"])
+
+cc_library(
+    name = "constexpr_testing",
+    testonly = 1,
+    hdrs = ["internal/constexpr_testing.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl:__subpackages__",
+        "@do_not_use_for_gloop_visibility_only//gloop/util/gtl:__subpackages__",
+    ],
+    deps = [
+        "//absl/base:config",
+    ],
+)
 
-licenses(["notice"])  # Apache 2.0
+cc_test(
+    name = "constexpr_testing_test",
+    srcs = ["internal/constexpr_testing_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":constexpr_testing",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_library(
+    name = "requires",
+    hdrs = ["internal/requires.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl:__subpackages__",
+        "@do_not_use_for_gloop_visibility_only//gloop/util/gtl:__subpackages__",
+    ],
+    deps = [
+        "//absl/base:config",
+    ],
+)
+
+cc_test(
+    name = "requires_test",
+    srcs = ["internal/requires_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":requires",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
 
 cc_library(
     name = "type_traits",
     hdrs = ["type_traits.h"],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
     deps = [
         "//absl/base:config",
+        "//absl/base:core_headers",
     ],
 )
 
@@ -43,6 +106,10 @@ cc_test(
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":type_traits",
-        "@com_google_googletest//:gtest_main",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/time",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
diff --git a/absl/meta/CMakeLists.txt b/absl/meta/CMakeLists.txt
index 672ead2fd0a..c98c360439a 100644
--- a/absl/meta/CMakeLists.txt
+++ b/absl/meta/CMakeLists.txt
@@ -14,6 +14,52 @@
 # limitations under the License.
 #
 
+absl_cc_library(
+  NAME
+    constexpr_testing_internal
+  HDRS
+    "internal/constexpr_testing.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::config
+)
+
+absl_cc_test(
+  NAME
+    constexpr_testing_test
+  SRCS
+    "internal/constexpr_testing_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::constexpr_testing_internal
+    GTest::gmock_main
+)
+
+absl_cc_library(
+  NAME
+    requires_internal
+  HDRS
+    "internal/requires.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::config
+)
+
+absl_cc_test(
+  NAME
+    requires_test
+  SRCS
+    "internal/requires_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::requires_internal
+    GTest::gmock_main
+)
+
 absl_cc_library(
   NAME
     type_traits
@@ -23,6 +69,7 @@ absl_cc_library(
     ${ABSL_DEFAULT_COPTS}
   DEPS
     absl::config
+    absl::core_headers
   PUBLIC
 )
 
@@ -34,8 +81,11 @@ absl_cc_test(
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
+    absl::config
+    absl::time
+    absl::core_headers
     absl::type_traits
-    gmock_main
+    GTest::gmock_main
 )
 
 # component target
diff --git a/absl/meta/internal/constexpr_testing.h b/absl/meta/internal/constexpr_testing.h
new file mode 100644
index 00000000000..bb10b379688
--- /dev/null
+++ b/absl/meta/internal/constexpr_testing.h
@@ -0,0 +1,72 @@
+// Copyright 2025 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_META_INTERNAL_CONSTEXPR_TESTING_H_
+#define ABSL_META_INTERNAL_CONSTEXPR_TESTING_H_
+
+#include <type_traits>
+
+#include "absl/base/config.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace meta_internal {
+
+// HasConstexprEvaluation([] { ... }) will evaluate to `true` if the
+// lambda can be evaluated in a constant expression and `false`
+// otherwise.
+// The return type of the lambda is not relevant, as long as the whole
+// evaluation works in a constant expression.
+template <typename F>
+constexpr bool HasConstexprEvaluation(F f);
+
+/// Implementation details below ///
+
+namespace internal_constexpr_evaluation {
+
+#ifdef __clang__
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wuninitialized"
+#endif
+// This will give a constexpr instance of `F`.
+// This works for captureless lambdas because they have no state and the copy
+// constructor does not look at the input reference.
+template <typename F>
+constexpr F default_instance = default_instance<F>;
+#ifdef __clang__
+#pragma clang diagnostic pop
+#endif
+
+template <typename F>
+constexpr std::bool_constant<(default_instance<F>(), true)> Tester(int) {
+  return {};
+}
+
+template <typename S>
+constexpr std::false_type Tester(char) {
+  return {};
+}
+
+}  // namespace internal_constexpr_evaluation
+
+template <typename F>
+constexpr bool HasConstexprEvaluation(F) {
+  return internal_constexpr_evaluation::Tester<F>(0);
+}
+
+}  // namespace meta_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_META_INTERNAL_CONSTEXPR_TESTING_H_
diff --git a/absl/meta/internal/constexpr_testing_test.cc b/absl/meta/internal/constexpr_testing_test.cc
new file mode 100644
index 00000000000..50c8c53d0ae
--- /dev/null
+++ b/absl/meta/internal/constexpr_testing_test.cc
@@ -0,0 +1,40 @@
+// Copyright 2025 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/meta/internal/constexpr_testing.h"
+
+#include <map>
+#include <string_view>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+
+namespace {
+
+TEST(ConstexprTesting, Basic) {
+  using absl::meta_internal::HasConstexprEvaluation;
+
+  EXPECT_TRUE(HasConstexprEvaluation([] {}));
+  static constexpr int const_global = 7;
+  EXPECT_TRUE(HasConstexprEvaluation([] { return const_global; }));
+  EXPECT_TRUE(HasConstexprEvaluation([] { return 0; }));
+  EXPECT_TRUE(HasConstexprEvaluation([] { return std::string_view{}; }));
+
+  static int nonconst_global;
+  EXPECT_FALSE(HasConstexprEvaluation([] { return nonconst_global; }));
+  EXPECT_FALSE(HasConstexprEvaluation([] { std::abort(); }));
+  EXPECT_FALSE(HasConstexprEvaluation([] { return std::map<int, int>(); }));
+}
+
+}  // namespace
diff --git a/absl/meta/internal/requires.h b/absl/meta/internal/requires.h
new file mode 100644
index 00000000000..14f185e976c
--- /dev/null
+++ b/absl/meta/internal/requires.h
@@ -0,0 +1,67 @@
+// Copyright 2017 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_META_INTERNAL_REQUIRES_H_
+#define ABSL_META_INTERNAL_REQUIRES_H_
+
+#include <type_traits>
+
+#include "absl/base/config.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace meta_internal {
+
+// C++17 port of the C++20 `requires` expressions.
+// It allows easy inline test of properties of types in template code.
+// https://en.cppreference.com/w/cpp/language/constraints#Requires_expressions
+//
+// Example usage:
+//
+// if constexpr (Requires<T>([](auto&& x) -> decltype(x.foo()) {})) {
+//   // T has foo()
+//   return t.foo();
+// } else if constexpr (Requires<T>([](auto&& x) -> decltype(Bar(x)) {})) {
+//   // Can call Bar with T
+//   return Bar(t);
+// } else if constexpr (Requires<T, U>(
+//     // Can test expression with multiple inputs
+//     [](auto&& x, auto&& y) -> decltype(x + y) {})) {
+//   return t + t2;
+// }
+//
+// The `Requires` function takes a list of types and a generic lambda where all
+// arguments are of type `auto&&`. The lambda is never actually invoked and the
+// body must be empty.
+// When used this way, `Requires` returns whether the expression inside
+// `decltype` is well-formed, when the lambda parameters have the types that
+// are specified by the corresponding template arguments.
+//
+// NOTE: C++17 does not allow lambdas in template parameters, which means that
+// code like the following is _not_ valid in C++17:
+//
+//  template <typename T,
+//            typename = std::enable_if_t<Requires<T>(
+//              [] (auto&& v) -> decltype(<expr>) {})>>
+//
+template <typename... T, typename F>
+constexpr bool Requires(F) {
+  return std::is_invocable_v<F, T...>;
+}
+
+}  // namespace meta_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_META_INTERNAL_REQUIRES_H_
diff --git a/absl/meta/internal/requires_test.cc b/absl/meta/internal/requires_test.cc
new file mode 100644
index 00000000000..046d8f304f9
--- /dev/null
+++ b/absl/meta/internal/requires_test.cc
@@ -0,0 +1,66 @@
+// Copyright 2017 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/meta/internal/requires.h"
+
+#include <string>
+#include <vector>
+
+#include "gtest/gtest.h"
+
+namespace {
+
+TEST(RequiresTest, SimpleLambdasWork) {
+  static_assert(absl::meta_internal::Requires([] {}));
+  static_assert(absl::meta_internal::Requires<int>([](auto&&) {}));
+  static_assert(
+      absl::meta_internal::Requires<int, char>([](auto&&, auto&&) {}));
+}
+
+template <typename T>
+inline constexpr bool has_cstr =
+    absl::meta_internal::Requires<T>([](auto&& x) -> decltype(x.c_str()) {});
+
+template <typename T, typename U>
+inline constexpr bool have_plus = absl::meta_internal::Requires<T, U>(
+    [](auto&& x, auto&& y) -> decltype(x + y) {});
+
+TEST(RequiresTest, CanTestProperties) {
+  static_assert(has_cstr<std::string>);
+  static_assert(!has_cstr<std::vector<int>>);
+
+  static_assert(have_plus<int, double>);
+  static_assert(have_plus<std::string, std::string>);
+  static_assert(!have_plus<std::string, double>);
+}
+
+TEST(RequiresTest, WorksWithUnmovableTypes) {
+  struct S {
+    S(const S&) = delete;
+    int foo() { return 0; }
+  };
+  static_assert(
+      absl::meta_internal::Requires<S>([](auto&& x) -> decltype(x.foo()) {}));
+  static_assert(
+      !absl::meta_internal::Requires<S>([](auto&& x) -> decltype(x.bar()) {}));
+}
+
+TEST(RequiresTest, WorksWithArrays) {
+  static_assert(
+      absl::meta_internal::Requires<int[2]>([](auto&& x) -> decltype(x[1]) {}));
+  static_assert(
+      !absl::meta_internal::Requires<int[2]>([](auto&& x) -> decltype(-x) {}));
+}
+
+}  // namespace
diff --git a/absl/meta/type_traits.h b/absl/meta/type_traits.h
index 76959cdef60..0bccd0b146d 100644
--- a/absl/meta/type_traits.h
+++ b/absl/meta/type_traits.h
@@ -17,7 +17,7 @@
 // type_traits.h
 // -----------------------------------------------------------------------------
 //
-// This file contains C++11-compatible versions of standard <type_traits> API
+// This file contains C++17-compatible versions of standard <type_traits> API
 // functions for determining the characteristics of types. Such traits can
 // support type inference, classification, and transformation, as well as
 // make it easier to write templates based on generic type behavior.
@@ -35,64 +35,43 @@
 #ifndef ABSL_META_TYPE_TRAITS_H_
 #define ABSL_META_TYPE_TRAITS_H_
 
-#include <stddef.h>
+#include <array>
+#include <cstddef>
 #include <functional>
+#include <string>
+#include <string_view>
+#include <tuple>
 #include <type_traits>
+#include <utility>
+#include <variant>
+#include <vector>
 
+#include "absl/base/attributes.h"
 #include "absl/base/config.h"
+#include "absl/base/macros.h"
 
-namespace absl {
-
-// Defined and documented later on in this file.
-template <typename T>
-struct is_trivially_move_assignable;
-
-namespace type_traits_internal {
-
-// Silence MSVC warnings about the destructor being defined as deleted.
-#if defined(_MSC_VER) && !defined(__GNUC__)
-#pragma warning(push)
-#pragma warning(disable : 4624)
-#endif  // defined(_MSC_VER) && !defined(__GNUC__)
-
-template <class T>
-union SingleMemberUnion {
-  T t;
-};
-
-// Restore the state of the destructor warning that was silenced above.
-#if defined(_MSC_VER) && !defined(__GNUC__)
-#pragma warning(pop)
-#endif  // defined(_MSC_VER) && !defined(__GNUC__)
+#ifdef __cpp_lib_span
+#include <span>  // NOLINT(build/c++20)
+#endif
 
-template <class T>
-struct IsTriviallyMoveAssignableReference : std::false_type {};
+// Defines the default alignment. `__STDCPP_DEFAULT_NEW_ALIGNMENT__` is a C++17
+// feature, but GCC leaves it undefined under -fno-aligned-new.
+#if defined(__STDCPP_DEFAULT_NEW_ALIGNMENT__)
+#define ABSL_INTERNAL_DEFAULT_NEW_ALIGNMENT __STDCPP_DEFAULT_NEW_ALIGNMENT__
+#else  // defined(__STDCPP_DEFAULT_NEW_ALIGNMENT__)
+#define ABSL_INTERNAL_DEFAULT_NEW_ALIGNMENT alignof(std::max_align_t)
+#endif  // defined(__STDCPP_DEFAULT_NEW_ALIGNMENT__)
 
-template <class T>
-struct IsTriviallyMoveAssignableReference<T&>
-    : absl::is_trivially_move_assignable<T>::type {};
+namespace absl {
+ABSL_NAMESPACE_BEGIN
 
-template <class T>
-struct IsTriviallyMoveAssignableReference<T&&>
-    : absl::is_trivially_move_assignable<T>::type {};
+namespace type_traits_internal {
 
 template <typename... Ts>
 struct VoidTImpl {
   using type = void;
 };
 
-// This trick to retrieve a default alignment is necessary for our
-// implementation of aligned_storage_t to be consistent with any implementation
-// of std::aligned_storage.
-template <size_t Len, typename T = std::aligned_storage<Len>>
-struct default_alignment_of_aligned_storage;
-
-template <size_t Len, size_t Align>
-struct default_alignment_of_aligned_storage<Len,
-                                            std::aligned_storage<Len, Align>> {
-  static constexpr size_t value = Align;
-};
-
 ////////////////////////////////
 // Library Fundamentals V2 TS //
 ////////////////////////////////
@@ -120,487 +99,205 @@ struct is_detected_impl<typename VoidTImpl<Op<Args...>>::type, Op, Args...> {
 template <template <class...> class Op, class... Args>
 struct is_detected : is_detected_impl<void, Op, Args...>::type {};
 
-template <class Enabler, class To, template <class...> class Op, class... Args>
-struct is_detected_convertible_impl {
-  using type = std::false_type;
-};
-
-template <class To, template <class...> class Op, class... Args>
-struct is_detected_convertible_impl<
-    typename std::enable_if<std::is_convertible<Op<Args...>, To>::value>::type,
-    To, Op, Args...> {
-  using type = std::true_type;
-};
-
-template <class To, template <class...> class Op, class... Args>
-struct is_detected_convertible
-    : is_detected_convertible_impl<void, To, Op, Args...>::type {};
-
-template <typename T>
-using IsCopyAssignableImpl =
-    decltype(std::declval<T&>() = std::declval<const T&>());
-
-template <typename T>
-using IsMoveAssignableImpl = decltype(std::declval<T&>() = std::declval<T&&>());
-
 }  // namespace type_traits_internal
 
-template <typename T>
-struct is_copy_assignable : type_traits_internal::is_detected<
-                                type_traits_internal::IsCopyAssignableImpl, T> {
-};
-
-template <typename T>
-struct is_move_assignable : type_traits_internal::is_detected<
-                                type_traits_internal::IsMoveAssignableImpl, T> {
-};
-
 // void_t()
 //
 // Ignores the type of any its arguments and returns `void`. In general, this
 // metafunction allows you to create a general case that maps to `void` while
 // allowing specializations that map to specific types.
 //
-// This metafunction is designed to be a drop-in replacement for the C++17
-// `std::void_t` metafunction.
-//
-// NOTE: `absl::void_t` does not use the standard-specified implementation so
-// that it can remain compatible with gcc < 5.1. This can introduce slightly
-// different behavior, such as when ordering partial specializations.
+// This metafunction is a workaround for some implementations of `std::void_t`
+// that evaluate to `void` prematurely, causing partial specializations to
+// appear duplicated (and thus invalid) to the compiler prior to substitution
+// taking place. Whenever possible, use `std::void_t` instead.
 template <typename... Ts>
-using void_t = typename type_traits_internal::VoidTImpl<Ts...>::type;
+using void_t [[deprecated("Use std::void_t instead.")]] =
+    typename type_traits_internal::VoidTImpl<Ts...>::type;
 
-// conjunction
-//
-// Performs a compile-time logical AND operation on the passed types (which
-// must have  `::value` members convertible to `bool`. Short-circuits if it
-// encounters any `false` members (and does not compare the `::value` members
-// of any remaining arguments).
+// Historical note: Abseil once provided implementations of these type traits
+// for platforms that lacked full support. New code should prefer to use the
+// std variants.
 //
-// This metafunction is designed to be a drop-in replacement for the C++17
-// `std::conjunction` metafunction.
-template <typename... Ts>
-struct conjunction;
+// See the documentation for the STL <type_traits> header for more information:
+// https://en.cppreference.com/w/cpp/header/type_traits
 
-template <typename T, typename... Ts>
-struct conjunction<T, Ts...>
-    : std::conditional<T::value, conjunction<Ts...>, T>::type {};
+template <class T>
+using add_const_t ABSL_DEPRECATE_AND_INLINE() = std::add_const_t<T>;
 
-template <typename T>
-struct conjunction<T> : T {};
+template <class T>
+using add_cv_t ABSL_DEPRECATE_AND_INLINE() = std::add_cv_t<T>;
 
-template <>
-struct conjunction<> : std::true_type {};
+template <class T>
+using add_lvalue_reference_t ABSL_DEPRECATE_AND_INLINE() =
+    std::add_lvalue_reference_t<T>;
 
-// disjunction
-//
-// Performs a compile-time logical OR operation on the passed types (which
-// must have  `::value` members convertible to `bool`. Short-circuits if it
-// encounters any `true` members (and does not compare the `::value` members
-// of any remaining arguments).
-//
-// This metafunction is designed to be a drop-in replacement for the C++17
-// `std::disjunction` metafunction.
-template <typename... Ts>
-struct disjunction;
+template <class T>
+using add_pointer_t ABSL_DEPRECATE_AND_INLINE() = std::add_pointer_t<T>;
 
-template <typename T, typename... Ts>
-struct disjunction<T, Ts...> :
-      std::conditional<T::value, T, disjunction<Ts...>>::type {};
+template <class T>
+using add_rvalue_reference_t ABSL_DEPRECATE_AND_INLINE() =
+    std::add_rvalue_reference_t<T>;
 
-template <typename T>
-struct disjunction<T> : T {};
+template <class T>
+using add_volatile_t ABSL_DEPRECATE_AND_INLINE() = std::add_volatile_t<T>;
 
-template <>
-struct disjunction<> : std::false_type {};
+template <class... T>
+using common_type_t ABSL_DEPRECATE_AND_INLINE() = std::common_type_t<T...>;
 
-// negation
-//
-// Performs a compile-time logical NOT operation on the passed type (which
-// must have  `::value` members convertible to `bool`.
-//
-// This metafunction is designed to be a drop-in replacement for the C++17
-// `std::negation` metafunction.
-template <typename T>
-struct negation : std::integral_constant<bool, !T::value> {};
+template <bool C, class T, class F>
+using conditional_t ABSL_DEPRECATE_AND_INLINE() = std::conditional_t<C, T, F>;
 
-// is_function()
-//
-// Determines whether the passed type `T` is a function type.
-//
-// This metafunction is designed to be a drop-in replacement for the C++11
-// `std::is_function()` metafunction for platforms that have incomplete C++11
-// support (such as libstdc++ 4.x).
-//
-// This metafunction works because appending `const` to a type does nothing to
-// function types and reference types (and forms a const-qualified type
-// otherwise).
-template <typename T>
-struct is_function
-    : std::integral_constant<
-          bool, !(std::is_reference<T>::value ||
-                  std::is_const<typename std::add_const<T>::type>::value)> {};
+template <class... T>
+using conjunction ABSL_DEPRECATE_AND_INLINE() = std::conjunction<T...>;
 
-// is_trivially_destructible()
-//
-// Determines whether the passed type `T` is trivially destructible.
-//
-// This metafunction is designed to be a drop-in replacement for the C++11
-// `std::is_trivially_destructible()` metafunction for platforms that have
-// incomplete C++11 support (such as libstdc++ 4.x). On any platforms that do
-// fully support C++11, we check whether this yields the same result as the std
-// implementation.
-//
-// NOTE: the extensions (__has_trivial_xxx) are implemented in gcc (version >=
-// 4.3) and clang. Since we are supporting libstdc++ > 4.7, they should always
-// be present. These  extensions are documented at
-// https://gcc.gnu.org/onlinedocs/gcc/Type-Traits.html#Type-Traits.
-template <typename T>
-struct is_trivially_destructible
-    : std::integral_constant<bool, __has_trivial_destructor(T) &&
-                                   std::is_destructible<T>::value> {
-#ifdef ABSL_HAVE_STD_IS_TRIVIALLY_DESTRUCTIBLE
- private:
-  static constexpr bool compliant = std::is_trivially_destructible<T>::value ==
-                                    is_trivially_destructible::value;
-  static_assert(compliant || std::is_trivially_destructible<T>::value,
-                "Not compliant with std::is_trivially_destructible; "
-                "Standard: false, Implementation: true");
-  static_assert(compliant || !std::is_trivially_destructible<T>::value,
-                "Not compliant with std::is_trivially_destructible; "
-                "Standard: true, Implementation: false");
-#endif  // ABSL_HAVE_STD_IS_TRIVIALLY_DESTRUCTIBLE
-};
+template <class T>
+using decay_t ABSL_DEPRECATE_AND_INLINE() = std::decay_t<T>;
 
-// is_trivially_default_constructible()
-//
-// Determines whether the passed type `T` is trivially default constructible.
-//
-// This metafunction is designed to be a drop-in replacement for the C++11
-// `std::is_trivially_default_constructible()` metafunction for platforms that
-// have incomplete C++11 support (such as libstdc++ 4.x). On any platforms that
-// do fully support C++11, we check whether this yields the same result as the
-// std implementation.
-//
-// NOTE: according to the C++ standard, Section: 20.15.4.3 [meta.unary.prop]
-// "The predicate condition for a template specialization is_constructible<T,
-// Args...> shall be satisfied if and only if the following variable
-// definition would be well-formed for some invented variable t:
-//
-// T t(declval<Args>()...);
-//
-// is_trivially_constructible<T, Args...> additionally requires that the
-// variable definition does not call any operation that is not trivial.
-// For the purposes of this check, the call to std::declval is considered
-// trivial."
-//
-// Notes from https://en.cppreference.com/w/cpp/types/is_constructible:
-// In many implementations, is_nothrow_constructible also checks if the
-// destructor throws because it is effectively noexcept(T(arg)). Same
-// applies to is_trivially_constructible, which, in these implementations, also
-// requires that the destructor is trivial.
-// GCC bug 51452: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51452
-// LWG issue 2116: http://cplusplus.github.io/LWG/lwg-active.html#2116.
-//
-// "T obj();" need to be well-formed and not call any nontrivial operation.
-// Nontrivially destructible types will cause the expression to be nontrivial.
-template <typename T>
-struct is_trivially_default_constructible
-    : std::integral_constant<bool, __has_trivial_constructor(T) &&
-                                   std::is_default_constructible<T>::value &&
-                                   is_trivially_destructible<T>::value> {
-#ifdef ABSL_HAVE_STD_IS_TRIVIALLY_CONSTRUCTIBLE
- private:
-  static constexpr bool compliant =
-      std::is_trivially_default_constructible<T>::value ==
-      is_trivially_default_constructible::value;
-  static_assert(compliant || std::is_trivially_default_constructible<T>::value,
-                "Not compliant with std::is_trivially_default_constructible; "
-                "Standard: false, Implementation: true");
-  static_assert(compliant || !std::is_trivially_default_constructible<T>::value,
-                "Not compliant with std::is_trivially_default_constructible; "
-                "Standard: true, Implementation: false");
-#endif  // ABSL_HAVE_STD_IS_TRIVIALLY_CONSTRUCTIBLE
-};
+// Avoid inlining since the inliner cannot handle default arguments well.
+template <bool C, class T = void>
+using enable_if_t [[deprecated("Use std::enable_if_t instead.")]] =
+    std::enable_if_t<C, T>;
 
-// is_trivially_move_constructible()
-//
-// Determines whether the passed type `T` is trivially move constructible.
-//
-// This metafunction is designed to be a drop-in replacement for the C++11
-// `std::is_trivially_move_constructible()` metafunction for platforms that have
-// incomplete C++11 support (such as libstdc++ 4.x). On any platforms that do
-// fully support C++11, we check whether this yields the same result as the std
-// implementation.
-//
-// NOTE: `T obj(declval<T>());` needs to be well-formed and not call any
-// nontrivial operation.  Nontrivially destructible types will cause the
-// expression to be nontrivial.
-template <typename T>
-struct is_trivially_move_constructible
-    : std::conditional<
-          std::is_object<T>::value && !std::is_array<T>::value,
-          std::is_move_constructible<
-              type_traits_internal::SingleMemberUnion<T>>,
-          std::is_reference<T>>::type::type {
-#ifdef ABSL_HAVE_STD_IS_TRIVIALLY_CONSTRUCTIBLE
- private:
-  static constexpr bool compliant =
-      std::is_trivially_move_constructible<T>::value ==
-      is_trivially_move_constructible::value;
-  static_assert(compliant || std::is_trivially_move_constructible<T>::value,
-                "Not compliant with std::is_trivially_move_constructible; "
-                "Standard: false, Implementation: true");
-  static_assert(compliant || !std::is_trivially_move_constructible<T>::value,
-                "Not compliant with std::is_trivially_move_constructible; "
-                "Standard: true, Implementation: false");
-#endif  // ABSL_HAVE_STD_IS_TRIVIALLY_CONSTRUCTIBLE
-};
+template <class... T>
+using disjunction ABSL_DEPRECATE_AND_INLINE() = std::disjunction<T...>;
 
-// is_trivially_copy_constructible()
-//
-// Determines whether the passed type `T` is trivially copy constructible.
-//
-// This metafunction is designed to be a drop-in replacement for the C++11
-// `std::is_trivially_copy_constructible()` metafunction for platforms that have
-// incomplete C++11 support (such as libstdc++ 4.x). On any platforms that do
-// fully support C++11, we check whether this yields the same result as the std
-// implementation.
-//
-// NOTE: `T obj(declval<const T&>());` needs to be well-formed and not call any
-// nontrivial operation.  Nontrivially destructible types will cause the
-// expression to be nontrivial.
-template <typename T>
-struct is_trivially_copy_constructible
-    : std::conditional<
-          std::is_object<T>::value && !std::is_array<T>::value,
-          std::is_copy_constructible<
-              type_traits_internal::SingleMemberUnion<T>>,
-          std::is_lvalue_reference<T>>::type::type {
-#ifdef ABSL_HAVE_STD_IS_TRIVIALLY_CONSTRUCTIBLE
- private:
-  static constexpr bool compliant =
-      std::is_trivially_copy_constructible<T>::value ==
-      is_trivially_copy_constructible::value;
-  static_assert(compliant || std::is_trivially_copy_constructible<T>::value,
-                "Not compliant with std::is_trivially_copy_constructible; "
-                "Standard: false, Implementation: true");
-  static_assert(compliant || !std::is_trivially_copy_constructible<T>::value,
-                "Not compliant with std::is_trivially_copy_constructible; "
-                "Standard: true, Implementation: false");
-#endif  // ABSL_HAVE_STD_IS_TRIVIALLY_CONSTRUCTIBLE
-};
+template <class T>
+using is_copy_assignable ABSL_DEPRECATE_AND_INLINE() =
+    std::is_copy_assignable<T>;
 
-// is_trivially_move_assignable()
-//
-// Determines whether the passed type `T` is trivially move assignable.
-//
-// This metafunction is designed to be a drop-in replacement for the C++11
-// `std::is_trivially_move_assignable()` metafunction for platforms that have
-// incomplete C++11 support (such as libstdc++ 4.x). On any platforms that do
-// fully support C++11, we check whether this yields the same result as the std
-// implementation.
-//
-// NOTE: `is_assignable<T, U>::value` is `true` if the expression
-// `declval<T>() = declval<U>()` is well-formed when treated as an unevaluated
-// operand. `is_trivially_assignable<T, U>` requires the assignment to call no
-// operation that is not trivial. `is_trivially_copy_assignable<T>` is simply
-// `is_trivially_assignable<T&, T>`.
-template <typename T>
-struct is_trivially_move_assignable
-    : std::conditional<
-          std::is_object<T>::value && !std::is_array<T>::value,
-          std::is_move_assignable<type_traits_internal::SingleMemberUnion<T>>,
-          type_traits_internal::IsTriviallyMoveAssignableReference<T>>::type::
-          type {
-#ifdef ABSL_HAVE_STD_IS_TRIVIALLY_ASSIGNABLE
- private:
-  static constexpr bool compliant =
-      std::is_trivially_move_assignable<T>::value ==
-      is_trivially_move_assignable::value;
-  static_assert(compliant || std::is_trivially_move_assignable<T>::value,
-                "Not compliant with std::is_trivially_move_assignable; "
-                "Standard: false, Implementation: true");
-  static_assert(compliant || !std::is_trivially_move_assignable<T>::value,
-                "Not compliant with std::is_trivially_move_assignable; "
-                "Standard: true, Implementation: false");
-#endif  // ABSL_HAVE_STD_IS_TRIVIALLY_ASSIGNABLE
-};
+template <class T>
+using is_function ABSL_DEPRECATE_AND_INLINE() = std::is_function<T>;
 
-// is_trivially_copy_assignable()
-//
-// Determines whether the passed type `T` is trivially copy assignable.
-//
-// This metafunction is designed to be a drop-in replacement for the C++11
-// `std::is_trivially_copy_assignable()` metafunction for platforms that have
-// incomplete C++11 support (such as libstdc++ 4.x). On any platforms that do
-// fully support C++11, we check whether this yields the same result as the std
-// implementation.
-//
-// NOTE: `is_assignable<T, U>::value` is `true` if the expression
-// `declval<T>() = declval<U>()` is well-formed when treated as an unevaluated
-// operand. `is_trivially_assignable<T, U>` requires the assignment to call no
-// operation that is not trivial. `is_trivially_copy_assignable<T>` is simply
-// `is_trivially_assignable<T&, const T&>`.
-template <typename T>
-struct is_trivially_copy_assignable
-    : std::integral_constant<
-          bool, __has_trivial_assign(typename std::remove_reference<T>::type) &&
-                    absl::is_copy_assignable<T>::value> {
-#ifdef ABSL_HAVE_STD_IS_TRIVIALLY_ASSIGNABLE
- private:
-  static constexpr bool compliant =
-      std::is_trivially_copy_assignable<T>::value ==
-      is_trivially_copy_assignable::value;
-  static_assert(compliant || std::is_trivially_copy_assignable<T>::value,
-                "Not compliant with std::is_trivially_copy_assignable; "
-                "Standard: false, Implementation: true");
-  static_assert(compliant || !std::is_trivially_copy_assignable<T>::value,
-                "Not compliant with std::is_trivially_copy_assignable; "
-                "Standard: true, Implementation: false");
-#endif  // ABSL_HAVE_STD_IS_TRIVIALLY_ASSIGNABLE
-};
+template <class T>
+using is_move_assignable ABSL_DEPRECATE_AND_INLINE() =
+    std::is_move_assignable<T>;
 
-namespace type_traits_internal {
-// is_trivially_copyable()
-//
-// Determines whether the passed type `T` is trivially copyable.
-//
-// This metafunction is designed to be a drop-in replacement for the C++11
-// `std::is_trivially_copyable()` metafunction for platforms that have
-// incomplete C++11 support (such as libstdc++ 4.x). We use the C++17 definition
-// of TriviallyCopyable.
-//
-// NOTE: `is_trivially_copyable<T>::value` is `true` if all of T's copy/move
-// constructors/assignment operators are trivial or deleted, T has at least
-// one non-deleted copy/move constructor/assignment operator, and T is trivially
-// destructible. Arrays of trivially copyable types are trivially copyable.
-//
-// We expose this metafunction only for internal use within absl.
-template <typename T>
-class is_trivially_copyable_impl {
-  using ExtentsRemoved = typename std::remove_all_extents<T>::type;
-  static constexpr bool kIsCopyOrMoveConstructible =
-      std::is_copy_constructible<ExtentsRemoved>::value ||
-      std::is_move_constructible<ExtentsRemoved>::value;
-  static constexpr bool kIsCopyOrMoveAssignable =
-      absl::is_copy_assignable<ExtentsRemoved>::value ||
-      absl::is_move_assignable<ExtentsRemoved>::value;
-
- public:
-  static constexpr bool kValue =
-      (__has_trivial_copy(ExtentsRemoved) || !kIsCopyOrMoveConstructible) &&
-      (__has_trivial_assign(ExtentsRemoved) || !kIsCopyOrMoveAssignable) &&
-      (kIsCopyOrMoveConstructible || kIsCopyOrMoveAssignable) &&
-      is_trivially_destructible<ExtentsRemoved>::value &&
-      // We need to check for this explicitly because otherwise we'll say
-      // references are trivial copyable when compiled by MSVC.
-      !std::is_reference<ExtentsRemoved>::value;
-};
+template <class T>
+using is_trivially_copy_assignable ABSL_DEPRECATE_AND_INLINE() =
+    std::is_trivially_copy_assignable<T>;
 
-template <typename T>
-struct is_trivially_copyable
-    : std::integral_constant<
-          bool, type_traits_internal::is_trivially_copyable_impl<T>::kValue> {};
-}  // namespace type_traits_internal
+template <class T>
+using is_trivially_copy_constructible ABSL_DEPRECATE_AND_INLINE() =
+    std::is_trivially_copy_constructible<T>;
 
-// -----------------------------------------------------------------------------
-// C++14 "_t" trait aliases
-// -----------------------------------------------------------------------------
+template <class T>
+using is_trivially_default_constructible ABSL_DEPRECATE_AND_INLINE() =
+    std::is_trivially_default_constructible<T>;
 
-template <typename T>
-using remove_cv_t = typename std::remove_cv<T>::type;
+template <class T>
+using is_trivially_destructible ABSL_DEPRECATE_AND_INLINE() =
+    std::is_trivially_destructible<T>;
 
-template <typename T>
-using remove_const_t = typename std::remove_const<T>::type;
+template <class T>
+using is_trivially_move_assignable ABSL_DEPRECATE_AND_INLINE() =
+    std::is_trivially_move_assignable<T>;
 
-template <typename T>
-using remove_volatile_t = typename std::remove_volatile<T>::type;
+template <class T>
+using is_trivially_move_constructible ABSL_DEPRECATE_AND_INLINE() =
+    std::is_trivially_move_constructible<T>;
 
-template <typename T>
-using add_cv_t = typename std::add_cv<T>::type;
+template <class T>
+using make_signed_t ABSL_DEPRECATE_AND_INLINE() = std::make_signed_t<T>;
 
-template <typename T>
-using add_const_t = typename std::add_const<T>::type;
+template <class T>
+using make_unsigned_t ABSL_DEPRECATE_AND_INLINE() = std::make_unsigned_t<T>;
 
-template <typename T>
-using add_volatile_t = typename std::add_volatile<T>::type;
+template <class T>
+using negation ABSL_DEPRECATE_AND_INLINE() = std::negation<T>;
 
-template <typename T>
-using remove_reference_t = typename std::remove_reference<T>::type;
+template <class T>
+using remove_all_extents_t ABSL_DEPRECATE_AND_INLINE() =
+    std::remove_all_extents_t<T>;
 
-template <typename T>
-using add_lvalue_reference_t = typename std::add_lvalue_reference<T>::type;
+template <class T>
+using remove_const_t ABSL_DEPRECATE_AND_INLINE() = std::remove_const_t<T>;
 
-template <typename T>
-using add_rvalue_reference_t = typename std::add_rvalue_reference<T>::type;
+template <class T>
+using remove_cv_t ABSL_DEPRECATE_AND_INLINE() = std::remove_cv_t<T>;
 
-template <typename T>
-using remove_pointer_t = typename std::remove_pointer<T>::type;
+template <class T>
+using remove_extent_t ABSL_DEPRECATE_AND_INLINE() = std::remove_extent_t<T>;
 
-template <typename T>
-using add_pointer_t = typename std::add_pointer<T>::type;
+template <class T>
+using remove_pointer_t ABSL_DEPRECATE_AND_INLINE() = std::remove_pointer_t<T>;
 
-template <typename T>
-using make_signed_t = typename std::make_signed<T>::type;
+template <class T>
+using remove_reference_t ABSL_DEPRECATE_AND_INLINE() =
+    std::remove_reference_t<T>;
+
+template <class T>
+using remove_volatile_t ABSL_DEPRECATE_AND_INLINE() = std::remove_volatile_t<T>;
+
+template <class T>
+using underlying_type_t ABSL_DEPRECATE_AND_INLINE() = std::underlying_type_t<T>;
 
+#if defined(__cpp_lib_remove_cvref) && __cpp_lib_remove_cvref >= 201711L
 template <typename T>
-using make_unsigned_t = typename std::make_unsigned<T>::type;
+using remove_cvref = std::remove_cvref<T>;
 
 template <typename T>
-using remove_extent_t = typename std::remove_extent<T>::type;
+using remove_cvref_t = std::remove_cvref_t<T>;
+#else
+// remove_cvref()
+//
+// C++17 compatible implementation of std::remove_cvref which was added in
+// C++20.
+template <typename T>
+struct remove_cvref {
+  using type = std::remove_cv_t<std::remove_reference_t<T>>;
+};
 
 template <typename T>
-using remove_all_extents_t = typename std::remove_all_extents<T>::type;
+using remove_cvref_t = typename remove_cvref<T>::type;
+#endif
 
-template <size_t Len, size_t Align = type_traits_internal::
-                          default_alignment_of_aligned_storage<Len>::value>
-using aligned_storage_t = typename std::aligned_storage<Len, Align>::type;
+#if defined(__cpp_lib_type_identity) && __cpp_lib_type_identity >= 201806L
+template <typename T>
+using type_identity = std::type_identity<T>;
 
 template <typename T>
-using decay_t = typename std::decay<T>::type;
+using type_identity_t = std::type_identity_t<T>;
+#else
+// type_identity
+//
+// Back-fill of C++20's `std::type_identity`.
+template <typename T>
+struct type_identity {
+  typedef T type;
+};
 
-template <bool B, typename T = void>
-using enable_if_t = typename std::enable_if<B, T>::type;
+// type_identity_t
+//
+// Back-fill of C++20's `std::type_identity_t`.
+template <typename T>
+using type_identity_t = typename type_identity<T>::type;
+#endif
 
-template <bool B, typename T, typename F>
-using conditional_t = typename std::conditional<B, T, F>::type;
+namespace type_traits_internal {
 
-template <typename... T>
-using common_type_t = typename std::common_type<T...>::type;
+template <typename>
+struct result_of;
+template <typename F, typename... Args>
+struct result_of<F(Args...)> : std::invoke_result<F, Args...> {};
 
-template <typename T>
-using underlying_type_t = typename std::underlying_type<T>::type;
+}  // namespace type_traits_internal
 
-template <typename T>
-using result_of_t = typename std::result_of<T>::type;
+template <typename F>
+using result_of_t [[deprecated("Use std::invoke_result_t instead.")]] =
+    typename type_traits_internal::result_of<F>::type;
 
 namespace type_traits_internal {
-// In MSVC we can't probe std::hash or stdext::hash because it triggers a
-// static_assert instead of failing substitution. Libc++ prior to 4.0
-// also used a static_assert.
-//
-#if defined(_MSC_VER) || (defined(_LIBCPP_VERSION) && \
-                          _LIBCPP_VERSION < 4000 && _LIBCPP_STD_VER > 11)
-#define ABSL_META_INTERNAL_STD_HASH_SFINAE_FRIENDLY_ 0
-#else
-#define ABSL_META_INTERNAL_STD_HASH_SFINAE_FRIENDLY_ 1
-#endif
 
-#if !ABSL_META_INTERNAL_STD_HASH_SFINAE_FRIENDLY_
-template <typename Key, typename = size_t>
-struct IsHashable : std::true_type {};
-#else   // ABSL_META_INTERNAL_STD_HASH_SFINAE_FRIENDLY_
 template <typename Key, typename = void>
 struct IsHashable : std::false_type {};
 
 template <typename Key>
 struct IsHashable<
     Key,
-    absl::enable_if_t<std::is_convertible<
+    std::enable_if_t<std::is_convertible_v<
         decltype(std::declval<std::hash<Key>&>()(std::declval<Key const&>())),
-        std::size_t>::value>> : std::true_type {};
-#endif  // !ABSL_META_INTERNAL_STD_HASH_SFINAE_FRIENDLY_
+        std::size_t>>> : std::true_type {};
 
 struct AssertHashEnabledHelper {
  private:
@@ -618,19 +315,19 @@ struct AssertHashEnabledHelper {
     static_assert(IsHashable<Key>::value,
                   "std::hash<Key> does not provide a call operator");
     static_assert(
-        std::is_default_constructible<std::hash<Key>>::value,
+        std::is_default_constructible_v<std::hash<Key>>,
         "std::hash<Key> must be default constructible when it is enabled");
     static_assert(
-        std::is_copy_constructible<std::hash<Key>>::value,
+        std::is_copy_constructible_v<std::hash<Key>>,
         "std::hash<Key> must be copy constructible when it is enabled");
-    static_assert(absl::is_copy_assignable<std::hash<Key>>::value,
+    static_assert(std::is_copy_assignable_v<std::hash<Key>>,
                   "std::hash<Key> must be copy assignable when it is enabled");
     // is_destructible is unchecked as it's implied by each of the
     // is_constructible checks.
     using ReturnType = decltype(GetReturnType<Key>(0));
-    static_assert(std::is_same<ReturnType, NAT>::value ||
-                      std::is_same<ReturnType, size_t>::value,
-                  "std::hash<Key> must return size_t");
+    static_assert(
+        std::is_same_v<ReturnType, NAT> || std::is_same_v<ReturnType, size_t>,
+        "std::hash<Key> must return size_t");
     return nullptr;
   }
 
@@ -644,71 +341,302 @@ inline void AssertHashEnabled() {
   Helper::Sink(Helper::DoIt<Ts>()...);
 }
 
-}  // namespace type_traits_internal
-
-// An internal namespace that is required to implement the C++17 swap traits.
-// It is not further nested in type_traits_internal to avoid long symbol names.
-namespace swap_internal {
-
-// Necessary for the traits.
-using std::swap;
-
-// This declaration prevents global `swap` and `absl::swap` overloads from being
-// considered unless ADL picks them up.
-void swap();
+template <class T>
+using IsSwappable ABSL_DEPRECATE_AND_INLINE() = std::is_swappable<T>;
 
 template <class T>
-using IsSwappableImpl = decltype(swap(std::declval<T&>(), std::declval<T&>()));
+using IsNothrowSwappable ABSL_DEPRECATE_AND_INLINE() =
+    std::is_nothrow_swappable<T>;
 
-// NOTE: This dance with the default template parameter is for MSVC.
-template <class T,
-          class IsNoexcept = std::integral_constant<
-              bool, noexcept(swap(std::declval<T&>(), std::declval<T&>()))>>
-using IsNothrowSwappableImpl = typename std::enable_if<IsNoexcept::value>::type;
+}  // namespace type_traits_internal
 
-// IsSwappable
+// absl::is_trivially_relocatable<T>
 //
-// Determines whether the standard swap idiom is a valid expression for
-// arguments of type `T`.
-template <class T>
-struct IsSwappable
-    : absl::type_traits_internal::is_detected<IsSwappableImpl, T> {};
-
-// IsNothrowSwappable
+// https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2024/p2786r11.html
+//
+// Detects whether a type is known to be "trivially relocatable" -- meaning it
+// can be relocated from one place to another as if by memcpy/memmove.
+// This implies that its object representation doesn't depend on its address,
+// and also none of its special member functions do anything strange.
+//
+// Note that when relocating the caller code should ensure that if the object is
+// polymorphic, the dynamic type is of the most derived type. Padding bytes
+// should not be copied.
 //
-// Determines whether the standard swap idiom is a valid expression for
-// arguments of type `T` and is noexcept.
+// This trait is conservative. If it's true then the type is definitely
+// trivially relocatable, but if it's false then the type may or may not be. For
+// example, std::vector<int> is trivially relocatable on every known STL
+// implementation, but absl::is_trivially_relocatable<std::vector<int>> remains
+// false.
+//
+// Example:
+//
+// if constexpr (absl::is_trivially_relocatable<T>::value) {
+//   memcpy(new_location, old_location, sizeof(T));
+// } else {
+//   new(new_location) T(std::move(*old_location));
+//   old_location->~T();
+// }
+//
+// Upstream documentation:
+//
+// https://clang.llvm.org/docs/LanguageExtensions.html#:~:text=__builtin_is_cpp_trivially_relocatable
+//
+// Clang on Windows has the builtin, but it falsely claims types with a
+// user-provided destructor are trivial (http://b/275003464). So we opt out
+// there.
+//
+// TODO(b/275003464): remove the opt-out once the bug is fixed.
+//
+// Starting with Xcode 15, the Apple compiler will falsely say a type
+// with a user-provided move constructor is trivially relocatable
+// (b/324278148). We will opt out without a version check, due to
+// the fluidity of Apple versions.
+//
+// TODO(b/324278148): If all versions we use have the bug fixed, then
+// remove the condition.
+//
+// Clang on all platforms fails to detect that a type with a user-provided
+// move-assignment operator is not trivially relocatable so we also check for
+// is_trivially_move_assignable for Clang.
+//
+// TODO(b/325479096): Remove the Clang is_trivially_move_assignable version once
+// Clang's behavior is fixed.
+//
+// According to https://github.com/abseil/abseil-cpp/issues/1479, this does not
+// work with NVCC either.
+#if ABSL_HAVE_BUILTIN(__builtin_is_cpp_trivially_relocatable)
+// https://github.com/llvm/llvm-project/pull/127636#pullrequestreview-2637005293
+// In the current implementation, __builtin_is_cpp_trivially_relocatable will
+// only return true for types that are trivially relocatable according to the
+// standard. Notably, this means that marking a type [[clang::trivial_abi]] aka
+// ABSL_HAVE_ATTRIBUTE_TRIVIAL_ABI will have no effect on this trait.
+template <class T>
+struct is_trivially_relocatable
+    : std::bool_constant<__builtin_is_cpp_trivially_relocatable(T)> {};
+#elif ABSL_HAVE_BUILTIN(__is_trivially_relocatable) && defined(__clang__) && \
+    !(defined(_WIN32) || defined(_WIN64)) && !defined(__APPLE__) &&          \
+    !defined(__NVCC__)
+// https://github.com/llvm/llvm-project/pull/139061
+//  __is_trivially_relocatable is deprecated.
+// TODO(b/325479096): Remove this case.
 template <class T>
-struct IsNothrowSwappable
-    : absl::type_traits_internal::is_detected<IsNothrowSwappableImpl, T> {};
+struct is_trivially_relocatable
+    : std::bool_constant<std::is_trivially_copyable_v<T> ||
+                         (__is_trivially_relocatable(T) &&
+                          std::is_trivially_move_assignable_v<T>)> {};
+#else
+// Otherwise we use a fallback that detects only those types we can feasibly
+// detect. Any type that is trivially copyable is by definition trivially
+// relocatable.
+template <class T>
+struct is_trivially_relocatable : std::is_trivially_copyable<T> {};
+#endif
 
-// Swap()
+// absl::is_constant_evaluated()
 //
-// Performs the swap idiom from a namespace where valid candidates may only be
-// found in `std` or via ADL.
-template <class T, absl::enable_if_t<IsSwappable<T>::value, int> = 0>
-void Swap(T& lhs, T& rhs) noexcept(IsNothrowSwappable<T>::value) {
-  swap(lhs, rhs);
+// Detects whether the function call occurs within a constant-evaluated context.
+// Returns true if the evaluation of the call occurs within the evaluation of an
+// expression or conversion that is manifestly constant-evaluated; otherwise
+// returns false.
+//
+// This function is implemented in terms of `std::is_constant_evaluated` for
+// c++20 and up. For older c++ versions, the function is implemented in terms
+// of `__builtin_is_constant_evaluated` if available, otherwise the function
+// will fail to compile.
+//
+// Applications can inspect `ABSL_HAVE_CONSTANT_EVALUATED` at compile time
+// to check if this function is supported.
+//
+// Example:
+//
+// constexpr MyClass::MyClass(int param) {
+// #ifdef ABSL_HAVE_CONSTANT_EVALUATED
+//   if (!absl::is_constant_evaluated()) {
+//     ABSL_LOG(INFO) << "MyClass(" << param << ")";
+//   }
+// #endif  // ABSL_HAVE_CONSTANT_EVALUATED
+// }
+//
+// Upstream documentation:
+//
+// http://en.cppreference.com/w/cpp/types/is_constant_evaluated
+// http://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html#:~:text=__builtin_is_constant_evaluated
+//
+#if defined(ABSL_HAVE_CONSTANT_EVALUATED)
+constexpr bool is_constant_evaluated() noexcept {
+#ifdef __cpp_lib_is_constant_evaluated
+  return std::is_constant_evaluated();
+#elif ABSL_HAVE_BUILTIN(__builtin_is_constant_evaluated)
+  return __builtin_is_constant_evaluated();
+#endif
 }
+#endif  // ABSL_HAVE_CONSTANT_EVALUATED
 
-// StdSwapIsUnconstrained
-//
-// Some standard library implementations are broken in that they do not
-// constrain `std::swap`. This will effectively tell us if we are dealing with
-// one of those implementations.
-using StdSwapIsUnconstrained = IsSwappable<void()>;
+namespace type_traits_internal {
 
-}  // namespace swap_internal
+// Detects if a class's definition has declared itself to be an owner by
+// declaring
+//   using absl_internal_is_view = std::false_type;
+// as a member.
+// Types that don't want either must either omit this declaration entirely, or
+// (if e.g. inheriting from a base class) define the member to something that
+// isn't a Boolean trait class, such as `void`.
+// Do not specialize or use this directly. It's an implementation detail.
+template <typename T, typename = void>
+struct IsOwnerImpl : std::false_type {
+  static_assert(std::is_same_v<T, absl::remove_cvref_t<T>>,
+                "type must lack qualifiers");
+};
 
-namespace type_traits_internal {
+template <typename T>
+struct IsOwnerImpl<
+    T, std::enable_if_t<std::is_class_v<typename T::absl_internal_is_view>>>
+    : std::negation<typename T::absl_internal_is_view> {};
+
+// A trait to determine whether a type is an owner.
+// Do *not* depend on the correctness of this trait for correct code behavior.
+// It is only a safety feature and its value may change in the future.
+// Do not specialize this; instead, define the member trait inside your type so
+// that it can be auto-detected, and to prevent ODR violations.
+// If it ever becomes possible to detect [[gsl::Owner]], we should leverage it:
+// https://wg21.link/p1179
+template <typename T>
+struct IsOwner : IsOwnerImpl<T> {};
+
+template <typename T>
+struct IsOwner<T&> : std::false_type {};
+
+template <typename T>
+struct IsOwner<T&&> : std::false_type {};
+
+template <typename T>
+struct IsOwner<const T> : IsOwner<T> {};
+
+template <typename T>
+struct IsOwner<volatile T> : IsOwner<T> {};
+
+template <typename T>
+struct IsOwner<const volatile T> : IsOwner<T> {};
+
+template <typename T>
+struct IsOwner<std::reference_wrapper<T>> : std::false_type {};
+
+template <typename T, std::size_t N>
+struct IsOwner<std::array<T, N>>
+    : std::conditional_t<N != 0, IsOwner<T>, std::false_type> {};
 
-// Make the swap-related traits/function accessible from this namespace.
-using swap_internal::IsNothrowSwappable;
-using swap_internal::IsSwappable;
-using swap_internal::Swap;
-using swap_internal::StdSwapIsUnconstrained;
+// This allows incomplete types to be used for associative containers, and also
+// expands the set of types we can handle to include std::pair.
+template <typename T1, typename T2>
+struct IsOwner<std::pair<T1, T2>> : IsOwner<std::tuple<T1, T2>> {};
+
+template <typename T, typename Traits, typename Alloc>
+struct IsOwner<std::basic_string<T, Traits, Alloc>> : std::true_type {};
+
+template <typename T, typename Alloc>
+struct IsOwner<std::vector<T, Alloc>> : std::true_type {};
+
+template <typename... T>
+struct IsOwner<std::tuple<T...>>
+    : std::bool_constant<(sizeof...(T) > 0) &&
+                         // Uses a C++17 fold expression where '...' unpacks the
+                         // parameter pack T, and 'true &&' provides the base
+                         // case for the logical AND operation across all types.
+                         (true && ... && IsOwner<T>::value)> {};
+
+template <typename... T>
+struct IsOwner<std::variant<T...>> : IsOwner<std::tuple<T...>> {};
+
+// Detects if a class's definition has declared itself to be a view by declaring
+//   using absl_internal_is_view = std::true_type;
+// as a member.
+// Do not specialize or use this directly.
+template <typename T, typename = void>
+struct IsViewImpl : std::false_type {
+  static_assert(std::is_same_v<T, absl::remove_cvref_t<T>>,
+                "type must lack qualifiers");
+};
+
+template <typename T>
+struct IsViewImpl<
+    T, std::enable_if_t<std::is_class_v<typename T::absl_internal_is_view>>>
+    : T::absl_internal_is_view {};
+
+// A trait to determine whether a type is a view.
+// Do *not* depend on the correctness of this trait for correct code behavior.
+// It is only a safety feature, and its value may change in the future.
+// Do not specialize this trait. Instead, define the member
+//   using absl_internal_is_view = std::true_type;
+// in your class to allow its detection while preventing ODR violations.
+// If it ever becomes possible to detect [[gsl::Pointer]], we should leverage
+// it: https://wg21.link/p1179
+template <typename T>
+struct IsView
+    : std::bool_constant<std::is_pointer_v<T> || IsViewImpl<T>::value> {};
+
+template <typename T>
+struct IsView<T&> : std::true_type {};
+
+template <typename T>
+struct IsView<T&&> : std::true_type {};
+
+template <typename T>
+struct IsView<const T> : IsView<T> {};
+
+template <typename T>
+struct IsView<volatile T> : IsView<T> {};
+
+template <typename T>
+struct IsView<const volatile T> : IsView<T> {};
+
+template <typename T>
+struct IsView<std::reference_wrapper<T>> : std::true_type {};
+
+template <typename T, std::size_t N>
+struct IsView<std::array<T, N>>
+    : std::conditional_t<N != 0, IsView<T>, std::false_type> {};
+
+// This allows incomplete types to be used for associative containers, and also
+// expands the set of types we can handle to include std::pair.
+template <typename T1, typename T2>
+struct IsView<std::pair<T1, T2>> : IsView<std::tuple<T1, T2>> {};
+
+template <typename Char, typename Traits>
+struct IsView<std::basic_string_view<Char, Traits>> : std::true_type {};
+
+template <typename... T>
+struct IsView<std::tuple<T...>>
+    : std::bool_constant<(sizeof...(T) > 0) &&
+                         // Uses a C++17 fold expression where '...' unpacks the
+                         // parameter pack T, and 'true &&' provides the base
+                         // case for the logical AND operation across all types.
+                         (true && ... && IsView<T>::value)> {};
+
+template <typename... T>
+struct IsView<std::variant<T...>> : IsView<std::tuple<T...>> {};
+
+#ifdef __cpp_lib_span
+template <typename T>
+struct IsView<std::span<T>> : std::true_type {};
+#endif
+
+// Determines whether the assignment of the given types is lifetime-bound.
+// Do *not* depend on the correctness of this trait for correct code behavior.
+// It is only a safety feature and its value may change in the future.
+// If it ever becomes possible to detect [[clang::lifetimebound]] directly,
+// we should change the implementation to leverage that.
+// Until then, we consider an assignment from an "owner" (such as std::string)
+// to a "view" (such as std::string_view) to be a lifetime-bound assignment.
+template <typename T, typename U>
+using IsLifetimeBoundAssignment =
+    std::conjunction<std::bool_constant<!std::is_lvalue_reference_v<U>>,
+                     IsOwner<absl::remove_cvref_t<U>>,
+                     IsView<absl::remove_cvref_t<T>>>;
 
 }  // namespace type_traits_internal
+
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_META_TYPE_TRAITS_H_
diff --git a/absl/meta/type_traits_test.cc b/absl/meta/type_traits_test.cc
index 6fbb42f82e0..a11049c6ba7 100644
--- a/absl/meta/type_traits_test.cc
+++ b/absl/meta/type_traits_test.cc
@@ -14,18 +14,51 @@
 
 #include "absl/meta/type_traits.h"
 
+#include <array>
 #include <cstdint>
+#include <functional>
 #include <string>
+#include <string_view>
+#include <tuple>
 #include <type_traits>
 #include <utility>
+#include <variant>
 #include <vector>
 
 #include "gtest/gtest.h"
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/time/clock.h"
+#include "absl/time/time.h"
 
 namespace {
 
 using ::testing::StaticAssertTypeEq;
 
+template <typename T>
+using IsViewAndNotOwner =
+    std::conjunction<absl::type_traits_internal::IsView<T>,
+                     std::negation<absl::type_traits_internal::IsOwner<T>>>;
+
+template <typename T>
+using IsOwnerAndNotView =
+    std::conjunction<absl::type_traits_internal::IsOwner<T>,
+                      std::negation<absl::type_traits_internal::IsView<T>>>;
+
+static_assert(
+    IsOwnerAndNotView<std::pair<std::vector<int>, std::string>>::value,
+    "pair of owners is an owner, not a view");
+static_assert(IsOwnerAndNotView<std::vector<int>>::value,
+              "vector is an owner, not a view");
+static_assert(IsOwnerAndNotView<std::string>::value,
+              "string is an owner, not a view");
+static_assert(IsOwnerAndNotView<std::wstring>::value,
+              "wstring is an owner, not a view");
+static_assert(IsViewAndNotOwner<std::string_view>::value,
+              "string_view is a view, not an owner");
+static_assert(IsViewAndNotOwner<std::wstring_view>::value,
+              "wstring_view is a view, not an owner");
+
 template <class T, class U>
 struct simple_pair {
   T first;
@@ -45,14 +78,12 @@ struct StructB {};
 struct StructC {};
 
 struct TypeWithBarFunction {
-  template <class T,
-            absl::enable_if_t<std::is_same<T&&, StructA&>::value, int> = 0>
+  template <class T, std::enable_if_t<std::is_same_v<T&&, StructA&>, int> = 0>
   ReturnType bar(T&&, const StructB&, StructC&&) &&;  // NOLINT
 };
 
 struct TypeWithBarFunctionAndConvertibleReturnType {
-  template <class T,
-            absl::enable_if_t<std::is_same<T&&, StructA&>::value, int> = 0>
+  template <class T, std::enable_if_t<std::is_same_v<T&&, StructA&>, int> = 0>
   ConvertibleToReturnType bar(T&&, const StructB&, StructC&&) &&;  // NOLINT
 };
 
@@ -64,10 +95,6 @@ template <class Class, class... T>
 using BarIsCallable =
     absl::type_traits_internal::is_detected<BarIsCallableImpl, Class, T...>;
 
-template <class Class, class... T>
-using BarIsCallableConv = absl::type_traits_internal::is_detected_convertible<
-    ReturnType, BarIsCallableImpl, Class, T...>;
-
 // NOTE: Test of detail type_traits_internal::is_detected.
 TEST(IsDetectedTest, BasicUsage) {
   EXPECT_TRUE((BarIsCallable<TypeWithBarFunction, StructA&, const StructB&,
@@ -84,1071 +111,46 @@ TEST(IsDetectedTest, BasicUsage) {
                               StructC>::value));
 }
 
-// NOTE: Test of detail type_traits_internal::is_detected_convertible.
-TEST(IsDetectedConvertibleTest, BasicUsage) {
-  EXPECT_TRUE((BarIsCallableConv<TypeWithBarFunction, StructA&, const StructB&,
-                                 StructC>::value));
-  EXPECT_TRUE((BarIsCallableConv<TypeWithBarFunction, StructA&, StructB&,
-                                 StructC>::value));
-  EXPECT_TRUE((BarIsCallableConv<TypeWithBarFunction, StructA&, StructB,
-                                 StructC>::value));
-  EXPECT_TRUE((BarIsCallableConv<TypeWithBarFunctionAndConvertibleReturnType,
-                                 StructA&, const StructB&, StructC>::value));
-  EXPECT_TRUE((BarIsCallableConv<TypeWithBarFunctionAndConvertibleReturnType,
-                                 StructA&, StructB&, StructC>::value));
-  EXPECT_TRUE((BarIsCallableConv<TypeWithBarFunctionAndConvertibleReturnType,
-                                 StructA&, StructB, StructC>::value));
-
-  EXPECT_FALSE(
-      (BarIsCallableConv<int, StructA&, const StructB&, StructC>::value));
-  EXPECT_FALSE((BarIsCallableConv<TypeWithBarFunction&, StructA&,
-                                  const StructB&, StructC>::value));
-  EXPECT_FALSE((BarIsCallableConv<TypeWithBarFunction, StructA, const StructB&,
-                                  StructC>::value));
-  EXPECT_FALSE((BarIsCallableConv<TypeWithBarFunctionAndConvertibleReturnType&,
-                                  StructA&, const StructB&, StructC>::value));
-  EXPECT_FALSE((BarIsCallableConv<TypeWithBarFunctionAndConvertibleReturnType,
-                                  StructA, const StructB&, StructC>::value));
-}
-
 TEST(VoidTTest, BasicUsage) {
   StaticAssertTypeEq<void, absl::void_t<Dummy>>();
   StaticAssertTypeEq<void, absl::void_t<Dummy, Dummy, Dummy>>();
 }
 
-TEST(ConjunctionTest, BasicBooleanLogic) {
-  EXPECT_TRUE(absl::conjunction<>::value);
-  EXPECT_TRUE(absl::conjunction<std::true_type>::value);
-  EXPECT_TRUE((absl::conjunction<std::true_type, std::true_type>::value));
-  EXPECT_FALSE((absl::conjunction<std::true_type, std::false_type>::value));
-  EXPECT_FALSE((absl::conjunction<std::false_type, std::true_type>::value));
-  EXPECT_FALSE((absl::conjunction<std::false_type, std::false_type>::value));
-}
-
-struct MyTrueType {
-  static constexpr bool value = true;
-};
-
-struct MyFalseType {
-  static constexpr bool value = false;
-};
-
-TEST(ConjunctionTest, ShortCircuiting) {
-  EXPECT_FALSE(
-      (absl::conjunction<std::true_type, std::false_type, Dummy>::value));
-  EXPECT_TRUE((std::is_base_of<MyFalseType,
-                               absl::conjunction<std::true_type, MyFalseType,
-                                                 std::false_type>>::value));
+TEST(TypeTraitsTest, TestRemoveCVRef) {
+  EXPECT_TRUE((std::is_same_v<typename absl::remove_cvref<int>::type, int>));
+  EXPECT_TRUE((std::is_same_v<typename absl::remove_cvref<int&>::type, int>));
+  EXPECT_TRUE((std::is_same_v<typename absl::remove_cvref<int&&>::type, int>));
   EXPECT_TRUE(
-      (std::is_base_of<MyTrueType,
-                       absl::conjunction<std::true_type, MyTrueType>>::value));
-}
-
-TEST(DisjunctionTest, BasicBooleanLogic) {
-  EXPECT_FALSE(absl::disjunction<>::value);
-  EXPECT_FALSE(absl::disjunction<std::false_type>::value);
-  EXPECT_TRUE((absl::disjunction<std::true_type, std::true_type>::value));
-  EXPECT_TRUE((absl::disjunction<std::true_type, std::false_type>::value));
-  EXPECT_TRUE((absl::disjunction<std::false_type, std::true_type>::value));
-  EXPECT_FALSE((absl::disjunction<std::false_type, std::false_type>::value));
-}
-
-TEST(DisjunctionTest, ShortCircuiting) {
+      (std::is_same_v<typename absl::remove_cvref<const int&>::type, int>));
+  EXPECT_TRUE((std::is_same_v<typename absl::remove_cvref<int*>::type, int*>));
+  // Does not remove const in this case.
+  EXPECT_TRUE((std::is_same_v<typename absl::remove_cvref<const int*>::type,
+                              const int*>));
   EXPECT_TRUE(
-      (absl::disjunction<std::false_type, std::true_type, Dummy>::value));
-  EXPECT_TRUE((
-      std::is_base_of<MyTrueType, absl::disjunction<std::false_type, MyTrueType,
-                                                    std::true_type>>::value));
-  EXPECT_TRUE((
-      std::is_base_of<MyFalseType,
-                      absl::disjunction<std::false_type, MyFalseType>>::value));
-}
-
-TEST(NegationTest, BasicBooleanLogic) {
-  EXPECT_FALSE(absl::negation<std::true_type>::value);
-  EXPECT_FALSE(absl::negation<MyTrueType>::value);
-  EXPECT_TRUE(absl::negation<std::false_type>::value);
-  EXPECT_TRUE(absl::negation<MyFalseType>::value);
-}
-
-// all member functions are trivial
-class Trivial {
-  int n_;
-};
-
-struct TrivialDestructor {
-  ~TrivialDestructor() = default;
-};
-
-struct NontrivialDestructor {
-  ~NontrivialDestructor() {}
-};
-
-struct DeletedDestructor {
-  ~DeletedDestructor() = delete;
-};
-
-class TrivialDefaultCtor {
- public:
-  TrivialDefaultCtor() = default;
-  explicit TrivialDefaultCtor(int n) : n_(n) {}
-
- private:
-  int n_;
-};
-
-class NontrivialDefaultCtor {
- public:
-  NontrivialDefaultCtor() : n_(1) {}
-
- private:
-  int n_;
-};
-
-class DeletedDefaultCtor {
- public:
-  DeletedDefaultCtor() = delete;
-  explicit DeletedDefaultCtor(int n) : n_(n) {}
-
- private:
-  int n_;
-};
-
-class TrivialMoveCtor {
- public:
-  explicit TrivialMoveCtor(int n) : n_(n) {}
-  TrivialMoveCtor(TrivialMoveCtor&&) = default;
-  TrivialMoveCtor& operator=(const TrivialMoveCtor& t) {
-    n_ = t.n_;
-    return *this;
-  }
-
- private:
-  int n_;
-};
-
-class NontrivialMoveCtor {
- public:
-  explicit NontrivialMoveCtor(int n) : n_(n) {}
-  NontrivialMoveCtor(NontrivialMoveCtor&& t) noexcept : n_(t.n_) {}
-  NontrivialMoveCtor& operator=(const NontrivialMoveCtor&) = default;
-
- private:
-  int n_;
-};
-
-class TrivialCopyCtor {
- public:
-  explicit TrivialCopyCtor(int n) : n_(n) {}
-  TrivialCopyCtor(const TrivialCopyCtor&) = default;
-  TrivialCopyCtor& operator=(const TrivialCopyCtor& t) {
-    n_ = t.n_;
-    return *this;
-  }
-
- private:
-  int n_;
-};
-
-class NontrivialCopyCtor {
- public:
-  explicit NontrivialCopyCtor(int n) : n_(n) {}
-  NontrivialCopyCtor(const NontrivialCopyCtor& t) : n_(t.n_) {}
-  NontrivialCopyCtor& operator=(const NontrivialCopyCtor&) = default;
-
- private:
-  int n_;
-};
-
-class DeletedCopyCtor {
- public:
-  explicit DeletedCopyCtor(int n) : n_(n) {}
-  DeletedCopyCtor(const DeletedCopyCtor&) = delete;
-  DeletedCopyCtor& operator=(const DeletedCopyCtor&) = default;
-
- private:
-  int n_;
-};
-
-class TrivialMoveAssign {
- public:
-  explicit TrivialMoveAssign(int n) : n_(n) {}
-  TrivialMoveAssign(const TrivialMoveAssign& t) : n_(t.n_) {}
-  TrivialMoveAssign& operator=(TrivialMoveAssign&&) = default;
-  ~TrivialMoveAssign() {}  // can have nontrivial destructor
- private:
-  int n_;
-};
-
-class NontrivialMoveAssign {
- public:
-  explicit NontrivialMoveAssign(int n) : n_(n) {}
-  NontrivialMoveAssign(const NontrivialMoveAssign&) = default;
-  NontrivialMoveAssign& operator=(NontrivialMoveAssign&& t) noexcept {
-    n_ = t.n_;
-    return *this;
-  }
-
- private:
-  int n_;
-};
-
-class TrivialCopyAssign {
- public:
-  explicit TrivialCopyAssign(int n) : n_(n) {}
-  TrivialCopyAssign(const TrivialCopyAssign& t) : n_(t.n_) {}
-  TrivialCopyAssign& operator=(const TrivialCopyAssign& t) = default;
-  ~TrivialCopyAssign() {}  // can have nontrivial destructor
- private:
-  int n_;
-};
-
-class NontrivialCopyAssign {
- public:
-  explicit NontrivialCopyAssign(int n) : n_(n) {}
-  NontrivialCopyAssign(const NontrivialCopyAssign&) = default;
-  NontrivialCopyAssign& operator=(const NontrivialCopyAssign& t) {
-    n_ = t.n_;
-    return *this;
-  }
-
- private:
-  int n_;
-};
-
-class DeletedCopyAssign {
- public:
-  explicit DeletedCopyAssign(int n) : n_(n) {}
-  DeletedCopyAssign(const DeletedCopyAssign&) = default;
-  DeletedCopyAssign& operator=(const DeletedCopyAssign&) = delete;
-
- private:
-  int n_;
-};
-
-struct MovableNonCopyable {
-  MovableNonCopyable() = default;
-  MovableNonCopyable(const MovableNonCopyable&) = delete;
-  MovableNonCopyable(MovableNonCopyable&&) = default;
-  MovableNonCopyable& operator=(const MovableNonCopyable&) = delete;
-  MovableNonCopyable& operator=(MovableNonCopyable&&) = default;
-};
-
-struct NonCopyableOrMovable {
-  NonCopyableOrMovable() = default;
-  NonCopyableOrMovable(const NonCopyableOrMovable&) = delete;
-  NonCopyableOrMovable(NonCopyableOrMovable&&) = delete;
-  NonCopyableOrMovable& operator=(const NonCopyableOrMovable&) = delete;
-  NonCopyableOrMovable& operator=(NonCopyableOrMovable&&) = delete;
-};
-
-class Base {
- public:
-  virtual ~Base() {}
-};
-
-// In GCC/Clang, std::is_trivially_constructible requires that the destructor is
-// trivial. However, MSVC doesn't require that. This results in different
-// behavior when checking is_trivially_constructible on any type with
-// nontrivial destructor. Since absl::is_trivially_default_constructible and
-// absl::is_trivially_copy_constructible both follows Clang/GCC's interpretation
-// and check is_trivially_destructible, it results in inconsistency with
-// std::is_trivially_xxx_constructible on MSVC. This macro is used to work
-// around this issue in test. In practice, a trivially constructible type
-// should also be trivially destructible.
-// GCC bug 51452: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51452
-// LWG issue 2116: http://cplusplus.github.io/LWG/lwg-active.html#2116
-#ifndef _MSC_VER
-#define ABSL_TRIVIALLY_CONSTRUCTIBLE_VERIFY_TRIVIALLY_DESTRUCTIBLE 1
-#endif
-
-// Old versions of libc++, around Clang 3.5 to 3.6, consider deleted destructors
-// as also being trivial. With the resolution of CWG 1928 and CWG 1734, this
-// is no longer considered true and has thus been amended.
-// Compiler Explorer: https://godbolt.org/g/zT59ZL
-// CWG issue 1734: http://open-std.org/JTC1/SC22/WG21/docs/cwg_defects.html#1734
-// CWG issue 1928: http://open-std.org/JTC1/SC22/WG21/docs/cwg_closed.html#1928
-#if !defined(_LIBCPP_VERSION) || _LIBCPP_VERSION >= 3700
-#define ABSL_TRIVIALLY_DESTRUCTIBLE_CONSIDER_DELETED_DESTRUCTOR_NOT_TRIVIAL 1
-#endif
-
-// As of the moment, GCC versions >5.1 have a problem compiling for
-// std::is_trivially_default_constructible<NontrivialDestructor[10]>, where
-// NontrivialDestructor is a struct with a custom nontrivial destructor. Note
-// that this problem only occurs for arrays of a known size, so something like
-// std::is_trivially_default_constructible<NontrivialDestructor[]> does not
-// have any problems.
-// Compiler Explorer: https://godbolt.org/g/dXRbdK
-// GCC bug 83689: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83689
-#if defined(__clang__) || defined(_MSC_VER) || \
-    (defined(__GNUC__) && __GNUC__ < 5)
-#define ABSL_GCC_BUG_TRIVIALLY_CONSTRUCTIBLE_ON_ARRAY_OF_NONTRIVIAL 1
-#endif
-
-TEST(TypeTraitsTest, TestIsFunction) {
-  struct Callable {
-    void operator()() {}
-  };
-  EXPECT_TRUE(absl::is_function<void()>::value);
-  EXPECT_TRUE(absl::is_function<void()&>::value);
-  EXPECT_TRUE(absl::is_function<void() const>::value);
-  EXPECT_TRUE(absl::is_function<void() noexcept>::value);
-  EXPECT_TRUE(absl::is_function<void(...) noexcept>::value);
-
-  EXPECT_FALSE(absl::is_function<void(*)()>::value);
-  EXPECT_FALSE(absl::is_function<void(&)()>::value);
-  EXPECT_FALSE(absl::is_function<int>::value);
-  EXPECT_FALSE(absl::is_function<Callable>::value);
-}
-
-TEST(TypeTraitsTest, TestTrivialDestructor) {
-  // Verify that arithmetic types and pointers have trivial destructors.
-  EXPECT_TRUE(absl::is_trivially_destructible<bool>::value);
-  EXPECT_TRUE(absl::is_trivially_destructible<char>::value);
-  EXPECT_TRUE(absl::is_trivially_destructible<unsigned char>::value);
-  EXPECT_TRUE(absl::is_trivially_destructible<signed char>::value);
-  EXPECT_TRUE(absl::is_trivially_destructible<wchar_t>::value);
-  EXPECT_TRUE(absl::is_trivially_destructible<int>::value);
-  EXPECT_TRUE(absl::is_trivially_destructible<unsigned int>::value);
-  EXPECT_TRUE(absl::is_trivially_destructible<int16_t>::value);
-  EXPECT_TRUE(absl::is_trivially_destructible<uint16_t>::value);
-  EXPECT_TRUE(absl::is_trivially_destructible<int64_t>::value);
-  EXPECT_TRUE(absl::is_trivially_destructible<uint64_t>::value);
-  EXPECT_TRUE(absl::is_trivially_destructible<float>::value);
-  EXPECT_TRUE(absl::is_trivially_destructible<double>::value);
-  EXPECT_TRUE(absl::is_trivially_destructible<long double>::value);
-  EXPECT_TRUE(absl::is_trivially_destructible<std::string*>::value);
-  EXPECT_TRUE(absl::is_trivially_destructible<Trivial*>::value);
-  EXPECT_TRUE(absl::is_trivially_destructible<const std::string*>::value);
-  EXPECT_TRUE(absl::is_trivially_destructible<const Trivial*>::value);
-  EXPECT_TRUE(absl::is_trivially_destructible<std::string**>::value);
-  EXPECT_TRUE(absl::is_trivially_destructible<Trivial**>::value);
-
-  // classes with destructors
-  EXPECT_TRUE(absl::is_trivially_destructible<Trivial>::value);
-  EXPECT_TRUE(absl::is_trivially_destructible<TrivialDestructor>::value);
-
-  // Verify that types with a nontrivial or deleted destructor
-  // are marked as such.
-  EXPECT_FALSE(absl::is_trivially_destructible<NontrivialDestructor>::value);
-#ifdef ABSL_TRIVIALLY_DESTRUCTIBLE_CONSIDER_DELETED_DESTRUCTOR_NOT_TRIVIAL
-  EXPECT_FALSE(absl::is_trivially_destructible<DeletedDestructor>::value);
-#endif
-
-  // simple_pair of such types is trivial
-  EXPECT_TRUE((absl::is_trivially_destructible<simple_pair<int, int>>::value));
-  EXPECT_TRUE((absl::is_trivially_destructible<
-               simple_pair<Trivial, TrivialDestructor>>::value));
-
-  // Verify that types without trivial destructors are correctly marked as such.
-  EXPECT_FALSE(absl::is_trivially_destructible<std::string>::value);
-  EXPECT_FALSE(absl::is_trivially_destructible<std::vector<int>>::value);
-
-  // Verify that simple_pairs of types without trivial destructors
-  // are not marked as trivial.
-  EXPECT_FALSE((absl::is_trivially_destructible<
-                simple_pair<int, std::string>>::value));
-  EXPECT_FALSE((absl::is_trivially_destructible<
-                simple_pair<std::string, int>>::value));
-
-  // array of such types is trivial
-  using int10 = int[10];
-  EXPECT_TRUE(absl::is_trivially_destructible<int10>::value);
-  using Trivial10 = Trivial[10];
-  EXPECT_TRUE(absl::is_trivially_destructible<Trivial10>::value);
-  using TrivialDestructor10 = TrivialDestructor[10];
-  EXPECT_TRUE(absl::is_trivially_destructible<TrivialDestructor10>::value);
-
-  // Conversely, the opposite also holds.
-  using NontrivialDestructor10 = NontrivialDestructor[10];
-  EXPECT_FALSE(absl::is_trivially_destructible<NontrivialDestructor10>::value);
-}
-
-TEST(TypeTraitsTest, TestTrivialDefaultCtor) {
-  // arithmetic types and pointers have trivial default constructors.
-  EXPECT_TRUE(absl::is_trivially_default_constructible<bool>::value);
-  EXPECT_TRUE(absl::is_trivially_default_constructible<char>::value);
-  EXPECT_TRUE(absl::is_trivially_default_constructible<unsigned char>::value);
-  EXPECT_TRUE(absl::is_trivially_default_constructible<signed char>::value);
-  EXPECT_TRUE(absl::is_trivially_default_constructible<wchar_t>::value);
-  EXPECT_TRUE(absl::is_trivially_default_constructible<int>::value);
-  EXPECT_TRUE(absl::is_trivially_default_constructible<unsigned int>::value);
-  EXPECT_TRUE(absl::is_trivially_default_constructible<int16_t>::value);
-  EXPECT_TRUE(absl::is_trivially_default_constructible<uint16_t>::value);
-  EXPECT_TRUE(absl::is_trivially_default_constructible<int64_t>::value);
-  EXPECT_TRUE(absl::is_trivially_default_constructible<uint64_t>::value);
-  EXPECT_TRUE(absl::is_trivially_default_constructible<float>::value);
-  EXPECT_TRUE(absl::is_trivially_default_constructible<double>::value);
-  EXPECT_TRUE(absl::is_trivially_default_constructible<long double>::value);
-  EXPECT_TRUE(absl::is_trivially_default_constructible<std::string*>::value);
-  EXPECT_TRUE(absl::is_trivially_default_constructible<Trivial*>::value);
-  EXPECT_TRUE(
-      absl::is_trivially_default_constructible<const std::string*>::value);
-  EXPECT_TRUE(absl::is_trivially_default_constructible<const Trivial*>::value);
-  EXPECT_TRUE(absl::is_trivially_default_constructible<std::string**>::value);
-  EXPECT_TRUE(absl::is_trivially_default_constructible<Trivial**>::value);
-
-  // types with compiler generated default ctors
-  EXPECT_TRUE(absl::is_trivially_default_constructible<Trivial>::value);
-  EXPECT_TRUE(
-      absl::is_trivially_default_constructible<TrivialDefaultCtor>::value);
-
-  // Verify that types without them are not.
-  EXPECT_FALSE(
-      absl::is_trivially_default_constructible<NontrivialDefaultCtor>::value);
-  EXPECT_FALSE(
-      absl::is_trivially_default_constructible<DeletedDefaultCtor>::value);
-
-#ifdef ABSL_TRIVIALLY_CONSTRUCTIBLE_VERIFY_TRIVIALLY_DESTRUCTIBLE
-  // types with nontrivial destructor are nontrivial
-  EXPECT_FALSE(
-      absl::is_trivially_default_constructible<NontrivialDestructor>::value);
-#endif
-
-  // types with vtables
-  EXPECT_FALSE(absl::is_trivially_default_constructible<Base>::value);
-
-  // Verify that simple_pair has trivial constructors where applicable.
-  EXPECT_TRUE((absl::is_trivially_default_constructible<
-               simple_pair<int, char*>>::value));
-  EXPECT_TRUE((absl::is_trivially_default_constructible<
-               simple_pair<int, Trivial>>::value));
-  EXPECT_TRUE((absl::is_trivially_default_constructible<
-               simple_pair<int, TrivialDefaultCtor>>::value));
-
-  // Verify that types without trivial constructors are
-  // correctly marked as such.
-  EXPECT_FALSE(absl::is_trivially_default_constructible<std::string>::value);
-  EXPECT_FALSE(
-      absl::is_trivially_default_constructible<std::vector<int>>::value);
-
-  // Verify that simple_pairs of types without trivial constructors
-  // are not marked as trivial.
-  EXPECT_FALSE((absl::is_trivially_default_constructible<
-                simple_pair<int, std::string>>::value));
-  EXPECT_FALSE((absl::is_trivially_default_constructible<
-                simple_pair<std::string, int>>::value));
-
-  // Verify that arrays of such types are trivially default constructible
-  using int10 = int[10];
-  EXPECT_TRUE(absl::is_trivially_default_constructible<int10>::value);
-  using Trivial10 = Trivial[10];
-  EXPECT_TRUE(absl::is_trivially_default_constructible<Trivial10>::value);
-  using TrivialDefaultCtor10 = TrivialDefaultCtor[10];
+      (std::is_same_v<typename absl::remove_cvref<int[2]>::type, int[2]>));
   EXPECT_TRUE(
-      absl::is_trivially_default_constructible<TrivialDefaultCtor10>::value);
-
-  // Conversely, the opposite also holds.
-#ifdef ABSL_GCC_BUG_TRIVIALLY_CONSTRUCTIBLE_ON_ARRAY_OF_NONTRIVIAL
-  using NontrivialDefaultCtor10 = NontrivialDefaultCtor[10];
-  EXPECT_FALSE(
-      absl::is_trivially_default_constructible<NontrivialDefaultCtor10>::value);
-#endif
-}
-
-// GCC prior to 7.4 had a bug in its trivially-constructible traits
-// (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80654).
-// This test makes sure that we do not depend on the trait in these cases when
-// implementing absl triviality traits.
-
-template <class T>
-struct BadConstructors {
-  BadConstructors() { static_assert(T::value, ""); }
-
-  BadConstructors(BadConstructors&&) { static_assert(T::value, ""); }
-
-  BadConstructors(const BadConstructors&) { static_assert(T::value, ""); }
-};
-
-TEST(TypeTraitsTest, TestTrivialityBadConstructors) {
-  using BadType = BadConstructors<int>;
-
-  EXPECT_FALSE(absl::is_trivially_default_constructible<BadType>::value);
-  EXPECT_FALSE(absl::is_trivially_move_constructible<BadType>::value);
-  EXPECT_FALSE(absl::is_trivially_copy_constructible<BadType>::value);
-}
-
-TEST(TypeTraitsTest, TestTrivialMoveCtor) {
-  // Verify that arithmetic types and pointers have trivial move
-  // constructors.
-  EXPECT_TRUE(absl::is_trivially_move_constructible<bool>::value);
-  EXPECT_TRUE(absl::is_trivially_move_constructible<char>::value);
-  EXPECT_TRUE(absl::is_trivially_move_constructible<unsigned char>::value);
-  EXPECT_TRUE(absl::is_trivially_move_constructible<signed char>::value);
-  EXPECT_TRUE(absl::is_trivially_move_constructible<wchar_t>::value);
-  EXPECT_TRUE(absl::is_trivially_move_constructible<int>::value);
-  EXPECT_TRUE(absl::is_trivially_move_constructible<unsigned int>::value);
-  EXPECT_TRUE(absl::is_trivially_move_constructible<int16_t>::value);
-  EXPECT_TRUE(absl::is_trivially_move_constructible<uint16_t>::value);
-  EXPECT_TRUE(absl::is_trivially_move_constructible<int64_t>::value);
-  EXPECT_TRUE(absl::is_trivially_move_constructible<uint64_t>::value);
-  EXPECT_TRUE(absl::is_trivially_move_constructible<float>::value);
-  EXPECT_TRUE(absl::is_trivially_move_constructible<double>::value);
-  EXPECT_TRUE(absl::is_trivially_move_constructible<long double>::value);
-  EXPECT_TRUE(absl::is_trivially_move_constructible<std::string*>::value);
-  EXPECT_TRUE(absl::is_trivially_move_constructible<Trivial*>::value);
-  EXPECT_TRUE(absl::is_trivially_move_constructible<const std::string*>::value);
-  EXPECT_TRUE(absl::is_trivially_move_constructible<const Trivial*>::value);
-  EXPECT_TRUE(absl::is_trivially_move_constructible<std::string**>::value);
-  EXPECT_TRUE(absl::is_trivially_move_constructible<Trivial**>::value);
-
-  // Reference types
-  EXPECT_TRUE(absl::is_trivially_move_constructible<int&>::value);
-  EXPECT_TRUE(absl::is_trivially_move_constructible<int&&>::value);
-
-  // types with compiler generated move ctors
-  EXPECT_TRUE(absl::is_trivially_move_constructible<Trivial>::value);
-  EXPECT_TRUE(absl::is_trivially_move_constructible<TrivialMoveCtor>::value);
-
-  // Verify that types without them (i.e. nontrivial or deleted) are not.
-  EXPECT_FALSE(
-      absl::is_trivially_move_constructible<NontrivialCopyCtor>::value);
-  EXPECT_FALSE(absl::is_trivially_move_constructible<DeletedCopyCtor>::value);
-  EXPECT_FALSE(
-      absl::is_trivially_move_constructible<NonCopyableOrMovable>::value);
-
-#ifdef ABSL_TRIVIALLY_CONSTRUCTIBLE_VERIFY_TRIVIALLY_DESTRUCTIBLE
-  // type with nontrivial destructor are nontrivial move construbtible
-  EXPECT_FALSE(
-      absl::is_trivially_move_constructible<NontrivialDestructor>::value);
-#endif
-
-  // types with vtables
-  EXPECT_FALSE(absl::is_trivially_move_constructible<Base>::value);
-
-  // Verify that simple_pair of such types is trivially move constructible
-  EXPECT_TRUE(
-      (absl::is_trivially_move_constructible<simple_pair<int, char*>>::value));
-  EXPECT_TRUE((
-      absl::is_trivially_move_constructible<simple_pair<int, Trivial>>::value));
-  EXPECT_TRUE((absl::is_trivially_move_constructible<
-               simple_pair<int, TrivialMoveCtor>>::value));
-
-  // Verify that types without trivial move constructors are
-  // correctly marked as such.
-  EXPECT_FALSE(absl::is_trivially_move_constructible<std::string>::value);
-  EXPECT_FALSE(absl::is_trivially_move_constructible<std::vector<int>>::value);
-
-  // Verify that simple_pairs of types without trivial move constructors
-  // are not marked as trivial.
-  EXPECT_FALSE((absl::is_trivially_move_constructible<
-                simple_pair<int, std::string>>::value));
-  EXPECT_FALSE((absl::is_trivially_move_constructible<
-                simple_pair<std::string, int>>::value));
-
-  // Verify that arrays are not
-  using int10 = int[10];
-  EXPECT_FALSE(absl::is_trivially_move_constructible<int10>::value);
-}
-
-TEST(TypeTraitsTest, TestTrivialCopyCtor) {
-  // Verify that arithmetic types and pointers have trivial copy
-  // constructors.
-  EXPECT_TRUE(absl::is_trivially_copy_constructible<bool>::value);
-  EXPECT_TRUE(absl::is_trivially_copy_constructible<char>::value);
-  EXPECT_TRUE(absl::is_trivially_copy_constructible<unsigned char>::value);
-  EXPECT_TRUE(absl::is_trivially_copy_constructible<signed char>::value);
-  EXPECT_TRUE(absl::is_trivially_copy_constructible<wchar_t>::value);
-  EXPECT_TRUE(absl::is_trivially_copy_constructible<int>::value);
-  EXPECT_TRUE(absl::is_trivially_copy_constructible<unsigned int>::value);
-  EXPECT_TRUE(absl::is_trivially_copy_constructible<int16_t>::value);
-  EXPECT_TRUE(absl::is_trivially_copy_constructible<uint16_t>::value);
-  EXPECT_TRUE(absl::is_trivially_copy_constructible<int64_t>::value);
-  EXPECT_TRUE(absl::is_trivially_copy_constructible<uint64_t>::value);
-  EXPECT_TRUE(absl::is_trivially_copy_constructible<float>::value);
-  EXPECT_TRUE(absl::is_trivially_copy_constructible<double>::value);
-  EXPECT_TRUE(absl::is_trivially_copy_constructible<long double>::value);
-  EXPECT_TRUE(absl::is_trivially_copy_constructible<std::string*>::value);
-  EXPECT_TRUE(absl::is_trivially_copy_constructible<Trivial*>::value);
-  EXPECT_TRUE(absl::is_trivially_copy_constructible<const std::string*>::value);
-  EXPECT_TRUE(absl::is_trivially_copy_constructible<const Trivial*>::value);
-  EXPECT_TRUE(absl::is_trivially_copy_constructible<std::string**>::value);
-  EXPECT_TRUE(absl::is_trivially_copy_constructible<Trivial**>::value);
-
-  // Reference types
-  EXPECT_TRUE(absl::is_trivially_copy_constructible<int&>::value);
-  EXPECT_FALSE(absl::is_trivially_copy_constructible<int&&>::value);
-
-  // types with compiler generated copy ctors
-  EXPECT_TRUE(absl::is_trivially_copy_constructible<Trivial>::value);
-  EXPECT_TRUE(absl::is_trivially_copy_constructible<TrivialCopyCtor>::value);
-
-  // Verify that types without them (i.e. nontrivial or deleted) are not.
-  EXPECT_FALSE(
-      absl::is_trivially_copy_constructible<NontrivialCopyCtor>::value);
-  EXPECT_FALSE(absl::is_trivially_copy_constructible<DeletedCopyCtor>::value);
-  EXPECT_FALSE(
-      absl::is_trivially_copy_constructible<MovableNonCopyable>::value);
-  EXPECT_FALSE(
-      absl::is_trivially_copy_constructible<NonCopyableOrMovable>::value);
-
-#ifdef ABSL_TRIVIALLY_CONSTRUCTIBLE_VERIFY_TRIVIALLY_DESTRUCTIBLE
-  // type with nontrivial destructor are nontrivial copy construbtible
-  EXPECT_FALSE(
-      absl::is_trivially_copy_constructible<NontrivialDestructor>::value);
-#endif
-
-  // types with vtables
-  EXPECT_FALSE(absl::is_trivially_copy_constructible<Base>::value);
-
-  // Verify that simple_pair of such types is trivially copy constructible
+      (std::is_same_v<typename absl::remove_cvref<int (&)[2]>::type, int[2]>));
   EXPECT_TRUE(
-      (absl::is_trivially_copy_constructible<simple_pair<int, char*>>::value));
+      (std::is_same_v<typename absl::remove_cvref<int (&&)[2]>::type, int[2]>));
   EXPECT_TRUE((
-      absl::is_trivially_copy_constructible<simple_pair<int, Trivial>>::value));
-  EXPECT_TRUE((absl::is_trivially_copy_constructible<
-               simple_pair<int, TrivialCopyCtor>>::value));
-
-  // Verify that types without trivial copy constructors are
-  // correctly marked as such.
-  EXPECT_FALSE(absl::is_trivially_copy_constructible<std::string>::value);
-  EXPECT_FALSE(absl::is_trivially_copy_constructible<std::vector<int>>::value);
-
-  // Verify that simple_pairs of types without trivial copy constructors
-  // are not marked as trivial.
-  EXPECT_FALSE((absl::is_trivially_copy_constructible<
-                simple_pair<int, std::string>>::value));
-  EXPECT_FALSE((absl::is_trivially_copy_constructible<
-                simple_pair<std::string, int>>::value));
-
-  // Verify that arrays are not
-  using int10 = int[10];
-  EXPECT_FALSE(absl::is_trivially_copy_constructible<int10>::value);
-}
-
-TEST(TypeTraitsTest, TestTrivialMoveAssign) {
-  // Verify that arithmetic types and pointers have trivial move
-  // assignment operators.
-  EXPECT_TRUE(absl::is_trivially_move_assignable<bool>::value);
-  EXPECT_TRUE(absl::is_trivially_move_assignable<char>::value);
-  EXPECT_TRUE(absl::is_trivially_move_assignable<unsigned char>::value);
-  EXPECT_TRUE(absl::is_trivially_move_assignable<signed char>::value);
-  EXPECT_TRUE(absl::is_trivially_move_assignable<wchar_t>::value);
-  EXPECT_TRUE(absl::is_trivially_move_assignable<int>::value);
-  EXPECT_TRUE(absl::is_trivially_move_assignable<unsigned int>::value);
-  EXPECT_TRUE(absl::is_trivially_move_assignable<int16_t>::value);
-  EXPECT_TRUE(absl::is_trivially_move_assignable<uint16_t>::value);
-  EXPECT_TRUE(absl::is_trivially_move_assignable<int64_t>::value);
-  EXPECT_TRUE(absl::is_trivially_move_assignable<uint64_t>::value);
-  EXPECT_TRUE(absl::is_trivially_move_assignable<float>::value);
-  EXPECT_TRUE(absl::is_trivially_move_assignable<double>::value);
-  EXPECT_TRUE(absl::is_trivially_move_assignable<long double>::value);
-  EXPECT_TRUE(absl::is_trivially_move_assignable<std::string*>::value);
-  EXPECT_TRUE(absl::is_trivially_move_assignable<Trivial*>::value);
-  EXPECT_TRUE(absl::is_trivially_move_assignable<const std::string*>::value);
-  EXPECT_TRUE(absl::is_trivially_move_assignable<const Trivial*>::value);
-  EXPECT_TRUE(absl::is_trivially_move_assignable<std::string**>::value);
-  EXPECT_TRUE(absl::is_trivially_move_assignable<Trivial**>::value);
-
-  // const qualified types are not assignable
-  EXPECT_FALSE(absl::is_trivially_move_assignable<const int>::value);
-
-  // types with compiler generated move assignment
-  EXPECT_TRUE(absl::is_trivially_move_assignable<Trivial>::value);
-  EXPECT_TRUE(absl::is_trivially_move_assignable<TrivialMoveAssign>::value);
-
-  // Verify that types without them (i.e. nontrivial or deleted) are not.
-  EXPECT_FALSE(absl::is_trivially_move_assignable<NontrivialCopyAssign>::value);
-  EXPECT_FALSE(absl::is_trivially_move_assignable<DeletedCopyAssign>::value);
-  EXPECT_FALSE(absl::is_trivially_move_assignable<NonCopyableOrMovable>::value);
-
-  // types with vtables
-  EXPECT_FALSE(absl::is_trivially_move_assignable<Base>::value);
-
-  // Verify that simple_pair is trivially assignable
+      std::is_same_v<typename absl::remove_cvref<const int[2]>::type, int[2]>));
   EXPECT_TRUE(
-      (absl::is_trivially_move_assignable<simple_pair<int, char*>>::value));
+      (std::is_same_v<typename absl::remove_cvref<const int (&)[2]>::type,
+                      int[2]>));
   EXPECT_TRUE(
-      (absl::is_trivially_move_assignable<simple_pair<int, Trivial>>::value));
-  EXPECT_TRUE((absl::is_trivially_move_assignable<
-               simple_pair<int, TrivialMoveAssign>>::value));
-
-  // Verify that types not trivially move assignable are
-  // correctly marked as such.
-  EXPECT_FALSE(absl::is_trivially_move_assignable<std::string>::value);
-  EXPECT_FALSE(absl::is_trivially_move_assignable<std::vector<int>>::value);
-
-  // Verify that simple_pairs of types not trivially move assignable
-  // are not marked as trivial.
-  EXPECT_FALSE((absl::is_trivially_move_assignable<
-                simple_pair<int, std::string>>::value));
-  EXPECT_FALSE((absl::is_trivially_move_assignable<
-                simple_pair<std::string, int>>::value));
-
-  // Verify that arrays are not trivially move assignable
-  using int10 = int[10];
-  EXPECT_FALSE(absl::is_trivially_move_assignable<int10>::value);
-
-  // Verify that references are handled correctly
-  EXPECT_TRUE(absl::is_trivially_move_assignable<Trivial&&>::value);
-  EXPECT_TRUE(absl::is_trivially_move_assignable<Trivial&>::value);
+      (std::is_same_v<typename absl::remove_cvref<const int (&&)[2]>::type,
+                      int[2]>));
 }
 
-TEST(TypeTraitsTest, TestTrivialCopyAssign) {
-  // Verify that arithmetic types and pointers have trivial copy
-  // assignment operators.
-  EXPECT_TRUE(absl::is_trivially_copy_assignable<bool>::value);
-  EXPECT_TRUE(absl::is_trivially_copy_assignable<char>::value);
-  EXPECT_TRUE(absl::is_trivially_copy_assignable<unsigned char>::value);
-  EXPECT_TRUE(absl::is_trivially_copy_assignable<signed char>::value);
-  EXPECT_TRUE(absl::is_trivially_copy_assignable<wchar_t>::value);
-  EXPECT_TRUE(absl::is_trivially_copy_assignable<int>::value);
-  EXPECT_TRUE(absl::is_trivially_copy_assignable<unsigned int>::value);
-  EXPECT_TRUE(absl::is_trivially_copy_assignable<int16_t>::value);
-  EXPECT_TRUE(absl::is_trivially_copy_assignable<uint16_t>::value);
-  EXPECT_TRUE(absl::is_trivially_copy_assignable<int64_t>::value);
-  EXPECT_TRUE(absl::is_trivially_copy_assignable<uint64_t>::value);
-  EXPECT_TRUE(absl::is_trivially_copy_assignable<float>::value);
-  EXPECT_TRUE(absl::is_trivially_copy_assignable<double>::value);
-  EXPECT_TRUE(absl::is_trivially_copy_assignable<long double>::value);
-  EXPECT_TRUE(absl::is_trivially_copy_assignable<std::string*>::value);
-  EXPECT_TRUE(absl::is_trivially_copy_assignable<Trivial*>::value);
-  EXPECT_TRUE(absl::is_trivially_copy_assignable<const std::string*>::value);
-  EXPECT_TRUE(absl::is_trivially_copy_assignable<const Trivial*>::value);
-  EXPECT_TRUE(absl::is_trivially_copy_assignable<std::string**>::value);
-  EXPECT_TRUE(absl::is_trivially_copy_assignable<Trivial**>::value);
-
-  // const qualified types are not assignable
-  EXPECT_FALSE(absl::is_trivially_copy_assignable<const int>::value);
-
-  // types with compiler generated copy assignment
-  EXPECT_TRUE(absl::is_trivially_copy_assignable<Trivial>::value);
-  EXPECT_TRUE(absl::is_trivially_copy_assignable<TrivialCopyAssign>::value);
-
-  // Verify that types without them (i.e. nontrivial or deleted) are not.
-  EXPECT_FALSE(absl::is_trivially_copy_assignable<NontrivialCopyAssign>::value);
-  EXPECT_FALSE(absl::is_trivially_copy_assignable<DeletedCopyAssign>::value);
-  EXPECT_FALSE(absl::is_trivially_copy_assignable<MovableNonCopyable>::value);
-  EXPECT_FALSE(absl::is_trivially_copy_assignable<NonCopyableOrMovable>::value);
-
-  // types with vtables
-  EXPECT_FALSE(absl::is_trivially_copy_assignable<Base>::value);
-
-  // Verify that simple_pair is trivially assignable
-  EXPECT_TRUE(
-      (absl::is_trivially_copy_assignable<simple_pair<int, char*>>::value));
-  EXPECT_TRUE(
-      (absl::is_trivially_copy_assignable<simple_pair<int, Trivial>>::value));
-  EXPECT_TRUE((absl::is_trivially_copy_assignable<
-               simple_pair<int, TrivialCopyAssign>>::value));
-
-  // Verify that types not trivially copy assignable are
-  // correctly marked as such.
-  EXPECT_FALSE(absl::is_trivially_copy_assignable<std::string>::value);
-  EXPECT_FALSE(absl::is_trivially_copy_assignable<std::vector<int>>::value);
-
-  // Verify that simple_pairs of types not trivially copy assignable
-  // are not marked as trivial.
-  EXPECT_FALSE((absl::is_trivially_copy_assignable<
-                simple_pair<int, std::string>>::value));
-  EXPECT_FALSE((absl::is_trivially_copy_assignable<
-                simple_pair<std::string, int>>::value));
-
-  // Verify that arrays are not trivially copy assignable
-  using int10 = int[10];
-  EXPECT_FALSE(absl::is_trivially_copy_assignable<int10>::value);
-
-  // Verify that references are handled correctly
-  EXPECT_TRUE(absl::is_trivially_copy_assignable<Trivial&&>::value);
-  EXPECT_TRUE(absl::is_trivially_copy_assignable<Trivial&>::value);
-}
-
-TEST(TypeTraitsTest, TestTriviallyCopyable) {
-  // Verify that arithmetic types and pointers are trivially copyable.
-  EXPECT_TRUE(absl::type_traits_internal::is_trivially_copyable<bool>::value);
-  EXPECT_TRUE(absl::type_traits_internal::is_trivially_copyable<char>::value);
-  EXPECT_TRUE(
-      absl::type_traits_internal::is_trivially_copyable<unsigned char>::value);
-  EXPECT_TRUE(
-      absl::type_traits_internal::is_trivially_copyable<signed char>::value);
-  EXPECT_TRUE(
-      absl::type_traits_internal::is_trivially_copyable<wchar_t>::value);
-  EXPECT_TRUE(absl::type_traits_internal::is_trivially_copyable<int>::value);
-  EXPECT_TRUE(
-      absl::type_traits_internal::is_trivially_copyable<unsigned int>::value);
-  EXPECT_TRUE(
-      absl::type_traits_internal::is_trivially_copyable<int16_t>::value);
-  EXPECT_TRUE(
-      absl::type_traits_internal::is_trivially_copyable<uint16_t>::value);
-  EXPECT_TRUE(
-      absl::type_traits_internal::is_trivially_copyable<int64_t>::value);
-  EXPECT_TRUE(
-      absl::type_traits_internal::is_trivially_copyable<uint64_t>::value);
-  EXPECT_TRUE(absl::type_traits_internal::is_trivially_copyable<float>::value);
-  EXPECT_TRUE(absl::type_traits_internal::is_trivially_copyable<double>::value);
-  EXPECT_TRUE(
-      absl::type_traits_internal::is_trivially_copyable<long double>::value);
-  EXPECT_TRUE(
-      absl::type_traits_internal::is_trivially_copyable<std::string*>::value);
-  EXPECT_TRUE(
-      absl::type_traits_internal::is_trivially_copyable<Trivial*>::value);
-  EXPECT_TRUE(absl::type_traits_internal::is_trivially_copyable<
-              const std::string*>::value);
-  EXPECT_TRUE(
-      absl::type_traits_internal::is_trivially_copyable<const Trivial*>::value);
-  EXPECT_TRUE(
-      absl::type_traits_internal::is_trivially_copyable<std::string**>::value);
-  EXPECT_TRUE(
-      absl::type_traits_internal::is_trivially_copyable<Trivial**>::value);
-
-  // const qualified types are not assignable but are constructible
-  EXPECT_TRUE(
-      absl::type_traits_internal::is_trivially_copyable<const int>::value);
+TEST(TypeTraitsTest, TestTypeIdentity) {
+  EXPECT_TRUE((std::is_same_v<typename absl::type_identity<int>::type, int>));
+  EXPECT_TRUE((std::is_same_v<absl::type_identity_t<int>, int>));
+  EXPECT_TRUE((std::is_same_v<typename absl::type_identity<int&>::type, int&>));
+  EXPECT_TRUE((std::is_same_v<absl::type_identity_t<int&>, int&>));
 
-  // Trivial copy constructor/assignment and destructor.
-  EXPECT_TRUE(
-      absl::type_traits_internal::is_trivially_copyable<Trivial>::value);
-  // Trivial copy assignment, but non-trivial copy constructor/destructor.
-  EXPECT_FALSE(absl::type_traits_internal::is_trivially_copyable<
-               TrivialCopyAssign>::value);
-  // Trivial copy constructor, but non-trivial assignment.
-  EXPECT_FALSE(absl::type_traits_internal::is_trivially_copyable<
-               TrivialCopyCtor>::value);
-
-  // Types with a non-trivial copy constructor/assignment
-  EXPECT_FALSE(absl::type_traits_internal::is_trivially_copyable<
-               NontrivialCopyCtor>::value);
-  EXPECT_FALSE(absl::type_traits_internal::is_trivially_copyable<
-               NontrivialCopyAssign>::value);
-
-  // Types without copy constructor/assignment, but with move
-  // MSVC disagrees with other compilers about this:
-  // EXPECT_TRUE(absl::type_traits_internal::is_trivially_copyable<
-  //             MovableNonCopyable>::value);
-
-  // Types without copy/move constructor/assignment
-  EXPECT_FALSE(absl::type_traits_internal::is_trivially_copyable<
-               NonCopyableOrMovable>::value);
-
-  // No copy assign, but has trivial copy constructor.
-  EXPECT_TRUE(absl::type_traits_internal::is_trivially_copyable<
-              DeletedCopyAssign>::value);
-
-  // types with vtables
-  EXPECT_FALSE(absl::type_traits_internal::is_trivially_copyable<Base>::value);
-
-  // Verify that simple_pair is trivially copyable if members are
-  EXPECT_TRUE((absl::type_traits_internal::is_trivially_copyable<
-               simple_pair<int, char*>>::value));
-  EXPECT_TRUE((absl::type_traits_internal::is_trivially_copyable<
-               simple_pair<int, Trivial>>::value));
-
-  // Verify that types not trivially copyable are
-  // correctly marked as such.
-  EXPECT_FALSE(
-      absl::type_traits_internal::is_trivially_copyable<std::string>::value);
-  EXPECT_FALSE(absl::type_traits_internal::is_trivially_copyable<
-               std::vector<int>>::value);
-
-  // Verify that simple_pairs of types not trivially copyable
-  // are not marked as trivial.
-  EXPECT_FALSE((absl::type_traits_internal::is_trivially_copyable<
-                simple_pair<int, std::string>>::value));
-  EXPECT_FALSE((absl::type_traits_internal::is_trivially_copyable<
-                simple_pair<std::string, int>>::value));
-  EXPECT_FALSE((absl::type_traits_internal::is_trivially_copyable<
-                simple_pair<int, TrivialCopyAssign>>::value));
-
-  // Verify that arrays of trivially copyable types are trivially copyable
-  using int10 = int[10];
-  EXPECT_TRUE(absl::type_traits_internal::is_trivially_copyable<int10>::value);
-  using int10x10 = int[10][10];
-  EXPECT_TRUE(
-      absl::type_traits_internal::is_trivially_copyable<int10x10>::value);
-
-  // Verify that references are handled correctly
   EXPECT_FALSE(
-      absl::type_traits_internal::is_trivially_copyable<Trivial&&>::value);
-  EXPECT_FALSE(
-      absl::type_traits_internal::is_trivially_copyable<Trivial&>::value);
-}
-
-#define ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(trait_name, ...)          \
-  EXPECT_TRUE((std::is_same<typename std::trait_name<__VA_ARGS__>::type, \
-                            absl::trait_name##_t<__VA_ARGS__>>::value))
-
-TEST(TypeTraitsTest, TestRemoveCVAliases) {
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(remove_cv, int);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(remove_cv, const int);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(remove_cv, volatile int);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(remove_cv, const volatile int);
-
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(remove_const, int);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(remove_const, const int);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(remove_const, volatile int);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(remove_const, const volatile int);
-
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(remove_volatile, int);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(remove_volatile, const int);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(remove_volatile, volatile int);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(remove_volatile, const volatile int);
-}
-
-TEST(TypeTraitsTest, TestAddCVAliases) {
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(add_cv, int);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(add_cv, const int);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(add_cv, volatile int);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(add_cv, const volatile int);
-
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(add_const, int);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(add_const, const int);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(add_const, volatile int);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(add_const, const volatile int);
-
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(add_volatile, int);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(add_volatile, const int);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(add_volatile, volatile int);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(add_volatile, const volatile int);
-}
-
-TEST(TypeTraitsTest, TestReferenceAliases) {
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(remove_reference, int);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(remove_reference, volatile int);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(remove_reference, int&);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(remove_reference, volatile int&);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(remove_reference, int&&);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(remove_reference, volatile int&&);
-
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(add_lvalue_reference, int);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(add_lvalue_reference, volatile int);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(add_lvalue_reference, int&);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(add_lvalue_reference, volatile int&);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(add_lvalue_reference, int&&);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(add_lvalue_reference, volatile int&&);
-
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(add_rvalue_reference, int);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(add_rvalue_reference, volatile int);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(add_rvalue_reference, int&);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(add_rvalue_reference, volatile int&);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(add_rvalue_reference, int&&);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(add_rvalue_reference, volatile int&&);
-}
-
-TEST(TypeTraitsTest, TestPointerAliases) {
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(remove_pointer, int*);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(remove_pointer, volatile int*);
-
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(add_pointer, int);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(add_pointer, volatile int);
-}
-
-TEST(TypeTraitsTest, TestSignednessAliases) {
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(make_signed, int);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(make_signed, volatile int);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(make_signed, unsigned);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(make_signed, volatile unsigned);
-
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(make_unsigned, int);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(make_unsigned, volatile int);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(make_unsigned, unsigned);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(make_unsigned, volatile unsigned);
-}
-
-TEST(TypeTraitsTest, TestExtentAliases) {
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(remove_extent, int[]);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(remove_extent, int[1]);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(remove_extent, int[1][1]);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(remove_extent, int[][1]);
-
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(remove_all_extents, int[]);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(remove_all_extents, int[1]);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(remove_all_extents, int[1][1]);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(remove_all_extents, int[][1]);
-}
-
-TEST(TypeTraitsTest, TestAlignedStorageAlias) {
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 1);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 2);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 3);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 4);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 5);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 6);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 7);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 8);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 9);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 10);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 11);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 12);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 13);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 14);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 15);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 16);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 17);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 18);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 19);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 20);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 21);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 22);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 23);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 24);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 25);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 26);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 27);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 28);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 29);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 30);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 31);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 32);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 33);
-
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 1, 128);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 2, 128);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 3, 128);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 4, 128);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 5, 128);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 6, 128);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 7, 128);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 8, 128);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 9, 128);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 10, 128);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 11, 128);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 12, 128);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 13, 128);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 14, 128);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 15, 128);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 16, 128);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 17, 128);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 18, 128);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 19, 128);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 20, 128);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 21, 128);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 22, 128);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 23, 128);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 24, 128);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 25, 128);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 26, 128);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 27, 128);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 28, 128);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 29, 128);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 30, 128);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 31, 128);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 32, 128);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(aligned_storage, 33, 128);
-}
-
-TEST(TypeTraitsTest, TestDecay) {
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(decay, int);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(decay, const int);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(decay, volatile int);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(decay, const volatile int);
-
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(decay, int&);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(decay, const int&);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(decay, volatile int&);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(decay, const volatile int&);
-
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(decay, int&);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(decay, const int&);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(decay, volatile int&);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(decay, const volatile int&);
-
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(decay, int[1]);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(decay, int[1][1]);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(decay, int[][1]);
-
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(decay, int());
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(decay, int(float));  // NOLINT
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(decay, int(char, ...));  // NOLINT
+      (std::is_same_v<typename absl::type_identity<int64_t>::type, int32_t>));
+  EXPECT_FALSE((std::is_same_v<absl::type_identity_t<int64_t>, int32_t>));
 }
 
 struct TypeA {};
@@ -1162,20 +164,17 @@ struct Wrap {};
 enum class TypeEnum { A, B, C, D };
 
 struct GetTypeT {
-  template <typename T,
-            absl::enable_if_t<std::is_same<T, TypeA>::value, int> = 0>
+  template <typename T, std::enable_if_t<std::is_same_v<T, TypeA>, int> = 0>
   TypeEnum operator()(Wrap<T>) const {
     return TypeEnum::A;
   }
 
-  template <typename T,
-            absl::enable_if_t<std::is_same<T, TypeB>::value, int> = 0>
+  template <typename T, std::enable_if_t<std::is_same_v<T, TypeB>, int> = 0>
   TypeEnum operator()(Wrap<T>) const {
     return TypeEnum::B;
   }
 
-  template <typename T,
-            absl::enable_if_t<std::is_same<T, TypeC>::value, int> = 0>
+  template <typename T, std::enable_if_t<std::is_same_v<T, TypeC>, int> = 0>
   TypeEnum operator()(Wrap<T>) const {
     return TypeEnum::C;
   }
@@ -1183,39 +182,9 @@ struct GetTypeT {
   // NOTE: TypeD is intentionally not handled
 } constexpr GetType = {};
 
-TEST(TypeTraitsTest, TestEnableIf) {
-  EXPECT_EQ(TypeEnum::A, GetType(Wrap<TypeA>()));
-  EXPECT_EQ(TypeEnum::B, GetType(Wrap<TypeB>()));
-  EXPECT_EQ(TypeEnum::C, GetType(Wrap<TypeC>()));
-}
-
-TEST(TypeTraitsTest, TestConditional) {
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(conditional, true, int, char);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(conditional, false, int, char);
-}
-
-// TODO(calabrese) Check with specialized std::common_type
-TEST(TypeTraitsTest, TestCommonType) {
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(common_type, int);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(common_type, int, char);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(common_type, int, char, int);
-
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(common_type, int&);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(common_type, int, char&);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(common_type, int, char, int&);
-}
-
-TEST(TypeTraitsTest, TestUnderlyingType) {
-  enum class enum_char : char {};
-  enum class enum_long_long : long long {};  // NOLINT(runtime/int)
-
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(underlying_type, enum_char);
-  ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(underlying_type, enum_long_long);
-}
-
 struct GetTypeExtT {
   template <typename T>
-  absl::result_of_t<const GetTypeT&(T)> operator()(T&& arg) const {
+  std::invoke_result_t<const GetTypeT&, T> operator()(T&& arg) const {
     return GetType(std::forward<T>(arg));
   }
 
@@ -1229,86 +198,9 @@ TEST(TypeTraitsTest, TestResultOf) {
   EXPECT_EQ(TypeEnum::D, GetTypeExt(Wrap<TypeD>()));
 }
 
-template <typename T>
-bool TestCopyAssign() {
-  return absl::is_copy_assignable<T>::value ==
-         std::is_copy_assignable<T>::value;
-}
-
-TEST(TypeTraitsTest, IsCopyAssignable) {
-  EXPECT_TRUE(TestCopyAssign<int>());
-  EXPECT_TRUE(TestCopyAssign<int&>());
-  EXPECT_TRUE(TestCopyAssign<int&&>());
-
-  struct S {};
-  EXPECT_TRUE(TestCopyAssign<S>());
-  EXPECT_TRUE(TestCopyAssign<S&>());
-  EXPECT_TRUE(TestCopyAssign<S&&>());
-
-  class C {
-   public:
-    explicit C(C* c) : c_(c) {}
-    ~C() { delete c_; }
-
-   private:
-    C* c_;
-  };
-  EXPECT_TRUE(TestCopyAssign<C>());
-  EXPECT_TRUE(TestCopyAssign<C&>());
-  EXPECT_TRUE(TestCopyAssign<C&&>());
-
-  // Reason for ifndef: add_lvalue_reference<T> in libc++ breaks for these cases
-#ifndef _LIBCPP_VERSION
-  EXPECT_TRUE(TestCopyAssign<int()>());
-  EXPECT_TRUE(TestCopyAssign<int(int) const>());
-  EXPECT_TRUE(TestCopyAssign<int(...) volatile&>());
-  EXPECT_TRUE(TestCopyAssign<int(int, ...) const volatile&&>());
-#endif  // _LIBCPP_VERSION
-}
-
-template <typename T>
-bool TestMoveAssign() {
-  return absl::is_move_assignable<T>::value ==
-         std::is_move_assignable<T>::value;
-}
-
-TEST(TypeTraitsTest, IsMoveAssignable) {
-  EXPECT_TRUE(TestMoveAssign<int>());
-  EXPECT_TRUE(TestMoveAssign<int&>());
-  EXPECT_TRUE(TestMoveAssign<int&&>());
-
-  struct S {};
-  EXPECT_TRUE(TestMoveAssign<S>());
-  EXPECT_TRUE(TestMoveAssign<S&>());
-  EXPECT_TRUE(TestMoveAssign<S&&>());
-
-  class C {
-   public:
-    explicit C(C* c) : c_(c) {}
-    ~C() { delete c_; }
-    void operator=(const C&) = delete;
-    void operator=(C&&) = delete;
-
-   private:
-    C* c_;
-  };
-  EXPECT_TRUE(TestMoveAssign<C>());
-  EXPECT_TRUE(TestMoveAssign<C&>());
-  EXPECT_TRUE(TestMoveAssign<C&&>());
-
-  // Reason for ifndef: add_lvalue_reference<T> in libc++ breaks for these cases
-#ifndef _LIBCPP_VERSION
-  EXPECT_TRUE(TestMoveAssign<int()>());
-  EXPECT_TRUE(TestMoveAssign<int(int) const>());
-  EXPECT_TRUE(TestMoveAssign<int(...) volatile&>());
-  EXPECT_TRUE(TestMoveAssign<int(int, ...) const volatile&&>());
-#endif  // _LIBCPP_VERSION
-}
-
 namespace adl_namespace {
 
-struct DeletedSwap {
-};
+struct DeletedSwap {};
 
 void swap(DeletedSwap&, DeletedSwap&) = delete;
 
@@ -1324,7 +216,6 @@ void swap(SpecialNoexceptSwap&, SpecialNoexceptSwap&) noexcept {}
 
 TEST(TypeTraitsTest, IsSwappable) {
   using absl::type_traits_internal::IsSwappable;
-  using absl::type_traits_internal::StdSwapIsUnconstrained;
 
   EXPECT_TRUE(IsSwappable<int>::value);
 
@@ -1337,14 +228,14 @@ TEST(TypeTraitsTest, IsSwappable) {
     ~NoConstruct() = default;
   };
 
-  EXPECT_EQ(IsSwappable<NoConstruct>::value, StdSwapIsUnconstrained::value);
+  EXPECT_FALSE(IsSwappable<NoConstruct>::value);
   struct NoAssign {
     NoAssign(NoAssign&&) {}
     NoAssign& operator=(NoAssign&&) = delete;
     ~NoAssign() = default;
   };
 
-  EXPECT_EQ(IsSwappable<NoAssign>::value, StdSwapIsUnconstrained::value);
+  EXPECT_FALSE(IsSwappable<NoAssign>::value);
 
   EXPECT_FALSE(IsSwappable<adl_namespace::DeletedSwap>::value);
 
@@ -1353,7 +244,6 @@ TEST(TypeTraitsTest, IsSwappable) {
 
 TEST(TypeTraitsTest, IsNothrowSwappable) {
   using absl::type_traits_internal::IsNothrowSwappable;
-  using absl::type_traits_internal::StdSwapIsUnconstrained;
 
   EXPECT_TRUE(IsNothrowSwappable<int>::value);
 
@@ -1386,4 +276,201 @@ TEST(TypeTraitsTest, IsNothrowSwappable) {
   EXPECT_TRUE(IsNothrowSwappable<adl_namespace::SpecialNoexceptSwap>::value);
 }
 
+TEST(TriviallyRelocatable, PrimitiveTypes) {
+  static_assert(absl::is_trivially_relocatable<int>::value);
+  static_assert(absl::is_trivially_relocatable<char>::value);
+  static_assert(absl::is_trivially_relocatable<void*>::value);
+}
+
+// User-defined types can be trivially relocatable as long as they don't have a
+// user-provided move constructor or destructor.
+TEST(TriviallyRelocatable, UserDefinedTriviallyRelocatable) {
+  struct S {
+    int x;
+    int y;
+  };
+
+  static_assert(absl::is_trivially_relocatable<S>::value);
+}
+
+// A user-provided move constructor disqualifies a type from being trivially
+// relocatable.
+TEST(TriviallyRelocatable, UserProvidedMoveConstructor) {
+  struct S {
+    S(S&&) {}  // NOLINT(modernize-use-equals-default)
+  };
+
+  static_assert(!absl::is_trivially_relocatable<S>::value);
+}
+
+// A user-provided copy constructor disqualifies a type from being trivially
+// relocatable.
+TEST(TriviallyRelocatable, UserProvidedCopyConstructor) {
+  struct S {
+    S(const S&) {}  // NOLINT(modernize-use-equals-default)
+  };
+
+  static_assert(!absl::is_trivially_relocatable<S>::value);
+}
+
+// A user-provided copy assignment operator disqualifies a type from
+// being trivially relocatable.
+TEST(TriviallyRelocatable, UserProvidedCopyAssignment) {
+  struct S {
+    S(const S&) = default;
+    S& operator=(const S&) {  // NOLINT(modernize-use-equals-default)
+      return *this;
+    }
+  };
+
+  static_assert(!absl::is_trivially_relocatable<S>::value);
+}
+
+// A user-provided move assignment operator disqualifies a type from
+// being trivially relocatable.
+TEST(TriviallyRelocatable, UserProvidedMoveAssignment) {
+  struct S {
+    S(S&&) = default;
+    S& operator=(S&&) { return *this; }  // NOLINT(modernize-use-equals-default)
+  };
+
+  static_assert(!absl::is_trivially_relocatable<S>::value);
+}
+
+// A user-provided destructor disqualifies a type from being trivially
+// relocatable.
+TEST(TriviallyRelocatable, UserProvidedDestructor) {
+  struct S {
+    ~S() {}  // NOLINT(modernize-use-equals-default)
+  };
+
+  static_assert(!absl::is_trivially_relocatable<S>::value);
+}
+
+TEST(Ownership, References) {
+  static_assert(IsViewAndNotOwner<std::string&>::value,
+                "std::string& is a view, not an owner");
+  static_assert(IsViewAndNotOwner<int&>::value, "int& is a view, not an owner");
+
+  static_assert(IsViewAndNotOwner<std::string&&>::value,
+                "std::string&& is a view, not an owner");
+  static_assert(IsViewAndNotOwner<int&&>::value,
+                "int&& is a view, not an owner");
+
+  static_assert(IsViewAndNotOwner<std::reference_wrapper<std::string>>::value,
+                "std::reference_wrapper<std::string> is a view, not an owner");
+  static_assert(IsViewAndNotOwner<std::reference_wrapper<int>>::value,
+                "std::reference_wrapper<int> is a view, not an owner");
+}
+
+TEST(Ownership, CVQualifiers) {
+  static_assert(IsOwnerAndNotView<const std::string>::value,
+                "const std::string is an owner, not a view");
+  static_assert(IsViewAndNotOwner<const std::string_view>::value,
+                "const std::string_view is a view, not an owner");
+
+  static_assert(IsOwnerAndNotView<volatile std::string>::value,
+                "volatile std::string is an owner, not a view");
+  static_assert(IsViewAndNotOwner<volatile std::string_view>::value,
+                "volatile std::string_view is a view, not an owner");
+
+  static_assert(IsOwnerAndNotView<const volatile std::string>::value,
+                "const volatile std::string is an owner, not a view");
+  static_assert(IsViewAndNotOwner<const volatile std::string_view>::value,
+                "const volatile std::string_view is a view, not an owner");
+}
+
+TEST(Ownership, Array) {
+  static_assert(!IsOwnerAndNotView<std::array<std::string, 0>>::value,
+                "empty array is not an owner");
+  static_assert(!IsViewAndNotOwner<std::array<std::string, 0>>::value,
+                "empty array is not a view");
+
+  static_assert(!IsOwnerAndNotView<std::array<std::string_view, 0>>::value,
+                "empty array is not an owner");
+  static_assert(!IsViewAndNotOwner<std::array<std::string_view, 0>>::value,
+                "empty array is not a view");
+
+  static_assert(IsOwnerAndNotView<std::array<std::string, 5>>::value,
+                "array of owners is an owner");
+  static_assert(IsViewAndNotOwner<std::array<std::string_view, 5>>::value,
+                "array of views is a view");
+}
+
+TEST(Ownership, Variant) {
+  static_assert(!IsOwnerAndNotView<std::variant<>>::value,
+                "empty variant is not an owner");
+  static_assert(!IsViewAndNotOwner<std::variant<>>::value,
+                "empty variant is not a view");
+
+  static_assert(IsOwnerAndNotView<std::variant<std::string, std::vector<char>,
+                                               std::vector<int>>>::value,
+                "aggregate of owners is an owner");
+  static_assert(IsViewAndNotOwner<
+                    std::variant<std::wstring_view, std::string_view>>::value,
+                "aggregate of views is a view");
+
+  static_assert(
+      !IsOwnerAndNotView<std::variant<const char*, std::string>>::value,
+      "variant of mixed-ownership types is not considered an owner");
+  static_assert(
+      !IsViewAndNotOwner<std::variant<const char*, std::string>>::value,
+      "variant of mixed-ownership types is not considered a view");
+
+  static_assert(
+      IsViewAndNotOwner<std::variant<std::reference_wrapper<const int>>>::value,
+      "variant of reference is considered a view");
+}
+
+TEST(Ownership, Tuple) {
+  static_assert(!IsOwnerAndNotView<std::tuple<>>::value,
+                "empty tuple is not an owner");
+  static_assert(!IsViewAndNotOwner<std::tuple<>>::value,
+                "empty tuple is not a view");
+
+  static_assert(
+      IsOwnerAndNotView<
+          std::tuple<std::string, std::vector<char>, std::vector<int>>>::value,
+      "aggregate of owners is an owner");
+  static_assert(
+      IsViewAndNotOwner<std::tuple<std::wstring_view, std::string_view>>::value,
+      "aggregate of views is a view");
+
+  static_assert(!IsOwnerAndNotView<std::tuple<const char*, std::string>>::value,
+                "tuple of mixed-ownership types is not considered an owner");
+  static_assert(!IsViewAndNotOwner<std::tuple<const char*, std::string>>::value,
+                "tuple of mixed-ownership types is not considered a view");
+
+  static_assert(IsViewAndNotOwner<std::tuple<const int&>>::value,
+                "tuple of reference is considered a view");
+}
+
+#ifdef ABSL_HAVE_CONSTANT_EVALUATED
+
+constexpr int64_t NegateIfConstantEvaluated(int64_t i) {
+  if (absl::is_constant_evaluated()) {
+    return -i;
+  } else {
+    return i;
+  }
+}
+
+#endif  // ABSL_HAVE_CONSTANT_EVALUATED
+
+TEST(IsConstantEvaluated, is_constant_evaluated) {
+#ifdef ABSL_HAVE_CONSTANT_EVALUATED
+  constexpr int64_t constant = NegateIfConstantEvaluated(42);
+  EXPECT_EQ(constant, -42);
+
+  int64_t now = absl::ToUnixSeconds(absl::Now());
+  int64_t not_constant = NegateIfConstantEvaluated(now);
+  EXPECT_EQ(not_constant, now);
+
+  static int64_t const_init = NegateIfConstantEvaluated(42);
+  EXPECT_EQ(const_init, -42);
+#else
+  GTEST_SKIP() << "absl::is_constant_evaluated is not defined";
+#endif  // ABSL_HAVE_CONSTANT_EVALUATED
+}
+
 }  // namespace
diff --git a/absl/numeric/BUILD.bazel b/absl/numeric/BUILD.bazel
index e09e52d21fe..d61606c0f5d 100644
--- a/absl/numeric/BUILD.bazel
+++ b/absl/numeric/BUILD.bazel
@@ -12,7 +12,9 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-load("@rules_cc//cc:defs.bzl", "cc_library", "cc_test")
+load("@rules_cc//cc:cc_binary.bzl", "cc_binary")
+load("@rules_cc//cc:cc_library.bzl", "cc_library")
+load("@rules_cc//cc:cc_test.bzl", "cc_test")
 load(
     "//absl:copts/configure_copts.bzl",
     "ABSL_DEFAULT_COPTS",
@@ -20,9 +22,62 @@ load(
     "ABSL_TEST_COPTS",
 )
 
-package(default_visibility = ["//visibility:public"])
+package(
+    default_visibility = ["//visibility:private"],
+    features = [
+        "header_modules",
+        "layering_check",
+        "parse_headers",
+    ],
+)
+
+licenses(["notice"])
+
+cc_library(
+    name = "bits",
+    hdrs = [
+        "bits.h",
+        "internal/bits.h",
+    ],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
+    deps = [
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:endian",
+    ],
+)
+
+cc_binary(
+    name = "bits_benchmark",
+    testonly = True,
+    srcs = ["bits_benchmark.cc"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":bits",
+        "//absl/base:core_headers",
+        "//absl/random",
+        "@google_benchmark//:benchmark_main",
+    ],
+)
 
-licenses(["notice"])  # Apache 2.0
+cc_test(
+    name = "bits_test",
+    size = "small",
+    srcs = [
+        "bits_test.cc",
+    ],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":bits",
+        "//absl/random",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
 
 cc_library(
     name = "int128",
@@ -34,9 +89,12 @@ cc_library(
     hdrs = ["int128.h"],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
     deps = [
+        ":bits",
         "//absl/base:config",
         "//absl/base:core_headers",
+        "//absl/types:compare",
     ],
 )
 
@@ -52,15 +110,18 @@ cc_test(
     deps = [
         ":int128",
         "//absl/base",
-        "//absl/base:core_headers",
         "//absl/hash:hash_testing",
         "//absl/meta:type_traits",
-        "@com_google_googletest//:gtest_main",
+        "//absl/strings",
+        "//absl/types:compare",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
-cc_test(
+cc_binary(
     name = "int128_benchmark",
+    testonly = True,
     srcs = ["int128_benchmark.cc"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
@@ -68,6 +129,20 @@ cc_test(
     deps = [
         ":int128",
         "//absl/base:config",
-        "@com_github_google_benchmark//:benchmark_main",
+        "//absl/random",
+        "@google_benchmark//:benchmark_main",
+    ],
+)
+
+cc_library(
+    name = "representation",
+    hdrs = [
+        "internal/representation.h",
+    ],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//absl:__subpackages__"],
+    deps = [
+        "//absl/base:config",
     ],
 )
diff --git a/absl/numeric/CMakeLists.txt b/absl/numeric/CMakeLists.txt
index 242889f088a..68446b012b5 100644
--- a/absl/numeric/CMakeLists.txt
+++ b/absl/numeric/CMakeLists.txt
@@ -14,6 +14,35 @@
 # limitations under the License.
 #
 
+absl_cc_library(
+  NAME
+    bits
+  HDRS
+    "bits.h"
+    "internal/bits.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::config
+    absl::core_headers
+    absl::endian
+  PUBLIC
+)
+
+absl_cc_test(
+  NAME
+    bits_test
+  SRCS
+    "bits_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::bits
+    absl::core_headers
+    absl::random_random
+    GTest::gmock_main
+)
+
 absl_cc_library(
   NAME
     int128
@@ -26,8 +55,10 @@ absl_cc_library(
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
+    absl::compare
     absl::config
     absl::core_headers
+    absl::bits
   PUBLIC
 )
 
@@ -42,10 +73,11 @@ absl_cc_test(
   DEPS
     absl::int128
     absl::base
-    absl::core_headers
+    absl::compare
     absl::hash_testing
     absl::type_traits
-    gmock_main
+    absl::strings
+    GTest::gmock_main
 )
 
 # component target
@@ -58,3 +90,15 @@ absl_cc_library(
     absl::int128
   PUBLIC
 )
+
+absl_cc_library(
+  NAME
+    numeric_representation
+  HDRS
+    "internal/representation.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::config
+  PUBLIC
+)
diff --git a/absl/numeric/bits.h b/absl/numeric/bits.h
new file mode 100644
index 00000000000..8eca3927ffd
--- /dev/null
+++ b/absl/numeric/bits.h
@@ -0,0 +1,254 @@
+// Copyright 2020 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: bits.h
+// -----------------------------------------------------------------------------
+//
+// This file contains implementations of C++20's bitwise math functions, as
+// defined by:
+//
+// P0553R4:
+//  http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p0553r4.html
+// P0556R3:
+//  http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0556r3.html
+// P1355R2:
+//  http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1355r2.html
+// P1956R1:
+//  http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p1956r1.pdf
+// P0463R1
+//  https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0463r1.html
+// P1272R4
+//  https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p1272r4.html
+//
+// When using a standard library that implements these functions, we use the
+// standard library's implementation.
+
+#ifndef ABSL_NUMERIC_BITS_H_
+#define ABSL_NUMERIC_BITS_H_
+
+#include <cstdint>
+#include <limits>
+#include <type_traits>
+#include <version>
+
+#include "absl/base/config.h"
+
+#if ABSL_INTERNAL_CPLUSPLUS_LANG >= 202002L
+#include <bit>
+#endif
+
+#include "absl/base/attributes.h"
+#include "absl/base/internal/endian.h"
+#include "absl/numeric/internal/bits.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+// https://github.com/llvm/llvm-project/issues/64544
+// libc++ had the wrong signature for std::rotl and std::rotr
+// prior to libc++ 18.0.
+//
+#if (defined(__cpp_lib_bitops) && __cpp_lib_bitops >= 201907L) &&     \
+    (!defined(_LIBCPP_VERSION) || _LIBCPP_VERSION >= 180000)
+using std::rotl;
+using std::rotr;
+
+#else
+
+// Rotating functions
+template <class T>
+[[nodiscard]] constexpr std::enable_if_t<std::is_unsigned_v<T>, T> rotl(
+    T x, int s) noexcept {
+  return numeric_internal::RotateLeft(x, s);
+}
+
+template <class T>
+[[nodiscard]] constexpr std::enable_if_t<std::is_unsigned_v<T>, T> rotr(
+    T x, int s) noexcept {
+  return numeric_internal::RotateRight(x, s);
+}
+
+#endif
+
+// https://github.com/llvm/llvm-project/issues/64544
+// libc++ had the wrong signature for std::rotl and std::rotr
+// prior to libc++ 18.0.
+//
+#if (defined(__cpp_lib_bitops) && __cpp_lib_bitops >= 201907L)
+
+using std::countl_one;
+using std::countl_zero;
+using std::countr_one;
+using std::countr_zero;
+using std::popcount;
+
+#else
+
+// Counting functions
+//
+// While these functions are typically constexpr, on some platforms, they may
+// not be marked as constexpr due to constraints of the compiler/available
+// intrinsics.
+template <class T>
+ABSL_INTERNAL_CONSTEXPR_CLZ inline std::enable_if_t<std::is_unsigned_v<T>, int>
+countl_zero(T x) noexcept {
+  return numeric_internal::CountLeadingZeroes(x);
+}
+
+template <class T>
+ABSL_INTERNAL_CONSTEXPR_CLZ inline std::enable_if_t<std::is_unsigned_v<T>, int>
+countl_one(T x) noexcept {
+  // Avoid integer promotion to a wider type
+  return countl_zero(static_cast<T>(~x));
+}
+
+template <class T>
+ABSL_INTERNAL_CONSTEXPR_CTZ inline std::enable_if_t<std::is_unsigned_v<T>, int>
+countr_zero(T x) noexcept {
+  return numeric_internal::CountTrailingZeroes(x);
+}
+
+template <class T>
+ABSL_INTERNAL_CONSTEXPR_CTZ inline std::enable_if_t<std::is_unsigned_v<T>, int>
+countr_one(T x) noexcept {
+  // Avoid integer promotion to a wider type
+  return countr_zero(static_cast<T>(~x));
+}
+
+template <class T>
+ABSL_INTERNAL_CONSTEXPR_POPCOUNT inline std::enable_if_t<std::is_unsigned_v<T>,
+                                                         int>
+popcount(T x) noexcept {
+  return numeric_internal::Popcount(x);
+}
+
+#endif
+
+#if (defined(__cpp_lib_int_pow2) && __cpp_lib_int_pow2 >= 202002L)
+
+using std::bit_ceil;
+using std::bit_floor;
+using std::bit_width;
+using std::has_single_bit;
+
+#else
+
+// Returns: true if x is an integral power of two; false otherwise.
+template <class T>
+constexpr inline std::enable_if_t<std::is_unsigned_v<T>, bool> has_single_bit(
+    T x) noexcept {
+  return x != 0 && (x & (x - 1)) == 0;
+}
+
+// Returns: If x == 0, 0; otherwise one plus the base-2 logarithm of x, with any
+// fractional part discarded.
+template <class T>
+ABSL_INTERNAL_CONSTEXPR_CLZ inline std::enable_if_t<std::is_unsigned_v<T>, int>
+bit_width(T x) noexcept {
+  return std::numeric_limits<T>::digits - countl_zero(x);
+}
+
+// Returns: If x == 0, 0; otherwise the maximal value y such that
+// has_single_bit(y) is true and y <= x.
+template <class T>
+ABSL_INTERNAL_CONSTEXPR_CLZ inline std::enable_if_t<std::is_unsigned_v<T>, T>
+bit_floor(T x) noexcept {
+  return x == 0 ? 0 : T{1} << (bit_width(x) - 1);
+}
+
+// Returns: N, where N is the smallest power of 2 greater than or equal to x.
+//
+// Preconditions: N is representable as a value of type T.
+template <class T>
+ABSL_INTERNAL_CONSTEXPR_CLZ inline std::enable_if_t<std::is_unsigned_v<T>, T>
+bit_ceil(T x) {
+  // If T is narrower than unsigned, T{1} << bit_width will be promoted.  We
+  // want to force it to wraparound so that bit_ceil of an invalid value are not
+  // core constant expressions.
+  //
+  // BitCeilNonPowerOf2 triggers an overflow in constexpr contexts if we would
+  // undergo promotion to unsigned but not fit the result into T without
+  // truncation.
+  return has_single_bit(x) ? T{1} << (bit_width(x) - 1)
+                           : numeric_internal::BitCeilNonPowerOf2(x);
+}
+
+#endif
+
+#if defined(__cpp_lib_endian) && __cpp_lib_endian >= 201907L
+
+// https://en.cppreference.com/w/cpp/types/endian
+//
+// Indicates the endianness of all scalar types:
+//   * If all scalar types are little-endian, `absl::endian::native` equals
+//     absl::endian::little.
+//   * If all scalar types are big-endian, `absl::endian::native` equals
+//     `absl::endian::big`.
+//   * Platforms that use anything else are unsupported.
+using std::endian;
+
+#else
+
+enum class endian {
+  little,
+  big,
+#if defined(ABSL_IS_LITTLE_ENDIAN)
+  native = little
+#elif defined(ABSL_IS_BIG_ENDIAN)
+  native = big
+#else
+#error "Endian detection needs to be set up for this platform"
+#endif
+};
+
+#endif  // defined(__cpp_lib_endian) && __cpp_lib_endian >= 201907L
+
+#if defined(__cpp_lib_byteswap) && __cpp_lib_byteswap >= 202110L
+
+// https://en.cppreference.com/w/cpp/numeric/byteswap
+//
+// Reverses the bytes in the given integer value `x`.
+//
+// `absl::byteswap` participates in overload resolution only if `T` satisfies
+// integral, i.e., `T` is an integer type. The program is ill-formed if `T` has
+// padding bits.
+using std::byteswap;
+
+#else
+
+template <class T>
+[[nodiscard]] constexpr T byteswap(T x) noexcept {
+  static_assert(std::is_integral_v<T>,
+                "byteswap requires an integral argument");
+  static_assert(
+      sizeof(T) == 1 || sizeof(T) == 2 || sizeof(T) == 4 || sizeof(T) == 8,
+      "byteswap works only with 8, 16, 32, or 64-bit integers");
+  if constexpr (sizeof(T) == 1) {
+    return x;
+  } else if constexpr (sizeof(T) == 2) {
+    return static_cast<T>(gbswap_16(static_cast<uint16_t>(x)));
+  } else if constexpr (sizeof(T) == 4) {
+    return static_cast<T>(gbswap_32(static_cast<uint32_t>(x)));
+  } else if constexpr (sizeof(T) == 8) {
+    return static_cast<T>(gbswap_64(static_cast<uint64_t>(x)));
+  }
+}
+
+#endif  // defined(__cpp_lib_byteswap) && __cpp_lib_byteswap >= 202110L
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_NUMERIC_BITS_H_
diff --git a/absl/numeric/bits_benchmark.cc b/absl/numeric/bits_benchmark.cc
new file mode 100644
index 00000000000..3f8bed08178
--- /dev/null
+++ b/absl/numeric/bits_benchmark.cc
@@ -0,0 +1,75 @@
+// Copyright 2022 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <cstddef>
+#include <cstdint>
+#include <limits>
+#include <vector>
+
+#include "absl/base/optimization.h"
+#include "absl/numeric/bits.h"
+#include "absl/random/random.h"
+#include "benchmark/benchmark.h"
+
+namespace absl {
+namespace {
+
+template <typename T>
+static void BM_bit_width(benchmark::State& state) {
+  const auto count = static_cast<size_t>(state.range(0));
+
+  absl::BitGen rng;
+  std::vector<T> values;
+  values.reserve(count);
+  for (size_t i = 0; i < count; ++i) {
+    values.push_back(absl::Uniform<T>(rng, 0, std::numeric_limits<T>::max()));
+  }
+
+  while (state.KeepRunningBatch(static_cast<int64_t>(count))) {
+    for (size_t i = 0; i < count; ++i) {
+      benchmark::DoNotOptimize(absl::bit_width(values[i]));
+    }
+  }
+}
+BENCHMARK_TEMPLATE(BM_bit_width, uint8_t)->Range(1, 1 << 20);
+BENCHMARK_TEMPLATE(BM_bit_width, uint16_t)->Range(1, 1 << 20);
+BENCHMARK_TEMPLATE(BM_bit_width, uint32_t)->Range(1, 1 << 20);
+BENCHMARK_TEMPLATE(BM_bit_width, uint64_t)->Range(1, 1 << 20);
+
+template <typename T>
+static void BM_bit_width_nonzero(benchmark::State& state) {
+  const auto count = static_cast<size_t>(state.range(0));
+
+  absl::BitGen rng;
+  std::vector<T> values;
+  values.reserve(count);
+  for (size_t i = 0; i < count; ++i) {
+    values.push_back(absl::Uniform<T>(rng, 1, std::numeric_limits<T>::max()));
+  }
+
+  while (state.KeepRunningBatch(static_cast<int64_t>(count))) {
+    for (size_t i = 0; i < count; ++i) {
+      const T value = values[i];
+      ABSL_ASSUME(value > 0);
+      benchmark::DoNotOptimize(absl::bit_width(value));
+    }
+  }
+}
+BENCHMARK_TEMPLATE(BM_bit_width_nonzero, uint8_t)->Range(1, 1 << 20);
+BENCHMARK_TEMPLATE(BM_bit_width_nonzero, uint16_t)->Range(1, 1 << 20);
+BENCHMARK_TEMPLATE(BM_bit_width_nonzero, uint32_t)->Range(1, 1 << 20);
+BENCHMARK_TEMPLATE(BM_bit_width_nonzero, uint64_t)->Range(1, 1 << 20);
+
+}  // namespace
+}  // namespace absl
diff --git a/absl/numeric/bits_test.cc b/absl/numeric/bits_test.cc
new file mode 100644
index 00000000000..30b59671cd7
--- /dev/null
+++ b/absl/numeric/bits_test.cc
@@ -0,0 +1,720 @@
+// Copyright 2020 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/numeric/bits.h"
+
+#include <cstdint>
+#include <limits>
+#include <type_traits>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/numeric/internal/bits.h"
+#include "absl/random/random.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace {
+
+template <typename IntT>
+class UnsignedIntegerTypesTest : public ::testing::Test {};
+template <typename IntT>
+class IntegerTypesTest : public ::testing::Test {};
+
+using UnsignedIntegerTypes =
+    ::testing::Types<uint8_t, uint16_t, uint32_t, uint64_t>;
+using OneByteIntegerTypes = ::testing::Types<
+    unsigned char,
+    uint8_t
+    >;
+
+TYPED_TEST_SUITE(UnsignedIntegerTypesTest, UnsignedIntegerTypes);
+TYPED_TEST_SUITE(IntegerTypesTest, OneByteIntegerTypes);
+
+TYPED_TEST(UnsignedIntegerTypesTest, ReturnTypes) {
+  using UIntType = TypeParam;
+
+  static_assert(std::is_same_v<decltype(byteswap(UIntType{0})), UIntType>);
+  static_assert(std::is_same_v<decltype(rotl(UIntType{0}, 0)), UIntType>);
+  static_assert(std::is_same_v<decltype(rotr(UIntType{0}, 0)), UIntType>);
+  static_assert(std::is_same_v<decltype(countl_zero(UIntType{0})), int>);
+  static_assert(std::is_same_v<decltype(countl_one(UIntType{0})), int>);
+  static_assert(std::is_same_v<decltype(countr_zero(UIntType{0})), int>);
+  static_assert(std::is_same_v<decltype(countr_one(UIntType{0})), int>);
+  static_assert(std::is_same_v<decltype(popcount(UIntType{0})), int>);
+  static_assert(std::is_same_v<decltype(bit_ceil(UIntType{0})), UIntType>);
+  static_assert(std::is_same_v<decltype(bit_floor(UIntType{0})), UIntType>);
+  static_assert(std::is_same_v<decltype(bit_width(UIntType{0})), int>);
+}
+
+TYPED_TEST(IntegerTypesTest, HandlesTypes) {
+  using UIntType = TypeParam;
+
+  EXPECT_EQ(rotl(UIntType{0x12}, 0), uint8_t{0x12});
+  EXPECT_EQ(rotr(UIntType{0x12}, -4), uint8_t{0x21});
+  static_assert(rotl(UIntType{0x12}, 0) == uint8_t{0x12});
+
+  static_assert(rotr(UIntType{0x12}, 0) == uint8_t{0x12});
+  EXPECT_EQ(rotr(UIntType{0x12}, 0), uint8_t{0x12});
+
+#if ABSL_INTERNAL_HAS_CONSTEXPR_CLZ
+  static_assert(countl_zero(UIntType{}) == 8);
+  static_assert(countl_zero(static_cast<UIntType>(-1)) == 0);
+
+  static_assert(countl_one(UIntType{}) == 0);
+  static_assert(countl_one(static_cast<UIntType>(-1)) == 8);
+
+  static_assert(countr_zero(UIntType{}) == 8);
+  static_assert(countr_zero(static_cast<UIntType>(-1)) == 0);
+
+  static_assert(countr_one(UIntType{}) == 0);
+  static_assert(countr_one(static_cast<UIntType>(-1)) == 8);
+
+  static_assert(popcount(UIntType{}) == 0);
+  static_assert(popcount(UIntType{1}) == 1);
+  static_assert(popcount(static_cast<UIntType>(-1)) == 8);
+
+  static_assert(bit_width(UIntType{}) == 0);
+  static_assert(bit_width(UIntType{1}) == 1);
+  static_assert(bit_width(UIntType{3}) == 2);
+  static_assert(bit_width(static_cast<UIntType>(-1)) == 8);
+#endif
+
+  EXPECT_EQ(countl_zero(UIntType{}), 8);
+  EXPECT_EQ(countl_zero(static_cast<UIntType>(-1)), 0);
+
+  EXPECT_EQ(countl_one(UIntType{}), 0);
+  EXPECT_EQ(countl_one(static_cast<UIntType>(-1)), 8);
+
+  EXPECT_EQ(countr_zero(UIntType{}), 8);
+  EXPECT_EQ(countr_zero(static_cast<UIntType>(-1)), 0);
+
+  EXPECT_EQ(countr_one(UIntType{}), 0);
+  EXPECT_EQ(countr_one(static_cast<UIntType>(-1)), 8);
+
+  EXPECT_EQ(popcount(UIntType{}), 0);
+  EXPECT_EQ(popcount(UIntType{1}), 1);
+
+  EXPECT_FALSE(has_single_bit(UIntType{}));
+  EXPECT_FALSE(has_single_bit(static_cast<UIntType>(-1)));
+
+  EXPECT_EQ(bit_width(UIntType{}), 0);
+  EXPECT_EQ(bit_width(UIntType{1}), 1);
+  EXPECT_EQ(bit_width(UIntType{3}), 2);
+  EXPECT_EQ(bit_width(static_cast<UIntType>(-1)), 8);
+}
+
+TEST(Rotate, Left) {
+  static_assert(rotl(uint8_t{0x12}, 0) == uint8_t{0x12});
+  static_assert(rotl(uint16_t{0x1234}, 0) == uint16_t{0x1234});
+  static_assert(rotl(uint32_t{0x12345678UL}, 0) == uint32_t{0x12345678UL});
+  static_assert(rotl(uint64_t{0x12345678ABCDEF01ULL}, 0) ==
+                uint64_t{0x12345678ABCDEF01ULL});
+
+  EXPECT_EQ(rotl(uint8_t{0x12}, 0), uint8_t{0x12});
+  EXPECT_EQ(rotl(uint16_t{0x1234}, 0), uint16_t{0x1234});
+  EXPECT_EQ(rotl(uint32_t{0x12345678UL}, 0), uint32_t{0x12345678UL});
+  EXPECT_EQ(rotl(uint64_t{0x12345678ABCDEF01ULL}, 0),
+            uint64_t{0x12345678ABCDEF01ULL});
+
+  EXPECT_EQ(rotl(uint8_t{0x12}, 8), uint8_t{0x12});
+  EXPECT_EQ(rotl(uint16_t{0x1234}, 16), uint16_t{0x1234});
+  EXPECT_EQ(rotl(uint32_t{0x12345678UL}, 32), uint32_t{0x12345678UL});
+  EXPECT_EQ(rotl(uint64_t{0x12345678ABCDEF01ULL}, 64),
+            uint64_t{0x12345678ABCDEF01ULL});
+
+  EXPECT_EQ(rotl(uint8_t{0x12}, -8), uint8_t{0x12});
+  EXPECT_EQ(rotl(uint16_t{0x1234}, -16), uint16_t{0x1234});
+  EXPECT_EQ(rotl(uint32_t{0x12345678UL}, -32), uint32_t{0x12345678UL});
+  EXPECT_EQ(rotl(uint64_t{0x12345678ABCDEF01ULL}, -64),
+            uint64_t{0x12345678ABCDEF01ULL});
+
+  EXPECT_EQ(rotl(uint8_t{0x12}, 4), uint8_t{0x21});
+  EXPECT_EQ(rotl(uint16_t{0x1234}, 4), uint16_t{0x2341});
+  EXPECT_EQ(rotl(uint32_t{0x12345678UL}, 4), uint32_t{0x23456781UL});
+  EXPECT_EQ(rotl(uint64_t{0x12345678ABCDEF01ULL}, 4),
+            uint64_t{0x2345678ABCDEF011ULL});
+
+  EXPECT_EQ(rotl(uint8_t{0x12}, -4), uint8_t{0x21});
+  EXPECT_EQ(rotl(uint16_t{0x1234}, -4), uint16_t{0x4123});
+  EXPECT_EQ(rotl(uint32_t{0x12345678UL}, -4), uint32_t{0x81234567UL});
+  EXPECT_EQ(rotl(uint64_t{0x12345678ABCDEF01ULL}, -4),
+            uint64_t{0x112345678ABCDEF0ULL});
+
+  EXPECT_EQ(rotl(uint32_t{1234}, std::numeric_limits<int>::min()),
+            uint32_t{1234});
+}
+
+TEST(Rotate, Right) {
+  static_assert(rotr(uint8_t{0x12}, 0) == uint8_t{0x12});
+  static_assert(rotr(uint16_t{0x1234}, 0) == uint16_t{0x1234});
+  static_assert(rotr(uint32_t{0x12345678UL}, 0) == uint32_t{0x12345678UL});
+  static_assert(rotr(uint64_t{0x12345678ABCDEF01ULL}, 0) ==
+                uint64_t{0x12345678ABCDEF01ULL});
+
+  EXPECT_EQ(rotr(uint8_t{0x12}, 0), uint8_t{0x12});
+  EXPECT_EQ(rotr(uint16_t{0x1234}, 0), uint16_t{0x1234});
+  EXPECT_EQ(rotr(uint32_t{0x12345678UL}, 0), uint32_t{0x12345678UL});
+  EXPECT_EQ(rotr(uint64_t{0x12345678ABCDEF01ULL}, 0),
+            uint64_t{0x12345678ABCDEF01ULL});
+
+  EXPECT_EQ(rotr(uint8_t{0x12}, 8), uint8_t{0x12});
+  EXPECT_EQ(rotr(uint16_t{0x1234}, 16), uint16_t{0x1234});
+  EXPECT_EQ(rotr(uint32_t{0x12345678UL}, 32), uint32_t{0x12345678UL});
+  EXPECT_EQ(rotr(uint64_t{0x12345678ABCDEF01ULL}, 64),
+            uint64_t{0x12345678ABCDEF01ULL});
+
+  EXPECT_EQ(rotr(uint8_t{0x12}, -8), uint8_t{0x12});
+  EXPECT_EQ(rotr(uint16_t{0x1234}, -16), uint16_t{0x1234});
+  EXPECT_EQ(rotr(uint32_t{0x12345678UL}, -32), uint32_t{0x12345678UL});
+  EXPECT_EQ(rotr(uint64_t{0x12345678ABCDEF01ULL}, -64),
+            uint64_t{0x12345678ABCDEF01ULL});
+
+  EXPECT_EQ(rotr(uint8_t{0x12}, 4), uint8_t{0x21});
+  EXPECT_EQ(rotr(uint16_t{0x1234}, 4), uint16_t{0x4123});
+  EXPECT_EQ(rotr(uint32_t{0x12345678UL}, 4), uint32_t{0x81234567UL});
+  EXPECT_EQ(rotr(uint64_t{0x12345678ABCDEF01ULL}, 4),
+            uint64_t{0x112345678ABCDEF0ULL});
+
+  EXPECT_EQ(rotr(uint8_t{0x12}, -4), uint8_t{0x21});
+  EXPECT_EQ(rotr(uint16_t{0x1234}, -4), uint16_t{0x2341});
+  EXPECT_EQ(rotr(uint32_t{0x12345678UL}, -4), uint32_t{0x23456781UL});
+  EXPECT_EQ(rotr(uint64_t{0x12345678ABCDEF01ULL}, -4),
+            uint64_t{0x2345678ABCDEF011ULL});
+
+  EXPECT_EQ(rotl(uint32_t{1234}, std::numeric_limits<int>::min()),
+            uint32_t{1234});
+}
+
+TEST(Rotate, Symmetry) {
+  // rotr(x, s) is equivalent to rotl(x, -s)
+  absl::BitGen rng;
+  constexpr int kTrials = 100;
+
+  for (int i = 0; i < kTrials; ++i) {
+    uint8_t value = absl::Uniform(rng, std::numeric_limits<uint8_t>::min(),
+                                  std::numeric_limits<uint8_t>::max());
+    int shift = absl::Uniform(rng, -2 * std::numeric_limits<uint8_t>::digits,
+                              2 * std::numeric_limits<uint8_t>::digits);
+
+    EXPECT_EQ(rotl(value, shift), rotr(value, -shift));
+  }
+
+  for (int i = 0; i < kTrials; ++i) {
+    uint16_t value = absl::Uniform(rng, std::numeric_limits<uint16_t>::min(),
+                                   std::numeric_limits<uint16_t>::max());
+    int shift = absl::Uniform(rng, -2 * std::numeric_limits<uint16_t>::digits,
+                              2 * std::numeric_limits<uint16_t>::digits);
+
+    EXPECT_EQ(rotl(value, shift), rotr(value, -shift));
+  }
+
+  for (int i = 0; i < kTrials; ++i) {
+    uint32_t value = absl::Uniform(rng, std::numeric_limits<uint32_t>::min(),
+                                   std::numeric_limits<uint32_t>::max());
+    int shift = absl::Uniform(rng, -2 * std::numeric_limits<uint32_t>::digits,
+                              2 * std::numeric_limits<uint32_t>::digits);
+
+    EXPECT_EQ(rotl(value, shift), rotr(value, -shift));
+  }
+
+  for (int i = 0; i < kTrials; ++i) {
+    uint64_t value = absl::Uniform(rng, std::numeric_limits<uint64_t>::min(),
+                                   std::numeric_limits<uint64_t>::max());
+    int shift = absl::Uniform(rng, -2 * std::numeric_limits<uint64_t>::digits,
+                              2 * std::numeric_limits<uint64_t>::digits);
+
+    EXPECT_EQ(rotl(value, shift), rotr(value, -shift));
+  }
+}
+
+TEST(Counting, LeadingZeroes) {
+#if ABSL_INTERNAL_HAS_CONSTEXPR_CLZ
+  static_assert(countl_zero(uint8_t{}) == 8);
+  static_assert(countl_zero(static_cast<uint8_t>(-1)) == 0);
+  static_assert(countl_zero(uint16_t{}) == 16);
+  static_assert(countl_zero(static_cast<uint16_t>(-1)) == 0);
+  static_assert(countl_zero(uint32_t{}) == 32);
+  static_assert(countl_zero(~uint32_t{}) == 0);
+  static_assert(countl_zero(uint64_t{}) == 64);
+  static_assert(countl_zero(~uint64_t{}) == 0);
+#endif
+
+  EXPECT_EQ(countl_zero(uint8_t{}), 8);
+  EXPECT_EQ(countl_zero(static_cast<uint8_t>(-1)), 0);
+  EXPECT_EQ(countl_zero(uint16_t{}), 16);
+  EXPECT_EQ(countl_zero(static_cast<uint16_t>(-1)), 0);
+  EXPECT_EQ(countl_zero(uint32_t{}), 32);
+  EXPECT_EQ(countl_zero(~uint32_t{}), 0);
+  EXPECT_EQ(countl_zero(uint64_t{}), 64);
+  EXPECT_EQ(countl_zero(~uint64_t{}), 0);
+
+  for (int i = 0; i < 8; i++) {
+    EXPECT_EQ(countl_zero(static_cast<uint8_t>(1u << i)), 7 - i);
+  }
+
+  for (int i = 0; i < 16; i++) {
+    EXPECT_EQ(countl_zero(static_cast<uint16_t>(1u << i)), 15 - i);
+  }
+
+  for (int i = 0; i < 32; i++) {
+    EXPECT_EQ(countl_zero(uint32_t{1} << i), 31 - i);
+  }
+
+  for (int i = 0; i < 64; i++) {
+    EXPECT_EQ(countl_zero(uint64_t{1} << i), 63 - i);
+  }
+}
+
+TEST(Counting, LeadingOnes) {
+#if ABSL_INTERNAL_HAS_CONSTEXPR_CLZ
+  static_assert(countl_one(uint8_t{}) == 0);
+  static_assert(countl_one(static_cast<uint8_t>(-1)) == 8);
+  static_assert(countl_one(uint16_t{}) == 0);
+  static_assert(countl_one(static_cast<uint16_t>(-1)) == 16);
+  static_assert(countl_one(uint32_t{}) == 0);
+  static_assert(countl_one(~uint32_t{}) == 32);
+  static_assert(countl_one(uint64_t{}) == 0);
+  static_assert(countl_one(~uint64_t{}) == 64);
+#endif
+
+  EXPECT_EQ(countl_one(uint8_t{}), 0);
+  EXPECT_EQ(countl_one(static_cast<uint8_t>(-1)), 8);
+  EXPECT_EQ(countl_one(uint16_t{}), 0);
+  EXPECT_EQ(countl_one(static_cast<uint16_t>(-1)), 16);
+  EXPECT_EQ(countl_one(uint32_t{}), 0);
+  EXPECT_EQ(countl_one(~uint32_t{}), 32);
+  EXPECT_EQ(countl_one(uint64_t{}), 0);
+  EXPECT_EQ(countl_one(~uint64_t{}), 64);
+}
+
+TEST(Counting, TrailingZeroes) {
+#if ABSL_INTERNAL_HAS_CONSTEXPR_CTZ
+  static_assert(countr_zero(uint8_t{}) == 8);
+  static_assert(countr_zero(static_cast<uint8_t>(-1)) == 0);
+  static_assert(countr_zero(uint16_t{}) == 16);
+  static_assert(countr_zero(static_cast<uint16_t>(-1)) == 0);
+  static_assert(countr_zero(uint32_t{}) == 32);
+  static_assert(countr_zero(~uint32_t{}) == 0);
+  static_assert(countr_zero(uint64_t{}) == 64);
+  static_assert(countr_zero(~uint64_t{}) == 0);
+#endif
+
+  EXPECT_EQ(countr_zero(uint8_t{}), 8);
+  EXPECT_EQ(countr_zero(static_cast<uint8_t>(-1)), 0);
+  EXPECT_EQ(countr_zero(uint16_t{}), 16);
+  EXPECT_EQ(countr_zero(static_cast<uint16_t>(-1)), 0);
+  EXPECT_EQ(countr_zero(uint32_t{}), 32);
+  EXPECT_EQ(countr_zero(~uint32_t{}), 0);
+  EXPECT_EQ(countr_zero(uint64_t{}), 64);
+  EXPECT_EQ(countr_zero(~uint64_t{}), 0);
+}
+
+TEST(Counting, TrailingOnes) {
+#if ABSL_INTERNAL_HAS_CONSTEXPR_CTZ
+  static_assert(countr_one(uint8_t{}) == 0);
+  static_assert(countr_one(static_cast<uint8_t>(-1)) == 8);
+  static_assert(countr_one(uint16_t{}) == 0);
+  static_assert(countr_one(static_cast<uint16_t>(-1)) == 16);
+  static_assert(countr_one(uint32_t{}) == 0);
+  static_assert(countr_one(~uint32_t{}) == 32);
+  static_assert(countr_one(uint64_t{}) == 0);
+  static_assert(countr_one(~uint64_t{}) == 64);
+#endif
+
+  EXPECT_EQ(countr_one(uint8_t{}), 0);
+  EXPECT_EQ(countr_one(static_cast<uint8_t>(-1)), 8);
+  EXPECT_EQ(countr_one(uint16_t{}), 0);
+  EXPECT_EQ(countr_one(static_cast<uint16_t>(-1)), 16);
+  EXPECT_EQ(countr_one(uint32_t{}), 0);
+  EXPECT_EQ(countr_one(~uint32_t{}), 32);
+  EXPECT_EQ(countr_one(uint64_t{}), 0);
+  EXPECT_EQ(countr_one(~uint64_t{}), 64);
+}
+
+TEST(Counting, Popcount) {
+#if ABSL_INTERNAL_HAS_CONSTEXPR_POPCOUNT
+  static_assert(popcount(uint8_t{}) == 0);
+  static_assert(popcount(uint8_t{1}) == 1);
+  static_assert(popcount(static_cast<uint8_t>(-1)) == 8);
+  static_assert(popcount(uint16_t{}) == 0);
+  static_assert(popcount(uint16_t{1}) == 1);
+  static_assert(popcount(static_cast<uint16_t>(-1)) == 16);
+  static_assert(popcount(uint32_t{}) == 0);
+  static_assert(popcount(uint32_t{1}) == 1);
+  static_assert(popcount(~uint32_t{}) == 32);
+  static_assert(popcount(uint64_t{}) == 0);
+  static_assert(popcount(uint64_t{1}) == 1);
+  static_assert(popcount(~uint64_t{}) == 64);
+#endif  // ABSL_INTERNAL_HAS_CONSTEXPR_POPCOUNT
+
+  EXPECT_EQ(popcount(uint8_t{}), 0);
+  EXPECT_EQ(popcount(uint8_t{1}), 1);
+  EXPECT_EQ(popcount(static_cast<uint8_t>(-1)), 8);
+  EXPECT_EQ(popcount(uint16_t{}), 0);
+  EXPECT_EQ(popcount(uint16_t{1}), 1);
+  EXPECT_EQ(popcount(static_cast<uint16_t>(-1)), 16);
+  EXPECT_EQ(popcount(uint32_t{}), 0);
+  EXPECT_EQ(popcount(uint32_t{1}), 1);
+  EXPECT_EQ(popcount(~uint32_t{}), 32);
+  EXPECT_EQ(popcount(uint64_t{}), 0);
+  EXPECT_EQ(popcount(uint64_t{1}), 1);
+  EXPECT_EQ(popcount(~uint64_t{}), 64);
+
+  for (int i = 0; i < 8; i++) {
+    EXPECT_EQ(popcount(static_cast<uint8_t>(uint8_t{1} << i)), 1);
+    EXPECT_EQ(popcount(static_cast<uint8_t>(static_cast<uint8_t>(-1) ^
+                                            (uint8_t{1} << i))),
+              7);
+  }
+
+  for (int i = 0; i < 16; i++) {
+    EXPECT_EQ(popcount(static_cast<uint16_t>(uint16_t{1} << i)), 1);
+    EXPECT_EQ(popcount(static_cast<uint16_t>(static_cast<uint16_t>(-1) ^
+                                             (uint16_t{1} << i))),
+              15);
+  }
+
+  for (int i = 0; i < 32; i++) {
+    EXPECT_EQ(popcount(uint32_t{1} << i), 1);
+    EXPECT_EQ(popcount(static_cast<uint32_t>(-1) ^ (uint32_t{1} << i)), 31);
+  }
+
+  for (int i = 0; i < 64; i++) {
+    EXPECT_EQ(popcount(uint64_t{1} << i), 1);
+    EXPECT_EQ(popcount(static_cast<uint64_t>(-1) ^ (uint64_t{1} << i)), 63);
+  }
+}
+
+template <typename T>
+struct PopcountInput {
+  T value = 0;
+  int expected = 0;
+};
+
+template <typename T>
+PopcountInput<T> GeneratePopcountInput(absl::BitGen& gen) {
+  PopcountInput<T> ret;
+  for (int i = 0; i < std::numeric_limits<T>::digits; i++) {
+    bool coin = absl::Bernoulli(gen, 0.2);
+    if (coin) {
+      ret.value |= T{1} << i;
+      ret.expected++;
+    }
+  }
+  return ret;
+}
+
+TEST(Counting, PopcountFuzz) {
+  absl::BitGen rng;
+  constexpr int kTrials = 100;
+
+  for (int i = 0; i < kTrials; ++i) {
+    auto input = GeneratePopcountInput<uint8_t>(rng);
+    EXPECT_EQ(popcount(input.value), input.expected);
+  }
+
+  for (int i = 0; i < kTrials; ++i) {
+    auto input = GeneratePopcountInput<uint16_t>(rng);
+    EXPECT_EQ(popcount(input.value), input.expected);
+  }
+
+  for (int i = 0; i < kTrials; ++i) {
+    auto input = GeneratePopcountInput<uint32_t>(rng);
+    EXPECT_EQ(popcount(input.value), input.expected);
+  }
+
+  for (int i = 0; i < kTrials; ++i) {
+    auto input = GeneratePopcountInput<uint64_t>(rng);
+    EXPECT_EQ(popcount(input.value), input.expected);
+  }
+}
+
+TEST(IntegralPowersOfTwo, SingleBit) {
+  EXPECT_FALSE(has_single_bit(uint8_t{}));
+  EXPECT_FALSE(has_single_bit(static_cast<uint8_t>(-1)));
+  EXPECT_FALSE(has_single_bit(uint16_t{}));
+  EXPECT_FALSE(has_single_bit(static_cast<uint16_t>(-1)));
+  EXPECT_FALSE(has_single_bit(uint32_t{}));
+  EXPECT_FALSE(has_single_bit(~uint32_t{}));
+  EXPECT_FALSE(has_single_bit(uint64_t{}));
+  EXPECT_FALSE(has_single_bit(~uint64_t{}));
+
+  static_assert(!has_single_bit(0u));
+  static_assert(has_single_bit(1u));
+  static_assert(has_single_bit(2u));
+  static_assert(!has_single_bit(3u));
+  static_assert(has_single_bit(4u));
+  static_assert(!has_single_bit(1337u));
+  static_assert(has_single_bit(65536u));
+  static_assert(has_single_bit(uint32_t{1} << 30));
+  static_assert(has_single_bit(uint64_t{1} << 42));
+
+  EXPECT_FALSE(has_single_bit(0u));
+  EXPECT_TRUE(has_single_bit(1u));
+  EXPECT_TRUE(has_single_bit(2u));
+  EXPECT_FALSE(has_single_bit(3u));
+  EXPECT_TRUE(has_single_bit(4u));
+  EXPECT_FALSE(has_single_bit(1337u));
+  EXPECT_TRUE(has_single_bit(65536u));
+  EXPECT_TRUE(has_single_bit(uint32_t{1} << 30));
+  EXPECT_TRUE(has_single_bit(uint64_t{1} << 42));
+
+  EXPECT_TRUE(has_single_bit(
+      static_cast<uint8_t>(std::numeric_limits<uint8_t>::max() / 2 + 1)));
+  EXPECT_TRUE(has_single_bit(
+      static_cast<uint16_t>(std::numeric_limits<uint16_t>::max() / 2 + 1)));
+  EXPECT_TRUE(has_single_bit(
+      static_cast<uint32_t>(std::numeric_limits<uint32_t>::max() / 2 + 1)));
+  EXPECT_TRUE(has_single_bit(
+      static_cast<uint64_t>(std::numeric_limits<uint64_t>::max() / 2 + 1)));
+}
+
+template <typename T, T arg, T = bit_ceil(arg)>
+bool IsBitCeilConstantExpression(int) {
+  return true;
+}
+template <typename T, T arg>
+bool IsBitCeilConstantExpression(char) {
+  return false;
+}
+
+TEST(IntegralPowersOfTwo, Ceiling) {
+#if ABSL_INTERNAL_HAS_CONSTEXPR_CLZ
+  static_assert(bit_ceil(0u) == 1);
+  static_assert(bit_ceil(1u) == 1);
+  static_assert(bit_ceil(2u) == 2);
+  static_assert(bit_ceil(3u) == 4);
+  static_assert(bit_ceil(4u) == 4);
+  static_assert(bit_ceil(1337u) == 2048);
+  static_assert(bit_ceil(65536u) == 65536);
+  static_assert(bit_ceil(65536u - 1337u) == 65536);
+  static_assert(bit_ceil(uint32_t{0x80000000}) == uint32_t{0x80000000});
+  static_assert(bit_ceil(uint64_t{0x40000000000}) == uint64_t{0x40000000000});
+  static_assert(bit_ceil(uint64_t{0x8000000000000000}) ==
+                uint64_t{0x8000000000000000});
+
+  EXPECT_TRUE((IsBitCeilConstantExpression<uint8_t, uint8_t{0x0}>(0)));
+  EXPECT_TRUE((IsBitCeilConstantExpression<uint8_t, uint8_t{0x80}>(0)));
+  EXPECT_FALSE((IsBitCeilConstantExpression<uint8_t, uint8_t{0x81}>(0)));
+  EXPECT_FALSE((IsBitCeilConstantExpression<uint8_t, uint8_t{0xff}>(0)));
+
+  EXPECT_TRUE((IsBitCeilConstantExpression<uint16_t, uint16_t{0x0}>(0)));
+  EXPECT_TRUE((IsBitCeilConstantExpression<uint16_t, uint16_t{0x8000}>(0)));
+  EXPECT_FALSE((IsBitCeilConstantExpression<uint16_t, uint16_t{0x8001}>(0)));
+  EXPECT_FALSE((IsBitCeilConstantExpression<uint16_t, uint16_t{0xffff}>(0)));
+
+  EXPECT_TRUE((IsBitCeilConstantExpression<uint32_t, uint32_t{0x0}>(0)));
+  EXPECT_TRUE((IsBitCeilConstantExpression<uint32_t, uint32_t{0x80000000}>(0)));
+  EXPECT_FALSE(
+      (IsBitCeilConstantExpression<uint32_t, uint32_t{0x80000001}>(0)));
+  EXPECT_FALSE(
+      (IsBitCeilConstantExpression<uint32_t, uint32_t{0xffffffff}>(0)));
+
+  EXPECT_TRUE((IsBitCeilConstantExpression<uint64_t, uint64_t{0x0}>(0)));
+  EXPECT_TRUE(
+      (IsBitCeilConstantExpression<uint64_t, uint64_t{0x8000000000000000}>(0)));
+  EXPECT_FALSE(
+      (IsBitCeilConstantExpression<uint64_t, uint64_t{0x8000000000000001}>(0)));
+  EXPECT_FALSE(
+      (IsBitCeilConstantExpression<uint64_t, uint64_t{0xffffffffffffffff}>(0)));
+#endif
+
+  EXPECT_EQ(bit_ceil(0u), 1);
+  EXPECT_EQ(bit_ceil(1u), 1);
+  EXPECT_EQ(bit_ceil(2u), 2);
+  EXPECT_EQ(bit_ceil(3u), 4);
+  EXPECT_EQ(bit_ceil(4u), 4);
+  EXPECT_EQ(bit_ceil(1337u), 2048);
+  EXPECT_EQ(bit_ceil(65536u), 65536);
+  EXPECT_EQ(bit_ceil(65536u - 1337u), 65536);
+  EXPECT_EQ(bit_ceil(uint64_t{0x40000000000}), uint64_t{0x40000000000});
+}
+
+TEST(IntegralPowersOfTwo, Floor) {
+#if ABSL_INTERNAL_HAS_CONSTEXPR_CLZ
+  static_assert(bit_floor(0u) == 0);
+  static_assert(bit_floor(1u) == 1);
+  static_assert(bit_floor(2u) == 2);
+  static_assert(bit_floor(3u) == 2);
+  static_assert(bit_floor(4u) == 4);
+  static_assert(bit_floor(1337u) == 1024);
+  static_assert(bit_floor(65536u) == 65536);
+  static_assert(bit_floor(65536u - 1337u) == 32768);
+  static_assert(bit_floor(uint64_t{0x40000000000}) == uint64_t{0x40000000000});
+#endif
+
+  EXPECT_EQ(bit_floor(0u), 0);
+  EXPECT_EQ(bit_floor(1u), 1);
+  EXPECT_EQ(bit_floor(2u), 2);
+  EXPECT_EQ(bit_floor(3u), 2);
+  EXPECT_EQ(bit_floor(4u), 4);
+  EXPECT_EQ(bit_floor(1337u), 1024);
+  EXPECT_EQ(bit_floor(65536u), 65536);
+  EXPECT_EQ(bit_floor(65536u - 1337u), 32768);
+  EXPECT_EQ(bit_floor(uint64_t{0x40000000000}), uint64_t{0x40000000000});
+
+  for (int i = 0; i < 8; i++) {
+    uint8_t input = uint8_t{1} << i;
+    EXPECT_EQ(bit_floor(input), input);
+    if (i > 0) {
+      EXPECT_EQ(bit_floor(static_cast<uint8_t>(input + 1)), input);
+    }
+  }
+
+  for (int i = 0; i < 16; i++) {
+    uint16_t input = uint16_t{1} << i;
+    EXPECT_EQ(bit_floor(input), input);
+    if (i > 0) {
+      EXPECT_EQ(bit_floor(static_cast<uint16_t>(input + 1)), input);
+    }
+  }
+
+  for (int i = 0; i < 32; i++) {
+    uint32_t input = uint32_t{1} << i;
+    EXPECT_EQ(bit_floor(input), input);
+    if (i > 0) {
+      EXPECT_EQ(bit_floor(input + 1), input);
+    }
+  }
+
+  for (int i = 0; i < 64; i++) {
+    uint64_t input = uint64_t{1} << i;
+    EXPECT_EQ(bit_floor(input), input);
+    if (i > 0) {
+      EXPECT_EQ(bit_floor(input + 1), input);
+    }
+  }
+}
+
+TEST(IntegralPowersOfTwo, Width) {
+#if ABSL_INTERNAL_HAS_CONSTEXPR_CLZ
+  static_assert(bit_width(uint8_t{}) == 0);
+  static_assert(bit_width(uint8_t{1}) == 1);
+  static_assert(bit_width(uint8_t{3}) == 2);
+  static_assert(bit_width(static_cast<uint8_t>(-1)) == 8);
+  static_assert(bit_width(uint16_t{}) == 0);
+  static_assert(bit_width(uint16_t{1}) == 1);
+  static_assert(bit_width(uint16_t{3}) == 2);
+  static_assert(bit_width(static_cast<uint16_t>(-1)) == 16);
+  static_assert(bit_width(uint32_t{}) == 0);
+  static_assert(bit_width(uint32_t{1}) == 1);
+  static_assert(bit_width(uint32_t{3}) == 2);
+  static_assert(bit_width(~uint32_t{}) == 32);
+  static_assert(bit_width(uint64_t{}) == 0);
+  static_assert(bit_width(uint64_t{1}) == 1);
+  static_assert(bit_width(uint64_t{3}) == 2);
+  static_assert(bit_width(~uint64_t{}) == 64);
+#endif
+
+  EXPECT_EQ(bit_width(uint8_t{}), 0);
+  EXPECT_EQ(bit_width(uint8_t{1}), 1);
+  EXPECT_EQ(bit_width(uint8_t{3}), 2);
+  EXPECT_EQ(bit_width(static_cast<uint8_t>(-1)), 8);
+  EXPECT_EQ(bit_width(uint16_t{}), 0);
+  EXPECT_EQ(bit_width(uint16_t{1}), 1);
+  EXPECT_EQ(bit_width(uint16_t{3}), 2);
+  EXPECT_EQ(bit_width(static_cast<uint16_t>(-1)), 16);
+  EXPECT_EQ(bit_width(uint32_t{}), 0);
+  EXPECT_EQ(bit_width(uint32_t{1}), 1);
+  EXPECT_EQ(bit_width(uint32_t{3}), 2);
+  EXPECT_EQ(bit_width(~uint32_t{}), 32);
+  EXPECT_EQ(bit_width(uint64_t{}), 0);
+  EXPECT_EQ(bit_width(uint64_t{1}), 1);
+  EXPECT_EQ(bit_width(uint64_t{3}), 2);
+  EXPECT_EQ(bit_width(~uint64_t{}), 64);
+
+  for (int i = 0; i < 8; i++) {
+    EXPECT_EQ(bit_width(static_cast<uint8_t>(uint8_t{1} << i)), i + 1);
+  }
+
+  for (int i = 0; i < 16; i++) {
+    EXPECT_EQ(bit_width(static_cast<uint16_t>(uint16_t{1} << i)), i + 1);
+  }
+
+  for (int i = 0; i < 32; i++) {
+    EXPECT_EQ(bit_width(uint32_t{1} << i), i + 1);
+  }
+
+  for (int i = 0; i < 64; i++) {
+    EXPECT_EQ(bit_width(uint64_t{1} << i), i + 1);
+  }
+}
+
+// On GCC and Clang, anticiapte that implementations will be constexpr
+#if defined(__GNUC__)
+static_assert(ABSL_INTERNAL_HAS_CONSTEXPR_POPCOUNT,
+              "popcount should be constexpr");
+static_assert(ABSL_INTERNAL_HAS_CONSTEXPR_CLZ, "clz should be constexpr");
+static_assert(ABSL_INTERNAL_HAS_CONSTEXPR_CTZ, "ctz should be constexpr");
+#endif
+
+TEST(Endian, Comparison) {
+#if defined(ABSL_IS_LITTLE_ENDIAN)
+  static_assert(absl::endian::native == absl::endian::little);
+  static_assert(absl::endian::native != absl::endian::big);
+#endif
+#if defined(ABSL_IS_BIG_ENDIAN)
+  static_assert(absl::endian::native != absl::endian::little);
+  static_assert(absl::endian::native == absl::endian::big);
+#endif
+}
+
+TEST(Byteswap, Constexpr) {
+  static_assert(absl::byteswap<int8_t>(0x12) == 0x12);
+  static_assert(absl::byteswap<int16_t>(0x1234) == 0x3412);
+  static_assert(absl::byteswap<int32_t>(0x12345678) == 0x78563412);
+  static_assert(absl::byteswap<int64_t>(0x123456789abcdef0) ==
+                static_cast<int64_t>(0xf0debc9a78563412));
+  static_assert(absl::byteswap<uint8_t>(0x21) == 0x21);
+  static_assert(absl::byteswap<uint16_t>(0x4321) == 0x2143);
+  static_assert(absl::byteswap<uint32_t>(0x87654321) == 0x21436587);
+  static_assert(absl::byteswap<uint64_t>(0xfedcba9876543210) ==
+                static_cast<uint64_t>(0x1032547698badcfe));
+  static_assert(absl::byteswap<int32_t>(static_cast<int32_t>(0xdeadbeef)) ==
+                static_cast<int32_t>(0xefbeadde));
+}
+
+TEST(Byteswap, NotConstexpr) {
+  int8_t a = 0x12;
+  int16_t b = 0x1234;
+  int32_t c = 0x12345678;
+  int64_t d = 0x123456789abcdef0;
+  uint8_t e = 0x21;
+  uint16_t f = 0x4321;
+  uint32_t g = 0x87654321;
+  uint64_t h = 0xfedcba9876543210;
+  EXPECT_EQ(absl::byteswap<int8_t>(a), 0x12);
+  EXPECT_EQ(absl::byteswap<int16_t>(b), 0x3412);
+  EXPECT_EQ(absl::byteswap(c), 0x78563412);
+  EXPECT_EQ(absl::byteswap(d), 0xf0debc9a78563412);
+  EXPECT_EQ(absl::byteswap<uint8_t>(e), 0x21);
+  EXPECT_EQ(absl::byteswap<uint16_t>(f), 0x2143);
+  EXPECT_EQ(absl::byteswap(g), 0x21436587);
+  EXPECT_EQ(absl::byteswap(h), 0x1032547698badcfe);
+  EXPECT_EQ(absl::byteswap(absl::byteswap<int8_t>(a)), a);
+  EXPECT_EQ(absl::byteswap(absl::byteswap<int16_t>(b)), b);
+  EXPECT_EQ(absl::byteswap(absl::byteswap(c)), c);
+  EXPECT_EQ(absl::byteswap(absl::byteswap(d)), d);
+  EXPECT_EQ(absl::byteswap(absl::byteswap<uint8_t>(e)), e);
+  EXPECT_EQ(absl::byteswap(absl::byteswap<uint16_t>(f)), f);
+  EXPECT_EQ(absl::byteswap(absl::byteswap(g)), g);
+  EXPECT_EQ(absl::byteswap(absl::byteswap(h)), h);
+  EXPECT_EQ(absl::byteswap<uint32_t>(0xdeadbeef), 0xefbeadde);
+  EXPECT_EQ(absl::byteswap<const uint32_t>(0xdeadbeef), 0xefbeadde);
+}
+
+}  // namespace
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/numeric/int128.cc b/absl/numeric/int128.cc
index 93b62c52901..dc01c0fae8d 100644
--- a/absl/numeric/int128.cc
+++ b/absl/numeric/int128.cc
@@ -15,59 +15,47 @@
 #include "absl/numeric/int128.h"
 
 #include <stddef.h>
+
 #include <cassert>
+#include <cstdint>
 #include <iomanip>
+#include <ios>
+#include <limits>
 #include <ostream>  // NOLINT(readability/streams)
 #include <sstream>
 #include <string>
 #include <type_traits>
 
-namespace absl {
+#include "absl/base/attributes.h"
+#include "absl/base/optimization.h"
+#include "absl/numeric/bits.h"
 
-const uint128 kuint128max = MakeUint128(std::numeric_limits<uint64_t>::max(),
-                                        std::numeric_limits<uint64_t>::max());
+namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 namespace {
 
 // Returns the 0-based position of the last set bit (i.e., most significant bit)
-// in the given uint64_t. The argument may not be 0.
+// in the given uint128. The argument is not 0.
 //
 // For example:
 //   Given: 5 (decimal) == 101 (binary)
 //   Returns: 2
-#define STEP(T, n, pos, sh)                   \
-  do {                                        \
-    if ((n) >= (static_cast<T>(1) << (sh))) { \
-      (n) = (n) >> (sh);                      \
-      (pos) |= (sh);                          \
-    }                                         \
-  } while (0)
-static inline int Fls64(uint64_t n) {
-  assert(n != 0);
-  int pos = 0;
-  STEP(uint64_t, n, pos, 0x20);
-  uint32_t n32 = static_cast<uint32_t>(n);
-  STEP(uint32_t, n32, pos, 0x10);
-  STEP(uint32_t, n32, pos, 0x08);
-  STEP(uint32_t, n32, pos, 0x04);
-  return pos + ((uint64_t{0x3333333322221100} >> (n32 << 2)) & 0x3);
-}
-#undef STEP
-
-// Like Fls64() above, but returns the 0-based position of the last set bit
-// (i.e., most significant bit) in the given uint128. The argument may not be 0.
-static inline int Fls128(uint128 n) {
+inline ABSL_ATTRIBUTE_ALWAYS_INLINE int Fls128(uint128 n) {
   if (uint64_t hi = Uint128High64(n)) {
-    return Fls64(hi) + 64;
+    ABSL_ASSUME(hi != 0);
+    return 127 - countl_zero(hi);
   }
-  return Fls64(Uint128Low64(n));
+  const uint64_t low = Uint128Low64(n);
+  ABSL_ASSUME(low != 0);
+  return 63 - countl_zero(low);
 }
 
 // Long division/modulo for uint128 implemented using the shift-subtract
 // division algorithm adapted from:
 // https://stackoverflow.com/questions/5386377/division-without-using
-void DivModImpl(uint128 dividend, uint128 divisor, uint128* quotient_ret,
-                uint128* remainder_ret) {
+inline void DivModImpl(uint128 dividend, uint128 divisor, uint128* quotient_ret,
+                       uint128* remainder_ret) {
   assert(divisor != 0);
 
   if (divisor > dividend) {
@@ -106,7 +94,7 @@ void DivModImpl(uint128 dividend, uint128 divisor, uint128* quotient_ret,
 
 template <typename T>
 uint128 MakeUint128FromFloat(T v) {
-  static_assert(std::is_floating_point<T>::value, "");
+  static_assert(std::is_floating_point_v<T>);
 
   // Rounding behavior is towards zero, same as for built-in types.
 
@@ -124,14 +112,14 @@ uint128 MakeUint128FromFloat(T v) {
   return MakeUint128(0, static_cast<uint64_t>(v));
 }
 
-#if defined(__clang__) && !defined(__SSE3__)
+#if defined(__clang__) && (__clang_major__ < 9) && !defined(__SSE3__)
 // Workaround for clang bug: https://bugs.llvm.org/show_bug.cgi?id=38289
 // Casting from long double to uint64_t is miscompiled and drops bits.
 // It is more work, so only use when we need the workaround.
 uint128 MakeUint128FromFloat(long double v) {
   // Go 50 bits at a time, that fits in a double
-  static_assert(std::numeric_limits<double>::digits >= 50, "");
-  static_assert(std::numeric_limits<long double>::digits <= 150, "");
+  static_assert(std::numeric_limits<double>::digits >= 50);
+  static_assert(std::numeric_limits<long double>::digits <= 150);
   // Undefined behavior if v is not finite or cannot fit into uint128.
   assert(std::isfinite(v) && v > -1 && v < std::ldexp(1.0L, 128));
 
@@ -144,35 +132,28 @@ uint128 MakeUint128FromFloat(long double v) {
   return (static_cast<uint128>(w0) << 100) | (static_cast<uint128>(w1) << 50) |
          static_cast<uint128>(w2);
 }
-#endif  // __clang__ && !__SSE3__
+#endif  // __clang__ && (__clang_major__ < 9) && !__SSE3__
 }  // namespace
 
 uint128::uint128(float v) : uint128(MakeUint128FromFloat(v)) {}
 uint128::uint128(double v) : uint128(MakeUint128FromFloat(v)) {}
 uint128::uint128(long double v) : uint128(MakeUint128FromFloat(v)) {}
 
+#if !defined(ABSL_HAVE_INTRINSIC_INT128)
 uint128 operator/(uint128 lhs, uint128 rhs) {
-#if defined(ABSL_HAVE_INTRINSIC_INT128)
-  return static_cast<unsigned __int128>(lhs) /
-         static_cast<unsigned __int128>(rhs);
-#else  // ABSL_HAVE_INTRINSIC_INT128
   uint128 quotient = 0;
   uint128 remainder = 0;
   DivModImpl(lhs, rhs, &quotient, &remainder);
   return quotient;
-#endif  // ABSL_HAVE_INTRINSIC_INT128
 }
+
 uint128 operator%(uint128 lhs, uint128 rhs) {
-#if defined(ABSL_HAVE_INTRINSIC_INT128)
-  return static_cast<unsigned __int128>(lhs) %
-         static_cast<unsigned __int128>(rhs);
-#else  // ABSL_HAVE_INTRINSIC_INT128
   uint128 quotient = 0;
   uint128 remainder = 0;
   DivModImpl(lhs, rhs, &quotient, &remainder);
   return remainder;
-#endif  // ABSL_HAVE_INTRINSIC_INT128
 }
+#endif  // !defined(ABSL_HAVE_INTRINSIC_INT128)
 
 namespace {
 
@@ -222,6 +203,10 @@ std::string Uint128ToFormattedString(uint128 v, std::ios_base::fmtflags flags) {
 
 }  // namespace
 
+std::string uint128::ToString() const {
+  return Uint128ToFormattedString(*this, std::ios_base::dec);
+}
+
 std::ostream& operator<<(std::ostream& os, uint128 v) {
   std::ios_base::fmtflags flags = os.flags();
   std::string rep = Uint128ToFormattedString(v, flags);
@@ -229,45 +214,135 @@ std::ostream& operator<<(std::ostream& os, uint128 v) {
   // Add the requisite padding.
   std::streamsize width = os.width(0);
   if (static_cast<size_t>(width) > rep.size()) {
+    const size_t count = static_cast<size_t>(width) - rep.size();
     std::ios::fmtflags adjustfield = flags & std::ios::adjustfield;
     if (adjustfield == std::ios::left) {
-      rep.append(width - rep.size(), os.fill());
+      rep.append(count, os.fill());
     } else if (adjustfield == std::ios::internal &&
                (flags & std::ios::showbase) &&
                (flags & std::ios::basefield) == std::ios::hex && v != 0) {
-      rep.insert(2, width - rep.size(), os.fill());
+      rep.insert(size_t{2}, count, os.fill());
     } else {
-      rep.insert(0, width - rep.size(), os.fill());
+      rep.insert(size_t{0}, count, os.fill());
     }
   }
 
   return os << rep;
 }
 
-}  // namespace absl
+namespace {
+
+uint128 UnsignedAbsoluteValue(int128 v) {
+  // Cast to uint128 before possibly negating because -Int128Min() is undefined.
+  return Int128High64(v) < 0 ? -uint128(v) : uint128(v);
+}
+
+}  // namespace
+
+#if !defined(ABSL_HAVE_INTRINSIC_INT128)
+namespace {
+
+template <typename T>
+int128 MakeInt128FromFloat(T v) {
+  // Conversion when v is NaN or cannot fit into int128 would be undefined
+  // behavior if using an intrinsic 128-bit integer.
+  assert(std::isfinite(v) && (std::numeric_limits<T>::max_exponent <= 127 ||
+                              (v >= -std::ldexp(static_cast<T>(1), 127) &&
+                               v < std::ldexp(static_cast<T>(1), 127))));
+
+  // We must convert the absolute value and then negate as needed, because
+  // floating point types are typically sign-magnitude. Otherwise, the
+  // difference between the high and low 64 bits when interpreted as two's
+  // complement overwhelms the precision of the mantissa.
+  uint128 result = v < 0 ? -MakeUint128FromFloat(-v) : MakeUint128FromFloat(v);
+  return MakeInt128(int128_internal::BitCastToSigned(Uint128High64(result)),
+                    Uint128Low64(result));
+}
+
+}  // namespace
+
+int128::int128(float v) : int128(MakeInt128FromFloat(v)) {}
+int128::int128(double v) : int128(MakeInt128FromFloat(v)) {}
+int128::int128(long double v) : int128(MakeInt128FromFloat(v)) {}
+
+int128 operator/(int128 lhs, int128 rhs) {
+  assert(lhs != Int128Min() || rhs != -1);  // UB on two's complement.
+
+  uint128 quotient = 0;
+  uint128 remainder = 0;
+  DivModImpl(UnsignedAbsoluteValue(lhs), UnsignedAbsoluteValue(rhs),
+             &quotient, &remainder);
+  if ((Int128High64(lhs) < 0) != (Int128High64(rhs) < 0)) quotient = -quotient;
+  return MakeInt128(int128_internal::BitCastToSigned(Uint128High64(quotient)),
+                    Uint128Low64(quotient));
+}
+
+int128 operator%(int128 lhs, int128 rhs) {
+  assert(lhs != Int128Min() || rhs != -1);  // UB on two's complement.
+
+  uint128 quotient = 0;
+  uint128 remainder = 0;
+  DivModImpl(UnsignedAbsoluteValue(lhs), UnsignedAbsoluteValue(rhs),
+             &quotient, &remainder);
+  if (Int128High64(lhs) < 0) remainder = -remainder;
+  return MakeInt128(int128_internal::BitCastToSigned(Uint128High64(remainder)),
+                    Uint128Low64(remainder));
+}
+#endif  // ABSL_HAVE_INTRINSIC_INT128
+
+std::string int128::ToString() const {
+  std::string rep;
+  if (Int128High64(*this) < 0) rep = "-";
+  rep.append(Uint128ToFormattedString(UnsignedAbsoluteValue(*this),
+                                      std::ios_base::dec));
+  return rep;
+}
+
+std::ostream& operator<<(std::ostream& os, int128 v) {
+  std::ios_base::fmtflags flags = os.flags();
+  std::string rep;
+
+  // Add the sign if needed.
+  bool print_as_decimal =
+      (flags & std::ios::basefield) == std::ios::dec ||
+      (flags & std::ios::basefield) == std::ios_base::fmtflags();
+  if (print_as_decimal) {
+    if (Int128High64(v) < 0) {
+      rep = "-";
+    } else if (flags & std::ios::showpos) {
+      rep = "+";
+    }
+  }
 
-namespace std {
-constexpr bool numeric_limits<absl::uint128>::is_specialized;
-constexpr bool numeric_limits<absl::uint128>::is_signed;
-constexpr bool numeric_limits<absl::uint128>::is_integer;
-constexpr bool numeric_limits<absl::uint128>::is_exact;
-constexpr bool numeric_limits<absl::uint128>::has_infinity;
-constexpr bool numeric_limits<absl::uint128>::has_quiet_NaN;
-constexpr bool numeric_limits<absl::uint128>::has_signaling_NaN;
-constexpr float_denorm_style numeric_limits<absl::uint128>::has_denorm;
-constexpr bool numeric_limits<absl::uint128>::has_denorm_loss;
-constexpr float_round_style numeric_limits<absl::uint128>::round_style;
-constexpr bool numeric_limits<absl::uint128>::is_iec559;
-constexpr bool numeric_limits<absl::uint128>::is_bounded;
-constexpr bool numeric_limits<absl::uint128>::is_modulo;
-constexpr int numeric_limits<absl::uint128>::digits;
-constexpr int numeric_limits<absl::uint128>::digits10;
-constexpr int numeric_limits<absl::uint128>::max_digits10;
-constexpr int numeric_limits<absl::uint128>::radix;
-constexpr int numeric_limits<absl::uint128>::min_exponent;
-constexpr int numeric_limits<absl::uint128>::min_exponent10;
-constexpr int numeric_limits<absl::uint128>::max_exponent;
-constexpr int numeric_limits<absl::uint128>::max_exponent10;
-constexpr bool numeric_limits<absl::uint128>::traps;
-constexpr bool numeric_limits<absl::uint128>::tinyness_before;
-}  // namespace std
+  rep.append(Uint128ToFormattedString(
+      print_as_decimal ? UnsignedAbsoluteValue(v) : uint128(v), os.flags()));
+
+  // Add the requisite padding.
+  std::streamsize width = os.width(0);
+  if (static_cast<size_t>(width) > rep.size()) {
+    const size_t count = static_cast<size_t>(width) - rep.size();
+    switch (flags & std::ios::adjustfield) {
+      case std::ios::left:
+        rep.append(count, os.fill());
+        break;
+      case std::ios::internal:
+        if (print_as_decimal && (rep[0] == '+' || rep[0] == '-')) {
+          rep.insert(size_t{1}, count, os.fill());
+        } else if ((flags & std::ios::basefield) == std::ios::hex &&
+                   (flags & std::ios::showbase) && v != 0) {
+          rep.insert(size_t{2}, count, os.fill());
+        } else {
+          rep.insert(size_t{0}, count, os.fill());
+        }
+        break;
+      default:  // std::ios::right
+        rep.insert(size_t{0}, count, os.fill());
+        break;
+    }
+  }
+
+  return os << rep;
+}
+
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/numeric/int128.h b/absl/numeric/int128.h
index 10be8eca42e..0078ab051e3 100644
--- a/absl/numeric/int128.h
+++ b/absl/numeric/int128.h
@@ -17,10 +17,11 @@
 // File: int128.h
 // -----------------------------------------------------------------------------
 //
-// This header file defines 128-bit integer types.
+// This header file defines 128-bit integer types, `uint128` and `int128`.
 //
-// Currently, this file defines `uint128`, an unsigned 128-bit integer;
-// a signed 128-bit integer is forthcoming.
+// TODO(absl-team): This module is inconsistent as many inline `uint128` methods
+// are defined in this file, while many inline `int128` methods are defined in
+// the `int128_*_intrinsic.inc` files.
 
 #ifndef ABSL_NUMERIC_INT128_H_
 #define ABSL_NUMERIC_INT128_H_
@@ -31,11 +32,14 @@
 #include <cstring>
 #include <iosfwd>
 #include <limits>
+#include <string>
 #include <utility>
 
+#include "absl/base/attributes.h"
 #include "absl/base/config.h"
 #include "absl/base/macros.h"
 #include "absl/base/port.h"
+#include "absl/types/compare.h"
 
 #if defined(_MSC_VER)
 // In very old versions of MSVC and when the /Zc:wchar_t flag is off, wchar_t is
@@ -43,7 +47,7 @@
 // builtin type.  We need to make sure not to define operator wchar_t()
 // alongside operator unsigned short() in these instances.
 #define ABSL_INTERNAL_WCHAR_T __wchar_t
-#if defined(_M_X64)
+#if defined(_M_X64) && !defined(_M_ARM64EC)
 #include <intrin.h>
 #pragma intrinsic(_umul128)
 #endif  // defined(_M_X64)
@@ -52,6 +56,9 @@
 #endif  // defined(_MSC_VER)
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+class int128;
 
 // uint128
 //
@@ -115,7 +122,8 @@ class
 #ifdef ABSL_HAVE_INTRINSIC_INT128
   constexpr uint128(__int128 v);           // NOLINT(runtime/explicit)
   constexpr uint128(unsigned __int128 v);  // NOLINT(runtime/explicit)
-#endif  // ABSL_HAVE_INTRINSIC_INT128
+#endif                                     // ABSL_HAVE_INTRINSIC_INT128
+  constexpr uint128(int128 v);             // NOLINT(runtime/explicit)
   explicit uint128(float v);
   explicit uint128(double v);
   explicit uint128(long double v);
@@ -131,6 +139,7 @@ class
   uint128& operator=(__int128 v);
   uint128& operator=(unsigned __int128 v);
 #endif  // ABSL_HAVE_INTRINSIC_INT128
+  uint128& operator=(int128 v);
 
   // Conversion operators to other arithmetic types
   constexpr explicit operator bool() const;
@@ -156,9 +165,9 @@ class
   constexpr explicit operator __int128() const;
   constexpr explicit operator unsigned __int128() const;
 #endif  // ABSL_HAVE_INTRINSIC_INT128
-  explicit operator float() const;
-  explicit operator double() const;
-  explicit operator long double() const;
+  constexpr explicit operator float() const;
+  constexpr explicit operator double() const;
+  constexpr explicit operator long double() const;
 
   // Trivial copy constructor, assignment operator and destructor.
 
@@ -208,12 +217,24 @@ class
   // Support for absl::Hash.
   template <typename H>
   friend H AbslHashValue(H h, uint128 v) {
+#if defined(ABSL_HAVE_INTRINSIC_INT128)
+    return H::combine(std::move(h), static_cast<unsigned __int128>(v));
+#else
     return H::combine(std::move(h), Uint128High64(v), Uint128Low64(v));
+#endif
+  }
+
+  // Support for absl::StrCat() etc.
+  template <typename Sink>
+  friend void AbslStringify(Sink& sink, uint128 v) {
+    sink.Append(v.ToString());
   }
 
  private:
   constexpr uint128(uint64_t high, uint64_t low);
 
+  std::string ToString() const;
+
   // TODO(strel) Update implementation to use __int128 once all users of
   // uint128 are fixed to not depend on alignof(uint128) == 8. Also add
   // alignas(16) to class definition to keep alignment consistent across
@@ -229,11 +250,6 @@ class
 #endif  // byte order
 };
 
-// Prefer to use the constexpr `Uint128Max()`.
-//
-// TODO(absl-team) deprecate kuint128max once migration tool is released.
-extern const uint128 kuint128max;
-
 // allow uint128 to be logged
 std::ostream& operator<<(std::ostream& os, uint128 v);
 
@@ -244,6 +260,7 @@ constexpr uint128 Uint128Max() {
                  (std::numeric_limits<uint64_t>::max)());
 }
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 // Specialized numeric_limits for uint128.
@@ -258,7 +275,9 @@ class numeric_limits<absl::uint128> {
   static constexpr bool has_infinity = false;
   static constexpr bool has_quiet_NaN = false;
   static constexpr bool has_signaling_NaN = false;
+  ABSL_INTERNAL_DISABLE_DEPRECATED_DECLARATION_WARNING
   static constexpr float_denorm_style has_denorm = denorm_absent;
+  ABSL_INTERNAL_RESTORE_DEPRECATED_DECLARATION_WARNING
   static constexpr bool has_denorm_loss = false;
   static constexpr float_round_style round_style = round_toward_zero;
   static constexpr bool is_iec559 = false;
@@ -279,9 +298,9 @@ class numeric_limits<absl::uint128> {
 #endif  // ABSL_HAVE_INTRINSIC_INT128
   static constexpr bool tinyness_before = false;
 
-  static constexpr absl::uint128 (min)() { return 0; }
+  static constexpr absl::uint128(min)() { return 0; }
   static constexpr absl::uint128 lowest() { return 0; }
-  static constexpr absl::uint128 (max)() { return absl::Uint128Max(); }
+  static constexpr absl::uint128(max)() { return absl::Uint128Max(); }
   static constexpr absl::uint128 epsilon() { return 0; }
   static constexpr absl::uint128 round_error() { return 0; }
   static constexpr absl::uint128 infinity() { return 0; }
@@ -291,12 +310,264 @@ class numeric_limits<absl::uint128> {
 };
 }  // namespace std
 
-// TODO(absl-team): Implement signed 128-bit type
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+// int128
+//
+// A signed 128-bit integer type. The API is meant to mimic an intrinsic
+// integral type as closely as is practical, including exhibiting undefined
+// behavior in analogous cases (e.g. division by zero).
+//
+// An `int128` supports the following:
+//
+//   * Implicit construction from integral types
+//   * Explicit conversion to integral types
+//
+// However, an `int128` differs from intrinsic integral types in the following
+// ways:
+//
+//   * It is not implicitly convertible to other integral types.
+//   * Requires explicit construction from and conversion to floating point
+//     types.
+
+// Additionally, if your compiler supports `__int128`, `int128` is
+// interoperable with that type. (Abseil checks for this compatibility through
+// the `ABSL_HAVE_INTRINSIC_INT128` macro.)
+//
+// The design goal for `int128` is that it will be compatible with a future
+// `int128_t`, if that type becomes a part of the standard.
+//
+// Example:
+//
+//     float y = absl::int128(17);  // Error. int128 cannot be implicitly
+//                                  // converted to float.
+//
+//     absl::int128 v;
+//     int64_t i = v;                        // Error
+//     int64_t i = static_cast<int64_t>(v);  // OK
+//
+class int128 {
+ public:
+  int128() = default;
+
+  // Constructors from arithmetic types
+  constexpr int128(int v);                 // NOLINT(runtime/explicit)
+  constexpr int128(unsigned int v);        // NOLINT(runtime/explicit)
+  constexpr int128(long v);                // NOLINT(runtime/int)
+  constexpr int128(unsigned long v);       // NOLINT(runtime/int)
+  constexpr int128(long long v);           // NOLINT(runtime/int)
+  constexpr int128(unsigned long long v);  // NOLINT(runtime/int)
+  constexpr explicit int128(uint128 v);
+#ifdef ABSL_HAVE_INTRINSIC_INT128
+  constexpr int128(__int128 v);  // NOLINT(runtime/explicit)
+  constexpr explicit int128(unsigned __int128 v);
+  constexpr explicit int128(float v);
+  constexpr explicit int128(double v);
+  constexpr explicit int128(long double v);
+#else
+  explicit int128(float v);
+  explicit int128(double v);
+  explicit int128(long double v);
+#endif  // ABSL_HAVE_INTRINSIC_INT128
+
+  // Assignment operators from arithmetic types
+  int128& operator=(int v);
+  int128& operator=(unsigned int v);
+  int128& operator=(long v);                // NOLINT(runtime/int)
+  int128& operator=(unsigned long v);       // NOLINT(runtime/int)
+  int128& operator=(long long v);           // NOLINT(runtime/int)
+  int128& operator=(unsigned long long v);  // NOLINT(runtime/int)
+#ifdef ABSL_HAVE_INTRINSIC_INT128
+  int128& operator=(__int128 v);
+#endif  // ABSL_HAVE_INTRINSIC_INT128
+
+  // Conversion operators to other arithmetic types
+  constexpr explicit operator bool() const;
+  constexpr explicit operator char() const;
+  constexpr explicit operator signed char() const;
+  constexpr explicit operator unsigned char() const;
+  constexpr explicit operator char16_t() const;
+  constexpr explicit operator char32_t() const;
+  constexpr explicit operator ABSL_INTERNAL_WCHAR_T() const;
+  constexpr explicit operator short() const;  // NOLINT(runtime/int)
+  // NOLINTNEXTLINE(runtime/int)
+  constexpr explicit operator unsigned short() const;
+  constexpr explicit operator int() const;
+  constexpr explicit operator unsigned int() const;
+  constexpr explicit operator long() const;  // NOLINT(runtime/int)
+  // NOLINTNEXTLINE(runtime/int)
+  constexpr explicit operator unsigned long() const;
+  // NOLINTNEXTLINE(runtime/int)
+  constexpr explicit operator long long() const;
+  // NOLINTNEXTLINE(runtime/int)
+  constexpr explicit operator unsigned long long() const;
+#ifdef ABSL_HAVE_INTRINSIC_INT128
+  constexpr explicit operator __int128() const;
+  constexpr explicit operator unsigned __int128() const;
+#endif  // ABSL_HAVE_INTRINSIC_INT128
+  constexpr explicit operator float() const;
+  constexpr explicit operator double() const;
+  constexpr explicit operator long double() const;
+
+  // Trivial copy constructor, assignment operator and destructor.
+
+  // Arithmetic operators
+  int128& operator+=(int128 other);
+  int128& operator-=(int128 other);
+  int128& operator*=(int128 other);
+  int128& operator/=(int128 other);
+  int128& operator%=(int128 other);
+  int128 operator++(int);  // postfix increment: i++
+  int128 operator--(int);  // postfix decrement: i--
+  int128& operator++();    // prefix increment:  ++i
+  int128& operator--();    // prefix decrement:  --i
+  int128& operator&=(int128 other);
+  int128& operator|=(int128 other);
+  int128& operator^=(int128 other);
+  int128& operator<<=(int amount);
+  int128& operator>>=(int amount);
+
+  // Int128Low64()
+  //
+  // Returns the lower 64-bit value of a `int128` value.
+  friend constexpr uint64_t Int128Low64(int128 v);
+
+  // Int128High64()
+  //
+  // Returns the higher 64-bit value of a `int128` value.
+  friend constexpr int64_t Int128High64(int128 v);
+
+  // MakeInt128()
+  //
+  // Constructs a `int128` numeric value from two 64-bit integers. Note that
+  // signedness is conveyed in the upper `high` value.
+  //
+  //   (absl::int128(1) << 64) * high + low
+  //
+  // Note that this factory function is the only way to construct a `int128`
+  // from integer values greater than 2^64 or less than -2^64.
+  //
+  // Example:
+  //
+  //   absl::int128 big = absl::MakeInt128(1, 0);
+  //   absl::int128 big_n = absl::MakeInt128(-1, 0);
+  friend constexpr int128 MakeInt128(int64_t high, uint64_t low);
+
+  // Int128Max()
+  //
+  // Returns the maximum value for a 128-bit signed integer.
+  friend constexpr int128 Int128Max();
+
+  // Int128Min()
+  //
+  // Returns the minimum value for a 128-bit signed integer.
+  friend constexpr int128 Int128Min();
+
+  // Support for absl::Hash.
+  template <typename H>
+  friend H AbslHashValue(H h, int128 v) {
+#if defined(ABSL_HAVE_INTRINSIC_INT128)
+    return H::combine(std::move(h), v.v_);
+#else
+    return H::combine(std::move(h), Int128High64(v), Int128Low64(v));
+#endif
+  }
+
+  // Support for absl::StrCat() etc.
+  template <typename Sink>
+  friend void AbslStringify(Sink& sink, int128 v) {
+    sink.Append(v.ToString());
+  }
+
+ private:
+  constexpr int128(int64_t high, uint64_t low);
+
+  std::string ToString() const;
+
+#if defined(ABSL_HAVE_INTRINSIC_INT128)
+  __int128 v_;
+#else  // ABSL_HAVE_INTRINSIC_INT128
+#if defined(ABSL_IS_LITTLE_ENDIAN)
+  uint64_t lo_;
+  int64_t hi_;
+#elif defined(ABSL_IS_BIG_ENDIAN)
+  int64_t hi_;
+  uint64_t lo_;
+#else  // byte order
+#error "Unsupported byte order: must be little-endian or big-endian."
+#endif  // byte order
+#endif  // ABSL_HAVE_INTRINSIC_INT128
+};
+
+std::ostream& operator<<(std::ostream& os, int128 v);
+
+// TODO(absl-team) add operator>>(std::istream&, int128)
+
+constexpr int128 Int128Max() {
+  return int128((std::numeric_limits<int64_t>::max)(),
+                (std::numeric_limits<uint64_t>::max)());
+}
+
+constexpr int128 Int128Min() {
+  return int128((std::numeric_limits<int64_t>::min)(), 0);
+}
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+// Specialized numeric_limits for int128.
+namespace std {
+template <>
+class numeric_limits<absl::int128> {
+ public:
+  static constexpr bool is_specialized = true;
+  static constexpr bool is_signed = true;
+  static constexpr bool is_integer = true;
+  static constexpr bool is_exact = true;
+  static constexpr bool has_infinity = false;
+  static constexpr bool has_quiet_NaN = false;
+  static constexpr bool has_signaling_NaN = false;
+  ABSL_INTERNAL_DISABLE_DEPRECATED_DECLARATION_WARNING
+  static constexpr float_denorm_style has_denorm = denorm_absent;
+  ABSL_INTERNAL_RESTORE_DEPRECATED_DECLARATION_WARNING
+  static constexpr bool has_denorm_loss = false;
+  static constexpr float_round_style round_style = round_toward_zero;
+  static constexpr bool is_iec559 = false;
+  static constexpr bool is_bounded = true;
+  static constexpr bool is_modulo = false;
+  static constexpr int digits = 127;
+  static constexpr int digits10 = 38;
+  static constexpr int max_digits10 = 0;
+  static constexpr int radix = 2;
+  static constexpr int min_exponent = 0;
+  static constexpr int min_exponent10 = 0;
+  static constexpr int max_exponent = 0;
+  static constexpr int max_exponent10 = 0;
+#ifdef ABSL_HAVE_INTRINSIC_INT128
+  static constexpr bool traps = numeric_limits<__int128>::traps;
+#else   // ABSL_HAVE_INTRINSIC_INT128
+  static constexpr bool traps = numeric_limits<uint64_t>::traps;
+#endif  // ABSL_HAVE_INTRINSIC_INT128
+  static constexpr bool tinyness_before = false;
+
+  static constexpr absl::int128(min)() { return absl::Int128Min(); }
+  static constexpr absl::int128 lowest() { return absl::Int128Min(); }
+  static constexpr absl::int128(max)() { return absl::Int128Max(); }
+  static constexpr absl::int128 epsilon() { return 0; }
+  static constexpr absl::int128 round_error() { return 0; }
+  static constexpr absl::int128 infinity() { return 0; }
+  static constexpr absl::int128 quiet_NaN() { return 0; }
+  static constexpr absl::int128 signaling_NaN() { return 0; }
+  static constexpr absl::int128 denorm_min() { return 0; }
+};
+}  // namespace std
 
 // --------------------------------------------------------------------------
 //                      Implementation details follow
 // --------------------------------------------------------------------------
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 constexpr uint128 MakeUint128(uint64_t high, uint64_t low) {
   return uint128(high, low);
@@ -320,9 +591,7 @@ inline uint128& uint128::operator=(unsigned long v) {
 }
 
 // NOLINTNEXTLINE(runtime/int)
-inline uint128& uint128::operator=(long long v) {
-  return *this = uint128(v);
-}
+inline uint128& uint128::operator=(long long v) { return *this = uint128(v); }
 
 // NOLINTNEXTLINE(runtime/int)
 inline uint128& uint128::operator=(unsigned long long v) {
@@ -330,24 +599,30 @@ inline uint128& uint128::operator=(unsigned long long v) {
 }
 
 #ifdef ABSL_HAVE_INTRINSIC_INT128
-inline uint128& uint128::operator=(__int128 v) {
-  return *this = uint128(v);
-}
+inline uint128& uint128::operator=(__int128 v) { return *this = uint128(v); }
 
 inline uint128& uint128::operator=(unsigned __int128 v) {
   return *this = uint128(v);
 }
 #endif  // ABSL_HAVE_INTRINSIC_INT128
 
+inline uint128& uint128::operator=(int128 v) { return *this = uint128(v); }
+
 // Arithmetic operators.
 
-uint128 operator<<(uint128 lhs, int amount);
-uint128 operator>>(uint128 lhs, int amount);
-uint128 operator+(uint128 lhs, uint128 rhs);
-uint128 operator-(uint128 lhs, uint128 rhs);
+constexpr uint128 operator<<(uint128 lhs, int amount);
+constexpr uint128 operator>>(uint128 lhs, int amount);
+constexpr uint128 operator+(uint128 lhs, uint128 rhs);
+constexpr uint128 operator-(uint128 lhs, uint128 rhs);
+#if defined(ABSL_HAVE_INTRINSIC_INT128)
+constexpr uint128 operator*(uint128 lhs, uint128 rhs);
+constexpr uint128 operator/(uint128 lhs, uint128 rhs);
+constexpr uint128 operator%(uint128 lhs, uint128 rhs);
+#else   // ABSL_HAVE_INTRINSIC_INT128
 uint128 operator*(uint128 lhs, uint128 rhs);
 uint128 operator/(uint128 lhs, uint128 rhs);
 uint128 operator%(uint128 lhs, uint128 rhs);
+#endif  // ABSL_HAVE_INTRINSIC_INT128
 
 inline uint128& uint128::operator<<=(int amount) {
   *this = *this << amount;
@@ -392,8 +667,7 @@ constexpr uint64_t Uint128High64(uint128 v) { return v.hi_; }
 
 #if defined(ABSL_IS_LITTLE_ENDIAN)
 
-constexpr uint128::uint128(uint64_t high, uint64_t low)
-    : lo_{low}, hi_{high} {}
+constexpr uint128::uint128(uint64_t high, uint64_t low) : lo_{low}, hi_{high} {}
 
 constexpr uint128::uint128(int v)
     : lo_{static_cast<uint64_t>(v)},
@@ -420,10 +694,12 @@ constexpr uint128::uint128(unsigned __int128 v)
       hi_{static_cast<uint64_t>(v >> 64)} {}
 #endif  // ABSL_HAVE_INTRINSIC_INT128
 
+constexpr uint128::uint128(int128 v)
+    : lo_{Int128Low64(v)}, hi_{static_cast<uint64_t>(Int128High64(v))} {}
+
 #elif defined(ABSL_IS_BIG_ENDIAN)
 
-constexpr uint128::uint128(uint64_t high, uint64_t low)
-    : hi_{high}, lo_{low} {}
+constexpr uint128::uint128(uint64_t high, uint64_t low) : hi_{high}, lo_{low} {}
 
 constexpr uint128::uint128(int v)
     : hi_{v < 0 ? (std::numeric_limits<uint64_t>::max)() : 0},
@@ -450,6 +726,9 @@ constexpr uint128::uint128(unsigned __int128 v)
       lo_{static_cast<uint64_t>(v & ~uint64_t{0})} {}
 #endif  // ABSL_HAVE_INTRINSIC_INT128
 
+constexpr uint128::uint128(int128 v)
+    : hi_{static_cast<uint64_t>(Int128High64(v))}, lo_{Int128Low64(v)} {}
+
 #else  // byte order
 #error "Unsupported byte order: must be little-endian or big-endian."
 #endif  // byte order
@@ -520,167 +799,248 @@ constexpr uint128::operator unsigned __int128() const {
 
 // Conversion operators to floating point types.
 
-inline uint128::operator float() const {
-  return static_cast<float>(lo_) + std::ldexp(static_cast<float>(hi_), 64);
+constexpr uint128::operator float() const {
+  // Note: This method might return Inf.
+  constexpr float pow_2_64 = 18446744073709551616.0f;
+  return static_cast<float>(lo_) + static_cast<float>(hi_) * pow_2_64;
 }
 
-inline uint128::operator double() const {
-  return static_cast<double>(lo_) + std::ldexp(static_cast<double>(hi_), 64);
+constexpr uint128::operator double() const {
+  constexpr double pow_2_64 = 18446744073709551616.0;
+  return static_cast<double>(lo_) + static_cast<double>(hi_) * pow_2_64;
 }
 
-inline uint128::operator long double() const {
+constexpr uint128::operator long double() const {
+  constexpr long double pow_2_64 = 18446744073709551616.0L;
   return static_cast<long double>(lo_) +
-         std::ldexp(static_cast<long double>(hi_), 64);
+         static_cast<long double>(hi_) * pow_2_64;
 }
 
 // Comparison operators.
 
-inline bool operator==(uint128 lhs, uint128 rhs) {
+constexpr bool operator==(uint128 lhs, uint128 rhs) {
+#if defined(ABSL_HAVE_INTRINSIC_INT128)
+  return static_cast<unsigned __int128>(lhs) ==
+         static_cast<unsigned __int128>(rhs);
+#else
   return (Uint128Low64(lhs) == Uint128Low64(rhs) &&
           Uint128High64(lhs) == Uint128High64(rhs));
+#endif
 }
 
-inline bool operator!=(uint128 lhs, uint128 rhs) {
-  return !(lhs == rhs);
-}
+constexpr bool operator!=(uint128 lhs, uint128 rhs) { return !(lhs == rhs); }
 
-inline bool operator<(uint128 lhs, uint128 rhs) {
+constexpr bool operator<(uint128 lhs, uint128 rhs) {
+#ifdef ABSL_HAVE_INTRINSIC_INT128
+  return static_cast<unsigned __int128>(lhs) <
+         static_cast<unsigned __int128>(rhs);
+#else
   return (Uint128High64(lhs) == Uint128High64(rhs))
              ? (Uint128Low64(lhs) < Uint128Low64(rhs))
              : (Uint128High64(lhs) < Uint128High64(rhs));
+#endif
 }
 
-inline bool operator>(uint128 lhs, uint128 rhs) {
-  return (Uint128High64(lhs) == Uint128High64(rhs))
-             ? (Uint128Low64(lhs) > Uint128Low64(rhs))
-             : (Uint128High64(lhs) > Uint128High64(rhs));
-}
+constexpr bool operator>(uint128 lhs, uint128 rhs) { return rhs < lhs; }
 
-inline bool operator<=(uint128 lhs, uint128 rhs) {
-  return (Uint128High64(lhs) == Uint128High64(rhs))
-             ? (Uint128Low64(lhs) <= Uint128Low64(rhs))
-             : (Uint128High64(lhs) <= Uint128High64(rhs));
-}
+constexpr bool operator<=(uint128 lhs, uint128 rhs) { return !(rhs < lhs); }
 
-inline bool operator>=(uint128 lhs, uint128 rhs) {
-  return (Uint128High64(lhs) == Uint128High64(rhs))
-             ? (Uint128Low64(lhs) >= Uint128Low64(rhs))
-             : (Uint128High64(lhs) >= Uint128High64(rhs));
+constexpr bool operator>=(uint128 lhs, uint128 rhs) { return !(lhs < rhs); }
+
+#ifdef __cpp_impl_three_way_comparison
+constexpr absl::strong_ordering operator<=>(uint128 lhs, uint128 rhs) {
+#if defined(ABSL_HAVE_INTRINSIC_INT128)
+  if (auto lhs_128 = static_cast<unsigned __int128>(lhs),
+      rhs_128 = static_cast<unsigned __int128>(rhs);
+      lhs_128 < rhs_128) {
+    return absl::strong_ordering::less;
+  } else if (lhs_128 > rhs_128) {
+    return absl::strong_ordering::greater;
+  } else {
+    return absl::strong_ordering::equal;
+  }
+#else
+  if (uint64_t lhs_high = Uint128High64(lhs), rhs_high = Uint128High64(rhs);
+      lhs_high < rhs_high) {
+    return absl::strong_ordering::less;
+  } else if (lhs_high > rhs_high) {
+    return absl::strong_ordering::greater;
+  } else if (uint64_t lhs_low = Uint128Low64(lhs), rhs_low = Uint128Low64(rhs);
+             lhs_low < rhs_low) {
+    return absl::strong_ordering::less;
+  } else if (lhs_low > rhs_low) {
+    return absl::strong_ordering::greater;
+  } else {
+    return absl::strong_ordering::equal;
+  }
+#endif
 }
+#endif
 
 // Unary operators.
 
-inline uint128 operator-(uint128 val) {
-  uint64_t hi = ~Uint128High64(val);
-  uint64_t lo = ~Uint128Low64(val) + 1;
-  if (lo == 0) ++hi;  // carry
-  return MakeUint128(hi, lo);
+constexpr inline uint128 operator+(uint128 val) { return val; }
+
+constexpr inline int128 operator+(int128 val) { return val; }
+
+constexpr uint128 operator-(uint128 val) {
+#if defined(ABSL_HAVE_INTRINSIC_INT128)
+  return -static_cast<unsigned __int128>(val);
+#else
+  return MakeUint128(
+      ~Uint128High64(val) + static_cast<unsigned long>(Uint128Low64(val) == 0),
+      ~Uint128Low64(val) + 1);
+#endif
 }
 
-inline bool operator!(uint128 val) {
+constexpr inline bool operator!(uint128 val) {
+#if defined(ABSL_HAVE_INTRINSIC_INT128)
+  return !static_cast<unsigned __int128>(val);
+#else
   return !Uint128High64(val) && !Uint128Low64(val);
+#endif
 }
 
 // Logical operators.
 
-inline uint128 operator~(uint128 val) {
+constexpr inline uint128 operator~(uint128 val) {
+#if defined(ABSL_HAVE_INTRINSIC_INT128)
+  return ~static_cast<unsigned __int128>(val);
+#else
   return MakeUint128(~Uint128High64(val), ~Uint128Low64(val));
+#endif
 }
 
-inline uint128 operator|(uint128 lhs, uint128 rhs) {
+constexpr inline uint128 operator|(uint128 lhs, uint128 rhs) {
+#if defined(ABSL_HAVE_INTRINSIC_INT128)
+  return static_cast<unsigned __int128>(lhs) |
+         static_cast<unsigned __int128>(rhs);
+#else
   return MakeUint128(Uint128High64(lhs) | Uint128High64(rhs),
-                           Uint128Low64(lhs) | Uint128Low64(rhs));
+                     Uint128Low64(lhs) | Uint128Low64(rhs));
+#endif
 }
 
-inline uint128 operator&(uint128 lhs, uint128 rhs) {
+constexpr inline uint128 operator&(uint128 lhs, uint128 rhs) {
+#if defined(ABSL_HAVE_INTRINSIC_INT128)
+  return static_cast<unsigned __int128>(lhs) &
+         static_cast<unsigned __int128>(rhs);
+#else
   return MakeUint128(Uint128High64(lhs) & Uint128High64(rhs),
-                           Uint128Low64(lhs) & Uint128Low64(rhs));
+                     Uint128Low64(lhs) & Uint128Low64(rhs));
+#endif
 }
 
-inline uint128 operator^(uint128 lhs, uint128 rhs) {
+constexpr inline uint128 operator^(uint128 lhs, uint128 rhs) {
+#if defined(ABSL_HAVE_INTRINSIC_INT128)
+  return static_cast<unsigned __int128>(lhs) ^
+         static_cast<unsigned __int128>(rhs);
+#else
   return MakeUint128(Uint128High64(lhs) ^ Uint128High64(rhs),
-                           Uint128Low64(lhs) ^ Uint128Low64(rhs));
+                     Uint128Low64(lhs) ^ Uint128Low64(rhs));
+#endif
 }
 
 inline uint128& uint128::operator|=(uint128 other) {
-  hi_ |= other.hi_;
-  lo_ |= other.lo_;
+  *this = *this | other;
   return *this;
 }
 
 inline uint128& uint128::operator&=(uint128 other) {
-  hi_ &= other.hi_;
-  lo_ &= other.lo_;
+  *this = *this & other;
   return *this;
 }
 
 inline uint128& uint128::operator^=(uint128 other) {
-  hi_ ^= other.hi_;
-  lo_ ^= other.lo_;
+  *this = *this ^ other;
   return *this;
 }
 
 // Arithmetic operators.
 
-inline uint128 operator<<(uint128 lhs, int amount) {
+constexpr uint128 operator<<(uint128 lhs, int amount) {
+#ifdef ABSL_HAVE_INTRINSIC_INT128
+  return static_cast<unsigned __int128>(lhs) << amount;
+#else
   // uint64_t shifts of >= 64 are undefined, so we will need some
   // special-casing.
-  if (amount < 64) {
-    if (amount != 0) {
-      return MakeUint128(
-          (Uint128High64(lhs) << amount) | (Uint128Low64(lhs) >> (64 - amount)),
-          Uint128Low64(lhs) << amount);
-    }
-    return lhs;
-  }
-  return MakeUint128(Uint128Low64(lhs) << (amount - 64), 0);
+  return amount >= 64  ? MakeUint128(Uint128Low64(lhs) << (amount - 64), 0)
+         : amount == 0 ? lhs
+                       : MakeUint128((Uint128High64(lhs) << amount) |
+                                         (Uint128Low64(lhs) >> (64 - amount)),
+                                     Uint128Low64(lhs) << amount);
+#endif
 }
 
-inline uint128 operator>>(uint128 lhs, int amount) {
+constexpr uint128 operator>>(uint128 lhs, int amount) {
+#ifdef ABSL_HAVE_INTRINSIC_INT128
+  return static_cast<unsigned __int128>(lhs) >> amount;
+#else
   // uint64_t shifts of >= 64 are undefined, so we will need some
   // special-casing.
-  if (amount < 64) {
-    if (amount != 0) {
-      return MakeUint128(Uint128High64(lhs) >> amount,
-                         (Uint128Low64(lhs) >> amount) |
-                             (Uint128High64(lhs) << (64 - amount)));
-    }
-    return lhs;
-  }
-  return MakeUint128(0, Uint128High64(lhs) >> (amount - 64));
+  return amount >= 64  ? MakeUint128(0, Uint128High64(lhs) >> (amount - 64))
+         : amount == 0 ? lhs
+                       : MakeUint128(Uint128High64(lhs) >> amount,
+                                     (Uint128Low64(lhs) >> amount) |
+                                         (Uint128High64(lhs) << (64 - amount)));
+#endif
 }
 
-inline uint128 operator+(uint128 lhs, uint128 rhs) {
-  uint128 result = MakeUint128(Uint128High64(lhs) + Uint128High64(rhs),
-                               Uint128Low64(lhs) + Uint128Low64(rhs));
-  if (Uint128Low64(result) < Uint128Low64(lhs)) {  // check for carry
-    return MakeUint128(Uint128High64(result) + 1, Uint128Low64(result));
-  }
-  return result;
+#if !defined(ABSL_HAVE_INTRINSIC_INT128)
+namespace int128_internal {
+constexpr uint128 AddResult(uint128 result, uint128 lhs) {
+  // check for carry
+  return (Uint128Low64(result) < Uint128Low64(lhs))
+             ? MakeUint128(Uint128High64(result) + 1, Uint128Low64(result))
+             : result;
 }
+}  // namespace int128_internal
+#endif
 
-inline uint128 operator-(uint128 lhs, uint128 rhs) {
-  uint128 result = MakeUint128(Uint128High64(lhs) - Uint128High64(rhs),
-                               Uint128Low64(lhs) - Uint128Low64(rhs));
-  if (Uint128Low64(lhs) < Uint128Low64(rhs)) {  // check for carry
-    return MakeUint128(Uint128High64(result) - 1, Uint128Low64(result));
-  }
-  return result;
+constexpr uint128 operator+(uint128 lhs, uint128 rhs) {
+#if defined(ABSL_HAVE_INTRINSIC_INT128)
+  return static_cast<unsigned __int128>(lhs) +
+         static_cast<unsigned __int128>(rhs);
+#else
+  return int128_internal::AddResult(
+      MakeUint128(Uint128High64(lhs) + Uint128High64(rhs),
+                  Uint128Low64(lhs) + Uint128Low64(rhs)),
+      lhs);
+#endif
 }
 
-inline uint128 operator*(uint128 lhs, uint128 rhs) {
+#if !defined(ABSL_HAVE_INTRINSIC_INT128)
+namespace int128_internal {
+constexpr uint128 SubstructResult(uint128 result, uint128 lhs, uint128 rhs) {
+  // check for carry
+  return (Uint128Low64(lhs) < Uint128Low64(rhs))
+             ? MakeUint128(Uint128High64(result) - 1, Uint128Low64(result))
+             : result;
+}
+}  // namespace int128_internal
+#endif
+
+constexpr uint128 operator-(uint128 lhs, uint128 rhs) {
 #if defined(ABSL_HAVE_INTRINSIC_INT128)
-  // TODO(strel) Remove once alignment issues are resolved and unsigned __int128
-  // can be used for uint128 storage.
-  return static_cast<unsigned __int128>(lhs) *
+  return static_cast<unsigned __int128>(lhs) -
          static_cast<unsigned __int128>(rhs);
-#elif defined(_MSC_VER) && defined(_M_X64)
+#else
+  return int128_internal::SubstructResult(
+      MakeUint128(Uint128High64(lhs) - Uint128High64(rhs),
+                  Uint128Low64(lhs) - Uint128Low64(rhs)),
+      lhs, rhs);
+#endif
+}
+
+#if !defined(ABSL_HAVE_INTRINSIC_INT128)
+inline uint128 operator*(uint128 lhs, uint128 rhs) {
+#if defined(_MSC_VER) && defined(_M_X64) && !defined(_M_ARM64EC)
   uint64_t carry;
   uint64_t low = _umul128(Uint128Low64(lhs), Uint128Low64(rhs), &carry);
   return MakeUint128(Uint128Low64(lhs) * Uint128High64(rhs) +
                          Uint128High64(lhs) * Uint128Low64(rhs) + carry,
                      low);
-#else   // ABSL_HAVE_INTRINSIC128
+#else   // _MSC_VER
   uint64_t a32 = Uint128Low64(lhs) >> 32;
   uint64_t a00 = Uint128Low64(lhs) & 0xffffffff;
   uint64_t b32 = Uint128Low64(rhs) >> 32;
@@ -692,8 +1052,28 @@ inline uint128 operator*(uint128 lhs, uint128 rhs) {
   result += uint128(a32 * b00) << 32;
   result += uint128(a00 * b32) << 32;
   return result;
-#endif  // ABSL_HAVE_INTRINSIC128
+#endif  // _MSC_VER
 }
+#endif  // ABSL_HAVE_INTRINSIC_INT128
+
+#if defined(ABSL_HAVE_INTRINSIC_INT128)
+constexpr uint128 operator*(uint128 lhs, uint128 rhs) {
+  // TODO(strel) Remove once alignment issues are resolved and unsigned __int128
+  // can be used for uint128 storage.
+  return static_cast<unsigned __int128>(lhs) *
+         static_cast<unsigned __int128>(rhs);
+}
+
+constexpr uint128 operator/(uint128 lhs, uint128 rhs) {
+  return static_cast<unsigned __int128>(lhs) /
+         static_cast<unsigned __int128>(rhs);
+}
+
+constexpr uint128 operator%(uint128 lhs, uint128 rhs) {
+  return static_cast<unsigned __int128>(lhs) %
+         static_cast<unsigned __int128>(rhs);
+}
+#endif
 
 // Increment/decrement operators.
 
@@ -719,12 +1099,125 @@ inline uint128& uint128::operator--() {
   return *this;
 }
 
+constexpr int128 MakeInt128(int64_t high, uint64_t low) {
+  return int128(high, low);
+}
+
+// Assignment from integer types.
+inline int128& int128::operator=(int v) { return *this = int128(v); }
+
+inline int128& int128::operator=(unsigned int v) { return *this = int128(v); }
+
+inline int128& int128::operator=(long v) {  // NOLINT(runtime/int)
+  return *this = int128(v);
+}
+
+// NOLINTNEXTLINE(runtime/int)
+inline int128& int128::operator=(unsigned long v) { return *this = int128(v); }
+
+// NOLINTNEXTLINE(runtime/int)
+inline int128& int128::operator=(long long v) { return *this = int128(v); }
+
+// NOLINTNEXTLINE(runtime/int)
+inline int128& int128::operator=(unsigned long long v) {
+  return *this = int128(v);
+}
+
+// Arithmetic operators.
+constexpr int128 operator-(int128 v);
+constexpr int128 operator+(int128 lhs, int128 rhs);
+constexpr int128 operator-(int128 lhs, int128 rhs);
+#if defined(ABSL_HAVE_INTRINSIC_INT128)
+constexpr int128 operator*(int128 lhs, int128 rhs);
+constexpr int128 operator/(int128 lhs, int128 rhs);
+constexpr int128 operator%(int128 lhs, int128 rhs);
+#else
+int128 operator*(int128 lhs, int128 rhs);
+int128 operator/(int128 lhs, int128 rhs);
+int128 operator%(int128 lhs, int128 rhs);
+#endif  // ABSL_HAVE_INTRINSIC_INT128
+constexpr int128 operator|(int128 lhs, int128 rhs);
+constexpr int128 operator&(int128 lhs, int128 rhs);
+constexpr int128 operator^(int128 lhs, int128 rhs);
+constexpr int128 operator<<(int128 lhs, int amount);
+constexpr int128 operator>>(int128 lhs, int amount);
+
+inline int128& int128::operator+=(int128 other) {
+  *this = *this + other;
+  return *this;
+}
+
+inline int128& int128::operator-=(int128 other) {
+  *this = *this - other;
+  return *this;
+}
+
+inline int128& int128::operator*=(int128 other) {
+  *this = *this * other;
+  return *this;
+}
+
+inline int128& int128::operator/=(int128 other) {
+  *this = *this / other;
+  return *this;
+}
+
+inline int128& int128::operator%=(int128 other) {
+  *this = *this % other;
+  return *this;
+}
+
+inline int128& int128::operator|=(int128 other) {
+  *this = *this | other;
+  return *this;
+}
+
+inline int128& int128::operator&=(int128 other) {
+  *this = *this & other;
+  return *this;
+}
+
+inline int128& int128::operator^=(int128 other) {
+  *this = *this ^ other;
+  return *this;
+}
+
+inline int128& int128::operator<<=(int amount) {
+  *this = *this << amount;
+  return *this;
+}
+
+inline int128& int128::operator>>=(int amount) {
+  *this = *this >> amount;
+  return *this;
+}
+
+// Forward declaration for comparison operators.
+constexpr bool operator!=(int128 lhs, int128 rhs);
+
+namespace int128_internal {
+
+// Casts from unsigned to signed while preserving the underlying binary
+// representation.
+constexpr int64_t BitCastToSigned(uint64_t v) {
+  // Casting an unsigned integer to a signed integer of the same
+  // width is implementation defined behavior if the source value would not fit
+  // in the destination type. We step around it with a roundtrip bitwise not
+  // operation to make sure this function remains constexpr. Clang, GCC, and
+  // MSVC optimize this to a no-op on x86-64.
+  return v & (uint64_t{1} << 63) ? ~static_cast<int64_t>(~v)
+                                 : static_cast<int64_t>(v);
+}
+
+}  // namespace int128_internal
+
 #if defined(ABSL_HAVE_INTRINSIC_INT128)
-#include "absl/numeric/int128_have_intrinsic.inc"
+#include "absl/numeric/int128_have_intrinsic.inc"  // IWYU pragma: export
 #else  // ABSL_HAVE_INTRINSIC_INT128
-#include "absl/numeric/int128_no_intrinsic.inc"
+#include "absl/numeric/int128_no_intrinsic.inc"  // IWYU pragma: export
 #endif  // ABSL_HAVE_INTRINSIC_INT128
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #undef ABSL_INTERNAL_WCHAR_T
diff --git a/absl/numeric/int128_benchmark.cc b/absl/numeric/int128_benchmark.cc
index a5502d927c0..3fee6be05d4 100644
--- a/absl/numeric/int128_benchmark.cc
+++ b/absl/numeric/int128_benchmark.cc
@@ -12,81 +12,105 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-#include "absl/numeric/int128.h"
-
 #include <algorithm>
+#include <cstddef>
 #include <cstdint>
+#include <limits>
 #include <random>
+#include <type_traits>
+#include <utility>
 #include <vector>
 
-#include "benchmark/benchmark.h"
 #include "absl/base/config.h"
+#include "absl/numeric/int128.h"
+#include "absl/random/random.h"
+#include "benchmark/benchmark.h"
 
 namespace {
 
 constexpr size_t kSampleSize = 1000000;
 
-std::mt19937 MakeRandomEngine() {
-  std::random_device r;
-  std::seed_seq seed({r(), r(), r(), r(), r(), r(), r(), r()});
-  return std::mt19937(seed);
-}
-
-std::vector<std::pair<absl::uint128, absl::uint128>>
-GetRandomClass128SampleUniformDivisor() {
-  std::vector<std::pair<absl::uint128, absl::uint128>> values;
-  std::mt19937 random = MakeRandomEngine();
-  std::uniform_int_distribution<uint64_t> uniform_uint64;
+template <typename T, typename H = std::conditional_t<
+                          std::numeric_limits<T>::is_signed, int64_t, uint64_t>>
+std::vector<std::pair<T, T>> GetRandomClass128SampleUniformDivisor() {
+  std::vector<std::pair<T, T>> values;
+  absl::InsecureBitGen random;
+  std::uniform_int_distribution<H> uniform_h;
   values.reserve(kSampleSize);
   for (size_t i = 0; i < kSampleSize; ++i) {
-    absl::uint128 a =
-        absl::MakeUint128(uniform_uint64(random), uniform_uint64(random));
-    absl::uint128 b =
-        absl::MakeUint128(uniform_uint64(random), uniform_uint64(random));
-    values.emplace_back(std::max(a, b),
-                        std::max(absl::uint128(2), std::min(a, b)));
+    T a{absl::MakeUint128(uniform_h(random), uniform_h(random))};
+    T b{absl::MakeUint128(uniform_h(random), uniform_h(random))};
+    values.emplace_back(std::max(a, b), std::max(T(2), std::min(a, b)));
   }
   return values;
 }
 
+template <typename T>
 void BM_DivideClass128UniformDivisor(benchmark::State& state) {
-  auto values = GetRandomClass128SampleUniformDivisor();
+  auto values = GetRandomClass128SampleUniformDivisor<T>();
   while (state.KeepRunningBatch(values.size())) {
     for (const auto& pair : values) {
       benchmark::DoNotOptimize(pair.first / pair.second);
     }
   }
 }
-BENCHMARK(BM_DivideClass128UniformDivisor);
+BENCHMARK_TEMPLATE(BM_DivideClass128UniformDivisor, absl::uint128);
+BENCHMARK_TEMPLATE(BM_DivideClass128UniformDivisor, absl::int128);
 
-std::vector<std::pair<absl::uint128, uint64_t>>
-GetRandomClass128SampleSmallDivisor() {
-  std::vector<std::pair<absl::uint128, uint64_t>> values;
-  std::mt19937 random = MakeRandomEngine();
-  std::uniform_int_distribution<uint64_t> uniform_uint64;
+template <typename T>
+void BM_RemainderClass128UniformDivisor(benchmark::State& state) {
+  auto values = GetRandomClass128SampleUniformDivisor<T>();
+  while (state.KeepRunningBatch(values.size())) {
+    for (const auto& pair : values) {
+      benchmark::DoNotOptimize(pair.first % pair.second);
+    }
+  }
+}
+BENCHMARK_TEMPLATE(BM_RemainderClass128UniformDivisor, absl::uint128);
+BENCHMARK_TEMPLATE(BM_RemainderClass128UniformDivisor, absl::int128);
+
+template <typename T, typename H = std::conditional_t<
+                          std::numeric_limits<T>::is_signed, int64_t, uint64_t>>
+std::vector<std::pair<T, H>> GetRandomClass128SampleSmallDivisor() {
+  std::vector<std::pair<T, H>> values;
+  absl::InsecureBitGen random;
+  std::uniform_int_distribution<H> uniform_h;
   values.reserve(kSampleSize);
   for (size_t i = 0; i < kSampleSize; ++i) {
-    absl::uint128 a =
-        absl::MakeUint128(uniform_uint64(random), uniform_uint64(random));
-    uint64_t b = std::max(uint64_t{2}, uniform_uint64(random));
-    values.emplace_back(std::max(a, absl::uint128(b)), b);
+    T a{absl::MakeUint128(uniform_h(random), uniform_h(random))};
+    H b{std::max(H{2}, uniform_h(random))};
+    values.emplace_back(std::max(a, T(b)), b);
   }
   return values;
 }
 
+template <typename T>
 void BM_DivideClass128SmallDivisor(benchmark::State& state) {
-  auto values = GetRandomClass128SampleSmallDivisor();
+  auto values = GetRandomClass128SampleSmallDivisor<T>();
   while (state.KeepRunningBatch(values.size())) {
     for (const auto& pair : values) {
       benchmark::DoNotOptimize(pair.first / pair.second);
     }
   }
 }
-BENCHMARK(BM_DivideClass128SmallDivisor);
+BENCHMARK_TEMPLATE(BM_DivideClass128SmallDivisor, absl::uint128);
+BENCHMARK_TEMPLATE(BM_DivideClass128SmallDivisor, absl::int128);
+
+template <typename T>
+void BM_RemainderClass128SmallDivisor(benchmark::State& state) {
+  auto values = GetRandomClass128SampleSmallDivisor<T>();
+  while (state.KeepRunningBatch(values.size())) {
+    for (const auto& pair : values) {
+      benchmark::DoNotOptimize(pair.first % pair.second);
+    }
+  }
+}
+BENCHMARK_TEMPLATE(BM_RemainderClass128SmallDivisor, absl::uint128);
+BENCHMARK_TEMPLATE(BM_RemainderClass128SmallDivisor, absl::int128);
 
 std::vector<std::pair<absl::uint128, absl::uint128>> GetRandomClass128Sample() {
   std::vector<std::pair<absl::uint128, absl::uint128>> values;
-  std::mt19937 random = MakeRandomEngine();
+  absl::InsecureBitGen random;
   std::uniform_int_distribution<uint64_t> uniform_uint64;
   values.reserve(kSampleSize);
   for (size_t i = 0; i < kSampleSize; ++i) {
@@ -121,74 +145,105 @@ BENCHMARK(BM_AddClass128);
 
 // Some implementations of <random> do not support __int128 when it is
 // available, so we make our own uniform_int_distribution-like type.
+template <typename T, typename H = std::conditional_t<
+                          std::is_same_v<T, __int128>, int64_t, uint64_t>>
 class UniformIntDistribution128 {
  public:
   // NOLINTNEXTLINE: mimicking std::uniform_int_distribution API
-  unsigned __int128 operator()(std::mt19937& generator) {
-    return (static_cast<unsigned __int128>(dist64_(generator)) << 64) |
-           dist64_(generator);
+  template <class URBG>
+  T operator()(URBG& generator) {
+    return (static_cast<T>(dist64_(generator)) << 64) | dist64_(generator);
   }
 
  private:
-  std::uniform_int_distribution<uint64_t> dist64_;
+  std::uniform_int_distribution<H> dist64_;
 };
 
-std::vector<std::pair<unsigned __int128, unsigned __int128>>
-GetRandomIntrinsic128SampleUniformDivisor() {
-  std::vector<std::pair<unsigned __int128, unsigned __int128>> values;
-  std::mt19937 random = MakeRandomEngine();
-  UniformIntDistribution128 uniform_uint128;
+template <typename T, typename H = std::conditional_t<
+                          std::is_same_v<T, __int128>, int64_t, uint64_t>>
+std::vector<std::pair<T, T>> GetRandomIntrinsic128SampleUniformDivisor() {
+  std::vector<std::pair<T, T>> values;
+  absl::InsecureBitGen random;
+  UniformIntDistribution128<T> uniform_128;
   values.reserve(kSampleSize);
   for (size_t i = 0; i < kSampleSize; ++i) {
-    unsigned __int128 a = uniform_uint128(random);
-    unsigned __int128 b = uniform_uint128(random);
-    values.emplace_back(
-        std::max(a, b),
-        std::max(static_cast<unsigned __int128>(2), std::min(a, b)));
+    T a = uniform_128(random);
+    T b = uniform_128(random);
+    values.emplace_back(std::max(a, b),
+                        std::max(static_cast<T>(2), std::min(a, b)));
   }
   return values;
 }
 
+template <typename T>
 void BM_DivideIntrinsic128UniformDivisor(benchmark::State& state) {
-  auto values = GetRandomIntrinsic128SampleUniformDivisor();
+  auto values = GetRandomIntrinsic128SampleUniformDivisor<T>();
   while (state.KeepRunningBatch(values.size())) {
     for (const auto& pair : values) {
       benchmark::DoNotOptimize(pair.first / pair.second);
     }
   }
 }
-BENCHMARK(BM_DivideIntrinsic128UniformDivisor);
+BENCHMARK_TEMPLATE(BM_DivideIntrinsic128UniformDivisor, unsigned __int128);
+BENCHMARK_TEMPLATE(BM_DivideIntrinsic128UniformDivisor, __int128);
 
-std::vector<std::pair<unsigned __int128, uint64_t>>
-GetRandomIntrinsic128SampleSmallDivisor() {
-  std::vector<std::pair<unsigned __int128, uint64_t>> values;
-  std::mt19937 random = MakeRandomEngine();
-  UniformIntDistribution128 uniform_uint128;
-  std::uniform_int_distribution<uint64_t> uniform_uint64;
+template <typename T>
+void BM_RemainderIntrinsic128UniformDivisor(benchmark::State& state) {
+  auto values = GetRandomIntrinsic128SampleUniformDivisor<T>();
+  while (state.KeepRunningBatch(values.size())) {
+    for (const auto& pair : values) {
+      benchmark::DoNotOptimize(pair.first % pair.second);
+    }
+  }
+}
+BENCHMARK_TEMPLATE(BM_RemainderIntrinsic128UniformDivisor, unsigned __int128);
+BENCHMARK_TEMPLATE(BM_RemainderIntrinsic128UniformDivisor, __int128);
+
+template <typename T, typename H = std::conditional_t<
+                          std::is_same_v<T, __int128>, int64_t, uint64_t>>
+std::vector<std::pair<T, H>> GetRandomIntrinsic128SampleSmallDivisor() {
+  std::vector<std::pair<T, H>> values;
+  absl::InsecureBitGen random;
+  UniformIntDistribution128<T> uniform_int128;
+  std::uniform_int_distribution<H> uniform_int64;
   values.reserve(kSampleSize);
   for (size_t i = 0; i < kSampleSize; ++i) {
-    unsigned __int128 a = uniform_uint128(random);
-    uint64_t b = std::max(uint64_t{2}, uniform_uint64(random));
-    values.emplace_back(std::max(a, static_cast<unsigned __int128>(b)), b);
+    T a = uniform_int128(random);
+    H b = std::max(H{2}, uniform_int64(random));
+    values.emplace_back(std::max(a, static_cast<T>(b)), b);
   }
   return values;
 }
 
+template <typename T>
 void BM_DivideIntrinsic128SmallDivisor(benchmark::State& state) {
-  auto values = GetRandomIntrinsic128SampleSmallDivisor();
+  auto values = GetRandomIntrinsic128SampleSmallDivisor<T>();
   while (state.KeepRunningBatch(values.size())) {
     for (const auto& pair : values) {
       benchmark::DoNotOptimize(pair.first / pair.second);
     }
   }
 }
-BENCHMARK(BM_DivideIntrinsic128SmallDivisor);
+BENCHMARK_TEMPLATE(BM_DivideIntrinsic128SmallDivisor, unsigned __int128);
+BENCHMARK_TEMPLATE(BM_DivideIntrinsic128SmallDivisor, __int128);
+
+template <typename T>
+void BM_RemainderIntrinsic128SmallDivisor(benchmark::State& state) {
+  auto values = GetRandomIntrinsic128SampleSmallDivisor<T>();
+  while (state.KeepRunningBatch(values.size())) {
+    for (const auto& pair : values) {
+      benchmark::DoNotOptimize(pair.first % pair.second);
+    }
+  }
+}
+BENCHMARK_TEMPLATE(BM_RemainderIntrinsic128SmallDivisor, unsigned __int128);
+BENCHMARK_TEMPLATE(BM_RemainderIntrinsic128SmallDivisor, __int128);
 
 std::vector<std::pair<unsigned __int128, unsigned __int128>>
-      GetRandomIntrinsic128Sample() {
+GetRandomIntrinsic128Sample() {
   std::vector<std::pair<unsigned __int128, unsigned __int128>> values;
-  std::mt19937 random = MakeRandomEngine();
-  UniformIntDistribution128 uniform_uint128;
+  absl::InsecureBitGen random;
+  UniformIntDistribution128<unsigned __int128> uniform_uint128;
   values.reserve(kSampleSize);
   for (size_t i = 0; i < kSampleSize; ++i) {
     values.emplace_back(uniform_uint128(random), uniform_uint128(random));
diff --git a/absl/numeric/int128_have_intrinsic.inc b/absl/numeric/int128_have_intrinsic.inc
index c7ea6834530..dea1d213c9f 100644
--- a/absl/numeric/int128_have_intrinsic.inc
+++ b/absl/numeric/int128_have_intrinsic.inc
@@ -16,3 +16,288 @@
 // This file contains :int128 implementation details that depend on internal
 // representation when ABSL_HAVE_INTRINSIC_INT128 is defined. This file is
 // included by int128.h and relies on ABSL_INTERNAL_WCHAR_T being defined.
+
+namespace int128_internal {
+
+// Casts from unsigned to signed while preserving the underlying binary
+// representation.
+constexpr __int128 BitCastToSigned(unsigned __int128 v) {
+  // Casting an unsigned integer to a signed integer of the same
+  // width is implementation defined behavior if the source value would not fit
+  // in the destination type. We step around it with a roundtrip bitwise not
+  // operation to make sure this function remains constexpr. Clang and GCC
+  // optimize this to a no-op on x86-64.
+  return v & (static_cast<unsigned __int128>(1) << 127)
+             ? ~static_cast<__int128>(~v)
+             : static_cast<__int128>(v);
+}
+
+}  // namespace int128_internal
+
+inline int128& int128::operator=(__int128 v) {
+  v_ = v;
+  return *this;
+}
+
+constexpr uint64_t Int128Low64(int128 v) {
+  return static_cast<uint64_t>(v.v_ & ~uint64_t{0});
+}
+
+constexpr int64_t Int128High64(int128 v) {
+  // Initially cast to unsigned to prevent a right shift on a negative value.
+  return int128_internal::BitCastToSigned(
+      static_cast<uint64_t>(static_cast<unsigned __int128>(v.v_) >> 64));
+}
+
+constexpr int128::int128(int64_t high, uint64_t low)
+    // Initially cast to unsigned to prevent a left shift that overflows.
+    : v_(int128_internal::BitCastToSigned(static_cast<unsigned __int128>(high)
+                                           << 64) |
+         low) {}
+
+
+constexpr int128::int128(int v) : v_{v} {}
+
+constexpr int128::int128(long v) : v_{v} {}       // NOLINT(runtime/int)
+
+constexpr int128::int128(long long v) : v_{v} {}  // NOLINT(runtime/int)
+
+constexpr int128::int128(__int128 v) : v_{v} {}
+
+constexpr int128::int128(unsigned int v) : v_{v} {}
+
+constexpr int128::int128(unsigned long v) : v_{v} {}  // NOLINT(runtime/int)
+
+// NOLINTNEXTLINE(runtime/int)
+constexpr int128::int128(unsigned long long v) : v_{v} {}
+
+constexpr int128::int128(unsigned __int128 v) : v_{static_cast<__int128>(v)} {}
+
+constexpr int128::int128(float v) : v_{static_cast<__int128>(v)} {}
+
+constexpr int128::int128(double v) : v_{static_cast<__int128>(v)} {}
+
+constexpr int128::int128(long double v) : v_{static_cast<__int128>(v)} {}
+
+constexpr int128::int128(uint128 v) : v_{static_cast<__int128>(v)} {}
+
+constexpr int128::operator bool() const { return static_cast<bool>(v_); }
+
+constexpr int128::operator char() const { return static_cast<char>(v_); }
+
+constexpr int128::operator signed char() const {
+  return static_cast<signed char>(v_);
+}
+
+constexpr int128::operator unsigned char() const {
+  return static_cast<unsigned char>(v_);
+}
+
+constexpr int128::operator char16_t() const {
+  return static_cast<char16_t>(v_);
+}
+
+constexpr int128::operator char32_t() const {
+  return static_cast<char32_t>(v_);
+}
+
+constexpr int128::operator ABSL_INTERNAL_WCHAR_T() const {
+  return static_cast<ABSL_INTERNAL_WCHAR_T>(v_);
+}
+
+constexpr int128::operator short() const {  // NOLINT(runtime/int)
+  return static_cast<short>(v_);            // NOLINT(runtime/int)
+}
+
+constexpr int128::operator unsigned short() const {  // NOLINT(runtime/int)
+  return static_cast<unsigned short>(v_);            // NOLINT(runtime/int)
+}
+
+constexpr int128::operator int() const { return static_cast<int>(v_); }
+
+constexpr int128::operator unsigned int() const {
+  return static_cast<unsigned int>(v_);
+}
+
+constexpr int128::operator long() const {  // NOLINT(runtime/int)
+  return static_cast<long>(v_);            // NOLINT(runtime/int)
+}
+
+constexpr int128::operator unsigned long() const {  // NOLINT(runtime/int)
+  return static_cast<unsigned long>(v_);            // NOLINT(runtime/int)
+}
+
+constexpr int128::operator long long() const {  // NOLINT(runtime/int)
+  return static_cast<long long>(v_);            // NOLINT(runtime/int)
+}
+
+constexpr int128::operator unsigned long long() const {  // NOLINT(runtime/int)
+  return static_cast<unsigned long long>(v_);            // NOLINT(runtime/int)
+}
+
+constexpr int128::operator __int128() const { return v_; }
+
+constexpr int128::operator unsigned __int128() const {
+  return static_cast<unsigned __int128>(v_);
+}
+
+// Clang on PowerPC sometimes produces incorrect __int128 to floating point
+// conversions. In that case, we do the conversion with a similar implementation
+// to the conversion operators in int128_no_intrinsic.inc.
+#if defined(__clang__) && !defined(__ppc64__)
+constexpr int128::operator float() const { return static_cast<float>(v_); }
+
+constexpr int128::operator double() const { return static_cast<double>(v_); }
+
+constexpr int128::operator long double() const {
+  return static_cast<long double>(v_);
+}
+
+#else   // Clang on PowerPC
+
+constexpr int128::operator float() const {
+  // We must convert the absolute value and then negate as needed, because
+  // floating point types are typically sign-magnitude. Otherwise, the
+  // difference between the high and low 64 bits when interpreted as two's
+  // complement overwhelms the precision of the mantissa.
+  //
+  // Also check to make sure we don't negate Int128Min()
+  constexpr float pow_2_64 = 18446744073709551616.0f;
+  return v_ < 0 && *this != Int128Min()
+             ? -static_cast<float>(-*this)
+             : static_cast<float>(Int128Low64(*this)) +
+                   static_cast<float>(Int128High64(*this)) * pow_2_64;
+}
+
+constexpr int128::operator double() const {
+  // See comment in int128::operator float() above.
+  constexpr double pow_2_64 = 18446744073709551616.0;
+  return v_ < 0 && *this != Int128Min()
+             ? -static_cast<double>(-*this)
+             : static_cast<double>(Int128Low64(*this)) +
+                   static_cast<double>(Int128High64(*this)) * pow_2_64;
+}
+
+constexpr int128::operator long double() const {
+  // See comment in int128::operator float() above.
+  constexpr long double pow_2_64 = 18446744073709551616.0L;
+  return v_ < 0 && *this != Int128Min()
+             ? -static_cast<long double>(-*this)
+             : static_cast<long double>(Int128Low64(*this)) +
+                   static_cast<long double>(Int128High64(*this)) * pow_2_64;
+}
+#endif  // Clang on PowerPC
+
+// Comparison operators.
+
+constexpr bool operator==(int128 lhs, int128 rhs) {
+  return static_cast<__int128>(lhs) == static_cast<__int128>(rhs);
+}
+
+constexpr bool operator!=(int128 lhs, int128 rhs) {
+  return static_cast<__int128>(lhs) != static_cast<__int128>(rhs);
+}
+
+constexpr bool operator<(int128 lhs, int128 rhs) {
+  return static_cast<__int128>(lhs) < static_cast<__int128>(rhs);
+}
+
+constexpr bool operator>(int128 lhs, int128 rhs) {
+  return static_cast<__int128>(lhs) > static_cast<__int128>(rhs);
+}
+
+constexpr bool operator<=(int128 lhs, int128 rhs) {
+  return static_cast<__int128>(lhs) <= static_cast<__int128>(rhs);
+}
+
+constexpr bool operator>=(int128 lhs, int128 rhs) {
+  return static_cast<__int128>(lhs) >= static_cast<__int128>(rhs);
+}
+
+#ifdef __cpp_impl_three_way_comparison
+constexpr absl::strong_ordering operator<=>(int128 lhs, int128 rhs) {
+  if (auto lhs_128 = static_cast<__int128>(lhs),
+      rhs_128 = static_cast<__int128>(rhs);
+      lhs_128 < rhs_128) {
+    return absl::strong_ordering::less;
+  } else if (lhs_128 > rhs_128) {
+    return absl::strong_ordering::greater;
+  } else {
+    return absl::strong_ordering::equal;
+  }
+}
+#endif
+
+// Unary operators.
+
+constexpr int128 operator-(int128 v) { return -static_cast<__int128>(v); }
+
+constexpr bool operator!(int128 v) { return !static_cast<__int128>(v); }
+
+constexpr int128 operator~(int128 val) { return ~static_cast<__int128>(val); }
+
+// Arithmetic operators.
+
+constexpr int128 operator+(int128 lhs, int128 rhs) {
+  return static_cast<__int128>(lhs) + static_cast<__int128>(rhs);
+}
+
+constexpr int128 operator-(int128 lhs, int128 rhs) {
+  return static_cast<__int128>(lhs) - static_cast<__int128>(rhs);
+}
+
+#if defined(ABSL_HAVE_INTRINSIC_INT128)
+constexpr int128 operator*(int128 lhs, int128 rhs) {
+  return static_cast<__int128>(lhs) * static_cast<__int128>(rhs);
+}
+
+constexpr int128 operator/(int128 lhs, int128 rhs) {
+  return static_cast<__int128>(lhs) / static_cast<__int128>(rhs);
+}
+
+constexpr int128 operator%(int128 lhs, int128 rhs) {
+  return static_cast<__int128>(lhs) % static_cast<__int128>(rhs);
+}
+#endif  // ABSL_HAVE_INTRINSIC_INT128
+
+inline int128 int128::operator++(int) {
+  int128 tmp(*this);
+  ++v_;
+  return tmp;
+}
+
+inline int128 int128::operator--(int) {
+  int128 tmp(*this);
+  --v_;
+  return tmp;
+}
+
+inline int128& int128::operator++() {
+  ++v_;
+  return *this;
+}
+
+inline int128& int128::operator--() {
+  --v_;
+  return *this;
+}
+
+constexpr int128 operator|(int128 lhs, int128 rhs) {
+  return static_cast<__int128>(lhs) | static_cast<__int128>(rhs);
+}
+
+constexpr int128 operator&(int128 lhs, int128 rhs) {
+  return static_cast<__int128>(lhs) & static_cast<__int128>(rhs);
+}
+
+constexpr int128 operator^(int128 lhs, int128 rhs) {
+  return static_cast<__int128>(lhs) ^ static_cast<__int128>(rhs);
+}
+
+constexpr int128 operator<<(int128 lhs, int amount) {
+  return static_cast<__int128>(lhs) << amount;
+}
+
+constexpr int128 operator>>(int128 lhs, int amount) {
+  return static_cast<__int128>(lhs) >> amount;
+}
diff --git a/absl/numeric/int128_no_intrinsic.inc b/absl/numeric/int128_no_intrinsic.inc
index 046cb9b3a40..48bec2cc8fc 100644
--- a/absl/numeric/int128_no_intrinsic.inc
+++ b/absl/numeric/int128_no_intrinsic.inc
@@ -16,3 +16,332 @@
 // This file contains :int128 implementation details that depend on internal
 // representation when ABSL_HAVE_INTRINSIC_INT128 is *not* defined. This file
 // is included by int128.h and relies on ABSL_INTERNAL_WCHAR_T being defined.
+
+constexpr uint64_t Int128Low64(int128 v) { return v.lo_; }
+
+constexpr int64_t Int128High64(int128 v) { return v.hi_; }
+
+#if defined(ABSL_IS_LITTLE_ENDIAN)
+
+constexpr int128::int128(int64_t high, uint64_t low) : lo_(low), hi_(high) {}
+
+constexpr int128::int128(int v)
+    : lo_{static_cast<uint64_t>(v)}, hi_{v < 0 ? ~int64_t{0} : 0} {}
+constexpr int128::int128(long v)  // NOLINT(runtime/int)
+    : lo_{static_cast<uint64_t>(v)}, hi_{v < 0 ? ~int64_t{0} : 0} {}
+constexpr int128::int128(long long v)  // NOLINT(runtime/int)
+    : lo_{static_cast<uint64_t>(v)}, hi_{v < 0 ? ~int64_t{0} : 0} {}
+
+constexpr int128::int128(unsigned int v) : lo_{v}, hi_{0} {}
+// NOLINTNEXTLINE(runtime/int)
+constexpr int128::int128(unsigned long v) : lo_{v}, hi_{0} {}
+// NOLINTNEXTLINE(runtime/int)
+constexpr int128::int128(unsigned long long v) : lo_{v}, hi_{0} {}
+
+constexpr int128::int128(uint128 v)
+    : lo_{Uint128Low64(v)}, hi_{static_cast<int64_t>(Uint128High64(v))} {}
+
+#elif defined(ABSL_IS_BIG_ENDIAN)
+
+constexpr int128::int128(int64_t high, uint64_t low) : hi_{high}, lo_{low} {}
+
+constexpr int128::int128(int v)
+    : hi_{v < 0 ? ~int64_t{0} : 0}, lo_{static_cast<uint64_t>(v)} {}
+constexpr int128::int128(long v)  // NOLINT(runtime/int)
+    : hi_{v < 0 ? ~int64_t{0} : 0}, lo_{static_cast<uint64_t>(v)} {}
+constexpr int128::int128(long long v)  // NOLINT(runtime/int)
+    : hi_{v < 0 ? ~int64_t{0} : 0}, lo_{static_cast<uint64_t>(v)} {}
+
+constexpr int128::int128(unsigned int v) : hi_{0}, lo_{v} {}
+// NOLINTNEXTLINE(runtime/int)
+constexpr int128::int128(unsigned long v) : hi_{0}, lo_{v} {}
+// NOLINTNEXTLINE(runtime/int)
+constexpr int128::int128(unsigned long long v) : hi_{0}, lo_{v} {}
+
+constexpr int128::int128(uint128 v)
+    : hi_{static_cast<int64_t>(Uint128High64(v))}, lo_{Uint128Low64(v)} {}
+
+#else  // byte order
+#error "Unsupported byte order: must be little-endian or big-endian."
+#endif  // byte order
+
+constexpr int128::operator bool() const { return lo_ || hi_; }
+
+constexpr int128::operator char() const {
+  // NOLINTNEXTLINE(runtime/int)
+  return static_cast<char>(static_cast<long long>(*this));
+}
+
+constexpr int128::operator signed char() const {
+  // NOLINTNEXTLINE(runtime/int)
+  return static_cast<signed char>(static_cast<long long>(*this));
+}
+
+constexpr int128::operator unsigned char() const {
+  return static_cast<unsigned char>(lo_);
+}
+
+constexpr int128::operator char16_t() const {
+  return static_cast<char16_t>(lo_);
+}
+
+constexpr int128::operator char32_t() const {
+  return static_cast<char32_t>(lo_);
+}
+
+constexpr int128::operator ABSL_INTERNAL_WCHAR_T() const {
+  // NOLINTNEXTLINE(runtime/int)
+  return static_cast<ABSL_INTERNAL_WCHAR_T>(static_cast<long long>(*this));
+}
+
+constexpr int128::operator short() const {  // NOLINT(runtime/int)
+  // NOLINTNEXTLINE(runtime/int)
+  return static_cast<short>(static_cast<long long>(*this));
+}
+
+constexpr int128::operator unsigned short() const {  // NOLINT(runtime/int)
+  return static_cast<unsigned short>(lo_);           // NOLINT(runtime/int)
+}
+
+constexpr int128::operator int() const {
+  // NOLINTNEXTLINE(runtime/int)
+  return static_cast<int>(static_cast<long long>(*this));
+}
+
+constexpr int128::operator unsigned int() const {
+  return static_cast<unsigned int>(lo_);
+}
+
+constexpr int128::operator long() const {  // NOLINT(runtime/int)
+  // NOLINTNEXTLINE(runtime/int)
+  return static_cast<long>(static_cast<long long>(*this));
+}
+
+constexpr int128::operator unsigned long() const {  // NOLINT(runtime/int)
+  return static_cast<unsigned long>(lo_);           // NOLINT(runtime/int)
+}
+
+constexpr int128::operator long long() const {  // NOLINT(runtime/int)
+  // We don't bother checking the value of hi_. If *this < 0, lo_'s high bit
+  // must be set in order for the value to fit into a long long. Conversely, if
+  // lo_'s high bit is set, *this must be < 0 for the value to fit.
+  return int128_internal::BitCastToSigned(lo_);
+}
+
+constexpr int128::operator unsigned long long() const {  // NOLINT(runtime/int)
+  return static_cast<unsigned long long>(lo_);           // NOLINT(runtime/int)
+}
+
+constexpr int128::operator float() const {
+  // We must convert the absolute value and then negate as needed, because
+  // floating point types are typically sign-magnitude. Otherwise, the
+  // difference between the high and low 64 bits when interpreted as two's
+  // complement overwhelms the precision of the mantissa.
+  //
+  // Also check to make sure we don't negate Int128Min()
+  constexpr float pow_2_64 = 18446744073709551616.0f;
+  return hi_ < 0 && *this != Int128Min()
+             ? -static_cast<float>(-*this)
+             : static_cast<float>(lo_) + static_cast<float>(hi_) * pow_2_64;
+}
+
+constexpr int128::operator double() const {
+  // See comment in int128::operator float() above.
+  constexpr double pow_2_64 = 18446744073709551616.0;
+  return hi_ < 0 && *this != Int128Min()
+             ? -static_cast<double>(-*this)
+             : static_cast<double>(lo_) + static_cast<double>(hi_) * pow_2_64;
+}
+
+constexpr int128::operator long double() const {
+  // See comment in int128::operator float() above.
+  constexpr long double pow_2_64 = 18446744073709551616.0L;
+  return hi_ < 0 && *this != Int128Min()
+             ? -static_cast<long double>(-*this)
+             : static_cast<long double>(lo_) +
+                   static_cast<long double>(hi_) * pow_2_64;
+}
+
+// Comparison operators.
+
+constexpr bool operator==(int128 lhs, int128 rhs) {
+  return (Int128Low64(lhs) == Int128Low64(rhs) &&
+          Int128High64(lhs) == Int128High64(rhs));
+}
+
+constexpr bool operator!=(int128 lhs, int128 rhs) { return !(lhs == rhs); }
+
+constexpr bool operator<(int128 lhs, int128 rhs) {
+  return (Int128High64(lhs) == Int128High64(rhs))
+             ? (Int128Low64(lhs) < Int128Low64(rhs))
+             : (Int128High64(lhs) < Int128High64(rhs));
+}
+
+constexpr bool operator>(int128 lhs, int128 rhs) {
+  return (Int128High64(lhs) == Int128High64(rhs))
+             ? (Int128Low64(lhs) > Int128Low64(rhs))
+             : (Int128High64(lhs) > Int128High64(rhs));
+}
+
+constexpr bool operator<=(int128 lhs, int128 rhs) { return !(lhs > rhs); }
+
+constexpr bool operator>=(int128 lhs, int128 rhs) { return !(lhs < rhs); }
+
+#ifdef __cpp_impl_three_way_comparison
+constexpr absl::strong_ordering operator<=>(int128 lhs, int128 rhs) {
+  if (int64_t lhs_high = Int128High64(lhs), rhs_high = Int128High64(rhs);
+      lhs_high < rhs_high) {
+    return absl::strong_ordering::less;
+  } else if (lhs_high > rhs_high) {
+    return absl::strong_ordering::greater;
+  } else if (uint64_t lhs_low = Uint128Low64(lhs), rhs_low = Uint128Low64(rhs);
+             lhs_low < rhs_low) {
+    return absl::strong_ordering::less;
+  } else if (lhs_low > rhs_low) {
+    return absl::strong_ordering::greater;
+  } else {
+    return absl::strong_ordering::equal;
+  }
+}
+#endif
+
+// Unary operators.
+
+constexpr int128 operator-(int128 v) {
+  return MakeInt128(~Int128High64(v) + (Int128Low64(v) == 0),
+                    ~Int128Low64(v) + 1);
+}
+
+constexpr bool operator!(int128 v) {
+  return !Int128Low64(v) && !Int128High64(v);
+}
+
+constexpr int128 operator~(int128 val) {
+  return MakeInt128(~Int128High64(val), ~Int128Low64(val));
+}
+
+// Arithmetic operators.
+
+namespace int128_internal {
+constexpr int128 SignedAddResult(int128 result, int128 lhs) {
+  // check for carry
+  return (Int128Low64(result) < Int128Low64(lhs))
+             ? MakeInt128(Int128High64(result) + 1, Int128Low64(result))
+             : result;
+}
+}  // namespace int128_internal
+constexpr int128 operator+(int128 lhs, int128 rhs) {
+  return int128_internal::SignedAddResult(
+      MakeInt128(Int128High64(lhs) + Int128High64(rhs),
+                 Int128Low64(lhs) + Int128Low64(rhs)),
+      lhs);
+}
+
+namespace int128_internal {
+constexpr int128 SignedSubstructResult(int128 result, int128 lhs, int128 rhs) {
+  // check for carry
+  return (Int128Low64(lhs) < Int128Low64(rhs))
+             ? MakeInt128(Int128High64(result) - 1, Int128Low64(result))
+             : result;
+}
+}  // namespace int128_internal
+constexpr int128 operator-(int128 lhs, int128 rhs) {
+  return int128_internal::SignedSubstructResult(
+      MakeInt128(Int128High64(lhs) - Int128High64(rhs),
+                 Int128Low64(lhs) - Int128Low64(rhs)),
+      lhs, rhs);
+}
+
+inline int128 operator*(int128 lhs, int128 rhs) {
+  return MakeInt128(
+      int128_internal::BitCastToSigned(Uint128High64(uint128(lhs) * rhs)),
+      Uint128Low64(uint128(lhs) * rhs));
+}
+
+inline int128 int128::operator++(int) {
+  int128 tmp(*this);
+  *this += 1;
+  return tmp;
+}
+
+inline int128 int128::operator--(int) {
+  int128 tmp(*this);
+  *this -= 1;
+  return tmp;
+}
+
+inline int128& int128::operator++() {
+  *this += 1;
+  return *this;
+}
+
+inline int128& int128::operator--() {
+  *this -= 1;
+  return *this;
+}
+
+constexpr int128 operator|(int128 lhs, int128 rhs) {
+  return MakeInt128(Int128High64(lhs) | Int128High64(rhs),
+                    Int128Low64(lhs) | Int128Low64(rhs));
+}
+
+constexpr int128 operator&(int128 lhs, int128 rhs) {
+  return MakeInt128(Int128High64(lhs) & Int128High64(rhs),
+                    Int128Low64(lhs) & Int128Low64(rhs));
+}
+
+constexpr int128 operator^(int128 lhs, int128 rhs) {
+  return MakeInt128(Int128High64(lhs) ^ Int128High64(rhs),
+                    Int128Low64(lhs) ^ Int128Low64(rhs));
+}
+
+constexpr int128 operator<<(int128 lhs, int amount) {
+  // int64_t shifts of >= 63 are undefined, so we need some special-casing.
+  assert(amount >= 0 && amount < 127);
+  if (amount <= 0) {
+    return lhs;
+  } else if (amount < 63) {
+    return MakeInt128(
+        (Int128High64(lhs) << amount) |
+            static_cast<int64_t>(Int128Low64(lhs) >> (64 - amount)),
+        Int128Low64(lhs) << amount);
+  } else if (amount == 63) {
+    return MakeInt128(((Int128High64(lhs) << 32) << 31) |
+                          static_cast<int64_t>(Int128Low64(lhs) >> 1),
+                      (Int128Low64(lhs) << 32) << 31);
+  } else if (amount == 127) {
+    return MakeInt128(static_cast<int64_t>(Int128Low64(lhs) << 63), 0);
+  } else if (amount > 127) {
+    return MakeInt128(0, 0);
+  } else {
+    // amount >= 64 && amount < 127
+    return MakeInt128(static_cast<int64_t>(Int128Low64(lhs) << (amount - 64)),
+                      0);
+  }
+}
+
+constexpr int128 operator>>(int128 lhs, int amount) {
+  // int64_t shifts of >= 63 are undefined, so we need some special-casing.
+  assert(amount >= 0 && amount < 127);
+  if (amount <= 0) {
+    return lhs;
+  } else if (amount < 63) {
+    return MakeInt128(
+        Int128High64(lhs) >> amount,
+        Int128Low64(lhs) >> amount | static_cast<uint64_t>(Int128High64(lhs))
+                                         << (64 - amount));
+  } else if (amount == 63) {
+    return MakeInt128((Int128High64(lhs) >> 32) >> 31,
+                      static_cast<uint64_t>(Int128High64(lhs) << 1) |
+                          (Int128Low64(lhs) >> 32) >> 31);
+
+  } else if (amount >= 127) {
+    return MakeInt128((Int128High64(lhs) >> 32) >> 31,
+                      static_cast<uint64_t>((Int128High64(lhs) >> 32) >> 31));
+  } else {
+    // amount >= 64 && amount < 127
+    return MakeInt128(
+        (Int128High64(lhs) >> 32) >> 31,
+        static_cast<uint64_t>(Int128High64(lhs) >> (amount - 64)));
+  }
+}
diff --git a/absl/numeric/int128_stream_test.cc b/absl/numeric/int128_stream_test.cc
index 3cfa9dc1835..f8eb431204a 100644
--- a/absl/numeric/int128_stream_test.cc
+++ b/absl/numeric/int128_stream_test.cc
@@ -12,12 +12,17 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-#include "absl/numeric/int128.h"
-
+#include <cstddef>
+#include <cstdint>
+#include <ios>
+#include <limits>
 #include <sstream>
 #include <string>
+#include <vector>
 
 #include "gtest/gtest.h"
+#include "absl/numeric/int128.h"
+#include "absl/strings/str_cat.h"
 
 namespace {
 
@@ -76,16 +81,6 @@ std::string StreamFormatToString(std::ios_base::fmtflags flags,
   return msg.str();
 }
 
-void CheckUint128Case(const Uint128TestCase& test_case) {
-  std::ostringstream os;
-  os.flags(test_case.flags);
-  os.width(test_case.width);
-  os.fill(kFill);
-  os << test_case.value;
-  SCOPED_TRACE(StreamFormatToString(test_case.flags, test_case.width));
-  EXPECT_EQ(test_case.expected, os.str());
-}
-
 constexpr std::ios::fmtflags kDec = std::ios::dec;
 constexpr std::ios::fmtflags kOct = std::ios::oct;
 constexpr std::ios::fmtflags kHex = std::ios::hex;
@@ -96,6 +91,19 @@ constexpr std::ios::fmtflags kUpper = std::ios::uppercase;
 constexpr std::ios::fmtflags kBase = std::ios::showbase;
 constexpr std::ios::fmtflags kPos = std::ios::showpos;
 
+void CheckUint128Case(const Uint128TestCase& test_case) {
+  if (test_case.flags == kDec && test_case.width == 0) {
+    EXPECT_EQ(absl::StrCat(test_case.value), test_case.expected);
+  }
+  std::ostringstream os;
+  os.flags(test_case.flags);
+  os.width(test_case.width);
+  os.fill(kFill);
+  os << test_case.value;
+  SCOPED_TRACE(StreamFormatToString(test_case.flags, test_case.width));
+  EXPECT_EQ(os.str(), test_case.expected);
+}
+
 TEST(Uint128, OStreamValueTest) {
   CheckUint128Case({1, kDec, /*width = */ 0, "1"});
   CheckUint128Case({1, kOct, /*width = */ 0, "1"});
@@ -131,11 +139,11 @@ TEST(Uint128, OStreamValueTest) {
                     "2000000000000000000000000000000000000000000"});
   CheckUint128Case({absl::MakeUint128(0x8000000000000000, 0), kHex,
                     /*width = */ 0, "80000000000000000000000000000000"});
-  CheckUint128Case({absl::kuint128max, kDec, /*width = */ 0,
+  CheckUint128Case({absl::Uint128Max(), kDec, /*width = */ 0,
                     "340282366920938463463374607431768211455"});
-  CheckUint128Case({absl::kuint128max, kOct, /*width = */ 0,
+  CheckUint128Case({absl::Uint128Max(), kOct, /*width = */ 0,
                     "3777777777777777777777777777777777777777777"});
-  CheckUint128Case({absl::kuint128max, kHex, /*width = */ 0,
+  CheckUint128Case({absl::Uint128Max(), kHex, /*width = */ 0,
                     "ffffffffffffffffffffffffffffffff"});
 }
 
@@ -147,6 +155,736 @@ TEST(Uint128, OStreamFormatTest) {
   }
 }
 
+struct Int128TestCase {
+  absl::int128 value;
+  std::ios_base::fmtflags flags;
+  std::streamsize width;
+  const char* expected;
+};
+
+void CheckInt128Case(const Int128TestCase& test_case) {
+  if (test_case.flags == kDec && test_case.width == 0) {
+    EXPECT_EQ(absl::StrCat(test_case.value), test_case.expected);
+  }
+  std::ostringstream os;
+  os.flags(test_case.flags);
+  os.width(test_case.width);
+  os.fill(kFill);
+  os << test_case.value;
+  SCOPED_TRACE(StreamFormatToString(test_case.flags, test_case.width));
+  EXPECT_EQ(os.str(), test_case.expected);
+}
+
+TEST(Int128, OStreamValueTest) {
+  CheckInt128Case({1, kDec, /*width = */ 0, "1"});
+  CheckInt128Case({1, kOct, /*width = */ 0, "1"});
+  CheckInt128Case({1, kHex, /*width = */ 0, "1"});
+  CheckInt128Case({9, kDec, /*width = */ 0, "9"});
+  CheckInt128Case({9, kOct, /*width = */ 0, "11"});
+  CheckInt128Case({9, kHex, /*width = */ 0, "9"});
+  CheckInt128Case({12345, kDec, /*width = */ 0, "12345"});
+  CheckInt128Case({12345, kOct, /*width = */ 0, "30071"});
+  CheckInt128Case({12345, kHex, /*width = */ 0, "3039"});
+  CheckInt128Case(
+      {0x8000000000000000, kDec, /*width = */ 0, "9223372036854775808"});
+  CheckInt128Case(
+      {0x8000000000000000, kOct, /*width = */ 0, "1000000000000000000000"});
+  CheckInt128Case(
+      {0x8000000000000000, kHex, /*width = */ 0, "8000000000000000"});
+  CheckInt128Case({std::numeric_limits<uint64_t>::max(), kDec,
+                   /*width = */ 0, "18446744073709551615"});
+  CheckInt128Case({std::numeric_limits<uint64_t>::max(), kOct,
+                   /*width = */ 0, "1777777777777777777777"});
+  CheckInt128Case({std::numeric_limits<uint64_t>::max(), kHex,
+                   /*width = */ 0, "ffffffffffffffff"});
+  CheckInt128Case(
+      {absl::MakeInt128(1, 0), kDec, /*width = */ 0, "18446744073709551616"});
+  CheckInt128Case(
+      {absl::MakeInt128(1, 0), kOct, /*width = */ 0, "2000000000000000000000"});
+  CheckInt128Case(
+      {absl::MakeInt128(1, 0), kHex, /*width = */ 0, "10000000000000000"});
+  CheckInt128Case({absl::MakeInt128(std::numeric_limits<int64_t>::max(),
+                                    std::numeric_limits<uint64_t>::max()),
+                   kDec, /*width = */ 0,
+                   "170141183460469231731687303715884105727"});
+  CheckInt128Case({absl::MakeInt128(std::numeric_limits<int64_t>::max(),
+                                    std::numeric_limits<uint64_t>::max()),
+                   kOct, /*width = */ 0,
+                   "1777777777777777777777777777777777777777777"});
+  CheckInt128Case({absl::MakeInt128(std::numeric_limits<int64_t>::max(),
+                                    std::numeric_limits<uint64_t>::max()),
+                   kHex, /*width = */ 0, "7fffffffffffffffffffffffffffffff"});
+  CheckInt128Case({absl::MakeInt128(std::numeric_limits<int64_t>::min(), 0),
+                   kDec, /*width = */ 0,
+                   "-170141183460469231731687303715884105728"});
+  CheckInt128Case({absl::MakeInt128(std::numeric_limits<int64_t>::min(), 0),
+                   kOct, /*width = */ 0,
+                   "2000000000000000000000000000000000000000000"});
+  CheckInt128Case({absl::MakeInt128(std::numeric_limits<int64_t>::min(), 0),
+                   kHex, /*width = */ 0, "80000000000000000000000000000000"});
+  CheckInt128Case({-1, kDec, /*width = */ 0, "-1"});
+  CheckInt128Case({-1, kOct, /*width = */ 0,
+                   "3777777777777777777777777777777777777777777"});
+  CheckInt128Case(
+      {-1, kHex, /*width = */ 0, "ffffffffffffffffffffffffffffffff"});
+  CheckInt128Case({-12345, kDec, /*width = */ 0, "-12345"});
+  CheckInt128Case({-12345, kOct, /*width = */ 0,
+                   "3777777777777777777777777777777777777747707"});
+  CheckInt128Case(
+      {-12345, kHex, /*width = */ 0, "ffffffffffffffffffffffffffffcfc7"});
+}
+
+std::vector<Int128TestCase> GetInt128FormatCases();
+TEST(Int128, OStreamFormatTest) {
+  for (const Int128TestCase& test_case : GetInt128FormatCases()) {
+    CheckInt128Case(test_case);
+  }
+}
+
+std::vector<Int128TestCase> GetInt128FormatCases() {
+  return {
+      {0, std::ios_base::fmtflags(), /*width = */ 0, "0"},
+      {0, std::ios_base::fmtflags(), /*width = */ 6, "_____0"},
+      {0, kPos, /*width = */ 0, "+0"},
+      {0, kPos, /*width = */ 6, "____+0"},
+      {0, kBase, /*width = */ 0, "0"},
+      {0, kBase, /*width = */ 6, "_____0"},
+      {0, kBase | kPos, /*width = */ 0, "+0"},
+      {0, kBase | kPos, /*width = */ 6, "____+0"},
+      {0, kUpper, /*width = */ 0, "0"},
+      {0, kUpper, /*width = */ 6, "_____0"},
+      {0, kUpper | kPos, /*width = */ 0, "+0"},
+      {0, kUpper | kPos, /*width = */ 6, "____+0"},
+      {0, kUpper | kBase, /*width = */ 0, "0"},
+      {0, kUpper | kBase, /*width = */ 6, "_____0"},
+      {0, kUpper | kBase | kPos, /*width = */ 0, "+0"},
+      {0, kUpper | kBase | kPos, /*width = */ 6, "____+0"},
+      {0, kLeft, /*width = */ 0, "0"},
+      {0, kLeft, /*width = */ 6, "0_____"},
+      {0, kLeft | kPos, /*width = */ 0, "+0"},
+      {0, kLeft | kPos, /*width = */ 6, "+0____"},
+      {0, kLeft | kBase, /*width = */ 0, "0"},
+      {0, kLeft | kBase, /*width = */ 6, "0_____"},
+      {0, kLeft | kBase | kPos, /*width = */ 0, "+0"},
+      {0, kLeft | kBase | kPos, /*width = */ 6, "+0____"},
+      {0, kLeft | kUpper, /*width = */ 0, "0"},
+      {0, kLeft | kUpper, /*width = */ 6, "0_____"},
+      {0, kLeft | kUpper | kPos, /*width = */ 0, "+0"},
+      {0, kLeft | kUpper | kPos, /*width = */ 6, "+0____"},
+      {0, kLeft | kUpper | kBase, /*width = */ 0, "0"},
+      {0, kLeft | kUpper | kBase, /*width = */ 6, "0_____"},
+      {0, kLeft | kUpper | kBase | kPos, /*width = */ 0, "+0"},
+      {0, kLeft | kUpper | kBase | kPos, /*width = */ 6, "+0____"},
+      {0, kInt, /*width = */ 0, "0"},
+      {0, kInt, /*width = */ 6, "_____0"},
+      {0, kInt | kPos, /*width = */ 0, "+0"},
+      {0, kInt | kPos, /*width = */ 6, "+____0"},
+      {0, kInt | kBase, /*width = */ 0, "0"},
+      {0, kInt | kBase, /*width = */ 6, "_____0"},
+      {0, kInt | kBase | kPos, /*width = */ 0, "+0"},
+      {0, kInt | kBase | kPos, /*width = */ 6, "+____0"},
+      {0, kInt | kUpper, /*width = */ 0, "0"},
+      {0, kInt | kUpper, /*width = */ 6, "_____0"},
+      {0, kInt | kUpper | kPos, /*width = */ 0, "+0"},
+      {0, kInt | kUpper | kPos, /*width = */ 6, "+____0"},
+      {0, kInt | kUpper | kBase, /*width = */ 0, "0"},
+      {0, kInt | kUpper | kBase, /*width = */ 6, "_____0"},
+      {0, kInt | kUpper | kBase | kPos, /*width = */ 0, "+0"},
+      {0, kInt | kUpper | kBase | kPos, /*width = */ 6, "+____0"},
+      {0, kRight, /*width = */ 0, "0"},
+      {0, kRight, /*width = */ 6, "_____0"},
+      {0, kRight | kPos, /*width = */ 0, "+0"},
+      {0, kRight | kPos, /*width = */ 6, "____+0"},
+      {0, kRight | kBase, /*width = */ 0, "0"},
+      {0, kRight | kBase, /*width = */ 6, "_____0"},
+      {0, kRight | kBase | kPos, /*width = */ 0, "+0"},
+      {0, kRight | kBase | kPos, /*width = */ 6, "____+0"},
+      {0, kRight | kUpper, /*width = */ 0, "0"},
+      {0, kRight | kUpper, /*width = */ 6, "_____0"},
+      {0, kRight | kUpper | kPos, /*width = */ 0, "+0"},
+      {0, kRight | kUpper | kPos, /*width = */ 6, "____+0"},
+      {0, kRight | kUpper | kBase, /*width = */ 0, "0"},
+      {0, kRight | kUpper | kBase, /*width = */ 6, "_____0"},
+      {0, kRight | kUpper | kBase | kPos, /*width = */ 0, "+0"},
+      {0, kRight | kUpper | kBase | kPos, /*width = */ 6, "____+0"},
+      {0, kDec, /*width = */ 0, "0"},
+      {0, kDec, /*width = */ 6, "_____0"},
+      {0, kDec | kPos, /*width = */ 0, "+0"},
+      {0, kDec | kPos, /*width = */ 6, "____+0"},
+      {0, kDec | kBase, /*width = */ 0, "0"},
+      {0, kDec | kBase, /*width = */ 6, "_____0"},
+      {0, kDec | kBase | kPos, /*width = */ 0, "+0"},
+      {0, kDec | kBase | kPos, /*width = */ 6, "____+0"},
+      {0, kDec | kUpper, /*width = */ 0, "0"},
+      {0, kDec | kUpper, /*width = */ 6, "_____0"},
+      {0, kDec | kUpper | kPos, /*width = */ 0, "+0"},
+      {0, kDec | kUpper | kPos, /*width = */ 6, "____+0"},
+      {0, kDec | kUpper | kBase, /*width = */ 0, "0"},
+      {0, kDec | kUpper | kBase, /*width = */ 6, "_____0"},
+      {0, kDec | kUpper | kBase | kPos, /*width = */ 0, "+0"},
+      {0, kDec | kUpper | kBase | kPos, /*width = */ 6, "____+0"},
+      {0, kDec | kLeft, /*width = */ 0, "0"},
+      {0, kDec | kLeft, /*width = */ 6, "0_____"},
+      {0, kDec | kLeft | kPos, /*width = */ 0, "+0"},
+      {0, kDec | kLeft | kPos, /*width = */ 6, "+0____"},
+      {0, kDec | kLeft | kBase, /*width = */ 0, "0"},
+      {0, kDec | kLeft | kBase, /*width = */ 6, "0_____"},
+      {0, kDec | kLeft | kBase | kPos, /*width = */ 0, "+0"},
+      {0, kDec | kLeft | kBase | kPos, /*width = */ 6, "+0____"},
+      {0, kDec | kLeft | kUpper, /*width = */ 0, "0"},
+      {0, kDec | kLeft | kUpper, /*width = */ 6, "0_____"},
+      {0, kDec | kLeft | kUpper | kPos, /*width = */ 0, "+0"},
+      {0, kDec | kLeft | kUpper | kPos, /*width = */ 6, "+0____"},
+      {0, kDec | kLeft | kUpper | kBase, /*width = */ 0, "0"},
+      {0, kDec | kLeft | kUpper | kBase, /*width = */ 6, "0_____"},
+      {0, kDec | kLeft | kUpper | kBase | kPos, /*width = */ 0, "+0"},
+      {0, kDec | kLeft | kUpper | kBase | kPos, /*width = */ 6, "+0____"},
+      {0, kDec | kInt, /*width = */ 0, "0"},
+      {0, kDec | kInt, /*width = */ 6, "_____0"},
+      {0, kDec | kInt | kPos, /*width = */ 0, "+0"},
+      {0, kDec | kInt | kPos, /*width = */ 6, "+____0"},
+      {0, kDec | kInt | kBase, /*width = */ 0, "0"},
+      {0, kDec | kInt | kBase, /*width = */ 6, "_____0"},
+      {0, kDec | kInt | kBase | kPos, /*width = */ 0, "+0"},
+      {0, kDec | kInt | kBase | kPos, /*width = */ 6, "+____0"},
+      {0, kDec | kInt | kUpper, /*width = */ 0, "0"},
+      {0, kDec | kInt | kUpper, /*width = */ 6, "_____0"},
+      {0, kDec | kInt | kUpper | kPos, /*width = */ 0, "+0"},
+      {0, kDec | kInt | kUpper | kPos, /*width = */ 6, "+____0"},
+      {0, kDec | kInt | kUpper | kBase, /*width = */ 0, "0"},
+      {0, kDec | kInt | kUpper | kBase, /*width = */ 6, "_____0"},
+      {0, kDec | kInt | kUpper | kBase | kPos, /*width = */ 0, "+0"},
+      {0, kDec | kInt | kUpper | kBase | kPos, /*width = */ 6, "+____0"},
+      {0, kDec | kRight, /*width = */ 0, "0"},
+      {0, kDec | kRight, /*width = */ 6, "_____0"},
+      {0, kDec | kRight | kPos, /*width = */ 0, "+0"},
+      {0, kDec | kRight | kPos, /*width = */ 6, "____+0"},
+      {0, kDec | kRight | kBase, /*width = */ 0, "0"},
+      {0, kDec | kRight | kBase, /*width = */ 6, "_____0"},
+      {0, kDec | kRight | kBase | kPos, /*width = */ 0, "+0"},
+      {0, kDec | kRight | kBase | kPos, /*width = */ 6, "____+0"},
+      {0, kDec | kRight | kUpper, /*width = */ 0, "0"},
+      {0, kDec | kRight | kUpper, /*width = */ 6, "_____0"},
+      {0, kDec | kRight | kUpper | kPos, /*width = */ 0, "+0"},
+      {0, kDec | kRight | kUpper | kPos, /*width = */ 6, "____+0"},
+      {0, kDec | kRight | kUpper | kBase, /*width = */ 0, "0"},
+      {0, kDec | kRight | kUpper | kBase, /*width = */ 6, "_____0"},
+      {0, kDec | kRight | kUpper | kBase | kPos, /*width = */ 0, "+0"},
+      {0, kDec | kRight | kUpper | kBase | kPos, /*width = */ 6, "____+0"},
+      {0, kOct, /*width = */ 0, "0"},
+      {0, kOct, /*width = */ 6, "_____0"},
+      {0, kOct | kPos, /*width = */ 0, "0"},
+      {0, kOct | kPos, /*width = */ 6, "_____0"},
+      {0, kOct | kBase, /*width = */ 0, "0"},
+      {0, kOct | kBase, /*width = */ 6, "_____0"},
+      {0, kOct | kBase | kPos, /*width = */ 0, "0"},
+      {0, kOct | kBase | kPos, /*width = */ 6, "_____0"},
+      {0, kOct | kUpper, /*width = */ 0, "0"},
+      {0, kOct | kUpper, /*width = */ 6, "_____0"},
+      {0, kOct | kUpper | kPos, /*width = */ 0, "0"},
+      {0, kOct | kUpper | kPos, /*width = */ 6, "_____0"},
+      {0, kOct | kUpper | kBase, /*width = */ 0, "0"},
+      {0, kOct | kUpper | kBase, /*width = */ 6, "_____0"},
+      {0, kOct | kUpper | kBase | kPos, /*width = */ 0, "0"},
+      {0, kOct | kUpper | kBase | kPos, /*width = */ 6, "_____0"},
+      {0, kOct | kLeft, /*width = */ 0, "0"},
+      {0, kOct | kLeft, /*width = */ 6, "0_____"},
+      {0, kOct | kLeft | kPos, /*width = */ 0, "0"},
+      {0, kOct | kLeft | kPos, /*width = */ 6, "0_____"},
+      {0, kOct | kLeft | kBase, /*width = */ 0, "0"},
+      {0, kOct | kLeft | kBase, /*width = */ 6, "0_____"},
+      {0, kOct | kLeft | kBase | kPos, /*width = */ 0, "0"},
+      {0, kOct | kLeft | kBase | kPos, /*width = */ 6, "0_____"},
+      {0, kOct | kLeft | kUpper, /*width = */ 0, "0"},
+      {0, kOct | kLeft | kUpper, /*width = */ 6, "0_____"},
+      {0, kOct | kLeft | kUpper | kPos, /*width = */ 0, "0"},
+      {0, kOct | kLeft | kUpper | kPos, /*width = */ 6, "0_____"},
+      {0, kOct | kLeft | kUpper | kBase, /*width = */ 0, "0"},
+      {0, kOct | kLeft | kUpper | kBase, /*width = */ 6, "0_____"},
+      {0, kOct | kLeft | kUpper | kBase | kPos, /*width = */ 0, "0"},
+      {0, kOct | kLeft | kUpper | kBase | kPos, /*width = */ 6, "0_____"},
+      {0, kOct | kInt, /*width = */ 0, "0"},
+      {0, kOct | kInt, /*width = */ 6, "_____0"},
+      {0, kOct | kInt | kPos, /*width = */ 0, "0"},
+      {0, kOct | kInt | kPos, /*width = */ 6, "_____0"},
+      {0, kOct | kInt | kBase, /*width = */ 0, "0"},
+      {0, kOct | kInt | kBase, /*width = */ 6, "_____0"},
+      {0, kOct | kInt | kBase | kPos, /*width = */ 0, "0"},
+      {0, kOct | kInt | kBase | kPos, /*width = */ 6, "_____0"},
+      {0, kOct | kInt | kUpper, /*width = */ 0, "0"},
+      {0, kOct | kInt | kUpper, /*width = */ 6, "_____0"},
+      {0, kOct | kInt | kUpper | kPos, /*width = */ 0, "0"},
+      {0, kOct | kInt | kUpper | kPos, /*width = */ 6, "_____0"},
+      {0, kOct | kInt | kUpper | kBase, /*width = */ 0, "0"},
+      {0, kOct | kInt | kUpper | kBase, /*width = */ 6, "_____0"},
+      {0, kOct | kInt | kUpper | kBase | kPos, /*width = */ 0, "0"},
+      {0, kOct | kInt | kUpper | kBase | kPos, /*width = */ 6, "_____0"},
+      {0, kOct | kRight, /*width = */ 0, "0"},
+      {0, kOct | kRight, /*width = */ 6, "_____0"},
+      {0, kOct | kRight | kPos, /*width = */ 0, "0"},
+      {0, kOct | kRight | kPos, /*width = */ 6, "_____0"},
+      {0, kOct | kRight | kBase, /*width = */ 0, "0"},
+      {0, kOct | kRight | kBase, /*width = */ 6, "_____0"},
+      {0, kOct | kRight | kBase | kPos, /*width = */ 0, "0"},
+      {0, kOct | kRight | kBase | kPos, /*width = */ 6, "_____0"},
+      {0, kOct | kRight | kUpper, /*width = */ 0, "0"},
+      {0, kOct | kRight | kUpper, /*width = */ 6, "_____0"},
+      {0, kOct | kRight | kUpper | kPos, /*width = */ 0, "0"},
+      {0, kOct | kRight | kUpper | kPos, /*width = */ 6, "_____0"},
+      {0, kOct | kRight | kUpper | kBase, /*width = */ 0, "0"},
+      {0, kOct | kRight | kUpper | kBase, /*width = */ 6, "_____0"},
+      {0, kOct | kRight | kUpper | kBase | kPos, /*width = */ 0, "0"},
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+      {42, kHex | kRight | kBase, /*width = */ 0, "0x2a"},
+      {42, kHex | kRight | kBase, /*width = */ 6, "__0x2a"},
+      {42, kHex | kRight | kBase | kPos, /*width = */ 0, "0x2a"},
+      {42, kHex | kRight | kBase | kPos, /*width = */ 6, "__0x2a"},
+      {42, kHex | kRight | kUpper, /*width = */ 0, "2A"},
+      {42, kHex | kRight | kUpper, /*width = */ 6, "____2A"},
+      {42, kHex | kRight | kUpper | kPos, /*width = */ 0, "2A"},
+      {42, kHex | kRight | kUpper | kPos, /*width = */ 6, "____2A"},
+      {42, kHex | kRight | kUpper | kBase, /*width = */ 0, "0X2A"},
+      {42, kHex | kRight | kUpper | kBase, /*width = */ 6, "__0X2A"},
+      {42, kHex | kRight | kUpper | kBase | kPos, /*width = */ 0, "0X2A"},
+      {42, kHex | kRight | kUpper | kBase | kPos, /*width = */ 6, "__0X2A"},
+      {-321, std::ios_base::fmtflags(), /*width = */ 0, "-321"},
+      {-321, std::ios_base::fmtflags(), /*width = */ 6, "__-321"},
+      {-321, kPos, /*width = */ 0, "-321"},
+      {-321, kPos, /*width = */ 6, "__-321"},
+      {-321, kBase, /*width = */ 0, "-321"},
+      {-321, kBase, /*width = */ 6, "__-321"},
+      {-321, kBase | kPos, /*width = */ 0, "-321"},
+      {-321, kBase | kPos, /*width = */ 6, "__-321"},
+      {-321, kUpper, /*width = */ 0, "-321"},
+      {-321, kUpper, /*width = */ 6, "__-321"},
+      {-321, kUpper | kPos, /*width = */ 0, "-321"},
+      {-321, kUpper | kPos, /*width = */ 6, "__-321"},
+      {-321, kUpper | kBase, /*width = */ 0, "-321"},
+      {-321, kUpper | kBase, /*width = */ 6, "__-321"},
+      {-321, kUpper | kBase | kPos, /*width = */ 0, "-321"},
+      {-321, kUpper | kBase | kPos, /*width = */ 6, "__-321"},
+      {-321, kLeft, /*width = */ 0, "-321"},
+      {-321, kLeft, /*width = */ 6, "-321__"},
+      {-321, kLeft | kPos, /*width = */ 0, "-321"},
+      {-321, kLeft | kPos, /*width = */ 6, "-321__"},
+      {-321, kLeft | kBase, /*width = */ 0, "-321"},
+      {-321, kLeft | kBase, /*width = */ 6, "-321__"},
+      {-321, kLeft | kBase | kPos, /*width = */ 0, "-321"},
+      {-321, kLeft | kBase | kPos, /*width = */ 6, "-321__"},
+      {-321, kLeft | kUpper, /*width = */ 0, "-321"},
+      {-321, kLeft | kUpper, /*width = */ 6, "-321__"},
+      {-321, kLeft | kUpper | kPos, /*width = */ 0, "-321"},
+      {-321, kLeft | kUpper | kPos, /*width = */ 6, "-321__"},
+      {-321, kLeft | kUpper | kBase, /*width = */ 0, "-321"},
+      {-321, kLeft | kUpper | kBase, /*width = */ 6, "-321__"},
+      {-321, kLeft | kUpper | kBase | kPos, /*width = */ 0, "-321"},
+      {-321, kLeft | kUpper | kBase | kPos, /*width = */ 6, "-321__"},
+      {-321, kInt, /*width = */ 0, "-321"},
+      {-321, kInt, /*width = */ 6, "-__321"},
+      {-321, kInt | kPos, /*width = */ 0, "-321"},
+      {-321, kInt | kPos, /*width = */ 6, "-__321"},
+      {-321, kInt | kBase, /*width = */ 0, "-321"},
+      {-321, kInt | kBase, /*width = */ 6, "-__321"},
+      {-321, kInt | kBase | kPos, /*width = */ 0, "-321"},
+      {-321, kInt | kBase | kPos, /*width = */ 6, "-__321"},
+      {-321, kInt | kUpper, /*width = */ 0, "-321"},
+      {-321, kInt | kUpper, /*width = */ 6, "-__321"},
+      {-321, kInt | kUpper | kPos, /*width = */ 0, "-321"},
+      {-321, kInt | kUpper | kPos, /*width = */ 6, "-__321"},
+      {-321, kInt | kUpper | kBase, /*width = */ 0, "-321"},
+      {-321, kInt | kUpper | kBase, /*width = */ 6, "-__321"},
+      {-321, kInt | kUpper | kBase | kPos, /*width = */ 0, "-321"},
+      {-321, kInt | kUpper | kBase | kPos, /*width = */ 6, "-__321"},
+      {-321, kRight, /*width = */ 0, "-321"},
+      {-321, kRight, /*width = */ 6, "__-321"},
+      {-321, kRight | kPos, /*width = */ 0, "-321"},
+      {-321, kRight | kPos, /*width = */ 6, "__-321"},
+      {-321, kRight | kBase, /*width = */ 0, "-321"},
+      {-321, kRight | kBase, /*width = */ 6, "__-321"},
+      {-321, kRight | kBase | kPos, /*width = */ 0, "-321"},
+      {-321, kRight | kBase | kPos, /*width = */ 6, "__-321"},
+      {-321, kRight | kUpper, /*width = */ 0, "-321"},
+      {-321, kRight | kUpper, /*width = */ 6, "__-321"},
+      {-321, kRight | kUpper | kPos, /*width = */ 0, "-321"},
+      {-321, kRight | kUpper | kPos, /*width = */ 6, "__-321"},
+      {-321, kRight | kUpper | kBase, /*width = */ 0, "-321"},
+      {-321, kRight | kUpper | kBase, /*width = */ 6, "__-321"},
+      {-321, kRight | kUpper | kBase | kPos, /*width = */ 0, "-321"},
+      {-321, kRight | kUpper | kBase | kPos, /*width = */ 6, "__-321"},
+      {-321, kDec, /*width = */ 0, "-321"},
+      {-321, kDec, /*width = */ 6, "__-321"},
+      {-321, kDec | kPos, /*width = */ 0, "-321"},
+      {-321, kDec | kPos, /*width = */ 6, "__-321"},
+      {-321, kDec | kBase, /*width = */ 0, "-321"},
+      {-321, kDec | kBase, /*width = */ 6, "__-321"},
+      {-321, kDec | kBase | kPos, /*width = */ 0, "-321"},
+      {-321, kDec | kBase | kPos, /*width = */ 6, "__-321"},
+      {-321, kDec | kUpper, /*width = */ 0, "-321"},
+      {-321, kDec | kUpper, /*width = */ 6, "__-321"},
+      {-321, kDec | kUpper | kPos, /*width = */ 0, "-321"},
+      {-321, kDec | kUpper | kPos, /*width = */ 6, "__-321"},
+      {-321, kDec | kUpper | kBase, /*width = */ 0, "-321"},
+      {-321, kDec | kUpper | kBase, /*width = */ 6, "__-321"},
+      {-321, kDec | kUpper | kBase | kPos, /*width = */ 0, "-321"},
+      {-321, kDec | kUpper | kBase | kPos, /*width = */ 6, "__-321"},
+      {-321, kDec | kLeft, /*width = */ 0, "-321"},
+      {-321, kDec | kLeft, /*width = */ 6, "-321__"},
+      {-321, kDec | kLeft | kPos, /*width = */ 0, "-321"},
+      {-321, kDec | kLeft | kPos, /*width = */ 6, "-321__"},
+      {-321, kDec | kLeft | kBase, /*width = */ 0, "-321"},
+      {-321, kDec | kLeft | kBase, /*width = */ 6, "-321__"},
+      {-321, kDec | kLeft | kBase | kPos, /*width = */ 0, "-321"},
+      {-321, kDec | kLeft | kBase | kPos, /*width = */ 6, "-321__"},
+      {-321, kDec | kLeft | kUpper, /*width = */ 0, "-321"},
+      {-321, kDec | kLeft | kUpper, /*width = */ 6, "-321__"},
+      {-321, kDec | kLeft | kUpper | kPos, /*width = */ 0, "-321"},
+      {-321, kDec | kLeft | kUpper | kPos, /*width = */ 6, "-321__"},
+      {-321, kDec | kLeft | kUpper | kBase, /*width = */ 0, "-321"},
+      {-321, kDec | kLeft | kUpper | kBase, /*width = */ 6, "-321__"},
+      {-321, kDec | kLeft | kUpper | kBase | kPos, /*width = */ 0, "-321"},
+      {-321, kDec | kLeft | kUpper | kBase | kPos, /*width = */ 6, "-321__"},
+      {-321, kDec | kInt, /*width = */ 0, "-321"},
+      {-321, kDec | kInt, /*width = */ 6, "-__321"},
+      {-321, kDec | kInt | kPos, /*width = */ 0, "-321"},
+      {-321, kDec | kInt | kPos, /*width = */ 6, "-__321"},
+      {-321, kDec | kInt | kBase, /*width = */ 0, "-321"},
+      {-321, kDec | kInt | kBase, /*width = */ 6, "-__321"},
+      {-321, kDec | kInt | kBase | kPos, /*width = */ 0, "-321"},
+      {-321, kDec | kInt | kBase | kPos, /*width = */ 6, "-__321"},
+      {-321, kDec | kInt | kUpper, /*width = */ 0, "-321"},
+      {-321, kDec | kInt | kUpper, /*width = */ 6, "-__321"},
+      {-321, kDec | kInt | kUpper | kPos, /*width = */ 0, "-321"},
+      {-321, kDec | kInt | kUpper | kPos, /*width = */ 6, "-__321"},
+      {-321, kDec | kInt | kUpper | kBase, /*width = */ 0, "-321"},
+      {-321, kDec | kInt | kUpper | kBase, /*width = */ 6, "-__321"},
+      {-321, kDec | kInt | kUpper | kBase | kPos, /*width = */ 0, "-321"},
+      {-321, kDec | kInt | kUpper | kBase | kPos, /*width = */ 6, "-__321"},
+      {-321, kDec | kRight, /*width = */ 0, "-321"},
+      {-321, kDec | kRight, /*width = */ 6, "__-321"},
+      {-321, kDec | kRight | kPos, /*width = */ 0, "-321"},
+      {-321, kDec | kRight | kPos, /*width = */ 6, "__-321"},
+      {-321, kDec | kRight | kBase, /*width = */ 0, "-321"},
+      {-321, kDec | kRight | kBase, /*width = */ 6, "__-321"},
+      {-321, kDec | kRight | kBase | kPos, /*width = */ 0, "-321"},
+      {-321, kDec | kRight | kBase | kPos, /*width = */ 6, "__-321"},
+      {-321, kDec | kRight | kUpper, /*width = */ 0, "-321"},
+      {-321, kDec | kRight | kUpper, /*width = */ 6, "__-321"},
+      {-321, kDec | kRight | kUpper | kPos, /*width = */ 0, "-321"},
+      {-321, kDec | kRight | kUpper | kPos, /*width = */ 6, "__-321"},
+      {-321, kDec | kRight | kUpper | kBase, /*width = */ 0, "-321"},
+      {-321, kDec | kRight | kUpper | kBase, /*width = */ 6, "__-321"},
+      {-321, kDec | kRight | kUpper | kBase | kPos, /*width = */ 0, "-321"},
+      {-321, kDec | kRight | kUpper | kBase | kPos, /*width = */ 6, "__-321"}};
+}
+
 std::vector<Uint128TestCase> GetUint128FormatCases() {
   return {
       {0, std::ios_base::fmtflags(), /*width = */ 0, "0"},
diff --git a/absl/numeric/int128_test.cc b/absl/numeric/int128_test.cc
index 5e1b5ec352b..93f0d73ee03 100644
--- a/absl/numeric/int128_test.cc
+++ b/absl/numeric/int128_test.cc
@@ -15,22 +15,23 @@
 #include "absl/numeric/int128.h"
 
 #include <algorithm>
+#include <cmath>
+#include <cstdint>
 #include <limits>
 #include <random>
+#include <tuple>
 #include <type_traits>
 #include <utility>
 #include <vector>
 
 #include "gtest/gtest.h"
+#include "absl/base/casts.h"
 #include "absl/base/internal/cycleclock.h"
 #include "absl/hash/hash_testing.h"
 #include "absl/meta/type_traits.h"
+#include "absl/types/compare.h"
 
-#if defined(_MSC_VER) && _MSC_VER == 1900
-// Disable "unary minus operator applied to unsigned type" warnings in Microsoft
-// Visual C++ 14 (2015).
-#pragma warning(disable:4146)
-#endif
+#define MAKE_INT128(HI, LO) absl::MakeInt128(static_cast<int64_t>(HI), LO)
 
 namespace {
 
@@ -54,22 +55,22 @@ typedef ::testing::Types<float, double, long double> FloatingPointTypes;
 TYPED_TEST_SUITE(Uint128IntegerTraitsTest, IntegerTypes);
 
 TYPED_TEST(Uint128IntegerTraitsTest, ConstructAssignTest) {
-  static_assert(std::is_constructible<absl::uint128, TypeParam>::value,
+  static_assert(std::is_constructible_v<absl::uint128, TypeParam>,
                 "absl::uint128 must be constructible from TypeParam");
-  static_assert(std::is_assignable<absl::uint128&, TypeParam>::value,
+  static_assert(std::is_assignable_v<absl::uint128&, TypeParam>,
                 "absl::uint128 must be assignable from TypeParam");
-  static_assert(!std::is_assignable<TypeParam&, absl::uint128>::value,
+  static_assert(!std::is_assignable_v<TypeParam&, absl::uint128>,
                 "TypeParam must not be assignable from absl::uint128");
 }
 
 TYPED_TEST_SUITE(Uint128FloatTraitsTest, FloatingPointTypes);
 
 TYPED_TEST(Uint128FloatTraitsTest, ConstructAssignTest) {
-  static_assert(std::is_constructible<absl::uint128, TypeParam>::value,
+  static_assert(std::is_constructible_v<absl::uint128, TypeParam>,
                 "absl::uint128 must be constructible from TypeParam");
-  static_assert(!std::is_assignable<absl::uint128&, TypeParam>::value,
+  static_assert(!std::is_assignable_v<absl::uint128&, TypeParam>,
                 "absl::uint128 must not be assignable from TypeParam");
-  static_assert(!std::is_assignable<TypeParam&, absl::uint128>::value,
+  static_assert(!std::is_assignable_v<TypeParam&, absl::uint128>,
                 "TypeParam must not be assignable from absl::uint128");
 }
 
@@ -77,29 +78,27 @@ TYPED_TEST(Uint128FloatTraitsTest, ConstructAssignTest) {
 // These type traits done separately as TYPED_TEST requires typeinfo, and not
 // all platforms have this for __int128 even though they define the type.
 TEST(Uint128, IntrinsicTypeTraitsTest) {
-  static_assert(std::is_constructible<absl::uint128, __int128>::value,
+  static_assert(std::is_constructible_v<absl::uint128, __int128>,
                 "absl::uint128 must be constructible from __int128");
-  static_assert(std::is_assignable<absl::uint128&, __int128>::value,
+  static_assert(std::is_assignable_v<absl::uint128&, __int128>,
                 "absl::uint128 must be assignable from __int128");
-  static_assert(!std::is_assignable<__int128&, absl::uint128>::value,
+  static_assert(!std::is_assignable_v<__int128&, absl::uint128>,
                 "__int128 must not be assignable from absl::uint128");
 
-  static_assert(std::is_constructible<absl::uint128, unsigned __int128>::value,
+  static_assert(std::is_constructible_v<absl::uint128, unsigned __int128>,
                 "absl::uint128 must be constructible from unsigned __int128");
-  static_assert(std::is_assignable<absl::uint128&, unsigned __int128>::value,
+  static_assert(std::is_assignable_v<absl::uint128&, unsigned __int128>,
                 "absl::uint128 must be assignable from unsigned __int128");
-  static_assert(!std::is_assignable<unsigned __int128&, absl::uint128>::value,
+  static_assert(!std::is_assignable_v<unsigned __int128&, absl::uint128>,
                 "unsigned __int128 must not be assignable from absl::uint128");
 }
 #endif  // ABSL_HAVE_INTRINSIC_INT128
 
 TEST(Uint128, TrivialTraitsTest) {
-  static_assert(absl::is_trivially_default_constructible<absl::uint128>::value,
-                "");
-  static_assert(absl::is_trivially_copy_constructible<absl::uint128>::value,
-                "");
-  static_assert(absl::is_trivially_copy_assignable<absl::uint128>::value, "");
-  static_assert(std::is_trivially_destructible<absl::uint128>::value, "");
+  static_assert(std::is_trivially_default_constructible_v<absl::uint128>);
+  static_assert(std::is_trivially_copy_constructible_v<absl::uint128>);
+  static_assert(std::is_trivially_copy_assignable_v<absl::uint128>);
+  static_assert(std::is_trivially_destructible_v<absl::uint128>);
 }
 
 TEST(Uint128, AllTests) {
@@ -226,12 +225,33 @@ TEST(Uint128, AllTests) {
   EXPECT_EQ(test >>= 1, one);
   EXPECT_EQ(test <<= 1, two);
 
+  EXPECT_EQ(big, +big);
+  EXPECT_EQ(two, +two);
+  EXPECT_EQ(absl::Uint128Max(), +absl::Uint128Max());
+  EXPECT_EQ(zero, +zero);
+
   EXPECT_EQ(big, -(-big));
   EXPECT_EQ(two, -((-one) - 1));
   EXPECT_EQ(absl::Uint128Max(), -one);
   EXPECT_EQ(zero, -zero);
+}
 
-  EXPECT_EQ(absl::Uint128Max(), absl::kuint128max);
+TEST(Int128, RightShiftOfNegativeNumbers) {
+  absl::int128 minus_six = -6;
+  absl::int128 minus_three = -3;
+  absl::int128 minus_two = -2;
+  absl::int128 minus_one = -1;
+  if ((-6 >> 1) == -3) {
+    // Right shift is arithmetic (sign propagates)
+    EXPECT_EQ(minus_six >> 1, minus_three);
+    EXPECT_EQ(minus_six >> 2, minus_two);
+    EXPECT_EQ(minus_six >> 65, minus_one);
+  } else {
+    // Right shift is logical (zeros shifted in at MSB)
+    EXPECT_EQ(minus_six >> 1, absl::int128(absl::uint128(minus_six) >> 1));
+    EXPECT_EQ(minus_six >> 2, absl::int128(absl::uint128(minus_six) >> 2));
+    EXPECT_EQ(minus_six >> 65, absl::int128(absl::uint128(minus_six) >> 65));
+  }
 }
 
 TEST(Uint128, ConversionTests) {
@@ -260,8 +280,9 @@ TEST(Uint128, ConversionTests) {
   EXPECT_EQ(from_precise_double, from_precise_ints);
   EXPECT_DOUBLE_EQ(static_cast<double>(from_precise_ints), precise_double);
 
-  double approx_double = 0xffffeeeeddddcccc * std::pow(2.0, 64.0) +
-                         0xbbbbaaaa99998888;
+  double approx_double =
+      static_cast<double>(0xffffeeeeddddcccc) * std::pow(2.0, 64.0) +
+      static_cast<double>(0xbbbbaaaa99998888);
   absl::uint128 from_approx_double(approx_double);
   EXPECT_DOUBLE_EQ(static_cast<double>(from_approx_double), approx_double);
 
@@ -329,7 +350,7 @@ TEST(Uint128, Multiply) {
   c = a * b;
   EXPECT_EQ(absl::MakeUint128(0x530EDA741C71D4C3, 0xBF25975319080000), c);
   EXPECT_EQ(0, c - b * a);
-  EXPECT_EQ(a*a - b*b, (a+b) * (a-b));
+  EXPECT_EQ(a * a - b * b, (a + b) * (a - b));
 
   // Verified with dc.
   a = absl::MakeUint128(0x0123456789abcdef, 0xfedcba9876543210);
@@ -337,7 +358,7 @@ TEST(Uint128, Multiply) {
   c = a * b;
   EXPECT_EQ(absl::MakeUint128(0x97a87f4f261ba3f2, 0x342d0bbf48948200), c);
   EXPECT_EQ(0, c - b * a);
-  EXPECT_EQ(a*a - b*b, (a+b) * (a-b));
+  EXPECT_EQ(a * a - b * b, (a + b) * (a - b));
 }
 
 TEST(Uint128, AliasTests) {
@@ -441,12 +462,32 @@ TEST(Uint128, ConstexprTest) {
   EXPECT_EQ(zero, absl::uint128(0));
   EXPECT_EQ(one, absl::uint128(1));
   EXPECT_EQ(minus_two, absl::MakeUint128(-1, -2));
+
+  constexpr double f = static_cast<float>(absl::uint128(123));
+  EXPECT_EQ(f, 123.0f);
+
+  constexpr double d = static_cast<double>(absl::uint128(123));
+  EXPECT_EQ(d, 123.0);
+
+  constexpr long double ld = static_cast<long double>(absl::uint128(123));
+  EXPECT_EQ(ld, 123.0);
+
+#ifdef ABSL_HAVE_INTRINSIC_INT128
+  constexpr absl::uint128 division = absl::uint128(10) / absl::uint128(2);
+  EXPECT_EQ(division, absl::uint128(5));
+
+  constexpr absl::uint128 modulus = absl::int128(10) % absl::int128(3);
+  EXPECT_EQ(modulus, absl::uint128(1));
+
+  constexpr absl::uint128 multiplication = absl::uint128(10) * absl::uint128(3);
+  EXPECT_EQ(multiplication, absl::uint128(30));
+#endif  // ABSL_HAVE_INTRINSIC_INT128
 }
 
 TEST(Uint128, NumericLimitsTest) {
-  static_assert(std::numeric_limits<absl::uint128>::is_specialized, "");
-  static_assert(!std::numeric_limits<absl::uint128>::is_signed, "");
-  static_assert(std::numeric_limits<absl::uint128>::is_integer, "");
+  static_assert(std::numeric_limits<absl::uint128>::is_specialized);
+  static_assert(!std::numeric_limits<absl::uint128>::is_signed);
+  static_assert(std::numeric_limits<absl::uint128>::is_integer);
   EXPECT_EQ(static_cast<int>(128 * std::log10(2)),
             std::numeric_limits<absl::uint128>::digits10);
   EXPECT_EQ(0, std::numeric_limits<absl::uint128>::min());
@@ -454,29 +495,912 @@ TEST(Uint128, NumericLimitsTest) {
   EXPECT_EQ(absl::Uint128Max(), std::numeric_limits<absl::uint128>::max());
 }
 
+// Some arbitrary constant to test hashing. The first hex digits of pi.
+constexpr absl::uint128 kPi = (absl::uint128(0x3243f6a8885a308d) << 64) |
+                              absl::uint128(0x313198a2e0370734);
+
 TEST(Uint128, Hash) {
-  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly({
+#if defined(ABSL_HAVE_INTRINSIC_INT128)
+  using Ext128 = unsigned __int128;
+#endif
+  // Make the tuple outside the EXPECT_TRUE because putting the #if inside the
+  // macro argument is not ok.
+  const auto values = std::make_tuple(
       // Some simple values
-      absl::uint128{0},
-      absl::uint128{1},
-      ~absl::uint128{},
+      absl::uint128{0}, absl::uint128{1}, ~absl::uint128{},
       // 64 bit limits
       absl::uint128{std::numeric_limits<int64_t>::max()},
       absl::uint128{std::numeric_limits<uint64_t>::max()} + 0,
       absl::uint128{std::numeric_limits<uint64_t>::max()} + 1,
       absl::uint128{std::numeric_limits<uint64_t>::max()} + 2,
       // Keeping high same
-      absl::uint128{1} << 62,
-      absl::uint128{1} << 63,
+      absl::uint128{1} << 62, absl::uint128{1} << 63,
       // Keeping low same
-      absl::uint128{1} << 64,
-      absl::uint128{1} << 65,
+      absl::uint128{1} << 64, absl::uint128{1} << 65,
       // 128 bit limits
       std::numeric_limits<absl::uint128>::max(),
       std::numeric_limits<absl::uint128>::max() - 1,
       std::numeric_limits<absl::uint128>::min() + 1,
       std::numeric_limits<absl::uint128>::min(),
-  }));
+      // arbitrary constant
+      kPi
+#if defined(ABSL_HAVE_INTRINSIC_INT128)
+      // Same but with the intrinsic to verify that they match
+      ,
+      Ext128{0}, Ext128{1}, ~Ext128{},
+      Ext128{std::numeric_limits<int64_t>::max()},
+      Ext128{std::numeric_limits<uint64_t>::max()} + 0,
+      Ext128{std::numeric_limits<uint64_t>::max()} + 1,
+      Ext128{std::numeric_limits<uint64_t>::max()} + 2, Ext128{1} << 62,
+      Ext128{1} << 63, Ext128{1} << 64, Ext128{1} << 65,
+      std::numeric_limits<Ext128>::max(),
+      std::numeric_limits<Ext128>::max() - 1,
+      std::numeric_limits<Ext128>::min() + 1,
+      std::numeric_limits<Ext128>::min(), static_cast<Ext128>(kPi)
+#endif
+  );
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(values));
+}
+
+TEST(Int128Uint128, ConversionTest) {
+  absl::int128 nonnegative_signed_values[] = {
+      0,
+      1,
+      0xffeeddccbbaa9988,
+      absl::MakeInt128(0x7766554433221100, 0),
+      absl::MakeInt128(0x1234567890abcdef, 0xfedcba0987654321),
+      absl::Int128Max()};
+  for (absl::int128 value : nonnegative_signed_values) {
+    EXPECT_EQ(value, absl::int128(absl::uint128(value)));
+
+    absl::uint128 assigned_value;
+    assigned_value = value;
+    EXPECT_EQ(value, absl::int128(assigned_value));
+  }
+
+  absl::int128 negative_values[] = {
+      -1, -0x1234567890abcdef, absl::MakeInt128(-0x5544332211ffeedd, 0),
+      -absl::MakeInt128(0x76543210fedcba98, 0xabcdef0123456789)};
+  for (absl::int128 value : negative_values) {
+    EXPECT_EQ(absl::uint128(-value), -absl::uint128(value));
+
+    absl::uint128 assigned_value;
+    assigned_value = value;
+    EXPECT_EQ(absl::uint128(-value), -assigned_value);
+  }
+}
+
+template <typename T>
+class Int128IntegerTraitsTest : public ::testing::Test {};
+
+TYPED_TEST_SUITE(Int128IntegerTraitsTest, IntegerTypes);
+
+TYPED_TEST(Int128IntegerTraitsTest, ConstructAssignTest) {
+  static_assert(std::is_constructible_v<absl::int128, TypeParam>,
+                "absl::int128 must be constructible from TypeParam");
+  static_assert(std::is_assignable_v<absl::int128&, TypeParam>,
+                "absl::int128 must be assignable from TypeParam");
+  static_assert(!std::is_assignable_v<TypeParam&, absl::int128>,
+                "TypeParam must not be assignable from absl::int128");
+}
+
+template <typename T>
+class Int128FloatTraitsTest : public ::testing::Test {};
+
+TYPED_TEST_SUITE(Int128FloatTraitsTest, FloatingPointTypes);
+
+TYPED_TEST(Int128FloatTraitsTest, ConstructAssignTest) {
+  static_assert(std::is_constructible_v<absl::int128, TypeParam>,
+                "absl::int128 must be constructible from TypeParam");
+  static_assert(!std::is_assignable_v<absl::int128&, TypeParam>,
+                "absl::int128 must not be assignable from TypeParam");
+  static_assert(!std::is_assignable_v<TypeParam&, absl::int128>,
+                "TypeParam must not be assignable from absl::int128");
+}
+
+#ifdef ABSL_HAVE_INTRINSIC_INT128
+// These type traits done separately as TYPED_TEST requires typeinfo, and not
+// all platforms have this for __int128 even though they define the type.
+TEST(Int128, IntrinsicTypeTraitsTest) {
+  static_assert(std::is_constructible_v<absl::int128, __int128>,
+                "absl::int128 must be constructible from __int128");
+  static_assert(std::is_assignable_v<absl::int128&, __int128>,
+                "absl::int128 must be assignable from __int128");
+  static_assert(!std::is_assignable_v<__int128&, absl::int128>,
+                "__int128 must not be assignable from absl::int128");
+
+  static_assert(std::is_constructible_v<absl::int128, unsigned __int128>,
+                "absl::int128 must be constructible from unsigned __int128");
+  static_assert(!std::is_assignable_v<absl::int128&, unsigned __int128>,
+                "absl::int128 must be assignable from unsigned __int128");
+  static_assert(!std::is_assignable_v<unsigned __int128&, absl::int128>,
+                "unsigned __int128 must not be assignable from absl::int128");
+}
+#endif  // ABSL_HAVE_INTRINSIC_INT128
+
+TEST(Int128, TrivialTraitsTest) {
+  static_assert(std::is_trivially_default_constructible_v<absl::int128>);
+  static_assert(std::is_trivially_copy_constructible_v<absl::int128>);
+  static_assert(std::is_trivially_copy_assignable_v<absl::int128>);
+  static_assert(std::is_trivially_destructible_v<absl::int128>);
+}
+
+TEST(Int128, BoolConversionTest) {
+  EXPECT_FALSE(absl::int128(0));
+  for (int i = 0; i < 64; ++i) {
+    EXPECT_TRUE(absl::MakeInt128(0, uint64_t{1} << i));
+  }
+  for (int i = 0; i < 63; ++i) {
+    EXPECT_TRUE(absl::MakeInt128(int64_t{1} << i, 0));
+  }
+  EXPECT_TRUE(absl::Int128Min());
+
+  EXPECT_EQ(absl::int128(1), absl::int128(true));
+  EXPECT_EQ(absl::int128(0), absl::int128(false));
+}
+
+template <typename T>
+class Int128IntegerConversionTest : public ::testing::Test {};
+
+TYPED_TEST_SUITE(Int128IntegerConversionTest, IntegerTypes);
+
+TYPED_TEST(Int128IntegerConversionTest, RoundTripTest) {
+  EXPECT_EQ(TypeParam{0}, static_cast<TypeParam>(absl::int128(0)));
+  EXPECT_EQ(std::numeric_limits<TypeParam>::min(),
+            static_cast<TypeParam>(
+                absl::int128(std::numeric_limits<TypeParam>::min())));
+  EXPECT_EQ(std::numeric_limits<TypeParam>::max(),
+            static_cast<TypeParam>(
+                absl::int128(std::numeric_limits<TypeParam>::max())));
+}
+
+template <typename T>
+class Int128FloatConversionTest : public ::testing::Test {};
+
+TYPED_TEST_SUITE(Int128FloatConversionTest, FloatingPointTypes);
+
+TYPED_TEST(Int128FloatConversionTest, ConstructAndCastTest) {
+  // Conversions where the floating point values should be exactly the same.
+  // 0x9f5b is a randomly chosen small value.
+  for (int i = 0; i < 110; ++i) {  // 110 = 126 - #bits in 0x9f5b
+    SCOPED_TRACE(::testing::Message() << "i = " << i);
+
+    TypeParam float_value = std::ldexp(static_cast<TypeParam>(0x9f5b), i);
+    absl::int128 int_value = absl::int128(0x9f5b) << i;
+
+    EXPECT_EQ(float_value, static_cast<TypeParam>(int_value));
+    EXPECT_EQ(-float_value, static_cast<TypeParam>(-int_value));
+    EXPECT_EQ(int_value, absl::int128(float_value));
+    EXPECT_EQ(-int_value, absl::int128(-float_value));
+  }
+
+  // Round trip conversions with a small sample of randomly generated uint64_t
+  // values (less than int64_t max so that value * 2^64 fits into int128).
+  uint64_t values[] = {0x6d4492c24fb86199, 0x26ead65e4cb359b5,
+                       0x2c43407433ba3fd1, 0x3b574ec668df6b55,
+                       0x1c750e55a29f4f0f};
+  for (uint64_t value : values) {
+    for (int i = 0; i <= 64; ++i) {
+      SCOPED_TRACE(::testing::Message()
+                   << "value = " << value << "; i = " << i);
+
+      TypeParam fvalue = std::ldexp(static_cast<TypeParam>(value), i);
+      EXPECT_DOUBLE_EQ(fvalue, static_cast<TypeParam>(absl::int128(fvalue)));
+      EXPECT_DOUBLE_EQ(-fvalue, static_cast<TypeParam>(-absl::int128(fvalue)));
+      EXPECT_DOUBLE_EQ(-fvalue, static_cast<TypeParam>(absl::int128(-fvalue)));
+      EXPECT_DOUBLE_EQ(fvalue, static_cast<TypeParam>(-absl::int128(-fvalue)));
+    }
+  }
+
+  // Round trip conversions with a small sample of random large positive values.
+  absl::int128 large_values[] = {
+      absl::MakeInt128(0x5b0640d96c7b3d9f, 0xb7a7189e51d18622),
+      absl::MakeInt128(0x34bed042c6f65270, 0x73b236570669a089),
+      absl::MakeInt128(0x43deba9e6da12724, 0xf7f0f83da686797d),
+      absl::MakeInt128(0x71e8d383be4e5589, 0x75c3f96fb00752b6)};
+  for (absl::int128 value : large_values) {
+    // Make value have as many significant bits as can be represented by
+    // the mantissa, also making sure the highest and lowest bit in the range
+    // are set.
+    value >>= (127 - std::numeric_limits<TypeParam>::digits);
+    value |= absl::int128(1) << (std::numeric_limits<TypeParam>::digits - 1);
+    value |= 1;
+    for (int i = 0; i < 127 - std::numeric_limits<TypeParam>::digits; ++i) {
+      absl::int128 int_value = value << i;
+      EXPECT_EQ(int_value,
+                static_cast<absl::int128>(static_cast<TypeParam>(int_value)));
+      EXPECT_EQ(-int_value,
+                static_cast<absl::int128>(static_cast<TypeParam>(-int_value)));
+    }
+  }
+
+  // Small sample of checks that rounding is toward zero
+  EXPECT_EQ(0, absl::int128(TypeParam(0.1)));
+  EXPECT_EQ(17, absl::int128(TypeParam(17.8)));
+  EXPECT_EQ(0, absl::int128(TypeParam(-0.8)));
+  EXPECT_EQ(-53, absl::int128(TypeParam(-53.1)));
+  EXPECT_EQ(0, absl::int128(TypeParam(0.5)));
+  EXPECT_EQ(0, absl::int128(TypeParam(-0.5)));
+  TypeParam just_lt_one = std::nexttoward(TypeParam(1), TypeParam(0));
+  EXPECT_EQ(0, absl::int128(just_lt_one));
+  TypeParam just_gt_minus_one = std::nexttoward(TypeParam(-1), TypeParam(0));
+  EXPECT_EQ(0, absl::int128(just_gt_minus_one));
+
+  // Check limits
+  EXPECT_DOUBLE_EQ(std::ldexp(static_cast<TypeParam>(1), 127),
+                   static_cast<TypeParam>(absl::Int128Max()));
+  EXPECT_DOUBLE_EQ(-std::ldexp(static_cast<TypeParam>(1), 127),
+                   static_cast<TypeParam>(absl::Int128Min()));
+}
+
+TEST(Int128, FactoryTest) {
+  EXPECT_EQ(absl::int128(-1), absl::MakeInt128(-1, -1));
+  EXPECT_EQ(absl::int128(-31), absl::MakeInt128(-1, -31));
+  EXPECT_EQ(absl::int128(std::numeric_limits<int64_t>::min()),
+            absl::MakeInt128(-1, std::numeric_limits<int64_t>::min()));
+  EXPECT_EQ(absl::int128(0), absl::MakeInt128(0, 0));
+  EXPECT_EQ(absl::int128(1), absl::MakeInt128(0, 1));
+  EXPECT_EQ(absl::int128(std::numeric_limits<int64_t>::max()),
+            absl::MakeInt128(0, std::numeric_limits<int64_t>::max()));
+}
+
+TEST(Int128, HighLowTest) {
+  struct HighLowPair {
+    int64_t high;
+    uint64_t low;
+  };
+  HighLowPair values[]{{0, 0}, {0, 1}, {1, 0}, {123, 456}, {-654, 321}};
+  for (const HighLowPair& pair : values) {
+    absl::int128 value = absl::MakeInt128(pair.high, pair.low);
+    EXPECT_EQ(pair.low, absl::Int128Low64(value));
+    EXPECT_EQ(pair.high, absl::Int128High64(value));
+  }
+}
+
+TEST(Int128, LimitsTest) {
+  EXPECT_EQ(absl::MakeInt128(0x7fffffffffffffff, 0xffffffffffffffff),
+            absl::Int128Max());
+  EXPECT_EQ(absl::Int128Max(), ~absl::Int128Min());
+}
+
+#if defined(ABSL_HAVE_INTRINSIC_INT128)
+TEST(Int128, IntrinsicConversionTest) {
+  __int128 intrinsic =
+      (static_cast<__int128>(0x3a5b76c209de76f6) << 64) + 0x1f25e1d63a2b46c5;
+  absl::int128 custom =
+      absl::MakeInt128(0x3a5b76c209de76f6, 0x1f25e1d63a2b46c5);
+
+  EXPECT_EQ(custom, absl::int128(intrinsic));
+  EXPECT_EQ(intrinsic, static_cast<__int128>(custom));
+}
+#endif  // ABSL_HAVE_INTRINSIC_INT128
+
+TEST(Int128, ConstexprTest) {
+  constexpr absl::int128 zero = absl::int128();
+  constexpr absl::int128 one = 1;
+  constexpr absl::int128 minus_two = -2;
+  constexpr absl::int128 min = absl::Int128Min();
+  constexpr absl::int128 max = absl::Int128Max();
+  EXPECT_EQ(zero, absl::int128(0));
+  EXPECT_EQ(one, absl::int128(1));
+  EXPECT_EQ(minus_two, absl::MakeInt128(-1, -2));
+  EXPECT_GT(max, one);
+  EXPECT_LT(min, minus_two);
+
+  constexpr double f = static_cast<float>(absl::int128(123));
+  EXPECT_EQ(f, 123.0f);
+
+  constexpr double d = static_cast<double>(absl::int128(123));
+  EXPECT_EQ(d, 123.0);
+
+  constexpr long double ld = static_cast<long double>(absl::int128(123));
+  EXPECT_EQ(ld, 123.0);
+
+#ifdef ABSL_HAVE_INTRINSIC_INT128
+  constexpr absl::int128 f_int128(static_cast<float>(123.0));
+  EXPECT_EQ(f_int128, absl::int128(123));
+
+  constexpr absl::int128 d_int128(static_cast<double>(123.0));
+  EXPECT_EQ(d_int128, absl::int128(123));
+
+  constexpr absl::int128 ld_int128(static_cast<long double>(123.0));
+  EXPECT_EQ(ld_int128, absl::int128(123));
+
+  constexpr absl::int128 division = absl::int128(10) / absl::int128(2);
+  EXPECT_EQ(division, absl::int128(5));
+
+  constexpr absl::int128 modulus = absl::int128(10) % absl::int128(3);
+  EXPECT_EQ(modulus, absl::int128(1));
+
+  constexpr absl::int128 multiplication = absl::int128(10) * absl::int128(3);
+  EXPECT_EQ(multiplication, absl::int128(30));
+#endif  // ABSL_HAVE_INTRINSIC_INT128
+}
+
+TEST(Int128, ComparisonTest) {
+  struct TestCase {
+    absl::int128 smaller;
+    absl::int128 larger;
+  };
+  TestCase cases[] = {
+      {absl::int128(0), absl::int128(123)},
+      {absl::MakeInt128(-12, 34), absl::MakeInt128(12, 34)},
+      {absl::MakeInt128(1, 1000), absl::MakeInt128(1000, 1)},
+      {absl::MakeInt128(-1000, 1000), absl::MakeInt128(-1, 1)},
+  };
+  for (const TestCase& pair : cases) {
+    SCOPED_TRACE(::testing::Message() << "pair.smaller = " << pair.smaller
+                                      << "; pair.larger = " << pair.larger);
+
+    EXPECT_TRUE(pair.smaller == pair.smaller);  // NOLINT(readability/check)
+    EXPECT_TRUE(pair.larger == pair.larger);    // NOLINT(readability/check)
+    EXPECT_FALSE(pair.smaller == pair.larger);  // NOLINT(readability/check)
+
+    EXPECT_TRUE(pair.smaller != pair.larger);    // NOLINT(readability/check)
+    EXPECT_FALSE(pair.smaller != pair.smaller);  // NOLINT(readability/check)
+    EXPECT_FALSE(pair.larger != pair.larger);    // NOLINT(readability/check)
+
+    EXPECT_TRUE(pair.smaller < pair.larger);   // NOLINT(readability/check)
+    EXPECT_FALSE(pair.larger < pair.smaller);  // NOLINT(readability/check)
+
+    EXPECT_TRUE(pair.larger > pair.smaller);   // NOLINT(readability/check)
+    EXPECT_FALSE(pair.smaller > pair.larger);  // NOLINT(readability/check)
+
+    EXPECT_TRUE(pair.smaller <= pair.larger);   // NOLINT(readability/check)
+    EXPECT_FALSE(pair.larger <= pair.smaller);  // NOLINT(readability/check)
+    EXPECT_TRUE(pair.smaller <= pair.smaller);  // NOLINT(readability/check)
+    EXPECT_TRUE(pair.larger <= pair.larger);    // NOLINT(readability/check)
+
+    EXPECT_TRUE(pair.larger >= pair.smaller);   // NOLINT(readability/check)
+    EXPECT_FALSE(pair.smaller >= pair.larger);  // NOLINT(readability/check)
+    EXPECT_TRUE(pair.smaller >= pair.smaller);  // NOLINT(readability/check)
+    EXPECT_TRUE(pair.larger >= pair.larger);    // NOLINT(readability/check)
+
+#ifdef __cpp_impl_three_way_comparison
+    EXPECT_EQ(pair.smaller <=> pair.larger, absl::strong_ordering::less);
+    EXPECT_EQ(pair.larger <=> pair.smaller, absl::strong_ordering::greater);
+    EXPECT_EQ(pair.smaller <=> pair.smaller, absl::strong_ordering::equal);
+    EXPECT_EQ(pair.larger <=> pair.larger, absl::strong_ordering::equal);
+#endif
+  }
+}
+
+TEST(Int128, UnaryPlusTest) {
+  int64_t values64[] = {0, 1, 12345, 0x4000000000000000,
+                        std::numeric_limits<int64_t>::max()};
+  for (int64_t value : values64) {
+    SCOPED_TRACE(::testing::Message() << "value = " << value);
+
+    EXPECT_EQ(absl::int128(value), +absl::int128(value));
+    EXPECT_EQ(absl::int128(-value), +absl::int128(-value));
+    EXPECT_EQ(absl::MakeInt128(value, 0), +absl::MakeInt128(value, 0));
+    EXPECT_EQ(absl::MakeInt128(-value, 0), +absl::MakeInt128(-value, 0));
+  }
+}
+
+TEST(Int128, UnaryNegationTest) {
+  int64_t values64[] = {0, 1, 12345, 0x4000000000000000,
+                        std::numeric_limits<int64_t>::max()};
+  for (int64_t value : values64) {
+    SCOPED_TRACE(::testing::Message() << "value = " << value);
+
+    EXPECT_EQ(absl::int128(-value), -absl::int128(value));
+    EXPECT_EQ(absl::int128(value), -absl::int128(-value));
+    EXPECT_EQ(absl::MakeInt128(-value, 0), -absl::MakeInt128(value, 0));
+    EXPECT_EQ(absl::MakeInt128(value, 0), -absl::MakeInt128(-value, 0));
+  }
+}
+
+TEST(Int128, LogicalNotTest) {
+  EXPECT_TRUE(!absl::int128(0));
+  for (int i = 0; i < 64; ++i) {
+    EXPECT_FALSE(!absl::MakeInt128(0, uint64_t{1} << i));
+  }
+  for (int i = 0; i < 63; ++i) {
+    EXPECT_FALSE(!absl::MakeInt128(int64_t{1} << i, 0));
+  }
+}
+
+TEST(Int128, AdditionSubtractionTest) {
+  // 64 bit pairs that will not cause overflow / underflow. These test negative
+  // carry; positive carry must be checked separately.
+  std::pair<int64_t, int64_t> cases[]{
+      {0, 0},                              // 0, 0
+      {0, 2945781290834},                  // 0, +
+      {1908357619234, 0},                  // +, 0
+      {0, -1204895918245},                 // 0, -
+      {-2957928523560, 0},                 // -, 0
+      {89023982312461, 98346012567134},    // +, +
+      {-63454234568239, -23456235230773},  // -, -
+      {98263457263502, -21428561935925},   // +, -
+      {-88235237438467, 15923659234573},   // -, +
+  };
+  for (const auto& pair : cases) {
+    SCOPED_TRACE(::testing::Message()
+                 << "pair = {" << pair.first << ", " << pair.second << '}');
+
+    EXPECT_EQ(absl::int128(pair.first + pair.second),
+              absl::int128(pair.first) + absl::int128(pair.second));
+    EXPECT_EQ(absl::int128(pair.second + pair.first),
+              absl::int128(pair.second) += absl::int128(pair.first));
+
+    EXPECT_EQ(absl::int128(pair.first - pair.second),
+              absl::int128(pair.first) - absl::int128(pair.second));
+    EXPECT_EQ(absl::int128(pair.second - pair.first),
+              absl::int128(pair.second) -= absl::int128(pair.first));
+
+    EXPECT_EQ(
+        absl::MakeInt128(pair.second + pair.first, 0),
+        absl::MakeInt128(pair.second, 0) + absl::MakeInt128(pair.first, 0));
+    EXPECT_EQ(
+        absl::MakeInt128(pair.first + pair.second, 0),
+        absl::MakeInt128(pair.first, 0) += absl::MakeInt128(pair.second, 0));
+
+    EXPECT_EQ(
+        absl::MakeInt128(pair.second - pair.first, 0),
+        absl::MakeInt128(pair.second, 0) - absl::MakeInt128(pair.first, 0));
+    EXPECT_EQ(
+        absl::MakeInt128(pair.first - pair.second, 0),
+        absl::MakeInt128(pair.first, 0) -= absl::MakeInt128(pair.second, 0));
+  }
+
+  // check positive carry
+  EXPECT_EQ(absl::MakeInt128(31, 0),
+            absl::MakeInt128(20, 1) +
+                absl::MakeInt128(10, std::numeric_limits<uint64_t>::max()));
+}
+
+TEST(Int128, IncrementDecrementTest) {
+  absl::int128 value = 0;
+  EXPECT_EQ(0, value++);
+  EXPECT_EQ(1, value);
+  EXPECT_EQ(1, value--);
+  EXPECT_EQ(0, value);
+  EXPECT_EQ(-1, --value);
+  EXPECT_EQ(-1, value);
+  EXPECT_EQ(0, ++value);
+  EXPECT_EQ(0, value);
+}
+
+TEST(Int128, MultiplicationTest) {
+  // 1 bit x 1 bit, and negative combinations
+  for (int i = 0; i < 64; ++i) {
+    for (int j = 0; j < 127 - i; ++j) {
+      SCOPED_TRACE(::testing::Message() << "i = " << i << "; j = " << j);
+      absl::int128 a = absl::int128(1) << i;
+      absl::int128 b = absl::int128(1) << j;
+      absl::int128 c = absl::int128(1) << (i + j);
+
+      EXPECT_EQ(c, a * b);
+      EXPECT_EQ(-c, -a * b);
+      EXPECT_EQ(-c, a * -b);
+      EXPECT_EQ(c, -a * -b);
+
+      EXPECT_EQ(c, absl::int128(a) *= b);
+      EXPECT_EQ(-c, absl::int128(-a) *= b);
+      EXPECT_EQ(-c, absl::int128(a) *= -b);
+      EXPECT_EQ(c, absl::int128(-a) *= -b);
+    }
+  }
+
+  // Pairs of random values that will not overflow signed 64-bit multiplication
+  std::pair<int64_t, int64_t> small_values[] = {
+      {0x5e61, 0xf29f79ca14b4},    // +, +
+      {0x3e033b, -0x612c0ee549},   // +, -
+      {-0x052ce7e8, 0x7c728f0f},   // -, +
+      {-0x3af7054626, -0xfb1e1d},  // -, -
+  };
+  for (const std::pair<int64_t, int64_t>& pair : small_values) {
+    SCOPED_TRACE(::testing::Message()
+                 << "pair = {" << pair.first << ", " << pair.second << '}');
+
+    EXPECT_EQ(absl::int128(pair.first * pair.second),
+              absl::int128(pair.first) * absl::int128(pair.second));
+    EXPECT_EQ(absl::int128(pair.first * pair.second),
+              absl::int128(pair.first) *= absl::int128(pair.second));
+
+    EXPECT_EQ(absl::MakeInt128(pair.first * pair.second, 0),
+              absl::MakeInt128(pair.first, 0) * absl::int128(pair.second));
+    EXPECT_EQ(absl::MakeInt128(pair.first * pair.second, 0),
+              absl::MakeInt128(pair.first, 0) *= absl::int128(pair.second));
+  }
+
+  // Pairs of positive random values that will not overflow 64-bit
+  // multiplication and can be left shifted by 32 without overflow
+  std::pair<int64_t, int64_t> small_values2[] = {
+      {0x1bb0a110, 0x31487671},
+      {0x4792784e, 0x28add7d7},
+      {0x7b66553a, 0x11dff8ef},
+  };
+  for (const std::pair<int64_t, int64_t>& pair : small_values2) {
+    SCOPED_TRACE(::testing::Message()
+                 << "pair = {" << pair.first << ", " << pair.second << '}');
+
+    absl::int128 a = absl::int128(pair.first << 32);
+    absl::int128 b = absl::int128(pair.second << 32);
+    absl::int128 c = absl::MakeInt128(pair.first * pair.second, 0);
+
+    EXPECT_EQ(c, a * b);
+    EXPECT_EQ(-c, -a * b);
+    EXPECT_EQ(-c, a * -b);
+    EXPECT_EQ(c, -a * -b);
+
+    EXPECT_EQ(c, absl::int128(a) *= b);
+    EXPECT_EQ(-c, absl::int128(-a) *= b);
+    EXPECT_EQ(-c, absl::int128(a) *= -b);
+    EXPECT_EQ(c, absl::int128(-a) *= -b);
+  }
+
+  // check 0, 1, and -1 behavior with large values
+  absl::int128 large_values[] = {
+      {absl::MakeInt128(0xd66f061af02d0408, 0x727d2846cb475b53)},
+      {absl::MakeInt128(0x27b8d5ed6104452d, 0x03f8a33b0ee1df4f)},
+      {-absl::MakeInt128(0x621b6626b9e8d042, 0x27311ac99df00938)},
+      {-absl::MakeInt128(0x34e0656f1e95fb60, 0x4281cfd731257a47)},
+  };
+  for (absl::int128 value : large_values) {
+    EXPECT_EQ(0, 0 * value);
+    EXPECT_EQ(0, value * 0);
+    EXPECT_EQ(0, absl::int128(0) *= value);
+    EXPECT_EQ(0, value *= 0);
+
+    EXPECT_EQ(value, 1 * value);
+    EXPECT_EQ(value, value * 1);
+    EXPECT_EQ(value, absl::int128(1) *= value);
+    EXPECT_EQ(value, value *= 1);
+
+    EXPECT_EQ(-value, -1 * value);
+    EXPECT_EQ(-value, value * -1);
+    EXPECT_EQ(-value, absl::int128(-1) *= value);
+    EXPECT_EQ(-value, value *= -1);
+  }
+
+  // Manually calculated random large value cases
+  EXPECT_EQ(absl::MakeInt128(0xcd0efd3442219bb, 0xde47c05bcd9df6e1),
+            absl::MakeInt128(0x7c6448, 0x3bc4285c47a9d253) * 0x1a6037537b);
+  EXPECT_EQ(-absl::MakeInt128(0x1f8f149850b1e5e6, 0x1e50d6b52d272c3e),
+            -absl::MakeInt128(0x23, 0x2e68a513ca1b8859) * 0xe5a434cd14866e);
+  EXPECT_EQ(-absl::MakeInt128(0x55cae732029d1fce, 0xca6474b6423263e4),
+            0xa9b98a8ddf66bc * -absl::MakeInt128(0x81, 0x672e58231e2469d7));
+  EXPECT_EQ(absl::MakeInt128(0x19c8b7620b507dc4, 0xfec042b71a5f29a4),
+            -0x3e39341147 * -absl::MakeInt128(0x6a14b2, 0x5ed34cca42327b3c));
+
+  EXPECT_EQ(absl::MakeInt128(0xcd0efd3442219bb, 0xde47c05bcd9df6e1),
+            absl::MakeInt128(0x7c6448, 0x3bc4285c47a9d253) *= 0x1a6037537b);
+  EXPECT_EQ(-absl::MakeInt128(0x1f8f149850b1e5e6, 0x1e50d6b52d272c3e),
+            -absl::MakeInt128(0x23, 0x2e68a513ca1b8859) *= 0xe5a434cd14866e);
+  EXPECT_EQ(-absl::MakeInt128(0x55cae732029d1fce, 0xca6474b6423263e4),
+            absl::int128(0xa9b98a8ddf66bc) *=
+            -absl::MakeInt128(0x81, 0x672e58231e2469d7));
+  EXPECT_EQ(absl::MakeInt128(0x19c8b7620b507dc4, 0xfec042b71a5f29a4),
+            absl::int128(-0x3e39341147) *=
+            -absl::MakeInt128(0x6a14b2, 0x5ed34cca42327b3c));
+}
+
+TEST(Int128, DivisionAndModuloTest) {
+  // Check against 64 bit division and modulo operators with a sample of
+  // randomly generated pairs.
+  std::pair<int64_t, int64_t> small_pairs[] = {
+      {0x15f2a64138, 0x67da05},    {0x5e56d194af43045f, 0xcf1543fb99},
+      {0x15e61ed052036a, -0xc8e6}, {0x88125a341e85, -0xd23fb77683},
+      {-0xc06e20, 0x5a},           {-0x4f100219aea3e85d, 0xdcc56cb4efe993},
+      {-0x168d629105, -0xa7},      {-0x7b44e92f03ab2375, -0x6516},
+  };
+  for (const std::pair<int64_t, int64_t>& pair : small_pairs) {
+    SCOPED_TRACE(::testing::Message()
+                 << "pair = {" << pair.first << ", " << pair.second << '}');
+
+    absl::int128 dividend = pair.first;
+    absl::int128 divisor = pair.second;
+    int64_t quotient = pair.first / pair.second;
+    int64_t remainder = pair.first % pair.second;
+
+    EXPECT_EQ(quotient, dividend / divisor);
+    EXPECT_EQ(quotient, absl::int128(dividend) /= divisor);
+    EXPECT_EQ(remainder, dividend % divisor);
+    EXPECT_EQ(remainder, absl::int128(dividend) %= divisor);
+  }
+
+  // Test behavior with 0, 1, and -1 with a sample of randomly generated large
+  // values.
+  absl::int128 values[] = {
+      absl::MakeInt128(0x63d26ee688a962b2, 0x9e1411abda5c1d70),
+      absl::MakeInt128(0x152f385159d6f986, 0xbf8d48ef63da395d),
+      -absl::MakeInt128(0x3098d7567030038c, 0x14e7a8a098dc2164),
+      -absl::MakeInt128(0x49a037aca35c809f, 0xa6a87525480ef330),
+  };
+  for (absl::int128 value : values) {
+    SCOPED_TRACE(::testing::Message() << "value = " << value);
+
+    EXPECT_EQ(0, 0 / value);
+    EXPECT_EQ(0, absl::int128(0) /= value);
+    EXPECT_EQ(0, 0 % value);
+    EXPECT_EQ(0, absl::int128(0) %= value);
+
+    EXPECT_EQ(value, value / 1);
+    EXPECT_EQ(value, absl::int128(value) /= 1);
+    EXPECT_EQ(0, value % 1);
+    EXPECT_EQ(0, absl::int128(value) %= 1);
+
+    EXPECT_EQ(-value, value / -1);
+    EXPECT_EQ(-value, absl::int128(value) /= -1);
+    EXPECT_EQ(0, value % -1);
+    EXPECT_EQ(0, absl::int128(value) %= -1);
+  }
+
+  // Min and max values
+  EXPECT_EQ(0, absl::Int128Max() / absl::Int128Min());
+  EXPECT_EQ(absl::Int128Max(), absl::Int128Max() % absl::Int128Min());
+  EXPECT_EQ(-1, absl::Int128Min() / absl::Int128Max());
+  EXPECT_EQ(-1, absl::Int128Min() % absl::Int128Max());
+
+  // Power of two division and modulo of random large dividends
+  absl::int128 positive_values[] = {
+      absl::MakeInt128(0x21e1a1cc69574620, 0xe7ac447fab2fc869),
+      absl::MakeInt128(0x32c2ff3ab89e66e8, 0x03379a613fd1ce74),
+      absl::MakeInt128(0x6f32ca786184dcaf, 0x046f9c9ecb3a9ce1),
+      absl::MakeInt128(0x1aeb469dd990e0ee, 0xda2740f243cd37eb),
+  };
+  for (absl::int128 value : positive_values) {
+    for (int i = 0; i < 127; ++i) {
+      SCOPED_TRACE(::testing::Message()
+                   << "value = " << value << "; i = " << i);
+      absl::int128 power_of_two = absl::int128(1) << i;
+
+      EXPECT_EQ(value >> i, value / power_of_two);
+      EXPECT_EQ(value >> i, absl::int128(value) /= power_of_two);
+      EXPECT_EQ(value & (power_of_two - 1), value % power_of_two);
+      EXPECT_EQ(value & (power_of_two - 1),
+                absl::int128(value) %= power_of_two);
+    }
+  }
+
+  // Manually calculated cases with random large dividends
+  struct DivisionModCase {
+    absl::int128 dividend;
+    absl::int128 divisor;
+    absl::int128 quotient;
+    absl::int128 remainder;
+  };
+  DivisionModCase manual_cases[] = {
+      {absl::MakeInt128(0x6ada48d489007966, 0x3c9c5c98150d5d69),
+       absl::MakeInt128(0x8bc308fb, 0x8cb9cc9a3b803344), 0xc3b87e08,
+       absl::MakeInt128(0x1b7db5e1, 0xd9eca34b7af04b49)},
+      {absl::MakeInt128(0xd6946511b5b, 0x4886c5c96546bf5f),
+       -absl::MakeInt128(0x263b, 0xfd516279efcfe2dc), -0x59cbabf0,
+       absl::MakeInt128(0x622, 0xf462909155651d1f)},
+      {-absl::MakeInt128(0x33db734f9e8d1399, 0x8447ac92482bca4d), 0x37495078240,
+       -absl::MakeInt128(0xf01f1, 0xbc0368bf9a77eae8), -0x21a508f404d},
+      {-absl::MakeInt128(0x13f837b409a07e7d, 0x7fc8e248a7d73560), -0x1b9f,
+       absl::MakeInt128(0xb9157556d724, 0xb14f635714d7563e), -0x1ade},
+  };
+  for (const DivisionModCase test_case : manual_cases) {
+    EXPECT_EQ(test_case.quotient, test_case.dividend / test_case.divisor);
+    EXPECT_EQ(test_case.quotient,
+              absl::int128(test_case.dividend) /= test_case.divisor);
+    EXPECT_EQ(test_case.remainder, test_case.dividend % test_case.divisor);
+    EXPECT_EQ(test_case.remainder,
+              absl::int128(test_case.dividend) %= test_case.divisor);
+  }
+}
+
+TEST(Int128, BitwiseLogicTest) {
+  EXPECT_EQ(absl::int128(-1), ~absl::int128(0));
+
+  absl::int128 values[]{
+      0, -1, 0xde400bee05c3ff6b, absl::MakeInt128(0x7f32178dd81d634a, 0),
+      absl::MakeInt128(0xaf539057055613a9, 0x7d104d7d946c2e4d)};
+  for (absl::int128 value : values) {
+    EXPECT_EQ(value, ~~value);
+
+    EXPECT_EQ(value, value | value);
+    EXPECT_EQ(value, value & value);
+    EXPECT_EQ(0, value ^ value);
+
+    EXPECT_EQ(value, absl::int128(value) |= value);
+    EXPECT_EQ(value, absl::int128(value) &= value);
+    EXPECT_EQ(0, absl::int128(value) ^= value);
+
+    EXPECT_EQ(value, value | 0);
+    EXPECT_EQ(0, value & 0);
+    EXPECT_EQ(value, value ^ 0);
+
+    EXPECT_EQ(absl::int128(-1), value | absl::int128(-1));
+    EXPECT_EQ(value, value & absl::int128(-1));
+    EXPECT_EQ(~value, value ^ absl::int128(-1));
+  }
+
+  // small sample of randomly generated int64_t's
+  std::pair<int64_t, int64_t> pairs64[]{
+      {0x7f86797f5e991af4, 0x1ee30494fb007c97},
+      {0x0b278282bacf01af, 0x58780e0a57a49e86},
+      {0x059f266ccb93a666, 0x3d5b731bae9286f5},
+      {0x63c0c4820f12108c, 0x58166713c12e1c3a},
+      {0x381488bb2ed2a66e, 0x2220a3eb76a3698c},
+      {0x2a0a0dfb81e06f21, 0x4b60585927f5523c},
+      {0x555b1c3a03698537, 0x25478cd19d8e53cb},
+      {0x4750f6f27d779225, 0x16397553c6ff05fc},
+  };
+  for (const std::pair<int64_t, int64_t>& pair : pairs64) {
+    SCOPED_TRACE(::testing::Message()
+                 << "pair = {" << pair.first << ", " << pair.second << '}');
+
+    EXPECT_EQ(absl::MakeInt128(~pair.first, ~pair.second),
+              ~absl::MakeInt128(pair.first, pair.second));
+
+    EXPECT_EQ(absl::int128(pair.first & pair.second),
+              absl::int128(pair.first) & absl::int128(pair.second));
+    EXPECT_EQ(absl::int128(pair.first | pair.second),
+              absl::int128(pair.first) | absl::int128(pair.second));
+    EXPECT_EQ(absl::int128(pair.first ^ pair.second),
+              absl::int128(pair.first) ^ absl::int128(pair.second));
+
+    EXPECT_EQ(absl::int128(pair.first & pair.second),
+              absl::int128(pair.first) &= absl::int128(pair.second));
+    EXPECT_EQ(absl::int128(pair.first | pair.second),
+              absl::int128(pair.first) |= absl::int128(pair.second));
+    EXPECT_EQ(absl::int128(pair.first ^ pair.second),
+              absl::int128(pair.first) ^= absl::int128(pair.second));
+
+    EXPECT_EQ(
+        absl::MakeInt128(pair.first & pair.second, 0),
+        absl::MakeInt128(pair.first, 0) & absl::MakeInt128(pair.second, 0));
+    EXPECT_EQ(
+        absl::MakeInt128(pair.first | pair.second, 0),
+        absl::MakeInt128(pair.first, 0) | absl::MakeInt128(pair.second, 0));
+    EXPECT_EQ(
+        absl::MakeInt128(pair.first ^ pair.second, 0),
+        absl::MakeInt128(pair.first, 0) ^ absl::MakeInt128(pair.second, 0));
+
+    EXPECT_EQ(
+        absl::MakeInt128(pair.first & pair.second, 0),
+        absl::MakeInt128(pair.first, 0) &= absl::MakeInt128(pair.second, 0));
+    EXPECT_EQ(
+        absl::MakeInt128(pair.first | pair.second, 0),
+        absl::MakeInt128(pair.first, 0) |= absl::MakeInt128(pair.second, 0));
+    EXPECT_EQ(
+        absl::MakeInt128(pair.first ^ pair.second, 0),
+        absl::MakeInt128(pair.first, 0) ^= absl::MakeInt128(pair.second, 0));
+  }
+}
+
+TEST(Int128, BitwiseShiftTest) {
+  for (int i = 0; i < 64; ++i) {
+    for (int j = 0; j <= i; ++j) {
+      // Left shift from j-th bit to i-th bit.
+      SCOPED_TRACE(::testing::Message() << "i = " << i << "; j = " << j);
+      EXPECT_EQ(uint64_t{1} << i, absl::int128(uint64_t{1} << j) << (i - j));
+      EXPECT_EQ(uint64_t{1} << i, absl::int128(uint64_t{1} << j) <<= (i - j));
+    }
+  }
+  for (int i = 0; i < 63; ++i) {
+    for (int j = 0; j < 64; ++j) {
+      // Left shift from j-th bit to (i + 64)-th bit.
+      SCOPED_TRACE(::testing::Message() << "i = " << i << "; j = " << j);
+      EXPECT_EQ(absl::MakeInt128(uint64_t{1} << i, 0),
+                absl::int128(uint64_t{1} << j) << (i + 64 - j));
+      EXPECT_EQ(absl::MakeInt128(uint64_t{1} << i, 0),
+                absl::int128(uint64_t{1} << j) <<= (i + 64 - j));
+    }
+    for (int j = 0; j <= i; ++j) {
+      // Left shift from (j + 64)-th bit to (i + 64)-th bit.
+      SCOPED_TRACE(::testing::Message() << "i = " << i << "; j = " << j);
+      EXPECT_EQ(absl::MakeInt128(uint64_t{1} << i, 0),
+                absl::MakeInt128(uint64_t{1} << j, 0) << (i - j));
+      EXPECT_EQ(absl::MakeInt128(uint64_t{1} << i, 0),
+                absl::MakeInt128(uint64_t{1} << j, 0) <<= (i - j));
+    }
+  }
+
+  for (int i = 0; i < 64; ++i) {
+    for (int j = i; j < 64; ++j) {
+      // Right shift from j-th bit to i-th bit.
+      SCOPED_TRACE(::testing::Message() << "i = " << i << "; j = " << j);
+      EXPECT_EQ(uint64_t{1} << i, absl::int128(uint64_t{1} << j) >> (j - i));
+      EXPECT_EQ(uint64_t{1} << i, absl::int128(uint64_t{1} << j) >>= (j - i));
+    }
+    for (int j = 0; j < 63; ++j) {
+      // Right shift from (j + 64)-th bit to i-th bit.
+      SCOPED_TRACE(::testing::Message() << "i = " << i << "; j = " << j);
+      EXPECT_EQ(uint64_t{1} << i,
+                absl::MakeInt128(uint64_t{1} << j, 0) >> (j + 64 - i));
+      EXPECT_EQ(uint64_t{1} << i,
+                absl::MakeInt128(uint64_t{1} << j, 0) >>= (j + 64 - i));
+    }
+  }
+  for (int i = 0; i < 63; ++i) {
+    for (int j = i; j < 63; ++j) {
+      // Right shift from (j + 64)-th bit to (i + 64)-th bit.
+      SCOPED_TRACE(::testing::Message() << "i = " << i << "; j = " << j);
+      EXPECT_EQ(absl::MakeInt128(uint64_t{1} << i, 0),
+                absl::MakeInt128(uint64_t{1} << j, 0) >> (j - i));
+      EXPECT_EQ(absl::MakeInt128(uint64_t{1} << i, 0),
+                absl::MakeInt128(uint64_t{1} << j, 0) >>= (j - i));
+    }
+  }
+
+  // Signed integer overflow is undefined behavior, so in these cases enough
+  // high bits must be zero to avoid over-shifting.
+  EXPECT_EQ(MAKE_INT128(0x0, 0x123456789abcdef0) << 63,
+            MAKE_INT128(0x91a2b3c4d5e6f78, 0x0));
+  EXPECT_EQ(MAKE_INT128(0x0, 0x123456789abcdef0) << 64,
+            MAKE_INT128(0x123456789abcdef0, 0x0));
+  EXPECT_EQ(MAKE_INT128(0x1, 0xfedcba0987654321) << 63,
+            MAKE_INT128(0xff6e5d04c3b2a190, 0x8000000000000000));
+  EXPECT_EQ(MAKE_INT128(0x0, 0xfedcba0987654321) << 64,
+            MAKE_INT128(0xfedcba0987654321, 0x0));
+  EXPECT_EQ(MAKE_INT128(0x0, 0x0) << 126, MAKE_INT128(0x0, 0x0));
+  EXPECT_EQ(MAKE_INT128(0x0, 0x1) << 126, MAKE_INT128(0x4000000000000000, 0x0));
+
+  // Manually calculated cases with shift count for positive (val1) and negative
+  // (val2) values
+  absl::int128 val1 = MAKE_INT128(0x123456789abcdef0, 0x123456789abcdef0);
+  absl::int128 val2 = MAKE_INT128(0xfedcba0987654321, 0xfedcba0987654321);
+
+  EXPECT_EQ(val1 >> 63, MAKE_INT128(0x0, 0x2468acf13579bde0));
+  EXPECT_EQ(val1 >> 64, MAKE_INT128(0x0, 0x123456789abcdef0));
+  EXPECT_EQ(val2 >> 63, MAKE_INT128(0xffffffffffffffff, 0xfdb974130eca8643));
+  EXPECT_EQ(val2 >> 64, MAKE_INT128(0xffffffffffffffff, 0xfedcba0987654321));
+
+  EXPECT_EQ(val1 >> 126, MAKE_INT128(0x0, 0x0));
+  EXPECT_EQ(val2 >> 126, MAKE_INT128(0xffffffffffffffff, 0xffffffffffffffff));
+}
+
+TEST(Int128, NumericLimitsTest) {
+  static_assert(std::numeric_limits<absl::int128>::is_specialized);
+  static_assert(std::numeric_limits<absl::int128>::is_signed);
+  static_assert(std::numeric_limits<absl::int128>::is_integer);
+  EXPECT_EQ(static_cast<int>(127 * std::log10(2)),
+            std::numeric_limits<absl::int128>::digits10);
+  EXPECT_EQ(absl::Int128Min(), std::numeric_limits<absl::int128>::min());
+  EXPECT_EQ(absl::Int128Min(), std::numeric_limits<absl::int128>::lowest());
+  EXPECT_EQ(absl::Int128Max(), std::numeric_limits<absl::int128>::max());
+}
+
+TEST(Int128, BitCastable) {
+  // NOTE: This test is not intended to be an example that demonstrate usages of
+  // `static_cast` and `std::bit_cast`, rather it is here simply to verify
+  // behavior. When deciding whether you should use `static_cast` or
+  // `std::bit_cast` when converting between `absl::int128` and `absl::uint128`,
+  // use your best judgement. As a rule of thumb, use the same cast that you
+  // would use when converting between the signed and unsigned counterparts of a
+  // builtin integral type.
+
+  // Verify bit casting between signed and unsigned works with regards to two's
+  // complement. This verifies we exhibit the same behavior as a theoretical
+  // builtin int128_t and uint128_t in C++20 onwards.
+  EXPECT_EQ(absl::bit_cast<absl::uint128>(absl::int128(-1)),
+            std::numeric_limits<absl::uint128>::max());
+  EXPECT_EQ(
+      absl::bit_cast<absl::int128>(std::numeric_limits<absl::uint128>::max()),
+      absl::int128(-1));
+  EXPECT_EQ(
+      absl::bit_cast<absl::uint128>(std::numeric_limits<absl::int128>::min()),
+      absl::uint128(1) << 127);
+  EXPECT_EQ(absl::bit_cast<absl::int128>(absl::uint128(1) << 127),
+            std::numeric_limits<absl::int128>::min());
+  EXPECT_EQ(
+      absl::bit_cast<absl::uint128>(std::numeric_limits<absl::int128>::max()),
+      (absl::uint128(1) << 127) - 1);
+  EXPECT_EQ(absl::bit_cast<absl::int128>((absl::uint128(1) << 127) - 1),
+            std::numeric_limits<absl::int128>::max());
+
+  // Also verify static casting has the same behavior as bit casting.
+  EXPECT_EQ(static_cast<absl::uint128>(absl::int128(-1)),
+            std::numeric_limits<absl::uint128>::max());
+  EXPECT_EQ(
+      static_cast<absl::int128>(std::numeric_limits<absl::uint128>::max()),
+      absl::int128(-1));
+  EXPECT_EQ(
+      static_cast<absl::uint128>(std::numeric_limits<absl::int128>::min()),
+      absl::uint128(1) << 127);
+  EXPECT_EQ(static_cast<absl::int128>(absl::uint128(1) << 127),
+            std::numeric_limits<absl::int128>::min());
+  EXPECT_EQ(
+      static_cast<absl::uint128>(std::numeric_limits<absl::int128>::max()),
+      (absl::uint128(1) << 127) - 1);
+  EXPECT_EQ(static_cast<absl::int128>((absl::uint128(1) << 127) - 1),
+            std::numeric_limits<absl::int128>::max());
 }
 
 }  // namespace
diff --git a/absl/numeric/internal/bits.h b/absl/numeric/internal/bits.h
new file mode 100644
index 00000000000..58351868110
--- /dev/null
+++ b/absl/numeric/internal/bits.h
@@ -0,0 +1,396 @@
+// Copyright 2020 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_NUMERIC_INTERNAL_BITS_H_
+#define ABSL_NUMERIC_INTERNAL_BITS_H_
+
+#include <cstdint>
+#include <limits>
+#include <type_traits>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+
+// Clang on Windows has __builtin_clzll; otherwise we need to use the
+// windows intrinsic functions.
+#if defined(_MSC_VER) && !defined(__clang__)
+#include <intrin.h>
+#endif
+
+#if defined(__GNUC__) && !defined(__clang__)
+// GCC
+#define ABSL_NUMERIC_INTERNAL_HAVE_BUILTIN_OR_GCC(x) 1
+#else
+#define ABSL_NUMERIC_INTERNAL_HAVE_BUILTIN_OR_GCC(x) ABSL_HAVE_BUILTIN(x)
+#endif
+
+#if ABSL_NUMERIC_INTERNAL_HAVE_BUILTIN_OR_GCC(__builtin_popcountl) && \
+    ABSL_NUMERIC_INTERNAL_HAVE_BUILTIN_OR_GCC(__builtin_popcountll)
+#define ABSL_INTERNAL_CONSTEXPR_POPCOUNT constexpr
+#define ABSL_INTERNAL_HAS_CONSTEXPR_POPCOUNT 1
+#else
+#define ABSL_INTERNAL_CONSTEXPR_POPCOUNT
+#define ABSL_INTERNAL_HAS_CONSTEXPR_POPCOUNT 0
+#endif
+
+#if ABSL_NUMERIC_INTERNAL_HAVE_BUILTIN_OR_GCC(__builtin_clz) && \
+    ABSL_NUMERIC_INTERNAL_HAVE_BUILTIN_OR_GCC(__builtin_clzll)
+#define ABSL_INTERNAL_CONSTEXPR_CLZ constexpr
+#define ABSL_INTERNAL_HAS_CONSTEXPR_CLZ 1
+#else
+#define ABSL_INTERNAL_CONSTEXPR_CLZ
+#define ABSL_INTERNAL_HAS_CONSTEXPR_CLZ 0
+#endif
+
+#if ABSL_NUMERIC_INTERNAL_HAVE_BUILTIN_OR_GCC(__builtin_ctz) && \
+    ABSL_NUMERIC_INTERNAL_HAVE_BUILTIN_OR_GCC(__builtin_ctzll)
+#define ABSL_INTERNAL_CONSTEXPR_CTZ constexpr
+#define ABSL_INTERNAL_HAS_CONSTEXPR_CTZ 1
+#else
+#define ABSL_INTERNAL_CONSTEXPR_CTZ
+#define ABSL_INTERNAL_HAS_CONSTEXPR_CTZ 0
+#endif
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace numeric_internal {
+
+constexpr bool IsPowerOf2(unsigned int x) noexcept {
+  return x != 0 && (x & (x - 1)) == 0;
+}
+
+template <class T>
+[[nodiscard]] ABSL_ATTRIBUTE_ALWAYS_INLINE constexpr T RotateRight(
+    T x, int s) noexcept {
+  static_assert(std::is_unsigned_v<T>, "T must be unsigned");
+  static_assert(IsPowerOf2(std::numeric_limits<T>::digits),
+                "T must have a power-of-2 size");
+
+  // Rotate by s mod the number of digits to avoid unnecessary rotations.
+  //
+  // A negative s represents a left rotation instead of a right rotation.
+  // We compute it as an equivalent complementary right rotation by leveraging
+  // its two's complement representation.
+  //
+  // For example, suppose we rotate a 3-bit number by -2.
+  // In that case:
+  //   * s = 0b11111111111111111111111111111110
+  //   * n = 8
+  //   * r = (0b11111111111111111111111111111110 & 0b111) = 0b110
+  //
+  // Instead of rotating by 2 to the left, we rotate by 6 to the right, which
+  // is equivalent.
+  const int n = std::numeric_limits<T>::digits;
+  const int r = s & (n - 1);
+
+  if (r == 0) {
+    return x;
+  } else {
+    return (x >> r) | (x << (n - r));
+  }
+}
+
+template <class T>
+[[nodiscard]] ABSL_ATTRIBUTE_ALWAYS_INLINE constexpr T RotateLeft(
+    T x, int s) noexcept {
+  static_assert(std::is_unsigned_v<T>, "T must be unsigned");
+  static_assert(IsPowerOf2(std::numeric_limits<T>::digits),
+                "T must have a power-of-2 size");
+
+  // Rotate by s mod the number of digits to avoid unnecessary rotations.
+  // See comment in RotateRight for a detailed explanation of the logic below.
+  const int n = std::numeric_limits<T>::digits;
+  const int r = s & (n - 1);
+
+  if (r == 0) {
+    return x;
+  } else {
+    return (x << r) | (x >> (n - r));
+  }
+}
+
+ABSL_ATTRIBUTE_ALWAYS_INLINE ABSL_INTERNAL_CONSTEXPR_POPCOUNT inline int
+Popcount32(uint32_t x) noexcept {
+#if ABSL_NUMERIC_INTERNAL_HAVE_BUILTIN_OR_GCC(__builtin_popcount)
+  static_assert(sizeof(unsigned int) == sizeof(x),
+                "__builtin_popcount does not take 32-bit arg");
+  return __builtin_popcount(x);
+#else
+  x -= ((x >> 1) & 0x55555555);
+  x = ((x >> 2) & 0x33333333) + (x & 0x33333333);
+  return static_cast<int>((((x + (x >> 4)) & 0xF0F0F0F) * 0x1010101) >> 24);
+#endif
+}
+
+ABSL_ATTRIBUTE_ALWAYS_INLINE ABSL_INTERNAL_CONSTEXPR_POPCOUNT inline int
+Popcount64(uint64_t x) noexcept {
+#if ABSL_NUMERIC_INTERNAL_HAVE_BUILTIN_OR_GCC(__builtin_popcountll)
+  static_assert(sizeof(unsigned long long) == sizeof(x),  // NOLINT(runtime/int)
+                "__builtin_popcount does not take 64-bit arg");
+  return __builtin_popcountll(x);
+#else
+  x -= (x >> 1) & 0x5555555555555555ULL;
+  x = ((x >> 2) & 0x3333333333333333ULL) + (x & 0x3333333333333333ULL);
+  return static_cast<int>(
+      (((x + (x >> 4)) & 0xF0F0F0F0F0F0F0FULL) * 0x101010101010101ULL) >> 56);
+#endif
+}
+
+template <class T>
+ABSL_ATTRIBUTE_ALWAYS_INLINE ABSL_INTERNAL_CONSTEXPR_POPCOUNT inline int
+Popcount(T x) noexcept {
+  static_assert(std::is_unsigned_v<T>, "T must be unsigned");
+  static_assert(IsPowerOf2(std::numeric_limits<T>::digits),
+                "T must have a power-of-2 size");
+  static_assert(sizeof(x) <= sizeof(uint64_t), "T is too large");
+  if constexpr (sizeof(x) <= sizeof(uint32_t)) {
+    return Popcount32(x);
+  } else {
+    return Popcount64(x);
+  }
+}
+
+ABSL_ATTRIBUTE_ALWAYS_INLINE ABSL_INTERNAL_CONSTEXPR_CLZ inline int
+CountLeadingZeroes32(uint32_t x) {
+#if ABSL_NUMERIC_INTERNAL_HAVE_BUILTIN_OR_GCC(__builtin_clz)
+  // Use __builtin_clz, which uses the following instructions:
+  //  x86: bsr, lzcnt
+  //  ARM64: clz
+  //  PPC: cntlzd
+
+  static_assert(sizeof(unsigned int) == sizeof(x),
+                "__builtin_clz does not take 32-bit arg");
+  // Handle 0 as a special case because __builtin_clz(0) is undefined.
+  return x == 0 ? 32 : __builtin_clz(x);
+#elif defined(_MSC_VER) && !defined(__clang__)
+  unsigned long result = 0;  // NOLINT(runtime/int)
+  if (_BitScanReverse(&result, x)) {
+    return 31 - result;
+  }
+  return 32;
+#else
+  int zeroes = 28;
+  if (x >> 16) {
+    zeroes -= 16;
+    x >>= 16;
+  }
+  if (x >> 8) {
+    zeroes -= 8;
+    x >>= 8;
+  }
+  if (x >> 4) {
+    zeroes -= 4;
+    x >>= 4;
+  }
+  return "\4\3\2\2\1\1\1\1\0\0\0\0\0\0\0"[x] + zeroes;
+#endif
+}
+
+ABSL_ATTRIBUTE_ALWAYS_INLINE ABSL_INTERNAL_CONSTEXPR_CLZ inline int
+CountLeadingZeroes16(uint16_t x) {
+#if ABSL_HAVE_BUILTIN(__builtin_clzg)
+  return x == 0 ? 16 : __builtin_clzg(x);
+#elif ABSL_HAVE_BUILTIN(__builtin_clzs)
+  static_assert(sizeof(unsigned short) == sizeof(x),  // NOLINT(runtime/int)
+                "__builtin_clzs does not take 16-bit arg");
+  return x == 0 ? 16 : __builtin_clzs(x);
+#else
+  return CountLeadingZeroes32(x) - 16;
+#endif
+}
+
+ABSL_ATTRIBUTE_ALWAYS_INLINE ABSL_INTERNAL_CONSTEXPR_CLZ inline int
+CountLeadingZeroes64(uint64_t x) {
+#if ABSL_NUMERIC_INTERNAL_HAVE_BUILTIN_OR_GCC(__builtin_clzll)
+  // Use __builtin_clzll, which uses the following instructions:
+  //  x86: bsr, lzcnt
+  //  ARM64: clz
+  //  PPC: cntlzd
+  static_assert(sizeof(unsigned long long) == sizeof(x),  // NOLINT(runtime/int)
+                "__builtin_clzll does not take 64-bit arg");
+
+  // Handle 0 as a special case because __builtin_clzll(0) is undefined.
+  return x == 0 ? 64 : __builtin_clzll(x);
+#elif defined(_MSC_VER) && !defined(__clang__) && \
+    (defined(_M_X64) || defined(_M_ARM64))
+  // MSVC does not have __buitin_clzll. Use _BitScanReverse64.
+  unsigned long result = 0;  // NOLINT(runtime/int)
+  if (_BitScanReverse64(&result, x)) {
+    return 63 - result;
+  }
+  return 64;
+#elif defined(_MSC_VER) && !defined(__clang__)
+  // MSVC does not have __buitin_clzll. Compose two calls to _BitScanReverse
+  unsigned long result = 0;  // NOLINT(runtime/int)
+  if ((x >> 32) &&
+      _BitScanReverse(&result, static_cast<unsigned long>(x >> 32))) {
+    return 31 - result;
+  }
+  if (_BitScanReverse(&result, static_cast<unsigned long>(x))) {
+    return 63 - result;
+  }
+  return 64;
+#else
+  int zeroes = 60;
+  if (x >> 32) {
+    zeroes -= 32;
+    x >>= 32;
+  }
+  if (x >> 16) {
+    zeroes -= 16;
+    x >>= 16;
+  }
+  if (x >> 8) {
+    zeroes -= 8;
+    x >>= 8;
+  }
+  if (x >> 4) {
+    zeroes -= 4;
+    x >>= 4;
+  }
+  return "\4\3\2\2\1\1\1\1\0\0\0\0\0\0\0"[x] + zeroes;
+#endif
+}
+
+template <typename T>
+ABSL_ATTRIBUTE_ALWAYS_INLINE ABSL_INTERNAL_CONSTEXPR_CLZ inline int
+CountLeadingZeroes(T x) {
+  static_assert(std::is_unsigned_v<T>, "T must be unsigned");
+  static_assert(IsPowerOf2(std::numeric_limits<T>::digits),
+                "T must have a power-of-2 size");
+  static_assert(sizeof(T) <= sizeof(uint64_t), "T too large");
+  return sizeof(T) <= sizeof(uint16_t)
+             ? CountLeadingZeroes16(static_cast<uint16_t>(x)) -
+                   (std::numeric_limits<uint16_t>::digits -
+                    std::numeric_limits<T>::digits)
+             : (sizeof(T) <= sizeof(uint32_t)
+                    ? CountLeadingZeroes32(static_cast<uint32_t>(x)) -
+                          (std::numeric_limits<uint32_t>::digits -
+                           std::numeric_limits<T>::digits)
+                    : CountLeadingZeroes64(x));
+}
+
+ABSL_ATTRIBUTE_ALWAYS_INLINE ABSL_INTERNAL_CONSTEXPR_CTZ inline int
+CountTrailingZeroesNonzero32(uint32_t x) {
+#if ABSL_NUMERIC_INTERNAL_HAVE_BUILTIN_OR_GCC(__builtin_ctz)
+  static_assert(sizeof(unsigned int) == sizeof(x),
+                "__builtin_ctz does not take 32-bit arg");
+  return __builtin_ctz(x);
+#elif defined(_MSC_VER) && !defined(__clang__)
+  unsigned long result = 0;  // NOLINT(runtime/int)
+  _BitScanForward(&result, x);
+  return result;
+#else
+  int c = 31;
+  x &= ~x + 1;
+  if (x & 0x0000FFFF) c -= 16;
+  if (x & 0x00FF00FF) c -= 8;
+  if (x & 0x0F0F0F0F) c -= 4;
+  if (x & 0x33333333) c -= 2;
+  if (x & 0x55555555) c -= 1;
+  return c;
+#endif
+}
+
+ABSL_ATTRIBUTE_ALWAYS_INLINE ABSL_INTERNAL_CONSTEXPR_CTZ inline int
+CountTrailingZeroesNonzero64(uint64_t x) {
+#if ABSL_NUMERIC_INTERNAL_HAVE_BUILTIN_OR_GCC(__builtin_ctzll)
+  static_assert(sizeof(unsigned long long) == sizeof(x),  // NOLINT(runtime/int)
+                "__builtin_ctzll does not take 64-bit arg");
+  return __builtin_ctzll(x);
+#elif defined(_MSC_VER) && !defined(__clang__) && \
+    (defined(_M_X64) || defined(_M_ARM64))
+  unsigned long result = 0;  // NOLINT(runtime/int)
+  _BitScanForward64(&result, x);
+  return result;
+#elif defined(_MSC_VER) && !defined(__clang__)
+  unsigned long result = 0;  // NOLINT(runtime/int)
+  if (static_cast<uint32_t>(x) == 0) {
+    _BitScanForward(&result, static_cast<unsigned long>(x >> 32));
+    return result + 32;
+  }
+  _BitScanForward(&result, static_cast<unsigned long>(x));
+  return result;
+#else
+  int c = 63;
+  x &= ~x + 1;
+  if (x & 0x00000000FFFFFFFF) c -= 32;
+  if (x & 0x0000FFFF0000FFFF) c -= 16;
+  if (x & 0x00FF00FF00FF00FF) c -= 8;
+  if (x & 0x0F0F0F0F0F0F0F0F) c -= 4;
+  if (x & 0x3333333333333333) c -= 2;
+  if (x & 0x5555555555555555) c -= 1;
+  return c;
+#endif
+}
+
+ABSL_ATTRIBUTE_ALWAYS_INLINE ABSL_INTERNAL_CONSTEXPR_CTZ inline int
+CountTrailingZeroesNonzero16(uint16_t x) {
+#if ABSL_HAVE_BUILTIN(__builtin_ctzg)
+  return __builtin_ctzg(x);
+#elif ABSL_HAVE_BUILTIN(__builtin_ctzs)
+  static_assert(sizeof(unsigned short) == sizeof(x),  // NOLINT(runtime/int)
+                "__builtin_ctzs does not take 16-bit arg");
+  return __builtin_ctzs(x);
+#else
+  return CountTrailingZeroesNonzero32(x);
+#endif
+}
+
+template <class T>
+ABSL_ATTRIBUTE_ALWAYS_INLINE ABSL_INTERNAL_CONSTEXPR_CTZ inline int
+CountTrailingZeroes(T x) noexcept {
+  static_assert(std::is_unsigned_v<T>, "T must be unsigned");
+  static_assert(IsPowerOf2(std::numeric_limits<T>::digits),
+                "T must have a power-of-2 size");
+  static_assert(sizeof(T) <= sizeof(uint64_t), "T too large");
+  return x == 0 ? std::numeric_limits<T>::digits
+                : (sizeof(T) <= sizeof(uint16_t)
+                       ? CountTrailingZeroesNonzero16(static_cast<uint16_t>(x))
+                       : (sizeof(T) <= sizeof(uint32_t)
+                              ? CountTrailingZeroesNonzero32(
+                                    static_cast<uint32_t>(x))
+                              : CountTrailingZeroesNonzero64(x)));
+}
+
+// If T is narrower than unsigned, T{1} << bit_width will be promoted.  We
+// want to force it to wraparound so that bit_ceil of an invalid value are not
+// core constant expressions.
+template <class T>
+ABSL_ATTRIBUTE_ALWAYS_INLINE
+    ABSL_INTERNAL_CONSTEXPR_CLZ inline std::enable_if_t<std::is_unsigned_v<T>,
+                                                        T>
+    BitCeilPromotionHelper(T x, T promotion) {
+  return (T{1} << (x + promotion)) >> promotion;
+}
+
+template <class T>
+ABSL_ATTRIBUTE_ALWAYS_INLINE
+    ABSL_INTERNAL_CONSTEXPR_CLZ inline std::enable_if_t<std::is_unsigned_v<T>,
+                                                        T>
+    BitCeilNonPowerOf2(T x) {
+  // If T is narrower than unsigned, it undergoes promotion to unsigned when we
+  // shift.  We calculate the number of bits added by the wider type.
+  return BitCeilPromotionHelper(
+      static_cast<T>(std::numeric_limits<T>::digits - CountLeadingZeroes(x)),
+      T{sizeof(T) >= sizeof(unsigned) ? 0
+                                      : std::numeric_limits<unsigned>::digits -
+                                            std::numeric_limits<T>::digits});
+}
+
+}  // namespace numeric_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_NUMERIC_INTERNAL_BITS_H_
diff --git a/absl/numeric/internal/representation.h b/absl/numeric/internal/representation.h
new file mode 100644
index 00000000000..82d332fdde3
--- /dev/null
+++ b/absl/numeric/internal/representation.h
@@ -0,0 +1,55 @@
+// Copyright 2021 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_NUMERIC_INTERNAL_REPRESENTATION_H_
+#define ABSL_NUMERIC_INTERNAL_REPRESENTATION_H_
+
+#include <limits>
+
+#include "absl/base/config.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace numeric_internal {
+
+// Returns true iff long double is represented as a pair of doubles added
+// together.
+inline constexpr bool IsDoubleDouble() {
+  // A double-double value always has exactly twice the precision of a double
+  // value--one double carries the high digits and one double carries the low
+  // digits. This property is not shared with any other common floating-point
+  // representation, so this test won't trigger false positives. For reference,
+  // this table gives the number of bits of precision of each common
+  // floating-point representation:
+  //
+  //                type     precision
+  //         IEEE single          24 b
+  //         IEEE double          53
+  //     x86 long double          64
+  //       double-double         106
+  //      IEEE quadruple         113
+  //
+  // Note in particular that a quadruple-precision float has greater precision
+  // than a double-double float despite taking up the same amount of memory; the
+  // quad has more of its bits allocated to the mantissa than the double-double
+  // has.
+  return std::numeric_limits<long double>::digits ==
+         2 * std::numeric_limits<double>::digits;
+}
+
+}  // namespace numeric_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_NUMERIC_INTERNAL_REPRESENTATION_H_
diff --git a/absl/profiling/BUILD.bazel b/absl/profiling/BUILD.bazel
new file mode 100644
index 00000000000..e958307e4ec
--- /dev/null
+++ b/absl/profiling/BUILD.bazel
@@ -0,0 +1,190 @@
+# Copyright 2021 The Abseil Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+load("@rules_cc//cc:cc_binary.bzl", "cc_binary")
+load("@rules_cc//cc:cc_library.bzl", "cc_library")
+load("@rules_cc//cc:cc_test.bzl", "cc_test")
+load(
+    "//absl:copts/configure_copts.bzl",
+    "ABSL_DEFAULT_COPTS",
+    "ABSL_DEFAULT_LINKOPTS",
+    "ABSL_TEST_COPTS",
+)
+
+package(
+    default_visibility = ["//visibility:private"],
+    features = [
+        "header_modules",
+        "layering_check",
+        "parse_headers",
+    ],
+)
+
+licenses(["notice"])
+
+cc_library(
+    name = "sample_recorder",
+    hdrs = ["internal/sample_recorder.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl:__subpackages__",
+    ],
+    deps = [
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/synchronization",
+        "//absl/time",
+    ],
+)
+
+cc_test(
+    name = "sample_recorder_test",
+    srcs = ["internal/sample_recorder_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    tags = [
+        "no_test_wasm",
+    ],
+    deps = [
+        ":sample_recorder",
+        "//absl/base:core_headers",
+        "//absl/random",
+        "//absl/synchronization",
+        "//absl/synchronization:thread_pool",
+        "//absl/time",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_library(
+    name = "exponential_biased",
+    srcs = ["internal/exponential_biased.cc"],
+    hdrs = ["internal/exponential_biased.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl:__subpackages__",
+    ],
+    deps = [
+        "//absl/base:config",
+        "//absl/base:core_headers",
+    ],
+)
+
+cc_test(
+    name = "exponential_biased_test",
+    size = "small",
+    srcs = ["internal/exponential_biased_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:private"],
+    deps = [
+        ":exponential_biased",
+        "//absl/base:core_headers",
+        "//absl/strings",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_library(
+    name = "periodic_sampler",
+    srcs = ["internal/periodic_sampler.cc"],
+    hdrs = ["internal/periodic_sampler.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl:__subpackages__",
+    ],
+    deps = [
+        ":exponential_biased",
+        "//absl/base:core_headers",
+    ],
+)
+
+cc_test(
+    name = "periodic_sampler_test",
+    size = "small",
+    srcs = ["internal/periodic_sampler_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:private"],
+    deps = [
+        ":periodic_sampler",
+        "//absl/base:core_headers",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_binary(
+    name = "periodic_sampler_benchmark",
+    testonly = True,
+    srcs = ["internal/periodic_sampler_benchmark.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    tags = ["benchmark"],
+    visibility = ["//visibility:private"],
+    deps = [
+        ":periodic_sampler",
+        "//absl/base:core_headers",
+        "@google_benchmark//:benchmark_main",
+    ],
+)
+
+cc_library(
+    name = "profile_builder",
+    srcs = ["internal/profile_builder.cc"],
+    hdrs = ["internal/profile_builder.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl:__subpackages__",
+    ],
+    deps = [
+        "//absl/base",
+        "//absl/base:config",
+        "//absl/base:raw_logging_internal",
+        "//absl/container:btree",
+        "//absl/container:flat_hash_map",
+        "//absl/container:hash_container_defaults",
+        "//absl/strings",
+        "//absl/strings:str_format",
+        "//absl/types:span",
+    ],
+)
+
+cc_library(
+    name = "hashtable",
+    srcs = ["hashtable.cc"],
+    hdrs = ["hashtable.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl:__subpackages__",
+    ],
+    deps = [
+        ":profile_builder",
+        "//absl/base",
+        "//absl/base:config",
+        "//absl/container:hashtablez_sampler",
+        "//absl/status:statusor",
+        "//absl/strings",
+        "//absl/strings:string_view",
+        "//absl/time",
+        "//absl/types:span",
+    ],
+)
diff --git a/absl/profiling/CMakeLists.txt b/absl/profiling/CMakeLists.txt
new file mode 100644
index 00000000000..6441daec852
--- /dev/null
+++ b/absl/profiling/CMakeLists.txt
@@ -0,0 +1,132 @@
+# Copyright 2021 The Abseil Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+absl_cc_library(
+  NAME
+    sample_recorder
+  HDRS
+    "internal/sample_recorder.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::base
+    absl::synchronization
+)
+
+absl_cc_test(
+  NAME
+    sample_recorder_test
+  SRCS
+    "internal/sample_recorder_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::random_random
+    absl::sample_recorder
+    absl::time
+    GTest::gmock_main
+)
+
+absl_cc_library(
+  NAME
+    exponential_biased
+  SRCS
+    "internal/exponential_biased.cc"
+  HDRS
+    "internal/exponential_biased.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::config
+    absl::core_headers
+)
+
+absl_cc_test(
+  NAME
+    exponential_biased_test
+  SRCS
+    "internal/exponential_biased_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::exponential_biased
+    absl::strings
+    GTest::gmock_main
+)
+
+absl_cc_library(
+  NAME
+    periodic_sampler
+  SRCS
+    "internal/periodic_sampler.cc"
+  HDRS
+    "internal/periodic_sampler.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::core_headers
+    absl::exponential_biased
+)
+
+absl_cc_test(
+  NAME
+    periodic_sampler_test
+  SRCS
+    "internal/periodic_sampler_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::core_headers
+    absl::periodic_sampler
+    GTest::gmock_main
+)
+
+# Internal-only target, do not depend on directly
+absl_cc_library(
+  NAME
+    profile_builder
+  HDRS
+    "internal/profile_builder.h"
+  SRCS
+    "internal/profile_builder.cc"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::config
+    absl::core_headers
+    absl::raw_logging_internal
+    absl::flat_hash_map
+    absl::btree
+    absl::strings
+    absl::str_format
+    absl::span
+)
+
+absl_cc_library(
+  NAME
+    hashtable_profiler
+  HDRS
+    "hashtable.h"
+  SRCS
+    "hashtable.cc"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::profile_builder
+    absl::config
+    absl::core_headers
+    absl::strings
+    absl::span
+    absl::hashtablez_sampler
+)
diff --git a/absl/profiling/hashtable.cc b/absl/profiling/hashtable.cc
new file mode 100644
index 00000000000..7e5e4ea1fff
--- /dev/null
+++ b/absl/profiling/hashtable.cc
@@ -0,0 +1,132 @@
+// Copyright 2025 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may
+// obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/profiling/hashtable.h"
+
+#include <atomic>
+#include <cstddef>
+#include <cstdint>
+#include <string>
+#include <utility>
+#include <vector>
+
+#include "absl/base/casts.h"
+#include "absl/base/config.h"
+#include "absl/container/internal/hashtablez_sampler.h"
+#include "absl/profiling/internal/profile_builder.h"
+#include "absl/status/statusor.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/string_view.h"
+#include "absl/time/clock.h"
+#include "absl/time/time.h"
+#include "absl/types/span.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+StatusOr<std::string> MarshalHashtableProfile() {
+  return debugging_internal::MarshalHashtableProfile(
+      container_internal::GlobalHashtablezSampler(), Now());
+}
+
+namespace debugging_internal {
+
+static void DroppedHashtableSample() {}
+
+StatusOr<std::string> MarshalHashtableProfile(
+    container_internal::HashtablezSampler& sampler, Time now) {
+  static constexpr absl::string_view kDropFrames =
+      "(::)?absl::container_internal::.*|"
+      "(::)?absl::(flat|node)_hash_(map|set).*";
+
+  ProfileBuilder builder;
+  StringId drop_frames_id = builder.InternString(kDropFrames);
+  builder.set_drop_frames_id(drop_frames_id);
+  builder.AddSampleType(builder.InternString("capacity"),
+                        builder.InternString("count"));
+  builder.set_default_sample_type_id(builder.InternString("capacity"));
+
+  const auto capacity_id = builder.InternString("capacity");
+  const auto size_id = builder.InternString("size");
+  const auto num_erases_id = builder.InternString("num_erases");
+  const auto num_rehashes_id = builder.InternString("num_rehashes");
+  const auto max_probe_length_id = builder.InternString("max_probe_length");
+  const auto total_probe_length_id = builder.InternString("total_probe_length");
+  const auto stuck_bits_id = builder.InternString("stuck_bits");
+  const auto inline_element_size_id =
+      builder.InternString("inline_element_size");
+  const auto key_size_id = builder.InternString("key_size");
+  const auto value_size_id = builder.InternString("value_size");
+  const auto soo_capacity_id = builder.InternString("soo_capacity");
+  const auto table_age_id = builder.InternString("table_age");
+  const auto max_reserve_id = builder.InternString("max_reserve");
+
+  size_t dropped =
+      sampler.Iterate([&](const container_internal::HashtablezInfo& info) {
+        const size_t capacity = info.capacity.load(std::memory_order_relaxed);
+        std::vector<std::pair<StringId, int64_t>> labels;
+
+        auto add_label = [&](StringId tag, uint64_t value) {
+          if (value == 0) {
+            return;
+          }
+          labels.emplace_back(tag, static_cast<int64_t>(value));
+        };
+
+        add_label(capacity_id, capacity);
+        add_label(size_id, info.size.load(std::memory_order_relaxed));
+        add_label(num_erases_id,
+                  info.num_erases.load(std::memory_order_relaxed));
+        add_label(num_rehashes_id,
+                  info.num_rehashes.load(std::memory_order_relaxed));
+        add_label(max_probe_length_id,
+                  info.max_probe_length.load(std::memory_order_relaxed));
+        add_label(total_probe_length_id,
+                  info.total_probe_length.load(std::memory_order_relaxed));
+        add_label(stuck_bits_id,
+                  (info.hashes_bitwise_and.load(std::memory_order_relaxed) |
+                   ~info.hashes_bitwise_or.load(std::memory_order_relaxed)));
+        add_label(inline_element_size_id, info.inline_element_size);
+        add_label(key_size_id, info.key_size);
+        add_label(value_size_id, info.value_size);
+        add_label(soo_capacity_id, info.soo_capacity);
+        add_label(
+            table_age_id,
+            static_cast<uint64_t>(ToInt64Microseconds(now - info.create_time)));
+        add_label(max_reserve_id,
+                  info.max_reserve.load(std::memory_order_relaxed));
+        builder.AddSample(static_cast<int64_t>(capacity) * info.weight,
+                          MakeSpan(info.stack, info.depth), labels);
+      });
+
+  if (dropped > 0) {
+    // If we dropped samples, we don't have information for them, including
+    // their sizes.  The non-zero weight allows it to be noticed in the profile
+    // and examined more closely.
+    //
+    // We compensate for the fixup done by AddSample by adjusting the address
+    // here.
+    const void* kFakeStack[] = {
+      absl::bit_cast<void*>(
+          reinterpret_cast<uintptr_t>(&DroppedHashtableSample)+1)};
+    builder.AddSample(static_cast<int64_t>(dropped), kFakeStack, {});
+  }
+  builder.AddCurrentMappings();
+  return std::move(builder).Emit();
+}
+
+}  // namespace debugging_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/profiling/hashtable.h b/absl/profiling/hashtable.h
new file mode 100644
index 00000000000..9e490dcdb8a
--- /dev/null
+++ b/absl/profiling/hashtable.h
@@ -0,0 +1,40 @@
+// Copyright 2025 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_PROFILING_HASHTABLE_H_
+#define ABSL_PROFILING_HASHTABLE_H_
+
+#include <cstdint>
+#include <string>
+
+#include "absl/container/internal/hashtablez_sampler.h"
+#include "absl/status/statusor.h"
+#include "absl/strings/string_view.h"
+#include "absl/time/time.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+absl::StatusOr<std::string> MarshalHashtableProfile();
+
+namespace debugging_internal {
+
+absl::StatusOr<std::string> MarshalHashtableProfile(
+    container_internal::HashtablezSampler& sampler, absl::Time now);
+
+}  // namespace debugging_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_PROFILING_HASHTABLE_H_
diff --git a/absl/profiling/internal/exponential_biased.cc b/absl/profiling/internal/exponential_biased.cc
new file mode 100644
index 00000000000..918d063aaae
--- /dev/null
+++ b/absl/profiling/internal/exponential_biased.cc
@@ -0,0 +1,93 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/profiling/internal/exponential_biased.h"
+
+#include <stdint.h>
+
+#include <algorithm>
+#include <atomic>
+#include <cmath>
+#include <limits>
+
+#include "absl/base/attributes.h"
+#include "absl/base/optimization.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace profiling_internal {
+
+// The algorithm generates a random number between 0 and 1 and applies the
+// inverse cumulative distribution function for an exponential. Specifically:
+// Let m be the inverse of the sample period, then the probability
+// distribution function is m*exp(-mx) so the CDF is
+// p = 1 - exp(-mx), so
+// q = 1 - p = exp(-mx)
+// log_e(q) = -mx
+// -log_e(q)/m = x
+// log_2(q) * (-log_e(2) * 1/m) = x
+// In the code, q is actually in the range 1 to 2**26, hence the -26 below
+int64_t ExponentialBiased::GetSkipCount(int64_t mean) {
+  if (ABSL_PREDICT_FALSE(!initialized_)) {
+    Initialize();
+  }
+
+  uint64_t rng = NextRandom(rng_);
+  rng_ = rng;
+
+  // Take the top 26 bits as the random number
+  // (This plus the 1<<58 sampling bound give a max possible step of
+  // 5194297183973780480 bytes.)
+  // The uint32_t cast is to prevent a (hard-to-reproduce) NAN
+  // under piii debug for some binaries.
+  double q = static_cast<uint32_t>(rng >> (kPrngNumBits - 26)) + 1.0;
+  // Put the computed p-value through the CDF of a geometric.
+  double interval = bias_ + (std::log2(q) - 26) * (-std::log(2.0) * mean);
+  // Very large values of interval overflow int64_t. To avoid that, we will
+  // cheat and clamp any huge values to (int64_t max)/2. This is a potential
+  // source of bias, but the mean would need to be such a large value that it's
+  // not likely to come up. For example, with a mean of 1e18, the probability of
+  // hitting this condition is about 1/1000. For a mean of 1e17, standard
+  // calculators claim that this event won't happen.
+  if (interval > static_cast<double>(std::numeric_limits<int64_t>::max() / 2)) {
+    // Assume huge values are bias neutral, retain bias for next call.
+    return std::numeric_limits<int64_t>::max() / 2;
+  }
+  double value = std::rint(interval);
+  bias_ = interval - value;
+  return static_cast<int64_t>(value);
+}
+
+int64_t ExponentialBiased::GetStride(int64_t mean) {
+  return GetSkipCount(mean - 1) + 1;
+}
+
+void ExponentialBiased::Initialize() {
+  // We don't get well distributed numbers from `this` so we call NextRandom() a
+  // bunch to mush the bits around. We use a global_rand to handle the case
+  // where the same thread (by memory address) gets created and destroyed
+  // repeatedly.
+  ABSL_CONST_INIT static std::atomic<uint32_t> global_rand(0);
+  uint64_t r = reinterpret_cast<uint64_t>(this) +
+               global_rand.fetch_add(1, std::memory_order_relaxed);
+  for (int i = 0; i < 20; ++i) {
+    r = NextRandom(r);
+  }
+  rng_ = r;
+  initialized_ = true;
+}
+
+}  // namespace profiling_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/profiling/internal/exponential_biased.h b/absl/profiling/internal/exponential_biased.h
new file mode 100644
index 00000000000..d31f7782e83
--- /dev/null
+++ b/absl/profiling/internal/exponential_biased.h
@@ -0,0 +1,130 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_PROFILING_INTERNAL_EXPONENTIAL_BIASED_H_
+#define ABSL_PROFILING_INTERNAL_EXPONENTIAL_BIASED_H_
+
+#include <stdint.h>
+
+#include "absl/base/config.h"
+#include "absl/base/macros.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace profiling_internal {
+
+// ExponentialBiased provides a small and fast random number generator for a
+// rounded exponential distribution. This generator manages very little state,
+// and imposes no synchronization overhead. This makes it useful in specialized
+// scenarios requiring minimum overhead, such as stride based periodic sampling.
+//
+// ExponentialBiased provides two closely related functions, GetSkipCount() and
+// GetStride(), both returning a rounded integer defining a number of events
+// required before some event with a given mean probability occurs.
+//
+// The distribution is useful to generate a random wait time or some periodic
+// event with a given mean probability. For example, if an action is supposed to
+// happen on average once every 'N' events, then we can get a random 'stride'
+// counting down how long before the event to happen. For example, if we'd want
+// to sample one in every 1000 'Frobber' calls, our code could look like this:
+//
+//   Frobber::Frobber() {
+//     stride_ = exponential_biased_.GetStride(1000);
+//   }
+//
+//   void Frobber::Frob(int arg) {
+//     if (--stride == 0) {
+//       SampleFrob(arg);
+//       stride_ = exponential_biased_.GetStride(1000);
+//     }
+//     ...
+//   }
+//
+// The rounding of the return value creates a bias, especially for smaller means
+// where the distribution of the fraction is not evenly distributed. We correct
+// this bias by tracking the fraction we rounded up or down on each iteration,
+// effectively tracking the distance between the cumulative value, and the
+// rounded cumulative value. For example, given a mean of 2:
+//
+//   raw = 1.63076, cumulative = 1.63076, rounded = 2, bias = -0.36923
+//   raw = 0.14624, cumulative = 1.77701, rounded = 2, bias =  0.14624
+//   raw = 4.93194, cumulative = 6.70895, rounded = 7, bias = -0.06805
+//   raw = 0.24206, cumulative = 6.95101, rounded = 7, bias =  0.24206
+//   etc...
+//
+// Adjusting with rounding bias is relatively trivial:
+//
+//    double value = bias_ + exponential_distribution(mean)();
+//    double rounded_value = std::rint(value);
+//    bias_ = value - rounded_value;
+//    return rounded_value;
+//
+// This class is thread-compatible.
+class ExponentialBiased {
+ public:
+  // The number of bits set by NextRandom.
+  static constexpr int kPrngNumBits = 48;
+
+  // `GetSkipCount()` returns the number of events to skip before some chosen
+  // event happens. For example, randomly tossing a coin, we will on average
+  // throw heads once before we get tails. We can simulate random coin tosses
+  // using GetSkipCount() as:
+  //
+  //   ExponentialBiased eb;
+  //   for (...) {
+  //     int number_of_heads_before_tail = eb.GetSkipCount(1);
+  //     for (int flips = 0; flips < number_of_heads_before_tail; ++flips) {
+  //       printf("head...");
+  //     }
+  //     printf("tail\n");
+  //   }
+  //
+  int64_t GetSkipCount(int64_t mean);
+
+  // GetStride() returns the number of events required for a specific event to
+  // happen. See the class comments for a usage example. `GetStride()` is
+  // equivalent to `GetSkipCount(mean - 1) + 1`. When to use `GetStride()` or
+  // `GetSkipCount()` depends mostly on what best fits the use case.
+  int64_t GetStride(int64_t mean);
+
+  // Computes a random number in the range [0, 1<<(kPrngNumBits+1) - 1]
+  //
+  // This is public to enable testing.
+  static uint64_t NextRandom(uint64_t rnd);
+
+ private:
+  void Initialize();
+
+  uint64_t rng_{0};
+  double bias_{0};
+  bool initialized_{false};
+};
+
+// Returns the next prng value.
+// pRNG is: aX+b mod c with a = 0x5DEECE66D, b =  0xB, c = 1<<48
+// This is the lrand64 generator.
+inline uint64_t ExponentialBiased::NextRandom(uint64_t rnd) {
+  const uint64_t prng_mult = uint64_t{0x5DEECE66D};
+  const uint64_t prng_add = 0xB;
+  const uint64_t prng_mod_power = 48;
+  const uint64_t prng_mod_mask =
+      ~((~static_cast<uint64_t>(0)) << prng_mod_power);
+  return (prng_mult * rnd + prng_add) & prng_mod_mask;
+}
+
+}  // namespace profiling_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_PROFILING_INTERNAL_EXPONENTIAL_BIASED_H_
diff --git a/absl/profiling/internal/exponential_biased_test.cc b/absl/profiling/internal/exponential_biased_test.cc
new file mode 100644
index 00000000000..103b7d543d8
--- /dev/null
+++ b/absl/profiling/internal/exponential_biased_test.cc
@@ -0,0 +1,250 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/profiling/internal/exponential_biased.h"
+
+#include <stddef.h>
+
+#include <algorithm>
+#include <cmath>
+#include <cstdint>
+#include <cstdio>
+#include <string>
+#include <vector>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/attributes.h"
+#include "absl/strings/str_cat.h"
+
+using ::testing::Ge;
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace profiling_internal {
+namespace {
+
+MATCHER_P2(IsBetween, a, b,
+           absl::StrCat(std::string(negation ? "isn't" : "is"), " between ", a,
+                        " and ", b)) {
+  return a <= arg && arg <= b;
+}
+
+// Tests of the quality of the random numbers generated
+// This uses the Anderson Darling test for uniformity.
+// See "Evaluating the Anderson-Darling Distribution" by Marsaglia
+// for details.
+
+// Short cut version of ADinf(z), z>0 (from Marsaglia)
+// This returns the p-value for Anderson Darling statistic in
+// the limit as n-> infinity. For finite n, apply the error fix below.
+double AndersonDarlingInf(double z) {
+  if (z < 2) {
+    return exp(-1.2337141 / z) / sqrt(z) *
+           (2.00012 +
+            (0.247105 -
+             (0.0649821 - (0.0347962 - (0.011672 - 0.00168691 * z) * z) * z) *
+                 z) *
+                z);
+  }
+  return exp(
+      -exp(1.0776 -
+           (2.30695 -
+            (0.43424 - (0.082433 - (0.008056 - 0.0003146 * z) * z) * z) * z) *
+               z));
+}
+
+// Corrects the approximation error in AndersonDarlingInf for small values of n
+// Add this to AndersonDarlingInf to get a better approximation
+// (from Marsaglia)
+double AndersonDarlingErrFix(int n, double x) {
+  if (x > 0.8) {
+    return (-130.2137 +
+            (745.2337 -
+             (1705.091 - (1950.646 - (1116.360 - 255.7844 * x) * x) * x) * x) *
+                x) /
+           n;
+  }
+  double cutoff = 0.01265 + 0.1757 / n;
+  if (x < cutoff) {
+    double t = x / cutoff;
+    t = sqrt(t) * (1 - t) * (49 * t - 102);
+    return t * (0.0037 / (n * n) + 0.00078 / n + 0.00006) / n;
+  } else {
+    double t = (x - cutoff) / (0.8 - cutoff);
+    t = -0.00022633 +
+        (6.54034 - (14.6538 - (14.458 - (8.259 - 1.91864 * t) * t) * t) * t) *
+            t;
+    return t * (0.04213 + 0.01365 / n) / n;
+  }
+}
+
+// Returns the AndersonDarling p-value given n and the value of the statistic
+double AndersonDarlingPValue(int n, double z) {
+  double ad = AndersonDarlingInf(z);
+  double errfix = AndersonDarlingErrFix(n, ad);
+  return ad + errfix;
+}
+
+double AndersonDarlingStatistic(const std::vector<double>& random_sample) {
+  size_t n = random_sample.size();
+  double ad_sum = 0;
+  for (size_t i = 0; i < n; i++) {
+    ad_sum += (2 * i + 1) *
+              std::log(random_sample[i] * (1 - random_sample[n - 1 - i]));
+  }
+  const auto n_as_double = static_cast<double>(n);
+  double ad_statistic = -n_as_double - 1 / n_as_double * ad_sum;
+  return ad_statistic;
+}
+
+// Tests if the array of doubles is uniformly distributed.
+// Returns the p-value of the Anderson Darling Statistic
+// for the given set of sorted random doubles
+// See "Evaluating the Anderson-Darling Distribution" by
+// Marsaglia and Marsaglia for details.
+double AndersonDarlingTest(const std::vector<double>& random_sample) {
+  double ad_statistic = AndersonDarlingStatistic(random_sample);
+  double p = AndersonDarlingPValue(static_cast<int>(random_sample.size()),
+                                   ad_statistic);
+  return p;
+}
+
+TEST(ExponentialBiasedTest, CoinTossDemoWithGetSkipCount) {
+  ExponentialBiased eb;
+  for (int runs = 0; runs < 10; ++runs) {
+    for (int64_t flips = eb.GetSkipCount(1); flips > 0; --flips) {
+      printf("head...");
+    }
+    printf("tail\n");
+  }
+  int heads = 0;
+  for (int i = 0; i < 10000000; i += 1 + eb.GetSkipCount(1)) {
+    ++heads;
+  }
+  printf("Heads = %d (%f%%)\n", heads, 100.0 * heads / 10000000);
+}
+
+// In simplified idealized world, `GetSkipCount(m) == 0` translates to
+// `-log(X) * m + B <= 1/2`, where X is a uniform random number in (0, 1],
+// and B is uniform random number in [-0.5, 0.5). Let U = 0.5 - B, that makes
+// U a uniform random number in (0, 1]. After exponentiation we get
+// `X >= exp(-U / m)`. Probability of this is `1 - exp(-U / m)`, so we have to
+// integrate for all possible values of U: `1 - m * (1 - exp(-1 / m))`. For
+// simplicity take just two terms of Taylor series expansion:
+// `1/(2*m) - 1/(6*m^2)`. Reciprocal of that is `2*m + 2/3 + O(1/m)`. If we use
+// more math tricks we can get "exact" approximation, but result will still be
+// `2*m + 2/3 + O(1/m)`.
+TEST(ExponentialBiasedTest, OnePerNDemoWithGetSkipCount) {
+  ExponentialBiased eb;
+  int n = 22;
+  int rounds = 220000;
+  int hits = 0;
+  for (int i = 0; i < rounds; i++) {
+    if (eb.GetSkipCount(n / 2) == 0) ++hits;
+  }
+  double inverse_hit_rate = rounds / static_cast<double>(hits);
+  printf("N = %d, rounds = %d, inverse hit rate = %f (should be ~= N + 0.67)\n",
+         n, rounds, inverse_hit_rate);
+}
+
+// Large requested mean flattens density function near zero, making it almost
+// linear.
+TEST(ExponentialBiasedTest, OneOfNDemoWithGetSkipCount) {
+  ExponentialBiased eb;
+  int n = 6;
+  int large_mean = 10000000;
+  int rounds = n * 10000;
+  std::vector<int> hits(static_cast<size_t>(n), 0);
+  for (int i = 0; i < rounds; i++) {
+    int64_t v = eb.GetSkipCount(large_mean);
+    int64_t a = v % n;
+    ++hits[static_cast<size_t>(a)];
+  }
+  printf("N = %d, rounds = %d, mean = %d\n", n, rounds, large_mean);
+  for (int i = 0; i < n; i++) {
+    printf("  inverse hit rate for %d: %f (should be ~= N)\n", i,
+           rounds / static_cast<double>(hits[static_cast<size_t>(i)]));
+  }
+}
+
+TEST(ExponentialBiasedTest, SampleDemoWithStride) {
+  ExponentialBiased eb;
+  int64_t stride = eb.GetStride(10);
+  int samples = 0;
+  for (int i = 0; i < 10000000; ++i) {
+    if (--stride == 0) {
+      ++samples;
+      stride = eb.GetStride(10);
+    }
+  }
+  printf("Samples = %d (%f%%)\n", samples, 100.0 * samples / 10000000);
+}
+
+
+// Testing that NextRandom generates uniform random numbers. Applies the
+// Anderson-Darling test for uniformity
+TEST(ExponentialBiasedTest, TestNextRandom) {
+  for (auto n : std::vector<size_t>({
+           10,  // Check short-range correlation
+           100, 1000,
+           10000  // Make sure there's no systemic error
+       })) {
+    uint64_t x = 1;
+    // This assumes that the prng returns 48 bit numbers
+    uint64_t max_prng_value = static_cast<uint64_t>(1) << 48;
+    // Initialize.
+    for (int i = 1; i <= 20; i++) {
+      x = ExponentialBiased::NextRandom(x);
+    }
+    std::vector<uint64_t> int_random_sample(n);
+    // Collect samples
+    for (size_t i = 0; i < n; i++) {
+      int_random_sample[i] = x;
+      x = ExponentialBiased::NextRandom(x);
+    }
+    // First sort them...
+    std::sort(int_random_sample.begin(), int_random_sample.end());
+    std::vector<double> random_sample(n);
+    // Convert them to uniform randoms (in the range [0,1])
+    for (size_t i = 0; i < n; i++) {
+      random_sample[i] =
+          static_cast<double>(int_random_sample[i]) / max_prng_value;
+    }
+    // Now compute the Anderson-Darling statistic
+    double ad_pvalue = AndersonDarlingTest(random_sample);
+    EXPECT_GT(std::min(ad_pvalue, 1 - ad_pvalue), 0.0001)
+        << "prng is not uniform: n = " << n << " p = " << ad_pvalue;
+  }
+}
+
+// The generator needs to be available as a thread_local and as a static
+// variable.
+TEST(ExponentialBiasedTest, InitializationModes) {
+  ABSL_CONST_INIT static ExponentialBiased eb_static;
+  EXPECT_THAT(eb_static.GetSkipCount(2), Ge(0));
+
+#ifdef ABSL_HAVE_THREAD_LOCAL
+  thread_local ExponentialBiased eb_thread;
+  EXPECT_THAT(eb_thread.GetSkipCount(2), Ge(0));
+#endif
+
+  ExponentialBiased eb_stack;
+  EXPECT_THAT(eb_stack.GetSkipCount(2), Ge(0));
+}
+
+}  // namespace
+}  // namespace profiling_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/profiling/internal/periodic_sampler.cc b/absl/profiling/internal/periodic_sampler.cc
new file mode 100644
index 00000000000..3e0fc2f13d2
--- /dev/null
+++ b/absl/profiling/internal/periodic_sampler.cc
@@ -0,0 +1,55 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/profiling/internal/periodic_sampler.h"
+
+#include <atomic>
+#include <cstdint>
+
+#include "absl/base/optimization.h"
+#include "absl/profiling/internal/exponential_biased.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace profiling_internal {
+
+int64_t PeriodicSamplerBase::GetExponentialBiased(int period) noexcept {
+  return rng_.GetStride(period);
+}
+
+bool PeriodicSamplerBase::SubtleConfirmSample() noexcept {
+  int current_period = period();
+
+  // Deal with period case 0 (always off) and 1 (always on)
+  if (ABSL_PREDICT_FALSE(current_period < 2)) {
+    stride_ = 0;
+    return current_period == 1;
+  }
+
+  // Check if this is the first call to Sample()
+  if (ABSL_PREDICT_FALSE(stride_ == 1)) {
+    stride_ = static_cast<uint64_t>(-GetExponentialBiased(current_period));
+    if (static_cast<int64_t>(stride_) < -1) {
+      ++stride_;
+      return false;
+    }
+  }
+
+  stride_ = static_cast<uint64_t>(-GetExponentialBiased(current_period));
+  return true;
+}
+
+}  // namespace profiling_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/profiling/internal/periodic_sampler.h b/absl/profiling/internal/periodic_sampler.h
new file mode 100644
index 00000000000..f5d847ab254
--- /dev/null
+++ b/absl/profiling/internal/periodic_sampler.h
@@ -0,0 +1,211 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_PROFILING_INTERNAL_PERIODIC_SAMPLER_H_
+#define ABSL_PROFILING_INTERNAL_PERIODIC_SAMPLER_H_
+
+#include <stdint.h>
+
+#include <atomic>
+
+#include "absl/base/optimization.h"
+#include "absl/profiling/internal/exponential_biased.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace profiling_internal {
+
+// PeriodicSamplerBase provides the basic period sampler implementation.
+//
+// This is the base class for the templated PeriodicSampler class, which holds
+// a global std::atomic value identified by a user defined tag, such that
+// each specific PeriodSampler implementation holds its own global period.
+//
+// PeriodicSamplerBase is thread-compatible except where stated otherwise.
+class PeriodicSamplerBase {
+ public:
+  // PeriodicSamplerBase is trivial / copyable / movable / destructible.
+  PeriodicSamplerBase() = default;
+  PeriodicSamplerBase(PeriodicSamplerBase&&) = default;
+  PeriodicSamplerBase(const PeriodicSamplerBase&) = default;
+
+  // Returns true roughly once every `period` calls. This is established by a
+  // randomly picked `stride` that is counted down on each call to `Sample`.
+  // This stride is picked such that the probability of `Sample()` returning
+  // true is 1 in `period`.
+  inline bool Sample() noexcept;
+
+  // The below methods are intended for optimized use cases where the
+  // size of the inlined fast path code is highly important. Applications
+  // should use the `Sample()` method unless they have proof that their
+  // specific use case requires the optimizations offered by these methods.
+  //
+  // An example of such a use case is SwissTable sampling. All sampling checks
+  // are in inlined SwissTable methods, and the number of call sites is huge.
+  // In this case, the inlined code size added to each translation unit calling
+  // SwissTable methods is non-trivial.
+  //
+  // The `SubtleMaybeSample()` function spuriously returns true even if the
+  // function should not be sampled, applications MUST match each call to
+  // 'SubtleMaybeSample()' returning true with a `SubtleConfirmSample()` call,
+  // and use the result of the latter as the sampling decision.
+  // In other words: the code should logically be equivalent to:
+  //
+  //    if (SubtleMaybeSample() && SubtleConfirmSample()) {
+  //      // Sample this call
+  //    }
+  //
+  // In the 'inline-size' optimized case, the `SubtleConfirmSample()` call can
+  // be placed out of line, for example, the typical use case looks as follows:
+  //
+  //   // --- frobber.h -----------
+  //   void FrobberSampled();
+  //
+  //   inline void FrobberImpl() {
+  //     // ...
+  //   }
+  //
+  //   inline void Frobber() {
+  //     if (ABSL_PREDICT_FALSE(sampler.SubtleMaybeSample())) {
+  //       FrobberSampled();
+  //     } else {
+  //       FrobberImpl();
+  //     }
+  //   }
+  //
+  //   // --- frobber.cc -----------
+  //   void FrobberSampled() {
+  //     if (!sampler.SubtleConfirmSample())) {
+  //       // Spurious false positive
+  //       FrobberImpl();
+  //       return;
+  //     }
+  //
+  //     // Sampled execution
+  //     // ...
+  //   }
+  inline bool SubtleMaybeSample() noexcept;
+  bool SubtleConfirmSample() noexcept;
+
+ protected:
+  // We explicitly don't use a virtual destructor as this class is never
+  // virtually destroyed, and it keeps the class trivial, which avoids TLS
+  // prologue and epilogue code for our TLS instances.
+  ~PeriodicSamplerBase() = default;
+
+  // Returns the next stride for our sampler.
+  // This function is virtual for testing purposes only.
+  virtual int64_t GetExponentialBiased(int period) noexcept;
+
+ private:
+  // Returns the current period of this sampler. Thread-safe.
+  virtual int period() const noexcept = 0;
+
+  // Keep and decrement stride_ as an unsigned integer, but compare the value
+  // to zero casted as a signed int. clang and msvc do not create optimum code
+  // if we use signed for the combined decrement and sign comparison.
+  //
+  // Below 3 alternative options, all compiles generate the best code
+  // using the unsigned increment <---> signed int comparison option.
+  //
+  // Option 1:
+  //   int64_t stride_;
+  //   if (ABSL_PREDICT_TRUE(++stride_ < 0)) { ... }
+  //
+  //   GCC   x64 (OK) : https://gcc.godbolt.org/z/R5MzzA
+  //   GCC   ppc (OK) : https://gcc.godbolt.org/z/z7NZAt
+  //   Clang x64 (BAD): https://gcc.godbolt.org/z/t4gPsd
+  //   ICC   x64 (OK) : https://gcc.godbolt.org/z/rE6s8W
+  //   MSVC  x64 (OK) : https://gcc.godbolt.org/z/ARMXqS
+  //
+  // Option 2:
+  //   int64_t stride_ = 0;
+  //   if (ABSL_PREDICT_TRUE(--stride_ >= 0)) { ... }
+  //
+  //   GCC   x64 (OK) : https://gcc.godbolt.org/z/jSQxYK
+  //   GCC   ppc (OK) : https://gcc.godbolt.org/z/VJdYaA
+  //   Clang x64 (BAD): https://gcc.godbolt.org/z/Xm4NjX
+  //   ICC   x64 (OK) : https://gcc.godbolt.org/z/4snaFd
+  //   MSVC  x64 (BAD): https://gcc.godbolt.org/z/BgnEKE
+  //
+  // Option 3:
+  //   uint64_t stride_;
+  //   if (ABSL_PREDICT_TRUE(static_cast<int64_t>(++stride_) < 0)) { ... }
+  //
+  //   GCC   x64 (OK) : https://gcc.godbolt.org/z/bFbfPy
+  //   GCC   ppc (OK) : https://gcc.godbolt.org/z/S9KkUE
+  //   Clang x64 (OK) : https://gcc.godbolt.org/z/UYzRb4
+  //   ICC   x64 (OK) : https://gcc.godbolt.org/z/ptTNfD
+  //   MSVC  x64 (OK) : https://gcc.godbolt.org/z/76j4-5
+  uint64_t stride_ = 0;
+  absl::profiling_internal::ExponentialBiased rng_;
+};
+
+inline bool PeriodicSamplerBase::SubtleMaybeSample() noexcept {
+  // See comments on `stride_` for the unsigned increment / signed compare.
+  if (ABSL_PREDICT_TRUE(static_cast<int64_t>(++stride_) < 0)) {
+    return false;
+  }
+  return true;
+}
+
+inline bool PeriodicSamplerBase::Sample() noexcept {
+  return ABSL_PREDICT_FALSE(SubtleMaybeSample()) ? SubtleConfirmSample()
+                                                 : false;
+}
+
+// PeriodicSampler is a concreted periodic sampler implementation.
+// The user provided Tag identifies the implementation, and is required to
+// isolate the global state of this instance from other instances.
+//
+// Typical use case:
+//
+//   struct HashTablezTag {};
+//   thread_local PeriodicSampler<HashTablezTag, 100> sampler;
+//
+//   void HashTableSamplingLogic(...) {
+//     if (sampler.Sample()) {
+//       HashTableSlowSamplePath(...);
+//     }
+//   }
+//
+template <typename Tag, int default_period = 0>
+class PeriodicSampler final : public PeriodicSamplerBase {
+ public:
+  ~PeriodicSampler() = default;
+
+  int period() const noexcept final {
+    return period_.load(std::memory_order_relaxed);
+  }
+
+  // Sets the global period for this sampler. Thread-safe.
+  // Setting a period of 0 disables the sampler, i.e., every call to Sample()
+  // will return false. Setting a period of 1 puts the sampler in 'always on'
+  // mode, i.e., every call to Sample() returns true.
+  static void SetGlobalPeriod(int period) {
+    period_.store(period, std::memory_order_relaxed);
+  }
+
+ private:
+  static std::atomic<int> period_;
+};
+
+template <typename Tag, int default_period>
+std::atomic<int> PeriodicSampler<Tag, default_period>::period_(default_period);
+
+}  // namespace profiling_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_PROFILING_INTERNAL_PERIODIC_SAMPLER_H_
diff --git a/absl/profiling/internal/periodic_sampler_benchmark.cc b/absl/profiling/internal/periodic_sampler_benchmark.cc
new file mode 100644
index 00000000000..2d6beca8d64
--- /dev/null
+++ b/absl/profiling/internal/periodic_sampler_benchmark.cc
@@ -0,0 +1,80 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/base/optimization.h"
+#include "absl/profiling/internal/periodic_sampler.h"
+#include "benchmark/benchmark.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace profiling_internal {
+namespace {
+
+template <typename Sampler>
+void BM_Sample(Sampler* sampler, benchmark::State& state) {
+  for (auto _ : state) {
+    benchmark::DoNotOptimize(sampler);
+    benchmark::DoNotOptimize(sampler->Sample());
+  }
+}
+
+template <typename Sampler>
+void BM_SampleMinunumInlined(Sampler* sampler, benchmark::State& state) {
+  for (auto _ : state) {
+    benchmark::DoNotOptimize(sampler);
+    if (ABSL_PREDICT_FALSE(sampler->SubtleMaybeSample())) {
+      benchmark::DoNotOptimize(sampler->SubtleConfirmSample());
+    }
+  }
+}
+
+void BM_PeriodicSampler_TinySample(benchmark::State& state) {
+  struct Tag {};
+  PeriodicSampler<Tag, 10> sampler;
+  BM_Sample(&sampler, state);
+}
+BENCHMARK(BM_PeriodicSampler_TinySample);
+
+void BM_PeriodicSampler_ShortSample(benchmark::State& state) {
+  struct Tag {};
+  PeriodicSampler<Tag, 1024> sampler;
+  BM_Sample(&sampler, state);
+}
+BENCHMARK(BM_PeriodicSampler_ShortSample);
+
+void BM_PeriodicSampler_LongSample(benchmark::State& state) {
+  struct Tag {};
+  PeriodicSampler<Tag, 1024 * 1024> sampler;
+  BM_Sample(&sampler, state);
+}
+BENCHMARK(BM_PeriodicSampler_LongSample);
+
+void BM_PeriodicSampler_LongSampleMinunumInlined(benchmark::State& state) {
+  struct Tag {};
+  PeriodicSampler<Tag, 1024 * 1024> sampler;
+  BM_SampleMinunumInlined(&sampler, state);
+}
+BENCHMARK(BM_PeriodicSampler_LongSampleMinunumInlined);
+
+void BM_PeriodicSampler_Disabled(benchmark::State& state) {
+  struct Tag {};
+  PeriodicSampler<Tag, 0> sampler;
+  BM_Sample(&sampler, state);
+}
+BENCHMARK(BM_PeriodicSampler_Disabled);
+
+}  // namespace
+}  // namespace profiling_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/profiling/internal/periodic_sampler_test.cc b/absl/profiling/internal/periodic_sampler_test.cc
new file mode 100644
index 00000000000..d89b21724cd
--- /dev/null
+++ b/absl/profiling/internal/periodic_sampler_test.cc
@@ -0,0 +1,178 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/profiling/internal/periodic_sampler.h"
+
+#include <cstdint>
+#include <thread>  // NOLINT(build/c++11)
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/attributes.h"
+#include "absl/base/macros.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace profiling_internal {
+namespace {
+
+using testing::Eq;
+using testing::Return;
+using testing::StrictMock;
+
+class MockPeriodicSampler : public PeriodicSamplerBase {
+ public:
+  virtual ~MockPeriodicSampler() = default;
+
+  MOCK_METHOD(int, period, (), (const, noexcept));
+  MOCK_METHOD(int64_t, GetExponentialBiased, (int), (noexcept));
+};
+
+TEST(PeriodicSamplerBaseTest, Sample) {
+  StrictMock<MockPeriodicSampler> sampler;
+
+  EXPECT_CALL(sampler, period()).Times(3).WillRepeatedly(Return(16));
+  EXPECT_CALL(sampler, GetExponentialBiased(16))
+      .WillOnce(Return(2))
+      .WillOnce(Return(3))
+      .WillOnce(Return(4));
+
+  EXPECT_FALSE(sampler.Sample());
+  EXPECT_TRUE(sampler.Sample());
+
+  EXPECT_FALSE(sampler.Sample());
+  EXPECT_FALSE(sampler.Sample());
+  EXPECT_TRUE(sampler.Sample());
+
+  EXPECT_FALSE(sampler.Sample());
+  EXPECT_FALSE(sampler.Sample());
+  EXPECT_FALSE(sampler.Sample());
+}
+
+TEST(PeriodicSamplerBaseTest, ImmediatelySample) {
+  StrictMock<MockPeriodicSampler> sampler;
+
+  EXPECT_CALL(sampler, period()).Times(2).WillRepeatedly(Return(16));
+  EXPECT_CALL(sampler, GetExponentialBiased(16))
+      .WillOnce(Return(1))
+      .WillOnce(Return(2))
+      .WillOnce(Return(3));
+
+  EXPECT_TRUE(sampler.Sample());
+
+  EXPECT_FALSE(sampler.Sample());
+  EXPECT_TRUE(sampler.Sample());
+
+  EXPECT_FALSE(sampler.Sample());
+  EXPECT_FALSE(sampler.Sample());
+}
+
+TEST(PeriodicSamplerBaseTest, Disabled) {
+  StrictMock<MockPeriodicSampler> sampler;
+
+  EXPECT_CALL(sampler, period()).Times(3).WillRepeatedly(Return(0));
+
+  EXPECT_FALSE(sampler.Sample());
+  EXPECT_FALSE(sampler.Sample());
+  EXPECT_FALSE(sampler.Sample());
+}
+
+TEST(PeriodicSamplerBaseTest, AlwaysOn) {
+  StrictMock<MockPeriodicSampler> sampler;
+
+  EXPECT_CALL(sampler, period()).Times(3).WillRepeatedly(Return(1));
+
+  EXPECT_TRUE(sampler.Sample());
+  EXPECT_TRUE(sampler.Sample());
+  EXPECT_TRUE(sampler.Sample());
+}
+
+TEST(PeriodicSamplerBaseTest, Disable) {
+  StrictMock<MockPeriodicSampler> sampler;
+
+  EXPECT_CALL(sampler, period()).WillOnce(Return(16));
+  EXPECT_CALL(sampler, GetExponentialBiased(16)).WillOnce(Return(3));
+  EXPECT_FALSE(sampler.Sample());
+  EXPECT_FALSE(sampler.Sample());
+
+  EXPECT_CALL(sampler, period()).Times(2).WillRepeatedly(Return(0));
+
+  EXPECT_FALSE(sampler.Sample());
+  EXPECT_FALSE(sampler.Sample());
+}
+
+TEST(PeriodicSamplerBaseTest, Enable) {
+  StrictMock<MockPeriodicSampler> sampler;
+
+  EXPECT_CALL(sampler, period()).WillOnce(Return(0));
+  EXPECT_FALSE(sampler.Sample());
+
+  EXPECT_CALL(sampler, period()).Times(2).WillRepeatedly(Return(16));
+  EXPECT_CALL(sampler, GetExponentialBiased(16))
+      .Times(2)
+      .WillRepeatedly(Return(3));
+
+  EXPECT_FALSE(sampler.Sample());
+  EXPECT_FALSE(sampler.Sample());
+  EXPECT_TRUE(sampler.Sample());
+
+  EXPECT_FALSE(sampler.Sample());
+  EXPECT_FALSE(sampler.Sample());
+}
+
+TEST(PeriodicSamplerTest, ConstructConstInit) {
+  struct Tag {};
+  ABSL_CONST_INIT static PeriodicSampler<Tag> sampler;
+  (void)sampler;
+}
+
+TEST(PeriodicSamplerTest, DefaultPeriod0) {
+  struct Tag {};
+  PeriodicSampler<Tag> sampler;
+  EXPECT_THAT(sampler.period(), Eq(0));
+}
+
+TEST(PeriodicSamplerTest, DefaultPeriod) {
+  struct Tag {};
+  PeriodicSampler<Tag, 100> sampler;
+  EXPECT_THAT(sampler.period(), Eq(100));
+}
+
+TEST(PeriodicSamplerTest, SetGlobalPeriod) {
+  struct Tag1 {};
+  struct Tag2 {};
+  PeriodicSampler<Tag1, 25> sampler1;
+  PeriodicSampler<Tag2, 50> sampler2;
+
+  EXPECT_THAT(sampler1.period(), Eq(25));
+  EXPECT_THAT(sampler2.period(), Eq(50));
+
+  std::thread thread([] {
+    PeriodicSampler<Tag1, 25> sampler1;
+    PeriodicSampler<Tag2, 50> sampler2;
+    EXPECT_THAT(sampler1.period(), Eq(25));
+    EXPECT_THAT(sampler2.period(), Eq(50));
+    sampler1.SetGlobalPeriod(10);
+    sampler2.SetGlobalPeriod(20);
+  });
+  thread.join();
+
+  EXPECT_THAT(sampler1.period(), Eq(10));
+  EXPECT_THAT(sampler2.period(), Eq(20));
+}
+
+}  // namespace
+}  // namespace profiling_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/profiling/internal/profile_builder.cc b/absl/profiling/internal/profile_builder.cc
new file mode 100644
index 00000000000..6dc067791bc
--- /dev/null
+++ b/absl/profiling/internal/profile_builder.cc
@@ -0,0 +1,463 @@
+// Copyright 2025 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may
+// obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/profiling/internal/profile_builder.h"
+
+#include <cassert>
+#include <cstdint>
+#include <cstring>
+#include <string>
+#include <utility>
+#include <vector>
+
+#include "absl/base/casts.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/strings/escaping.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/str_format.h"
+#include "absl/strings/string_view.h"
+#include "absl/types/span.h"
+
+#ifdef __linux__
+#include <elf.h>
+#include <link.h>
+#endif  // __linux__
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace debugging_internal {
+
+namespace {
+
+// This file contains a simplified implementation of the pprof profile builder,
+// which avoids a dependency on protobuf.
+//
+// The canonical profile proto definition is at
+// https://github.com/google/pprof/blob/master/proto/profile.proto
+//
+// Wire-format encoding is a simple sequence of (tag, value) pairs. The tag
+// is a varint-encoded integer, where the low 3 bits are the wire type, and the
+// high bits are the field number.
+//
+// For the fields we care about, we'll be using the following wire types:
+//
+// Wire Type 0: Varint-encoded integer.
+// Wire Type 2: Length-delimited. Used for strings and sub-messages.
+enum class WireType {
+  kVarint = 0,
+  kLengthDelimited = 2,
+};
+
+#ifdef __linux__
+// Returns the Phdr of the first segment of the given type.
+const ElfW(Phdr) * GetFirstSegment(const dl_phdr_info* const info,
+                                   const ElfW(Word) segment_type) {
+  for (int i = 0; i < info->dlpi_phnum; ++i) {
+    if (info->dlpi_phdr[i].p_type == segment_type) {
+      return &info->dlpi_phdr[i];
+    }
+  }
+  return nullptr;
+}
+
+// Return DT_SONAME for the given image.  If there is no PT_DYNAMIC or if
+// PT_DYNAMIC does not contain DT_SONAME, return nullptr.
+static const char* GetSoName(const dl_phdr_info* const info) {
+  const ElfW(Phdr)* const pt_dynamic = GetFirstSegment(info, PT_DYNAMIC);
+  if (pt_dynamic == nullptr) {
+    return nullptr;
+  }
+  const ElfW(Dyn)* dyn =
+      reinterpret_cast<ElfW(Dyn)*>(info->dlpi_addr + pt_dynamic->p_vaddr);
+  const ElfW(Dyn)* dt_strtab = nullptr;
+  const ElfW(Dyn)* dt_strsz = nullptr;
+  const ElfW(Dyn)* dt_soname = nullptr;
+  for (; dyn->d_tag != DT_NULL; ++dyn) {
+    if (dyn->d_tag == DT_SONAME) {
+      dt_soname = dyn;
+    } else if (dyn->d_tag == DT_STRTAB) {
+      dt_strtab = dyn;
+    } else if (dyn->d_tag == DT_STRSZ) {
+      dt_strsz = dyn;
+    }
+  }
+  if (dt_soname == nullptr) {
+    return nullptr;
+  }
+  ABSL_RAW_CHECK(dt_strtab != nullptr, "Unexpected nullptr");
+  ABSL_RAW_CHECK(dt_strsz != nullptr, "Unexpected nullptr");
+  const char* const strtab = reinterpret_cast<char*>(
+      info->dlpi_addr + static_cast<ElfW(Word)>(dt_strtab->d_un.d_val));
+  ABSL_RAW_CHECK(dt_soname->d_un.d_val < dt_strsz->d_un.d_val,
+                 "Unexpected order");
+  return strtab + dt_soname->d_un.d_val;
+}
+
+// Helper function to get the build ID of a shared object.
+std::string GetBuildId(const dl_phdr_info* const info) {
+  std::string result;
+
+  // pt_note contains entries (of type ElfW(Nhdr)) starting at
+  //   info->dlpi_addr + pt_note->p_vaddr
+  // with length
+  //   pt_note->p_memsz
+  //
+  // The length of each entry is given by
+  //   Align(sizeof(ElfW(Nhdr)) + nhdr->n_namesz) + Align(nhdr->n_descsz)
+  for (int i = 0; i < info->dlpi_phnum; ++i) {
+    const ElfW(Phdr)* pt_note = &info->dlpi_phdr[i];
+    if (pt_note->p_type != PT_NOTE) continue;
+
+    const char* note =
+        reinterpret_cast<char*>(info->dlpi_addr + pt_note->p_vaddr);
+    const char* const last = note + pt_note->p_filesz;
+    const ElfW(Xword) align = pt_note->p_align;
+    while (note < last) {
+      const ElfW(Nhdr)* const nhdr = reinterpret_cast<const ElfW(Nhdr)*>(note);
+      if (note + sizeof(*nhdr) > last) {
+        // Corrupt PT_NOTE
+        break;
+      }
+
+      // Both the start and end of the descriptor are aligned by sh_addralign
+      // (= p_align).
+      const ElfW(Xword) desc_start =
+          (sizeof(*nhdr) + nhdr->n_namesz + align - 1) & -align;
+      const ElfW(Xword) size =
+          desc_start + ((nhdr->n_descsz + align - 1) & -align);
+
+      // Beware of wrap-around.
+      if (nhdr->n_namesz >= static_cast<ElfW(Word)>(-align) ||
+          nhdr->n_descsz >= static_cast<ElfW(Word)>(-align) ||
+          desc_start < sizeof(*nhdr) || size < desc_start ||
+          size > static_cast<ElfW(Xword)>(last - note)) {
+        // Corrupt PT_NOTE
+        break;
+      }
+
+      if (nhdr->n_type == NT_GNU_BUILD_ID) {
+        const char* const note_name = note + sizeof(*nhdr);
+        // n_namesz is the length of note_name.
+        if (nhdr->n_namesz == 4 && memcmp(note_name, "GNU\0", 4) == 0) {
+          if (!result.empty()) {
+            // Repeated build-ids.  Ignore them.
+            return "";
+          }
+          result = absl::BytesToHexString(
+              absl::string_view(note + desc_start, nhdr->n_descsz));
+        }
+      }
+      note += size;
+    }
+  }
+
+  return result;
+}
+#endif  // __linux__
+
+// A varint-encoded integer.
+struct Varint {
+  explicit Varint(uint64_t v) : value(v) {}
+  explicit Varint(StringId v) : value(static_cast<uint64_t>(v)) {}
+  explicit Varint(LocationId v) : value(static_cast<uint64_t>(v)) {}
+  explicit Varint(MappingId v) : value(static_cast<uint64_t>(v)) {}
+
+  uint64_t value;
+
+  template <typename Sink>
+  friend void AbslStringify(Sink& sink, const Varint& v) {
+    char buf[10];
+    char* p = buf;
+    uint64_t u = v.value;
+    while (u >= 0x80) {
+      *p++ = static_cast<char>((u & 0x7f) | 0x80);
+      u >>= 7;
+    }
+    *p++ = static_cast<char>(u);
+    sink.Append(absl::string_view(buf, static_cast<size_t>(p - buf)));
+  }
+};
+
+struct Tag {
+  int field_number;
+  WireType wire_type;
+
+  template <typename Sink>
+  friend void AbslStringify(Sink& sink, const Tag& t) {
+    absl::Format(&sink, "%v",
+                 Varint((static_cast<uint64_t>(t.field_number) << 3) |
+                        static_cast<uint64_t>(t.wire_type)));
+  }
+};
+
+struct LengthDelimited {
+  int field_number;
+  absl::string_view value;
+
+  template <typename Sink>
+  friend void AbslStringify(Sink& sink, const LengthDelimited& ld) {
+    absl::Format(&sink, "%v%v%v",
+                 Tag{ld.field_number, WireType::kLengthDelimited},
+                 Varint(ld.value.size()), ld.value);
+  }
+};
+
+struct VarintField {
+  int field_number;
+  Varint value;
+
+  template <typename Sink>
+  friend void AbslStringify(Sink& sink, const VarintField& vf) {
+    absl::Format(&sink, "%v%v", Tag{vf.field_number, WireType::kVarint},
+                 vf.value);
+  }
+};
+
+}  // namespace
+
+StringId ProfileBuilder::InternString(absl::string_view str) {
+  if (str.empty()) return StringId(0);
+  return string_table_.emplace(str, StringId(string_table_.size()))
+      .first->second;
+}
+
+LocationId ProfileBuilder::InternLocation(const void* address) {
+  return location_table_
+      .emplace(absl::bit_cast<uintptr_t>(address),
+               LocationId(location_table_.size() + 1))
+      .first->second;
+}
+
+void ProfileBuilder::AddSample(
+    int64_t value, absl::Span<const void* const> stack,
+    absl::Span<const std::pair<StringId, int64_t>> labels) {
+  std::string sample_proto;
+  absl::StrAppend(
+      &sample_proto,
+      VarintField{SampleProto::kValue, Varint(static_cast<uint64_t>(value))});
+
+  for (const void* addr : stack) {
+    // Profile addresses are raw stack unwind addresses, so they should be
+    // adjusted by -1 to land inside the call instruction (although potentially
+    // misaligned).
+    absl::StrAppend(
+        &sample_proto,
+        VarintField{SampleProto::kLocationId,
+                    Varint(InternLocation(absl::bit_cast<const void*>(
+                        absl::bit_cast<uintptr_t>(addr) - 1)))});
+  }
+
+  for (const auto& label : labels) {
+    std::string label_proto =
+        absl::StrCat(VarintField{LabelProto::kKey, Varint(label.first)},
+                     VarintField{LabelProto::kNum,
+                                 Varint(static_cast<uint64_t>(label.second))});
+    absl::StrAppend(&sample_proto,
+                    LengthDelimited{SampleProto::kLabel, label_proto});
+  }
+  samples_.push_back(std::move(sample_proto));
+}
+
+void ProfileBuilder::AddSampleType(StringId type, StringId unit) {
+  std::string sample_type_proto =
+      absl::StrCat(VarintField{ValueTypeProto::kType, Varint(type)},
+                   VarintField{ValueTypeProto::kUnit, Varint(unit)});
+  sample_types_.push_back(std::move(sample_type_proto));
+}
+
+MappingId ProfileBuilder::AddMapping(uintptr_t memory_start,
+                                     uintptr_t memory_limit,
+                                     uintptr_t file_offset,
+                                     absl::string_view filename,
+                                     absl::string_view build_id) {
+  size_t index = mappings_.size() + 1;
+  auto [it, inserted] = mapping_table_.emplace(memory_start, index);
+  if (!inserted) {
+    return static_cast<MappingId>(it->second);
+  }
+
+  Mapping m;
+  m.start = memory_start;
+  m.limit = memory_limit;
+  m.offset = file_offset;
+  m.filename = std::string(filename);
+  m.build_id = std::string(build_id);
+
+  mappings_.push_back(std::move(m));
+  return static_cast<MappingId>(index);
+}
+
+std::string ProfileBuilder::Emit() && {
+  std::string profile_proto;
+  for (const auto& sample_type : sample_types_) {
+    absl::StrAppend(&profile_proto,
+                    LengthDelimited{ProfileProto::kSampleType, sample_type});
+  }
+  for (const auto& sample : samples_) {
+    absl::StrAppend(&profile_proto,
+                    LengthDelimited{ProfileProto::kSample, sample});
+  }
+
+  // Build mapping table.
+  for (size_t i = 0, n = mappings_.size(); i < n; ++i) {
+    const auto& mapping = mappings_[i];
+    std::string mapping_proto = absl::StrCat(
+        VarintField{MappingProto::kId, Varint(static_cast<uint64_t>(i + 1))},
+        VarintField{MappingProto::kMemoryStart, Varint(mapping.start)},
+        VarintField{MappingProto::kMemoryLimit, Varint(mapping.limit)},
+        VarintField{MappingProto::kFileOffset, Varint(mapping.offset)},
+        VarintField{MappingProto::kFilename,
+                    Varint(InternString(mapping.filename))},
+        VarintField{MappingProto::kBuildId,
+                    Varint(InternString(mapping.build_id))});
+
+    absl::StrAppend(&profile_proto,
+                    LengthDelimited{ProfileProto::kMapping, mapping_proto});
+  }
+
+  // Build location table.
+  for (const auto& [address, id] : location_table_) {
+    std::string location =
+        absl::StrCat(VarintField{LocationProto::kId, Varint(id)},
+                     VarintField{LocationProto::kAddress, Varint(address)});
+
+    if (!mappings_.empty()) {
+      // Find the mapping ID.
+      auto it = mapping_table_.upper_bound(address);
+      if (it != mapping_table_.begin()) {
+        --it;
+      }
+
+      // If *it contains address, add mapping to location.
+      const size_t mapping_index = it->second;
+      const Mapping& mapping = mappings_[mapping_index - 1];
+
+      if (it->first <= address && address < mapping.limit) {
+        absl::StrAppend(
+            &location,
+            VarintField{LocationProto::kMappingId,
+                        Varint(static_cast<uint64_t>(mapping_index))});
+      }
+    }
+
+    absl::StrAppend(&profile_proto,
+                    LengthDelimited{ProfileProto::kLocation, location});
+  }
+
+  std::string string_table_proto;
+  std::vector<absl::string_view> sorted_strings(string_table_.size());
+  for (const auto& p : string_table_) {
+    sorted_strings[static_cast<size_t>(p.second)] = p.first;
+  }
+  for (const auto& s : sorted_strings) {
+    absl::StrAppend(&string_table_proto,
+                    LengthDelimited{ProfileProto::kStringTable, s});
+  }
+  absl::StrAppend(&profile_proto, VarintField{ProfileProto::kDropFrames,
+                                              Varint(drop_frames_id_)});
+  absl::StrAppend(&profile_proto,
+                  VarintField{ProfileProto::kComment, Varint(comment_id_)});
+  absl::StrAppend(&profile_proto, VarintField{ProfileProto::kDefaultSampleType,
+                                              Varint(default_sample_type_id_)});
+  return absl::StrCat(string_table_proto, profile_proto);
+}
+
+void ProfileBuilder::set_drop_frames_id(StringId drop_frames_id) {
+  drop_frames_id_ = drop_frames_id;
+}
+
+void ProfileBuilder::set_comment_id(StringId comment_id) {
+  comment_id_ = comment_id;
+}
+
+void ProfileBuilder::set_default_sample_type_id(
+    StringId default_sample_type_id) {
+  default_sample_type_id_ = default_sample_type_id;
+}
+
+void ProfileBuilder::AddCurrentMappings() {
+#ifdef __linux__
+  dl_iterate_phdr(
+      +[](dl_phdr_info* info, size_t, void* data) {
+        auto& builder = *reinterpret_cast<ProfileBuilder*>(data);
+
+        // Skip dummy entry introduced since glibc 2.18.
+        if (info->dlpi_phdr == nullptr && info->dlpi_phnum == 0) {
+          return 0;
+        }
+
+        const bool is_main_executable = builder.mappings_.empty();
+
+        // Storage for path to executable as dlpi_name isn't populated for the
+        // main executable.  +1 to allow for the null terminator that readlink
+        // does not add.
+        char self_filename[PATH_MAX + 1];
+        const char* filename = info->dlpi_name;
+        if (filename == nullptr || filename[0] == '\0') {
+          // This is either the main executable or the VDSO.  The main
+          // executable is always the first entry processed by callbacks.
+          if (is_main_executable) {
+            // This is the main executable.
+            ssize_t ret = readlink("/proc/self/exe", self_filename,
+                                   sizeof(self_filename) - 1);
+            if (ret >= 0 && static_cast<size_t>(ret) < sizeof(self_filename)) {
+              self_filename[ret] = '\0';
+              filename = self_filename;
+            }
+          } else {
+            // This is the VDSO.
+            filename = GetSoName(info);
+          }
+        }
+
+        char resolved_path[PATH_MAX];
+        absl::string_view resolved_filename;
+        if (realpath(filename, resolved_path)) {
+          resolved_filename = resolved_path;
+        } else {
+          resolved_filename = filename;
+        }
+
+        const std::string build_id = GetBuildId(info);
+
+        // Evaluate all the loadable segments.
+        for (int i = 0; i < info->dlpi_phnum; ++i) {
+          if (info->dlpi_phdr[i].p_type != PT_LOAD) {
+            continue;
+          }
+          const ElfW(Phdr)* pt_load = &info->dlpi_phdr[i];
+
+          ABSL_RAW_CHECK(pt_load != nullptr, "Unexpected nullptr");
+
+          // Extract data.
+          const size_t memory_start = info->dlpi_addr + pt_load->p_vaddr;
+          const size_t memory_limit = memory_start + pt_load->p_memsz;
+          const size_t file_offset = pt_load->p_offset;
+
+          // Add to profile.
+          builder.AddMapping(memory_start, memory_limit, file_offset,
+                             resolved_filename, build_id);
+        }
+        // Keep going.
+        return 0;
+      },
+      this);
+#endif  // __linux__
+}
+
+}  // namespace debugging_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/profiling/internal/profile_builder.h b/absl/profiling/internal/profile_builder.h
new file mode 100644
index 00000000000..243b927c541
--- /dev/null
+++ b/absl/profiling/internal/profile_builder.h
@@ -0,0 +1,164 @@
+// Copyright 2025 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may
+// obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_PROFILING_INTERNAL_PROFILE_BUILDER_H_
+#define ABSL_PROFILING_INTERNAL_PROFILE_BUILDER_H_
+
+#include <cstddef>
+#include <cstdint>
+#include <memory>
+#include <string>
+#include <utility>
+#include <vector>
+
+#include "absl/container/btree_map.h"
+#include "absl/container/flat_hash_map.h"
+#include "absl/container/hash_container_defaults.h"
+#include "absl/strings/string_view.h"
+#include "absl/types/span.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace debugging_internal {
+
+// Field numbers for perftools.profiles.Profile.
+// https://github.com/google/pprof/blob/master/proto/profile.proto
+struct ProfileProto {
+  static constexpr int kSampleType = 1;
+  static constexpr int kSample = 2;
+  static constexpr int kMapping = 3;
+  static constexpr int kLocation = 4;
+  static constexpr int kStringTable = 6;
+  static constexpr int kDropFrames = 7;
+  static constexpr int kComment = 13;
+  static constexpr int kDefaultSampleType = 14;
+};
+
+struct ValueTypeProto {
+  static constexpr int kType = 1;
+  static constexpr int kUnit = 2;
+};
+
+struct SampleProto {
+  static constexpr int kLocationId = 1;
+  static constexpr int kValue = 2;
+  static constexpr int kLabel = 3;
+};
+
+struct LabelProto {
+  static constexpr int kKey = 1;
+  static constexpr int kStr = 2;
+  static constexpr int kNum = 3;
+  static constexpr int kNumUnit = 4;
+};
+
+struct MappingProto {
+  static constexpr int kId = 1;
+  static constexpr int kMemoryStart = 2;
+  static constexpr int kMemoryLimit = 3;
+  static constexpr int kFileOffset = 4;
+  static constexpr int kFilename = 5;
+  static constexpr int kBuildId = 6;
+};
+
+struct LocationProto {
+  static constexpr int kId = 1;
+  static constexpr int kMappingId = 2;
+  static constexpr int kAddress = 3;
+};
+
+enum class StringId : size_t {};
+enum class LocationId : size_t {};
+enum class MappingId : size_t {};
+
+// A helper class to build a profile protocol buffer.
+class ProfileBuilder {
+ public:
+  struct Mapping {
+    uint64_t start;
+    uint64_t limit;
+    uint64_t offset;
+    std::string filename;
+    std::string build_id;
+  };
+
+  StringId InternString(absl::string_view str);
+
+  LocationId InternLocation(const void* address);
+
+  void AddSample(int64_t value, absl::Span<const void* const> stack,
+                 absl::Span<const std::pair<StringId, int64_t>> labels);
+
+  void AddSampleType(StringId type, StringId unit);
+
+  // Adds the current process mappings to the profile.
+  void AddCurrentMappings();
+
+  // Adds a single mapping to the profile and to lookup cache and returns the
+  // resulting ID.
+  MappingId AddMapping(uintptr_t memory_start, uintptr_t memory_limit,
+                       uintptr_t file_offset, absl::string_view filename,
+                       absl::string_view build_id);
+
+  std::string Emit() &&;
+
+  void set_drop_frames_id(StringId drop_frames_id);
+  void set_comment_id(StringId comment_id);
+  void set_default_sample_type_id(StringId default_sample_type_id);
+
+ private:
+  // We turn off hashtable profiling for the ProfileBuilder's own tables.
+  //
+  // This is necessary since we use this class to construct hashtable profiles,
+  // which entails walking the hashtable profiling data and we don't want to
+  // encounter it reentrantly.
+  template <typename T>
+  struct HashtablezBarrier : std::allocator<T> {
+    HashtablezBarrier() = default;
+
+    template <typename U>
+    HashtablezBarrier(const HashtablezBarrier<U>&) {}
+
+    template <class U>
+    struct rebind {
+      using other = HashtablezBarrier<U>;
+    };
+  };
+
+  template <typename K, typename V>
+  using UnprofiledHashMap = absl::flat_hash_map<
+      K, V, DefaultHashContainerHash<K>, DefaultHashContainerEq<K>,
+      HashtablezBarrier<std::pair<const K, V>>>;
+
+  UnprofiledHashMap<std::string, StringId> string_table_{{"", StringId(0)}};
+  UnprofiledHashMap<uintptr_t, LocationId> location_table_;
+  // mapping_table_ stores the start address of each mapping in mapping_
+  // to its index.
+  absl::btree_map<uintptr_t, size_t> mapping_table_;
+  std::vector<Mapping> mappings_;
+
+  std::vector<std::string> sample_types_;
+  std::vector<std::string> samples_;
+
+  StringId drop_frames_id_{};
+  StringId comment_id_{};
+  StringId default_sample_type_id_{};
+};
+
+}  // namespace debugging_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_PROFILING_INTERNAL_PROFILE_BUILDER_H_
diff --git a/absl/profiling/internal/sample_recorder.h b/absl/profiling/internal/sample_recorder.h
new file mode 100644
index 00000000000..0d14027957e
--- /dev/null
+++ b/absl/profiling/internal/sample_recorder.h
@@ -0,0 +1,259 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: sample_recorder.h
+// -----------------------------------------------------------------------------
+//
+// This header file defines a lock-free linked list for recording samples
+// collected from a random/stochastic process.
+//
+// This utility is internal-only. Use at your own risk.
+
+#ifndef ABSL_PROFILING_INTERNAL_SAMPLE_RECORDER_H_
+#define ABSL_PROFILING_INTERNAL_SAMPLE_RECORDER_H_
+
+#include <atomic>
+#include <cstddef>
+#include <cstdint>
+#include <functional>
+
+#include "absl/base/config.h"
+#include "absl/base/thread_annotations.h"
+#include "absl/synchronization/mutex.h"
+#include "absl/time/time.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace profiling_internal {
+
+// Sample<T> that has members required for linking samples in the linked list of
+// samples maintained by the SampleRecorder.  Type T defines the sampled data.
+template <typename T>
+struct Sample {
+  // Guards the ability to restore the sample to a pristine state.  This
+  // prevents races with sampling and resurrecting an object.
+  absl::Mutex init_mu;
+  T* next = nullptr;
+  T* dead ABSL_GUARDED_BY(init_mu) = nullptr;
+  int64_t weight;  // How many sampling events were required to sample this one.
+};
+
+// Holds samples and their associated stack traces with a soft limit of
+// `SetHashtablezMaxSamples()`.
+//
+// Thread safe.
+template <typename T>
+class SampleRecorder {
+ public:
+  SampleRecorder();
+  ~SampleRecorder();
+
+  // Registers for sampling.  Returns an opaque registration info.
+  template <typename... Targs>
+  T* Register(Targs&&... args);
+
+  // Unregisters the sample.
+  void Unregister(T* sample);
+
+  // The dispose callback will be called on all samples the moment they are
+  // being unregistered. Only affects samples that are unregistered after the
+  // callback has been set.
+  // Returns the previous callback.
+  using DisposeCallback = void (*)(const T&);
+  DisposeCallback SetDisposeCallback(DisposeCallback f);
+
+  // Iterates over all the registered `StackInfo`s.  Returning the number of
+  // samples that have been dropped.
+  size_t Iterate(const std::function<void(const T& stack)>& f);
+
+  size_t GetMaxSamples() const;
+  void SetMaxSamples(size_t max);
+
+ private:
+  void PushNew(T* sample);
+  void PushDead(T* sample);
+  template <typename... Targs>
+  T* PopDead(Targs... args);
+
+  std::atomic<size_t> dropped_samples_;
+  std::atomic<size_t> size_estimate_;
+  std::atomic<size_t> max_samples_{1 << 20};
+
+  // Intrusive lock free linked lists for tracking samples.
+  //
+  // `all_` records all samples (they are never removed from this list) and is
+  // terminated with a `nullptr`.
+  //
+  // `graveyard_.dead` is a circular linked list.  When it is empty,
+  // `graveyard_.dead == &graveyard`.  The list is circular so that
+  // every item on it (even the last) has a non-null dead pointer.  This allows
+  // `Iterate` to determine if a given sample is live or dead using only
+  // information on the sample itself.
+  //
+  // For example, nodes [A, B, C, D, E] with [A, C, E] alive and [B, D] dead
+  // looks like this (G is the Graveyard):
+  //
+  //           +---+    +---+    +---+    +---+    +---+
+  //    all -->| A |--->| B |--->| C |--->| D |--->| E |
+  //           |   |    |   |    |   |    |   |    |   |
+  //   +---+   |   | +->|   |-+  |   | +->|   |-+  |   |
+  //   | G |   +---+ |  +---+ |  +---+ |  +---+ |  +---+
+  //   |   |         |        |        |        |
+  //   |   | --------+        +--------+        |
+  //   +---+                                    |
+  //     ^                                      |
+  //     +--------------------------------------+
+  //
+  std::atomic<T*> all_;
+  T graveyard_;
+
+  std::atomic<DisposeCallback> dispose_;
+};
+
+template <typename T>
+typename SampleRecorder<T>::DisposeCallback
+SampleRecorder<T>::SetDisposeCallback(DisposeCallback f) {
+  return dispose_.exchange(f, std::memory_order_relaxed);
+}
+
+template <typename T>
+SampleRecorder<T>::SampleRecorder()
+    : dropped_samples_(0), size_estimate_(0), all_(nullptr), dispose_(nullptr) {
+  absl::MutexLock l(graveyard_.init_mu);
+  graveyard_.dead = &graveyard_;
+}
+
+template <typename T>
+SampleRecorder<T>::~SampleRecorder() {
+  T* s = all_.load(std::memory_order_acquire);
+  while (s != nullptr) {
+    T* next = s->next;
+    delete s;
+    s = next;
+  }
+}
+
+template <typename T>
+void SampleRecorder<T>::PushNew(T* sample) {
+  sample->next = all_.load(std::memory_order_relaxed);
+  while (!all_.compare_exchange_weak(sample->next, sample,
+                                     std::memory_order_release,
+                                     std::memory_order_relaxed)) {
+  }
+}
+
+template <typename T>
+void SampleRecorder<T>::PushDead(T* sample) {
+  if (auto* dispose = dispose_.load(std::memory_order_relaxed)) {
+    dispose(*sample);
+  }
+
+  absl::MutexLock graveyard_lock(graveyard_.init_mu);
+  absl::MutexLock sample_lock(sample->init_mu);
+  sample->dead = graveyard_.dead;
+  graveyard_.dead = sample;
+}
+
+template <typename T>
+template <typename... Targs>
+T* SampleRecorder<T>::PopDead(Targs... args) {
+  absl::ReleasableMutexLock graveyard_lock(graveyard_.init_mu);
+
+  // The list is circular, so eventually it collapses down to
+  //   graveyard_.dead == &graveyard_
+  // when it is empty.
+  T* sample = graveyard_.dead;
+  if (sample == &graveyard_) return nullptr;
+
+  absl::MutexLock sample_lock(sample->init_mu);
+  graveyard_.dead = sample->dead;
+  // Release the global graveyard lock early, before the potentially slow
+  // preparation.
+  graveyard_lock.Release();
+  // Prepare the sample while still holding the per-sample lock.
+  // `Iterate` will wait for the lock to be released.
+  sample->dead = nullptr;
+  sample->PrepareForSampling(std::forward<Targs>(args)...);
+  return sample;
+}
+
+template <typename T>
+template <typename... Targs>
+T* SampleRecorder<T>::Register(Targs&&... args) {
+  size_t size = size_estimate_.fetch_add(1, std::memory_order_relaxed);
+  if (size > max_samples_.load(std::memory_order_relaxed)) {
+    size_estimate_.fetch_sub(1, std::memory_order_relaxed);
+    dropped_samples_.fetch_add(1, std::memory_order_relaxed);
+    return nullptr;
+  }
+
+  T* sample = PopDead(args...);
+  if (sample == nullptr) {
+    // Resurrection failed.  Hire a new warlock.
+    sample = new T();
+    {
+      absl::MutexLock sample_lock(sample->init_mu);
+      // If flag initialization happens to occur (perhaps in another thread)
+      // while in this block, it will lock `graveyard_` which is usually always
+      // locked before any sample. This will appear as a lock inversion.
+      // However, this code is run exactly once per sample, and this sample
+      // cannot be accessed until after it is returned from this method.  This
+      // means that this lock state can never be recreated, so we can safely
+      // inform the deadlock detector to ignore it.
+      sample->init_mu.ForgetDeadlockInfo();
+      sample->PrepareForSampling(std::forward<Targs>(args)...);
+    }
+    PushNew(sample);
+  }
+
+  return sample;
+}
+
+template <typename T>
+void SampleRecorder<T>::Unregister(T* sample) {
+  PushDead(sample);
+  size_estimate_.fetch_sub(1, std::memory_order_relaxed);
+}
+
+template <typename T>
+size_t SampleRecorder<T>::Iterate(
+    const std::function<void(const T& stack)>& f) {
+  T* s = all_.load(std::memory_order_acquire);
+  while (s != nullptr) {
+    absl::MutexLock l(s->init_mu);
+    if (s->dead == nullptr) {
+      f(*s);
+    }
+    s = s->next;
+  }
+
+  return dropped_samples_.load(std::memory_order_relaxed);
+}
+
+template <typename T>
+void SampleRecorder<T>::SetMaxSamples(size_t max) {
+  max_samples_.store(max, std::memory_order_release);
+}
+
+template <typename T>
+size_t SampleRecorder<T>::GetMaxSamples() const {
+  return max_samples_.load(std::memory_order_acquire);
+}
+
+}  // namespace profiling_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_PROFILING_INTERNAL_SAMPLE_RECORDER_H_
diff --git a/absl/profiling/internal/sample_recorder_test.cc b/absl/profiling/internal/sample_recorder_test.cc
new file mode 100644
index 00000000000..2401149c8e0
--- /dev/null
+++ b/absl/profiling/internal/sample_recorder_test.cc
@@ -0,0 +1,192 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/profiling/internal/sample_recorder.h"
+
+#include <algorithm>
+#include <atomic>
+#include <cassert>
+#include <cstddef>
+#include <cstdint>
+#include <random>
+#include <vector>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/thread_annotations.h"
+#include "absl/random/random.h"
+#include "absl/synchronization/internal/thread_pool.h"
+#include "absl/synchronization/mutex.h"
+#include "absl/synchronization/notification.h"
+#include "absl/time/clock.h"
+#include "absl/time/time.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace profiling_internal {
+
+namespace {
+using ::absl::synchronization_internal::ThreadPool;
+using ::testing::IsEmpty;
+using ::testing::UnorderedElementsAre;
+
+struct Info : public Sample<Info> {
+ public:
+  void PrepareForSampling(int64_t w) { weight = w; }
+  std::atomic<size_t> size;
+  absl::Time create_time;
+};
+
+std::vector<size_t> GetSizes(SampleRecorder<Info>* s) {
+  std::vector<size_t> res;
+  EXPECT_EQ(s->Iterate([&](const Info& info) {
+    res.push_back(info.size.load(std::memory_order_acquire));
+  }),
+            0);
+  return res;
+}
+
+std::vector<int64_t> GetWeights(SampleRecorder<Info>* s) {
+  std::vector<int64_t> res;
+  EXPECT_EQ(s->Iterate([&](const Info& info) { res.push_back(info.weight); }),
+            0);
+  return res;
+}
+
+Info* Register(SampleRecorder<Info>* s, int64_t weight, size_t size) {
+  auto* info = s->Register(weight);
+  assert(info != nullptr);
+  info->size.store(size);
+  return info;
+}
+
+TEST(SampleRecorderTest, Registration) {
+  SampleRecorder<Info> sampler;
+  auto* info1 = Register(&sampler, 31, 1);
+  EXPECT_THAT(GetSizes(&sampler), UnorderedElementsAre(1));
+  EXPECT_THAT(GetWeights(&sampler), UnorderedElementsAre(31));
+
+  auto* info2 = Register(&sampler, 32, 2);
+  EXPECT_THAT(GetSizes(&sampler), UnorderedElementsAre(1, 2));
+  info1->size.store(3);
+  EXPECT_THAT(GetSizes(&sampler), UnorderedElementsAre(3, 2));
+  EXPECT_THAT(GetWeights(&sampler), UnorderedElementsAre(31, 32));
+
+  sampler.Unregister(info1);
+  sampler.Unregister(info2);
+}
+
+TEST(SampleRecorderTest, Unregistration) {
+  SampleRecorder<Info> sampler;
+  std::vector<Info*> infos;
+  for (size_t i = 0; i < 3; ++i) {
+    infos.push_back(Register(&sampler, 33 + i, i));
+  }
+  EXPECT_THAT(GetSizes(&sampler), UnorderedElementsAre(0, 1, 2));
+  EXPECT_THAT(GetWeights(&sampler), UnorderedElementsAre(33, 34, 35));
+
+  sampler.Unregister(infos[1]);
+  EXPECT_THAT(GetSizes(&sampler), UnorderedElementsAre(0, 2));
+  EXPECT_THAT(GetWeights(&sampler), UnorderedElementsAre(33, 35));
+
+  infos.push_back(Register(&sampler, 36, 3));
+  infos.push_back(Register(&sampler, 37, 4));
+  EXPECT_THAT(GetSizes(&sampler), UnorderedElementsAre(0, 2, 3, 4));
+  EXPECT_THAT(GetWeights(&sampler), UnorderedElementsAre(33, 35, 36, 37));
+  sampler.Unregister(infos[3]);
+  EXPECT_THAT(GetSizes(&sampler), UnorderedElementsAre(0, 2, 4));
+  EXPECT_THAT(GetWeights(&sampler), UnorderedElementsAre(33, 35, 37));
+
+  sampler.Unregister(infos[0]);
+  sampler.Unregister(infos[2]);
+  sampler.Unregister(infos[4]);
+  EXPECT_THAT(GetSizes(&sampler), IsEmpty());
+}
+
+TEST(SampleRecorderTest, MultiThreaded) {
+  SampleRecorder<Info> sampler;
+  Notification stop;
+  ThreadPool pool(10);
+
+  for (int i = 0; i < 10; ++i) {
+    pool.Schedule([&sampler, &stop, i]() {
+      absl::InsecureBitGen gen;
+      std::vector<Info*> infoz;
+      while (!stop.HasBeenNotified()) {
+        if (infoz.empty()) {
+          infoz.push_back(sampler.Register(i));
+        }
+        switch (std::uniform_int_distribution<>(0, 2)(gen)) {
+          case 0: {
+            infoz.push_back(sampler.Register(i));
+            break;
+          }
+          case 1: {
+            size_t p =
+                std::uniform_int_distribution<>(0, infoz.size() - 1)(gen);
+            Info* info = infoz[p];
+            infoz[p] = infoz.back();
+            infoz.pop_back();
+            EXPECT_EQ(info->weight, i);
+            sampler.Unregister(info);
+            break;
+          }
+          case 2: {
+            absl::Duration oldest = absl::ZeroDuration();
+            EXPECT_EQ(sampler.Iterate([&](const Info& info) {
+              oldest = std::max(oldest, absl::Now() - info.create_time);
+            }),
+                      0);
+            ASSERT_GE(oldest, absl::ZeroDuration());
+            break;
+          }
+        }
+      }
+    });
+  }
+  // The threads will hammer away.  Give it a little bit of time for tsan to
+  // spot errors.
+  absl::SleepFor(absl::Seconds(3));
+  stop.Notify();
+}
+
+TEST(SampleRecorderTest, Callback) {
+  SampleRecorder<Info> sampler;
+
+  auto* info1 = Register(&sampler, 39, 1);
+  auto* info2 = Register(&sampler, 40, 2);
+
+  static const Info* expected;
+
+  auto callback = [](const Info& info) {
+    // We can't use `info` outside of this callback because the object will be
+    // disposed as soon as we return from here.
+    EXPECT_EQ(&info, expected);
+  };
+
+  // Set the callback.
+  EXPECT_EQ(sampler.SetDisposeCallback(callback), nullptr);
+  expected = info1;
+  sampler.Unregister(info1);
+
+  // Unset the callback.
+  EXPECT_EQ(callback, sampler.SetDisposeCallback(nullptr));
+  expected = nullptr;  // no more calls.
+  sampler.Unregister(info2);
+}
+
+}  // namespace
+}  // namespace profiling_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/random/BUILD.bazel b/absl/random/BUILD.bazel
index c904618da2a..7d62fa51932 100644
--- a/absl/random/BUILD.bazel
+++ b/absl/random/BUILD.bazel
@@ -16,7 +16,9 @@
 
 # ABSL random-number generation libraries.
 
-load("@rules_cc//cc:defs.bzl", "cc_binary", "cc_library", "cc_test")
+load("@rules_cc//cc:cc_binary.bzl", "cc_binary")
+load("@rules_cc//cc:cc_library.bzl", "cc_library")
+load("@rules_cc//cc:cc_test.bzl", "cc_test")
 load(
     "//absl:copts/configure_copts.bzl",
     "ABSL_DEFAULT_COPTS",
@@ -24,21 +26,29 @@ load(
     "ABSL_TEST_COPTS",
 )
 
-package(default_visibility = ["//visibility:public"])
+package(
+    default_visibility = ["//visibility:private"],
+    features = [
+        "header_modules",
+        "layering_check",
+        "parse_headers",
+    ],
+)
 
-licenses(["notice"])  # Apache 2.0
+licenses(["notice"])
 
 cc_library(
     name = "random",
     hdrs = ["random.h"],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
     deps = [
         ":distributions",
         ":seed_sequences",
+        "//absl/base:config",
         "//absl/random/internal:nonsecure_base",
         "//absl/random/internal:pcg_engine",
-        "//absl/random/internal:pool_urbg",
         "//absl/random/internal:randen_engine",
     ],
 )
@@ -53,7 +63,6 @@ cc_library(
         "bernoulli_distribution.h",
         "beta_distribution.h",
         "discrete_distribution.h",
-        "distribution_format_traits.h",
         "distributions.h",
         "exponential_distribution.h",
         "gaussian_distribution.h",
@@ -65,19 +74,21 @@ cc_library(
     ],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
     deps = [
         "//absl/base:base_internal",
+        "//absl/base:config",
         "//absl/base:core_headers",
         "//absl/meta:type_traits",
-        "//absl/random/internal:distribution_impl",
-        "//absl/random/internal:distributions",
+        "//absl/random/internal:distribution_caller",
         "//absl/random/internal:fast_uniform_bits",
         "//absl/random/internal:fastmath",
+        "//absl/random/internal:generate_real",
         "//absl/random/internal:iostream_state_saver",
         "//absl/random/internal:traits",
         "//absl/random/internal:uniform_helper",
+        "//absl/random/internal:wide_multiply",
         "//absl/strings",
-        "//absl/types:span",
     ],
 )
 
@@ -87,7 +98,11 @@ cc_library(
     hdrs = ["seed_gen_exception.h"],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
-    deps = ["//absl/base:config"],
+    visibility = ["//visibility:public"],
+    deps = [
+        "//absl/base:config",
+        "//absl/base:raw_logging_internal",
+    ],
 )
 
 cc_library(
@@ -98,17 +113,84 @@ cc_library(
     ],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
     deps = [
         ":seed_gen_exception",
-        "//absl/container:inlined_vector",
-        "//absl/random/internal:nonsecure_base",
-        "//absl/random/internal:pool_urbg",
+        "//absl/base:config",
+        "//absl/base:nullability",
+        "//absl/random/internal:entropy_pool",
         "//absl/random/internal:salted_seed_seq",
         "//absl/random/internal:seed_material",
+        "//absl/strings:string_view",
         "//absl/types:span",
     ],
 )
 
+cc_library(
+    name = "bit_gen_ref",
+    hdrs = ["bit_gen_ref.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
+    deps = [
+        ":mocking_access",
+        ":random",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:fast_type_id",
+        "//absl/meta:type_traits",
+        "//absl/random/internal:fast_uniform_bits",
+        "//absl/random/internal:traits",
+    ],
+)
+
+cc_library(
+    name = "mock_distributions",
+    testonly = True,
+    hdrs = ["mock_distributions.h"],
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
+    deps = [
+        ":distributions",
+        ":mocking_bit_gen",
+        "//absl/base:config",
+        "//absl/random/internal:mock_overload_set",
+        "//absl/random/internal:mock_validators",
+    ],
+)
+
+cc_library(
+    name = "mocking_bit_gen",
+    testonly = True,
+    hdrs = [
+        "mocking_bit_gen.h",
+    ],
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
+    deps = [
+        ":mocking_access",
+        ":random",
+        "//absl/base:config",
+        "//absl/base:fast_type_id",
+        "//absl/container:flat_hash_map",
+        "//absl/meta:type_traits",
+        "//absl/random/internal:mock_helpers",
+        "//absl/utility",
+        "@googletest//:gtest",
+    ],
+)
+
+cc_library(
+    name = "mocking_access",
+    hdrs = ["mocking_access.h"],
+    visibility = ["//visibility:public"],
+    deps = [
+        "//absl/base:config",
+        "//absl/base:fast_type_id",
+        "//absl/meta:type_traits",
+    ],
+)
+
 cc_test(
     name = "bernoulli_distribution_test",
     size = "small",
@@ -119,8 +201,10 @@ cc_test(
     deps = [
         ":distributions",
         ":random",
+        "//absl/random/internal:pcg_engine",
         "//absl/random/internal:sequence_urbg",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -130,22 +214,30 @@ cc_test(
     timeout = "eternal",  # Android can take a very long time
     srcs = ["beta_distribution_test.cc"],
     copts = ABSL_TEST_COPTS,
+    flaky = 1,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    tags = [
+        "no_test_wasm",
+    ],
     deps = [
         ":distributions",
         ":random",
-        "//absl/base:raw_logging_internal",
+        "//absl/log",
+        "//absl/numeric:representation",
         "//absl/random/internal:distribution_test_util",
+        "//absl/random/internal:pcg_engine",
         "//absl/random/internal:sequence_urbg",
         "//absl/strings",
         "//absl/strings:str_format",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
 cc_test(
     name = "distributions_test",
     size = "small",
+    timeout = "moderate",
     srcs = [
         "distributions_test.cc",
     ],
@@ -154,8 +246,11 @@ cc_test(
     deps = [
         ":distributions",
         ":random",
+        "//absl/meta:type_traits",
+        "//absl/numeric:int128",
         "//absl/random/internal:distribution_test_util",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -168,7 +263,9 @@ cc_test(
     deps = [
         ":distributions",
         ":random",
-        "@com_google_googletest//:gtest_main",
+        "//absl/numeric:int128",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -180,16 +277,21 @@ cc_test(
     ],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    tags = [
+        "no_test_wasm",  # Does not converge on WASM.
+    ],
     deps = [
         ":distributions",
         ":random",
         "//absl/base:core_headers",
-        "//absl/base:raw_logging_internal",
+        "//absl/log",
         "//absl/random/internal:distribution_test_util",
+        "//absl/random/internal:pcg_engine",
         "//absl/random/internal:sequence_urbg",
         "//absl/strings",
         "//absl/strings:str_format",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -204,11 +306,13 @@ cc_test(
     deps = [
         ":distributions",
         ":random",
-        "//absl/base:raw_logging_internal",
+        "//absl/log",
         "//absl/random/internal:distribution_test_util",
+        "//absl/random/internal:pcg_engine",
         "//absl/random/internal:sequence_urbg",
         "//absl/strings",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -230,19 +334,22 @@ cc_test(
         ":distributions",
         ":random",
         "//absl/base:core_headers",
-        "//absl/base:raw_logging_internal",
         "//absl/container:flat_hash_map",
+        "//absl/log",
         "//absl/random/internal:distribution_test_util",
+        "//absl/random/internal:pcg_engine",
         "//absl/random/internal:sequence_urbg",
         "//absl/strings",
         "//absl/strings:str_format",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
 cc_test(
     name = "exponential_distribution_test",
     size = "small",
+    timeout = "moderate",
     srcs = ["exponential_distribution_test.cc"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
@@ -250,12 +357,15 @@ cc_test(
         ":distributions",
         ":random",
         "//absl/base:core_headers",
-        "//absl/base:raw_logging_internal",
+        "//absl/log",
+        "//absl/numeric:representation",
         "//absl/random/internal:distribution_test_util",
+        "//absl/random/internal:pcg_engine",
         "//absl/random/internal:sequence_urbg",
         "//absl/strings",
         "//absl/strings:str_format",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -272,12 +382,14 @@ cc_test(
         ":distributions",
         ":random",
         "//absl/base:core_headers",
-        "//absl/base:raw_logging_internal",
+        "//absl/log",
+        "//absl/numeric:representation",
         "//absl/random/internal:distribution_test_util",
         "//absl/random/internal:sequence_urbg",
         "//absl/strings",
         "//absl/strings:str_format",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -293,11 +405,13 @@ cc_test(
     deps = [
         ":distributions",
         ":random",
-        "//absl/base:raw_logging_internal",
+        "//absl/log",
         "//absl/random/internal:distribution_test_util",
+        "//absl/random/internal:pcg_engine",
         "//absl/random/internal:sequence_urbg",
         "//absl/strings",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -317,11 +431,14 @@ cc_test(
     deps = [
         ":distributions",
         ":random",
-        "//absl/base:raw_logging_internal",
+        "//absl/log",
+        "//absl/numeric:representation",
         "//absl/random/internal:distribution_test_util",
+        "//absl/random/internal:pcg_engine",
         "//absl/random/internal:sequence_urbg",
         "//absl/strings",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -336,11 +453,70 @@ cc_test(
     deps = [
         ":distributions",
         ":random",
-        "//absl/base:raw_logging_internal",
+        "//absl/log",
         "//absl/random/internal:distribution_test_util",
+        "//absl/random/internal:pcg_engine",
         "//absl/random/internal:sequence_urbg",
         "//absl/strings",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "bit_gen_ref_test",
+    size = "small",
+    srcs = ["bit_gen_ref_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":bit_gen_ref",
+        ":random",
+        "//absl/base:config",
+        "//absl/base:fast_type_id",
+        "//absl/random/internal:sequence_urbg",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "mocking_bit_gen_test",
+    size = "small",
+    srcs = ["mocking_bit_gen_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    tags = [
+        "no_test_wasm",
+    ],
+    deps = [
+        ":bit_gen_ref",
+        ":mock_distributions",
+        ":mocking_bit_gen",
+        ":random",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "mock_distributions_test",
+    size = "small",
+    timeout = "moderate",
+    srcs = ["mock_distributions_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    tags = [
+        "no_test_wasm",
+    ],
+    deps = [
+        ":distributions",
+        ":mock_distributions",
+        ":mocking_bit_gen",
+        ":random",
+        "//absl/numeric:int128",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -350,9 +526,13 @@ cc_test(
     srcs = ["examples_test.cc"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    tags = [
+        "no_test_wasm",
+    ],
     deps = [
         ":random",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -366,40 +546,27 @@ cc_test(
         ":random",
         ":seed_sequences",
         "//absl/random/internal:nonsecure_base",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
-BENCHMARK_TAGS = [
-    "benchmark",
-    "no_test_android_arm",
-    "no_test_android_arm64",
-    "no_test_android_x86",
-    "no_test_darwin_x86_64",
-    "no_test_ios_x86_64",
-    "no_test_loonix",
-    "no_test_msvc_x64",
-    "no_test_wasm",
-]
-
 # Benchmarks for various methods / test utilities
 cc_binary(
     name = "benchmarks",
-    testonly = 1,
+    testonly = True,
     srcs = [
         "benchmarks.cc",
     ],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
-    tags = BENCHMARK_TAGS,
+    tags = ["benchmark"],
     deps = [
         ":distributions",
         ":random",
-        ":seed_sequences",
         "//absl/base:core_headers",
-        "//absl/meta:type_traits",
         "//absl/random/internal:fast_uniform_bits",
         "//absl/random/internal:randen_engine",
-        "@com_github_google_benchmark//:benchmark_main",
+        "@google_benchmark//:benchmark_main",
     ],
 )
diff --git a/absl/random/CMakeLists.txt b/absl/random/CMakeLists.txt
index 151cefa4bf1..4a13e84e7db 100644
--- a/absl/random/CMakeLists.txt
+++ b/absl/random/CMakeLists.txt
@@ -24,14 +24,161 @@ absl_cc_library(
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
+    absl::config
     absl::random_distributions
     absl::random_internal_nonsecure_base
     absl::random_internal_pcg_engine
-    absl::random_internal_pool_urbg
     absl::random_internal_randen_engine
     absl::random_seed_sequences
 )
 
+absl_cc_library(
+  NAME
+    random_bit_gen_ref
+  HDRS
+    "bit_gen_ref.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::config
+    absl::core_headers
+    absl::random_internal_fast_uniform_bits
+    absl::type_traits
+    absl::random_mocking_access
+    absl::random_internal_traits
+)
+
+absl_cc_test(
+  NAME
+    random_bit_gen_ref_test
+  SRCS
+    "bit_gen_ref_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::config
+    absl::random_bit_gen_ref
+    absl::random_random
+    absl::random_internal_sequence_urbg
+    absl::fast_type_id
+    GTest::gmock
+    GTest::gtest_main
+)
+
+absl_cc_library(
+  NAME
+    random_mocking_access
+  HDRS
+    "mocking_access.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::config
+    absl::fast_type_id
+    absl::type_traits
+)
+
+# Internal-only target, do not depend on directly.
+absl_cc_library(
+  NAME
+    random_internal_mock_helpers
+  HDRS
+    "internal/mock_helpers.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::config
+    absl::fast_type_id
+    absl::optional
+    absl::random_mocking_access
+)
+
+# Internal-only target, do not depend on directly.
+absl_cc_library(
+  NAME
+    random_internal_mock_overload_set
+  HDRS
+    "internal/mock_overload_set.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::config
+    absl::random_mocking_bit_gen
+    absl::random_internal_mock_helpers
+  TESTONLY
+)
+
+absl_cc_library(
+  NAME
+    random_mocking_bit_gen
+  HDRS
+    "mock_distributions.h"
+    "mocking_bit_gen.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::config
+    absl::core_headers
+    absl::fast_type_id
+    absl::flat_hash_map
+    absl::raw_logging_internal
+    absl::random_internal_mock_helpers
+    absl::random_mocking_access
+    absl::random_random
+    absl::type_traits
+    absl::utility
+    GTest::gmock
+    GTest::gtest
+  PUBLIC
+  TESTONLY
+)
+
+absl_cc_test(
+  NAME
+    random_mock_distributions_test
+  SRCS
+    "mock_distributions_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::random_distributions
+    absl::random_mocking_bit_gen
+    absl::random_random
+    GTest::gmock
+    GTest::gtest_main
+)
+
+absl_cc_test(
+  NAME
+    random_mocking_bit_gen_test
+  SRCS
+    "mocking_bit_gen_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::random_bit_gen_ref
+    absl::random_mocking_bit_gen
+    absl::random_random
+    GTest::gmock
+    GTest::gtest_main
+)
+
 absl_cc_library(
   NAME
     random_distributions
@@ -42,7 +189,6 @@ absl_cc_library(
     "bernoulli_distribution.h"
     "beta_distribution.h"
     "discrete_distribution.h"
-    "distribution_format_traits.h"
     "distributions.h"
     "exponential_distribution.h"
     "gaussian_distribution.h"
@@ -57,16 +203,17 @@ absl_cc_library(
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
     absl::base_internal
+    absl::config
     absl::core_headers
-    absl::random_internal_distribution_impl
-    absl::random_internal_distributions
+    absl::random_internal_generate_real
+    absl::random_internal_distribution_caller
     absl::random_internal_fast_uniform_bits
     absl::random_internal_fastmath
     absl::random_internal_iostream_state_saver
     absl::random_internal_traits
     absl::random_internal_uniform_helper
+    absl::random_internal_wide_multiply
     absl::strings
-    absl::span
     absl::type_traits
 )
 
@@ -83,6 +230,7 @@ absl_cc_library(
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
     absl::config
+    absl::raw_logging_internal
 )
 
 absl_cc_library(
@@ -97,13 +245,15 @@ absl_cc_library(
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
+    absl::config
     absl::inlined_vector
-    absl::random_internal_nonsecure_base
-    absl::random_internal_pool_urbg
+    absl::nullability
+    absl::random_internal_entropy_pool
     absl::random_internal_salted_seed_seq
     absl::random_internal_seed_material
     absl::random_seed_gen_exception
     absl::span
+    absl::string_view
 )
 
 absl_cc_test(
@@ -119,8 +269,9 @@ absl_cc_test(
     absl::random_distributions
     absl::random_random
     absl::random_internal_sequence_urbg
-    gmock
-    gtest_main
+    absl::random_internal_pcg_engine
+    GTest::gmock
+    GTest::gtest_main
 )
 
 absl_cc_test(
@@ -133,15 +284,17 @@ absl_cc_test(
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
+    absl::log
+    absl::numeric_representation
     absl::random_distributions
     absl::random_random
     absl::random_internal_distribution_test_util
     absl::random_internal_sequence_urbg
-    absl::raw_logging_internal
+    absl::random_internal_pcg_engine
     absl::strings
     absl::str_format
-    gmock
-    gtest_main
+    GTest::gmock
+    GTest::gtest_main
 )
 
 absl_cc_test(
@@ -156,9 +309,11 @@ absl_cc_test(
   DEPS
     absl::random_distributions
     absl::random_random
+    absl::type_traits
+    absl::int128
     absl::random_internal_distribution_test_util
-    gmock
-    gtest_main
+    GTest::gmock
+    GTest::gtest_main
 )
 
 absl_cc_test(
@@ -170,11 +325,12 @@ absl_cc_test(
     ${ABSL_TEST_COPTS}
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
     absl::random_distributions
     absl::random_random
     absl::raw_logging_internal
-    gmock
-    gtest_main
+    GTest::gmock
+    GTest::gtest_main
 )
 
 absl_cc_test(
@@ -186,16 +342,17 @@ absl_cc_test(
     ${ABSL_TEST_COPTS}
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
-    absl::core_headers
+  DEPS
+    absl::log
     absl::random_distributions
     absl::random_internal_distribution_test_util
+    absl::random_internal_pcg_engine
     absl::random_internal_sequence_urbg
     absl::random_random
-    absl::raw_logging_internal
     absl::strings
     absl::str_format
-    gmock
-    gtest_main
+    GTest::gmock
+    GTest::gtest_main
 )
 
 absl_cc_test(
@@ -208,14 +365,15 @@ absl_cc_test(
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
+    absl::log
     absl::random_distributions
     absl::random_internal_distribution_test_util
+    absl::random_internal_pcg_engine
     absl::random_internal_sequence_urbg
     absl::random_random
-    absl::raw_logging_internal
     absl::strings
-    gmock
-    gtest_main
+    GTest::gmock
+    GTest::gtest_main
 )
 
 absl_cc_test(
@@ -232,13 +390,14 @@ absl_cc_test(
     absl::random_random
     absl::core_headers
     absl::flat_hash_map
+    absl::log
     absl::random_internal_distribution_test_util
+    absl::random_internal_pcg_engine
     absl::random_internal_sequence_urbg
-    absl::raw_logging_internal
     absl::strings
     absl::str_format
-    gmock
-    gtest_main
+    GTest::gmock
+    GTest::gtest_main
 )
 
 absl_cc_test(
@@ -252,15 +411,17 @@ absl_cc_test(
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
     absl::core_headers
+    absl::log
+    absl::numeric_representation
     absl::random_distributions
     absl::random_internal_distribution_test_util
+    absl::random_internal_pcg_engine
     absl::random_internal_sequence_urbg
     absl::random_random
-    absl::raw_logging_internal
     absl::strings
     absl::str_format
-    gmock
-    gtest_main
+    GTest::gmock
+    GTest::gtest_main
 )
 
 absl_cc_test(
@@ -274,15 +435,16 @@ absl_cc_test(
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
     absl::core_headers
+    absl::log
+    absl::numeric_representation
     absl::random_distributions
     absl::random_internal_distribution_test_util
     absl::random_internal_sequence_urbg
     absl::random_random
-    absl::raw_logging_internal
     absl::strings
     absl::str_format
-    gmock
-    gtest_main
+    GTest::gmock
+    GTest::gtest_main
 )
 
 absl_cc_test(
@@ -295,14 +457,15 @@ absl_cc_test(
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
+    absl::log
     absl::random_distributions
     absl::random_internal_distribution_test_util
+    absl::random_internal_pcg_engine
     absl::random_internal_sequence_urbg
     absl::random_random
-    absl::raw_logging_internal
     absl::strings
-    gmock
-    gtest_main
+    GTest::gmock
+    GTest::gtest_main
 )
 
 absl_cc_test(
@@ -315,13 +478,16 @@ absl_cc_test(
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
+    absl::log
+    absl::numeric_representation
     absl::random_distributions
     absl::random_internal_distribution_test_util
+    absl::random_internal_pcg_engine
     absl::random_internal_sequence_urbg
     absl::random_random
     absl::strings
-    gmock
-    gtest_main
+    GTest::gmock
+    GTest::gtest_main
 )
 
 absl_cc_test(
@@ -334,14 +500,15 @@ absl_cc_test(
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
+    absl::log
     absl::random_distributions
     absl::random_internal_distribution_test_util
+    absl::random_internal_pcg_engine
     absl::random_internal_sequence_urbg
     absl::random_random
-    absl::raw_logging_internal
     absl::strings
-    gmock
-    gtest_main
+    GTest::gmock
+    GTest::gtest_main
 )
 
 absl_cc_test(
@@ -355,7 +522,7 @@ absl_cc_test(
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
     absl::random_random
-    gtest_main
+    GTest::gtest_main
 )
 
 absl_cc_test(
@@ -371,8 +538,8 @@ absl_cc_test(
     absl::random_seed_sequences
     absl::random_internal_nonsecure_base
     absl::random_random
-    gmock
-    gtest_main
+    GTest::gmock
+    GTest::gtest_main
 )
 
 # Internal-only target, do not depend on directly.
@@ -387,6 +554,9 @@ absl_cc_library(
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
     absl::config
+    absl::type_traits
+    absl::bits
+    absl::int128
 )
 
 # Internal-only target, do not depend on directly.
@@ -399,27 +569,12 @@ absl_cc_library(
     ${ABSL_DEFAULT_COPTS}
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
-)
-
-# Internal-only target, do not depend on directly.
-absl_cc_library(
-  NAME
-    random_internal_distributions
-  HDRS
-    "internal/distributions.h"
-  COPTS
-    ${ABSL_DEFAULT_COPTS}
-  LINKOPTS
-    ${ABSL_DEFAULT_LINKOPTS}
   DEPS
-    absl::random_internal_distribution_caller
-    absl::random_internal_fast_uniform_bits
-    absl::random_internal_fastmath
-    absl::random_internal_traits
-    absl::random_internal_uniform_helper
-    absl::span
-    absl::strings
+    absl::config
+    absl::utility
+    absl::fast_type_id
     absl::type_traits
+    absl::random_mocking_access
 )
 
 # Internal-only target, do not depend on directly.
@@ -432,6 +587,8 @@ absl_cc_library(
     ${ABSL_DEFAULT_COPTS}
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::config
 )
 
 # Internal-only target, do not depend on directly.
@@ -446,8 +603,9 @@ absl_cc_library(
     ${ABSL_DEFAULT_COPTS}
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
+    $<$<BOOL:${MINGW}>:-lbcrypt>
   DEPS
-    absl::core_headers
+    absl::config
     absl::optional
     absl::random_internal_fast_uniform_bits
     absl::raw_logging_internal
@@ -458,11 +616,11 @@ absl_cc_library(
 # Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
-    random_internal_pool_urbg
+    random_internal_entropy_pool
   SRCS
-    "internal/pool_urbg.cc"
+    "internal/entropy_pool.cc"
   HDRS
-    "internal/pool_urbg.h"
+    "internal/entropy_pool.h"
   COPTS
     ${ABSL_DEFAULT_COPTS}
   LINKOPTS
@@ -471,13 +629,12 @@ absl_cc_library(
     absl::base
     absl::config
     absl::core_headers
-    absl::endian
+    absl::random_internal_platform
     absl::random_internal_randen
     absl::random_internal_seed_material
-    absl::random_internal_traits
     absl::random_seed_gen_exception
-    absl::raw_logging_internal
     absl::span
+    absl::synchronization
 )
 
 # Internal-only target, do not depend on directly.
@@ -490,6 +647,9 @@ absl_cc_library(
     ${ABSL_DEFAULT_COPTS}
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::config
+    absl::endian
   TESTONLY
 )
 
@@ -503,6 +663,8 @@ absl_cc_library(
     ${ABSL_DEFAULT_COPTS}
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::config
   TESTONLY
 )
 
@@ -535,6 +697,7 @@ absl_cc_library(
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
+    absl::config
     absl::int128
     absl::type_traits
 )
@@ -542,19 +705,34 @@ absl_cc_library(
 # Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
-    random_internal_distribution_impl
+    random_internal_generate_real
   HDRS
-    "internal/distribution_impl.h"
+    "internal/generate_real.h"
   COPTS
     ${ABSL_DEFAULT_COPTS}
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
     absl::bits
-    absl::config
-    absl::int128
     absl::random_internal_fastmath
     absl::random_internal_traits
+    absl::type_traits
+)
+
+# Internal-only target, do not depend on directly.
+absl_cc_library(
+  NAME
+    random_internal_wide_multiply
+  HDRS
+    "internal/wide_multiply.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::bits
+    absl::config
+    absl::int128
 )
 
 # Internal-only target, do not depend on directly.
@@ -582,13 +760,11 @@ absl_cc_library(
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
-    absl::core_headers
-    absl::optional
-    absl::random_internal_pool_urbg
+    absl::config
+    absl::inlined_vector
+    absl::random_internal_entropy_pool
     absl::random_internal_salted_seed_seq
     absl::random_internal_seed_material
-    absl::span
-    absl::strings
     absl::type_traits
 )
 
@@ -621,6 +797,7 @@ absl_cc_library(
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
+    absl::endian
     absl::random_internal_iostream_state_saver
     absl::random_internal_randen
     absl::raw_logging_internal
@@ -633,12 +810,15 @@ absl_cc_library(
     random_internal_platform
   HDRS
     "internal/randen_traits.h"
-    "internal/randen-keys.inc"
     "internal/platform.h"
+  SRCS
+    "internal/randen_round_keys.cc"
   COPTS
     ${ABSL_DEFAULT_COPTS}
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::config
 )
 
 # Internal-only target, do not depend on directly.
@@ -673,6 +853,7 @@ absl_cc_library(
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
     absl::random_internal_platform
+    absl::config
 )
 
 # Internal-only target, do not depend on directly.
@@ -692,6 +873,7 @@ absl_cc_library(
   DEPS
     absl::random_internal_platform
     absl::random_internal_randen_hwaes_impl
+    absl::config
 )
 
 # Internal-only target, do not depend on directly.
@@ -708,6 +890,7 @@ absl_cc_library(
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
     absl::random_internal_platform
+    absl::config
 )
 
 # Internal-only target, do not depend on directly.
@@ -725,6 +908,7 @@ absl_cc_library(
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
+    absl::config
     absl::core_headers
     absl::raw_logging_internal
     absl::strings
@@ -744,15 +928,15 @@ absl_cc_test(
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
     absl::random_internal_traits
-    gtest_main
+    GTest::gtest_main
 )
 
 # Internal-only target, do not depend on directly.
 absl_cc_test(
   NAME
-    random_internal_distribution_impl_test
+    random_internal_generate_real_test
   SRCS
-    "internal/distribution_impl_test.cc"
+    "internal/generate_real_test.cc"
   COPTS
     ${ABSL_TEST_COPTS}
   LINKOPTS
@@ -760,9 +944,8 @@ absl_cc_test(
   DEPS
     absl::bits
     absl::flags
-    absl::int128
-    absl::random_internal_distribution_impl
-    gtest_main
+    absl::random_internal_generate_real
+    GTest::gtest_main
 )
 
 # Internal-only target, do not depend on directly.
@@ -777,7 +960,7 @@ absl_cc_test(
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
     absl::random_internal_distribution_test_util
-    gtest_main
+    GTest::gtest_main
 )
 
 # Internal-only target, do not depend on directly.
@@ -792,7 +975,7 @@ absl_cc_test(
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
     absl::random_internal_fastmath
-    gtest_main
+    GTest::gtest_main
 )
 
 # Internal-only target, do not depend on directly.
@@ -808,8 +991,8 @@ absl_cc_test(
   DEPS
     absl::random_internal_explicit_seed_seq
     absl::random_seed_sequences
-    gmock
-    gtest_main
+    GTest::gmock
+    GTest::gtest_main
 )
 
 # Internal-only target, do not depend on directly.
@@ -824,8 +1007,8 @@ absl_cc_test(
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
     absl::random_internal_salted_seed_seq
-    gmock
-    gtest_main
+    GTest::gmock
+    GTest::gtest_main
 )
 
 # Internal-only target, do not depend on directly.
@@ -841,7 +1024,7 @@ absl_cc_test(
   DEPS
     absl::core_headers
     absl::random_internal_distribution_test_util
-    gtest_main
+    GTest::gtest_main
 )
 
 # Internal-only target, do not depend on directly.
@@ -856,7 +1039,7 @@ absl_cc_test(
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
     absl::random_internal_fast_uniform_bits
-    gtest_main
+    GTest::gtest_main
 )
 
 # Internal-only target, do not depend on directly.
@@ -870,12 +1053,13 @@ absl_cc_test(
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
+    absl::flat_hash_map
+    absl::random_distributions
     absl::random_internal_nonsecure_base
     absl::random_random
-    absl::random_distributions
-    absl::random_seed_sequences
-    absl::strings
-    gtest_main
+    absl::synchronization
+    absl::type_traits
+    GTest::gtest_main
 )
 
 # Internal-only target, do not depend on directly.
@@ -890,25 +1074,26 @@ absl_cc_test(
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
     absl::random_internal_seed_material
-    gmock
-    gtest_main
+    absl::span
+    GTest::gmock
+    GTest::gtest_main
 )
 
 # Internal-only target, do not depend on directly.
 absl_cc_test(
   NAME
-    random_internal_pool_urbg_test
+    random_internal_entropy_pool_test
   SRCS
-    "internal/pool_urbg_test.cc"
+    "internal/entropy_pool_test.cc"
   COPTS
     ${ABSL_TEST_COPTS}
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
-    absl::random_internal_pool_urbg
-    absl::span
-    absl::type_traits
-    gtest_main
+    absl::flat_hash_map
+    absl::random_internal_entropy_pool
+    absl::synchronization
+    GTest::gtest_main
 )
 
 # Internal-only target, do not depend on directly.
@@ -925,8 +1110,8 @@ absl_cc_test(
     absl::random_internal_explicit_seed_seq
     absl::random_internal_pcg_engine
     absl::time
-    gmock
-    gtest_main
+    GTest::gmock
+    GTest::gtest_main
 )
 
 # Internal-only target, do not depend on directly.
@@ -940,13 +1125,13 @@ absl_cc_test(
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
+    absl::log
     absl::random_internal_explicit_seed_seq
     absl::random_internal_randen_engine
-    absl::raw_logging_internal
     absl::strings
     absl::time
-    gmock
-    gtest_main
+    GTest::gmock
+    GTest::gtest_main
 )
 
 # Internal-only target, do not depend on directly.
@@ -962,7 +1147,7 @@ absl_cc_test(
   DEPS
     absl::random_internal_randen
     absl::type_traits
-    gtest_main
+    GTest::gtest_main
 )
 
 # Internal-only target, do not depend on directly.
@@ -976,8 +1161,9 @@ absl_cc_test(
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
+    absl::endian
     absl::random_internal_randen_slow
-    gtest_main
+    GTest::gtest_main
 )
 
 # Internal-only target, do not depend on directly.
@@ -991,13 +1177,13 @@ absl_cc_test(
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
+    absl::log
     absl::random_internal_platform
     absl::random_internal_randen_hwaes
     absl::random_internal_randen_hwaes_impl
-    absl::raw_logging_internal
     absl::str_format
-    gmock
-    gtest
+    GTest::gmock
+    GTest::gtest
 )
 
 # Internal-only target, do not depend on directly.
@@ -1011,14 +1197,47 @@ absl_cc_library(
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
-    absl::core_headers
-    absl::random_internal_distribution_impl
-    absl::random_internal_fast_uniform_bits
-    absl::random_internal_iostream_state_saver
+    absl::config
     absl::random_internal_traits
     absl::type_traits
 )
 
+# Internal-only target, do not depend on directly.
+absl_cc_library(
+  NAME
+    random_internal_mock_validators
+  HDRS
+    "internal/mock_validators.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::random_internal_iostream_state_saver
+    absl::random_internal_uniform_helper
+    absl::config
+    absl::raw_logging_internal
+    absl::strings
+    absl::string_view
+  TESTONLY
+)
+
+# Internal-only target, do not depend on directly.
+absl_cc_test(
+  NAME
+    random_internal_uniform_helper_test
+  SRCS
+    "internal/uniform_helper_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::random_internal_uniform_helper
+    GTest::gtest_main
+    absl::int128
+)
+
 # Internal-only target, do not depend on directly.
 absl_cc_test(
   NAME
@@ -1031,5 +1250,22 @@ absl_cc_test(
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
     absl::random_internal_iostream_state_saver
-    gtest_main
+    GTest::gtest_main
+)
+
+# Internal-only target, do not depend on directly.
+absl_cc_test(
+  NAME
+    random_internal_wide_multiply_test
+  SRCS
+      internal/wide_multiply_test.cc
+  COPTS
+    ${ABSL_TEST_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::random_internal_wide_multiply
+    absl::int128
+    GTest::gmock
+    GTest::gtest_main
 )
diff --git a/absl/random/benchmarks.cc b/absl/random/benchmarks.cc
index 87bbb9810a7..c89388bf429 100644
--- a/absl/random/benchmarks.cc
+++ b/absl/random/benchmarks.cc
@@ -26,7 +26,6 @@
 #include <vector>
 
 #include "absl/base/macros.h"
-#include "absl/meta/type_traits.h"
 #include "absl/random/bernoulli_distribution.h"
 #include "absl/random/beta_distribution.h"
 #include "absl/random/exponential_distribution.h"
@@ -62,26 +61,26 @@ class PrecompiledSeedSeq {
  public:
   using result_type = uint32_t;
 
-  PrecompiledSeedSeq() {}
+  PrecompiledSeedSeq() = default;
 
   template <typename Iterator>
-  PrecompiledSeedSeq(Iterator begin, Iterator end) {}
+  PrecompiledSeedSeq(Iterator, Iterator) {}
 
   template <typename T>
-  PrecompiledSeedSeq(std::initializer_list<T> il) {}
+  PrecompiledSeedSeq(std::initializer_list<T>) {}
 
   template <typename OutIterator>
   void generate(OutIterator begin, OutIterator end) {
     static size_t idx = 0;
     for (; begin != end; begin++) {
       *begin = kSeedData[idx++];
-      if (idx >= ABSL_ARRAYSIZE(kSeedData)) {
+      if (idx >= std::size(kSeedData)) {
         idx = 0;
       }
     }
   }
 
-  size_t size() const { return ABSL_ARRAYSIZE(kSeedData); }
+  size_t size() const { return std::size(kSeedData); }
 
   template <typename OutIterator>
   void param(OutIterator out) const {
@@ -89,30 +88,23 @@ class PrecompiledSeedSeq {
   }
 };
 
-// use_default_initialization<T> indicates whether the random engine
-// T must be default initialized, or whether we may initialize it using
-// a seed sequence. This is used because some engines do not accept seed
-// sequence-based initialization.
-template <typename E>
-using use_default_initialization = std::false_type;
+// Triggers default constructor initialization.
+class DefaultConstructorSeedSeq {};
 
 // make_engine<T, SSeq> returns a random_engine which is initialized,
 // either via the default constructor, when use_default_initialization<T>
 // is true, or via the indicated seed sequence, SSeq.
-template <typename Engine, typename SSeq = PrecompiledSeedSeq>
-typename absl::enable_if_t<!use_default_initialization<Engine>::value, Engine>
-make_engine() {
-  // Initialize the random engine using the seed sequence SSeq, which
-  // is constructed from the precompiled seed data.
-  SSeq seq(std::begin(kSeedData), std::end(kSeedData));
-  return Engine(seq);
-}
-
-template <typename Engine, typename SSeq = PrecompiledSeedSeq>
-typename absl::enable_if_t<use_default_initialization<Engine>::value, Engine>
-make_engine() {
-  // Initialize the random engine using the default constructor.
-  return Engine();
+template <typename Engine, typename SSeq = DefaultConstructorSeedSeq>
+Engine make_engine() {
+  constexpr bool use_default_initialization =
+      std::is_same_v<SSeq, DefaultConstructorSeedSeq>;
+  if constexpr (use_default_initialization) {
+    return Engine();
+  } else {
+    // Otherwise, use the provided seed sequence.
+    SSeq seq(std::begin(kSeedData), std::end(kSeedData));
+    return Engine(seq);
+  }
 }
 
 template <typename Engine, typename SSeq>
@@ -248,6 +240,7 @@ void BM_Thread(benchmark::State& state) {
 
 // Normal benchmark suite
 #define BM_BASIC(Engine)                                                       \
+  BENCHMARK_TEMPLATE(BM_Construct, Engine, DefaultConstructorSeedSeq);         \
   BENCHMARK_TEMPLATE(BM_Construct, Engine, PrecompiledSeedSeq);                \
   BENCHMARK_TEMPLATE(BM_Construct, Engine, std::seed_seq);                     \
   BENCHMARK_TEMPLATE(BM_Direct, Engine);                                       \
@@ -291,7 +284,7 @@ void BM_Thread(benchmark::State& state) {
   BENCHMARK_TEMPLATE(BM_Shuffle, Engine, 100)->ThreadPerCpu();      \
   BENCHMARK_TEMPLATE(BM_Shuffle, Engine, 1000)->ThreadPerCpu();     \
   BENCHMARK_TEMPLATE(BM_ShuffleReuse, Engine, 100)->ThreadPerCpu(); \
-  BENCHMARK_TEMPLATE(BM_ShuffleReuse, Engine, 1000)->ThreadPerCpu();
+  BENCHMARK_TEMPLATE(BM_ShuffleReuse, Engine, 1000)->ThreadPerCpu()
 
 #define BM_EXTENDED(Engine)                                                    \
   /* -------------- Extended Uniform -----------------------*/                 \
@@ -355,11 +348,11 @@ void BM_Thread(benchmark::State& state) {
   BENCHMARK_TEMPLATE(BM_Beta, Engine, absl::beta_distribution<float>, 410,     \
                      580);                                                     \
   BENCHMARK_TEMPLATE(BM_Gamma, Engine, std::gamma_distribution<float>, 199);   \
-  BENCHMARK_TEMPLATE(BM_Gamma, Engine, std::gamma_distribution<double>, 199);
+  BENCHMARK_TEMPLATE(BM_Gamma, Engine, std::gamma_distribution<double>, 199)
 
 // ABSL Recommended interfaces.
 BM_BASIC(absl::InsecureBitGen);  // === pcg64_2018_engine
-BM_BASIC(absl::BitGen);    // === randen_engine<uint64_t>.
+BM_BASIC(absl::BitGen);          // === randen_engine<uint64_t>.
 BM_THREAD(absl::BitGen);
 BM_EXTENDED(absl::BitGen);
 
diff --git a/absl/random/bernoulli_distribution.h b/absl/random/bernoulli_distribution.h
index 326fcb6e012..10c24e62212 100644
--- a/absl/random/bernoulli_distribution.h
+++ b/absl/random/bernoulli_distribution.h
@@ -15,15 +15,18 @@
 #ifndef ABSL_RANDOM_BERNOULLI_DISTRIBUTION_H_
 #define ABSL_RANDOM_BERNOULLI_DISTRIBUTION_H_
 
+#include <cassert>
 #include <cstdint>
 #include <istream>
-#include <limits>
+#include <ostream>
 
+#include "absl/base/config.h"
 #include "absl/base/optimization.h"
 #include "absl/random/internal/fast_uniform_bits.h"
 #include "absl/random/internal/iostream_state_saver.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 // absl::bernoulli_distribution is a drop in replacement for
 // std::bernoulli_distribution. It guarantees that (given a perfect
@@ -137,16 +140,16 @@ bool bernoulli_distribution::Generate(double p,
     // 64 bits.
     //
     // Second, `c` is constructed by first casting explicitly to a signed
-    // integer and then converting implicitly to an unsigned integer of the same
+    // integer and then casting explicitly to an unsigned integer of the same
     // size.  This is done because the hardware conversion instructions produce
     // signed integers from double; if taken as a uint64_t the conversion would
     // be wrong for doubles greater than 2^63 (not relevant in this use-case).
     // If converted directly to an unsigned integer, the compiler would end up
     // emitting code to handle such large values that are not relevant due to
     // the known bounds on `c`.  To avoid these extra instructions this
-    // implementation converts first to the signed type and then use the
-    // implicit conversion to unsigned (which is a no-op).
-    const uint64_t c = static_cast<int64_t>(p * kP32);
+    // implementation converts first to the signed type and then convert to
+    // unsigned (which is a no-op).
+    const uint64_t c = static_cast<uint64_t>(static_cast<int64_t>(p * kP32));
     const uint32_t v = fast_u32(g);
     // FAST PATH: this path fails with probability 1/2^32.  Note that simply
     // returning v <= c would approximate P very well (up to an absolute error
@@ -193,6 +196,7 @@ bool bernoulli_distribution::Generate(double p,
   }
 }
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_BERNOULLI_DISTRIBUTION_H_
diff --git a/absl/random/bernoulli_distribution_test.cc b/absl/random/bernoulli_distribution_test.cc
index f2c3b99cd4f..9c04bf6c657 100644
--- a/absl/random/bernoulli_distribution_test.cc
+++ b/absl/random/bernoulli_distribution_test.cc
@@ -16,11 +16,14 @@
 
 #include <cmath>
 #include <cstddef>
+#include <limits>
 #include <random>
 #include <sstream>
+#include <string>
 #include <utility>
 
 #include "gtest/gtest.h"
+#include "absl/random/internal/pcg_engine.h"
 #include "absl/random/internal/sequence_urbg.h"
 #include "absl/random/random.h"
 
@@ -63,7 +66,10 @@ TEST_P(BernoulliTest, Accuracy) {
   size_t trials = para.second;
   double p = para.first;
 
-  absl::InsecureBitGen rng;
+  // We use a fixed bit generator for distribution accuracy tests.  This allows
+  // these tests to be deterministic, while still testing the qualify of the
+  // implementation.
+  absl::random_internal::pcg64_2018_engine rng(0x2B7E151628AED2A6);
 
   size_t yes = 0;
   absl::bernoulli_distribution dist(p);
@@ -131,7 +137,7 @@ TEST(BernoulliTest, StabilityTest) {
       0x275b0dc7e0a18acfull, 0x36cebe0d2653682eull, 0x0361e9b23861596bull,
   });
 
-  // Generate a std::string of '0' and '1' for the distribution output.
+  // Generate a string of '0' and '1' for the distribution output.
   auto generate = [&urbg](absl::bernoulli_distribution& dist) {
     std::string output;
     output.reserve(36);
@@ -176,7 +182,7 @@ TEST(BernoulliTest, StabilityTest2) {
        0xECDD4775619F1510ull, 0x13CCA830EB61BD96ull, 0x0334FE1EAA0363CFull,
        0xB5735C904C70A239ull, 0xD59E9E0BCBAADE14ull, 0xEECC86BC60622CA7ull});
 
-  // Generate a std::string of '0' and '1' for the distribution output.
+  // Generate a string of '0' and '1' for the distribution output.
   auto generate = [&urbg](absl::bernoulli_distribution& dist) {
     std::string output;
     output.reserve(13);
diff --git a/absl/random/beta_distribution.h b/absl/random/beta_distribution.h
index e29894f2037..a2cdd255496 100644
--- a/absl/random/beta_distribution.h
+++ b/absl/random/beta_distribution.h
@@ -17,17 +17,20 @@
 
 #include <cassert>
 #include <cmath>
+#include <cstdint>
 #include <istream>
 #include <limits>
 #include <ostream>
 #include <type_traits>
 
-#include "absl/random/internal/distribution_impl.h"
+#include "absl/base/config.h"
+#include "absl/meta/type_traits.h"
 #include "absl/random/internal/fast_uniform_bits.h"
-#include "absl/random/internal/fastmath.h"
+#include "absl/random/internal/generate_real.h"
 #include "absl/random/internal/iostream_state_saver.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 // absl::beta_distribution:
 // Generate a floating-point variate conforming to a Beta distribution:
@@ -179,20 +182,20 @@ class beta_distribution {
     result_type alpha_;
     result_type beta_;
 
-    result_type a_;  // the smaller of {alpha, beta}, or 1.0/alpha_ in JOEHNK
-    result_type b_;  // the larger of {alpha, beta}, or 1.0/beta_ in JOEHNK
-    result_type x_;  // alpha + beta, or the result in degenerate cases
-    result_type log_x_;  // log(x_)
-    result_type y_;      // "beta" in Cheng
-    result_type gamma_;  // "gamma" in Cheng
+    result_type a_{};  // the smaller of {alpha, beta}, or 1.0/alpha_ in JOEHNK
+    result_type b_{};  // the larger of {alpha, beta}, or 1.0/beta_ in JOEHNK
+    result_type x_{};  // alpha + beta, or the result in degenerate cases
+    result_type log_x_{};  // log(x_)
+    result_type y_{};      // "beta" in Cheng
+    result_type gamma_{};  // "gamma" in Cheng
 
-    Method method_;
+    Method method_{};
 
     // Placing this last for optimal alignment.
     // Whether alpha_ != a_, i.e. true iff alpha_ > beta_.
-    bool inverted_;
+    bool inverted_{};
 
-    static_assert(std::is_floating_point<RealType>::value,
+    static_assert(std::is_floating_point_v<RealType>,
                   "Class-template absl::beta_distribution<> must be "
                   "parameterized using a floating-point type.");
   };
@@ -275,19 +278,25 @@ typename beta_distribution<RealType>::result_type
 beta_distribution<RealType>::AlgorithmJoehnk(
     URBG& g,  // NOLINT(runtime/references)
     const param_type& p) {
+  using random_internal::GeneratePositiveTag;
+  using random_internal::GenerateRealFromBits;
+  using real_type =
+      std::conditional_t<std::is_same_v<RealType, float>, float, double>;
+
   // Based on Joehnk, M. D. Erzeugung von betaverteilten und gammaverteilten
   // Zufallszahlen. Metrika 8.1 (1964): 5-15.
   // This method is described in Knuth, Vol 2 (Third Edition), pp 134.
-  using RandU64ToReal = typename random_internal::RandU64ToReal<result_type>;
-  using random_internal::PositiveValueT;
+
   result_type u, v, x, y, z;
   for (;;) {
-    u = RandU64ToReal::template Value<PositiveValueT, false>(fast_u64_(g));
-    v = RandU64ToReal::template Value<PositiveValueT, false>(fast_u64_(g));
+    u = GenerateRealFromBits<real_type, GeneratePositiveTag, false>(
+        fast_u64_(g));
+    v = GenerateRealFromBits<real_type, GeneratePositiveTag, false>(
+        fast_u64_(g));
 
     // Direct method. std::pow is slow for float, so rely on the optimizer to
     // remove the std::pow() path for that case.
-    if (!std::is_same<float, result_type>::value) {
+    if (!std::is_same_v<float, result_type>) {
       x = std::pow(u, p.a_);
       y = std::pow(v, p.b_);
       z = x + y;
@@ -327,12 +336,14 @@ typename beta_distribution<RealType>::result_type
 beta_distribution<RealType>::AlgorithmCheng(
     URBG& g,  // NOLINT(runtime/references)
     const param_type& p) {
+  using random_internal::GeneratePositiveTag;
+  using random_internal::GenerateRealFromBits;
+  using real_type =
+      std::conditional_t<std::is_same_v<RealType, float>, float, double>;
+
   // Based on Cheng, Russell CH. Generating beta variates with nonintegral
   // shape parameters. Communications of the ACM 21.4 (1978): 317-322.
   // (https://dl.acm.org/citation.cfm?id=359482).
-  using RandU64ToReal = typename random_internal::RandU64ToReal<result_type>;
-  using random_internal::PositiveValueT;
-
   static constexpr result_type kLogFour =
       result_type(1.3862943611198906188344642429163531361);  // log(4)
   static constexpr result_type kS =
@@ -341,8 +352,10 @@ beta_distribution<RealType>::AlgorithmCheng(
   const bool use_algorithm_ba = (p.method_ == param_type::CHENG_BA);
   result_type u1, u2, v, w, z, r, s, t, bw_inv, lhs;
   for (;;) {
-    u1 = RandU64ToReal::template Value<PositiveValueT, false>(fast_u64_(g));
-    u2 = RandU64ToReal::template Value<PositiveValueT, false>(fast_u64_(g));
+    u1 = GenerateRealFromBits<real_type, GeneratePositiveTag, false>(
+        fast_u64_(g));
+    u2 = GenerateRealFromBits<real_type, GeneratePositiveTag, false>(
+        fast_u64_(g));
     v = p.y_ * std::log(u1 / (1 - u1));
     w = p.a_ * std::exp(v);
     bw_inv = result_type(1) / (p.b_ + w);
@@ -373,7 +386,6 @@ beta_distribution<RealType>::operator()(URBG& g,  // NOLINT(runtime/references)
     case param_type::JOEHNK:
       return AlgorithmJoehnk(g, p);
     case param_type::CHENG_BA:
-      ABSL_FALLTHROUGH_INTENDED;
     case param_type::CHENG_BB:
       return AlgorithmCheng(g, p);
     default:
@@ -409,6 +421,7 @@ std::basic_istream<CharT, Traits>& operator>>(
   return is;
 }
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_BETA_DISTRIBUTION_H_
diff --git a/absl/random/beta_distribution_test.cc b/absl/random/beta_distribution_test.cc
index 966ad08b5c9..7e06cb28354 100644
--- a/absl/random/beta_distribution_test.cc
+++ b/absl/random/beta_distribution_test.cc
@@ -15,20 +15,28 @@
 #include "absl/random/beta_distribution.h"
 
 #include <algorithm>
+#include <cfloat>
+#include <cmath>
 #include <cstddef>
 #include <cstdint>
+#include <cstdlib>
 #include <iterator>
+#include <limits>
 #include <random>
 #include <sstream>
 #include <string>
+#include <type_traits>
 #include <unordered_map>
+#include <utility>
 #include <vector>
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
-#include "absl/base/internal/raw_logging.h"
+#include "absl/log/log.h"
+#include "absl/numeric/internal/representation.h"
 #include "absl/random/internal/chi_square.h"
 #include "absl/random/internal/distribution_test_util.h"
+#include "absl/random/internal/pcg_engine.h"
 #include "absl/random/internal/sequence_urbg.h"
 #include "absl/random/random.h"
 #include "absl/strings/str_cat.h"
@@ -41,8 +49,27 @@ namespace {
 template <typename IntType>
 class BetaDistributionInterfaceTest : public ::testing::Test {};
 
-using RealTypes = ::testing::Types<float, double, long double>;
-TYPED_TEST_CASE(BetaDistributionInterfaceTest, RealTypes);
+constexpr bool ShouldExerciseLongDoubleTests() {
+  // long double arithmetic is not supported well by either GCC or Clang on
+  // most platforms specifically not when implemented in terms of double-double;
+  // see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99048,
+  // https://bugs.llvm.org/show_bug.cgi?id=49131, and
+  // https://bugs.llvm.org/show_bug.cgi?id=49132.
+  // So a conservative choice here is to disable long-double tests pretty much
+  // everywhere except on x64 but only if long double is not implemented as
+  // double-double.
+#if defined(__i686__) && defined(__x86_64__)
+  return !absl::numeric_internal::IsDoubleDouble();
+#else
+  return false;
+#endif
+}
+
+using RealTypes =
+    std::conditional_t<ShouldExerciseLongDoubleTests(),
+                       ::testing::Types<float, double, long double>,
+                       ::testing::Types<float, double>>;
+TYPED_TEST_SUITE(BetaDistributionInterfaceTest, RealTypes);
 
 TYPED_TEST(BetaDistributionInterfaceTest, SerializeTest) {
   // The threshold for whether std::exp(1/a) is finite.
@@ -52,9 +79,6 @@ TYPED_TEST(BetaDistributionInterfaceTest, SerializeTest) {
   const TypeParam kLargeA =
       std::exp(std::log((std::numeric_limits<TypeParam>::max)()) -
                std::log(std::log((std::numeric_limits<TypeParam>::max)())));
-  const TypeParam kLargeAPPC = std::exp(
-      std::log((std::numeric_limits<TypeParam>::max)()) -
-      std::log(std::log((std::numeric_limits<TypeParam>::max)())) - 10.0f);
   using param_type = typename absl::beta_distribution<TypeParam>::param_type;
 
   constexpr int kCount = 1000;
@@ -75,9 +99,6 @@ TYPED_TEST(BetaDistributionInterfaceTest, SerializeTest) {
       kLargeA,                                //
       std::nextafter(kLargeA, TypeParam(0)),  //
       std::nextafter(kLargeA, std::numeric_limits<TypeParam>::max()),
-      kLargeAPPC,  //
-      std::nextafter(kLargeAPPC, TypeParam(0)),
-      std::nextafter(kLargeAPPC, std::numeric_limits<TypeParam>::max()),
       // Boundary cases.
       std::numeric_limits<TypeParam>::max(),
       std::numeric_limits<TypeParam>::epsilon(),
@@ -91,8 +112,8 @@ TYPED_TEST(BetaDistributionInterfaceTest, SerializeTest) {
   };
   for (TypeParam alpha : kValues) {
     for (TypeParam beta : kValues) {
-      ABSL_INTERNAL_LOG(
-          INFO, absl::StrFormat("Smoke test for Beta(%f, %f)", alpha, beta));
+      LOG(INFO) << absl::StreamFormat("Smoke test for Beta(%a, %a)", alpha,
+                                      beta);
 
       param_type param(alpha, beta);
       absl::beta_distribution<TypeParam> before(alpha, beta);
@@ -124,28 +145,6 @@ TYPED_TEST(BetaDistributionInterfaceTest, SerializeTest) {
 
       ss >> after;
 
-#if defined(__powerpc64__) || defined(__PPC64__) || defined(__powerpc__) || \
-    defined(__ppc__) || defined(__PPC__)
-      if (std::is_same<TypeParam, long double>::value) {
-        // Roundtripping floating point values requires sufficient precision
-        // to reconstruct the exact value. It turns out that long double
-        // has some errors doing this on ppc.
-        if (alpha <= std::numeric_limits<double>::max() &&
-            alpha >= std::numeric_limits<double>::lowest()) {
-          EXPECT_EQ(static_cast<double>(before.alpha()),
-                    static_cast<double>(after.alpha()))
-              << ss.str();
-        }
-        if (beta <= std::numeric_limits<double>::max() &&
-            beta >= std::numeric_limits<double>::lowest()) {
-          EXPECT_EQ(static_cast<double>(before.beta()),
-                    static_cast<double>(after.beta()))
-              << ss.str();
-        }
-        continue;
-      }
-#endif
-
       EXPECT_EQ(before.alpha(), after.alpha());
       EXPECT_EQ(before.beta(), after.beta());
       EXPECT_EQ(before, after)           //
@@ -159,8 +158,12 @@ TYPED_TEST(BetaDistributionInterfaceTest, SerializeTest) {
 }
 
 TYPED_TEST(BetaDistributionInterfaceTest, DegenerateCases) {
+  // We use a fixed bit generator for distribution accuracy tests.  This allows
+  // these tests to be deterministic, while still testing the qualify of the
+  // implementation.
+  absl::random_internal::pcg64_2018_engine rng(0x2B7E151628AED2A6);
+
   // Extreme cases when the params are abnormal.
-  absl::InsecureBitGen gen;
   constexpr int kCount = 1000;
   const TypeParam kSmallValues[] = {
       std::numeric_limits<TypeParam>::min(),
@@ -186,7 +189,7 @@ TYPED_TEST(BetaDistributionInterfaceTest, DegenerateCases) {
         int ones = 0;
         absl::beta_distribution<TypeParam> d(alpha, beta);
         for (int i = 0; i < kCount; ++i) {
-          TypeParam x = d(gen);
+          TypeParam x = d(rng);
           if (x == 0.0) {
             zeros++;
           } else if (x == 1.0) {
@@ -212,7 +215,7 @@ TYPED_TEST(BetaDistributionInterfaceTest, DegenerateCases) {
       for (TypeParam beta : kLargeValues) {
         absl::beta_distribution<TypeParam> d(alpha, beta);
         for (int i = 0; i < kCount; ++i) {
-          EXPECT_EQ(d(gen), 0.0);
+          EXPECT_EQ(d(rng), 0.0);
         }
       }
     }
@@ -227,7 +230,7 @@ TYPED_TEST(BetaDistributionInterfaceTest, DegenerateCases) {
       for (TypeParam beta : kSmallValues) {
         absl::beta_distribution<TypeParam> d(alpha, beta);
         for (int i = 0; i < kCount; ++i) {
-          EXPECT_EQ(d(gen), 1.0);
+          EXPECT_EQ(d(rng), 1.0);
         }
       }
     }
@@ -237,7 +240,7 @@ TYPED_TEST(BetaDistributionInterfaceTest, DegenerateCases) {
     absl::beta_distribution<TypeParam> d(std::numeric_limits<TypeParam>::max(),
                                          std::numeric_limits<TypeParam>::max());
     for (int i = 0; i < kCount; ++i) {
-      EXPECT_EQ(d(gen), 0.5);
+      EXPECT_EQ(d(rng), 0.5);
     }
   }
   {
@@ -246,7 +249,7 @@ TYPED_TEST(BetaDistributionInterfaceTest, DegenerateCases) {
         std::numeric_limits<TypeParam>::max(),
         std::numeric_limits<TypeParam>::max() * 0.9999);
     for (int i = 0; i < kCount; ++i) {
-      TypeParam x = d(gen);
+      TypeParam x = d(rng);
       EXPECT_NE(x, 0.5f);
       EXPECT_FLOAT_EQ(x, 0.500025f);
     }
@@ -329,15 +332,13 @@ bool BetaDistributionTest::SingleZTestOnMeanAndVariance(double p,
       absl::random_internal::Near("z", z_mean, 0.0, max_err) &&
       absl::random_internal::Near("z_variance", z_variance, 0.0, max_err);
   if (!pass) {
-    ABSL_INTERNAL_LOG(
-        INFO,
-        absl::StrFormat(
-            "Beta(%f, %f), "
-            "mean: sample %f, expect %f, which is %f stddevs away, "
-            "variance: sample %f, expect %f, which is %f stddevs away.",
-            alpha_, beta_, m.mean, Mean(),
-            std::abs(m.mean - Mean()) / mean_stddev, m.variance, Variance(),
-            std::abs(m.variance - Variance()) / variance_stddev));
+    LOG(INFO) << "Beta(" << alpha_ << ", " << beta_ << "), mean: sample "
+              << m.mean << ", expect " << Mean() << ", which is "
+              << std::abs(m.mean - Mean()) / mean_stddev
+              << " stddevs away, variance: sample " << m.variance << ", expect "
+              << Variance() << ", which is "
+              << std::abs(m.variance - Variance()) / variance_stddev
+              << " stddevs away.";
   }
   return pass;
 }
@@ -398,18 +399,15 @@ bool BetaDistributionTest::SingleChiSquaredTest(double p, size_t samples,
   const bool pass =
       (absl::random_internal::ChiSquarePValue(chi_square, dof) >= p);
   if (!pass) {
-    for (int i = 0; i < cutoffs.size(); i++) {
-      ABSL_INTERNAL_LOG(
-          INFO, absl::StrFormat("cutoff[%d] = %f, actual count %d, expected %d",
-                                i, cutoffs[i], counts[i],
-                                static_cast<int>(expected[i])));
+    for (size_t i = 0; i < cutoffs.size(); i++) {
+      LOG(INFO) << "cutoff[" << i << "] = " << cutoffs[i] << ", actual count "
+                << counts[i] << ", expected " << static_cast<int>(expected[i]);
     }
 
-    ABSL_INTERNAL_LOG(
-        INFO, absl::StrFormat(
-                  "Beta(%f, %f) %s %f, p = %f", alpha_, beta_,
-                  absl::random_internal::kChiSquared, chi_square,
-                  absl::random_internal::ChiSquarePValue(chi_square, dof)));
+    LOG(INFO) << "Beta(" << alpha_ << ", " << beta_ << ") "
+              << absl::random_internal::kChiSquared << " " << chi_square
+              << ", p = "
+              << absl::random_internal::ChiSquarePValue(chi_square, dof);
   }
   return pass;
 }
@@ -443,13 +441,13 @@ std::string ParamName(
   return absl::StrReplaceAll(name, {{"+", "_"}, {"-", "_"}, {".", "_"}});
 }
 
-INSTANTIATE_TEST_CASE_P(
+INSTANTIATE_TEST_SUITE_P(
     TestSampleStatisticsCombinations, BetaDistributionTest,
     ::testing::Combine(::testing::Values(0.1, 0.2, 0.9, 1.1, 2.5, 10.0, 123.4),
                        ::testing::Values(0.1, 0.2, 0.9, 1.1, 2.5, 10.0, 123.4)),
     ParamName);
 
-INSTANTIATE_TEST_CASE_P(
+INSTANTIATE_TEST_SUITE_P(
     TestSampleStatistics_SelectedPairs, BetaDistributionTest,
     ::testing::Values(std::make_pair(0.5, 1000), std::make_pair(1000, 0.5),
                       std::make_pair(900, 1000), std::make_pair(10000, 20000),
@@ -571,6 +569,14 @@ TEST(BetaDistributionTest, StabilityTest) {
 // dependencies of the distribution change, such as RandU64ToDouble, then this
 // is also likely to change.
 TEST(BetaDistributionTest, AlgorithmBounds) {
+#if (defined(__i386__) || defined(_M_IX86)) && FLT_EVAL_METHOD != 0
+  // We're using an x87-compatible FPU, and intermediate operations are
+  // performed with 80-bit floats. This produces slightly different results from
+  // what we expect below.
+  GTEST_SKIP()
+      << "Skipping the test because we detected x87 floating-point semantics";
+#endif
+
   {
     absl::random_internal::sequence_urbg urbg(
         {0x7fbe76c8b4395800ull, 0x8000000000000000ull});
diff --git a/absl/random/bit_gen_ref.h b/absl/random/bit_gen_ref.h
new file mode 100644
index 00000000000..df8675b1234
--- /dev/null
+++ b/absl/random/bit_gen_ref.h
@@ -0,0 +1,160 @@
+//
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: bit_gen_ref.h
+// -----------------------------------------------------------------------------
+//
+// This header defines a bit generator "reference" class, for use in interfaces
+// that take both Abseil (e.g. `absl::BitGen`) and standard library (e.g.
+// `std::mt19937`) bit generators.
+
+#ifndef ABSL_RANDOM_BIT_GEN_REF_H_
+#define ABSL_RANDOM_BIT_GEN_REF_H_
+
+#include <cstdint>
+#include <limits>
+#include <type_traits>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/fast_type_id.h"
+#include "absl/meta/type_traits.h"
+#include "absl/random/internal/fast_uniform_bits.h"
+#include "absl/random/internal/traits.h"
+#include "absl/random/mocking_access.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+// -----------------------------------------------------------------------------
+// absl::BitGenRef
+// -----------------------------------------------------------------------------
+//
+// `absl::BitGenRef` is a type-erasing class that provides a generator-agnostic
+// non-owning "reference" interface for use in place of any specific uniform
+// random bit generator (URBG). This class may be used for both Abseil
+// (e.g. `absl::BitGen`, `absl::InsecureBitGen`) and Standard library (e.g
+// `std::mt19937`, `std::minstd_rand`) bit generators.
+//
+// Like other reference classes, `absl::BitGenRef` does not own the
+// underlying bit generator, and the underlying instance must outlive the
+// `absl::BitGenRef`.
+//
+// `absl::BitGenRef` is particularly useful when used with an
+// `absl::MockingBitGen` to test specific paths in functions which use random
+// values.
+//
+// Example:
+//    void TakesBitGenRef(absl::BitGenRef gen) {
+//      int x = absl::Uniform<int>(gen, 0, 1000);
+//    }
+//
+class BitGenRef {
+  template <template <class...> class Trait, class AlwaysVoid, class... Args>
+  struct detector : std::false_type {};
+  template <template <class...> class Trait, class... Args>
+  struct detector<Trait, std::void_t<Trait<Args...>>, Args...>
+      : std::true_type {};
+
+  template <typename T>
+  using has_conversion_operator_t =
+      decltype(std::declval<T>().operator BitGenRef());
+
+  template <typename T>
+  using HasConversionOperator = detector<has_conversion_operator_t, void, T>;
+
+ public:
+  BitGenRef(const BitGenRef&) = default;
+  BitGenRef(BitGenRef&&) = default;
+  BitGenRef& operator=(const BitGenRef&) = default;
+  BitGenRef& operator=(BitGenRef&&) = default;
+
+  template <typename URBGRef, typename URBG = absl::remove_cvref_t<URBGRef>,
+            typename std::enable_if_t<
+                (!std::is_same_v<URBG, BitGenRef> &&
+                 !std::is_base_of_v<BitGenRef, URBG> &&
+                 !HasConversionOperator<URBG>::value &&
+                 random_internal::is_urbg<URBG>::value &&
+                 !RandomMockingAccess::HasInvokeMock<URBG>::value)>* = nullptr>
+  BitGenRef(URBGRef&& gen ABSL_ATTRIBUTE_LIFETIME_BOUND)  // NOLINT
+      : t_erased_gen_ptr_(reinterpret_cast<uintptr_t>(&gen)),
+        mock_call_(NotAMock),
+        generate_impl_fn_(ImplFn<URBG>) {}
+
+  template <typename URBGRef, typename URBG = absl::remove_cvref_t<URBGRef>,
+            typename std::enable_if_t<
+                (!std::is_same_v<URBG, BitGenRef> &&
+                 !std::is_base_of_v<BitGenRef, URBG> &&
+                 !HasConversionOperator<URBG>::value &&
+                 random_internal::is_urbg<URBG>::value &&
+                 RandomMockingAccess::HasInvokeMock<URBG>::value)>* = nullptr>
+  BitGenRef(URBGRef&& gen ABSL_ATTRIBUTE_LIFETIME_BOUND)  // NOLINT
+      : t_erased_gen_ptr_(reinterpret_cast<uintptr_t>(&gen)),
+        mock_call_(MockCall<URBG>),
+        generate_impl_fn_(ImplFn<URBG>) {}
+
+  using result_type = uint64_t;
+
+  static constexpr result_type(min)() {
+    return (std::numeric_limits<result_type>::min)();
+  }
+
+  static constexpr result_type(max)() {
+    return (std::numeric_limits<result_type>::max)();
+  }
+
+  result_type operator()() { return generate_impl_fn_(t_erased_gen_ptr_); }
+
+ private:
+  using impl_fn = result_type (*)(uintptr_t);
+  using mock_call_fn = bool (*)(uintptr_t, FastTypeIdType, void*, void*);
+
+  template <typename URBG>
+  static result_type ImplFn(uintptr_t ptr) {
+    // Ensure that the return values from operator() fill the entire
+    // range promised by result_type, min() and max().
+    absl::random_internal::FastUniformBits<result_type> fast_uniform_bits;
+    return fast_uniform_bits(*reinterpret_cast<URBG*>(ptr));
+  }
+
+  // Get a type-erased InvokeMock pointer.
+  template <typename URBG>
+  static bool MockCall(uintptr_t gen_ptr, FastTypeIdType key_id,
+                       void* args_tuple, void* result) {
+    return RandomMockingAccess::InvokeMock(reinterpret_cast<URBG*>(gen_ptr),
+                                           key_id, args_tuple, result);
+  }
+  static bool NotAMock(uintptr_t, FastTypeIdType, void*, void*) {
+    return false;
+  }
+
+  inline bool InvokeMock(FastTypeIdType key_id, void* args_tuple,
+                         void* result) {
+    if (mock_call_ == NotAMock) return false;  // avoids an indirect call.
+    return mock_call_(t_erased_gen_ptr_, key_id, args_tuple, result);
+  }
+
+  uintptr_t t_erased_gen_ptr_;
+  mock_call_fn mock_call_;
+  impl_fn generate_impl_fn_;
+
+  friend class ::absl::RandomMockingAccess;  // for InvokeMock
+};
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_RANDOM_BIT_GEN_REF_H_
diff --git a/absl/random/bit_gen_ref_test.cc b/absl/random/bit_gen_ref_test.cc
new file mode 100644
index 00000000000..ae5c1db80a8
--- /dev/null
+++ b/absl/random/bit_gen_ref_test.cc
@@ -0,0 +1,150 @@
+//
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+#include "absl/random/bit_gen_ref.h"
+
+#include <cstdint>
+#include <random>
+#include <type_traits>
+#include <vector>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+#include "absl/base/fast_type_id.h"
+#include "absl/random/internal/sequence_urbg.h"
+#include "absl/random/random.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+class ConstBitGen {
+ public:
+  // URBG interface
+  using result_type = absl::BitGen::result_type;
+
+  static constexpr result_type(min)() { return (absl::BitGen::min)(); }
+  static constexpr result_type(max)() { return (absl::BitGen::max)(); }
+  result_type operator()() { return 1; }
+
+  // InvokeMock method
+  bool InvokeMock(FastTypeIdType, void*, void* result) {
+    *static_cast<int*>(result) = 42;
+    return true;
+  }
+};
+
+namespace {
+
+int FnTest(absl::BitGenRef gen_ref) { return absl::Uniform(gen_ref, 1, 7); }
+
+template <typename T>
+class BitGenRefTest : public testing::Test {};
+
+using BitGenTypes =
+    ::testing::Types<absl::BitGen, absl::InsecureBitGen, std::mt19937,
+                     std::mt19937_64, std::minstd_rand>;
+TYPED_TEST_SUITE(BitGenRefTest, BitGenTypes);
+
+TYPED_TEST(BitGenRefTest, BasicTest) {
+  TypeParam gen;
+  auto x = FnTest(gen);
+  EXPECT_NEAR(x, 4, 3);
+}
+
+TYPED_TEST(BitGenRefTest, Copyable) {
+  TypeParam gen;
+  absl::BitGenRef gen_ref(gen);
+  FnTest(gen_ref);  // Copy
+}
+
+TEST(BitGenRefTest, PassThroughEquivalence) {
+  // sequence_urbg returns 64-bit results.
+  absl::random_internal::sequence_urbg urbg(
+      {0x0003eb76f6f7f755ull, 0xFFCEA50FDB2F953Bull, 0xC332DDEFBE6C5AA5ull,
+       0x6558218568AB9702ull, 0x2AEF7DAD5B6E2F84ull, 0x1521B62829076170ull,
+       0xECDD4775619F1510ull, 0x13CCA830EB61BD96ull, 0x0334FE1EAA0363CFull,
+       0xB5735C904C70A239ull, 0xD59E9E0BCBAADE14ull, 0xEECC86BC60622CA7ull});
+
+  std::vector<uint64_t> output(12);
+
+  {
+    absl::BitGenRef view(urbg);
+    for (auto& v : output) {
+      v = view();
+    }
+  }
+
+  std::vector<uint64_t> expected(
+      {0x0003eb76f6f7f755ull, 0xFFCEA50FDB2F953Bull, 0xC332DDEFBE6C5AA5ull,
+       0x6558218568AB9702ull, 0x2AEF7DAD5B6E2F84ull, 0x1521B62829076170ull,
+       0xECDD4775619F1510ull, 0x13CCA830EB61BD96ull, 0x0334FE1EAA0363CFull,
+       0xB5735C904C70A239ull, 0xD59E9E0BCBAADE14ull, 0xEECC86BC60622CA7ull});
+
+  EXPECT_THAT(output, testing::Eq(expected));
+}
+
+TEST(BitGenRefTest, MockingBitGenBaseOverrides) {
+  ConstBitGen const_gen;
+  EXPECT_EQ(FnTest(const_gen), 42);
+
+  absl::BitGenRef gen_ref(const_gen);
+  EXPECT_EQ(FnTest(gen_ref), 42);  // Copy
+}
+
+struct MinStdRand {
+  // The URBG just returns 0.
+  using result_type = absl::BitGen::result_type;
+  static constexpr result_type(min)() { return (absl::BitGen::min)(); }
+  static constexpr result_type(max)() { return (absl::BitGen::max)(); }
+  result_type operator()() { return 0; }
+
+  // Implicit conversions allow passing MinStdRand to functions taking
+  // absl::BitGenRef as well as explicitly constructing an absl::BitGenRef from
+  // a MinStdRand.
+  operator absl::BitGenRef() const {
+    conversion_count++;
+    return absl::BitGenRef(minstd_gen);
+  }
+
+  std::minstd_rand minstd_gen;
+  mutable int conversion_count = 0;
+};
+
+TEST(BitGenRefTest, IsConvertibleTest) {
+  // Verify that MinStdRandBitGen is convertible to absl::BitGenRef.
+  EXPECT_TRUE((std::is_convertible_v<MinStdRand, absl::BitGenRef>));
+
+  // Explicit construction should trigger the conversion.
+  {
+    MinStdRand minstd;
+    absl::BitGenRef gen_ref(minstd);
+    EXPECT_EQ(minstd.conversion_count, 1);
+    (void)gen_ref;
+  }
+
+  // Calling a function that accepts absl::BitGenRef should trigger the
+  // conversion. This tests implicit conversion.
+  {
+    MinStdRand minstd;
+    auto result = FnTest(minstd);
+    EXPECT_EQ(minstd.conversion_count, 1);
+    EXPECT_GE(result, 1);
+  }
+}
+
+}  // namespace
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/random/discrete_distribution.cc b/absl/random/discrete_distribution.cc
index e6c09c5180b..247faa84fb1 100644
--- a/absl/random/discrete_distribution.cc
+++ b/absl/random/discrete_distribution.cc
@@ -14,7 +14,18 @@
 
 #include "absl/random/discrete_distribution.h"
 
+#include <cassert>
+#include <cmath>
+#include <cstddef>
+#include <iterator>
+#include <numeric>
+#include <utility>
+#include <vector>
+
+#include "absl/base/config.h"
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
 // Initializes the distribution table for Walker's Aliasing algorithm, described
@@ -93,4 +104,5 @@ std::vector<std::pair<double, size_t>> InitDiscreteDistribution(
 }
 
 }  // namespace random_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/random/discrete_distribution.h b/absl/random/discrete_distribution.h
index 1560f03c5fd..35d42c80b6a 100644
--- a/absl/random/discrete_distribution.h
+++ b/absl/random/discrete_distribution.h
@@ -16,26 +16,29 @@
 #define ABSL_RANDOM_DISCRETE_DISTRIBUTION_H_
 
 #include <cassert>
-#include <cmath>
+#include <cstddef>
+#include <initializer_list>
 #include <istream>
 #include <limits>
-#include <numeric>
+#include <ostream>
 #include <type_traits>
 #include <utility>
 #include <vector>
 
+#include "absl/base/config.h"
 #include "absl/random/bernoulli_distribution.h"
 #include "absl/random/internal/iostream_state_saver.h"
 #include "absl/random/uniform_int_distribution.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 // absl::discrete_distribution
 //
 // A discrete distribution produces random integers i, where 0 <= i < n
 // distributed according to the discrete probability function:
 //
-//     P(i|p0,...,pn−1)=pi
+//     P(i|p0,...,pn-1)=pi
 //
 // This class is an implementation of discrete_distribution (see
 // [rand.dist.samp.discrete]).
@@ -102,7 +105,7 @@ class discrete_distribution {
     std::vector<double> p_;                     // normalized probabilities
     std::vector<std::pair<double, size_t>> q_;  // (acceptance, alternate) pairs
 
-    static_assert(std::is_integral<result_type>::value,
+    static_assert(std::is_integral_v<result_type>,
                   "Class-template absl::discrete_distribution<> must be "
                   "parameterized using an integral type.");
   };
@@ -240,6 +243,7 @@ std::basic_istream<CharT, Traits>& operator>>(
   return is;
 }
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_DISCRETE_DISTRIBUTION_H_
diff --git a/absl/random/discrete_distribution_test.cc b/absl/random/discrete_distribution_test.cc
index 7296f0ac226..9d961f779da 100644
--- a/absl/random/discrete_distribution_test.cc
+++ b/absl/random/discrete_distribution_test.cc
@@ -22,13 +22,15 @@
 #include <random>
 #include <sstream>
 #include <string>
+#include <utility>
 #include <vector>
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
-#include "absl/base/internal/raw_logging.h"
+#include "absl/log/log.h"
 #include "absl/random/internal/chi_square.h"
 #include "absl/random/internal/distribution_test_util.h"
+#include "absl/random/internal/pcg_engine.h"
 #include "absl/random/internal/sequence_urbg.h"
 #include "absl/random/random.h"
 #include "absl/strings/str_cat.h"
@@ -98,6 +100,7 @@ TYPED_TEST(DiscreteDistributionTypeTest, Constructor) {
 }
 
 TEST(DiscreteDistributionTest, InitDiscreteDistribution) {
+  using testing::_;
   using testing::Pair;
 
   {
@@ -110,8 +113,8 @@ TEST(DiscreteDistributionTest, InitDiscreteDistribution) {
     // Each bucket is p=1/3, so bucket 0 will send half it's traffic
     // to bucket 2, while the rest will retain all of their traffic.
     EXPECT_THAT(q, testing::ElementsAre(Pair(0.5, 2),  //
-                                        Pair(1.0, 1),  //
-                                        Pair(1.0, 2)));
+                                        Pair(1.0, _),  //
+                                        Pair(1.0, _)));
   }
 
   {
@@ -134,7 +137,7 @@ TEST(DiscreteDistributionTest, InitDiscreteDistribution) {
 
     EXPECT_THAT(q, testing::ElementsAre(Pair(b0, 3),   //
                                         Pair(b1, 3),   //
-                                        Pair(1.0, 2),  //
+                                        Pair(1.0, _),  //
                                         Pair(b3, 2),   //
                                         Pair(b1, 3)));
   }
@@ -144,7 +147,7 @@ TEST(DiscreteDistributionTest, ChiSquaredTest50) {
   using absl::random_internal::kChiSquared;
 
   constexpr size_t kTrials = 10000;
-  constexpr int kBuckets = 50;  // inclusive, so actally +1
+  constexpr int kBuckets = 50;  // inclusive, so actually +1
 
   // 1-in-100000 threshold, but remember, there are about 8 tests
   // in this file. And the test could fail for other reasons.
@@ -156,7 +159,10 @@ TEST(DiscreteDistributionTest, ChiSquaredTest50) {
   std::iota(std::begin(weights), std::end(weights), 1);
   absl::discrete_distribution<int> dist(std::begin(weights), std::end(weights));
 
-  absl::InsecureBitGen rng;
+  // We use a fixed bit generator for distribution accuracy tests.  This allows
+  // these tests to be deterministic, while still testing the qualify of the
+  // implementation.
+  absl::random_internal::pcg64_2018_engine rng(0x2B7E151628AED2A6);
 
   std::vector<int32_t> counts(kBuckets, 0);
   for (size_t i = 0; i < kTrials; i++) {
@@ -189,13 +195,13 @@ TEST(DiscreteDistributionTest, ChiSquaredTest50) {
     absl::StrAppend(&msg, kChiSquared, " p-value ", p_value, "\n");
     absl::StrAppend(&msg, "High ", kChiSquared, " value: ", chi_square, " > ",
                     kThreshold);
-    ABSL_RAW_LOG(INFO, "%s", msg.c_str());
+    LOG(INFO) << msg;
     FAIL() << msg;
   }
 }
 
 TEST(DiscreteDistributionTest, StabilityTest) {
-  // absl::discrete_distribution stabilitiy relies on
+  // absl::discrete_distribution stability relies on
   // absl::uniform_int_distribution and absl::bernoulli_distribution.
   absl::random_internal::sequence_urbg urbg(
       {0x0003eb76f6f7f755ull, 0xFFCEA50FDB2F953Bull, 0xC332DDEFBE6C5AA5ull,
diff --git a/absl/random/distribution_format_traits.h b/absl/random/distribution_format_traits.h
deleted file mode 100644
index 3298c2cdb64..00000000000
--- a/absl/random/distribution_format_traits.h
+++ /dev/null
@@ -1,288 +0,0 @@
-//
-// Copyright 2018 The Abseil Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-//
-#ifndef ABSL_RANDOM_DISTRIBUTION_FORMAT_TRAITS_H_
-#define ABSL_RANDOM_DISTRIBUTION_FORMAT_TRAITS_H_
-
-#include <string>
-#include <tuple>
-#include <typeinfo>
-
-#include "absl/meta/type_traits.h"
-#include "absl/random/bernoulli_distribution.h"
-#include "absl/random/beta_distribution.h"
-#include "absl/random/exponential_distribution.h"
-#include "absl/random/gaussian_distribution.h"
-#include "absl/random/log_uniform_int_distribution.h"
-#include "absl/random/poisson_distribution.h"
-#include "absl/random/uniform_int_distribution.h"
-#include "absl/random/uniform_real_distribution.h"
-#include "absl/random/zipf_distribution.h"
-#include "absl/strings/str_cat.h"
-#include "absl/strings/str_join.h"
-#include "absl/strings/string_view.h"
-#include "absl/types/span.h"
-
-namespace absl {
-
-struct IntervalClosedClosedTag;
-struct IntervalClosedOpenTag;
-struct IntervalOpenClosedTag;
-struct IntervalOpenOpenTag;
-
-namespace random_internal {
-
-// ScalarTypeName defines a preferred hierarchy of preferred type names for
-// scalars, and is evaluated at compile time for the specific type
-// specialization.
-template <typename T>
-constexpr const char* ScalarTypeName() {
-  static_assert(std::is_integral<T>() || std::is_floating_point<T>(), "");
-  // clang-format off
-    return
-        std::is_same<T, float>::value ? "float" :
-        std::is_same<T, double>::value ? "double" :
-        std::is_same<T, long double>::value ? "long double" :
-        std::is_same<T, bool>::value ? "bool" :
-        std::is_signed<T>::value && sizeof(T) == 1 ? "int8_t" :
-        std::is_signed<T>::value && sizeof(T) == 2 ? "int16_t" :
-        std::is_signed<T>::value && sizeof(T) == 4 ? "int32_t" :
-        std::is_signed<T>::value && sizeof(T) == 8 ? "int64_t" :
-        std::is_unsigned<T>::value && sizeof(T) == 1 ? "uint8_t" :
-        std::is_unsigned<T>::value && sizeof(T) == 2 ? "uint16_t" :
-        std::is_unsigned<T>::value && sizeof(T) == 4 ? "uint32_t" :
-        std::is_unsigned<T>::value && sizeof(T) == 8 ? "uint64_t" :
-            "undefined";
-  // clang-format on
-
-  // NOTE: It would be nice to use typeid(T).name(), but that's an
-  // implementation-defined attribute which does not necessarily
-  // correspond to a name. We could potentially demangle it
-  // using, e.g. abi::__cxa_demangle.
-}
-
-// Distribution traits used by DistributionCaller and internal implementation
-// details of the mocking framework.
-/*
-struct DistributionFormatTraits {
-   // Returns the parameterized name of the distribution function.
-   static constexpr const char* FunctionName()
-   // Format DistrT parameters.
-   static std::string FormatArgs(DistrT& dist);
-   // Format DistrT::result_type results.
-   static std::string FormatResults(DistrT& dist);
-};
-*/
-template <typename DistrT>
-struct DistributionFormatTraits;
-
-template <typename R>
-struct DistributionFormatTraits<absl::uniform_int_distribution<R>> {
-  using distribution_t = absl::uniform_int_distribution<R>;
-  using result_t = typename distribution_t::result_type;
-
-  static constexpr const char* Name() { return "Uniform"; }
-
-  static std::string FunctionName() {
-    return absl::StrCat(Name(), "<", ScalarTypeName<R>(), ">");
-  }
-  static std::string FormatArgs(const distribution_t& d) {
-    return absl::StrCat("absl::IntervalClosedClosed, ", (d.min)(), ", ",
-                        (d.max)());
-  }
-  static std::string FormatResults(absl::Span<const result_t> results) {
-    return absl::StrJoin(results, ", ");
-  }
-};
-
-template <typename R>
-struct DistributionFormatTraits<absl::uniform_real_distribution<R>> {
-  using distribution_t = absl::uniform_real_distribution<R>;
-  using result_t = typename distribution_t::result_type;
-
-  static constexpr const char* Name() { return "Uniform"; }
-
-  static std::string FunctionName() {
-    return absl::StrCat(Name(), "<", ScalarTypeName<R>(), ">");
-  }
-  static std::string FormatArgs(const distribution_t& d) {
-    return absl::StrCat((d.min)(), ", ", (d.max)());
-  }
-  static std::string FormatResults(absl::Span<const result_t> results) {
-    return absl::StrJoin(results, ", ");
-  }
-};
-
-template <typename R>
-struct DistributionFormatTraits<absl::exponential_distribution<R>> {
-  using distribution_t = absl::exponential_distribution<R>;
-  using result_t = typename distribution_t::result_type;
-
-  static constexpr const char* Name() { return "Exponential"; }
-
-  static std::string FunctionName() {
-    return absl::StrCat(Name(), "<", ScalarTypeName<R>(), ">");
-  }
-  static std::string FormatArgs(const distribution_t& d) {
-    return absl::StrCat(d.lambda());
-  }
-  static std::string FormatResults(absl::Span<const result_t> results) {
-    return absl::StrJoin(results, ", ");
-  }
-};
-
-template <typename R>
-struct DistributionFormatTraits<absl::poisson_distribution<R>> {
-  using distribution_t = absl::poisson_distribution<R>;
-  using result_t = typename distribution_t::result_type;
-
-  static constexpr const char* Name() { return "Poisson"; }
-
-  static std::string FunctionName() {
-    return absl::StrCat(Name(), "<", ScalarTypeName<R>(), ">");
-  }
-  static std::string FormatArgs(const distribution_t& d) {
-    return absl::StrCat(d.mean());
-  }
-  static std::string FormatResults(absl::Span<const result_t> results) {
-    return absl::StrJoin(results, ", ");
-  }
-};
-
-template <>
-struct DistributionFormatTraits<absl::bernoulli_distribution> {
-  using distribution_t = absl::bernoulli_distribution;
-  using result_t = typename distribution_t::result_type;
-
-  static constexpr const char* Name() { return "Bernoulli"; }
-
-  static constexpr const char* FunctionName() { return Name(); }
-  static std::string FormatArgs(const distribution_t& d) {
-    return absl::StrCat(d.p());
-  }
-  static std::string FormatResults(absl::Span<const result_t> results) {
-    return absl::StrJoin(results, ", ");
-  }
-};
-
-template <typename R>
-struct DistributionFormatTraits<absl::beta_distribution<R>> {
-  using distribution_t = absl::beta_distribution<R>;
-  using result_t = typename distribution_t::result_type;
-
-  static constexpr const char* Name() { return "Beta"; }
-
-  static std::string FunctionName() {
-    return absl::StrCat(Name(), "<", ScalarTypeName<R>(), ">");
-  }
-  static std::string FormatArgs(const distribution_t& d) {
-    return absl::StrCat(d.alpha(), ", ", d.beta());
-  }
-  static std::string FormatResults(absl::Span<const result_t> results) {
-    return absl::StrJoin(results, ", ");
-  }
-};
-
-template <typename R>
-struct DistributionFormatTraits<absl::zipf_distribution<R>> {
-  using distribution_t = absl::zipf_distribution<R>;
-  using result_t = typename distribution_t::result_type;
-
-  static constexpr const char* Name() { return "Zipf"; }
-
-  static std::string FunctionName() {
-    return absl::StrCat(Name(), "<", ScalarTypeName<R>(), ">");
-  }
-  static std::string FormatArgs(const distribution_t& d) {
-    return absl::StrCat(d.k(), ", ", d.v(), ", ", d.q());
-  }
-  static std::string FormatResults(absl::Span<const result_t> results) {
-    return absl::StrJoin(results, ", ");
-  }
-};
-
-template <typename R>
-struct DistributionFormatTraits<absl::gaussian_distribution<R>> {
-  using distribution_t = absl::gaussian_distribution<R>;
-  using result_t = typename distribution_t::result_type;
-
-  static constexpr const char* Name() { return "Gaussian"; }
-
-  static std::string FunctionName() {
-    return absl::StrCat(Name(), "<", ScalarTypeName<R>(), ">");
-  }
-  static std::string FormatArgs(const distribution_t& d) {
-    return absl::StrJoin(std::make_tuple(d.mean(), d.stddev()), ", ");
-  }
-  static std::string FormatResults(absl::Span<const result_t> results) {
-    return absl::StrJoin(results, ", ");
-  }
-};
-
-template <typename R>
-struct DistributionFormatTraits<absl::log_uniform_int_distribution<R>> {
-  using distribution_t = absl::log_uniform_int_distribution<R>;
-  using result_t = typename distribution_t::result_type;
-
-  static constexpr const char* Name() { return "LogUniform"; }
-
-  static std::string FunctionName() {
-    return absl::StrCat(Name(), "<", ScalarTypeName<R>(), ">");
-  }
-  static std::string FormatArgs(const distribution_t& d) {
-    return absl::StrJoin(std::make_tuple((d.min)(), (d.max)(), d.base()), ", ");
-  }
-  static std::string FormatResults(absl::Span<const result_t> results) {
-    return absl::StrJoin(results, ", ");
-  }
-};
-
-template <typename TagType, typename NumType>
-struct UniformDistributionWrapper;
-
-template <typename TagType, typename NumType>
-struct DistributionFormatTraits<UniformDistributionWrapper<TagType, NumType>> {
-  using distribution_t = UniformDistributionWrapper<TagType, NumType>;
-  using result_t = NumType;
-
-  static constexpr const char* Name() { return "Uniform"; }
-
-  static std::string FunctionName() {
-    return absl::StrCat(Name(), "<", ScalarTypeName<NumType>(), ">");
-  }
-  static std::string FormatArgs(const distribution_t& d) {
-    absl::string_view tag;
-    if (std::is_same<TagType, IntervalClosedClosedTag>::value) {
-      tag = "IntervalClosedClosed";
-    } else if (std::is_same<TagType, IntervalClosedOpenTag>::value) {
-      tag = "IntervalClosedOpen";
-    } else if (std::is_same<TagType, IntervalOpenClosedTag>::value) {
-      tag = "IntervalOpenClosed";
-    } else if (std::is_same<TagType, IntervalOpenOpenTag>::value) {
-      tag = "IntervalOpenOpen";
-    } else {
-      tag = "[[unknown tag type]]";
-    }
-    return absl::StrCat(tag, ", ", (d.min)(), ", ", (d.max)());
-  }
-  static std::string FormatResults(absl::Span<const result_t> results) {
-    return absl::StrJoin(results, ", ");
-  }
-};
-
-}  // namespace random_internal
-}  // namespace absl
-
-#endif  // ABSL_RANDOM_DISTRIBUTION_FORMAT_TRAITS_H_
diff --git a/absl/random/distributions.h b/absl/random/distributions.h
index 3a4e93abfa2..8a032df60ca 100644
--- a/absl/random/distributions.h
+++ b/absl/random/distributions.h
@@ -32,8 +32,8 @@
 //     continuously and independently at a constant average rate
 //   * `absl::Gaussian` (also known as "normal distributions") for continuous
 //     distributions using an associated quadratic function
-//   * `absl::LogUniform` for continuous uniform distributions where the log
-//     to the given base of all values is uniform
+//   * `absl::LogUniform` for discrete distributions where the log to the given
+//     base of all values is uniform
 //   * `absl::Poisson` for discrete probability distributions that express the
 //     probability of a given number of events occurring within a fixed interval
 //   * `absl::Zipf` for discrete probability distributions commonly used for
@@ -46,35 +46,33 @@
 #ifndef ABSL_RANDOM_DISTRIBUTIONS_H_
 #define ABSL_RANDOM_DISTRIBUTIONS_H_
 
-#include <algorithm>
-#include <cmath>
 #include <limits>
-#include <random>
 #include <type_traits>
 
-#include "absl/base/internal/inline_variable.h"
+#include "absl/base/config.h"
+#include "absl/meta/type_traits.h"
 #include "absl/random/bernoulli_distribution.h"
 #include "absl/random/beta_distribution.h"
-#include "absl/random/distribution_format_traits.h"
 #include "absl/random/exponential_distribution.h"
 #include "absl/random/gaussian_distribution.h"
-#include "absl/random/internal/distributions.h"  // IWYU pragma: export
+#include "absl/random/internal/distribution_caller.h"  // IWYU pragma: export
+#include "absl/random/internal/traits.h"
 #include "absl/random/internal/uniform_helper.h"  // IWYU pragma: export
 #include "absl/random/log_uniform_int_distribution.h"
 #include "absl/random/poisson_distribution.h"
-#include "absl/random/uniform_int_distribution.h"
-#include "absl/random/uniform_real_distribution.h"
+#include "absl/random/uniform_int_distribution.h"  // IWYU pragma: export
+#include "absl/random/uniform_real_distribution.h"  // IWYU pragma: export
 #include "absl/random/zipf_distribution.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
-ABSL_INTERNAL_INLINE_CONSTEXPR(IntervalClosedClosedTag, IntervalClosedClosed,
-                               {});
-ABSL_INTERNAL_INLINE_CONSTEXPR(IntervalClosedClosedTag, IntervalClosed, {});
-ABSL_INTERNAL_INLINE_CONSTEXPR(IntervalClosedOpenTag, IntervalClosedOpen, {});
-ABSL_INTERNAL_INLINE_CONSTEXPR(IntervalOpenOpenTag, IntervalOpenOpen, {});
-ABSL_INTERNAL_INLINE_CONSTEXPR(IntervalOpenOpenTag, IntervalOpen, {});
-ABSL_INTERNAL_INLINE_CONSTEXPR(IntervalOpenClosedTag, IntervalOpenClosed, {});
+inline constexpr IntervalClosedClosedTag IntervalClosedClosed = {};
+inline constexpr IntervalClosedClosedTag IntervalClosed = {};
+inline constexpr IntervalClosedOpenTag IntervalClosedOpen = {};
+inline constexpr IntervalOpenOpenTag IntervalOpenOpen = {};
+inline constexpr IntervalOpenOpenTag IntervalOpen = {};
+inline constexpr IntervalOpenClosedTag IntervalOpenClosed = {};
 
 // -----------------------------------------------------------------------------
 // absl::Uniform<T>(tag, bitgen, lo, hi)
@@ -119,12 +117,19 @@ ABSL_INTERNAL_INLINE_CONSTEXPR(IntervalOpenClosedTag, IntervalOpenClosed, {});
 //   auto x = absl::Uniform<float>(bitgen, 0, 1);
 //
 template <typename R = void, typename TagType, typename URBG>
-typename absl::enable_if_t<!std::is_same<R, void>::value, R>  //
+typename std::enable_if_t<!std::is_same_v<R, void>, R>  //
 Uniform(TagType tag,
         URBG&& urbg,  // NOLINT(runtime/references)
         R lo, R hi) {
-  using gen_t = absl::decay_t<URBG>;
-  return random_internal::UniformImpl<R, TagType, gen_t>(tag, urbg, lo, hi);
+  using gen_t = std::decay_t<URBG>;
+  using distribution_t = random_internal::UniformDistributionWrapper<R>;
+
+  auto a = random_internal::uniform_lower_bound(tag, lo, hi);
+  auto b = random_internal::uniform_upper_bound(tag, lo, hi);
+  if (!random_internal::is_uniform_range_valid(a, b)) return lo;
+
+  return random_internal::DistributionCaller<gen_t>::template Call<
+      distribution_t>(&urbg, tag, lo, hi);
 }
 
 // absl::Uniform<T>(bitgen, lo, hi)
@@ -132,14 +137,19 @@ Uniform(TagType tag,
 // Overload of `Uniform()` using the default closed-open interval of [lo, hi),
 // and returning values of type `T`
 template <typename R = void, typename URBG>
-typename absl::enable_if_t<!std::is_same<R, void>::value, R>  //
+typename std::enable_if_t<!std::is_same_v<R, void>, R>  //
 Uniform(URBG&& urbg,  // NOLINT(runtime/references)
         R lo, R hi) {
+  using gen_t = std::decay_t<URBG>;
+  using distribution_t = random_internal::UniformDistributionWrapper<R>;
   constexpr auto tag = absl::IntervalClosedOpen;
-  using tag_t = decltype(tag);
-  using gen_t = absl::decay_t<URBG>;
 
-  return random_internal::UniformImpl<R, tag_t, gen_t>(tag, urbg, lo, hi);
+  auto a = random_internal::uniform_lower_bound(tag, lo, hi);
+  auto b = random_internal::uniform_upper_bound(tag, lo, hi);
+  if (!random_internal::is_uniform_range_valid(a, b)) return lo;
+
+  return random_internal::DistributionCaller<gen_t>::template Call<
+      distribution_t>(&urbg, lo, hi);
 }
 
 // absl::Uniform(tag, bitgen, lo, hi)
@@ -149,16 +159,22 @@ Uniform(URBG&& urbg,  // NOLINT(runtime/references)
 // correctly from the passed types.
 template <typename R = void, typename TagType, typename URBG, typename A,
           typename B>
-typename absl::enable_if_t<std::is_same<R, void>::value,
-                           random_internal::uniform_inferred_return_t<A, B>>
+typename std::enable_if_t<std::is_same_v<R, void>,
+                          random_internal::uniform_inferred_return_t<A, B>>
 Uniform(TagType tag,
         URBG&& urbg,  // NOLINT(runtime/references)
         A lo, B hi) {
-  using gen_t = absl::decay_t<URBG>;
+  using gen_t = std::decay_t<URBG>;
   using return_t = typename random_internal::uniform_inferred_return_t<A, B>;
+  using distribution_t = random_internal::UniformDistributionWrapper<return_t>;
+
+  auto a = random_internal::uniform_lower_bound<return_t>(tag, lo, hi);
+  auto b = random_internal::uniform_upper_bound<return_t>(tag, lo, hi);
+  if (!random_internal::is_uniform_range_valid(a, b)) return lo;
 
-  return random_internal::UniformImpl<return_t, TagType, gen_t>(tag, urbg, lo,
-                                                                hi);
+  return random_internal::DistributionCaller<gen_t>::template Call<
+      distribution_t>(&urbg, tag, static_cast<return_t>(lo),
+                      static_cast<return_t>(hi));
 }
 
 // absl::Uniform(bitgen, lo, hi)
@@ -167,17 +183,22 @@ Uniform(TagType tag,
 // default closed-open interval of [lo, hi). Note that a compile-error will
 // result if the return type cannot be deduced correctly from the passed types.
 template <typename R = void, typename URBG, typename A, typename B>
-typename absl::enable_if_t<std::is_same<R, void>::value,
-                           random_internal::uniform_inferred_return_t<A, B>>
+typename std::enable_if_t<std::is_same_v<R, void>,
+                          random_internal::uniform_inferred_return_t<A, B>>
 Uniform(URBG&& urbg,  // NOLINT(runtime/references)
         A lo, B hi) {
-  constexpr auto tag = absl::IntervalClosedOpen;
-  using tag_t = decltype(tag);
-  using gen_t = absl::decay_t<URBG>;
+  using gen_t = std::decay_t<URBG>;
   using return_t = typename random_internal::uniform_inferred_return_t<A, B>;
+  using distribution_t = random_internal::UniformDistributionWrapper<return_t>;
+
+  constexpr auto tag = absl::IntervalClosedOpen;
+  auto a = random_internal::uniform_lower_bound<return_t>(tag, lo, hi);
+  auto b = random_internal::uniform_upper_bound<return_t>(tag, lo, hi);
+  if (!random_internal::is_uniform_range_valid(a, b)) return lo;
 
-  return random_internal::UniformImpl<return_t, tag_t, gen_t>(tag, urbg, lo,
-                                                              hi);
+  return random_internal::DistributionCaller<gen_t>::template Call<
+      distribution_t>(&urbg, static_cast<return_t>(lo),
+                      static_cast<return_t>(hi));
 }
 
 // absl::Uniform<unsigned T>(bitgen)
@@ -185,15 +206,13 @@ Uniform(URBG&& urbg,  // NOLINT(runtime/references)
 // Overload of Uniform() using the minimum and maximum values of a given type
 // `T` (which must be unsigned), returning a value of type `unsigned T`
 template <typename R, typename URBG>
-typename absl::enable_if_t<!std::is_signed<R>::value, R>  //
+typename std::enable_if_t<!std::numeric_limits<R>::is_signed, R>  //
 Uniform(URBG&& urbg) {  // NOLINT(runtime/references)
-  constexpr auto tag = absl::IntervalClosedClosed;
-  constexpr auto lo = std::numeric_limits<R>::lowest();
-  constexpr auto hi = (std::numeric_limits<R>::max)();
-  using tag_t = decltype(tag);
-  using gen_t = absl::decay_t<URBG>;
+  using gen_t = std::decay_t<URBG>;
+  using distribution_t = random_internal::UniformDistributionWrapper<R>;
 
-  return random_internal::UniformImpl<R, tag_t, gen_t>(tag, urbg, lo, hi);
+  return random_internal::DistributionCaller<gen_t>::template Call<
+      distribution_t>(&urbg);
 }
 
 // -----------------------------------------------------------------------------
@@ -219,12 +238,11 @@ Uniform(URBG&& urbg) {  // NOLINT(runtime/references)
 template <typename URBG>
 bool Bernoulli(URBG&& urbg,  // NOLINT(runtime/references)
                double p) {
-  using gen_t = absl::decay_t<URBG>;
+  using gen_t = std::decay_t<URBG>;
   using distribution_t = absl::bernoulli_distribution;
-  using format_t = random_internal::DistributionFormatTraits<distribution_t>;
 
   return random_internal::DistributionCaller<gen_t>::template Call<
-      distribution_t, format_t>(&urbg, p);
+      distribution_t>(&urbg, p);
 }
 
 // -----------------------------------------------------------------------------
@@ -248,26 +266,28 @@ template <typename RealType, typename URBG>
 RealType Beta(URBG&& urbg,  // NOLINT(runtime/references)
               RealType alpha, RealType beta) {
   static_assert(
-      std::is_floating_point<RealType>::value,
+      std::is_floating_point_v<RealType>,
       "Template-argument 'RealType' must be a floating-point type, in "
       "absl::Beta<RealType, URBG>(...)");
 
-  using gen_t = absl::decay_t<URBG>;
+  using gen_t = std::decay_t<URBG>;
   using distribution_t = typename absl::beta_distribution<RealType>;
-  using format_t = random_internal::DistributionFormatTraits<distribution_t>;
 
   return random_internal::DistributionCaller<gen_t>::template Call<
-      distribution_t, format_t>(&urbg, alpha, beta);
+      distribution_t>(&urbg, alpha, beta);
 }
 
 // -----------------------------------------------------------------------------
 // absl::Exponential<T>(bitgen, lambda = 1)
 // -----------------------------------------------------------------------------
 //
-// `absl::Exponential` produces a floating point number for discrete
-// distributions of events occurring continuously and independently at a
-// constant average rate. `T` must be a floating point type, but may be inferred
-// from the type of `lambda`.
+// `absl::Exponential` produces a floating point number representing the
+// distance (time) between two consecutive events in a point process of events
+// occurring continuously and independently at a constant average rate `lambda`.
+// `T` must be a floating point type, but may be inferred from the type of
+// `lambda`.
+//
+// The mean of the distribution is 1/`lambda`.
 //
 // See https://en.wikipedia.org/wiki/Exponential_distribution.
 //
@@ -281,16 +301,15 @@ template <typename RealType, typename URBG>
 RealType Exponential(URBG&& urbg,  // NOLINT(runtime/references)
                      RealType lambda = 1) {
   static_assert(
-      std::is_floating_point<RealType>::value,
+      std::is_floating_point_v<RealType>,
       "Template-argument 'RealType' must be a floating-point type, in "
       "absl::Exponential<RealType, URBG>(...)");
 
-  using gen_t = absl::decay_t<URBG>;
+  using gen_t = std::decay_t<URBG>;
   using distribution_t = typename absl::exponential_distribution<RealType>;
-  using format_t = random_internal::DistributionFormatTraits<distribution_t>;
 
   return random_internal::DistributionCaller<gen_t>::template Call<
-      distribution_t, format_t>(&urbg, lambda);
+      distribution_t>(&urbg, lambda);
 }
 
 // -----------------------------------------------------------------------------
@@ -313,16 +332,15 @@ template <typename RealType, typename URBG>
 RealType Gaussian(URBG&& urbg,  // NOLINT(runtime/references)
                   RealType mean = 0, RealType stddev = 1) {
   static_assert(
-      std::is_floating_point<RealType>::value,
+      std::is_floating_point_v<RealType>,
       "Template-argument 'RealType' must be a floating-point type, in "
       "absl::Gaussian<RealType, URBG>(...)");
 
-  using gen_t = absl::decay_t<URBG>;
+  using gen_t = std::decay_t<URBG>;
   using distribution_t = typename absl::gaussian_distribution<RealType>;
-  using format_t = random_internal::DistributionFormatTraits<distribution_t>;
 
   return random_internal::DistributionCaller<gen_t>::template Call<
-      distribution_t, format_t>(&urbg, mean, stddev);
+      distribution_t>(&urbg, mean, stddev);
 }
 
 // -----------------------------------------------------------------------------
@@ -345,7 +363,7 @@ RealType Gaussian(URBG&& urbg,  // NOLINT(runtime/references)
 // If `lo` is nonzero then this distribution is shifted to the desired interval,
 // so LogUniform(lo, hi, b) is equivalent to LogUniform(0, hi-lo, b)+lo.
 //
-// See http://ecolego.facilia.se/ecolego/show/Log-Uniform%20Distribution
+// See https://en.wikipedia.org/wiki/Reciprocal_distribution
 //
 // Example:
 //
@@ -356,16 +374,15 @@ RealType Gaussian(URBG&& urbg,  // NOLINT(runtime/references)
 template <typename IntType, typename URBG>
 IntType LogUniform(URBG&& urbg,  // NOLINT(runtime/references)
                    IntType lo, IntType hi, IntType base = 2) {
-  static_assert(std::is_integral<IntType>::value,
+  static_assert(random_internal::IsIntegral<IntType>::value,
                 "Template-argument 'IntType' must be an integral type, in "
                 "absl::LogUniform<IntType, URBG>(...)");
 
-  using gen_t = absl::decay_t<URBG>;
+  using gen_t = std::decay_t<URBG>;
   using distribution_t = typename absl::log_uniform_int_distribution<IntType>;
-  using format_t = random_internal::DistributionFormatTraits<distribution_t>;
 
   return random_internal::DistributionCaller<gen_t>::template Call<
-      distribution_t, format_t>(&urbg, lo, hi, base);
+      distribution_t>(&urbg, lo, hi, base);
 }
 
 // -----------------------------------------------------------------------------
@@ -387,16 +404,15 @@ IntType LogUniform(URBG&& urbg,  // NOLINT(runtime/references)
 template <typename IntType, typename URBG>
 IntType Poisson(URBG&& urbg,  // NOLINT(runtime/references)
                 double mean = 1.0) {
-  static_assert(std::is_integral<IntType>::value,
+  static_assert(random_internal::IsIntegral<IntType>::value,
                 "Template-argument 'IntType' must be an integral type, in "
                 "absl::Poisson<IntType, URBG>(...)");
 
-  using gen_t = absl::decay_t<URBG>;
+  using gen_t = std::decay_t<URBG>;
   using distribution_t = typename absl::poisson_distribution<IntType>;
-  using format_t = random_internal::DistributionFormatTraits<distribution_t>;
 
   return random_internal::DistributionCaller<gen_t>::template Call<
-      distribution_t, format_t>(&urbg, mean);
+      distribution_t>(&urbg, mean);
 }
 
 // -----------------------------------------------------------------------------
@@ -420,18 +436,18 @@ template <typename IntType, typename URBG>
 IntType Zipf(URBG&& urbg,  // NOLINT(runtime/references)
              IntType hi = (std::numeric_limits<IntType>::max)(), double q = 2.0,
              double v = 1.0) {
-  static_assert(std::is_integral<IntType>::value,
+  static_assert(random_internal::IsIntegral<IntType>::value,
                 "Template-argument 'IntType' must be an integral type, in "
                 "absl::Zipf<IntType, URBG>(...)");
 
-  using gen_t = absl::decay_t<URBG>;
+  using gen_t = std::decay_t<URBG>;
   using distribution_t = typename absl::zipf_distribution<IntType>;
-  using format_t = random_internal::DistributionFormatTraits<distribution_t>;
 
   return random_internal::DistributionCaller<gen_t>::template Call<
-      distribution_t, format_t>(&urbg, hi, q, v);
+      distribution_t>(&urbg, hi, q, v);
 }
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_DISTRIBUTIONS_H_
diff --git a/absl/random/distributions_test.cc b/absl/random/distributions_test.cc
index 2d92723ad2b..9b081e955ce 100644
--- a/absl/random/distributions_test.cc
+++ b/absl/random/distributions_test.cc
@@ -14,12 +14,17 @@
 
 #include "absl/random/distributions.h"
 
+#include <cfloat>
 #include <cmath>
 #include <cstdint>
-#include <random>
+#include <limits>
+#include <type_traits>
+#include <utility>
 #include <vector>
 
 #include "gtest/gtest.h"
+#include "absl/meta/type_traits.h"
+#include "absl/numeric/int128.h"
 #include "absl/random/internal/distribution_test_util.h"
 #include "absl/random/random.h"
 
@@ -29,95 +34,6 @@ constexpr int kSize = 400000;
 
 class RandomDistributionsTest : public testing::Test {};
 
-TEST_F(RandomDistributionsTest, UniformBoundFunctions) {
-  using absl::IntervalClosedClosed;
-  using absl::IntervalClosedOpen;
-  using absl::IntervalOpenClosed;
-  using absl::IntervalOpenOpen;
-  using absl::random_internal::uniform_lower_bound;
-  using absl::random_internal::uniform_upper_bound;
-
-  // absl::uniform_int_distribution natively assumes IntervalClosedClosed
-  // absl::uniform_real_distribution natively assumes IntervalClosedOpen
-
-  EXPECT_EQ(uniform_lower_bound(IntervalOpenClosed, 0, 100), 1);
-  EXPECT_EQ(uniform_lower_bound(IntervalOpenOpen, 0, 100), 1);
-  EXPECT_GT(uniform_lower_bound<float>(IntervalOpenClosed, 0, 1.0), 0);
-  EXPECT_GT(uniform_lower_bound<float>(IntervalOpenOpen, 0, 1.0), 0);
-  EXPECT_GT(uniform_lower_bound<double>(IntervalOpenClosed, 0, 1.0), 0);
-  EXPECT_GT(uniform_lower_bound<double>(IntervalOpenOpen, 0, 1.0), 0);
-
-  EXPECT_EQ(uniform_lower_bound(IntervalClosedClosed, 0, 100), 0);
-  EXPECT_EQ(uniform_lower_bound(IntervalClosedOpen, 0, 100), 0);
-  EXPECT_EQ(uniform_lower_bound<float>(IntervalClosedClosed, 0, 1.0), 0);
-  EXPECT_EQ(uniform_lower_bound<float>(IntervalClosedOpen, 0, 1.0), 0);
-  EXPECT_EQ(uniform_lower_bound<double>(IntervalClosedClosed, 0, 1.0), 0);
-  EXPECT_EQ(uniform_lower_bound<double>(IntervalClosedOpen, 0, 1.0), 0);
-
-  EXPECT_EQ(uniform_upper_bound(IntervalOpenOpen, 0, 100), 99);
-  EXPECT_EQ(uniform_upper_bound(IntervalClosedOpen, 0, 100), 99);
-  EXPECT_EQ(uniform_upper_bound<float>(IntervalOpenOpen, 0, 1.0), 1.0);
-  EXPECT_EQ(uniform_upper_bound<float>(IntervalClosedOpen, 0, 1.0), 1.0);
-  EXPECT_EQ(uniform_upper_bound<double>(IntervalOpenOpen, 0, 1.0), 1.0);
-  EXPECT_EQ(uniform_upper_bound<double>(IntervalClosedOpen, 0, 1.0), 1.0);
-
-  EXPECT_EQ(uniform_upper_bound(IntervalOpenClosed, 0, 100), 100);
-  EXPECT_EQ(uniform_upper_bound(IntervalClosedClosed, 0, 100), 100);
-  EXPECT_GT(uniform_upper_bound<float>(IntervalOpenClosed, 0, 1.0), 1.0);
-  EXPECT_GT(uniform_upper_bound<float>(IntervalClosedClosed, 0, 1.0), 1.0);
-  EXPECT_GT(uniform_upper_bound<double>(IntervalOpenClosed, 0, 1.0), 1.0);
-  EXPECT_GT(uniform_upper_bound<double>(IntervalClosedClosed, 0, 1.0), 1.0);
-
-  // Negative value tests
-  EXPECT_EQ(uniform_lower_bound(IntervalOpenClosed, -100, -1), -99);
-  EXPECT_EQ(uniform_lower_bound(IntervalOpenOpen, -100, -1), -99);
-  EXPECT_GT(uniform_lower_bound<float>(IntervalOpenClosed, -2.0, -1.0), -2.0);
-  EXPECT_GT(uniform_lower_bound<float>(IntervalOpenOpen, -2.0, -1.0), -2.0);
-  EXPECT_GT(uniform_lower_bound<double>(IntervalOpenClosed, -2.0, -1.0), -2.0);
-  EXPECT_GT(uniform_lower_bound<double>(IntervalOpenOpen, -2.0, -1.0), -2.0);
-
-  EXPECT_EQ(uniform_lower_bound(IntervalClosedClosed, -100, -1), -100);
-  EXPECT_EQ(uniform_lower_bound(IntervalClosedOpen, -100, -1), -100);
-  EXPECT_EQ(uniform_lower_bound<float>(IntervalClosedClosed, -2.0, -1.0), -2.0);
-  EXPECT_EQ(uniform_lower_bound<float>(IntervalClosedOpen, -2.0, -1.0), -2.0);
-  EXPECT_EQ(uniform_lower_bound<double>(IntervalClosedClosed, -2.0, -1.0),
-            -2.0);
-  EXPECT_EQ(uniform_lower_bound<double>(IntervalClosedOpen, -2.0, -1.0), -2.0);
-
-  EXPECT_EQ(uniform_upper_bound(IntervalOpenOpen, -100, -1), -2);
-  EXPECT_EQ(uniform_upper_bound(IntervalClosedOpen, -100, -1), -2);
-  EXPECT_EQ(uniform_upper_bound<float>(IntervalOpenOpen, -2.0, -1.0), -1.0);
-  EXPECT_EQ(uniform_upper_bound<float>(IntervalClosedOpen, -2.0, -1.0), -1.0);
-  EXPECT_EQ(uniform_upper_bound<double>(IntervalOpenOpen, -2.0, -1.0), -1.0);
-  EXPECT_EQ(uniform_upper_bound<double>(IntervalClosedOpen, -2.0, -1.0), -1.0);
-
-  EXPECT_EQ(uniform_upper_bound(IntervalOpenClosed, -100, -1), -1);
-  EXPECT_EQ(uniform_upper_bound(IntervalClosedClosed, -100, -1), -1);
-  EXPECT_GT(uniform_upper_bound<float>(IntervalOpenClosed, -2.0, -1.0), -1.0);
-  EXPECT_GT(uniform_upper_bound<float>(IntervalClosedClosed, -2.0, -1.0), -1.0);
-  EXPECT_GT(uniform_upper_bound<double>(IntervalOpenClosed, -2.0, -1.0), -1.0);
-  EXPECT_GT(uniform_upper_bound<double>(IntervalClosedClosed, -2.0, -1.0),
-            -1.0);
-
-  // Edge cases: the next value toward itself is itself.
-  const double d = 1.0;
-  const float f = 1.0;
-  EXPECT_EQ(uniform_lower_bound(IntervalOpenClosed, d, d), d);
-  EXPECT_EQ(uniform_lower_bound(IntervalOpenClosed, f, f), f);
-
-  EXPECT_GT(uniform_lower_bound(IntervalOpenClosed, 1.0, 2.0), 1.0);
-  EXPECT_LT(uniform_lower_bound(IntervalOpenClosed, 1.0, +0.0), 1.0);
-  EXPECT_LT(uniform_lower_bound(IntervalOpenClosed, 1.0, -0.0), 1.0);
-  EXPECT_LT(uniform_lower_bound(IntervalOpenClosed, 1.0, -1.0), 1.0);
-
-  EXPECT_EQ(uniform_upper_bound(IntervalClosedClosed, 0.0f,
-                                std::numeric_limits<float>::max()),
-            std::numeric_limits<float>::max());
-  EXPECT_EQ(uniform_upper_bound(IntervalClosedClosed, 0.0,
-                                std::numeric_limits<double>::max()),
-            std::numeric_limits<double>::max());
-}
-
 struct Invalid {};
 
 template <typename A, typename B>
@@ -164,32 +80,28 @@ Invalid InferredTaggedUniformReturnT(...);
 template <typename A, typename B, typename Expect>
 void CheckArgsInferType() {
   static_assert(
-      absl::conjunction<
+      std::conjunction_v<
           std::is_same<Expect, decltype(InferredUniformReturnT<A, B>(0))>,
-          std::is_same<Expect,
-                       decltype(InferredUniformReturnT<B, A>(0))>>::value,
-      "");
+          std::is_same<Expect, decltype(InferredUniformReturnT<B, A>(0))>>);
   static_assert(
-      absl::conjunction<
+      std::conjunction_v<
           std::is_same<Expect, decltype(InferredTaggedUniformReturnT<
                                         absl::IntervalOpenOpenTag, A, B>(0))>,
-          std::is_same<Expect,
-                       decltype(InferredTaggedUniformReturnT<
-                                absl::IntervalOpenOpenTag, B, A>(0))>>::value,
-      "");
+          std::is_same<Expect, decltype(InferredTaggedUniformReturnT<
+                                        absl::IntervalOpenOpenTag, B, A>(0))>>);
 }
 
 template <typename A, typename B, typename ExplicitRet>
-auto ExplicitUniformReturnT(int) -> decltype(
-    absl::Uniform<ExplicitRet>(*std::declval<absl::InsecureBitGen*>(),
-                               std::declval<A>(), std::declval<B>()));
+auto ExplicitUniformReturnT(int) -> decltype(absl::Uniform<ExplicitRet>(
+    std::declval<absl::InsecureBitGen&>(), std::declval<A>(),
+    std::declval<B>()));
 
 template <typename, typename, typename ExplicitRet>
 Invalid ExplicitUniformReturnT(...);
 
 template <typename TagType, typename A, typename B, typename ExplicitRet>
 auto ExplicitTaggedUniformReturnT(int) -> decltype(absl::Uniform<ExplicitRet>(
-    std::declval<TagType>(), *std::declval<absl::InsecureBitGen*>(),
+    std::declval<TagType>(), std::declval<absl::InsecureBitGen&>(),
     std::declval<A>(), std::declval<B>()));
 
 template <typename, typename, typename, typename ExplicitRet>
@@ -205,23 +117,29 @@ Invalid ExplicitTaggedUniformReturnT(...);
 template <typename A, typename B, typename Expect>
 void CheckArgsReturnExpectedType() {
   static_assert(
-      absl::conjunction<
+      std::conjunction_v<
           std::is_same<Expect,
                        decltype(ExplicitUniformReturnT<A, B, Expect>(0))>,
-          std::is_same<Expect, decltype(ExplicitUniformReturnT<B, A, Expect>(
-                                   0))>>::value,
-      "");
+          std::is_same<Expect,
+                       decltype(ExplicitUniformReturnT<B, A, Expect>(0))>>);
   static_assert(
-      absl::conjunction<
+      std::conjunction_v<
           std::is_same<Expect,
                        decltype(ExplicitTaggedUniformReturnT<
                                 absl::IntervalOpenOpenTag, A, B, Expect>(0))>,
-          std::is_same<Expect, decltype(ExplicitTaggedUniformReturnT<
-                                        absl::IntervalOpenOpenTag, B, A,
-                                        Expect>(0))>>::value,
-      "");
+          std::is_same<Expect,
+                       decltype(ExplicitTaggedUniformReturnT<
+                                absl::IntervalOpenOpenTag, B, A, Expect>(0))>>);
 }
 
+// Takes the type of `absl::Uniform<R>(gen)` if valid or `Invalid` otherwise.
+template <typename R>
+auto UniformNoBoundsReturnT(int)
+    -> decltype(absl::Uniform<R>(std::declval<absl::InsecureBitGen&>()));
+
+template <typename>
+Invalid UniformNoBoundsReturnT(...);
+
 TEST_F(RandomDistributionsTest, UniformTypeInference) {
   // Infers common types.
   CheckArgsInferType<uint16_t, uint16_t, uint16_t>();
@@ -284,7 +202,9 @@ TEST_F(RandomDistributionsTest, UniformTypeInference) {
 
   // Properly promotes float.
   CheckArgsInferType<float, double, double>();
+}
 
+TEST_F(RandomDistributionsTest, UniformExamples) {
   // Examples.
   absl::InsecureBitGen gen;
   EXPECT_NE(1, absl::Uniform(gen, static_cast<uint16_t>(0), 1.0f));
@@ -305,6 +225,100 @@ TEST_F(RandomDistributionsTest, UniformNoBounds) {
   absl::Uniform<uint16_t>(gen);
   absl::Uniform<uint32_t>(gen);
   absl::Uniform<uint64_t>(gen);
+  absl::Uniform<absl::uint128>(gen);
+
+  // Compile-time validity tests.
+
+  // Allows unsigned ints.
+  testing::StaticAssertTypeEq<uint8_t,
+                              decltype(UniformNoBoundsReturnT<uint8_t>(0))>();
+  testing::StaticAssertTypeEq<uint16_t,
+                              decltype(UniformNoBoundsReturnT<uint16_t>(0))>();
+  testing::StaticAssertTypeEq<uint32_t,
+                              decltype(UniformNoBoundsReturnT<uint32_t>(0))>();
+  testing::StaticAssertTypeEq<uint64_t,
+                              decltype(UniformNoBoundsReturnT<uint64_t>(0))>();
+  testing::StaticAssertTypeEq<
+      absl::uint128, decltype(UniformNoBoundsReturnT<absl::uint128>(0))>();
+
+  // Disallows signed ints.
+  testing::StaticAssertTypeEq<Invalid,
+                              decltype(UniformNoBoundsReturnT<int8_t>(0))>();
+  testing::StaticAssertTypeEq<Invalid,
+                              decltype(UniformNoBoundsReturnT<int16_t>(0))>();
+  testing::StaticAssertTypeEq<Invalid,
+                              decltype(UniformNoBoundsReturnT<int32_t>(0))>();
+  testing::StaticAssertTypeEq<Invalid,
+                              decltype(UniformNoBoundsReturnT<int64_t>(0))>();
+  testing::StaticAssertTypeEq<
+      Invalid, decltype(UniformNoBoundsReturnT<absl::int128>(0))>();
+
+  // Disallows float types.
+  testing::StaticAssertTypeEq<Invalid,
+                              decltype(UniformNoBoundsReturnT<float>(0))>();
+  testing::StaticAssertTypeEq<Invalid,
+                              decltype(UniformNoBoundsReturnT<double>(0))>();
+}
+
+TEST_F(RandomDistributionsTest, UniformNonsenseRanges) {
+  // The ranges used in this test are undefined behavior.
+  // The results are arbitrary and subject to future changes.
+
+#if (defined(__i386__) || defined(_M_IX86)) && FLT_EVAL_METHOD != 0
+  // We're using an x87-compatible FPU, and intermediate operations can be
+  // performed with 80-bit floats. This produces slightly different results from
+  // what we expect below.
+  GTEST_SKIP()
+      << "Skipping the test because we detected x87 floating-point semantics";
+#endif
+
+  absl::InsecureBitGen gen;
+
+  // <uint>
+  EXPECT_EQ(0, absl::Uniform<uint64_t>(gen, 0, 0));
+  EXPECT_EQ(1, absl::Uniform<uint64_t>(gen, 1, 0));
+  EXPECT_EQ(0, absl::Uniform<uint64_t>(absl::IntervalOpenOpen, gen, 0, 0));
+  EXPECT_EQ(1, absl::Uniform<uint64_t>(absl::IntervalOpenOpen, gen, 1, 0));
+
+  constexpr auto m = (std::numeric_limits<uint64_t>::max)();
+
+  EXPECT_EQ(m, absl::Uniform(gen, m, m));
+  EXPECT_EQ(m, absl::Uniform(gen, m, m - 1));
+  EXPECT_EQ(m - 1, absl::Uniform(gen, m - 1, m));
+  EXPECT_EQ(m, absl::Uniform(absl::IntervalOpenOpen, gen, m, m));
+  EXPECT_EQ(m, absl::Uniform(absl::IntervalOpenOpen, gen, m, m - 1));
+  EXPECT_EQ(m - 1, absl::Uniform(absl::IntervalOpenOpen, gen, m - 1, m));
+
+  // <int>
+  EXPECT_EQ(0, absl::Uniform<int64_t>(gen, 0, 0));
+  EXPECT_EQ(1, absl::Uniform<int64_t>(gen, 1, 0));
+  EXPECT_EQ(0, absl::Uniform<int64_t>(absl::IntervalOpenOpen, gen, 0, 0));
+  EXPECT_EQ(1, absl::Uniform<int64_t>(absl::IntervalOpenOpen, gen, 1, 0));
+
+  constexpr auto l = (std::numeric_limits<int64_t>::min)();
+  constexpr auto r = (std::numeric_limits<int64_t>::max)();
+
+  EXPECT_EQ(l, absl::Uniform(gen, l, l));
+  EXPECT_EQ(r, absl::Uniform(gen, r, r));
+  EXPECT_EQ(r, absl::Uniform(gen, r, r - 1));
+  EXPECT_EQ(r - 1, absl::Uniform(gen, r - 1, r));
+  EXPECT_EQ(l, absl::Uniform(absl::IntervalOpenOpen, gen, l, l));
+  EXPECT_EQ(r, absl::Uniform(absl::IntervalOpenOpen, gen, r, r));
+  EXPECT_EQ(r, absl::Uniform(absl::IntervalOpenOpen, gen, r, r - 1));
+  EXPECT_EQ(r - 1, absl::Uniform(absl::IntervalOpenOpen, gen, r - 1, r));
+
+  // <double>
+  const double e = std::nextafter(1.0, 2.0);  // 1 + epsilon
+  const double f = std::nextafter(1.0, 0.0);  // 1 - epsilon
+  const double g = std::numeric_limits<double>::denorm_min();
+
+  EXPECT_EQ(1.0, absl::Uniform(gen, 1.0, e));
+  EXPECT_EQ(1.0, absl::Uniform(gen, 1.0, f));
+  EXPECT_EQ(0.0, absl::Uniform(gen, 0.0, g));
+
+  EXPECT_EQ(e, absl::Uniform(absl::IntervalOpenOpen, gen, 1.0, e));
+  EXPECT_EQ(f, absl::Uniform(absl::IntervalOpenOpen, gen, 1.0, f));
+  EXPECT_EQ(g, absl::Uniform(absl::IntervalOpenOpen, gen, 0.0, g));
 }
 
 // TODO(lar): Validate properties of non-default interval-semantics.
@@ -450,6 +464,13 @@ TEST_F(RandomDistributionsTest, Zipf) {
   EXPECT_NEAR(6.5944, moments.mean, 2000) << moments;
 }
 
+TEST_F(RandomDistributionsTest, ZipfWithZeroMax) {
+  absl::InsecureBitGen gen;
+  for (int i = 0; i < 100; ++i) {
+    EXPECT_EQ(0, absl::Zipf(gen, 0));
+  }
+}
+
 TEST_F(RandomDistributionsTest, Gaussian) {
   std::vector<double> values(kSize);
 
diff --git a/absl/random/examples_test.cc b/absl/random/examples_test.cc
index 1dcb5146f3d..25c673f0f1a 100644
--- a/absl/random/examples_test.cc
+++ b/absl/random/examples_test.cc
@@ -12,7 +12,12 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+#include <algorithm>
 #include <cinttypes>
+#include <cstddef>
+#include <cstdint>
+#include <iostream>
+#include <ostream>
 #include <random>
 #include <sstream>
 #include <vector>
@@ -96,4 +101,3 @@ TEST(Examples, CreateingCorrelatedVariateSequences) {
     EXPECT_EQ(absl::Uniform<uint32_t>(gen_1), absl::Uniform<uint32_t>(gen_2));
   }
 }
-
diff --git a/absl/random/exponential_distribution.h b/absl/random/exponential_distribution.h
index c8af197575c..21a02793911 100644
--- a/absl/random/exponential_distribution.h
+++ b/absl/random/exponential_distribution.h
@@ -17,15 +17,20 @@
 
 #include <cassert>
 #include <cmath>
+#include <cstdint>
 #include <istream>
 #include <limits>
+#include <ostream>
 #include <type_traits>
 
-#include "absl/random/internal/distribution_impl.h"
+#include "absl/base/config.h"
+#include "absl/meta/type_traits.h"
 #include "absl/random/internal/fast_uniform_bits.h"
+#include "absl/random/internal/generate_real.h"
 #include "absl/random/internal/iostream_state_saver.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 // absl::exponential_distribution:
 // Generates a number conforming to an exponential distribution and is
@@ -61,7 +66,7 @@ class exponential_distribution {
     result_type neg_inv_lambda_;
 
     static_assert(
-        std::is_floating_point<RealType>::value,
+        std::is_floating_point_v<RealType>,
         "Class-template absl::exponential_distribution<> must be parameterized "
         "using a floating-point type.");
   };
@@ -118,9 +123,14 @@ typename exponential_distribution<RealType>::result_type
 exponential_distribution<RealType>::operator()(
     URBG& g,  // NOLINT(runtime/references)
     const param_type& p) {
-  using random_internal::NegativeValueT;
-  const result_type u = random_internal::RandU64ToReal<
-      result_type>::template Value<NegativeValueT, false>(fast_u64_(g));
+  using random_internal::GenerateNegativeTag;
+  using random_internal::GenerateRealFromBits;
+  using real_type =
+      std::conditional_t<std::is_same_v<RealType, float>, float, double>;
+
+  const result_type u = GenerateRealFromBits<real_type, GenerateNegativeTag,
+                                             false>(fast_u64_(g));  // U(-1, 0)
+
   // log1p(-x) is mathematically equivalent to log(1 - x) but has more
   // accuracy for x near zero.
   return p.neg_inv_lambda_ * std::log1p(u);
@@ -152,6 +162,7 @@ std::basic_istream<CharT, Traits>& operator>>(
   return is;
 }
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_EXPONENTIAL_DISTRIBUTION_H_
diff --git a/absl/random/exponential_distribution_test.cc b/absl/random/exponential_distribution_test.cc
index dc49044d3b7..71be99ec0cb 100644
--- a/absl/random/exponential_distribution_test.cc
+++ b/absl/random/exponential_distribution_test.cc
@@ -15,6 +15,7 @@
 #include "absl/random/exponential_distribution.h"
 
 #include <algorithm>
+#include <cfloat>
 #include <cmath>
 #include <cstddef>
 #include <cstdint>
@@ -28,10 +29,12 @@
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
-#include "absl/base/internal/raw_logging.h"
 #include "absl/base/macros.h"
+#include "absl/log/log.h"
+#include "absl/numeric/internal/representation.h"
 #include "absl/random/internal/chi_square.h"
 #include "absl/random/internal/distribution_test_util.h"
+#include "absl/random/internal/pcg_engine.h"
 #include "absl/random/internal/sequence_urbg.h"
 #include "absl/random/random.h"
 #include "absl/strings/str_cat.h"
@@ -46,8 +49,16 @@ using absl::random_internal::kChiSquared;
 template <typename RealType>
 class ExponentialDistributionTypedTest : public ::testing::Test {};
 
-using RealTypes = ::testing::Types<float, double, long double>;
-TYPED_TEST_CASE(ExponentialDistributionTypedTest, RealTypes);
+// double-double arithmetic is not supported well by either GCC or Clang; see
+// https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99048,
+// https://bugs.llvm.org/show_bug.cgi?id=49131, and
+// https://bugs.llvm.org/show_bug.cgi?id=49132. Don't bother running these tests
+// with double doubles until compiler support is better.
+using RealTypes =
+    std::conditional_t<absl::numeric_internal::IsDoubleDouble(),
+                       ::testing::Types<float, double>,
+                       ::testing::Types<float, double, long double>>;
+TYPED_TEST_SUITE(ExponentialDistributionTypedTest, RealTypes);
 
 TYPED_TEST(ExponentialDistributionTypedTest, SerializeTest) {
   using param_type =
@@ -103,10 +114,9 @@ TYPED_TEST(ExponentialDistributionTypedTest, SerializeTest) {
       if (sample > sample_max) sample_max = sample;
       if (sample < sample_min) sample_min = sample;
     }
-    if (!std::is_same<TypeParam, long double>::value) {
-      ABSL_INTERNAL_LOG(INFO,
-                        absl::StrFormat("Range {%f}: %f, %f, lambda=%f", lambda,
-                                        sample_min, sample_max, lambda));
+    if (!std::is_same_v<TypeParam, long double>) {
+      LOG(INFO) << "Range {" << lambda << "}: " << sample_min << ", "
+                << sample_max << ", lambda=" << lambda;
     }
 
     std::stringstream ss;
@@ -125,23 +135,6 @@ TYPED_TEST(ExponentialDistributionTypedTest, SerializeTest) {
 
     ss >> after;
 
-#if defined(__powerpc64__) || defined(__PPC64__) || defined(__powerpc__) || \
-    defined(__ppc__) || defined(__PPC__)
-    if (std::is_same<TypeParam, long double>::value) {
-      // Roundtripping floating point values requires sufficient precision to
-      // reconstruct the exact value. It turns out that long double has some
-      // errors doing this on ppc, particularly for values
-      // near {1.0 +/- epsilon}.
-      if (lambda <= std::numeric_limits<double>::max() &&
-          lambda >= std::numeric_limits<double>::lowest()) {
-        EXPECT_EQ(static_cast<double>(before.lambda()),
-                  static_cast<double>(after.lambda()))
-            << ss.str();
-      }
-      continue;
-    }
-#endif
-
     EXPECT_EQ(before.lambda(), after.lambda())  //
         << ss.str() << " "                      //
         << (ss.good() ? "good " : "")           //
@@ -201,7 +194,10 @@ class ExponentialDistributionTests : public testing::TestWithParam<Param>,
   template <typename D>
   double SingleChiSquaredTest();
 
-  absl::InsecureBitGen rng_;
+  // We use a fixed bit generator for distribution accuracy tests.  This allows
+  // these tests to be deterministic, while still testing the qualify of the
+  // implementation.
+  absl::random_internal::pcg64_2018_engine rng_{0x2B7E151628AED2A6};
 };
 
 template <typename D>
@@ -222,17 +218,16 @@ bool ExponentialDistributionTests::SingleZTest(const double p,
   const bool pass = absl::random_internal::Near("z", z, 0.0, max_err);
 
   if (!pass) {
-    ABSL_INTERNAL_LOG(
-        INFO, absl::StrFormat("p=%f max_err=%f\n"
-                              " lambda=%f\n"
-                              " mean=%f vs. %f\n"
-                              " stddev=%f vs. %f\n"
-                              " skewness=%f vs. %f\n"
-                              " kurtosis=%f vs. %f\n"
-                              " z=%f vs. 0",
-                              p, max_err, lambda(), m.mean, mean(),
-                              std::sqrt(m.variance), stddev(), m.skewness,
-                              skew(), m.kurtosis, kurtosis(), z));
+    // clang-format off
+    LOG(INFO)
+        << "p=" << p << " max_err=" << max_err << "\n"
+           " lambda=" << lambda() << "\n"
+           " mean=" << m.mean << " vs. " << mean() << "\n"
+           " stddev=" << std::sqrt(m.variance) << " vs. " << stddev() << "\n"
+           " skewness=" << m.skewness << " vs. " << skew() << "\n"
+           " kurtosis=" << m.kurtosis << " vs. " << kurtosis() << "\n"
+           " z=" << z << " vs. 0";
+    // clang-format on
   }
   return pass;
 }
@@ -277,16 +272,16 @@ double ExponentialDistributionTests::SingleChiSquaredTest() {
   double p = absl::random_internal::ChiSquarePValue(chi_square, dof);
 
   if (chi_square > threshold) {
-    for (int i = 0; i < cutoffs.size(); i++) {
-      ABSL_INTERNAL_LOG(
-          INFO, absl::StrFormat("%d : (%f) = %d", i, cutoffs[i], counts[i]));
+    for (size_t i = 0; i < cutoffs.size(); i++) {
+      LOG(INFO) << i << " : (" << cutoffs[i] << ") = " << counts[i];
     }
 
-    ABSL_INTERNAL_LOG(INFO,
-                      absl::StrCat("lambda ", lambda(), "\n",     //
-                                   " expected ", expected, "\n",  //
-                                   kChiSquared, " ", chi_square, " (", p, ")\n",
-                                   kChiSquared, " @ 0.98 = ", threshold));
+    // clang-format off
+    LOG(INFO) << "lambda " << lambda() << "\n"
+                 " expected " << expected << "\n"
+              << kChiSquared << " " << chi_square << " (" << p << ")\n"
+              << kChiSquared << " @ 0.98 = " << threshold;
+    // clang-format on
   }
   return p;
 }
@@ -346,8 +341,8 @@ std::string ParamName(const ::testing::TestParamInfo<Param>& info) {
   return absl::StrReplaceAll(name, {{"+", "_"}, {"-", "_"}, {".", "_"}});
 }
 
-INSTANTIATE_TEST_CASE_P(All, ExponentialDistributionTests,
-                        ::testing::ValuesIn(GenParams()), ParamName);
+INSTANTIATE_TEST_SUITE_P(All, ExponentialDistributionTests,
+                         ::testing::ValuesIn(GenParams()), ParamName);
 
 // NOTE: absl::exponential_distribution is not guaranteed to be stable.
 TEST(ExponentialDistributionTest, StabilityTest) {
@@ -388,6 +383,15 @@ TEST(ExponentialDistributionTest, StabilityTest) {
 TEST(ExponentialDistributionTest, AlgorithmBounds) {
   // Relies on absl::uniform_real_distribution, so some of these comments
   // reference that.
+
+#if (defined(__i386__) || defined(_M_IX86)) && FLT_EVAL_METHOD != 0
+  // We're using an x87-compatible FPU, and intermediate operations can be
+  // performed with 80-bit floats. This produces slightly different results from
+  // what we expect below.
+  GTEST_SKIP()
+      << "Skipping the test because we detected x87 floating-point semantics";
+#endif
+
   absl::exponential_distribution<double> dist;
 
   {
diff --git a/absl/random/gaussian_distribution.cc b/absl/random/gaussian_distribution.cc
index 5dd84619414..c7a72cb2f61 100644
--- a/absl/random/gaussian_distribution.cc
+++ b/absl/random/gaussian_distribution.cc
@@ -4,6 +4,7 @@
 #include "absl/random/gaussian_distribution.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
 const gaussian_distribution_base::Tables
@@ -96,6 +97,7 @@ const gaussian_distribution_base::Tables
      0.9362826816850632339, 0.9635996931270905952, 1}};
 
 }  // namespace random_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 // clang-format on
diff --git a/absl/random/gaussian_distribution.h b/absl/random/gaussian_distribution.h
index 1d1347bce0a..d14de0b2640 100644
--- a/absl/random/gaussian_distribution.h
+++ b/absl/random/gaussian_distribution.h
@@ -26,13 +26,16 @@
 #include <cstdint>
 #include <istream>
 #include <limits>
+#include <ostream>
 #include <type_traits>
 
-#include "absl/random/internal/distribution_impl.h"
+#include "absl/base/config.h"
 #include "absl/random/internal/fast_uniform_bits.h"
+#include "absl/random/internal/generate_real.h"
 #include "absl/random/internal/iostream_state_saver.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
 // absl::gaussian_distribution_base implements the underlying ziggurat algorithm
@@ -42,7 +45,7 @@ namespace random_internal {
 // The specific algorithm has some of the improvements suggested by the
 // 2005 paper, "An Improved Ziggurat Method to Generate Normal Random Samples",
 // Jurgen A Doornik.  (https://www.doornik.com/research/ziggurat.pdf)
-class gaussian_distribution_base {
+class ABSL_DLL gaussian_distribution_base {
  public:
   template <typename URBG>
   inline double zignor(URBG& g);  // NOLINT(runtime/references)
@@ -55,7 +58,7 @@ class gaussian_distribution_base {
                                 bool neg);
 
   // Constants used for the gaussian distribution.
-  static constexpr double kR = 3.442619855899;  // Start of the tail.
+  static constexpr double kR = 3.442619855899;          // Start of the tail.
   static constexpr double kRInv = 0.29047645161474317;  // ~= (1.0 / kR) .
   static constexpr double kV = 9.91256303526217e-3;
   static constexpr uint64_t kMask = 0x07f;
@@ -113,7 +116,7 @@ class gaussian_distribution : random_internal::gaussian_distribution_base {
     result_type stddev_;
 
     static_assert(
-        std::is_floating_point<RealType>::value,
+        std::is_floating_point_v<RealType>,
         "Class-template absl::gaussian_distribution<> must be parameterized "
         "using a floating-point type.");
   };
@@ -207,12 +210,18 @@ namespace random_internal {
 
 template <typename URBG>
 inline double gaussian_distribution_base::zignor_fallback(URBG& g, bool neg) {
+  using random_internal::GeneratePositiveTag;
+  using random_internal::GenerateRealFromBits;
+
   // This fallback path happens approximately 0.05% of the time.
   double x, y;
   do {
     // kRInv = 1/r, U(0, 1)
-    x = kRInv * std::log(RandU64ToDouble<PositiveValueT, false>(fast_u64_(g)));
-    y = -std::log(RandU64ToDouble<PositiveValueT, false>(fast_u64_(g)));
+    x = kRInv *
+        std::log(GenerateRealFromBits<double, GeneratePositiveTag, false>(
+            fast_u64_(g)));
+    y = -std::log(
+        GenerateRealFromBits<double, GeneratePositiveTag, false>(fast_u64_(g)));
   } while ((y + y) < (x * x));
   return neg ? (x - kR) : (kR - x);
 }
@@ -220,6 +229,10 @@ inline double gaussian_distribution_base::zignor_fallback(URBG& g, bool neg) {
 template <typename URBG>
 inline double gaussian_distribution_base::zignor(
     URBG& g) {  // NOLINT(runtime/references)
+  using random_internal::GeneratePositiveTag;
+  using random_internal::GenerateRealFromBits;
+  using random_internal::GenerateSignedTag;
+
   while (true) {
     // We use a single uint64_t to generate both a double and a strip.
     // These bits are unused when the generated double is > 1/2^5.
@@ -227,10 +240,11 @@ inline double gaussian_distribution_base::zignor(
     // values (those smaller than 1/2^5, which all end up on the left tail).
     uint64_t bits = fast_u64_(g);
     int i = static_cast<int>(bits & kMask);  // pick a random strip
-    double j = RandU64ToDouble<SignedValueT, false>(bits);  // U(-1, 1)
+    double j = GenerateRealFromBits<double, GenerateSignedTag, false>(
+        bits);  // U(-1, 1)
     const double x = j * zg_.x[i];
 
-    // Retangular box. Handles >97% of all cases.
+    // Rectangular box. Handles >97% of all cases.
     // For any given box, this handles between 75% and 99% of values.
     // Equivalent to U(01) < (x[i+1] / x[i]), and when i == 0, ~93.5%
     if (std::abs(x) < zg_.x[i + 1]) {
@@ -244,7 +258,8 @@ inline double gaussian_distribution_base::zignor(
     }
 
     // i > 0: Wedge samples using precomputed values.
-    double v = RandU64ToDouble<PositiveValueT, false>(fast_u64_(g));  // U(0, 1)
+    double v = GenerateRealFromBits<double, GeneratePositiveTag, false>(
+        fast_u64_(g));  // U(0, 1)
     if ((zg_.f[i + 1] + v * (zg_.f[i] - zg_.f[i + 1])) <
         std::exp(-0.5 * x * x)) {
       return x;
@@ -255,6 +270,7 @@ inline double gaussian_distribution_base::zignor(
 }
 
 }  // namespace random_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_GAUSSIAN_DISTRIBUTION_H_
diff --git a/absl/random/gaussian_distribution_test.cc b/absl/random/gaussian_distribution_test.cc
index 49c07513bf2..65fd85a1bdd 100644
--- a/absl/random/gaussian_distribution_test.cc
+++ b/absl/random/gaussian_distribution_test.cc
@@ -17,16 +17,21 @@
 #include <algorithm>
 #include <cmath>
 #include <cstddef>
+#include <cstdint>
 #include <ios>
 #include <iterator>
+#include <limits>
 #include <random>
+#include <sstream>
 #include <string>
+#include <type_traits>
 #include <vector>
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
-#include "absl/base/internal/raw_logging.h"
 #include "absl/base/macros.h"
+#include "absl/log/log.h"
+#include "absl/numeric/internal/representation.h"
 #include "absl/random/internal/chi_square.h"
 #include "absl/random/internal/distribution_test_util.h"
 #include "absl/random/internal/sequence_urbg.h"
@@ -43,8 +48,16 @@ using absl::random_internal::kChiSquared;
 template <typename RealType>
 class GaussianDistributionInterfaceTest : public ::testing::Test {};
 
-using RealTypes = ::testing::Types<float, double, long double>;
-TYPED_TEST_CASE(GaussianDistributionInterfaceTest, RealTypes);
+// double-double arithmetic is not supported well by either GCC or Clang; see
+// https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99048,
+// https://bugs.llvm.org/show_bug.cgi?id=49131, and
+// https://bugs.llvm.org/show_bug.cgi?id=49132. Don't bother running these tests
+// with double doubles until compiler support is better.
+using RealTypes =
+    std::conditional_t<absl::numeric_internal::IsDoubleDouble(),
+                       ::testing::Types<float, double>,
+                       ::testing::Types<float, double, long double>>;
+TYPED_TEST_SUITE(GaussianDistributionInterfaceTest, RealTypes);
 
 TYPED_TEST(GaussianDistributionInterfaceTest, SerializeTest) {
   using param_type =
@@ -105,10 +118,9 @@ TYPED_TEST(GaussianDistributionInterfaceTest, SerializeTest) {
           EXPECT_GE(sample, before.min()) << before;
           EXPECT_LE(sample, before.max()) << before;
         }
-        if (!std::is_same<TypeParam, long double>::value) {
-          ABSL_INTERNAL_LOG(
-              INFO, absl::StrFormat("Range{%f, %f}: %f, %f", mean, stddev,
-                                    sample_min, sample_max));
+        if (!std::is_same_v<TypeParam, long double>) {
+          LOG(INFO) << "Range{" << mean << ", " << stddev << "}: " << sample_min
+                    << ", " << sample_max;
         }
 
         std::stringstream ss;
@@ -129,29 +141,6 @@ TYPED_TEST(GaussianDistributionInterfaceTest, SerializeTest) {
 
         ss >> after;
 
-#if defined(__powerpc64__) || defined(__PPC64__) || defined(__powerpc__) || \
-    defined(__ppc__) || defined(__PPC__)
-        if (std::is_same<TypeParam, long double>::value) {
-          // Roundtripping floating point values requires sufficient precision
-          // to reconstruct the exact value.  It turns out that long double
-          // has some errors doing this on ppc, particularly for values
-          // near {1.0 +/- epsilon}.
-          if (mean <= std::numeric_limits<double>::max() &&
-              mean >= std::numeric_limits<double>::lowest()) {
-            EXPECT_EQ(static_cast<double>(before.mean()),
-                      static_cast<double>(after.mean()))
-                << ss.str();
-          }
-          if (stddev <= std::numeric_limits<double>::max() &&
-              stddev >= std::numeric_limits<double>::lowest()) {
-            EXPECT_EQ(static_cast<double>(before.stddev()),
-                      static_cast<double>(after.stddev()))
-                << ss.str();
-          }
-          continue;
-        }
-#endif
-
         EXPECT_EQ(before.mean(), after.mean());
         EXPECT_EQ(before.stddev(), after.stddev())  //
             << ss.str() << " "                      //
@@ -213,7 +202,10 @@ class GaussianDistributionTests : public testing::TestWithParam<Param>,
   template <typename D>
   double SingleChiSquaredTest();
 
-  absl::InsecureBitGen rng_;
+  // We use a fixed bit generator for distribution accuracy tests.  This allows
+  // these tests to be deterministic, while still testing the qualify of the
+  // implementation.
+  absl::random_internal::pcg64_2018_engine rng_{0x2B7E151628AED2A6};
 };
 
 template <typename D>
@@ -250,17 +242,16 @@ bool GaussianDistributionTests::SingleZTest(const double p,
       (std::pow(m.skewness, 2.0) + std::pow(m.kurtosis - 3.0, 2.0) / 4.0);
 
   if (!pass || jb > 9.21) {
-    ABSL_INTERNAL_LOG(
-        INFO, absl::StrFormat("p=%f max_err=%f\n"
-                              " mean=%f vs. %f\n"
-                              " stddev=%f vs. %f\n"
-                              " skewness=%f vs. %f\n"
-                              " kurtosis=%f vs. %f\n"
-                              " z=%f vs. 0\n"
-                              " jb=%f vs. 9.21",
-                              p, max_err, m.mean, mean(), std::sqrt(m.variance),
-                              stddev(), m.skewness, skew(), m.kurtosis,
-                              kurtosis(), z, jb));
+    // clang-format off
+    LOG(INFO)
+        << "p=" << p << " max_err=" << max_err << "\n"
+           " mean=" << m.mean << " vs. " << mean() << "\n"
+           " stddev=" << std::sqrt(m.variance) << " vs. " << stddev() << "\n"
+           " skewness=" << m.skewness << " vs. " << skew() << "\n"
+           " kurtosis=" << m.kurtosis << " vs. " << kurtosis() << "\n"
+           " z=" << z << " vs. 0\n"
+           " jb=" << jb << " vs. 9.21";
+    // clang-format on
   }
   return pass;
 }
@@ -307,16 +298,16 @@ double GaussianDistributionTests::SingleChiSquaredTest() {
 
   // Log if the chi_square value is above the threshold.
   if (chi_square > threshold) {
-    for (int i = 0; i < cutoffs.size(); i++) {
-      ABSL_INTERNAL_LOG(
-          INFO, absl::StrFormat("%d : (%f) = %d", i, cutoffs[i], counts[i]));
+    for (size_t i = 0; i < cutoffs.size(); i++) {
+      LOG(INFO) << i << " : (" << cutoffs[i] << ") = " << counts[i];
     }
 
-    ABSL_INTERNAL_LOG(
-        INFO, absl::StrCat("mean=", mean(), " stddev=", stddev(), "\n",   //
-                           " expected ", expected, "\n",                  //
-                           kChiSquared, " ", chi_square, " (", p, ")\n",  //
-                           kChiSquared, " @ 0.98 = ", threshold));
+    // clang-format off
+    LOG(INFO) << "mean=" << mean() << " stddev=" << stddev() << "\n"
+                 " expected " << expected << "\n"
+              << kChiSquared << " " << chi_square << " (" << p << ")\n"
+              << kChiSquared << " @ 0.98 = " << threshold;
+    // clang-format on
   }
   return p;
 }
diff --git a/absl/random/generators_test.cc b/absl/random/generators_test.cc
index 41725f139cd..3217a42edb0 100644
--- a/absl/random/generators_test.cc
+++ b/absl/random/generators_test.cc
@@ -18,6 +18,7 @@
 #include <vector>
 
 #include "gtest/gtest.h"
+#include "absl/numeric/int128.h"
 #include "absl/random/distributions.h"
 #include "absl/random/random.h"
 
@@ -49,7 +50,7 @@ void TestUniform(URBG* gen) {
   // (a, b) semantics, inferred types.
   absl::Uniform(absl::IntervalOpenOpen, *gen, 0, 1.0);  // Promoted to double
 
-  // Explict overriding of types.
+  // Explicit overriding of types.
   absl::Uniform<int>(*gen, 0, 100);
   absl::Uniform<int8_t>(*gen, 0, 100);
   absl::Uniform<int16_t>(*gen, 0, 100);
@@ -107,6 +108,8 @@ void TestPoisson(URBG* gen) {
   absl::Poisson<int64_t>(*gen);
   absl::Poisson<uint64_t>(*gen);
   absl::Poisson<uint64_t>(URBG());
+  absl::Poisson<absl::int128>(*gen);
+  absl::Poisson<absl::uint128>(*gen);
 }
 
 template <typename URBG>
@@ -126,6 +129,8 @@ void TestZipf(URBG* gen) {
   absl::Zipf<int64_t>(*gen, 1 << 10);
   absl::Zipf<uint64_t>(*gen, 1 << 10);
   absl::Zipf<uint64_t>(URBG(), 1 << 10);
+  absl::Zipf<absl::int128>(*gen, 1 << 10);
+  absl::Zipf<absl::uint128>(*gen, 1 << 10);
 }
 
 template <typename URBG>
@@ -146,6 +151,8 @@ void TestLogNormal(URBG* gen) {
   absl::LogUniform<int64_t>(*gen, 0, 1 << 10);
   absl::LogUniform<uint64_t>(*gen, 0, 1 << 10);
   absl::LogUniform<uint64_t>(URBG(), 0, 1 << 10);
+  absl::LogUniform<absl::int128>(*gen, 0, 1 << 10);
+  absl::LogUniform<absl::uint128>(*gen, 0, 1 << 10);
 }
 
 template <typename URBG>
diff --git a/absl/random/internal/BUILD.bazel b/absl/random/internal/BUILD.bazel
index cd50982d8b3..3ba8d9c541a 100644
--- a/absl/random/internal/BUILD.bazel
+++ b/absl/random/internal/BUILD.bazel
@@ -14,33 +14,112 @@
 # limitations under the License.
 #
 
-load("@rules_cc//cc:defs.bzl", "cc_binary", "cc_library", "cc_test")
+load("@bazel_skylib//lib:selects.bzl", "selects")
+load("@rules_cc//cc:cc_binary.bzl", "cc_binary")
+load("@rules_cc//cc:cc_library.bzl", "cc_library")
+load("@rules_cc//cc:cc_test.bzl", "cc_test")
 
 # Internal-only implementation classes for Abseil Random
 load(
     "//absl:copts/configure_copts.bzl",
     "ABSL_DEFAULT_COPTS",
     "ABSL_DEFAULT_LINKOPTS",
-    "ABSL_RANDOM_RANDEN_COPTS",
     "ABSL_TEST_COPTS",
-    "absl_random_randen_copts_init",
 )
 
-package(default_visibility = [
+default_package_visibility = [
     "//absl/random:__pkg__",
-])
+    "@do_not_use_for_gloop_visibility_only//gloop/util/random:__subpackages__",
+]
+
+package(
+    default_visibility = default_package_visibility,
+    features = [
+        "header_modules",
+        "layering_check",
+        "parse_headers",
+    ],
+)
+
+licenses(["notice"])
+
+# Used to select on compilers that support GCC-compatible options
+# (e.g. "-maes").
+selects.config_setting_group(
+    name = "gcc_compatible",
+    match_any = [
+        "@rules_cc//cc/compiler:clang",
+        "@rules_cc//cc/compiler:gcc",
+    ],
+)
 
-licenses(["notice"])  # Apache 2.0
+selects.config_setting_group(
+    name = "gcc_compatible-aarch32",
+    match_all = [
+        ":gcc_compatible",
+        "@platforms//cpu:aarch32",
+    ],
+)
+
+selects.config_setting_group(
+    name = "gcc_compatible-aarch64",
+    match_all = [
+        ":gcc_compatible",
+        "@platforms//cpu:aarch64",
+    ],
+)
+
+selects.config_setting_group(
+    name = "ppc_crypto",
+    match_any = [
+        "@platforms//cpu:ppc",
+        "@platforms//cpu:ppc32",
+        "@platforms//cpu:ppc64le",
+    ],
+)
+
+selects.config_setting_group(
+    name = "gcc_compatible-ppc_crypto",
+    match_all = [
+        ":gcc_compatible",
+        ":ppc_crypto",
+    ],
+)
+
+selects.config_setting_group(
+    name = "gcc_compatible-x86_64",
+    match_all = [
+        ":gcc_compatible",
+        "@platforms//cpu:x86_64",
+    ],
+)
+
+# Some libraries are compiled with options to generate AES-NI
+# instructions, and runtime dispatch is used to determine if the host
+# microarchitecture supports AES-NI or if a portable fallback library
+# should be called.
+ABSL_RANDOM_RANDEN_COPTS = select({
+    ":gcc_compatible-aarch32": ["-mfpu=neon"],
+    ":gcc_compatible-aarch64": ["-march=armv8-a+crypto"],
+    ":gcc_compatible-ppc_crypto": ["-mcrypto"],
+    ":gcc_compatible-x86_64": [
+        "-maes",
+        "-msse4.1",
+    ],
+    "//conditions:default": [],
+})
 
 cc_library(
     name = "traits",
     hdrs = ["traits.h"],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
-    visibility = [
-        "//absl/random:__pkg__",
+    deps = [
+        "//absl/base:config",
+        "//absl/meta:type_traits",
+        "//absl/numeric:bits",
+        "//absl/numeric:int128",
     ],
-    deps = ["//absl/base:config"],
 )
 
 cc_library(
@@ -48,23 +127,12 @@ cc_library(
     hdrs = ["distribution_caller.h"],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
-    visibility = [
-        "//absl/random:__pkg__",
-    ],
-)
-
-cc_library(
-    name = "distributions",
-    hdrs = ["distributions.h"],
-    copts = ABSL_DEFAULT_COPTS,
-    linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
-        ":distribution_caller",
-        ":traits",
-        ":uniform_helper",
-        "//absl/base",
+        "//absl/base:config",
+        "//absl/base:fast_type_id",
         "//absl/meta:type_traits",
-        "//absl/strings",
+        "//absl/random:mocking_access",
+        "//absl/utility",
     ],
 )
 
@@ -75,8 +143,10 @@ cc_library(
     ],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
-    visibility = [
-        "//absl/random:__pkg__",
+    deps = [
+        ":traits",
+        "//absl/base:config",
+        "//absl/meta:type_traits",
     ],
 )
 
@@ -89,10 +159,19 @@ cc_library(
         "seed_material.h",
     ],
     copts = ABSL_DEFAULT_COPTS,
-    linkopts = ABSL_DEFAULT_LINKOPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS + select({
+        "@rules_cc//cc/compiler:msvc-cl": ["-DEFAULTLIB:bcrypt.lib"],
+        "@rules_cc//cc/compiler:clang-cl": ["-DEFAULTLIB:bcrypt.lib"],
+        "//absl:mingw_compiler": [
+            "-DEFAULTLIB:bcrypt.lib",
+            "-lbcrypt",
+        ],
+        "//conditions:default": [],
+    }),
     deps = [
         ":fast_uniform_bits",
-        "//absl/base:core_headers",
+        "//absl/base:config",
+        "//absl/base:dynamic_annotations",
         "//absl/base:raw_logging_internal",
         "//absl/strings",
         "//absl/types:optional",
@@ -101,27 +180,24 @@ cc_library(
 )
 
 cc_library(
-    name = "pool_urbg",
-    srcs = [
-        "pool_urbg.cc",
-    ],
-    hdrs = [
-        "pool_urbg.h",
-    ],
+    name = "entropy_pool",
+    srcs = ["entropy_pool.cc"],
+    hdrs = ["entropy_pool.h"],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = select({
-        "//absl:windows": [],
+        "@rules_cc//cc/compiler:msvc-cl": [],
+        "@rules_cc//cc/compiler:clang-cl": [],
+        "@rules_cc//cc/compiler:emscripten": [],
         "//conditions:default": ["-pthread"],
     }) + ABSL_DEFAULT_LINKOPTS,
     deps = [
+        ":platform",
         ":randen",
         ":seed_material",
-        ":traits",
+        "//absl/algorithm:container",
         "//absl/base",
         "//absl/base:config",
         "//absl/base:core_headers",
-        "//absl/base:endian",
-        "//absl/base:raw_logging_internal",
         "//absl/random:seed_gen_exception",
         "//absl/types:span",
     ],
@@ -129,22 +205,27 @@ cc_library(
 
 cc_library(
     name = "explicit_seed_seq",
-    testonly = 1,
+    testonly = True,
     hdrs = [
         "explicit_seed_seq.h",
     ],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        "//absl/base:config",
+        "//absl/base:endian",
+    ],
 )
 
 cc_library(
     name = "sequence_urbg",
-    testonly = 1,
+    testonly = True,
     hdrs = [
         "sequence_urbg.h",
     ],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = ["//absl/base:config"],
 )
 
 cc_library(
@@ -169,24 +250,24 @@ cc_library(
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
+        "//absl/base:config",
         "//absl/meta:type_traits",
         "//absl/numeric:int128",
     ],
 )
 
 cc_library(
-    name = "distribution_impl",
+    name = "generate_real",
     hdrs = [
-        "distribution_impl.h",
+        "generate_real.h",
     ],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":fastmath",
         ":traits",
-        "//absl/base:bits",
-        "//absl/base:config",
-        "//absl/numeric:int128",
+        "//absl/meta:type_traits",
+        "//absl/numeric:bits",
     ],
 )
 
@@ -197,7 +278,20 @@ cc_library(
     ],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
-    deps = ["//absl/base:bits"],
+    deps = ["//absl/numeric:bits"],
+)
+
+cc_library(
+    name = "wide_multiply",
+    hdrs = ["wide_multiply.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":traits",
+        "//absl/base:config",
+        "//absl/numeric:bits",
+        "//absl/numeric:int128",
+    ],
 )
 
 cc_library(
@@ -206,14 +300,12 @@ cc_library(
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
-        ":pool_urbg",
+        ":entropy_pool",
         ":salted_seed_seq",
         ":seed_material",
-        "//absl/base:core_headers",
+        "//absl/base:config",
+        "//absl/container:inlined_vector",
         "//absl/meta:type_traits",
-        "//absl/strings",
-        "//absl/types:optional",
-        "//absl/types:span",
     ],
 )
 
@@ -227,6 +319,7 @@ cc_library(
         ":iostream_state_saver",
         "//absl/base:config",
         "//absl/meta:type_traits",
+        "//absl/numeric:bits",
         "//absl/numeric:int128",
     ],
 )
@@ -236,24 +329,30 @@ cc_library(
     hdrs = ["randen_engine.h"],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = default_package_visibility + [
+    ],
     deps = [
         ":iostream_state_saver",
         ":randen",
+        "//absl/base:endian",
         "//absl/meta:type_traits",
     ],
 )
 
 cc_library(
     name = "platform",
+    srcs = [
+        "randen_round_keys.cc",
+    ],
     hdrs = [
         "randen_traits.h",
     ],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     textual_hdrs = [
-        "randen-keys.inc",
         "platform.h",
     ],
+    deps = ["//absl/base:config"],
 )
 
 cc_library(
@@ -269,8 +368,9 @@ cc_library(
     deps = [
         ":platform",
         ":randen_hwaes",
+        ":randen_hwaes_impl",
         ":randen_slow",
-        "//absl/base:raw_logging_internal",
+        "//absl/base:config",
     ],
 )
 
@@ -282,11 +382,13 @@ cc_library(
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":platform",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:endian",
+        "//absl/numeric:int128",
     ],
 )
 
-absl_random_randen_copts_init()
-
 cc_library(
     name = "randen_hwaes",
     srcs = [
@@ -294,13 +396,12 @@ cc_library(
     ],
     hdrs = [
         "randen_detect.h",
-        "randen_hwaes.h",
     ],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":platform",
-        ":randen_hwaes_impl",
+        "//absl/base:config",
     ],
 )
 
@@ -309,20 +410,20 @@ cc_library(
     name = "randen_hwaes_impl",
     srcs = [
         "randen_hwaes.cc",
+    ],
+    hdrs = [
         "randen_hwaes.h",
     ],
     copts = ABSL_DEFAULT_COPTS + ABSL_RANDOM_RANDEN_COPTS + select({
-        "//absl:windows": [],
+        "@rules_cc//cc/compiler:msvc-cl": [],
+        "@rules_cc//cc/compiler:clang-cl": [],
         "//conditions:default": ["-Wno-pass-failed"],
     }),
-    # copts in RANDEN_HWAES_COPTS can make this target unusable as a module
-    # leading to a Clang diagnostic. Furthermore, it only has a private header
-    # anyway and thus there wouldn't be any gain from using it as a module.
-    features = ["-header_modules"],
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":platform",
-        "//absl/base:core_headers",
+        "//absl/base:config",
+        "//absl/numeric:int128",
     ],
 )
 
@@ -333,6 +434,7 @@ cc_binary(
     ],
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
+        "//absl/base:config",
         "//absl/base:core_headers",
         "//absl/random:distributions",
     ],
@@ -340,7 +442,7 @@ cc_binary(
 
 cc_library(
     name = "distribution_test_util",
-    testonly = 1,
+    testonly = True,
     srcs = [
         "chi_square.cc",
         "distribution_test_util.cc",
@@ -352,6 +454,7 @@ cc_library(
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
+        "//absl/base:config",
         "//absl/base:core_headers",
         "//absl/base:raw_logging_internal",
         "//absl/strings",
@@ -368,7 +471,7 @@ ABSL_RANDOM_NONPORTABLE_TAGS = [
     "no_test_darwin_x86_64",
     "no_test_ios_x86_64",
     "no_test_loonix",
-    "no_test_msvc_x64",
+    "no_test_lexan",
     "no_test_wasm",
 ]
 
@@ -380,22 +483,25 @@ cc_test(
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":traits",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
 cc_test(
-    name = "distribution_impl_test",
+    name = "generate_real_test",
     size = "small",
-    srcs = ["distribution_impl_test.cc"],
+    srcs = [
+        "generate_real_test.cc",
+    ],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
-        ":distribution_impl",
-        "//absl/base:bits",
+        ":generate_real",
         "//absl/flags:flag",
-        "//absl/numeric:int128",
-        "@com_google_googletest//:gtest_main",
+        "//absl/numeric:bits",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -407,7 +513,8 @@ cc_test(
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":distribution_test_util",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -419,7 +526,8 @@ cc_test(
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":fastmath",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -431,8 +539,10 @@ cc_test(
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":explicit_seed_seq",
-        "//absl/random:seed_sequences",
-        "@com_google_googletest//:gtest_main",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -444,7 +554,10 @@ cc_test(
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":salted_seed_seq",
-        "@com_google_googletest//:gtest_main",
+        ":seed_material",
+        "//absl/base:core_headers",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -458,8 +571,10 @@ cc_test(
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":distribution_test_util",
+        "//absl/algorithm:container",
         "//absl/base:core_headers",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -473,7 +588,33 @@ cc_test(
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":fast_uniform_bits",
-        "@com_google_googletest//:gtest_main",
+        "//absl/base:config",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_library(
+    name = "mock_helpers",
+    hdrs = ["mock_helpers.h"],
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        "//absl/base:config",
+        "//absl/base:fast_type_id",
+        "//absl/random:mocking_access",
+    ],
+)
+
+cc_library(
+    name = "mock_overload_set",
+    testonly = True,
+    hdrs = ["mock_overload_set.h"],
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":mock_helpers",
+        "//absl/base:config",
+        "//absl/random:mocking_bit_gen",
+        "@googletest//:gtest",
     ],
 )
 
@@ -487,11 +628,13 @@ cc_test(
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":nonsecure_base",
+        "//absl/algorithm:container",
+        "//absl/container:flat_hash_set",
         "//absl/random",
         "//absl/random:distributions",
-        "//absl/random:seed_sequences",
-        "//absl/strings",
-        "@com_google_googletest//:gtest_main",
+        "//absl/synchronization",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -503,23 +646,26 @@ cc_test(
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":seed_material",
-        "@com_google_googletest//:gtest_main",
+        "//absl/types:span",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
 cc_test(
-    name = "pool_urbg_test",
+    name = "entropy_pool_test",
     size = "small",
     srcs = [
-        "pool_urbg_test.cc",
+        "entropy_pool_test.cc",
     ],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
-        ":pool_urbg",
-        "//absl/meta:type_traits",
-        "//absl/types:span",
-        "@com_google_googletest//:gtest_main",
+        ":entropy_pool",
+        "//absl/container:flat_hash_set",
+        "//absl/synchronization",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -533,8 +679,8 @@ cc_test(
     deps = [
         ":explicit_seed_seq",
         ":pcg_engine",
-        "//absl/time",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -549,10 +695,10 @@ cc_test(
     deps = [
         ":explicit_seed_seq",
         ":randen_engine",
-        "//absl/base:raw_logging_internal",
-        "//absl/strings",
+        "//absl/log",
         "//absl/time",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -564,8 +710,8 @@ cc_test(
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":randen",
-        "//absl/meta:type_traits",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -576,8 +722,10 @@ cc_test(
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
+        ":platform",
         ":randen_slow",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -585,16 +733,30 @@ cc_test(
     name = "randen_hwaes_test",
     size = "small",
     srcs = ["randen_hwaes_test.cc"],
-    copts = ABSL_TEST_COPTS,
+    copts = ABSL_TEST_COPTS + ABSL_RANDOM_RANDEN_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     tags = ABSL_RANDOM_NONPORTABLE_TAGS,
     deps = [
         ":platform",
         ":randen_hwaes",
         ":randen_hwaes_impl",  # build_cleaner: keep
-        "//absl/base:raw_logging_internal",
-        "//absl/strings:str_format",
-        "@com_google_googletest//:gtest",
+        "//absl/log",
+        "@googletest//:gtest",
+    ],
+)
+
+cc_test(
+    name = "wide_multiply_test",
+    size = "small",
+    srcs = ["wide_multiply_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":traits",
+        ":wide_multiply",
+        "//absl/numeric:int128",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -606,6 +768,9 @@ cc_library(
     deps = [
         ":platform",
         ":randen_engine",
+        "//absl/algorithm:container",
+        "//absl/base:config",
+        "//absl/base:core_headers",
         "//absl/base:raw_logging_internal",
     ],
 )
@@ -616,10 +781,25 @@ cc_library(
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
+        ":traits",
+        "//absl/base:config",
         "//absl/meta:type_traits",
     ],
 )
 
+cc_library(
+    name = "mock_validators",
+    hdrs = ["mock_validators.h"],
+    deps = [
+        ":iostream_state_saver",
+        ":uniform_helper",
+        "//absl/base:config",
+        "//absl/base:raw_logging_internal",
+        "//absl/strings",
+        "//absl/strings:string_view",
+    ],
+)
+
 cc_test(
     name = "nanobenchmark_test",
     size = "small",
@@ -628,19 +808,25 @@ cc_test(
     linkopts = ABSL_DEFAULT_LINKOPTS,
     tags = [
         "benchmark",
+        "no_test_ios_sim_arm64",
         "no_test_ios_x86_64",
         "no_test_loonix",  # Crashing.
+        "no_test_wasm",
     ],
     deps = [
         ":nanobenchmark",
-        "//absl/base:raw_logging_internal",
+        "//absl/base:config",
+        "//absl/log",
+        "//absl/log:check",
         "//absl/strings",
+        "//absl/strings:str_format",
     ],
 )
 
 cc_test(
     name = "randen_benchmarks",
     size = "medium",
+    timeout = "long",
     srcs = ["randen_benchmarks.cc"],
     copts = ABSL_TEST_COPTS + ABSL_RANDOM_RANDEN_COPTS,
     flaky = 1,
@@ -648,7 +834,6 @@ cc_test(
     tags = ABSL_RANDOM_NONPORTABLE_TAGS + ["benchmark"],
     deps = [
         ":nanobenchmark",
-        ":platform",
         ":randen",
         ":randen_engine",
         ":randen_hwaes",
@@ -661,11 +846,24 @@ cc_test(
 
 cc_test(
     name = "iostream_state_saver_test",
-    size = "small",
     srcs = ["iostream_state_saver_test.cc"],
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":iostream_state_saver",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "uniform_helper_test",
+    size = "small",
+    srcs = ["uniform_helper_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":uniform_helper",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
diff --git a/absl/random/internal/chi_square.cc b/absl/random/internal/chi_square.cc
index c0acc947788..481c1c3da32 100644
--- a/absl/random/internal/chi_square.cc
+++ b/absl/random/internal/chi_square.cc
@@ -16,17 +16,17 @@
 
 #include <cmath>
 
+#include "absl/base/config.h"
 #include "absl/random/internal/distribution_test_util.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 namespace {
 
 #if defined(__EMSCRIPTEN__)
 // Workaround __EMSCRIPTEN__ error: llvm_fma_f64 not found.
-inline double fma(double x, double y, double z) {
-  return (x * y) + z;
-}
+inline double fma(double x, double y, double z) { return (x * y) + z; }
 #endif
 
 // Use Horner's method to evaluate a polynomial.
@@ -104,9 +104,8 @@ double normal_survival(double z) {
 // p-value, usually using bisection. Also known by the name CRITCHI.
 double ChiSquareValue(int dof, double p) {
   static constexpr double kChiEpsilon =
-      0.000001;  // Accuracy of the approximation.
-  static constexpr double kChiMax =
-      99999.0;  // Maximum chi-squared value.
+      0.000001;                               // Accuracy of the approximation.
+  static constexpr double kChiMax = 99999.0;  // Maximum chi-squared value.
 
   const double p_value = 1.0 - p;
   if (dof < 1 || p_value > 1.0) {
@@ -124,7 +123,8 @@ double ChiSquareValue(int dof, double p) {
     const double variance = 2.0 / (9 * dof);
     // Cannot use this method if the variance is 0.
     if (variance != 0) {
-      return std::pow(z * std::sqrt(variance) + mean, 3.0) * dof;
+      double term = z * std::sqrt(variance) + mean;
+      return dof * (term * term * term);
     }
   }
 
@@ -227,4 +227,5 @@ double ChiSquarePValue(double chi_square, int dof) {
 }
 
 }  // namespace random_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/random/internal/chi_square.h b/absl/random/internal/chi_square.h
index fa8646f274c..07f4fbe5229 100644
--- a/absl/random/internal/chi_square.h
+++ b/absl/random/internal/chi_square.h
@@ -26,7 +26,10 @@
 
 #include <cassert>
 
+#include "absl/base/config.h"
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
 constexpr const char kChiSquared[] = "chi-squared";
@@ -80,6 +83,7 @@ double ChiSquareValue(int dof, double p);
 double ChiSquarePValue(double chi_square, int dof);
 
 }  // namespace random_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_INTERNAL_CHI_SQUARE_H_
diff --git a/absl/random/internal/chi_square_test.cc b/absl/random/internal/chi_square_test.cc
index 5025defac12..964cd222ad6 100644
--- a/absl/random/internal/chi_square_test.cc
+++ b/absl/random/internal/chi_square_test.cc
@@ -22,6 +22,7 @@
 #include <vector>
 
 #include "gtest/gtest.h"
+#include "absl/algorithm/container.h"
 #include "absl/base/macros.h"
 
 using absl::random_internal::ChiSquare;
@@ -160,7 +161,7 @@ TEST(ChiSquareTest, CalcChiSquare) {
   for (const auto& spec : specs) {
     SCOPED_TRACE(spec.line);
     double chi_square = 0;
-    for (int i = 0; i < spec.expected.size(); ++i) {
+    for (size_t i = 0; i < spec.expected.size(); ++i) {
       const double diff = spec.actual[i] - spec.expected[i];
       chi_square += (diff * diff) / spec.expected[i];
     }
@@ -176,8 +177,8 @@ TEST(ChiSquareTest, CalcChiSquareInt64) {
   // $ python -c "import scipy.stats
   // > print scipy.stats.chisquare([910293487, 910292491, 910216780])[0]"
   // 4.25410123524
-  double sum = std::accumulate(std::begin(data), std::end(data), double{0});
-  size_t n = std::distance(std::begin(data), std::end(data));
+  double sum = absl::c_accumulate(data, double{0});
+  size_t n = absl::c_distance(data);
   double a = ChiSquareWithExpected(std::begin(data), std::end(data), sum / n);
   EXPECT_NEAR(4.254101, a, 1e-6);
 
@@ -291,11 +292,10 @@ TEST(ChiSquareTest, TableData) {
       /*97*/ {115.223, 120.990, 126.141, 132.309, 145.789},
       /*98*/ {116.315, 122.108, 127.282, 133.476, 147.010},
       /*99*/ {117.407, 123.225, 128.422, 134.642, 148.230},
-      /*100*/ {118.498, 124.342, 129.561, 135.807, 149.449}
-      /**/};
+      /*100*/ {118.498, 124.342, 129.561, 135.807, 149.449} /**/};
 
   //    0.90      0.95     0.975      0.99     0.999
-  for (int i = 0; i < ABSL_ARRAYSIZE(data); i++) {
+  for (size_t i = 0; i < std::size(data); i++) {
     const double E = 0.0001;
     EXPECT_NEAR(ChiSquarePValue(data[i][0], i + 1), 0.10, E)
         << i << " " << data[i][0];
@@ -344,8 +344,8 @@ TEST(ChiSquareTest, DiceRolls) {
   // The dof value of 4, @95% = 9.488 (see above test)
   // The dof value of 5, @95% = 11.070
   const int rolls[6] = {22, 11, 17, 14, 20, 18};
-  double sum = std::accumulate(std::begin(rolls), std::end(rolls), double{0});
-  size_t n = std::distance(std::begin(rolls), std::end(rolls));
+  double sum = absl::c_accumulate(rolls, double{0});
+  size_t n = absl::c_distance(rolls);
 
   double a = ChiSquareWithExpected(std::begin(rolls), std::end(rolls), sum / n);
   EXPECT_NEAR(a, 4.70588, 1e-5);
diff --git a/absl/random/internal/distribution_caller.h b/absl/random/internal/distribution_caller.h
index 0318e1f8072..03ace5afabe 100644
--- a/absl/random/internal/distribution_caller.h
+++ b/absl/random/internal/distribution_caller.h
@@ -17,9 +17,18 @@
 #ifndef ABSL_RANDOM_INTERNAL_DISTRIBUTION_CALLER_H_
 #define ABSL_RANDOM_INTERNAL_DISTRIBUTION_CALLER_H_
 
+#include <tuple>
+#include <type_traits>
 #include <utility>
 
+#include "absl/base/config.h"
+#include "absl/base/fast_type_id.h"
+#include "absl/meta/type_traits.h"
+#include "absl/random/mocking_access.h"
+#include "absl/utility/utility.h"
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
 // DistributionCaller provides an opportunity to overload the general
@@ -27,30 +36,48 @@ namespace random_internal {
 // to intercept such calls.
 template <typename URBG>
 struct DistributionCaller {
-  // Call the provided distribution type. The parameters are expected
-  // to be explicitly specified.
-  // DistrT is the distribution type.
-  // FormatT is the formatter type:
-  //
-  // struct FormatT {
-  //   using result_type = distribution_t::result_type;
-  //   static std::string FormatCall(
-  //       const distribution_t& distr,
-  //       absl::Span<const result_type>);
-  //
-  //   static std::string FormatExpectation(
-  //       absl::string_view match_args,
-  //       absl::Span<const result_t> results);
-  // }
-  //
-  template <typename DistrT, typename FormatT, typename... Args>
-  static typename DistrT::result_type Call(URBG* urbg, Args&&... args) {
+  static_assert(!std::is_pointer_v<URBG>,
+                "You must pass a reference, not a pointer.");
+
+  using RandomMockingAccess = ::absl::RandomMockingAccess;
+
+  // Default implementation of distribution caller.
+  template <typename DistrT, typename... Args>
+  static typename DistrT::result_type Impl(std::false_type, URBG* urbg,
+                                           Args&&... args) {
     DistrT dist(std::forward<Args>(args)...);
     return dist(*urbg);
   }
+
+  // Mock implementation of distribution caller.
+  // The underlying KeyT must match the KeyT constructed by MockOverloadSet.
+  template <typename DistrT, typename... Args>
+  static typename DistrT::result_type Impl(std::true_type, URBG* urbg,
+                                           Args&&... args) {
+    using ResultT = typename DistrT::result_type;
+    using ArgTupleT = std::tuple<std::decay_t<Args>...>;
+    using KeyT = ResultT(DistrT, ArgTupleT);
+
+    ArgTupleT arg_tuple(std::forward<Args>(args)...);
+    ResultT result;
+    if (!RandomMockingAccess::InvokeMock<URBG>(urbg, FastTypeId<KeyT>(),
+                                               &arg_tuple, &result)) {
+      auto dist = std::make_from_tuple<DistrT>(arg_tuple);
+      result = dist(*urbg);
+    }
+    return result;
+  }
+
+  // Default implementation of distribution caller.
+  template <typename DistrT, typename... Args>
+  static typename DistrT::result_type Call(URBG* urbg, Args&&... args) {
+    return Impl<DistrT, Args...>(RandomMockingAccess::HasInvokeMock<URBG>{},
+                                 urbg, std::forward<Args>(args)...);
+  }
 };
 
 }  // namespace random_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_INTERNAL_DISTRIBUTION_CALLER_H_
diff --git a/absl/random/internal/distribution_impl.h b/absl/random/internal/distribution_impl.h
deleted file mode 100644
index 9b6ffb0fb50..00000000000
--- a/absl/random/internal/distribution_impl.h
+++ /dev/null
@@ -1,260 +0,0 @@
-// Copyright 2017 The Abseil Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-#ifndef ABSL_RANDOM_INTERNAL_DISTRIBUTION_IMPL_H_
-#define ABSL_RANDOM_INTERNAL_DISTRIBUTION_IMPL_H_
-
-// This file contains some implementation details which are used by one or more
-// of the absl random number distributions.
-
-#include <cfloat>
-#include <cstddef>
-#include <cstdint>
-#include <cstring>
-#include <limits>
-#include <type_traits>
-
-#if (defined(_WIN32) || defined(_WIN64)) && defined(_M_IA64)
-#include <intrin.h>  // NOLINT(build/include_order)
-#pragma intrinsic(_umul128)
-#define ABSL_INTERNAL_USE_UMUL128 1
-#endif
-
-#include "absl/base/config.h"
-#include "absl/base/internal/bits.h"
-#include "absl/numeric/int128.h"
-#include "absl/random/internal/fastmath.h"
-#include "absl/random/internal/traits.h"
-
-namespace absl {
-namespace random_internal {
-
-// Creates a double from `bits`, with the template fields controlling the
-// output.
-//
-// RandU64To is both more efficient and generates more unique values in the
-// result interval than known implementations of std::generate_canonical().
-//
-// The `Signed` parameter controls whether positive, negative, or both are
-// returned (thus affecting the output interval).
-//   When Signed == SignedValueT, range is U(-1, 1)
-//   When Signed == NegativeValueT, range is U(-1, 0)
-//   When Signed == PositiveValueT, range is U(0, 1)
-//
-// When the `IncludeZero` parameter is true, the function may return 0 for some
-// inputs, otherwise it never returns 0.
-//
-// The `ExponentBias` parameter determines the scale of the output range by
-// adjusting the exponent.
-//
-// When a value in U(0,1) is required, use:
-//   RandU64ToDouble<PositiveValueT, true, 0>();
-//
-// When a value in U(-1,1) is required, use:
-//   RandU64ToDouble<SignedValueT, false, 0>() => U(-1, 1)
-// This generates more distinct values than the mathematically equivalent
-// expression `U(0, 1) * 2.0 - 1.0`, and is preferable.
-//
-// Scaling the result by powers of 2 (and avoiding a multiply) is also possible:
-//   RandU64ToDouble<PositiveValueT, false, 1>();  => U(0, 2)
-//   RandU64ToDouble<PositiveValueT, false, -1>();  => U(0, 0.5)
-//
-
-// Tristate types controlling the output.
-struct PositiveValueT {};
-struct NegativeValueT {};
-struct SignedValueT {};
-
-// RandU64ToDouble is the double-result variant of RandU64To, described above.
-template <typename Signed, bool IncludeZero, int ExponentBias = 0>
-inline double RandU64ToDouble(uint64_t bits) {
-  static_assert(std::is_same<Signed, PositiveValueT>::value ||
-                    std::is_same<Signed, NegativeValueT>::value ||
-                    std::is_same<Signed, SignedValueT>::value,
-                "");
-
-  // Maybe use the left-most bit for a sign bit.
-  uint64_t sign = std::is_same<Signed, NegativeValueT>::value
-                      ? 0x8000000000000000ull
-                      : 0;  // Sign bits.
-
-  if (std::is_same<Signed, SignedValueT>::value) {
-    sign = bits & 0x8000000000000000ull;
-    bits = bits & 0x7FFFFFFFFFFFFFFFull;
-  }
-  if (IncludeZero) {
-    if (bits == 0u) return 0;
-  }
-
-  // Number of leading zeros is mapped to the exponent: 2^-clz
-  int clz = base_internal::CountLeadingZeros64(bits);
-  // Shift number left to erase leading zeros.
-  bits <<= IncludeZero ? clz : (clz & 63);
-
-  // Shift number right to remove bits that overflow double mantissa.  The
-  // direction of the shift depends on `clz`.
-  bits >>= (64 - DBL_MANT_DIG);
-
-  // Compute IEEE 754 double exponent.
-  // In the Signed case, bits is a 63-bit number with a 0 msb.  Adjust the
-  // exponent to account for that.
-  const uint64_t exp =
-      (std::is_same<Signed, SignedValueT>::value ? 1023U : 1022U) +
-      static_cast<uint64_t>(ExponentBias - clz);
-  constexpr int kExp = DBL_MANT_DIG - 1;
-  // Construct IEEE 754 double from exponent and mantissa.
-  const uint64_t val = sign | (exp << kExp) | (bits & ((1ULL << kExp) - 1U));
-
-  double res;
-  static_assert(sizeof(res) == sizeof(val), "double is not 64 bit");
-  // Memcpy value from "val" to "res" to avoid aliasing problems.  Assumes that
-  // endian-ness is same for double and uint64_t.
-  std::memcpy(&res, &val, sizeof(res));
-
-  return res;
-}
-
-// RandU64ToFloat is the float-result variant of RandU64To, described above.
-template <typename Signed, bool IncludeZero, int ExponentBias = 0>
-inline float RandU64ToFloat(uint64_t bits) {
-  static_assert(std::is_same<Signed, PositiveValueT>::value ||
-                    std::is_same<Signed, NegativeValueT>::value ||
-                    std::is_same<Signed, SignedValueT>::value,
-                "");
-
-  // Maybe use the left-most bit for a sign bit.
-  uint64_t sign = std::is_same<Signed, NegativeValueT>::value
-                      ? 0x80000000ul
-                      : 0;  // Sign bits.
-
-  if (std::is_same<Signed, SignedValueT>::value) {
-    uint64_t a = bits & 0x8000000000000000ull;
-    sign = static_cast<uint32_t>(a >> 32);
-    bits = bits & 0x7FFFFFFFFFFFFFFFull;
-  }
-  if (IncludeZero) {
-    if (bits == 0u) return 0;
-  }
-
-  // Number of leading zeros is mapped to the exponent: 2^-clz
-  int clz = base_internal::CountLeadingZeros64(bits);
-  // Shift number left to erase leading zeros.
-  bits <<= IncludeZero ? clz : (clz & 63);
-  // Shift number right to remove bits that overflow double mantissa.  The
-  // direction of the shift depends on `clz`.
-  bits >>= (64 - FLT_MANT_DIG);
-
-  // Construct IEEE 754 float exponent.
-  // In the Signed case, bits is a 63-bit number with a 0 msb.  Adjust the
-  // exponent to account for that.
-  const uint32_t exp =
-      (std::is_same<Signed, SignedValueT>::value ? 127U : 126U) +
-      static_cast<uint32_t>(ExponentBias - clz);
-  constexpr int kExp = FLT_MANT_DIG - 1;
-  const uint32_t val = sign | (exp << kExp) | (bits & ((1U << kExp) - 1U));
-
-  float res;
-  static_assert(sizeof(res) == sizeof(val), "float is not 32 bit");
-  // Assumes that endian-ness is same for float and uint32_t.
-  std::memcpy(&res, &val, sizeof(res));
-
-  return res;
-}
-
-template <typename Result>
-struct RandU64ToReal {
-  template <typename Signed, bool IncludeZero, int ExponentBias = 0>
-  static inline Result Value(uint64_t bits) {
-    return RandU64ToDouble<Signed, IncludeZero, ExponentBias>(bits);
-  }
-};
-
-template <>
-struct RandU64ToReal<float> {
-  template <typename Signed, bool IncludeZero, int ExponentBias = 0>
-  static inline float Value(uint64_t bits) {
-    return RandU64ToFloat<Signed, IncludeZero, ExponentBias>(bits);
-  }
-};
-
-inline uint128 MultiplyU64ToU128(uint64_t a, uint64_t b) {
-#if defined(ABSL_HAVE_INTRINSIC_INT128)
-  return uint128(static_cast<__uint128_t>(a) * b);
-#elif defined(ABSL_INTERNAL_USE_UMUL128)
-  // uint64_t * uint64_t => uint128 multiply using imul intrinsic on MSVC.
-  uint64_t high = 0;
-  const uint64_t low = _umul128(a, b, &high);
-  return absl::MakeUint128(high, low);
-#else
-  // uint128(a) * uint128(b) in emulated mode computes a full 128-bit x 128-bit
-  // multiply.  However there are many cases where that is not necessary, and it
-  // is only necessary to support a 64-bit x 64-bit = 128-bit multiply.  This is
-  // for those cases.
-  const uint64_t a00 = static_cast<uint32_t>(a);
-  const uint64_t a32 = a >> 32;
-  const uint64_t b00 = static_cast<uint32_t>(b);
-  const uint64_t b32 = b >> 32;
-
-  const uint64_t c00 = a00 * b00;
-  const uint64_t c32a = a00 * b32;
-  const uint64_t c32b = a32 * b00;
-  const uint64_t c64 = a32 * b32;
-
-  const uint32_t carry =
-      static_cast<uint32_t>(((c00 >> 32) + static_cast<uint32_t>(c32a) +
-                             static_cast<uint32_t>(c32b)) >>
-                            32);
-
-  return absl::MakeUint128(c64 + (c32a >> 32) + (c32b >> 32) + carry,
-                           c00 + (c32a << 32) + (c32b << 32));
-#endif
-}
-
-// wide_multiply<T> multiplies two N-bit values to a 2N-bit result.
-template <typename UIntType>
-struct wide_multiply {
-  static constexpr size_t kN = std::numeric_limits<UIntType>::digits;
-  using input_type = UIntType;
-  using result_type = typename random_internal::unsigned_bits<kN * 2>::type;
-
-  static result_type multiply(input_type a, input_type b) {
-    return static_cast<result_type>(a) * b;
-  }
-
-  static input_type hi(result_type r) { return r >> kN; }
-  static input_type lo(result_type r) { return r; }
-
-  static_assert(std::is_unsigned<UIntType>::value,
-                "Class-template wide_multiply<> argument must be unsigned.");
-};
-
-#ifndef ABSL_HAVE_INTRINSIC_INT128
-template <>
-struct wide_multiply<uint64_t> {
-  using input_type = uint64_t;
-  using result_type = uint128;
-
-  static result_type multiply(uint64_t a, uint64_t b) {
-    return MultiplyU64ToU128(a, b);
-  }
-
-  static uint64_t hi(result_type r) { return Uint128High64(r); }
-  static uint64_t lo(result_type r) { return Uint128Low64(r); }
-};
-#endif
-
-}  // namespace random_internal
-}  // namespace absl
-
-#endif  // ABSL_RANDOM_INTERNAL_DISTRIBUTION_IMPL_H_
diff --git a/absl/random/internal/distribution_test_util.cc b/absl/random/internal/distribution_test_util.cc
index 85c8d596ebf..4ae6fcbfd75 100644
--- a/absl/random/internal/distribution_test_util.cc
+++ b/absl/random/internal/distribution_test_util.cc
@@ -14,17 +14,23 @@
 
 #include "absl/random/internal/distribution_test_util.h"
 
+#include <algorithm>
 #include <cassert>
 #include <cmath>
+#include <limits>
+#include <ostream>
 #include <string>
-#include <vector>
 
+#include "absl/base/config.h"
 #include "absl/base/internal/raw_logging.h"
 #include "absl/base/macros.h"
 #include "absl/strings/str_cat.h"
 #include "absl/strings/str_format.h"
+#include "absl/strings/string_view.h"
+#include "absl/types/span.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 namespace {
 
@@ -212,7 +218,7 @@ double BetaIncompleteImpl(const double x, const double p, const double q,
   double result = 1.;
   int ns = static_cast<int>(q + xc * psq);
 
-  // Use the soper reduction forumla.
+  // Use the soper reduction formula.
   double rx = (ns == 0) ? x : x / xc;
   double temp = q - ai;
   for (;;) {
@@ -235,7 +241,7 @@ double BetaIncompleteImpl(const double x, const double p, const double q,
     }
   }
 
-  // NOTE: See also TOMS Alogrithm 708.
+  // NOTE: See also TOMS Algorithm 708.
   // http://www.netlib.org/toms/index.html
   //
   // NOTE: The NWSC library also includes BRATIO / ISUBX (p87)
@@ -246,7 +252,7 @@ double BetaIncompleteImpl(const double x, const double p, const double q,
 // https://www.jstor.org/stable/2346798?read-now=1&seq=4#page_scan_tab_contents
 // https://www.jstor.org/stable/2346887?seq=1#page_scan_tab_contents
 //
-// XINBTA(p, q, beta, alhpa)
+// XINBTA(p, q, beta, alpha)
 //  p:     the value of the parameter p.
 //  q:     the value of the parameter q.
 //  beta:  the value of ln B(p, q)
@@ -413,4 +419,5 @@ double MaxErrorTolerance(double acceptance_probability) {
 }
 
 }  // namespace random_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/random/internal/distribution_test_util.h b/absl/random/internal/distribution_test_util.h
index b5ba49fa7e1..1189340e3a7 100644
--- a/absl/random/internal/distribution_test_util.h
+++ b/absl/random/internal/distribution_test_util.h
@@ -16,9 +16,9 @@
 #define ABSL_RANDOM_INTERNAL_DISTRIBUTION_TEST_UTIL_H_
 
 #include <cstddef>
-#include <iostream>
-#include <vector>
+#include <ostream>
 
+#include "absl/base/config.h"
 #include "absl/strings/string_view.h"
 #include "absl/types/span.h"
 
@@ -26,6 +26,7 @@
 // non-test code.
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
 // http://webspace.ship.edu/pgmarr/Geo441/Lectures/Lec%205%20-%20Normality%20Testing.pdf
@@ -95,7 +96,7 @@ double BetaIncomplete(double x, double p, double q);
 //   https://www.jstor.org/stable/2346798
 //   https://www.jstor.org/stable/2346887
 //
-// BetaIncompleteInv(p, q, beta, alhpa)
+// BetaIncompleteInv(p, q, beta, alpha)
 //   `p` is beta parameter p, `q` is beta parameter q.
 //   `alpha` is the value of the lower tail area.
 //
@@ -106,6 +107,7 @@ double BetaIncomplete(double x, double p, double q);
 double BetaIncompleteInv(double p, double q, double alpha);
 
 }  // namespace random_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_INTERNAL_DISTRIBUTION_TEST_UTIL_H_
diff --git a/absl/random/internal/distribution_test_util_test.cc b/absl/random/internal/distribution_test_util_test.cc
index c49d44fb479..1b66879ffcb 100644
--- a/absl/random/internal/distribution_test_util_test.cc
+++ b/absl/random/internal/distribution_test_util_test.cc
@@ -14,6 +14,9 @@
 
 #include "absl/random/internal/distribution_test_util.h"
 
+#include <utility>
+#include <vector>
+
 #include "gtest/gtest.h"
 
 namespace {
diff --git a/absl/random/internal/distributions.h b/absl/random/internal/distributions.h
deleted file mode 100644
index 96f8bae3918..00000000000
--- a/absl/random/internal/distributions.h
+++ /dev/null
@@ -1,80 +0,0 @@
-// Copyright 2019 The Abseil Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-#ifndef ABSL_RANDOM_INTERNAL_DISTRIBUTIONS_H_
-#define ABSL_RANDOM_INTERNAL_DISTRIBUTIONS_H_
-
-#include <type_traits>
-
-#include "absl/meta/type_traits.h"
-#include "absl/random/internal/distribution_caller.h"
-#include "absl/random/internal/traits.h"
-#include "absl/random/internal/uniform_helper.h"
-
-namespace absl {
-namespace random_internal {
-template <typename D>
-struct DistributionFormatTraits;
-
-// UniformImpl implements the core logic of the Uniform<T> call, which is to
-// select the correct distribution type, compute the bounds based on the
-// interval tag, and then generate a value.
-template <typename NumType, typename TagType, typename URBG>
-NumType UniformImpl(TagType tag,
-                    URBG& urbg,  // NOLINT(runtime/references)
-                    NumType lo, NumType hi) {
-  static_assert(
-      std::is_arithmetic<NumType>::value,
-      "absl::Uniform<T>() must use an integer or real parameter type.");
-
-  using distribution_t =
-      UniformDistributionWrapper<absl::decay_t<TagType>, NumType>;
-  using format_t = random_internal::DistributionFormatTraits<distribution_t>;
-  auto a = uniform_lower_bound(tag, lo, hi);
-  auto b = uniform_upper_bound(tag, lo, hi);
-
-  // TODO(lar): it doesn't make a lot of sense to ask for a random number in an
-  // empty range.  Right now we just return a boundary--even though that
-  // boundary is not an acceptable value!  Is there something better we can do
-  // here?
-  if (a > b) return a;
-
-  using gen_t = absl::decay_t<URBG>;
-  return DistributionCaller<gen_t>::template Call<distribution_t, format_t>(
-      &urbg, tag, lo, hi);
-}
-
-// In the absence of an explicitly provided return-type, the template
-// "uniform_inferred_return_t<A, B>" is used to derive a suitable type, based on
-// the data-types of the endpoint-arguments {A lo, B hi}.
-//
-// Given endpoints {A lo, B hi}, one of {A, B} will be chosen as the
-// return-type, if one type can be implicitly converted into the other, in a
-// lossless way. The template "is_widening_convertible" implements the
-// compile-time logic for deciding if such a conversion is possible.
-//
-// If no such conversion between {A, B} exists, then the overload for
-// absl::Uniform() will be discarded, and the call will be ill-formed.
-// Return-type for absl::Uniform() when the return-type is inferred.
-template <typename A, typename B>
-using uniform_inferred_return_t =
-    absl::enable_if_t<absl::disjunction<is_widening_convertible<A, B>,
-                                        is_widening_convertible<B, A>>::value,
-                      typename std::conditional<
-                          is_widening_convertible<A, B>::value, B, A>::type>;
-
-}  // namespace random_internal
-}  // namespace absl
-
-#endif  // ABSL_RANDOM_INTERNAL_DISTRIBUTIONS_H_
diff --git a/absl/random/internal/pool_urbg.cc b/absl/random/internal/entropy_pool.cc
similarity index 56%
rename from absl/random/internal/pool_urbg.cc
rename to absl/random/internal/entropy_pool.cc
index f2e1c1f6fe2..21f83052362 100644
--- a/absl/random/internal/pool_urbg.cc
+++ b/absl/random/internal/entropy_pool.cc
@@ -12,7 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-#include "absl/random/internal/pool_urbg.h"
+#include "absl/random/internal/entropy_pool.h"
 
 #include <algorithm>
 #include <atomic>
@@ -20,23 +20,24 @@
 #include <cstring>
 #include <iterator>
 
+#include "absl/algorithm/container.h"
 #include "absl/base/attributes.h"
 #include "absl/base/call_once.h"
 #include "absl/base/config.h"
-#include "absl/base/internal/endian.h"
-#include "absl/base/internal/raw_logging.h"
 #include "absl/base/internal/spinlock.h"
-#include "absl/base/internal/sysinfo.h"
-#include "absl/base/internal/unaligned_access.h"
 #include "absl/base/optimization.h"
+#include "absl/base/thread_annotations.h"
 #include "absl/random/internal/randen.h"
+#include "absl/random/internal/randen_traits.h"
 #include "absl/random/internal/seed_material.h"
 #include "absl/random/seed_gen_exception.h"
+#include "absl/types/span.h"
 
 using absl::base_internal::SpinLock;
 using absl::base_internal::SpinLockHolder;
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 namespace {
 
@@ -44,83 +45,50 @@ namespace {
 // single generator within a RandenPool<T>. It is an internal implementation
 // detail, and does not aim to conform to [rand.req.urng].
 //
-// NOTE: There are alignment issues when used on ARM, for instance.
-// See the allocation code in PoolAlignedAlloc().
-class RandenPoolEntry {
+// At least 32-byte alignment is required for the state_ array on some ARM
+// platforms.  We also want this aligned to a cacheline to eliminate false
+// sharing.
+class alignas(std::max(size_t{ABSL_CACHELINE_SIZE}, size_t{32}))
+    RandenPoolEntry {
  public:
   static constexpr size_t kState = RandenTraits::kStateBytes / sizeof(uint32_t);
   static constexpr size_t kCapacity =
       RandenTraits::kCapacityBytes / sizeof(uint32_t);
 
   void Init(absl::Span<const uint32_t> data) {
-    SpinLockHolder l(&mu_);  // Always uncontested.
-    std::copy(data.begin(), data.end(), std::begin(state_));
+    SpinLockHolder l(mu_);  // Always uncontested.
+    absl::c_copy(data, std::begin(state_));
     next_ = kState;
   }
 
   // Copy bytes into out.
   void Fill(uint8_t* out, size_t bytes) ABSL_LOCKS_EXCLUDED(mu_);
 
-  // Returns random bits from the buffer in units of T.
-  template <typename T>
-  inline T Generate() ABSL_LOCKS_EXCLUDED(mu_);
-
-  inline void MaybeRefill() ABSL_EXCLUSIVE_LOCKS_REQUIRED(mu_) {
+  void MaybeRefill() ABSL_EXCLUSIVE_LOCKS_REQUIRED(mu_) {
     if (next_ >= kState) {
       next_ = kCapacity;
       impl_.Generate(state_);
     }
   }
 
+  size_t available() const ABSL_SHARED_LOCKS_REQUIRED(mu_) {
+    return kState - next_;
+  }
+
  private:
   // Randen URBG state.
-  uint32_t state_[kState] ABSL_GUARDED_BY(mu_);  // First to satisfy alignment.
+  // At least 32-byte alignment is required by ARM platform code.
+  alignas(32) uint32_t state_[kState] ABSL_GUARDED_BY(mu_);
   SpinLock mu_;
   const Randen impl_;
   size_t next_ ABSL_GUARDED_BY(mu_);
 };
 
-template <>
-inline uint8_t RandenPoolEntry::Generate<uint8_t>() {
-  SpinLockHolder l(&mu_);
-  MaybeRefill();
-  return static_cast<uint8_t>(state_[next_++]);
-}
-
-template <>
-inline uint16_t RandenPoolEntry::Generate<uint16_t>() {
-  SpinLockHolder l(&mu_);
-  MaybeRefill();
-  return static_cast<uint16_t>(state_[next_++]);
-}
-
-template <>
-inline uint32_t RandenPoolEntry::Generate<uint32_t>() {
-  SpinLockHolder l(&mu_);
-  MaybeRefill();
-  return state_[next_++];
-}
-
-template <>
-inline uint64_t RandenPoolEntry::Generate<uint64_t>() {
-  SpinLockHolder l(&mu_);
-  if (next_ >= kState - 1) {
-    next_ = kCapacity;
-    impl_.Generate(state_);
-  }
-  auto p = state_ + next_;
-  next_ += 2;
-
-  uint64_t result;
-  std::memcpy(&result, p, sizeof(result));
-  return result;
-}
-
 void RandenPoolEntry::Fill(uint8_t* out, size_t bytes) {
-  SpinLockHolder l(&mu_);
+  SpinLockHolder l(mu_);
   while (bytes > 0) {
     MaybeRefill();
-    size_t remaining = (kState - next_) * sizeof(state_[0]);
+    size_t remaining = available() * sizeof(state_[0]);
     size_t to_copy = std::min(bytes, remaining);
     std::memcpy(out, &state_[next_], to_copy);
     out += to_copy;
@@ -130,7 +98,7 @@ void RandenPoolEntry::Fill(uint8_t* out, size_t bytes) {
 }
 
 // Number of pooled urbg entries.
-static constexpr int kPoolSize = 8;
+static constexpr size_t kPoolSize = 8;
 
 // Shared pool entries.
 static absl::once_flag pool_once;
@@ -146,15 +114,15 @@ ABSL_CACHELINE_ALIGNED static RandenPoolEntry* shared_pools[kPoolSize];
 // on subsequent runs the order within the same program may be significantly
 // different. However, as other thread IDs are not assigned sequentially,
 // this is not expected to matter.
-int GetPoolID() {
+size_t GetPoolID() {
   static_assert(kPoolSize >= 1,
                 "At least one urbg instance is required for PoolURBG");
 
-  ABSL_CONST_INIT static std::atomic<int64_t> sequence{0};
+  ABSL_CONST_INIT static std::atomic<uint64_t> sequence{0};
 
 #ifdef ABSL_HAVE_THREAD_LOCAL
-  static thread_local int my_pool_id = -1;
-  if (ABSL_PREDICT_FALSE(my_pool_id < 0)) {
+  static thread_local size_t my_pool_id = kPoolSize;
+  if (ABSL_PREDICT_FALSE(my_pool_id == kPoolSize)) {
     my_pool_id = (sequence++ % kPoolSize);
   }
   return my_pool_id;
@@ -170,8 +138,8 @@ int GetPoolID() {
 
   // Store the value in the pthread_{get/set}specific. However an uninitialized
   // value is 0, so add +1 to distinguish from the null value.
-  intptr_t my_pool_id =
-      reinterpret_cast<intptr_t>(pthread_getspecific(tid_key));
+  uintptr_t my_pool_id =
+      reinterpret_cast<uintptr_t>(pthread_getspecific(tid_key));
   if (ABSL_PREDICT_FALSE(my_pool_id == 0)) {
     // No allocated ID, allocate the next value, cache it, and return.
     my_pool_id = (sequence++ % kPoolSize) + 1;
@@ -184,39 +152,17 @@ int GetPoolID() {
 #endif
 }
 
-// Allocate a RandenPoolEntry with at least 32-byte alignment, which is required
-// by ARM platform code.
-RandenPoolEntry* PoolAlignedAlloc() {
-  constexpr size_t kAlignment =
-      ABSL_CACHELINE_SIZE > 32 ? ABSL_CACHELINE_SIZE : 32;
-
-  // Not all the platforms that we build for have std::aligned_alloc, however
-  // since we never free these objects, we can over allocate and munge the
-  // pointers to the correct alignment.
-  void* memory = std::malloc(sizeof(RandenPoolEntry) + kAlignment);
-  auto x = reinterpret_cast<intptr_t>(memory);
-  auto y = x % kAlignment;
-  void* aligned =
-      (y == 0) ? memory : reinterpret_cast<void*>(x + kAlignment - y);
-  return new (aligned) RandenPoolEntry();
-}
-
 // Allocate and initialize kPoolSize objects of type RandenPoolEntry.
-//
-// The initialization strategy is to initialize one object directly from
-// OS entropy, then to use that object to seed all of the individual
-// pool instances.
 void InitPoolURBG() {
   static constexpr size_t kSeedSize =
       RandenTraits::kStateBytes / sizeof(uint32_t);
-  // Read the seed data from OS entropy once.
+  // Read OS entropy once, and use it to initialize each pool entry.
   uint32_t seed_material[kPoolSize * kSeedSize];
-  if (!random_internal::ReadSeedMaterialFromOSEntropy(
-          absl::MakeSpan(seed_material))) {
-    random_internal::ThrowSeedGenException();
+  if (!ReadSeedMaterialFromOSEntropy(absl::MakeSpan(seed_material))) {
+    ThrowSeedGenException();
   }
-  for (int i = 0; i < kPoolSize; i++) {
-    shared_pools[i] = PoolAlignedAlloc();
+  for (size_t i = 0; i < kPoolSize; i++) {
+    shared_pools[i] = new RandenPoolEntry();
     shared_pools[i]->Init(
         absl::MakeSpan(&seed_material[i * kSeedSize], kSeedSize));
   }
@@ -230,23 +176,11 @@ RandenPoolEntry* GetPoolForCurrentThread() {
 
 }  // namespace
 
-template <typename T>
-typename RandenPool<T>::result_type RandenPool<T>::Generate() {
+void GetEntropyFromRandenPool(void* dest, size_t bytes) {
   auto* pool = GetPoolForCurrentThread();
-  return pool->Generate<T>();
+  pool->Fill(reinterpret_cast<uint8_t*>(dest), bytes);
 }
 
-template <typename T>
-void RandenPool<T>::Fill(absl::Span<result_type> data) {
-  auto* pool = GetPoolForCurrentThread();
-  pool->Fill(reinterpret_cast<uint8_t*>(data.data()),
-             data.size() * sizeof(result_type));
-}
-
-template class RandenPool<uint8_t>;
-template class RandenPool<uint16_t>;
-template class RandenPool<uint32_t>;
-template class RandenPool<uint64_t>;
-
 }  // namespace random_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/types/bad_any_cast.cc b/absl/random/internal/entropy_pool.h
similarity index 56%
rename from absl/types/bad_any_cast.cc
rename to absl/random/internal/entropy_pool.h
index 505919a5583..970ef8744cc 100644
--- a/absl/types/bad_any_cast.cc
+++ b/absl/random/internal/entropy_pool.h
@@ -12,33 +12,24 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-#include "absl/types/bad_any_cast.h"
+#ifndef ABSL_RANDOM_INTERNAL_ENTROPY_POOL_H_
+#define ABSL_RANDOM_INTERNAL_ENTROPY_POOL_H_
 
-#ifndef ABSL_HAVE_STD_ANY
-
-#include <cstdlib>
+#include <cstddef>
 
 #include "absl/base/config.h"
-#include "absl/base/internal/raw_logging.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace random_internal {
 
-bad_any_cast::~bad_any_cast() = default;
-
-const char* bad_any_cast::what() const noexcept { return "Bad any cast"; }
-
-namespace any_internal {
-
-void ThrowBadAnyCast() {
-#ifdef ABSL_HAVE_EXCEPTIONS
-  throw bad_any_cast();
-#else
-  ABSL_RAW_LOG(FATAL, "Bad any cast");
-  std::abort();
-#endif
-}
+// GetEntropyFromRandenPool() is a helper function that fills a memory region
+// with random bytes from the RandenPool.  This is used by the absl::BitGen
+// implementation to fill the internal buffer.
+void GetEntropyFromRandenPool(void* dest, size_t bytes);
 
-}  // namespace any_internal
+}  // namespace random_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
-#endif  // ABSL_HAVE_STD_ANY
+#endif  // ABSL_RANDOM_INTERNAL_ENTROPY_POOL_H_
diff --git a/absl/random/internal/entropy_pool_test.cc b/absl/random/internal/entropy_pool_test.cc
new file mode 100644
index 00000000000..7739f19dfb6
--- /dev/null
+++ b/absl/random/internal/entropy_pool_test.cc
@@ -0,0 +1,119 @@
+// Copyright 2017 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/random/internal/entropy_pool.h"
+
+#include <bitset>
+#include <cmath>
+#include <cstddef>
+#include <cstdint>
+#include <thread>  // NOLINT
+#include <utility>
+#include <vector>
+
+#include "gtest/gtest.h"
+#include "absl/container/flat_hash_set.h"
+#include "absl/synchronization/mutex.h"
+
+namespace {
+
+using ::absl::random_internal::GetEntropyFromRandenPool;
+
+TEST(EntropyPoolTest, DistinctSequencesPerThread) {
+  using result_type = uint32_t;
+  constexpr int kNumThreads = 12;
+  constexpr size_t kValuesPerThread = 32;
+
+  // Acquire entropy from multiple threads.
+  std::vector<std::vector<result_type>> data;
+  {
+    absl::Mutex mu;
+    std::vector<std::thread> threads;
+    for (int i = 0; i < kNumThreads; i++) {
+      threads.emplace_back([&]() {
+        std::vector<result_type> v(kValuesPerThread);
+        GetEntropyFromRandenPool(v.data(), sizeof(result_type) * v.size());
+        absl::MutexLock l(mu);
+        data.push_back(std::move(v));
+      });
+    }
+    for (auto& t : threads) t.join();
+  }
+
+  EXPECT_EQ(data.size(), kNumThreads);
+
+  // There should be essentially no duplicates in the sequences.
+  size_t expected_size = 0;
+  absl::flat_hash_set<result_type> seen;
+  for (const auto& v : data) {
+    expected_size += v.size();
+    for (result_type x : v) seen.insert(x);
+  }
+  EXPECT_GE(seen.size(), expected_size - 1);
+}
+
+// This validates that sequences are independent.
+TEST(EntropyPoolTest, ValidateDistribution) {
+  using result_type = uint32_t;
+  constexpr int kNumOutputs = 16;
+  std::vector<result_type> a(kNumOutputs);
+  std::vector<result_type> b(kNumOutputs);
+
+  GetEntropyFromRandenPool(a.data(), sizeof(a[0]) * a.size());
+  GetEntropyFromRandenPool(b.data(), sizeof(b[0]) * b.size());
+
+  // Compare the two sequences, counting the number of bits that are different,
+  // then verify using a normal-approximation of the binomial distribution.
+  size_t changed_bits = 0;
+  size_t total_set = 0;
+  size_t equal_count = 0;
+  size_t zero_count = 0;
+  for (size_t i = 0; i < a.size(); ++i) {
+    std::bitset<sizeof(result_type) * 8> changed_set(a[i] ^ b[i]);
+    changed_bits += changed_set.count();
+
+    std::bitset<sizeof(result_type) * 8> a_set(a[i]);
+    std::bitset<sizeof(result_type) * 8> b_set(b[i]);
+    total_set += a_set.count() + b_set.count();
+
+    equal_count += (a[i] == b[i]) ? 1 : 0;
+
+    zero_count += (a[i] == 0) ? 1 : 0;
+    zero_count += (b[i] == 0) ? 1 : 0;
+  }
+
+  constexpr size_t kNBits = kNumOutputs * sizeof(result_type) * 8;
+
+  // This should be a binomial distribution with:
+  //    p = 0.5
+  //    n = kNBits
+  //    sigma =~ 11.3 (sqrt(n * 0.5 * 0.5))
+  // So we expect the number of changed bits to be within 5 standard deviations
+  // of the mean; this should fail less than one in 3 million times.
+  EXPECT_NEAR(changed_bits, kNBits * 0.5, 5 * std::sqrt(kNBits))
+      << "@" << changed_bits / static_cast<double>(kNBits);
+
+  // Verify that the number of set bits is also within the expected range;
+  // Note that this is summed over the two sequences, so the number of trials
+  // is twice the number of bits.
+  EXPECT_NEAR(total_set, kNBits, 5 * std::sqrt(2 * kNBits))
+      << "@" << total_set / static_cast<double>(2 * kNBits);
+
+  // A[i] == B[i] with probability ~= 16 * 1/2^32; certainly less than 1.
+  EXPECT_LE(equal_count, 1);
+
+  // Zeros values must be rare; 32 / 2^32 is certainly less than 1.
+  EXPECT_LE(zero_count, 1);
+}
+}  // namespace
diff --git a/absl/random/internal/explicit_seed_seq.h b/absl/random/internal/explicit_seed_seq.h
index b660ece5001..25f791535f8 100644
--- a/absl/random/internal/explicit_seed_seq.h
+++ b/absl/random/internal/explicit_seed_seq.h
@@ -22,7 +22,11 @@
 #include <iterator>
 #include <vector>
 
+#include "absl/base/config.h"
+#include "absl/base/internal/endian.h"
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
 // This class conforms to the C++ Standard "Seed Sequence" concept
@@ -82,6 +86,7 @@ class ExplicitSeedSeq {
 };
 
 }  // namespace random_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_INTERNAL_EXPLICIT_SEED_SEQ_H_
diff --git a/absl/random/internal/explicit_seed_seq_test.cc b/absl/random/internal/explicit_seed_seq_test.cc
index a55ad73948b..0052d41d43d 100644
--- a/absl/random/internal/explicit_seed_seq_test.cc
+++ b/absl/random/internal/explicit_seed_seq_test.cc
@@ -14,37 +14,47 @@
 
 #include "absl/random/internal/explicit_seed_seq.h"
 
+#include <cstddef>
+#include <cstdint>
 #include <iterator>
 #include <random>
 #include <utility>
+#include <vector>
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
-#include "absl/random/seed_sequences.h"
+#include "absl/base/config.h"
+#include "absl/base/macros.h"
 
 namespace {
 
+using ::absl::random_internal::ExplicitSeedSeq;
+
 template <typename Sseq>
 bool ConformsToInterface() {
   // Check that the SeedSequence can be default-constructed.
-  { Sseq default_constructed_seq; }
+  {
+    Sseq default_constructed_seq;
+  }
   // Check that the SeedSequence can be constructed with two iterators.
   {
     uint32_t init_array[] = {1, 3, 5, 7, 9};
     Sseq iterator_constructed_seq(init_array, &init_array[5]);
   }
   // Check that the SeedSequence can be std::initializer_list-constructed.
-  { Sseq list_constructed_seq = {1, 3, 5, 7, 9, 11, 13}; }
+  {
+    Sseq list_constructed_seq = {1, 3, 5, 7, 9, 11, 13};
+  }
   // Check that param() and size() return state provided to constructor.
   {
     uint32_t init_array[] = {1, 2, 3, 4, 5};
-    Sseq seq(init_array, &init_array[ABSL_ARRAYSIZE(init_array)]);
-    EXPECT_EQ(seq.size(), ABSL_ARRAYSIZE(init_array));
+    Sseq seq(init_array, &init_array[std::size(init_array)]);
+    EXPECT_EQ(seq.size(), std::size(init_array));
 
-    uint32_t state_array[ABSL_ARRAYSIZE(init_array)];
+    uint32_t state_array[std::size(init_array)];
     seq.param(state_array);
 
-    for (int i = 0; i < ABSL_ARRAYSIZE(state_array); i++) {
+    for (int i = 0; i < std::size(state_array); i++) {
       EXPECT_EQ(state_array[i], i + 1);
     }
   }
@@ -53,7 +63,7 @@ bool ConformsToInterface() {
     Sseq seq;
     uint32_t seeds[5];
 
-    seq.generate(seeds, &seeds[ABSL_ARRAYSIZE(seeds)]);
+    seq.generate(seeds, &seeds[std::size(seeds)]);
   }
   return true;
 }
@@ -64,14 +74,14 @@ TEST(SeedSequences, CheckInterfaces) {
   EXPECT_TRUE(ConformsToInterface<std::seed_seq>());
 
   // Abseil classes
-  EXPECT_TRUE(ConformsToInterface<absl::random_internal::ExplicitSeedSeq>());
+  EXPECT_TRUE(ConformsToInterface<ExplicitSeedSeq>());
 }
 
 TEST(ExplicitSeedSeq, DefaultConstructorGeneratesZeros) {
   const size_t kNumBlocks = 128;
 
   uint32_t outputs[kNumBlocks];
-  absl::random_internal::ExplicitSeedSeq seq;
+  ExplicitSeedSeq seq;
   seq.generate(outputs, &outputs[kNumBlocks]);
 
   for (uint32_t& seed : outputs) {
@@ -87,8 +97,7 @@ TEST(ExplicitSeeqSeq, SeedMaterialIsForwardedIdentically) {
   for (uint32_t& seed : seed_material) {
     seed = urandom();
   }
-  absl::random_internal::ExplicitSeedSeq seq(seed_material,
-                                             &seed_material[kNumBlocks]);
+  ExplicitSeedSeq seq(seed_material, &seed_material[kNumBlocks]);
 
   // Check that output is same as seed-material provided to constructor.
   {
@@ -133,17 +142,14 @@ TEST(ExplicitSeedSeq, CopyAndMoveConstructors) {
   for (uint32_t& entry : entropy) {
     entry = urandom();
   }
-  absl::random_internal::ExplicitSeedSeq seq_from_entropy(std::begin(entropy),
-                                                          std::end(entropy));
+  ExplicitSeedSeq seq_from_entropy(std::begin(entropy), std::end(entropy));
   // Copy constructor.
   {
-    absl::random_internal::ExplicitSeedSeq seq_copy(seq_from_entropy);
+    ExplicitSeedSeq seq_copy(seq_from_entropy);
     EXPECT_EQ(seq_copy.size(), seq_from_entropy.size());
 
-    std::vector<uint32_t> seeds_1;
-    seeds_1.resize(1000, 0);
-    std::vector<uint32_t> seeds_2;
-    seeds_2.resize(1000, 1);
+    std::vector<uint32_t> seeds_1(1000, 0);
+    std::vector<uint32_t> seeds_2(1000, 1);
 
     seq_from_entropy.generate(seeds_1.begin(), seeds_1.end());
     seq_copy.generate(seeds_2.begin(), seeds_2.end());
@@ -155,13 +161,10 @@ TEST(ExplicitSeedSeq, CopyAndMoveConstructors) {
     for (uint32_t& entry : entropy) {
       entry = urandom();
     }
-    absl::random_internal::ExplicitSeedSeq another_seq(std::begin(entropy),
-                                                       std::end(entropy));
+    ExplicitSeedSeq another_seq(std::begin(entropy), std::end(entropy));
 
-    std::vector<uint32_t> seeds_1;
-    seeds_1.resize(1000, 0);
-    std::vector<uint32_t> seeds_2;
-    seeds_2.resize(1000, 0);
+    std::vector<uint32_t> seeds_1(1000, 0);
+    std::vector<uint32_t> seeds_2(1000, 0);
 
     seq_from_entropy.generate(seeds_1.begin(), seeds_1.end());
     another_seq.generate(seeds_2.begin(), seeds_2.end());
@@ -170,7 +173,15 @@ TEST(ExplicitSeedSeq, CopyAndMoveConstructors) {
     EXPECT_THAT(seeds_1, Not(Pointwise(Eq(), seeds_2)));
 
     // Apply the assignment-operator.
+    // GCC 12 has a false-positive -Wstringop-overflow warning here.
+#if ABSL_INTERNAL_HAVE_MIN_GNUC_VERSION(12, 0)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wstringop-overflow"
+#endif
     another_seq = seq_from_entropy;
+#if ABSL_INTERNAL_HAVE_MIN_GNUC_VERSION(12, 0)
+#pragma GCC diagnostic pop
+#endif
 
     // Re-generate seeds.
     seq_from_entropy.generate(seeds_1.begin(), seeds_1.end());
@@ -182,15 +193,13 @@ TEST(ExplicitSeedSeq, CopyAndMoveConstructors) {
   // Move constructor.
   {
     // Get seeds from seed-sequence constructed from entropy.
-    std::vector<uint32_t> seeds_1;
-    seeds_1.resize(1000, 0);
+    std::vector<uint32_t> seeds_1(1000, 0);
     seq_from_entropy.generate(seeds_1.begin(), seeds_1.end());
 
     // Apply move-constructor move the sequence to another instance.
     absl::random_internal::ExplicitSeedSeq moved_seq(
         std::move(seq_from_entropy));
-    std::vector<uint32_t> seeds_2;
-    seeds_2.resize(1000, 1);
+    std::vector<uint32_t> seeds_2(1000, 1);
     moved_seq.generate(seeds_2.begin(), seeds_2.end());
     // Verify that seeds produced by moved-instance are the same as original.
     EXPECT_THAT(seeds_1, Pointwise(Eq(), seeds_2));
@@ -202,3 +211,35 @@ TEST(ExplicitSeedSeq, CopyAndMoveConstructors) {
     EXPECT_THAT(seeds_1, Each(Eq(0)));
   }
 }
+
+TEST(ExplicitSeedSeq, StdURBGGoldenTests) {
+  // Verify that for std::- URBG instances the results are stable across
+  // platforms (these should have deterministic output).
+  {
+    ExplicitSeedSeq seed_sequence{12, 34, 56};
+    std::minstd_rand rng(seed_sequence);
+
+    std::minstd_rand::result_type values[4] = {rng(), rng(), rng(), rng()};
+    EXPECT_THAT(values,
+                testing::ElementsAre(579252, 43785881, 464353103, 1501811174));
+  }
+
+  {
+    ExplicitSeedSeq seed_sequence{12, 34, 56};
+    std::mt19937 rng(seed_sequence);
+
+    std::mt19937::result_type values[4] = {rng(), rng(), rng(), rng()};
+    EXPECT_THAT(values, testing::ElementsAre(138416803, 151130212, 33817739,
+                                             138416803));
+  }
+
+  {
+    ExplicitSeedSeq seed_sequence{12, 34, 56};
+    std::mt19937_64 rng(seed_sequence);
+
+    std::mt19937_64::result_type values[4] = {rng(), rng(), rng(), rng()};
+    EXPECT_THAT(values,
+                testing::ElementsAre(19738651785169348, 1464811352364190456,
+                                     18054685302720800, 19738651785169348));
+  }
+}
diff --git a/absl/random/internal/fast_uniform_bits.h b/absl/random/internal/fast_uniform_bits.h
index e8df92f3d99..7292d1f0f76 100644
--- a/absl/random/internal/fast_uniform_bits.h
+++ b/absl/random/internal/fast_uniform_bits.h
@@ -20,7 +20,12 @@
 #include <limits>
 #include <type_traits>
 
+#include "absl/base/config.h"
+#include "absl/meta/type_traits.h"
+#include "absl/random/internal/traits.h"
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 // Returns true if the input value is zero or a power of two. Useful for
 // determining if the range of output values in a URBG
@@ -35,48 +40,43 @@ constexpr bool IsPowerOfTwoOrZero(UIntType n) {
 template <typename URBG>
 constexpr typename URBG::result_type RangeSize() {
   using result_type = typename URBG::result_type;
+  static_assert((URBG::max)() != (URBG::min)(), "URBG range cannot be 0.");
   return ((URBG::max)() == (std::numeric_limits<result_type>::max)() &&
           (URBG::min)() == std::numeric_limits<result_type>::lowest())
              ? result_type{0}
-             : (URBG::max)() - (URBG::min)() + result_type{1};
-}
-
-template <typename UIntType>
-constexpr UIntType LargestPowerOfTwoLessThanOrEqualTo(UIntType n) {
-  return n < 2 ? n : 2 * LargestPowerOfTwoLessThanOrEqualTo(n / 2);
-}
-
-// Given a URBG generating values in the closed interval [Lo, Hi], returns the
-// largest power of two less than or equal to `Hi - Lo + 1`.
-template <typename URBG>
-constexpr typename URBG::result_type PowerOfTwoSubRangeSize() {
-  return LargestPowerOfTwoLessThanOrEqualTo(RangeSize<URBG>());
+             : ((URBG::max)() - (URBG::min)() + result_type{1});
 }
 
 // Computes the floor of the log. (i.e., std::floor(std::log2(N));
 template <typename UIntType>
 constexpr UIntType IntegerLog2(UIntType n) {
-  return (n <= 1) ? 0 : 1 + IntegerLog2(n / 2);
+  return (n <= 1) ? 0 : 1 + IntegerLog2(n >> 1);
 }
 
 // Returns the number of bits of randomness returned through
 // `PowerOfTwoVariate(urbg)`.
 template <typename URBG>
 constexpr size_t NumBits() {
-  return RangeSize<URBG>() == 0
-             ? std::numeric_limits<typename URBG::result_type>::digits
-             : IntegerLog2(PowerOfTwoSubRangeSize<URBG>());
+  return static_cast<size_t>(
+      RangeSize<URBG>() == 0
+          ? std::numeric_limits<typename URBG::result_type>::digits
+          : IntegerLog2(RangeSize<URBG>()));
 }
 
 // Given a shift value `n`, constructs a mask with exactly the low `n` bits set.
 // If `n == 0`, all bits are set.
 template <typename UIntType>
-constexpr UIntType MaskFromShift(UIntType n) {
+constexpr UIntType MaskFromShift(size_t n) {
   return ((n % std::numeric_limits<UIntType>::digits) == 0)
              ? ~UIntType{0}
              : (UIntType{1} << n) - UIntType{1};
 }
 
+// Tags used to dispatch FastUniformBits::generate to the simple or more complex
+// entropy extraction algorithm.
+struct SimplifiedLoopTag {};
+struct RejectionLoopTag {};
+
 // FastUniformBits implements a fast path to acquire uniform independent bits
 // from a type which conforms to the [rand.req.urbg] concept.
 // Parameterized by:
@@ -100,54 +100,20 @@ class FastUniformBits {
   result_type operator()(URBG& g);  // NOLINT(runtime/references)
 
  private:
-  static_assert(std::is_unsigned<UIntType>::value,
+  static_assert(IsUnsigned<UIntType>::value,
                 "Class-template FastUniformBits<> must be parameterized using "
                 "an unsigned type.");
 
-  // PowerOfTwoVariate() generates a single random variate, always returning a
-  // value in the half-open interval `[0, PowerOfTwoSubRangeSize<URBG>())`. If
-  // the URBG already generates values in a power-of-two range, the generator
-  // itself is used. Otherwise, we use rejection sampling on the largest
-  // possible power-of-two-sized subrange.
-  struct PowerOfTwoTag {};
-  struct RejectionSamplingTag {};
-  template <typename URBG>
-  static typename URBG::result_type PowerOfTwoVariate(
-      URBG& g) {  // NOLINT(runtime/references)
-    using tag =
-        typename std::conditional<IsPowerOfTwoOrZero(RangeSize<URBG>()),
-                                  PowerOfTwoTag, RejectionSamplingTag>::type;
-    return PowerOfTwoVariate(g, tag{});
-  }
-
-  template <typename URBG>
-  static typename URBG::result_type PowerOfTwoVariate(
-      URBG& g,  // NOLINT(runtime/references)
-      PowerOfTwoTag) {
-    return g() - (URBG::min)();
-  }
-
-  template <typename URBG>
-  static typename URBG::result_type PowerOfTwoVariate(
-      URBG& g,  // NOLINT(runtime/references)
-      RejectionSamplingTag) {
-    // Use rejection sampling to ensure uniformity across the range.
-    typename URBG::result_type u;
-    do {
-      u = g() - (URBG::min)();
-    } while (u >= PowerOfTwoSubRangeSize<URBG>());
-    return u;
-  }
-
   // Generate() generates a random value, dispatched on whether
-  // the underlying URBG must loop over multiple calls or not.
+  // the underlying URBG must use rejection sampling to generate a value,
+  // or whether a simplified loop will suffice.
   template <typename URBG>
   result_type Generate(URBG& g,  // NOLINT(runtime/references)
-                       std::true_type /* avoid_looping */);
+                       SimplifiedLoopTag);
 
   template <typename URBG>
   result_type Generate(URBG& g,  // NOLINT(runtime/references)
-                       std::false_type /* avoid_looping */);
+                       RejectionLoopTag);
 };
 
 template <typename UIntType>
@@ -159,31 +125,48 @@ FastUniformBits<UIntType>::operator()(URBG& g) {  // NOLINT(runtime/references)
   // Y = (2 ^ kRange) - 1
   static_assert((URBG::max)() > (URBG::min)(),
                 "URBG::max and URBG::min may not be equal.");
-  using urbg_result_type = typename URBG::result_type;
-  constexpr urbg_result_type kRangeMask =
-      RangeSize<URBG>() == 0
-          ? (std::numeric_limits<urbg_result_type>::max)()
-          : static_cast<urbg_result_type>(PowerOfTwoSubRangeSize<URBG>() - 1);
-  return Generate(g, std::integral_constant<bool, (kRangeMask >= (max)())>{});
+
+  using tag = std::conditional_t<IsPowerOfTwoOrZero(RangeSize<URBG>()),
+                                  SimplifiedLoopTag, RejectionLoopTag>;
+  return Generate(g, tag{});
 }
 
 template <typename UIntType>
 template <typename URBG>
 typename FastUniformBits<UIntType>::result_type
 FastUniformBits<UIntType>::Generate(URBG& g,  // NOLINT(runtime/references)
-                                    std::true_type /* avoid_looping */) {
-  // The width of the result_type is less than than the width of the random bits
-  // provided by URBG.  Thus, generate a single value and then simply mask off
-  // the required bits.
+                                    SimplifiedLoopTag) {
+  // The simplified version of FastUniformBits works only on URBGs that have
+  // a range that is a power of 2. In this case we simply loop and shift without
+  // attempting to balance the bits across calls.
+  static_assert(IsPowerOfTwoOrZero(RangeSize<URBG>()),
+                "incorrect Generate tag for URBG instance");
 
-  return PowerOfTwoVariate(g) & (max)();
+  static constexpr size_t kResultBits =
+      std::numeric_limits<result_type>::digits;
+  static constexpr size_t kUrbgBits = NumBits<URBG>();
+  static constexpr size_t kIters =
+      (kResultBits / kUrbgBits) + (kResultBits % kUrbgBits != 0);
+  static constexpr size_t kShift = (kIters == 1) ? 0 : kUrbgBits;
+  static constexpr auto kMin = (URBG::min)();
+
+  result_type r = static_cast<result_type>(g() - kMin);
+  for (size_t n = 1; n < kIters; ++n) {
+    r = static_cast<result_type>(r << kShift) +
+        static_cast<result_type>(g() - kMin);
+  }
+  return r;
 }
 
 template <typename UIntType>
 template <typename URBG>
 typename FastUniformBits<UIntType>::result_type
 FastUniformBits<UIntType>::Generate(URBG& g,  // NOLINT(runtime/references)
-                                    std::false_type /* avoid_looping */) {
+                                    RejectionLoopTag) {
+  static_assert(!IsPowerOfTwoOrZero(RangeSize<URBG>()),
+                "incorrect Generate tag for URBG instance");
+  using urbg_result_type = typename URBG::result_type;
+
   // See [rand.adapt.ibits] for more details on the constants calculated below.
   //
   // It is preferable to use roughly the same number of bits from each generator
@@ -196,21 +179,44 @@ FastUniformBits<UIntType>::Generate(URBG& g,  // NOLINT(runtime/references)
   // `kSmallIters` and `kLargeIters` times respectively such
   // that
   //
-  //    `kTotalWidth == kSmallIters * kSmallWidth
-  //                    + kLargeIters * kLargeWidth`
+  //    `kResultBits == kSmallIters * kSmallBits
+  //                    + kLargeIters * kLargeBits`
   //
-  // where `kTotalWidth` is the total number of bits in `result_type`.
+  // where `kResultBits` is the total number of bits in `result_type`.
   //
-  constexpr size_t kTotalWidth = std::numeric_limits<result_type>::digits;
-  constexpr size_t kUrbgWidth = NumBits<URBG>();
-  constexpr size_t kTotalIters =
-      kTotalWidth / kUrbgWidth + (kTotalWidth % kUrbgWidth != 0);
-  constexpr size_t kSmallWidth = kTotalWidth / kTotalIters;
-  constexpr size_t kLargeWidth = kSmallWidth + 1;
+  static constexpr size_t kResultBits =
+      std::numeric_limits<result_type>::digits;                      // w
+  static constexpr urbg_result_type kUrbgRange = RangeSize<URBG>();  // R
+  static constexpr size_t kUrbgBits = NumBits<URBG>();               // m
+
+  // compute the initial estimate of the bits used.
+  // [rand.adapt.ibits] 2 (c)
+  static constexpr size_t kA =  // ceil(w/m)
+      (kResultBits / kUrbgBits) + ((kResultBits % kUrbgBits) != 0);  // n'
+
+  static constexpr size_t kABits = kResultBits / kA;  // w0'
+  static constexpr urbg_result_type kARejection =
+      ((kUrbgRange >> kABits) << kABits);  // y0'
+
+  // refine the selection to reduce the rejection frequency.
+  static constexpr size_t kTotalIters =
+      ((kUrbgRange - kARejection) <= (kARejection / kA)) ? kA : (kA + 1);  // n
+
+  // [rand.adapt.ibits] 2 (b)
+  static constexpr size_t kSmallIters =
+      kTotalIters - (kResultBits % kTotalIters);                   // n0
+  static constexpr size_t kSmallBits = kResultBits / kTotalIters;  // w0
+  static constexpr urbg_result_type kSmallRejection =
+      ((kUrbgRange >> kSmallBits) << kSmallBits);  // y0
+
+  static constexpr size_t kLargeBits = kSmallBits + 1;  // w0+1
+  static constexpr urbg_result_type kLargeRejection =
+      ((kUrbgRange >> kLargeBits) << kLargeBits);  // y1
+
   //
-  // Because `kLargeWidth == kSmallWidth + 1`, it follows that
+  // Because `kLargeBits == kSmallBits + 1`, it follows that
   //
-  //     `kTotalWidth == kTotalIters * kSmallWidth + kLargeIters`
+  //     `kResultBits == kSmallIters * kSmallBits + kLargeIters`
   //
   // and therefore
   //
@@ -221,40 +227,45 @@ FastUniformBits<UIntType>::Generate(URBG& g,  // NOLINT(runtime/references)
   // mentioned above, if the URBG width is a divisor of `kTotalWidth`, then
   // there would be no need for any large iterations (i.e., one loop would
   // suffice), and indeed, in this case, `kLargeIters` would be zero.
-  constexpr size_t kLargeIters = kTotalWidth % kSmallWidth;
-  constexpr size_t kSmallIters =
-      (kTotalWidth - (kLargeWidth * kLargeIters)) / kSmallWidth;
+  static_assert(kResultBits == kSmallIters * kSmallBits +
+                                   (kTotalIters - kSmallIters) * kLargeBits,
+                "Error in looping constant calculations.");
 
-  static_assert(
-      kTotalWidth == kSmallIters * kSmallWidth + kLargeIters * kLargeWidth,
-      "Error in looping constant calculations.");
+  // The small shift is essentially small bits, but due to the potential
+  // of generating a smaller result_type from a larger urbg type, the actual
+  // shift might be 0.
+  static constexpr size_t kSmallShift = kSmallBits % kResultBits;
+  static constexpr auto kSmallMask =
+      MaskFromShift<urbg_result_type>(kSmallShift);
+  static constexpr size_t kLargeShift = kLargeBits % kResultBits;
+  static constexpr auto kLargeMask =
+      MaskFromShift<urbg_result_type>(kLargeShift);
 
-  result_type s = 0;
+  static constexpr auto kMin = (URBG::min)();
 
-  constexpr size_t kSmallShift = kSmallWidth % kTotalWidth;
-  constexpr result_type kSmallMask = MaskFromShift(result_type{kSmallShift});
+  result_type s = 0;
   for (size_t n = 0; n < kSmallIters; ++n) {
-    s = (s << kSmallShift) +
-        (static_cast<result_type>(PowerOfTwoVariate(g)) & kSmallMask);
-  }
+    urbg_result_type v;
+    do {
+      v = g() - kMin;
+    } while (v >= kSmallRejection);
 
-  constexpr size_t kLargeShift = kLargeWidth % kTotalWidth;
-  constexpr result_type kLargeMask = MaskFromShift(result_type{kLargeShift});
-  for (size_t n = 0; n < kLargeIters; ++n) {
-    s = (s << kLargeShift) +
-        (static_cast<result_type>(PowerOfTwoVariate(g)) & kLargeMask);
+    s = (s << kSmallShift) + static_cast<result_type>(v & kSmallMask);
   }
 
-  static_assert(
-      kLargeShift == kSmallShift + 1 ||
-          (kLargeShift == 0 &&
-           kSmallShift == std::numeric_limits<result_type>::digits - 1),
-      "Error in looping constant calculations");
+  for (size_t n = kSmallIters; n < kTotalIters; ++n) {
+    urbg_result_type v;
+    do {
+      v = g() - kMin;
+    } while (v >= kLargeRejection);
 
+    s = (s << kLargeShift) + static_cast<result_type>(v & kLargeMask);
+  }
   return s;
 }
 
 }  // namespace random_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_INTERNAL_FAST_UNIFORM_BITS_H_
diff --git a/absl/random/internal/fast_uniform_bits_test.cc b/absl/random/internal/fast_uniform_bits_test.cc
index 9f2e82687dd..3c33d417505 100644
--- a/absl/random/internal/fast_uniform_bits_test.cc
+++ b/absl/random/internal/fast_uniform_bits_test.cc
@@ -14,11 +14,16 @@
 
 #include "absl/random/internal/fast_uniform_bits.h"
 
+#include <cstddef>
+#include <cstdint>
+#include <limits>
 #include <random>
 
 #include "gtest/gtest.h"
+#include "absl/base/config.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 namespace {
 
@@ -33,8 +38,8 @@ TYPED_TEST(FastUniformBitsTypedTest, BasicTest) {
   using Limits = std::numeric_limits<TypeParam>;
   using FastBits = FastUniformBits<TypeParam>;
 
-  EXPECT_EQ(0, FastBits::min());
-  EXPECT_EQ(Limits::max(), FastBits::max());
+  EXPECT_EQ(0, (FastBits::min)());
+  EXPECT_EQ((Limits::max)(), (FastBits::max)());
 
   constexpr int kIters = 10000;
   std::random_device rd;
@@ -42,8 +47,8 @@ TYPED_TEST(FastUniformBitsTypedTest, BasicTest) {
   FastBits fast;
   for (int i = 0; i < kIters; i++) {
     const auto v = fast(gen);
-    EXPECT_LE(v, FastBits::max());
-    EXPECT_GE(v, FastBits::min());
+    EXPECT_LE(v, (FastBits::max)());
+    EXPECT_GE(v, (FastBits::min)());
   }
 }
 
@@ -51,21 +56,26 @@ template <typename UIntType, UIntType Lo, UIntType Hi, UIntType Val = Lo>
 struct FakeUrbg {
   using result_type = UIntType;
 
+  FakeUrbg() = default;
+  explicit FakeUrbg(bool r) : reject(r) {}
+
   static constexpr result_type(max)() { return Hi; }
   static constexpr result_type(min)() { return Lo; }
-  result_type operator()() { return Val; }
-};
+  result_type operator()() {
+    // when reject is set, return Hi half the time.
+    return ((++calls % 2) == 1 && reject) ? Hi : Val;
+  }
 
-using UrngOddbits = FakeUrbg<uint8_t, 1, 0xfe, 0x73>;
-using Urng4bits = FakeUrbg<uint8_t, 1, 0x10, 2>;
-using Urng31bits = FakeUrbg<uint32_t, 1, 0xfffffffe, 0x60070f03>;
-using Urng32bits = FakeUrbg<uint32_t, 0, 0xffffffff, 0x74010f01>;
+  bool reject = false;
+  size_t calls = 0;
+};
 
 TEST(FastUniformBitsTest, IsPowerOfTwoOrZero) {
   EXPECT_TRUE(IsPowerOfTwoOrZero(uint8_t{0}));
   EXPECT_TRUE(IsPowerOfTwoOrZero(uint8_t{1}));
   EXPECT_TRUE(IsPowerOfTwoOrZero(uint8_t{2}));
   EXPECT_FALSE(IsPowerOfTwoOrZero(uint8_t{3}));
+  EXPECT_TRUE(IsPowerOfTwoOrZero(uint8_t{4}));
   EXPECT_TRUE(IsPowerOfTwoOrZero(uint8_t{16}));
   EXPECT_FALSE(IsPowerOfTwoOrZero(uint8_t{17}));
   EXPECT_FALSE(IsPowerOfTwoOrZero((std::numeric_limits<uint8_t>::max)()));
@@ -74,6 +84,7 @@ TEST(FastUniformBitsTest, IsPowerOfTwoOrZero) {
   EXPECT_TRUE(IsPowerOfTwoOrZero(uint16_t{1}));
   EXPECT_TRUE(IsPowerOfTwoOrZero(uint16_t{2}));
   EXPECT_FALSE(IsPowerOfTwoOrZero(uint16_t{3}));
+  EXPECT_TRUE(IsPowerOfTwoOrZero(uint16_t{4}));
   EXPECT_TRUE(IsPowerOfTwoOrZero(uint16_t{16}));
   EXPECT_FALSE(IsPowerOfTwoOrZero(uint16_t{17}));
   EXPECT_FALSE(IsPowerOfTwoOrZero((std::numeric_limits<uint16_t>::max)()));
@@ -90,183 +101,240 @@ TEST(FastUniformBitsTest, IsPowerOfTwoOrZero) {
   EXPECT_TRUE(IsPowerOfTwoOrZero(uint64_t{1}));
   EXPECT_TRUE(IsPowerOfTwoOrZero(uint64_t{2}));
   EXPECT_FALSE(IsPowerOfTwoOrZero(uint64_t{3}));
+  EXPECT_TRUE(IsPowerOfTwoOrZero(uint64_t{4}));
   EXPECT_TRUE(IsPowerOfTwoOrZero(uint64_t{64}));
   EXPECT_FALSE(IsPowerOfTwoOrZero(uint64_t{17}));
   EXPECT_FALSE(IsPowerOfTwoOrZero((std::numeric_limits<uint64_t>::max)()));
 }
 
 TEST(FastUniformBitsTest, IntegerLog2) {
-  EXPECT_EQ(IntegerLog2(uint16_t{0}), 0);
-  EXPECT_EQ(IntegerLog2(uint16_t{1}), 0);
-  EXPECT_EQ(IntegerLog2(uint16_t{2}), 1);
-  EXPECT_EQ(IntegerLog2(uint16_t{3}), 1);
-  EXPECT_EQ(IntegerLog2(uint16_t{4}), 2);
-  EXPECT_EQ(IntegerLog2(uint16_t{5}), 2);
-  EXPECT_EQ(IntegerLog2(std::numeric_limits<uint64_t>::max()), 63);
+  EXPECT_EQ(0, IntegerLog2(uint16_t{0}));
+  EXPECT_EQ(0, IntegerLog2(uint16_t{1}));
+  EXPECT_EQ(1, IntegerLog2(uint16_t{2}));
+  EXPECT_EQ(1, IntegerLog2(uint16_t{3}));
+  EXPECT_EQ(2, IntegerLog2(uint16_t{4}));
+  EXPECT_EQ(2, IntegerLog2(uint16_t{5}));
+  EXPECT_EQ(2, IntegerLog2(uint16_t{7}));
+  EXPECT_EQ(3, IntegerLog2(uint16_t{8}));
+  EXPECT_EQ(63, IntegerLog2((std::numeric_limits<uint64_t>::max)()));
 }
 
 TEST(FastUniformBitsTest, RangeSize) {
-  EXPECT_EQ((RangeSize<FakeUrbg<uint8_t, 0, 3>>()), 4);
-  EXPECT_EQ((RangeSize<FakeUrbg<uint8_t, 2, 2>>()), 1);
-  EXPECT_EQ((RangeSize<FakeUrbg<uint8_t, 2, 5>>()), 4);
-  EXPECT_EQ((RangeSize<FakeUrbg<uint8_t, 2, 6>>()), 5);
-  EXPECT_EQ((RangeSize<FakeUrbg<uint8_t, 2, 10>>()), 9);
+  EXPECT_EQ(2, (RangeSize<FakeUrbg<uint8_t, 0, 1>>()));
+  EXPECT_EQ(3, (RangeSize<FakeUrbg<uint8_t, 0, 2>>()));
+  EXPECT_EQ(4, (RangeSize<FakeUrbg<uint8_t, 0, 3>>()));
+  //  EXPECT_EQ(0, (RangeSize<FakeUrbg<uint8_t, 2, 2>>()));
+  EXPECT_EQ(4, (RangeSize<FakeUrbg<uint8_t, 2, 5>>()));
+  EXPECT_EQ(5, (RangeSize<FakeUrbg<uint8_t, 2, 6>>()));
+  EXPECT_EQ(9, (RangeSize<FakeUrbg<uint8_t, 2, 10>>()));
   EXPECT_EQ(
-      (RangeSize<FakeUrbg<uint8_t, 0, std::numeric_limits<uint8_t>::max()>>()),
-      0);
-
-  EXPECT_EQ((RangeSize<FakeUrbg<uint16_t, 0, 3>>()), 4);
-  EXPECT_EQ((RangeSize<FakeUrbg<uint16_t, 2, 2>>()), 1);
-  EXPECT_EQ((RangeSize<FakeUrbg<uint16_t, 2, 5>>()), 4);
-  EXPECT_EQ((RangeSize<FakeUrbg<uint16_t, 2, 6>>()), 5);
-  EXPECT_EQ((RangeSize<FakeUrbg<uint16_t, 1000, 1017>>()), 18);
-  EXPECT_EQ((RangeSize<
-                FakeUrbg<uint16_t, 0, std::numeric_limits<uint16_t>::max()>>()),
-            0);
-
-  EXPECT_EQ((RangeSize<FakeUrbg<uint32_t, 0, 3>>()), 4);
-  EXPECT_EQ((RangeSize<FakeUrbg<uint32_t, 2, 2>>()), 1);
-  EXPECT_EQ((RangeSize<FakeUrbg<uint32_t, 2, 5>>()), 4);
-  EXPECT_EQ((RangeSize<FakeUrbg<uint32_t, 2, 6>>()), 5);
-  EXPECT_EQ((RangeSize<FakeUrbg<uint32_t, 1000, 1017>>()), 18);
-  EXPECT_EQ((RangeSize<FakeUrbg<uint32_t, 0, 0xffffffff>>()), 0);
-  EXPECT_EQ((RangeSize<FakeUrbg<uint32_t, 1, 0xffffffff>>()), 0xffffffff);
-  EXPECT_EQ((RangeSize<FakeUrbg<uint32_t, 1, 0xfffffffe>>()), 0xfffffffe);
-  EXPECT_EQ((RangeSize<FakeUrbg<uint32_t, 2, 0xfffffffe>>()), 0xfffffffd);
-  EXPECT_EQ((RangeSize<
-                FakeUrbg<uint32_t, 0, std::numeric_limits<uint32_t>::max()>>()),
-            0);
-
-  EXPECT_EQ((RangeSize<FakeUrbg<uint64_t, 0, 3>>()), 4);
-  EXPECT_EQ((RangeSize<FakeUrbg<uint64_t, 2, 2>>()), 1);
-  EXPECT_EQ((RangeSize<FakeUrbg<uint64_t, 2, 5>>()), 4);
-  EXPECT_EQ((RangeSize<FakeUrbg<uint64_t, 2, 6>>()), 5);
-  EXPECT_EQ((RangeSize<FakeUrbg<uint64_t, 1000, 1017>>()), 18);
-  EXPECT_EQ((RangeSize<FakeUrbg<uint64_t, 0, 0xffffffff>>()), 0x100000000ull);
-  EXPECT_EQ((RangeSize<FakeUrbg<uint64_t, 1, 0xffffffff>>()), 0xffffffffull);
-  EXPECT_EQ((RangeSize<FakeUrbg<uint64_t, 1, 0xfffffffe>>()), 0xfffffffeull);
-  EXPECT_EQ((RangeSize<FakeUrbg<uint64_t, 2, 0xfffffffe>>()), 0xfffffffdull);
-  EXPECT_EQ((RangeSize<FakeUrbg<uint64_t, 0, 0xffffffffffffffffull>>()), 0ull);
-  EXPECT_EQ((RangeSize<FakeUrbg<uint64_t, 1, 0xffffffffffffffffull>>()),
-            0xffffffffffffffffull);
-  EXPECT_EQ((RangeSize<FakeUrbg<uint64_t, 1, 0xfffffffffffffffeull>>()),
-            0xfffffffffffffffeull);
-  EXPECT_EQ((RangeSize<FakeUrbg<uint64_t, 2, 0xfffffffffffffffeull>>()),
-            0xfffffffffffffffdull);
-  EXPECT_EQ((RangeSize<
-                FakeUrbg<uint64_t, 0, std::numeric_limits<uint64_t>::max()>>()),
-            0);
-}
+      0, (RangeSize<
+             FakeUrbg<uint8_t, 0, (std::numeric_limits<uint8_t>::max)()>>()));
 
-TEST(FastUniformBitsTest, PowerOfTwoSubRangeSize) {
-  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint8_t, 0, 3>>()), 4);
-  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint8_t, 2, 2>>()), 1);
-  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint8_t, 2, 5>>()), 4);
-  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint8_t, 2, 6>>()), 4);
-  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint8_t, 2, 10>>()), 8);
-  EXPECT_EQ((PowerOfTwoSubRangeSize<
-                FakeUrbg<uint8_t, 0, std::numeric_limits<uint8_t>::max()>>()),
-            0);
-
-  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint16_t, 0, 3>>()), 4);
-  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint16_t, 2, 2>>()), 1);
-  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint16_t, 2, 5>>()), 4);
-  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint16_t, 2, 6>>()), 4);
-  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint16_t, 1000, 1017>>()), 16);
-  EXPECT_EQ((PowerOfTwoSubRangeSize<
-                FakeUrbg<uint16_t, 0, std::numeric_limits<uint16_t>::max()>>()),
-            0);
-
-  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint32_t, 0, 3>>()), 4);
-  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint32_t, 2, 2>>()), 1);
-  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint32_t, 2, 5>>()), 4);
-  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint32_t, 2, 6>>()), 4);
-  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint32_t, 1000, 1017>>()), 16);
-  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint32_t, 0, 0xffffffff>>()), 0);
-  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint32_t, 1, 0xffffffff>>()),
-            0x80000000);
-  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint32_t, 1, 0xfffffffe>>()),
-            0x80000000);
-  EXPECT_EQ((PowerOfTwoSubRangeSize<
-                FakeUrbg<uint32_t, 0, std::numeric_limits<uint32_t>::max()>>()),
-            0);
-
-  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint64_t, 0, 3>>()), 4);
-  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint64_t, 2, 2>>()), 1);
-  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint64_t, 2, 5>>()), 4);
-  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint64_t, 2, 6>>()), 4);
-  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint64_t, 1000, 1017>>()), 16);
-  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint64_t, 0, 0xffffffff>>()),
-            0x100000000ull);
-  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint64_t, 1, 0xffffffff>>()),
-            0x80000000ull);
-  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint64_t, 1, 0xfffffffe>>()),
-            0x80000000ull);
+  EXPECT_EQ(4, (RangeSize<FakeUrbg<uint16_t, 0, 3>>()));
+  EXPECT_EQ(4, (RangeSize<FakeUrbg<uint16_t, 2, 5>>()));
+  EXPECT_EQ(5, (RangeSize<FakeUrbg<uint16_t, 2, 6>>()));
+  EXPECT_EQ(18, (RangeSize<FakeUrbg<uint16_t, 1000, 1017>>()));
   EXPECT_EQ(
-      (PowerOfTwoSubRangeSize<FakeUrbg<uint64_t, 0, 0xffffffffffffffffull>>()),
-      0);
+      0, (RangeSize<
+             FakeUrbg<uint16_t, 0, (std::numeric_limits<uint16_t>::max)()>>()));
+
+  EXPECT_EQ(4, (RangeSize<FakeUrbg<uint32_t, 0, 3>>()));
+  EXPECT_EQ(4, (RangeSize<FakeUrbg<uint32_t, 2, 5>>()));
+  EXPECT_EQ(5, (RangeSize<FakeUrbg<uint32_t, 2, 6>>()));
+  EXPECT_EQ(18, (RangeSize<FakeUrbg<uint32_t, 1000, 1017>>()));
+  EXPECT_EQ(0, (RangeSize<FakeUrbg<uint32_t, 0, 0xffffffff>>()));
+  EXPECT_EQ(0xffffffff, (RangeSize<FakeUrbg<uint32_t, 1, 0xffffffff>>()));
+  EXPECT_EQ(0xfffffffe, (RangeSize<FakeUrbg<uint32_t, 1, 0xfffffffe>>()));
+  EXPECT_EQ(0xfffffffd, (RangeSize<FakeUrbg<uint32_t, 2, 0xfffffffe>>()));
   EXPECT_EQ(
-      (PowerOfTwoSubRangeSize<FakeUrbg<uint64_t, 1, 0xffffffffffffffffull>>()),
-      0x8000000000000000ull);
+      0, (RangeSize<
+             FakeUrbg<uint32_t, 0, (std::numeric_limits<uint32_t>::max)()>>()));
+
+  EXPECT_EQ(4, (RangeSize<FakeUrbg<uint64_t, 0, 3>>()));
+  EXPECT_EQ(4, (RangeSize<FakeUrbg<uint64_t, 2, 5>>()));
+  EXPECT_EQ(5, (RangeSize<FakeUrbg<uint64_t, 2, 6>>()));
+  EXPECT_EQ(18, (RangeSize<FakeUrbg<uint64_t, 1000, 1017>>()));
+  EXPECT_EQ(0x100000000, (RangeSize<FakeUrbg<uint64_t, 0, 0xffffffff>>()));
+  EXPECT_EQ(0xffffffff, (RangeSize<FakeUrbg<uint64_t, 1, 0xffffffff>>()));
+  EXPECT_EQ(0xfffffffe, (RangeSize<FakeUrbg<uint64_t, 1, 0xfffffffe>>()));
+  EXPECT_EQ(0xfffffffd, (RangeSize<FakeUrbg<uint64_t, 2, 0xfffffffe>>()));
+  EXPECT_EQ(0, (RangeSize<FakeUrbg<uint64_t, 0, 0xffffffffffffffff>>()));
+  EXPECT_EQ(0xffffffffffffffff,
+            (RangeSize<FakeUrbg<uint64_t, 1, 0xffffffffffffffff>>()));
+  EXPECT_EQ(0xfffffffffffffffe,
+            (RangeSize<FakeUrbg<uint64_t, 1, 0xfffffffffffffffe>>()));
+  EXPECT_EQ(0xfffffffffffffffd,
+            (RangeSize<FakeUrbg<uint64_t, 2, 0xfffffffffffffffe>>()));
   EXPECT_EQ(
-      (PowerOfTwoSubRangeSize<FakeUrbg<uint64_t, 1, 0xfffffffffffffffeull>>()),
-      0x8000000000000000ull);
-  EXPECT_EQ((PowerOfTwoSubRangeSize<
-                FakeUrbg<uint64_t, 0, std::numeric_limits<uint64_t>::max()>>()),
-            0);
+      0, (RangeSize<
+             FakeUrbg<uint64_t, 0, (std::numeric_limits<uint64_t>::max)()>>()));
 }
 
-TEST(FastUniformBitsTest, Urng4_VariousOutputs) {
+// The constants need to be chosen so that an infinite rejection loop doesn't
+// happen...
+using Urng1_5bit = FakeUrbg<uint8_t, 0, 2, 0>;  // ~1.5 bits (range 3)
+using Urng4bits = FakeUrbg<uint8_t, 1, 0x10, 2>;
+using Urng22bits = FakeUrbg<uint32_t, 0, 0x3fffff, 0x301020>;
+using Urng31bits = FakeUrbg<uint32_t, 1, 0xfffffffe, 0x60070f03>;  // ~31.9 bits
+using Urng32bits = FakeUrbg<uint32_t, 0, 0xffffffff, 0x74010f01>;
+using Urng33bits =
+    FakeUrbg<uint64_t, 1, 0x1ffffffff, 0x013301033>;  // ~32.9 bits
+using Urng63bits = FakeUrbg<uint64_t, 1, 0xfffffffffffffffe,
+                            0xfedcba9012345678>;  // ~63.9 bits
+using Urng64bits =
+    FakeUrbg<uint64_t, 0, 0xffffffffffffffff, 0x123456780fedcba9>;
+
+TEST(FastUniformBitsTest, OutputsUpTo32Bits) {
   // Tests that how values are composed; the single-bit deltas should be spread
   // across each invocation.
+  Urng1_5bit urng1_5;
   Urng4bits urng4;
+  Urng22bits urng22;
   Urng31bits urng31;
   Urng32bits urng32;
+  Urng33bits urng33;
+  Urng63bits urng63;
+  Urng64bits urng64;
 
   // 8-bit types
   {
     FastUniformBits<uint8_t> fast8;
+    EXPECT_EQ(0x0, fast8(urng1_5));
     EXPECT_EQ(0x11, fast8(urng4));
+    EXPECT_EQ(0x20, fast8(urng22));
     EXPECT_EQ(0x2, fast8(urng31));
     EXPECT_EQ(0x1, fast8(urng32));
+    EXPECT_EQ(0x32, fast8(urng33));
+    EXPECT_EQ(0x77, fast8(urng63));
+    EXPECT_EQ(0xa9, fast8(urng64));
   }
 
   // 16-bit types
   {
     FastUniformBits<uint16_t> fast16;
+    EXPECT_EQ(0x0, fast16(urng1_5));
     EXPECT_EQ(0x1111, fast16(urng4));
-    EXPECT_EQ(0xf02, fast16(urng31));
-    EXPECT_EQ(0xf01, fast16(urng32));
+    EXPECT_EQ(0x1020, fast16(urng22));
+    EXPECT_EQ(0x0f02, fast16(urng31));
+    EXPECT_EQ(0x0f01, fast16(urng32));
+    EXPECT_EQ(0x1032, fast16(urng33));
+    EXPECT_EQ(0x5677, fast16(urng63));
+    EXPECT_EQ(0xcba9, fast16(urng64));
   }
 
   // 32-bit types
   {
     FastUniformBits<uint32_t> fast32;
+    EXPECT_EQ(0x0, fast32(urng1_5));
     EXPECT_EQ(0x11111111, fast32(urng4));
+    EXPECT_EQ(0x08301020, fast32(urng22));
     EXPECT_EQ(0x0f020f02, fast32(urng31));
     EXPECT_EQ(0x74010f01, fast32(urng32));
+    EXPECT_EQ(0x13301032, fast32(urng33));
+    EXPECT_EQ(0x12345677, fast32(urng63));
+    EXPECT_EQ(0x0fedcba9, fast32(urng64));
   }
+}
+
+TEST(FastUniformBitsTest, Outputs64Bits) {
+  // Tests that how values are composed; the single-bit deltas should be spread
+  // across each invocation.
+  FastUniformBits<uint64_t> fast64;
 
-  // 64-bit types
   {
-    FastUniformBits<uint64_t> fast64;
+    FakeUrbg<uint8_t, 0, 1, 0> urng0;
+    FakeUrbg<uint8_t, 0, 1, 1> urng1;
+    Urng4bits urng4;
+    Urng22bits urng22;
+    Urng31bits urng31;
+    Urng32bits urng32;
+    Urng33bits urng33;
+    Urng63bits urng63;
+    Urng64bits urng64;
+
+    // somewhat degenerate cases only create a single bit.
+    EXPECT_EQ(0x0, fast64(urng0));
+    EXPECT_EQ(64, urng0.calls);
+    EXPECT_EQ(0xffffffffffffffff, fast64(urng1));
+    EXPECT_EQ(64, urng1.calls);
+
+    // less degenerate cases.
     EXPECT_EQ(0x1111111111111111, fast64(urng4));
+    EXPECT_EQ(16, urng4.calls);
+    EXPECT_EQ(0x01020c0408301020, fast64(urng22));
+    EXPECT_EQ(3, urng22.calls);
     EXPECT_EQ(0x387811c3c0870f02, fast64(urng31));
+    EXPECT_EQ(3, urng31.calls);
     EXPECT_EQ(0x74010f0174010f01, fast64(urng32));
+    EXPECT_EQ(2, urng32.calls);
+    EXPECT_EQ(0x808194040cb01032, fast64(urng33));
+    EXPECT_EQ(3, urng33.calls);
+    EXPECT_EQ(0x1234567712345677, fast64(urng63));
+    EXPECT_EQ(2, urng63.calls);
+    EXPECT_EQ(0x123456780fedcba9, fast64(urng64));
+    EXPECT_EQ(1, urng64.calls);
+  }
+
+  // The 1.5 bit case is somewhat interesting in that the algorithm refinement
+  // causes one extra small sample. Comments here reference the names used in
+  // [rand.adapt.ibits] that correspond to this case.
+  {
+    Urng1_5bit urng1_5;
+
+    // w = 64
+    // R = 3
+    // m = 1
+    // n' = 64
+    // w0' = 1
+    // y0' = 2
+    // n = (1 <= 0) > 64 : 65 = 65
+    // n0 = 65 - (64%65) = 1
+    // n1 = 64
+    // w0 = 0
+    // y0 = 3
+    // w1 = 1
+    // y1 = 2
+    EXPECT_EQ(0x0, fast64(urng1_5));
+    EXPECT_EQ(65, urng1_5.calls);
+  }
+
+  // Validate rejections for non-power-of-2 cases.
+  {
+    Urng1_5bit urng1_5(true);
+    Urng31bits urng31(true);
+    Urng33bits urng33(true);
+    Urng63bits urng63(true);
+
+    // For 1.5 bits, there would be 1+2*64, except the first
+    // value was accepted and shifted off the end.
+    EXPECT_EQ(0, fast64(urng1_5));
+    EXPECT_EQ(128, urng1_5.calls);
+    EXPECT_EQ(0x387811c3c0870f02, fast64(urng31));
+    EXPECT_EQ(6, urng31.calls);
+    EXPECT_EQ(0x808194040cb01032, fast64(urng33));
+    EXPECT_EQ(6, urng33.calls);
+    EXPECT_EQ(0x1234567712345677, fast64(urng63));
+    EXPECT_EQ(4, urng63.calls);
   }
 }
 
 TEST(FastUniformBitsTest, URBG32bitRegression) {
   // Validate with deterministic 32-bit std::minstd_rand
   // to ensure that operator() performs as expected.
+
+  EXPECT_EQ(2147483646, RangeSize<std::minstd_rand>());
+  EXPECT_EQ(30, IntegerLog2(RangeSize<std::minstd_rand>()));
+
   std::minstd_rand gen(1);
   FastUniformBits<uint64_t> fast64;
 
-  EXPECT_EQ(0x05e47095f847c122ull, fast64(gen));
-  EXPECT_EQ(0x8f82c1ba30b64d22ull, fast64(gen));
-  EXPECT_EQ(0x3b971a3558155039ull, fast64(gen));
+  EXPECT_EQ(0x05e47095f8791f45, fast64(gen));
+  EXPECT_EQ(0x028be17e3c07c122, fast64(gen));
+  EXPECT_EQ(0x55d2847c1626e8c2, fast64(gen));
 }
 
 }  // namespace
 }  // namespace random_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/random/internal/fastmath.h b/absl/random/internal/fastmath.h
index 4bd18410539..963b7690f1a 100644
--- a/absl/random/internal/fastmath.h
+++ b/absl/random/internal/fastmath.h
@@ -22,26 +22,22 @@
 #include <cmath>
 #include <cstdint>
 
-#include "absl/base/internal/bits.h"
+#include "absl/numeric/bits.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
-// Returns the position of the first bit set.
-inline int LeadingSetBit(uint64_t n) {
-  return 64 - base_internal::CountLeadingZeros64(n);
-}
-
 // Compute log2(n) using integer operations.
 // While std::log2 is more accurate than std::log(n) / std::log(2), for
 // very large numbers--those close to std::numeric_limits<uint64_t>::max() - 2,
 // for instance--std::log2 rounds up rather than down, which introduces
 // definite skew in the results.
 inline int IntLog2Floor(uint64_t n) {
-  return (n <= 1) ? 0 : (63 - base_internal::CountLeadingZeros64(n));
+  return (n <= 1) ? 0 : (63 - countl_zero(n));
 }
 inline int IntLog2Ceil(uint64_t n) {
-  return (n <= 1) ? 0 : (64 - base_internal::CountLeadingZeros64(n - 1));
+  return (n <= 1) ? 0 : (64 - countl_zero(n - 1));
 }
 
 inline double StirlingLogFactorial(double n) {
@@ -54,19 +50,8 @@ inline double StirlingLogFactorial(double n) {
          (1.0 / 360.0) * ninv * ninv * ninv;
 }
 
-// Rotate value right.
-//
-// We only implement the uint32_t / uint64_t versions because
-// 1) those are the only ones we use, and
-// 2) those are the only ones where clang detects the rotate idiom correctly.
-inline constexpr uint32_t rotr(uint32_t value, uint8_t bits) {
-  return (value >> (bits & 31)) | (value << ((-bits) & 31));
-}
-inline constexpr uint64_t rotr(uint64_t value, uint8_t bits) {
-  return (value >> (bits & 63)) | (value << ((-bits) & 63));
-}
-
 }  // namespace random_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_INTERNAL_FASTMATH_H_
diff --git a/absl/random/internal/fastmath_test.cc b/absl/random/internal/fastmath_test.cc
index 65859c25de4..cd33d7a2110 100644
--- a/absl/random/internal/fastmath_test.cc
+++ b/absl/random/internal/fastmath_test.cc
@@ -14,32 +14,20 @@
 
 #include "absl/random/internal/fastmath.h"
 
+#include <cmath>
+#include <cstdint>
+
 #include "gtest/gtest.h"
 
-#if defined(__native_client__) || defined(__EMSCRIPTEN__)
-// NACL has a less accurate implementation of std::log2 than most of
+#if defined(__EMSCRIPTEN__)
+// Emscripten has a less accurate implementation of std::log2 than most of
 // the other platforms. For some values which should have integral results,
-// sometimes NACL returns slightly larger values.
-//
-// The MUSL libc used by emscripten also has a similar bug.
+// sometimes Emscripten returns slightly larger values.
 #define ABSL_RANDOM_INACCURATE_LOG2
 #endif
 
 namespace {
 
-TEST(DistributionImplTest, LeadingSetBit) {
-  using absl::random_internal::LeadingSetBit;
-  constexpr uint64_t kZero = 0;
-  EXPECT_EQ(0, LeadingSetBit(kZero));
-  EXPECT_EQ(64, LeadingSetBit(~kZero));
-
-  for (int index = 0; index < 64; index++) {
-    uint64_t x = static_cast<uint64_t>(1) << index;
-    EXPECT_EQ(index + 1, LeadingSetBit(x)) << index;
-    EXPECT_EQ(index + 1, LeadingSetBit(x + x - 1)) << index;
-  }
-}
-
 TEST(FastMathTest, IntLog2FloorTest) {
   using absl::random_internal::IntLog2Floor;
   constexpr uint64_t kZero = 0;
diff --git a/absl/random/internal/gaussian_distribution_gentables.cc b/absl/random/internal/gaussian_distribution_gentables.cc
index 16a23cb2f91..967a327b7f0 100644
--- a/absl/random/internal/gaussian_distribution_gentables.cc
+++ b/absl/random/internal/gaussian_distribution_gentables.cc
@@ -16,17 +16,19 @@
 //
 // $ blaze run :gaussian_distribution_gentables > gaussian_distribution.cc
 //
-#include "absl/random/gaussian_distribution.h"
-
 #include <cmath>
 #include <cstddef>
 #include <iostream>
 #include <limits>
 #include <string>
+#include <type_traits>
 
+#include "absl/base/config.h"
 #include "absl/base/macros.h"
+#include "absl/random/gaussian_distribution.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 namespace {
 
@@ -79,10 +81,11 @@ TableGenerator::TableGenerator() {
   // The constants here should match the values in gaussian_distribution.h
   static constexpr int kC = kMask + 1;
 
-  static_assert((ABSL_ARRAYSIZE(tables_.x) == kC + 1),
+  static_assert((std::extent_v<decltype(tables_.x)> == kC + 1),
                 "xArray must be length kMask + 2");
 
-  static_assert((ABSL_ARRAYSIZE(tables_.x) == ABSL_ARRAYSIZE(tables_.f)),
+  static_assert((std::extent_v<decltype(tables_.x)> ==
+                 std::extent_v<decltype(tables_.f)>),
                 "fx and x arrays must be identical length");
 
   auto f = [](double x) { return std::exp(-0.5 * x * x); };
@@ -110,12 +113,9 @@ void TableGenerator::Print(std::ostream* os) {
          "\n"
          "#include \"absl/random/gaussian_distribution.h\"\n"
          "\n"
-         // "namespace " and "absl" are broken apart so as not to conflict with
-         // script that adds the LTS inline namespace.
-         "namespace "
-         "absl {\n"
-         "namespace "
-         "random_internal {\n"
+         "namespace absl {\n"
+         "ABSL_NAMESPACE_BEGIN\n"
+         "namespace random_internal {\n"
          "\n"
          "const gaussian_distribution_base::Tables\n"
          "    gaussian_distribution_base::zg_ = {\n";
@@ -124,10 +124,9 @@ void TableGenerator::Print(std::ostream* os) {
   FormatArrayContents(os, tables_.f);
   *os << "};\n"
          "\n"
-         "}  // namespace "
-         "random_internal\n"
-         "}  // namespace "
-         "absl\n"
+         "}  // namespace random_internal\n"
+         "ABSL_NAMESPACE_END\n"
+         "}  // namespace absl\n"
          "\n"
          "// clang-format on\n"
          "// END GENERATED CODE";
@@ -135,6 +134,7 @@ void TableGenerator::Print(std::ostream* os) {
 }
 
 }  // namespace random_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 int main(int, char**) {
diff --git a/absl/random/internal/generate_real.h b/absl/random/internal/generate_real.h
new file mode 100644
index 00000000000..6af0455e345
--- /dev/null
+++ b/absl/random/internal/generate_real.h
@@ -0,0 +1,142 @@
+// Copyright 2017 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_RANDOM_INTERNAL_GENERATE_REAL_H_
+#define ABSL_RANDOM_INTERNAL_GENERATE_REAL_H_
+
+// This file contains some implementation details which are used by one or more
+// of the absl random number distributions.
+
+#include <cstdint>
+#include <cstring>
+#include <limits>
+#include <type_traits>
+
+#include "absl/meta/type_traits.h"
+#include "absl/numeric/bits.h"
+#include "absl/random/internal/fastmath.h"
+#include "absl/random/internal/traits.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace random_internal {
+
+// Tristate tag types controlling the output of GenerateRealFromBits.
+struct GeneratePositiveTag {};
+struct GenerateNegativeTag {};
+struct GenerateSignedTag {};
+
+// GenerateRealFromBits generates a single real value from a single 64-bit
+// `bits` with template fields controlling the output.
+//
+// The `SignedTag` parameter controls whether positive, negative,
+// or either signed/unsigned may be returned.
+//   When SignedTag == GeneratePositiveTag, range is U(0, 1)
+//   When SignedTag == GenerateNegativeTag, range is U(-1, 0)
+//   When SignedTag == GenerateSignedTag, range is U(-1, 1)
+//
+// When the `IncludeZero` parameter is true, the function may return 0 for some
+// inputs, otherwise it never returns 0.
+//
+// When a value in U(0,1) is required, use:
+//   GenerateRealFromBits<double, PositiveValueT, true>;
+//
+// When a value in U(-1,1) is required, use:
+//   GenerateRealFromBits<double, SignedValueT, false>;
+//
+//   This generates more distinct values than the mathematical equivalent
+//   `U(0, 1) * 2.0 - 1.0`.
+//
+// Scaling the result by powers of 2 (and avoiding a multiply) is also possible:
+//   GenerateRealFromBits<double>(..., -1);  => U(0, 0.5)
+//   GenerateRealFromBits<double>(..., 1);   => U(0, 2)
+//
+template <typename RealType,  // Real type, either float or double.
+          typename SignedTag = GeneratePositiveTag,  // Whether a positive,
+                                                     // negative, or signed
+                                                     // value is generated.
+          bool IncludeZero = true>
+inline RealType GenerateRealFromBits(uint64_t bits, int exp_bias = 0) {
+  using real_type = RealType;
+  using uint_type =
+      std::conditional_t<std::is_same_v<real_type, float>, uint32_t, uint64_t>;
+
+  static_assert(
+      (std::is_same_v<double, real_type> || std::is_same_v<float, real_type>),
+      "GenerateRealFromBits must be parameterized by either float or double.");
+
+  static_assert(sizeof(uint_type) == sizeof(real_type),
+                "Mismatched unsigned and real types.");
+
+  static_assert((std::numeric_limits<real_type>::is_iec559 &&
+                 std::numeric_limits<real_type>::radix == 2),
+                "RealType representation is not IEEE 754 binary.");
+
+  static_assert((std::is_same_v<SignedTag, GeneratePositiveTag> ||
+                 std::is_same_v<SignedTag, GenerateNegativeTag> ||
+                 std::is_same_v<SignedTag, GenerateSignedTag>));
+
+  static constexpr int kExp = std::numeric_limits<real_type>::digits - 1;
+  static constexpr uint_type kMask = (static_cast<uint_type>(1) << kExp) - 1u;
+  static constexpr int kUintBits = sizeof(uint_type) * 8;
+
+  int exp = exp_bias + int{std::numeric_limits<real_type>::max_exponent - 2};
+
+  // Determine the sign bit.
+  // Depending on the SignedTag, this may use the left-most bit
+  // or it may be a constant value.
+  uint_type sign = std::is_same_v<SignedTag, GenerateNegativeTag>
+                       ? (static_cast<uint_type>(1) << (kUintBits - 1))
+                       : 0;
+  if (std::is_same_v<SignedTag, GenerateSignedTag>) {
+    if (std::is_same_v<uint_type, uint64_t>) {
+      sign = bits & uint64_t{0x8000000000000000};
+    }
+    if (std::is_same_v<uint_type, uint32_t>) {
+      const uint64_t tmp = bits & uint64_t{0x8000000000000000};
+      sign = static_cast<uint32_t>(tmp >> 32);
+    }
+    // adjust the bits and the exponent to account for removing
+    // the leading bit.
+    bits = bits & uint64_t{0x7FFFFFFFFFFFFFFF};
+    exp++;
+  }
+  if (IncludeZero) {
+    if (bits == 0u) return 0;
+  }
+
+  // Number of leading zeros is mapped to the exponent: 2^-clz
+  // bits is 0..01xxxxxx. After shifting, we're left with 1xxx...0..0
+  int clz = countl_zero(bits);
+  bits <<= (IncludeZero ? clz : (clz & 63));  // remove 0-bits.
+  exp -= clz;                                 // set the exponent.
+  bits >>= (63 - kExp);
+
+  // Construct the 32-bit or 64-bit IEEE 754 floating-point value from
+  // the individual fields: sign, exp, mantissa(bits).
+  uint_type val = sign | (static_cast<uint_type>(exp) << kExp) |
+                  (static_cast<uint_type>(bits) & kMask);
+
+  // bit_cast to the output-type
+  real_type result;
+  memcpy(static_cast<void*>(&result), static_cast<const void*>(&val),
+         sizeof(result));
+  return result;
+}
+
+}  // namespace random_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_RANDOM_INTERNAL_GENERATE_REAL_H_
diff --git a/absl/random/internal/distribution_impl_test.cc b/absl/random/internal/generate_real_test.cc
similarity index 79%
rename from absl/random/internal/distribution_impl_test.cc
rename to absl/random/internal/generate_real_test.cc
index 09e7a318ccc..b099dbf35fe 100644
--- a/absl/random/internal/distribution_impl_test.cc
+++ b/absl/random/internal/generate_real_test.cc
@@ -12,57 +12,74 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-#include "absl/random/internal/distribution_impl.h"
+#include "absl/random/internal/generate_real.h"
+
+#include <cfloat>
+#include <cstddef>
+#include <cstdint>
+#include <string>
 
 #include "gtest/gtest.h"
-#include "absl/base/internal/bits.h"
 #include "absl/flags/flag.h"
-#include "absl/numeric/int128.h"
+#include "absl/numeric/bits.h"
 
 ABSL_FLAG(int64_t, absl_random_test_trials, 50000,
           "Number of trials for the probability tests.");
 
-using absl::random_internal::NegativeValueT;
-using absl::random_internal::PositiveValueT;
-using absl::random_internal::RandU64ToDouble;
-using absl::random_internal::RandU64ToFloat;
-using absl::random_internal::SignedValueT;
+using absl::random_internal::GenerateNegativeTag;
+using absl::random_internal::GeneratePositiveTag;
+using absl::random_internal::GenerateRealFromBits;
+using absl::random_internal::GenerateSignedTag;
 
 namespace {
 
-TEST(DistributionImplTest, U64ToFloat_Positive_NoZero_Test) {
+TEST(GenerateRealTest, U64ToFloat_Positive_NoZero_Test) {
   auto ToFloat = [](uint64_t a) {
-    return RandU64ToFloat<PositiveValueT, false>(a);
+    return GenerateRealFromBits<float, GeneratePositiveTag, false>(a);
   };
   EXPECT_EQ(ToFloat(0x0000000000000000), 2.710505431e-20f);
   EXPECT_EQ(ToFloat(0x0000000000000001), 5.421010862e-20f);
   EXPECT_EQ(ToFloat(0x8000000000000000), 0.5);
+  EXPECT_EQ(ToFloat(0x8000000000000001), 0.5);
   EXPECT_EQ(ToFloat(0xFFFFFFFFFFFFFFFF), 0.9999999404f);
 }
 
-TEST(DistributionImplTest, U64ToFloat_Positive_Zero_Test) {
+TEST(GenerateRealTest, U64ToFloat_Positive_Zero_Test) {
   auto ToFloat = [](uint64_t a) {
-    return RandU64ToFloat<PositiveValueT, true>(a);
+    return GenerateRealFromBits<float, GeneratePositiveTag, true>(a);
   };
   EXPECT_EQ(ToFloat(0x0000000000000000), 0.0);
   EXPECT_EQ(ToFloat(0x0000000000000001), 5.421010862e-20f);
   EXPECT_EQ(ToFloat(0x8000000000000000), 0.5);
+  EXPECT_EQ(ToFloat(0x8000000000000001), 0.5);
   EXPECT_EQ(ToFloat(0xFFFFFFFFFFFFFFFF), 0.9999999404f);
 }
 
-TEST(DistributionImplTest, U64ToFloat_Negative_NoZero_Test) {
+TEST(GenerateRealTest, U64ToFloat_Negative_NoZero_Test) {
   auto ToFloat = [](uint64_t a) {
-    return RandU64ToFloat<NegativeValueT, false>(a);
+    return GenerateRealFromBits<float, GenerateNegativeTag, false>(a);
   };
   EXPECT_EQ(ToFloat(0x0000000000000000), -2.710505431e-20f);
   EXPECT_EQ(ToFloat(0x0000000000000001), -5.421010862e-20f);
   EXPECT_EQ(ToFloat(0x8000000000000000), -0.5);
+  EXPECT_EQ(ToFloat(0x8000000000000001), -0.5);
+  EXPECT_EQ(ToFloat(0xFFFFFFFFFFFFFFFF), -0.9999999404f);
+}
+
+TEST(GenerateRealTest, U64ToFloat_Negative_Zero_Test) {
+  auto ToFloat = [](uint64_t a) {
+    return GenerateRealFromBits<float, GenerateNegativeTag, true>(a);
+  };
+  EXPECT_EQ(ToFloat(0x0000000000000000), 0.0);
+  EXPECT_EQ(ToFloat(0x0000000000000001), -5.421010862e-20f);
+  EXPECT_EQ(ToFloat(0x8000000000000000), -0.5);
+  EXPECT_EQ(ToFloat(0x8000000000000001), -0.5);
   EXPECT_EQ(ToFloat(0xFFFFFFFFFFFFFFFF), -0.9999999404f);
 }
 
-TEST(DistributionImplTest, U64ToFloat_Signed_NoZero_Test) {
+TEST(GenerateRealTest, U64ToFloat_Signed_NoZero_Test) {
   auto ToFloat = [](uint64_t a) {
-    return RandU64ToFloat<SignedValueT, false>(a);
+    return GenerateRealFromBits<float, GenerateSignedTag, false>(a);
   };
   EXPECT_EQ(ToFloat(0x0000000000000000), 5.421010862e-20f);
   EXPECT_EQ(ToFloat(0x0000000000000001), 1.084202172e-19f);
@@ -72,9 +89,9 @@ TEST(DistributionImplTest, U64ToFloat_Signed_NoZero_Test) {
   EXPECT_EQ(ToFloat(0xFFFFFFFFFFFFFFFF), -0.9999999404f);
 }
 
-TEST(DistributionImplTest, U64ToFloat_Signed_Zero_Test) {
+TEST(GenerateRealTest, U64ToFloat_Signed_Zero_Test) {
   auto ToFloat = [](uint64_t a) {
-    return RandU64ToFloat<SignedValueT, true>(a);
+    return GenerateRealFromBits<float, GenerateSignedTag, true>(a);
   };
   EXPECT_EQ(ToFloat(0x0000000000000000), 0);
   EXPECT_EQ(ToFloat(0x0000000000000001), 1.084202172e-19f);
@@ -84,9 +101,9 @@ TEST(DistributionImplTest, U64ToFloat_Signed_Zero_Test) {
   EXPECT_EQ(ToFloat(0xFFFFFFFFFFFFFFFF), -0.9999999404f);
 }
 
-TEST(DistributionImplTest, U64ToFloat_Signed_Bias_Test) {
+TEST(GenerateRealTest, U64ToFloat_Signed_Bias_Test) {
   auto ToFloat = [](uint64_t a) {
-    return RandU64ToFloat<SignedValueT, true, 1>(a);
+    return GenerateRealFromBits<float, GenerateSignedTag, true>(a, 1);
   };
   EXPECT_EQ(ToFloat(0x0000000000000000), 0);
   EXPECT_EQ(ToFloat(0x0000000000000001), 2 * 1.084202172e-19f);
@@ -96,9 +113,9 @@ TEST(DistributionImplTest, U64ToFloat_Signed_Bias_Test) {
   EXPECT_EQ(ToFloat(0xFFFFFFFFFFFFFFFF), 2 * -0.9999999404f);
 }
 
-TEST(DistributionImplTest, U64ToFloatTest) {
+TEST(GenerateRealTest, U64ToFloatTest) {
   auto ToFloat = [](uint64_t a) -> float {
-    return RandU64ToFloat<PositiveValueT, true>(a);
+    return GenerateRealFromBits<float, GeneratePositiveTag, true>(a);
   };
 
   EXPECT_EQ(ToFloat(0x0000000000000000), 0.0f);
@@ -150,44 +167,60 @@ TEST(DistributionImplTest, U64ToFloatTest) {
   }
 }
 
-TEST(DistributionImplTest, U64ToDouble_Positive_NoZero_Test) {
+TEST(GenerateRealTest, U64ToDouble_Positive_NoZero_Test) {
   auto ToDouble = [](uint64_t a) {
-    return RandU64ToDouble<PositiveValueT, false>(a);
+    return GenerateRealFromBits<double, GeneratePositiveTag, false>(a);
   };
 
   EXPECT_EQ(ToDouble(0x0000000000000000), 2.710505431213761085e-20);
   EXPECT_EQ(ToDouble(0x0000000000000001), 5.42101086242752217004e-20);
   EXPECT_EQ(ToDouble(0x0000000000000002), 1.084202172485504434e-19);
   EXPECT_EQ(ToDouble(0x8000000000000000), 0.5);
+  EXPECT_EQ(ToDouble(0x8000000000000001), 0.5);
   EXPECT_EQ(ToDouble(0xFFFFFFFFFFFFFFFF), 0.999999999999999888978);
 }
 
-TEST(DistributionImplTest, U64ToDouble_Positive_Zero_Test) {
+TEST(GenerateRealTest, U64ToDouble_Positive_Zero_Test) {
   auto ToDouble = [](uint64_t a) {
-    return RandU64ToDouble<PositiveValueT, true>(a);
+    return GenerateRealFromBits<double, GeneratePositiveTag, true>(a);
   };
 
   EXPECT_EQ(ToDouble(0x0000000000000000), 0.0);
   EXPECT_EQ(ToDouble(0x0000000000000001), 5.42101086242752217004e-20);
   EXPECT_EQ(ToDouble(0x8000000000000000), 0.5);
+  EXPECT_EQ(ToDouble(0x8000000000000001), 0.5);
   EXPECT_EQ(ToDouble(0xFFFFFFFFFFFFFFFF), 0.999999999999999888978);
 }
 
-TEST(DistributionImplTest, U64ToDouble_Negative_NoZero_Test) {
+TEST(GenerateRealTest, U64ToDouble_Negative_NoZero_Test) {
   auto ToDouble = [](uint64_t a) {
-    return RandU64ToDouble<NegativeValueT, false>(a);
+    return GenerateRealFromBits<double, GenerateNegativeTag, false>(a);
   };
 
   EXPECT_EQ(ToDouble(0x0000000000000000), -2.710505431213761085e-20);
   EXPECT_EQ(ToDouble(0x0000000000000001), -5.42101086242752217004e-20);
   EXPECT_EQ(ToDouble(0x0000000000000002), -1.084202172485504434e-19);
   EXPECT_EQ(ToDouble(0x8000000000000000), -0.5);
+  EXPECT_EQ(ToDouble(0x8000000000000001), -0.5);
+  EXPECT_EQ(ToDouble(0xFFFFFFFFFFFFFFFF), -0.999999999999999888978);
+}
+
+TEST(GenerateRealTest, U64ToDouble_Negative_Zero_Test) {
+  auto ToDouble = [](uint64_t a) {
+    return GenerateRealFromBits<double, GenerateNegativeTag, true>(a);
+  };
+
+  EXPECT_EQ(ToDouble(0x0000000000000000), 0.0);
+  EXPECT_EQ(ToDouble(0x0000000000000001), -5.42101086242752217004e-20);
+  EXPECT_EQ(ToDouble(0x0000000000000002), -1.084202172485504434e-19);
+  EXPECT_EQ(ToDouble(0x8000000000000000), -0.5);
+  EXPECT_EQ(ToDouble(0x8000000000000001), -0.5);
   EXPECT_EQ(ToDouble(0xFFFFFFFFFFFFFFFF), -0.999999999999999888978);
 }
 
-TEST(DistributionImplTest, U64ToDouble_Signed_NoZero_Test) {
+TEST(GenerateRealTest, U64ToDouble_Signed_NoZero_Test) {
   auto ToDouble = [](uint64_t a) {
-    return RandU64ToDouble<SignedValueT, false>(a);
+    return GenerateRealFromBits<double, GenerateSignedTag, false>(a);
   };
 
   EXPECT_EQ(ToDouble(0x0000000000000000), 5.42101086242752217004e-20);
@@ -198,9 +231,9 @@ TEST(DistributionImplTest, U64ToDouble_Signed_NoZero_Test) {
   EXPECT_EQ(ToDouble(0xFFFFFFFFFFFFFFFF), -0.999999999999999888978);
 }
 
-TEST(DistributionImplTest, U64ToDouble_Signed_Zero_Test) {
+TEST(GenerateRealTest, U64ToDouble_Signed_Zero_Test) {
   auto ToDouble = [](uint64_t a) {
-    return RandU64ToDouble<SignedValueT, true>(a);
+    return GenerateRealFromBits<double, GenerateSignedTag, true>(a);
   };
   EXPECT_EQ(ToDouble(0x0000000000000000), 0);
   EXPECT_EQ(ToDouble(0x0000000000000001), 1.084202172485504434e-19);
@@ -210,9 +243,9 @@ TEST(DistributionImplTest, U64ToDouble_Signed_Zero_Test) {
   EXPECT_EQ(ToDouble(0xFFFFFFFFFFFFFFFF), -0.999999999999999888978);
 }
 
-TEST(DistributionImplTest, U64ToDouble_Signed_Bias_Test) {
+TEST(GenerateRealTest, U64ToDouble_GenerateSignedTag_Bias_Test) {
   auto ToDouble = [](uint64_t a) {
-    return RandU64ToDouble<SignedValueT, true, -1>(a);
+    return GenerateRealFromBits<double, GenerateSignedTag, true>(a, -1);
   };
   EXPECT_EQ(ToDouble(0x0000000000000000), 0);
   EXPECT_EQ(ToDouble(0x0000000000000001), 1.084202172485504434e-19 / 2);
@@ -222,9 +255,9 @@ TEST(DistributionImplTest, U64ToDouble_Signed_Bias_Test) {
   EXPECT_EQ(ToDouble(0xFFFFFFFFFFFFFFFF), -0.999999999999999888978 / 2);
 }
 
-TEST(DistributionImplTest, U64ToDoubleTest) {
+TEST(GenerateRealTest, U64ToDoubleTest) {
   auto ToDouble = [](uint64_t a) {
-    return RandU64ToDouble<PositiveValueT, true>(a);
+    return GenerateRealFromBits<double, GeneratePositiveTag, true>(a);
   };
 
   EXPECT_EQ(ToDouble(0x0000000000000000), 0.0);
@@ -296,9 +329,9 @@ TEST(DistributionImplTest, U64ToDoubleTest) {
   }
 }
 
-TEST(DistributionImplTest, U64ToDoubleSignedTest) {
+TEST(GenerateRealTest, U64ToDoubleSignedTest) {
   auto ToDouble = [](uint64_t a) {
-    return RandU64ToDouble<SignedValueT, false>(a);
+    return GenerateRealFromBits<double, GenerateSignedTag, false>(a);
   };
 
   EXPECT_EQ(ToDouble(0x0000000000000000), 5.42101086242752217004e-20);
@@ -379,15 +412,14 @@ TEST(DistributionImplTest, U64ToDoubleSignedTest) {
   }
 }
 
-TEST(DistributionImplTest, ExhaustiveFloat) {
-  using absl::base_internal::CountLeadingZeros64;
+TEST(GenerateRealTest, ExhaustiveFloat) {
   auto ToFloat = [](uint64_t a) {
-    return RandU64ToFloat<PositiveValueT, true>(a);
+    return GenerateRealFromBits<float, GeneratePositiveTag, true>(a);
   };
 
   // Rely on RandU64ToFloat generating values from greatest to least when
-  // supplied with uint64_t values from greatest (0xfff...) to least (0x0).  Thus,
-  // this algorithm stores the previous value, and if the new value is at
+  // supplied with uint64_t values from greatest (0xfff...) to least (0x0).
+  // Thus, this algorithm stores the previous value, and if the new value is at
   // greater than or equal to the previous value, then there is a collision in
   // the generation algorithm.
   //
@@ -431,7 +463,7 @@ TEST(DistributionImplTest, ExhaustiveFloat) {
 
     // Adjust decrement and check value based on how many leading 0
     // bits are set in the current value.
-    const int clz = CountLeadingZeros64(x);
+    const int clz = absl::countl_zero(x);
     if (clz < kDig) {
       dec <<= (kDig - clz);
       chk = (~uint64_t(0)) >> (clz + 1);
@@ -461,46 +493,4 @@ TEST(DistributionImplTest, ExhaustiveFloat) {
   }
 }
 
-TEST(DistributionImplTest, MultiplyU64ToU128Test) {
-  using absl::random_internal::MultiplyU64ToU128;
-  constexpr uint64_t k1 = 1;
-  constexpr uint64_t kMax = ~static_cast<uint64_t>(0);
-
-  EXPECT_EQ(absl::uint128(0), MultiplyU64ToU128(0, 0));
-
-  // Max uint64
-  EXPECT_EQ(MultiplyU64ToU128(kMax, kMax),
-            absl::MakeUint128(0xfffffffffffffffe, 0x0000000000000001));
-  EXPECT_EQ(absl::MakeUint128(0, kMax), MultiplyU64ToU128(kMax, 1));
-  EXPECT_EQ(absl::MakeUint128(0, kMax), MultiplyU64ToU128(1, kMax));
-  for (int i = 0; i < 64; ++i) {
-    EXPECT_EQ(absl::MakeUint128(0, kMax) << i,
-              MultiplyU64ToU128(kMax, k1 << i));
-    EXPECT_EQ(absl::MakeUint128(0, kMax) << i,
-              MultiplyU64ToU128(k1 << i, kMax));
-  }
-
-  // 1-bit x 1-bit.
-  for (int i = 0; i < 64; ++i) {
-    for (int j = 0; j < 64; ++j) {
-      EXPECT_EQ(absl::MakeUint128(0, 1) << (i + j),
-                MultiplyU64ToU128(k1 << i, k1 << j));
-      EXPECT_EQ(absl::MakeUint128(0, 1) << (i + j),
-                MultiplyU64ToU128(k1 << i, k1 << j));
-    }
-  }
-
-  // Verified multiplies
-  EXPECT_EQ(MultiplyU64ToU128(0xffffeeeeddddcccc, 0xbbbbaaaa99998888),
-            absl::MakeUint128(0xbbbb9e2692c5dddc, 0xc28f7531048d2c60));
-  EXPECT_EQ(MultiplyU64ToU128(0x0123456789abcdef, 0xfedcba9876543210),
-            absl::MakeUint128(0x0121fa00ad77d742, 0x2236d88fe5618cf0));
-  EXPECT_EQ(MultiplyU64ToU128(0x0123456789abcdef, 0xfdb97531eca86420),
-            absl::MakeUint128(0x0120ae99d26725fc, 0xce197f0ecac319e0));
-  EXPECT_EQ(MultiplyU64ToU128(0x97a87f4f261ba3f2, 0xfedcba9876543210),
-            absl::MakeUint128(0x96fbf1a8ae78d0ba, 0x5a6dd4b71f278320));
-  EXPECT_EQ(MultiplyU64ToU128(0xfedcba9876543210, 0xfdb97531eca86420),
-            absl::MakeUint128(0xfc98c6981a413e22, 0x342d0bbf48948200));
-}
-
 }  // namespace
diff --git a/absl/random/internal/iostream_state_saver.h b/absl/random/internal/iostream_state_saver.h
index df88fa76bcd..05d9f839e1a 100644
--- a/absl/random/internal/iostream_state_saver.h
+++ b/absl/random/internal/iostream_state_saver.h
@@ -16,14 +16,19 @@
 #define ABSL_RANDOM_INTERNAL_IOSTREAM_STATE_SAVER_H_
 
 #include <cmath>
-#include <iostream>
+#include <cstdint>
+#include <ios>
+#include <istream>
 #include <limits>
+#include <ostream>
 #include <type_traits>
 
+#include "absl/base/config.h"
 #include "absl/meta/type_traits.h"
 #include "absl/numeric/int128.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
 // The null_state_saver does nothing.
@@ -34,7 +39,7 @@ class null_state_saver {
   using flags_type = std::ios_base::fmtflags;
 
   null_state_saver(T&, flags_type) {}
-  ~null_state_saver() {}
+  ~null_state_saver() = default;
 };
 
 // ostream_state_saver is a RAII object to save and restore the common
@@ -90,11 +95,10 @@ ostream_state_saver<std::basic_ostream<CharT, Traits>> make_ostream_state_saver(
 }
 
 template <typename T>
-typename absl::enable_if_t<!std::is_base_of<std::ios_base, T>::value,
-                           null_state_saver<T>>
+typename std::enable_if_t<!std::is_base_of_v<std::ios_base, T>,
+                          null_state_saver<T>>
 make_ostream_state_saver(T& is,  // NOLINT(runtime/references)
                          std::ios_base::fmtflags flags = std::ios_base::dec) {
-  std::cerr << "null_state_saver";
   using result_type = null_state_saver<T>;
   return result_type(is, flags);
 }
@@ -155,8 +159,8 @@ istream_state_saver<std::basic_istream<CharT, Traits>> make_istream_state_saver(
 }
 
 template <typename T>
-typename absl::enable_if_t<!std::is_base_of<std::ios_base, T>::value,
-                           null_state_saver<T>>
+typename std::enable_if_t<!std::is_base_of_v<std::ios_base, T>,
+                          null_state_saver<T>>
 make_istream_state_saver(T& is,  // NOLINT(runtime/references)
                          std::ios_base::fmtflags flags = std::ios_base::dec) {
   using result_type = null_state_saver<T>;
@@ -182,7 +186,7 @@ struct stream_u128_helper;
 template <>
 struct stream_u128_helper<absl::uint128> {
   template <typename IStream>
-  inline absl::uint128 read(IStream& in) {
+  absl::uint128 read(IStream& in) {
     uint64_t h = 0;
     uint64_t l = 0;
     in >> h >> l;
@@ -190,9 +194,9 @@ struct stream_u128_helper<absl::uint128> {
   }
 
   template <typename OStream>
-  inline void write(absl::uint128 val, OStream& out) {
-    uint64_t h = Uint128High64(val);
-    uint64_t l = Uint128Low64(val);
+  void write(absl::uint128 val, OStream& out) {
+    uint64_t h = absl::Uint128High64(val);
+    uint64_t l = absl::Uint128Low64(val);
     out << h << out.fill() << l;
   }
 };
@@ -201,7 +205,7 @@ struct stream_u128_helper<absl::uint128> {
 template <>
 struct stream_u128_helper<__uint128_t> {
   template <typename IStream>
-  inline __uint128_t read(IStream& in) {
+  __uint128_t read(IStream& in) {
     uint64_t h = 0;
     uint64_t l = 0;
     in >> h >> l;
@@ -209,7 +213,7 @@ struct stream_u128_helper<__uint128_t> {
   }
 
   template <typename OStream>
-  inline void write(__uint128_t val, OStream& out) {
+  void write(__uint128_t val, OStream& out) {
     uint64_t h = static_cast<uint64_t>(val >> 64u);
     uint64_t l = static_cast<uint64_t>(val);
     out << h << out.fill() << l;
@@ -219,7 +223,7 @@ struct stream_u128_helper<__uint128_t> {
 
 template <typename FloatType, typename IStream>
 inline FloatType read_floating_point(IStream& is) {
-  static_assert(std::is_floating_point<FloatType>::value, "");
+  static_assert(std::is_floating_point_v<FloatType>);
   FloatType dest;
   is >> dest;
   // Parsing a double value may report a subnormal value as an error
@@ -238,6 +242,7 @@ inline FloatType read_floating_point(IStream& is) {
 }
 
 }  // namespace random_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_INTERNAL_IOSTREAM_STATE_SAVER_H_
diff --git a/absl/random/internal/iostream_state_saver_test.cc b/absl/random/internal/iostream_state_saver_test.cc
index 2ecbaac143d..a196ace76ec 100644
--- a/absl/random/internal/iostream_state_saver_test.cc
+++ b/absl/random/internal/iostream_state_saver_test.cc
@@ -14,8 +14,16 @@
 
 #include "absl/random/internal/iostream_state_saver.h"
 
+#include <stdio.h>
+
+#include <cmath>
+#include <cstdint>
+#include <cstdlib>
+#include <ios>
+#include <limits>
 #include <sstream>
 #include <string>
+#include <type_traits>
 
 #include "gtest/gtest.h"
 
@@ -26,7 +34,7 @@ using absl::random_internal::make_ostream_state_saver;
 using absl::random_internal::stream_precision_helper;
 
 template <typename T>
-typename absl::enable_if_t<std::is_integral<T>::value, T>  //
+typename std::enable_if_t<std::is_integral_v<T>, T>  //
 StreamRoundTrip(T t) {
   std::stringstream ss;
   {
@@ -50,7 +58,7 @@ StreamRoundTrip(T t) {
 }
 
 template <typename T>
-typename absl::enable_if_t<std::is_floating_point<T>::value, T>  //
+typename std::enable_if_t<std::is_floating_point_v<T>, T>  //
 StreamRoundTrip(T t) {
   std::stringstream ss;
   {
@@ -196,8 +204,8 @@ TEST(IOStreamStateSaver, RoundTripFloats) {
     EXPECT_EQ(-d, StreamRoundTrip<double>(-d));
 
     // Avoid undefined behavior (overflow/underflow).
-    if (d <= std::numeric_limits<int64_t>::max() &&
-        d >= std::numeric_limits<int64_t>::lowest()) {
+    if (f <= static_cast<float>(std::numeric_limits<int64_t>::max()) &&
+        f >= static_cast<float>(std::numeric_limits<int64_t>::lowest())) {
       int64_t x = static_cast<int64_t>(f);
       EXPECT_EQ(x, StreamRoundTrip<int64_t>(x));
     }
@@ -264,8 +272,8 @@ TEST(IOStreamStateSaver, RoundTripDoubles) {
     }
 
     // Avoid undefined behavior (overflow/underflow).
-    if (d <= std::numeric_limits<int64_t>::max() &&
-        d >= std::numeric_limits<int64_t>::lowest()) {
+    if (d <= static_cast<double>(std::numeric_limits<int64_t>::max()) &&
+        d >= static_cast<double>(std::numeric_limits<int64_t>::lowest())) {
       int64_t x = static_cast<int64_t>(d);
       EXPECT_EQ(x, StreamRoundTrip<int64_t>(x));
     }
@@ -342,8 +350,9 @@ TEST(IOStreamStateSaver, RoundTripLongDoubles) {
     }
 
     // Avoid undefined behavior (overflow/underflow).
-    if (dd <= std::numeric_limits<int64_t>::max() &&
-        dd >= std::numeric_limits<int64_t>::lowest()) {
+    if (dd <= static_cast<long double>(std::numeric_limits<int64_t>::max()) &&
+        dd >=
+            static_cast<long double>(std::numeric_limits<int64_t>::lowest())) {
       int64_t x = static_cast<int64_t>(dd);
       EXPECT_EQ(x, StreamRoundTrip<int64_t>(x));
     }
diff --git a/absl/random/internal/mock_helpers.h b/absl/random/internal/mock_helpers.h
new file mode 100644
index 00000000000..b73b9cbac0a
--- /dev/null
+++ b/absl/random/internal/mock_helpers.h
@@ -0,0 +1,134 @@
+//
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_RANDOM_INTERNAL_MOCK_HELPERS_H_
+#define ABSL_RANDOM_INTERNAL_MOCK_HELPERS_H_
+
+#include <optional>
+#include <utility>
+
+#include "absl/base/config.h"
+#include "absl/base/fast_type_id.h"
+#include "absl/random/mocking_access.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace random_internal {
+
+// A no-op validator meeting the ValidatorT requirements for MockHelpers.
+//
+// Custom validators should follow a similar structure, passing the type to
+// MockHelpers::MockFor<KeyT>(m, CustomValidatorT()).
+struct NoOpValidator {
+  // Default validation: do nothing.
+  template <typename ResultT, typename... Args>
+  static void Validate(ResultT, Args&&...) {}
+};
+
+// MockHelpers works in conjunction with MockOverloadSet, MockingBitGen, and
+// BitGenRef to enable the mocking capability for absl distribution functions.
+//
+// MockingBitGen registers mocks based on the typeid of a mock signature, KeyT,
+// which is used to generate a unique id.
+//
+// KeyT is a signature of the form:
+//   result_type(discriminator_type, std::tuple<args...>)
+// The mocked function signature will be composed from KeyT as:
+//   result_type(args...)
+//
+class MockHelpers {
+  using IdType = ::absl::FastTypeIdType;
+  using RandomMockingAccess = ::absl::RandomMockingAccess;
+
+  // Given a key signature type used to index the mock, extract the components.
+  // KeyT is expected to have the form:
+  //   result_type(discriminator_type, arg_tuple_type)
+  template <typename KeyT>
+  struct KeySignature;
+
+  template <typename ResultT, typename DiscriminatorT, typename ArgTupleT>
+  struct KeySignature<ResultT(DiscriminatorT, ArgTupleT)> {
+    using result_type = ResultT;
+    using discriminator_type = DiscriminatorT;
+    using arg_tuple_type = ArgTupleT;
+  };
+
+ public:
+  // Invoke a mock for the KeyT (may or may not be a signature).
+  //
+  // KeyT is used to generate a typeid-based lookup key for the mock.
+  // KeyT is a signature of the form:
+  //   result_type(discriminator_type, std::tuple<args...>)
+  // The mocked function signature will be composed from KeyT as:
+  //   result_type(args...)
+  //
+  // An instance of arg_tuple_type must be constructable from Args..., since
+  // the underlying mechanism requires a pointer to an argument tuple.
+  template <typename KeyT, typename URBG, typename... Args>
+  static auto MaybeInvokeMock(URBG* urbg, Args&&... args)
+      -> std::optional<typename KeySignature<KeyT>::result_type> {
+    if constexpr (RandomMockingAccess::HasInvokeMock<URBG>::value) {
+      typename KeySignature<KeyT>::arg_tuple_type arg_tuple(
+          std::forward<Args>(args)...);
+      typename KeySignature<KeyT>::result_type result;
+      if (RandomMockingAccess::InvokeMock(urbg, FastTypeId<KeyT>(), &arg_tuple,
+                                          &result)) {
+        return result;
+      }
+    }
+    return std::nullopt;
+  }
+
+  // Acquire a mock for the KeyT (may or may not be a signature), set up to use
+  // the ValidatorT to verify that the result is in the range of the RNG
+  // function.
+  //
+  // KeyT is used to generate a typeid-based lookup for the mock.
+  // KeyT is a signature of the form:
+  //   result_type(discriminator_type, std::tuple<args...>)
+  // The mocked function signature will be composed from KeyT as:
+  //   result_type(args...)
+  // ValidatorT::Validate will be called after the result of the RNG. The
+  //   signature is expected to be of the form:
+  //      ValidatorT::Validate(result, args...)
+  template <typename KeyT, typename ValidatorT, typename MockURBG>
+  static auto MockFor(MockURBG& m, ValidatorT)
+      -> decltype(m.template RegisterMock<
+                  typename KeySignature<KeyT>::result_type,
+                  typename KeySignature<KeyT>::arg_tuple_type>(
+          m, std::declval<IdType>(), ValidatorT())) {
+    return m.template RegisterMock<typename KeySignature<KeyT>::result_type,
+                                   typename KeySignature<KeyT>::arg_tuple_type>(
+        m, ::absl::FastTypeId<KeyT>(), ValidatorT());
+  }
+
+  // Acquire a mock for the KeyT (may or may not be a signature).
+  //
+  // KeyT is used to generate a typeid-based lookup for the mock.
+  // KeyT is a signature of the form:
+  //   result_type(discriminator_type, std::tuple<args...>)
+  // The mocked function signature will be composed from KeyT as:
+  //   result_type(args...)
+  template <typename KeyT, typename MockURBG>
+  static decltype(auto) MockFor(MockURBG& m) {
+    return MockFor<KeyT>(m, NoOpValidator());
+  }
+};
+
+}  // namespace random_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_RANDOM_INTERNAL_MOCK_HELPERS_H_
diff --git a/absl/random/internal/mock_overload_set.h b/absl/random/internal/mock_overload_set.h
new file mode 100644
index 00000000000..a3b15ee10cf
--- /dev/null
+++ b/absl/random/internal/mock_overload_set.h
@@ -0,0 +1,122 @@
+//
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_RANDOM_INTERNAL_MOCK_OVERLOAD_SET_H_
+#define ABSL_RANDOM_INTERNAL_MOCK_OVERLOAD_SET_H_
+
+#include <tuple>
+#include <type_traits>
+
+#include "gmock/gmock.h"
+#include "absl/base/config.h"
+#include "absl/random/internal/mock_helpers.h"
+#include "absl/random/mocking_bit_gen.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace random_internal {
+
+template <typename DistrT, typename ValidatorT, typename Fn>
+struct MockSingleOverload;
+
+// MockSingleOverload
+//
+// MockSingleOverload hooks in to gMock's `ON_CALL` and `EXPECT_CALL` macros.
+// EXPECT_CALL(mock_single_overload, Call(...))` will expand to a call to
+// `mock_single_overload.gmock_Call(...)`. Because expectations are stored on
+// the MockingBitGen (an argument passed inside `Call(...)`), this forwards to
+// arguments to MockingBitGen::Register.
+//
+// The underlying KeyT must match the KeyT constructed by DistributionCaller.
+template <typename DistrT, typename ValidatorT, typename Ret, typename... Args>
+struct MockSingleOverload<DistrT, ValidatorT, Ret(MockingBitGen&, Args...)> {
+  static_assert(std::is_same_v<typename DistrT::result_type, Ret>,
+                "Overload signature must have return type matching the "
+                "distribution result_type.");
+  using KeyT = Ret(DistrT, std::tuple<Args...>);
+
+  template <typename MockURBG>
+  auto gmock_Call(MockURBG& gen, const ::testing::Matcher<Args>&... matchers)
+      -> decltype(MockHelpers::MockFor<KeyT>(gen, ValidatorT())
+                      .gmock_Call(matchers...)) {
+    static_assert(std::is_base_of_v<MockingBitGen, MockURBG>,
+                  "Mocking requires an absl::MockingBitGen");
+    return MockHelpers::MockFor<KeyT>(gen, ValidatorT())
+        .gmock_Call(matchers...);
+  }
+};
+
+template <typename DistrT, typename ValidatorT, typename Ret, typename Arg,
+          typename... Args>
+struct MockSingleOverload<DistrT, ValidatorT,
+                          Ret(Arg, MockingBitGen&, Args...)> {
+  static_assert(std::is_same_v<typename DistrT::result_type, Ret>,
+                "Overload signature must have return type matching the "
+                "distribution result_type.");
+  using KeyT = Ret(DistrT, std::tuple<Arg, Args...>);
+
+  template <typename MockURBG>
+  auto gmock_Call(const ::testing::Matcher<Arg>& matcher, MockURBG& gen,
+                  const ::testing::Matcher<Args>&... matchers)
+      -> decltype(MockHelpers::MockFor<KeyT>(gen, ValidatorT())
+                      .gmock_Call(matcher, matchers...)) {
+    static_assert(std::is_base_of_v<MockingBitGen, MockURBG>,
+                  "Mocking requires an absl::MockingBitGen");
+    return MockHelpers::MockFor<KeyT>(gen, ValidatorT())
+        .gmock_Call(matcher, matchers...);
+  }
+};
+
+// MockOverloadSetWithValidator
+//
+// MockOverloadSetWithValidator is a wrapper around MockOverloadSet which takes
+// an additional Validator parameter, allowing for customization of the mock
+// behavior.
+//
+// `ValidatorT::Validate(result, args...)` will be called after the mock
+// distribution returns a value in `result`, allowing for validation against the
+// args.
+template <typename DistrT, typename ValidatorT, typename... Fns>
+struct MockOverloadSetWithValidator;
+
+template <typename DistrT, typename ValidatorT, typename Sig>
+struct MockOverloadSetWithValidator<DistrT, ValidatorT, Sig>
+    : public MockSingleOverload<DistrT, ValidatorT, Sig> {
+  using MockSingleOverload<DistrT, ValidatorT, Sig>::gmock_Call;
+};
+
+template <typename DistrT, typename ValidatorT, typename FirstSig,
+          typename... Rest>
+struct MockOverloadSetWithValidator<DistrT, ValidatorT, FirstSig, Rest...>
+    : public MockSingleOverload<DistrT, ValidatorT, FirstSig>,
+      public MockOverloadSetWithValidator<DistrT, ValidatorT, Rest...> {
+  using MockSingleOverload<DistrT, ValidatorT, FirstSig>::gmock_Call;
+  using MockOverloadSetWithValidator<DistrT, ValidatorT, Rest...>::gmock_Call;
+};
+
+// MockOverloadSet
+//
+// MockOverloadSet takes a distribution and a collection of signatures and
+// performs overload resolution amongst all the overloads. This makes
+// `EXPECT_CALL(mock_overload_set, Call(...))` expand and do overload resolution
+// correctly.
+template <typename DistrT, typename... Signatures>
+using MockOverloadSet =
+    MockOverloadSetWithValidator<DistrT, NoOpValidator, Signatures...>;
+
+}  // namespace random_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+#endif  // ABSL_RANDOM_INTERNAL_MOCK_OVERLOAD_SET_H_
diff --git a/absl/random/internal/mock_validators.h b/absl/random/internal/mock_validators.h
new file mode 100644
index 00000000000..d76d169c250
--- /dev/null
+++ b/absl/random/internal/mock_validators.h
@@ -0,0 +1,98 @@
+// Copyright 2024 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_RANDOM_INTERNAL_MOCK_VALIDATORS_H_
+#define ABSL_RANDOM_INTERNAL_MOCK_VALIDATORS_H_
+
+#include <type_traits>
+
+#include "absl/base/config.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/random/internal/iostream_state_saver.h"
+#include "absl/random/internal/uniform_helper.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace random_internal {
+
+template <typename NumType>
+class UniformDistributionValidator {
+ public:
+  // Handle absl::Uniform<NumType>(gen, absl::IntervalTag, lo, hi).
+  template <typename TagType>
+  static void Validate(NumType x, TagType tag, NumType lo, NumType hi) {
+    // For invalid ranges, absl::Uniform() simply returns one of the bounds.
+    if (x == lo && lo == hi) return;
+
+    ValidateImpl(std::is_floating_point<NumType>{}, x, tag, lo, hi);
+  }
+
+  // Handle absl::Uniform<NumType>(gen, lo, hi).
+  static void Validate(NumType x, NumType lo, NumType hi) {
+    Validate(x, IntervalClosedOpenTag(), lo, hi);
+  }
+
+  // Handle absl::Uniform<NumType>(gen).
+  static void Validate(NumType) {
+    // absl::Uniform<NumType>(gen) spans the entire range of `NumType`, so any
+    // value is okay. This overload exists because the validation logic attempts
+    // to call it anyway rather than adding extra SFINAE.
+  }
+
+ private:
+  static absl::string_view TagLbBound(IntervalClosedOpenTag) { return "["; }
+  static absl::string_view TagLbBound(IntervalOpenOpenTag) { return "("; }
+  static absl::string_view TagLbBound(IntervalClosedClosedTag) { return "["; }
+  static absl::string_view TagLbBound(IntervalOpenClosedTag) { return "("; }
+  static absl::string_view TagUbBound(IntervalClosedOpenTag) { return ")"; }
+  static absl::string_view TagUbBound(IntervalOpenOpenTag) { return ")"; }
+  static absl::string_view TagUbBound(IntervalClosedClosedTag) { return "]"; }
+  static absl::string_view TagUbBound(IntervalOpenClosedTag) { return "]"; }
+
+  template <typename TagType>
+  static void ValidateImpl(std::true_type /* is_floating_point */, NumType x,
+                           TagType tag, NumType lo, NumType hi) {
+    UniformDistributionWrapper<NumType> dist(tag, lo, hi);
+    NumType lb = dist.a();
+    NumType ub = dist.b();
+    // uniform_real_distribution is always closed-open, so the upper bound is
+    // always non-inclusive.
+    ABSL_INTERNAL_CHECK(lb <= x && x < ub,
+                        absl::StrCat(x, " is not in ", TagLbBound(tag), lo,
+                                     ", ", hi, TagUbBound(tag)));
+  }
+
+  template <typename TagType>
+  static void ValidateImpl(std::false_type /* is_floating_point */, NumType x,
+                           TagType tag, NumType lo, NumType hi) {
+    using stream_type =
+        typename random_internal::stream_format_type<NumType>::type;
+
+    UniformDistributionWrapper<NumType> dist(tag, lo, hi);
+    NumType lb = dist.a();
+    NumType ub = dist.b();
+    ABSL_INTERNAL_CHECK(
+        lb <= x && x <= ub,
+        absl::StrCat(stream_type{x}, " is not in ", TagLbBound(tag),
+                     stream_type{lo}, ", ", stream_type{hi}, TagUbBound(tag)));
+  }
+};
+
+}  // namespace random_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_RANDOM_INTERNAL_MOCK_VALIDATORS_H_
diff --git a/absl/random/internal/nanobenchmark.cc b/absl/random/internal/nanobenchmark.cc
index 7f37800c683..0cf657b4851 100644
--- a/absl/random/internal/nanobenchmark.cc
+++ b/absl/random/internal/nanobenchmark.cc
@@ -17,7 +17,6 @@
 #include <sys/types.h>
 
 #include <algorithm>  // sort
-#include <atomic>
 #include <cstddef>
 #include <cstdint>
 #include <cstdlib>
@@ -27,6 +26,9 @@
 #include <utility>
 #include <vector>
 
+#include "absl/algorithm/container.h"
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
 #include "absl/base/internal/raw_logging.h"
 #include "absl/random/internal/platform.h"
 #include "absl/random/internal/randen_engine.h"
@@ -59,15 +61,6 @@
 #include <time.h>  // NOLINT
 #endif
 
-// ABSL_HAVE_ATTRIBUTE
-#if !defined(ABSL_HAVE_ATTRIBUTE)
-#ifdef __has_attribute
-#define ABSL_HAVE_ATTRIBUTE(x) __has_attribute(x)
-#else
-#define ABSL_HAVE_ATTRIBUTE(x) 0
-#endif
-#endif
-
 // ABSL_RANDOM_INTERNAL_ATTRIBUTE_NEVER_INLINE prevents inlining of the method.
 #if ABSL_HAVE_ATTRIBUTE(noinline) || (defined(__GNUC__) && !defined(__clang__))
 #define ABSL_RANDOM_INTERNAL_ATTRIBUTE_NEVER_INLINE __attribute__((noinline))
@@ -78,6 +71,7 @@
 #endif
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal_nanobenchmark {
 namespace {
 
@@ -108,7 +102,7 @@ std::string BrandString() {
   char brand_string[49];
   uint32_t abcd[4];
 
-  // Check if brand std::string is supported (it is on all reasonable Intel/AMD)
+  // Check if brand string is supported (it is on all reasonable Intel/AMD)
   Cpuid(0x80000000U, 0, abcd);
   if (abcd[0] < 0x80000004U) {
     return std::string();
@@ -368,7 +362,7 @@ void CountingSort(T* values, size_t num_values) {
   // Write that many copies of each unique value to the array.
   T* ABSL_RANDOM_INTERNAL_RESTRICT p = values;
   for (const auto& value_count : unique) {
-    std::fill(p, p + value_count.second, value_count.first);
+    std::fill_n(p, value_count.second, value_count.first);
     p += value_count.second;
   }
   ABSL_RAW_CHECK(p == values + num_values, "Did not produce enough output");
@@ -555,7 +549,7 @@ using InputVec = std::vector<FuncInput>;
 // Returns vector of unique input values.
 InputVec UniqueInputs(const FuncInput* inputs, const size_t num_inputs) {
   InputVec unique(inputs, inputs + num_inputs);
-  std::sort(unique.begin(), unique.end());
+  absl::c_sort(unique);
   unique.erase(std::unique(unique.begin(), unique.end()), unique.end());
   return unique;
 }
@@ -614,7 +608,7 @@ InputVec ReplicateInputs(const FuncInput* inputs, const size_t num_inputs,
     full.insert(full.end(), inputs, inputs + num_inputs);
   }
   absl::random_internal::randen_engine<uint32_t> rng;
-  std::shuffle(full.begin(), full.end(), rng);
+  absl::c_shuffle(full, rng);
   return full;
 }
 
@@ -622,20 +616,20 @@ InputVec ReplicateInputs(const FuncInput* inputs, const size_t num_inputs,
 // randomly selected occurrences of "input_to_skip" removed.
 void FillSubset(const InputVec& full, const FuncInput input_to_skip,
                 const size_t num_skip, InputVec* subset) {
-  const size_t count = std::count(full.begin(), full.end(), input_to_skip);
+  const ptrdiff_t count = absl::c_count(full, input_to_skip);
   // Generate num_skip random indices: which occurrence to skip.
   std::vector<uint32_t> omit;
   // Replacement for std::iota, not yet available in MSVC builds.
   omit.reserve(count);
-  for (size_t i = 0; i < count; ++i) {
+  for (ptrdiff_t i = 0; i < count; ++i) {
     omit.push_back(i);
   }
   // omit[] is the same on every call, but that's OK because they identify the
   // Nth instance of input_to_skip, so the position within full[] differs.
   absl::random_internal::randen_engine<uint32_t> rng;
-  std::shuffle(omit.begin(), omit.end(), rng);
+  absl::c_shuffle(omit, rng);
   omit.resize(num_skip);
-  std::sort(omit.begin(), omit.end());
+  absl::c_sort(omit);
 
   uint32_t occurrence = ~0u;  // 0 after preincrement
   size_t idx_omit = 0;        // cursor within omit[]
@@ -807,4 +801,5 @@ size_t Measure(const Func func, const void* arg, const FuncInput* inputs,
 }
 
 }  // namespace random_internal_nanobenchmark
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/random/internal/nanobenchmark.h b/absl/random/internal/nanobenchmark.h
index c2b650d191b..9a1cb590894 100644
--- a/absl/random/internal/nanobenchmark.h
+++ b/absl/random/internal/nanobenchmark.h
@@ -50,7 +50,10 @@
 #include <stddef.h>
 #include <stdint.h>
 
+#include "absl/base/config.h"
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal_nanobenchmark {
 
 // Input influencing the function being measured (e.g. number of bytes to copy).
@@ -121,7 +124,7 @@ struct Result {
 // Ensures the thread is running on the specified cpu, and no others.
 // Reduces noise due to desynchronized socket RDTSC and context switches.
 // If "cpu" is negative, pin to the currently running core.
-void PinThreadToCPU(const int cpu = -1);
+void PinThreadToCPU(int cpu = -1);
 
 // Returns tick rate, useful for converting measurements to seconds. Invariant
 // means the tick counter frequency is independent of CPU throttling or sleep.
@@ -140,9 +143,8 @@ double InvariantTicksPerSecond();
 //   uniform distribution over [0, 4) could be represented as {3,0,2,1}.
 // Returns how many Result were written to "results": one per unique input, or
 //   zero if the measurement failed (an error message goes to stderr).
-size_t Measure(const Func func, const void* arg, const FuncInput* inputs,
-               const size_t num_inputs, Result* results,
-               const Params& p = Params());
+size_t Measure(Func func, const void* arg, const FuncInput* inputs,
+               size_t num_inputs, Result* results, const Params& p = Params());
 
 // Calls operator() of the given closure (lambda function).
 template <class Closure>
@@ -163,6 +165,7 @@ static inline size_t MeasureClosure(const Closure& closure,
 }
 
 }  // namespace random_internal_nanobenchmark
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_INTERNAL_NANOBENCHMARK_H_
diff --git a/absl/random/internal/nanobenchmark_test.cc b/absl/random/internal/nanobenchmark_test.cc
index 383345a8b47..fc3f996c437 100644
--- a/absl/random/internal/nanobenchmark_test.cc
+++ b/absl/random/internal/nanobenchmark_test.cc
@@ -14,10 +14,17 @@
 
 #include "absl/random/internal/nanobenchmark.h"
 
-#include "absl/base/internal/raw_logging.h"
+#include <cstddef>
+#include <cstdint>
+
+#include "absl/base/config.h"
+#include "absl/log/check.h"
+#include "absl/log/log.h"
 #include "absl/strings/numbers.h"
+#include "absl/strings/str_format.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal_nanobenchmark {
 namespace {
 
@@ -35,16 +42,16 @@ void MeasureDiv(const FuncInput (&inputs)[N]) {
   params.max_evals = 6;  // avoid test timeout
   const size_t num_results = Measure(&Div, nullptr, inputs, N, results, params);
   if (num_results == 0) {
-    ABSL_RAW_LOG(
-        WARNING,
-        "WARNING: Measurement failed, should not happen when using "
-        "PinThreadToCPU unless the region to measure takes > 1 second.\n");
+    LOG(WARNING)
+        << "WARNING: Measurement failed, should not happen when using "
+           "PinThreadToCPU unless the region to measure takes > 1 second.";
     return;
   }
   for (size_t i = 0; i < num_results; ++i) {
-    ABSL_RAW_LOG(INFO, "%5zu: %6.2f ticks; MAD=%4.2f%%\n", results[i].input,
-                 results[i].ticks, results[i].variability * 100.0);
-    ABSL_RAW_CHECK(results[i].ticks != 0.0f, "Zero duration");
+    LOG(INFO) << absl::StreamFormat("%5u: %6.2f ticks; MAD=%4.2f%%\n",
+                                    results[i].input, results[i].ticks,
+                                    results[i].variability * 100.0);
+    CHECK_NE(results[i].ticks, 0.0f) << "Zero duration";
   }
 }
 
@@ -52,8 +59,8 @@ void RunAll(const int argc, char* argv[]) {
   // Avoid migrating between cores - important on multi-socket systems.
   int cpu = -1;
   if (argc == 2) {
-    if (!SimpleAtoi(argv[1], &cpu)) {
-      ABSL_RAW_LOG(FATAL, "The optional argument must be a CPU number >= 0.\n");
+    if (!absl::SimpleAtoi(argv[1], &cpu)) {
+      LOG(FATAL) << "The optional argument must be a CPU number >= 0.";
     }
   }
   PinThreadToCPU(cpu);
@@ -67,6 +74,7 @@ void RunAll(const int argc, char* argv[]) {
 
 }  // namespace
 }  // namespace random_internal_nanobenchmark
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 int main(int argc, char* argv[]) {
diff --git a/absl/random/internal/nonsecure_base.h b/absl/random/internal/nonsecure_base.h
index 8847e74bef6..d1442f8037c 100644
--- a/absl/random/internal/nonsecure_base.h
+++ b/absl/random/internal/nonsecure_base.h
@@ -16,28 +16,82 @@
 #define ABSL_RANDOM_INTERNAL_NONSECURE_BASE_H_
 
 #include <algorithm>
+#include <cstddef>
 #include <cstdint>
-#include <iostream>
 #include <iterator>
-#include <random>
-#include <string>
 #include <type_traits>
+#include <utility>
 #include <vector>
 
-#include "absl/base/macros.h"
+#include "absl/base/config.h"
+#include "absl/container/inlined_vector.h"
 #include "absl/meta/type_traits.h"
-#include "absl/random/internal/pool_urbg.h"
+#include "absl/random/internal/entropy_pool.h"
 #include "absl/random/internal/salted_seed_seq.h"
 #include "absl/random/internal/seed_material.h"
-#include "absl/types/optional.h"
-#include "absl/types/span.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
+// RandenPoolSeedSeq is a custom seed sequence type where generate() fills the
+// provided buffer via the RandenPool entropy source.
+class RandenPoolSeedSeq {
+ private:
+  struct ContiguousTag {};
+  struct BufferTag {};
+
+  // Generate random unsigned values directly into the buffer.
+  template <typename Contiguous>
+  void generate_impl(ContiguousTag, Contiguous begin, Contiguous end) {
+    const size_t n = static_cast<size_t>(std::distance(begin, end));
+    auto* a = &(*begin);
+    GetEntropyFromRandenPool(a, sizeof(*a) * n);
+  }
+
+  // Construct a buffer of size n and fill it with values, then copy
+  // those values into the seed iterators.
+  template <typename RandomAccessIterator>
+  void generate_impl(BufferTag, RandomAccessIterator begin,
+                     RandomAccessIterator end) {
+    const size_t n = std::distance(begin, end);
+    absl::InlinedVector<uint32_t, 8> data(n, 0);
+    GetEntropyFromRandenPool(data.begin(), sizeof(data[0]) * n);
+    std::copy(std::begin(data), std::end(data), begin);
+  }
+
+ public:
+  using result_type = uint32_t;
+
+  size_t size() { return 0; }
+
+  template <typename OutIterator>
+  void param(OutIterator) const {}
+
+  template <typename RandomAccessIterator>
+  void generate(RandomAccessIterator begin, RandomAccessIterator end) {
+    // RandomAccessIterator must be assignable from uint32_t
+    if (begin != end) {
+      using U = typename std::iterator_traits<RandomAccessIterator>::value_type;
+      // ContiguousTag indicates the common case of a known contiguous buffer,
+      // which allows directly filling the buffer. In C++20,
+      // std::contiguous_iterator_tag provides a mechanism for testing this
+      // capability, however until Abseil's support requirements allow us to
+      // assume C++20, limit checks to a few common cases.
+      using TagType = std::conditional_t<
+          (std::is_pointer_v<RandomAccessIterator> ||
+           std::is_same_v<RandomAccessIterator,
+                          typename std::vector<U>::iterator>),
+          ContiguousTag, BufferTag>;
+
+      generate_impl(TagType{}, begin, end);
+    }
+  }
+};
+
 // Each instance of NonsecureURBGBase<URBG> will be seeded by variates produced
 // by a thread-unique URBG-instance.
-template <typename URBG>
+template <typename URBG, typename Seeder = RandenPoolSeedSeq>
 class NonsecureURBGBase {
  public:
   using result_type = typename URBG::result_type;
@@ -52,8 +106,8 @@ class NonsecureURBGBase {
   NonsecureURBGBase& operator=(NonsecureURBGBase&&) = default;
 
   // Constructor using a seed
-  template <class SSeq, typename = typename absl::enable_if_t<
-                            !std::is_same<SSeq, NonsecureURBGBase>::value>>
+  template <class SSeq, typename = typename std::enable_if_t<
+                            !std::is_same_v<SSeq, NonsecureURBGBase>>>
   explicit NonsecureURBGBase(SSeq&& seq)
       : urbg_(ConstructURBG(std::forward<SSeq>(seq))) {}
 
@@ -84,49 +138,6 @@ class NonsecureURBGBase {
   }
 
  private:
-  // Seeder is a custom seed sequence type where generate() fills the provided
-  // buffer via the RandenPool entropy source.
-  struct Seeder {
-    using result_type = uint32_t;
-
-    size_t size() { return 0; }
-
-    template <typename OutIterator>
-    void param(OutIterator) const {}
-
-    template <typename RandomAccessIterator>
-    void generate(RandomAccessIterator begin, RandomAccessIterator end) {
-      if (begin != end) {
-        // begin, end must be random access iterators assignable from uint32_t.
-        generate_impl(
-            std::integral_constant<bool, sizeof(*begin) == sizeof(uint32_t)>{},
-            begin, end);
-      }
-    }
-
-    // Commonly, generate is invoked with a pointer to a buffer which
-    // can be cast to a uint32_t.
-    template <typename RandomAccessIterator>
-    void generate_impl(std::integral_constant<bool, true>,
-                       RandomAccessIterator begin, RandomAccessIterator end) {
-      auto buffer = absl::MakeSpan(begin, end);
-      auto target = absl::MakeSpan(reinterpret_cast<uint32_t*>(buffer.data()),
-                                   buffer.size());
-      RandenPool<uint32_t>::Fill(target);
-    }
-
-    // The non-uint32_t case should be uncommon, and involves an extra copy,
-    // filling the uint32_t buffer and then mixing into the output.
-    template <typename RandomAccessIterator>
-    void generate_impl(std::integral_constant<bool, false>,
-                       RandomAccessIterator begin, RandomAccessIterator end) {
-      const size_t n = std::distance(begin, end);
-      absl::InlinedVector<uint32_t, 8> data(n, 0);
-      RandenPool<uint32_t>::Fill(absl::MakeSpan(data.begin(), data.end()));
-      std::copy(std::begin(data), std::end(data), begin);
-    }
-  };
-
   static URBG ConstructURBG() {
     Seeder seeder;
     return URBG(seeder);
@@ -143,6 +154,7 @@ class NonsecureURBGBase {
 };
 
 }  // namespace random_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_INTERNAL_NONSECURE_BASE_H_
diff --git a/absl/random/internal/nonsecure_base_test.cc b/absl/random/internal/nonsecure_base_test.cc
index d9de99019ce..61f49cdfdb9 100644
--- a/absl/random/internal/nonsecure_base_test.cc
+++ b/absl/random/internal/nonsecure_base_test.cc
@@ -15,15 +15,21 @@
 #include "absl/random/internal/nonsecure_base.h"
 
 #include <algorithm>
-#include <iostream>
-#include <memory>
+#include <cstddef>
+#include <cstdint>
+#include <iterator>
 #include <random>
-#include <sstream>
+#include <thread>  // NOLINT
+#include <type_traits>
+#include <utility>
+#include <vector>
 
 #include "gtest/gtest.h"
+#include "absl/algorithm/container.h"
+#include "absl/container/flat_hash_set.h"
 #include "absl/random/distributions.h"
 #include "absl/random/random.h"
-#include "absl/strings/str_cat.h"
+#include "absl/synchronization/mutex.h"
 
 namespace {
 
@@ -68,19 +74,19 @@ TEST(NonsecureURBGBase, StandardInterface) {
 
   using T = typename E::result_type;
 
-  static_assert(!std::is_copy_constructible<E>::value,
+  static_assert(!std::is_copy_constructible_v<E>,
                 "NonsecureURBGBase should not be copy constructible");
 
-  static_assert(!absl::is_copy_assignable<E>::value,
+  static_assert(!std::is_copy_assignable_v<E>,
                 "NonsecureURBGBase should not be copy assignable");
 
-  static_assert(std::is_move_constructible<E>::value,
+  static_assert(std::is_move_constructible_v<E>,
                 "NonsecureURBGBase should be move constructible");
 
-  static_assert(absl::is_move_assignable<E>::value,
+  static_assert(std::is_move_assignable_v<E>,
                 "NonsecureURBGBase should be move assignable");
 
-  static_assert(std::is_same<decltype(std::declval<E>()()), T>::value,
+  static_assert(std::is_same_v<decltype(std::declval<E>()()), T>,
                 "return type of operator() must be result_type");
 
   {
@@ -88,10 +94,10 @@ TEST(NonsecureURBGBase, StandardInterface) {
     Use(x);
     Use(y);
 
-    static_assert(std::is_same<decltype(x == y), bool>::value,
+    static_assert(std::is_same_v<decltype(x == y), bool>,
                   "return type of operator== must be bool");
 
-    static_assert(std::is_same<decltype(x != y), bool>::value,
+    static_assert(std::is_same_v<decltype(x != y), bool>,
                   "return type of operator== must be bool");
   }
 
@@ -154,9 +160,10 @@ TEST(NonsecureURBGBase, CompatibleWithDistributionUtils) {
 TEST(NonsecureURBGBase, CompatibleWithStdDistributions) {
   ExampleNonsecureURBG rbg;
 
-  std::uniform_int_distribution<uint32_t>(0, 100)(rbg);
-  std::uniform_real_distribution<float>()(rbg);
-  std::bernoulli_distribution(0.2)(rbg);
+  // Cast to void to suppress [[nodiscard]] warnings
+  static_cast<void>(std::uniform_int_distribution<uint32_t>(0, 100)(rbg));
+  static_cast<void>(std::uniform_real_distribution<float>()(rbg));
+  static_cast<void>(std::bernoulli_distribution(0.2)(rbg));
 }
 
 TEST(NonsecureURBGBase, ConsecutiveDefaultInstancesYieldUniqueVariates) {
@@ -191,54 +198,70 @@ TEST(NonsecureURBGBase, EqualSeedSequencesYieldEqualVariates) {
   }
 }
 
-// This is a PRNG-compatible type specifically designed to test
-// that NonsecureURBGBase::Seeder can correctly handle iterators
-// to arbitrary non-uint32_t size types.
-template <typename T>
-struct SeederTestEngine {
-  using result_type = T;
+TEST(NonsecureURBGBase, DistinctSequencesPerThread) {
+  constexpr int kNumThreads = 12;
+  constexpr size_t kValuesPerThread = 32;
+  using result_type = absl::BitGen::result_type;
 
-  static constexpr result_type(min)() {
-    return (std::numeric_limits<result_type>::min)();
-  }
-  static constexpr result_type(max)() {
-    return (std::numeric_limits<result_type>::max)();
+  // Acquire initial sequences from multiple threads.
+  std::vector<std::vector<result_type>> data;
+  {
+    absl::Mutex mu;
+    std::vector<std::thread> threads;
+    for (int i = 0; i < kNumThreads; i++) {
+      threads.emplace_back([&]() {
+        absl::BitGen gen;
+
+        std::vector<result_type> v(kValuesPerThread);
+        absl::c_generate(v, [&]() { return gen(); });
+        absl::MutexLock l(mu);
+        data.push_back(std::move(v));
+      });
+    }
+    for (auto& t : threads) t.join();
   }
 
-  template <class SeedSequence,
-            typename = typename absl::enable_if_t<
-                !std::is_same<SeedSequence, SeederTestEngine>::value>>
-  explicit SeederTestEngine(SeedSequence&& seq) {
-    seed(seq);
+  EXPECT_EQ(data.size(), kNumThreads);
+
+  // There should be essentially no duplicates in the sequences.
+  size_t expected_size = 0;
+  absl::flat_hash_set<result_type> seen;
+  for (const auto& v : data) {
+    expected_size += v.size();
+    for (result_type x : v) seen.insert(x);
   }
+  EXPECT_GE(seen.size(), expected_size - 1);
+}
 
-  SeederTestEngine(const SeederTestEngine&) = default;
-  SeederTestEngine& operator=(const SeederTestEngine&) = default;
-  SeederTestEngine(SeederTestEngine&&) = default;
-  SeederTestEngine& operator=(SeederTestEngine&&) = default;
+TEST(RandenPoolSeedSeqTest, SeederWorksForU32) {
+  absl::random_internal::RandenPoolSeedSeq seeder;
 
-  result_type operator()() { return state[0]; }
+  uint32_t state[2] = {0, 0};
+  seeder.generate(std::begin(state), std::end(state));
+  EXPECT_FALSE(state[0] == 0 && state[1] == 0);
+}
 
-  template <class SeedSequence>
-  void seed(SeedSequence&& seq) {
-    std::fill(std::begin(state), std::end(state), T(0));
-    seq.generate(std::begin(state), std::end(state));
-  }
+TEST(RandenPoolSeedSeqTest, SeederWorksForU64) {
+  absl::random_internal::RandenPoolSeedSeq seeder;
+
+  uint64_t state[2] = {0, 0};
+  seeder.generate(std::begin(state), std::end(state));
+  EXPECT_FALSE(state[0] == 0 && state[1] == 0);
+  EXPECT_FALSE((state[0] >> 32) == 0 && (state[1] >> 32) == 0);
+}
 
-  T state[2];
-};
+TEST(RandenPoolSeedSeqTest, SeederWorksForS32) {
+  absl::random_internal::RandenPoolSeedSeq seeder;
 
-TEST(NonsecureURBGBase, SeederWorksForU32) {
-  using U32 =
-      absl::random_internal::NonsecureURBGBase<SeederTestEngine<uint32_t>>;
-  U32 x;
-  EXPECT_NE(0, x());
+  int32_t state[2] = {0, 0};
+  seeder.generate(std::begin(state), std::end(state));
+  EXPECT_FALSE(state[0] == 0 && state[1] == 0);
 }
 
-TEST(NonsecureURBGBase, SeederWorksForU64) {
-  using U64 =
-      absl::random_internal::NonsecureURBGBase<SeederTestEngine<uint64_t>>;
+TEST(RandenPoolSeedSeqTest, SeederWorksForVector) {
+  absl::random_internal::RandenPoolSeedSeq seeder;
 
-  U64 x;
-  EXPECT_NE(0, x());
+  std::vector<uint32_t> state(2);
+  seeder.generate(std::begin(state), std::end(state));
+  EXPECT_FALSE(state[0] == 0 && state[1] == 0);
 }
diff --git a/absl/random/internal/pcg_engine.h b/absl/random/internal/pcg_engine.h
index b5df4eafffa..f85a1acdd22 100644
--- a/absl/random/internal/pcg_engine.h
+++ b/absl/random/internal/pcg_engine.h
@@ -15,15 +15,23 @@
 #ifndef ABSL_RANDOM_INTERNAL_PCG_ENGINE_H_
 #define ABSL_RANDOM_INTERNAL_PCG_ENGINE_H_
 
+#include <cstddef>
+#include <cstdint>
+#include <ios>
+#include <istream>
+#include <limits>
+#include <ostream>
 #include <type_traits>
 
 #include "absl/base/config.h"
 #include "absl/meta/type_traits.h"
+#include "absl/numeric/bits.h"
 #include "absl/numeric/int128.h"
 #include "absl/random/internal/fastmath.h"
 #include "absl/random/internal/iostream_state_saver.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
 // pcg_engine is a simplified implementation of Melissa O'Neil's PCG engine in
@@ -37,12 +45,12 @@ namespace random_internal {
 //
 template <typename Params, typename Mix>
 class pcg_engine {
-  static_assert(std::is_same<typename Params::state_type,
-                             typename Mix::state_type>::value,
-                "Class-template absl::pcg_engine must be parameterized by "
-                "Params and Mix with identical state_type");
+  static_assert(
+      std::is_same_v<typename Params::state_type, typename Mix::state_type>,
+      "Class-template absl::pcg_engine must be parameterized by "
+      "Params and Mix with identical state_type");
 
-  static_assert(std::is_unsigned<typename Mix::result_type>::value,
+  static_assert(std::is_unsigned_v<typename Mix::result_type>,
                 "Class-template absl::pcg_engine must be parameterized by "
                 "an unsigned Mix::result_type");
 
@@ -64,9 +72,8 @@ class pcg_engine {
 
   explicit pcg_engine(uint64_t seed_value = 0) { seed(seed_value); }
 
-  template <class SeedSequence,
-            typename = typename absl::enable_if_t<
-                !std::is_same<SeedSequence, pcg_engine>::value>>
+  template <class SeedSequence, typename = typename std::enable_if_t<
+                                    !std::is_same_v<SeedSequence, pcg_engine>>>
   explicit pcg_engine(SeedSequence&& seq) {
     seed(seq);
   }
@@ -88,8 +95,8 @@ class pcg_engine {
   }
 
   template <class SeedSequence>
-  typename absl::enable_if_t<
-      !std::is_convertible<SeedSequence, uint64_t>::value, void>
+  typename std::enable_if_t<!std::is_convertible_v<SeedSequence, uint64_t>,
+                            void>
   seed(SeedSequence&& seq) {
     reseed(seq);
   }
@@ -103,7 +110,7 @@ class pcg_engine {
   bool operator!=(const pcg_engine& other) const { return !(*this == other); }
 
   template <class CharT, class Traits>
-  friend typename absl::enable_if_t<(sizeof(state_type) == 16),
+  friend typename std::enable_if_t<(sizeof(state_type) == 16),
                                     std::basic_ostream<CharT, Traits>&>
   operator<<(
       std::basic_ostream<CharT, Traits>& os,  // NOLINT(runtime/references)
@@ -119,7 +126,7 @@ class pcg_engine {
   }
 
   template <class CharT, class Traits>
-  friend typename absl::enable_if_t<(sizeof(state_type) <= 8),
+  friend typename std::enable_if_t<(sizeof(state_type) <= 8),
                                     std::basic_ostream<CharT, Traits>&>
   operator<<(
       std::basic_ostream<CharT, Traits>& os,  // NOLINT(runtime/references)
@@ -132,7 +139,7 @@ class pcg_engine {
   }
 
   template <class CharT, class Traits>
-  friend typename absl::enable_if_t<(sizeof(state_type) == 16),
+  friend typename std::enable_if_t<(sizeof(state_type) == 16),
                                     std::basic_istream<CharT, Traits>&>
   operator>>(
       std::basic_istream<CharT, Traits>& is,  // NOLINT(runtime/references)
@@ -153,7 +160,7 @@ class pcg_engine {
   }
 
   template <class CharT, class Traits>
-  friend typename absl::enable_if_t<(sizeof(state_type) <= 8),
+  friend typename std::enable_if_t<(sizeof(state_type) <= 8),
                                     std::basic_istream<CharT, Traits>&>
   operator>>(
       std::basic_istream<CharT, Traits>& is,  // NOLINT(runtime/references)
@@ -175,12 +182,12 @@ class pcg_engine {
   state_type state_;
 
   // Returns the linear-congruential generator next state.
-  static inline constexpr state_type lcg(state_type s) {
+  static constexpr state_type lcg(state_type s) {
     return s * Params::multiplier() + Params::increment();
   }
 
   // Returns the linear-congruential arbitrary seek state.
-  inline state_type advance(state_type s, uint64_t n) const {
+  state_type advance(state_type s, uint64_t n) const {
     state_type mult = Params::multiplier();
     state_type inc = Params::increment();
     state_type m = 1;
@@ -219,48 +226,27 @@ class pcg_engine {
 template <uint64_t kMultA, uint64_t kMultB, uint64_t kIncA, uint64_t kIncB>
 class pcg128_params {
  public:
-#if ABSL_HAVE_INTRINSIC_INT128
-  using state_type = __uint128_t;
-  static inline constexpr state_type make_u128(uint64_t a, uint64_t b) {
-    return (static_cast<__uint128_t>(a) << 64) | b;
-  }
-#else
   using state_type = absl::uint128;
-  static inline constexpr state_type make_u128(uint64_t a, uint64_t b) {
-    return absl::MakeUint128(a, b);
-  }
-#endif
-
-  static inline constexpr state_type multiplier() {
-    return make_u128(kMultA, kMultB);
+  static constexpr state_type multiplier() {
+    return absl::MakeUint128(kMultA, kMultB);
   }
-  static inline constexpr state_type increment() {
-    return make_u128(kIncA, kIncB);
+  static constexpr state_type increment() {
+    return absl::MakeUint128(kIncA, kIncB);
   }
 };
 
 // Implementation of the PCG xsl_rr_128_64 128-bit mixing function, which
 // accepts an input of state_type and mixes it into an output of result_type.
 struct pcg_xsl_rr_128_64 {
-#if ABSL_HAVE_INTRINSIC_INT128
-  using state_type = __uint128_t;
-#else
   using state_type = absl::uint128;
-#endif
   using result_type = uint64_t;
 
-  inline uint64_t operator()(state_type state) {
+  uint64_t operator()(state_type state) {
     // This is equivalent to the xsl_rr_128_64 mixing function.
-#if ABSL_HAVE_INTRINSIC_INT128
     uint64_t rotate = static_cast<uint64_t>(state >> 122u);
     state ^= state >> 64;
     uint64_t s = static_cast<uint64_t>(state);
-#else
-    uint64_t h = Uint128High64(state);
-    uint64_t rotate = h >> 58u;
-    uint64_t s = Uint128Low64(state) ^ h;
-#endif
-    return random_internal::rotr(s, rotate);
+    return rotr(s, static_cast<int>(rotate));
   }
 };
 
@@ -270,8 +256,8 @@ template <uint64_t kMult, uint64_t kInc>
 class pcg64_params {
  public:
   using state_type = uint64_t;
-  static inline constexpr state_type multiplier() { return kMult; }
-  static inline constexpr state_type increment() { return kInc; }
+  static constexpr state_type multiplier() { return kMult; }
+  static constexpr state_type increment() { return kInc; }
 };
 
 // Implementation of the PCG xsh_rr_64_32 64-bit mixing function, which accepts
@@ -279,9 +265,9 @@ class pcg64_params {
 struct pcg_xsh_rr_64_32 {
   using state_type = uint64_t;
   using result_type = uint32_t;
-  inline uint32_t operator()(uint64_t state) {
-    return random_internal::rotr(
-        static_cast<uint32_t>(((state >> 18) ^ state) >> 27), state >> 59);
+  uint32_t operator()(uint64_t state) {
+    return rotr(static_cast<uint32_t>(((state >> 18) ^ state) >> 27),
+                state >> 59);
   }
 };
 
@@ -300,6 +286,7 @@ using pcg32_2018_engine = pcg_engine<
     random_internal::pcg_xsh_rr_64_32>;
 
 }  // namespace random_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_INTERNAL_PCG_ENGINE_H_
diff --git a/absl/random/internal/pcg_engine_test.cc b/absl/random/internal/pcg_engine_test.cc
index 4d763e89eb5..9b34cfe7eb3 100644
--- a/absl/random/internal/pcg_engine_test.cc
+++ b/absl/random/internal/pcg_engine_test.cc
@@ -16,13 +16,15 @@
 
 #include <algorithm>
 #include <bitset>
+#include <cmath>
+#include <cstddef>
+#include <cstdint>
 #include <random>
 #include <sstream>
+#include <type_traits>
 
-#include "gmock/gmock.h"
 #include "gtest/gtest.h"
 #include "absl/random/internal/explicit_seed_seq.h"
-#include "absl/time/clock.h"
 
 #define UPDATE_GOLDEN 0
 
@@ -166,19 +168,19 @@ TYPED_TEST(PCGEngineTest, RandomNumberEngineInterface) {
   using E = engine_type;
   using T = typename E::result_type;
 
-  static_assert(std::is_copy_constructible<E>::value,
+  static_assert(std::is_copy_constructible_v<E>,
                 "engine_type must be copy constructible");
 
-  static_assert(absl::is_copy_assignable<E>::value,
+  static_assert(std::is_copy_assignable_v<E>,
                 "engine_type must be copy assignable");
 
-  static_assert(std::is_move_constructible<E>::value,
+  static_assert(std::is_move_constructible_v<E>,
                 "engine_type must be move constructible");
 
-  static_assert(absl::is_move_assignable<E>::value,
+  static_assert(std::is_move_assignable_v<E>,
                 "engine_type must be move assignable");
 
-  static_assert(std::is_same<decltype(std::declval<E>()()), T>::value,
+  static_assert(std::is_same_v<decltype(std::declval<E>()()), T>,
                 "return type of operator() must be result_type");
 
   // Names after definition of [rand.req.urbg] in C++ standard.
@@ -224,10 +226,10 @@ TYPED_TEST(PCGEngineTest, RandomNumberEngineInterface) {
 
   e.discard(z);
 
-  static_assert(std::is_same<decltype(x == y), bool>::value,
+  static_assert(std::is_same_v<decltype(x == y), bool>,
                 "return type of operator== must be bool");
 
-  static_assert(std::is_same<decltype(x != y), bool>::value,
+  static_assert(std::is_same_v<decltype(x != y), bool>,
                 "return type of operator== must be bool");
 }
 
diff --git a/absl/random/internal/platform.h b/absl/random/internal/platform.h
index a5a42cbb80e..a6fde4b2775 100644
--- a/absl/random/internal/platform.h
+++ b/absl/random/internal/platform.h
@@ -12,6 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+// SKIP_ABSL_INLINE_NAMESPACE_CHECK
+
 #ifndef ABSL_RANDOM_INTERNAL_PLATFORM_H_
 #define ABSL_RANDOM_INTERNAL_PLATFORM_H_
 
@@ -33,7 +35,6 @@
 //   Darwin (macOS and iOS)            __APPLE__
 //   Akaros (http://akaros.org)        __ros__
 //   Windows                           _WIN32
-//   NaCL                              __native_client__
 //   AsmJS                             __asmjs__
 //   WebAssembly                       __wasm__
 //   Fuchsia                           __Fuchsia__
@@ -123,18 +124,19 @@
 
 #endif
 
-// NaCl does not allow AES.
-#if defined(__native_client__)
-#undef ABSL_HAVE_ACCELERATED_AES
-#define ABSL_HAVE_ACCELERATED_AES 0
-#endif
-
 // ABSL_RANDOM_INTERNAL_AES_DISPATCH indicates whether the currently active
 // platform has, or should use run-time dispatch for selecting the
-// acclerated Randen implementation.
+// accelerated Randen implementation.
 #define ABSL_RANDOM_INTERNAL_AES_DISPATCH 0
 
-#if defined(ABSL_ARCH_X86_64)
+// iOS does not support dispatch, even on x86, since applications
+// should be bundled as fat binaries, with a different build tailored for
+// each specific supported platform/architecture.
+#if (defined(TARGET_OS_IPHONE) && TARGET_OS_IPHONE) || \
+    (defined(TARGET_OS_IPHONE_SIMULATOR) && TARGET_OS_IPHONE_SIMULATOR)
+#undef ABSL_RANDOM_INTERNAL_AES_DISPATCH
+#define ABSL_RANDOM_INTERNAL_AES_DISPATCH 0
+#elif defined(ABSL_ARCH_X86_64)
 // Dispatch is available on x86_64
 #undef ABSL_RANDOM_INTERNAL_AES_DISPATCH
 #define ABSL_RANDOM_INTERNAL_AES_DISPATCH 1
@@ -142,8 +144,8 @@
 // Or when running linux PPC
 #undef ABSL_RANDOM_INTERNAL_AES_DISPATCH
 #define ABSL_RANDOM_INTERNAL_AES_DISPATCH 1
-#elif defined(__linux__) && defined(ABSL_ARCH_AARCH64)
-// Or when running linux AArch64
+#elif (defined(__linux__) || defined(__APPLE__)) && defined(ABSL_ARCH_AARCH64)
+// Or when running linux or macOS AArch64
 #undef ABSL_RANDOM_INTERNAL_AES_DISPATCH
 #define ABSL_RANDOM_INTERNAL_AES_DISPATCH 1
 #elif defined(__linux__) && defined(ABSL_ARCH_ARM) && (__ARM_ARCH >= 8)
@@ -153,18 +155,4 @@
 #define ABSL_RANDOM_INTERNAL_AES_DISPATCH 1
 #endif
 
-// NaCl does not allow dispatch.
-#if defined(__native_client__)
-#undef ABSL_RANDOM_INTERNAL_AES_DISPATCH
-#define ABSL_RANDOM_INTERNAL_AES_DISPATCH 0
-#endif
-
-// iOS does not support dispatch, even on x86, since applications
-// should be bundled as fat binaries, with a different build tailored for
-// each specific supported platform/architecture.
-#if defined(__APPLE__) && (TARGET_OS_IPHONE || TARGET_OS_IPHONE_SIMULATOR)
-#undef ABSL_RANDOM_INTERNAL_AES_DISPATCH
-#define ABSL_RANDOM_INTERNAL_AES_DISPATCH 0
-#endif
-
 #endif  // ABSL_RANDOM_INTERNAL_PLATFORM_H_
diff --git a/absl/random/internal/pool_urbg.h b/absl/random/internal/pool_urbg.h
deleted file mode 100644
index 9b2dd4bfcee..00000000000
--- a/absl/random/internal/pool_urbg.h
+++ /dev/null
@@ -1,129 +0,0 @@
-// Copyright 2017 The Abseil Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-#ifndef ABSL_RANDOM_INTERNAL_POOL_URBG_H_
-#define ABSL_RANDOM_INTERNAL_POOL_URBG_H_
-
-#include <cinttypes>
-#include <limits>
-
-#include "absl/random/internal/traits.h"
-#include "absl/types/span.h"
-
-namespace absl {
-namespace random_internal {
-
-// RandenPool is a thread-safe random number generator [random.req.urbg] that
-// uses an underlying pool of Randen generators to generate values.  Each thread
-// has affinity to one instance of the underlying pool generators.  Concurrent
-// access is guarded by a spin-lock.
-template <typename T>
-class RandenPool {
- public:
-  using result_type = T;
-  static_assert(std::is_unsigned<result_type>::value,
-                "RandenPool template argument must be a built-in unsigned "
-                "integer type");
-
-  static constexpr result_type(min)() {
-    return (std::numeric_limits<result_type>::min)();
-  }
-
-  static constexpr result_type(max)() {
-    return (std::numeric_limits<result_type>::max)();
-  }
-
-  RandenPool() {}
-
-  // Returns a single value.
-  inline result_type operator()() { return Generate(); }
-
-  // Fill data with random values.
-  static void Fill(absl::Span<result_type> data);
-
- protected:
-  // Generate returns a single value.
-  static result_type Generate();
-};
-
-extern template class RandenPool<uint8_t>;
-extern template class RandenPool<uint16_t>;
-extern template class RandenPool<uint32_t>;
-extern template class RandenPool<uint64_t>;
-
-// PoolURBG uses an underlying pool of random generators to implement a
-// thread-compatible [random.req.urbg] interface with an internal cache of
-// values.
-template <typename T, size_t kBufferSize>
-class PoolURBG {
-  // Inheritance to access the protected static members of RandenPool.
-  using unsigned_type = typename make_unsigned_bits<T>::type;
-  using PoolType = RandenPool<unsigned_type>;
-  using SpanType = absl::Span<unsigned_type>;
-
-  static constexpr size_t kInitialBuffer = kBufferSize + 1;
-  static constexpr size_t kHalfBuffer = kBufferSize / 2;
-
- public:
-  using result_type = T;
-
-  static_assert(std::is_unsigned<result_type>::value,
-                "PoolURBG must be parameterized by an unsigned integer type");
-
-  static_assert(kBufferSize > 1,
-                "PoolURBG must be parameterized by a buffer-size > 1");
-
-  static_assert(kBufferSize <= 256,
-                "PoolURBG must be parameterized by a buffer-size <= 256");
-
-  static constexpr result_type(min)() {
-    return (std::numeric_limits<result_type>::min)();
-  }
-
-  static constexpr result_type(max)() {
-    return (std::numeric_limits<result_type>::max)();
-  }
-
-  PoolURBG() : next_(kInitialBuffer) {}
-
-  // copy-constructor does not copy cache.
-  PoolURBG(const PoolURBG&) : next_(kInitialBuffer) {}
-  const PoolURBG& operator=(const PoolURBG&) {
-    next_ = kInitialBuffer;
-    return *this;
-  }
-
-  // move-constructor does move cache.
-  PoolURBG(PoolURBG&&) = default;
-  PoolURBG& operator=(PoolURBG&&) = default;
-
-  inline result_type operator()() {
-    if (next_ >= kBufferSize) {
-      next_ = (kBufferSize > 2 && next_ > kBufferSize) ? kHalfBuffer : 0;
-      PoolType::Fill(SpanType(reinterpret_cast<unsigned_type*>(state_ + next_),
-                              kBufferSize - next_));
-    }
-    return state_[next_++];
-  }
-
- private:
-  // Buffer size.
-  size_t next_;  // index within state_
-  result_type state_[kBufferSize];
-};
-
-}  // namespace random_internal
-}  // namespace absl
-
-#endif  // ABSL_RANDOM_INTERNAL_POOL_URBG_H_
diff --git a/absl/random/internal/pool_urbg_test.cc b/absl/random/internal/pool_urbg_test.cc
deleted file mode 100644
index 53f4eacf16f..00000000000
--- a/absl/random/internal/pool_urbg_test.cc
+++ /dev/null
@@ -1,182 +0,0 @@
-// Copyright 2017 The Abseil Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-#include "absl/random/internal/pool_urbg.h"
-
-#include <algorithm>
-#include <bitset>
-#include <cmath>
-#include <cstdint>
-#include <iterator>
-
-#include "gtest/gtest.h"
-#include "absl/meta/type_traits.h"
-#include "absl/types/span.h"
-
-using absl::random_internal::PoolURBG;
-using absl::random_internal::RandenPool;
-
-namespace {
-
-// is_randen_pool trait is true when parameterized by an RandenPool
-template <typename T>
-using is_randen_pool = typename absl::disjunction<  //
-    std::is_same<T, RandenPool<uint8_t>>,           //
-    std::is_same<T, RandenPool<uint16_t>>,          //
-    std::is_same<T, RandenPool<uint32_t>>,          //
-    std::is_same<T, RandenPool<uint64_t>>>;         //
-
-// MyFill either calls RandenPool::Fill() or std::generate(..., rng)
-template <typename T, typename V>
-typename absl::enable_if_t<absl::negation<is_randen_pool<T>>::value, void>  //
-MyFill(T& rng, absl::Span<V> data) {  // NOLINT(runtime/references)
-  std::generate(std::begin(data), std::end(data), rng);
-}
-
-template <typename T, typename V>
-typename absl::enable_if_t<is_randen_pool<T>::value, void>  //
-MyFill(T& rng, absl::Span<V> data) {  // NOLINT(runtime/references)
-  rng.Fill(data);
-}
-
-template <typename EngineType>
-class PoolURBGTypedTest : public ::testing::Test {};
-
-using EngineTypes = ::testing::Types<  //
-    RandenPool<uint8_t>,               //
-    RandenPool<uint16_t>,              //
-    RandenPool<uint32_t>,              //
-    RandenPool<uint64_t>,              //
-    PoolURBG<uint8_t, 2>,              //
-    PoolURBG<uint16_t, 2>,             //
-    PoolURBG<uint32_t, 2>,             //
-    PoolURBG<uint64_t, 2>,             //
-    PoolURBG<unsigned int, 8>,         // NOLINT(runtime/int)
-    PoolURBG<unsigned long, 8>,        // NOLINT(runtime/int)
-    PoolURBG<unsigned long int, 4>,    // NOLINT(runtime/int)
-    PoolURBG<unsigned long long, 4>>;  // NOLINT(runtime/int)
-
-TYPED_TEST_SUITE(PoolURBGTypedTest, EngineTypes);
-
-// This test is checks that the engines meet the URBG interface requirements
-// defined in [rand.req.urbg].
-TYPED_TEST(PoolURBGTypedTest, URBGInterface) {
-  using E = TypeParam;
-  using T = typename E::result_type;
-
-  static_assert(std::is_copy_constructible<E>::value,
-                "engine must be copy constructible");
-
-  static_assert(absl::is_copy_assignable<E>::value,
-                "engine must be copy assignable");
-
-  E e;
-  const E x;
-
-  e();
-
-  static_assert(std::is_same<decltype(e()), T>::value,
-                "return type of operator() must be result_type");
-
-  E u0(x);
-  u0();
-
-  E u1 = e;
-  u1();
-}
-
-// This validates that sequences are independent.
-TYPED_TEST(PoolURBGTypedTest, VerifySequences) {
-  using E = TypeParam;
-  using result_type = typename E::result_type;
-
-  E rng;
-  (void)rng();  // Discard one value.
-
-  constexpr int kNumOutputs = 64;
-  result_type a[kNumOutputs];
-  result_type b[kNumOutputs];
-  std::fill(std::begin(b), std::end(b), 0);
-
-  // Fill a using Fill or generate, depending on the engine type.
-  {
-    E x = rng;
-    MyFill(x, absl::MakeSpan(a));
-  }
-
-  // Fill b using std::generate().
-  {
-    E x = rng;
-    std::generate(std::begin(b), std::end(b), x);
-  }
-
-  // Test that generated sequence changed as sequence of bits, i.e. if about
-  // half of the bites were flipped between two non-correlated values.
-  size_t changed_bits = 0;
-  size_t unchanged_bits = 0;
-  size_t total_set = 0;
-  size_t total_bits = 0;
-  size_t equal_count = 0;
-  for (size_t i = 0; i < kNumOutputs; ++i) {
-    equal_count += (a[i] == b[i]) ? 1 : 0;
-    std::bitset<sizeof(result_type) * 8> bitset(a[i] ^ b[i]);
-    changed_bits += bitset.count();
-    unchanged_bits += bitset.size() - bitset.count();
-
-    std::bitset<sizeof(result_type) * 8> a_set(a[i]);
-    std::bitset<sizeof(result_type) * 8> b_set(b[i]);
-    total_set += a_set.count() + b_set.count();
-    total_bits += 2 * 8 * sizeof(result_type);
-  }
-  // On average, half the bits are changed between two calls.
-  EXPECT_LE(changed_bits, 0.60 * (changed_bits + unchanged_bits));
-  EXPECT_GE(changed_bits, 0.40 * (changed_bits + unchanged_bits));
-
-  // verify using a quick normal-approximation to the binomial.
-  EXPECT_NEAR(total_set, total_bits * 0.5, 4 * std::sqrt(total_bits))
-      << "@" << total_set / static_cast<double>(total_bits);
-
-  // Also, A[i] == B[i] with probability (1/range) * N.
-  // Give this a pretty wide latitude, though.
-  const double kExpected = kNumOutputs / (1.0 * sizeof(result_type) * 8);
-  EXPECT_LE(equal_count, 1.0 + kExpected);
-}
-
-}  // namespace
-
-/*
-$ nanobenchmarks 1 RandenPool construct
-$ nanobenchmarks 1 PoolURBG construct
-
-RandenPool<uint32_t> | 1    | 1000 |    48482.00 ticks | 48.48 ticks | 13.9 ns
-RandenPool<uint32_t> | 10   | 2000 |  1028795.00 ticks | 51.44 ticks | 14.7 ns
-RandenPool<uint32_t> | 100  | 1000 |  5119968.00 ticks | 51.20 ticks | 14.6 ns
-RandenPool<uint32_t> | 1000 |  500 | 25867936.00 ticks | 51.74 ticks | 14.8 ns
-
-RandenPool<uint64_t> | 1    | 1000 |    49921.00 ticks | 49.92 ticks | 14.3 ns
-RandenPool<uint64_t> | 10   | 2000 |  1208269.00 ticks | 60.41 ticks | 17.3 ns
-RandenPool<uint64_t> | 100  | 1000 |  5844955.00 ticks | 58.45 ticks | 16.7 ns
-RandenPool<uint64_t> | 1000 |  500 | 28767404.00 ticks | 57.53 ticks | 16.4 ns
-
-PoolURBG<uint32_t,8> | 1    | 1000 |    86431.00 ticks | 86.43 ticks | 24.7 ns
-PoolURBG<uint32_t,8> | 10   | 1000 |   206191.00 ticks | 20.62 ticks |  5.9 ns
-PoolURBG<uint32_t,8> | 100  | 1000 |  1516049.00 ticks | 15.16 ticks |  4.3 ns
-PoolURBG<uint32_t,8> | 1000 |  500 |  7613936.00 ticks | 15.23 ticks |  4.4 ns
-
-PoolURBG<uint64_t,4> | 1    | 1000 |    96668.00 ticks | 96.67 ticks | 27.6 ns
-PoolURBG<uint64_t,4> | 10   | 1000 |   282423.00 ticks | 28.24 ticks |  8.1 ns
-PoolURBG<uint64_t,4> | 100  | 1000 |  2609587.00 ticks | 26.10 ticks |  7.5 ns
-PoolURBG<uint64_t,4> | 1000 |  500 | 12408757.00 ticks | 24.82 ticks |  7.1 ns
-
-*/
diff --git a/absl/random/internal/randen-keys.inc b/absl/random/internal/randen-keys.inc
deleted file mode 100644
index fa4b1668353..00000000000
--- a/absl/random/internal/randen-keys.inc
+++ /dev/null
@@ -1,207 +0,0 @@
-// Copyright 2017 The Abseil Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-#ifndef ABSL_RANDOM_INTERNAL_RANDEN_KEYS_INC_
-#define ABSL_RANDOM_INTERNAL_RANDEN_KEYS_INC_
-
-// Textual header to include the randen_keys where necessary.
-// REQUIRES: struct u64x2{}
-//
-// PROVIDES: kKeys
-// PROVIDES: round_keys[]
-
-// "Nothing up my sleeve" numbers from the first hex digits of Pi, obtained
-// from http://hexpi.sourceforge.net/. The array was generated by following
-// Python script:
-/*
-python << EOF
-"""Generates Randen round keys array from pi-hex.62500.txt file."""
-import binascii
-
-KEYS = 136
-
-def chunks(l, n):
-    """Yield successive n-sized chunks from l."""
-    for i in range(0, len(l), n):
-        yield l[i:i + n]
-
-def pairwise(t):
-    """Transforms sequence into sequence of pairs."""
-    it = iter(t)
-    return zip(it,it)
-
-def digits_from_pi():
-  """Reads digits from hexpi.sourceforge.net file."""
-  with open("pi-hex.62500.txt") as file:
-    return file.read()
-
-def digits_from_urandom():
-  """Reads digits from /dev/urandom."""
-  with open("/dev/urandom") as file:
-    return binascii.hexlify(file.read(KEYS * 16))
-
-digits = digits_from_pi()
-print("static constexpr const size_t kRoundKeys = {0};\n".format(KEYS))
-print("alignas(16) constexpr const u64x2 round_keys[kRoundKeys] = {")
-
-for i, (hi, lo) in zip(range(KEYS), pairwise(chunks(digits, 16))):
-  hi = "0x{0}ull".format(hi)
-  lo = "0x{0}ull".format(lo)
-  print("  u64x2({0}, {1}){2}".format(hi, lo, ',' if i+1 < KEYS else ''))
-
-print("};")
-EOF
-*/
-
-static constexpr const size_t kRoundKeys = 136;
-
-alignas(16) constexpr u64x2 round_keys[kRoundKeys] = {
-    u64x2(0x243F6A8885A308D3ull, 0x13198A2E03707344ull),
-    u64x2(0xA4093822299F31D0ull, 0x082EFA98EC4E6C89ull),
-    u64x2(0x452821E638D01377ull, 0xBE5466CF34E90C6Cull),
-    u64x2(0xC0AC29B7C97C50DDull, 0x3F84D5B5B5470917ull),
-    u64x2(0x9216D5D98979FB1Bull, 0xD1310BA698DFB5ACull),
-    u64x2(0x2FFD72DBD01ADFB7ull, 0xB8E1AFED6A267E96ull),
-    u64x2(0xBA7C9045F12C7F99ull, 0x24A19947B3916CF7ull),
-    u64x2(0x0801F2E2858EFC16ull, 0x636920D871574E69ull),
-    u64x2(0xA458FEA3F4933D7Eull, 0x0D95748F728EB658ull),
-    u64x2(0x718BCD5882154AEEull, 0x7B54A41DC25A59B5ull),
-    u64x2(0x9C30D5392AF26013ull, 0xC5D1B023286085F0ull),
-    u64x2(0xCA417918B8DB38EFull, 0x8E79DCB0603A180Eull),
-    u64x2(0x6C9E0E8BB01E8A3Eull, 0xD71577C1BD314B27ull),
-    u64x2(0x78AF2FDA55605C60ull, 0xE65525F3AA55AB94ull),
-    u64x2(0x5748986263E81440ull, 0x55CA396A2AAB10B6ull),
-    u64x2(0xB4CC5C341141E8CEull, 0xA15486AF7C72E993ull),
-    u64x2(0xB3EE1411636FBC2Aull, 0x2BA9C55D741831F6ull),
-    u64x2(0xCE5C3E169B87931Eull, 0xAFD6BA336C24CF5Cull),
-    u64x2(0x7A32538128958677ull, 0x3B8F48986B4BB9AFull),
-    u64x2(0xC4BFE81B66282193ull, 0x61D809CCFB21A991ull),
-    u64x2(0x487CAC605DEC8032ull, 0xEF845D5DE98575B1ull),
-    u64x2(0xDC262302EB651B88ull, 0x23893E81D396ACC5ull),
-    u64x2(0x0F6D6FF383F44239ull, 0x2E0B4482A4842004ull),
-    u64x2(0x69C8F04A9E1F9B5Eull, 0x21C66842F6E96C9Aull),
-    u64x2(0x670C9C61ABD388F0ull, 0x6A51A0D2D8542F68ull),
-    u64x2(0x960FA728AB5133A3ull, 0x6EEF0B6C137A3BE4ull),
-    u64x2(0xBA3BF0507EFB2A98ull, 0xA1F1651D39AF0176ull),
-    u64x2(0x66CA593E82430E88ull, 0x8CEE8619456F9FB4ull),
-    u64x2(0x7D84A5C33B8B5EBEull, 0xE06F75D885C12073ull),
-    u64x2(0x401A449F56C16AA6ull, 0x4ED3AA62363F7706ull),
-    u64x2(0x1BFEDF72429B023Dull, 0x37D0D724D00A1248ull),
-    u64x2(0xDB0FEAD349F1C09Bull, 0x075372C980991B7Bull),
-    u64x2(0x25D479D8F6E8DEF7ull, 0xE3FE501AB6794C3Bull),
-    u64x2(0x976CE0BD04C006BAull, 0xC1A94FB6409F60C4ull),
-    u64x2(0x5E5C9EC2196A2463ull, 0x68FB6FAF3E6C53B5ull),
-    u64x2(0x1339B2EB3B52EC6Full, 0x6DFC511F9B30952Cull),
-    u64x2(0xCC814544AF5EBD09ull, 0xBEE3D004DE334AFDull),
-    u64x2(0x660F2807192E4BB3ull, 0xC0CBA85745C8740Full),
-    u64x2(0xD20B5F39B9D3FBDBull, 0x5579C0BD1A60320Aull),
-    u64x2(0xD6A100C6402C7279ull, 0x679F25FEFB1FA3CCull),
-    u64x2(0x8EA5E9F8DB3222F8ull, 0x3C7516DFFD616B15ull),
-    u64x2(0x2F501EC8AD0552ABull, 0x323DB5FAFD238760ull),
-    u64x2(0x53317B483E00DF82ull, 0x9E5C57BBCA6F8CA0ull),
-    u64x2(0x1A87562EDF1769DBull, 0xD542A8F6287EFFC3ull),
-    u64x2(0xAC6732C68C4F5573ull, 0x695B27B0BBCA58C8ull),
-    u64x2(0xE1FFA35DB8F011A0ull, 0x10FA3D98FD2183B8ull),
-    u64x2(0x4AFCB56C2DD1D35Bull, 0x9A53E479B6F84565ull),
-    u64x2(0xD28E49BC4BFB9790ull, 0xE1DDF2DAA4CB7E33ull),
-    u64x2(0x62FB1341CEE4C6E8ull, 0xEF20CADA36774C01ull),
-    u64x2(0xD07E9EFE2BF11FB4ull, 0x95DBDA4DAE909198ull),
-    u64x2(0xEAAD8E716B93D5A0ull, 0xD08ED1D0AFC725E0ull),
-    u64x2(0x8E3C5B2F8E7594B7ull, 0x8FF6E2FBF2122B64ull),
-    u64x2(0x8888B812900DF01Cull, 0x4FAD5EA0688FC31Cull),
-    u64x2(0xD1CFF191B3A8C1ADull, 0x2F2F2218BE0E1777ull),
-    u64x2(0xEA752DFE8B021FA1ull, 0xE5A0CC0FB56F74E8ull),
-    u64x2(0x18ACF3D6CE89E299ull, 0xB4A84FE0FD13E0B7ull),
-    u64x2(0x7CC43B81D2ADA8D9ull, 0x165FA26680957705ull),
-    u64x2(0x93CC7314211A1477ull, 0xE6AD206577B5FA86ull),
-    u64x2(0xC75442F5FB9D35CFull, 0xEBCDAF0C7B3E89A0ull),
-    u64x2(0xD6411BD3AE1E7E49ull, 0x00250E2D2071B35Eull),
-    u64x2(0x226800BB57B8E0AFull, 0x2464369BF009B91Eull),
-    u64x2(0x5563911D59DFA6AAull, 0x78C14389D95A537Full),
-    u64x2(0x207D5BA202E5B9C5ull, 0x832603766295CFA9ull),
-    u64x2(0x11C819684E734A41ull, 0xB3472DCA7B14A94Aull),
-    u64x2(0x1B5100529A532915ull, 0xD60F573FBC9BC6E4ull),
-    u64x2(0x2B60A47681E67400ull, 0x08BA6FB5571BE91Full),
-    u64x2(0xF296EC6B2A0DD915ull, 0xB6636521E7B9F9B6ull),
-    u64x2(0xFF34052EC5855664ull, 0x53B02D5DA99F8FA1ull),
-    u64x2(0x08BA47996E85076Aull, 0x4B7A70E9B5B32944ull),
-    u64x2(0xDB75092EC4192623ull, 0xAD6EA6B049A7DF7Dull),
-    u64x2(0x9CEE60B88FEDB266ull, 0xECAA8C71699A18FFull),
-    u64x2(0x5664526CC2B19EE1ull, 0x193602A575094C29ull),
-    u64x2(0xA0591340E4183A3Eull, 0x3F54989A5B429D65ull),
-    u64x2(0x6B8FE4D699F73FD6ull, 0xA1D29C07EFE830F5ull),
-    u64x2(0x4D2D38E6F0255DC1ull, 0x4CDD20868470EB26ull),
-    u64x2(0x6382E9C6021ECC5Eull, 0x09686B3F3EBAEFC9ull),
-    u64x2(0x3C9718146B6A70A1ull, 0x687F358452A0E286ull),
-    u64x2(0xB79C5305AA500737ull, 0x3E07841C7FDEAE5Cull),
-    u64x2(0x8E7D44EC5716F2B8ull, 0xB03ADA37F0500C0Dull),
-    u64x2(0xF01C1F040200B3FFull, 0xAE0CF51A3CB574B2ull),
-    u64x2(0x25837A58DC0921BDull, 0xD19113F97CA92FF6ull),
-    u64x2(0x9432477322F54701ull, 0x3AE5E58137C2DADCull),
-    u64x2(0xC8B576349AF3DDA7ull, 0xA94461460FD0030Eull),
-    u64x2(0xECC8C73EA4751E41ull, 0xE238CD993BEA0E2Full),
-    u64x2(0x3280BBA1183EB331ull, 0x4E548B384F6DB908ull),
-    u64x2(0x6F420D03F60A04BFull, 0x2CB8129024977C79ull),
-    u64x2(0x5679B072BCAF89AFull, 0xDE9A771FD9930810ull),
-    u64x2(0xB38BAE12DCCF3F2Eull, 0x5512721F2E6B7124ull),
-    u64x2(0x501ADDE69F84CD87ull, 0x7A5847187408DA17ull),
-    u64x2(0xBC9F9ABCE94B7D8Cull, 0xEC7AEC3ADB851DFAull),
-    u64x2(0x63094366C464C3D2ull, 0xEF1C18473215D808ull),
-    u64x2(0xDD433B3724C2BA16ull, 0x12A14D432A65C451ull),
-    u64x2(0x50940002133AE4DDull, 0x71DFF89E10314E55ull),
-    u64x2(0x81AC77D65F11199Bull, 0x043556F1D7A3C76Bull),
-    u64x2(0x3C11183B5924A509ull, 0xF28FE6ED97F1FBFAull),
-    u64x2(0x9EBABF2C1E153C6Eull, 0x86E34570EAE96FB1ull),
-    u64x2(0x860E5E0A5A3E2AB3ull, 0x771FE71C4E3D06FAull),
-    u64x2(0x2965DCB999E71D0Full, 0x803E89D65266C825ull),
-    u64x2(0x2E4CC9789C10B36Aull, 0xC6150EBA94E2EA78ull),
-    u64x2(0xA6FC3C531E0A2DF4ull, 0xF2F74EA7361D2B3Dull),
-    u64x2(0x1939260F19C27960ull, 0x5223A708F71312B6ull),
-    u64x2(0xEBADFE6EEAC31F66ull, 0xE3BC4595A67BC883ull),
-    u64x2(0xB17F37D1018CFF28ull, 0xC332DDEFBE6C5AA5ull),
-    u64x2(0x6558218568AB9702ull, 0xEECEA50FDB2F953Bull),
-    u64x2(0x2AEF7DAD5B6E2F84ull, 0x1521B62829076170ull),
-    u64x2(0xECDD4775619F1510ull, 0x13CCA830EB61BD96ull),
-    u64x2(0x0334FE1EAA0363CFull, 0xB5735C904C70A239ull),
-    u64x2(0xD59E9E0BCBAADE14ull, 0xEECC86BC60622CA7ull),
-    u64x2(0x9CAB5CABB2F3846Eull, 0x648B1EAF19BDF0CAull),
-    u64x2(0xA02369B9655ABB50ull, 0x40685A323C2AB4B3ull),
-    u64x2(0x319EE9D5C021B8F7ull, 0x9B540B19875FA099ull),
-    u64x2(0x95F7997E623D7DA8ull, 0xF837889A97E32D77ull),
-    u64x2(0x11ED935F16681281ull, 0x0E358829C7E61FD6ull),
-    u64x2(0x96DEDFA17858BA99ull, 0x57F584A51B227263ull),
-    u64x2(0x9B83C3FF1AC24696ull, 0xCDB30AEB532E3054ull),
-    u64x2(0x8FD948E46DBC3128ull, 0x58EBF2EF34C6FFEAull),
-    u64x2(0xFE28ED61EE7C3C73ull, 0x5D4A14D9E864B7E3ull),
-    u64x2(0x42105D14203E13E0ull, 0x45EEE2B6A3AAABEAull),
-    u64x2(0xDB6C4F15FACB4FD0ull, 0xC742F442EF6ABBB5ull),
-    u64x2(0x654F3B1D41CD2105ull, 0xD81E799E86854DC7ull),
-    u64x2(0xE44B476A3D816250ull, 0xCF62A1F25B8D2646ull),
-    u64x2(0xFC8883A0C1C7B6A3ull, 0x7F1524C369CB7492ull),
-    u64x2(0x47848A0B5692B285ull, 0x095BBF00AD19489Dull),
-    u64x2(0x1462B17423820D00ull, 0x58428D2A0C55F5EAull),
-    u64x2(0x1DADF43E233F7061ull, 0x3372F0928D937E41ull),
-    u64x2(0xD65FECF16C223BDBull, 0x7CDE3759CBEE7460ull),
-    u64x2(0x4085F2A7CE77326Eull, 0xA607808419F8509Eull),
-    u64x2(0xE8EFD85561D99735ull, 0xA969A7AAC50C06C2ull),
-    u64x2(0x5A04ABFC800BCADCull, 0x9E447A2EC3453484ull),
-    u64x2(0xFDD567050E1E9EC9ull, 0xDB73DBD3105588CDull),
-    u64x2(0x675FDA79E3674340ull, 0xC5C43465713E38D8ull),
-    u64x2(0x3D28F89EF16DFF20ull, 0x153E21E78FB03D4Aull),
-    u64x2(0xE6E39F2BDB83ADF7ull, 0xE93D5A68948140F7ull),
-    u64x2(0xF64C261C94692934ull, 0x411520F77602D4F7ull),
-    u64x2(0xBCF46B2ED4A10068ull, 0xD40824713320F46Aull),
-    u64x2(0x43B7D4B7500061AFull, 0x1E39F62E97244546ull)};
-
-#endif  // ABSL_RANDOM_INTERNAL_RANDEN_KEYS_INC_
diff --git a/absl/random/internal/randen.cc b/absl/random/internal/randen.cc
index bab8075a54f..e42013c9e27 100644
--- a/absl/random/internal/randen.cc
+++ b/absl/random/internal/randen.cc
@@ -14,10 +14,13 @@
 
 #include "absl/random/internal/randen.h"
 
-#include "absl/base/internal/raw_logging.h"
+#include "absl/base/config.h"
+#include "absl/random/internal/platform.h"
 #include "absl/random/internal/randen_detect.h"
+#include "absl/random/internal/randen_hwaes.h"
+#include "absl/random/internal/randen_slow.h"
 
-// RANDen = RANDom generator or beetroots in Swiss German.
+// RANDen = RANDom generator or beetroots in Swiss High German.
 // 'Strong' (well-distributed, unpredictable, backtracking-resistant) random
 // generator, faster in some benchmarks than std::mt19937_64 and pcg64_c32.
 //
@@ -41,6 +44,7 @@
 // structured/low-entropy counters to digits of Pi.
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 namespace {
 
@@ -86,4 +90,5 @@ Randen::Randen() {
 }
 
 }  // namespace random_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/random/internal/randen.h b/absl/random/internal/randen.h
index a4ff2545968..619233337c2 100644
--- a/absl/random/internal/randen.h
+++ b/absl/random/internal/randen.h
@@ -23,9 +23,10 @@
 #include "absl/random/internal/randen_traits.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
-// RANDen = RANDom generator or beetroots in Swiss German.
+// RANDen = RANDom generator or beetroots in Swiss High German.
 // 'Strong' (well-distributed, unpredictable, backtracking-resistant) random
 // generator, faster in some benchmarks than std::mt19937_64 and pcg64_c32.
 //
@@ -42,10 +43,8 @@ class Randen {
 
   // Generate updates the randen sponge. The outer portion of the sponge
   // (kCapacityBytes .. kStateBytes) may be consumed as PRNG state.
-  template <typename T, size_t N>
-  void Generate(T (&state)[N]) const {
-    static_assert(N * sizeof(T) == kStateBytes,
-                  "Randen::Generate() requires kStateBytes of state");
+  // REQUIRES: state points to kStateBytes of state.
+  void Generate(void* state) const {
 #if ABSL_RANDOM_INTERNAL_AES_DISPATCH
     // HW AES Dispatch.
     if (has_crypto_) {
@@ -64,13 +63,9 @@ class Randen {
 
   // Absorb incorporates additional seed material into the randen sponge.  After
   // absorb returns, Generate must be called before the state may be consumed.
-  template <typename S, size_t M, typename T, size_t N>
-  void Absorb(const S (&seed)[M], T (&state)[N]) const {
-    static_assert(M * sizeof(S) == RandenTraits::kSeedBytes,
-                  "Randen::Absorb() requires kSeedBytes of seed");
-
-    static_assert(N * sizeof(T) == RandenTraits::kStateBytes,
-                  "Randen::Absorb() requires kStateBytes of state");
+  // REQUIRES: seed points to kSeedBytes of seed.
+  // REQUIRES: state points to kStateBytes of state.
+  void Absorb(const void* seed, void* state) const {
 #if ABSL_RANDOM_INTERNAL_AES_DISPATCH
     // HW AES Dispatch.
     if (has_crypto_) {
@@ -95,6 +90,7 @@ class Randen {
 };
 
 }  // namespace random_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_INTERNAL_RANDEN_H_
diff --git a/absl/random/internal/randen_benchmarks.cc b/absl/random/internal/randen_benchmarks.cc
index f589172c046..51e6a502de4 100644
--- a/absl/random/internal/randen_benchmarks.cc
+++ b/absl/random/internal/randen_benchmarks.cc
@@ -12,15 +12,14 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 //
-#include "absl/random/internal/randen.h"
-
 #include <cstdint>
 #include <cstdio>
 #include <cstring>
 
 #include "absl/base/internal/raw_logging.h"
 #include "absl/random/internal/nanobenchmark.h"
-#include "absl/random/internal/platform.h"
+#include "absl/random/internal/randen.h"
+#include "absl/random/internal/randen_detect.h"
 #include "absl/random/internal/randen_engine.h"
 #include "absl/random/internal/randen_hwaes.h"
 #include "absl/random/internal/randen_slow.h"
@@ -28,6 +27,7 @@
 
 namespace {
 
+using absl::random_internal::CPUSupportsRandenHwAes;
 using absl::random_internal::Randen;
 using absl::random_internal::RandenHwAes;
 using absl::random_internal::RandenSlow;
@@ -47,8 +47,10 @@ static constexpr size_t kSeedSizeT = Randen::kSeedBytes / sizeof(uint32_t);
 // Randen implementation benchmarks.
 template <typename T>
 struct AbsorbFn : public T {
-  mutable uint64_t state[kStateSizeT] = {};
-  mutable uint32_t seed[kSeedSizeT] = {};
+  // These are both cast to uint128* in the RandenHwAes implementation, so
+  // ensure they are 16 byte aligned.
+  alignas(16) mutable uint64_t state[kStateSizeT] = {};
+  alignas(16) mutable uint32_t seed[kSeedSizeT] = {};
 
   static constexpr size_t bytes() { return sizeof(seed); }
 
@@ -149,14 +151,14 @@ void RunAll(const int argc, char* argv[]) {
   const FuncInput unpredictable = (argc != 999);
   static const FuncInput inputs[] = {unpredictable * 100, unpredictable * 1000};
 
-#if !defined(ABSL_INTERNAL_DISABLE_AES) && ABSL_HAVE_ACCELERATED_AES
-  Measure<AbsorbFn<RandenHwAes>>("Absorb (HwAes)", inputs);
-#endif
+  if (CPUSupportsRandenHwAes()) {
+    Measure<AbsorbFn<RandenHwAes>>("Absorb (HwAes)", inputs);
+  }
   Measure<AbsorbFn<RandenSlow>>("Absorb (Slow)", inputs);
 
-#if !defined(ABSL_INTERNAL_DISABLE_AES) && ABSL_HAVE_ACCELERATED_AES
-  Measure<GenerateFn<RandenHwAes>>("Generate (HwAes)", inputs);
-#endif
+  if (CPUSupportsRandenHwAes()) {
+    Measure<GenerateFn<RandenHwAes>>("Generate (HwAes)", inputs);
+  }
   Measure<GenerateFn<RandenSlow>>("Generate (Slow)", inputs);
 
   // Measure the production engine.
diff --git a/absl/random/internal/randen_detect.cc b/absl/random/internal/randen_detect.cc
index d5946b219cc..83bde75f3c8 100644
--- a/absl/random/internal/randen_detect.cc
+++ b/absl/random/internal/randen_detect.cc
@@ -1,13 +1,13 @@
 // Copyright 2017 The Abseil Authors.
 //
-// Licensed under the Apache License, Version 2.0 (the"License");
+// Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
 // You may obtain a copy of the License at
 //
 //      https://www.apache.org/licenses/LICENSE-2.0
 //
 // Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an"AS IS" BASIS,
+// distributed under the License is distributed on an "AS IS" BASIS,
 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 // See the License for the specific language governing permissions and
 // limitations under the License.
@@ -19,11 +19,24 @@
 
 #include "absl/random/internal/randen_detect.h"
 
-#include <cstdint>
-#include <cstring>
+#include <optional>  // IWYU pragma: keep
 
+#include "absl/base/config.h"
 #include "absl/random/internal/platform.h"
 
+#if defined(__APPLE__) && defined(__aarch64__)
+#if __has_include(<arm/cpu_capabilities_public.h>)
+#include <arm/cpu_capabilities_public.h>
+#endif
+#include <sys/sysctl.h>
+#include <sys/types.h>
+#endif
+
+#if !defined(__UCLIBC__) && defined(__GLIBC__) && \
+    (__GLIBC__ > 2 || (__GLIBC__ == 2 && __GLIBC_MINOR__ >= 16))
+#define ABSL_HAVE_GETAUXVAL
+#endif
+
 #if defined(ABSL_ARCH_X86_64)
 #define ABSL_INTERNAL_USE_X86_CPUID
 #elif defined(ABSL_ARCH_PPC) || defined(ABSL_ARCH_ARM) || \
@@ -31,7 +44,7 @@
 #if defined(__ANDROID__)
 #define ABSL_INTERNAL_USE_ANDROID_GETAUXVAL
 #define ABSL_INTERNAL_USE_GETAUXVAL
-#elif defined(__linux__)
+#elif defined(__linux__) && defined(ABSL_HAVE_GETAUXVAL)
 #define ABSL_INTERNAL_USE_LINUX_GETAUXVAL
 #define ABSL_INTERNAL_USE_GETAUXVAL
 #endif
@@ -40,7 +53,10 @@
 #if defined(ABSL_INTERNAL_USE_X86_CPUID)
 #if defined(_WIN32) || defined(_WIN64)
 #include <intrin.h>  // NOLINT(build/include_order)
-#pragma intrinsic(__cpuid)
+#elif ABSL_HAVE_BUILTIN(__cpuid)
+// MSVC-equivalent __cpuid intrinsic declaration for clang-like compilers
+// for non-Windows build environments.
+extern void __cpuid(int[4], int);
 #else
 // MSVC-equivalent __cpuid intrinsic function.
 static void __cpuid(int cpu_info[4], int info_type) {
@@ -55,7 +71,7 @@ static void __cpuid(int cpu_info[4], int info_type) {
 // On linux, just use the c-library getauxval call.
 #if defined(ABSL_INTERNAL_USE_LINUX_GETAUXVAL)
 
-extern "C" unsigned long getauxval(unsigned long type);  // NOLINT(runtime/int)
+#include <sys/auxv.h>
 
 static uint32_t GetAuxval(uint32_t hwcap_type) {
   return static_cast<uint32_t>(getauxval(hwcap_type));
@@ -72,6 +88,8 @@ static uint32_t GetAuxval(uint32_t hwcap_type) {
 #if defined(ABSL_INTERNAL_USE_ANDROID_GETAUXVAL)
 #include <dlfcn.h>
 
+#include <cstring>
+
 static uint32_t GetAuxval(uint32_t hwcap_type) {
   // NOLINTNEXTLINE(runtime/int)
   typedef unsigned long (*getauxval_func_t)(unsigned long);
@@ -94,7 +112,21 @@ static uint32_t GetAuxval(uint32_t hwcap_type) {
 
 #endif
 
+#if defined(__APPLE__) && defined(ABSL_ARCH_AARCH64)
+template <typename T>
+static std::optional<T> ReadSysctlByName(const char* name) {
+  T val;
+  size_t val_size = sizeof(T);
+  int ret = sysctlbyname(name, &val, &val_size, nullptr, 0);
+  if (ret == -1) {
+    return std::nullopt;
+  }
+  return val;
+}
+#endif
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
 // The default return at the end of the function might be unreachable depending
@@ -120,7 +152,9 @@ namespace random_internal {
 //    cpu capabilities, and should allow us to enable crypto in the android
 //    builds where it is supported.
 //
-// 3. Use the default for the compiler architecture.
+// 3. When __APPLE__ is defined on AARCH64, use sysctlbyname().
+//
+// 4. Use the default for the compiler architecture.
 //
 
 bool CPUSupportsRandenHwAes() {
@@ -169,8 +203,36 @@ bool CPUSupportsRandenHwAes() {
   return ((hwcap & kNEON) != 0) && ((hwcap & kAES) != 0);
 #endif
 
+#elif defined(__APPLE__) && defined(ABSL_ARCH_AARCH64)
+  // 3. Use sysctlbyname.
+
+  // Newer XNU kernels support querying all capabilities in a single
+  // sysctlbyname.
+#if defined(CAP_BIT_AdvSIMD) && defined(CAP_BIT_FEAT_AES)
+  static const std::optional<uint64_t> caps =
+      ReadSysctlByName<uint64_t>("hw.optional.arm.caps");
+  if (caps.has_value()) {
+    constexpr uint64_t kNeonAndAesCaps =
+        (uint64_t{1} << CAP_BIT_AdvSIMD) | (uint64_t{1} << CAP_BIT_FEAT_AES);
+    return (*caps & kNeonAndAesCaps) == kNeonAndAesCaps;
+  }
+#endif
+
+  // https://developer.apple.com/documentation/kernel/1387446-sysctlbyname/determining_instruction_set_characteristics#overview
+  static const std::optional<int> adv_simd =
+      ReadSysctlByName<int>("hw.optional.AdvSIMD");
+  if (adv_simd.value_or(0) == 0) {
+    return false;
+  }
+  // https://developer.apple.com/documentation/kernel/1387446-sysctlbyname/determining_instruction_set_characteristics#3918855
+  static const std::optional<int> feat_aes =
+      ReadSysctlByName<int>("hw.optional.arm.FEAT_AES");
+  if (feat_aes.value_or(0) == 0) {
+    return false;
+  }
+  return true;
 #else  // ABSL_INTERNAL_USE_GETAUXVAL
-  // 3. By default, assume that the compiler default.
+  // 4. By default, assume that the compiler default.
   return ABSL_HAVE_ACCELERATED_AES ? true : false;
 
 #endif
@@ -206,9 +268,6 @@ bool CPUSupportsRandenHwAes() {
   //   __asm __volatile("mrs %0, id_aa64isar0_el1" :"=&r" (val));
   //
   // * Use a CPUID-style heuristic database.
-  //
-  // * On Apple (__APPLE__), AES is available on Arm v8.
-  //   https://stackoverflow.com/questions/45637888/how-to-determine-armv8-features-at-runtime-on-ios
 }
 
 #if defined(__clang__)
@@ -216,4 +275,5 @@ bool CPUSupportsRandenHwAes() {
 #endif
 
 }  // namespace random_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/random/internal/randen_detect.h b/absl/random/internal/randen_detect.h
index 44c5c667239..f283f432264 100644
--- a/absl/random/internal/randen_detect.h
+++ b/absl/random/internal/randen_detect.h
@@ -15,7 +15,10 @@
 #ifndef ABSL_RANDOM_INTERNAL_RANDEN_DETECT_H_
 #define ABSL_RANDOM_INTERNAL_RANDEN_DETECT_H_
 
+#include "absl/base/config.h"
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
 // Returns whether the current CPU supports RandenHwAes implementation.
@@ -24,6 +27,7 @@ namespace random_internal {
 bool CPUSupportsRandenHwAes();
 
 }  // namespace random_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_INTERNAL_RANDEN_DETECT_H_
diff --git a/absl/random/internal/randen_engine.h b/absl/random/internal/randen_engine.h
index 02212a137c3..a33736eca52 100644
--- a/absl/random/internal/randen_engine.h
+++ b/absl/random/internal/randen_engine.h
@@ -17,17 +17,22 @@
 
 #include <algorithm>
 #include <cinttypes>
+#include <cstdint>
 #include <cstdlib>
-#include <iostream>
+#include <cstring>
+#include <istream>
 #include <iterator>
 #include <limits>
+#include <ostream>
 #include <type_traits>
 
+#include "absl/base/internal/endian.h"
 #include "absl/meta/type_traits.h"
 #include "absl/random/internal/iostream_state_saver.h"
 #include "absl/random/internal/randen.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
 // Deterministic pseudorandom byte generator with backtracking resistance
@@ -40,11 +45,11 @@ namespace random_internal {
 // 'Strong' (well-distributed, unpredictable, backtracking-resistant) random
 // generator, faster in some benchmarks than std::mt19937_64 and pcg64_c32.
 template <typename T>
-class alignas(16) randen_engine {
+class alignas(8) randen_engine {
  public:
   // C++11 URBG interface:
   using result_type = T;
-  static_assert(std::is_unsigned<result_type>::value,
+  static_assert(std::is_unsigned_v<result_type>,
                 "randen_engine template argument must be a built-in unsigned "
                 "integer type");
 
@@ -56,31 +61,41 @@ class alignas(16) randen_engine {
     return (std::numeric_limits<result_type>::max)();
   }
 
-  explicit randen_engine(result_type seed_value = 0) { seed(seed_value); }
+  randen_engine() : randen_engine(0) {}
+  explicit randen_engine(result_type seed_value) { seed(seed_value); }
 
   template <class SeedSequence,
-            typename = typename absl::enable_if_t<
-                !std::is_same<SeedSequence, randen_engine>::value>>
+            typename = typename std::enable_if_t<
+                !std::is_same_v<SeedSequence, randen_engine>>>
   explicit randen_engine(SeedSequence&& seq) {
     seed(seq);
   }
 
-  randen_engine(const randen_engine&) = default;
+  // alignment requirements dictate custom copy and move constructors.
+  randen_engine(const randen_engine& other)
+      : next_(other.next_), impl_(other.impl_) {
+    std::memcpy(state(), other.state(), kStateSizeT * sizeof(result_type));
+  }
+  randen_engine& operator=(const randen_engine& other) {
+    next_ = other.next_;
+    impl_ = other.impl_;
+    std::memcpy(state(), other.state(), kStateSizeT * sizeof(result_type));
+    return *this;
+  }
 
   // Returns random bits from the buffer in units of result_type.
   result_type operator()() {
     // Refill the buffer if needed (unlikely).
+    auto* begin = state();
     if (next_ >= kStateSizeT) {
       next_ = kCapacityT;
-      impl_.Generate(state_);
+      impl_.Generate(begin);
     }
-
-    return state_[next_++];
+    return little_endian::ToHost(begin[next_++]);
   }
 
   template <class SeedSequence>
-  typename absl::enable_if_t<
-      !std::is_convertible<SeedSequence, result_type>::value>
+  typename std::enable_if_t<!std::is_convertible_v<SeedSequence, result_type>>
   seed(SeedSequence&& seq) {
     // Zeroes the state.
     seed();
@@ -90,9 +105,10 @@ class alignas(16) randen_engine {
   void seed(result_type seed_value = 0) {
     next_ = kStateSizeT;
     // Zeroes the inner state and fills the outer state with seed_value to
-    // mimics behaviour of reseed
-    std::fill(std::begin(state_), std::begin(state_) + kCapacityT, 0);
-    std::fill(std::begin(state_) + kCapacityT, std::end(state_), seed_value);
+    // mimic the behaviour of reseed
+    auto* begin = state();
+    std::fill(begin, begin + kCapacityT, 0);
+    std::fill(begin + kCapacityT, begin + kStateSizeT, seed_value);
   }
 
   // Inserts entropy into (part of) the state. Calling this periodically with
@@ -103,7 +119,6 @@ class alignas(16) randen_engine {
     using sequence_result_type = typename SeedSequence::result_type;
     static_assert(sizeof(sequence_result_type) == 4,
                   "SeedSequence::result_type must be 32-bit");
-
     constexpr size_t kBufferSize =
         Randen::kSeedBytes / sizeof(sequence_result_type);
     alignas(16) sequence_result_type buffer[kBufferSize];
@@ -117,12 +132,19 @@ class alignas(16) randen_engine {
     if (entropy_size < kBufferSize) {
       // ... and only request that many values, or 256-bits, when unspecified.
       const size_t requested_entropy = (entropy_size == 0) ? 8u : entropy_size;
-      std::fill(std::begin(buffer) + requested_entropy, std::end(buffer), 0);
-      seq.generate(std::begin(buffer), std::begin(buffer) + requested_entropy);
+      std::fill(buffer + requested_entropy, buffer + kBufferSize, 0);
+      seq.generate(buffer, buffer + requested_entropy);
+#ifdef ABSL_IS_BIG_ENDIAN
+      // Randen expects the seed buffer to be in Little Endian; reverse it on
+      // Big Endian platforms.
+      for (sequence_result_type& e : buffer) {
+        e = absl::little_endian::FromHost(e);
+      }
+#endif
       // The Randen paper suggests preferentially initializing even-numbered
       // 128-bit vectors of the randen state (there are 16 such vectors).
       // The seed data is merged into the state offset by 128-bits, which
-      // implies prefering seed bytes [16..31, ..., 208..223]. Since the
+      // implies preferring seed bytes [16..31, ..., 208..223]. Since the
       // buffer is 32-bit values, we swap the corresponding buffer positions in
       // 128-bit chunks.
       size_t dst = kBufferSize;
@@ -137,9 +159,9 @@ class alignas(16) randen_engine {
         std::swap(buffer[--dst], buffer[--src]);
       }
     } else {
-      seq.generate(std::begin(buffer), std::end(buffer));
+      seq.generate(buffer, buffer + kBufferSize);
     }
-    impl_.Absorb(buffer, state_);
+    impl_.Absorb(buffer, state());
 
     // Generate will be called when operator() is called
     next_ = kStateSizeT;
@@ -150,9 +172,10 @@ class alignas(16) randen_engine {
     count -= step;
 
     constexpr uint64_t kRateT = kStateSizeT - kCapacityT;
+    auto* begin = state();
     while (count > 0) {
       next_ = kCapacityT;
-      impl_.Generate(state_);
+      impl_.Generate(*reinterpret_cast<result_type(*)[kStateSizeT]>(begin));
       step = std::min<uint64_t>(kRateT, count);
       count -= step;
     }
@@ -160,9 +183,9 @@ class alignas(16) randen_engine {
   }
 
   bool operator==(const randen_engine& other) const {
+    const auto* begin = state();
     return next_ == other.next_ &&
-           std::equal(std::begin(state_), std::end(state_),
-                      std::begin(other.state_));
+           std::equal(begin, begin + kStateSizeT, other.state());
   }
 
   bool operator!=(const randen_engine& other) const {
@@ -176,11 +199,13 @@ class alignas(16) randen_engine {
     using numeric_type =
         typename random_internal::stream_format_type<result_type>::type;
     auto saver = random_internal::make_ostream_state_saver(os);
-    for (const auto& elem : engine.state_) {
+    auto* it = engine.state();
+    for (auto* end = it + kStateSizeT; it < end; ++it) {
       // In the case that `elem` is `uint8_t`, it must be cast to something
       // larger so that it prints as an integer rather than a character. For
       // simplicity, apply the cast all circumstances.
-      os << static_cast<numeric_type>(elem) << os.fill();
+      os << static_cast<numeric_type>(little_endian::FromHost(*it))
+         << os.fill();
     }
     os << engine.next_;
     return os;
@@ -199,13 +224,13 @@ class alignas(16) randen_engine {
       // necessary to read a wider type and then cast it to uint8_t.
       numeric_type value;
       is >> value;
-      elem = static_cast<result_type>(value);
+      elem = little_endian::ToHost(static_cast<result_type>(value));
     }
     is >> next;
     if (is.fail()) {
       return is;
     }
-    std::memcpy(engine.state_, state, sizeof(engine.state_));
+    std::memcpy(engine.state(), state, sizeof(state));
     engine.next_ = next;
     return is;
   }
@@ -216,13 +241,26 @@ class alignas(16) randen_engine {
   static constexpr size_t kCapacityT =
       Randen::kCapacityBytes / sizeof(result_type);
 
-  // First kCapacityT are `inner', the others are accessible random bits.
-  alignas(16) result_type state_[kStateSizeT];
-  size_t next_;  // index within state_
+  // Returns the state array pointer, which is aligned to 16 bytes.
+  // The first kCapacityT are the `inner' sponge; the remainder are available.
+  result_type* state() {
+    return reinterpret_cast<result_type*>(
+        (reinterpret_cast<uintptr_t>(&raw_state_) & 0xf) ? (raw_state_ + 8)
+                                                         : raw_state_);
+  }
+  const result_type* state() const {
+    return const_cast<randen_engine*>(this)->state();
+  }
+
+  // raw state array, manually aligned in state(). This overallocates
+  // by 8 bytes since C++ does not guarantee extended heap alignment.
+  alignas(8) char raw_state_[Randen::kStateBytes + 8];
+  size_t next_;  // index within state()
   Randen impl_;
 };
 
 }  // namespace random_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_INTERNAL_RANDEN_ENGINE_H_
diff --git a/absl/random/internal/randen_engine_test.cc b/absl/random/internal/randen_engine_test.cc
index c8e7685bdda..5488bd522b6 100644
--- a/absl/random/internal/randen_engine_test.cc
+++ b/absl/random/internal/randen_engine_test.cc
@@ -16,14 +16,16 @@
 
 #include <algorithm>
 #include <bitset>
+#include <cmath>
+#include <cstddef>
+#include <cstdint>
 #include <random>
 #include <sstream>
+#include <type_traits>
 
-#include "gmock/gmock.h"
 #include "gtest/gtest.h"
-#include "absl/base/internal/raw_logging.h"
+#include "absl/log/log.h"
 #include "absl/random/internal/explicit_seed_seq.h"
-#include "absl/strings/str_cat.h"
 #include "absl/time/clock.h"
 
 #define UPDATE_GOLDEN 0
@@ -167,19 +169,19 @@ TYPED_TEST(RandenEngineTypedTest, RandomNumberEngineInterface) {
   using E = randen;
   using T = typename E::result_type;
 
-  static_assert(std::is_copy_constructible<E>::value,
+  static_assert(std::is_copy_constructible_v<E>,
                 "randen_engine must be copy constructible");
 
-  static_assert(absl::is_copy_assignable<E>::value,
+  static_assert(std::is_copy_assignable_v<E>,
                 "randen_engine must be copy assignable");
 
-  static_assert(std::is_move_constructible<E>::value,
+  static_assert(std::is_move_constructible_v<E>,
                 "randen_engine must be move constructible");
 
-  static_assert(absl::is_move_assignable<E>::value,
+  static_assert(std::is_move_assignable_v<E>,
                 "randen_engine must be move assignable");
 
-  static_assert(std::is_same<decltype(std::declval<E>()()), T>::value,
+  static_assert(std::is_same_v<decltype(std::declval<E>()()), T>,
                 "return type of operator() must be result_type");
 
   // Names after definition of [rand.req.urbg] in C++ standard.
@@ -225,10 +227,10 @@ TYPED_TEST(RandenEngineTypedTest, RandomNumberEngineInterface) {
 
   e.discard(z);
 
-  static_assert(std::is_same<decltype(x == y), bool>::value,
+  static_assert(std::is_same_v<decltype(x == y), bool>,
                 "return type of operator== must be bool");
 
-  static_assert(std::is_same<decltype(x != y), bool>::value,
+  static_assert(std::is_same_v<decltype(x != y), bool>,
                 "return type of operator== must be bool");
 }
 
@@ -632,7 +634,6 @@ TEST(RandenTest, IsFastOrSlow) {
   //
   // linux, optimized ~5ns
   // ppc, optimized ~7ns
-  // nacl (slow), ~1100ns
   //
   // `kCount` is chosen below so that, in debug builds and without hardware
   // acceleration, the test (assuming ~1us per call) should finish in ~0.1s
@@ -645,9 +646,8 @@ TEST(RandenTest, IsFastOrSlow) {
   }
   auto duration = absl::GetCurrentTimeNanos() - start;
 
-  ABSL_INTERNAL_LOG(INFO, absl::StrCat(static_cast<double>(duration) /
-                                           static_cast<double>(kCount),
-                                       "ns"));
+  LOG(INFO) << static_cast<double>(duration) / static_cast<double>(kCount)
+            << "ns";
 
   EXPECT_GT(sum, 0);
   EXPECT_GE(duration, kCount);  // Should be slower than 1ns per call.
diff --git a/absl/random/internal/randen_hwaes.cc b/absl/random/internal/randen_hwaes.cc
index 7d5b2b74128..f8dd358c8f6 100644
--- a/absl/random/internal/randen_hwaes.cc
+++ b/absl/random/internal/randen_hwaes.cc
@@ -19,52 +19,23 @@
 
 #include "absl/random/internal/randen_hwaes.h"
 
-#include <cstdint>
 #include <cstring>
 
-#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/numeric/int128.h"
 #include "absl/random/internal/platform.h"
+#include "absl/random/internal/randen_traits.h"
 
 // ABSL_RANDEN_HWAES_IMPL indicates whether this file will contain
 // a hardware accelerated implementation of randen, or whether it
 // will contain stubs that exit the process.
-#if defined(ABSL_ARCH_X86_64) || defined(ABSL_ARCH_X86_32)
-// The platform.h directives are sufficient to indicate whether
-// we should build accelerated implementations for x86.
-#if (ABSL_HAVE_ACCELERATED_AES || ABSL_RANDOM_INTERNAL_AES_DISPATCH)
-#define ABSL_RANDEN_HWAES_IMPL 1
-#endif
-#elif defined(ABSL_ARCH_PPC)
-// The platform.h directives are sufficient to indicate whether
-// we should build accelerated implementations for PPC.
-//
-// NOTE: This has mostly been tested on 64-bit Power variants,
-// and not embedded cpus such as powerpc32-8540
 #if ABSL_HAVE_ACCELERATED_AES
+// The following platforms have implemented RandenHwAes.
+#if defined(ABSL_ARCH_X86_64) || defined(ABSL_ARCH_X86_32) || \
+    defined(ABSL_ARCH_PPC) || defined(ABSL_ARCH_ARM) ||       \
+    defined(ABSL_ARCH_AARCH64)
 #define ABSL_RANDEN_HWAES_IMPL 1
 #endif
-#elif defined(ABSL_ARCH_ARM) || defined(ABSL_ARCH_AARCH64)
-// ARM is somewhat more complicated. We might support crypto natively...
-#if ABSL_HAVE_ACCELERATED_AES || \
-    (defined(__ARM_NEON) && defined(__ARM_FEATURE_CRYPTO))
-#define ABSL_RANDEN_HWAES_IMPL 1
-
-#elif ABSL_RANDOM_INTERNAL_AES_DISPATCH && !defined(__APPLE__) && \
-    (defined(__GNUC__) && __GNUC__ > 4 || __GNUC__ == 4 && __GNUC_MINOR__ > 9)
-// ...or, on GCC, we can use an ASM directive to
-// instruct the assember to allow crypto instructions.
-#define ABSL_RANDEN_HWAES_IMPL 1
-#define ABSL_RANDEN_HWAES_IMPL_CRYPTO_DIRECTIVE 1
-#endif
-#else
-// HWAES is unsupported by these architectures / platforms:
-//   __myriad2__
-//   __mips__
-//
-// Other architectures / platforms are unknown.
-//
-// See the Abseil documentation on supported macros at:
-// https://abseil.io/docs/cpp/platforms/macros
 #endif
 
 #if !defined(ABSL_RANDEN_HWAES_IMPL)
@@ -75,6 +46,7 @@
 #include <cstdlib>
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
 // No accelerated implementation.
@@ -106,6 +78,7 @@ void RandenHwAes::Generate(const void*, void*) {
 }
 
 }  // namespace random_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #else  // defined(ABSL_RANDEN_HWAES_IMPL)
@@ -113,8 +86,11 @@ void RandenHwAes::Generate(const void*, void*) {
 // Accelerated implementations are supported.
 // We need the per-architecture includes and defines.
 //
+namespace {
 
-#include "absl/random/internal/randen_traits.h"
+using absl::random_internal::RandenTraits;
+
+}  // namespace
 
 // TARGET_CRYPTO defines a crypto attribute for each architecture.
 //
@@ -139,6 +115,7 @@ void RandenHwAes::Generate(const void*, void*) {
 #include <altivec.h>
 // <altivec.h> #defines vector __vector; in C++, this is bad form.
 #undef vector
+#undef bool
 
 // Rely on the PowerPC AltiVec vector operations for accelerated AES
 // instructions. GCC support of the PPC vector types is described in:
@@ -148,7 +125,6 @@ void RandenHwAes::Generate(const void*, void*) {
 using Vector128 = __vector unsigned long long;  // NOLINT(runtime/int)
 
 namespace {
-
 inline ABSL_TARGET_CRYPTO Vector128 ReverseBytes(const Vector128& v) {
   // Reverses the bytes of the vector.
   const __vector unsigned char perm = {15, 14, 13, 12, 11, 10, 9, 8,
@@ -159,13 +135,11 @@ inline ABSL_TARGET_CRYPTO Vector128 ReverseBytes(const Vector128& v) {
 // WARNING: these load/store in native byte order. It is OK to load and then
 // store an unchanged vector, but interpreting the bits as a number or input
 // to AES will have undefined results.
-inline ABSL_TARGET_CRYPTO Vector128
-Vector128Load(const void* ABSL_RANDOM_INTERNAL_RESTRICT from) {
+inline ABSL_TARGET_CRYPTO Vector128 Vector128Load(const void* from) {
   return vec_vsx_ld(0, reinterpret_cast<const Vector128*>(from));
 }
 
-inline ABSL_TARGET_CRYPTO void Vector128Store(
-    const Vector128& v, void* ABSL_RANDOM_INTERNAL_RESTRICT to) {
+inline ABSL_TARGET_CRYPTO void Vector128Store(const Vector128& v, void* to) {
   vec_vsx_st(v, 0, reinterpret_cast<Vector128*>(to));
 }
 
@@ -177,15 +151,9 @@ inline ABSL_TARGET_CRYPTO Vector128 AesRound(const Vector128& state,
 }
 
 // Enables native loads in the round loop by pre-swapping.
-inline ABSL_TARGET_CRYPTO void SwapEndian(
-    uint64_t* ABSL_RANDOM_INTERNAL_RESTRICT state) {
-  using absl::random_internal::RandenTraits;
-  constexpr size_t kLanes = 2;
-  constexpr size_t kFeistelBlocks = RandenTraits::kFeistelBlocks;
-
-  for (uint32_t branch = 0; branch < kFeistelBlocks; ++branch) {
-    const Vector128 v = ReverseBytes(Vector128Load(state + kLanes * branch));
-    Vector128Store(v, state + kLanes * branch);
+inline ABSL_TARGET_CRYPTO void SwapEndian(absl::uint128* state) {
+  for (uint32_t block = 0; block < RandenTraits::kFeistelBlocks; ++block) {
+    Vector128Store(ReverseBytes(Vector128Load(state + block)), state + block);
   }
 }
 
@@ -193,22 +161,6 @@ inline ABSL_TARGET_CRYPTO void SwapEndian(
 
 #elif defined(ABSL_ARCH_ARM) || defined(ABSL_ARCH_AARCH64)
 
-// This asm directive will cause the file to be compiled with crypto extensions
-// whether or not the cpu-architecture supports it.
-#if ABSL_RANDEN_HWAES_IMPL_CRYPTO_DIRECTIVE
-asm(".arch_extension  crypto\n");
-
-// Override missing defines.
-#if !defined(__ARM_NEON)
-#define __ARM_NEON 1
-#endif
-
-#if !defined(__ARM_FEATURE_CRYPTO)
-#define __ARM_FEATURE_CRYPTO 1
-#endif
-
-#endif
-
 // Rely on the ARM NEON+Crypto advanced simd types, defined in <arm_neon.h>.
 // uint8x16_t is the user alias for underlying __simd128_uint8_t type.
 // http://infocenter.arm.com/help/topic/com.arm.doc.ihi0073a/IHI0073A_arm_neon_intrinsics_ref.pdf
@@ -230,13 +182,11 @@ using Vector128 = uint8x16_t;
 
 namespace {
 
-inline ABSL_TARGET_CRYPTO Vector128
-Vector128Load(const void* ABSL_RANDOM_INTERNAL_RESTRICT from) {
+inline ABSL_TARGET_CRYPTO Vector128 Vector128Load(const void* from) {
   return vld1q_u8(reinterpret_cast<const uint8_t*>(from));
 }
 
-inline ABSL_TARGET_CRYPTO void Vector128Store(
-    const Vector128& v, void* ABSL_RANDOM_INTERNAL_RESTRICT to) {
+inline ABSL_TARGET_CRYPTO void Vector128Store(const Vector128& v, void* to) {
   vst1q_u8(reinterpret_cast<uint8_t*>(to), v);
 }
 
@@ -254,14 +204,13 @@ inline ABSL_TARGET_CRYPTO Vector128 AesRound(const Vector128& state,
   return vaesmcq_u8(vaeseq_u8(state, uint8x16_t{})) ^ round_key;
 }
 
-inline ABSL_TARGET_CRYPTO void SwapEndian(
-    uint64_t* ABSL_RANDOM_INTERNAL_RESTRICT) {}
+inline ABSL_TARGET_CRYPTO void SwapEndian(void*) {}
 
 }  // namespace
 
 #elif defined(ABSL_ARCH_X86_64) || defined(ABSL_ARCH_X86_32)
 // On x86 we rely on the aesni instructions
-#include <wmmintrin.h>
+#include <immintrin.h>
 
 namespace {
 
@@ -270,11 +219,11 @@ namespace {
 class Vector128 {
  public:
   // Convert from/to intrinsics.
-  inline explicit Vector128(const __m128i& Vector128) : data_(Vector128) {}
+  explicit Vector128(const __m128i& v) : data_(v) {}
 
-  inline __m128i data() const { return data_; }
+  __m128i data() const { return data_; }
 
-  inline Vector128& operator^=(const Vector128& other) {
+  Vector128& operator^=(const Vector128& other) {
     data_ = _mm_xor_si128(data_, other.data());
     return *this;
   }
@@ -283,15 +232,12 @@ class Vector128 {
   __m128i data_;
 };
 
-inline ABSL_TARGET_CRYPTO Vector128
-Vector128Load(const void* ABSL_RANDOM_INTERNAL_RESTRICT from) {
+inline ABSL_TARGET_CRYPTO Vector128 Vector128Load(const void* from) {
   return Vector128(_mm_load_si128(reinterpret_cast<const __m128i*>(from)));
 }
 
-inline ABSL_TARGET_CRYPTO void Vector128Store(
-    const Vector128& v, void* ABSL_RANDOM_INTERNAL_RESTRICT to) {
-  _mm_store_si128(reinterpret_cast<__m128i * ABSL_RANDOM_INTERNAL_RESTRICT>(to),
-                  v.data());
+inline ABSL_TARGET_CRYPTO void Vector128Store(const Vector128& v, void* to) {
+  _mm_store_si128(reinterpret_cast<__m128i*>(to), v.data());
 }
 
 // One round of AES. "round_key" is a public constant for breaking the
@@ -304,40 +250,12 @@ inline ABSL_TARGET_CRYPTO Vector128 AesRound(const Vector128& state,
   return Vector128(_mm_aesenc_si128(state.data(), round_key.data()));
 }
 
-inline ABSL_TARGET_CRYPTO void SwapEndian(
-    uint64_t* ABSL_RANDOM_INTERNAL_RESTRICT) {}
+inline ABSL_TARGET_CRYPTO void SwapEndian(void*) {}
 
 }  // namespace
 
 #endif
 
-namespace {
-
-// u64x2 is a 128-bit, (2 x uint64_t lanes) struct used to store
-// the randen_keys.
-struct alignas(16) u64x2 {
-  constexpr u64x2(uint64_t hi, uint64_t lo)
-#if defined(ABSL_ARCH_PPC)
-      // This has been tested with PPC running in little-endian mode;
-      // We byte-swap the u64x2 structure from little-endian to big-endian
-      // because altivec always runs in big-endian mode.
-      : v{__builtin_bswap64(hi), __builtin_bswap64(lo)} {
-#else
-      : v{lo, hi} {
-#endif
-  }
-
-  constexpr bool operator==(const u64x2& other) const {
-    return v[0] == other.v[0] && v[1] == other.v[1];
-  }
-
-  constexpr bool operator!=(const u64x2& other) const {
-    return !(*this == other);
-  }
-
-  uint64_t v[2];
-};  // namespace
-
 #ifdef __clang__
 #pragma clang diagnostic push
 #pragma clang diagnostic ignored "-Wunknown-pragmas"
@@ -346,7 +264,6 @@ struct alignas(16) u64x2 {
 // At this point, all of the platform-specific features have been defined /
 // implemented.
 //
-// REQUIRES: using u64x2 = ...
 // REQUIRES: using Vector128 = ...
 // REQUIRES: Vector128 Vector128Load(void*) {...}
 // REQUIRES: void Vector128Store(Vector128, void*) {...}
@@ -355,95 +272,50 @@ struct alignas(16) u64x2 {
 //
 // PROVIDES: absl::random_internal::RandenHwAes::Absorb
 // PROVIDES: absl::random_internal::RandenHwAes::Generate
-
-// RANDen = RANDom generator or beetroots in Swiss German.
-// 'Strong' (well-distributed, unpredictable, backtracking-resistant) random
-// generator, faster in some benchmarks than std::mt19937_64 and pcg64_c32.
-//
-// High-level summary:
-// 1) Reverie (see "A Robust and Sponge-Like PRNG with Improved Efficiency") is
-//    a sponge-like random generator that requires a cryptographic permutation.
-//    It improves upon "Provably Robust Sponge-Based PRNGs and KDFs" by
-//    achieving backtracking resistance with only one Permute() per buffer.
-//
-// 2) "Simpira v2: A Family of Efficient Permutations Using the AES Round
-//    Function" constructs up to 1024-bit permutations using an improved
-//    Generalized Feistel network with 2-round AES-128 functions. This Feistel
-//    block shuffle achieves diffusion faster and is less vulnerable to
-//    sliced-biclique attacks than the Type-2 cyclic shuffle.
-//
-// 3) "Improving the Generalized Feistel" and "New criterion for diffusion
-//    property" extends the same kind of improved Feistel block shuffle to 16
-//    branches, which enables a 2048-bit permutation.
-//
-// We combine these three ideas and also change Simpira's subround keys from
-// structured/low-entropy counters to digits of Pi.
-
-// Randen constants.
-using absl::random_internal::RandenTraits;
-constexpr size_t kStateBytes = RandenTraits::kStateBytes;
-constexpr size_t kCapacityBytes = RandenTraits::kCapacityBytes;
-constexpr size_t kFeistelBlocks = RandenTraits::kFeistelBlocks;
-constexpr size_t kFeistelRounds = RandenTraits::kFeistelRounds;
-constexpr size_t kFeistelFunctions = RandenTraits::kFeistelFunctions;
-
-// Independent keys (272 = 2.1 KiB) for the first AES subround of each function.
-constexpr size_t kKeys = kFeistelRounds * kFeistelFunctions;
-
-// INCLUDE keys.
-#include "absl/random/internal/randen-keys.inc"
-
-static_assert(kKeys == kRoundKeys, "kKeys and kRoundKeys must be equal");
-static_assert(round_keys[kKeys - 1] != u64x2(0, 0),
-              "Too few round_keys initializers");
-
-// Number of uint64_t lanes per 128-bit vector;
-constexpr size_t kLanes = 2;
+namespace {
 
 // Block shuffles applies a shuffle to the entire state between AES rounds.
 // Improved odd-even shuffle from "New criterion for diffusion property".
-inline ABSL_TARGET_CRYPTO void BlockShuffle(
-    uint64_t* ABSL_RANDOM_INTERNAL_RESTRICT state) {
-  static_assert(kFeistelBlocks == 16, "Expecting 16 FeistelBlocks.");
-
-  constexpr size_t shuffle[kFeistelBlocks] = {7,  2, 13, 4,  11, 8,  3, 6,
-                                              15, 0, 9,  10, 1,  14, 5, 12};
-
-  // The fully unrolled loop without the memcpy improves the speed by about
-  // 30% over the equivalent loop.
-  const Vector128 v0 = Vector128Load(state + kLanes * shuffle[0]);
-  const Vector128 v1 = Vector128Load(state + kLanes * shuffle[1]);
-  const Vector128 v2 = Vector128Load(state + kLanes * shuffle[2]);
-  const Vector128 v3 = Vector128Load(state + kLanes * shuffle[3]);
-  const Vector128 v4 = Vector128Load(state + kLanes * shuffle[4]);
-  const Vector128 v5 = Vector128Load(state + kLanes * shuffle[5]);
-  const Vector128 v6 = Vector128Load(state + kLanes * shuffle[6]);
-  const Vector128 v7 = Vector128Load(state + kLanes * shuffle[7]);
-  const Vector128 w0 = Vector128Load(state + kLanes * shuffle[8]);
-  const Vector128 w1 = Vector128Load(state + kLanes * shuffle[9]);
-  const Vector128 w2 = Vector128Load(state + kLanes * shuffle[10]);
-  const Vector128 w3 = Vector128Load(state + kLanes * shuffle[11]);
-  const Vector128 w4 = Vector128Load(state + kLanes * shuffle[12]);
-  const Vector128 w5 = Vector128Load(state + kLanes * shuffle[13]);
-  const Vector128 w6 = Vector128Load(state + kLanes * shuffle[14]);
-  const Vector128 w7 = Vector128Load(state + kLanes * shuffle[15]);
-
-  Vector128Store(v0, state + kLanes * 0);
-  Vector128Store(v1, state + kLanes * 1);
-  Vector128Store(v2, state + kLanes * 2);
-  Vector128Store(v3, state + kLanes * 3);
-  Vector128Store(v4, state + kLanes * 4);
-  Vector128Store(v5, state + kLanes * 5);
-  Vector128Store(v6, state + kLanes * 6);
-  Vector128Store(v7, state + kLanes * 7);
-  Vector128Store(w0, state + kLanes * 8);
-  Vector128Store(w1, state + kLanes * 9);
-  Vector128Store(w2, state + kLanes * 10);
-  Vector128Store(w3, state + kLanes * 11);
-  Vector128Store(w4, state + kLanes * 12);
-  Vector128Store(w5, state + kLanes * 13);
-  Vector128Store(w6, state + kLanes * 14);
-  Vector128Store(w7, state + kLanes * 15);
+inline ABSL_TARGET_CRYPTO void BlockShuffle(absl::uint128* state) {
+  static_assert(RandenTraits::kFeistelBlocks == 16,
+                "Expecting 16 FeistelBlocks.");
+
+  constexpr size_t shuffle[RandenTraits::kFeistelBlocks] = {
+      7, 2, 13, 4, 11, 8, 3, 6, 15, 0, 9, 10, 1, 14, 5, 12};
+
+  const Vector128 v0 = Vector128Load(state + shuffle[0]);
+  const Vector128 v1 = Vector128Load(state + shuffle[1]);
+  const Vector128 v2 = Vector128Load(state + shuffle[2]);
+  const Vector128 v3 = Vector128Load(state + shuffle[3]);
+  const Vector128 v4 = Vector128Load(state + shuffle[4]);
+  const Vector128 v5 = Vector128Load(state + shuffle[5]);
+  const Vector128 v6 = Vector128Load(state + shuffle[6]);
+  const Vector128 v7 = Vector128Load(state + shuffle[7]);
+  const Vector128 w0 = Vector128Load(state + shuffle[8]);
+  const Vector128 w1 = Vector128Load(state + shuffle[9]);
+  const Vector128 w2 = Vector128Load(state + shuffle[10]);
+  const Vector128 w3 = Vector128Load(state + shuffle[11]);
+  const Vector128 w4 = Vector128Load(state + shuffle[12]);
+  const Vector128 w5 = Vector128Load(state + shuffle[13]);
+  const Vector128 w6 = Vector128Load(state + shuffle[14]);
+  const Vector128 w7 = Vector128Load(state + shuffle[15]);
+
+  Vector128Store(v0, state + 0);
+  Vector128Store(v1, state + 1);
+  Vector128Store(v2, state + 2);
+  Vector128Store(v3, state + 3);
+  Vector128Store(v4, state + 4);
+  Vector128Store(v5, state + 5);
+  Vector128Store(v6, state + 6);
+  Vector128Store(v7, state + 7);
+  Vector128Store(w0, state + 8);
+  Vector128Store(w1, state + 9);
+  Vector128Store(w2, state + 10);
+  Vector128Store(w3, state + 11);
+  Vector128Store(w4, state + 12);
+  Vector128Store(w5, state + 13);
+  Vector128Store(w6, state + 14);
+  Vector128Store(w7, state + 15);
 }
 
 // Feistel round function using two AES subrounds. Very similar to F()
@@ -451,29 +323,30 @@ inline ABSL_TARGET_CRYPTO void BlockShuffle(
 // per 16 bytes (vs. 10 for AES-CTR). Computing eight round functions in
 // parallel hides the 7-cycle AESNI latency on HSW. Note that the Feistel
 // XORs are 'free' (included in the second AES instruction).
-inline ABSL_TARGET_CRYPTO const u64x2* FeistelRound(
-    uint64_t* ABSL_RANDOM_INTERNAL_RESTRICT state,
-    const u64x2* ABSL_RANDOM_INTERNAL_RESTRICT keys) {
-  static_assert(kFeistelBlocks == 16, "Expecting 16 FeistelBlocks.");
+inline ABSL_TARGET_CRYPTO const absl::uint128* FeistelRound(
+    absl::uint128* state,
+    const absl::uint128* ABSL_RANDOM_INTERNAL_RESTRICT keys) {
+  static_assert(RandenTraits::kFeistelBlocks == 16,
+                "Expecting 16 FeistelBlocks.");
 
   // MSVC does a horrible job at unrolling loops.
   // So we unroll the loop by hand to improve the performance.
-  const Vector128 s0 = Vector128Load(state + kLanes * 0);
-  const Vector128 s1 = Vector128Load(state + kLanes * 1);
-  const Vector128 s2 = Vector128Load(state + kLanes * 2);
-  const Vector128 s3 = Vector128Load(state + kLanes * 3);
-  const Vector128 s4 = Vector128Load(state + kLanes * 4);
-  const Vector128 s5 = Vector128Load(state + kLanes * 5);
-  const Vector128 s6 = Vector128Load(state + kLanes * 6);
-  const Vector128 s7 = Vector128Load(state + kLanes * 7);
-  const Vector128 s8 = Vector128Load(state + kLanes * 8);
-  const Vector128 s9 = Vector128Load(state + kLanes * 9);
-  const Vector128 s10 = Vector128Load(state + kLanes * 10);
-  const Vector128 s11 = Vector128Load(state + kLanes * 11);
-  const Vector128 s12 = Vector128Load(state + kLanes * 12);
-  const Vector128 s13 = Vector128Load(state + kLanes * 13);
-  const Vector128 s14 = Vector128Load(state + kLanes * 14);
-  const Vector128 s15 = Vector128Load(state + kLanes * 15);
+  const Vector128 s0 = Vector128Load(state + 0);
+  const Vector128 s1 = Vector128Load(state + 1);
+  const Vector128 s2 = Vector128Load(state + 2);
+  const Vector128 s3 = Vector128Load(state + 3);
+  const Vector128 s4 = Vector128Load(state + 4);
+  const Vector128 s5 = Vector128Load(state + 5);
+  const Vector128 s6 = Vector128Load(state + 6);
+  const Vector128 s7 = Vector128Load(state + 7);
+  const Vector128 s8 = Vector128Load(state + 8);
+  const Vector128 s9 = Vector128Load(state + 9);
+  const Vector128 s10 = Vector128Load(state + 10);
+  const Vector128 s11 = Vector128Load(state + 11);
+  const Vector128 s12 = Vector128Load(state + 12);
+  const Vector128 s13 = Vector128Load(state + 13);
+  const Vector128 s14 = Vector128Load(state + 14);
+  const Vector128 s15 = Vector128Load(state + 15);
 
   // Encode even blocks with keys.
   const Vector128 e0 = AesRound(s0, Vector128Load(keys + 0));
@@ -496,14 +369,14 @@ inline ABSL_TARGET_CRYPTO const u64x2* FeistelRound(
   const Vector128 o15 = AesRound(e14, s15);
 
   // Store odd blocks. (These will be shuffled later).
-  Vector128Store(o1, state + kLanes * 1);
-  Vector128Store(o3, state + kLanes * 3);
-  Vector128Store(o5, state + kLanes * 5);
-  Vector128Store(o7, state + kLanes * 7);
-  Vector128Store(o9, state + kLanes * 9);
-  Vector128Store(o11, state + kLanes * 11);
-  Vector128Store(o13, state + kLanes * 13);
-  Vector128Store(o15, state + kLanes * 15);
+  Vector128Store(o1, state + 1);
+  Vector128Store(o3, state + 3);
+  Vector128Store(o5, state + 5);
+  Vector128Store(o7, state + 7);
+  Vector128Store(o9, state + 9);
+  Vector128Store(o11, state + 11);
+  Vector128Store(o13, state + 13);
+  Vector128Store(o15, state + 15);
 
   return keys + 8;
 }
@@ -513,17 +386,14 @@ inline ABSL_TARGET_CRYPTO const u64x2* FeistelRound(
 // 2^64 queries if the round function is a PRF. This is similar to the b=8 case
 // of Simpira v2, but more efficient than its generic construction for b=16.
 inline ABSL_TARGET_CRYPTO void Permute(
-    const void* ABSL_RANDOM_INTERNAL_RESTRICT keys,
-    uint64_t* ABSL_RANDOM_INTERNAL_RESTRICT state) {
-  const u64x2* ABSL_RANDOM_INTERNAL_RESTRICT keys128 =
-      static_cast<const u64x2*>(keys);
-
+    absl::uint128* state,
+    const absl::uint128* ABSL_RANDOM_INTERNAL_RESTRICT keys) {
   // (Successfully unrolled; the first iteration jumps into the second half)
 #ifdef __clang__
 #pragma clang loop unroll_count(2)
 #endif
-  for (size_t round = 0; round < kFeistelRounds; ++round) {
-    keys128 = FeistelRound(state, keys128);
+  for (size_t round = 0; round < RandenTraits::kFeistelRounds; ++round) {
+    keys = FeistelRound(state, keys);
     BlockShuffle(state);
   }
 }
@@ -531,6 +401,7 @@ inline ABSL_TARGET_CRYPTO void Permute(
 }  // namespace
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
 bool HasRandenHwAesImplementation() { return true; }
@@ -538,99 +409,102 @@ bool HasRandenHwAesImplementation() { return true; }
 const void* ABSL_TARGET_CRYPTO RandenHwAes::GetKeys() {
   // Round keys for one AES per Feistel round and branch.
   // The canonical implementation uses first digits of Pi.
-  return round_keys;
+#if defined(ABSL_ARCH_PPC)
+  return kRandenRoundKeysBE;
+#else
+  return kRandenRoundKeys;
+#endif
 }
 
 // NOLINTNEXTLINE
 void ABSL_TARGET_CRYPTO RandenHwAes::Absorb(const void* seed_void,
                                             void* state_void) {
-  uint64_t* ABSL_RANDOM_INTERNAL_RESTRICT state =
-      reinterpret_cast<uint64_t*>(state_void);
-  const uint64_t* ABSL_RANDOM_INTERNAL_RESTRICT seed =
-      reinterpret_cast<const uint64_t*>(seed_void);
-
-  constexpr size_t kCapacityBlocks = kCapacityBytes / sizeof(Vector128);
-  constexpr size_t kStateBlocks = kStateBytes / sizeof(Vector128);
-
-  static_assert(kCapacityBlocks * sizeof(Vector128) == kCapacityBytes,
-                "Not i*V");
-  static_assert(kCapacityBlocks == 1, "Unexpected Randen kCapacityBlocks");
-  static_assert(kStateBlocks == 16, "Unexpected Randen kStateBlocks");
-
-  Vector128 b1 = Vector128Load(state + kLanes * 1);
-  b1 ^= Vector128Load(seed + kLanes * 0);
-  Vector128Store(b1, state + kLanes * 1);
-
-  Vector128 b2 = Vector128Load(state + kLanes * 2);
-  b2 ^= Vector128Load(seed + kLanes * 1);
-  Vector128Store(b2, state + kLanes * 2);
-
-  Vector128 b3 = Vector128Load(state + kLanes * 3);
-  b3 ^= Vector128Load(seed + kLanes * 2);
-  Vector128Store(b3, state + kLanes * 3);
-
-  Vector128 b4 = Vector128Load(state + kLanes * 4);
-  b4 ^= Vector128Load(seed + kLanes * 3);
-  Vector128Store(b4, state + kLanes * 4);
-
-  Vector128 b5 = Vector128Load(state + kLanes * 5);
-  b5 ^= Vector128Load(seed + kLanes * 4);
-  Vector128Store(b5, state + kLanes * 5);
-
-  Vector128 b6 = Vector128Load(state + kLanes * 6);
-  b6 ^= Vector128Load(seed + kLanes * 5);
-  Vector128Store(b6, state + kLanes * 6);
-
-  Vector128 b7 = Vector128Load(state + kLanes * 7);
-  b7 ^= Vector128Load(seed + kLanes * 6);
-  Vector128Store(b7, state + kLanes * 7);
-
-  Vector128 b8 = Vector128Load(state + kLanes * 8);
-  b8 ^= Vector128Load(seed + kLanes * 7);
-  Vector128Store(b8, state + kLanes * 8);
-
-  Vector128 b9 = Vector128Load(state + kLanes * 9);
-  b9 ^= Vector128Load(seed + kLanes * 8);
-  Vector128Store(b9, state + kLanes * 9);
-
-  Vector128 b10 = Vector128Load(state + kLanes * 10);
-  b10 ^= Vector128Load(seed + kLanes * 9);
-  Vector128Store(b10, state + kLanes * 10);
-
-  Vector128 b11 = Vector128Load(state + kLanes * 11);
-  b11 ^= Vector128Load(seed + kLanes * 10);
-  Vector128Store(b11, state + kLanes * 11);
-
-  Vector128 b12 = Vector128Load(state + kLanes * 12);
-  b12 ^= Vector128Load(seed + kLanes * 11);
-  Vector128Store(b12, state + kLanes * 12);
-
-  Vector128 b13 = Vector128Load(state + kLanes * 13);
-  b13 ^= Vector128Load(seed + kLanes * 12);
-  Vector128Store(b13, state + kLanes * 13);
-
-  Vector128 b14 = Vector128Load(state + kLanes * 14);
-  b14 ^= Vector128Load(seed + kLanes * 13);
-  Vector128Store(b14, state + kLanes * 14);
-
-  Vector128 b15 = Vector128Load(state + kLanes * 15);
-  b15 ^= Vector128Load(seed + kLanes * 14);
-  Vector128Store(b15, state + kLanes * 15);
+  static_assert(RandenTraits::kCapacityBytes / sizeof(Vector128) == 1,
+                "Unexpected Randen kCapacityBlocks");
+  static_assert(RandenTraits::kStateBytes / sizeof(Vector128) == 16,
+                "Unexpected Randen kStateBlocks");
+
+  auto* state = reinterpret_cast<absl::uint128 * ABSL_RANDOM_INTERNAL_RESTRICT>(
+      state_void);
+  const auto* seed =
+      reinterpret_cast<const absl::uint128 * ABSL_RANDOM_INTERNAL_RESTRICT>(
+          seed_void);
+
+  Vector128 b1 = Vector128Load(state + 1);
+  b1 ^= Vector128Load(seed + 0);
+  Vector128Store(b1, state + 1);
+
+  Vector128 b2 = Vector128Load(state + 2);
+  b2 ^= Vector128Load(seed + 1);
+  Vector128Store(b2, state + 2);
+
+  Vector128 b3 = Vector128Load(state + 3);
+  b3 ^= Vector128Load(seed + 2);
+  Vector128Store(b3, state + 3);
+
+  Vector128 b4 = Vector128Load(state + 4);
+  b4 ^= Vector128Load(seed + 3);
+  Vector128Store(b4, state + 4);
+
+  Vector128 b5 = Vector128Load(state + 5);
+  b5 ^= Vector128Load(seed + 4);
+  Vector128Store(b5, state + 5);
+
+  Vector128 b6 = Vector128Load(state + 6);
+  b6 ^= Vector128Load(seed + 5);
+  Vector128Store(b6, state + 6);
+
+  Vector128 b7 = Vector128Load(state + 7);
+  b7 ^= Vector128Load(seed + 6);
+  Vector128Store(b7, state + 7);
+
+  Vector128 b8 = Vector128Load(state + 8);
+  b8 ^= Vector128Load(seed + 7);
+  Vector128Store(b8, state + 8);
+
+  Vector128 b9 = Vector128Load(state + 9);
+  b9 ^= Vector128Load(seed + 8);
+  Vector128Store(b9, state + 9);
+
+  Vector128 b10 = Vector128Load(state + 10);
+  b10 ^= Vector128Load(seed + 9);
+  Vector128Store(b10, state + 10);
+
+  Vector128 b11 = Vector128Load(state + 11);
+  b11 ^= Vector128Load(seed + 10);
+  Vector128Store(b11, state + 11);
+
+  Vector128 b12 = Vector128Load(state + 12);
+  b12 ^= Vector128Load(seed + 11);
+  Vector128Store(b12, state + 12);
+
+  Vector128 b13 = Vector128Load(state + 13);
+  b13 ^= Vector128Load(seed + 12);
+  Vector128Store(b13, state + 13);
+
+  Vector128 b14 = Vector128Load(state + 14);
+  b14 ^= Vector128Load(seed + 13);
+  Vector128Store(b14, state + 14);
+
+  Vector128 b15 = Vector128Load(state + 15);
+  b15 ^= Vector128Load(seed + 14);
+  Vector128Store(b15, state + 15);
 }
 
 // NOLINTNEXTLINE
-void ABSL_TARGET_CRYPTO RandenHwAes::Generate(const void* keys,
+void ABSL_TARGET_CRYPTO RandenHwAes::Generate(const void* keys_void,
                                               void* state_void) {
-  static_assert(kCapacityBytes == sizeof(Vector128), "Capacity mismatch");
+  static_assert(RandenTraits::kCapacityBytes == sizeof(Vector128),
+                "Capacity mismatch");
 
-  uint64_t* ABSL_RANDOM_INTERNAL_RESTRICT state =
-      reinterpret_cast<uint64_t*>(state_void);
+  auto* state = reinterpret_cast<absl::uint128*>(state_void);
+  const auto* keys = reinterpret_cast<const absl::uint128*>(keys_void);
 
   const Vector128 prev_inner = Vector128Load(state);
 
   SwapEndian(state);
 
-  Permute(keys, state);
+  Permute(state, keys);
 
   SwapEndian(state);
 
@@ -645,6 +519,7 @@ void ABSL_TARGET_CRYPTO RandenHwAes::Generate(const void* keys,
 #endif
 
 }  // namespace random_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // (ABSL_RANDEN_HWAES_IMPL)
diff --git a/absl/random/internal/randen_hwaes.h b/absl/random/internal/randen_hwaes.h
index d8e6055f01a..71a7f69f25b 100644
--- a/absl/random/internal/randen_hwaes.h
+++ b/absl/random/internal/randen_hwaes.h
@@ -15,15 +15,18 @@
 #ifndef ABSL_RANDOM_INTERNAL_RANDEN_HWAES_H_
 #define ABSL_RANDOM_INTERNAL_RANDEN_HWAES_H_
 
+#include "absl/base/config.h"
+
 // HERMETIC NOTE: The randen_hwaes target must not introduce duplicate
 // symbols from arbitrary system and other headers, since it may be built
 // with different flags from other targets, using different levels of
 // optimization, potentially introducing ODR violations.
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
-// RANDen = RANDom generator or beetroots in Swiss German.
+// RANDen = RANDom generator or beetroots in Swiss High German.
 // 'Strong' (well-distributed, unpredictable, backtracking-resistant) random
 // generator, faster in some benchmarks than std::mt19937_64 and pcg64_c32.
 //
@@ -41,6 +44,7 @@ class RandenHwAes {
 bool HasRandenHwAesImplementation();
 
 }  // namespace random_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_INTERNAL_RANDEN_HWAES_H_
diff --git a/absl/random/internal/randen_hwaes_test.cc b/absl/random/internal/randen_hwaes_test.cc
index a7cbd46b2dd..956e9d69049 100644
--- a/absl/random/internal/randen_hwaes_test.cc
+++ b/absl/random/internal/randen_hwaes_test.cc
@@ -14,55 +14,53 @@
 
 #include "absl/random/internal/randen_hwaes.h"
 
-#include "gmock/gmock.h"
+#include <cstdint>
+#include <cstring>
+
 #include "gtest/gtest.h"
-#include "absl/base/internal/raw_logging.h"
+#include "absl/log/log.h"
 #include "absl/random/internal/platform.h"
 #include "absl/random/internal/randen_detect.h"
 #include "absl/random/internal/randen_traits.h"
-#include "absl/strings/str_format.h"
 
 namespace {
 
 using absl::random_internal::RandenHwAes;
 using absl::random_internal::RandenTraits;
 
-struct randen {
-  static constexpr size_t kStateSizeT =
-      RandenTraits::kStateBytes / sizeof(uint64_t);
-  uint64_t state[kStateSizeT];
-  static constexpr size_t kSeedSizeT =
-      RandenTraits::kSeedBytes / sizeof(uint32_t);
-  uint32_t seed[kSeedSizeT];
-};
-
 TEST(RandenHwAesTest, Default) {
   EXPECT_TRUE(absl::random_internal::CPUSupportsRandenHwAes());
 
-  constexpr uint64_t kGolden[] = {
-      0x6c6534090ee6d3ee, 0x044e2b9b9d5333c6, 0xc3c14f134e433977,
-      0xdda9f47cd90410ee, 0x887bf3087fd8ca10, 0xf0b780f545c72912,
-      0x15dbb1d37696599f, 0x30ec63baff3c6d59, 0xb29f73606f7f20a6,
-      0x02808a316f49a54c, 0x3b8feaf9d5c8e50e, 0x9cbf605e3fd9de8a,
-      0xc970ae1a78183bbb, 0xd8b2ffd356301ed5, 0xf4b327fe0fc73c37,
-      0xcdfd8d76eb8f9a19, 0xc3a506eb91420c9d, 0xd5af05dd3eff9556,
-      0x48db1bb78f83c4a1, 0x7023920e0d6bfe8c, 0x58d3575834956d42,
-      0xed1ef4c26b87b840, 0x8eef32a23e0b2df3, 0x497cabf3431154fc,
-      0x4e24370570029a8b, 0xd88b5749f090e5ea, 0xc651a582a970692f,
-      0x78fcec2cbb6342f5, 0x463cb745612f55db, 0x352ee4ad1816afe3,
-      0x026ff374c101da7e, 0x811ef0821c3de851,
+  constexpr uint8_t kGolden[] = {
+      0xee, 0xd3, 0xe6, 0x0e, 0x09, 0x34, 0x65, 0x6c, 0xc6, 0x33, 0x53, 0x9d,
+      0x9b, 0x2b, 0x4e, 0x04, 0x77, 0x39, 0x43, 0x4e, 0x13, 0x4f, 0xc1, 0xc3,
+      0xee, 0x10, 0x04, 0xd9, 0x7c, 0xf4, 0xa9, 0xdd, 0x10, 0xca, 0xd8, 0x7f,
+      0x08, 0xf3, 0x7b, 0x88, 0x12, 0x29, 0xc7, 0x45, 0xf5, 0x80, 0xb7, 0xf0,
+      0x9f, 0x59, 0x96, 0x76, 0xd3, 0xb1, 0xdb, 0x15, 0x59, 0x6d, 0x3c, 0xff,
+      0xba, 0x63, 0xec, 0x30, 0xa6, 0x20, 0x7f, 0x6f, 0x60, 0x73, 0x9f, 0xb2,
+      0x4c, 0xa5, 0x49, 0x6f, 0x31, 0x8a, 0x80, 0x02, 0x0e, 0xe5, 0xc8, 0xd5,
+      0xf9, 0xea, 0x8f, 0x3b, 0x8a, 0xde, 0xd9, 0x3f, 0x5e, 0x60, 0xbf, 0x9c,
+      0xbb, 0x3b, 0x18, 0x78, 0x1a, 0xae, 0x70, 0xc9, 0xd5, 0x1e, 0x30, 0x56,
+      0xd3, 0xff, 0xb2, 0xd8, 0x37, 0x3c, 0xc7, 0x0f, 0xfe, 0x27, 0xb3, 0xf4,
+      0x19, 0x9a, 0x8f, 0xeb, 0x76, 0x8d, 0xfd, 0xcd, 0x9d, 0x0c, 0x42, 0x91,
+      0xeb, 0x06, 0xa5, 0xc3, 0x56, 0x95, 0xff, 0x3e, 0xdd, 0x05, 0xaf, 0xd5,
+      0xa1, 0xc4, 0x83, 0x8f, 0xb7, 0x1b, 0xdb, 0x48, 0x8c, 0xfe, 0x6b, 0x0d,
+      0x0e, 0x92, 0x23, 0x70, 0x42, 0x6d, 0x95, 0x34, 0x58, 0x57, 0xd3, 0x58,
+      0x40, 0xb8, 0x87, 0x6b, 0xc2, 0xf4, 0x1e, 0xed, 0xf3, 0x2d, 0x0b, 0x3e,
+      0xa2, 0x32, 0xef, 0x8e, 0xfc, 0x54, 0x11, 0x43, 0xf3, 0xab, 0x7c, 0x49,
+      0x8b, 0x9a, 0x02, 0x70, 0x05, 0x37, 0x24, 0x4e, 0xea, 0xe5, 0x90, 0xf0,
+      0x49, 0x57, 0x8b, 0xd8, 0x2f, 0x69, 0x70, 0xa9, 0x82, 0xa5, 0x51, 0xc6,
+      0xf5, 0x42, 0x63, 0xbb, 0x2c, 0xec, 0xfc, 0x78, 0xdb, 0x55, 0x2f, 0x61,
+      0x45, 0xb7, 0x3c, 0x46, 0xe3, 0xaf, 0x16, 0x18, 0xad, 0xe4, 0x2e, 0x35,
+      0x7e, 0xda, 0x01, 0xc1, 0x74, 0xf3, 0x6f, 0x02, 0x51, 0xe8, 0x3d, 0x1c,
+      0x82, 0xf0, 0x1e, 0x81,
   };
 
-  alignas(16) randen d;
-  memset(d.state, 0, sizeof(d.state));
-  RandenHwAes::Generate(RandenHwAes::GetKeys(), d.state);
+  alignas(16) uint8_t state[RandenTraits::kStateBytes];
+  std::memset(state, 0, sizeof(state));
 
-  uint64_t* id = d.state;
-  for (const auto& elem : kGolden) {
-    auto a = absl::StrFormat("%#x", elem);
-    auto b = absl::StrFormat("%#x", *id++);
-    EXPECT_EQ(a, b);
-  }
+  RandenHwAes::Generate(RandenHwAes::GetKeys(), state);
+  EXPECT_EQ(0, std::memcmp(state, kGolden, sizeof(state)));
 }
 
 }  // namespace
@@ -70,32 +68,32 @@ TEST(RandenHwAesTest, Default) {
 int main(int argc, char* argv[]) {
   testing::InitGoogleTest(&argc, argv);
 
-  ABSL_RAW_LOG(INFO, "ABSL_HAVE_ACCELERATED_AES=%d", ABSL_HAVE_ACCELERATED_AES);
-  ABSL_RAW_LOG(INFO, "ABSL_RANDOM_INTERNAL_AES_DISPATCH=%d",
-               ABSL_RANDOM_INTERNAL_AES_DISPATCH);
+  LOG(INFO) << "ABSL_HAVE_ACCELERATED_AES=" << ABSL_HAVE_ACCELERATED_AES;
+  LOG(INFO) << "ABSL_RANDOM_INTERNAL_AES_DISPATCH="
+            << ABSL_RANDOM_INTERNAL_AES_DISPATCH;
 
 #if defined(ABSL_ARCH_X86_64)
-  ABSL_RAW_LOG(INFO, "ABSL_ARCH_X86_64");
+  LOG(INFO) << "ABSL_ARCH_X86_64";
 #elif defined(ABSL_ARCH_X86_32)
-  ABSL_RAW_LOG(INFO, "ABSL_ARCH_X86_32");
+  LOG(INFO) << "ABSL_ARCH_X86_32";
 #elif defined(ABSL_ARCH_AARCH64)
-  ABSL_RAW_LOG(INFO, "ABSL_ARCH_AARCH64");
+  LOG(INFO) << "ABSL_ARCH_AARCH64";
 #elif defined(ABSL_ARCH_ARM)
-  ABSL_RAW_LOG(INFO, "ABSL_ARCH_ARM");
+  LOG(INFO) << "ABSL_ARCH_ARM";
 #elif defined(ABSL_ARCH_PPC)
-  ABSL_RAW_LOG(INFO, "ABSL_ARCH_PPC");
+  LOG(INFO) << "ABSL_ARCH_PPC";
 #else
-  ABSL_RAW_LOG(INFO, "ARCH Unknown");
+  LOG(INFO) << "ARCH Unknown";
 #endif
 
   int x = absl::random_internal::HasRandenHwAesImplementation();
-  ABSL_RAW_LOG(INFO, "HasRandenHwAesImplementation = %d", x);
+  LOG(INFO) << "HasRandenHwAesImplementation = " << x;
 
   int y = absl::random_internal::CPUSupportsRandenHwAes();
-  ABSL_RAW_LOG(INFO, "CPUSupportsRandenHwAes = %d", x);
+  LOG(INFO) << "CPUSupportsRandenHwAes = " << y;
 
   if (!x || !y) {
-    ABSL_RAW_LOG(INFO, "Skipping Randen HWAES tests.");
+    LOG(INFO) << "Skipping Randen HWAES tests.";
     return 0;
   }
   return RUN_ALL_TESTS();
diff --git a/absl/random/internal/randen_round_keys.cc b/absl/random/internal/randen_round_keys.cc
new file mode 100644
index 00000000000..27a97b56451
--- /dev/null
+++ b/absl/random/internal/randen_round_keys.cc
@@ -0,0 +1,465 @@
+// Copyright 2017 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <cstddef>
+
+#include "absl/base/config.h"
+#include "absl/random/internal/randen_traits.h"
+
+// This file contains only the round keys for randen.
+//
+// "Nothing up my sleeve" numbers from the first hex digits of Pi, obtained
+// from http://hexpi.sourceforge.net/. The array was generated by following
+// Python script:
+
+/*
+python >tmp.cc << EOF
+"""Generates Randen round keys array from pi-hex.62500.txt file."""
+import binascii
+
+KEYS = 17 * 8
+
+def chunks(l, n):
+    """Yield successive n-sized chunks from l."""
+    for i in range(0, len(l), n):
+        yield l[i:i + n]
+
+def pairwise(t):
+    """Transforms sequence into sequence of pairs."""
+    it = iter(t)
+    return zip(it,it)
+
+def digits_from_pi():
+  """Reads digits from hexpi.sourceforge.net file."""
+  with open("pi-hex.62500.txt") as file:
+    return file.read()
+
+def digits_from_urandom():
+  """Reads digits from /dev/urandom."""
+  with open("/dev/urandom") as file:
+    return binascii.hexlify(file.read(KEYS * 16))
+
+def print_row(b)
+  print("  0x{0}, 0x{1}, 0x{2}, 0x{3}, 0x{4}, 0x{5}, 0x{6}, 0x{7}, 0x{8}, 0x{9},
+0x{10}, 0x{11}, 0x{12}, 0x{13}, 0x{14}, 0x{15},".format(*b))
+
+
+digits = digits_from_pi()
+#digits = digits_from_urandom()
+
+print("namespace {")
+print("static constexpr size_t kKeyBytes = {0};\n".format(KEYS * 16))
+print("}")
+
+print("alignas(16) const unsigned char kRandenRoundKeysBE[kKeyBytes] = {")
+
+for i, u16 in zip(range(KEYS), chunks(digits, 32)):
+  b = list(chunks(u16, 2))
+  print_row(b)
+
+print("};")
+
+print("alignas(16) const unsigned char kRandenRoundKeys[kKeyBytes] = {")
+
+for i, u16 in zip(range(KEYS), chunks(digits, 32)):
+  b = list(chunks(u16, 2))
+  b.reverse()
+  print_row(b)
+
+print("};")
+
+EOF
+
+*/
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace random_internal {
+namespace {
+static constexpr size_t kKeyBytes = 2176;
+}
+
+alignas(16) const unsigned char kRandenRoundKeysBE[kKeyBytes] = {
+    0x24, 0x3F, 0x6A, 0x88, 0x85, 0xA3, 0x08, 0xD3, 0x13, 0x19, 0x8A, 0x2E,
+    0x03, 0x70, 0x73, 0x44, 0xA4, 0x09, 0x38, 0x22, 0x29, 0x9F, 0x31, 0xD0,
+    0x08, 0x2E, 0xFA, 0x98, 0xEC, 0x4E, 0x6C, 0x89, 0x45, 0x28, 0x21, 0xE6,
+    0x38, 0xD0, 0x13, 0x77, 0xBE, 0x54, 0x66, 0xCF, 0x34, 0xE9, 0x0C, 0x6C,
+    0xC0, 0xAC, 0x29, 0xB7, 0xC9, 0x7C, 0x50, 0xDD, 0x3F, 0x84, 0xD5, 0xB5,
+    0xB5, 0x47, 0x09, 0x17, 0x92, 0x16, 0xD5, 0xD9, 0x89, 0x79, 0xFB, 0x1B,
+    0xD1, 0x31, 0x0B, 0xA6, 0x98, 0xDF, 0xB5, 0xAC, 0x2F, 0xFD, 0x72, 0xDB,
+    0xD0, 0x1A, 0xDF, 0xB7, 0xB8, 0xE1, 0xAF, 0xED, 0x6A, 0x26, 0x7E, 0x96,
+    0xBA, 0x7C, 0x90, 0x45, 0xF1, 0x2C, 0x7F, 0x99, 0x24, 0xA1, 0x99, 0x47,
+    0xB3, 0x91, 0x6C, 0xF7, 0x08, 0x01, 0xF2, 0xE2, 0x85, 0x8E, 0xFC, 0x16,
+    0x63, 0x69, 0x20, 0xD8, 0x71, 0x57, 0x4E, 0x69, 0xA4, 0x58, 0xFE, 0xA3,
+    0xF4, 0x93, 0x3D, 0x7E, 0x0D, 0x95, 0x74, 0x8F, 0x72, 0x8E, 0xB6, 0x58,
+    0x71, 0x8B, 0xCD, 0x58, 0x82, 0x15, 0x4A, 0xEE, 0x7B, 0x54, 0xA4, 0x1D,
+    0xC2, 0x5A, 0x59, 0xB5, 0x9C, 0x30, 0xD5, 0x39, 0x2A, 0xF2, 0x60, 0x13,
+    0xC5, 0xD1, 0xB0, 0x23, 0x28, 0x60, 0x85, 0xF0, 0xCA, 0x41, 0x79, 0x18,
+    0xB8, 0xDB, 0x38, 0xEF, 0x8E, 0x79, 0xDC, 0xB0, 0x60, 0x3A, 0x18, 0x0E,
+    0x6C, 0x9E, 0x0E, 0x8B, 0xB0, 0x1E, 0x8A, 0x3E, 0xD7, 0x15, 0x77, 0xC1,
+    0xBD, 0x31, 0x4B, 0x27, 0x78, 0xAF, 0x2F, 0xDA, 0x55, 0x60, 0x5C, 0x60,
+    0xE6, 0x55, 0x25, 0xF3, 0xAA, 0x55, 0xAB, 0x94, 0x57, 0x48, 0x98, 0x62,
+    0x63, 0xE8, 0x14, 0x40, 0x55, 0xCA, 0x39, 0x6A, 0x2A, 0xAB, 0x10, 0xB6,
+    0xB4, 0xCC, 0x5C, 0x34, 0x11, 0x41, 0xE8, 0xCE, 0xA1, 0x54, 0x86, 0xAF,
+    0x7C, 0x72, 0xE9, 0x93, 0xB3, 0xEE, 0x14, 0x11, 0x63, 0x6F, 0xBC, 0x2A,
+    0x2B, 0xA9, 0xC5, 0x5D, 0x74, 0x18, 0x31, 0xF6, 0xCE, 0x5C, 0x3E, 0x16,
+    0x9B, 0x87, 0x93, 0x1E, 0xAF, 0xD6, 0xBA, 0x33, 0x6C, 0x24, 0xCF, 0x5C,
+    0x7A, 0x32, 0x53, 0x81, 0x28, 0x95, 0x86, 0x77, 0x3B, 0x8F, 0x48, 0x98,
+    0x6B, 0x4B, 0xB9, 0xAF, 0xC4, 0xBF, 0xE8, 0x1B, 0x66, 0x28, 0x21, 0x93,
+    0x61, 0xD8, 0x09, 0xCC, 0xFB, 0x21, 0xA9, 0x91, 0x48, 0x7C, 0xAC, 0x60,
+    0x5D, 0xEC, 0x80, 0x32, 0xEF, 0x84, 0x5D, 0x5D, 0xE9, 0x85, 0x75, 0xB1,
+    0xDC, 0x26, 0x23, 0x02, 0xEB, 0x65, 0x1B, 0x88, 0x23, 0x89, 0x3E, 0x81,
+    0xD3, 0x96, 0xAC, 0xC5, 0x0F, 0x6D, 0x6F, 0xF3, 0x83, 0xF4, 0x42, 0x39,
+    0x2E, 0x0B, 0x44, 0x82, 0xA4, 0x84, 0x20, 0x04, 0x69, 0xC8, 0xF0, 0x4A,
+    0x9E, 0x1F, 0x9B, 0x5E, 0x21, 0xC6, 0x68, 0x42, 0xF6, 0xE9, 0x6C, 0x9A,
+    0x67, 0x0C, 0x9C, 0x61, 0xAB, 0xD3, 0x88, 0xF0, 0x6A, 0x51, 0xA0, 0xD2,
+    0xD8, 0x54, 0x2F, 0x68, 0x96, 0x0F, 0xA7, 0x28, 0xAB, 0x51, 0x33, 0xA3,
+    0x6E, 0xEF, 0x0B, 0x6C, 0x13, 0x7A, 0x3B, 0xE4, 0xBA, 0x3B, 0xF0, 0x50,
+    0x7E, 0xFB, 0x2A, 0x98, 0xA1, 0xF1, 0x65, 0x1D, 0x39, 0xAF, 0x01, 0x76,
+    0x66, 0xCA, 0x59, 0x3E, 0x82, 0x43, 0x0E, 0x88, 0x8C, 0xEE, 0x86, 0x19,
+    0x45, 0x6F, 0x9F, 0xB4, 0x7D, 0x84, 0xA5, 0xC3, 0x3B, 0x8B, 0x5E, 0xBE,
+    0xE0, 0x6F, 0x75, 0xD8, 0x85, 0xC1, 0x20, 0x73, 0x40, 0x1A, 0x44, 0x9F,
+    0x56, 0xC1, 0x6A, 0xA6, 0x4E, 0xD3, 0xAA, 0x62, 0x36, 0x3F, 0x77, 0x06,
+    0x1B, 0xFE, 0xDF, 0x72, 0x42, 0x9B, 0x02, 0x3D, 0x37, 0xD0, 0xD7, 0x24,
+    0xD0, 0x0A, 0x12, 0x48, 0xDB, 0x0F, 0xEA, 0xD3, 0x49, 0xF1, 0xC0, 0x9B,
+    0x07, 0x53, 0x72, 0xC9, 0x80, 0x99, 0x1B, 0x7B, 0x25, 0xD4, 0x79, 0xD8,
+    0xF6, 0xE8, 0xDE, 0xF7, 0xE3, 0xFE, 0x50, 0x1A, 0xB6, 0x79, 0x4C, 0x3B,
+    0x97, 0x6C, 0xE0, 0xBD, 0x04, 0xC0, 0x06, 0xBA, 0xC1, 0xA9, 0x4F, 0xB6,
+    0x40, 0x9F, 0x60, 0xC4, 0x5E, 0x5C, 0x9E, 0xC2, 0x19, 0x6A, 0x24, 0x63,
+    0x68, 0xFB, 0x6F, 0xAF, 0x3E, 0x6C, 0x53, 0xB5, 0x13, 0x39, 0xB2, 0xEB,
+    0x3B, 0x52, 0xEC, 0x6F, 0x6D, 0xFC, 0x51, 0x1F, 0x9B, 0x30, 0x95, 0x2C,
+    0xCC, 0x81, 0x45, 0x44, 0xAF, 0x5E, 0xBD, 0x09, 0xBE, 0xE3, 0xD0, 0x04,
+    0xDE, 0x33, 0x4A, 0xFD, 0x66, 0x0F, 0x28, 0x07, 0x19, 0x2E, 0x4B, 0xB3,
+    0xC0, 0xCB, 0xA8, 0x57, 0x45, 0xC8, 0x74, 0x0F, 0xD2, 0x0B, 0x5F, 0x39,
+    0xB9, 0xD3, 0xFB, 0xDB, 0x55, 0x79, 0xC0, 0xBD, 0x1A, 0x60, 0x32, 0x0A,
+    0xD6, 0xA1, 0x00, 0xC6, 0x40, 0x2C, 0x72, 0x79, 0x67, 0x9F, 0x25, 0xFE,
+    0xFB, 0x1F, 0xA3, 0xCC, 0x8E, 0xA5, 0xE9, 0xF8, 0xDB, 0x32, 0x22, 0xF8,
+    0x3C, 0x75, 0x16, 0xDF, 0xFD, 0x61, 0x6B, 0x15, 0x2F, 0x50, 0x1E, 0xC8,
+    0xAD, 0x05, 0x52, 0xAB, 0x32, 0x3D, 0xB5, 0xFA, 0xFD, 0x23, 0x87, 0x60,
+    0x53, 0x31, 0x7B, 0x48, 0x3E, 0x00, 0xDF, 0x82, 0x9E, 0x5C, 0x57, 0xBB,
+    0xCA, 0x6F, 0x8C, 0xA0, 0x1A, 0x87, 0x56, 0x2E, 0xDF, 0x17, 0x69, 0xDB,
+    0xD5, 0x42, 0xA8, 0xF6, 0x28, 0x7E, 0xFF, 0xC3, 0xAC, 0x67, 0x32, 0xC6,
+    0x8C, 0x4F, 0x55, 0x73, 0x69, 0x5B, 0x27, 0xB0, 0xBB, 0xCA, 0x58, 0xC8,
+    0xE1, 0xFF, 0xA3, 0x5D, 0xB8, 0xF0, 0x11, 0xA0, 0x10, 0xFA, 0x3D, 0x98,
+    0xFD, 0x21, 0x83, 0xB8, 0x4A, 0xFC, 0xB5, 0x6C, 0x2D, 0xD1, 0xD3, 0x5B,
+    0x9A, 0x53, 0xE4, 0x79, 0xB6, 0xF8, 0x45, 0x65, 0xD2, 0x8E, 0x49, 0xBC,
+    0x4B, 0xFB, 0x97, 0x90, 0xE1, 0xDD, 0xF2, 0xDA, 0xA4, 0xCB, 0x7E, 0x33,
+    0x62, 0xFB, 0x13, 0x41, 0xCE, 0xE4, 0xC6, 0xE8, 0xEF, 0x20, 0xCA, 0xDA,
+    0x36, 0x77, 0x4C, 0x01, 0xD0, 0x7E, 0x9E, 0xFE, 0x2B, 0xF1, 0x1F, 0xB4,
+    0x95, 0xDB, 0xDA, 0x4D, 0xAE, 0x90, 0x91, 0x98, 0xEA, 0xAD, 0x8E, 0x71,
+    0x6B, 0x93, 0xD5, 0xA0, 0xD0, 0x8E, 0xD1, 0xD0, 0xAF, 0xC7, 0x25, 0xE0,
+    0x8E, 0x3C, 0x5B, 0x2F, 0x8E, 0x75, 0x94, 0xB7, 0x8F, 0xF6, 0xE2, 0xFB,
+    0xF2, 0x12, 0x2B, 0x64, 0x88, 0x88, 0xB8, 0x12, 0x90, 0x0D, 0xF0, 0x1C,
+    0x4F, 0xAD, 0x5E, 0xA0, 0x68, 0x8F, 0xC3, 0x1C, 0xD1, 0xCF, 0xF1, 0x91,
+    0xB3, 0xA8, 0xC1, 0xAD, 0x2F, 0x2F, 0x22, 0x18, 0xBE, 0x0E, 0x17, 0x77,
+    0xEA, 0x75, 0x2D, 0xFE, 0x8B, 0x02, 0x1F, 0xA1, 0xE5, 0xA0, 0xCC, 0x0F,
+    0xB5, 0x6F, 0x74, 0xE8, 0x18, 0xAC, 0xF3, 0xD6, 0xCE, 0x89, 0xE2, 0x99,
+    0xB4, 0xA8, 0x4F, 0xE0, 0xFD, 0x13, 0xE0, 0xB7, 0x7C, 0xC4, 0x3B, 0x81,
+    0xD2, 0xAD, 0xA8, 0xD9, 0x16, 0x5F, 0xA2, 0x66, 0x80, 0x95, 0x77, 0x05,
+    0x93, 0xCC, 0x73, 0x14, 0x21, 0x1A, 0x14, 0x77, 0xE6, 0xAD, 0x20, 0x65,
+    0x77, 0xB5, 0xFA, 0x86, 0xC7, 0x54, 0x42, 0xF5, 0xFB, 0x9D, 0x35, 0xCF,
+    0xEB, 0xCD, 0xAF, 0x0C, 0x7B, 0x3E, 0x89, 0xA0, 0xD6, 0x41, 0x1B, 0xD3,
+    0xAE, 0x1E, 0x7E, 0x49, 0x00, 0x25, 0x0E, 0x2D, 0x20, 0x71, 0xB3, 0x5E,
+    0x22, 0x68, 0x00, 0xBB, 0x57, 0xB8, 0xE0, 0xAF, 0x24, 0x64, 0x36, 0x9B,
+    0xF0, 0x09, 0xB9, 0x1E, 0x55, 0x63, 0x91, 0x1D, 0x59, 0xDF, 0xA6, 0xAA,
+    0x78, 0xC1, 0x43, 0x89, 0xD9, 0x5A, 0x53, 0x7F, 0x20, 0x7D, 0x5B, 0xA2,
+    0x02, 0xE5, 0xB9, 0xC5, 0x83, 0x26, 0x03, 0x76, 0x62, 0x95, 0xCF, 0xA9,
+    0x11, 0xC8, 0x19, 0x68, 0x4E, 0x73, 0x4A, 0x41, 0xB3, 0x47, 0x2D, 0xCA,
+    0x7B, 0x14, 0xA9, 0x4A, 0x1B, 0x51, 0x00, 0x52, 0x9A, 0x53, 0x29, 0x15,
+    0xD6, 0x0F, 0x57, 0x3F, 0xBC, 0x9B, 0xC6, 0xE4, 0x2B, 0x60, 0xA4, 0x76,
+    0x81, 0xE6, 0x74, 0x00, 0x08, 0xBA, 0x6F, 0xB5, 0x57, 0x1B, 0xE9, 0x1F,
+    0xF2, 0x96, 0xEC, 0x6B, 0x2A, 0x0D, 0xD9, 0x15, 0xB6, 0x63, 0x65, 0x21,
+    0xE7, 0xB9, 0xF9, 0xB6, 0xFF, 0x34, 0x05, 0x2E, 0xC5, 0x85, 0x56, 0x64,
+    0x53, 0xB0, 0x2D, 0x5D, 0xA9, 0x9F, 0x8F, 0xA1, 0x08, 0xBA, 0x47, 0x99,
+    0x6E, 0x85, 0x07, 0x6A, 0x4B, 0x7A, 0x70, 0xE9, 0xB5, 0xB3, 0x29, 0x44,
+    0xDB, 0x75, 0x09, 0x2E, 0xC4, 0x19, 0x26, 0x23, 0xAD, 0x6E, 0xA6, 0xB0,
+    0x49, 0xA7, 0xDF, 0x7D, 0x9C, 0xEE, 0x60, 0xB8, 0x8F, 0xED, 0xB2, 0x66,
+    0xEC, 0xAA, 0x8C, 0x71, 0x69, 0x9A, 0x18, 0xFF, 0x56, 0x64, 0x52, 0x6C,
+    0xC2, 0xB1, 0x9E, 0xE1, 0x19, 0x36, 0x02, 0xA5, 0x75, 0x09, 0x4C, 0x29,
+    0xA0, 0x59, 0x13, 0x40, 0xE4, 0x18, 0x3A, 0x3E, 0x3F, 0x54, 0x98, 0x9A,
+    0x5B, 0x42, 0x9D, 0x65, 0x6B, 0x8F, 0xE4, 0xD6, 0x99, 0xF7, 0x3F, 0xD6,
+    0xA1, 0xD2, 0x9C, 0x07, 0xEF, 0xE8, 0x30, 0xF5, 0x4D, 0x2D, 0x38, 0xE6,
+    0xF0, 0x25, 0x5D, 0xC1, 0x4C, 0xDD, 0x20, 0x86, 0x84, 0x70, 0xEB, 0x26,
+    0x63, 0x82, 0xE9, 0xC6, 0x02, 0x1E, 0xCC, 0x5E, 0x09, 0x68, 0x6B, 0x3F,
+    0x3E, 0xBA, 0xEF, 0xC9, 0x3C, 0x97, 0x18, 0x14, 0x6B, 0x6A, 0x70, 0xA1,
+    0x68, 0x7F, 0x35, 0x84, 0x52, 0xA0, 0xE2, 0x86, 0xB7, 0x9C, 0x53, 0x05,
+    0xAA, 0x50, 0x07, 0x37, 0x3E, 0x07, 0x84, 0x1C, 0x7F, 0xDE, 0xAE, 0x5C,
+    0x8E, 0x7D, 0x44, 0xEC, 0x57, 0x16, 0xF2, 0xB8, 0xB0, 0x3A, 0xDA, 0x37,
+    0xF0, 0x50, 0x0C, 0x0D, 0xF0, 0x1C, 0x1F, 0x04, 0x02, 0x00, 0xB3, 0xFF,
+    0xAE, 0x0C, 0xF5, 0x1A, 0x3C, 0xB5, 0x74, 0xB2, 0x25, 0x83, 0x7A, 0x58,
+    0xDC, 0x09, 0x21, 0xBD, 0xD1, 0x91, 0x13, 0xF9, 0x7C, 0xA9, 0x2F, 0xF6,
+    0x94, 0x32, 0x47, 0x73, 0x22, 0xF5, 0x47, 0x01, 0x3A, 0xE5, 0xE5, 0x81,
+    0x37, 0xC2, 0xDA, 0xDC, 0xC8, 0xB5, 0x76, 0x34, 0x9A, 0xF3, 0xDD, 0xA7,
+    0xA9, 0x44, 0x61, 0x46, 0x0F, 0xD0, 0x03, 0x0E, 0xEC, 0xC8, 0xC7, 0x3E,
+    0xA4, 0x75, 0x1E, 0x41, 0xE2, 0x38, 0xCD, 0x99, 0x3B, 0xEA, 0x0E, 0x2F,
+    0x32, 0x80, 0xBB, 0xA1, 0x18, 0x3E, 0xB3, 0x31, 0x4E, 0x54, 0x8B, 0x38,
+    0x4F, 0x6D, 0xB9, 0x08, 0x6F, 0x42, 0x0D, 0x03, 0xF6, 0x0A, 0x04, 0xBF,
+    0x2C, 0xB8, 0x12, 0x90, 0x24, 0x97, 0x7C, 0x79, 0x56, 0x79, 0xB0, 0x72,
+    0xBC, 0xAF, 0x89, 0xAF, 0xDE, 0x9A, 0x77, 0x1F, 0xD9, 0x93, 0x08, 0x10,
+    0xB3, 0x8B, 0xAE, 0x12, 0xDC, 0xCF, 0x3F, 0x2E, 0x55, 0x12, 0x72, 0x1F,
+    0x2E, 0x6B, 0x71, 0x24, 0x50, 0x1A, 0xDD, 0xE6, 0x9F, 0x84, 0xCD, 0x87,
+    0x7A, 0x58, 0x47, 0x18, 0x74, 0x08, 0xDA, 0x17, 0xBC, 0x9F, 0x9A, 0xBC,
+    0xE9, 0x4B, 0x7D, 0x8C, 0xEC, 0x7A, 0xEC, 0x3A, 0xDB, 0x85, 0x1D, 0xFA,
+    0x63, 0x09, 0x43, 0x66, 0xC4, 0x64, 0xC3, 0xD2, 0xEF, 0x1C, 0x18, 0x47,
+    0x32, 0x15, 0xD8, 0x08, 0xDD, 0x43, 0x3B, 0x37, 0x24, 0xC2, 0xBA, 0x16,
+    0x12, 0xA1, 0x4D, 0x43, 0x2A, 0x65, 0xC4, 0x51, 0x50, 0x94, 0x00, 0x02,
+    0x13, 0x3A, 0xE4, 0xDD, 0x71, 0xDF, 0xF8, 0x9E, 0x10, 0x31, 0x4E, 0x55,
+    0x81, 0xAC, 0x77, 0xD6, 0x5F, 0x11, 0x19, 0x9B, 0x04, 0x35, 0x56, 0xF1,
+    0xD7, 0xA3, 0xC7, 0x6B, 0x3C, 0x11, 0x18, 0x3B, 0x59, 0x24, 0xA5, 0x09,
+    0xF2, 0x8F, 0xE6, 0xED, 0x97, 0xF1, 0xFB, 0xFA, 0x9E, 0xBA, 0xBF, 0x2C,
+    0x1E, 0x15, 0x3C, 0x6E, 0x86, 0xE3, 0x45, 0x70, 0xEA, 0xE9, 0x6F, 0xB1,
+    0x86, 0x0E, 0x5E, 0x0A, 0x5A, 0x3E, 0x2A, 0xB3, 0x77, 0x1F, 0xE7, 0x1C,
+    0x4E, 0x3D, 0x06, 0xFA, 0x29, 0x65, 0xDC, 0xB9, 0x99, 0xE7, 0x1D, 0x0F,
+    0x80, 0x3E, 0x89, 0xD6, 0x52, 0x66, 0xC8, 0x25, 0x2E, 0x4C, 0xC9, 0x78,
+    0x9C, 0x10, 0xB3, 0x6A, 0xC6, 0x15, 0x0E, 0xBA, 0x94, 0xE2, 0xEA, 0x78,
+    0xA6, 0xFC, 0x3C, 0x53, 0x1E, 0x0A, 0x2D, 0xF4, 0xF2, 0xF7, 0x4E, 0xA7,
+    0x36, 0x1D, 0x2B, 0x3D, 0x19, 0x39, 0x26, 0x0F, 0x19, 0xC2, 0x79, 0x60,
+    0x52, 0x23, 0xA7, 0x08, 0xF7, 0x13, 0x12, 0xB6, 0xEB, 0xAD, 0xFE, 0x6E,
+    0xEA, 0xC3, 0x1F, 0x66, 0xE3, 0xBC, 0x45, 0x95, 0xA6, 0x7B, 0xC8, 0x83,
+    0xB1, 0x7F, 0x37, 0xD1, 0x01, 0x8C, 0xFF, 0x28, 0xC3, 0x32, 0xDD, 0xEF,
+    0xBE, 0x6C, 0x5A, 0xA5, 0x65, 0x58, 0x21, 0x85, 0x68, 0xAB, 0x97, 0x02,
+    0xEE, 0xCE, 0xA5, 0x0F, 0xDB, 0x2F, 0x95, 0x3B, 0x2A, 0xEF, 0x7D, 0xAD,
+    0x5B, 0x6E, 0x2F, 0x84, 0x15, 0x21, 0xB6, 0x28, 0x29, 0x07, 0x61, 0x70,
+    0xEC, 0xDD, 0x47, 0x75, 0x61, 0x9F, 0x15, 0x10, 0x13, 0xCC, 0xA8, 0x30,
+    0xEB, 0x61, 0xBD, 0x96, 0x03, 0x34, 0xFE, 0x1E, 0xAA, 0x03, 0x63, 0xCF,
+    0xB5, 0x73, 0x5C, 0x90, 0x4C, 0x70, 0xA2, 0x39, 0xD5, 0x9E, 0x9E, 0x0B,
+    0xCB, 0xAA, 0xDE, 0x14, 0xEE, 0xCC, 0x86, 0xBC, 0x60, 0x62, 0x2C, 0xA7,
+    0x9C, 0xAB, 0x5C, 0xAB, 0xB2, 0xF3, 0x84, 0x6E, 0x64, 0x8B, 0x1E, 0xAF,
+    0x19, 0xBD, 0xF0, 0xCA, 0xA0, 0x23, 0x69, 0xB9, 0x65, 0x5A, 0xBB, 0x50,
+    0x40, 0x68, 0x5A, 0x32, 0x3C, 0x2A, 0xB4, 0xB3, 0x31, 0x9E, 0xE9, 0xD5,
+    0xC0, 0x21, 0xB8, 0xF7, 0x9B, 0x54, 0x0B, 0x19, 0x87, 0x5F, 0xA0, 0x99,
+    0x95, 0xF7, 0x99, 0x7E, 0x62, 0x3D, 0x7D, 0xA8, 0xF8, 0x37, 0x88, 0x9A,
+    0x97, 0xE3, 0x2D, 0x77, 0x11, 0xED, 0x93, 0x5F, 0x16, 0x68, 0x12, 0x81,
+    0x0E, 0x35, 0x88, 0x29, 0xC7, 0xE6, 0x1F, 0xD6, 0x96, 0xDE, 0xDF, 0xA1,
+    0x78, 0x58, 0xBA, 0x99, 0x57, 0xF5, 0x84, 0xA5, 0x1B, 0x22, 0x72, 0x63,
+    0x9B, 0x83, 0xC3, 0xFF, 0x1A, 0xC2, 0x46, 0x96, 0xCD, 0xB3, 0x0A, 0xEB,
+    0x53, 0x2E, 0x30, 0x54, 0x8F, 0xD9, 0x48, 0xE4, 0x6D, 0xBC, 0x31, 0x28,
+    0x58, 0xEB, 0xF2, 0xEF, 0x34, 0xC6, 0xFF, 0xEA, 0xFE, 0x28, 0xED, 0x61,
+    0xEE, 0x7C, 0x3C, 0x73, 0x5D, 0x4A, 0x14, 0xD9, 0xE8, 0x64, 0xB7, 0xE3,
+    0x42, 0x10, 0x5D, 0x14, 0x20, 0x3E, 0x13, 0xE0, 0x45, 0xEE, 0xE2, 0xB6,
+    0xA3, 0xAA, 0xAB, 0xEA, 0xDB, 0x6C, 0x4F, 0x15, 0xFA, 0xCB, 0x4F, 0xD0,
+    0xC7, 0x42, 0xF4, 0x42, 0xEF, 0x6A, 0xBB, 0xB5, 0x65, 0x4F, 0x3B, 0x1D,
+    0x41, 0xCD, 0x21, 0x05, 0xD8, 0x1E, 0x79, 0x9E, 0x86, 0x85, 0x4D, 0xC7,
+    0xE4, 0x4B, 0x47, 0x6A, 0x3D, 0x81, 0x62, 0x50, 0xCF, 0x62, 0xA1, 0xF2,
+    0x5B, 0x8D, 0x26, 0x46, 0xFC, 0x88, 0x83, 0xA0, 0xC1, 0xC7, 0xB6, 0xA3,
+    0x7F, 0x15, 0x24, 0xC3, 0x69, 0xCB, 0x74, 0x92, 0x47, 0x84, 0x8A, 0x0B,
+    0x56, 0x92, 0xB2, 0x85, 0x09, 0x5B, 0xBF, 0x00, 0xAD, 0x19, 0x48, 0x9D,
+    0x14, 0x62, 0xB1, 0x74, 0x23, 0x82, 0x0D, 0x00, 0x58, 0x42, 0x8D, 0x2A,
+    0x0C, 0x55, 0xF5, 0xEA, 0x1D, 0xAD, 0xF4, 0x3E, 0x23, 0x3F, 0x70, 0x61,
+    0x33, 0x72, 0xF0, 0x92, 0x8D, 0x93, 0x7E, 0x41, 0xD6, 0x5F, 0xEC, 0xF1,
+    0x6C, 0x22, 0x3B, 0xDB, 0x7C, 0xDE, 0x37, 0x59, 0xCB, 0xEE, 0x74, 0x60,
+    0x40, 0x85, 0xF2, 0xA7, 0xCE, 0x77, 0x32, 0x6E, 0xA6, 0x07, 0x80, 0x84,
+    0x19, 0xF8, 0x50, 0x9E, 0xE8, 0xEF, 0xD8, 0x55, 0x61, 0xD9, 0x97, 0x35,
+    0xA9, 0x69, 0xA7, 0xAA, 0xC5, 0x0C, 0x06, 0xC2, 0x5A, 0x04, 0xAB, 0xFC,
+    0x80, 0x0B, 0xCA, 0xDC, 0x9E, 0x44, 0x7A, 0x2E, 0xC3, 0x45, 0x34, 0x84,
+    0xFD, 0xD5, 0x67, 0x05, 0x0E, 0x1E, 0x9E, 0xC9, 0xDB, 0x73, 0xDB, 0xD3,
+    0x10, 0x55, 0x88, 0xCD, 0x67, 0x5F, 0xDA, 0x79, 0xE3, 0x67, 0x43, 0x40,
+    0xC5, 0xC4, 0x34, 0x65, 0x71, 0x3E, 0x38, 0xD8, 0x3D, 0x28, 0xF8, 0x9E,
+    0xF1, 0x6D, 0xFF, 0x20, 0x15, 0x3E, 0x21, 0xE7, 0x8F, 0xB0, 0x3D, 0x4A,
+    0xE6, 0xE3, 0x9F, 0x2B, 0xDB, 0x83, 0xAD, 0xF7, 0xE9, 0x3D, 0x5A, 0x68,
+    0x94, 0x81, 0x40, 0xF7, 0xF6, 0x4C, 0x26, 0x1C, 0x94, 0x69, 0x29, 0x34,
+    0x41, 0x15, 0x20, 0xF7, 0x76, 0x02, 0xD4, 0xF7, 0xBC, 0xF4, 0x6B, 0x2E,
+    0xD4, 0xA1, 0x00, 0x68, 0xD4, 0x08, 0x24, 0x71, 0x33, 0x20, 0xF4, 0x6A,
+    0x43, 0xB7, 0xD4, 0xB7, 0x50, 0x00, 0x61, 0xAF, 0x1E, 0x39, 0xF6, 0x2E,
+    0x97, 0x24, 0x45, 0x46,
+};
+
+alignas(16) const unsigned char kRandenRoundKeys[kKeyBytes] = {
+    0x44, 0x73, 0x70, 0x03, 0x2E, 0x8A, 0x19, 0x13, 0xD3, 0x08, 0xA3, 0x85,
+    0x88, 0x6A, 0x3F, 0x24, 0x89, 0x6C, 0x4E, 0xEC, 0x98, 0xFA, 0x2E, 0x08,
+    0xD0, 0x31, 0x9F, 0x29, 0x22, 0x38, 0x09, 0xA4, 0x6C, 0x0C, 0xE9, 0x34,
+    0xCF, 0x66, 0x54, 0xBE, 0x77, 0x13, 0xD0, 0x38, 0xE6, 0x21, 0x28, 0x45,
+    0x17, 0x09, 0x47, 0xB5, 0xB5, 0xD5, 0x84, 0x3F, 0xDD, 0x50, 0x7C, 0xC9,
+    0xB7, 0x29, 0xAC, 0xC0, 0xAC, 0xB5, 0xDF, 0x98, 0xA6, 0x0B, 0x31, 0xD1,
+    0x1B, 0xFB, 0x79, 0x89, 0xD9, 0xD5, 0x16, 0x92, 0x96, 0x7E, 0x26, 0x6A,
+    0xED, 0xAF, 0xE1, 0xB8, 0xB7, 0xDF, 0x1A, 0xD0, 0xDB, 0x72, 0xFD, 0x2F,
+    0xF7, 0x6C, 0x91, 0xB3, 0x47, 0x99, 0xA1, 0x24, 0x99, 0x7F, 0x2C, 0xF1,
+    0x45, 0x90, 0x7C, 0xBA, 0x69, 0x4E, 0x57, 0x71, 0xD8, 0x20, 0x69, 0x63,
+    0x16, 0xFC, 0x8E, 0x85, 0xE2, 0xF2, 0x01, 0x08, 0x58, 0xB6, 0x8E, 0x72,
+    0x8F, 0x74, 0x95, 0x0D, 0x7E, 0x3D, 0x93, 0xF4, 0xA3, 0xFE, 0x58, 0xA4,
+    0xB5, 0x59, 0x5A, 0xC2, 0x1D, 0xA4, 0x54, 0x7B, 0xEE, 0x4A, 0x15, 0x82,
+    0x58, 0xCD, 0x8B, 0x71, 0xF0, 0x85, 0x60, 0x28, 0x23, 0xB0, 0xD1, 0xC5,
+    0x13, 0x60, 0xF2, 0x2A, 0x39, 0xD5, 0x30, 0x9C, 0x0E, 0x18, 0x3A, 0x60,
+    0xB0, 0xDC, 0x79, 0x8E, 0xEF, 0x38, 0xDB, 0xB8, 0x18, 0x79, 0x41, 0xCA,
+    0x27, 0x4B, 0x31, 0xBD, 0xC1, 0x77, 0x15, 0xD7, 0x3E, 0x8A, 0x1E, 0xB0,
+    0x8B, 0x0E, 0x9E, 0x6C, 0x94, 0xAB, 0x55, 0xAA, 0xF3, 0x25, 0x55, 0xE6,
+    0x60, 0x5C, 0x60, 0x55, 0xDA, 0x2F, 0xAF, 0x78, 0xB6, 0x10, 0xAB, 0x2A,
+    0x6A, 0x39, 0xCA, 0x55, 0x40, 0x14, 0xE8, 0x63, 0x62, 0x98, 0x48, 0x57,
+    0x93, 0xE9, 0x72, 0x7C, 0xAF, 0x86, 0x54, 0xA1, 0xCE, 0xE8, 0x41, 0x11,
+    0x34, 0x5C, 0xCC, 0xB4, 0xF6, 0x31, 0x18, 0x74, 0x5D, 0xC5, 0xA9, 0x2B,
+    0x2A, 0xBC, 0x6F, 0x63, 0x11, 0x14, 0xEE, 0xB3, 0x5C, 0xCF, 0x24, 0x6C,
+    0x33, 0xBA, 0xD6, 0xAF, 0x1E, 0x93, 0x87, 0x9B, 0x16, 0x3E, 0x5C, 0xCE,
+    0xAF, 0xB9, 0x4B, 0x6B, 0x98, 0x48, 0x8F, 0x3B, 0x77, 0x86, 0x95, 0x28,
+    0x81, 0x53, 0x32, 0x7A, 0x91, 0xA9, 0x21, 0xFB, 0xCC, 0x09, 0xD8, 0x61,
+    0x93, 0x21, 0x28, 0x66, 0x1B, 0xE8, 0xBF, 0xC4, 0xB1, 0x75, 0x85, 0xE9,
+    0x5D, 0x5D, 0x84, 0xEF, 0x32, 0x80, 0xEC, 0x5D, 0x60, 0xAC, 0x7C, 0x48,
+    0xC5, 0xAC, 0x96, 0xD3, 0x81, 0x3E, 0x89, 0x23, 0x88, 0x1B, 0x65, 0xEB,
+    0x02, 0x23, 0x26, 0xDC, 0x04, 0x20, 0x84, 0xA4, 0x82, 0x44, 0x0B, 0x2E,
+    0x39, 0x42, 0xF4, 0x83, 0xF3, 0x6F, 0x6D, 0x0F, 0x9A, 0x6C, 0xE9, 0xF6,
+    0x42, 0x68, 0xC6, 0x21, 0x5E, 0x9B, 0x1F, 0x9E, 0x4A, 0xF0, 0xC8, 0x69,
+    0x68, 0x2F, 0x54, 0xD8, 0xD2, 0xA0, 0x51, 0x6A, 0xF0, 0x88, 0xD3, 0xAB,
+    0x61, 0x9C, 0x0C, 0x67, 0xE4, 0x3B, 0x7A, 0x13, 0x6C, 0x0B, 0xEF, 0x6E,
+    0xA3, 0x33, 0x51, 0xAB, 0x28, 0xA7, 0x0F, 0x96, 0x76, 0x01, 0xAF, 0x39,
+    0x1D, 0x65, 0xF1, 0xA1, 0x98, 0x2A, 0xFB, 0x7E, 0x50, 0xF0, 0x3B, 0xBA,
+    0xB4, 0x9F, 0x6F, 0x45, 0x19, 0x86, 0xEE, 0x8C, 0x88, 0x0E, 0x43, 0x82,
+    0x3E, 0x59, 0xCA, 0x66, 0x73, 0x20, 0xC1, 0x85, 0xD8, 0x75, 0x6F, 0xE0,
+    0xBE, 0x5E, 0x8B, 0x3B, 0xC3, 0xA5, 0x84, 0x7D, 0x06, 0x77, 0x3F, 0x36,
+    0x62, 0xAA, 0xD3, 0x4E, 0xA6, 0x6A, 0xC1, 0x56, 0x9F, 0x44, 0x1A, 0x40,
+    0x48, 0x12, 0x0A, 0xD0, 0x24, 0xD7, 0xD0, 0x37, 0x3D, 0x02, 0x9B, 0x42,
+    0x72, 0xDF, 0xFE, 0x1B, 0x7B, 0x1B, 0x99, 0x80, 0xC9, 0x72, 0x53, 0x07,
+    0x9B, 0xC0, 0xF1, 0x49, 0xD3, 0xEA, 0x0F, 0xDB, 0x3B, 0x4C, 0x79, 0xB6,
+    0x1A, 0x50, 0xFE, 0xE3, 0xF7, 0xDE, 0xE8, 0xF6, 0xD8, 0x79, 0xD4, 0x25,
+    0xC4, 0x60, 0x9F, 0x40, 0xB6, 0x4F, 0xA9, 0xC1, 0xBA, 0x06, 0xC0, 0x04,
+    0xBD, 0xE0, 0x6C, 0x97, 0xB5, 0x53, 0x6C, 0x3E, 0xAF, 0x6F, 0xFB, 0x68,
+    0x63, 0x24, 0x6A, 0x19, 0xC2, 0x9E, 0x5C, 0x5E, 0x2C, 0x95, 0x30, 0x9B,
+    0x1F, 0x51, 0xFC, 0x6D, 0x6F, 0xEC, 0x52, 0x3B, 0xEB, 0xB2, 0x39, 0x13,
+    0xFD, 0x4A, 0x33, 0xDE, 0x04, 0xD0, 0xE3, 0xBE, 0x09, 0xBD, 0x5E, 0xAF,
+    0x44, 0x45, 0x81, 0xCC, 0x0F, 0x74, 0xC8, 0x45, 0x57, 0xA8, 0xCB, 0xC0,
+    0xB3, 0x4B, 0x2E, 0x19, 0x07, 0x28, 0x0F, 0x66, 0x0A, 0x32, 0x60, 0x1A,
+    0xBD, 0xC0, 0x79, 0x55, 0xDB, 0xFB, 0xD3, 0xB9, 0x39, 0x5F, 0x0B, 0xD2,
+    0xCC, 0xA3, 0x1F, 0xFB, 0xFE, 0x25, 0x9F, 0x67, 0x79, 0x72, 0x2C, 0x40,
+    0xC6, 0x00, 0xA1, 0xD6, 0x15, 0x6B, 0x61, 0xFD, 0xDF, 0x16, 0x75, 0x3C,
+    0xF8, 0x22, 0x32, 0xDB, 0xF8, 0xE9, 0xA5, 0x8E, 0x60, 0x87, 0x23, 0xFD,
+    0xFA, 0xB5, 0x3D, 0x32, 0xAB, 0x52, 0x05, 0xAD, 0xC8, 0x1E, 0x50, 0x2F,
+    0xA0, 0x8C, 0x6F, 0xCA, 0xBB, 0x57, 0x5C, 0x9E, 0x82, 0xDF, 0x00, 0x3E,
+    0x48, 0x7B, 0x31, 0x53, 0xC3, 0xFF, 0x7E, 0x28, 0xF6, 0xA8, 0x42, 0xD5,
+    0xDB, 0x69, 0x17, 0xDF, 0x2E, 0x56, 0x87, 0x1A, 0xC8, 0x58, 0xCA, 0xBB,
+    0xB0, 0x27, 0x5B, 0x69, 0x73, 0x55, 0x4F, 0x8C, 0xC6, 0x32, 0x67, 0xAC,
+    0xB8, 0x83, 0x21, 0xFD, 0x98, 0x3D, 0xFA, 0x10, 0xA0, 0x11, 0xF0, 0xB8,
+    0x5D, 0xA3, 0xFF, 0xE1, 0x65, 0x45, 0xF8, 0xB6, 0x79, 0xE4, 0x53, 0x9A,
+    0x5B, 0xD3, 0xD1, 0x2D, 0x6C, 0xB5, 0xFC, 0x4A, 0x33, 0x7E, 0xCB, 0xA4,
+    0xDA, 0xF2, 0xDD, 0xE1, 0x90, 0x97, 0xFB, 0x4B, 0xBC, 0x49, 0x8E, 0xD2,
+    0x01, 0x4C, 0x77, 0x36, 0xDA, 0xCA, 0x20, 0xEF, 0xE8, 0xC6, 0xE4, 0xCE,
+    0x41, 0x13, 0xFB, 0x62, 0x98, 0x91, 0x90, 0xAE, 0x4D, 0xDA, 0xDB, 0x95,
+    0xB4, 0x1F, 0xF1, 0x2B, 0xFE, 0x9E, 0x7E, 0xD0, 0xE0, 0x25, 0xC7, 0xAF,
+    0xD0, 0xD1, 0x8E, 0xD0, 0xA0, 0xD5, 0x93, 0x6B, 0x71, 0x8E, 0xAD, 0xEA,
+    0x64, 0x2B, 0x12, 0xF2, 0xFB, 0xE2, 0xF6, 0x8F, 0xB7, 0x94, 0x75, 0x8E,
+    0x2F, 0x5B, 0x3C, 0x8E, 0x1C, 0xC3, 0x8F, 0x68, 0xA0, 0x5E, 0xAD, 0x4F,
+    0x1C, 0xF0, 0x0D, 0x90, 0x12, 0xB8, 0x88, 0x88, 0x77, 0x17, 0x0E, 0xBE,
+    0x18, 0x22, 0x2F, 0x2F, 0xAD, 0xC1, 0xA8, 0xB3, 0x91, 0xF1, 0xCF, 0xD1,
+    0xE8, 0x74, 0x6F, 0xB5, 0x0F, 0xCC, 0xA0, 0xE5, 0xA1, 0x1F, 0x02, 0x8B,
+    0xFE, 0x2D, 0x75, 0xEA, 0xB7, 0xE0, 0x13, 0xFD, 0xE0, 0x4F, 0xA8, 0xB4,
+    0x99, 0xE2, 0x89, 0xCE, 0xD6, 0xF3, 0xAC, 0x18, 0x05, 0x77, 0x95, 0x80,
+    0x66, 0xA2, 0x5F, 0x16, 0xD9, 0xA8, 0xAD, 0xD2, 0x81, 0x3B, 0xC4, 0x7C,
+    0x86, 0xFA, 0xB5, 0x77, 0x65, 0x20, 0xAD, 0xE6, 0x77, 0x14, 0x1A, 0x21,
+    0x14, 0x73, 0xCC, 0x93, 0xA0, 0x89, 0x3E, 0x7B, 0x0C, 0xAF, 0xCD, 0xEB,
+    0xCF, 0x35, 0x9D, 0xFB, 0xF5, 0x42, 0x54, 0xC7, 0x5E, 0xB3, 0x71, 0x20,
+    0x2D, 0x0E, 0x25, 0x00, 0x49, 0x7E, 0x1E, 0xAE, 0xD3, 0x1B, 0x41, 0xD6,
+    0x1E, 0xB9, 0x09, 0xF0, 0x9B, 0x36, 0x64, 0x24, 0xAF, 0xE0, 0xB8, 0x57,
+    0xBB, 0x00, 0x68, 0x22, 0x7F, 0x53, 0x5A, 0xD9, 0x89, 0x43, 0xC1, 0x78,
+    0xAA, 0xA6, 0xDF, 0x59, 0x1D, 0x91, 0x63, 0x55, 0xA9, 0xCF, 0x95, 0x62,
+    0x76, 0x03, 0x26, 0x83, 0xC5, 0xB9, 0xE5, 0x02, 0xA2, 0x5B, 0x7D, 0x20,
+    0x4A, 0xA9, 0x14, 0x7B, 0xCA, 0x2D, 0x47, 0xB3, 0x41, 0x4A, 0x73, 0x4E,
+    0x68, 0x19, 0xC8, 0x11, 0xE4, 0xC6, 0x9B, 0xBC, 0x3F, 0x57, 0x0F, 0xD6,
+    0x15, 0x29, 0x53, 0x9A, 0x52, 0x00, 0x51, 0x1B, 0x1F, 0xE9, 0x1B, 0x57,
+    0xB5, 0x6F, 0xBA, 0x08, 0x00, 0x74, 0xE6, 0x81, 0x76, 0xA4, 0x60, 0x2B,
+    0xB6, 0xF9, 0xB9, 0xE7, 0x21, 0x65, 0x63, 0xB6, 0x15, 0xD9, 0x0D, 0x2A,
+    0x6B, 0xEC, 0x96, 0xF2, 0xA1, 0x8F, 0x9F, 0xA9, 0x5D, 0x2D, 0xB0, 0x53,
+    0x64, 0x56, 0x85, 0xC5, 0x2E, 0x05, 0x34, 0xFF, 0x44, 0x29, 0xB3, 0xB5,
+    0xE9, 0x70, 0x7A, 0x4B, 0x6A, 0x07, 0x85, 0x6E, 0x99, 0x47, 0xBA, 0x08,
+    0x7D, 0xDF, 0xA7, 0x49, 0xB0, 0xA6, 0x6E, 0xAD, 0x23, 0x26, 0x19, 0xC4,
+    0x2E, 0x09, 0x75, 0xDB, 0xFF, 0x18, 0x9A, 0x69, 0x71, 0x8C, 0xAA, 0xEC,
+    0x66, 0xB2, 0xED, 0x8F, 0xB8, 0x60, 0xEE, 0x9C, 0x29, 0x4C, 0x09, 0x75,
+    0xA5, 0x02, 0x36, 0x19, 0xE1, 0x9E, 0xB1, 0xC2, 0x6C, 0x52, 0x64, 0x56,
+    0x65, 0x9D, 0x42, 0x5B, 0x9A, 0x98, 0x54, 0x3F, 0x3E, 0x3A, 0x18, 0xE4,
+    0x40, 0x13, 0x59, 0xA0, 0xF5, 0x30, 0xE8, 0xEF, 0x07, 0x9C, 0xD2, 0xA1,
+    0xD6, 0x3F, 0xF7, 0x99, 0xD6, 0xE4, 0x8F, 0x6B, 0x26, 0xEB, 0x70, 0x84,
+    0x86, 0x20, 0xDD, 0x4C, 0xC1, 0x5D, 0x25, 0xF0, 0xE6, 0x38, 0x2D, 0x4D,
+    0xC9, 0xEF, 0xBA, 0x3E, 0x3F, 0x6B, 0x68, 0x09, 0x5E, 0xCC, 0x1E, 0x02,
+    0xC6, 0xE9, 0x82, 0x63, 0x86, 0xE2, 0xA0, 0x52, 0x84, 0x35, 0x7F, 0x68,
+    0xA1, 0x70, 0x6A, 0x6B, 0x14, 0x18, 0x97, 0x3C, 0x5C, 0xAE, 0xDE, 0x7F,
+    0x1C, 0x84, 0x07, 0x3E, 0x37, 0x07, 0x50, 0xAA, 0x05, 0x53, 0x9C, 0xB7,
+    0x0D, 0x0C, 0x50, 0xF0, 0x37, 0xDA, 0x3A, 0xB0, 0xB8, 0xF2, 0x16, 0x57,
+    0xEC, 0x44, 0x7D, 0x8E, 0xB2, 0x74, 0xB5, 0x3C, 0x1A, 0xF5, 0x0C, 0xAE,
+    0xFF, 0xB3, 0x00, 0x02, 0x04, 0x1F, 0x1C, 0xF0, 0xF6, 0x2F, 0xA9, 0x7C,
+    0xF9, 0x13, 0x91, 0xD1, 0xBD, 0x21, 0x09, 0xDC, 0x58, 0x7A, 0x83, 0x25,
+    0xDC, 0xDA, 0xC2, 0x37, 0x81, 0xE5, 0xE5, 0x3A, 0x01, 0x47, 0xF5, 0x22,
+    0x73, 0x47, 0x32, 0x94, 0x0E, 0x03, 0xD0, 0x0F, 0x46, 0x61, 0x44, 0xA9,
+    0xA7, 0xDD, 0xF3, 0x9A, 0x34, 0x76, 0xB5, 0xC8, 0x2F, 0x0E, 0xEA, 0x3B,
+    0x99, 0xCD, 0x38, 0xE2, 0x41, 0x1E, 0x75, 0xA4, 0x3E, 0xC7, 0xC8, 0xEC,
+    0x08, 0xB9, 0x6D, 0x4F, 0x38, 0x8B, 0x54, 0x4E, 0x31, 0xB3, 0x3E, 0x18,
+    0xA1, 0xBB, 0x80, 0x32, 0x79, 0x7C, 0x97, 0x24, 0x90, 0x12, 0xB8, 0x2C,
+    0xBF, 0x04, 0x0A, 0xF6, 0x03, 0x0D, 0x42, 0x6F, 0x10, 0x08, 0x93, 0xD9,
+    0x1F, 0x77, 0x9A, 0xDE, 0xAF, 0x89, 0xAF, 0xBC, 0x72, 0xB0, 0x79, 0x56,
+    0x24, 0x71, 0x6B, 0x2E, 0x1F, 0x72, 0x12, 0x55, 0x2E, 0x3F, 0xCF, 0xDC,
+    0x12, 0xAE, 0x8B, 0xB3, 0x17, 0xDA, 0x08, 0x74, 0x18, 0x47, 0x58, 0x7A,
+    0x87, 0xCD, 0x84, 0x9F, 0xE6, 0xDD, 0x1A, 0x50, 0xFA, 0x1D, 0x85, 0xDB,
+    0x3A, 0xEC, 0x7A, 0xEC, 0x8C, 0x7D, 0x4B, 0xE9, 0xBC, 0x9A, 0x9F, 0xBC,
+    0x08, 0xD8, 0x15, 0x32, 0x47, 0x18, 0x1C, 0xEF, 0xD2, 0xC3, 0x64, 0xC4,
+    0x66, 0x43, 0x09, 0x63, 0x51, 0xC4, 0x65, 0x2A, 0x43, 0x4D, 0xA1, 0x12,
+    0x16, 0xBA, 0xC2, 0x24, 0x37, 0x3B, 0x43, 0xDD, 0x55, 0x4E, 0x31, 0x10,
+    0x9E, 0xF8, 0xDF, 0x71, 0xDD, 0xE4, 0x3A, 0x13, 0x02, 0x00, 0x94, 0x50,
+    0x6B, 0xC7, 0xA3, 0xD7, 0xF1, 0x56, 0x35, 0x04, 0x9B, 0x19, 0x11, 0x5F,
+    0xD6, 0x77, 0xAC, 0x81, 0xFA, 0xFB, 0xF1, 0x97, 0xED, 0xE6, 0x8F, 0xF2,
+    0x09, 0xA5, 0x24, 0x59, 0x3B, 0x18, 0x11, 0x3C, 0xB1, 0x6F, 0xE9, 0xEA,
+    0x70, 0x45, 0xE3, 0x86, 0x6E, 0x3C, 0x15, 0x1E, 0x2C, 0xBF, 0xBA, 0x9E,
+    0xFA, 0x06, 0x3D, 0x4E, 0x1C, 0xE7, 0x1F, 0x77, 0xB3, 0x2A, 0x3E, 0x5A,
+    0x0A, 0x5E, 0x0E, 0x86, 0x25, 0xC8, 0x66, 0x52, 0xD6, 0x89, 0x3E, 0x80,
+    0x0F, 0x1D, 0xE7, 0x99, 0xB9, 0xDC, 0x65, 0x29, 0x78, 0xEA, 0xE2, 0x94,
+    0xBA, 0x0E, 0x15, 0xC6, 0x6A, 0xB3, 0x10, 0x9C, 0x78, 0xC9, 0x4C, 0x2E,
+    0x3D, 0x2B, 0x1D, 0x36, 0xA7, 0x4E, 0xF7, 0xF2, 0xF4, 0x2D, 0x0A, 0x1E,
+    0x53, 0x3C, 0xFC, 0xA6, 0xB6, 0x12, 0x13, 0xF7, 0x08, 0xA7, 0x23, 0x52,
+    0x60, 0x79, 0xC2, 0x19, 0x0F, 0x26, 0x39, 0x19, 0x83, 0xC8, 0x7B, 0xA6,
+    0x95, 0x45, 0xBC, 0xE3, 0x66, 0x1F, 0xC3, 0xEA, 0x6E, 0xFE, 0xAD, 0xEB,
+    0xA5, 0x5A, 0x6C, 0xBE, 0xEF, 0xDD, 0x32, 0xC3, 0x28, 0xFF, 0x8C, 0x01,
+    0xD1, 0x37, 0x7F, 0xB1, 0x3B, 0x95, 0x2F, 0xDB, 0x0F, 0xA5, 0xCE, 0xEE,
+    0x02, 0x97, 0xAB, 0x68, 0x85, 0x21, 0x58, 0x65, 0x70, 0x61, 0x07, 0x29,
+    0x28, 0xB6, 0x21, 0x15, 0x84, 0x2F, 0x6E, 0x5B, 0xAD, 0x7D, 0xEF, 0x2A,
+    0x96, 0xBD, 0x61, 0xEB, 0x30, 0xA8, 0xCC, 0x13, 0x10, 0x15, 0x9F, 0x61,
+    0x75, 0x47, 0xDD, 0xEC, 0x39, 0xA2, 0x70, 0x4C, 0x90, 0x5C, 0x73, 0xB5,
+    0xCF, 0x63, 0x03, 0xAA, 0x1E, 0xFE, 0x34, 0x03, 0xA7, 0x2C, 0x62, 0x60,
+    0xBC, 0x86, 0xCC, 0xEE, 0x14, 0xDE, 0xAA, 0xCB, 0x0B, 0x9E, 0x9E, 0xD5,
+    0xCA, 0xF0, 0xBD, 0x19, 0xAF, 0x1E, 0x8B, 0x64, 0x6E, 0x84, 0xF3, 0xB2,
+    0xAB, 0x5C, 0xAB, 0x9C, 0xB3, 0xB4, 0x2A, 0x3C, 0x32, 0x5A, 0x68, 0x40,
+    0x50, 0xBB, 0x5A, 0x65, 0xB9, 0x69, 0x23, 0xA0, 0x99, 0xA0, 0x5F, 0x87,
+    0x19, 0x0B, 0x54, 0x9B, 0xF7, 0xB8, 0x21, 0xC0, 0xD5, 0xE9, 0x9E, 0x31,
+    0x77, 0x2D, 0xE3, 0x97, 0x9A, 0x88, 0x37, 0xF8, 0xA8, 0x7D, 0x3D, 0x62,
+    0x7E, 0x99, 0xF7, 0x95, 0xD6, 0x1F, 0xE6, 0xC7, 0x29, 0x88, 0x35, 0x0E,
+    0x81, 0x12, 0x68, 0x16, 0x5F, 0x93, 0xED, 0x11, 0x63, 0x72, 0x22, 0x1B,
+    0xA5, 0x84, 0xF5, 0x57, 0x99, 0xBA, 0x58, 0x78, 0xA1, 0xDF, 0xDE, 0x96,
+    0x54, 0x30, 0x2E, 0x53, 0xEB, 0x0A, 0xB3, 0xCD, 0x96, 0x46, 0xC2, 0x1A,
+    0xFF, 0xC3, 0x83, 0x9B, 0xEA, 0xFF, 0xC6, 0x34, 0xEF, 0xF2, 0xEB, 0x58,
+    0x28, 0x31, 0xBC, 0x6D, 0xE4, 0x48, 0xD9, 0x8F, 0xE3, 0xB7, 0x64, 0xE8,
+    0xD9, 0x14, 0x4A, 0x5D, 0x73, 0x3C, 0x7C, 0xEE, 0x61, 0xED, 0x28, 0xFE,
+    0xEA, 0xAB, 0xAA, 0xA3, 0xB6, 0xE2, 0xEE, 0x45, 0xE0, 0x13, 0x3E, 0x20,
+    0x14, 0x5D, 0x10, 0x42, 0xB5, 0xBB, 0x6A, 0xEF, 0x42, 0xF4, 0x42, 0xC7,
+    0xD0, 0x4F, 0xCB, 0xFA, 0x15, 0x4F, 0x6C, 0xDB, 0xC7, 0x4D, 0x85, 0x86,
+    0x9E, 0x79, 0x1E, 0xD8, 0x05, 0x21, 0xCD, 0x41, 0x1D, 0x3B, 0x4F, 0x65,
+    0x46, 0x26, 0x8D, 0x5B, 0xF2, 0xA1, 0x62, 0xCF, 0x50, 0x62, 0x81, 0x3D,
+    0x6A, 0x47, 0x4B, 0xE4, 0x92, 0x74, 0xCB, 0x69, 0xC3, 0x24, 0x15, 0x7F,
+    0xA3, 0xB6, 0xC7, 0xC1, 0xA0, 0x83, 0x88, 0xFC, 0x9D, 0x48, 0x19, 0xAD,
+    0x00, 0xBF, 0x5B, 0x09, 0x85, 0xB2, 0x92, 0x56, 0x0B, 0x8A, 0x84, 0x47,
+    0xEA, 0xF5, 0x55, 0x0C, 0x2A, 0x8D, 0x42, 0x58, 0x00, 0x0D, 0x82, 0x23,
+    0x74, 0xB1, 0x62, 0x14, 0x41, 0x7E, 0x93, 0x8D, 0x92, 0xF0, 0x72, 0x33,
+    0x61, 0x70, 0x3F, 0x23, 0x3E, 0xF4, 0xAD, 0x1D, 0x60, 0x74, 0xEE, 0xCB,
+    0x59, 0x37, 0xDE, 0x7C, 0xDB, 0x3B, 0x22, 0x6C, 0xF1, 0xEC, 0x5F, 0xD6,
+    0x9E, 0x50, 0xF8, 0x19, 0x84, 0x80, 0x07, 0xA6, 0x6E, 0x32, 0x77, 0xCE,
+    0xA7, 0xF2, 0x85, 0x40, 0xC2, 0x06, 0x0C, 0xC5, 0xAA, 0xA7, 0x69, 0xA9,
+    0x35, 0x97, 0xD9, 0x61, 0x55, 0xD8, 0xEF, 0xE8, 0x84, 0x34, 0x45, 0xC3,
+    0x2E, 0x7A, 0x44, 0x9E, 0xDC, 0xCA, 0x0B, 0x80, 0xFC, 0xAB, 0x04, 0x5A,
+    0xCD, 0x88, 0x55, 0x10, 0xD3, 0xDB, 0x73, 0xDB, 0xC9, 0x9E, 0x1E, 0x0E,
+    0x05, 0x67, 0xD5, 0xFD, 0xD8, 0x38, 0x3E, 0x71, 0x65, 0x34, 0xC4, 0xC5,
+    0x40, 0x43, 0x67, 0xE3, 0x79, 0xDA, 0x5F, 0x67, 0x4A, 0x3D, 0xB0, 0x8F,
+    0xE7, 0x21, 0x3E, 0x15, 0x20, 0xFF, 0x6D, 0xF1, 0x9E, 0xF8, 0x28, 0x3D,
+    0xF7, 0x40, 0x81, 0x94, 0x68, 0x5A, 0x3D, 0xE9, 0xF7, 0xAD, 0x83, 0xDB,
+    0x2B, 0x9F, 0xE3, 0xE6, 0xF7, 0xD4, 0x02, 0x76, 0xF7, 0x20, 0x15, 0x41,
+    0x34, 0x29, 0x69, 0x94, 0x1C, 0x26, 0x4C, 0xF6, 0x6A, 0xF4, 0x20, 0x33,
+    0x71, 0x24, 0x08, 0xD4, 0x68, 0x00, 0xA1, 0xD4, 0x2E, 0x6B, 0xF4, 0xBC,
+    0x46, 0x45, 0x24, 0x97, 0x2E, 0xF6, 0x39, 0x1E, 0xAF, 0x61, 0x00, 0x50,
+    0xB7, 0xD4, 0xB7, 0x43,
+};
+
+}  // namespace random_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/random/internal/randen_slow.cc b/absl/random/internal/randen_slow.cc
index 7a2e2daa53f..d2b7da3c2cb 100644
--- a/absl/random/internal/randen_slow.cc
+++ b/absl/random/internal/randen_slow.cc
@@ -18,16 +18,12 @@
 #include <cstdint>
 #include <cstring>
 
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/endian.h"  // IWYU pragma: keep
+#include "absl/numeric/int128.h"
 #include "absl/random/internal/platform.h"
-
-// ABSL_HAVE_ATTRIBUTE
-#if !defined(ABSL_HAVE_ATTRIBUTE)
-#ifdef __has_attribute
-#define ABSL_HAVE_ATTRIBUTE(x) __has_attribute(x)
-#else
-#define ABSL_HAVE_ATTRIBUTE(x) 0
-#endif
-#endif
+#include "absl/random/internal/randen_traits.h"
 
 #if ABSL_HAVE_ATTRIBUTE(always_inline) || \
     (defined(__GNUC__) && !defined(__clang__))
@@ -45,369 +41,303 @@
 namespace {
 
 // AES portions based on rijndael-alg-fst.c,
-// https://fastcrypto.org/front/misc/rijndael-alg-fst.c
+// https://fastcrypto.org/front/misc/rijndael-alg-fst.c, and modified for
+// platform-endianness.
 //
 // Implementation of
 // http://www.csrc.nist.gov/publications/fips/fips197/fips-197.pdf
 constexpr uint32_t te0[256] = {
-    0xc66363a5, 0xf87c7c84, 0xee777799, 0xf67b7b8d, 0xfff2f20d, 0xd66b6bbd,
-    0xde6f6fb1, 0x91c5c554, 0x60303050, 0x02010103, 0xce6767a9, 0x562b2b7d,
-    0xe7fefe19, 0xb5d7d762, 0x4dababe6, 0xec76769a, 0x8fcaca45, 0x1f82829d,
-    0x89c9c940, 0xfa7d7d87, 0xeffafa15, 0xb25959eb, 0x8e4747c9, 0xfbf0f00b,
-    0x41adadec, 0xb3d4d467, 0x5fa2a2fd, 0x45afafea, 0x239c9cbf, 0x53a4a4f7,
-    0xe4727296, 0x9bc0c05b, 0x75b7b7c2, 0xe1fdfd1c, 0x3d9393ae, 0x4c26266a,
-    0x6c36365a, 0x7e3f3f41, 0xf5f7f702, 0x83cccc4f, 0x6834345c, 0x51a5a5f4,
-    0xd1e5e534, 0xf9f1f108, 0xe2717193, 0xabd8d873, 0x62313153, 0x2a15153f,
-    0x0804040c, 0x95c7c752, 0x46232365, 0x9dc3c35e, 0x30181828, 0x379696a1,
-    0x0a05050f, 0x2f9a9ab5, 0x0e070709, 0x24121236, 0x1b80809b, 0xdfe2e23d,
-    0xcdebeb26, 0x4e272769, 0x7fb2b2cd, 0xea75759f, 0x1209091b, 0x1d83839e,
-    0x582c2c74, 0x341a1a2e, 0x361b1b2d, 0xdc6e6eb2, 0xb45a5aee, 0x5ba0a0fb,
-    0xa45252f6, 0x763b3b4d, 0xb7d6d661, 0x7db3b3ce, 0x5229297b, 0xdde3e33e,
-    0x5e2f2f71, 0x13848497, 0xa65353f5, 0xb9d1d168, 0x00000000, 0xc1eded2c,
-    0x40202060, 0xe3fcfc1f, 0x79b1b1c8, 0xb65b5bed, 0xd46a6abe, 0x8dcbcb46,
-    0x67bebed9, 0x7239394b, 0x944a4ade, 0x984c4cd4, 0xb05858e8, 0x85cfcf4a,
-    0xbbd0d06b, 0xc5efef2a, 0x4faaaae5, 0xedfbfb16, 0x864343c5, 0x9a4d4dd7,
-    0x66333355, 0x11858594, 0x8a4545cf, 0xe9f9f910, 0x04020206, 0xfe7f7f81,
-    0xa05050f0, 0x783c3c44, 0x259f9fba, 0x4ba8a8e3, 0xa25151f3, 0x5da3a3fe,
-    0x804040c0, 0x058f8f8a, 0x3f9292ad, 0x219d9dbc, 0x70383848, 0xf1f5f504,
-    0x63bcbcdf, 0x77b6b6c1, 0xafdada75, 0x42212163, 0x20101030, 0xe5ffff1a,
-    0xfdf3f30e, 0xbfd2d26d, 0x81cdcd4c, 0x180c0c14, 0x26131335, 0xc3ecec2f,
-    0xbe5f5fe1, 0x359797a2, 0x884444cc, 0x2e171739, 0x93c4c457, 0x55a7a7f2,
-    0xfc7e7e82, 0x7a3d3d47, 0xc86464ac, 0xba5d5de7, 0x3219192b, 0xe6737395,
-    0xc06060a0, 0x19818198, 0x9e4f4fd1, 0xa3dcdc7f, 0x44222266, 0x542a2a7e,
-    0x3b9090ab, 0x0b888883, 0x8c4646ca, 0xc7eeee29, 0x6bb8b8d3, 0x2814143c,
-    0xa7dede79, 0xbc5e5ee2, 0x160b0b1d, 0xaddbdb76, 0xdbe0e03b, 0x64323256,
-    0x743a3a4e, 0x140a0a1e, 0x924949db, 0x0c06060a, 0x4824246c, 0xb85c5ce4,
-    0x9fc2c25d, 0xbdd3d36e, 0x43acacef, 0xc46262a6, 0x399191a8, 0x319595a4,
-    0xd3e4e437, 0xf279798b, 0xd5e7e732, 0x8bc8c843, 0x6e373759, 0xda6d6db7,
-    0x018d8d8c, 0xb1d5d564, 0x9c4e4ed2, 0x49a9a9e0, 0xd86c6cb4, 0xac5656fa,
-    0xf3f4f407, 0xcfeaea25, 0xca6565af, 0xf47a7a8e, 0x47aeaee9, 0x10080818,
-    0x6fbabad5, 0xf0787888, 0x4a25256f, 0x5c2e2e72, 0x381c1c24, 0x57a6a6f1,
-    0x73b4b4c7, 0x97c6c651, 0xcbe8e823, 0xa1dddd7c, 0xe874749c, 0x3e1f1f21,
-    0x964b4bdd, 0x61bdbddc, 0x0d8b8b86, 0x0f8a8a85, 0xe0707090, 0x7c3e3e42,
-    0x71b5b5c4, 0xcc6666aa, 0x904848d8, 0x06030305, 0xf7f6f601, 0x1c0e0e12,
-    0xc26161a3, 0x6a35355f, 0xae5757f9, 0x69b9b9d0, 0x17868691, 0x99c1c158,
-    0x3a1d1d27, 0x279e9eb9, 0xd9e1e138, 0xebf8f813, 0x2b9898b3, 0x22111133,
-    0xd26969bb, 0xa9d9d970, 0x078e8e89, 0x339494a7, 0x2d9b9bb6, 0x3c1e1e22,
-    0x15878792, 0xc9e9e920, 0x87cece49, 0xaa5555ff, 0x50282878, 0xa5dfdf7a,
-    0x038c8c8f, 0x59a1a1f8, 0x09898980, 0x1a0d0d17, 0x65bfbfda, 0xd7e6e631,
-    0x844242c6, 0xd06868b8, 0x824141c3, 0x299999b0, 0x5a2d2d77, 0x1e0f0f11,
-    0x7bb0b0cb, 0xa85454fc, 0x6dbbbbd6, 0x2c16163a,
+    0xa56363c6, 0x847c7cf8, 0x997777ee, 0x8d7b7bf6, 0x0df2f2ff, 0xbd6b6bd6,
+    0xb16f6fde, 0x54c5c591, 0x50303060, 0x03010102, 0xa96767ce, 0x7d2b2b56,
+    0x19fefee7, 0x62d7d7b5, 0xe6abab4d, 0x9a7676ec, 0x45caca8f, 0x9d82821f,
+    0x40c9c989, 0x877d7dfa, 0x15fafaef, 0xeb5959b2, 0xc947478e, 0x0bf0f0fb,
+    0xecadad41, 0x67d4d4b3, 0xfda2a25f, 0xeaafaf45, 0xbf9c9c23, 0xf7a4a453,
+    0x967272e4, 0x5bc0c09b, 0xc2b7b775, 0x1cfdfde1, 0xae93933d, 0x6a26264c,
+    0x5a36366c, 0x413f3f7e, 0x02f7f7f5, 0x4fcccc83, 0x5c343468, 0xf4a5a551,
+    0x34e5e5d1, 0x08f1f1f9, 0x937171e2, 0x73d8d8ab, 0x53313162, 0x3f15152a,
+    0x0c040408, 0x52c7c795, 0x65232346, 0x5ec3c39d, 0x28181830, 0xa1969637,
+    0x0f05050a, 0xb59a9a2f, 0x0907070e, 0x36121224, 0x9b80801b, 0x3de2e2df,
+    0x26ebebcd, 0x6927274e, 0xcdb2b27f, 0x9f7575ea, 0x1b090912, 0x9e83831d,
+    0x742c2c58, 0x2e1a1a34, 0x2d1b1b36, 0xb26e6edc, 0xee5a5ab4, 0xfba0a05b,
+    0xf65252a4, 0x4d3b3b76, 0x61d6d6b7, 0xceb3b37d, 0x7b292952, 0x3ee3e3dd,
+    0x712f2f5e, 0x97848413, 0xf55353a6, 0x68d1d1b9, 0x00000000, 0x2cededc1,
+    0x60202040, 0x1ffcfce3, 0xc8b1b179, 0xed5b5bb6, 0xbe6a6ad4, 0x46cbcb8d,
+    0xd9bebe67, 0x4b393972, 0xde4a4a94, 0xd44c4c98, 0xe85858b0, 0x4acfcf85,
+    0x6bd0d0bb, 0x2aefefc5, 0xe5aaaa4f, 0x16fbfbed, 0xc5434386, 0xd74d4d9a,
+    0x55333366, 0x94858511, 0xcf45458a, 0x10f9f9e9, 0x06020204, 0x817f7ffe,
+    0xf05050a0, 0x443c3c78, 0xba9f9f25, 0xe3a8a84b, 0xf35151a2, 0xfea3a35d,
+    0xc0404080, 0x8a8f8f05, 0xad92923f, 0xbc9d9d21, 0x48383870, 0x04f5f5f1,
+    0xdfbcbc63, 0xc1b6b677, 0x75dadaaf, 0x63212142, 0x30101020, 0x1affffe5,
+    0x0ef3f3fd, 0x6dd2d2bf, 0x4ccdcd81, 0x140c0c18, 0x35131326, 0x2fececc3,
+    0xe15f5fbe, 0xa2979735, 0xcc444488, 0x3917172e, 0x57c4c493, 0xf2a7a755,
+    0x827e7efc, 0x473d3d7a, 0xac6464c8, 0xe75d5dba, 0x2b191932, 0x957373e6,
+    0xa06060c0, 0x98818119, 0xd14f4f9e, 0x7fdcdca3, 0x66222244, 0x7e2a2a54,
+    0xab90903b, 0x8388880b, 0xca46468c, 0x29eeeec7, 0xd3b8b86b, 0x3c141428,
+    0x79dedea7, 0xe25e5ebc, 0x1d0b0b16, 0x76dbdbad, 0x3be0e0db, 0x56323264,
+    0x4e3a3a74, 0x1e0a0a14, 0xdb494992, 0x0a06060c, 0x6c242448, 0xe45c5cb8,
+    0x5dc2c29f, 0x6ed3d3bd, 0xefacac43, 0xa66262c4, 0xa8919139, 0xa4959531,
+    0x37e4e4d3, 0x8b7979f2, 0x32e7e7d5, 0x43c8c88b, 0x5937376e, 0xb76d6dda,
+    0x8c8d8d01, 0x64d5d5b1, 0xd24e4e9c, 0xe0a9a949, 0xb46c6cd8, 0xfa5656ac,
+    0x07f4f4f3, 0x25eaeacf, 0xaf6565ca, 0x8e7a7af4, 0xe9aeae47, 0x18080810,
+    0xd5baba6f, 0x887878f0, 0x6f25254a, 0x722e2e5c, 0x241c1c38, 0xf1a6a657,
+    0xc7b4b473, 0x51c6c697, 0x23e8e8cb, 0x7cdddda1, 0x9c7474e8, 0x211f1f3e,
+    0xdd4b4b96, 0xdcbdbd61, 0x868b8b0d, 0x858a8a0f, 0x907070e0, 0x423e3e7c,
+    0xc4b5b571, 0xaa6666cc, 0xd8484890, 0x05030306, 0x01f6f6f7, 0x120e0e1c,
+    0xa36161c2, 0x5f35356a, 0xf95757ae, 0xd0b9b969, 0x91868617, 0x58c1c199,
+    0x271d1d3a, 0xb99e9e27, 0x38e1e1d9, 0x13f8f8eb, 0xb398982b, 0x33111122,
+    0xbb6969d2, 0x70d9d9a9, 0x898e8e07, 0xa7949433, 0xb69b9b2d, 0x221e1e3c,
+    0x92878715, 0x20e9e9c9, 0x49cece87, 0xff5555aa, 0x78282850, 0x7adfdfa5,
+    0x8f8c8c03, 0xf8a1a159, 0x80898909, 0x170d0d1a, 0xdabfbf65, 0x31e6e6d7,
+    0xc6424284, 0xb86868d0, 0xc3414182, 0xb0999929, 0x772d2d5a, 0x110f0f1e,
+    0xcbb0b07b, 0xfc5454a8, 0xd6bbbb6d, 0x3a16162c,
 };
 
 constexpr uint32_t te1[256] = {
-    0xa5c66363, 0x84f87c7c, 0x99ee7777, 0x8df67b7b, 0x0dfff2f2, 0xbdd66b6b,
-    0xb1de6f6f, 0x5491c5c5, 0x50603030, 0x03020101, 0xa9ce6767, 0x7d562b2b,
-    0x19e7fefe, 0x62b5d7d7, 0xe64dabab, 0x9aec7676, 0x458fcaca, 0x9d1f8282,
-    0x4089c9c9, 0x87fa7d7d, 0x15effafa, 0xebb25959, 0xc98e4747, 0x0bfbf0f0,
-    0xec41adad, 0x67b3d4d4, 0xfd5fa2a2, 0xea45afaf, 0xbf239c9c, 0xf753a4a4,
-    0x96e47272, 0x5b9bc0c0, 0xc275b7b7, 0x1ce1fdfd, 0xae3d9393, 0x6a4c2626,
-    0x5a6c3636, 0x417e3f3f, 0x02f5f7f7, 0x4f83cccc, 0x5c683434, 0xf451a5a5,
-    0x34d1e5e5, 0x08f9f1f1, 0x93e27171, 0x73abd8d8, 0x53623131, 0x3f2a1515,
-    0x0c080404, 0x5295c7c7, 0x65462323, 0x5e9dc3c3, 0x28301818, 0xa1379696,
-    0x0f0a0505, 0xb52f9a9a, 0x090e0707, 0x36241212, 0x9b1b8080, 0x3ddfe2e2,
-    0x26cdebeb, 0x694e2727, 0xcd7fb2b2, 0x9fea7575, 0x1b120909, 0x9e1d8383,
-    0x74582c2c, 0x2e341a1a, 0x2d361b1b, 0xb2dc6e6e, 0xeeb45a5a, 0xfb5ba0a0,
-    0xf6a45252, 0x4d763b3b, 0x61b7d6d6, 0xce7db3b3, 0x7b522929, 0x3edde3e3,
-    0x715e2f2f, 0x97138484, 0xf5a65353, 0x68b9d1d1, 0x00000000, 0x2cc1eded,
-    0x60402020, 0x1fe3fcfc, 0xc879b1b1, 0xedb65b5b, 0xbed46a6a, 0x468dcbcb,
-    0xd967bebe, 0x4b723939, 0xde944a4a, 0xd4984c4c, 0xe8b05858, 0x4a85cfcf,
-    0x6bbbd0d0, 0x2ac5efef, 0xe54faaaa, 0x16edfbfb, 0xc5864343, 0xd79a4d4d,
-    0x55663333, 0x94118585, 0xcf8a4545, 0x10e9f9f9, 0x06040202, 0x81fe7f7f,
-    0xf0a05050, 0x44783c3c, 0xba259f9f, 0xe34ba8a8, 0xf3a25151, 0xfe5da3a3,
-    0xc0804040, 0x8a058f8f, 0xad3f9292, 0xbc219d9d, 0x48703838, 0x04f1f5f5,
-    0xdf63bcbc, 0xc177b6b6, 0x75afdada, 0x63422121, 0x30201010, 0x1ae5ffff,
-    0x0efdf3f3, 0x6dbfd2d2, 0x4c81cdcd, 0x14180c0c, 0x35261313, 0x2fc3ecec,
-    0xe1be5f5f, 0xa2359797, 0xcc884444, 0x392e1717, 0x5793c4c4, 0xf255a7a7,
-    0x82fc7e7e, 0x477a3d3d, 0xacc86464, 0xe7ba5d5d, 0x2b321919, 0x95e67373,
-    0xa0c06060, 0x98198181, 0xd19e4f4f, 0x7fa3dcdc, 0x66442222, 0x7e542a2a,
-    0xab3b9090, 0x830b8888, 0xca8c4646, 0x29c7eeee, 0xd36bb8b8, 0x3c281414,
-    0x79a7dede, 0xe2bc5e5e, 0x1d160b0b, 0x76addbdb, 0x3bdbe0e0, 0x56643232,
-    0x4e743a3a, 0x1e140a0a, 0xdb924949, 0x0a0c0606, 0x6c482424, 0xe4b85c5c,
-    0x5d9fc2c2, 0x6ebdd3d3, 0xef43acac, 0xa6c46262, 0xa8399191, 0xa4319595,
-    0x37d3e4e4, 0x8bf27979, 0x32d5e7e7, 0x438bc8c8, 0x596e3737, 0xb7da6d6d,
-    0x8c018d8d, 0x64b1d5d5, 0xd29c4e4e, 0xe049a9a9, 0xb4d86c6c, 0xfaac5656,
-    0x07f3f4f4, 0x25cfeaea, 0xafca6565, 0x8ef47a7a, 0xe947aeae, 0x18100808,
-    0xd56fbaba, 0x88f07878, 0x6f4a2525, 0x725c2e2e, 0x24381c1c, 0xf157a6a6,
-    0xc773b4b4, 0x5197c6c6, 0x23cbe8e8, 0x7ca1dddd, 0x9ce87474, 0x213e1f1f,
-    0xdd964b4b, 0xdc61bdbd, 0x860d8b8b, 0x850f8a8a, 0x90e07070, 0x427c3e3e,
-    0xc471b5b5, 0xaacc6666, 0xd8904848, 0x05060303, 0x01f7f6f6, 0x121c0e0e,
-    0xa3c26161, 0x5f6a3535, 0xf9ae5757, 0xd069b9b9, 0x91178686, 0x5899c1c1,
-    0x273a1d1d, 0xb9279e9e, 0x38d9e1e1, 0x13ebf8f8, 0xb32b9898, 0x33221111,
-    0xbbd26969, 0x70a9d9d9, 0x89078e8e, 0xa7339494, 0xb62d9b9b, 0x223c1e1e,
-    0x92158787, 0x20c9e9e9, 0x4987cece, 0xffaa5555, 0x78502828, 0x7aa5dfdf,
-    0x8f038c8c, 0xf859a1a1, 0x80098989, 0x171a0d0d, 0xda65bfbf, 0x31d7e6e6,
-    0xc6844242, 0xb8d06868, 0xc3824141, 0xb0299999, 0x775a2d2d, 0x111e0f0f,
-    0xcb7bb0b0, 0xfca85454, 0xd66dbbbb, 0x3a2c1616,
+    0x6363c6a5, 0x7c7cf884, 0x7777ee99, 0x7b7bf68d, 0xf2f2ff0d, 0x6b6bd6bd,
+    0x6f6fdeb1, 0xc5c59154, 0x30306050, 0x01010203, 0x6767cea9, 0x2b2b567d,
+    0xfefee719, 0xd7d7b562, 0xabab4de6, 0x7676ec9a, 0xcaca8f45, 0x82821f9d,
+    0xc9c98940, 0x7d7dfa87, 0xfafaef15, 0x5959b2eb, 0x47478ec9, 0xf0f0fb0b,
+    0xadad41ec, 0xd4d4b367, 0xa2a25ffd, 0xafaf45ea, 0x9c9c23bf, 0xa4a453f7,
+    0x7272e496, 0xc0c09b5b, 0xb7b775c2, 0xfdfde11c, 0x93933dae, 0x26264c6a,
+    0x36366c5a, 0x3f3f7e41, 0xf7f7f502, 0xcccc834f, 0x3434685c, 0xa5a551f4,
+    0xe5e5d134, 0xf1f1f908, 0x7171e293, 0xd8d8ab73, 0x31316253, 0x15152a3f,
+    0x0404080c, 0xc7c79552, 0x23234665, 0xc3c39d5e, 0x18183028, 0x969637a1,
+    0x05050a0f, 0x9a9a2fb5, 0x07070e09, 0x12122436, 0x80801b9b, 0xe2e2df3d,
+    0xebebcd26, 0x27274e69, 0xb2b27fcd, 0x7575ea9f, 0x0909121b, 0x83831d9e,
+    0x2c2c5874, 0x1a1a342e, 0x1b1b362d, 0x6e6edcb2, 0x5a5ab4ee, 0xa0a05bfb,
+    0x5252a4f6, 0x3b3b764d, 0xd6d6b761, 0xb3b37dce, 0x2929527b, 0xe3e3dd3e,
+    0x2f2f5e71, 0x84841397, 0x5353a6f5, 0xd1d1b968, 0x00000000, 0xededc12c,
+    0x20204060, 0xfcfce31f, 0xb1b179c8, 0x5b5bb6ed, 0x6a6ad4be, 0xcbcb8d46,
+    0xbebe67d9, 0x3939724b, 0x4a4a94de, 0x4c4c98d4, 0x5858b0e8, 0xcfcf854a,
+    0xd0d0bb6b, 0xefefc52a, 0xaaaa4fe5, 0xfbfbed16, 0x434386c5, 0x4d4d9ad7,
+    0x33336655, 0x85851194, 0x45458acf, 0xf9f9e910, 0x02020406, 0x7f7ffe81,
+    0x5050a0f0, 0x3c3c7844, 0x9f9f25ba, 0xa8a84be3, 0x5151a2f3, 0xa3a35dfe,
+    0x404080c0, 0x8f8f058a, 0x92923fad, 0x9d9d21bc, 0x38387048, 0xf5f5f104,
+    0xbcbc63df, 0xb6b677c1, 0xdadaaf75, 0x21214263, 0x10102030, 0xffffe51a,
+    0xf3f3fd0e, 0xd2d2bf6d, 0xcdcd814c, 0x0c0c1814, 0x13132635, 0xececc32f,
+    0x5f5fbee1, 0x979735a2, 0x444488cc, 0x17172e39, 0xc4c49357, 0xa7a755f2,
+    0x7e7efc82, 0x3d3d7a47, 0x6464c8ac, 0x5d5dbae7, 0x1919322b, 0x7373e695,
+    0x6060c0a0, 0x81811998, 0x4f4f9ed1, 0xdcdca37f, 0x22224466, 0x2a2a547e,
+    0x90903bab, 0x88880b83, 0x46468cca, 0xeeeec729, 0xb8b86bd3, 0x1414283c,
+    0xdedea779, 0x5e5ebce2, 0x0b0b161d, 0xdbdbad76, 0xe0e0db3b, 0x32326456,
+    0x3a3a744e, 0x0a0a141e, 0x494992db, 0x06060c0a, 0x2424486c, 0x5c5cb8e4,
+    0xc2c29f5d, 0xd3d3bd6e, 0xacac43ef, 0x6262c4a6, 0x919139a8, 0x959531a4,
+    0xe4e4d337, 0x7979f28b, 0xe7e7d532, 0xc8c88b43, 0x37376e59, 0x6d6ddab7,
+    0x8d8d018c, 0xd5d5b164, 0x4e4e9cd2, 0xa9a949e0, 0x6c6cd8b4, 0x5656acfa,
+    0xf4f4f307, 0xeaeacf25, 0x6565caaf, 0x7a7af48e, 0xaeae47e9, 0x08081018,
+    0xbaba6fd5, 0x7878f088, 0x25254a6f, 0x2e2e5c72, 0x1c1c3824, 0xa6a657f1,
+    0xb4b473c7, 0xc6c69751, 0xe8e8cb23, 0xdddda17c, 0x7474e89c, 0x1f1f3e21,
+    0x4b4b96dd, 0xbdbd61dc, 0x8b8b0d86, 0x8a8a0f85, 0x7070e090, 0x3e3e7c42,
+    0xb5b571c4, 0x6666ccaa, 0x484890d8, 0x03030605, 0xf6f6f701, 0x0e0e1c12,
+    0x6161c2a3, 0x35356a5f, 0x5757aef9, 0xb9b969d0, 0x86861791, 0xc1c19958,
+    0x1d1d3a27, 0x9e9e27b9, 0xe1e1d938, 0xf8f8eb13, 0x98982bb3, 0x11112233,
+    0x6969d2bb, 0xd9d9a970, 0x8e8e0789, 0x949433a7, 0x9b9b2db6, 0x1e1e3c22,
+    0x87871592, 0xe9e9c920, 0xcece8749, 0x5555aaff, 0x28285078, 0xdfdfa57a,
+    0x8c8c038f, 0xa1a159f8, 0x89890980, 0x0d0d1a17, 0xbfbf65da, 0xe6e6d731,
+    0x424284c6, 0x6868d0b8, 0x414182c3, 0x999929b0, 0x2d2d5a77, 0x0f0f1e11,
+    0xb0b07bcb, 0x5454a8fc, 0xbbbb6dd6, 0x16162c3a,
 };
 
 constexpr uint32_t te2[256] = {
-    0x63a5c663, 0x7c84f87c, 0x7799ee77, 0x7b8df67b, 0xf20dfff2, 0x6bbdd66b,
-    0x6fb1de6f, 0xc55491c5, 0x30506030, 0x01030201, 0x67a9ce67, 0x2b7d562b,
-    0xfe19e7fe, 0xd762b5d7, 0xabe64dab, 0x769aec76, 0xca458fca, 0x829d1f82,
-    0xc94089c9, 0x7d87fa7d, 0xfa15effa, 0x59ebb259, 0x47c98e47, 0xf00bfbf0,
-    0xadec41ad, 0xd467b3d4, 0xa2fd5fa2, 0xafea45af, 0x9cbf239c, 0xa4f753a4,
-    0x7296e472, 0xc05b9bc0, 0xb7c275b7, 0xfd1ce1fd, 0x93ae3d93, 0x266a4c26,
-    0x365a6c36, 0x3f417e3f, 0xf702f5f7, 0xcc4f83cc, 0x345c6834, 0xa5f451a5,
-    0xe534d1e5, 0xf108f9f1, 0x7193e271, 0xd873abd8, 0x31536231, 0x153f2a15,
-    0x040c0804, 0xc75295c7, 0x23654623, 0xc35e9dc3, 0x18283018, 0x96a13796,
-    0x050f0a05, 0x9ab52f9a, 0x07090e07, 0x12362412, 0x809b1b80, 0xe23ddfe2,
-    0xeb26cdeb, 0x27694e27, 0xb2cd7fb2, 0x759fea75, 0x091b1209, 0x839e1d83,
-    0x2c74582c, 0x1a2e341a, 0x1b2d361b, 0x6eb2dc6e, 0x5aeeb45a, 0xa0fb5ba0,
-    0x52f6a452, 0x3b4d763b, 0xd661b7d6, 0xb3ce7db3, 0x297b5229, 0xe33edde3,
-    0x2f715e2f, 0x84971384, 0x53f5a653, 0xd168b9d1, 0x00000000, 0xed2cc1ed,
-    0x20604020, 0xfc1fe3fc, 0xb1c879b1, 0x5bedb65b, 0x6abed46a, 0xcb468dcb,
-    0xbed967be, 0x394b7239, 0x4ade944a, 0x4cd4984c, 0x58e8b058, 0xcf4a85cf,
-    0xd06bbbd0, 0xef2ac5ef, 0xaae54faa, 0xfb16edfb, 0x43c58643, 0x4dd79a4d,
-    0x33556633, 0x85941185, 0x45cf8a45, 0xf910e9f9, 0x02060402, 0x7f81fe7f,
-    0x50f0a050, 0x3c44783c, 0x9fba259f, 0xa8e34ba8, 0x51f3a251, 0xa3fe5da3,
-    0x40c08040, 0x8f8a058f, 0x92ad3f92, 0x9dbc219d, 0x38487038, 0xf504f1f5,
-    0xbcdf63bc, 0xb6c177b6, 0xda75afda, 0x21634221, 0x10302010, 0xff1ae5ff,
-    0xf30efdf3, 0xd26dbfd2, 0xcd4c81cd, 0x0c14180c, 0x13352613, 0xec2fc3ec,
-    0x5fe1be5f, 0x97a23597, 0x44cc8844, 0x17392e17, 0xc45793c4, 0xa7f255a7,
-    0x7e82fc7e, 0x3d477a3d, 0x64acc864, 0x5de7ba5d, 0x192b3219, 0x7395e673,
-    0x60a0c060, 0x81981981, 0x4fd19e4f, 0xdc7fa3dc, 0x22664422, 0x2a7e542a,
-    0x90ab3b90, 0x88830b88, 0x46ca8c46, 0xee29c7ee, 0xb8d36bb8, 0x143c2814,
-    0xde79a7de, 0x5ee2bc5e, 0x0b1d160b, 0xdb76addb, 0xe03bdbe0, 0x32566432,
-    0x3a4e743a, 0x0a1e140a, 0x49db9249, 0x060a0c06, 0x246c4824, 0x5ce4b85c,
-    0xc25d9fc2, 0xd36ebdd3, 0xacef43ac, 0x62a6c462, 0x91a83991, 0x95a43195,
-    0xe437d3e4, 0x798bf279, 0xe732d5e7, 0xc8438bc8, 0x37596e37, 0x6db7da6d,
-    0x8d8c018d, 0xd564b1d5, 0x4ed29c4e, 0xa9e049a9, 0x6cb4d86c, 0x56faac56,
-    0xf407f3f4, 0xea25cfea, 0x65afca65, 0x7a8ef47a, 0xaee947ae, 0x08181008,
-    0xbad56fba, 0x7888f078, 0x256f4a25, 0x2e725c2e, 0x1c24381c, 0xa6f157a6,
-    0xb4c773b4, 0xc65197c6, 0xe823cbe8, 0xdd7ca1dd, 0x749ce874, 0x1f213e1f,
-    0x4bdd964b, 0xbddc61bd, 0x8b860d8b, 0x8a850f8a, 0x7090e070, 0x3e427c3e,
-    0xb5c471b5, 0x66aacc66, 0x48d89048, 0x03050603, 0xf601f7f6, 0x0e121c0e,
-    0x61a3c261, 0x355f6a35, 0x57f9ae57, 0xb9d069b9, 0x86911786, 0xc15899c1,
-    0x1d273a1d, 0x9eb9279e, 0xe138d9e1, 0xf813ebf8, 0x98b32b98, 0x11332211,
-    0x69bbd269, 0xd970a9d9, 0x8e89078e, 0x94a73394, 0x9bb62d9b, 0x1e223c1e,
-    0x87921587, 0xe920c9e9, 0xce4987ce, 0x55ffaa55, 0x28785028, 0xdf7aa5df,
-    0x8c8f038c, 0xa1f859a1, 0x89800989, 0x0d171a0d, 0xbfda65bf, 0xe631d7e6,
-    0x42c68442, 0x68b8d068, 0x41c38241, 0x99b02999, 0x2d775a2d, 0x0f111e0f,
-    0xb0cb7bb0, 0x54fca854, 0xbbd66dbb, 0x163a2c16,
+    0x63c6a563, 0x7cf8847c, 0x77ee9977, 0x7bf68d7b, 0xf2ff0df2, 0x6bd6bd6b,
+    0x6fdeb16f, 0xc59154c5, 0x30605030, 0x01020301, 0x67cea967, 0x2b567d2b,
+    0xfee719fe, 0xd7b562d7, 0xab4de6ab, 0x76ec9a76, 0xca8f45ca, 0x821f9d82,
+    0xc98940c9, 0x7dfa877d, 0xfaef15fa, 0x59b2eb59, 0x478ec947, 0xf0fb0bf0,
+    0xad41ecad, 0xd4b367d4, 0xa25ffda2, 0xaf45eaaf, 0x9c23bf9c, 0xa453f7a4,
+    0x72e49672, 0xc09b5bc0, 0xb775c2b7, 0xfde11cfd, 0x933dae93, 0x264c6a26,
+    0x366c5a36, 0x3f7e413f, 0xf7f502f7, 0xcc834fcc, 0x34685c34, 0xa551f4a5,
+    0xe5d134e5, 0xf1f908f1, 0x71e29371, 0xd8ab73d8, 0x31625331, 0x152a3f15,
+    0x04080c04, 0xc79552c7, 0x23466523, 0xc39d5ec3, 0x18302818, 0x9637a196,
+    0x050a0f05, 0x9a2fb59a, 0x070e0907, 0x12243612, 0x801b9b80, 0xe2df3de2,
+    0xebcd26eb, 0x274e6927, 0xb27fcdb2, 0x75ea9f75, 0x09121b09, 0x831d9e83,
+    0x2c58742c, 0x1a342e1a, 0x1b362d1b, 0x6edcb26e, 0x5ab4ee5a, 0xa05bfba0,
+    0x52a4f652, 0x3b764d3b, 0xd6b761d6, 0xb37dceb3, 0x29527b29, 0xe3dd3ee3,
+    0x2f5e712f, 0x84139784, 0x53a6f553, 0xd1b968d1, 0x00000000, 0xedc12ced,
+    0x20406020, 0xfce31ffc, 0xb179c8b1, 0x5bb6ed5b, 0x6ad4be6a, 0xcb8d46cb,
+    0xbe67d9be, 0x39724b39, 0x4a94de4a, 0x4c98d44c, 0x58b0e858, 0xcf854acf,
+    0xd0bb6bd0, 0xefc52aef, 0xaa4fe5aa, 0xfbed16fb, 0x4386c543, 0x4d9ad74d,
+    0x33665533, 0x85119485, 0x458acf45, 0xf9e910f9, 0x02040602, 0x7ffe817f,
+    0x50a0f050, 0x3c78443c, 0x9f25ba9f, 0xa84be3a8, 0x51a2f351, 0xa35dfea3,
+    0x4080c040, 0x8f058a8f, 0x923fad92, 0x9d21bc9d, 0x38704838, 0xf5f104f5,
+    0xbc63dfbc, 0xb677c1b6, 0xdaaf75da, 0x21426321, 0x10203010, 0xffe51aff,
+    0xf3fd0ef3, 0xd2bf6dd2, 0xcd814ccd, 0x0c18140c, 0x13263513, 0xecc32fec,
+    0x5fbee15f, 0x9735a297, 0x4488cc44, 0x172e3917, 0xc49357c4, 0xa755f2a7,
+    0x7efc827e, 0x3d7a473d, 0x64c8ac64, 0x5dbae75d, 0x19322b19, 0x73e69573,
+    0x60c0a060, 0x81199881, 0x4f9ed14f, 0xdca37fdc, 0x22446622, 0x2a547e2a,
+    0x903bab90, 0x880b8388, 0x468cca46, 0xeec729ee, 0xb86bd3b8, 0x14283c14,
+    0xdea779de, 0x5ebce25e, 0x0b161d0b, 0xdbad76db, 0xe0db3be0, 0x32645632,
+    0x3a744e3a, 0x0a141e0a, 0x4992db49, 0x060c0a06, 0x24486c24, 0x5cb8e45c,
+    0xc29f5dc2, 0xd3bd6ed3, 0xac43efac, 0x62c4a662, 0x9139a891, 0x9531a495,
+    0xe4d337e4, 0x79f28b79, 0xe7d532e7, 0xc88b43c8, 0x376e5937, 0x6ddab76d,
+    0x8d018c8d, 0xd5b164d5, 0x4e9cd24e, 0xa949e0a9, 0x6cd8b46c, 0x56acfa56,
+    0xf4f307f4, 0xeacf25ea, 0x65caaf65, 0x7af48e7a, 0xae47e9ae, 0x08101808,
+    0xba6fd5ba, 0x78f08878, 0x254a6f25, 0x2e5c722e, 0x1c38241c, 0xa657f1a6,
+    0xb473c7b4, 0xc69751c6, 0xe8cb23e8, 0xdda17cdd, 0x74e89c74, 0x1f3e211f,
+    0x4b96dd4b, 0xbd61dcbd, 0x8b0d868b, 0x8a0f858a, 0x70e09070, 0x3e7c423e,
+    0xb571c4b5, 0x66ccaa66, 0x4890d848, 0x03060503, 0xf6f701f6, 0x0e1c120e,
+    0x61c2a361, 0x356a5f35, 0x57aef957, 0xb969d0b9, 0x86179186, 0xc19958c1,
+    0x1d3a271d, 0x9e27b99e, 0xe1d938e1, 0xf8eb13f8, 0x982bb398, 0x11223311,
+    0x69d2bb69, 0xd9a970d9, 0x8e07898e, 0x9433a794, 0x9b2db69b, 0x1e3c221e,
+    0x87159287, 0xe9c920e9, 0xce8749ce, 0x55aaff55, 0x28507828, 0xdfa57adf,
+    0x8c038f8c, 0xa159f8a1, 0x89098089, 0x0d1a170d, 0xbf65dabf, 0xe6d731e6,
+    0x4284c642, 0x68d0b868, 0x4182c341, 0x9929b099, 0x2d5a772d, 0x0f1e110f,
+    0xb07bcbb0, 0x54a8fc54, 0xbb6dd6bb, 0x162c3a16,
 };
 
 constexpr uint32_t te3[256] = {
-    0x6363a5c6, 0x7c7c84f8, 0x777799ee, 0x7b7b8df6, 0xf2f20dff, 0x6b6bbdd6,
-    0x6f6fb1de, 0xc5c55491, 0x30305060, 0x01010302, 0x6767a9ce, 0x2b2b7d56,
-    0xfefe19e7, 0xd7d762b5, 0xababe64d, 0x76769aec, 0xcaca458f, 0x82829d1f,
-    0xc9c94089, 0x7d7d87fa, 0xfafa15ef, 0x5959ebb2, 0x4747c98e, 0xf0f00bfb,
-    0xadadec41, 0xd4d467b3, 0xa2a2fd5f, 0xafafea45, 0x9c9cbf23, 0xa4a4f753,
-    0x727296e4, 0xc0c05b9b, 0xb7b7c275, 0xfdfd1ce1, 0x9393ae3d, 0x26266a4c,
-    0x36365a6c, 0x3f3f417e, 0xf7f702f5, 0xcccc4f83, 0x34345c68, 0xa5a5f451,
-    0xe5e534d1, 0xf1f108f9, 0x717193e2, 0xd8d873ab, 0x31315362, 0x15153f2a,
-    0x04040c08, 0xc7c75295, 0x23236546, 0xc3c35e9d, 0x18182830, 0x9696a137,
-    0x05050f0a, 0x9a9ab52f, 0x0707090e, 0x12123624, 0x80809b1b, 0xe2e23ddf,
-    0xebeb26cd, 0x2727694e, 0xb2b2cd7f, 0x75759fea, 0x09091b12, 0x83839e1d,
-    0x2c2c7458, 0x1a1a2e34, 0x1b1b2d36, 0x6e6eb2dc, 0x5a5aeeb4, 0xa0a0fb5b,
-    0x5252f6a4, 0x3b3b4d76, 0xd6d661b7, 0xb3b3ce7d, 0x29297b52, 0xe3e33edd,
-    0x2f2f715e, 0x84849713, 0x5353f5a6, 0xd1d168b9, 0x00000000, 0xeded2cc1,
-    0x20206040, 0xfcfc1fe3, 0xb1b1c879, 0x5b5bedb6, 0x6a6abed4, 0xcbcb468d,
-    0xbebed967, 0x39394b72, 0x4a4ade94, 0x4c4cd498, 0x5858e8b0, 0xcfcf4a85,
-    0xd0d06bbb, 0xefef2ac5, 0xaaaae54f, 0xfbfb16ed, 0x4343c586, 0x4d4dd79a,
-    0x33335566, 0x85859411, 0x4545cf8a, 0xf9f910e9, 0x02020604, 0x7f7f81fe,
-    0x5050f0a0, 0x3c3c4478, 0x9f9fba25, 0xa8a8e34b, 0x5151f3a2, 0xa3a3fe5d,
-    0x4040c080, 0x8f8f8a05, 0x9292ad3f, 0x9d9dbc21, 0x38384870, 0xf5f504f1,
-    0xbcbcdf63, 0xb6b6c177, 0xdada75af, 0x21216342, 0x10103020, 0xffff1ae5,
-    0xf3f30efd, 0xd2d26dbf, 0xcdcd4c81, 0x0c0c1418, 0x13133526, 0xecec2fc3,
-    0x5f5fe1be, 0x9797a235, 0x4444cc88, 0x1717392e, 0xc4c45793, 0xa7a7f255,
-    0x7e7e82fc, 0x3d3d477a, 0x6464acc8, 0x5d5de7ba, 0x19192b32, 0x737395e6,
-    0x6060a0c0, 0x81819819, 0x4f4fd19e, 0xdcdc7fa3, 0x22226644, 0x2a2a7e54,
-    0x9090ab3b, 0x8888830b, 0x4646ca8c, 0xeeee29c7, 0xb8b8d36b, 0x14143c28,
-    0xdede79a7, 0x5e5ee2bc, 0x0b0b1d16, 0xdbdb76ad, 0xe0e03bdb, 0x32325664,
-    0x3a3a4e74, 0x0a0a1e14, 0x4949db92, 0x06060a0c, 0x24246c48, 0x5c5ce4b8,
-    0xc2c25d9f, 0xd3d36ebd, 0xacacef43, 0x6262a6c4, 0x9191a839, 0x9595a431,
-    0xe4e437d3, 0x79798bf2, 0xe7e732d5, 0xc8c8438b, 0x3737596e, 0x6d6db7da,
-    0x8d8d8c01, 0xd5d564b1, 0x4e4ed29c, 0xa9a9e049, 0x6c6cb4d8, 0x5656faac,
-    0xf4f407f3, 0xeaea25cf, 0x6565afca, 0x7a7a8ef4, 0xaeaee947, 0x08081810,
-    0xbabad56f, 0x787888f0, 0x25256f4a, 0x2e2e725c, 0x1c1c2438, 0xa6a6f157,
-    0xb4b4c773, 0xc6c65197, 0xe8e823cb, 0xdddd7ca1, 0x74749ce8, 0x1f1f213e,
-    0x4b4bdd96, 0xbdbddc61, 0x8b8b860d, 0x8a8a850f, 0x707090e0, 0x3e3e427c,
-    0xb5b5c471, 0x6666aacc, 0x4848d890, 0x03030506, 0xf6f601f7, 0x0e0e121c,
-    0x6161a3c2, 0x35355f6a, 0x5757f9ae, 0xb9b9d069, 0x86869117, 0xc1c15899,
-    0x1d1d273a, 0x9e9eb927, 0xe1e138d9, 0xf8f813eb, 0x9898b32b, 0x11113322,
-    0x6969bbd2, 0xd9d970a9, 0x8e8e8907, 0x9494a733, 0x9b9bb62d, 0x1e1e223c,
-    0x87879215, 0xe9e920c9, 0xcece4987, 0x5555ffaa, 0x28287850, 0xdfdf7aa5,
-    0x8c8c8f03, 0xa1a1f859, 0x89898009, 0x0d0d171a, 0xbfbfda65, 0xe6e631d7,
-    0x4242c684, 0x6868b8d0, 0x4141c382, 0x9999b029, 0x2d2d775a, 0x0f0f111e,
-    0xb0b0cb7b, 0x5454fca8, 0xbbbbd66d, 0x16163a2c,
-};
-
-struct alignas(16) u64x2 {
-  constexpr u64x2() : v{0, 0} {};
-  constexpr u64x2(uint64_t hi, uint64_t lo) : v{lo, hi} {}
-
-  uint64_t v[2];
+    0xc6a56363, 0xf8847c7c, 0xee997777, 0xf68d7b7b, 0xff0df2f2, 0xd6bd6b6b,
+    0xdeb16f6f, 0x9154c5c5, 0x60503030, 0x02030101, 0xcea96767, 0x567d2b2b,
+    0xe719fefe, 0xb562d7d7, 0x4de6abab, 0xec9a7676, 0x8f45caca, 0x1f9d8282,
+    0x8940c9c9, 0xfa877d7d, 0xef15fafa, 0xb2eb5959, 0x8ec94747, 0xfb0bf0f0,
+    0x41ecadad, 0xb367d4d4, 0x5ffda2a2, 0x45eaafaf, 0x23bf9c9c, 0x53f7a4a4,
+    0xe4967272, 0x9b5bc0c0, 0x75c2b7b7, 0xe11cfdfd, 0x3dae9393, 0x4c6a2626,
+    0x6c5a3636, 0x7e413f3f, 0xf502f7f7, 0x834fcccc, 0x685c3434, 0x51f4a5a5,
+    0xd134e5e5, 0xf908f1f1, 0xe2937171, 0xab73d8d8, 0x62533131, 0x2a3f1515,
+    0x080c0404, 0x9552c7c7, 0x46652323, 0x9d5ec3c3, 0x30281818, 0x37a19696,
+    0x0a0f0505, 0x2fb59a9a, 0x0e090707, 0x24361212, 0x1b9b8080, 0xdf3de2e2,
+    0xcd26ebeb, 0x4e692727, 0x7fcdb2b2, 0xea9f7575, 0x121b0909, 0x1d9e8383,
+    0x58742c2c, 0x342e1a1a, 0x362d1b1b, 0xdcb26e6e, 0xb4ee5a5a, 0x5bfba0a0,
+    0xa4f65252, 0x764d3b3b, 0xb761d6d6, 0x7dceb3b3, 0x527b2929, 0xdd3ee3e3,
+    0x5e712f2f, 0x13978484, 0xa6f55353, 0xb968d1d1, 0x00000000, 0xc12ceded,
+    0x40602020, 0xe31ffcfc, 0x79c8b1b1, 0xb6ed5b5b, 0xd4be6a6a, 0x8d46cbcb,
+    0x67d9bebe, 0x724b3939, 0x94de4a4a, 0x98d44c4c, 0xb0e85858, 0x854acfcf,
+    0xbb6bd0d0, 0xc52aefef, 0x4fe5aaaa, 0xed16fbfb, 0x86c54343, 0x9ad74d4d,
+    0x66553333, 0x11948585, 0x8acf4545, 0xe910f9f9, 0x04060202, 0xfe817f7f,
+    0xa0f05050, 0x78443c3c, 0x25ba9f9f, 0x4be3a8a8, 0xa2f35151, 0x5dfea3a3,
+    0x80c04040, 0x058a8f8f, 0x3fad9292, 0x21bc9d9d, 0x70483838, 0xf104f5f5,
+    0x63dfbcbc, 0x77c1b6b6, 0xaf75dada, 0x42632121, 0x20301010, 0xe51affff,
+    0xfd0ef3f3, 0xbf6dd2d2, 0x814ccdcd, 0x18140c0c, 0x26351313, 0xc32fecec,
+    0xbee15f5f, 0x35a29797, 0x88cc4444, 0x2e391717, 0x9357c4c4, 0x55f2a7a7,
+    0xfc827e7e, 0x7a473d3d, 0xc8ac6464, 0xbae75d5d, 0x322b1919, 0xe6957373,
+    0xc0a06060, 0x19988181, 0x9ed14f4f, 0xa37fdcdc, 0x44662222, 0x547e2a2a,
+    0x3bab9090, 0x0b838888, 0x8cca4646, 0xc729eeee, 0x6bd3b8b8, 0x283c1414,
+    0xa779dede, 0xbce25e5e, 0x161d0b0b, 0xad76dbdb, 0xdb3be0e0, 0x64563232,
+    0x744e3a3a, 0x141e0a0a, 0x92db4949, 0x0c0a0606, 0x486c2424, 0xb8e45c5c,
+    0x9f5dc2c2, 0xbd6ed3d3, 0x43efacac, 0xc4a66262, 0x39a89191, 0x31a49595,
+    0xd337e4e4, 0xf28b7979, 0xd532e7e7, 0x8b43c8c8, 0x6e593737, 0xdab76d6d,
+    0x018c8d8d, 0xb164d5d5, 0x9cd24e4e, 0x49e0a9a9, 0xd8b46c6c, 0xacfa5656,
+    0xf307f4f4, 0xcf25eaea, 0xcaaf6565, 0xf48e7a7a, 0x47e9aeae, 0x10180808,
+    0x6fd5baba, 0xf0887878, 0x4a6f2525, 0x5c722e2e, 0x38241c1c, 0x57f1a6a6,
+    0x73c7b4b4, 0x9751c6c6, 0xcb23e8e8, 0xa17cdddd, 0xe89c7474, 0x3e211f1f,
+    0x96dd4b4b, 0x61dcbdbd, 0x0d868b8b, 0x0f858a8a, 0xe0907070, 0x7c423e3e,
+    0x71c4b5b5, 0xccaa6666, 0x90d84848, 0x06050303, 0xf701f6f6, 0x1c120e0e,
+    0xc2a36161, 0x6a5f3535, 0xaef95757, 0x69d0b9b9, 0x17918686, 0x9958c1c1,
+    0x3a271d1d, 0x27b99e9e, 0xd938e1e1, 0xeb13f8f8, 0x2bb39898, 0x22331111,
+    0xd2bb6969, 0xa970d9d9, 0x07898e8e, 0x33a79494, 0x2db69b9b, 0x3c221e1e,
+    0x15928787, 0xc920e9e9, 0x8749cece, 0xaaff5555, 0x50782828, 0xa57adfdf,
+    0x038f8c8c, 0x59f8a1a1, 0x09808989, 0x1a170d0d, 0x65dabfbf, 0xd731e6e6,
+    0x84c64242, 0xd0b86868, 0x82c34141, 0x29b09999, 0x5a772d2d, 0x1e110f0f,
+    0x7bcbb0b0, 0xa8fc5454, 0x6dd6bbbb, 0x2c3a1616,
 };
 
 // Software implementation of the Vector128 class, using uint32_t
 // as an underlying vector register.
-//
-struct Vector128 {
-  inline ABSL_RANDOM_INTERNAL_ATTRIBUTE_ALWAYS_INLINE Vector128& operator^=(
-      const Vector128& other) {
-    s[0] ^= other.s[0];
-    s[1] ^= other.s[1];
-    s[2] ^= other.s[2];
-    s[3] ^= other.s[3];
-    return *this;
-  }
-
+struct alignas(16) Vector128 {
   uint32_t s[4];
 };
 
 inline ABSL_RANDOM_INTERNAL_ATTRIBUTE_ALWAYS_INLINE Vector128
-Vector128Load(const void* ABSL_RANDOM_INTERNAL_RESTRICT from) {
+Vector128Load(const void* from) {
   Vector128 result;
-  const uint8_t* ABSL_RANDOM_INTERNAL_RESTRICT src =
-      reinterpret_cast<const uint8_t*>(from);
-
-  result.s[0] = static_cast<uint32_t>(src[0]) << 24 |
-                static_cast<uint32_t>(src[1]) << 16 |
-                static_cast<uint32_t>(src[2]) << 8 |
-                static_cast<uint32_t>(src[3]);
-  result.s[1] = static_cast<uint32_t>(src[4]) << 24 |
-                static_cast<uint32_t>(src[5]) << 16 |
-                static_cast<uint32_t>(src[6]) << 8 |
-                static_cast<uint32_t>(src[7]);
-  result.s[2] = static_cast<uint32_t>(src[8]) << 24 |
-                static_cast<uint32_t>(src[9]) << 16 |
-                static_cast<uint32_t>(src[10]) << 8 |
-                static_cast<uint32_t>(src[11]);
-  result.s[3] = static_cast<uint32_t>(src[12]) << 24 |
-                static_cast<uint32_t>(src[13]) << 16 |
-                static_cast<uint32_t>(src[14]) << 8 |
-                static_cast<uint32_t>(src[15]);
+  std::memcpy(result.s, from, sizeof(Vector128));
   return result;
 }
 
 inline ABSL_RANDOM_INTERNAL_ATTRIBUTE_ALWAYS_INLINE void Vector128Store(
-    const Vector128& v, void* ABSL_RANDOM_INTERNAL_RESTRICT to) {
-  uint8_t* dst = reinterpret_cast<uint8_t*>(to);
-  dst[0] = static_cast<uint8_t>(v.s[0] >> 24);
-  dst[1] = static_cast<uint8_t>(v.s[0] >> 16);
-  dst[2] = static_cast<uint8_t>(v.s[0] >> 8);
-  dst[3] = static_cast<uint8_t>(v.s[0]);
-  dst[4] = static_cast<uint8_t>(v.s[1] >> 24);
-  dst[5] = static_cast<uint8_t>(v.s[1] >> 16);
-  dst[6] = static_cast<uint8_t>(v.s[1] >> 8);
-  dst[7] = static_cast<uint8_t>(v.s[1]);
-  dst[8] = static_cast<uint8_t>(v.s[2] >> 24);
-  dst[9] = static_cast<uint8_t>(v.s[2] >> 16);
-  dst[10] = static_cast<uint8_t>(v.s[2] >> 8);
-  dst[11] = static_cast<uint8_t>(v.s[2]);
-  dst[12] = static_cast<uint8_t>(v.s[3] >> 24);
-  dst[13] = static_cast<uint8_t>(v.s[3] >> 16);
-  dst[14] = static_cast<uint8_t>(v.s[3] >> 8);
-  dst[15] = static_cast<uint8_t>(v.s[3]);
+    const Vector128& v, void* to) {
+  std::memcpy(to, v.s, sizeof(Vector128));
 }
 
 // One round of AES. "round_key" is a public constant for breaking the
 // symmetry of AES (ensures previously equal columns differ afterwards).
 inline ABSL_RANDOM_INTERNAL_ATTRIBUTE_ALWAYS_INLINE Vector128
 AesRound(const Vector128& state, const Vector128& round_key) {
-  // clang-format off
   Vector128 result;
-  result.s[0] = round_key.s[0] ^
-                te0[uint8_t(state.s[0] >> 24)] ^
-                te1[uint8_t(state.s[1] >> 16)] ^
-                te2[uint8_t(state.s[2] >> 8)] ^
-                te3[uint8_t(state.s[3])];
-  result.s[1] = round_key.s[1] ^
-                te0[uint8_t(state.s[1] >> 24)] ^
-                te1[uint8_t(state.s[2] >> 16)] ^
-                te2[uint8_t(state.s[3] >> 8)] ^
-                te3[uint8_t(state.s[0])];
-  result.s[2] = round_key.s[2] ^
-                te0[uint8_t(state.s[2] >> 24)] ^
-                te1[uint8_t(state.s[3] >> 16)] ^
-                te2[uint8_t(state.s[0] >> 8)] ^
-                te3[uint8_t(state.s[1])];
-  result.s[3] = round_key.s[3] ^
-                te0[uint8_t(state.s[3] >> 24)] ^
-                te1[uint8_t(state.s[0] >> 16)] ^
-                te2[uint8_t(state.s[1] >> 8)] ^
-                te3[uint8_t(state.s[2])];
+#ifdef ABSL_IS_LITTLE_ENDIAN
+  result.s[0] = round_key.s[0] ^                  //
+                te0[uint8_t(state.s[0])] ^        //
+                te1[uint8_t(state.s[1] >> 8)] ^   //
+                te2[uint8_t(state.s[2] >> 16)] ^  //
+                te3[uint8_t(state.s[3] >> 24)];
+  result.s[1] = round_key.s[1] ^                  //
+                te0[uint8_t(state.s[1])] ^        //
+                te1[uint8_t(state.s[2] >> 8)] ^   //
+                te2[uint8_t(state.s[3] >> 16)] ^  //
+                te3[uint8_t(state.s[0] >> 24)];
+  result.s[2] = round_key.s[2] ^                  //
+                te0[uint8_t(state.s[2])] ^        //
+                te1[uint8_t(state.s[3] >> 8)] ^   //
+                te2[uint8_t(state.s[0] >> 16)] ^  //
+                te3[uint8_t(state.s[1] >> 24)];
+  result.s[3] = round_key.s[3] ^                  //
+                te0[uint8_t(state.s[3])] ^        //
+                te1[uint8_t(state.s[0] >> 8)] ^   //
+                te2[uint8_t(state.s[1] >> 16)] ^  //
+                te3[uint8_t(state.s[2] >> 24)];
+#else
+  result.s[0] = round_key.s[0] ^                  //
+                te0[uint8_t(state.s[0])] ^        //
+                te1[uint8_t(state.s[3] >> 8)] ^   //
+                te2[uint8_t(state.s[2] >> 16)] ^  //
+                te3[uint8_t(state.s[1] >> 24)];
+  result.s[1] = round_key.s[1] ^                  //
+                te0[uint8_t(state.s[1])] ^        //
+                te1[uint8_t(state.s[0] >> 8)] ^   //
+                te2[uint8_t(state.s[3] >> 16)] ^  //
+                te3[uint8_t(state.s[2] >> 24)];
+  result.s[2] = round_key.s[2] ^                  //
+                te0[uint8_t(state.s[2])] ^        //
+                te1[uint8_t(state.s[1] >> 8)] ^   //
+                te2[uint8_t(state.s[0] >> 16)] ^  //
+                te3[uint8_t(state.s[3] >> 24)];
+  result.s[3] = round_key.s[3] ^                  //
+                te0[uint8_t(state.s[3])] ^        //
+                te1[uint8_t(state.s[2] >> 8)] ^   //
+                te2[uint8_t(state.s[1] >> 16)] ^  //
+                te3[uint8_t(state.s[0] >> 24)];
+#endif
   return result;
-  // clang-format on
 }
 
-// RANDen = RANDom generator or beetroots in Swiss German.
-// 'Strong' (well-distributed, unpredictable, backtracking-resistant) random
-// generator, faster in some benchmarks than std::mt19937_64 and pcg64_c32.
-//
-// High-level summary:
-// 1) Reverie (see "A Robust and Sponge-Like PRNG with Improved Efficiency") is
-//    a sponge-like random generator that requires a cryptographic permutation.
-//    It improves upon "Provably Robust Sponge-Based PRNGs and KDFs" by
-//    achieving backtracking resistance with only one Permute() per buffer.
-//
-// 2) "Simpira v2: A Family of Efficient Permutations Using the AES Round
-//    Function" constructs up to 1024-bit permutations using an improved
-//    Generalized Feistel network with 2-round AES-128 functions. This Feistel
-//    block shuffle achieves diffusion faster and is less vulnerable to
-//    sliced-biclique attacks than the Type-2 cyclic shuffle.
-//
-// 3) "Improving the Generalized Feistel" and "New criterion for diffusion
-//    property" extends the same kind of improved Feistel block shuffle to 16
-//    branches, which enables a 2048-bit permutation.
-//
-// Combine these three ideas and also change Simpira's subround keys from
-// structured/low-entropy counters to digits of Pi.
-
-// Randen constants.
-constexpr size_t kFeistelBlocks = 16;
-constexpr size_t kFeistelFunctions = kFeistelBlocks / 2;  // = 8
-constexpr size_t kFeistelRounds = 16 + 1;  // > 4 * log2(kFeistelBlocks)
-constexpr size_t kKeys = kFeistelRounds * kFeistelFunctions;
-
-// INCLUDE keys.
-#include "absl/random/internal/randen-keys.inc"
-
-static_assert(kKeys == kRoundKeys, "kKeys and kRoundKeys must be equal");
-
-// 2 uint64_t lanes per Vector128
-static constexpr size_t kLanes = 2;
+using ::absl::random_internal::RandenTraits;
 
 // The improved Feistel block shuffle function for 16 blocks.
 inline ABSL_RANDOM_INTERNAL_ATTRIBUTE_ALWAYS_INLINE void BlockShuffle(
-    uint64_t* ABSL_RANDOM_INTERNAL_RESTRICT state_u64) {
-  static_assert(kFeistelBlocks == 16,
+    absl::uint128* state) {
+  static_assert(RandenTraits::kFeistelBlocks == 16,
                 "Feistel block shuffle only works for 16 blocks.");
 
-  constexpr size_t shuffle[kFeistelBlocks] = {7,  2, 13, 4,  11, 8,  3, 6,
-                                              15, 0, 9,  10, 1,  14, 5, 12};
-
-  u64x2* ABSL_RANDOM_INTERNAL_RESTRICT state =
-      reinterpret_cast<u64x2*>(state_u64);
+  constexpr size_t shuffle[RandenTraits::kFeistelBlocks] = {
+      7, 2, 13, 4, 11, 8, 3, 6, 15, 0, 9, 10, 1, 14, 5, 12};
 
   // The fully unrolled loop without the memcpy improves the speed by about
-  // 30% over the equivalent (leaving code here as a comment):
-  if (false) {
-    u64x2 source[kFeistelBlocks];
-    std::memcpy(source, state, sizeof(source));
-    for (size_t i = 0; i < kFeistelBlocks; i++) {
-      const u64x2 v0 = source[shuffle[i]];
-      state[i] = v0;
-    }
+  // 30% over the equivalent:
+#if 0
+  absl::uint128 source[RandenTraits::kFeistelBlocks];
+  std::memcpy(source, state, sizeof(source));
+  for (size_t i = 0; i < RandenTraits::kFeistelBlocks; i++) {
+    const absl::uint128 v0 = source[shuffle[i]];
+    state[i] = v0;
   }
+  return;
+#endif
 
-  const u64x2 v0 = state[shuffle[0]];
-  const u64x2 v1 = state[shuffle[1]];
-  const u64x2 v2 = state[shuffle[2]];
-  const u64x2 v3 = state[shuffle[3]];
-  const u64x2 v4 = state[shuffle[4]];
-  const u64x2 v5 = state[shuffle[5]];
-  const u64x2 v6 = state[shuffle[6]];
-  const u64x2 v7 = state[shuffle[7]];
-  const u64x2 w0 = state[shuffle[8]];
-  const u64x2 w1 = state[shuffle[9]];
-  const u64x2 w2 = state[shuffle[10]];
-  const u64x2 w3 = state[shuffle[11]];
-  const u64x2 w4 = state[shuffle[12]];
-  const u64x2 w5 = state[shuffle[13]];
-  const u64x2 w6 = state[shuffle[14]];
-  const u64x2 w7 = state[shuffle[15]];
+  const absl::uint128 v0 = state[shuffle[0]];
+  const absl::uint128 v1 = state[shuffle[1]];
+  const absl::uint128 v2 = state[shuffle[2]];
+  const absl::uint128 v3 = state[shuffle[3]];
+  const absl::uint128 v4 = state[shuffle[4]];
+  const absl::uint128 v5 = state[shuffle[5]];
+  const absl::uint128 v6 = state[shuffle[6]];
+  const absl::uint128 v7 = state[shuffle[7]];
+  const absl::uint128 w0 = state[shuffle[8]];
+  const absl::uint128 w1 = state[shuffle[9]];
+  const absl::uint128 w2 = state[shuffle[10]];
+  const absl::uint128 w3 = state[shuffle[11]];
+  const absl::uint128 w4 = state[shuffle[12]];
+  const absl::uint128 w5 = state[shuffle[13]];
+  const absl::uint128 w6 = state[shuffle[14]];
+  const absl::uint128 w7 = state[shuffle[15]];
   state[0] = v0;
   state[1] = v1;
   state[2] = v2;
@@ -431,24 +361,24 @@ inline ABSL_RANDOM_INTERNAL_ATTRIBUTE_ALWAYS_INLINE void BlockShuffle(
 // per 16 bytes (vs. 10 for AES-CTR). Computing eight round functions in
 // parallel hides the 7-cycle AESNI latency on HSW. Note that the Feistel
 // XORs are 'free' (included in the second AES instruction).
-inline ABSL_RANDOM_INTERNAL_ATTRIBUTE_ALWAYS_INLINE const u64x2* FeistelRound(
-    uint64_t* ABSL_RANDOM_INTERNAL_RESTRICT state,
-    const u64x2* ABSL_RANDOM_INTERNAL_RESTRICT keys) {
-  for (size_t branch = 0; branch < kFeistelBlocks; branch += 4) {
-    const Vector128 s0 = Vector128Load(state + kLanes * branch);
-    const Vector128 s1 = Vector128Load(state + kLanes * (branch + 1));
+inline ABSL_RANDOM_INTERNAL_ATTRIBUTE_ALWAYS_INLINE const absl::uint128*
+FeistelRound(absl::uint128* ABSL_RANDOM_INTERNAL_RESTRICT state,
+             const absl::uint128* ABSL_RANDOM_INTERNAL_RESTRICT keys) {
+  for (size_t branch = 0; branch < RandenTraits::kFeistelBlocks; branch += 4) {
+    const Vector128 s0 = Vector128Load(state + branch);
+    const Vector128 s1 = Vector128Load(state + branch + 1);
     const Vector128 f0 = AesRound(s0, Vector128Load(keys));
     keys++;
     const Vector128 o1 = AesRound(f0, s1);
-    Vector128Store(o1, state + kLanes * (branch + 1));
+    Vector128Store(o1, state + branch + 1);
 
     // Manually unroll this loop once. about 10% better than not unrolled.
-    const Vector128 s2 = Vector128Load(state + kLanes * (branch + 2));
-    const Vector128 s3 = Vector128Load(state + kLanes * (branch + 3));
+    const Vector128 s2 = Vector128Load(state + branch + 2);
+    const Vector128 s3 = Vector128Load(state + branch + 3);
     const Vector128 f2 = AesRound(s2, Vector128Load(keys));
     keys++;
     const Vector128 o3 = AesRound(f2, s3);
-    Vector128Store(o3, state + kLanes * (branch + 3));
+    Vector128Store(o3, state + branch + 3);
   }
   return keys;
 }
@@ -458,55 +388,85 @@ inline ABSL_RANDOM_INTERNAL_ATTRIBUTE_ALWAYS_INLINE const u64x2* FeistelRound(
 // 2^64 queries if the round function is a PRF. This is similar to the b=8 case
 // of Simpira v2, but more efficient than its generic construction for b=16.
 inline ABSL_RANDOM_INTERNAL_ATTRIBUTE_ALWAYS_INLINE void Permute(
-    const void* keys, uint64_t* ABSL_RANDOM_INTERNAL_RESTRICT state) {
-  const u64x2* ABSL_RANDOM_INTERNAL_RESTRICT keys128 =
-      static_cast<const u64x2*>(keys);
-  for (size_t round = 0; round < kFeistelRounds; ++round) {
-    keys128 = FeistelRound(state, keys128);
+    absl::uint128* state,
+    const absl::uint128* ABSL_RANDOM_INTERNAL_RESTRICT keys) {
+  for (size_t round = 0; round < RandenTraits::kFeistelRounds; ++round) {
+    keys = FeistelRound(state, keys);
     BlockShuffle(state);
   }
 }
 
+// Enables native loads in the round loop by pre-swapping.
+inline ABSL_RANDOM_INTERNAL_ATTRIBUTE_ALWAYS_INLINE void SwapEndian(
+    absl::uint128* state) {
+#ifdef ABSL_IS_BIG_ENDIAN
+  for (uint32_t block = 0; block < RandenTraits::kFeistelBlocks; ++block) {
+    uint64_t new_lo = absl::little_endian::ToHost64(
+        static_cast<uint64_t>(state[block] >> 64));
+    uint64_t new_hi = absl::little_endian::ToHost64(
+        static_cast<uint64_t>((state[block] << 64) >> 64));
+    state[block] = (static_cast<absl::uint128>(new_hi) << 64) | new_lo;
+  }
+#else
+  // Avoid warning about unused variable.
+  (void)state;
+#endif
+}
+
 }  // namespace
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
 const void* RandenSlow::GetKeys() {
   // Round keys for one AES per Feistel round and branch.
   // The canonical implementation uses first digits of Pi.
-  return round_keys;
+#ifdef ABSL_IS_LITTLE_ENDIAN
+  return kRandenRoundKeys;
+#else
+  return kRandenRoundKeysBE;
+#endif
 }
 
 void RandenSlow::Absorb(const void* seed_void, void* state_void) {
-  uint64_t* ABSL_RANDOM_INTERNAL_RESTRICT state =
-      reinterpret_cast<uint64_t*>(state_void);
-  const uint64_t* ABSL_RANDOM_INTERNAL_RESTRICT seed =
-      reinterpret_cast<const uint64_t*>(seed_void);
-
-  constexpr size_t kCapacityBlocks = kCapacityBytes / sizeof(uint64_t);
-  static_assert(kCapacityBlocks * sizeof(uint64_t) == kCapacityBytes,
-                "Not i*V");
-  for (size_t i = kCapacityBlocks; i < kStateBytes / sizeof(uint64_t); ++i) {
+  auto* state =
+      reinterpret_cast<uint64_t * ABSL_RANDOM_INTERNAL_RESTRICT>(state_void);
+  const auto* seed =
+      reinterpret_cast<const uint64_t * ABSL_RANDOM_INTERNAL_RESTRICT>(
+          seed_void);
+
+  constexpr size_t kCapacityBlocks =
+      RandenTraits::kCapacityBytes / sizeof(uint64_t);
+  static_assert(
+      kCapacityBlocks * sizeof(uint64_t) == RandenTraits::kCapacityBytes,
+      "Not i*V");
+
+  for (size_t i = kCapacityBlocks;
+       i < RandenTraits::kStateBytes / sizeof(uint64_t); ++i) {
     state[i] ^= seed[i - kCapacityBlocks];
   }
 }
 
-void RandenSlow::Generate(const void* keys, void* state_void) {
-  static_assert(kCapacityBytes == sizeof(Vector128), "Capacity mismatch");
+void RandenSlow::Generate(const void* keys_void, void* state_void) {
+  static_assert(RandenTraits::kCapacityBytes == sizeof(absl::uint128),
+                "Capacity mismatch");
+
+  auto* state = reinterpret_cast<absl::uint128*>(state_void);
+  const auto* keys = reinterpret_cast<const absl::uint128*>(keys_void);
+
+  const absl::uint128 prev_inner = state[0];
 
-  uint64_t* ABSL_RANDOM_INTERNAL_RESTRICT state =
-      reinterpret_cast<uint64_t*>(state_void);
+  SwapEndian(state);
 
-  const Vector128 prev_inner = Vector128Load(state);
+  Permute(state, keys);
 
-  Permute(keys, state);
+  SwapEndian(state);
 
   // Ensure backtracking resistance.
-  Vector128 inner = Vector128Load(state);
-  inner ^= prev_inner;
-  Vector128Store(inner, state);
+  *state ^= prev_inner;
 }
 
 }  // namespace random_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/random/internal/randen_slow.h b/absl/random/internal/randen_slow.h
index 30586130800..532c3a8991e 100644
--- a/absl/random/internal/randen_slow.h
+++ b/absl/random/internal/randen_slow.h
@@ -17,27 +17,24 @@
 
 #include <cstddef>
 
+#include "absl/base/config.h"
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
-// RANDen = RANDom generator or beetroots in Swiss German.
+// RANDen = RANDom generator or beetroots in Swiss High German.
 // RandenSlow implements the basic state manipulation methods for
 // architectures lacking AES hardware acceleration intrinsics.
 class RandenSlow {
  public:
-  // Size of the entire sponge / state for the randen PRNG.
-  static constexpr size_t kStateBytes = 256;  // 2048-bit
-
-  // Size of the 'inner' (inaccessible) part of the sponge. Larger values would
-  // require more frequent calls to RandenGenerate.
-  static constexpr size_t kCapacityBytes = 16;  // 128-bit
-
   static void Generate(const void* keys, void* state_void);
   static void Absorb(const void* seed_void, void* state_void);
   static const void* GetKeys();
 };
 
 }  // namespace random_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_INTERNAL_RANDEN_SLOW_H_
diff --git a/absl/random/internal/randen_slow_test.cc b/absl/random/internal/randen_slow_test.cc
index c07155d8dd0..b9d54aac751 100644
--- a/absl/random/internal/randen_slow_test.cc
+++ b/absl/random/internal/randen_slow_test.cc
@@ -14,48 +14,48 @@
 
 #include "absl/random/internal/randen_slow.h"
 
+#include <cstdint>
 #include <cstring>
 
 #include "gtest/gtest.h"
+#include "absl/random/internal/randen_traits.h"
 
 namespace {
 
 using absl::random_internal::RandenSlow;
-
-// Local state parameters.
-constexpr size_t kSeedBytes =
-    RandenSlow::kStateBytes - RandenSlow::kCapacityBytes;
-constexpr size_t kStateSizeT = RandenSlow::kStateBytes / sizeof(uint64_t);
-constexpr size_t kSeedSizeT = kSeedBytes / sizeof(uint32_t);
-
-struct randen {
-  uint64_t state[kStateSizeT];
-  uint32_t seed[kSeedSizeT];
-};
+using absl::random_internal::RandenTraits;
 
 TEST(RandenSlowTest, Default) {
-  constexpr uint64_t kGolden[] = {
-      0x6c6534090ee6d3ee, 0x044e2b9b9d5333c6, 0xc3c14f134e433977,
-      0xdda9f47cd90410ee, 0x887bf3087fd8ca10, 0xf0b780f545c72912,
-      0x15dbb1d37696599f, 0x30ec63baff3c6d59, 0xb29f73606f7f20a6,
-      0x02808a316f49a54c, 0x3b8feaf9d5c8e50e, 0x9cbf605e3fd9de8a,
-      0xc970ae1a78183bbb, 0xd8b2ffd356301ed5, 0xf4b327fe0fc73c37,
-      0xcdfd8d76eb8f9a19, 0xc3a506eb91420c9d, 0xd5af05dd3eff9556,
-      0x48db1bb78f83c4a1, 0x7023920e0d6bfe8c, 0x58d3575834956d42,
-      0xed1ef4c26b87b840, 0x8eef32a23e0b2df3, 0x497cabf3431154fc,
-      0x4e24370570029a8b, 0xd88b5749f090e5ea, 0xc651a582a970692f,
-      0x78fcec2cbb6342f5, 0x463cb745612f55db, 0x352ee4ad1816afe3,
-      0x026ff374c101da7e, 0x811ef0821c3de851,
+  constexpr uint8_t kGolden[] = {
+      0xee, 0xd3, 0xe6, 0x0e, 0x09, 0x34, 0x65, 0x6c, 0xc6, 0x33, 0x53, 0x9d,
+      0x9b, 0x2b, 0x4e, 0x04, 0x77, 0x39, 0x43, 0x4e, 0x13, 0x4f, 0xc1, 0xc3,
+      0xee, 0x10, 0x04, 0xd9, 0x7c, 0xf4, 0xa9, 0xdd, 0x10, 0xca, 0xd8, 0x7f,
+      0x08, 0xf3, 0x7b, 0x88, 0x12, 0x29, 0xc7, 0x45, 0xf5, 0x80, 0xb7, 0xf0,
+      0x9f, 0x59, 0x96, 0x76, 0xd3, 0xb1, 0xdb, 0x15, 0x59, 0x6d, 0x3c, 0xff,
+      0xba, 0x63, 0xec, 0x30, 0xa6, 0x20, 0x7f, 0x6f, 0x60, 0x73, 0x9f, 0xb2,
+      0x4c, 0xa5, 0x49, 0x6f, 0x31, 0x8a, 0x80, 0x02, 0x0e, 0xe5, 0xc8, 0xd5,
+      0xf9, 0xea, 0x8f, 0x3b, 0x8a, 0xde, 0xd9, 0x3f, 0x5e, 0x60, 0xbf, 0x9c,
+      0xbb, 0x3b, 0x18, 0x78, 0x1a, 0xae, 0x70, 0xc9, 0xd5, 0x1e, 0x30, 0x56,
+      0xd3, 0xff, 0xb2, 0xd8, 0x37, 0x3c, 0xc7, 0x0f, 0xfe, 0x27, 0xb3, 0xf4,
+      0x19, 0x9a, 0x8f, 0xeb, 0x76, 0x8d, 0xfd, 0xcd, 0x9d, 0x0c, 0x42, 0x91,
+      0xeb, 0x06, 0xa5, 0xc3, 0x56, 0x95, 0xff, 0x3e, 0xdd, 0x05, 0xaf, 0xd5,
+      0xa1, 0xc4, 0x83, 0x8f, 0xb7, 0x1b, 0xdb, 0x48, 0x8c, 0xfe, 0x6b, 0x0d,
+      0x0e, 0x92, 0x23, 0x70, 0x42, 0x6d, 0x95, 0x34, 0x58, 0x57, 0xd3, 0x58,
+      0x40, 0xb8, 0x87, 0x6b, 0xc2, 0xf4, 0x1e, 0xed, 0xf3, 0x2d, 0x0b, 0x3e,
+      0xa2, 0x32, 0xef, 0x8e, 0xfc, 0x54, 0x11, 0x43, 0xf3, 0xab, 0x7c, 0x49,
+      0x8b, 0x9a, 0x02, 0x70, 0x05, 0x37, 0x24, 0x4e, 0xea, 0xe5, 0x90, 0xf0,
+      0x49, 0x57, 0x8b, 0xd8, 0x2f, 0x69, 0x70, 0xa9, 0x82, 0xa5, 0x51, 0xc6,
+      0xf5, 0x42, 0x63, 0xbb, 0x2c, 0xec, 0xfc, 0x78, 0xdb, 0x55, 0x2f, 0x61,
+      0x45, 0xb7, 0x3c, 0x46, 0xe3, 0xaf, 0x16, 0x18, 0xad, 0xe4, 0x2e, 0x35,
+      0x7e, 0xda, 0x01, 0xc1, 0x74, 0xf3, 0x6f, 0x02, 0x51, 0xe8, 0x3d, 0x1c,
+      0x82, 0xf0, 0x1e, 0x81,
   };
 
-  alignas(16) randen d;
-  std::memset(d.state, 0, sizeof(d.state));
-  RandenSlow::Generate(RandenSlow::GetKeys(), d.state);
+  alignas(16) uint8_t state[RandenTraits::kStateBytes];
+  std::memset(state, 0, sizeof(state));
 
-  uint64_t* id = d.state;
-  for (const auto& elem : kGolden) {
-    EXPECT_EQ(elem, *id++);
-  }
+  RandenSlow::Generate(RandenSlow::GetKeys(), state);
+  EXPECT_EQ(0, std::memcmp(state, kGolden, sizeof(state)));
 }
 
 }  // namespace
diff --git a/absl/random/internal/randen_test.cc b/absl/random/internal/randen_test.cc
index c186fe0d686..ff7365b3b29 100644
--- a/absl/random/internal/randen_test.cc
+++ b/absl/random/internal/randen_test.cc
@@ -14,57 +14,63 @@
 
 #include "absl/random/internal/randen.h"
 
+#include <cstdint>
 #include <cstring>
+#include <type_traits>
 
 #include "gtest/gtest.h"
-#include "absl/meta/type_traits.h"
 
 namespace {
 
 using absl::random_internal::Randen;
 
-// Local state parameters.
-constexpr size_t kStateSizeT = Randen::kStateBytes / sizeof(uint64_t);
-
 TEST(RandenTest, CopyAndMove) {
-  static_assert(std::is_copy_constructible<Randen>::value,
+  static_assert(std::is_copy_constructible_v<Randen>,
                 "Randen must be copy constructible");
 
-  static_assert(absl::is_copy_assignable<Randen>::value,
+  static_assert(std::is_copy_assignable_v<Randen>,
                 "Randen must be copy assignable");
 
-  static_assert(std::is_move_constructible<Randen>::value,
+  static_assert(std::is_move_constructible_v<Randen>,
                 "Randen must be move constructible");
 
-  static_assert(absl::is_move_assignable<Randen>::value,
+  static_assert(std::is_move_assignable_v<Randen>,
                 "Randen must be move assignable");
 }
 
 TEST(RandenTest, Default) {
-  constexpr uint64_t kGolden[] = {
-      0x6c6534090ee6d3ee, 0x044e2b9b9d5333c6, 0xc3c14f134e433977,
-      0xdda9f47cd90410ee, 0x887bf3087fd8ca10, 0xf0b780f545c72912,
-      0x15dbb1d37696599f, 0x30ec63baff3c6d59, 0xb29f73606f7f20a6,
-      0x02808a316f49a54c, 0x3b8feaf9d5c8e50e, 0x9cbf605e3fd9de8a,
-      0xc970ae1a78183bbb, 0xd8b2ffd356301ed5, 0xf4b327fe0fc73c37,
-      0xcdfd8d76eb8f9a19, 0xc3a506eb91420c9d, 0xd5af05dd3eff9556,
-      0x48db1bb78f83c4a1, 0x7023920e0d6bfe8c, 0x58d3575834956d42,
-      0xed1ef4c26b87b840, 0x8eef32a23e0b2df3, 0x497cabf3431154fc,
-      0x4e24370570029a8b, 0xd88b5749f090e5ea, 0xc651a582a970692f,
-      0x78fcec2cbb6342f5, 0x463cb745612f55db, 0x352ee4ad1816afe3,
-      0x026ff374c101da7e, 0x811ef0821c3de851,
+  constexpr uint8_t kGolden[] = {
+      0xee, 0xd3, 0xe6, 0x0e, 0x09, 0x34, 0x65, 0x6c, 0xc6, 0x33, 0x53, 0x9d,
+      0x9b, 0x2b, 0x4e, 0x04, 0x77, 0x39, 0x43, 0x4e, 0x13, 0x4f, 0xc1, 0xc3,
+      0xee, 0x10, 0x04, 0xd9, 0x7c, 0xf4, 0xa9, 0xdd, 0x10, 0xca, 0xd8, 0x7f,
+      0x08, 0xf3, 0x7b, 0x88, 0x12, 0x29, 0xc7, 0x45, 0xf5, 0x80, 0xb7, 0xf0,
+      0x9f, 0x59, 0x96, 0x76, 0xd3, 0xb1, 0xdb, 0x15, 0x59, 0x6d, 0x3c, 0xff,
+      0xba, 0x63, 0xec, 0x30, 0xa6, 0x20, 0x7f, 0x6f, 0x60, 0x73, 0x9f, 0xb2,
+      0x4c, 0xa5, 0x49, 0x6f, 0x31, 0x8a, 0x80, 0x02, 0x0e, 0xe5, 0xc8, 0xd5,
+      0xf9, 0xea, 0x8f, 0x3b, 0x8a, 0xde, 0xd9, 0x3f, 0x5e, 0x60, 0xbf, 0x9c,
+      0xbb, 0x3b, 0x18, 0x78, 0x1a, 0xae, 0x70, 0xc9, 0xd5, 0x1e, 0x30, 0x56,
+      0xd3, 0xff, 0xb2, 0xd8, 0x37, 0x3c, 0xc7, 0x0f, 0xfe, 0x27, 0xb3, 0xf4,
+      0x19, 0x9a, 0x8f, 0xeb, 0x76, 0x8d, 0xfd, 0xcd, 0x9d, 0x0c, 0x42, 0x91,
+      0xeb, 0x06, 0xa5, 0xc3, 0x56, 0x95, 0xff, 0x3e, 0xdd, 0x05, 0xaf, 0xd5,
+      0xa1, 0xc4, 0x83, 0x8f, 0xb7, 0x1b, 0xdb, 0x48, 0x8c, 0xfe, 0x6b, 0x0d,
+      0x0e, 0x92, 0x23, 0x70, 0x42, 0x6d, 0x95, 0x34, 0x58, 0x57, 0xd3, 0x58,
+      0x40, 0xb8, 0x87, 0x6b, 0xc2, 0xf4, 0x1e, 0xed, 0xf3, 0x2d, 0x0b, 0x3e,
+      0xa2, 0x32, 0xef, 0x8e, 0xfc, 0x54, 0x11, 0x43, 0xf3, 0xab, 0x7c, 0x49,
+      0x8b, 0x9a, 0x02, 0x70, 0x05, 0x37, 0x24, 0x4e, 0xea, 0xe5, 0x90, 0xf0,
+      0x49, 0x57, 0x8b, 0xd8, 0x2f, 0x69, 0x70, 0xa9, 0x82, 0xa5, 0x51, 0xc6,
+      0xf5, 0x42, 0x63, 0xbb, 0x2c, 0xec, 0xfc, 0x78, 0xdb, 0x55, 0x2f, 0x61,
+      0x45, 0xb7, 0x3c, 0x46, 0xe3, 0xaf, 0x16, 0x18, 0xad, 0xe4, 0x2e, 0x35,
+      0x7e, 0xda, 0x01, 0xc1, 0x74, 0xf3, 0x6f, 0x02, 0x51, 0xe8, 0x3d, 0x1c,
+      0x82, 0xf0, 0x1e, 0x81,
   };
 
-  alignas(16) uint64_t state[kStateSizeT];
+  alignas(16) uint8_t state[Randen::kStateBytes];
   std::memset(state, 0, sizeof(state));
 
   Randen r;
   r.Generate(state);
 
-  auto id = std::begin(state);
-  for (const auto& elem : kGolden) {
-    EXPECT_EQ(elem, *id++);
-  }
+  EXPECT_EQ(0, std::memcmp(state, kGolden, sizeof(state)));
 }
 
 }  // namespace
diff --git a/absl/random/internal/randen_traits.h b/absl/random/internal/randen_traits.h
index 4f1f408d191..120022c9fb8 100644
--- a/absl/random/internal/randen_traits.h
+++ b/absl/random/internal/randen_traits.h
@@ -22,13 +22,35 @@
 
 #include <cstddef>
 
+#include "absl/base/config.h"
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
-// RANDen = RANDom generator or beetroots in Swiss German.
+// RANDen = RANDom generator or beetroots in Swiss High German.
 // 'Strong' (well-distributed, unpredictable, backtracking-resistant) random
 // generator, faster in some benchmarks than std::mt19937_64 and pcg64_c32.
 //
+// High-level summary:
+// 1) Reverie (see "A Robust and Sponge-Like PRNG with Improved Efficiency") is
+//    a sponge-like random generator that requires a cryptographic permutation.
+//    It improves upon "Provably Robust Sponge-Based PRNGs and KDFs" by
+//    achieving backtracking resistance with only one Permute() per buffer.
+//
+// 2) "Simpira v2: A Family of Efficient Permutations Using the AES Round
+//    Function" constructs up to 1024-bit permutations using an improved
+//    Generalized Feistel network with 2-round AES-128 functions. This Feistel
+//    block shuffle achieves diffusion faster and is less vulnerable to
+//    sliced-biclique attacks than the Type-2 cyclic shuffle.
+//
+// 3) "Improving the Generalized Feistel" and "New criterion for diffusion
+//    property" extends the same kind of improved Feistel block shuffle to 16
+//    branches, which enables a 2048-bit permutation.
+//
+// Combine these three ideas and also change Simpira's subround keys from
+// structured/low-entropy counters to digits of Pi (or other random source).
+
 // RandenTraits contains the basic algorithm traits, such as the size of the
 // state, seed, sponge, etc.
 struct RandenTraits {
@@ -42,18 +64,25 @@ struct RandenTraits {
   // Size of the default seed consumed by the sponge.
   static constexpr size_t kSeedBytes = kStateBytes - kCapacityBytes;
 
+  // Assuming 128-bit blocks, the number of blocks in the state.
   // Largest size for which security proofs are known.
   static constexpr size_t kFeistelBlocks = 16;
 
-  // Type-2 generalized Feistel => one round function for every two blocks.
-  static constexpr size_t kFeistelFunctions = kFeistelBlocks / 2;  // = 8
-
   // Ensures SPRP security and two full subblock diffusions.
   // Must be > 4 * log2(kFeistelBlocks).
   static constexpr size_t kFeistelRounds = 16 + 1;
+
+  // Size of the key. A 128-bit key block is used for every-other
+  // feistel block (Type-2 generalized Feistel network) in each round.
+  static constexpr size_t kKeyBytes = 16 * kFeistelRounds * kFeistelBlocks / 2;
 };
 
+// Randen key arrays. In randen_round_keys.cc
+extern const unsigned char kRandenRoundKeys[RandenTraits::kKeyBytes];
+extern const unsigned char kRandenRoundKeysBE[RandenTraits::kKeyBytes];
+
 }  // namespace random_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_INTERNAL_RANDEN_TRAITS_H_
diff --git a/absl/random/internal/salted_seed_seq.h b/absl/random/internal/salted_seed_seq.h
index 86487006925..68296e85ed5 100644
--- a/absl/random/internal/salted_seed_seq.h
+++ b/absl/random/internal/salted_seed_seq.h
@@ -20,8 +20,10 @@
 #include <initializer_list>
 #include <iterator>
 #include <memory>
+#include <optional>
 #include <type_traits>
 #include <utility>
+#include <vector>
 
 #include "absl/container/inlined_vector.h"
 #include "absl/meta/type_traits.h"
@@ -30,6 +32,7 @@
 #include "absl/types/span.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
 // This class conforms to the C++ Standard "Seed Sequence" concept
@@ -46,11 +49,11 @@ class SaltedSeedSeq {
   using inner_sequence_type = SSeq;
   using result_type = typename SSeq::result_type;
 
-  SaltedSeedSeq() : seq_(absl::make_unique<SSeq>()) {}
+  SaltedSeedSeq() : seq_(std::make_unique<SSeq>()) {}
 
   template <typename Iterator>
   SaltedSeedSeq(Iterator begin, Iterator end)
-      : seq_(absl::make_unique<SSeq>(begin, end)) {}
+      : seq_(std::make_unique<SSeq>(begin, end)) {}
 
   template <typename T>
   SaltedSeedSeq(std::initializer_list<T> il)
@@ -64,15 +67,19 @@ class SaltedSeedSeq {
 
   template <typename RandomAccessIterator>
   void generate(RandomAccessIterator begin, RandomAccessIterator end) {
+    using U = typename std::iterator_traits<RandomAccessIterator>::value_type;
+
     // The common case is that generate is called with ContiguousIterators
     // to uint arrays. Such contiguous memory regions may be optimized,
     // which we detect here.
-    using tag = absl::conditional_t<
-        (std::is_pointer<RandomAccessIterator>::value &&
-         std::is_same<absl::decay_t<decltype(*begin)>, uint32_t>::value),
+    using TagType = std::conditional_t<
+        (std::is_same_v<U, uint32_t> &&
+         (std::is_pointer_v<RandomAccessIterator> ||
+          std::is_same_v<RandomAccessIterator,
+                         typename std::vector<U>::iterator>)),
         ContiguousAndUint32Tag, DefaultTag>;
     if (begin != end) {
-      generate_impl(begin, end, tag{});
+      generate_impl(TagType{}, begin, end, std::distance(begin, end));
     }
   }
 
@@ -88,8 +95,15 @@ class SaltedSeedSeq {
   struct DefaultTag {};
 
   // Generate which requires the iterators are contiguous pointers to uint32_t.
-  void generate_impl(uint32_t* begin, uint32_t* end, ContiguousAndUint32Tag) {
-    generate_contiguous(absl::MakeSpan(begin, end));
+  // Fills the initial seed buffer the underlying SSeq::generate() call,
+  // then mixes in the salt material.
+  template <typename Contiguous>
+  void generate_impl(ContiguousAndUint32Tag, Contiguous begin, Contiguous end,
+                     size_t n) {
+    seq_->generate(begin, end);
+    const uint32_t salt = absl::random_internal::GetSaltMaterial().value_or(0);
+    auto span = absl::Span<uint32_t>(&*begin, n);
+    MixIntoSeedMaterial(absl::MakeConstSpan(&salt, 1), span);
   }
 
   // The uncommon case for generate is that it is called with iterators over
@@ -97,27 +111,13 @@ class SaltedSeedSeq {
   // case we allocate a temporary 32-bit buffer and then copy-assign back
   // to the initial inputs.
   template <typename RandomAccessIterator>
-  void generate_impl(RandomAccessIterator begin, RandomAccessIterator end,
-                     DefaultTag) {
-    return generate_and_copy(std::distance(begin, end), begin);
-  }
-
-  // Fills the initial seed buffer the underlying SSeq::generate() call,
-  // mixing in the salt material.
-  void generate_contiguous(absl::Span<uint32_t> buffer) {
-    seq_->generate(buffer.begin(), buffer.end());
-    const uint32_t salt = absl::random_internal::GetSaltMaterial().value_or(0);
-    MixIntoSeedMaterial(absl::MakeConstSpan(&salt, 1), buffer);
-  }
-
-  // Allocates a seed buffer of `n` elements, generates the seed, then
-  // copies the result into the `out` iterator.
-  template <typename Iterator>
-  void generate_and_copy(size_t n, Iterator out) {
-    // Allocate a temporary buffer, generate, and then copy.
+  void generate_impl(DefaultTag, RandomAccessIterator begin,
+                     RandomAccessIterator, size_t n) {
+    // Allocates a seed buffer of `n` elements, generates the seed, then
+    // copies the result into the `out` iterator.
     absl::InlinedVector<uint32_t, 8> data(n, 0);
-    generate_contiguous(absl::MakeSpan(data.data(), data.size()));
-    std::copy(data.begin(), data.end(), out);
+    generate_impl(ContiguousAndUint32Tag{}, data.begin(), data.end(), n);
+    std::copy(data.begin(), data.end(), begin);
   }
 
   // Because [rand.req.seedseq] is not required to be copy-constructible,
@@ -132,8 +132,8 @@ struct is_salted_seed_seq : public std::false_type {};
 
 template <typename T>
 struct is_salted_seed_seq<
-    T, typename std::enable_if<std::is_same<
-           T, SaltedSeedSeq<typename T::inner_sequence_type>>::value>::type>
+    T, std::enable_if_t<
+           std::is_same_v<T, SaltedSeedSeq<typename T::inner_sequence_type>>>>
     : public std::true_type {};
 
 // MakeSaltedSeedSeq returns a salted variant of the seed sequence.
@@ -142,16 +142,16 @@ struct is_salted_seed_seq<
 // non-salted seed parameters.
 template <
     typename SSeq,  //
-    typename EnableIf = absl::enable_if_t<is_salted_seed_seq<SSeq>::value>>
+    typename EnableIf = std::enable_if_t<is_salted_seed_seq<SSeq>::value>>
 SSeq MakeSaltedSeedSeq(SSeq&& seq) {
   return SSeq(std::forward<SSeq>(seq));
 }
 
 template <
     typename SSeq,  //
-    typename EnableIf = absl::enable_if_t<!is_salted_seed_seq<SSeq>::value>>
-SaltedSeedSeq<typename std::decay<SSeq>::type> MakeSaltedSeedSeq(SSeq&& seq) {
-  using sseq_type = typename std::decay<SSeq>::type;
+    typename EnableIf = std::enable_if_t<!is_salted_seed_seq<SSeq>::value>>
+SaltedSeedSeq<std::decay_t<SSeq>> MakeSaltedSeedSeq(SSeq&& seq) {
+  using sseq_type = std::decay_t<SSeq>;
   using result_type = typename sseq_type::result_type;
 
   absl::InlinedVector<result_type, 8> data;
@@ -160,6 +160,7 @@ SaltedSeedSeq<typename std::decay<SSeq>::type> MakeSaltedSeedSeq(SSeq&& seq) {
 }
 
 }  // namespace random_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_INTERNAL_SALTED_SEED_SEQ_H_
diff --git a/absl/random/internal/salted_seed_seq_test.cc b/absl/random/internal/salted_seed_seq_test.cc
index 0bf19a63ef8..3e8dfeb8e26 100644
--- a/absl/random/internal/salted_seed_seq_test.cc
+++ b/absl/random/internal/salted_seed_seq_test.cc
@@ -14,6 +14,8 @@
 
 #include "absl/random/internal/salted_seed_seq.h"
 
+#include <cstddef>
+#include <cstdint>
 #include <iterator>
 #include <random>
 #include <utility>
@@ -21,6 +23,8 @@
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
+#include "absl/base/macros.h"
+#include "absl/random/internal/seed_material.h"
 
 using absl::random_internal::GetSaltMaterial;
 using absl::random_internal::MakeSaltedSeedSeq;
@@ -33,24 +37,28 @@ namespace {
 template <typename Sseq>
 void ConformsToInterface() {
   // Check that the SeedSequence can be default-constructed.
-  { Sseq default_constructed_seq; }
+  {
+    Sseq default_constructed_seq;
+  }
   // Check that the SeedSequence can be constructed with two iterators.
   {
     uint32_t init_array[] = {1, 3, 5, 7, 9};
     Sseq iterator_constructed_seq(std::begin(init_array), std::end(init_array));
   }
   // Check that the SeedSequence can be std::initializer_list-constructed.
-  { Sseq list_constructed_seq = {1, 3, 5, 7, 9, 11, 13}; }
+  {
+    Sseq list_constructed_seq = {1, 3, 5, 7, 9, 11, 13};
+  }
   // Check that param() and size() return state provided to constructor.
   {
     uint32_t init_array[] = {1, 2, 3, 4, 5};
     Sseq seq(std::begin(init_array), std::end(init_array));
-    EXPECT_EQ(seq.size(), ABSL_ARRAYSIZE(init_array));
+    EXPECT_EQ(seq.size(), std::size(init_array));
 
     std::vector<uint32_t> state_vector;
     seq.param(std::back_inserter(state_vector));
 
-    EXPECT_EQ(state_vector.size(), ABSL_ARRAYSIZE(init_array));
+    EXPECT_EQ(state_vector.size(), std::size(init_array));
     for (int i = 0; i < state_vector.size(); i++) {
       EXPECT_EQ(state_vector[i], i + 1);
     }
diff --git a/absl/random/internal/seed_material.cc b/absl/random/internal/seed_material.cc
index 85dd535f9c0..75b9e295e92 100644
--- a/absl/random/internal/seed_material.cc
+++ b/absl/random/internal/seed_material.cc
@@ -16,35 +16,50 @@
 
 #include <fcntl.h>
 
-#ifndef _WIN32
-#include <unistd.h>
-#else
-#include <io.h>
-#endif
-
 #include <algorithm>
+#include <cassert>
 #include <cerrno>
 #include <cstdint>
 #include <cstdlib>
 #include <cstring>
+#include <optional>
+#include <string>
+#include <vector>
 
+#include "absl/base/config.h"
+#include "absl/base/dynamic_annotations.h"
 #include "absl/base/internal/raw_logging.h"
-#include "absl/strings/ascii.h"
 #include "absl/strings/escaping.h"
 #include "absl/strings/string_view.h"
-#include "absl/strings/strip.h"
-
-#if defined(__native_client__)
+#include "absl/types/span.h"
 
-#include <nacl/nacl_random.h>
-#define ABSL_RANDOM_USE_NACL_SECURE_RANDOM 1
+#ifndef _WIN32
+#include <unistd.h>
+#else
+#include <io.h>
+#endif
 
-#elif defined(_WIN32)
+#if defined(_WIN32)
 
 #include <windows.h>
 #define ABSL_RANDOM_USE_BCRYPT 1
 #pragma comment(lib, "bcrypt.lib")
 
+#elif defined(__Fuchsia__)
+#include <zircon/syscalls.h>
+
+#endif
+
+#if defined(__GLIBC__) && \
+    (__GLIBC__ > 2 || (__GLIBC__ == 2 && __GLIBC_MINOR__ >= 25))
+// glibc >= 2.25 has getentropy()
+#define ABSL_RANDOM_USE_GET_ENTROPY 1
+#endif
+
+#if defined(__EMSCRIPTEN__)
+#include <sys/random.h>
+// Emscripten has getentropy, but it resides in a different header.
+#define ABSL_RANDOM_USE_GET_ENTROPY 1
 #endif
 
 #if defined(ABSL_RANDOM_USE_BCRYPT)
@@ -58,6 +73,7 @@
 #endif
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 namespace {
 
@@ -86,55 +102,81 @@ bool ReadSeedMaterialFromOSEntropyImpl(absl::Span<uint32_t> values) {
   return BCRYPT_SUCCESS(ret);
 }
 
-#elif defined(ABSL_RANDOM_USE_NACL_SECURE_RANDOM)
+#elif defined(__Fuchsia__)
 
-// On NaCL use nacl_secure_random to acquire bytes.
 bool ReadSeedMaterialFromOSEntropyImpl(absl::Span<uint32_t> values) {
   auto buffer = reinterpret_cast<uint8_t*>(values.data());
   size_t buffer_size = sizeof(uint32_t) * values.size();
+  zx_cprng_draw(buffer, buffer_size);
+  return true;
+}
 
-  uint8_t* output_ptr = buffer;
+#else
+
+#if defined(ABSL_RANDOM_USE_GET_ENTROPY)
+// On *nix, use getentropy() if supported. Note that libc may support
+// getentropy(), but the kernel may not, in which case this function will return
+// false.
+bool ReadSeedMaterialFromGetEntropy(absl::Span<uint32_t> values) {
+  auto buffer = reinterpret_cast<uint8_t*>(values.data());
+  size_t buffer_size = sizeof(uint32_t) * values.size();
   while (buffer_size > 0) {
-    size_t nread = 0;
-    const int error = nacl_secure_random(output_ptr, buffer_size, &nread);
-    if (error != 0 || nread > buffer_size) {
-      ABSL_RAW_LOG(ERROR, "Failed to read secure_random seed data: %d", error);
+    // getentropy() has a maximum permitted length of 256.
+    size_t to_read = std::min<size_t>(buffer_size, 256);
+    int result = getentropy(buffer, to_read);
+    if (result < 0) {
       return false;
     }
-    output_ptr += nread;
-    buffer_size -= nread;
+    // https://github.com/google/sanitizers/issues/1173
+    // MemorySanitizer can't see through getentropy().
+    ABSL_ANNOTATE_MEMORY_IS_INITIALIZED(buffer, to_read);
+    buffer += to_read;
+    buffer_size -= to_read;
   }
   return true;
 }
-
-#else
+#endif  // defined(ABSL_RANDOM_GETENTROPY)
 
 // On *nix, read entropy from /dev/urandom.
-bool ReadSeedMaterialFromOSEntropyImpl(absl::Span<uint32_t> values) {
+bool ReadSeedMaterialFromDevURandom(absl::Span<uint32_t> values) {
   const char kEntropyFile[] = "/dev/urandom";
 
   auto buffer = reinterpret_cast<uint8_t*>(values.data());
   size_t buffer_size = sizeof(uint32_t) * values.size();
 
   int dev_urandom = open(kEntropyFile, O_RDONLY);
-  bool success = (-1 != dev_urandom);
-  if (!success) {
+  if (dev_urandom < 0) {
+    ABSL_RAW_LOG(ERROR, "Failed to open /dev/urandom.");
     return false;
   }
 
-  while (success && buffer_size > 0) {
-    int bytes_read = read(dev_urandom, buffer, buffer_size);
+  while (buffer_size > 0) {
+    ssize_t bytes_read = read(dev_urandom, buffer, buffer_size);
     int read_error = errno;
-    success = (bytes_read > 0);
-    if (success) {
-      buffer += bytes_read;
-      buffer_size -= bytes_read;
-    } else if (bytes_read == -1 && read_error == EINTR) {
-      success = true;  // Need to try again.
+    if (bytes_read == -1 && read_error == EINTR) {
+      // Interrupted, try again.
+      continue;
+    } else if (bytes_read <= 0) {
+      // EOF, or error.
+      break;
     }
+    buffer += bytes_read;
+    buffer_size -= static_cast<size_t>(bytes_read);
   }
+
   close(dev_urandom);
-  return success;
+  return buffer_size == 0;
+}
+
+bool ReadSeedMaterialFromOSEntropyImpl(absl::Span<uint32_t> values) {
+#if defined(ABSL_RANDOM_USE_GET_ENTROPY)
+  if (ReadSeedMaterialFromGetEntropy(values)) {
+    return true;
+  }
+#endif
+  // Libc may support getentropy, but the kernel may not, so we still have
+  // to fallback to ReadSeedMaterialFromDevURandom().
+  return ReadSeedMaterialFromDevURandom(values);
 }
 
 #endif
@@ -184,22 +226,22 @@ void MixIntoSeedMaterial(absl::Span<const uint32_t> sequence,
   }
 }
 
-absl::optional<uint32_t> GetSaltMaterial() {
+std::optional<uint32_t> GetSaltMaterial() {
   // Salt must be common for all generators within the same process so read it
   // only once and store in static variable.
-  static const auto salt_material = []() -> absl::optional<uint32_t> {
+  static const auto salt_material = []() -> std::optional<uint32_t> {
     uint32_t salt_value = 0;
 
-    if (random_internal::ReadSeedMaterialFromOSEntropy(
-            MakeSpan(&salt_value, 1))) {
+    if (ReadSeedMaterialFromOSEntropy(absl::MakeSpan(&salt_value, 1))) {
       return salt_value;
     }
 
-    return absl::nullopt;
+    return std::nullopt;
   }();
 
   return salt_material;
 }
 
 }  // namespace random_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/random/internal/seed_material.h b/absl/random/internal/seed_material.h
index 57de8a24d5e..23bf4fac301 100644
--- a/absl/random/internal/seed_material.h
+++ b/absl/random/internal/seed_material.h
@@ -18,15 +18,17 @@
 #include <cassert>
 #include <cstdint>
 #include <cstdlib>
+#include <optional>
 #include <string>
 #include <vector>
 
-#include "absl/base/attributes.h"
+#include "absl/base/config.h"
 #include "absl/random/internal/fast_uniform_bits.h"
 #include "absl/types/optional.h"
 #include "absl/types/span.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
 // Returns the number of 32-bit blocks needed to contain the given number of
@@ -53,7 +55,7 @@ static_assert(kEntropyBlocksNeeded > 0,
 // to the C++ Standard "Seed Sequence" concept [rand.req.seedseq].
 //
 // If values.data() == nullptr, the behavior is undefined.
-ABSL_MUST_USE_RESULT
+[[nodiscard]]
 bool ReadSeedMaterialFromOSEntropy(absl::Span<uint32_t> values);
 
 // Attempts to fill a span of uint32_t-values using variates generated by an
@@ -64,8 +66,8 @@ bool ReadSeedMaterialFromOSEntropy(absl::Span<uint32_t> values);
 //
 // If urbg == nullptr or values.data() == nullptr, the behavior is undefined.
 template <typename URBG>
-ABSL_MUST_USE_RESULT bool ReadSeedMaterialFromURBG(
-    URBG* urbg, absl::Span<uint32_t> values) {
+[[nodiscard]] bool ReadSeedMaterialFromURBG(URBG* urbg,
+                                            absl::Span<uint32_t> values) {
   random_internal::FastUniformBits<uint32_t> distr;
 
   assert(urbg != nullptr && values.data() != nullptr);
@@ -93,10 +95,11 @@ void MixIntoSeedMaterial(absl::Span<const uint32_t> sequence,
 //
 // Salt is obtained only once and stored in static variable.
 //
-// May return empty value if optaining the salt was not possible.
-absl::optional<uint32_t> GetSaltMaterial();
+// May return empty value if obtaining the salt was not possible.
+std::optional<uint32_t> GetSaltMaterial();
 
 }  // namespace random_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_INTERNAL_SEED_MATERIAL_H_
diff --git a/absl/random/internal/seed_material_test.cc b/absl/random/internal/seed_material_test.cc
index 6db2820ec70..45db76552f6 100644
--- a/absl/random/internal/seed_material_test.cc
+++ b/absl/random/internal/seed_material_test.cc
@@ -15,12 +15,15 @@
 #include "absl/random/internal/seed_material.h"
 
 #include <bitset>
+#include <cstdint>
 #include <cstdlib>
 #include <cstring>
 #include <random>
+#include <vector>
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
+#include "absl/types/span.h"
 
 #ifdef __ANDROID__
 // Android assert messages only go to system log, so death tests cannot inspect
diff --git a/absl/random/internal/sequence_urbg.h b/absl/random/internal/sequence_urbg.h
index 9a9b57738da..a7acd9e6deb 100644
--- a/absl/random/internal/sequence_urbg.h
+++ b/absl/random/internal/sequence_urbg.h
@@ -17,11 +17,15 @@
 
 #include <cstdint>
 #include <cstring>
+#include <initializer_list>
 #include <limits>
 #include <type_traits>
 #include <vector>
 
+#include "absl/base/config.h"
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
 // `sequence_urbg` is a simple random number generator which meets the
@@ -51,6 +55,7 @@ class sequence_urbg {
 };
 
 }  // namespace random_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_INTERNAL_SEQUENCE_URBG_H_
diff --git a/absl/random/internal/traits.h b/absl/random/internal/traits.h
index 40eb011f195..8404d93fb70 100644
--- a/absl/random/internal/traits.h
+++ b/absl/random/internal/traits.h
@@ -15,15 +15,38 @@
 #ifndef ABSL_RANDOM_INTERNAL_TRAITS_H_
 #define ABSL_RANDOM_INTERNAL_TRAITS_H_
 
+#include <cstddef>
 #include <cstdint>
 #include <limits>
 #include <type_traits>
 
 #include "absl/base/config.h"
+#include "absl/meta/type_traits.h"
+#include "absl/numeric/bits.h"
+#include "absl/numeric/int128.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
+// is_urbg<URBG>
+//
+// Indicates whether a type URBG is a Uniform Random Bit Generator.
+template <typename URBG, typename = void, typename = void, typename = void>
+struct is_urbg : std::false_type {};
+
+template <typename URBG>
+struct is_urbg<
+    URBG,
+    std::enable_if_t<std::is_same_v<typename URBG::result_type,
+                                    std::decay_t<decltype((URBG::min)())>>>,
+    std::enable_if_t<std::is_same_v<typename URBG::result_type,
+                                    std::decay_t<decltype((URBG::max)())>>>,
+    std::enable_if_t<
+        std::is_same_v<typename URBG::result_type,
+                       std::decay_t<decltype(std::declval<URBG>()())>>>>
+    : std::true_type {};
+
 // random_internal::is_widening_convertible<A, B>
 //
 // Returns whether a type A is widening-convertible to a type B.
@@ -58,6 +81,31 @@ class is_widening_convertible {
       rank<A>() <= rank<B>();
 };
 
+template <typename T>
+struct IsIntegral : std::is_integral<T> {};
+template <>
+struct IsIntegral<absl::int128> : std::true_type {};
+template <>
+struct IsIntegral<absl::uint128> : std::true_type {};
+
+template <typename T>
+struct MakeUnsigned : std::make_unsigned<T> {};
+template <>
+struct MakeUnsigned<absl::int128> {
+  using type = absl::uint128;
+};
+template <>
+struct MakeUnsigned<absl::uint128> {
+  using type = absl::uint128;
+};
+
+template <typename T>
+struct IsUnsigned : std::is_unsigned<T> {};
+template <>
+struct IsUnsigned<absl::int128> : std::false_type {};
+template <>
+struct IsUnsigned<absl::uint128> : std::true_type {};
+
 // unsigned_bits<N>::type returns the unsigned int type with the indicated
 // number of bits.
 template <size_t N>
@@ -80,20 +128,42 @@ struct unsigned_bits<64> {
   using type = uint64_t;
 };
 
-#ifdef ABSL_HAVE_INTRINSIC_INT128
 template <>
 struct unsigned_bits<128> {
-  using type = __uint128_t;
+  using type = absl::uint128;
+};
+
+// 256-bit wrapper for wide multiplications.
+struct U256 {
+  uint128 hi;
+  uint128 lo;
+};
+template <>
+struct unsigned_bits<256> {
+  using type = U256;
 };
-#endif
 
 template <typename IntType>
 struct make_unsigned_bits {
-  using type = typename unsigned_bits<std::numeric_limits<
-      typename std::make_unsigned<IntType>::type>::digits>::type;
+  using type = typename unsigned_bits<
+      std::numeric_limits<typename MakeUnsigned<IntType>::type>::digits>::type;
 };
 
+template <typename T>
+int BitWidth(T v) {
+  // Workaround for bit_width not supporting int128.
+  // Don't hardcode `64` to make sure this code does not trigger compiler
+  // warnings in smaller types.
+  constexpr int half_bits = sizeof(T) * 8 / 2;
+  if (sizeof(T) == 16 && (v >> half_bits) != 0) {
+    return bit_width(static_cast<uint64_t>(v >> half_bits)) + half_bits;
+  } else {
+    return bit_width(static_cast<uint64_t>(v));
+  }
+}
+
 }  // namespace random_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_INTERNAL_TRAITS_H_
diff --git a/absl/random/internal/traits_test.cc b/absl/random/internal/traits_test.cc
index a844887d3e4..08361a4727c 100644
--- a/absl/random/internal/traits_test.cc
+++ b/absl/random/internal/traits_test.cc
@@ -15,14 +15,19 @@
 #include "absl/random/internal/traits.h"
 
 #include <cstdint>
+#include <random>
 #include <type_traits>
 
 #include "gtest/gtest.h"
 
 namespace {
 
+using absl::random_internal::is_urbg;
 using absl::random_internal::is_widening_convertible;
 
+static_assert(is_urbg<std::minstd_rand>::value);
+static_assert(!is_urbg<uint64_t>::value);
+
 // CheckWideningConvertsToSelf<T1, T2, ...>()
 //
 // For each type T, checks:
@@ -48,7 +53,7 @@ void CheckWideningConvertsToSelf() {
 //
 template <typename T>
 void CheckNotWideningConvertibleWithSigned() {
-  using signed_t = typename std::make_signed<T>::type;
+  using signed_t = std::make_signed_t<T>;
 
   static_assert(!is_widening_convertible<T, signed_t>::value,
                 "Unsigned type is convertible to same-sized signed-type!");
@@ -71,9 +76,9 @@ void CheckNotWideningConvertibleWithSigned() {
 // - Signed(Ti) IS widening-convertible to Ti+1
 template <typename T, typename Higher>
 void CheckWideningConvertsToLargerTypes() {
-  using signed_t = typename std::make_signed<T>::type;
+  using signed_t = std::make_signed_t<T>;
   using higher_t = Higher;
-  using signed_higher_t = typename std::make_signed<Higher>::type;
+  using signed_higher_t = std::make_signed_t<Higher>;
 
   static_assert(is_widening_convertible<T, higher_t>::value,
                 "Type not embeddable into larger type!");
@@ -104,14 +109,11 @@ void CheckWideningConvertsTo() {
 TEST(TraitsTest, IsWideningConvertibleTest) {
   constexpr bool kInvalid = false;
 
-  CheckWideningConvertsToSelf<
-      uint8_t, uint16_t, uint32_t, uint64_t,
-      int8_t,  int16_t,  int32_t,  int64_t,
-      float,   double>();
-  CheckNotWideningConvertibleWithSigned<
-      uint8_t, uint16_t, uint32_t, uint64_t>();
-  CheckWideningConvertsToLargerTypes<
-      uint8_t, uint16_t, uint32_t, uint64_t>();
+  CheckWideningConvertsToSelf<uint8_t, uint16_t, uint32_t, uint64_t, int8_t,
+                              int16_t, int32_t, int64_t, float, double>();
+  CheckNotWideningConvertibleWithSigned<uint8_t, uint16_t, uint32_t,
+                                        uint64_t>();
+  CheckWideningConvertsToLargerTypes<uint8_t, uint16_t, uint32_t, uint64_t>();
 
   CheckWideningConvertsTo<float, double>();
   CheckWideningConvertsTo<uint16_t, float>();
diff --git a/absl/random/internal/uniform_helper.h b/absl/random/internal/uniform_helper.h
index 2929407e149..aefbd007352 100644
--- a/absl/random/internal/uniform_helper.h
+++ b/absl/random/internal/uniform_helper.h
@@ -19,9 +19,13 @@
 #include <limits>
 #include <type_traits>
 
+#include "absl/base/config.h"
 #include "absl/meta/type_traits.h"
+#include "absl/random/internal/traits.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
+
 template <typename IntType>
 class uniform_int_distribution;
 
@@ -57,6 +61,25 @@ struct IntervalOpenOpenTag
     : public random_internal::TagTypeCompare<IntervalOpenOpenTag> {};
 
 namespace random_internal {
+
+// In the absence of an explicitly provided return-type, the template
+// "uniform_inferred_return_t<A, B>" is used to derive a suitable type, based on
+// the data-types of the endpoint-arguments {A lo, B hi}.
+//
+// Given endpoints {A lo, B hi}, one of {A, B} will be chosen as the
+// return-type, if one type can be implicitly converted into the other, in a
+// lossless way. The template "is_widening_convertible" implements the
+// compile-time logic for deciding if such a conversion is possible.
+//
+// If no such conversion between {A, B} exists, then the overload for
+// absl::Uniform() will be discarded, and the call will be ill-formed.
+// Return-type for absl::Uniform() when the return-type is inferred.
+template <typename A, typename B>
+using uniform_inferred_return_t = std::enable_if_t<
+    std::disjunction_v<is_widening_convertible<A, B>,
+                       is_widening_convertible<B, A>>,
+    std::conditional_t<is_widening_convertible<A, B>::value, B, A>>;
+
 // The functions
 //    uniform_lower_bound(tag, a, b)
 // and
@@ -74,95 +97,147 @@ namespace random_internal {
 //               uniform_upper_bound(IntervalOpenClosed, a, b)]
 //
 template <typename IntType, typename Tag>
-typename absl::enable_if_t<
-    absl::conjunction<
-        std::is_integral<IntType>,
-        absl::disjunction<std::is_same<Tag, IntervalOpenClosedTag>,
-                          std::is_same<Tag, IntervalOpenOpenTag>>>::value,
+typename std::enable_if_t<
+    std::conjunction_v<
+        IsIntegral<IntType>,
+        std::disjunction<std::is_same<Tag, IntervalOpenClosedTag>,
+                         std::is_same<Tag, IntervalOpenOpenTag>>>,
     IntType>
 uniform_lower_bound(Tag, IntType a, IntType) {
-  return a + 1;
+  return a < (std::numeric_limits<IntType>::max)() ? (a + 1) : a;
 }
 
 template <typename FloatType, typename Tag>
-typename absl::enable_if_t<
-    absl::conjunction<
+typename std::enable_if_t<
+    std::conjunction_v<
         std::is_floating_point<FloatType>,
-        absl::disjunction<std::is_same<Tag, IntervalOpenClosedTag>,
-                          std::is_same<Tag, IntervalOpenOpenTag>>>::value,
+        std::disjunction<std::is_same<Tag, IntervalOpenClosedTag>,
+                         std::is_same<Tag, IntervalOpenOpenTag>>>,
     FloatType>
 uniform_lower_bound(Tag, FloatType a, FloatType b) {
   return std::nextafter(a, b);
 }
 
 template <typename NumType, typename Tag>
-typename absl::enable_if_t<
-    absl::disjunction<std::is_same<Tag, IntervalClosedClosedTag>,
-                      std::is_same<Tag, IntervalClosedOpenTag>>::value,
+typename std::enable_if_t<
+    std::disjunction_v<std::is_same<Tag, IntervalClosedClosedTag>,
+                       std::is_same<Tag, IntervalClosedOpenTag>>,
     NumType>
 uniform_lower_bound(Tag, NumType a, NumType) {
   return a;
 }
 
 template <typename IntType, typename Tag>
-typename absl::enable_if_t<
-    absl::conjunction<
-        std::is_integral<IntType>,
-        absl::disjunction<std::is_same<Tag, IntervalClosedOpenTag>,
-                          std::is_same<Tag, IntervalOpenOpenTag>>>::value,
+typename std::enable_if_t<
+    std::conjunction_v<
+        IsIntegral<IntType>,
+        std::disjunction<std::is_same<Tag, IntervalClosedOpenTag>,
+                         std::is_same<Tag, IntervalOpenOpenTag>>>,
     IntType>
 uniform_upper_bound(Tag, IntType, IntType b) {
-  return b - 1;
+  return b > (std::numeric_limits<IntType>::min)() ? (b - 1) : b;
 }
 
 template <typename FloatType, typename Tag>
-typename absl::enable_if_t<
-    absl::conjunction<
+typename std::enable_if_t<
+    std::conjunction_v<
         std::is_floating_point<FloatType>,
-        absl::disjunction<std::is_same<Tag, IntervalClosedOpenTag>,
-                          std::is_same<Tag, IntervalOpenOpenTag>>>::value,
+        std::disjunction<std::is_same<Tag, IntervalClosedOpenTag>,
+                         std::is_same<Tag, IntervalOpenOpenTag>>>,
     FloatType>
 uniform_upper_bound(Tag, FloatType, FloatType b) {
   return b;
 }
 
 template <typename IntType, typename Tag>
-typename absl::enable_if_t<
-    absl::conjunction<
-        std::is_integral<IntType>,
-        absl::disjunction<std::is_same<Tag, IntervalClosedClosedTag>,
-                          std::is_same<Tag, IntervalOpenClosedTag>>>::value,
+typename std::enable_if_t<
+    std::conjunction_v<
+        IsIntegral<IntType>,
+        std::disjunction<std::is_same<Tag, IntervalClosedClosedTag>,
+                         std::is_same<Tag, IntervalOpenClosedTag>>>,
     IntType>
 uniform_upper_bound(Tag, IntType, IntType b) {
   return b;
 }
 
 template <typename FloatType, typename Tag>
-typename absl::enable_if_t<
-    absl::conjunction<
+typename std::enable_if_t<
+    std::conjunction_v<
         std::is_floating_point<FloatType>,
-        absl::disjunction<std::is_same<Tag, IntervalClosedClosedTag>,
-                          std::is_same<Tag, IntervalOpenClosedTag>>>::value,
+        std::disjunction<std::is_same<Tag, IntervalClosedClosedTag>,
+                         std::is_same<Tag, IntervalOpenClosedTag>>>,
     FloatType>
 uniform_upper_bound(Tag, FloatType, FloatType b) {
   return std::nextafter(b, (std::numeric_limits<FloatType>::max)());
 }
 
+// Returns whether the bounds are valid for the underlying distribution.
+// Inputs must have already been resolved via uniform_*_bound calls.
+//
+// The c++ standard constraints in [rand.dist.uni.int] are listed as:
+//    requires: lo <= hi.
+//
+// In the uniform_int_distrubtion, {lo, hi} are closed, closed. Thus:
+// [0, 0] is legal.
+// [0, 0) is not legal, but [0, 1) is, which translates to [0, 0].
+// (0, 1) is not legal, but (0, 2) is, which translates to [1, 1].
+// (0, 0] is not legal, but (0, 1] is, which translates to [1, 1].
+//
+// The c++ standard constraints in [rand.dist.uni.real] are listed as:
+//    requires: lo <= hi.
+//    requires: (hi - lo) <= numeric_limits<T>::max()
+//
+// In the uniform_real_distribution, {lo, hi} are closed, open, Thus:
+// [0, 0] is legal, which is [0, 0+epsilon).
+// [0, 0) is legal.
+// (0, 0) is not legal, but (0-epsilon, 0+epsilon) is.
+// (0, 0] is not legal, but (0, 0+epsilon] is.
+//
+template <typename FloatType>
+std::enable_if_t<std::is_floating_point_v<FloatType>, bool>
+is_uniform_range_valid(FloatType a, FloatType b) {
+  return a <= b && std::isfinite(b - a);
+}
+
+template <typename IntType>
+std::enable_if_t<IsIntegral<IntType>::value, bool> is_uniform_range_valid(
+    IntType a, IntType b) {
+  return a <= b;
+}
+
+// UniformDistribution selects either absl::uniform_int_distribution
+// or absl::uniform_real_distribution depending on the NumType parameter.
 template <typename NumType>
 using UniformDistribution =
-    typename std::conditional<std::is_integral<NumType>::value,
-                              absl::uniform_int_distribution<NumType>,
-                              absl::uniform_real_distribution<NumType>>::type;
-
-template <typename TagType, typename NumType>
+    std::conditional_t<IsIntegral<NumType>::value,
+                       absl::uniform_int_distribution<NumType>,
+                       absl::uniform_real_distribution<NumType>>;
+
+// UniformDistributionWrapper is used as the underlying distribution type
+// by the absl::Uniform template function. It selects the proper Abseil
+// uniform distribution and provides constructor overloads that match the
+// expected parameter order as well as adjusting distribution bounds based
+// on the tag.
+template <typename NumType>
 struct UniformDistributionWrapper : public UniformDistribution<NumType> {
+  template <typename TagType>
   explicit UniformDistributionWrapper(TagType, NumType lo, NumType hi)
       : UniformDistribution<NumType>(
             uniform_lower_bound<NumType>(TagType{}, lo, hi),
             uniform_upper_bound<NumType>(TagType{}, lo, hi)) {}
+
+  explicit UniformDistributionWrapper(NumType lo, NumType hi)
+      : UniformDistribution<NumType>(
+            uniform_lower_bound<NumType>(IntervalClosedOpenTag(), lo, hi),
+            uniform_upper_bound<NumType>(IntervalClosedOpenTag(), lo, hi)) {}
+
+  explicit UniformDistributionWrapper()
+      : UniformDistribution<NumType>(std::numeric_limits<NumType>::lowest(),
+                                     (std::numeric_limits<NumType>::max)()) {}
 };
 
 }  // namespace random_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_INTERNAL_UNIFORM_HELPER_H_
diff --git a/absl/random/internal/uniform_helper_test.cc b/absl/random/internal/uniform_helper_test.cc
new file mode 100644
index 00000000000..e38b3b1cb8f
--- /dev/null
+++ b/absl/random/internal/uniform_helper_test.cc
@@ -0,0 +1,278 @@
+// Copyright 2017 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/random/internal/uniform_helper.h"
+
+#include <cmath>
+#include <cstdint>
+#include <limits>
+#include <type_traits>
+
+#include "gtest/gtest.h"
+
+namespace {
+
+using absl::IntervalClosedClosedTag;
+using absl::IntervalClosedOpenTag;
+using absl::IntervalOpenClosedTag;
+using absl::IntervalOpenOpenTag;
+using absl::random_internal::uniform_inferred_return_t;
+using absl::random_internal::uniform_lower_bound;
+using absl::random_internal::uniform_upper_bound;
+
+class UniformHelperTest : public testing::Test {};
+
+TEST_F(UniformHelperTest, UniformBoundFunctionsGeneral) {
+  constexpr IntervalClosedClosedTag IntervalClosedClosed;
+  constexpr IntervalClosedOpenTag IntervalClosedOpen;
+  constexpr IntervalOpenClosedTag IntervalOpenClosed;
+  constexpr IntervalOpenOpenTag IntervalOpenOpen;
+
+  // absl::uniform_int_distribution natively assumes IntervalClosedClosed
+  // absl::uniform_real_distribution natively assumes IntervalClosedOpen
+
+  EXPECT_EQ(uniform_lower_bound(IntervalOpenClosed, 0, 100), 1);
+  EXPECT_EQ(uniform_lower_bound(IntervalOpenOpen, 0, 100), 1);
+  EXPECT_GT(uniform_lower_bound<float>(IntervalOpenClosed, 0, 1.0), 0);
+  EXPECT_GT(uniform_lower_bound<float>(IntervalOpenOpen, 0, 1.0), 0);
+  EXPECT_GT(uniform_lower_bound<double>(IntervalOpenClosed, 0, 1.0), 0);
+  EXPECT_GT(uniform_lower_bound<double>(IntervalOpenOpen, 0, 1.0), 0);
+
+  EXPECT_EQ(uniform_lower_bound(IntervalClosedClosed, 0, 100), 0);
+  EXPECT_EQ(uniform_lower_bound(IntervalClosedOpen, 0, 100), 0);
+  EXPECT_EQ(uniform_lower_bound<float>(IntervalClosedClosed, 0, 1.0), 0);
+  EXPECT_EQ(uniform_lower_bound<float>(IntervalClosedOpen, 0, 1.0), 0);
+  EXPECT_EQ(uniform_lower_bound<double>(IntervalClosedClosed, 0, 1.0), 0);
+  EXPECT_EQ(uniform_lower_bound<double>(IntervalClosedOpen, 0, 1.0), 0);
+
+  EXPECT_EQ(uniform_upper_bound(IntervalOpenOpen, 0, 100), 99);
+  EXPECT_EQ(uniform_upper_bound(IntervalClosedOpen, 0, 100), 99);
+  EXPECT_EQ(uniform_upper_bound<float>(IntervalOpenOpen, 0, 1.0), 1.0);
+  EXPECT_EQ(uniform_upper_bound<float>(IntervalClosedOpen, 0, 1.0), 1.0);
+  EXPECT_EQ(uniform_upper_bound<double>(IntervalOpenOpen, 0, 1.0), 1.0);
+  EXPECT_EQ(uniform_upper_bound<double>(IntervalClosedOpen, 0, 1.0), 1.0);
+
+  EXPECT_EQ(uniform_upper_bound(IntervalOpenClosed, 0, 100), 100);
+  EXPECT_EQ(uniform_upper_bound(IntervalClosedClosed, 0, 100), 100);
+  EXPECT_GT(uniform_upper_bound<float>(IntervalOpenClosed, 0, 1.0), 1.0);
+  EXPECT_GT(uniform_upper_bound<float>(IntervalClosedClosed, 0, 1.0), 1.0);
+  EXPECT_GT(uniform_upper_bound<double>(IntervalOpenClosed, 0, 1.0), 1.0);
+  EXPECT_GT(uniform_upper_bound<double>(IntervalClosedClosed, 0, 1.0), 1.0);
+
+  // Negative value tests
+  EXPECT_EQ(uniform_lower_bound(IntervalOpenClosed, -100, -1), -99);
+  EXPECT_EQ(uniform_lower_bound(IntervalOpenOpen, -100, -1), -99);
+  EXPECT_GT(uniform_lower_bound<float>(IntervalOpenClosed, -2.0, -1.0), -2.0);
+  EXPECT_GT(uniform_lower_bound<float>(IntervalOpenOpen, -2.0, -1.0), -2.0);
+  EXPECT_GT(uniform_lower_bound<double>(IntervalOpenClosed, -2.0, -1.0), -2.0);
+  EXPECT_GT(uniform_lower_bound<double>(IntervalOpenOpen, -2.0, -1.0), -2.0);
+
+  EXPECT_EQ(uniform_lower_bound(IntervalClosedClosed, -100, -1), -100);
+  EXPECT_EQ(uniform_lower_bound(IntervalClosedOpen, -100, -1), -100);
+  EXPECT_EQ(uniform_lower_bound<float>(IntervalClosedClosed, -2.0, -1.0), -2.0);
+  EXPECT_EQ(uniform_lower_bound<float>(IntervalClosedOpen, -2.0, -1.0), -2.0);
+  EXPECT_EQ(uniform_lower_bound<double>(IntervalClosedClosed, -2.0, -1.0),
+            -2.0);
+  EXPECT_EQ(uniform_lower_bound<double>(IntervalClosedOpen, -2.0, -1.0), -2.0);
+
+  EXPECT_EQ(uniform_upper_bound(IntervalOpenOpen, -100, -1), -2);
+  EXPECT_EQ(uniform_upper_bound(IntervalClosedOpen, -100, -1), -2);
+  EXPECT_EQ(uniform_upper_bound<float>(IntervalOpenOpen, -2.0, -1.0), -1.0);
+  EXPECT_EQ(uniform_upper_bound<float>(IntervalClosedOpen, -2.0, -1.0), -1.0);
+  EXPECT_EQ(uniform_upper_bound<double>(IntervalOpenOpen, -2.0, -1.0), -1.0);
+  EXPECT_EQ(uniform_upper_bound<double>(IntervalClosedOpen, -2.0, -1.0), -1.0);
+
+  EXPECT_EQ(uniform_upper_bound(IntervalOpenClosed, -100, -1), -1);
+  EXPECT_EQ(uniform_upper_bound(IntervalClosedClosed, -100, -1), -1);
+  EXPECT_GT(uniform_upper_bound<float>(IntervalOpenClosed, -2.0, -1.0), -1.0);
+  EXPECT_GT(uniform_upper_bound<float>(IntervalClosedClosed, -2.0, -1.0), -1.0);
+  EXPECT_GT(uniform_upper_bound<double>(IntervalOpenClosed, -2.0, -1.0), -1.0);
+  EXPECT_GT(uniform_upper_bound<double>(IntervalClosedClosed, -2.0, -1.0),
+            -1.0);
+
+  EXPECT_GT(uniform_lower_bound(IntervalOpenClosed, 1.0, 2.0), 1.0);
+  EXPECT_LT(uniform_lower_bound(IntervalOpenClosed, 1.0, +0.0), 1.0);
+  EXPECT_LT(uniform_lower_bound(IntervalOpenClosed, 1.0, -0.0), 1.0);
+  EXPECT_LT(uniform_lower_bound(IntervalOpenClosed, 1.0, -1.0), 1.0);
+}
+
+TEST_F(UniformHelperTest, UniformBoundFunctionsIntBounds) {
+  // Verifies the saturating nature of uniform_lower_bound and
+  // uniform_upper_bound
+  constexpr IntervalOpenOpenTag IntervalOpenOpen;
+
+  // uint max.
+  constexpr auto m = (std::numeric_limits<uint64_t>::max)();
+
+  EXPECT_EQ(1, uniform_lower_bound(IntervalOpenOpen, 0u, 0u));
+  EXPECT_EQ(m, uniform_lower_bound(IntervalOpenOpen, m, m));
+  EXPECT_EQ(m, uniform_lower_bound(IntervalOpenOpen, m - 1, m - 1));
+  EXPECT_EQ(0, uniform_upper_bound(IntervalOpenOpen, 0u, 0u));
+  EXPECT_EQ(m - 1, uniform_upper_bound(IntervalOpenOpen, m, m));
+
+  // int min/max
+  constexpr auto l = (std::numeric_limits<int64_t>::min)();
+  constexpr auto r = (std::numeric_limits<int64_t>::max)();
+  EXPECT_EQ(1, uniform_lower_bound(IntervalOpenOpen, 0, 0));
+  EXPECT_EQ(l + 1, uniform_lower_bound(IntervalOpenOpen, l, l));
+  EXPECT_EQ(r, uniform_lower_bound(IntervalOpenOpen, r - 1, r - 1));
+  EXPECT_EQ(r, uniform_lower_bound(IntervalOpenOpen, r, r));
+  EXPECT_EQ(-1, uniform_upper_bound(IntervalOpenOpen, 0, 0));
+  EXPECT_EQ(l, uniform_upper_bound(IntervalOpenOpen, l, l));
+  EXPECT_EQ(r - 1, uniform_upper_bound(IntervalOpenOpen, r, r));
+}
+
+TEST_F(UniformHelperTest, UniformBoundFunctionsRealBounds) {
+  // absl::uniform_real_distribution natively assumes IntervalClosedOpen;
+  // use the inverse here so each bound has to change.
+  constexpr IntervalOpenClosedTag IntervalOpenClosed;
+
+  // Edge cases: the next value toward itself is itself.
+  EXPECT_EQ(1.0, uniform_lower_bound(IntervalOpenClosed, 1.0, 1.0));
+  EXPECT_EQ(1.0f, uniform_lower_bound(IntervalOpenClosed, 1.0f, 1.0f));
+
+  // rightmost and leftmost finite values.
+  constexpr auto r = (std::numeric_limits<double>::max)();
+  const auto re = std::nexttoward(r, 0.0);
+  constexpr auto l = -r;
+  const auto le = std::nexttoward(l, 0.0);
+
+  EXPECT_EQ(l, uniform_lower_bound(IntervalOpenClosed, l, l));     // (l,l)
+  EXPECT_EQ(r, uniform_lower_bound(IntervalOpenClosed, r, r));     // (r,r)
+  EXPECT_EQ(le, uniform_lower_bound(IntervalOpenClosed, l, r));    // (l,r)
+  EXPECT_EQ(le, uniform_lower_bound(IntervalOpenClosed, l, 0.0));  // (l, 0)
+  EXPECT_EQ(le, uniform_lower_bound(IntervalOpenClosed, l, le));   // (l, le)
+  EXPECT_EQ(r, uniform_lower_bound(IntervalOpenClosed, re, r));    // (re, r)
+
+  EXPECT_EQ(le, uniform_upper_bound(IntervalOpenClosed, l, l));   // (l,l)
+  EXPECT_EQ(r, uniform_upper_bound(IntervalOpenClosed, r, r));    // (r,r)
+  EXPECT_EQ(r, uniform_upper_bound(IntervalOpenClosed, l, r));    // (l,r)
+  EXPECT_EQ(r, uniform_upper_bound(IntervalOpenClosed, l, re));   // (l,re)
+  EXPECT_EQ(r, uniform_upper_bound(IntervalOpenClosed, 0.0, r));  // (0, r)
+  EXPECT_EQ(r, uniform_upper_bound(IntervalOpenClosed, re, r));   // (re, r)
+  EXPECT_EQ(r, uniform_upper_bound(IntervalOpenClosed, le, re));  // (le, re)
+
+  const double e = std::nextafter(1.0, 2.0);  // 1 + epsilon
+  const double f = std::nextafter(1.0, 0.0);  // 1 - epsilon
+
+  // (1.0, 1.0 + epsilon)
+  EXPECT_EQ(e, uniform_lower_bound(IntervalOpenClosed, 1.0, e));
+  EXPECT_EQ(std::nextafter(e, 2.0),
+            uniform_upper_bound(IntervalOpenClosed, 1.0, e));
+
+  // (1.0-epsilon, 1.0)
+  EXPECT_EQ(1.0, uniform_lower_bound(IntervalOpenClosed, f, 1.0));
+  EXPECT_EQ(e, uniform_upper_bound(IntervalOpenClosed, f, 1.0));
+
+  // denorm cases.
+  const double g = std::numeric_limits<double>::denorm_min();
+  const double h = std::nextafter(g, 1.0);
+
+  // (0, denorm_min)
+  EXPECT_EQ(g, uniform_lower_bound(IntervalOpenClosed, 0.0, g));
+  EXPECT_EQ(h, uniform_upper_bound(IntervalOpenClosed, 0.0, g));
+
+  // (denorm_min, 1.0)
+  EXPECT_EQ(h, uniform_lower_bound(IntervalOpenClosed, g, 1.0));
+  EXPECT_EQ(e, uniform_upper_bound(IntervalOpenClosed, g, 1.0));
+
+  // Edge cases: invalid bounds.
+  EXPECT_EQ(f, uniform_lower_bound(IntervalOpenClosed, 1.0, -1.0));
+}
+
+struct Invalid {};
+
+template <typename A, typename B>
+auto InferredUniformReturnT(int) -> uniform_inferred_return_t<A, B>;
+
+template <typename, typename>
+Invalid InferredUniformReturnT(...);
+
+// Given types <A, B, Expect>, CheckArgsInferType() verifies that
+//
+//   uniform_inferred_return_t<A, B> and
+//   uniform_inferred_return_t<B, A>
+//
+// returns the type "Expect".
+//
+// This interface can also be used to assert that a given inferred return types
+// are invalid. Writing:
+//
+//   CheckArgsInferType<float, int, Invalid>()
+//
+// will assert that this overload does not exist.
+template <typename A, typename B, typename Expect>
+void CheckArgsInferType() {
+  static_assert(
+      std::conjunction_v<
+          std::is_same<Expect, decltype(InferredUniformReturnT<A, B>(0))>,
+          std::is_same<Expect, decltype(InferredUniformReturnT<B, A>(0))>>);
+}
+
+TEST_F(UniformHelperTest, UniformTypeInference) {
+  // Infers common types.
+  CheckArgsInferType<uint16_t, uint16_t, uint16_t>();
+  CheckArgsInferType<uint32_t, uint32_t, uint32_t>();
+  CheckArgsInferType<uint64_t, uint64_t, uint64_t>();
+  CheckArgsInferType<int16_t, int16_t, int16_t>();
+  CheckArgsInferType<int32_t, int32_t, int32_t>();
+  CheckArgsInferType<int64_t, int64_t, int64_t>();
+  CheckArgsInferType<float, float, float>();
+  CheckArgsInferType<double, double, double>();
+
+  // Properly promotes uint16_t.
+  CheckArgsInferType<uint16_t, uint32_t, uint32_t>();
+  CheckArgsInferType<uint16_t, uint64_t, uint64_t>();
+  CheckArgsInferType<uint16_t, int32_t, int32_t>();
+  CheckArgsInferType<uint16_t, int64_t, int64_t>();
+  CheckArgsInferType<uint16_t, float, float>();
+  CheckArgsInferType<uint16_t, double, double>();
+
+  // Properly promotes int16_t.
+  CheckArgsInferType<int16_t, int32_t, int32_t>();
+  CheckArgsInferType<int16_t, int64_t, int64_t>();
+  CheckArgsInferType<int16_t, float, float>();
+  CheckArgsInferType<int16_t, double, double>();
+
+  // Invalid (u)int16_t-pairings do not compile.
+  // See "CheckArgsInferType" comments above, for how this is achieved.
+  CheckArgsInferType<uint16_t, int16_t, Invalid>();
+  CheckArgsInferType<int16_t, uint32_t, Invalid>();
+  CheckArgsInferType<int16_t, uint64_t, Invalid>();
+
+  // Properly promotes uint32_t.
+  CheckArgsInferType<uint32_t, uint64_t, uint64_t>();
+  CheckArgsInferType<uint32_t, int64_t, int64_t>();
+  CheckArgsInferType<uint32_t, double, double>();
+
+  // Properly promotes int32_t.
+  CheckArgsInferType<int32_t, int64_t, int64_t>();
+  CheckArgsInferType<int32_t, double, double>();
+
+  // Invalid (u)int32_t-pairings do not compile.
+  CheckArgsInferType<uint32_t, int32_t, Invalid>();
+  CheckArgsInferType<int32_t, uint64_t, Invalid>();
+  CheckArgsInferType<int32_t, float, Invalid>();
+  CheckArgsInferType<uint32_t, float, Invalid>();
+
+  // Invalid (u)int64_t-pairings do not compile.
+  CheckArgsInferType<uint64_t, int64_t, Invalid>();
+  CheckArgsInferType<int64_t, float, Invalid>();
+  CheckArgsInferType<int64_t, double, Invalid>();
+
+  // Properly promotes float.
+  CheckArgsInferType<float, double, double>();
+}
+
+}  // namespace
diff --git a/absl/random/internal/wide_multiply.h b/absl/random/internal/wide_multiply.h
new file mode 100644
index 00000000000..9cf17a980ad
--- /dev/null
+++ b/absl/random/internal/wide_multiply.h
@@ -0,0 +1,96 @@
+// Copyright 2017 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_RANDOM_INTERNAL_WIDE_MULTIPLY_H_
+#define ABSL_RANDOM_INTERNAL_WIDE_MULTIPLY_H_
+
+#include <cstddef>
+#include <cstdint>
+#include <limits>
+#include <type_traits>
+
+#include "absl/base/config.h"
+#include "absl/numeric/bits.h"
+#include "absl/numeric/int128.h"
+#include "absl/random/internal/traits.h"
+
+#if (defined(_WIN32) || defined(_WIN64)) && defined(_M_IA64)
+#include <intrin.h>  // NOLINT(build/include_order)
+#pragma intrinsic(_umul128)
+#define ABSL_INTERNAL_USE_UMUL128 1
+#endif
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace random_internal {
+
+// wide_multiply<T> multiplies two N-bit values to a 2N-bit result.
+template <typename UIntType>
+struct wide_multiply {
+  static constexpr size_t kN = std::numeric_limits<UIntType>::digits;
+  using input_type = UIntType;
+  using result_type = typename random_internal::unsigned_bits<kN * 2>::type;
+
+  static result_type multiply(input_type a, input_type b) {
+    return static_cast<result_type>(a) * b;
+  }
+
+  static input_type hi(result_type r) {
+    return static_cast<input_type>(r >> kN);
+  }
+  static input_type lo(result_type r) { return static_cast<input_type>(r); }
+
+  static_assert(std::is_unsigned_v<UIntType>,
+                "Class-template wide_multiply<> argument must be unsigned.");
+};
+
+// MultiplyU128ToU256 multiplies two 128-bit values to a 256-bit value.
+inline U256 MultiplyU128ToU256(uint128 a, uint128 b) {
+  const uint128 a00 = static_cast<uint64_t>(a);
+  const uint128 a64 = a >> 64;
+  const uint128 b00 = static_cast<uint64_t>(b);
+  const uint128 b64 = b >> 64;
+
+  const uint128 c00 = a00 * b00;
+  const uint128 c64a = a00 * b64;
+  const uint128 c64b = a64 * b00;
+  const uint128 c128 = a64 * b64;
+
+  const uint64_t carry =
+      static_cast<uint64_t>(((c00 >> 64) + static_cast<uint64_t>(c64a) +
+                             static_cast<uint64_t>(c64b)) >>
+                            64);
+
+  return {c128 + (c64a >> 64) + (c64b >> 64) + carry,
+          c00 + (c64a << 64) + (c64b << 64)};
+}
+
+template <>
+struct wide_multiply<uint128> {
+  using input_type = uint128;
+  using result_type = U256;
+
+  static result_type multiply(input_type a, input_type b) {
+    return MultiplyU128ToU256(a, b);
+  }
+
+  static input_type hi(result_type r) { return r.hi; }
+  static input_type lo(result_type r) { return r.lo; }
+};
+
+}  // namespace random_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_RANDOM_INTERNAL_WIDE_MULTIPLY_H_
diff --git a/absl/random/internal/wide_multiply_test.cc b/absl/random/internal/wide_multiply_test.cc
new file mode 100644
index 00000000000..caf5ab429f3
--- /dev/null
+++ b/absl/random/internal/wide_multiply_test.cc
@@ -0,0 +1,120 @@
+// Copyright 2017 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/random/internal/wide_multiply.h"
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/numeric/int128.h"
+#include "absl/random/internal/traits.h"
+
+using absl::random_internal::MultiplyU128ToU256;
+using absl::random_internal::U256;
+
+namespace {
+
+U256 LeftShift(U256 v, int s) {
+  if (s == 0) {
+    return v;
+  } else if (s < 128) {
+    return {(v.hi << s) | (v.lo >> (128 - s)), v.lo << s};
+  } else {
+    return {v.lo << (s - 128), 0};
+  }
+}
+
+MATCHER_P2(Eq256, hi, lo, "") { return arg.hi == hi && arg.lo == lo; }
+MATCHER_P(Eq256, v, "") { return arg.hi == v.hi && arg.lo == v.lo; }
+
+TEST(WideMultiplyTest, MultiplyU128ToU256Test) {
+  using absl::uint128;
+  constexpr uint128 k1 = 1;
+  constexpr uint128 kMax = ~static_cast<uint128>(0);
+
+  EXPECT_THAT(MultiplyU128ToU256(0, 0), Eq256(0, 0));
+
+  // Max uin128_t
+  EXPECT_THAT(MultiplyU128ToU256(kMax, kMax), Eq256(kMax << 1, 1));
+  EXPECT_THAT(MultiplyU128ToU256(kMax, 1), Eq256(0, kMax));
+  EXPECT_THAT(MultiplyU128ToU256(1, kMax), Eq256(0, kMax));
+  for (int i = 0; i < 64; ++i) {
+    SCOPED_TRACE(i);
+    EXPECT_THAT(MultiplyU128ToU256(kMax, k1 << i),
+                Eq256(LeftShift({0, kMax}, i)));
+    EXPECT_THAT(MultiplyU128ToU256(k1 << i, kMax),
+                Eq256(LeftShift({0, kMax}, i)));
+  }
+
+  // 1-bit x 1-bit.
+  for (int i = 0; i < 64; ++i) {
+    for (int j = 0; j < 64; ++j) {
+      EXPECT_THAT(MultiplyU128ToU256(k1 << i, k1 << j),
+                  Eq256(LeftShift({0, 1}, i + j)));
+    }
+  }
+
+  // Verified multiplies
+  EXPECT_THAT(MultiplyU128ToU256(
+                  absl::MakeUint128(0xc502da0d6ea99fe8, 0xfa3c9141a1f50912),
+                  absl::MakeUint128(0x96bcf1ac37f97bd6, 0x27e2cdeb5fb2299e)),
+              Eq256(absl::MakeUint128(0x740113d838f96a64, 0x22e8cfa4d71f89ea),
+                    absl::MakeUint128(0x19184a345c62e993, 0x237871b630337b1c)));
+  EXPECT_THAT(MultiplyU128ToU256(
+                  absl::MakeUint128(0x6f29e670cee07230, 0xc3d8e6c3e4d86759),
+                  absl::MakeUint128(0x3227d29fa6386db1, 0x231682bb1e4b764f)),
+              Eq256(absl::MakeUint128(0x15c779d9d5d3b07c, 0xd7e6c827f0c81cbe),
+                    absl::MakeUint128(0xf88e3914f7fa287a, 0x15b79975137dea77)));
+  EXPECT_THAT(MultiplyU128ToU256(
+                  absl::MakeUint128(0xafb77107215646e1, 0x3b844cb1ac5769e7),
+                  absl::MakeUint128(0x1ff7b2d888b62479, 0x92f758ae96fcba0b)),
+              Eq256(absl::MakeUint128(0x15f13b70181f6985, 0x2adb36bbabce7d02),
+                    absl::MakeUint128(0x6c470d72e13aad04, 0x63fba3f5841762ed)));
+  EXPECT_THAT(MultiplyU128ToU256(
+                  absl::MakeUint128(0xd85d5558d67ac905, 0xf88c70654dae19b1),
+                  absl::MakeUint128(0x17252c6727db3738, 0x399ff658c511eedc)),
+              Eq256(absl::MakeUint128(0x138fcdaf8b0421ee, 0x1b465ddf2a0d03f6),
+                    absl::MakeUint128(0x8f573ba68296860f, 0xf327d2738741a21c)));
+  EXPECT_THAT(MultiplyU128ToU256(
+                  absl::MakeUint128(0x46f0421a37ff6bee, 0xa61df89f09d140b1),
+                  absl::MakeUint128(0x3d712ec9f37ca2e1, 0x9658a2cba47ef4b1)),
+              Eq256(absl::MakeUint128(0x11069cc48ee7c95d, 0xd35fb1c7aa91c978),
+                    absl::MakeUint128(0xbe2f4a6de874b015, 0xd2f7ac1b76746e61)));
+  EXPECT_THAT(MultiplyU128ToU256(
+                  absl::MakeUint128(0x730d27c72d58fa49, 0x3ebeda7498f8827c),
+                  absl::MakeUint128(0xa2c959eca9f503af, 0x189c687eb842bbd8)),
+              Eq256(absl::MakeUint128(0x4928d0ea356ba022, 0x1546d34a2963393),
+                    absl::MakeUint128(0x7481531e1e0a16d1, 0xdd8025015cf6aca0)));
+  EXPECT_THAT(MultiplyU128ToU256(
+                  absl::MakeUint128(0x6ca41020f856d2f1, 0xb9b0838c04a7f4aa),
+                  absl::MakeUint128(0x9cf41d28a8396f54, 0x1d681695e377ffe6)),
+              Eq256(absl::MakeUint128(0x429b92934d9be6f1, 0xea182877157c1e7),
+                    absl::MakeUint128(0x7135c23f0a4a475, 0xc1adc366f4a126bc)));
+  EXPECT_THAT(MultiplyU128ToU256(
+                  absl::MakeUint128(0x57472833797c332, 0x6c79272fdec4687a),
+                  absl::MakeUint128(0xb5f022ea3838e46b, 0x16face2f003e27a6)),
+              Eq256(absl::MakeUint128(0x3e072e0962b3400, 0x5d9fe8fdc3d0e1f4),
+                    absl::MakeUint128(0x7dc0df47cedafd62, 0xbe6501f1acd2551c)));
+  EXPECT_THAT(MultiplyU128ToU256(
+                  absl::MakeUint128(0xf0fb4198322eb1c2, 0xfe7f5f31f3885938),
+                  absl::MakeUint128(0xd99012b71bb7aa31, 0xac7a6f9eb190789)),
+              Eq256(absl::MakeUint128(0xcccc998cf075ca01, 0x642d144322fb873a),
+                    absl::MakeUint128(0xc79dc12b69d91ed4, 0xa83459132ce046f8)));
+  EXPECT_THAT(MultiplyU128ToU256(
+                  absl::MakeUint128(0xb5c04120848cdb47, 0x8aa62a827bf52635),
+                  absl::MakeUint128(0x8d07a359be2f1380, 0x467bb90d59da0dea)),
+              Eq256(absl::MakeUint128(0x64205019d139a9ce, 0x99425c5fb6e7a977),
+                    absl::MakeUint128(0xd3e99628a9e5fca7, 0x9c7824cb7279d72)));
+}
+
+}  // namespace
diff --git a/absl/random/log_uniform_int_distribution.h b/absl/random/log_uniform_int_distribution.h
index ac43416ee57..cbd5e0ca2ba 100644
--- a/absl/random/log_uniform_int_distribution.h
+++ b/absl/random/log_uniform_int_distribution.h
@@ -21,15 +21,14 @@
 #include <istream>
 #include <limits>
 #include <ostream>
-#include <type_traits>
 
-#include "absl/random/internal/distribution_impl.h"
-#include "absl/random/internal/fastmath.h"
+#include "absl/base/config.h"
 #include "absl/random/internal/iostream_state_saver.h"
 #include "absl/random/internal/traits.h"
 #include "absl/random/uniform_int_distribution.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 // log_uniform_int_distribution:
 //
@@ -67,7 +66,7 @@ class log_uniform_int_distribution {
       if (base_ == 2) {
         // Determine where the first set bit is on range(), giving a log2(range)
         // value which can be used to construct bounds.
-        log_range_ = (std::min)(random_internal::LeadingSetBit(range()),
+        log_range_ = (std::min)(random_internal::BitWidth(range()),
                                 std::numeric_limits<unsigned_type>::digits);
       } else {
         // NOTE: Computing the logN(x) introduces error from 2 sources:
@@ -79,7 +78,7 @@ class log_uniform_int_distribution {
         //
         // Thus a result which should equal K may equal K +/- epsilon,
         // which can eliminate some values depending on where the bounds fall.
-        const double inv_log_base = 1.0 / std::log(base_);
+        const double inv_log_base = 1.0 / std::log(static_cast<double>(base_));
         const double log_range = std::log(static_cast<double>(range()) + 0.5);
         log_range_ = static_cast<int>(std::ceil(inv_log_base * log_range));
       }
@@ -109,7 +108,7 @@ class log_uniform_int_distribution {
     unsigned_type range_;  // max - min
     int log_range_;        // ceil(logN(range_))
 
-    static_assert(std::is_integral<IntType>::value,
+    static_assert(random_internal::IsIntegral<IntType>::value,
                   "Class-template absl::log_uniform_int_distribution<> must be "
                   "parameterized using an integral type.");
   };
@@ -135,7 +134,7 @@ class log_uniform_int_distribution {
   template <typename URBG>
   result_type operator()(URBG& g,  // NOLINT(runtime/references)
                          const param_type& p) {
-    return (p.min)() + Generate(g, p);
+    return static_cast<result_type>((p.min)() + Generate(g, p));
   }
 
   result_type(min)() const { return (param_.min)(); }
@@ -189,23 +188,26 @@ log_uniform_int_distribution<IntType>::Generate(
                 ? (std::numeric_limits<unsigned_type>::max)()
                 : (static_cast<unsigned_type>(1) << e) - 1;
   } else {
-    const double r = std::pow(p.base(), d);
-    const double s = (r * p.base()) - 1.0;
-
-    base_e = (r > (std::numeric_limits<unsigned_type>::max)())
-                 ? (std::numeric_limits<unsigned_type>::max)()
-                 : static_cast<unsigned_type>(r);
-
-    top_e = (s > (std::numeric_limits<unsigned_type>::max)())
-                ? (std::numeric_limits<unsigned_type>::max)()
-                : static_cast<unsigned_type>(s);
+    const double r = std::pow(static_cast<double>(p.base()), d);
+    const double s = (r * static_cast<double>(p.base())) - 1.0;
+
+    base_e =
+        (r > static_cast<double>((std::numeric_limits<unsigned_type>::max)()))
+            ? (std::numeric_limits<unsigned_type>::max)()
+            : static_cast<unsigned_type>(r);
+
+    top_e =
+        (s > static_cast<double>((std::numeric_limits<unsigned_type>::max)()))
+            ? (std::numeric_limits<unsigned_type>::max)()
+            : static_cast<unsigned_type>(s);
   }
 
   const unsigned_type lo = (base_e >= p.range()) ? p.range() : base_e;
   const unsigned_type hi = (top_e >= p.range()) ? p.range() : top_e;
 
   // choose uniformly over [lo, hi]
-  return absl::uniform_int_distribution<result_type>(lo, hi)(g);
+  return absl::uniform_int_distribution<result_type>(
+      static_cast<result_type>(lo), static_cast<result_type>(hi))(g);
 }
 
 template <typename CharT, typename Traits, typename IntType>
@@ -245,6 +247,7 @@ std::basic_istream<CharT, Traits>& operator>>(
   return is;
 }
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_LOG_UNIFORM_INT_DISTRIBUTION_H_
diff --git a/absl/random/log_uniform_int_distribution_test.cc b/absl/random/log_uniform_int_distribution_test.cc
index 5270531d203..591b5b37e48 100644
--- a/absl/random/log_uniform_int_distribution_test.cc
+++ b/absl/random/log_uniform_int_distribution_test.cc
@@ -14,9 +14,12 @@
 
 #include "absl/random/log_uniform_int_distribution.h"
 
+#include <algorithm>
+#include <cmath>
 #include <cstddef>
 #include <cstdint>
 #include <iterator>
+#include <limits>
 #include <random>
 #include <sstream>
 #include <string>
@@ -24,9 +27,11 @@
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
-#include "absl/base/internal/raw_logging.h"
+#include "absl/base/macros.h"
+#include "absl/log/log.h"
 #include "absl/random/internal/chi_square.h"
 #include "absl/random/internal/distribution_test_util.h"
+#include "absl/random/internal/pcg_engine.h"
 #include "absl/random/internal/sequence_urbg.h"
 #include "absl/random/random.h"
 #include "absl/strings/str_cat.h"
@@ -41,7 +46,7 @@ class LogUniformIntDistributionTypeTest : public ::testing::Test {};
 
 using IntTypes = ::testing::Types<int8_t, int16_t, int32_t, int64_t,  //
                                   uint8_t, uint16_t, uint32_t, uint64_t>;
-TYPED_TEST_CASE(LogUniformIntDistributionTypeTest, IntTypes);
+TYPED_TEST_SUITE(LogUniformIntDistributionTypeTest, IntTypes);
 
 TYPED_TEST(LogUniformIntDistributionTypeTest, SerializeTest) {
   using param_type =
@@ -107,8 +112,7 @@ TYPED_TEST(LogUniformIntDistributionTypeTest, SerializeTest) {
       if (sample > sample_max) sample_max = sample;
       if (sample < sample_min) sample_min = sample;
     }
-    ABSL_INTERNAL_LOG(INFO,
-                      absl::StrCat("Range: ", +sample_min, ", ", +sample_max));
+    LOG(INFO) << "Range: " << sample_min << ", " << sample_max;
   }
 }
 
@@ -121,7 +125,10 @@ class LogUniformIntChiSquaredTest
   // data generated by the log-uniform-int distribution.
   double ChiSquaredTestImpl();
 
-  absl::InsecureBitGen rng_;
+  // We use a fixed bit generator for distribution accuracy tests.  This allows
+  // these tests to be deterministic, while still testing the qualify of the
+  // implementation.
+  absl::random_internal::pcg64_2018_engine rng_{0x2B7E151628AED2A6};
 };
 
 double LogUniformIntChiSquaredTest::ChiSquaredTestImpl() {
@@ -145,8 +152,8 @@ double LogUniformIntChiSquaredTest::ChiSquaredTestImpl() {
   const auto bucket_index = [base_log, is_2, &param](int32_t x) {
     uint64_t y = static_cast<uint64_t>(x) - param.min();
     return (y == 0) ? 0
-                    : is_2 ? static_cast<int>(1 + std::log2(y))
-                           : static_cast<int>(1 + std::log(y) * base_log);
+           : is_2   ? static_cast<int>(1 + std::log2(y))
+                    : static_cast<int>(1 + std::log(y) * base_log);
   };
   const int max_bucket = bucket_index(param.max());  // inclusive
   const size_t trials = 15 + (max_bucket + 1) * 10;
@@ -178,23 +185,20 @@ double LogUniformIntChiSquaredTest::ChiSquaredTestImpl() {
   const double p = absl::random_internal::ChiSquarePValue(chi_square, dof);
 
   if (chi_square > threshold) {
-    ABSL_INTERNAL_LOG(INFO, "values");
+    LOG(INFO) << "values";
     for (size_t i = 0; i < buckets.size(); i++) {
-      ABSL_INTERNAL_LOG(INFO, absl::StrCat(i, ": ", buckets[i]));
+      LOG(INFO) << i << ": " << buckets[i];
     }
-    ABSL_INTERNAL_LOG(INFO,
-                      absl::StrFormat("trials=%d\n"
-                                      "%s(data, %d) = %f (%f)\n"
-                                      "%s @ 0.98 = %f",
-                                      trials, kChiSquared, dof, chi_square, p,
-                                      kChiSquared, threshold));
+    LOG(INFO) << "trials=" << trials << "\n"
+              << kChiSquared << "(data, " << dof << ") = " << chi_square << " ("
+              << p << ")\n"
+              << kChiSquared << " @ 0.98 = " << threshold;
   }
   return p;
 }
 
 TEST_P(LogUniformIntChiSquaredTest, MultiTest) {
   const int kTrials = 5;
-
   int failures = 0;
   for (int i = 0; i < kTrials; i++) {
     double p_value = ChiSquaredTestImpl();
diff --git a/absl/random/mock_distributions.h b/absl/random/mock_distributions.h
new file mode 100644
index 00000000000..1983547327c
--- /dev/null
+++ b/absl/random/mock_distributions.h
@@ -0,0 +1,269 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: mock_distributions.h
+// -----------------------------------------------------------------------------
+//
+// This file contains mock distribution functions for use alongside an
+// `absl::MockingBitGen` object within the GoogleTest testing framework. Such
+// mocks are useful to provide deterministic values as return values within
+// (otherwise random) Abseil distribution functions.
+//
+// The return type of each function is a mock expectation object which
+// is used to set the match result.
+//
+// More information about the GoogleTest testing framework is available at
+// https://github.com/google/googletest
+//
+// EXPECT_CALL and ON_CALL need to be made within the same DLL component as
+// the call to absl::Uniform and related methods, otherwise mocking will fail
+// since the  underlying implementation creates a type-specific pointer which
+// will be distinct across different DLL boundaries.
+//
+// Example:
+//
+//   absl::MockingBitGen mock;
+//   EXPECT_CALL(absl::MockUniform<int>(), Call(mock, 1, 1000))
+//     .WillRepeatedly(testing::ReturnRoundRobin({20, 40}));
+//
+//   EXPECT_EQ(absl::Uniform<int>(gen, 1, 1000), 20);
+//   EXPECT_EQ(absl::Uniform<int>(gen, 1, 1000), 40);
+//   EXPECT_EQ(absl::Uniform<int>(gen, 1, 1000), 20);
+//   EXPECT_EQ(absl::Uniform<int>(gen, 1, 1000), 40);
+
+#ifndef ABSL_RANDOM_MOCK_DISTRIBUTIONS_H_
+#define ABSL_RANDOM_MOCK_DISTRIBUTIONS_H_
+
+#include "absl/base/config.h"
+#include "absl/random/bernoulli_distribution.h"
+#include "absl/random/beta_distribution.h"
+#include "absl/random/distributions.h"
+#include "absl/random/exponential_distribution.h"
+#include "absl/random/gaussian_distribution.h"
+#include "absl/random/internal/mock_overload_set.h"
+#include "absl/random/internal/mock_validators.h"
+#include "absl/random/log_uniform_int_distribution.h"
+#include "absl/random/mocking_bit_gen.h"
+#include "absl/random/poisson_distribution.h"
+#include "absl/random/zipf_distribution.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+// -----------------------------------------------------------------------------
+// absl::MockUniform
+// -----------------------------------------------------------------------------
+//
+// Matches calls to absl::Uniform.
+//
+// `absl::MockUniform` is a class template used in conjunction with Googletest's
+// `ON_CALL()` and `EXPECT_CALL()` macros. To use it, default-construct an
+// instance of it inside `ON_CALL()` or `EXPECT_CALL()`, and use `Call(...)` the
+// same way one would define mocks on a Googletest `MockFunction()`.
+//
+// Example:
+//
+//  absl::MockingBitGen mock;
+//  EXPECT_CALL(absl::MockUniform<uint32_t>(), Call(mock))
+//     .WillOnce(Return(123456));
+//  auto x = absl::Uniform<uint32_t>(mock);
+//  assert(x == 123456)
+//
+template <typename R>
+using MockUniform = random_internal::MockOverloadSetWithValidator<
+    random_internal::UniformDistributionWrapper<R>,
+    random_internal::UniformDistributionValidator<R>,
+    R(IntervalClosedOpenTag, MockingBitGen&, R, R),
+    R(IntervalClosedClosedTag, MockingBitGen&, R, R),
+    R(IntervalOpenOpenTag, MockingBitGen&, R, R),
+    R(IntervalOpenClosedTag, MockingBitGen&, R, R), R(MockingBitGen&, R, R),
+    R(MockingBitGen&)>;
+
+// -----------------------------------------------------------------------------
+// absl::MockBernoulli
+// -----------------------------------------------------------------------------
+//
+// Matches calls to absl::Bernoulli.
+//
+// `absl::MockBernoulli` is a class used in conjunction with Googletest's
+// `ON_CALL()` and `EXPECT_CALL()` macros. To use it, default-construct an
+// instance of it inside `ON_CALL()` or `EXPECT_CALL()`, and use `Call(...)` the
+// same way one would define mocks on a Googletest `MockFunction()`.
+//
+// Example:
+//
+//  absl::MockingBitGen mock;
+//  EXPECT_CALL(absl::MockBernoulli(), Call(mock, testing::_))
+//     .WillOnce(Return(false));
+//  assert(absl::Bernoulli(mock, 0.5) == false);
+//
+using MockBernoulli =
+    random_internal::MockOverloadSet<absl::bernoulli_distribution,
+                                     bool(MockingBitGen&, double)>;
+
+// -----------------------------------------------------------------------------
+// absl::MockBeta
+// -----------------------------------------------------------------------------
+//
+// Matches calls to absl::Beta.
+//
+// `absl::MockBeta` is a class used in conjunction with Googletest's `ON_CALL()`
+// and `EXPECT_CALL()` macros. To use it, default-construct an instance of it
+// inside `ON_CALL()` or `EXPECT_CALL()`, and use `Call(...)` the same way one
+// would define mocks on a Googletest `MockFunction()`.
+//
+// Example:
+//
+//  absl::MockingBitGen mock;
+//  EXPECT_CALL(absl::MockBeta(), Call(mock, 3.0, 2.0))
+//     .WillOnce(Return(0.567));
+//  auto x = absl::Beta<double>(mock, 3.0, 2.0);
+//  assert(x == 0.567);
+//
+template <typename RealType>
+using MockBeta =
+    random_internal::MockOverloadSet<absl::beta_distribution<RealType>,
+                                     RealType(MockingBitGen&, RealType,
+                                              RealType)>;
+
+// -----------------------------------------------------------------------------
+// absl::MockExponential
+// -----------------------------------------------------------------------------
+//
+// Matches calls to absl::Exponential.
+//
+// `absl::MockExponential` is a class template used in conjunction with
+// Googletest's `ON_CALL()` and `EXPECT_CALL()` macros. To use it,
+// default-construct an instance of it inside `ON_CALL()` or `EXPECT_CALL()`,
+// and use `Call(...)` the same way one would define mocks on a
+// Googletest `MockFunction()`.
+//
+// Example:
+//
+//  absl::MockingBitGen mock;
+//  EXPECT_CALL(absl::MockExponential<double>(), Call(mock, 0.5))
+//     .WillOnce(Return(12.3456789));
+//  auto x = absl::Exponential<double>(mock, 0.5);
+//  assert(x == 12.3456789)
+//
+template <typename RealType>
+using MockExponential =
+    random_internal::MockOverloadSet<absl::exponential_distribution<RealType>,
+                                     RealType(MockingBitGen&, RealType)>;
+
+// -----------------------------------------------------------------------------
+// absl::MockGaussian
+// -----------------------------------------------------------------------------
+//
+// Matches calls to absl::Gaussian.
+//
+// `absl::MockGaussian` is a class template used in conjunction with
+// Googletest's `ON_CALL()` and `EXPECT_CALL()` macros. To use it,
+// default-construct an instance of it inside `ON_CALL()` or `EXPECT_CALL()`,
+// and use `Call(...)` the same way one would define mocks on a
+// Googletest `MockFunction()`.
+//
+// Example:
+//
+//  absl::MockingBitGen mock;
+//  EXPECT_CALL(absl::MockGaussian<double>(), Call(mock, 16.3, 3.3))
+//     .WillOnce(Return(12.3456789));
+//  auto x = absl::Gaussian<double>(mock, 16.3, 3.3);
+//  assert(x == 12.3456789)
+//
+template <typename RealType>
+using MockGaussian =
+    random_internal::MockOverloadSet<absl::gaussian_distribution<RealType>,
+                                     RealType(MockingBitGen&, RealType,
+                                              RealType)>;
+
+// -----------------------------------------------------------------------------
+// absl::MockLogUniform
+// -----------------------------------------------------------------------------
+//
+// Matches calls to absl::LogUniform.
+//
+// `absl::MockLogUniform` is a class template used in conjunction with
+// Googletest's `ON_CALL()` and `EXPECT_CALL()` macros. To use it,
+// default-construct an instance of it inside `ON_CALL()` or `EXPECT_CALL()`,
+// and use `Call(...)` the same way one would define mocks on a
+// Googletest `MockFunction()`.
+//
+// Example:
+//
+//  absl::MockingBitGen mock;
+//  EXPECT_CALL(absl::MockLogUniform<int>(), Call(mock, 10, 10000, 10))
+//     .WillOnce(Return(1221));
+//  auto x = absl::LogUniform<int>(mock, 10, 10000, 10);
+//  assert(x == 1221)
+//
+template <typename IntType>
+using MockLogUniform = random_internal::MockOverloadSet<
+    absl::log_uniform_int_distribution<IntType>,
+    IntType(MockingBitGen&, IntType, IntType, IntType)>;
+
+// -----------------------------------------------------------------------------
+// absl::MockPoisson
+// -----------------------------------------------------------------------------
+//
+// Matches calls to absl::Poisson.
+//
+// `absl::MockPoisson` is a class template used in conjunction with Googletest's
+// `ON_CALL()` and `EXPECT_CALL()` macros. To use it, default-construct an
+// instance of it inside `ON_CALL()` or `EXPECT_CALL()`, and use `Call(...)` the
+// same way one would define mocks on a Googletest `MockFunction()`.
+//
+// Example:
+//
+//  absl::MockingBitGen mock;
+//  EXPECT_CALL(absl::MockPoisson<int>(), Call(mock, 2.0))
+//     .WillOnce(Return(1221));
+//  auto x = absl::Poisson<int>(mock, 2.0);
+//  assert(x == 1221)
+//
+template <typename IntType>
+using MockPoisson =
+    random_internal::MockOverloadSet<absl::poisson_distribution<IntType>,
+                                     IntType(MockingBitGen&, double)>;
+
+// -----------------------------------------------------------------------------
+// absl::MockZipf
+// -----------------------------------------------------------------------------
+//
+// Matches calls to absl::Zipf.
+//
+// `absl::MockZipf` is a class template used in conjunction with Googletest's
+// `ON_CALL()` and `EXPECT_CALL()` macros. To use it, default-construct an
+// instance of it inside `ON_CALL()` or `EXPECT_CALL()`, and use `Call(...)` the
+// same way one would define mocks on a Googletest `MockFunction()`.
+//
+// Example:
+//
+//  absl::MockingBitGen mock;
+//  EXPECT_CALL(absl::MockZipf<int>(), Call(mock, 1000000, 2.0, 1.0))
+//     .WillOnce(Return(1221));
+//  auto x = absl::Zipf<int>(mock, 1000000, 2.0, 1.0);
+//  assert(x == 1221)
+//
+template <typename IntType>
+using MockZipf =
+    random_internal::MockOverloadSet<absl::zipf_distribution<IntType>,
+                                     IntType(MockingBitGen&, IntType, double,
+                                             double)>;
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_RANDOM_MOCK_DISTRIBUTIONS_H_
diff --git a/absl/random/mock_distributions_test.cc b/absl/random/mock_distributions_test.cc
new file mode 100644
index 00000000000..93af3f92ecf
--- /dev/null
+++ b/absl/random/mock_distributions_test.cc
@@ -0,0 +1,281 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/random/mock_distributions.h"
+
+#include <cmath>
+#include <limits>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/numeric/int128.h"
+#include "absl/random/distributions.h"
+#include "absl/random/mocking_bit_gen.h"
+#include "absl/random/random.h"
+
+namespace {
+using ::testing::Return;
+
+TEST(MockDistributions, Examples) {
+  absl::MockingBitGen gen;
+
+  EXPECT_NE(absl::Uniform<int>(gen, 1, 1000000), 20);
+  EXPECT_CALL(absl::MockUniform<int>(), Call(gen, 1, 1000000))
+      .WillOnce(Return(20));
+  EXPECT_EQ(absl::Uniform<int>(gen, 1, 1000000), 20);
+
+  EXPECT_NE(absl::Uniform<double>(gen, 0.0, 100.0), 5.0);
+  EXPECT_CALL(absl::MockUniform<double>(), Call(gen, 0.0, 100.0))
+      .WillOnce(Return(5.0));
+  EXPECT_EQ(absl::Uniform<double>(gen, 0.0, 100.0), 5.0);
+
+  EXPECT_NE(absl::Exponential<double>(gen, 1.0), 42);
+  EXPECT_CALL(absl::MockExponential<double>(), Call(gen, 1.0))
+      .WillOnce(Return(42));
+  EXPECT_EQ(absl::Exponential<double>(gen, 1.0), 42);
+
+  EXPECT_NE(absl::Poisson<int>(gen, 1.0), 500);
+  EXPECT_CALL(absl::MockPoisson<int>(), Call(gen, 1.0)).WillOnce(Return(500));
+  EXPECT_EQ(absl::Poisson<int>(gen, 1.0), 500);
+
+  EXPECT_NE(absl::Bernoulli(gen, 0.000001), true);
+  EXPECT_CALL(absl::MockBernoulli(), Call(gen, 0.000001))
+      .WillOnce(Return(true));
+  EXPECT_EQ(absl::Bernoulli(gen, 0.000001), true);
+
+  EXPECT_NE(absl::Beta<double>(gen, 3.0, 2.0), 0.567);
+  EXPECT_CALL(absl::MockBeta<double>(), Call(gen, 3.0, 2.0))
+      .WillOnce(Return(0.567));
+  EXPECT_EQ(absl::Beta<double>(gen, 3.0, 2.0), 0.567);
+
+  EXPECT_NE(absl::Zipf<int>(gen, 1000000, 2.0, 1.0), 1221);
+  EXPECT_CALL(absl::MockZipf<int>(), Call(gen, 1000000, 2.0, 1.0))
+      .WillOnce(Return(1221));
+  EXPECT_EQ(absl::Zipf<int>(gen, 1000000, 2.0, 1.0), 1221);
+
+  EXPECT_NE(absl::Gaussian<double>(gen, 0.0, 1.0), 0.001);
+  EXPECT_CALL(absl::MockGaussian<double>(), Call(gen, 0.0, 1.0))
+      .WillOnce(Return(0.001));
+  EXPECT_EQ(absl::Gaussian<double>(gen, 0.0, 1.0), 0.001);
+
+  const int kHigh = (1 << 30) - 1;
+  EXPECT_NE(absl::LogUniform<int>(gen, 0, kHigh, 2), kHigh);
+  EXPECT_CALL(absl::MockLogUniform<int>(), Call(gen, 0, kHigh, 2))
+      .WillOnce(Return(kHigh));
+  EXPECT_EQ(absl::LogUniform<int>(gen, 0, kHigh, 2), kHigh);
+}
+
+TEST(MockDistributions, UniformUInt128BoundariesAreAllowed) {
+  absl::MockingBitGen gen;
+
+  EXPECT_CALL(absl::MockUniform<absl::uint128>(), Call(gen))
+      .WillOnce(Return(absl::Uint128Max()));
+  EXPECT_EQ(absl::Uniform<absl::uint128>(gen), absl::Uint128Max());
+}
+
+TEST(MockDistributions, UniformDoubleBoundaryCasesAreAllowed) {
+  absl::MockingBitGen gen;
+
+  EXPECT_CALL(absl::MockUniform<double>(), Call(gen, 1.0, 10.0))
+      .WillOnce(Return(
+          std::nextafter(10.0, -std::numeric_limits<double>::infinity())));
+  EXPECT_EQ(absl::Uniform<double>(gen, 1.0, 10.0),
+            std::nextafter(10.0, -std::numeric_limits<double>::infinity()));
+
+  EXPECT_CALL(absl::MockUniform<double>(),
+              Call(absl::IntervalOpen, gen, 1.0, 10.0))
+      .WillOnce(Return(
+          std::nextafter(10.0, -std::numeric_limits<double>::infinity())));
+  EXPECT_EQ(absl::Uniform<double>(absl::IntervalOpen, gen, 1.0, 10.0),
+            std::nextafter(10.0, -std::numeric_limits<double>::infinity()));
+
+  EXPECT_CALL(absl::MockUniform<double>(),
+              Call(absl::IntervalOpen, gen, 1.0, 10.0))
+      .WillOnce(
+          Return(std::nextafter(1.0, std::numeric_limits<double>::infinity())));
+  EXPECT_EQ(absl::Uniform<double>(absl::IntervalOpen, gen, 1.0, 10.0),
+            std::nextafter(1.0, std::numeric_limits<double>::infinity()));
+}
+
+TEST(MockDistributions, UniformDoubleEmptyRangesAllowTheBoundary) {
+  absl::MockingBitGen gen;
+
+  ON_CALL(absl::MockUniform<double>(), Call(absl::IntervalOpen, gen, 1.0, 1.0))
+      .WillByDefault(Return(1.0));
+  EXPECT_EQ(absl::Uniform<double>(absl::IntervalOpen, gen, 1.0, 1.0), 1.0);
+
+  ON_CALL(absl::MockUniform<double>(),
+          Call(absl::IntervalOpenClosed, gen, 1.0, 1.0))
+      .WillByDefault(Return(1.0));
+  EXPECT_EQ(absl::Uniform<double>(absl::IntervalOpenClosed, gen, 1.0, 1.0),
+            1.0);
+
+  ON_CALL(absl::MockUniform<double>(),
+          Call(absl::IntervalClosedOpen, gen, 1.0, 1.0))
+      .WillByDefault(Return(1.0));
+  EXPECT_EQ(absl::Uniform<double>(absl::IntervalClosedOpen, gen, 1.0, 1.0),
+            1.0);
+}
+
+TEST(MockDistributions, UniformIntEmptyRangeCasesAllowTheBoundary) {
+  absl::MockingBitGen gen;
+
+  ON_CALL(absl::MockUniform<int>(), Call(absl::IntervalOpen, gen, 1, 1))
+      .WillByDefault(Return(1));
+  EXPECT_EQ(absl::Uniform<int>(absl::IntervalOpen, gen, 1, 1), 1);
+
+  ON_CALL(absl::MockUniform<int>(), Call(absl::IntervalOpenClosed, gen, 1, 1))
+      .WillByDefault(Return(1));
+  EXPECT_EQ(absl::Uniform<int>(absl::IntervalOpenClosed, gen, 1, 1), 1);
+
+  ON_CALL(absl::MockUniform<int>(), Call(absl::IntervalClosedOpen, gen, 1, 1))
+      .WillByDefault(Return(1));
+  EXPECT_EQ(absl::Uniform<int>(absl::IntervalClosedOpen, gen, 1, 1), 1);
+}
+
+TEST(MockUniformDeathTest, OutOfBoundsValuesAreRejected) {
+  absl::MockingBitGen gen;
+
+  EXPECT_DEATH_IF_SUPPORTED(
+      {
+        EXPECT_CALL(absl::MockUniform<int>(), Call(gen, 1, 100))
+            .WillOnce(Return(0));
+        absl::Uniform<int>(gen, 1, 100);
+      },
+      " 0 is not in \\[1, 100\\)");
+  EXPECT_DEATH_IF_SUPPORTED(
+      {
+        EXPECT_CALL(absl::MockUniform<int>(), Call(gen, 1, 100))
+            .WillOnce(Return(101));
+        absl::Uniform<int>(gen, 1, 100);
+      },
+      " 101 is not in \\[1, 100\\)");
+  EXPECT_DEATH_IF_SUPPORTED(
+      {
+        EXPECT_CALL(absl::MockUniform<int>(), Call(gen, 1, 100))
+            .WillOnce(Return(100));
+        absl::Uniform<int>(gen, 1, 100);
+      },
+      " 100 is not in \\[1, 100\\)");
+
+  EXPECT_DEATH_IF_SUPPORTED(
+      {
+        EXPECT_CALL(absl::MockUniform<int>(),
+                    Call(absl::IntervalOpen, gen, 1, 100))
+            .WillOnce(Return(1));
+        absl::Uniform<int>(absl::IntervalOpen, gen, 1, 100);
+      },
+      " 1 is not in \\(1, 100\\)");
+  EXPECT_DEATH_IF_SUPPORTED(
+      {
+        EXPECT_CALL(absl::MockUniform<int>(),
+                    Call(absl::IntervalOpen, gen, 1, 100))
+            .WillOnce(Return(101));
+        absl::Uniform<int>(absl::IntervalOpen, gen, 1, 100);
+      },
+      " 101 is not in \\(1, 100\\)");
+  EXPECT_DEATH_IF_SUPPORTED(
+      {
+        EXPECT_CALL(absl::MockUniform<int>(),
+                    Call(absl::IntervalOpen, gen, 1, 100))
+            .WillOnce(Return(100));
+        absl::Uniform<int>(absl::IntervalOpen, gen, 1, 100);
+      },
+      " 100 is not in \\(1, 100\\)");
+
+  EXPECT_DEATH_IF_SUPPORTED(
+      {
+        EXPECT_CALL(absl::MockUniform<int>(),
+                    Call(absl::IntervalOpenClosed, gen, 1, 100))
+            .WillOnce(Return(1));
+        absl::Uniform<int>(absl::IntervalOpenClosed, gen, 1, 100);
+      },
+      " 1 is not in \\(1, 100\\]");
+  EXPECT_DEATH_IF_SUPPORTED(
+      {
+        EXPECT_CALL(absl::MockUniform<int>(),
+                    Call(absl::IntervalOpenClosed, gen, 1, 100))
+            .WillOnce(Return(101));
+        absl::Uniform<int>(absl::IntervalOpenClosed, gen, 1, 100);
+      },
+      " 101 is not in \\(1, 100\\]");
+
+  EXPECT_DEATH_IF_SUPPORTED(
+      {
+        EXPECT_CALL(absl::MockUniform<int>(),
+                    Call(absl::IntervalOpenClosed, gen, 1, 100))
+            .WillOnce(Return(0));
+        absl::Uniform<int>(absl::IntervalOpenClosed, gen, 1, 100);
+      },
+      " 0 is not in \\(1, 100\\]");
+  EXPECT_DEATH_IF_SUPPORTED(
+      {
+        EXPECT_CALL(absl::MockUniform<int>(),
+                    Call(absl::IntervalOpenClosed, gen, 1, 100))
+            .WillOnce(Return(101));
+        absl::Uniform<int>(absl::IntervalOpenClosed, gen, 1, 100);
+      },
+      " 101 is not in \\(1, 100\\]");
+
+  EXPECT_DEATH_IF_SUPPORTED(
+      {
+        EXPECT_CALL(absl::MockUniform<int>(),
+                    Call(absl::IntervalClosed, gen, 1, 100))
+            .WillOnce(Return(0));
+        absl::Uniform<int>(absl::IntervalClosed, gen, 1, 100);
+      },
+      " 0 is not in \\[1, 100\\]");
+  EXPECT_DEATH_IF_SUPPORTED(
+      {
+        EXPECT_CALL(absl::MockUniform<int>(),
+                    Call(absl::IntervalClosed, gen, 1, 100))
+            .WillOnce(Return(101));
+        absl::Uniform<int>(absl::IntervalClosed, gen, 1, 100);
+      },
+      " 101 is not in \\[1, 100\\]");
+}
+
+TEST(MockUniformDeathTest, OutOfBoundsDoublesAreRejected) {
+  absl::MockingBitGen gen;
+
+  EXPECT_DEATH_IF_SUPPORTED(
+      {
+        EXPECT_CALL(absl::MockUniform<double>(), Call(gen, 1.0, 10.0))
+            .WillOnce(Return(10.0));
+        EXPECT_EQ(absl::Uniform<double>(gen, 1.0, 10.0), 10.0);
+      },
+      " 10 is not in \\[1, 10\\)");
+
+  EXPECT_DEATH_IF_SUPPORTED(
+      {
+        EXPECT_CALL(absl::MockUniform<double>(),
+                    Call(absl::IntervalOpen, gen, 1.0, 10.0))
+            .WillOnce(Return(10.0));
+        EXPECT_EQ(absl::Uniform<double>(absl::IntervalOpen, gen, 1.0, 10.0),
+                  10.0);
+      },
+      " 10 is not in \\(1, 10\\)");
+
+  EXPECT_DEATH_IF_SUPPORTED(
+      {
+        EXPECT_CALL(absl::MockUniform<double>(),
+                    Call(absl::IntervalOpen, gen, 1.0, 10.0))
+            .WillOnce(Return(1.0));
+        EXPECT_EQ(absl::Uniform<double>(absl::IntervalOpen, gen, 1.0, 10.0),
+                  1.0);
+      },
+      " 1 is not in \\(1, 10\\)");
+}
+
+}  // namespace
diff --git a/absl/random/mocking_access.h b/absl/random/mocking_access.h
new file mode 100644
index 00000000000..f204c95f71e
--- /dev/null
+++ b/absl/random/mocking_access.h
@@ -0,0 +1,74 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_RANDOM_MOCKING_ACCESS_H_
+#define ABSL_RANDOM_MOCKING_ACCESS_H_
+
+#include <type_traits>
+#include <utility>
+
+#include "absl/base/config.h"
+#include "absl/base/fast_type_id.h"
+#include "absl/meta/type_traits.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+// RandomMockingAccess must be a friend of any class which exposes an InvokeMock
+// method. All calls to InvokeMock should be made through RandomMockingAccess.
+//
+// Any URBG type which wants to participate in mocking should declare
+// RandomMockingAccess as a friend and have a protected or private method with
+// the signature:
+//
+// bool InvokeMock(absl::FastTypeIdType key_id, void* args_tuple, void* result)
+//
+// This method returns false when mocking is not enabled, otherwise it will
+// apply the mock.  Typically this will involve forwarding to an underlying
+// URBG such as absl::MockingBitGen by calling RandomMockingAccess::InvokeMock
+// after checking that RandomMockingAccess::HasInvokeMock<URBG> is true for the
+// underlying URBG type.
+class RandomMockingAccess {
+  template <template <class...> class Trait, class AlwaysVoid, class... Args>
+  struct detector : std::false_type {};
+  template <template <class...> class Trait, class... Args>
+  struct detector<Trait, std::void_t<Trait<Args...>>, Args...>
+      : std::true_type {};
+
+  using IdType = ::absl::FastTypeIdType;
+
+  // Detector for `bool InvokeMock(key_id, args_tuple*, result*)`
+  // Lives inside RandomMockingAccess so that it has friend access to private
+  // members of URBG types.
+  template <class T>
+  using invoke_mock_t = decltype(std::declval<T*>()->InvokeMock(
+      std::declval<IdType>(), std::declval<void*>(), std::declval<void*>()));
+
+ public:
+  // Returns true if the URBG type has an InvokeMock method.
+  template <typename T>
+  using HasInvokeMock = typename detector<invoke_mock_t, void, T>::type;
+
+  // InvokeMock is private; calls to InvokeMock are proxied by MockingAccess.
+  template <typename URBG>
+  static inline bool InvokeMock(URBG* urbg, IdType key_id, void* args_tuple,
+                                void* result) {
+    return urbg->InvokeMock(key_id, args_tuple, result);
+  }
+};
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_RANDOM_MOCKING_ACCESS_H_
diff --git a/absl/random/mocking_bit_gen.h b/absl/random/mocking_bit_gen.h
new file mode 100644
index 00000000000..2e3c606f60f
--- /dev/null
+++ b/absl/random/mocking_bit_gen.h
@@ -0,0 +1,226 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// mocking_bit_gen.h
+// -----------------------------------------------------------------------------
+//
+// This file includes an `absl::MockingBitGen` class to use as a mock within the
+// GoogleTest testing framework. Such a mock is useful to provide deterministic
+// values as return values within (otherwise random) Abseil distribution
+// functions. Such determinism within a mock is useful within testing frameworks
+// to test otherwise indeterminate APIs.
+//
+// More information about the GoogleTest testing framework is available at
+// https://github.com/google/googletest
+
+#ifndef ABSL_RANDOM_MOCKING_BIT_GEN_H_
+#define ABSL_RANDOM_MOCKING_BIT_GEN_H_
+
+#include <memory>
+#include <tuple>
+#include <type_traits>
+#include <utility>
+
+#include "gmock/gmock.h"
+#include "absl/base/config.h"
+#include "absl/base/fast_type_id.h"
+#include "absl/container/flat_hash_map.h"
+#include "absl/meta/type_traits.h"
+#include "absl/random/internal/mock_helpers.h"
+#include "absl/random/mocking_access.h"
+#include "absl/random/random.h"
+#include "absl/utility/utility.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+// MockingBitGen
+//
+// `absl::MockingBitGen` is a mock Uniform Random Bit Generator (URBG) class
+// which can act in place of an `absl::BitGen` URBG within tests using the
+// GoogleTest testing framework.
+//
+// Usage:
+//
+// Use an `absl::MockingBitGen` along with a mock distribution object (within
+// mock_distributions.h) inside Googletest constructs such as ON_CALL(),
+// EXPECT_TRUE(), etc. to produce deterministic results conforming to the
+// distribution's API contract.
+//
+// Example:
+//
+//  // Mock a call to an `absl::Bernoulli` distribution using Googletest
+//   absl::MockingBitGen bitgen;
+//
+//   ON_CALL(absl::MockBernoulli(), Call(bitgen, 0.5))
+//       .WillByDefault(testing::Return(true));
+//   EXPECT_TRUE(absl::Bernoulli(bitgen, 0.5));
+//
+//  // Mock a call to an `absl::Uniform` distribution within Googletest
+//  absl::MockingBitGen bitgen;
+//
+//   ON_CALL(absl::MockUniform<int>(), Call(bitgen, testing::_, testing::_))
+//       .WillByDefault([] (int low, int high) {
+//           return low + (high - low) / 2;
+//       });
+//
+//   EXPECT_EQ(absl::Uniform<int>(gen, 0, 10), 5);
+//   EXPECT_EQ(absl::Uniform<int>(gen, 30, 40), 35);
+//
+// At this time, only mock distributions supplied within the Abseil random
+// library are officially supported.
+//
+// EXPECT_CALL and ON_CALL need to be made within the same DLL component as
+// the call to absl::Uniform and related methods, otherwise mocking will fail
+// since the  underlying implementation creates a type-specific pointer which
+// will be distinct across different DLL boundaries.
+//
+class MockingBitGen {
+ public:
+  MockingBitGen() = default;
+  ~MockingBitGen() = default;
+
+  // URBG interface
+  using result_type = absl::BitGen::result_type;
+
+  static constexpr result_type(min)() { return (absl::BitGen::min)(); }
+  static constexpr result_type(max)() { return (absl::BitGen::max)(); }
+  result_type operator()() { return gen_(); }
+
+ private:
+  // GetMockFnType returns the testing::MockFunction for a result and tuple.
+  // This method only exists for type deduction and is otherwise unimplemented.
+  template <typename ResultT, typename... Args>
+  static auto GetMockFnType(ResultT, std::tuple<Args...>)
+      -> ::testing::MockFunction<ResultT(Args...)>;
+
+  // MockFnCaller is a helper method for use with std::apply to
+  // apply an ArgTupleT to a compatible MockFunction.
+  // NOTE: MockFnCaller is essentially equivalent to the lambda:
+  // [fn](auto... args) { return fn->Call(std::move(args)...)}
+  // however that fails to build on some supported platforms.
+  template <typename MockFnType, typename ValidatorT, typename ResultT,
+            typename Tuple>
+  struct MockFnCaller;
+
+  // specialization for std::tuple.
+  template <typename MockFnType, typename ValidatorT, typename ResultT,
+            typename... Args>
+  struct MockFnCaller<MockFnType, ValidatorT, ResultT, std::tuple<Args...>> {
+    MockFnType* fn;
+    inline ResultT operator()(Args... args) {
+      ResultT result = fn->Call(args...);
+      ValidatorT::Validate(result, args...);
+      return result;
+    }
+  };
+
+  // FunctionHolder owns a particular ::testing::MockFunction associated with
+  // a mocked type signature, and implement the type-erased Apply call, which
+  // applies type-erased arguments to the mock.
+  class FunctionHolder {
+   public:
+    virtual ~FunctionHolder() = default;
+
+    // Call is a dispatch function which converts the
+    // generic type-erased parameters into a specific mock invocation call.
+    virtual void Apply(/*ArgTupleT*/ void* args_tuple,
+                       /*ResultT*/ void* result) = 0;
+  };
+
+  template <typename MockFnType, typename ValidatorT, typename ResultT,
+            typename ArgTupleT>
+  class FunctionHolderImpl final : public FunctionHolder {
+   public:
+    void Apply(void* args_tuple, void* result) final {
+      // Requires tuple_args to point to a ArgTupleT, which is a
+      // std::tuple<Args...> used to invoke the mock function. Requires result
+      // to point to a ResultT, which is the result of the call.
+      *static_cast<ResultT*>(result) = std::apply(
+          MockFnCaller<MockFnType, ValidatorT, ResultT, ArgTupleT>{&mock_fn_},
+          *static_cast<ArgTupleT*>(args_tuple));
+    }
+
+    MockFnType mock_fn_;
+  };
+
+  // MockingBitGen::RegisterMock
+  //
+  // RegisterMock<ResultT, ArgTupleT>(FastTypeIdType) is the main extension
+  // point for extending the MockingBitGen framework. It provides a mechanism to
+  // install a mock expectation for a function like ResultT(Args...) keyed by
+  // type_idex onto the MockingBitGen context. The key is that the type_index
+  // used to register must match the type index used to call the mock.
+  //
+  // The returned MockFunction<...> type can be used to setup additional
+  // distribution parameters of the expectation.
+  template <typename ResultT, typename ArgTupleT, typename SelfT,
+            typename ValidatorT>
+  auto RegisterMock(SelfT&, FastTypeIdType type, ValidatorT)
+      -> decltype(GetMockFnType(std::declval<ResultT>(),
+                                std::declval<ArgTupleT>()))& {
+    using MockFnType = decltype(GetMockFnType(std::declval<ResultT>(),
+                                              std::declval<ArgTupleT>()));
+
+    using WrappedFnType = std::conditional_t<
+        std::is_same_v<SelfT, ::testing::NiceMock<MockingBitGen>>,
+        ::testing::NiceMock<MockFnType>,
+        std::conditional_t<
+            std::is_same_v<SelfT, ::testing::NaggyMock<MockingBitGen>>,
+            ::testing::NaggyMock<MockFnType>,
+            std::conditional_t<
+                std::is_same_v<SelfT, ::testing::StrictMock<MockingBitGen>>,
+                ::testing::StrictMock<MockFnType>, MockFnType>>>;
+
+    using ImplT =
+        FunctionHolderImpl<WrappedFnType, ValidatorT, ResultT, ArgTupleT>;
+    auto& mock = mocks_[type];
+    if (!mock) {
+      mock = std::make_unique<ImplT>();
+    }
+    return static_cast<ImplT*>(mock.get())->mock_fn_;
+  }
+
+  // MockingBitGen::InvokeMock
+  //
+  // bool InvokeMock(key_id, args_tuple*, result*) is the entrypoint
+  // for invoking mocks registered on MockingBitGen.
+  //
+  // When no mocks are registered on the provided FastTypeIdType, returns false.
+  // Otherwise attempts to invoke the mock function ResultT(Args...) that
+  // was previously registered via the type_index.
+  // Requires tuple_args to point to a ArgTupleT, which is a std::tuple<Args...>
+  // used to invoke the mock function.
+  // Requires result to point to a ResultT, which is the result of the call.
+  inline bool InvokeMock(FastTypeIdType key_id, void* args_tuple,
+                         void* result) {
+    // Trigger a mock, if there exists one that matches `param`.
+    auto it = mocks_.find(key_id);
+    if (it == mocks_.end()) return false;
+    it->second->Apply(args_tuple, result);
+    return true;
+  }
+
+  absl::flat_hash_map<FastTypeIdType, std::unique_ptr<FunctionHolder>> mocks_;
+  absl::BitGen gen_;
+
+  friend class ::absl::RandomMockingAccess;           // for InvokeMock
+  friend class ::absl::random_internal::MockHelpers;  // for RegisterMock
+};
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_RANDOM_MOCKING_BIT_GEN_H_
diff --git a/absl/random/mocking_bit_gen_test.cc b/absl/random/mocking_bit_gen_test.cc
new file mode 100644
index 00000000000..26e673ac63e
--- /dev/null
+++ b/absl/random/mocking_bit_gen_test.cc
@@ -0,0 +1,403 @@
+//
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+#include "absl/random/mocking_bit_gen.h"
+
+#include <cmath>
+#include <cstddef>
+#include <cstdint>
+#include <iterator>
+#include <numeric>
+#include <vector>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest-spi.h"
+#include "gtest/gtest.h"
+#include "absl/random/bit_gen_ref.h"
+#include "absl/random/mock_distributions.h"
+#include "absl/random/random.h"
+
+namespace {
+
+using ::testing::_;
+using ::testing::Ne;
+using ::testing::Return;
+
+TEST(BasicMocking, AllDistributionsAreOverridable) {
+  absl::MockingBitGen gen;
+
+  EXPECT_NE(absl::Uniform<int>(gen, 1, 1000000), 20);
+  EXPECT_CALL(absl::MockUniform<int>(), Call(gen, 1, 1000000))
+      .WillOnce(Return(20));
+  EXPECT_EQ(absl::Uniform<int>(gen, 1, 1000000), 20);
+
+  EXPECT_NE(absl::Uniform<double>(gen, 0.0, 100.0), 5.0);
+  EXPECT_CALL(absl::MockUniform<double>(), Call(gen, 0.0, 100.0))
+      .WillOnce(Return(5.0));
+  EXPECT_EQ(absl::Uniform<double>(gen, 0.0, 100.0), 5.0);
+
+  EXPECT_NE(absl::Exponential<double>(gen, 1.0), 42);
+  EXPECT_CALL(absl::MockExponential<double>(), Call(gen, 1.0))
+      .WillOnce(Return(42));
+  EXPECT_EQ(absl::Exponential<double>(gen, 1.0), 42);
+
+  EXPECT_NE(absl::Poisson<int>(gen, 1.0), 500);
+  EXPECT_CALL(absl::MockPoisson<int>(), Call(gen, 1.0)).WillOnce(Return(500));
+  EXPECT_EQ(absl::Poisson<int>(gen, 1.0), 500);
+
+  EXPECT_NE(absl::Bernoulli(gen, 0.000001), true);
+  EXPECT_CALL(absl::MockBernoulli(), Call(gen, 0.000001))
+      .WillOnce(Return(true));
+  EXPECT_EQ(absl::Bernoulli(gen, 0.000001), true);
+
+  EXPECT_NE(absl::Zipf<int>(gen, 1000000, 2.0, 1.0), 1221);
+  EXPECT_CALL(absl::MockZipf<int>(), Call(gen, 1000000, 2.0, 1.0))
+      .WillOnce(Return(1221));
+  EXPECT_EQ(absl::Zipf<int>(gen, 1000000, 2.0, 1.0), 1221);
+
+  EXPECT_NE(absl::Gaussian<double>(gen, 0.0, 1.0), 0.001);
+  EXPECT_CALL(absl::MockGaussian<double>(), Call(gen, 0.0, 1.0))
+      .WillOnce(Return(0.001));
+  EXPECT_EQ(absl::Gaussian<double>(gen, 0.0, 1.0), 0.001);
+
+  EXPECT_NE(absl::LogUniform<int>(gen, 0, 1000000, 2), 500000);
+  EXPECT_CALL(absl::MockLogUniform<int>(), Call(gen, 0, 1000000, 2))
+      .WillOnce(Return(500000));
+  EXPECT_EQ(absl::LogUniform<int>(gen, 0, 1000000, 2), 500000);
+}
+
+TEST(BasicMocking, OnDistribution) {
+  absl::MockingBitGen gen;
+
+  EXPECT_NE(absl::Uniform<int>(gen, 1, 1000000), 20);
+  ON_CALL(absl::MockUniform<int>(), Call(gen, 1, 1000000))
+      .WillByDefault(Return(20));
+  EXPECT_EQ(absl::Uniform<int>(gen, 1, 1000000), 20);
+
+  EXPECT_NE(absl::Uniform<double>(gen, 0.0, 100.0), 5.0);
+  ON_CALL(absl::MockUniform<double>(), Call(gen, 0.0, 100.0))
+      .WillByDefault(Return(5.0));
+  EXPECT_EQ(absl::Uniform<double>(gen, 0.0, 100.0), 5.0);
+
+  EXPECT_NE(absl::Exponential<double>(gen, 1.0), 42);
+  ON_CALL(absl::MockExponential<double>(), Call(gen, 1.0))
+      .WillByDefault(Return(42));
+  EXPECT_EQ(absl::Exponential<double>(gen, 1.0), 42);
+
+  EXPECT_NE(absl::Poisson<int>(gen, 1.0), 500);
+  ON_CALL(absl::MockPoisson<int>(), Call(gen, 1.0)).WillByDefault(Return(500));
+  EXPECT_EQ(absl::Poisson<int>(gen, 1.0), 500);
+
+  EXPECT_NE(absl::Bernoulli(gen, 0.000001), true);
+  ON_CALL(absl::MockBernoulli(), Call(gen, 0.000001))
+      .WillByDefault(Return(true));
+  EXPECT_EQ(absl::Bernoulli(gen, 0.000001), true);
+
+  EXPECT_NE(absl::Zipf<int>(gen, 1000000, 2.0, 1.0), 1221);
+  ON_CALL(absl::MockZipf<int>(), Call(gen, 1000000, 2.0, 1.0))
+      .WillByDefault(Return(1221));
+  EXPECT_EQ(absl::Zipf<int>(gen, 1000000, 2.0, 1.0), 1221);
+
+  EXPECT_NE(absl::Gaussian<double>(gen, 0.0, 1.0), 0.001);
+  ON_CALL(absl::MockGaussian<double>(), Call(gen, 0.0, 1.0))
+      .WillByDefault(Return(0.001));
+  EXPECT_EQ(absl::Gaussian<double>(gen, 0.0, 1.0), 0.001);
+
+  EXPECT_NE(absl::LogUniform<int>(gen, 0, 1000000, 2), 2040);
+  ON_CALL(absl::MockLogUniform<int>(), Call(gen, 0, 1000000, 2))
+      .WillByDefault(Return(2040));
+  EXPECT_EQ(absl::LogUniform<int>(gen, 0, 1000000, 2), 2040);
+}
+
+TEST(BasicMocking, GMockMatchers) {
+  absl::MockingBitGen gen;
+
+  EXPECT_NE(absl::Zipf<int>(gen, 1000000, 2.0, 1.0), 1221);
+  ON_CALL(absl::MockZipf<int>(), Call(gen, 1000000, 2.0, 1.0))
+      .WillByDefault(Return(1221));
+  EXPECT_EQ(absl::Zipf<int>(gen, 1000000, 2.0, 1.0), 1221);
+}
+
+TEST(BasicMocking, OverridesWithMultipleGMockExpectations) {
+  absl::MockingBitGen gen;
+
+  EXPECT_CALL(absl::MockUniform<int>(), Call(gen, 1, 10000))
+      .WillOnce(Return(20))
+      .WillOnce(Return(40))
+      .WillOnce(Return(60));
+  EXPECT_EQ(absl::Uniform(gen, 1, 10000), 20);
+  EXPECT_EQ(absl::Uniform(gen, 1, 10000), 40);
+  EXPECT_EQ(absl::Uniform(gen, 1, 10000), 60);
+}
+
+TEST(BasicMocking, DefaultArgument) {
+  absl::MockingBitGen gen;
+
+  ON_CALL(absl::MockExponential<double>(), Call(gen, 1.0))
+      .WillByDefault(Return(200));
+
+  EXPECT_EQ(absl::Exponential<double>(gen), 200);
+  EXPECT_EQ(absl::Exponential<double>(gen, 1.0), 200);
+}
+
+TEST(BasicMocking, MultipleGenerators) {
+  auto get_value = [](absl::BitGenRef gen_ref) {
+    return absl::Uniform(gen_ref, 1, 1000000);
+  };
+  absl::MockingBitGen unmocked_generator;
+  absl::MockingBitGen mocked_with_3;
+  absl::MockingBitGen mocked_with_11;
+
+  EXPECT_CALL(absl::MockUniform<int>(), Call(mocked_with_3, 1, 1000000))
+      .WillOnce(Return(3))
+      .WillRepeatedly(Return(17));
+  EXPECT_CALL(absl::MockUniform<int>(), Call(mocked_with_11, 1, 1000000))
+      .WillOnce(Return(11))
+      .WillRepeatedly(Return(17));
+
+  // Ensure that unmocked generator generates neither value.
+  int unmocked_value = get_value(unmocked_generator);
+  EXPECT_NE(unmocked_value, 3);
+  EXPECT_NE(unmocked_value, 11);
+  // Mocked generators should generate their mocked values.
+  EXPECT_EQ(get_value(mocked_with_3), 3);
+  EXPECT_EQ(get_value(mocked_with_11), 11);
+  // Ensure that the mocks have expired.
+  EXPECT_NE(get_value(mocked_with_3), 3);
+  EXPECT_NE(get_value(mocked_with_11), 11);
+}
+
+TEST(BasicMocking, MocksNotTriggeredForIncorrectTypes) {
+  absl::MockingBitGen gen;
+  EXPECT_CALL(absl::MockUniform<uint32_t>(), Call(gen))
+      .WillRepeatedly(Return(42));
+
+  bool uint16_always42 = true;
+  for (int i = 0; i < 10000; i++) {
+    EXPECT_EQ(absl::Uniform<uint32_t>(gen), 42);  // Mock triggered.
+    // uint16_t not mocked.
+    uint16_always42 = uint16_always42 && absl::Uniform<uint16_t>(gen) == 42;
+  }
+  EXPECT_FALSE(uint16_always42);
+}
+
+TEST(BasicMocking, FailsOnUnsatisfiedMocks) {
+  EXPECT_NONFATAL_FAILURE(
+      []() {
+        absl::MockingBitGen gen;
+        EXPECT_CALL(absl::MockExponential<double>(), Call(gen, 1.0))
+            .WillOnce(Return(3.0));
+        // Does not call absl::Exponential().
+      }(),
+      "unsatisfied and active");
+}
+
+TEST(OnUniform, RespectsUniformIntervalSemantics) {
+  absl::MockingBitGen gen;
+
+  EXPECT_CALL(absl::MockUniform<int>(),
+              Call(absl::IntervalClosed, gen, 1, 1000000))
+      .WillOnce(Return(301));
+  EXPECT_NE(absl::Uniform(gen, 1, 1000000), 301);  // Not mocked
+  EXPECT_EQ(absl::Uniform(absl::IntervalClosed, gen, 1, 1000000), 301);
+}
+
+TEST(OnUniform, RespectsNoArgUnsignedShorthand) {
+  absl::MockingBitGen gen;
+  EXPECT_CALL(absl::MockUniform<uint32_t>(), Call(gen)).WillOnce(Return(42));
+  EXPECT_EQ(absl::Uniform<uint32_t>(gen), 42);
+}
+
+TEST(RepeatedlyModifier, ForceSnakeEyesForManyDice) {
+  auto roll_some_dice = [](absl::BitGenRef gen_ref) {
+    std::vector<int> results(16);
+    for (auto& r : results) {
+      r = absl::Uniform(absl::IntervalClosed, gen_ref, 1, 6);
+    }
+    return results;
+  };
+  std::vector<int> results;
+  absl::MockingBitGen gen;
+
+  // Without any mocked calls, not all dice roll a "6".
+  results = roll_some_dice(gen);
+  EXPECT_LT(std::accumulate(std::begin(results), std::end(results), 0),
+            results.size() * 6);
+
+  // Verify that we can force all "6"-rolls, with mocking.
+  ON_CALL(absl::MockUniform<int>(), Call(absl::IntervalClosed, gen, 1, 6))
+      .WillByDefault(Return(6));
+  results = roll_some_dice(gen);
+  EXPECT_EQ(std::accumulate(std::begin(results), std::end(results), 0),
+            results.size() * 6);
+}
+
+TEST(WillOnce, DistinctCounters) {
+  absl::MockingBitGen gen;
+  EXPECT_CALL(absl::MockUniform<int>(), Call(gen, 1, 1000000))
+      .Times(3)
+      .WillRepeatedly(Return(1));
+  EXPECT_CALL(absl::MockUniform<int>(), Call(gen, 1000001, 2000000))
+      .Times(3)
+      .WillRepeatedly(Return(1000001));
+  EXPECT_EQ(absl::Uniform(gen, 1000001, 2000000), 1000001);
+  EXPECT_EQ(absl::Uniform(gen, 1, 1000000), 1);
+  EXPECT_EQ(absl::Uniform(gen, 1000001, 2000000), 1000001);
+  EXPECT_EQ(absl::Uniform(gen, 1, 1000000), 1);
+  EXPECT_EQ(absl::Uniform(gen, 1000001, 2000000), 1000001);
+  EXPECT_EQ(absl::Uniform(gen, 1, 1000000), 1);
+}
+
+TEST(TimesModifier, ModifierSaturatesAndExpires) {
+  EXPECT_NONFATAL_FAILURE(
+      []() {
+        absl::MockingBitGen gen;
+        EXPECT_CALL(absl::MockUniform<int>(), Call(gen, 0, 1000000))
+            .Times(3)
+            .WillRepeatedly(Return(15))
+            .RetiresOnSaturation();
+
+        EXPECT_EQ(absl::Uniform(gen, 0, 1000000), 15);
+        EXPECT_EQ(absl::Uniform(gen, 0, 1000000), 15);
+        EXPECT_EQ(absl::Uniform(gen, 0, 1000000), 15);
+        // Times(3) has expired - Should get a different value now.
+
+        EXPECT_NE(absl::Uniform(gen, 0, 1000000), 15);
+      }(),
+      "");
+}
+
+TEST(TimesModifier, Times0) {
+  absl::MockingBitGen gen;
+  EXPECT_CALL(absl::MockBernoulli(), Call(gen, 0.0)).Times(0);
+  EXPECT_CALL(absl::MockPoisson<int>(), Call(gen, 1.0)).Times(0);
+}
+
+TEST(AnythingMatcher, MatchesAnyArgument) {
+  using testing::_;
+
+  {
+    absl::MockingBitGen gen;
+    ON_CALL(absl::MockUniform<int>(), Call(absl::IntervalClosed, gen, _, 1000))
+        .WillByDefault(Return(11));
+    ON_CALL(absl::MockUniform<int>(),
+            Call(absl::IntervalClosed, gen, _, Ne(1000)))
+        .WillByDefault(Return(99));
+
+    EXPECT_EQ(absl::Uniform(absl::IntervalClosed, gen, 10, 1000000), 99);
+    EXPECT_EQ(absl::Uniform(absl::IntervalClosed, gen, 10, 1000), 11);
+  }
+
+  {
+    absl::MockingBitGen gen;
+    ON_CALL(absl::MockUniform<int>(), Call(gen, 1, _))
+        .WillByDefault(Return(25));
+    ON_CALL(absl::MockUniform<int>(), Call(gen, Ne(1), _))
+        .WillByDefault(Return(99));
+    EXPECT_EQ(absl::Uniform(gen, 3, 1000000), 99);
+    EXPECT_EQ(absl::Uniform(gen, 1, 1000000), 25);
+  }
+
+  {
+    absl::MockingBitGen gen;
+    ON_CALL(absl::MockUniform<int>(), Call(gen, _, _))
+        .WillByDefault(Return(145));
+    EXPECT_EQ(absl::Uniform(gen, 1, 1000), 145);
+    EXPECT_EQ(absl::Uniform(gen, 10, 1000), 145);
+    EXPECT_EQ(absl::Uniform(gen, 100, 1000), 145);
+  }
+}
+
+TEST(AnythingMatcher, WithWillByDefault) {
+  using testing::_;
+  absl::MockingBitGen gen;
+  std::vector<int> values = {11, 22, 33, 44, 55, 66, 77, 88, 99, 1010};
+
+  ON_CALL(absl::MockUniform<size_t>(), Call(gen, 0, _))
+      .WillByDefault(Return(0));
+  for (int i = 0; i < 100; i++) {
+    auto& elem = values[absl::Uniform(gen, 0u, values.size())];
+    EXPECT_EQ(elem, 11);
+  }
+}
+
+TEST(BasicMocking, WillByDefaultWithArgs) {
+  using testing::_;
+
+  absl::MockingBitGen gen;
+  ON_CALL(absl::MockPoisson<int>(), Call(gen, _))
+      .WillByDefault([](double lambda) {
+        return static_cast<int>(std::rint(lambda * 10));
+      });
+  EXPECT_EQ(absl::Poisson<int>(gen, 1.7), 17);
+  EXPECT_EQ(absl::Poisson<int>(gen, 0.03), 0);
+}
+
+TEST(MockingBitGen, InSequenceSucceedsInOrder) {
+  absl::MockingBitGen gen;
+
+  testing::InSequence seq;
+
+  EXPECT_CALL(absl::MockPoisson<int>(), Call(gen, 1.0)).WillOnce(Return(3));
+  EXPECT_CALL(absl::MockPoisson<int>(), Call(gen, 2.0)).WillOnce(Return(4));
+
+  EXPECT_EQ(absl::Poisson<int>(gen, 1.0), 3);
+  EXPECT_EQ(absl::Poisson<int>(gen, 2.0), 4);
+}
+
+TEST(MockingBitGen, NiceMock) {
+  ::testing::NiceMock<absl::MockingBitGen> gen;
+  ON_CALL(absl::MockUniform<int>(), Call(gen, _, _)).WillByDefault(Return(145));
+
+  ON_CALL(absl::MockPoisson<int>(), Call(gen, _)).WillByDefault(Return(3));
+
+  EXPECT_EQ(absl::Uniform(gen, 1, 1000), 145);
+  EXPECT_EQ(absl::Uniform(gen, 10, 1000), 145);
+  EXPECT_EQ(absl::Uniform(gen, 100, 1000), 145);
+}
+
+TEST(MockingBitGen, NaggyMock) {
+  // This is difficult to test, as only the output matters, so just verify
+  // that ON_CALL can be installed. Anything else requires log inspection.
+  ::testing::NaggyMock<absl::MockingBitGen> gen;
+
+  ON_CALL(absl::MockUniform<int>(), Call(gen, _, _)).WillByDefault(Return(145));
+  ON_CALL(absl::MockPoisson<int>(), Call(gen, _)).WillByDefault(Return(3));
+
+  EXPECT_EQ(absl::Uniform(gen, 1, 1000), 145);
+}
+
+TEST(MockingBitGen, StrictMock_NotEnough) {
+  EXPECT_NONFATAL_FAILURE(
+      []() {
+        ::testing::StrictMock<absl::MockingBitGen> gen;
+        EXPECT_CALL(absl::MockUniform<int>(), Call(gen, _, _))
+            .WillOnce(Return(145));
+      }(),
+      "unsatisfied and active");
+}
+
+TEST(MockingBitGen, StrictMock_TooMany) {
+  ::testing::StrictMock<absl::MockingBitGen> gen;
+
+  EXPECT_CALL(absl::MockUniform<int>(), Call(gen, _, _)).WillOnce(Return(145));
+  EXPECT_EQ(absl::Uniform(gen, 1, 1000), 145);
+
+  EXPECT_NONFATAL_FAILURE(
+      [&]() { EXPECT_EQ(absl::Uniform(gen, 0, 1000), 0); }(),
+      "over-saturated and active");
+}
+
+}  // namespace
diff --git a/absl/random/poisson_distribution.h b/absl/random/poisson_distribution.h
index 7750b1c9ba5..ae2e095be37 100644
--- a/absl/random/poisson_distribution.h
+++ b/absl/random/poisson_distribution.h
@@ -17,17 +17,20 @@
 
 #include <cassert>
 #include <cmath>
+#include <cstdint>
 #include <istream>
 #include <limits>
 #include <ostream>
-#include <type_traits>
 
-#include "absl/random/internal/distribution_impl.h"
+#include "absl/base/config.h"
 #include "absl/random/internal/fast_uniform_bits.h"
 #include "absl/random/internal/fastmath.h"
+#include "absl/random/internal/generate_real.h"
 #include "absl/random/internal/iostream_state_saver.h"
+#include "absl/random/internal/traits.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 // absl::poisson_distribution:
 // Generates discrete variates conforming to a Poisson distribution.
@@ -46,8 +49,8 @@ namespace absl {
 // the distribution results are limited to the max() value.
 //
 // The goals of this implementation are to provide good performance while still
-// beig thread-safe: This limits the implementation to not using lgamma provided
-// by <math.h>.
+// being thread-safe: This limits the implementation to not using lgamma
+// provided by <math.h>.
 //
 template <typename IntType = int>
 class poisson_distribution {
@@ -79,7 +82,7 @@ class poisson_distribution {
     double log_k_;
     int split_;
 
-    static_assert(std::is_integral<IntType>::value,
+    static_assert(random_internal::IsIntegral<IntType>::value,
                   "Class-template absl::poisson_distribution<> must be "
                   "parameterized using an integral type.");
   };
@@ -132,7 +135,8 @@ template <typename IntType>
 poisson_distribution<IntType>::param_type::param_type(double mean)
     : mean_(mean), split_(0) {
   assert(mean >= 0);
-  assert(mean <= (std::numeric_limits<result_type>::max)());
+  assert(mean <=
+         static_cast<double>((std::numeric_limits<result_type>::max)()));
   // As a defensive measure, avoid large values of the mean.  The rejection
   // algorithm used does not support very large values well.  It my be worth
   // changing algorithms to better deal with these cases.
@@ -164,9 +168,9 @@ typename poisson_distribution<IntType>::result_type
 poisson_distribution<IntType>::operator()(
     URBG& g,  // NOLINT(runtime/references)
     const param_type& p) {
-  using random_internal::PositiveValueT;
-  using random_internal::RandU64ToDouble;
-  using random_internal::SignedValueT;
+  using random_internal::GeneratePositiveTag;
+  using random_internal::GenerateRealFromBits;
+  using random_internal::GenerateSignedTag;
 
   if (p.split_ != 0) {
     // Use Knuth's algorithm with range splitting to avoid floating-point
@@ -186,7 +190,8 @@ poisson_distribution<IntType>::operator()(
     for (int split = p.split_; split > 0; --split) {
       double r = 1.0;
       do {
-        r *= RandU64ToDouble<PositiveValueT, true>(fast_u64_(g));
+        r *= GenerateRealFromBits<double, GeneratePositiveTag, true>(
+            fast_u64_(g));  // U(-1, 0)
         ++n;
       } while (r > p.emu_);
       --n;
@@ -205,10 +210,11 @@ poisson_distribution<IntType>::operator()(
   // and k = max(f).
   const double a = p.mean_ + 0.5;
   for (;;) {
-    const double u =
-        RandU64ToDouble<PositiveValueT, false>(fast_u64_(g));  // (0, 1)
-    const double v =
-        RandU64ToDouble<SignedValueT, false>(fast_u64_(g));  // (-1, 1)
+    const double u = GenerateRealFromBits<double, GeneratePositiveTag, false>(
+        fast_u64_(g));  // U(0, 1)
+    const double v = GenerateRealFromBits<double, GenerateSignedTag, false>(
+        fast_u64_(g));  // U(-1, 1)
+
     const double x = std::floor(p.s_ * v / u + a);
     if (x < 0) continue;  // f(negative) = 0
     const double rhs = x * p.lmu_;
@@ -219,8 +225,9 @@ poisson_distribution<IntType>::operator()(
     // clang-format on
     const double lhs = 2.0 * std::log(u) + p.log_k_ + s;
     if (lhs < rhs) {
-      return x > (max)() ? (max)()
-                         : static_cast<result_type>(x);  // f(x)/k >= u^2
+      return x > static_cast<double>((max)())
+                 ? (max)()
+                 : static_cast<result_type>(x);  // f(x)/k >= u^2
     }
   }
 }
@@ -249,6 +256,7 @@ std::basic_istream<CharT, Traits>& operator>>(
   return is;
 }
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_POISSON_DISTRIBUTION_H_
diff --git a/absl/random/poisson_distribution_test.cc b/absl/random/poisson_distribution_test.cc
index 9d215fbc70f..e8ee3643628 100644
--- a/absl/random/poisson_distribution_test.cc
+++ b/absl/random/poisson_distribution_test.cc
@@ -15,9 +15,11 @@
 #include "absl/random/poisson_distribution.h"
 
 #include <algorithm>
+#include <cmath>
 #include <cstddef>
 #include <cstdint>
 #include <iterator>
+#include <limits>
 #include <random>
 #include <sstream>
 #include <string>
@@ -25,11 +27,12 @@
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
-#include "absl/base/internal/raw_logging.h"
 #include "absl/base/macros.h"
 #include "absl/container/flat_hash_map.h"
+#include "absl/log/log.h"
 #include "absl/random/internal/chi_square.h"
 #include "absl/random/internal/distribution_test_util.h"
+#include "absl/random/internal/pcg_engine.h"
 #include "absl/random/internal/sequence_urbg.h"
 #include "absl/random/random.h"
 #include "absl/strings/str_cat.h"
@@ -72,7 +75,7 @@ class PoissonDistributionInterfaceTest : public ::testing::Test {};
 
 using IntTypes = ::testing::Types<int, int8_t, int16_t, int32_t, int64_t,
                                   uint8_t, uint16_t, uint32_t, uint64_t>;
-TYPED_TEST_CASE(PoissonDistributionInterfaceTest, IntTypes);
+TYPED_TEST_SUITE(PoissonDistributionInterfaceTest, IntTypes);
 
 TYPED_TEST(PoissonDistributionInterfaceTest, SerializeTest) {
   using param_type = typename absl::poisson_distribution<TypeParam>::param_type;
@@ -105,7 +108,6 @@ TYPED_TEST(PoissonDistributionInterfaceTest, SerializeTest) {
                      0.0),  // denorm_max
   };
 
-
   constexpr int kCount = 1000;
   absl::InsecureBitGen gen;
   for (const double m : kParams) {
@@ -133,8 +135,8 @@ TYPED_TEST(PoissonDistributionInterfaceTest, SerializeTest) {
       if (sample < sample_min) sample_min = sample;
     }
 
-    ABSL_INTERNAL_LOG(INFO, absl::StrCat("Range {", param.mean(), "}: ",
-                                         +sample_min, ", ", +sample_max));
+    LOG(INFO) << "Range {" << param.mean() << "}: " << sample_min << ", "
+              << sample_max;
 
     // Validate stream serialization.
     std::stringstream ss;
@@ -187,10 +189,9 @@ class PoissonModel {
   }
 
   void LogCDF() {
-    ABSL_INTERNAL_LOG(INFO, absl::StrCat("CDF (mean = ", mean_, ")"));
+    LOG(INFO) << "CDF (mean = " << mean_ << ")";
     for (const auto c : cdf_) {
-      ABSL_INTERNAL_LOG(INFO,
-                        absl::StrCat(c.index, ": pmf=", c.pmf, " cdf=", c.cdf));
+      LOG(INFO) << c.index << ": pmf=" << c.pmf << " cdf=" << c.cdf;
     }
   }
 
@@ -257,7 +258,10 @@ class PoissonDistributionZTest : public testing::TestWithParam<ZParam>,
   template <typename D>
   bool SingleZTest(const double p, const size_t samples);
 
-  absl::InsecureBitGen rng_;
+  // We use a fixed bit generator for distribution accuracy tests.  This allows
+  // these tests to be deterministic, while still testing the qualify of the
+  // implementation.
+  absl::random_internal::pcg64_2018_engine rng_{0x2B7E151628AED2A6};
 };
 
 template <typename D>
@@ -282,16 +286,15 @@ bool PoissonDistributionZTest::SingleZTest(const double p,
   const bool pass = absl::random_internal::Near("z", z, 0.0, max_err);
 
   if (!pass) {
-    ABSL_INTERNAL_LOG(
-        INFO, absl::StrFormat("p=%f max_err=%f\n"
-                              " mean=%f vs. %f\n"
-                              " stddev=%f vs. %f\n"
-                              " skewness=%f vs. %f\n"
-                              " kurtosis=%f vs. %f\n"
-                              " z=%f",
-                              p, max_err, m.mean, mean(), std::sqrt(m.variance),
-                              stddev(), m.skewness, skew(), m.kurtosis,
-                              kurtosis(), z));
+    // clang-format off
+    LOG(INFO)
+        << "p=" << p << " max_err=" << max_err << "\n"
+           " mean=" << m.mean << " vs. " << mean() << "\n"
+           " stddev=" << std::sqrt(m.variance) << " vs. " << stddev() << "\n"
+           " skewness=" << m.skewness << " vs. " << skew() << "\n"
+           " kurtosis=" << m.kurtosis << " vs. " << kurtosis() << "\n"
+           " z=" << z;
+    // clang-format on
   }
   return pass;
 }
@@ -357,9 +360,13 @@ class PoissonDistributionChiSquaredTest : public testing::TestWithParam<double>,
  private:
   void InitChiSquaredTest(const double buckets);
 
-  absl::InsecureBitGen rng_;
   std::vector<size_t> cutoffs_;
   std::vector<double> expected_;
+
+  // We use a fixed bit generator for distribution accuracy tests.  This allows
+  // these tests to be deterministic, while still testing the qualify of the
+  // implementation.
+  absl::random_internal::pcg64_2018_engine rng_{0x2B7E151628AED2A6};
 };
 
 void PoissonDistributionChiSquaredTest::InitChiSquaredTest(
@@ -431,17 +438,16 @@ double PoissonDistributionChiSquaredTest::ChiSquaredTestImpl() {
   if (chi_square > threshold) {
     LogCDF();
 
-    ABSL_INTERNAL_LOG(INFO, absl::StrCat("VALUES  buckets=", counts.size(),
-                                         "  samples=", kSamples));
+    LOG(INFO) << "VALUES  buckets=" << counts.size()
+              << "  samples=" << kSamples;
     for (size_t i = 0; i < counts.size(); i++) {
-      ABSL_INTERNAL_LOG(
-          INFO, absl::StrCat(cutoffs_[i], ": ", counts[i], " vs. E=", e[i]));
+      LOG(INFO) << cutoffs_[i] << ": " << counts[i] << " vs. E=" << e[i];
     }
 
-    ABSL_INTERNAL_LOG(
-        INFO,
-        absl::StrCat(kChiSquared, "(data, dof=", dof, ") = ", chi_square, " (",
-                     p, ")\n", " vs.\n", kChiSquared, " @ 0.98 = ", threshold));
+    LOG(INFO) << kChiSquared << "(data, dof=" << dof << ") = " << chi_square
+              << " (" << p << ")\n"
+              << " vs.\n"
+              << kChiSquared << " @ 0.98 = " << threshold;
   }
   return p;
 }
diff --git a/absl/random/random.h b/absl/random/random.h
index dc6852f4adf..b55b3612876 100644
--- a/absl/random/random.h
+++ b/absl/random/random.h
@@ -31,16 +31,18 @@
 #ifndef ABSL_RANDOM_RANDOM_H_
 #define ABSL_RANDOM_RANDOM_H_
 
+#include <cstdint>
 #include <random>
 
+#include "absl/base/config.h"
 #include "absl/random/distributions.h"  // IWYU pragma: export
-#include "absl/random/internal/nonsecure_base.h"  // IWYU pragma: export
-#include "absl/random/internal/pcg_engine.h"  // IWYU pragma: export
-#include "absl/random/internal/pool_urbg.h"
+#include "absl/random/internal/nonsecure_base.h"
+#include "absl/random/internal/pcg_engine.h"
 #include "absl/random/internal/randen_engine.h"
 #include "absl/random/seed_sequences.h"  // IWYU pragma: export
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 // -----------------------------------------------------------------------------
 // absl::BitGen
@@ -67,7 +69,7 @@ namespace absl {
 //
 // `absl::BitGen` may be constructed with an optional seed sequence type,
 // conforming to [rand.req.seed_seq], which will be mixed with additional
-// non-deterministic data.
+// non-deterministic data as detailed below.
 //
 // Example:
 //
@@ -78,46 +80,61 @@ namespace absl {
 //  // Generate an integer value in the closed interval [1,6]
 //  int die_roll2 = absl::uniform_int_distribution<int>(1, 6)(gen_with_seed);
 //
+// Constructing two `absl::BitGen`s with the same seed sequence in the same
+// process will produce the same sequence of variates, but need not do so across
+// multiple processes even if they're executing the same binary.
+//
 // `absl::BitGen` meets the requirements of the Uniform Random Bit Generator
 // (URBG) concept as per the C++17 standard [rand.req.urng] though differs
 // slightly with [rand.req.eng]. Like its standard library equivalents (e.g.
 // `std::mersenne_twister_engine`) `absl::BitGen` is not cryptographically
 // secure.
 //
-// Constructing two `absl::BitGen`s with the same seed sequence in the same
-// binary will produce the same sequence of variates within the same binary, but
-// need not do so across multiple binary invocations.
-//
 // This type has been optimized to perform better than Mersenne Twister
 // (https://en.wikipedia.org/wiki/Mersenne_Twister) and many other complex URBG
 // types on modern x86, ARM, and PPC architectures.
 //
 // This type is thread-compatible, but not thread-safe.
-
-// ---------------------------------------------------------------------------
-// absl::BitGen member functions
-// ---------------------------------------------------------------------------
-
-// absl::BitGen::operator()()
-//
-// Calls the BitGen, returning a generated value.
-
-// absl::BitGen::min()
-//
-// Returns the smallest possible value from this bit generator.
-
-// absl::BitGen::max()
-//
-// Returns the largest possible value from this bit generator., and
-
-// absl::BitGen::discard(num)
-//
-// Advances the internal state of this bit generator by `num` times, and
-// discards the intermediate results.
-// ---------------------------------------------------------------------------
-
-using BitGen = random_internal::NonsecureURBGBase<
-    random_internal::randen_engine<uint64_t>>;
+class BitGen : private random_internal::NonsecureURBGBase<
+                   random_internal::randen_engine<uint64_t>> {
+  using Base = random_internal::NonsecureURBGBase<
+      random_internal::randen_engine<uint64_t>>;
+
+ public:
+  using result_type = typename Base::result_type;
+
+  // BitGen()
+  // BitGen(SeedSequence seed_seq)
+  //
+  // Copy disallowed.
+  // Move allowed.
+  using Base::Base;
+  using Base::operator=;
+
+  // BitGen::min()
+  //
+  // Returns the smallest possible value from this bit generator.
+  using Base::min;
+
+  // BitGen::max()
+  //
+  // Returns the largest possible value from this bit generator.
+  using Base::max;
+
+  // BitGen::discard(num)
+  //
+  // Advances the internal state of this bit generator by `num` times, and
+  // discards the intermediate results.
+  using Base::discard;
+
+  // BitGen::operator()()
+  //
+  // Invoke the URBG, returning a generated value.
+  using Base::operator();
+
+  using Base::operator==;
+  using Base::operator!=;
+};
 
 // -----------------------------------------------------------------------------
 // absl::InsecureBitGen
@@ -146,7 +163,7 @@ using BitGen = random_internal::NonsecureURBGBase<
 //
 // `absl::InsecureBitGen` may be constructed with an optional seed sequence
 // type, conforming to [rand.req.seed_seq], which will be mixed with additional
-// non-deterministic data. (See std_seed_seq.h for more information.)
+// non-deterministic data, as detailed in the `absl::BitGen` comment.
 //
 // `absl::InsecureBitGen` meets the requirements of the Uniform Random Bit
 // Generator (URBG) concept as per the C++17 standard [rand.req.urng] though
@@ -155,33 +172,53 @@ using BitGen = random_internal::NonsecureURBGBase<
 // `absl::InsecureBitGen` is not cryptographically secure.
 //
 // Prefer `absl::BitGen` over `absl::InsecureBitGen` as the general type is
-// often fast enough for the vast majority of applications.
-
-using InsecureBitGen =
-    random_internal::NonsecureURBGBase<random_internal::pcg64_2018_engine>;
-
-// ---------------------------------------------------------------------------
-// absl::InsecureBitGen member functions
-// ---------------------------------------------------------------------------
-
-// absl::InsecureBitGen::operator()()
-//
-// Calls the InsecureBitGen, returning a generated value.
-
-// absl::InsecureBitGen::min()
+// often fast enough for the vast majority of applications. However, it is
+// reasonable to use `absl::InsecureBitGen` in tests or when using a URBG
+// in small isolated tasks such as in `std::shuffle`.
 //
-// Returns the smallest possible value from this bit generator.
-
-// absl::InsecureBitGen::max()
-//
-// Returns the largest possible value from this bit generator.
-
-// absl::InsecureBitGen::discard(num)
-//
-// Advances the internal state of this bit generator by `num` times, and
-// discards the intermediate results.
-// ---------------------------------------------------------------------------
-
+// This type is thread-compatible, but not thread-safe.
+class InsecureBitGen : private random_internal::NonsecureURBGBase<
+                           random_internal::pcg64_2018_engine> {
+  using Base =
+      random_internal::NonsecureURBGBase<random_internal::pcg64_2018_engine>;
+
+ public:
+  using result_type = typename Base::result_type;
+
+  // InsecureBitGen()
+  // InsecureBitGen(SeedSequence seed_seq)
+  //
+  // Copy disallowed.
+  // Move allowed.
+  using Base::Base;
+  using Base::operator=;
+
+  // InsecureBitGen::min()
+  //
+  // Returns the smallest possible value from this bit generator.
+  using Base::min;
+
+  // InsecureBitGen::max()
+  //
+  // Returns the largest possible value from this bit generator.
+  using Base::max;
+
+  // InsecureBitGen::discard(num)
+  //
+  // Advances the internal state of this bit generator by `num` times, and
+  // discards the intermediate results.
+  using Base::discard;
+
+  // InsecureBitGen::operator()()
+  //
+  // Invoke the URBG, returning a generated value.
+  using Base::operator();
+
+  using Base::operator==;
+  using Base::operator!=;
+};
+
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_RANDOM_H_
diff --git a/absl/random/seed_gen_exception.cc b/absl/random/seed_gen_exception.cc
index e4271baaa07..2b6f0901837 100644
--- a/absl/random/seed_gen_exception.cc
+++ b/absl/random/seed_gen_exception.cc
@@ -14,11 +14,13 @@
 
 #include "absl/random/seed_gen_exception.h"
 
-#include <iostream>
+#include <exception>
 
 #include "absl/base/config.h"
+#include "absl/base/internal/raw_logging.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 static constexpr const char kExceptionMessage[] =
     "Failed generating seed-material for URBG.";
@@ -35,10 +37,11 @@ void ThrowSeedGenException() {
 #ifdef ABSL_HAVE_EXCEPTIONS
   throw absl::SeedGenException();
 #else
-  std::cerr << kExceptionMessage << std::endl;
+  ABSL_RAW_LOG(FATAL, "%s", kExceptionMessage);
   std::terminate();
 #endif
 }
 
 }  // namespace random_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/random/seed_gen_exception.h b/absl/random/seed_gen_exception.h
index b464d52fb95..53539005643 100644
--- a/absl/random/seed_gen_exception.h
+++ b/absl/random/seed_gen_exception.h
@@ -28,7 +28,10 @@
 
 #include <exception>
 
+#include "absl/base/config.h"
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 //------------------------------------------------------------------------------
 // SeedGenException
@@ -46,6 +49,7 @@ namespace random_internal {
 [[noreturn]] void ThrowSeedGenException();
 
 }  // namespace random_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_SEED_GEN_EXCEPTION_H_
diff --git a/absl/random/seed_sequences.cc b/absl/random/seed_sequences.cc
index 9f319615289..316381ba971 100644
--- a/absl/random/seed_sequences.cc
+++ b/absl/random/seed_sequences.cc
@@ -14,14 +14,20 @@
 
 #include "absl/random/seed_sequences.h"
 
-#include "absl/random/internal/pool_urbg.h"
+#include <iterator>
+
+#include "absl/base/config.h"
+#include "absl/random/internal/entropy_pool.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 SeedSeq MakeSeedSeq() {
   SeedSeq::result_type seed_material[8];
-  random_internal::RandenPool<uint32_t>::Fill(absl::MakeSpan(seed_material));
+  random_internal::GetEntropyFromRandenPool(&seed_material[0],
+                                            sizeof(seed_material[0]) * 8);
   return SeedSeq(std::begin(seed_material), std::end(seed_material));
 }
 
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/random/seed_sequences.h b/absl/random/seed_sequences.h
index 631d1ecd4ea..a8d953b8111 100644
--- a/absl/random/seed_sequences.h
+++ b/absl/random/seed_sequences.h
@@ -26,14 +26,19 @@
 #define ABSL_RANDOM_SEED_SEQUENCES_H_
 
 #include <iterator>
+#include <ostream>
 #include <random>
 
+#include "absl/base/config.h"
+#include "absl/base/nullability.h"
 #include "absl/random/internal/salted_seed_seq.h"
 #include "absl/random/internal/seed_material.h"
 #include "absl/random/seed_gen_exception.h"
+#include "absl/strings/string_view.h"
 #include "absl/types/span.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 // -----------------------------------------------------------------------------
 // absl::SeedSeq
@@ -76,8 +81,7 @@ using SeedSeq = random_internal::SaltedSeedSeq<std::seed_seq>;
 //
 template <typename URBG>
 SeedSeq CreateSeedSeqFrom(URBG* urbg) {
-  SeedSeq::result_type
-      seed_material[random_internal::kEntropyBlocksNeeded];
+  SeedSeq::result_type seed_material[random_internal::kEntropyBlocksNeeded];
 
   if (!random_internal::ReadSeedMaterialFromURBG(
           urbg, absl::MakeSpan(seed_material))) {
@@ -103,6 +107,7 @@ SeedSeq CreateSeedSeqFrom(URBG* urbg) {
 //
 SeedSeq MakeSeedSeq();
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_SEED_SEQUENCES_H_
diff --git a/absl/random/seed_sequences_test.cc b/absl/random/seed_sequences_test.cc
index 2cc8b0e6f2b..1a218a71d9d 100644
--- a/absl/random/seed_sequences_test.cc
+++ b/absl/random/seed_sequences_test.cc
@@ -14,8 +14,12 @@
 
 #include "absl/random/seed_sequences.h"
 
+#include <cstddef>
+#include <cstdint>
+#include <iostream>
 #include <iterator>
 #include <random>
+#include <vector>
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
@@ -96,7 +100,6 @@ template <typename URBG>
 void TestReproducibleVariateSequencesForNonsecureURBG() {
   const size_t kNumVariates = 1000;
 
-  // Master RNG instance.
   URBG rng;
   // Reused for both RNG instances.
   auto reusable_seed = absl::CreateSeedSeqFrom(&rng);
diff --git a/absl/random/uniform_int_distribution.h b/absl/random/uniform_int_distribution.h
index 4970486a561..0dc7c62c3e3 100644
--- a/absl/random/uniform_int_distribution.h
+++ b/absl/random/uniform_int_distribution.h
@@ -31,15 +31,17 @@
 #include <cassert>
 #include <istream>
 #include <limits>
-#include <type_traits>
+#include <ostream>
 
+#include "absl/base/config.h"
 #include "absl/base/optimization.h"
-#include "absl/random/internal/distribution_impl.h"
 #include "absl/random/internal/fast_uniform_bits.h"
 #include "absl/random/internal/iostream_state_saver.h"
 #include "absl/random/internal/traits.h"
+#include "absl/random/internal/wide_multiply.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 // absl::uniform_int_distribution<T>
 //
@@ -96,7 +98,7 @@ class uniform_int_distribution {
     result_type lo_;
     unsigned_type range_;
 
-    static_assert(std::is_integral<result_type>::value,
+    static_assert(random_internal::IsIntegral<result_type>::value,
                   "Class-template absl::uniform_int_distribution<> must be "
                   "parameterized using an integral type.");
   };  // param_type
@@ -124,7 +126,7 @@ class uniform_int_distribution {
   template <typename URBG>
   result_type operator()(
       URBG& gen, const param_type& param) {  // NOLINT(runtime/references)
-    return param.a() + Generate(gen, param.range());
+    return static_cast<result_type>(param.a() + Generate(gen, param.range()));
   }
 
   result_type a() const { return param_.a(); }
@@ -195,7 +197,7 @@ typename random_internal::make_unsigned_bits<IntType>::type
 uniform_int_distribution<IntType>::Generate(
     URBG& g,  // NOLINT(runtime/references)
     typename random_internal::make_unsigned_bits<IntType>::type R) {
-    random_internal::FastUniformBits<unsigned_type> fast_bits;
+  random_internal::FastUniformBits<unsigned_type> fast_bits;
   unsigned_type bits = fast_bits(g);
   const unsigned_type Lim = R + 1;
   if ((R & Lim) == 0) {
@@ -268,6 +270,7 @@ uniform_int_distribution<IntType>::Generate(
   return helper::hi(product);
 }
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_UNIFORM_INT_DISTRIBUTION_H_
diff --git a/absl/random/uniform_int_distribution_test.cc b/absl/random/uniform_int_distribution_test.cc
index aacff88d386..b055833c23e 100644
--- a/absl/random/uniform_int_distribution_test.cc
+++ b/absl/random/uniform_int_distribution_test.cc
@@ -15,17 +15,21 @@
 #include "absl/random/uniform_int_distribution.h"
 
 #include <cmath>
+#include <cstddef>
 #include <cstdint>
 #include <iterator>
+#include <limits>
 #include <random>
 #include <sstream>
+#include <string>
 #include <vector>
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
-#include "absl/base/internal/raw_logging.h"
+#include "absl/log/log.h"
 #include "absl/random/internal/chi_square.h"
 #include "absl/random/internal/distribution_test_util.h"
+#include "absl/random/internal/pcg_engine.h"
 #include "absl/random/internal/sequence_urbg.h"
 #include "absl/random/random.h"
 #include "absl/strings/str_cat.h"
@@ -45,8 +49,8 @@ TYPED_TEST(UniformIntDistributionTest, ParamSerializeTest) {
   using Limits = std::numeric_limits<TypeParam>;
   using param_type =
       typename absl::uniform_int_distribution<TypeParam>::param_type;
-  const TypeParam kMin = std::is_unsigned<TypeParam>::value ? 37 : -105;
-  const TypeParam kNegOneOrZero = std::is_unsigned<TypeParam>::value ? 0 : -1;
+  const TypeParam kMin = std::is_unsigned_v<TypeParam> ? 37 : -105;
+  const TypeParam kNegOneOrZero = std::is_unsigned_v<TypeParam> ? 0 : -1;
 
   constexpr int kCount = 1000;
   absl::InsecureBitGen gen;
@@ -105,16 +109,15 @@ TYPED_TEST(UniformIntDistributionTest, ParamSerializeTest) {
         sample_min = sample;
       }
     }
-    std::string msg = absl::StrCat("Range: ", +sample_min, ", ", +sample_max);
-    ABSL_RAW_LOG(INFO, "%s", msg.c_str());
+    LOG(INFO) << "Range: " << sample_min << ", " << sample_max;
   }
 }
 
 TYPED_TEST(UniformIntDistributionTest, ViolatesPreconditionsDeathTest) {
 #if GTEST_HAS_DEATH_TEST
   // Hi < Lo
-  EXPECT_DEBUG_DEATH({ absl::uniform_int_distribution<TypeParam> dist(10, 1); },
-                     "");
+  EXPECT_DEBUG_DEATH(
+      { absl::uniform_int_distribution<TypeParam> dist(10, 1); }, "");
 #endif  // GTEST_HAS_DEATH_TEST
 #if defined(NDEBUG)
   // opt-mode, for invalid parameters, will generate a garbage value,
@@ -123,7 +126,7 @@ TYPED_TEST(UniformIntDistributionTest, ViolatesPreconditionsDeathTest) {
   absl::uniform_int_distribution<TypeParam> dist(10, 1);
   auto x = dist(gen);
 
-  // Any value will generate a non-empty std::string.
+  // Any value will generate a non-empty string.
   EXPECT_FALSE(absl::StrCat(+x).empty()) << x;
 #endif  // NDEBUG
 }
@@ -134,7 +137,11 @@ TYPED_TEST(UniformIntDistributionTest, TestMoments) {
   using param_type =
       typename absl::uniform_int_distribution<TypeParam>::param_type;
 
-  absl::InsecureBitGen rng;
+  // We use a fixed bit generator for distribution accuracy tests.  This allows
+  // these tests to be deterministic, while still testing the quality of the
+  // implementation.
+  absl::random_internal::pcg64_2018_engine rng{0x2B7E151628AED2A6};
+
   std::vector<double> values(kSize);
   for (const auto& param :
        {param_type(0, Limits::max()), param_type(13, 127)}) {
@@ -167,7 +174,7 @@ TYPED_TEST(UniformIntDistributionTest, ChiSquaredTest50) {
   using absl::random_internal::kChiSquared;
 
   constexpr size_t kTrials = 1000;
-  constexpr int kBuckets = 50;  // inclusive, so actally +1
+  constexpr int kBuckets = 50;  // inclusive, so actually +1
   constexpr double kExpected =
       static_cast<double>(kTrials) / static_cast<double>(kBuckets);
 
@@ -175,10 +182,14 @@ TYPED_TEST(UniformIntDistributionTest, ChiSquaredTest50) {
   const int kThreshold =
       absl::random_internal::ChiSquareValue(kBuckets, 0.999999);
 
-  const TypeParam min = std::is_unsigned<TypeParam>::value ? 37 : -37;
+  const TypeParam min = std::is_unsigned_v<TypeParam> ? 37 : -37;
   const TypeParam max = min + kBuckets;
 
-  absl::InsecureBitGen rng;
+  // We use a fixed bit generator for distribution accuracy tests.  This allows
+  // these tests to be deterministic, while still testing the quality of the
+  // implementation.
+  absl::random_internal::pcg64_2018_engine rng{0x2B7E151628AED2A6};
+
   absl::uniform_int_distribution<TypeParam> dist(min, max);
 
   std::vector<int32_t> counts(kBuckets + 1, 0);
@@ -200,7 +211,7 @@ TYPED_TEST(UniformIntDistributionTest, ChiSquaredTest50) {
     absl::StrAppend(&msg, kChiSquared, " p-value ", p_value, "\n");
     absl::StrAppend(&msg, "High ", kChiSquared, " value: ", chi_square, " > ",
                     kThreshold);
-    ABSL_RAW_LOG(INFO, "%s", msg.c_str());
+    LOG(INFO) << msg;
     FAIL() << msg;
   }
 }
diff --git a/absl/random/uniform_real_distribution.h b/absl/random/uniform_real_distribution.h
index 336abb3907b..79cf79336e8 100644
--- a/absl/random/uniform_real_distribution.h
+++ b/absl/random/uniform_real_distribution.h
@@ -37,13 +37,17 @@
 #include <cstdint>
 #include <istream>
 #include <limits>
+#include <ostream>
 #include <type_traits>
 
-#include "absl/random/internal/distribution_impl.h"
+#include "absl/base/config.h"
+#include "absl/meta/type_traits.h"
 #include "absl/random/internal/fast_uniform_bits.h"
+#include "absl/random/internal/generate_real.h"
 #include "absl/random/internal/iostream_state_saver.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 // absl::uniform_real_distribution<T>
 //
@@ -71,6 +75,7 @@ class uniform_real_distribution {
         : lo_(lo), hi_(hi), range_(hi - lo) {
       // [rand.dist.uni.real] preconditions 2 & 3
       assert(lo <= hi);
+
       // NOTE: For integral types, we can promote the range to an unsigned type,
       // which gives full width of the range. However for real (fp) types, this
       // is not possible, so value generation cannot use the full range of the
@@ -93,7 +98,7 @@ class uniform_real_distribution {
     friend class uniform_real_distribution;
     result_type lo_, hi_, range_;
 
-    static_assert(std::is_floating_point<RealType>::value,
+    static_assert(std::is_floating_point_v<RealType>,
                   "Class-template absl::uniform_real_distribution<> must be "
                   "parameterized using a floating-point type.");
   };
@@ -151,10 +156,15 @@ template <typename URBG>
 typename uniform_real_distribution<RealType>::result_type
 uniform_real_distribution<RealType>::operator()(
     URBG& gen, const param_type& p) {  // NOLINT(runtime/references)
-  using random_internal::PositiveValueT;
+  using random_internal::GeneratePositiveTag;
+  using random_internal::GenerateRealFromBits;
+  using real_type =
+      std::conditional_t<std::is_same_v<RealType, float>, float, double>;
+
   while (true) {
-    const result_type sample = random_internal::RandU64ToReal<
-        result_type>::template Value<PositiveValueT, true>(fast_u64_(gen));
+    const result_type sample =
+        GenerateRealFromBits<real_type, GeneratePositiveTag, true>(
+            fast_u64_(gen));
     const result_type res = p.a() + (sample * p.range_);
     if (res < p.b() || p.range_ <= 0 || !std::isfinite(p.range_)) {
       return res;
@@ -188,6 +198,7 @@ std::basic_istream<CharT, Traits>& operator>>(
   }
   return is;
 }
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_UNIFORM_REAL_DISTRIBUTION_H_
diff --git a/absl/random/uniform_real_distribution_test.cc b/absl/random/uniform_real_distribution_test.cc
index 597f0ee5efa..11e1d6581bd 100644
--- a/absl/random/uniform_real_distribution_test.cc
+++ b/absl/random/uniform_real_distribution_test.cc
@@ -14,19 +14,25 @@
 
 #include "absl/random/uniform_real_distribution.h"
 
+#include <cfloat>
 #include <cmath>
+#include <cstddef>
 #include <cstdint>
 #include <iterator>
+#include <limits>
 #include <random>
 #include <sstream>
 #include <string>
+#include <type_traits>
 #include <vector>
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
-#include "absl/base/internal/raw_logging.h"
+#include "absl/log/log.h"
+#include "absl/numeric/internal/representation.h"
 #include "absl/random/internal/chi_square.h"
 #include "absl/random/internal/distribution_test_util.h"
+#include "absl/random/internal/pcg_engine.h"
 #include "absl/random/internal/sequence_urbg.h"
 #include "absl/random/random.h"
 #include "absl/strings/str_cat.h"
@@ -54,65 +60,94 @@ namespace {
 template <typename RealType>
 class UniformRealDistributionTest : public ::testing::Test {};
 
-using RealTypes = ::testing::Types<float, double, long double>;
+// double-double arithmetic is not supported well by either GCC or Clang; see
+// https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99048,
+// https://bugs.llvm.org/show_bug.cgi?id=49131, and
+// https://bugs.llvm.org/show_bug.cgi?id=49132. Don't bother running these tests
+// with double doubles until compiler support is better.
+using RealTypes =
+    std::conditional_t<absl::numeric_internal::IsDoubleDouble(),
+                       ::testing::Types<float, double>,
+                       ::testing::Types<float, double, long double>>;
+
 TYPED_TEST_SUITE(UniformRealDistributionTest, RealTypes);
 
 TYPED_TEST(UniformRealDistributionTest, ParamSerializeTest) {
-  using param_type =
-      typename absl::uniform_real_distribution<TypeParam>::param_type;
-
-  constexpr const TypeParam a{1152921504606846976};
+#if (defined(__i386__) || defined(_M_IX86)) && FLT_EVAL_METHOD != 0
+  // We're using an x87-compatible FPU, and intermediate operations are
+  // performed with 80-bit floats. This produces slightly different results from
+  // what we expect below.
+  GTEST_SKIP()
+      << "Skipping the test because we detected x87 floating-point semantics";
+#endif
+  using DistributionType = absl::uniform_real_distribution<TypeParam>;
+  using real_type = TypeParam;
+  using param_type = typename DistributionType::param_type;
+
+  constexpr const real_type kMax = std::numeric_limits<real_type>::max();
+  constexpr const real_type kMin = std::numeric_limits<real_type>::min();
+  constexpr const real_type kEpsilon =
+      std::numeric_limits<real_type>::epsilon();
+  constexpr const real_type kLowest =
+      std::numeric_limits<real_type>::lowest();  // -max
+
+  const real_type kDenormMax = std::nextafter(kMin, real_type{0});
+  const real_type kOneMinusE =
+      std::nextafter(real_type{1}, real_type{0});  // 1 - epsilon
+
+  constexpr const real_type kTwo60{1152921504606846976};  // 2^60
 
   constexpr int kCount = 1000;
   absl::InsecureBitGen gen;
   for (const auto& param : {
            param_type(),
-           param_type(TypeParam(2.0), TypeParam(2.0)),  // Same
-           param_type(TypeParam(-0.1), TypeParam(0.1)),
-           param_type(TypeParam(0.05), TypeParam(0.12)),
-           param_type(TypeParam(-0.05), TypeParam(0.13)),
-           param_type(TypeParam(-0.05), TypeParam(-0.02)),
+           param_type(real_type{0}, real_type{1}),
+           param_type(real_type(-0.1), real_type(0.1)),
+           param_type(real_type(0.05), real_type(0.12)),
+           param_type(real_type(-0.05), real_type(0.13)),
+           param_type(real_type(-0.05), real_type(-0.02)),
+           // range = 0
+           param_type(real_type(2.0), real_type(2.0)),  // Same
            // double range = 0
            // 2^60 , 2^60 + 2^6
-           param_type(a, TypeParam(1152921504606847040)),
+           param_type(kTwo60, real_type(1152921504606847040)),
            // 2^60 , 2^60 + 2^7
-           param_type(a, TypeParam(1152921504606847104)),
+           param_type(kTwo60, real_type(1152921504606847104)),
            // double range = 2^8
            // 2^60 , 2^60 + 2^8
-           param_type(a, TypeParam(1152921504606847232)),
+           param_type(kTwo60, real_type(1152921504606847232)),
            // float range = 0
            // 2^60 , 2^60 + 2^36
-           param_type(a, TypeParam(1152921573326323712)),
+           param_type(kTwo60, real_type(1152921573326323712)),
            // 2^60 , 2^60 + 2^37
-           param_type(a, TypeParam(1152921642045800448)),
+           param_type(kTwo60, real_type(1152921642045800448)),
            // float range = 2^38
            // 2^60 , 2^60 + 2^38
-           param_type(a, TypeParam(1152921779484753920)),
+           param_type(kTwo60, real_type(1152921779484753920)),
            // Limits
-           param_type(0, std::numeric_limits<TypeParam>::max()),
-           param_type(std::numeric_limits<TypeParam>::lowest(), 0),
-           param_type(0, std::numeric_limits<TypeParam>::epsilon()),
-           param_type(-std::numeric_limits<TypeParam>::epsilon(),
-                      std::numeric_limits<TypeParam>::epsilon()),
-           param_type(std::numeric_limits<TypeParam>::epsilon(),
-                      2 * std::numeric_limits<TypeParam>::epsilon()),
+           param_type(0, kMax),
+           param_type(kLowest, 0),
+           param_type(0, kMin),
+           param_type(0, kEpsilon),
+           param_type(-kEpsilon, kEpsilon),
+           param_type(0, kOneMinusE),
+           param_type(0, kDenormMax),
        }) {
     // Validate parameters.
     const auto a = param.a();
     const auto b = param.b();
-    absl::uniform_real_distribution<TypeParam> before(a, b);
+    DistributionType before(a, b);
     EXPECT_EQ(before.a(), param.a());
     EXPECT_EQ(before.b(), param.b());
 
     {
-      absl::uniform_real_distribution<TypeParam> via_param(param);
+      DistributionType via_param(param);
       EXPECT_EQ(via_param, before);
     }
 
     std::stringstream ss;
     ss << before;
-    absl::uniform_real_distribution<TypeParam> after(TypeParam(1.0),
-                                                     TypeParam(3.1));
+    DistributionType after(real_type(1.0), real_type(3.1));
 
     EXPECT_NE(before.a(), after.a());
     EXPECT_NE(before.b(), after.b());
@@ -147,56 +182,87 @@ TYPED_TEST(UniformRealDistributionTest, ParamSerializeTest) {
       }
     }
 
-    if (!std::is_same<TypeParam, long double>::value) {
+    if (!std::is_same_v<real_type, long double>) {
       // static_cast<double>(long double) can overflow.
-      std::string msg = absl::StrCat("Range: ", static_cast<double>(sample_min),
-                                     ", ", static_cast<double>(sample_max));
-      ABSL_RAW_LOG(INFO, "%s", msg.c_str());
+      LOG(INFO) << "Range: " << static_cast<double>(sample_min) << ", "
+                << static_cast<double>(sample_max);
     }
   }
 }
 
+#ifdef _MSC_VER
+#pragma warning(push)
+#pragma warning(disable : 4756)  // Constant arithmetic overflow.
+#endif
 TYPED_TEST(UniformRealDistributionTest, ViolatesPreconditionsDeathTest) {
+  using DistributionType = absl::uniform_real_distribution<TypeParam>;
+  using real_type = TypeParam;
+
 #if GTEST_HAS_DEATH_TEST
   // Hi < Lo
-  EXPECT_DEBUG_DEATH(
-      { absl::uniform_real_distribution<TypeParam> dist(10.0, 1.0); }, "");
+  EXPECT_DEBUG_DEATH({ DistributionType dist(10.0, 1.0); }, "");
 
   // Hi - Lo > numeric_limits<>::max()
   EXPECT_DEBUG_DEATH(
       {
-        absl::uniform_real_distribution<TypeParam> dist(
-            std::numeric_limits<TypeParam>::lowest(),
-            std::numeric_limits<TypeParam>::max());
+        DistributionType dist(std::numeric_limits<real_type>::lowest(),
+                              std::numeric_limits<real_type>::max());
+      },
+      "");
+
+  // kEpsilon guarantees that max + kEpsilon = inf.
+  const auto kEpsilon = std::nexttoward(
+      (std::numeric_limits<real_type>::max() -
+       std::nexttoward(std::numeric_limits<real_type>::max(), 0.0)) /
+          2,
+      std::numeric_limits<real_type>::max());
+  EXPECT_DEBUG_DEATH(
+      {
+        DistributionType dist(-kEpsilon, std::numeric_limits<real_type>::max());
       },
       "");
+  EXPECT_DEBUG_DEATH(
+      {
+        DistributionType dist(std::numeric_limits<real_type>::lowest(),
+                              kEpsilon);
+      },
+      "");
+
 #endif  // GTEST_HAS_DEATH_TEST
 #if defined(NDEBUG)
   // opt-mode, for invalid parameters, will generate a garbage value,
   // but should not enter an infinite loop.
   absl::InsecureBitGen gen;
   {
-    absl::uniform_real_distribution<TypeParam> dist(10.0, 1.0);
+    DistributionType dist(10.0, 1.0);
     auto x = dist(gen);
     EXPECT_FALSE(std::isnan(x)) << x;
   }
   {
-    absl::uniform_real_distribution<TypeParam> dist(
-        std::numeric_limits<TypeParam>::lowest(),
-        std::numeric_limits<TypeParam>::max());
+    DistributionType dist(std::numeric_limits<real_type>::lowest(),
+                          std::numeric_limits<real_type>::max());
     auto x = dist(gen);
     // Infinite result.
     EXPECT_FALSE(std::isfinite(x)) << x;
   }
 #endif  // NDEBUG
 }
+#ifdef _MSC_VER
+#pragma warning(pop)  // warning(disable:4756)
+#endif
 
 TYPED_TEST(UniformRealDistributionTest, TestMoments) {
+  using DistributionType = absl::uniform_real_distribution<TypeParam>;
+
   constexpr int kSize = 1000000;
   std::vector<double> values(kSize);
 
-  absl::InsecureBitGen rng;
-  absl::uniform_real_distribution<TypeParam> dist;
+  // We use a fixed bit generator for distribution accuracy tests.  This allows
+  // these tests to be deterministic, while still testing the qualify of the
+  // implementation.
+  absl::random_internal::pcg64_2018_engine rng{0x2B7E151628AED2A6};
+
+  DistributionType dist;
   for (int i = 0; i < kSize; i++) {
     values[i] = dist(rng);
   }
@@ -210,9 +276,10 @@ TYPED_TEST(UniformRealDistributionTest, TestMoments) {
 }
 
 TYPED_TEST(UniformRealDistributionTest, ChiSquaredTest50) {
+  using DistributionType = absl::uniform_real_distribution<TypeParam>;
+  using param_type = typename DistributionType::param_type;
+
   using absl::random_internal::kChiSquared;
-  using param_type =
-      typename absl::uniform_real_distribution<TypeParam>::param_type;
 
   constexpr size_t kTrials = 100000;
   constexpr int kBuckets = 50;
@@ -225,7 +292,11 @@ TYPED_TEST(UniformRealDistributionTest, ChiSquaredTest50) {
   const int kThreshold =
       absl::random_internal::ChiSquareValue(kBuckets - 1, 0.999999);
 
-  absl::InsecureBitGen rng;
+  // We use a fixed bit generator for distribution accuracy tests.  This allows
+  // these tests to be deterministic, while still testing the qualify of the
+  // implementation.
+  absl::random_internal::pcg64_2018_engine rng{0x2B7E151628AED2A6};
+
   for (const auto& param : {param_type(0, 1), param_type(5, 12),
                             param_type(-5, 13), param_type(-5, -2)}) {
     const double min_val = param.a();
@@ -233,7 +304,7 @@ TYPED_TEST(UniformRealDistributionTest, ChiSquaredTest50) {
     const double factor = kBuckets / (max_val - min_val);
 
     std::vector<int32_t> counts(kBuckets, 0);
-    absl::uniform_real_distribution<TypeParam> dist(param);
+    DistributionType dist(param);
     for (size_t i = 0; i < kTrials; i++) {
       auto x = dist(rng);
       auto bucket = static_cast<size_t>((x - min_val) * factor);
@@ -254,15 +325,18 @@ TYPED_TEST(UniformRealDistributionTest, ChiSquaredTest50) {
       absl::StrAppend(&msg, kChiSquared, " p-value ", p_value, "\n");
       absl::StrAppend(&msg, "High ", kChiSquared, " value: ", chi_square, " > ",
                       kThreshold);
-      ABSL_RAW_LOG(INFO, "%s", msg.c_str());
+      LOG(INFO) << msg;
       FAIL() << msg;
     }
   }
 }
 
 TYPED_TEST(UniformRealDistributionTest, StabilityTest) {
+  using DistributionType = absl::uniform_real_distribution<TypeParam>;
+  using real_type = TypeParam;
+
   // absl::uniform_real_distribution stability relies only on
-  // random_internal::RandU64ToDouble and random_internal::RandU64ToFloat.
+  // random_internal::GenerateRealFromBits.
   absl::random_internal::sequence_urbg urbg(
       {0x0003eb76f6f7f755ull, 0xFFCEA50FDB2F953Bull, 0xC332DDEFBE6C5AA5ull,
        0x6558218568AB9702ull, 0x2AEF7DAD5B6E2F84ull, 0x1521B62829076170ull,
@@ -271,9 +345,9 @@ TYPED_TEST(UniformRealDistributionTest, StabilityTest) {
 
   std::vector<int> output(12);
 
-  absl::uniform_real_distribution<TypeParam> dist;
+  DistributionType dist;
   std::generate(std::begin(output), std::end(output), [&] {
-    return static_cast<int>(TypeParam(1000000) * dist(urbg));
+    return static_cast<int>(real_type(1000000) * dist(urbg));
   });
 
   EXPECT_THAT(
diff --git a/absl/random/zipf_distribution.h b/absl/random/zipf_distribution.h
index d7b4ac38eda..21e3b709b05 100644
--- a/absl/random/zipf_distribution.h
+++ b/absl/random/zipf_distribution.h
@@ -22,13 +22,16 @@
 #include <ostream>
 #include <type_traits>
 
+#include "absl/base/config.h"
 #include "absl/random/internal/iostream_state_saver.h"
+#include "absl/random/internal/traits.h"
 #include "absl/random/uniform_real_distribution.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 // absl::zipf_distribution produces random integer-values in the range [0, k],
-// distributed according to the discrete probability function:
+// distributed according to the unnormalized discrete probability function:
 //
 //  P(x) = (v + x) ^ -q
 //
@@ -55,8 +58,8 @@ class zipf_distribution {
    public:
     using distribution_type = zipf_distribution;
 
-    // Preconditions: k > 0, v > 0, q > 1
-    // The precondidtions are validated when NDEBUG is not defined via
+    // Preconditions: k >= 0, v > 0, q > 1
+    // The preconditions are validated when NDEBUG is not defined via
     // a pair of assert() directives.
     // If NDEBUG is defined and either or both of these parameters take invalid
     // values, the behavior of the class is undefined.
@@ -93,7 +96,7 @@ class zipf_distribution {
     double hxm_;              // h(k + 0.5)
     double hx0_minus_hxm_;    // h(x0) - h(k + 0.5)
 
-    static_assert(std::is_integral<IntType>::value,
+    static_assert(random_internal::IsIntegral<IntType>::value,
                   "Class-template absl::zipf_distribution<> must be "
                   "parameterized using an integral type.");
   };
@@ -150,7 +153,7 @@ zipf_distribution<IntType>::param_type::param_type(
     : k_(k), q_(q), v_(v), one_minus_q_(1 - q) {
   assert(q > 1);
   assert(v > 0);
-  assert(k > 0);
+  assert(k >= 0);
   one_minus_q_inv_ = 1 / one_minus_q_;
 
   // Setup for the ZRI algorithm (pg 17 of the paper).
@@ -219,8 +222,8 @@ zipf_distribution<IntType>::operator()(
     const double v = uniform_double(g);
     const double u = p.hxm_ + v * p.hx0_minus_hxm_;
     const double x = p.hinv(u);
-    k = rint(x);              // std::floor(x + 0.5);
-    if (k > p.k()) continue;  // reject k > max_k
+    k = rint(x);                                   // std::floor(x + 0.5);
+    if (k > static_cast<double>(p.k())) continue;  // reject k > max_k
     if (k - x <= p.s_) break;
     const double h = p.h(k + 0.5);
     const double r = p.pow_negative_q(p.v_ + k);
@@ -264,6 +267,7 @@ std::basic_istream<CharT, Traits>& operator>>(
   return is;
 }
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_ZIPF_DISTRIBUTION_H_
diff --git a/absl/random/zipf_distribution_test.cc b/absl/random/zipf_distribution_test.cc
index 4d4a0fcf793..f29aca5935a 100644
--- a/absl/random/zipf_distribution_test.cc
+++ b/absl/random/zipf_distribution_test.cc
@@ -15,18 +15,23 @@
 #include "absl/random/zipf_distribution.h"
 
 #include <algorithm>
+#include <cassert>
+#include <cmath>
 #include <cstddef>
 #include <cstdint>
 #include <iterator>
+#include <limits>
 #include <random>
+#include <sstream>
 #include <string>
 #include <utility>
 #include <vector>
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
-#include "absl/base/internal/raw_logging.h"
+#include "absl/log/log.h"
 #include "absl/random/internal/chi_square.h"
+#include "absl/random/internal/pcg_engine.h"
 #include "absl/random/internal/sequence_urbg.h"
 #include "absl/random/random.h"
 #include "absl/strings/str_cat.h"
@@ -43,7 +48,7 @@ class ZipfDistributionTypedTest : public ::testing::Test {};
 
 using IntTypes = ::testing::Types<int, int8_t, int16_t, int32_t, int64_t,
                                   uint8_t, uint16_t, uint32_t, uint64_t>;
-TYPED_TEST_CASE(ZipfDistributionTypedTest, IntTypes);
+TYPED_TEST_SUITE(ZipfDistributionTypedTest, IntTypes);
 
 TYPED_TEST(ZipfDistributionTypedTest, SerializeTest) {
   using param_type = typename absl::zipf_distribution<TypeParam>::param_type;
@@ -101,8 +106,7 @@ TYPED_TEST(ZipfDistributionTypedTest, SerializeTest) {
       if (sample > sample_max) sample_max = sample;
       if (sample < sample_min) sample_min = sample;
     }
-    ABSL_INTERNAL_LOG(INFO,
-                      absl::StrCat("Range: ", +sample_min, ", ", +sample_max));
+    LOG(INFO) << "Range: " << sample_min << ", " << sample_max;
   }
 }
 
@@ -176,15 +180,15 @@ class ZipfModel {
       const double x = v_ + i;
 
       // H(n, q-1)
-      const double hnqm1 =
-          (q_ == 2.0) ? (1.0 / x)
-                      : (q_ == 3.0) ? (1.0 / (x * x)) : std::pow(x, -qm1);
+      const double hnqm1 = (q_ == 2.0)   ? (1.0 / x)
+                           : (q_ == 3.0) ? (1.0 / (x * x))
+                                         : std::pow(x, -qm1);
       sum_hnq_m1 += hnqm1;
 
       // H(n, q)
-      const double hnq =
-          (q_ == 2.0) ? (1.0 / (x * x))
-                      : (q_ == 3.0) ? (1.0 / (x * x * x)) : std::pow(x, -q_);
+      const double hnq = (q_ == 2.0)   ? (1.0 / (x * x))
+                         : (q_ == 3.0) ? (1.0 / (x * x * x))
+                                       : std::pow(x, -q_);
       sum_hnq_ += hnq;
       hnq_.push_back(hnq);
       if (i > 1000 && hnq <= 1e-10) {
@@ -213,7 +217,10 @@ class ZipfTest : public testing::TestWithParam<zipf_u64::param_type>,
  public:
   ZipfTest() : ZipfModel(GetParam().k(), GetParam().q(), GetParam().v()) {}
 
-  absl::InsecureBitGen rng_;
+  // We use a fixed bit generator for distribution accuracy tests.  This allows
+  // these tests to be deterministic, while still testing the qualify of the
+  // implementation.
+  absl::random_internal::pcg64_2018_engine rng_{0x2B7E151628AED2A6};
 };
 
 TEST_P(ZipfTest, ChiSquaredTest) {
@@ -299,18 +306,15 @@ TEST_P(ZipfTest, ChiSquaredTest) {
 
   // Log if the chi_squared value is above the threshold.
   if (chi_square > threshold) {
-    ABSL_INTERNAL_LOG(INFO, "values");
+    LOG(INFO) << "values";
     for (size_t i = 0; i < expected.size(); i++) {
-      ABSL_INTERNAL_LOG(INFO, absl::StrCat(points[i], ": ", buckets[i],
-                                           " vs. E=", expected[i]));
+      LOG(INFO) << points[i] << ": " << buckets[i] << " vs. E=" << expected[i];
     }
-    ABSL_INTERNAL_LOG(INFO, absl::StrCat("trials ", trials));
-    ABSL_INTERNAL_LOG(INFO,
-                      absl::StrCat("mean ", avg, " vs. expected ", mean()));
-    ABSL_INTERNAL_LOG(INFO, absl::StrCat(kChiSquared, "(data, ", dof, ") = ",
-                                         chi_square, " (", p_actual, ")"));
-    ABSL_INTERNAL_LOG(INFO,
-                      absl::StrCat(kChiSquared, " @ 0.9995 = ", threshold));
+    LOG(INFO) << "trials " << trials;
+    LOG(INFO) << "mean " << avg << " vs. expected " << mean();
+    LOG(INFO) << kChiSquared << "(data, " << dof << ") = " << chi_square << " ("
+              << p_actual << ")";
+    LOG(INFO) << kChiSquared << " @ 0.9995 = " << threshold;
     FAIL() << kChiSquared << " value of " << chi_square
            << " is above the threshold.";
   }
diff --git a/absl/status/BUILD.bazel b/absl/status/BUILD.bazel
new file mode 100644
index 00000000000..5a611ec9f9e
--- /dev/null
+++ b/absl/status/BUILD.bazel
@@ -0,0 +1,297 @@
+#
+# Copyright 2017 The Abseil Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# This package contains `absl::Status`.
+# It will expand later to have utilities around `Status` like `StatusOr`,
+# `StatusBuilder` and macros.
+
+load("@rules_cc//cc:cc_binary.bzl", "cc_binary")
+load("@rules_cc//cc:cc_library.bzl", "cc_library")
+load("@rules_cc//cc:cc_test.bzl", "cc_test")
+load(
+    "//absl:copts/configure_copts.bzl",
+    "ABSL_DEFAULT_COPTS",
+    "ABSL_DEFAULT_LINKOPTS",
+    "ABSL_TEST_COPTS",
+)
+
+package(
+    default_visibility = ["//visibility:private"],
+    features = [
+        "header_modules",
+        "layering_check",
+        "parse_headers",
+    ],
+)
+
+licenses(["notice"])
+
+cc_library(
+    name = "status",
+    srcs = [
+        "internal/status_internal.cc",
+        "internal/status_internal.h",
+        "status.cc",
+        "status_payload_printer.cc",
+    ],
+    hdrs = [
+        "status.h",
+        "status_payload_printer.h",
+    ],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
+    deps = [
+        "//absl/base:atomic_hook",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:no_destructor",
+        "//absl/base:nullability",
+        "//absl/base:raw_logging_internal",
+        "//absl/base:strerror",
+        "//absl/container:inlined_vector",
+        "//absl/debugging:leak_check",
+        "//absl/debugging:stacktrace",
+        "//absl/debugging:symbolize",
+        "//absl/functional:function_ref",
+        "//absl/hash",
+        "//absl/memory",
+        "//absl/strings",
+        "//absl/strings:cord",
+        "//absl/strings:str_format",
+        "//absl/types:optional",
+        "//absl/types:optional_ref",
+        "//absl/types:source_location",
+        "//absl/types:span",
+    ],
+)
+
+cc_test(
+    name = "status_test",
+    srcs = ["status_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":status",
+        "//absl/strings",
+        "//absl/strings:cord",
+        "//absl/strings:str_format",
+        "//absl/types:source_location",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_binary(
+    name = "status_benchmark",
+    testonly = True,
+    srcs = ["status_benchmark.cc"],
+    tags = ["benchmark"],
+    deps = [
+        ":status",
+        "//absl/types:source_location",
+        "@google_benchmark//:benchmark_main",
+    ],
+)
+
+cc_library(
+    name = "statusor",
+    srcs = [
+        "internal/statusor_internal.h",
+        "statusor.cc",
+    ],
+    hdrs = [
+        "statusor.h",
+    ],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
+    deps = [
+        ":status",
+        "//absl/base",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:nullability",
+        "//absl/base:raw_logging_internal",
+        "//absl/meta:type_traits",
+        "//absl/strings",
+        "//absl/strings:has_ostream_operator",
+        "//absl/strings:str_format",
+        "//absl/types:source_location",
+        "//absl/types:span",
+        "//absl/types:variant",
+        "//absl/utility",
+    ],
+)
+
+cc_test(
+    name = "statusor_test",
+    size = "small",
+    srcs = ["statusor_test.cc"],
+    deps = [
+        ":status",
+        ":status_matchers",
+        ":statusor",
+        "//absl/base",
+        "//absl/memory",
+        "//absl/strings",
+        "//absl/types:any",
+        "//absl/types:source_location",
+        "//absl/types:variant",
+        "//absl/utility",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_binary(
+    name = "statusor_benchmark",
+    testonly = True,
+    srcs = ["statusor_benchmark.cc"],
+    tags = ["benchmark"],
+    deps = [
+        ":status",
+        ":statusor",
+        "//absl/base:core_headers",
+        "@google_benchmark//:benchmark_main",
+    ],
+)
+
+cc_library(
+    name = "status_builder",
+    srcs = ["status_builder.cc"],
+    hdrs = ["status_builder.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
+    deps = [
+        ":status",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:log_severity",
+        "//absl/strings",
+        "//absl/strings:cord",
+        "//absl/strings:internal",
+        "//absl/strings:stringify_stream",
+        "//absl/time",
+        "//absl/types:source_location",
+    ],
+)
+
+cc_test(
+    name = "status_builder_test",
+    srcs = ["status_builder_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":status",
+        ":status_builder",
+        ":status_matchers",
+        ":statusor",
+        "//absl/base:config",
+        "//absl/log",
+        "//absl/log:flags",
+        "//absl/log:log_entry",
+        "//absl/log:log_sink",
+        "//absl/strings",
+        "//absl/types:source_location",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_library(
+    name = "status_macros",
+    hdrs = ["status_macros.h"],
+    visibility = ["//visibility:public"],
+    deps = [
+        ":status",
+        ":status_builder",
+        ":statusor",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/types:source_location",
+    ],
+)
+
+cc_test(
+    name = "status_macros_test",
+    size = "small",
+    srcs = ["status_macros_test.cc"],
+    deps = [
+        ":status",
+        ":status_builder",
+        ":status_macros",
+        ":statusor",
+        "//absl/strings:string_view",
+        "//absl/types:source_location",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_library(
+    name = "status_matchers",
+    testonly = 1,
+    srcs = [
+        "internal/status_matchers.cc",
+        "internal/status_matchers.h",
+    ],
+    hdrs = ["status_matchers.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
+    deps = [
+        ":status",
+        ":statusor",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/strings:string_view",
+        "@googletest//:gtest",
+    ],
+)
+
+cc_test(
+    name = "status_matchers_test",
+    size = "small",
+    srcs = ["status_matchers_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":status",
+        ":status_matchers",
+        ":statusor",
+        "//absl/strings",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "status_matchers_with_unqualified_macros_test",
+    size = "small",
+    srcs = ["status_matchers_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    local_defines = ["ABSL_DEFINE_UNQUALIFIED_STATUS_TESTING_MACROS"],
+    deps = [
+        ":status",
+        ":status_matchers",
+        ":statusor",
+        "//absl/strings",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
diff --git a/absl/status/CMakeLists.txt b/absl/status/CMakeLists.txt
new file mode 100644
index 00000000000..90d4baf1bc6
--- /dev/null
+++ b/absl/status/CMakeLists.txt
@@ -0,0 +1,250 @@
+#
+# Copyright 2020 The Abseil Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+absl_cc_library(
+  NAME
+    status
+  HDRS
+    "status.h"
+  SRCS
+    "internal/status_internal.h"
+    "internal/status_internal.cc"
+    "status.cc"
+    "status_payload_printer.h"
+    "status_payload_printer.cc"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEFINES
+    "$<$<PLATFORM_ID:AIX>:_LINUX_SOURCE_COMPAT>"
+  DEPS
+    absl::atomic_hook
+    absl::config
+    absl::cord
+    absl::core_headers
+    absl::function_ref
+    absl::inlined_vector
+    absl::internal_optional_ref
+    absl::leak_check
+    absl::memory
+    absl::no_destructor
+    absl::nullability
+    absl::optional
+    absl::raw_logging_internal
+    absl::source_location
+    absl::span
+    absl::stacktrace
+    absl::str_format
+    absl::strerror
+    absl::strings
+    absl::symbolize
+  PUBLIC
+)
+
+absl_cc_test(
+  NAME
+    status_test
+  SRCS
+   "status_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::status
+    absl::source_location
+    absl::str_format
+    absl::strings
+    GTest::gmock_main
+)
+
+absl_cc_library(
+  NAME
+    statusor
+  HDRS
+    "statusor.h"
+  SRCS
+    "statusor.cc"
+    "internal/statusor_internal.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::base
+    absl::config
+    absl::core_headers
+    absl::has_ostream_operator
+    absl::nullability
+    absl::raw_logging_internal
+    absl::status
+    absl::source_location
+    absl::str_format
+    absl::strings
+    absl::type_traits
+    absl::utility
+    absl::variant
+  PUBLIC
+)
+
+absl_cc_test(
+  NAME
+    statusor_test
+  SRCS
+   "statusor_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::status
+    absl::status_matchers
+    absl::statusor
+    absl::source_location
+    absl::strings
+    GTest::gmock_main
+)
+
+absl_cc_library(
+  NAME
+    status_builder
+  HDRS
+    "status_builder.h"
+  SRCS
+    "status_builder.cc"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::config
+    absl::cord
+    absl::core_headers
+    absl::nullability
+    absl::optional
+    absl::source_location
+    absl::span
+    absl::status
+    absl::strings
+    absl::time
+  PUBLIC
+)
+
+absl_cc_test(
+  NAME
+    status_builder_test
+  SRCS
+   "status_builder_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::core_headers
+    absl::flags
+    GTest::gmock_main
+    absl::log
+    absl::log_entry
+    absl::log_flags
+    absl::log_internal_flags
+    absl::log_severity
+    absl::log_sink
+    absl::scoped_mock_log
+    absl::source_location
+    absl::span
+    absl::status
+    absl::statusor
+    absl::status_builder
+    absl::status_matchers
+    absl::strings
+    absl::time
+)
+
+absl_cc_library(
+  NAME
+    status_macros
+  HDRS
+    "status_macros.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::core_headers
+    absl::source_location
+    absl::status
+    absl::status_builder
+    absl::statusor
+  PUBLIC
+)
+
+absl_cc_test(
+  NAME
+    status_macros_test
+  SRCS
+   "status_macros_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::core_headers
+    absl::source_location
+    absl::status
+    absl::status_builder
+    absl::status_macros
+    absl::statusor
+    absl::string_view
+    GTest::gmock_main
+)
+
+absl_cc_library(
+  NAME
+    status_matchers
+  HDRS
+    "status_matchers.h"
+  SRCS
+    "internal/status_matchers.h"
+    "internal/status_matchers.cc"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::base
+    absl::core_headers
+    absl::status
+    absl::statusor
+    absl::strings
+    GTest::gmock
+    GTest::gtest
+  PUBLIC
+  TESTONLY
+)
+
+absl_cc_test(
+  NAME
+    status_matchers_test
+  SRCS
+   "status_matchers_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::status
+    absl::statusor
+    absl::status_matchers
+    GTest::gmock_main
+)
+
+absl_cc_test(
+  NAME
+    status_matchers_with_unqualified_macros_test
+  SRCS
+   "status_matchers_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEFINES
+    "ABSL_DEFINE_UNQUALIFIED_STATUS_TESTING_MACROS"
+  DEPS
+    absl::status
+    absl::statusor
+    absl::status_matchers
+    GTest::gmock_main
+)
diff --git a/absl/status/internal/status_internal.cc b/absl/status/internal/status_internal.cc
new file mode 100644
index 00000000000..77f3ff78ff0
--- /dev/null
+++ b/absl/status/internal/status_internal.cc
@@ -0,0 +1,297 @@
+// Copyright 2023 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/status/internal/status_internal.h"
+
+#include <atomic>
+#include <cassert>
+#include <cstddef>
+#include <cstdint>
+#include <cstdio>
+#include <cstring>
+#include <iterator>
+#include <memory>
+#include <optional>
+#include <string>
+#include <utility>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/macros.h"
+#include "absl/base/nullability.h"
+#include "absl/debugging/leak_check.h"
+#include "absl/debugging/stacktrace.h"
+#include "absl/debugging/symbolize.h"
+#include "absl/functional/function_ref.h"
+#include "absl/hash/hash.h"
+#include "absl/status/status.h"
+#include "absl/status/status_payload_printer.h"
+#include "absl/strings/cord.h"
+#include "absl/strings/escaping.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/str_format.h"
+#include "absl/strings/str_split.h"
+#include "absl/strings/string_view.h"
+#include "absl/types/optional_ref.h"
+#include "absl/types/source_location.h"
+#include "absl/types/span.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace status_internal {
+
+void StatusRep::Unref() const {
+  // Fast path: if ref==1, there is no need for a RefCountDec (since
+  // this is the only reference and therefore no other thread is
+  // allowed to be mucking with r).
+  if (ref_.load(std::memory_order_acquire) == 1 ||
+      ref_.fetch_sub(1, std::memory_order_acq_rel) - 1 == 0) {
+    delete this;
+  }
+}
+
+static std::optional<size_t> FindPayloadIndexByUrl(const Payloads* payloads,
+                                                   absl::string_view type_url) {
+  if (payloads == nullptr) return std::nullopt;
+
+  for (size_t i = 0; i < payloads->size(); ++i) {
+    if ((*payloads)[i].type_url == type_url) return i;
+  }
+
+  return std::nullopt;
+}
+
+std::optional<absl::Cord> StatusRep::GetPayload(
+    absl::string_view type_url) const {
+  std::optional<size_t> index =
+      status_internal::FindPayloadIndexByUrl(payloads_.get(), type_url);
+  if (index.has_value()) return (*payloads_)[index.value()].payload;
+
+  return std::nullopt;
+}
+
+void StatusRep::SetPayload(absl::string_view type_url, absl::Cord payload) {
+  if (payloads_ == nullptr) {
+    payloads_ = std::make_unique<status_internal::Payloads>();
+  }
+
+  std::optional<size_t> index =
+      status_internal::FindPayloadIndexByUrl(payloads_.get(), type_url);
+  if (index.has_value()) {
+    (*payloads_)[index.value()].payload = std::move(payload);
+    return;
+  }
+
+  payloads_->push_back({std::string(type_url), std::move(payload)});
+}
+
+StatusRep::EraseResult StatusRep::ErasePayload(absl::string_view type_url) {
+  std::optional<size_t> index =
+      status_internal::FindPayloadIndexByUrl(payloads_.get(), type_url);
+  if (!index.has_value()) return {false, Status::PointerToRep(this)};
+  payloads_->erase(payloads_->begin() + index.value());
+  if (payloads_->empty() && message_.empty()) {
+    // Special case: If this can be represented inlined, it MUST be inlined
+    // (== depends on this behavior).
+    EraseResult result = {true, Status::CodeToInlinedRep(code_)};
+    Unref();
+    return result;
+  }
+  return {true, Status::PointerToRep(this)};
+}
+
+void StatusRep::ForEachPayload(
+    absl::FunctionRef<void(absl::string_view, const absl::Cord&)> visitor)
+    const {
+  if (auto* payloads = payloads_.get()) {
+    bool in_reverse =
+        payloads->size() > 1 && reinterpret_cast<uintptr_t>(payloads) % 13 > 6;
+
+    for (size_t index = 0; index < payloads->size(); ++index) {
+      const auto& elem =
+          (*payloads)[in_reverse ? payloads->size() - 1 - index : index];
+
+#ifdef NDEBUG
+      visitor(elem.type_url, elem.payload);
+#else
+      // In debug mode invalidate the type url to prevent users from relying on
+      // this string lifetime.
+
+      // NOLINTNEXTLINE intentional extra conversion to force temporary.
+      visitor(std::string(elem.type_url), elem.payload);
+#endif  // NDEBUG
+    }
+  }
+}
+
+absl::Span<const SourceLocation> StatusRep::GetSourceLocations() const {
+  return absl::MakeSpan(source_locations_);
+}
+
+void StatusRep::AddSourceLocation(absl::SourceLocation loc) {
+  source_locations_.push_back(loc);
+}
+
+std::string StatusRep::ToString(StatusToStringMode mode) const {
+  std::string text;
+  absl::StrAppend(&text, absl::StatusCodeToString(code()), ": ", message());
+
+  const bool with_payload = (mode & StatusToStringMode::kWithPayload) ==
+                            StatusToStringMode::kWithPayload;
+
+  if (with_payload) {
+    status_internal::StatusPayloadPrinter printer =
+        status_internal::GetStatusPayloadPrinter();
+    this->ForEachPayload([&](absl::string_view type_url,
+                             const absl::Cord& payload) {
+      std::optional<std::string> result;
+      if (printer) result = printer(type_url, payload);
+      absl::StrAppend(
+          &text, " [", type_url, "='",
+          result.has_value() ? *result : absl::CHexEscape(std::string(payload)),
+          "']");
+    });
+  }
+  const bool with_source_location =
+      (mode & StatusToStringMode::kWithSourceLocation) ==
+      StatusToStringMode::kWithSourceLocation;
+  if (with_source_location && !source_locations_.empty()) {
+    absl::string_view whitespace = (absl::Hash<int>{}(42) % 2 == 0) ? "" : " ";
+    absl::StrAppend(&text, "\n=== Source Location Trace: ===", whitespace,
+                    "\n");
+    for (const absl::SourceLocation loc : GetSourceLocations()) {
+      absl::StrAppend(&text, loc.file_name(), ":", loc.line(), "\n");
+    }
+  }
+
+  return text;
+}
+
+bool StatusRep::operator==(const StatusRep& other) const {
+  assert(this != &other);
+  if (code_ != other.code_) return false;
+  if (message_ != other.message_) return false;
+  const status_internal::Payloads* this_payloads = payloads_.get();
+  const status_internal::Payloads* other_payloads = other.payloads_.get();
+
+  const status_internal::Payloads no_payloads;
+  const status_internal::Payloads* larger_payloads =
+      this_payloads ? this_payloads : &no_payloads;
+  const status_internal::Payloads* smaller_payloads =
+      other_payloads ? other_payloads : &no_payloads;
+  if (larger_payloads->size() < smaller_payloads->size()) {
+    std::swap(larger_payloads, smaller_payloads);
+  }
+  if ((larger_payloads->size() - smaller_payloads->size()) > 1) return false;
+  // Payloads can be ordered differently, so we can't just compare payload
+  // vectors.
+  for (const auto& payload : *larger_payloads) {
+
+    bool found = false;
+    for (const auto& other_payload : *smaller_payloads) {
+      if (payload.type_url == other_payload.type_url) {
+        if (payload.payload != other_payload.payload) {
+          return false;
+        }
+        found = true;
+        break;
+      }
+    }
+    if (!found) return false;
+  }
+  return true;
+}
+
+StatusRep* absl_nonnull StatusRep::Clone(
+    absl::optional_ref<absl::string_view> new_message, bool include_payloads,
+    bool include_source_locations) const {
+  // Optimization: no need to create a clone if we already have a refcount of 1.
+  if (ref_.load(std::memory_order_acquire) == 1 && !new_message.has_value() &&
+      include_payloads && include_source_locations) {
+    // All StatusRep instances are heap allocated and mutable, therefore this
+    // const_cast will never cast away const from a stack instance.
+    //
+    // CloneAndUnref is the only method that doesn't involve an external cast to
+    // get a mutable StatusRep* from the uintptr_t rep stored in Status.
+    return const_cast<StatusRep*>(this);
+  }
+  std::unique_ptr<status_internal::Payloads> payloads;
+  if (include_payloads && payloads_) {
+    payloads = std::make_unique<status_internal::Payloads>(*payloads_);
+  }
+  auto* new_rep =
+      new StatusRep(code_, new_message.value_or(message_), std::move(payloads));
+  if (include_source_locations) {
+    new_rep->source_locations_ = source_locations_;
+  }
+  return new_rep;
+}
+
+StatusRep* absl_nonnull StatusRep::CloneAndUnref() const {
+  return CloneAndUnref(std::nullopt, true, true);
+}
+
+StatusRep* absl_nonnull StatusRep::CloneAndUnref(
+    absl::optional_ref<absl::string_view> new_message, bool include_payloads,
+    bool include_source_locations) const {
+  StatusRep* new_rep =
+      Clone(new_message, /*include_payloads=*/include_payloads,
+            /*include_source_locations=*/include_source_locations);
+  if (new_rep != this) {
+    Unref();
+  }
+  return new_rep;
+}
+
+// Convert canonical code to a value known to this binary.
+absl::StatusCode MapToLocalCode(int value) {
+  absl::StatusCode code = static_cast<absl::StatusCode>(value);
+  switch (code) {
+    case absl::StatusCode::kOk:
+    case absl::StatusCode::kCancelled:
+    case absl::StatusCode::kUnknown:
+    case absl::StatusCode::kInvalidArgument:
+    case absl::StatusCode::kDeadlineExceeded:
+    case absl::StatusCode::kNotFound:
+    case absl::StatusCode::kAlreadyExists:
+    case absl::StatusCode::kPermissionDenied:
+    case absl::StatusCode::kResourceExhausted:
+    case absl::StatusCode::kFailedPrecondition:
+    case absl::StatusCode::kAborted:
+    case absl::StatusCode::kOutOfRange:
+    case absl::StatusCode::kUnimplemented:
+    case absl::StatusCode::kInternal:
+    case absl::StatusCode::kUnavailable:
+    case absl::StatusCode::kDataLoss:
+    case absl::StatusCode::kUnauthenticated:
+      return code;
+    default:
+      return absl::StatusCode::kUnknown;
+  }
+}
+
+const char* absl_nonnull MakeCheckFailString(
+    const absl::Status* absl_nonnull status, const char* absl_nonnull prefix) {
+  // There's no need to free this string since the process is crashing.
+  return absl::IgnoreLeak(
+             new std::string(absl::StrCat(
+                 prefix, " (",
+                 status->ToString(StatusToStringMode::kWithEverything), ")")))
+      ->c_str();
+}
+
+}  // namespace status_internal
+
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/status/internal/status_internal.h b/absl/status/internal/status_internal.h
new file mode 100644
index 00000000000..1cc87aba4ea
--- /dev/null
+++ b/absl/status/internal/status_internal.h
@@ -0,0 +1,186 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+#ifndef ABSL_STATUS_INTERNAL_STATUS_INTERNAL_H_
+#define ABSL_STATUS_INTERNAL_STATUS_INTERNAL_H_
+
+// IWYU pragma: private, include "absl/status/status.h"
+
+#include <atomic>
+#include <cstdint>
+#include <memory>
+#include <optional>
+#include <string>
+#include <type_traits>
+#include <utility>
+#include <vector>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/nullability.h"
+#include "absl/container/inlined_vector.h"
+#include "absl/functional/function_ref.h"
+#include "absl/strings/cord.h"
+#include "absl/strings/string_view.h"
+#include "absl/types/optional.h"
+#include "absl/types/optional_ref.h"
+#include "absl/types/source_location.h"
+#include "absl/types/span.h"
+
+#ifndef SWIG
+// Disabled for SWIG as it doesn't parse attributes correctly.
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+// Returned Status objects may not be ignored. Codesearch doesn't handle ifdefs
+// as part of a class definitions (b/6995610), so we use a forward declaration.
+//
+// TODO(b/176172494): ABSL_MUST_USE_RESULT should expand to the more strict
+// [[nodiscard]]. For now, just use [[nodiscard]] directly when it is available.
+#if ABSL_HAVE_CPP_ATTRIBUTE(nodiscard)
+class [[nodiscard]] ABSL_ATTRIBUTE_TRIVIAL_ABI
+    Status;
+#else
+class ABSL_MUST_USE_RESULT ABSL_ATTRIBUTE_TRIVIAL_ABI
+    Status;
+#endif
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+#endif  // !SWIG
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+enum class StatusCode : int;
+enum class StatusToStringMode : int;
+
+// Forward declaration of StatusOr for Status friendship.
+template <typename T>
+class StatusOr;
+
+namespace status_internal {
+#ifndef SWIG
+class StatusPrivateAccessor;
+class StatusPrivateAccessorForStatusBuilder;
+#endif  // !SWIG
+
+#ifndef SWIG
+// Container for status payloads.
+struct Payload {
+  std::string type_url;
+  absl::Cord payload;
+};
+
+using Payloads = absl::InlinedVector<Payload, 1>;
+
+template <typename T>
+using EnableIfString = std::enable_if_t<std::is_same_v<T, std::string>>;
+
+// Reference-counted representation of Status data.
+class StatusRep {
+ public:
+  StatusRep(absl::StatusCode code_arg, absl::string_view message_arg,
+            std::unique_ptr<status_internal::Payloads> payloads_arg)
+      : ref_(int32_t{1}),
+        code_(code_arg),
+        message_(message_arg),
+        payloads_(std::move(payloads_arg)) {}
+
+  template <typename String, typename = EnableIfString<String>>
+  StatusRep(absl::StatusCode code_arg, String&& message_arg,
+            std::unique_ptr<status_internal::Payloads> payloads_arg)
+      : ref_(int32_t{1}),
+        code_(code_arg),
+        message_(std::forward<String>(message_arg)),
+        payloads_(std::move(payloads_arg)) {}
+
+  absl::StatusCode code() const { return code_; }
+  const std::string& message() const { return message_; }
+
+  // Ref and unref are const to allow access through a const pointer, and are
+  // used during copying operations.
+  void Ref() const { ref_.fetch_add(1, std::memory_order_relaxed); }
+  void Unref() const;
+
+  // Payload methods correspond to the same methods in absl::Status.
+  std::optional<absl::Cord> GetPayload(absl::string_view type_url) const;
+  void SetPayload(absl::string_view type_url, absl::Cord payload);
+  struct EraseResult {
+    bool erased;
+    uintptr_t new_rep;
+  };
+  EraseResult ErasePayload(absl::string_view type_url);
+  void ForEachPayload(
+      absl::FunctionRef<void(absl::string_view, const absl::Cord&)> visitor)
+      const;
+
+  absl::Span<const SourceLocation> GetSourceLocations() const;
+  void AddSourceLocation(absl::SourceLocation loc);
+
+  std::string ToString(StatusToStringMode mode) const;
+
+  bool operator==(const StatusRep& other) const;
+  bool operator!=(const StatusRep& other) const { return !(*this == other); }
+
+  // Returns an equivalent heap allocated StatusRep with refcount 1.
+  //
+  // If `new_message` is provided, the message will be replaced with the new
+  // message.
+  StatusRep* absl_nonnull Clone(
+      absl::optional_ref<absl::string_view> new_message, bool include_payloads,
+      bool include_source_locations) const;
+
+  // Same as Clone(), but also removes a reference to `this`. `this` is not safe
+  // to be used after calling as it may have been deleted.
+  StatusRep* absl_nonnull CloneAndUnref(
+      absl::optional_ref<absl::string_view> new_message, bool include_payloads,
+      bool include_source_locations) const;
+  StatusRep* absl_nonnull CloneAndUnref() const;
+
+ private:
+  friend class absl::Status;
+
+  mutable std::atomic<int32_t> ref_;
+  absl::StatusCode code_;
+
+  // As an internal implementation detail, we guarantee that if status.message()
+  // is non-empty, then the resulting string_view is null terminated.
+  // This is required to implement 'StatusMessageAsCStr(...)'
+  //
+  // NOTE: if most statuses are constructed with messages that are either empty
+  // or so long they don't fit in the std::string's local storage (small string
+  // optimization), replacing std::string with an entirely heap-allocated
+  // string might save memory at scale.
+  std::string message_;
+
+  absl::InlinedVector<absl::SourceLocation, 1> source_locations_;
+  std::unique_ptr<status_internal::Payloads> payloads_;
+};
+
+absl::StatusCode MapToLocalCode(int value);
+
+// Returns a pointer to a newly-allocated string with the given `prefix`,
+// suitable for output as an error message in assertion/`CHECK()` failures.
+//
+// This is an internal implementation detail for Abseil logging.
+ABSL_ATTRIBUTE_PURE_FUNCTION
+const char* absl_nonnull MakeCheckFailString(
+    const absl::Status* absl_nonnull status, const char* absl_nonnull prefix);
+
+}  // namespace status_internal
+#endif  // SWIG
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STATUS_INTERNAL_STATUS_INTERNAL_H_
diff --git a/absl/status/internal/status_matchers.cc b/absl/status/internal/status_matchers.cc
new file mode 100644
index 00000000000..7252d187ee8
--- /dev/null
+++ b/absl/status/internal/status_matchers.cc
@@ -0,0 +1,69 @@
+// Copyright 2024 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: status_matchers.cc
+// -----------------------------------------------------------------------------
+
+#include "absl/status/internal/status_matchers.h"
+
+#include <ostream>
+#include <string>
+
+#include "gmock/gmock.h"  // gmock_for_status_matchers.h
+#include "absl/base/config.h"
+#include "absl/status/status.h"
+
+namespace absl_testing {
+ABSL_NAMESPACE_BEGIN
+namespace status_internal {
+
+void StatusIsMatcherCommonImpl::DescribeTo(std::ostream* os) const {
+  *os << "has a status code that ";
+  code_matcher_.DescribeTo(os);
+  *os << ", and has an error message that ";
+  message_matcher_.DescribeTo(os);
+}
+
+void StatusIsMatcherCommonImpl::DescribeNegationTo(std::ostream* os) const {
+  *os << "either has a status code that ";
+  code_matcher_.DescribeNegationTo(os);
+  *os << ", or has an error message that ";
+  message_matcher_.DescribeNegationTo(os);
+}
+
+bool StatusIsMatcherCommonImpl::MatchAndExplain(
+    const ::absl::Status& status,
+    ::testing::MatchResultListener* result_listener) const {
+  ::testing::StringMatchResultListener inner_listener;
+  if (!code_matcher_.MatchAndExplain(status.code(), &inner_listener)) {
+    *result_listener << (inner_listener.str().empty()
+                             ? status.ok() ? "which is OK"
+                                           : "whose status code is wrong"
+                             : "which has a status code " +
+                                   inner_listener.str());
+    return false;
+  }
+
+  if (!message_matcher_.Matches(status.message())) {
+    *result_listener << "whose error message is wrong";
+    return false;
+  }
+
+  return true;
+}
+
+}  // namespace status_internal
+ABSL_NAMESPACE_END
+}  // namespace absl_testing
diff --git a/absl/status/internal/status_matchers.h b/absl/status/internal/status_matchers.h
new file mode 100644
index 00000000000..b7425e1f57c
--- /dev/null
+++ b/absl/status/internal/status_matchers.h
@@ -0,0 +1,251 @@
+// Copyright 2024 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_STATUS_INTERNAL_STATUS_MATCHERS_H_
+#define ABSL_STATUS_INTERNAL_STATUS_MATCHERS_H_
+
+#include <ostream>  // NOLINT
+#include <type_traits>
+#include <utility>
+
+#include "gmock/gmock.h"  // gmock_for_status_matchers.h
+#include "absl/base/config.h"
+#include "absl/base/macros.h"
+#include "absl/status/status.h"
+#include "absl/status/statusor.h"
+#include "absl/strings/string_view.h"
+
+namespace absl_testing {
+ABSL_NAMESPACE_BEGIN
+namespace status_internal {
+
+inline const absl::Status& GetStatus(const absl::Status& status) {
+  return status;
+}
+
+template <typename T>
+const absl::Status& GetStatus(const absl::StatusOr<T>& status) {
+  return status.status();
+}
+
+////////////////////////////////////////////////////////////
+// Implementation of IsOkAndHolds().
+
+// Monomorphic implementation of matcher IsOkAndHolds(m).  StatusOrType is a
+// reference to StatusOr<T>.
+template <typename StatusOrType>
+class IsOkAndHoldsMatcherImpl
+    : public ::testing::MatcherInterface<StatusOrType> {
+ public:
+  typedef typename std::remove_reference_t<StatusOrType>::value_type value_type;
+
+  template <typename InnerMatcher>
+  explicit IsOkAndHoldsMatcherImpl(InnerMatcher&& inner_matcher)
+      : inner_matcher_(::testing::SafeMatcherCast<const value_type&>(
+            std::forward<InnerMatcher>(inner_matcher))) {}
+
+  void DescribeTo(std::ostream* os) const override {
+    *os << "is OK and has a value that ";
+    inner_matcher_.DescribeTo(os);
+  }
+
+  void DescribeNegationTo(std::ostream* os) const override {
+    *os << "isn't OK or has a value that ";
+    inner_matcher_.DescribeNegationTo(os);
+  }
+
+  bool MatchAndExplain(
+      StatusOrType actual_value,
+      ::testing::MatchResultListener* result_listener) const override {
+    if (!actual_value.ok()) {
+      *result_listener << "which has status " << actual_value.status();
+      return false;
+    }
+
+    // Call through to the inner matcher.
+    return inner_matcher_.MatchAndExplain(*actual_value, result_listener);
+  }
+
+ private:
+  const ::testing::Matcher<const value_type&> inner_matcher_;
+};
+
+// Implements IsOkAndHolds(m) as a polymorphic matcher.
+template <typename InnerMatcher>
+class IsOkAndHoldsMatcher {
+ public:
+  explicit IsOkAndHoldsMatcher(InnerMatcher inner_matcher)
+      : inner_matcher_(std::forward<InnerMatcher>(inner_matcher)) {}
+
+  // Converts this polymorphic matcher to a monomorphic matcher of the
+  // given type.  StatusOrType can be either StatusOr<T> or a
+  // reference to StatusOr<T>.
+  template <typename StatusOrType>
+  operator ::testing::Matcher<StatusOrType>() const {  // NOLINT
+    return ::testing::Matcher<StatusOrType>(
+        new IsOkAndHoldsMatcherImpl<const StatusOrType&>(inner_matcher_));
+  }
+
+ private:
+  const InnerMatcher inner_matcher_;
+};
+
+////////////////////////////////////////////////////////////
+// Implementation of StatusIs().
+
+// `StatusCode` is implicitly convertible from `int`, `absl::StatusCode`, and
+//  is explicitly convertible to these types as well.
+//
+// We need this class because `absl::StatusCode` (as a scoped enum) is not
+// implicitly convertible to `int`. In order to handle use cases like
+// ```
+// StatusIs(Anyof(absl::StatusCode::kUnknown, absl::StatusCode::kCancelled))
+// ```
+// which uses polymorphic matchers, we need to unify the interfaces into
+// `Matcher<StatusCode>`.
+class StatusCode {
+ public:
+  /*implicit*/ StatusCode(int code)  // NOLINT
+      : code_(code) {}
+  /*implicit*/ StatusCode(::absl::StatusCode code)  // NOLINT
+      : code_(static_cast<int>(code)) {}
+
+  explicit operator int() const { return static_cast<int>(code_); }
+
+  friend void PrintTo(const StatusCode& code, std::ostream* os) {
+    absl::string_view text =
+        absl::StatusCodeToStringView(static_cast<absl::StatusCode>(code.code_));
+    if (!text.empty()) {
+      *os << text;
+    } else {
+      *os << code.code_;
+    }
+  }
+
+ private:
+  int code_;
+};
+
+// Relational operators to handle matchers like Eq, Lt, etc..
+inline bool operator==(const StatusCode& lhs, const StatusCode& rhs) {
+  return static_cast<int>(lhs) == static_cast<int>(rhs);
+}
+inline bool operator!=(const StatusCode& lhs, const StatusCode& rhs) {
+  return static_cast<int>(lhs) != static_cast<int>(rhs);
+}
+
+// StatusIs() is a polymorphic matcher.  This class is the common
+// implementation of it shared by all types T where StatusIs() can be
+// used as a Matcher<T>.
+class StatusIsMatcherCommonImpl {
+ public:
+  StatusIsMatcherCommonImpl(
+      ::testing::Matcher<StatusCode> code_matcher,
+      ::testing::Matcher<absl::string_view> message_matcher)
+      : code_matcher_(std::move(code_matcher)),
+        message_matcher_(std::move(message_matcher)) {}
+
+  void DescribeTo(std::ostream* os) const;
+
+  void DescribeNegationTo(std::ostream* os) const;
+
+  bool MatchAndExplain(const absl::Status& status,
+                       ::testing::MatchResultListener* result_listener) const;
+
+ private:
+  const ::testing::Matcher<StatusCode> code_matcher_;
+  const ::testing::Matcher<absl::string_view> message_matcher_;
+};
+
+// Monomorphic implementation of matcher StatusIs() for a given type
+// T.  T can be Status, StatusOr<>, or a reference to either of them.
+template <typename T>
+class MonoStatusIsMatcherImpl : public ::testing::MatcherInterface<T> {
+ public:
+  explicit MonoStatusIsMatcherImpl(StatusIsMatcherCommonImpl common_impl)
+      : common_impl_(std::move(common_impl)) {}
+
+  void DescribeTo(std::ostream* os) const override {
+    common_impl_.DescribeTo(os);
+  }
+
+  void DescribeNegationTo(std::ostream* os) const override {
+    common_impl_.DescribeNegationTo(os);
+  }
+
+  bool MatchAndExplain(
+      T actual_value,
+      ::testing::MatchResultListener* result_listener) const override {
+    return common_impl_.MatchAndExplain(GetStatus(actual_value),
+                                        result_listener);
+  }
+
+ private:
+  StatusIsMatcherCommonImpl common_impl_;
+};
+
+// Implements StatusIs() as a polymorphic matcher.
+class StatusIsMatcher {
+ public:
+  template <typename StatusCodeMatcher, typename StatusMessageMatcher>
+  StatusIsMatcher(StatusCodeMatcher&& code_matcher,
+                  StatusMessageMatcher&& message_matcher)
+      : common_impl_(::testing::MatcherCast<StatusCode>(
+                         std::forward<StatusCodeMatcher>(code_matcher)),
+                     ::testing::MatcherCast<absl::string_view>(
+                         std::forward<StatusMessageMatcher>(message_matcher))) {
+  }
+
+  // Converts this polymorphic matcher to a monomorphic matcher of the
+  // given type.  T can be StatusOr<>, Status, or a reference to
+  // either of them.
+  template <typename T>
+  /*implicit*/ operator ::testing::Matcher<T>() const {  // NOLINT
+    return ::testing::Matcher<T>(
+        new MonoStatusIsMatcherImpl<const T&>(common_impl_));
+  }
+
+ private:
+  const StatusIsMatcherCommonImpl common_impl_;
+};
+
+// Monomorphic implementation of matcher IsOk() for a given type T.
+// T can be Status, StatusOr<>, or a reference to either of them.
+template <typename T>
+class MonoIsOkMatcherImpl : public ::testing::MatcherInterface<T> {
+ public:
+  void DescribeTo(std::ostream* os) const override { *os << "is OK"; }
+  void DescribeNegationTo(std::ostream* os) const override {
+    *os << "is not OK";
+  }
+  bool MatchAndExplain(T actual_value,
+                       ::testing::MatchResultListener*) const override {
+    return GetStatus(actual_value).ok();
+  }
+};
+
+// Implements IsOk() as a polymorphic matcher.
+class IsOkMatcher {
+ public:
+  template <typename T>
+  /*implicit*/ operator ::testing::Matcher<T>() const {  // NOLINT
+    return ::testing::Matcher<T>(new MonoIsOkMatcherImpl<const T&>());
+  }
+};
+
+}  // namespace status_internal
+ABSL_NAMESPACE_END
+}  // namespace absl_testing
+
+#endif  // ABSL_STATUS_INTERNAL_STATUS_MATCHERS_H_
diff --git a/absl/status/internal/statusor_internal.h b/absl/status/internal/statusor_internal.h
new file mode 100644
index 00000000000..b0862339b09
--- /dev/null
+++ b/absl/status/internal/statusor_internal.h
@@ -0,0 +1,658 @@
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+#ifndef ABSL_STATUS_INTERNAL_STATUSOR_INTERNAL_H_
+#define ABSL_STATUS_INTERNAL_STATUSOR_INTERNAL_H_
+
+#include <cstdint>
+#include <type_traits>
+#include <utility>
+
+#include "absl/base/attributes.h"
+#include "absl/base/nullability.h"
+#include "absl/base/optimization.h"
+#include "absl/meta/type_traits.h"
+#include "absl/status/status.h"
+#include "absl/strings/string_view.h"
+#include "absl/utility/utility.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+template <typename T>
+class ABSL_MUST_USE_RESULT
+    StatusOr;
+
+namespace internal_statusor {
+
+// Detects whether `U` has conversion operator to `StatusOr<T>`, i.e. `operator
+// StatusOr<T>()`.
+template <typename T, typename U, typename = void>
+struct HasConversionOperatorToStatusOr : std::false_type {};
+
+template <typename T, typename U>
+void test(char (*absl_nullable)[sizeof(
+    std::declval<U>().operator absl::StatusOr<T>())]);
+
+template <typename T, typename U>
+struct HasConversionOperatorToStatusOr<T, U, decltype(test<T, U>(0))>
+    : std::true_type {};
+
+// Detects whether `T` is equality-comparable.
+template <typename T, typename = void>
+struct IsEqualityComparable : std::false_type {};
+
+template <typename T>
+struct IsEqualityComparable<
+    T, std::enable_if_t<std::is_convertible_v<
+           decltype(std::declval<T>() == std::declval<T>()), bool>>>
+    : std::true_type {};
+
+// Detects whether `T` is constructible or convertible from `StatusOr<U>`.
+template <typename T, typename U>
+using IsConstructibleOrConvertibleFromStatusOr =
+    std::disjunction<std::is_constructible<T, StatusOr<U>&>,
+                      std::is_constructible<T, const StatusOr<U>&>,
+                      std::is_constructible<T, StatusOr<U>&&>,
+                      std::is_constructible<T, const StatusOr<U>&&>,
+                      std::is_convertible<StatusOr<U>&, T>,
+                      std::is_convertible<const StatusOr<U>&, T>,
+                      std::is_convertible<StatusOr<U>&&, T>,
+                      std::is_convertible<const StatusOr<U>&&, T>>;
+
+// Detects whether `T` is constructible or convertible or assignable from
+// `StatusOr<U>`.
+template <typename T, typename U>
+using IsConstructibleOrConvertibleOrAssignableFromStatusOr =
+    std::disjunction<IsConstructibleOrConvertibleFromStatusOr<T, U>,
+                      std::is_assignable<T&, StatusOr<U>&>,
+                      std::is_assignable<T&, const StatusOr<U>&>,
+                      std::is_assignable<T&, StatusOr<U>&&>,
+                      std::is_assignable<T&, const StatusOr<U>&&>>;
+
+// Detects whether direct initializing `StatusOr<T>` from `U` is ambiguous, i.e.
+// when `U` is `StatusOr<V>` and `T` is constructible or convertible from `V`.
+template <typename T, typename U>
+struct IsDirectInitializationAmbiguous
+    : public std::conditional_t<
+          std::is_same_v<absl::remove_cvref_t<U>, U>, std::false_type,
+          IsDirectInitializationAmbiguous<T, absl::remove_cvref_t<U>>> {};
+
+template <typename T, typename V>
+struct IsDirectInitializationAmbiguous<T, absl::StatusOr<V>>
+    : public IsConstructibleOrConvertibleFromStatusOr<T, V> {};
+
+// Checks whether the conversion from U to T can be done without dangling
+// temporaries.
+// REQUIRES: T and U are references.
+template <typename T, typename U>
+using IsReferenceConversionValid = std::conjunction<  //
+    std::is_reference<T>, std::is_reference<U>,
+    // The references are convertible. This checks for
+    // lvalue/rvalue compatibility.
+    std::is_convertible<U, T>,
+    // The pointers are convertible. This checks we don't have
+    // a temporary.
+    std::is_convertible<std::remove_reference_t<U>*,
+                        std::remove_reference_t<T>*>>;
+
+// Checks against the constraints of the direction initialization, i.e. when
+// `StatusOr<T>::StatusOr(U&&)` should participate in overload resolution.
+template <typename T, typename U>
+using IsDirectInitializationValid = std::disjunction<
+    // Short circuits if T is basically U.
+    std::is_same<T, absl::remove_cvref_t<U>>,  //
+    std::conditional_t<
+        std::is_reference_v<T>,  //
+        IsReferenceConversionValid<T, U>,
+        std::negation<std::disjunction<
+            std::is_same<absl::StatusOr<T>, absl::remove_cvref_t<U>>,
+            std::is_same<absl::Status, absl::remove_cvref_t<U>>,
+            std::is_same<std::in_place_t, absl::remove_cvref_t<U>>,
+            IsDirectInitializationAmbiguous<T, U>>>>>;
+
+// This trait detects whether `StatusOr<T>::operator=(U&&)` is ambiguous, which
+// is equivalent to whether all the following conditions are met:
+// 1. `U` is `StatusOr<V>`.
+// 2. `T` is constructible and assignable from `V`.
+// 3. `T` is constructible and assignable from `U` (i.e. `StatusOr<V>`).
+// For example, the following code is considered ambiguous:
+// (`T` is `bool`, `U` is `StatusOr<bool>`, `V` is `bool`)
+//   StatusOr<bool> s1 = true;  // s1.ok() && s1.ValueOrDie() == true
+//   StatusOr<bool> s2 = false;  // s2.ok() && s2.ValueOrDie() == false
+//   s1 = s2;  // ambiguous, `s1 = s2.ValueOrDie()` or `s1 = bool(s2)`?
+template <typename T, typename U>
+struct IsForwardingAssignmentAmbiguous
+    : public std::conditional_t<
+          std::is_same_v<absl::remove_cvref_t<U>, U>, std::false_type,
+          IsForwardingAssignmentAmbiguous<T, absl::remove_cvref_t<U>>> {};
+
+template <typename T, typename U>
+struct IsForwardingAssignmentAmbiguous<T, absl::StatusOr<U>>
+    : public IsConstructibleOrConvertibleOrAssignableFromStatusOr<T, U> {};
+
+// Checks against the constraints of the forwarding assignment, i.e. whether
+// `StatusOr<T>::operator(U&&)` should participate in overload resolution.
+template <typename T, typename U>
+using IsForwardingAssignmentValid = std::disjunction<
+    // Short circuits if T is basically U.
+    std::is_same<T, absl::remove_cvref_t<U>>,
+    std::negation<std::disjunction<
+        std::is_same<absl::StatusOr<T>, absl::remove_cvref_t<U>>,
+        std::is_same<absl::Status, absl::remove_cvref_t<U>>,
+        std::is_same<std::in_place_t, absl::remove_cvref_t<U>>,
+        IsForwardingAssignmentAmbiguous<T, U>>>>;
+
+template <bool Value, typename T>
+using Equality = std::conditional_t<Value, T, std::negation<T>>;
+
+template <bool Explicit, typename T, typename U, bool Lifetimebound>
+using IsConstructionValid = std::conjunction<
+    Equality<Lifetimebound,
+             std::disjunction<
+                 std::is_reference<T>,
+                 type_traits_internal::IsLifetimeBoundAssignment<T, U>>>,
+    IsDirectInitializationValid<T, U&&>, std::is_constructible<T, U&&>,
+    Equality<!Explicit, std::is_convertible<U&&, T>>,
+    std::disjunction<
+        std::is_same<T, absl::remove_cvref_t<U>>,
+        std::conjunction<
+            std::conditional_t<
+                Explicit,
+                std::negation<std::is_constructible<absl::Status, U&&>>,
+                std::negation<std::is_convertible<U&&, absl::Status>>>,
+            std::negation<
+                internal_statusor::HasConversionOperatorToStatusOr<T, U&&>>>>>;
+
+template <typename T, typename U, bool Lifetimebound>
+using IsAssignmentValid = std::conjunction<
+    Equality<Lifetimebound,
+             std::disjunction<
+                 std::is_reference<T>,
+                 type_traits_internal::IsLifetimeBoundAssignment<T, U>>>,
+    std::conditional_t<std::is_reference_v<T>,
+                       IsReferenceConversionValid<T, U&&>,
+                       std::conjunction<std::is_constructible<T, U&&>,
+                                         std::is_assignable<T&, U&&>>>,
+    std::disjunction<
+        std::is_same<T, absl::remove_cvref_t<U>>,
+        std::conjunction<
+            std::negation<std::is_convertible<U&&, absl::Status>>,
+            std::negation<HasConversionOperatorToStatusOr<T, U&&>>>>,
+    IsForwardingAssignmentValid<T, U&&>>;
+
+template <bool Explicit, typename T, typename U>
+using IsConstructionFromStatusValid = std::conjunction<
+    std::negation<std::is_same<absl::StatusOr<T>, absl::remove_cvref_t<U>>>,
+    std::negation<std::is_same<T, absl::remove_cvref_t<U>>>,
+    std::negation<std::is_same<std::in_place_t, absl::remove_cvref_t<U>>>,
+    Equality<!Explicit, std::is_convertible<U, absl::Status>>,
+    std::is_constructible<absl::Status, U>,
+    std::negation<HasConversionOperatorToStatusOr<T, U>>>;
+
+template <bool Explicit, typename T, typename U, bool Lifetimebound,
+          typename UQ>
+using IsConstructionFromStatusOrValid = std::conjunction<
+    std::negation<std::is_same<T, U>>,
+    // If `T` is a reference, then U must be a compatible one.
+    std::disjunction<std::negation<std::is_reference<T>>,
+                      IsReferenceConversionValid<T, U>>,
+    Equality<Lifetimebound,
+             type_traits_internal::IsLifetimeBoundAssignment<T, U>>,
+    std::is_constructible<T, UQ>,
+    Equality<!Explicit, std::is_convertible<UQ, T>>,
+    std::negation<IsConstructibleOrConvertibleFromStatusOr<T, U>>>;
+
+template <typename T, typename U, bool Lifetimebound>
+using IsStatusOrAssignmentValid = std::conjunction<
+    std::negation<std::is_same<T, absl::remove_cvref_t<U>>>,
+    Equality<Lifetimebound,
+             type_traits_internal::IsLifetimeBoundAssignment<T, U>>,
+    std::is_constructible<T, U>, std::is_assignable<T, U>,
+    std::negation<IsConstructibleOrConvertibleOrAssignableFromStatusOr<
+        T, absl::remove_cvref_t<U>>>>;
+
+template <typename T, typename U, bool Lifetimebound>
+using IsValueOrValid = std::conjunction<
+    // If `T` is a reference, then U must be a compatible one.
+    std::disjunction<std::negation<std::is_reference<T>>,
+                      IsReferenceConversionValid<T, U>>,
+    Equality<Lifetimebound,
+             std::disjunction<
+                 std::is_reference<T>,
+                 type_traits_internal::IsLifetimeBoundAssignment<T, U>>>>;
+
+class Helper {
+ public:
+  // Move type-agnostic error handling to the .cc.
+  static void HandleInvalidStatusCtorArg(Status* absl_nonnull);
+  [[noreturn]] static void Crash(const absl::Status& status);
+};
+
+// Construct an instance of T in `p` through placement new, passing Args... to
+// the constructor.
+// This abstraction is here mostly for the gcc performance fix.
+template <typename T, typename... Args>
+ABSL_ATTRIBUTE_NONNULL(1)
+void PlacementNew(void* absl_nonnull p, Args&&... args) {
+  new (p) T(std::forward<Args>(args)...);
+}
+
+template <typename T>
+class Reference {
+ public:
+  constexpr explicit Reference(T ref ABSL_ATTRIBUTE_LIFETIME_BOUND)
+      : payload_(std::addressof(ref)) {}
+
+  Reference(const Reference&) = default;
+  Reference& operator=(const Reference&) = default;
+  Reference& operator=(T value) {
+    payload_ = std::addressof(value);
+    return *this;
+  }
+
+  operator T() const { return static_cast<T>(*payload_); }  // NOLINT
+  T get() const { return *this; }
+
+ private:
+  std::remove_reference_t<T>* absl_nonnull payload_;
+};
+
+// Helper base class to hold the data and all operations.
+// We move all this to a base class to allow mixing with the appropriate
+// TraitsBase specialization.
+template <typename T>
+class StatusOrData {
+  template <typename U>
+  friend class StatusOrData;
+
+  decltype(auto) MaybeMoveData() {
+    if constexpr (std::is_reference_v<T>) {
+      return data_.get();
+    } else {
+      return std::move(data_);
+    }
+  }
+
+ public:
+  StatusOrData() = delete;
+
+  StatusOrData(const StatusOrData& other) {
+    if (other.ok()) {
+      MakeValue(other.data_);
+      MakeStatus();
+    } else {
+      MakeStatus(other.status_);
+    }
+  }
+
+  StatusOrData(StatusOrData&& other) noexcept {
+    if (other.ok()) {
+      MakeValue(other.MaybeMoveData());
+      MakeStatus();
+    } else {
+      MakeStatus(std::move(other.status_));
+    }
+  }
+
+  template <typename U>
+  explicit StatusOrData(const StatusOrData<U>& other) {
+    if (other.ok()) {
+      MakeValue(other.data_);
+      MakeStatus();
+    } else {
+      MakeStatus(other.status_);
+    }
+  }
+
+  template <typename U>
+  explicit StatusOrData(StatusOrData<U>&& other) {
+    if (other.ok()) {
+      MakeValue(other.MaybeMoveData());
+      MakeStatus();
+    } else {
+      MakeStatus(std::move(other.status_));
+    }
+  }
+
+  template <typename... Args>
+  explicit StatusOrData(std::in_place_t, Args&&... args)
+      : data_(std::forward<Args>(args)...) {
+    MakeStatus();
+  }
+
+  template <
+      typename U,
+      std::enable_if_t<std::is_constructible_v<absl::Status, U&&>, int> = 0>
+  explicit StatusOrData(U&& v) : status_(std::forward<U>(v)) {
+    EnsureNotOk();
+  }
+
+  StatusOrData& operator=(const StatusOrData& other) {
+    if (this == &other) return *this;
+    if (other.ok())
+      Assign(other.data_);
+    else
+      AssignStatus(other.status_);
+    return *this;
+  }
+
+  StatusOrData& operator=(StatusOrData&& other) {
+    if (this == &other) return *this;
+    if (other.ok())
+      Assign(other.MaybeMoveData());
+    else
+      AssignStatus(std::move(other.status_));
+    return *this;
+  }
+
+  ~StatusOrData() {
+    if (ok()) {
+      status_.~Status();
+      if constexpr (!std::is_trivially_destructible_v<T>) {
+        data_.~T();
+      }
+    } else {
+      status_.~Status();
+    }
+  }
+
+  template <typename U>
+  void Assign(U&& value) {
+    if (ok()) {
+      data_ = std::forward<U>(value);
+    } else {
+      MakeValue(std::forward<U>(value));
+      status_ = OkStatus();
+    }
+  }
+
+  template <typename U>
+  void AssignStatus(U&& v) {
+    Clear();
+    status_ = static_cast<absl::Status>(std::forward<U>(v));
+    EnsureNotOk();
+  }
+
+  bool ok() const { return status_.ok(); }
+
+ protected:
+  // status_ will always be active after the constructor.
+  // We make it a union to be able to initialize exactly how we need without
+  // waste.
+  // Eg. in the copy constructor we use the default constructor of Status in
+  // the ok() path to avoid an extra Ref call.
+  union {
+    Status status_;
+  };
+
+  // data_ is active iff status_.ok()==true
+  struct Dummy {};
+  union {
+    // When T is const, we need some non-const object we can cast to void* for
+    // the placement new. dummy_ is that object.
+    Dummy dummy_;
+    std::conditional_t<std::is_reference_v<T>, Reference<T>, T> data_;
+  };
+
+  void Clear() {
+    if constexpr (!std::is_trivially_destructible_v<T>) {
+      if (ok()) data_.~T();
+    }
+  }
+
+  void EnsureOk() const {
+    if (ABSL_PREDICT_FALSE(!ok())) Helper::Crash(status_);
+  }
+
+  void EnsureNotOk() {
+    if (ABSL_PREDICT_FALSE(ok())) Helper::HandleInvalidStatusCtorArg(&status_);
+  }
+
+  // Construct the value (ie. data_) through placement new with the passed
+  // argument.
+  template <typename... Arg>
+  void MakeValue(Arg&&... arg) {
+    internal_statusor::PlacementNew<decltype(data_)>(&dummy_,
+                                                     std::forward<Arg>(arg)...);
+  }
+
+  // Construct the status (ie. status_) through placement new with the passed
+  // argument.
+  template <typename... Args>
+  void MakeStatus(Args&&... args) {
+    internal_statusor::PlacementNew<Status>(&status_,
+                                            std::forward<Args>(args)...);
+  }
+
+  template <typename U>
+  T ValueOrImpl(U&& default_value) const& {
+    if (ok()) {
+      return data_;
+    }
+    return std::forward<U>(default_value);
+  }
+
+  template <typename U>
+  T ValueOrImpl(U&& default_value) && {
+    if (ok()) {
+      return std::move(data_);
+    }
+    return std::forward<U>(default_value);
+  }
+};
+
+[[noreturn]] void ThrowBadStatusOrAccess(absl::Status status);
+
+template <typename T>
+struct OperatorBase {
+  auto& self() const { return static_cast<const StatusOr<T>&>(*this); }
+  auto& self() { return static_cast<StatusOr<T>&>(*this); }
+
+  const T& operator*() const& ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    self().EnsureOk();
+    return self().data_;
+  }
+  T& operator*() & ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    self().EnsureOk();
+    return self().data_;
+  }
+  const T&& operator*() const&& ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    self().EnsureOk();
+    return std::move(self().data_);
+  }
+  T&& operator*() && ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    self().EnsureOk();
+    return std::move(self().data_);
+  }
+
+  const T& value() const& ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    if (!self().ok()) internal_statusor::ThrowBadStatusOrAccess(self().status_);
+    return self().data_;
+  }
+  T& value() & ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    if (!self().ok()) internal_statusor::ThrowBadStatusOrAccess(self().status_);
+    return self().data_;
+  }
+  const T&& value() const&& ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    if (!self().ok()) {
+      internal_statusor::ThrowBadStatusOrAccess(std::move(self().status_));
+    }
+    return std::move(self().data_);
+  }
+  T&& value() && ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    if (!self().ok()) {
+      internal_statusor::ThrowBadStatusOrAccess(std::move(self().status_));
+    }
+    return std::move(self().data_);
+  }
+
+  const T* absl_nonnull operator->() const ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return std::addressof(**this);
+  }
+  T* absl_nonnull operator->() ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return std::addressof(**this);
+  }
+};
+
+template <typename T>
+struct OperatorBase<T&> {
+  auto& self() const { return static_cast<const StatusOr<T&>&>(*this); }
+
+  T& operator*() const {
+    self().EnsureOk();
+    return self().data_;
+  }
+
+  T& value() const {
+    if (!self().ok()) internal_statusor::ThrowBadStatusOrAccess(self().status_);
+    return self().data_;
+  }
+
+  T* absl_nonnull operator->() const {
+    return std::addressof(**this);
+  }
+};
+
+// Helper base classes to allow implicitly deleted constructors and assignment
+// operators in `StatusOr`. For example, `CopyCtorBase` will explicitly delete
+// the copy constructor when T is not copy constructible and `StatusOr` will
+// inherit that behavior implicitly.
+template <typename T, bool = std::is_copy_constructible_v<T>>
+struct CopyCtorBase {
+  CopyCtorBase() = default;
+  CopyCtorBase(const CopyCtorBase&) = default;
+  CopyCtorBase(CopyCtorBase&&) = default;
+  CopyCtorBase& operator=(const CopyCtorBase&) = default;
+  CopyCtorBase& operator=(CopyCtorBase&&) = default;
+};
+
+template <typename T>
+struct CopyCtorBase<T, false> {
+  CopyCtorBase() = default;
+  CopyCtorBase(const CopyCtorBase&) = delete;
+  CopyCtorBase(CopyCtorBase&&) = default;
+  CopyCtorBase& operator=(const CopyCtorBase&) = default;
+  CopyCtorBase& operator=(CopyCtorBase&&) = default;
+};
+
+template <typename T, bool = std::is_move_constructible_v<T>>
+struct MoveCtorBase {
+  MoveCtorBase() = default;
+  MoveCtorBase(const MoveCtorBase&) = default;
+  MoveCtorBase(MoveCtorBase&&) = default;
+  MoveCtorBase& operator=(const MoveCtorBase&) = default;
+  MoveCtorBase& operator=(MoveCtorBase&&) = default;
+};
+
+template <typename T>
+struct MoveCtorBase<T, false> {
+  MoveCtorBase() = default;
+  MoveCtorBase(const MoveCtorBase&) = default;
+  MoveCtorBase(MoveCtorBase&&) = delete;
+  MoveCtorBase& operator=(const MoveCtorBase&) = default;
+  MoveCtorBase& operator=(MoveCtorBase&&) = default;
+};
+
+template <typename T, bool = (std::is_copy_constructible_v<T> &&
+                              std::is_copy_assignable_v<T>) ||
+                             std::is_reference_v<T>>
+struct CopyAssignBase {
+  CopyAssignBase() = default;
+  CopyAssignBase(const CopyAssignBase&) = default;
+  CopyAssignBase(CopyAssignBase&&) = default;
+  CopyAssignBase& operator=(const CopyAssignBase&) = default;
+  CopyAssignBase& operator=(CopyAssignBase&&) = default;
+};
+
+template <typename T>
+struct CopyAssignBase<T, false> {
+  CopyAssignBase() = default;
+  CopyAssignBase(const CopyAssignBase&) = default;
+  CopyAssignBase(CopyAssignBase&&) = default;
+  CopyAssignBase& operator=(const CopyAssignBase&) = delete;
+  CopyAssignBase& operator=(CopyAssignBase&&) = default;
+};
+
+template <typename T, bool = (std::is_move_constructible_v<T> &&
+                              std::is_move_assignable_v<T>) ||
+                             std::is_reference_v<T>>
+struct MoveAssignBase {
+  MoveAssignBase() = default;
+  MoveAssignBase(const MoveAssignBase&) = default;
+  MoveAssignBase(MoveAssignBase&&) = default;
+  MoveAssignBase& operator=(const MoveAssignBase&) = default;
+  MoveAssignBase& operator=(MoveAssignBase&&) = default;
+};
+
+template <typename T>
+struct MoveAssignBase<T, false> {
+  MoveAssignBase() = default;
+  MoveAssignBase(const MoveAssignBase&) = default;
+  MoveAssignBase(MoveAssignBase&&) = default;
+  MoveAssignBase& operator=(const MoveAssignBase&) = default;
+  MoveAssignBase& operator=(MoveAssignBase&&) = delete;
+};
+
+// Used to introduce jitter into the output of printing functions for
+// `StatusOr` (i.e. `AbslStringify` and `operator<<`).
+class StringifyRandom {
+  enum BracesType {
+    kBareParens = 0,
+    kSpaceParens,
+    kBareBrackets,
+    kSpaceBrackets,
+  };
+
+  // Returns a random `BracesType` determined once per binary load.
+  static BracesType RandomBraces() {
+    static const BracesType kRandomBraces = static_cast<BracesType>(
+        (reinterpret_cast<uintptr_t>(&kRandomBraces) >> 4) % 4);
+    return kRandomBraces;
+  }
+
+ public:
+  static absl::string_view OpenBrackets() {
+    switch (RandomBraces()) {
+      case kBareParens:
+        return "(";
+      case kSpaceParens:
+        return "( ";
+      case kBareBrackets:
+        return "[";
+      case kSpaceBrackets:
+        return "[ ";
+    }
+    return "(";
+  }
+
+  static absl::string_view CloseBrackets() {
+    switch (RandomBraces()) {
+      case kBareParens:
+        return ")";
+      case kSpaceParens:
+        return " )";
+      case kBareBrackets:
+        return "]";
+      case kSpaceBrackets:
+        return " ]";
+    }
+    return ")";
+  }
+};
+
+}  // namespace internal_statusor
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STATUS_INTERNAL_STATUSOR_INTERNAL_H_
diff --git a/absl/status/status.cc b/absl/status/status.cc
new file mode 100644
index 00000000000..80e84791f4b
--- /dev/null
+++ b/absl/status/status.cc
@@ -0,0 +1,464 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+#include "absl/status/status.h"
+
+#include <errno.h>
+
+#include <cstdint>
+#include <memory>
+#include <ostream>
+#include <string>
+#include <type_traits>
+#include <utility>
+
+#include "absl/base/config.h"
+#include "absl/base/internal/strerror.h"
+#include "absl/base/no_destructor.h"
+#include "absl/base/nullability.h"
+#include "absl/status/internal/status_internal.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/string_view.h"
+#include "absl/types/source_location.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+static_assert(
+    alignof(status_internal::StatusRep) >= 4,
+    "absl::Status assumes it can use the bottom 2 bits of a StatusRep*.");
+
+std::string StatusCodeToString(StatusCode code) {
+  return std::string(absl::StatusCodeToStringView(code));
+}
+
+absl::string_view StatusCodeToStringView(StatusCode code) {
+  switch (code) {
+    case StatusCode::kOk:
+      return "OK";
+    case StatusCode::kCancelled:
+      return "CANCELLED";
+    case StatusCode::kUnknown:
+      return "UNKNOWN";
+    case StatusCode::kInvalidArgument:
+      return "INVALID_ARGUMENT";
+    case StatusCode::kDeadlineExceeded:
+      return "DEADLINE_EXCEEDED";
+    case StatusCode::kNotFound:
+      return "NOT_FOUND";
+    case StatusCode::kAlreadyExists:
+      return "ALREADY_EXISTS";
+    case StatusCode::kPermissionDenied:
+      return "PERMISSION_DENIED";
+    case StatusCode::kUnauthenticated:
+      return "UNAUTHENTICATED";
+    case StatusCode::kResourceExhausted:
+      return "RESOURCE_EXHAUSTED";
+    case StatusCode::kFailedPrecondition:
+      return "FAILED_PRECONDITION";
+    case StatusCode::kAborted:
+      return "ABORTED";
+    case StatusCode::kOutOfRange:
+      return "OUT_OF_RANGE";
+    case StatusCode::kUnimplemented:
+      return "UNIMPLEMENTED";
+    case StatusCode::kInternal:
+      return "INTERNAL";
+    case StatusCode::kUnavailable:
+      return "UNAVAILABLE";
+    case StatusCode::kDataLoss:
+      return "DATA_LOSS";
+    default:
+      return "";
+  }
+}
+
+std::ostream& operator<<(std::ostream& os, StatusCode code) {
+  return os << StatusCodeToString(code);
+}
+
+const std::string* absl_nonnull Status::EmptyString() {
+  static const absl::NoDestructor<std::string> kEmpty;
+  return kEmpty.get();
+}
+
+const std::string* absl_nonnull Status::MovedFromString() {
+  static const absl::NoDestructor<std::string> kMovedFrom(kMovedFromString);
+  return kMovedFrom.get();
+}
+
+absl::Status absl::Status::MakeNonOkStatusWithOkCode(
+    absl::string_view message) {
+  return absl::Status(
+      absl::Status::PointerToRep(new absl::status_internal::StatusRep(
+          absl::StatusCode::kOk, message, nullptr)));
+}
+
+template <typename StringOrView>
+uintptr_t MakeStatusRepImpl(uintptr_t inlined_rep, StringOrView msg,
+                            absl::SourceLocation loc) {
+  static_assert(std::is_same_v<StringOrView, absl::string_view> ||
+                std::is_same_v<StringOrView, std::string&&>);
+  bool ok = inlined_rep == Status::CodeToInlinedRep(absl::StatusCode::kOk);
+  if (ok) return inlined_rep;
+  if (msg.empty()
+  ) {
+    return inlined_rep;
+  }
+  auto* rep =
+      new status_internal::StatusRep(Status::InlinedRepToCode(inlined_rep),
+                                     std::forward<StringOrView>(msg), nullptr);
+  if (loc.file_name()[0] != '\0') {
+    rep->AddSourceLocation(loc);
+  }
+  return Status::PointerToRep(rep);
+}
+
+uintptr_t Status::MakeRepFromStringView(uintptr_t inlined_rep,
+                                        absl::string_view msg,
+                                        absl::SourceLocation loc) {
+  return MakeStatusRepImpl<absl::string_view>(inlined_rep, msg, loc);
+}
+
+#ifndef SWIG
+uintptr_t Status::MakeRepFromStringRvalue(uintptr_t inlined_rep,
+                                          std::string&& msg,
+                                          absl::SourceLocation loc) {
+  return MakeStatusRepImpl<std::string&&>(inlined_rep, std::move(msg), loc);
+}
+#endif  // SWIG
+
+uintptr_t Status::AddSourceLocationImpl(uintptr_t rep,
+                                        absl::SourceLocation loc) {
+  if (IsInlined(rep)) return rep;
+  if (loc.file_name()[0] == '\0') return rep;
+  status_internal::StatusRep* rep_ptr = PrepareToModify(rep);
+  rep_ptr->AddSourceLocation(loc);
+  return PointerToRep(rep_ptr);
+}
+
+uintptr_t Status::WithContextImpl(uintptr_t rep, absl::string_view context) {
+  if (context.empty()) return rep;
+  status_internal::StatusRep* rep_ptr = PrepareToModify(rep);
+  if (rep_ptr->message_.empty()) {
+    rep_ptr->message_ = std::string(context);
+  } else {
+    absl::StrAppend(&rep_ptr->message_, "; ", context);
+  }
+  return PointerToRep(rep_ptr);
+}
+
+status_internal::StatusRep* absl_nonnull Status::PrepareToModify(
+    uintptr_t rep) {
+  if (IsInlined(rep)) {
+    return new status_internal::StatusRep(InlinedRepToCode(rep),
+                                          absl::string_view(), nullptr);
+  }
+  return RepToPointer(rep)->CloneAndUnref();
+}
+
+std::string Status::ToStringSlow(uintptr_t rep, StatusToStringMode mode) {
+  if (IsInlined(rep)) {
+    return absl::StrCat(absl::StatusCodeToString(InlinedRepToCode(rep)), ": ");
+  }
+  return RepToPointer(rep)->ToString(mode);
+}
+
+std::ostream& operator<<(std::ostream& os, const Status& x) {
+  os << x.ToString(StatusToStringMode::kWithEverything);
+  return os;
+}
+
+namespace status_internal {
+// We use an int in the template parameter to shorten mangled names.
+template <int error_code>
+Status MakeErrorStringViewImpl(string_view message, SourceLocation loc) {
+  return Status(static_cast<StatusCode>(error_code), message, loc);
+}
+
+// Explicit instantiation for all the error codes.
+// If we add more error code, we need to add their values on this list.
+// Using ints here instead of static_cast<int>(StatusCode::kFoo) makes it easier
+// to see that the list is complete.
+template Status MakeErrorStringViewImpl<1>(string_view, SourceLocation);
+template Status MakeErrorStringViewImpl<2>(string_view, SourceLocation);
+template Status MakeErrorStringViewImpl<3>(string_view, SourceLocation);
+template Status MakeErrorStringViewImpl<4>(string_view, SourceLocation);
+template Status MakeErrorStringViewImpl<5>(string_view, SourceLocation);
+template Status MakeErrorStringViewImpl<6>(string_view, SourceLocation);
+template Status MakeErrorStringViewImpl<7>(string_view, SourceLocation);
+template Status MakeErrorStringViewImpl<8>(string_view, SourceLocation);
+template Status MakeErrorStringViewImpl<9>(string_view, SourceLocation);
+template Status MakeErrorStringViewImpl<10>(string_view, SourceLocation);
+template Status MakeErrorStringViewImpl<11>(string_view, SourceLocation);
+template Status MakeErrorStringViewImpl<12>(string_view, SourceLocation);
+template Status MakeErrorStringViewImpl<13>(string_view, SourceLocation);
+template Status MakeErrorStringViewImpl<14>(string_view, SourceLocation);
+template Status MakeErrorStringViewImpl<15>(string_view, SourceLocation);
+template Status MakeErrorStringViewImpl<16>(string_view, SourceLocation);
+
+// Same as above, but for rvalue strings.
+#ifndef SWIG
+// We use an int in the template parameter to shorten mangled names.
+template <int error_code>
+Status MakeErrorStringRvalueImpl(std::string&& message, SourceLocation loc) {
+  return Status(static_cast<StatusCode>(error_code), std::move(message), loc);
+}
+
+// Explicit instantiation for all the error codes.
+// If we add more error code, we need to add their values on this list.
+// Using ints here instead of static_cast<int>(StatusCode::kFoo) makes it easier
+// to see that the list is complete.
+template Status MakeErrorStringRvalueImpl<1>(std::string&&, SourceLocation);
+template Status MakeErrorStringRvalueImpl<2>(std::string&&, SourceLocation);
+template Status MakeErrorStringRvalueImpl<3>(std::string&&, SourceLocation);
+template Status MakeErrorStringRvalueImpl<4>(std::string&&, SourceLocation);
+template Status MakeErrorStringRvalueImpl<5>(std::string&&, SourceLocation);
+template Status MakeErrorStringRvalueImpl<6>(std::string&&, SourceLocation);
+template Status MakeErrorStringRvalueImpl<7>(std::string&&, SourceLocation);
+template Status MakeErrorStringRvalueImpl<8>(std::string&&, SourceLocation);
+template Status MakeErrorStringRvalueImpl<9>(std::string&&, SourceLocation);
+template Status MakeErrorStringRvalueImpl<10>(std::string&&, SourceLocation);
+template Status MakeErrorStringRvalueImpl<11>(std::string&&, SourceLocation);
+template Status MakeErrorStringRvalueImpl<12>(std::string&&, SourceLocation);
+template Status MakeErrorStringRvalueImpl<13>(std::string&&, SourceLocation);
+template Status MakeErrorStringRvalueImpl<14>(std::string&&, SourceLocation);
+template Status MakeErrorStringRvalueImpl<15>(std::string&&, SourceLocation);
+template Status MakeErrorStringRvalueImpl<16>(std::string&&, SourceLocation);
+#endif  // SWIG
+
+}  // namespace status_internal
+
+bool IsAborted(const Status& status) {
+  return status.code() == absl::StatusCode::kAborted;
+}
+
+bool IsAlreadyExists(const Status& status) {
+  return status.code() == absl::StatusCode::kAlreadyExists;
+}
+
+bool IsCancelled(const Status& status) {
+  return status.code() == absl::StatusCode::kCancelled;
+}
+
+bool IsDataLoss(const Status& status) {
+  return status.code() == absl::StatusCode::kDataLoss;
+}
+
+bool IsDeadlineExceeded(const Status& status) {
+  return status.code() == absl::StatusCode::kDeadlineExceeded;
+}
+
+bool IsFailedPrecondition(const Status& status) {
+  return status.code() == absl::StatusCode::kFailedPrecondition;
+}
+
+bool IsInternal(const Status& status) {
+  return status.code() == absl::StatusCode::kInternal;
+}
+
+bool IsInvalidArgument(const Status& status) {
+  return status.code() == absl::StatusCode::kInvalidArgument;
+}
+
+bool IsNotFound(const Status& status) {
+  return status.code() == absl::StatusCode::kNotFound;
+}
+
+bool IsOutOfRange(const Status& status) {
+  return status.code() == absl::StatusCode::kOutOfRange;
+}
+
+bool IsPermissionDenied(const Status& status) {
+  return status.code() == absl::StatusCode::kPermissionDenied;
+}
+
+bool IsResourceExhausted(const Status& status) {
+  return status.code() == absl::StatusCode::kResourceExhausted;
+}
+
+bool IsUnauthenticated(const Status& status) {
+  return status.code() == absl::StatusCode::kUnauthenticated;
+}
+
+bool IsUnavailable(const Status& status) {
+  return status.code() == absl::StatusCode::kUnavailable;
+}
+
+bool IsUnimplemented(const Status& status) {
+  return status.code() == absl::StatusCode::kUnimplemented;
+}
+
+bool IsUnknown(const Status& status) {
+  return status.code() == absl::StatusCode::kUnknown;
+}
+
+StatusCode ErrnoToStatusCode(int error_number) {
+  switch (error_number) {
+    case 0:
+      return StatusCode::kOk;
+    case EINVAL:        // Invalid argument
+    case ENAMETOOLONG:  // Filename too long
+    case E2BIG:         // Argument list too long
+    case EDESTADDRREQ:  // Destination address required
+    case EDOM:          // Mathematics argument out of domain of function
+    case EFAULT:        // Bad address
+    case EILSEQ:        // Illegal byte sequence
+    case ENOPROTOOPT:   // Protocol not available
+    case ENOTSOCK:      // Not a socket
+    case ENOTTY:        // Inappropriate I/O control operation
+    case EPROTOTYPE:    // Protocol wrong type for socket
+    case ESPIPE:        // Invalid seek
+      return StatusCode::kInvalidArgument;
+    case ETIMEDOUT:  // Connection timed out
+      return StatusCode::kDeadlineExceeded;
+    case ENODEV:  // No such device
+    case ENOENT:  // No such file or directory
+#ifdef ENOMEDIUM
+    case ENOMEDIUM:  // No medium found
+#endif
+    case ENXIO:  // No such device or address
+    case ESRCH:  // No such process
+      return StatusCode::kNotFound;
+    case EEXIST:         // File exists
+    case EADDRNOTAVAIL:  // Address not available
+    case EALREADY:       // Connection already in progress
+#ifdef ENOTUNIQ
+    case ENOTUNIQ:  // Name not unique on network
+#endif
+      return StatusCode::kAlreadyExists;
+    case EPERM:   // Operation not permitted
+    case EACCES:  // Permission denied
+#ifdef ENOKEY
+    case ENOKEY:  // Required key not available
+#endif
+    case EROFS:  // Read only file system
+      return StatusCode::kPermissionDenied;
+    case ENOTEMPTY:   // Directory not empty
+    case EISDIR:      // Is a directory
+    case ENOTDIR:     // Not a directory
+    case EADDRINUSE:  // Address already in use
+    case EBADF:       // Invalid file descriptor
+#ifdef EBADFD
+    case EBADFD:  // File descriptor in bad state
+#endif
+    case EBUSY:    // Device or resource busy
+    case ECHILD:   // No child processes
+    case EISCONN:  // Socket is connected
+#ifdef EISNAM
+    case EISNAM:  // Is a named type file
+#endif
+#ifdef ENOTBLK
+    case ENOTBLK:  // Block device required
+#endif
+    case ENOTCONN:  // The socket is not connected
+    case EPIPE:     // Broken pipe
+#ifdef ESHUTDOWN
+    case ESHUTDOWN:  // Cannot send after transport endpoint shutdown
+#endif
+    case ETXTBSY:  // Text file busy
+#ifdef EUNATCH
+    case EUNATCH:  // Protocol driver not attached
+#endif
+      return StatusCode::kFailedPrecondition;
+    case ENOSPC:  // No space left on device
+#ifdef EDQUOT
+    case EDQUOT:  // Disk quota exceeded
+#endif
+    case EMFILE:   // Too many open files
+    case EMLINK:   // Too many links
+    case ENFILE:   // Too many open files in system
+    case ENOBUFS:  // No buffer space available
+    case ENOMEM:   // Not enough space
+#ifdef EUSERS
+    case EUSERS:  // Too many users
+#endif
+      return StatusCode::kResourceExhausted;
+#ifdef ECHRNG
+    case ECHRNG:  // Channel number out of range
+#endif
+    case EFBIG:      // File too large
+    case EOVERFLOW:  // Value too large to be stored in data type
+    case ERANGE:     // Result too large
+      return StatusCode::kOutOfRange;
+#ifdef ENOPKG
+    case ENOPKG:  // Package not installed
+#endif
+    case ENOSYS:        // Function not implemented
+    case ENOTSUP:       // Operation not supported
+    case EAFNOSUPPORT:  // Address family not supported
+#ifdef EPFNOSUPPORT
+    case EPFNOSUPPORT:  // Protocol family not supported
+#endif
+    case EPROTONOSUPPORT:  // Protocol not supported
+#ifdef ESOCKTNOSUPPORT
+    case ESOCKTNOSUPPORT:  // Socket type not supported
+#endif
+    case EXDEV:  // Improper link
+      return StatusCode::kUnimplemented;
+    case EAGAIN:  // Resource temporarily unavailable
+#ifdef ECOMM
+    case ECOMM:  // Communication error on send
+#endif
+    case ECONNREFUSED:  // Connection refused
+    case ECONNABORTED:  // Connection aborted
+    case ECONNRESET:    // Connection reset
+    case EINTR:         // Interrupted function call
+#ifdef EHOSTDOWN
+    case EHOSTDOWN:  // Host is down
+#endif
+    case EHOSTUNREACH:  // Host is unreachable
+    case ENETDOWN:      // Network is down
+    case ENETRESET:     // Connection aborted by network
+    case ENETUNREACH:   // Network unreachable
+    case ENOLCK:        // No locks available
+    case ENOLINK:       // Link has been severed
+#ifdef ENONET
+    case ENONET:  // Machine is not on the network
+#endif
+      return StatusCode::kUnavailable;
+    case EDEADLK:  // Resource deadlock avoided
+#ifdef ESTALE
+    case ESTALE:  // Stale file handle
+#endif
+      return StatusCode::kAborted;
+    case ECANCELED:  // Operation cancelled
+      return StatusCode::kCancelled;
+    default:
+      return StatusCode::kUnknown;
+  }
+}
+
+namespace {
+std::string MessageForErrnoToStatus(int error_number,
+                                    absl::string_view message) {
+  return absl::StrCat(message, ": ",
+                      absl::base_internal::StrError(error_number));
+}
+}  // namespace
+
+Status ErrnoToStatus(int error_number, absl::string_view message,
+                     absl::SourceLocation loc) {
+  return Status(ErrnoToStatusCode(error_number),
+                MessageForErrnoToStatus(error_number, message), loc);
+}
+
+const char* absl_nonnull StatusMessageAsCStr(const Status& status) {
+  // As an internal implementation detail, we guarantee that if status.message()
+  // is non-empty, then the resulting string_view is null terminated.
+  auto sv_message = status.message();
+  return sv_message.empty() ? "" : sv_message.data();
+}
+
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/status/status.h b/absl/status/status.h
new file mode 100644
index 00000000000..4158de1c8b3
--- /dev/null
+++ b/absl/status/status.h
@@ -0,0 +1,1612 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: status.h
+// -----------------------------------------------------------------------------
+//
+// This header file defines the Abseil `status` library, consisting of:
+//
+//   * An `absl::Status` class for holding error handling information
+//   * A set of canonical `absl::StatusCode` error codes, and associated
+//     utilities for generating and propagating status codes.
+//   * A set of helper functions for creating status codes and checking their
+//     values
+//
+// Within Google, `absl::Status` is the primary mechanism for communicating
+// errors in C++, and is used to represent error state in both in-process
+// library calls as well as RPC calls. Some of these errors may be recoverable,
+// but others may not. Most functions that can produce a recoverable error
+// should be designed to return an `absl::Status` (or `absl::StatusOr`).
+//
+// Example:
+//
+// absl::Status myFunction(absl::string_view fname, ...) {
+//   ...
+//   // encounter error
+//   if (error condition) {
+//     return absl::InvalidArgumentError("bad mode");
+//   }
+//   // else, return OK
+//   return absl::OkStatus();
+// }
+//
+// An `absl::Status` is designed to either return "OK" or one of a number of
+// different error codes, corresponding to typical error conditions.
+// In almost all cases, when using `absl::Status` you should use the canonical
+// error codes (of type `absl::StatusCode`) enumerated in this header file.
+// These canonical codes are understood across the codebase and will be
+// accepted across all API and RPC boundaries.
+#ifndef ABSL_STATUS_STATUS_H_
+#define ABSL_STATUS_STATUS_H_
+
+#include <cassert>
+#include <cstdint>
+#include <optional>
+#include <ostream>
+#include <string>
+#include <type_traits>
+#include <utility>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/macros.h"
+#include "absl/base/nullability.h"
+#include "absl/base/optimization.h"
+#include "absl/functional/function_ref.h"
+#include "absl/status/internal/status_internal.h"
+#include "absl/strings/cord.h"
+#include "absl/strings/string_view.h"
+#include "absl/types/optional.h"
+#include "absl/types/source_location.h"
+#include "absl/types/span.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+#ifndef SWIG  // SWIG chokes on enum class
+// The following canonical error codes should always be in sync with
+// https://github.com/googleapis/googleapis/blob/master/google/rpc/code.proto.
+
+// absl::StatusCode
+//
+// An `absl::StatusCode` is an enumerated type indicating either no error ("OK")
+// or an error condition. In most cases, an `absl::Status` indicates a
+// recoverable error, and the purpose of signalling an error is to indicate what
+// action to take in response to that error. These error codes map to the proto
+// RPC error codes indicated in https://cloud.google.com/apis/design/errors.
+//
+// The errors listed below are the canonical errors associated with
+// `absl::Status` and are used throughout the codebase. As a result, these
+// error codes are somewhat generic.
+//
+// In general, try to return the most specific error that applies if more than
+// one error may pertain. For example, prefer `kOutOfRange` over
+// `kFailedPrecondition` if both codes apply. Similarly prefer `kNotFound` or
+// `kAlreadyExists` over `kFailedPrecondition`.
+//
+// Because these errors may cross RPC boundaries, these codes are tied to the
+// `google.rpc.Code` definitions within
+// https://github.com/googleapis/googleapis/blob/master/google/rpc/code.proto
+// The string value of these RPC codes is denoted within each enum below.
+//
+// If your error handling code requires more context, you can attach payloads
+// to your status. See `absl::Status::SetPayload()` and
+// `absl::Status::GetPayload()` below.
+enum class StatusCode : int {
+  // StatusCode::kOk
+  //
+  // kOK (gRPC code "OK") does not indicate an error; this value is returned on
+  // success. It is typical to check for this value before proceeding on any
+  // given call across an API or RPC boundary. To check this value, use the
+  // `absl::Status::ok()` member function rather than inspecting the raw code.
+  kOk = 0,
+
+  // StatusCode::kCancelled
+  //
+  // kCancelled (gRPC code "CANCELLED") indicates the operation was cancelled,
+  // typically by the caller.
+  kCancelled = 1,
+
+  // StatusCode::kUnknown
+  //
+  // kUnknown (gRPC code "UNKNOWN") indicates an unknown error occurred. In
+  // general, more specific errors should be raised, if possible. Errors raised
+  // by APIs that do not return enough error information may be converted to
+  // this error.
+  kUnknown = 2,
+
+  // StatusCode::kInvalidArgument
+  //
+  // kInvalidArgument (gRPC code "INVALID_ARGUMENT") indicates the caller
+  // specified an invalid argument, such as a malformed filename. Note that use
+  // of such errors should be narrowly limited to indicate the invalid nature of
+  // the arguments themselves. Errors with validly formed arguments that may
+  // cause errors with the state of the receiving system should be denoted with
+  // `kFailedPrecondition` instead.
+  kInvalidArgument = 3,
+
+  // StatusCode::kDeadlineExceeded
+  //
+  // kDeadlineExceeded (gRPC code "DEADLINE_EXCEEDED") indicates a deadline
+  // expired before the operation could complete. For operations that may change
+  // state within a system, this error may be returned even if the operation has
+  // completed successfully. For example, a successful response from a server
+  // could have been delayed long enough for the deadline to expire.
+  kDeadlineExceeded = 4,
+
+  // StatusCode::kNotFound
+  //
+  // kNotFound (gRPC code "NOT_FOUND") indicates some requested entity (such as
+  // a file or directory) was not found.
+  //
+  // `kNotFound` is useful if a request should be denied for an entire class of
+  // users, such as during a gradual feature rollout or undocumented allow list.
+  // If a request should be denied for specific sets of users, such as through
+  // user-based access control, use `kPermissionDenied` instead.
+  kNotFound = 5,
+
+  // StatusCode::kAlreadyExists
+  //
+  // kAlreadyExists (gRPC code "ALREADY_EXISTS") indicates that the entity a
+  // caller attempted to create (such as a file or directory) is already
+  // present.
+  kAlreadyExists = 6,
+
+  // StatusCode::kPermissionDenied
+  //
+  // kPermissionDenied (gRPC code "PERMISSION_DENIED") indicates that the caller
+  // does not have permission to execute the specified operation. Note that this
+  // error is different than an error due to an *un*authenticated user. This
+  // error code does not imply the request is valid or the requested entity
+  // exists or satisfies any other pre-conditions.
+  //
+  // `kPermissionDenied` must not be used for rejections caused by exhausting
+  // some resource. Instead, use `kResourceExhausted` for those errors.
+  // `kPermissionDenied` must not be used if the caller cannot be identified.
+  // Instead, use `kUnauthenticated` for those errors.
+  kPermissionDenied = 7,
+
+  // StatusCode::kResourceExhausted
+  //
+  // kResourceExhausted (gRPC code "RESOURCE_EXHAUSTED") indicates some resource
+  // has been exhausted, perhaps a per-user quota, or perhaps the entire file
+  // system is out of space.
+  kResourceExhausted = 8,
+
+  // StatusCode::kFailedPrecondition
+  //
+  // kFailedPrecondition (gRPC code "FAILED_PRECONDITION") indicates that the
+  // operation was rejected because the system is not in a state required for
+  // the operation's execution. For example, a directory to be deleted may be
+  // non-empty, an "rmdir" operation is applied to a non-directory, etc.
+  //
+  // Some guidelines that may help a service implementer in deciding between
+  // `kFailedPrecondition`, `kAborted`, and `kUnavailable`:
+  //
+  //  (a) Use `kUnavailable` if the client can retry just the failing call.
+  //  (b) Use `kAborted` if the client should retry at a higher transaction
+  //      level (such as when a client-specified test-and-set fails, indicating
+  //      the client should restart a read-modify-write sequence).
+  //  (c) Use `kFailedPrecondition` if the client should not retry until
+  //      the system state has been explicitly fixed. For example, if a "rmdir"
+  //      fails because the directory is non-empty, `kFailedPrecondition`
+  //      should be returned since the client should not retry unless
+  //      the files are deleted from the directory.
+  kFailedPrecondition = 9,
+
+  // StatusCode::kAborted
+  //
+  // kAborted (gRPC code "ABORTED") indicates the operation was aborted,
+  // typically due to a concurrency issue such as a sequencer check failure or a
+  // failed transaction.
+  //
+  // See the guidelines above for deciding between `kFailedPrecondition`,
+  // `kAborted`, and `kUnavailable`.
+  kAborted = 10,
+
+  // StatusCode::kOutOfRange
+  //
+  // kOutOfRange (gRPC code "OUT_OF_RANGE") indicates the operation was
+  // attempted past the valid range, such as seeking or reading past an
+  // end-of-file.
+  //
+  // Unlike `kInvalidArgument`, this error indicates a problem that may
+  // be fixed if the system state changes. For example, a 32-bit file
+  // system will generate `kInvalidArgument` if asked to read at an
+  // offset that is not in the range [0,2^32-1], but it will generate
+  // `kOutOfRange` if asked to read from an offset past the current
+  // file size.
+  //
+  // There is a fair bit of overlap between `kFailedPrecondition` and
+  // `kOutOfRange`.  We recommend using `kOutOfRange` (the more specific
+  // error) when it applies so that callers who are iterating through
+  // a space can easily look for an `kOutOfRange` error to detect when
+  // they are done.
+  kOutOfRange = 11,
+
+  // StatusCode::kUnimplemented
+  //
+  // kUnimplemented (gRPC code "UNIMPLEMENTED") indicates the operation is not
+  // implemented or supported in this service. In this case, the operation
+  // should not be re-attempted.
+  kUnimplemented = 12,
+
+  // StatusCode::kInternal
+  //
+  // kInternal (gRPC code "INTERNAL") indicates an internal error has occurred
+  // and some invariants expected by the underlying system have not been
+  // satisfied. This error code is reserved for serious errors.
+  kInternal = 13,
+
+  // StatusCode::kUnavailable
+  //
+  // kUnavailable (gRPC code "UNAVAILABLE") indicates the service is currently
+  // unavailable and that this is most likely a transient condition. An error
+  // such as this can be corrected by retrying with a backoff scheme. Note that
+  // it is not always safe to retry non-idempotent operations.
+  //
+  // See the guidelines above for deciding between `kFailedPrecondition`,
+  // `kAborted`, and `kUnavailable`.
+  kUnavailable = 14,
+
+  // StatusCode::kDataLoss
+  //
+  // kDataLoss (gRPC code "DATA_LOSS") indicates that unrecoverable data loss or
+  // corruption has occurred. As this error is serious, proper alerting should
+  // be attached to errors such as this.
+  kDataLoss = 15,
+
+  // StatusCode::kUnauthenticated
+  //
+  // kUnauthenticated (gRPC code "UNAUTHENTICATED") indicates that the request
+  // does not have valid authentication credentials for the operation. Correct
+  // the authentication and try again.
+  kUnauthenticated = 16,
+
+  // StatusCode::DoNotUseReservedForFutureExpansionUseDefaultInSwitchInstead_
+  //
+  // NOTE: this error code entry should not be used and you should not rely on
+  // its value, which may change.
+  //
+  // The purpose of this enumerated value is to force people who handle status
+  // codes with `switch()` statements to *not* simply enumerate all possible
+  // values, but instead provide a "default:" case. Providing such a default
+  // case ensures that code will compile when new codes are added.
+  kDoNotUseReservedForFutureExpansionUseDefaultInSwitchInstead_ = 20
+};
+
+// StatusCodeToString()
+//
+// Returns the name for the status code, or "" if it is an unknown value.
+std::string StatusCodeToString(StatusCode code);
+
+// StatusCodeToStringView()
+//
+// Same as StatusCodeToString(), but returns a string_view.
+absl::string_view StatusCodeToStringView(StatusCode code);
+
+// operator<<
+//
+// Streams StatusCodeToString(code) to `os`.
+std::ostream& operator<<(std::ostream& os, StatusCode code);
+
+// absl::StatusToStringMode
+//
+// An `absl::StatusToStringMode` is an enumerated type indicating how
+// `absl::Status::ToString()` should construct the output string for a non-ok
+// status.
+enum class StatusToStringMode : int {
+  // ToString will not contain any extra data (such as payloads). It will only
+  // contain the error code and message, if any.
+  kWithNoExtraData = 0,
+  // ToString will contain the payloads.
+  kWithPayload = 1 << 0,
+  // ToString will contain the source locations.
+  kWithSourceLocation = 1 << 1,
+  // ToString will include all the extra data this Status has.
+  kWithEverything = ~kWithNoExtraData,
+  // Default mode used by ToString. Its exact value might change in the future.
+  kDefault = kWithPayload,
+};
+
+// absl::StatusToStringMode is specified as a bitmask type, which means the
+// following operations must be provided:
+constexpr StatusToStringMode operator&(StatusToStringMode lhs,
+                                       StatusToStringMode rhs) {
+  return static_cast<StatusToStringMode>(static_cast<int>(lhs) &
+                                         static_cast<int>(rhs));
+}
+constexpr StatusToStringMode operator|(StatusToStringMode lhs,
+                                       StatusToStringMode rhs) {
+  return static_cast<StatusToStringMode>(static_cast<int>(lhs) |
+                                         static_cast<int>(rhs));
+}
+constexpr StatusToStringMode operator^(StatusToStringMode lhs,
+                                       StatusToStringMode rhs) {
+  return static_cast<StatusToStringMode>(static_cast<int>(lhs) ^
+                                         static_cast<int>(rhs));
+}
+constexpr StatusToStringMode operator~(StatusToStringMode arg) {
+  return static_cast<StatusToStringMode>(~static_cast<int>(arg));
+}
+inline StatusToStringMode& operator&=(StatusToStringMode& lhs,
+                                      StatusToStringMode rhs) {
+  lhs = lhs & rhs;
+  return lhs;
+}
+inline StatusToStringMode& operator|=(StatusToStringMode& lhs,
+                                      StatusToStringMode rhs) {
+  lhs = lhs | rhs;
+  return lhs;
+}
+inline StatusToStringMode& operator^=(StatusToStringMode& lhs,
+                                      StatusToStringMode rhs) {
+  lhs = lhs ^ rhs;
+  return lhs;
+}
+#endif  // SWIG
+
+// absl::Status
+//
+// The `absl::Status` class is generally used to gracefully handle errors
+// across API boundaries (and in particular across RPC boundaries). Some of
+// these errors may be recoverable, but others may not. Most
+// functions which can produce a recoverable error should be designed to return
+// either an `absl::Status` (or the similar `absl::StatusOr<T>`, which holds
+// either an object of type `T` or an error).
+//
+// API developers should construct their functions to return `absl::OkStatus()`
+// upon success, or an `absl::StatusCode` upon another type of error (e.g
+// an `absl::StatusCode::kInvalidArgument` error). The API provides convenience
+// functions to construct each status code.
+//
+// Example:
+//
+// absl::Status myFunction(absl::string_view fname, ...) {
+//   ...
+//   // encounter error
+//   if (error condition) {
+//     // Construct an absl::StatusCode::kInvalidArgument error
+//     return absl::InvalidArgumentError("bad mode");
+//   }
+//   // else, return OK
+//   return absl::OkStatus();
+// }
+//
+// Users handling status error codes should prefer checking for an OK status
+// using the `ok()` member function. Handling multiple error codes may justify
+// use of switch statement, but only check for error codes you know how to
+// handle; do not try to exhaustively match against all canonical error codes.
+// Errors that cannot be handled should be logged and/or propagated for higher
+// levels to deal with. If you do use a switch statement, make sure that you
+// also provide a `default:` switch case, so that code does not break as other
+// canonical codes are added to the API.
+//
+// Example:
+//
+//   absl::Status result = DoSomething();
+//   if (!result.ok()) {
+//     LOG(ERROR) << result;
+//   }
+//
+//   // Provide a default if switching on multiple error codes
+//   switch (result.code()) {
+//     // The user hasn't authenticated. Ask them to reauth
+//     case absl::StatusCode::kUnauthenticated:
+//       DoReAuth();
+//       break;
+//     // The user does not have permission. Log an error.
+//     case absl::StatusCode::kPermissionDenied:
+//       LOG(ERROR) << result;
+//       break;
+//     // Propagate the error otherwise.
+//     default:
+//       return true;
+//   }
+//
+// An `absl::Status` can optionally include a payload with more information
+// about the error. Typically, this payload serves one of several purposes:
+//
+//   * It may provide more fine-grained semantic information about the error to
+//     facilitate actionable remedies.
+//   * It may provide human-readable contextual information that is more
+//     appropriate to display to an end user.
+//
+// Example:
+//
+//   absl::Status result = DoSomething();
+//   // Inform user to retry after 30 seconds
+//   // See more error details in googleapis/google/rpc/error_details.proto
+//   if (absl::IsResourceExhausted(result)) {
+//     google::rpc::RetryInfo info;
+//     info.retry_delay().seconds() = 30;
+//     // Payloads require a unique key (a URL to ensure no collisions with
+//     // other payloads), and an `absl::Cord` to hold the encoded data.
+//     absl::string_view url = "type.googleapis.com/google.rpc.RetryInfo";
+//     result.SetPayload(url, info.SerializeAsCord());
+//     return result;
+//   }
+//
+// For documentation see https://abseil.io/docs/cpp/guides/status.
+//
+// Returned Status objects may not be ignored. status_internal.h has a forward
+// declaration of the form
+// class ABSL_MUST_USE_RESULT Status;
+class ABSL_ATTRIBUTE_TRIVIAL_ABI Status final {
+ public:
+  // Constructors
+
+  // This default constructor creates an OK status with no message or payload.
+  // Avoid this constructor and prefer explicit construction of an OK status
+  // with `absl::OkStatus()`.
+  Status();
+
+  // Creates a status in the canonical error space with the specified
+  // `absl::StatusCode` and error message.  If `code == absl::StatusCode::kOk`,
+  // `msg` is ignored and an object identical to an OK status is constructed.
+  //
+  // The `msg` string must be in UTF-8. The implementation may complain (e.g.,
+  // by printing a warning) if it is not.
+  //
+  // The `loc` is the SourceLocation of the callsite. It will be stored in the
+  // Status iff `code != absl::StatusCode::kOk` and `!msg.empty()`.
+  Status(absl::StatusCode code, absl::string_view msg,
+         absl::SourceLocation loc = SourceLocation::current());
+
+#ifndef SWIG
+  // Same as above but for rvalue string.
+  // Note: using a template to disambiguate the case of matching string_view and
+  // string&& (e.g. char*) as a template lowers the priority of the overload.
+  template <typename String, typename = status_internal::EnableIfString<String>>
+  Status(absl::StatusCode code, String&& msg,
+         absl::SourceLocation loc = SourceLocation::current());
+#endif  // SWIG
+
+  // Create a status from a `base_status` and a `loc`. The `loc` will be
+  // appended to the location chain of the new status, iff the `base_status` is
+  // not ok and has non-empty msg.
+  Status(const Status& base_status, absl::SourceLocation loc)
+      : Status(base_status) {
+    AddSourceLocation(loc);
+  }
+#ifndef SWIG
+  Status(Status&& base_status, absl::SourceLocation loc)
+      : Status(std::move(base_status)) {
+    AddSourceLocation(loc);
+  }
+#endif  // SWIG
+
+  Status(const Status&);
+  Status& operator=(const Status& x);
+
+#ifndef SWIG
+  // Move operators
+
+  // The moved-from state is valid but unspecified.
+  Status(Status&&) noexcept;
+  Status& operator=(Status&&) noexcept;
+#endif  // SWIG
+
+  ~Status();
+
+  // Status::Update()
+  //
+  // Updates the existing status with `new_status` provided that `this->ok()`.
+  // If the existing status already contains a non-OK error, this update has no
+  // effect and preserves the current data. Note that this behavior may change
+  // in the future to augment a current non-ok status with additional
+  // information about `new_status`.
+  //
+  // `Update()` provides a convenient way of keeping track of the first error
+  // encountered.
+  //
+  // Example:
+  //   // Instead of "if (overall_status.ok()) overall_status = new_status"
+  //   overall_status.Update(new_status);
+  //
+  void Update(const Status& new_status);
+#ifndef SWIG
+  void Update(Status&& new_status);
+#endif  // SWIG
+
+  // Status::ok()
+  //
+  // Returns `true` if `this->code()` == `absl::StatusCode::kOk`,
+  // indicating the absence of an error.
+  // Prefer checking for an OK status using this member function.
+  ABSL_MUST_USE_RESULT bool ok() const;
+
+  // Status::code()
+  //
+  // Returns the canonical error code of type `absl::StatusCode` of this status.
+  absl::StatusCode code() const;
+
+  // Status::raw_code()
+  //
+  // Returns a raw (canonical) error code corresponding to the enum value of
+  // `google.rpc.Code` definitions within
+  // https://github.com/googleapis/googleapis/blob/master/google/rpc/code.proto.
+  // These values could be out of the range of canonical `absl::StatusCode`
+  // enum values.
+  //
+  // NOTE: This function should only be called when converting to an associated
+  // wire format. Use `Status::code()` for error handling.
+  int raw_code() const;
+
+#ifndef SWIG
+
+  // Status::message()
+  //
+  // Returns the error message associated with this error code, if available.
+  // Note that this message rarely describes the error code.  It is not unusual
+  // for the error message to be the empty string. As a result, prefer
+  // `operator<<` or `Status::ToString()` for debug logging.
+  absl::string_view message() const;
+#endif  // SWIG
+
+  friend bool operator==(const Status&, const Status&);
+  friend bool operator!=(const Status&, const Status&);
+
+#ifndef SWIG
+  // `ToString` has stubs in SWIG to remain backward compatible with the old
+  // format by calling `util::StatusToString`, to save migration cost.
+
+  // Status::ToString()
+  //
+  // Returns a string based on the `mode`. By default, it returns combination of
+  // the error code name, the message and any associated payload messages. This
+  // string is designed simply to be human readable and its exact format should
+  // not be load bearing. Do not depend on the exact format of the result of
+  // `ToString()` which is subject to change.
+  //
+  // The printed code name and the message are generally substrings of the
+  // result, and the payloads to be printed use the status payload printer
+  // mechanism (which is internal).
+  std::string ToString(
+      StatusToStringMode mode = StatusToStringMode::kDefault) const;
+
+  // Support `absl::StrCat`, `absl::StrFormat`, etc.
+  template <typename Sink>
+  friend void AbslStringify(Sink& sink, const Status& status) {
+    sink.Append(status.ToString(StatusToStringMode::kWithEverything));
+  }
+#endif  // SWIG
+
+  // Status::IgnoreError()
+  //
+  // Ignores any errors. This method does nothing except potentially suppress
+  // complaints from any tools that are checking that errors are not dropped on
+  // the floor.
+  void IgnoreError() const;
+
+  // swap()
+  //
+  // Swap the contents of one status with another.
+#ifndef SWIG
+  friend void swap(Status& a, Status& b) noexcept;
+#else
+  friend void swap(Status& a, Status& b);
+#endif  // SWIG
+
+  //----------------------------------------------------------------------------
+  // Payload Management APIs
+  //----------------------------------------------------------------------------
+
+  // A payload may be attached to a status to provide additional context to an
+  // error that may not be satisfied by an existing `absl::StatusCode`.
+  // Typically, this payload serves one of several purposes:
+  //
+  //   * It may provide more fine-grained semantic information about the error
+  //     to facilitate actionable remedies.
+  //   * It may provide human-readable contextual information that is more
+  //     appropriate to display to an end user.
+  //
+  // A payload consists of a [key,value] pair, where the key is a string
+  // referring to a unique "type URL" and the value is an object of type
+  // `absl::Cord` to hold the contextual data.
+  //
+  // The "type URL" should be unique and follow the format of a URL
+  // (https://en.wikipedia.org/wiki/URL) and, ideally, provide some
+  // documentation or schema on how to interpret its associated data. For
+  // example, the default type URL for a protobuf message type is
+  // "type.googleapis.com/packagename.messagename". Other custom wire formats
+  // should define the format of type URL in a similar practice so as to
+  // minimize the chance of conflict between type URLs.
+  // Users should ensure that the type URL can be mapped to a concrete
+  // C++ type if they want to deserialize the payload and read it effectively.
+  //
+  // To attach a payload to a status object, call `Status::SetPayload()`,
+  // passing it the type URL and an `absl::Cord` of associated data. Similarly,
+  // to extract the payload from a status, call `Status::GetPayload()`. You
+  // may attach multiple payloads (with differing type URLs) to any given
+  // status object, provided that the status is currently exhibiting an error
+  // code (i.e. is not OK).
+
+  // Status::GetPayload()
+  //
+  // Gets the payload of a status given its unique `type_url` key, if present.
+  std::optional<absl::Cord> GetPayload(absl::string_view type_url) const;
+
+  // Status::SetPayload()
+  //
+  // Sets the payload for a non-ok status using a `type_url` key, overwriting
+  // any existing payload for that `type_url`.
+  //
+  // NOTE: This function does nothing if the Status is ok.
+  void SetPayload(absl::string_view type_url, absl::Cord payload);
+
+  // Status::ErasePayload()
+  //
+  // Erases the payload corresponding to the `type_url` key.  Returns `true` if
+  // the payload was present.
+  bool ErasePayload(absl::string_view type_url);
+
+  // Status::ForEachPayload()
+  //
+  // Iterates over the stored payloads and calls the
+  // `visitor(type_key, payload)` callable for each one.
+  //
+  // NOTE: The order of calls to `visitor()` is not specified and may change at
+  // any time.
+  //
+  // NOTE: Any mutation on the same 'absl::Status' object during visitation is
+  // forbidden and could result in undefined behavior.
+  // FunctionRef doesn't work nicely with Swig.
+  // TODO(b/189736749): Consider making this available once FunctionRef is
+  // supported.
+#ifndef SWIG
+  void ForEachPayload(
+      absl::FunctionRef<void(absl::string_view, const absl::Cord&)> visitor)
+      const;
+#endif  // SWIG
+
+  absl::Span<const absl::SourceLocation> GetSourceLocations() const {
+    if (IsInlined(rep_)) return {};
+    return RepToPointer(rep_)->GetSourceLocations();
+  }
+  // Appends the `loc` to the current location chain inside the status, iff the
+  // status is non-ok and contains a non-empty message.
+  void AddSourceLocation(
+      absl::SourceLocation loc = absl::SourceLocation::current()) {
+    if (ok()) return;
+    rep_ = AddSourceLocationImpl(rep_, loc);
+    [[maybe_unused]] bool okay = ok();
+    // This hint tells the optimizer that the status is still not ok after the
+    // AddSourceLocation() call. This is useful when passing a known !ok status
+    // to StatusOr. StatusOr checks for ok() on its constructor and this assume
+    // helps the optimizer remove that check.
+    ABSL_ASSUME(!okay);
+  }
+
+#ifndef SWIG
+
+  // Status::WithSourceLocation()
+  //
+  // Returns a copy of the current status, with `loc` appended to its location
+  // chain iff the status is non-ok and contains a non-empty message.
+  //
+  // Example:
+  //
+  //   if (Status status = Foo(); !status.ok()) {
+  //     return status.WithSourceLocation();
+  //   }
+  Status WithSourceLocation(
+      absl::SourceLocation loc = absl::SourceLocation::current()) const& {
+    return Status(*this, loc);
+  }
+
+  // Status::WithSourceLocation()
+  //
+  // Appends the `loc` to the current location chain inside the status iff the
+  // status is non-ok and contains a non-empty message, and returns an rvalue
+  // reference to `*this`.
+  //
+  // Example:
+  //
+  //   Status Finalize(...);
+  //
+  //   Status DoSomething(...) {
+  //     ...
+  //     return Finalize().WithSourceLocation();
+  //   }
+  ABSL_MUST_USE_RESULT Status&& WithSourceLocation(
+      absl::SourceLocation loc = absl::SourceLocation::current()) && {
+    AddSourceLocation(loc);
+    return std::move(*this);
+  }
+#endif  // SWIG
+
+ private:
+  friend Status CancelledError();
+
+#ifndef SWIG
+  // Returns a `Status` object which is not `ok()` but
+  // `code() == absl::StatusCode::kOk`. This is necessary to be compatible with
+  // `Status` objects created with an error code in a custom `ErrorSpace` that
+  // is mapped to the canonical code `absl::StatusCode::kOk`.
+  static Status MakeNonOkStatusWithOkCode(absl::string_view message);
+
+  friend class absl::status_internal::StatusPrivateAccessor;
+  friend class absl::status_internal::StatusPrivateAccessorForStatusBuilder;
+  template <typename T>
+  friend class absl::StatusOr;
+#endif  // !SWIG
+
+  // Creates a status in the canonical error space with the specified
+  // code, and an empty error message.
+  explicit Status(absl::StatusCode code);
+
+  // Delegate factory in header that ensures CodeToInlinedRep is inlined
+  // where possible.
+  static uintptr_t MakeRepFromStringView(uintptr_t inlined_rep,
+                                         absl::string_view msg,
+                                         absl::SourceLocation loc);
+
+#ifndef SWIG
+  // Same as above but for rvalue string.
+  static uintptr_t MakeRepFromStringRvalue(uintptr_t inlined_rep,
+                                           std::string&& msg,
+                                           absl::SourceLocation loc);
+#endif  // SWIG
+
+  template <typename StringOrView>
+  friend uintptr_t MakeStatusRepImpl(uintptr_t inlined_rep, StringOrView msg,
+                                     absl::SourceLocation loc);
+
+  // Underlying constructor for status from a rep_.
+  explicit Status(uintptr_t rep) : rep_(rep) {}
+
+  // An out-of-line AddSourceLocation that mutates rep directly.
+  static uintptr_t AddSourceLocationImpl(uintptr_t rep,
+                                         absl::SourceLocation loc);
+  static uintptr_t WithContextImpl(uintptr_t rep, absl::string_view context);
+
+  static void Ref(uintptr_t rep);
+  static void Unref(uintptr_t rep);
+
+  // REQUIRES: !ok()
+  // Ensures rep is not inlined or shared with any other Status.
+  static status_internal::StatusRep* absl_nonnull PrepareToModify(
+      uintptr_t rep);
+
+  // MSVC 14.0 limitation requires the const.
+  static constexpr const char kMovedFromString[] =
+      "Status accessed after move.";
+
+  static const std::string* absl_nonnull EmptyString();
+  static const std::string* absl_nonnull MovedFromString();
+
+  // Returns whether rep contains an inlined representation.
+  // See rep_ for details.
+  static constexpr bool IsInlined(uintptr_t rep);
+
+  // Indicates whether this Status was the rhs of a move operation. See rep_
+  // for details.
+  static constexpr bool IsMovedFrom(uintptr_t rep);
+  static constexpr uintptr_t MovedFromRep();
+
+  // Convert between error::Code and the inlined uintptr_t representation used
+  // by rep_. See rep_ for details.
+  static constexpr uintptr_t CodeToInlinedRep(absl::StatusCode code);
+  static constexpr absl::StatusCode InlinedRepToCode(uintptr_t rep);
+
+  // Converts between StatusRep* and the external uintptr_t representation used
+  // by rep_. See rep_ for details.
+  static uintptr_t PointerToRep(status_internal::StatusRep* absl_nonnull rep);
+  static const status_internal::StatusRep* absl_nonnull RepToPointer(
+      uintptr_t rep);
+
+  static std::string ToStringSlow(uintptr_t rep, StatusToStringMode mode);
+
+  // Status supports two different representations.
+  //  - When the low bit is set it is an inlined representation.
+  //    It uses the canonical error space, no message or payload.
+  //    The error code is (rep_ >> 2).
+  //    The (rep_ & 2) bit is the "moved from" indicator, used in IsMovedFrom().
+  //  - When the low bit is off it is an external representation.
+  //    In this case all the data comes from a heap allocated Rep object.
+  //    rep_ is a status_internal::StatusRep* pointer to that structure.
+  uintptr_t rep_;
+
+  friend class status_internal::StatusRep;
+};
+
+// OkStatus()
+//
+// Returns an OK status, equivalent to a default constructed instance. Prefer
+// usage of `absl::OkStatus()` when constructing such an OK status.
+Status OkStatus();
+
+// operator<<()
+//
+// Prints a human-readable representation of `x` to `os`.
+std::ostream& operator<<(std::ostream& os, const Status& x);
+
+// IsAborted()
+// IsAlreadyExists()
+// IsCancelled()
+// IsDataLoss()
+// IsDeadlineExceeded()
+// IsFailedPrecondition()
+// IsInternal()
+// IsInvalidArgument()
+// IsNotFound()
+// IsOutOfRange()
+// IsPermissionDenied()
+// IsResourceExhausted()
+// IsUnauthenticated()
+// IsUnavailable()
+// IsUnimplemented()
+// IsUnknown()
+//
+// These convenience functions return `true` if a given status matches the
+// `absl::StatusCode` error code of its associated function.
+ABSL_MUST_USE_RESULT bool IsAborted(const Status& status);
+ABSL_MUST_USE_RESULT bool IsAlreadyExists(const Status& status);
+ABSL_MUST_USE_RESULT bool IsCancelled(const Status& status);
+ABSL_MUST_USE_RESULT bool IsDataLoss(const Status& status);
+ABSL_MUST_USE_RESULT bool IsDeadlineExceeded(const Status& status);
+ABSL_MUST_USE_RESULT bool IsFailedPrecondition(const Status& status);
+ABSL_MUST_USE_RESULT bool IsInternal(const Status& status);
+ABSL_MUST_USE_RESULT bool IsInvalidArgument(const Status& status);
+ABSL_MUST_USE_RESULT bool IsNotFound(const Status& status);
+ABSL_MUST_USE_RESULT bool IsOutOfRange(const Status& status);
+ABSL_MUST_USE_RESULT bool IsPermissionDenied(const Status& status);
+ABSL_MUST_USE_RESULT bool IsResourceExhausted(const Status& status);
+ABSL_MUST_USE_RESULT bool IsUnauthenticated(const Status& status);
+ABSL_MUST_USE_RESULT bool IsUnavailable(const Status& status);
+ABSL_MUST_USE_RESULT bool IsUnimplemented(const Status& status);
+ABSL_MUST_USE_RESULT bool IsUnknown(const Status& status);
+
+// AbortedError()
+// AlreadyExistsError()
+// CancelledError()
+// DataLossError()
+// DeadlineExceededError()
+// FailedPreconditionError()
+// InternalError()
+// InvalidArgumentError()
+// NotFoundError()
+// OutOfRangeError()
+// PermissionDeniedError()
+// ResourceExhaustedError()
+// UnauthenticatedError()
+// UnavailableError()
+// UnimplementedError()
+// UnknownError()
+//
+// These convenience functions create an `absl::Status` object with an error
+// code as indicated by the associated function name, using the error message
+// passed in `message`.
+// Note: using a template for the rvalue message overload to disambiguate the
+// case of matching string_view and string&& (e.g. char*) as a template lowers
+// the priority of the overload.
+
+// AbortedError
+Status AbortedError(absl::string_view message,
+                    absl::SourceLocation loc = SourceLocation::current());
+
+#ifndef SWIG
+template <typename String, typename = status_internal::EnableIfString<String>>
+Status AbortedError(String&& message,
+                    absl::SourceLocation loc = SourceLocation::current());
+#endif  // SWIG
+
+// AlreadyExistsError
+Status AlreadyExistsError(absl::string_view message,
+                          absl::SourceLocation loc = SourceLocation::current());
+
+#ifndef SWIG
+template <typename String, typename = status_internal::EnableIfString<String>>
+Status AlreadyExistsError(String&& message,
+                          absl::SourceLocation loc = SourceLocation::current());
+#endif  // SWIG
+
+// CancelledError
+Status CancelledError(absl::string_view message,
+                      absl::SourceLocation loc = SourceLocation::current());
+
+#ifndef SWIG
+template <typename String, typename = status_internal::EnableIfString<String>>
+Status CancelledError(String&& message,
+                      absl::SourceLocation loc = SourceLocation::current());
+#endif  // SWIG
+
+// DataLossError
+Status DataLossError(absl::string_view message,
+                     absl::SourceLocation loc = SourceLocation::current());
+
+#ifndef SWIG
+template <typename String, typename = status_internal::EnableIfString<String>>
+Status DataLossError(String&& message,
+                     absl::SourceLocation loc = SourceLocation::current());
+#endif  // SWIG
+
+// DeadlineExceededError
+Status DeadlineExceededError(
+    absl::string_view message,
+    absl::SourceLocation loc = SourceLocation::current());
+
+#ifndef SWIG
+template <typename String, typename = status_internal::EnableIfString<String>>
+Status DeadlineExceededError(
+    String&& message, absl::SourceLocation loc = SourceLocation::current());
+#endif  // SWIG
+
+// FailedPreconditionError
+Status FailedPreconditionError(
+    absl::string_view message,
+    absl::SourceLocation loc = SourceLocation::current());
+
+#ifndef SWIG
+template <typename String, typename = status_internal::EnableIfString<String>>
+Status FailedPreconditionError(
+    String&& message, absl::SourceLocation loc = SourceLocation::current());
+#endif  // SWIG
+
+// InternalError
+Status InternalError(absl::string_view message,
+                     absl::SourceLocation loc = SourceLocation::current());
+
+#ifndef SWIG
+template <typename String, typename = status_internal::EnableIfString<String>>
+Status InternalError(String&& message,
+                     absl::SourceLocation loc = SourceLocation::current());
+#endif  // SWIG
+
+// InvalidArgumentError
+Status InvalidArgumentError(
+    absl::string_view message,
+    absl::SourceLocation loc = SourceLocation::current());
+
+#ifndef SWIG
+template <typename String, typename = status_internal::EnableIfString<String>>
+Status InvalidArgumentError(
+    String&& message, absl::SourceLocation loc = SourceLocation::current());
+#endif  // SWIG
+
+// NotFoundError
+Status NotFoundError(absl::string_view message,
+                     absl::SourceLocation loc = SourceLocation::current());
+
+#ifndef SWIG
+template <typename String, typename = status_internal::EnableIfString<String>>
+Status NotFoundError(String&& message,
+                     absl::SourceLocation loc = SourceLocation::current());
+#endif  // SWIG
+
+// OutOfRangeError
+Status OutOfRangeError(absl::string_view message,
+                       absl::SourceLocation loc = SourceLocation::current());
+
+#ifndef SWIG
+template <typename String, typename = status_internal::EnableIfString<String>>
+Status OutOfRangeError(String&& message,
+                       absl::SourceLocation loc = SourceLocation::current());
+#endif  // SWIG
+
+// PermissionDeniedError
+Status PermissionDeniedError(
+    absl::string_view message,
+    absl::SourceLocation loc = SourceLocation::current());
+
+#ifndef SWIG
+template <typename String, typename = status_internal::EnableIfString<String>>
+Status PermissionDeniedError(
+    String&& message, absl::SourceLocation loc = SourceLocation::current());
+#endif  // SWIG
+
+// ResourceExhaustedError
+Status ResourceExhaustedError(
+    absl::string_view message,
+    absl::SourceLocation loc = SourceLocation::current());
+
+#ifndef SWIG
+template <typename String, typename = status_internal::EnableIfString<String>>
+Status ResourceExhaustedError(
+    String&& message, absl::SourceLocation loc = SourceLocation::current());
+#endif  // SWIG
+
+// UnauthenticatedError
+Status UnauthenticatedError(
+    absl::string_view message,
+    absl::SourceLocation loc = SourceLocation::current());
+
+#ifndef SWIG
+template <typename String, typename = status_internal::EnableIfString<String>>
+Status UnauthenticatedError(
+    String&& message, absl::SourceLocation loc = SourceLocation::current());
+#endif  // SWIG
+
+// UnavailableError
+Status UnavailableError(absl::string_view message,
+                        absl::SourceLocation loc = SourceLocation::current());
+
+#ifndef SWIG
+template <typename String, typename = status_internal::EnableIfString<String>>
+Status UnavailableError(String&& message,
+                        absl::SourceLocation loc = SourceLocation::current());
+#endif  // SWIG
+
+// UnimplementedError
+Status UnimplementedError(absl::string_view message,
+                          absl::SourceLocation loc = SourceLocation::current());
+
+#ifndef SWIG
+template <typename String, typename = status_internal::EnableIfString<String>>
+Status UnimplementedError(String&& message,
+                          absl::SourceLocation loc = SourceLocation::current());
+#endif  // SWIG
+
+// UnknownError
+Status UnknownError(absl::string_view message,
+                    absl::SourceLocation loc = SourceLocation::current());
+
+#ifndef SWIG
+template <typename String, typename = status_internal::EnableIfString<String>>
+Status UnknownError(String&& message,
+                    absl::SourceLocation loc = SourceLocation::current());
+#endif  // SWIG
+
+// ErrnoToStatusCode()
+//
+// Returns the StatusCode for `error_number`, which should be an `errno` value.
+// See https://en.cppreference.com/w/cpp/error/errno_macros and similar
+// references.
+absl::StatusCode ErrnoToStatusCode(int error_number);
+
+// ErrnoToStatus()
+//
+// Convenience function that creates a `absl::Status` using an `error_number`,
+// which should be an `errno` value.
+Status ErrnoToStatus(int error_number, absl::string_view message,
+                     absl::SourceLocation loc = SourceLocation::current());
+
+//------------------------------------------------------------------------------
+// Implementation details follow
+//------------------------------------------------------------------------------
+
+inline Status::Status() : Status(absl::StatusCode::kOk) {}
+
+inline Status::Status(absl::StatusCode code) : Status(CodeToInlinedRep(code)) {}
+
+inline Status::Status(absl::StatusCode code, absl::string_view msg,
+                      absl::SourceLocation loc)
+    : Status(MakeRepFromStringView(CodeToInlinedRep(code), msg, loc)) {}
+
+#ifndef SWIG
+template <typename String, typename>
+inline Status::Status(absl::StatusCode code, String&& msg,
+                      absl::SourceLocation loc)
+    : Status(MakeRepFromStringRvalue(CodeToInlinedRep(code),
+                                     std::forward<String>(msg), loc)) {}
+#endif  // SWIG
+
+inline Status::Status(const Status& x) : Status(x.rep_) { Ref(rep_); }
+
+inline Status& Status::operator=(const Status& x) {
+  uintptr_t old_rep = rep_;
+  if (x.rep_ != old_rep) {
+    Ref(x.rep_);
+    rep_ = x.rep_;
+    Unref(old_rep);
+  }
+  return *this;
+}
+
+#ifndef SWIG
+inline Status::Status(Status&& x) noexcept : Status(x.rep_) {
+  x.rep_ = MovedFromRep();
+}
+
+inline Status& Status::operator=(Status&& x) noexcept {
+  uintptr_t old_rep = rep_;
+  if (x.rep_ != old_rep) {
+    rep_ = x.rep_;
+    x.rep_ = MovedFromRep();
+    Unref(old_rep);
+  }
+  return *this;
+}
+#endif  // SWIG
+
+inline void Status::Update(const Status& new_status) {
+  if (ok()) {
+    *this = new_status;
+  }
+}
+
+#ifndef SWIG
+inline void Status::Update(Status&& new_status) {
+  if (ok()) {
+    *this = std::move(new_status);
+  }
+}
+#endif  // SWIG
+
+inline Status::~Status() { Unref(rep_); }
+
+inline bool Status::ok() const {
+  return rep_ == CodeToInlinedRep(absl::StatusCode::kOk);
+}
+
+inline absl::StatusCode Status::code() const {
+  return status_internal::MapToLocalCode(raw_code());
+}
+
+inline int Status::raw_code() const {
+  if (IsInlined(rep_)) return static_cast<int>(InlinedRepToCode(rep_));
+  return static_cast<int>(RepToPointer(rep_)->code());
+}
+
+inline absl::string_view Status::message() const {
+  return !IsInlined(rep_)
+             ? RepToPointer(rep_)->message()
+             : (IsMovedFrom(rep_) ? absl::string_view(kMovedFromString)
+                                  : absl::string_view());
+}
+
+inline bool operator==(const Status& lhs, const Status& rhs) {
+  if (lhs.rep_ == rhs.rep_) return true;
+  if (Status::IsInlined(lhs.rep_)) return false;
+  if (Status::IsInlined(rhs.rep_)) return false;
+  return *Status::RepToPointer(lhs.rep_) == *Status::RepToPointer(rhs.rep_);
+}
+
+inline bool operator!=(const Status& lhs, const Status& rhs) {
+  return !(lhs == rhs);
+}
+
+inline std::string Status::ToString(StatusToStringMode mode) const {
+  return ok() ? "OK" : ToStringSlow(rep_, mode);
+}
+
+inline void Status::IgnoreError() const {
+  // no-op
+}
+
+#ifndef SWIG
+inline void swap(absl::Status& a, absl::Status& b) noexcept {
+  using std::swap;
+  swap(a.rep_, b.rep_);
+}
+#else
+inline void swap(absl::Status& a, absl::Status& b) {
+  using std::swap;
+  swap(a.rep_, b.rep_);
+}
+#endif  // SWIG
+
+inline std::optional<absl::Cord> Status::GetPayload(
+    absl::string_view type_url) const {
+  if (IsInlined(rep_)) return std::nullopt;
+  return RepToPointer(rep_)->GetPayload(type_url);
+}
+
+inline void Status::SetPayload(absl::string_view type_url, absl::Cord payload) {
+  if (ok()) return;
+  status_internal::StatusRep* rep = PrepareToModify(rep_);
+  rep->SetPayload(type_url, std::move(payload));
+  rep_ = PointerToRep(rep);
+}
+
+inline bool Status::ErasePayload(absl::string_view type_url) {
+  if (IsInlined(rep_)) return false;
+  status_internal::StatusRep* rep = PrepareToModify(rep_);
+  auto res = rep->ErasePayload(type_url);
+  rep_ = res.new_rep;
+  return res.erased;
+}
+
+inline void Status::ForEachPayload(
+    absl::FunctionRef<void(absl::string_view, const absl::Cord&)> visitor)
+    const {
+  if (IsInlined(rep_)) return;
+  RepToPointer(rep_)->ForEachPayload(visitor);
+}
+
+constexpr bool Status::IsInlined(uintptr_t rep) { return (rep & 1) != 0; }
+
+constexpr bool Status::IsMovedFrom(uintptr_t rep) { return (rep & 2) != 0; }
+
+constexpr uintptr_t Status::CodeToInlinedRep(absl::StatusCode code) {
+  return (static_cast<uintptr_t>(code) << 2) + 1;
+}
+
+constexpr absl::StatusCode Status::InlinedRepToCode(uintptr_t rep) {
+  ABSL_ASSERT(IsInlined(rep));
+  return static_cast<absl::StatusCode>(rep >> 2);
+}
+
+constexpr uintptr_t Status::MovedFromRep() {
+  return CodeToInlinedRep(absl::StatusCode::kInternal) | 2;
+}
+
+inline const status_internal::StatusRep* absl_nonnull Status::RepToPointer(
+    uintptr_t rep) {
+  assert(!IsInlined(rep));
+  return reinterpret_cast<const status_internal::StatusRep*>(rep);
+}
+
+inline uintptr_t Status::PointerToRep(
+    status_internal::StatusRep* absl_nonnull rep) {
+  return reinterpret_cast<uintptr_t>(rep);
+}
+
+inline void Status::Ref(uintptr_t rep) {
+  if (!IsInlined(rep)) RepToPointer(rep)->Ref();
+}
+
+inline void Status::Unref(uintptr_t rep) {
+  if (!IsInlined(rep)) RepToPointer(rep)->Unref();
+}
+
+#ifndef SWIG
+inline Status OkStatus() { return Status(); }
+#endif  // SWIG
+
+// Creates a `Status` object with the `absl::StatusCode::kCancelled` error code
+// and an empty message. It is provided only for efficiency, given that
+// message-less kCancelled errors are common in the infrastructure.
+inline Status CancelledError() { return Status(absl::StatusCode::kCancelled); }
+
+// Retrieves a message's status as a null terminated C string. The lifetime of
+// this string is tied to the lifetime of the status object itself.
+//
+// If the status's message is empty, the empty string is returned.
+//
+// StatusMessageAsCStr exists for C support. Use `status.message()` in C++.
+const char* absl_nonnull StatusMessageAsCStr(
+    const Status& status ABSL_ATTRIBUTE_LIFETIME_BOUND);
+
+namespace status_internal {
+// We use an int in the template parameter to shorten mangled names.
+template <int error_code>
+Status MakeErrorStringViewImpl(string_view message, SourceLocation loc);
+// Make the instantiations extern to reduce bloat on callers.
+#ifndef SWIG
+extern template Status MakeErrorStringViewImpl<1>(string_view, SourceLocation);
+extern template Status MakeErrorStringViewImpl<2>(string_view, SourceLocation);
+extern template Status MakeErrorStringViewImpl<3>(string_view, SourceLocation);
+extern template Status MakeErrorStringViewImpl<4>(string_view, SourceLocation);
+extern template Status MakeErrorStringViewImpl<5>(string_view, SourceLocation);
+extern template Status MakeErrorStringViewImpl<6>(string_view, SourceLocation);
+extern template Status MakeErrorStringViewImpl<7>(string_view, SourceLocation);
+extern template Status MakeErrorStringViewImpl<8>(string_view, SourceLocation);
+extern template Status MakeErrorStringViewImpl<9>(string_view, SourceLocation);
+extern template Status MakeErrorStringViewImpl<10>(string_view, SourceLocation);
+extern template Status MakeErrorStringViewImpl<11>(string_view, SourceLocation);
+extern template Status MakeErrorStringViewImpl<12>(string_view, SourceLocation);
+extern template Status MakeErrorStringViewImpl<13>(string_view, SourceLocation);
+extern template Status MakeErrorStringViewImpl<14>(string_view, SourceLocation);
+extern template Status MakeErrorStringViewImpl<15>(string_view, SourceLocation);
+extern template Status MakeErrorStringViewImpl<16>(string_view, SourceLocation);
+#endif  // SWIG
+
+template <StatusCode error_code>
+Status MakeErrorFromStringView(string_view message, SourceLocation loc) {
+  static_assert(error_code != StatusCode::kOk, "expecting non-OK code");
+  Status out =
+      MakeErrorStringViewImpl<static_cast<int>(error_code)>(message, loc);
+  // -Wassume warning complains about potential side effects of `ok()`, so use a
+  // local to avoid that.
+  [[maybe_unused]] bool ok = out.ok();
+  ABSL_ASSUME(!ok);
+  return out;
+}
+
+// Same as above, but for rvalue std::string.
+#ifndef SWIG
+// We use an int in the template parameter to shorten mangled names.
+template <int error_code>
+Status MakeErrorStringRvalueImpl(std::string&& message, SourceLocation loc);
+// Make the instantiations extern to reduce bloat on callers.
+extern template Status MakeErrorStringRvalueImpl<1>(std::string&&,
+                                                    SourceLocation);
+extern template Status MakeErrorStringRvalueImpl<2>(std::string&&,
+                                                    SourceLocation);
+extern template Status MakeErrorStringRvalueImpl<3>(std::string&&,
+                                                    SourceLocation);
+extern template Status MakeErrorStringRvalueImpl<4>(std::string&&,
+                                                    SourceLocation);
+extern template Status MakeErrorStringRvalueImpl<5>(std::string&&,
+                                                    SourceLocation);
+extern template Status MakeErrorStringRvalueImpl<6>(std::string&&,
+                                                    SourceLocation);
+extern template Status MakeErrorStringRvalueImpl<7>(std::string&&,
+                                                    SourceLocation);
+extern template Status MakeErrorStringRvalueImpl<8>(std::string&&,
+                                                    SourceLocation);
+extern template Status MakeErrorStringRvalueImpl<9>(std::string&&,
+                                                    SourceLocation);
+extern template Status MakeErrorStringRvalueImpl<10>(std::string&&,
+                                                     SourceLocation);
+extern template Status MakeErrorStringRvalueImpl<11>(std::string&&,
+                                                     SourceLocation);
+extern template Status MakeErrorStringRvalueImpl<12>(std::string&&,
+                                                     SourceLocation);
+extern template Status MakeErrorStringRvalueImpl<13>(std::string&&,
+                                                     SourceLocation);
+extern template Status MakeErrorStringRvalueImpl<14>(std::string&&,
+                                                     SourceLocation);
+extern template Status MakeErrorStringRvalueImpl<15>(std::string&&,
+                                                     SourceLocation);
+extern template Status MakeErrorStringRvalueImpl<16>(std::string&&,
+                                                     SourceLocation);
+
+template <StatusCode error_code, typename String>
+Status MakeErrorFromStringRvalue(String&& message, SourceLocation loc) {
+  static_assert(error_code != StatusCode::kOk, "expecting non-OK code");
+  static_assert(std::is_same_v<String&&, std::string&&>,
+                "`message` should be std::string&&");
+  Status out = MakeErrorStringRvalueImpl<static_cast<int>(error_code)>(
+      std::forward<String>(message), loc);
+  // -Wassume warning complains about potential side effects of `ok()`, so use a
+  // local to avoid that.
+  [[maybe_unused]] bool ok = out.ok();
+  ABSL_ASSUME(!ok);
+  return out;
+}
+#endif  // SWIG
+
+}  // namespace status_internal
+
+// Inline implementations to give the compiler static knowledge about the
+// objects.
+
+// AbortedError
+inline Status AbortedError(absl::string_view message,
+                           absl::SourceLocation loc) {
+  return status_internal::MakeErrorFromStringView<StatusCode::kAborted>(message,
+                                                                        loc);
+}
+
+#ifndef SWIG
+template <typename String, typename>
+Status AbortedError(String&& message, absl::SourceLocation loc) {
+  return status_internal::MakeErrorFromStringRvalue<StatusCode::kAborted>(
+      std::forward<String>(message), loc);
+}
+#endif  // SWIG
+
+// AlreadyExistsError
+inline Status AlreadyExistsError(absl::string_view message,
+                                 absl::SourceLocation loc) {
+  return status_internal::MakeErrorFromStringView<StatusCode::kAlreadyExists>(
+      message, loc);
+}
+
+#ifndef SWIG
+template <typename String, typename>
+Status AlreadyExistsError(String&& message, absl::SourceLocation loc) {
+  return status_internal::MakeErrorFromStringRvalue<StatusCode::kAlreadyExists>(
+      std::forward<String>(message), loc);
+}
+#endif  // SWIG
+
+// CancelledError
+inline Status CancelledError(absl::string_view message,
+                             absl::SourceLocation loc) {
+  return status_internal::MakeErrorFromStringView<StatusCode::kCancelled>(
+      message, loc);
+}
+
+#ifndef SWIG
+template <typename String, typename>
+Status CancelledError(String&& message, absl::SourceLocation loc) {
+  return status_internal::MakeErrorFromStringRvalue<StatusCode::kCancelled>(
+      std::forward<String>(message), loc);
+}
+#endif  // SWIG
+
+// DataLossError
+inline Status DataLossError(absl::string_view message,
+                            absl::SourceLocation loc) {
+  return status_internal::MakeErrorFromStringView<StatusCode::kDataLoss>(
+      message, loc);
+}
+
+#ifndef SWIG
+template <typename String, typename>
+Status DataLossError(String&& message, absl::SourceLocation loc) {
+  return status_internal::MakeErrorFromStringRvalue<StatusCode::kDataLoss>(
+      std::forward<String>(message), loc);
+}
+#endif  // SWIG
+
+// DeadlineExceededError
+inline Status DeadlineExceededError(absl::string_view message,
+                                    absl::SourceLocation loc) {
+  return status_internal::MakeErrorFromStringView<
+      StatusCode::kDeadlineExceeded>(message, loc);
+}
+
+#ifndef SWIG
+template <typename String, typename>
+Status DeadlineExceededError(String&& message, absl::SourceLocation loc) {
+  return status_internal::MakeErrorFromStringRvalue<
+      StatusCode::kDeadlineExceeded>(std::forward<String>(message), loc);
+}
+#endif  // SWIG
+
+// FailedPreconditionError
+inline Status FailedPreconditionError(absl::string_view message,
+                                      absl::SourceLocation loc) {
+  return status_internal::MakeErrorFromStringView<
+      StatusCode::kFailedPrecondition>(message, loc);
+}
+
+#ifndef SWIG
+template <typename String, typename>
+Status FailedPreconditionError(String&& message, absl::SourceLocation loc) {
+  return status_internal::MakeErrorFromStringRvalue<
+      StatusCode::kFailedPrecondition>(std::forward<String>(message), loc);
+}
+#endif  // SWIG
+
+// InternalError
+inline Status InternalError(absl::string_view message,
+                            absl::SourceLocation loc) {
+  return status_internal::MakeErrorFromStringView<StatusCode::kInternal>(
+      message, loc);
+}
+
+#ifndef SWIG
+template <typename String, typename>
+Status InternalError(String&& message, absl::SourceLocation loc) {
+  return status_internal::MakeErrorFromStringRvalue<StatusCode::kInternal>(
+      std::forward<String>(message), loc);
+}
+#endif  // SWIG
+
+// InvalidArgumentError
+inline Status InvalidArgumentError(absl::string_view message,
+                                   absl::SourceLocation loc) {
+  return status_internal::MakeErrorFromStringView<StatusCode::kInvalidArgument>(
+      message, loc);
+}
+
+#ifndef SWIG
+template <typename String, typename>
+Status InvalidArgumentError(String&& message, absl::SourceLocation loc) {
+  return status_internal::MakeErrorFromStringRvalue<
+      StatusCode::kInvalidArgument>(std::forward<String>(message), loc);
+}
+#endif  // SWIG
+
+// NotFoundError
+inline Status NotFoundError(absl::string_view message,
+                            absl::SourceLocation loc) {
+  return status_internal::MakeErrorFromStringView<StatusCode::kNotFound>(
+      message, loc);
+}
+
+#ifndef SWIG
+template <typename String, typename>
+Status NotFoundError(String&& message, absl::SourceLocation loc) {
+  return status_internal::MakeErrorFromStringRvalue<StatusCode::kNotFound>(
+      std::forward<String>(message), loc);
+}
+#endif  // SWIG
+
+// OutOfRangeError
+inline Status OutOfRangeError(absl::string_view message,
+                              absl::SourceLocation loc) {
+  return status_internal::MakeErrorFromStringView<StatusCode::kOutOfRange>(
+      message, loc);
+}
+
+#ifndef SWIG
+template <typename String, typename>
+Status OutOfRangeError(String&& message, absl::SourceLocation loc) {
+  return status_internal::MakeErrorFromStringRvalue<StatusCode::kOutOfRange>(
+      std::forward<String>(message), loc);
+}
+#endif  // SWIG
+
+// PermissionDeniedError
+inline Status PermissionDeniedError(absl::string_view message,
+                                    absl::SourceLocation loc) {
+  return status_internal::MakeErrorFromStringView<
+      StatusCode::kPermissionDenied>(message, loc);
+}
+
+#ifndef SWIG
+template <typename String, typename>
+Status PermissionDeniedError(String&& message, absl::SourceLocation loc) {
+  return status_internal::MakeErrorFromStringRvalue<
+      StatusCode::kPermissionDenied>(std::forward<String>(message), loc);
+}
+#endif  // SWIG
+
+// ResourceExhaustedError
+inline Status ResourceExhaustedError(absl::string_view message,
+                                     absl::SourceLocation loc) {
+  return status_internal::MakeErrorFromStringView<
+      StatusCode::kResourceExhausted>(message, loc);
+}
+
+#ifndef SWIG
+template <typename String, typename>
+Status ResourceExhaustedError(String&& message, absl::SourceLocation loc) {
+  return status_internal::MakeErrorFromStringRvalue<
+      StatusCode::kResourceExhausted>(std::forward<String>(message), loc);
+}
+#endif  // SWIG
+
+// UnauthenticatedError
+inline Status UnauthenticatedError(absl::string_view message,
+                                   absl::SourceLocation loc) {
+  return status_internal::MakeErrorFromStringView<StatusCode::kUnauthenticated>(
+      message, loc);
+}
+
+#ifndef SWIG
+template <typename String, typename>
+Status UnauthenticatedError(String&& message, absl::SourceLocation loc) {
+  return status_internal::MakeErrorFromStringRvalue<
+      StatusCode::kUnauthenticated>(std::forward<String>(message), loc);
+}
+#endif  // SWIG
+
+// UnavailableError
+inline Status UnavailableError(absl::string_view message,
+                               absl::SourceLocation loc) {
+  return status_internal::MakeErrorFromStringView<StatusCode::kUnavailable>(
+      message, loc);
+}
+
+#ifndef SWIG
+template <typename String, typename>
+Status UnavailableError(String&& message, absl::SourceLocation loc) {
+  return status_internal::MakeErrorFromStringRvalue<StatusCode::kUnavailable>(
+      std::forward<String>(message), loc);
+}
+#endif  // SWIG
+
+// UnimplementedError
+inline Status UnimplementedError(absl::string_view message,
+                                 absl::SourceLocation loc) {
+  return status_internal::MakeErrorFromStringView<StatusCode::kUnimplemented>(
+      message, loc);
+}
+
+#ifndef SWIG
+template <typename String, typename>
+Status UnimplementedError(String&& message, absl::SourceLocation loc) {
+  return status_internal::MakeErrorFromStringRvalue<StatusCode::kUnimplemented>(
+      std::forward<String>(message), loc);
+}
+#endif  // SWIG
+
+// UnknownError
+inline Status UnknownError(absl::string_view message,
+                           absl::SourceLocation loc) {
+  return status_internal::MakeErrorFromStringView<StatusCode::kUnknown>(message,
+                                                                        loc);
+}
+
+#ifndef SWIG
+template <typename String, typename>
+Status UnknownError(String&& message, absl::SourceLocation loc) {
+  return status_internal::MakeErrorFromStringRvalue<StatusCode::kUnknown>(
+      std::forward<String>(message), loc);
+}
+#endif  // SWIG
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STATUS_STATUS_H_
diff --git a/absl/status/status_benchmark.cc b/absl/status/status_benchmark.cc
new file mode 100644
index 00000000000..b001e6310ef
--- /dev/null
+++ b/absl/status/status_benchmark.cc
@@ -0,0 +1,70 @@
+// Copyright 2025 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <string>
+#include <utility>
+
+#include "absl/status/status.h"
+#include "absl/types/source_location.h"
+#include "benchmark/benchmark.h"
+
+namespace {
+
+void BM_CreateOk(benchmark::State& state) {
+  for (auto _ : state) {
+    absl::Status s;  // ok.
+    benchmark::DoNotOptimize(s);
+  }
+}
+BENCHMARK(BM_CreateOk);
+
+void BM_CreateBad(benchmark::State& state) {
+  for (auto _ : state) {
+    absl::Status s(absl::StatusCode::kInvalidArgument, "message");
+    benchmark::DoNotOptimize(s);
+  }
+}
+BENCHMARK(BM_CreateBad);
+
+void BM_AppendSourceLocation(benchmark::State& state) {
+  for (auto _ : state) {
+    absl::Status s(absl::StatusCode::kInvalidArgument, "message");
+    benchmark::DoNotOptimize(s);
+    absl::Status s2(std::move(s), absl::SourceLocation::current());
+    benchmark::DoNotOptimize(s2);
+  }
+}
+BENCHMARK(BM_AppendSourceLocation);
+
+void BM_LongMessageRValue(benchmark::State& state) {
+  for (auto _ : state) {
+    std::string msg(100, 'X');
+    benchmark::DoNotOptimize(msg);
+    absl::Status s(absl::StatusCode::kInvalidArgument, std::move(msg));
+    benchmark::DoNotOptimize(s);
+  }
+}
+BENCHMARK(BM_LongMessageRValue);
+
+void BM_LongMessageRValueConvenienceFn(benchmark::State& state) {
+  for (auto _ : state) {
+    std::string msg(100, 'X');
+    benchmark::DoNotOptimize(msg);
+    absl::Status s = absl::InvalidArgumentError(std::move(msg));
+    benchmark::DoNotOptimize(s);
+  }
+}
+BENCHMARK(BM_LongMessageRValueConvenienceFn);
+
+}  // namespace
diff --git a/absl/status/status_builder.cc b/absl/status/status_builder.cc
new file mode 100644
index 00000000000..8b30f618c53
--- /dev/null
+++ b/absl/status/status_builder.cc
@@ -0,0 +1,196 @@
+// Copyright 2026 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/status/status_builder.h"
+
+#include <cstdint>
+#include <memory>
+#include <ostream>
+#include <string>
+#include <type_traits>
+#include <utility>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/macros.h"
+#include "absl/status/status.h"
+#include "absl/strings/cord.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/string_view.h"
+#include "absl/time/time.h"
+#include "absl/types/source_location.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+void StatusBuilder::Destroy(std::unique_ptr<Rep>) {
+  // nothing to do. The unique_ptr will do the cleanup.
+}
+
+// These constructors are not-inlined and defined in the .cc file to reduce
+// binary size. See cl/354351433 for a quantification.
+StatusBuilder::StatusBuilder() = default;
+
+StatusBuilder::StatusBuilder(const absl::Status& original_status,
+                             absl::SourceLocation location)
+    : loc_(location), rep_(InitRep(original_status)) {}
+
+StatusBuilder::operator absl::Status() const& {
+  if (rep_ == nullptr) return absl::Status();
+  return CreateStatusAndConditionallyLog(loc_, std::make_unique<Rep>(*rep_));
+}
+
+StatusBuilder::Rep::Rep(const absl::Status& s) : status(s) {}
+StatusBuilder::Rep::Rep(absl::Status&& s) : status(std::move(s)) {}
+StatusBuilder::Rep::~Rep() = default;
+
+StatusBuilder::Rep* StatusBuilder::InitRepImpl(absl::Status s) {
+  if (s.ok()) {
+    return nullptr;
+  } else {
+    return new Rep(std::move(s));
+  }
+}
+
+StatusBuilder::Rep::Rep(const Rep& r)
+    : status(r.status),
+      logging_mode(r.logging_mode),
+      log_severity(r.log_severity),
+      verbose_level(r.verbose_level),
+      n(r.n),
+      period(r.period),
+      stream_message(r.stream_message),
+      sink(r.sink),
+      message_join_style(r.message_join_style),
+      should_log_stack_trace(r.should_log_stack_trace),
+      also_send_to_log(r.also_send_to_log) {
+  if (r.stream.has_value()) {
+    InitStream();
+  }
+}
+
+void StatusBuilder::Rep::InitStream() { stream.emplace(stream_message); }
+
+bool StatusBuilder::HasPayload() const {
+  static constexpr absl::string_view kMessageSetUrl =
+      "type.googleapis.com/util.MessageSetPayload";
+  return rep_ != nullptr && rep_->status.GetPayload(kMessageSetUrl).has_value();
+}
+
+ABSL_ATTRIBUTE_WEAK StatusBuilder& StatusBuilder::SetCode(
+    absl::StatusCode code) & {
+  if (rep_ == nullptr) {
+    rep_ = std::make_unique<StatusBuilder::Rep>(
+        absl::Status(code, absl::string_view(), absl::SourceLocation()));
+  } else {
+    absl::Status status(code, absl::string_view(), absl::SourceLocation());
+    rep_->status.ForEachPayload(
+        [&status](absl::string_view type_url, const absl::Cord& payload) {
+          status.SetPayload(type_url, payload);
+        });
+    rep_->status = std::move(status);
+  }
+  return *this;
+}
+
+ABSL_ATTRIBUTE_WEAK void AbslInternalSetErrorCode(StatusBuilder& builder,
+                                                  absl::StatusCode code) {
+  builder.SetCode(code);
+}
+
+class status_internal::StatusPrivateAccessorForStatusBuilder {
+ public:
+  static absl::Status SetMessage(const absl::Status& status,
+                                 absl::string_view message) {
+    ABSL_ASSERT(!status.ok());
+
+    if (message.empty()) {
+      return absl::Status(status.code(), message, absl::SourceLocation());
+    }
+
+    using StatusRep =
+        std::remove_cv_t<std::remove_pointer_t<decltype(Status::RepToPointer(
+            std::declval<uintptr_t>()))>>;
+    StatusRep* rep;
+    if (Status::IsInlined(status.rep_)) {
+      rep = new StatusRep(Status::InlinedRepToCode(status.rep_), message,
+                          nullptr);
+    } else {
+      rep = Status::RepToPointer(status.rep_)->Clone(message, true, true);
+    }
+    return absl::Status(Status::PointerToRep(rep));
+  }
+
+  static absl::Status JoinMessageToStatus(absl::Status s, absl::string_view msg,
+                                          MessageJoinStyle style) {
+    if (s.ok() || msg.empty()) return s;
+    const absl::string_view original_message = s.message();
+    switch (style) {
+      case MessageJoinStyle::kAnnotate: {
+        std::string annotated;
+        if (!original_message.empty()) {
+          absl::StrAppend(&annotated, original_message, "; ", msg);
+          msg = annotated;
+        }
+        return SetMessage(s, msg);
+      }
+      case MessageJoinStyle::kPrepend:
+        return SetMessage(s, absl::StrCat(msg, original_message));
+      case MessageJoinStyle::kAppend:
+        return SetMessage(s, absl::StrCat(original_message, msg));
+      default:
+        return absl::InternalError("Unknown MessageJoinStyle");
+    }
+  }
+};
+
+ABSL_ATTRIBUTE_WEAK std::string StatusBuilder::CurrentStackTrace() {
+  return std::string();
+}
+
+ABSL_ATTRIBUTE_WEAK absl::Status StatusBuilder::CreateStatusAndConditionallyLog(
+    absl::SourceLocation loc, std::unique_ptr<Rep> rep) {
+  if (rep == nullptr) return absl::OkStatus();
+  absl::Status result = status_internal::StatusPrivateAccessorForStatusBuilder::
+      JoinMessageToStatus(std::move(rep->status), rep->stream_message,
+                          rep->message_join_style);
+  // Passing in the `loc` last to ensure the sequence of the source locations.
+  result.AddSourceLocation(loc);
+  return result;
+}
+
+ABSL_ATTRIBUTE_WEAK std::string StatusBuilder::ToString() const {
+  if (rep_ == nullptr) {
+    return absl::OkStatus().ToString();
+  }
+
+  return status_internal::StatusPrivateAccessorForStatusBuilder::
+      JoinMessageToStatus(rep_->status, rep_->stream_message,
+                          rep_->message_join_style)
+          .WithSourceLocation(loc_)
+          .ToString();
+}
+
+ABSL_ATTRIBUTE_WEAK std::ostream& operator<<(std::ostream& os,
+                                             const StatusBuilder& builder) {
+  return os << static_cast<absl::Status>(builder);
+}
+
+ABSL_ATTRIBUTE_WEAK std::ostream& operator<<(std::ostream& os,
+                                             StatusBuilder&& builder) {
+  return os << static_cast<absl::Status>(std::move(builder));
+}
+
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/status/status_builder.h b/absl/status/status_builder.h
new file mode 100644
index 00000000000..55ab7a8355e
--- /dev/null
+++ b/absl/status/status_builder.h
@@ -0,0 +1,978 @@
+// Copyright 2026 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: status_builder.h
+// -----------------------------------------------------------------------------
+
+#ifndef ABSL_STATUS_STATUS_BUILDER_H_
+#define ABSL_STATUS_STATUS_BUILDER_H_
+
+#include <memory>
+#include <optional>
+#include <ostream>
+#include <string>
+#include <type_traits>
+#include <utility>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/log_severity.h"
+#include "absl/base/optimization.h"
+#include "absl/status/status.h"
+#include "absl/strings/cord.h"
+#include "absl/strings/internal/ostringstream.h"
+#include "absl/strings/internal/stringify_stream.h"
+#include "absl/strings/string_view.h"
+#include "absl/time/time.h"
+#include "absl/types/source_location.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+class LogSink;
+class StatusBuilder;
+
+namespace status_internal {
+
+// StatusBuilder's operator<< overloads use an AbslStringifyStream to allow us
+// to use AbslStringify. This wraps (but does not own) an OStringStream, which
+// we use for speed. We bundle them together in Stream here, partly for
+// convenience in StatusBuilder's implementation, and partly to help make sure
+// their lifetimes are managed correctly.
+class Stream {
+ public:
+  explicit Stream(std::string& message)
+      : ostringstream_(&message), absl_stringify_stream_(ostringstream_) {}
+
+  template <typename T>
+  friend Stream& operator<<(Stream& stream, const T& t) {
+    stream.absl_stringify_stream_ << t;
+    return stream;
+  }
+
+ private:
+  absl::strings_internal::OStringStream ostringstream_;
+  absl::strings_internal::StringifyStream absl_stringify_stream_;
+};
+
+// StatusBuilder::With() adaptors can be classified as either "pure policy" or
+// "terminal". Terminal adaptors are a mix of "side effect" or "conversion".
+// We differentiate between these types by the functor's return type.
+//
+// This is currently for analysis only, as part of an ongoing LSC investigation.
+template <typename Fn, typename Arg, typename Expected>
+inline constexpr bool kResultMatches =
+    std::is_same_v<std::decay_t<std::invoke_result_t<Fn, Arg>>, Expected>;
+
+template <typename Adaptor, typename Builder>
+using PurePolicy =
+    std::enable_if_t<kResultMatches<Adaptor, Builder, StatusBuilder>,
+                     std::invoke_result_t<Adaptor, Builder>>;
+
+template <typename Adaptor, typename Builder>
+using SideEffect =
+    std::enable_if_t<kResultMatches<Adaptor, Builder, absl::Status>,
+                     std::invoke_result_t<Adaptor, Builder>>;
+
+template <typename Adaptor, typename Builder>
+using Conversion =
+    std::enable_if_t<!kResultMatches<Adaptor, Builder, StatusBuilder> &&
+                         !kResultMatches<Adaptor, Builder, absl::Status>,
+                     std::invoke_result_t<Adaptor, Builder>>;
+
+class StatusBuilderPrivateAccessor;
+
+}  // namespace status_internal
+
+// Internal argument-dependent lookup extension point for StatusBuilder.
+// Necessary for allowing StatusBuilder to be open-sourced into Abseil without
+// introducing hard dependencies on non-canonical error spaces or protobuf.
+//
+// What we would have preferred here is to just leave
+// StatusBuilder::SetErrorCode() undeclared for OSS users, but defined for
+// internal users. However, C++ doesn't provide a great way to do that directly,
+// since merely omitting a definition still leaves it declared and thus
+// accessible.
+//
+// We therefore use this ADL extension point as the closest moral equivalent:
+// this allows use to define a simple shell for the method in OSS, but to
+// prevent its actual usage (via SFINAE) unless this extension point is also
+// defined, which is only the case in our internal libraries.
+void AbslInternalSetErrorCode(StatusBuilder&, absl::StatusCode);
+
+// Specifies how to join the error message in the original status and any
+// additional message that has been streamed into the builder.
+enum class MessageJoinStyle {
+  kAnnotate,
+  kAppend,
+  kPrepend,
+};
+
+// Creates a status based on an original_status, but enriched with additional
+// information.  The builder implicitly converts to Status and StatusOr<T>
+// allowing for it to be returned directly.
+//
+//   StatusBuilder builder(original);
+//   builder.SetPayload(proto);
+//   builder << "info about error";
+//   return builder;
+//
+// It provides method chaining to simplify typical usage:
+//
+//   return StatusBuilder(original)
+//       .Log(absl::LogSeverity::kWarning) << "oh no!";
+//
+// In more detail:
+// - When the original status is OK, all methods become no-ops and nothing will
+//   be logged.
+// - Messages streamed into the status builder are collected into a single
+//   additional message string.
+// - The original Status's message and the additional message are joined
+//   together when the result status is built.
+// - By default, the messages will be joined as if by `util::Annotate`, which
+//   includes a convenience separator between the original message and the
+//   additional one.  This behavior can be changed with the `SetAppend()` and
+//   `SetPrepend()` methods of the builder.
+// - By default, the result status is not logged.  The `Log` and
+//   `EmitStackTrace` methods will cause the builder to log the result status
+//   when it is built.
+// - All side effects (like logging or constructing a stack trace) happen when
+//   the builder is converted to a status.
+class ABSL_MUST_USE_RESULT StatusBuilder {
+ public:
+  explicit StatusBuilder();
+  ~StatusBuilder();
+
+  // Creates a `StatusBuilder` based on an original status.  If logging is
+  // enabled, it will use `location` as the location from which the log message
+  // occurs.  A typical user will not specify `location`, allowing it to default
+  // to the current location.
+  explicit StatusBuilder(
+      const absl::Status& original_status,
+      absl::SourceLocation location = absl::SourceLocation::current());
+  explicit StatusBuilder(
+      absl::Status&& original_status,
+      absl::SourceLocation location = absl::SourceLocation::current());
+
+  // Creates a `StatusBuilder` from a status code.  If logging is enabled, it
+  // will use `location` as the location from which the log message occurs.  A
+  // typical user will not specify `location`, allowing it to default to the
+  // current location.
+  explicit StatusBuilder(
+      absl::StatusCode code,
+      absl::SourceLocation location = absl::SourceLocation::current());
+
+  StatusBuilder(const StatusBuilder& sb);
+  StatusBuilder& operator=(const StatusBuilder& sb);
+  StatusBuilder(StatusBuilder&&) = default;
+  StatusBuilder& operator=(StatusBuilder&&) = default;
+
+  // Mutates the builder so that the final additional message is prepended to
+  // the original error message in the status.  A convenience separator is not
+  // placed between the messages.
+  //
+  // NOTE: Multiple calls to `SetPrepend` and `SetAppend` just adjust the
+  // behavior of the final join of the original status with the extra message.
+  //
+  // Returns `*this` to allow method chaining.
+  StatusBuilder& SetPrepend() &;
+  ABSL_MUST_USE_RESULT StatusBuilder&& SetPrepend() &&;
+
+  // Mutates the builder so that the final additional message is appended to the
+  // original error message in the status.  A convenience separator is not
+  // placed between the messages.
+  //
+  // NOTE: Multiple calls to `SetPrepend` and `SetAppend` just adjust the
+  // behavior of the final join of the original status with the extra message.
+  //
+  // Returns `*this` to allow method chaining.
+  StatusBuilder& SetAppend() &;
+  ABSL_MUST_USE_RESULT StatusBuilder&& SetAppend() &&;
+
+  // Mutates the builder to disable any logging that was set using any of the
+  // logging functions below.  Returns `*this` to allow method chaining.
+  StatusBuilder& SetNoLogging() &;
+  ABSL_MUST_USE_RESULT StatusBuilder&& SetNoLogging() &&;
+
+  // Mutates the builder so that the result status will be logged (without a
+  // stack trace) when this builder is converted to a Status.  This overrides
+  // the logging settings from earlier calls to any of the logging mutator
+  // functions.  Returns `*this` to allow method chaining.
+  StatusBuilder& Log(absl::LogSeverity level) &;
+  ABSL_MUST_USE_RESULT StatusBuilder&& Log(absl::LogSeverity level) &&;
+
+  StatusBuilder& LogError() & { return Log(absl::LogSeverity::kError); }
+  ABSL_MUST_USE_RESULT StatusBuilder&& LogError() && {
+    return std::move(LogError());
+  }
+  StatusBuilder& LogWarning() & { return Log(absl::LogSeverity::kWarning); }
+  ABSL_MUST_USE_RESULT StatusBuilder&& LogWarning() && {
+    return std::move(LogWarning());
+  }
+  StatusBuilder& LogInfo() & { return Log(absl::LogSeverity::kInfo); }
+  ABSL_MUST_USE_RESULT StatusBuilder&& LogInfo() && {
+    return std::move(LogInfo());
+  }
+
+  // Mutates the builder so that the result status will be logged every N
+  // invocations (without a stack trace) when this builder is converted to a
+  // Status.  This overrides the logging settings from earlier calls to any of
+  // the logging mutator functions.  Returns `*this` to allow method chaining.
+  StatusBuilder& LogEveryN(absl::LogSeverity level, int n) &;
+  ABSL_MUST_USE_RESULT StatusBuilder&& LogEveryN(absl::LogSeverity level,
+                                                 int n) &&;
+
+  // Mutates the builder so that the result status will be logged once per
+  // period (without a stack trace) when this builder is converted to a Status.
+  // This overrides the logging settings from earlier calls to any of the
+  // logging mutator functions.  Returns `*this` to allow method chaining.
+  // If period is absl::ZeroDuration() or less, then this is equivalent to
+  // calling the Log() method.
+  StatusBuilder& LogEvery(absl::LogSeverity level, absl::Duration period) &;
+  ABSL_MUST_USE_RESULT StatusBuilder&& LogEvery(absl::LogSeverity level,
+                                                absl::Duration period) &&;
+
+  // Mutates the builder so that the result status will be VLOGged (without a
+  // stack trace) when this builder is converted to a Status.  `verbose_level`
+  // indicates the verbosity level that would be passed to VLOG().  This
+  // overrides the logging settings from earlier calls to any of the logging
+  // mutator functions.  Returns `*this` to allow method chaining.
+  StatusBuilder& VLog(int verbose_level) &;
+  ABSL_MUST_USE_RESULT StatusBuilder&& VLog(int verbose_level) &&;
+
+  // Mutates the builder so that a stack trace will be logged if the status is
+  // logged. One of the logging setters above should be called as well. If
+  // logging is not yet enabled this behaves as if LogInfo().EmitStackTrace()
+  // was called. Returns `*this` to allow method chaining.
+  StatusBuilder& EmitStackTrace() &;
+  ABSL_MUST_USE_RESULT StatusBuilder&& EmitStackTrace() &&;
+
+  // Mutates the builder so that the result status will also be logged to the
+  // provided `sink` when this builder is converted to a status. Overwrites any
+  // sink set prior. The provided `sink` must point to a valid object by the
+  // time this builder is converted to a status. Has no effect if this builder
+  // is not configured to log by calling any of the LogXXX methods. Returns
+  // `*this` to allow method chaining.
+  StatusBuilder& AlsoOutputToSink(absl::LogSink* sink) &;
+  ABSL_MUST_USE_RESULT StatusBuilder&& AlsoOutputToSink(absl::LogSink* sink) &&;
+
+  // Mutates the builder so that the result status will only be logged to the
+  // provided `sink` when this builder is converted to a status. Overwrites any
+  // sink set prior. The provided `sink` must point to a valid object by the
+  // time this builder is converted to a status. Has no effect if this builder
+  // is not configured to log by calling any of the LogXXX methods. Returns
+  // `*this` to allow method chaining.
+  StatusBuilder& OnlyOutputToSink(absl::LogSink* sink) &;
+  ABSL_MUST_USE_RESULT StatusBuilder&& OnlyOutputToSink(absl::LogSink* sink) &&;
+
+  // Appends to the extra message that will be added to the original status.  By
+  // default, the extra message is added to the original message as if by
+  // `util::Annotate`, which includes a convenience separator between the
+  // original message and the enriched one.
+  template <typename T>
+  StatusBuilder& operator<<(const T& value) &;
+
+  template <typename T>
+  ABSL_MUST_USE_RESULT StatusBuilder&& operator<<(const T& value) &&;
+
+  // Adds a payload for the status that will be returned by this StatusBuilder.
+  // Note that this is equivalent to `Status::SetPayload`, to attach protos to
+  // the MessageSet payload, use `util::AttachPayload`. Returns '*this' to allow
+  // method chaining.
+  StatusBuilder& SetPayload(absl::string_view type_url, absl::Cord payload) &;
+  ABSL_MUST_USE_RESULT StatusBuilder&& SetPayload(absl::string_view type_url,
+                                                  absl::Cord payload) && {
+    return std::move(SetPayload(type_url, std::move(payload)));
+  }
+
+  std::optional<absl::Cord> GetPayload(absl::string_view type_url) const;
+
+  // INTERNAL API. NOT FOR PUBLIC USE.
+  template <typename MessageSetExtension, typename ExtensionIdentifier>
+  auto AttachPayload(const MessageSetExtension& obj,
+                     const ExtensionIdentifier& id) &  //
+      -> std::enable_if_t<
+          std::is_void_v<decltype(AbslInternalAttachPayload(*this, obj, id))>,
+          StatusBuilder&> {
+    AbslInternalAttachPayload(*this, obj, id);
+    return *this;
+  }
+
+  // As above, but &&-qualified.
+  //
+  // Uses decltype() to propagate template constraints (SFINAE).
+  template <typename MessageSetExtension, typename ExtensionIdentifier>
+  ABSL_MUST_USE_RESULT auto AttachPayload(const MessageSetExtension& obj,
+                                          const ExtensionIdentifier& id) &&  //
+      -> decltype(std::move(AttachPayload(obj, id))) {
+    return std::move(AttachPayload(obj, id));
+  }
+
+  // Like `AttachPayload() &`, but with the default extension ID.
+  //
+  // INTERNAL API. NOT FOR PUBLIC USE.
+  template <typename MessageSetExtension>
+  auto AttachPayload(const MessageSetExtension& obj) &  //
+      -> std::enable_if_t<
+          std::is_void_v<decltype(AbslInternalAttachPayload(*this, obj))>,
+          StatusBuilder&> {
+    AbslInternalAttachPayload(*this, obj);
+    return *this;
+  }
+
+  // As above, but &&-qualified.
+  //
+  // Uses decltype() to propagate template constraints (SFINAE).
+  template <typename MessageSetExtension>
+  ABSL_MUST_USE_RESULT auto AttachPayload(const MessageSetExtension& obj) &&  //
+      -> decltype(std::move(AttachPayload(obj))) {
+    return std::move(AttachPayload(obj));
+  }
+
+  // HasPayload()
+  //
+  // Indicates whether the Status object that will be returned by the
+  // StatusBuilder contains any payloads with a type extending proto2's
+  // `MessageSet`, returning `true` if so. Having a payload does not guarantee
+  // the presence of a payload with a specific type. Note that returning `false`
+  // does not necessarily indicate the absence of a payload, but only the
+  // absence on one which extends `MessageSet`.
+  bool HasPayload() const;
+
+  // INTERNAL API. NOT FOR PUBLIC USE.
+  template <typename Enum>
+  auto SetErrorCode(Enum code) &  //
+      -> std::enable_if_t<
+          std::is_void_v<decltype(AbslInternalSetErrorCode(*this, code))>,
+          StatusBuilder&> {
+    AbslInternalSetErrorCode(*this, code);
+    return *this;
+  }
+
+  // As above, but &&-qualified.
+  //
+  // Uses decltype() to propagate template constraints (SFINAE).
+  template <typename Enum>
+  ABSL_MUST_USE_RESULT auto SetErrorCode(Enum code) &&  //
+      -> decltype(std::move(SetErrorCode(code))) {
+    return std::move(SetErrorCode(code));
+  }
+
+  // Sets the status code for the status that will be returned by this
+  // StatusBuilder. Returns `*this` to allow method chaining.
+  StatusBuilder& SetCode(absl::StatusCode code) &;
+  ABSL_MUST_USE_RESULT StatusBuilder&& SetCode(absl::StatusCode code) && {
+    return std::move(SetCode(code));
+  }
+
+  ///////////////////////////////// Adaptors /////////////////////////////////
+  //
+  // A StatusBuilder `adaptor` is a functor which can be included in a builder
+  // method chain. There are two common variants:
+  //
+  // 1. `Pure policy` adaptors modify the StatusBuilder and return the modified
+  //    object, which can then be chained with further adaptors or mutations.
+  //
+  // 2. `Terminal` adaptors consume the builder's Status and return some
+  //    other type of object. Alternatively, the consumed Status may be used
+  //    for side effects, e.g. by passing it to a side channel. A terminal
+  //    adaptor cannot be chained.
+  //
+  // Careful: The conversion of StatusBuilder to Status has side effects!
+  // Adaptors must ensure that this conversion happens at most once in the
+  // builder chain. The easiest way to do this is to determine the adaptor type
+  // and follow the corresponding guidelines:
+  //
+  // Pure policy adaptors should:
+  // (a) Take a StatusBuilder as input parameter.
+  // (b) NEVER convert the StatusBuilder to Status:
+  //     - Never assign the builder to a Status variable.
+  //     - Never pass the builder to a function whose parameter type is Status,
+  //       including by reference (e.g. const Status&).
+  //     - Never pass the builder to any function which might convert the
+  //       builder to Status (i.e. this restriction is viral).
+  // (c) Return a StatusBuilder (usually the input parameter).
+  //
+  // Terminal adaptors should:
+  // (a) Take a Status as input parameter (not a StatusBuilder!).
+  // (b) Return a type matching the enclosing function. (This can be `void`.)
+  //
+  // Adaptors do not necessarily fit into one of these categories. However, any
+  // which satisfies the conversion rule can always be decomposed into a pure
+  // adaptor chained into a terminal adaptor. (This is recommended.)
+  //
+  // Examples
+  //
+  // Pure adaptors allow teams to configure team-specific error handling
+  // policies.  For example:
+  //
+  //   StatusBuilder TeamPolicy(StatusBuilder builder) {
+  //     builder.SetPayload(...);
+  //     return std::move(builder).Log(absl::LogSeverity::kWarning);
+  //   }
+  //
+  //   ABSL_RETURN_IF_ERROR(foo()).With(TeamPolicy);
+  //
+  // Because pure policy adaptors return the modified StatusBuilder, they
+  // can be chained with further adaptors, e.g.:
+  //
+  //   ABSL_RETURN_IF_ERROR(foo()).With(TeamPolicy).With(OtherTeamPolicy);
+  //
+  // Terminal adaptors are often used for type conversion. This allows
+  // ABSL_RETURN_IF_ERROR to be used in functions which do not return Status.
+  // For example, a function might want to return some default value on error:
+  //
+  //   int GetSysCounter() {
+  //     int value;
+  //     ABSL_RETURN_IF_ERROR(ReadCounterFile(filename, &value))
+  //         .LogInfo()
+  //         .With([](const absl::Status& unused) { return 0; });
+  //     return value;
+  //   }
+  //
+  // For the simple case of returning a constant (e.g. zero, false, nullptr) on
+  // error, consider `status_macros::Return` or `status_macros::ReturnVoid`:
+  //
+  //   #include "util/task/contrib/status_macros/return.h"
+  //
+  //   bool DoMyThing() {
+  //     ABSL_RETURN_IF_ERROR(foo()).LogWarning().With(status_macros::Return(false));
+  //     ...
+  //   }
+  //
+  // A terminal adaptor may instead (or additionally) be used to create side
+  // effects that are not supported natively by `StatusBuilder`, such as
+  // returning the Status through a side channel. For example,
+  // `util::TaskReturn` returns the Status through the `util::Task` that it was
+  // initialized with. This adaptor then returns `void`, to match the typical
+  // return type of functions that maintain state through `util::Task`:
+  //
+  //   class TaskReturn {
+  //    public:
+  //     explicit TaskReturn(Task* t) : task_(t) {}
+  //     void operator()(const Status& status) const { task_->Return(status); }
+  //     // ...
+  //   };
+  //
+  //   void Read(absl::string_view name, util::Task* task) {
+  //     int64 id;
+  //     ABSL_RETURN_IF_ERROR(GetIdForName(name, &id)).With(TaskReturn(task));
+  //     ABSL_RETURN_IF_ERROR(ReadForId(id)).With(TaskReturn(task));
+  //     task->Return();
+  //   }
+
+  // Calls `adaptor` on this status builder to apply policies, type conversions,
+  // and/or side effects on the StatusBuilder. Returns the value returned by
+  // `adaptor`, which may be any type including `void`. See comments above.
+  //
+  // Style guide exception for Ref qualified methods and rvalue refs
+  // (cl/128258530).  This allows us to avoid a copy in the common case.
+  //
+  // Note: All With() overrides are equivalent, and return Adaptor(this). They
+  // are part of an ongoing LSC investigation.
+  template <typename Adaptor>
+  auto With(Adaptor&& adaptor) & -> status_internal::PurePolicy<
+      Adaptor, StatusBuilder&> {
+    return std::forward<Adaptor>(adaptor)(*this);
+  }
+  template <typename Adaptor>
+  ABSL_MUST_USE_RESULT auto With(
+      Adaptor&&
+          adaptor) && -> status_internal::PurePolicy<Adaptor, StatusBuilder&&> {
+    return std::forward<Adaptor>(adaptor)(std::move(*this));
+  }
+
+  template <typename Adaptor>
+  auto With(Adaptor&& adaptor) & -> status_internal::SideEffect<
+      Adaptor, StatusBuilder&> {
+    return std::forward<Adaptor>(adaptor)(*this);
+  }
+  template <typename Adaptor>
+  ABSL_MUST_USE_RESULT auto With(
+      Adaptor&&
+          adaptor) && -> status_internal::SideEffect<Adaptor, StatusBuilder&&> {
+    return std::forward<Adaptor>(adaptor)(std::move(*this));
+  }
+
+  template <typename Adaptor>
+  auto With(Adaptor&& adaptor) & -> status_internal::Conversion<
+      Adaptor, StatusBuilder&> {
+    return std::forward<Adaptor>(adaptor)(*this);
+  }
+  template <typename Adaptor>
+  ABSL_MUST_USE_RESULT auto With(
+      Adaptor&&
+          adaptor) && -> status_internal::Conversion<Adaptor, StatusBuilder&&> {
+    return std::forward<Adaptor>(adaptor)(std::move(*this));
+  }
+
+  // Returns true if the Status created by this builder will be ok().
+  ABSL_MUST_USE_RESULT bool ok() const;
+
+  // Returns the (canonical) error code for the Status created by this builder.
+  absl::StatusCode code() const;
+
+  // Implicit conversion to Status.
+  //
+  // Careful: this operator has side effects, so it should be called at
+  // most once. In particular, do NOT use this conversion operator to inspect
+  // the status from an adapter object passed into With().
+  //
+  // Style guide exception for using Ref qualified methods and for implicit
+  // conversions (cl/124566728).  This override allows us to implement
+  // ABSL_RETURN_IF_ERROR with 2 move operations in the common case.
+  operator absl::Status() const&;  // NOLINT: Builder converts implicitly.
+  operator absl::Status() &&;      // NOLINT: Builder converts implicitly.
+
+  // Returns the source location used to create this builder. This differs from
+  // `GetPreviousSourceLocations()` as this location is the single location for
+  // the creation of this builder.
+  ABSL_MUST_USE_RESULT absl::SourceLocation source_location() const;
+
+  // Returns the source locations previously recorded in the status. This will
+  // not include the source location of the `StatusBuilder` itself (which is
+  // available via `source_location()`). When the builder is converted to a
+  // Status, the builder's location will be appended to this list of previous
+  // locations. For `StatusBuilder`s created without an original status (e.g.,
+  // from a status code), this will be empty.
+  decltype(auto) GetPreviousSourceLocations() const {
+    if (rep_ == nullptr) {
+      return absl::OkStatus().GetSourceLocations();
+    }
+    return rep_->status.GetSourceLocations();
+  }
+
+  // Returns a string based on the `mode`. This produces the same string as
+  // would converting to a Status and calling operator<<, but it does so without
+  // side effects (e.g., logging). Therefore, it is safe to use from within an
+  // adapter object passed into With().
+  std::string ToString() const;
+
+ private:
+  friend class status_internal::StatusBuilderPrivateAccessor;
+
+  // Returns true if the compiler can determine that the instance is empty. That
+  // is, `rep_ == nullptr`.
+  // A `false` return does not necessarily indicate that it has a rep. It just
+  // can't prove it doesn't.
+  ABSL_ATTRIBUTE_ALWAYS_INLINE bool IsKnownToBeEmpty() const {
+#if ABSL_HAVE_BUILTIN(__builtin_constant_p)
+    // __builtin_constant_p does not like it when it has non trivial expressions
+    // in it, and `rep_==nullptr` is a user-defined operator.
+    // Do it out of the builtin and pass the bool instead.
+    bool is_empty = rep_ == nullptr;
+    return __builtin_constant_p(is_empty) && is_empty;
+#else
+    return false;
+#endif
+  }
+
+  // Tells the compiler that it can assume that `rep_` is null.
+  // This is verified in non-opt mode.
+  void AssumeEmpty() const {
+    if (rep_ != nullptr) ABSL_UNREACHABLE();
+  }
+
+  static std::string CurrentStackTrace();
+
+  struct Rep;
+  // Destroy the `Rep` object. It is just calling the destructor of the
+  // `unique_ptr`, but out of line.
+  static void Destroy(std::unique_ptr<Rep>);
+
+  // Creates a Status from this builder and logs it if the builder has been
+  // configured to log itself.
+  // NOTE: This function is `static` to prevent escaping the `this` pointer. We
+  //       transfer the `Rep` to delegate the destruction.
+  static absl::Status CreateStatusAndConditionallyLog(absl::SourceLocation loc,
+                                                      std::unique_ptr<Rep> rep);
+
+  // Infrequently set builder options, instantiated lazily. This reduces
+  // average construction/destruction time (e.g. the `stream` is fairly
+  // expensive). Stacks can also be blown if StatusBuilder grows too large.
+  // This is primarily an issue for debug builds, which do not necessarily
+  // re-use stack space within a function across the sub-scopes used by
+  // status macros.
+  struct Rep {
+    explicit Rep(const absl::Status& s);
+    explicit Rep(absl::Status&& s);
+    Rep(const Rep& r);
+    ~Rep();
+    void InitStream();
+
+    // The status that the result will be based on.  Can be modified by
+    // SetPayload().
+    absl::Status status;
+
+    enum class LoggingMode {
+      kDisabled,
+      kLog,
+      kVLog,
+      kLogEveryN,
+      kLogEveryPeriod
+    };
+    LoggingMode logging_mode = LoggingMode::kDisabled;
+
+    // The severity level at which the Status should be logged. Note that
+    // `logging_mode == LoggingMode::kVLog` always logs at severity INFO.
+    absl::LogSeverity log_severity;
+
+    // The level at which the Status should be VLOGged.
+    // Only used when `logging_mode == LoggingMode::kVLog`.
+    int verbose_level;
+
+    // Only log every N invocations.
+    // Only used when `logging_mode == LoggingMode::kLogEveryN`.
+    int n;
+
+    // Only log once per period.
+    // Only used when `logging_mode == LoggingMode::kLogEveryPeriod`.
+    absl::Duration period;
+
+    // Gathers additional messages added with `<<` for use in the final status.
+    std::string stream_message;
+    std::optional<status_internal::Stream> stream;
+
+    // If not nullptr, specifies the log sink where log output should be
+    // sent to.  Only used when `logging_mode != LoggingMode::kDisabled`.
+    absl::LogSink* sink = nullptr;
+
+    // Specifies how to join the message in `status` and `stream`.
+    MessageJoinStyle message_join_style = MessageJoinStyle::kAnnotate;
+
+    // Whether to log stack trace.  Only used when `logging_mode !=
+    // LoggingMode::kDisabled`.
+    bool should_log_stack_trace = false;
+
+    // When this is true, will send both to `sink` and will log. When this is
+    // false, will only send to `sink`.
+    bool also_send_to_log = true;
+  };
+
+  static Rep* InitRep(const absl::Status& s) {
+    if (s.ok()) {
+      return nullptr;
+    } else {
+      return new Rep(s);
+    }
+  }
+
+  static Rep* InitRep(absl::Status&& s) {
+    if (s.ok()) {
+      return nullptr;
+    } else {
+      Rep* rep = InitRepImpl(std::move(s));
+      ABSL_ASSUME(rep != nullptr);
+      return rep;
+    }
+  }
+
+  // Out of line function that constructs the Rep.
+  // Note that `s` is by value because of the TRIVIAL_ABI.
+  static Rep* InitRepImpl(absl::Status s);
+
+  // The location to record if this status is logged.
+  absl::SourceLocation loc_;
+
+  // nullptr if the result status will be OK.  Extra fields moved to the heap to
+  // minimize stack space.
+  std::unique_ptr<Rep> rep_;
+
+  // For testing Rep's implememntation.
+  friend class StatusBuilderTest;
+};
+
+// Implicitly converts `builder` to `Status` and write it to `os`.
+std::ostream& operator<<(std::ostream& os, const StatusBuilder& builder);
+std::ostream& operator<<(std::ostream& os, StatusBuilder&& builder);
+
+// StatusBuilder policy to append an extra message to the original status.
+//
+// This is most useful with adaptors such as util::TaskReturn that otherwise
+// would prevent use of operator<<.  For example:
+//
+//   ABSL_RETURN_IF_ERROR(foo(val))
+//       .With(absl::ExtraMessage("when calling foo()"))
+//       .With(util::TaskReturn(task));
+//
+// or
+//
+//   ABSL_RETURN_IF_ERROR(foo(val))
+//       .With(absl::ExtraMessage() << "val: " << val)
+//       .With(util::TaskReturn(task));
+//
+// Note in the above example, the ABSL_RETURN_IF_ERROR macro ensures the
+// ExtraMessage expression is evaluated only in the error case, so efficiency of
+// constructing the message is not a concern in the success case.
+class ExtraMessage {
+ public:
+  ExtraMessage() : ExtraMessage(std::string()) {}
+  explicit ExtraMessage(std::string msg)
+      : msg_(std::move(msg)), stream_(msg_) {}
+
+  ExtraMessage(
+      ExtraMessage&& other) noexcept  // strings::OStringStream is stateless
+                                      // so we can simply move over the message.
+      : ExtraMessage(std::move(other.msg_)) {}
+
+  // Appends to the extra message that will be added to the original status.  By
+  // default, the extra message is added to the original message as if by
+  // `util::Annotate`, which includes a convenience separator between the
+  // original message and the enriched one.
+  template <typename T>
+  ExtraMessage& operator<<(const T& value) & {
+    stream_ << value;
+    return *this;
+  }
+
+  // As above, preserving the rvalue-ness of the ExtraMessage object.
+  template <typename T>
+  ExtraMessage&& operator<<(const T& value) && {
+    *this << value;
+    return std::move(*this);
+  }
+
+  // Appends to the extra message that will be added to the original status.  By
+  // default, the extra message is added to the original message as if by
+  // `util::Annotate`, which includes a convenience separator between the
+  // original message and the enriched one.
+  StatusBuilder operator()(StatusBuilder builder) const {
+    builder << msg_;
+    return builder;
+  }
+
+ private:
+  std::string msg_;
+  status_internal::Stream stream_;
+};
+
+// Implementation details follow; clients should ignore.
+
+inline StatusBuilder::StatusBuilder(absl::StatusCode code,
+                                    absl::SourceLocation location)
+    : loc_(location), rep_(InitRep(absl::Status(code, ""))) {}
+
+inline StatusBuilder::StatusBuilder(const StatusBuilder& sb) : loc_(sb.loc_) {
+  if (sb.rep_ != nullptr) {
+    rep_ = std::make_unique<Rep>(*sb.rep_);
+  }
+}
+
+inline StatusBuilder::StatusBuilder(absl::Status&& original_status,
+                                    absl::SourceLocation location)
+    : loc_(location), rep_(InitRep(std::move(original_status))) {}
+
+inline StatusBuilder::~StatusBuilder() {
+  if (IsKnownToBeEmpty()) {
+    // Nothing to do.
+    return;
+  }
+  // We will run the destructor logic, so move it out of line.
+  // The destructor of the unique_ptr runs ~Rep() and then ::operator delete.
+  // We don't want that bloat on the caller.
+  Destroy(std::move(rep_));
+  // Tell the compiler that `rep_` was not filled again even if `this` escaped.
+  AssumeEmpty();
+}
+
+inline StatusBuilder& StatusBuilder::operator=(const StatusBuilder& sb) {
+  loc_ = sb.loc_;
+  if (sb.rep_ != nullptr) {
+    rep_ = std::make_unique<Rep>(*sb.rep_);
+  } else {
+    rep_ = nullptr;
+  }
+  return *this;
+}
+
+inline StatusBuilder& StatusBuilder::SetPrepend() & {
+  if (rep_ == nullptr) return *this;
+  rep_->message_join_style = MessageJoinStyle::kPrepend;
+  return *this;
+}
+inline StatusBuilder&& StatusBuilder::SetPrepend() && {
+  return std::move(SetPrepend());
+}
+
+inline StatusBuilder& StatusBuilder::SetAppend() & {
+  if (rep_ == nullptr) return *this;
+  rep_->message_join_style = MessageJoinStyle::kAppend;
+  return *this;
+}
+inline StatusBuilder&& StatusBuilder::SetAppend() && {
+  return std::move(SetAppend());
+}
+
+inline StatusBuilder& StatusBuilder::SetNoLogging() & {
+  if (rep_ != nullptr) {
+    rep_->logging_mode = Rep::LoggingMode::kDisabled;
+    rep_->should_log_stack_trace = false;
+  }
+  return *this;
+}
+inline StatusBuilder&& StatusBuilder::SetNoLogging() && {
+  return std::move(SetNoLogging());
+}
+
+inline StatusBuilder& StatusBuilder::Log(absl::LogSeverity level) & {
+  if (rep_ == nullptr) return *this;
+  rep_->logging_mode = Rep::LoggingMode::kLog;
+  rep_->log_severity = level;
+  return *this;
+}
+inline StatusBuilder&& StatusBuilder::Log(absl::LogSeverity level) && {
+  return std::move(Log(level));
+}
+
+inline StatusBuilder& StatusBuilder::LogEveryN(absl::LogSeverity level,
+                                               int n) & {
+  if (rep_ == nullptr) return *this;
+  if (n < 1) return Log(level);
+  rep_->logging_mode = Rep::LoggingMode::kLogEveryN;
+  rep_->log_severity = level;
+  rep_->n = n;
+  return *this;
+}
+inline StatusBuilder&& StatusBuilder::LogEveryN(absl::LogSeverity level,
+                                                int n) && {
+  return std::move(LogEveryN(level, n));
+}
+
+inline StatusBuilder& StatusBuilder::LogEvery(absl::LogSeverity level,
+                                              absl::Duration period) & {
+  if (rep_ == nullptr) return *this;
+  if (period <= absl::ZeroDuration()) return Log(level);
+  rep_->logging_mode = Rep::LoggingMode::kLogEveryPeriod;
+  rep_->log_severity = level;
+  rep_->period = period;
+  return *this;
+}
+inline StatusBuilder&& StatusBuilder::LogEvery(absl::LogSeverity level,
+                                               absl::Duration period) && {
+  return std::move(LogEvery(level, period));
+}
+
+inline StatusBuilder& StatusBuilder::VLog(int verbose_level) & {
+  if (rep_ == nullptr) return *this;
+  rep_->logging_mode = Rep::LoggingMode::kVLog;
+  rep_->verbose_level = verbose_level;
+  return *this;
+}
+inline StatusBuilder&& StatusBuilder::VLog(int verbose_level) && {
+  return std::move(VLog(verbose_level));
+}
+
+inline StatusBuilder& StatusBuilder::EmitStackTrace() & {
+  if (rep_ == nullptr) return *this;
+  if (rep_->logging_mode == Rep::LoggingMode::kDisabled) {
+    // Default to INFO logging, otherwise nothing would be emitted.
+    rep_->logging_mode = Rep::LoggingMode::kLog;
+    rep_->log_severity = absl::LogSeverity::kInfo;
+  }
+  rep_->should_log_stack_trace = true;
+  return *this;
+}
+inline StatusBuilder&& StatusBuilder::EmitStackTrace() && {
+  return std::move(EmitStackTrace());
+}
+
+inline StatusBuilder& StatusBuilder::AlsoOutputToSink(absl::LogSink* sink) & {
+  if (rep_ == nullptr) return *this;
+  rep_->sink = sink;
+  rep_->also_send_to_log = true;
+  return *this;
+}
+inline StatusBuilder&& StatusBuilder::AlsoOutputToSink(absl::LogSink* sink) && {
+  return std::move(AlsoOutputToSink(sink));
+}
+inline StatusBuilder& StatusBuilder::OnlyOutputToSink(absl::LogSink* sink) & {
+  if (rep_ == nullptr) return *this;
+  rep_->sink = sink;
+  rep_->also_send_to_log = false;
+  return *this;
+}
+inline StatusBuilder&& StatusBuilder::OnlyOutputToSink(absl::LogSink* sink) && {
+  return std::move(OnlyOutputToSink(sink));
+}
+
+template <typename T>
+StatusBuilder& StatusBuilder::operator<<(const T& value) & {
+  if (rep_ == nullptr) return *this;
+  if (!rep_->stream.has_value()) {
+    rep_->InitStream();
+  }
+  *rep_->stream << value;
+  return *this;
+}
+
+template <typename T>
+StatusBuilder&& StatusBuilder::operator<<(const T& value) && {
+  return std::move(operator<<(value));
+}
+
+inline StatusBuilder& StatusBuilder::SetPayload(absl::string_view type_url,
+                                                absl::Cord payload) & {
+  if (rep_ != nullptr) {
+    rep_->status.SetPayload(type_url, std::move(payload));
+  }
+  return *this;
+}
+
+inline std::optional<absl::Cord> StatusBuilder::GetPayload(
+    absl::string_view type_url) const {
+  return rep_ == nullptr ? std::nullopt : rep_->status.GetPayload(type_url);
+}
+
+inline bool StatusBuilder::ok() const {
+  return rep_ == nullptr ? true : rep_->status.ok();
+}
+
+inline absl::StatusCode StatusBuilder::code() const {
+  return rep_ == nullptr ? absl::StatusCode::kOk : rep_->status.code();
+}
+
+inline StatusBuilder::operator absl::Status() && {
+  // Tell the compiler that the `ok()` path will return an `Ok` status, but do
+  // it only if the compiler can determine it at compile time.
+  // When it can't, we delegate this check into the out of line function.
+  if (IsKnownToBeEmpty()) {
+    return absl::OkStatus();
+  }
+  absl::Status result = CreateStatusAndConditionallyLog(loc_, std::move(rep_));
+  // Tell the compiler that `rep_` was not filled again even if `this` escaped.
+  AssumeEmpty();
+  return result;
+}
+
+inline absl::SourceLocation StatusBuilder::source_location() const {
+  return loc_;
+}
+
+// HasPayload()
+//
+// Indicates whether the Status object that will be returned by the
+// StatusBuilder contains any payloads with a type extending proto2's
+// `MessageSet`, returning `true` if so. Having a payload does not guarantee the
+// presence of a payload with a specific type. Note that returning `false` does
+// not necessarily indicate the absence of a payload, but only the absence on
+// one which extends `MessageSet`.
+inline bool HasPayload(const StatusBuilder& builder) {
+  return builder.HasPayload();
+}
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STATUS_STATUS_BUILDER_H_
diff --git a/absl/status/status_builder_test.cc b/absl/status/status_builder_test.cc
new file mode 100644
index 00000000000..28fd6b945d7
--- /dev/null
+++ b/absl/status/status_builder_test.cc
@@ -0,0 +1,380 @@
+// Copyright 2026 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/status/status_builder.h"
+
+#include <stddef.h>
+
+#include <memory>
+#include <string>
+#include <type_traits>
+#include <utility>
+#include <vector>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+#include "absl/log/log.h"
+#include "absl/log/log_entry.h"
+#include "absl/log/log_sink.h"
+#include "absl/status/status.h"
+#include "absl/status/status_matchers.h"
+#include "absl/status/statusor.h"
+#include "absl/strings/string_view.h"
+#include "absl/types/source_location.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace {
+
+using ::absl_testing::StatusIs;
+using ::testing::AllOf;
+using ::testing::AnyOf;
+using ::testing::ElementsAre;
+using ::testing::Eq;
+using ::testing::IsEmpty;
+using ::testing::Pointee;
+using ::testing::Property;
+
+// Converts a StatusBuilder to a Status.
+absl::Status ToStatus(const StatusBuilder& s) { return s; }
+
+// Converts a StatusBuilder to a StatusOr<T>.
+template <typename T>
+absl::StatusOr<T> ToStatusOr(const StatusBuilder& s) {
+  return s;
+}
+
+void CheckSourceLocation(
+    const absl::Status& status, std::vector<int> lines = {},
+    absl::SourceLocation loc = absl::SourceLocation::current()) {
+  ASSERT_EQ(status.GetSourceLocations().size(), lines.size())
+      << "Size check failed at " << loc.line();
+  for (size_t i = 0; i < lines.size(); ++i) {
+    EXPECT_EQ(absl::string_view(status.GetSourceLocations()[i].file_name()),
+              absl::string_view(loc.file_name()))
+        << "File name check failed at " << loc.line();
+    EXPECT_EQ(status.GetSourceLocations()[i].line(), lines[i])
+        << "Line check failed at " << loc.line();
+  }
+}
+
+class StatusBuilderTest : public ::testing::Test {};
+
+TEST_F(StatusBuilderTest, Size) {
+  EXPECT_LE(sizeof(StatusBuilder), 40)
+      << "Relax this test with caution and thorough testing. If StatusBuilder "
+         "is too large it can potentially blow stacks, especially in debug "
+         "builds. See the comments for StatusBuilder::Rep.";
+}
+
+TEST_F(StatusBuilderTest, ExplicitSourceLocation) {
+  const absl::SourceLocation kLocation = absl::SourceLocation::current();
+
+  {
+    const StatusBuilder builder(absl::OkStatus(), kLocation);
+    EXPECT_THAT(builder.source_location().file_name(),
+                Eq(kLocation.file_name()));
+    EXPECT_THAT(builder.source_location().line(), Eq(kLocation.line()));
+  }
+}
+
+TEST_F(StatusBuilderTest, ImplicitSourceLocation) {
+  const StatusBuilder builder(absl::OkStatus());
+  auto loc = absl::SourceLocation::current();
+  EXPECT_THAT(builder.source_location().file_name(),
+              AnyOf(Eq(absl::string_view(loc.file_name())),
+                    Eq(absl::string_view("<source_location>"))));
+  EXPECT_THAT(builder.source_location().line(),
+              AnyOf(Eq(1), Eq(loc.line() - 1)));
+}
+
+testing::Matcher<absl::SourceLocation> SourceLocationIs(
+    absl::SourceLocation loc) {
+  return AnyOf(
+      AllOf(Property(&absl::SourceLocation::file_name, Eq(loc.file_name())),
+            Property(&absl::SourceLocation::line, Eq(loc.line()))),
+      // Fallback for platforms that don't support source locations.
+      AllOf(Property(&absl::SourceLocation::file_name, Eq("<source_location>")),
+            Property(&absl::SourceLocation::line, Eq(1))));
+}
+
+TEST_F(StatusBuilderTest, GetPreviousSourceLocations) {
+  const absl::SourceLocation loc0 = absl::SourceLocation::current();
+  absl::Status status = absl::InvalidArgumentError("hi", loc0);
+  const absl::SourceLocation loc1 = absl::SourceLocation::current();
+  status.AddSourceLocation(loc1);
+  const absl::SourceLocation loc2 = absl::SourceLocation::current();
+  status.AddSourceLocation(loc2);
+
+  // The builder's location is not included.
+  const StatusBuilder builder(status);
+  EXPECT_THAT(builder.GetPreviousSourceLocations(),
+              ElementsAre(SourceLocationIs(loc0), SourceLocationIs(loc1),
+                          SourceLocationIs(loc2)));
+}
+
+TEST_F(StatusBuilderTest, EmptyGetPreviousSourceLocationsForNewFromStatusCode) {
+  const StatusBuilder builder(absl::StatusCode::kInvalidArgument);
+  EXPECT_THAT(builder.GetPreviousSourceLocations(), IsEmpty());
+}
+
+TEST_F(StatusBuilderTest, StatusCode) {
+  // OK
+  {
+    const StatusBuilder builder(absl::StatusCode::kOk);
+    EXPECT_TRUE(builder.ok());
+    EXPECT_THAT(builder.code(), Eq(absl::StatusCode::kOk));
+  }
+  // Non-OK code
+  {
+    const StatusBuilder builder(absl::StatusCode::kInvalidArgument);
+    EXPECT_FALSE(builder.ok());
+    EXPECT_THAT(builder.code(), Eq(absl::StatusCode::kInvalidArgument));
+  }
+}
+
+TEST_F(StatusBuilderTest, OkIgnoresStuff) {
+  EXPECT_THAT(ToStatus(StatusBuilder(absl::OkStatus(), absl::SourceLocation())
+                       << "booyah"),
+              Eq(absl::OkStatus()));
+}
+
+TEST_F(StatusBuilderTest, Streaming) {
+  EXPECT_THAT(
+      ToStatus(StatusBuilder(absl::CancelledError(), absl::SourceLocation())
+               << "booyah"),
+      Eq(absl::CancelledError("booyah")));
+  EXPECT_THAT(
+      ToStatus(
+          StatusBuilder(absl::AbortedError("hello"), absl::SourceLocation())
+          << "world"),
+      Eq(absl::AbortedError("hello; world")));
+}
+
+TEST_F(StatusBuilderTest, PrependLvalue) {
+  {
+    StatusBuilder builder(absl::CancelledError(), absl::SourceLocation());
+    EXPECT_THAT(ToStatus(builder.SetPrepend() << "booyah"),
+                Eq(absl::CancelledError("booyah")));
+  }
+  {
+    StatusBuilder builder(absl::AbortedError(" hello"), absl::SourceLocation());
+    EXPECT_THAT(ToStatus(builder.SetPrepend() << "world"),
+                Eq(absl::AbortedError("world hello")));
+  }
+}
+
+TEST_F(StatusBuilderTest, PrependRvalue) {
+  EXPECT_THAT(
+      ToStatus(StatusBuilder(absl::CancelledError(), absl::SourceLocation())
+                   .SetPrepend()
+               << "booyah"),
+      Eq(absl::CancelledError("booyah")));
+  EXPECT_THAT(ToStatus(StatusBuilder(absl::AbortedError(" hello"),
+                                     absl::SourceLocation())
+                           .SetPrepend()
+                       << "world"),
+              Eq(absl::AbortedError("world hello")));
+}
+
+TEST_F(StatusBuilderTest, AppendLvalue) {
+  {
+    StatusBuilder builder(absl::CancelledError(), absl::SourceLocation());
+    EXPECT_THAT(ToStatus(builder.SetAppend() << "booyah"),
+                Eq(absl::CancelledError("booyah")));
+  }
+  {
+    StatusBuilder builder(absl::AbortedError("hello"), absl::SourceLocation());
+    EXPECT_THAT(ToStatus(builder.SetAppend() << " world"),
+                Eq(absl::AbortedError("hello world")));
+  }
+}
+
+TEST_F(StatusBuilderTest, AppendRvalue) {
+  EXPECT_THAT(
+      ToStatus(StatusBuilder(absl::CancelledError(), absl::SourceLocation())
+                   .SetAppend()
+               << "booyah"),
+      Eq(absl::CancelledError("booyah")));
+  EXPECT_THAT(ToStatus(StatusBuilder(absl::AbortedError("hello"),
+                                     absl::SourceLocation())
+                           .SetAppend()
+                       << " world"),
+              Eq(absl::AbortedError("hello world")));
+}
+
+TEST_F(StatusBuilderTest, WithRvalueRef) {
+  auto policy = [](StatusBuilder sb) { return sb << "policy"; };
+  EXPECT_THAT(ToStatus(StatusBuilder(absl::AbortedError("hello"),
+                                     absl::SourceLocation())
+                           .With(policy)),
+              Eq(absl::AbortedError("hello; policy")));
+}
+
+TEST_F(StatusBuilderTest, WithRef) {
+  auto policy = [](StatusBuilder sb) { return sb << "policy"; };
+  StatusBuilder sb(absl::AbortedError("zomg"), absl::SourceLocation());
+  EXPECT_THAT(ToStatus(sb.With(policy)),
+              Eq(absl::AbortedError("zomg; policy")));
+}
+
+TEST_F(StatusBuilderTest, WithTypeChange) {
+  auto policy = [](StatusBuilder sb) -> std::string {
+    return sb.ok() ? "true" : "false";
+  };
+  EXPECT_EQ(StatusBuilder(absl::CancelledError(), absl::SourceLocation())
+                .With(policy),
+            "false");
+  EXPECT_EQ(
+      StatusBuilder(absl::OkStatus(), absl::SourceLocation()).With(policy),
+      "true");
+}
+
+struct MoveOnlyAdaptor {
+  std::unique_ptr<int> value;
+  std::unique_ptr<int> operator()(const absl::Status&) && {
+    return std::move(value);
+  }
+};
+
+TEST_F(StatusBuilderTest, WithMoveOnlyAdaptor) {
+  StatusBuilder sb(absl::AbortedError("zomg"), absl::SourceLocation());
+  EXPECT_THAT(sb.With(MoveOnlyAdaptor{std::make_unique<int>(100)}),
+              Pointee(100));
+  EXPECT_THAT(StatusBuilder(absl::AbortedError("zomg"), absl::SourceLocation())
+                  .With(MoveOnlyAdaptor{std::make_unique<int>(100)}),
+              Pointee(100));
+}
+
+struct StringifiableType {
+  absl::string_view message;
+
+  template <typename Sink>
+  friend void AbslStringify(Sink& sink, const StringifiableType& o) {
+    sink.Append(o.message);
+  }
+};
+
+class MockLogSink : public absl::LogSink {
+ public:
+  MOCK_METHOD(void, Send, (const absl::LogEntry&), (override));
+};
+
+TEST(WithExtraMessagePolicyTest, AppendsToExtraMessage) {
+  // The policy simply calls operator<< on the builder; the following examples
+  // demonstrate that, without duplicating all of the above tests.
+  EXPECT_THAT(ToStatus(StatusBuilder(absl::AbortedError("hello"),
+                                     absl::SourceLocation())
+                           .With(ExtraMessage("world"))),
+              Eq(absl::AbortedError("hello; world")));
+  EXPECT_THAT(ToStatus(StatusBuilder(absl::AbortedError("hello"),
+                                     absl::SourceLocation())
+                           .With(ExtraMessage() << "world")),
+              Eq(absl::AbortedError("hello; world")));
+  EXPECT_THAT(ToStatus(StatusBuilder(absl::AbortedError("hello"),
+                                     absl::SourceLocation())
+                           .With(ExtraMessage("world"))
+                           .With(ExtraMessage("!"))),
+              Eq(absl::AbortedError("hello; world!")));
+  EXPECT_THAT(ToStatus(StatusBuilder(absl::AbortedError("hello"),
+                                     absl::SourceLocation())
+                           .With(ExtraMessage("world, "))
+                           .SetPrepend()),
+              Eq(absl::AbortedError("world, hello")));
+  EXPECT_THAT(ToStatus(StatusBuilder(absl::AbortedError("hello"),
+                                     absl::SourceLocation())
+                           .With(ExtraMessage() << StringifiableType{"world"})),
+              Eq(absl::AbortedError("hello; world")));
+
+  // The above examples use temporary StatusBuilder rvalues; verify things also
+  // work fine when StatusBuilder is an lvalue.
+  StatusBuilder builder(absl::AbortedError("hello"), absl::SourceLocation());
+  EXPECT_THAT(
+      ToStatus(builder.With(ExtraMessage("world")).With(ExtraMessage("!"))),
+      Eq(absl::AbortedError("hello; world!")));
+}
+
+TEST(WithExtraMessagePolicyTest,
+     ExtraMessageStreamOperatorPreservesRvalueness) {
+  static_assert(
+      std::is_same_v<ExtraMessage&&, decltype(ExtraMessage() << "foo")>);
+}
+
+TEST_F(StatusBuilderTest, StatusSourceLocationChaining) {
+  {
+    absl::Status src = absl::OkStatus();
+    CheckSourceLocation(src);
+    CheckSourceLocation(ToStatus(StatusBuilder(src, absl::SourceLocation())));
+    CheckSourceLocation(
+        ToStatus(StatusBuilder(src, absl::SourceLocation::current())));
+    CheckSourceLocation(
+        ToStatus(StatusBuilder(src, absl::SourceLocation::current()) << "hmm"));
+  }
+  {
+    absl::Status src = absl::Status(absl::StatusCode::kCancelled, "");
+    CheckSourceLocation(src);
+    CheckSourceLocation(ToStatus(StatusBuilder(src, absl::SourceLocation())));
+    CheckSourceLocation(
+        ToStatus(StatusBuilder(src, absl::SourceLocation::current())));
+    CheckSourceLocation(
+        ToStatus(StatusBuilder(src, absl::SourceLocation::current()) << ""));
+    CheckSourceLocation(
+        ToStatus(StatusBuilder(src, absl::SourceLocation::current()) << "hmm"),
+        {__builtin_LINE() - 1});
+  }
+  {
+    absl::Status src = absl::Status(absl::StatusCode::kCancelled, "msg",
+                                    absl::SourceLocation());
+    CheckSourceLocation(src);
+    CheckSourceLocation(ToStatus(StatusBuilder(src, absl::SourceLocation())));
+    CheckSourceLocation(
+        ToStatus(StatusBuilder(src, absl::SourceLocation::current())),
+        {__builtin_LINE() - 1});
+    CheckSourceLocation(
+        ToStatus(StatusBuilder(src, absl::SourceLocation::current()) << "hmm"),
+        {__builtin_LINE() - 1});
+  }
+  {
+    absl::Status src = absl::Status(absl::StatusCode::kCancelled, "msg");
+    int src_line = __builtin_LINE() - 1;
+    CheckSourceLocation(src, {src_line});
+    CheckSourceLocation(ToStatus(StatusBuilder(src, absl::SourceLocation())),
+                        {src_line});
+    CheckSourceLocation(
+        ToStatus(StatusBuilder(src, absl::SourceLocation::current())),
+        {src_line, __builtin_LINE() - 1});
+    CheckSourceLocation(
+        ToStatus(StatusBuilder(src, absl::SourceLocation::current()) << "hmm"),
+        {src_line, __builtin_LINE() - 1});
+  }
+}
+
+TEST_F(StatusBuilderTest, SetErrorCode) {
+  StatusBuilder builder;
+  builder.SetCode(absl::StatusCode::kResourceExhausted);
+  LOG(INFO) << "Builder code: " << builder;
+  EXPECT_FALSE(builder.ok());
+  EXPECT_EQ(builder.code(), absl::StatusCode::kResourceExhausted);
+}
+
+TEST_F(StatusBuilderTest, BuilderToStatusOrStatusShouldGiveErrorStatusOr) {
+  absl::StatusOr<absl::Status> value = StatusBuilder(absl::CancelledError());
+  ASSERT_FALSE(value.ok());
+  EXPECT_THAT(value.status(), StatusIs(absl::StatusCode::kCancelled));
+}
+
+}  // namespace
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/status/status_macros.h b/absl/status/status_macros.h
new file mode 100644
index 00000000000..b823e0dd89f
--- /dev/null
+++ b/absl/status/status_macros.h
@@ -0,0 +1,484 @@
+// Copyright 2026 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: status_macros.h
+// -----------------------------------------------------------------------------
+//
+// Helper macros and methods to return and propagate errors with `absl::Status`.
+
+#ifndef ABSL_STATUS_STATUS_MACROS_H_
+#define ABSL_STATUS_STATUS_MACROS_H_
+
+#include <cstddef>
+#include <type_traits>
+#include <utility>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/optimization.h"
+#include "absl/status/status.h"
+#include "absl/status/status_builder.h"  // IWYU pragma: export
+#include "absl/status/statusor.h"
+#include "absl/types/source_location.h"
+
+// Evaluates an expression that produces a `absl::Status`. If the status is not
+// ok, returns it from the current function.
+//
+// For example:
+//   absl::Status MultiStepFunction() {
+//     ABSL_RETURN_IF_ERROR(Function(args...));
+//     ABSL_RETURN_IF_ERROR(foo.Method(args...));
+//     return absl::OkStatus();
+//   }
+//
+// The macro ends with a `absl::StatusBuilder` which allows the returned status
+// to be extended with more details.  Any chained expressions after the macro
+// will not be evaluated unless there is an error.
+//
+// For example:
+//   absl::Status MultiStepFunction() {
+//     ABSL_RETURN_IF_ERROR(Function(args...)) << "in MultiStepFunction";
+//     ABSL_RETURN_IF_ERROR(foo.Method(args...)).Log(absl::LogSeverity::kError)
+//         << "while processing query: " << query.DebugString();
+//     return absl::OkStatus();
+//   }
+//
+// `absl::StatusBuilder` supports adapting the builder chain using a `With`
+// method and a functor.  This allows for powerful extensions to the macro.
+//
+// For example, teams can define local policies to use across their code:
+//
+//   StatusBuilder TeamPolicy(StatusBuilder builder) {
+//     return std::move(builder.Log(absl::LogSeverity::kWarning).Attach(...));
+//   }
+//
+//   ABSL_RETURN_IF_ERROR(foo()).With(TeamPolicy);
+//   ABSL_RETURN_IF_ERROR(bar()).With(TeamPolicy);
+//
+// Changing the return type allows the macro to be used with Task and Rpc
+// interfaces.  See `util::TaskReturn` and `rpc::RpcSetStatus` for details.
+//
+//   void Read(StringPiece name, util::Task* task) {
+//     int64 id;
+//     ABSL_RETURN_IF_ERROR(GetIdForName(name, &id)).With(TaskReturn(task));
+//     ABSL_RETURN_IF_ERROR(ReadForId(id)).With(TaskReturn(task));
+//     task->Return();
+//   }
+//
+// If using this macro inside a lambda, you need to annotate the return type
+// to avoid confusion between a `absl::StatusBuilder` and a `absl::Status` type.
+// E.g.
+//
+//   []() -> absl::Status {
+//     ABSL_RETURN_IF_ERROR(Function(args...));
+//     ABSL_RETURN_IF_ERROR(foo.Method(args...));
+//     return absl::OkStatus();
+//   }
+#define ABSL_RETURN_IF_ERROR(expr) \
+  ABSL_INTERNAL_STATUS_MACROS_RETURN_IF_ERROR_IMPL_(return, expr)
+
+// Executes an expression `rexpr` that returns an `absl::StatusOr<T>`. On OK,
+// moves its value into the variable defined by `lhs`, otherwise returns
+// from the current function. By default the error status is returned
+// unchanged, but it may be modified by an `error_expression`. If there is an
+// error, `lhs` is not evaluated; thus any side effects that `lhs` may have
+// only occur in the success case.
+//
+// Interface:
+//
+//   ABSL_ASSIGN_OR_RETURN(lhs, rexpr)
+//   ABSL_ASSIGN_OR_RETURN(lhs, rexpr, error_expression);
+//
+// WARNING: if lhs is parenthesized, the parentheses are removed. See examples
+// for more details.
+//
+// WARNING: expands into multiple statements; it cannot be used in a single
+// statement (e.g. as the body of an if statement without {})!
+//
+// Example: Declaring and initializing a new variable (ValueType can be anything
+//          that can be initialized with assignment--including a const
+//          reference, although that's discouraged by go/totw/107):
+//   ABSL_ASSIGN_OR_RETURN(ValueType value, MaybeGetValue(arg));
+//
+// Example: Assigning to an existing variable:
+//   ValueType value;
+//   ABSL_ASSIGN_OR_RETURN(value, MaybeGetValue(arg));
+//
+// Example: Assigning to an expression with side effects:
+//   MyProto data;
+//   ABSL_ASSIGN_OR_RETURN(*data.mutable_str(), MaybeGetValue(arg));
+//   // No field "str" is added on error.
+//
+// Example: Initializing a `std::unique_ptr`.
+//   ABSL_ASSIGN_OR_RETURN(std::unique_ptr<T> ptr, MaybeGetPtr(arg));
+//
+// Example: Initializing a map. Because of C++ preprocessor limitations,
+// the type used in ABSL_ASSIGN_OR_RETURN cannot contain commas, so wrap the
+// lhs in parentheses:
+//   ABSL_ASSIGN_OR_RETURN((absl::flat_hash_map<Foo, Bar> my_map), GetMap());
+// Or use `auto` if the type is obvious enough:
+//   ABSL_ASSIGN_OR_RETURN(auto my_map, GetMap());
+//
+// Example: Assigning to structured bindings (go/totw/169). The same situation
+// with comma as in map, so wrap the statement in parentheses.
+//   ABSL_ASSIGN_OR_RETURN((auto [first, second]), GetPair());
+//
+// If passed, the `error_expression` is evaluated to produce the return
+// value. The expression may reference any variable visible in scope, as
+// well as a `absl::StatusBuilder` object populated with the error and
+// named by a single underscore `_`. The expression typically uses the
+// builder to modify the status and is returned directly in manner similar
+// to ABSL_RETURN_IF_ERROR. The expression may, however, evaluate to any type
+// returnable by the function, including (void). For example:
+//
+// Example: Adjusting the error message.
+//   ABSL_ASSIGN_OR_RETURN(ValueType value, MaybeGetValue(query),
+//                    _ << "while processing query " << query.DebugString());
+//
+// Example: Logging the error on failure.
+//   ABSL_ASSIGN_OR_RETURN(ValueType value, MaybeGetValue(query), _.LogError());
+//
+#define ABSL_ASSIGN_OR_RETURN(...) \
+  ABSL_INTERNAL_STATUS_MACROS_ASSIGN_OR_RETURN_IMPL_(return, __VA_ARGS__)
+
+// =================================================================
+// == Implementation details, do not rely on anything below here. ==
+// =================================================================
+
+#define ABSL_INTERNAL_STATUS_MACROS_RETURN_IF_ERROR_IMPL_(return_keyword, \
+                                                          expr)           \
+  ABSL_INTERNAL_STATUS_MACROS_IMPL_ELSE_BLOCKER_                          \
+  if (auto status_macro_internal_adaptor =                                \
+          absl::status_macro_internal::MacroAdaptor(                      \
+              (expr), absl::SourceLocation::current())) {                 \
+  } else /* NOLINT */                                                     \
+    return_keyword status_macro_internal_adaptor.Consume()
+
+#define ABSL_INTERNAL_STATUS_MACROS_ASSIGN_OR_RETURN_IMPL_(return_keyword, \
+                                                           ...)            \
+  ABSL_INTERNAL_STATUS_MACROS_IMPL_GET_VARIADIC_(                          \
+      (return_keyword, __VA_ARGS__,                                        \
+       ABSL_INTERNAL_STATUS_MACROS_IMPL_ASSIGN_OR_RETURN_3_,               \
+       ABSL_INTERNAL_STATUS_MACROS_IMPL_ASSIGN_OR_RETURN_2_))              \
+  (return_keyword, __VA_ARGS__)
+
+constexpr bool HasPotentialConditionalOperator(const char* lhs, int size) {
+  for (int i = 0; i < size; ++i) {
+    if (lhs[i] == '?') {
+      return true;
+    }
+  }
+  return false;
+}
+
+template <std::size_t N>
+constexpr bool IsEnclosedByParentheses(const char (&lhs)[N]) {
+  if (N < 2) {
+    return false;
+  }
+  return lhs[0] == '(' && lhs[N - 2] == ')';
+}
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace status_macro_internal {
+
+template <typename T, typename EnableIf = void>
+struct IsAllowedStatusOrMacroType : std::false_type {};
+
+template <typename T>
+struct IsAllowedStatusOrMacroType<
+    T, std::enable_if_t<std::is_convertible_v<
+           T*, typename absl::StatusOr<typename T::value_type>*>>>
+    : std::true_type {};
+
+}  // namespace status_macro_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+// MSVC incorrectly expands variadic macros, splice together a macro call to
+// work around the bug.
+#define ABSL_INTERNAL_STATUS_MACROS_IMPL_GET_VARIADIC_HELPER_(_1, _2, _3, _4, \
+                                                              NAME, ...)      \
+  NAME
+#define ABSL_INTERNAL_STATUS_MACROS_IMPL_GET_VARIADIC_(args) \
+  /* NOLINTNEXTLINE(clang-diagnostic-pre-c++20-compat) */    \
+  ABSL_INTERNAL_STATUS_MACROS_IMPL_GET_VARIADIC_HELPER_ args
+
+#define ABSL_INTERNAL_STATUS_MACROS_IMPL_ASSIGN_OR_RETURN_2_(return_keyword,   \
+                                                             lhs, rexpr)       \
+  ABSL_INTERNAL_STATUS_MACROS_IMPL_ASSIGN_OR_RETURN_(                          \
+      ABSL_INTERNAL_STATUS_MACROS_IMPL_CONCAT_(_status_or_value, __LINE__),    \
+      lhs, rexpr,                                                              \
+      return_keyword absl::Status(                                             \
+          std::move(ABSL_INTERNAL_STATUS_MACROS_IMPL_CONCAT_(_status_or_value, \
+                                                             __LINE__))        \
+              .status(),                                                       \
+          absl::SourceLocation::current()))
+
+#define ABSL_INTERNAL_STATUS_MACROS_IMPL_ASSIGN_OR_RETURN_3_(                  \
+    return_keyword, lhs, rexpr, error_expression)                              \
+  ABSL_INTERNAL_STATUS_MACROS_IMPL_ASSIGN_OR_RETURN_(                          \
+      ABSL_INTERNAL_STATUS_MACROS_IMPL_CONCAT_(_status_or_value, __LINE__),    \
+      lhs, rexpr, /* NOLINTNEXTLINE(misc-const-correctness) */                 \
+      absl::StatusBuilder _(                                                   \
+          std::move(ABSL_INTERNAL_STATUS_MACROS_IMPL_CONCAT_(_status_or_value, \
+                                                             __LINE__))        \
+              .status(),                                                       \
+          absl::SourceLocation::current());                                    \
+      (void)_; /* error_expression is allowed to not use this variable */      \
+      return_keyword(error_expression))
+
+#define ABSL_INTERNAL_STATUS_MACROS_IMPL_ASSIGN_OR_RETURN_(                 \
+    statusor, lhs, rexpr, error_expression)                                 \
+  auto statusor = (rexpr);                                                  \
+  if (ABSL_PREDICT_FALSE(!statusor.ok())) {                                 \
+    error_expression;                                                       \
+  }                                                                         \
+  {                                                                         \
+    static_assert(                                                          \
+        !IsEnclosedByParentheses(#lhs) ||                                   \
+            !HasPotentialConditionalOperator(#lhs, sizeof(#lhs) - 2),       \
+        "Identified potential conditional operator, consider not "          \
+        "using ABSL_ASSIGN_OR_RETURN");                                     \
+  }                                                                         \
+  {                                                                         \
+    static_assert(                                                          \
+        absl::status_macro_internal::IsAllowedStatusOrMacroType<            \
+            std::remove_const_t<decltype(statusor)>>(),                     \
+        "ABSL_ASSIGN_OR_RETURN should only be used with absl::StatusOr<>"); \
+  }                                                                         \
+  ABSL_INTERNAL_STATUS_MACROS_IMPL_UNPARENTHESIZE_IF_PARENTHESIZED(lhs) =   \
+      (*std::move(statusor))
+
+// Internal helpers for macro expansion.
+#define ABSL_INTERNAL_STATUS_MACROS_IMPL_EAT(...)
+#define ABSL_INTERNAL_STATUS_MACROS_IMPL_REM(...) __VA_ARGS__
+#define ABSL_INTERNAL_STATUS_MACROS_IMPL_COMMA(...) ,
+#define ABSL_INTERNAL_STATUS_MACROS_IMPL_ARG_3(a, b, c, ...) c
+
+#define ABSL_INTERNAL_STATUS_MACROS_IMPL_HAS_COMMA(...) \
+  ABSL_INTERNAL_STATUS_MACROS_IMPL_REM(                 \
+      ABSL_INTERNAL_STATUS_MACROS_IMPL_ARG_3(__VA_ARGS__, 1, 10))
+
+// Concatenates five macro arguments.
+#define ABSL_INTERNAL_STATUS_MACROS_IMPL_CONCAT_5(a, b, c, d, e) a##b##c##d##e
+
+// Identifies (below) the case where all arguments are empty, except the last.
+#define ABSL_INTERNAL_STATUS_MACROS_IMPL_IS_EMPTY_CASE_1010101 ,
+
+// Evaluates to 1 if the arguments concatenate to 1010101, and 10 otherwise.
+#define ABSL_INTERNAL_STATUS_MACROS_1_IF_1010101_ELSE_10(a, b, c, d) \
+  ABSL_INTERNAL_STATUS_MACROS_IMPL_HAS_COMMA(                        \
+      ABSL_INTERNAL_STATUS_MACROS_IMPL_CONCAT_5(                     \
+          ABSL_INTERNAL_STATUS_MACROS_IMPL_IS_EMPTY_CASE_, a, b, c, d))
+
+// Evaluates to 1 if __VA_OPT__ is supported and the argument list is non-empty,
+// and 0 otherwise.
+#define ABSL_INTERNAL_STATUS_MACROS_IMPL_HAVE_VA_OPT(...) \
+  ABSL_INTERNAL_STATUS_MACROS_IMPL_ARG_3(__VA_OPT__(, ), 1, 0, )
+
+#if ABSL_INTERNAL_STATUS_MACROS_IMPL_HAVE_VA_OPT(.)
+// Evaluates to 1 if the argument list is empty, and 10 otherwise.
+#define ABSL_INTERNAL_STATUS_MACROS_1_IF_EMPTY_ELSE_10(...) 1##__VA_OPT__(0)
+#else
+#define ABSL_INTERNAL_STATUS_MACROS_1_IF_EMPTY_ELSE_10(...)      \
+  ABSL_INTERNAL_STATUS_MACROS_1_IF_1010101_ELSE_10(              \
+      ABSL_INTERNAL_STATUS_MACROS_IMPL_HAS_COMMA(__VA_ARGS__),   \
+      ABSL_INTERNAL_STATUS_MACROS_IMPL_HAS_COMMA(                \
+          ABSL_INTERNAL_STATUS_MACROS_IMPL_COMMA __VA_ARGS__),   \
+      ABSL_INTERNAL_STATUS_MACROS_IMPL_HAS_COMMA(__VA_ARGS__()), \
+      ABSL_INTERNAL_STATUS_MACROS_IMPL_HAS_COMMA(                \
+          ABSL_INTERNAL_STATUS_MACROS_IMPL_COMMA __VA_ARGS__()))
+
+#endif
+
+// Parenthesized case: strips the initial pair of parentheses from the input.
+//
+// Note that this may not result in an equivalent expression. For example,
+// (*a)[b] would evaluate to a[b]. The caller below is required to ensure
+// this is only called when the input is fully parenthesized to avoid this
+// issue.
+#define ABSL_INTERNAL_STATUS_MACROS_IMPL_UNPARENTHESIZE_IF_PARENTHESIZED_1(x) \
+  ABSL_INTERNAL_STATUS_MACROS_IMPL_REM x
+
+// Unparenthesized case: evaluates to the argument list as-is.
+#define ABSL_INTERNAL_STATUS_MACROS_IMPL_UNPARENTHESIZE_IF_PARENTHESIZED_10(x) \
+  ABSL_INTERNAL_STATUS_MACROS_IMPL_REM(x)
+
+// If the input is parenthesized, removes the parentheses. Otherwise expands to
+// the input unchanged.
+#define ABSL_INTERNAL_STATUS_MACROS_IMPL_UNPARENTHESIZE_IF_PARENTHESIZED(...) \
+  ABSL_INTERNAL_STATUS_MACROS_IMPL_REM(                                       \
+      ABSL_INTERNAL_STATUS_MACROS_IMPL_CONCAT_(                               \
+          ABSL_INTERNAL_STATUS_MACROS_IMPL_UNPARENTHESIZE_IF_PARENTHESIZED_,  \
+          ABSL_INTERNAL_STATUS_MACROS_1_IF_EMPTY_ELSE_10(                     \
+              ABSL_INTERNAL_STATUS_MACROS_IMPL_EAT __VA_ARGS__))(             \
+          ABSL_INTERNAL_STATUS_MACROS_IMPL_REM(__VA_ARGS__)))
+
+// Internal helper for concatenating macro values.
+#define ABSL_INTERNAL_STATUS_MACROS_IMPL_CONCAT_INNER_(x, y) x##y
+
+#define ABSL_INTERNAL_STATUS_MACROS_IMPL_CONCAT_(x, y) \
+  ABSL_INTERNAL_STATUS_MACROS_IMPL_CONCAT_INNER_(x, y)
+
+// The GNU compiler emits a warning for code like:
+//
+//   if (foo)
+//     if (bar) { } else baz;
+//
+// because it thinks you might want the else to bind to the first if.  This
+// leads to problems with code like:
+//
+//   if (do_expr) ABSL_RETURN_IF_ERROR(expr) << "Some message";
+//
+// The "switch (0) case 0:" idiom is used to suppress this.
+#define ABSL_INTERNAL_STATUS_MACROS_IMPL_ELSE_BLOCKER_ \
+  switch (0)                                           \
+  case 0:                                              \
+  default:  // NOLINT
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace status_macro_internal {
+
+// Provides a conversion to bool so that it can be used inside an if statement
+// that declares a variable.
+class StatusAdaptorForMacros {
+ public:
+  StatusAdaptorForMacros(
+      const absl::Status& status,
+      absl::SourceLocation loc = absl::SourceLocation::current())
+      : builder_(status, loc) {}
+
+  StatusAdaptorForMacros(
+      absl::Status&& status,
+      absl::SourceLocation loc = absl::SourceLocation::current())
+      : builder_(std::move(status), loc) {}
+
+  StatusAdaptorForMacros(const StatusBuilder& builder,
+                         absl::SourceLocation = absl::SourceLocation())
+      : builder_(builder) {}
+
+  StatusAdaptorForMacros(StatusBuilder&& builder,
+                         absl::SourceLocation = absl::SourceLocation())
+      : builder_(std::move(builder)) {}
+
+  StatusAdaptorForMacros(const StatusAdaptorForMacros&) = delete;
+  StatusAdaptorForMacros& operator=(const StatusAdaptorForMacros&) = delete;
+
+  explicit operator bool() const { return ABSL_PREDICT_TRUE(builder_.ok()); }
+
+  StatusBuilder&& Consume() { return std::move(builder_); }
+
+ private:
+  StatusBuilder builder_;
+};
+
+// Special adaptor for use by ABSL_RETURN_IF_ERROR for absl::Status arguments.
+// This one avoids constructing a StatusBuilder on the fast path.
+//
+// REQUIRES: Only used by ABSL_RETURN_IF_ERROR implementation.
+class ReturnIfErrorAdaptor {
+ public:
+  explicit ReturnIfErrorAdaptor(
+      const absl::Status& status,
+      absl::SourceLocation loc = absl::SourceLocation::current())
+      : status_(status), loc_(loc) {}
+
+  explicit ReturnIfErrorAdaptor(
+      absl::Status&& status,
+      absl::SourceLocation loc = absl::SourceLocation::current())
+      : status_(std::move(status)), loc_(loc) {}
+
+  ~ReturnIfErrorAdaptor() {
+    // WARNING! WARNING! WARNING!
+    //
+    // We play fast and loose here and avoid destroying status_. This should be
+    // safe because status_ will never own memory at destruction time. The two
+    // cases to consider are:
+    //  (1) OK: OkStatus() representation needs no cleanup
+    //  (2) Not-OK: we take the else branch in ABSL_RETURN_IF_ERROR and move
+    //      status_ into StatusBuilder which leaves status_ with a MovedFromRep
+    //      that needs no cleanup.
+    // If the absl::Status implementation changes, leaks should be caught by
+    // the various tests we have when run under lsan or debug leak checker.
+  }
+
+  explicit operator bool() const { return ABSL_PREDICT_TRUE(status_.ok()); }
+
+  StatusBuilder Consume() { return StatusBuilder(std::move(status_), loc_); }
+
+ private:
+  // Place the status inside a union so we can avoid generating unnecessary code
+  // to call the Status destructor.
+  union {
+    absl::Status status_;
+    char nothing_[1];
+  };
+  absl::SourceLocation loc_;
+};
+
+// Overloads of MacroAdaptor that pick the right adaptor class
+// for each argument type.
+//
+// TODO(b/501387693): Replace these with deduction guides of template
+// specializations once the bug is fixed. (See prior file revision.)
+inline ReturnIfErrorAdaptor MacroAdaptor(const absl::Status& s,
+                                         absl::SourceLocation loc) {
+  return ReturnIfErrorAdaptor(s, loc);
+}
+inline ReturnIfErrorAdaptor MacroAdaptor(absl::Status&& s,
+                                         absl::SourceLocation loc) {
+  return ReturnIfErrorAdaptor(std::move(s), loc);
+}
+inline StatusAdaptorForMacros MacroAdaptor(const StatusBuilder& s,
+                                           absl::SourceLocation loc) {
+  return StatusAdaptorForMacros(s, loc);
+}
+inline StatusAdaptorForMacros MacroAdaptor(StatusBuilder&& s,
+                                           absl::SourceLocation loc) {
+  return StatusAdaptorForMacros(std::move(s), loc);
+}
+
+}  // namespace status_macro_internal
+
+// By defining ABSL_DEFINE_UNQUALIFIED_STATUS_MACROS, this library also provides
+// unqualified versions of its macros.
+//
+// Unqualified macro names are likely to collide with those in other projects,
+// and so are not recommended.  Furthermore, this is true for any transitive
+// dependencies of Abseil: it is impossible to be confident that downstream
+// libraries will neither define these macros themselves nor depend on a
+// different library that also defines them.
+//
+// To enable unqualified names despite the caveats, define
+// `ABSL_DEFINE_UNQUALIFIED_STATUS_MACROS` preferably at the command line, e.g.
+// `-DABSL_DEFINE_UNQUALIFIED_STATUS_MACROS`, or `local_defines =
+// ["ABSL_DEFINE_UNQUALIFIED_STATUS_MACROS"]` if using Bazel.
+//
+// These are turned on by default inside Google's internal codebase where their
+// use is historically ubiquitous.  Other OSS Google projects should use the
+// qualified versions.
+//
+#ifdef ABSL_DEFINE_UNQUALIFIED_STATUS_MACROS
+#define ASSIGN_OR_RETURN(...) ABSL_ASSIGN_OR_RETURN(__VA_ARGS__)
+#define RETURN_IF_ERROR(...) ABSL_RETURN_IF_ERROR(__VA_ARGS__)
+#endif  // ABSL_DEFINE_UNQUALIFIED_STATUS_MACROS
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STATUS_STATUS_MACROS_H_
diff --git a/absl/status/status_macros_test.cc b/absl/status/status_macros_test.cc
new file mode 100644
index 00000000000..7b5751d1522
--- /dev/null
+++ b/absl/status/status_macros_test.cc
@@ -0,0 +1,632 @@
+// Copyright 2026 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/status/status_macros.h"
+
+#include <cstddef>
+#include <memory>
+#include <string>
+#include <tuple>
+#include <type_traits>
+#include <utility>
+#include <vector>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/status/status.h"
+#include "absl/status/status_builder.h"
+#include "absl/status/statusor.h"
+#include "absl/strings/string_view.h"
+#include "absl/types/source_location.h"
+
+namespace {
+
+using ::testing::AllOf;
+using ::testing::Eq;
+using ::testing::HasSubstr;
+
+using absl::status_macro_internal::ReturnIfErrorAdaptor;
+using absl::status_macro_internal::StatusAdaptorForMacros;
+static_assert(!std::is_default_constructible_v<ReturnIfErrorAdaptor>);
+static_assert(!std::is_default_constructible_v<StatusAdaptorForMacros>);
+static_assert(!std::is_copy_constructible_v<ReturnIfErrorAdaptor>);
+static_assert(!std::is_copy_constructible_v<StatusAdaptorForMacros>);
+static_assert(!std::is_copy_assignable_v<ReturnIfErrorAdaptor>);
+static_assert(!std::is_copy_assignable_v<StatusAdaptorForMacros>);
+
+absl::Status ReturnOk() { return absl::OkStatus(); }
+
+absl::StatusBuilder ReturnOkBuilder() {
+  return absl::StatusBuilder(absl::OkStatus());
+}
+
+absl::Status ReturnError(absl::string_view msg) {
+  return absl::Status(absl::StatusCode::kUnknown, msg);
+}
+
+absl::StatusBuilder ReturnErrorBuilder(absl::string_view msg) {
+  return absl::StatusBuilder(absl::Status(absl::StatusCode::kUnknown, msg));
+}
+
+absl::StatusOr<int> ReturnStatusOrValue(int v) { return v; }
+
+absl::StatusOr<int> ReturnStatusOrError(absl::string_view msg) {
+  return absl::Status(absl::StatusCode::kUnknown, msg);
+}
+
+template <class... Args>
+absl::StatusOr<std::tuple<Args...>> ReturnStatusOrTupleValue(Args&&... v) {
+  return std::tuple<Args...>(std::forward<Args>(v)...);
+}
+
+template <class... Args>
+absl::StatusOr<std::tuple<Args...>> ReturnStatusOrTupleError(
+    absl::string_view msg) {
+  return absl::Status(absl::StatusCode::kUnknown, msg);
+}
+
+absl::StatusOr<int&> ReturnStatusOrRef(int& v) { return v; }
+
+absl::StatusOr<std::unique_ptr<int>> ReturnStatusOrPtrValue(int v) {
+  return std::make_unique<int>(v);
+}
+void CheckSourceLocation(
+    const absl::Status& status, std::vector<int> lines = {},
+    absl::SourceLocation loc = absl::SourceLocation::current()) {
+  ASSERT_EQ(status.GetSourceLocations().size(), lines.size())
+      << "Size check failed at " << loc.line();
+  for (size_t i = 0; i < lines.size(); ++i) {
+    EXPECT_EQ(absl::string_view(status.GetSourceLocations()[i].file_name()),
+              absl::string_view(loc.file_name()))
+        << "File name check failed at " << loc.line();
+    EXPECT_EQ(status.GetSourceLocations()[i].line(), lines[i])
+        << "Line check failed at " << loc.line();
+  }
+}
+TEST(AssignOrReturn, Works) {
+  auto func = []() -> absl::Status {
+    ABSL_ASSIGN_OR_RETURN(int value1, ReturnStatusOrValue(1));
+    EXPECT_EQ(1, value1);
+    ABSL_ASSIGN_OR_RETURN(const int value2, ReturnStatusOrValue(2));
+    EXPECT_EQ(2, value2);
+    ABSL_ASSIGN_OR_RETURN(const int& value3, ReturnStatusOrValue(3));
+    EXPECT_EQ(3, value3);
+    ABSL_ASSIGN_OR_RETURN(int value4 [[maybe_unused]],
+                          ReturnStatusOrError("EXPECTED"));
+    return ReturnError("ERROR");
+  };
+
+  EXPECT_THAT(func().message(), Eq("EXPECTED"));
+}
+
+TEST(AssignOrReturn, WorksWithReferences) {
+  int value = 17;
+  auto func = [&]() -> absl::Status {
+    ABSL_ASSIGN_OR_RETURN(int& value1, ReturnStatusOrRef(value));
+    EXPECT_EQ(&value1, &value);
+
+    ABSL_ASSIGN_OR_RETURN(const int& value2, ReturnStatusOrRef(value));
+    EXPECT_EQ(&value2, &value);
+
+    ABSL_ASSIGN_OR_RETURN(int value3, ReturnStatusOrRef(value));
+    EXPECT_EQ(value3, value);
+
+    value = 11;
+    EXPECT_NE(value3, value);
+
+    ABSL_ASSIGN_OR_RETURN(int value4 [[maybe_unused]],
+                          ReturnStatusOrError("EXPECTED"));
+    return ReturnError("ERROR");
+  };
+
+  EXPECT_THAT(func().message(), Eq("EXPECTED"));
+}
+
+TEST(AssignOrReturn, WorksWithCommasInType) {
+  auto func = []() -> absl::Status {
+    ABSL_ASSIGN_OR_RETURN((std::tuple<int, int> t1),
+                          ReturnStatusOrTupleValue(1, 1));
+    EXPECT_EQ((std::tuple{1, 1}), t1);
+    ABSL_ASSIGN_OR_RETURN((const std::tuple<int, std::tuple<int, int>, int> t2),
+                          ReturnStatusOrTupleValue(1, std::tuple{1, 1}, 1));
+    EXPECT_EQ((std::tuple{1, std::tuple{1, 1}, 1}), t2);
+    ABSL_ASSIGN_OR_RETURN(
+        (std::tuple<int, std::tuple<int, int>, int> t3),
+        (ReturnStatusOrTupleError<int, std::tuple<int, int>, int>("EXPECTED")));
+    t3 = {};  // fix unused error
+    return ReturnError("ERROR");
+  };
+
+  EXPECT_THAT(func().message(), Eq("EXPECTED"));
+}
+
+TEST(AssignOrReturn, WorksWithStructureBindings) {
+  auto func = []() -> absl::Status {
+    ABSL_ASSIGN_OR_RETURN(
+        (const auto& [t1, t2, t3, t4, t5]),
+        ReturnStatusOrTupleValue(std::tuple{1, 1}, 1, 2, 3, 4));
+    EXPECT_EQ((std::tuple{1, 1}), t1);
+    EXPECT_EQ(1, t2);
+    EXPECT_EQ(2, t3);
+    EXPECT_EQ(3, t4);
+    EXPECT_EQ(4, t5);
+    ABSL_ASSIGN_OR_RETURN(int t6 [[maybe_unused]],
+                          ReturnStatusOrError("EXPECTED"));
+    return ReturnError("ERROR");
+  };
+
+  EXPECT_THAT(func().message(), Eq("EXPECTED"));
+}
+
+TEST(AssignOrReturn, WorksWithParenthesesAndDereference) {
+  auto func = []() -> absl::Status {
+    int integer;
+    int* pointer_to_integer = &integer;
+    ABSL_ASSIGN_OR_RETURN((*pointer_to_integer), ReturnStatusOrValue(1));
+    EXPECT_EQ(1, integer);
+    ABSL_ASSIGN_OR_RETURN(*pointer_to_integer, ReturnStatusOrValue(2));
+    EXPECT_EQ(2, integer);
+    // Make the test where the order of dereference matters and treat the
+    // parentheses.
+    pointer_to_integer--;
+    int** pointer_to_pointer_to_integer = &pointer_to_integer;
+    ABSL_ASSIGN_OR_RETURN((*pointer_to_pointer_to_integer)[1],
+                          ReturnStatusOrValue(3));
+    EXPECT_EQ(3, integer);
+    ABSL_ASSIGN_OR_RETURN(int t1 [[maybe_unused]],
+                          ReturnStatusOrError("EXPECTED"));
+    return ReturnError("ERROR");
+  };
+
+  EXPECT_THAT(func().message(), Eq("EXPECTED"));
+}
+
+TEST(AssignOrReturn, WorksWithAppend) {
+  auto fail_test_if_called = []() -> std::string {
+    ADD_FAILURE();
+    return "FAILURE";
+  };
+  auto func = [&]() -> absl::Status {
+    int value [[maybe_unused]];
+    ABSL_ASSIGN_OR_RETURN(value, ReturnStatusOrValue(1),
+                          _ << fail_test_if_called());
+    ABSL_ASSIGN_OR_RETURN(value, ReturnStatusOrError("EXPECTED A"),
+                          _ << "EXPECTED B");
+    return ReturnOk();
+  };
+
+  EXPECT_THAT(func().message(),
+              AllOf(HasSubstr("EXPECTED A"), HasSubstr("EXPECTED B")));
+}
+
+TEST(AssignOrReturn, WorksWithAdaptorFunc) {
+  auto fail_test_if_called = [](absl::StatusBuilder builder) {
+    ADD_FAILURE();
+    return builder;
+  };
+  auto adaptor = [](absl::StatusBuilder builder) {
+    return builder << "EXPECTED B";
+  };
+  auto func = [&]() -> absl::Status {
+    int value [[maybe_unused]];
+    ABSL_ASSIGN_OR_RETURN(value, ReturnStatusOrValue(1),
+                          fail_test_if_called(_));
+    ABSL_ASSIGN_OR_RETURN(value, ReturnStatusOrError("EXPECTED A"), adaptor(_));
+    return ReturnOk();
+  };
+
+  EXPECT_THAT(func().message(),
+              AllOf(HasSubstr("EXPECTED A"), HasSubstr("EXPECTED B")));
+}
+
+TEST(AssignOrReturn, WorksWithThirdArgumentAndCommas) {
+  auto fail_test_if_called = [](absl::StatusBuilder builder) {
+    ADD_FAILURE();
+    return builder;
+  };
+  auto adaptor = [](absl::StatusBuilder builder) {
+    return builder << "EXPECTED B";
+  };
+  auto func = [&]() -> absl::Status {
+    ABSL_ASSIGN_OR_RETURN((const auto& [t1, t2, t3]),
+                          ReturnStatusOrTupleValue(1, 2, 3),
+                          fail_test_if_called(_));
+    EXPECT_EQ(t1, 1);
+    EXPECT_EQ(t2, 2);
+    EXPECT_EQ(t3, 3);
+    ABSL_ASSIGN_OR_RETURN(
+        (const auto& [t4, t5, t6]),
+        (ReturnStatusOrTupleError<int, int, int>("EXPECTED A")), adaptor(_));
+    // Silence errors about the unused values.
+    static_cast<void>(t4);
+    static_cast<void>(t5);
+    static_cast<void>(t6);
+    return ReturnOk();
+  };
+
+  EXPECT_THAT(func().message(),
+              AllOf(HasSubstr("EXPECTED A"), HasSubstr("EXPECTED B")));
+}
+
+TEST(AssignOrReturn, WorksWithAppendIncludingLocals) {
+  auto func = [&](absl::string_view str) -> absl::Status {
+    int value [[maybe_unused]];
+    ABSL_ASSIGN_OR_RETURN(value, ReturnStatusOrError("EXPECTED A"), _ << str);
+    return ReturnOk();
+  };
+
+  EXPECT_THAT(func("EXPECTED B").message(),
+              AllOf(HasSubstr("EXPECTED A"), HasSubstr("EXPECTED B")));
+}
+
+TEST(AssignOrReturn, WorksForExistingVariable) {
+  auto func = []() -> absl::Status {
+    int value = 1;
+    ABSL_ASSIGN_OR_RETURN(value, ReturnStatusOrValue(2));
+    EXPECT_EQ(2, value);
+    ABSL_ASSIGN_OR_RETURN(value, ReturnStatusOrValue(3));
+    EXPECT_EQ(3, value);
+    ABSL_ASSIGN_OR_RETURN(value, ReturnStatusOrError("EXPECTED"));
+    return ReturnError("ERROR");
+  };
+
+  EXPECT_THAT(func().message(), Eq("EXPECTED"));
+}
+
+TEST(AssignOrReturn, UniquePtrWorks) {
+  auto func = []() -> absl::Status {
+    ABSL_ASSIGN_OR_RETURN(std::unique_ptr<int> ptr, ReturnStatusOrPtrValue(1));
+    EXPECT_EQ(*ptr, 1);
+    return ReturnError("EXPECTED");
+  };
+
+  EXPECT_THAT(func().message(), Eq("EXPECTED"));
+}
+
+TEST(AssignOrReturn, UniquePtrWorksForExistingVariable) {
+  auto func = []() -> absl::Status {
+    std::unique_ptr<int> ptr;
+    ABSL_ASSIGN_OR_RETURN(ptr, ReturnStatusOrPtrValue(1));
+    EXPECT_EQ(*ptr, 1);
+
+    ABSL_ASSIGN_OR_RETURN(ptr, ReturnStatusOrPtrValue(2));
+    EXPECT_EQ(*ptr, 2);
+    return ReturnError("EXPECTED");
+  };
+
+  EXPECT_THAT(func().message(), Eq("EXPECTED"));
+}
+
+TEST(AssignOrReturn, ChainSourceLocation) {
+  auto func1 = []() -> absl::StatusOr<std::unique_ptr<int>> {
+    std::unique_ptr<int> ptr;
+    ABSL_ASSIGN_OR_RETURN(ptr, ReturnStatusOrPtrValue(1));
+    return ptr;
+  };
+  auto func2 = []() -> absl::StatusOr<std::unique_ptr<int>> {
+    return absl::Status(absl::StatusCode::kInternal, "msg");
+  };
+  int func2_line = __builtin_LINE() - 2;
+
+  auto func3 = [=]() -> absl::StatusOr<std::unique_ptr<int>> {
+    std::unique_ptr<int> ptr;
+    ABSL_ASSIGN_OR_RETURN(ptr, func1());
+    ABSL_ASSIGN_OR_RETURN(ptr, func2());
+    return ptr;
+  };
+  int func3_line = __builtin_LINE() - 3;
+
+  auto func4 = [=]() -> absl::StatusOr<std::unique_ptr<int>> {
+    std::unique_ptr<int> ptr;
+    ABSL_ASSIGN_OR_RETURN(ptr, func3());
+    ABSL_ASSIGN_OR_RETURN(ptr, func2());
+    return ptr;
+  };
+  int func4_line = __builtin_LINE() - 4;
+
+  absl::StatusOr<std::unique_ptr<int>> result = func4();
+  EXPECT_EQ(absl::StatusCode::kInternal, result.status().code());
+  CheckSourceLocation(result.status(), {func2_line, func3_line, func4_line});
+}
+
+TEST(AssignOrReturn, NotChainSourceLocationWithEmptyMsg) {
+  auto func1 = []() -> absl::StatusOr<std::unique_ptr<int>> {
+    std::unique_ptr<int> ptr;
+    ABSL_ASSIGN_OR_RETURN(ptr, ReturnStatusOrPtrValue(1));
+    return ptr;
+  };
+  auto func2 = []() -> absl::StatusOr<std::unique_ptr<int>> {
+    return absl::Status(absl::StatusCode::kInternal, "");
+  };
+
+  auto func3 = [=]() -> absl::StatusOr<std::unique_ptr<int>> {
+    std::unique_ptr<int> ptr;
+    ABSL_ASSIGN_OR_RETURN(ptr, func1());
+    ABSL_ASSIGN_OR_RETURN(ptr, func2());
+    return ptr;
+  };
+
+  auto func4 = [=]() -> absl::StatusOr<std::unique_ptr<int>> {
+    std::unique_ptr<int> ptr;
+    ABSL_ASSIGN_OR_RETURN(ptr, func3());
+    ABSL_ASSIGN_OR_RETURN(ptr, func2());
+    return ptr;
+  };
+
+  absl::StatusOr<std::unique_ptr<int>> result = func4();
+  EXPECT_EQ(absl::StatusCode::kInternal, result.status().code());
+  CheckSourceLocation(result.status());
+}
+
+TEST(AssignOrReturn, ChainSourceLocationWith3ArgStatusMacro) {
+  auto func1 = []() -> absl::StatusOr<std::unique_ptr<int>> {
+    std::unique_ptr<int> ptr;
+    ABSL_ASSIGN_OR_RETURN(ptr, ReturnStatusOrPtrValue(1));
+    return ptr;
+  };
+  auto func2 = []() -> absl::StatusOr<std::unique_ptr<int>> {
+    return absl::Status(absl::StatusCode::kInternal, "");
+  };
+
+  auto func3 = [=]() -> absl::StatusOr<std::unique_ptr<int>> {
+    std::unique_ptr<int> ptr;
+    ABSL_ASSIGN_OR_RETURN(ptr, func1());
+    ABSL_ASSIGN_OR_RETURN(ptr, func2(), _ << "hmm");
+    return ptr;
+  };
+  int func3_line = __builtin_LINE() - 3;
+
+  auto func4 = [=]() -> absl::StatusOr<std::unique_ptr<int>> {
+    std::unique_ptr<int> ptr;
+    ABSL_ASSIGN_OR_RETURN(ptr, func3());
+    ABSL_ASSIGN_OR_RETURN(ptr, func2());
+    return ptr;
+  };
+  int func4_line = __builtin_LINE() - 4;
+
+  absl::StatusOr<std::unique_ptr<int>> result = func4();
+  EXPECT_EQ(absl::StatusCode::kInternal, result.status().code());
+  CheckSourceLocation(result.status(), {func3_line, func4_line});
+}
+
+TEST(ReturnIfError, Works) {
+  auto func = []() -> absl::Status {
+    ABSL_RETURN_IF_ERROR(ReturnOk());
+    ABSL_RETURN_IF_ERROR(ReturnOk());
+    ABSL_RETURN_IF_ERROR(ReturnError("EXPECTED"));
+    return ReturnError("ERROR");
+  };
+
+  EXPECT_THAT(func().message(), Eq("EXPECTED"));
+}
+
+TEST(ReturnIfError, WorksWithSourceLocation) {
+  using StatusCode = absl::StatusCode;
+  {
+    auto func = []() -> absl::Status {
+      ABSL_RETURN_IF_ERROR(ReturnOk());
+      ABSL_RETURN_IF_ERROR(ReturnOk());
+      ABSL_RETURN_IF_ERROR(absl::Status(StatusCode::kUnknown, "EXPECTED"));
+      return ReturnError("ERROR");
+    };
+    int error_line = __builtin_LINE() - 3;
+    int func_line = error_line;
+
+    EXPECT_THAT(func().message(), Eq("EXPECTED"));
+    CheckSourceLocation(func(), {error_line, func_line});
+  }
+  {
+    auto func1 = []() -> absl::Status {
+      ABSL_RETURN_IF_ERROR(ReturnOk());
+      ABSL_RETURN_IF_ERROR(ReturnOk());
+      ABSL_RETURN_IF_ERROR(absl::Status(StatusCode::kUnknown, "EXPECTED"));
+      return ReturnError("ERROR");
+    };
+    int error_line = __builtin_LINE() - 3;
+    int func_line = error_line;
+
+    auto func3 = [=]() -> absl::Status {
+      ABSL_RETURN_IF_ERROR(func1());
+      return ReturnError("ERROR_Func3");
+    };
+    int func3_line = __builtin_LINE() - 3;
+
+    auto func4 = [=]() -> absl::Status {
+      ABSL_RETURN_IF_ERROR(func3());
+      return ReturnError("ERROR_Func4");
+    };
+    int func4_line = __builtin_LINE() - 3;
+    CheckSourceLocation(func4(),
+                        {error_line, func_line, func3_line, func4_line});
+  }
+  {
+    auto func1 = []() -> absl::Status {
+      ABSL_RETURN_IF_ERROR(ReturnOk());
+      ABSL_RETURN_IF_ERROR(ReturnOk());
+      ABSL_RETURN_IF_ERROR(absl::Status(StatusCode::kUnknown, ""));
+      return ReturnError("ERROR");
+    };
+
+    auto func3 = [=]() -> absl::Status {
+      ABSL_RETURN_IF_ERROR(func1());
+      return ReturnError("ERROR_Func3");
+    };
+
+    auto func4 = [=]() -> absl::Status {
+      ABSL_RETURN_IF_ERROR(func3());
+      return ReturnError("ERROR_Func4");
+    };
+    CheckSourceLocation(func4());
+  }
+}
+
+TEST(ReturnIfError, WorksWithSourceLocationOn2Arg) {
+  {
+    auto func = []() -> absl::Status {
+      ABSL_RETURN_IF_ERROR(ReturnOk());
+      ABSL_RETURN_IF_ERROR(ReturnOk());
+      ABSL_RETURN_IF_ERROR(absl::Status(absl::StatusCode::kUnknown, "EXPECTED"))
+          << "foo";
+      return ReturnError("ERROR");
+    };
+    int error_line = __builtin_LINE() - 4;
+    int func_line = error_line;
+
+    EXPECT_THAT(func().message(), Eq("EXPECTED; foo"));
+    CheckSourceLocation(func(), {error_line, func_line});
+  }
+  {
+    auto func1 = []() -> absl::Status {
+      ABSL_RETURN_IF_ERROR(ReturnOk());
+      ABSL_RETURN_IF_ERROR(ReturnOk());
+      ABSL_RETURN_IF_ERROR(absl::Status(absl::StatusCode::kUnknown, "EXPECTED"))
+          << "foo";
+      return ReturnError("ERROR");
+    };
+    int error_line = __builtin_LINE() - 4;
+    int func_line = error_line;
+
+    auto func3 = [=]() -> absl::Status {
+      ABSL_RETURN_IF_ERROR(func1());
+      return ReturnError("ERROR_Func3");
+    };
+    int func3_line = __builtin_LINE() - 3;
+
+    auto func4 = [=]() -> absl::Status {
+      ABSL_RETURN_IF_ERROR(func3());
+      return ReturnError("ERROR_Func4");
+    };
+    int func4_line = __builtin_LINE() - 3;
+    CheckSourceLocation(func4(),
+                        {error_line, func_line, func3_line, func4_line});
+  }
+  {
+    auto func1 = []() -> absl::Status {
+      ABSL_RETURN_IF_ERROR(ReturnOk());
+      ABSL_RETURN_IF_ERROR(ReturnOk());
+      ABSL_RETURN_IF_ERROR(absl::Status(absl::StatusCode::kUnknown, ""));
+      return ReturnError("ERROR");
+    };
+
+    auto func3 = [=]() -> absl::Status {
+      ABSL_RETURN_IF_ERROR(func1()) << "foo";
+      return ReturnError("ERROR_Func3");
+    };
+    int func3_line = __builtin_LINE() - 3;
+
+    auto func4 = [=]() -> absl::Status {
+      ABSL_RETURN_IF_ERROR(func3());
+      return ReturnError("ERROR_Func4");
+    };
+    int func4_line = __builtin_LINE() - 3;
+    EXPECT_THAT(func4().message(), Eq("foo"));
+    CheckSourceLocation(func4(), {func3_line, func4_line});
+  }
+  {
+    auto func1 = []() -> absl::Status {
+      ABSL_RETURN_IF_ERROR(ReturnOk());
+      ABSL_RETURN_IF_ERROR(ReturnOk());
+      ABSL_RETURN_IF_ERROR(absl::Status(absl::StatusCode::kUnknown, "")) << "";
+      return ReturnError("ERROR");
+    };
+
+    auto func3 = [=]() -> absl::Status {
+      ABSL_RETURN_IF_ERROR(func1());
+      return ReturnError("ERROR_Func3");
+    };
+
+    auto func4 = [=]() -> absl::Status {
+      ABSL_RETURN_IF_ERROR(func3());
+      return ReturnError("ERROR_Func4");
+    };
+    CheckSourceLocation(func4());
+  }
+}
+
+TEST(ReturnIfError, WorksWithBuilder) {
+  auto func = []() -> absl::Status {
+    ABSL_RETURN_IF_ERROR(ReturnOkBuilder());
+    ABSL_RETURN_IF_ERROR(ReturnOkBuilder());
+    ABSL_RETURN_IF_ERROR(ReturnErrorBuilder("EXPECTED"));
+    return ReturnErrorBuilder("ERROR");
+  };
+
+  EXPECT_THAT(func().message(), Eq("EXPECTED"));
+
+  // Test that ABSL_RETURN_IF_ERROR can also return the StatusBuilder directly.
+  auto func2 = []() -> absl::StatusBuilder {
+    ABSL_RETURN_IF_ERROR(ReturnOkBuilder());
+    ABSL_RETURN_IF_ERROR(ReturnOkBuilder());
+    ABSL_RETURN_IF_ERROR(ReturnErrorBuilder("EXPECTED"));
+    return ReturnErrorBuilder("ERROR");
+  };
+  EXPECT_THAT(static_cast<absl::Status>(func2()).message(), Eq("EXPECTED"));
+}
+
+TEST(ReturnIfError, IfInputIsBuilderDoesNotEagerlyConvertToStatus) {
+  auto func = []() -> absl::Status {
+    auto builder = ReturnErrorBuilder("FIRST");
+    builder.SetPrepend();
+    // If this call decays the builder to Status first and then makes another
+    // builder it will forget about the SetPrepend and the streaming will happen
+    // in the wrong order.
+    ABSL_RETURN_IF_ERROR(builder) << "SECOND ";
+    return absl::OkStatus();
+  };
+  EXPECT_THAT(func().message(), Eq("SECOND FIRST"));
+}
+
+TEST(ReturnIfError, WorksWithLambda) {
+  auto func = []() -> absl::Status {
+    ABSL_RETURN_IF_ERROR([] { return ReturnOk(); }());
+    ABSL_RETURN_IF_ERROR([] { return ReturnError("EXPECTED"); }());
+    return ReturnError("ERROR");
+  };
+
+  EXPECT_THAT(func().message(), Eq("EXPECTED"));
+}
+
+TEST(ReturnIfError, WorksWithAppend) {
+  auto fail_test_if_called = []() -> std::string {
+    ADD_FAILURE();
+    return "FAILURE";
+  };
+  auto func = [&]() -> absl::Status {
+    ABSL_RETURN_IF_ERROR(ReturnOk()) << fail_test_if_called();
+    ABSL_RETURN_IF_ERROR(ReturnError("EXPECTED A")) << "EXPECTED B";
+    return absl::OkStatus();
+  };
+
+  EXPECT_THAT(func().message(),
+              AllOf(HasSubstr("EXPECTED A"), HasSubstr("EXPECTED B")));
+}
+
+TEST(ReturnIfError, WorksWithVoidReturnAdaptor) {
+  int code = 0;
+  int phase = 0;
+  auto adaptor = [&](absl::Status status) -> void { code = phase; };
+  auto func = [&]() -> void {
+    phase = 1;
+    ABSL_RETURN_IF_ERROR(ReturnOk()).With(adaptor);
+    phase = 2;
+    ABSL_RETURN_IF_ERROR(ReturnError("EXPECTED A")).With(adaptor);
+    phase = 3;
+  };
+
+  func();
+  EXPECT_EQ(phase, 2);
+  EXPECT_EQ(code, 2);
+}
+
+}  // namespace
diff --git a/absl/status/status_matchers.h b/absl/status/status_matchers.h
new file mode 100644
index 00000000000..a7d5634f67c
--- /dev/null
+++ b/absl/status/status_matchers.h
@@ -0,0 +1,166 @@
+// Copyright 2024 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: status_matchers.h
+// -----------------------------------------------------------------------------
+//
+// Testing utilities for working with `absl::Status` and `absl::StatusOr`.
+//
+// Defines the following utilities:
+//
+//   =================
+//   ABSL_EXPECT_OK(s)
+//
+//   ABSL_ASSERT_OK(s)
+//   =================
+//   Convenience macros for `EXPECT_THAT(s, IsOk())`, where `s` is either
+//   a `Status` or a `StatusOr<T>`.
+//
+//   There are no EXPECT_NOT_OK/ASSERT_NOT_OK macros.
+//   Prefer to check for the specific expected error:
+//
+//   EXPECT_THAT(s, StatusIs(expected_error));
+//   EXPECT_THAT(s, StatusIs(_, _, HasSubstr("expected error")));
+//
+//
+//   ===============
+//   `IsOkAndHolds(m)`
+//   ===============
+//
+//   This gMock matcher matches a StatusOr<T> value whose status is OK
+//   and whose inner value matches matcher m.  Example:
+//
+//   ```
+//   using ::testing::MatchesRegex;
+//   using ::absl_testing::IsOkAndHolds;
+//   ...
+//   absl::StatusOr<string> maybe_name = ...;
+//   EXPECT_THAT(maybe_name, IsOkAndHolds(MatchesRegex("John .*")));
+//   ```
+//
+//   ===============================
+//   `StatusIs(status_code_matcher)`
+//   ===============================
+//
+//   This is a shorthand for
+//     `StatusIs(status_code_matcher, ::testing::_)`
+//   In other words, it's like the two-argument `StatusIs()`, except that it
+//   ignores error message.
+//
+//   ===============
+//   `IsOk()`
+//   ===============
+//
+//   Matches an `absl::Status` or `absl::StatusOr<T>` value whose status value
+//   is `absl::StatusCode::kOk.`
+//
+//   Equivalent to 'StatusIs(absl::StatusCode::kOk)'.
+//   Example:
+//   ```
+//   using ::absl_testing::IsOk;
+//   ...
+//   absl::StatusOr<string> maybe_name = ...;
+//   EXPECT_THAT(maybe_name, IsOk());
+//   Status s = ...;
+//   EXPECT_THAT(s, IsOk());
+//   ```
+//
+//   There is no NotOk() matcher. Prefer to check for the specific expected
+//   error.
+
+#ifndef ABSL_STATUS_STATUS_MATCHERS_H_
+#define ABSL_STATUS_STATUS_MATCHERS_H_
+
+#include <ostream>  // NOLINT
+#include <type_traits>
+#include <utility>
+
+#include "gmock/gmock.h"  // gmock_for_status_matchers.h
+#include "absl/base/config.h"
+#include "absl/status/internal/status_matchers.h"
+
+namespace absl_testing {
+ABSL_NAMESPACE_BEGIN
+
+// Macros for testing the results of functions that return absl::Status or
+// absl::StatusOr<T> (for any type T).
+#define ABSL_EXPECT_OK(expression) \
+  EXPECT_THAT(expression, ::absl_testing::IsOk())
+#define ABSL_ASSERT_OK(expression) \
+  ASSERT_THAT(expression, ::absl_testing::IsOk())
+
+// Returns a gMock matcher that matches a StatusOr<> whose status is
+// OK and whose value matches the inner matcher.
+template <typename InnerMatcherT>
+status_internal::IsOkAndHoldsMatcher<std::decay_t<InnerMatcherT>> IsOkAndHolds(
+    InnerMatcherT&& inner_matcher) {
+  return status_internal::IsOkAndHoldsMatcher<std::decay_t<InnerMatcherT>>(
+      std::forward<InnerMatcherT>(inner_matcher));
+}
+
+// Returns a gMock matcher that matches a Status or StatusOr<> whose status code
+// matches code_matcher and whose error message matches message_matcher.
+// Typically, code_matcher will be an absl::StatusCode, e.g.
+//
+// StatusIs(absl::StatusCode::kInvalidArgument, "...")
+template <typename StatusCodeMatcherT, typename StatusMessageMatcherT>
+status_internal::StatusIsMatcher StatusIs(
+    StatusCodeMatcherT&& code_matcher,
+    StatusMessageMatcherT&& message_matcher) {
+  return status_internal::StatusIsMatcher(
+      std::forward<StatusCodeMatcherT>(code_matcher),
+      std::forward<StatusMessageMatcherT>(message_matcher));
+}
+
+// Returns a gMock matcher that matches a Status or StatusOr<> and whose status
+// code matches code_matcher.  See above for details.
+template <typename StatusCodeMatcherT>
+status_internal::StatusIsMatcher StatusIs(StatusCodeMatcherT&& code_matcher) {
+  return absl_testing::StatusIs(std::forward<StatusCodeMatcherT>(code_matcher),
+                                ::testing::_);
+}
+
+// Returns a gMock matcher that matches a Status or StatusOr<> which is OK.
+inline status_internal::IsOkMatcher IsOk() {
+  return status_internal::IsOkMatcher();
+}
+
+// By defining ABSL_DEFINE_UNQUALIFIED_STATUS_TESTING_MACROS, this library also
+// provides unqualified versions of macros
+//
+// Unqualified macro names are likely to collide with those other projects, and
+// so are not recommended.  Further, this is true of any transitive dependency
+// of Abseil; it is impossible to be confident no downstream library will not
+// also define these macros itself nor depend on a different library that also
+// defines them.
+//
+// To enable this, define `ABSL_DEFINE_UNQUALIFIED_STATUS_TESTING_MACROS`
+// preferably at the command line, e.g.
+// `-DABSL_DEFINE_UNQUALIFIED_STATUS_TESTING_MACROS` or
+// `local_defines = ["ABSL_DEFINE_UNQUALIFIED_STATUS_TESTING_MACROS"]` if using
+// Bazel.
+//
+// These are turned on by default inside Google's internal codebase where their
+// use is historically ubiquitous.  Other OSS Google projects should use the
+// qualified versions or the `EXPECT_THAT(..., IsOk())` form.
+#ifdef ABSL_DEFINE_UNQUALIFIED_STATUS_TESTING_MACROS
+#define EXPECT_OK(expression) ABSL_EXPECT_OK(expression)
+#define ASSERT_OK(expression) ABSL_ASSERT_OK(expression)
+#endif  // ABSL_DEFINE_UNQUALIFIED_STATUS_TESTING_MACROS
+
+ABSL_NAMESPACE_END
+}  // namespace absl_testing
+
+#endif  // ABSL_STATUS_STATUS_MATCHERS_H_
diff --git a/absl/status/status_matchers_test.cc b/absl/status/status_matchers_test.cc
new file mode 100644
index 00000000000..54cc7333e53
--- /dev/null
+++ b/absl/status/status_matchers_test.cc
@@ -0,0 +1,210 @@
+// Copyright 2024 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: status_matchers_test.cc
+// -----------------------------------------------------------------------------
+#include "absl/status/status_matchers.h"
+
+#include <string>
+#include <vector>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest-spi.h"
+#include "gtest/gtest.h"
+#include "absl/status/status.h"
+#include "absl/status/statusor.h"
+#include "absl/strings/string_view.h"
+
+namespace {
+
+using ::absl_testing::IsOk;
+using ::absl_testing::IsOkAndHolds;
+using ::absl_testing::StatusIs;
+using ::testing::ElementsAre;
+using ::testing::Eq;
+using ::testing::Gt;
+using ::testing::MatchesRegex;
+using ::testing::Not;
+using ::testing::Ref;
+
+TEST(StatusMatcherTest, AbslExpectAssertOk) {
+  ABSL_EXPECT_OK(absl::OkStatus());
+  ABSL_ASSERT_OK(absl::OkStatus());
+  EXPECT_NONFATAL_FAILURE(ABSL_EXPECT_OK(absl::InternalError("Smigla error")),
+                          "Smigla error");
+  EXPECT_FATAL_FAILURE(ABSL_ASSERT_OK(absl::InternalError("Smigla error")),
+                       "Smigla error");
+}
+
+TEST(StatusMatcherTest, ExpectAssertOk) {
+#ifdef ABSL_DEFINE_UNQUALIFIED_STATUS_TESTING_MACROS
+  EXPECT_OK(absl::OkStatus());
+  ASSERT_OK(absl::OkStatus());
+  EXPECT_NONFATAL_FAILURE(EXPECT_OK(absl::InternalError("Smigla error")),
+                          "Smigla error");
+  EXPECT_FATAL_FAILURE(ASSERT_OK(absl::InternalError("Smigla error")),
+                       "Smigla error");
+#else
+#ifdef EXPECT_OK
+  static_assert(false, "EXPECT_OK defined despite being turned off.");
+#endif  // EXPECT_OK
+#ifdef ASSERT_OK
+  static_assert(false, "ASSERT_OK defined despite being turned off.");
+#endif  // ASSERT_OK
+#endif  // ABSL_DEFINE_UNQUALIFIED_STATUS_TESTING_MACROS
+}
+
+TEST(StatusMatcherTest, StatusIsOk) { EXPECT_THAT(absl::OkStatus(), IsOk()); }
+
+TEST(StatusMatcherTest, StatusOrIsOk) {
+  absl::StatusOr<int> ok_int = {0};
+  EXPECT_THAT(ok_int, IsOk());
+}
+
+TEST(StatusMatcherTest, StatusIsNotOk) {
+  absl::Status error = absl::UnknownError("Smigla");
+  EXPECT_NONFATAL_FAILURE(EXPECT_THAT(error, IsOk()), "Smigla");
+}
+
+TEST(StatusMatcherTest, StatusOrIsNotOk) {
+  absl::StatusOr<int> error = absl::UnknownError("Smigla");
+  EXPECT_NONFATAL_FAILURE(EXPECT_THAT(error, IsOk()), "Smigla");
+}
+
+TEST(StatusMatcherTest, IsOkAndHolds) {
+  absl::StatusOr<int> ok_int = {4};
+  absl::StatusOr<absl::string_view> ok_str = {"text"};
+  EXPECT_THAT(ok_int, IsOkAndHolds(4));
+  EXPECT_THAT(ok_int, IsOkAndHolds(Gt(0)));
+  EXPECT_THAT(ok_str, IsOkAndHolds("text"));
+}
+
+TEST(StatusMatcherTest, IsOkAndHoldsFailure) {
+  absl::StatusOr<int> ok_int = {502};
+  absl::StatusOr<int> error = absl::UnknownError("Smigla");
+  absl::StatusOr<absl::string_view> ok_str = {"actual"};
+  EXPECT_NONFATAL_FAILURE(EXPECT_THAT(ok_int, IsOkAndHolds(0)), "502");
+  EXPECT_NONFATAL_FAILURE(EXPECT_THAT(error, IsOkAndHolds(0)), "Smigla");
+  EXPECT_NONFATAL_FAILURE(EXPECT_THAT(ok_str, IsOkAndHolds("expected")),
+                          "actual");
+}
+
+template <typename MatcherType, typename Value>
+std::string Explain(const MatcherType& m, const Value& x) {
+  ::testing::StringMatchResultListener listener;
+  ExplainMatchResult(m, x, &listener);
+  return listener.str();
+}
+
+TEST(StatusMatcherTest, StatusIs) {
+  absl::Status unknown = absl::UnknownError("unbekannt");
+  absl::Status invalid = absl::InvalidArgumentError("ungueltig");
+  EXPECT_THAT(absl::OkStatus(), StatusIs(absl::StatusCode::kOk));
+  EXPECT_THAT(absl::OkStatus(), StatusIs(0));
+  EXPECT_THAT(unknown, StatusIs(absl::StatusCode::kUnknown));
+  EXPECT_THAT(unknown, StatusIs(2));
+  EXPECT_THAT(unknown, StatusIs(absl::StatusCode::kUnknown, "unbekannt"));
+  EXPECT_THAT(invalid, StatusIs(absl::StatusCode::kInvalidArgument));
+  EXPECT_THAT(invalid, StatusIs(3));
+  EXPECT_THAT(invalid,
+              StatusIs(absl::StatusCode::kInvalidArgument, "ungueltig"));
+
+  auto m = StatusIs(absl::StatusCode::kInternal, "internal error");
+  EXPECT_THAT(::testing::DescribeMatcher<absl::Status>(m),
+              MatchesRegex("has a status code that is equal to INTERNAL, and "
+                           "has an error message that .*"));
+  EXPECT_THAT(
+      ::testing::DescribeMatcher<absl::Status>(m, /*negation=*/true),
+      MatchesRegex(
+          "either has a status code that .*, or has an error message that .*"));
+  EXPECT_THAT(Explain(m, absl::InvalidArgumentError("internal error")),
+              Eq("whose status code is wrong"));
+  EXPECT_THAT(Explain(m, absl::InternalError("unexpected error")),
+              Eq("whose error message is wrong"));
+}
+
+TEST(StatusMatcherTest, StatusOrIs) {
+  absl::StatusOr<int> ok = {42};
+  absl::StatusOr<int> unknown = absl::UnknownError("unbekannt");
+  absl::StatusOr<absl::string_view> invalid =
+      absl::InvalidArgumentError("ungueltig");
+  EXPECT_THAT(ok, StatusIs(absl::StatusCode::kOk));
+  EXPECT_THAT(ok, StatusIs(0));
+  EXPECT_THAT(unknown, StatusIs(absl::StatusCode::kUnknown));
+  EXPECT_THAT(unknown, StatusIs(2));
+  EXPECT_THAT(unknown, StatusIs(absl::StatusCode::kUnknown, "unbekannt"));
+  EXPECT_THAT(invalid, StatusIs(absl::StatusCode::kInvalidArgument));
+  EXPECT_THAT(invalid, StatusIs(3));
+  EXPECT_THAT(invalid,
+              StatusIs(absl::StatusCode::kInvalidArgument, "ungueltig"));
+
+  auto m = StatusIs(absl::StatusCode::kInternal, "internal error");
+  EXPECT_THAT(
+      ::testing::DescribeMatcher<absl::StatusOr<int>>(m),
+      MatchesRegex(
+          "has a status code that .*, and has an error message that .*"));
+  EXPECT_THAT(
+      ::testing::DescribeMatcher<absl::StatusOr<int>>(m, /*negation=*/true),
+      MatchesRegex(
+          "either has a status code that .*, or has an error message that .*"));
+  EXPECT_THAT(Explain(m, absl::StatusOr<int>(57)), Eq("which is OK"));
+  EXPECT_THAT(Explain(m, absl::StatusOr<int>(
+                             absl::InvalidArgumentError("internal error"))),
+              Eq("whose status code is wrong"));
+  EXPECT_THAT(
+      Explain(m, absl::StatusOr<int>(absl::InternalError("unexpected error"))),
+      Eq("whose error message is wrong"));
+}
+
+TEST(StatusMatcherTest, StatusIsFailure) {
+  absl::Status unknown = absl::UnknownError("unbekannt");
+  absl::Status invalid = absl::InvalidArgumentError("ungueltig");
+  EXPECT_NONFATAL_FAILURE(
+      EXPECT_THAT(absl::OkStatus(),
+                  StatusIs(absl::StatusCode::kInvalidArgument)),
+      "OK");
+  EXPECT_NONFATAL_FAILURE(
+      EXPECT_THAT(unknown, StatusIs(absl::StatusCode::kCancelled)), "UNKNOWN");
+  EXPECT_NONFATAL_FAILURE(
+      EXPECT_THAT(unknown, StatusIs(absl::StatusCode::kUnknown, "inconnu")),
+      "unbekannt");
+  EXPECT_NONFATAL_FAILURE(
+      EXPECT_THAT(invalid, StatusIs(absl::StatusCode::kOutOfRange)), "INVALID");
+  EXPECT_NONFATAL_FAILURE(
+      EXPECT_THAT(invalid,
+                  StatusIs(absl::StatusCode::kInvalidArgument, "invalide")),
+      "ungueltig");
+}
+
+TEST(StatusMatcherTest, ReferencesWork) {
+  int i = 17;
+  int j = 19;
+  EXPECT_THAT(absl::StatusOr<int&>(i), IsOkAndHolds(17));
+  EXPECT_THAT(absl::StatusOr<int&>(i), Not(IsOkAndHolds(19)));
+  EXPECT_THAT(absl::StatusOr<const int&>(i), IsOkAndHolds(17));
+
+  // Reference testing works as expected.
+  EXPECT_THAT(absl::StatusOr<int&>(i), IsOkAndHolds(Ref(i)));
+  EXPECT_THAT(absl::StatusOr<int&>(i), Not(IsOkAndHolds(Ref(j))));
+
+  // Try a more complex one.
+  std::vector<std::string> vec = {"A", "B", "C"};
+  EXPECT_THAT(absl::StatusOr<std::vector<std::string>&>(vec),
+              IsOkAndHolds(ElementsAre("A", "B", "C")));
+  EXPECT_THAT(absl::StatusOr<std::vector<std::string>&>(vec),
+              Not(IsOkAndHolds(ElementsAre("A", "X", "C"))));
+}
+
+}  // namespace
diff --git a/absl/status/status_payload_printer.cc b/absl/status/status_payload_printer.cc
new file mode 100644
index 00000000000..98401e90722
--- /dev/null
+++ b/absl/status/status_payload_printer.cc
@@ -0,0 +1,36 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+#include "absl/status/status_payload_printer.h"
+
+#include "absl/base/config.h"
+#include "absl/base/internal/atomic_hook.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace status_internal {
+
+ABSL_INTERNAL_ATOMIC_HOOK_ATTRIBUTES
+static absl::base_internal::AtomicHook<StatusPayloadPrinter> storage;
+
+void SetStatusPayloadPrinter(StatusPayloadPrinter printer) {
+  storage.Store(printer);
+}
+
+StatusPayloadPrinter GetStatusPayloadPrinter() {
+  return storage.Load();
+}
+
+}  // namespace status_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/status/status_payload_printer.h b/absl/status/status_payload_printer.h
new file mode 100644
index 00000000000..7e2c8c8bbf8
--- /dev/null
+++ b/absl/status/status_payload_printer.h
@@ -0,0 +1,53 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+#ifndef ABSL_STATUS_STATUS_PAYLOAD_PRINTER_H_
+#define ABSL_STATUS_STATUS_PAYLOAD_PRINTER_H_
+
+#include <optional>
+#include <string>
+
+#include "absl/base/nullability.h"
+#include "absl/strings/cord.h"
+#include "absl/strings/string_view.h"
+#include "absl/types/optional.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace status_internal {
+
+// By default, `Status::ToString` and `operator<<(Status)` print a payload by
+// dumping the type URL and the raw bytes. To help debugging, we provide an
+// extension point, which is a global printer function that can be set by users
+// to specify how to print payloads. The function takes the type URL and the
+// payload as input, and should return a valid human-readable string on success
+// or `std::nullopt` on failure (in which case it falls back to the default
+// approach of printing the raw bytes).
+// NOTE: This is an internal API and the design is subject to change in the
+// future in a non-backward-compatible way. Since it's only meant for debugging
+// purpose, you should not rely on it in any critical logic.
+using StatusPayloadPrinter = std::optional<std::string> (*absl_nullable)(
+    absl::string_view, const absl::Cord&);
+
+// Sets the global payload printer. Only one printer should be set per process.
+// If multiple printers are set, it's undefined which one will be used.
+void SetStatusPayloadPrinter(StatusPayloadPrinter);
+
+// Returns the global payload printer if previously set, otherwise `nullptr`.
+StatusPayloadPrinter GetStatusPayloadPrinter();
+
+}  // namespace status_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STATUS_STATUS_PAYLOAD_PRINTER_H_
diff --git a/absl/status/status_test.cc b/absl/status/status_test.cc
new file mode 100644
index 00000000000..c4679d68169
--- /dev/null
+++ b/absl/status/status_test.cc
@@ -0,0 +1,1088 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/status/status.h"
+
+#include <errno.h>
+
+#include <array>
+#include <cstddef>
+#include <sstream>
+#include <string>
+#include <utility>
+#include <vector>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/strings/cord.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/str_format.h"
+#include "absl/strings/string_view.h"
+#include "absl/types/source_location.h"
+
+namespace {
+
+using ::testing::Eq;
+using ::testing::HasSubstr;
+using ::testing::Optional;
+using ::testing::UnorderedElementsAreArray;
+
+TEST(StatusCode, InsertionOperator) {
+  const absl::StatusCode code = absl::StatusCode::kUnknown;
+  std::ostringstream oss;
+  oss << code;
+  EXPECT_EQ(oss.str(), absl::StatusCodeToString(code));
+  EXPECT_EQ(oss.str(), absl::StatusCodeToStringView(code));
+}
+
+// This structure holds the details for testing a single error code,
+// its creator, its rvalue creator, and its classifier.
+struct ErrorTest {
+  absl::StatusCode code;
+  using Creator = absl::Status (*)(absl::string_view, absl::SourceLocation);
+  Creator creator;
+  using CreatorRValue = absl::Status (*)(std::string&&, absl::SourceLocation);
+  CreatorRValue creator_rvalue;
+  using Classifier = bool (*)(const absl::Status&);
+  Classifier classifier;
+};
+
+constexpr ErrorTest kErrorTests[]{
+    {absl::StatusCode::kCancelled,           //
+     absl::CancelledError,                   //
+     absl::CancelledError,                   //
+     absl::IsCancelled},                     //
+    {absl::StatusCode::kUnknown,             //
+     absl::UnknownError,                     //
+     absl::UnknownError,                     //
+     absl::IsUnknown},                       //
+    {absl::StatusCode::kInvalidArgument,     //
+     absl::InvalidArgumentError,             //
+     absl::InvalidArgumentError,             //
+     absl::IsInvalidArgument},               //
+    {absl::StatusCode::kDeadlineExceeded,    //
+     absl::DeadlineExceededError,            //
+     absl::DeadlineExceededError,            //
+     absl::IsDeadlineExceeded},              //
+    {absl::StatusCode::kNotFound,            //
+     absl::NotFoundError,                    //
+     absl::NotFoundError,                    //
+     absl::IsNotFound},                      //
+    {absl::StatusCode::kAlreadyExists,       //
+     absl::AlreadyExistsError,               //
+     absl::AlreadyExistsError,               //
+     absl::IsAlreadyExists},                 //
+    {absl::StatusCode::kPermissionDenied,    //
+     absl::PermissionDeniedError,            //
+     absl::PermissionDeniedError,            //
+     absl::IsPermissionDenied},              //
+    {absl::StatusCode::kResourceExhausted,   //
+     absl::ResourceExhaustedError,           //
+     absl::ResourceExhaustedError,           //
+     absl::IsResourceExhausted},             //
+    {absl::StatusCode::kFailedPrecondition,  //
+     absl::FailedPreconditionError,          //
+     absl::FailedPreconditionError,          //
+     absl::IsFailedPrecondition},            //
+    {absl::StatusCode::kAborted,             //
+     absl::AbortedError,                     //
+     absl::AbortedError,                     //
+     absl::IsAborted},                       //
+    {absl::StatusCode::kOutOfRange,          //
+     absl::OutOfRangeError,                  //
+     absl::OutOfRangeError,                  //
+     absl::IsOutOfRange},                    //
+    {absl::StatusCode::kUnimplemented,       //
+     absl::UnimplementedError,               //
+     absl::UnimplementedError,               //
+     absl::IsUnimplemented},                 //
+    {absl::StatusCode::kInternal,            //
+     absl::InternalError,                    //
+     absl::InternalError,                    //
+     absl::IsInternal},                      //
+    {absl::StatusCode::kUnavailable,         //
+     absl::UnavailableError,                 //
+     absl::UnavailableError,                 //
+     absl::IsUnavailable},                   //
+    {absl::StatusCode::kDataLoss,            //
+     absl::DataLossError,                    //
+     absl::DataLossError,                    //
+     absl::IsDataLoss},                      //
+    {absl::StatusCode::kUnauthenticated,     //
+     absl::UnauthenticatedError,             //
+     absl::UnauthenticatedError,             //
+     absl::IsUnauthenticated},               //
+};
+
+TEST(Status, CreateAndClassify) {
+  for (const auto& test : kErrorTests) {
+    SCOPED_TRACE(absl::StatusCodeToString(test.code));
+
+    // Ensure that the creator does, in fact, create status objects with the
+    // expected error code and message.
+    std::string message =
+        absl::StrCat("error code ", test.code, " test message");
+    absl::Status status =
+        test.creator(message, absl::SourceLocation::current());
+    EXPECT_EQ(test.code, status.code());
+    EXPECT_EQ(message, status.message());
+
+    // Testing the rvalue creator.
+    absl::Status status_rvalue = test.creator_rvalue(
+        std::string(message), absl::SourceLocation::current());
+    EXPECT_EQ(test.code, status_rvalue.code());
+    EXPECT_EQ(message, status_rvalue.message());
+
+    // Ensure that the classifier returns true for a status produced by the
+    // creator.
+    EXPECT_TRUE(test.classifier(status));
+    EXPECT_TRUE(test.classifier(status_rvalue));
+
+    // Ensure that the classifier returns false for status with a different
+    // code.
+    for (const auto& other : kErrorTests) {
+      if (other.code != test.code) {
+        EXPECT_FALSE(test.classifier(absl::Status(other.code, "")))
+            << " other.code = " << other.code;
+      }
+    }
+  }
+}
+
+// Test that status convenience functions can be called with a string literal.
+// Doing so to make sure it's not ambiguous between the string_view and string&&
+// overloads.
+TEST(Status, StringLiteralCreation) {
+  EXPECT_EQ(absl::AbortedError("msg").code(), absl::StatusCode::kAborted);
+  EXPECT_EQ(absl::AlreadyExistsError("msg").code(),
+            absl::StatusCode::kAlreadyExists);
+  EXPECT_EQ(absl::CancelledError("msg").code(), absl::StatusCode::kCancelled);
+  EXPECT_EQ(absl::DataLossError("msg").code(), absl::StatusCode::kDataLoss);
+  EXPECT_EQ(absl::DeadlineExceededError("msg").code(),
+            absl::StatusCode::kDeadlineExceeded);
+  EXPECT_EQ(absl::FailedPreconditionError("msg").code(),
+            absl::StatusCode::kFailedPrecondition);
+  EXPECT_EQ(absl::InternalError("msg").code(), absl::StatusCode::kInternal);
+  EXPECT_EQ(absl::InvalidArgumentError("msg").code(),
+            absl::StatusCode::kInvalidArgument);
+  EXPECT_EQ(absl::NotFoundError("msg").code(), absl::StatusCode::kNotFound);
+  EXPECT_EQ(absl::OutOfRangeError("msg").code(), absl::StatusCode::kOutOfRange);
+  EXPECT_EQ(absl::PermissionDeniedError("msg").code(),
+            absl::StatusCode::kPermissionDenied);
+  EXPECT_EQ(absl::ResourceExhaustedError("msg").code(),
+            absl::StatusCode::kResourceExhausted);
+  EXPECT_EQ(absl::UnauthenticatedError("msg").code(),
+            absl::StatusCode::kUnauthenticated);
+  EXPECT_EQ(absl::UnavailableError("msg").code(),
+            absl::StatusCode::kUnavailable);
+  EXPECT_EQ(absl::UnimplementedError("msg").code(),
+            absl::StatusCode::kUnimplemented);
+  EXPECT_EQ(absl::UnknownError("msg").code(), absl::StatusCode::kUnknown);
+}
+
+TEST(Status, DefaultConstructor) {
+  absl::Status status;
+  EXPECT_TRUE(status.ok());
+  EXPECT_EQ(absl::StatusCode::kOk, status.code());
+  EXPECT_EQ("", status.message());
+}
+
+TEST(Status, OkStatus) {
+  absl::Status status = absl::OkStatus();
+  EXPECT_TRUE(status.ok());
+  EXPECT_EQ(absl::StatusCode::kOk, status.code());
+  EXPECT_EQ("", status.message());
+}
+
+TEST(Status, ConstructorWithCodeMessage) {
+  {
+    absl::Status status(absl::StatusCode::kCancelled, "");
+    EXPECT_FALSE(status.ok());
+    EXPECT_EQ(absl::StatusCode::kCancelled, status.code());
+    EXPECT_EQ("", status.message());
+  }
+  {
+    absl::Status status(absl::StatusCode::kInternal, "message");
+    EXPECT_FALSE(status.ok());
+    EXPECT_EQ(absl::StatusCode::kInternal, status.code());
+    EXPECT_EQ("message", status.message());
+  }
+}
+
+TEST(Status, StatusMessageCStringTest) {
+  {
+    absl::Status status = absl::OkStatus();
+    EXPECT_EQ(status.message(), "");
+    EXPECT_STREQ(absl::StatusMessageAsCStr(status), "");
+    EXPECT_EQ(status.message(), absl::StatusMessageAsCStr(status));
+    EXPECT_NE(absl::StatusMessageAsCStr(status), nullptr);
+  }
+  {
+    absl::Status status;
+    EXPECT_EQ(status.message(), "");
+    EXPECT_NE(absl::StatusMessageAsCStr(status), nullptr);
+    EXPECT_STREQ(absl::StatusMessageAsCStr(status), "");
+  }
+  {
+    absl::Status status(absl::StatusCode::kInternal, "message");
+    EXPECT_FALSE(status.ok());
+    EXPECT_EQ(absl::StatusCode::kInternal, status.code());
+    EXPECT_EQ("message", status.message());
+    EXPECT_STREQ("message", absl::StatusMessageAsCStr(status));
+  }
+}
+
+TEST(Status, ConstructOutOfRangeCode) {
+  const int kRawCode = 9999;
+  absl::Status status(static_cast<absl::StatusCode>(kRawCode), "");
+  EXPECT_EQ(absl::StatusCode::kUnknown, status.code());
+  EXPECT_EQ(kRawCode, status.raw_code());
+}
+
+constexpr char kUrl1[] = "url.payload.1";
+constexpr char kUrl2[] = "url.payload.2";
+constexpr char kUrl3[] = "url.payload.3";
+constexpr char kUrl4[] = "url.payload.xx";
+
+constexpr char kPayload1[] = "aaaaa";
+constexpr char kPayload2[] = "bbbbb";
+constexpr char kPayload3[] = "ccccc";
+
+using PayloadsVec = std::vector<std::pair<std::string, absl::Cord>>;
+
+TEST(Status, TestGetSetPayload) {
+  absl::Status ok_status = absl::OkStatus();
+  ok_status.SetPayload(kUrl1, absl::Cord(kPayload1));
+  ok_status.SetPayload(kUrl2, absl::Cord(kPayload2));
+
+  EXPECT_FALSE(ok_status.GetPayload(kUrl1));
+  EXPECT_FALSE(ok_status.GetPayload(kUrl2));
+
+  absl::Status bad_status(absl::StatusCode::kInternal, "fail");
+  bad_status.SetPayload(kUrl1, absl::Cord(kPayload1));
+  bad_status.SetPayload(kUrl2, absl::Cord(kPayload2));
+
+  EXPECT_THAT(bad_status.GetPayload(kUrl1), Optional(Eq(kPayload1)));
+  EXPECT_THAT(bad_status.GetPayload(kUrl2), Optional(Eq(kPayload2)));
+
+  EXPECT_FALSE(bad_status.GetPayload(kUrl3));
+
+  bad_status.SetPayload(kUrl1, absl::Cord(kPayload3));
+  EXPECT_THAT(bad_status.GetPayload(kUrl1), Optional(Eq(kPayload3)));
+
+  // Testing dynamically generated type_url
+  bad_status.SetPayload(absl::StrCat(kUrl1, ".1"), absl::Cord(kPayload1));
+  EXPECT_THAT(bad_status.GetPayload(absl::StrCat(kUrl1, ".1")),
+              Optional(Eq(kPayload1)));
+}
+
+TEST(Status, TestErasePayload) {
+  absl::Status bad_status(absl::StatusCode::kInternal, "fail");
+  bad_status.SetPayload(kUrl1, absl::Cord(kPayload1));
+  bad_status.SetPayload(kUrl2, absl::Cord(kPayload2));
+  bad_status.SetPayload(kUrl3, absl::Cord(kPayload3));
+
+  EXPECT_FALSE(bad_status.ErasePayload(kUrl4));
+
+  EXPECT_TRUE(bad_status.GetPayload(kUrl2));
+  EXPECT_TRUE(bad_status.ErasePayload(kUrl2));
+  EXPECT_FALSE(bad_status.GetPayload(kUrl2));
+  EXPECT_FALSE(bad_status.ErasePayload(kUrl2));
+
+  EXPECT_TRUE(bad_status.ErasePayload(kUrl1));
+  EXPECT_TRUE(bad_status.ErasePayload(kUrl3));
+
+  bad_status.SetPayload(kUrl1, absl::Cord(kPayload1));
+  EXPECT_TRUE(bad_status.ErasePayload(kUrl1));
+}
+
+TEST(Status, TestComparePayloads) {
+  absl::Status bad_status1(absl::StatusCode::kInternal, "fail");
+  bad_status1.SetPayload(kUrl1, absl::Cord(kPayload1));
+  bad_status1.SetPayload(kUrl2, absl::Cord(kPayload2));
+  bad_status1.SetPayload(kUrl3, absl::Cord(kPayload3));
+
+  absl::Status bad_status2(absl::StatusCode::kInternal, "fail");
+  bad_status2.SetPayload(kUrl2, absl::Cord(kPayload2));
+  bad_status2.SetPayload(kUrl3, absl::Cord(kPayload3));
+  bad_status2.SetPayload(kUrl1, absl::Cord(kPayload1));
+
+  EXPECT_EQ(bad_status1, bad_status2);
+}
+
+TEST(Status, TestComparePayloadsAfterErase) {
+  absl::Status payload_status(absl::StatusCode::kInternal, "");
+  payload_status.SetPayload(kUrl1, absl::Cord(kPayload1));
+  payload_status.SetPayload(kUrl2, absl::Cord(kPayload2));
+
+  absl::Status empty_status(absl::StatusCode::kInternal, "");
+
+  // Different payloads, not equal
+  EXPECT_NE(payload_status, empty_status);
+  EXPECT_TRUE(payload_status.ErasePayload(kUrl1));
+
+  // Still Different payloads, still not equal.
+  EXPECT_NE(payload_status, empty_status);
+  EXPECT_TRUE(payload_status.ErasePayload(kUrl2));
+
+  // Both empty payloads, should be equal
+  EXPECT_EQ(payload_status, empty_status);
+}
+
+PayloadsVec AllVisitedPayloads(const absl::Status& s) {
+  PayloadsVec result;
+
+  s.ForEachPayload([&](absl::string_view type_url, const absl::Cord& payload) {
+    result.push_back(std::make_pair(std::string(type_url), payload));
+  });
+
+  return result;
+}
+
+TEST(Status, TestForEachPayload) {
+  absl::Status bad_status(absl::StatusCode::kInternal, "fail");
+  bad_status.SetPayload(kUrl1, absl::Cord(kPayload1));
+  bad_status.SetPayload(kUrl2, absl::Cord(kPayload2));
+  bad_status.SetPayload(kUrl3, absl::Cord(kPayload3));
+
+  int count = 0;
+
+  bad_status.ForEachPayload(
+      [&count](absl::string_view, const absl::Cord&) { ++count; });
+
+  EXPECT_EQ(count, 3);
+
+  PayloadsVec expected_payloads = {{kUrl1, absl::Cord(kPayload1)},
+                                   {kUrl2, absl::Cord(kPayload2)},
+                                   {kUrl3, absl::Cord(kPayload3)}};
+
+  // Test that we visit all the payloads in the status.
+  PayloadsVec visited_payloads = AllVisitedPayloads(bad_status);
+  EXPECT_THAT(visited_payloads, UnorderedElementsAreArray(expected_payloads));
+
+  // Test that visitation order is not consistent between run.
+  std::vector<absl::Status> scratch;
+  while (true) {
+    scratch.emplace_back(absl::StatusCode::kInternal, "fail");
+
+    scratch.back().SetPayload(kUrl1, absl::Cord(kPayload1));
+    scratch.back().SetPayload(kUrl2, absl::Cord(kPayload2));
+    scratch.back().SetPayload(kUrl3, absl::Cord(kPayload3));
+
+    if (AllVisitedPayloads(scratch.back()) != visited_payloads) {
+      break;
+    }
+  }
+}
+
+TEST(Status, ToString) {
+  absl::Status status(absl::StatusCode::kInternal, "fail");
+  EXPECT_EQ("INTERNAL: fail", status.ToString());
+  status.SetPayload("foo", absl::Cord("bar"));
+  EXPECT_EQ("INTERNAL: fail [foo='bar']", status.ToString());
+  status.SetPayload("bar", absl::Cord("\377"));
+  EXPECT_THAT(status.ToString(),
+              AllOf(HasSubstr("INTERNAL: fail"), HasSubstr("[foo='bar']"),
+                    HasSubstr("[bar='\\xff']")));
+}
+
+TEST(Status, ToStringMode) {
+  absl::Status status(absl::StatusCode::kInternal, "fail");
+  status.SetPayload("foo", absl::Cord("bar"));
+  status.SetPayload("bar", absl::Cord("\377"));
+
+  EXPECT_EQ("INTERNAL: fail",
+            status.ToString(absl::StatusToStringMode::kWithNoExtraData));
+
+  EXPECT_THAT(status.ToString(absl::StatusToStringMode::kWithPayload),
+              AllOf(HasSubstr("INTERNAL: fail"), HasSubstr("[foo='bar']"),
+                    HasSubstr("[bar='\\xff']")));
+
+  EXPECT_THAT(status.ToString(absl::StatusToStringMode::kWithEverything),
+              AllOf(HasSubstr("INTERNAL: fail"), HasSubstr("[foo='bar']"),
+                    HasSubstr("[bar='\\xff']")));
+
+  EXPECT_THAT(status.ToString(~absl::StatusToStringMode::kWithPayload),
+              AllOf(HasSubstr("INTERNAL: fail"), Not(HasSubstr("[foo='bar']")),
+                    Not(HasSubstr("[bar='\\xff']"))));
+}
+
+TEST(Status, OstreamOperator) {
+  absl::Status status(absl::StatusCode::kInternal, "fail");
+  {
+    std::stringstream stream;
+    stream << status;
+    EXPECT_THAT(stream.str(), AllOf(HasSubstr("INTERNAL: fail"),
+                                    HasSubstr("status_test.cc:")));
+  }
+  status.SetPayload("foo", absl::Cord("bar"));
+  {
+    std::stringstream stream;
+    stream << status;
+    EXPECT_THAT(stream.str(),
+                AllOf(HasSubstr("INTERNAL: fail"), HasSubstr("[foo='bar']"),
+                      HasSubstr("status_test.cc:")));
+  }
+  status.SetPayload("bar", absl::Cord("\377"));
+  {
+    std::stringstream stream;
+    stream << status;
+    EXPECT_THAT(
+        stream.str(),
+        AllOf(HasSubstr("INTERNAL: fail"), HasSubstr("[foo='bar']"),
+              HasSubstr("[bar='\\xff']"), HasSubstr("status_test.cc:")));
+  }
+}
+
+TEST(Status, AbslStringify) {
+  absl::Status status(absl::StatusCode::kInternal, "fail");
+  EXPECT_THAT(absl::StrCat(status),
+              AllOf(HasSubstr("INTERNAL: fail"), HasSubstr("status_test.cc:")));
+  EXPECT_THAT(absl::StrFormat("%v", status),
+              AllOf(HasSubstr("INTERNAL: fail"), HasSubstr("status_test.cc:")));
+  EXPECT_EQ(absl::StrCat(status), absl::StrFormat("%v", status));
+  status.SetPayload("foo", absl::Cord("bar"));
+  EXPECT_THAT(absl::StrCat(status),
+              AllOf(HasSubstr("INTERNAL: fail"), HasSubstr("[foo='bar']"),
+                    HasSubstr("status_test.cc:")));
+  status.SetPayload("bar", absl::Cord("\377"));
+  EXPECT_THAT(absl::StrCat(status),
+              AllOf(HasSubstr("INTERNAL: fail"), HasSubstr("[foo='bar']"),
+                    HasSubstr("[bar='\\xff']"), HasSubstr("status_test.cc:")));
+}
+
+TEST(Status, OstreamEqStringify) {
+  absl::Status status(absl::StatusCode::kUnknown, "fail");
+  status.SetPayload("foo", absl::Cord("bar"));
+  std::stringstream stream;
+  stream << status;
+  EXPECT_EQ(stream.str(), absl::StrCat(status));
+}
+
+absl::Status EraseAndReturn(const absl::Status& base) {
+  absl::Status copy = base;
+  EXPECT_TRUE(copy.ErasePayload(kUrl1));
+  return copy;
+}
+
+TEST(Status, CopyOnWriteForErasePayload) {
+  {
+    absl::Status base(absl::StatusCode::kInvalidArgument, "fail");
+    base.SetPayload(kUrl1, absl::Cord(kPayload1));
+    EXPECT_TRUE(base.GetPayload(kUrl1).has_value());
+    absl::Status copy = EraseAndReturn(base);
+    EXPECT_TRUE(base.GetPayload(kUrl1).has_value());
+    EXPECT_FALSE(copy.GetPayload(kUrl1).has_value());
+  }
+  {
+    absl::Status base(absl::StatusCode::kInvalidArgument, "fail");
+    base.SetPayload(kUrl1, absl::Cord(kPayload1));
+    absl::Status copy = base;
+
+    EXPECT_TRUE(base.GetPayload(kUrl1).has_value());
+    EXPECT_TRUE(copy.GetPayload(kUrl1).has_value());
+
+    EXPECT_TRUE(base.ErasePayload(kUrl1));
+
+    EXPECT_FALSE(base.GetPayload(kUrl1).has_value());
+    EXPECT_TRUE(copy.GetPayload(kUrl1).has_value());
+  }
+}
+
+TEST(Status, CopyConstructor) {
+  {
+    absl::Status status;
+    absl::Status copy(status);
+    EXPECT_EQ(copy, status);
+  }
+  {
+    absl::Status status(absl::StatusCode::kInvalidArgument, "message");
+    absl::Status copy(status);
+    EXPECT_EQ(copy, status);
+  }
+  {
+    absl::Status status(absl::StatusCode::kInvalidArgument, "message");
+    status.SetPayload(kUrl1, absl::Cord(kPayload1));
+    absl::Status copy(status);
+    EXPECT_EQ(copy, status);
+  }
+}
+
+TEST(Status, CopyAssignment) {
+  absl::Status assignee;
+  {
+    absl::Status status;
+    assignee = status;
+    EXPECT_EQ(assignee, status);
+  }
+  {
+    absl::Status status(absl::StatusCode::kInvalidArgument, "message");
+    assignee = status;
+    EXPECT_EQ(assignee, status);
+  }
+  {
+    absl::Status status(absl::StatusCode::kInvalidArgument, "message");
+    status.SetPayload(kUrl1, absl::Cord(kPayload1));
+    assignee = status;
+    EXPECT_EQ(assignee, status);
+  }
+}
+
+TEST(Status, CopyAssignmentIsNotRef) {
+  const absl::Status status_orig(absl::StatusCode::kInvalidArgument, "message");
+  absl::Status status_copy = status_orig;
+  EXPECT_EQ(status_orig, status_copy);
+  status_copy.SetPayload(kUrl1, absl::Cord(kPayload1));
+  EXPECT_NE(status_orig, status_copy);
+}
+
+TEST(Status, MoveConstructor) {
+  {
+    absl::Status status;
+    absl::Status copy(absl::Status{});
+    EXPECT_EQ(copy, status);
+  }
+  {
+    absl::Status status(absl::StatusCode::kInvalidArgument, "message");
+    absl::Status copy(
+        absl::Status(absl::StatusCode::kInvalidArgument, "message"));
+    EXPECT_EQ(copy, status);
+  }
+  {
+    absl::Status status(absl::StatusCode::kInvalidArgument, "message");
+    status.SetPayload(kUrl1, absl::Cord(kPayload1));
+    absl::Status copy1(status);
+    absl::Status copy2(std::move(status));
+    EXPECT_EQ(copy1, copy2);
+  }
+}
+
+TEST(Status, MoveAssignment) {
+  absl::Status assignee;
+  {
+    absl::Status status;
+    assignee = absl::Status();
+    EXPECT_EQ(assignee, status);
+  }
+  {
+    absl::Status status(absl::StatusCode::kInvalidArgument, "message");
+    assignee = absl::Status(absl::StatusCode::kInvalidArgument, "message");
+    EXPECT_EQ(assignee, status);
+  }
+  {
+    absl::Status status(absl::StatusCode::kInvalidArgument, "message");
+    status.SetPayload(kUrl1, absl::Cord(kPayload1));
+    absl::Status copy(status);
+    assignee = std::move(status);
+    EXPECT_EQ(assignee, copy);
+  }
+  {
+    absl::Status status(absl::StatusCode::kInvalidArgument, "message");
+    absl::Status copy(status);
+    assignee = static_cast<absl::Status&&>(status);
+    EXPECT_EQ(assignee, copy);
+  }
+}
+
+TEST(Status, Update) {
+  absl::Status s;
+  s.Update(absl::OkStatus());
+  EXPECT_TRUE(s.ok());
+  const absl::Status a(absl::StatusCode::kCancelled, "message");
+  s.Update(a);
+  EXPECT_EQ(s, a);
+  const absl::Status b(absl::StatusCode::kInternal, "other message");
+  s.Update(b);
+  EXPECT_EQ(s, a);
+  s.Update(absl::OkStatus());
+  EXPECT_EQ(s, a);
+  EXPECT_FALSE(s.ok());
+}
+
+TEST(Status, Equality) {
+  absl::Status ok;
+  absl::Status no_payload = absl::CancelledError("no payload");
+  absl::Status one_payload = absl::InvalidArgumentError("one payload");
+  one_payload.SetPayload(kUrl1, absl::Cord(kPayload1));
+  absl::Status two_payloads = one_payload;
+  two_payloads.SetPayload(kUrl2, absl::Cord(kPayload2));
+  const std::array<absl::Status, 4> status_arr = {ok, no_payload, one_payload,
+                                                  two_payloads};
+  for (int i = 0; i < status_arr.size(); i++) {
+    for (int j = 0; j < status_arr.size(); j++) {
+      if (i == j) {
+        EXPECT_TRUE(status_arr[i] == status_arr[j]);
+        EXPECT_FALSE(status_arr[i] != status_arr[j]);
+      } else {
+        EXPECT_TRUE(status_arr[i] != status_arr[j]);
+        EXPECT_FALSE(status_arr[i] == status_arr[j]);
+      }
+    }
+  }
+}
+
+TEST(Status, Swap) {
+  auto test_swap = [](const absl::Status& s1, const absl::Status& s2) {
+    absl::Status copy1 = s1, copy2 = s2;
+    swap(copy1, copy2);
+    EXPECT_EQ(copy1, s2);
+    EXPECT_EQ(copy2, s1);
+  };
+  const absl::Status ok;
+  const absl::Status no_payload(absl::StatusCode::kAlreadyExists, "no payload");
+  absl::Status with_payload(absl::StatusCode::kInternal, "with payload");
+  with_payload.SetPayload(kUrl1, absl::Cord(kPayload1));
+  test_swap(ok, no_payload);
+  test_swap(no_payload, ok);
+  test_swap(ok, with_payload);
+  test_swap(with_payload, ok);
+  test_swap(no_payload, with_payload);
+  test_swap(with_payload, no_payload);
+}
+
+TEST(StatusErrno, ErrnoToStatusCode) {
+  EXPECT_EQ(absl::ErrnoToStatusCode(0), absl::StatusCode::kOk);
+
+  // Spot-check a few errno values.
+  EXPECT_EQ(absl::ErrnoToStatusCode(EINVAL),
+            absl::StatusCode::kInvalidArgument);
+  EXPECT_EQ(absl::ErrnoToStatusCode(ENOENT), absl::StatusCode::kNotFound);
+
+  // We'll pick a very large number so it hopefully doesn't collide to errno.
+  EXPECT_EQ(absl::ErrnoToStatusCode(19980927), absl::StatusCode::kUnknown);
+}
+
+TEST(StatusErrno, ErrnoToStatus) {
+  absl::Status status = absl::ErrnoToStatus(ENOENT, "Cannot open 'path'");
+  EXPECT_EQ(status.code(), absl::StatusCode::kNotFound);
+  EXPECT_EQ(status.message(), "Cannot open 'path': No such file or directory");
+}
+
+#ifdef ABSL_INTERNAL_HAVE_BUILTIN_LINE_FILE
+#define GET_SOURCE_LOCATION(offset) __builtin_LINE() - offset
+#else
+#define GET_SOURCE_LOCATION(offset) 1
+#endif
+
+void CheckSourceLocation(
+    const absl::Status& status, std::vector<int> lines = {},
+    absl::SourceLocation loc = absl::SourceLocation::current()) {
+  ASSERT_EQ(status.GetSourceLocations().size(), lines.size())
+      << "Size check failed at " << loc.line();
+  for (size_t i = 0; i < lines.size(); ++i) {
+    EXPECT_EQ(absl::string_view(status.GetSourceLocations()[i].file_name()),
+              absl::string_view(loc.file_name()))
+        << "File name check failed at " << loc.line();
+    EXPECT_EQ(status.GetSourceLocations()[i].line(), lines[i])
+        << "Line check failed at " << loc.line();
+  }
+}
+
+TEST(Status, ConstructorCheckSourceLocation) {
+  {
+    const absl::Status a;
+    const absl::Status b = a;
+    for (const absl::Status& status : {a, b}) {
+      EXPECT_TRUE(status.ok());
+      EXPECT_EQ(absl::StatusCode::kOk, status.code());
+      CheckSourceLocation(status);
+    }
+  }
+  {
+    const absl::Status a(absl::StatusCode::kInternal, "message",
+                         absl::SourceLocation::current());
+    int line = GET_SOURCE_LOCATION(1);
+    const absl::Status b = a;
+    for (const absl::Status& status : {a, b}) {
+      EXPECT_FALSE(status.ok());
+      EXPECT_EQ(absl::StatusCode::kInternal, status.code());
+      CheckSourceLocation(status, {line});
+    }
+  }
+  {
+    const absl::Status a(absl::StatusCode::kInternal, "message",
+                         absl::SourceLocation());
+    const absl::Status b = a;
+    for (const absl::Status& status : {a, b}) {
+      EXPECT_FALSE(status.ok());
+      EXPECT_EQ(absl::StatusCode::kInternal, status.code());
+      CheckSourceLocation(status);
+    }
+  }
+  {
+    const absl::Status a(absl::StatusCode::kInternal, "",
+                         absl::SourceLocation::current());
+    const absl::Status b = a;
+    for (const absl::Status& status : {a, b}) {
+      EXPECT_FALSE(status.ok());
+      EXPECT_EQ(absl::StatusCode::kInternal, status.code());
+      CheckSourceLocation(status);
+    }
+  }
+  {
+    const absl::Status a(absl::StatusCode::kInternal, "",
+                         absl::SourceLocation());
+    const absl::Status b = a;
+    for (const absl::Status& status : {a, b}) {
+      EXPECT_FALSE(status.ok());
+      EXPECT_EQ(absl::StatusCode::kInternal, status.code());
+      CheckSourceLocation(status);
+    }
+  }
+}
+
+TEST(Status, SourceLocationConstructor) {
+  {
+    // OK status doesn't save source locations.
+    const absl::Status original;
+    const absl::Status status(original, absl::SourceLocation());
+    EXPECT_TRUE(status.ok());
+    EXPECT_EQ(absl::StatusCode::kOk, status.code());
+    CheckSourceLocation(status);
+  }
+  {
+    // OK status doesn't save source locations.
+    const absl::Status original;
+    const absl::Status status(original, absl::SourceLocation::current());
+    EXPECT_TRUE(status.ok());
+    EXPECT_EQ(absl::StatusCode::kOk, status.code());
+    CheckSourceLocation(status);
+  }
+  {
+    // Non-ok Status with non-empty msg can save source locations with
+    // non-nullptr filename.
+    const absl::Status original(absl::StatusCode::kInternal, "message",
+                                absl::SourceLocation::current());
+    int line = GET_SOURCE_LOCATION(1);
+    // Default absl::SourceLocation cannot be saved into the chain.
+    const absl::Status status(original, absl::SourceLocation());
+    EXPECT_FALSE(status.ok());
+    EXPECT_EQ(absl::StatusCode::kInternal, status.code());
+    CheckSourceLocation(status, {line});
+  }
+  {
+    const absl::Status original(absl::StatusCode::kInternal, "message",
+                                absl::SourceLocation::current());
+    int line = GET_SOURCE_LOCATION(1);
+
+    const absl::Status status(original, absl::SourceLocation::current());
+    int line2 = GET_SOURCE_LOCATION(1);
+
+    EXPECT_FALSE(status.ok());
+    EXPECT_EQ(absl::StatusCode::kInternal, status.code());
+    CheckSourceLocation(status, {line, line2});
+  }
+  {
+    // Non-OK status with empty msg doesn't save source locations.
+    const absl::Status original(absl::StatusCode::kInternal, "",
+                                absl::SourceLocation::current());
+    const absl::Status status(original, absl::SourceLocation());
+    EXPECT_FALSE(status.ok());
+    EXPECT_EQ(absl::StatusCode::kInternal, status.code());
+    CheckSourceLocation(status);
+  }
+  {
+    // Non-OK status with empty msg doesn't save source locations.
+    const absl::Status original(absl::StatusCode::kInternal, "",
+                                absl::SourceLocation::current());
+    const absl::Status status(original, absl::SourceLocation::current());
+    EXPECT_FALSE(status.ok());
+    EXPECT_EQ(absl::StatusCode::kInternal, status.code());
+    CheckSourceLocation(status);
+  }
+  {
+    // Non-OK status with empty msg doesn't save source locations from default
+    // constructor.
+    const absl::Status original(absl::StatusCode::kInternal, "",
+                                absl::SourceLocation());
+    const absl::Status status(original, absl::SourceLocation());
+    EXPECT_FALSE(status.ok());
+    EXPECT_EQ(absl::StatusCode::kInternal, status.code());
+    CheckSourceLocation(status);
+  }
+  {
+    // Non-OK status with empty msg doesn't save source locations.
+    const absl::Status original(absl::StatusCode::kInternal, "",
+                                absl::SourceLocation());
+    const absl::Status status(original, absl::SourceLocation::current());
+    EXPECT_FALSE(status.ok());
+    EXPECT_EQ(absl::StatusCode::kInternal, status.code());
+    CheckSourceLocation(status);
+  }
+  {
+    const absl::Status original(absl::StatusCode::kInternal, "message",
+                                absl::SourceLocation());
+    const absl::Status status(original, absl::SourceLocation());
+    EXPECT_FALSE(status.ok());
+    EXPECT_EQ(absl::StatusCode::kInternal, status.code());
+    CheckSourceLocation(status);
+  }
+  {
+    const absl::Status original(absl::StatusCode::kInternal, "message",
+                                absl::SourceLocation());
+    const absl::Status status(original, absl::SourceLocation::current());
+    int line = GET_SOURCE_LOCATION(1);
+    EXPECT_FALSE(status.ok());
+    EXPECT_EQ(absl::StatusCode::kInternal, status.code());
+    CheckSourceLocation(status, {line});
+  }
+}
+
+TEST(Status, SourceLocationWithMoveConstructor) {
+  {
+    // OK status doesn't save source locations.
+    absl::Status original;
+    const absl::Status status(std::move(original), absl::SourceLocation());
+    EXPECT_TRUE(status.ok());
+    EXPECT_EQ(absl::StatusCode::kOk, status.code());
+    CheckSourceLocation(status);
+  }
+  {
+    // OK status doesn't save source locations.
+    absl::Status original;
+    const absl::Status status(std::move(original),
+                              absl::SourceLocation::current());
+    EXPECT_TRUE(status.ok());
+    EXPECT_EQ(absl::StatusCode::kOk, status.code());
+    CheckSourceLocation(status);
+  }
+  {
+    // Non-ok Status with non-empty msg can save source locations with
+    // non-nullptr filename.
+    absl::Status original(absl::StatusCode::kInternal, "message",
+                          absl::SourceLocation::current());
+    int line = GET_SOURCE_LOCATION(1);
+    // Default absl::SourceLocation cannot be saved into the chain.
+    const absl::Status status(std::move(original), absl::SourceLocation());
+    EXPECT_FALSE(status.ok());
+    EXPECT_EQ(absl::StatusCode::kInternal, status.code());
+    CheckSourceLocation(status, {line});
+  }
+  {
+    absl::Status original(absl::StatusCode::kInternal, "message",
+                          absl::SourceLocation::current());
+    int line = GET_SOURCE_LOCATION(1);
+
+    const absl::Status status(std::move(original),
+                              absl::SourceLocation::current());
+    int line2 = GET_SOURCE_LOCATION(1);
+
+    EXPECT_FALSE(status.ok());
+    EXPECT_EQ(absl::StatusCode::kInternal, status.code());
+    CheckSourceLocation(status, {line, line2});
+  }
+  {
+    // Non-OK status with empty msg doesn't save source locations.
+    absl::Status original(absl::StatusCode::kInternal, "",
+                          absl::SourceLocation::current());
+    const absl::Status status(std::move(original), absl::SourceLocation());
+    EXPECT_FALSE(status.ok());
+    EXPECT_EQ(absl::StatusCode::kInternal, status.code());
+    CheckSourceLocation(status);
+  }
+  {
+    // Non-OK status with empty msg doesn't save source locations.
+    absl::Status original(absl::StatusCode::kInternal, "",
+                          absl::SourceLocation::current());
+    const absl::Status status(std::move(original),
+                              absl::SourceLocation::current());
+    EXPECT_FALSE(status.ok());
+    EXPECT_EQ(absl::StatusCode::kInternal, status.code());
+    CheckSourceLocation(status);
+  }
+  {
+    // Non-OK status with empty msg doesn't save source locations from default
+    // constructor.
+    absl::Status original(absl::StatusCode::kInternal, "",
+                          absl::SourceLocation());
+    const absl::Status status(std::move(original), absl::SourceLocation());
+    EXPECT_FALSE(status.ok());
+    EXPECT_EQ(absl::StatusCode::kInternal, status.code());
+    CheckSourceLocation(status);
+  }
+  {
+    // Non-OK status with empty msg doesn't save source locations.
+    absl::Status original(absl::StatusCode::kInternal, "",
+                          absl::SourceLocation());
+    const absl::Status status(std::move(original),
+                              absl::SourceLocation::current());
+    EXPECT_FALSE(status.ok());
+    EXPECT_EQ(absl::StatusCode::kInternal, status.code());
+    CheckSourceLocation(status);
+  }
+  {
+    absl::Status original(absl::StatusCode::kInternal, "message",
+                          absl::SourceLocation());
+    const absl::Status status(std::move(original), absl::SourceLocation());
+    EXPECT_FALSE(status.ok());
+    EXPECT_EQ(absl::StatusCode::kInternal, status.code());
+    CheckSourceLocation(status);
+  }
+  {
+    absl::Status original(absl::StatusCode::kInternal, "message",
+                          absl::SourceLocation());
+    const absl::Status status(std::move(original),
+                              absl::SourceLocation::current());
+    int line = GET_SOURCE_LOCATION(1);
+    EXPECT_FALSE(status.ok());
+    EXPECT_EQ(absl::StatusCode::kInternal, status.code());
+    CheckSourceLocation(status, {line});
+  }
+}
+
+TEST(Status, AddSourceLocation) {
+  int max_iter = 10;
+  {
+    // Status that ignores source location.
+    absl::Status status_ignores_source_location[] = {
+        absl::Status(), absl::Status(absl::StatusCode::kInternal, "")};
+    for (absl::Status& s : status_ignores_source_location) {
+      for (int i = 0; i < max_iter; ++i) {
+        s.AddSourceLocation(absl::SourceLocation::current());
+        s.AddSourceLocation(absl::SourceLocation());
+      }
+      CheckSourceLocation(s);
+    }
+  }
+  {
+    // Default SourceLocation is not added.
+    absl::Status status(absl::StatusCode::kInternal, "foo",
+                        absl::SourceLocation::current());
+    int line = GET_SOURCE_LOCATION(1);
+    for (int i = 0; i < max_iter; ++i) {
+      status.AddSourceLocation(absl::SourceLocation());
+    }
+    CheckSourceLocation(status, {line});
+  }
+  {
+    // Default SourceLocation is not added.
+    absl::Status status(absl::StatusCode::kInternal, "foo",
+                        absl::SourceLocation::current());
+    int line = GET_SOURCE_LOCATION(1);
+    std::vector<int> lines = {line};
+    lines.reserve(1 + max_iter);
+    for (int i = 0; i < max_iter; ++i) {
+      status.AddSourceLocation(absl::SourceLocation::current());
+      lines.push_back(GET_SOURCE_LOCATION(1));
+    }
+    CheckSourceLocation(status, lines);
+  }
+}
+
+TEST(Status, WithSourceLocationCopy) {
+  absl::Status original(absl::StatusCode::kInternal, "message",
+                        absl::SourceLocation::current());
+  int line = GET_SOURCE_LOCATION(1);
+
+  const absl::Status status =
+      original.WithSourceLocation(absl::SourceLocation::current());
+  int line2 = GET_SOURCE_LOCATION(1);
+
+  CheckSourceLocation(original, {line});
+  CheckSourceLocation(status, {line, line2});
+  EXPECT_EQ(original, status);
+}
+
+absl::Status&& IsRvalueStatus(absl::Status&& s) { return std::move(s); }
+
+TEST(Status, WithSourceLocationMove) {
+  absl::Status original(absl::StatusCode::kInternal, "message",
+                        absl::SourceLocation::current());
+  int line = GET_SOURCE_LOCATION(1);
+
+  const absl::Status status = IsRvalueStatus(
+      std::move(original).WithSourceLocation(absl::SourceLocation::current()));
+  int line2 = GET_SOURCE_LOCATION(1);
+
+  CheckSourceLocation(status, {line, line2});
+  EXPECT_FALSE(status.ok());
+  EXPECT_EQ(absl::StatusCode::kInternal, status.code());
+}
+
+TEST(Status, CopyOnWriteSourceLocations) {
+  absl::Status source(absl::StatusCode::kInvalidArgument, "fail",
+                      absl::SourceLocation::current());
+  EXPECT_EQ(source.GetSourceLocations().size(), 1);
+  absl::Status copy = source;
+  EXPECT_EQ(copy.GetSourceLocations().size(), 1);
+  copy.AddSourceLocation(absl::SourceLocation::current());  // Copy rep.
+  EXPECT_EQ(copy.GetSourceLocations().size(), 2);
+  EXPECT_EQ(source.GetSourceLocations().size(), 1);
+}
+
+TEST(Status, SourceLocationToStringMode) {
+  absl::Status s(absl::StatusCode::kInternal, "fail",
+                 absl::SourceLocation::current());
+  int line = GET_SOURCE_LOCATION(1);
+  std::string source_location_string = "\n=== Source Location Trace: ===";
+  std::string source_location_stack = absl::StrCat(
+      absl::SourceLocation::current().file_name(), ":", line, "\n");
+
+  s.SetPayload("foo", absl::Cord("bar"));
+
+  EXPECT_EQ("INTERNAL: fail",
+            s.ToString(absl::StatusToStringMode::kWithNoExtraData));
+
+  EXPECT_EQ("INTERNAL: fail",
+            s.ToString(~absl::StatusToStringMode::kWithSourceLocation &
+                       ~absl::StatusToStringMode::kWithPayload));
+  EXPECT_THAT(s.ToString(absl::StatusToStringMode::kWithSourceLocation |
+                         absl::StatusToStringMode::kWithPayload),
+              AllOf(HasSubstr("INTERNAL: fail [foo='bar']"),
+                    HasSubstr(source_location_string),
+                    HasSubstr(source_location_stack)));
+
+  s.SetPayload("bar", absl::Cord("\377"));
+
+  EXPECT_THAT(
+      s.ToString(absl::StatusToStringMode::kWithEverything),
+      AllOf(HasSubstr("INTERNAL: fail"), HasSubstr("[foo='bar']"),
+            HasSubstr("[bar='\\xff']"), HasSubstr(source_location_string),
+            HasSubstr(source_location_stack)));
+  EXPECT_THAT(
+      s.ToString(absl::StatusToStringMode::kWithPayload |
+                 absl::StatusToStringMode::kWithSourceLocation),
+      AllOf(HasSubstr("INTERNAL: fail"), HasSubstr("[foo='bar']"),
+            HasSubstr("[bar='\\xff']"), HasSubstr(source_location_string),
+            HasSubstr(source_location_stack)));
+  EXPECT_THAT(
+      s.ToString(absl::StatusToStringMode::kWithSourceLocation),
+      AllOf(HasSubstr("INTERNAL: fail"), Not(HasSubstr("[foo='bar']")),
+            Not(HasSubstr("[bar='\\xff']")), HasSubstr(source_location_string),
+            HasSubstr(source_location_stack)));
+  EXPECT_THAT(
+      s.ToString(absl::StatusToStringMode::kWithPayload),
+      AllOf(HasSubstr("INTERNAL: fail"), HasSubstr("[foo='bar']"),
+            HasSubstr("[bar='\\xff']"), Not(HasSubstr(source_location_string)),
+            Not(HasSubstr(source_location_stack))));
+}
+
+TEST(Status, StackTracePayloadOverflow) {
+  // Stack must have the same layout as status_internal::StackTracePayload.
+  struct Stack {
+    size_t size;
+    void* frames[20];
+  } stack;
+  stack.size = 200;  // Overflows frames.
+
+  absl::Status status = absl::CancelledError();
+  status.SetPayload("AbslStatusStackTracePayload",
+                    absl::Cord(absl::string_view(
+                        reinterpret_cast<const char*>(&stack), sizeof(stack))));
+
+  // An unchecked overflow should be detected by ASAN/MSAN on the next line.
+  static_cast<void>(status.ToString(absl::StatusToStringMode::kWithEverything));
+}
+
+}  // namespace
diff --git a/absl/status/statusor.cc b/absl/status/statusor.cc
new file mode 100644
index 00000000000..d8f66a64619
--- /dev/null
+++ b/absl/status/statusor.cc
@@ -0,0 +1,106 @@
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+#include "absl/status/statusor.h"
+
+#include <cstdlib>
+#include <utility>
+
+#include "absl/base/call_once.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/base/nullability.h"
+#include "absl/status/internal/statusor_internal.h"
+#include "absl/status/status.h"
+#include "absl/strings/str_cat.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+BadStatusOrAccess::BadStatusOrAccess(absl::Status status)
+    : status_(std::move(status)) {}
+
+BadStatusOrAccess::BadStatusOrAccess(const BadStatusOrAccess& other)
+    : status_(other.status_) {}
+
+BadStatusOrAccess& BadStatusOrAccess::operator=(
+    const BadStatusOrAccess& other) {
+  // Ensure assignment is correct regardless of whether this->InitWhat() has
+  // already been called.
+  other.InitWhat();
+  status_ = other.status_;
+  what_ = other.what_;
+  return *this;
+}
+
+BadStatusOrAccess& BadStatusOrAccess::operator=(BadStatusOrAccess&& other) {
+  // Ensure assignment is correct regardless of whether this->InitWhat() has
+  // already been called.
+  other.InitWhat();
+  status_ = std::move(other.status_);
+  what_ = std::move(other.what_);
+  return *this;
+}
+
+BadStatusOrAccess::BadStatusOrAccess(BadStatusOrAccess&& other)
+    : status_(std::move(other.status_)) {}
+
+const char* absl_nonnull BadStatusOrAccess::what() const noexcept {
+  InitWhat();
+  return what_.c_str();
+}
+
+const absl::Status& BadStatusOrAccess::status() const { return status_; }
+
+void BadStatusOrAccess::InitWhat() const {
+  absl::call_once(init_what_, [this] {
+    what_ = absl::StrCat("Bad StatusOr access: ", status_.ToString());
+  });
+}
+
+namespace internal_statusor {
+
+void Helper::HandleInvalidStatusCtorArg(absl::Status* absl_nonnull status) {
+  const char* kMessage =
+      "An OK status is not a valid constructor argument to StatusOr<T>";
+#ifdef NDEBUG
+  ABSL_INTERNAL_LOG(ERROR, kMessage);
+#else
+  ABSL_INTERNAL_LOG(FATAL, kMessage);
+#endif
+  // In optimized builds, we will fall back to InternalError.
+  *status = absl::InternalError(kMessage);
+}
+
+void Helper::Crash(const absl::Status& status) {
+  ABSL_INTERNAL_LOG(
+      FATAL,
+      absl::StrCat("Attempting to fetch value instead of handling error ",
+                   status.ToString()));
+}
+
+void ThrowBadStatusOrAccess(absl::Status status) {
+#ifdef ABSL_HAVE_EXCEPTIONS
+  throw absl::BadStatusOrAccess(std::move(status));
+#else
+  ABSL_INTERNAL_LOG(
+      FATAL,
+      absl::StrCat("Attempting to fetch value instead of handling error ",
+                   status.ToString()));
+  std::abort();
+#endif
+}
+
+}  // namespace internal_statusor
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/status/statusor.h b/absl/status/statusor.h
new file mode 100644
index 00000000000..3d7f8c48832
--- /dev/null
+++ b/absl/status/statusor.h
@@ -0,0 +1,810 @@
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: statusor.h
+// -----------------------------------------------------------------------------
+//
+// An `absl::StatusOr<T>` represents a union of an `absl::Status` object
+// and an object of type `T`. The `absl::StatusOr<T>` will either contain an
+// object of type `T` (indicating a successful operation), or an error (of type
+// `absl::Status`) explaining why such a value is not present.
+//
+// In general, check the success of an operation returning an
+// `absl::StatusOr<T>` like you would an `absl::Status` by using the `ok()`
+// member function.
+//
+// Example:
+//
+//   StatusOr<Foo> result = Calculation();
+//   if (result.ok()) {
+//     result->DoSomethingCool();
+//   } else {
+//     LOG(ERROR) << result.status();
+//   }
+#ifndef ABSL_STATUS_STATUSOR_H_
+#define ABSL_STATUS_STATUSOR_H_
+
+#include <exception>
+#include <initializer_list>
+#include <new>
+#include <ostream>
+#include <string>
+#include <type_traits>
+#include <utility>
+#include <variant>
+
+#include "absl/base/attributes.h"
+#include "absl/base/call_once.h"
+#include "absl/base/nullability.h"
+#include "absl/meta/type_traits.h"
+#include "absl/status/internal/statusor_internal.h"
+#include "absl/status/status.h"
+#include "absl/strings/has_absl_stringify.h"
+#include "absl/strings/has_ostream_operator.h"
+#include "absl/strings/str_format.h"
+#include "absl/types/source_location.h"
+#include "absl/types/span.h"
+#include "absl/types/variant.h"
+#include "absl/utility/utility.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+#ifndef SWIG
+
+// BadStatusOrAccess
+//
+// This class defines the type of object to throw (if exceptions are enabled),
+// when accessing the value of an `absl::StatusOr<T>` object that does not
+// contain a value. This behavior is analogous to that of
+// `std::bad_optional_access` in the case of accessing an invalid
+// `std::optional` value.
+//
+// Example:
+//
+// try {
+//   absl::StatusOr<int> v = FetchInt();
+//   DoWork(v.value());  // Accessing value() when not "OK" may throw
+// } catch (absl::BadStatusOrAccess& ex) {
+//   LOG(ERROR) << ex.status();
+// }
+class BadStatusOrAccess : public std::exception {
+ public:
+  explicit BadStatusOrAccess(absl::Status status);
+  ~BadStatusOrAccess() override = default;
+
+  BadStatusOrAccess(const BadStatusOrAccess& other);
+  BadStatusOrAccess& operator=(const BadStatusOrAccess& other);
+  BadStatusOrAccess(BadStatusOrAccess&& other);
+  BadStatusOrAccess& operator=(BadStatusOrAccess&& other);
+
+  // BadStatusOrAccess::what()
+  //
+  // Returns the associated explanatory string of the `absl::StatusOr<T>`
+  // object's error code. This function contains information about the failing
+  // status, but its exact formatting may change and should not be depended on.
+  //
+  // The pointer of this string is guaranteed to be valid until any non-const
+  // function is invoked on the exception object.
+  const char* absl_nonnull what() const noexcept override;
+
+  // BadStatusOrAccess::status()
+  //
+  // Returns the associated `absl::Status` of the `absl::StatusOr<T>` object's
+  // error.
+  const absl::Status& status() const;
+
+ private:
+  void InitWhat() const;
+
+  absl::Status status_;
+  mutable absl::once_flag init_what_;
+  mutable std::string what_;
+};
+#endif  // !SWIG
+
+// Returned StatusOr objects may not be ignored.
+#ifndef SWIG
+template <typename T>
+#if ABSL_HAVE_CPP_ATTRIBUTE(nodiscard)
+// TODO(b/176172494): ABSL_MUST_USE_RESULT should expand to the more strict
+// [[nodiscard]]. For now, just use [[nodiscard]] directly when it is available.
+class [[nodiscard]] StatusOr;
+#else
+class ABSL_MUST_USE_RESULT StatusOr;
+#endif  // ABSL_HAVE_CPP_ATTRIBUTE(nodiscard)
+#endif  // !SWIG
+
+// absl::StatusOr<T>
+//
+// The `absl::StatusOr<T>` class template is a union of an `absl::Status` object
+// and an object of type `T`. The `absl::StatusOr<T>` models an object that is
+// either a usable object, or an error (of type `absl::Status`) explaining why
+// such an object is not present. An `absl::StatusOr<T>` is typically the return
+// value of a function which may fail.
+//
+// An `absl::StatusOr<T>` can never hold an "OK" status (an
+// `absl::StatusCode::kOk` value); instead, the presence of an object of type
+// `T` indicates success. Instead of checking for a `kOk` value, use the
+// `absl::StatusOr<T>::ok()` member function. (It is for this reason, and code
+// readability, that using the `ok()` function is preferred for `absl::Status`
+// as well.)
+//
+// Example:
+//
+//   StatusOr<Foo> result = DoBigCalculationThatCouldFail();
+//   if (result.ok()) {
+//     result->DoSomethingCool();
+//   } else {
+//     LOG(ERROR) << result.status();
+//   }
+//
+// Accessing the object held by an `absl::StatusOr<T>` should be performed via
+// `operator*` or `operator->`, after a call to `ok()` confirms that the
+// `absl::StatusOr<T>` holds an object of type `T`:
+//
+// Example:
+//
+//   absl::StatusOr<int> i = GetCount();
+//   if (i.ok()) {
+//     updated_total += *i;
+//   }
+//
+// NOTE: using `absl::StatusOr<T>::value()` when no valid value is present will
+// throw an exception if exceptions are enabled or terminate the process when
+// exceptions are not enabled.
+//
+// Example:
+//
+//   StatusOr<Foo> result = DoBigCalculationThatCouldFail();
+//   const Foo& foo = result.value();    // Crash/exception if no value present
+//   foo.DoSomethingCool();
+//
+// A `absl::StatusOr<T*>` can be constructed from a null pointer like any other
+// pointer value, and the result will be that `ok()` returns `true` and
+// `value()` returns `nullptr`. Checking the value of pointer in an
+// `absl::StatusOr<T*>` generally requires a bit more care, to ensure both that
+// a value is present and that value is not null:
+//
+//  StatusOr<std::unique_ptr<Foo>> result = FooFactory::MakeNewFoo(arg);
+//  if (!result.ok()) {
+//    LOG(ERROR) << result.status();
+//  } else if (*result == nullptr) {
+//    LOG(ERROR) << "Unexpected null pointer";
+//  } else {
+//    (*result)->DoSomethingCool();
+//  }
+//
+// Example factory implementation returning StatusOr<T>:
+//
+//  StatusOr<Foo> FooFactory::MakeFoo(int arg) {
+//    if (arg <= 0) {
+//      return absl::Status(absl::StatusCode::kInvalidArgument,
+//                          "Arg must be positive");
+//    }
+//    return Foo(arg);
+//  }
+template <typename T>
+class StatusOr : private internal_statusor::OperatorBase<T>,
+                 private internal_statusor::StatusOrData<T>,
+                 private internal_statusor::CopyCtorBase<T>,
+                 private internal_statusor::MoveCtorBase<T>,
+                 private internal_statusor::CopyAssignBase<T>,
+                 private internal_statusor::MoveAssignBase<T> {
+#ifndef SWIG
+  static_assert(!std::is_rvalue_reference_v<T>,
+                "rvalue references are not yet supported.");
+#endif  // SWIG
+
+  template <typename U>
+  friend class StatusOr;
+
+  friend internal_statusor::OperatorBase<T>;
+
+  typedef internal_statusor::StatusOrData<T> Base;
+
+ public:
+  // StatusOr<T>::value_type
+  //
+  // This instance data provides a generic `value_type` member for use within
+  // generic programming. This usage is analogous to that of
+  // `optional::value_type` in the case of `std::optional`.
+  typedef T value_type;
+
+  // Constructors
+
+  // Constructs a new `absl::StatusOr` with an `absl::StatusCode::kUnknown`
+  // status. This constructor is marked 'explicit' to prevent usages in return
+  // values such as 'return {};', under the misconception that
+  // `absl::StatusOr<std::vector<int>>` will be initialized with an empty
+  // vector, instead of an `absl::StatusCode::kUnknown` error code.
+  explicit StatusOr();
+
+  // `StatusOr<T>` is copy constructible if `T` is copy constructible.
+  StatusOr(const StatusOr&) = default;
+  // `StatusOr<T>` is copy assignable if `T` is copy constructible and copy
+  // assignable.
+  StatusOr& operator=(const StatusOr&) = default;
+
+#ifndef SWIG
+
+  // `StatusOr<T>` is move constructible if `T` is move constructible.
+  StatusOr(StatusOr&&) = default;
+  // `StatusOr<T>` is moveAssignable if `T` is move constructible and move
+  // assignable.
+  StatusOr& operator=(StatusOr&&) = default;
+
+  // Converting Constructors
+
+  // Constructs a new `absl::StatusOr<T>` from an `absl::StatusOr<U>`, when `T`
+  // is constructible from `U`. To avoid ambiguity, these constructors are
+  // disabled if `T` is also constructible from `StatusOr<U>.`. This constructor
+  // is explicit if and only if the corresponding construction of `T` from `U`
+  // is explicit. (This constructor inherits its explicitness from the
+  // underlying constructor.)
+  template <typename U, std::enable_if_t<
+                            internal_statusor::IsConstructionFromStatusOrValid<
+                                false, T, U, false, const U&>::value,
+                            int> = 0>
+  StatusOr(const StatusOr<U>& other)  // NOLINT
+      : Base(static_cast<const typename StatusOr<U>::Base&>(other)) {}
+  template <typename U, std::enable_if_t<
+                            internal_statusor::IsConstructionFromStatusOrValid<
+                                false, T, U, true, const U&>::value,
+                            int> = 0>
+  StatusOr(const StatusOr<U>& other ABSL_ATTRIBUTE_LIFETIME_BOUND)  // NOLINT
+      : Base(static_cast<const typename StatusOr<U>::Base&>(other)) {}
+  template <typename U, std::enable_if_t<
+                            internal_statusor::IsConstructionFromStatusOrValid<
+                                true, T, U, false, const U&>::value,
+                            int> = 0>
+  explicit StatusOr(const StatusOr<U>& other)
+      : Base(static_cast<const typename StatusOr<U>::Base&>(other)) {}
+  template <typename U, std::enable_if_t<
+                            internal_statusor::IsConstructionFromStatusOrValid<
+                                true, T, U, true, const U&>::value,
+                            int> = 0>
+  explicit StatusOr(const StatusOr<U>& other ABSL_ATTRIBUTE_LIFETIME_BOUND)
+      : Base(static_cast<const typename StatusOr<U>::Base&>(other)) {}
+
+  template <typename U, std::enable_if_t<
+                            internal_statusor::IsConstructionFromStatusOrValid<
+                                false, T, U, false, U&&>::value,
+                            int> = 0>
+  StatusOr(StatusOr<U>&& other)  // NOLINT
+      : Base(static_cast<typename StatusOr<U>::Base&&>(other)) {}
+  template <typename U, std::enable_if_t<
+                            internal_statusor::IsConstructionFromStatusOrValid<
+                                false, T, U, true, U&&>::value,
+                            int> = 0>
+  StatusOr(StatusOr<U>&& other ABSL_ATTRIBUTE_LIFETIME_BOUND)  // NOLINT
+      : Base(static_cast<typename StatusOr<U>::Base&&>(other)) {}
+  template <typename U, std::enable_if_t<
+                            internal_statusor::IsConstructionFromStatusOrValid<
+                                true, T, U, false, U&&>::value,
+                            int> = 0>
+  explicit StatusOr(StatusOr<U>&& other)
+      : Base(static_cast<typename StatusOr<U>::Base&&>(other)) {}
+  template <typename U, std::enable_if_t<
+                            internal_statusor::IsConstructionFromStatusOrValid<
+                                true, T, U, true, U&&>::value,
+                            int> = 0>
+  explicit StatusOr(StatusOr<U>&& other ABSL_ATTRIBUTE_LIFETIME_BOUND)
+      : Base(static_cast<typename StatusOr<U>::Base&&>(other)) {}
+
+  // Converting Assignment Operators
+
+  // Creates an `absl::StatusOr<T>` through assignment from an
+  // `absl::StatusOr<U>` when:
+  //
+  //   * Both `absl::StatusOr<T>` and `absl::StatusOr<U>` are OK by assigning
+  //     `U` to `T` directly.
+  //   * `absl::StatusOr<T>` is OK and `absl::StatusOr<U>` contains an error
+  //      code by destroying `absl::StatusOr<T>`'s value and assigning from
+  //      `absl::StatusOr<U>'
+  //   * `absl::StatusOr<T>` contains an error code and `absl::StatusOr<U>` is
+  //      OK by directly initializing `T` from `U`.
+  //   * Both `absl::StatusOr<T>` and `absl::StatusOr<U>` contain an error
+  //     code by assigning the `Status` in `absl::StatusOr<U>` to
+  //     `absl::StatusOr<T>`
+  //
+  // These overloads only apply if `absl::StatusOr<T>` is constructible and
+  // assignable from `absl::StatusOr<U>` and `StatusOr<T>` cannot be directly
+  // assigned from `StatusOr<U>`.
+  template <typename U,
+            std::enable_if_t<internal_statusor::IsStatusOrAssignmentValid<
+                                  T, const U&, false>::value,
+                              int> = 0>
+  StatusOr& operator=(const StatusOr<U>& other) {
+    this->Assign(other);
+    return *this;
+  }
+  template <typename U,
+            std::enable_if_t<internal_statusor::IsStatusOrAssignmentValid<
+                                  T, const U&, true>::value,
+                              int> = 0>
+  StatusOr& operator=(const StatusOr<U>& other ABSL_ATTRIBUTE_LIFETIME_BOUND) {
+    this->Assign(other);
+    return *this;
+  }
+  template <typename U,
+            std::enable_if_t<internal_statusor::IsStatusOrAssignmentValid<
+                                  T, U&&, false>::value,
+                              int> = 0>
+  StatusOr& operator=(StatusOr<U>&& other) {
+    this->Assign(std::move(other));
+    return *this;
+  }
+  template <typename U,
+            std::enable_if_t<internal_statusor::IsStatusOrAssignmentValid<
+                                  T, U&&, true>::value,
+                              int> = 0>
+  StatusOr& operator=(StatusOr<U>&& other ABSL_ATTRIBUTE_LIFETIME_BOUND) {
+    this->Assign(std::move(other));
+    return *this;
+  }
+
+  // Constructs a new `absl::StatusOr<T>` with a non-ok status. After calling
+  // this constructor, `this->ok()` will be `false` and calls to `value()` will
+  // crash, or produce an exception if exceptions are enabled.
+  //
+  // The constructor also takes any type `U` that is convertible to
+  // `absl::Status`. This constructor is explicit if an only if `U` is not of
+  // type `absl::Status` and the conversion from `U` to `Status` is explicit.
+  //
+  // REQUIRES: !Status(std::forward<U>(v)).ok(). This requirement is DCHECKed.
+  // In optimized builds, passing absl::OkStatus() here will have the effect
+  // of passing absl::StatusCode::kInternal as a fallback.
+  template <typename U = absl::Status,
+            std::enable_if_t<internal_statusor::IsConstructionFromStatusValid<
+                                 false, T, U>::value,
+                             int> = 0>
+  StatusOr(U&& v) : Base(std::forward<U>(v)) {}
+
+  template <typename U = absl::Status,
+            std::enable_if_t<internal_statusor::IsConstructionFromStatusValid<
+                                  true, T, U>::value,
+                              int> = 0>
+  explicit StatusOr(U&& v) : Base(std::forward<U>(v)) {}
+  template <typename U = absl::Status,
+            std::enable_if_t<internal_statusor::IsConstructionFromStatusValid<
+                                  false, T, U>::value,
+                              int> = 0>
+  StatusOr& operator=(U&& v) {
+    this->AssignStatus(std::forward<U>(v));
+    return *this;
+  }
+
+  // Perfect-forwarding value assignment operator.
+
+  // If `*this` contains a `T` value before the call, the contained value is
+  // assigned from `std::forward<U>(v)`; Otherwise, it is directly-initialized
+  // from `std::forward<U>(v)`.
+  // This function does not participate in overload unless:
+  // 1. `std::is_constructible_v<T, U>` is true,
+  // 2. `std::is_assignable_v<T&, U>` is true.
+  // 3. `std::is_same_v<StatusOr<T>, std::remove_cvref_t<U>>` is false.
+  // 4. Assigning `U` to `T` is not ambiguous:
+  //  If `U` is `StatusOr<V>` and `T` is constructible and assignable from
+  //  both `StatusOr<V>` and `V`, the assignment is considered bug-prone and
+  //  ambiguous thus will fail to compile. For example:
+  //    StatusOr<bool> s1 = true;  // s1.ok() && *s1 == true
+  //    StatusOr<bool> s2 = false;  // s2.ok() && *s2 == false
+  //    s1 = s2;  // ambiguous, `s1 = *s2` or `s1 = bool(s2)`?
+  template <
+      typename U = T,
+      std::enable_if_t<internal_statusor::IsAssignmentValid<T, U, false>::value,
+                       int> = 0>
+  StatusOr& operator=(U&& v) {
+    this->Assign(std::forward<U>(v));
+    return *this;
+  }
+  template <
+      typename U = T,
+      std::enable_if_t<internal_statusor::IsAssignmentValid<T, U, true>::value,
+                       int> = 0>
+  StatusOr& operator=(U&& v ABSL_INTERNAL_ATTRIBUTE_CAPTURED_BY_THIS) {
+    this->Assign(std::forward<U>(v));
+    return *this;
+  }
+
+  // Constructs the inner value `T` in-place using the provided args, using the
+  // `T(args...)` constructor.
+  template <typename... Args>
+  explicit StatusOr(std::in_place_t, Args&&... args);
+  template <typename U, typename... Args>
+  explicit StatusOr(std::in_place_t, std::initializer_list<U> ilist,
+                    Args&&... args);
+
+  // Constructs the inner value `T` in-place using the provided args, using the
+  // `T(U)` (direct-initialization) constructor. This constructor is only valid
+  // if `T` can be constructed from a `U`. Can accept move or copy constructors.
+  //
+  // This constructor is explicit if `U` is not convertible to `T`. To avoid
+  // ambiguity, this constructor is disabled if `U` is a `StatusOr<J>`, where
+  // `J` is convertible to `T`.
+  template <typename U = T,
+            std::enable_if_t<internal_statusor::IsConstructionValid<
+                                 false, T, U, false>::value,
+                             int> = 0>
+  StatusOr(U&& u)  // NOLINT
+      : StatusOr(std::in_place, std::forward<U>(u)) {}
+  template <typename U = T,
+            std::enable_if_t<internal_statusor::IsConstructionValid<
+                                 false, T, U, true>::value,
+                             int> = 0>
+  StatusOr(U&& u ABSL_ATTRIBUTE_LIFETIME_BOUND)  // NOLINT
+      : StatusOr(std::in_place, std::forward<U>(u)) {}
+
+  template <typename U = T,
+            std::enable_if_t<internal_statusor::IsConstructionValid<
+                                 true, T, U, false>::value,
+                             int> = 0>
+  explicit StatusOr(U&& u)  // NOLINT
+      : StatusOr(std::in_place, std::forward<U>(u)) {}
+  template <typename U = T,
+            std::enable_if_t<
+                internal_statusor::IsConstructionValid<true, T, U, true>::value,
+                int> = 0>
+  explicit StatusOr(U&& u ABSL_ATTRIBUTE_LIFETIME_BOUND)  // NOLINT
+      : StatusOr(std::in_place, std::forward<U>(u)) {}
+
+#endif  // SWIG
+
+  // StatusOr<T>::ok()
+  //
+  // Returns whether or not this `absl::StatusOr<T>` holds a `T` value. This
+  // member function is analogous to `absl::Status::ok()` and should be used
+  // similarly to check the status of return values.
+  //
+  // Example:
+  //
+  // StatusOr<Foo> result = DoBigCalculationThatCouldFail();
+  // if (result.ok()) {
+  //    // Handle result
+  // else {
+  //    // Handle error
+  // }
+  ABSL_MUST_USE_RESULT bool ok() const { return this->status_.ok(); }
+
+  // StatusOr<T>::status()
+  //
+  // Returns a reference to the current `absl::Status` contained within the
+  // `absl::StatusOr<T>`. If `absl::StatusOr<T>` contains a `T`, then this
+  // function returns `absl::OkStatus()`.
+#ifdef SWIG
+  ABSL_MUST_USE_RESULT const absl::Status& status() const;
+#else  // SWIG
+  ABSL_MUST_USE_RESULT const Status& status() const&;
+  Status status() &&;
+#endif  // SWIG
+
+  absl::Span<const absl::SourceLocation> GetSourceLocations() const {
+    return this->status_.GetSourceLocations();
+  }
+  // Appends the `loc` to the current location chain inside the status, iff the
+  // status-or is non-ok and contains a non-empty message.
+  void AddSourceLocation(
+      absl::SourceLocation loc = absl::SourceLocation::current()) {
+    this->status_.AddSourceLocation(loc);
+  }
+
+#ifndef SWIG
+
+  // StatusOr<T>::WithSourceLocation()
+  //
+  // Appends the `loc` to the current location chain inside the status iff the
+  // status-or is non-ok and contains a non-empty message, and returns an rvalue
+  // reference to `*this`.
+  //
+  // Example:
+  //
+  //   StatusOr<int> Finalize(...);
+  //
+  //   StatusOr<int> DoSomething(...) {
+  //     ...
+  //     return Finalize().WithSourceLocation();
+  //   }
+  ABSL_MUST_USE_RESULT StatusOr<T>&& WithSourceLocation(
+      absl::SourceLocation loc = absl::SourceLocation::current()) && {
+    AddSourceLocation(loc);
+    return std::move(*this);
+  }
+#endif  // SWIG
+
+  // StatusOr<T>::value()
+  //
+  // Returns a reference to the held value if `this->ok()`. Otherwise, throws
+  // `absl::BadStatusOrAccess` if exceptions are enabled, or is guaranteed to
+  // terminate the process if exceptions are disabled.
+  //
+  // If you have already checked the status using `this->ok()`, you probably
+  // want to use `operator*()` or `operator->()` to access the value instead of
+  // `value`.
+  //
+  // Note: for value types that are cheap to copy, prefer simple code:
+  //
+  //   T value = statusor.value();
+  //
+  // Otherwise, if the value type is expensive to copy, but can be left
+  // in the StatusOr, simply assign to a reference:
+  //
+  //   T& value = statusor.value();  // or `const T&`
+  //
+  // Otherwise, if the value type supports an efficient move, it can be
+  // used as follows:
+  //
+  //   T value = std::move(statusor).value();
+  //
+  // The `std::move` on statusor instead of on the whole expression enables
+  // warnings about possible uses of the statusor object after the move.
+#ifdef SWIG
+  const T& value() const ABSL_ATTRIBUTE_LIFETIME_BOUND;
+#else  // SWIG
+  using StatusOr::OperatorBase::value;
+#endif  // SWIG
+
+#ifndef SWIG
+
+  // StatusOr<T>:: operator*()
+  //
+  // Returns a reference to the current value.
+  //
+  // REQUIRES: `this->ok() == true`, otherwise the behavior is undefined.
+  //
+  // Use `this->ok()` to verify that there is a current value within the
+  // `absl::StatusOr<T>`. Alternatively, see the `value()` member function for a
+  // similar API that guarantees crashing or throwing an exception if there is
+  // no current value.
+  using StatusOr::OperatorBase::operator*;
+
+  // StatusOr<T>::operator->()
+  //
+  // Returns a pointer to the current value.
+  //
+  // REQUIRES: `this->ok() == true`, otherwise the behavior is undefined.
+  //
+  // Use `this->ok()` to verify that there is a current value.
+  using StatusOr::OperatorBase::operator->;
+
+  // StatusOr<T>::value_or()
+  //
+  // Returns the current value if `this->ok() == true`. Otherwise constructs a
+  // value using the provided `default_value`.
+  //
+  // Unlike `value`, this function returns by value, copying the current value
+  // if necessary. If the value type supports an efficient move, it can be used
+  // as follows:
+  //
+  //   T value = std::move(statusor).value_or(def);
+  //
+  // Unlike with `value`, calling `std::move()` on the result of `value_or` will
+  // still trigger a copy.
+  template <
+      typename U,
+      std::enable_if_t<internal_statusor::IsValueOrValid<T, U&&, false>::value,
+                       int> = 0>
+  T value_or(U&& default_value) const& {
+    return this->ValueOrImpl(std::forward<U>(default_value));
+  }
+  template <
+      typename U,
+      std::enable_if_t<internal_statusor::IsValueOrValid<T, U&&, false>::value,
+                       int> = 0>
+  T value_or(U&& default_value) && {
+    return std::move(*this).ValueOrImpl(std::forward<U>(default_value));
+  }
+  template <
+      typename U,
+      std::enable_if_t<internal_statusor::IsValueOrValid<T, U&&, true>::value,
+                       int> = 0>
+  T value_or(U&& default_value ABSL_ATTRIBUTE_LIFETIME_BOUND) const& {
+    return this->ValueOrImpl(std::forward<U>(default_value));
+  }
+  template <
+      typename U,
+      std::enable_if_t<internal_statusor::IsValueOrValid<T, U&&, true>::value,
+                       int> = 0>
+  T value_or(U&& default_value ABSL_ATTRIBUTE_LIFETIME_BOUND) && {
+    return std::move(*this).ValueOrImpl(std::forward<U>(default_value));
+  }
+#endif  // SWIG
+
+  // StatusOr<T>::IgnoreError()
+  //
+  // Ignores any errors. This method does nothing except potentially suppress
+  // complaints from any tools that are checking that errors are not dropped on
+  // the floor.
+  void IgnoreError() const;
+
+#ifndef SWIG
+
+  // StatusOr<T>::emplace()
+  //
+  // Reconstructs the inner value T in-place using the provided args, using the
+  // T(args...) constructor. Returns reference to the reconstructed `T`.
+  template <typename... Args>
+  T& emplace(Args&&... args) ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    if (ok()) {
+      this->Clear();
+      // Temporarily transition to a non-ok status (using the zero-allocation
+      // inlined representation) so that if MakeValue() throws an exception,
+      // ok() returns false during stack unwinding and ~StatusOrData() does not
+      // attempt to destroy uninitialized memory.
+      this->status_ = absl::Status(absl::StatusCode::kInternal);
+    }
+    this->MakeValue(std::forward<Args>(args)...);
+    this->status_ = absl::OkStatus();
+    return this->data_;
+  }
+
+  template <typename U, typename... Args,
+            std::enable_if_t<std::is_constructible_v<
+                                 T, std::initializer_list<U>&, Args&&...>,
+                             int> = 0>
+  T& emplace(std::initializer_list<U> ilist,
+             Args&&... args) ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    if (ok()) {
+      this->Clear();
+      // Temporarily transition to a non-ok status (using the zero-allocation
+      // inlined representation) so that if MakeValue() throws an exception,
+      // ok() returns false during stack unwinding and ~StatusOrData() does not
+      // attempt to destroy uninitialized memory.
+      this->status_ = absl::Status(absl::StatusCode::kInternal);
+    }
+    this->MakeValue(ilist, std::forward<Args>(args)...);
+    this->status_ = absl::OkStatus();
+    return this->data_;
+  }
+
+  // StatusOr<T>::AssignStatus()
+  //
+  // Sets the status of `absl::StatusOr<T>` to the given non-ok status value.
+  //
+  // NOTE: We recommend using the constructor and `operator=` where possible.
+  // This method is intended for use in generic programming, to enable setting
+  // the status of a `StatusOr<T>` when `T` may be `Status`. In that case, the
+  // constructor and `operator=` would assign into the inner value of type
+  // `Status`, rather than status of the `StatusOr` (b/280392796).
+  //
+  // REQUIRES: !Status(std::forward<U>(v)).ok(). This requirement is DCHECKed.
+  // In optimized builds, passing absl::OkStatus() here will have the effect
+  // of passing absl::StatusCode::kInternal as a fallback.
+  using internal_statusor::StatusOrData<T>::AssignStatus;
+#endif  // SWIG
+
+ private:
+#ifndef SWIG
+  using internal_statusor::StatusOrData<T>::Assign;
+  template <typename U>
+  void Assign(const absl::StatusOr<U>& other);
+  template <typename U>
+  void Assign(absl::StatusOr<U>&& other);
+#endif  // SWIG
+};
+
+// operator==()
+//
+// This operator checks the equality of two `absl::StatusOr<T>` objects.
+template <typename T,
+          std::enable_if_t<internal_statusor::IsEqualityComparable<T>::value,
+                           int> = 0>
+bool operator==(const StatusOr<T>& lhs, const StatusOr<T>& rhs) {
+  if (lhs.ok() && rhs.ok()) return *lhs == *rhs;
+  return lhs.status() == rhs.status();
+}
+
+// operator!=()
+//
+// This operator checks the inequality of two `absl::StatusOr<T>` objects.
+template <typename T,
+          std::enable_if_t<internal_statusor::IsEqualityComparable<T>::value,
+                           int> = 0>
+bool operator!=(const StatusOr<T>& lhs, const StatusOr<T>& rhs) {
+  return !(lhs == rhs);
+}
+
+// Prints the `value` or the status in brackets to `os`.
+//
+// Requires `T` supports `operator<<`.  Do not rely on the output format which
+// may change without notice.
+template <typename T,
+          std::enable_if_t<absl::HasOstreamOperator<T>::value, int> = 0>
+std::ostream& operator<<(std::ostream& os, const StatusOr<T>& status_or) {
+  if (status_or.ok()) {
+    os << status_or.value();
+  } else {
+    os << internal_statusor::StringifyRandom::OpenBrackets()
+       << status_or.status()
+       << internal_statusor::StringifyRandom::CloseBrackets();
+  }
+  return os;
+}
+
+// As above, but supports `StrCat`, `StrFormat`, etc.
+//
+// Requires `T` has `AbslStringify`.  Do not rely on the output format which
+// may change without notice.
+template <typename Sink, typename T,
+          std::enable_if_t<absl::HasAbslStringify<T>::value, int> = 0>
+void AbslStringify(Sink& sink, const StatusOr<T>& status_or) {
+  if (status_or.ok()) {
+    absl::Format(&sink, "%v", status_or.value());
+  } else {
+    absl::Format(&sink, "%s%v%s",
+                 internal_statusor::StringifyRandom::OpenBrackets(),
+                 status_or.status(),
+                 internal_statusor::StringifyRandom::CloseBrackets());
+  }
+}
+
+#ifndef SWIG
+//------------------------------------------------------------------------------
+// Implementation details for StatusOr<T>
+//------------------------------------------------------------------------------
+
+// TODO(sbenza): avoid the string here completely.
+template <typename T>
+StatusOr<T>::StatusOr() : Base(Status(absl::StatusCode::kUnknown, "")) {}
+
+template <typename T>
+template <typename U>
+inline void StatusOr<T>::Assign(const StatusOr<U>& other) {
+  if (other.ok()) {
+    this->Assign(*other);
+  } else {
+    this->AssignStatus(other.status());
+  }
+}
+
+template <typename T>
+template <typename U>
+inline void StatusOr<T>::Assign(StatusOr<U>&& other) {
+  if (other.ok()) {
+    this->Assign(*std::move(other));
+  } else {
+    this->AssignStatus(std::move(other).status());
+  }
+}
+template <typename T>
+template <typename... Args>
+StatusOr<T>::StatusOr(std::in_place_t, Args&&... args)
+    : Base(std::in_place, std::forward<Args>(args)...) {}
+
+template <typename T>
+template <typename U, typename... Args>
+StatusOr<T>::StatusOr(std::in_place_t, std::initializer_list<U> ilist,
+                      Args&&... args)
+    : Base(std::in_place, ilist, std::forward<Args>(args)...) {}
+
+template <typename T>
+const Status& StatusOr<T>::status() const& {
+  return this->status_;
+}
+template <typename T>
+Status StatusOr<T>::status() && {
+  return ok() ? OkStatus() : std::move(this->status_);
+}
+
+template <typename T>
+void StatusOr<T>::IgnoreError() const {
+  // no-op
+}
+
+#endif  // SWIG
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STATUS_STATUSOR_H_
diff --git a/absl/status/statusor_benchmark.cc b/absl/status/statusor_benchmark.cc
new file mode 100644
index 00000000000..ac353bb1562
--- /dev/null
+++ b/absl/status/statusor_benchmark.cc
@@ -0,0 +1,483 @@
+// Copyright 2025 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <cstdint>
+#include <string>
+#include <utility>
+
+#include "absl/base/attributes.h"
+#include "absl/status/status.h"
+#include "absl/status/statusor.h"
+#include "benchmark/benchmark.h"
+
+namespace {
+
+void BM_StatusOrInt_CtorStatus(benchmark::State& state) {
+  for (auto _ : state) {
+    absl::StatusOr<int> status(absl::CancelledError());
+    benchmark::DoNotOptimize(status);
+  }
+}
+BENCHMARK(BM_StatusOrInt_CtorStatus);
+
+void BM_StatusOrInt_CtorStatusWithMessage(benchmark::State& state) {
+  for (auto _ : state) {
+    absl::StatusOr<int> status(
+        absl::UnknownError("This string is 28 characters"));
+    benchmark::DoNotOptimize(status);
+  }
+}
+BENCHMARK(BM_StatusOrInt_CtorStatusWithMessage);
+
+void BM_StatusOrInt_CopyCtor_Error(benchmark::State& state) {
+  for (auto _ : state) {
+    absl::StatusOr<int> original(
+        absl::UnknownError("This string is 28 characters"));
+    benchmark::DoNotOptimize(original);
+    absl::StatusOr<int> status(original);
+    benchmark::DoNotOptimize(status);
+  }
+}
+BENCHMARK(BM_StatusOrInt_CopyCtor_Error);
+
+void BM_StatusOrInt_CopyCtor_Ok(benchmark::State& state) {
+  for (auto _ : state) {
+    absl::StatusOr<int> original(42);
+    benchmark::DoNotOptimize(original);
+    absl::StatusOr<int> status(original);
+    benchmark::DoNotOptimize(status);
+  }
+}
+BENCHMARK(BM_StatusOrInt_CopyCtor_Ok);
+
+void BM_StatusOrInt_MoveCtor_Error(benchmark::State& state) {
+  for (auto _ : state) {
+    absl::StatusOr<int> original(
+        absl::UnknownError("This string is 28 characters"));
+    benchmark::DoNotOptimize(original);
+    absl::StatusOr<int> status(std::move(original));
+    benchmark::DoNotOptimize(status);
+  }
+}
+BENCHMARK(BM_StatusOrInt_MoveCtor_Error);
+
+void BM_StatusOrInt_MoveCtor_Ok(benchmark::State& state) {
+  for (auto _ : state) {
+    absl::StatusOr<int> original(42);
+    benchmark::DoNotOptimize(original);
+    absl::StatusOr<int> status(std::move(original));
+    benchmark::DoNotOptimize(status);
+  }
+}
+BENCHMARK(BM_StatusOrInt_MoveCtor_Ok);
+
+void BM_StatusOrInt_CopyAssign_Error(benchmark::State& state) {
+  for (auto _ : state) {
+    absl::StatusOr<int> original(
+        absl::UnknownError("This string is 28 characters"));
+    benchmark::DoNotOptimize(original);
+    absl::StatusOr<int> status(42);
+    benchmark::DoNotOptimize(status);
+    benchmark::DoNotOptimize(status = original);
+    benchmark::DoNotOptimize(status);
+  }
+}
+BENCHMARK(BM_StatusOrInt_CopyAssign_Error);
+
+void BM_StatusOrInt_CopyAssign_Ok(benchmark::State& state) {
+  for (auto _ : state) {
+    absl::StatusOr<int> original(42);
+    benchmark::DoNotOptimize(original);
+    absl::StatusOr<int> status(42);
+    benchmark::DoNotOptimize(status);
+    benchmark::DoNotOptimize(status = original);
+    benchmark::DoNotOptimize(status);
+  }
+}
+BENCHMARK(BM_StatusOrInt_CopyAssign_Ok);
+
+void BM_StatusOrInt_MoveAssign_Error(benchmark::State& state) {
+  for (auto _ : state) {
+    absl::StatusOr<int> original(
+        absl::UnknownError("This string is 28 characters"));
+    benchmark::DoNotOptimize(original);
+    absl::StatusOr<int> status(42);
+    benchmark::DoNotOptimize(status);
+    benchmark::DoNotOptimize(status = std::move(original));
+    benchmark::DoNotOptimize(status);
+  }
+}
+BENCHMARK(BM_StatusOrInt_MoveAssign_Error);
+
+void BM_StatusOrInt_MoveAssign_Ok(benchmark::State& state) {
+  for (auto _ : state) {
+    absl::StatusOr<int> original(42);
+    benchmark::DoNotOptimize(original);
+    absl::StatusOr<int> status(42);
+    benchmark::DoNotOptimize(status);
+    benchmark::DoNotOptimize(status = std::move(original));
+    benchmark::DoNotOptimize(status);
+  }
+}
+BENCHMARK(BM_StatusOrInt_MoveAssign_Ok);
+
+void BM_StatusOrInt_OkMethod_Error(benchmark::State& state) {
+  absl::StatusOr<int> status(
+      absl::UnknownError("This string is 28 characters"));
+  for (auto _ : state) {
+    benchmark::DoNotOptimize(status);
+    benchmark::DoNotOptimize(status.ok());
+  }
+}
+BENCHMARK(BM_StatusOrInt_OkMethod_Error);
+
+void BM_StatusOrInt_OkMethod_Ok(benchmark::State& state) {
+  absl::StatusOr<int> status(42);
+  for (auto _ : state) {
+    benchmark::DoNotOptimize(status);
+    benchmark::DoNotOptimize(status.ok());
+  }
+}
+BENCHMARK(BM_StatusOrInt_OkMethod_Ok);
+
+void BM_StatusOrInt_StatusMethod_Error(benchmark::State& state) {
+  for (auto _ : state) {
+    absl::StatusOr<int> status(
+        absl::UnknownError("This string is 28 characters"));
+    benchmark::DoNotOptimize(status);
+    benchmark::DoNotOptimize(status.status().ok());
+  }
+}
+BENCHMARK(BM_StatusOrInt_StatusMethod_Error);
+
+void BM_StatusOrInt_StatusMethod_Ok(benchmark::State& state) {
+  for (auto _ : state) {
+    absl::StatusOr<int> status(42);
+    benchmark::DoNotOptimize(status);
+    benchmark::DoNotOptimize(std::move(status).status().ok());
+  }
+}
+BENCHMARK(BM_StatusOrInt_StatusMethod_Ok);
+
+void BM_StatusOrInt_StatusMethodRvalue_Error(benchmark::State& state) {
+  for (auto _ : state) {
+    absl::StatusOr<int> status(
+        absl::UnknownError("This string is 28 characters"));
+    benchmark::DoNotOptimize(status);
+    benchmark::DoNotOptimize(std::move(status).status().ok());
+  }
+}
+BENCHMARK(BM_StatusOrInt_StatusMethodRvalue_Error);
+
+void BM_StatusOrInt_StatusMethodRvalue_Ok(benchmark::State& state) {
+  for (auto _ : state) {
+    absl::StatusOr<int> status(42);
+    benchmark::DoNotOptimize(status);
+    benchmark::DoNotOptimize(std::move(status).status());
+  }
+}
+BENCHMARK(BM_StatusOrInt_StatusMethodRvalue_Ok);
+
+void BM_StatusOrString_CtorStatus(benchmark::State& state) {
+  for (auto _ : state) {
+    absl::StatusOr<std::string> status(absl::CancelledError());
+    benchmark::DoNotOptimize(status);
+  }
+}
+BENCHMARK(BM_StatusOrString_CtorStatus);
+
+void BM_StatusOrString_CtorStatusWithMessage(benchmark::State& state) {
+  for (auto _ : state) {
+    absl::StatusOr<std::string> status(
+        absl::UnknownError("This string is 28 characters"));
+    benchmark::DoNotOptimize(status);
+  }
+}
+BENCHMARK(BM_StatusOrString_CtorStatusWithMessage);
+
+void BM_StatusOrString_CopyCtor_Error(benchmark::State& state) {
+  for (auto _ : state) {
+    absl::StatusOr<std::string> original(
+        absl::UnknownError("This string is 28 characters"));
+    benchmark::DoNotOptimize(original);
+    absl::StatusOr<std::string> status(original);
+    benchmark::DoNotOptimize(status);
+  }
+}
+BENCHMARK(BM_StatusOrString_CopyCtor_Error);
+
+void BM_StatusOrString_CopyCtor_Ok(benchmark::State& state) {
+  for (auto _ : state) {
+    absl::StatusOr<std::string> original("This string is 28 characters");
+    benchmark::DoNotOptimize(original);
+    absl::StatusOr<std::string> status(original);
+    benchmark::DoNotOptimize(status);
+  }
+}
+BENCHMARK(BM_StatusOrString_CopyCtor_Ok);
+
+void BM_StatusOrString_MoveCtor_Error(benchmark::State& state) {
+  for (auto _ : state) {
+    absl::StatusOr<std::string> original(
+        absl::UnknownError("This string is 28 characters"));
+    benchmark::DoNotOptimize(original);
+    absl::StatusOr<std::string> status(std::move(original));
+    benchmark::DoNotOptimize(status);
+  }
+}
+BENCHMARK(BM_StatusOrString_MoveCtor_Error);
+
+void BM_StatusOrString_MoveCtor_Ok(benchmark::State& state) {
+  for (auto _ : state) {
+    absl::StatusOr<std::string> original("This string is 28 characters");
+    benchmark::DoNotOptimize(original);
+    absl::StatusOr<std::string> status(std::move(original));
+    benchmark::DoNotOptimize(status);
+  }
+}
+BENCHMARK(BM_StatusOrString_MoveCtor_Ok);
+
+void BM_StatusOrString_CopyAssign_Error(benchmark::State& state) {
+  for (auto _ : state) {
+    absl::StatusOr<std::string> original(
+        absl::UnknownError("This string is 28 characters"));
+    benchmark::DoNotOptimize(original);
+    absl::StatusOr<std::string> status("This string is 28 characters");
+    benchmark::DoNotOptimize(status);
+    benchmark::DoNotOptimize(status = original);
+    benchmark::DoNotOptimize(status);
+  }
+}
+BENCHMARK(BM_StatusOrString_CopyAssign_Error);
+
+void BM_StatusOrString_CopyAssign_Ok(benchmark::State& state) {
+  for (auto _ : state) {
+    absl::StatusOr<std::string> original("This string is 28 characters");
+    benchmark::DoNotOptimize(original);
+    absl::StatusOr<std::string> status("This string is 28 characters");
+    benchmark::DoNotOptimize(status);
+    benchmark::DoNotOptimize(status = original);
+    benchmark::DoNotOptimize(status);
+  }
+}
+BENCHMARK(BM_StatusOrString_CopyAssign_Ok);
+
+void BM_StatusOrString_MoveAssign_Error(benchmark::State& state) {
+  for (auto _ : state) {
+    absl::StatusOr<std::string> original(
+        absl::UnknownError("This string is 28 characters"));
+    benchmark::DoNotOptimize(original);
+    absl::StatusOr<std::string> status("This string is 28 characters");
+    benchmark::DoNotOptimize(status);
+    benchmark::DoNotOptimize(status = std::move(original));
+    benchmark::DoNotOptimize(status);
+  }
+}
+BENCHMARK(BM_StatusOrString_MoveAssign_Error);
+
+void BM_StatusOrString_MoveAssign_Ok(benchmark::State& state) {
+  for (auto _ : state) {
+    absl::StatusOr<std::string> original("This string is 28 characters");
+    benchmark::DoNotOptimize(original);
+    absl::StatusOr<std::string> status("This string is 28 characters");
+    benchmark::DoNotOptimize(status);
+    benchmark::DoNotOptimize(status = std::move(original));
+    benchmark::DoNotOptimize(status);
+  }
+}
+BENCHMARK(BM_StatusOrString_MoveAssign_Ok);
+
+void BM_StatusOrString_OkMethod_Error(benchmark::State& state) {
+  for (auto _ : state) {
+    absl::StatusOr<std::string> status(
+        absl::UnknownError("This string is 28 characters"));
+    benchmark::DoNotOptimize(status);
+    benchmark::DoNotOptimize(status.ok());
+  }
+}
+BENCHMARK(BM_StatusOrString_OkMethod_Error);
+
+void BM_StatusOrString_OkMethod_Ok(benchmark::State& state) {
+  for (auto _ : state) {
+    absl::StatusOr<std::string> status("This string is 28 characters");
+    benchmark::DoNotOptimize(status);
+    benchmark::DoNotOptimize(status.ok());
+  }
+}
+BENCHMARK(BM_StatusOrString_OkMethod_Ok);
+
+void BM_StatusOrString_StatusMethod_Error(benchmark::State& state) {
+  for (auto _ : state) {
+    absl::StatusOr<std::string> status(
+        absl::UnknownError("This string is 28 characters"));
+    benchmark::DoNotOptimize(status);
+    benchmark::DoNotOptimize(status.status().ok());
+  }
+}
+BENCHMARK(BM_StatusOrString_StatusMethod_Error);
+
+void BM_StatusOrString_StatusMethod_Ok(benchmark::State& state) {
+  for (auto _ : state) {
+    absl::StatusOr<std::string> status("This string is 28 characters");
+    benchmark::DoNotOptimize(status);
+    benchmark::DoNotOptimize(status.status().ok());
+  }
+}
+BENCHMARK(BM_StatusOrString_StatusMethod_Ok);
+
+void BM_StatusOrString_StatusMethodRvalue_Error(benchmark::State& state) {
+  for (auto _ : state) {
+    absl::StatusOr<std::string> status(
+        absl::UnknownError("This string is 28 characters"));
+    benchmark::DoNotOptimize(status);
+    benchmark::DoNotOptimize(std::move(status).status());
+  }
+}
+BENCHMARK(BM_StatusOrString_StatusMethodRvalue_Error);
+
+void BM_StatusOrString_StatusMethodRvalue_Ok(benchmark::State& state) {
+  for (auto _ : state) {
+    absl::StatusOr<std::string> status("This string is 28 characters");
+    benchmark::DoNotOptimize(status);
+    benchmark::DoNotOptimize(std::move(status).status());
+  }
+}
+BENCHMARK(BM_StatusOrString_StatusMethodRvalue_Ok);
+
+// Benchmarks comparing a few alternative ways of structuring an interface
+// for returning an int64 on success or an error.  See (a), (b), (c), (d)
+// below for the variants.
+bool bm_cond = true;
+
+bool SimpleIntInterface(int64_t* v) ABSL_ATTRIBUTE_NOINLINE;
+bool SimpleIntInterfaceWithErrorMessage(int64_t* v, std::string* msg)
+    ABSL_ATTRIBUTE_NOINLINE;
+absl::Status SimpleIntInterfaceWithErrorStatus(int64_t* v)
+    ABSL_ATTRIBUTE_NOINLINE;
+absl::StatusOr<int64_t> SimpleIntStatusOrInterface() ABSL_ATTRIBUTE_NOINLINE;
+
+// (a): Just a boolean return value with an out int64* parameter
+bool SimpleIntInterface(int64_t* v) {
+  benchmark::DoNotOptimize(bm_cond);
+  if (bm_cond) {
+    *v = 42;
+    return true;
+  } else {
+    return false;
+  }
+}
+
+// (b): A boolean return value and a string error message filled in on failure
+// and an out int64* parameter filled on success
+bool SimpleIntInterfaceWithErrorMessage(int64_t* v, std::string* msg) {
+  benchmark::DoNotOptimize(bm_cond);
+  if (bm_cond) {
+    *v = 42;
+    return true;
+  } else {
+    *msg = "This is an error message";
+    return false;
+  }
+}
+
+// (c): A Status return value with an out int64* parameter on success
+absl::Status SimpleIntInterfaceWithErrorStatus(int64_t* v) {
+  benchmark::DoNotOptimize(bm_cond);
+  if (bm_cond) {
+    *v = 42;
+    return absl::OkStatus();
+  } else {
+    return absl::UnknownError("This is an error message");
+  }
+}
+
+// (d): A StatusOr<int64> return value
+absl::StatusOr<int64_t> SimpleIntStatusOrInterface() {
+  benchmark::DoNotOptimize(bm_cond);
+  if (bm_cond) {
+    return 42;
+  } else {
+    return absl::StatusOr<int64_t>(
+        absl::UnknownError("This is an error message"));
+  }
+}
+
+void SetCondition(benchmark::State& state) {
+  bm_cond = (state.range(0) == 0);
+  state.SetLabel(bm_cond ? "Success" : "Failure");
+}
+
+void BM_SimpleIntInterface(benchmark::State& state) {
+  SetCondition(state);
+  int64_t sum = 0;
+  for (auto s : state) {
+    int64_t v;
+    if (SimpleIntInterface(&v)) {
+      sum += v;
+    }
+    benchmark::DoNotOptimize(sum);
+  }
+}
+
+void BM_SimpleIntInterfaceMsg(benchmark::State& state) {
+  SetCondition(state);
+  int64_t sum = 0;
+  std::string msg;
+  for (auto s : state) {
+    int64_t v;
+    if (SimpleIntInterfaceWithErrorMessage(&v, &msg)) {
+      sum += v;
+    }
+    benchmark::DoNotOptimize(sum);
+    benchmark::DoNotOptimize(msg);
+  }
+}
+
+void BM_SimpleIntInterfaceStatus(benchmark::State& state) {
+  SetCondition(state);
+  int64_t sum = 0;
+  for (auto s : state) {
+    int64_t v;
+    auto result = SimpleIntInterfaceWithErrorStatus(&v);
+    if (result.ok()) {
+      sum += v;
+    }
+    benchmark::DoNotOptimize(sum);
+  }
+}
+
+void BM_SimpleIntStatusOrInterface(benchmark::State& state) {
+  SetCondition(state);
+  int64_t sum = 0;
+  for (auto s : state) {
+    auto v_s = SimpleIntStatusOrInterface();
+    if (v_s.ok()) {
+      sum += *v_s;
+    }
+    benchmark::DoNotOptimize(sum);
+  }
+}
+
+// Ordered like this so all the success path benchmarks (Arg(0)) show up,
+// then all the failure benchmarks (Arg(1))
+BENCHMARK(BM_SimpleIntInterface)->Arg(0);
+BENCHMARK(BM_SimpleIntInterfaceMsg)->Arg(0);
+BENCHMARK(BM_SimpleIntInterfaceStatus)->Arg(0);
+BENCHMARK(BM_SimpleIntStatusOrInterface)->Arg(0);
+BENCHMARK(BM_SimpleIntInterface)->Arg(1);
+BENCHMARK(BM_SimpleIntInterfaceMsg)->Arg(1);
+BENCHMARK(BM_SimpleIntInterfaceStatus)->Arg(1);
+BENCHMARK(BM_SimpleIntStatusOrInterface)->Arg(1);
+
+}  // namespace
diff --git a/absl/status/statusor_test.cc b/absl/status/statusor_test.cc
new file mode 100644
index 00000000000..8a9504909ec
--- /dev/null
+++ b/absl/status/statusor_test.cc
@@ -0,0 +1,2265 @@
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/status/statusor.h"
+
+#include <any>
+#include <array>
+#include <cstddef>
+#include <cstdint>
+#include <initializer_list>
+#include <map>
+#include <memory>
+#include <ostream>
+#include <sstream>
+#include <string>
+#include <type_traits>
+#include <utility>
+#include <variant>
+#include <vector>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/casts.h"
+#include "absl/memory/memory.h"
+#include "absl/status/status.h"
+#include "absl/status/status_matchers.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/string_view.h"
+#include "absl/types/source_location.h"
+
+namespace {
+
+using ::absl_testing::IsOk;
+using ::absl_testing::IsOkAndHolds;
+using ::testing::AllOf;
+using ::testing::AnyOf;
+using ::testing::AnyWith;
+using ::testing::ElementsAre;
+using ::testing::EndsWith;
+using ::testing::Field;
+using ::testing::HasSubstr;
+using ::testing::Ne;
+using ::testing::Not;
+using ::testing::Pointee;
+using ::testing::StartsWith;
+using ::testing::VariantWith;
+
+struct CopyDetector {
+  CopyDetector() = default;
+  explicit CopyDetector(int xx) : x(xx) {}
+  CopyDetector(CopyDetector&& d) noexcept
+      : x(d.x), copied(false), moved(true) {}
+  CopyDetector(const CopyDetector& d) : x(d.x), copied(true), moved(false) {}
+  CopyDetector& operator=(const CopyDetector& c) {
+    x = c.x;
+    copied = true;
+    moved = false;
+    return *this;
+  }
+  CopyDetector& operator=(CopyDetector&& c) noexcept {
+    x = c.x;
+    copied = false;
+    moved = true;
+    return *this;
+  }
+  int x = 0;
+  bool copied = false;
+  bool moved = false;
+};
+
+testing::Matcher<const CopyDetector&> CopyDetectorHas(int a, bool b, bool c) {
+  return AllOf(Field(&CopyDetector::x, a), Field(&CopyDetector::moved, b),
+               Field(&CopyDetector::copied, c));
+}
+
+class Base1 {
+ public:
+  virtual ~Base1() = default;
+  int pad;
+};
+
+class Base2 {
+ public:
+  virtual ~Base2() = default;
+  int yetotherpad;
+};
+
+class Derived : public Base1, public Base2 {
+ public:
+  ~Derived() override = default;
+  int evenmorepad;
+};
+
+class CopyNoAssign {
+ public:
+  explicit CopyNoAssign(int value) : foo(value) {}
+  CopyNoAssign(const CopyNoAssign& other) = default;
+  int foo;
+
+ private:
+  const CopyNoAssign& operator=(const CopyNoAssign&);
+};
+
+absl::StatusOr<std::unique_ptr<int>> ReturnUniquePtr() {
+  // Uses implicit constructor from T&&
+  return std::make_unique<int>(0);
+}
+
+TEST(StatusOr, ElementType) {
+  static_assert(std::is_same<absl::StatusOr<int>::value_type, int>());
+  static_assert(std::is_same<absl::StatusOr<char>::value_type, char>());
+}
+
+TEST(StatusOr, TestMoveOnlyInitialization) {
+  absl::StatusOr<std::unique_ptr<int>> thing(ReturnUniquePtr());
+  ASSERT_TRUE(thing.ok());
+  EXPECT_EQ(0, **thing);
+  int* previous = thing->get();
+
+  thing = ReturnUniquePtr();
+  EXPECT_TRUE(thing.ok());
+  EXPECT_EQ(0, **thing);
+  EXPECT_NE(previous, thing->get());
+}
+
+TEST(StatusOr, TestMoveOnlyValueExtraction) {
+  absl::StatusOr<std::unique_ptr<int>> thing(ReturnUniquePtr());
+  ASSERT_TRUE(thing.ok());
+  std::unique_ptr<int> ptr = *std::move(thing);
+  EXPECT_EQ(0, *ptr);
+
+  thing = std::move(ptr);
+  ptr = std::move(*thing);
+  EXPECT_EQ(0, *ptr);
+}
+
+TEST(StatusOr, TestMoveOnlyInitializationFromTemporaryByValueOrDie) {
+  std::unique_ptr<int> ptr(*ReturnUniquePtr());
+  EXPECT_EQ(0, *ptr);
+}
+
+TEST(StatusOr, TestValueOrDieOverloadForConstTemporary) {
+  static_assert(
+      std::is_same<
+          const int&&,
+          decltype(std::declval<const absl::StatusOr<int>&&>().value())>(),
+      "value() for const temporaries should return const T&&");
+}
+
+TEST(StatusOr, TestMoveOnlyConversion) {
+  absl::StatusOr<std::unique_ptr<const int>> const_thing(ReturnUniquePtr());
+  EXPECT_TRUE(const_thing.ok());
+  EXPECT_EQ(0, **const_thing);
+
+  // Test rvalue converting assignment
+  const int* const_previous = const_thing->get();
+  const_thing = ReturnUniquePtr();
+  EXPECT_TRUE(const_thing.ok());
+  EXPECT_EQ(0, **const_thing);
+  EXPECT_NE(const_previous, const_thing->get());
+}
+
+TEST(StatusOr, TestMoveOnlyVector) {
+  // Sanity check that absl::StatusOr<MoveOnly> works in vector.
+  std::vector<absl::StatusOr<std::unique_ptr<int>>> vec;
+  vec.push_back(ReturnUniquePtr());
+  vec.resize(2);
+  auto another_vec = std::move(vec);
+  EXPECT_EQ(0, **another_vec[0]);
+  EXPECT_EQ(absl::UnknownError(""), another_vec[1].status());
+}
+
+TEST(StatusOr, TestDefaultCtor) {
+  absl::StatusOr<int> thing;
+  EXPECT_FALSE(thing.ok());
+  EXPECT_EQ(thing.status().code(), absl::StatusCode::kUnknown);
+}
+
+TEST(StatusOr, StatusCtorForwards) {
+  absl::Status status(absl::StatusCode::kInternal, "Some error");
+
+  EXPECT_EQ(absl::StatusOr<int>(status).status().message(), "Some error");
+  EXPECT_EQ(status.message(), "Some error");
+
+  EXPECT_EQ(absl::StatusOr<int>(std::move(status)).status().message(),
+            "Some error");
+  EXPECT_NE(status.message(), "Some error");
+}
+
+TEST(BadStatusOrAccessTest, CopyConstructionWhatOk) {
+  absl::Status error =
+      absl::InternalError("some arbitrary message too big for the sso buffer");
+  absl::BadStatusOrAccess e1{error};
+  absl::BadStatusOrAccess e2{e1};
+  EXPECT_THAT(e1.what(), HasSubstr(error.ToString()));
+  EXPECT_THAT(e2.what(), HasSubstr(error.ToString()));
+}
+
+TEST(BadStatusOrAccessTest, CopyAssignmentWhatOk) {
+  absl::Status error =
+      absl::InternalError("some arbitrary message too big for the sso buffer");
+  absl::BadStatusOrAccess e1{error};
+  absl::BadStatusOrAccess e2{absl::InternalError("other")};
+  e2 = e1;
+  EXPECT_THAT(e1.what(), HasSubstr(error.ToString()));
+  EXPECT_THAT(e2.what(), HasSubstr(error.ToString()));
+}
+
+TEST(BadStatusOrAccessTest, MoveConstructionWhatOk) {
+  absl::Status error =
+      absl::InternalError("some arbitrary message too big for the sso buffer");
+  absl::BadStatusOrAccess e1{error};
+  absl::BadStatusOrAccess e2{std::move(e1)};
+  EXPECT_THAT(e2.what(), HasSubstr(error.ToString()));
+}
+
+TEST(BadStatusOrAccessTest, MoveAssignmentWhatOk) {
+  absl::Status error =
+      absl::InternalError("some arbitrary message too big for the sso buffer");
+  absl::BadStatusOrAccess e1{error};
+  absl::BadStatusOrAccess e2{absl::InternalError("other")};
+  e2 = std::move(e1);
+  EXPECT_THAT(e2.what(), HasSubstr(error.ToString()));
+}
+
+// Define `EXPECT_DEATH_OR_THROW` to test the behavior of `StatusOr::value`,
+// which either throws `BadStatusOrAccess` or `LOG(FATAL)` based on whether
+// exceptions are enabled.
+#ifdef ABSL_HAVE_EXCEPTIONS
+#define EXPECT_DEATH_OR_THROW(statement, status_)                  \
+  EXPECT_THROW(                                                    \
+      {                                                            \
+        try {                                                      \
+          statement;                                               \
+        } catch (const absl::BadStatusOrAccess& e) {               \
+          EXPECT_EQ(e.status(), status_);                          \
+          EXPECT_THAT(e.what(), HasSubstr(e.status().ToString())); \
+          throw;                                                   \
+        }                                                          \
+      },                                                           \
+      absl::BadStatusOrAccess);
+#else  // ABSL_HAVE_EXCEPTIONS
+#define EXPECT_DEATH_OR_THROW(statement, status) \
+  EXPECT_DEATH_IF_SUPPORTED(statement, status.ToString());
+#endif  // ABSL_HAVE_EXCEPTIONS
+
+TEST(StatusOrDeathTest, TestDefaultCtorValue) {
+  absl::StatusOr<int> thing;
+  EXPECT_DEATH_OR_THROW(thing.value(), absl::UnknownError(""));
+  const absl::StatusOr<int> thing2;
+  EXPECT_DEATH_OR_THROW(thing2.value(), absl::UnknownError(""));
+}
+
+TEST(StatusOrDeathTest, TestValueNotOk) {
+  absl::StatusOr<int> thing(absl::CancelledError());
+  EXPECT_DEATH_OR_THROW(thing.value(), absl::CancelledError());
+}
+
+TEST(StatusOrDeathTest, TestValueNotOkConst) {
+  const absl::StatusOr<int> thing(absl::UnknownError(""));
+  EXPECT_DEATH_OR_THROW(thing.value(), absl::UnknownError(""));
+}
+
+TEST(StatusOrDeathTest, TestPointerDefaultCtorValue) {
+  absl::StatusOr<int*> thing;
+  EXPECT_DEATH_OR_THROW(thing.value(), absl::UnknownError(""));
+}
+
+TEST(StatusOrDeathTest, TestPointerValueNotOk) {
+  absl::StatusOr<int*> thing(absl::CancelledError());
+  EXPECT_DEATH_OR_THROW(thing.value(), absl::CancelledError());
+}
+
+TEST(StatusOrDeathTest, TestPointerValueNotOkConst) {
+  const absl::StatusOr<int*> thing(absl::CancelledError());
+  EXPECT_DEATH_OR_THROW(thing.value(), absl::CancelledError());
+}
+
+#if GTEST_HAS_DEATH_TEST
+TEST(StatusOrDeathTest, TestStatusCtorStatusOk) {
+  EXPECT_DEBUG_DEATH(
+      {
+        // This will DCHECK
+        absl::StatusOr<int> thing(absl::OkStatus());
+        // In optimized mode, we are actually going to get error::INTERNAL for
+        // status here, rather than crashing, so check that.
+        EXPECT_FALSE(thing.ok());
+        EXPECT_EQ(thing.status().code(), absl::StatusCode::kInternal);
+      },
+      "An OK status is not a valid constructor argument");
+}
+
+TEST(StatusOrDeathTest, TestPointerStatusCtorStatusOk) {
+  EXPECT_DEBUG_DEATH(
+      {
+        absl::StatusOr<int*> thing(absl::OkStatus());
+        // In optimized mode, we are actually going to get error::INTERNAL for
+        // status here, rather than crashing, so check that.
+        EXPECT_FALSE(thing.ok());
+        EXPECT_EQ(thing.status().code(), absl::StatusCode::kInternal);
+      },
+      "An OK status is not a valid constructor argument");
+}
+#endif
+
+TEST(StatusOr, ValueAccessor) {
+  const int kIntValue = 110;
+  {
+    absl::StatusOr<int> status_or(kIntValue);
+    EXPECT_EQ(kIntValue, status_or.value());
+    EXPECT_EQ(kIntValue, std::move(status_or).value());
+  }
+  {
+    absl::StatusOr<CopyDetector> status_or(kIntValue);
+    EXPECT_THAT(status_or,
+                IsOkAndHolds(CopyDetectorHas(kIntValue, false, false)));
+    CopyDetector copy_detector = status_or.value();
+    EXPECT_THAT(copy_detector, CopyDetectorHas(kIntValue, false, true));
+    copy_detector = std::move(status_or).value();
+    EXPECT_THAT(copy_detector, CopyDetectorHas(kIntValue, true, false));
+  }
+}
+
+TEST(StatusOr, BadValueAccess) {
+  const absl::Status kError = absl::CancelledError("message");
+  absl::StatusOr<int> status_or(kError);
+  EXPECT_DEATH_OR_THROW(status_or.value(), kError);
+}
+
+TEST(StatusOr, TestStatusCtor) {
+  absl::StatusOr<int> thing(absl::CancelledError());
+  EXPECT_FALSE(thing.ok());
+  EXPECT_EQ(thing.status().code(), absl::StatusCode::kCancelled);
+}
+
+TEST(StatusOr, TestValueCtor) {
+  const int kI = 4;
+  const absl::StatusOr<int> thing(kI);
+  EXPECT_TRUE(thing.ok());
+  EXPECT_EQ(kI, *thing);
+}
+
+struct Foo {
+  const int x;
+  explicit Foo(int y) : x(y) {}
+};
+
+TEST(StatusOr, InPlaceConstruction) {
+  EXPECT_THAT(absl::StatusOr<Foo>(std::in_place, 10),
+              IsOkAndHolds(Field(&Foo::x, 10)));
+}
+
+struct InPlaceHelper {
+  InPlaceHelper(std::initializer_list<int> xs, std::unique_ptr<int> yy)
+      : x(xs), y(std::move(yy)) {}
+  const std::vector<int> x;
+  std::unique_ptr<int> y;
+};
+
+TEST(StatusOr, InPlaceInitListConstruction) {
+  absl::StatusOr<InPlaceHelper> status_or(std::in_place, {10, 11, 12},
+                                          std::make_unique<int>(13));
+  EXPECT_THAT(status_or, IsOkAndHolds(AllOf(
+                             Field(&InPlaceHelper::x, ElementsAre(10, 11, 12)),
+                             Field(&InPlaceHelper::y, Pointee(13)))));
+}
+
+TEST(StatusOr, Emplace) {
+  absl::StatusOr<Foo> status_or_foo(10);
+  status_or_foo.emplace(20);
+  EXPECT_THAT(status_or_foo, IsOkAndHolds(Field(&Foo::x, 20)));
+  status_or_foo = absl::InvalidArgumentError("msg");
+  EXPECT_FALSE(status_or_foo.ok());
+  EXPECT_EQ(status_or_foo.status().code(), absl::StatusCode::kInvalidArgument);
+  EXPECT_EQ(status_or_foo.status().message(), "msg");
+  status_or_foo.emplace(20);
+  EXPECT_THAT(status_or_foo, IsOkAndHolds(Field(&Foo::x, 20)));
+}
+
+TEST(StatusOr, EmplaceInitializerList) {
+  absl::StatusOr<InPlaceHelper> status_or(std::in_place, {10, 11, 12},
+                                          std::make_unique<int>(13));
+  status_or.emplace({1, 2, 3}, std::make_unique<int>(4));
+  EXPECT_THAT(status_or,
+              IsOkAndHolds(AllOf(Field(&InPlaceHelper::x, ElementsAre(1, 2, 3)),
+                                 Field(&InPlaceHelper::y, Pointee(4)))));
+  status_or = absl::InvalidArgumentError("msg");
+  EXPECT_FALSE(status_or.ok());
+  EXPECT_EQ(status_or.status().code(), absl::StatusCode::kInvalidArgument);
+  EXPECT_EQ(status_or.status().message(), "msg");
+  status_or.emplace({1, 2, 3}, std::make_unique<int>(4));
+  EXPECT_THAT(status_or,
+              IsOkAndHolds(AllOf(Field(&InPlaceHelper::x, ElementsAre(1, 2, 3)),
+                                 Field(&InPlaceHelper::y, Pointee(4)))));
+}
+
+#ifdef ABSL_HAVE_EXCEPTIONS
+class ThrowOnEmplace {
+ public:
+  explicit ThrowOnEmplace(int* counter, int val) : destructor_calls_(counter) {
+    if (val < 0) {
+      throw std::runtime_error("expected");
+    }
+    // While destructor_calls tracks the logic, ptr_ ensures that a double
+    // destruction actually results in a reliable crash. Performing a real heap
+    // allocation and deallocation (new/delete) guarantees that AddressSanitizer
+    // (ASAN) or the heap allocator will instantly catch the double-free if the
+    // bug regresses, rather than relying solely on the integer check.
+    ptr_ = new int(val);
+  }
+
+  ThrowOnEmplace(const ThrowOnEmplace&) = delete;
+  ThrowOnEmplace& operator=(const ThrowOnEmplace&) = delete;
+
+  ~ThrowOnEmplace() {
+    if (destructor_calls_) {
+      ++(*destructor_calls_);
+    }
+    delete ptr_;
+  }
+
+ private:
+  int* destructor_calls_ = nullptr;
+  int* ptr_ = nullptr;
+};
+
+TEST(StatusOr, EmplaceThrowsExceptionSafety) {
+  int destructor_calls = 0;
+  {
+    absl::StatusOr<ThrowOnEmplace> status_or(std::in_place, &destructor_calls,
+                                             1);
+    EXPECT_TRUE(status_or.ok());
+    EXPECT_THROW(status_or.emplace(&destructor_calls, -1), std::runtime_error);
+    EXPECT_FALSE(status_or.ok());
+    EXPECT_EQ(status_or.status().code(), absl::StatusCode::kInternal);
+  }
+  // Verifies that the initial object is properly destroyed by Clear() (count is
+  // 1), and that the exception thrown during replacement does not cause a
+  // second destruction (double-free) during stack unwinding.
+  EXPECT_EQ(destructor_calls, 1);
+}
+#endif  // ABSL_HAVE_EXCEPTIONS
+
+TEST(StatusOr, TestCopyCtorStatusOk) {
+  const int kI = 4;
+  const absl::StatusOr<int> original(kI);
+  const absl::StatusOr<int> copy(original);
+  EXPECT_THAT(copy.status(), IsOk());
+  EXPECT_EQ(*original, *copy);
+}
+
+TEST(StatusOr, TestCopyCtorStatusNotOk) {
+  absl::StatusOr<int> original(absl::CancelledError());
+  absl::StatusOr<int> copy(original);
+  EXPECT_EQ(copy.status().code(), absl::StatusCode::kCancelled);
+}
+
+TEST(StatusOr, TestCopyCtorNonAssignable) {
+  const int kI = 4;
+  CopyNoAssign value(kI);
+  absl::StatusOr<CopyNoAssign> original(value);
+  absl::StatusOr<CopyNoAssign> copy(original);
+  EXPECT_THAT(copy.status(), IsOk());
+  EXPECT_EQ(original->foo, copy->foo);
+}
+
+TEST(StatusOr, TestCopyCtorStatusOKConverting) {
+  const int kI = 4;
+  absl::StatusOr<int> original(kI);
+  absl::StatusOr<double> copy(original);
+  EXPECT_THAT(copy.status(), IsOk());
+  EXPECT_DOUBLE_EQ(*original, *copy);
+}
+
+TEST(StatusOr, TestCopyCtorStatusNotOkConverting) {
+  absl::StatusOr<int> original(absl::CancelledError());
+  absl::StatusOr<double> copy(original);
+  EXPECT_EQ(copy.status(), original.status());
+}
+
+TEST(StatusOr, TestAssignmentStatusOk) {
+  // Copy assignmment
+  {
+    const auto p = std::make_shared<int>(17);
+    absl::StatusOr<std::shared_ptr<int>> source(p);
+
+    absl::StatusOr<std::shared_ptr<int>> target;
+    target = source;
+
+    ASSERT_TRUE(target.ok());
+    EXPECT_THAT(target.status(), IsOk());
+    EXPECT_EQ(p, *target);
+
+    ASSERT_TRUE(source.ok());
+    EXPECT_THAT(source.status(), IsOk());
+    EXPECT_EQ(p, *source);
+  }
+
+  // Move assignment
+  {
+    const auto p = std::make_shared<int>(17);
+    absl::StatusOr<std::shared_ptr<int>> source(p);
+
+    absl::StatusOr<std::shared_ptr<int>> target;
+    target = std::move(source);
+
+    ASSERT_TRUE(target.ok());
+    EXPECT_THAT(target.status(), IsOk());
+    EXPECT_EQ(p, *target);
+
+    ASSERT_TRUE(source.ok());
+    EXPECT_THAT(source.status(), IsOk());
+    EXPECT_EQ(nullptr, *source);
+  }
+}
+
+TEST(StatusOr, TestAssignmentStatusNotOk) {
+  // Copy assignment
+  {
+    const absl::Status expected = absl::CancelledError();
+    absl::StatusOr<int> source(expected);
+
+    absl::StatusOr<int> target;
+    target = source;
+
+    EXPECT_FALSE(target.ok());
+    EXPECT_EQ(expected, target.status());
+
+    EXPECT_FALSE(source.ok());
+    EXPECT_EQ(expected, source.status());
+  }
+
+  // Move assignment
+  {
+    const absl::Status expected = absl::CancelledError();
+    absl::StatusOr<int> source(expected);
+
+    absl::StatusOr<int> target;
+    target = std::move(source);
+
+    EXPECT_FALSE(target.ok());
+    EXPECT_EQ(expected, target.status());
+
+    EXPECT_FALSE(source.ok());
+    EXPECT_EQ(source.status().code(), absl::StatusCode::kInternal);
+  }
+}
+
+TEST(StatusOr, TestAssignmentStatusOKConverting) {
+  // Copy assignment
+  {
+    const int kI = 4;
+    absl::StatusOr<int> source(kI);
+
+    absl::StatusOr<double> target;
+    target = source;
+
+    ASSERT_TRUE(target.ok());
+    EXPECT_THAT(target.status(), IsOk());
+    EXPECT_DOUBLE_EQ(kI, *target);
+
+    ASSERT_TRUE(source.ok());
+    EXPECT_THAT(source.status(), IsOk());
+    EXPECT_DOUBLE_EQ(kI, *source);
+  }
+
+  // Move assignment
+  {
+    const auto p = new int(17);
+    absl::StatusOr<std::unique_ptr<int>> source(absl::WrapUnique(p));
+
+    absl::StatusOr<std::shared_ptr<int>> target;
+    target = std::move(source);
+
+    ASSERT_TRUE(target.ok());
+    EXPECT_THAT(target.status(), IsOk());
+    EXPECT_EQ(p, target->get());
+
+    ASSERT_TRUE(source.ok());
+    EXPECT_THAT(source.status(), IsOk());
+    EXPECT_EQ(nullptr, source->get());
+  }
+}
+
+struct A {
+  int x;
+};
+
+struct ImplicitConstructibleFromA {
+  int x;
+  bool moved;
+  ImplicitConstructibleFromA(const A& a)  // NOLINT
+      : x(a.x), moved(false) {}
+  ImplicitConstructibleFromA(A&& a)  // NOLINT
+      : x(a.x), moved(true) {}
+};
+
+TEST(StatusOr, ImplicitConvertingConstructor) {
+  EXPECT_THAT(
+      absl::implicit_cast<absl::StatusOr<ImplicitConstructibleFromA>>(
+          absl::StatusOr<A>(A{11})),
+      IsOkAndHolds(AllOf(Field(&ImplicitConstructibleFromA::x, 11),
+                         Field(&ImplicitConstructibleFromA::moved, true))));
+  absl::StatusOr<A> a(A{12});
+  EXPECT_THAT(
+      absl::implicit_cast<absl::StatusOr<ImplicitConstructibleFromA>>(a),
+      IsOkAndHolds(AllOf(Field(&ImplicitConstructibleFromA::x, 12),
+                         Field(&ImplicitConstructibleFromA::moved, false))));
+}
+
+struct ExplicitConstructibleFromA {
+  int x;
+  bool moved;
+  explicit ExplicitConstructibleFromA(const A& a) : x(a.x), moved(false) {}
+  explicit ExplicitConstructibleFromA(A&& a) : x(a.x), moved(true) {}
+};
+
+TEST(StatusOr, ExplicitConvertingConstructor) {
+  EXPECT_FALSE(
+      (std::is_convertible_v<const absl::StatusOr<A>&,
+                             absl::StatusOr<ExplicitConstructibleFromA>>));
+  EXPECT_FALSE(
+      (std::is_convertible_v<absl::StatusOr<A>&&,
+                             absl::StatusOr<ExplicitConstructibleFromA>>));
+  EXPECT_THAT(
+      absl::StatusOr<ExplicitConstructibleFromA>(absl::StatusOr<A>(A{11})),
+      IsOkAndHolds(AllOf(Field(&ExplicitConstructibleFromA::x, 11),
+                         Field(&ExplicitConstructibleFromA::moved, true))));
+  absl::StatusOr<A> a(A{12});
+  EXPECT_THAT(
+      absl::StatusOr<ExplicitConstructibleFromA>(a),
+      IsOkAndHolds(AllOf(Field(&ExplicitConstructibleFromA::x, 12),
+                         Field(&ExplicitConstructibleFromA::moved, false))));
+}
+
+struct ImplicitConstructibleFromBool {
+  ImplicitConstructibleFromBool(bool y) : x(y) {}  // NOLINT
+  bool x = false;
+};
+
+struct ConvertibleToBool {
+  explicit ConvertibleToBool(bool y) : x(y) {}
+  operator bool() const { return x; }  // NOLINT
+  bool x = false;
+};
+
+TEST(StatusOr, ImplicitBooleanConstructionWithImplicitCasts) {
+  EXPECT_THAT(absl::StatusOr<bool>(absl::StatusOr<ConvertibleToBool>(true)),
+              IsOkAndHolds(true));
+  EXPECT_THAT(absl::StatusOr<bool>(absl::StatusOr<ConvertibleToBool>(false)),
+              IsOkAndHolds(false));
+  EXPECT_THAT(
+      absl::implicit_cast<absl::StatusOr<ImplicitConstructibleFromBool>>(
+          absl::StatusOr<bool>(false)),
+      IsOkAndHolds(Field(&ImplicitConstructibleFromBool::x, false)));
+  EXPECT_FALSE(
+      (std::is_convertible_v<absl::StatusOr<ConvertibleToBool>,
+                             absl::StatusOr<ImplicitConstructibleFromBool>>));
+}
+
+TEST(StatusOr, BooleanConstructionWithImplicitCasts) {
+  EXPECT_THAT(absl::StatusOr<bool>(absl::StatusOr<ConvertibleToBool>(true)),
+              IsOkAndHolds(true));
+  EXPECT_THAT(absl::StatusOr<bool>(absl::StatusOr<ConvertibleToBool>(false)),
+              IsOkAndHolds(false));
+  EXPECT_THAT(
+      absl::StatusOr<ImplicitConstructibleFromBool>{
+          absl::StatusOr<bool>(false)},
+      IsOkAndHolds(Field(&ImplicitConstructibleFromBool::x, false)));
+  EXPECT_THAT(
+      absl::StatusOr<ImplicitConstructibleFromBool>{
+          absl::StatusOr<bool>(absl::InvalidArgumentError(""))},
+      Not(IsOk()));
+
+  EXPECT_THAT(
+      absl::StatusOr<ImplicitConstructibleFromBool>{
+          absl::StatusOr<ConvertibleToBool>(ConvertibleToBool{false})},
+      IsOkAndHolds(Field(&ImplicitConstructibleFromBool::x, false)));
+  EXPECT_THAT(
+      absl::StatusOr<ImplicitConstructibleFromBool>{
+          absl::StatusOr<ConvertibleToBool>(absl::InvalidArgumentError(""))},
+      Not(IsOk()));
+}
+
+TEST(StatusOr, ConstImplicitCast) {
+  EXPECT_THAT(absl::implicit_cast<absl::StatusOr<bool>>(
+                  absl::StatusOr<const bool>(true)),
+              IsOkAndHolds(true));
+  EXPECT_THAT(absl::implicit_cast<absl::StatusOr<bool>>(
+                  absl::StatusOr<const bool>(false)),
+              IsOkAndHolds(false));
+  EXPECT_THAT(absl::implicit_cast<absl::StatusOr<const bool>>(
+                  absl::StatusOr<bool>(true)),
+              IsOkAndHolds(true));
+  EXPECT_THAT(absl::implicit_cast<absl::StatusOr<const bool>>(
+                  absl::StatusOr<bool>(false)),
+              IsOkAndHolds(false));
+  EXPECT_THAT(absl::implicit_cast<absl::StatusOr<const std::string>>(
+                  absl::StatusOr<std::string>("foo")),
+              IsOkAndHolds("foo"));
+  EXPECT_THAT(absl::implicit_cast<absl::StatusOr<std::string>>(
+                  absl::StatusOr<const std::string>("foo")),
+              IsOkAndHolds("foo"));
+  EXPECT_THAT(
+      absl::implicit_cast<absl::StatusOr<std::shared_ptr<const std::string>>>(
+          absl::StatusOr<std::shared_ptr<std::string>>(
+              std::make_shared<std::string>("foo"))),
+      IsOkAndHolds(Pointee(std::string("foo"))));
+}
+
+TEST(StatusOr, ConstExplicitConstruction) {
+  EXPECT_THAT(absl::StatusOr<bool>(absl::StatusOr<const bool>(true)),
+              IsOkAndHolds(true));
+  EXPECT_THAT(absl::StatusOr<bool>(absl::StatusOr<const bool>(false)),
+              IsOkAndHolds(false));
+  EXPECT_THAT(absl::StatusOr<const bool>(absl::StatusOr<bool>(true)),
+              IsOkAndHolds(true));
+  EXPECT_THAT(absl::StatusOr<const bool>(absl::StatusOr<bool>(false)),
+              IsOkAndHolds(false));
+}
+
+struct ExplicitConstructibleFromInt {
+  int x;
+  explicit ExplicitConstructibleFromInt(int y) : x(y) {}
+};
+
+TEST(StatusOr, ExplicitConstruction) {
+  EXPECT_THAT(absl::StatusOr<ExplicitConstructibleFromInt>(10),
+              IsOkAndHolds(Field(&ExplicitConstructibleFromInt::x, 10)));
+}
+
+TEST(StatusOr, ImplicitConstruction) {
+  // Check implicit casting works.
+  auto status_or =
+      absl::implicit_cast<absl::StatusOr<std::variant<int, std::string>>>(10);
+  EXPECT_THAT(status_or, IsOkAndHolds(VariantWith<int>(10)));
+}
+
+TEST(StatusOr, ImplicitConstructionFromInitliazerList) {
+  // Note: dropping the explicit std::initializer_list<int> is not supported
+  // by absl::StatusOr or std::optional.
+  auto status_or =
+      absl::implicit_cast<absl::StatusOr<std::vector<int>>>({{10, 20, 30}});
+  EXPECT_THAT(status_or, IsOkAndHolds(ElementsAre(10, 20, 30)));
+}
+
+TEST(StatusOr, UniquePtrImplicitConstruction) {
+  auto status_or = absl::implicit_cast<absl::StatusOr<std::unique_ptr<Base1>>>(
+      std::make_unique<Derived>());
+  EXPECT_THAT(status_or, IsOkAndHolds(Ne(nullptr)));
+}
+
+TEST(StatusOr, NestedStatusOrCopyAndMoveConstructorTests) {
+  absl::StatusOr<absl::StatusOr<CopyDetector>> status_or = CopyDetector(10);
+  absl::StatusOr<absl::StatusOr<CopyDetector>> status_error =
+      absl::InvalidArgumentError("foo");
+  EXPECT_THAT(status_or,
+              IsOkAndHolds(IsOkAndHolds(CopyDetectorHas(10, true, false))));
+  absl::StatusOr<absl::StatusOr<CopyDetector>> a = status_or;
+  EXPECT_THAT(a, IsOkAndHolds(IsOkAndHolds(CopyDetectorHas(10, false, true))));
+  absl::StatusOr<absl::StatusOr<CopyDetector>> a_err = status_error;
+  EXPECT_THAT(a_err, Not(IsOk()));
+
+  const absl::StatusOr<absl::StatusOr<CopyDetector>>& cref = status_or;
+  absl::StatusOr<absl::StatusOr<CopyDetector>> b = cref;  // NOLINT
+  EXPECT_THAT(b, IsOkAndHolds(IsOkAndHolds(CopyDetectorHas(10, false, true))));
+  const absl::StatusOr<absl::StatusOr<CopyDetector>>& cref_err = status_error;
+  absl::StatusOr<absl::StatusOr<CopyDetector>> b_err = cref_err;  // NOLINT
+  EXPECT_THAT(b_err, Not(IsOk()));
+
+  absl::StatusOr<absl::StatusOr<CopyDetector>> c = std::move(status_or);
+  EXPECT_THAT(c, IsOkAndHolds(IsOkAndHolds(CopyDetectorHas(10, true, false))));
+  absl::StatusOr<absl::StatusOr<CopyDetector>> c_err = std::move(status_error);
+  EXPECT_THAT(c_err, Not(IsOk()));
+}
+
+TEST(StatusOr, NestedStatusOrCopyAndMoveAssignment) {
+  absl::StatusOr<absl::StatusOr<CopyDetector>> status_or = CopyDetector(10);
+  absl::StatusOr<absl::StatusOr<CopyDetector>> status_error =
+      absl::InvalidArgumentError("foo");
+  absl::StatusOr<absl::StatusOr<CopyDetector>> a;
+  a = status_or;
+  EXPECT_THAT(a, IsOkAndHolds(IsOkAndHolds(CopyDetectorHas(10, false, true))));
+  a = status_error;
+  EXPECT_THAT(a, Not(IsOk()));
+
+  const absl::StatusOr<absl::StatusOr<CopyDetector>>& cref = status_or;
+  a = cref;
+  EXPECT_THAT(a, IsOkAndHolds(IsOkAndHolds(CopyDetectorHas(10, false, true))));
+  const absl::StatusOr<absl::StatusOr<CopyDetector>>& cref_err = status_error;
+  a = cref_err;
+  EXPECT_THAT(a, Not(IsOk()));
+  a = std::move(status_or);
+  EXPECT_THAT(a, IsOkAndHolds(IsOkAndHolds(CopyDetectorHas(10, true, false))));
+  a = std::move(status_error);
+  EXPECT_THAT(a, Not(IsOk()));
+}
+
+struct Copyable {
+  Copyable() = default;
+  Copyable(const Copyable&) = default;
+  Copyable& operator=(const Copyable&) = default;
+};
+
+struct MoveOnly {
+  MoveOnly() = default;
+  MoveOnly(MoveOnly&&) {}
+  MoveOnly& operator=(MoveOnly&&) { return *this; }
+};
+
+struct NonMovable {
+  NonMovable() = default;
+  NonMovable(const NonMovable&) = delete;
+  NonMovable(NonMovable&&) = delete;
+  NonMovable& operator=(const NonMovable&) = delete;
+  NonMovable& operator=(NonMovable&&) = delete;
+};
+
+TEST(StatusOr, CopyAndMoveAbility) {
+  EXPECT_TRUE(std::is_copy_constructible_v<Copyable>);
+  EXPECT_TRUE(std::is_copy_assignable_v<Copyable>);
+  EXPECT_TRUE(std::is_move_constructible_v<Copyable>);
+  EXPECT_TRUE(std::is_move_assignable_v<Copyable>);
+  EXPECT_FALSE(std::is_copy_constructible_v<MoveOnly>);
+  EXPECT_FALSE(std::is_copy_assignable_v<MoveOnly>);
+  EXPECT_TRUE(std::is_move_constructible_v<MoveOnly>);
+  EXPECT_TRUE(std::is_move_assignable_v<MoveOnly>);
+  EXPECT_FALSE(std::is_copy_constructible_v<NonMovable>);
+  EXPECT_FALSE(std::is_copy_assignable_v<NonMovable>);
+  EXPECT_FALSE(std::is_move_constructible_v<NonMovable>);
+  EXPECT_FALSE(std::is_move_assignable_v<NonMovable>);
+}
+
+TEST(StatusOr, StatusOrAnyCopyAndMoveConstructorTests) {
+  absl::StatusOr<std::any> status_or = CopyDetector(10);
+  absl::StatusOr<std::any> status_error = absl::InvalidArgumentError("foo");
+  EXPECT_THAT(
+      status_or,
+      IsOkAndHolds(AnyWith<CopyDetector>(CopyDetectorHas(10, true, false))));
+  absl::StatusOr<std::any> a = status_or;
+  EXPECT_THAT(
+      a, IsOkAndHolds(AnyWith<CopyDetector>(CopyDetectorHas(10, false, true))));
+  absl::StatusOr<std::any> a_err = status_error;
+  EXPECT_THAT(a_err, Not(IsOk()));
+
+  const absl::StatusOr<std::any>& cref = status_or;
+  // No lint for no-change copy.
+  absl::StatusOr<std::any> b = cref;  // NOLINT
+  EXPECT_THAT(
+      b, IsOkAndHolds(AnyWith<CopyDetector>(CopyDetectorHas(10, false, true))));
+  const absl::StatusOr<std::any>& cref_err = status_error;
+  // No lint for no-change copy.
+  absl::StatusOr<std::any> b_err = cref_err;  // NOLINT
+  EXPECT_THAT(b_err, Not(IsOk()));
+
+  absl::StatusOr<std::any> c = std::move(status_or);
+  EXPECT_THAT(
+      c, IsOkAndHolds(AnyWith<CopyDetector>(CopyDetectorHas(10, true, false))));
+  absl::StatusOr<std::any> c_err = std::move(status_error);
+  EXPECT_THAT(c_err, Not(IsOk()));
+}
+
+TEST(StatusOr, StatusOrAnyCopyAndMoveAssignment) {
+  absl::StatusOr<std::any> status_or = CopyDetector(10);
+  absl::StatusOr<std::any> status_error = absl::InvalidArgumentError("foo");
+  absl::StatusOr<std::any> a;
+  a = status_or;
+  EXPECT_THAT(
+      a, IsOkAndHolds(AnyWith<CopyDetector>(CopyDetectorHas(10, false, true))));
+  a = status_error;
+  EXPECT_THAT(a, Not(IsOk()));
+
+  const absl::StatusOr<std::any>& cref = status_or;
+  a = cref;
+  EXPECT_THAT(
+      a, IsOkAndHolds(AnyWith<CopyDetector>(CopyDetectorHas(10, false, true))));
+  const absl::StatusOr<std::any>& cref_err = status_error;
+  a = cref_err;
+  EXPECT_THAT(a, Not(IsOk()));
+  a = std::move(status_or);
+  EXPECT_THAT(
+      a, IsOkAndHolds(AnyWith<CopyDetector>(CopyDetectorHas(10, true, false))));
+  a = std::move(status_error);
+  EXPECT_THAT(a, Not(IsOk()));
+}
+
+TEST(StatusOr, StatusOrCopyAndMoveTestsConstructor) {
+  absl::StatusOr<CopyDetector> status_or(10);
+  ASSERT_THAT(status_or, IsOkAndHolds(CopyDetectorHas(10, false, false)));
+  absl::StatusOr<CopyDetector> a(status_or);
+  EXPECT_THAT(a, IsOkAndHolds(CopyDetectorHas(10, false, true)));
+  const absl::StatusOr<CopyDetector>& cref = status_or;
+  absl::StatusOr<CopyDetector> b(cref);  // NOLINT
+  EXPECT_THAT(b, IsOkAndHolds(CopyDetectorHas(10, false, true)));
+  absl::StatusOr<CopyDetector> c(std::move(status_or));
+  EXPECT_THAT(c, IsOkAndHolds(CopyDetectorHas(10, true, false)));
+}
+
+TEST(StatusOr, StatusOrCopyAndMoveTestsAssignment) {
+  absl::StatusOr<CopyDetector> status_or(10);
+  ASSERT_THAT(status_or, IsOkAndHolds(CopyDetectorHas(10, false, false)));
+  absl::StatusOr<CopyDetector> a;
+  a = status_or;
+  EXPECT_THAT(a, IsOkAndHolds(CopyDetectorHas(10, false, true)));
+  const absl::StatusOr<CopyDetector>& cref = status_or;
+  absl::StatusOr<CopyDetector> b;
+  b = cref;
+  EXPECT_THAT(b, IsOkAndHolds(CopyDetectorHas(10, false, true)));
+  absl::StatusOr<CopyDetector> c;
+  c = std::move(status_or);
+  EXPECT_THAT(c, IsOkAndHolds(CopyDetectorHas(10, true, false)));
+}
+
+TEST(StatusOr, AbslAnyAssignment) {
+  EXPECT_FALSE(
+      (std::is_assignable_v<absl::StatusOr<std::any>, absl::StatusOr<int>>));
+  absl::StatusOr<std::any> status_or;
+  status_or = absl::InvalidArgumentError("foo");
+  EXPECT_THAT(status_or, Not(IsOk()));
+}
+
+TEST(StatusOr, ImplicitAssignment) {
+  absl::StatusOr<std::variant<int, std::string>> status_or;
+  status_or = 10;
+  EXPECT_THAT(status_or, IsOkAndHolds(VariantWith<int>(10)));
+}
+
+TEST(StatusOr, SelfDirectInitAssignment) {
+  absl::StatusOr<std::vector<int>> status_or = {{10, 20, 30}};
+  status_or = *status_or;
+  EXPECT_THAT(status_or, IsOkAndHolds(ElementsAre(10, 20, 30)));
+}
+
+TEST(StatusOr, ImplicitCastFromInitializerList) {
+  absl::StatusOr<std::vector<int>> status_or = {{10, 20, 30}};
+  EXPECT_THAT(status_or, IsOkAndHolds(ElementsAre(10, 20, 30)));
+}
+
+TEST(StatusOr, UniquePtrImplicitAssignment) {
+  absl::StatusOr<std::unique_ptr<Base1>> status_or;
+  status_or = std::make_unique<Derived>();
+  EXPECT_THAT(status_or, IsOkAndHolds(Ne(nullptr)));
+}
+
+TEST(StatusOr, Pointer) {
+  struct A {};
+  struct B : public A {};
+  struct C : private A {};
+
+  EXPECT_TRUE((std::is_constructible_v<absl::StatusOr<A*>, B*>));
+  EXPECT_TRUE((std::is_convertible_v<B*, absl::StatusOr<A*>>));
+  EXPECT_FALSE((std::is_constructible_v<absl::StatusOr<A*>, C*>));
+  EXPECT_FALSE((std::is_convertible_v<C*, absl::StatusOr<A*>>));
+}
+
+TEST(StatusOr, TestAssignmentStatusNotOkConverting) {
+  // Copy assignment
+  {
+    const absl::Status expected = absl::CancelledError();
+    absl::StatusOr<int> source(expected);
+
+    absl::StatusOr<double> target;
+    target = source;
+
+    EXPECT_FALSE(target.ok());
+    EXPECT_EQ(expected, target.status());
+
+    EXPECT_FALSE(source.ok());
+    EXPECT_EQ(expected, source.status());
+  }
+
+  // Move assignment
+  {
+    const absl::Status expected = absl::CancelledError();
+    absl::StatusOr<int> source(expected);
+
+    absl::StatusOr<double> target;
+    target = std::move(source);
+
+    EXPECT_FALSE(target.ok());
+    EXPECT_EQ(expected, target.status());
+
+    EXPECT_FALSE(source.ok());
+    EXPECT_EQ(source.status().code(), absl::StatusCode::kInternal);
+  }
+}
+
+TEST(StatusOr, SelfAssignment) {
+  // Copy-assignment, status OK
+  {
+    // A string long enough that it's likely to defeat any inline representation
+    // optimization.
+    const std::string long_str(128, 'a');
+
+    absl::StatusOr<std::string> so = long_str;
+    so = *&so;
+
+    ASSERT_TRUE(so.ok());
+    EXPECT_THAT(so.status(), IsOk());
+    EXPECT_EQ(long_str, *so);
+  }
+
+  // Copy-assignment, error status
+  {
+    absl::StatusOr<int> so = absl::NotFoundError("taco");
+    so = *&so;
+
+    EXPECT_FALSE(so.ok());
+    EXPECT_EQ(so.status().code(), absl::StatusCode::kNotFound);
+    EXPECT_EQ(so.status().message(), "taco");
+  }
+
+  // Move-assignment with copyable type, status OK
+  {
+    absl::StatusOr<int> so = 17;
+
+    // Fool the compiler, which otherwise complains.
+    auto& same = so;
+    so = std::move(same);
+
+    ASSERT_TRUE(so.ok());
+    EXPECT_THAT(so.status(), IsOk());
+    EXPECT_EQ(17, *so);
+  }
+
+  // Move-assignment with copyable type, error status
+  {
+    absl::StatusOr<int> so = absl::NotFoundError("taco");
+
+    // Fool the compiler, which otherwise complains.
+    auto& same = so;
+    so = std::move(same);
+
+    EXPECT_FALSE(so.ok());
+    EXPECT_EQ(so.status().code(), absl::StatusCode::kNotFound);
+    EXPECT_EQ(so.status().message(), "taco");
+  }
+
+  // Move-assignment with non-copyable type, status OK
+  {
+    const auto raw = new int(17);
+    absl::StatusOr<std::unique_ptr<int>> so = absl::WrapUnique(raw);
+
+    // Fool the compiler, which otherwise complains.
+    auto& same = so;
+    so = std::move(same);
+
+    ASSERT_TRUE(so.ok());
+    EXPECT_THAT(so.status(), IsOk());
+    EXPECT_EQ(raw, so->get());
+  }
+
+  // Move-assignment with non-copyable type, error status
+  {
+    absl::StatusOr<std::unique_ptr<int>> so = absl::NotFoundError("taco");
+
+    // Fool the compiler, which otherwise complains.
+    auto& same = so;
+    so = std::move(same);
+
+    EXPECT_FALSE(so.ok());
+    EXPECT_EQ(so.status().code(), absl::StatusCode::kNotFound);
+    EXPECT_EQ(so.status().message(), "taco");
+  }
+}
+
+// These types form the overload sets of the constructors and the assignment
+// operators of `MockValue`. They distinguish construction from assignment,
+// lvalue from rvalue.
+struct FromConstructibleAssignableLvalue {};
+struct FromConstructibleAssignableRvalue {};
+struct FromImplicitConstructibleOnly {};
+struct FromAssignableOnly {};
+
+// This class is for testing the forwarding value assignments of `StatusOr`.
+// `from_rvalue` indicates whether the constructor or the assignment taking
+// rvalue reference is called. `from_assignment` indicates whether any
+// assignment is called.
+struct MockValue {
+  // Constructs `MockValue` from `FromConstructibleAssignableLvalue`.
+  MockValue(const FromConstructibleAssignableLvalue&)  // NOLINT
+      : from_rvalue(false), assigned(false) {}
+  // Constructs `MockValue` from `FromConstructibleAssignableRvalue`.
+  MockValue(FromConstructibleAssignableRvalue&&)  // NOLINT
+      : from_rvalue(true), assigned(false) {}
+  // Constructs `MockValue` from `FromImplicitConstructibleOnly`.
+  // `MockValue` is not assignable from `FromImplicitConstructibleOnly`.
+  MockValue(const FromImplicitConstructibleOnly&)  // NOLINT
+      : from_rvalue(false), assigned(false) {}
+  // Assigns `FromConstructibleAssignableLvalue`.
+  MockValue& operator=(const FromConstructibleAssignableLvalue&) {
+    from_rvalue = false;
+    assigned = true;
+    return *this;
+  }
+  // Assigns `FromConstructibleAssignableRvalue` (rvalue only).
+  MockValue& operator=(FromConstructibleAssignableRvalue&&) {
+    from_rvalue = true;
+    assigned = true;
+    return *this;
+  }
+  // Assigns `FromAssignableOnly`, but not constructible from
+  // `FromAssignableOnly`.
+  MockValue& operator=(const FromAssignableOnly&) {
+    from_rvalue = false;
+    assigned = true;
+    return *this;
+  }
+  bool from_rvalue;
+  bool assigned;
+};
+
+// operator=(U&&)
+TEST(StatusOr, PerfectForwardingAssignment) {
+  // U == T
+  constexpr int kValue1 = 10, kValue2 = 20;
+  absl::StatusOr<CopyDetector> status_or;
+  CopyDetector lvalue(kValue1);
+  status_or = lvalue;
+  EXPECT_THAT(status_or, IsOkAndHolds(CopyDetectorHas(kValue1, false, true)));
+  status_or = CopyDetector(kValue2);
+  EXPECT_THAT(status_or, IsOkAndHolds(CopyDetectorHas(kValue2, true, false)));
+
+  // U != T
+  EXPECT_TRUE((std::is_assignable_v<absl::StatusOr<MockValue>&,
+                                    const FromConstructibleAssignableLvalue&>));
+  EXPECT_TRUE((std::is_assignable_v<absl::StatusOr<MockValue>&,
+                                    FromConstructibleAssignableLvalue&&>));
+  EXPECT_FALSE(
+      (std::is_assignable_v<absl::StatusOr<MockValue>&,
+                            const FromConstructibleAssignableRvalue&>));
+  EXPECT_TRUE((std::is_assignable_v<absl::StatusOr<MockValue>&,
+                                    FromConstructibleAssignableRvalue&&>));
+  EXPECT_TRUE((std::is_assignable_v<absl::StatusOr<MockValue>&,
+                                    const FromImplicitConstructibleOnly&>));
+  EXPECT_FALSE((std::is_assignable_v<absl::StatusOr<MockValue>&,
+                                     const FromAssignableOnly&>));
+
+  absl::StatusOr<MockValue> from_lvalue(FromConstructibleAssignableLvalue{});
+  EXPECT_FALSE(from_lvalue->from_rvalue);
+  EXPECT_FALSE(from_lvalue->assigned);
+  from_lvalue = FromConstructibleAssignableLvalue{};
+  EXPECT_FALSE(from_lvalue->from_rvalue);
+  EXPECT_TRUE(from_lvalue->assigned);
+
+  absl::StatusOr<MockValue> from_rvalue(FromConstructibleAssignableRvalue{});
+  EXPECT_TRUE(from_rvalue->from_rvalue);
+  EXPECT_FALSE(from_rvalue->assigned);
+  from_rvalue = FromConstructibleAssignableRvalue{};
+  EXPECT_TRUE(from_rvalue->from_rvalue);
+  EXPECT_TRUE(from_rvalue->assigned);
+
+  absl::StatusOr<MockValue> from_implicit_constructible(
+      FromImplicitConstructibleOnly{});
+  EXPECT_FALSE(from_implicit_constructible->from_rvalue);
+  EXPECT_FALSE(from_implicit_constructible->assigned);
+  // construct a temporary `StatusOr` object and invoke the `StatusOr` move
+  // assignment operator.
+  from_implicit_constructible = FromImplicitConstructibleOnly{};
+  EXPECT_FALSE(from_implicit_constructible->from_rvalue);
+  EXPECT_FALSE(from_implicit_constructible->assigned);
+}
+
+TEST(StatusOr, TestStatus) {
+  absl::StatusOr<int> good(4);
+  EXPECT_TRUE(good.ok());
+  absl::StatusOr<int> bad(absl::CancelledError());
+  EXPECT_FALSE(bad.ok());
+  EXPECT_EQ(bad.status().code(), absl::StatusCode::kCancelled);
+}
+
+TEST(StatusOr, OperatorStarRefQualifiers) {
+  static_assert(
+      std::is_same<const int&,
+                   decltype(*std::declval<const absl::StatusOr<int>&>())>(),
+      "Unexpected ref-qualifiers");
+  static_assert(
+      std::is_same<int&, decltype(*std::declval<absl::StatusOr<int>&>())>(),
+      "Unexpected ref-qualifiers");
+  static_assert(
+      std::is_same<const int&&,
+                   decltype(*std::declval<const absl::StatusOr<int>&&>())>(),
+      "Unexpected ref-qualifiers");
+  static_assert(
+      std::is_same<int&&, decltype(*std::declval<absl::StatusOr<int>&&>())>(),
+      "Unexpected ref-qualifiers");
+}
+
+TEST(StatusOr, OperatorStar) {
+  const absl::StatusOr<std::string> const_lvalue("hello");
+  EXPECT_EQ("hello", *const_lvalue);
+
+  absl::StatusOr<std::string> lvalue("hello");
+  EXPECT_EQ("hello", *lvalue);
+
+  // Note: Recall that std::move() is equivalent to a static_cast to an rvalue
+  // reference type.
+  const absl::StatusOr<std::string> const_rvalue("hello");
+  EXPECT_EQ("hello", *std::move(const_rvalue));  // NOLINT
+
+  absl::StatusOr<std::string> rvalue("hello");
+  EXPECT_EQ("hello", *std::move(rvalue));
+}
+
+TEST(StatusOr, OperatorArrowQualifiers) {
+  static_assert(
+      std::is_same<
+          const int*,
+          decltype(std::declval<const absl::StatusOr<int>&>().operator->())>(),
+      "Unexpected qualifiers");
+  static_assert(
+      std::is_same<
+          int*, decltype(std::declval<absl::StatusOr<int>&>().operator->())>(),
+      "Unexpected qualifiers");
+  static_assert(
+      std::is_same<
+          const int*,
+          decltype(std::declval<const absl::StatusOr<int>&&>().operator->())>(),
+      "Unexpected qualifiers");
+  static_assert(
+      std::is_same<
+          int*, decltype(std::declval<absl::StatusOr<int>&&>().operator->())>(),
+      "Unexpected qualifiers");
+}
+
+TEST(StatusOr, OperatorArrow) {
+  const absl::StatusOr<std::string> const_lvalue("hello");
+  EXPECT_EQ(std::string("hello"), const_lvalue->c_str());
+
+  absl::StatusOr<std::string> lvalue("hello");
+  EXPECT_EQ(std::string("hello"), lvalue->c_str());
+}
+
+TEST(StatusOr, RValueStatus) {
+  absl::StatusOr<int> so(absl::NotFoundError("taco"));
+  const absl::Status s = std::move(so).status();
+
+  EXPECT_EQ(s.code(), absl::StatusCode::kNotFound);
+  EXPECT_EQ(s.message(), "taco");
+
+  // Check that !ok() still implies !status().ok(), even after moving out of the
+  // object. See the note on the rvalue ref-qualified status method.
+  EXPECT_FALSE(so.ok());  // NOLINT
+  EXPECT_FALSE(so.status().ok());
+  EXPECT_EQ(so.status().code(), absl::StatusCode::kInternal);
+  EXPECT_EQ(so.status().message(), "Status accessed after move.");
+}
+
+TEST(StatusOr, TestValue) {
+  const int kI = 4;
+  absl::StatusOr<int> thing(kI);
+  EXPECT_EQ(kI, *thing);
+}
+
+TEST(StatusOr, TestValueConst) {
+  const int kI = 4;
+  const absl::StatusOr<int> thing(kI);
+  EXPECT_EQ(kI, *thing);
+}
+
+TEST(StatusOr, TestPointerDefaultCtor) {
+  absl::StatusOr<int*> thing;
+  EXPECT_FALSE(thing.ok());
+  EXPECT_EQ(thing.status().code(), absl::StatusCode::kUnknown);
+}
+
+TEST(StatusOr, TestPointerStatusCtor) {
+  absl::StatusOr<int*> thing(absl::CancelledError());
+  EXPECT_FALSE(thing.ok());
+  EXPECT_EQ(thing.status().code(), absl::StatusCode::kCancelled);
+}
+
+TEST(StatusOr, TestPointerValueCtor) {
+  const int kI = 4;
+
+  // Construction from a non-null pointer
+  {
+    absl::StatusOr<const int*> so(&kI);
+    EXPECT_TRUE(so.ok());
+    EXPECT_THAT(so.status(), IsOk());
+    EXPECT_EQ(&kI, *so);
+  }
+
+  // Construction from a null pointer constant
+  {
+    absl::StatusOr<const int*> so(nullptr);
+    EXPECT_TRUE(so.ok());
+    EXPECT_THAT(so.status(), IsOk());
+    EXPECT_EQ(nullptr, *so);
+  }
+
+  // Construction from a non-literal null pointer
+  {
+    const int* const p = nullptr;
+
+    absl::StatusOr<const int*> so(p);
+    EXPECT_TRUE(so.ok());
+    EXPECT_THAT(so.status(), IsOk());
+    EXPECT_EQ(nullptr, *so);
+  }
+}
+
+TEST(StatusOr, TestPointerCopyCtorStatusOk) {
+  const int kI = 0;
+  absl::StatusOr<const int*> original(&kI);
+  absl::StatusOr<const int*> copy(original);
+  EXPECT_THAT(copy.status(), IsOk());
+  EXPECT_EQ(*original, *copy);
+}
+
+TEST(StatusOr, TestPointerCopyCtorStatusNotOk) {
+  absl::StatusOr<int*> original(absl::CancelledError());
+  absl::StatusOr<int*> copy(original);
+  EXPECT_EQ(copy.status().code(), absl::StatusCode::kCancelled);
+}
+
+TEST(StatusOr, TestPointerCopyCtorStatusOKConverting) {
+  Derived derived;
+  absl::StatusOr<Derived*> original(&derived);
+  absl::StatusOr<Base2*> copy(original);
+  EXPECT_THAT(copy.status(), IsOk());
+  EXPECT_EQ(static_cast<const Base2*>(*original), *copy);
+}
+
+TEST(StatusOr, TestPointerCopyCtorStatusNotOkConverting) {
+  absl::StatusOr<Derived*> original(absl::CancelledError());
+  absl::StatusOr<Base2*> copy(original);
+  EXPECT_EQ(copy.status().code(), absl::StatusCode::kCancelled);
+}
+
+TEST(StatusOr, TestPointerAssignmentStatusOk) {
+  const int kI = 0;
+  absl::StatusOr<const int*> source(&kI);
+  absl::StatusOr<const int*> target;
+  target = source;
+  EXPECT_THAT(target.status(), IsOk());
+  EXPECT_EQ(*source, *target);
+}
+
+TEST(StatusOr, TestPointerAssignmentStatusNotOk) {
+  absl::StatusOr<int*> source(absl::CancelledError());
+  absl::StatusOr<int*> target;
+  target = source;
+  EXPECT_EQ(target.status().code(), absl::StatusCode::kCancelled);
+}
+
+TEST(StatusOr, TestPointerAssignmentStatusOKConverting) {
+  Derived derived;
+  absl::StatusOr<Derived*> source(&derived);
+  absl::StatusOr<Base2*> target;
+  target = source;
+  EXPECT_THAT(target.status(), IsOk());
+  EXPECT_EQ(static_cast<const Base2*>(*source), *target);
+}
+
+TEST(StatusOr, TestPointerAssignmentStatusNotOkConverting) {
+  absl::StatusOr<Derived*> source(absl::CancelledError());
+  absl::StatusOr<Base2*> target;
+  target = source;
+  EXPECT_EQ(target.status(), source.status());
+}
+
+TEST(StatusOr, TestPointerStatus) {
+  const int kI = 0;
+  absl::StatusOr<const int*> good(&kI);
+  EXPECT_TRUE(good.ok());
+  absl::StatusOr<const int*> bad(absl::CancelledError());
+  EXPECT_EQ(bad.status().code(), absl::StatusCode::kCancelled);
+}
+
+TEST(StatusOr, TestPointerValue) {
+  const int kI = 0;
+  absl::StatusOr<const int*> thing(&kI);
+  EXPECT_EQ(&kI, *thing);
+}
+
+TEST(StatusOr, TestPointerValueConst) {
+  const int kI = 0;
+  const absl::StatusOr<const int*> thing(&kI);
+  EXPECT_EQ(&kI, *thing);
+}
+
+TEST(StatusOr, StatusOrVectorOfUniquePointerCanReserveAndResize) {
+  using EvilType = std::vector<std::unique_ptr<int>>;
+  static_assert(std::is_copy_constructible_v<EvilType>);
+  std::vector<::absl::StatusOr<EvilType>> v(5);
+  v.reserve(v.capacity() + 10);
+  v.resize(v.capacity() + 10);
+}
+
+TEST(StatusOr, ConstPayload) {
+  // A reduced version of a problematic type found in the wild. All of the
+  // operations below should compile.
+  absl::StatusOr<const int> a;
+
+  // Copy-construction
+  absl::StatusOr<const int> b(a);
+
+  // Copy-assignment
+  EXPECT_FALSE(std::is_copy_assignable_v<absl::StatusOr<const int>>);
+
+  // Move-construction
+  absl::StatusOr<const int> c(std::move(a));
+
+  // Move-assignment
+  EXPECT_FALSE(std::is_move_assignable_v<absl::StatusOr<const int>>);
+}
+
+TEST(StatusOr, MapToStatusOrUniquePtr) {
+  // A reduced version of a problematic type found in the wild. All of the
+  // operations below should compile.
+  using MapType = std::map<std::string, absl::StatusOr<std::unique_ptr<int>>>;
+
+  MapType a;
+
+  // Move-construction
+  MapType b(std::move(a));
+
+  // Move-assignment
+  a = std::move(b);
+}
+
+TEST(StatusOr, ValueOrOk) {
+  const absl::StatusOr<int> status_or = 0;
+  EXPECT_EQ(status_or.value_or(-1), 0);
+}
+
+TEST(StatusOr, ValueOrDefault) {
+  const absl::StatusOr<int> status_or = absl::CancelledError();
+  EXPECT_EQ(status_or.value_or(-1), -1);
+}
+
+TEST(StatusOr, MoveOnlyValueOrOk) {
+  EXPECT_THAT(absl::StatusOr<std::unique_ptr<int>>(std::make_unique<int>(0))
+                  .value_or(std::make_unique<int>(-1)),
+              Pointee(0));
+}
+
+TEST(StatusOr, MoveOnlyValueOrDefault) {
+  EXPECT_THAT(absl::StatusOr<std::unique_ptr<int>>(absl::CancelledError())
+                  .value_or(std::make_unique<int>(-1)),
+              Pointee(-1));
+}
+
+static absl::StatusOr<int> MakeStatus() { return 100; }
+
+TEST(StatusOr, TestIgnoreError) { MakeStatus().IgnoreError(); }
+
+TEST(StatusOr, EqualityOperator) {
+  constexpr size_t kNumCases = 4;
+  std::array<absl::StatusOr<int>, kNumCases> group1 = {
+      absl::StatusOr<int>(1), absl::StatusOr<int>(2),
+      absl::StatusOr<int>(absl::InvalidArgumentError("msg")),
+      absl::StatusOr<int>(absl::InternalError("msg"))};
+  std::array<absl::StatusOr<int>, kNumCases> group2 = {
+      absl::StatusOr<int>(1), absl::StatusOr<int>(2),
+      absl::StatusOr<int>(absl::InvalidArgumentError("msg")),
+      absl::StatusOr<int>(absl::InternalError("msg"))};
+  for (size_t i = 0; i < kNumCases; ++i) {
+    for (size_t j = 0; j < kNumCases; ++j) {
+      if (i == j) {
+        EXPECT_TRUE(group1[i] == group2[j]);
+        EXPECT_FALSE(group1[i] != group2[j]);
+      } else {
+        EXPECT_FALSE(group1[i] == group2[j]);
+        EXPECT_TRUE(group1[i] != group2[j]);
+      }
+    }
+  }
+}
+
+struct MyType {
+  bool operator==(const MyType&) const { return true; }
+};
+
+enum class ConvTraits { kNone = 0, kImplicit = 1, kExplicit = 2 };
+
+// This class has conversion operator to `StatusOr<T>` based on value of
+// `conv_traits`.
+template <typename T, ConvTraits conv_traits = ConvTraits::kNone>
+struct StatusOrConversionBase {};
+
+template <typename T>
+struct StatusOrConversionBase<T, ConvTraits::kImplicit> {
+  operator absl::StatusOr<T>() const& {  // NOLINT
+    return absl::InvalidArgumentError("conversion to absl::StatusOr");
+  }
+  operator absl::StatusOr<T>() && {  // NOLINT
+    return absl::InvalidArgumentError("conversion to absl::StatusOr");
+  }
+};
+
+template <typename T>
+struct StatusOrConversionBase<T, ConvTraits::kExplicit> {
+  explicit operator absl::StatusOr<T>() const& {
+    return absl::InvalidArgumentError("conversion to absl::StatusOr");
+  }
+  explicit operator absl::StatusOr<T>() && {
+    return absl::InvalidArgumentError("conversion to absl::StatusOr");
+  }
+};
+
+// This class has conversion operator to `T` based on the value of
+// `conv_traits`.
+template <typename T, ConvTraits conv_traits = ConvTraits::kNone>
+struct ConversionBase {};
+
+template <typename T>
+struct ConversionBase<T, ConvTraits::kImplicit> {
+  operator T() const& { return t; }         // NOLINT
+  operator T() && { return std::move(t); }  // NOLINT
+  T t;
+};
+
+template <typename T>
+struct ConversionBase<T, ConvTraits::kExplicit> {
+  explicit operator T() const& { return t; }
+  explicit operator T() && { return std::move(t); }
+  T t;
+};
+
+// This class has conversion operator to `absl::Status` based on the value of
+// `conv_traits`.
+template <ConvTraits conv_traits = ConvTraits::kNone>
+struct StatusConversionBase {};
+
+template <>
+struct StatusConversionBase<ConvTraits::kImplicit> {
+  operator absl::Status() const& {  // NOLINT
+    return absl::InternalError("conversion to Status");
+  }
+  operator absl::Status() && {  // NOLINT
+    return absl::InternalError("conversion to Status");
+  }
+};
+
+template <>
+struct StatusConversionBase<ConvTraits::kExplicit> {
+  explicit operator absl::Status() const& {  // NOLINT
+    return absl::InternalError("conversion to Status");
+  }
+  explicit operator absl::Status() && {  // NOLINT
+    return absl::InternalError("conversion to Status");
+  }
+};
+
+static constexpr int kConvToStatus = 1;
+static constexpr int kConvToStatusOr = 2;
+static constexpr int kConvToT = 4;
+static constexpr int kConvExplicit = 8;
+
+constexpr ConvTraits GetConvTraits(int bit, int config) {
+  return (config & bit) == 0
+             ? ConvTraits::kNone
+             : ((config & kConvExplicit) == 0 ? ConvTraits::kImplicit
+                                              : ConvTraits::kExplicit);
+}
+
+// This class conditionally has conversion operator to `absl::Status`, `T`,
+// `StatusOr<T>`, based on values of the template parameters.
+template <typename T, int config>
+struct CustomType
+    : StatusOrConversionBase<T, GetConvTraits(kConvToStatusOr, config)>,
+      ConversionBase<T, GetConvTraits(kConvToT, config)>,
+      StatusConversionBase<GetConvTraits(kConvToStatus, config)> {};
+
+struct ConvertibleToAnyStatusOr {
+  template <typename T>
+  operator absl::StatusOr<T>() const {  // NOLINT
+    return absl::InvalidArgumentError("Conversion to absl::StatusOr");
+  }
+};
+
+// Test the rank of overload resolution for `StatusOr<T>` constructor and
+// assignment, from highest to lowest:
+// 1. T/Status
+// 2. U that has conversion operator to absl::StatusOr<T>
+// 3. U that is convertible to Status
+// 4. U that is convertible to T
+TEST(StatusOr, ConstructionFromT) {
+  // Construct absl::StatusOr<T> from T when T is convertible to
+  // absl::StatusOr<T>
+  {
+    ConvertibleToAnyStatusOr v;
+    absl::StatusOr<ConvertibleToAnyStatusOr> statusor(v);
+    EXPECT_TRUE(statusor.ok());
+  }
+  {
+    ConvertibleToAnyStatusOr v;
+    absl::StatusOr<ConvertibleToAnyStatusOr> statusor = v;
+    EXPECT_TRUE(statusor.ok());
+  }
+  // Construct absl::StatusOr<T> from T when T is explicitly convertible to
+  // Status
+  {
+    CustomType<MyType, kConvToStatus | kConvExplicit> v;
+    absl::StatusOr<CustomType<MyType, kConvToStatus | kConvExplicit>> statusor(
+        v);
+    EXPECT_TRUE(statusor.ok());
+  }
+  {
+    CustomType<MyType, kConvToStatus | kConvExplicit> v;
+    absl::StatusOr<CustomType<MyType, kConvToStatus | kConvExplicit>> statusor =
+        v;
+    EXPECT_TRUE(statusor.ok());
+  }
+}
+
+// Construct absl::StatusOr<T> from U when U is explicitly convertible to T
+TEST(StatusOr, ConstructionFromTypeConvertibleToT) {
+  {
+    CustomType<MyType, kConvToT | kConvExplicit> v;
+    absl::StatusOr<MyType> statusor(v);
+    EXPECT_TRUE(statusor.ok());
+  }
+  {
+    CustomType<MyType, kConvToT> v;
+    absl::StatusOr<MyType> statusor = v;
+    EXPECT_TRUE(statusor.ok());
+  }
+}
+
+// Construct absl::StatusOr<T> from U when U has explicit conversion operator to
+// absl::StatusOr<T>
+TEST(StatusOr, ConstructionFromTypeWithConversionOperatorToStatusOrT) {
+  {
+    CustomType<MyType, kConvToStatusOr | kConvExplicit> v;
+    absl::StatusOr<MyType> statusor(v);
+    EXPECT_EQ(statusor, v.operator absl::StatusOr<MyType>());
+  }
+  {
+    CustomType<MyType, kConvToT | kConvToStatusOr | kConvExplicit> v;
+    absl::StatusOr<MyType> statusor(v);
+    EXPECT_EQ(statusor, v.operator absl::StatusOr<MyType>());
+  }
+  {
+    CustomType<MyType, kConvToStatusOr | kConvToStatus | kConvExplicit> v;
+    absl::StatusOr<MyType> statusor(v);
+    EXPECT_EQ(statusor, v.operator absl::StatusOr<MyType>());
+  }
+  {
+    CustomType<MyType,
+               kConvToT | kConvToStatusOr | kConvToStatus | kConvExplicit>
+        v;
+    absl::StatusOr<MyType> statusor(v);
+    EXPECT_EQ(statusor, v.operator absl::StatusOr<MyType>());
+  }
+  {
+    CustomType<MyType, kConvToStatusOr> v;
+    absl::StatusOr<MyType> statusor = v;
+    EXPECT_EQ(statusor, v.operator absl::StatusOr<MyType>());
+  }
+  {
+    CustomType<MyType, kConvToT | kConvToStatusOr> v;
+    absl::StatusOr<MyType> statusor = v;
+    EXPECT_EQ(statusor, v.operator absl::StatusOr<MyType>());
+  }
+  {
+    CustomType<MyType, kConvToStatusOr | kConvToStatus> v;
+    absl::StatusOr<MyType> statusor = v;
+    EXPECT_EQ(statusor, v.operator absl::StatusOr<MyType>());
+  }
+  {
+    CustomType<MyType, kConvToT | kConvToStatusOr | kConvToStatus> v;
+    absl::StatusOr<MyType> statusor = v;
+    EXPECT_EQ(statusor, v.operator absl::StatusOr<MyType>());
+  }
+}
+
+TEST(StatusOr, ConstructionFromTypeConvertibleToStatus) {
+  // Construction fails because conversion to `Status` is explicit.
+  {
+    CustomType<MyType, kConvToStatus | kConvExplicit> v;
+    absl::StatusOr<MyType> statusor(v);
+    EXPECT_FALSE(statusor.ok());
+    EXPECT_EQ(statusor.status(), static_cast<absl::Status>(v));
+  }
+  {
+    CustomType<MyType, kConvToT | kConvToStatus | kConvExplicit> v;
+    absl::StatusOr<MyType> statusor(v);
+    EXPECT_FALSE(statusor.ok());
+    EXPECT_EQ(statusor.status(), static_cast<absl::Status>(v));
+  }
+  {
+    CustomType<MyType, kConvToStatus> v;
+    absl::StatusOr<MyType> statusor = v;
+    EXPECT_FALSE(statusor.ok());
+    EXPECT_EQ(statusor.status(), static_cast<absl::Status>(v));
+  }
+  {
+    CustomType<MyType, kConvToT | kConvToStatus> v;
+    absl::StatusOr<MyType> statusor = v;
+    EXPECT_FALSE(statusor.ok());
+    EXPECT_EQ(statusor.status(), static_cast<absl::Status>(v));
+  }
+}
+
+TEST(StatusOr, AssignmentFromT) {
+  // Assign to absl::StatusOr<T> from T when T is convertible to
+  // absl::StatusOr<T>
+  {
+    ConvertibleToAnyStatusOr v;
+    absl::StatusOr<ConvertibleToAnyStatusOr> statusor;
+    statusor = v;
+    EXPECT_TRUE(statusor.ok());
+  }
+  // Assign to absl::StatusOr<T> from T when T is convertible to Status
+  {
+    CustomType<MyType, kConvToStatus> v;
+    absl::StatusOr<CustomType<MyType, kConvToStatus>> statusor;
+    statusor = v;
+    EXPECT_TRUE(statusor.ok());
+  }
+}
+
+TEST(StatusOr, AssignmentFromTypeConvertibleToT) {
+  // Assign to absl::StatusOr<T> from U when U is convertible to T
+  {
+    CustomType<MyType, kConvToT> v;
+    absl::StatusOr<MyType> statusor;
+    statusor = v;
+    EXPECT_TRUE(statusor.ok());
+  }
+}
+
+TEST(StatusOr, AssignmentFromTypeWithConversionOperatortoStatusOrT) {
+  // Assign to absl::StatusOr<T> from U when U has conversion operator to
+  // absl::StatusOr<T>
+  {
+    CustomType<MyType, kConvToStatusOr> v;
+    absl::StatusOr<MyType> statusor;
+    statusor = v;
+    EXPECT_EQ(statusor, v.operator absl::StatusOr<MyType>());
+  }
+  {
+    CustomType<MyType, kConvToT | kConvToStatusOr> v;
+    absl::StatusOr<MyType> statusor;
+    statusor = v;
+    EXPECT_EQ(statusor, v.operator absl::StatusOr<MyType>());
+  }
+  {
+    CustomType<MyType, kConvToStatusOr | kConvToStatus> v;
+    absl::StatusOr<MyType> statusor;
+    statusor = v;
+    EXPECT_EQ(statusor, v.operator absl::StatusOr<MyType>());
+  }
+  {
+    CustomType<MyType, kConvToT | kConvToStatusOr | kConvToStatus> v;
+    absl::StatusOr<MyType> statusor;
+    statusor = v;
+    EXPECT_EQ(statusor, v.operator absl::StatusOr<MyType>());
+  }
+}
+
+TEST(StatusOr, AssignmentFromTypeConvertibleToStatus) {
+  // Assign to absl::StatusOr<T> from U when U is convertible to Status
+  {
+    CustomType<MyType, kConvToStatus> v;
+    absl::StatusOr<MyType> statusor;
+    statusor = v;
+    EXPECT_FALSE(statusor.ok());
+    EXPECT_EQ(statusor.status(), static_cast<absl::Status>(v));
+  }
+  {
+    CustomType<MyType, kConvToT | kConvToStatus> v;
+    absl::StatusOr<MyType> statusor;
+    statusor = v;
+    EXPECT_FALSE(statusor.ok());
+    EXPECT_EQ(statusor.status(), static_cast<absl::Status>(v));
+  }
+}
+
+TEST(StatusOr, StatusAssignmentFromStatusError) {
+  absl::StatusOr<absl::Status> statusor;
+  statusor.AssignStatus(absl::CancelledError());
+
+  EXPECT_FALSE(statusor.ok());
+  EXPECT_EQ(statusor.status(), absl::CancelledError());
+}
+
+#if GTEST_HAS_DEATH_TEST
+TEST(StatusOr, StatusAssignmentFromStatusOk) {
+  EXPECT_DEBUG_DEATH(
+      {
+        absl::StatusOr<absl::Status> statusor;
+        // This will DCHECK.
+        statusor.AssignStatus(absl::OkStatus());
+        // In optimized mode, we are actually going to get error::INTERNAL for
+        // status here, rather than crashing, so check that.
+        EXPECT_FALSE(statusor.ok());
+        EXPECT_EQ(statusor.status().code(), absl::StatusCode::kInternal);
+      },
+      "An OK status is not a valid constructor argument to StatusOr<T>");
+}
+#endif
+
+TEST(StatusOr, StatusAssignmentFromTypeConvertibleToStatus) {
+  CustomType<MyType, kConvToStatus> v;
+  absl::StatusOr<MyType> statusor;
+  statusor.AssignStatus(v);
+
+  EXPECT_FALSE(statusor.ok());
+  EXPECT_EQ(statusor.status(), static_cast<absl::Status>(v));
+}
+
+struct PrintTestStruct {
+  friend std::ostream& operator<<(std::ostream& os, const PrintTestStruct&) {
+    return os << "ostream";
+  }
+
+  template <typename Sink>
+  friend void AbslStringify(Sink& sink, const PrintTestStruct&) {
+    sink.Append("stringify");
+  }
+};
+
+TEST(StatusOr, OkPrinting) {
+  absl::StatusOr<PrintTestStruct> print_me = PrintTestStruct{};
+  std::stringstream stream;
+  stream << print_me;
+  EXPECT_EQ(stream.str(), "ostream");
+  EXPECT_EQ(absl::StrCat(print_me), "stringify");
+}
+
+TEST(StatusOr, ErrorPrinting) {
+  absl::StatusOr<PrintTestStruct> print_me = absl::UnknownError("error");
+  std::stringstream stream;
+  stream << print_me;
+  const auto error_matcher =
+      AllOf(HasSubstr("UNKNOWN"), HasSubstr("error"),
+            AnyOf(AllOf(StartsWith("("), EndsWith(")")),
+                  AllOf(StartsWith("["), EndsWith("]"))));
+  EXPECT_THAT(stream.str(), error_matcher);
+  EXPECT_THAT(absl::StrCat(print_me), error_matcher);
+}
+
+#ifdef ABSL_INTERNAL_HAVE_BUILTIN_LINE_FILE
+#define GET_SOURCE_LOCATION(offset) __builtin_LINE() - offset
+#else
+#define GET_SOURCE_LOCATION(offset) 1
+#endif
+
+template <typename T>
+void CheckSourceLocation(
+    const absl::StatusOr<T>& status_or, std::vector<int> lines = {},
+    absl::SourceLocation loc = absl::SourceLocation::current()) {
+  ASSERT_EQ(status_or.GetSourceLocations().size(), lines.size())
+      << "Size check failed at " << loc.line();
+  for (size_t i = 0; i < lines.size(); ++i) {
+    EXPECT_EQ(absl::string_view(status_or.GetSourceLocations()[i].file_name()),
+              absl::string_view(loc.file_name()))
+        << "File name check failed at " << loc.line();
+    EXPECT_EQ(status_or.GetSourceLocations()[i].line(), lines[i])
+        << "Line check failed at " << loc.line();
+  }
+}
+
+TEST(StatusOr, AddSourceLocation) {
+  constexpr int kMaxIter = 10;
+  {
+    // Status that ignores source location.
+    absl::StatusOr<int> status_ignores_source_location[] = {
+        123, absl::Status(absl::StatusCode::kInternal, "")};
+    for (absl::StatusOr<int>& s : status_ignores_source_location) {
+      for (int i = 0; i < kMaxIter; ++i) {
+        s.AddSourceLocation(absl::SourceLocation::current());
+        s.AddSourceLocation(absl::SourceLocation());
+      }
+      CheckSourceLocation(s);
+    }
+  }
+  {
+    // Default SourceLocation is not added.
+    absl::StatusOr<int> status = absl::Status(
+        absl::StatusCode::kInternal, "foo", absl::SourceLocation::current());
+    int line = GET_SOURCE_LOCATION(1);
+    for (int i = 0; i < kMaxIter; ++i) {
+      status.AddSourceLocation(absl::SourceLocation());
+    }
+    CheckSourceLocation(status, {line});
+  }
+  {
+    // Default SourceLocation is not added.
+    absl::StatusOr<int> status = absl::Status(
+        absl::StatusCode::kInternal, "foo", absl::SourceLocation::current());
+    int line = GET_SOURCE_LOCATION(1);
+    std::vector<int> lines = {line};
+    lines.reserve(1 + kMaxIter);
+    for (int i = 0; i < kMaxIter; ++i) {
+      status.AddSourceLocation(absl::SourceLocation::current());
+      lines.push_back(GET_SOURCE_LOCATION(1));
+    }
+    CheckSourceLocation(status, lines);
+  }
+}
+
+absl::StatusOr<int>&& IsRvalueStatus(absl::StatusOr<int>&& s) {
+  return std::move(s);
+}
+
+TEST(StatusOr, WithSourceLocationMove) {
+  absl::StatusOr<int> original = absl::Status(
+      absl::StatusCode::kInternal, "message", absl::SourceLocation::current());
+  int line = GET_SOURCE_LOCATION(1);
+
+  const absl::StatusOr<int> status_or = IsRvalueStatus(
+      std::move(original).WithSourceLocation(absl::SourceLocation::current()));
+  int line2 = GET_SOURCE_LOCATION(1);
+
+  CheckSourceLocation(status_or, {line, line2});
+  EXPECT_FALSE(status_or.ok());
+}
+
+TEST(StatusOr, WithSourceLocationReturn) {
+  absl::SourceLocation loc1 = absl::SourceLocation::current();
+  int line1 = GET_SOURCE_LOCATION(1);
+  absl::SourceLocation loc2 = absl::SourceLocation::current();
+  int line2 = GET_SOURCE_LOCATION(1);
+
+  const auto return_error = [&loc1]() -> absl::StatusOr<int> {
+    return absl::InvalidArgumentError("I am error", loc1);
+  };
+  const auto return_error_with_source_location =
+      [&return_error, &loc2]() -> absl::StatusOr<int> {
+    return return_error().WithSourceLocation(loc2);
+  };
+
+  absl::StatusOr<int> status_or = return_error_with_source_location();
+  CheckSourceLocation(status_or, {line1, line2});
+  EXPECT_FALSE(status_or.ok());
+}
+
+TEST(StatusOr, SupportsReferenceTypes) {
+  int i = 1;
+  absl::StatusOr<int&> s = i;
+  EXPECT_EQ(&i, &*s);
+  *s = 10;
+  EXPECT_EQ(i, 10);
+}
+
+TEST(StatusOr, ReferenceFromStatus) {
+  int i = 10;
+  absl::StatusOr<int&> s = i;
+  s = absl::InternalError("foo");
+  EXPECT_EQ(s.status().message(), "foo");
+
+  absl::StatusOr<int&> s2 = absl::InternalError("foo2");
+  EXPECT_EQ(s2.status().message(), "foo2");
+}
+
+TEST(StatusOr, SupportReferenceValueConstructor) {
+  int i = 1;
+  absl::StatusOr<int&> s = i;
+  absl::StatusOr<const int&> cs = i;
+  absl::StatusOr<const int&> cs2 = std::move(i);  // `T&&` to `const T&` is ok.
+
+  EXPECT_EQ(&i, &*s);
+  EXPECT_EQ(&i, &*cs);
+
+  Derived d;
+  absl::StatusOr<const Base1&> b = d;
+  EXPECT_EQ(&d, &*b);
+
+  // We disallow constructions that cause temporaries.
+  EXPECT_FALSE((std::is_constructible_v<absl::StatusOr<const int&>, double>));
+  EXPECT_FALSE(
+      (std::is_constructible_v<absl::StatusOr<const int&>, const double&>));
+  EXPECT_FALSE(
+      (std::is_constructible_v<absl::StatusOr<const absl::string_view&>,
+                               std::string>));
+
+  // We disallow constructions with wrong reference.
+  EXPECT_FALSE((std::is_constructible_v<absl::StatusOr<int&>, int&&>));
+  EXPECT_FALSE((std::is_constructible_v<absl::StatusOr<int&>, const int&>));
+}
+
+TEST(StatusOr, SupportReferenceConvertingConstructor) {
+  int i = 1;
+  absl::StatusOr<int&> s = i;
+  absl::StatusOr<const int&> cs = s;
+
+  EXPECT_EQ(&i, &*s);
+  EXPECT_EQ(&i, &*cs);
+
+  // The other direction is not allowed.
+  EXPECT_FALSE((std::is_constructible_v<absl::StatusOr<int&>,
+                                        absl::StatusOr<const int&>>));
+
+  Derived d;
+  absl::StatusOr<const Base1&> b = absl::StatusOr<const Derived&>(d);
+  EXPECT_EQ(&d, &*b);
+
+  // The other direction is not allowed.
+  EXPECT_FALSE((std::is_constructible_v<absl::StatusOr<const Derived&>,
+                                        absl::StatusOr<const Base1&>>));
+
+  // We disallow conversions that cause temporaries.
+  EXPECT_FALSE((std::is_constructible_v<absl::StatusOr<const int&>,
+                                        absl::StatusOr<int>>));
+  EXPECT_FALSE((std::is_constructible_v<absl::StatusOr<const int&>,
+                                        absl::StatusOr<double>>));
+  EXPECT_FALSE((std::is_constructible_v<absl::StatusOr<const int&>,
+                                        absl::StatusOr<const double&>>));
+  EXPECT_FALSE((std::is_constructible_v<absl::StatusOr<const double&>,
+                                        absl::StatusOr<const int&>>));
+  EXPECT_FALSE(
+      (std::is_constructible_v<absl::StatusOr<const absl::string_view&>,
+                               absl::StatusOr<std::string>>));
+
+  // We disallow constructions with wrong reference.
+  EXPECT_FALSE((std::is_constructible_v<absl::StatusOr<int&>,
+                                        absl::StatusOr<const int&>>));
+}
+
+TEST(StatusOr, SupportReferenceValueAssignment) {
+  int i = 1;
+  absl::StatusOr<int&> s = i;
+  absl::StatusOr<const int&> cs;
+  cs = i;
+  absl::StatusOr<const int&> cs2;
+  cs2 = std::move(i);  // `T&&` to `const T&` is ok.
+
+  EXPECT_EQ(&i, &*s);
+  EXPECT_EQ(&i, &*cs);
+
+  Derived d;
+  absl::StatusOr<const Base1&> b;
+  b = d;
+  EXPECT_EQ(&d, &*b);
+
+  // We disallow constructions that cause temporaries.
+  EXPECT_FALSE((std::is_assignable_v<absl::StatusOr<const int&>, double>));
+  EXPECT_FALSE(
+      (std::is_assignable_v<absl::StatusOr<const int&>, const double&>));
+  EXPECT_FALSE((std::is_assignable_v<absl::StatusOr<const absl::string_view&>,
+                                     std::string>));
+
+  // We disallow constructions with wrong reference.
+  EXPECT_FALSE((std::is_assignable_v<absl::StatusOr<int&>, int&&>));
+  EXPECT_FALSE((std::is_assignable_v<absl::StatusOr<int&>, const int&>));
+}
+
+TEST(StatusOr, SupportReferenceConvertingAssignment) {
+  int i = 1;
+  absl::StatusOr<int&> s;
+  s = i;
+  absl::StatusOr<const int&> cs;
+  cs = s;
+
+  EXPECT_EQ(&i, &*s);
+  EXPECT_EQ(&i, &*cs);
+
+  // The other direction is not allowed.
+  EXPECT_FALSE(
+      (std::is_assignable_v<absl::StatusOr<int&>, absl::StatusOr<const int&>>));
+
+  Derived d;
+  absl::StatusOr<const Base1&> b;
+  b = absl::StatusOr<const Derived&>(d);
+  EXPECT_EQ(&d, &*b);
+
+  // The other direction is not allowed.
+  EXPECT_FALSE((std::is_assignable_v<absl::StatusOr<const Derived&>,
+                                     absl::StatusOr<const Base1&>>));
+
+  // We disallow conversions that cause temporaries.
+  EXPECT_FALSE((std::is_assignable_v<absl::StatusOr<const int&>,
+                                     absl::StatusOr<const double&>>));
+  EXPECT_FALSE((std::is_assignable_v<absl::StatusOr<const int&>,
+                                     absl::StatusOr<double>>));
+  EXPECT_FALSE((std::is_assignable_v<absl::StatusOr<const absl::string_view&>,
+                                     absl::StatusOr<std::string>>));
+
+  // We disallow constructions with wrong reference.
+  EXPECT_FALSE(
+      (std::is_assignable_v<absl::StatusOr<int&>, absl::StatusOr<const int&>>));
+}
+
+TEST(StatusOr, SupportReferenceToNonReferenceConversions) {
+  int i = 17;
+  absl::StatusOr<int&> si = i;
+  absl::StatusOr<float> sf = si;
+  EXPECT_THAT(sf, IsOkAndHolds(17.));
+
+  i = 20;
+  sf = si;
+  EXPECT_THAT(sf, IsOkAndHolds(20.));
+
+  EXPECT_THAT(absl::StatusOr<int64_t>(absl::StatusOr<int&>(i)),
+              IsOkAndHolds(20));
+  EXPECT_THAT(absl::StatusOr<int64_t>(absl::StatusOr<const int&>(i)),
+              IsOkAndHolds(20));
+
+  std::string str = "str";
+  absl::StatusOr<std::string> sos = absl::StatusOr<std::string&>(str);
+  EXPECT_THAT(sos, IsOkAndHolds("str"));
+  str = "str2";
+  EXPECT_THAT(sos, IsOkAndHolds("str"));
+  sos = absl::StatusOr<std::string&>(str);
+  EXPECT_THAT(sos, IsOkAndHolds("str2"));
+
+  absl::StatusOr<absl::string_view> sosv = absl::StatusOr<std::string&>(str);
+  EXPECT_THAT(sosv, IsOkAndHolds("str2"));
+  str = "str3";
+  sosv = absl::StatusOr<std::string&>(str);
+  EXPECT_THAT(sosv, IsOkAndHolds("str3"));
+
+  absl::string_view view = "view";
+  // This way it is constructible, but not convertible because
+  // string_view->string is explicit
+  EXPECT_THAT(
+      absl::StatusOr<std::string>(absl::StatusOr<absl::string_view&>(view)),
+      IsOkAndHolds("view"));
+#if defined(ABSL_USES_STD_STRING_VIEW)
+  // The assignment doesn't work with normal absl::string_view because
+  // std::string doesn't know about it.
+  sos = absl::StatusOr<absl::string_view&>(view);
+  EXPECT_THAT(sos, IsOkAndHolds("view"));
+#endif
+
+  EXPECT_FALSE((std::is_convertible_v<absl::StatusOr<absl::string_view&>,
+                                      absl::StatusOr<std::string>>));
+}
+
+TEST(StatusOr, ReferenceOperatorStarAndArrow) {
+  std::string str = "Foo";
+  absl::StatusOr<std::string&> s = str;
+  s->assign("Bar");
+  EXPECT_EQ(str, "Bar");
+
+  *s = "Baz";
+  EXPECT_EQ(str, "Baz");
+
+  const absl::StatusOr<std::string&> cs = str;
+  // Even if the StatusOr is const, the reference it gives is non-const so we
+  // can still assign.
+  *cs = "Finally";
+  EXPECT_EQ(str, "Finally");
+
+  cs->clear();
+  EXPECT_EQ(cs.value(), str);
+  EXPECT_EQ(str, "");
+}
+
+TEST(StatusOr, ReferenceValueOr) {
+  int i = 17;
+  absl::StatusOr<int&> si = i;
+
+  int other = 20;
+  EXPECT_EQ(&i, &si.value_or(other));
+
+  si = absl::UnknownError("");
+  EXPECT_EQ(&other, &si.value_or(other));
+
+  absl::StatusOr<const int&> csi = i;
+  EXPECT_EQ(&i, &csi.value_or(1));
+
+  const auto value_or_call = [](auto&& sor, auto&& v)
+      -> decltype(std::forward<decltype(sor)>(sor).value_or(
+          std::forward<decltype(v)>(v))) {};
+  using Probe = decltype(value_or_call);
+  // Just to verify that Probe works as expected in the good cases.
+  EXPECT_TRUE((std::is_invocable_v<Probe, absl::StatusOr<const int&>, int&&>));
+  // Causes temporary conversion.
+  EXPECT_FALSE(
+      (std::is_invocable_v<Probe, absl::StatusOr<const int&>, double&&>));
+  // Const invalid.
+  EXPECT_FALSE((std::is_invocable_v<Probe, absl::StatusOr<int&>, const int&>));
+}
+
+TEST(StatusOr, ReferenceAssignmentFromStatusOr) {
+  std::vector<int> v = {1, 2, 3};
+  absl::StatusOr<int&> si = v[0];
+  absl::StatusOr<int&> si2 = v[1];
+
+  EXPECT_THAT(v, ElementsAre(1, 2, 3));
+  EXPECT_THAT(si, IsOkAndHolds(1));
+  EXPECT_THAT(si2, IsOkAndHolds(2));
+
+  // This rebinds the reference.
+  si = si2;
+  EXPECT_THAT(v, ElementsAre(1, 2, 3));
+  EXPECT_THAT(si, IsOkAndHolds(2));
+  EXPECT_THAT(si2, IsOkAndHolds(2));
+  EXPECT_EQ(&*si, &*si2);
+}
+
+TEST(StatusOr, ReferenceAssignFromReference) {
+  std::vector<int> v = {1, 2, 3};
+  absl::StatusOr<int&> si = v[0];
+
+  EXPECT_THAT(v, ElementsAre(1, 2, 3));
+  EXPECT_THAT(si, IsOkAndHolds(1));
+
+  // This rebinds the reference.
+  si = v[2];
+  EXPECT_THAT(v, ElementsAre(1, 2, 3));
+  EXPECT_THAT(si, IsOkAndHolds(3));
+  EXPECT_EQ(&*si, &v[2]);
+}
+
+TEST(StatusOr, ReferenceIsNotLifetimeBoundForStarValue) {
+  int i = 0;
+
+  // op*/value should not be LIFETIME_BOUND because the ref is not limited to
+  // the lifetime of the StatusOr.
+  int& r = *absl::StatusOr<int&>(i);
+  EXPECT_EQ(&r, &i);
+  int& r2 = absl::StatusOr<int&>(i).value();
+  EXPECT_EQ(&r2, &i);
+
+  struct S {
+    int i;
+  };
+  S s;
+  // op-> should also not be LIFETIME_BOUND for refs.
+  int& r3 = absl::StatusOr<S&>(s)->i;
+  EXPECT_EQ(&r3, &s.i);
+}
+
+template <typename Expected, typename T>
+void TestReferenceDeref() {
+  static_assert(std::is_same_v<Expected, decltype(*std::declval<T>())>);
+  static_assert(std::is_same_v<Expected, decltype(std::declval<T>().value())>);
+}
+
+TEST(StatusOr, ReferenceTypeIsMaintainedOnDeref) {
+  TestReferenceDeref<int&, absl::StatusOr<int&>&>();
+  TestReferenceDeref<int&, absl::StatusOr<int&>&&>();
+  TestReferenceDeref<int&, const absl::StatusOr<int&>&>();
+  TestReferenceDeref<int&, const absl::StatusOr<int&>&&>();
+
+  TestReferenceDeref<const int&, absl::StatusOr<const int&>&>();
+  TestReferenceDeref<const int&, absl::StatusOr<const int&>&&>();
+  TestReferenceDeref<const int&, const absl::StatusOr<const int&>&>();
+  TestReferenceDeref<const int&, const absl::StatusOr<const int&>&&>();
+
+  struct Struct {
+    int value;
+  };
+  EXPECT_TRUE(
+      (std::is_same_v<
+          int&, decltype((std::declval<absl::StatusOr<Struct&>>()->value))>));
+  EXPECT_TRUE(
+      (std::is_same_v<
+          int&,
+          decltype((std::declval<const absl::StatusOr<Struct&>>()->value))>));
+  EXPECT_TRUE(
+      (std::is_same_v<
+          const int&,
+          decltype((std::declval<absl::StatusOr<const Struct&>>()->value))>));
+}
+
+}  // namespace
diff --git a/absl/strings/BUILD.bazel b/absl/strings/BUILD.bazel
index 729bdd761f0..7f8ca5f090d 100644
--- a/absl/strings/BUILD.bazel
+++ b/absl/strings/BUILD.bazel
@@ -13,20 +13,41 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-load("@rules_cc//cc:defs.bzl", "cc_library", "cc_test")
+load("@rules_cc//cc:cc_binary.bzl", "cc_binary")
+load("@rules_cc//cc:cc_library.bzl", "cc_library")
+load("@rules_cc//cc:cc_test.bzl", "cc_test")
 load(
     "//absl:copts/configure_copts.bzl",
     "ABSL_DEFAULT_COPTS",
+    "ABSL_DEFAULT_LINKOPTS",
     "ABSL_TEST_COPTS",
 )
 
 package(
-    default_visibility = ["//visibility:public"],
-    features = ["parse_headers"],
+    default_visibility = ["//visibility:private"],
+    features = [
+        "header_modules",
+        "layering_check",
+        "parse_headers",
+    ],
 )
 
 licenses(["notice"])
 
+cc_library(
+    name = "string_view",
+    hdrs = ["string_view.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
+    deps = [
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:hardening",
+        "//absl/base:nullability",
+    ],
+)
+
 cc_library(
     name = "strings",
     srcs = [
@@ -37,23 +58,27 @@ cc_library(
         "internal/charconv_bigint.h",
         "internal/charconv_parse.cc",
         "internal/charconv_parse.h",
+        "internal/damerau_levenshtein_distance.cc",
         "internal/memutil.cc",
         "internal/memutil.h",
-        "internal/stl_type_traits.h",
         "internal/str_join_internal.h",
         "internal/str_split_internal.h",
+        "internal/stringify_sink.cc",
+        "internal/stringify_sink.h",
         "match.cc",
         "numbers.cc",
         "str_cat.cc",
         "str_replace.cc",
         "str_split.cc",
-        "string_view.cc",
         "substitute.cc",
     ],
     hdrs = [
         "ascii.h",
         "charconv.h",
         "escaping.h",
+        "has_absl_stringify.h",
+        "internal/damerau_levenshtein_distance.h",
+        "internal/string_constant.h",
         "match.h",
         "numbers.h",
         "str_cat.h",
@@ -65,202 +90,1036 @@ cc_library(
         "substitute.h",
     ],
     copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    textual_hdrs = [
+        # string_view.h was once part of :strings, so string_view.h is
+        # re-exported for backwards compatibility.
+        # New code should directly depend on :string_view.
+        "string_view.h",
+    ],
+    visibility = ["//visibility:public"],
+    deps = [
+        ":append_and_overwrite",
+        ":charset",
+        ":internal",
+        ":resize_and_overwrite",
+        ":string_view",
+        "//absl/base",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:endian",
+        "//absl/base:hardening",
+        "//absl/base:iterator_traits_internal",
+        "//absl/base:nullability",
+        "//absl/base:raw_logging_internal",
+        "//absl/base:throw_delegate",
+        "//absl/memory",
+        "//absl/meta:type_traits",
+        "//absl/numeric:bits",
+        "//absl/numeric:int128",
+        "//absl/types:source_location",
+    ],
+)
+
+cc_library(
+    name = "internal",
+    srcs = [
+        "internal/escaping.cc",
+        "internal/ostringstream.cc",
+        "internal/utf8.cc",
+    ],
+    hdrs = [
+        "internal/escaping.h",
+        "internal/ostringstream.h",
+        "internal/resize_uninitialized.h",
+        "internal/utf8.h",
+    ],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],  # Logically private. Do not depend on this.
+    deps = [
+        ":resize_and_overwrite",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:endian",
+        "//absl/base:raw_logging_internal",
+        "//absl/meta:type_traits",
+    ],
+)
+
+cc_library(
+    name = "resize_and_overwrite",
+    hdrs = ["resize_and_overwrite.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
+    deps = [
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:dynamic_annotations",
+        "//absl/base:hardening",
+        "//absl/base:throw_delegate",
+    ],
+)
+
+cc_test(
+    name = "resize_and_overwrite_test",
+    srcs = ["resize_and_overwrite_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":resize_and_overwrite",
+        "//absl/base:dynamic_annotations",
+        "//absl/log:absl_check",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_library(
+    name = "append_and_overwrite",
+    hdrs = ["internal/append_and_overwrite.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:private"],
+    deps = [
+        ":resize_and_overwrite",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:hardening",
+        "//absl/base:throw_delegate",
+    ],
+)
+
+cc_test(
+    name = "append_and_overwrite_test",
+    srcs = ["internal/append_and_overwrite_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":append_and_overwrite",
+        "//absl/log:absl_check",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "match_test",
+    size = "small",
+    srcs = ["match_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    visibility = ["//visibility:private"],
+    deps = [
+        ":strings",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_binary(
+    name = "match_benchmark",
+    testonly = True,
+    srcs = ["match_benchmark.cc"],
+    copts = ABSL_TEST_COPTS,
+    tags = ["benchmark"],
+    visibility = ["//visibility:private"],
+    deps = [
+        ":strings",
+        "//absl/base:raw_logging_internal",
+        "@google_benchmark//:benchmark_main",
+    ],
+)
+
+cc_test(
+    name = "escaping_test",
+    size = "small",
+    srcs = [
+        "escaping_test.cc",
+        "internal/escaping_test_common.h",
+    ],
+    copts = ABSL_TEST_COPTS,
+    tags = ["no_test_chromiumos_x86_64"],
+    visibility = ["//visibility:private"],
+    deps = [
+        ":charset",
+        ":cord",
+        ":str_format",
+        ":strings",
+        "//absl/base:core_headers",
+        "//absl/container:fixed_array",
+        "//absl/log:check",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_binary(
+    name = "escaping_benchmark",
+    testonly = True,
+    srcs = [
+        "escaping_benchmark.cc",
+        "internal/escaping_test_common.h",
+    ],
+    copts = ABSL_TEST_COPTS,
+    tags = ["benchmark"],
+    visibility = ["//visibility:private"],
+    deps = [
+        ":strings",
+        "//absl/base:raw_logging_internal",
+        "@google_benchmark//:benchmark_main",
+    ],
+)
+
+cc_test(
+    name = "has_absl_stringify_test",
+    size = "small",
+    srcs = ["has_absl_stringify_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    visibility = ["//visibility:private"],
+    deps = [
+        ":strings",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_library(
+    name = "has_ostream_operator",
+    hdrs = ["has_ostream_operator.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
+    deps = [
+        "//absl/base:config",
+    ],
+)
+
+cc_test(
+    name = "has_ostream_operator_test",
+    size = "small",
+    srcs = ["has_ostream_operator_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    visibility = ["//visibility:private"],
+    deps = [
+        ":has_ostream_operator",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "ascii_test",
+    size = "small",
+    srcs = ["ascii_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    visibility = ["//visibility:private"],
+    deps = [
+        ":strings",
+        "//absl/base:core_headers",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_binary(
+    name = "ascii_benchmark",
+    testonly = True,
+    srcs = ["ascii_benchmark.cc"],
+    copts = ABSL_TEST_COPTS,
+    tags = ["benchmark"],
+    visibility = ["//visibility:private"],
+    deps = [
+        ":strings",
+        "//absl/random",
+        "@google_benchmark//:benchmark_main",
+    ],
+)
+
+cc_test(
+    name = "damerau_levenshtein_distance_test",
+    size = "small",
+    srcs = [
+        "internal/damerau_levenshtein_distance_test.cc",
+    ],
+    copts = ABSL_TEST_COPTS,
+    deps = [
+        ":strings",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_binary(
+    name = "damerau_levenshtein_distance_benchmark",
+    testonly = True,
+    srcs = [
+        "internal/damerau_levenshtein_distance_benchmark.cc",
+    ],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    tags = ["benchmark"],
+    visibility = ["//visibility:private"],
+    deps = [
+        ":strings",
+        "@google_benchmark//:benchmark_main",
+    ],
+)
+
+cc_binary(
+    name = "memutil_benchmark",
+    testonly = True,
+    srcs = [
+        "internal/memutil.h",
+        "internal/memutil_benchmark.cc",
+    ],
+    copts = ABSL_TEST_COPTS,
+    tags = ["benchmark"],
+    visibility = ["//visibility:private"],
+    deps = [
+        ":strings",
+        "//absl/base:core_headers",
+        "@google_benchmark//:benchmark_main",
+    ],
+)
+
+cc_test(
+    name = "memutil_test",
+    size = "small",
+    srcs = [
+        "internal/memutil.h",
+        "internal/memutil_test.cc",
+    ],
+    copts = ABSL_TEST_COPTS,
+    visibility = ["//visibility:private"],
+    deps = [
+        ":strings",
+        "//absl/base:core_headers",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "utf8_test",
+    size = "small",
+    srcs = [
+        "internal/utf8_test.cc",
+    ],
+    copts = ABSL_TEST_COPTS,
+    visibility = ["//visibility:private"],
+    deps = [
+        ":internal",
+        ":string_view",
+        "//absl/base:core_headers",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "string_constant_test",
+    size = "small",
+    srcs = ["internal/string_constant_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    visibility = ["//visibility:private"],
+    deps = [
+        ":strings",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "string_view_test",
+    size = "small",
+    srcs = ["string_view_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    visibility = ["//visibility:private"],
+    deps = [
+        ":string_view",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:dynamic_annotations",
+        "//absl/meta:type_traits",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_library(
+    name = "stringify_stream",
+    hdrs = ["internal/stringify_stream.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl/status:__pkg__",
+        "@do_not_use_for_gloop_visibility_only//gloop/util/status:__pkg__",
+    ],
+    deps = [
+        ":string_view",
+        "//absl/base:config",
+    ],
+)
+
+cc_test(
+    name = "stringify_stream_test",
+    srcs = ["internal/stringify_stream_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":str_format",
+        ":string_view",
+        ":stringify_stream",
+        "//absl/base:config",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_binary(
+    name = "charset_benchmark",
+    testonly = True,
+    srcs = [
+        "charset_benchmark.cc",
+    ],
+    copts = ABSL_TEST_COPTS,
+    tags = [
+        "benchmark",
+    ],
+    visibility = ["//visibility:private"],
+    deps = [
+        ":charset",
+        "//absl/log:check",
+        "@google_benchmark//:benchmark_main",
+    ],
+)
+
+cc_library(
+    name = "charset",
+    hdrs = [
+        "charset.h",
+    ],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
+    deps = [
+        ":string_view",
+        "//absl/base:config",
+    ],
+)
+
+cc_test(
+    name = "charset_test",
+    size = "small",
+    srcs = ["charset_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    visibility = ["//visibility:private"],
+    deps = [
+        ":charset",
+        ":strings",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_library(
+    name = "cord_internal",
+    srcs = [
+        "internal/cord_internal.cc",
+        "internal/cord_rep_btree.cc",
+        "internal/cord_rep_btree_navigator.cc",
+        "internal/cord_rep_btree_reader.cc",
+        "internal/cord_rep_consume.cc",
+        "internal/cord_rep_crc.cc",
+    ],
+    hdrs = [
+        "internal/cord_data_edge.h",
+        "internal/cord_internal.h",
+        "internal/cord_rep_btree.h",
+        "internal/cord_rep_btree_navigator.h",
+        "internal/cord_rep_btree_reader.h",
+        "internal/cord_rep_consume.h",
+        "internal/cord_rep_crc.h",
+        "internal/cord_rep_flat.h",
+    ],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//visibility:private",
+    ],
+    deps = [
+        ":strings",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:endian",
+        "//absl/base:nullability",
+        "//absl/base:raw_logging_internal",
+        "//absl/container:compressed_tuple",
+        "//absl/container:container_memory",
+        "//absl/container:inlined_vector",
+        "//absl/crc:crc_cord_state",
+        "//absl/functional:function_ref",
+        "//absl/types:span",
+    ],
+)
+
+cc_test(
+    name = "cord_data_edge_test",
+    size = "small",
+    srcs = ["internal/cord_data_edge_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    visibility = ["//visibility:private"],
+    deps = [
+        ":cord_internal",
+        ":cord_rep_test_util",
+        ":strings",
+        "//absl/base:config",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "cord_rep_btree_test",
+    size = "medium",
+    timeout = "long",
+    srcs = ["internal/cord_rep_btree_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    visibility = ["//visibility:private"],
+    deps = [
+        ":cord_internal",
+        ":cord_rep_test_util",
+        ":strings",
+        "//absl/base:config",
+        "//absl/base:raw_logging_internal",
+        "//absl/cleanup",
+        "//absl/types:span",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "cord_rep_btree_navigator_test",
+    size = "medium",
+    srcs = ["internal/cord_rep_btree_navigator_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    visibility = ["//visibility:private"],
+    deps = [
+        ":cord_internal",
+        ":cord_rep_test_util",
+        ":strings",
+        "//absl/base:config",
+        "//absl/base:raw_logging_internal",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "cord_rep_btree_reader_test",
+    size = "medium",
+    srcs = ["internal/cord_rep_btree_reader_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    visibility = ["//visibility:private"],
+    deps = [
+        ":cord",
+        ":cord_internal",
+        ":cord_rep_test_util",
+        ":strings",
+        "//absl/base:config",
+        "//absl/base:raw_logging_internal",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "cord_rep_crc_test",
+    size = "small",
+    srcs = ["internal/cord_rep_crc_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    visibility = ["//visibility:private"],
+    deps = [
+        ":cord_internal",
+        ":cord_rep_test_util",
+        "//absl/base:config",
+        "//absl/crc:crc32c",
+        "//absl/crc:crc_cord_state",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_library(
+    name = "cordz_update_tracker",
+    hdrs = ["internal/cordz_update_tracker.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl:__subpackages__",
+    ],
+    deps = ["//absl/base:config"],
+)
+
+cc_test(
+    name = "cordz_update_tracker_test",
+    srcs = ["internal/cordz_update_tracker_test.cc"],
+    deps = [
+        ":cordz_update_tracker",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/synchronization",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_library(
+    name = "cord",
+    srcs = [
+        "cord.cc",
+        "cord_analysis.cc",
+        "cord_analysis.h",
+    ],
+    hdrs = [
+        "cord.h",
+        "cord_buffer.h",
+    ],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
+    deps = [
+        ":append_and_overwrite",
+        ":cord_internal",
+        ":cordz_info",
+        ":cordz_update_scope",
+        ":cordz_update_tracker",
+        ":internal",
+        ":resize_and_overwrite",
+        ":strings",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:endian",
+        "//absl/base:hardening",
+        "//absl/base:nullability",
+        "//absl/base:raw_logging_internal",
+        "//absl/cleanup",
+        "//absl/container:inlined_vector",
+        "//absl/crc:crc32c",
+        "//absl/crc:crc_cord_state",
+        "//absl/functional:function_ref",
+        "//absl/hash:weakly_mixed_integer",
+        "//absl/meta:type_traits",
+        "//absl/numeric:bits",
+        "//absl/types:compare",
+        "//absl/types:optional",
+        "//absl/types:span",
+        "@do_not_use_for_gloop_visibility_only//gloop/base:fprint",
+    ],
+)
+
+cc_library(
+    name = "cordz_handle",
+    srcs = ["internal/cordz_handle.cc"],
+    hdrs = ["internal/cordz_handle.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//absl:__subpackages__"],
+    deps = [
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:no_destructor",
+        "//absl/base:raw_logging_internal",
+        "//absl/synchronization",
+    ],
+)
+
+cc_library(
+    name = "cordz_info",
+    srcs = ["internal/cordz_info.cc"],
+    hdrs = ["internal/cordz_info.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl:__subpackages__",
+    ],
+    deps = [
+        ":cord_internal",
+        ":cordz_functions",
+        ":cordz_handle",
+        ":cordz_statistics",
+        ":cordz_update_tracker",
+        "//absl/base",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:no_destructor",
+        "//absl/base:raw_logging_internal",
+        "//absl/container:inlined_vector",
+        "//absl/debugging:stacktrace",
+        "//absl/synchronization",
+        "//absl/time",
+        "//absl/types:span",
+    ],
+)
+
+cc_library(
+    name = "cordz_update_scope",
+    hdrs = ["internal/cordz_update_scope.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//absl:__subpackages__"],
+    deps = [
+        ":cord_internal",
+        ":cordz_info",
+        ":cordz_update_tracker",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+    ],
+)
+
+cc_test(
+    name = "cordz_update_scope_test",
+    srcs = ["internal/cordz_update_scope_test.cc"],
+    copts = ABSL_DEFAULT_COPTS,
+    deps = [
+        ":cord_internal",
+        ":cordz_info",
+        ":cordz_test_helpers",
+        ":cordz_update_scope",
+        ":cordz_update_tracker",
+        "//absl/base:config",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_library(
+    name = "cordz_sample_token",
+    srcs = ["internal/cordz_sample_token.cc"],
+    hdrs = ["internal/cordz_sample_token.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl:__subpackages__",
+    ],
+    deps = [
+        ":cordz_handle",
+        ":cordz_info",
+        "//absl/base:config",
+    ],
+)
+
+cc_library(
+    name = "cordz_functions",
+    srcs = ["internal/cordz_functions.cc"],
+    hdrs = ["internal/cordz_functions.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl:__subpackages__",
+    ],
     deps = [
-        ":internal",
-        "//absl/base",
-        "//absl/base:bits",
         "//absl/base:config",
         "//absl/base:core_headers",
-        "//absl/base:endian",
         "//absl/base:raw_logging_internal",
-        "//absl/base:throw_delegate",
-        "//absl/memory",
-        "//absl/meta:type_traits",
-        "//absl/numeric:int128",
+        "//absl/profiling:exponential_biased",
     ],
 )
 
 cc_library(
-    name = "internal",
-    srcs = [
-        "internal/ostringstream.cc",
-        "internal/utf8.cc",
+    name = "cordz_statistics",
+    hdrs = ["internal/cordz_statistics.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl:__subpackages__",
     ],
-    hdrs = [
-        "internal/char_map.h",
-        "internal/ostringstream.h",
-        "internal/resize_uninitialized.h",
-        "internal/utf8.h",
+    deps = [
+        ":cordz_update_tracker",
+        "//absl/base:config",
+    ],
+)
+
+cc_test(
+    name = "cordz_functions_test",
+    srcs = [
+        "internal/cordz_functions_test.cc",
     ],
-    copts = ABSL_DEFAULT_COPTS,
     deps = [
-        "//absl/base:core_headers",
-        "//absl/base:endian",
-        "//absl/meta:type_traits",
+        ":cordz_functions",
+        ":cordz_test_helpers",
+        "//absl/base:config",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
 cc_test(
-    name = "match_test",
-    size = "small",
-    srcs = ["match_test.cc"],
-    copts = ABSL_TEST_COPTS,
-    visibility = ["//visibility:private"],
+    name = "cordz_handle_test",
+    srcs = [
+        "internal/cordz_handle_test.cc",
+    ],
     deps = [
-        ":strings",
-        "@com_google_googletest//:gtest_main",
+        ":cordz_handle",
+        "//absl/base:config",
+        "//absl/memory",
+        "//absl/random",
+        "//absl/random:distributions",
+        "//absl/synchronization",
+        "//absl/synchronization:thread_pool",
+        "//absl/time",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
 cc_test(
-    name = "escaping_test",
-    size = "small",
+    name = "cordz_info_test",
     srcs = [
-        "escaping_test.cc",
-        "internal/escaping_test_common.h",
+        "internal/cordz_info_test.cc",
     ],
-    copts = ABSL_TEST_COPTS,
-    visibility = ["//visibility:private"],
     deps = [
+        ":cord_internal",
+        ":cordz_handle",
+        ":cordz_info",
+        ":cordz_statistics",
+        ":cordz_test_helpers",
+        ":cordz_update_tracker",
         ":strings",
-        "//absl/base:core_headers",
-        "//absl/container:fixed_array",
-        "@com_google_googletest//:gtest_main",
+        "//absl/base:config",
+        "//absl/debugging:stacktrace",
+        "//absl/debugging:symbolize",
+        "//absl/types:span",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
 cc_test(
-    name = "escaping_benchmark",
+    name = "cordz_info_statistics_test",
     srcs = [
-        "escaping_benchmark.cc",
-        "internal/escaping_test_common.h",
+        "internal/cordz_info_statistics_test.cc",
     ],
-    copts = ABSL_TEST_COPTS,
-    tags = ["benchmark"],
-    visibility = ["//visibility:private"],
     deps = [
-        ":strings",
-        "//absl/base:raw_logging_internal",
-        "@com_github_google_benchmark//:benchmark_main",
+        ":cord",
+        ":cord_internal",
+        ":cordz_info",
+        ":cordz_sample_token",
+        ":cordz_statistics",
+        ":cordz_update_scope",
+        ":cordz_update_tracker",
+        ":string_view",
+        "//absl/base:config",
+        "//absl/crc:crc_cord_state",
+        "//absl/synchronization",
+        "//absl/synchronization:thread_pool",
+        "//absl/time",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
 cc_test(
-    name = "ascii_test",
-    size = "small",
-    srcs = ["ascii_test.cc"],
-    copts = ABSL_TEST_COPTS,
-    visibility = ["//visibility:private"],
+    name = "cordz_sample_token_test",
+    srcs = [
+        "internal/cordz_sample_token_test.cc",
+    ],
+    deps = [
+        ":cord_internal",
+        ":cordz_handle",
+        ":cordz_info",
+        ":cordz_sample_token",
+        ":cordz_test_helpers",
+        ":cordz_update_tracker",
+        "//absl/base:config",
+        "//absl/memory",
+        "//absl/random",
+        "//absl/synchronization",
+        "//absl/synchronization:thread_pool",
+        "//absl/time",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_library(
+    name = "cord_test_helpers",
+    testonly = True,
+    hdrs = [
+        "cord_test_helpers.h",
+    ],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
     deps = [
+        ":cord",
+        ":cord_internal",
         ":strings",
-        "//absl/base:core_headers",
-        "@com_google_googletest//:gtest_main",
+        "//absl/base:config",
     ],
 )
 
-cc_test(
-    name = "ascii_benchmark",
-    srcs = ["ascii_benchmark.cc"],
-    copts = ABSL_TEST_COPTS,
-    tags = ["benchmark"],
-    visibility = ["//visibility:private"],
+cc_library(
+    name = "cord_rep_test_util",
+    testonly = True,
+    hdrs = ["internal/cord_rep_test_util.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
+        ":cord_internal",
         ":strings",
-        "@com_github_google_benchmark//:benchmark_main",
+        "//absl/base:config",
+        "//absl/base:raw_logging_internal",
+        "//absl/types:span",
     ],
 )
 
-cc_test(
-    name = "memutil_benchmark",
-    srcs = [
-        "internal/memutil.h",
-        "internal/memutil_benchmark.cc",
+cc_library(
+    name = "cordz_test_helpers",
+    testonly = True,
+    hdrs = ["cordz_test_helpers.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl/strings:__pkg__",
     ],
-    copts = ABSL_TEST_COPTS,
-    tags = ["benchmark"],
-    visibility = ["//visibility:private"],
     deps = [
+        ":cord",
+        ":cord_internal",
+        ":cordz_functions",
+        ":cordz_info",
+        ":cordz_sample_token",
+        ":cordz_statistics",
+        ":cordz_update_tracker",
         ":strings",
+        "//absl/base:config",
         "//absl/base:core_headers",
-        "@com_github_google_benchmark//:benchmark_main",
+        "//absl/base:nullability",
+        "@googletest//:gtest",
     ],
 )
 
 cc_test(
-    name = "memutil_test",
+    name = "cord_buffer_test",
     size = "small",
-    srcs = [
-        "internal/memutil.h",
-        "internal/memutil_test.cc",
-    ],
+    srcs = ["cord_buffer_test.cc"],
     copts = ABSL_TEST_COPTS,
     visibility = ["//visibility:private"],
     deps = [
-        ":strings",
-        "//absl/base:core_headers",
-        "@com_google_googletest//:gtest_main",
+        ":cord",
+        ":cord_internal",
+        ":cord_rep_test_util",
+        ":string_view",
+        "//absl/base:config",
+        "//absl/types:span",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
 cc_test(
-    name = "utf8_test",
-    size = "small",
-    srcs = [
-        "internal/utf8_test.cc",
-    ],
+    name = "cord_test",
+    size = "medium",
+    timeout = "long",
+    srcs = ["cord_test.cc"],
     copts = ABSL_TEST_COPTS,
     visibility = ["//visibility:private"],
     deps = [
-        ":internal",
+        ":cord",
+        ":cord_internal",
+        ":cord_test_helpers",
+        ":cordz_functions",
+        ":cordz_statistics",
+        ":cordz_test_helpers",
+        ":cordz_update_tracker",
+        ":str_format",
+        ":strings",
+        "//absl/base:config",
         "//absl/base:core_headers",
-        "@com_google_googletest//:gtest_main",
+        "//absl/base:endian",
+        "//absl/base:no_destructor",
+        "//absl/container:fixed_array",
+        "//absl/functional:function_ref",
+        "//absl/hash",
+        "//absl/hash:hash_testing",
+        "//absl/log",
+        "//absl/log:check",
+        "//absl/random",
+        "//absl/types:compare",
+        "//absl/types:span",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
 cc_test(
-    name = "string_view_benchmark",
-    srcs = ["string_view_benchmark.cc"],
+    name = "cordz_test",
+    size = "medium",
+    srcs = ["cordz_test.cc"],
     copts = ABSL_TEST_COPTS,
-    tags = ["benchmark"],
+    tags = [
+        "no_test_android_arm",
+        "no_test_android_arm64",
+        "no_test_android_x86",
+        "no_test_ios_x86_64",
+        "no_test_lexan",
+        "no_test_loonix",
+    ],
     visibility = ["//visibility:private"],
     deps = [
+        ":cord",
+        ":cord_internal",
+        ":cord_test_helpers",
+        ":cordz_functions",
+        ":cordz_info",
+        ":cordz_sample_token",
+        ":cordz_statistics",
+        ":cordz_test_helpers",
+        ":cordz_update_tracker",
         ":strings",
+        "//absl/base:config",
         "//absl/base:core_headers",
         "//absl/base:raw_logging_internal",
-        "@com_github_google_benchmark//:benchmark_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
 cc_test(
-    name = "string_view_test",
+    name = "substitute_test",
     size = "small",
-    srcs = ["string_view_test.cc"],
+    srcs = ["substitute_test.cc"],
     copts = ABSL_TEST_COPTS,
     visibility = ["//visibility:private"],
     deps = [
         ":strings",
-        "//absl/base:config",
         "//absl/base:core_headers",
-        "//absl/base:dynamic_annotations",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
-cc_test(
-    name = "substitute_test",
-    size = "small",
-    srcs = ["substitute_test.cc"],
+cc_binary(
+    name = "substitute_benchmark",
+    testonly = True,
+    srcs = ["substitute_benchmark.cc"],
     copts = ABSL_TEST_COPTS,
+    tags = [
+        "benchmark",
+    ],
     visibility = ["//visibility:private"],
     deps = [
         ":strings",
         "//absl/base:core_headers",
-        "@com_google_googletest//:gtest_main",
+        "@google_benchmark//:benchmark_main",
     ],
 )
 
-cc_test(
+cc_binary(
     name = "str_replace_benchmark",
+    testonly = True,
     srcs = ["str_replace_benchmark.cc"],
     copts = ABSL_TEST_COPTS,
     tags = ["benchmark"],
@@ -268,7 +1127,7 @@ cc_test(
     deps = [
         ":strings",
         "//absl/base:raw_logging_internal",
-        "@com_github_google_benchmark//:benchmark_main",
+        "@google_benchmark//:benchmark_main",
     ],
 )
 
@@ -280,7 +1139,8 @@ cc_test(
     visibility = ["//visibility:private"],
     deps = [
         ":strings",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -293,12 +1153,18 @@ cc_test(
         ":strings",
         "//absl/base:core_headers",
         "//absl/base:dynamic_annotations",
-        "@com_google_googletest//:gtest_main",
+        "//absl/container:btree",
+        "//absl/container:flat_hash_map",
+        "//absl/container:node_hash_map",
+        "//absl/hash",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
-cc_test(
+cc_binary(
     name = "str_split_benchmark",
+    testonly = True,
     srcs = ["str_split_benchmark.cc"],
     copts = ABSL_TEST_COPTS,
     tags = ["benchmark"],
@@ -306,7 +1172,7 @@ cc_test(
     deps = [
         ":strings",
         "//absl/base:raw_logging_internal",
-        "@com_github_google_benchmark//:benchmark_main",
+        "@google_benchmark//:benchmark_main",
     ],
 )
 
@@ -318,19 +1184,21 @@ cc_test(
     visibility = ["//visibility:private"],
     deps = [
         ":internal",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
-cc_test(
+cc_binary(
     name = "ostringstream_benchmark",
+    testonly = True,
     srcs = ["internal/ostringstream_benchmark.cc"],
     copts = ABSL_TEST_COPTS,
     tags = ["benchmark"],
     visibility = ["//visibility:private"],
     deps = [
         ":internal",
-        "@com_github_google_benchmark//:benchmark_main",
+        "@google_benchmark//:benchmark_main",
     ],
 )
 
@@ -338,15 +1206,16 @@ cc_test(
     name = "resize_uninitialized_test",
     size = "small",
     srcs = [
-        "internal/resize_uninitialized.h",
         "internal/resize_uninitialized_test.cc",
     ],
     copts = ABSL_TEST_COPTS,
     visibility = ["//visibility:private"],
     deps = [
+        ":internal",
         "//absl/base:core_headers",
         "//absl/meta:type_traits",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -360,19 +1229,21 @@ cc_test(
         ":strings",
         "//absl/base:core_headers",
         "//absl/memory",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
-cc_test(
+cc_binary(
     name = "str_join_benchmark",
+    testonly = True,
     srcs = ["str_join_benchmark.cc"],
     copts = ABSL_TEST_COPTS,
     tags = ["benchmark"],
     visibility = ["//visibility:private"],
     deps = [
         ":strings",
-        "@com_github_google_benchmark//:benchmark_main",
+        "@google_benchmark//:benchmark_main",
     ],
 )
 
@@ -383,21 +1254,27 @@ cc_test(
     copts = ABSL_TEST_COPTS,
     visibility = ["//visibility:private"],
     deps = [
+        ":str_format",
         ":strings",
-        "//absl/base:core_headers",
-        "@com_google_googletest//:gtest_main",
+        "//absl/base:config",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
-cc_test(
+cc_binary(
     name = "str_cat_benchmark",
+    testonly = True,
     srcs = ["str_cat_benchmark.cc"],
     copts = ABSL_TEST_COPTS,
     tags = ["benchmark"],
     visibility = ["//visibility:private"],
     deps = [
+        ":str_format",
         ":strings",
-        "@com_github_google_benchmark//:benchmark_main",
+        "//absl/random",
+        "//absl/random:distributions",
+        "@google_benchmark//:benchmark_main",
     ],
 )
 
@@ -411,16 +1288,23 @@ cc_test(
     copts = ABSL_TEST_COPTS,
     visibility = ["//visibility:private"],
     deps = [
+        ":internal",
         ":pow10_helper",
         ":strings",
-        "//absl/base:core_headers",
-        "//absl/base:raw_logging_internal",
-        "@com_google_googletest//:gtest_main",
+        "//absl/base:config",
+        "//absl/cleanup",
+        "//absl/log",
+        "//absl/numeric:int128",
+        "//absl/random",
+        "//absl/random:distributions",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
-cc_test(
+cc_binary(
     name = "numbers_benchmark",
+    testonly = True,
     srcs = ["numbers_benchmark.cc"],
     copts = ABSL_TEST_COPTS,
     tags = ["benchmark"],
@@ -428,7 +1312,9 @@ cc_test(
     deps = [
         ":strings",
         "//absl/base:raw_logging_internal",
-        "@com_github_google_benchmark//:benchmark_main",
+        "//absl/random",
+        "//absl/random:distributions",
+        "@google_benchmark//:benchmark_main",
     ],
 )
 
@@ -440,28 +1326,8 @@ cc_test(
     visibility = ["//visibility:private"],
     deps = [
         ":strings",
-        "@com_google_googletest//:gtest_main",
-    ],
-)
-
-cc_test(
-    name = "char_map_test",
-    srcs = ["internal/char_map_test.cc"],
-    copts = ABSL_TEST_COPTS,
-    deps = [
-        ":internal",
-        "@com_google_googletest//:gtest_main",
-    ],
-)
-
-cc_test(
-    name = "char_map_benchmark",
-    srcs = ["internal/char_map_benchmark.cc"],
-    copts = ABSL_TEST_COPTS,
-    tags = ["benchmark"],
-    deps = [
-        ":internal",
-        "@com_github_google_benchmark//:benchmark_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -473,7 +1339,8 @@ cc_test(
         ":pow10_helper",
         ":str_format",
         ":strings",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -486,8 +1353,10 @@ cc_test(
     copts = ABSL_TEST_COPTS,
     deps = [
         ":strings",
-        "//absl/base:raw_logging_internal",
-        "@com_google_googletest//:gtest_main",
+        "//absl/base:config",
+        "//absl/log:check",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -501,12 +1370,15 @@ cc_test(
     copts = ABSL_TEST_COPTS,
     deps = [
         ":strings",
-        "@com_google_googletest//:gtest_main",
+        "//absl/base:config",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
-cc_test(
+cc_binary(
     name = "charconv_benchmark",
+    testonly = True,
     srcs = [
         "charconv_benchmark.cc",
     ],
@@ -515,7 +1387,7 @@ cc_test(
     ],
     deps = [
         ":strings",
-        "@com_github_google_benchmark//:benchmark_main",
+        "@google_benchmark//:benchmark_main",
     ],
 )
 
@@ -525,8 +1397,15 @@ cc_library(
         "str_format.h",
     ],
     copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
     deps = [
         ":str_format_internal",
+        ":string_view",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:nullability",
+        "//absl/types:span",
     ],
 )
 
@@ -544,20 +1423,29 @@ cc_library(
         "internal/str_format/arg.h",
         "internal/str_format/bind.h",
         "internal/str_format/checker.h",
+        "internal/str_format/constexpr_parser.h",
         "internal/str_format/extension.h",
         "internal/str_format/float_conversion.h",
         "internal/str_format/output.h",
         "internal/str_format/parser.h",
     ],
     copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
     visibility = ["//visibility:private"],
     deps = [
+        ":internal",
         ":strings",
         "//absl/base:config",
         "//absl/base:core_headers",
+        "//absl/container:fixed_array",
+        "//absl/container:inlined_vector",
+        "//absl/functional:function_ref",
         "//absl/meta:type_traits",
+        "//absl/numeric:bits",
         "//absl/numeric:int128",
+        "//absl/numeric:representation",
         "//absl/types:span",
+        "//absl/utility",
     ],
 )
 
@@ -567,10 +1455,15 @@ cc_test(
     copts = ABSL_TEST_COPTS,
     visibility = ["//visibility:private"],
     deps = [
+        ":cord",
         ":str_format",
+        ":str_format_internal",
         ":strings",
+        "//absl/base:config",
         "//absl/base:core_headers",
-        "@com_google_googletest//:gtest_main",
+        "//absl/types:span",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -584,7 +1477,10 @@ cc_test(
     deps = [
         ":str_format",
         ":str_format_internal",
-        "@com_google_googletest//:gtest_main",
+        ":strings",
+        "//absl/random",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -596,7 +1492,9 @@ cc_test(
     deps = [
         ":str_format",
         ":str_format_internal",
-        "@com_google_googletest//:gtest_main",
+        "//absl/base:config",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -607,7 +1505,10 @@ cc_test(
     visibility = ["//visibility:private"],
     deps = [
         ":str_format_internal",
-        "@com_google_googletest//:gtest_main",
+        ":string_view",
+        "//absl/types:span",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -618,20 +1519,33 @@ cc_test(
     visibility = ["//visibility:private"],
     deps = [
         ":str_format",
-        "@com_google_googletest//:gtest_main",
+        ":string_view",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
 cc_test(
     name = "str_format_convert_test",
     size = "medium",
+    timeout = "long",
     srcs = ["internal/str_format/convert_test.cc"],
     copts = ABSL_TEST_COPTS,
     visibility = ["//visibility:private"],
     deps = [
+        ":str_format",
         ":str_format_internal",
+        ":strings",
+        "//absl/base",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:raw_logging_internal",
+        "//absl/log",
+        "//absl/log:absl_check",
         "//absl/numeric:int128",
-        "@com_google_googletest//:gtest_main",
+        "//absl/types:span",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -641,8 +1555,10 @@ cc_test(
     copts = ABSL_TEST_COPTS,
     visibility = ["//visibility:private"],
     deps = [
+        ":cord",
         ":str_format_internal",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -653,8 +1569,11 @@ cc_test(
     visibility = ["//visibility:private"],
     deps = [
         ":str_format_internal",
+        ":string_view",
+        "//absl/base:config",
         "//absl/base:core_headers",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -663,7 +1582,10 @@ cc_library(
     testonly = True,
     srcs = ["internal/pow10_helper.cc"],
     hdrs = ["internal/pow10_helper.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
     visibility = ["//visibility:private"],
+    deps = ["//absl/base:config"],
 )
 
 cc_test(
@@ -674,6 +1596,76 @@ cc_test(
     deps = [
         ":pow10_helper",
         ":str_format",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_binary(
+    name = "atod_manual_test",
+    testonly = True,
+    srcs = ["atod_manual_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:private"],
+    deps = [
+        ":str_format",
+        ":strings",
+        "//absl/base",
+    ],
+)
+
+cc_test(
+    name = "char_formatting_test",
+    srcs = [
+        "char_formatting_test.cc",
+    ],
+    deps = [
+        ":str_format",
+        ":strings",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_library(
+    name = "generic_printer",
+    srcs = [
+        "internal/generic_printer.cc",
+        "internal/generic_printer_internal.h",
+    ],
+    hdrs = ["internal/generic_printer.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl:__subpackages__",
+        "@do_not_use_for_gloop_visibility_only//gloop/util/gtl:__pkg__",
+    ],
+    deps = [
+        ":str_format",
+        ":strings",
+        "//absl/base:config",
+        "//absl/log/internal:container",
+        "//absl/meta:requires",
+    ],
+)
+
+cc_test(
+    name = "generic_printer_test",
+    srcs = ["internal/generic_printer_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":generic_printer",
+        ":str_format",
+        ":strings",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/cleanup",
+        "//absl/container:flat_hash_map",
+        "//absl/status",
+        "//absl/status:statusor",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
diff --git a/absl/strings/CMakeLists.txt b/absl/strings/CMakeLists.txt
index ccff4441934..3ea24fea105 100644
--- a/absl/strings/CMakeLists.txt
+++ b/absl/strings/CMakeLists.txt
@@ -14,6 +14,20 @@
 # limitations under the License.
 #
 
+absl_cc_library(
+  NAME
+    string_view
+  HDRS
+    "string_view.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::config
+    absl::core_headers
+    absl::nullability
+  PUBLIC
+)
+
 absl_cc_library(
   NAME
     strings
@@ -21,13 +35,15 @@ absl_cc_library(
     "ascii.h"
     "charconv.h"
     "escaping.h"
+    "has_absl_stringify.h"
+    "internal/damerau_levenshtein_distance.h"
+    "internal/string_constant.h"
     "match.h"
     "numbers.h"
     "str_cat.h"
     "str_join.h"
     "str_replace.h"
     "str_split.h"
-    "string_view.h"
     "strip.h"
     "substitute.h"
   SRCS
@@ -38,9 +54,11 @@ absl_cc_library(
     "internal/charconv_bigint.h"
     "internal/charconv_parse.cc"
     "internal/charconv_parse.h"
+    "internal/damerau_levenshtein_distance.cc"
     "internal/memutil.cc"
     "internal/memutil.h"
-    "internal/stl_type_traits.h"
+    "internal/stringify_sink.h"
+    "internal/stringify_sink.cc"
     "internal/str_join_internal.h"
     "internal/str_split_internal.h"
     "match.cc"
@@ -48,30 +66,90 @@ absl_cc_library(
     "str_cat.cc"
     "str_replace.cc"
     "str_split.cc"
-    "string_view.cc"
     "substitute.cc"
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
+    absl::string_view
+    absl::strings_append_and_overwrite
     absl::strings_internal
+    absl::strings_resize_and_overwrite
     absl::base
     absl::bits
+    absl::charset
     absl::config
     absl::core_headers
     absl::endian
+    absl::hardening
     absl::int128
+    absl::iterator_traits_internal
     absl::memory
+    absl::nullability
     absl::raw_logging_internal
+    absl::source_location
     absl::throw_delegate
     absl::type_traits
   PUBLIC
 )
 
+absl_cc_library(
+  NAME
+    charset
+  HDRS
+    charset.h
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::config
+    absl::string_view
+  PUBLIC
+)
+
+absl_cc_library(
+  NAME
+    has_ostream_operator
+  HDRS
+    "has_ostream_operator.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::config
+  PUBLIC
+)
+
+absl_cc_library(
+  NAME
+    stringify_stream
+  HDRS
+    "internal/stringify_stream.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::string_view
+    absl::config
+)
+
+absl_cc_test(
+  NAME
+    stringify_stream_test
+  SRCS
+    "internal/stringify_stream_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::string_view
+    absl::stringify_stream
+    absl::str_format
+    GTest::gmock_main
+)
+
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     strings_internal
   HDRS
-    "internal/char_map.h"
+    "internal/escaping.cc"
+    "internal/escaping.h"
     "internal/ostringstream.h"
     "internal/resize_uninitialized.h"
     "internal/utf8.h"
@@ -81,11 +159,71 @@ absl_cc_library(
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
+    absl::config
     absl::core_headers
     absl::endian
+    absl::raw_logging_internal
+    absl::strings_resize_and_overwrite
     absl::type_traits
 )
 
+absl_cc_library(
+  NAME
+    strings_resize_and_overwrite
+  HDRS
+    "resize_and_overwrite.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::config
+    absl::core_headers
+    absl::dynamic_annotations
+    absl::hardening
+    absl::throw_delegate
+)
+
+absl_cc_test(
+  NAME
+    strings_resize_and_overwrite_test
+  SRCS
+    "resize_and_overwrite_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::strings_resize_and_overwrite
+    absl::absl_check
+    absl::dynamic_annotations
+    GTest::gmock_main
+)
+
+absl_cc_library(
+  NAME
+    strings_append_and_overwrite
+  HDRS
+    "internal/append_and_overwrite.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::config
+    absl::core_headers
+    absl::hardening
+    absl::strings_resize_and_overwrite
+    absl::throw_delegate
+)
+
+absl_cc_test(
+  NAME
+    strings_append_and_overwrite_test
+  SRCS
+    "internal/append_and_overwrite_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::strings_resize_and_overwrite
+    absl::absl_check
+    GTest::gmock_main
+)
+
 absl_cc_test(
   NAME
     match_test
@@ -96,7 +234,7 @@ absl_cc_test(
   DEPS
     absl::strings
     absl::base
-    gmock_main
+    GTest::gmock_main
 )
 
 absl_cc_test(
@@ -107,10 +245,37 @@ absl_cc_test(
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
+    absl::charset
     absl::strings
+    absl::str_format
     absl::core_headers
     absl::fixed_array
-    gmock_main
+    GTest::gmock_main
+    absl::check
+)
+
+absl_cc_test(
+  NAME
+    has_absl_stringify_test
+  SRCS
+    "has_absl_stringify_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::strings
+    GTest::gmock_main
+)
+
+absl_cc_test(
+  NAME
+    has_ostream_operator_test
+  SRCS
+    "has_ostream_operator_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::has_ostream_operator
+    GTest::gmock_main
 )
 
 absl_cc_test(
@@ -123,7 +288,20 @@ absl_cc_test(
   DEPS
     absl::strings
     absl::core_headers
-    gmock_main
+    GTest::gmock_main
+)
+
+absl_cc_test(
+  NAME
+    damerau_levenshtein_distance_test
+  SRCS
+    "internal/damerau_levenshtein_distance_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::strings
+    absl::base
+    GTest::gmock_main
 )
 
 absl_cc_test(
@@ -137,7 +315,7 @@ absl_cc_test(
   DEPS
     absl::strings
     absl::core_headers
-    gmock_main
+    GTest::gmock_main
 )
 
 absl_cc_test(
@@ -148,10 +326,23 @@ absl_cc_test(
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
+    absl::string_view
     absl::strings_internal
     absl::base
     absl::core_headers
-    gmock_main
+    GTest::gmock_main
+)
+
+absl_cc_test(
+  NAME
+    string_constant_test
+  SRCS
+    "internal/string_constant_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::strings
+    GTest::gmock_main
 )
 
 absl_cc_test(
@@ -166,7 +357,8 @@ absl_cc_test(
     absl::config
     absl::core_headers
     absl::dynamic_annotations
-    gmock_main
+    absl::type_traits
+    GTest::gmock_main
 )
 
 absl_cc_test(
@@ -179,7 +371,7 @@ absl_cc_test(
   DEPS
     absl::strings
     absl::core_headers
-    gmock_main
+    GTest::gmock_main
 )
 
 absl_cc_test(
@@ -191,7 +383,7 @@ absl_cc_test(
     ${ABSL_TEST_COPTS}
   DEPS
     absl::strings
-    gmock_main
+    GTest::gmock_main
 )
 
 absl_cc_test(
@@ -203,10 +395,13 @@ absl_cc_test(
     ${ABSL_TEST_COPTS}
   DEPS
     absl::strings
-    absl::base
     absl::core_headers
     absl::dynamic_annotations
-    gmock_main
+    absl::btree
+    absl::flat_hash_map
+    absl::hash
+    absl::node_hash_map
+    GTest::gmock_main
 )
 
 absl_cc_test(
@@ -218,22 +413,22 @@ absl_cc_test(
     ${ABSL_TEST_COPTS}
   DEPS
     absl::strings_internal
-    gmock_main
+    GTest::gmock_main
 )
 
 absl_cc_test(
   NAME
     resize_uninitialized_test
   SRCS
-    "internal/resize_uninitialized.h"
     "internal/resize_uninitialized_test.cc"
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
+    absl::strings_internal
     absl::base
     absl::core_headers
     absl::type_traits
-    gmock_main
+    GTest::gmock_main
 )
 
 absl_cc_test(
@@ -248,7 +443,7 @@ absl_cc_test(
     absl::base
     absl::core_headers
     absl::memory
-    gmock_main
+    GTest::gmock_main
 )
 
 absl_cc_test(
@@ -260,8 +455,10 @@ absl_cc_test(
     ${ABSL_TEST_COPTS}
   DEPS
     absl::strings
+    absl::str_format
+    absl::config
     absl::core_headers
-    gmock_main
+    GTest::gmock_main
 )
 
 absl_cc_test(
@@ -273,11 +470,17 @@ absl_cc_test(
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
-    absl::strings
+    absl::cleanup
+    absl::config
     absl::core_headers
+    absl::int128
+    absl::log
     absl::pow10_helper
-    absl::raw_logging_internal
-    gmock_main
+    absl::random_distributions
+    absl::random_random
+    absl::strings
+    absl::strings_internal
+    GTest::gmock_main
 )
 
 absl_cc_test(
@@ -290,19 +493,19 @@ absl_cc_test(
   DEPS
     absl::strings
     absl::base
-    gmock_main
+    GTest::gmock_main
 )
 
 absl_cc_test(
   NAME
-    char_map_test
+    charset_test
   SRCS
-    "internal/char_map_test.cc"
+    "charset_test.cc"
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
-    absl::strings_internal
-    gmock_main
+    absl::strings
+    GTest::gmock_main
 )
 
 absl_cc_test(
@@ -316,7 +519,7 @@ absl_cc_test(
     absl::strings
     absl::str_format
     absl::pow10_helper
-    gmock_main
+    GTest::gmock_main
 )
 
 absl_cc_test(
@@ -328,9 +531,10 @@ absl_cc_test(
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
+    absl::check
+    absl::config
     absl::strings
-    absl::raw_logging_internal
-    gmock_main
+    GTest::gmock_main
 )
 
 absl_cc_test(
@@ -344,7 +548,8 @@ absl_cc_test(
     ${ABSL_TEST_COPTS}
   DEPS
     absl::strings
-    gmock_main
+    absl::config
+    GTest::gmock_main
 )
 
 absl_cc_library(
@@ -355,10 +560,16 @@ absl_cc_library(
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
+    absl::config
+    absl::core_headers
+    absl::nullability
+    absl::span
     absl::str_format_internal
+    absl::string_view
   PUBLIC
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     str_format_internal
@@ -366,6 +577,7 @@ absl_cc_library(
     "internal/str_format/arg.h"
     "internal/str_format/bind.h"
     "internal/str_format/checker.h"
+    "internal/str_format/constexpr_parser.h"
     "internal/str_format/extension.h"
     "internal/str_format/float_conversion.h"
     "internal/str_format/output.h"
@@ -380,12 +592,18 @@ absl_cc_library(
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
+    absl::bits
     absl::strings
     absl::config
     absl::core_headers
+    absl::fixed_array
+    absl::inlined_vector
+    absl::numeric_representation
     absl::type_traits
+    absl::utility
     absl::int128
     absl::span
+    absl::strings_internal
 )
 
 absl_cc_test(
@@ -396,10 +614,13 @@ absl_cc_test(
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
+    absl::config
+    absl::cord
+    absl::core_headers
+    absl::span
     absl::str_format
     absl::strings
-    absl::core_headers
-    gmock_main
+    GTest::gmock_main
 )
 
 absl_cc_test(
@@ -410,9 +631,11 @@ absl_cc_test(
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
+    absl::random_random
     absl::str_format
     absl::str_format_internal
-    gmock_main
+    absl::strings
+    GTest::gmock_main
 )
 
 absl_cc_test(
@@ -423,9 +646,10 @@ absl_cc_test(
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
+    absl::config
     absl::str_format
     absl::str_format_internal
-    gmock_main
+    GTest::gmock_main
 )
 
 absl_cc_test(
@@ -437,7 +661,7 @@ absl_cc_test(
     ${ABSL_TEST_COPTS}
   DEPS
     absl::str_format_internal
-    gmock_main
+    GTest::gmock_main
 )
 
 absl_cc_test(
@@ -449,7 +673,7 @@ absl_cc_test(
     ${ABSL_TEST_COPTS}
   DEPS
     absl::str_format
-    gmock_main
+    GTest::gmock_main
 )
 
 absl_cc_test(
@@ -460,9 +684,16 @@ absl_cc_test(
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
-    absl::str_format_internal
+    absl::config
+    absl::core_headers
     absl::int128
-    gmock_main
+    absl::log
+    absl::raw_logging_internal
+    absl::span
+    absl::str_format
+    absl::str_format_internal
+    absl::strings
+    GTest::gmock_main
 )
 
 absl_cc_test(
@@ -474,7 +705,8 @@ absl_cc_test(
     ${ABSL_TEST_COPTS}
   DEPS
     absl::str_format_internal
-    gmock_main
+    absl::cord
+    GTest::gmock_main
 )
 
 absl_cc_test(
@@ -486,10 +718,27 @@ absl_cc_test(
     ${ABSL_TEST_COPTS}
   DEPS
     absl::str_format_internal
+    absl::string_view
+    absl::config
     absl::core_headers
-    gmock_main
+    GTest::gmock_main
 )
 
+absl_cc_test(
+  NAME
+    char_formatting_test
+  SRCS
+    "char_formatting_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::base
+    absl::str_format
+    absl::strings
+    GTest::gmock_main
+)
+
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     pow10_helper
@@ -498,7 +747,9 @@ absl_cc_library(
   SRCS
     "internal/pow10_helper.cc"
   COPTS
-    ${ABSL_TEST_COPTS}
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::config
   TESTONLY
 )
 
@@ -512,5 +763,564 @@ absl_cc_test(
   DEPS
     absl::pow10_helper
     absl::str_format
-    gmock_main
+    GTest::gmock_main
+)
+
+# Internal-only target, do not depend on directly.
+absl_cc_library(
+  NAME
+    cord_internal
+  HDRS
+    "internal/cord_data_edge.h"
+    "internal/cord_internal.h"
+    "internal/cord_rep_btree.h"
+    "internal/cord_rep_btree_navigator.h"
+    "internal/cord_rep_btree_reader.h"
+    "internal/cord_rep_crc.h"
+    "internal/cord_rep_consume.h"
+    "internal/cord_rep_flat.h"
+  SRCS
+    "internal/cord_internal.cc"
+    "internal/cord_rep_btree.cc"
+    "internal/cord_rep_btree_navigator.cc"
+    "internal/cord_rep_btree_reader.cc"
+    "internal/cord_rep_crc.cc"
+    "internal/cord_rep_consume.cc"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::compressed_tuple
+    absl::config
+    absl::container_memory
+    absl::compare
+    absl::core_headers
+    absl::crc_cord_state
+    absl::endian
+    absl::inlined_vector
+    absl::layout
+    absl::raw_logging_internal
+    absl::strings
+    absl::throw_delegate
+    absl::type_traits
+)
+
+# Internal-only target, do not depend on directly.
+absl_cc_library(
+  NAME
+    cordz_update_tracker
+  HDRS
+    "internal/cordz_update_tracker.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::config
+)
+
+absl_cc_test(
+  NAME
+    cordz_update_tracker_test
+  SRCS
+    "internal/cordz_update_tracker_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::config
+    absl::cordz_update_tracker
+    absl::core_headers
+    absl::synchronization
+    GTest::gmock_main
+)
+
+# Internal-only target, do not depend on directly.
+absl_cc_library(
+  NAME
+    cordz_functions
+  HDRS
+    "internal/cordz_functions.h"
+  SRCS
+    "internal/cordz_functions.cc"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::config
+    absl::core_headers
+    absl::exponential_biased
+    absl::raw_logging_internal
+)
+
+absl_cc_test(
+  NAME
+    cordz_functions_test
+  SRCS
+    "internal/cordz_functions_test.cc"
+  DEPS
+    absl::config
+    absl::cordz_functions
+    absl::cordz_test_helpers
+    GTest::gmock_main
+)
+
+# Internal-only target, do not depend on directly.
+absl_cc_library(
+  NAME
+    cordz_statistics
+  HDRS
+    "internal/cordz_statistics.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::config
+    absl::core_headers
+    absl::cordz_update_tracker
+    absl::synchronization
+)
+
+# Internal-only target, do not depend on directly.
+absl_cc_library(
+  NAME
+    cordz_handle
+  HDRS
+    "internal/cordz_handle.h"
+  SRCS
+    "internal/cordz_handle.cc"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::base
+    absl::config
+    absl::no_destructor
+    absl::raw_logging_internal
+    absl::synchronization
+)
+
+absl_cc_test(
+  NAME
+    cordz_handle_test
+  SRCS
+    "internal/cordz_handle_test.cc"
+  DEPS
+    absl::config
+    absl::cordz_handle
+    absl::cordz_test_helpers
+    absl::memory
+    absl::random_random
+    absl::random_distributions
+    absl::synchronization
+    absl::time
+    GTest::gmock_main
+)
+
+# Internal-only target, do not depend on directly.
+absl_cc_library(
+  NAME
+    cordz_info
+  HDRS
+    "internal/cordz_info.h"
+  SRCS
+    "internal/cordz_info.cc"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::base
+    absl::config
+    absl::cord_internal
+    absl::cordz_functions
+    absl::cordz_handle
+    absl::cordz_statistics
+    absl::cordz_update_tracker
+    absl::core_headers
+    absl::no_destructor
+    absl::inlined_vector
+    absl::span
+    absl::raw_logging_internal
+    absl::stacktrace
+    absl::synchronization
+    absl::time
+)
+
+absl_cc_test(
+  NAME
+    cordz_info_test
+  SRCS
+    "internal/cordz_info_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::config
+    absl::cord_internal
+    absl::cordz_test_helpers
+    absl::cordz_handle
+    absl::cordz_info
+    absl::cordz_statistics
+    absl::cordz_test_helpers
+    absl::cordz_update_tracker
+    absl::span
+    absl::stacktrace
+    absl::symbolize
+    GTest::gmock_main
+)
+
+absl_cc_test(
+  NAME
+    cordz_info_statistics_test
+  SRCS
+    "internal/cordz_info_statistics_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::config
+    absl::cord
+    absl::cord_internal
+    absl::cordz_info
+    absl::cordz_sample_token
+    absl::cordz_statistics
+    absl::cordz_update_scope
+    absl::cordz_update_tracker
+    absl::crc_cord_state
+    absl::thread_pool
+    GTest::gmock_main
+)
+
+# Internal-only target, do not depend on directly.
+absl_cc_library(
+  NAME
+    cordz_sample_token
+  HDRS
+    "internal/cordz_sample_token.h"
+  SRCS
+    "internal/cordz_sample_token.cc"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::config
+    absl::cordz_handle
+    absl::cordz_info
+)
+
+absl_cc_test(
+  NAME
+    cordz_sample_token_test
+  SRCS
+    "internal/cordz_sample_token_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::config
+    absl::cord_internal
+    absl::cordz_handle
+    absl::cordz_info
+    absl::cordz_info
+    absl::cordz_sample_token
+    absl::cordz_test_helpers
+    absl::memory
+    absl::random_random
+    absl::synchronization
+    absl::thread_pool
+    absl::time
+    GTest::gmock_main
+)
+
+# Internal-only target, do not depend on directly.
+absl_cc_library(
+  NAME
+    cordz_update_scope
+  HDRS
+    "internal/cordz_update_scope.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::config
+    absl::cord_internal
+    absl::cordz_info
+    absl::cordz_update_tracker
+    absl::core_headers
+)
+
+absl_cc_test(
+  NAME
+    cordz_update_scope_test
+  SRCS
+    "internal/cordz_update_scope_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::config
+    absl::cord_internal
+    absl::cordz_info
+    absl::cordz_test_helpers
+    absl::cordz_update_scope
+    absl::cordz_update_tracker
+    absl::core_headers
+    GTest::gmock_main
+)
+
+absl_cc_library(
+  NAME
+    cord
+  HDRS
+    "cord.h"
+    "cord_buffer.h"
+  SRCS
+    "cord.cc"
+    "cord_analysis.cc"
+    "cord_analysis.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::base
+    absl::config
+    absl::cord_internal
+    absl::cordz_functions
+    absl::cordz_info
+    absl::cordz_update_scope
+    absl::cordz_update_tracker
+    absl::core_headers
+    absl::crc32c
+    absl::crc_cord_state
+    absl::endian
+    absl::function_ref
+    absl::hardening
+    absl::inlined_vector
+    absl::nullability
+    absl::optional
+    absl::raw_logging_internal
+    absl::span
+    absl::strings
+    absl::strings_append_and_overwrite
+    absl::strings_resize_and_overwrite
+    absl::type_traits
+    absl::weakly_mixed_integer
+  PUBLIC
+)
+
+# Internal-only target, do not depend on directly.
+absl_cc_library(
+  NAME
+    cord_rep_test_util
+  HDRS
+    "internal/cord_rep_test_util.h"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::config
+    absl::cord_internal
+    absl::raw_logging_internal
+    absl::strings
+  TESTONLY
+)
+
+absl_cc_library(
+  NAME
+    cord_test_helpers
+  HDRS
+    "cord_test_helpers.h"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::config
+    absl::cord
+    absl::cord_internal
+    absl::strings
+  TESTONLY
+  PUBLIC
+)
+
+# Internal-only target, do not depend on directly.
+absl_cc_library(
+  NAME
+    cordz_test_helpers
+  HDRS
+    "cordz_test_helpers.h"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::config
+    absl::cord
+    absl::cord_internal
+    absl::cordz_info
+    absl::cordz_sample_token
+    absl::cordz_statistics
+    absl::cordz_update_tracker
+    absl::core_headers
+    absl::nullability
+    absl::strings
+  TESTONLY
+)
+
+absl_cc_test(
+  NAME
+    cord_test
+  SRCS
+    "cord_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::base
+    absl::check
+    absl::config
+    absl::cord
+    absl::cord_test_helpers
+    absl::cordz_test_helpers
+    absl::core_headers
+    absl::endian
+    absl::fixed_array
+    absl::function_ref
+    absl::hash
+    absl::hash_testing
+    absl::no_destructor
+    absl::log
+    absl::random_random
+    absl::str_format
+    absl::strings
+    GTest::gmock_main
+)
+
+absl_cc_test(
+  NAME
+    cord_data_edge_test
+  SRCS
+    "internal/cord_data_edge_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::base
+    absl::config
+    absl::cord_internal
+    absl::cord_rep_test_util
+    absl::core_headers
+    absl::strings
+    GTest::gmock_main
+)
+
+absl_cc_test(
+  NAME
+    cord_rep_btree_test
+  SRCS
+    "internal/cord_rep_btree_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::base
+    absl::cleanup
+    absl::config
+    absl::cord_internal
+    absl::cord_rep_test_util
+    absl::core_headers
+    absl::raw_logging_internal
+    absl::strings
+    GTest::gmock_main
+)
+
+absl_cc_test(
+  NAME
+    cord_rep_btree_navigator_test
+  SRCS
+    "internal/cord_rep_btree_navigator_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::base
+    absl::config
+    absl::cord_internal
+    absl::cord_rep_test_util
+    absl::core_headers
+    absl::raw_logging_internal
+    absl::strings
+    GTest::gmock_main
+)
+
+absl_cc_test(
+  NAME
+    cord_rep_btree_reader_test
+  SRCS
+    "internal/cord_rep_btree_reader_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::base
+    absl::config
+    absl::cord_internal
+    absl::cord_rep_test_util
+    absl::core_headers
+    absl::raw_logging_internal
+    absl::strings
+    GTest::gmock_main
+)
+
+absl_cc_test(
+  NAME
+    cord_rep_crc_test
+  SRCS
+    "internal/cord_rep_crc_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::config
+    absl::cord_internal
+    absl::cord_rep_test_util
+    absl::crc_cord_state
+    GTest::gmock_main
+)
+
+absl_cc_test(
+  NAME
+    cordz_test
+  SRCS
+    "cordz_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::cord
+    absl::cord_internal
+    absl::cord_test_helpers
+    absl::cordz_test_helpers
+    absl::cordz_functions
+    absl::cordz_info
+    absl::cordz_sample_token
+    absl::cordz_statistics
+    absl::cordz_update_tracker
+    absl::base
+    absl::config
+    absl::core_headers
+    absl::raw_logging_internal
+    absl::strings
+    GTest::gmock_main
+)
+
+absl_cc_library(
+  NAME
+    generic_printer_internal
+  SRCS
+    "internal/generic_printer.cc"
+    "internal/generic_printer_internal.h"
+  HDRS
+    "internal/generic_printer.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::config
+    absl::strings
+    absl::str_format
+    absl::log_internal_container
+    absl::requires_internal
+)
+
+absl_cc_test(
+  NAME
+    generic_printer_test
+  SRCS
+    "internal/generic_printer_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::base
+    absl::cleanup
+    absl::config
+    absl::flat_hash_map
+    absl::generic_printer_internal
+    absl::status
+    absl::statusor
+    absl::str_format
+    absl::strings
+    GTest::gmock_main
 )
diff --git a/absl/strings/ascii.cc b/absl/strings/ascii.cc
index 3f7c581f139..b5c7e195b7c 100644
--- a/absl/strings/ascii.cc
+++ b/absl/strings/ascii.cc
@@ -14,7 +14,16 @@
 
 #include "absl/strings/ascii.h"
 
+#include <climits>
+#include <cstddef>
+#include <cstring>
+#include <string>
+
+#include "absl/base/config.h"
+#include "absl/base/nullability.h"
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace ascii_internal {
 
 // # Table generated by this Python code (bit 0x02 is currently unused):
@@ -56,7 +65,7 @@ namespace ascii_internal {
 // of these bits is tightly coupled to this implementation, the individual bits
 // are not named. Note that bitfields for all characters above ASCII 127 are
 // zero-initialized.
-const unsigned char kPropertyBits[256] = {
+ABSL_DLL const unsigned char kPropertyBits[256] = {
     0x40, 0x40, 0x40, 0x40, 0x40, 0x40, 0x40, 0x40,  // 0x00
     0x40, 0x68, 0x48, 0x48, 0x48, 0x48, 0x40, 0x40,
     0x40, 0x40, 0x40, 0x40, 0x40, 0x40, 0x40, 0x40,  // 0x10
@@ -78,7 +87,7 @@ const unsigned char kPropertyBits[256] = {
 // Array of characters for the ascii_tolower() function. For values 'A'
 // through 'Z', return the lower-case character; otherwise, return the
 // identity of the passed character.
-const char kToLower[256] = {
+ABSL_DLL const char kToLower[256] = {
   '\x00', '\x01', '\x02', '\x03', '\x04', '\x05', '\x06', '\x07',
   '\x08', '\x09', '\x0a', '\x0b', '\x0c', '\x0d', '\x0e', '\x0f',
   '\x10', '\x11', '\x12', '\x13', '\x14', '\x15', '\x16', '\x17',
@@ -116,7 +125,7 @@ const char kToLower[256] = {
 // Array of characters for the ascii_toupper() function. For values 'a'
 // through 'z', return the upper-case character; otherwise, return the
 // identity of the passed character.
-const char kToUpper[256] = {
+ABSL_DLL const char kToUpper[256] = {
   '\x00', '\x01', '\x02', '\x03', '\x04', '\x05', '\x06', '\x07',
   '\x08', '\x09', '\x0a', '\x0b', '\x0c', '\x0d', '\x0e', '\x0f',
   '\x10', '\x11', '\x12', '\x13', '\x14', '\x15', '\x16', '\x17',
@@ -152,21 +161,109 @@ const char kToUpper[256] = {
 };
 // clang-format on
 
-}  // namespace ascii_internal
+// Returns whether `c` is in the a-z/A-Z range (w.r.t. `ToUpper`).
+// Implemented by:
+//  1. Pushing the a-z/A-Z range to [SCHAR_MIN, SCHAR_MIN + 26).
+//  2. Comparing to SCHAR_MIN + 26.
+template <bool ToUpper>
+constexpr bool AsciiInAZRange(unsigned char c) {
+  constexpr unsigned char sub = (ToUpper ? 'a' : 'A') - SCHAR_MIN;
+  constexpr signed char threshold = SCHAR_MIN + 26;  // 26 = alphabet size.
+  // Using unsigned arithmetic as overflows/underflows are well defined.
+  unsigned char u = c - sub;
+  // Using signed cmp, as SIMD unsigned cmp isn't available in many platforms.
+  return static_cast<signed char>(u) < threshold;
+}
+
+template <bool ToUpper>
+constexpr bool AsciiInAZRangeNaive(unsigned char c) {
+  constexpr unsigned char a = (ToUpper ? 'a' : 'A');
+  constexpr unsigned char z = (ToUpper ? 'z' : 'Z');
+  return a <= c && c <= z;
+}
 
-void AsciiStrToLower(std::string* s) {
-  for (auto& ch : *s) {
-    ch = absl::ascii_tolower(ch);
+template <bool ToUpper, bool Naive>
+constexpr void AsciiStrCaseFoldImpl(char* absl_nonnull dst,
+                                    const char* absl_nullable src,
+                                    size_t size) {
+  // The upper- and lowercase versions of ASCII characters differ by only 1 bit.
+  // When we need to flip the case, we can xor with this bit to achieve the
+  // desired result. Note that the choice of 'a' and 'A' here is arbitrary. We
+  // could have chosen 'z' and 'Z', or any other pair of characters as they all
+  // have the same single bit difference.
+  constexpr unsigned char kAsciiCaseBitFlip = 'a' ^ 'A';
+
+  for (size_t i = 0; i < size; ++i) {
+    unsigned char v = static_cast<unsigned char>(src[i]);
+    if constexpr (Naive) {
+      v ^= AsciiInAZRangeNaive<ToUpper>(v) ? kAsciiCaseBitFlip : 0;
+    } else {
+      v ^= AsciiInAZRange<ToUpper>(v) ? kAsciiCaseBitFlip : 0;
+    }
+    dst[i] = static_cast<char>(v);
   }
 }
 
-void AsciiStrToUpper(std::string* s) {
-  for (auto& ch : *s) {
-    ch = absl::ascii_toupper(ch);
+// Splitting to short and long strings to allow vectorization decisions
+// to be made separately in the long and short cases.
+// Using slightly different implementations so the compiler won't optimize them
+// into the same code (the non-naive version is needed for SIMD, so for short
+// strings it's not important).
+// `src` may be null iff `size` is zero.
+template <bool ToUpper>
+constexpr void AsciiStrCaseFold(char* absl_nonnull dst,
+                                const char* absl_nullable src, size_t size) {
+  size < 16 ? AsciiStrCaseFoldImpl<ToUpper, /*Naive=*/true>(dst, src, size)
+            : AsciiStrCaseFoldImpl<ToUpper, /*Naive=*/false>(dst, src, size);
+}
+
+void AsciiStrToLower(char* absl_nonnull dst, const char* absl_nullable src,
+                     size_t n) {
+  return AsciiStrCaseFold<false>(dst, src, n);
+}
+
+void AsciiStrToUpper(char* absl_nonnull dst, const char* absl_nullable src,
+                     size_t n) {
+  return AsciiStrCaseFold<true>(dst, src, n);
+}
+
+static constexpr size_t ValidateAsciiCasefold() {
+  constexpr size_t num_chars = 1 + CHAR_MAX - CHAR_MIN;
+  size_t incorrect_index = 0;
+  char lowered[num_chars] = {};
+  char uppered[num_chars] = {};
+  for (unsigned int i = 0; i < num_chars; ++i) {
+    uppered[i] = lowered[i] = static_cast<char>(i);
   }
+  AsciiStrCaseFold<false>(&lowered[0], &lowered[0], num_chars);
+  AsciiStrCaseFold<true>(&uppered[0], &uppered[0], num_chars);
+  for (size_t i = 0; i < num_chars; ++i) {
+    const char ch = static_cast<char>(i),
+               ch_upper = ('a' <= ch && ch <= 'z' ? 'A' + (ch - 'a') : ch),
+               ch_lower = ('A' <= ch && ch <= 'Z' ? 'a' + (ch - 'A') : ch);
+    if (uppered[i] != ch_upper || lowered[i] != ch_lower) {
+      incorrect_index = i > 0 ? i : num_chars;
+      break;
+    }
+  }
+  return incorrect_index;
+}
+
+static_assert(ValidateAsciiCasefold() == 0, "error in case conversion");
+
+}  // namespace ascii_internal
+
+void AsciiStrToLower(std::string* absl_nonnull s) {
+  char* p = &(*s)[0];
+  return ascii_internal::AsciiStrCaseFold<false>(p, p, s->size());
+}
+
+void AsciiStrToUpper(std::string* absl_nonnull s) {
+  char* p = &(*s)[0];
+  return ascii_internal::AsciiStrCaseFold<true>(p, p, s->size());
 }
 
-void RemoveExtraAsciiWhitespace(std::string* str) {
+void RemoveExtraAsciiWhitespace(std::string* absl_nonnull str) {
   auto stripped = StripAsciiWhitespace(*str);
 
   if (stripped.empty()) {
@@ -182,17 +279,18 @@ void RemoveExtraAsciiWhitespace(std::string* str) {
   for (; input_it < input_end; ++input_it) {
     if (is_ws) {
       // Consecutive whitespace?  Keep only the last.
-      is_ws = absl::ascii_isspace(*input_it);
+      is_ws = absl::ascii_isspace(static_cast<unsigned char>(*input_it));
       if (is_ws) --output_it;
     } else {
-      is_ws = absl::ascii_isspace(*input_it);
+      is_ws = absl::ascii_isspace(static_cast<unsigned char>(*input_it));
     }
 
     *output_it = *input_it;
     ++output_it;
   }
 
-  str->erase(output_it - &(*str)[0]);
+  str->erase(static_cast<size_t>(output_it - &(*str)[0]));
 }
 
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/ascii.h b/absl/strings/ascii.h
index f9e4fd1dbca..d6252d63caa 100644
--- a/absl/strings/ascii.h
+++ b/absl/strings/ascii.h
@@ -53,22 +53,34 @@
 #define ABSL_STRINGS_ASCII_H_
 
 #include <algorithm>
+#include <cstddef>
 #include <string>
+#include <utility>
 
 #include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/nullability.h"
+#include "absl/strings/resize_and_overwrite.h"
 #include "absl/strings/string_view.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace ascii_internal {
 
 // Declaration for an array of bitfields holding character information.
-extern const unsigned char kPropertyBits[256];
+ABSL_DLL extern const unsigned char kPropertyBits[256];
 
 // Declaration for the array of characters to upper-case characters.
-extern const char kToUpper[256];
+ABSL_DLL extern const char kToUpper[256];
 
 // Declaration for the array of characters to lower-case characters.
-extern const char kToLower[256];
+ABSL_DLL extern const char kToLower[256];
+
+void AsciiStrToLower(char* absl_nonnull dst, const char* absl_nullable src,
+                     size_t n);
+
+void AsciiStrToUpper(char* absl_nonnull dst, const char* absl_nullable src,
+                     size_t n);
 
 }  // namespace ascii_internal
 
@@ -118,7 +130,7 @@ inline bool ascii_iscntrl(unsigned char c) {
 // ascii_isxdigit()
 //
 // Determines whether the given character can be represented as a hexadecimal
-// digit character (i.e. {0-9} or {A-F}).
+// digit character (i.e. {0-9} or {A-F} or {a-f}).
 inline bool ascii_isxdigit(unsigned char c) {
   return (ascii_internal::kPropertyBits[c] & 0x80) != 0;
 }
@@ -127,32 +139,42 @@ inline bool ascii_isxdigit(unsigned char c) {
 //
 // Determines whether the given character can be represented as a decimal
 // digit character (i.e. {0-9}).
-inline bool ascii_isdigit(unsigned char c) { return c >= '0' && c <= '9'; }
+inline constexpr bool ascii_isdigit(unsigned char c) {
+  return c >= '0' && c <= '9';
+}
 
 // ascii_isprint()
 //
-// Determines whether the given character is printable, including whitespace.
-inline bool ascii_isprint(unsigned char c) { return c >= 32 && c < 127; }
+// Determines whether the given character is printable, including spaces.
+inline constexpr bool ascii_isprint(unsigned char c) {
+  return c >= 32 && c < 127;
+}
 
 // ascii_isgraph()
 //
 // Determines whether the given character has a graphical representation.
-inline bool ascii_isgraph(unsigned char c) { return c > 32 && c < 127; }
+inline constexpr bool ascii_isgraph(unsigned char c) {
+  return c > 32 && c < 127;
+}
 
 // ascii_isupper()
 //
 // Determines whether the given character is uppercase.
-inline bool ascii_isupper(unsigned char c) { return c >= 'A' && c <= 'Z'; }
+inline constexpr bool ascii_isupper(unsigned char c) {
+  return c >= 'A' && c <= 'Z';
+}
 
 // ascii_islower()
 //
 // Determines whether the given character is lowercase.
-inline bool ascii_islower(unsigned char c) { return c >= 'a' && c <= 'z'; }
+inline constexpr bool ascii_islower(unsigned char c) {
+  return c >= 'a' && c <= 'z';
+}
 
 // ascii_isascii()
 //
 // Determines whether the given character is ASCII.
-inline bool ascii_isascii(unsigned char c) { return c < 128; }
+inline constexpr bool ascii_isascii(unsigned char c) { return c < 128; }
 
 // ascii_tolower()
 //
@@ -163,11 +185,24 @@ inline char ascii_tolower(unsigned char c) {
 }
 
 // Converts the characters in `s` to lowercase, changing the contents of `s`.
-void AsciiStrToLower(std::string* s);
+void AsciiStrToLower(std::string* absl_nonnull s);
 
 // Creates a lowercase string from a given absl::string_view.
-ABSL_MUST_USE_RESULT inline std::string AsciiStrToLower(absl::string_view s) {
-  std::string result(s);
+[[nodiscard]] inline std::string AsciiStrToLower(absl::string_view s) {
+  std::string result;
+  StringResizeAndOverwrite(result, s.size(), [s](char* buf, size_t buf_size) {
+    ascii_internal::AsciiStrToLower(buf, s.data(), s.size());
+    return buf_size;
+  });
+  return result;
+}
+
+// Creates a lowercase string from a given std::string&&.
+//
+// (Template is used to lower priority of this overload.)
+template <int&... DoNotSpecify>
+[[nodiscard]] inline std::string AsciiStrToLower(std::string&& s) {
+  std::string result = std::move(s);
   absl::AsciiStrToLower(&result);
   return result;
 }
@@ -181,59 +216,73 @@ inline char ascii_toupper(unsigned char c) {
 }
 
 // Converts the characters in `s` to uppercase, changing the contents of `s`.
-void AsciiStrToUpper(std::string* s);
+void AsciiStrToUpper(std::string* absl_nonnull s);
 
 // Creates an uppercase string from a given absl::string_view.
-ABSL_MUST_USE_RESULT inline std::string AsciiStrToUpper(absl::string_view s) {
-  std::string result(s);
+[[nodiscard]] inline std::string AsciiStrToUpper(absl::string_view s) {
+  std::string result;
+  StringResizeAndOverwrite(result, s.size(), [s](char* buf, size_t buf_size) {
+    ascii_internal::AsciiStrToUpper(buf, s.data(), s.size());
+    return buf_size;
+  });
+  return result;
+}
+
+// Creates an uppercase string from a given std::string&&.
+//
+// (Template is used to lower priority of this overload.)
+template <int&... DoNotSpecify>
+[[nodiscard]] inline std::string AsciiStrToUpper(std::string&& s) {
+  std::string result = std::move(s);
   absl::AsciiStrToUpper(&result);
   return result;
 }
 
 // Returns absl::string_view with whitespace stripped from the beginning of the
 // given string_view.
-ABSL_MUST_USE_RESULT inline absl::string_view StripLeadingAsciiWhitespace(
-    absl::string_view str) {
+[[nodiscard]] inline absl::string_view StripLeadingAsciiWhitespace(
+    absl::string_view str ABSL_ATTRIBUTE_LIFETIME_BOUND) {
   auto it = std::find_if_not(str.begin(), str.end(), absl::ascii_isspace);
-  return str.substr(it - str.begin());
+  return str.substr(static_cast<size_t>(it - str.begin()));
 }
 
 // Strips in place whitespace from the beginning of the given string.
-inline void StripLeadingAsciiWhitespace(std::string* str) {
+inline void StripLeadingAsciiWhitespace(std::string* absl_nonnull str) {
   auto it = std::find_if_not(str->begin(), str->end(), absl::ascii_isspace);
   str->erase(str->begin(), it);
 }
 
 // Returns absl::string_view with whitespace stripped from the end of the given
 // string_view.
-ABSL_MUST_USE_RESULT inline absl::string_view StripTrailingAsciiWhitespace(
-    absl::string_view str) {
+[[nodiscard]] inline absl::string_view StripTrailingAsciiWhitespace(
+    absl::string_view str ABSL_ATTRIBUTE_LIFETIME_BOUND) {
   auto it = std::find_if_not(str.rbegin(), str.rend(), absl::ascii_isspace);
-  return str.substr(0, str.rend() - it);
+  return str.substr(0, static_cast<size_t>(str.rend() - it));
 }
 
 // Strips in place whitespace from the end of the given string
-inline void StripTrailingAsciiWhitespace(std::string* str) {
+inline void StripTrailingAsciiWhitespace(std::string* absl_nonnull str) {
   auto it = std::find_if_not(str->rbegin(), str->rend(), absl::ascii_isspace);
-  str->erase(str->rend() - it);
+  str->erase(static_cast<size_t>(str->rend() - it));
 }
 
 // Returns absl::string_view with whitespace stripped from both ends of the
 // given string_view.
-ABSL_MUST_USE_RESULT inline absl::string_view StripAsciiWhitespace(
-    absl::string_view str) {
+[[nodiscard]] inline absl::string_view StripAsciiWhitespace(
+    absl::string_view str ABSL_ATTRIBUTE_LIFETIME_BOUND) {
   return StripTrailingAsciiWhitespace(StripLeadingAsciiWhitespace(str));
 }
 
 // Strips in place whitespace from both ends of the given string
-inline void StripAsciiWhitespace(std::string* str) {
+inline void StripAsciiWhitespace(std::string* absl_nonnull str) {
   StripTrailingAsciiWhitespace(str);
   StripLeadingAsciiWhitespace(str);
 }
 
 // Removes leading, trailing, and consecutive internal whitespace.
-void RemoveExtraAsciiWhitespace(std::string*);
+void RemoveExtraAsciiWhitespace(std::string* absl_nonnull str);
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_ASCII_H_
diff --git a/absl/strings/ascii_benchmark.cc b/absl/strings/ascii_benchmark.cc
index aca458c8042..435090d380d 100644
--- a/absl/strings/ascii_benchmark.cc
+++ b/absl/strings/ascii_benchmark.cc
@@ -12,13 +12,14 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-#include "absl/strings/ascii.h"
-
+#include <algorithm>
+#include <array>
 #include <cctype>
+#include <cstddef>
 #include <string>
-#include <array>
-#include <random>
 
+#include "absl/random/random.h"
+#include "absl/strings/ascii.h"
 #include "benchmark/benchmark.h"
 
 namespace {
@@ -26,10 +27,8 @@ namespace {
 std::array<unsigned char, 256> MakeShuffledBytes() {
   std::array<unsigned char, 256> bytes;
   for (size_t i = 0; i < 256; ++i) bytes[i] = static_cast<unsigned char>(i);
-  std::random_device rd;
-  std::seed_seq seed({rd(), rd(), rd(), rd(), rd(), rd(), rd(), rd()});
-  std::mt19937 g(seed);
-  std::shuffle(bytes.begin(), bytes.end(), g);
+  absl::InsecureBitGen gen;
+  std::shuffle(bytes.begin(), bytes.end(), gen);
   return bytes;
 }
 
@@ -100,21 +99,59 @@ BENCHMARK_TEMPLATE(BM_Ascii, std::toupper);
 BENCHMARK_TEMPLATE(BM_Ascii, absl::ascii_toupper);
 
 static void BM_StrToLower(benchmark::State& state) {
-  const int size = state.range(0);
+  const size_t size = static_cast<size_t>(state.range(0));
   std::string s(size, 'X');
   for (auto _ : state) {
-    benchmark::DoNotOptimize(absl::AsciiStrToLower(s));
+    benchmark::DoNotOptimize(s);
+    std::string res = absl::AsciiStrToLower(s);
+    benchmark::DoNotOptimize(res);
   }
 }
-BENCHMARK(BM_StrToLower)->Range(1, 1 << 20);
+BENCHMARK(BM_StrToLower)
+    ->DenseRange(0, 32)
+    ->RangeMultiplier(2)
+    ->Range(64, 1 << 26);
 
 static void BM_StrToUpper(benchmark::State& state) {
-  const int size = state.range(0);
+  const size_t size = static_cast<size_t>(state.range(0));
+  std::string s(size, 'x');
+  for (auto _ : state) {
+    benchmark::DoNotOptimize(s);
+    std::string res = absl::AsciiStrToUpper(s);
+    benchmark::DoNotOptimize(res);
+  }
+}
+BENCHMARK(BM_StrToUpper)
+    ->DenseRange(0, 32)
+    ->RangeMultiplier(2)
+    ->Range(64, 1 << 26);
+
+static void BM_StrToUpperFromRvalref(benchmark::State& state) {
+  const size_t size = static_cast<size_t>(state.range(0));
+  std::string s(size, 'X');
+  for (auto _ : state) {
+    benchmark::DoNotOptimize(s);
+    std::string res = absl::AsciiStrToUpper(std::string(s));
+    benchmark::DoNotOptimize(res);
+  }
+}
+BENCHMARK(BM_StrToUpperFromRvalref)
+    ->DenseRange(0, 32)
+    ->RangeMultiplier(2)
+    ->Range(64, 1 << 26);
+
+static void BM_StrToLowerFromRvalref(benchmark::State& state) {
+  const size_t size = static_cast<size_t>(state.range(0));
   std::string s(size, 'x');
   for (auto _ : state) {
-    benchmark::DoNotOptimize(absl::AsciiStrToUpper(s));
+    benchmark::DoNotOptimize(s);
+    std::string res = absl::AsciiStrToLower(std::string(s));
+    benchmark::DoNotOptimize(res);
   }
 }
-BENCHMARK(BM_StrToUpper)->Range(1, 1 << 20);
+BENCHMARK(BM_StrToLowerFromRvalref)
+    ->DenseRange(0, 32)
+    ->RangeMultiplier(2)
+    ->Range(64, 1 << 26);
 
 }  // namespace
diff --git a/absl/strings/ascii_test.cc b/absl/strings/ascii_test.cc
index 5ecd23f8697..6afae493790 100644
--- a/absl/strings/ascii_test.cc
+++ b/absl/strings/ascii_test.cc
@@ -14,116 +14,114 @@
 
 #include "absl/strings/ascii.h"
 
+#include <algorithm>
 #include <cctype>
 #include <clocale>
 #include <cstring>
+#include <iterator>
 #include <string>
 
 #include "gtest/gtest.h"
 #include "absl/base/macros.h"
-#include "absl/base/port.h"
+#include "absl/strings/string_view.h"
 
 namespace {
 
 TEST(AsciiIsFoo, All) {
   for (int i = 0; i < 256; i++) {
-    if ((i >= 'a' && i <= 'z') || (i >= 'A' && i <= 'Z'))
-      EXPECT_TRUE(absl::ascii_isalpha(i)) << ": failed on " << i;
+    const auto c = static_cast<unsigned char>(i);
+    if ((c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z'))
+      EXPECT_TRUE(absl::ascii_isalpha(c)) << ": failed on " << c;
     else
-      EXPECT_TRUE(!absl::ascii_isalpha(i)) << ": failed on " << i;
+      EXPECT_TRUE(!absl::ascii_isalpha(c)) << ": failed on " << c;
   }
   for (int i = 0; i < 256; i++) {
-    if ((i >= '0' && i <= '9'))
-      EXPECT_TRUE(absl::ascii_isdigit(i)) << ": failed on " << i;
+    const auto c = static_cast<unsigned char>(i);
+    if ((c >= '0' && c <= '9'))
+      EXPECT_TRUE(absl::ascii_isdigit(c)) << ": failed on " << c;
     else
-      EXPECT_TRUE(!absl::ascii_isdigit(i)) << ": failed on " << i;
+      EXPECT_TRUE(!absl::ascii_isdigit(c)) << ": failed on " << c;
   }
   for (int i = 0; i < 256; i++) {
-    if (absl::ascii_isalpha(i) || absl::ascii_isdigit(i))
-      EXPECT_TRUE(absl::ascii_isalnum(i)) << ": failed on " << i;
+    const auto c = static_cast<unsigned char>(i);
+    if (absl::ascii_isalpha(c) || absl::ascii_isdigit(c))
+      EXPECT_TRUE(absl::ascii_isalnum(c)) << ": failed on " << c;
     else
-      EXPECT_TRUE(!absl::ascii_isalnum(i)) << ": failed on " << i;
+      EXPECT_TRUE(!absl::ascii_isalnum(c)) << ": failed on " << c;
   }
   for (int i = 0; i < 256; i++) {
+    const auto c = static_cast<unsigned char>(i);
     if (i != '\0' && strchr(" \r\n\t\v\f", i))
-      EXPECT_TRUE(absl::ascii_isspace(i)) << ": failed on " << i;
+      EXPECT_TRUE(absl::ascii_isspace(c)) << ": failed on " << c;
     else
-      EXPECT_TRUE(!absl::ascii_isspace(i)) << ": failed on " << i;
+      EXPECT_TRUE(!absl::ascii_isspace(c)) << ": failed on " << c;
   }
   for (int i = 0; i < 256; i++) {
+    const auto c = static_cast<unsigned char>(i);
     if (i >= 32 && i < 127)
-      EXPECT_TRUE(absl::ascii_isprint(i)) << ": failed on " << i;
+      EXPECT_TRUE(absl::ascii_isprint(c)) << ": failed on " << c;
     else
-      EXPECT_TRUE(!absl::ascii_isprint(i)) << ": failed on " << i;
+      EXPECT_TRUE(!absl::ascii_isprint(c)) << ": failed on " << c;
   }
   for (int i = 0; i < 256; i++) {
-    if (absl::ascii_isprint(i) && !absl::ascii_isspace(i) &&
-        !absl::ascii_isalnum(i))
-      EXPECT_TRUE(absl::ascii_ispunct(i)) << ": failed on " << i;
-    else
-      EXPECT_TRUE(!absl::ascii_ispunct(i)) << ": failed on " << i;
+    const auto c = static_cast<unsigned char>(i);
+    if (absl::ascii_isprint(c) && !absl::ascii_isspace(c) &&
+        !absl::ascii_isalnum(c)) {
+      EXPECT_TRUE(absl::ascii_ispunct(c)) << ": failed on " << c;
+    } else {
+      EXPECT_TRUE(!absl::ascii_ispunct(c)) << ": failed on " << c;
+    }
   }
   for (int i = 0; i < 256; i++) {
+    const auto c = static_cast<unsigned char>(i);
     if (i == ' ' || i == '\t')
-      EXPECT_TRUE(absl::ascii_isblank(i)) << ": failed on " << i;
+      EXPECT_TRUE(absl::ascii_isblank(c)) << ": failed on " << c;
     else
-      EXPECT_TRUE(!absl::ascii_isblank(i)) << ": failed on " << i;
+      EXPECT_TRUE(!absl::ascii_isblank(c)) << ": failed on " << c;
   }
   for (int i = 0; i < 256; i++) {
+    const auto c = static_cast<unsigned char>(i);
     if (i < 32 || i == 127)
-      EXPECT_TRUE(absl::ascii_iscntrl(i)) << ": failed on " << i;
+      EXPECT_TRUE(absl::ascii_iscntrl(c)) << ": failed on " << c;
     else
-      EXPECT_TRUE(!absl::ascii_iscntrl(i)) << ": failed on " << i;
+      EXPECT_TRUE(!absl::ascii_iscntrl(c)) << ": failed on " << c;
   }
   for (int i = 0; i < 256; i++) {
-    if (absl::ascii_isdigit(i) || (i >= 'A' && i <= 'F') ||
-        (i >= 'a' && i <= 'f'))
-      EXPECT_TRUE(absl::ascii_isxdigit(i)) << ": failed on " << i;
-    else
-      EXPECT_TRUE(!absl::ascii_isxdigit(i)) << ": failed on " << i;
+    const auto c = static_cast<unsigned char>(i);
+    if (absl::ascii_isdigit(c) || (i >= 'A' && i <= 'F') ||
+        (i >= 'a' && i <= 'f')) {
+      EXPECT_TRUE(absl::ascii_isxdigit(c)) << ": failed on " << c;
+    } else {
+      EXPECT_TRUE(!absl::ascii_isxdigit(c)) << ": failed on " << c;
+    }
   }
   for (int i = 0; i < 256; i++) {
+    const auto c = static_cast<unsigned char>(i);
     if (i > 32 && i < 127)
-      EXPECT_TRUE(absl::ascii_isgraph(i)) << ": failed on " << i;
+      EXPECT_TRUE(absl::ascii_isgraph(c)) << ": failed on " << c;
     else
-      EXPECT_TRUE(!absl::ascii_isgraph(i)) << ": failed on " << i;
+      EXPECT_TRUE(!absl::ascii_isgraph(c)) << ": failed on " << c;
   }
   for (int i = 0; i < 256; i++) {
+    const auto c = static_cast<unsigned char>(i);
     if (i >= 'A' && i <= 'Z')
-      EXPECT_TRUE(absl::ascii_isupper(i)) << ": failed on " << i;
+      EXPECT_TRUE(absl::ascii_isupper(c)) << ": failed on " << c;
     else
-      EXPECT_TRUE(!absl::ascii_isupper(i)) << ": failed on " << i;
+      EXPECT_TRUE(!absl::ascii_isupper(c)) << ": failed on " << c;
   }
   for (int i = 0; i < 256; i++) {
+    const auto c = static_cast<unsigned char>(i);
     if (i >= 'a' && i <= 'z')
-      EXPECT_TRUE(absl::ascii_islower(i)) << ": failed on " << i;
+      EXPECT_TRUE(absl::ascii_islower(c)) << ": failed on " << c;
     else
-      EXPECT_TRUE(!absl::ascii_islower(i)) << ": failed on " << i;
+      EXPECT_TRUE(!absl::ascii_islower(c)) << ": failed on " << c;
   }
-  for (int i = 0; i < 128; i++) {
-    EXPECT_TRUE(absl::ascii_isascii(i)) << ": failed on " << i;
+  for (unsigned char c = 0; c < 128; c++) {
+    EXPECT_TRUE(absl::ascii_isascii(c)) << ": failed on " << c;
   }
   for (int i = 128; i < 256; i++) {
-    EXPECT_TRUE(!absl::ascii_isascii(i)) << ": failed on " << i;
-  }
-
-  // The official is* functions don't accept negative signed chars, but
-  // our absl::ascii_is* functions do.
-  for (int i = 0; i < 256; i++) {
-    signed char sc = static_cast<signed char>(static_cast<unsigned char>(i));
-    EXPECT_EQ(absl::ascii_isalpha(i), absl::ascii_isalpha(sc)) << i;
-    EXPECT_EQ(absl::ascii_isdigit(i), absl::ascii_isdigit(sc)) << i;
-    EXPECT_EQ(absl::ascii_isalnum(i), absl::ascii_isalnum(sc)) << i;
-    EXPECT_EQ(absl::ascii_isspace(i), absl::ascii_isspace(sc)) << i;
-    EXPECT_EQ(absl::ascii_ispunct(i), absl::ascii_ispunct(sc)) << i;
-    EXPECT_EQ(absl::ascii_isblank(i), absl::ascii_isblank(sc)) << i;
-    EXPECT_EQ(absl::ascii_iscntrl(i), absl::ascii_iscntrl(sc)) << i;
-    EXPECT_EQ(absl::ascii_isxdigit(i), absl::ascii_isxdigit(sc)) << i;
-    EXPECT_EQ(absl::ascii_isprint(i), absl::ascii_isprint(sc)) << i;
-    EXPECT_EQ(absl::ascii_isgraph(i), absl::ascii_isgraph(sc)) << i;
-    EXPECT_EQ(absl::ascii_isupper(i), absl::ascii_isupper(sc)) << i;
-    EXPECT_EQ(absl::ascii_islower(i), absl::ascii_islower(sc)) << i;
-    EXPECT_EQ(absl::ascii_isascii(i), absl::ascii_isascii(sc)) << i;
+    const auto c = static_cast<unsigned char>(i);
+    EXPECT_TRUE(!absl::ascii_isascii(c)) << ": failed on " << c;
   }
 }
 
@@ -137,19 +135,20 @@ TEST(AsciiIsFoo, SameAsIsFoo) {
 #endif
 
   for (int i = 0; i < 256; i++) {
-    EXPECT_EQ(isalpha(i) != 0, absl::ascii_isalpha(i)) << i;
-    EXPECT_EQ(isdigit(i) != 0, absl::ascii_isdigit(i)) << i;
-    EXPECT_EQ(isalnum(i) != 0, absl::ascii_isalnum(i)) << i;
-    EXPECT_EQ(isspace(i) != 0, absl::ascii_isspace(i)) << i;
-    EXPECT_EQ(ispunct(i) != 0, absl::ascii_ispunct(i)) << i;
-    EXPECT_EQ(isblank(i) != 0, absl::ascii_isblank(i)) << i;
-    EXPECT_EQ(iscntrl(i) != 0, absl::ascii_iscntrl(i)) << i;
-    EXPECT_EQ(isxdigit(i) != 0, absl::ascii_isxdigit(i)) << i;
-    EXPECT_EQ(isprint(i) != 0, absl::ascii_isprint(i)) << i;
-    EXPECT_EQ(isgraph(i) != 0, absl::ascii_isgraph(i)) << i;
-    EXPECT_EQ(isupper(i) != 0, absl::ascii_isupper(i)) << i;
-    EXPECT_EQ(islower(i) != 0, absl::ascii_islower(i)) << i;
-    EXPECT_EQ(isascii(i) != 0, absl::ascii_isascii(i)) << i;
+    const auto c = static_cast<unsigned char>(i);
+    EXPECT_EQ(isalpha(c) != 0, absl::ascii_isalpha(c)) << c;
+    EXPECT_EQ(isdigit(c) != 0, absl::ascii_isdigit(c)) << c;
+    EXPECT_EQ(isalnum(c) != 0, absl::ascii_isalnum(c)) << c;
+    EXPECT_EQ(isspace(c) != 0, absl::ascii_isspace(c)) << c;
+    EXPECT_EQ(ispunct(c) != 0, absl::ascii_ispunct(c)) << c;
+    EXPECT_EQ(isblank(c) != 0, absl::ascii_isblank(c)) << c;
+    EXPECT_EQ(iscntrl(c) != 0, absl::ascii_iscntrl(c)) << c;
+    EXPECT_EQ(isxdigit(c) != 0, absl::ascii_isxdigit(c)) << c;
+    EXPECT_EQ(isprint(c) != 0, absl::ascii_isprint(c)) << c;
+    EXPECT_EQ(isgraph(c) != 0, absl::ascii_isgraph(c)) << c;
+    EXPECT_EQ(isupper(c) != 0, absl::ascii_isupper(c)) << c;
+    EXPECT_EQ(islower(c) != 0, absl::ascii_islower(c)) << c;
+    EXPECT_EQ(isascii(c) != 0, absl::ascii_isascii(c)) << c;
   }
 
 #ifndef __ANDROID__
@@ -166,25 +165,20 @@ TEST(AsciiToFoo, All) {
 #endif
 
   for (int i = 0; i < 256; i++) {
-    if (absl::ascii_islower(i))
-      EXPECT_EQ(absl::ascii_toupper(i), 'A' + (i - 'a')) << i;
+    const auto c = static_cast<unsigned char>(i);
+    if (absl::ascii_islower(c))
+      EXPECT_EQ(absl::ascii_toupper(c), 'A' + (i - 'a')) << c;
     else
-      EXPECT_EQ(absl::ascii_toupper(i), static_cast<char>(i)) << i;
+      EXPECT_EQ(absl::ascii_toupper(c), static_cast<char>(i)) << c;
 
-    if (absl::ascii_isupper(i))
-      EXPECT_EQ(absl::ascii_tolower(i), 'a' + (i - 'A')) << i;
+    if (absl::ascii_isupper(c))
+      EXPECT_EQ(absl::ascii_tolower(c), 'a' + (i - 'A')) << c;
     else
-      EXPECT_EQ(absl::ascii_tolower(i), static_cast<char>(i)) << i;
+      EXPECT_EQ(absl::ascii_tolower(c), static_cast<char>(i)) << c;
 
     // These CHECKs only hold in a C locale.
-    EXPECT_EQ(static_cast<char>(tolower(i)), absl::ascii_tolower(i)) << i;
-    EXPECT_EQ(static_cast<char>(toupper(i)), absl::ascii_toupper(i)) << i;
-
-    // The official to* functions don't accept negative signed chars, but
-    // our absl::ascii_to* functions do.
-    signed char sc = static_cast<signed char>(static_cast<unsigned char>(i));
-    EXPECT_EQ(absl::ascii_tolower(i), absl::ascii_tolower(sc)) << i;
-    EXPECT_EQ(absl::ascii_toupper(i), absl::ascii_toupper(sc)) << i;
+    EXPECT_EQ(static_cast<char>(tolower(i)), absl::ascii_tolower(c)) << c;
+    EXPECT_EQ(static_cast<char>(toupper(i)), absl::ascii_toupper(c)) << c;
   }
 #ifndef __ANDROID__
   // restore the old locale.
@@ -197,10 +191,22 @@ TEST(AsciiStrTo, Lower) {
   const std::string str("GHIJKL");
   const std::string str2("MNOPQR");
   const absl::string_view sp(str2);
+  const std::string long_str("ABCDEFGHIJKLMNOPQRSTUVWXYZ1!a");
+  std::string mutable_str("_`?@[{AMNOPQRSTUVWXYZ");
+  auto fun = []() -> std::string { return "PQRSTU"; };
 
   EXPECT_EQ("abcdef", absl::AsciiStrToLower(buf));
   EXPECT_EQ("ghijkl", absl::AsciiStrToLower(str));
   EXPECT_EQ("mnopqr", absl::AsciiStrToLower(sp));
+  EXPECT_EQ("abcdefghijklmnopqrstuvwxyz1!a", absl::AsciiStrToLower(long_str));
+  EXPECT_EQ("pqrstu", absl::AsciiStrToLower(fun()));
+
+  // An empty `string_view` specifically exercises the case where a null data
+  // pointer is passed to internal functions.
+  EXPECT_EQ("", absl::AsciiStrToLower(absl::string_view()));
+
+  absl::AsciiStrToLower(&mutable_str);
+  EXPECT_EQ("_`?@[{amnopqrstuvwxyz", mutable_str);
 
   char mutable_buf[] = "Mutable";
   std::transform(mutable_buf, mutable_buf + strlen(mutable_buf),
@@ -211,12 +217,20 @@ TEST(AsciiStrTo, Lower) {
 TEST(AsciiStrTo, Upper) {
   const char buf[] = "abcdef";
   const std::string str("ghijkl");
-  const std::string str2("mnopqr");
+  const std::string str2("_`?@[{amnopqrstuvwxyz");
   const absl::string_view sp(str2);
+  const std::string long_str("abcdefghijklmnopqrstuvwxyz1!A");
+  auto fun = []() -> std::string { return "pqrstu"; };
 
   EXPECT_EQ("ABCDEF", absl::AsciiStrToUpper(buf));
   EXPECT_EQ("GHIJKL", absl::AsciiStrToUpper(str));
-  EXPECT_EQ("MNOPQR", absl::AsciiStrToUpper(sp));
+  EXPECT_EQ("_`?@[{AMNOPQRSTUVWXYZ", absl::AsciiStrToUpper(sp));
+  EXPECT_EQ("ABCDEFGHIJKLMNOPQRSTUVWXYZ1!A", absl::AsciiStrToUpper(long_str));
+  EXPECT_EQ("PQRSTU", absl::AsciiStrToUpper(fun()));
+
+  // An empty `string_view` specifically exercises the case where a null data
+  // pointer is passed to internal functions.
+  EXPECT_EQ("", absl::AsciiStrToUpper(absl::string_view()));
 
   char mutable_buf[] = "Mutable";
   std::transform(mutable_buf, mutable_buf + strlen(mutable_buf),
@@ -349,7 +363,7 @@ TEST(RemoveExtraAsciiWhitespace, InPlace) {
       "",
       "a\nb",
   };
-  const int NUM_TESTS = ABSL_ARRAYSIZE(inputs);
+  const int NUM_TESTS = std::size(inputs);
 
   for (int i = 0; i < NUM_TESTS; i++) {
     std::string s(inputs[i]);
diff --git a/absl/strings/atod_manual_test.cc b/absl/strings/atod_manual_test.cc
new file mode 100644
index 00000000000..b22657a5daf
--- /dev/null
+++ b/absl/strings/atod_manual_test.cc
@@ -0,0 +1,193 @@
+// Copyright 2022 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// This program tests the absl::SimpleAtod and absl::SimpleAtof functions. Run
+// it as "atod_manual_test pnftd/data/*.txt" where the pnftd directory is a
+// local checkout of the https://github.com/nigeltao/parse-number-fxx-test-data
+// repository. The test suite lives in a separate repository because its more
+// than 5 million test cases weigh over several hundred megabytes and because
+// the test cases are also useful to other software projects, not just Abseil.
+// Its data/*.txt files contain one test case per line, like:
+//
+// 3C00 3F800000 3FF0000000000000 1
+// 3D00 3FA00000 3FF4000000000000 1.25
+// 3D9A 3FB33333 3FF6666666666666 1.4
+// 57B7 42F6E979 405EDD2F1A9FBE77 123.456
+// 622A 44454000 4088A80000000000 789
+// 7C00 7F800000 7FF0000000000000 123.456e789
+//
+// For each line (and using 0-based column indexes), columns [5..13] and
+// [14..30] contain the 32-bit float and 64-bit double result of parsing
+// columns [31..].
+//
+// For example, parsing "1.4" as a float gives the bits 0x3FB33333.
+//
+// In this 6-line example, the final line's float and double values are all
+// infinity. The largest finite float and double values are approximately
+// 3.40e+38 and 1.80e+308.
+
+#include <cstdint>
+#include <cstdio>
+#include <optional>
+#include <string>
+
+#include "absl/base/casts.h"
+#include "absl/strings/numbers.h"
+#include "absl/strings/str_format.h"
+#include "absl/strings/string_view.h"
+
+static constexpr uint8_t kUnhex[256] = {
+    0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,  //
+    0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,  //
+    0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,  //
+    0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,  //
+    0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,  //
+    0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,  //
+    0x0, 0x1, 0x2, 0x3, 0x4, 0x5, 0x6, 0x7,  // '0' ..= '7'
+    0x8, 0x9, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,  // '8' ..= '9'
+
+    0x0, 0xA, 0xB, 0xC, 0xD, 0xE, 0xF, 0x0,  // 'A' ..= 'F'
+    0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,  //
+    0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,  //
+    0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,  //
+    0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,  //
+    0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,  //
+    0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,  //
+    0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,  //
+
+    0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,  //
+    0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,  //
+    0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,  //
+    0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,  //
+    0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,  //
+    0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,  //
+    0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,  //
+    0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,  //
+
+    0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,  //
+    0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,  //
+    0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,  //
+    0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,  //
+    0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,  //
+    0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,  //
+    0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,  //
+    0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,  //
+};
+
+static std::optional<std::string> ReadFileToString(const char* filename) {
+  FILE* f = fopen(filename, "rb");
+  if (!f) {
+    return std::nullopt;
+  }
+  fseek(f, 0, SEEK_END);
+  size_t size = ftell(f);
+  fseek(f, 0, SEEK_SET);
+  std::string s(size, '\x00');
+  size_t n = fread(&s[0], 1, size, f);
+  fclose(f);
+  if (n != size) {
+    return std::nullopt;
+  }
+  return s;
+}
+
+static bool ProcessOneTestFile(const char* filename) {
+  std::optional<std::string> contents = ReadFileToString(filename);
+  if (!contents) {
+    absl::FPrintF(stderr, "Invalid file: %s\n", filename);
+    return false;
+  }
+
+  int num_cases = 0;
+  for (absl::string_view v(*contents); !v.empty();) {
+    size_t new_line = v.find('\n');
+    if ((new_line == absl::string_view::npos) || (new_line < 32)) {
+      break;
+    }
+    absl::string_view input = v.substr(31, new_line - 31);
+
+    // Test absl::SimpleAtof.
+    {
+      float f;
+      if (!absl::SimpleAtof(input, &f)) {
+        absl::FPrintF(stderr, "Could not parse \"%s\" in %s\n", input,
+                      filename);
+        return false;
+      }
+      uint32_t have32 = absl::bit_cast<uint32_t>(f);
+
+      uint32_t want32 = 0;
+      for (int i = 0; i < 8; i++) {
+        want32 = (want32 << 4) | kUnhex[static_cast<unsigned char>(v[5 + i])];
+      }
+
+      if (have32 != want32) {
+        absl::FPrintF(stderr,
+                      "absl::SimpleAtof failed parsing \"%s\" in %s\n  have  "
+                      "%08X\n  want  %08X\n",
+                      input, filename, have32, want32);
+        return false;
+      }
+    }
+
+    // Test absl::SimpleAtod.
+    {
+      double d;
+      if (!absl::SimpleAtod(input, &d)) {
+        absl::FPrintF(stderr, "Could not parse \"%s\" in %s\n", input,
+                      filename);
+        return false;
+      }
+      uint64_t have64 = absl::bit_cast<uint64_t>(d);
+
+      uint64_t want64 = 0;
+      for (int i = 0; i < 16; i++) {
+        want64 = (want64 << 4) | kUnhex[static_cast<unsigned char>(v[14 + i])];
+      }
+
+      if (have64 != want64) {
+        absl::FPrintF(stderr,
+                      "absl::SimpleAtod failed parsing \"%s\" in %s\n  have  "
+                      "%016X\n  want  %016X\n",
+                      input, filename, have64, want64);
+        return false;
+      }
+    }
+
+    num_cases++;
+    v = v.substr(new_line + 1);
+  }
+  printf("%8d OK in %s\n", num_cases, filename);
+  return true;
+}
+
+int main(int argc, char** argv) {
+  if (argc < 2) {
+    absl::FPrintF(
+        stderr,
+        "Usage: %s pnftd/data/*.txt\nwhere the pnftd directory is a local "
+        "checkout of "
+        "the\nhttps://github.com/nigeltao/parse-number-fxx-test-data "
+        "repository.\n",
+        argv[0]);
+    return 1;
+  }
+
+  for (int i = 1; i < argc; i++) {
+    if (!ProcessOneTestFile(argv[i])) {
+      return 1;
+    }
+  }
+  return 0;
+}
diff --git a/absl/strings/char_formatting_test.cc b/absl/strings/char_formatting_test.cc
new file mode 100644
index 00000000000..1692da7028f
--- /dev/null
+++ b/absl/strings/char_formatting_test.cc
@@ -0,0 +1,169 @@
+// Copyright 2023 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <cstddef>
+
+#include "gtest/gtest.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/str_format.h"
+#include "absl/strings/substitute.h"
+
+namespace {
+
+TEST(CharFormatting, Char) {
+  const char v = 'A';
+
+  // Desired behavior: does not compile:
+  // EXPECT_EQ(absl::StrCat(v, "B"), "AB");
+  // EXPECT_EQ(absl::StrFormat("%vB", v), "AB");
+
+  // Legacy behavior: format as char:
+  EXPECT_EQ(absl::Substitute("$0B", v), "AB");
+}
+
+enum CharEnum : char {};
+TEST(CharFormatting, CharEnum) {
+  auto v = static_cast<CharEnum>('A');
+
+  // Desired behavior: format as decimal
+  EXPECT_EQ(absl::StrFormat("%vB", v), "65B");
+  EXPECT_EQ(absl::StrCat(v, "B"), "65B");
+
+  // Legacy behavior: format as character:
+
+  // Some older versions of gcc behave differently in this one case
+#if !defined(__GNUC__) || defined(__clang__)
+  EXPECT_EQ(absl::Substitute("$0B", v), "AB");
+#endif
+}
+
+enum class CharEnumClass: char {};
+TEST(CharFormatting, CharEnumClass) {
+  auto v = static_cast<CharEnumClass>('A');
+
+  // Desired behavior: format as decimal:
+  EXPECT_EQ(absl::StrFormat("%vB", v), "65B");
+  EXPECT_EQ(absl::StrCat(v, "B"), "65B");
+
+  // Legacy behavior: format as character:
+  EXPECT_EQ(absl::Substitute("$0B", v), "AB");
+}
+
+TEST(CharFormatting, UnsignedChar) {
+  const unsigned char v = 'A';
+
+  // Desired behavior: format as decimal:
+  EXPECT_EQ(absl::StrCat(v, "B"), "65B");
+  EXPECT_EQ(absl::Substitute("$0B", v), "65B");
+  EXPECT_EQ(absl::StrFormat("%vB", v), "65B");
+
+  // Signedness check
+  const unsigned char w = 255;
+  EXPECT_EQ(absl::StrCat(w, "B"), "255B");
+  EXPECT_EQ(absl::Substitute("$0B", w), "255B");
+  // EXPECT_EQ(absl::StrFormat("%vB", v), "255B");
+}
+
+TEST(CharFormatting, SignedChar) {
+  const signed char v = 'A';
+
+  // Desired behavior: format as decimal:
+  EXPECT_EQ(absl::StrCat(v, "B"), "65B");
+  EXPECT_EQ(absl::Substitute("$0B", v), "65B");
+  EXPECT_EQ(absl::StrFormat("%vB", v), "65B");
+
+  // Signedness check
+  const signed char w = -128;
+  EXPECT_EQ(absl::StrCat(w, "B"), "-128B");
+  EXPECT_EQ(absl::Substitute("$0B", w), "-128B");
+}
+
+enum UnsignedCharEnum : unsigned char {};
+TEST(CharFormatting, UnsignedCharEnum) {
+  auto v = static_cast<UnsignedCharEnum>('A');
+
+  // Desired behavior: format as decimal:
+  EXPECT_EQ(absl::StrCat(v, "B"), "65B");
+  EXPECT_EQ(absl::Substitute("$0B", v), "65B");
+  EXPECT_EQ(absl::StrFormat("%vB", v), "65B");
+
+  // Signedness check
+  auto w = static_cast<UnsignedCharEnum>(255);
+  EXPECT_EQ(absl::StrCat(w, "B"), "255B");
+  EXPECT_EQ(absl::Substitute("$0B", w), "255B");
+  EXPECT_EQ(absl::StrFormat("%vB", w), "255B");
+}
+
+enum SignedCharEnum : signed char {};
+TEST(CharFormatting, SignedCharEnum) {
+  auto v = static_cast<SignedCharEnum>('A');
+
+  // Desired behavior: format as decimal:
+  EXPECT_EQ(absl::StrCat(v, "B"), "65B");
+  EXPECT_EQ(absl::Substitute("$0B", v), "65B");
+  EXPECT_EQ(absl::StrFormat("%vB", v), "65B");
+
+  // Signedness check
+  auto w = static_cast<SignedCharEnum>(-128);
+  EXPECT_EQ(absl::StrCat(w, "B"), "-128B");
+  EXPECT_EQ(absl::Substitute("$0B", w), "-128B");
+  EXPECT_EQ(absl::StrFormat("%vB", w), "-128B");
+}
+
+enum class UnsignedCharEnumClass : unsigned char {};
+TEST(CharFormatting, UnsignedCharEnumClass) {
+  auto v = static_cast<UnsignedCharEnumClass>('A');
+
+  // Desired behavior: format as decimal:
+  EXPECT_EQ(absl::StrCat(v, "B"), "65B");
+  EXPECT_EQ(absl::Substitute("$0B", v), "65B");
+  EXPECT_EQ(absl::StrFormat("%vB", v), "65B");
+
+  // Signedness check
+  auto w = static_cast<UnsignedCharEnumClass>(255);
+  EXPECT_EQ(absl::StrCat(w, "B"), "255B");
+  EXPECT_EQ(absl::Substitute("$0B", w), "255B");
+  EXPECT_EQ(absl::StrFormat("%vB", w), "255B");
+}
+
+enum SignedCharEnumClass : signed char {};
+TEST(CharFormatting, SignedCharEnumClass) {
+  auto v = static_cast<SignedCharEnumClass>('A');
+
+  // Desired behavior: format as decimal:
+  EXPECT_EQ(absl::StrCat(v, "B"), "65B");
+  EXPECT_EQ(absl::Substitute("$0B", v), "65B");
+  EXPECT_EQ(absl::StrFormat("%vB", v), "65B");
+
+  // Signedness check
+  auto w = static_cast<SignedCharEnumClass>(-128);
+  EXPECT_EQ(absl::StrCat(w, "B"), "-128B");
+  EXPECT_EQ(absl::Substitute("$0B", w), "-128B");
+  EXPECT_EQ(absl::StrFormat("%vB", w), "-128B");
+}
+
+#ifdef __cpp_lib_byte
+TEST(CharFormatting, StdByte) {
+  auto v = static_cast<std::byte>('A');
+  // Desired behavior: format as 0xff
+  // (No APIs do this today.)
+
+  // Legacy behavior: format as decimal:
+  EXPECT_EQ(absl::StrCat(v, "B"), "65B");
+  EXPECT_EQ(absl::Substitute("$0B", v), "65B");
+  EXPECT_EQ(absl::StrFormat("%vB", v), "65B");
+}
+#endif  // _cpp_lib_byte
+
+}  // namespace
diff --git a/absl/strings/charconv.cc b/absl/strings/charconv.cc
index bc07e7ab9e4..6f367739e9d 100644
--- a/absl/strings/charconv.cc
+++ b/absl/strings/charconv.cc
@@ -16,11 +16,15 @@
 
 #include <algorithm>
 #include <cassert>
-#include <cmath>
-#include <cstring>
+#include <cstddef>
+#include <cstdint>
+#include <limits>
+#include <system_error>  // NOLINT(build/c++11)
 
 #include "absl/base/casts.h"
-#include "absl/base/internal/bits.h"
+#include "absl/base/config.h"
+#include "absl/base/nullability.h"
+#include "absl/numeric/bits.h"
 #include "absl/numeric/int128.h"
 #include "absl/strings/internal/charconv_bigint.h"
 #include "absl/strings/internal/charconv_parse.h"
@@ -57,6 +61,7 @@
 // narrower mantissas.
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace {
 
 template <typename FloatType>
@@ -64,6 +69,14 @@ struct FloatTraits;
 
 template <>
 struct FloatTraits<double> {
+  using mantissa_t = uint64_t;
+
+  // The number of bits in the given float type.
+  static constexpr int kTargetBits = 64;
+
+  // The number of exponent bits in the given float type.
+  static constexpr int kTargetExponentBits = 11;
+
   // The number of mantissa bits in the given float type.  This includes the
   // implied high bit.
   static constexpr int kTargetMantissaBits = 53;
@@ -82,11 +95,43 @@ struct FloatTraits<double> {
   // m * 2**kMinNormalExponent is exactly equal to DBL_MIN.
   static constexpr int kMinNormalExponent = -1074;
 
-  static double MakeNan(const char* tagp) {
+  // The IEEE exponent bias.  It equals ((1 << (kTargetExponentBits - 1)) - 1).
+  static constexpr int kExponentBias = 1023;
+
+  // The Eisel-Lemire "Shifting to 54/25 Bits" adjustment.  It equals (63 - 1 -
+  // kTargetMantissaBits).
+  static constexpr int kEiselLemireShift = 9;
+
+  // The Eisel-Lemire high64_mask.  It equals ((1 << kEiselLemireShift) - 1).
+  static constexpr uint64_t kEiselLemireMask = uint64_t{0x1FF};
+
+  // The smallest negative integer N (smallest negative means furthest from
+  // zero) such that parsing 9999999999999999999eN, with 19 nines, is still
+  // positive. Parsing a smaller (more negative) N will produce zero.
+  //
+  // Adjusting the decimal point and exponent, without adjusting the value,
+  // 9999999999999999999eN equals 9.999999999999999999eM where M = N + 18.
+  //
+  // 9999999999999999999, with 19 nines but no decimal point, is the largest
+  // "repeated nines" integer that fits in a uint64_t.
+  static constexpr int kEiselLemireMinInclusiveExp10 = -324 - 18;
+
+  // The smallest positive integer N such that parsing 1eN produces infinity.
+  // Parsing a smaller N will produce something finite.
+  static constexpr int kEiselLemireMaxExclusiveExp10 = 309;
+
+  static double MakeNan(const char* absl_nonnull tagp) {
+#if ABSL_HAVE_BUILTIN(__builtin_nan)
+    // Use __builtin_nan() if available since it has a fix for
+    // https://bugs.llvm.org/show_bug.cgi?id=37778
+    // std::nan may use the glibc implementation.
+    return __builtin_nan(tagp);
+#else
     // Support nan no matter which namespace it's in.  Some platforms
     // incorrectly don't put it in namespace std.
     using namespace std;  // NOLINT
     return nan(tagp);
+#endif
   }
 
   // Builds a nonzero floating point number out of the provided parts.
@@ -102,7 +147,7 @@ struct FloatTraits<double> {
   // a normal value is made, or it must be less narrow than that, in which case
   // `exponent` must be exactly kMinNormalExponent, and a subnormal value is
   // made.
-  static double Make(uint64_t mantissa, int exponent, bool sign) {
+  static double Make(mantissa_t mantissa, int exponent, bool sign) {
 #ifndef ABSL_BIT_PACK_FLOATS
     // Support ldexp no matter which namespace it's in.  Some platforms
     // incorrectly don't put it in namespace std.
@@ -110,13 +155,15 @@ struct FloatTraits<double> {
     return sign ? -ldexp(mantissa, exponent) : ldexp(mantissa, exponent);
 #else
     constexpr uint64_t kMantissaMask =
-        (uint64_t(1) << (kTargetMantissaBits - 1)) - 1;
+        (uint64_t{1} << (kTargetMantissaBits - 1)) - 1;
     uint64_t dbl = static_cast<uint64_t>(sign) << 63;
     if (mantissa > kMantissaMask) {
       // Normal value.
       // Adjust by 1023 for the exponent representation bias, and an additional
-      // 52 due to the implied decimal point in the IEEE mantissa represenation.
-      dbl += uint64_t{exponent + 1023u + kTargetMantissaBits - 1} << 52;
+      // 52 due to the implied decimal point in the IEEE mantissa
+      // representation.
+      dbl += static_cast<uint64_t>(exponent + 1023 + kTargetMantissaBits - 1)
+             << 52;
       mantissa &= kMantissaMask;
     } else {
       // subnormal value
@@ -133,16 +180,34 @@ struct FloatTraits<double> {
 // members and methods.
 template <>
 struct FloatTraits<float> {
+  using mantissa_t = uint32_t;
+
+  static constexpr int kTargetBits = 32;
+  static constexpr int kTargetExponentBits = 8;
   static constexpr int kTargetMantissaBits = 24;
   static constexpr int kMaxExponent = 104;
   static constexpr int kMinNormalExponent = -149;
-  static float MakeNan(const char* tagp) {
+  static constexpr int kExponentBias = 127;
+  static constexpr int kEiselLemireShift = 38;
+  static constexpr uint64_t kEiselLemireMask = uint64_t{0x3FFFFFFFFF};
+  static constexpr int kEiselLemireMinInclusiveExp10 = -46 - 18;
+  static constexpr int kEiselLemireMaxExclusiveExp10 = 39;
+
+  static float MakeNan(const char* absl_nonnull tagp) {
+#if ABSL_HAVE_BUILTIN(__builtin_nanf)
+    // Use __builtin_nanf() if available since it has a fix for
+    // https://bugs.llvm.org/show_bug.cgi?id=37778
+    // std::nanf may use the glibc implementation.
+    return __builtin_nanf(tagp);
+#else
     // Support nanf no matter which namespace it's in.  Some platforms
     // incorrectly don't put it in namespace std.
     using namespace std;  // NOLINT
-    return nanf(tagp);
+    return std::nanf(tagp);
+#endif
   }
-  static float Make(uint32_t mantissa, int exponent, bool sign) {
+
+  static float Make(mantissa_t mantissa, int exponent, bool sign) {
 #ifndef ABSL_BIT_PACK_FLOATS
     // Support ldexpf no matter which namespace it's in.  Some platforms
     // incorrectly don't put it in namespace std.
@@ -150,13 +215,15 @@ struct FloatTraits<float> {
     return sign ? -ldexpf(mantissa, exponent) : ldexpf(mantissa, exponent);
 #else
     constexpr uint32_t kMantissaMask =
-        (uint32_t(1) << (kTargetMantissaBits - 1)) - 1;
+        (uint32_t{1} << (kTargetMantissaBits - 1)) - 1;
     uint32_t flt = static_cast<uint32_t>(sign) << 31;
     if (mantissa > kMantissaMask) {
       // Normal value.
       // Adjust by 127 for the exponent representation bias, and an additional
-      // 23 due to the implied decimal point in the IEEE mantissa represenation.
-      flt += uint32_t{exponent + 127u + kTargetMantissaBits - 1} << 23;
+      // 23 due to the implied decimal point in the IEEE mantissa
+      // representation.
+      flt += static_cast<uint32_t>(exponent + 127 + kTargetMantissaBits - 1)
+             << 23;
       mantissa &= kMantissaMask;
     } else {
       // subnormal value
@@ -180,39 +247,45 @@ struct FloatTraits<float> {
 //
 //   2**63 <= Power10Mantissa(n) < 2**64.
 //
+// See the "Table of powers of 10" comment below for a "1e60" example.
+//
 // Lookups into the power-of-10 table must first check the Power10Overflow() and
 // Power10Underflow() functions, to avoid out-of-bounds table access.
 //
-// Indexes into these tables are biased by -kPower10TableMin, and the table has
-// values in the range [kPower10TableMin, kPower10TableMax].
-extern const uint64_t kPower10MantissaTable[];
-extern const int16_t kPower10ExponentTable[];
+// Indexes into these tables are biased by -kPower10TableMinInclusive. Valid
+// indexes range from kPower10TableMinInclusive to kPower10TableMaxExclusive.
+extern const uint64_t kPower10MantissaHighTable[];  // High 64 of 128 bits.
+extern const uint64_t kPower10MantissaLowTable[];   // Low  64 of 128 bits.
 
-// The smallest allowed value for use with the Power10Mantissa() and
-// Power10Exponent() functions below.  (If a smaller exponent is needed in
+// The smallest (inclusive) allowed value for use with the Power10Mantissa()
+// and Power10Exponent() functions below.  (If a smaller exponent is needed in
 // calculations, the end result is guaranteed to underflow.)
-constexpr int kPower10TableMin = -342;
+constexpr int kPower10TableMinInclusive = -342;
 
-// The largest allowed value for use with the Power10Mantissa() and
-// Power10Exponent() functions below.  (If a smaller exponent is needed in
-// calculations, the end result is guaranteed to overflow.)
-constexpr int kPower10TableMax = 308;
+// The largest (exclusive) allowed value for use with the Power10Mantissa() and
+// Power10Exponent() functions below.  (If a larger-or-equal exponent is needed
+// in calculations, the end result is guaranteed to overflow.)
+constexpr int kPower10TableMaxExclusive = 309;
 
 uint64_t Power10Mantissa(int n) {
-  return kPower10MantissaTable[n - kPower10TableMin];
+  return kPower10MantissaHighTable[n - kPower10TableMinInclusive];
 }
 
 int Power10Exponent(int n) {
-  return kPower10ExponentTable[n - kPower10TableMin];
+  // The 217706 etc magic numbers encode the results as a formula instead of a
+  // table. Their equivalence (over the kPower10TableMinInclusive ..
+  // kPower10TableMaxExclusive range) is confirmed by
+  // https://github.com/google/wuffs/blob/315b2e52625ebd7b02d8fac13e3cd85ea374fb80/script/print-mpb-powers-of-10.go
+  return (217706 * n >> 16) - 63;
 }
 
 // Returns true if n is large enough that 10**n always results in an IEEE
 // overflow.
-bool Power10Overflow(int n) { return n > kPower10TableMax; }
+bool Power10Overflow(int n) { return n >= kPower10TableMaxExclusive; }
 
 // Returns true if n is small enough that 10**n times a ParsedFloat mantissa
 // always results in an IEEE underflow.
-bool Power10Underflow(int n) { return n < kPower10TableMin; }
+bool Power10Underflow(int n) { return n < kPower10TableMinInclusive; }
 
 // Returns true if Power10Mantissa(n) * 2**Power10Exponent(n) is exactly equal
 // to 10**n numerically.  Put another way, this returns true if there is no
@@ -243,9 +316,11 @@ struct CalculatedFloat {
 // minus the number of leading zero bits.)
 int BitWidth(uint128 value) {
   if (Uint128High64(value) == 0) {
-    return 64 - base_internal::CountLeadingZeros64(Uint128Low64(value));
+    // This static_cast is only needed when using a std::bit_width()
+    // implementation that does not have the fix for LWG 3656 applied.
+    return static_cast<int>(bit_width(Uint128Low64(value)));
   }
-  return 128 - base_internal::CountLeadingZeros64(Uint128High64(value));
+  return 128 - countl_zero(Uint128High64(value));
 }
 
 // Calculates how far to the right a mantissa needs to be shifted to create a
@@ -270,7 +345,7 @@ int NormalizedShiftSize(int mantissa_width, int binary_exponent) {
 // `value` must be wider than the requested bit width.
 //
 // Returns the number of bits shifted.
-int TruncateToBitWidth(int bit_width, uint128* value) {
+int TruncateToBitWidth(int bit_width, uint128* absl_nonnull value) {
   const int current_bit_width = BitWidth(*value);
   const int shift = current_bit_width - bit_width;
   *value >>= shift;
@@ -282,16 +357,19 @@ int TruncateToBitWidth(int bit_width, uint128* value) {
 // the appropriate double, and returns true.
 template <typename FloatType>
 bool HandleEdgeCase(const strings_internal::ParsedFloat& input, bool negative,
-                    FloatType* value) {
+                    FloatType* absl_nonnull value) {
   if (input.type == strings_internal::FloatType::kNan) {
-    // A bug in both clang and gcc would cause the compiler to optimize away the
-    // buffer we are building below.  Declaring the buffer volatile avoids the
-    // issue, and has no measurable performance impact in microbenchmarks.
+    // A bug in gcc would cause the compiler to optimize away the buffer we are
+    // building below.  Declaring the buffer volatile avoids the issue, and has
+    // no measurable performance impact in microbenchmarks.
     //
-    // https://bugs.llvm.org/show_bug.cgi?id=37778
     // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86113
     constexpr ptrdiff_t kNanBufferSize = 128;
+#if (defined(__GNUC__) && !defined(__clang__))
     volatile char n_char_sequence[kNanBufferSize];
+#else
+    char n_char_sequence[kNanBufferSize];
+#endif
     if (input.subrange_begin == nullptr) {
       n_char_sequence[0] = '\0';
     } else {
@@ -311,7 +389,7 @@ bool HandleEdgeCase(const strings_internal::ParsedFloat& input, bool negative,
     return true;
   }
   if (input.mantissa == 0) {
-    *value = negative ? -0.0 : 0.0;
+    *value = negative ? -0.0f : 0.0f;
     return true;
   }
   return false;
@@ -325,7 +403,8 @@ bool HandleEdgeCase(const strings_internal::ParsedFloat& input, bool negative,
 // number is stored in *value.
 template <typename FloatType>
 void EncodeResult(const CalculatedFloat& calculated, bool negative,
-                  absl::from_chars_result* result, FloatType* value) {
+                  absl::from_chars_result* absl_nonnull result,
+                  FloatType* absl_nonnull value) {
   if (calculated.exponent == kOverflow) {
     result->ec = std::errc::result_out_of_range;
     *value = negative ? -std::numeric_limits<FloatType>::max()
@@ -333,11 +412,13 @@ void EncodeResult(const CalculatedFloat& calculated, bool negative,
     return;
   } else if (calculated.mantissa == 0 || calculated.exponent == kUnderflow) {
     result->ec = std::errc::result_out_of_range;
-    *value = negative ? -0.0 : 0.0;
+    *value = negative ? -0.0f : 0.0f;
     return;
   }
-  *value = FloatTraits<FloatType>::Make(calculated.mantissa,
-                                        calculated.exponent, negative);
+  *value = FloatTraits<FloatType>::Make(
+      static_cast<typename FloatTraits<FloatType>::mantissa_t>(
+          calculated.mantissa),
+      calculated.exponent, negative);
 }
 
 // Returns the given uint128 shifted to the right by `shift` bits, and rounds
@@ -369,7 +450,7 @@ void EncodeResult(const CalculatedFloat& calculated, bool negative,
 // Zero and negative values of `shift` are accepted, in which case the word is
 // shifted left, as necessary.
 uint64_t ShiftRightAndRound(uint128 value, int shift, bool input_exact,
-                            bool* output_exact) {
+                            bool* absl_nonnull output_exact) {
   if (shift <= 0) {
     *output_exact = input_exact;
     return static_cast<uint64_t>(value << -shift);
@@ -398,7 +479,7 @@ uint64_t ShiftRightAndRound(uint128 value, int shift, bool input_exact,
     // the low bit of `value` is set.
     //
     // In inexact mode, the nonzero error means the actual value is greater
-    // than the halfway point and we must alway round up.
+    // than the halfway point and we must always round up.
     if ((value & 1) == 1 || !input_exact) {
       ++value;
     }
@@ -498,7 +579,7 @@ bool MustRoundUp(uint64_t guess_mantissa, int guess_exponent,
 template <typename FloatType>
 CalculatedFloat CalculatedFloatFromRawValues(uint64_t mantissa, int exponent) {
   CalculatedFloat result;
-  if (mantissa == uint64_t(1) << FloatTraits<FloatType>::kTargetMantissaBits) {
+  if (mantissa == uint64_t{1} << FloatTraits<FloatType>::kTargetMantissaBits) {
     mantissa >>= 1;
     exponent += 1;
   }
@@ -518,7 +599,9 @@ CalculatedFloat CalculateFromParsedHexadecimal(
     const strings_internal::ParsedFloat& parsed_hex) {
   uint64_t mantissa = parsed_hex.mantissa;
   int exponent = parsed_hex.exponent;
-  int mantissa_width = 64 - base_internal::CountLeadingZeros64(mantissa);
+  // This static_cast is only needed when using a std::bit_width()
+  // implementation that does not have the fix for LWG 3656 applied.
+  int mantissa_width = static_cast<int>(bit_width(mantissa));
   const int shift = NormalizedShiftSize<FloatType>(mantissa_width, exponent);
   bool result_exact;
   exponent += shift;
@@ -594,9 +677,189 @@ CalculatedFloat CalculateFromParsedDecimal(
                                                  binary_exponent);
 }
 
+// As discussed in https://nigeltao.github.io/blog/2020/eisel-lemire.html the
+// primary goal of the Eisel-Lemire algorithm is speed, for 99+% of the cases,
+// not 100% coverage. As long as Eisel-Lemire doesn’t claim false positives,
+// the combined approach (falling back to an alternative implementation when
+// this function returns false) is both fast and correct.
+template <typename FloatType>
+bool EiselLemire(const strings_internal::ParsedFloat& input, bool negative,
+                 FloatType* absl_nonnull value, std::errc* absl_nonnull ec) {
+  uint64_t man = input.mantissa;
+  int exp10 = input.exponent;
+  if (exp10 < FloatTraits<FloatType>::kEiselLemireMinInclusiveExp10) {
+    *value = negative ? -0.0f : 0.0f;
+    *ec = std::errc::result_out_of_range;
+    return true;
+  } else if (exp10 >= FloatTraits<FloatType>::kEiselLemireMaxExclusiveExp10) {
+    // Return max (a finite value) consistent with from_chars and DR 3081. For
+    // SimpleAtod and SimpleAtof, post-processing will return infinity.
+    *value = negative ? -std::numeric_limits<FloatType>::max()
+                      : std::numeric_limits<FloatType>::max();
+    *ec = std::errc::result_out_of_range;
+    return true;
+  }
+
+  // Assert kPower10TableMinInclusive <= exp10 < kPower10TableMaxExclusive.
+  // Equivalently, !Power10Underflow(exp10) and !Power10Overflow(exp10).
+  static_assert(
+      FloatTraits<FloatType>::kEiselLemireMinInclusiveExp10 >=
+          kPower10TableMinInclusive,
+      "(exp10-kPower10TableMinInclusive) in kPower10MantissaHighTable bounds");
+  static_assert(
+      FloatTraits<FloatType>::kEiselLemireMaxExclusiveExp10 <=
+          kPower10TableMaxExclusive,
+      "(exp10-kPower10TableMinInclusive) in kPower10MantissaHighTable bounds");
+
+  // The terse (+) comments in this function body refer to sections of the
+  // https://nigeltao.github.io/blog/2020/eisel-lemire.html blog post.
+  //
+  // That blog post discusses double precision (11 exponent bits with a -1023
+  // bias, 52 mantissa bits), but the same approach applies to single precision
+  // (8 exponent bits with a -127 bias, 23 mantissa bits). Either way, the
+  // computation here happens with 64-bit values (e.g. man) or 128-bit values
+  // (e.g. x) before finally converting to 64- or 32-bit floating point.
+  //
+  // See also "Number Parsing at a Gigabyte per Second, Software: Practice and
+  // Experience 51 (8), 2021" (https://arxiv.org/abs/2101.11408) for detail.
+
+  // (+) Normalization.
+  int clz = countl_zero(man);
+  man <<= static_cast<unsigned int>(clz);
+  // The 217706 etc magic numbers are from the Power10Exponent function.
+  uint64_t ret_exp2 =
+      static_cast<uint64_t>((217706 * exp10 >> 16) + 64 +
+                            FloatTraits<FloatType>::kExponentBias - clz);
+
+  // (+) Multiplication.
+  uint128 x = static_cast<uint128>(man) *
+              static_cast<uint128>(
+                  kPower10MantissaHighTable[exp10 - kPower10TableMinInclusive]);
+
+  // (+) Wider Approximation.
+  static constexpr uint64_t high64_mask =
+      FloatTraits<FloatType>::kEiselLemireMask;
+  if (((Uint128High64(x) & high64_mask) == high64_mask) &&
+      (man > (std::numeric_limits<uint64_t>::max() - Uint128Low64(x)))) {
+    uint128 y =
+        static_cast<uint128>(man) *
+        static_cast<uint128>(
+            kPower10MantissaLowTable[exp10 - kPower10TableMinInclusive]);
+    x += Uint128High64(y);
+    // For example, parsing "4503599627370497.5" will take the if-true
+    // branch here (for double precision), since:
+    //  - x   = 0x8000000000000BFF_FFFFFFFFFFFFFFFF
+    //  - y   = 0x8000000000000BFF_7FFFFFFFFFFFF400
+    //  - man = 0xA000000000000F00
+    // Likewise, when parsing "0.0625" for single precision:
+    //  - x   = 0x7FFFFFFFFFFFFFFF_FFFFFFFFFFFFFFFF
+    //  - y   = 0x813FFFFFFFFFFFFF_8A00000000000000
+    //  - man = 0x9C40000000000000
+    if (((Uint128High64(x) & high64_mask) == high64_mask) &&
+        ((Uint128Low64(x) + 1) == 0) &&
+        (man > (std::numeric_limits<uint64_t>::max() - Uint128Low64(y)))) {
+      return false;
+    }
+  }
+
+  // (+) Shifting to 54 Bits (or for single precision, to 25 bits).
+  uint64_t msb = Uint128High64(x) >> 63;
+  uint64_t ret_man =
+      Uint128High64(x) >> (msb + FloatTraits<FloatType>::kEiselLemireShift);
+  ret_exp2 -= 1 ^ msb;
+
+  // (+) Half-way Ambiguity.
+  //
+  // For example, parsing "1e+23" will take the if-true branch here (for double
+  // precision), since:
+  //  - x       = 0x54B40B1F852BDA00_0000000000000000
+  //  - ret_man = 0x002A5A058FC295ED
+  // Likewise, when parsing "20040229.0" for single precision:
+  //  - x       = 0x4C72894000000000_0000000000000000
+  //  - ret_man = 0x000000000131CA25
+  if ((Uint128Low64(x) == 0) && ((Uint128High64(x) & high64_mask) == 0) &&
+      ((ret_man & 3) == 1)) {
+    return false;
+  }
+
+  // (+) From 54 to 53 Bits (or for single precision, from 25 to 24 bits).
+  ret_man += ret_man & 1;  // Line From54a.
+  ret_man >>= 1;           // Line From54b.
+  // Incrementing ret_man (at line From54a) may have overflowed 54 bits (53
+  // bits after the right shift by 1 at line From54b), so adjust for that.
+  //
+  // For example, parsing "9223372036854775807" will take the if-true branch
+  // here (for double precision), since:
+  //  - ret_man = 0x0020000000000000 = (1 << 53)
+  // Likewise, when parsing "2147483647.0" for single precision:
+  //  - ret_man = 0x0000000001000000 = (1 << 24)
+  if ((ret_man >> FloatTraits<FloatType>::kTargetMantissaBits) > 0) {
+    ret_exp2 += 1;
+    // Conceptually, we need a "ret_man >>= 1" in this if-block to balance
+    // incrementing ret_exp2 in the line immediately above. However, we only
+    // get here when line From54a overflowed (after adding a 1), so ret_man
+    // here is (1 << 53). Its low 53 bits are therefore all zeroes. The only
+    // remaining use of ret_man is to mask it with ((1 << 52) - 1), so only its
+    // low 52 bits matter. A "ret_man >>= 1" would have no effect in practice.
+    //
+    // We omit the "ret_man >>= 1", even if it is cheap (and this if-branch is
+    // rarely taken) and technically 'more correct', so that mutation tests
+    // that would otherwise modify or omit that "ret_man >>= 1" don't complain
+    // that such code mutations have no observable effect.
+  }
+
+  // ret_exp2 is a uint64_t. Zero or underflow means that we're in subnormal
+  // space. max_exp2 (0x7FF for double precision, 0xFF for single precision) or
+  // above means that we're in Inf/NaN space.
+  //
+  // The if block is equivalent to (but has fewer branches than):
+  //   if ((ret_exp2 <= 0) || (ret_exp2 >= max_exp2)) { etc }
+  //
+  // For example, parsing "4.9406564584124654e-324" will take the if-true
+  // branch here, since ret_exp2 = -51.
+  static constexpr uint64_t max_exp2 =
+      (1 << FloatTraits<FloatType>::kTargetExponentBits) - 1;
+  if ((ret_exp2 - 1) >= (max_exp2 - 1)) {
+    return false;
+  }
+
+#ifndef ABSL_BIT_PACK_FLOATS
+  if (FloatTraits<FloatType>::kTargetBits == 64) {
+    *value = FloatTraits<FloatType>::Make(
+        (ret_man & 0x000FFFFFFFFFFFFFu) | 0x0010000000000000u,
+        static_cast<int>(ret_exp2) - 1023 - 52, negative);
+    return true;
+  } else if (FloatTraits<FloatType>::kTargetBits == 32) {
+    *value = FloatTraits<FloatType>::Make(
+        (static_cast<uint32_t>(ret_man) & 0x007FFFFFu) | 0x00800000u,
+        static_cast<int>(ret_exp2) - 127 - 23, negative);
+    return true;
+  }
+#else
+  if (FloatTraits<FloatType>::kTargetBits == 64) {
+    uint64_t ret_bits = (ret_exp2 << 52) | (ret_man & 0x000FFFFFFFFFFFFFu);
+    if (negative) {
+      ret_bits |= 0x8000000000000000u;
+    }
+    *value = static_cast<FloatType>(absl::bit_cast<double>(ret_bits));
+    return true;
+  } else if (FloatTraits<FloatType>::kTargetBits == 32) {
+    uint32_t ret_bits = (static_cast<uint32_t>(ret_exp2) << 23) |
+                        (static_cast<uint32_t>(ret_man) & 0x007FFFFFu);
+    if (negative) {
+      ret_bits |= 0x80000000u;
+    }
+    *value = static_cast<FloatType>(absl::bit_cast<float>(ret_bits));
+    return true;
+  }
+#endif  // ABSL_BIT_PACK_FLOATS
+  return false;
+}
+
 template <typename FloatType>
-from_chars_result FromCharsImpl(const char* first, const char* last,
-                                FloatType& value, chars_format fmt_flags) {
+from_chars_result FromCharsImpl(const char* absl_nonnull first,
+                                const char* absl_nonnull last, FloatType& value,
+                                chars_format fmt_flags) {
   from_chars_result result;
   result.ptr = first;  // overwritten on successful parse
   result.ec = std::errc();
@@ -618,15 +881,15 @@ from_chars_result FromCharsImpl(const char* first, const char* last,
       // Either we failed to parse a hex float after the "0x", or we read
       // "0xinf" or "0xnan" which we don't want to match.
       //
-      // However, a std::string that begins with "0x" also begins with "0", which
+      // However, a string that begins with "0x" also begins with "0", which
       // is normally a valid match for the number zero.  So we want these
       // strings to match zero unless fmt_flags is `scientific`.  (This flag
-      // means an exponent is required, which the std::string "0" does not have.)
+      // means an exponent is required, which the string "0" does not have.)
       if (fmt_flags == chars_format::scientific) {
         result.ec = std::errc::invalid_argument;
       } else {
         result.ptr = first + 1;
-        value = negative ? -0.0 : 0.0;
+        value = negative ? -0.0f : 0.0f;
       }
       return result;
     }
@@ -667,36 +930,74 @@ from_chars_result FromCharsImpl(const char* first, const char* last,
     if (HandleEdgeCase(decimal_parse, negative, &value)) {
       return result;
     }
+    // A nullptr subrange_begin means that the decimal_parse.mantissa is exact
+    // (not truncated), a precondition of the Eisel-Lemire algorithm.
+    if ((decimal_parse.subrange_begin == nullptr) &&
+        EiselLemire<FloatType>(decimal_parse, negative, &value, &result.ec)) {
+      return result;
+    }
     CalculatedFloat calculated =
         CalculateFromParsedDecimal<FloatType>(decimal_parse);
     EncodeResult(calculated, negative, &result, &value);
     return result;
   }
-  return result;
 }
 }  // namespace
 
-from_chars_result from_chars(const char* first, const char* last, double& value,
+from_chars_result from_chars(const char* absl_nonnull first,
+                             const char* absl_nonnull last, double& value,
                              chars_format fmt) {
   return FromCharsImpl(first, last, value, fmt);
 }
 
-from_chars_result from_chars(const char* first, const char* last, float& value,
+from_chars_result from_chars(const char* absl_nonnull first,
+                             const char* absl_nonnull last, float& value,
                              chars_format fmt) {
   return FromCharsImpl(first, last, value, fmt);
 }
 
 namespace {
 
-// Table of powers of 10, from kPower10TableMin to kPower10TableMax.
+// Table of powers of 10, from kPower10TableMinInclusive to
+// kPower10TableMaxExclusive.
+//
+// kPower10MantissaHighTable[i - kPower10TableMinInclusive] stores the 64-bit
+// mantissa. The high bit is always on.
+//
+// kPower10MantissaLowTable extends that 64-bit mantissa to 128 bits.
+//
+// Power10Exponent(i) calculates the power-of-two exponent.
+//
+// For a number i, this gives the unique mantissaHigh and exponent such that
+// (mantissaHigh * 2**exponent) <= 10**i < ((mantissaHigh + 1) * 2**exponent).
+//
+// For example, Python can confirm that the exact hexadecimal value of 1e60 is:
+//    >>> a = 1000000000000000000000000000000000000000000000000000000000000
+//    >>> hex(a)
+//    '0x9f4f2726179a224501d762422c946590d91000000000000000'
+// Adding underscores at every 8th hex digit shows 50 hex digits:
+//    '0x9f4f2726_179a2245_01d76242_2c946590_d9100000_00000000_00'.
+// In this case, the high bit of the first hex digit, 9, is coincidentally set,
+// so we do not have to do further shifting to deduce the 128-bit mantissa:
+//   - kPower10MantissaHighTable[60 - kP10TMI] = 0x9f4f2726179a2245U
+//   - kPower10MantissaLowTable[ 60 - kP10TMI] = 0x01d762422c946590U
+// where kP10TMI is kPower10TableMinInclusive. The low 18 of those 50 hex
+// digits are truncated.
+//
+// 50 hex digits (with the high bit set) is 200 bits and mantissaHigh holds 64
+// bits, so Power10Exponent(60) = 200 - 64 = 136. Again, Python can confirm:
+//    >>> b = 0x9f4f2726179a2245
+//    >>> ((b+0)<<136) <= a
+//    True
+//    >>> ((b+1)<<136) <= a
+//    False
 //
-// kPower10MantissaTable[i - kPower10TableMin] stores the 64-bit mantissa (high
-// bit always on), and kPower10ExponentTable[i - kPower10TableMin] stores the
-// power-of-two exponent.  For a given number i, this gives the unique mantissa
-// and exponent such that mantissa * 2**exponent <= 10**i < (mantissa + 1) *
-// 2**exponent.
+// The tables were generated by
+// https://github.com/google/wuffs/blob/315b2e52625ebd7b02d8fac13e3cd85ea374fb80/script/print-mpb-powers-of-10.go
+// after re-formatting its output into two arrays of N uint64_t values (instead
+// of an N element array of uint64_t pairs).
 
-const uint64_t kPower10MantissaTable[] = {
+const uint64_t kPower10MantissaHighTable[] = {
     0xeef453d6923bd65aU, 0x9558b4661b6565f8U, 0xbaaee17fa23ebf76U,
     0xe95a99df8ace6f53U, 0x91d8a02bb6c10594U, 0xb64ec836a47146f9U,
     0xe3e27a444d8d98b7U, 0x8e6d8c6ab0787f72U, 0xb208ef855c969f4fU,
@@ -916,68 +1217,226 @@ const uint64_t kPower10MantissaTable[] = {
     0xb6472e511c81471dU, 0xe3d8f9e563a198e5U, 0x8e679c2f5e44ff8fU,
 };
 
-const int16_t kPower10ExponentTable[] = {
-    -1200, -1196, -1193, -1190, -1186, -1183, -1180, -1176, -1173, -1170, -1166,
-    -1163, -1160, -1156, -1153, -1150, -1146, -1143, -1140, -1136, -1133, -1130,
-    -1127, -1123, -1120, -1117, -1113, -1110, -1107, -1103, -1100, -1097, -1093,
-    -1090, -1087, -1083, -1080, -1077, -1073, -1070, -1067, -1063, -1060, -1057,
-    -1053, -1050, -1047, -1043, -1040, -1037, -1034, -1030, -1027, -1024, -1020,
-    -1017, -1014, -1010, -1007, -1004, -1000, -997,  -994,  -990,  -987,  -984,
-    -980,  -977,  -974,  -970,  -967,  -964,  -960,  -957,  -954,  -950,  -947,
-    -944,  -940,  -937,  -934,  -931,  -927,  -924,  -921,  -917,  -914,  -911,
-    -907,  -904,  -901,  -897,  -894,  -891,  -887,  -884,  -881,  -877,  -874,
-    -871,  -867,  -864,  -861,  -857,  -854,  -851,  -847,  -844,  -841,  -838,
-    -834,  -831,  -828,  -824,  -821,  -818,  -814,  -811,  -808,  -804,  -801,
-    -798,  -794,  -791,  -788,  -784,  -781,  -778,  -774,  -771,  -768,  -764,
-    -761,  -758,  -754,  -751,  -748,  -744,  -741,  -738,  -735,  -731,  -728,
-    -725,  -721,  -718,  -715,  -711,  -708,  -705,  -701,  -698,  -695,  -691,
-    -688,  -685,  -681,  -678,  -675,  -671,  -668,  -665,  -661,  -658,  -655,
-    -651,  -648,  -645,  -642,  -638,  -635,  -632,  -628,  -625,  -622,  -618,
-    -615,  -612,  -608,  -605,  -602,  -598,  -595,  -592,  -588,  -585,  -582,
-    -578,  -575,  -572,  -568,  -565,  -562,  -558,  -555,  -552,  -549,  -545,
-    -542,  -539,  -535,  -532,  -529,  -525,  -522,  -519,  -515,  -512,  -509,
-    -505,  -502,  -499,  -495,  -492,  -489,  -485,  -482,  -479,  -475,  -472,
-    -469,  -465,  -462,  -459,  -455,  -452,  -449,  -446,  -442,  -439,  -436,
-    -432,  -429,  -426,  -422,  -419,  -416,  -412,  -409,  -406,  -402,  -399,
-    -396,  -392,  -389,  -386,  -382,  -379,  -376,  -372,  -369,  -366,  -362,
-    -359,  -356,  -353,  -349,  -346,  -343,  -339,  -336,  -333,  -329,  -326,
-    -323,  -319,  -316,  -313,  -309,  -306,  -303,  -299,  -296,  -293,  -289,
-    -286,  -283,  -279,  -276,  -273,  -269,  -266,  -263,  -259,  -256,  -253,
-    -250,  -246,  -243,  -240,  -236,  -233,  -230,  -226,  -223,  -220,  -216,
-    -213,  -210,  -206,  -203,  -200,  -196,  -193,  -190,  -186,  -183,  -180,
-    -176,  -173,  -170,  -166,  -163,  -160,  -157,  -153,  -150,  -147,  -143,
-    -140,  -137,  -133,  -130,  -127,  -123,  -120,  -117,  -113,  -110,  -107,
-    -103,  -100,  -97,   -93,   -90,   -87,   -83,   -80,   -77,   -73,   -70,
-    -67,   -63,   -60,   -57,   -54,   -50,   -47,   -44,   -40,   -37,   -34,
-    -30,   -27,   -24,   -20,   -17,   -14,   -10,   -7,    -4,    0,     3,
-    6,     10,    13,    16,    20,    23,    26,    30,    33,    36,    39,
-    43,    46,    49,    53,    56,    59,    63,    66,    69,    73,    76,
-    79,    83,    86,    89,    93,    96,    99,    103,   106,   109,   113,
-    116,   119,   123,   126,   129,   132,   136,   139,   142,   146,   149,
-    152,   156,   159,   162,   166,   169,   172,   176,   179,   182,   186,
-    189,   192,   196,   199,   202,   206,   209,   212,   216,   219,   222,
-    226,   229,   232,   235,   239,   242,   245,   249,   252,   255,   259,
-    262,   265,   269,   272,   275,   279,   282,   285,   289,   292,   295,
-    299,   302,   305,   309,   312,   315,   319,   322,   325,   328,   332,
-    335,   338,   342,   345,   348,   352,   355,   358,   362,   365,   368,
-    372,   375,   378,   382,   385,   388,   392,   395,   398,   402,   405,
-    408,   412,   415,   418,   422,   425,   428,   431,   435,   438,   441,
-    445,   448,   451,   455,   458,   461,   465,   468,   471,   475,   478,
-    481,   485,   488,   491,   495,   498,   501,   505,   508,   511,   515,
-    518,   521,   524,   528,   531,   534,   538,   541,   544,   548,   551,
-    554,   558,   561,   564,   568,   571,   574,   578,   581,   584,   588,
-    591,   594,   598,   601,   604,   608,   611,   614,   617,   621,   624,
-    627,   631,   634,   637,   641,   644,   647,   651,   654,   657,   661,
-    664,   667,   671,   674,   677,   681,   684,   687,   691,   694,   697,
-    701,   704,   707,   711,   714,   717,   720,   724,   727,   730,   734,
-    737,   740,   744,   747,   750,   754,   757,   760,   764,   767,   770,
-    774,   777,   780,   784,   787,   790,   794,   797,   800,   804,   807,
-    810,   813,   817,   820,   823,   827,   830,   833,   837,   840,   843,
-    847,   850,   853,   857,   860,   863,   867,   870,   873,   877,   880,
-    883,   887,   890,   893,   897,   900,   903,   907,   910,   913,   916,
-    920,   923,   926,   930,   933,   936,   940,   943,   946,   950,   953,
-    956,   960,
+const uint64_t kPower10MantissaLowTable[] = {
+    0x113faa2906a13b3fU, 0x4ac7ca59a424c507U, 0x5d79bcf00d2df649U,
+    0xf4d82c2c107973dcU, 0x79071b9b8a4be869U, 0x9748e2826cdee284U,
+    0xfd1b1b2308169b25U, 0xfe30f0f5e50e20f7U, 0xbdbd2d335e51a935U,
+    0xad2c788035e61382U, 0x4c3bcb5021afcc31U, 0xdf4abe242a1bbf3dU,
+    0xd71d6dad34a2af0dU, 0x8672648c40e5ad68U, 0x680efdaf511f18c2U,
+    0x0212bd1b2566def2U, 0x014bb630f7604b57U, 0x419ea3bd35385e2dU,
+    0x52064cac828675b9U, 0x7343efebd1940993U, 0x1014ebe6c5f90bf8U,
+    0xd41a26e077774ef6U, 0x8920b098955522b4U, 0x55b46e5f5d5535b0U,
+    0xeb2189f734aa831dU, 0xa5e9ec7501d523e4U, 0x47b233c92125366eU,
+    0x999ec0bb696e840aU, 0xc00670ea43ca250dU, 0x380406926a5e5728U,
+    0xc605083704f5ecf2U, 0xf7864a44c633682eU, 0x7ab3ee6afbe0211dU,
+    0x5960ea05bad82964U, 0x6fb92487298e33bdU, 0xa5d3b6d479f8e056U,
+    0x8f48a4899877186cU, 0x331acdabfe94de87U, 0x9ff0c08b7f1d0b14U,
+    0x07ecf0ae5ee44dd9U, 0xc9e82cd9f69d6150U, 0xbe311c083a225cd2U,
+    0x6dbd630a48aaf406U, 0x092cbbccdad5b108U, 0x25bbf56008c58ea5U,
+    0xaf2af2b80af6f24eU, 0x1af5af660db4aee1U, 0x50d98d9fc890ed4dU,
+    0xe50ff107bab528a0U, 0x1e53ed49a96272c8U, 0x25e8e89c13bb0f7aU,
+    0x77b191618c54e9acU, 0xd59df5b9ef6a2417U, 0x4b0573286b44ad1dU,
+    0x4ee367f9430aec32U, 0x229c41f793cda73fU, 0x6b43527578c1110fU,
+    0x830a13896b78aaa9U, 0x23cc986bc656d553U, 0x2cbfbe86b7ec8aa8U,
+    0x7bf7d71432f3d6a9U, 0xdaf5ccd93fb0cc53U, 0xd1b3400f8f9cff68U,
+    0x23100809b9c21fa1U, 0xabd40a0c2832a78aU, 0x16c90c8f323f516cU,
+    0xae3da7d97f6792e3U, 0x99cd11cfdf41779cU, 0x40405643d711d583U,
+    0x482835ea666b2572U, 0xda3243650005eecfU, 0x90bed43e40076a82U,
+    0x5a7744a6e804a291U, 0x711515d0a205cb36U, 0x0d5a5b44ca873e03U,
+    0xe858790afe9486c2U, 0x626e974dbe39a872U, 0xfb0a3d212dc8128fU,
+    0x7ce66634bc9d0b99U, 0x1c1fffc1ebc44e80U, 0xa327ffb266b56220U,
+    0x4bf1ff9f0062baa8U, 0x6f773fc3603db4a9U, 0xcb550fb4384d21d3U,
+    0x7e2a53a146606a48U, 0x2eda7444cbfc426dU, 0xfa911155fefb5308U,
+    0x793555ab7eba27caU, 0x4bc1558b2f3458deU, 0x9eb1aaedfb016f16U,
+    0x465e15a979c1cadcU, 0x0bfacd89ec191ec9U, 0xcef980ec671f667bU,
+    0x82b7e12780e7401aU, 0xd1b2ecb8b0908810U, 0x861fa7e6dcb4aa15U,
+    0x67a791e093e1d49aU, 0xe0c8bb2c5c6d24e0U, 0x58fae9f773886e18U,
+    0xaf39a475506a899eU, 0x6d8406c952429603U, 0xc8e5087ba6d33b83U,
+    0xfb1e4a9a90880a64U, 0x5cf2eea09a55067fU, 0xf42faa48c0ea481eU,
+    0xf13b94daf124da26U, 0x76c53d08d6b70858U, 0x54768c4b0c64ca6eU,
+    0xa9942f5dcf7dfd09U, 0xd3f93b35435d7c4cU, 0xc47bc5014a1a6dafU,
+    0x359ab6419ca1091bU, 0xc30163d203c94b62U, 0x79e0de63425dcf1dU,
+    0x985915fc12f542e4U, 0x3e6f5b7b17b2939dU, 0xa705992ceecf9c42U,
+    0x50c6ff782a838353U, 0xa4f8bf5635246428U, 0x871b7795e136be99U,
+    0x28e2557b59846e3fU, 0x331aeada2fe589cfU, 0x3ff0d2c85def7621U,
+    0x0fed077a756b53a9U, 0xd3e8495912c62894U, 0x64712dd7abbbd95cU,
+    0xbd8d794d96aacfb3U, 0xecf0d7a0fc5583a0U, 0xf41686c49db57244U,
+    0x311c2875c522ced5U, 0x7d633293366b828bU, 0xae5dff9c02033197U,
+    0xd9f57f830283fdfcU, 0xd072df63c324fd7bU, 0x4247cb9e59f71e6dU,
+    0x52d9be85f074e608U, 0x67902e276c921f8bU, 0x00ba1cd8a3db53b6U,
+    0x80e8a40eccd228a4U, 0x6122cd128006b2cdU, 0x796b805720085f81U,
+    0xcbe3303674053bb0U, 0xbedbfc4411068a9cU, 0xee92fb5515482d44U,
+    0x751bdd152d4d1c4aU, 0xd262d45a78a0635dU, 0x86fb897116c87c34U,
+    0xd45d35e6ae3d4da0U, 0x8974836059cca109U, 0x2bd1a438703fc94bU,
+    0x7b6306a34627ddcfU, 0x1a3bc84c17b1d542U, 0x20caba5f1d9e4a93U,
+    0x547eb47b7282ee9cU, 0xe99e619a4f23aa43U, 0x6405fa00e2ec94d4U,
+    0xde83bc408dd3dd04U, 0x9624ab50b148d445U, 0x3badd624dd9b0957U,
+    0xe54ca5d70a80e5d6U, 0x5e9fcf4ccd211f4cU, 0x7647c3200069671fU,
+    0x29ecd9f40041e073U, 0xf468107100525890U, 0x7182148d4066eeb4U,
+    0xc6f14cd848405530U, 0xb8ada00e5a506a7cU, 0xa6d90811f0e4851cU,
+    0x908f4a166d1da663U, 0x9a598e4e043287feU, 0x40eff1e1853f29fdU,
+    0xd12bee59e68ef47cU, 0x82bb74f8301958ceU, 0xe36a52363c1faf01U,
+    0xdc44e6c3cb279ac1U, 0x29ab103a5ef8c0b9U, 0x7415d448f6b6f0e7U,
+    0x111b495b3464ad21U, 0xcab10dd900beec34U, 0x3d5d514f40eea742U,
+    0x0cb4a5a3112a5112U, 0x47f0e785eaba72abU, 0x59ed216765690f56U,
+    0x306869c13ec3532cU, 0x1e414218c73a13fbU, 0xe5d1929ef90898faU,
+    0xdf45f746b74abf39U, 0x6b8bba8c328eb783U, 0x066ea92f3f326564U,
+    0xc80a537b0efefebdU, 0xbd06742ce95f5f36U, 0x2c48113823b73704U,
+    0xf75a15862ca504c5U, 0x9a984d73dbe722fbU, 0xc13e60d0d2e0ebbaU,
+    0x318df905079926a8U, 0xfdf17746497f7052U, 0xfeb6ea8bedefa633U,
+    0xfe64a52ee96b8fc0U, 0x3dfdce7aa3c673b0U, 0x06bea10ca65c084eU,
+    0x486e494fcff30a62U, 0x5a89dba3c3efccfaU, 0xf89629465a75e01cU,
+    0xf6bbb397f1135823U, 0x746aa07ded582e2cU, 0xa8c2a44eb4571cdcU,
+    0x92f34d62616ce413U, 0x77b020baf9c81d17U, 0x0ace1474dc1d122eU,
+    0x0d819992132456baU, 0x10e1fff697ed6c69U, 0xca8d3ffa1ef463c1U,
+    0xbd308ff8a6b17cb2U, 0xac7cb3f6d05ddbdeU, 0x6bcdf07a423aa96bU,
+    0x86c16c98d2c953c6U, 0xe871c7bf077ba8b7U, 0x11471cd764ad4972U,
+    0xd598e40d3dd89bcfU, 0x4aff1d108d4ec2c3U, 0xcedf722a585139baU,
+    0xc2974eb4ee658828U, 0x733d226229feea32U, 0x0806357d5a3f525fU,
+    0xca07c2dcb0cf26f7U, 0xfc89b393dd02f0b5U, 0xbbac2078d443ace2U,
+    0xd54b944b84aa4c0dU, 0x0a9e795e65d4df11U, 0x4d4617b5ff4a16d5U,
+    0x504bced1bf8e4e45U, 0xe45ec2862f71e1d6U, 0x5d767327bb4e5a4cU,
+    0x3a6a07f8d510f86fU, 0x890489f70a55368bU, 0x2b45ac74ccea842eU,
+    0x3b0b8bc90012929dU, 0x09ce6ebb40173744U, 0xcc420a6a101d0515U,
+    0x9fa946824a12232dU, 0x47939822dc96abf9U, 0x59787e2b93bc56f7U,
+    0x57eb4edb3c55b65aU, 0xede622920b6b23f1U, 0xe95fab368e45ecedU,
+    0x11dbcb0218ebb414U, 0xd652bdc29f26a119U, 0x4be76d3346f0495fU,
+    0x6f70a4400c562ddbU, 0xcb4ccd500f6bb952U, 0x7e2000a41346a7a7U,
+    0x8ed400668c0c28c8U, 0x728900802f0f32faU, 0x4f2b40a03ad2ffb9U,
+    0xe2f610c84987bfa8U, 0x0dd9ca7d2df4d7c9U, 0x91503d1c79720dbbU,
+    0x75a44c6397ce912aU, 0xc986afbe3ee11abaU, 0xfbe85badce996168U,
+    0xfae27299423fb9c3U, 0xdccd879fc967d41aU, 0x5400e987bbc1c920U,
+    0x290123e9aab23b68U, 0xf9a0b6720aaf6521U, 0xf808e40e8d5b3e69U,
+    0xb60b1d1230b20e04U, 0xb1c6f22b5e6f48c2U, 0x1e38aeb6360b1af3U,
+    0x25c6da63c38de1b0U, 0x579c487e5a38ad0eU, 0x2d835a9df0c6d851U,
+    0xf8e431456cf88e65U, 0x1b8e9ecb641b58ffU, 0xe272467e3d222f3fU,
+    0x5b0ed81dcc6abb0fU, 0x98e947129fc2b4e9U, 0x3f2398d747b36224U,
+    0x8eec7f0d19a03aadU, 0x1953cf68300424acU, 0x5fa8c3423c052dd7U,
+    0x3792f412cb06794dU, 0xe2bbd88bbee40bd0U, 0x5b6aceaeae9d0ec4U,
+    0xf245825a5a445275U, 0xeed6e2f0f0d56712U, 0x55464dd69685606bU,
+    0xaa97e14c3c26b886U, 0xd53dd99f4b3066a8U, 0xe546a8038efe4029U,
+    0xde98520472bdd033U, 0x963e66858f6d4440U, 0xdde7001379a44aa8U,
+    0x5560c018580d5d52U, 0xaab8f01e6e10b4a6U, 0xcab3961304ca70e8U,
+    0x3d607b97c5fd0d22U, 0x8cb89a7db77c506aU, 0x77f3608e92adb242U,
+    0x55f038b237591ed3U, 0x6b6c46dec52f6688U, 0x2323ac4b3b3da015U,
+    0xabec975e0a0d081aU, 0x96e7bd358c904a21U, 0x7e50d64177da2e54U,
+    0xdde50bd1d5d0b9e9U, 0x955e4ec64b44e864U, 0xbd5af13bef0b113eU,
+    0xecb1ad8aeacdd58eU, 0x67de18eda5814af2U, 0x80eacf948770ced7U,
+    0xa1258379a94d028dU, 0x096ee45813a04330U, 0x8bca9d6e188853fcU,
+    0x775ea264cf55347dU, 0x95364afe032a819dU, 0x3a83ddbd83f52204U,
+    0xc4926a9672793542U, 0x75b7053c0f178293U, 0x5324c68b12dd6338U,
+    0xd3f6fc16ebca5e03U, 0x88f4bb1ca6bcf584U, 0x2b31e9e3d06c32e5U,
+    0x3aff322e62439fcfU, 0x09befeb9fad487c2U, 0x4c2ebe687989a9b3U,
+    0x0f9d37014bf60a10U, 0x538484c19ef38c94U, 0x2865a5f206b06fb9U,
+    0xf93f87b7442e45d3U, 0xf78f69a51539d748U, 0xb573440e5a884d1bU,
+    0x31680a88f8953030U, 0xfdc20d2b36ba7c3dU, 0x3d32907604691b4cU,
+    0xa63f9a49c2c1b10fU, 0x0fcf80dc33721d53U, 0xd3c36113404ea4a8U,
+    0x645a1cac083126e9U, 0x3d70a3d70a3d70a3U, 0xccccccccccccccccU,
+    0x0000000000000000U, 0x0000000000000000U, 0x0000000000000000U,
+    0x0000000000000000U, 0x0000000000000000U, 0x0000000000000000U,
+    0x0000000000000000U, 0x0000000000000000U, 0x0000000000000000U,
+    0x0000000000000000U, 0x0000000000000000U, 0x0000000000000000U,
+    0x0000000000000000U, 0x0000000000000000U, 0x0000000000000000U,
+    0x0000000000000000U, 0x0000000000000000U, 0x0000000000000000U,
+    0x0000000000000000U, 0x0000000000000000U, 0x0000000000000000U,
+    0x0000000000000000U, 0x0000000000000000U, 0x0000000000000000U,
+    0x0000000000000000U, 0x0000000000000000U, 0x0000000000000000U,
+    0x0000000000000000U, 0x4000000000000000U, 0x5000000000000000U,
+    0xa400000000000000U, 0x4d00000000000000U, 0xf020000000000000U,
+    0x6c28000000000000U, 0xc732000000000000U, 0x3c7f400000000000U,
+    0x4b9f100000000000U, 0x1e86d40000000000U, 0x1314448000000000U,
+    0x17d955a000000000U, 0x5dcfab0800000000U, 0x5aa1cae500000000U,
+    0xf14a3d9e40000000U, 0x6d9ccd05d0000000U, 0xe4820023a2000000U,
+    0xdda2802c8a800000U, 0xd50b2037ad200000U, 0x4526f422cc340000U,
+    0x9670b12b7f410000U, 0x3c0cdd765f114000U, 0xa5880a69fb6ac800U,
+    0x8eea0d047a457a00U, 0x72a4904598d6d880U, 0x47a6da2b7f864750U,
+    0x999090b65f67d924U, 0xfff4b4e3f741cf6dU, 0xbff8f10e7a8921a4U,
+    0xaff72d52192b6a0dU, 0x9bf4f8a69f764490U, 0x02f236d04753d5b4U,
+    0x01d762422c946590U, 0x424d3ad2b7b97ef5U, 0xd2e0898765a7deb2U,
+    0x63cc55f49f88eb2fU, 0x3cbf6b71c76b25fbU, 0x8bef464e3945ef7aU,
+    0x97758bf0e3cbb5acU, 0x3d52eeed1cbea317U, 0x4ca7aaa863ee4bddU,
+    0x8fe8caa93e74ef6aU, 0xb3e2fd538e122b44U, 0x60dbbca87196b616U,
+    0xbc8955e946fe31cdU, 0x6babab6398bdbe41U, 0xc696963c7eed2dd1U,
+    0xfc1e1de5cf543ca2U, 0x3b25a55f43294bcbU, 0x49ef0eb713f39ebeU,
+    0x6e3569326c784337U, 0x49c2c37f07965404U, 0xdc33745ec97be906U,
+    0x69a028bb3ded71a3U, 0xc40832ea0d68ce0cU, 0xf50a3fa490c30190U,
+    0x792667c6da79e0faU, 0x577001b891185938U, 0xed4c0226b55e6f86U,
+    0x544f8158315b05b4U, 0x696361ae3db1c721U, 0x03bc3a19cd1e38e9U,
+    0x04ab48a04065c723U, 0x62eb0d64283f9c76U, 0x3ba5d0bd324f8394U,
+    0xca8f44ec7ee36479U, 0x7e998b13cf4e1ecbU, 0x9e3fedd8c321a67eU,
+    0xc5cfe94ef3ea101eU, 0xbba1f1d158724a12U, 0x2a8a6e45ae8edc97U,
+    0xf52d09d71a3293bdU, 0x593c2626705f9c56U, 0x6f8b2fb00c77836cU,
+    0x0b6dfb9c0f956447U, 0x4724bd4189bd5eacU, 0x58edec91ec2cb657U,
+    0x2f2967b66737e3edU, 0xbd79e0d20082ee74U, 0xecd8590680a3aa11U,
+    0xe80e6f4820cc9495U, 0x3109058d147fdcddU, 0xbd4b46f0599fd415U,
+    0x6c9e18ac7007c91aU, 0x03e2cf6bc604ddb0U, 0x84db8346b786151cU,
+    0xe612641865679a63U, 0x4fcb7e8f3f60c07eU, 0xe3be5e330f38f09dU,
+    0x5cadf5bfd3072cc5U, 0x73d9732fc7c8f7f6U, 0x2867e7fddcdd9afaU,
+    0xb281e1fd541501b8U, 0x1f225a7ca91a4226U, 0x3375788de9b06958U,
+    0x0052d6b1641c83aeU, 0xc0678c5dbd23a49aU, 0xf840b7ba963646e0U,
+    0xb650e5a93bc3d898U, 0xa3e51f138ab4cebeU, 0xc66f336c36b10137U,
+    0xb80b0047445d4184U, 0xa60dc059157491e5U, 0x87c89837ad68db2fU,
+    0x29babe4598c311fbU, 0xf4296dd6fef3d67aU, 0x1899e4a65f58660cU,
+    0x5ec05dcff72e7f8fU, 0x76707543f4fa1f73U, 0x6a06494a791c53a8U,
+    0x0487db9d17636892U, 0x45a9d2845d3c42b6U, 0x0b8a2392ba45a9b2U,
+    0x8e6cac7768d7141eU, 0x3207d795430cd926U, 0x7f44e6bd49e807b8U,
+    0x5f16206c9c6209a6U, 0x36dba887c37a8c0fU, 0xc2494954da2c9789U,
+    0xf2db9baa10b7bd6cU, 0x6f92829494e5acc7U, 0xcb772339ba1f17f9U,
+    0xff2a760414536efbU, 0xfef5138519684abaU, 0x7eb258665fc25d69U,
+    0xef2f773ffbd97a61U, 0xaafb550ffacfd8faU, 0x95ba2a53f983cf38U,
+    0xdd945a747bf26183U, 0x94f971119aeef9e4U, 0x7a37cd5601aab85dU,
+    0xac62e055c10ab33aU, 0x577b986b314d6009U, 0xed5a7e85fda0b80bU,
+    0x14588f13be847307U, 0x596eb2d8ae258fc8U, 0x6fca5f8ed9aef3bbU,
+    0x25de7bb9480d5854U, 0xaf561aa79a10ae6aU, 0x1b2ba1518094da04U,
+    0x90fb44d2f05d0842U, 0x353a1607ac744a53U, 0x42889b8997915ce8U,
+    0x69956135febada11U, 0x43fab9837e699095U, 0x94f967e45e03f4bbU,
+    0x1d1be0eebac278f5U, 0x6462d92a69731732U, 0x7d7b8f7503cfdcfeU,
+    0x5cda735244c3d43eU, 0x3a0888136afa64a7U, 0x088aaa1845b8fdd0U,
+    0x8aad549e57273d45U, 0x36ac54e2f678864bU, 0x84576a1bb416a7ddU,
+    0x656d44a2a11c51d5U, 0x9f644ae5a4b1b325U, 0x873d5d9f0dde1feeU,
+    0xa90cb506d155a7eaU, 0x09a7f12442d588f2U, 0x0c11ed6d538aeb2fU,
+    0x8f1668c8a86da5faU, 0xf96e017d694487bcU, 0x37c981dcc395a9acU,
+    0x85bbe253f47b1417U, 0x93956d7478ccec8eU, 0x387ac8d1970027b2U,
+    0x06997b05fcc0319eU, 0x441fece3bdf81f03U, 0xd527e81cad7626c3U,
+    0x8a71e223d8d3b074U, 0xf6872d5667844e49U, 0xb428f8ac016561dbU,
+    0xe13336d701beba52U, 0xecc0024661173473U, 0x27f002d7f95d0190U,
+    0x31ec038df7b441f4U, 0x7e67047175a15271U, 0x0f0062c6e984d386U,
+    0x52c07b78a3e60868U, 0xa7709a56ccdf8a82U, 0x88a66076400bb691U,
+    0x6acff893d00ea435U, 0x0583f6b8c4124d43U, 0xc3727a337a8b704aU,
+    0x744f18c0592e4c5cU, 0x1162def06f79df73U, 0x8addcb5645ac2ba8U,
+    0x6d953e2bd7173692U, 0xc8fa8db6ccdd0437U, 0x1d9c9892400a22a2U,
+    0x2503beb6d00cab4bU, 0x2e44ae64840fd61dU, 0x5ceaecfed289e5d2U,
+    0x7425a83e872c5f47U, 0xd12f124e28f77719U, 0x82bd6b70d99aaa6fU,
+    0x636cc64d1001550bU, 0x3c47f7e05401aa4eU, 0x65acfaec34810a71U,
+    0x7f1839a741a14d0dU, 0x1ede48111209a050U, 0x934aed0aab460432U,
+    0xf81da84d5617853fU, 0x36251260ab9d668eU, 0xc1d72b7c6b426019U,
+    0xb24cf65b8612f81fU, 0xdee033f26797b627U, 0x169840ef017da3b1U,
+    0x8e1f289560ee864eU, 0xf1a6f2bab92a27e2U, 0xae10af696774b1dbU,
+    0xacca6da1e0a8ef29U, 0x17fd090a58d32af3U, 0xddfc4b4cef07f5b0U,
+    0x4abdaf101564f98eU, 0x9d6d1ad41abe37f1U, 0x84c86189216dc5edU,
+    0x32fd3cf5b4e49bb4U, 0x3fbc8c33221dc2a1U, 0x0fabaf3feaa5334aU,
+    0x29cb4d87f2a7400eU, 0x743e20e9ef511012U, 0x914da9246b255416U,
+    0x1ad089b6c2f7548eU, 0xa184ac2473b529b1U, 0xc9e5d72d90a2741eU,
+    0x7e2fa67c7a658892U, 0xddbb901b98feeab7U, 0x552a74227f3ea565U,
+    0xd53a88958f87275fU, 0x8a892abaf368f137U, 0x2d2b7569b0432d85U,
+    0x9c3b29620e29fc73U, 0x8349f3ba91b47b8fU, 0x241c70a936219a73U,
+    0xed238cd383aa0110U, 0xf4363804324a40aaU, 0xb143c6053edcd0d5U,
+    0xdd94b7868e94050aU, 0xca7cf2b4191c8326U, 0xfd1c2f611f63a3f0U,
+    0xbc633b39673c8cecU, 0xd5be0503e085d813U, 0x4b2d8644d8a74e18U,
+    0xddf8e7d60ed1219eU, 0xcabb90e5c942b503U, 0x3d6a751f3b936243U,
+    0x0cc512670a783ad4U, 0x27fb2b80668b24c5U, 0xb1f9f660802dedf6U,
+    0x5e7873f8a0396973U, 0xdb0b487b6423e1e8U, 0x91ce1a9a3d2cda62U,
+    0x7641a140cc7810fbU, 0xa9e904c87fcb0a9dU, 0x546345fa9fbdcd44U,
+    0xa97c177947ad4095U, 0x49ed8eabcccc485dU, 0x5c68f256bfff5a74U,
+    0x73832eec6fff3111U, 0xc831fd53c5ff7eabU, 0xba3e7ca8b77f5e55U,
+    0x28ce1bd2e55f35ebU, 0x7980d163cf5b81b3U, 0xd7e105bcc332621fU,
+    0x8dd9472bf3fefaa7U, 0xb14f98f6f0feb951U, 0x6ed1bf9a569f33d3U,
+    0x0a862f80ec4700c8U, 0xcd27bb612758c0faU, 0x8038d51cb897789cU,
+    0xe0470a63e6bd56c3U, 0x1858ccfce06cac74U, 0x0f37801e0c43ebc8U,
+    0xd30560258f54e6baU, 0x47c6b82ef32a2069U, 0x4cdc331d57fa5441U,
+    0xe0133fe4adf8e952U, 0x58180fddd97723a6U, 0x570f09eaa7ea7648U,
 };
 
 }  // namespace
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/charconv.h b/absl/strings/charconv.h
index 3f5891ba7f3..e5733f8c251 100644
--- a/absl/strings/charconv.h
+++ b/absl/strings/charconv.h
@@ -17,9 +17,13 @@
 
 #include <system_error>  // NOLINT(build/c++11)
 
+#include "absl/base/config.h"
+#include "absl/base/nullability.h"
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
-// Workalike compatibilty version of std::chars_format from C++17.
+// Workalike compatibility version of std::chars_format from C++17.
 //
 // This is an bitfield enumerator which can be passed to absl::from_chars to
 // configure the string-to-float conversion.
@@ -41,11 +45,11 @@ enum class chars_format {
 // characters that were successfully parsed.  If none was found, `ptr` is set
 // to the `first` argument to from_chars.
 struct from_chars_result {
-  const char* ptr;
+  const char* absl_nonnull ptr;
   std::errc ec;
 };
 
-// Workalike compatibilty version of std::from_chars from C++17.  Currently
+// Workalike compatibility version of std::from_chars from C++17.  Currently
 // this only supports the `double` and `float` types.
 //
 // This interface incorporates the proposed resolutions for library issues
@@ -61,8 +65,9 @@ struct from_chars_result {
 // the result in `value`.
 //
 // The matching pattern format is almost the same as that of strtod(), except
-// that C locale is not respected, and an initial '+' character in the input
-// range will never be matched.
+// that (1) C locale is not respected, (2) an initial '+' character in the
+// input range will never be matched, and (3) leading whitespaces are not
+// ignored.
 //
 // If `fmt` is set, it must be one of the enumerator values of the chars_format.
 // (This is despite the fact that chars_format is a bitmask type.)  If set to
@@ -72,11 +77,13 @@ struct from_chars_result {
 // format that strtod() accepts, except that a "0x" prefix is NOT matched.
 // (In particular, in `hex` mode, the input "0xff" results in the largest
 // matching pattern "0".)
-absl::from_chars_result from_chars(const char* first, const char* last,
+absl::from_chars_result from_chars(const char* absl_nonnull first,
+                                   const char* absl_nonnull last,
                                    double& value,  // NOLINT
                                    chars_format fmt = chars_format::general);
 
-absl::from_chars_result from_chars(const char* first, const char* last,
+absl::from_chars_result from_chars(const char* absl_nonnull first,
+                                   const char* absl_nonnull last,
                                    float& value,  // NOLINT
                                    chars_format fmt = chars_format::general);
 
@@ -110,6 +117,7 @@ inline chars_format& operator^=(chars_format& lhs, chars_format rhs) {
   return lhs;
 }
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_CHARCONV_H_
diff --git a/absl/strings/charconv_benchmark.cc b/absl/strings/charconv_benchmark.cc
index 644b2abdf68..0ee9363d17f 100644
--- a/absl/strings/charconv_benchmark.cc
+++ b/absl/strings/charconv_benchmark.cc
@@ -12,12 +12,11 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-#include "absl/strings/charconv.h"
-
 #include <cstdlib>
 #include <cstring>
 #include <string>
 
+#include "absl/strings/charconv.h"
 #include "benchmark/benchmark.h"
 
 namespace {
@@ -132,7 +131,7 @@ BENCHMARK(BM_Absl_HugeMantissa);
 std::string MakeHardCase(int length) {
   // The number 1.1521...e-297 is exactly halfway between 12345 * 2**-1000 and
   // the next larger representable number.  The digits of this number are in
-  // the std::string below.
+  // the string below.
   const std::string digits =
       "1."
       "152113937042223790993097181572444900347587985074226836242307364987727724"
diff --git a/absl/strings/charconv_test.cc b/absl/strings/charconv_test.cc
index b58fad269d8..c16c735c24b 100644
--- a/absl/strings/charconv_test.cc
+++ b/absl/strings/charconv_test.cc
@@ -14,14 +14,19 @@
 
 #include "absl/strings/charconv.h"
 
+#include <cfloat>
+#include <cmath>
 #include <cstdlib>
+#include <functional>
+#include <limits>
 #include <string>
+#include <system_error>  // NOLINT(build/c++11)
 
-#include "gmock/gmock.h"
 #include "gtest/gtest.h"
 #include "absl/strings/internal/pow10_helper.h"
 #include "absl/strings/str_cat.h"
 #include "absl/strings/str_format.h"
+#include "absl/strings/string_view.h"
 
 #ifdef _MSC_FULL_VER
 #define ABSL_COMPILER_DOES_EXACT_ROUNDING 0
@@ -511,6 +516,13 @@ TEST(FromChars, Overflow) {
   EXPECT_EQ(f, std::numeric_limits<float>::max());
 }
 
+TEST(FromChars, RegressionTestsFromFuzzer) {
+  absl::string_view src = "0x21900000p00000000099";
+  float f;
+  auto result = absl::from_chars(src.data(), src.data() + src.size(), f);
+  EXPECT_EQ(result.ec, std::errc::result_out_of_range);
+}
+
 TEST(FromChars, ReturnValuePtr) {
   // Check that `ptr` points one past the number scanned, even if that number
   // is not representable.
@@ -646,7 +658,9 @@ TEST(FromChars, NaNFloats) {
                      negative_from_chars_float);
     EXPECT_TRUE(std::signbit(negative_from_chars_float));
     EXPECT_FALSE(Identical(negative_from_chars_float, from_chars_float));
-    from_chars_float = std::copysign(from_chars_float, -1.0);
+    // Use the (float, float) overload of std::copysign to prevent narrowing;
+    // see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98251.
+    from_chars_float = std::copysign(from_chars_float, -1.0f);
     EXPECT_TRUE(Identical(negative_from_chars_float, from_chars_float));
   }
 }
diff --git a/absl/strings/charset.h b/absl/strings/charset.h
new file mode 100644
index 00000000000..04ad459bc69
--- /dev/null
+++ b/absl/strings/charset.h
@@ -0,0 +1,163 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: charset.h
+// -----------------------------------------------------------------------------
+//
+// This file contains absl::CharSet, a fast, bit-vector set of 8-bit unsigned
+// characters.
+//
+// Instances can be initialized as constexpr constants. For example:
+//
+//   constexpr absl::CharSet kJustX = absl::CharSet::Char('x');
+//   constexpr absl::CharSet kMySymbols = absl::CharSet("$@!");
+//   constexpr absl::CharSet kLetters = absl::CharSet::Range('a', 'z');
+//
+// Multiple instances can be combined that still forms a constexpr expression.
+// For example:
+//
+//   constexpr absl::CharSet kLettersAndNumbers =
+//       absl::CharSet::Range('a', 'z') | absl::CharSet::Range('0', '9');
+//
+// Several pre-defined character classes are available that mirror the methods
+// from <cctype>. For example:
+//
+//   constexpr absl::CharSet kLettersAndWhitespace =
+//       absl::CharSet::AsciiAlphabet() | absl::CharSet::AsciiWhitespace();
+//
+// To check membership, use the .contains method, e.g.
+//
+//   absl::CharSet hex_letters("abcdef");
+//   hex_letters.contains('a');  // true
+//   hex_letters.contains('g');  // false
+
+#ifndef ABSL_STRINGS_CHARSET_H_
+#define ABSL_STRINGS_CHARSET_H_
+
+#include <cstdint>
+
+#include "absl/base/config.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+class CharSet {
+ public:
+  constexpr CharSet() : m_() {}
+
+  // Initializes with a given string_view.
+  constexpr explicit CharSet(absl::string_view str) : m_() {
+    for (char c : str) {
+      SetChar(static_cast<unsigned char>(c));
+    }
+  }
+
+  constexpr bool contains(char c) const {
+    return ((m_[static_cast<unsigned char>(c) / 64] >>
+             (static_cast<unsigned char>(c) % 64)) &
+            0x1) == 0x1;
+  }
+
+  constexpr bool empty() const {
+    for (uint64_t c : m_) {
+      if (c != 0) return false;
+    }
+    return true;
+  }
+
+  // Containing only a single specified char.
+  static constexpr CharSet Char(char x) {
+    return CharSet(CharMaskForWord(x, 0), CharMaskForWord(x, 1),
+                   CharMaskForWord(x, 2), CharMaskForWord(x, 3));
+  }
+
+  // Containing all the chars in the closed interval [lo,hi].
+  static constexpr CharSet Range(char lo, char hi) {
+    return CharSet(RangeForWord(lo, hi, 0), RangeForWord(lo, hi, 1),
+                   RangeForWord(lo, hi, 2), RangeForWord(lo, hi, 3));
+  }
+
+  friend constexpr CharSet operator&(const CharSet& a, const CharSet& b) {
+    return CharSet(a.m_[0] & b.m_[0], a.m_[1] & b.m_[1], a.m_[2] & b.m_[2],
+                   a.m_[3] & b.m_[3]);
+  }
+
+  friend constexpr CharSet operator|(const CharSet& a, const CharSet& b) {
+    return CharSet(a.m_[0] | b.m_[0], a.m_[1] | b.m_[1], a.m_[2] | b.m_[2],
+                   a.m_[3] | b.m_[3]);
+  }
+
+  friend constexpr CharSet operator~(const CharSet& a) {
+    return CharSet(~a.m_[0], ~a.m_[1], ~a.m_[2], ~a.m_[3]);
+  }
+
+  // Mirrors the char-classifying predicates in <cctype>.
+  static constexpr CharSet AsciiUppercase() { return CharSet::Range('A', 'Z'); }
+  static constexpr CharSet AsciiLowercase() { return CharSet::Range('a', 'z'); }
+  static constexpr CharSet AsciiDigits() { return CharSet::Range('0', '9'); }
+  static constexpr CharSet AsciiAlphabet() {
+    return AsciiLowercase() | AsciiUppercase();
+  }
+  static constexpr CharSet AsciiAlphanumerics() {
+    return AsciiDigits() | AsciiAlphabet();
+  }
+  static constexpr CharSet AsciiHexDigits() {
+    return AsciiDigits() | CharSet::Range('A', 'F') | CharSet::Range('a', 'f');
+  }
+  static constexpr CharSet AsciiPrintable() {
+    return CharSet::Range(0x20, 0x7e);
+  }
+  static constexpr CharSet AsciiWhitespace() { return CharSet("\t\n\v\f\r "); }
+  static constexpr CharSet AsciiPunctuation() {
+    return AsciiPrintable() & ~AsciiWhitespace() & ~AsciiAlphanumerics();
+  }
+
+ private:
+  constexpr CharSet(uint64_t b0, uint64_t b1, uint64_t b2, uint64_t b3)
+      : m_{b0, b1, b2, b3} {}
+
+  static constexpr uint64_t RangeForWord(char lo, char hi, uint64_t word) {
+    return OpenRangeFromZeroForWord(static_cast<unsigned char>(hi) + 1, word) &
+           ~OpenRangeFromZeroForWord(static_cast<unsigned char>(lo), word);
+  }
+
+  // All the chars in the specified word of the range [0, upper).
+  static constexpr uint64_t OpenRangeFromZeroForWord(uint64_t upper,
+                                                     uint64_t word) {
+    return (upper <= 64 * word) ? 0
+           : (upper >= 64 * (word + 1))
+               ? ~static_cast<uint64_t>(0)
+               : (~static_cast<uint64_t>(0) >> (64 - upper % 64));
+  }
+
+  static constexpr uint64_t CharMaskForWord(char x, uint64_t word) {
+    return (static_cast<unsigned char>(x) / 64 == word)
+               ? (static_cast<uint64_t>(1)
+                  << (static_cast<unsigned char>(x) % 64))
+               : 0;
+  }
+
+  constexpr void SetChar(unsigned char c) {
+    m_[c / 64] |= static_cast<uint64_t>(1) << (c % 64);
+  }
+
+  uint64_t m_[4];
+};
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STRINGS_CHARSET_H_
diff --git a/absl/strings/internal/char_map_benchmark.cc b/absl/strings/charset_benchmark.cc
similarity index 71%
rename from absl/strings/internal/char_map_benchmark.cc
rename to absl/strings/charset_benchmark.cc
index 5cef967b308..713398555bf 100644
--- a/absl/strings/internal/char_map_benchmark.cc
+++ b/absl/strings/charset_benchmark.cc
@@ -1,4 +1,4 @@
-// Copyright 2017 The Abseil Authors.
+// Copyright 2020 The Abseil Authors.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
@@ -12,30 +12,30 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-#include "absl/strings/internal/char_map.h"
-
 #include <cstdint>
 
+#include "absl/log/check.h"
+#include "absl/strings/charset.h"
 #include "benchmark/benchmark.h"
 
 namespace {
 
-absl::strings_internal::Charmap MakeBenchmarkMap() {
-  absl::strings_internal::Charmap m;
+absl::CharSet MakeBenchmarkMap() {
+  absl::CharSet m;
   uint32_t x[] = {0x0, 0x1, 0x2, 0x3, 0xf, 0xe, 0xd, 0xc};
   for (uint32_t& t : x) t *= static_cast<uint32_t>(0x11111111UL);
   for (uint32_t i = 0; i < 256; ++i) {
-    if ((x[i / 32] >> (i % 32)) & 1)
-      m = m | absl::strings_internal::Charmap::Char(i);
+    if ((x[i / 32] >> (i % 32)) & 1) m = m | absl::CharSet::Char(i);
   }
   return m;
 }
 
 // Micro-benchmark for Charmap::contains.
-void BM_Contains(benchmark::State& state) {
+static void BM_Contains(benchmark::State& state) {
   // Loop-body replicated 10 times to increase time per iteration.
   // Argument continuously changed to avoid generating common subexpressions.
-  const absl::strings_internal::Charmap benchmark_map = MakeBenchmarkMap();
+  // Final CHECK used to discourage unwanted optimization.
+  const absl::CharSet benchmark_map = MakeBenchmarkMap();
   unsigned char c = 0;
   int ops = 0;
   for (auto _ : state) {
@@ -50,12 +50,8 @@ void BM_Contains(benchmark::State& state) {
     ops += benchmark_map.contains(c++);
     ops += benchmark_map.contains(c++);
   }
-  benchmark::DoNotOptimize(ops);
+  CHECK_NE(ops, -1);
 }
 BENCHMARK(BM_Contains);
 
-// We don't bother benchmarking Charmap::IsZero or Charmap::IntersectsWith;
-// their running time is data-dependent and it is not worth characterizing
-// "typical" data.
-
 }  // namespace
diff --git a/absl/strings/charset_test.cc b/absl/strings/charset_test.cc
new file mode 100644
index 00000000000..fff943ae647
--- /dev/null
+++ b/absl/strings/charset_test.cc
@@ -0,0 +1,181 @@
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/strings/charset.h"
+
+#include <stdio.h>
+#include <stdlib.h>
+
+#include <string>
+#include <vector>
+
+#include "gtest/gtest.h"
+#include "absl/strings/ascii.h"
+#include "absl/strings/string_view.h"
+
+namespace {
+
+constexpr absl::CharSet everything_map = ~absl::CharSet();
+constexpr absl::CharSet nothing_map = absl::CharSet();
+
+TEST(Charmap, AllTests) {
+  const absl::CharSet also_nothing_map("");
+  EXPECT_TRUE(everything_map.contains('\0'));
+  EXPECT_FALSE(nothing_map.contains('\0'));
+  EXPECT_FALSE(also_nothing_map.contains('\0'));
+  for (unsigned char ch = 1; ch != 0; ++ch) {
+    SCOPED_TRACE(ch);
+    EXPECT_TRUE(everything_map.contains(ch));
+    EXPECT_FALSE(nothing_map.contains(ch));
+    EXPECT_FALSE(also_nothing_map.contains(ch));
+  }
+
+  const absl::CharSet symbols(absl::string_view("&@#@^!@?", 5));
+  EXPECT_TRUE(symbols.contains('&'));
+  EXPECT_TRUE(symbols.contains('@'));
+  EXPECT_TRUE(symbols.contains('#'));
+  EXPECT_TRUE(symbols.contains('^'));
+  EXPECT_FALSE(symbols.contains('!'));
+  EXPECT_FALSE(symbols.contains('?'));
+  int cnt = 0;
+  for (unsigned char ch = 1; ch != 0; ++ch) cnt += symbols.contains(ch);
+  EXPECT_EQ(cnt, 4);
+
+  const absl::CharSet lets(absl::string_view("^abcde", 3));
+  const absl::CharSet lets2(absl::string_view("fghij\0klmnop", 10));
+  const absl::CharSet lets3("fghij\0klmnop");
+  EXPECT_TRUE(lets2.contains('k'));
+  EXPECT_FALSE(lets3.contains('k'));
+
+  EXPECT_FALSE((symbols & lets).empty());
+  EXPECT_TRUE((lets2 & lets).empty());
+  EXPECT_FALSE((lets & symbols).empty());
+  EXPECT_TRUE((lets & lets2).empty());
+
+  EXPECT_TRUE(nothing_map.empty());
+  EXPECT_FALSE(lets.empty());
+}
+
+std::string Members(const absl::CharSet& m) {
+  std::string r;
+  for (size_t i = 0; i < 256; ++i)
+    if (m.contains(i)) r.push_back(i);
+  return r;
+}
+
+std::string ClosedRangeString(unsigned char lo, unsigned char hi) {
+  // Don't depend on lo<hi. Just increment until lo==hi.
+  std::string s;
+  while (true) {
+    s.push_back(lo);
+    if (lo == hi) break;
+    ++lo;
+  }
+  return s;
+}
+
+TEST(Charmap, Constexpr) {
+  constexpr absl::CharSet kEmpty = absl::CharSet();
+  EXPECT_EQ(Members(kEmpty), "");
+  constexpr absl::CharSet kA = absl::CharSet::Char('A');
+  EXPECT_EQ(Members(kA), "A");
+  constexpr absl::CharSet kAZ = absl::CharSet::Range('A', 'Z');
+  EXPECT_EQ(Members(kAZ), "ABCDEFGHIJKLMNOPQRSTUVWXYZ");
+  constexpr absl::CharSet kIdentifier =
+      absl::CharSet::Range('0', '9') | absl::CharSet::Range('A', 'Z') |
+      absl::CharSet::Range('a', 'z') | absl::CharSet::Char('_');
+  EXPECT_EQ(Members(kIdentifier),
+            "0123456789"
+            "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+            "_"
+            "abcdefghijklmnopqrstuvwxyz");
+  constexpr absl::CharSet kAll = ~absl::CharSet();
+  for (size_t i = 0; i < 256; ++i) {
+    SCOPED_TRACE(i);
+    EXPECT_TRUE(kAll.contains(i));
+  }
+  constexpr absl::CharSet kHello = absl::CharSet("Hello, world!");
+  EXPECT_EQ(Members(kHello), " !,Hdelorw");
+
+  // test negation and intersection
+  constexpr absl::CharSet kABC =
+      absl::CharSet::Range('A', 'Z') & ~absl::CharSet::Range('D', 'Z');
+  EXPECT_EQ(Members(kABC), "ABC");
+
+  // contains
+  constexpr bool kContainsA = absl::CharSet("abc").contains('a');
+  EXPECT_TRUE(kContainsA);
+  constexpr bool kContainsD = absl::CharSet("abc").contains('d');
+  EXPECT_FALSE(kContainsD);
+
+  // empty
+  constexpr bool kEmptyIsEmpty = absl::CharSet().empty();
+  EXPECT_TRUE(kEmptyIsEmpty);
+  constexpr bool kNotEmptyIsEmpty = absl::CharSet("abc").empty();
+  EXPECT_FALSE(kNotEmptyIsEmpty);
+}
+
+TEST(Charmap, Range) {
+  // Exhaustive testing takes too long, so test some of the boundaries that
+  // are perhaps going to cause trouble.
+  std::vector<size_t> poi = {0,   1,   2,   3,   4,   7,   8,   9,  15,
+                             16,  17,  30,  31,  32,  33,  63,  64, 65,
+                             127, 128, 129, 223, 224, 225, 254, 255};
+  for (auto lo = poi.begin(); lo != poi.end(); ++lo) {
+    SCOPED_TRACE(*lo);
+    for (auto hi = lo; hi != poi.end(); ++hi) {
+      SCOPED_TRACE(*hi);
+      EXPECT_EQ(Members(absl::CharSet::Range(*lo, *hi)),
+                ClosedRangeString(*lo, *hi));
+    }
+  }
+}
+
+TEST(Charmap, NullByteWithStringView) {
+  char characters[5] = {'a', 'b', '\0', 'd', 'x'};
+  absl::string_view view(characters, 5);
+  absl::CharSet tester(view);
+  EXPECT_TRUE(tester.contains('a'));
+  EXPECT_TRUE(tester.contains('b'));
+  EXPECT_TRUE(tester.contains('\0'));
+  EXPECT_TRUE(tester.contains('d'));
+  EXPECT_TRUE(tester.contains('x'));
+  EXPECT_FALSE(tester.contains('c'));
+}
+
+TEST(CharmapCtype, Match) {
+  for (int c = 0; c < 256; ++c) {
+    SCOPED_TRACE(c);
+    SCOPED_TRACE(static_cast<char>(c));
+    EXPECT_EQ(absl::ascii_isupper(c),
+              absl::CharSet::AsciiUppercase().contains(c));
+    EXPECT_EQ(absl::ascii_islower(c),
+              absl::CharSet::AsciiLowercase().contains(c));
+    EXPECT_EQ(absl::ascii_isdigit(c), absl::CharSet::AsciiDigits().contains(c));
+    EXPECT_EQ(absl::ascii_isalpha(c),
+              absl::CharSet::AsciiAlphabet().contains(c));
+    EXPECT_EQ(absl::ascii_isalnum(c),
+              absl::CharSet::AsciiAlphanumerics().contains(c));
+    EXPECT_EQ(absl::ascii_isxdigit(c),
+              absl::CharSet::AsciiHexDigits().contains(c));
+    EXPECT_EQ(absl::ascii_isprint(c),
+              absl::CharSet::AsciiPrintable().contains(c));
+    EXPECT_EQ(absl::ascii_isspace(c),
+              absl::CharSet::AsciiWhitespace().contains(c));
+    EXPECT_EQ(absl::ascii_ispunct(c),
+              absl::CharSet::AsciiPunctuation().contains(c));
+  }
+}
+
+}  // namespace
diff --git a/absl/strings/cord.cc b/absl/strings/cord.cc
new file mode 100644
index 00000000000..5827d8974d5
--- /dev/null
+++ b/absl/strings/cord.cc
@@ -0,0 +1,1576 @@
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/strings/cord.h"
+
+#include <algorithm>
+#include <cassert>
+#include <cstddef>
+#include <cstdint>
+#include <cstdio>
+#include <cstdlib>
+#include <cstring>
+#include <iomanip>
+#include <ios>
+#include <iostream>
+#include <limits>
+#include <memory>
+#include <optional>
+#include <ostream>
+#include <sstream>
+#include <string>
+#include <utility>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/endian.h"
+#include "absl/base/internal/hardening.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/base/macros.h"
+#include "absl/base/nullability.h"
+#include "absl/base/optimization.h"
+#include "absl/container/inlined_vector.h"
+#include "absl/crc/crc32c.h"
+#include "absl/crc/internal/crc_cord_state.h"
+#include "absl/functional/function_ref.h"
+#include "absl/strings/cord_buffer.h"
+#include "absl/strings/escaping.h"
+#include "absl/strings/internal/append_and_overwrite.h"
+#include "absl/strings/internal/cord_data_edge.h"
+#include "absl/strings/internal/cord_internal.h"
+#include "absl/strings/internal/cord_rep_btree.h"
+#include "absl/strings/internal/cord_rep_crc.h"
+#include "absl/strings/internal/cord_rep_flat.h"
+#include "absl/strings/internal/cordz_update_tracker.h"
+#include "absl/strings/match.h"
+#include "absl/strings/resize_and_overwrite.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/string_view.h"
+#include "absl/strings/strip.h"
+#include "absl/types/span.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+using ::absl::cord_internal::CordRep;
+using ::absl::cord_internal::CordRepBtree;
+using ::absl::cord_internal::CordRepCrc;
+using ::absl::cord_internal::CordRepExternal;
+using ::absl::cord_internal::CordRepFlat;
+using ::absl::cord_internal::CordRepSubstring;
+using ::absl::cord_internal::CordzUpdateTracker;
+using ::absl::cord_internal::InlineData;
+using ::absl::cord_internal::kMaxBytesToCopy;
+using ::absl::cord_internal::kMaxFlatLength;
+using ::absl::cord_internal::kMinFlatLength;
+
+static void DumpNode(CordRep* absl_nonnull nonnull_rep, bool include_data,
+                     std::ostream* absl_nonnull os, int indent = 0);
+static bool VerifyNode(CordRep* absl_nonnull root,
+                       CordRep* absl_nonnull start_node);
+
+static inline CordRep* absl_nullable VerifyTree(CordRep* absl_nullable node) {
+  assert(node == nullptr || VerifyNode(node, node));
+  static_cast<void>(&VerifyNode);
+  return node;
+}
+
+static CordRepFlat* absl_nonnull CreateFlat(const char* absl_nonnull data,
+                                            size_t length, size_t alloc_hint) {
+  CordRepFlat* flat = CordRepFlat::New(length + alloc_hint);
+  flat->length = length;
+  memcpy(flat->Data(), data, length);
+  return flat;
+}
+
+// Creates a new flat or Btree out of the specified array.
+// The returned node has a refcount of 1.
+static CordRep* absl_nonnull NewBtree(const char* absl_nonnull data,
+                                      size_t length, size_t alloc_hint) {
+  if (length <= kMaxFlatLength) {
+    return CreateFlat(data, length, alloc_hint);
+  }
+  CordRepFlat* flat = CreateFlat(data, kMaxFlatLength, 0);
+  data += kMaxFlatLength;
+  length -= kMaxFlatLength;
+  auto* root = CordRepBtree::Create(flat);
+  return CordRepBtree::Append(root, {data, length}, alloc_hint);
+}
+
+// Create a new tree out of the specified array.
+// The returned node has a refcount of 1.
+static CordRep* absl_nullable NewTree(const char* absl_nullable data,
+                                      size_t length, size_t alloc_hint) {
+  if (length == 0) return nullptr;
+  return NewBtree(data, length, alloc_hint);
+}
+
+namespace cord_internal {
+
+void InitializeCordRepExternal(absl::string_view data,
+                               CordRepExternal* absl_nonnull rep) {
+  assert(!data.empty());
+  rep->length = data.size();
+  rep->tag = EXTERNAL;
+  rep->base = data.data();
+  VerifyTree(rep);
+}
+
+}  // namespace cord_internal
+
+// Creates a CordRep from the provided string. If the string is large enough,
+// and not wasteful, we move the string into an external cord rep, preserving
+// the already allocated string contents.
+// Requires the provided string length to be larger than `kMaxInline`.
+static CordRep* absl_nonnull CordRepFromString(std::string&& src) {
+  assert(src.length() > cord_internal::kMaxInline);
+  if (
+      // String is short: copy data to avoid external block overhead.
+      src.size() <= kMaxBytesToCopy ||
+      // String is wasteful: copy data to avoid pinning too much unused memory.
+      src.size() < src.capacity() / 2
+  ) {
+    return NewTree(src.data(), src.size(), 0);
+  }
+
+  struct StringReleaser {
+    void operator()(absl::string_view /* data */) {}
+    std::string data;
+  };
+  const absl::string_view original_data = src;
+  auto* rep =
+      static_cast<::absl::cord_internal::CordRepExternalImpl<StringReleaser>*>(
+          absl::cord_internal::NewExternalRep(original_data,
+                                              StringReleaser{std::move(src)}));
+  // Moving src may have invalidated its data pointer, so adjust it.
+  rep->base = rep->template get<0>().data.data();
+  return rep;
+}
+
+// --------------------------------------------------------------------
+// Cord::InlineRep functions
+
+// Returns `rep` converted into a CordRepBtree.
+// Directly returns `rep` if `rep` is already a CordRepBtree.
+static CordRepBtree* absl_nonnull ForceBtree(CordRep* rep) {
+  return rep->IsBtree()
+             ? rep->btree()
+             : CordRepBtree::Create(cord_internal::RemoveCrcNode(rep));
+}
+
+void Cord::InlineRep::AppendTreeToInlined(CordRep* absl_nonnull tree,
+                                          MethodIdentifier method) {
+  assert(!is_tree());
+  if (!data_.is_empty()) {
+    CordRepFlat* flat = MakeFlatWithExtraCapacity(0);
+    tree = CordRepBtree::Append(CordRepBtree::Create(flat), tree);
+  }
+  EmplaceTree(tree, method);
+}
+
+void Cord::InlineRep::AppendTreeToTree(CordRep* absl_nonnull tree,
+                                       MethodIdentifier method) {
+  assert(is_tree());
+  const CordzUpdateScope scope(data_.cordz_info(), method);
+  tree = CordRepBtree::Append(ForceBtree(data_.as_tree()), tree);
+  SetTree(tree, scope);
+}
+
+void Cord::InlineRep::AppendTree(CordRep* absl_nonnull tree,
+                                 MethodIdentifier method) {
+  assert(tree != nullptr);
+  assert(tree->length != 0);
+  assert(!tree->IsCrc());
+  if (data_.is_tree()) {
+    AppendTreeToTree(tree, method);
+  } else {
+    AppendTreeToInlined(tree, method);
+  }
+}
+
+void Cord::InlineRep::PrependTreeToInlined(CordRep* absl_nonnull tree,
+                                           MethodIdentifier method) {
+  assert(!is_tree());
+  if (!data_.is_empty()) {
+    CordRepFlat* flat = MakeFlatWithExtraCapacity(0);
+    tree = CordRepBtree::Prepend(CordRepBtree::Create(flat), tree);
+  }
+  EmplaceTree(tree, method);
+}
+
+void Cord::InlineRep::PrependTreeToTree(CordRep* absl_nonnull tree,
+                                        MethodIdentifier method) {
+  assert(is_tree());
+  const CordzUpdateScope scope(data_.cordz_info(), method);
+  tree = CordRepBtree::Prepend(ForceBtree(data_.as_tree()), tree);
+  SetTree(tree, scope);
+}
+
+void Cord::InlineRep::PrependTree(CordRep* absl_nonnull tree,
+                                  MethodIdentifier method) {
+  assert(tree != nullptr);
+  assert(tree->length != 0);
+  assert(!tree->IsCrc());
+  if (data_.is_tree()) {
+    PrependTreeToTree(tree, method);
+  } else {
+    PrependTreeToInlined(tree, method);
+  }
+}
+
+// Searches for a non-full flat node at the rightmost leaf of the tree. If a
+// suitable leaf is found, the function will update the length field for all
+// nodes to account for the size increase. The append region address will be
+// written to region and the actual size increase will be written to size.
+static inline bool PrepareAppendRegion(CordRep* absl_nonnull root,
+                                       char* absl_nullable* absl_nonnull region,
+                                       size_t* absl_nonnull size,
+                                       size_t max_length) {
+  if (root->IsBtree() && root->refcount.IsOne()) {
+    Span<char> span = root->btree()->GetAppendBuffer(max_length);
+    if (!span.empty()) {
+      *region = span.data();
+      *size = span.size();
+      return true;
+    }
+  }
+
+  CordRep* dst = root;
+  if (!dst->IsFlat() || !dst->refcount.IsOne()) {
+    *region = nullptr;
+    *size = 0;
+    return false;
+  }
+
+  const size_t in_use = dst->length;
+  const size_t capacity = dst->flat()->Capacity();
+  if (in_use == capacity) {
+    *region = nullptr;
+    *size = 0;
+    return false;
+  }
+
+  const size_t size_increase = std::min(capacity - in_use, max_length);
+  dst->length += size_increase;
+
+  *region = dst->flat()->Data() + in_use;
+  *size = size_increase;
+  return true;
+}
+
+void Cord::InlineRep::AssignSlow(const Cord::InlineRep& src) {
+  assert(&src != this);
+  assert(is_tree() || src.is_tree());
+  auto constexpr method = CordzUpdateTracker::kAssignCord;
+  if (ABSL_PREDICT_TRUE(!is_tree())) {
+    EmplaceTree(CordRep::Ref(src.as_tree()), src.data_, method);
+    return;
+  }
+
+  CordRep* tree = as_tree();
+  if (CordRep* src_tree = src.tree()) {
+    // Leave any existing `cordz_info` in place, and let MaybeTrackCord()
+    // decide if this cord should be (or remains to be) sampled or not.
+    data_.set_tree(CordRep::Ref(src_tree));
+    CordzInfo::MaybeTrackCord(data_, src.data_, method);
+  } else {
+    CordzInfo::MaybeUntrackCord(data_.cordz_info());
+    data_ = src.data_;
+  }
+  CordRep::Unref(tree);
+}
+
+void Cord::InlineRep::UnrefTree() {
+  if (is_tree()) {
+    CordzInfo::MaybeUntrackCord(data_.cordz_info());
+    CordRep::Unref(tree());
+  }
+}
+
+// --------------------------------------------------------------------
+// Constructors and destructors
+
+Cord::Cord(absl::string_view src, MethodIdentifier method)
+    : contents_(InlineData::kDefaultInit) {
+  const size_t n = src.size();
+  if (n <= InlineRep::kMaxInline) {
+    contents_.set_data(src.data(), n);
+  } else {
+    CordRep* rep = NewTree(src.data(), n, 0);
+    contents_.EmplaceTree(rep, method);
+  }
+}
+
+template <typename T, Cord::EnableIfString<T>>
+Cord::Cord(T&& src) : contents_(InlineData::kDefaultInit) {
+  if (src.size() <= InlineRep::kMaxInline) {
+    contents_.set_data(src.data(), src.size());
+  } else {
+    CordRep* rep = CordRepFromString(std::forward<T>(src));
+    contents_.EmplaceTree(rep, CordzUpdateTracker::kConstructorString);
+  }
+}
+
+template Cord::Cord(std::string&& src);
+
+// The destruction code is separate so that the compiler can determine
+// that it does not need to call the destructor on a moved-from Cord.
+void Cord::DestroyCordSlow() {
+  assert(contents_.is_tree());
+  CordzInfo::MaybeUntrackCord(contents_.cordz_info());
+  CordRep::Unref(VerifyTree(contents_.as_tree()));
+}
+
+// --------------------------------------------------------------------
+// Mutators
+
+void Cord::Clear() {
+  if (CordRep* tree = contents_.clear()) {
+    CordRep::Unref(tree);
+  }
+}
+
+Cord& Cord::AssignLargeString(std::string&& src) {
+  auto constexpr method = CordzUpdateTracker::kAssignString;
+  assert(src.size() > kMaxBytesToCopy);
+  CordRep* rep = CordRepFromString(std::move(src));
+  if (CordRep* tree = contents_.tree()) {
+    CordzUpdateScope scope(contents_.cordz_info(), method);
+    contents_.SetTree(rep, scope);
+    CordRep::Unref(tree);
+  } else {
+    contents_.EmplaceTree(rep, method);
+  }
+  return *this;
+}
+
+Cord& Cord::operator=(absl::string_view src) {
+  auto constexpr method = CordzUpdateTracker::kAssignString;
+  const char* data = src.data();
+  size_t length = src.size();
+  CordRep* tree = contents_.tree();
+  if (length <= InlineRep::kMaxInline) {
+    // Embed into this->contents_, which is somewhat subtle:
+    // - MaybeUntrackCord must be called before Unref(tree).
+    // - MaybeUntrackCord must be called before set_data() clobbers cordz_info.
+    // - set_data() must be called before Unref(tree) as it may reference tree.
+    if (tree != nullptr) CordzInfo::MaybeUntrackCord(contents_.cordz_info());
+    contents_.set_data(data, length);
+    if (tree != nullptr) CordRep::Unref(tree);
+    return *this;
+  }
+  if (tree != nullptr) {
+    CordzUpdateScope scope(contents_.cordz_info(), method);
+    if (tree->IsFlat() && tree->flat()->Capacity() >= length &&
+        tree->refcount.IsOne()) {
+      // Copy in place if the existing FLAT node is reusable.
+      memmove(tree->flat()->Data(), data, length);
+      tree->length = length;
+      VerifyTree(tree);
+      return *this;
+    }
+    contents_.SetTree(NewTree(data, length, 0), scope);
+    CordRep::Unref(tree);
+  } else {
+    contents_.EmplaceTree(NewTree(data, length, 0), method);
+  }
+  return *this;
+}
+
+// TODO(sanjay): Move to Cord::InlineRep section of file.  For now,
+// we keep it here to make diffs easier.
+void Cord::InlineRep::AppendArray(absl::string_view src,
+                                  MethodIdentifier method) {
+  if (src.empty()) return;  // memcpy(_, nullptr, 0) is undefined.
+  MaybeRemoveEmptyCrcNode();
+
+  size_t appended = 0;
+  CordRep* rep = tree();
+  const CordRep* const root = rep;
+  CordzUpdateScope scope(root ? cordz_info() : nullptr, method);
+  if (root != nullptr) {
+    rep = cord_internal::RemoveCrcNode(rep);
+    char* region;
+    if (PrepareAppendRegion(rep, &region, &appended, src.size())) {
+      memcpy(region, src.data(), appended);
+    }
+  } else {
+    // Try to fit in the inline buffer if possible.
+    size_t inline_length = inline_size();
+    if (src.size() <= kMaxInline - inline_length) {
+      // Append new data to embedded array
+      set_inline_size(inline_length + src.size());
+      memcpy(data_.as_chars() + inline_length, src.data(), src.size());
+      return;
+    }
+
+    // Allocate flat to be a perfect fit on first append exceeding inlined size.
+    // Subsequent growth will use amortized growth until we reach maximum flat
+    // size.
+    rep = CordRepFlat::New(inline_length + src.size());
+    appended = std::min(src.size(), rep->flat()->Capacity() - inline_length);
+    memcpy(rep->flat()->Data(), data_.as_chars(), inline_length);
+    memcpy(rep->flat()->Data() + inline_length, src.data(), appended);
+    rep->length = inline_length + appended;
+  }
+
+  src.remove_prefix(appended);
+  if (src.empty()) {
+    CommitTree(root, rep, scope, method);
+    return;
+  }
+
+  // TODO(b/192061034): keep legacy 10% growth rate: consider other rates.
+  rep = ForceBtree(rep);
+  const size_t min_growth = std::max<size_t>(rep->length / 10, src.size());
+  rep = CordRepBtree::Append(rep->btree(), src, min_growth - src.size());
+
+  CommitTree(root, rep, scope, method);
+}
+
+inline CordRep* absl_nonnull Cord::TakeRep() const& {
+  return CordRep::Ref(contents_.tree());
+}
+
+inline CordRep* absl_nonnull Cord::TakeRep() && {
+  CordRep* rep = contents_.tree();
+  contents_.clear();
+  return rep;
+}
+
+template <typename C>
+inline void Cord::AppendImpl(C&& src) {
+  auto constexpr method = CordzUpdateTracker::kAppendCord;
+
+  contents_.MaybeRemoveEmptyCrcNode();
+  if (src.empty()) return;
+  ABSL_RAW_CHECK(src.contents_.size() <=
+                     std::numeric_limits<size_t>::max() - contents_.size(),
+                 "Cord length overflow");
+
+  if (empty()) {
+    // Since destination is empty, we can avoid allocating a node,
+    if (src.contents_.is_tree()) {
+      // by taking the tree directly
+      CordRep* rep =
+          cord_internal::RemoveCrcNode(std::forward<C>(src).TakeRep());
+      contents_.EmplaceTree(rep, method);
+    } else {
+      // or copying over inline data
+      contents_.data_ = src.contents_.data_;
+    }
+    return;
+  }
+
+  // For short cords, it is faster to copy data if there is room in dst.
+  const size_t src_size = src.contents_.size();
+  if (src_size <= kMaxBytesToCopy) {
+    CordRep* src_tree = src.contents_.tree();
+    if (src_tree == nullptr) {
+      // src has embedded data.
+      contents_.AppendArray({src.contents_.data(), src_size}, method);
+      return;
+    }
+    if (src_tree->IsFlat()) {
+      // src tree just has one flat node.
+      contents_.AppendArray({src_tree->flat()->Data(), src_size}, method);
+      return;
+    }
+    if (&src == this) {
+      // ChunkIterator below assumes that src is not modified during traversal.
+      Append(Cord(src));
+      return;
+    }
+    // TODO(mec): Should we only do this if "dst" has space?
+    for (absl::string_view chunk : src.Chunks()) {
+      Append(chunk);
+    }
+    return;
+  }
+
+  // Guaranteed to be a tree (kMaxBytesToCopy > kInlinedSize)
+  CordRep* rep = cord_internal::RemoveCrcNode(std::forward<C>(src).TakeRep());
+  contents_.AppendTree(rep, CordzUpdateTracker::kAppendCord);
+}
+
+static CordRep::ExtractResult ExtractAppendBuffer(CordRep* absl_nonnull rep,
+                                                  size_t min_capacity) {
+  switch (rep->tag) {
+    case cord_internal::BTREE:
+      return CordRepBtree::ExtractAppendBuffer(rep->btree(), min_capacity);
+    default:
+      if (rep->IsFlat() && rep->refcount.IsOne() &&
+          rep->flat()->Capacity() - rep->length >= min_capacity) {
+        return {nullptr, rep};
+      }
+      return {rep, nullptr};
+  }
+}
+
+static CordBuffer CreateAppendBuffer(InlineData& data, size_t block_size,
+                                     size_t capacity) {
+  // Watch out for overflow, people can ask for size_t::max().
+  const size_t size = data.inline_size();
+  const size_t max_capacity = std::numeric_limits<size_t>::max() - size;
+  capacity = (std::min)(max_capacity, capacity) + size;
+  CordBuffer buffer =
+      block_size ? CordBuffer::CreateWithCustomLimit(block_size, capacity)
+                 : CordBuffer::CreateWithDefaultLimit(capacity);
+  cord_internal::SmallMemmove(buffer.data(), data.as_chars(), size);
+  buffer.SetLength(size);
+  data = {};
+  return buffer;
+}
+
+CordBuffer Cord::GetAppendBufferSlowPath(size_t block_size, size_t capacity,
+                                         size_t min_capacity) {
+  auto constexpr method = CordzUpdateTracker::kGetAppendBuffer;
+  CordRep* tree = contents_.tree();
+  if (tree != nullptr) {
+    CordzUpdateScope scope(contents_.cordz_info(), method);
+    CordRep::ExtractResult result = ExtractAppendBuffer(tree, min_capacity);
+    if (result.extracted != nullptr) {
+      contents_.SetTreeOrEmpty(result.tree, scope);
+      return CordBuffer(result.extracted->flat());
+    }
+    return block_size ? CordBuffer::CreateWithCustomLimit(block_size, capacity)
+                      : CordBuffer::CreateWithDefaultLimit(capacity);
+  }
+  return CreateAppendBuffer(contents_.data_, block_size, capacity);
+}
+
+void Cord::Append(const Cord& src) { AppendImpl(src); }
+
+void Cord::Append(Cord&& src) { AppendImpl(std::move(src)); }
+
+template <typename T, Cord::EnableIfString<T>>
+void Cord::Append(T&& src) {
+  if (src.size() <= kMaxBytesToCopy) {
+    Append(absl::string_view(src));
+  } else {
+    CordRep* rep = CordRepFromString(std::forward<T>(src));
+    contents_.AppendTree(rep, CordzUpdateTracker::kAppendString);
+  }
+}
+
+template void Cord::Append(std::string&& src);
+
+void Cord::Prepend(const Cord& src) {
+  contents_.MaybeRemoveEmptyCrcNode();
+  if (src.empty()) return;
+  ABSL_RAW_CHECK(src.contents_.size() <=
+                     std::numeric_limits<size_t>::max() - contents_.size(),
+                 "Cord length overflow");
+
+  CordRep* src_tree = src.contents_.tree();
+  if (src_tree != nullptr) {
+    CordRep::Ref(src_tree);
+    contents_.PrependTree(cord_internal::RemoveCrcNode(src_tree),
+                          CordzUpdateTracker::kPrependCord);
+    return;
+  }
+
+  // `src` cord is inlined.
+  absl::string_view src_contents(src.contents_.data(), src.contents_.size());
+  return Prepend(src_contents);
+}
+
+void Cord::PrependArray(absl::string_view src, MethodIdentifier method) {
+  contents_.MaybeRemoveEmptyCrcNode();
+  if (src.empty()) return;  // memcpy(_, nullptr, 0) is undefined.
+
+  if (!contents_.is_tree()) {
+    size_t cur_size = contents_.inline_size();
+    if (cur_size + src.size() <= InlineRep::kMaxInline) {
+      // Use embedded storage.
+      InlineData data;
+      data.set_inline_size(cur_size + src.size());
+      memcpy(data.as_chars(), src.data(), src.size());
+      memcpy(data.as_chars() + src.size(), contents_.data(), cur_size);
+      contents_.data_ = data;
+      return;
+    }
+  }
+  CordRep* rep = NewTree(src.data(), src.size(), 0);
+  contents_.PrependTree(rep, method);
+}
+
+void Cord::AppendPrecise(absl::string_view src, MethodIdentifier method) {
+  assert(!src.empty());
+  assert(src.size() <= cord_internal::kMaxFlatLength);
+  if (contents_.remaining_inline_capacity() >= src.size()) {
+    const size_t inline_length = contents_.inline_size();
+    contents_.set_inline_size(inline_length + src.size());
+    memcpy(contents_.data_.as_chars() + inline_length, src.data(), src.size());
+  } else {
+    contents_.AppendTree(CordRepFlat::Create(src), method);
+  }
+}
+
+void Cord::PrependPrecise(absl::string_view src, MethodIdentifier method) {
+  assert(!src.empty());
+  assert(src.size() <= cord_internal::kMaxFlatLength);
+  if (contents_.remaining_inline_capacity() >= src.size()) {
+    const size_t cur_size = contents_.inline_size();
+    InlineData data;
+    data.set_inline_size(cur_size + src.size());
+    memcpy(data.as_chars(), src.data(), src.size());
+    memcpy(data.as_chars() + src.size(), contents_.data(), cur_size);
+    contents_.data_ = data;
+  } else {
+    contents_.PrependTree(CordRepFlat::Create(src), method);
+  }
+}
+
+template <typename T, Cord::EnableIfString<T>>
+inline void Cord::Prepend(T&& src) {
+  if (src.size() <= kMaxBytesToCopy) {
+    Prepend(absl::string_view(src));
+  } else {
+    CordRep* rep = CordRepFromString(std::forward<T>(src));
+    contents_.PrependTree(rep, CordzUpdateTracker::kPrependString);
+  }
+}
+
+template void Cord::Prepend(std::string&& src);
+
+void Cord::RemovePrefix(size_t n) {
+  ABSL_INTERNAL_CHECK(n <= size(),
+                      absl::StrCat("Requested prefix size ", n,
+                                   " exceeds Cord's size ", size()));
+  contents_.MaybeRemoveEmptyCrcNode();
+  CordRep* tree = contents_.tree();
+  if (tree == nullptr) {
+    contents_.remove_prefix(n);
+  } else {
+    auto constexpr method = CordzUpdateTracker::kRemovePrefix;
+    CordzUpdateScope scope(contents_.cordz_info(), method);
+    tree = cord_internal::RemoveCrcNode(tree);
+    if (n >= tree->length) {
+      CordRep::Unref(tree);
+      tree = nullptr;
+    } else if (tree->IsBtree()) {
+      CordRep* old = tree;
+      tree = tree->btree()->SubTree(n, tree->length - n);
+      CordRep::Unref(old);
+    } else if (tree->IsSubstring() && tree->refcount.IsOne()) {
+      tree->substring()->start += n;
+      tree->length -= n;
+    } else {
+      CordRep* rep = CordRepSubstring::Substring(tree, n, tree->length - n);
+      CordRep::Unref(tree);
+      tree = rep;
+    }
+    contents_.SetTreeOrEmpty(tree, scope);
+  }
+}
+
+void Cord::RemoveSuffix(size_t n) {
+  ABSL_INTERNAL_CHECK(n <= size(),
+                      absl::StrCat("Requested suffix size ", n,
+                                   " exceeds Cord's size ", size()));
+  contents_.MaybeRemoveEmptyCrcNode();
+  CordRep* tree = contents_.tree();
+  if (tree == nullptr) {
+    contents_.reduce_size(n);
+  } else {
+    auto constexpr method = CordzUpdateTracker::kRemoveSuffix;
+    CordzUpdateScope scope(contents_.cordz_info(), method);
+    tree = cord_internal::RemoveCrcNode(tree);
+    if (n >= tree->length) {
+      CordRep::Unref(tree);
+      tree = nullptr;
+    } else if (tree->IsBtree()) {
+      tree = CordRepBtree::RemoveSuffix(tree->btree(), n);
+    } else if (!tree->IsExternal() && tree->refcount.IsOne()) {
+      assert(tree->IsFlat() || tree->IsSubstring());
+      tree->length -= n;
+    } else {
+      CordRep* rep = CordRepSubstring::Substring(tree, 0, tree->length - n);
+      CordRep::Unref(tree);
+      tree = rep;
+    }
+    contents_.SetTreeOrEmpty(tree, scope);
+  }
+}
+
+Cord Cord::Subcord(size_t pos, size_t new_size) const {
+  Cord sub_cord;
+  size_t length = size();
+  if (pos > length) pos = length;
+  if (new_size > length - pos) new_size = length - pos;
+  if (new_size == 0) return sub_cord;
+
+  CordRep* tree = contents_.tree();
+  if (tree == nullptr) {
+    sub_cord.contents_.set_data(contents_.data() + pos, new_size);
+    return sub_cord;
+  }
+
+  if (new_size <= InlineRep::kMaxInline) {
+    sub_cord.contents_.set_inline_size(new_size);
+    char* dest = sub_cord.contents_.data_.as_chars();
+    Cord::ChunkIterator it = chunk_begin();
+    it.AdvanceBytes(pos);
+    size_t remaining_size = new_size;
+    while (remaining_size > it->size()) {
+      cord_internal::SmallMemmove(dest, it->data(), it->size());
+      remaining_size -= it->size();
+      dest += it->size();
+      ++it;
+    }
+    cord_internal::SmallMemmove(dest, it->data(), remaining_size);
+    return sub_cord;
+  }
+
+  tree = cord_internal::SkipCrcNode(tree);
+  if (tree->IsBtree()) {
+    tree = tree->btree()->SubTree(pos, new_size);
+  } else {
+    tree = CordRepSubstring::Substring(tree, pos, new_size);
+  }
+  sub_cord.contents_.EmplaceTree(tree, contents_.data_,
+                                 CordzUpdateTracker::kSubCord);
+  return sub_cord;
+}
+
+// --------------------------------------------------------------------
+// Comparators
+
+namespace {
+
+int ClampResult(int memcmp_res) {
+  return static_cast<int>(memcmp_res > 0) - static_cast<int>(memcmp_res < 0);
+}
+
+int CompareChunks(absl::string_view* absl_nonnull lhs,
+                  absl::string_view* absl_nonnull rhs,
+                  size_t* absl_nonnull size_to_compare) {
+  size_t compared_size = std::min(lhs->size(), rhs->size());
+  assert(*size_to_compare >= compared_size);
+  *size_to_compare -= compared_size;
+
+  int memcmp_res = ::memcmp(lhs->data(), rhs->data(), compared_size);
+  if (memcmp_res != 0) return memcmp_res;
+
+  lhs->remove_prefix(compared_size);
+  rhs->remove_prefix(compared_size);
+
+  return 0;
+}
+
+// This overload set computes comparison results from memcmp result. This
+// interface is used inside GenericCompare below. Different implementations
+// are specialized for int and bool. For int we clamp result to {-1, 0, 1}
+// set. For bool we just interested in "value == 0".
+template <typename ResultType>
+ResultType ComputeCompareResult(int memcmp_res) {
+  return ClampResult(memcmp_res);
+}
+template <>
+bool ComputeCompareResult<bool>(int memcmp_res) {
+  return memcmp_res == 0;
+}
+
+}  // namespace
+
+// Helper routine. Locates the first flat or external chunk of the Cord without
+// initializing the iterator, and returns a string_view referencing the data.
+inline absl::string_view Cord::InlineRep::FindFlatStartPiece() const {
+  if (!is_tree()) {
+    return absl::string_view(data_.as_chars(), data_.inline_size());
+  }
+
+  CordRep* node = cord_internal::SkipCrcNode(tree());
+  if (node->IsFlat()) {
+    return absl::string_view(node->flat()->Data(), node->length);
+  }
+
+  if (node->IsExternal()) {
+    return absl::string_view(node->external()->base, node->length);
+  }
+
+  if (node->IsBtree()) {
+    CordRepBtree* tree = node->btree();
+    int height = tree->height();
+    while (--height >= 0) {
+      tree = tree->Edge(CordRepBtree::kFront)->btree();
+    }
+    return tree->Data(tree->begin());
+  }
+
+  // Get the child node if we encounter a SUBSTRING.
+  size_t offset = 0;
+  size_t length = node->length;
+  assert(length != 0);
+
+  if (node->IsSubstring()) {
+    offset = node->substring()->start;
+    node = node->substring()->child;
+  }
+
+  if (node->IsFlat()) {
+    return absl::string_view(node->flat()->Data() + offset, length);
+  }
+
+  assert(node->IsExternal() && "Expect FLAT or EXTERNAL node here");
+
+  return absl::string_view(node->external()->base + offset, length);
+}
+
+void Cord::SetCrcCordState(crc_internal::CrcCordState state) {
+  auto constexpr method = CordzUpdateTracker::kSetExpectedChecksum;
+  if (empty()) {
+    contents_.MaybeRemoveEmptyCrcNode();
+    CordRep* rep = CordRepCrc::New(nullptr, std::move(state));
+    contents_.EmplaceTree(rep, method);
+  } else if (!contents_.is_tree()) {
+    CordRep* rep = contents_.MakeFlatWithExtraCapacity(0);
+    rep = CordRepCrc::New(rep, std::move(state));
+    contents_.EmplaceTree(rep, method);
+  } else {
+    const CordzUpdateScope scope(contents_.data_.cordz_info(), method);
+    CordRep* rep = CordRepCrc::New(contents_.data_.as_tree(), std::move(state));
+    contents_.SetTree(rep, scope);
+  }
+}
+
+void Cord::SetExpectedChecksum(uint32_t crc) {
+  // Construct a CrcCordState with a single chunk.
+  crc_internal::CrcCordState state;
+  state.mutable_rep()->prefix_crc.push_back(
+      crc_internal::CrcCordState::PrefixCrc(size(), absl::crc32c_t{crc}));
+  SetCrcCordState(std::move(state));
+}
+
+const crc_internal::CrcCordState* absl_nullable Cord::MaybeGetCrcCordState()
+    const {
+  if (!contents_.is_tree() || !contents_.tree()->IsCrc()) {
+    return nullptr;
+  }
+  return &contents_.tree()->crc()->crc_cord_state;
+}
+
+std::optional<uint32_t> Cord::ExpectedChecksum() const {
+  if (!contents_.is_tree() || !contents_.tree()->IsCrc()) {
+    return std::nullopt;
+  }
+  return static_cast<uint32_t>(
+      contents_.tree()->crc()->crc_cord_state.Checksum());
+}
+
+inline int Cord::CompareSlowPath(absl::string_view rhs, size_t compared_size,
+                                 size_t size_to_compare) const {
+  auto advance = [](Cord::ChunkIterator* absl_nonnull it,
+                    absl::string_view* absl_nonnull chunk) {
+    if (!chunk->empty()) return true;
+    ++*it;
+    if (it->bytes_remaining_ == 0) return false;
+    *chunk = **it;
+    return true;
+  };
+
+  Cord::ChunkIterator lhs_it = chunk_begin();
+
+  // compared_size is inside first chunk.
+  absl::string_view lhs_chunk =
+      (lhs_it.bytes_remaining_ != 0) ? *lhs_it : absl::string_view();
+  assert(compared_size <= lhs_chunk.size());
+  assert(compared_size <= rhs.size());
+  lhs_chunk.remove_prefix(compared_size);
+  rhs.remove_prefix(compared_size);
+  size_to_compare -= compared_size;  // skip already compared size.
+
+  while (advance(&lhs_it, &lhs_chunk) && !rhs.empty()) {
+    int comparison_result = CompareChunks(&lhs_chunk, &rhs, &size_to_compare);
+    if (comparison_result != 0) return comparison_result;
+    if (size_to_compare == 0) return 0;
+  }
+
+  return static_cast<int>(rhs.empty()) - static_cast<int>(lhs_chunk.empty());
+}
+
+inline int Cord::CompareSlowPath(const Cord& rhs, size_t compared_size,
+                                 size_t size_to_compare) const {
+  auto advance = [](Cord::ChunkIterator* absl_nonnull it,
+                    absl::string_view* absl_nonnull chunk) {
+    if (!chunk->empty()) return true;
+    ++*it;
+    if (it->bytes_remaining_ == 0) return false;
+    *chunk = **it;
+    return true;
+  };
+
+  Cord::ChunkIterator lhs_it = chunk_begin();
+  Cord::ChunkIterator rhs_it = rhs.chunk_begin();
+
+  // compared_size is inside both first chunks.
+  absl::string_view lhs_chunk =
+      (lhs_it.bytes_remaining_ != 0) ? *lhs_it : absl::string_view();
+  absl::string_view rhs_chunk =
+      (rhs_it.bytes_remaining_ != 0) ? *rhs_it : absl::string_view();
+  assert(compared_size <= lhs_chunk.size());
+  assert(compared_size <= rhs_chunk.size());
+  lhs_chunk.remove_prefix(compared_size);
+  rhs_chunk.remove_prefix(compared_size);
+  size_to_compare -= compared_size;  // skip already compared size.
+
+  while (advance(&lhs_it, &lhs_chunk) && advance(&rhs_it, &rhs_chunk)) {
+    int memcmp_res = CompareChunks(&lhs_chunk, &rhs_chunk, &size_to_compare);
+    if (memcmp_res != 0) return memcmp_res;
+    if (size_to_compare == 0) return 0;
+  }
+
+  return static_cast<int>(rhs_chunk.empty()) -
+         static_cast<int>(lhs_chunk.empty());
+}
+
+inline absl::string_view Cord::GetFirstChunk(const Cord& c) {
+  if (c.empty()) return {};
+  return c.contents_.FindFlatStartPiece();
+}
+inline absl::string_view Cord::GetFirstChunk(absl::string_view sv) {
+  return sv;
+}
+
+// Compares up to 'size_to_compare' bytes of 'lhs' with 'rhs'. It is assumed
+// that 'size_to_compare' is greater that size of smallest of first chunks.
+template <typename ResultType, typename RHS>
+ResultType GenericCompare(const Cord& lhs, const RHS& rhs,
+                          size_t size_to_compare) {
+  absl::string_view lhs_chunk = Cord::GetFirstChunk(lhs);
+  absl::string_view rhs_chunk = Cord::GetFirstChunk(rhs);
+
+  size_t compared_size = std::min(lhs_chunk.size(), rhs_chunk.size());
+  assert(size_to_compare >= compared_size);
+  int memcmp_res = compared_size > 0 ? ::memcmp(lhs_chunk.data(),
+                                                rhs_chunk.data(), compared_size)
+                                     : 0;
+  if (compared_size == size_to_compare || memcmp_res != 0) {
+    return ComputeCompareResult<ResultType>(memcmp_res);
+  }
+
+  return ComputeCompareResult<ResultType>(
+      lhs.CompareSlowPath(rhs, compared_size, size_to_compare));
+}
+
+bool Cord::EqualsImpl(absl::string_view rhs, size_t size_to_compare) const {
+  return GenericCompare<bool>(*this, rhs, size_to_compare);
+}
+
+bool Cord::EqualsImpl(const Cord& rhs, size_t size_to_compare) const {
+  return GenericCompare<bool>(*this, rhs, size_to_compare);
+}
+
+template <typename RHS>
+inline int SharedCompareImpl(const Cord& lhs, const RHS& rhs) {
+  size_t lhs_size = lhs.size();
+  size_t rhs_size = rhs.size();
+  if (lhs_size == rhs_size) {
+    return GenericCompare<int>(lhs, rhs, lhs_size);
+  }
+  if (lhs_size < rhs_size) {
+    auto data_comp_res = GenericCompare<int>(lhs, rhs, lhs_size);
+    return data_comp_res == 0 ? -1 : data_comp_res;
+  }
+
+  auto data_comp_res = GenericCompare<int>(lhs, rhs, rhs_size);
+  return data_comp_res == 0 ? +1 : data_comp_res;
+}
+
+int Cord::Compare(absl::string_view rhs) const {
+  return SharedCompareImpl(*this, rhs);
+}
+
+int Cord::CompareImpl(const Cord& rhs) const {
+  return SharedCompareImpl(*this, rhs);
+}
+
+bool Cord::EndsWith(absl::string_view rhs) const {
+  size_t my_size = size();
+  size_t rhs_size = rhs.size();
+
+  if (my_size < rhs_size) return false;
+
+  Cord tmp(*this);
+  tmp.RemovePrefix(my_size - rhs_size);
+  return tmp.EqualsImpl(rhs, rhs_size);
+}
+
+bool Cord::EndsWith(const Cord& rhs) const {
+  size_t my_size = size();
+  size_t rhs_size = rhs.size();
+
+  if (my_size < rhs_size) return false;
+
+  Cord tmp(*this);
+  tmp.RemovePrefix(my_size - rhs_size);
+  return tmp.EqualsImpl(rhs, rhs_size);
+}
+
+// --------------------------------------------------------------------
+// Misc.
+
+Cord::operator std::string() const {
+  std::string s;
+  absl::CopyCordToString(*this, &s);
+  return s;
+}
+
+void CopyCordToString(const Cord& src, std::string* absl_nonnull dst) {
+  if (!src.contents_.is_tree()) {
+    src.contents_.CopyTo(dst);
+  } else {
+    StringResizeAndOverwrite(*dst, src.size(),
+                             [&src](char* buf, size_t buf_size) {
+                               src.CopyToArraySlowPath(buf);
+                               return buf_size;
+                             });
+  }
+}
+
+void AppendCordToString(const Cord& src, std::string* absl_nonnull dst) {
+  strings_internal::StringAppendAndOverwrite(
+      *dst, src.size(), [&src](char* buf, size_t buf_size) {
+        src.CopyToArrayImpl(buf);
+        return buf_size;
+      });
+}
+
+void Cord::CopyToArraySlowPath(char* absl_nonnull dst) const {
+  assert(contents_.is_tree());
+  absl::string_view fragment;
+  if (GetFlatAux(contents_.tree(), &fragment) && !fragment.empty()) {
+    memcpy(dst, fragment.data(), fragment.size());
+    return;
+  }
+  for (absl::string_view chunk : Chunks()) {
+    memcpy(dst, chunk.data(), chunk.size());
+    dst += chunk.size();
+  }
+}
+
+size_t CopyCordToSpan(const Cord& src, absl::Span<char> dst) {
+  if (src.size() <= dst.size()) {
+    src.CopyToArrayImpl(dst.data());
+    return src.size();
+  }
+
+  const size_t result = dst.size();
+  for (absl::string_view chunk : src.Chunks()) {
+    size_t n = std::min(chunk.size(), dst.size());
+    if (n == 0) {
+      break;
+    }
+    memcpy(dst.data(), chunk.data(), n);
+    dst.remove_prefix(n);
+  }
+  return result;
+}
+
+Cord Cord::ChunkIterator::AdvanceAndReadBytes(size_t n) {
+  // Failure of this assertion indicates an attempt to iterate past `end()`.
+  absl::base_internal::HardeningAssertGE(bytes_remaining_, n);
+  Cord subcord;
+  auto constexpr method = CordzUpdateTracker::kCordReader;
+
+  if (n <= InlineRep::kMaxInline) {
+    // Range to read fits in inline data. Flatten it.
+    char* data = subcord.contents_.set_data(n);
+    while (n > current_chunk_.size()) {
+      memcpy(data, current_chunk_.data(), current_chunk_.size());
+      data += current_chunk_.size();
+      n -= current_chunk_.size();
+      ++*this;
+    }
+    memcpy(data, current_chunk_.data(), n);
+    if (n < current_chunk_.size()) {
+      RemoveChunkPrefix(n);
+    } else if (n > 0) {
+      ++*this;
+    }
+    return subcord;
+  }
+
+  if (btree_reader_) {
+    size_t chunk_size = current_chunk_.size();
+    if (n <= chunk_size && n <= kMaxBytesToCopy) {
+      subcord = Cord(current_chunk_.substr(0, n), method);
+      if (n < chunk_size) {
+        current_chunk_.remove_prefix(n);
+      } else {
+        current_chunk_ = btree_reader_.Next();
+      }
+    } else {
+      CordRep* rep;
+      current_chunk_ = btree_reader_.Read(n, chunk_size, rep);
+      subcord.contents_.EmplaceTree(rep, method);
+    }
+    bytes_remaining_ -= n;
+    return subcord;
+  }
+
+  // Short circuit if reading the entire data edge.
+  assert(current_leaf_ != nullptr);
+  if (n == current_leaf_->length) {
+    bytes_remaining_ = 0;
+    current_chunk_ = {};
+    CordRep* tree = CordRep::Ref(current_leaf_);
+    subcord.contents_.EmplaceTree(VerifyTree(tree), method);
+    return subcord;
+  }
+
+  // From this point on, we need a partial substring node.
+  // Get pointer to the underlying flat or external data payload and
+  // compute data pointer and offset into current flat or external.
+  CordRep* payload = current_leaf_->IsSubstring()
+                         ? current_leaf_->substring()->child
+                         : current_leaf_;
+  const char* data = payload->IsExternal() ? payload->external()->base
+                                           : payload->flat()->Data();
+  const size_t offset = static_cast<size_t>(current_chunk_.data() - data);
+
+  auto* tree = CordRepSubstring::Substring(payload, offset, n);
+  subcord.contents_.EmplaceTree(VerifyTree(tree), method);
+  bytes_remaining_ -= n;
+  current_chunk_.remove_prefix(n);
+  return subcord;
+}
+
+char Cord::operator[](size_t i) const {
+  absl::base_internal::HardeningAssertLT(i, size());
+  size_t offset = i;
+  const CordRep* rep = contents_.tree();
+  if (rep == nullptr) {
+    return contents_.data()[i];
+  }
+  rep = cord_internal::SkipCrcNode(rep);
+  while (true) {
+    assert(rep != nullptr);
+    assert(offset < rep->length);
+    if (rep->IsFlat()) {
+      // Get the "i"th character directly from the flat array.
+      return rep->flat()->Data()[offset];
+    } else if (rep->IsBtree()) {
+      return rep->btree()->GetCharacter(offset);
+    } else if (rep->IsExternal()) {
+      // Get the "i"th character from the external array.
+      return rep->external()->base[offset];
+    } else {
+      // This must be a substring a node, so bypass it to get to the child.
+      assert(rep->IsSubstring());
+      offset += rep->substring()->start;
+      rep = rep->substring()->child;
+    }
+  }
+}
+
+namespace {
+
+// Tests whether the sequence of chunks beginning at `position` starts with
+// `needle`.
+//
+// REQUIRES: remaining `absl::Cord` starting at `position` is greater than or
+// equal to `needle.size()`.
+bool IsSubstringInCordAt(absl::Cord::CharIterator position,
+                         absl::string_view needle) {
+  auto haystack_chunk = absl::Cord::ChunkRemaining(position);
+  while (true) {
+    // Precondition is that `absl::Cord::ChunkRemaining(position)` is not
+    // empty. This assert will trigger if that is not true.
+    assert(!haystack_chunk.empty());
+    auto min_length = std::min(haystack_chunk.size(), needle.size());
+    if (!absl::ConsumePrefix(&needle, haystack_chunk.substr(0, min_length))) {
+      return false;
+    }
+    if (needle.empty()) {
+      return true;
+    }
+    absl::Cord::Advance(&position, min_length);
+    haystack_chunk = absl::Cord::ChunkRemaining(position);
+  }
+}
+
+}  // namespace
+
+// A few options how this could be implemented:
+// (a) Flatten the Cord and find, i.e.
+//       haystack.Flatten().find(needle)
+//     For large 'haystack' (where Cord makes sense to be used), this copies
+//     the whole 'haystack' and can be slow.
+// (b) Use std::search, i.e.
+//       std::search(haystack.char_begin(), haystack.char_end(),
+//                   needle.begin(), needle.end())
+//     This avoids the copy, but compares one byte at a time, and branches a
+//     lot every time it has to advance. It is also not possible to use
+//     std::search as is, because CharIterator is only an input iterator, not a
+//     forward iterator.
+// (c) Use string_view::find in each fragment, and specifically handle fragment
+//     boundaries.
+//
+// This currently implements option (b).
+absl::Cord::CharIterator absl::Cord::FindImpl(CharIterator it,
+                                              absl::string_view needle) const {
+  // Ensure preconditions are met by callers first.
+
+  // Needle must not be empty.
+  assert(!needle.empty());
+  // Haystack must be at least as large as needle.
+  assert(it.chunk_iterator_.bytes_remaining_ >= needle.size());
+
+  // Cord is a sequence of chunks. To find `needle` we go chunk by chunk looking
+  // for the first char of needle, up until we have advanced `N` defined as
+  // `haystack.size() - needle.size()`. If we find the first char of needle at
+  // `P` and `P` is less than `N`, we then call `IsSubstringInCordAt` to
+  // see if this is the needle. If not, we advance to `P + 1` and try again.
+  while (it.chunk_iterator_.bytes_remaining_ >= needle.size()) {
+    auto haystack_chunk = Cord::ChunkRemaining(it);
+    assert(!haystack_chunk.empty());
+    // Look for the first char of `needle` in the current chunk.
+    auto idx = haystack_chunk.find(needle.front());
+    if (idx == absl::string_view::npos) {
+      // No potential match in this chunk, advance past it.
+      Cord::Advance(&it, haystack_chunk.size());
+      continue;
+    }
+    // We found the start of a potential match in the chunk. Advance the
+    // iterator and haystack chunk to the match the position.
+    Cord::Advance(&it, idx);
+    // Check if there is enough haystack remaining to actually have a match.
+    if (it.chunk_iterator_.bytes_remaining_ < needle.size()) {
+      break;
+    }
+    // Check if this is `needle`.
+    if (IsSubstringInCordAt(it, needle)) {
+      return it;
+    }
+    // No match, increment the iterator for the next attempt.
+    Cord::Advance(&it, 1);
+  }
+  // If we got here, we did not find `needle`.
+  return char_end();
+}
+
+absl::Cord::CharIterator absl::Cord::Find(absl::string_view needle) const {
+  if (needle.empty()) {
+    return char_begin();
+  }
+  if (needle.size() > size()) {
+    return char_end();
+  }
+  if (needle.size() == size()) {
+    return *this == needle ? char_begin() : char_end();
+  }
+  return FindImpl(char_begin(), needle);
+}
+
+namespace {
+
+// Tests whether the sequence of chunks beginning at `haystack` starts with the
+// sequence of chunks beginning at `needle_begin` and extending to `needle_end`.
+//
+// REQUIRES: remaining `absl::Cord` starting at `position` is greater than or
+// equal to `needle_end - needle_begin` and `advance`.
+bool IsSubcordInCordAt(absl::Cord::CharIterator haystack,
+                       absl::Cord::CharIterator needle_begin,
+                       absl::Cord::CharIterator needle_end) {
+  while (needle_begin != needle_end) {
+    auto haystack_chunk = absl::Cord::ChunkRemaining(haystack);
+    assert(!haystack_chunk.empty());
+    auto needle_chunk = absl::Cord::ChunkRemaining(needle_begin);
+    auto min_length = std::min(haystack_chunk.size(), needle_chunk.size());
+    if (haystack_chunk.substr(0, min_length) !=
+        needle_chunk.substr(0, min_length)) {
+      return false;
+    }
+    absl::Cord::Advance(&haystack, min_length);
+    absl::Cord::Advance(&needle_begin, min_length);
+  }
+  return true;
+}
+
+// Tests whether the sequence of chunks beginning at `position` starts with the
+// cord `needle`.
+//
+// REQUIRES: remaining `absl::Cord` starting at `position` is greater than or
+// equal to `needle.size()`.
+bool IsSubcordInCordAt(absl::Cord::CharIterator position,
+                       const absl::Cord& needle) {
+  return IsSubcordInCordAt(position, needle.char_begin(), needle.char_end());
+}
+
+}  // namespace
+
+absl::Cord::CharIterator absl::Cord::Find(const absl::Cord& needle) const {
+  if (needle.empty()) {
+    return char_begin();
+  }
+  const auto needle_size = needle.size();
+  if (needle_size > size()) {
+    return char_end();
+  }
+  if (needle_size == size()) {
+    return *this == needle ? char_begin() : char_end();
+  }
+  const auto needle_chunk = Cord::ChunkRemaining(needle.char_begin());
+  auto haystack_it = char_begin();
+  while (true) {
+    haystack_it = FindImpl(haystack_it, needle_chunk);
+    if (haystack_it == char_end() ||
+        haystack_it.chunk_iterator_.bytes_remaining_ < needle_size) {
+      break;
+    }
+    // We found the first chunk of `needle` at `haystack_it` but not the entire
+    // subcord. Advance past the first chunk and check for the remainder.
+    auto haystack_advanced_it = haystack_it;
+    auto needle_it = needle.char_begin();
+    Cord::Advance(&haystack_advanced_it, needle_chunk.size());
+    Cord::Advance(&needle_it, needle_chunk.size());
+    if (IsSubcordInCordAt(haystack_advanced_it, needle_it, needle.char_end())) {
+      return haystack_it;
+    }
+    Cord::Advance(&haystack_it, 1);
+    if (haystack_it.chunk_iterator_.bytes_remaining_ < needle_size) {
+      break;
+    }
+    if (haystack_it.chunk_iterator_.bytes_remaining_ == needle_size) {
+      // Special case, if there is exactly `needle_size` bytes remaining, the
+      // subcord is either at `haystack_it` or not at all.
+      if (IsSubcordInCordAt(haystack_it, needle)) {
+        return haystack_it;
+      }
+      break;
+    }
+  }
+  return char_end();
+}
+
+bool Cord::Contains(absl::string_view rhs) const {
+  return rhs.empty() || Find(rhs) != char_end();
+}
+
+bool Cord::Contains(const absl::Cord& rhs) const {
+  return rhs.empty() || Find(rhs) != char_end();
+}
+
+absl::string_view Cord::FlattenSlowPath() {
+  assert(contents_.is_tree());
+  size_t total_size = size();
+  CordRep* new_rep;
+  char* new_buffer;
+
+  // Try to put the contents into a new flat rep. If they won't fit in the
+  // biggest possible flat node, use an external rep instead.
+  if (total_size <= kMaxFlatLength) {
+    new_rep = CordRepFlat::New(total_size);
+    new_rep->length = total_size;
+    new_buffer = new_rep->flat()->Data();
+    CopyToArraySlowPath(new_buffer);
+  } else {
+    new_buffer = std::allocator<char>().allocate(total_size);
+    CopyToArraySlowPath(new_buffer);
+    new_rep = absl::cord_internal::NewExternalRep(
+        absl::string_view(new_buffer, total_size), [](absl::string_view s) {
+          std::allocator<char>().deallocate(const_cast<char*>(s.data()),
+                                            s.size());
+        });
+  }
+  CordzUpdateScope scope(contents_.cordz_info(), CordzUpdateTracker::kFlatten);
+  CordRep::Unref(contents_.as_tree());
+  contents_.SetTree(new_rep, scope);
+  return absl::string_view(new_buffer, total_size);
+}
+
+/* static */ bool Cord::GetFlatAux(CordRep* absl_nonnull rep,
+                                   absl::string_view* absl_nonnull fragment) {
+  assert(rep != nullptr);
+  if (rep->length == 0) {
+    *fragment = absl::string_view();
+    return true;
+  }
+  rep = cord_internal::SkipCrcNode(rep);
+  if (rep->IsFlat()) {
+    *fragment = absl::string_view(rep->flat()->Data(), rep->length);
+    return true;
+  } else if (rep->IsExternal()) {
+    *fragment = absl::string_view(rep->external()->base, rep->length);
+    return true;
+  } else if (rep->IsBtree()) {
+    return rep->btree()->IsFlat(fragment);
+  } else if (rep->IsSubstring()) {
+    CordRep* child = rep->substring()->child;
+    if (child->IsFlat()) {
+      *fragment = absl::string_view(
+          child->flat()->Data() + rep->substring()->start, rep->length);
+      return true;
+    } else if (child->IsExternal()) {
+      *fragment = absl::string_view(
+          child->external()->base + rep->substring()->start, rep->length);
+      return true;
+    } else if (child->IsBtree()) {
+      return child->btree()->IsFlat(rep->substring()->start, rep->length,
+                                    fragment);
+    }
+  }
+  return false;
+}
+
+/* static */ void Cord::ForEachChunkAux(
+    absl::cord_internal::CordRep* absl_nonnull rep,
+    absl::FunctionRef<void(absl::string_view)> callback) {
+  assert(rep != nullptr);
+  if (rep->length == 0) return;
+  rep = cord_internal::SkipCrcNode(rep);
+
+  if (rep->IsBtree()) {
+    ChunkIterator it(rep), end;
+    while (it != end) {
+      callback(*it);
+      ++it;
+    }
+    return;
+  }
+
+  // This is a leaf node, so invoke our callback.
+  absl::cord_internal::CordRep* current_node = cord_internal::SkipCrcNode(rep);
+  absl::string_view chunk;
+  bool success = GetFlatAux(current_node, &chunk);
+  assert(success);
+  if (success) {
+    callback(chunk);
+  }
+}
+
+static void DumpNode(CordRep* absl_nonnull nonnull_rep, bool include_data,
+                     std::ostream* absl_nonnull os, int indent) {
+  CordRep* rep = nonnull_rep;
+  const int kIndentStep = 1;
+  for (;;) {
+    *os << std::setw(3) << (rep == nullptr ? 0 : rep->refcount.Get());
+    *os << " " << std::setw(7) << (rep == nullptr ? 0 : rep->length);
+    *os << " [";
+    if (include_data) *os << static_cast<void*>(rep);
+    *os << "]";
+    *os << " " << std::setw(indent) << "";
+    bool leaf = false;
+    if (rep == nullptr) {
+      *os << "NULL\n";
+      leaf = true;
+    } else if (rep->IsCrc()) {
+      *os << "CRC crc=" << rep->crc()->crc_cord_state.Checksum() << "\n";
+      indent += kIndentStep;
+      rep = rep->crc()->child;
+    } else if (rep->IsSubstring()) {
+      *os << "SUBSTRING @ " << rep->substring()->start << "\n";
+      indent += kIndentStep;
+      rep = rep->substring()->child;
+    } else {  // Leaf or ring
+      leaf = true;
+      if (rep->IsExternal()) {
+        *os << "EXTERNAL [";
+        if (include_data)
+          *os << absl::CEscape(
+              absl::string_view(rep->external()->base, rep->length));
+        *os << "]\n";
+      } else if (rep->IsFlat()) {
+        *os << "FLAT cap=" << rep->flat()->Capacity() << " [";
+        if (include_data)
+          *os << absl::CEscape(
+              absl::string_view(rep->flat()->Data(), rep->length));
+        *os << "]\n";
+      } else {
+        CordRepBtree::Dump(rep, /*label=*/"", include_data, *os);
+      }
+    }
+    if (leaf) {
+      break;
+    }
+  }
+}
+
+static std::string ReportError(CordRep* absl_nonnull root,
+                               CordRep* absl_nonnull node) {
+  std::ostringstream buf;
+  buf << "Error at node " << node << " in:";
+  DumpNode(root, true, &buf);
+  return buf.str();
+}
+
+static bool VerifyNode(CordRep* absl_nonnull root,
+                       CordRep* absl_nonnull start_node) {
+  absl::InlinedVector<CordRep* absl_nonnull, 2> worklist;
+  worklist.push_back(start_node);
+  do {
+    CordRep* node = worklist.back();
+    worklist.pop_back();
+
+    ABSL_INTERNAL_CHECK(node != nullptr, ReportError(root, node));
+    if (node != root) {
+      ABSL_INTERNAL_CHECK(node->length != 0, ReportError(root, node));
+      ABSL_INTERNAL_CHECK(!node->IsCrc(), ReportError(root, node));
+    }
+
+    if (node->IsFlat()) {
+      ABSL_INTERNAL_CHECK(node->length <= node->flat()->Capacity(),
+                          ReportError(root, node));
+    } else if (node->IsExternal()) {
+      ABSL_INTERNAL_CHECK(node->external()->base != nullptr,
+                          ReportError(root, node));
+    } else if (node->IsSubstring()) {
+      ABSL_INTERNAL_CHECK(
+          node->substring()->start < node->substring()->child->length,
+          ReportError(root, node));
+      ABSL_INTERNAL_CHECK(node->substring()->start + node->length <=
+                              node->substring()->child->length,
+                          ReportError(root, node));
+    } else if (node->IsCrc()) {
+      ABSL_INTERNAL_CHECK(
+          node->crc()->child != nullptr || node->crc()->length == 0,
+          ReportError(root, node));
+      if (node->crc()->child != nullptr) {
+        ABSL_INTERNAL_CHECK(node->crc()->length == node->crc()->child->length,
+                            ReportError(root, node));
+        worklist.push_back(node->crc()->child);
+      }
+    }
+  } while (!worklist.empty());
+  return true;
+}
+
+std::ostream& operator<<(std::ostream& out, const Cord& cord) {
+  for (absl::string_view chunk : cord.Chunks()) {
+    out.write(chunk.data(), static_cast<std::streamsize>(chunk.size()));
+  }
+  return out;
+}
+
+namespace strings_internal {
+size_t CordTestAccess::FlatOverhead() { return cord_internal::kFlatOverhead; }
+size_t CordTestAccess::MaxFlatLength() { return cord_internal::kMaxFlatLength; }
+size_t CordTestAccess::FlatTagToLength(uint8_t tag) {
+  return cord_internal::TagToLength(tag);
+}
+uint8_t CordTestAccess::LengthToTag(size_t s) {
+  ABSL_INTERNAL_CHECK(s <= kMaxFlatLength, absl::StrCat("Invalid length ", s));
+  return cord_internal::AllocatedSizeToTag(s + cord_internal::kFlatOverhead);
+}
+size_t CordTestAccess::SizeofCordRepExternal() {
+  return sizeof(CordRepExternal);
+}
+size_t CordTestAccess::SizeofCordRepSubstring() {
+  return sizeof(CordRepSubstring);
+}
+}  // namespace strings_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/strings/cord.h b/absl/strings/cord.h
new file mode 100644
index 00000000000..fad5d976f86
--- /dev/null
+++ b/absl/strings/cord.h
@@ -0,0 +1,1802 @@
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: cord.h
+// -----------------------------------------------------------------------------
+//
+// This file defines the `absl::Cord` data structure and operations on that data
+// structure. A Cord is a string-like sequence of characters optimized for
+// specific use cases. Unlike a `std::string`, which stores an array of
+// contiguous characters, Cord data is stored in a structure consisting of
+// separate, reference-counted "chunks."
+//
+// Because a Cord consists of these chunks, data can be added to or removed from
+// a Cord during its lifetime. Chunks may also be shared between Cords. Unlike a
+// `std::string`, a Cord can therefore accommodate data that changes over its
+// lifetime, though it's not quite "mutable"; it can change only in the
+// attachment, detachment, or rearrangement of chunks of its constituent data.
+//
+// A Cord provides some benefit over `std::string` under the following (albeit
+// narrow) circumstances:
+//
+//   * Cord data is designed to grow and shrink over a Cord's lifetime. Cord
+//     provides efficient insertions and deletions at the start and end of the
+//     character sequences, avoiding copies in those cases. Static data should
+//     generally be stored as strings.
+//   * External memory consisting of string-like data can be directly added to
+//     a Cord without requiring copies or allocations.
+//   * Cord data may be shared and copied cheaply. Cord provides a copy-on-write
+//     implementation and cheap sub-Cord operations. Copying a Cord is an O(1)
+//     operation.
+//
+// As a consequence to the above, Cord data is generally large. Small data
+// should generally use strings, as construction of a Cord requires some
+// overhead. Small Cords (<= 15 bytes) are represented inline, but most small
+// Cords are expected to grow over their lifetimes.
+//
+// Note that because a Cord is made up of separate chunked data, random access
+// to character data within a Cord is slower than within a `std::string`.
+//
+// Thread Safety
+//
+// Cord has the same thread-safety properties as many other types like
+// std::string, std::vector<>, int, etc -- it is thread-compatible. In
+// particular, if threads do not call non-const methods, then it is safe to call
+// const methods without synchronization. Copying a Cord produces a new instance
+// that can be used concurrently with the original in arbitrary ways.
+
+#ifndef ABSL_STRINGS_CORD_H_
+#define ABSL_STRINGS_CORD_H_
+
+#include <algorithm>
+#include <cassert>
+#include <cstddef>
+#include <cstdint>
+#include <cstring>
+#include <iosfwd>
+#include <iterator>
+#include <optional>
+#include <string>
+#include <type_traits>
+#include <utility>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/endian.h"
+#include "absl/base/internal/hardening.h"
+#include "absl/base/macros.h"
+#include "absl/base/nullability.h"
+#include "absl/base/optimization.h"
+#include "absl/crc/internal/crc_cord_state.h"
+#include "absl/functional/function_ref.h"
+#include "absl/hash/internal/weakly_mixed_integer.h"
+#include "absl/meta/type_traits.h"
+#include "absl/strings/cord_analysis.h"
+#include "absl/strings/cord_buffer.h"
+#include "absl/strings/internal/cord_data_edge.h"
+#include "absl/strings/internal/cord_internal.h"
+#include "absl/strings/internal/cord_rep_btree.h"
+#include "absl/strings/internal/cord_rep_btree_reader.h"
+#include "absl/strings/internal/cord_rep_crc.h"
+#include "absl/strings/internal/cord_rep_flat.h"
+#include "absl/strings/internal/cordz_info.h"
+#include "absl/strings/internal/cordz_update_scope.h"
+#include "absl/strings/internal/cordz_update_tracker.h"
+#include "absl/strings/internal/string_constant.h"
+#include "absl/strings/string_view.h"
+#include "absl/types/compare.h"
+#include "absl/types/optional.h"
+#include "absl/types/span.h"
+
+namespace strings {
+class CordReader;
+}  // namespace strings
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+class Cord;
+class CordTestPeer;
+template <typename Releaser>
+Cord MakeCordFromExternal(absl::string_view, Releaser&&);
+void CopyCordToString(const Cord& src, std::string* absl_nonnull dst);
+void AppendCordToString(const Cord& src, std::string* absl_nonnull dst);
+[[nodiscard]] size_t CopyCordToSpan(const Cord& src, absl::Span<char> dst);
+
+// Cord memory accounting modes
+enum class CordMemoryAccounting {
+  // Counts the *approximate* number of bytes held in full or in part by this
+  // Cord (which may not remain the same between invocations). Cords that share
+  // memory could each be "charged" independently for the same shared memory.
+  // See also comment on `kTotalMorePrecise` on internally shared memory.
+  kTotal,
+
+  // Counts the *approximate* number of bytes held in full or in part by this
+  // Cord for the distinct memory held by this cord. This option is similar
+  // to `kTotal`, except that if the cord has multiple references to the same
+  // memory, that memory is only counted once.
+  //
+  // For example:
+  //   absl::Cord cord;
+  //   cord.Append(some_other_cord);
+  //   cord.Append(some_other_cord);
+  //   // Counts `some_other_cord` twice:
+  //   cord.EstimatedMemoryUsage(kTotal);
+  //   // Counts `some_other_cord` once:
+  //   cord.EstimatedMemoryUsage(kTotalMorePrecise);
+  //
+  // The `kTotalMorePrecise` number is more expensive to compute as it requires
+  // deduplicating all memory references. Applications should prefer to use
+  // `kFairShare` or `kTotal` unless they really need a more precise estimate
+  // on "how much memory is potentially held / kept alive by this cord?"
+  kTotalMorePrecise,
+
+  // Counts the *approximate* number of bytes held in full or in part by this
+  // Cord weighted by the sharing ratio of that data. For example, if some data
+  // edge is shared by 4 different Cords, then each cord is attributed 1/4th of
+  // the total memory usage as a 'fair share' of the total memory usage.
+  kFairShare,
+};
+
+// Cord
+//
+// A Cord is a sequence of characters, designed to be more efficient than a
+// `std::string` in certain circumstances: namely, large string data that needs
+// to change over its lifetime or shared, especially when such data is shared
+// across API boundaries.
+//
+// A Cord stores its character data in a structure that allows efficient prepend
+// and append operations. This makes a Cord useful for large string data sent
+// over in a wire format that may need to be prepended or appended at some point
+// during the data exchange (e.g. HTTP, protocol buffers). For example, a
+// Cord is useful for storing an HTTP request, and prepending an HTTP header to
+// such a request.
+//
+// Cords should not be used for storing general string data, however. They
+// require overhead to construct and are slower than strings for random access.
+//
+// The Cord API provides the following common API operations:
+//
+// * Create or assign Cords out of existing string data, memory, or other Cords
+// * Append and prepend data to an existing Cord
+// * Create new Sub-Cords from existing Cord data
+// * Swap Cord data and compare Cord equality
+// * Write out Cord data by constructing a `std::string`
+//
+// Additionally, the API provides iterator utilities to iterate through Cord
+// data via chunks or character bytes.
+//
+class ABSL_ATTRIBUTE_TRIVIAL_ABI Cord {
+ private:
+  template <typename T>
+  using EnableIfString = std::enable_if_t<std::is_same_v<T, std::string>, int>;
+
+ public:
+  // Cord::Cord() Constructors.
+
+  // Creates an empty Cord.
+  constexpr Cord() noexcept;
+
+  // Creates a Cord from an existing Cord. Cord is copyable and efficiently
+  // movable. The moved-from state is valid but unspecified.
+  Cord(const Cord& src);
+  Cord(Cord&& src) noexcept;
+  Cord& operator=(const Cord& x);
+  Cord& operator=(Cord&& x) noexcept;
+
+  // Creates a Cord from a `src` string. This constructor is marked explicit to
+  // prevent implicit Cord constructions from arguments convertible to an
+  // `absl::string_view`.
+  explicit Cord(absl::string_view src);
+  Cord& operator=(absl::string_view src);
+
+  // Creates a Cord from a `std::string&&` rvalue. These constructors are
+  // templated to avoid ambiguities for types that are convertible to both
+  // `absl::string_view` and `std::string`, such as `const char*`.
+  template <typename T, EnableIfString<T> = 0>
+  explicit Cord(T&& src);
+  template <typename T, EnableIfString<T> = 0>
+  Cord& operator=(T&& src);
+
+  // Cord::~Cord()
+  //
+  // Destructs the Cord.
+  ~Cord() {
+    if (contents_.is_tree()) DestroyCordSlow();
+  }
+
+  // MakeCordFromExternal()
+  //
+  // Creates a Cord that takes ownership of external string memory. The
+  // contents of `data` are not copied to the Cord; instead, the external
+  // memory is added to the Cord and reference-counted. This data may not be
+  // changed for the life of the Cord, though it may be prepended or appended
+  // to.
+  //
+  // `MakeCordFromExternal()` takes a callable "releaser" that is invoked when
+  // the reference count for `data` reaches zero. As noted above, this data must
+  // remain live until the releaser is invoked. The callable releaser also must:
+  //
+  //   * be move constructible
+  //   * support `void operator()(absl::string_view)` or `void operator()()`
+  //
+  // Example:
+  //
+  // Cord MakeCord(BlockPool* pool) {
+  //   Block* block = pool->NewBlock();
+  //   FillBlock(block);
+  //   return absl::MakeCordFromExternal(
+  //       block->ToStringView(),
+  //       [pool, block](absl::string_view v) {
+  //         pool->FreeBlock(block, v);
+  //       });
+  // }
+  //
+  // WARNING: Because a Cord can be reference-counted, it's likely a bug if your
+  // releaser doesn't do anything. For example, consider the following:
+  //
+  // void Foo(const char* buffer, int len) {
+  //   auto c = absl::MakeCordFromExternal(absl::string_view(buffer, len),
+  //                                       [](absl::string_view) {});
+  //
+  //   // BUG: If Bar() copies its cord for any reason, including keeping a
+  //   // substring of it, the lifetime of buffer might be extended beyond
+  //   // when Foo() returns.
+  //   Bar(c);
+  // }
+  template <typename Releaser>
+  friend Cord MakeCordFromExternal(absl::string_view data, Releaser&& releaser);
+
+  // Cord::Clear()
+  //
+  // Releases the Cord data. Any nodes that share data with other Cords, if
+  // applicable, will have their reference counts reduced by 1.
+  ABSL_ATTRIBUTE_REINITIALIZES void Clear();
+
+  // Cord::Append()
+  //
+  // Appends data to the Cord, which may come from another Cord or other string
+  // data.
+  void Append(const Cord& src);
+  void Append(Cord&& src);
+  void Append(absl::string_view src);
+  template <typename T, EnableIfString<T> = 0>
+  void Append(T&& src);
+
+  // Appends `buffer` to this cord, unless `buffer` has a zero length in which
+  // case this method has no effect on this cord instance.
+  // This method is guaranteed to consume `buffer`.
+  void Append(CordBuffer buffer);
+
+  // Returns a CordBuffer, re-using potential existing capacity in this cord.
+  //
+  // Cord instances may have additional unused capacity in the last (or first)
+  // nodes of the underlying tree to facilitate amortized growth. This method
+  // allows applications to explicitly use this spare capacity if available,
+  // or create a new CordBuffer instance otherwise.
+  // If this cord has a final non-shared node with at least `min_capacity`
+  // available, then this method will return that buffer including its data
+  // contents. I.e.; the returned buffer will have a non-zero length, and
+  // a capacity of at least `buffer.length + min_capacity`. Otherwise, this
+  // method will return `CordBuffer::CreateWithDefaultLimit(capacity)`.
+  //
+  // Below an example of using GetAppendBuffer. Notice that in this example we
+  // use `GetAppendBuffer()` only on the first iteration. As we know nothing
+  // about any initial extra capacity in `cord`, we may be able to use the extra
+  // capacity. But as we add new buffers with fully utilized contents after that
+  // we avoid calling `GetAppendBuffer()` on subsequent iterations: while this
+  // works fine, it results in an unnecessary inspection of cord contents:
+  //
+  //   void AppendRandomDataToCord(absl::Cord &cord, size_t n) {
+  //     bool first = true;
+  //     while (n > 0) {
+  //       CordBuffer buffer = first ? cord.GetAppendBuffer(n)
+  //                                 : CordBuffer::CreateWithDefaultLimit(n);
+  //       absl::Span<char> data = buffer.available_up_to(n);
+  //       FillRandomValues(data.data(), data.size());
+  //       buffer.IncreaseLengthBy(data.size());
+  //       cord.Append(std::move(buffer));
+  //       n -= data.size();
+  //       first = false;
+  //     }
+  //   }
+  CordBuffer GetAppendBuffer(size_t capacity, size_t min_capacity = 16);
+
+  // Returns a CordBuffer, re-using potential existing capacity in this cord.
+  //
+  // This function is identical to `GetAppendBuffer`, except that in the case
+  // where a new `CordBuffer` is allocated, it is allocated using the provided
+  // custom limit instead of the default limit. `GetAppendBuffer` will default
+  // to `CordBuffer::CreateWithDefaultLimit(capacity)` whereas this method
+  // will default to `CordBuffer::CreateWithCustomLimit(block_size, capacity)`.
+  // This method is equivalent to `GetAppendBuffer` if `block_size` is zero.
+  // See the documentation for `CreateWithCustomLimit` for more details on the
+  // restrictions and legal values for `block_size`.
+  CordBuffer GetCustomAppendBuffer(size_t block_size, size_t capacity,
+                                   size_t min_capacity = 16);
+
+  // Cord::Prepend()
+  //
+  // Prepends data to the Cord, which may come from another Cord or other string
+  // data.
+  void Prepend(const Cord& src);
+  void Prepend(absl::string_view src);
+  template <typename T, EnableIfString<T> = 0>
+  void Prepend(T&& src);
+
+  // Prepends `buffer` to this cord, unless `buffer` has a zero length in which
+  // case this method has no effect on this cord instance.
+  // This method is guaranteed to consume `buffer`.
+  void Prepend(CordBuffer buffer);
+
+  // Cord::RemovePrefix()
+  //
+  // Removes the first `n` bytes of a Cord.
+  void RemovePrefix(size_t n);
+  void RemoveSuffix(size_t n);
+
+  // Cord::Subcord()
+  //
+  // Returns a new Cord representing the subrange [pos, pos + new_size) of
+  // *this. If pos >= size(), the result is empty(). If
+  // (pos + new_size) >= size(), the result is the subrange [pos, size()).
+  Cord Subcord(size_t pos, size_t new_size) const;
+
+  // Cord::swap()
+  //
+  // Swaps the contents of the Cord with `other`.
+  void swap(Cord& other) noexcept;
+
+  // swap()
+  //
+  // Swaps the contents of two Cords.
+  friend void swap(Cord& x, Cord& y) noexcept { x.swap(y); }
+
+  // Cord::size()
+  //
+  // Returns the size of the Cord.
+  size_t size() const;
+
+  // Cord::empty()
+  //
+  // Determines whether the given Cord is empty, returning `true` if so.
+  bool empty() const;
+
+  // Cord::EstimatedMemoryUsage()
+  //
+  // Returns the *approximate* number of bytes held by this cord.
+  // See CordMemoryAccounting for more information on the accounting method.
+  size_t EstimatedMemoryUsage(CordMemoryAccounting accounting_method =
+                                  CordMemoryAccounting::kTotal) const;
+
+  // Cord::Compare()
+  //
+  // Compares 'this' Cord with rhs. This function and its relatives treat Cords
+  // as sequences of unsigned bytes. The comparison is a straightforward
+  // lexicographic comparison. `Cord::Compare()` returns values as follows:
+  //
+  //   -1  'this' Cord is smaller
+  //    0  two Cords are equal
+  //    1  'this' Cord is larger
+  int Compare(absl::string_view rhs) const;
+  int Compare(const Cord& rhs) const;
+
+  // Cord::StartsWith()
+  //
+  // Determines whether the Cord starts with the passed string data `rhs`.
+  bool StartsWith(const Cord& rhs) const;
+  bool StartsWith(absl::string_view rhs) const;
+
+  // Cord::EndsWith()
+  //
+  // Determines whether the Cord ends with the passed string data `rhs`.
+  bool EndsWith(absl::string_view rhs) const;
+  bool EndsWith(const Cord& rhs) const;
+
+  // Cord::Contains()
+  //
+  // Determines whether the Cord contains the passed string data `rhs`.
+  bool Contains(absl::string_view rhs) const;
+  bool Contains(const Cord& rhs) const;
+
+  // Cord::operator std::string()
+  //
+  // Converts a Cord into a `std::string()`. This operator is marked explicit to
+  // prevent unintended Cord usage in functions that take a string.
+  explicit operator std::string() const;
+
+  // CopyCordToString()
+  //
+  // Copies the contents of a `src` Cord into a `*dst` string.
+  //
+  // This function optimizes the case of reusing the destination string since it
+  // can reuse previously allocated capacity. However, this function does not
+  // guarantee that pointers previously returned by `dst->data()` remain valid
+  // even if `*dst` had enough capacity to hold `src`. If `*dst` is a new
+  // object, prefer to simply use the conversion operator to `std::string`.
+  friend void CopyCordToString(const Cord& src, std::string* absl_nonnull dst);
+
+  // AppendCordToString()
+  //
+  // Appends the contents of a `src` Cord to a `*dst` string.
+  //
+  // This function optimizes the case of appending to a non-empty destination
+  // string. If `*dst` already has capacity to store the contents of the cord,
+  // this function does not invalidate pointers previously returned by
+  // `dst->data()`. If `*dst` is a new object, prefer to simply use the
+  // conversion operator to `std::string`.
+  friend void AppendCordToString(const Cord& src,
+                                 std::string* absl_nonnull dst);
+
+  // CopyCordToSpan()
+  //
+  // Copies up to `dst.size()` bytes starting from the beginning of `src` to
+  // `dst`.  Returns the number of bytes copied.
+  friend size_t CopyCordToSpan(const Cord& src, absl::Span<char> dst);
+
+  class CharIterator;
+
+  //----------------------------------------------------------------------------
+  // Cord::ChunkIterator
+  //----------------------------------------------------------------------------
+  //
+  // A `Cord::ChunkIterator` allows iteration over the constituent chunks of its
+  // Cord. Such iteration allows you to perform non-const operations on the data
+  // of a Cord without modifying it.
+  //
+  // Generally, you do not instantiate a `Cord::ChunkIterator` directly;
+  // instead, you create one implicitly through use of the `Cord::Chunks()`
+  // member function.
+  //
+  // The `Cord::ChunkIterator` has the following properties:
+  //
+  //   * The iterator is invalidated after any non-const operation on the
+  //     Cord object over which it iterates.
+  //   * The `string_view` returned by dereferencing a valid, non-`end()`
+  //     iterator is guaranteed to be non-empty.
+  //   * Two `ChunkIterator` objects can be compared equal if and only if they
+  //     remain valid and iterate over the same Cord.
+  //   * The iterator in this case is a proxy iterator; the `string_view`
+  //     returned by the iterator does not live inside the Cord, and its
+  //     lifetime is limited to the lifetime of the iterator itself. To help
+  //     prevent lifetime issues, `ChunkIterator::reference` is not a true
+  //     reference type and is equivalent to `value_type`.
+  //   * The iterator keeps state that can grow for Cords that contain many
+  //     nodes and are imbalanced due to sharing. Prefer to pass this type by
+  //     const reference instead of by value.
+  class ChunkIterator {
+   public:
+    using iterator_category = std::input_iterator_tag;
+    using value_type = absl::string_view;
+    using difference_type = ptrdiff_t;
+    using pointer = const value_type* absl_nonnull;
+    using reference = value_type;
+
+    ChunkIterator() = default;
+
+    ChunkIterator& operator++();
+    ChunkIterator operator++(int);
+    bool operator==(const ChunkIterator& other) const;
+    bool operator!=(const ChunkIterator& other) const;
+    reference operator*() const;
+    pointer operator->() const;
+
+    friend class Cord;
+    friend class CharIterator;
+
+   private:
+    using CordRep = absl::cord_internal::CordRep;
+    using CordRepBtree = absl::cord_internal::CordRepBtree;
+    using CordRepBtreeReader = absl::cord_internal::CordRepBtreeReader;
+
+    // Constructs a `begin()` iterator from `tree`.
+    explicit ChunkIterator(cord_internal::CordRep* absl_nonnull tree);
+
+    // Constructs a `begin()` iterator from `cord`.
+    explicit ChunkIterator(const Cord* absl_nonnull cord);
+
+    // Initializes this instance from a tree. Invoked by constructors.
+    void InitTree(cord_internal::CordRep* absl_nonnull tree);
+
+    // Removes `n` bytes from `current_chunk_`. Expects `n` to be smaller than
+    // `current_chunk_.size()`.
+    void RemoveChunkPrefix(size_t n);
+    Cord AdvanceAndReadBytes(size_t n);
+    void AdvanceBytes(size_t n);
+
+    // Btree specific operator++
+    ChunkIterator& AdvanceBtree();
+    void AdvanceBytesBtree(size_t n);
+
+    // A view into bytes of the current `CordRep`. It may only be a view to a
+    // suffix of bytes if this is being used by `CharIterator`.
+    absl::string_view current_chunk_;
+    // The current leaf, or `nullptr` if the iterator points to short data.
+    // If the current chunk is a substring node, current_leaf_ points to the
+    // underlying flat or external node.
+    absl::cord_internal::CordRep* absl_nullable current_leaf_ = nullptr;
+    // The number of bytes left in the `Cord` over which we are iterating.
+    size_t bytes_remaining_ = 0;
+
+    // Cord reader for cord btrees. Empty if not traversing a btree.
+    CordRepBtreeReader btree_reader_;
+  };
+
+  // Cord::chunk_begin()
+  //
+  // Returns an iterator to the first chunk of the `Cord`.
+  //
+  // Generally, prefer using `Cord::Chunks()` within a range-based for loop for
+  // iterating over the chunks of a Cord. This method may be useful for getting
+  // a `ChunkIterator` where range-based for-loops are not useful.
+  //
+  // Example:
+  //
+  //   absl::Cord::ChunkIterator FindAsChunk(const absl::Cord& c,
+  //                                         absl::string_view s) {
+  //     return std::find(c.chunk_begin(), c.chunk_end(), s);
+  //   }
+  ChunkIterator chunk_begin() const ABSL_ATTRIBUTE_LIFETIME_BOUND;
+
+  // Cord::chunk_end()
+  //
+  // Returns an iterator one increment past the last chunk of the `Cord`.
+  //
+  // Generally, prefer using `Cord::Chunks()` within a range-based for loop for
+  // iterating over the chunks of a Cord. This method may be useful for getting
+  // a `ChunkIterator` where range-based for-loops may not be available.
+  ChunkIterator chunk_end() const ABSL_ATTRIBUTE_LIFETIME_BOUND;
+
+  //----------------------------------------------------------------------------
+  // Cord::ChunkRange
+  //----------------------------------------------------------------------------
+  //
+  // `ChunkRange` is a helper class for iterating over the chunks of the `Cord`,
+  // producing an iterator which can be used within a range-based for loop.
+  // Construction of a `ChunkRange` will return an iterator pointing to the
+  // first chunk of the Cord. Generally, do not construct a `ChunkRange`
+  // directly; instead, prefer to use the `Cord::Chunks()` method.
+  //
+  // Implementation note: `ChunkRange` is simply a convenience wrapper over
+  // `Cord::chunk_begin()` and `Cord::chunk_end()`.
+  class ChunkRange {
+   public:
+    // Fulfill minimum c++ container requirements [container.requirements]
+    // These (partial) container type definitions allow ChunkRange to be used
+    // in various utilities expecting a subset of [container.requirements].
+    // For example, the below enables using `::testing::ElementsAre(...)`
+    using value_type = absl::string_view;
+    using reference = value_type&;
+    using const_reference = const value_type&;
+    using iterator = ChunkIterator;
+    using const_iterator = ChunkIterator;
+
+    explicit ChunkRange(const Cord* absl_nonnull cord) : cord_(cord) {}
+
+    ChunkIterator begin() const;
+    ChunkIterator end() const;
+
+   private:
+    const Cord* absl_nonnull cord_;
+  };
+
+  // Cord::Chunks()
+  //
+  // Returns a `Cord::ChunkRange` for iterating over the chunks of a `Cord` with
+  // a range-based for-loop. For most iteration tasks on a Cord, use
+  // `Cord::Chunks()` to retrieve this iterator.
+  //
+  // Example:
+  //
+  //   void ProcessChunks(const Cord& cord) {
+  //     for (absl::string_view chunk : cord.Chunks()) { ... }
+  //   }
+  //
+  // Note that the ordinary caveats of temporary lifetime extension apply:
+  //
+  //   void Process() {
+  //     for (absl::string_view chunk : CordFactory().Chunks()) {
+  //       // The temporary Cord returned by CordFactory has been destroyed!
+  //     }
+  //   }
+  ChunkRange Chunks() const ABSL_ATTRIBUTE_LIFETIME_BOUND;
+
+  //----------------------------------------------------------------------------
+  // Cord::CharIterator
+  //----------------------------------------------------------------------------
+  //
+  // A `Cord::CharIterator` allows iteration over the constituent characters of
+  // a `Cord`.
+  //
+  // Generally, you do not instantiate a `Cord::CharIterator` directly; instead,
+  // you create one implicitly through use of the `Cord::Chars()` member
+  // function.
+  //
+  // A `Cord::CharIterator` has the following properties:
+  //
+  //   * The iterator is invalidated after any non-const operation on the
+  //     Cord object over which it iterates.
+  //   * Two `CharIterator` objects can be compared equal if and only if they
+  //     remain valid and iterate over the same Cord.
+  //   * The iterator keeps state that can grow for Cords that contain many
+  //     nodes and are imbalanced due to sharing. Prefer to pass this type by
+  //     const reference instead of by value.
+  //   * This type cannot act as a forward iterator because a `Cord` can reuse
+  //     sections of memory. This fact violates the requirement for forward
+  //     iterators to compare equal if dereferencing them returns the same
+  //     object.
+  class CharIterator {
+   public:
+    using iterator_category = std::input_iterator_tag;
+    using value_type = char;
+    using difference_type = ptrdiff_t;
+    using pointer = const char* absl_nonnull;
+    using reference = const char&;
+
+    CharIterator() = default;
+
+    CharIterator& operator++();
+    CharIterator operator++(int);
+    bool operator==(const CharIterator& other) const;
+    bool operator!=(const CharIterator& other) const;
+    reference operator*() const;
+
+    friend Cord;
+
+   private:
+    explicit CharIterator(const Cord* absl_nonnull cord)
+        : chunk_iterator_(cord) {}
+
+    ChunkIterator chunk_iterator_;
+  };
+
+  // Cord::AdvanceAndRead()
+  //
+  // Advances the `Cord::CharIterator` by `n_bytes` and returns the bytes
+  // advanced as a separate `Cord`. `n_bytes` must be less than or equal to the
+  // number of bytes within the Cord; otherwise, behavior is undefined. It is
+  // valid to pass `char_end()` and `0`.
+  static Cord AdvanceAndRead(CharIterator* absl_nonnull it, size_t n_bytes);
+
+  // Cord::Advance()
+  //
+  // Advances the `Cord::CharIterator` by `n_bytes`. `n_bytes` must be less than
+  // or equal to the number of bytes remaining within the Cord; otherwise,
+  // behavior is undefined. It is valid to pass `char_end()` and `0`.
+  static void Advance(CharIterator* absl_nonnull it, size_t n_bytes);
+
+  // Cord::ChunkRemaining()
+  //
+  // Returns the longest contiguous view starting at the iterator's position.
+  //
+  // `it` must be dereferenceable.
+  static absl::string_view ChunkRemaining(const CharIterator& it);
+
+  // Cord::Distance()
+  //
+  // Returns the distance between `first` and `last`, as if
+  // `std::distance(first, last)` was called.
+  static ptrdiff_t Distance(const CharIterator& first,
+                            const CharIterator& last);
+
+  // Cord::char_begin()
+  //
+  // Returns an iterator to the first character of the `Cord`.
+  //
+  // Generally, prefer using `Cord::Chars()` within a range-based for loop for
+  // iterating over the chunks of a Cord. This method may be useful for getting
+  // a `CharIterator` where range-based for-loops may not be available.
+  CharIterator char_begin() const ABSL_ATTRIBUTE_LIFETIME_BOUND;
+
+  // Cord::char_end()
+  //
+  // Returns an iterator to one past the last character of the `Cord`.
+  //
+  // Generally, prefer using `Cord::Chars()` within a range-based for loop for
+  // iterating over the chunks of a Cord. This method may be useful for getting
+  // a `CharIterator` where range-based for-loops are not useful.
+  CharIterator char_end() const ABSL_ATTRIBUTE_LIFETIME_BOUND;
+
+  // Cord::CharRange
+  //
+  // `CharRange` is a helper class for iterating over the characters of a
+  // producing an iterator which can be used within a range-based for loop.
+  // Construction of a `CharRange` will return an iterator pointing to the first
+  // character of the Cord. Generally, do not construct a `CharRange` directly;
+  // instead, prefer to use the `Cord::Chars()` method shown below.
+  //
+  // Implementation note: `CharRange` is simply a convenience wrapper over
+  // `Cord::char_begin()` and `Cord::char_end()`.
+  class CharRange {
+   public:
+    // Fulfill minimum c++ container requirements [container.requirements]
+    // These (partial) container type definitions allow CharRange to be used
+    // in various utilities expecting a subset of [container.requirements].
+    // For example, the below enables using `::testing::ElementsAre(...)`
+    using value_type = char;
+    using reference = value_type&;
+    using const_reference = const value_type&;
+    using iterator = CharIterator;
+    using const_iterator = CharIterator;
+
+    explicit CharRange(const Cord* absl_nonnull cord) : cord_(cord) {}
+
+    CharIterator begin() const;
+    CharIterator end() const;
+
+   private:
+    const Cord* absl_nonnull cord_;
+  };
+
+  // Cord::Chars()
+  //
+  // Returns a `Cord::CharRange` for iterating over the characters of a `Cord`
+  // with a range-based for-loop. For most character-based iteration tasks on a
+  // Cord, use `Cord::Chars()` to retrieve this iterator.
+  //
+  // Example:
+  //
+  //   void ProcessCord(const Cord& cord) {
+  //     for (char c : cord.Chars()) { ... }
+  //   }
+  //
+  // Note that the ordinary caveats of temporary lifetime extension apply:
+  //
+  //   void Process() {
+  //     for (char c : CordFactory().Chars()) {
+  //       // The temporary Cord returned by CordFactory has been destroyed!
+  //     }
+  //   }
+  CharRange Chars() const ABSL_ATTRIBUTE_LIFETIME_BOUND;
+
+  // Cord::operator[]
+  //
+  // Gets the "i"th character of the Cord and returns it, provided that
+  // 0 <= i < Cord.size().
+  //
+  // NOTE: This routine is reasonably efficient. It is roughly
+  // logarithmic based on the number of chunks that make up the cord. Still,
+  // if you need to iterate over the contents of a cord, you should
+  // use a CharIterator/ChunkIterator rather than call operator[]
+  // repeatedly in a loop.
+  char operator[](size_t i) const;
+
+  // Cord::TryFlat()
+  //
+  // If this cord's representation is a single flat array, returns a
+  // string_view referencing that array.  Otherwise returns nullopt.
+  std::optional<absl::string_view> TryFlat() const
+      ABSL_ATTRIBUTE_LIFETIME_BOUND;
+
+  // Cord::Flatten()
+  //
+  // Flattens the cord into a single array and returns a view of the data.
+  //
+  // If the cord was already flat, the contents are not modified.
+  absl::string_view Flatten() ABSL_ATTRIBUTE_LIFETIME_BOUND;
+
+  // Cord::Find()
+  //
+  // Returns an iterator to the first occurrence of the substring `needle`.
+  //
+  // If the substring `needle` does not occur, `Cord::char_end()` is returned.
+  CharIterator Find(absl::string_view needle) const;
+  CharIterator Find(const absl::Cord& needle) const;
+
+  // Supports absl::Cord as a sink object for absl::Format().
+  friend void AbslFormatFlush(absl::Cord* absl_nonnull cord,
+                              absl::string_view part) {
+    cord->Append(part);
+  }
+
+  // Support automatic stringification with absl::StrCat and absl::StrFormat.
+  template <typename Sink>
+  friend void AbslStringify(Sink& sink, const absl::Cord& cord) {
+    for (absl::string_view chunk : cord.Chunks()) {
+      sink.Append(chunk);
+    }
+  }
+
+  // Cord::SetExpectedChecksum()
+  //
+  // Stores a checksum value with this non-empty cord instance, for later
+  // retrieval.
+  //
+  // The expected checksum is a number stored out-of-band, alongside the data.
+  // It is preserved across copies and assignments, but any mutations to a cord
+  // will cause it to lose its expected checksum.
+  //
+  // The expected checksum is not part of a Cord's value, and does not affect
+  // operations such as equality or hashing.
+  //
+  // This field is intended to store a CRC32C checksum for later validation, to
+  // help support end-to-end checksum workflows.  However, the Cord API itself
+  // does no CRC validation, and assigns no meaning to this number.
+  //
+  // This call has no effect if this cord is empty.
+  void SetExpectedChecksum(uint32_t crc);
+
+  // Returns this cord's expected checksum, if it has one.  Otherwise, returns
+  // nullopt.
+  std::optional<uint32_t> ExpectedChecksum() const;
+
+  template <typename H>
+  friend H AbslHashValue(H hash_state, const absl::Cord& c) {
+    std::optional<absl::string_view> maybe_flat = c.TryFlat();
+    if (maybe_flat.has_value()) {
+      return H::combine(std::move(hash_state), *maybe_flat);
+    }
+    return c.HashFragmented(std::move(hash_state));
+  }
+
+  // Create a Cord with the contents of StringConstant<T>::value.
+  // No allocations will be done and no data will be copied.
+  // This is an INTERNAL API and subject to change or removal. This API can only
+  // be used by spelling absl::strings_internal::MakeStringConstant, which is
+  // also an internal API.
+  template <typename T>
+  // NOLINTNEXTLINE(google-explicit-constructor)
+  constexpr Cord(strings_internal::StringConstant<T>);
+
+ private:
+  using CordRep = absl::cord_internal::CordRep;
+  using CordRepFlat = absl::cord_internal::CordRepFlat;
+  using CordzInfo = cord_internal::CordzInfo;
+  using CordzUpdateScope = cord_internal::CordzUpdateScope;
+  using CordzUpdateTracker = cord_internal::CordzUpdateTracker;
+  using InlineData = cord_internal::InlineData;
+  using MethodIdentifier = CordzUpdateTracker::MethodIdentifier;
+
+  // Creates a cord instance with `method` representing the originating
+  // public API call causing the cord to be created.
+  explicit Cord(absl::string_view src, MethodIdentifier method);
+
+  friend class ::strings::CordReader;
+  friend class CordTestPeer;
+  friend bool operator==(const Cord& lhs, const Cord& rhs);
+  friend bool operator==(const Cord& lhs, absl::string_view rhs);
+
+#ifdef __cpp_impl_three_way_comparison
+
+  // Cords support comparison with other Cords and string_views via operator<
+  // and others; here we provide a wrapper for the C++20 three-way comparison
+  // <=> operator.
+
+  static inline std::strong_ordering ConvertCompareResultToStrongOrdering(
+      int c) {
+    if (c == 0) {
+      return std::strong_ordering::equal;
+    } else if (c < 0) {
+      return std::strong_ordering::less;
+    } else {
+      return std::strong_ordering::greater;
+    }
+  }
+
+  friend inline std::strong_ordering operator<=>(const Cord& x, const Cord& y) {
+    return ConvertCompareResultToStrongOrdering(x.Compare(y));
+  }
+
+  friend inline std::strong_ordering operator<=>(const Cord& lhs,
+                                                 absl::string_view rhs) {
+    return ConvertCompareResultToStrongOrdering(lhs.Compare(rhs));
+  }
+
+  friend inline std::strong_ordering operator<=>(absl::string_view lhs,
+                                                 const Cord& rhs) {
+    return ConvertCompareResultToStrongOrdering(-rhs.Compare(lhs));
+  }
+#endif
+
+  friend const CordzInfo* absl_nullable GetCordzInfoForTesting(
+      const Cord& cord);
+
+  // Calls the provided function once for each cord chunk, in order.  Unlike
+  // Chunks(), this API will not allocate memory.
+  void ForEachChunk(absl::FunctionRef<void(absl::string_view)>) const;
+
+  // Allocates new contiguous storage for the contents of the cord. This is
+  // called by Flatten() when the cord was not already flat.
+  absl::string_view FlattenSlowPath();
+
+  // Actual cord contents are hidden inside the following simple
+  // class so that we can isolate the bulk of cord.cc from changes
+  // to the representation.
+  //
+  // InlineRep holds either a tree pointer, or an array of kMaxInline bytes.
+  class ABSL_ATTRIBUTE_TRIVIAL_ABI InlineRep {
+   public:
+    static constexpr unsigned char kMaxInline = cord_internal::kMaxInline;
+    static_assert(kMaxInline >= sizeof(absl::cord_internal::CordRep*));
+
+    constexpr InlineRep() : data_() {}
+    explicit InlineRep(InlineData::DefaultInitType init) : data_(init) {}
+    InlineRep(const InlineRep& src);
+    InlineRep(InlineRep&& src);
+    InlineRep& operator=(const InlineRep& src);
+    InlineRep& operator=(InlineRep&& src) noexcept;
+
+    explicit constexpr InlineRep(absl::string_view sv,
+                                 CordRep* absl_nullable rep);
+
+    void Swap(InlineRep* absl_nonnull rhs);
+    size_t size() const;
+    // Returns nullptr if holding pointer
+    const char* absl_nullable data() const;
+    // Discards pointer, if any
+    void set_data(const char* absl_nullable data, size_t n);
+    char* absl_nonnull set_data(size_t n);  // Write data to the result
+    // Returns nullptr if holding bytes
+    absl::cord_internal::CordRep* absl_nullable tree() const;
+    absl::cord_internal::CordRep* absl_nonnull as_tree() const;
+    const char* absl_nonnull as_chars() const;
+    // Returns non-null iff was holding a pointer
+    absl::cord_internal::CordRep* absl_nullable clear();
+    // Converts to pointer if necessary.
+    void reduce_size(size_t n);    // REQUIRES: holding data
+    void remove_prefix(size_t n);  // REQUIRES: holding data
+    void AppendArray(absl::string_view src, MethodIdentifier method);
+    absl::string_view FindFlatStartPiece() const;
+
+    // Creates a CordRepFlat instance from the current inlined data with `extra'
+    // bytes of desired additional capacity.
+    CordRepFlat* absl_nonnull MakeFlatWithExtraCapacity(size_t extra);
+
+    // Sets the tree value for this instance. `rep` must not be null.
+    // Requires the current instance to hold a tree, and a lock to be held on
+    // any CordzInfo referenced by this instance. The latter is enforced through
+    // the CordzUpdateScope argument. If the current instance is sampled, then
+    // the CordzInfo instance is updated to reference the new `rep` value.
+    void SetTree(CordRep* absl_nonnull rep, const CordzUpdateScope& scope);
+
+    // Identical to SetTree(), except that `rep` is allowed to be null, in
+    // which case the current instance is reset to an empty value.
+    void SetTreeOrEmpty(CordRep* absl_nullable rep,
+                        const CordzUpdateScope& scope);
+
+    // Sets the tree value for this instance, and randomly samples this cord.
+    // This function disregards existing contents in `data_`, and should be
+    // called when a Cord is 'promoted' from an 'uninitialized' or 'inlined'
+    // value to a non-inlined (tree / ring) value.
+    void EmplaceTree(CordRep* absl_nonnull rep, MethodIdentifier method);
+
+    // Identical to EmplaceTree, except that it copies the parent stack from
+    // the provided `parent` data if the parent is sampled.
+    void EmplaceTree(CordRep* absl_nonnull rep, const InlineData& parent,
+                     MethodIdentifier method);
+
+    // Commits the change of a newly created, or updated `rep` root value into
+    // this cord. `old_rep` indicates the old (inlined or tree) value of the
+    // cord, and determines if the commit invokes SetTree() or EmplaceTree().
+    void CommitTree(const CordRep* absl_nullable old_rep,
+                    CordRep* absl_nonnull rep, const CordzUpdateScope& scope,
+                    MethodIdentifier method);
+
+    void AppendTreeToInlined(CordRep* absl_nonnull tree,
+                             MethodIdentifier method);
+    void AppendTreeToTree(CordRep* absl_nonnull tree, MethodIdentifier method);
+    void AppendTree(CordRep* absl_nonnull tree, MethodIdentifier method);
+    void PrependTreeToInlined(CordRep* absl_nonnull tree,
+                              MethodIdentifier method);
+    void PrependTreeToTree(CordRep* absl_nonnull tree, MethodIdentifier method);
+    void PrependTree(CordRep* absl_nonnull tree, MethodIdentifier method);
+
+    bool IsSame(const InlineRep& other) const { return data_ == other.data_; }
+
+    // Copies the inline contents into `dst`. Assumes the cord is not empty.
+    void CopyTo(std::string* absl_nonnull dst) const {
+      data_.CopyInlineToString(dst);
+    }
+
+    // Copies the inline contents into `dst`. Assumes the cord is not empty.
+    void CopyToArray(char* absl_nonnull dst) const;
+
+    bool is_tree() const { return data_.is_tree(); }
+
+    // Returns true if the Cord is being profiled by cordz.
+    bool is_profiled() const { return data_.is_tree() && data_.is_profiled(); }
+
+    // Returns the available inlined capacity, or 0 if is_tree() == true.
+    size_t remaining_inline_capacity() const {
+      return data_.is_tree() ? 0 : kMaxInline - data_.inline_size();
+    }
+
+    // Returns the profiled CordzInfo, or nullptr if not sampled.
+    absl::cord_internal::CordzInfo* absl_nullable cordz_info() const {
+      return data_.cordz_info();
+    }
+
+    // Sets the profiled CordzInfo.
+    void set_cordz_info(cord_internal::CordzInfo* absl_nonnull cordz_info) {
+      assert(cordz_info != nullptr);
+      data_.set_cordz_info(cordz_info);
+    }
+
+    // Resets the current cordz_info to null / empty.
+    void clear_cordz_info() { data_.clear_cordz_info(); }
+
+   private:
+    friend class Cord;
+
+    void AssignSlow(const InlineRep& src);
+    // Unrefs the tree and stops profiling.
+    void UnrefTree();
+
+    void ResetToEmpty() { data_ = {}; }
+
+    void set_inline_size(size_t size) { data_.set_inline_size(size); }
+    size_t inline_size() const { return data_.inline_size(); }
+
+    // Empty cords that carry a checksum have a CordRepCrc node with a null
+    // child node. The code can avoid lots of special cases where it would
+    // otherwise transition from tree to inline storage if we just remove the
+    // CordRepCrc node before mutations. Must never be called inside a
+    // CordzUpdateScope since it untracks the cordz info.
+    void MaybeRemoveEmptyCrcNode();
+
+    cord_internal::InlineData data_;
+  };
+  InlineRep contents_;
+
+  // Helper for GetFlat() and TryFlat().
+  static bool GetFlatAux(absl::cord_internal::CordRep* absl_nonnull rep,
+                         absl::string_view* absl_nonnull fragment);
+
+  // Helper for ForEachChunk().
+  static void ForEachChunkAux(
+      absl::cord_internal::CordRep* absl_nonnull rep,
+      absl::FunctionRef<void(absl::string_view)> callback);
+
+  // The destructor for non-empty Cords.
+  void DestroyCordSlow();
+
+  // Out-of-line implementation of slower parts of logic.
+  void CopyToArraySlowPath(char* absl_nonnull dst) const;
+  int CompareSlowPath(absl::string_view rhs, size_t compared_size,
+                      size_t size_to_compare) const;
+  int CompareSlowPath(const Cord& rhs, size_t compared_size,
+                      size_t size_to_compare) const;
+  bool EqualsImpl(absl::string_view rhs, size_t size_to_compare) const;
+  bool EqualsImpl(const Cord& rhs, size_t size_to_compare) const;
+  int CompareImpl(const Cord& rhs) const;
+
+  template <typename ResultType, typename RHS>
+  friend ResultType GenericCompare(const Cord& lhs, const RHS& rhs,
+                                   size_t size_to_compare);
+  static absl::string_view GetFirstChunk(const Cord& c);
+  static absl::string_view GetFirstChunk(absl::string_view sv);
+
+  // Returns a new reference to contents_.tree(), or steals an existing
+  // reference if called on an rvalue.
+  absl::cord_internal::CordRep* absl_nonnull TakeRep() const&;
+  absl::cord_internal::CordRep* absl_nonnull TakeRep() &&;
+
+  // Helper for Append().
+  template <typename C>
+  void AppendImpl(C&& src);
+
+  // Appends / Prepends `src` to this instance, using precise sizing.
+  // This method does explicitly not attempt to use any spare capacity
+  // in any pending last added private owned flat.
+  // Requires `src` to be <= kMaxFlatLength.
+  void AppendPrecise(absl::string_view src, MethodIdentifier method);
+  void PrependPrecise(absl::string_view src, MethodIdentifier method);
+
+  CordBuffer GetAppendBufferSlowPath(size_t block_size, size_t capacity,
+                                     size_t min_capacity);
+
+  // Prepends the provided data to this instance. `method` contains the public
+  // API method for this action which is tracked for Cordz sampling purposes.
+  void PrependArray(absl::string_view src, MethodIdentifier method);
+
+  // Assigns the value in 'src' to this instance, 'stealing' its contents.
+  // Requires src.length() > kMaxBytesToCopy.
+  Cord& AssignLargeString(std::string&& src);
+
+  // Helper for AbslHashValue().
+  template <typename H>
+  H HashFragmented(H hash_state) const {
+    typename H::AbslInternalPiecewiseCombiner combiner;
+    ForEachChunk([&combiner, &hash_state](absl::string_view chunk) {
+      hash_state = combiner.add_buffer(std::move(hash_state), chunk.data(),
+                                       chunk.size());
+    });
+    return combiner.finalize(std::move(hash_state));
+  }
+
+  friend class CrcCord;
+  void SetCrcCordState(crc_internal::CrcCordState state);
+  const crc_internal::CrcCordState* absl_nullable MaybeGetCrcCordState() const;
+
+  CharIterator FindImpl(CharIterator it, absl::string_view needle) const;
+
+  void CopyToArrayImpl(char* absl_nonnull dst) const;
+};
+
+// allow a Cord to be logged
+extern std::ostream& operator<<(std::ostream& out, const Cord& cord);
+
+// ------------------------------------------------------------------
+// Internal details follow.  Clients should ignore.
+
+namespace cord_internal {
+
+// Does non-template-specific `CordRepExternal` initialization.
+// Requires `data` to be non-empty.
+void InitializeCordRepExternal(absl::string_view data,
+                               CordRepExternal* absl_nonnull rep);
+
+// Creates a new `CordRep` that owns `data` and `releaser` and returns a pointer
+// to it. Requires `data` to be non-empty.
+template <typename Releaser>
+// NOLINTNEXTLINE - suppress clang-tidy raw pointer return.
+CordRep* absl_nonnull NewExternalRep(absl::string_view data,
+                                     Releaser&& releaser) {
+  assert(!data.empty());
+  using ReleaserType = std::decay_t<Releaser>;
+  CordRepExternal* rep = new CordRepExternalImpl<ReleaserType>(
+      std::forward<Releaser>(releaser), 0);
+  InitializeCordRepExternal(data, rep);
+  return rep;
+}
+
+// Overload for function reference types that dispatches using a function
+// pointer because there are no `alignof()` or `sizeof()` a function reference.
+// NOLINTNEXTLINE - suppress clang-tidy raw pointer return.
+inline CordRep* absl_nonnull NewExternalRep(
+    absl::string_view data, void (&releaser)(absl::string_view)) {
+  return NewExternalRep(data, &releaser);
+}
+
+}  // namespace cord_internal
+
+template <typename Releaser>
+Cord MakeCordFromExternal(absl::string_view data, Releaser&& releaser) {
+  Cord cord;
+  if (ABSL_PREDICT_TRUE(!data.empty())) {
+    cord.contents_.EmplaceTree(::absl::cord_internal::NewExternalRep(
+                                   data, std::forward<Releaser>(releaser)),
+                               Cord::MethodIdentifier::kMakeCordFromExternal);
+  } else {
+    using ReleaserType = std::decay_t<Releaser>;
+    cord_internal::InvokeReleaser(
+        cord_internal::Rank1{}, ReleaserType(std::forward<Releaser>(releaser)),
+        data);
+  }
+  return cord;
+}
+
+constexpr Cord::InlineRep::InlineRep(absl::string_view sv,
+                                     CordRep* absl_nullable rep)
+    : data_(sv, rep) {}
+
+inline Cord::InlineRep::InlineRep(const Cord::InlineRep& src)
+    : data_(InlineData::kDefaultInit) {
+  if (CordRep* tree = src.tree()) {
+    EmplaceTree(CordRep::Ref(tree), src.data_,
+                CordzUpdateTracker::kConstructorCord);
+  } else {
+    data_ = src.data_;
+  }
+}
+
+inline Cord::InlineRep::InlineRep(Cord::InlineRep&& src) : data_(src.data_) {
+  src.ResetToEmpty();
+}
+
+inline Cord::InlineRep& Cord::InlineRep::operator=(const Cord::InlineRep& src) {
+  if (this == &src) {
+    return *this;
+  }
+  if (!is_tree() && !src.is_tree()) {
+    data_ = src.data_;
+    return *this;
+  }
+  AssignSlow(src);
+  return *this;
+}
+
+inline Cord::InlineRep& Cord::InlineRep::operator=(
+    Cord::InlineRep&& src) noexcept {
+  if (is_tree()) {
+    UnrefTree();
+  }
+  data_ = src.data_;
+  src.ResetToEmpty();
+  return *this;
+}
+
+inline void Cord::InlineRep::Swap(Cord::InlineRep* absl_nonnull rhs) {
+  if (rhs == this) {
+    return;
+  }
+  using std::swap;
+  swap(data_, rhs->data_);
+}
+
+inline const char* absl_nullable Cord::InlineRep::data() const {
+  return is_tree() ? nullptr : data_.as_chars();
+}
+
+inline char* absl_nonnull Cord::InlineRep::set_data(size_t n) {
+  assert(n <= kMaxInline);
+  ResetToEmpty();
+  set_inline_size(n);
+  return data_.as_chars();
+}
+
+inline void Cord::InlineRep::set_data(const char* absl_nullable data,
+                                      size_t n) {
+  static_assert(kMaxInline == 15, "set_data is hard-coded for a length of 15");
+  assert(data != nullptr || n == 0);
+  data_.set_inline_data(data, n);
+}
+
+inline void Cord::InlineRep::reduce_size(size_t n) {
+  size_t tag = inline_size();
+  assert(tag <= kMaxInline);
+  assert(tag >= n);
+  tag -= n;
+  memset(data_.as_chars() + tag, 0, n);
+  set_inline_size(tag);
+}
+
+inline void Cord::InlineRep::remove_prefix(size_t n) {
+  cord_internal::SmallMemmove(data_.as_chars(), data_.as_chars() + n,
+                              inline_size() - n);
+  reduce_size(n);
+}
+
+inline const char* absl_nonnull Cord::InlineRep::as_chars() const {
+  assert(!data_.is_tree());
+  return data_.as_chars();
+}
+
+inline absl::cord_internal::CordRep* absl_nonnull Cord::InlineRep::as_tree()
+    const {
+  assert(data_.is_tree());
+  return data_.as_tree();
+}
+
+inline absl::cord_internal::CordRep* absl_nullable Cord::InlineRep::tree()
+    const {
+  if (is_tree()) {
+    return as_tree();
+  } else {
+    return nullptr;
+  }
+}
+
+inline size_t Cord::InlineRep::size() const {
+  return is_tree() ? as_tree()->length : inline_size();
+}
+
+inline cord_internal::CordRepFlat* absl_nonnull
+Cord::InlineRep::MakeFlatWithExtraCapacity(size_t extra) {
+  static_assert(cord_internal::kMinFlatLength >= sizeof(data_));
+  size_t len = data_.inline_size();
+  auto* result = CordRepFlat::New(len + extra);
+  result->length = len;
+  data_.copy_max_inline_to(result->Data());
+  return result;
+}
+
+inline void Cord::InlineRep::EmplaceTree(CordRep* absl_nonnull rep,
+                                         MethodIdentifier method) {
+  assert(rep);
+  data_.make_tree(rep);
+  CordzInfo::MaybeTrackCord(data_, method);
+}
+
+inline void Cord::InlineRep::EmplaceTree(CordRep* absl_nonnull rep,
+                                         const InlineData& parent,
+                                         MethodIdentifier method) {
+  data_.make_tree(rep);
+  CordzInfo::MaybeTrackCord(data_, parent, method);
+}
+
+inline void Cord::InlineRep::SetTree(CordRep* absl_nonnull rep,
+                                     const CordzUpdateScope& scope) {
+  assert(rep);
+  assert(data_.is_tree());
+  data_.set_tree(rep);
+  scope.SetCordRep(rep);
+}
+
+inline void Cord::InlineRep::SetTreeOrEmpty(CordRep* absl_nullable rep,
+                                            const CordzUpdateScope& scope) {
+  assert(data_.is_tree());
+  if (rep) {
+    data_.set_tree(rep);
+  } else {
+    data_ = {};
+  }
+  scope.SetCordRep(rep);
+}
+
+inline void Cord::InlineRep::CommitTree(const CordRep* absl_nullable old_rep,
+                                        CordRep* absl_nonnull rep,
+                                        const CordzUpdateScope& scope,
+                                        MethodIdentifier method) {
+  if (old_rep) {
+    SetTree(rep, scope);
+  } else {
+    EmplaceTree(rep, method);
+  }
+}
+
+inline absl::cord_internal::CordRep* absl_nullable Cord::InlineRep::clear() {
+  if (is_tree()) {
+    CordzInfo::MaybeUntrackCord(cordz_info());
+  }
+  absl::cord_internal::CordRep* result = tree();
+  ResetToEmpty();
+  return result;
+}
+
+inline void Cord::InlineRep::CopyToArray(char* absl_nonnull dst) const {
+  assert(!is_tree());
+  size_t n = inline_size();
+  assert(n != 0);
+  cord_internal::SmallMemmove(dst, data_.as_chars(), n);
+}
+
+inline void Cord::InlineRep::MaybeRemoveEmptyCrcNode() {
+  CordRep* rep = tree();
+  if (rep == nullptr || ABSL_PREDICT_TRUE(rep->length > 0)) {
+    return;
+  }
+  assert(rep->IsCrc());
+  assert(rep->crc()->child == nullptr);
+  CordzInfo::MaybeUntrackCord(cordz_info());
+  CordRep::Unref(rep);
+  ResetToEmpty();
+}
+
+constexpr inline Cord::Cord() noexcept {}
+
+inline Cord::Cord(absl::string_view src)
+    : Cord(src, CordzUpdateTracker::kConstructorString) {}
+
+template <typename T>
+constexpr Cord::Cord(strings_internal::StringConstant<T>)
+    : contents_(strings_internal::StringConstant<T>::value,
+                strings_internal::StringConstant<T>::value.size() <=
+                        cord_internal::kMaxInline
+                    ? nullptr
+                    : &cord_internal::ConstInitExternalStorage<
+                          strings_internal::StringConstant<T>>::value) {}
+
+inline Cord& Cord::operator=(const Cord& x) {
+  contents_ = x.contents_;
+  return *this;
+}
+
+template <typename T, Cord::EnableIfString<T>>
+Cord& Cord::operator=(T&& src) {
+  if (src.size() <= cord_internal::kMaxBytesToCopy) {
+    return operator=(absl::string_view(src));
+  } else {
+    return AssignLargeString(std::forward<T>(src));
+  }
+}
+
+inline Cord::Cord(const Cord& src) : contents_(src.contents_) {}
+
+inline Cord::Cord(Cord&& src) noexcept : contents_(std::move(src.contents_)) {}
+
+inline void Cord::swap(Cord& other) noexcept {
+  contents_.Swap(&other.contents_);
+}
+
+inline Cord& Cord::operator=(Cord&& x) noexcept {
+  contents_ = std::move(x.contents_);
+  return *this;
+}
+
+extern template Cord::Cord(std::string&& src);
+
+inline size_t Cord::size() const {
+  // Length is 1st field in str.rep_
+  return contents_.size();
+}
+
+inline bool Cord::empty() const { return size() == 0; }
+
+inline size_t Cord::EstimatedMemoryUsage(
+    CordMemoryAccounting accounting_method) const {
+  size_t result = sizeof(Cord);
+  if (const absl::cord_internal::CordRep* rep = contents_.tree()) {
+    switch (accounting_method) {
+      case CordMemoryAccounting::kFairShare:
+        result += cord_internal::GetEstimatedFairShareMemoryUsage(rep);
+        break;
+      case CordMemoryAccounting::kTotalMorePrecise:
+        result += cord_internal::GetMorePreciseMemoryUsage(rep);
+        break;
+      case CordMemoryAccounting::kTotal:
+        result += cord_internal::GetEstimatedMemoryUsage(rep);
+        break;
+    }
+  }
+  return result;
+}
+
+inline std::optional<absl::string_view> Cord::TryFlat() const
+    ABSL_ATTRIBUTE_LIFETIME_BOUND {
+  absl::cord_internal::CordRep* rep = contents_.tree();
+  if (rep == nullptr) {
+    return absl::string_view(contents_.data(), contents_.size());
+  }
+  absl::string_view fragment;
+  if (GetFlatAux(rep, &fragment)) {
+    return fragment;
+  }
+  return std::nullopt;
+}
+
+inline absl::string_view Cord::Flatten() ABSL_ATTRIBUTE_LIFETIME_BOUND {
+  absl::cord_internal::CordRep* rep = contents_.tree();
+  if (rep == nullptr) {
+    return absl::string_view(contents_.data(), contents_.size());
+  } else {
+    absl::string_view already_flat_contents;
+    if (GetFlatAux(rep, &already_flat_contents)) {
+      return already_flat_contents;
+    }
+  }
+  return FlattenSlowPath();
+}
+
+inline void Cord::Append(absl::string_view src) {
+  contents_.AppendArray(src, CordzUpdateTracker::kAppendString);
+}
+
+inline void Cord::Prepend(absl::string_view src) {
+  PrependArray(src, CordzUpdateTracker::kPrependString);
+}
+
+inline void Cord::Append(CordBuffer buffer) {
+  if (ABSL_PREDICT_FALSE(buffer.length() == 0)) return;
+  contents_.MaybeRemoveEmptyCrcNode();
+  absl::string_view short_value;
+  if (CordRep* rep = buffer.ConsumeValue(short_value)) {
+    contents_.AppendTree(rep, CordzUpdateTracker::kAppendCordBuffer);
+  } else {
+    AppendPrecise(short_value, CordzUpdateTracker::kAppendCordBuffer);
+  }
+}
+
+inline void Cord::Prepend(CordBuffer buffer) {
+  if (ABSL_PREDICT_FALSE(buffer.length() == 0)) return;
+  contents_.MaybeRemoveEmptyCrcNode();
+  absl::string_view short_value;
+  if (CordRep* rep = buffer.ConsumeValue(short_value)) {
+    contents_.PrependTree(rep, CordzUpdateTracker::kPrependCordBuffer);
+  } else {
+    PrependPrecise(short_value, CordzUpdateTracker::kPrependCordBuffer);
+  }
+}
+
+inline CordBuffer Cord::GetAppendBuffer(size_t capacity, size_t min_capacity) {
+  if (empty()) return CordBuffer::CreateWithDefaultLimit(capacity);
+  return GetAppendBufferSlowPath(0, capacity, min_capacity);
+}
+
+inline CordBuffer Cord::GetCustomAppendBuffer(size_t block_size,
+                                              size_t capacity,
+                                              size_t min_capacity) {
+  if (empty()) {
+    return block_size ? CordBuffer::CreateWithCustomLimit(block_size, capacity)
+                      : CordBuffer::CreateWithDefaultLimit(capacity);
+  }
+  return GetAppendBufferSlowPath(block_size, capacity, min_capacity);
+}
+
+extern template void Cord::Append(std::string&& src);
+extern template void Cord::Prepend(std::string&& src);
+
+inline int Cord::Compare(const Cord& rhs) const {
+  if (!contents_.is_tree() && !rhs.contents_.is_tree()) {
+    return contents_.data_.Compare(rhs.contents_.data_);
+  }
+
+  return CompareImpl(rhs);
+}
+
+// Does 'this' cord start/end with rhs
+inline bool Cord::StartsWith(const Cord& rhs) const {
+  if (contents_.IsSame(rhs.contents_)) return true;
+  size_t rhs_size = rhs.size();
+  if (size() < rhs_size) return false;
+  return EqualsImpl(rhs, rhs_size);
+}
+
+inline bool Cord::StartsWith(absl::string_view rhs) const {
+  size_t rhs_size = rhs.size();
+  if (size() < rhs_size) return false;
+  return EqualsImpl(rhs, rhs_size);
+}
+
+inline void Cord::CopyToArrayImpl(char* absl_nonnull dst) const {
+  if (!contents_.is_tree()) {
+    if (!empty()) contents_.CopyToArray(dst);
+  } else {
+    CopyToArraySlowPath(dst);
+  }
+}
+
+inline void Cord::ChunkIterator::InitTree(
+    cord_internal::CordRep* absl_nonnull tree) {
+  tree = cord_internal::SkipCrcNode(tree);
+  if (tree->tag == cord_internal::BTREE) {
+    current_chunk_ = btree_reader_.Init(tree->btree());
+  } else {
+    current_leaf_ = tree;
+    current_chunk_ = cord_internal::EdgeData(tree);
+  }
+}
+
+inline Cord::ChunkIterator::ChunkIterator(
+    cord_internal::CordRep* absl_nonnull tree) {
+  bytes_remaining_ = tree->length;
+  InitTree(tree);
+}
+
+inline Cord::ChunkIterator::ChunkIterator(const Cord* absl_nonnull cord) {
+  if (CordRep* tree = cord->contents_.tree()) {
+    bytes_remaining_ = tree->length;
+    if (ABSL_PREDICT_TRUE(bytes_remaining_ != 0)) {
+      InitTree(tree);
+    } else {
+      current_chunk_ = {};
+    }
+  } else {
+    bytes_remaining_ = cord->contents_.inline_size();
+    current_chunk_ = {cord->contents_.data(), bytes_remaining_};
+  }
+}
+
+inline Cord::ChunkIterator& Cord::ChunkIterator::AdvanceBtree() {
+  current_chunk_ = btree_reader_.Next();
+  return *this;
+}
+
+inline void Cord::ChunkIterator::AdvanceBytesBtree(size_t n) {
+  assert(n >= current_chunk_.size());
+  bytes_remaining_ -= n;
+  if (bytes_remaining_) {
+    if (n == current_chunk_.size()) {
+      current_chunk_ = btree_reader_.Next();
+    } else {
+      size_t offset = btree_reader_.length() - bytes_remaining_;
+      current_chunk_ = btree_reader_.Seek(offset);
+    }
+  } else {
+    current_chunk_ = {};
+  }
+}
+
+inline Cord::ChunkIterator& Cord::ChunkIterator::operator++() {
+  // Failure of this assertion indicates an attempt to iterate past `end()`.
+  absl::base_internal::HardeningAssertGT(bytes_remaining_, size_t{0});
+  assert(bytes_remaining_ >= current_chunk_.size());
+  bytes_remaining_ -= current_chunk_.size();
+  if (bytes_remaining_ > 0) {
+    if (btree_reader_) {
+      return AdvanceBtree();
+    } else {
+      assert(!current_chunk_.empty());  // Called on invalid iterator.
+    }
+    current_chunk_ = {};
+  }
+  return *this;
+}
+
+inline Cord::ChunkIterator Cord::ChunkIterator::operator++(int) {
+  ChunkIterator tmp(*this);
+  operator++();
+  return tmp;
+}
+
+inline bool Cord::ChunkIterator::operator==(const ChunkIterator& other) const {
+  return bytes_remaining_ == other.bytes_remaining_;
+}
+
+inline bool Cord::ChunkIterator::operator!=(const ChunkIterator& other) const {
+  return !(*this == other);
+}
+
+inline Cord::ChunkIterator::reference Cord::ChunkIterator::operator*() const {
+  absl::base_internal::HardeningAssertGT(bytes_remaining_, size_t{0});
+  ABSL_ASSERT(bytes_remaining_ >= current_chunk_.size());
+  return current_chunk_;
+}
+
+inline Cord::ChunkIterator::pointer Cord::ChunkIterator::operator->() const {
+  absl::base_internal::HardeningAssertGT(bytes_remaining_, size_t{0});
+  ABSL_ASSERT(bytes_remaining_ >= current_chunk_.size());
+  return &current_chunk_;
+}
+
+inline void Cord::ChunkIterator::RemoveChunkPrefix(size_t n) {
+  assert(n < current_chunk_.size());
+  current_chunk_.remove_prefix(n);
+  bytes_remaining_ -= n;
+}
+
+inline void Cord::ChunkIterator::AdvanceBytes(size_t n) {
+  assert(bytes_remaining_ >= n);
+  if (ABSL_PREDICT_TRUE(n < current_chunk_.size())) {
+    RemoveChunkPrefix(n);
+  } else if (n != 0) {
+    if (btree_reader_) {
+      AdvanceBytesBtree(n);
+    } else {
+      bytes_remaining_ = 0;
+    }
+  }
+}
+
+inline Cord::ChunkIterator Cord::chunk_begin() const {
+  return ChunkIterator(this);
+}
+
+inline Cord::ChunkIterator Cord::chunk_end() const { return ChunkIterator(); }
+
+inline Cord::ChunkIterator Cord::ChunkRange::begin() const {
+  return cord_->chunk_begin();
+}
+
+inline Cord::ChunkIterator Cord::ChunkRange::end() const {
+  return cord_->chunk_end();
+}
+
+inline Cord::ChunkRange Cord::Chunks() const { return ChunkRange(this); }
+
+inline Cord::CharIterator& Cord::CharIterator::operator++() {
+  if (ABSL_PREDICT_TRUE(chunk_iterator_->size() > 1)) {
+    chunk_iterator_.RemoveChunkPrefix(1);
+  } else {
+    ++chunk_iterator_;
+  }
+  return *this;
+}
+
+inline Cord::CharIterator Cord::CharIterator::operator++(int) {
+  CharIterator tmp(*this);
+  operator++();
+  return tmp;
+}
+
+inline bool Cord::CharIterator::operator==(const CharIterator& other) const {
+  return chunk_iterator_ == other.chunk_iterator_;
+}
+
+inline bool Cord::CharIterator::operator!=(const CharIterator& other) const {
+  return !(*this == other);
+}
+
+inline Cord::CharIterator::reference Cord::CharIterator::operator*() const {
+  return *chunk_iterator_->data();
+}
+
+inline Cord Cord::AdvanceAndRead(CharIterator* absl_nonnull it,
+                                 size_t n_bytes) {
+  assert(it != nullptr);
+  return it->chunk_iterator_.AdvanceAndReadBytes(n_bytes);
+}
+
+inline void Cord::Advance(CharIterator* absl_nonnull it, size_t n_bytes) {
+  assert(it != nullptr);
+  it->chunk_iterator_.AdvanceBytes(n_bytes);
+}
+
+inline absl::string_view Cord::ChunkRemaining(const CharIterator& it) {
+  return *it.chunk_iterator_;
+}
+
+inline ptrdiff_t Cord::Distance(const CharIterator& first,
+                                const CharIterator& last) {
+  return static_cast<ptrdiff_t>(first.chunk_iterator_.bytes_remaining_ -
+                                last.chunk_iterator_.bytes_remaining_);
+}
+
+inline Cord::CharIterator Cord::char_begin() const {
+  return CharIterator(this);
+}
+
+inline Cord::CharIterator Cord::char_end() const { return CharIterator(); }
+
+inline Cord::CharIterator Cord::CharRange::begin() const {
+  return cord_->char_begin();
+}
+
+inline Cord::CharIterator Cord::CharRange::end() const {
+  return cord_->char_end();
+}
+
+inline Cord::CharRange Cord::Chars() const { return CharRange(this); }
+
+inline void Cord::ForEachChunk(
+    absl::FunctionRef<void(absl::string_view)> callback) const {
+  absl::cord_internal::CordRep* rep = contents_.tree();
+  if (rep == nullptr) {
+    callback(absl::string_view(contents_.data(), contents_.size()));
+  } else {
+    ForEachChunkAux(rep, callback);
+  }
+}
+
+// Nonmember Cord-to-Cord relational operators.
+inline bool operator==(const Cord& lhs, const Cord& rhs) {
+  if (lhs.contents_.IsSame(rhs.contents_)) return true;
+  size_t rhs_size = rhs.size();
+  if (lhs.size() != rhs_size) return false;
+  return lhs.EqualsImpl(rhs, rhs_size);
+}
+
+inline bool operator!=(const Cord& x, const Cord& y) { return !(x == y); }
+inline bool operator<(const Cord& x, const Cord& y) { return x.Compare(y) < 0; }
+inline bool operator>(const Cord& x, const Cord& y) { return x.Compare(y) > 0; }
+inline bool operator<=(const Cord& x, const Cord& y) {
+  return x.Compare(y) <= 0;
+}
+inline bool operator>=(const Cord& x, const Cord& y) {
+  return x.Compare(y) >= 0;
+}
+
+// Nonmember Cord-to-absl::string_view relational operators.
+//
+// Due to implicit conversions, these also enable comparisons of Cord with
+// std::string and const char*.
+inline bool operator==(const Cord& lhs, absl::string_view rhs) {
+  size_t lhs_size = lhs.size();
+  size_t rhs_size = rhs.size();
+  if (lhs_size != rhs_size) return false;
+  return lhs.EqualsImpl(rhs, rhs_size);
+}
+
+inline bool operator==(absl::string_view x, const Cord& y) { return y == x; }
+inline bool operator!=(const Cord& x, absl::string_view y) { return !(x == y); }
+inline bool operator!=(absl::string_view x, const Cord& y) { return !(x == y); }
+inline bool operator<(const Cord& x, absl::string_view y) {
+  return x.Compare(y) < 0;
+}
+inline bool operator<(absl::string_view x, const Cord& y) {
+  return y.Compare(x) > 0;
+}
+inline bool operator>(const Cord& x, absl::string_view y) { return y < x; }
+inline bool operator>(absl::string_view x, const Cord& y) { return y < x; }
+inline bool operator<=(const Cord& x, absl::string_view y) { return !(y < x); }
+inline bool operator<=(absl::string_view x, const Cord& y) { return !(y < x); }
+inline bool operator>=(const Cord& x, absl::string_view y) { return !(x < y); }
+inline bool operator>=(absl::string_view x, const Cord& y) { return !(x < y); }
+
+// Some internals exposed to test code.
+namespace strings_internal {
+class CordTestAccess {
+ public:
+  static size_t FlatOverhead();
+  static size_t MaxFlatLength();
+  static size_t SizeofCordRepExternal();
+  static size_t SizeofCordRepSubstring();
+  static size_t FlatTagToLength(uint8_t tag);
+  static uint8_t LengthToTag(size_t s);
+};
+}  // namespace strings_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STRINGS_CORD_H_
diff --git a/absl/strings/cord_analysis.cc b/absl/strings/cord_analysis.cc
new file mode 100644
index 00000000000..dcbc826c6e6
--- /dev/null
+++ b/absl/strings/cord_analysis.cc
@@ -0,0 +1,196 @@
+// Copyright 2021 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/strings/cord_analysis.h"
+
+#include <cassert>
+#include <cstddef>
+#include <cstdint>
+#include <unordered_set>
+
+#include "absl/base/config.h"
+#include "absl/base/nullability.h"
+#include "absl/strings/internal/cord_data_edge.h"
+#include "absl/strings/internal/cord_internal.h"
+#include "absl/strings/internal/cord_rep_btree.h"
+#include "absl/strings/internal/cord_rep_crc.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+namespace {
+
+// Accounting mode for analyzing memory usage.
+enum class Mode { kFairShare, kTotal, kTotalMorePrecise };
+
+// CordRepRef holds a `const CordRep*` reference in rep, and depending on mode,
+// holds a 'fraction' representing a cumulative inverse refcount weight.
+template <Mode mode>
+struct CordRepRef {
+  // Instantiates a CordRepRef instance.
+  explicit CordRepRef(const CordRep* absl_nonnull r) : rep(r) {}
+
+  // Creates a child reference holding the provided child.
+  // Overloaded to add cumulative reference count for kFairShare.
+  CordRepRef Child(const CordRep* absl_nonnull child) const {
+    return CordRepRef(child);
+  }
+
+  const CordRep* absl_nonnull rep;
+};
+
+// RawUsage holds the computed total number of bytes.
+template <Mode mode>
+struct RawUsage {
+  size_t total = 0;
+
+  // Add 'size' to total, ignoring the CordRepRef argument.
+  void Add(size_t size, CordRepRef<mode>) { total += size; }
+};
+
+// Overloaded representation of RawUsage that tracks the set of objects
+// counted, and avoids double-counting objects referenced more than once
+// by the same Cord.
+template <>
+struct RawUsage<Mode::kTotalMorePrecise> {
+  size_t total = 0;
+  // TODO(b/289250880): Replace this with a flat_hash_set.
+  std::unordered_set<const CordRep* absl_nonnull> counted;
+
+  void Add(size_t size, CordRepRef<Mode::kTotalMorePrecise> repref) {
+    if (counted.insert(repref.rep).second) {
+      total += size;
+    }
+  }
+};
+
+// Returns n / refcount avoiding a div for the common refcount == 1.
+template <typename refcount_t>
+double MaybeDiv(double d, refcount_t refcount) {
+  return refcount == 1 ? d : d / refcount;
+}
+
+// Overloaded 'kFairShare' specialization for CordRepRef. This class holds a
+// `fraction` value which represents a cumulative inverse refcount weight.
+// For example, a top node with a reference count of 2 will have a fraction
+// value of 1/2 = 0.5, representing the 'fair share' of memory it references.
+// A node below such a node with a reference count of 5 then has a fraction of
+// 0.5 / 5 = 0.1 representing the fair share of memory below that node, etc.
+template <>
+struct CordRepRef<Mode::kFairShare> {
+  // Creates a CordRepRef with the provided rep and top (parent) fraction.
+  explicit CordRepRef(const CordRep* absl_nonnull r, double frac = 1.0)
+      : rep(r), fraction(MaybeDiv(frac, r->refcount.Get())) {}
+
+  // Returns a CordRepRef with a fraction of `this->fraction / child.refcount`
+  CordRepRef Child(const CordRep* absl_nonnull child) const {
+    return CordRepRef(child, fraction);
+  }
+
+  const CordRep* absl_nonnull rep;
+  double fraction;
+};
+
+// Overloaded 'kFairShare' specialization for RawUsage
+template <>
+struct RawUsage<Mode::kFairShare> {
+  double total = 0;
+
+  // Adds `size` multiplied by `rep.fraction` to the total size.
+  void Add(size_t size, CordRepRef<Mode::kFairShare> rep) {
+    total += static_cast<double>(size) * rep.fraction;
+  }
+};
+
+// Computes the estimated memory size of the provided data edge.
+// External reps are assumed 'heap allocated at their exact size'.
+template <Mode mode>
+void AnalyzeDataEdge(CordRepRef<mode> rep, RawUsage<mode>& raw_usage) {
+  assert(IsDataEdge(rep.rep));
+
+  // Consume all substrings
+  if (rep.rep->tag == SUBSTRING) {
+    raw_usage.Add(sizeof(CordRepSubstring), rep);
+    rep = rep.Child(rep.rep->substring()->child);
+  }
+
+  // Consume FLAT / EXTERNAL
+  const size_t size =
+      rep.rep->tag >= FLAT
+          ? rep.rep->flat()->AllocatedSize()
+          : rep.rep->length + sizeof(CordRepExternalImpl<intptr_t>);
+  raw_usage.Add(size, rep);
+}
+
+// Computes the memory size of the provided Btree tree.
+template <Mode mode>
+void AnalyzeBtree(CordRepRef<mode> rep, RawUsage<mode>& raw_usage) {
+  raw_usage.Add(sizeof(CordRepBtree), rep);
+  const CordRepBtree* tree = rep.rep->btree();
+  if (tree->height() > 0) {
+    for (CordRep* edge : tree->Edges()) {
+      AnalyzeBtree(rep.Child(edge), raw_usage);
+    }
+  } else {
+    for (CordRep* edge : tree->Edges()) {
+      AnalyzeDataEdge(rep.Child(edge), raw_usage);
+    }
+  }
+}
+
+template <Mode mode>
+size_t GetEstimatedUsage(const CordRep* absl_nonnull rep) {
+  // Zero initialized memory usage totals.
+  RawUsage<mode> raw_usage;
+
+  // Capture top level node and refcount into a CordRepRef.
+  CordRepRef<mode> repref(rep);
+
+  // Consume the top level CRC node if present.
+  if (repref.rep->tag == CRC) {
+    raw_usage.Add(sizeof(CordRepCrc), repref);
+    if (repref.rep->crc()->child == nullptr) {
+      return static_cast<size_t>(raw_usage.total);
+    }
+    repref = repref.Child(repref.rep->crc()->child);
+  }
+
+  if (IsDataEdge(repref.rep)) {
+    AnalyzeDataEdge(repref, raw_usage);
+  } else if (repref.rep->tag == BTREE) {
+    AnalyzeBtree(repref, raw_usage);
+  } else {
+    assert(false);
+  }
+
+  return static_cast<size_t>(raw_usage.total);
+}
+
+}  // namespace
+
+size_t GetEstimatedMemoryUsage(const CordRep* absl_nonnull rep) {
+  return GetEstimatedUsage<Mode::kTotal>(rep);
+}
+
+size_t GetEstimatedFairShareMemoryUsage(const CordRep* absl_nonnull rep) {
+  return GetEstimatedUsage<Mode::kFairShare>(rep);
+}
+
+size_t GetMorePreciseMemoryUsage(const CordRep* absl_nonnull rep) {
+  return GetEstimatedUsage<Mode::kTotalMorePrecise>(rep);
+}
+
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/strings/cord_analysis.h b/absl/strings/cord_analysis.h
new file mode 100644
index 00000000000..db50f3a19ce
--- /dev/null
+++ b/absl/strings/cord_analysis.h
@@ -0,0 +1,63 @@
+// Copyright 2021 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_STRINGS_CORD_ANALYSIS_H_
+#define ABSL_STRINGS_CORD_ANALYSIS_H_
+
+#include <cstddef>
+#include <cstdint>
+
+#include "absl/base/config.h"
+#include "absl/base/nullability.h"
+#include "absl/strings/internal/cord_internal.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+
+// Returns the *approximate* number of bytes held in full or in part by this
+// Cord (which may not remain the same between invocations). Cords that share
+// memory could each be "charged" independently for the same shared memory.
+size_t GetEstimatedMemoryUsage(const CordRep* absl_nonnull rep);
+
+// Returns the *approximate* number of bytes held in full or in part by this
+// Cord for the distinct memory held by this cord. This is similar to
+// `GetEstimatedMemoryUsage()`, except that if the cord has multiple references
+// to the same memory, that memory is only counted once.
+//
+// For example:
+//   absl::Cord cord;
+//   cord.append(some_other_cord);
+//   cord.append(some_other_cord);
+//    // Calls GetEstimatedMemoryUsage() and counts `other_cord` twice:
+//   cord.EstimatedMemoryUsage(kTotal);
+//    // Calls GetMorePreciseMemoryUsage() and counts `other_cord` once:
+//   cord.EstimatedMemoryUsage(kTotalMorePrecise);
+//
+// This is more expensive than `GetEstimatedMemoryUsage()` as it requires
+// deduplicating all memory references.
+size_t GetMorePreciseMemoryUsage(const CordRep* absl_nonnull rep);
+
+// Returns the *approximate* number of bytes held in full or in part by this
+// CordRep weighted by the sharing ratio of that data. For example, if some data
+// edge is shared by 4 different Cords, then each cord is attribute 1/4th of
+// the total memory usage as a 'fair share' of the total memory usage.
+size_t GetEstimatedFairShareMemoryUsage(const CordRep* absl_nonnull rep);
+
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+
+#endif  // ABSL_STRINGS_CORD_ANALYSIS_H_
diff --git a/absl/strings/cord_buffer.h b/absl/strings/cord_buffer.h
new file mode 100644
index 00000000000..c3803eb1fcd
--- /dev/null
+++ b/absl/strings/cord_buffer.h
@@ -0,0 +1,575 @@
+// Copyright 2021 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: cord_buffer.h
+// -----------------------------------------------------------------------------
+//
+// This file defines an `absl::CordBuffer` data structure to hold data for
+// eventual inclusion within an existing `Cord` data structure. Cord buffers are
+// useful for building large Cords that may require custom allocation of its
+// associated memory.
+//
+#ifndef ABSL_STRINGS_CORD_BUFFER_H_
+#define ABSL_STRINGS_CORD_BUFFER_H_
+
+#include <algorithm>
+#include <cassert>
+#include <cstddef>
+#include <cstdint>
+#include <memory>
+#include <utility>
+
+#include "absl/base/config.h"
+#include "absl/base/internal/hardening.h"
+#include "absl/base/macros.h"
+#include "absl/numeric/bits.h"
+#include "absl/strings/internal/cord_internal.h"
+#include "absl/strings/internal/cord_rep_flat.h"
+#include "absl/strings/string_view.h"
+#include "absl/types/span.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+class Cord;
+class CordBufferTestPeer;
+
+// CordBuffer
+//
+// CordBuffer manages memory buffers for purposes such as zero-copy APIs as well
+// as applications building cords with large data requiring granular control
+// over the allocation and size of cord data. For example, a function creating
+// a cord of random data could use a CordBuffer as follows:
+//
+//   absl::Cord CreateRandomCord(size_t length) {
+//     absl::Cord cord;
+//     while (length > 0) {
+//       CordBuffer buffer = CordBuffer::CreateWithDefaultLimit(length);
+//       absl::Span<char> data = buffer.available_up_to(length);
+//       FillRandomValues(data.data(), data.size());
+//       buffer.IncreaseLengthBy(data.size());
+//       cord.Append(std::move(buffer));
+//       length -= data.size();
+//     }
+//     return cord;
+//   }
+//
+// CordBuffer instances are by default limited to a capacity of `kDefaultLimit`
+// bytes. `kDefaultLimit` is currently just under 4KiB, but this default may
+// change in the future and/or for specific architectures. The default limit is
+// aimed to provide a good trade-off between performance and memory overhead.
+// Smaller buffers typically incur more compute cost while larger buffers are
+// more CPU efficient but create significant memory overhead because of such
+// allocations being less granular. Using larger buffers may also increase the
+// risk of memory fragmentation.
+//
+// Applications create a buffer using one of the `CreateWithDefaultLimit()` or
+// `CreateWithCustomLimit()` methods. The returned instance will have a non-zero
+// capacity and a zero length. Applications use the `data()` method to set the
+// contents of the managed memory, and once done filling the buffer, use the
+// `IncreaseLengthBy()` or 'SetLength()' method to specify the length of the
+// initialized data before adding the buffer to a Cord.
+//
+// The `CreateWithCustomLimit()` method is intended for applications needing
+// larger buffers than the default memory limit, allowing the allocation of up
+// to a capacity of `kCustomLimit` bytes minus some minimum internal overhead.
+// The usage of `CreateWithCustomLimit()` should be limited to only those use
+// cases where the distribution of the input is relatively well known, and/or
+// where the trade-off between the efficiency gains outweigh the risk of memory
+// fragmentation. See the documentation for `CreateWithCustomLimit()` for more
+// information on using larger custom limits.
+//
+// The capacity of a `CordBuffer` returned by one of the `Create` methods may
+// be larger than the requested capacity due to rounding, alignment and
+// granularity of the memory allocator. Applications should use the `capacity`
+// method to obtain the effective capacity of the returned instance as
+// demonstrated in the provided example above.
+//
+// CordBuffer is a move-only class. All references into the managed memory are
+// invalidated when an instance is moved into either another CordBuffer instance
+// or a Cord. Writing to a location obtained by a previous call to `data()`
+// after an instance was moved will lead to undefined behavior.
+//
+// A `moved from` CordBuffer instance will have a valid, but empty state.
+// CordBuffer is thread compatible.
+class CordBuffer {
+ public:
+  // kDefaultLimit
+  //
+  // Default capacity limits of allocated CordBuffers.
+  // See the class comments for more information on allocation limits.
+  static constexpr size_t kDefaultLimit = cord_internal::kMaxFlatLength;
+
+  // kCustomLimit
+  //
+  // Maximum size for CreateWithCustomLimit() allocated buffers.
+  // Note that the effective capacity may be slightly less
+  // because of internal overhead of internal cord buffers.
+  static constexpr size_t kCustomLimit = 64U << 10;
+
+  // Constructors, Destructors and Assignment Operators
+
+  // Creates an empty CordBuffer.
+  CordBuffer() = default;
+
+  // Destroys this CordBuffer instance and, if not empty, releases any memory
+  // managed by this instance, invalidating previously returned references.
+  ~CordBuffer();
+
+  // CordBuffer is move-only
+  CordBuffer(CordBuffer&& rhs) noexcept;
+  CordBuffer& operator=(CordBuffer&&) noexcept;
+  CordBuffer(const CordBuffer&) = delete;
+  CordBuffer& operator=(const CordBuffer&) = delete;
+
+  // CordBuffer::MaximumPayload()
+  //
+  // Returns the guaranteed maximum payload for a CordBuffer returned by the
+  // `CreateWithDefaultLimit()` method. While small, each internal buffer inside
+  // a Cord incurs an overhead to manage the length, type and reference count
+  // for the buffer managed inside the cord tree. Applications can use this
+  // method to get approximate number of buffers required for a given byte
+  // size, etc.
+  //
+  // For example:
+  //   const size_t payload = absl::CordBuffer::MaximumPayload();
+  //   const size_t buffer_count = (total_size + payload - 1) / payload;
+  //   buffers.reserve(buffer_count);
+  static constexpr size_t MaximumPayload();
+
+  // Overload to the above `MaximumPayload()` except that it returns the
+  // maximum payload for a CordBuffer returned by the `CreateWithCustomLimit()`
+  // method given the provided `block_size`.
+  static constexpr size_t MaximumPayload(size_t block_size);
+
+  // CordBuffer::CreateWithDefaultLimit()
+  //
+  // Creates a CordBuffer instance of the desired `capacity`, capped at the
+  // default limit `kDefaultLimit`. The returned buffer has a guaranteed
+  // capacity of at least `min(kDefaultLimit, capacity)`. See the class comments
+  // for more information on buffer capacities and intended usage.
+  static CordBuffer CreateWithDefaultLimit(size_t capacity);
+
+  // CordBuffer::CreateWithCustomLimit()
+  //
+  // Creates a CordBuffer instance of the desired `capacity` rounded to an
+  // appropriate power of 2 size less than, or equal to `block_size`.
+  // Requires `block_size` to be a power of 2.
+  //
+  // If `capacity` is less than or equal to `kDefaultLimit`, then this method
+  // behaves identical to `CreateWithDefaultLimit`, which means that the caller
+  // is guaranteed to get a buffer of at least the requested capacity.
+  //
+  // If `capacity` is greater than or equal to `block_size`, then this method
+  // returns a buffer with an `allocated size` of `block_size` bytes. Otherwise,
+  // this methods returns a buffer with a suitable smaller power of 2 block size
+  // to satisfy the request. The actual size depends on a number of factors, and
+  // is typically (but not necessarily) the highest or second highest power of 2
+  // value less than or equal to `capacity`.
+  //
+  // The 'allocated size' includes a small amount of overhead required for
+  // internal state, which is currently 13 bytes on 64-bit platforms. For
+  // example: a buffer created with `block_size` and `capacity' set to 8KiB
+  // will have an allocated size of 8KiB, and an effective internal `capacity`
+  // of 8KiB - 13 = 8179 bytes.
+  //
+  // To demonstrate this in practice, let's assume we want to read data from
+  // somewhat larger files using approximately 64KiB buffers:
+  //
+  //   absl::Cord ReadFromFile(int fd, size_t n) {
+  //     absl::Cord cord;
+  //     while (n > 0) {
+  //       CordBuffer buffer = CordBuffer::CreateWithCustomLimit(64 << 10, n);
+  //       absl::Span<char> data = buffer.available_up_to(n);
+  //       ReadFileDataOrDie(fd, data.data(), data.size());
+  //       buffer.IncreaseLengthBy(data.size());
+  //       cord.Append(std::move(buffer));
+  //       n -= data.size();
+  //     }
+  //     return cord;
+  //   }
+  //
+  // If we'd use this function to read a file of 659KiB, we may get the
+  // following pattern of allocated cord buffer sizes:
+  //
+  //   CreateWithCustomLimit(64KiB, 674816) --> ~64KiB (65523)
+  //   CreateWithCustomLimit(64KiB, 674816) --> ~64KiB (65523)
+  //   ...
+  //   CreateWithCustomLimit(64KiB,  19586) --> ~16KiB (16371)
+  //   CreateWithCustomLimit(64KiB,   3215) -->   3215 (at least 3215)
+  //
+  // The reason the method returns a 16K buffer instead of a roughly 19K buffer
+  // is to reduce memory overhead and fragmentation risks. Using carefully
+  // chosen power of 2 values reduces the entropy of allocated memory sizes.
+  //
+  // Additionally, let's assume we'd use the above function on files that are
+  // generally smaller than 64K. If we'd use 'precise' sized buffers for such
+  // files, than we'd get a very wide distribution of allocated memory sizes
+  // rounded to 4K page sizes, and we'd end up with a lot of unused capacity.
+  //
+  // In general, application should only use custom sizes if the data they are
+  // consuming or storing is expected to be many times the chosen block size,
+  // and be based on objective data and performance metrics. For example, a
+  // compress function may work faster and consume less CPU when using larger
+  // buffers. Such an application should pick a size offering a reasonable
+  // trade-off between expected data size, compute savings with larger buffers,
+  // and the cost or fragmentation effect of larger buffers.
+  // Applications must pick a reasonable spot on that curve, and make sure their
+  // data meets their expectations in size distributions such as "mostly large".
+  static CordBuffer CreateWithCustomLimit(size_t block_size, size_t capacity);
+
+  // CordBuffer::available()
+  //
+  // Returns the span delineating the available capacity in this buffer
+  // which is defined as `{ data() + length(), capacity() - length() }`.
+  absl::Span<char> available();
+
+  // CordBuffer::available_up_to()
+  //
+  // Returns the span delineating the available capacity in this buffer limited
+  // to `size` bytes. This is equivalent to `available().subspan(0, size)`.
+  absl::Span<char> available_up_to(size_t size);
+
+  // CordBuffer::data()
+  //
+  // Returns a non-null reference to the data managed by this instance.
+  // Applications are allowed to write up to `capacity` bytes of instance data.
+  // CordBuffer data is uninitialized by default. Reading data from an instance
+  // that has not yet been initialized will lead to undefined behavior.
+  char* data();
+  const char* data() const;
+
+  // CordBuffer::length()
+  //
+  // Returns the length of this instance. The default length of a CordBuffer is
+  // 0, indicating an 'empty' CordBuffer. Applications must specify the length
+  // of the data in a CordBuffer before adding it to a Cord.
+  size_t length() const;
+
+  // CordBuffer::capacity()
+  //
+  // Returns the capacity of this instance. All instances have a non-zero
+  // capacity: default and `moved from` instances have a small internal buffer.
+  size_t capacity() const;
+
+  // CordBuffer::IncreaseLengthBy()
+  //
+  // Increases the length of this buffer by the specified 'n' bytes.
+  // Applications must make sure all data in this buffer up to the new length
+  // has been initialized before adding a CordBuffer to a Cord: failure to do so
+  // will lead to undefined behavior.  Requires `length() + n <= capacity()`.
+  // Typically, applications will use 'available_up_to()` to get a span of the
+  // desired capacity, and use `span.size()` to increase the length as in:
+  //   absl::Span<char> span = buffer.available_up_to(desired);
+  //   buffer.IncreaseLengthBy(span.size());
+  //   memcpy(span.data(), src, span.size());
+  //   etc...
+  void IncreaseLengthBy(size_t n);
+
+  // CordBuffer::SetLength()
+  //
+  // Sets the data length of this instance. Applications must make sure all data
+  // of the specified length has been initialized before adding a CordBuffer to
+  // a Cord: failure to do so will lead to undefined behavior.
+  // Setting the length to a small value or zero does not release any memory
+  // held by this CordBuffer instance. Requires `length <= capacity()`.
+  // Applications should preferably use the `IncreaseLengthBy()` method above
+  // in combination with the 'available()` or `available_up_to()` methods.
+  void SetLength(size_t length);
+
+ private:
+  // Make sure we don't accidentally over promise.
+  static_assert(kCustomLimit <= cord_internal::kMaxLargeFlatSize);
+
+  // Assume the cost of an 'uprounded' allocation to CeilPow2(size) versus
+  // the cost of allocating at least 1 extra flat <= 4KB:
+  // - Flat overhead = 13 bytes
+  // - Btree amortized cost / node =~ 13 bytes
+  // - 64 byte granularity of tcmalloc at 4K =~ 32 byte average
+  // CPU cost and efficiency requires we should at least 'save' something by
+  // splitting, as a poor man's measure, we say the slop needs to be
+  // at least double the cost offset to make it worth splitting: ~128 bytes.
+  static constexpr size_t kMaxPageSlop = 128;
+
+  // Overhead for allocation a flat.
+  static constexpr size_t kOverhead = cord_internal::kFlatOverhead;
+
+  using CordRepFlat = cord_internal::CordRepFlat;
+
+  // `Rep` is the internal data representation of a CordBuffer. The internal
+  // representation has an internal small size optimization similar to
+  // std::string (SSO).
+  struct Rep {
+    // Inline SSO size of a CordBuffer
+    static constexpr size_t kInlineCapacity = sizeof(intptr_t) * 2 - 1;
+
+    // Creates a default instance with kInlineCapacity.
+    Rep() : short_rep{} {}
+
+    // Creates an instance managing an allocated non zero CordRep.
+    explicit Rep(cord_internal::CordRepFlat* rep) : long_rep{rep} {
+      assert(rep != nullptr);
+    }
+
+    // Returns true if this instance manages the SSO internal buffer.
+    bool is_short() const {
+      constexpr size_t offset = offsetof(Short, raw_size);
+      return (reinterpret_cast<const char*>(this)[offset] & 1) != 0;
+    }
+
+    // Returns the available area of the internal SSO data
+    absl::Span<char> short_available() {
+      const size_t length = short_length();
+      return absl::Span<char>(short_rep.data + length,
+                              kInlineCapacity - length);
+    }
+
+    // Returns the available area of the internal SSO data
+    absl::Span<char> long_available() const {
+      assert(!is_short());
+      const size_t length = long_rep.rep->length;
+      return absl::Span<char>(long_rep.rep->Data() + length,
+                              long_rep.rep->Capacity() - length);
+    }
+
+    // Returns the length of the internal SSO data.
+    size_t short_length() const {
+      assert(is_short());
+      return static_cast<size_t>(short_rep.raw_size >> 1);
+    }
+
+    // Sets the length of the internal SSO data.
+    // Disregards any previously set CordRep instance.
+    void set_short_length(size_t length) {
+      short_rep.raw_size = static_cast<char>((length << 1) + 1);
+    }
+
+    // Adds `n` to the current short length.
+    void add_short_length(size_t n) {
+      assert(is_short());
+      short_rep.raw_size += static_cast<char>(n << 1);
+    }
+
+    // Returns reference to the internal SSO data buffer.
+    char* data() {
+      assert(is_short());
+      return short_rep.data;
+    }
+    const char* data() const {
+      assert(is_short());
+      return short_rep.data;
+    }
+
+    // Returns a pointer the external CordRep managed by this instance.
+    cord_internal::CordRepFlat* rep() const {
+      assert(!is_short());
+      return long_rep.rep;
+    }
+
+    // The internal representation takes advantage of the fact that allocated
+    // memory is always on an even address, and uses the least significant bit
+    // of the first or last byte (depending on endianness) as the inline size
+    // indicator overlapping with the least significant byte of the CordRep*.
+#if defined(ABSL_IS_BIG_ENDIAN)
+    struct Long {
+      explicit Long(cord_internal::CordRepFlat* rep_arg) : rep(rep_arg) {}
+      void* padding;
+      cord_internal::CordRepFlat* rep;
+    };
+    struct Short {
+      char data[sizeof(Long) - 1];
+      char raw_size = 1;
+    };
+#else
+    struct Long {
+      explicit Long(cord_internal::CordRepFlat* rep_arg) : rep(rep_arg) {}
+      cord_internal::CordRepFlat* rep;
+      void* padding;
+    };
+    struct Short {
+      char raw_size = 1;
+      char data[sizeof(Long) - 1];
+    };
+#endif
+
+    union {
+      Long long_rep;
+      Short short_rep;
+    };
+  };
+
+  // Power2 functions
+  static bool IsPow2(size_t size) { return absl::has_single_bit(size); }
+  static size_t Log2Floor(size_t size) {
+    return static_cast<size_t>(absl::bit_width(size) - 1);
+  }
+  static size_t Log2Ceil(size_t size) {
+    return static_cast<size_t>(absl::bit_width(size - 1));
+  }
+
+  // Implementation of `CreateWithCustomLimit()`.
+  // This implementation allows for future memory allocation hints to
+  // be passed down into the CordRepFlat allocation function.
+  template <typename... AllocationHints>
+  static CordBuffer CreateWithCustomLimitImpl(size_t block_size,
+                                              size_t capacity,
+                                              AllocationHints... hints);
+
+  // Consumes the value contained in this instance and resets the instance.
+  // This method returns a non-null Cordrep* if the current instances manages a
+  // CordRep*, and resets the instance to an empty SSO instance. If the current
+  // instance is an SSO instance, then this method returns nullptr and sets
+  // `short_value` to the inlined data value. In either case, the current
+  // instance length is reset to zero.
+  // This method is intended to be used by Cord internal functions only.
+  cord_internal::CordRep* ConsumeValue(absl::string_view& short_value) {
+    cord_internal::CordRep* rep = nullptr;
+    if (rep_.is_short()) {
+      short_value = absl::string_view(rep_.data(), rep_.short_length());
+    } else {
+      rep = rep_.rep();
+    }
+    rep_.set_short_length(0);
+    return rep;
+  }
+
+  // Internal constructor.
+  explicit CordBuffer(cord_internal::CordRepFlat* rep) : rep_(rep) {
+    assert(rep != nullptr);
+  }
+
+  Rep rep_;
+
+  friend class Cord;
+  friend class CordBufferTestPeer;
+};
+
+inline constexpr size_t CordBuffer::MaximumPayload() {
+  return cord_internal::kMaxFlatLength;
+}
+
+inline constexpr size_t CordBuffer::MaximumPayload(size_t block_size) {
+  return (std::min)(kCustomLimit, block_size) - cord_internal::kFlatOverhead;
+}
+
+inline CordBuffer CordBuffer::CreateWithDefaultLimit(size_t capacity) {
+  if (capacity > Rep::kInlineCapacity) {
+    auto* rep = cord_internal::CordRepFlat::New(capacity);
+    rep->length = 0;
+    return CordBuffer(rep);
+  }
+  return CordBuffer();
+}
+
+template <typename... AllocationHints>
+inline CordBuffer CordBuffer::CreateWithCustomLimitImpl(
+    size_t block_size, size_t capacity, AllocationHints... hints) {
+  assert(IsPow2(block_size));
+  capacity = (std::min)(capacity, kCustomLimit);
+  block_size = (std::min)(block_size, kCustomLimit);
+  if (capacity + kOverhead >= block_size) {
+    capacity = block_size;
+  } else if (capacity <= kDefaultLimit) {
+    capacity = capacity + kOverhead;
+  } else if (!IsPow2(capacity)) {
+    // Check if rounded up to next power 2 is a good enough fit
+    // with limited waste making it an acceptable direct fit.
+    const size_t rounded_up = size_t{1} << Log2Ceil(capacity);
+    const size_t slop = rounded_up - capacity;
+    if (slop >= kOverhead && slop <= kMaxPageSlop + kOverhead) {
+      capacity = rounded_up;
+    } else {
+      // Round down to highest power of 2 <= capacity.
+      // Consider a more aggressive step down if that may reduce the
+      // risk of fragmentation where 'people are holding it wrong'.
+      const size_t rounded_down = size_t{1} << Log2Floor(capacity);
+      capacity = rounded_down;
+    }
+  }
+  const size_t length = capacity - kOverhead;
+  auto* rep = CordRepFlat::New(CordRepFlat::Large(), length, hints...);
+  rep->length = 0;
+  return CordBuffer(rep);
+}
+
+inline CordBuffer CordBuffer::CreateWithCustomLimit(size_t block_size,
+                                                    size_t capacity) {
+  return CreateWithCustomLimitImpl(block_size, capacity);
+}
+
+inline CordBuffer::~CordBuffer() {
+  if (!rep_.is_short()) {
+    cord_internal::CordRepFlat::Delete(rep_.rep());
+  }
+}
+
+inline CordBuffer::CordBuffer(CordBuffer&& rhs) noexcept : rep_(rhs.rep_) {
+  rhs.rep_.set_short_length(0);
+}
+
+inline CordBuffer& CordBuffer::operator=(CordBuffer&& rhs) noexcept {
+  if (!rep_.is_short()) cord_internal::CordRepFlat::Delete(rep_.rep());
+  rep_ = rhs.rep_;
+  rhs.rep_.set_short_length(0);
+  return *this;
+}
+
+inline absl::Span<char> CordBuffer::available() {
+  return rep_.is_short() ? rep_.short_available() : rep_.long_available();
+}
+
+inline absl::Span<char> CordBuffer::available_up_to(size_t size) {
+  return available().subspan(0, size);
+}
+
+inline char* CordBuffer::data() {
+  return rep_.is_short() ? rep_.data() : rep_.rep()->Data();
+}
+
+inline const char* CordBuffer::data() const {
+  return rep_.is_short() ? rep_.data() : rep_.rep()->Data();
+}
+
+inline size_t CordBuffer::capacity() const {
+  return rep_.is_short() ? Rep::kInlineCapacity : rep_.rep()->Capacity();
+}
+
+inline size_t CordBuffer::length() const {
+  return rep_.is_short() ? rep_.short_length() : rep_.rep()->length;
+}
+
+inline void CordBuffer::SetLength(size_t length) {
+  absl::base_internal::HardeningAssertLE(length, capacity());
+  if (rep_.is_short()) {
+    rep_.set_short_length(length);
+  } else {
+    rep_.rep()->length = length;
+  }
+}
+
+inline void CordBuffer::IncreaseLengthBy(size_t n) {
+  absl::base_internal::HardeningAssertLE(n, capacity());
+  absl::base_internal::HardeningAssertLE(length() + n, capacity());
+  if (rep_.is_short()) {
+    rep_.add_short_length(n);
+  } else {
+    rep_.rep()->length += n;
+  }
+}
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STRINGS_CORD_BUFFER_H_
diff --git a/absl/strings/cord_buffer_test.cc b/absl/strings/cord_buffer_test.cc
new file mode 100644
index 00000000000..c1b2396536e
--- /dev/null
+++ b/absl/strings/cord_buffer_test.cc
@@ -0,0 +1,321 @@
+// Copyright 2021 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/strings/cord_buffer.h"
+
+#include <algorithm>
+#include <cstring>
+#include <limits>
+#include <string>
+#include <utility>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+#include "absl/strings/internal/cord_internal.h"
+#include "absl/strings/internal/cord_rep_flat.h"
+#include "absl/strings/internal/cord_rep_test_util.h"
+#include "absl/strings/string_view.h"
+#include "absl/types/span.h"
+
+using testing::Eq;
+using testing::Ge;
+using testing::Le;
+using testing::Ne;
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+class CordBufferTestPeer {
+ public:
+  static cord_internal::CordRep* ConsumeValue(CordBuffer& buffer,
+                                              absl::string_view& short_value) {
+    return buffer.ConsumeValue(short_value);
+  }
+};
+
+namespace {
+
+using ::absl::cordrep_testing::CordToString;
+
+constexpr size_t kInlinedSize = sizeof(CordBuffer) - 1;
+constexpr size_t kDefaultLimit = CordBuffer::kDefaultLimit;
+constexpr size_t kCustomLimit = CordBuffer::kCustomLimit;
+constexpr size_t kMaxFlatSize = cord_internal::kMaxFlatSize;
+constexpr size_t kMaxFlatLength = cord_internal::kMaxFlatLength;
+constexpr size_t kFlatOverhead = cord_internal::kFlatOverhead;
+
+constexpr size_t k8KiB = 8 << 10;
+constexpr size_t k16KiB = 16 << 10;
+constexpr size_t k64KiB = 64 << 10;
+constexpr size_t k1MB = 1 << 20;
+
+class CordBufferTest : public testing::TestWithParam<size_t> {};
+
+INSTANTIATE_TEST_SUITE_P(MediumSize, CordBufferTest,
+                         testing::Values(1, kInlinedSize - 1, kInlinedSize,
+                                         kInlinedSize + 1, kDefaultLimit - 1,
+                                         kDefaultLimit));
+
+TEST_P(CordBufferTest, MaximumPayload) {
+  EXPECT_THAT(CordBuffer::MaximumPayload(), Eq(kMaxFlatLength));
+  EXPECT_THAT(CordBuffer::MaximumPayload(512), Eq(512 - kFlatOverhead));
+  EXPECT_THAT(CordBuffer::MaximumPayload(k64KiB), Eq(k64KiB - kFlatOverhead));
+  EXPECT_THAT(CordBuffer::MaximumPayload(k1MB), Eq(k64KiB - kFlatOverhead));
+}
+
+TEST(CordBufferTest, ConstructDefault) {
+  CordBuffer buffer;
+  EXPECT_THAT(buffer.capacity(), Eq(sizeof(CordBuffer) - 1));
+  EXPECT_THAT(buffer.length(), Eq(0));
+  EXPECT_THAT(buffer.data(), Ne(nullptr));
+  EXPECT_THAT(buffer.available().data(), Eq(buffer.data()));
+  EXPECT_THAT(buffer.available().size(), Eq(buffer.capacity()));
+  memset(buffer.data(), 0xCD, buffer.capacity());
+}
+
+TEST(CordBufferTest, CreateSsoWithDefaultLimit) {
+  CordBuffer buffer = CordBuffer::CreateWithDefaultLimit(3);
+  EXPECT_THAT(buffer.capacity(), Ge(3));
+  EXPECT_THAT(buffer.capacity(), Le(sizeof(CordBuffer)));
+  EXPECT_THAT(buffer.length(), Eq(0));
+  memset(buffer.data(), 0xCD, buffer.capacity());
+
+  memcpy(buffer.data(), "Abc", 3);
+  buffer.SetLength(3);
+  EXPECT_THAT(buffer.length(), Eq(3));
+  absl::string_view short_value;
+  EXPECT_THAT(CordBufferTestPeer::ConsumeValue(buffer, short_value),
+              Eq(nullptr));
+  EXPECT_THAT(absl::string_view(buffer.data(), 3), Eq("Abc"));
+  EXPECT_THAT(short_value, Eq("Abc"));
+}
+
+TEST_P(CordBufferTest, Available) {
+  const size_t requested = GetParam();
+  CordBuffer buffer = CordBuffer::CreateWithDefaultLimit(requested);
+  EXPECT_THAT(buffer.available().data(), Eq(buffer.data()));
+  EXPECT_THAT(buffer.available().size(), Eq(buffer.capacity()));
+
+  buffer.SetLength(2);
+  EXPECT_THAT(buffer.available().data(), Eq(buffer.data() + 2));
+  EXPECT_THAT(buffer.available().size(), Eq(buffer.capacity() - 2));
+}
+
+TEST_P(CordBufferTest, IncreaseLengthBy) {
+  const size_t requested = GetParam();
+  CordBuffer buffer = CordBuffer::CreateWithDefaultLimit(requested);
+  buffer.IncreaseLengthBy(2);
+  EXPECT_THAT(buffer.length(), Eq(2));
+  buffer.IncreaseLengthBy(5);
+  EXPECT_THAT(buffer.length(), Eq(7));
+}
+
+TEST_P(CordBufferTest, AvailableUpTo) {
+  const size_t requested = GetParam();
+  CordBuffer buffer = CordBuffer::CreateWithDefaultLimit(requested);
+  size_t expected_up_to = std::min<size_t>(3, buffer.capacity());
+  EXPECT_THAT(buffer.available_up_to(3).data(), Eq(buffer.data()));
+  EXPECT_THAT(buffer.available_up_to(3).size(), Eq(expected_up_to));
+
+  buffer.SetLength(2);
+  expected_up_to = std::min<size_t>(3, buffer.capacity() - 2);
+  EXPECT_THAT(buffer.available_up_to(3).data(), Eq(buffer.data() + 2));
+  EXPECT_THAT(buffer.available_up_to(3).size(), Eq(expected_up_to));
+}
+
+// Returns the maximum capacity for a given block_size and requested size.
+size_t MaxCapacityFor(size_t block_size, size_t requested) {
+  requested = (std::min)(requested, cord_internal::kMaxLargeFlatSize);
+  // Maximum returned size is always capped at block_size - kFlatOverhead.
+  return block_size - kFlatOverhead;
+}
+
+TEST_P(CordBufferTest, CreateWithDefaultLimit) {
+  const size_t requested = GetParam();
+  CordBuffer buffer = CordBuffer::CreateWithDefaultLimit(requested);
+  EXPECT_THAT(buffer.capacity(), Ge(requested));
+  EXPECT_THAT(buffer.capacity(), Le(MaxCapacityFor(kMaxFlatSize, requested)));
+  EXPECT_THAT(buffer.length(), Eq(0));
+
+  memset(buffer.data(), 0xCD, buffer.capacity());
+
+  std::string data(requested - 1, 'x');
+  memcpy(buffer.data(), data.c_str(), requested);
+  buffer.SetLength(requested);
+
+  EXPECT_THAT(buffer.length(), Eq(requested));
+  EXPECT_THAT(absl::string_view(buffer.data()), Eq(data));
+}
+
+TEST(CordBufferTest, CreateWithDefaultLimitAskingFor2GB) {
+  constexpr size_t k2GiB = 1U << 31;
+  CordBuffer buffer = CordBuffer::CreateWithDefaultLimit(k2GiB);
+  // Expect to never be awarded more than a reasonable memory size, even in
+  // cases where a (debug) memory allocator may grant us somewhat more memory
+  // than `kDefaultLimit` which should be no more than `2 * kDefaultLimit`
+  EXPECT_THAT(buffer.capacity(), Le(2 * CordBuffer::kDefaultLimit));
+  EXPECT_THAT(buffer.length(), Eq(0));
+  EXPECT_THAT(buffer.data(), Ne(nullptr));
+  memset(buffer.data(), 0xCD, buffer.capacity());
+}
+
+TEST_P(CordBufferTest, MoveConstruct) {
+  const size_t requested = GetParam();
+  CordBuffer from = CordBuffer::CreateWithDefaultLimit(requested);
+  const size_t capacity = from.capacity();
+  memcpy(from.data(), "Abc", 4);
+  from.SetLength(4);
+
+  CordBuffer to(std::move(from));
+  EXPECT_THAT(to.capacity(), Eq(capacity));
+  EXPECT_THAT(to.length(), Eq(4));
+  EXPECT_THAT(absl::string_view(to.data()), Eq("Abc"));
+
+  EXPECT_THAT(from.length(), Eq(0));  // NOLINT
+}
+
+TEST_P(CordBufferTest, MoveAssign) {
+  const size_t requested = GetParam();
+  CordBuffer from = CordBuffer::CreateWithDefaultLimit(requested);
+  const size_t capacity = from.capacity();
+  memcpy(from.data(), "Abc", 4);
+  from.SetLength(4);
+
+  CordBuffer to;
+  to = std::move(from);
+  EXPECT_THAT(to.capacity(), Eq(capacity));
+  EXPECT_THAT(to.length(), Eq(4));
+  EXPECT_THAT(absl::string_view(to.data()), Eq("Abc"));
+
+  EXPECT_THAT(from.length(), Eq(0));  // NOLINT
+}
+
+TEST_P(CordBufferTest, ConsumeValue) {
+  const size_t requested = GetParam();
+  CordBuffer buffer = CordBuffer::CreateWithDefaultLimit(requested);
+  memcpy(buffer.data(), "Abc", 4);
+  buffer.SetLength(3);
+
+  absl::string_view short_value;
+  if (cord_internal::CordRep* rep =
+          CordBufferTestPeer::ConsumeValue(buffer, short_value)) {
+    EXPECT_THAT(CordToString(rep), Eq("Abc"));
+    cord_internal::CordRep::Unref(rep);
+  } else {
+    EXPECT_THAT(short_value, Eq("Abc"));
+  }
+  EXPECT_THAT(buffer.length(), Eq(0));
+}
+
+TEST_P(CordBufferTest, CreateWithCustomLimitWithinDefaultLimit) {
+  const size_t requested = GetParam();
+  CordBuffer buffer =
+      CordBuffer::CreateWithCustomLimit(kMaxFlatSize, requested);
+  EXPECT_THAT(buffer.capacity(), Ge(requested));
+  EXPECT_THAT(buffer.capacity(), Le(MaxCapacityFor(kMaxFlatSize, requested)));
+  EXPECT_THAT(buffer.length(), Eq(0));
+
+  memset(buffer.data(), 0xCD, buffer.capacity());
+
+  std::string data(requested - 1, 'x');
+  memcpy(buffer.data(), data.c_str(), requested);
+  buffer.SetLength(requested);
+
+  EXPECT_THAT(buffer.length(), Eq(requested));
+  EXPECT_THAT(absl::string_view(buffer.data()), Eq(data));
+}
+
+TEST(CordLargeBufferTest, CreateAtOrBelowDefaultLimit) {
+  CordBuffer buffer = CordBuffer::CreateWithCustomLimit(k64KiB, kDefaultLimit);
+  EXPECT_THAT(buffer.capacity(), Ge(kDefaultLimit));
+  EXPECT_THAT(buffer.capacity(),
+              Le(MaxCapacityFor(kMaxFlatSize, kDefaultLimit)));
+
+  buffer = CordBuffer::CreateWithCustomLimit(k64KiB, 3178);
+  EXPECT_THAT(buffer.capacity(), Ge(3178));
+}
+
+TEST(CordLargeBufferTest, CreateWithCustomLimit) {
+  ASSERT_THAT((kMaxFlatSize & (kMaxFlatSize - 1)) == 0, "Must be power of 2");
+
+  for (size_t size = kMaxFlatSize; size <= kCustomLimit; size *= 2) {
+    CordBuffer buffer = CordBuffer::CreateWithCustomLimit(size, size);
+    size_t expected = size - kFlatOverhead;
+    ASSERT_THAT(buffer.capacity(), Ge(expected));
+    EXPECT_THAT(buffer.capacity(), Le(MaxCapacityFor(size, expected)));
+  }
+}
+
+TEST(CordLargeBufferTest, CreateWithTooLargeLimit) {
+  CordBuffer buffer = CordBuffer::CreateWithCustomLimit(k64KiB, k1MB);
+  ASSERT_THAT(buffer.capacity(), Ge(k64KiB - kFlatOverhead));
+  EXPECT_THAT(buffer.capacity(), Le(MaxCapacityFor(k64KiB, k1MB)));
+}
+
+TEST(CordLargeBufferTest, CreateWithHugeValueForOverFlowHardening) {
+  for (size_t dist_from_max = 0; dist_from_max <= 32; ++dist_from_max) {
+    size_t capacity = std::numeric_limits<size_t>::max() - dist_from_max;
+
+    CordBuffer buffer = CordBuffer::CreateWithDefaultLimit(capacity);
+    ASSERT_THAT(buffer.capacity(), Ge(kDefaultLimit));
+    EXPECT_THAT(buffer.capacity(), Le(MaxCapacityFor(kMaxFlatSize, capacity)));
+
+    for (size_t limit = kMaxFlatSize; limit <= kCustomLimit; limit *= 2) {
+      CordBuffer buffer = CordBuffer::CreateWithCustomLimit(limit, capacity);
+      ASSERT_THAT(buffer.capacity(), Ge(limit - kFlatOverhead));
+      EXPECT_THAT(buffer.capacity(), Le(MaxCapacityFor(limit, capacity)));
+    }
+  }
+}
+
+TEST(CordLargeBufferTest, CreateWithSmallLimit) {
+  CordBuffer buffer = CordBuffer::CreateWithCustomLimit(512, 1024);
+  ASSERT_THAT(buffer.capacity(), Ge(512 - kFlatOverhead));
+  EXPECT_THAT(buffer.capacity(), Le(MaxCapacityFor(512, 1024)));
+
+  // Ask for precise block size, should return size - kOverhead
+  buffer = CordBuffer::CreateWithCustomLimit(512, 512);
+  ASSERT_THAT(buffer.capacity(), Ge(512 - kFlatOverhead));
+  EXPECT_THAT(buffer.capacity(), Le(MaxCapacityFor(512, 512)));
+
+  // Corner case: 511 < block_size, but 511 + kOverhead is above
+  buffer = CordBuffer::CreateWithCustomLimit(512, 511);
+  ASSERT_THAT(buffer.capacity(), Ge(512 - kFlatOverhead));
+  EXPECT_THAT(buffer.capacity(), Le(MaxCapacityFor(512, 511)));
+
+  // Corner case: 498 + kOverhead < block_size
+  buffer = CordBuffer::CreateWithCustomLimit(512, 498);
+  ASSERT_THAT(buffer.capacity(), Ge(512 - kFlatOverhead));
+  EXPECT_THAT(buffer.capacity(), Le(MaxCapacityFor(512, 498)));
+}
+
+TEST(CordLargeBufferTest, CreateWasteFull) {
+  // 15 KiB gets rounded down to next pow2 value.
+  const size_t requested = (15 << 10);
+  CordBuffer buffer = CordBuffer::CreateWithCustomLimit(k16KiB, requested);
+  ASSERT_THAT(buffer.capacity(), Ge(k8KiB - kFlatOverhead));
+  EXPECT_THAT(buffer.capacity(), Le(MaxCapacityFor(k8KiB, requested)));
+}
+
+TEST(CordLargeBufferTest, CreateSmallSlop) {
+  const size_t requested = k16KiB - 2 * kFlatOverhead;
+  CordBuffer buffer = CordBuffer::CreateWithCustomLimit(k16KiB, requested);
+  ASSERT_THAT(buffer.capacity(), Ge(k16KiB - kFlatOverhead));
+  EXPECT_THAT(buffer.capacity(), Le(MaxCapacityFor(k16KiB, requested)));
+}
+
+}  // namespace
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/strings/cord_test.cc b/absl/strings/cord_test.cc
new file mode 100644
index 00000000000..1ff172ee244
--- /dev/null
+++ b/absl/strings/cord_test.cc
@@ -0,0 +1,3414 @@
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/strings/cord.h"
+
+#include <algorithm>
+#include <array>
+#include <cassert>
+#include <cstddef>
+#include <cstdint>
+#include <cstdio>
+#include <cstring>
+#include <iostream>
+#include <iterator>
+#include <limits>
+#include <optional>
+#include <random>
+#include <set>
+#include <sstream>
+#include <string>
+#include <type_traits>
+#include <utility>
+#include <vector>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/endian.h"
+#include "absl/base/macros.h"
+#include "absl/base/no_destructor.h"
+#include "absl/base/options.h"
+#include "absl/container/fixed_array.h"
+#include "absl/functional/function_ref.h"
+#include "absl/hash/hash.h"
+#include "absl/hash/hash_testing.h"
+#include "absl/log/check.h"
+#include "absl/log/log.h"
+#include "absl/random/random.h"
+#include "absl/strings/cord_buffer.h"
+#include "absl/strings/cord_test_helpers.h"
+#include "absl/strings/cordz_test_helpers.h"
+#include "absl/strings/internal/cord_internal.h"
+#include "absl/strings/internal/cord_rep_crc.h"
+#include "absl/strings/internal/cord_rep_flat.h"
+#include "absl/strings/internal/cordz_statistics.h"
+#include "absl/strings/internal/cordz_update_tracker.h"
+#include "absl/strings/internal/string_constant.h"
+#include "absl/strings/match.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/str_format.h"
+#include "absl/strings/string_view.h"
+#include "absl/types/compare.h"
+#include "absl/types/span.h"
+
+// convenience local constants
+static constexpr auto FLAT = absl::cord_internal::FLAT;
+static constexpr auto MAX_FLAT_TAG = absl::cord_internal::MAX_FLAT_TAG;
+
+typedef std::mt19937_64 RandomEngine;
+
+using absl::cord_internal::CordRep;
+using absl::cord_internal::CordRepBtree;
+using absl::cord_internal::CordRepConcat;
+using absl::cord_internal::CordRepCrc;
+using absl::cord_internal::CordRepExternal;
+using absl::cord_internal::CordRepFlat;
+using absl::cord_internal::CordRepSubstring;
+using absl::cord_internal::CordzUpdateTracker;
+using absl::cord_internal::kFlatOverhead;
+using absl::cord_internal::kMaxFlatLength;
+using ::testing::ElementsAre;
+using ::testing::Le;
+
+static std::string RandomLowercaseString(RandomEngine* rng);
+static std::string RandomLowercaseString(RandomEngine* rng, size_t length);
+
+static int GetUniformRandomUpTo(RandomEngine* rng, int upper_bound) {
+  if (upper_bound > 0) {
+    std::uniform_int_distribution<int> uniform(0, upper_bound - 1);
+    return uniform(*rng);
+  } else {
+    return 0;
+  }
+}
+
+static size_t GetUniformRandomUpTo(RandomEngine* rng, size_t upper_bound) {
+  if (upper_bound > 0) {
+    std::uniform_int_distribution<size_t> uniform(0, upper_bound - 1);
+    return uniform(*rng);
+  } else {
+    return 0;
+  }
+}
+
+static int32_t GenerateSkewedRandom(RandomEngine* rng, int max_log) {
+  const uint32_t base = (*rng)() % (max_log + 1);
+  const uint32_t mask = ((base < 32) ? (1u << base) : 0u) - 1u;
+  return (*rng)() & mask;
+}
+
+static std::string RandomLowercaseString(RandomEngine* rng) {
+  int length;
+  std::bernoulli_distribution one_in_1k(0.001);
+  std::bernoulli_distribution one_in_10k(0.0001);
+  // With low probability, make a large fragment
+  if (one_in_10k(*rng)) {
+    length = GetUniformRandomUpTo(rng, 1048576);
+  } else if (one_in_1k(*rng)) {
+    length = GetUniformRandomUpTo(rng, 10000);
+  } else {
+    length = GenerateSkewedRandom(rng, 10);
+  }
+  return RandomLowercaseString(rng, length);
+}
+
+static std::string RandomLowercaseString(RandomEngine* rng, size_t length) {
+  std::string result(length, '\0');
+  std::uniform_int_distribution<int> chars('a', 'z');
+  std::generate(result.begin(), result.end(),
+                [&]() { return static_cast<char>(chars(*rng)); });
+  return result;
+}
+
+static void DoNothing(absl::string_view /* data */, void* /* arg */) {}
+
+static void DeleteExternalString(absl::string_view data, void* arg) {
+  std::string* s = reinterpret_cast<std::string*>(arg);
+  EXPECT_EQ(data, *s);
+  delete s;
+}
+
+// Add "s" to *dst via `MakeCordFromExternal`
+static void AddExternalMemory(absl::string_view s, absl::Cord* dst) {
+  std::string* str = new std::string(s.data(), s.size());
+  dst->Append(absl::MakeCordFromExternal(*str, [str](absl::string_view data) {
+    DeleteExternalString(data, str);
+  }));
+}
+
+static void DumpGrowth() {
+  absl::Cord str;
+  for (int i = 0; i < 1000; i++) {
+    char c = 'a' + i % 26;
+    str.Append(absl::string_view(&c, 1));
+  }
+}
+
+// Make a Cord with some number of fragments.  Return the size (in bytes)
+// of the smallest fragment.
+static size_t AppendWithFragments(const std::string& s, RandomEngine* rng,
+                                  absl::Cord* cord) {
+  size_t j = 0;
+  const size_t max_size = s.size() / 5;  // Make approx. 10 fragments
+  size_t min_size = max_size;            // size of smallest fragment
+  while (j < s.size()) {
+    size_t N = 1 + GetUniformRandomUpTo(rng, max_size);
+    if (N > (s.size() - j)) {
+      N = s.size() - j;
+    }
+    if (N < min_size) {
+      min_size = N;
+    }
+
+    std::bernoulli_distribution coin_flip(0.5);
+    if (coin_flip(*rng)) {
+      // Grow by adding an external-memory.
+      AddExternalMemory(absl::string_view(s.data() + j, N), cord);
+    } else {
+      cord->Append(absl::string_view(s.data() + j, N));
+    }
+    j += N;
+  }
+  return min_size;
+}
+
+// Add an external memory that contains the specified std::string to cord
+static void AddNewStringBlock(const std::string& str, absl::Cord* dst) {
+  char* data = new char[str.size()];
+  memcpy(data, str.data(), str.size());
+  dst->Append(absl::MakeCordFromExternal(
+      absl::string_view(data, str.size()),
+      [](absl::string_view s) { delete[] s.data(); }));
+}
+
+// Make a Cord out of many different types of nodes.
+static absl::Cord MakeComposite() {
+  absl::Cord cord;
+  cord.Append("the");
+  AddExternalMemory(" quick brown", &cord);
+  AddExternalMemory(" fox jumped", &cord);
+
+  absl::Cord full(" over");
+  AddExternalMemory(" the lazy", &full);
+  AddNewStringBlock(" dog slept the whole day away", &full);
+  absl::Cord substring = full.Subcord(0, 18);
+
+  // Make substring long enough to defeat the copying fast path in Append.
+  substring.Append(std::string(1000, '.'));
+  cord.Append(substring);
+  cord = cord.Subcord(0, cord.size() - 998);  // Remove most of extra junk
+
+  return cord;
+}
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+class CordTestPeer {
+ public:
+  static void ForEachChunk(
+      const Cord& c, absl::FunctionRef<void(absl::string_view)> callback) {
+    c.ForEachChunk(callback);
+  }
+
+  static bool IsTree(const Cord& c) { return c.contents_.is_tree(); }
+  static CordRep* Tree(const Cord& c) { return c.contents_.tree(); }
+
+  static cord_internal::CordzInfo* GetCordzInfo(const Cord& c) {
+    return c.contents_.cordz_info();
+  }
+
+  static Cord MakeSubstring(Cord src, size_t offset, size_t length) {
+    CHECK(src.contents_.is_tree()) << "Can not be inlined";
+    CHECK(!src.ExpectedChecksum().has_value()) << "Can not be hardened";
+    Cord cord;
+    auto* tree = cord_internal::SkipCrcNode(src.contents_.tree());
+    auto* rep = CordRepSubstring::Create(CordRep::Ref(tree), offset, length);
+    cord.contents_.EmplaceTree(rep, CordzUpdateTracker::kSubCord);
+    return cord;
+  }
+};
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+// The CordTest fixture runs all tests with and without expected CRCs being set
+// on the subject Cords.
+class CordTest : public testing::TestWithParam<bool /*useCrc*/> {
+ public:
+  // Returns true if test is running with Crc enabled.
+  bool UseCrc() const { return GetParam(); }
+  void MaybeHarden(absl::Cord& c) {
+    if (UseCrc()) {
+      c.SetExpectedChecksum(1);
+    }
+  }
+  absl::Cord MaybeHardened(absl::Cord c) {
+    MaybeHarden(c);
+    return c;
+  }
+
+  // Returns human readable string representation of the test parameter.
+  static std::string ToString(testing::TestParamInfo<bool> useCrc) {
+    if (useCrc.param) {
+      return "BtreeHardened";
+    } else {
+      return "Btree";
+    }
+  }
+};
+
+INSTANTIATE_TEST_SUITE_P(WithParam, CordTest, testing::Bool(),
+                         CordTest::ToString);
+
+TEST(CordRepFlat, AllFlatCapacities) {
+  // Explicitly and redundantly assert built-in min/max limits
+  static_assert(absl::cord_internal::kFlatOverhead < 32);
+  static_assert(absl::cord_internal::kMinFlatSize == 32);
+  static_assert(absl::cord_internal::kMaxLargeFlatSize == 256 << 10);
+  EXPECT_EQ(absl::cord_internal::TagToAllocatedSize(FLAT), 32);
+  EXPECT_EQ(absl::cord_internal::TagToAllocatedSize(MAX_FLAT_TAG), 256 << 10);
+
+  // Verify all tags to map perfectly back and forth, and
+  // that sizes are monotonically increasing.
+  size_t last_size = 0;
+  for (int tag = FLAT; tag <= MAX_FLAT_TAG; ++tag) {
+    size_t size = absl::cord_internal::TagToAllocatedSize(tag);
+    ASSERT_GT(size, last_size);
+    ASSERT_EQ(absl::cord_internal::TagToAllocatedSize(tag), size);
+    last_size = size;
+  }
+
+  // All flat size from 32 - 512 are 8 byte granularity
+  for (size_t size = 32; size <= 512; size += 8) {
+    ASSERT_EQ(absl::cord_internal::RoundUpForTag(size), size);
+    uint8_t tag = absl::cord_internal::AllocatedSizeToTag(size);
+    ASSERT_EQ(absl::cord_internal::TagToAllocatedSize(tag), size);
+  }
+
+  // All flat sizes from 512 - 8192 are 64 byte granularity
+  for (size_t size = 512; size <= 8192; size += 64) {
+    ASSERT_EQ(absl::cord_internal::RoundUpForTag(size), size);
+    uint8_t tag = absl::cord_internal::AllocatedSizeToTag(size);
+    ASSERT_EQ(absl::cord_internal::TagToAllocatedSize(tag), size);
+  }
+
+  // All flat sizes from 8KB to 256KB are 4KB granularity
+  for (size_t size = 8192; size <= 256 * 1024; size += 4 * 1024) {
+    ASSERT_EQ(absl::cord_internal::RoundUpForTag(size), size);
+    uint8_t tag = absl::cord_internal::AllocatedSizeToTag(size);
+    ASSERT_EQ(absl::cord_internal::TagToAllocatedSize(tag), size);
+  }
+}
+
+TEST(CordRepFlat, MaxFlatSize) {
+  CordRepFlat* flat = CordRepFlat::New(kMaxFlatLength);
+  EXPECT_EQ(flat->Capacity(), kMaxFlatLength);
+  CordRep::Unref(flat);
+
+  flat = CordRepFlat::New(kMaxFlatLength * 4);
+  EXPECT_EQ(flat->Capacity(), kMaxFlatLength);
+  CordRep::Unref(flat);
+}
+
+TEST(CordRepFlat, MaxLargeFlatSize) {
+  const size_t size = 256 * 1024 - kFlatOverhead;
+  CordRepFlat* flat = CordRepFlat::New(CordRepFlat::Large(), size);
+  EXPECT_GE(flat->Capacity(), size);
+  CordRep::Unref(flat);
+}
+
+TEST(CordRepFlat, AllFlatSizes) {
+  const size_t kMaxSize = 256 * 1024;
+  for (size_t size = 32; size <= kMaxSize; size *=2) {
+    const size_t length = size - kFlatOverhead - 1;
+    CordRepFlat* flat = CordRepFlat::New(CordRepFlat::Large(), length);
+    EXPECT_GE(flat->Capacity(), length);
+    memset(flat->Data(), 0xCD, flat->Capacity());
+    CordRep::Unref(flat);
+  }
+}
+
+TEST_P(CordTest, AllFlatSizes) {
+  using absl::strings_internal::CordTestAccess;
+
+  for (size_t s = 0; s < CordTestAccess::MaxFlatLength(); s++) {
+    // Make a string of length s.
+    std::string src;
+    while (src.size() < s) {
+      src.push_back('a' + (src.size() % 26));
+    }
+
+    absl::Cord dst(src);
+    MaybeHarden(dst);
+    EXPECT_EQ(std::string(dst), src) << s;
+  }
+}
+
+// We create a Cord at least 128GB in size using the fact that Cords can
+// internally reference-count; thus the Cord is enormous without actually
+// consuming very much memory.
+TEST_P(CordTest, GigabyteCordFromExternal) {
+  const size_t one_gig = 1024U * 1024U * 1024U;
+  size_t max_size = 2 * one_gig;
+  if (sizeof(max_size) > 4) max_size = 128 * one_gig;
+
+  size_t length = 128 * 1024;
+  char* data = new char[length];
+  absl::Cord from = absl::MakeCordFromExternal(
+      absl::string_view(data, length),
+      [](absl::string_view sv) { delete[] sv.data(); });
+
+  // This loop may seem odd due to its combination of exponential doubling of
+  // size and incremental size increases.  We do it incrementally to be sure the
+  // Cord will need rebalancing and will exercise code that, in the past, has
+  // caused crashes in production.  We grow exponentially so that the code will
+  // execute in a reasonable amount of time.
+  absl::Cord c;
+  c.Append(from);
+  while (c.size() < max_size) {
+    c.Append(c);
+    c.Append(from);
+    c.Append(from);
+    c.Append(from);
+    c.Append(from);
+    MaybeHarden(c);
+  }
+
+  for (int i = 0; i < 1024; ++i) {
+    c.Append(from);
+  }
+  LOG(INFO) << "Made a Cord with " << c.size() << " bytes!";
+  // Note: on a 32-bit build, this comes out to   2,818,048,000 bytes.
+  // Note: on a 64-bit build, this comes out to 171,932,385,280 bytes.
+}
+
+static absl::Cord MakeExternalCord(int size) {
+  char* buffer = new char[size];
+  memset(buffer, 'x', size);
+  absl::Cord cord;
+  cord.Append(absl::MakeCordFromExternal(
+      absl::string_view(buffer, size),
+      [](absl::string_view s) { delete[] s.data(); }));
+  return cord;
+}
+
+// Extern to fool clang that this is not constant. Needed to suppress
+// a warning of unsafe code we want to test.
+extern bool my_unique_true_boolean;
+bool my_unique_true_boolean = true;
+
+TEST_P(CordTest, Assignment) {
+  absl::Cord x(absl::string_view("hi there"));
+  absl::Cord y(x);
+  MaybeHarden(y);
+  ASSERT_EQ(x.ExpectedChecksum(), std::nullopt);
+  ASSERT_EQ(std::string(x), "hi there");
+  ASSERT_EQ(std::string(y), "hi there");
+  ASSERT_TRUE(x == y);
+  ASSERT_TRUE(x <= y);
+  ASSERT_TRUE(y <= x);
+
+  x = absl::string_view("foo");
+  ASSERT_EQ(std::string(x), "foo");
+  ASSERT_EQ(std::string(y), "hi there");
+  ASSERT_TRUE(x < y);
+  ASSERT_TRUE(y > x);
+  ASSERT_TRUE(x != y);
+  ASSERT_TRUE(x <= y);
+  ASSERT_TRUE(y >= x);
+
+  x = "foo";
+  ASSERT_EQ(x, "foo");
+
+  // Test that going from inline rep to tree we don't leak memory.
+  std::vector<std::pair<absl::string_view, absl::string_view>>
+      test_string_pairs = {{"hi there", "foo"},
+                           {"loooooong coooooord", "short cord"},
+                           {"short cord", "loooooong coooooord"},
+                           {"loooooong coooooord1", "loooooong coooooord2"}};
+  for (std::pair<absl::string_view, absl::string_view> test_strings :
+       test_string_pairs) {
+    absl::Cord tmp(test_strings.first);
+    absl::Cord z(std::move(tmp));
+    ASSERT_EQ(std::string(z), test_strings.first);
+    tmp = test_strings.second;
+    z = std::move(tmp);
+    ASSERT_EQ(std::string(z), test_strings.second);
+  }
+  {
+    // Test that self-move assignment doesn't crash/leak.
+    // Do not write such code!
+    absl::Cord my_small_cord("foo");
+    absl::Cord my_big_cord("loooooong coooooord");
+    // Bypass clang's warning on self move-assignment.
+    absl::Cord* my_small_alias =
+        my_unique_true_boolean ? &my_small_cord : &my_big_cord;
+    absl::Cord* my_big_alias =
+        !my_unique_true_boolean ? &my_small_cord : &my_big_cord;
+
+    *my_small_alias = std::move(my_small_cord);
+    *my_big_alias = std::move(my_big_cord);
+    // my_small_cord and my_big_cord are in an unspecified but valid
+    // state, and will be correctly destroyed here.
+  }
+}
+
+TEST_P(CordTest, StartsEndsWith) {
+  absl::Cord x(absl::string_view("abcde"));
+  MaybeHarden(x);
+  absl::Cord empty("");
+
+  ASSERT_TRUE(x.StartsWith(absl::Cord("abcde")));
+  ASSERT_TRUE(x.StartsWith(absl::Cord("abc")));
+  ASSERT_TRUE(x.StartsWith(absl::Cord("")));
+  ASSERT_TRUE(empty.StartsWith(absl::Cord("")));
+  ASSERT_TRUE(x.EndsWith(absl::Cord("abcde")));
+  ASSERT_TRUE(x.EndsWith(absl::Cord("cde")));
+  ASSERT_TRUE(x.EndsWith(absl::Cord("")));
+  ASSERT_TRUE(empty.EndsWith(absl::Cord("")));
+
+  ASSERT_TRUE(!x.StartsWith(absl::Cord("xyz")));
+  ASSERT_TRUE(!empty.StartsWith(absl::Cord("xyz")));
+  ASSERT_TRUE(!x.EndsWith(absl::Cord("xyz")));
+  ASSERT_TRUE(!empty.EndsWith(absl::Cord("xyz")));
+
+  ASSERT_TRUE(x.StartsWith("abcde"));
+  ASSERT_TRUE(x.StartsWith("abc"));
+  ASSERT_TRUE(x.StartsWith(""));
+  ASSERT_TRUE(empty.StartsWith(""));
+  ASSERT_TRUE(x.EndsWith("abcde"));
+  ASSERT_TRUE(x.EndsWith("cde"));
+  ASSERT_TRUE(x.EndsWith(""));
+  ASSERT_TRUE(empty.EndsWith(""));
+
+  ASSERT_TRUE(!x.StartsWith("xyz"));
+  ASSERT_TRUE(!empty.StartsWith("xyz"));
+  ASSERT_TRUE(!x.EndsWith("xyz"));
+  ASSERT_TRUE(!empty.EndsWith("xyz"));
+}
+
+TEST_P(CordTest, Contains) {
+  auto flat_haystack = absl::Cord("this is a flat cord");
+  auto fragmented_haystack = absl::MakeFragmentedCord(
+      {"this", " ", "is", " ", "a", " ", "fragmented", " ", "cord"});
+
+  EXPECT_TRUE(flat_haystack.Contains(""));
+  EXPECT_TRUE(fragmented_haystack.Contains(""));
+  EXPECT_TRUE(flat_haystack.Contains(absl::Cord("")));
+  EXPECT_TRUE(fragmented_haystack.Contains(absl::Cord("")));
+  EXPECT_TRUE(absl::Cord("").Contains(""));
+  EXPECT_TRUE(absl::Cord("").Contains(absl::Cord("")));
+  EXPECT_FALSE(absl::Cord("").Contains(flat_haystack));
+  EXPECT_FALSE(absl::Cord("").Contains(fragmented_haystack));
+
+  EXPECT_FALSE(flat_haystack.Contains("z"));
+  EXPECT_FALSE(fragmented_haystack.Contains("z"));
+  EXPECT_FALSE(flat_haystack.Contains(absl::Cord("z")));
+  EXPECT_FALSE(fragmented_haystack.Contains(absl::Cord("z")));
+
+  EXPECT_FALSE(flat_haystack.Contains("is an"));
+  EXPECT_FALSE(fragmented_haystack.Contains("is an"));
+  EXPECT_FALSE(flat_haystack.Contains(absl::Cord("is an")));
+  EXPECT_FALSE(fragmented_haystack.Contains(absl::Cord("is an")));
+  EXPECT_FALSE(
+      flat_haystack.Contains(absl::MakeFragmentedCord({"is", " ", "an"})));
+  EXPECT_FALSE(fragmented_haystack.Contains(
+      absl::MakeFragmentedCord({"is", " ", "an"})));
+
+  EXPECT_TRUE(flat_haystack.Contains("is a"));
+  EXPECT_TRUE(fragmented_haystack.Contains("is a"));
+  EXPECT_TRUE(flat_haystack.Contains(absl::Cord("is a")));
+  EXPECT_TRUE(fragmented_haystack.Contains(absl::Cord("is a")));
+  EXPECT_TRUE(
+      flat_haystack.Contains(absl::MakeFragmentedCord({"is", " ", "a"})));
+  EXPECT_TRUE(
+      fragmented_haystack.Contains(absl::MakeFragmentedCord({"is", " ", "a"})));
+}
+
+TEST_P(CordTest, Find) {
+  auto flat_haystack = absl::Cord("this is a flat cord");
+  auto fragmented_haystack = absl::MakeFragmentedCord(
+      {"this", " ", "is", " ", "a", " ", "fragmented", " ", "cord"});
+  auto empty_haystack = absl::Cord("");
+
+  EXPECT_EQ(flat_haystack.Find(""), flat_haystack.char_begin());
+  EXPECT_EQ(fragmented_haystack.Find(""), fragmented_haystack.char_begin());
+  EXPECT_EQ(flat_haystack.Find(absl::Cord("")), flat_haystack.char_begin());
+  EXPECT_EQ(fragmented_haystack.Find(absl::Cord("")),
+            fragmented_haystack.char_begin());
+  EXPECT_EQ(empty_haystack.Find(""), empty_haystack.char_begin());
+  EXPECT_EQ(empty_haystack.Find(absl::Cord("")), empty_haystack.char_begin());
+  EXPECT_EQ(empty_haystack.Find(flat_haystack), empty_haystack.char_end());
+  EXPECT_EQ(empty_haystack.Find(fragmented_haystack),
+            empty_haystack.char_end());
+
+  EXPECT_EQ(flat_haystack.Find("z"), flat_haystack.char_end());
+  EXPECT_EQ(fragmented_haystack.Find("z"), fragmented_haystack.char_end());
+  EXPECT_EQ(flat_haystack.Find(absl::Cord("z")), flat_haystack.char_end());
+  EXPECT_EQ(fragmented_haystack.Find(absl::Cord("z")),
+            fragmented_haystack.char_end());
+
+  EXPECT_EQ(flat_haystack.Find("is an"), flat_haystack.char_end());
+  EXPECT_EQ(fragmented_haystack.Find("is an"), fragmented_haystack.char_end());
+  EXPECT_EQ(flat_haystack.Find(absl::Cord("is an")), flat_haystack.char_end());
+  EXPECT_EQ(fragmented_haystack.Find(absl::Cord("is an")),
+            fragmented_haystack.char_end());
+  EXPECT_EQ(flat_haystack.Find(absl::MakeFragmentedCord({"is", " ", "an"})),
+            flat_haystack.char_end());
+  EXPECT_EQ(
+      fragmented_haystack.Find(absl::MakeFragmentedCord({"is", " ", "an"})),
+      fragmented_haystack.char_end());
+
+  EXPECT_EQ(flat_haystack.Find("is a"),
+            std::next(flat_haystack.char_begin(), 5));
+  EXPECT_EQ(fragmented_haystack.Find("is a"),
+            std::next(fragmented_haystack.char_begin(), 5));
+  EXPECT_EQ(flat_haystack.Find(absl::Cord("is a")),
+            std::next(flat_haystack.char_begin(), 5));
+  EXPECT_EQ(fragmented_haystack.Find(absl::Cord("is a")),
+            std::next(fragmented_haystack.char_begin(), 5));
+  EXPECT_EQ(flat_haystack.Find(absl::MakeFragmentedCord({"is", " ", "a"})),
+            std::next(flat_haystack.char_begin(), 5));
+  EXPECT_EQ(
+      fragmented_haystack.Find(absl::MakeFragmentedCord({"is", " ", "a"})),
+      std::next(fragmented_haystack.char_begin(), 5));
+}
+
+TEST_P(CordTest, Subcord) {
+  RandomEngine rng(GTEST_FLAG_GET(random_seed));
+  const std::string s = RandomLowercaseString(&rng, 1024);
+
+  absl::Cord a;
+  AppendWithFragments(s, &rng, &a);
+  MaybeHarden(a);
+  ASSERT_EQ(s, std::string(a));
+
+  // Check subcords of a, from a variety of interesting points.
+  std::set<size_t> positions;
+  for (int i = 0; i <= 32; ++i) {
+    positions.insert(i);
+    positions.insert(i * 32 - 1);
+    positions.insert(i * 32);
+    positions.insert(i * 32 + 1);
+    positions.insert(a.size() - i);
+  }
+  positions.insert(237);
+  positions.insert(732);
+  for (size_t pos : positions) {
+    if (pos > a.size()) continue;
+    for (size_t end_pos : positions) {
+      if (end_pos < pos || end_pos > a.size()) continue;
+      absl::Cord sa = a.Subcord(pos, end_pos - pos);
+      ASSERT_EQ(absl::string_view(s).substr(pos, end_pos - pos),
+                std::string(sa))
+          << a;
+      if (pos != 0 || end_pos != a.size()) {
+        ASSERT_EQ(sa.ExpectedChecksum(), std::nullopt);
+      }
+    }
+  }
+
+  // Do the same thing for an inline cord.
+  const std::string sh = "short";
+  absl::Cord c(sh);
+  for (size_t pos = 0; pos <= sh.size(); ++pos) {
+    for (size_t n = 0; n <= sh.size() - pos; ++n) {
+      absl::Cord sc = c.Subcord(pos, n);
+      ASSERT_EQ(sh.substr(pos, n), std::string(sc)) << c;
+    }
+  }
+
+  // Check subcords of subcords.
+  absl::Cord sa = a.Subcord(0, a.size());
+  std::string ss = s.substr(0, s.size());
+  while (sa.size() > 1) {
+    sa = sa.Subcord(1, sa.size() - 2);
+    ss = ss.substr(1, ss.size() - 2);
+    ASSERT_EQ(ss, std::string(sa)) << a;
+    if (HasFailure()) break;  // halt cascade
+  }
+
+  // It is OK to ask for too much.
+  sa = a.Subcord(0, a.size() + 1);
+  EXPECT_EQ(s, std::string(sa));
+
+  // It is OK to ask for something beyond the end.
+  sa = a.Subcord(a.size() + 1, 0);
+  EXPECT_TRUE(sa.empty());
+  sa = a.Subcord(a.size() + 1, 1);
+  EXPECT_TRUE(sa.empty());
+}
+
+TEST_P(CordTest, Swap) {
+  absl::string_view a("Dexter");
+  absl::string_view b("Mandark");
+  absl::Cord x(a);
+  absl::Cord y(b);
+  MaybeHarden(x);
+  swap(x, y);
+  if (UseCrc()) {
+    ASSERT_EQ(x.ExpectedChecksum(), std::nullopt);
+    ASSERT_EQ(y.ExpectedChecksum(), 1);
+  }
+  ASSERT_EQ(x, absl::Cord(b));
+  ASSERT_EQ(y, absl::Cord(a));
+  x.swap(y);
+  if (UseCrc()) {
+    ASSERT_EQ(x.ExpectedChecksum(), 1);
+    ASSERT_EQ(y.ExpectedChecksum(), std::nullopt);
+  }
+  ASSERT_EQ(x, absl::Cord(a));
+  ASSERT_EQ(y, absl::Cord(b));
+}
+
+static void VerifyCopyToString(const absl::Cord& cord) {
+  std::string initially_empty;
+  absl::CopyCordToString(cord, &initially_empty);
+  EXPECT_EQ(initially_empty, cord);
+
+  constexpr size_t kInitialLength = 1024;
+  std::string has_initial_contents(kInitialLength, 'x');
+  const char* address_before_copy = has_initial_contents.data();
+  absl::CopyCordToString(cord, &has_initial_contents);
+  EXPECT_EQ(has_initial_contents, cord);
+
+  if (cord.size() <= kInitialLength) {
+    EXPECT_EQ(has_initial_contents.data(), address_before_copy)
+        << "CopyCordToString allocated new string storage; "
+           "has_initial_contents = \""
+        << has_initial_contents << "\"";
+  }
+}
+
+TEST_P(CordTest, CopyToString) {
+  VerifyCopyToString(absl::Cord());  // empty cords cannot carry CRCs
+  VerifyCopyToString(MaybeHardened(absl::Cord("small cord")));
+  VerifyCopyToString(MaybeHardened(
+      absl::MakeFragmentedCord({"fragmented ", "cord ", "to ", "test ",
+                                "copying ", "to ", "a ", "string."})));
+}
+
+static void VerifyAppendCordToString(const absl::Cord& cord) {
+  std::string initially_empty;
+  absl::AppendCordToString(cord, &initially_empty);
+  EXPECT_EQ(initially_empty, cord);
+
+  const absl::string_view kInitialContents = "initial contents.";
+  std::string expected_after_append =
+      absl::StrCat(kInitialContents, std::string(cord));
+
+  std::string no_reserve(kInitialContents);
+  absl::AppendCordToString(cord, &no_reserve);
+  EXPECT_EQ(no_reserve, expected_after_append);
+
+  std::string has_reserved_capacity(kInitialContents);
+  has_reserved_capacity.reserve(has_reserved_capacity.size() + cord.size());
+  const char* address_before_copy = has_reserved_capacity.data();
+  absl::AppendCordToString(cord, &has_reserved_capacity);
+  EXPECT_EQ(has_reserved_capacity, expected_after_append);
+  EXPECT_EQ(has_reserved_capacity.data(), address_before_copy)
+      << "AppendCordToString allocated new string storage; "
+         "has_reserved_capacity = \""
+      << has_reserved_capacity << "\"";
+}
+
+TEST_P(CordTest, AppendToString) {
+  VerifyAppendCordToString(absl::Cord());  // empty cords cannot carry CRCs
+  VerifyAppendCordToString(MaybeHardened(absl::Cord("small cord")));
+  VerifyAppendCordToString(MaybeHardened(
+      absl::MakeFragmentedCord({"fragmented ", "cord ", "to ", "test ",
+                                "appending ", "to ", "a ", "string."})));
+}
+
+static void VerifyCopyToSpan(const absl::Cord& cord) {
+  // Test with span exactly the same size as the cord.
+  {
+    std::string dst(cord.size(), '\0');
+    size_t copied = absl::CopyCordToSpan(cord, absl::MakeSpan(dst));
+    EXPECT_EQ(copied, cord.size());
+    EXPECT_EQ(dst, cord);
+  }
+
+  // Test with span larger than the cord.
+  {
+    std::string dst(cord.size() + 10, 'x');
+    size_t copied = absl::CopyCordToSpan(cord, absl::MakeSpan(dst));
+    EXPECT_EQ(copied, cord.size());
+    EXPECT_EQ(absl::string_view(dst).substr(0, copied), cord);
+    if (cord.size() < dst.size()) {
+      absl::string_view tail = absl::string_view(dst).substr(copied);
+      EXPECT_EQ(tail, std::string(tail.size(), 'x'));
+    }
+  }
+
+  // Test with span smaller than the cord.
+  {
+    size_t target_size = cord.size() / 2;
+    std::string dst(target_size, '\0');
+    size_t copied = absl::CopyCordToSpan(cord, absl::MakeSpan(dst));
+    EXPECT_EQ(copied, target_size);
+    EXPECT_EQ(dst, std::string(cord).substr(0, target_size));
+  }
+
+  // Test with empty span.
+  {
+    char c = 'x';
+    size_t copied = absl::CopyCordToSpan(cord, absl::MakeSpan(&c, 0));
+    EXPECT_EQ(copied, 0);
+    EXPECT_EQ(c, 'x');
+  }
+}
+
+TEST_P(CordTest, CopyToSpan) {
+  VerifyCopyToSpan(absl::Cord());  // Empty cords cannot be hardened.
+  VerifyCopyToSpan(MaybeHardened(absl::Cord("small cord")));
+  VerifyCopyToSpan(MaybeHardened(
+      absl::MakeFragmentedCord({"fragmented ", "cord ", "to ", "test ",
+                                "copying ", "to ", "a ", "span."})));
+}
+
+TEST_P(CordTest, AppendEmptyBuffer) {
+  absl::Cord cord;
+  cord.Append(absl::CordBuffer());
+  cord.Append(absl::CordBuffer::CreateWithDefaultLimit(2000));
+}
+
+TEST_P(CordTest, AppendEmptyBufferToFlat) {
+  absl::Cord cord(std::string(2000, 'x'));
+  cord.Append(absl::CordBuffer());
+  cord.Append(absl::CordBuffer::CreateWithDefaultLimit(2000));
+}
+
+TEST_P(CordTest, AppendEmptyBufferToTree) {
+  absl::Cord cord(std::string(2000, 'x'));
+  cord.Append(std::string(2000, 'y'));
+  cord.Append(absl::CordBuffer());
+  cord.Append(absl::CordBuffer::CreateWithDefaultLimit(2000));
+}
+
+TEST_P(CordTest, AppendSmallBuffer) {
+  absl::Cord cord;
+  absl::CordBuffer buffer = absl::CordBuffer::CreateWithDefaultLimit(3);
+  ASSERT_THAT(buffer.capacity(), Le(15));
+  memcpy(buffer.data(), "Abc", 3);
+  buffer.SetLength(3);
+  cord.Append(std::move(buffer));
+  EXPECT_EQ(buffer.length(), 0);    // NOLINT
+  EXPECT_GT(buffer.capacity(), 0);  // NOLINT
+
+  buffer = absl::CordBuffer::CreateWithDefaultLimit(3);
+  memcpy(buffer.data(), "defgh", 5);
+  buffer.SetLength(5);
+  cord.Append(std::move(buffer));
+  EXPECT_EQ(buffer.length(), 0);    // NOLINT
+  EXPECT_GT(buffer.capacity(), 0);  // NOLINT
+
+  EXPECT_THAT(cord.Chunks(), ElementsAre("Abcdefgh"));
+}
+
+TEST_P(CordTest, AppendAndPrependBufferArePrecise) {
+  // Create a cord large enough to force 40KB flats.
+  std::string test_data(absl::cord_internal::kMaxFlatLength * 10, 'x');
+  absl::Cord cord1(test_data);
+  absl::Cord cord2(test_data);
+  const size_t size1 = cord1.EstimatedMemoryUsage();
+  const size_t size2 = cord2.EstimatedMemoryUsage();
+
+  absl::CordBuffer buffer = absl::CordBuffer::CreateWithDefaultLimit(3);
+  memcpy(buffer.data(), "Abc", 3);
+  buffer.SetLength(3);
+  cord1.Append(std::move(buffer));
+
+  buffer = absl::CordBuffer::CreateWithDefaultLimit(3);
+  memcpy(buffer.data(), "Abc", 3);
+  buffer.SetLength(3);
+  cord2.Prepend(std::move(buffer));
+
+#ifndef NDEBUG
+  // Allow 32 bytes new CordRepFlat, and 128 bytes for 'glue nodes'
+  constexpr size_t kMaxDelta = 128 + 32;
+#else
+  // Allow 256 bytes extra for 'allocation debug overhead'
+  constexpr size_t kMaxDelta = 128 + 32 + 256;
+#endif
+
+  EXPECT_LE(cord1.EstimatedMemoryUsage() - size1, kMaxDelta);
+  EXPECT_LE(cord2.EstimatedMemoryUsage() - size2, kMaxDelta);
+
+  EXPECT_EQ(cord1, absl::StrCat(test_data, "Abc"));
+  EXPECT_EQ(cord2, absl::StrCat("Abc", test_data));
+}
+
+TEST_P(CordTest, PrependSmallBuffer) {
+  absl::Cord cord;
+  absl::CordBuffer buffer = absl::CordBuffer::CreateWithDefaultLimit(3);
+  ASSERT_THAT(buffer.capacity(), Le(15));
+  memcpy(buffer.data(), "Abc", 3);
+  buffer.SetLength(3);
+  cord.Prepend(std::move(buffer));
+  EXPECT_EQ(buffer.length(), 0);    // NOLINT
+  EXPECT_GT(buffer.capacity(), 0);  // NOLINT
+
+  buffer = absl::CordBuffer::CreateWithDefaultLimit(3);
+  memcpy(buffer.data(), "defgh", 5);
+  buffer.SetLength(5);
+  cord.Prepend(std::move(buffer));
+  EXPECT_EQ(buffer.length(), 0);    // NOLINT
+  EXPECT_GT(buffer.capacity(), 0);  // NOLINT
+
+  EXPECT_THAT(cord.Chunks(), ElementsAre("defghAbc"));
+}
+
+TEST_P(CordTest, AppendLargeBuffer) {
+  absl::Cord cord;
+
+  std::string s1(700, '1');
+  absl::CordBuffer buffer = absl::CordBuffer::CreateWithDefaultLimit(s1.size());
+  memcpy(buffer.data(), s1.data(), s1.size());
+  buffer.SetLength(s1.size());
+  cord.Append(std::move(buffer));
+  EXPECT_EQ(buffer.length(), 0);    // NOLINT
+  EXPECT_GT(buffer.capacity(), 0);  // NOLINT
+
+  std::string s2(1000, '2');
+  buffer = absl::CordBuffer::CreateWithDefaultLimit(s2.size());
+  memcpy(buffer.data(), s2.data(), s2.size());
+  buffer.SetLength(s2.size());
+  cord.Append(std::move(buffer));
+  EXPECT_EQ(buffer.length(), 0);    // NOLINT
+  EXPECT_GT(buffer.capacity(), 0);  // NOLINT
+
+  EXPECT_THAT(cord.Chunks(), ElementsAre(s1, s2));
+}
+
+TEST_P(CordTest, PrependLargeBuffer) {
+  absl::Cord cord;
+
+  std::string s1(700, '1');
+  absl::CordBuffer buffer = absl::CordBuffer::CreateWithDefaultLimit(s1.size());
+  memcpy(buffer.data(), s1.data(), s1.size());
+  buffer.SetLength(s1.size());
+  cord.Prepend(std::move(buffer));
+  EXPECT_EQ(buffer.length(), 0);    // NOLINT
+  EXPECT_GT(buffer.capacity(), 0);  // NOLINT
+
+  std::string s2(1000, '2');
+  buffer = absl::CordBuffer::CreateWithDefaultLimit(s2.size());
+  memcpy(buffer.data(), s2.data(), s2.size());
+  buffer.SetLength(s2.size());
+  cord.Prepend(std::move(buffer));
+  EXPECT_EQ(buffer.length(), 0);    // NOLINT
+  EXPECT_GT(buffer.capacity(), 0);  // NOLINT
+
+  EXPECT_THAT(cord.Chunks(), ElementsAre(s2, s1));
+}
+
+class CordAppendBufferTest : public testing::TestWithParam<bool> {
+ public:
+  size_t is_default() const { return GetParam(); }
+
+  // Returns human readable string representation of the test parameter.
+  static std::string ToString(testing::TestParamInfo<bool> param) {
+    return param.param ? "DefaultLimit" : "CustomLimit";
+  }
+
+  size_t limit() const {
+    return is_default() ? absl::CordBuffer::kDefaultLimit
+                        : absl::CordBuffer::kCustomLimit;
+  }
+
+  size_t maximum_payload() const {
+    return is_default() ? absl::CordBuffer::MaximumPayload()
+                        : absl::CordBuffer::MaximumPayload(limit());
+  }
+
+  absl::CordBuffer GetAppendBuffer(absl::Cord& cord, size_t capacity,
+                                   size_t min_capacity = 16) {
+    return is_default()
+               ? cord.GetAppendBuffer(capacity, min_capacity)
+               : cord.GetCustomAppendBuffer(limit(), capacity, min_capacity);
+  }
+};
+
+INSTANTIATE_TEST_SUITE_P(WithParam, CordAppendBufferTest, testing::Bool(),
+                         CordAppendBufferTest::ToString);
+
+TEST_P(CordAppendBufferTest, GetAppendBufferOnEmptyCord) {
+  absl::Cord cord;
+  absl::CordBuffer buffer = GetAppendBuffer(cord, 1000);
+  EXPECT_GE(buffer.capacity(), 1000);
+  EXPECT_EQ(buffer.length(), 0);
+}
+
+TEST_P(CordAppendBufferTest, GetAppendBufferOnInlinedCord) {
+  static constexpr int kInlinedSize = sizeof(absl::CordBuffer) - 1;
+  for (int size : {6, kInlinedSize - 3, kInlinedSize - 2, 1000}) {
+    absl::Cord cord("Abc");
+    absl::CordBuffer buffer = GetAppendBuffer(cord, size, 1);
+    EXPECT_GE(buffer.capacity(), 3 + size);
+    EXPECT_EQ(buffer.length(), 3);
+    EXPECT_EQ(absl::string_view(buffer.data(), buffer.length()), "Abc");
+    EXPECT_TRUE(cord.empty());
+  }
+}
+
+TEST_P(CordAppendBufferTest, GetAppendBufferOnInlinedCordCapacityCloseToMax) {
+  // Cover the use case where we have a non empty inlined cord with some size
+  // 'n', and ask for something like 'uint64_max - k', assuming internal logic
+  // could overflow on 'uint64_max - k + size', and return a valid, but
+  // inefficiently smaller buffer if it would provide is the max allowed size.
+  for (size_t dist_from_max = 0; dist_from_max <= 4; ++dist_from_max) {
+    absl::Cord cord("Abc");
+    size_t size = std::numeric_limits<size_t>::max() - dist_from_max;
+    absl::CordBuffer buffer = GetAppendBuffer(cord, size, 1);
+    EXPECT_GE(buffer.capacity(), maximum_payload());
+    EXPECT_EQ(buffer.length(), 3);
+    EXPECT_EQ(absl::string_view(buffer.data(), buffer.length()), "Abc");
+    EXPECT_TRUE(cord.empty());
+  }
+}
+
+TEST_P(CordAppendBufferTest, GetAppendBufferOnFlat) {
+  // Create a cord with a single flat and extra capacity
+  absl::Cord cord;
+  absl::CordBuffer buffer = absl::CordBuffer::CreateWithDefaultLimit(500);
+  const size_t expected_capacity = buffer.capacity();
+  buffer.SetLength(3);
+  memcpy(buffer.data(), "Abc", 3);
+  cord.Append(std::move(buffer));
+
+  buffer = GetAppendBuffer(cord, 6);
+  EXPECT_EQ(buffer.capacity(), expected_capacity);
+  EXPECT_EQ(buffer.length(), 3);
+  EXPECT_EQ(absl::string_view(buffer.data(), buffer.length()), "Abc");
+  EXPECT_TRUE(cord.empty());
+}
+
+TEST_P(CordAppendBufferTest, GetAppendBufferOnFlatWithoutMinCapacity) {
+  // Create a cord with a single flat and extra capacity
+  absl::Cord cord;
+  absl::CordBuffer buffer = absl::CordBuffer::CreateWithDefaultLimit(500);
+  buffer.SetLength(30);
+  memset(buffer.data(), 'x', 30);
+  cord.Append(std::move(buffer));
+
+  buffer = GetAppendBuffer(cord, 1000, 900);
+  EXPECT_GE(buffer.capacity(), 1000);
+  EXPECT_EQ(buffer.length(), 0);
+  EXPECT_EQ(cord, std::string(30, 'x'));
+}
+
+TEST_P(CordAppendBufferTest, GetAppendBufferOnTree) {
+  RandomEngine rng;
+  for (int num_flats : {2, 3, 100}) {
+    // Create a cord with `num_flats` flats and extra capacity
+    absl::Cord cord;
+    std::string prefix;
+    std::string last;
+    for (int i = 0; i < num_flats - 1; ++i) {
+      prefix += last;
+      last = RandomLowercaseString(&rng, 10);
+      absl::CordBuffer buffer = absl::CordBuffer::CreateWithDefaultLimit(500);
+      buffer.SetLength(10);
+      memcpy(buffer.data(), last.data(), 10);
+      cord.Append(std::move(buffer));
+    }
+    absl::CordBuffer buffer = GetAppendBuffer(cord, 6);
+    EXPECT_GE(buffer.capacity(), 500);
+    EXPECT_EQ(buffer.length(), 10);
+    EXPECT_EQ(absl::string_view(buffer.data(), buffer.length()), last);
+    EXPECT_EQ(cord, prefix);
+  }
+}
+
+TEST_P(CordAppendBufferTest, GetAppendBufferOnTreeWithoutMinCapacity) {
+  absl::Cord cord;
+  for (int i = 0; i < 2; ++i) {
+    absl::CordBuffer buffer = absl::CordBuffer::CreateWithDefaultLimit(500);
+    buffer.SetLength(3);
+    memcpy(buffer.data(), i ? "def" : "Abc", 3);
+    cord.Append(std::move(buffer));
+  }
+  absl::CordBuffer buffer = GetAppendBuffer(cord, 1000, 900);
+  EXPECT_GE(buffer.capacity(), 1000);
+  EXPECT_EQ(buffer.length(), 0);
+  EXPECT_EQ(cord, "Abcdef");
+}
+
+TEST_P(CordAppendBufferTest, GetAppendBufferOnSubstring) {
+  // Create a large cord with a single flat and some extra capacity
+  absl::Cord cord;
+  absl::CordBuffer buffer = absl::CordBuffer::CreateWithDefaultLimit(500);
+  buffer.SetLength(450);
+  memset(buffer.data(), 'x', 450);
+  cord.Append(std::move(buffer));
+  cord.RemovePrefix(1);
+
+  // Deny on substring
+  buffer = GetAppendBuffer(cord, 6);
+  EXPECT_EQ(buffer.length(), 0);
+  EXPECT_EQ(cord, std::string(449, 'x'));
+}
+
+TEST_P(CordAppendBufferTest, GetAppendBufferOnSharedCord) {
+  // Create a shared cord with a single flat and extra capacity
+  absl::Cord cord;
+  absl::CordBuffer buffer = absl::CordBuffer::CreateWithDefaultLimit(500);
+  buffer.SetLength(3);
+  memcpy(buffer.data(), "Abc", 3);
+  cord.Append(std::move(buffer));
+  absl::Cord shared_cord = cord;
+
+  // Deny on flat
+  buffer = GetAppendBuffer(cord, 6);
+  EXPECT_EQ(buffer.length(), 0);
+  EXPECT_EQ(cord, "Abc");
+
+  buffer = absl::CordBuffer::CreateWithDefaultLimit(500);
+  buffer.SetLength(3);
+  memcpy(buffer.data(), "def", 3);
+  cord.Append(std::move(buffer));
+  shared_cord = cord;
+
+  // Deny on tree
+  buffer = GetAppendBuffer(cord, 6);
+  EXPECT_EQ(buffer.length(), 0);
+  EXPECT_EQ(cord, "Abcdef");
+}
+
+TEST_P(CordTest, TryFlatEmpty) {
+  absl::Cord c;
+  EXPECT_EQ(c.TryFlat(), "");
+}
+
+TEST_P(CordTest, TryFlatFlat) {
+  absl::Cord c("hello");
+  MaybeHarden(c);
+  EXPECT_EQ(c.TryFlat(), "hello");
+}
+
+TEST_P(CordTest, TryFlatSubstrInlined) {
+  absl::Cord c("hello");
+  c.RemovePrefix(1);
+  MaybeHarden(c);
+  EXPECT_EQ(c.TryFlat(), "ello");
+}
+
+TEST_P(CordTest, TryFlatSubstrFlat) {
+  absl::Cord c("longer than 15 bytes");
+  absl::Cord sub = absl::CordTestPeer::MakeSubstring(c, 1, c.size() - 1);
+  MaybeHarden(sub);
+  EXPECT_EQ(sub.TryFlat(), "onger than 15 bytes");
+}
+
+TEST_P(CordTest, TryFlatConcat) {
+  absl::Cord c = absl::MakeFragmentedCord({"hel", "lo"});
+  MaybeHarden(c);
+  EXPECT_EQ(c.TryFlat(), std::nullopt);
+}
+
+TEST_P(CordTest, TryFlatExternal) {
+  absl::Cord c = absl::MakeCordFromExternal("hell", [](absl::string_view) {});
+  MaybeHarden(c);
+  EXPECT_EQ(c.TryFlat(), "hell");
+}
+
+TEST_P(CordTest, TryFlatSubstrExternal) {
+  absl::Cord c = absl::MakeCordFromExternal("hell", [](absl::string_view) {});
+  absl::Cord sub = absl::CordTestPeer::MakeSubstring(c, 1, c.size() - 1);
+  MaybeHarden(sub);
+  EXPECT_EQ(sub.TryFlat(), "ell");
+}
+
+TEST_P(CordTest, TryFlatCommonlyAssumedInvariants) {
+  // The behavior tested below is not part of the API contract of Cord, but it's
+  // something we intend to be true in our current implementation.  This test
+  // exists to detect and prevent accidental breakage of the implementation.
+  absl::string_view fragments[] = {"A fragmented test",
+                                   " cord",
+                                   " to test subcords",
+                                   " of ",
+                                   "a",
+                                   " cord for",
+                                   " each chunk "
+                                   "returned by the ",
+                                   "iterator"};
+  absl::Cord c = absl::MakeFragmentedCord(fragments);
+  MaybeHarden(c);
+  int fragment = 0;
+  int offset = 0;
+  absl::Cord::CharIterator itc = c.char_begin();
+  for (absl::string_view sv : c.Chunks()) {
+    absl::string_view expected = fragments[fragment];
+    absl::Cord subcord1 = c.Subcord(offset, sv.length());
+    absl::Cord subcord2 = absl::Cord::AdvanceAndRead(&itc, sv.size());
+    EXPECT_EQ(subcord1.TryFlat(), expected);
+    EXPECT_EQ(subcord2.TryFlat(), expected);
+    ++fragment;
+    offset += sv.length();
+  }
+}
+
+static bool IsFlat(const absl::Cord& c) {
+  return c.chunk_begin() == c.chunk_end() || ++c.chunk_begin() == c.chunk_end();
+}
+
+static void VerifyFlatten(absl::Cord c) {
+  std::string old_contents(c);
+  absl::string_view old_flat;
+  bool already_flat_and_non_empty = IsFlat(c) && !c.empty();
+  if (already_flat_and_non_empty) {
+    old_flat = *c.chunk_begin();
+  }
+  absl::string_view new_flat = c.Flatten();
+
+  // Verify that the contents of the flattened Cord are correct.
+  EXPECT_EQ(new_flat, old_contents);
+  EXPECT_EQ(std::string(c), old_contents);
+
+  // If the Cord contained data and was already flat, verify that the data
+  // wasn't copied.
+  if (already_flat_and_non_empty) {
+    EXPECT_EQ(old_flat.data(), new_flat.data())
+        << "Allocated new memory even though the Cord was already flat.";
+  }
+
+  // Verify that the flattened Cord is in fact flat.
+  EXPECT_TRUE(IsFlat(c));
+}
+
+TEST_P(CordTest, Flatten) {
+  VerifyFlatten(absl::Cord());
+  VerifyFlatten(MaybeHardened(absl::Cord("small cord")));
+  VerifyFlatten(
+      MaybeHardened(absl::Cord("larger than small buffer optimization")));
+  VerifyFlatten(MaybeHardened(
+      absl::MakeFragmentedCord({"small ", "fragmented ", "cord"})));
+
+  // Test with a cord that is longer than the largest flat buffer
+  RandomEngine rng(GTEST_FLAG_GET(random_seed));
+  VerifyFlatten(MaybeHardened(absl::Cord(RandomLowercaseString(&rng, 8192))));
+}
+
+// Test data
+namespace {
+class TestData {
+ private:
+  std::vector<std::string> data_;
+
+  // Return a std::string of the specified length.
+  static std::string MakeString(int length) {
+    std::string result;
+    char buf[30];
+    snprintf(buf, sizeof(buf), "(%d)", length);
+    while (result.size() < length) {
+      result += buf;
+    }
+    result.resize(length);
+    return result;
+  }
+
+ public:
+  TestData() {
+    // short strings increasing in length by one
+    for (int i = 0; i < 30; i++) {
+      data_.push_back(MakeString(i));
+    }
+
+    // strings around half kMaxFlatLength
+    static const int kMaxFlatLength = 4096 - 9;
+    static const int kHalf = kMaxFlatLength / 2;
+
+    for (int i = -10; i <= +10; i++) {
+      data_.push_back(MakeString(kHalf + i));
+    }
+
+    for (int i = -10; i <= +10; i++) {
+      data_.push_back(MakeString(kMaxFlatLength + i));
+    }
+  }
+
+  size_t size() const { return data_.size(); }
+  const std::string& data(size_t i) const { return data_[i]; }
+};
+}  // namespace
+
+TEST_P(CordTest, MultipleLengths) {
+  TestData d;
+  for (size_t i = 0; i < d.size(); i++) {
+    std::string a = d.data(i);
+
+    {  // Construct from Cord
+      absl::Cord tmp(a);
+      absl::Cord x(tmp);
+      MaybeHarden(x);
+      EXPECT_EQ(a, std::string(x)) << "'" << a << "'";
+    }
+
+    {  // Construct from absl::string_view
+      absl::Cord x(a);
+      MaybeHarden(x);
+      EXPECT_EQ(a, std::string(x)) << "'" << a << "'";
+    }
+
+    {  // Append cord to self
+      absl::Cord self(a);
+      MaybeHarden(self);
+      self.Append(self);
+      EXPECT_EQ(a + a, std::string(self)) << "'" << a << "' + '" << a << "'";
+    }
+
+    {  // Prepend cord to self
+      absl::Cord self(a);
+      MaybeHarden(self);
+      self.Prepend(self);
+      EXPECT_EQ(a + a, std::string(self)) << "'" << a << "' + '" << a << "'";
+    }
+
+    // Try to append/prepend others
+    for (size_t j = 0; j < d.size(); j++) {
+      std::string b = d.data(j);
+
+      {  // CopyFrom Cord
+        absl::Cord x(a);
+        absl::Cord y(b);
+        MaybeHarden(x);
+        x = y;
+        EXPECT_EQ(b, std::string(x)) << "'" << a << "' + '" << b << "'";
+      }
+
+      {  // CopyFrom absl::string_view
+        absl::Cord x(a);
+        MaybeHarden(x);
+        x = b;
+        EXPECT_EQ(b, std::string(x)) << "'" << a << "' + '" << b << "'";
+      }
+
+      {  // Cord::Append(Cord)
+        absl::Cord x(a);
+        absl::Cord y(b);
+        MaybeHarden(x);
+        x.Append(y);
+        EXPECT_EQ(a + b, std::string(x)) << "'" << a << "' + '" << b << "'";
+      }
+
+      {  // Cord::Append(absl::string_view)
+        absl::Cord x(a);
+        MaybeHarden(x);
+        x.Append(b);
+        EXPECT_EQ(a + b, std::string(x)) << "'" << a << "' + '" << b << "'";
+      }
+
+      {  // Cord::Prepend(Cord)
+        absl::Cord x(a);
+        absl::Cord y(b);
+        MaybeHarden(x);
+        x.Prepend(y);
+        EXPECT_EQ(b + a, std::string(x)) << "'" << b << "' + '" << a << "'";
+      }
+
+      {  // Cord::Prepend(absl::string_view)
+        absl::Cord x(a);
+        MaybeHarden(x);
+        x.Prepend(b);
+        EXPECT_EQ(b + a, std::string(x)) << "'" << b << "' + '" << a << "'";
+      }
+    }
+  }
+}
+
+namespace {
+
+TEST_P(CordTest, RemoveSuffixWithExternalOrSubstring) {
+  absl::Cord cord = absl::MakeCordFromExternal(
+      "foo bar baz", [](absl::string_view s) { DoNothing(s, nullptr); });
+  EXPECT_EQ("foo bar baz", std::string(cord));
+
+  MaybeHarden(cord);
+
+  // This RemoveSuffix() will wrap the EXTERNAL node in a SUBSTRING node.
+  cord.RemoveSuffix(4);
+  EXPECT_EQ("foo bar", std::string(cord));
+
+  MaybeHarden(cord);
+
+  // This RemoveSuffix() will adjust the SUBSTRING node in-place.
+  cord.RemoveSuffix(4);
+  EXPECT_EQ("foo", std::string(cord));
+}
+
+TEST_P(CordTest, RemoveSuffixMakesZeroLengthNode) {
+  absl::Cord c;
+  c.Append(absl::Cord(std::string(100, 'x')));
+  absl::Cord other_ref = c;  // Prevent inplace appends
+  EXPECT_THAT(other_ref, testing::Eq(c));
+  MaybeHarden(c);
+  c.Append(absl::Cord(std::string(200, 'y')));
+  c.RemoveSuffix(200);
+  EXPECT_EQ(std::string(100, 'x'), std::string(c));
+}
+
+}  // namespace
+
+// CordSpliceTest contributed by hendrie.
+namespace {
+
+// Create a cord with an external memory block filled with 'z'
+absl::Cord CordWithZedBlock(size_t size) {
+  char* data = new char[size];
+  if (size > 0) {
+    memset(data, 'z', size);
+  }
+  absl::Cord cord = absl::MakeCordFromExternal(
+      absl::string_view(data, size),
+      [](absl::string_view s) { delete[] s.data(); });
+  return cord;
+}
+
+// Establish that ZedBlock does what we think it does.
+TEST_P(CordTest, CordSpliceTestZedBlock) {
+  absl::Cord blob = CordWithZedBlock(10);
+  MaybeHarden(blob);
+  EXPECT_EQ(10, blob.size());
+  std::string s;
+  absl::CopyCordToString(blob, &s);
+  EXPECT_EQ("zzzzzzzzzz", s);
+}
+
+TEST_P(CordTest, CordSpliceTestZedBlock0) {
+  absl::Cord blob = CordWithZedBlock(0);
+  MaybeHarden(blob);
+  EXPECT_EQ(0, blob.size());
+  std::string s;
+  absl::CopyCordToString(blob, &s);
+  EXPECT_EQ("", s);
+}
+
+TEST_P(CordTest, CordSpliceTestZedBlockSuffix1) {
+  absl::Cord blob = CordWithZedBlock(10);
+  MaybeHarden(blob);
+  EXPECT_EQ(10, blob.size());
+  absl::Cord suffix(blob);
+  suffix.RemovePrefix(9);
+  EXPECT_EQ(1, suffix.size());
+  std::string s;
+  absl::CopyCordToString(suffix, &s);
+  EXPECT_EQ("z", s);
+}
+
+// Remove all of a prefix block
+TEST_P(CordTest, CordSpliceTestZedBlockSuffix0) {
+  absl::Cord blob = CordWithZedBlock(10);
+  MaybeHarden(blob);
+  EXPECT_EQ(10, blob.size());
+  absl::Cord suffix(blob);
+  suffix.RemovePrefix(10);
+  EXPECT_EQ(0, suffix.size());
+  std::string s;
+  absl::CopyCordToString(suffix, &s);
+  EXPECT_EQ("", s);
+}
+
+absl::Cord BigCord(size_t len, char v) {
+  std::string s(len, v);
+  return absl::Cord(s);
+}
+
+// Splice block into cord.
+absl::Cord SpliceCord(const absl::Cord& blob, int64_t offset,
+                      const absl::Cord& block) {
+  CHECK_GE(offset, 0);
+  CHECK_LE(static_cast<size_t>(offset) + block.size(), blob.size());
+  absl::Cord result(blob);
+  result.RemoveSuffix(blob.size() - offset);
+  result.Append(block);
+  absl::Cord suffix(blob);
+  suffix.RemovePrefix(offset + block.size());
+  result.Append(suffix);
+  CHECK_EQ(blob.size(), result.size());
+  return result;
+}
+
+// Taking an empty suffix of a block breaks appending.
+TEST_P(CordTest, CordSpliceTestRemoveEntireBlock1) {
+  absl::Cord zero = CordWithZedBlock(10);
+  MaybeHarden(zero);
+  absl::Cord suffix(zero);
+  suffix.RemovePrefix(10);
+  absl::Cord result;
+  result.Append(suffix);
+}
+
+TEST_P(CordTest, CordSpliceTestRemoveEntireBlock2) {
+  absl::Cord zero = CordWithZedBlock(10);
+  MaybeHarden(zero);
+  absl::Cord prefix(zero);
+  prefix.RemoveSuffix(10);
+  absl::Cord suffix(zero);
+  suffix.RemovePrefix(10);
+  absl::Cord result(prefix);
+  result.Append(suffix);
+}
+
+TEST_P(CordTest, CordSpliceTestRemoveEntireBlock3) {
+  absl::Cord blob = CordWithZedBlock(10);
+  absl::Cord block = BigCord(10, 'b');
+  MaybeHarden(blob);
+  MaybeHarden(block);
+  blob = SpliceCord(blob, 0, block);
+}
+
+struct CordCompareTestCase {
+  template <typename LHS, typename RHS>
+  CordCompareTestCase(const LHS& lhs, const RHS& rhs, bool use_crc)
+      : lhs_cord(lhs), rhs_cord(rhs) {
+    if (use_crc) {
+      lhs_cord.SetExpectedChecksum(1);
+    }
+  }
+
+  absl::Cord lhs_cord;
+  absl::Cord rhs_cord;
+};
+
+const auto sign = [](int x) { return x == 0 ? 0 : (x > 0 ? 1 : -1); };
+
+void VerifyComparison(const CordCompareTestCase& test_case) {
+  std::string lhs_string(test_case.lhs_cord);
+  std::string rhs_string(test_case.rhs_cord);
+  int expected = sign(lhs_string.compare(rhs_string));
+  EXPECT_EQ(expected, test_case.lhs_cord.Compare(test_case.rhs_cord))
+      << "LHS=" << lhs_string << "; RHS=" << rhs_string;
+  EXPECT_EQ(expected, test_case.lhs_cord.Compare(rhs_string))
+      << "LHS=" << lhs_string << "; RHS=" << rhs_string;
+  EXPECT_EQ(-expected, test_case.rhs_cord.Compare(test_case.lhs_cord))
+      << "LHS=" << rhs_string << "; RHS=" << lhs_string;
+  EXPECT_EQ(-expected, test_case.rhs_cord.Compare(lhs_string))
+      << "LHS=" << rhs_string << "; RHS=" << lhs_string;
+}
+
+TEST_P(CordTest, Compare) {
+  absl::Cord subcord("aaaaaBBBBBcccccDDDDD");
+  subcord = subcord.Subcord(3, 10);
+
+  absl::Cord tmp("aaaaaaaaaaaaaaaa");
+  tmp.Append("BBBBBBBBBBBBBBBB");
+  absl::Cord concat = absl::Cord("cccccccccccccccc");
+  concat.Append("DDDDDDDDDDDDDDDD");
+  concat.Prepend(tmp);
+
+  absl::Cord concat2("aaaaaaaaaaaaa");
+  concat2.Append("aaaBBBBBBBBBBBBBBBBccccc");
+  concat2.Append("cccccccccccDDDDDDDDDDDDDD");
+  concat2.Append("DD");
+
+  const bool use_crc = UseCrc();
+
+  std::vector<CordCompareTestCase> test_cases = {{
+      // Inline cords
+      {"abcdef", "abcdef", use_crc},
+      {"abcdef", "abcdee", use_crc},
+      {"abcdef", "abcdeg", use_crc},
+      {"bbcdef", "abcdef", use_crc},
+      {"bbcdef", "abcdeg", use_crc},
+      {"abcdefa", "abcdef", use_crc},
+      {"abcdef", "abcdefa", use_crc},
+
+      // Small flat cords
+      {"aaaaaBBBBBcccccDDDDD", "aaaaaBBBBBcccccDDDDD", use_crc},
+      {"aaaaaBBBBBcccccDDDDD", "aaaaaBBBBBxccccDDDDD", use_crc},
+      {"aaaaaBBBBBcxcccDDDDD", "aaaaaBBBBBcccccDDDDD", use_crc},
+      {"aaaaaBBBBBxccccDDDDD", "aaaaaBBBBBcccccDDDDX", use_crc},
+      {"aaaaaBBBBBcccccDDDDDa", "aaaaaBBBBBcccccDDDDD", use_crc},
+      {"aaaaaBBBBBcccccDDDDD", "aaaaaBBBBBcccccDDDDDa", use_crc},
+
+      // Subcords
+      {subcord, subcord, use_crc},
+      {subcord, "aaBBBBBccc", use_crc},
+      {subcord, "aaBBBBBccd", use_crc},
+      {subcord, "aaBBBBBccb", use_crc},
+      {subcord, "aaBBBBBxcb", use_crc},
+      {subcord, "aaBBBBBccca", use_crc},
+      {subcord, "aaBBBBBcc", use_crc},
+
+      // Concats
+      {concat, concat, use_crc},
+      {concat,
+       "aaaaaaaaaaaaaaaaBBBBBBBBBBBBBBBBccccccccccccccccDDDDDDDDDDDDDDDD",
+       use_crc},
+      {concat,
+       "aaaaaaaaaaaaaaaaBBBBBBBBBBBBBBBBcccccccccccccccxDDDDDDDDDDDDDDDD",
+       use_crc},
+      {concat,
+       "aaaaaaaaaaaaaaaaBBBBBBBBBBBBBBBBacccccccccccccccDDDDDDDDDDDDDDDD",
+       use_crc},
+      {concat,
+       "aaaaaaaaaaaaaaaaBBBBBBBBBBBBBBBBccccccccccccccccDDDDDDDDDDDDDDD",
+       use_crc},
+      {concat,
+       "aaaaaaaaaaaaaaaaBBBBBBBBBBBBBBBBccccccccccccccccDDDDDDDDDDDDDDDDe",
+       use_crc},
+
+      {concat, concat2, use_crc},
+  }};
+
+  for (const auto& tc : test_cases) {
+    VerifyComparison(tc);
+  }
+}
+
+TEST_P(CordTest, CompareAfterAssign) {
+  absl::Cord a("aaaaaa1111111");
+  absl::Cord b("aaaaaa2222222");
+  MaybeHarden(a);
+  a = "cccccc";
+  b = "cccccc";
+  EXPECT_EQ(a, b);
+  EXPECT_FALSE(a < b);
+
+  a = "aaaa";
+  b = "bbbbb";
+  a = "";
+  b = "";
+  EXPECT_EQ(a, b);
+  EXPECT_FALSE(a < b);
+}
+
+// Test CompareTo() and ComparePrefix() against string and substring
+// comparison methods from basic_string.
+static void TestCompare(const absl::Cord& c, const absl::Cord& d,
+                        RandomEngine* rng) {
+  // char_traits<char>::lt is guaranteed to do an unsigned comparison:
+  // https://en.cppreference.com/w/cpp/string/char_traits/cmp. We also expect
+  // Cord comparisons to be based on unsigned byte comparisons regardless of
+  // whether char is signed.
+  int expected = sign(std::string(c).compare(std::string(d)));
+  EXPECT_EQ(expected, sign(c.Compare(d))) << c << ", " << d;
+}
+
+TEST_P(CordTest, CompareComparisonIsUnsigned) {
+  RandomEngine rng(GTEST_FLAG_GET(random_seed));
+  std::uniform_int_distribution<uint32_t> uniform_uint8(0, 255);
+  char x = static_cast<char>(uniform_uint8(rng));
+  TestCompare(
+      absl::Cord(std::string(GetUniformRandomUpTo(&rng, 100), x)),
+      absl::Cord(std::string(GetUniformRandomUpTo(&rng, 100), x ^ 0x80)), &rng);
+}
+
+TEST_P(CordTest, CompareRandomComparisons) {
+  const int kIters = 5000;
+  RandomEngine rng(GTEST_FLAG_GET(random_seed));
+
+  int n = GetUniformRandomUpTo(&rng, 5000);
+  absl::Cord a[] = {MakeExternalCord(n),
+                    absl::Cord("ant"),
+                    absl::Cord("elephant"),
+                    absl::Cord("giraffe"),
+                    absl::Cord(std::string(GetUniformRandomUpTo(&rng, 100),
+                                           GetUniformRandomUpTo(&rng, 100))),
+                    absl::Cord(""),
+                    absl::Cord("x"),
+                    absl::Cord("A"),
+                    absl::Cord("B"),
+                    absl::Cord("C")};
+  for (int i = 0; i < kIters; i++) {
+    absl::Cord c, d;
+    for (int j = 0; j < (i % 7) + 1; j++) {
+      c.Append(a[GetUniformRandomUpTo(&rng, std::size(a))]);
+      d.Append(a[GetUniformRandomUpTo(&rng, std::size(a))]);
+    }
+    std::bernoulli_distribution coin_flip(0.5);
+    MaybeHarden(c);
+    MaybeHarden(d);
+    TestCompare(coin_flip(rng) ? c : absl::Cord(std::string(c)),
+                coin_flip(rng) ? d : absl::Cord(std::string(d)), &rng);
+  }
+}
+
+template <typename T1, typename T2>
+void CompareOperators() {
+  const T1 a("a");
+  const T2 b("b");
+
+  EXPECT_TRUE(a == a);
+  // For pointer type (i.e. `const char*`), operator== compares the address
+  // instead of the string, so `a == const char*("a")` isn't necessarily true.
+  EXPECT_TRUE(std::is_pointer_v<T1> || a == T1("a"));
+  EXPECT_TRUE(std::is_pointer_v<T2> || a == T2("a"));
+  EXPECT_FALSE(a == b);
+
+  EXPECT_TRUE(a != b);
+  EXPECT_FALSE(a != a);
+
+  EXPECT_TRUE(a < b);
+  EXPECT_FALSE(b < a);
+
+  EXPECT_TRUE(b > a);
+  EXPECT_FALSE(a > b);
+
+  EXPECT_TRUE(a >= a);
+  EXPECT_TRUE(b >= a);
+  EXPECT_FALSE(a >= b);
+
+  EXPECT_TRUE(a <= a);
+  EXPECT_TRUE(a <= b);
+  EXPECT_FALSE(b <= a);
+}
+
+TEST_P(CordTest, ComparisonOperators_Cord_Cord) {
+  CompareOperators<absl::Cord, absl::Cord>();
+}
+
+TEST_P(CordTest, ComparisonOperators_Cord_StringPiece) {
+  CompareOperators<absl::Cord, absl::string_view>();
+}
+
+TEST_P(CordTest, ComparisonOperators_StringPiece_Cord) {
+  CompareOperators<absl::string_view, absl::Cord>();
+}
+
+TEST_P(CordTest, ComparisonOperators_Cord_string) {
+  CompareOperators<absl::Cord, std::string>();
+}
+
+TEST_P(CordTest, ComparisonOperators_string_Cord) {
+  CompareOperators<std::string, absl::Cord>();
+}
+
+TEST_P(CordTest, ComparisonOperators_stdstring_Cord) {
+  CompareOperators<std::string, absl::Cord>();
+}
+
+TEST_P(CordTest, ComparisonOperators_Cord_stdstring) {
+  CompareOperators<absl::Cord, std::string>();
+}
+
+TEST_P(CordTest, ComparisonOperators_charstar_Cord) {
+  CompareOperators<const char*, absl::Cord>();
+}
+
+TEST_P(CordTest, ComparisonOperators_Cord_charstar) {
+  CompareOperators<absl::Cord, const char*>();
+}
+
+TEST_P(CordTest, ConstructFromExternalReleaserInvoked) {
+  // Empty external memory means the releaser should be called immediately.
+  {
+    bool invoked = false;
+    auto releaser = [&invoked](absl::string_view) { invoked = true; };
+    {
+      auto c = absl::MakeCordFromExternal("", releaser);
+      EXPECT_THAT(c, testing::Eq(""));
+      EXPECT_TRUE(invoked);
+    }
+  }
+
+  // If the size of the data is small enough, a future constructor
+  // implementation may copy the bytes and immediately invoke the releaser
+  // instead of creating an external node. We make a large dummy std::string to
+  // make this test independent of such an optimization.
+  std::string large_dummy(2048, 'c');
+  {
+    bool invoked = false;
+    auto releaser = [&invoked](absl::string_view) { invoked = true; };
+    {
+      auto c = absl::MakeCordFromExternal(large_dummy, releaser);
+      EXPECT_THAT(c, testing::Eq(large_dummy));
+      EXPECT_FALSE(invoked);
+    }
+    EXPECT_TRUE(invoked);
+  }
+
+  {
+    bool invoked = false;
+    auto releaser = [&invoked](absl::string_view) { invoked = true; };
+    {
+      absl::Cord copy;
+      {
+        auto c = absl::MakeCordFromExternal(large_dummy, releaser);
+        copy = c;
+        EXPECT_FALSE(invoked);
+      }
+      EXPECT_FALSE(invoked);
+    }
+    EXPECT_TRUE(invoked);
+  }
+}
+
+TEST_P(CordTest, ConstructFromExternalCompareContents) {
+  RandomEngine rng(GTEST_FLAG_GET(random_seed));
+
+  for (int length = 1; length <= 2048; length *= 2) {
+    std::string data = RandomLowercaseString(&rng, length);
+    auto* external = new std::string(data);
+    auto cord =
+        absl::MakeCordFromExternal(*external, [external](absl::string_view sv) {
+          EXPECT_EQ(external->data(), sv.data());
+          EXPECT_EQ(external->size(), sv.size());
+          delete external;
+        });
+    MaybeHarden(cord);
+    EXPECT_EQ(data, cord);
+  }
+}
+
+TEST_P(CordTest, ConstructFromExternalLargeReleaser) {
+  RandomEngine rng(GTEST_FLAG_GET(random_seed));
+  constexpr size_t kLength = 256;
+  std::string data = RandomLowercaseString(&rng, kLength);
+  std::array<char, kLength> data_array;
+  for (size_t i = 0; i < kLength; ++i) data_array[i] = data[i];
+  bool invoked = false;
+  auto releaser = [data_array, &invoked](absl::string_view data) {
+    EXPECT_EQ(data, absl::string_view(data_array.data(), data_array.size()));
+    invoked = true;
+  };
+  (void)MaybeHardened(absl::MakeCordFromExternal(data, releaser));
+  EXPECT_TRUE(invoked);
+}
+
+TEST_P(CordTest, ConstructFromExternalFunctionPointerReleaser) {
+  static absl::string_view data("hello world");
+  static bool invoked;
+  auto* releaser =
+      static_cast<void (*)(absl::string_view)>([](absl::string_view sv) {
+        EXPECT_EQ(data, sv);
+        invoked = true;
+      });
+  invoked = false;
+  (void)MaybeHardened(absl::MakeCordFromExternal(data, releaser));
+  EXPECT_TRUE(invoked);
+
+  invoked = false;
+  (void)MaybeHardened(absl::MakeCordFromExternal(data, *releaser));
+  EXPECT_TRUE(invoked);
+}
+
+TEST_P(CordTest, ConstructFromExternalMoveOnlyReleaser) {
+  struct Releaser {
+    explicit Releaser(bool* invoked) : invoked(invoked) {}
+    Releaser(Releaser&& other) noexcept : invoked(other.invoked) {}
+    void operator()(absl::string_view) const { *invoked = true; }
+
+    bool* invoked;
+  };
+
+  bool invoked = false;
+  (void)MaybeHardened(absl::MakeCordFromExternal("dummy", Releaser(&invoked)));
+  EXPECT_TRUE(invoked);
+}
+
+TEST_P(CordTest, ConstructFromExternalNonConstReleaser) {
+  struct Releaser {
+    explicit Releaser(bool* invoked) : invoked(invoked) {}
+    // Non const method.
+    void operator()(absl::string_view) { *invoked = true; }
+
+    bool* invoked;
+  };
+
+  bool invoked = false;
+  (void)MaybeHardened(absl::MakeCordFromExternal("dummy", Releaser(&invoked)));
+  EXPECT_TRUE(invoked);
+}
+
+TEST_P(CordTest, ConstructFromExternalNonConstNoArgReleaser) {
+  struct Releaser {
+    explicit Releaser(bool* invoked) : invoked(invoked) {}
+    // Non const method.
+    void operator()() { *invoked = true; }
+
+    bool* invoked;
+  };
+
+  bool invoked = false;
+  (void)MaybeHardened(absl::MakeCordFromExternal("dummy", Releaser(&invoked)));
+  EXPECT_TRUE(invoked);
+}
+
+TEST_P(CordTest, ConstructFromExternalNoArgLambda) {
+  bool invoked = false;
+  (void)MaybeHardened(
+      absl::MakeCordFromExternal("dummy", [&invoked]() { invoked = true; }));
+  EXPECT_TRUE(invoked);
+}
+
+TEST_P(CordTest, ConstructFromExternalStringViewArgLambda) {
+  bool invoked = false;
+  (void)MaybeHardened(absl::MakeCordFromExternal(
+      "dummy", [&invoked](absl::string_view) { invoked = true; }));
+  EXPECT_TRUE(invoked);
+}
+
+TEST_P(CordTest, ConstructFromExternalNonTrivialReleaserDestructor) {
+  struct Releaser {
+    explicit Releaser(bool* destroyed) : destroyed(destroyed) {}
+    ~Releaser() { *destroyed = true; }
+    void operator()(absl::string_view) const {}
+
+    bool* destroyed;
+  };
+
+  bool destroyed = false;
+  Releaser releaser(&destroyed);
+  (void)MaybeHardened(absl::MakeCordFromExternal("dummy", releaser));
+  EXPECT_TRUE(destroyed);
+}
+
+TEST_P(CordTest, ConstructFromExternalReferenceQualifierOverloads) {
+  enum InvokedAs { kMissing, kLValue, kRValue };
+  enum CopiedAs { kNone, kMove, kCopy };
+  struct Tracker {
+    CopiedAs copied_as = kNone;
+    InvokedAs invoked_as = kMissing;
+
+    void Record(InvokedAs rhs) {
+      ASSERT_EQ(invoked_as, kMissing);
+      invoked_as = rhs;
+    }
+
+    void Record(CopiedAs rhs) {
+      if (copied_as == kNone || rhs == kCopy) copied_as = rhs;
+    }
+  } tracker;
+
+  class Releaser {
+   public:
+    explicit Releaser(Tracker* tracker) : tr_(tracker) { *tracker = Tracker(); }
+    Releaser(Releaser&& rhs) : tr_(rhs.tr_) { tr_->Record(kMove); }
+    Releaser(const Releaser& rhs) : tr_(rhs.tr_) { tr_->Record(kCopy); }
+
+    void operator()(absl::string_view) & { tr_->Record(kLValue); }
+    void operator()(absl::string_view) && { tr_->Record(kRValue); }
+
+   private:
+    Tracker* tr_;
+  };
+
+  const Releaser releaser1(&tracker);
+  (void)MaybeHardened(absl::MakeCordFromExternal("", releaser1));
+  EXPECT_EQ(tracker.copied_as, kCopy);
+  EXPECT_EQ(tracker.invoked_as, kRValue);
+
+  const Releaser releaser2(&tracker);
+  (void)MaybeHardened(absl::MakeCordFromExternal("", releaser2));
+  EXPECT_EQ(tracker.copied_as, kCopy);
+  EXPECT_EQ(tracker.invoked_as, kRValue);
+
+  Releaser releaser3(&tracker);
+  (void)MaybeHardened(absl::MakeCordFromExternal("", std::move(releaser3)));
+  EXPECT_EQ(tracker.copied_as, kMove);
+  EXPECT_EQ(tracker.invoked_as, kRValue);
+
+  Releaser releaser4(&tracker);
+  (void)MaybeHardened(absl::MakeCordFromExternal("dummy", releaser4));
+  EXPECT_EQ(tracker.copied_as, kCopy);
+  EXPECT_EQ(tracker.invoked_as, kRValue);
+
+  const Releaser releaser5(&tracker);
+  (void)MaybeHardened(absl::MakeCordFromExternal("dummy", releaser5));
+  EXPECT_EQ(tracker.copied_as, kCopy);
+  EXPECT_EQ(tracker.invoked_as, kRValue);
+
+  Releaser releaser6(&tracker);
+  (void)MaybeHardened(absl::MakeCordFromExternal("foo", std::move(releaser6)));
+  EXPECT_EQ(tracker.copied_as, kMove);
+  EXPECT_EQ(tracker.invoked_as, kRValue);
+}
+
+TEST_P(CordTest, ExternalMemoryBasicUsage) {
+  static const char* strings[] = {"", "hello", "there"};
+  for (const char* str : strings) {
+    absl::Cord dst("(prefix)");
+    MaybeHarden(dst);
+    AddExternalMemory(str, &dst);
+    MaybeHarden(dst);
+    dst.Append("(suffix)");
+    EXPECT_EQ((std::string("(prefix)") + str + std::string("(suffix)")),
+              std::string(dst));
+  }
+}
+
+TEST_P(CordTest, ExternalMemoryRemovePrefixSuffix) {
+  // Exhaustively try all sub-strings.
+  absl::Cord cord = MakeComposite();
+  std::string s = std::string(cord);
+  for (int offset = 0; offset <= s.size(); offset++) {
+    for (int length = 0; length <= s.size() - offset; length++) {
+      absl::Cord result(cord);
+      MaybeHarden(result);
+      result.RemovePrefix(offset);
+      MaybeHarden(result);
+      result.RemoveSuffix(result.size() - length);
+      EXPECT_EQ(s.substr(offset, length), std::string(result))
+          << offset << " " << length;
+    }
+  }
+}
+
+TEST_P(CordTest, ExternalMemoryGet) {
+  absl::Cord cord("hello");
+  AddExternalMemory(" world!", &cord);
+  MaybeHarden(cord);
+  AddExternalMemory(" how are ", &cord);
+  cord.Append(" you?");
+  MaybeHarden(cord);
+  std::string s = std::string(cord);
+  for (int i = 0; i < s.size(); i++) {
+    EXPECT_EQ(s[i], cord[i]);
+  }
+}
+
+// CordMemoryUsage tests verify the correctness of the EstimatedMemoryUsage()
+// We use whiteboxed expectations based on our knowledge of the layout and size
+// of empty and inlined cords, and flat nodes.
+
+constexpr auto kFairShare = absl::CordMemoryAccounting::kFairShare;
+constexpr auto kTotalMorePrecise =
+    absl::CordMemoryAccounting::kTotalMorePrecise;
+
+// Creates a cord of `n` `c` values, making sure no string stealing occurs.
+absl::Cord MakeCord(size_t n, char c) {
+  const std::string s(n, c);
+  return absl::Cord(s);
+}
+
+TEST(CordTest, CordMemoryUsageEmpty) {
+  absl::Cord cord;
+  EXPECT_EQ(sizeof(absl::Cord), cord.EstimatedMemoryUsage());
+  EXPECT_EQ(sizeof(absl::Cord), cord.EstimatedMemoryUsage(kFairShare));
+  EXPECT_EQ(sizeof(absl::Cord), cord.EstimatedMemoryUsage(kTotalMorePrecise));
+}
+
+TEST(CordTest, CordMemoryUsageInlined) {
+  absl::Cord a("hello");
+  EXPECT_EQ(a.EstimatedMemoryUsage(), sizeof(absl::Cord));
+  EXPECT_EQ(a.EstimatedMemoryUsage(kFairShare), sizeof(absl::Cord));
+  EXPECT_EQ(a.EstimatedMemoryUsage(kTotalMorePrecise), sizeof(absl::Cord));
+}
+
+TEST(CordTest, CordMemoryUsageExternalMemory) {
+  absl::Cord cord;
+  AddExternalMemory(std::string(1000, 'x'), &cord);
+  const size_t expected =
+      sizeof(absl::Cord) + 1000 + sizeof(CordRepExternal) + sizeof(intptr_t);
+  EXPECT_EQ(cord.EstimatedMemoryUsage(), expected);
+  EXPECT_EQ(cord.EstimatedMemoryUsage(kFairShare), expected);
+  EXPECT_EQ(cord.EstimatedMemoryUsage(kTotalMorePrecise), expected);
+}
+
+TEST(CordTest, CordMemoryUsageFlat) {
+  absl::Cord cord = MakeCord(1000, 'a');
+  const size_t flat_size =
+      absl::CordTestPeer::Tree(cord)->flat()->AllocatedSize();
+  EXPECT_EQ(cord.EstimatedMemoryUsage(), sizeof(absl::Cord) + flat_size);
+  EXPECT_EQ(cord.EstimatedMemoryUsage(kFairShare),
+            sizeof(absl::Cord) + flat_size);
+  EXPECT_EQ(cord.EstimatedMemoryUsage(kTotalMorePrecise),
+            sizeof(absl::Cord) + flat_size);
+}
+
+TEST(CordTest, CordMemoryUsageSubStringSharedFlat) {
+  absl::Cord flat = MakeCord(2000, 'a');
+  const size_t flat_size =
+      absl::CordTestPeer::Tree(flat)->flat()->AllocatedSize();
+  absl::Cord cord = flat.Subcord(500, 1000);
+  EXPECT_EQ(cord.EstimatedMemoryUsage(),
+            sizeof(absl::Cord) + sizeof(CordRepSubstring) + flat_size);
+  EXPECT_EQ(cord.EstimatedMemoryUsage(kTotalMorePrecise),
+            sizeof(absl::Cord) + sizeof(CordRepSubstring) + flat_size);
+  EXPECT_EQ(cord.EstimatedMemoryUsage(kFairShare),
+            sizeof(absl::Cord) + sizeof(CordRepSubstring) + flat_size / 2);
+}
+
+TEST(CordTest, CordMemoryUsageFlatShared) {
+  absl::Cord shared = MakeCord(1000, 'a');
+  absl::Cord cord(shared);
+  const size_t flat_size =
+      absl::CordTestPeer::Tree(cord)->flat()->AllocatedSize();
+  EXPECT_EQ(cord.EstimatedMemoryUsage(), sizeof(absl::Cord) + flat_size);
+  EXPECT_EQ(cord.EstimatedMemoryUsage(kTotalMorePrecise),
+            sizeof(absl::Cord) + flat_size);
+  EXPECT_EQ(cord.EstimatedMemoryUsage(kFairShare),
+            sizeof(absl::Cord) + flat_size / 2);
+}
+
+TEST(CordTest, CordMemoryUsageFlatHardenedAndShared) {
+  absl::Cord shared = MakeCord(1000, 'a');
+  absl::Cord cord(shared);
+  const size_t flat_size =
+      absl::CordTestPeer::Tree(cord)->flat()->AllocatedSize();
+  cord.SetExpectedChecksum(1);
+  EXPECT_EQ(cord.EstimatedMemoryUsage(),
+            sizeof(absl::Cord) + sizeof(CordRepCrc) + flat_size);
+  EXPECT_EQ(cord.EstimatedMemoryUsage(kFairShare),
+            sizeof(absl::Cord) + sizeof(CordRepCrc) + flat_size / 2);
+
+  absl::Cord cord2(cord);
+  EXPECT_EQ(cord2.EstimatedMemoryUsage(),
+            sizeof(absl::Cord) + sizeof(CordRepCrc) + flat_size);
+  EXPECT_EQ(cord2.EstimatedMemoryUsage(kTotalMorePrecise),
+            sizeof(absl::Cord) + sizeof(CordRepCrc) + flat_size);
+  EXPECT_EQ(cord2.EstimatedMemoryUsage(kFairShare),
+            sizeof(absl::Cord) + (sizeof(CordRepCrc) + flat_size / 2) / 2);
+}
+
+TEST(CordTest, CordMemoryUsageBTree) {
+  absl::Cord cord1;
+  size_t flats1_size = 0;
+  absl::Cord flats1[4] = {MakeCord(1000, 'a'), MakeCord(1100, 'a'),
+                          MakeCord(1200, 'a'), MakeCord(1300, 'a')};
+  for (absl::Cord flat : flats1) {
+    flats1_size += absl::CordTestPeer::Tree(flat)->flat()->AllocatedSize();
+    cord1.Append(std::move(flat));
+  }
+
+  // Make sure the created cord is a BTREE tree. Under some builds such as
+  // windows DLL, we may have ODR like effects on the flag, meaning the DLL
+  // code will run with the picked up default.
+  if (!absl::CordTestPeer::Tree(cord1)->IsBtree()) {
+    LOG(WARNING) << "Cord library code not respecting btree flag";
+    return;
+  }
+
+  size_t rep1_size = sizeof(CordRepBtree) + flats1_size;
+  size_t rep1_shared_size = sizeof(CordRepBtree) + flats1_size / 2;
+
+  EXPECT_EQ(cord1.EstimatedMemoryUsage(), sizeof(absl::Cord) + rep1_size);
+  EXPECT_EQ(cord1.EstimatedMemoryUsage(kTotalMorePrecise),
+            sizeof(absl::Cord) + rep1_size);
+  EXPECT_EQ(cord1.EstimatedMemoryUsage(kFairShare),
+            sizeof(absl::Cord) + rep1_shared_size);
+
+  absl::Cord cord2;
+  size_t flats2_size = 0;
+  absl::Cord flats2[4] = {MakeCord(600, 'a'), MakeCord(700, 'a'),
+                          MakeCord(800, 'a'), MakeCord(900, 'a')};
+  for (absl::Cord& flat : flats2) {
+    flats2_size += absl::CordTestPeer::Tree(flat)->flat()->AllocatedSize();
+    cord2.Append(std::move(flat));
+  }
+  size_t rep2_size = sizeof(CordRepBtree) + flats2_size;
+
+  EXPECT_EQ(cord2.EstimatedMemoryUsage(), sizeof(absl::Cord) + rep2_size);
+  EXPECT_EQ(cord2.EstimatedMemoryUsage(kTotalMorePrecise),
+            sizeof(absl::Cord) + rep2_size);
+  EXPECT_EQ(cord2.EstimatedMemoryUsage(kFairShare),
+            sizeof(absl::Cord) + rep2_size);
+
+  absl::Cord cord(cord1);
+  cord.Append(std::move(cord2));
+
+  EXPECT_EQ(cord.EstimatedMemoryUsage(),
+            sizeof(absl::Cord) + sizeof(CordRepBtree) + rep1_size + rep2_size);
+  EXPECT_EQ(cord.EstimatedMemoryUsage(kTotalMorePrecise),
+            sizeof(absl::Cord) + sizeof(CordRepBtree) + rep1_size + rep2_size);
+  EXPECT_EQ(cord.EstimatedMemoryUsage(kFairShare),
+            sizeof(absl::Cord) + sizeof(CordRepBtree) + rep1_shared_size / 2 +
+                rep2_size);
+}
+
+TEST(CordTest, TestHashFragmentation) {
+  // Make sure we hit these boundary cases precisely.
+  EXPECT_EQ(1024, absl::hash_internal::PiecewiseChunkSize());
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly({
+      absl::Cord(),
+      absl::MakeFragmentedCord({std::string(600, 'a'), std::string(600, 'a')}),
+      absl::MakeFragmentedCord({std::string(1200, 'a')}),
+      absl::MakeFragmentedCord({std::string(900, 'b'), std::string(900, 'b')}),
+      absl::MakeFragmentedCord({std::string(1800, 'b')}),
+      absl::MakeFragmentedCord(
+          {std::string(2000, 'c'), std::string(2000, 'c')}),
+      absl::MakeFragmentedCord({std::string(4000, 'c')}),
+      absl::MakeFragmentedCord({std::string(1024, 'd')}),
+      absl::MakeFragmentedCord({std::string(1023, 'd'), "d"}),
+      absl::MakeFragmentedCord({std::string(1025, 'e')}),
+      absl::MakeFragmentedCord({std::string(1024, 'e'), "e"}),
+      absl::MakeFragmentedCord({std::string(1023, 'e'), "e", "e"}),
+  }));
+}
+
+// Regtest for a change that had to be rolled back because it expanded out
+// of the InlineRep too soon, which was observable through MemoryUsage().
+TEST_P(CordTest, CordMemoryUsageInlineRep) {
+  constexpr size_t kMaxInline = 15;  // Cord::InlineRep::N
+  const std::string small_string(kMaxInline, 'x');
+  absl::Cord c1(small_string);
+
+  absl::Cord c2;
+  c2.Append(small_string);
+  EXPECT_EQ(c1, c2);
+  EXPECT_EQ(c1.EstimatedMemoryUsage(), c2.EstimatedMemoryUsage());
+}
+
+TEST_P(CordTest, CordMemoryUsageTotalMorePreciseMode) {
+  constexpr size_t kChunkSize = 2000;
+  std::string tmp_str(kChunkSize, 'x');
+  const absl::Cord flat(std::move(tmp_str));
+
+  // Construct `fragmented` with two references into the same
+  // underlying buffer shared with `flat`:
+  absl::Cord fragmented(flat);
+  fragmented.Append(flat);
+
+  // Memory usage of `flat`, minus the top-level Cord object:
+  const size_t flat_internal_usage =
+      flat.EstimatedMemoryUsage() - sizeof(absl::Cord);
+
+  // `fragmented` holds a Cord and a CordRepBtree. That tree points to two
+  // copies of flat's internals, which we expect to dedup:
+  EXPECT_EQ(fragmented.EstimatedMemoryUsage(kTotalMorePrecise),
+            sizeof(absl::Cord) +
+            sizeof(CordRepBtree) +
+            flat_internal_usage);
+
+  // This is a case where kTotal produces an overestimate:
+  EXPECT_EQ(fragmented.EstimatedMemoryUsage(),
+            sizeof(absl::Cord) +
+            sizeof(CordRepBtree) +
+            2 * flat_internal_usage);
+}
+
+TEST_P(CordTest, CordMemoryUsageTotalMorePreciseModeWithSubstring) {
+  constexpr size_t kChunkSize = 2000;
+  std::string tmp_str(kChunkSize, 'x');
+  const absl::Cord flat(std::move(tmp_str));
+
+  // Construct `fragmented` with two references into the same
+  // underlying buffer shared with `flat`.
+  //
+  // This time, each reference is through a Subcord():
+  absl::Cord fragmented;
+  fragmented.Append(flat.Subcord(1, kChunkSize - 2));
+  fragmented.Append(flat.Subcord(1, kChunkSize - 2));
+
+  // Memory usage of `flat`, minus the top-level Cord object:
+  const size_t flat_internal_usage =
+      flat.EstimatedMemoryUsage() - sizeof(absl::Cord);
+
+  // `fragmented` holds a Cord and a CordRepBtree. That tree points to two
+  // CordRepSubstrings, each pointing at flat's internals.
+  EXPECT_EQ(fragmented.EstimatedMemoryUsage(kTotalMorePrecise),
+            sizeof(absl::Cord) +
+            sizeof(CordRepBtree) +
+            2 * sizeof(CordRepSubstring) +
+            flat_internal_usage);
+
+  // This is a case where kTotal produces an overestimate:
+  EXPECT_EQ(fragmented.EstimatedMemoryUsage(),
+            sizeof(absl::Cord) +
+            sizeof(CordRepBtree) +
+            2 * sizeof(CordRepSubstring) +
+            2 * flat_internal_usage);
+}
+}  // namespace
+
+// Regtest for 7510292 (fix a bug introduced by 7465150)
+TEST_P(CordTest, Concat_Append) {
+  // Create a rep of type CONCAT
+  absl::Cord s1("foobarbarbarbarbar");
+  MaybeHarden(s1);
+  s1.Append("abcdefgabcdefgabcdefgabcdefgabcdefgabcdefgabcdefg");
+  size_t size = s1.size();
+
+  // Create a copy of s1 and append to it.
+  absl::Cord s2 = s1;
+  MaybeHarden(s2);
+  s2.Append("x");
+
+  // 7465150 modifies s1 when it shouldn't.
+  EXPECT_EQ(s1.size(), size);
+  EXPECT_EQ(s2.size(), size + 1);
+}
+
+TEST_P(CordTest, DiabolicalGrowth) {
+  // This test exercises a diabolical Append(<one char>) on a cord, making the
+  // cord shared before each Append call resulting in a terribly fragmented
+  // resulting cord.
+  RandomEngine rng(GTEST_FLAG_GET(random_seed));
+  const std::string expected = RandomLowercaseString(&rng, 5000);
+  absl::Cord cord;
+  for (char c : expected) {
+    absl::Cord shared(cord);
+    EXPECT_THAT(cord, testing::Eq(shared));
+    cord.Append(absl::string_view(&c, 1));
+    MaybeHarden(cord);
+  }
+  std::string value;
+  absl::CopyCordToString(cord, &value);
+  EXPECT_EQ(value, expected);
+  LOG(INFO) << "Diabolical size allocated = " << cord.EstimatedMemoryUsage();
+}
+
+// The following tests check support for >4GB cords in 64-bit binaries, and
+// 2GB-4GB cords in 32-bit binaries.  This function returns the large cord size
+// that's appropriate for the binary.
+
+// Construct a huge cord with the specified valid prefix.
+static absl::Cord MakeHuge(absl::string_view prefix) {
+  absl::Cord cord;
+  if (sizeof(size_t) > 4) {
+    // In 64-bit binaries, test 64-bit Cord support.
+    const size_t size =
+        static_cast<size_t>(std::numeric_limits<uint32_t>::max()) + 314;
+    cord.Append(absl::MakeCordFromExternal(
+        absl::string_view(prefix.data(), size),
+        [](absl::string_view s) { DoNothing(s, nullptr); }));
+  } else {
+    // Cords are limited to 32-bit lengths in 32-bit binaries.  The following
+    // tests check for use of "signed int" to represent Cord length/offset.
+    // However absl::string_view does not allow lengths >= (1u<<31), so we need
+    // to append in two parts;
+    const size_t s1 = (1u << 31) - 1;
+    // For shorter cord, `Append` copies the data rather than allocating a new
+    // node. The threshold is currently set to 511, so `s2` needs to be bigger
+    // to not trigger the copy.
+    const size_t s2 = 600;
+    cord.Append(absl::MakeCordFromExternal(
+        absl::string_view(prefix.data(), s1),
+        [](absl::string_view s) { DoNothing(s, nullptr); }));
+    cord.Append(absl::MakeCordFromExternal(
+        absl::string_view("", s2),
+        [](absl::string_view s) { DoNothing(s, nullptr); }));
+  }
+  return cord;
+}
+
+TEST_P(CordTest, HugeCord) {
+  absl::Cord cord = MakeHuge("huge cord");
+  MaybeHarden(cord);
+
+  const size_t acceptable_delta =
+      100 + (UseCrc() ? sizeof(absl::cord_internal::CordRepCrc) : 0);
+  EXPECT_LE(cord.size(), cord.EstimatedMemoryUsage());
+  EXPECT_GE(cord.size() + acceptable_delta, cord.EstimatedMemoryUsage());
+}
+
+// Tests that Append() works ok when handed a self reference
+TEST_P(CordTest, AppendSelf) {
+  // Test the empty case.
+  absl::Cord empty;
+  MaybeHarden(empty);
+  empty.Append(empty);
+  ASSERT_EQ(empty, "");
+
+  // We run the test until data is ~16K
+  // This guarantees it covers small, medium and large data.
+  std::string control_data = "Abc";
+  absl::Cord data(control_data);
+  while (control_data.length() < 0x4000) {
+    MaybeHarden(data);
+    data.Append(data);
+    control_data.append(control_data);
+    ASSERT_EQ(control_data, data);
+  }
+}
+
+TEST_P(CordTest, MakeFragmentedCordFromInitializerList) {
+  absl::Cord fragmented =
+      absl::MakeFragmentedCord({"A ", "fragmented ", "Cord"});
+
+  MaybeHarden(fragmented);
+
+  EXPECT_EQ("A fragmented Cord", fragmented);
+
+  auto chunk_it = fragmented.chunk_begin();
+
+  ASSERT_TRUE(chunk_it != fragmented.chunk_end());
+  EXPECT_EQ("A ", *chunk_it);
+
+  ASSERT_TRUE(++chunk_it != fragmented.chunk_end());
+  EXPECT_EQ("fragmented ", *chunk_it);
+
+  ASSERT_TRUE(++chunk_it != fragmented.chunk_end());
+  EXPECT_EQ("Cord", *chunk_it);
+
+  ASSERT_TRUE(++chunk_it == fragmented.chunk_end());
+}
+
+TEST_P(CordTest, MakeFragmentedCordFromVector) {
+  std::vector<absl::string_view> chunks = {"A ", "fragmented ", "Cord"};
+  absl::Cord fragmented = absl::MakeFragmentedCord(chunks);
+
+  MaybeHarden(fragmented);
+
+  EXPECT_EQ("A fragmented Cord", fragmented);
+
+  auto chunk_it = fragmented.chunk_begin();
+
+  ASSERT_TRUE(chunk_it != fragmented.chunk_end());
+  EXPECT_EQ("A ", *chunk_it);
+
+  ASSERT_TRUE(++chunk_it != fragmented.chunk_end());
+  EXPECT_EQ("fragmented ", *chunk_it);
+
+  ASSERT_TRUE(++chunk_it != fragmented.chunk_end());
+  EXPECT_EQ("Cord", *chunk_it);
+
+  ASSERT_TRUE(++chunk_it == fragmented.chunk_end());
+}
+
+TEST_P(CordTest, CordChunkIteratorTraits) {
+  static_assert(std::is_copy_constructible_v<absl::Cord::ChunkIterator>);
+  static_assert(std::is_copy_assignable_v<absl::Cord::ChunkIterator>);
+
+  // Move semantics to satisfy swappable via std::swap
+  static_assert(std::is_move_constructible_v<absl::Cord::ChunkIterator>);
+  static_assert(std::is_move_assignable_v<absl::Cord::ChunkIterator>);
+
+  static_assert(
+      std::is_same_v<
+          std::iterator_traits<absl::Cord::ChunkIterator>::iterator_category,
+          std::input_iterator_tag>);
+  static_assert(std::is_same_v<
+                std::iterator_traits<absl::Cord::ChunkIterator>::value_type,
+                absl::string_view>);
+  static_assert(
+      std::is_same_v<
+          std::iterator_traits<absl::Cord::ChunkIterator>::difference_type,
+          ptrdiff_t>);
+  static_assert(
+      std::is_same_v<std::iterator_traits<absl::Cord::ChunkIterator>::pointer,
+                     const absl::string_view*>);
+  static_assert(
+      std::is_same_v<std::iterator_traits<absl::Cord::ChunkIterator>::reference,
+                     absl::string_view>);
+}
+
+static void VerifyChunkIterator(const absl::Cord& cord,
+                                size_t expected_chunks) {
+  EXPECT_EQ(cord.chunk_begin() == cord.chunk_end(), cord.empty()) << cord;
+  EXPECT_EQ(cord.chunk_begin() != cord.chunk_end(), !cord.empty());
+
+  absl::Cord::ChunkRange range = cord.Chunks();
+  EXPECT_EQ(range.begin() == range.end(), cord.empty());
+  EXPECT_EQ(range.begin() != range.end(), !cord.empty());
+
+  std::string content(cord);
+  size_t pos = 0;
+  auto pre_iter = cord.chunk_begin(), post_iter = cord.chunk_begin();
+  size_t n_chunks = 0;
+  while (pre_iter != cord.chunk_end() && post_iter != cord.chunk_end()) {
+    EXPECT_FALSE(pre_iter == cord.chunk_end());   // NOLINT: explicitly test ==
+    EXPECT_FALSE(post_iter == cord.chunk_end());  // NOLINT
+
+    EXPECT_EQ(pre_iter, post_iter);
+    EXPECT_EQ(*pre_iter, *post_iter);
+
+    EXPECT_EQ(pre_iter->data(), (*pre_iter).data());
+    EXPECT_EQ(pre_iter->size(), (*pre_iter).size());
+
+    absl::string_view chunk = *pre_iter;
+    EXPECT_FALSE(chunk.empty());
+    EXPECT_LE(pos + chunk.size(), content.size());
+    EXPECT_EQ(absl::string_view(content.c_str() + pos, chunk.size()), chunk);
+
+    int n_equal_iterators = 0;
+    for (absl::Cord::ChunkIterator it = range.begin(); it != range.end();
+         ++it) {
+      n_equal_iterators += static_cast<int>(it == pre_iter);
+    }
+    EXPECT_EQ(n_equal_iterators, 1);
+
+    ++pre_iter;
+    EXPECT_EQ(*post_iter++, chunk);
+
+    pos += chunk.size();
+    ++n_chunks;
+  }
+  EXPECT_EQ(expected_chunks, n_chunks);
+  EXPECT_EQ(pos, content.size());
+  EXPECT_TRUE(pre_iter == cord.chunk_end());   // NOLINT: explicitly test ==
+  EXPECT_TRUE(post_iter == cord.chunk_end());  // NOLINT
+}
+
+TEST_P(CordTest, CordChunkIteratorOperations) {
+  absl::Cord empty_cord;
+  VerifyChunkIterator(empty_cord, 0);
+
+  absl::Cord small_buffer_cord("small cord");
+  MaybeHarden(small_buffer_cord);
+  VerifyChunkIterator(small_buffer_cord, 1);
+
+  absl::Cord flat_node_cord("larger than small buffer optimization");
+  MaybeHarden(flat_node_cord);
+  VerifyChunkIterator(flat_node_cord, 1);
+
+  VerifyChunkIterator(MaybeHardened(absl::MakeFragmentedCord(
+                          {"a ", "small ", "fragmented ", "cord ", "for ",
+                           "testing ", "chunk ", "iterations."})),
+                      8);
+
+  absl::Cord reused_nodes_cord(std::string(40, 'c'));
+  reused_nodes_cord.Prepend(absl::Cord(std::string(40, 'b')));
+  MaybeHarden(reused_nodes_cord);
+  reused_nodes_cord.Prepend(absl::Cord(std::string(40, 'a')));
+  size_t expected_chunks = 3;
+  for (int i = 0; i < 8; ++i) {
+    reused_nodes_cord.Prepend(reused_nodes_cord);
+    MaybeHarden(reused_nodes_cord);
+    expected_chunks *= 2;
+    VerifyChunkIterator(reused_nodes_cord, expected_chunks);
+  }
+
+  RandomEngine rng(GTEST_FLAG_GET(random_seed));
+  absl::Cord flat_cord(RandomLowercaseString(&rng, 256));
+  absl::Cord subcords;
+  for (int i = 0; i < 128; ++i) subcords.Prepend(flat_cord.Subcord(i, 128));
+  VerifyChunkIterator(subcords, 128);
+}
+
+
+TEST_P(CordTest, AdvanceAndReadOnDataEdge) {
+  RandomEngine rng(GTEST_FLAG_GET(random_seed));
+  const std::string data = RandomLowercaseString(&rng, 2000);
+  for (bool as_flat : {true, false}) {
+    SCOPED_TRACE(as_flat ? "Flat" : "External");
+
+    absl::Cord cord =
+        as_flat ? absl::Cord(data)
+                : absl::MakeCordFromExternal(data, [](absl::string_view) {});
+    auto it = cord.Chars().begin();
+#if !defined(NDEBUG) || ABSL_OPTION_HARDENED
+    EXPECT_DEATH_IF_SUPPORTED(cord.AdvanceAndRead(&it, 2001), ".*");
+#endif
+
+    it = cord.Chars().begin();
+    absl::Cord frag = cord.AdvanceAndRead(&it, 2000);
+    EXPECT_EQ(frag, data);
+    EXPECT_TRUE(it == cord.Chars().end());
+
+    it = cord.Chars().begin();
+    frag = cord.AdvanceAndRead(&it, 200);
+    EXPECT_EQ(frag, data.substr(0, 200));
+    EXPECT_FALSE(it == cord.Chars().end());
+
+    frag = cord.AdvanceAndRead(&it, 1500);
+    EXPECT_EQ(frag, data.substr(200, 1500));
+    EXPECT_FALSE(it == cord.Chars().end());
+
+    frag = cord.AdvanceAndRead(&it, 300);
+    EXPECT_EQ(frag, data.substr(1700, 300));
+    EXPECT_TRUE(it == cord.Chars().end());
+  }
+}
+
+TEST_P(CordTest, AdvanceAndReadOnSubstringDataEdge) {
+  RandomEngine rng(GTEST_FLAG_GET(random_seed));
+  const std::string data = RandomLowercaseString(&rng, 2500);
+  for (bool as_flat : {true, false}) {
+    SCOPED_TRACE(as_flat ? "Flat" : "External");
+
+    absl::Cord cord =
+        as_flat ? absl::Cord(data)
+                : absl::MakeCordFromExternal(data, [](absl::string_view) {});
+    cord = cord.Subcord(200, 2000);
+    const std::string substr = data.substr(200, 2000);
+
+    auto it = cord.Chars().begin();
+#if !defined(NDEBUG) || ABSL_OPTION_HARDENED
+    EXPECT_DEATH_IF_SUPPORTED(cord.AdvanceAndRead(&it, 2001), ".*");
+#endif
+
+    it = cord.Chars().begin();
+    absl::Cord frag = cord.AdvanceAndRead(&it, 2000);
+    EXPECT_EQ(frag, substr);
+    EXPECT_TRUE(it == cord.Chars().end());
+
+    it = cord.Chars().begin();
+    frag = cord.AdvanceAndRead(&it, 200);
+    EXPECT_EQ(frag, substr.substr(0, 200));
+    EXPECT_FALSE(it == cord.Chars().end());
+
+    frag = cord.AdvanceAndRead(&it, 1500);
+    EXPECT_EQ(frag, substr.substr(200, 1500));
+    EXPECT_FALSE(it == cord.Chars().end());
+
+    frag = cord.AdvanceAndRead(&it, 300);
+    EXPECT_EQ(frag, substr.substr(1700, 300));
+    EXPECT_TRUE(it == cord.Chars().end());
+  }
+}
+
+TEST_P(CordTest, CharIteratorTraits) {
+  static_assert(std::is_copy_constructible_v<absl::Cord::CharIterator>);
+  static_assert(std::is_copy_assignable_v<absl::Cord::CharIterator>);
+
+  // Move semantics to satisfy swappable via std::swap
+  static_assert(std::is_move_constructible_v<absl::Cord::CharIterator>);
+  static_assert(std::is_move_assignable_v<absl::Cord::CharIterator>);
+
+  static_assert(
+      std::is_same_v<
+          std::iterator_traits<absl::Cord::CharIterator>::iterator_category,
+          std::input_iterator_tag>);
+  static_assert(
+      std::is_same_v<std::iterator_traits<absl::Cord::CharIterator>::value_type,
+                     char>);
+  static_assert(std::is_same_v<
+                std::iterator_traits<absl::Cord::CharIterator>::difference_type,
+                ptrdiff_t>);
+  static_assert(
+      std::is_same_v<std::iterator_traits<absl::Cord::CharIterator>::pointer,
+                     const char*>);
+  static_assert(
+      std::is_same_v<std::iterator_traits<absl::Cord::CharIterator>::reference,
+                     const char&>);
+}
+
+static void VerifyCharIterator(const absl::Cord& cord) {
+  EXPECT_EQ(cord.char_begin() == cord.char_end(), cord.empty());
+  EXPECT_EQ(cord.char_begin() != cord.char_end(), !cord.empty());
+
+  absl::Cord::CharRange range = cord.Chars();
+  EXPECT_EQ(range.begin() == range.end(), cord.empty());
+  EXPECT_EQ(range.begin() != range.end(), !cord.empty());
+  EXPECT_EQ(absl::Cord::Distance(range.begin(), range.end()),
+            static_cast<ptrdiff_t>(cord.size()));
+  EXPECT_EQ(absl::Cord::Distance(range.end(), range.begin()),
+            -static_cast<ptrdiff_t>(cord.size()));
+
+  size_t i = 0;
+  absl::Cord::CharIterator pre_iter = cord.char_begin();
+  absl::Cord::CharIterator post_iter = cord.char_begin();
+  std::string content(cord);
+  while (pre_iter != cord.char_end() && post_iter != cord.char_end()) {
+    EXPECT_FALSE(pre_iter == cord.char_end());   // NOLINT: explicitly test ==
+    EXPECT_FALSE(post_iter == cord.char_end());  // NOLINT
+
+    EXPECT_LT(i, cord.size());
+    EXPECT_EQ(content[i], *pre_iter);
+
+    EXPECT_EQ(pre_iter, post_iter);
+    EXPECT_EQ(*pre_iter, *post_iter);
+    EXPECT_EQ(&*pre_iter, &*post_iter);
+
+    const char* character_address = &*pre_iter;
+    absl::Cord::CharIterator copy = pre_iter;
+    ++copy;
+    EXPECT_EQ(character_address, &*pre_iter);
+
+    int n_equal_iterators = 0;
+    for (absl::Cord::CharIterator it = range.begin(); it != range.end(); ++it) {
+      n_equal_iterators += static_cast<int>(it == pre_iter);
+    }
+    EXPECT_EQ(n_equal_iterators, 1);
+
+    absl::Cord::CharIterator advance_iter = range.begin();
+    absl::Cord::Advance(&advance_iter, i);
+    EXPECT_EQ(pre_iter, advance_iter);
+    EXPECT_EQ(absl::Cord::Distance(range.begin(), advance_iter),
+              static_cast<ptrdiff_t>(i));
+
+    advance_iter = range.begin();
+    EXPECT_EQ(absl::Cord::AdvanceAndRead(&advance_iter, i), cord.Subcord(0, i));
+    EXPECT_EQ(pre_iter, advance_iter);
+    EXPECT_EQ(absl::Cord::Distance(range.begin(), advance_iter),
+              static_cast<ptrdiff_t>(i));
+
+    advance_iter = pre_iter;
+    absl::Cord::Advance(&advance_iter, cord.size() - i);
+    EXPECT_EQ(range.end(), advance_iter);
+    EXPECT_EQ(absl::Cord::Distance(range.begin(), advance_iter),
+              static_cast<ptrdiff_t>(cord.size()));
+    EXPECT_EQ(absl::Cord::Distance(advance_iter, range.end()), 0);
+
+    advance_iter = pre_iter;
+    EXPECT_EQ(absl::Cord::AdvanceAndRead(&advance_iter, cord.size() - i),
+              cord.Subcord(i, cord.size() - i));
+    EXPECT_EQ(range.end(), advance_iter);
+    EXPECT_EQ(absl::Cord::Distance(range.begin(), advance_iter),
+              static_cast<ptrdiff_t>(cord.size()));
+    EXPECT_EQ(absl::Cord::Distance(advance_iter, range.end()), 0);
+
+    ++i;
+    ++pre_iter;
+    post_iter++;
+  }
+  EXPECT_EQ(i, cord.size());
+  EXPECT_TRUE(pre_iter == cord.char_end());   // NOLINT: explicitly test ==
+  EXPECT_TRUE(post_iter == cord.char_end());  // NOLINT
+
+  absl::Cord::CharIterator zero_advanced_end = cord.char_end();
+  absl::Cord::Advance(&zero_advanced_end, 0);
+  EXPECT_EQ(zero_advanced_end, cord.char_end());
+
+  absl::Cord::CharIterator it = cord.char_begin();
+  for (absl::string_view chunk : cord.Chunks()) {
+    while (!chunk.empty()) {
+      EXPECT_EQ(absl::Cord::ChunkRemaining(it), chunk);
+      chunk.remove_prefix(1);
+      ++it;
+    }
+  }
+}
+
+TEST_P(CordTest, CharIteratorOperations) {
+  absl::Cord empty_cord;
+  VerifyCharIterator(empty_cord);
+
+  absl::Cord small_buffer_cord("small cord");
+  MaybeHarden(small_buffer_cord);
+  VerifyCharIterator(small_buffer_cord);
+
+  absl::Cord flat_node_cord("larger than small buffer optimization");
+  MaybeHarden(flat_node_cord);
+  VerifyCharIterator(flat_node_cord);
+
+  VerifyCharIterator(MaybeHardened(
+      absl::MakeFragmentedCord({"a ", "small ", "fragmented ", "cord ", "for ",
+                                "testing ", "character ", "iteration."})));
+
+  absl::Cord reused_nodes_cord("ghi");
+  reused_nodes_cord.Prepend(absl::Cord("def"));
+  reused_nodes_cord.Prepend(absl::Cord("abc"));
+  for (int i = 0; i < 4; ++i) {
+    reused_nodes_cord.Prepend(reused_nodes_cord);
+    MaybeHarden(reused_nodes_cord);
+    VerifyCharIterator(reused_nodes_cord);
+  }
+
+  RandomEngine rng(GTEST_FLAG_GET(random_seed));
+  absl::Cord flat_cord(RandomLowercaseString(&rng, 256));
+  absl::Cord subcords;
+  for (int i = 0; i < 4; ++i) {
+    subcords.Prepend(flat_cord.Subcord(16 * i, 128));
+    MaybeHarden(subcords);
+  }
+  VerifyCharIterator(subcords);
+}
+
+TEST_P(CordTest, CharIteratorAdvanceAndRead) {
+  // Create a Cord holding 6 flats of 2500 bytes each, and then iterate over it
+  // reading 150, 1500, 2500 and 3000 bytes. This will result in all possible
+  // partial, full and straddled read combinations including reads below
+  // kMaxBytesToCopy. b/197776822 surfaced a bug for a specific partial, small
+  // read 'at end' on Cord which caused a failure on attempting to read past the
+  // end in CordRepBtreeReader which was not covered by any existing test.
+  constexpr int kBlocks = 6;
+  constexpr size_t kBlockSize = 2500;
+  constexpr size_t kChunkSize1 = 1500;
+  constexpr size_t kChunkSize2 = 2500;
+  constexpr size_t kChunkSize3 = 3000;
+  constexpr size_t kChunkSize4 = 150;
+  RandomEngine rng;
+  std::string data = RandomLowercaseString(&rng, kBlocks * kBlockSize);
+  absl::Cord cord;
+  for (int i = 0; i < kBlocks; ++i) {
+    const std::string block = data.substr(i * kBlockSize, kBlockSize);
+    cord.Append(absl::Cord(block));
+  }
+
+  MaybeHarden(cord);
+
+
+  for (size_t chunk_size :
+       {kChunkSize1, kChunkSize2, kChunkSize3, kChunkSize4}) {
+    absl::Cord::CharIterator it = cord.char_begin();
+    size_t it_remaining = cord.size();
+    size_t it_advanced = 0;
+    size_t offset = 0;
+    while (offset < data.length()) {
+      EXPECT_EQ(absl::Cord::Distance(it, cord.char_end()), it_remaining);
+      EXPECT_EQ(absl::Cord::Distance(cord.char_begin(), it), it_advanced);
+      const size_t n = std::min<size_t>(data.length() - offset, chunk_size);
+      absl::Cord chunk = cord.AdvanceAndRead(&it, n);
+      ASSERT_EQ(chunk.size(), n);
+      ASSERT_EQ(chunk.Compare(data.substr(offset, n)), 0);
+      offset += n;
+      it_remaining -= n;
+      it_advanced += n;
+      EXPECT_EQ(absl::Cord::Distance(it, cord.char_end()), it_remaining);
+      EXPECT_EQ(absl::Cord::Distance(cord.char_begin(), it), it_advanced);
+    }
+  }
+}
+
+TEST_P(CordTest, StreamingOutput) {
+  absl::Cord c =
+      absl::MakeFragmentedCord({"A ", "small ", "fragmented ", "Cord", "."});
+  MaybeHarden(c);
+  std::stringstream output;
+  output << c;
+  EXPECT_EQ("A small fragmented Cord.", output.str());
+}
+
+TEST_P(CordTest, ForEachChunk) {
+  for (int num_elements : {1, 10, 200}) {
+    SCOPED_TRACE(num_elements);
+    std::vector<std::string> cord_chunks;
+    for (int i = 0; i < num_elements; ++i) {
+      cord_chunks.push_back(absl::StrCat("[", i, "]"));
+    }
+    absl::Cord c = absl::MakeFragmentedCord(cord_chunks);
+    MaybeHarden(c);
+
+    std::vector<std::string> iterated_chunks;
+    absl::CordTestPeer::ForEachChunk(c,
+                                     [&iterated_chunks](absl::string_view sv) {
+                                       iterated_chunks.emplace_back(sv);
+                                     });
+    EXPECT_EQ(iterated_chunks, cord_chunks);
+  }
+}
+
+TEST_P(CordTest, SmallBufferAssignFromOwnData) {
+  constexpr size_t kMaxInline = 15;
+  std::string contents = "small buff cord";
+  EXPECT_EQ(contents.size(), kMaxInline);
+  for (size_t pos = 0; pos < contents.size(); ++pos) {
+    for (size_t count = contents.size() - pos; count > 0; --count) {
+      absl::Cord c(contents);
+      MaybeHarden(c);
+      absl::string_view flat = c.Flatten();
+      c = flat.substr(pos, count);
+      EXPECT_EQ(c, contents.substr(pos, count))
+          << "pos = " << pos << "; count = " << count;
+    }
+  }
+}
+
+TEST_P(CordTest, Format) {
+  absl::Cord c;
+  absl::Format(&c, "There were %04d little %s.", 3, "pigs");
+  EXPECT_EQ(c, "There were 0003 little pigs.");
+  MaybeHarden(c);
+  absl::Format(&c, "And %-3llx bad wolf!", 1);
+  MaybeHarden(c);
+  EXPECT_EQ(c, "There were 0003 little pigs.And 1   bad wolf!");
+}
+
+TEST_P(CordTest, Stringify) {
+  absl::Cord c =
+      absl::MakeFragmentedCord({"A ", "small ", "fragmented ", "Cord", "."});
+  MaybeHarden(c);
+  EXPECT_EQ(absl::StrCat(c), "A small fragmented Cord.");
+}
+
+TEST_P(CordTest, Hardening) {
+  absl::Cord cord("hello");
+  MaybeHarden(cord);
+
+  // These statement should abort the program in all builds modes.
+  EXPECT_DEATH_IF_SUPPORTED(cord.RemovePrefix(6), "");
+  EXPECT_DEATH_IF_SUPPORTED(cord.RemoveSuffix(6), "");
+
+  bool test_hardening = false;
+  ABSL_HARDENING_ASSERT([&]() {
+    // This only runs when ABSL_HARDENING_ASSERT is active.
+    test_hardening = true;
+    return true;
+  }());
+  if (!test_hardening) return;
+
+  EXPECT_DEATH_IF_SUPPORTED(cord[5], "");
+  EXPECT_DEATH_IF_SUPPORTED(*cord.chunk_end(), "");
+  EXPECT_DEATH_IF_SUPPORTED(static_cast<void>(cord.chunk_end()->empty()), "");
+  EXPECT_DEATH_IF_SUPPORTED(++cord.chunk_end(), "");
+}
+
+// This test mimics a specific (and rare) application repeatedly splitting a
+// cord, inserting (overwriting) a string value, and composing a new cord from
+// the three pieces. This is hostile towards a Btree implementation: A split of
+// a node at any level is likely to have the right-most edge of the left split,
+// and the left-most edge of the right split shared. For example, splitting a
+// leaf node with 6 edges will result likely in a 1-6, 2-5, 3-4, etc. split,
+// sharing the 'split node'. When recomposing such nodes, we 'injected' an edge
+// in that node. As this happens with some probability on each level of the
+// tree, this will quickly grow the tree until it reaches maximum height.
+TEST_P(CordTest, BtreeHostileSplitInsertJoin) {
+  absl::BitGen bitgen;
+
+  // Start with about 1GB of data
+  std::string data(1 << 10, 'x');
+  absl::Cord buffer(data);
+  absl::Cord cord;
+  for (int i = 0; i < 1000000; ++i) {
+    cord.Append(buffer);
+  }
+
+  for (int j = 0; j < 1000; ++j) {
+    MaybeHarden(cord);
+    size_t offset = absl::Uniform(bitgen, 0u, cord.size());
+    size_t length = absl::Uniform(bitgen, 100u, data.size());
+    if (cord.size() == offset) {
+      cord.Append(absl::string_view(data.data(), length));
+    } else {
+      absl::Cord suffix;
+      if (offset + length < cord.size()) {
+        suffix = cord;
+        suffix.RemovePrefix(offset + length);
+      }
+      if (cord.size() > offset) {
+        cord.RemoveSuffix(cord.size() - offset);
+      }
+      cord.Append(absl::string_view(data.data(), length));
+      if (!suffix.empty()) {
+        cord.Append(suffix);
+      }
+    }
+  }
+}
+
+class AfterExitCordTester {
+ public:
+  bool Set(absl::Cord* cord, absl::string_view expected) {
+    cord_ = cord;
+    expected_ = expected;
+    return true;
+  }
+
+  ~AfterExitCordTester() {
+    EXPECT_EQ(*cord_, expected_);
+  }
+ private:
+  absl::Cord* cord_;
+  absl::string_view expected_;
+};
+
+template <typename Str>
+void TestAfterExit(Str) {
+  const auto expected = Str::value;
+  // Defined before `cord` to be destroyed after it.
+  static AfterExitCordTester exit_tester;  // NOLINT
+  static absl::NoDestructor<absl::Cord> cord_leaker(Str{});
+  // cord_leaker is static, so this reference will remain valid through the end
+  // of program execution.
+  static absl::Cord& cord = *cord_leaker;
+  static bool init_exit_tester = exit_tester.Set(&cord, expected);
+  (void)init_exit_tester;
+
+  EXPECT_EQ(cord, expected);
+  // Copy the object and test the copy, and the original.
+  {
+    absl::Cord copy = cord;
+    EXPECT_EQ(copy, expected);
+  }
+  // The original still works
+  EXPECT_EQ(cord, expected);
+
+  // Try making adding more structure to the tree.
+  {
+    absl::Cord copy = cord;
+    std::string expected_copy(expected);
+    for (int i = 0; i < 10; ++i) {
+      copy.Append(cord);
+      absl::StrAppend(&expected_copy, expected);
+      EXPECT_EQ(copy, expected_copy);
+    }
+  }
+
+  // Make sure we are using the right branch during constant evaluation.
+  EXPECT_EQ(absl::CordTestPeer::IsTree(cord), cord.size() >= 16);
+
+  for (int i = 0; i < 10; ++i) {
+    // Make a few more Cords from the same global rep.
+    // This tests what happens when the refcount for it gets below 1.
+    EXPECT_EQ(expected, absl::Cord(Str{}));
+  }
+}
+
+constexpr int SimpleStrlen(const char* p) {
+  return *p ? 1 + SimpleStrlen(p + 1) : 0;
+}
+
+struct ShortView {
+  constexpr absl::string_view operator()() const {
+    return absl::string_view("SSO string", SimpleStrlen("SSO string"));
+  }
+};
+
+struct LongView {
+  constexpr absl::string_view operator()() const {
+    return absl::string_view("String that does not fit SSO.",
+                             SimpleStrlen("String that does not fit SSO."));
+  }
+};
+
+
+TEST_P(CordTest, AfterExit) {
+  TestAfterExit(absl::strings_internal::MakeStringConstant(ShortView{}));
+  TestAfterExit(absl::strings_internal::MakeStringConstant(LongView{}));
+}
+
+namespace {
+
+// Test helper that generates a populated cord for future manipulation.
+//
+// By test convention, all generated cords begin with the characters "abcde" at
+// the start of the first chunk.
+class PopulatedCordFactory {
+ public:
+  constexpr PopulatedCordFactory(absl::string_view name,
+                                 absl::Cord (*generator)())
+      : name_(name), generator_(generator) {}
+
+  absl::string_view Name() const { return name_; }
+  absl::Cord Generate() const { return generator_(); }
+
+ private:
+  absl::string_view name_;
+  absl::Cord (*generator_)();
+};
+
+// clang-format off
+// This array is constant-initialized in conformant compilers.
+PopulatedCordFactory cord_factories[] = {
+  {"sso", [] { return absl::Cord("abcde"); }},
+  {"flat", [] {
+    // Too large to live in SSO space, but small enough to be a simple FLAT.
+    absl::Cord flat(absl::StrCat("abcde", std::string(1000, 'x')));
+    flat.Flatten();
+    return flat;
+  }},
+  {"external", [] {
+    // A cheat: we are using a string literal as the external storage, so a
+    // no-op releaser is correct here.
+    return absl::MakeCordFromExternal("abcde External!", []{});
+  }},
+  {"external substring", [] {
+    // A cheat: we are using a string literal as the external storage, so a
+    // no-op releaser is correct here.
+    absl::Cord ext = absl::MakeCordFromExternal("-abcde External!", []{});
+    return absl::CordTestPeer::MakeSubstring(ext, 1, ext.size() - 1);
+  }},
+  {"substring", [] {
+    absl::Cord flat(absl::StrCat("-abcde", std::string(1000, 'x')));
+    flat.Flatten();
+    return flat.Subcord(1, 998);
+  }},
+  {"fragmented", [] {
+    std::string fragment = absl::StrCat("abcde", std::string(195, 'x'));
+    std::vector<std::string> fragments(200, fragment);
+    absl::Cord cord = absl::MakeFragmentedCord(fragments);
+    assert(cord.size() == 40000);
+    return cord;
+  }},
+};
+// clang-format on
+
+// Test helper that can mutate a cord, and possibly undo the mutation, for
+// testing.
+class CordMutator {
+ public:
+  constexpr CordMutator(absl::string_view name, void (*mutate)(absl::Cord&),
+                        void (*undo)(absl::Cord&) = nullptr)
+      : name_(name), mutate_(mutate), undo_(undo) {}
+
+  absl::string_view Name() const { return name_; }
+  void Mutate(absl::Cord& cord) const { mutate_(cord); }
+  bool CanUndo() const { return undo_ != nullptr; }
+  void Undo(absl::Cord& cord) const { undo_(cord); }
+
+ private:
+  absl::string_view name_;
+  void (*mutate_)(absl::Cord&);
+  void (*undo_)(absl::Cord&);
+};
+
+// clang-format off
+// This array is constant-initialized in conformant compilers.
+CordMutator cord_mutators[] = {
+  {"clear", [](absl::Cord& c) { c.Clear(); }},
+  {"overwrite", [](absl::Cord& c) { c = "overwritten"; }},
+  {
+    "append string",
+    [](absl::Cord& c) { c.Append("0123456789"); },
+    [](absl::Cord& c) { c.RemoveSuffix(10); }
+  },
+  {
+    "append cord",
+    [](absl::Cord& c) {
+      c.Append(absl::MakeFragmentedCord({"12345", "67890"}));
+    },
+    [](absl::Cord& c) { c.RemoveSuffix(10); }
+  },
+  {
+    "append checksummed cord",
+    [](absl::Cord& c) {
+      absl::Cord to_append = absl::MakeFragmentedCord({"12345", "67890"});
+      to_append.SetExpectedChecksum(999);
+      c.Append(to_append);
+    },
+    [](absl::Cord& c) { c.RemoveSuffix(10); }
+  },
+  {
+    "append self",
+    [](absl::Cord& c) { c.Append(c); },
+    [](absl::Cord& c) { c.RemoveSuffix(c.size() / 2); }
+  },
+  {
+    "append empty string",
+    [](absl::Cord& c) { c.Append(""); },
+    [](absl::Cord& c) { }
+  },
+  {
+    "append empty cord",
+    [](absl::Cord& c) { c.Append(absl::Cord()); },
+    [](absl::Cord& c) { }
+  },
+  {
+    "append empty checksummed cord",
+    [](absl::Cord& c) {
+      absl::Cord to_append;
+      to_append.SetExpectedChecksum(999);
+      c.Append(to_append);
+    },
+    [](absl::Cord& c) { }
+  },
+  {
+    "prepend string",
+    [](absl::Cord& c) { c.Prepend("9876543210"); },
+    [](absl::Cord& c) { c.RemovePrefix(10); }
+  },
+  {
+    "prepend cord",
+    [](absl::Cord& c) {
+      c.Prepend(absl::MakeFragmentedCord({"98765", "43210"}));
+    },
+    [](absl::Cord& c) { c.RemovePrefix(10); }
+  },
+  {
+    "prepend checksummed cord",
+    [](absl::Cord& c) {
+      absl::Cord to_prepend = absl::MakeFragmentedCord({"98765", "43210"});
+      to_prepend.SetExpectedChecksum(999);
+      c.Prepend(to_prepend);
+    },
+    [](absl::Cord& c) { c.RemovePrefix(10); }
+  },
+  {
+    "prepend empty string",
+    [](absl::Cord& c) { c.Prepend(""); },
+    [](absl::Cord& c) { }
+  },
+  {
+    "prepend empty cord",
+    [](absl::Cord& c) { c.Prepend(absl::Cord()); },
+    [](absl::Cord& c) { }
+  },
+  {
+    "prepend empty checksummed cord",
+    [](absl::Cord& c) {
+      absl::Cord to_prepend;
+      to_prepend.SetExpectedChecksum(999);
+      c.Prepend(to_prepend);
+    },
+    [](absl::Cord& c) { }
+  },
+  {
+    "prepend self",
+    [](absl::Cord& c) { c.Prepend(c); },
+    [](absl::Cord& c) { c.RemovePrefix(c.size() / 2); }
+  },
+  {"remove prefix", [](absl::Cord& c) { c.RemovePrefix(c.size() / 2); }},
+  {"remove suffix", [](absl::Cord& c) { c.RemoveSuffix(c.size() / 2); }},
+  {"remove 0-prefix", [](absl::Cord& c) { c.RemovePrefix(0); }},
+  {"remove 0-suffix", [](absl::Cord& c) { c.RemoveSuffix(0); }},
+  {"subcord", [](absl::Cord& c) { c = c.Subcord(1, c.size() - 2); }},
+  {
+    "swap inline",
+    [](absl::Cord& c) {
+      absl::Cord other("swap");
+      c.swap(other);
+    }
+  },
+  {
+    "swap tree",
+    [](absl::Cord& c) {
+      absl::Cord other(std::string(10000, 'x'));
+      c.swap(other);
+    }
+  },
+};
+// clang-format on
+}  // namespace
+
+TEST_P(CordTest, ExpectedChecksum) {
+  for (const PopulatedCordFactory& factory : cord_factories) {
+    SCOPED_TRACE(factory.Name());
+    for (bool shared : {false, true}) {
+      SCOPED_TRACE(shared);
+
+      absl::Cord shared_cord_source = factory.Generate();
+      auto make_instance = [=] {
+        return shared ? shared_cord_source : factory.Generate();
+      };
+
+      const absl::Cord base_value = factory.Generate();
+      const std::string base_value_as_string(factory.Generate().Flatten());
+
+      absl::Cord c1 = make_instance();
+      EXPECT_FALSE(c1.ExpectedChecksum().has_value());
+
+      // Setting an expected checksum works, and retains the cord's bytes
+      c1.SetExpectedChecksum(12345);
+      EXPECT_EQ(c1.ExpectedChecksum().value_or(0), 12345);
+      EXPECT_EQ(c1, base_value);
+
+      // Test that setting an expected checksum again doesn't crash or leak
+      // memory.
+      c1.SetExpectedChecksum(12345);
+      EXPECT_EQ(c1.ExpectedChecksum().value_or(0), 12345);
+      EXPECT_EQ(c1, base_value);
+
+      // CRC persists through copies, assignments, and moves:
+      absl::Cord c1_copy_construct = c1;
+      EXPECT_EQ(c1_copy_construct.ExpectedChecksum().value_or(0), 12345);
+
+      absl::Cord c1_copy_assign;
+      c1_copy_assign = c1;
+      EXPECT_EQ(c1_copy_assign.ExpectedChecksum().value_or(0), 12345);
+
+      absl::Cord c1_move(std::move(c1_copy_assign));
+      EXPECT_EQ(c1_move.ExpectedChecksum().value_or(0), 12345);
+
+      EXPECT_EQ(c1.ExpectedChecksum().value_or(0), 12345);
+
+      // A CRC Cord compares equal to its non-CRC value.
+      EXPECT_EQ(c1, make_instance());
+
+      for (const CordMutator& mutator : cord_mutators) {
+        SCOPED_TRACE(mutator.Name());
+
+        // Test that mutating a cord removes its stored checksum
+        absl::Cord c2 = make_instance();
+        c2.SetExpectedChecksum(24680);
+
+        mutator.Mutate(c2);
+
+        if (c1 == c2) {
+          // Not a mutation (for example, appending the empty string).
+          // Whether the checksum is removed is not defined.
+          continue;
+        }
+
+        EXPECT_EQ(c2.ExpectedChecksum(), std::nullopt);
+
+        if (mutator.CanUndo()) {
+          // Undoing an operation should not restore the checksum
+          mutator.Undo(c2);
+          EXPECT_EQ(c2, base_value);
+          EXPECT_EQ(c2.ExpectedChecksum(), std::nullopt);
+        }
+      }
+
+      absl::Cord c3 = make_instance();
+      c3.SetExpectedChecksum(999);
+      const absl::Cord& cc3 = c3;
+
+      // Test that all cord reading operations function in the face of an
+      // expected checksum.
+
+      // Test data precondition
+      ASSERT_TRUE(cc3.StartsWith("abcde"));
+
+      EXPECT_EQ(cc3.size(), base_value_as_string.size());
+      EXPECT_FALSE(cc3.empty());
+      EXPECT_EQ(cc3.Compare(base_value), 0);
+      EXPECT_EQ(cc3.Compare(base_value_as_string), 0);
+      EXPECT_EQ(cc3.Compare("wxyz"), -1);
+      EXPECT_EQ(cc3.Compare(absl::Cord("wxyz")), -1);
+      EXPECT_EQ(cc3.Compare("aaaa"), 1);
+      EXPECT_EQ(cc3.Compare(absl::Cord("aaaa")), 1);
+      EXPECT_EQ(absl::Cord("wxyz").Compare(cc3), 1);
+      EXPECT_EQ(absl::Cord("aaaa").Compare(cc3), -1);
+      EXPECT_TRUE(cc3.StartsWith("abcd"));
+      EXPECT_EQ(std::string(cc3), base_value_as_string);
+
+      std::string dest;
+      absl::CopyCordToString(cc3, &dest);
+      EXPECT_EQ(dest, base_value_as_string);
+
+      bool first_pass = true;
+      for (absl::string_view chunk : cc3.Chunks()) {
+        if (first_pass) {
+          EXPECT_TRUE(absl::StartsWith(chunk, "abcde"));
+        }
+        first_pass = false;
+      }
+      first_pass = true;
+      for (char ch : cc3.Chars()) {
+        if (first_pass) {
+          EXPECT_EQ(ch, 'a');
+        }
+        first_pass = false;
+      }
+      EXPECT_TRUE(absl::StartsWith(*cc3.chunk_begin(), "abcde"));
+      EXPECT_EQ(*cc3.char_begin(), 'a');
+
+      auto char_it = cc3.char_begin();
+      absl::Cord::Advance(&char_it, 2);
+      EXPECT_EQ(absl::Cord::AdvanceAndRead(&char_it, 2), "cd");
+      EXPECT_EQ(*char_it, 'e');
+      char_it = cc3.char_begin();
+      absl::Cord::Advance(&char_it, 2);
+      EXPECT_TRUE(absl::StartsWith(absl::Cord::ChunkRemaining(char_it), "cde"));
+
+      EXPECT_EQ(cc3[0], 'a');
+      EXPECT_EQ(cc3[4], 'e');
+      EXPECT_EQ(absl::HashOf(cc3), absl::HashOf(base_value));
+      EXPECT_EQ(absl::HashOf(cc3), absl::HashOf(base_value_as_string));
+    }
+  }
+}
+
+// Test the special cases encountered with an empty checksummed cord.
+TEST_P(CordTest, ChecksummedEmptyCord) {
+  absl::Cord c1;
+  EXPECT_FALSE(c1.ExpectedChecksum().has_value());
+
+  // Setting an expected checksum works.
+  c1.SetExpectedChecksum(12345);
+  EXPECT_EQ(c1.ExpectedChecksum().value_or(0), 12345);
+  EXPECT_EQ(c1, "");
+  EXPECT_TRUE(c1.empty());
+
+  // Test that setting an expected checksum again doesn't crash or leak memory.
+  c1.SetExpectedChecksum(12345);
+  EXPECT_EQ(c1.ExpectedChecksum().value_or(0), 12345);
+  EXPECT_EQ(c1, "");
+  EXPECT_TRUE(c1.empty());
+
+  // CRC persists through copies, assignments, and moves:
+  absl::Cord c1_copy_construct = c1;
+  EXPECT_EQ(c1_copy_construct.ExpectedChecksum().value_or(0), 12345);
+
+  absl::Cord c1_copy_assign;
+  c1_copy_assign = c1;
+  EXPECT_EQ(c1_copy_assign.ExpectedChecksum().value_or(0), 12345);
+
+  absl::Cord c1_move(std::move(c1_copy_assign));
+  EXPECT_EQ(c1_move.ExpectedChecksum().value_or(0), 12345);
+
+  EXPECT_EQ(c1.ExpectedChecksum().value_or(0), 12345);
+
+  // A CRC Cord compares equal to its non-CRC value.
+  EXPECT_EQ(c1, absl::Cord());
+
+  for (const CordMutator& mutator : cord_mutators) {
+    SCOPED_TRACE(mutator.Name());
+
+    // Exercise mutating an empty checksummed cord to catch crashes and exercise
+    // memory sanitizers.
+    absl::Cord c2;
+    c2.SetExpectedChecksum(24680);
+    mutator.Mutate(c2);
+
+    if (c2.empty()) {
+      // Not a mutation
+      continue;
+    }
+    EXPECT_EQ(c2.ExpectedChecksum(), std::nullopt);
+
+    if (mutator.CanUndo()) {
+      mutator.Undo(c2);
+    }
+  }
+
+  absl::Cord c3;
+  c3.SetExpectedChecksum(999);
+  const absl::Cord& cc3 = c3;
+
+  // Test that all cord reading operations function in the face of an
+  // expected checksum.
+  EXPECT_TRUE(cc3.StartsWith(""));
+  EXPECT_TRUE(cc3.EndsWith(""));
+  EXPECT_TRUE(cc3.empty());
+  EXPECT_EQ(cc3, "");
+  EXPECT_EQ(cc3, absl::Cord());
+  EXPECT_EQ(cc3.size(), 0);
+  EXPECT_EQ(cc3.Compare(absl::Cord()), 0);
+  EXPECT_EQ(cc3.Compare(c1), 0);
+  EXPECT_EQ(cc3.Compare(cc3), 0);
+  EXPECT_EQ(cc3.Compare(""), 0);
+  EXPECT_EQ(cc3.Compare("wxyz"), -1);
+  EXPECT_EQ(cc3.Compare(absl::Cord("wxyz")), -1);
+  EXPECT_EQ(absl::Cord("wxyz").Compare(cc3), 1);
+  EXPECT_EQ(std::string(cc3), "");
+
+  std::string dest;
+  absl::CopyCordToString(cc3, &dest);
+  EXPECT_EQ(dest, "");
+
+  for (absl::string_view chunk : cc3.Chunks()) {  // NOLINT(unreachable loop)
+    static_cast<void>(chunk);
+    GTEST_FAIL() << "no chunks expected";
+  }
+  EXPECT_TRUE(cc3.chunk_begin() == cc3.chunk_end());
+
+  for (char ch : cc3.Chars()) {  // NOLINT(unreachable loop)
+    static_cast<void>(ch);
+    GTEST_FAIL() << "no chars expected";
+  }
+  EXPECT_TRUE(cc3.char_begin() == cc3.char_end());
+
+  EXPECT_EQ(cc3.TryFlat(), "");
+  EXPECT_EQ(absl::HashOf(c3), absl::HashOf(absl::Cord()));
+  EXPECT_EQ(absl::HashOf(c3), absl::HashOf(absl::string_view()));
+}
+
+// This must not be static to avoid aggressive optimizations.
+ABSL_ATTRIBUTE_WEAK
+size_t FalseReport(const absl::Cord& a, bool f);
+
+ABSL_ATTRIBUTE_NOINLINE
+size_t FalseReport(const absl::Cord& a, bool f) {
+  absl::Cord b;
+  const absl::Cord& ref = f ? b : a;
+  // Test that sanitizers report nothing here. Without
+  // InlineData::Rep::annotated_this() compiler can unconditionally load
+  // poisoned parts, assuming that local variable is fully accessible.
+  return ref.size();
+}
+
+TEST(CordSanitizerTest, SanitizesCordFalseReport) {
+  absl::Cord c;
+  for (int i = 0; i < 1000; ++i) c.Append("a");
+  FalseReport(c, false);
+}
+
+TEST(CrcCordTest, ChecksummedEmptyCordEstimateMemoryUsage) {
+  absl::Cord cord;
+  cord.SetExpectedChecksum(0);
+  EXPECT_NE(cord.EstimatedMemoryUsage(), 0);
+}
+
+TEST(CordThreeWayComparisonTest, CompareCords) {
+#ifndef __cpp_impl_three_way_comparison
+  GTEST_SKIP() << "C++20 three-way <=> comparison not supported";
+#else
+  EXPECT_EQ(absl::Cord("a") <=> absl::Cord("a"), std::strong_ordering::equal);
+  EXPECT_EQ(absl::Cord("aaaa") <=> absl::Cord("aaab"),
+            std::strong_ordering::less);
+  EXPECT_EQ(absl::Cord("baaa") <=> absl::Cord("a"),
+            std::strong_ordering::greater);
+#endif
+}
+
+TEST(CordThreeWayComparisonTest, CompareCordsAndStringViews) {
+#ifndef __cpp_impl_three_way_comparison
+  GTEST_SKIP() << "C++20 three-way <=> comparison not supported";
+#else
+  EXPECT_EQ(absl::string_view("a") <=> absl::Cord("a"),
+            std::strong_ordering::equal);
+  EXPECT_EQ(absl::Cord("a") <=> absl::string_view("b"),
+            std::strong_ordering::less);
+  EXPECT_EQ(absl::string_view("b") <=> absl::Cord("a"),
+            std::strong_ordering::greater);
+#endif
+}
diff --git a/absl/strings/cord_test_helpers.h b/absl/strings/cord_test_helpers.h
new file mode 100644
index 00000000000..715e980a0b7
--- /dev/null
+++ b/absl/strings/cord_test_helpers.h
@@ -0,0 +1,125 @@
+//
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+#ifndef ABSL_STRINGS_CORD_TEST_HELPERS_H_
+#define ABSL_STRINGS_CORD_TEST_HELPERS_H_
+
+#include <cstddef>
+#include <cstdint>
+#include <initializer_list>
+#include <iostream>
+#include <string>
+
+#include "absl/base/config.h"
+#include "absl/strings/cord.h"
+#include "absl/strings/internal/cord_internal.h"
+#include "absl/strings/internal/cord_rep_flat.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+// Cord sizes relevant for testing
+enum class TestCordSize {
+  // An empty value
+  kEmpty = 0,
+
+  // An inlined string value
+  kInlined = cord_internal::kMaxInline / 2 + 1,
+
+  // 'Well known' SSO lengths (excluding terminating zero).
+  // libstdcxx has a maximum SSO of 15, libc++ has a maximum SSO of 22.
+  kStringSso1 = 15,
+  kStringSso2 = 22,
+
+  // A string value which is too large to fit in inlined data, but small enough
+  // such that Cord prefers copying the value if possible, i.e.: not stealing
+  // std::string inputs, or referencing existing CordReps on Append, etc.
+  kSmall = cord_internal::kMaxBytesToCopy / 2 + 1,
+
+  // A string value large enough that Cord prefers to reference or steal from
+  // existing inputs rather than copying contents of the input.
+  kMedium = cord_internal::kMaxFlatLength / 2 + 1,
+
+  // A string value large enough to cause it to be stored in multiple flats.
+  kLarge = cord_internal::kMaxFlatLength * 4
+};
+
+// To string helper
+inline absl::string_view ToString(TestCordSize size) {
+  switch (size) {
+    case TestCordSize::kEmpty:
+      return "Empty";
+    case TestCordSize::kInlined:
+      return "Inlined";
+    case TestCordSize::kSmall:
+      return "Small";
+    case TestCordSize::kStringSso1:
+      return "StringSso1";
+    case TestCordSize::kStringSso2:
+      return "StringSso2";
+    case TestCordSize::kMedium:
+      return "Medium";
+    case TestCordSize::kLarge:
+      return "Large";
+  }
+  return "???";
+}
+
+// Returns the length matching the specified size
+inline size_t Length(TestCordSize size) { return static_cast<size_t>(size); }
+
+// Stream output helper
+inline std::ostream& operator<<(std::ostream& stream, TestCordSize size) {
+  return stream << ToString(size);
+}
+
+// Creates a multi-segment Cord from an iterable container of strings.  The
+// resulting Cord is guaranteed to have one segment for every string in the
+// container.  This allows code to be unit tested with multi-segment Cord
+// inputs.
+//
+// Example:
+//
+//   absl::Cord c = absl::MakeFragmentedCord({"A ", "fragmented ", "Cord"});
+//   EXPECT_FALSE(c.GetFlat(&unused));
+//
+// The mechanism by which this Cord is created is an implementation detail.  Any
+// implementation that produces a multi-segment Cord may produce a flat Cord in
+// the future as new optimizations are added to the Cord class.
+// MakeFragmentedCord will, however, always be updated to return a multi-segment
+// Cord.
+template <typename Container>
+Cord MakeFragmentedCord(const Container& c) {
+  Cord result;
+  for (const auto& s : c) {
+    auto* external = new std::string(s);
+    Cord tmp = absl::MakeCordFromExternal(
+        *external, [external](absl::string_view) { delete external; });
+    tmp.Prepend(result);
+    result = tmp;
+  }
+  return result;
+}
+
+inline Cord MakeFragmentedCord(std::initializer_list<absl::string_view> list) {
+  return MakeFragmentedCord<std::initializer_list<absl::string_view>>(list);
+}
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STRINGS_CORD_TEST_HELPERS_H_
diff --git a/absl/strings/cordz_test.cc b/absl/strings/cordz_test.cc
new file mode 100644
index 00000000000..a35493e3fdc
--- /dev/null
+++ b/absl/strings/cordz_test.cc
@@ -0,0 +1,468 @@
+// Copyright 2021 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <cstddef>
+#include <cstring>
+#include <ostream>
+#include <string>
+#include <utility>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+#include "absl/strings/cord.h"
+#include "absl/strings/cord_buffer.h"
+#include "absl/strings/cord_test_helpers.h"
+#include "absl/strings/cordz_test_helpers.h"
+#include "absl/strings/internal/cord_internal.h"
+#include "absl/strings/internal/cordz_info.h"
+#include "absl/strings/internal/cordz_sample_token.h"
+#include "absl/strings/internal/cordz_statistics.h"
+#include "absl/strings/internal/cordz_update_tracker.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/string_view.h"
+
+#ifdef ABSL_INTERNAL_CORDZ_ENABLED
+
+using testing::Eq;
+using testing::AnyOf;
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+using cord_internal::CordzInfo;
+using cord_internal::CordzSampleToken;
+using cord_internal::CordzStatistics;
+using cord_internal::CordzUpdateTracker;
+using Method = CordzUpdateTracker::MethodIdentifier;
+
+// Do not print cord contents, we only care about 'size' perhaps.
+// Note that this method must be inside the named namespace.
+inline void PrintTo(const Cord& cord, std::ostream* s) {
+  if (s) *s << "Cord[" << cord.size() << "]";
+}
+
+namespace {
+
+auto constexpr kMaxInline = cord_internal::kMaxInline;
+
+// Returns a string_view value of the specified length
+// We do this to avoid 'consuming' large strings in Cord by default.
+absl::string_view MakeString(size_t size) {
+  thread_local std::string str;
+  str = std::string(size, '.');
+  return str;
+}
+
+absl::string_view MakeString(TestCordSize size) {
+  return MakeString(Length(size));
+}
+
+// Returns a cord with a sampled method of kAppendString.
+absl::Cord MakeAppendStringCord(TestCordSize size) {
+  CordzSamplingIntervalHelper always(1);
+  absl::Cord cord;
+  cord.Append(MakeString(size));
+  return cord;
+}
+
+std::string TestParamToString(::testing::TestParamInfo<TestCordSize> size) {
+  return absl::StrCat("On", ToString(size.param), "Cord");
+}
+
+class CordzUpdateTest : public testing::TestWithParam<TestCordSize> {
+ public:
+  Cord& cord() { return cord_; }
+
+  Method InitialOr(Method method) const {
+    return (GetParam() > TestCordSize::kInlined) ? Method::kConstructorString
+                                                 : method;
+  }
+
+ private:
+  CordzSamplingIntervalHelper sample_every_{1};
+  Cord cord_{MakeString(GetParam())};
+};
+
+template <typename T>
+std::string ParamToString(::testing::TestParamInfo<T> param) {
+  return std::string(ToString(param.param));
+}
+
+INSTANTIATE_TEST_SUITE_P(WithParam, CordzUpdateTest,
+                         testing::Values(TestCordSize::kEmpty,
+                                         TestCordSize::kInlined,
+                                         TestCordSize::kLarge),
+                         TestParamToString);
+
+class CordzStringTest : public testing::TestWithParam<TestCordSize> {
+ private:
+  CordzSamplingIntervalHelper sample_every_{1};
+};
+
+INSTANTIATE_TEST_SUITE_P(WithParam, CordzStringTest,
+                         testing::Values(TestCordSize::kInlined,
+                                         TestCordSize::kStringSso1,
+                                         TestCordSize::kStringSso2,
+                                         TestCordSize::kSmall,
+                                         TestCordSize::kLarge),
+                         ParamToString<TestCordSize>);
+
+TEST(CordzTest, ConstructSmallArray) {
+  CordzSamplingIntervalHelper sample_every{1};
+  Cord cord(MakeString(TestCordSize::kSmall));
+  EXPECT_THAT(cord, HasValidCordzInfoOf(Method::kConstructorString));
+}
+
+TEST(CordzTest, ConstructLargeArray) {
+  CordzSamplingIntervalHelper sample_every{1};
+  Cord cord(MakeString(TestCordSize::kLarge));
+  EXPECT_THAT(cord, HasValidCordzInfoOf(Method::kConstructorString));
+}
+
+TEST_P(CordzStringTest, ConstructString) {
+  CordzSamplingIntervalHelper sample_every{1};
+  Cord cord(std::string(Length(GetParam()), '.'));
+  if (Length(GetParam()) > kMaxInline) {
+    EXPECT_THAT(cord, HasValidCordzInfoOf(Method::kConstructorString));
+  }
+}
+
+TEST(CordzTest, CopyConstructFromUnsampled) {
+  CordzSamplingIntervalHelper sample_every{1};
+  Cord src = UnsampledCord(MakeString(TestCordSize::kLarge));
+  Cord cord(src);
+  EXPECT_THAT(GetCordzInfoForTesting(cord), Eq(nullptr));
+}
+
+TEST(CordzTest, CopyConstructFromSampled) {
+  CordzSamplingIntervalHelper sample_never{99999};
+  Cord src = MakeAppendStringCord(TestCordSize::kLarge);
+  Cord cord(src);
+  ASSERT_THAT(cord, HasValidCordzInfoOf(Method::kConstructorCord));
+  CordzStatistics stats = GetCordzInfoForTesting(cord)->GetCordzStatistics();
+  EXPECT_THAT(stats.parent_method, Eq(Method::kAppendString));
+  EXPECT_THAT(stats.update_tracker.Value(Method::kAppendString), Eq(1));
+}
+
+TEST(CordzTest, MoveConstruct) {
+  CordzSamplingIntervalHelper sample_every{1};
+  Cord src(MakeString(TestCordSize::kLarge));
+  Cord cord(std::move(src));
+  EXPECT_THAT(cord, HasValidCordzInfoOf(Method::kConstructorString));
+}
+
+TEST_P(CordzUpdateTest, AssignUnsampledCord) {
+  Cord src = UnsampledCord(MakeString(TestCordSize::kLarge));
+  const CordzInfo* info = GetCordzInfoForTesting(cord());
+  cord() = src;
+  EXPECT_THAT(GetCordzInfoForTesting(cord()), Eq(nullptr));
+  EXPECT_FALSE(CordzInfoIsListed(info));
+}
+
+TEST_P(CordzUpdateTest, AssignSampledCord) {
+  Cord src = MakeAppendStringCord(TestCordSize::kLarge);
+  cord() = src;
+  ASSERT_THAT(cord(), HasValidCordzInfoOf(Method::kAssignCord));
+  CordzStatistics stats = GetCordzInfoForTesting(cord())->GetCordzStatistics();
+  EXPECT_THAT(stats.parent_method, Eq(Method::kAppendString));
+  EXPECT_THAT(stats.update_tracker.Value(Method::kAppendString), Eq(1));
+  EXPECT_THAT(stats.update_tracker.Value(Method::kConstructorString), Eq(0));
+}
+
+TEST(CordzUpdateTest, AssignSampledCordToInlined) {
+  CordzSamplingIntervalHelper sample_never{99999};
+  Cord cord;
+  Cord src = MakeAppendStringCord(TestCordSize::kLarge);
+  cord = src;
+  ASSERT_THAT(cord, HasValidCordzInfoOf(Method::kAssignCord));
+  CordzStatistics stats = GetCordzInfoForTesting(cord)->GetCordzStatistics();
+  EXPECT_THAT(stats.parent_method, Eq(Method::kAppendString));
+  EXPECT_THAT(stats.update_tracker.Value(Method::kAppendString), Eq(1));
+  EXPECT_THAT(stats.update_tracker.Value(Method::kConstructorString), Eq(0));
+}
+
+TEST(CordzUpdateTest, AssignSampledCordToUnsampledCord) {
+  CordzSamplingIntervalHelper sample_never{99999};
+  Cord cord = UnsampledCord(MakeString(TestCordSize::kLarge));
+  Cord src = MakeAppendStringCord(TestCordSize::kLarge);
+  cord = src;
+  ASSERT_THAT(cord, HasValidCordzInfoOf(Method::kAssignCord));
+  CordzStatistics stats = GetCordzInfoForTesting(cord)->GetCordzStatistics();
+  EXPECT_THAT(stats.parent_method, Eq(Method::kAppendString));
+  EXPECT_THAT(stats.update_tracker.Value(Method::kAppendString), Eq(1));
+  EXPECT_THAT(stats.update_tracker.Value(Method::kConstructorString), Eq(0));
+}
+
+TEST(CordzUpdateTest, AssignUnsampledCordToSampledCordWithoutSampling) {
+  CordzSamplingIntervalHelper sample_never{99999};
+  Cord cord = MakeAppendStringCord(TestCordSize::kLarge);
+  const CordzInfo* info = GetCordzInfoForTesting(cord);
+  Cord src = UnsampledCord(MakeString(TestCordSize::kLarge));
+  cord = src;
+  EXPECT_THAT(GetCordzInfoForTesting(cord), Eq(nullptr));
+  EXPECT_FALSE(CordzInfoIsListed(info));
+}
+
+TEST(CordzUpdateTest, AssignUnsampledCordToSampledCordWithSampling) {
+  CordzSamplingIntervalHelper sample_every{1};
+  Cord cord = MakeAppendStringCord(TestCordSize::kLarge);
+  const CordzInfo* info = GetCordzInfoForTesting(cord);
+  Cord src = UnsampledCord(MakeString(TestCordSize::kLarge));
+  cord = src;
+  EXPECT_THAT(GetCordzInfoForTesting(cord), Eq(nullptr));
+  EXPECT_FALSE(CordzInfoIsListed(info));
+}
+
+TEST(CordzUpdateTest, AssignSampledCordToSampledCord) {
+  CordzSamplingIntervalHelper sample_every{1};
+  Cord src = MakeAppendStringCord(TestCordSize::kLarge);
+  Cord cord(MakeString(TestCordSize::kLarge));
+  cord = src;
+  ASSERT_THAT(cord, HasValidCordzInfoOf(Method::kAssignCord));
+  CordzStatistics stats = GetCordzInfoForTesting(cord)->GetCordzStatistics();
+  EXPECT_THAT(stats.parent_method, Eq(Method::kAppendString));
+  EXPECT_THAT(stats.update_tracker.Value(Method::kAppendString), Eq(1));
+  EXPECT_THAT(stats.update_tracker.Value(Method::kConstructorString), Eq(0));
+}
+
+TEST(CordzUpdateTest, AssignUnsampledCordToSampledCord) {
+  CordzSamplingIntervalHelper sample_every{1};
+  Cord src = MakeAppendStringCord(TestCordSize::kLarge);
+  Cord cord(MakeString(TestCordSize::kLarge));
+  cord = src;
+  ASSERT_THAT(cord, HasValidCordzInfoOf(Method::kAssignCord));
+  CordzStatistics stats = GetCordzInfoForTesting(cord)->GetCordzStatistics();
+  EXPECT_THAT(stats.parent_method, Eq(Method::kAppendString));
+  EXPECT_THAT(stats.update_tracker.Value(Method::kAppendString), Eq(1));
+  EXPECT_THAT(stats.update_tracker.Value(Method::kConstructorString), Eq(0));
+}
+
+TEST(CordzTest, AssignInlinedCordToSampledCord) {
+  CordzSampleToken token;
+  CordzSamplingIntervalHelper sample_every{1};
+  Cord cord(MakeString(TestCordSize::kLarge));
+  const CordzInfo* info = GetCordzInfoForTesting(cord);
+  Cord src = UnsampledCord(MakeString(TestCordSize::kInlined));
+  cord = src;
+  EXPECT_THAT(GetCordzInfoForTesting(cord), Eq(nullptr));
+  EXPECT_FALSE(CordzInfoIsListed(info));
+}
+
+TEST(CordzUpdateTest, MoveAssignCord) {
+  CordzSamplingIntervalHelper sample_every{1};
+  Cord cord;
+  Cord src(MakeString(TestCordSize::kLarge));
+  cord = std::move(src);
+  EXPECT_THAT(cord, HasValidCordzInfoOf(Method::kConstructorString));
+}
+
+TEST_P(CordzUpdateTest, AssignLargeArray) {
+  cord() = MakeString(TestCordSize::kSmall);
+  EXPECT_THAT(cord(), HasValidCordzInfoOf(Method::kAssignString));
+}
+
+TEST_P(CordzUpdateTest, AssignSmallArray) {
+  cord() = MakeString(TestCordSize::kSmall);
+  EXPECT_THAT(cord(), HasValidCordzInfoOf(Method::kAssignString));
+}
+
+TEST_P(CordzUpdateTest, AssignInlinedArray) {
+  cord() = MakeString(TestCordSize::kInlined);
+  EXPECT_THAT(GetCordzInfoForTesting(cord()), Eq(nullptr));
+}
+
+TEST_P(CordzStringTest, AssignStringToInlined) {
+  Cord cord;
+  cord = std::string(Length(GetParam()), '.');
+  if (Length(GetParam()) > kMaxInline) {
+    EXPECT_THAT(cord, HasValidCordzInfoOf(Method::kAssignString));
+  }
+}
+
+TEST_P(CordzStringTest, AssignStringToCord) {
+  Cord cord(MakeString(TestCordSize::kLarge));
+  cord = std::string(Length(GetParam()), '.');
+  if (Length(GetParam()) > kMaxInline) {
+    EXPECT_THAT(cord, HasValidCordzInfoOf(Method::kConstructorString));
+    EXPECT_THAT(cord, CordzMethodCountEq(Method::kAssignString, 1));
+  }
+}
+
+TEST_P(CordzUpdateTest, AssignInlinedString) {
+  cord() = std::string(Length(TestCordSize::kInlined), '.');
+  EXPECT_THAT(GetCordzInfoForTesting(cord()), Eq(nullptr));
+}
+
+TEST_P(CordzUpdateTest, AppendCord) {
+  Cord src = UnsampledCord(MakeString(TestCordSize::kLarge));
+  cord().Append(src);
+  EXPECT_THAT(cord(), HasValidCordzInfoOf(InitialOr(Method::kAppendCord)));
+}
+
+TEST_P(CordzUpdateTest, MoveAppendCord) {
+  cord().Append(UnsampledCord(MakeString(TestCordSize::kLarge)));
+  EXPECT_THAT(cord(), HasValidCordzInfoOf(InitialOr(Method::kAppendCord)));
+}
+
+TEST_P(CordzUpdateTest, AppendSmallArray) {
+  cord().Append(MakeString(TestCordSize::kSmall));
+  EXPECT_THAT(cord(), HasValidCordzInfoOf(InitialOr(Method::kAppendString)));
+}
+
+TEST_P(CordzUpdateTest, AppendLargeArray) {
+  cord().Append(MakeString(TestCordSize::kLarge));
+  EXPECT_THAT(cord(), HasValidCordzInfoOf(InitialOr(Method::kAppendString)));
+}
+
+TEST_P(CordzStringTest, AppendStringToEmpty) {
+  Cord cord;
+  cord.Append(std::string(Length(GetParam()), '.'));
+  if (Length(GetParam()) > kMaxInline) {
+    EXPECT_THAT(cord, HasValidCordzInfoOf(Method::kAppendString));
+  }
+}
+
+TEST_P(CordzStringTest, AppendStringToInlined) {
+  Cord cord(MakeString(TestCordSize::kInlined));
+  cord.Append(std::string(Length(GetParam()), '.'));
+  if (Length(TestCordSize::kInlined) + Length(GetParam()) > kMaxInline) {
+    EXPECT_THAT(cord, HasValidCordzInfoOf(Method::kAppendString));
+  }
+}
+
+TEST_P(CordzStringTest, AppendStringToCord) {
+  Cord cord(MakeString(TestCordSize::kLarge));
+  cord.Append(std::string(Length(GetParam()), '.'));
+  EXPECT_THAT(cord, HasValidCordzInfoOf(Method::kConstructorString));
+  EXPECT_THAT(cord, CordzMethodCountEq(Method::kAppendString, 1));
+}
+
+TEST(CordzTest, MakeCordFromExternal) {
+  CordzSamplingIntervalHelper sample_every{1};
+  Cord cord = MakeCordFromExternal("Hello world", [](absl::string_view) {});
+  EXPECT_THAT(cord, HasValidCordzInfoOf(Method::kMakeCordFromExternal));
+}
+
+TEST(CordzTest, MakeCordFromEmptyExternal) {
+  CordzSamplingIntervalHelper sample_every{1};
+  Cord cord = MakeCordFromExternal({}, [](absl::string_view) {});
+  EXPECT_THAT(GetCordzInfoForTesting(cord), Eq(nullptr));
+}
+
+TEST_P(CordzUpdateTest, PrependCord) {
+  Cord src = UnsampledCord(MakeString(TestCordSize::kLarge));
+  cord().Prepend(src);
+  EXPECT_THAT(cord(), HasValidCordzInfoOf(InitialOr(Method::kPrependCord)));
+}
+
+TEST_P(CordzUpdateTest, PrependSmallArray) {
+  cord().Prepend(MakeString(TestCordSize::kSmall));
+  EXPECT_THAT(cord(), HasValidCordzInfoOf(InitialOr(Method::kPrependString)));
+}
+
+TEST_P(CordzUpdateTest, PrependLargeArray) {
+  cord().Prepend(MakeString(TestCordSize::kLarge));
+  EXPECT_THAT(cord(), HasValidCordzInfoOf(InitialOr(Method::kPrependString)));
+}
+
+TEST_P(CordzStringTest, PrependStringToEmpty) {
+  Cord cord;
+  cord.Prepend(std::string(Length(GetParam()), '.'));
+  if (Length(GetParam()) > kMaxInline) {
+    EXPECT_THAT(cord, HasValidCordzInfoOf(Method::kPrependString));
+  }
+}
+
+TEST_P(CordzStringTest, PrependStringToInlined) {
+  Cord cord(MakeString(TestCordSize::kInlined));
+  cord.Prepend(std::string(Length(GetParam()), '.'));
+  if (Length(TestCordSize::kInlined) + Length(GetParam()) > kMaxInline) {
+    EXPECT_THAT(cord, HasValidCordzInfoOf(Method::kPrependString));
+  }
+}
+
+TEST_P(CordzStringTest, PrependStringToCord) {
+  Cord cord(MakeString(TestCordSize::kLarge));
+  cord.Prepend(std::string(Length(GetParam()), '.'));
+  EXPECT_THAT(cord, HasValidCordzInfoOf(Method::kConstructorString));
+  EXPECT_THAT(cord, CordzMethodCountEq(Method::kPrependString, 1));
+}
+
+TEST(CordzTest, RemovePrefix) {
+  CordzSamplingIntervalHelper sample_every(1);
+  Cord cord(MakeString(TestCordSize::kLarge));
+
+  // Half the cord
+  cord.RemovePrefix(cord.size() / 2);
+  EXPECT_THAT(cord, HasValidCordzInfoOf(Method::kConstructorString));
+  EXPECT_THAT(cord, CordzMethodCountEq(Method::kRemovePrefix, 1));
+
+  // TODO(mvels): RemovePrefix does not reset to inlined, except if empty?
+  cord.RemovePrefix(cord.size() - kMaxInline);
+  EXPECT_THAT(cord, HasValidCordzInfoOf(Method::kConstructorString));
+  EXPECT_THAT(cord, CordzMethodCountEq(Method::kRemovePrefix, 2));
+
+  cord.RemovePrefix(cord.size());
+  EXPECT_THAT(GetCordzInfoForTesting(cord), Eq(nullptr));
+}
+
+TEST(CordzTest, RemoveSuffix) {
+  CordzSamplingIntervalHelper sample_every(1);
+  Cord cord(MakeString(TestCordSize::kLarge));
+
+  // Half the cord
+  cord.RemoveSuffix(cord.size() / 2);
+  EXPECT_THAT(cord, HasValidCordzInfoOf(Method::kConstructorString));
+  EXPECT_THAT(cord, CordzMethodCountEq(Method::kRemoveSuffix, 1));
+
+  // TODO(mvels): RemoveSuffix does not reset to inlined, except if empty?
+  cord.RemoveSuffix(cord.size() - kMaxInline);
+  EXPECT_THAT(cord, HasValidCordzInfoOf(Method::kConstructorString));
+  EXPECT_THAT(cord, CordzMethodCountEq(Method::kRemoveSuffix, 2));
+
+  cord.RemoveSuffix(cord.size());
+  EXPECT_THAT(GetCordzInfoForTesting(cord), Eq(nullptr));
+}
+
+TEST(CordzTest, SubCordFromUnsampledCord) {
+  CordzSamplingIntervalHelper sample_every{1};
+  Cord src = UnsampledCord(MakeString(TestCordSize::kLarge));
+  Cord cord = src.Subcord(10, src.size() / 2);
+  EXPECT_THAT(GetCordzInfoForTesting(cord), Eq(nullptr));
+}
+
+TEST(CordzTest, SubCordFromSampledCord) {
+  CordzSamplingIntervalHelper sample_never{99999};
+  Cord src = MakeAppendStringCord(TestCordSize::kLarge);
+  Cord cord = src.Subcord(10, src.size() / 2);
+  ASSERT_THAT(cord, HasValidCordzInfoOf(Method::kSubCord));
+  CordzStatistics stats = GetCordzInfoForTesting(cord)->GetCordzStatistics();
+  EXPECT_THAT(stats.parent_method, Eq(Method::kAppendString));
+  EXPECT_THAT(stats.update_tracker.Value(Method::kAppendString), Eq(1));
+}
+
+TEST(CordzTest, SmallSubCord) {
+  CordzSamplingIntervalHelper sample_never{99999};
+  Cord src = MakeAppendStringCord(TestCordSize::kLarge);
+  Cord cord = src.Subcord(10, kMaxInline + 1);
+  EXPECT_THAT(cord, HasValidCordzInfoOf(Method::kSubCord));
+}
+
+}  // namespace
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_INTERNAL_CORDZ_ENABLED
diff --git a/absl/strings/cordz_test_helpers.h b/absl/strings/cordz_test_helpers.h
new file mode 100644
index 00000000000..6a290307c50
--- /dev/null
+++ b/absl/strings/cordz_test_helpers.h
@@ -0,0 +1,155 @@
+// Copyright 2021 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_STRINGS_CORDZ_TEST_HELPERS_H_
+#define ABSL_STRINGS_CORDZ_TEST_HELPERS_H_
+
+#include <cstdint>
+#include <cstring>
+#include <utility>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+#include "absl/base/macros.h"
+#include "absl/base/nullability.h"
+#include "absl/strings/cord.h"
+#include "absl/strings/internal/cord_internal.h"
+#include "absl/strings/internal/cordz_functions.h"
+#include "absl/strings/internal/cordz_info.h"
+#include "absl/strings/internal/cordz_sample_token.h"
+#include "absl/strings/internal/cordz_statistics.h"
+#include "absl/strings/internal/cordz_update_tracker.h"
+#include "absl/strings/str_cat.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+// Returns the CordzInfo for the cord, or nullptr if the cord is not sampled.
+inline const cord_internal::CordzInfo* GetCordzInfoForTesting(
+    const Cord& cord) {
+  if (!cord.contents_.is_tree()) return nullptr;
+  return cord.contents_.cordz_info();
+}
+
+// Returns true if the provided cordz_info is in the list of sampled cords.
+inline bool CordzInfoIsListed(const cord_internal::CordzInfo* cordz_info,
+                              cord_internal::CordzSampleToken token = {}) {
+  for (const cord_internal::CordzInfo& info : token) {
+    if (cordz_info == &info) return true;
+  }
+  return false;
+}
+
+// Matcher on Cord that verifies all of:
+// - the cord is sampled
+// - the CordzInfo of the cord is listed / discoverable.
+// - the reported CordzStatistics match the cord's actual properties
+// - the cord has an (initial) UpdateTracker count of 1 for `method`
+MATCHER_P(HasValidCordzInfoOf, method, "CordzInfo matches cord") {
+  const cord_internal::CordzInfo* cord_info = GetCordzInfoForTesting(arg);
+  if (cord_info == nullptr) {
+    *result_listener << "cord is not sampled";
+    return false;
+  }
+  if (!CordzInfoIsListed(cord_info)) {
+    *result_listener << "cord is sampled, but not listed";
+    return false;
+  }
+  cord_internal::CordzStatistics stat = cord_info->GetCordzStatistics();
+  if (stat.size != arg.size()) {
+    *result_listener << "cordz size " << stat.size
+                     << " does not match cord size " << arg.size();
+    return false;
+  }
+  if (stat.update_tracker.Value(method) != 1) {
+    *result_listener << "Expected method count 1 for " << method << ", found "
+                     << stat.update_tracker.Value(method);
+    return false;
+  }
+  return true;
+}
+
+// Matcher on Cord that verifies that the cord is sampled and that the CordzInfo
+// update tracker has 'method' with a call count of 'n'
+MATCHER_P2(CordzMethodCountEq, method, n,
+           absl::StrCat("CordzInfo method count equals ", n)) {
+  const cord_internal::CordzInfo* cord_info = GetCordzInfoForTesting(arg);
+  if (cord_info == nullptr) {
+    *result_listener << "cord is not sampled";
+    return false;
+  }
+  cord_internal::CordzStatistics stat = cord_info->GetCordzStatistics();
+  if (stat.update_tracker.Value(method) != n) {
+    *result_listener << "Expected method count " << n << " for " << method
+                     << ", found " << stat.update_tracker.Value(method);
+    return false;
+  }
+  return true;
+}
+
+// Cordz will only update with a new rate once the previously scheduled event
+// has fired. When we disable Cordz, a long delay takes place where we won't
+// consider profiling new Cords. CordzSampleIntervalHelper will burn through
+// that interval and allow for testing that assumes that the average sampling
+// interval is a particular value.
+class CordzSamplingIntervalHelper {
+ public:
+  explicit CordzSamplingIntervalHelper(int32_t interval)
+      : orig_mean_interval_(absl::cord_internal::get_cordz_mean_interval()) {
+    absl::cord_internal::set_cordz_mean_interval(interval);
+    absl::cord_internal::cordz_set_next_sample_for_testing(interval);
+  }
+
+  ~CordzSamplingIntervalHelper() {
+    absl::cord_internal::set_cordz_mean_interval(orig_mean_interval_);
+    absl::cord_internal::cordz_set_next_sample_for_testing(orig_mean_interval_);
+  }
+
+ private:
+  int32_t orig_mean_interval_;
+};
+
+// Wrapper struct managing a small CordRep `rep`
+struct TestCordRep {
+  cord_internal::CordRepFlat* rep;
+
+  TestCordRep() {
+    rep = cord_internal::CordRepFlat::New(100);
+    rep->length = 100;
+    memset(rep->Data(), 1, 100);
+  }
+  ~TestCordRep() { cord_internal::CordRep::Unref(rep); }
+};
+
+// Wrapper struct managing a small CordRep `rep`, and
+// an InlineData `data` initialized with that CordRep.
+struct TestCordData {
+  TestCordRep rep;
+  cord_internal::InlineData data{rep.rep};
+};
+
+// Creates a Cord that is not sampled
+template <typename... Args>
+Cord UnsampledCord(Args... args) {
+  CordzSamplingIntervalHelper never(9999);
+  Cord cord(std::forward<Args>(args)...);
+  ABSL_ASSERT(GetCordzInfoForTesting(cord) == nullptr);
+  return cord;
+}
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STRINGS_CORDZ_TEST_HELPERS_H_
diff --git a/absl/strings/escaping.cc b/absl/strings/escaping.cc
index 0d336e3ff69..efe90412e6f 100644
--- a/absl/strings/escaping.cc
+++ b/absl/strings/escaping.cc
@@ -15,64 +15,62 @@
 #include "absl/strings/escaping.h"
 
 #include <algorithm>
+#include <array>
 #include <cassert>
+#include <cstddef>
 #include <cstdint>
 #include <cstring>
 #include <iterator>
 #include <limits>
+#include <optional>
 #include <string>
+#include <utility>
 
+#include "absl/base/config.h"
 #include "absl/base/internal/endian.h"
 #include "absl/base/internal/raw_logging.h"
 #include "absl/base/internal/unaligned_access.h"
-#include "absl/strings/internal/char_map.h"
-#include "absl/strings/internal/resize_uninitialized.h"
+#include "absl/base/macros.h"
+#include "absl/base/nullability.h"
+#include "absl/base/optimization.h"
+#include "absl/strings/ascii.h"
+#include "absl/strings/charset.h"
+#include "absl/strings/internal/append_and_overwrite.h"
+#include "absl/strings/internal/escaping.h"
 #include "absl/strings/internal/utf8.h"
+#include "absl/strings/numbers.h"
+#include "absl/strings/resize_and_overwrite.h"
 #include "absl/strings/str_cat.h"
-#include "absl/strings/str_join.h"
 #include "absl/strings/string_view.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace {
 
-// Digit conversion.
-constexpr char kHexChar[] = "0123456789abcdef";
-
-constexpr char kHexTable[513] =
-    "000102030405060708090a0b0c0d0e0f"
-    "101112131415161718191a1b1c1d1e1f"
-    "202122232425262728292a2b2c2d2e2f"
-    "303132333435363738393a3b3c3d3e3f"
-    "404142434445464748494a4b4c4d4e4f"
-    "505152535455565758595a5b5c5d5e5f"
-    "606162636465666768696a6b6c6d6e6f"
-    "707172737475767778797a7b7c7d7e7f"
-    "808182838485868788898a8b8c8d8e8f"
-    "909192939495969798999a9b9c9d9e9f"
-    "a0a1a2a3a4a5a6a7a8a9aaabacadaeaf"
-    "b0b1b2b3b4b5b6b7b8b9babbbcbdbebf"
-    "c0c1c2c3c4c5c6c7c8c9cacbcccdcecf"
-    "d0d1d2d3d4d5d6d7d8d9dadbdcdddedf"
-    "e0e1e2e3e4e5e6e7e8e9eaebecedeeef"
-    "f0f1f2f3f4f5f6f7f8f9fafbfcfdfeff";
-
 // These are used for the leave_nulls_escaped argument to CUnescapeInternal().
 constexpr bool kUnescapeNulls = false;
 
 inline bool is_octal_digit(char c) { return ('0' <= c) && (c <= '7'); }
 
-inline int hex_digit_to_int(char c) {
+inline unsigned int hex_digit_to_int(char c) {
   static_assert('0' == 0x30 && 'A' == 0x41 && 'a' == 0x61,
                 "Character set must be ASCII.");
-  assert(absl::ascii_isxdigit(c));
-  int x = static_cast<unsigned char>(c);
+  assert(absl::ascii_isxdigit(static_cast<unsigned char>(c)));
+  unsigned int x = static_cast<unsigned char>(c);
   if (x > '9') {
     x += 9;
   }
   return x & 0xf;
 }
 
-inline bool IsSurrogate(char32_t c, absl::string_view src, std::string* error) {
+inline char int_to_hex_digit(int i) {
+  assert(i >= 0 && i <= 15);
+  return ((i < 10) ? (static_cast<char>(i) + '0')
+                   : (static_cast<char>(i - 10) + 'A'));
+}
+
+inline bool IsSurrogate(char32_t c, absl::string_view src,
+                        std::string* absl_nullable error) {
   if (c >= 0xD800 && c <= 0xDFFF) {
     if (error) {
       *error = absl::StrCat("invalid surrogate character (0xD800-DFFF): \\",
@@ -89,47 +87,52 @@ inline bool IsSurrogate(char32_t c, absl::string_view src, std::string* error) {
 //
 //    Unescapes C escape sequences and is the reverse of CEscape().
 //
-//    If 'source' is valid, stores the unescaped string and its size in
-//    'dest' and 'dest_len' respectively, and returns true. Otherwise
-//    returns false and optionally stores the error description in
-//    'error'. Set 'error' to nullptr to disable error reporting.
+//    If `src` is valid, stores the unescaped string in `dst` and the length of
+//    unescaped string in `dst_size`, and returns true. Otherwise returns false
+//    and optionally stores the error description in `error`. Set `error` to
+//    nullptr to disable error reporting.
 //
-//    'dest' should point to a buffer that is at least as big as 'source'.
-//    'source' and 'dest' may be the same.
-//
-//     NOTE: any changes to this function must also be reflected in the older
-//     UnescapeCEscapeSequences().
+//    `src` and `dst` may use the same underlying buffer (but keep in mind
+//    that if this returns an error, it will leave both `src` and `dst` in
+//    an unspecified state because they are using the same underlying buffer.)
+//    `dst` must have at least as much space as `src`.
 // ----------------------------------------------------------------------
-bool CUnescapeInternal(absl::string_view source, bool leave_nulls_escaped,
-                       char* dest, ptrdiff_t* dest_len, std::string* error) {
-  char* d = dest;
-  const char* p = source.data();
-  const char* end = p + source.size();
-  const char* last_byte = end - 1;
-
-  // Small optimization for case where source = dest and there's no escaping
-  while (p == d && p < end && *p != '\\') p++, d++;
-
-  while (p < end) {
-    if (*p != '\\') {
-      *d++ = *p++;
+
+bool CUnescapeInternal(absl::string_view src, bool leave_nulls_escaped,
+                       char* absl_nonnull dst, size_t* absl_nonnull dst_size,
+                       std::string* absl_nullable error) {
+  absl::string_view::size_type p = 0;  // Current src position.
+  size_t d = 0;                        // Current dst position.
+
+  // When unescaping in-place, skip any prefix that does not have escaping.
+  if (src.data() == dst) {
+    while (p < src.size() && src[p] != '\\') p++, d++;
+  }
+
+  while (p < src.size()) {
+    if (src[p] != '\\') {
+      dst[d++] = src[p++];
     } else {
-      if (++p > last_byte) {  // skip past the '\\'
-        if (error) *error = "String cannot end with \\";
+      if (++p >= src.size()) {  // skip past the '\\'
+        if (error != nullptr) {
+          *error = "String cannot end with \\";
+        }
         return false;
       }
-      switch (*p) {
-        case 'a':  *d++ = '\a';  break;
-        case 'b':  *d++ = '\b';  break;
-        case 'f':  *d++ = '\f';  break;
-        case 'n':  *d++ = '\n';  break;
-        case 'r':  *d++ = '\r';  break;
-        case 't':  *d++ = '\t';  break;
-        case 'v':  *d++ = '\v';  break;
-        case '\\': *d++ = '\\';  break;
-        case '?':  *d++ = '\?';  break;    // \?  Who knew?
-        case '\'': *d++ = '\'';  break;
-        case '"':  *d++ = '\"';  break;
+      switch (src[p]) {
+          // clang-format off
+        case 'a':  dst[d++] = '\a';  break;
+        case 'b':  dst[d++] = '\b';  break;
+        case 'f':  dst[d++] = '\f';  break;
+        case 'n':  dst[d++] = '\n';  break;
+        case 'r':  dst[d++] = '\r';  break;
+        case 't':  dst[d++] = '\t';  break;
+        case 'v':  dst[d++] = '\v';  break;
+        case '\\': dst[d++] = '\\';  break;
+        case '?':  dst[d++] = '\?';  break;
+        case '\'': dst[d++] = '\'';  break;
+        case '"':  dst[d++] = '\"';  break;
+        // clang-format on
         case '0':
         case '1':
         case '2':
@@ -139,178 +142,173 @@ bool CUnescapeInternal(absl::string_view source, bool leave_nulls_escaped,
         case '6':
         case '7': {
           // octal digit: 1 to 3 digits
-          const char* octal_start = p;
-          unsigned int ch = *p - '0';
-          if (p < last_byte && is_octal_digit(p[1])) ch = ch * 8 + *++p - '0';
-          if (p < last_byte && is_octal_digit(p[1]))
-            ch = ch * 8 + *++p - '0';      // now points at last digit
+          auto octal_start = p;
+          unsigned int ch = static_cast<unsigned int>(src[p] - '0');  // digit 1
+          if (p + 1 < src.size() && is_octal_digit(src[p + 1]))
+            ch = ch * 8 + static_cast<unsigned int>(src[++p] - '0');  // digit 2
+          if (p + 1 < src.size() && is_octal_digit(src[p + 1]))
+            ch = ch * 8 + static_cast<unsigned int>(src[++p] - '0');  // digit 3
           if (ch > 0xff) {
-            if (error) {
-              *error = "Value of \\" +
-                       std::string(octal_start, p + 1 - octal_start) +
-                       " exceeds 0xff";
+            if (error != nullptr) {
+              *error =
+                  "Value of \\" +
+                  std::string(src.substr(octal_start, p + 1 - octal_start)) +
+                  " exceeds 0xff";
             }
             return false;
           }
           if ((ch == 0) && leave_nulls_escaped) {
             // Copy the escape sequence for the null character
-            const ptrdiff_t octal_size = p + 1 - octal_start;
-            *d++ = '\\';
-            memcpy(d, octal_start, octal_size);
-            d += octal_size;
+            dst[d++] = '\\';
+            while (octal_start <= p) {
+              dst[d++] = src[octal_start++];
+            }
             break;
           }
-          *d++ = ch;
+          dst[d++] = static_cast<char>(ch);
           break;
         }
         case 'x':
         case 'X': {
-          if (p >= last_byte) {
-            if (error) *error = "String cannot end with \\x";
+          if (p + 1 >= src.size()) {
+            if (error != nullptr) {
+              *error = "String cannot end with \\x";
+            }
             return false;
-          } else if (!absl::ascii_isxdigit(p[1])) {
-            if (error) *error = "\\x cannot be followed by a non-hex digit";
+          } else if (!absl::ascii_isxdigit(
+              static_cast<unsigned char>(src[p + 1]))) {
+            if (error != nullptr) {
+              *error = "\\x cannot be followed by a non-hex digit";
+            }
             return false;
           }
           unsigned int ch = 0;
-          const char* hex_start = p;
-          while (p < last_byte && absl::ascii_isxdigit(p[1]))
+          auto hex_start = p;
+          while (p + 1 < src.size() &&
+                 absl::ascii_isxdigit(static_cast<unsigned char>(src[p + 1]))) {
             // Arbitrarily many hex digits
-            ch = (ch << 4) + hex_digit_to_int(*++p);
-          if (ch > 0xFF) {
-            if (error) {
-              *error = "Value of \\" +
-                       std::string(hex_start, p + 1 - hex_start) +
-                       " exceeds 0xff";
+            ch = (ch << 4) + hex_digit_to_int(src[++p]);
+            // If ch was 0xFF at the start of this loop, the most can it can be
+            // here is (0xFF << 4) + 0xF, which is 4095, thus ch cannot overflow
+            // 32-bits here. The check below is sufficient.
+            if (ch > 0xFF) {
+              if (error != nullptr) {
+                *error = "Value of \\" +
+                         std::string(src.substr(hex_start, p + 1 - hex_start)) +
+                         " exceeds 0xff";
+              }
+              return false;
             }
-            return false;
           }
           if ((ch == 0) && leave_nulls_escaped) {
             // Copy the escape sequence for the null character
-            const ptrdiff_t hex_size = p + 1 - hex_start;
-            *d++ = '\\';
-            memcpy(d, hex_start, hex_size);
-            d += hex_size;
+            dst[d++] = '\\';
+            while (hex_start <= p) {
+              dst[d++] = src[hex_start++];
+            }
             break;
           }
-          *d++ = ch;
+          dst[d++] = static_cast<char>(ch);
           break;
         }
         case 'u': {
           // \uhhhh => convert 4 hex digits to UTF-8
           char32_t rune = 0;
-          const char* hex_start = p;
-          if (p + 4 >= end) {
-            if (error) {
-              *error = "\\u must be followed by 4 hex digits: \\" +
-                       std::string(hex_start, p + 1 - hex_start);
+          auto hex_start = p;
+          if (p + 4 >= src.size()) {
+            if (error != nullptr) {
+              *error = "\\u must be followed by 4 hex digits";
             }
             return false;
           }
           for (int i = 0; i < 4; ++i) {
             // Look one char ahead.
-            if (absl::ascii_isxdigit(p[1])) {
-              rune = (rune << 4) + hex_digit_to_int(*++p);  // Advance p.
+            if (absl::ascii_isxdigit(static_cast<unsigned char>(src[p + 1]))) {
+              rune = (rune << 4) + hex_digit_to_int(src[++p]);
             } else {
-              if (error) {
+              if (error != nullptr) {
                 *error = "\\u must be followed by 4 hex digits: \\" +
-                         std::string(hex_start, p + 1 - hex_start);
+                         std::string(src.substr(hex_start, p + 1 - hex_start));
               }
               return false;
             }
           }
           if ((rune == 0) && leave_nulls_escaped) {
             // Copy the escape sequence for the null character
-            *d++ = '\\';
-            memcpy(d, hex_start, 5);  // u0000
-            d += 5;
+            dst[d++] = '\\';
+            while (hex_start <= p) {
+              dst[d++] = src[hex_start++];
+            }
             break;
           }
-          if (IsSurrogate(rune, absl::string_view(hex_start, 5), error)) {
+          if (IsSurrogate(rune, src.substr(hex_start, 5), error)) {
             return false;
           }
-          d += strings_internal::EncodeUTF8Char(d, rune);
+          d += strings_internal::EncodeUTF8Char(dst + d, rune);
           break;
         }
         case 'U': {
           // \Uhhhhhhhh => convert 8 hex digits to UTF-8
           char32_t rune = 0;
-          const char* hex_start = p;
-          if (p + 8 >= end) {
-            if (error) {
-              *error = "\\U must be followed by 8 hex digits: \\" +
-                       std::string(hex_start, p + 1 - hex_start);
+          auto hex_start = p;
+          if (p + 8 >= src.size()) {
+            if (error != nullptr) {
+              *error = "\\U must be followed by 8 hex digits";
             }
             return false;
           }
           for (int i = 0; i < 8; ++i) {
             // Look one char ahead.
-            if (absl::ascii_isxdigit(p[1])) {
+            if (absl::ascii_isxdigit(static_cast<unsigned char>(src[p + 1]))) {
               // Don't change rune until we're sure this
               // is within the Unicode limit, but do advance p.
-              uint32_t newrune = (rune << 4) + hex_digit_to_int(*++p);
+              uint32_t newrune = (rune << 4) + hex_digit_to_int(src[++p]);
               if (newrune > 0x10FFFF) {
-                if (error) {
-                  *error = "Value of \\" +
-                           std::string(hex_start, p + 1 - hex_start) +
-                           " exceeds Unicode limit (0x10FFFF)";
+                if (error != nullptr) {
+                  *error =
+                      "Value of \\" +
+                      std::string(src.substr(hex_start, p + 1 - hex_start)) +
+                      " exceeds Unicode limit (0x10FFFF)";
                 }
                 return false;
               } else {
                 rune = newrune;
               }
             } else {
-              if (error) {
+              if (error != nullptr) {
                 *error = "\\U must be followed by 8 hex digits: \\" +
-                         std::string(hex_start, p + 1 - hex_start);
+                         std::string(src.substr(hex_start, p + 1 - hex_start));
               }
               return false;
             }
           }
           if ((rune == 0) && leave_nulls_escaped) {
             // Copy the escape sequence for the null character
-            *d++ = '\\';
-            memcpy(d, hex_start, 9);  // U00000000
-            d += 9;
+            dst[d++] = '\\';
+            // U00000000
+            while (hex_start <= p) {
+              dst[d++] = src[hex_start++];
+            }
             break;
           }
-          if (IsSurrogate(rune, absl::string_view(hex_start, 9), error)) {
+          if (IsSurrogate(rune, src.substr(hex_start, 9), error)) {
             return false;
           }
-          d += strings_internal::EncodeUTF8Char(d, rune);
+          d += strings_internal::EncodeUTF8Char(dst + d, rune);
           break;
         }
         default: {
-          if (error) *error = std::string("Unknown escape sequence: \\") + *p;
+          if (error != nullptr) {
+            *error = std::string("Unknown escape sequence: \\") + src[p];
+          }
           return false;
         }
       }
-      p++;                                 // read past letter we escaped
+      p++;  // Read past letter we escaped.
     }
   }
-  *dest_len = d - dest;
-  return true;
-}
 
-// ----------------------------------------------------------------------
-// CUnescapeInternal()
-//
-//    Same as above but uses a std::string for output. 'source' and 'dest'
-//    may be the same.
-// ----------------------------------------------------------------------
-bool CUnescapeInternal(absl::string_view source, bool leave_nulls_escaped,
-                       std::string* dest, std::string* error) {
-  strings_internal::STLStringResizeUninitialized(dest, source.size());
-
-  ptrdiff_t dest_size;
-  if (!CUnescapeInternal(source,
-                         leave_nulls_escaped,
-                         &(*dest)[0],
-                         &dest_size,
-                         error)) {
-    return false;
-  }
-  dest->erase(dest_size);
+  *dst_size = d;
   return true;
 }
 
@@ -330,7 +328,7 @@ std::string CEscapeInternal(absl::string_view src, bool use_hex,
   std::string dest;
   bool last_hex_escape = false;  // true if last output char was \xNN.
 
-  for (unsigned char c : src) {
+  for (char c : src) {
     bool is_hex_escape = false;
     switch (c) {
       case '\n': dest.append("\\" "n"); break;
@@ -339,28 +337,30 @@ std::string CEscapeInternal(absl::string_view src, bool use_hex,
       case '\"': dest.append("\\" "\""); break;
       case '\'': dest.append("\\" "'"); break;
       case '\\': dest.append("\\" "\\"); break;
-      default:
+      default: {
         // Note that if we emit \xNN and the src character after that is a hex
         // digit then that digit must be escaped too to prevent it being
         // interpreted as part of the character code by C.
-        if ((!utf8_safe || c < 0x80) &&
-            (!absl::ascii_isprint(c) ||
-             (last_hex_escape && absl::ascii_isxdigit(c)))) {
+        const unsigned char uc = static_cast<unsigned char>(c);
+        if ((!utf8_safe || uc < 0x80) &&
+            (!absl::ascii_isprint(uc) ||
+             (last_hex_escape && absl::ascii_isxdigit(uc)))) {
           if (use_hex) {
             dest.append("\\" "x");
-            dest.push_back(kHexChar[c / 16]);
-            dest.push_back(kHexChar[c % 16]);
+            dest.push_back(numbers_internal::kHexChar[uc / 16]);
+            dest.push_back(numbers_internal::kHexChar[uc % 16]);
             is_hex_escape = true;
           } else {
             dest.append("\\");
-            dest.push_back(kHexChar[c / 64]);
-            dest.push_back(kHexChar[(c % 64) / 8]);
-            dest.push_back(kHexChar[c % 8]);
+            dest.push_back(numbers_internal::kHexChar[uc / 64]);
+            dest.push_back(numbers_internal::kHexChar[(uc % 64) / 8]);
+            dest.push_back(numbers_internal::kHexChar[uc % 8]);
           }
         } else {
           dest.push_back(c);
           break;
         }
+      }
     }
     last_hex_escape = is_hex_escape;
   }
@@ -369,7 +369,7 @@ std::string CEscapeInternal(absl::string_view src, bool use_hex,
 }
 
 /* clang-format off */
-constexpr char c_escaped_len[256] = {
+constexpr std::array<unsigned char, 256> kCEscapedLen = {
     4, 4, 4, 4, 4, 4, 4, 4, 4, 2, 2, 4, 4, 2, 4, 4,  // \t, \n, \r
     4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4,
     1, 1, 2, 1, 1, 1, 1, 2, 1, 1, 1, 1, 1, 1, 1, 1,  // ", '
@@ -389,77 +389,259 @@ constexpr char c_escaped_len[256] = {
 };
 /* clang-format on */
 
+constexpr std::array<std::array<char, 4>, 256> kCEscapedSequence = []() {
+  std::array<std::array<char, 4>, 256> a{};
+  for (size_t c = 0; c < 256; ++c) {
+    size_t char_len = kCEscapedLen[c];
+    if (char_len == 1) {
+      a[c][0] = static_cast<char>(c);
+    } else if (char_len == 2) {
+      a[c][0] = '\\';
+      // clang-format off
+      switch (c) {
+        case '\n': a[c][1] = 'n'; break;
+        case '\r': a[c][1] = 'r'; break;
+        case '\t': a[c][1] = 't'; break;
+        case '\"': a[c][1] = '\"'; break;
+        case '\'': a[c][1] = '\''; break;
+        case '\\': a[c][1] = '\\'; break;
+      }
+      // clang-format on
+    } else {
+      assert(char_len == 4);
+      // A backslash followed by the octal value of the byte.
+      a[c][0] = '\\';
+      a[c][1] = static_cast<char>('0' + (c / 64));
+      a[c][2] = static_cast<char>('0' + ((c % 64) / 8));
+      a[c][3] = static_cast<char>('0' + (c % 8));
+    }
+  }
+  return a;
+}();
+
 // Calculates the length of the C-style escaped version of 'src'.
 // Assumes that non-printable characters are escaped using octal sequences, and
 // that UTF-8 bytes are not handled specially.
 inline size_t CEscapedLength(absl::string_view src) {
   size_t escaped_len = 0;
-  for (unsigned char c : src) escaped_len += c_escaped_len[c];
+  // The maximum value of kCEscapedLen[x] is 4, so we can escape any string of
+  // length size_t_max/4 without checking for overflow.
+  size_t unchecked_limit =
+      std::min<size_t>(src.size(), std::numeric_limits<size_t>::max() / 4);
+  size_t i = 0;
+  while (i < unchecked_limit) {
+    // Common case: No need to check for overflow.
+    escaped_len += kCEscapedLen[static_cast<unsigned char>(src[i++])];
+  }
+  while (i < src.size()) {
+    // Beyond unchecked_limit we need to check for overflow before adding.
+    size_t char_len = kCEscapedLen[static_cast<unsigned char>(src[i++])];
+    ABSL_INTERNAL_CHECK(
+        escaped_len <= std::numeric_limits<size_t>::max() - char_len,
+        "escaped_len overflow");
+    escaped_len += char_len;
+  }
   return escaped_len;
 }
 
-void CEscapeAndAppendInternal(absl::string_view src, std::string* dest) {
+void CEscapeAndAppendInternal(absl::string_view src,
+                              std::string* absl_nonnull dest) {
   size_t escaped_len = CEscapedLength(src);
   if (escaped_len == src.size()) {
     dest->append(src.data(), src.size());
     return;
   }
 
-  size_t cur_dest_len = dest->size();
-  strings_internal::STLStringResizeUninitialized(dest,
-                                                 cur_dest_len + escaped_len);
-  char* append_ptr = &(*dest)[cur_dest_len];
+  // The small `memcpy` is faster when the size is a compile-time constant, so
+  // keep 3 slop bytes so that we can call memcpy with size=4.
+  constexpr size_t kSlopBytes = 3;
+  ABSL_INTERNAL_CHECK(
+      escaped_len <= std::numeric_limits<size_t>::max() - kSlopBytes,
+      "CEscape length overflow");
+  size_t append_buf_len = escaped_len + kSlopBytes;
+  strings_internal::StringAppendAndOverwrite(
+      *dest, append_buf_len, [src, escaped_len](char* append_ptr, size_t) {
+        for (char c : src) {
+          unsigned char uc = static_cast<unsigned char>(c);
+          memcpy(append_ptr, kCEscapedSequence[uc].data(), 4);
+          append_ptr += kCEscapedLen[uc];
+        }
+        return escaped_len;
+      });
+}
 
-  for (unsigned char c : src) {
-    int char_len = c_escaped_len[c];
-    if (char_len == 1) {
-      *append_ptr++ = c;
-    } else if (char_len == 2) {
-      switch (c) {
-        case '\n':
-          *append_ptr++ = '\\';
-          *append_ptr++ = 'n';
-          break;
-        case '\r':
-          *append_ptr++ = '\\';
-          *append_ptr++ = 'r';
-          break;
-        case '\t':
-          *append_ptr++ = '\\';
-          *append_ptr++ = 't';
-          break;
-        case '\"':
-          *append_ptr++ = '\\';
-          *append_ptr++ = '\"';
-          break;
-        case '\'':
-          *append_ptr++ = '\\';
-          *append_ptr++ = '\'';
-          break;
-        case '\\':
-          *append_ptr++ = '\\';
-          *append_ptr++ = '\\';
-          break;
+// The two strings below provide maps from normal 6-bit characters to their
+// base64-escaped equivalent.
+// For the inverse case, see kUn(WebSafe)Base64 in the external
+// escaping.cc.
+constexpr char kBase64Chars[] =
+    "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
+
+constexpr char kWebSafeBase64Chars[] =
+    "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_";
+
+// ----------------------------------------------------------------------
+//   Take the input in groups of 4 characters and turn each
+//   character into a code 0 to 63 thus:
+//           A-Z map to 0 to 25
+//           a-z map to 26 to 51
+//           0-9 map to 52 to 61
+//           +(- for WebSafe) maps to 62
+//           /(_ for WebSafe) maps to 63
+//   There will be four numbers, all less than 64 which can be represented
+//   by a 6 digit binary number (aaaaaa, bbbbbb, cccccc, dddddd respectively).
+//   Arrange the 6 digit binary numbers into three bytes as such:
+//   aaaaaabb bbbbcccc ccdddddd
+//   Equals signs (one or two) are used at the end of the encoded block to
+//   indicate that the text was not an integer multiple of three bytes long.
+// ----------------------------------------------------------------------
+size_t Base64EscapeInternal(const unsigned char* src, size_t szsrc, char* dest,
+                            size_t szdest, const char* base64,
+                            bool do_padding) {
+  constexpr char kPad64 = '=';
+
+  constexpr size_t kMaxSize = (std::numeric_limits<size_t>::max() - 1) / 4 * 3;
+  if (ABSL_PREDICT_FALSE(szsrc > kMaxSize || szsrc * 4 > szdest * 3)) return 0;
+
+  char* cur_dest = dest;
+  const unsigned char* cur_src = src;
+
+  char* const limit_dest = dest + szdest;
+  const unsigned char* const limit_src = src + szsrc;
+
+  // (from https://tools.ietf.org/html/rfc3548)
+  // Special processing is performed if fewer than 24 bits are available
+  // at the end of the data being encoded.  A full encoding quantum is
+  // always completed at the end of a quantity.  When fewer than 24 input
+  // bits are available in an input group, zero bits are added (on the
+  // right) to form an integral number of 6-bit groups.
+  //
+  // If do_padding is true, padding at the end of the data is performed. This
+  // output padding uses the '=' character.
+
+  // Three bytes of data encodes to four characters of cyphertext.
+  // So we can pump through three-byte chunks atomically.
+  if (szsrc >= 3) {                    // "limit_src - 3" is UB if szsrc < 3.
+    while (cur_src < limit_src - 3) {  // While we have >= 32 bits.
+      uint32_t in = absl::big_endian::Load32(cur_src) >> 8;
+
+      cur_dest[0] = base64[in >> 18];
+      in &= 0x3FFFF;
+      cur_dest[1] = base64[in >> 12];
+      in &= 0xFFF;
+      cur_dest[2] = base64[in >> 6];
+      in &= 0x3F;
+      cur_dest[3] = base64[in];
+
+      cur_dest += 4;
+      cur_src += 3;
+    }
+  }
+  // To save time, we didn't update szdest or szsrc in the loop.  So do it now.
+  szdest = static_cast<size_t>(limit_dest - cur_dest);
+  szsrc = static_cast<size_t>(limit_src - cur_src);
+
+  /* now deal with the tail (<=3 bytes) */
+  switch (szsrc) {
+    case 0:
+      // Nothing left; nothing more to do.
+      break;
+    case 1: {
+      // One byte left: this encodes to two characters, and (optionally)
+      // two pad characters to round out the four-character cypherblock.
+      if (szdest < 2) return 0;
+      uint32_t in = cur_src[0];
+      cur_dest[0] = base64[in >> 2];
+      in &= 0x3;
+      cur_dest[1] = base64[in << 4];
+      cur_dest += 2;
+      szdest -= 2;
+      if (do_padding) {
+        if (szdest < 2) return 0;
+        cur_dest[0] = kPad64;
+        cur_dest[1] = kPad64;
+        cur_dest += 2;
+        szdest -= 2;
       }
-    } else {
-      *append_ptr++ = '\\';
-      *append_ptr++ = '0' + c / 64;
-      *append_ptr++ = '0' + (c % 64) / 8;
-      *append_ptr++ = '0' + c % 8;
+      break;
+    }
+    case 2: {
+      // Two bytes left: this encodes to three characters, and (optionally)
+      // one pad character to round out the four-character cypherblock.
+      if (szdest < 3) return 0;
+      uint32_t in = absl::big_endian::Load16(cur_src);
+      cur_dest[0] = base64[in >> 10];
+      in &= 0x3FF;
+      cur_dest[1] = base64[in >> 4];
+      in &= 0x00F;
+      cur_dest[2] = base64[in << 2];
+      cur_dest += 3;
+      szdest -= 3;
+      if (do_padding) {
+        if (szdest < 1) return 0;
+        cur_dest[0] = kPad64;
+        cur_dest += 1;
+        szdest -= 1;
+      }
+      break;
+    }
+    case 3: {
+      // Three bytes left: same as in the big loop above.  We can't do this in
+      // the loop because the loop above always reads 4 bytes, and the fourth
+      // byte is past the end of the input.
+      if (szdest < 4) return 0;
+      uint32_t in =
+          (uint32_t{cur_src[0]} << 16) + absl::big_endian::Load16(cur_src + 1);
+      cur_dest[0] = base64[in >> 18];
+      in &= 0x3FFFF;
+      cur_dest[1] = base64[in >> 12];
+      in &= 0xFFF;
+      cur_dest[2] = base64[in >> 6];
+      in &= 0x3F;
+      cur_dest[3] = base64[in];
+      cur_dest += 4;
+      szdest -= 4;
+      break;
     }
+    default:
+      // Should not be reached: blocks of 4 bytes are handled
+      // in the while loop before this switch statement.
+      ABSL_RAW_LOG(FATAL, "Logic problem? szsrc = %zu", szsrc);
+      break;
   }
+  return static_cast<size_t>(cur_dest - dest);
+}
+
+std::string Base64EscapeToStringInternal(const unsigned char* src, size_t szsrc,
+                                         bool do_padding,
+                                         const char* base64_chars) {
+  std::string escaped;
+  const size_t calc_escaped_size =
+      strings_internal::CalculateBase64EscapedLenInternal(szsrc, do_padding);
+  StringResizeAndOverwrite(
+      escaped, calc_escaped_size,
+      [src, szsrc, base64_chars, do_padding](char* buf, size_t buf_size) {
+        const size_t escaped_len = Base64EscapeInternal(
+            src, szsrc, buf, buf_size, base64_chars, do_padding);
+        assert(escaped_len == buf_size);
+        return escaped_len;
+      });
+  return escaped;
 }
 
-bool Base64UnescapeInternal(const char* src_param, size_t szsrc, char* dest,
-                            size_t szdest, const signed char* unbase64,
-                            size_t* len) {
+// Reverses the mapping in Base64EscapeInternal; see that method's
+// documentation for details of the mapping.
+bool Base64UnescapeInternal(const char* absl_nullable src_param, size_t szsrc,
+                            char* absl_nullable dest, size_t szdest,
+                            const std::array<signed char, 256>& unbase64,
+                            size_t* absl_nonnull len) {
   static const char kPad64Equals = '=';
   static const char kPad64Dot = '.';
 
   size_t destidx = 0;
   int decode = 0;
   int state = 0;
-  unsigned int ch = 0;
+  unsigned char ch = 0;
   unsigned int temp = 0;
 
   // If "char" is signed by default, using *src as an array index results in
@@ -469,7 +651,7 @@ bool Base64UnescapeInternal(const char* src_param, size_t szsrc, char* dest,
 
   // The GET_INPUT macro gets the next input character, skipping
   // over any whitespace, and stopping when we reach the end of the
-  // std::string or when we read any non-data character.  The arguments are
+  // string or when we read any non-data character.  The arguments are
   // an arbitrary identifier (used as a label for goto) and the number
   // of data bytes that must remain in the input to avoid aborting the
   // loop.
@@ -492,18 +674,18 @@ bool Base64UnescapeInternal(const char* src_param, size_t szsrc, char* dest,
   if (dest) {
     // This loop consumes 4 input bytes and produces 3 output bytes
     // per iteration.  We can't know at the start that there is enough
-    // data left in the std::string for a full iteration, so the loop may
+    // data left in the string for a full iteration, so the loop may
     // break out in the middle; if so 'state' will be set to the
     // number of input bytes read.
 
     while (szsrc >= 4) {
       // We'll start by optimistically assuming that the next four
-      // bytes of the std::string (src[0..3]) are four good data bytes
+      // bytes of the string (src[0..3]) are four good data bytes
       // (that is, no nulls, whitespace, padding chars, or illegal
       // chars).  We need to test src[0..2] for nulls individually
       // before constructing temp to preserve the property that we
-      // never read past a null in the std::string (no matter how long
-      // szsrc claims the std::string is).
+      // never read past a null in the string (no matter how long
+      // szsrc claims the string is).
 
       if (!src[0] || !src[1] || !src[2] ||
           ((temp = ((unsigned(unbase64[src[0]]) << 18) |
@@ -519,16 +701,16 @@ bool Base64UnescapeInternal(const char* src_param, size_t szsrc, char* dest,
         // how to handle those cases.
 
         GET_INPUT(first, 4);
-        temp = decode;
+        temp = static_cast<unsigned char>(decode);
         GET_INPUT(second, 3);
-        temp = (temp << 6) | decode;
+        temp = (temp << 6) | static_cast<unsigned char>(decode);
         GET_INPUT(third, 2);
-        temp = (temp << 6) | decode;
+        temp = (temp << 6) | static_cast<unsigned char>(decode);
         GET_INPUT(fourth, 1);
-        temp = (temp << 6) | decode;
+        temp = (temp << 6) | static_cast<unsigned char>(decode);
       } else {
         // We really did have four good data bytes, so advance four
-        // characters in the std::string.
+        // characters in the string.
 
         szsrc -= 4;
         src += 4;
@@ -537,11 +719,11 @@ bool Base64UnescapeInternal(const char* src_param, size_t szsrc, char* dest,
       // temp has 24 bits of input, so write that out as three bytes.
 
       if (destidx + 3 > szdest) return false;
-      dest[destidx + 2] = temp;
+      dest[destidx + 2] = static_cast<char>(temp);
       temp >>= 8;
-      dest[destidx + 1] = temp;
+      dest[destidx + 1] = static_cast<char>(temp);
       temp >>= 8;
-      dest[destidx] = temp;
+      dest[destidx] = static_cast<char>(temp);
       destidx += 3;
     }
   } else {
@@ -602,18 +784,18 @@ bool Base64UnescapeInternal(const char* src_param, size_t szsrc, char* dest,
       }
 
       // Each input character gives us six bits of output.
-      temp = (temp << 6) | decode;
+      temp = (temp << 6) | static_cast<unsigned char>(decode);
       ++state;
       if (state == 4) {
         // If we've accumulated 24 bits of output, write that out as
         // three bytes.
         if (dest) {
           if (destidx + 3 > szdest) return false;
-          dest[destidx + 2] = temp;
+          dest[destidx + 2] = static_cast<char>(temp);
           temp >>= 8;
-          dest[destidx + 1] = temp;
+          dest[destidx + 1] = static_cast<char>(temp);
           temp >>= 8;
-          dest[destidx] = temp;
+          dest[destidx] = static_cast<char>(temp);
         }
         destidx += 3;
         state = 0;
@@ -638,7 +820,7 @@ bool Base64UnescapeInternal(const char* src_param, size_t szsrc, char* dest,
       if (dest) {
         if (destidx + 1 > szdest) return false;
         temp >>= 4;
-        dest[destidx] = temp;
+        dest[destidx] = static_cast<char>(temp);
       }
       ++destidx;
       expected_equals = 2;
@@ -649,9 +831,9 @@ bool Base64UnescapeInternal(const char* src_param, size_t szsrc, char* dest,
       if (dest) {
         if (destidx + 2 > szdest) return false;
         temp >>= 2;
-        dest[destidx + 1] = temp;
+        dest[destidx + 1] = static_cast<char>(temp);
         temp >>= 8;
-        dest[destidx] = temp;
+        dest[destidx] = static_cast<char>(temp);
       }
       destidx += 2;
       expected_equals = 1;
@@ -663,7 +845,7 @@ bool Base64UnescapeInternal(const char* src_param, size_t szsrc, char* dest,
                    state);
   }
 
-  // The remainder of the std::string should be all whitespace, mixed with
+  // The remainder of the string should be all whitespace, mixed with
   // exactly 0 equals signs, or exactly 'expected_equals' equals
   // signs.  (Always accepting 0 equals signs is an Abseil extension
   // not covered in the RFC, as is accepting dot as the pad character.)
@@ -683,7 +865,10 @@ bool Base64UnescapeInternal(const char* src_param, size_t szsrc, char* dest,
   return ok;
 }
 
-// The arrays below were generated by the following code
+// The arrays below map base64-escaped characters back to their original values.
+// For the inverse case, see k(WebSafe)Base64Chars in the internal
+// escaping.cc.
+// These arrays were generated by the following inversion code:
 // #include <sys/time.h>
 // #include <stdlib.h>
 // #include <string.h>
@@ -710,10 +895,10 @@ bool Base64UnescapeInternal(const char* src_param, size_t szsrc, char* dest,
 //   }
 // }
 //
-// where the value of "Base64[]" was replaced by one of the base-64 conversion
-// tables from the functions below.
+// where the value of "Base64[]" was replaced by one of k(WebSafe)Base64Chars
+// in the internal escaping.cc.
 /* clang-format off */
-constexpr signed char kUnBase64[] = {
+constexpr std::array<signed char, 256> kUnBase64 = {
     -1,      -1,      -1,      -1,      -1,      -1,      -1,      -1,
     -1,      -1,      -1,      -1,      -1,      -1,      -1,      -1,
     -1,      -1,      -1,      -1,      -1,      -1,      -1,      -1,
@@ -748,7 +933,7 @@ constexpr signed char kUnBase64[] = {
     -1,      -1,      -1,      -1,      -1,      -1,      -1,      -1
 };
 
-constexpr signed char kUnWebSafeBase64[] = {
+constexpr std::array<signed char, 256> kUnWebSafeBase64 = {
     -1,      -1,      -1,      -1,      -1,      -1,      -1,      -1,
     -1,      -1,      -1,      -1,      -1,      -1,      -1,      -1,
     -1,      -1,      -1,      -1,      -1,      -1,      -1,      -1,
@@ -784,205 +969,30 @@ constexpr signed char kUnWebSafeBase64[] = {
 };
 /* clang-format on */
 
-size_t CalculateBase64EscapedLenInternal(size_t input_len, bool do_padding) {
-  // Base64 encodes three bytes of input at a time. If the input is not
-  // divisible by three, we pad as appropriate.
-  //
-  // (from https://tools.ietf.org/html/rfc3548)
-  // Special processing is performed if fewer than 24 bits are available
-  // at the end of the data being encoded.  A full encoding quantum is
-  // always completed at the end of a quantity.  When fewer than 24 input
-  // bits are available in an input group, zero bits are added (on the
-  // right) to form an integral number of 6-bit groups.  Padding at the
-  // end of the data is performed using the '=' character.  Since all base
-  // 64 input is an integral number of octets, only the following cases
-  // can arise:
-
-  // Base64 encodes each three bytes of input into four bytes of output.
-  size_t len = (input_len / 3) * 4;
-
-  if (input_len % 3 == 0) {
-    // (from https://tools.ietf.org/html/rfc3548)
-    // (1) the final quantum of encoding input is an integral multiple of 24
-    // bits; here, the final unit of encoded output will be an integral
-    // multiple of 4 characters with no "=" padding,
-  } else if (input_len % 3 == 1) {
-    // (from https://tools.ietf.org/html/rfc3548)
-    // (2) the final quantum of encoding input is exactly 8 bits; here, the
-    // final unit of encoded output will be two characters followed by two
-    // "=" padding characters, or
-    len += 2;
-    if (do_padding) {
-      len += 2;
-    }
-  } else {  // (input_len % 3 == 2)
-    // (from https://tools.ietf.org/html/rfc3548)
-    // (3) the final quantum of encoding input is exactly 16 bits; here, the
-    // final unit of encoded output will be three characters followed by one
-    // "=" padding character.
-    len += 3;
-    if (do_padding) {
-      len += 1;
-    }
-  }
-
-  assert(len >= input_len);  // make sure we didn't overflow
-  return len;
-}
-
-size_t Base64EscapeInternal(const unsigned char* src, size_t szsrc, char* dest,
-                            size_t szdest, const char* base64,
-                            bool do_padding) {
-  static const char kPad64 = '=';
-
-  if (szsrc * 4 > szdest * 3) return 0;
-
-  char* cur_dest = dest;
-  const unsigned char* cur_src = src;
-
-  char* const limit_dest = dest + szdest;
-  const unsigned char* const limit_src = src + szsrc;
-
-  // Three bytes of data encodes to four characters of cyphertext.
-  // So we can pump through three-byte chunks atomically.
-  if (szsrc >= 3) {  // "limit_src - 3" is UB if szsrc < 3.
-    while (cur_src < limit_src - 3) {  // While we have >= 32 bits.
-      uint32_t in = absl::big_endian::Load32(cur_src) >> 8;
-
-      cur_dest[0] = base64[in >> 18];
-      in &= 0x3FFFF;
-      cur_dest[1] = base64[in >> 12];
-      in &= 0xFFF;
-      cur_dest[2] = base64[in >> 6];
-      in &= 0x3F;
-      cur_dest[3] = base64[in];
-
-      cur_dest += 4;
-      cur_src += 3;
-    }
-  }
-  // To save time, we didn't update szdest or szsrc in the loop.  So do it now.
-  szdest = limit_dest - cur_dest;
-  szsrc = limit_src - cur_src;
-
-  /* now deal with the tail (<=3 bytes) */
-  switch (szsrc) {
-    case 0:
-      // Nothing left; nothing more to do.
-      break;
-    case 1: {
-      // One byte left: this encodes to two characters, and (optionally)
-      // two pad characters to round out the four-character cypherblock.
-      if (szdest < 2) return 0;
-      uint32_t in = cur_src[0];
-      cur_dest[0] = base64[in >> 2];
-      in &= 0x3;
-      cur_dest[1] = base64[in << 4];
-      cur_dest += 2;
-      szdest -= 2;
-      if (do_padding) {
-        if (szdest < 2) return 0;
-        cur_dest[0] = kPad64;
-        cur_dest[1] = kPad64;
-        cur_dest += 2;
-        szdest -= 2;
-      }
-      break;
-    }
-    case 2: {
-      // Two bytes left: this encodes to three characters, and (optionally)
-      // one pad character to round out the four-character cypherblock.
-      if (szdest < 3) return 0;
-      uint32_t in = absl::big_endian::Load16(cur_src);
-      cur_dest[0] = base64[in >> 10];
-      in &= 0x3FF;
-      cur_dest[1] = base64[in >> 4];
-      in &= 0x00F;
-      cur_dest[2] = base64[in << 2];
-      cur_dest += 3;
-      szdest -= 3;
-      if (do_padding) {
-        if (szdest < 1) return 0;
-        cur_dest[0] = kPad64;
-        cur_dest += 1;
-        szdest -= 1;
-      }
-      break;
-    }
-    case 3: {
-      // Three bytes left: same as in the big loop above.  We can't do this in
-      // the loop because the loop above always reads 4 bytes, and the fourth
-      // byte is past the end of the input.
-      if (szdest < 4) return 0;
-      uint32_t in = (cur_src[0] << 16) + absl::big_endian::Load16(cur_src + 1);
-      cur_dest[0] = base64[in >> 18];
-      in &= 0x3FFFF;
-      cur_dest[1] = base64[in >> 12];
-      in &= 0xFFF;
-      cur_dest[2] = base64[in >> 6];
-      in &= 0x3F;
-      cur_dest[3] = base64[in];
-      cur_dest += 4;
-      szdest -= 4;
-      break;
-    }
-    default:
-      // Should not be reached: blocks of 4 bytes are handled
-      // in the while loop before this switch statement.
-      ABSL_RAW_LOG(FATAL, "Logic problem? szsrc = %zu", szsrc);
-      break;
-  }
-  return (cur_dest - dest);
-}
-
-constexpr char kBase64Chars[] =
-    "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
-
-constexpr char kWebSafeBase64Chars[] =
-    "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_";
-
-template <typename String>
-void Base64EscapeInternal(const unsigned char* src, size_t szsrc, String* dest,
-                          bool do_padding, const char* base64_chars) {
-  const size_t calc_escaped_size =
-      CalculateBase64EscapedLenInternal(szsrc, do_padding);
-  strings_internal::STLStringResizeUninitialized(dest, calc_escaped_size);
-
-  const size_t escaped_len = Base64EscapeInternal(
-      src, szsrc, &(*dest)[0], dest->size(), base64_chars, do_padding);
-  assert(calc_escaped_size == escaped_len);
-  dest->erase(escaped_len);
-}
-
 template <typename String>
-bool Base64UnescapeInternal(const char* src, size_t slen, String* dest,
-                            const signed char* unbase64) {
-  // Determine the size of the output std::string.  Base64 encodes every 3 bytes into
-  // 4 characters.  any leftover chars are added directly for good measure.
-  // This is documented in the base64 RFC: http://tools.ietf.org/html/rfc3548
+bool Base64UnescapeInternal(const char* absl_nullable src, size_t slen,
+                            String* absl_nonnull dest,
+                            const std::array<signed char, 256>& unbase64) {
+  // Determine the size of the output string.  Base64 encodes every 3 bytes into
+  // 4 characters.  Any leftover chars are added directly for good measure.
   const size_t dest_len = 3 * (slen / 4) + (slen % 4);
 
-  strings_internal::STLStringResizeUninitialized(dest, dest_len);
-
-  // We are getting the destination buffer by getting the beginning of the
-  // std::string and converting it into a char *.
-  size_t len;
-  const bool ok =
-      Base64UnescapeInternal(src, slen, &(*dest)[0], dest_len, unbase64, &len);
-  if (!ok) {
-    dest->clear();
-    return false;
-  }
-
-  // could be shorter if there was padding
-  assert(len <= dest_len);
-  dest->erase(len);
-
-  return true;
+  bool ok;
+  StringResizeAndOverwrite(
+      *dest, dest_len, [src, slen, unbase64, &ok](char* buf, size_t buf_size) {
+        size_t len;
+        ok = Base64UnescapeInternal(src, slen, buf, buf_size, unbase64, &len);
+        if (!ok) {
+          len = 0;
+        }
+        assert(len <= buf_size);  // Could be shorter if there was padding.
+        return len;
+      });
+  return ok;
 }
 
 /* clang-format off */
-constexpr char kHexValue[256] = {
+constexpr std::array<uint8_t, 256> kHexValueLenient = {
     0,  0,  0,  0,  0,  0,  0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
     0,  0,  0,  0,  0,  0,  0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
     0,  0,  0,  0,  0,  0,  0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
@@ -998,7 +1008,26 @@ constexpr char kHexValue[256] = {
     0,  0,  0,  0,  0,  0,  0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
     0,  0,  0,  0,  0,  0,  0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
     0,  0,  0,  0,  0,  0,  0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
-    0,  0,  0,  0,  0,  0,  0, 0, 0, 0, 0, 0, 0, 0, 0, 0
+    0,  0,  0,  0,  0,  0,  0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+};
+
+constexpr std::array<int8_t, 256> kHexValueStrict = {
+    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+     0,  1,  2,  3,  4,  5,  6,  7,  8,  9, -1, -1, -1, -1, -1, -1,  // '0'..'9'
+    -1, 10, 11, 12, 13, 14, 15, -1, -1, -1, -1, -1, -1, -1, -1, -1,  // 'A'..'F'
+    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+    -1, 10, 11, 12, 13, 14, 15, -1, -1, -1, -1, -1, -1, -1, -1, -1,  // 'a'..'f'
+    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
 };
 /* clang-format on */
 
@@ -1006,21 +1035,19 @@ constexpr char kHexValue[256] = {
 // or a string.  This works because we use the [] operator to access
 // individual characters at a time.
 template <typename T>
-void HexStringToBytesInternal(const char* from, T to, ptrdiff_t num) {
-  for (int i = 0; i < num; i++) {
-    to[i] = (kHexValue[from[i * 2] & 0xFF] << 4) +
-            (kHexValue[from[i * 2 + 1] & 0xFF]);
+void HexStringToBytesInternal(const char* absl_nullable from, T to,
+                              size_t num) {
+  for (size_t i = 0; i < num; i++) {
+    to[i] = static_cast<char>(kHexValueLenient[from[i * 2] & 0xFF] << 4) +
+            static_cast<char>(kHexValueLenient[from[i * 2 + 1] & 0xFF]);
   }
 }
 
-// This is a templated function so that T can be either a char* or a
-// std::string.
-template <typename T>
-void BytesToHexStringInternal(const unsigned char* src, T dest, ptrdiff_t num) {
-  auto dest_ptr = &dest[0];
-  for (auto src_ptr = src; src_ptr != (src + num); ++src_ptr, dest_ptr += 2) {
-    const char* hex_p = &kHexTable[*src_ptr * 2];
-    std::copy(hex_p, hex_p + 2, dest_ptr);
+void BytesToHexStringInternal(const unsigned char* absl_nullable src,
+                              char* dest, size_t num) {
+  for (auto src_ptr = src; src_ptr != (src + num); ++src_ptr, dest += 2) {
+    const char* hex_p = &numbers_internal::kHexTable[*src_ptr * 2];
+    std::copy(hex_p, hex_p + 2, dest);
   }
 }
 
@@ -1031,9 +1058,35 @@ void BytesToHexStringInternal(const unsigned char* src, T dest, ptrdiff_t num) {
 //
 // See CUnescapeInternal() for implementation details.
 // ----------------------------------------------------------------------
-bool CUnescape(absl::string_view source, std::string* dest,
-               std::string* error) {
-  return CUnescapeInternal(source, kUnescapeNulls, dest, error);
+
+bool CUnescape(absl::string_view source, std::string* absl_nonnull dest,
+               std::string* absl_nullable error) {
+  bool success;
+
+  // `CUnescape()` allows for in-place unescaping, which means `source` may
+  // alias `*dest`.  However, absl::StringResizeAndOverwrite() invalidates all
+  // iterators, pointers, and references into the string, regardless whether
+  // reallocation occurs. Therefore we need to avoid calling
+  // absl::StringResizeAndOverwrite() when `source.data() ==
+  // dest->data()`. Comparing the sizes is sufficient to cover this case.
+  if (dest->size() >= source.size()) {
+    size_t dest_size = 0;
+    success = CUnescapeInternal(source, kUnescapeNulls, dest->data(),
+                                &dest_size, error);
+    ABSL_ASSERT(dest_size <= dest->size());
+    dest->erase(dest_size);
+  } else {
+    StringResizeAndOverwrite(
+        *dest, source.size(),
+        [source, error, &success](char* buf, size_t buf_size) {
+          size_t dest_size = 0;
+          success =
+              CUnescapeInternal(source, kUnescapeNulls, buf, &dest_size, error);
+          ABSL_ASSERT(dest_size <= buf_size);
+          return dest_size;
+        });
+  }
+  return success;
 }
 
 std::string CEscape(absl::string_view src) {
@@ -1054,76 +1107,207 @@ std::string Utf8SafeCHexEscape(absl::string_view src) {
   return CEscapeInternal(src, true, true);
 }
 
-// ----------------------------------------------------------------------
-// Base64Unescape() - base64 decoder
-// Base64Escape() - base64 encoder
-// WebSafeBase64Unescape() - Google's variation of base64 decoder
-// WebSafeBase64Escape() - Google's variation of base64 encoder
-//
-// Check out
-// http://tools.ietf.org/html/rfc2045 for formal description, but what we
-// care about is that...
-//   Take the encoded stuff in groups of 4 characters and turn each
-//   character into a code 0 to 63 thus:
-//           A-Z map to 0 to 25
-//           a-z map to 26 to 51
-//           0-9 map to 52 to 61
-//           +(- for WebSafe) maps to 62
-//           /(_ for WebSafe) maps to 63
-//   There will be four numbers, all less than 64 which can be represented
-//   by a 6 digit binary number (aaaaaa, bbbbbb, cccccc, dddddd respectively).
-//   Arrange the 6 digit binary numbers into three bytes as such:
-//   aaaaaabb bbbbcccc ccdddddd
-//   Equals signs (one or two) are used at the end of the encoded block to
-//   indicate that the text was not an integer multiple of three bytes long.
-// ----------------------------------------------------------------------
-
-bool Base64Unescape(absl::string_view src, std::string* dest) {
+bool Base64Unescape(absl::string_view src, std::string* absl_nonnull dest) {
   return Base64UnescapeInternal(src.data(), src.size(), dest, kUnBase64);
 }
 
-bool WebSafeBase64Unescape(absl::string_view src, std::string* dest) {
+bool WebSafeBase64Unescape(absl::string_view src,
+                           std::string* absl_nonnull dest) {
   return Base64UnescapeInternal(src.data(), src.size(), dest, kUnWebSafeBase64);
 }
 
-void Base64Escape(absl::string_view src, std::string* dest) {
-  Base64EscapeInternal(reinterpret_cast<const unsigned char*>(src.data()),
-                       src.size(), dest, true, kBase64Chars);
+std::string Base64Escape(absl::string_view src) {
+  return Base64EscapeToStringInternal(
+      reinterpret_cast<const unsigned char*>(src.data()), src.size(), true,
+      kBase64Chars);
 }
 
-void WebSafeBase64Escape(absl::string_view src, std::string* dest) {
-  Base64EscapeInternal(reinterpret_cast<const unsigned char*>(src.data()),
-                       src.size(), dest, false, kWebSafeBase64Chars);
+std::string WebSafeBase64Escape(absl::string_view src) {
+  return Base64EscapeToStringInternal(
+      reinterpret_cast<const unsigned char*>(src.data()), src.size(), false,
+      kWebSafeBase64Chars);
 }
 
-std::string Base64Escape(absl::string_view src) {
-  std::string dest;
-  Base64EscapeInternal(reinterpret_cast<const unsigned char*>(src.data()),
-                       src.size(), &dest, true, kBase64Chars);
-  return dest;
-}
+bool HexStringToBytes(absl::string_view hex, std::string* absl_nonnull bytes) {
+  std::string output;
 
-std::string WebSafeBase64Escape(absl::string_view src) {
-  std::string dest;
-  Base64EscapeInternal(reinterpret_cast<const unsigned char*>(src.data()),
-                       src.size(), &dest, false, kWebSafeBase64Chars);
-  return dest;
+  size_t num_bytes = hex.size() / 2;
+  if (hex.size() != num_bytes * 2) {
+    return false;
+  }
+
+  StringResizeAndOverwrite(
+      output, num_bytes, [hex](char* buf, size_t buf_size) {
+        auto hex_p = hex.cbegin();
+        for (size_t i = 0; i < buf_size; ++i) {
+          int h1 = absl::kHexValueStrict[static_cast<size_t>(
+              static_cast<uint8_t>(*hex_p++))];
+          int h2 = absl::kHexValueStrict[static_cast<size_t>(
+              static_cast<uint8_t>(*hex_p++))];
+          if (h1 == -1 || h2 == -1) {
+            return size_t{0};
+          }
+          buf[i] = static_cast<char>((h1 << 4) + h2);
+        }
+        return buf_size;
+      });
+
+  if (output.size() != num_bytes) {
+    return false;
+  }
+  *bytes = std::move(output);
+  return true;
 }
 
 std::string HexStringToBytes(absl::string_view from) {
   std::string result;
   const auto num = from.size() / 2;
-  strings_internal::STLStringResizeUninitialized(&result, num);
-  absl::HexStringToBytesInternal<std::string&>(from.data(), result, num);
+  StringResizeAndOverwrite(result, num, [from](char* buf, size_t buf_size) {
+    absl::HexStringToBytesInternal<char*>(from.data(), buf, buf_size);
+    return buf_size;
+  });
   return result;
 }
 
 std::string BytesToHexString(absl::string_view from) {
   std::string result;
-  strings_internal::STLStringResizeUninitialized(&result, 2 * from.size());
-  absl::BytesToHexStringInternal<std::string&>(
-      reinterpret_cast<const unsigned char*>(from.data()), result, from.size());
+  ABSL_INTERNAL_CHECK(from.size() <= std::numeric_limits<size_t>::max() / 2,
+                      "BytesToHexString() overflow");
+  StringResizeAndOverwrite(
+      result, 2 * from.size(), [from](char* buf, size_t buf_size) {
+        absl::BytesToHexStringInternal(
+            reinterpret_cast<const unsigned char*>(from.data()), buf,
+            from.size());
+        return buf_size;
+      });
   return result;
 }
 
+static std::string UrlEscapeInternal(absl::string_view input,
+                                     const bool escape_space_to_plus) {
+  // Unreserved characters from RFC 3986.
+  // See https://www.rfc-editor.org/info/rfc3986/#section-2.3.
+  static constexpr absl::CharSet kRfc3986Unreserved =
+      absl::CharSet::AsciiAlphanumerics() | absl::CharSet("-._~");
+
+  std::string output;
+  absl::string_view::iterator in = input.begin();
+
+  // Fast path for when we don't need to do any escaping.
+  while (in < input.end() && kRfc3986Unreserved.contains(*in)) {
+    ++in;
+  }
+
+  std::size_t initial_portion =
+      static_cast<std::size_t>(std::distance(input.begin(), in));
+
+  if (initial_portion == input.size()) {
+    return std::string(input);
+  }
+
+  // We need a buffer with enough space to store at most the initial portion
+  // plus 3 bytes for each remaining character since escapes use 3 characters.
+  ABSL_INTERNAL_CHECK(
+      (input.size() - initial_portion) <=
+          (std::numeric_limits<size_t>::max() - initial_portion) / 3,
+      "UrlEscape() overflow");
+  StringResizeAndOverwrite(
+      output, initial_portion + 3 * (input.size() - initial_portion),
+      [&](char* buf, size_t) {
+        char* out = buf;
+
+        // Copy the initial portion that did not need escaping.
+        out = std::copy(input.begin(), in, out);
+
+        // Handle the rest of the string.
+        while (in < input.end()) {
+          char c = *in++;
+          if (kRfc3986Unreserved.contains(c)) {
+            *out++ = c;
+          } else if (escape_space_to_plus && c == ' ') {
+            *out++ = '+';
+          } else {
+            *out++ = '%';
+            *out++ = static_cast<char>(
+                int_to_hex_digit((static_cast<unsigned char>(c) >> 4) & 0xf));
+            *out++ = static_cast<char>(
+                int_to_hex_digit(static_cast<unsigned char>(c) & 0xf));
+          }
+        }
+        return static_cast<size_t>(std::distance(buf, out));
+      });
+
+  return output;
+}
+
+static std::optional<std::string> UrlUnescapeInternal(
+    absl::string_view input, const bool unescape_plus_to_space) {
+  std::string output;
+
+  // Fast path for when we don't need to do any unescaping.
+  // This case includes empty input, which allows us to return 0 from the
+  // lambda below to signal the error case.
+  size_t in =
+      unescape_plus_to_space ? input.find_first_of("%+") : input.find('%');
+  if (in == input.npos) {
+    return std::string(input);
+  }
+
+  StringResizeAndOverwrite(output, input.size(), [&](char* buf, size_t) {
+    char* out = buf;
+
+    // Copy the initial portion that did not need unescaping.
+    out = std::copy_n(input.data(), in, out);
+
+    // Handle the rest of the string.
+    while (in < input.size()) {
+      char c = input[in++];
+      if (unescape_plus_to_space && c == '+') {
+        *out++ = ' ';
+      } else if (c == '%') {
+        if (in + 1 >= input.size() ||
+            !absl::ascii_isxdigit(static_cast<unsigned char>(input[in])) ||
+            !absl::ascii_isxdigit(static_cast<unsigned char>(input[in + 1]))) {
+          return size_t{0};  // Error.
+        }
+        int x = static_cast<int>(hex_digit_to_int(input[in++])) << 4;
+        x += static_cast<int>(hex_digit_to_int(input[in++]));
+        *out++ = static_cast<char>(x);
+      } else {
+        *out++ = c;
+      }
+    }
+    return static_cast<size_t>(std::distance(buf, out));
+  });
+
+  if (output.empty()) {
+    // Empty output is only valid if the input was empty, and that case is
+    // handled above.
+    return std::nullopt;
+  }
+
+  return output;
+}
+
+std::string UrlEscape(absl::string_view input) {
+  constexpr bool kEscapeSpaceToPlus = false;
+  return UrlEscapeInternal(input, kEscapeSpaceToPlus);
+}
+
+std::optional<std::string> UrlUnescape(absl::string_view input) {
+  constexpr bool kUnescapePlusToSpace = false;
+  return UrlUnescapeInternal(input, kUnescapePlusToSpace);
+}
+
+std::string UrlEscapePlus(absl::string_view input) {
+  constexpr bool kEscapeSpaceToPlus = true;
+  return UrlEscapeInternal(input, kEscapeSpaceToPlus);
+}
+
+std::optional<std::string> UrlUnescapePlus(absl::string_view input) {
+  constexpr bool kUnescapePlusToSpace = true;
+  return UrlUnescapeInternal(input, kUnescapePlusToSpace);
+}
+
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/escaping.h b/absl/strings/escaping.h
index 198b934875e..0b0f8b279ec 100644
--- a/absl/strings/escaping.h
+++ b/absl/strings/escaping.h
@@ -23,16 +23,21 @@
 #ifndef ABSL_STRINGS_ESCAPING_H_
 #define ABSL_STRINGS_ESCAPING_H_
 
+#include <cassert>
 #include <cstddef>
+#include <optional>
 #include <string>
 #include <vector>
 
+#include "absl/base/attributes.h"
 #include "absl/base/macros.h"
+#include "absl/base/nullability.h"
 #include "absl/strings/ascii.h"
 #include "absl/strings/str_join.h"
 #include "absl/strings/string_view.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 // CUnescape()
 //
@@ -64,20 +69,22 @@ namespace absl {
 //
 //   std::string s = "foo\\rbar\\nbaz\\t";
 //   std::string unescaped_s;
-//   if (!absl::CUnescape(s, &unescaped_s) {
+//   if (!absl::CUnescape(s, &unescaped_s)) {
 //     ...
 //   }
 //   EXPECT_EQ(unescaped_s, "foo\rbar\nbaz\t");
-bool CUnescape(absl::string_view source, std::string* dest, std::string* error);
+bool CUnescape(absl::string_view source, std::string* absl_nonnull dest,
+               std::string* absl_nullable error);
 
 // Overload of `CUnescape()` with no error reporting.
-inline bool CUnescape(absl::string_view source, std::string* dest) {
+inline bool CUnescape(absl::string_view source,
+                      std::string* absl_nonnull dest) {
   return CUnescape(source, dest, nullptr);
 }
 
 // CEscape()
 //
-// Escapes a 'src' string using C-style escapes sequences
+// Escapes a `src` string using C-style escapes sequences
 // (https://en.cppreference.com/w/cpp/language/escape), escaping other
 // non-printable/non-whitespace bytes as octal sequences (e.g. "\377").
 //
@@ -90,7 +97,7 @@ std::string CEscape(absl::string_view src);
 
 // CHexEscape()
 //
-// Escapes a 'src' string using C-style escape sequences, escaping
+// Escapes a `src` string using C-style escape sequences, escaping
 // other non-printable/non-whitespace bytes as hexadecimal sequences (e.g.
 // "\xFF").
 //
@@ -103,7 +110,7 @@ std::string CHexEscape(absl::string_view src);
 
 // Utf8SafeCEscape()
 //
-// Escapes a 'src' string using C-style escape sequences, escaping bytes as
+// Escapes a `src` string using C-style escape sequences, escaping bytes as
 // octal sequences, and passing through UTF-8 characters without conversion.
 // I.e., when encountering any bytes with their high bit set, this function
 // will not escape those values, whether or not they are valid UTF-8.
@@ -111,44 +118,72 @@ std::string Utf8SafeCEscape(absl::string_view src);
 
 // Utf8SafeCHexEscape()
 //
-// Escapes a 'src' string using C-style escape sequences, escaping bytes as
+// Escapes a `src` string using C-style escape sequences, escaping bytes as
 // hexadecimal sequences, and passing through UTF-8 characters without
 // conversion.
 std::string Utf8SafeCHexEscape(absl::string_view src);
 
-// Base64Unescape()
-//
-// Converts a `src` string encoded in Base64 to its binary equivalent, writing
-// it to a `dest` buffer, returning `true` on success. If `src` contains invalid
-// characters, `dest` is cleared and returns `false`.
-bool Base64Unescape(absl::string_view src, std::string* dest);
-
-// WebSafeBase64Unescape()
-//
-// Converts a `src` string encoded in Base64 to its binary equivalent, writing
-// it to a `dest` buffer, but using '-' instead of '+', and '_' instead of '/'.
-// If `src` contains invalid characters, `dest` is cleared and returns `false`.
-bool WebSafeBase64Unescape(absl::string_view src, std::string* dest);
-
 // Base64Escape()
 //
-// Encodes a `src` string into a base64-encoded string, with padding characters.
-// This function conforms with RFC 4648 section 4 (base64).
-void Base64Escape(absl::string_view src, std::string* dest);
+// Encodes a `src` string into a base64-encoded `dest` string with padding
+// characters. This function conforms with RFC 4648 section 4 (base64) and RFC
+// 2045.
 std::string Base64Escape(absl::string_view src);
+[[deprecated(
+    "Use the string-returning version of "
+    "Base64Escape()")]] ABSL_REFACTOR_INLINE inline void
+Base64Escape(absl::string_view src, std::string* absl_nonnull dest) {
+  *dest = Base64Escape(src);
+}
 
 // WebSafeBase64Escape()
 //
-// Encodes a `src` string into a base64-like string, using '-' instead of '+'
-// and '_' instead of '/', and without padding. This function conforms with RFC
-// 4648 section 5 (base64url).
-void WebSafeBase64Escape(absl::string_view src, std::string* dest);
+// Encodes a `src` string into a base64 string, like Base64Escape() does, but
+// outputs '-' instead of '+' and '_' instead of '/', and does not pad `dest`.
+// This function conforms with RFC 4648 section 5 (base64url).
 std::string WebSafeBase64Escape(absl::string_view src);
+[[deprecated(
+    "Use the string-returning version of "
+    "WebSafeBase64Escape()")]] ABSL_REFACTOR_INLINE inline void
+WebSafeBase64Escape(absl::string_view src, std::string* absl_nonnull dest) {
+  *dest = WebSafeBase64Escape(src);
+}
+
+// Base64Unescape()
+//
+// Converts a `src` string encoded in Base64 (RFC 4648 section 4) to its binary
+// equivalent, writing it to a `dest` buffer, returning `true` on success. If
+// `src` contains invalid characters, `dest` is cleared and returns `false`.
+// If padding is included (note that `Base64Escape()` does produce it), it must
+// be correct. In the padding, '=' and '.' are treated identically.
+bool Base64Unescape(absl::string_view src, std::string* absl_nonnull dest);
+
+// WebSafeBase64Unescape()
+//
+// Converts a `src` string encoded in "web safe" Base64 (RFC 4648 section 5) to
+// its binary equivalent, writing it to a `dest` buffer, returning `true` on
+// success. If `src` contains invalid characters, `dest` is cleared and returns
+// `false`. If padding is included (note that `WebSafeBase64Escape()` does not
+// produce it), it must be correct. In the padding, '=' and '.' are treated
+// identically.
+bool WebSafeBase64Unescape(absl::string_view src,
+                           std::string* absl_nonnull dest);
+
+// HexStringToBytes()
+//
+// Converts the hexadecimal encoded data in `hex` into raw bytes in the `bytes`
+// output string.  If `hex` does not consist of valid hexadecimal data, this
+// function returns false and leaves `bytes` in an unspecified state. Returns
+// true on success.
+[[nodiscard]] bool HexStringToBytes(absl::string_view hex,
+                                    std::string* absl_nonnull bytes);
 
 // HexStringToBytes()
 //
 // Converts an ASCII hex string into bytes, returning binary data of length
-// `from.size()/2`.
+// `from.size()/2`. The input must be valid hexadecimal data, otherwise the
+// return value is unspecified.
+ABSL_DEPRECATED("Use the HexStringToBytes() that returns a bool")
 std::string HexStringToBytes(absl::string_view from);
 
 // BytesToHexString()
@@ -157,6 +192,75 @@ std::string HexStringToBytes(absl::string_view from);
 // `2*from.size()`.
 std::string BytesToHexString(absl::string_view from);
 
+// UrlEscape()
+//
+// Escapes a string so it can be safely used as a value in a URL component by
+// replacing all characters that are not "unreserved characters" with
+// percent-escapes. See https://tools.ietf.org/html/rfc3986
+//
+// Usage note: URLs use "reserved characters" (like ?, &, =, /) as structural
+// syntax. This function escapes these syntax characters. The correct use of
+// this function is to clean individual URL components *before* assembling them
+// into the final URL structure. Do not run it on a fully constructed URL, as
+// this will turn structural delimiters into URL component data.
+//
+// Example (encoding "gift for mom & dad" as a URL query parameter):
+//
+//   std::string url = absl::StrFormat("https://www.google.com/search?q=%s",
+//                                     absl::UrlEscape("gift for mom & dad"));
+//   assert(url ==
+//     "https://www.google.com/search?q=gift%20for%20mom%20%26%20dad");
+[[nodiscard]] std::string UrlEscape(absl::string_view input);
+
+// UrlUnescape()
+//
+// Performs the inverse transformation of UrlEscape(), converting each
+// percent-encoded sequence of the form "%AB" into the character with the
+// hexadecimal value 0xAB. It returns `std::nullopt` if any % is not followed by
+// two hexadecimal digits.
+//
+// UrlUnescape() is identical to UrlUnescapePlus() except that it does not
+// unescape '+' to ' '.
+[[nodiscard]] std::optional<std::string> UrlUnescape(absl::string_view input);
+
+// UrlEscapePlus()
+//
+// Escapes a string so it can be safely used as a value for
+// application/x-www-form-urlencoded (HTML form submissions).
+//
+// Historically web browsers have also used this form of escaping for query
+// parameters.
+//
+// UrlEscapePlus() differs from UrlEscape() in that space (' ') is encoded to
+// plus ("+") instead of "%20". According to the URI specification (RFC 3986),
+// the correct way to escape a space anywhere in a URL (including the query
+// string) is "%20". Using "%20" in a query parameter will work universally.
+//
+// Some strict URL parsers (especially outside of web browsers/web servers)
+// follow RFC 3986 strictly and will treat a literal '+' in the query string as
+// a literal plus sign, rather than decoding it to a space.
+//
+// Recommendation: Use UrlEscapePlus() only if you are specifically implementing
+// or interacting with a system that strictly expects
+// "application/x-www-form-urlencoded" formatting. For general URL construction,
+// UrlEscape() is the correct and safest choice.
+//
+// Example (encoding "gift for mom & dad" as a URL query parameter):
+//
+//   std::string url = absl::StrFormat("https://www.google.com/search?q=%s",
+//                                     absl::UrlEscapePlus(
+//                                         "gift for mom & dad"));
+//   assert(url == "https://www.google.com/search?q=gift+for+mom+%26+dad");
+[[nodiscard]] std::string UrlEscapePlus(absl::string_view input);
+
+// UrlUnescapePlus()
+//
+// Performs the inverse transformation of UrlEscapePlus(). It returns
+// `std::nullopt` if any % is not followed by two hexadecimal digits.
+[[nodiscard]] std::optional<std::string> UrlUnescapePlus(
+    absl::string_view input);
+
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_ESCAPING_H_
diff --git a/absl/strings/escaping_benchmark.cc b/absl/strings/escaping_benchmark.cc
index 10d5b033c52..1fd5b415011 100644
--- a/absl/strings/escaping_benchmark.cc
+++ b/absl/strings/escaping_benchmark.cc
@@ -12,15 +12,18 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-#include "absl/strings/escaping.h"
-
-#include <cstdio>
-#include <cstring>
+#include <cstdint>
+#include <memory>
 #include <random>
+#include <string>
 
-#include "benchmark/benchmark.h"
 #include "absl/base/internal/raw_logging.h"
+#include "absl/strings/ascii.h"
+#include "absl/strings/escaping.h"
 #include "absl/strings/internal/escaping_test_common.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/string_view.h"
+#include "benchmark/benchmark.h"
 
 namespace {
 
@@ -29,9 +32,12 @@ void BM_CUnescapeHexString(benchmark::State& state) {
   for (int i = 0; i < 50; i++) {
     src += "\\x55";
   }
-  std::string dest;
   for (auto _ : state) {
-    absl::CUnescape(src, &dest);
+    std::string dest;
+    benchmark::DoNotOptimize(src);
+    bool result = absl::CUnescape(src, &dest);
+    benchmark::DoNotOptimize(result);
+    benchmark::DoNotOptimize(dest);
   }
 }
 BENCHMARK(BM_CUnescapeHexString);
@@ -43,21 +49,78 @@ void BM_WebSafeBase64Escape_string(benchmark::State& state) {
       raw += std::string(test_set.plaintext);
     }
   }
+  for (auto _ : state) {
+    std::string escaped = absl::WebSafeBase64Escape(raw);
+    benchmark::DoNotOptimize(escaped);
+  }
+}
+BENCHMARK(BM_WebSafeBase64Escape_string);
 
-  // The actual benchmark loop is tiny...
-  std::string escaped;
+void BM_HexStringToBytes(benchmark::State& state) {
+  const int size = state.range(0);
+  std::string input, output;
+  for (int i = 0; i < size; ++i) input += "1c";
   for (auto _ : state) {
-    absl::WebSafeBase64Escape(raw, &escaped);
+    benchmark::DoNotOptimize(input);
+    bool result = absl::HexStringToBytes(input, &output);
+    benchmark::DoNotOptimize(result);
+    benchmark::DoNotOptimize(output);
   }
+}
+BENCHMARK(BM_HexStringToBytes)->Range(1, 1 << 8);
 
-  // We want to be sure the compiler doesn't throw away the loop above,
-  // and the easiest way to ensure that is to round-trip the results and verify
-  // them.
-  std::string round_trip;
-  absl::WebSafeBase64Unescape(escaped, &round_trip);
-  ABSL_RAW_CHECK(round_trip == raw, "");
+void BM_HexStringToBytes_Fail(benchmark::State& state) {
+  std::string binary;
+  absl::string_view hex_input1 = "1c2f003";
+  absl::string_view hex_input2 = "1c2f0032f40123456789abcdef**";
+  for (auto _ : state) {
+    benchmark::DoNotOptimize(hex_input1);
+    bool result1 = absl::HexStringToBytes(hex_input1, &binary);
+    benchmark::DoNotOptimize(result1);
+    benchmark::DoNotOptimize(binary);
+    benchmark::DoNotOptimize(hex_input2);
+    bool result2 = absl::HexStringToBytes(hex_input2, &binary);
+    benchmark::DoNotOptimize(result2);
+    benchmark::DoNotOptimize(binary);
+  }
 }
-BENCHMARK(BM_WebSafeBase64Escape_string);
+BENCHMARK(BM_HexStringToBytes_Fail);
+
+static void BM_UrlEscape(benchmark::State& state) {
+  std::string all;
+  std::string alnum;
+  all.reserve(256);
+  for (int c = 0; c < 256; ++c) {
+    all.push_back(c);
+    if (absl::ascii_isalnum(c)) {
+      alnum.push_back(c);
+    }
+  }
+
+  for (auto _ : state) {
+    benchmark::DoNotOptimize(absl::UrlEscape(all));
+    benchmark::DoNotOptimize(absl::UrlEscape(alnum));
+  }
+}
+BENCHMARK(BM_UrlEscape);
+
+static void BM_UrlEscapePlus(benchmark::State& state) {
+  std::string all;
+  std::string alnum;
+  all.reserve(256);
+  for (int c = 0; c < 256; ++c) {
+    all.push_back(c);
+    if (absl::ascii_isalnum(c)) {
+      alnum.push_back(c);
+    }
+  }
+
+  for (auto _ : state) {
+    benchmark::DoNotOptimize(absl::UrlEscapePlus(all));
+    benchmark::DoNotOptimize(absl::UrlEscapePlus(alnum));
+  }
+}
+BENCHMARK(BM_UrlEscapePlus);
 
 // Used for the CEscape benchmarks
 const char kStringValueNoEscape[] = "1234567890";
@@ -72,7 +135,9 @@ void CEscapeBenchmarkHelper(benchmark::State& state, const char* string_value,
   }
 
   for (auto _ : state) {
-    absl::CEscape(src);
+    benchmark::DoNotOptimize(src);
+    std::string result = absl::CEscape(src);
+    benchmark::DoNotOptimize(result);
   }
 }
 
diff --git a/absl/strings/escaping_test.cc b/absl/strings/escaping_test.cc
index 1967975b69a..e4a3868f7e6 100644
--- a/absl/strings/escaping_test.cc
+++ b/absl/strings/escaping_test.cc
@@ -15,20 +15,30 @@
 #include "absl/strings/escaping.h"
 
 #include <array>
+#include <cstddef>
 #include <cstdio>
 #include <cstring>
+#include <initializer_list>
+#include <iterator>
 #include <memory>
+#include <optional>
+#include <string>
 #include <vector>
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
-#include "absl/container/fixed_array.h"
-#include "absl/strings/str_cat.h"
-
+#include "absl/log/check.h"
+#include "absl/strings/charset.h"
 #include "absl/strings/internal/escaping_test_common.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/str_format.h"
+#include "absl/strings/string_view.h"
 
 namespace {
 
+using ::testing::Eq;
+using ::testing::Optional;
+
 struct epair {
   std::string escaped;
   std::string unescaped;
@@ -166,15 +176,26 @@ TEST(Unescape, BasicFunction) {
     EXPECT_TRUE(absl::CUnescape(val.escaped, &out));
     EXPECT_EQ(out, val.unescaped);
   }
-  std::string bad[] = {"\\u1",         // too short
-                       "\\U1",         // too short
-                       "\\Uffffff",    // exceeds 0x10ffff (largest Unicode)
-                       "\\U00110000",  // exceeds 0x10ffff (largest Unicode)
-                       "\\uD835",      // surrogate character (D800-DFFF)
-                       "\\U0000DD04",  // surrogate character (D800-DFFF)
-                       "\\777",        // exceeds 0xff
-                       "\\xABCD"};     // exceeds 0xff
-  for (const std::string& e : bad) {
+  constexpr absl::string_view bad[] = {
+      "\\u1",          // too short
+      "\\U1",          // too short
+      "\\Uffffff",     // exceeds 0x10ffff (largest Unicode)
+      "\\U00110000",   // exceeds 0x10ffff (largest Unicode)
+      "\\uD835",       // surrogate character (D800-DFFF)
+      "\\U0000DD04",   // surrogate character (D800-DFFF)
+      "\\777",         // exceeds 0xff
+      "\\xABCD",       // exceeds 0xff
+      "\\x100000041",  // overflows uint32_t
+      "endswith\\",    // ends with "\"
+      "endswith\\x",   // ends with "\x"
+      "endswith\\X",   // ends with "\X"
+      "\\x.2345678",   // non-hex follows "\x"
+      "\\X.2345678",   // non-hex follows "\X"
+      "\\u.2345678",   // non-hex follows "\U"
+      "\\U.2345678",   // non-hex follows "\U"
+      "\\.unknown",    // unknown escape sequence
+  };
+  for (const auto e : bad) {
     std::string error;
     std::string out;
     EXPECT_FALSE(absl::CUnescape(e, &out, &error));
@@ -300,7 +321,7 @@ static struct {
   absl::string_view plaintext;
   absl::string_view cyphertext;
 } const base64_tests[] = {
-    // Empty std::string.
+    // Empty string.
     {{"", 0}, {"", 0}},
     {{nullptr, 0},
      {"", 0}},  // if length is zero, plaintext ptr must be ignored!
@@ -562,6 +583,7 @@ template <typename StringType>
 void TestEscapeAndUnescape() {
   // Check the short strings; this tests the math (and boundaries)
   for (const auto& tc : base64_tests) {
+    // Test plain base64.
     StringType encoded("this junk should be ignored");
     absl::Base64Escape(tc.plaintext, &encoded);
     EXPECT_EQ(encoded, tc.cyphertext);
@@ -571,22 +593,26 @@ void TestEscapeAndUnescape() {
     EXPECT_TRUE(absl::Base64Unescape(encoded, &decoded));
     EXPECT_EQ(decoded, tc.plaintext);
 
-    StringType websafe(tc.cyphertext);
-    for (int c = 0; c < websafe.size(); ++c) {
-      if ('+' == websafe[c]) websafe[c] = '-';
-      if ('/' == websafe[c]) websafe[c] = '_';
+    StringType websafe_with_padding(tc.cyphertext);
+    for (unsigned int c = 0; c < websafe_with_padding.size(); ++c) {
+      if ('+' == websafe_with_padding[c]) websafe_with_padding[c] = '-';
+      if ('/' == websafe_with_padding[c]) websafe_with_padding[c] = '_';
+      // Intentionally keeping padding aka '='.
+    }
+
+    // Test plain websafe (aka without padding).
+    StringType websafe(websafe_with_padding);
+    for (unsigned int c = 0; c < websafe.size(); ++c) {
       if ('=' == websafe[c]) {
         websafe.resize(c);
         break;
       }
     }
-
     encoded = "this junk should be ignored";
     absl::WebSafeBase64Escape(tc.plaintext, &encoded);
     EXPECT_EQ(encoded, websafe);
     EXPECT_EQ(absl::WebSafeBase64Escape(tc.plaintext), websafe);
 
-    // Let's try the std::string version of the decoder
     decoded = "this junk should be ignored";
     EXPECT_TRUE(absl::WebSafeBase64Unescape(websafe, &decoded));
     EXPECT_EQ(decoded, tc.plaintext);
@@ -617,15 +643,56 @@ TEST(Base64, EscapeAndUnescape) {
   TestEscapeAndUnescape<std::string>();
 }
 
+TEST(Base64, Padding) {
+  // Padding is optional.
+  // '.' is an acceptable padding character, just like '='.
+  std::initializer_list<absl::string_view> good_padding = {
+    "YQ",
+    "YQ==",
+    "YQ=.",
+    "YQ.=",
+    "YQ..",
+  };
+  for (absl::string_view b64 : good_padding) {
+    std::string decoded;
+    EXPECT_TRUE(absl::Base64Unescape(b64, &decoded));
+    EXPECT_EQ(decoded, "a");
+    std::string websafe_decoded;
+    EXPECT_TRUE(absl::WebSafeBase64Unescape(b64, &websafe_decoded));
+    EXPECT_EQ(websafe_decoded, "a");
+  }
+  std::initializer_list<absl::string_view> bad_padding = {
+    "YQ=",
+    "YQ.",
+    "YQ===",
+    "YQ==.",
+    "YQ=.=",
+    "YQ=..",
+    "YQ.==",
+    "YQ.=.",
+    "YQ..=",
+    "YQ...",
+    "YQ====",
+    "YQ....",
+    "YQ=====",
+    "YQ.....",
+  };
+  for (absl::string_view b64 : bad_padding) {
+    std::string decoded;
+    EXPECT_FALSE(absl::Base64Unescape(b64, &decoded));
+    std::string websafe_decoded;
+    EXPECT_FALSE(absl::WebSafeBase64Unescape(b64, &websafe_decoded));
+  }
+}
+
 TEST(Base64, DISABLED_HugeData) {
   const size_t kSize = size_t(3) * 1000 * 1000 * 1000;
   static_assert(kSize % 3 == 0, "kSize must be divisible by 3");
   const std::string huge(kSize, 'x');
 
-  std::string escaped;
-  absl::Base64Escape(huge, &escaped);
+  std::string escaped = absl::Base64Escape(huge);
 
-  // Generates the std::string that should match a base64 encoded "xxx..." std::string.
+  // Generates the string that should match a base64 encoded "xxx..." string.
   // "xxx" in base64 is "eHh4".
   std::string expected_encoding;
   expected_encoding.reserve(kSize / 3 * 4);
@@ -639,6 +706,46 @@ TEST(Base64, DISABLED_HugeData) {
   EXPECT_EQ(huge, unescaped);
 }
 
+TEST(Escaping, HexStringToBytesBackToHex) {
+  std::string bytes, hex;
+
+  constexpr absl::string_view kTestHexLower =  "1c2f0032f40123456789abcdef";
+  constexpr absl::string_view kTestHexUpper =  "1C2F0032F40123456789ABCDEF";
+  constexpr absl::string_view kTestBytes = absl::string_view(
+      "\x1c\x2f\x00\x32\xf4\x01\x23\x45\x67\x89\xab\xcd\xef", 13);
+
+  EXPECT_TRUE(absl::HexStringToBytes(kTestHexLower, &bytes));
+  EXPECT_EQ(bytes, kTestBytes);
+
+  EXPECT_TRUE(absl::HexStringToBytes(kTestHexUpper, &bytes));
+  EXPECT_EQ(bytes, kTestBytes);
+
+  hex = absl::BytesToHexString(kTestBytes);
+  EXPECT_EQ(hex, kTestHexLower);
+
+  // Same buffer.
+  // We do not care if this works since we do not promise it in the contract.
+  // The purpose of this test is to to see if the program will crash or if
+  // sanitizers will catch anything.
+  bytes = std::string(kTestHexUpper);
+  (void)absl::HexStringToBytes(bytes, &bytes);
+
+  // Length not a multiple of two.
+  EXPECT_FALSE(absl::HexStringToBytes("1c2f003", &bytes));
+
+  // Not hex.
+  EXPECT_FALSE(absl::HexStringToBytes("1c2f00ft", &bytes));
+
+  // Empty input.
+  bytes = "abc";
+  EXPECT_TRUE(absl::HexStringToBytes("", &bytes));
+  EXPECT_EQ("", bytes);  // Results in empty output.
+
+  // Ensure there is no sign extension bug on a signed char.
+  hex.assign("\xC8" "b", 2);
+  EXPECT_FALSE(absl::HexStringToBytes(hex, &bytes));
+}
+
 TEST(HexAndBack, HexStringToBytes_and_BytesToHexString) {
   std::string hex_mixed = "0123456789abcdefABCDEF";
   std::string bytes_expected = "\x01\x23\x45\x67\x89\xab\xcd\xef\xAB\xCD\xEF";
@@ -661,4 +768,151 @@ TEST(HexAndBack, HexStringToBytes_and_BytesToHexString) {
   EXPECT_EQ(hex_only_lower, hex_result);
 }
 
+TEST(UrlEscape, Basics) {
+  EXPECT_EQ(absl::UrlEscape(""), "");
+  EXPECT_THAT(absl::UrlUnescape(""), Optional(Eq("")));
+
+  EXPECT_EQ(absl::UrlEscape("abc"), "abc");
+  EXPECT_THAT(absl::UrlUnescape("abc"), Optional(Eq("abc")));
+
+  EXPECT_EQ(absl::UrlEscape("a/b"), "a%2Fb");
+  EXPECT_THAT(absl::UrlUnescape("a%2Fb"), Optional(Eq("a/b")));
+
+  EXPECT_EQ(absl::UrlEscape("one two"), "one%20two");
+  EXPECT_THAT(absl::UrlUnescape("one%20two"), Optional(Eq("one two")));
+
+  EXPECT_EQ(absl::UrlEscape("10%"), "10%25");
+  EXPECT_THAT(absl::UrlUnescape("10%25"), Optional(Eq("10%")));
+
+  EXPECT_EQ(absl::UrlEscape(" ?&=#+%!<>#\"{}|\\^[]`☺\t:/@$'()*,;"),
+            "%20%3F%26%3D%23%2B%25%21%3C%3E%23%22%7B%7D%7C%5C%5E%5B%5D%60%E2%"
+            "98%BA%09%3A%2F%40%24%27%28%29%2A%2C%3B");
+  EXPECT_THAT(absl::UrlUnescape("%20%3F%26%3D%23%2B%25%21%3C%3E%23%22%7B%7D%7C%"
+                                "5C%5E%5B%5D%60%E2%98%BA%"
+                                "09%3A%2F%40%24%27%28%29%2A%2C%3B"),
+              Optional(Eq(" ?&=#+%!<>#\"{}|\\^[]`☺\t:/@$'()*,;")));
+
+  // Test all characters.
+  static constexpr absl::CharSet kDoNotEscape =
+      absl::CharSet::AsciiAlphanumerics() | absl::CharSet("-._~");
+  for (int i = 0; i < 256; ++i) {
+    char c = static_cast<char>(i);
+    std::string expected = kDoNotEscape.contains(c)
+                               ? std::string(1, c)
+                               : absl::StrFormat("%%%02X", c);
+    EXPECT_EQ(absl::UrlEscape(absl::string_view(&c, 1)), expected);
+    EXPECT_EQ(absl::UrlUnescape(expected), absl::string_view(&c, 1));
+  }
+}
+
+TEST(UrlUnescape, SuccessCases) {
+  EXPECT_THAT(absl::UrlUnescape(""), Optional(Eq("")));
+  EXPECT_THAT(absl::UrlUnescape("abc"), Optional(Eq("abc")));
+  EXPECT_THAT(absl::UrlUnescape("1%41"), Optional(Eq("1A")));
+  EXPECT_THAT(absl::UrlUnescape("1%41%42%43"), Optional(Eq("1ABC")));
+  EXPECT_THAT(absl::UrlUnescape("%4a"), Optional(Eq("J")));
+  EXPECT_THAT(absl::UrlUnescape("%6F"), Optional(Eq("o")));
+  EXPECT_THAT(absl::UrlUnescape("a%20b"), Optional(Eq("a b")));
+  EXPECT_THAT(absl::UrlUnescape("a+b"), Optional(Eq("a+b")));
+}
+
+TEST(UrlUnescape, NotEnoughCharsAfterPercent) {
+  EXPECT_EQ(absl::UrlUnescape("%"), std::nullopt);
+  EXPECT_EQ(absl::UrlUnescape("%a"), std::nullopt);
+  EXPECT_EQ(absl::UrlUnescape("%1"), std::nullopt);
+  EXPECT_EQ(absl::UrlUnescape("123%45%6"), std::nullopt);
+}
+
+TEST(UrlUnescape, InvalidHexDigits) {
+  EXPECT_EQ(absl::UrlUnescape("%zzzzz"), std::nullopt);
+}
+
+TEST(UrlUnescape, NoErrorWithNoEscapeSequence) {
+  // Any string that does not contain '%' should not produce an error, even if
+  // absl::UrlEscape() would never produce a string with certain characters.
+  std::string no_percent;
+  for (int c = 0; c < 256; ++c) {
+    if (c != '%') {
+      no_percent.push_back(static_cast<char>(c));
+    }
+  }
+  EXPECT_THAT(absl::UrlUnescape(no_percent), Optional(no_percent));
+}
+
+TEST(UrlEscapePlus, Basics) {
+  EXPECT_EQ(absl::UrlEscapePlus(""), "");
+  EXPECT_THAT(absl::UrlUnescapePlus(""), Optional(Eq("")));
+
+  EXPECT_EQ(absl::UrlEscapePlus("abc"), "abc");
+  EXPECT_THAT(absl::UrlUnescapePlus("abc"), Optional(Eq("abc")));
+
+  EXPECT_EQ(absl::UrlEscapePlus("one two"), "one+two");
+  EXPECT_THAT(absl::UrlUnescapePlus("one+two"), Optional(Eq("one two")));
+
+  EXPECT_EQ(absl::UrlEscapePlus("gift for mom & dad"), "gift+for+mom+%26+dad");
+  EXPECT_THAT(absl::UrlUnescapePlus("gift+for+mom+%26+dad"),
+              Optional(Eq("gift for mom & dad")));
+
+  EXPECT_EQ(absl::UrlEscapePlus("10%"), "10%25");
+  EXPECT_THAT(absl::UrlUnescapePlus("10%25"), Optional(Eq("10%")));
+
+  EXPECT_EQ(absl::UrlEscapePlus(" ?&=#+%!<>#\"{}|\\^[]`☺\t:/@$'()*,;"),
+            "+%3F%26%3D%23%2B%25%21%3C%3E%23%22%7B%7D%7C%5C%5E%5B%5D%60%E2%"
+            "98%BA%09%3A%2F%40%24%27%28%29%2A%2C%3B");
+  EXPECT_THAT(absl::UrlUnescapePlus("+%3F%26%3D%23%2B%25%21%3C%3E%23%22%7B%7D%"
+                                    "7C%5C%5E%5B%5D%60%E2%98%BA%"
+                                    "09%3A%2F%40%24%27%28%29%2A%2C%3B"),
+              Optional(Eq(" ?&=#+%!<>#\"{}|\\^[]`☺\t:/@$'()*,;")));
+
+  // Test all characters.
+  static constexpr absl::CharSet kDoNotEscape =
+      absl::CharSet::AsciiAlphanumerics() | absl::CharSet("-._~");
+  for (int i = 0; i < 256; ++i) {
+    char c = static_cast<char>(i);
+    std::string expected = kDoNotEscape.contains(c)
+                               ? std::string(1, c)
+                               : absl::StrFormat("%%%02X", c);
+    if (c == ' ') expected = '+';
+    EXPECT_EQ(absl::UrlEscapePlus(absl::string_view(&c, 1)), expected);
+    EXPECT_EQ(absl::UrlUnescapePlus(expected), absl::string_view(&c, 1));
+  }
+}
+
+TEST(UrlUnescapePlus, SuccessCases) {
+  EXPECT_THAT(absl::UrlUnescapePlus(""), Optional(Eq("")));
+  EXPECT_THAT(absl::UrlUnescapePlus("abc"), Optional(Eq("abc")));
+  EXPECT_THAT(absl::UrlUnescapePlus("1%41"), Optional(Eq("1A")));
+  EXPECT_THAT(absl::UrlUnescapePlus("1%41%42%43"), Optional(Eq("1ABC")));
+  EXPECT_THAT(absl::UrlUnescapePlus("%4a"), Optional(Eq("J")));
+  EXPECT_THAT(absl::UrlUnescapePlus("%6F"), Optional(Eq("o")));
+  EXPECT_THAT(absl::UrlUnescapePlus("a%20b"), Optional(Eq("a b")));
+  EXPECT_THAT(absl::UrlUnescapePlus("a+b"), Optional(Eq("a b")));
+}
+
+TEST(UrlUnescapePlus, NotEnoughCharsAfterPercent) {
+  EXPECT_EQ(absl::UrlUnescapePlus("%"), std::nullopt);
+  EXPECT_EQ(absl::UrlUnescapePlus("%a"), std::nullopt);
+  EXPECT_EQ(absl::UrlUnescapePlus("%1"), std::nullopt);
+  EXPECT_EQ(absl::UrlUnescapePlus("123%45%6"), std::nullopt);
+}
+
+TEST(UrlUnescapePlus, InvalidHexDigits) {
+  EXPECT_EQ(absl::UrlUnescapePlus("%zzzzz"), std::nullopt);
+}
+
+TEST(UrlUnescapePlus, NoErrorWithNoEscapeSequence) {
+  // Any string that does not contain '%' should not produce an error, even if
+  // absl::UrlEscapePlus() would never produce a string with certain
+  // characters.
+  std::string no_percent;
+  std::string no_percent_expected;
+  for (int c = 0; c < 256; ++c) {
+    if (c != '%') {
+      no_percent.push_back(static_cast<char>(c));
+      no_percent_expected.push_back(c != '+' ? static_cast<char>(c) : ' ');
+    }
+  }
+  EXPECT_THAT(absl::UrlUnescapePlus(no_percent), Optional(no_percent_expected));
+}
+
 }  // namespace
diff --git a/absl/strings/has_absl_stringify.h b/absl/strings/has_absl_stringify.h
new file mode 100644
index 00000000000..5bc87914f12
--- /dev/null
+++ b/absl/strings/has_absl_stringify.h
@@ -0,0 +1,65 @@
+// Copyright 2022 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_STRINGS_HAS_ABSL_STRINGIFY_H_
+#define ABSL_STRINGS_HAS_ABSL_STRINGIFY_H_
+
+#include <cstddef>
+#include <type_traits>
+#include <utility>
+
+#include "absl/base/config.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+namespace strings_internal {
+
+// This is an empty class not intended to be used. It exists so that
+// `HasAbslStringify` can reference a universal class rather than needing to be
+// copied for each new sink.
+class UnimplementedSink {
+ public:
+  void Append(size_t count, char ch);
+
+  void Append(string_view v);
+
+  // Support `absl::Format(&sink, format, args...)`.
+  friend void AbslFormatFlush(UnimplementedSink* sink, absl::string_view v);
+};
+
+}  // namespace strings_internal
+
+// `HasAbslStringify<T>` detects if type `T` supports the `AbslStringify()`
+// customization point (see
+// https://abseil.io/docs/cpp/guides/format#abslstringify for the
+// documentation).
+//
+// Note that there are types that can be `StrCat`-ed that do not use the
+// `AbslStringify` customization point (for example, `int`).
+
+template <typename T, typename = void>
+struct HasAbslStringify : std::false_type {};
+
+template <typename T>
+struct HasAbslStringify<
+    T, std::enable_if_t<std::is_void_v<decltype(AbslStringify(
+           std::declval<strings_internal::UnimplementedSink&>(),
+           std::declval<const T&>()))>>> : std::true_type {};
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STRINGS_HAS_ABSL_STRINGIFY_H_
diff --git a/absl/strings/has_absl_stringify_test.cc b/absl/strings/has_absl_stringify_test.cc
new file mode 100644
index 00000000000..1aee90410cf
--- /dev/null
+++ b/absl/strings/has_absl_stringify_test.cc
@@ -0,0 +1,40 @@
+// Copyright 2023 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/strings/has_absl_stringify.h"
+
+#include <optional>
+#include <string>
+
+#include "gtest/gtest.h"
+
+namespace {
+
+struct TypeWithoutAbslStringify {};
+
+struct TypeWithAbslStringify {
+  template <typename Sink>
+  friend void AbslStringify(Sink&, const TypeWithAbslStringify&) {}
+};
+
+TEST(HasAbslStringifyTest, Works) {
+  EXPECT_FALSE(absl::HasAbslStringify<int>::value);
+  EXPECT_FALSE(absl::HasAbslStringify<std::string>::value);
+  EXPECT_FALSE(absl::HasAbslStringify<TypeWithoutAbslStringify>::value);
+  EXPECT_TRUE(absl::HasAbslStringify<TypeWithAbslStringify>::value);
+  EXPECT_FALSE(
+      absl::HasAbslStringify<std::optional<TypeWithAbslStringify>>::value);
+}
+
+}  // namespace
diff --git a/absl/strings/has_ostream_operator.h b/absl/strings/has_ostream_operator.h
new file mode 100644
index 00000000000..156ffc743dd
--- /dev/null
+++ b/absl/strings/has_ostream_operator.h
@@ -0,0 +1,42 @@
+// Copyright 2023 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_STRINGS_HAS_OSTREAM_OPERATOR_H_
+#define ABSL_STRINGS_HAS_OSTREAM_OPERATOR_H_
+
+#include <ostream>
+#include <type_traits>
+#include <utility>
+
+#include "absl/base/config.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+// Detects if type `T` supports streaming to `std::ostream`s with `operator<<`.
+
+template <typename T, typename = void>
+struct HasOstreamOperator : std::false_type {};
+
+template <typename T>
+struct HasOstreamOperator<
+    T, std::enable_if_t<std::is_same<
+           std::ostream&, decltype(std::declval<std::ostream&>()
+                                   << std::declval<const T&>())>::value>>
+    : std::true_type {};
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STRINGS_HAS_OSTREAM_OPERATOR_H_
diff --git a/absl/strings/has_ostream_operator_test.cc b/absl/strings/has_ostream_operator_test.cc
new file mode 100644
index 00000000000..720ebd5e03f
--- /dev/null
+++ b/absl/strings/has_ostream_operator_test.cc
@@ -0,0 +1,41 @@
+// Copyright 2023 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/strings/has_ostream_operator.h"
+
+#include <optional>
+#include <ostream>
+#include <string>
+
+#include "gtest/gtest.h"
+
+namespace {
+
+struct TypeWithoutOstreamOp {};
+
+struct TypeWithOstreamOp {
+  friend std::ostream& operator<<(std::ostream& os, const TypeWithOstreamOp&) {
+    return os;
+  }
+};
+
+TEST(HasOstreamOperatorTest, Works) {
+  EXPECT_TRUE(absl::HasOstreamOperator<int>::value);
+  EXPECT_TRUE(absl::HasOstreamOperator<std::string>::value);
+  EXPECT_FALSE(absl::HasOstreamOperator<std::optional<int>>::value);
+  EXPECT_FALSE(absl::HasOstreamOperator<TypeWithoutOstreamOp>::value);
+  EXPECT_TRUE(absl::HasOstreamOperator<TypeWithOstreamOp>::value);
+}
+
+}  // namespace
diff --git a/absl/strings/internal/append_and_overwrite.h b/absl/strings/internal/append_and_overwrite.h
new file mode 100644
index 00000000000..833d70decab
--- /dev/null
+++ b/absl/strings/internal/append_and_overwrite.h
@@ -0,0 +1,93 @@
+// Copyright 2025 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_STRINGS_INTERNAL_APPEND_AND_OVERWRITE_H_
+#define ABSL_STRINGS_INTERNAL_APPEND_AND_OVERWRITE_H_
+
+#include "absl/base/config.h"
+#include "absl/base/internal/hardening.h"
+#include "absl/base/macros.h"
+#include "absl/base/optimization.h"
+#include "absl/base/throw_delegate.h"
+#include "absl/strings/resize_and_overwrite.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace strings_internal {
+
+// An internal-only variant similar to `absl::StringResizeAndOverwrite()`
+// optimized for repeated appends to a string that uses exponential growth so
+// that the amortized complexity of increasing the string size by a small amount
+// is O(1), in contrast to O(str.size()) in the case of precise growth. Use of
+// this function is subtle; see https://reviews.llvm.org/D102727 to understand
+// the tradeoffs.
+//
+// Appends at most `append_n` characters to `str`, using the user-provided
+// operation `append_op` to modify the possibly indeterminate
+// contents. `append_op` must return the length of the buffer appended to `str`.
+//
+// Invalidates all iterators, pointers, and references into `str`, regardless
+// of whether reallocation occurs.
+//
+// `append_op(value_type* buf, size_t buf_size)` is allowed to write
+// `value_type{}` to `buf[buf_size]`, which facilitiates interoperation with
+// functions that write a trailing NUL.
+template <typename T, typename Op>
+void StringAppendAndOverwrite(T& str, typename T::size_type append_n,
+                              Op append_op) {
+  if (ABSL_PREDICT_FALSE(append_n > str.max_size() - str.size())) {
+    ThrowStdLengthError("absl::strings_internal::StringAppendAndOverwrite");
+  }
+
+  auto old_size = str.size();
+  auto resize = old_size + append_n;
+
+  if (resize > str.capacity()) {
+    // Make sure to always grow by at least a factor of 2x.
+    const auto min_growth = str.capacity();
+    if (ABSL_PREDICT_FALSE(str.capacity() > str.max_size() - min_growth)) {
+      str.reserve(str.max_size());
+    } else if (resize < str.capacity() + min_growth) {
+      str.reserve(str.capacity() + min_growth);
+    }
+  }
+
+  // Avoid calling StringResizeAndOverwrite() here since it does an MSAN
+  // verification on the entire string. StringResizeAndOverwriteImpl() is
+  // StringResizeAndOverwrite() without the MSAN verification.
+  StringResizeAndOverwriteImpl(
+      str, resize,
+      [old_size, append_n, do_append = std::move(append_op)](
+          typename T::value_type* data_ptr, typename T::size_type) mutable {
+        typename T::size_type num_appended = static_cast<typename T::size_type>(
+            std::move(do_append)(data_ptr + old_size, append_n));
+        absl::base_internal::HardeningAssertGE(num_appended,
+                                               typename T::size_type{0});
+        absl::base_internal::HardeningAssertLE(num_appended, append_n);
+        return old_size + num_appended;
+      });
+
+#if defined(ABSL_HAVE_MEMORY_SANITIZER)
+  // Only check the region appended to. Checking the entire string would cause
+  // pathological quadratic verfication on repeated small appends.
+  __msan_check_mem_is_initialized(str.data() + old_size, str.size() - old_size);
+#endif
+}
+
+}  // namespace strings_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+
+#endif  // ABSL_STRINGS_INTERNAL_APPEND_AND_OVERWRITE_H_
diff --git a/absl/strings/internal/append_and_overwrite_test.cc b/absl/strings/internal/append_and_overwrite_test.cc
new file mode 100644
index 00000000000..aa9c7a173bc
--- /dev/null
+++ b/absl/strings/internal/append_and_overwrite_test.cc
@@ -0,0 +1,95 @@
+// Copyright 2025 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/strings/internal/append_and_overwrite.h"
+
+#include <algorithm>
+#include <cstddef>
+#include <string>
+
+#include "gtest/gtest.h"
+#include "absl/log/absl_check.h"
+
+namespace {
+
+struct AppendAndOverwriteParam {
+  size_t initial_size;
+  size_t append_capacity;
+  size_t append_size;
+};
+
+using StringAppendAndOverwriteTest =
+    ::testing::TestWithParam<AppendAndOverwriteParam>;
+
+TEST_P(StringAppendAndOverwriteTest, StringAppendAndOverwrite) {
+  const auto& param = GetParam();
+  std::string s(param.initial_size, 'a');
+  absl::strings_internal::StringAppendAndOverwrite(
+      s, param.append_capacity, [&](char* p, size_t n) {
+        ABSL_CHECK_EQ(n, param.append_capacity);
+        std::fill_n(p, param.append_size, 'b');
+        p[param.append_size] = '\0';
+        return param.append_size;
+      });
+
+  std::string expected =
+      std::string(param.initial_size, 'a') +
+      std::string(param.append_size,
+                  'b');
+
+  EXPECT_EQ(s, expected);
+  EXPECT_EQ(s.c_str()[s.size()], '\0');
+}
+
+// clang-format off
+INSTANTIATE_TEST_SUITE_P(StringAppendAndOverwriteTestSuite,
+                         StringAppendAndOverwriteTest,
+                         ::testing::ValuesIn<AppendAndOverwriteParam>({
+                             {0,  10,  5},
+                             {10, 10, 10},
+                             {10, 15, 15},
+                             {10, 20, 15},
+                             {10, 40, 40},
+                             {10, 50, 40},
+                             {30, 35, 35},
+                             {30, 45, 35},
+                             {10, 30, 15},
+                         }));
+// clang-format on
+
+TEST(StringAppendAndOverwrite, AmortizedComplexity) {
+  std::string str;
+  std::string expected;
+  size_t prev_cap = str.capacity();
+  int cap_increase_count = 0;
+  for (int i = 0; i < 1000; ++i) {
+    char c = static_cast<char>('a' + (i % 26));
+    absl::strings_internal::StringAppendAndOverwrite(
+        str, 1, [c](char* buf, size_t buf_size) {
+          ABSL_CHECK_EQ(buf_size, 1);
+          buf[0] = c;
+          return size_t{1};
+        });
+    expected.push_back(c);
+    EXPECT_EQ(str, expected);
+    size_t new_cap = str.capacity();
+    if (new_cap > prev_cap) {
+      ++cap_increase_count;
+    }
+    prev_cap = new_cap;
+  }
+  EXPECT_LT(cap_increase_count, 50);
+}
+
+}  // namespace
diff --git a/absl/strings/internal/char_map.h b/absl/strings/internal/char_map.h
deleted file mode 100644
index b9108b8cddd..00000000000
--- a/absl/strings/internal/char_map.h
+++ /dev/null
@@ -1,154 +0,0 @@
-// Copyright 2017 The Abseil Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-//
-// Character Map Class
-//
-// A fast, bit-vector map for 8-bit unsigned characters.
-// This class is useful for non-character purposes as well.
-
-#ifndef ABSL_STRINGS_INTERNAL_CHAR_MAP_H_
-#define ABSL_STRINGS_INTERNAL_CHAR_MAP_H_
-
-#include <cstddef>
-#include <cstdint>
-#include <cstring>
-
-#include "absl/base/macros.h"
-#include "absl/base/port.h"
-
-namespace absl {
-namespace strings_internal {
-
-class Charmap {
- public:
-  constexpr Charmap() : m_() {}
-
-  // Initializes with a given char*.  Note that NUL is not treated as
-  // a terminator, but rather a char to be flicked.
-  Charmap(const char* str, int len) : m_() {
-    while (len--) SetChar(*str++);
-  }
-
-  // Initializes with a given char*.  NUL is treated as a terminator
-  // and will not be in the charmap.
-  explicit Charmap(const char* str) : m_() {
-    while (*str) SetChar(*str++);
-  }
-
-  constexpr bool contains(unsigned char c) const {
-    return (m_[c / 64] >> (c % 64)) & 0x1;
-  }
-
-  // Returns true if and only if a character exists in both maps.
-  bool IntersectsWith(const Charmap& c) const {
-    for (size_t i = 0; i < ABSL_ARRAYSIZE(m_); ++i) {
-      if ((m_[i] & c.m_[i]) != 0) return true;
-    }
-    return false;
-  }
-
-  bool IsZero() const {
-    for (uint64_t c : m_) {
-      if (c != 0) return false;
-    }
-    return true;
-  }
-
-  // Containing only a single specified char.
-  static constexpr Charmap Char(char x) {
-    return Charmap(CharMaskForWord(x, 0), CharMaskForWord(x, 1),
-                   CharMaskForWord(x, 2), CharMaskForWord(x, 3));
-  }
-
-  // Containing all the chars in the C-std::string 's'.
-  // Note that this is expensively recursive because of the C++11 constexpr
-  // formulation. Use only in constexpr initializers.
-  static constexpr Charmap FromString(const char* s) {
-    return *s == 0 ? Charmap() : (Char(*s) | FromString(s + 1));
-  }
-
-  // Containing all the chars in the closed interval [lo,hi].
-  static constexpr Charmap Range(char lo, char hi) {
-    return Charmap(RangeForWord(lo, hi, 0), RangeForWord(lo, hi, 1),
-                   RangeForWord(lo, hi, 2), RangeForWord(lo, hi, 3));
-  }
-
-  friend constexpr Charmap operator&(const Charmap& a, const Charmap& b) {
-    return Charmap(a.m_[0] & b.m_[0], a.m_[1] & b.m_[1], a.m_[2] & b.m_[2],
-                   a.m_[3] & b.m_[3]);
-  }
-
-  friend constexpr Charmap operator|(const Charmap& a, const Charmap& b) {
-    return Charmap(a.m_[0] | b.m_[0], a.m_[1] | b.m_[1], a.m_[2] | b.m_[2],
-                   a.m_[3] | b.m_[3]);
-  }
-
-  friend constexpr Charmap operator~(const Charmap& a) {
-    return Charmap(~a.m_[0], ~a.m_[1], ~a.m_[2], ~a.m_[3]);
-  }
-
- private:
-  constexpr Charmap(uint64_t b0, uint64_t b1, uint64_t b2, uint64_t b3)
-      : m_{b0, b1, b2, b3} {}
-
-  static constexpr uint64_t RangeForWord(unsigned char lo, unsigned char hi,
-                                         uint64_t word) {
-    return OpenRangeFromZeroForWord(hi + 1, word) &
-           ~OpenRangeFromZeroForWord(lo, word);
-  }
-
-  // All the chars in the specified word of the range [0, upper).
-  static constexpr uint64_t OpenRangeFromZeroForWord(uint64_t upper,
-                                                     uint64_t word) {
-    return (upper <= 64 * word)
-               ? 0
-               : (upper >= 64 * (word + 1))
-                     ? ~static_cast<uint64_t>(0)
-                     : (~static_cast<uint64_t>(0) >> (64 - upper % 64));
-  }
-
-  static constexpr uint64_t CharMaskForWord(unsigned char x, uint64_t word) {
-    return (x / 64 == word) ? (static_cast<uint64_t>(1) << (x % 64)) : 0;
-  }
-
- private:
-  void SetChar(unsigned char c) {
-    m_[c / 64] |= static_cast<uint64_t>(1) << (c % 64);
-  }
-
-  uint64_t m_[4];
-};
-
-// Mirror the char-classifying predicates in <cctype>
-constexpr Charmap UpperCharmap() { return Charmap::Range('A', 'Z'); }
-constexpr Charmap LowerCharmap() { return Charmap::Range('a', 'z'); }
-constexpr Charmap DigitCharmap() { return Charmap::Range('0', '9'); }
-constexpr Charmap AlphaCharmap() { return LowerCharmap() | UpperCharmap(); }
-constexpr Charmap AlnumCharmap() { return DigitCharmap() | AlphaCharmap(); }
-constexpr Charmap XDigitCharmap() {
-  return DigitCharmap() | Charmap::Range('A', 'F') | Charmap::Range('a', 'f');
-}
-constexpr Charmap PrintCharmap() { return Charmap::Range(0x20, 0x7e); }
-constexpr Charmap SpaceCharmap() { return Charmap::FromString("\t\n\v\f\r "); }
-constexpr Charmap CntrlCharmap() {
-  return Charmap::Range(0, 0x7f) & ~PrintCharmap();
-}
-constexpr Charmap BlankCharmap() { return Charmap::FromString("\t "); }
-constexpr Charmap GraphCharmap() { return PrintCharmap() & ~SpaceCharmap(); }
-constexpr Charmap PunctCharmap() { return GraphCharmap() & ~AlnumCharmap(); }
-
-}  // namespace strings_internal
-}  // namespace absl
-
-#endif  // ABSL_STRINGS_INTERNAL_CHAR_MAP_H_
diff --git a/absl/strings/internal/char_map_test.cc b/absl/strings/internal/char_map_test.cc
deleted file mode 100644
index d3306241a40..00000000000
--- a/absl/strings/internal/char_map_test.cc
+++ /dev/null
@@ -1,172 +0,0 @@
-// Copyright 2017 The Abseil Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-#include "absl/strings/internal/char_map.h"
-
-#include <cctype>
-#include <string>
-#include <vector>
-
-#include "gmock/gmock.h"
-#include "gtest/gtest.h"
-
-namespace {
-
-constexpr absl::strings_internal::Charmap everything_map =
-    ~absl::strings_internal::Charmap();
-constexpr absl::strings_internal::Charmap nothing_map{};
-
-TEST(Charmap, AllTests) {
-  const absl::strings_internal::Charmap also_nothing_map("", 0);
-  ASSERT_TRUE(everything_map.contains('\0'));
-  ASSERT_TRUE(!nothing_map.contains('\0'));
-  ASSERT_TRUE(!also_nothing_map.contains('\0'));
-  for (unsigned char ch = 1; ch != 0; ++ch) {
-    ASSERT_TRUE(everything_map.contains(ch));
-    ASSERT_TRUE(!nothing_map.contains(ch));
-    ASSERT_TRUE(!also_nothing_map.contains(ch));
-  }
-
-  const absl::strings_internal::Charmap symbols("&@#@^!@?", 5);
-  ASSERT_TRUE(symbols.contains('&'));
-  ASSERT_TRUE(symbols.contains('@'));
-  ASSERT_TRUE(symbols.contains('#'));
-  ASSERT_TRUE(symbols.contains('^'));
-  ASSERT_TRUE(!symbols.contains('!'));
-  ASSERT_TRUE(!symbols.contains('?'));
-  int cnt = 0;
-  for (unsigned char ch = 1; ch != 0; ++ch)
-    cnt += symbols.contains(ch);
-  ASSERT_EQ(cnt, 4);
-
-  const absl::strings_internal::Charmap lets("^abcde", 3);
-  const absl::strings_internal::Charmap lets2("fghij\0klmnop", 10);
-  const absl::strings_internal::Charmap lets3("fghij\0klmnop");
-  ASSERT_TRUE(lets2.contains('k'));
-  ASSERT_TRUE(!lets3.contains('k'));
-
-  ASSERT_TRUE(symbols.IntersectsWith(lets));
-  ASSERT_TRUE(!lets2.IntersectsWith(lets));
-  ASSERT_TRUE(lets.IntersectsWith(symbols));
-  ASSERT_TRUE(!lets.IntersectsWith(lets2));
-
-  ASSERT_TRUE(nothing_map.IsZero());
-  ASSERT_TRUE(!lets.IsZero());
-}
-
-namespace {
-std::string Members(const absl::strings_internal::Charmap& m) {
-  std::string r;
-  for (size_t i = 0; i < 256; ++i)
-    if (m.contains(i)) r.push_back(i);
-  return r;
-}
-
-std::string ClosedRangeString(unsigned char lo, unsigned char hi) {
-  // Don't depend on lo<hi. Just increment until lo==hi.
-  std::string s;
-  while (true) {
-    s.push_back(lo);
-    if (lo == hi) break;
-    ++lo;
-  }
-  return s;
-}
-
-}  // namespace
-
-TEST(Charmap, Constexpr) {
-  constexpr absl::strings_internal::Charmap kEmpty = nothing_map;
-  EXPECT_THAT(Members(kEmpty), "");
-  constexpr absl::strings_internal::Charmap kA =
-      absl::strings_internal::Charmap::Char('A');
-  EXPECT_THAT(Members(kA), "A");
-  constexpr absl::strings_internal::Charmap kAZ =
-      absl::strings_internal::Charmap::Range('A', 'Z');
-  EXPECT_THAT(Members(kAZ), "ABCDEFGHIJKLMNOPQRSTUVWXYZ");
-  constexpr absl::strings_internal::Charmap kIdentifier =
-      absl::strings_internal::Charmap::Range('0', '9') |
-      absl::strings_internal::Charmap::Range('A', 'Z') |
-      absl::strings_internal::Charmap::Range('a', 'z') |
-      absl::strings_internal::Charmap::Char('_');
-  EXPECT_THAT(Members(kIdentifier),
-              "0123456789"
-              "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
-              "_"
-              "abcdefghijklmnopqrstuvwxyz");
-  constexpr absl::strings_internal::Charmap kAll = everything_map;
-  for (size_t i = 0; i < 256; ++i) {
-    EXPECT_TRUE(kAll.contains(i)) << i;
-  }
-  constexpr absl::strings_internal::Charmap kHello =
-      absl::strings_internal::Charmap::FromString("Hello, world!");
-  EXPECT_THAT(Members(kHello), " !,Hdelorw");
-
-  // test negation and intersection
-  constexpr absl::strings_internal::Charmap kABC =
-      absl::strings_internal::Charmap::Range('A', 'Z') &
-      ~absl::strings_internal::Charmap::Range('D', 'Z');
-  EXPECT_THAT(Members(kABC), "ABC");
-}
-
-TEST(Charmap, Range) {
-  // Exhaustive testing takes too long, so test some of the boundaries that
-  // are perhaps going to cause trouble.
-  std::vector<size_t> poi = {0,   1,   2,   3,   4,   7,   8,   9,  15,
-                             16,  17,  30,  31,  32,  33,  63,  64, 65,
-                             127, 128, 129, 223, 224, 225, 254, 255};
-  for (auto lo = poi.begin(); lo != poi.end(); ++lo) {
-    SCOPED_TRACE(*lo);
-    for (auto hi = lo; hi != poi.end(); ++hi) {
-      SCOPED_TRACE(*hi);
-      EXPECT_THAT(Members(absl::strings_internal::Charmap::Range(*lo, *hi)),
-                  ClosedRangeString(*lo, *hi));
-    }
-  }
-}
-
-bool AsBool(int x) { return static_cast<bool>(x); }
-
-TEST(CharmapCtype, Match) {
-  for (int c = 0; c < 256; ++c) {
-    SCOPED_TRACE(c);
-    SCOPED_TRACE(static_cast<char>(c));
-    EXPECT_EQ(AsBool(std::isupper(c)),
-              absl::strings_internal::UpperCharmap().contains(c));
-    EXPECT_EQ(AsBool(std::islower(c)),
-              absl::strings_internal::LowerCharmap().contains(c));
-    EXPECT_EQ(AsBool(std::isdigit(c)),
-              absl::strings_internal::DigitCharmap().contains(c));
-    EXPECT_EQ(AsBool(std::isalpha(c)),
-              absl::strings_internal::AlphaCharmap().contains(c));
-    EXPECT_EQ(AsBool(std::isalnum(c)),
-              absl::strings_internal::AlnumCharmap().contains(c));
-    EXPECT_EQ(AsBool(std::isxdigit(c)),
-              absl::strings_internal::XDigitCharmap().contains(c));
-    EXPECT_EQ(AsBool(std::isprint(c)),
-              absl::strings_internal::PrintCharmap().contains(c));
-    EXPECT_EQ(AsBool(std::isspace(c)),
-              absl::strings_internal::SpaceCharmap().contains(c));
-    EXPECT_EQ(AsBool(std::iscntrl(c)),
-              absl::strings_internal::CntrlCharmap().contains(c));
-    EXPECT_EQ(AsBool(std::isblank(c)),
-              absl::strings_internal::BlankCharmap().contains(c));
-    EXPECT_EQ(AsBool(std::isgraph(c)),
-              absl::strings_internal::GraphCharmap().contains(c));
-    EXPECT_EQ(AsBool(std::ispunct(c)),
-              absl::strings_internal::PunctCharmap().contains(c));
-  }
-}
-
-}  // namespace
diff --git a/absl/strings/internal/charconv_bigint.cc b/absl/strings/internal/charconv_bigint.cc
index 95d471d9e22..e91e90bd4ef 100644
--- a/absl/strings/internal/charconv_bigint.cc
+++ b/absl/strings/internal/charconv_bigint.cc
@@ -16,9 +16,14 @@
 
 #include <algorithm>
 #include <cassert>
+#include <cstdint>
+#include <iterator>
 #include <string>
 
+#include "absl/strings/internal/charconv_parse.h"
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace strings_internal {
 
 namespace {
@@ -157,12 +162,12 @@ const uint32_t* LargePowerOfFiveData(int i) {
 int LargePowerOfFiveSize(int i) { return 2 * i; }
 }  // namespace
 
-const uint32_t kFiveToNth[14] = {
+ABSL_DLL const uint32_t kFiveToNth[14] = {
     1,     5,      25,      125,     625,      3125,      15625,
     78125, 390625, 1953125, 9765625, 48828125, 244140625, 1220703125,
 };
 
-const uint32_t kTenToNth[10] = {
+ABSL_DLL const uint32_t kTenToNth[10] = {
     1, 10, 100, 1000, 10000, 100000, 1000000, 10000000, 100000000, 1000000000,
 };
 
@@ -207,7 +212,7 @@ int BigUnsigned<max_words>::ReadDigits(const char* begin, const char* end,
     ++dropped_digits;
   }
   if (begin < end && *std::prev(end) == '.') {
-    // If the std::string ends in '.', either before or after dropping zeroes, then
+    // If the string ends in '.', either before or after dropping zeroes, then
     // drop the decimal point and look for more digits to drop.
     dropped_digits = 0;
     --end;
@@ -241,7 +246,7 @@ int BigUnsigned<max_words>::ReadDigits(const char* begin, const char* end,
       // decimal exponent to compensate.
       --exponent_adjust;
     }
-    int digit = (*begin - '0');
+    char digit = (*begin - '0');
     --significant_digits;
     if (significant_digits == 0 && std::next(begin) != end &&
         (digit == 0 || digit == 5)) {
@@ -254,7 +259,7 @@ int BigUnsigned<max_words>::ReadDigits(const char* begin, const char* end,
       // 500000...000000000001 to correctly round up, rather than to nearest.
       ++digit;
     }
-    queued = 10 * queued + digit;
+    queued = 10 * queued + static_cast<uint32_t>(digit);
     ++digits_queued;
     if (digits_queued == kMaxSmallPowerOfTen) {
       MultiplyBy(kTenToNth[kMaxSmallPowerOfTen]);
@@ -278,7 +283,7 @@ int BigUnsigned<max_words>::ReadDigits(const char* begin, const char* end,
     // Either way, [begin, decimal_point) will contain the set of dropped digits
     // that require an exponent adjustment.
     const char* decimal_point = std::find(begin, end, '.');
-    exponent_adjust += (decimal_point - begin);
+    exponent_adjust += static_cast<int>(decimal_point - begin);
   }
   return exponent_adjust;
 }
@@ -295,10 +300,8 @@ template <int max_words>
         std::min(n / kLargePowerOfFiveStep, kLargestPowerOfFiveIndex);
     if (first_pass) {
       // just copy, rather than multiplying by 1
-      std::copy(
-          LargePowerOfFiveData(big_power),
-          LargePowerOfFiveData(big_power) + LargePowerOfFiveSize(big_power),
-          answer.words_);
+      std::copy_n(LargePowerOfFiveData(big_power),
+                  LargePowerOfFiveSize(big_power), answer.words_);
       answer.size_ = LargePowerOfFiveSize(big_power);
       first_pass = false;
     } else {
@@ -340,8 +343,8 @@ std::string BigUnsigned<max_words>::ToString() const {
   std::string result;
   // Build result in reverse order
   while (copy.size() > 0) {
-    int next_digit = copy.DivMod<10>();
-    result.push_back('0' + next_digit);
+    uint32_t next_digit = copy.DivMod<10>();
+    result.push_back('0' + static_cast<char>(next_digit));
   }
   if (result.empty()) {
     result.push_back('0');
@@ -354,4 +357,5 @@ template class BigUnsigned<4>;
 template class BigUnsigned<84>;
 
 }  // namespace strings_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/internal/charconv_bigint.h b/absl/strings/internal/charconv_bigint.h
index 7da9a7e75ac..2c7a3366464 100644
--- a/absl/strings/internal/charconv_bigint.h
+++ b/absl/strings/internal/charconv_bigint.h
@@ -17,14 +17,16 @@
 
 #include <algorithm>
 #include <cstdint>
-#include <iostream>
+#include <ostream>
 #include <string>
 
+#include "absl/base/config.h"
 #include "absl/strings/ascii.h"
 #include "absl/strings/internal/charconv_parse.h"
 #include "absl/strings/string_view.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace strings_internal {
 
 // The largest power that 5 that can be raised to, and still fit in a uint32_t.
@@ -32,8 +34,9 @@ constexpr int kMaxSmallPowerOfFive = 13;
 // The largest power that 10 that can be raised to, and still fit in a uint32_t.
 constexpr int kMaxSmallPowerOfTen = 9;
 
-extern const uint32_t kFiveToNth[kMaxSmallPowerOfFive + 1];
-extern const uint32_t kTenToNth[kMaxSmallPowerOfTen + 1];
+ABSL_DLL extern const uint32_t
+    kFiveToNth[kMaxSmallPowerOfFive + 1];
+ABSL_DLL extern const uint32_t kTenToNth[kMaxSmallPowerOfTen + 1];
 
 // Large, fixed-width unsigned integer.
 //
@@ -63,7 +66,7 @@ class BigUnsigned {
                static_cast<uint32_t>(v >> 32)} {}
 
   // Constructs a BigUnsigned from the given string_view containing a decimal
-  // value.  If the input std::string is not a decimal integer, constructs a 0
+  // value.  If the input string is not a decimal integer, constructs a 0
   // instead.
   explicit BigUnsigned(absl::string_view sv) : size_(0), words_{} {
     // Check for valid input, returning a 0 otherwise.  This is reasonable
@@ -89,7 +92,7 @@ class BigUnsigned {
   // numbers with this many decimal digits or fewer are representable by this
   // type.
   //
-  // Analagous to std::numeric_limits<BigUnsigned>::digits10.
+  // Analogous to std::numeric_limits<BigUnsigned>::digits10.
   static constexpr int Digits10() {
     // 9975007/1035508 is very slightly less than log10(2**32).
     return static_cast<uint64_t>(max_words) * 9975007 / 1035508;
@@ -106,7 +109,17 @@ class BigUnsigned {
       size_ = (std::min)(size_ + word_shift, max_words);
       count %= 32;
       if (count == 0) {
+// https://gcc.gnu.org/bugzilla/show_bug.cgi?id=warray-bounds
+// shows a lot of bogus -Warray-bounds warnings under GCC.
+// This is not the only one in Abseil.
+#if ABSL_INTERNAL_HAVE_MIN_GNUC_VERSION(14, 0)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Warray-bounds"
+#endif
         std::copy_backward(words_, words_ + size_ - word_shift, words_ + size_);
+#if ABSL_INTERNAL_HAVE_MIN_GNUC_VERSION(14, 0)
+#pragma GCC diagnostic pop
+#endif
       } else {
         for (int i = (std::min)(size_, max_words - 1); i > word_shift; --i) {
           words_[i] = (words_[i - word_shift] << count) |
@@ -118,7 +131,7 @@ class BigUnsigned {
           ++size_;
         }
       }
-      std::fill(words_, words_ + word_shift, 0u);
+      std::fill_n(words_, word_shift, 0u);
     }
   }
 
@@ -194,7 +207,7 @@ class BigUnsigned {
   }
 
   void SetToZero() {
-    std::fill(words_, words_ + size_, 0u);
+    std::fill_n(words_, size_, 0u);
     size_ = 0;
   }
 
@@ -207,7 +220,7 @@ class BigUnsigned {
     return words_[index];
   }
 
-  // Returns this integer as a decimal std::string.  This is not used in the decimal-
+  // Returns this integer as a decimal string.  This is not used in the decimal-
   // to-binary conversion; it is intended to aid in testing.
   std::string ToString() const;
 
@@ -414,6 +427,7 @@ extern template class BigUnsigned<4>;
 extern template class BigUnsigned<84>;
 
 }  // namespace strings_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_INTERNAL_CHARCONV_BIGINT_H_
diff --git a/absl/strings/internal/charconv_bigint_test.cc b/absl/strings/internal/charconv_bigint_test.cc
index 745714aca1b..8d9f62d5abb 100644
--- a/absl/strings/internal/charconv_bigint_test.cc
+++ b/absl/strings/internal/charconv_bigint_test.cc
@@ -14,11 +14,14 @@
 
 #include "absl/strings/internal/charconv_bigint.h"
 
+#include <cstdint>
+#include <limits>
 #include <string>
 
 #include "gtest/gtest.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace strings_internal {
 
 TEST(BigUnsigned, ShiftLeft) {
@@ -68,6 +71,61 @@ TEST(BigUnsigned, ShiftLeft) {
     // And we should have fully rotated all bits off by now:
     EXPECT_EQ(a, BigUnsigned<84>(0u));
   }
+  {
+    // Bit shifting large and small numbers by large and small offsets.
+    // Intended to exercise bounds-checking corner on ShiftLeft() (directly
+    // and under asan).
+
+    // 2**(32*84)-1
+    const BigUnsigned<84> all_bits_one(
+        "1474444211396924248063325089479706787923460402125687709454567433186613"
+        "6228083464060749874845919674257665016359189106695900028098437021384227"
+        "3285029708032466536084583113729486015826557532750465299832071590813090"
+        "2011853039837649252477307070509704043541368002938784757296893793903797"
+        "8180292336310543540677175225040919704702800559606097685920595947397024"
+        "8303316808753252115729411497720357971050627997031988036134171378490368"
+        "6008000778741115399296162550786288457245180872759047016734959330367829"
+        "5235612397427686310674725251378116268607113017720538636924549612987647"
+        "5767411074510311386444547332882472126067840027882117834454260409440463"
+        "9345147252664893456053258463203120637089916304618696601333953616715125"
+        "2115882482473279040772264257431663818610405673876655957323083702713344"
+        "4201105427930770976052393421467136557055");
+    const BigUnsigned<84> zero(0u);
+    const BigUnsigned<84> one(1u);
+    // in bounds shifts
+    for (int i = 1; i < 84*32; ++i) {
+      // shifting all_bits_one to the left should result in a smaller number,
+      // since the high bits rotate off and the low bits are replaced with
+      // zeroes.
+      BigUnsigned<84> big_shifted = all_bits_one;
+      big_shifted.ShiftLeft(i);
+      EXPECT_GT(all_bits_one, big_shifted);
+      // Shifting 1 to the left should instead result in a larger number.
+      BigUnsigned<84> small_shifted = one;
+      small_shifted.ShiftLeft(i);
+      EXPECT_LT(one, small_shifted);
+    }
+    // Shifting by zero or a negative number has no effect
+    for (int no_op_shift : {0, -1, -84 * 32, std::numeric_limits<int>::min()}) {
+      BigUnsigned<84> big_shifted = all_bits_one;
+      big_shifted.ShiftLeft(no_op_shift);
+      EXPECT_EQ(all_bits_one, big_shifted);
+      BigUnsigned<84> small_shifted = one;
+      big_shifted.ShiftLeft(no_op_shift);
+      EXPECT_EQ(one, small_shifted);
+    }
+    // Shifting by an amount greater than the number of bits should result in
+    // zero.
+    for (int out_of_bounds_shift :
+         {84 * 32, 84 * 32 + 1, std::numeric_limits<int>::max()}) {
+      BigUnsigned<84> big_shifted = all_bits_one;
+      big_shifted.ShiftLeft(out_of_bounds_shift);
+      EXPECT_EQ(zero, big_shifted);
+      BigUnsigned<84> small_shifted = one;
+      small_shifted.ShiftLeft(out_of_bounds_shift);
+      EXPECT_EQ(zero, small_shifted);
+    }
+  }
 }
 
 TEST(BigUnsigned, MultiplyByUint32) {
@@ -120,7 +178,7 @@ TEST(BigUnsigned, MultiplyByBigUnsigned) {
 
 TEST(BigUnsigned, MultiplyByOverflow) {
   {
-    // Check that multiplcation overflow predictably truncates.
+    // Check that multiplication overflow predictably truncates.
 
     // A big int with all bits on.
     BigUnsigned<4> all_bits_on("340282366920938463463374607431768211455");
@@ -200,4 +258,5 @@ TEST(BigUnsigned, TenToTheNth) {
 
 
 }  // namespace strings_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/internal/charconv_parse.cc b/absl/strings/internal/charconv_parse.cc
index f3c7232403a..2766cf3e2e5 100644
--- a/absl/strings/internal/charconv_parse.cc
+++ b/absl/strings/internal/charconv_parse.cc
@@ -13,15 +13,17 @@
 // limitations under the License.
 
 #include "absl/strings/internal/charconv_parse.h"
-#include "absl/strings/charconv.h"
 
 #include <cassert>
+#include <cstddef>
 #include <cstdint>
 #include <limits>
 
+#include "absl/strings/charconv.h"
 #include "absl/strings/internal/memutil.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace {
 
 // ParseFloat<10> will read the first 19 significant digits of the mantissa.
@@ -51,7 +53,7 @@ static_assert(std::numeric_limits<double>::digits == 53, "IEEE double fact");
 
 // The lowest valued 19-digit decimal mantissa we can read still contains
 // sufficient information to reconstruct a binary mantissa.
-static_assert(1000000000000000000u > (uint64_t(1) << (53 + 3)), "(b) above");
+static_assert(1000000000000000000u > (uint64_t{1} << (53 + 3)), "(b) above");
 
 // ParseFloat<16> will read the first 15 significant digits of the mantissa.
 //
@@ -189,11 +191,11 @@ bool IsDigit<16>(char ch) {
 
 template <>
 unsigned ToDigit<10>(char ch) {
-  return ch - '0';
+  return static_cast<unsigned>(ch - '0');
 }
 template <>
 unsigned ToDigit<16>(char ch) {
-  return kAsciiToInt[static_cast<unsigned char>(ch)];
+  return static_cast<unsigned>(kAsciiToInt[static_cast<unsigned char>(ch)]);
 }
 
 template <>
@@ -245,14 +247,21 @@ constexpr int DigitMagnitude<16>() {
 // ConsumeDigits does not protect against overflow on *out; max_digits must
 // be chosen with respect to type T to avoid the possibility of overflow.
 template <int base, typename T>
-std::size_t ConsumeDigits(const char* begin, const char* end, int max_digits,
-                          T* out, bool* dropped_nonzero_digit) {
+ptrdiff_t ConsumeDigits(const char* begin, const char* end,
+                        ptrdiff_t max_digits, T* out,
+                        bool* dropped_nonzero_digit) {
   if (base == 10) {
     assert(max_digits <= std::numeric_limits<T>::digits10);
   } else if (base == 16) {
     assert(max_digits * 4 <= std::numeric_limits<T>::digits);
   }
   const char* const original_begin = begin;
+
+  // Skip leading zeros, but only if *out is zero.
+  // They don't cause an overflow so we don't have to count them for
+  // `max_digits`.
+  while (!*out && end != begin && *begin == '0') ++begin;
+
   T accumulator = *out;
   const char* significant_digits_end =
       (end - begin > max_digits) ? begin + max_digits : end;
@@ -295,7 +304,7 @@ bool ParseInfinityOrNan(const char* begin, const char* end,
   switch (*begin) {
     case 'i':
     case 'I': {
-      // An infinity std::string consists of the characters "inf" or "infinity",
+      // An infinity string consists of the characters "inf" or "infinity",
       // case insensitive.
       if (strings_internal::memcasecmp(begin + 1, "nf", 2) != 0) {
         return false;
@@ -319,7 +328,7 @@ bool ParseInfinityOrNan(const char* begin, const char* end,
       }
       out->type = strings_internal::FloatType::kNan;
       out->end = begin + 3;
-      // NaN is allowed to be followed by a parenthesized std::string, consisting of
+      // NaN is allowed to be followed by a parenthesized string, consisting of
       // only the characters [a-zA-Z0-9_].  Match that if it's present.
       begin += 3;
       if (begin < end && *begin == '(') {
@@ -363,24 +372,22 @@ strings_internal::ParsedFloat ParseFloat(const char* begin, const char* end,
   }
   uint64_t mantissa = 0;
 
-  int exponent_adjustment = 0;
+  ptrdiff_t exponent_adjustment = 0;
   bool mantissa_is_inexact = false;
-  std::size_t pre_decimal_digits = ConsumeDigits<base>(
+  ptrdiff_t pre_decimal_digits = ConsumeDigits<base>(
       begin, end, MantissaDigitsMax<base>(), &mantissa, &mantissa_is_inexact);
   begin += pre_decimal_digits;
-  int digits_left;
+  ptrdiff_t digits_left;
   if (pre_decimal_digits >= DigitLimit<base>()) {
     // refuse to parse pathological inputs
     return result;
   } else if (pre_decimal_digits > MantissaDigitsMax<base>()) {
     // We dropped some non-fraction digits on the floor.  Adjust our exponent
     // to compensate.
-    exponent_adjustment =
-        static_cast<int>(pre_decimal_digits - MantissaDigitsMax<base>());
+    exponent_adjustment = pre_decimal_digits - MantissaDigitsMax<base>();
     digits_left = 0;
   } else {
-    digits_left =
-        static_cast<int>(MantissaDigitsMax<base>() - pre_decimal_digits);
+    digits_left = MantissaDigitsMax<base>() - pre_decimal_digits;
   }
   if (begin < end && *begin == '.') {
     ++begin;
@@ -391,14 +398,14 @@ strings_internal::ParsedFloat ParseFloat(const char* begin, const char* end,
       while (begin < end && *begin == '0') {
         ++begin;
       }
-      std::size_t zeros_skipped = begin - begin_zeros;
+      ptrdiff_t zeros_skipped = begin - begin_zeros;
       if (zeros_skipped >= DigitLimit<base>()) {
         // refuse to parse pathological inputs
         return result;
       }
-      exponent_adjustment -= static_cast<int>(zeros_skipped);
+      exponent_adjustment -= zeros_skipped;
     }
-    std::size_t post_decimal_digits = ConsumeDigits<base>(
+    ptrdiff_t post_decimal_digits = ConsumeDigits<base>(
         begin, end, digits_left, &mantissa, &mantissa_is_inexact);
     begin += post_decimal_digits;
 
@@ -475,15 +482,26 @@ strings_internal::ParsedFloat ParseFloat(const char* begin, const char* end,
     return result;
   }
 
-  // Success!
-  result.type = strings_internal::FloatType::kNumber;
   if (result.mantissa > 0) {
-    result.exponent = result.literal_exponent +
-                      (DigitMagnitude<base>() * exponent_adjustment);
+    const ptrdiff_t exponent = result.literal_exponent +
+                               (DigitMagnitude<base>() * exponent_adjustment);
+
+    if (exponent < (std::numeric_limits<int>::min)() ||
+        exponent > (std::numeric_limits<int>::max)()) {
+      // We cannot store the exponent in int. Fail by returning a result with
+      // end default-initialized to nullptr.
+      return result;
+    }
+
+    result.exponent = static_cast<int>(exponent);
   } else {
     result.exponent = 0;
   }
   result.end = begin;
+
+  // Success!
+  result.type = strings_internal::FloatType::kNumber;
+
   return result;
 }
 
@@ -493,4 +511,5 @@ template ParsedFloat ParseFloat<16>(const char* begin, const char* end,
                                     chars_format format_flags);
 
 }  // namespace strings_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/internal/charconv_parse.h b/absl/strings/internal/charconv_parse.h
index 44d06b2e9bd..505998b5394 100644
--- a/absl/strings/internal/charconv_parse.h
+++ b/absl/strings/internal/charconv_parse.h
@@ -17,9 +17,11 @@
 
 #include <cstdint>
 
+#include "absl/base/config.h"
 #include "absl/strings/charconv.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace strings_internal {
 
 // Enum indicating whether a parsed float is a number or special value.
@@ -92,5 +94,6 @@ extern template ParsedFloat ParseFloat<16>(const char* begin, const char* end,
                                            absl::chars_format format_flags);
 
 }  // namespace strings_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 #endif  // ABSL_STRINGS_INTERNAL_CHARCONV_PARSE_H_
diff --git a/absl/strings/internal/charconv_parse_test.cc b/absl/strings/internal/charconv_parse_test.cc
index 9511c987450..c10b77ba0c7 100644
--- a/absl/strings/internal/charconv_parse_test.cc
+++ b/absl/strings/internal/charconv_parse_test.cc
@@ -14,13 +14,16 @@
 
 #include "absl/strings/internal/charconv_parse.h"
 
+#include <cstdint>
 #include <string>
 #include <utility>
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
-#include "absl/base/internal/raw_logging.h"
+#include "absl/log/check.h"
+#include "absl/strings/charconv.h"
 #include "absl/strings/str_cat.h"
+#include "absl/strings/string_view.h"
 
 using absl::chars_format;
 using absl::strings_internal::FloatType;
@@ -56,14 +59,14 @@ void ExpectParsedFloat(std::string s, absl::chars_format format_flags,
     begin_subrange = static_cast<int>(open_bracket_pos);
     s.replace(open_bracket_pos, 1, "");
     std::string::size_type close_bracket_pos = s.find(']');
-    ABSL_RAW_CHECK(close_bracket_pos != absl::string_view::npos,
-                   "Test input contains [ without matching ]");
+    CHECK_NE(close_bracket_pos, absl::string_view::npos)
+        << "Test input contains [ without matching ]";
     end_subrange = static_cast<int>(close_bracket_pos);
     s.replace(close_bracket_pos, 1, "");
   }
   const std::string::size_type expected_characters_matched = s.find('$');
-  ABSL_RAW_CHECK(expected_characters_matched != std::string::npos,
-                 "Input std::string must contain $");
+  CHECK_NE(expected_characters_matched, std::string::npos)
+      << "Input string must contain $";
   s.replace(expected_characters_matched, 1, "");
 
   ParsedFloat parsed =
diff --git a/absl/strings/internal/cord_data_edge.h b/absl/strings/internal/cord_data_edge.h
new file mode 100644
index 00000000000..e18b33e1b0f
--- /dev/null
+++ b/absl/strings/internal/cord_data_edge.h
@@ -0,0 +1,63 @@
+// Copyright 2022 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_STRINGS_INTERNAL_CORD_DATA_EDGE_H_
+#define ABSL_STRINGS_INTERNAL_CORD_DATA_EDGE_H_
+
+#include <cassert>
+#include <cstddef>
+
+#include "absl/base/config.h"
+#include "absl/strings/internal/cord_internal.h"
+#include "absl/strings/internal/cord_rep_flat.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+
+// Returns true if the provided rep is a FLAT, EXTERNAL or a SUBSTRING node
+// holding a FLAT or EXTERNAL child rep. Requires `rep != nullptr`.
+inline bool IsDataEdge(const CordRep* edge) {
+  assert(edge != nullptr);
+
+  // The fast path is that `edge` is an EXTERNAL or FLAT node, making the below
+  // if a single, well predicted branch. We then repeat the FLAT or EXTERNAL
+  // check in the slow path of the SUBSTRING check to optimize for the hot path.
+  if (edge->tag == EXTERNAL || edge->tag >= FLAT) return true;
+  if (edge->tag == SUBSTRING) edge = edge->substring()->child;
+  return edge->tag == EXTERNAL || edge->tag >= FLAT;
+}
+
+// Returns the `absl::string_view` data reference for the provided data edge.
+// Requires 'IsDataEdge(edge) == true`.
+inline absl::string_view EdgeData(const CordRep* edge) {
+  assert(IsDataEdge(edge));
+
+  size_t offset = 0;
+  const size_t length = edge->length;
+  if (edge->IsSubstring()) {
+    offset = edge->substring()->start;
+    edge = edge->substring()->child;
+  }
+  return edge->tag >= FLAT
+             ? absl::string_view{edge->flat()->Data() + offset, length}
+             : absl::string_view{edge->external()->base + offset, length};
+}
+
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STRINGS_INTERNAL_CORD_DATA_EDGE_H_
diff --git a/absl/strings/internal/cord_data_edge_test.cc b/absl/strings/internal/cord_data_edge_test.cc
new file mode 100644
index 00000000000..58046febbb0
--- /dev/null
+++ b/absl/strings/internal/cord_data_edge_test.cc
@@ -0,0 +1,131 @@
+// Copyright 2022 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/strings/internal/cord_data_edge.h"
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/strings/internal/cord_internal.h"
+#include "absl/strings/internal/cord_rep_test_util.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+namespace {
+
+using ::absl::cordrep_testing::MakeExternal;
+using ::absl::cordrep_testing::MakeFlat;
+using ::absl::cordrep_testing::MakeSubstring;
+
+TEST(CordDataEdgeTest, IsDataEdgeOnFlat) {
+  CordRep* rep = MakeFlat("Lorem ipsum dolor sit amet, consectetur ...");
+  EXPECT_TRUE(IsDataEdge(rep));
+  CordRep::Unref(rep);
+}
+
+TEST(CordDataEdgeTest, IsDataEdgeOnExternal) {
+  CordRep* rep = MakeExternal("Lorem ipsum dolor sit amet, consectetur ...");
+  EXPECT_TRUE(IsDataEdge(rep));
+  CordRep::Unref(rep);
+}
+
+TEST(CordDataEdgeTest, IsDataEdgeOnSubstringOfFlat) {
+  CordRep* rep = MakeFlat("Lorem ipsum dolor sit amet, consectetur ...");
+  CordRep* substr = MakeSubstring(1, 20, rep);
+  EXPECT_TRUE(IsDataEdge(substr));
+  CordRep::Unref(substr);
+}
+
+TEST(CordDataEdgeTest, IsDataEdgeOnSubstringOfExternal) {
+  CordRep* rep = MakeExternal("Lorem ipsum dolor sit amet, consectetur ...");
+  CordRep* substr = MakeSubstring(1, 20, rep);
+  EXPECT_TRUE(IsDataEdge(substr));
+  CordRep::Unref(substr);
+}
+
+TEST(CordDataEdgeTest, IsDataEdgeOnBtree) {
+  CordRep* rep = MakeFlat("Lorem ipsum dolor sit amet, consectetur ...");
+  CordRepBtree* tree = CordRepBtree::New(rep);
+  EXPECT_FALSE(IsDataEdge(tree));
+  CordRep::Unref(tree);
+}
+
+TEST(CordDataEdgeTest, IsDataEdgeOnBadSubstr) {
+  CordRep* rep = MakeFlat("Lorem ipsum dolor sit amet, consectetur ...");
+  CordRep* substr = MakeSubstring(1, 18, MakeSubstring(1, 20, rep));
+  EXPECT_FALSE(IsDataEdge(substr));
+  CordRep::Unref(substr);
+}
+
+TEST(CordDataEdgeTest, EdgeDataOnFlat) {
+  absl::string_view value = "Lorem ipsum dolor sit amet, consectetur ...";
+  CordRep* rep = MakeFlat(value);
+  EXPECT_EQ(EdgeData(rep), value);
+  CordRep::Unref(rep);
+}
+
+TEST(CordDataEdgeTest, EdgeDataOnExternal) {
+  absl::string_view value = "Lorem ipsum dolor sit amet, consectetur ...";
+  CordRep* rep = MakeExternal(value);
+  EXPECT_EQ(EdgeData(rep), value);
+  CordRep::Unref(rep);
+}
+
+TEST(CordDataEdgeTest, EdgeDataOnSubstringOfFlat) {
+  absl::string_view value = "Lorem ipsum dolor sit amet, consectetur ...";
+  CordRep* rep = MakeFlat(value);
+  CordRep* substr = MakeSubstring(1, 20, rep);
+  EXPECT_EQ(EdgeData(substr), value.substr(1, 20));
+  CordRep::Unref(substr);
+}
+
+TEST(CordDataEdgeTest, EdgeDataOnSubstringOfExternal) {
+  absl::string_view value = "Lorem ipsum dolor sit amet, consectetur ...";
+  CordRep* rep = MakeExternal(value);
+  CordRep* substr = MakeSubstring(1, 20, rep);
+  EXPECT_EQ(EdgeData(substr), value.substr(1, 20));
+  CordRep::Unref(substr);
+}
+
+#if defined(GTEST_HAS_DEATH_TEST) && !defined(NDEBUG)
+
+TEST(CordDataEdgeTest, IsDataEdgeOnNullPtr) {
+  EXPECT_DEATH(IsDataEdge(nullptr), ".*");
+}
+
+TEST(CordDataEdgeTest, EdgeDataOnNullPtr) {
+  EXPECT_DEATH(EdgeData(nullptr), ".*");
+}
+
+TEST(CordDataEdgeTest, EdgeDataOnBtree) {
+  CordRep* rep = MakeFlat("Lorem ipsum dolor sit amet, consectetur ...");
+  CordRepBtree* tree = CordRepBtree::New(rep);
+  EXPECT_DEATH(EdgeData(tree), ".*");
+  CordRep::Unref(tree);
+}
+
+TEST(CordDataEdgeTest, EdgeDataOnBadSubstr) {
+  CordRep* rep = MakeFlat("Lorem ipsum dolor sit amet, consectetur ...");
+  CordRep* substr = MakeSubstring(1, 18, MakeSubstring(1, 20, rep));
+  EXPECT_DEATH(EdgeData(substr), ".*");
+  CordRep::Unref(substr);
+}
+
+#endif  // GTEST_HAS_DEATH_TEST && !NDEBUG
+
+}  // namespace
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/strings/internal/cord_internal.cc b/absl/strings/internal/cord_internal.cc
new file mode 100644
index 00000000000..79e446181b3
--- /dev/null
+++ b/absl/strings/internal/cord_internal.cc
@@ -0,0 +1,75 @@
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+#include "absl/strings/internal/cord_internal.h"
+
+#include <atomic>
+#include <cassert>
+#include <memory>
+
+#include "absl/base/attributes.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/container/inlined_vector.h"
+#include "absl/strings/internal/cord_rep_btree.h"
+#include "absl/strings/internal/cord_rep_crc.h"
+#include "absl/strings/internal/cord_rep_flat.h"
+#include "absl/strings/str_cat.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+
+ABSL_CONST_INIT std::atomic<bool> shallow_subcords_enabled(
+    kCordShallowSubcordsDefault);
+
+void RefcountAndFlags::IncrementOverflow() {
+  ABSL_INTERNAL_LOG(FATAL, "refcount is too large and vulnerable to overflow");
+}
+
+void LogFatalNodeType(CordRep* rep) {
+  ABSL_INTERNAL_LOG(FATAL, absl::StrCat("Unexpected node type: ",
+                                        static_cast<int>(rep->tag)));
+}
+
+void CordRep::Destroy(CordRep* rep) {
+  assert(rep != nullptr);
+
+  while (true) {
+    assert(!rep->refcount.IsImmortal());
+    if (rep->tag == BTREE) {
+      CordRepBtree::Destroy(rep->btree());
+      return;
+    } else if (rep->tag == EXTERNAL) {
+      CordRepExternal::Delete(rep);
+      return;
+    } else if (rep->tag == SUBSTRING) {
+      CordRepSubstring* rep_substring = rep->substring();
+      rep = rep_substring->child;
+      delete rep_substring;
+      if (rep->refcount.Decrement()) {
+        return;
+      }
+    } else if (rep->tag == CRC) {
+      CordRepCrc::Destroy(rep->crc());
+      return;
+    } else {
+      assert(rep->IsFlat());
+      CordRepFlat::Delete(rep);
+      return;
+    }
+  }
+}
+
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/strings/internal/cord_internal.h b/absl/strings/internal/cord_internal.h
new file mode 100644
index 00000000000..cf45907d109
--- /dev/null
+++ b/absl/strings/internal/cord_internal.h
@@ -0,0 +1,809 @@
+// Copyright 2021 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_STRINGS_INTERNAL_CORD_INTERNAL_H_
+#define ABSL_STRINGS_INTERNAL_CORD_INTERNAL_H_
+
+#include <atomic>
+#include <cassert>
+#include <cstddef>
+#include <cstdint>
+#include <cstring>
+#include <limits>
+#include <string>
+#include <type_traits>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/endian.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/base/macros.h"
+#include "absl/base/nullability.h"
+#include "absl/base/optimization.h"
+#include "absl/container/internal/compressed_tuple.h"
+#include "absl/container/internal/container_memory.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+
+// The overhead of a vtable is too much for Cord, so we roll our own subclasses
+// using only a single byte to differentiate classes from each other - the "tag"
+// byte.  Define the subclasses first so we can provide downcasting helper
+// functions in the base class.
+struct CordRep;
+struct CordRepConcat;
+struct CordRepExternal;
+struct CordRepFlat;
+struct CordRepSubstring;
+struct CordRepCrc;
+class CordRepBtree;
+
+class CordzInfo;
+
+// Default feature enable states for cord ring buffers
+enum CordFeatureDefaults { kCordShallowSubcordsDefault = false };
+
+extern std::atomic<bool> shallow_subcords_enabled;
+
+inline void enable_shallow_subcords(bool enable) {
+  shallow_subcords_enabled.store(enable, std::memory_order_relaxed);
+}
+
+enum Constants {
+  // Prefer copying blocks of at most this size, otherwise reference count.
+  kMaxBytesToCopy = 511
+};
+
+// Emits a fatal error "Unexpected node type: xyz" and aborts the program.
+[[noreturn]] void LogFatalNodeType(CordRep* rep);
+
+// Fast implementation of memmove for up to 15 bytes. This implementation is
+// safe for overlapping regions. If nullify_tail is true, the destination is
+// padded with '\0' up to 15 bytes.
+template <bool nullify_tail = false>
+inline void SmallMemmove(char* dst, const char* src, size_t n) {
+  if (n >= 8) {
+    assert(n <= 15);
+    uint64_t buf1;
+    uint64_t buf2;
+    memcpy(&buf1, src, 8);
+    memcpy(&buf2, src + n - 8, 8);
+    if (nullify_tail) {
+      memset(dst + 7, 0, 8);
+    }
+    // GCC 12 has a false-positive -Wstringop-overflow warning here.
+#if ABSL_INTERNAL_HAVE_MIN_GNUC_VERSION(12, 0)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wstringop-overflow"
+#endif
+    memcpy(dst, &buf1, 8);
+    memcpy(dst + n - 8, &buf2, 8);
+#if ABSL_INTERNAL_HAVE_MIN_GNUC_VERSION(12, 0)
+#pragma GCC diagnostic pop
+#endif
+  } else if (n >= 4) {
+    uint32_t buf1;
+    uint32_t buf2;
+    memcpy(&buf1, src, 4);
+    memcpy(&buf2, src + n - 4, 4);
+    if (nullify_tail) {
+      memset(dst + 4, 0, 4);
+      memset(dst + 7, 0, 8);
+    }
+    memcpy(dst, &buf1, 4);
+    memcpy(dst + n - 4, &buf2, 4);
+  } else {
+    if (n != 0) {
+      dst[0] = src[0];
+      dst[n / 2] = src[n / 2];
+      dst[n - 1] = src[n - 1];
+    }
+    if (nullify_tail) {
+      memset(dst + 7, 0, 8);
+      memset(dst + n, 0, 8);
+    }
+  }
+}
+
+// Compact class for tracking the reference count and state flags for CordRep
+// instances.  Data is stored in an atomic int32_t for compactness and speed.
+class RefcountAndFlags {
+ public:
+  constexpr RefcountAndFlags() : count_{kRefIncrement} {}
+  struct Immortal {};
+  explicit constexpr RefcountAndFlags(Immortal) : count_(kImmortalFlag) {}
+
+  static void IncrementOverflow();
+
+  // Increments the reference count. Imposes no memory ordering.
+  inline void Increment() {
+    const int32_t prev_count =
+        count_.fetch_add(kRefIncrement, std::memory_order_relaxed);
+    if (ABSL_PREDICT_FALSE(
+            prev_count >=
+            ((std::numeric_limits<decltype(count_)::value_type>::max)() / 3) *
+                2)) {
+      IncrementOverflow();
+    }
+  }
+
+  // Asserts that the current refcount is greater than 0. If the refcount is
+  // greater than 1, decrements the reference count.
+  //
+  // Returns false if there are no references outstanding; true otherwise.
+  // Inserts barriers to ensure that state written before this method returns
+  // false will be visible to a thread that just observed this method returning
+  // false.  Always returns false when the immortal bit is set.
+  inline bool Decrement() {
+    int32_t refcount = count_.load(std::memory_order_acquire);
+    assert(refcount > 0 || refcount & kImmortalFlag);
+    return refcount != kRefIncrement &&
+           count_.fetch_sub(kRefIncrement, std::memory_order_acq_rel) !=
+               kRefIncrement;
+  }
+
+  // Same as Decrement but expect that refcount is greater than 1.
+  inline bool DecrementExpectHighRefcount() {
+    int32_t refcount =
+        count_.fetch_sub(kRefIncrement, std::memory_order_acq_rel);
+    assert(refcount > 0 || refcount & kImmortalFlag);
+    return refcount != kRefIncrement;
+  }
+
+  // Returns the current reference count using acquire semantics.
+  inline size_t Get() const {
+    return static_cast<size_t>(count_.load(std::memory_order_acquire) >>
+                               kNumFlags);
+  }
+
+  // Returns whether the atomic integer is 1.
+  // If the reference count is used in the conventional way, a
+  // reference count of 1 implies that the current thread owns the
+  // reference and no other thread shares it.
+  // This call performs the test for a reference count of one, and
+  // performs the memory barrier needed for the owning thread
+  // to act on the object, knowing that it has exclusive access to the
+  // object. Always returns false when the immortal bit is set.
+  inline bool IsOne() {
+    return count_.load(std::memory_order_acquire) == kRefIncrement;
+  }
+
+  bool IsImmortal() const {
+    return (count_.load(std::memory_order_relaxed) & kImmortalFlag) != 0;
+  }
+
+ private:
+  // We reserve the bottom bit for flag.
+  // kImmortalBit indicates that this entity should never be collected; it is
+  // used for the StringConstant constructor to avoid collecting immutable
+  // constant cords.
+  enum Flags {
+    kNumFlags = 1,
+
+    kImmortalFlag = 0x1,
+    kRefIncrement = (1 << kNumFlags),
+  };
+
+  std::atomic<int32_t> count_;
+};
+
+// Various representations that we allow
+enum CordRepKind {
+  UNUSED_0 = 0,
+  SUBSTRING = 1,
+  CRC = 2,
+  BTREE = 3,
+  UNUSED_4 = 4,
+  EXTERNAL = 5,
+
+  // We have different tags for different sized flat arrays,
+  // starting with FLAT, and limited to MAX_FLAT_TAG. The below values map to an
+  // allocated range of 32 bytes to 256 KB. The current granularity is:
+  // - 8 byte granularity for flat sizes in [32 - 512]
+  // - 64 byte granularity for flat sizes in (512 - 8KiB]
+  // - 4KiB byte granularity for flat sizes in (8KiB, 256 KiB]
+  // If a new tag is needed in the future, then 'FLAT' and 'MAX_FLAT_TAG' should
+  // be adjusted as well as the Tag <---> Size mapping logic so that FLAT still
+  // represents the minimum flat allocation size. (32 bytes as of now).
+  FLAT = 6,
+  MAX_FLAT_TAG = 248
+};
+
+// There are various locations where we want to check if some rep is a 'plain'
+// data edge, i.e. an external or flat rep. By having FLAT == EXTERNAL + 1, we
+// can perform this check in a single branch as 'tag >= EXTERNAL'
+// Note that we can leave this optimization to the compiler. The compiler will
+// DTRT when it sees a condition like `tag == EXTERNAL || tag >= FLAT`.
+static_assert(FLAT == EXTERNAL + 1, "EXTERNAL and FLAT not consecutive");
+
+struct CordRep {
+  // Result from an `extract edge` operation. Contains the (possibly changed)
+  // tree node as well as the extracted edge, or {tree, nullptr} if no edge
+  // could be extracted.
+  // On success, the returned `tree` value is null if `extracted` was the only
+  // data edge inside the tree, a data edge if there were only two data edges in
+  // the tree, or the (possibly new / smaller) remaining tree with the extracted
+  // data edge removed.
+  struct ExtractResult {
+    CordRep* tree;
+    CordRep* extracted;
+  };
+
+  CordRep() = default;
+  constexpr CordRep(RefcountAndFlags::Immortal immortal, size_t l)
+      : length(l), refcount(immortal), tag(EXTERNAL), storage{} {}
+
+  // The following three fields have to be less than 32 bytes since
+  // that is the smallest supported flat node size. Some code optimizations rely
+  // on the specific layout of these fields. Notably: the non-trivial field
+  // `refcount` being preceded by `length`, and being tailed by POD data
+  // members only.
+  // LINT.IfChange
+  size_t length;
+  RefcountAndFlags refcount;
+  // If tag < FLAT, it represents CordRepKind and indicates the type of node.
+  // Otherwise, the node type is CordRepFlat and the tag is the encoded size.
+  uint8_t tag;
+
+  // `storage` provides two main purposes:
+  // - the starting point for FlatCordRep.Data() [flexible-array-member]
+  // - 3 bytes of additional storage for use by derived classes.
+  // The latter is used by CordrepConcat and CordRepBtree. CordRepConcat stores
+  // a 'depth' value in storage[0], and the (future) CordRepBtree class stores
+  // `height`, `begin` and `end` in the 3 entries. Otherwise we would need to
+  // allocate room for these in the derived class, as not all compilers reuse
+  // padding space from the base class (clang and gcc do, MSVC does not, etc)
+  uint8_t storage[3];
+  // LINT.ThenChange(cord_rep_btree.h:copy_raw)
+
+  // Returns true if this instance's tag matches the requested type.
+  constexpr bool IsSubstring() const { return tag == SUBSTRING; }
+  constexpr bool IsCrc() const { return tag == CRC; }
+  constexpr bool IsExternal() const { return tag == EXTERNAL; }
+  constexpr bool IsFlat() const { return tag >= FLAT; }
+  constexpr bool IsBtree() const { return tag == BTREE; }
+
+  inline CordRepSubstring* substring();
+  inline const CordRepSubstring* substring() const;
+  inline CordRepCrc* crc();
+  inline const CordRepCrc* crc() const;
+  inline CordRepExternal* external();
+  inline const CordRepExternal* external() const;
+  inline CordRepFlat* flat();
+  inline const CordRepFlat* flat() const;
+  inline CordRepBtree* btree();
+  inline const CordRepBtree* btree() const;
+
+  // --------------------------------------------------------------------
+  // Memory management
+
+  // Destroys the provided `rep`.
+  static void Destroy(CordRep* rep);
+
+  // Increments the reference count of `rep`.
+  // Requires `rep` to be a non-null pointer value.
+  static inline CordRep* Ref(CordRep* rep);
+
+  // Decrements the reference count of `rep`. Destroys rep if count reaches
+  // zero. Requires `rep` to be a non-null pointer value.
+  static inline void Unref(CordRep* rep);
+};
+
+struct CordRepSubstring : public CordRep {
+  size_t start;  // Starting offset of substring in child
+  CordRep* child;
+
+  // Creates a substring on `child`, adopting a reference on `child`.
+  // Requires `child` to be either a flat or external node, and `pos` and `n` to
+  // form a non-empty partial sub range of `'child`, i.e.:
+  // `n > 0 && n < length && n + pos <= length`
+  static inline CordRepSubstring* Create(CordRep* child, size_t pos, size_t n);
+
+  // Creates a substring of `rep`. Does not adopt a reference on `rep`.
+  // Requires `IsDataEdge(rep) && n > 0 && pos + n <= rep->length`.
+  // If `n == rep->length` then this method returns `CordRep::Ref(rep)`
+  // If `rep` is a substring of a flat or external node, then this method will
+  // return a new substring of that flat or external node with `pos` adjusted
+  // with the original `start` position.
+  static inline CordRep* Substring(CordRep* rep, size_t pos, size_t n);
+};
+
+// Type for function pointer that will invoke the releaser function and also
+// delete the `CordRepExternalImpl` corresponding to the passed in
+// `CordRepExternal`.
+using ExternalReleaserInvoker = void (*)(CordRepExternal*);
+
+// External CordReps are allocated together with a type erased releaser. The
+// releaser is stored in the memory directly following the CordRepExternal.
+struct CordRepExternal : public CordRep {
+  CordRepExternal() = default;
+  explicit constexpr CordRepExternal(absl::string_view str)
+      : CordRep(RefcountAndFlags::Immortal{}, str.size()),
+        base(str.data()),
+        releaser_invoker(nullptr) {}
+
+  const char* base;
+  // Pointer to function that knows how to call and destroy the releaser.
+  ExternalReleaserInvoker releaser_invoker;
+
+  // Deletes (releases) the external rep.
+  // Requires rep != nullptr and rep->IsExternal()
+  static void Delete(CordRep* rep);
+};
+
+// Use go/ranked-overloads for dispatching.
+struct Rank0 {};
+struct Rank1 : Rank0 {};
+
+template <typename Releaser,
+          typename = ::std::invoke_result_t<Releaser, absl::string_view>>
+void InvokeReleaser(Rank1, Releaser&& releaser, absl::string_view data) {
+  ::std::invoke(std::forward<Releaser>(releaser), data);
+}
+
+template <typename Releaser, typename = ::std::invoke_result_t<Releaser>>
+void InvokeReleaser(Rank0, Releaser&& releaser, absl::string_view) {
+  ::std::invoke(std::forward<Releaser>(releaser));
+}
+
+// We use CompressedTuple so that we can benefit from EBCO.
+template <typename Releaser>
+struct CordRepExternalImpl
+    : public CordRepExternal,
+      public ::absl::container_internal::CompressedTuple<Releaser> {
+  // The extra int arg is so that we can avoid interfering with copy/move
+  // constructors while still benefitting from perfect forwarding.
+  template <typename T>
+  CordRepExternalImpl(T&& releaser, int)
+      : CordRepExternalImpl::CompressedTuple(std::forward<T>(releaser)) {
+    this->releaser_invoker = &Release;
+  }
+
+  const Releaser* releaser() const { return &this->template get<0>(); }
+
+  ~CordRepExternalImpl() {
+    InvokeReleaser(Rank1{}, std::move(this->template get<0>()),
+                   absl::string_view(base, length));
+  }
+
+  static void Release(CordRepExternal* rep) {
+    delete static_cast<CordRepExternalImpl*>(rep);
+  }
+};
+
+inline CordRepSubstring* CordRepSubstring::Create(CordRep* child, size_t pos,
+                                                  size_t n) {
+  assert(child != nullptr);
+  assert(n > 0);
+  assert(n < child->length);
+  assert(pos < child->length);
+  assert(n <= child->length - pos);
+
+  // Move to strategical places inside the Cord logic and make this an assert.
+  if (ABSL_PREDICT_FALSE(!(child->IsExternal() || child->IsFlat()))) {
+    LogFatalNodeType(child);
+  }
+
+  CordRepSubstring* rep = new CordRepSubstring();
+  rep->length = n;
+  rep->tag = SUBSTRING;
+  rep->start = pos;
+  rep->child = child;
+  return rep;
+}
+
+inline CordRep* CordRepSubstring::Substring(CordRep* rep, size_t pos,
+                                            size_t n) {
+  assert(rep != nullptr);
+  assert(n != 0);
+  assert(pos < rep->length);
+  assert(n <= rep->length - pos);
+  if (n == rep->length) return CordRep::Ref(rep);
+  if (rep->IsSubstring()) {
+    pos += rep->substring()->start;
+    rep = rep->substring()->child;
+  }
+  CordRepSubstring* substr = new CordRepSubstring();
+  substr->length = n;
+  substr->tag = SUBSTRING;
+  substr->start = pos;
+  substr->child = CordRep::Ref(rep);
+  return substr;
+}
+
+inline void CordRepExternal::Delete(CordRep* rep) {
+  assert(rep != nullptr && rep->IsExternal());
+  auto* rep_external = static_cast<CordRepExternal*>(rep);
+  assert(rep_external->releaser_invoker != nullptr);
+  rep_external->releaser_invoker(rep_external);
+}
+
+template <typename Str>
+struct ConstInitExternalStorage {
+  ABSL_CONST_INIT static CordRepExternal value;
+};
+
+template <typename Str>
+ABSL_CONST_INIT CordRepExternal
+    ConstInitExternalStorage<Str>::value(Str::value);
+
+enum {
+  kMaxInline = 15,
+};
+
+constexpr char GetOrNull(absl::string_view data, size_t pos) {
+  return pos < data.size() ? data[pos] : '\0';
+}
+
+// We store cordz_info as 64 bit pointer value in little endian format. This
+// guarantees that the least significant byte of cordz_info matches the first
+// byte of the inline data representation in `data`, which holds the inlined
+// size or the 'is_tree' bit.
+using cordz_info_t = int64_t;
+
+// Assert that the `cordz_info` pointer value perfectly overlaps the last half
+// of `data` and can hold a pointer value.
+static_assert(sizeof(cordz_info_t) * 2 == kMaxInline + 1);
+static_assert(sizeof(cordz_info_t) >= sizeof(intptr_t));
+
+// LittleEndianByte() creates a little endian representation of 'value', i.e.:
+// a little endian value where the first byte in the host's representation
+// holds 'value`, with all other bytes being 0.
+static constexpr cordz_info_t LittleEndianByte(unsigned char value) {
+#if defined(ABSL_IS_BIG_ENDIAN)
+  return static_cast<cordz_info_t>(value) << ((sizeof(cordz_info_t) - 1) * 8);
+#else
+  return value;
+#endif
+}
+
+class ABSL_ATTRIBUTE_TRIVIAL_ABI InlineData {
+ public:
+  // DefaultInitType forces the use of the default initialization constructor.
+  enum DefaultInitType { kDefaultInit };
+
+  // kNullCordzInfo holds the little endian representation of intptr_t(1)
+  // This is the 'null' / initial value of 'cordz_info'. The null value
+  // is specifically big endian 1 as with 64-bit pointers, the last
+  // byte of cordz_info overlaps with the last byte holding the tag.
+  static constexpr cordz_info_t kNullCordzInfo = LittleEndianByte(1);
+
+  // kTagOffset contains the offset of the control byte / tag. This constant is
+  // intended mostly for debugging purposes: do not remove this constant as it
+  // is actively inspected and used by gdb pretty printing code.
+  static constexpr size_t kTagOffset = 0;
+
+  InlineData() = default;
+
+  explicit InlineData(DefaultInitType) noexcept : rep_(kDefaultInit) {}
+
+  explicit InlineData(CordRep* rep) noexcept : rep_(rep) {
+    ABSL_ASSERT(rep != nullptr);
+  }
+
+  // Explicit constexpr constructor to create a constexpr InlineData
+  // value. Creates an inlined SSO value if `rep` is null, otherwise
+  // creates a tree instance value.
+  constexpr InlineData(absl::string_view sv, CordRep* rep) noexcept
+      : rep_(rep != nullptr ? Rep(rep) : Rep(sv)) {}
+
+  constexpr InlineData(const InlineData& rhs) noexcept = default;
+  InlineData& operator=(const InlineData& rhs) noexcept = default;
+  friend void swap(InlineData& lhs, InlineData& rhs) noexcept;
+
+  friend bool operator==(const InlineData& lhs, const InlineData& rhs) {
+    return memcmp(&lhs, &rhs, sizeof(lhs)) == 0;
+  }
+
+  friend bool operator!=(const InlineData& lhs, const InlineData& rhs) {
+    return !operator==(lhs, rhs);
+  }
+
+  // Returns true if the current instance is empty.
+  // The 'empty value' is an inlined data value of zero length.
+  bool is_empty() const { return rep_.tag() == 0; }
+
+  // Returns true if the current instance holds a tree value.
+  bool is_tree() const { return (rep_.tag() & 1) != 0; }
+
+  // Returns true if the current instance holds a cordz_info value.
+  // Requires the current instance to hold a tree value.
+  bool is_profiled() const {
+    assert(is_tree());
+    return rep_.cordz_info() != kNullCordzInfo;
+  }
+
+  // Returns true if either of the provided instances hold a cordz_info value.
+  // This method is more efficient than the equivalent `data1.is_profiled() ||
+  // data2.is_profiled()`. Requires both arguments to hold a tree.
+  static bool is_either_profiled(const InlineData& data1,
+                                 const InlineData& data2) {
+    assert(data1.is_tree() && data2.is_tree());
+    return (data1.rep_.cordz_info() | data2.rep_.cordz_info()) !=
+           kNullCordzInfo;
+  }
+
+  // Returns the cordz_info sampling instance for this instance, or nullptr
+  // if the current instance is not sampled and does not have CordzInfo data.
+  // Requires the current instance to hold a tree value.
+  CordzInfo* cordz_info() const {
+    assert(is_tree());
+    intptr_t info = static_cast<intptr_t>(absl::little_endian::ToHost64(
+        static_cast<uint64_t>(rep_.cordz_info())));
+    assert(info & 1);
+    return reinterpret_cast<CordzInfo*>(info - 1);
+  }
+
+  // Sets the current cordz_info sampling instance for this instance, or nullptr
+  // if the current instance is not sampled and does not have CordzInfo data.
+  // Requires the current instance to hold a tree value.
+  void set_cordz_info(CordzInfo* cordz_info) {
+    assert(is_tree());
+    uintptr_t info = reinterpret_cast<uintptr_t>(cordz_info) | 1;
+    rep_.set_cordz_info(
+        static_cast<cordz_info_t>(absl::little_endian::FromHost64(info)));
+  }
+
+  // Resets the current cordz_info to null / empty.
+  void clear_cordz_info() {
+    assert(is_tree());
+    rep_.set_cordz_info(kNullCordzInfo);
+  }
+
+  // Returns a read only pointer to the character data inside this instance.
+  // Requires the current instance to hold inline data.
+  const char* as_chars() const {
+    assert(!is_tree());
+    return rep_.as_chars();
+  }
+
+  // Returns a mutable pointer to the character data inside this instance.
+  // Should be used for 'write only' operations setting an inlined value.
+  // Applications can set the value of inlined data either before or after
+  // setting the inlined size, i.e., both of the below are valid:
+  //
+  //   // Set inlined data and inline size
+  //   memcpy(data_.as_chars(), data, size);
+  //   data_.set_inline_size(size);
+  //
+  //   // Set inlined size and inline data
+  //   data_.set_inline_size(size);
+  //   memcpy(data_.as_chars(), data, size);
+  //
+  // It's an error to read from the returned pointer without a preceding write
+  // if the current instance does not hold inline data, i.e.: is_tree() == true.
+  char* as_chars() { return rep_.as_chars(); }
+
+  // Returns the tree value of this value.
+  // Requires the current instance to hold a tree value.
+  CordRep* as_tree() const {
+    assert(is_tree());
+    return rep_.tree();
+  }
+
+  void set_inline_data(const char* data, size_t n) {
+    ABSL_ASSERT(n <= kMaxInline);
+    rep_.set_tag(static_cast<int8_t>(n << 1));
+    SmallMemmove<true>(rep_.as_chars(), data, n);
+  }
+
+  void CopyInlineToString(std::string* dst) const {
+    assert(!is_tree());
+    // As Cord can store only 15 bytes it is smaller than std::string's
+    // small string optimization buffer size. Therefore we will always trigger
+    // the fast assign short path.
+    //
+    // Copying with a size equal to the maximum allows more efficient, wider
+    // stores to be used and no branching.
+    dst->assign(rep_.SanitizerSafeCopy().as_chars(), kMaxInline);
+    // After the copy we then change the size and put in a 0 byte.
+    dst->erase(inline_size());
+  }
+
+  void copy_max_inline_to(char* dst) const {
+    assert(!is_tree());
+    memcpy(dst, rep_.SanitizerSafeCopy().as_chars(), kMaxInline);
+  }
+
+  // Initialize this instance to holding the tree value `rep`,
+  // initializing the cordz_info to null, i.e.: 'not profiled'.
+  void make_tree(CordRep* rep) {
+    rep_.make_tree(rep);
+  }
+
+  // Set the tree value of this instance to 'rep`.
+  // Requires the current instance to already hold a tree value.
+  // Does not affect the value of cordz_info.
+  void set_tree(CordRep* rep) {
+    assert(is_tree());
+    rep_.set_tree(rep);
+  }
+
+  // Returns the size of the inlined character data inside this instance.
+  // Requires the current instance to hold inline data.
+  size_t inline_size() const { return rep_.inline_size(); }
+
+  // Sets the size of the inlined character data inside this instance.
+  // Requires `size` to be <= kMaxInline.
+  // See the documentation on 'as_chars()' for more information and examples.
+  void set_inline_size(size_t size) {
+    rep_.set_inline_size(size);
+  }
+
+  // Compares 'this' inlined data  with rhs. The comparison is a straightforward
+  // lexicographic comparison. `Compare()` returns values as follows:
+  //
+  //   -1  'this' InlineData instance is smaller
+  //    0  the InlineData instances are equal
+  //    1  'this' InlineData instance larger
+  int Compare(const InlineData& rhs) const {
+    return Compare(rep_.SanitizerSafeCopy(), rhs.rep_.SanitizerSafeCopy());
+  }
+
+ private:
+  struct Rep {
+    // See cordz_info_t for forced alignment and size of `cordz_info` details.
+    struct AsTree {
+      explicit constexpr AsTree(absl::cord_internal::CordRep* tree)
+          : rep(tree) {}
+      cordz_info_t cordz_info = kNullCordzInfo;
+      absl::cord_internal::CordRep* rep;
+    };
+
+    explicit Rep(DefaultInitType) {}
+    constexpr Rep() : data{0} {}
+    constexpr Rep(const Rep&) = default;
+    constexpr Rep& operator=(const Rep&) = default;
+
+    explicit constexpr Rep(CordRep* rep) : as_tree(rep) {}
+
+    explicit constexpr Rep(absl::string_view chars)
+        : data{static_cast<char>((chars.size() << 1)),
+               GetOrNull(chars, 0),
+               GetOrNull(chars, 1),
+               GetOrNull(chars, 2),
+               GetOrNull(chars, 3),
+               GetOrNull(chars, 4),
+               GetOrNull(chars, 5),
+               GetOrNull(chars, 6),
+               GetOrNull(chars, 7),
+               GetOrNull(chars, 8),
+               GetOrNull(chars, 9),
+               GetOrNull(chars, 10),
+               GetOrNull(chars, 11),
+               GetOrNull(chars, 12),
+               GetOrNull(chars, 13),
+               GetOrNull(chars, 14)} {}
+
+    int8_t tag() const { return reinterpret_cast<const int8_t*>(this)[0]; }
+    void set_tag(int8_t rhs) { reinterpret_cast<int8_t*>(this)[0] = rhs; }
+
+    char* as_chars() { return this->data + 1; }
+    const char* as_chars() const { return this->data + 1; }
+
+    bool is_tree() const { return (this->tag() & 1) != 0; }
+
+    size_t inline_size() const {
+      ABSL_ASSERT(!this->is_tree());
+      return static_cast<size_t>(this->tag()) >> 1;
+    }
+
+    void set_inline_size(size_t size) {
+      ABSL_ASSERT(size <= kMaxInline);
+      this->set_tag(static_cast<int8_t>(size << 1));
+    }
+
+    CordRep* tree() const { return this->as_tree.rep; }
+    void set_tree(CordRep* rhs) { this->as_tree.rep = rhs; }
+
+    cordz_info_t cordz_info() const { return this->as_tree.cordz_info; }
+    void set_cordz_info(cordz_info_t rhs) { this->as_tree.cordz_info = rhs; }
+
+    void make_tree(CordRep* tree) {
+      this->as_tree.rep = tree;
+      this->as_tree.cordz_info = kNullCordzInfo;
+    }
+
+    constexpr const Rep& SanitizerSafeCopy() const { return *this; }
+
+    // If the data has length <= kMaxInline, we store it in `data`, and
+    // store the size in the first char of `data` shifted left + 1.
+    // Else we store it in a tree and store a pointer to that tree in
+    // `as_tree.rep` with a tagged pointer to make `tag() & 1` non zero.
+    union {
+      char data[kMaxInline + 1];
+      AsTree as_tree;
+    };
+
+    // TODO(b/145829486): see swap(InlineData, InlineData) for more info.
+    inline void SwapValue(Rep rhs, Rep& refrhs) {
+      memcpy(&refrhs, this, sizeof(*this));
+      memcpy(this, &rhs, sizeof(*this));
+    }
+  };
+
+  // Private implementation of `Compare()`
+  static inline int Compare(const Rep& lhs, const Rep& rhs) {
+    uint64_t x, y;
+    memcpy(&x, lhs.as_chars(), sizeof(x));
+    memcpy(&y, rhs.as_chars(), sizeof(y));
+    if (x == y) {
+      memcpy(&x, lhs.as_chars() + 7, sizeof(x));
+      memcpy(&y, rhs.as_chars() + 7, sizeof(y));
+      if (x == y) {
+        if (lhs.inline_size() == rhs.inline_size()) return 0;
+        return lhs.inline_size() < rhs.inline_size() ? -1 : 1;
+      }
+    }
+    x = absl::big_endian::FromHost64(x);
+    y = absl::big_endian::FromHost64(y);
+    return x < y ? -1 : 1;
+  }
+
+  Rep rep_;
+};
+
+static_assert(sizeof(InlineData) == kMaxInline + 1);
+
+inline CordRepSubstring* CordRep::substring() {
+  assert(IsSubstring());
+  return static_cast<CordRepSubstring*>(this);
+}
+
+inline const CordRepSubstring* CordRep::substring() const {
+  assert(IsSubstring());
+  return static_cast<const CordRepSubstring*>(this);
+}
+
+inline CordRepExternal* CordRep::external() {
+  assert(IsExternal());
+  return static_cast<CordRepExternal*>(this);
+}
+
+inline const CordRepExternal* CordRep::external() const {
+  assert(IsExternal());
+  return static_cast<const CordRepExternal*>(this);
+}
+
+inline CordRep* CordRep::Ref(CordRep* rep) {
+  // ABSL_ASSUME is a workaround for
+  // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105585
+  ABSL_ASSUME(rep != nullptr);
+  rep->refcount.Increment();
+  return rep;
+}
+
+inline void CordRep::Unref(CordRep* rep) {
+  assert(rep != nullptr);
+  if (ABSL_PREDICT_FALSE(!rep->refcount.DecrementExpectHighRefcount())) {
+    Destroy(rep);
+  }
+}
+
+inline void swap(InlineData& lhs, InlineData& rhs) noexcept {
+  // TODO(b/145829486): `std::swap(lhs.rep_, rhs.rep_)` results in bad codegen
+  // on clang, spilling the temporary swap value on the stack. Since `Rep` is
+  // trivial, we can make clang DTRT by calling a hand-rolled `SwapValue` where
+  // we pass `rhs` both by value (register allocated) and by reference. The IR
+  // then folds and inlines correctly into an optimized swap without spill.
+  lhs.rep_.SwapValue(rhs.rep_, rhs.rep_);
+}
+
+}  // namespace cord_internal
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+#endif  // ABSL_STRINGS_INTERNAL_CORD_INTERNAL_H_
diff --git a/absl/strings/internal/cord_rep_btree.cc b/absl/strings/internal/cord_rep_btree.cc
new file mode 100644
index 00000000000..7de2a137894
--- /dev/null
+++ b/absl/strings/internal/cord_rep_btree.cc
@@ -0,0 +1,1244 @@
+// Copyright 2021 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/strings/internal/cord_rep_btree.h"
+
+#include <algorithm>
+#include <atomic>
+#include <cassert>
+#include <cstddef>
+#include <cstdint>
+#include <cstring>
+#include <iostream>
+#include <ostream>
+#include <string>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/base/optimization.h"
+#include "absl/strings/internal/cord_data_edge.h"
+#include "absl/strings/internal/cord_internal.h"
+#include "absl/strings/internal/cord_rep_consume.h"
+#include "absl/strings/internal/cord_rep_flat.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/string_view.h"
+#include "absl/types/span.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+
+namespace {
+
+using NodeStack = CordRepBtree * [CordRepBtree::kMaxDepth];
+using EdgeType = CordRepBtree::EdgeType;
+using OpResult = CordRepBtree::OpResult;
+using CopyResult = CordRepBtree::CopyResult;
+
+constexpr auto kFront = CordRepBtree::kFront;
+constexpr auto kBack = CordRepBtree::kBack;
+
+ABSL_CONST_INIT std::atomic<bool> cord_btree_exhaustive_validation(false);
+
+// Implementation of the various 'Dump' functions.
+// Prints the entire tree structure or 'rep'. External callers should
+// not specify 'depth' and leave it to its default (0) value.
+// Rep may be a CordRepBtree tree, or a SUBSTRING / EXTERNAL / FLAT node.
+void DumpAll(const CordRep* rep,
+             bool include_contents,
+             std::ostream& stream,
+             size_t depth = 0) {
+  // Allow for full height trees + substring -> flat / external nodes.
+  assert(depth <= CordRepBtree::kMaxDepth + 2);
+  std::string sharing = const_cast<CordRep*>(rep)->refcount.IsOne()
+                            ? std::string("Private")
+                            : absl::StrCat("Shared(", rep->refcount.Get(), ")");
+  std::string sptr = absl::StrCat("0x", absl::Hex(rep));
+
+  // Dumps the data contents of `rep` if `include_contents` is true.
+  // Always emits a new line character.
+  auto maybe_dump_data = [&stream, include_contents](const CordRep* r) {
+    if (include_contents) {
+      // Allow for up to 60 wide display of content data, which with some
+      // indentation and prefix / labels keeps us within roughly 80-100 wide.
+      constexpr size_t kMaxDataLength = 60;
+      stream << ", data = \""
+             << EdgeData(r).substr(0, kMaxDataLength)
+             << (r->length > kMaxDataLength ? "\"..." : "\"");
+    }
+    stream << '\n';
+  };
+
+  // For each level, we print the 'shared/private' state and the rep pointer,
+  // indented by two spaces per recursive depth.
+  stream << std::string(depth * 2, ' ') << sharing << " (" << sptr << ") ";
+
+  if (rep->IsBtree()) {
+    const CordRepBtree* node = rep->btree();
+    std::string label =
+        node->height() ? absl::StrCat("Node(", node->height(), ")") : "Leaf";
+    stream << label << ", len = " << node->length
+           << ", begin = " << node->begin() << ", end = " << node->end()
+           << "\n";
+    for (CordRep* edge : node->Edges()) {
+      DumpAll(edge, include_contents, stream, depth + 1);
+    }
+  } else if (rep->tag == SUBSTRING) {
+    const CordRepSubstring* substring = rep->substring();
+    stream << "Substring, len = " << rep->length
+           << ", start = " << substring->start;
+    maybe_dump_data(rep);
+    DumpAll(substring->child, include_contents, stream, depth + 1);
+  } else if (rep->tag >= FLAT) {
+    stream << "Flat, len = " << rep->length
+           << ", cap = " << rep->flat()->Capacity();
+    maybe_dump_data(rep);
+  } else if (rep->tag == EXTERNAL) {
+    stream << "Extn, len = " << rep->length;
+    maybe_dump_data(rep);
+  }
+}
+
+// TODO(b/192061034): add 'bytes to copy' logic to avoid large slop on substring
+// small data out of large reps, and general efficiency of 'always copy small
+// data'. Consider making this a cord rep internal library function.
+CordRepSubstring* CreateSubstring(CordRep* rep, size_t offset, size_t n) {
+  assert(n != 0);
+  assert(offset + n <= rep->length);
+  assert(offset != 0 || n != rep->length);
+
+  if (rep->tag == SUBSTRING) {
+    CordRepSubstring* substring = rep->substring();
+    offset += substring->start;
+    rep = CordRep::Ref(substring->child);
+    CordRep::Unref(substring);
+  }
+  assert(rep->IsExternal() || rep->IsFlat());
+  CordRepSubstring* substring = new CordRepSubstring();
+  substring->length = n;
+  substring->tag = SUBSTRING;
+  substring->start = offset;
+  substring->child = rep;
+  return substring;
+}
+
+// TODO(b/192061034): consider making this a cord rep library function.
+inline CordRep* MakeSubstring(CordRep* rep, size_t offset, size_t n) {
+  if (n == rep->length) return rep;
+  if (n == 0) return CordRep::Unref(rep), nullptr;
+  return CreateSubstring(rep, offset, n);
+}
+
+// TODO(b/192061034): consider making this a cord rep library function.
+inline CordRep* MakeSubstring(CordRep* rep, size_t offset) {
+  if (offset == 0) return rep;
+  return CreateSubstring(rep, offset, rep->length - offset);
+}
+
+// Resizes `edge` to the provided `length`. Adopts a reference on `edge`.
+// This method directly returns `edge` if `length` equals `edge->length`.
+// If `is_mutable` is set to true, this function may return `edge` with
+// `edge->length` set to the new length depending on the type and size of
+// `edge`. Otherwise, this function returns a new CordRepSubstring value.
+// Requires `length > 0 && length <= edge->length`.
+CordRep* ResizeEdge(CordRep* edge, size_t length, bool is_mutable) {
+  assert(length > 0);
+  assert(length <= edge->length);
+  assert(IsDataEdge(edge));
+  if (length >= edge->length) return edge;
+
+  if (is_mutable && (edge->tag >= FLAT || edge->tag == SUBSTRING)) {
+    edge->length = length;
+    return edge;
+  }
+
+  return CreateSubstring(edge, 0, length);
+}
+
+template <EdgeType edge_type>
+inline absl::string_view Consume(absl::string_view s, size_t n) {
+  return edge_type == kBack ? s.substr(n) : s.substr(0, s.size() - n);
+}
+
+template <EdgeType edge_type>
+inline absl::string_view Consume(char* dst, absl::string_view s, size_t n) {
+  if (edge_type == kBack) {
+    memcpy(dst, s.data(), n);
+    return s.substr(n);
+  } else {
+    const size_t offset = s.size() - n;
+    memcpy(dst, s.data() + offset, n);
+    return s.substr(0, offset);
+  }
+}
+
+// Known issue / optimization weirdness: the store associated with the
+// decrement introduces traffic between cpus (even if the result of that
+// traffic does nothing), making this faster than a single call to
+// refcount.Decrement() checking the zero refcount condition.
+template <typename R, typename Fn>
+inline void FastUnref(R* r, Fn&& fn) {
+  if (r->refcount.IsOne()) {
+    fn(r);
+  } else if (!r->refcount.DecrementExpectHighRefcount()) {
+    fn(r);
+  }
+}
+
+
+void DeleteSubstring(CordRepSubstring* substring) {
+  CordRep* rep = substring->child;
+  if (!rep->refcount.Decrement()) {
+    if (rep->tag >= FLAT) {
+      CordRepFlat::Delete(rep->flat());
+    } else {
+      assert(rep->tag == EXTERNAL);
+      CordRepExternal::Delete(rep->external());
+    }
+  }
+  delete substring;
+}
+
+// Deletes a leaf node data edge. Requires `IsDataEdge(rep)`.
+void DeleteLeafEdge(CordRep* rep) {
+  assert(IsDataEdge(rep));
+  if (rep->tag >= FLAT) {
+    CordRepFlat::Delete(rep->flat());
+  } else if (rep->tag == EXTERNAL) {
+    CordRepExternal::Delete(rep->external());
+  } else {
+    DeleteSubstring(rep->substring());
+  }
+}
+
+// StackOperations contains the logic to build a left-most or right-most stack
+// (leg) down to the leaf level of a btree, and 'unwind' / 'Finalize' methods to
+// propagate node changes up the stack.
+template <EdgeType edge_type>
+struct StackOperations {
+  // Returns true if the node at 'depth' is not shared, i.e. has a refcount
+  // of one and all of its parent nodes have a refcount of one.
+  inline bool owned(int depth) const { return depth < share_depth; }
+
+  // Returns the node at 'depth'.
+  inline CordRepBtree* node(int depth) const { return stack[depth]; }
+
+  // Builds a `depth` levels deep stack starting at `tree` recording which nodes
+  // are private in the form of the 'share depth' where nodes are shared.
+  inline CordRepBtree* BuildStack(CordRepBtree* tree, int depth) {
+    assert(depth <= tree->height());
+    int current_depth = 0;
+    while (current_depth < depth && tree->refcount.IsOne()) {
+      stack[current_depth++] = tree;
+      tree = tree->Edge(edge_type)->btree();
+    }
+    share_depth = current_depth + (tree->refcount.IsOne() ? 1 : 0);
+    while (current_depth < depth) {
+      stack[current_depth++] = tree;
+      tree = tree->Edge(edge_type)->btree();
+    }
+    return tree;
+  }
+
+  // Builds a stack with the invariant that all nodes are private owned / not
+  // shared. This is used in iterative updates where a previous propagation
+  // guaranteed all nodes are owned / private.
+  inline void BuildOwnedStack(CordRepBtree* tree, int height) {
+    assert(height <= CordRepBtree::kMaxHeight);
+    int depth = 0;
+    while (depth < height) {
+      assert(tree->refcount.IsOne());
+      stack[depth++] = tree;
+      tree = tree->Edge(edge_type)->btree();
+    }
+    assert(tree->refcount.IsOne());
+    share_depth = depth + 1;
+  }
+
+  // Processes the final 'top level' result action for the tree.
+  // See the 'Action' enum for the various action implications.
+  static inline CordRepBtree* Finalize(CordRepBtree* tree, OpResult result) {
+    switch (result.action) {
+      case CordRepBtree::kPopped:
+        tree = edge_type == kBack ? CordRepBtree::New(tree, result.tree)
+                                  : CordRepBtree::New(result.tree, tree);
+        if (ABSL_PREDICT_FALSE(tree->height() > CordRepBtree::kMaxHeight)) {
+          tree = CordRepBtree::Rebuild(tree);
+          ABSL_RAW_CHECK(tree->height() <= CordRepBtree::kMaxHeight,
+                         "Max height exceeded");
+        }
+        return tree;
+      case CordRepBtree::kCopied:
+        CordRep::Unref(tree);
+        [[fallthrough]];
+      case CordRepBtree::kSelf:
+        return result.tree;
+    }
+    ABSL_UNREACHABLE();
+    return result.tree;
+  }
+
+  // Propagate the action result in 'result' up into all nodes of the stack
+  // starting at depth 'depth'. 'length' contains the extra length of data that
+  // was added at the lowest level, and is updated into all nodes of the stack.
+  // See the 'Action' enum for the various action implications.
+  // If 'propagate' is true, then any copied node values are updated into the
+  // stack, which is used for iterative processing on the same stack.
+  template <bool propagate = false>
+  inline CordRepBtree* Unwind(CordRepBtree* tree, int depth, size_t length,
+                              OpResult result) {
+    // TODO(mvels): revisit the below code to check if 3 loops with 3
+    // (incremental) conditions is faster than 1 loop with a switch.
+    // Benchmarking and perf recordings indicate the loop with switch is
+    // fastest, likely because of indirect jumps on the tight case values and
+    // dense branches. But it's worth considering 3 loops, as the `action`
+    // transitions are mono directional. E.g.:
+    //   while (action == kPopped) {
+    //     ...
+    //   }
+    //   while (action == kCopied) {
+    //     ...
+    //   }
+    //   ...
+    // We also  found that an "if () do {}" loop here seems faster, possibly
+    // because it allows the branch predictor more granular heuristics on
+    // 'single leaf' (`depth` == 0) and 'single depth' (`depth` == 1) cases
+    // which appear to be the most common use cases.
+    if (depth != 0) {
+      do {
+        CordRepBtree* node = stack[--depth];
+        const bool owned = depth < share_depth;
+        switch (result.action) {
+          case CordRepBtree::kPopped:
+            assert(!propagate);
+            result = node->AddEdge<edge_type>(owned, result.tree, length);
+            break;
+          case CordRepBtree::kCopied:
+            result = node->SetEdge<edge_type>(owned, result.tree, length);
+            if (propagate) stack[depth] = result.tree;
+            break;
+          case CordRepBtree::kSelf:
+            node->length += length;
+            while (depth > 0) {
+              node = stack[--depth];
+              node->length += length;
+            }
+            return node;
+        }
+      } while (depth > 0);
+    }
+    return Finalize(tree, result);
+  }
+
+  // Invokes `Unwind` with `propagate=true` to update the stack node values.
+  inline CordRepBtree* Propagate(CordRepBtree* tree, int depth, size_t length,
+                                 OpResult result) {
+    return Unwind</*propagate=*/true>(tree, depth, length, result);
+  }
+
+  // `share_depth` contains the depth at which the nodes in the stack become
+  // shared. I.e., if the top most level is shared (i.e.: `!refcount.IsOne()`),
+  // then `share_depth` is 0. If the 2nd node is shared (and implicitly all
+  // nodes below that) then `share_depth` is 1, etc. A `share_depth` greater
+  // than the depth of the stack indicates that none of the nodes in the stack
+  // are shared.
+  int share_depth;
+
+  NodeStack stack;
+};
+
+}  // namespace
+
+void SetCordBtreeExhaustiveValidation(bool do_exaustive_validation) {
+  cord_btree_exhaustive_validation.store(do_exaustive_validation,
+                                         std::memory_order_relaxed);
+}
+
+bool IsCordBtreeExhaustiveValidationEnabled() {
+  return cord_btree_exhaustive_validation.load(std::memory_order_relaxed);
+}
+
+void CordRepBtree::Dump(const CordRep* rep, absl::string_view label,
+                        bool include_contents, std::ostream& stream) {
+  stream << "===================================\n";
+  if (!label.empty()) {
+    stream << label << '\n';
+    stream << "-----------------------------------\n";
+  }
+  if (rep) {
+    DumpAll(rep, include_contents, stream);
+  } else {
+    stream << "NULL\n";
+  }
+}
+
+void CordRepBtree::Dump(const CordRep* rep, absl::string_view label,
+                        std::ostream& stream) {
+  Dump(rep, label, false, stream);
+}
+
+void CordRepBtree::Dump(const CordRep* rep, std::ostream& stream) {
+  Dump(rep, absl::string_view(), false, stream);
+}
+
+template <size_t size>
+static void DestroyTree(CordRepBtree* tree) {
+  for (CordRep* node : tree->Edges()) {
+    if (node->refcount.Decrement()) continue;
+    for (CordRep* edge : node->btree()->Edges()) {
+      if (edge->refcount.Decrement()) continue;
+      if (size == 1) {
+        DeleteLeafEdge(edge);
+      } else {
+        CordRepBtree::Destroy(edge->btree());
+      }
+    }
+    CordRepBtree::Delete(node->btree());
+  }
+  CordRepBtree::Delete(tree);
+}
+
+void CordRepBtree::Destroy(CordRepBtree* tree) {
+  switch (tree->height()) {
+    case 0:
+      for (CordRep* edge : tree->Edges()) {
+        if (!edge->refcount.Decrement()) {
+          DeleteLeafEdge(edge);
+        }
+      }
+      return CordRepBtree::Delete(tree);
+    case 1:
+      return DestroyTree<1>(tree);
+    default:
+      return DestroyTree<2>(tree);
+  }
+}
+
+bool CordRepBtree::IsValid(const CordRepBtree* tree, bool shallow) {
+#define NODE_CHECK_VALID(x)                                           \
+  if (!(x)) {                                                         \
+    ABSL_RAW_LOG(ERROR, "CordRepBtree::CheckValid() FAILED: %s", #x); \
+    return false;                                                     \
+  }
+#define NODE_CHECK_EQ(x, y)                                                    \
+  if ((x) != (y)) {                                                            \
+    ABSL_RAW_LOG(ERROR,                                                        \
+                 "CordRepBtree::CheckValid() FAILED: %s != %s (%s vs %s)", #x, \
+                 #y, absl::StrCat(x).c_str(), absl::StrCat(y).c_str());        \
+    return false;                                                              \
+  }
+
+  NODE_CHECK_VALID(tree != nullptr);
+  NODE_CHECK_VALID(tree->IsBtree());
+  NODE_CHECK_VALID(tree->height() <= kMaxHeight);
+  NODE_CHECK_VALID(tree->begin() < tree->capacity());
+  NODE_CHECK_VALID(tree->end() <= tree->capacity());
+  NODE_CHECK_VALID(tree->begin() <= tree->end());
+  size_t child_length = 0;
+  for (CordRep* edge : tree->Edges()) {
+    NODE_CHECK_VALID(edge != nullptr);
+    if (tree->height() > 0) {
+      NODE_CHECK_VALID(edge->IsBtree());
+      NODE_CHECK_VALID(edge->btree()->height() == tree->height() - 1);
+    } else {
+      NODE_CHECK_VALID(IsDataEdge(edge));
+    }
+    child_length += edge->length;
+  }
+  NODE_CHECK_EQ(child_length, tree->length);
+  if ((!shallow || IsCordBtreeExhaustiveValidationEnabled()) &&
+      tree->height() > 0) {
+    for (CordRep* edge : tree->Edges()) {
+      if (!IsValid(edge->btree(), shallow)) return false;
+    }
+  }
+  return true;
+
+#undef NODE_CHECK_VALID
+#undef NODE_CHECK_EQ
+}
+
+#ifndef NDEBUG
+
+CordRepBtree* CordRepBtree::AssertValid(CordRepBtree* tree, bool shallow) {
+  if (!IsValid(tree, shallow)) {
+    Dump(tree, "CordRepBtree validation failed:", false, std::cout);
+    ABSL_RAW_LOG(FATAL, "CordRepBtree::CheckValid() FAILED");
+  }
+  return tree;
+}
+
+const CordRepBtree* CordRepBtree::AssertValid(const CordRepBtree* tree,
+                                              bool shallow) {
+  if (!IsValid(tree, shallow)) {
+    Dump(tree, "CordRepBtree validation failed:", false, std::cout);
+    ABSL_RAW_LOG(FATAL, "CordRepBtree::CheckValid() FAILED");
+  }
+  return tree;
+}
+
+#endif  // NDEBUG
+
+template <EdgeType edge_type>
+inline OpResult CordRepBtree::AddEdge(bool owned, CordRep* edge, size_t delta) {
+  if (size() >= kMaxCapacity) return {New(edge), kPopped};
+  OpResult result = ToOpResult(owned);
+  result.tree->Add<edge_type>(edge);
+  result.tree->length += delta;
+  return result;
+}
+
+template <EdgeType edge_type>
+OpResult CordRepBtree::SetEdge(bool owned, CordRep* edge, size_t delta) {
+  OpResult result;
+  const size_t idx = index(edge_type);
+  if (owned) {
+    result = {this, kSelf};
+    CordRep::Unref(edges_[idx]);
+  } else {
+    // Create a copy containing all unchanged edges. Unchanged edges are the
+    // open interval [begin, back) or [begin + 1, end) depending on `edge_type`.
+    // We conveniently cover both case using a constexpr `shift` being 0 or 1
+    // as `end :== back + 1`.
+    result = {CopyRaw(length), kCopied};
+    constexpr int shift = edge_type == kFront ? 1 : 0;
+    for (CordRep* r : Edges(begin() + shift, back() + shift)) {
+      CordRep::Ref(r);
+    }
+  }
+  result.tree->edges_[idx] = edge;
+  result.tree->length += delta;
+  return result;
+}
+
+template <EdgeType edge_type>
+CordRepBtree* CordRepBtree::AddCordRep(CordRepBtree* tree, CordRep* rep) {
+  const int depth = tree->height();
+  const size_t length = rep->length;
+  StackOperations<edge_type> ops;
+  CordRepBtree* leaf = ops.BuildStack(tree, depth);
+  const OpResult result =
+      leaf->AddEdge<edge_type>(ops.owned(depth), rep, length);
+  return ops.Unwind(tree, depth, length, result);
+}
+
+template <>
+CordRepBtree* CordRepBtree::NewLeaf<kBack>(absl::string_view data,
+                                           size_t extra) {
+  CordRepBtree* leaf = CordRepBtree::New(0);
+  size_t length = 0;
+  size_t end = 0;
+  const size_t cap = leaf->capacity();
+  while (!data.empty() && end != cap) {
+    auto* flat = CordRepFlat::New(data.length() + extra);
+    flat->length = (std::min)(data.length(), flat->Capacity());
+    length += flat->length;
+    leaf->edges_[end++] = flat;
+    data = Consume<kBack>(flat->Data(), data, flat->length);
+  }
+  leaf->length = length;
+  leaf->set_end(end);
+  return leaf;
+}
+
+template <>
+CordRepBtree* CordRepBtree::NewLeaf<kFront>(absl::string_view data,
+                                            size_t extra) {
+  CordRepBtree* leaf = CordRepBtree::New(0);
+  size_t length = 0;
+  size_t begin = leaf->capacity();
+  leaf->set_end(leaf->capacity());
+  while (!data.empty() && begin != 0) {
+    auto* flat = CordRepFlat::New(data.length() + extra);
+    flat->length = (std::min)(data.length(), flat->Capacity());
+    length += flat->length;
+    leaf->edges_[--begin] = flat;
+    data = Consume<kFront>(flat->Data(), data, flat->length);
+  }
+  leaf->length = length;
+  leaf->set_begin(begin);
+  return leaf;
+}
+
+template <>
+absl::string_view CordRepBtree::AddData<kBack>(absl::string_view data,
+                                               size_t extra) {
+  assert(!data.empty());
+  assert(size() < capacity());
+  AlignBegin();
+  const size_t cap = capacity();
+  do {
+    CordRepFlat* flat = CordRepFlat::New(data.length() + extra);
+    const size_t n = (std::min)(data.length(), flat->Capacity());
+    flat->length = n;
+    edges_[fetch_add_end(1)] = flat;
+    data = Consume<kBack>(flat->Data(), data, n);
+  } while (!data.empty() && end() != cap);
+  return data;
+}
+
+template <>
+absl::string_view CordRepBtree::AddData<kFront>(absl::string_view data,
+                                                size_t extra) {
+  assert(!data.empty());
+  assert(size() < capacity());
+  AlignEnd();
+  do {
+    CordRepFlat* flat = CordRepFlat::New(data.length() + extra);
+    const size_t n = (std::min)(data.length(), flat->Capacity());
+    flat->length = n;
+    edges_[sub_fetch_begin(1)] = flat;
+    data = Consume<kFront>(flat->Data(), data, n);
+  } while (!data.empty() && begin() != 0);
+  return data;
+}
+
+template <EdgeType edge_type>
+CordRepBtree* CordRepBtree::AddData(CordRepBtree* tree, absl::string_view data,
+                                    size_t extra) {
+  if (ABSL_PREDICT_FALSE(data.empty())) return tree;
+
+  const size_t original_data_size = data.size();
+  int depth = tree->height();
+  StackOperations<edge_type> ops;
+  CordRepBtree* leaf = ops.BuildStack(tree, depth);
+
+  // If there is capacity in the last edge, append as much data
+  // as possible into this last edge.
+  if (leaf->size() < leaf->capacity()) {
+    OpResult result = leaf->ToOpResult(ops.owned(depth));
+    data = result.tree->AddData<edge_type>(data, extra);
+    if (data.empty()) {
+      result.tree->length += original_data_size;
+      return ops.Unwind(tree, depth, original_data_size, result);
+    }
+
+    // We added some data into this leaf, but not all. Propagate the added
+    // length to the top most node, and rebuild the stack with any newly copied
+    // or updated nodes. From this point on, the path (leg) from the top most
+    // node to the right-most node towards the leaf node is privately owned.
+    size_t delta = original_data_size - data.size();
+    assert(delta > 0);
+    result.tree->length += delta;
+    tree = ops.Propagate(tree, depth, delta, result);
+    ops.share_depth = depth + 1;
+  }
+
+  // We were unable to append all data into the existing right-most leaf node.
+  // This means all remaining data must be put into (a) new leaf node(s) which
+  // we append to the tree. To make this efficient, we iteratively build full
+  // leaf nodes from `data` until the created leaf contains all remaining data.
+  // We utilize the `Unwind` method to merge the created leaf into the first
+  // level towards root that has capacity. On each iteration with remaining
+  // data, we rebuild the stack in the knowledge that right-most nodes are
+  // privately owned after the first `Unwind` completes.
+  for (;;) {
+    OpResult result = {CordRepBtree::NewLeaf<edge_type>(data, extra), kPopped};
+    if (result.tree->length == data.size()) {
+      return ops.Unwind(tree, depth, result.tree->length, result);
+    }
+    data = Consume<edge_type>(data, result.tree->length);
+    tree = ops.Unwind(tree, depth, result.tree->length, result);
+    depth = tree->height();
+    ops.BuildOwnedStack(tree, depth);
+  }
+}
+
+template <EdgeType edge_type>
+CordRepBtree* CordRepBtree::Merge(CordRepBtree* dst, CordRepBtree* src) {
+  assert(dst->height() >= src->height());
+
+  // Capture source length as we may consume / destroy `src`.
+  const size_t length = src->length;
+
+  // We attempt to merge `src` at its corresponding height in `dst`.
+  const int depth = dst->height() - src->height();
+  StackOperations<edge_type> ops;
+  CordRepBtree* merge_node = ops.BuildStack(dst, depth);
+
+  // If there is enough space in `merge_node` for all edges from `src`, add all
+  // edges to this node, making a fresh copy as needed if not privately owned.
+  // If `merge_node` does not have capacity for `src`, we rely on `Unwind` and
+  // `Finalize` to merge `src` into the first level towards `root` where there
+  // is capacity for another edge, or create a new top level node.
+  OpResult result;
+  if (merge_node->size() + src->size() <= kMaxCapacity) {
+    result = merge_node->ToOpResult(ops.owned(depth));
+    result.tree->Add<edge_type>(src->Edges());
+    result.tree->length += src->length;
+    if (src->refcount.IsOne()) {
+      Delete(src);
+    } else {
+      for (CordRep* edge : src->Edges()) CordRep::Ref(edge);
+      CordRepBtree::Unref(src);
+    }
+  } else {
+    result = {src, kPopped};
+  }
+
+  // Unless we merged at the top level (i.e.: src and dst are equal height),
+  // unwind the result towards the top level, and finalize the result.
+  if (depth) {
+    return ops.Unwind(dst, depth, length, result);
+  }
+  return ops.Finalize(dst, result);
+}
+
+CopyResult CordRepBtree::CopySuffix(size_t offset) {
+  assert(offset < this->length);
+
+  // As long as `offset` starts inside the last edge, we can 'drop' the current
+  // depth. For the most extreme example: if offset references the last data
+  // edge in the tree, there is only a single edge / path from the top of the
+  // tree to that last edge, so we can drop all the nodes except that edge.
+  // The fast path check for this is `back->length >= length - offset`.
+  int height = this->height();
+  CordRepBtree* node = this;
+  size_t len = node->length - offset;
+  CordRep* back = node->Edge(kBack);
+  while (back->length >= len) {
+    offset = back->length - len;
+    if (--height < 0) {
+      return {MakeSubstring(CordRep::Ref(back), offset), height};
+    }
+    node = back->btree();
+    back = node->Edge(kBack);
+  }
+  if (offset == 0) return {CordRep::Ref(node), height};
+
+  // Offset does not point into the last edge, so we span at least two edges.
+  // Find the index of offset with `IndexBeyond` which provides us the edge
+  // 'beyond' the offset if offset is not a clean starting point of an edge.
+  Position pos = node->IndexBeyond(offset);
+  CordRepBtree* sub = node->CopyToEndFrom(pos.index, len);
+  const CopyResult result = {sub, height};
+
+  // `pos.n` contains a non zero value if the offset is not an exact starting
+  // point of an edge. In this case, `pos.n` contains the 'trailing' amount of
+  // bytes of the edge preceding that in `pos.index`. We need to iteratively
+  // adjust the preceding edge with the 'broken' offset until we have a perfect
+  // start of the edge.
+  while (pos.n != 0) {
+    assert(pos.index >= 1);
+    const size_t begin = pos.index - 1;
+    sub->set_begin(begin);
+    CordRep* const edge = node->Edge(begin);
+
+    len = pos.n;
+    offset = edge->length - len;
+
+    if (--height < 0) {
+      sub->edges_[begin] = MakeSubstring(CordRep::Ref(edge), offset, len);
+      return result;
+    }
+
+    node = edge->btree();
+    pos = node->IndexBeyond(offset);
+
+    CordRepBtree* nsub = node->CopyToEndFrom(pos.index, len);
+    sub->edges_[begin] = nsub;
+    sub = nsub;
+  }
+  sub->set_begin(pos.index);
+  return result;
+}
+
+CopyResult CordRepBtree::CopyPrefix(size_t n, bool allow_folding) {
+  assert(n > 0);
+  assert(n <= this->length);
+
+  // As long as `n` does not exceed the length of the first edge, we can 'drop'
+  // the current depth. For the most extreme example: if we'd copy a 1 byte
+  // prefix from a tree, there is only a single edge / path from the top of the
+  // tree to the single data edge containing this byte, so we can drop all the
+  // nodes except the data node.
+  int height = this->height();
+  CordRepBtree* node = this;
+  CordRep* front = node->Edge(kFront);
+  if (allow_folding) {
+    while (front->length >= n) {
+      if (--height < 0) return {MakeSubstring(CordRep::Ref(front), 0, n), -1};
+      node = front->btree();
+      front = node->Edge(kFront);
+    }
+  }
+  if (node->length == n) return {CordRep::Ref(node), height};
+
+  // `n` spans at least two nodes, find the end point of the span.
+  Position pos = node->IndexOf(n);
+
+  // Create a partial copy of the node up to `pos.index`, with a defined length
+  // of `n`. Any 'partial last edge' is added further below as needed.
+  CordRepBtree* sub = node->CopyBeginTo(pos.index, n);
+  const CopyResult result = {sub, height};
+
+  // `pos.n` contains the 'offset inside the edge for IndexOf(n)'. As long as
+  // this is not zero, we don't have a 'clean cut', so we need to make a
+  // (partial) copy of that last edge, and repeat this until pos.n is zero.
+  while (pos.n != 0) {
+    size_t end = pos.index;
+    n = pos.n;
+
+    CordRep* edge = node->Edge(pos.index);
+    if (--height < 0) {
+      sub->edges_[end++] = MakeSubstring(CordRep::Ref(edge), 0, n);
+      sub->set_end(end);
+      AssertValid(result.edge->btree());
+      return result;
+    }
+
+    node = edge->btree();
+    pos = node->IndexOf(n);
+    CordRepBtree* nsub = node->CopyBeginTo(pos.index, n);
+    sub->edges_[end++] = nsub;
+    sub->set_end(end);
+    sub = nsub;
+  }
+  sub->set_end(pos.index);
+  AssertValid(result.edge->btree());
+  return result;
+}
+
+CordRep* CordRepBtree::ExtractFront(CordRepBtree* tree) {
+  CordRep* front = tree->Edge(tree->begin());
+  if (tree->refcount.IsOne()) {
+    Unref(tree->Edges(tree->begin() + 1, tree->end()));
+    CordRepBtree::Delete(tree);
+  } else {
+    CordRep::Ref(front);
+    CordRep::Unref(tree);
+  }
+  return front;
+}
+
+CordRepBtree* CordRepBtree::ConsumeBeginTo(CordRepBtree* tree, size_t end,
+                                           size_t new_length) {
+  assert(end <= tree->end());
+  if (tree->refcount.IsOne()) {
+    Unref(tree->Edges(end, tree->end()));
+    tree->set_end(end);
+    tree->length = new_length;
+  } else {
+    CordRepBtree* old = tree;
+    tree = tree->CopyBeginTo(end, new_length);
+    CordRep::Unref(old);
+  }
+  return tree;
+}
+
+CordRep* CordRepBtree::RemoveSuffix(CordRepBtree* tree, size_t n) {
+  // Check input and deal with trivial cases 'Remove all/none'
+  assert(tree != nullptr);
+  assert(n <= tree->length);
+  const size_t len = tree->length;
+  if (ABSL_PREDICT_FALSE(n == 0)) {
+    return tree;
+  }
+  if (ABSL_PREDICT_FALSE(n >= len)) {
+    CordRepBtree::Unref(tree);
+    return nullptr;
+  }
+
+  size_t length = len - n;
+  int height = tree->height();
+  bool is_mutable = tree->refcount.IsOne();
+
+  // Extract all top nodes which are reduced to size = 1
+  Position pos = tree->IndexOfLength(length);
+  while (pos.index == tree->begin()) {
+    CordRep* edge = ExtractFront(tree);
+    is_mutable &= edge->refcount.IsOne();
+    if (height-- == 0) return ResizeEdge(edge, length, is_mutable);
+    tree = edge->btree();
+    pos = tree->IndexOfLength(length);
+  }
+
+  // Repeat the following sequence traversing down the tree:
+  // - Crop the top node to the 'last remaining edge' adjusting length.
+  // - Set the length for down edges to the partial length in that last edge.
+  // - Repeat this until the last edge is 'included in full'
+  // - If we hit the data edge level, resize and return the last data edge
+  CordRepBtree* top = tree = ConsumeBeginTo(tree, pos.index + 1, length);
+  CordRep* edge = tree->Edge(pos.index);
+  length = pos.n;
+  while (length != edge->length) {
+    // ConsumeBeginTo guarantees `tree` is a clean, privately owned copy.
+    assert(tree->refcount.IsOne());
+    const bool edge_is_mutable = edge->refcount.IsOne();
+
+    if (height-- == 0) {
+      tree->edges_[pos.index] = ResizeEdge(edge, length, edge_is_mutable);
+      return AssertValid(top);
+    }
+
+    if (!edge_is_mutable) {
+      // We can't 'in place' remove any suffixes down this edge.
+      // Replace this edge with a prefix copy instead.
+      tree->edges_[pos.index] = edge->btree()->CopyPrefix(length, false).edge;
+      CordRep::Unref(edge);
+      return AssertValid(top);
+    }
+
+    // Move down one level, rinse repeat.
+    tree = edge->btree();
+    pos = tree->IndexOfLength(length);
+    tree = ConsumeBeginTo(edge->btree(), pos.index + 1, length);
+    edge = tree->Edge(pos.index);
+    length = pos.n;
+  }
+
+  return AssertValid(top);
+}
+
+CordRep* CordRepBtree::SubTree(size_t offset, size_t n) {
+  assert(n <= this->length);
+  assert(offset <= this->length - n);
+  if (ABSL_PREDICT_FALSE(n == 0)) return nullptr;
+
+  CordRepBtree* node = this;
+  int height = node->height();
+  Position front = node->IndexOf(offset);
+  CordRep* left = node->edges_[front.index];
+  while (front.n + n <= left->length) {
+    if (--height < 0) return MakeSubstring(CordRep::Ref(left), front.n, n);
+    node = left->btree();
+    front = node->IndexOf(front.n);
+    left = node->edges_[front.index];
+  }
+
+  const Position back = node->IndexBefore(front, n);
+  CordRep* const right = node->edges_[back.index];
+  assert(back.index > front.index);
+
+  // Get partial suffix and prefix entries.
+  CopyResult prefix;
+  CopyResult suffix;
+  if (height > 0) {
+    // Copy prefix and suffix of the boundary nodes.
+    prefix = left->btree()->CopySuffix(front.n);
+    suffix = right->btree()->CopyPrefix(back.n);
+
+    // If there is an edge between the prefix and suffix edges, then the tree
+    // must remain at its previous (full) height. If we have no edges between
+    // prefix and suffix edges, then the tree must be as high as either the
+    // suffix or prefix edges (which are collapsed to their minimum heights).
+    if (front.index + 1 == back.index) {
+      height = (std::max)(prefix.height, suffix.height) + 1;
+    }
+
+    // Raise prefix and suffixes to the new tree height.
+    for (int h = prefix.height + 1; h < height; ++h) {
+      prefix.edge = CordRepBtree::New(prefix.edge);
+    }
+    for (int h = suffix.height + 1; h < height; ++h) {
+      suffix.edge = CordRepBtree::New(suffix.edge);
+    }
+  } else {
+    // Leaf node, simply take substrings for prefix and suffix.
+    prefix = CopyResult{MakeSubstring(CordRep::Ref(left), front.n), -1};
+    suffix = CopyResult{MakeSubstring(CordRep::Ref(right), 0, back.n), -1};
+  }
+
+  // Compose resulting tree.
+  CordRepBtree* sub = CordRepBtree::New(height);
+  size_t end = 0;
+  sub->edges_[end++] = prefix.edge;
+  for (CordRep* r : node->Edges(front.index + 1, back.index)) {
+    sub->edges_[end++] = CordRep::Ref(r);
+  }
+  sub->edges_[end++] = suffix.edge;
+  sub->set_end(end);
+  sub->length = n;
+  return AssertValid(sub);
+}
+
+CordRepBtree* CordRepBtree::MergeTrees(CordRepBtree* left,
+                                       CordRepBtree* right) {
+  return left->height() >= right->height() ? Merge<kBack>(left, right)
+                                           : Merge<kFront>(right, left);
+}
+
+bool CordRepBtree::IsFlat(absl::string_view* fragment) const {
+  if (height() == 0 && size() == 1) {
+    if (fragment) *fragment = Data(begin());
+    return true;
+  }
+  return false;
+}
+
+bool CordRepBtree::IsFlat(size_t offset, const size_t n,
+                          absl::string_view* fragment) const {
+  assert(n <= this->length);
+  assert(offset <= this->length - n);
+  if (ABSL_PREDICT_FALSE(n == 0)) return false;
+  int height = this->height();
+  const CordRepBtree* node = this;
+  for (;;) {
+    const Position front = node->IndexOf(offset);
+    const CordRep* edge = node->Edge(front.index);
+    if (edge->length < front.n + n) return false;
+    if (--height < 0) {
+      if (fragment) *fragment = EdgeData(edge).substr(front.n, n);
+      return true;
+    }
+    offset = front.n;
+    node = node->Edge(front.index)->btree();
+  }
+}
+
+char CordRepBtree::GetCharacter(size_t offset) const {
+  assert(offset < length);
+  const CordRepBtree* node = this;
+  int height = node->height();
+  for (;;) {
+    Position front = node->IndexOf(offset);
+    if (--height < 0) return node->Data(front.index)[front.n];
+    offset = front.n;
+    node = node->Edge(front.index)->btree();
+  }
+}
+
+Span<char> CordRepBtree::GetAppendBufferSlow(size_t size) {
+  // The inlined version in `GetAppendBuffer()` deals with all heights <= 3.
+  assert(height() >= 4);
+  assert(refcount.IsOne());
+
+  // Build a stack of nodes we may potentially need to update if we find a
+  // non-shared FLAT with capacity at the leaf level.
+  const int depth = height();
+  CordRepBtree* node = this;
+  CordRepBtree* stack[kMaxDepth];
+  for (int i = 0; i < depth; ++i) {
+    node = node->Edge(kBack)->btree();
+    if (!node->refcount.IsOne()) return {};
+    stack[i] = node;
+  }
+
+  // Must be a privately owned, mutable flat.
+  CordRep* const edge = node->Edge(kBack);
+  if (!edge->refcount.IsOne() || edge->tag < FLAT) return {};
+
+  // Must have capacity.
+  const size_t avail = edge->flat()->Capacity() - edge->length;
+  if (avail == 0) return {};
+
+  // Build span on remaining capacity.
+  size_t delta = (std::min)(size, avail);
+  Span<char> span = {edge->flat()->Data() + edge->length, delta};
+  edge->length += delta;
+  this->length += delta;
+  for (int i = 0; i < depth; ++i) {
+    stack[i]->length += delta;
+  }
+  return span;
+}
+
+CordRepBtree* CordRepBtree::CreateSlow(CordRep* rep) {
+  if (rep->IsBtree()) return rep->btree();
+
+  CordRepBtree* node = nullptr;
+  auto consume = [&node](CordRep* r, size_t offset, size_t length) {
+    r = MakeSubstring(r, offset, length);
+    if (node == nullptr) {
+      node = New(r);
+    } else {
+      node = CordRepBtree::AddCordRep<kBack>(node, r);
+    }
+  };
+  Consume(rep, consume);
+  return node;
+}
+
+CordRepBtree* CordRepBtree::AppendSlow(CordRepBtree* tree, CordRep* rep) {
+  if (ABSL_PREDICT_TRUE(rep->IsBtree())) {
+    return MergeTrees(tree, rep->btree());
+  }
+  auto consume = [&tree](CordRep* r, size_t offset, size_t length) {
+    r = MakeSubstring(r, offset, length);
+    tree = CordRepBtree::AddCordRep<kBack>(tree, r);
+  };
+  Consume(rep, consume);
+  return tree;
+}
+
+CordRepBtree* CordRepBtree::PrependSlow(CordRepBtree* tree, CordRep* rep) {
+  if (ABSL_PREDICT_TRUE(rep->IsBtree())) {
+    return MergeTrees(rep->btree(), tree);
+  }
+  auto consume = [&tree](CordRep* r, size_t offset, size_t length) {
+    r = MakeSubstring(r, offset, length);
+    tree = CordRepBtree::AddCordRep<kFront>(tree, r);
+  };
+  ReverseConsume(rep, consume);
+  return tree;
+}
+
+CordRepBtree* CordRepBtree::Append(CordRepBtree* tree, absl::string_view data,
+                                   size_t extra) {
+  return CordRepBtree::AddData<kBack>(tree, data, extra);
+}
+
+CordRepBtree* CordRepBtree::Prepend(CordRepBtree* tree, absl::string_view data,
+                                    size_t extra) {
+  return CordRepBtree::AddData<kFront>(tree, data, extra);
+}
+
+template CordRepBtree* CordRepBtree::AddCordRep<kFront>(CordRepBtree* tree,
+                                                        CordRep* rep);
+template CordRepBtree* CordRepBtree::AddCordRep<kBack>(CordRepBtree* tree,
+                                                       CordRep* rep);
+template CordRepBtree* CordRepBtree::AddData<kFront>(CordRepBtree* tree,
+                                                     absl::string_view data,
+                                                     size_t extra);
+template CordRepBtree* CordRepBtree::AddData<kBack>(CordRepBtree* tree,
+                                                    absl::string_view data,
+                                                    size_t extra);
+
+void CordRepBtree::Rebuild(CordRepBtree** stack, CordRepBtree* tree,
+                           bool consume) {
+  bool owned = consume && tree->refcount.IsOne();
+  if (tree->height() == 0) {
+    for (CordRep* edge : tree->Edges()) {
+      if (!owned) edge = CordRep::Ref(edge);
+      size_t height = 0;
+      size_t length = edge->length;
+      CordRepBtree* node = stack[0];
+      OpResult result = node->AddEdge<kBack>(true, edge, length);
+      while (result.action == CordRepBtree::kPopped) {
+        stack[height] = result.tree;
+        if (ABSL_PREDICT_FALSE(++height >= kMaxDepth)) {
+          ABSL_RAW_LOG(FATAL, "CordRepBtree::Rebuild() exceeded max depth");
+        }
+        if (stack[height] == nullptr) {
+          result.action = CordRepBtree::kSelf;
+          stack[height] = CordRepBtree::New(node, result.tree);
+        } else {
+          node = stack[height];
+          result = node->AddEdge<kBack>(true, result.tree, length);
+        }
+      }
+      while (++height < kMaxDepth && stack[height] != nullptr) {
+        stack[height]->length += length;
+      }
+    }
+  } else {
+    for (CordRep* rep : tree->Edges()) {
+      Rebuild(stack, rep->btree(), owned);
+    }
+  }
+  if (consume) {
+    if (owned) {
+      CordRepBtree::Delete(tree);
+    } else {
+      CordRepBtree::Unref(tree);
+    }
+  }
+}
+
+CordRepBtree* CordRepBtree::Rebuild(CordRepBtree* tree) {
+  // Set up initial stack with empty leaf node.
+  CordRepBtree* node = CordRepBtree::New();
+  CordRepBtree* stack[CordRepBtree::kMaxDepth + 1] = {node};
+
+  // Recursively build the tree, consuming the input tree.
+  Rebuild(stack, tree, /* consume reference */ true);
+
+  // Return top most node
+  for (CordRepBtree* parent : stack) {
+    if (parent == nullptr) return node;
+    node = parent;
+  }
+
+  // Unreachable
+  assert(false);
+  return nullptr;
+}
+
+CordRepBtree::ExtractResult CordRepBtree::ExtractAppendBuffer(
+    CordRepBtree* tree, size_t extra_capacity) {
+  int depth = 0;
+  NodeStack stack;
+
+  // Set up default 'no success' result which is {tree, nullptr}.
+  ExtractResult result;
+  result.tree = tree;
+  result.extracted = nullptr;
+
+  // Dive down the right side of the tree, making sure no edges are shared.
+  while (tree->height() > 0) {
+    if (!tree->refcount.IsOne()) return result;
+    stack[depth++] = tree;
+    tree = tree->Edge(kBack)->btree();
+  }
+  if (!tree->refcount.IsOne()) return result;
+
+  // Validate we ended on a non shared flat.
+  CordRep* rep = tree->Edge(kBack);
+  if (!(rep->IsFlat() && rep->refcount.IsOne())) return result;
+
+  // Verify it has at least the requested extra capacity.
+  CordRepFlat* flat = rep->flat();
+  const size_t length = flat->length;
+  const size_t avail = flat->Capacity() - flat->length;
+  if (extra_capacity > avail) return result;
+
+  // Set the extracted flat in the result.
+  result.extracted = flat;
+
+  // Cascading delete all nodes that become empty.
+  while (tree->size() == 1) {
+    CordRepBtree::Delete(tree);
+    if (--depth < 0) {
+      // We consumed the entire tree: return nullptr for new tree.
+      result.tree = nullptr;
+      return result;
+    }
+    rep = tree;
+    tree = stack[depth];
+  }
+
+  // Remove the edge or cascaded up parent node.
+  tree->set_end(tree->end() - 1);
+  tree->length -= length;
+
+  // Adjust lengths up the tree.
+  while (depth > 0) {
+    tree = stack[--depth];
+    tree->length -= length;
+  }
+
+  // Remove unnecessary top nodes with size = 1. This may iterate all the way
+  // down to the leaf node in which case we simply return the remaining last
+  // edge in that node and the extracted flat.
+  while (tree->size() == 1) {
+    int height = tree->height();
+    rep = tree->Edge(kBack);
+    Delete(tree);
+    if (height == 0) {
+      // We consumed the leaf: return the sole data edge as the new tree.
+      result.tree = rep;
+      return result;
+    }
+    tree = rep->btree();
+  }
+
+  // Done: return the (new) top level node and extracted flat.
+  result.tree = tree;
+  return result;
+}
+
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/strings/internal/cord_rep_btree.h b/absl/strings/internal/cord_rep_btree.h
new file mode 100644
index 00000000000..f65373526ed
--- /dev/null
+++ b/absl/strings/internal/cord_rep_btree.h
@@ -0,0 +1,947 @@
+// Copyright 2021 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_STRINGS_INTERNAL_CORD_REP_BTREE_H_
+#define ABSL_STRINGS_INTERNAL_CORD_REP_BTREE_H_
+
+#include <algorithm>
+#include <cassert>
+#include <cstddef>
+#include <cstdint>
+#include <cstring>
+#include <iosfwd>
+
+#include "absl/base/config.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/base/optimization.h"
+#include "absl/strings/internal/cord_data_edge.h"
+#include "absl/strings/internal/cord_internal.h"
+#include "absl/strings/internal/cord_rep_flat.h"
+#include "absl/strings/string_view.h"
+#include "absl/types/span.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+
+// `SetCordBtreeExhaustiveValidation()` can be set to force exhaustive
+// validation in debug assertions, and code that calls `IsValid()`
+// explicitly. By default, assertions should be relatively cheap and
+// AssertValid() can easily lead to O(n^2) complexity as recursive / full tree
+// validation is O(n).
+void SetCordBtreeExhaustiveValidation(bool do_exaustive_validation);
+bool IsCordBtreeExhaustiveValidationEnabled();
+
+class CordRepBtreeNavigator;
+
+// CordRepBtree is as the name implies a btree implementation of a Cordrep tree.
+// Data is stored at the leaf level only, non leaf nodes contain down pointers
+// only. Allowed types of data edges are FLAT, EXTERNAL and SUBSTRINGs of FLAT
+// or EXTERNAL nodes. The implementation allows for data to be added to either
+// end of the tree only, it does not provide any 'insert' logic. This has the
+// benefit that we can expect good fill ratios: all nodes except the outer
+// 'legs' will have 100% fill ratios for trees built using Append/Prepend
+// methods. Merged trees will typically have a fill ratio well above 50% as in a
+// similar fashion, one side of the merged tree will typically have a 100% fill
+// ratio, and the 'open' end will average 50%. All operations are O(log(n)) or
+// better, and the tree never needs balancing.
+//
+// All methods accepting a CordRep* or CordRepBtree* adopt a reference on that
+// input unless explicitly stated otherwise. All functions returning a CordRep*
+// or CordRepBtree* instance transfer a reference back to the caller.
+// Simplified, callers both 'donate' and 'consume' a reference count on each
+// call, simplifying the API. An example of building a tree:
+//
+//   CordRepBtree* tree = CordRepBtree::Create(MakeFlat("Hello"));
+//   tree = CordRepBtree::Append(tree, MakeFlat("world"));
+//
+// In the above example, all inputs are consumed, making each call affecting
+// `tree` reference count neutral. The returned `tree` value can be different
+// from the input if the input is shared with other threads, or if the tree
+// grows in height, but callers typically never have to concern themselves with
+// that and trust that all methods DTRT at all times.
+class CordRepBtree : public CordRep {
+ public:
+  // EdgeType identifies `front` and `back` enum values.
+  // Various implementations in CordRepBtree such as `Add` and `Edge` are
+  // generic and templated on operating on either of the boundary edges.
+  // For more information on the possible edges contained in a CordRepBtree
+  // instance see the documentation for `edges_`.
+  enum class EdgeType { kFront, kBack };
+
+  // Convenience constants into `EdgeType`
+  static constexpr EdgeType kFront = EdgeType::kFront;
+  static constexpr EdgeType kBack = EdgeType::kBack;
+
+  // Maximum number of edges: based on experiments and performance data, we can
+  // pick suitable values resulting in optimum cacheline aligned values. The
+  // preferred values are based on 64-bit systems where we aim to align this
+  // class onto 64 bytes, i.e.:  6 = 64 bytes, 14 = 128 bytes, etc.
+  // TODO(b/192061034): experiment with alternative sizes.
+  static constexpr size_t kMaxCapacity = 6;
+
+  // Reasonable maximum height of the btree. We can expect a fill ratio of at
+  // least 50%: trees are always expanded at the front or back. Concatenating
+  // trees will then typically fold at the top most node, where the lower nodes
+  // are at least at capacity on one side of joined inputs. At a lower fill
+  // rate of 4 edges per node, we have capacity for ~16 million leaf nodes.
+  // We will fail / abort if an application ever exceeds this height, which
+  // should be extremely rare (near impossible) and be an indication of an
+  // application error: we do not assume it reasonable for any application to
+  // operate correctly with such monster trees.
+  // Another compelling reason for the number `12` is that any contextual stack
+  // required for navigation or insertion requires 12 words and 12 bytes, which
+  // fits inside 2 cache lines with some room to spare, and is reasonable as a
+  // local stack variable compared to Cord's current near 400 bytes stack use.
+  // The maximum `height` value of a node is then `kMaxDepth - 1` as node height
+  // values start with a value of 0 for leaf nodes.
+  static constexpr size_t kMaxDepth = 12;
+  // See comments on height() for why this is an int and not a size_t.
+  static constexpr int kMaxHeight = static_cast<int>(kMaxDepth - 1);
+
+  // `Action` defines the action for unwinding changes done at the btree's leaf
+  // level that need to be propagated up to the parent node(s). Each operation
+  // on a node has an effect / action defined as follows:
+  // - kSelf
+  //   The operation (add / update, etc) was performed directly on the node as
+  //   the node is private to the current thread (i.e.: not shared directly or
+  //   indirectly through a refcount > 1). Changes can be propagated directly to
+  //   all parent nodes as all parent nodes are also then private to the current
+  //   thread.
+  // - kCopied
+  //   The operation (add / update, etc) was performed on a copy of the original
+  //   node, as the node is (potentially) directly or indirectly shared with
+  //   other threads. Changes need to be propagated into the parent nodes where
+  //   the old down pointer must be unreffed and replaced with this new copy.
+  //   Such changes to parent nodes may themselves require a copy if the parent
+  //   node is also shared. A kCopied action can propagate all the way to the
+  //   top node where we then must unref the `tree` input provided by the
+  //   caller, and return the new copy.
+  // - kPopped
+  //   The operation (typically add) could not be satisfied due to insufficient
+  //   capacity in the targeted node, and a new 'leg' was created that needs to
+  //   be added into the parent node. For example, adding a FLAT inside a leaf
+  //   node that is at capacity will create a new leaf node containing that
+  //   FLAT, that needs to be 'popped' up the btree. Such 'pop' actions can
+  //   cascade up the tree if parent nodes are also at capacity. A 'Popped'
+  //   action propagating all the way to the top of the tree will result in
+  //   the tree becoming one level higher than the current tree through a final
+  //   `CordRepBtree::New(tree, popped)` call, resulting in a new top node
+  //   referencing the old tree and the new (fully popped upwards) 'leg'.
+  enum Action { kSelf, kCopied, kPopped };
+
+  // Result of an operation on a node. See the `Action` enum for details.
+  struct OpResult {
+    CordRepBtree* tree;
+    Action action;
+  };
+
+  // Return value of the CopyPrefix and CopySuffix methods which can
+  // return a node or data edge at any height inside the tree.
+  // A height of 0 defines the lowest (leaf) node, a height of -1 identifies
+  // `edge` as being a plain data node: EXTERNAL / FLAT or SUBSTRING thereof.
+  struct CopyResult {
+    CordRep* edge;
+    int height;
+  };
+
+  // Logical position inside a node:
+  // - index: index of the edge.
+  // - n: size or offset value depending on context.
+  struct Position {
+    size_t index;
+    size_t n;
+  };
+
+  // Creates a btree from the given input. Adopts a ref of `rep`.
+  // If the input `rep` is itself a btree, i.e., `IsBtree()`, then this
+  // function immediately returns `rep->btree()`. If the input is a valid data
+  // edge (see IsDataEdge()), then a new leaf node is returned containing `rep`
+  // as the sole data edge. Else, the input is assumed to be a (legacy) concat
+  // tree, and the input is consumed and transformed into a btree().
+  static CordRepBtree* Create(CordRep* rep);
+
+  // Destroys the provided tree. Should only be called by cord internal API's,
+  // typically after a ref_count.Decrement() on the last reference count.
+  static void Destroy(CordRepBtree* tree);
+
+  // Destruction
+  static void Delete(CordRepBtree* tree) { delete tree; }
+
+  // Use CordRep::Unref() as we overload for absl::Span<CordRep* const>.
+  using CordRep::Unref;
+
+  // Unrefs all edges in `edges` which are assumed to be 'likely one'.
+  static void Unref(absl::Span<CordRep* const> edges);
+
+  // Appends / Prepends an existing CordRep instance to this tree.
+  // The below methods accept three types of input:
+  // 1) `rep` is a data node (See `IsDataNode` for valid data edges).
+  // `rep` is appended or prepended to this tree 'as is'.
+  // 2) `rep` is a BTREE.
+  // `rep` is merged into `tree` respecting the Append/Prepend order.
+  // 3) `rep` is some other (legacy) type.
+  // `rep` is converted in place and added to `tree`
+  // Requires `tree` and `rep` to be not null.
+  static CordRepBtree* Append(CordRepBtree* tree, CordRep* rep);
+  static CordRepBtree* Prepend(CordRepBtree* tree, CordRep* rep);
+
+  // Append/Prepend the data in `data` to this tree.
+  // The `extra` parameter defines how much extra capacity should be allocated
+  // for any additional FLAT being allocated. This is an optimization hint from
+  // the caller. For example, a caller may need to add 2 string_views of data
+  // "abc" and "defghi" which are not consecutive. The caller can in this case
+  // invoke `AddData(tree, "abc", 6)`, and any newly added flat is allocated
+  // where possible with at least 6 bytes of extra capacity beyond `length`.
+  // This helps avoiding data getting fragmented over multiple flats.
+  // There is no limit on the size of `data`. If `data` can not be stored inside
+  // a single flat, then the function will iteratively add flats until all data
+  // has been consumed and appended or prepended to the tree.
+  static CordRepBtree* Append(CordRepBtree* tree, string_view data,
+                              size_t extra = 0);
+  static CordRepBtree* Prepend(CordRepBtree* tree, string_view data,
+                               size_t extra = 0);
+
+  // Returns a new tree, containing `n` bytes of data from this instance
+  // starting at offset `offset`. Where possible, the returned tree shares
+  // (re-uses) data edges and nodes with this instance to minimize the
+  // combined memory footprint of both trees.
+  // Requires `offset + n <= length`. Returns `nullptr` if `n` is zero.
+  CordRep* SubTree(size_t offset, size_t n);
+
+  // Removes `n` trailing bytes from `tree`, and returns the resulting tree
+  // or data edge. Returns `tree` if n is zero, and nullptr if n == length.
+  // This function is logically identical to:
+  //   result = tree->SubTree(0, tree->length - n);
+  //   Unref(tree);
+  //   return result;
+  // However, the actual implementation will as much as possible perform 'in
+  // place' modifications on the tree on all nodes and edges that are mutable.
+  // For example, in a fully privately owned tree with the last edge being a
+  // flat of length 12, RemoveSuffix(1) will simply set the length of that data
+  // edge to 11, and reduce the length of all nodes on the edge path by 1.
+  static CordRep* RemoveSuffix(CordRepBtree* tree, size_t n);
+
+  // Returns the character at the given offset.
+  char GetCharacter(size_t offset) const;
+
+  // Returns true if this node holds a single data edge, and if so, sets
+  // `fragment` to reference the contained data. `fragment` is an optional
+  // output parameter and allowed to be null.
+  bool IsFlat(absl::string_view* fragment) const;
+
+  // Returns true if the data of `n` bytes starting at offset `offset`
+  // is contained in a single data edge, and if so, sets fragment to reference
+  // the contained data. `fragment` is an optional output parameter and allowed
+  // to be null.
+  bool IsFlat(size_t offset, size_t n, absl::string_view* fragment) const;
+
+  // Returns a span (mutable range of bytes) of up to `size` bytes into the
+  // last FLAT data edge inside this tree under the following conditions:
+  // - none of the nodes down into the FLAT node are shared.
+  // - the last data edge in this tree is a non-shared FLAT.
+  // - the referenced FLAT has additional capacity available.
+  // If all these conditions are met, a non-empty span is returned, and the
+  // length of the flat node and involved tree nodes have been increased by
+  // `span.length()`. The caller is responsible for immediately assigning values
+  // to all uninitialized data reference by the returned span.
+  // Requires `this->refcount.IsOne()`: this function forces the caller to do
+  // this fast path check on the top level node, as this is the most commonly
+  // shared node of a cord tree.
+  Span<char> GetAppendBuffer(size_t size);
+
+  // Extracts the right-most data edge from this tree iff:
+  // - the tree and all internal edges to the right-most node are not shared.
+  // - the right-most node is a FLAT node and not shared.
+  // - the right-most node has at least the desired extra capacity.
+  //
+  // Returns {tree, nullptr} if any of the above conditions are not met.
+  // This method effectively removes data from the tree. The intent of this
+  // method is to allow applications appending small string data to use
+  // pre-existing capacity, and add the modified rep back to the tree.
+  //
+  // Simplified such code would look similar to this:
+  //   void MyTreeBuilder::Append(string_view data) {
+  //     ExtractResult result = CordRepBtree::ExtractAppendBuffer(tree_, 1);
+  //     if (CordRep* rep = result.extracted) {
+  //       size_t available = rep->Capacity() - rep->length;
+  //       size_t n = std::min(data.size(), n);
+  //       memcpy(rep->Data(), data.data(), n);
+  //       rep->length += n;
+  //       data.remove_prefix(n);
+  //       if (!result.tree->IsBtree()) {
+  //         tree_ = CordRepBtree::Create(result.tree);
+  //       }
+  //       tree_ = CordRepBtree::Append(tree_, rep);
+  //     }
+  //     ...
+  //     // Remaining edge in `result.tree`.
+  //   }
+  static ExtractResult ExtractAppendBuffer(CordRepBtree* tree,
+                                           size_t extra_capacity = 1);
+
+  // Returns the `height` of the tree. The height of a tree is limited to
+  // kMaxHeight. `height` is implemented as an `int` as in some places we
+  // use negative (-1) values for 'data edges'.
+  int height() const { return static_cast<int>(storage[0]); }
+
+  // Properties: begin, back, end, front/back boundary indexes.
+  size_t begin() const { return static_cast<size_t>(storage[1]); }
+  size_t back() const { return static_cast<size_t>(storage[2]) - 1; }
+  size_t end() const { return static_cast<size_t>(storage[2]); }
+  size_t index(EdgeType edge) const {
+    return edge == kFront ? begin() : back();
+  }
+
+  // Properties: size and capacity.
+  // `capacity` contains the current capacity of this instance, where
+  // `kMaxCapacity` contains the maximum capacity of a btree node.
+  // For now, `capacity` and `kMaxCapacity` return the same value, but this may
+  // change in the future if we see benefit in dynamically sizing 'small' nodes
+  // to 'large' nodes for large data trees.
+  size_t size() const { return end() - begin(); }
+  size_t capacity() const { return kMaxCapacity; }
+
+  // Edge access
+  inline CordRep* Edge(size_t index) const;
+  inline CordRep* Edge(EdgeType edge_type) const;
+  inline absl::Span<CordRep* const> Edges() const;
+  inline absl::Span<CordRep* const> Edges(size_t begin, size_t end) const;
+
+  // Returns reference to the data edge at `index`.
+  // Requires this instance to be a leaf node, and `index` to be valid index.
+  inline absl::string_view Data(size_t index) const;
+
+  // Diagnostics: returns true if `tree` is valid and internally consistent.
+  // If `shallow` is false, then the provided top level node and all child nodes
+  // below it are recursively checked. If `shallow` is true, only the provided
+  // node in `tree` and the cumulative length, type and height of the direct
+  // child nodes of `tree` are checked. The value of `shallow` is ignored if the
+  // internal `cord_btree_exhaustive_validation` diagnostics variable is true,
+  // in which case the performed validations works as if `shallow` were false.
+  // This function is intended for debugging and testing purposes only.
+  static bool IsValid(const CordRepBtree* tree, bool shallow = false);
+
+  // Diagnostics: asserts that the provided tree is valid.
+  // `AssertValid()` performs a shallow validation by default. `shallow` can be
+  // set to false in which case an exhaustive validation is performed. This
+  // function is implemented in terms of calling `IsValid()` and asserting the
+  // return value to be true. See `IsValid()` for more information.
+  // This function is intended for debugging and testing purposes only.
+  static CordRepBtree* AssertValid(CordRepBtree* tree, bool shallow = true);
+  static const CordRepBtree* AssertValid(const CordRepBtree* tree,
+                                         bool shallow = true);
+
+  // Diagnostics: dump the contents of this tree to `stream`.
+  // This function is intended for debugging and testing purposes only.
+  static void Dump(const CordRep* rep, std::ostream& stream);
+  static void Dump(const CordRep* rep, absl::string_view label,
+                   std::ostream& stream);
+  static void Dump(const CordRep* rep, absl::string_view label,
+                   bool include_contents, std::ostream& stream);
+
+  // Adds the edge `edge` to this node if possible. `owned` indicates if the
+  // current node is potentially shared or not with other threads. Returns:
+  // - {kSelf, <this>}
+  //   The edge was directly added to this node.
+  // - {kCopied, <node>}
+  //   The edge was added to a copy of this node.
+  // - {kPopped, New(edge, height())}
+  //   A new leg with the edge was created as this node has no extra capacity.
+  template <EdgeType edge_type>
+  inline OpResult AddEdge(bool owned, CordRep* edge, size_t delta);
+
+  // Replaces the front or back edge with the provided new edge. Returns:
+  // - {kSelf, <this>}
+  //   The edge was directly set in this node. The old edge is unreffed.
+  // - {kCopied, <node>}
+  //   A copy of this node was created with the new edge value.
+  // In both cases, the function adopts a reference on `edge`.
+  template <EdgeType edge_type>
+  OpResult SetEdge(bool owned, CordRep* edge, size_t delta);
+
+  // Creates a new empty node at the specified height.
+  static CordRepBtree* New(int height = 0);
+
+  // Creates a new node containing `rep`, with the height being computed
+  // automatically based on the type of `rep`.
+  static CordRepBtree* New(CordRep* rep);
+
+  // Creates a new node containing both `front` and `back` at height
+  // `front.height() + 1`. Requires `back.height() == front.height()`.
+  static CordRepBtree* New(CordRepBtree* front, CordRepBtree* back);
+
+  // Creates a fully balanced tree from the provided tree by rebuilding a new
+  // tree from all data edges in the input. This function is automatically
+  // invoked internally when the tree exceeds the maximum height.
+  static CordRepBtree* Rebuild(CordRepBtree* tree);
+
+ private:
+  CordRepBtree() = default;
+  ~CordRepBtree() = default;
+
+  // Initializes the main properties `tag`, `begin`, `end`, `height`.
+  inline void InitInstance(int height, size_t begin = 0, size_t end = 0);
+
+  // Direct property access begin / end
+  void set_begin(size_t begin) { storage[1] = static_cast<uint8_t>(begin); }
+  void set_end(size_t end) { storage[2] = static_cast<uint8_t>(end); }
+
+  // Decreases the value of `begin` by `n`, and returns the new value. Notice
+  // how this returns the new value unlike atomic::fetch_add which returns the
+  // old value. This is because this is used to prepend edges at 'begin - 1'.
+  size_t sub_fetch_begin(size_t n) {
+    storage[1] -= static_cast<uint8_t>(n);
+    return storage[1];
+  }
+
+  // Increases the value of `end` by `n`, and returns the previous value. This
+  // function is typically used to append edges at 'end'.
+  size_t fetch_add_end(size_t n) {
+    const uint8_t current = storage[2];
+    storage[2] = static_cast<uint8_t>(current + n);
+    return current;
+  }
+
+  // Returns the index of the last edge starting on, or before `offset`, with
+  // `n` containing the relative offset of `offset` inside that edge.
+  // Requires `offset` < length.
+  Position IndexOf(size_t offset) const;
+
+  // Returns the index of the last edge starting before `offset`, with `n`
+  // containing the relative offset of `offset` inside that edge.
+  // This function is useful to find the edges for some span of bytes ending at
+  // `offset` (i.e., `n` bytes). For example:
+  //
+  //   Position pos = IndexBefore(n)
+  //   edges = Edges(begin(), pos.index)     // All full edges (may be empty)
+  //   last = Sub(Edge(pos.index), 0, pos.n) // Last partial edge (may be empty)
+  //
+  // Requires 0 < `offset` <= length.
+  Position IndexBefore(size_t offset) const;
+
+  // Returns the index of the edge ending at (or on) length `length`, and the
+  // number of bytes inside that edge up to `length`. For example, if we have a
+  // Node with 2 edges, one of 10 and one of 20 long, then IndexOfLength(27)
+  // will return {1, 17}, and IndexOfLength(10) will return {0, 10}.
+  Position IndexOfLength(size_t n) const;
+
+  // Identical to the above function except starting from the position `front`.
+  // This function is equivalent to `IndexBefore(front.n + offset)`, with
+  // the difference that this function is optimized to start at `front.index`.
+  Position IndexBefore(Position front, size_t offset) const;
+
+  // Returns the index of the edge directly beyond the edge containing offset
+  // `offset`, with `n` containing the distance of that edge from `offset`.
+  // This function is useful for iteratively finding suffix nodes and remaining
+  // partial bytes in left-most suffix nodes as for example in CopySuffix.
+  // Requires `offset` < length.
+  Position IndexBeyond(size_t offset) const;
+
+  // Creates a new leaf node containing as much data as possible from `data`.
+  // The data is added either forwards or reversed depending on `edge_type`.
+  // Callers must check the length of the returned node to determine if all data
+  // was copied or not.
+  // See the `Append/Prepend` function for the meaning and purpose of `extra`.
+  template <EdgeType edge_type>
+  static CordRepBtree* NewLeaf(absl::string_view data, size_t extra);
+
+  // Creates a raw copy of this Btree node with the specified length, copying
+  // all properties, but without adding any references to existing edges.
+  CordRepBtree* CopyRaw(size_t new_length) const;
+
+  // Creates a full copy of this Btree node, adding a reference on all edges.
+  CordRepBtree* Copy() const;
+
+  // Creates a partial copy of this Btree node, copying all edges up to `end`,
+  // adding a reference on each copied edge, and sets the length of the newly
+  // created copy to `new_length`.
+  CordRepBtree* CopyBeginTo(size_t end, size_t new_length) const;
+
+  // Returns a tree containing the edges [tree->begin(), end) and length
+  // of `new_length`. This method consumes a reference on the provided
+  // tree, and logically performs the following operation:
+  //   result = tree->CopyBeginTo(end, new_length);
+  //   CordRep::Unref(tree);
+  //   return result;
+  static CordRepBtree* ConsumeBeginTo(CordRepBtree* tree, size_t end,
+                                      size_t new_length);
+
+  // Creates a partial copy of this Btree node, copying all edges starting at
+  // `begin`, adding a reference on each copied edge, and sets the length of
+  // the newly created copy to `new_length`.
+  CordRepBtree* CopyToEndFrom(size_t begin, size_t new_length) const;
+
+  // Extracts and returns the front edge from the provided tree.
+  // This method consumes a reference on the provided tree, and logically
+  // performs the following operation:
+  //   edge = CordRep::Ref(tree->Edge(kFront));
+  //   CordRep::Unref(tree);
+  //   return edge;
+  static CordRep* ExtractFront(CordRepBtree* tree);
+
+  // Returns a tree containing the result of appending `right` to `left`.
+  static CordRepBtree* MergeTrees(CordRepBtree* left, CordRepBtree* right);
+
+  // Fallback functions for `Create()`, `Append()` and `Prepend()` which
+  // deal with legacy / non conforming input, i.e.: CONCAT trees.
+  static CordRepBtree* CreateSlow(CordRep* rep);
+  static CordRepBtree* AppendSlow(CordRepBtree*, CordRep* rep);
+  static CordRepBtree* PrependSlow(CordRepBtree*, CordRep* rep);
+
+  // Recursively rebuilds `tree` into `stack`. If 'consume` is set to true, the
+  // function will consume a reference on `tree`. `stack` is a null terminated
+  // array containing the new tree's state, with the current leaf node at
+  // stack[0], and parent nodes above that, or null for 'top of tree'.
+  static void Rebuild(CordRepBtree** stack, CordRepBtree* tree, bool consume);
+
+  // Aligns existing edges to start at index 0, to allow for a new edge to be
+  // added to the back of the current edges.
+  inline void AlignBegin();
+
+  // Aligns existing edges to end at `capacity`, to allow for a new edge to be
+  // added in front of the current edges.
+  inline void AlignEnd();
+
+  // Adds the provided edge to this node.
+  // Requires this node to have capacity for the edge. Realigns / moves
+  // existing edges as needed to prepend or append the new edge.
+  template <EdgeType edge_type>
+  inline void Add(CordRep* rep);
+
+  // Adds the provided edges to this node.
+  // Requires this node to have capacity for the edges. Realigns / moves
+  // existing edges as needed to prepend or append the new edges.
+  template <EdgeType edge_type>
+  inline void Add(absl::Span<CordRep* const>);
+
+  // Adds data from `data` to this node until either all data has been consumed,
+  // or there is no more capacity for additional flat nodes inside this node.
+  // Requires the current node to be a leaf node, data to be non empty, and the
+  // current node to have capacity for at least one more data edge.
+  // Returns any remaining data from `data` that was not added, which is
+  // depending on the edge type (front / back) either the remaining prefix of
+  // suffix of the input.
+  // See the `Append/Prepend` function for the meaning and purpose of `extra`.
+  template <EdgeType edge_type>
+  absl::string_view AddData(absl::string_view data, size_t extra);
+
+  // Replace the front or back edge with the provided value.
+  // Adopts a reference on `edge` and unrefs the old edge.
+  template <EdgeType edge_type>
+  inline void SetEdge(CordRep* edge);
+
+  // Returns a partial copy of the current tree containing the first `n` bytes
+  // of data. `CopyResult` contains both the resulting edge and its height. The
+  // resulting tree may be less high than the current tree, or even be a single
+  // matching data edge if `allow_folding` is set to true.
+  // For example, if `n == 1`, then the result will be the single data edge, and
+  // height will be set to -1 (one below the owning leaf node). If n == 0, this
+  // function returns null. Requires `n <= length`
+  CopyResult CopyPrefix(size_t n, bool allow_folding = true);
+
+  // Returns a partial copy of the current tree containing all data starting
+  // after `offset`. `CopyResult` contains both the resulting edge and its
+  // height. The resulting tree may be less high than the current tree, or even
+  // be a single matching data edge. For example, if `n == length - 1`, then the
+  // result will be a single data edge, and height will be set to -1 (one below
+  // the owning leaf node).
+  // Requires `offset < length`
+  CopyResult CopySuffix(size_t offset);
+
+  // Returns a OpResult value of {this, kSelf} or {Copy(), kCopied}
+  // depending on the value of `owned`.
+  inline OpResult ToOpResult(bool owned);
+
+  // Adds `rep` to the specified tree, returning the modified tree.
+  template <EdgeType edge_type>
+  static CordRepBtree* AddCordRep(CordRepBtree* tree, CordRep* rep);
+
+  // Adds `data` to the specified tree, returning the modified tree.
+  // See the `Append/Prepend` function for the meaning and purpose of `extra`.
+  template <EdgeType edge_type>
+  static CordRepBtree* AddData(CordRepBtree* tree, absl::string_view data,
+                               size_t extra = 0);
+
+  // Merges `src` into `dst` with `src` being added either before (kFront) or
+  // after (kBack) `dst`. Requires the height of `dst` to be greater than or
+  // equal to the height of `src`.
+  template <EdgeType edge_type>
+  static CordRepBtree* Merge(CordRepBtree* dst, CordRepBtree* src);
+
+  // Fallback version of GetAppendBuffer for large trees: GetAppendBuffer()
+  // implements an inlined version for trees of limited height (3 levels),
+  // GetAppendBufferSlow implements the logic for large trees.
+  Span<char> GetAppendBufferSlow(size_t size);
+
+  // `edges_` contains all edges starting from this instance.
+  // These are explicitly `child` edges only, a cord btree (or any cord tree in
+  // that respect) does not store `parent` pointers anywhere: multiple trees /
+  // parents can reference the same shared child edge. The type of these edges
+  // depends on the height of the node. `Leaf nodes` (height == 0) contain `data
+  // edges` (external or flat nodes, or sub-strings thereof). All other nodes
+  // (height > 0) contain pointers to BTREE nodes with a height of `height - 1`.
+  CordRep* edges_[kMaxCapacity];
+
+  friend class CordRepBtreeTestPeer;
+  friend class CordRepBtreeNavigator;
+};
+
+inline CordRepBtree* CordRep::btree() {
+  assert(IsBtree());
+  return static_cast<CordRepBtree*>(this);
+}
+
+inline const CordRepBtree* CordRep::btree() const {
+  assert(IsBtree());
+  return static_cast<const CordRepBtree*>(this);
+}
+
+inline void CordRepBtree::InitInstance(int height, size_t begin, size_t end) {
+  tag = BTREE;
+  storage[0] = static_cast<uint8_t>(height);
+  storage[1] = static_cast<uint8_t>(begin);
+  storage[2] = static_cast<uint8_t>(end);
+}
+
+inline CordRep* CordRepBtree::Edge(size_t index) const {
+  assert(index >= begin());
+  assert(index < end());
+  return edges_[index];
+}
+
+inline CordRep* CordRepBtree::Edge(EdgeType edge_type) const {
+  return edges_[edge_type == kFront ? begin() : back()];
+}
+
+inline absl::Span<CordRep* const> CordRepBtree::Edges() const {
+  return {edges_ + begin(), size()};
+}
+
+inline absl::Span<CordRep* const> CordRepBtree::Edges(size_t begin,
+                                                      size_t end) const {
+  assert(begin <= end);
+  assert(begin >= this->begin());
+  assert(end <= this->end());
+  return {edges_ + begin, static_cast<size_t>(end - begin)};
+}
+
+inline absl::string_view CordRepBtree::Data(size_t index) const {
+  assert(height() == 0);
+  return EdgeData(Edge(index));
+}
+
+inline CordRepBtree* CordRepBtree::New(int height) {
+  CordRepBtree* tree = new CordRepBtree;
+  tree->length = 0;
+  tree->InitInstance(height);
+  return tree;
+}
+
+inline CordRepBtree* CordRepBtree::New(CordRep* rep) {
+  CordRepBtree* tree = new CordRepBtree;
+  int height = rep->IsBtree() ? rep->btree()->height() + 1 : 0;
+  tree->length = rep->length;
+  tree->InitInstance(height, /*begin=*/0, /*end=*/1);
+  tree->edges_[0] = rep;
+  return tree;
+}
+
+inline CordRepBtree* CordRepBtree::New(CordRepBtree* front,
+                                       CordRepBtree* back) {
+  assert(front->height() == back->height());
+  CordRepBtree* tree = new CordRepBtree;
+  tree->length = front->length + back->length;
+  tree->InitInstance(front->height() + 1, /*begin=*/0, /*end=*/2);
+  tree->edges_[0] = front;
+  tree->edges_[1] = back;
+  return tree;
+}
+
+inline void CordRepBtree::Unref(absl::Span<CordRep* const> edges) {
+  for (CordRep* edge : edges) {
+    if (ABSL_PREDICT_FALSE(!edge->refcount.Decrement())) {
+      CordRep::Destroy(edge);
+    }
+  }
+}
+
+inline CordRepBtree* CordRepBtree::CopyRaw(size_t new_length) const {
+  CordRepBtree* tree = new CordRepBtree;
+
+  // `length` and `refcount` are the first members of `CordRepBtree`.
+  // We initialize `length` using the given length, have `refcount` be set to
+  // ref = 1 through its default constructor, and copy all data beyond
+  // 'refcount' which starts with `tag` using a single memcpy: all contents
+  // except `refcount` is trivially copyable, and the compiler does not
+  // efficiently coalesce member-wise copy of these members.
+  // See https://gcc.godbolt.org/z/qY8zsca6z
+  // LINT.IfChange(copy_raw)
+  tree->length = new_length;
+  uint8_t* dst = &tree->tag;
+  const uint8_t* src = &tag;
+  const ptrdiff_t offset = src - reinterpret_cast<const uint8_t*>(this);
+  memcpy(dst, src, sizeof(CordRepBtree) - static_cast<size_t>(offset));
+  return tree;
+  // LINT.ThenChange()
+}
+
+inline CordRepBtree* CordRepBtree::Copy() const {
+  CordRepBtree* tree = CopyRaw(length);
+  for (CordRep* rep : Edges()) CordRep::Ref(rep);
+  return tree;
+}
+
+inline CordRepBtree* CordRepBtree::CopyToEndFrom(size_t begin,
+                                                 size_t new_length) const {
+  assert(begin >= this->begin());
+  assert(begin <= this->end());
+  CordRepBtree* tree = CopyRaw(new_length);
+  tree->set_begin(begin);
+  for (CordRep* edge : tree->Edges()) CordRep::Ref(edge);
+  return tree;
+}
+
+inline CordRepBtree* CordRepBtree::CopyBeginTo(size_t end,
+                                               size_t new_length) const {
+  assert(end <= capacity());
+  assert(end >= this->begin());
+  CordRepBtree* tree = CopyRaw(new_length);
+  tree->set_end(end);
+  for (CordRep* edge : tree->Edges()) CordRep::Ref(edge);
+  return tree;
+}
+
+inline void CordRepBtree::AlignBegin() {
+  // The below code itself does not need to be fast as typically we have
+  // mono-directional append/prepend calls, and `begin` / `end` are typically
+  // adjusted no more than once. But we want to avoid potential register clobber
+  // effects, making the compiler emit register save/store/spills, and minimize
+  // the size of code.
+  const size_t delta = begin();
+  if (ABSL_PREDICT_FALSE(delta != 0)) {
+    const size_t new_end = end() - delta;
+    set_begin(0);
+    set_end(new_end);
+    // TODO(mvels): we can write this using 2 loads / 2 stores depending on
+    // total size for the kMaxCapacity = 6 case. I.e., we can branch (switch) on
+    // size, and then do overlapping load/store of up to 4 pointers (inlined as
+    // XMM, YMM or ZMM load/store) and up to 2 pointers (XMM / YMM), which is a)
+    // compact and b) not clobbering any registers.
+    ABSL_ASSUME(new_end <= kMaxCapacity);
+#ifdef __clang__
+#pragma unroll 1
+#endif
+    for (size_t i = 0; i < new_end; ++i) {
+      edges_[i] = edges_[i + delta];
+    }
+  }
+}
+
+inline void CordRepBtree::AlignEnd() {
+  // See comments in `AlignBegin` for motivation on the hand-rolled for loops.
+  const size_t delta = capacity() - end();
+  if (delta != 0) {
+    const size_t new_begin = begin() + delta;
+    const size_t new_end = end() + delta;
+    set_begin(new_begin);
+    set_end(new_end);
+    ABSL_ASSUME(new_end <= kMaxCapacity);
+#ifdef __clang__
+#pragma unroll 1
+#endif
+    for (size_t i = new_end - 1; i >= new_begin; --i) {
+      edges_[i] = edges_[i - delta];
+    }
+  }
+}
+
+template <>
+inline void CordRepBtree::Add<CordRepBtree::kBack>(CordRep* rep) {
+  AlignBegin();
+  edges_[fetch_add_end(1)] = rep;
+}
+
+template <>
+inline void CordRepBtree::Add<CordRepBtree::kBack>(
+    absl::Span<CordRep* const> edges) {
+  AlignBegin();
+  size_t new_end = end();
+  for (CordRep* edge : edges) edges_[new_end++] = edge;
+  set_end(new_end);
+}
+
+template <>
+inline void CordRepBtree::Add<CordRepBtree::kFront>(CordRep* rep) {
+  AlignEnd();
+  edges_[sub_fetch_begin(1)] = rep;
+}
+
+template <>
+inline void CordRepBtree::Add<CordRepBtree::kFront>(
+    absl::Span<CordRep* const> edges) {
+  AlignEnd();
+  size_t new_begin = begin() - edges.size();
+  set_begin(new_begin);
+  for (CordRep* edge : edges) edges_[new_begin++] = edge;
+}
+
+template <CordRepBtree::EdgeType edge_type>
+inline void CordRepBtree::SetEdge(CordRep* edge) {
+  const int idx = edge_type == kFront ? begin() : back();
+  CordRep::Unref(edges_[idx]);
+  edges_[idx] = edge;
+}
+
+inline CordRepBtree::OpResult CordRepBtree::ToOpResult(bool owned) {
+  return owned ? OpResult{this, kSelf} : OpResult{Copy(), kCopied};
+}
+
+inline CordRepBtree::Position CordRepBtree::IndexOf(size_t offset) const {
+  assert(offset < length);
+  size_t index = begin();
+  while (offset >= edges_[index]->length) offset -= edges_[index++]->length;
+  return {index, offset};
+}
+
+inline CordRepBtree::Position CordRepBtree::IndexBefore(size_t offset) const {
+  assert(offset > 0);
+  assert(offset <= length);
+  size_t index = begin();
+  while (offset > edges_[index]->length) offset -= edges_[index++]->length;
+  return {index, offset};
+}
+
+inline CordRepBtree::Position CordRepBtree::IndexBefore(Position front,
+                                                        size_t offset) const {
+  size_t index = front.index;
+  offset = offset + front.n;
+  while (offset > edges_[index]->length) offset -= edges_[index++]->length;
+  return {index, offset};
+}
+
+inline CordRepBtree::Position CordRepBtree::IndexOfLength(size_t n) const {
+  assert(n <= length);
+  size_t index = back();
+  size_t strip = length - n;
+  while (strip >= edges_[index]->length) strip -= edges_[index--]->length;
+  return {index, edges_[index]->length - strip};
+}
+
+inline CordRepBtree::Position CordRepBtree::IndexBeyond(
+    const size_t offset) const {
+  // We need to find the edge which `starting offset` is beyond (>=)`offset`.
+  // For this we can't use the `offset -= length` logic of IndexOf. Instead, we
+  // track the offset of the `current edge` in `off`, which we increase as we
+  // iterate over the edges until we find the matching edge.
+  size_t off = 0;
+  size_t index = begin();
+  while (offset > off) off += edges_[index++]->length;
+  return {index, off - offset};
+}
+
+inline CordRepBtree* CordRepBtree::Create(CordRep* rep) {
+  if (IsDataEdge(rep)) return New(rep);
+  return CreateSlow(rep);
+}
+
+inline Span<char> CordRepBtree::GetAppendBuffer(size_t size) {
+  assert(refcount.IsOne());
+  CordRepBtree* tree = this;
+  const int height = this->height();
+  CordRepBtree* n1 = tree;
+  CordRepBtree* n2 = tree;
+  CordRepBtree* n3 = tree;
+  switch (height) {
+    case 3:
+      tree = tree->Edge(kBack)->btree();
+      if (!tree->refcount.IsOne()) return {};
+      n2 = tree;
+      [[fallthrough]];
+    case 2:
+      tree = tree->Edge(kBack)->btree();
+      if (!tree->refcount.IsOne()) return {};
+      n1 = tree;
+      [[fallthrough]];
+    case 1:
+      tree = tree->Edge(kBack)->btree();
+      if (!tree->refcount.IsOne()) return {};
+      [[fallthrough]];
+    case 0:
+      CordRep* edge = tree->Edge(kBack);
+      if (!edge->refcount.IsOne()) return {};
+      if (edge->tag < FLAT) return {};
+      size_t avail = edge->flat()->Capacity() - edge->length;
+      if (avail == 0) return {};
+      size_t delta = (std::min)(size, avail);
+      Span<char> span = {edge->flat()->Data() + edge->length, delta};
+      edge->length += delta;
+      switch (height) {
+        case 3:
+          n3->length += delta;
+          [[fallthrough]];
+        case 2:
+          n2->length += delta;
+          [[fallthrough]];
+        case 1:
+          n1->length += delta;
+          [[fallthrough]];
+        case 0:
+          tree->length += delta;
+          return span;
+      }
+      break;
+  }
+  return GetAppendBufferSlow(size);
+}
+
+extern template CordRepBtree* CordRepBtree::AddCordRep<CordRepBtree::kBack>(
+    CordRepBtree* tree, CordRep* rep);
+
+extern template CordRepBtree* CordRepBtree::AddCordRep<CordRepBtree::kFront>(
+    CordRepBtree* tree, CordRep* rep);
+
+inline CordRepBtree* CordRepBtree::Append(CordRepBtree* tree, CordRep* rep) {
+  if (ABSL_PREDICT_TRUE(IsDataEdge(rep))) {
+    return CordRepBtree::AddCordRep<kBack>(tree, rep);
+  }
+  return AppendSlow(tree, rep);
+}
+
+inline CordRepBtree* CordRepBtree::Prepend(CordRepBtree* tree, CordRep* rep) {
+  if (ABSL_PREDICT_TRUE(IsDataEdge(rep))) {
+    return CordRepBtree::AddCordRep<kFront>(tree, rep);
+  }
+  return PrependSlow(tree, rep);
+}
+
+#ifdef NDEBUG
+
+inline CordRepBtree* CordRepBtree::AssertValid(CordRepBtree* tree,
+                                               bool /* shallow */) {
+  return tree;
+}
+
+inline const CordRepBtree* CordRepBtree::AssertValid(const CordRepBtree* tree,
+                                                     bool /* shallow */) {
+  return tree;
+}
+
+#endif
+
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STRINGS_INTERNAL_CORD_REP_BTREE_H_
diff --git a/absl/strings/internal/cord_rep_btree_navigator.cc b/absl/strings/internal/cord_rep_btree_navigator.cc
new file mode 100644
index 00000000000..707e5c8effb
--- /dev/null
+++ b/absl/strings/internal/cord_rep_btree_navigator.cc
@@ -0,0 +1,189 @@
+// Copyright 2021 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/strings/internal/cord_rep_btree_navigator.h"
+
+#include <cassert>
+#include <cstddef>
+#include <cstdint>
+
+#include "absl/strings/internal/cord_data_edge.h"
+#include "absl/strings/internal/cord_internal.h"
+#include "absl/strings/internal/cord_rep_btree.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+
+using ReadResult = CordRepBtreeNavigator::ReadResult;
+
+namespace {
+
+// Returns a `CordRepSubstring` from `rep` starting at `offset` of size `n`.
+// If `rep` is already a `CordRepSubstring` instance, an adjusted instance is
+// created based on the old offset and new offset.
+// Adopts a reference on `rep`. Rep must be a valid data edge. Returns
+// nullptr if `n == 0`, `rep` if `n == rep->length`.
+// Requires `offset < rep->length` and `offset + n <= rep->length`.
+// TODO(192061034): move to utility library in internal and optimize for small
+// substrings of larger reps.
+inline CordRep* Substring(CordRep* rep, size_t offset, size_t n) {
+  assert(n <= rep->length);
+  assert(offset < rep->length);
+  assert(offset <= rep->length - n);
+  assert(IsDataEdge(rep));
+
+  if (n == 0) return nullptr;
+  if (n == rep->length) return CordRep::Ref(rep);
+
+  if (rep->tag == SUBSTRING) {
+    offset += rep->substring()->start;
+    rep = rep->substring()->child;
+  }
+
+  assert(rep->IsExternal() || rep->IsFlat());
+  CordRepSubstring* substring = new CordRepSubstring();
+  substring->length = n;
+  substring->tag = SUBSTRING;
+  substring->start = offset;
+  substring->child = CordRep::Ref(rep);
+  return substring;
+}
+
+inline CordRep* Substring(CordRep* rep, size_t offset) {
+  return Substring(rep, offset, rep->length - offset);
+}
+
+}  // namespace
+
+CordRepBtreeNavigator::Position CordRepBtreeNavigator::Skip(size_t n) {
+  int height = 0;
+  size_t index = index_[0];
+  CordRepBtree* node = node_[0];
+  CordRep* edge = node->Edge(index);
+
+  // Overall logic: Find an edge of at least the length we need to skip.
+  // We consume all edges which are smaller (i.e., must be 100% skipped).
+  // If we exhausted all edges on the current level, we move one level
+  // up the tree, and repeat until we either find the edge, or until we hit
+  // the top of the tree meaning the skip exceeds tree->length.
+  while (n >= edge->length) {
+    n -= edge->length;
+    while (++index == node->end()) {
+      if (++height > height_) return {nullptr, n};
+      node = node_[height];
+      index = index_[height];
+    }
+    edge = node->Edge(index);
+  }
+
+  // If we moved up the tree, descend down to the leaf level, consuming all
+  // edges that must be skipped.
+  while (height > 0) {
+    node = edge->btree();
+    index_[height] = static_cast<uint8_t>(index);
+    node_[--height] = node;
+    index = node->begin();
+    edge = node->Edge(index);
+    while (n >= edge->length) {
+      n -= edge->length;
+      ++index;
+      assert(index != node->end());
+      edge = node->Edge(index);
+    }
+  }
+  index_[0] = static_cast<uint8_t>(index);
+  return {edge, n};
+}
+
+ReadResult CordRepBtreeNavigator::Read(size_t edge_offset, size_t n) {
+  int height = 0;
+  size_t length = edge_offset + n;
+  size_t index = index_[0];
+  CordRepBtree* node = node_[0];
+  CordRep* edge = node->Edge(index);
+  assert(edge_offset < edge->length);
+
+  if (length < edge->length) {
+    return {Substring(edge, edge_offset, n), length};
+  }
+
+  // Similar to 'Skip', we consume all edges that are inside the 'length' of
+  // data that needs to be read. If we exhaust the current level, we move one
+  // level up the tree and repeat until we hit the final edge that must be
+  // (partially) read. We consume all edges into `subtree`.
+  CordRepBtree* subtree = CordRepBtree::New(Substring(edge, edge_offset));
+  size_t subtree_end = 1;
+  do {
+    length -= edge->length;
+    while (++index == node->end()) {
+      index_[height] = static_cast<uint8_t>(index);
+      if (++height > height_) {
+        subtree->set_end(subtree_end);
+        if (length == 0) return {subtree, 0};
+        CordRep::Unref(subtree);
+        return {nullptr, length};
+      }
+      if (length != 0) {
+        subtree->set_end(subtree_end);
+        subtree = CordRepBtree::New(subtree);
+        subtree_end = 1;
+      }
+      node = node_[height];
+      index = index_[height];
+    }
+    edge = node->Edge(index);
+    if (length >= edge->length) {
+      subtree->length += edge->length;
+      subtree->edges_[subtree_end++] = CordRep::Ref(edge);
+    }
+  } while (length >= edge->length);
+  CordRepBtree* tree = subtree;
+  subtree->length += length;
+
+  // If we moved up the tree, descend down to the leaf level, consuming all
+  // edges that must be read, adding 'down' nodes to `subtree`.
+  while (height > 0) {
+    node = edge->btree();
+    index_[height] = static_cast<uint8_t>(index);
+    node_[--height] = node;
+    index = node->begin();
+    edge = node->Edge(index);
+
+    if (length != 0) {
+      CordRepBtree* right = CordRepBtree::New(height);
+      right->length = length;
+      subtree->edges_[subtree_end++] = right;
+      subtree->set_end(subtree_end);
+      subtree = right;
+      subtree_end = 0;
+      while (length >= edge->length) {
+        subtree->edges_[subtree_end++] = CordRep::Ref(edge);
+        length -= edge->length;
+        edge = node->Edge(++index);
+      }
+    }
+  }
+  // Add any (partial) edge still remaining at the leaf level.
+  if (length != 0) {
+    subtree->edges_[subtree_end++] = Substring(edge, 0, length);
+  }
+  subtree->set_end(subtree_end);
+  index_[0] = static_cast<uint8_t>(index);
+  return {tree, length};
+}
+
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/strings/internal/cord_rep_btree_navigator.h b/absl/strings/internal/cord_rep_btree_navigator.h
new file mode 100644
index 00000000000..1024525d4a7
--- /dev/null
+++ b/absl/strings/internal/cord_rep_btree_navigator.h
@@ -0,0 +1,270 @@
+// Copyright 2021 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_STRINGS_INTERNAL_CORD_REP_BTREE_NAVIGATOR_H_
+#define ABSL_STRINGS_INTERNAL_CORD_REP_BTREE_NAVIGATOR_H_
+
+#include <cassert>
+#include <cstddef>
+#include <cstdint>
+#include <iostream>
+
+#include "absl/base/optimization.h"
+#include "absl/strings/internal/cord_internal.h"
+#include "absl/strings/internal/cord_rep_btree.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+
+// CordRepBtreeNavigator is a bi-directional navigator allowing callers to
+// navigate all the (leaf) data edges in a CordRepBtree instance.
+//
+// A CordRepBtreeNavigator instance is by default empty. Callers initialize a
+// navigator instance by calling one of `InitFirst()`, `InitLast()` or
+// `InitOffset()`, which establishes a current position. Callers can then
+// navigate using the `Next`, `Previous`, `Skip` and `Seek` methods.
+//
+// The navigator instance does not take or adopt a reference on the provided
+// `tree` on any of the initialization calls. Callers are responsible for
+// guaranteeing the lifecycle of the provided tree. A navigator instance can
+// be reset to the empty state by calling `Reset`.
+//
+// A navigator only keeps positional state on the 'current data edge', it does
+// explicitly not keep any 'offset' state. The class does accept and return
+// offsets in the `Read()`, `Skip()` and 'Seek()` methods as these would
+// otherwise put a big burden on callers. Callers are expected to maintain
+// (returned) offset info if they require such granular state.
+class CordRepBtreeNavigator {
+ public:
+  // The logical position as returned by the Seek() and Skip() functions.
+  // Returns the current leaf edge for the desired seek or skip position and
+  // the offset of that position inside that edge.
+  struct Position {
+    CordRep* edge;
+    size_t offset;
+  };
+
+  // The read result as returned by the Read() function.
+  // `tree` contains the resulting tree which is identical to the result
+  // of calling CordRepBtree::SubTree(...) on the tree being navigated.
+  // `n` contains the number of bytes used from the last navigated to
+  // edge of the tree.
+  struct ReadResult {
+    CordRep* tree;
+    size_t n;
+  };
+
+  // Returns true if this instance is not empty.
+  explicit operator bool() const;
+
+  // Returns the tree for this instance or nullptr if empty.
+  CordRepBtree* btree() const;
+
+  // Returns the data edge of the current position.
+  // Requires this instance to not be empty.
+  CordRep* Current() const;
+
+  // Resets this navigator to `tree`, returning the first data edge in the tree.
+  CordRep* InitFirst(CordRepBtree* tree);
+
+  // Resets this navigator to `tree`, returning the last data edge in the tree.
+  CordRep* InitLast(CordRepBtree* tree);
+
+  // Resets this navigator to `tree` returning the data edge at position
+  // `offset` and the relative offset of `offset` into that data edge.
+  // Returns `Position.edge = nullptr` if the provided offset is greater
+  // than or equal to the length of the tree, in which case the state of
+  // the navigator instance remains unchanged.
+  Position InitOffset(CordRepBtree* tree, size_t offset);
+
+  // Navigates to the next data edge.
+  // Returns the next data edge or nullptr if there is no next data edge, in
+  // which case the current position remains unchanged.
+  CordRep* Next();
+
+  // Navigates to the previous data edge.
+  // Returns the previous data edge or nullptr if there is no previous data
+  // edge, in which case the current position remains unchanged.
+  CordRep* Previous();
+
+  // Navigates to the data edge at position `offset`. Returns the navigated to
+  // data edge in `Position.edge` and the relative offset of `offset` into that
+  // data edge in `Position.offset`. Returns `Position.edge = nullptr` if the
+  // provide offset is greater than or equal to the tree's length.
+  Position Seek(size_t offset);
+
+  // Reads `n` bytes of data starting at offset `edge_offset` of the current
+  // data edge, and returns the result in `ReadResult.tree`. `ReadResult.n`
+  // contains the 'bytes used` from the last / current data edge in the tree.
+  // This allows users that mix regular navigation (using string views) and
+  // 'read into cord' navigation to keep track of the current state, and which
+  // bytes have been consumed from a navigator.
+  // This function returns `ReadResult.tree = nullptr` if the requested length
+  // exceeds the length of the tree starting at the current data edge.
+  ReadResult Read(size_t edge_offset, size_t n);
+
+  // Skips `n` bytes forward from the current data edge, returning the navigated
+  // to data edge in `Position.edge` and `Position.offset` containing the offset
+  // inside that data edge. Note that the state of the navigator is left
+  // unchanged if `n` is smaller than the length of the current data edge.
+  Position Skip(size_t n);
+
+  // Resets this instance to the default / empty state.
+  void Reset();
+
+ private:
+  // Slow path for Next() if Next() reached the end of a leaf node. Backtracks
+  // up the stack until it finds a node that has a 'next' position available,
+  // and then does a 'front dive' towards the next leaf node.
+  CordRep* NextUp();
+
+  // Slow path for Previous() if Previous() reached the beginning of a leaf
+  // node. Backtracks up the stack until it finds a node that has a 'previous'
+  // position available, and then does a 'back dive' towards the previous leaf
+  // node.
+  CordRep* PreviousUp();
+
+  // Generic implementation of InitFirst() and InitLast().
+  template <CordRepBtree::EdgeType edge_type>
+  CordRep* Init(CordRepBtree* tree);
+
+  // `height_` contains the height of the current tree, or -1 if empty.
+  int height_ = -1;
+
+  // `index_` and `node_` contain the navigation state as the 'path' to the
+  // current data edge which is at `node_[0]->Edge(index_[0])`. The contents
+  // of these are undefined until the instance is initialized (`height_ >= 0`).
+  uint8_t index_[CordRepBtree::kMaxDepth];
+  CordRepBtree* node_[CordRepBtree::kMaxDepth];
+};
+
+// Returns true if this instance is not empty.
+inline CordRepBtreeNavigator::operator bool() const { return height_ >= 0; }
+
+inline CordRepBtree* CordRepBtreeNavigator::btree() const {
+  return height_ >= 0 ? node_[height_] : nullptr;
+}
+
+inline CordRep* CordRepBtreeNavigator::Current() const {
+  assert(height_ >= 0);
+  return node_[0]->Edge(index_[0]);
+}
+
+inline void CordRepBtreeNavigator::Reset() { height_ = -1; }
+
+inline CordRep* CordRepBtreeNavigator::InitFirst(CordRepBtree* tree) {
+  return Init<CordRepBtree::kFront>(tree);
+}
+
+inline CordRep* CordRepBtreeNavigator::InitLast(CordRepBtree* tree) {
+  return Init<CordRepBtree::kBack>(tree);
+}
+
+template <CordRepBtree::EdgeType edge_type>
+inline CordRep* CordRepBtreeNavigator::Init(CordRepBtree* tree) {
+  assert(tree != nullptr);
+  assert(tree->size() > 0);
+  assert(tree->height() <= CordRepBtree::kMaxHeight);
+  int height = height_ = tree->height();
+  size_t index = tree->index(edge_type);
+  node_[height] = tree;
+  index_[height] = static_cast<uint8_t>(index);
+  while (--height >= 0) {
+    tree = tree->Edge(index)->btree();
+    node_[height] = tree;
+    index = tree->index(edge_type);
+    index_[height] = static_cast<uint8_t>(index);
+  }
+  return node_[0]->Edge(index);
+}
+
+inline CordRepBtreeNavigator::Position CordRepBtreeNavigator::Seek(
+    size_t offset) {
+  assert(btree() != nullptr);
+  int height = height_;
+  CordRepBtree* edge = node_[height];
+  if (ABSL_PREDICT_FALSE(offset >= edge->length)) return {nullptr, 0};
+  CordRepBtree::Position index = edge->IndexOf(offset);
+  index_[height] = static_cast<uint8_t>(index.index);
+  while (--height >= 0) {
+    edge = edge->Edge(index.index)->btree();
+    node_[height] = edge;
+    index = edge->IndexOf(index.n);
+    index_[height] = static_cast<uint8_t>(index.index);
+  }
+  return {edge->Edge(index.index), index.n};
+}
+
+inline CordRepBtreeNavigator::Position CordRepBtreeNavigator::InitOffset(
+    CordRepBtree* tree, size_t offset) {
+  assert(tree != nullptr);
+  assert(tree->height() <= CordRepBtree::kMaxHeight);
+  if (ABSL_PREDICT_FALSE(offset >= tree->length)) return {nullptr, 0};
+  height_ = tree->height();
+  node_[height_] = tree;
+  return Seek(offset);
+}
+
+inline CordRep* CordRepBtreeNavigator::Next() {
+  CordRepBtree* edge = node_[0];
+  return index_[0] == edge->back() ? NextUp() : edge->Edge(++index_[0]);
+}
+
+inline CordRep* CordRepBtreeNavigator::Previous() {
+  CordRepBtree* edge = node_[0];
+  return index_[0] == edge->begin() ? PreviousUp() : edge->Edge(--index_[0]);
+}
+
+inline CordRep* CordRepBtreeNavigator::NextUp() {
+  assert(index_[0] == node_[0]->back());
+  CordRepBtree* edge;
+  size_t index;
+  int height = 0;
+  do {
+    if (++height > height_) return nullptr;
+    edge = node_[height];
+    index = index_[height] + 1;
+  } while (index == edge->end());
+  index_[height] = static_cast<uint8_t>(index);
+  do {
+    node_[--height] = edge = edge->Edge(index)->btree();
+    index_[height] = static_cast<uint8_t>(index = edge->begin());
+  } while (height > 0);
+  return edge->Edge(index);
+}
+
+inline CordRep* CordRepBtreeNavigator::PreviousUp() {
+  assert(index_[0] == node_[0]->begin());
+  CordRepBtree* edge;
+  size_t index;
+  int height = 0;
+  do {
+    if (++height > height_) return nullptr;
+    edge = node_[height];
+    index = index_[height];
+  } while (index == edge->begin());
+  index_[height] = static_cast<uint8_t>(--index);
+  do {
+    node_[--height] = edge = edge->Edge(index)->btree();
+    index_[height] = static_cast<uint8_t>(index = edge->back());
+  } while (height > 0);
+  return edge->Edge(index);
+}
+
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STRINGS_INTERNAL_CORD_REP_BTREE_NAVIGATOR_H_
diff --git a/absl/strings/internal/cord_rep_btree_navigator_test.cc b/absl/strings/internal/cord_rep_btree_navigator_test.cc
new file mode 100644
index 00000000000..6e4a79b6536
--- /dev/null
+++ b/absl/strings/internal/cord_rep_btree_navigator_test.cc
@@ -0,0 +1,347 @@
+// Copyright 2021 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/strings/internal/cord_rep_btree_navigator.h"
+
+#include <cstddef>
+#include <string>
+#include <vector>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/strings/internal/cord_internal.h"
+#include "absl/strings/internal/cord_rep_btree.h"
+#include "absl/strings/internal/cord_rep_test_util.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+namespace {
+
+using ::testing::Eq;
+using ::testing::Ne;
+
+using ::absl::cordrep_testing::CordRepBtreeFromFlats;
+using ::absl::cordrep_testing::CordToString;
+using ::absl::cordrep_testing::CreateFlatsFromString;
+using ::absl::cordrep_testing::CreateRandomString;
+using ::absl::cordrep_testing::MakeFlat;
+using ::absl::cordrep_testing::MakeSubstring;
+
+using ReadResult = CordRepBtreeNavigator::ReadResult;
+using Position = CordRepBtreeNavigator::Position;
+
+// CordRepBtreeNavigatorTest is a test fixture which automatically creates a
+// tree to test navigation logic on. The parameter `count' defines the number of
+// data edges in the test tree.
+class CordRepBtreeNavigatorTest : public testing::TestWithParam<size_t> {
+ public:
+  using Flats = std::vector<CordRep*>;
+  static constexpr size_t kCharsPerFlat = 3;
+
+  CordRepBtreeNavigatorTest() {
+    data_ = CreateRandomString(count() * kCharsPerFlat);
+    flats_ = CreateFlatsFromString(data_, kCharsPerFlat);
+
+    // Turn flat 0 or 1 into a substring to cover partial reads on substrings.
+    if (count() > 1) {
+      CordRep::Unref(flats_[1]);
+      flats_[1] = MakeSubstring(kCharsPerFlat, kCharsPerFlat, MakeFlat(data_));
+    } else {
+      CordRep::Unref(flats_[0]);
+      flats_[0] = MakeSubstring(0, kCharsPerFlat, MakeFlat(data_));
+    }
+
+    tree_ = CordRepBtreeFromFlats(flats_);
+  }
+
+  ~CordRepBtreeNavigatorTest() override { CordRep::Unref(tree_); }
+
+  size_t count() const { return GetParam(); }
+  CordRepBtree* tree() { return tree_; }
+  const std::string& data() const { return data_; }
+  const std::vector<CordRep*>& flats() const { return flats_; }
+
+  static std::string ToString(testing::TestParamInfo<size_t> param) {
+    return absl::StrCat(param.param, "_Flats");
+  }
+
+ private:
+  std::string data_;
+  Flats flats_;
+  CordRepBtree* tree_;
+};
+
+INSTANTIATE_TEST_SUITE_P(
+    WithParam, CordRepBtreeNavigatorTest,
+    testing::Values(1, CordRepBtree::kMaxCapacity - 1,
+                    CordRepBtree::kMaxCapacity,
+                    CordRepBtree::kMaxCapacity* CordRepBtree::kMaxCapacity - 1,
+                    CordRepBtree::kMaxCapacity* CordRepBtree::kMaxCapacity,
+                    CordRepBtree::kMaxCapacity* CordRepBtree::kMaxCapacity + 1,
+                    CordRepBtree::kMaxCapacity* CordRepBtree::kMaxCapacity * 2 +
+                        17),
+    CordRepBtreeNavigatorTest::ToString);
+
+TEST(CordRepBtreeNavigatorTest, Uninitialized) {
+  CordRepBtreeNavigator nav;
+  EXPECT_FALSE(nav);
+  EXPECT_THAT(nav.btree(), Eq(nullptr));
+#if defined(GTEST_HAS_DEATH_TEST) && !defined(NDEBUG)
+  EXPECT_DEATH(nav.Current(), ".*");
+#endif
+}
+
+TEST_P(CordRepBtreeNavigatorTest, InitFirst) {
+  CordRepBtreeNavigator nav;
+  CordRep* edge = nav.InitFirst(tree());
+  EXPECT_TRUE(nav);
+  EXPECT_THAT(nav.btree(), Eq(tree()));
+  EXPECT_THAT(nav.Current(), Eq(flats().front()));
+  EXPECT_THAT(edge, Eq(flats().front()));
+}
+
+TEST_P(CordRepBtreeNavigatorTest, InitLast) {
+  CordRepBtreeNavigator nav;
+  CordRep* edge = nav.InitLast(tree());
+  EXPECT_TRUE(nav);
+  EXPECT_THAT(nav.btree(), Eq(tree()));
+  EXPECT_THAT(nav.Current(), Eq(flats().back()));
+  EXPECT_THAT(edge, Eq(flats().back()));
+}
+
+TEST_P(CordRepBtreeNavigatorTest, NextPrev) {
+  CordRepBtreeNavigator nav;
+  nav.InitFirst(tree());
+  const Flats& flats = this->flats();
+
+  EXPECT_THAT(nav.Previous(), Eq(nullptr));
+  EXPECT_THAT(nav.Current(), Eq(flats.front()));
+  for (size_t i = 1; i < flats.size(); ++i) {
+    ASSERT_THAT(nav.Next(), Eq(flats[i]));
+    EXPECT_THAT(nav.Current(), Eq(flats[i]));
+  }
+  EXPECT_THAT(nav.Next(), Eq(nullptr));
+  EXPECT_THAT(nav.Current(), Eq(flats.back()));
+  for (size_t i = flats.size() - 1; i > 0; --i) {
+    ASSERT_THAT(nav.Previous(), Eq(flats[i - 1]));
+    EXPECT_THAT(nav.Current(), Eq(flats[i - 1]));
+  }
+  EXPECT_THAT(nav.Previous(), Eq(nullptr));
+  EXPECT_THAT(nav.Current(), Eq(flats.front()));
+}
+
+TEST_P(CordRepBtreeNavigatorTest, PrevNext) {
+  CordRepBtreeNavigator nav;
+  nav.InitLast(tree());
+  const Flats& flats = this->flats();
+
+  EXPECT_THAT(nav.Next(), Eq(nullptr));
+  EXPECT_THAT(nav.Current(), Eq(flats.back()));
+  for (size_t i = flats.size() - 1; i > 0; --i) {
+    ASSERT_THAT(nav.Previous(), Eq(flats[i - 1]));
+    EXPECT_THAT(nav.Current(), Eq(flats[i - 1]));
+  }
+  EXPECT_THAT(nav.Previous(), Eq(nullptr));
+  EXPECT_THAT(nav.Current(), Eq(flats.front()));
+  for (size_t i = 1; i < flats.size(); ++i) {
+    ASSERT_THAT(nav.Next(), Eq(flats[i]));
+    EXPECT_THAT(nav.Current(), Eq(flats[i]));
+  }
+  EXPECT_THAT(nav.Next(), Eq(nullptr));
+  EXPECT_THAT(nav.Current(), Eq(flats.back()));
+}
+
+TEST(CordRepBtreeNavigatorTest, Reset) {
+  CordRepBtree* tree = CordRepBtree::Create(MakeFlat("abc"));
+  CordRepBtreeNavigator nav;
+  nav.InitFirst(tree);
+  nav.Reset();
+  EXPECT_FALSE(nav);
+  EXPECT_THAT(nav.btree(), Eq(nullptr));
+#if defined(GTEST_HAS_DEATH_TEST) && !defined(NDEBUG)
+  EXPECT_DEATH(nav.Current(), ".*");
+#endif
+  CordRep::Unref(tree);
+}
+
+TEST_P(CordRepBtreeNavigatorTest, Skip) {
+  size_t count = this->count();
+  const Flats& flats = this->flats();
+  CordRepBtreeNavigator nav;
+  nav.InitFirst(tree());
+
+  for (size_t char_offset = 0; char_offset < kCharsPerFlat; ++char_offset) {
+    Position pos = nav.Skip(char_offset);
+    EXPECT_THAT(pos.edge, Eq(nav.Current()));
+    EXPECT_THAT(pos.edge, Eq(flats[0]));
+    EXPECT_THAT(pos.offset, Eq(char_offset));
+  }
+
+  for (size_t index1 = 0; index1 < count; ++index1) {
+    for (size_t index2 = index1; index2 < count; ++index2) {
+      for (size_t char_offset = 0; char_offset < kCharsPerFlat; ++char_offset) {
+        CordRepBtreeNavigator nav;
+        nav.InitFirst(tree());
+
+        size_t length1 = index1 * kCharsPerFlat;
+        Position pos1 = nav.Skip(length1 + char_offset);
+        ASSERT_THAT(pos1.edge, Eq(flats[index1]));
+        ASSERT_THAT(pos1.edge, Eq(nav.Current()));
+        ASSERT_THAT(pos1.offset, Eq(char_offset));
+
+        size_t length2 = index2 * kCharsPerFlat;
+        Position pos2 = nav.Skip(length2 - length1 + char_offset);
+        ASSERT_THAT(pos2.edge, Eq(flats[index2]));
+        ASSERT_THAT(pos2.edge, Eq(nav.Current()));
+        ASSERT_THAT(pos2.offset, Eq(char_offset));
+      }
+    }
+  }
+}
+
+TEST_P(CordRepBtreeNavigatorTest, Seek) {
+  size_t count = this->count();
+  const Flats& flats = this->flats();
+  CordRepBtreeNavigator nav;
+  nav.InitFirst(tree());
+
+  for (size_t char_offset = 0; char_offset < kCharsPerFlat; ++char_offset) {
+    Position pos = nav.Seek(char_offset);
+    EXPECT_THAT(pos.edge, Eq(nav.Current()));
+    EXPECT_THAT(pos.edge, Eq(flats[0]));
+    EXPECT_THAT(pos.offset, Eq(char_offset));
+  }
+
+  for (size_t index = 0; index < count; ++index) {
+    for (size_t char_offset = 0; char_offset < kCharsPerFlat; ++char_offset) {
+      size_t offset = index * kCharsPerFlat + char_offset;
+      Position pos1 = nav.Seek(offset);
+      ASSERT_THAT(pos1.edge, Eq(flats[index]));
+      ASSERT_THAT(pos1.edge, Eq(nav.Current()));
+      ASSERT_THAT(pos1.offset, Eq(char_offset));
+    }
+  }
+}
+
+TEST(CordRepBtreeNavigatorTest, InitOffset) {
+  // Whitebox: InitOffset() is implemented in terms of Seek() which is
+  // exhaustively tested. Only test it initializes / forwards properly..
+  CordRepBtree* tree = CordRepBtree::Create(MakeFlat("abc"));
+  tree = CordRepBtree::Append(tree, MakeFlat("def"));
+  CordRepBtreeNavigator nav;
+  Position pos = nav.InitOffset(tree, 5);
+  EXPECT_TRUE(nav);
+  EXPECT_THAT(nav.btree(), Eq(tree));
+  EXPECT_THAT(pos.edge, Eq(tree->Edges()[1]));
+  EXPECT_THAT(pos.edge, Eq(nav.Current()));
+  EXPECT_THAT(pos.offset, Eq(2u));
+  CordRep::Unref(tree);
+}
+
+TEST(CordRepBtreeNavigatorTest, InitOffsetAndSeekBeyondLength) {
+  CordRepBtree* tree1 = CordRepBtree::Create(MakeFlat("abc"));
+  CordRepBtree* tree2 = CordRepBtree::Create(MakeFlat("def"));
+
+  CordRepBtreeNavigator nav;
+  nav.InitFirst(tree1);
+  EXPECT_THAT(nav.Seek(3).edge, Eq(nullptr));
+  EXPECT_THAT(nav.Seek(100).edge, Eq(nullptr));
+  EXPECT_THAT(nav.btree(), Eq(tree1));
+  EXPECT_THAT(nav.Current(), Eq(tree1->Edges().front()));
+
+  EXPECT_THAT(nav.InitOffset(tree2, 3).edge, Eq(nullptr));
+  EXPECT_THAT(nav.InitOffset(tree2, 100).edge, Eq(nullptr));
+  EXPECT_THAT(nav.btree(), Eq(tree1));
+  EXPECT_THAT(nav.Current(), Eq(tree1->Edges().front()));
+
+  CordRep::Unref(tree1);
+  CordRep::Unref(tree2);
+}
+
+TEST_P(CordRepBtreeNavigatorTest, Read) {
+  const Flats& flats = this->flats();
+  const std::string& data = this->data();
+
+  for (size_t offset = 0; offset < data.size(); ++offset) {
+    for (size_t length = 1; length <= data.size() - offset; ++length) {
+      CordRepBtreeNavigator nav;
+      nav.InitFirst(tree());
+
+      // Skip towards edge holding offset
+      size_t edge_offset = nav.Skip(offset).offset;
+
+      // Read node
+      ReadResult result = nav.Read(edge_offset, length);
+      ASSERT_THAT(result.tree, Ne(nullptr));
+      EXPECT_THAT(result.tree->length, Eq(length));
+      if (result.tree->tag == BTREE) {
+        ASSERT_TRUE(CordRepBtree::IsValid(result.tree->btree()));
+      }
+
+      // Verify contents
+      std::string value = CordToString(result.tree);
+      EXPECT_THAT(value, Eq(data.substr(offset, length)));
+
+      // Verify 'partial last edge' reads.
+      size_t partial = (offset + length) % kCharsPerFlat;
+      ASSERT_THAT(result.n, Eq(partial));
+
+      // Verify ending position if not EOF
+      if (offset + length < data.size()) {
+        size_t index = (offset + length) / kCharsPerFlat;
+        EXPECT_THAT(nav.Current(), Eq(flats[index]));
+      }
+
+      CordRep::Unref(result.tree);
+    }
+  }
+}
+
+TEST_P(CordRepBtreeNavigatorTest, ReadBeyondLengthOfTree) {
+  CordRepBtreeNavigator nav;
+  nav.InitFirst(tree());
+  ReadResult result = nav.Read(2, tree()->length);
+  ASSERT_THAT(result.tree, Eq(nullptr));
+}
+
+TEST(CordRepBtreeNavigatorTest, NavigateMaximumTreeDepth) {
+  CordRepFlat* flat1 = MakeFlat("Hello world");
+  CordRepFlat* flat2 = MakeFlat("World Hello");
+
+  CordRepBtree* node = CordRepBtree::Create(flat1);
+  node = CordRepBtree::Append(node, flat2);
+  while (node->height() < CordRepBtree::kMaxHeight) {
+    node = CordRepBtree::New(node);
+  }
+
+  CordRepBtreeNavigator nav;
+  CordRep* edge = nav.InitFirst(node);
+  EXPECT_THAT(edge, Eq(flat1));
+  EXPECT_THAT(nav.Next(), Eq(flat2));
+  EXPECT_THAT(nav.Next(), Eq(nullptr));
+  EXPECT_THAT(nav.Previous(), Eq(flat1));
+  EXPECT_THAT(nav.Previous(), Eq(nullptr));
+
+  CordRep::Unref(node);
+}
+
+}  // namespace
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/strings/internal/cord_rep_btree_reader.cc b/absl/strings/internal/cord_rep_btree_reader.cc
new file mode 100644
index 00000000000..660f2406728
--- /dev/null
+++ b/absl/strings/internal/cord_rep_btree_reader.cc
@@ -0,0 +1,71 @@
+// Copyright 2021 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/strings/internal/cord_rep_btree_reader.h"
+
+#include <cassert>
+#include <cstddef>
+
+#include "absl/base/config.h"
+#include "absl/strings/internal/cord_data_edge.h"
+#include "absl/strings/internal/cord_internal.h"
+#include "absl/strings/internal/cord_rep_btree.h"
+#include "absl/strings/internal/cord_rep_btree_navigator.h"
+#include "absl/strings/internal/cord_rep_flat.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+
+absl::string_view CordRepBtreeReader::Read(size_t n, size_t chunk_size,
+                                           CordRep*& tree) {
+  assert(chunk_size <= navigator_.Current()->length);
+
+  // If chunk_size is non-zero, we need to start inside last returned edge.
+  // Else we start reading at the next data edge of the tree.
+  CordRep* edge = chunk_size ? navigator_.Current() : navigator_.Next();
+  const size_t offset = chunk_size ? edge->length - chunk_size : 0;
+
+  // Read the sub tree and verify we got what we wanted.
+  ReadResult result = navigator_.Read(offset, n);
+  tree = result.tree;
+
+  // If the data returned in `tree` was covered entirely by `chunk_size`, i.e.,
+  // read from the 'previous' edge, we did not consume any additional data, and
+  // can directly return the substring into the current data edge as the next
+  // chunk. We can easily establish from the above code that `navigator_.Next()`
+  // has not been called as that requires `chunk_size` to be zero.
+  if (n < chunk_size) return EdgeData(edge).substr(result.n);
+
+  // The amount of data taken from the last edge is `chunk_size` and `result.n`
+  // contains the offset into the current edge trailing the read data (which can
+  // be 0). As the call to `navigator_.Read()` could have consumed all remaining
+  // data, calling `navigator_.Current()` is not safe before checking if we
+  // already consumed all remaining data.
+  const size_t consumed_by_read = n - chunk_size - result.n;
+  if (consumed_by_read >= remaining_) {
+    remaining_ = 0;
+    return {};
+  }
+
+  // We did not read all data, return remaining data from current edge.
+  edge = navigator_.Current();
+  remaining_ -= consumed_by_read + edge->length;
+  return EdgeData(edge).substr(result.n);
+}
+
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/strings/internal/cord_rep_btree_reader.h b/absl/strings/internal/cord_rep_btree_reader.h
new file mode 100644
index 00000000000..46ed3523d92
--- /dev/null
+++ b/absl/strings/internal/cord_rep_btree_reader.h
@@ -0,0 +1,215 @@
+// Copyright 2021 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_STRINGS_INTERNAL_CORD_REP_BTREE_READER_H_
+#define ABSL_STRINGS_INTERNAL_CORD_REP_BTREE_READER_H_
+
+#include <cassert>
+#include <cstddef>
+
+#include "absl/base/config.h"
+#include "absl/base/optimization.h"
+#include "absl/strings/internal/cord_data_edge.h"
+#include "absl/strings/internal/cord_internal.h"
+#include "absl/strings/internal/cord_rep_btree.h"
+#include "absl/strings/internal/cord_rep_btree_navigator.h"
+#include "absl/strings/internal/cord_rep_flat.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+
+// CordRepBtreeReader implements logic to iterate over cord btrees.
+// References to the underlying data are returned as absl::string_view values.
+// The most typical use case is a forward only iteration over tree data.
+// The class also provides `Skip()`, `Seek()` and `Read()` methods similar to
+// CordRepBtreeNavigator that allow more advanced navigation.
+//
+// Example: iterate over all data inside a cord btree:
+//
+//   CordRepBtreeReader reader;
+//   for (string_view sv = reader.Init(tree); !sv.Empty(); sv = sv.Next()) {
+//     DoSomethingWithDataIn(sv);
+//   }
+//
+// All navigation methods always return the next 'chunk' of data. The class
+// assumes that all data is directly 'consumed' by the caller. For example:
+// invoking `Skip()` will skip the desired number of bytes, and directly
+// read and return the next chunk of data directly after the skipped bytes.
+//
+// Example: iterate over all data inside a btree skipping the first 100 bytes:
+//
+//   CordRepBtreeReader reader;
+//   absl::string_view sv = reader.Init(tree);
+//   if (sv.length() > 100) {
+//     sv.RemovePrefix(100);
+//   } else {
+//     sv = reader.Skip(100 - sv.length());
+//   }
+//   while (!sv.empty()) {
+//     DoSomethingWithDataIn(sv);
+//     absl::string_view sv = reader.Next();
+//   }
+//
+// It is important to notice that `remaining` is based on the end position of
+// the last data edge returned to the caller, not the cumulative data returned
+// to the caller which can be less in cases of skipping or seeking over data.
+//
+// For example, consider a cord btree with five data edges: "abc", "def", "ghi",
+// "jkl" and "mno":
+//
+//   absl::string_view sv;
+//   CordRepBtreeReader reader;
+//
+//   sv = reader.Init(tree); // sv = "abc", remaining = 12
+//   sv = reader.Skip(4);    // sv = "hi",  remaining = 6
+//   sv = reader.Skip(2);    // sv = "l",   remaining = 3
+//   sv = reader.Next();     // sv = "mno", remaining = 0
+//   sv = reader.Seek(1);    // sv = "bc", remaining = 12
+//
+class CordRepBtreeReader {
+ public:
+  using ReadResult = CordRepBtreeNavigator::ReadResult;
+  using Position = CordRepBtreeNavigator::Position;
+
+  // Returns true if this instance is not empty.
+  explicit operator bool() const { return navigator_.btree() != nullptr; }
+
+  // Returns the tree referenced by this instance or nullptr if empty.
+  CordRepBtree* btree() const { return navigator_.btree(); }
+
+  // Returns the current data edge inside the referenced btree.
+  // Requires that the current instance is not empty.
+  CordRep* node() const { return navigator_.Current(); }
+
+  // Returns the length of the referenced tree.
+  // Requires that the current instance is not empty.
+  size_t length() const;
+
+  // Returns the number of remaining bytes available for iteration, which is the
+  // number of bytes directly following the end of the last chunk returned.
+  // This value will be zero if we iterated over the last edge in the bound
+  // tree, in which case any call to Next() or Skip() will return an empty
+  // string_view reflecting the EOF state.
+  // Note that a call to `Seek()` resets `remaining` to a value based on the
+  // end position of the chunk returned by that call.
+  size_t remaining() const { return remaining_; }
+
+  // Resets this instance to an empty value.
+  void Reset() { navigator_.Reset(); }
+
+  // Initializes this instance with `tree`. `tree` must not be null.
+  // Returns a reference to the first data edge of the provided tree.
+  absl::string_view Init(CordRepBtree* tree);
+
+  // Navigates to and returns the next data edge of the referenced tree.
+  // Returns an empty string_view if an attempt is made to read beyond the end
+  // of the tree, i.e.: if `remaining()` is zero indicating an EOF condition.
+  // Requires that the current instance is not empty.
+  absl::string_view Next();
+
+  // Skips the provided amount of bytes and returns a reference to the data
+  // directly following the skipped bytes.
+  absl::string_view Skip(size_t skip);
+
+  // Reads `n` bytes into `tree`.
+  // If `chunk_size` is zero, starts reading at the next data edge. If
+  // `chunk_size` is non zero, the read starts at the last `chunk_size` bytes of
+  // the last returned data edge. Effectively, this means that the read starts
+  // at offset `consumed() - chunk_size`.
+  // Requires that `chunk_size` is less than or equal to the length of the
+  // last returned data edge. The purpose of `chunk_size` is to simplify code
+  // partially consuming a returned chunk and wanting to include the remaining
+  // bytes in the Read call. For example, the below code will read 1000 bytes of
+  // data into a cord tree if the first chunk starts with "big:":
+  //
+  //   CordRepBtreeReader reader;
+  //   absl::string_view sv = reader.Init(tree);
+  //   if (absl::StartsWith(sv, "big:")) {
+  //     CordRepBtree tree;
+  //     sv = reader.Read(1000, sv.size() - 4 /* "big:" */, &tree);
+  //   }
+  //
+  // This method will return an empty string view if all remaining data was
+  // read. If `n` exceeded the amount of remaining data this function will
+  // return an empty string view and `tree` will be set to nullptr.
+  // In both cases, `consumed` will be set to `length`.
+  absl::string_view Read(size_t n, size_t chunk_size, CordRep*& tree);
+
+  // Navigates to the chunk at offset `offset`.
+  // Returns a reference into the navigated to chunk, adjusted for the relative
+  // position of `offset` into that chunk. For example, calling `Seek(13)` on a
+  // cord tree containing 2 chunks of 10 and 20 bytes respectively will return
+  // a string view into the second chunk starting at offset 3 with a size of 17.
+  // Returns an empty string view if `offset` is equal to or greater than the
+  // length of the referenced tree.
+  absl::string_view Seek(size_t offset);
+
+ private:
+  size_t remaining_ = 0;
+  CordRepBtreeNavigator navigator_;
+};
+
+inline size_t CordRepBtreeReader::length() const {
+  assert(btree() != nullptr);
+  return btree()->length;
+}
+
+inline absl::string_view CordRepBtreeReader::Init(CordRepBtree* tree) {
+  assert(tree != nullptr);
+  const CordRep* edge = navigator_.InitFirst(tree);
+  remaining_ = tree->length - edge->length;
+  return EdgeData(edge);
+}
+
+inline absl::string_view CordRepBtreeReader::Next() {
+  if (remaining_ == 0) return {};
+  const CordRep* edge = navigator_.Next();
+  assert(edge != nullptr);
+  remaining_ -= edge->length;
+  return EdgeData(edge);
+}
+
+inline absl::string_view CordRepBtreeReader::Skip(size_t skip) {
+  // As we are always positioned on the last 'consumed' edge, we
+  // need to skip the current edge as well as `skip`.
+  const size_t edge_length = navigator_.Current()->length;
+  CordRepBtreeNavigator::Position pos = navigator_.Skip(skip + edge_length);
+  if (ABSL_PREDICT_FALSE(pos.edge == nullptr)) {
+    remaining_ = 0;
+    return {};
+  }
+  // The combined length of all edges skipped before `pos.edge` is `skip -
+  // pos.offset`, all of which are 'consumed', as well as the current edge.
+  remaining_ -= skip - pos.offset + pos.edge->length;
+  return EdgeData(pos.edge).substr(pos.offset);
+}
+
+inline absl::string_view CordRepBtreeReader::Seek(size_t offset) {
+  const CordRepBtreeNavigator::Position pos = navigator_.Seek(offset);
+  if (ABSL_PREDICT_FALSE(pos.edge == nullptr)) {
+    remaining_ = 0;
+    return {};
+  }
+  absl::string_view chunk = EdgeData(pos.edge).substr(pos.offset);
+  remaining_ = length() - offset - chunk.length();
+  return chunk;
+}
+
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STRINGS_INTERNAL_CORD_REP_BTREE_READER_H_
diff --git a/absl/strings/internal/cord_rep_btree_reader_test.cc b/absl/strings/internal/cord_rep_btree_reader_test.cc
new file mode 100644
index 00000000000..684da35416e
--- /dev/null
+++ b/absl/strings/internal/cord_rep_btree_reader_test.cc
@@ -0,0 +1,295 @@
+// Copyright 2021 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/strings/internal/cord_rep_btree_reader.h"
+
+#include <algorithm>
+#include <cstddef>
+#include <iostream>
+#include <random>
+#include <string>
+#include <vector>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/strings/cord.h"
+#include "absl/strings/internal/cord_internal.h"
+#include "absl/strings/internal/cord_rep_btree.h"
+#include "absl/strings/internal/cord_rep_test_util.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+namespace {
+
+using ::testing::Eq;
+using ::testing::IsEmpty;
+using ::testing::Ne;
+using ::testing::Not;
+
+using ::absl::cordrep_testing::CordRepBtreeFromFlats;
+using ::absl::cordrep_testing::MakeFlat;
+using ::absl::cordrep_testing::CordToString;
+using ::absl::cordrep_testing::CreateFlatsFromString;
+using ::absl::cordrep_testing::CreateRandomString;
+
+using ReadResult = CordRepBtreeReader::ReadResult;
+
+TEST(CordRepBtreeReaderTest, Next) {
+  constexpr size_t kChars = 3;
+  const size_t cap = CordRepBtree::kMaxCapacity;
+  size_t counts[] = {1, 2, cap, cap * cap, cap * cap + 1, cap * cap * 2 + 17};
+
+  for (size_t count : counts) {
+    std::string data = CreateRandomString(count * kChars);
+    std::vector<CordRep*> flats = CreateFlatsFromString(data, kChars);
+    CordRepBtree* node = CordRepBtreeFromFlats(flats);
+
+    CordRepBtreeReader reader;
+    size_t remaining = data.length();
+    absl::string_view chunk = reader.Init(node);
+    EXPECT_THAT(chunk, Eq(data.substr(0, chunk.length())));
+
+    remaining -= chunk.length();
+    EXPECT_THAT(reader.remaining(), Eq(remaining));
+
+    while (remaining > 0) {
+      const size_t offset = data.length() - remaining;
+      chunk = reader.Next();
+      EXPECT_THAT(chunk, Eq(data.substr(offset, chunk.length())));
+
+      remaining -= chunk.length();
+      EXPECT_THAT(reader.remaining(), Eq(remaining));
+    }
+
+    EXPECT_THAT(reader.remaining(), Eq(0u));
+
+    // Verify trying to read beyond EOF returns empty string_view
+    EXPECT_THAT(reader.Next(), testing::IsEmpty());
+
+    CordRep::Unref(node);
+  }
+}
+
+TEST(CordRepBtreeReaderTest, Skip) {
+  constexpr size_t kChars = 3;
+  const size_t cap = CordRepBtree::kMaxCapacity;
+  size_t counts[] = {1, 2, cap, cap * cap, cap * cap + 1, cap * cap * 2 + 17};
+
+  for (size_t count : counts) {
+    std::string data = CreateRandomString(count * kChars);
+    std::vector<CordRep*> flats = CreateFlatsFromString(data, kChars);
+    CordRepBtree* node = CordRepBtreeFromFlats(flats);
+
+    for (size_t skip1 = 0; skip1 < data.length() - kChars; ++skip1) {
+      for (size_t skip2 = 0; skip2 < data.length() - kChars; ++skip2) {
+        CordRepBtreeReader reader;
+        size_t remaining = data.length();
+        absl::string_view chunk = reader.Init(node);
+        remaining -= chunk.length();
+
+        chunk = reader.Skip(skip1);
+        size_t offset = data.length() - remaining;
+        ASSERT_THAT(chunk, Eq(data.substr(offset + skip1, chunk.length())));
+        remaining -= chunk.length() + skip1;
+        ASSERT_THAT(reader.remaining(), Eq(remaining));
+
+        if (remaining == 0) continue;
+
+        size_t skip = std::min(remaining - 1, skip2);
+        chunk = reader.Skip(skip);
+        offset = data.length() - remaining;
+        ASSERT_THAT(chunk, Eq(data.substr(offset + skip, chunk.length())));
+      }
+    }
+
+    CordRep::Unref(node);
+  }
+}
+
+TEST(CordRepBtreeReaderTest, SkipBeyondLength) {
+  CordRepBtree* tree = CordRepBtree::Create(MakeFlat("abc"));
+  tree = CordRepBtree::Append(tree, MakeFlat("def"));
+  CordRepBtreeReader reader;
+  reader.Init(tree);
+  EXPECT_THAT(reader.Skip(100), IsEmpty());
+  EXPECT_THAT(reader.remaining(), Eq(0u));
+  CordRep::Unref(tree);
+}
+
+TEST(CordRepBtreeReaderTest, Seek) {
+  constexpr size_t kChars = 3;
+  const size_t cap = CordRepBtree::kMaxCapacity;
+  size_t counts[] = {1, 2, cap, cap * cap, cap * cap + 1, cap * cap * 2 + 17};
+
+  for (size_t count : counts) {
+    std::string data = CreateRandomString(count * kChars);
+    std::vector<CordRep*> flats = CreateFlatsFromString(data, kChars);
+    CordRepBtree* node = CordRepBtreeFromFlats(flats);
+
+    for (size_t seek = 0; seek < data.length() - 1; ++seek) {
+      CordRepBtreeReader reader;
+      reader.Init(node);
+      absl::string_view chunk = reader.Seek(seek);
+      ASSERT_THAT(chunk, Not(IsEmpty()));
+      ASSERT_THAT(chunk, Eq(data.substr(seek, chunk.length())));
+      ASSERT_THAT(reader.remaining(),
+                  Eq(data.length() - seek - chunk.length()));
+    }
+
+    CordRep::Unref(node);
+  }
+}
+
+TEST(CordRepBtreeReaderTest, SeekBeyondLength) {
+  CordRepBtree* tree = CordRepBtree::Create(MakeFlat("abc"));
+  tree = CordRepBtree::Append(tree, MakeFlat("def"));
+  CordRepBtreeReader reader;
+  reader.Init(tree);
+  EXPECT_THAT(reader.Seek(6), IsEmpty());
+  EXPECT_THAT(reader.remaining(), Eq(0u));
+  EXPECT_THAT(reader.Seek(100), IsEmpty());
+  EXPECT_THAT(reader.remaining(), Eq(0u));
+  CordRep::Unref(tree);
+}
+
+TEST(CordRepBtreeReaderTest, Read) {
+  std::string data = "abcdefghijklmno";
+  std::vector<CordRep*> flats = CreateFlatsFromString(data, 5);
+  CordRepBtree* node = CordRepBtreeFromFlats(flats);
+
+  CordRep* tree;
+  CordRepBtreeReader reader;
+  absl::string_view chunk;
+
+  // Read zero bytes
+  chunk = reader.Init(node);
+  chunk = reader.Read(0, chunk.length(), tree);
+  EXPECT_THAT(tree, Eq(nullptr));
+  EXPECT_THAT(chunk, Eq("abcde"));
+  EXPECT_THAT(reader.remaining(), Eq(10u));
+  EXPECT_THAT(reader.Next(), Eq("fghij"));
+
+  // Read in full
+  chunk = reader.Init(node);
+  chunk = reader.Read(15, chunk.length(), tree);
+  EXPECT_THAT(tree, Ne(nullptr));
+  EXPECT_THAT(CordToString(tree), Eq("abcdefghijklmno"));
+  EXPECT_THAT(chunk, Eq(""));
+  EXPECT_THAT(reader.remaining(), Eq(0u));
+  CordRep::Unref(tree);
+
+  // Read < chunk bytes
+  chunk = reader.Init(node);
+  chunk = reader.Read(3, chunk.length(), tree);
+  ASSERT_THAT(tree, Ne(nullptr));
+  EXPECT_THAT(CordToString(tree), Eq("abc"));
+  EXPECT_THAT(chunk, Eq("de"));
+  EXPECT_THAT(reader.remaining(), Eq(10u));
+  EXPECT_THAT(reader.Next(), Eq("fghij"));
+  CordRep::Unref(tree);
+
+  // Read < chunk bytes at offset
+  chunk = reader.Init(node);
+  chunk = reader.Read(2, chunk.length() - 2, tree);
+  ASSERT_THAT(tree, Ne(nullptr));
+  EXPECT_THAT(CordToString(tree), Eq("cd"));
+  EXPECT_THAT(chunk, Eq("e"));
+  EXPECT_THAT(reader.remaining(), Eq(10u));
+  EXPECT_THAT(reader.Next(), Eq("fghij"));
+  CordRep::Unref(tree);
+
+  // Read from consumed chunk
+  chunk = reader.Init(node);
+  chunk = reader.Read(3, 0, tree);
+  ASSERT_THAT(tree, Ne(nullptr));
+  EXPECT_THAT(CordToString(tree), Eq("fgh"));
+  EXPECT_THAT(chunk, Eq("ij"));
+  EXPECT_THAT(reader.remaining(), Eq(5u));
+  EXPECT_THAT(reader.Next(), Eq("klmno"));
+  CordRep::Unref(tree);
+
+  // Read across chunks
+  chunk = reader.Init(node);
+  chunk = reader.Read(12, chunk.length() - 2, tree);
+  ASSERT_THAT(tree, Ne(nullptr));
+  EXPECT_THAT(CordToString(tree), Eq("cdefghijklmn"));
+  EXPECT_THAT(chunk, Eq("o"));
+  EXPECT_THAT(reader.remaining(), Eq(0u));
+  CordRep::Unref(tree);
+
+  // Read across chunks landing on exact edge boundary
+  chunk = reader.Init(node);
+  chunk = reader.Read(10 - 2, chunk.length() - 2, tree);
+  ASSERT_THAT(tree, Ne(nullptr));
+  EXPECT_THAT(CordToString(tree), Eq("cdefghij"));
+  EXPECT_THAT(chunk, Eq("klmno"));
+  EXPECT_THAT(reader.remaining(), Eq(0u));
+  CordRep::Unref(tree);
+
+  CordRep::Unref(node);
+}
+
+TEST(CordRepBtreeReaderTest, ReadExhaustive) {
+  constexpr size_t kChars = 3;
+  const size_t cap = CordRepBtree::kMaxCapacity;
+  size_t counts[] = {1, 2, cap, cap * cap + 1, cap * cap * cap * 2 + 17};
+
+  for (size_t count : counts) {
+    std::string data = CreateRandomString(count * kChars);
+    std::vector<CordRep*> flats = CreateFlatsFromString(data, kChars);
+    CordRepBtree* node = CordRepBtreeFromFlats(flats);
+
+    for (size_t read_size : {kChars - 1, kChars, kChars + 7, cap * cap}) {
+      CordRepBtreeReader reader;
+      absl::string_view chunk = reader.Init(node);
+
+      // `consumed` tracks the end of last consumed chunk which is the start of
+      // the next chunk: we always read with `chunk_size = chunk.length()`.
+      size_t consumed = 0;
+      size_t remaining = data.length();
+      while (remaining > 0) {
+        CordRep* tree;
+        size_t n = (std::min)(remaining, read_size);
+        chunk = reader.Read(n, chunk.length(), tree);
+        EXPECT_THAT(tree, Ne(nullptr));
+        if (tree) {
+          EXPECT_THAT(CordToString(tree), Eq(data.substr(consumed, n)));
+          CordRep::Unref(tree);
+        }
+
+        consumed += n;
+        remaining -= n;
+        EXPECT_THAT(reader.remaining(), Eq(remaining - chunk.length()));
+
+        if (remaining > 0) {
+          ASSERT_FALSE(chunk.empty());
+          ASSERT_THAT(chunk, Eq(data.substr(consumed, chunk.length())));
+        } else {
+          ASSERT_TRUE(chunk.empty()) << chunk;
+        }
+      }
+    }
+
+    CordRep::Unref(node);
+  }
+}
+
+}  // namespace
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/strings/internal/cord_rep_btree_test.cc b/absl/strings/internal/cord_rep_btree_test.cc
new file mode 100644
index 00000000000..6b8e628806f
--- /dev/null
+++ b/absl/strings/internal/cord_rep_btree_test.cc
@@ -0,0 +1,1575 @@
+// Copyright 2021 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/strings/internal/cord_rep_btree.h"
+
+#include <cassert>
+#include <cmath>
+#include <cstddef>
+#include <cstdint>
+#include <deque>
+#include <iostream>
+#include <random>
+#include <sstream>
+#include <string>
+#include <vector>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/cleanup/cleanup.h"
+#include "absl/strings/internal/cord_data_edge.h"
+#include "absl/strings/internal/cord_internal.h"
+#include "absl/strings/internal/cord_rep_flat.h"
+#include "absl/strings/internal/cord_rep_test_util.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/string_view.h"
+#include "absl/types/span.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+
+class CordRepBtreeTestPeer {
+ public:
+  static void SetEdge(CordRepBtree* node, size_t idx, CordRep* edge) {
+    node->edges_[idx] = edge;
+  }
+  static void AddEdge(CordRepBtree* node, CordRep* edge) {
+    node->edges_[node->fetch_add_end(1)] = edge;
+  }
+};
+
+namespace {
+
+using ::absl::cordrep_testing::AutoUnref;
+using ::absl::cordrep_testing::CordCollectRepsIf;
+using ::absl::cordrep_testing::CordToString;
+using ::absl::cordrep_testing::CordVisitReps;
+using ::absl::cordrep_testing::CreateFlatsFromString;
+using ::absl::cordrep_testing::CreateRandomString;
+using ::absl::cordrep_testing::MakeExternal;
+using ::absl::cordrep_testing::MakeFlat;
+using ::absl::cordrep_testing::MakeSubstring;
+using ::testing::_;
+using ::testing::AllOf;
+using ::testing::AnyOf;
+using ::testing::Conditional;
+using ::testing::ElementsAre;
+using ::testing::ElementsAreArray;
+using ::testing::Eq;
+using ::testing::HasSubstr;
+using ::testing::Le;
+using ::testing::Ne;
+using ::testing::Not;
+using ::testing::SizeIs;
+using ::testing::TypedEq;
+
+MATCHER_P(EqFlatHolding, data, "Equals flat holding data") {
+  if (arg->tag < FLAT) {
+    *result_listener << "Expected FLAT, got tag " << static_cast<int>(arg->tag);
+    return false;
+  }
+  std::string actual = CordToString(arg);
+  if (actual != data) {
+    *result_listener << "Expected flat holding \"" << data
+                     << "\", got flat holding \"" << actual << "\"";
+    return false;
+  }
+  return true;
+}
+
+MATCHER_P(IsNode, height, absl::StrCat("Is a valid node of height ", height)) {
+  if (arg == nullptr) {
+    *result_listener << "Expected NODE, got nullptr";
+    return false;
+  }
+  if (arg->tag != BTREE) {
+    *result_listener << "Expected NODE, got " << static_cast<int>(arg->tag);
+    return false;
+  }
+  if (!CordRepBtree::IsValid(arg->btree())) {
+    CordRepBtree::Dump(arg->btree(), "Expected valid NODE, got:", false,
+                       *result_listener->stream());
+    return false;
+  }
+  if (arg->btree()->height() != height) {
+    *result_listener << "Expected NODE of height " << height << ", got "
+                     << arg->btree()->height();
+    return false;
+  }
+  return true;
+}
+
+MATCHER_P2(IsSubstring, start, length,
+           absl::StrCat("Is a substring(start = ", start, ", length = ", length,
+                        ")")) {
+  if (arg == nullptr) {
+    *result_listener << "Expected substring, got nullptr";
+    return false;
+  }
+  if (arg->tag != SUBSTRING) {
+    *result_listener << "Expected SUBSTRING, got "
+                     << static_cast<int>(arg->tag);
+    return false;
+  }
+  const CordRepSubstring* const substr = arg->substring();
+  if (substr->start != start || substr->length != length) {
+    *result_listener << "Expected substring(" << start << ", " << length
+                     << "), got substring(" << substr->start << ", "
+                     << substr->length << ")";
+    return false;
+  }
+  return true;
+}
+
+MATCHER_P2(EqExtractResult, tree, rep, "Equals ExtractResult") {
+  if (arg.tree != tree || arg.extracted != rep) {
+    *result_listener << "Expected {" << static_cast<const void*>(tree) << ", "
+                     << static_cast<const void*>(rep) << "}, got {" << arg.tree
+                     << ", " << arg.extracted << "}";
+    return false;
+  }
+  return true;
+}
+
+// DataConsumer is a simple helper class used by tests to 'consume' string
+// fragments from the provided input in forward or backward direction.
+class DataConsumer {
+ public:
+  // Starts consumption of `data`. Caller must make sure `data` outlives this
+  // instance. Consumes data starting at the front if `forward` is true, else
+  // consumes data from the back.
+  DataConsumer(absl::string_view data, bool forward)
+      : data_(data), forward_(forward) {}
+
+  // Return the next `n` bytes from referenced data.
+  absl::string_view Next(size_t n) {
+    assert(n <= data_.size() - consumed_);
+    consumed_ += n;
+    return data_.substr(forward_ ? consumed_ - n : data_.size() - consumed_, n);
+  }
+
+  // Returns all data consumed so far.
+  absl::string_view Consumed() const {
+    return forward_ ? data_.substr(0, consumed_)
+                    : data_.substr(data_.size() - consumed_);
+  }
+
+ private:
+  absl::string_view data_;
+  size_t consumed_ = 0;
+  bool forward_;
+};
+
+// BtreeAdd returns either CordRepBtree::Append or CordRepBtree::Prepend.
+CordRepBtree* BtreeAdd(CordRepBtree* node, bool append,
+                       absl::string_view data) {
+  return append ? CordRepBtree::Append(node, data)
+                : CordRepBtree::Prepend(node, data);
+}
+
+// Recursively collects all leaf edges from `tree` and appends them to `edges`.
+void GetLeafEdges(const CordRepBtree* tree, std::vector<CordRep*>& edges) {
+  if (tree->height() == 0) {
+    for (CordRep* edge : tree->Edges()) {
+      edges.push_back(edge);
+    }
+  } else {
+    for (CordRep* edge : tree->Edges()) {
+      GetLeafEdges(edge->btree(), edges);
+    }
+  }
+}
+
+// Recursively collects and returns all leaf edges from `tree`.
+std::vector<CordRep*> GetLeafEdges(const CordRepBtree* tree) {
+  std::vector<CordRep*> edges;
+  GetLeafEdges(tree, edges);
+  return edges;
+}
+
+// Creates a flat containing the hexadecimal value of `i` zero padded
+// to at least 4 digits prefixed with "0x", e.g.: "0x04AC".
+CordRepFlat* MakeHexFlat(size_t i) {
+  return MakeFlat(absl::StrCat("0x", absl::Hex(i, absl::kZeroPad4)));
+}
+
+CordRepBtree* MakeLeaf(size_t size = CordRepBtree::kMaxCapacity) {
+  assert(size <= CordRepBtree::kMaxCapacity);
+  CordRepBtree* leaf = CordRepBtree::Create(MakeHexFlat(0));
+  for (size_t i = 1; i < size; ++i) {
+    leaf = CordRepBtree::Append(leaf, MakeHexFlat(i));
+  }
+  return leaf;
+}
+
+CordRepBtree* MakeTree(size_t size, bool append = true) {
+  CordRepBtree* tree = CordRepBtree::Create(MakeHexFlat(0));
+  for (size_t i = 1; i < size; ++i) {
+    tree = append ? CordRepBtree::Append(tree, MakeHexFlat(i))
+                  : CordRepBtree::Prepend(tree, MakeHexFlat(i));
+  }
+  return tree;
+}
+
+CordRepBtree* CreateTree(absl::Span<CordRep* const> reps) {
+  auto it = reps.begin();
+  CordRepBtree* tree = CordRepBtree::Create(*it);
+  while (++it != reps.end()) tree = CordRepBtree::Append(tree, *it);
+  return tree;
+}
+
+CordRepBtree* CreateTree(absl::string_view data, size_t chunk_size) {
+  return CreateTree(CreateFlatsFromString(data, chunk_size));
+}
+
+CordRepBtree* CreateTreeReverse(absl::string_view data, size_t chunk_size) {
+  std::vector<CordRep*> flats = CreateFlatsFromString(data, chunk_size);
+  auto rit = flats.rbegin();
+  CordRepBtree* tree = CordRepBtree::Create(*rit);
+  while (++rit != flats.rend()) tree = CordRepBtree::Prepend(tree, *rit);
+  return tree;
+}
+
+class CordRepBtreeTest : public testing::TestWithParam<bool> {
+ public:
+  bool shared() const { return GetParam(); }
+
+  static std::string ToString(testing::TestParamInfo<bool> param) {
+    return param.param ? "Shared" : "Private";
+  }
+};
+
+INSTANTIATE_TEST_SUITE_P(WithParam, CordRepBtreeTest, testing::Bool(),
+                         CordRepBtreeTest::ToString);
+
+class CordRepBtreeHeightTest : public testing::TestWithParam<int> {
+ public:
+  int height() const { return GetParam(); }
+
+  static std::string ToString(testing::TestParamInfo<int> param) {
+    return absl::StrCat(param.param);
+  }
+};
+
+INSTANTIATE_TEST_SUITE_P(WithHeights, CordRepBtreeHeightTest,
+                         testing::Range(0, CordRepBtree::kMaxHeight),
+                         CordRepBtreeHeightTest::ToString);
+
+using TwoBools = testing::tuple<bool, bool>;
+
+class CordRepBtreeDualTest : public testing::TestWithParam<TwoBools> {
+ public:
+  bool first_shared() const { return std::get<0>(GetParam()); }
+  bool second_shared() const { return std::get<1>(GetParam()); }
+
+  static std::string ToString(testing::TestParamInfo<TwoBools> param) {
+    if (std::get<0>(param.param)) {
+      return std::get<1>(param.param) ? "BothShared" : "FirstShared";
+    }
+    return std::get<1>(param.param) ? "SecondShared" : "Private";
+  }
+};
+
+INSTANTIATE_TEST_SUITE_P(WithParam, CordRepBtreeDualTest,
+                         testing::Combine(testing::Bool(), testing::Bool()),
+                         CordRepBtreeDualTest::ToString);
+
+TEST(CordRepBtreeTest, SizeIsMultipleOf64) {
+  // Only enforce for fully 64-bit platforms.
+  if (sizeof(size_t) == 8 && sizeof(void*) == 8) {
+    EXPECT_THAT(sizeof(CordRepBtree) % 64, Eq(0u))
+        << "Should be multiple of 64";
+  }
+}
+
+TEST(CordRepBtreeTest, NewDestroyEmptyTree) {
+  auto* tree = CordRepBtree::New();
+  EXPECT_THAT(tree->size(), Eq(0u));
+  EXPECT_THAT(tree->height(), Eq(0));
+  EXPECT_THAT(tree->Edges(), ElementsAre());
+  CordRepBtree::Destroy(tree);
+}
+
+TEST(CordRepBtreeTest, NewDestroyEmptyTreeAtHeight) {
+  auto* tree = CordRepBtree::New(3);
+  EXPECT_THAT(tree->size(), Eq(0u));
+  EXPECT_THAT(tree->height(), Eq(3));
+  EXPECT_THAT(tree->Edges(), ElementsAre());
+  CordRepBtree::Destroy(tree);
+}
+
+TEST(CordRepBtreeTest, Btree) {
+  CordRep* rep = CordRepBtree::New();
+  EXPECT_THAT(rep->btree(), Eq(rep));
+  EXPECT_THAT(static_cast<const CordRep*>(rep)->btree(), Eq(rep));
+  CordRep::Unref(rep);
+#if defined(GTEST_HAS_DEATH_TEST) && !defined(NDEBUG)
+  rep = MakeFlat("Hello world");
+  EXPECT_DEATH(rep->btree(), ".*");
+  EXPECT_DEATH(static_cast<const CordRep*>(rep)->btree(), ".*");
+  CordRep::Unref(rep);
+#endif
+}
+
+TEST(CordRepBtreeTest, EdgeData) {
+  CordRepFlat* flat = MakeFlat("Hello world");
+  CordRepExternal* external = MakeExternal("Hello external");
+  CordRep* substr1 = MakeSubstring(1, 6, CordRep::Ref(flat));
+  CordRep* substr2 = MakeSubstring(1, 6, CordRep::Ref(external));
+  CordRep* bad_substr = MakeSubstring(1, 2, CordRep::Ref(substr1));
+
+  EXPECT_TRUE(IsDataEdge(flat));
+  EXPECT_THAT(EdgeData(flat).data(), TypedEq<const void*>(flat->Data()));
+  EXPECT_THAT(EdgeData(flat), Eq("Hello world"));
+
+  EXPECT_TRUE(IsDataEdge(external));
+  EXPECT_THAT(EdgeData(external).data(), TypedEq<const void*>(external->base));
+  EXPECT_THAT(EdgeData(external), Eq("Hello external"));
+
+  EXPECT_TRUE(IsDataEdge(substr1));
+  EXPECT_THAT(EdgeData(substr1).data(), TypedEq<const void*>(flat->Data() + 1));
+  EXPECT_THAT(EdgeData(substr1), Eq("ello w"));
+
+  EXPECT_TRUE(IsDataEdge(substr2));
+  EXPECT_THAT(EdgeData(substr2).data(),
+              TypedEq<const void*>(external->base + 1));
+  EXPECT_THAT(EdgeData(substr2), Eq("ello e"));
+
+  EXPECT_FALSE(IsDataEdge(bad_substr));
+#if defined(GTEST_HAS_DEATH_TEST) && !defined(NDEBUG)
+  EXPECT_DEATH(EdgeData(bad_substr), ".*");
+#endif
+
+  CordRep::Unref(bad_substr);
+  CordRep::Unref(substr2);
+  CordRep::Unref(substr1);
+  CordRep::Unref(external);
+  CordRep::Unref(flat);
+}
+
+TEST(CordRepBtreeTest, CreateUnrefLeaf) {
+  auto* flat = MakeFlat("a");
+  auto* leaf = CordRepBtree::Create(flat);
+  EXPECT_THAT(leaf->size(), Eq(1u));
+  EXPECT_THAT(leaf->height(), Eq(0));
+  EXPECT_THAT(leaf->Edges(), ElementsAre(flat));
+  CordRepBtree::Unref(leaf);
+}
+
+TEST(CordRepBtreeTest, NewUnrefNode) {
+  auto* leaf = CordRepBtree::Create(MakeFlat("a"));
+  CordRepBtree* tree = CordRepBtree::New(leaf);
+  EXPECT_THAT(tree->size(), Eq(1u));
+  EXPECT_THAT(tree->height(), Eq(1));
+  EXPECT_THAT(tree->Edges(), ElementsAre(leaf));
+  CordRepBtree::Unref(tree);
+}
+
+TEST_P(CordRepBtreeTest, AppendToLeafToCapacity) {
+  AutoUnref refs;
+  std::vector<CordRep*> flats;
+  flats.push_back(MakeHexFlat(0));
+  auto* leaf = CordRepBtree::Create(flats.back());
+
+  for (size_t i = 1; i < CordRepBtree::kMaxCapacity; ++i) {
+    refs.RefIf(shared(), leaf);
+    flats.push_back(MakeHexFlat(i));
+    auto* result = CordRepBtree::Append(leaf, flats.back());
+    EXPECT_THAT(result->height(), Eq(0));
+    EXPECT_THAT(result, Conditional(shared(), Ne(leaf), Eq(leaf)));
+    EXPECT_THAT(result->Edges(), ElementsAreArray(flats));
+    leaf = result;
+  }
+  CordRep::Unref(leaf);
+}
+
+TEST_P(CordRepBtreeTest, PrependToLeafToCapacity) {
+  AutoUnref refs;
+  std::deque<CordRep*> flats;
+  flats.push_front(MakeHexFlat(0));
+  auto* leaf = CordRepBtree::Create(flats.front());
+
+  for (size_t i = 1; i < CordRepBtree::kMaxCapacity; ++i) {
+    refs.RefIf(shared(), leaf);
+    flats.push_front(MakeHexFlat(i));
+    auto* result = CordRepBtree::Prepend(leaf, flats.front());
+    EXPECT_THAT(result->height(), Eq(0));
+    EXPECT_THAT(result, Conditional(shared(), Ne(leaf), Eq(leaf)));
+    EXPECT_THAT(result->Edges(), ElementsAreArray(flats));
+    leaf = result;
+  }
+  CordRep::Unref(leaf);
+}
+
+// This test specifically aims at code aligning data at either the front or the
+// back of the contained `edges[]` array, alternating Append and Prepend will
+// move `begin()` and `end()` values as needed for each added value.
+TEST_P(CordRepBtreeTest, AppendPrependToLeafToCapacity) {
+  AutoUnref refs;
+  std::deque<CordRep*> flats;
+  flats.push_front(MakeHexFlat(0));
+  auto* leaf = CordRepBtree::Create(flats.front());
+
+  for (size_t i = 1; i < CordRepBtree::kMaxCapacity; ++i) {
+    refs.RefIf(shared(), leaf);
+    CordRepBtree* result;
+    if (i % 2 != 0) {
+      flats.push_front(MakeHexFlat(i));
+      result = CordRepBtree::Prepend(leaf, flats.front());
+    } else {
+      flats.push_back(MakeHexFlat(i));
+      result = CordRepBtree::Append(leaf, flats.back());
+    }
+    EXPECT_THAT(result->height(), Eq(0));
+    EXPECT_THAT(result, Conditional(shared(), Ne(leaf), Eq(leaf)));
+    EXPECT_THAT(result->Edges(), ElementsAreArray(flats));
+    leaf = result;
+  }
+  CordRep::Unref(leaf);
+}
+
+TEST_P(CordRepBtreeTest, AppendToLeafBeyondCapacity) {
+  AutoUnref refs;
+  auto* leaf = MakeLeaf();
+  refs.RefIf(shared(), leaf);
+  CordRep* flat = MakeFlat("abc");
+  auto* result = CordRepBtree::Append(leaf, flat);
+  ASSERT_THAT(result, IsNode(1));
+  EXPECT_THAT(result, Ne(leaf));
+  absl::Span<CordRep* const> edges = result->Edges();
+  ASSERT_THAT(edges, ElementsAre(leaf, IsNode(0)));
+  EXPECT_THAT(edges[1]->btree()->Edges(), ElementsAre(flat));
+  CordRep::Unref(result);
+}
+
+TEST_P(CordRepBtreeTest, PrependToLeafBeyondCapacity) {
+  AutoUnref refs;
+  auto* leaf = MakeLeaf();
+  refs.RefIf(shared(), leaf);
+  CordRep* flat = MakeFlat("abc");
+  auto* result = CordRepBtree::Prepend(leaf, flat);
+  ASSERT_THAT(result, IsNode(1));
+  EXPECT_THAT(result, Ne(leaf));
+  absl::Span<CordRep* const> edges = result->Edges();
+  ASSERT_THAT(edges, ElementsAre(IsNode(0), leaf));
+  EXPECT_THAT(edges[0]->btree()->Edges(), ElementsAre(flat));
+  CordRep::Unref(result);
+}
+
+TEST_P(CordRepBtreeTest, AppendToTreeOneDeep) {
+  constexpr size_t max_cap = CordRepBtree::kMaxCapacity;
+  AutoUnref refs;
+  std::vector<CordRep*> flats;
+  flats.push_back(MakeHexFlat(0));
+  CordRepBtree* tree = CordRepBtree::Create(flats.back());
+  for (size_t i = 1; i <= max_cap; ++i) {
+    flats.push_back(MakeHexFlat(i));
+    tree = CordRepBtree::Append(tree, flats.back());
+  }
+  ASSERT_THAT(tree, IsNode(1));
+
+  for (size_t i = max_cap + 1; i < max_cap * max_cap; ++i) {
+    // Ref top level tree based on param.
+    // Ref leaf node once every 4 iterations, which should not have an
+    // observable effect other than that the leaf itself is copied.
+    refs.RefIf(shared(), tree);
+    refs.RefIf(i % 4 == 0, tree->Edges().back());
+
+    flats.push_back(MakeHexFlat(i));
+    CordRepBtree* result = CordRepBtree::Append(tree, flats.back());
+    ASSERT_THAT(result, IsNode(1));
+    ASSERT_THAT(result, Conditional(shared(), Ne(tree), Eq(tree)));
+    std::vector<CordRep*> edges = GetLeafEdges(result);
+    ASSERT_THAT(edges, ElementsAreArray(flats));
+    tree = result;
+  }
+  CordRep::Unref(tree);
+}
+
+TEST_P(CordRepBtreeTest, AppendToTreeTwoDeep) {
+  constexpr size_t max_cap = CordRepBtree::kMaxCapacity;
+  AutoUnref refs;
+  std::vector<CordRep*> flats;
+  flats.push_back(MakeHexFlat(0));
+  CordRepBtree* tree = CordRepBtree::Create(flats.back());
+  for (size_t i = 1; i <= max_cap * max_cap; ++i) {
+    flats.push_back(MakeHexFlat(i));
+    tree = CordRepBtree::Append(tree, flats.back());
+  }
+  ASSERT_THAT(tree, IsNode(2));
+  for (size_t i = max_cap * max_cap + 1; i < max_cap * max_cap * max_cap; ++i) {
+    // Ref top level tree based on param.
+    // Ref child node once every 16 iterations, and leaf node every 4
+    // iterations which  which should not have an observable effect other than
+    //  the node and/or the leaf below it being copied.
+    refs.RefIf(shared(), tree);
+    refs.RefIf(i % 16 == 0, tree->Edges().back());
+    refs.RefIf(i % 4 == 0, tree->Edges().back()->btree()->Edges().back());
+
+    flats.push_back(MakeHexFlat(i));
+    CordRepBtree* result = CordRepBtree::Append(tree, flats.back());
+    ASSERT_THAT(result, IsNode(2));
+    ASSERT_THAT(result, Conditional(shared(), Ne(tree), Eq(tree)));
+    std::vector<CordRep*> edges = GetLeafEdges(result);
+    ASSERT_THAT(edges, ElementsAreArray(flats));
+    tree = result;
+  }
+  CordRep::Unref(tree);
+}
+
+TEST_P(CordRepBtreeTest, PrependToTreeOneDeep) {
+  constexpr size_t max_cap = CordRepBtree::kMaxCapacity;
+  AutoUnref refs;
+  std::deque<CordRep*> flats;
+  flats.push_back(MakeHexFlat(0));
+  CordRepBtree* tree = CordRepBtree::Create(flats.back());
+  for (size_t i = 1; i <= max_cap; ++i) {
+    flats.push_front(MakeHexFlat(i));
+    tree = CordRepBtree::Prepend(tree, flats.front());
+  }
+  ASSERT_THAT(tree, IsNode(1));
+
+  for (size_t i = max_cap + 1; i < max_cap * max_cap; ++i) {
+    // Ref top level tree based on param.
+    // Ref leaf node once every 4 iterations which should not have an observable
+    // effect other than than the leaf itself is copied.
+    refs.RefIf(shared(), tree);
+    refs.RefIf(i % 4 == 0, tree->Edges().back());
+
+    flats.push_front(MakeHexFlat(i));
+    CordRepBtree* result = CordRepBtree::Prepend(tree, flats.front());
+    ASSERT_THAT(result, IsNode(1));
+    ASSERT_THAT(result, Conditional(shared(), Ne(tree), Eq(tree)));
+    std::vector<CordRep*> edges = GetLeafEdges(result);
+    ASSERT_THAT(edges, ElementsAreArray(flats));
+    tree = result;
+  }
+  CordRep::Unref(tree);
+}
+
+TEST_P(CordRepBtreeTest, PrependToTreeTwoDeep) {
+  constexpr size_t max_cap = CordRepBtree::kMaxCapacity;
+  AutoUnref refs;
+  std::deque<CordRep*> flats;
+  flats.push_back(MakeHexFlat(0));
+  CordRepBtree* tree = CordRepBtree::Create(flats.back());
+  for (size_t i = 1; i <= max_cap * max_cap; ++i) {
+    flats.push_front(MakeHexFlat(i));
+    tree = CordRepBtree::Prepend(tree, flats.front());
+  }
+  ASSERT_THAT(tree, IsNode(2));
+  for (size_t i = max_cap * max_cap + 1; i < max_cap * max_cap * max_cap; ++i) {
+    // Ref top level tree based on param.
+    // Ref child node once every 16 iterations, and leaf node every 4
+    // iterations which  which should not have an observable effect other than
+    //  the node and/or the leaf below it being copied.
+    refs.RefIf(shared(), tree);
+    refs.RefIf(i % 16 == 0, tree->Edges().back());
+    refs.RefIf(i % 4 == 0, tree->Edges().back()->btree()->Edges().back());
+
+    flats.push_front(MakeHexFlat(i));
+    CordRepBtree* result = CordRepBtree::Prepend(tree, flats.front());
+    ASSERT_THAT(result, IsNode(2));
+    ASSERT_THAT(result, Conditional(shared(), Ne(tree), Eq(tree)));
+    std::vector<CordRep*> edges = GetLeafEdges(result);
+    ASSERT_THAT(edges, ElementsAreArray(flats));
+    tree = result;
+  }
+  CordRep::Unref(tree);
+}
+
+TEST_P(CordRepBtreeDualTest, MergeLeafsNotExceedingCapacity) {
+  for (bool use_append : {false, true}) {
+    SCOPED_TRACE(use_append ? "Using Append" : "Using Prepend");
+
+    AutoUnref refs;
+    std::vector<CordRep*> flats;
+
+    // Build `left` side leaf appending all contained flats to `flats`
+    CordRepBtree* left = MakeLeaf(3);
+    GetLeafEdges(left, flats);
+    refs.RefIf(first_shared(), left);
+
+    // Build `right` side leaf appending all contained flats to `flats`
+    CordRepBtree* right = MakeLeaf(2);
+    GetLeafEdges(right, flats);
+    refs.RefIf(second_shared(), right);
+
+    CordRepBtree* tree = use_append ? CordRepBtree::Append(left, right)
+                                    : CordRepBtree::Prepend(right, left);
+    EXPECT_THAT(tree, IsNode(0));
+
+    // `tree` contains all flats originally belonging to `left` and `right`.
+    EXPECT_THAT(tree->Edges(), ElementsAreArray(flats));
+    CordRepBtree::Unref(tree);
+  }
+}
+
+TEST_P(CordRepBtreeDualTest, MergeLeafsExceedingCapacity) {
+  for (bool use_append : {false, true}) {
+    SCOPED_TRACE(use_append ? "Using Append" : "Using Prepend");
+
+    AutoUnref refs;
+
+    // Build `left` side tree appending all contained flats to `flats`
+    CordRepBtree* left = MakeLeaf(CordRepBtree::kMaxCapacity - 2);
+    refs.RefIf(first_shared(), left);
+
+    // Build `right` side tree appending all contained flats to `flats`
+    CordRepBtree* right = MakeLeaf(CordRepBtree::kMaxCapacity - 1);
+    refs.RefIf(second_shared(), right);
+
+    CordRepBtree* tree = use_append ? CordRepBtree::Append(left, right)
+                                    : CordRepBtree::Prepend(right, left);
+    EXPECT_THAT(tree, IsNode(1));
+    EXPECT_THAT(tree->Edges(), ElementsAre(left, right));
+    CordRepBtree::Unref(tree);
+  }
+}
+
+TEST_P(CordRepBtreeDualTest, MergeEqualHeightTrees) {
+  for (bool use_append : {false, true}) {
+    SCOPED_TRACE(use_append ? "Using Append" : "Using Prepend");
+
+    AutoUnref refs;
+    std::vector<CordRep*> flats;
+
+    // Build `left` side tree appending all contained flats to `flats`
+    CordRepBtree* left = MakeTree(CordRepBtree::kMaxCapacity * 3);
+    GetLeafEdges(left, flats);
+    refs.RefIf(first_shared(), left);
+
+    // Build `right` side tree appending all contained flats to `flats`
+    CordRepBtree* right = MakeTree(CordRepBtree::kMaxCapacity * 2);
+    GetLeafEdges(right, flats);
+    refs.RefIf(second_shared(), right);
+
+    CordRepBtree* tree = use_append ? CordRepBtree::Append(left, right)
+                                    : CordRepBtree::Prepend(right, left);
+    EXPECT_THAT(tree, IsNode(1));
+    EXPECT_THAT(tree->Edges(), SizeIs(5u));
+
+    // `tree` contains all flats originally belonging to `left` and `right`.
+    EXPECT_THAT(GetLeafEdges(tree), ElementsAreArray(flats));
+    CordRepBtree::Unref(tree);
+  }
+}
+
+TEST_P(CordRepBtreeDualTest, MergeLeafWithTreeNotExceedingLeafCapacity) {
+  for (bool use_append : {false, true}) {
+    SCOPED_TRACE(use_append ? "Using Append" : "Using Prepend");
+
+    AutoUnref refs;
+    std::vector<CordRep*> flats;
+
+    // Build `left` side tree appending all added flats to `flats`
+    CordRepBtree* left = MakeTree(CordRepBtree::kMaxCapacity * 2 + 2);
+    GetLeafEdges(left, flats);
+    refs.RefIf(first_shared(), left);
+
+    // Build `right` side tree appending all added flats to `flats`
+    CordRepBtree* right = MakeTree(3);
+    GetLeafEdges(right, flats);
+    refs.RefIf(second_shared(), right);
+
+    CordRepBtree* tree = use_append ? CordRepBtree::Append(left, right)
+                                    : CordRepBtree::Prepend(right, left);
+    EXPECT_THAT(tree, IsNode(1));
+    EXPECT_THAT(tree->Edges(), SizeIs(3u));
+
+    // `tree` contains all flats originally belonging to `left` and `right`.
+    EXPECT_THAT(GetLeafEdges(tree), ElementsAreArray(flats));
+    CordRepBtree::Unref(tree);
+  }
+}
+
+TEST_P(CordRepBtreeDualTest, MergeLeafWithTreeExceedingLeafCapacity) {
+  for (bool use_append : {false, true}) {
+    SCOPED_TRACE(use_append ? "Using Append" : "Using Prepend");
+
+    AutoUnref refs;
+    std::vector<CordRep*> flats;
+
+    // Build `left` side tree appending all added flats to `flats`
+    CordRepBtree* left = MakeTree(CordRepBtree::kMaxCapacity * 3 - 2);
+    GetLeafEdges(left, flats);
+    refs.RefIf(first_shared(), left);
+
+    // Build `right` side tree appending all added flats to `flats`
+    CordRepBtree* right = MakeTree(3);
+    GetLeafEdges(right, flats);
+    refs.RefIf(second_shared(), right);
+
+    CordRepBtree* tree = use_append ? CordRepBtree::Append(left, right)
+                                    : CordRepBtree::Prepend(right, left);
+    EXPECT_THAT(tree, IsNode(1));
+    EXPECT_THAT(tree->Edges(), SizeIs(4u));
+
+    // `tree` contains all flats originally belonging to `left` and `right`.
+    EXPECT_THAT(GetLeafEdges(tree), ElementsAreArray(flats));
+    CordRepBtree::Unref(tree);
+  }
+}
+
+void RefEdgesAt(size_t depth, AutoUnref& refs, CordRepBtree* tree) {
+  absl::Span<CordRep* const> edges = tree->Edges();
+  if (depth == 0) {
+    refs.Ref(edges.front());
+    refs.Ref(edges.back());
+  } else {
+    assert(tree->height() > 0);
+    RefEdgesAt(depth - 1, refs, edges.front()->btree());
+    RefEdgesAt(depth - 1, refs, edges.back()->btree());
+  }
+}
+
+TEST(CordRepBtreeTest, MergeFuzzTest) {
+  constexpr size_t max_cap = CordRepBtree::kMaxCapacity;
+  std::minstd_rand rnd;
+  std::uniform_int_distribution<int> coin_flip(0, 1);
+  std::uniform_int_distribution<int> dice_throw(1, 6);
+
+  auto random_leaf_count = [&]() {
+    std::uniform_int_distribution<int> dist_height(0, 3);
+    std::uniform_int_distribution<int> dist_leaf(0, max_cap - 1);
+    const int height = dist_height(rnd);
+    return (height ? pow(max_cap, height) : 0) + dist_leaf(rnd);
+  };
+
+  for (int i = 0; i < 10000; ++i) {
+    AutoUnref refs;
+    std::vector<CordRep*> flats;
+
+    CordRepBtree* left = MakeTree(random_leaf_count(), coin_flip(rnd));
+    GetLeafEdges(left, flats);
+    if (dice_throw(rnd) == 1) {
+      std::uniform_int_distribution<size_t> dist(
+          0, static_cast<size_t>(left->height()));
+      RefEdgesAt(dist(rnd), refs, left);
+    }
+
+    CordRepBtree* right = MakeTree(random_leaf_count(), coin_flip(rnd));
+    GetLeafEdges(right, flats);
+    if (dice_throw(rnd) == 1) {
+      std::uniform_int_distribution<size_t> dist(
+          0, static_cast<size_t>(right->height()));
+      RefEdgesAt(dist(rnd), refs, right);
+    }
+
+    CordRepBtree* tree = CordRepBtree::Append(left, right);
+    EXPECT_THAT(GetLeafEdges(tree), ElementsAreArray(flats));
+    CordRepBtree::Unref(tree);
+  }
+}
+
+TEST_P(CordRepBtreeTest, RemoveSuffix) {
+  // Create tree of 1, 2 and 3 levels high
+  constexpr size_t max_cap = CordRepBtree::kMaxCapacity;
+  for (size_t cap : {max_cap - 1, max_cap * 2, max_cap * max_cap * 2}) {
+    const std::string data = CreateRandomString(cap * 512);
+
+    {
+      // Verify RemoveSuffix(<all>)
+      AutoUnref refs;
+      CordRepBtree* node = refs.RefIf(shared(), CreateTree(data, 512));
+      EXPECT_THAT(CordRepBtree::RemoveSuffix(node, data.length()), Eq(nullptr));
+
+      // Verify RemoveSuffix(<none>)
+      node = refs.RefIf(shared(), CreateTree(data, 512));
+      EXPECT_THAT(CordRepBtree::RemoveSuffix(node, 0), Eq(node));
+      CordRep::Unref(node);
+    }
+
+    for (size_t n = 1; n < data.length(); ++n) {
+      AutoUnref refs;
+      auto flats = CreateFlatsFromString(data, 512);
+      CordRepBtree* node = refs.RefIf(shared(), CreateTree(flats));
+      CordRep* rep = refs.Add(CordRepBtree::RemoveSuffix(node, n));
+      EXPECT_THAT(CordToString(rep), Eq(data.substr(0, data.length() - n)));
+
+      // Collect all flats
+      auto is_flat = [](CordRep* rep) { return rep->tag >= FLAT; };
+      std::vector<CordRep*> edges = CordCollectRepsIf(is_flat, rep);
+      ASSERT_THAT(edges.size(), Le(flats.size()));
+
+      // Isolate last edge
+      CordRep* last_edge = edges.back();
+      edges.pop_back();
+      const size_t last_length = rep->length - edges.size() * 512;
+
+      // All flats except the last edge must be kept or copied 'as is'
+      size_t index = 0;
+      for (CordRep* edge : edges) {
+        ASSERT_THAT(edge, Eq(flats[index++]));
+        ASSERT_THAT(edge->length, Eq(512u));
+      }
+
+      // CordRepBtree may optimize small substrings to avoid waste, so only
+      // check for flat sharing / updates where the code should always do this.
+      if (last_length >= 500) {
+        EXPECT_THAT(last_edge, Eq(flats[index++]));
+        if (shared()) {
+          EXPECT_THAT(last_edge->length, Eq(512u));
+        } else {
+          EXPECT_TRUE(last_edge->refcount.IsOne());
+          EXPECT_THAT(last_edge->length, Eq(last_length));
+        }
+      }
+    }
+  }
+}
+
+TEST(CordRepBtreeTest, SubTree) {
+  // Create tree of at least 2 levels high
+  constexpr size_t max_cap = CordRepBtree::kMaxCapacity;
+  const size_t n = max_cap * max_cap * 2;
+  const std::string data = CreateRandomString(n * 3);
+  std::vector<CordRep*> flats;
+  for (absl::string_view s = data; !s.empty(); s.remove_prefix(3)) {
+    flats.push_back(MakeFlat(s.substr(0, 3)));
+  }
+  CordRepBtree* node = CordRepBtree::Create(CordRep::Ref(flats[0]));
+  for (size_t i = 1; i < flats.size(); ++i) {
+    node = CordRepBtree::Append(node, CordRep::Ref(flats[i]));
+  }
+
+  for (size_t offset = 0; offset < data.length(); ++offset) {
+    for (size_t length = 1; length <= data.length() - offset; ++length) {
+      CordRep* rep = node->SubTree(offset, length);
+      EXPECT_THAT(CordToString(rep), Eq(data.substr(offset, length)));
+      CordRep::Unref(rep);
+    }
+  }
+  CordRepBtree::Unref(node);
+  for (CordRep* rep : flats) {
+    CordRep::Unref(rep);
+  }
+}
+
+TEST(CordRepBtreeTest, SubTreeOnExistingSubstring) {
+  // This test verifies that a SubTree call on a pre-existing (large) substring
+  // adjusts the existing substring if not shared, and else rewrites the
+  // existing substring.
+  AutoUnref refs;
+  std::string data = CreateRandomString(1000);
+  CordRepBtree* leaf = CordRepBtree::Create(MakeFlat("abc"));
+  CordRep* flat = MakeFlat(data);
+  leaf = CordRepBtree::Append(leaf, flat);
+
+  // Setup tree containing substring.
+  CordRep* result = leaf->SubTree(0, 3 + 990);
+  ASSERT_THAT(result->tag, Eq(BTREE));
+  CordRep::Unref(leaf);
+  leaf = result->btree();
+  ASSERT_THAT(leaf->Edges(), ElementsAre(_, IsSubstring(0u, 990u)));
+  EXPECT_THAT(leaf->Edges()[1]->substring()->child, Eq(flat));
+
+  // Verify substring of substring.
+  result = leaf->SubTree(3 + 5, 970);
+  ASSERT_THAT(result, IsSubstring(5u, 970u));
+  EXPECT_THAT(result->substring()->child, Eq(flat));
+  CordRep::Unref(result);
+
+  CordRep::Unref(leaf);
+}
+
+TEST_P(CordRepBtreeTest, AddDataToLeaf) {
+  const size_t n = CordRepBtree::kMaxCapacity;
+  const std::string data = CreateRandomString(n * 3);
+
+  for (bool append : {true, false}) {
+    AutoUnref refs;
+    DataConsumer consumer(data, append);
+    SCOPED_TRACE(append ? "Append" : "Prepend");
+
+    CordRepBtree* leaf = CordRepBtree::Create(MakeFlat(consumer.Next(3)));
+    for (size_t i = 1; i < n; ++i) {
+      refs.RefIf(shared(), leaf);
+      CordRepBtree* result = BtreeAdd(leaf, append, consumer.Next(3));
+      EXPECT_THAT(result, Conditional(shared(), Ne(leaf), Eq(leaf)));
+      EXPECT_THAT(CordToString(result), Eq(consumer.Consumed()));
+      leaf = result;
+    }
+    CordRep::Unref(leaf);
+  }
+}
+
+TEST_P(CordRepBtreeTest, AppendDataToTree) {
+  AutoUnref refs;
+  size_t n = CordRepBtree::kMaxCapacity + CordRepBtree::kMaxCapacity / 2;
+  std::string data = CreateRandomString(n * 3);
+  CordRepBtree* tree = refs.RefIf(shared(), CreateTree(data, 3));
+  CordRepBtree* leaf0 = tree->Edges()[0]->btree();
+  CordRepBtree* leaf1 = tree->Edges()[1]->btree();
+  CordRepBtree* result = CordRepBtree::Append(tree, "123456789");
+  EXPECT_THAT(result, Conditional(shared(), Ne(tree), Eq(tree)));
+  EXPECT_THAT(result->Edges(),
+              ElementsAre(leaf0, Conditional(shared(), Ne(leaf1), Eq(leaf1))));
+  EXPECT_THAT(CordToString(result), Eq(data + "123456789"));
+  CordRep::Unref(result);
+}
+
+TEST_P(CordRepBtreeTest, PrependDataToTree) {
+  AutoUnref refs;
+  size_t n = CordRepBtree::kMaxCapacity + CordRepBtree::kMaxCapacity / 2;
+  std::string data = CreateRandomString(n * 3);
+  CordRepBtree* tree = refs.RefIf(shared(), CreateTreeReverse(data, 3));
+  CordRepBtree* leaf0 = tree->Edges()[0]->btree();
+  CordRepBtree* leaf1 = tree->Edges()[1]->btree();
+  CordRepBtree* result = CordRepBtree::Prepend(tree, "123456789");
+  EXPECT_THAT(result, Conditional(shared(), Ne(tree), Eq(tree)));
+  EXPECT_THAT(result->Edges(),
+              ElementsAre(Conditional(shared(), Ne(leaf0), Eq(leaf0)), leaf1));
+  EXPECT_THAT(CordToString(result), Eq("123456789" + data));
+  CordRep::Unref(result);
+}
+
+TEST_P(CordRepBtreeTest, AddDataToTreeThreeLevelsDeep) {
+  constexpr size_t max_cap = CordRepBtree::kMaxCapacity;
+  const size_t n = max_cap * max_cap * max_cap;
+  const std::string data = CreateRandomString(n * 3);
+
+  for (bool append : {true, false}) {
+    AutoUnref refs;
+    DataConsumer consumer(data, append);
+    SCOPED_TRACE(append ? "Append" : "Prepend");
+
+    // Fill leaf
+    CordRepBtree* tree = CordRepBtree::Create(MakeFlat(consumer.Next(3)));
+    for (size_t i = 1; i < max_cap; ++i) {
+      tree = BtreeAdd(tree, append, consumer.Next(3));
+    }
+    ASSERT_THAT(CordToString(tree), Eq(consumer.Consumed()));
+
+    // Fill to maximum at one deep
+    refs.RefIf(shared(), tree);
+    CordRepBtree* result = BtreeAdd(tree, append, consumer.Next(3));
+    ASSERT_THAT(result, IsNode(1));
+    ASSERT_THAT(result, Ne(tree));
+    ASSERT_THAT(CordToString(result), Eq(consumer.Consumed()));
+    tree = result;
+    for (size_t i = max_cap + 1; i < max_cap * max_cap; ++i) {
+      refs.RefIf(shared(), tree);
+      result = BtreeAdd(tree, append, consumer.Next(3));
+      ASSERT_THAT(result, Conditional(shared(), Ne(tree), Eq(tree)));
+      ASSERT_THAT(CordToString(result), Eq(consumer.Consumed()));
+      tree = result;
+    }
+
+    // Fill to maximum at two deep
+    refs.RefIf(shared(), tree);
+    result = BtreeAdd(tree, append, consumer.Next(3));
+    ASSERT_THAT(result, IsNode(2));
+    ASSERT_THAT(result, Ne(tree));
+    ASSERT_THAT(CordToString(result), Eq(consumer.Consumed()));
+    tree = result;
+    for (size_t i = max_cap * max_cap + 1; i < max_cap * max_cap * max_cap;
+         ++i) {
+      refs.RefIf(shared(), tree);
+      result = BtreeAdd(tree, append, consumer.Next(3));
+      ASSERT_THAT(result, Conditional(shared(), Ne(tree), Eq(tree)));
+      ASSERT_THAT(CordToString(result), Eq(consumer.Consumed()));
+      tree = result;
+    }
+
+    CordRep::Unref(tree);
+  }
+}
+
+TEST_P(CordRepBtreeTest, AddLargeDataToLeaf) {
+  const size_t max_cap = CordRepBtree::kMaxCapacity;
+  const size_t n = max_cap * max_cap * max_cap * 3 + 2;
+  const std::string data = CreateRandomString(n * kMaxFlatLength);
+
+  for (bool append : {true, false}) {
+    AutoUnref refs;
+    SCOPED_TRACE(append ? "Append" : "Prepend");
+
+    CordRepBtree* leaf = CordRepBtree::Create(MakeFlat("abc"));
+    refs.RefIf(shared(), leaf);
+    CordRepBtree* result = BtreeAdd(leaf, append, data);
+    EXPECT_THAT(CordToString(result), Eq(append ? "abc" + data : data + "abc"));
+    CordRep::Unref(result);
+  }
+}
+
+TEST_P(CordRepBtreeTest, CreateFromTreeReturnsTree) {
+  AutoUnref refs;
+  CordRepBtree* leaf = CordRepBtree::Create(MakeFlat("Hello world"));
+  refs.RefIf(shared(), leaf);
+  CordRepBtree* result = CordRepBtree::Create(leaf);
+  EXPECT_THAT(result, Eq(leaf));
+  CordRep::Unref(result);
+}
+
+TEST(CordRepBtreeTest, GetCharacter) {
+  size_t n = CordRepBtree::kMaxCapacity * CordRepBtree::kMaxCapacity + 2;
+  std::string data = CreateRandomString(n * 3);
+  CordRepBtree* tree = CreateTree(data, 3);
+  // Add a substring node for good measure.
+  tree = tree->Append(tree, MakeSubstring(4, 5, MakeFlat("abcdefghijklm")));
+  data += "efghi";
+  for (size_t i = 0; i < data.length(); ++i) {
+    ASSERT_THAT(tree->GetCharacter(i), Eq(data[i]));
+  }
+  CordRep::Unref(tree);
+}
+
+TEST_P(CordRepBtreeTest, IsFlatSingleFlat) {
+  CordRepBtree* leaf = CordRepBtree::Create(MakeFlat("Hello world"));
+
+  absl::string_view fragment;
+  EXPECT_TRUE(leaf->IsFlat(nullptr));
+  EXPECT_TRUE(leaf->IsFlat(&fragment));
+  EXPECT_THAT(fragment, Eq("Hello world"));
+  fragment = "";
+  EXPECT_TRUE(leaf->IsFlat(0, 11, nullptr));
+  EXPECT_TRUE(leaf->IsFlat(0, 11, &fragment));
+  EXPECT_THAT(fragment, Eq("Hello world"));
+
+  // Arbitrary ranges must check true as well.
+  EXPECT_TRUE(leaf->IsFlat(1, 4, &fragment));
+  EXPECT_THAT(fragment, Eq("ello"));
+  EXPECT_TRUE(leaf->IsFlat(6, 5, &fragment));
+  EXPECT_THAT(fragment, Eq("world"));
+
+  CordRep::Unref(leaf);
+}
+
+TEST(CordRepBtreeTest, IsFlatMultiFlat) {
+  size_t n = CordRepBtree::kMaxCapacity * CordRepBtree::kMaxCapacity + 2;
+  std::string data = CreateRandomString(n * 3);
+  CordRepBtree* tree = CreateTree(data, 3);
+  // Add substring nodes for good measure.
+  tree = tree->Append(tree, MakeSubstring(4, 3, MakeFlat("abcdefghijklm")));
+  tree = tree->Append(tree, MakeSubstring(8, 3, MakeFlat("abcdefghijklm")));
+  data += "efgijk";
+
+  EXPECT_FALSE(tree->IsFlat(nullptr));
+  absl::string_view fragment = "Can't touch this";
+  EXPECT_FALSE(tree->IsFlat(&fragment));
+  EXPECT_THAT(fragment, Eq("Can't touch this"));
+
+  for (size_t offset = 0; offset < data.size(); offset += 3) {
+    EXPECT_TRUE(tree->IsFlat(offset, 3, nullptr));
+    EXPECT_TRUE(tree->IsFlat(offset, 3, &fragment));
+    EXPECT_THAT(fragment, Eq(data.substr(offset, 3)));
+
+    fragment = "Can't touch this";
+    if (offset > 0) {
+      EXPECT_FALSE(tree->IsFlat(offset - 1, 4, nullptr));
+      EXPECT_FALSE(tree->IsFlat(offset - 1, 4, &fragment));
+      EXPECT_THAT(fragment, Eq("Can't touch this"));
+    }
+    if (offset < data.size() - 4) {
+      EXPECT_FALSE(tree->IsFlat(offset, 4, nullptr));
+      EXPECT_FALSE(tree->IsFlat(offset, 4, &fragment));
+      EXPECT_THAT(fragment, Eq("Can't touch this"));
+    }
+  }
+
+  CordRep::Unref(tree);
+}
+
+#if defined(GTEST_HAS_DEATH_TEST) && !defined(NDEBUG)
+
+TEST_P(CordRepBtreeHeightTest, GetAppendBufferNotPrivate) {
+  CordRepBtree* tree = CordRepBtree::Create(MakeExternal("Foo"));
+  CordRepBtree::Ref(tree);
+  EXPECT_DEATH(tree->GetAppendBuffer(1), ".*");
+  CordRepBtree::Unref(tree);
+  CordRepBtree::Unref(tree);
+}
+
+#endif  // defined(GTEST_HAS_DEATH_TEST) && !defined(NDEBUG)
+
+TEST_P(CordRepBtreeHeightTest, GetAppendBufferNotFlat) {
+  CordRepBtree* tree = CordRepBtree::Create(MakeExternal("Foo"));
+  for (int i = 1; i <= height(); ++i) {
+    tree = CordRepBtree::New(tree);
+  }
+  EXPECT_THAT(tree->GetAppendBuffer(1), SizeIs(0u));
+  CordRepBtree::Unref(tree);
+}
+
+TEST_P(CordRepBtreeHeightTest, GetAppendBufferFlatNotPrivate) {
+  CordRepFlat* flat = MakeFlat("abc");
+  CordRepBtree* tree = CordRepBtree::Create(CordRep::Ref(flat));
+  for (int i = 1; i <= height(); ++i) {
+    tree = CordRepBtree::New(tree);
+  }
+  EXPECT_THAT(tree->GetAppendBuffer(1), SizeIs(0u));
+  CordRepBtree::Unref(tree);
+  CordRep::Unref(flat);
+}
+
+TEST_P(CordRepBtreeHeightTest, GetAppendBufferTreeNotPrivate) {
+  if (height() == 0) return;
+  AutoUnref refs;
+  CordRepFlat* flat = MakeFlat("abc");
+  CordRepBtree* tree = CordRepBtree::Create(CordRep::Ref(flat));
+  for (int i = 1; i <= height(); ++i) {
+    if (i == (height() + 1) / 2) refs.Ref(tree);
+    tree = CordRepBtree::New(tree);
+  }
+  EXPECT_THAT(tree->GetAppendBuffer(1), SizeIs(0u));
+  CordRepBtree::Unref(tree);
+  CordRep::Unref(flat);
+}
+
+TEST_P(CordRepBtreeHeightTest, GetAppendBufferFlatNoCapacity) {
+  CordRepFlat* flat = MakeFlat("abc");
+  flat->length = flat->Capacity();
+  CordRepBtree* tree = CordRepBtree::Create(flat);
+  for (int i = 1; i <= height(); ++i) {
+    tree = CordRepBtree::New(tree);
+  }
+  EXPECT_THAT(tree->GetAppendBuffer(1), SizeIs(0u));
+  CordRepBtree::Unref(tree);
+}
+
+TEST_P(CordRepBtreeHeightTest, GetAppendBufferFlatWithCapacity) {
+  CordRepFlat* flat = MakeFlat("abc");
+  CordRepBtree* tree = CordRepBtree::Create(flat);
+  for (int i = 1; i <= height(); ++i) {
+    tree = CordRepBtree::New(tree);
+  }
+  absl::Span<char> span = tree->GetAppendBuffer(2);
+  EXPECT_THAT(span, SizeIs(2u));
+  EXPECT_THAT(span.data(), TypedEq<void*>(flat->Data() + 3));
+  EXPECT_THAT(tree->length, Eq(5u));
+
+  size_t avail = flat->Capacity() - 5;
+  span = tree->GetAppendBuffer(avail + 100);
+  EXPECT_THAT(span, SizeIs(avail));
+  EXPECT_THAT(span.data(), TypedEq<void*>(flat->Data() + 5));
+  EXPECT_THAT(tree->length, Eq(5 + avail));
+
+  CordRepBtree::Unref(tree);
+}
+
+TEST(CordRepBtreeTest, Dump) {
+  // Handles nullptr
+  std::stringstream ss;
+  CordRepBtree::Dump(nullptr, ss);
+  CordRepBtree::Dump(nullptr, "Once upon a label", ss);
+  CordRepBtree::Dump(nullptr, "Once upon a label", false, ss);
+  CordRepBtree::Dump(nullptr, "Once upon a label", true, ss);
+
+  // Cover legal edges
+  CordRepFlat* flat = MakeFlat("Hello world");
+  CordRepExternal* external = MakeExternal("Hello external");
+  CordRep* substr_flat = MakeSubstring(1, 6, CordRep::Ref(flat));
+  CordRep* substr_external = MakeSubstring(2, 7, CordRep::Ref(external));
+
+  // Build tree
+  CordRepBtree* tree = CordRepBtree::Create(flat);
+  tree = CordRepBtree::Append(tree, external);
+  tree = CordRepBtree::Append(tree, substr_flat);
+  tree = CordRepBtree::Append(tree, substr_external);
+
+  // Repeat until we have a tree
+  while (tree->height() == 0) {
+    tree = CordRepBtree::Append(tree, CordRep::Ref(flat));
+    tree = CordRepBtree::Append(tree, CordRep::Ref(external));
+    tree = CordRepBtree::Append(tree, CordRep::Ref(substr_flat));
+    tree = CordRepBtree::Append(tree, CordRep::Ref(substr_external));
+  }
+
+  for (int api = 0; api <= 3; ++api) {
+    absl::string_view api_scope;
+    std::stringstream ss;
+    switch (api) {
+      case 0:
+        api_scope = "Bare";
+        CordRepBtree::Dump(tree, ss);
+        break;
+      case 1:
+        api_scope = "Label only";
+        CordRepBtree::Dump(tree, "Once upon a label", ss);
+        break;
+      case 2:
+        api_scope = "Label no content";
+        CordRepBtree::Dump(tree, "Once upon a label", false, ss);
+        break;
+      default:
+        api_scope = "Label and content";
+        CordRepBtree::Dump(tree, "Once upon a label", true, ss);
+        break;
+    }
+    SCOPED_TRACE(api_scope);
+    std::string str = ss.str();
+
+    // Contains Node(depth) / Leaf and private / shared indicators
+    EXPECT_THAT(str, AllOf(HasSubstr("Node(1)"), HasSubstr("Leaf"),
+                           HasSubstr("Private"), HasSubstr("Shared")));
+
+    // Contains length and start offset of all data edges
+    EXPECT_THAT(str, AllOf(HasSubstr("len = 11"), HasSubstr("len = 14"),
+                           HasSubstr("len = 6"), HasSubstr("len = 7"),
+                           HasSubstr("start = 1"), HasSubstr("start = 2")));
+
+    // Contains address of all data edges
+    EXPECT_THAT(
+        str, AllOf(HasSubstr(absl::StrCat("0x", absl::Hex(flat))),
+                   HasSubstr(absl::StrCat("0x", absl::Hex(external))),
+                   HasSubstr(absl::StrCat("0x", absl::Hex(substr_flat))),
+                   HasSubstr(absl::StrCat("0x", absl::Hex(substr_external)))));
+
+    if (api != 0) {
+      // Contains label
+      EXPECT_THAT(str, HasSubstr("Once upon a label"));
+    }
+
+    if (api != 3) {
+      // Does not contain contents
+      EXPECT_THAT(str, Not(AnyOf(HasSubstr("data = \"Hello world\""),
+                                 HasSubstr("data = \"Hello external\""),
+                                 HasSubstr("data = \"ello w\""),
+                                 HasSubstr("data = \"llo ext\""))));
+    } else {
+      // Contains contents
+      EXPECT_THAT(str, AllOf(HasSubstr("data = \"Hello world\""),
+                             HasSubstr("data = \"Hello external\""),
+                             HasSubstr("data = \"ello w\""),
+                             HasSubstr("data = \"llo ext\"")));
+    }
+  }
+
+  CordRep::Unref(tree);
+}
+
+TEST(CordRepBtreeTest, IsValid) {
+  EXPECT_FALSE(CordRepBtree::IsValid(nullptr));
+
+  CordRepBtree* empty = CordRepBtree::New(0);
+  EXPECT_TRUE(CordRepBtree::IsValid(empty));
+  CordRep::Unref(empty);
+
+  for (bool as_tree : {false, true}) {
+    CordRepBtree* leaf = CordRepBtree::Create(MakeFlat("abc"));
+    CordRepBtree* tree = as_tree ? CordRepBtree::New(leaf) : nullptr;
+    CordRepBtree* check = as_tree ? tree : leaf;
+
+    ASSERT_TRUE(CordRepBtree::IsValid(check));
+    leaf->length--;
+    EXPECT_FALSE(CordRepBtree::IsValid(check));
+    leaf->length++;
+
+    ASSERT_TRUE(CordRepBtree::IsValid(check));
+    leaf->tag--;
+    EXPECT_FALSE(CordRepBtree::IsValid(check));
+    leaf->tag++;
+
+    // Height
+    ASSERT_TRUE(CordRepBtree::IsValid(check));
+    leaf->storage[0] = static_cast<uint8_t>(CordRepBtree::kMaxHeight + 1);
+    EXPECT_FALSE(CordRepBtree::IsValid(check));
+    leaf->storage[0] = 1;
+    EXPECT_FALSE(CordRepBtree::IsValid(check));
+    leaf->storage[0] = 0;
+
+    // Begin
+    ASSERT_TRUE(CordRepBtree::IsValid(check));
+    const uint8_t begin = leaf->storage[1];
+    leaf->storage[1] = static_cast<uint8_t>(CordRepBtree::kMaxCapacity);
+    EXPECT_FALSE(CordRepBtree::IsValid(check));
+    leaf->storage[1] = 2;
+    EXPECT_FALSE(CordRepBtree::IsValid(check));
+    leaf->storage[1] = begin;
+
+    // End
+    ASSERT_TRUE(CordRepBtree::IsValid(check));
+    const uint8_t end = leaf->storage[2];
+    leaf->storage[2] = static_cast<uint8_t>(CordRepBtree::kMaxCapacity + 1);
+    EXPECT_FALSE(CordRepBtree::IsValid(check));
+    leaf->storage[2] = end;
+
+    // DataEdge tag and value
+    ASSERT_TRUE(CordRepBtree::IsValid(check));
+    CordRep* const edge = leaf->Edges()[0];
+    const uint8_t tag = edge->tag;
+    CordRepBtreeTestPeer::SetEdge(leaf, begin, nullptr);
+    EXPECT_FALSE(CordRepBtree::IsValid(check));
+    CordRepBtreeTestPeer::SetEdge(leaf, begin, edge);
+    edge->tag = BTREE;
+    EXPECT_FALSE(CordRepBtree::IsValid(check));
+    edge->tag = tag;
+
+    if (as_tree) {
+      ASSERT_TRUE(CordRepBtree::IsValid(check));
+      leaf->length--;
+      EXPECT_FALSE(CordRepBtree::IsValid(check));
+      leaf->length++;
+
+      // Height
+      ASSERT_TRUE(CordRepBtree::IsValid(check));
+      tree->storage[0] = static_cast<uint8_t>(2);
+      EXPECT_FALSE(CordRepBtree::IsValid(check));
+      tree->storage[0] = 1;
+
+      // Btree edge
+      ASSERT_TRUE(CordRepBtree::IsValid(check));
+      CordRep* const edge = tree->Edges()[0];
+      const uint8_t tag = edge->tag;
+      edge->tag = FLAT;
+      EXPECT_FALSE(CordRepBtree::IsValid(check));
+      edge->tag = tag;
+    }
+
+    ASSERT_TRUE(CordRepBtree::IsValid(check));
+    CordRep::Unref(check);
+  }
+}
+
+TEST(CordRepBtreeTest, AssertValid) {
+  CordRepBtree* tree = CordRepBtree::Create(MakeFlat("abc"));
+  const CordRepBtree* ctree = tree;
+  EXPECT_THAT(CordRepBtree::AssertValid(tree), Eq(tree));
+  EXPECT_THAT(CordRepBtree::AssertValid(ctree), Eq(ctree));
+
+#if defined(GTEST_HAS_DEATH_TEST)
+  CordRepBtree* nulltree = nullptr;
+  const CordRepBtree* cnulltree = nullptr;
+  EXPECT_DEBUG_DEATH(
+      EXPECT_THAT(CordRepBtree::AssertValid(nulltree), Eq(nulltree)), ".*");
+  EXPECT_DEBUG_DEATH(
+      EXPECT_THAT(CordRepBtree::AssertValid(cnulltree), Eq(cnulltree)), ".*");
+
+  tree->length--;
+  EXPECT_DEBUG_DEATH(EXPECT_THAT(CordRepBtree::AssertValid(tree), Eq(tree)),
+                     ".*");
+  EXPECT_DEBUG_DEATH(EXPECT_THAT(CordRepBtree::AssertValid(ctree), Eq(ctree)),
+                     ".*");
+  tree->length++;
+#endif
+  CordRep::Unref(tree);
+}
+
+TEST(CordRepBtreeTest, CheckAssertValidShallowVsDeep) {
+  // Restore exhaustive validation on any exit.
+  const bool exhaustive_validation = IsCordBtreeExhaustiveValidationEnabled();
+  auto cleanup = absl::MakeCleanup([exhaustive_validation] {
+    SetCordBtreeExhaustiveValidation(exhaustive_validation);
+  });
+
+  // Create a tree of at least 2 levels, and mess with the original flat, which
+  // should go undetected in shallow mode as the flat is too far away, but
+  // should be detected in forced non-shallow mode.
+  CordRep* flat = MakeFlat("abc");
+  CordRepBtree* tree = CordRepBtree::Create(flat);
+  constexpr size_t max_cap = CordRepBtree::kMaxCapacity;
+  const size_t n = max_cap * max_cap * 2;
+  for (size_t i = 0; i < n; ++i) {
+    tree = CordRepBtree::Append(tree, MakeFlat("Hello world"));
+  }
+  flat->length = 100;
+
+  SetCordBtreeExhaustiveValidation(false);
+  EXPECT_FALSE(CordRepBtree::IsValid(tree));
+  EXPECT_TRUE(CordRepBtree::IsValid(tree, true));
+  EXPECT_FALSE(CordRepBtree::IsValid(tree, false));
+  CordRepBtree::AssertValid(tree);
+  CordRepBtree::AssertValid(tree, true);
+#if defined(GTEST_HAS_DEATH_TEST)
+  EXPECT_DEBUG_DEATH(CordRepBtree::AssertValid(tree, false), ".*");
+#endif
+
+  SetCordBtreeExhaustiveValidation(true);
+  EXPECT_FALSE(CordRepBtree::IsValid(tree));
+  EXPECT_FALSE(CordRepBtree::IsValid(tree, true));
+  EXPECT_FALSE(CordRepBtree::IsValid(tree, false));
+#if defined(GTEST_HAS_DEATH_TEST)
+  EXPECT_DEBUG_DEATH(CordRepBtree::AssertValid(tree), ".*");
+  EXPECT_DEBUG_DEATH(CordRepBtree::AssertValid(tree, true), ".*");
+#endif
+
+  flat->length = 3;
+  CordRep::Unref(tree);
+}
+
+TEST_P(CordRepBtreeTest, Rebuild) {
+  for (size_t size : {3u, 8u, 100u, 10000u, 1000000u}) {
+    SCOPED_TRACE(absl::StrCat("Rebuild @", size));
+
+    std::vector<CordRepFlat*> flats;
+    for (size_t i = 0; i < size; ++i) {
+      flats.push_back(CordRepFlat::New(2));
+      flats.back()->Data()[0] = 'x';
+      flats.back()->length = 1;
+    }
+
+    // Build the tree into 'right', and each so many 'split_limit' edges,
+    // combine 'left' + 'right' into a new 'left', and start a new 'right'.
+    // This guarantees we get a reasonable amount of chaos in the tree.
+    size_t split_count = 0;
+    size_t split_limit = 3;
+    auto it = flats.begin();
+    CordRepBtree* left = nullptr;
+    CordRepBtree* right = CordRepBtree::New(*it);
+    while (++it != flats.end()) {
+      if (++split_count >= split_limit) {
+        split_limit += split_limit / 16;
+        left = left ? CordRepBtree::Append(left, right) : right;
+        right = CordRepBtree::New(*it);
+      } else {
+        right = CordRepBtree::Append(right, *it);
+      }
+    }
+
+    // Finalize tree
+    left = left ? CordRepBtree::Append(left, right) : right;
+
+    // Rebuild
+    AutoUnref ref;
+    left = ref.Add(CordRepBtree::Rebuild(ref.RefIf(shared(), left)));
+    ASSERT_TRUE(CordRepBtree::IsValid(left));
+
+    // Verify we have the exact same edges in the exact same order.
+    bool ok = true;
+    it = flats.begin();
+    CordVisitReps(left, [&](CordRep* edge) {
+      if (edge->tag < FLAT) return;
+      ok = ok && (it != flats.end() && *it++ == edge);
+    });
+    EXPECT_TRUE(ok && it == flats.end()) << "Rebuild edges mismatch";
+  }
+}
+
+// Convenience helper for CordRepBtree::ExtractAppendBuffer
+CordRepBtree::ExtractResult ExtractLast(CordRepBtree* input, size_t cap = 1) {
+  return CordRepBtree::ExtractAppendBuffer(input, cap);
+}
+
+TEST(CordRepBtreeTest, ExtractAppendBufferLeafSingleFlat) {
+  CordRep* flat = MakeFlat("Abc");
+  CordRepBtree* leaf = CordRepBtree::Create(flat);
+  EXPECT_THAT(ExtractLast(leaf), EqExtractResult(nullptr, flat));
+  CordRep::Unref(flat);
+}
+
+TEST(CordRepBtreeTest, ExtractAppendBufferNodeSingleFlat) {
+  CordRep* flat = MakeFlat("Abc");
+  CordRepBtree* leaf = CordRepBtree::Create(flat);
+  CordRepBtree* node = CordRepBtree::New(leaf);
+  EXPECT_THAT(ExtractLast(node), EqExtractResult(nullptr, flat));
+  CordRep::Unref(flat);
+}
+
+TEST(CordRepBtreeTest, ExtractAppendBufferLeafTwoFlats) {
+  std::vector<CordRep*> flats = CreateFlatsFromString("abcdef", 3);
+  CordRepBtree* leaf = CreateTree(flats);
+  EXPECT_THAT(ExtractLast(leaf), EqExtractResult(flats[0], flats[1]));
+  CordRep::Unref(flats[0]);
+  CordRep::Unref(flats[1]);
+}
+
+TEST(CordRepBtreeTest, ExtractAppendBufferNodeTwoFlats) {
+  std::vector<CordRep*> flats = CreateFlatsFromString("abcdef", 3);
+  CordRepBtree* leaf = CreateTree(flats);
+  CordRepBtree* node = CordRepBtree::New(leaf);
+  EXPECT_THAT(ExtractLast(node), EqExtractResult(flats[0], flats[1]));
+  CordRep::Unref(flats[0]);
+  CordRep::Unref(flats[1]);
+}
+
+TEST(CordRepBtreeTest, ExtractAppendBufferNodeTwoFlatsInTwoLeafs) {
+  std::vector<CordRep*> flats = CreateFlatsFromString("abcdef", 3);
+  CordRepBtree* leaf1 = CordRepBtree::Create(flats[0]);
+  CordRepBtree* leaf2 = CordRepBtree::Create(flats[1]);
+  CordRepBtree* node = CordRepBtree::New(leaf1, leaf2);
+  EXPECT_THAT(ExtractLast(node), EqExtractResult(flats[0], flats[1]));
+  CordRep::Unref(flats[0]);
+  CordRep::Unref(flats[1]);
+}
+
+TEST(CordRepBtreeTest, ExtractAppendBufferLeafThreeFlats) {
+  std::vector<CordRep*> flats = CreateFlatsFromString("abcdefghi", 3);
+  CordRepBtree* leaf = CreateTree(flats);
+  EXPECT_THAT(ExtractLast(leaf), EqExtractResult(leaf, flats[2]));
+  CordRep::Unref(flats[2]);
+  CordRep::Unref(leaf);
+}
+
+TEST(CordRepBtreeTest, ExtractAppendBufferNodeThreeFlatsRightNoFolding) {
+  CordRep* flat = MakeFlat("Abc");
+  std::vector<CordRep*> flats = CreateFlatsFromString("defghi", 3);
+  CordRepBtree* leaf1 = CordRepBtree::Create(flat);
+  CordRepBtree* leaf2 = CreateTree(flats);
+  CordRepBtree* node = CordRepBtree::New(leaf1, leaf2);
+  EXPECT_THAT(ExtractLast(node), EqExtractResult(node, flats[1]));
+  EXPECT_THAT(node->Edges(), ElementsAre(leaf1, leaf2));
+  EXPECT_THAT(leaf1->Edges(), ElementsAre(flat));
+  EXPECT_THAT(leaf2->Edges(), ElementsAre(flats[0]));
+  CordRep::Unref(node);
+  CordRep::Unref(flats[1]);
+}
+
+TEST(CordRepBtreeTest, ExtractAppendBufferNodeThreeFlatsRightLeafFolding) {
+  CordRep* flat = MakeFlat("Abc");
+  std::vector<CordRep*> flats = CreateFlatsFromString("defghi", 3);
+  CordRepBtree* leaf1 = CreateTree(flats);
+  CordRepBtree* leaf2 = CordRepBtree::Create(flat);
+  CordRepBtree* node = CordRepBtree::New(leaf1, leaf2);
+  EXPECT_THAT(ExtractLast(node), EqExtractResult(leaf1, flat));
+  EXPECT_THAT(leaf1->Edges(), ElementsAreArray(flats));
+  CordRep::Unref(leaf1);
+  CordRep::Unref(flat);
+}
+
+TEST(CordRepBtreeTest, ExtractAppendBufferNoCapacity) {
+  std::vector<CordRep*> flats = CreateFlatsFromString("abcdef", 3);
+  CordRepBtree* leaf = CreateTree(flats);
+  size_t avail = flats[1]->flat()->Capacity() - flats[1]->length;
+  EXPECT_THAT(ExtractLast(leaf, avail + 1), EqExtractResult(leaf, nullptr));
+  EXPECT_THAT(ExtractLast(leaf, avail), EqExtractResult(flats[0], flats[1]));
+  CordRep::Unref(flats[0]);
+  CordRep::Unref(flats[1]);
+}
+
+TEST(CordRepBtreeTest, ExtractAppendBufferNotFlat) {
+  std::vector<CordRep*> flats = CreateFlatsFromString("abcdef", 3);
+  auto substr = MakeSubstring(1, 2, flats[1]);
+  CordRepBtree* leaf = CreateTree({flats[0], substr});
+  EXPECT_THAT(ExtractLast(leaf), EqExtractResult(leaf, nullptr));
+  CordRep::Unref(leaf);
+}
+
+TEST(CordRepBtreeTest, ExtractAppendBufferShared) {
+  std::vector<CordRep*> flats = CreateFlatsFromString("abcdef", 3);
+  CordRepBtree* leaf = CreateTree(flats);
+
+  CordRep::Ref(flats[1]);
+  EXPECT_THAT(ExtractLast(leaf), EqExtractResult(leaf, nullptr));
+  CordRep::Unref(flats[1]);
+
+  CordRep::Ref(leaf);
+  EXPECT_THAT(ExtractLast(leaf), EqExtractResult(leaf, nullptr));
+  CordRep::Unref(leaf);
+
+  CordRepBtree* node = CordRepBtree::New(leaf);
+  CordRep::Ref(node);
+  EXPECT_THAT(ExtractLast(node), EqExtractResult(node, nullptr));
+  CordRep::Unref(node);
+
+  CordRep::Unref(node);
+}
+
+}  // namespace
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/strings/internal/cord_rep_consume.cc b/absl/strings/internal/cord_rep_consume.cc
new file mode 100644
index 00000000000..8b6af152f8e
--- /dev/null
+++ b/absl/strings/internal/cord_rep_consume.cc
@@ -0,0 +1,65 @@
+// Copyright 2021 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/strings/internal/cord_rep_consume.h"
+
+#include <array>
+#include <cstddef>
+#include <utility>
+
+#include "absl/container/inlined_vector.h"
+#include "absl/functional/function_ref.h"
+#include "absl/strings/internal/cord_internal.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+
+namespace {
+
+// Unrefs the provided `substring`, and returns `substring->child`
+// Adds or assumes a reference on `substring->child`
+CordRep* ClipSubstring(CordRepSubstring* substring) {
+  CordRep* child = substring->child;
+  if (substring->refcount.IsOne()) {
+    delete substring;
+  } else {
+    CordRep::Ref(child);
+    CordRep::Unref(substring);
+  }
+  return child;
+}
+
+}  // namespace
+
+void Consume(CordRep* rep,
+             FunctionRef<void(CordRep*, size_t, size_t)> consume_fn) {
+  size_t offset = 0;
+  size_t length = rep->length;
+
+  if (rep->tag == SUBSTRING) {
+    offset += rep->substring()->start;
+    rep = ClipSubstring(rep->substring());
+  }
+  consume_fn(rep, offset, length);
+}
+
+void ReverseConsume(CordRep* rep,
+                    FunctionRef<void(CordRep*, size_t, size_t)> consume_fn) {
+  return Consume(rep, consume_fn);
+}
+
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/strings/internal/cord_rep_consume.h b/absl/strings/internal/cord_rep_consume.h
new file mode 100644
index 00000000000..cdb06ba17f5
--- /dev/null
+++ b/absl/strings/internal/cord_rep_consume.h
@@ -0,0 +1,48 @@
+// Copyright 2021 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_STRINGS_INTERNAL_CORD_REP_CONSUME_H_
+#define ABSL_STRINGS_INTERNAL_CORD_REP_CONSUME_H_
+
+#include <cstddef>
+#include <functional>
+
+#include "absl/functional/function_ref.h"
+#include "absl/strings/internal/cord_internal.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+
+// Consume() and ReverseConsume() consume CONCAT based trees and invoke the
+// provided functor with the contained nodes in the proper forward or reverse
+// order, which is used to convert CONCAT trees into other tree or cord data.
+// All CONCAT and SUBSTRING nodes are processed internally. The 'offset`
+// parameter of the functor is non-zero for any nodes below SUBSTRING nodes.
+// It's up to the caller to form these back into SUBSTRING nodes or otherwise
+// store offset / prefix information. These functions are intended to be used
+// only for migration / transitional code where due to factors such as ODR
+// violations, we can not 100% guarantee that all code respects 'new format'
+// settings and flags, so we need to be able to parse old data on the fly until
+// all old code is deprecated / no longer the default format.
+void Consume(CordRep* rep,
+             FunctionRef<void(CordRep*, size_t, size_t)> consume_fn);
+void ReverseConsume(CordRep* rep,
+                    FunctionRef<void(CordRep*, size_t, size_t)> consume_fn);
+
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STRINGS_INTERNAL_CORD_REP_CONSUME_H_
diff --git a/absl/strings/internal/cord_rep_crc.cc b/absl/strings/internal/cord_rep_crc.cc
new file mode 100644
index 00000000000..50e1cb99dcd
--- /dev/null
+++ b/absl/strings/internal/cord_rep_crc.cc
@@ -0,0 +1,57 @@
+// Copyright 2021 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/strings/internal/cord_rep_crc.h"
+
+#include <cassert>
+#include <cstdint>
+#include <utility>
+
+#include "absl/base/config.h"
+#include "absl/crc/internal/crc_cord_state.h"
+#include "absl/strings/internal/cord_internal.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+
+CordRepCrc* CordRepCrc::New(CordRep* child, crc_internal::CrcCordState state) {
+  if (child != nullptr && child->IsCrc()) {
+    if (child->refcount.IsOne()) {
+      child->crc()->crc_cord_state = std::move(state);
+      return child->crc();
+    }
+    CordRep* old = child;
+    child = old->crc()->child;
+    CordRep::Ref(child);
+    CordRep::Unref(old);
+  }
+  auto* new_cordrep = new CordRepCrc;
+  new_cordrep->length = child != nullptr ? child->length : 0;
+  new_cordrep->tag = cord_internal::CRC;
+  new_cordrep->child = child;
+  new_cordrep->crc_cord_state = std::move(state);
+  return new_cordrep;
+}
+
+void CordRepCrc::Destroy(CordRepCrc* node) {
+  if (node->child != nullptr) {
+    CordRep::Unref(node->child);
+  }
+  delete node;
+}
+
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/strings/internal/cord_rep_crc.h b/absl/strings/internal/cord_rep_crc.h
new file mode 100644
index 00000000000..379d7a604e9
--- /dev/null
+++ b/absl/strings/internal/cord_rep_crc.h
@@ -0,0 +1,103 @@
+// Copyright 2021 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_STRINGS_INTERNAL_CORD_REP_CRC_H_
+#define ABSL_STRINGS_INTERNAL_CORD_REP_CRC_H_
+
+#include <cassert>
+#include <cstdint>
+
+#include "absl/base/config.h"
+#include "absl/base/optimization.h"
+#include "absl/crc/internal/crc_cord_state.h"
+#include "absl/strings/internal/cord_internal.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+
+// CordRepCrc is a CordRep node intended only to appear at the top level of a
+// cord tree.  It associates an "expected CRC" with the contained data, to allow
+// for easy passage of checksum data in Cord data flows.
+//
+// From Cord's perspective, the crc value has no semantics; any validation of
+// the contained checksum is the user's responsibility.
+struct CordRepCrc : public CordRep {
+  CordRep* child;
+  absl::crc_internal::CrcCordState crc_cord_state;
+
+  // Consumes `child` and returns a CordRepCrc prefixed tree containing `child`.
+  // If the specified `child` is itself a CordRepCrc node, then this method
+  // either replaces the existing node, or directly updates the crc state in it
+  // depending on the node being shared or not, i.e.: refcount.IsOne().
+  // `child` must only be null if the Cord is empty. Never returns null.
+  static CordRepCrc* New(CordRep* child, crc_internal::CrcCordState state);
+
+  // Destroys (deletes) the provided node. `node` must not be null.
+  static void Destroy(CordRepCrc* node);
+};
+
+// Consumes `rep` and returns a CordRep* with any outer CordRepCrc wrapper
+// removed.  This is usually a no-op (returning `rep`), but this will remove and
+// unref an outer CordRepCrc node.
+inline CordRep* RemoveCrcNode(CordRep* rep) {
+  assert(rep != nullptr);
+  if (ABSL_PREDICT_FALSE(rep->IsCrc())) {
+    CordRep* child = rep->crc()->child;
+    if (rep->refcount.IsOne()) {
+      delete rep->crc();
+    } else {
+      CordRep::Ref(child);
+      CordRep::Unref(rep);
+    }
+    return child;
+  }
+  return rep;
+}
+
+// Returns `rep` if it is not a CordRepCrc node, or its child if it is.
+// Does not consume or create a reference on `rep` or the returned value.
+inline CordRep* SkipCrcNode(CordRep* rep) {
+  assert(rep != nullptr);
+  if (ABSL_PREDICT_FALSE(rep->IsCrc())) {
+    return rep->crc()->child;
+  } else {
+    return rep;
+  }
+}
+
+inline const CordRep* SkipCrcNode(const CordRep* rep) {
+  assert(rep != nullptr);
+  if (ABSL_PREDICT_FALSE(rep->IsCrc())) {
+    return rep->crc()->child;
+  } else {
+    return rep;
+  }
+}
+
+inline CordRepCrc* CordRep::crc() {
+  assert(IsCrc());
+  return static_cast<CordRepCrc*>(this);
+}
+
+inline const CordRepCrc* CordRep::crc() const {
+  assert(IsCrc());
+  return static_cast<const CordRepCrc*>(this);
+}
+
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STRINGS_INTERNAL_CORD_REP_CRC_H_
diff --git a/absl/strings/internal/cord_rep_crc_test.cc b/absl/strings/internal/cord_rep_crc_test.cc
new file mode 100644
index 00000000000..feeaac678bc
--- /dev/null
+++ b/absl/strings/internal/cord_rep_crc_test.cc
@@ -0,0 +1,133 @@
+// Copyright 2021 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/strings/internal/cord_rep_crc.h"
+
+#include <cstdint>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+#include "absl/crc/crc32c.h"
+#include "absl/crc/internal/crc_cord_state.h"
+#include "absl/strings/internal/cord_internal.h"
+#include "absl/strings/internal/cord_rep_test_util.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+namespace {
+
+using ::absl::cordrep_testing::MakeFlat;
+using ::testing::Eq;
+using ::testing::IsNull;
+using ::testing::Ne;
+
+#if !defined(NDEBUG) && GTEST_HAS_DEATH_TEST
+
+TEST(CordRepCrc, RemoveCrcWithNullptr) {
+  EXPECT_DEATH(RemoveCrcNode(nullptr), "");
+}
+
+#endif  // !NDEBUG && GTEST_HAS_DEATH_TEST
+
+absl::crc_internal::CrcCordState MakeCrcCordState(uint32_t crc) {
+  crc_internal::CrcCordState state;
+  state.mutable_rep()->prefix_crc.push_back(
+      crc_internal::CrcCordState::PrefixCrc(42, crc32c_t{crc}));
+  return state;
+}
+
+TEST(CordRepCrc, NewDestroy) {
+  CordRep* rep = cordrep_testing::MakeFlat("Hello world");
+  CordRepCrc* crc = CordRepCrc::New(rep, MakeCrcCordState(12345));
+  EXPECT_TRUE(crc->refcount.IsOne());
+  EXPECT_THAT(crc->child, Eq(rep));
+  EXPECT_THAT(crc->crc_cord_state.Checksum(), Eq(crc32c_t{12345u}));
+  EXPECT_TRUE(rep->refcount.IsOne());
+  CordRepCrc::Destroy(crc);
+}
+
+TEST(CordRepCrc, NewExistingCrcNotShared) {
+  CordRep* rep = cordrep_testing::MakeFlat("Hello world");
+  CordRepCrc* crc = CordRepCrc::New(rep, MakeCrcCordState(12345));
+  CordRepCrc* new_crc = CordRepCrc::New(crc, MakeCrcCordState(54321));
+  EXPECT_THAT(new_crc, Eq(crc));
+  EXPECT_TRUE(new_crc->refcount.IsOne());
+  EXPECT_THAT(new_crc->child, Eq(rep));
+  EXPECT_THAT(new_crc->crc_cord_state.Checksum(), Eq(crc32c_t{54321u}));
+  EXPECT_TRUE(rep->refcount.IsOne());
+  CordRepCrc::Destroy(new_crc);
+}
+
+TEST(CordRepCrc, NewExistingCrcShared) {
+  CordRep* rep = cordrep_testing::MakeFlat("Hello world");
+  CordRepCrc* crc = CordRepCrc::New(rep, MakeCrcCordState(12345));
+  CordRep::Ref(crc);
+  CordRepCrc* new_crc = CordRepCrc::New(crc, MakeCrcCordState(54321));
+
+  EXPECT_THAT(new_crc, Ne(crc));
+  EXPECT_TRUE(new_crc->refcount.IsOne());
+  EXPECT_TRUE(crc->refcount.IsOne());
+  EXPECT_FALSE(rep->refcount.IsOne());
+  EXPECT_THAT(crc->child, Eq(rep));
+  EXPECT_THAT(new_crc->child, Eq(rep));
+  EXPECT_THAT(crc->crc_cord_state.Checksum(), Eq(crc32c_t{12345u}));
+  EXPECT_THAT(new_crc->crc_cord_state.Checksum(), Eq(crc32c_t{54321u}));
+
+  CordRep::Unref(crc);
+  CordRep::Unref(new_crc);
+}
+
+TEST(CordRepCrc, NewEmpty) {
+  CordRepCrc* crc = CordRepCrc::New(nullptr, MakeCrcCordState(12345));
+  EXPECT_TRUE(crc->refcount.IsOne());
+  EXPECT_THAT(crc->child, IsNull());
+  EXPECT_THAT(crc->length, Eq(0u));
+  EXPECT_THAT(crc->crc_cord_state.Checksum(), Eq(crc32c_t{12345u}));
+  EXPECT_TRUE(crc->refcount.IsOne());
+  CordRepCrc::Destroy(crc);
+}
+
+TEST(CordRepCrc, RemoveCrcNotCrc) {
+  CordRep* rep = cordrep_testing::MakeFlat("Hello world");
+  CordRep* nocrc = RemoveCrcNode(rep);
+  EXPECT_THAT(nocrc, Eq(rep));
+  CordRep::Unref(nocrc);
+}
+
+TEST(CordRepCrc, RemoveCrcNotShared) {
+  CordRep* rep = cordrep_testing::MakeFlat("Hello world");
+  CordRepCrc* crc = CordRepCrc::New(rep, MakeCrcCordState(12345));
+  CordRep* nocrc = RemoveCrcNode(crc);
+  EXPECT_THAT(nocrc, Eq(rep));
+  EXPECT_TRUE(rep->refcount.IsOne());
+  CordRep::Unref(nocrc);
+}
+
+TEST(CordRepCrc, RemoveCrcShared) {
+  CordRep* rep = cordrep_testing::MakeFlat("Hello world");
+  CordRepCrc* crc = CordRepCrc::New(rep, MakeCrcCordState(12345));
+  CordRep::Ref(crc);
+  CordRep* nocrc = RemoveCrcNode(crc);
+  EXPECT_THAT(nocrc, Eq(rep));
+  EXPECT_FALSE(rep->refcount.IsOne());
+  CordRep::Unref(nocrc);
+  CordRep::Unref(crc);
+}
+
+}  // namespace
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/strings/internal/cord_rep_flat.h b/absl/strings/internal/cord_rep_flat.h
new file mode 100644
index 00000000000..34da171199e
--- /dev/null
+++ b/absl/strings/internal/cord_rep_flat.h
@@ -0,0 +1,196 @@
+// Copyright 2020 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_STRINGS_INTERNAL_CORD_REP_FLAT_H_
+#define ABSL_STRINGS_INTERNAL_CORD_REP_FLAT_H_
+
+#include <algorithm>
+#include <cassert>
+#include <cstddef>
+#include <cstdint>
+#include <cstring>
+#include <memory>
+
+#include "absl/base/config.h"
+#include "absl/base/macros.h"
+#include "absl/strings/internal/cord_internal.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+
+// Note: all constants below are never ODR used and internal to cord, we define
+// these as static constexpr to avoid 'in struct' definition and usage clutter.
+
+// Largest and smallest flat node lengths we are willing to allocate
+// Flat allocation size is stored in tag, which currently can encode sizes up
+// to 4K, encoded as multiple of either 8 or 32 bytes.
+// If we allow for larger sizes, we need to change this to 8/64, 16/128, etc.
+// kMinFlatSize is bounded by tag needing to be at least FLAT * 8 bytes, and
+// ideally a 'nice' size aligning with allocation and cacheline sizes like 32.
+// kMaxFlatSize is bounded by the size resulting in a computed tag no greater
+// than MAX_FLAT_TAG. MAX_FLAT_TAG provides for additional 'high' tag values.
+static constexpr size_t kFlatOverhead = offsetof(CordRep, storage);
+static constexpr size_t kMinFlatSize = 32;
+static constexpr size_t kMaxFlatSize = 4096;
+static constexpr size_t kMaxFlatLength = kMaxFlatSize - kFlatOverhead;
+static constexpr size_t kMinFlatLength = kMinFlatSize - kFlatOverhead;
+static constexpr size_t kMaxLargeFlatSize = 256 * 1024;
+static constexpr size_t kMaxLargeFlatLength = kMaxLargeFlatSize - kFlatOverhead;
+
+// kTagBase should make the Size <--> Tag computation resilient
+// against changes to the value of FLAT when we add a new tag..
+static constexpr uint8_t kTagBase = FLAT - 4;
+
+// Converts the provided rounded size to the corresponding tag
+constexpr uint8_t AllocatedSizeToTagUnchecked(size_t size) {
+  return static_cast<uint8_t>(size <= 512 ? kTagBase + size / 8
+                              : size <= 8192
+                                  ? kTagBase + 512 / 8 + size / 64 - 512 / 64
+                                  : kTagBase + 512 / 8 + ((8192 - 512) / 64) +
+                                        size / 4096 - 8192 / 4096);
+}
+
+// Converts the provided tag to the corresponding allocated size
+constexpr size_t TagToAllocatedSize(uint8_t tag) {
+  return (tag <= kTagBase + 512 / 8) ? tag * 8 - kTagBase * 8
+         : (tag <= kTagBase + (512 / 8) + ((8192 - 512) / 64))
+             ? 512 + tag * 64 - kTagBase * 64 - 512 / 8 * 64
+             : 8192 + tag * 4096 - kTagBase * 4096 -
+                   ((512 / 8) + ((8192 - 512) / 64)) * 4096;
+}
+
+static_assert(AllocatedSizeToTagUnchecked(kMinFlatSize) == FLAT);
+static_assert(AllocatedSizeToTagUnchecked(kMaxLargeFlatSize) == MAX_FLAT_TAG);
+
+// RoundUp logically performs `((n + m - 1) / m) * m` to round up to the nearest
+// multiple of `m`, optimized for the invariant that `m` is a power of 2.
+constexpr size_t RoundUp(size_t n, size_t m) {
+  return (n + m - 1) & (0 - m);
+}
+
+// Returns the size to the nearest equal or larger value that can be
+// expressed exactly as a tag value.
+inline size_t RoundUpForTag(size_t size) {
+  return RoundUp(size, (size <= 512) ? 8 : (size <= 8192 ? 64 : 4096));
+}
+
+// Converts the allocated size to a tag, rounding down if the size
+// does not exactly match a 'tag expressible' size value. The result is
+// undefined if the size exceeds the maximum size that can be encoded in
+// a tag, i.e., if size is larger than TagToAllocatedSize(<max tag>).
+inline uint8_t AllocatedSizeToTag(size_t size) {
+  const uint8_t tag = AllocatedSizeToTagUnchecked(size);
+  assert(tag <= MAX_FLAT_TAG);
+  return tag;
+}
+
+// Converts the provided tag to the corresponding available data length
+constexpr size_t TagToLength(uint8_t tag) {
+  return TagToAllocatedSize(tag) - kFlatOverhead;
+}
+
+// Enforce that kMaxFlatSize maps to a well-known exact tag value.
+static_assert(TagToAllocatedSize(MAX_FLAT_TAG) == kMaxLargeFlatSize,
+              "Bad tag logic");
+
+struct CordRepFlat : public CordRep {
+  // Tag for explicit 'large flat' allocation
+  struct Large {};
+
+  // Creates a new flat node.
+  template <size_t max_flat_size, typename... Args>
+  static CordRepFlat* NewImpl(size_t len, Args... args ABSL_ATTRIBUTE_UNUSED) {
+    if (len <= kMinFlatLength) {
+      len = kMinFlatLength;
+    } else if (len > max_flat_size - kFlatOverhead) {
+      len = max_flat_size - kFlatOverhead;
+    }
+
+    // Round size up so it matches a size we can exactly express in a tag.
+    const size_t size = RoundUpForTag(len + kFlatOverhead);
+    void* const raw_rep = ::operator new(size);
+    // GCC 13 has a false-positive -Wstringop-overflow warning here.
+    #if ABSL_INTERNAL_HAVE_MIN_GNUC_VERSION(13, 0)
+    #pragma GCC diagnostic push
+    #pragma GCC diagnostic ignored "-Wstringop-overflow"
+    #endif
+    CordRepFlat* rep = new (raw_rep) CordRepFlat();
+    rep->tag = AllocatedSizeToTag(size);
+    #if ABSL_INTERNAL_HAVE_MIN_GNUC_VERSION(13, 0)
+    #pragma GCC diagnostic pop
+    #endif
+    return rep;
+  }
+
+  static CordRepFlat* New(size_t len) { return NewImpl<kMaxFlatSize>(len); }
+
+  static CordRepFlat* New(Large, size_t len) {
+    return NewImpl<kMaxLargeFlatSize>(len);
+  }
+
+  // Deletes a CordRepFlat instance created previously through a call to New().
+  // Flat CordReps are allocated and constructed with raw ::operator new and
+  // placement new, and must be destructed and deallocated accordingly.
+  static void Delete(CordRep*rep) {
+    assert(rep->tag >= FLAT && rep->tag <= MAX_FLAT_TAG);
+
+#if defined(__cpp_sized_deallocation)
+    size_t size = TagToAllocatedSize(rep->tag);
+    rep->~CordRep();
+    ::operator delete(rep, size);
+#else
+    rep->~CordRep();
+    ::operator delete(rep);
+#endif
+  }
+
+  // Create a CordRepFlat containing `data`, with an optional additional
+  // extra capacity of up to `extra` bytes. Requires that `data.size()`
+  // is less than kMaxFlatLength.
+  static CordRepFlat* Create(absl::string_view data, size_t extra = 0) {
+    assert(data.size() <= kMaxFlatLength);
+    CordRepFlat* flat = New(data.size() + (std::min)(extra, kMaxFlatLength));
+    memcpy(flat->Data(), data.data(), data.size());
+    flat->length = data.size();
+    return flat;
+  }
+
+  // Returns a pointer to the data inside this flat rep.
+  char* Data() { return reinterpret_cast<char*>(storage); }
+  const char* Data() const { return reinterpret_cast<const char*>(storage); }
+
+  // Returns the maximum capacity (payload size) of this instance.
+  size_t Capacity() const { return TagToLength(tag); }
+
+  // Returns the allocated size (payload + overhead) of this instance.
+  size_t AllocatedSize() const { return TagToAllocatedSize(tag); }
+};
+
+// Now that CordRepFlat is defined, we can define CordRep's helper casts:
+inline CordRepFlat* CordRep::flat() {
+  assert(tag >= FLAT && tag <= MAX_FLAT_TAG);
+  return reinterpret_cast<CordRepFlat*>(this);
+}
+
+inline const CordRepFlat* CordRep::flat() const {
+  assert(tag >= FLAT && tag <= MAX_FLAT_TAG);
+  return reinterpret_cast<const CordRepFlat*>(this);
+}
+
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STRINGS_INTERNAL_CORD_REP_FLAT_H_
diff --git a/absl/strings/internal/cord_rep_test_util.h b/absl/strings/internal/cord_rep_test_util.h
new file mode 100644
index 00000000000..9a6744c7228
--- /dev/null
+++ b/absl/strings/internal/cord_rep_test_util.h
@@ -0,0 +1,208 @@
+// Copyright 2021 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_STRINGS_INTERNAL_CORD_REP_TEST_UTIL_H_
+#define ABSL_STRINGS_INTERNAL_CORD_REP_TEST_UTIL_H_
+
+#include <cassert>
+#include <cstddef>
+#include <cstring>
+#include <memory>
+#include <random>
+#include <string>
+#include <vector>
+
+#include "absl/base/config.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/strings/internal/cord_internal.h"
+#include "absl/strings/internal/cord_rep_btree.h"
+#include "absl/strings/internal/cord_rep_flat.h"
+#include "absl/strings/string_view.h"
+#include "absl/types/span.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cordrep_testing {
+
+inline cord_internal::CordRepSubstring* MakeSubstring(
+    size_t start, size_t len, cord_internal::CordRep* rep) {
+  auto* sub = new cord_internal::CordRepSubstring;
+  sub->tag = cord_internal::SUBSTRING;
+  sub->start = start;
+  sub->length = len <= 0 ? rep->length - start + len : len;
+  sub->child = rep;
+  return sub;
+}
+
+inline cord_internal::CordRepFlat* MakeFlat(absl::string_view value) {
+  assert(value.length() <= cord_internal::kMaxFlatLength);
+  auto* flat = cord_internal::CordRepFlat::New(value.length());
+  flat->length = value.length();
+  memcpy(flat->Data(), value.data(), value.length());
+  return flat;
+}
+
+// Creates an external node for testing
+inline cord_internal::CordRepExternal* MakeExternal(absl::string_view s) {
+  struct Rep : public cord_internal::CordRepExternal {
+    std::string s;
+    explicit Rep(absl::string_view sv) : s(sv) {
+      this->tag = cord_internal::EXTERNAL;
+      this->base = s.data();
+      this->length = s.length();
+      this->releaser_invoker = [](cord_internal::CordRepExternal* self) {
+        delete static_cast<Rep*>(self);
+      };
+    }
+  };
+  return new Rep(s);
+}
+
+inline std::string CreateRandomString(size_t n) {
+  absl::string_view data =
+      "abcdefghijklmnopqrstuvwxyz"
+      "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+      "0123456789~!@#$%^&*()_+=-<>?:\"{}[]|";
+  std::minstd_rand rnd;
+  std::uniform_int_distribution<size_t> dist(0, data.size() - 1);
+  std::string s(n, ' ');
+  for (size_t i = 0; i < n; ++i) {
+    s[i] = data[dist(rnd)];
+  }
+  return s;
+}
+
+// Creates an array of flats from the provided string, chopping
+// the provided string up into flats of size `chunk_size` characters
+// resulting in roughly `data.size() / chunk_size` total flats.
+inline std::vector<cord_internal::CordRep*> CreateFlatsFromString(
+    absl::string_view data, size_t chunk_size) {
+  assert(chunk_size > 0);
+  std::vector<cord_internal::CordRep*> flats;
+  for (absl::string_view s = data; !s.empty(); s.remove_prefix(chunk_size)) {
+    flats.push_back(MakeFlat(s.substr(0, chunk_size)));
+  }
+  return flats;
+}
+
+inline cord_internal::CordRepBtree* CordRepBtreeFromFlats(
+    absl::Span<cord_internal::CordRep* const> flats) {
+  assert(!flats.empty());
+  auto* node = cord_internal::CordRepBtree::Create(flats[0]);
+  for (size_t i = 1; i < flats.size(); ++i) {
+    node = cord_internal::CordRepBtree::Append(node, flats[i]);
+  }
+  return node;
+}
+
+template <typename Fn>
+inline void CordVisitReps(cord_internal::CordRep* rep, Fn&& fn) {
+  fn(rep);
+  while (rep->tag == cord_internal::SUBSTRING) {
+    rep = rep->substring()->child;
+    fn(rep);
+  }
+  if (rep->tag == cord_internal::BTREE) {
+    for (cord_internal::CordRep* edge : rep->btree()->Edges()) {
+      CordVisitReps(edge, fn);
+    }
+  }
+}
+
+template <typename Predicate>
+inline std::vector<cord_internal::CordRep*> CordCollectRepsIf(
+    Predicate&& predicate, cord_internal::CordRep* rep) {
+  std::vector<cord_internal::CordRep*> reps;
+  CordVisitReps(rep, [&reps, &predicate](cord_internal::CordRep* rep) {
+    if (predicate(rep)) reps.push_back(rep);
+  });
+  return reps;
+}
+
+inline std::vector<cord_internal::CordRep*> CordCollectReps(
+    cord_internal::CordRep* rep) {
+  std::vector<cord_internal::CordRep*> reps;
+  auto fn = [&reps](cord_internal::CordRep* rep) { reps.push_back(rep); };
+  CordVisitReps(rep, fn);
+  return reps;
+}
+
+inline void CordToString(cord_internal::CordRep* rep, std::string& s) {
+  size_t offset = 0;
+  size_t length = rep->length;
+  while (rep->tag == cord_internal::SUBSTRING) {
+    offset += rep->substring()->start;
+    rep = rep->substring()->child;
+  }
+  if (rep->tag == cord_internal::BTREE) {
+    for (cord_internal::CordRep* edge : rep->btree()->Edges()) {
+      CordToString(edge, s);
+    }
+  } else if (rep->tag >= cord_internal::FLAT) {
+    s.append(rep->flat()->Data() + offset, length);
+  } else if (rep->tag == cord_internal::EXTERNAL) {
+    s.append(rep->external()->base + offset, length);
+  } else {
+    ABSL_RAW_LOG(FATAL, "Unsupported tag %d", rep->tag);
+  }
+}
+
+inline std::string CordToString(cord_internal::CordRep* rep) {
+  std::string s;
+  s.reserve(rep->length);
+  CordToString(rep, s);
+  return s;
+}
+
+// RAII Helper class to automatically unref reps on destruction.
+class AutoUnref {
+ public:
+  ~AutoUnref() {
+    for (CordRep* rep : unrefs_) CordRep::Unref(rep);
+  }
+
+  // Adds `rep` to the list of reps to be unreffed at destruction.
+  template <typename CordRepType>
+  CordRepType* Add(CordRepType* rep) {
+    unrefs_.push_back(rep);
+    return rep;
+  }
+
+  // Increments the reference count of `rep` by one, and adds it to
+  // the list of reps to be unreffed at destruction.
+  template <typename CordRepType>
+  CordRepType* Ref(CordRepType* rep) {
+    unrefs_.push_back(CordRep::Ref(rep));
+    return rep;
+  }
+
+  // Increments the reference count of `rep` by one if `condition` is true,
+  // and adds it to the list of reps to be unreffed at destruction.
+  template <typename CordRepType>
+  CordRepType* RefIf(bool condition, CordRepType* rep) {
+    if (condition) unrefs_.push_back(CordRep::Ref(rep));
+    return rep;
+  }
+
+ private:
+  using CordRep = absl::cord_internal::CordRep;
+
+  std::vector<CordRep*> unrefs_;
+};
+
+}  // namespace cordrep_testing
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STRINGS_INTERNAL_CORD_REP_TEST_UTIL_H_
diff --git a/absl/strings/internal/cordz_functions.cc b/absl/strings/internal/cordz_functions.cc
new file mode 100644
index 00000000000..50948170db3
--- /dev/null
+++ b/absl/strings/internal/cordz_functions.cc
@@ -0,0 +1,103 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/strings/internal/cordz_functions.h"
+
+#include <atomic>
+#include <cmath>
+#include <cstdint>
+#include <limits>
+#include <random>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/profiling/internal/exponential_biased.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+namespace {
+
+// The average interval until the next sample. A value of 0 disables profiling
+// while a value of 1 will profile all Cords.
+std::atomic<int> g_cordz_mean_interval(50000);
+
+}  // namespace
+
+#ifdef ABSL_INTERNAL_CORDZ_ENABLED
+
+// Special negative 'not initialized' per thread value for cordz_next_sample.
+static constexpr int64_t kInitCordzNextSample = -1;
+
+ABSL_CONST_INIT thread_local SamplingState cordz_next_sample = {
+    kInitCordzNextSample, 1};
+
+// kIntervalIfDisabled is the number of profile-eligible events need to occur
+// before the code will confirm that cordz is still disabled.
+constexpr int64_t kIntervalIfDisabled = 1 << 16;
+
+ABSL_ATTRIBUTE_NOINLINE int64_t
+cordz_should_profile_slow(SamplingState& state) {
+
+  thread_local absl::profiling_internal::ExponentialBiased
+      exponential_biased_generator;
+  int32_t mean_interval = get_cordz_mean_interval();
+
+  // Check if we disabled profiling. If so, set the next sample to a "large"
+  // number to minimize the overhead of the should_profile codepath.
+  if (mean_interval <= 0) {
+    state = {kIntervalIfDisabled, kIntervalIfDisabled};
+    return 0;
+  }
+
+  // Check if we're always sampling.
+  if (mean_interval == 1) {
+    state = {1, 1};
+    return 1;
+  }
+
+  if (cordz_next_sample.next_sample <= 0) {
+    // If first check on current thread, check cordz_should_profile()
+    // again using the created (initial) stride in cordz_next_sample.
+    const bool initialized =
+        cordz_next_sample.next_sample != kInitCordzNextSample;
+    auto old_stride = state.sample_stride;
+    auto stride = exponential_biased_generator.GetStride(mean_interval);
+    state = {stride, stride};
+    bool should_sample = initialized || cordz_should_profile() > 0;
+    return should_sample ? old_stride : 0;
+  }
+
+  --state.next_sample;
+  return 0;
+}
+
+void cordz_set_next_sample_for_testing(int64_t next_sample) {
+  cordz_next_sample = {next_sample, next_sample};
+}
+
+#endif  // ABSL_INTERNAL_CORDZ_ENABLED
+
+int32_t get_cordz_mean_interval() {
+  return g_cordz_mean_interval.load(std::memory_order_acquire);
+}
+
+void set_cordz_mean_interval(int32_t mean_interval) {
+  g_cordz_mean_interval.store(mean_interval, std::memory_order_release);
+}
+
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/strings/internal/cordz_functions.h b/absl/strings/internal/cordz_functions.h
new file mode 100644
index 00000000000..84c185e4072
--- /dev/null
+++ b/absl/strings/internal/cordz_functions.h
@@ -0,0 +1,87 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_STRINGS_INTERNAL_CORDZ_FUNCTIONS_H_
+#define ABSL_STRINGS_INTERNAL_CORDZ_FUNCTIONS_H_
+
+#include <stdint.h>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/optimization.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+
+// Returns the current sample rate. This represents the average interval
+// between samples.
+int32_t get_cordz_mean_interval();
+
+// Sets the sample rate with the average interval between samples.
+void set_cordz_mean_interval(int32_t mean_interval);
+
+// Cordz is only enabled on Linux with thread_local support.
+#if defined(ABSL_INTERNAL_CORDZ_ENABLED)
+#error ABSL_INTERNAL_CORDZ_ENABLED cannot be set directly
+#elif defined(__linux__) && defined(ABSL_HAVE_THREAD_LOCAL)
+#define ABSL_INTERNAL_CORDZ_ENABLED 1
+#endif
+
+#ifdef ABSL_INTERNAL_CORDZ_ENABLED
+
+struct SamplingState {
+  int64_t next_sample;
+  int64_t sample_stride;
+};
+
+// cordz_next_sample is the number of events until the next sample event. If
+// the value is 1 or less, the code will check on the next event if cordz is
+// enabled, and if so, will sample the Cord. cordz is only enabled when we can
+// use thread locals.
+ABSL_CONST_INIT extern thread_local SamplingState cordz_next_sample;
+
+// Determines if the next sample should be profiled.
+// Returns:
+//   0: Do not sample
+//  >0: Sample with the stride of the last sampling period
+int64_t cordz_should_profile_slow(SamplingState& state);
+
+// Determines if the next sample should be profiled.
+// Returns:
+//   0: Do not sample
+//  >0: Sample with the stride of the last sampling period
+inline int64_t cordz_should_profile() {
+  if (ABSL_PREDICT_TRUE(cordz_next_sample.next_sample > 1)) {
+    cordz_next_sample.next_sample--;
+    return 0;
+  }
+  return cordz_should_profile_slow(cordz_next_sample);
+}
+
+// Sets the interval until the next sample (for testing only)
+void cordz_set_next_sample_for_testing(int64_t next_sample);
+
+#else  // ABSL_INTERNAL_CORDZ_ENABLED
+
+inline int64_t cordz_should_profile() { return 0; }
+inline void cordz_set_next_sample_for_testing(int64_t) {}
+
+#endif  // ABSL_INTERNAL_CORDZ_ENABLED
+
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STRINGS_INTERNAL_CORDZ_FUNCTIONS_H_
diff --git a/absl/strings/internal/cordz_functions_test.cc b/absl/strings/internal/cordz_functions_test.cc
new file mode 100644
index 00000000000..3949bf3ae7a
--- /dev/null
+++ b/absl/strings/internal/cordz_functions_test.cc
@@ -0,0 +1,148 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/strings/internal/cordz_functions.h"
+
+#include <cstdint>
+#include <thread>  // NOLINT we need real clean new threads
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+namespace {
+
+using ::testing::Eq;
+using ::testing::Ge;
+using ::testing::Le;
+
+TEST(CordzFunctionsTest, SampleRate) {
+  int32_t orig_sample_rate = get_cordz_mean_interval();
+  int32_t expected_sample_rate = 123;
+  set_cordz_mean_interval(expected_sample_rate);
+  EXPECT_THAT(get_cordz_mean_interval(), Eq(expected_sample_rate));
+  set_cordz_mean_interval(orig_sample_rate);
+}
+
+// Cordz is disabled when we don't have thread_local. All calls to
+// should_profile will return false when cordz is disabled, so we might want to
+// avoid those tests.
+#ifdef ABSL_INTERNAL_CORDZ_ENABLED
+
+TEST(CordzFunctionsTest, ShouldProfileDisable) {
+  int32_t orig_sample_rate = get_cordz_mean_interval();
+
+  set_cordz_mean_interval(0);
+  cordz_set_next_sample_for_testing(0);
+  EXPECT_EQ(cordz_should_profile(), 0);
+  // 1 << 16 is from kIntervalIfDisabled in cordz_functions.cc.
+  EXPECT_THAT(cordz_next_sample.next_sample, Eq(1 << 16));
+
+  set_cordz_mean_interval(orig_sample_rate);
+}
+
+TEST(CordzFunctionsTest, ShouldProfileAlways) {
+  int32_t orig_sample_rate = get_cordz_mean_interval();
+
+  set_cordz_mean_interval(1);
+  cordz_set_next_sample_for_testing(1);
+  EXPECT_GT(cordz_should_profile(), 0);
+  EXPECT_THAT(cordz_next_sample.next_sample, Le(1));
+
+  set_cordz_mean_interval(orig_sample_rate);
+}
+
+TEST(CordzFunctionsTest, DoesNotAlwaysSampleFirstCord) {
+  // Set large enough interval such that the chance of 'tons' of threads
+  // randomly sampling the first call is infinitely small.
+  set_cordz_mean_interval(10000);
+  int tries = 0;
+  bool sampled = false;
+  do {
+    ++tries;
+    ASSERT_THAT(tries, Le(1000));
+    std::thread thread([&sampled] { sampled = cordz_should_profile() > 0; });
+    thread.join();
+  } while (sampled);
+}
+
+TEST(CordzFunctionsTest, ShouldProfileRate) {
+  static constexpr int kDesiredMeanInterval = 1000;
+  static constexpr int kSamples = 10000;
+  int32_t orig_sample_rate = get_cordz_mean_interval();
+
+  set_cordz_mean_interval(kDesiredMeanInterval);
+
+  int64_t sum_of_intervals = 0;
+  for (int i = 0; i < kSamples; i++) {
+    // Setting next_sample to 0 will force cordz_should_profile to generate a
+    // new value for next_sample each iteration.
+    cordz_set_next_sample_for_testing(0);
+    cordz_should_profile();
+    sum_of_intervals += cordz_next_sample.next_sample;
+  }
+
+  // The sum of independent exponential variables is an Erlang distribution,
+  // which is a gamma distribution where the shape parameter is equal to the
+  // number of summands. The distribution used for cordz_should_profile is
+  // actually floor(Exponential(1/mean)) which introduces bias. However, we can
+  // apply the squint-really-hard correction factor. That is, when mean is
+  // large, then if we squint really hard the shape of the distribution between
+  // N and N+1 looks like a uniform distribution. On average, each value for
+  // next_sample will be about 0.5 lower than we would expect from an
+  // exponential distribution. This squint-really-hard correction approach won't
+  // work when mean is smaller than about 10 but works fine when mean is 1000.
+  //
+  // We can use R to calculate a confidence interval. This
+  // shows how to generate a confidence interval with a false positive rate of
+  // one in a billion.
+  //
+  // $ R -q
+  // > mean = 1000
+  // > kSamples = 10000
+  // > errorRate = 1e-9
+  // > correction = -kSamples / 2
+  // > low = qgamma(errorRate/2, kSamples, 1/mean) + correction
+  // > high = qgamma(1 - errorRate/2, kSamples, 1/mean) + correction
+  // > low
+  // [1] 9396115
+  // > high
+  // [1] 10618100
+  EXPECT_THAT(sum_of_intervals, Ge(9396115));
+  EXPECT_THAT(sum_of_intervals, Le(10618100));
+
+  set_cordz_mean_interval(orig_sample_rate);
+}
+
+#else  // ABSL_INTERNAL_CORDZ_ENABLED
+
+TEST(CordzFunctionsTest, ShouldProfileDisabled) {
+  int32_t orig_sample_rate = get_cordz_mean_interval();
+
+  set_cordz_mean_interval(1);
+  cordz_set_next_sample_for_testing(0);
+  EXPECT_FALSE(cordz_should_profile());
+
+  set_cordz_mean_interval(orig_sample_rate);
+}
+
+#endif  // ABSL_INTERNAL_CORDZ_ENABLED
+
+}  // namespace
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/strings/internal/cordz_handle.cc b/absl/strings/internal/cordz_handle.cc
new file mode 100644
index 00000000000..4fa8183898c
--- /dev/null
+++ b/absl/strings/internal/cordz_handle.cc
@@ -0,0 +1,169 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+#include "absl/strings/internal/cordz_handle.h"
+
+#include <atomic>
+#include <cassert>
+#include <vector>
+
+#include "absl/base/internal/raw_logging.h"  // For ABSL_RAW_CHECK
+#include "absl/base/macros.h"
+#include "absl/base/no_destructor.h"
+#include "absl/base/thread_annotations.h"
+#include "absl/synchronization/mutex.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+
+namespace {
+
+struct Queue {
+  Queue() = default;
+
+  absl::Mutex mutex;
+  std::atomic<CordzHandle*> dq_tail ABSL_GUARDED_BY(mutex){nullptr};
+
+  // Returns true if this delete queue is empty. This method does not acquire
+  // the lock, but does a 'load acquire' observation on the delete queue tail.
+  // It is used inside Delete() to check for the presence of a delete queue
+  // without holding the lock. The assumption is that the caller is in the
+  // state of 'being deleted', and can not be newly discovered by a concurrent
+  // 'being constructed' snapshot instance. Practically, this means that any
+  // such discovery (`find`, 'first' or 'next', etc) must have proper 'happens
+  // before / after' semantics and atomic fences.
+  bool IsEmpty() const ABSL_NO_THREAD_SAFETY_ANALYSIS {
+    return dq_tail.load(std::memory_order_acquire) == nullptr;
+  }
+};
+
+static Queue& GlobalQueue() {
+  static absl::NoDestructor<Queue> global_queue;
+  return *global_queue;
+}
+
+}  // namespace
+
+CordzHandle::CordzHandle(bool is_snapshot) : is_snapshot_(is_snapshot) {
+  Queue& global_queue = GlobalQueue();
+  if (is_snapshot) {
+    MutexLock lock(global_queue.mutex);
+    CordzHandle* dq_tail = global_queue.dq_tail.load(std::memory_order_acquire);
+    if (dq_tail != nullptr) {
+      dq_prev_ = dq_tail;
+      dq_tail->dq_next_ = this;
+    }
+    global_queue.dq_tail.store(this, std::memory_order_release);
+  }
+}
+
+CordzHandle::~CordzHandle() {
+  Queue& global_queue = GlobalQueue();
+  if (is_snapshot_) {
+    std::vector<CordzHandle*> to_delete;
+    {
+      MutexLock lock(global_queue.mutex);
+      CordzHandle* next = dq_next_;
+      if (dq_prev_ == nullptr) {
+        // We were head of the queue, delete every CordzHandle until we reach
+        // either the end of the list, or a snapshot handle.
+        while (next && !next->is_snapshot_) {
+          to_delete.push_back(next);
+          next = next->dq_next_;
+        }
+      } else {
+        // Another CordzHandle existed before this one, don't delete anything.
+        dq_prev_->dq_next_ = next;
+      }
+      if (next) {
+        next->dq_prev_ = dq_prev_;
+      } else {
+        global_queue.dq_tail.store(dq_prev_, std::memory_order_release);
+      }
+    }
+    for (CordzHandle* handle : to_delete) {
+      delete handle;
+    }
+  }
+}
+
+bool CordzHandle::SafeToDelete() const {
+  return is_snapshot_ || GlobalQueue().IsEmpty();
+}
+
+void CordzHandle::Delete(CordzHandle* handle) {
+  assert(handle);
+  if (handle) {
+    Queue& queue = GlobalQueue();
+    if (!handle->SafeToDelete()) {
+      MutexLock lock(queue.mutex);
+      CordzHandle* dq_tail = queue.dq_tail.load(std::memory_order_acquire);
+      if (dq_tail != nullptr) {
+        handle->dq_prev_ = dq_tail;
+        dq_tail->dq_next_ = handle;
+        queue.dq_tail.store(handle, std::memory_order_release);
+        return;
+      }
+    }
+    delete handle;
+  }
+}
+
+std::vector<const CordzHandle*> CordzHandle::DiagnosticsGetDeleteQueue() {
+  std::vector<const CordzHandle*> handles;
+  Queue& global_queue = GlobalQueue();
+  MutexLock lock(global_queue.mutex);
+  CordzHandle* dq_tail = global_queue.dq_tail.load(std::memory_order_acquire);
+  for (const CordzHandle* p = dq_tail; p; p = p->dq_prev_) {
+    handles.push_back(p);
+  }
+  return handles;
+}
+
+bool CordzHandle::DiagnosticsHandleIsSafeToInspect(
+    const CordzHandle* handle) const {
+  if (!is_snapshot_) return false;
+  if (handle == nullptr) return true;
+  if (handle->is_snapshot_) return false;
+  bool snapshot_found = false;
+  Queue& global_queue = GlobalQueue();
+  MutexLock lock(global_queue.mutex);
+  for (const CordzHandle* p = global_queue.dq_tail; p; p = p->dq_prev_) {
+    if (p == handle) return !snapshot_found;
+    if (p == this) snapshot_found = true;
+  }
+  ABSL_ASSERT(snapshot_found);  // Assert that 'this' is in delete queue.
+  return true;
+}
+
+std::vector<const CordzHandle*>
+CordzHandle::DiagnosticsGetSafeToInspectDeletedHandles() {
+  std::vector<const CordzHandle*> handles;
+  if (!is_snapshot()) {
+    return handles;
+  }
+
+  Queue& global_queue = GlobalQueue();
+  MutexLock lock(global_queue.mutex);
+  for (const CordzHandle* p = dq_next_; p != nullptr; p = p->dq_next_) {
+    if (!p->is_snapshot()) {
+      handles.push_back(p);
+    }
+  }
+  return handles;
+}
+
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/strings/internal/cordz_handle.h b/absl/strings/internal/cordz_handle.h
new file mode 100644
index 00000000000..08e3f0d3a63
--- /dev/null
+++ b/absl/strings/internal/cordz_handle.h
@@ -0,0 +1,98 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_STRINGS_INTERNAL_CORDZ_HANDLE_H_
+#define ABSL_STRINGS_INTERNAL_CORDZ_HANDLE_H_
+
+#include <atomic>
+#include <vector>
+
+#include "absl/base/config.h"
+#include "absl/base/internal/raw_logging.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+
+// This base class allows multiple types of object (CordzInfo and
+// CordzSampleToken) to exist simultaneously on the delete queue (pointed to by
+// global_dq_tail and traversed using dq_prev_ and dq_next_). The
+// delete queue guarantees that once a profiler creates a CordzSampleToken and
+// has gained visibility into a CordzInfo object, that CordzInfo object will not
+// be deleted prematurely. This allows the profiler to inspect all CordzInfo
+// objects that are alive without needing to hold a global lock.
+class ABSL_DLL CordzHandle {
+ public:
+  CordzHandle() : CordzHandle(false) {}
+
+  bool is_snapshot() const { return is_snapshot_; }
+
+  // Returns true if this instance is safe to be deleted because it is either a
+  // snapshot, which is always safe to delete, or not included in the global
+  // delete queue and thus not included in any snapshot.
+  // Callers are responsible for making sure this instance can not be newly
+  // discovered by other threads. For example, CordzInfo instances first de-list
+  // themselves from the global CordzInfo list before determining if they are
+  // safe to be deleted directly.
+  // If SafeToDelete returns false, callers MUST use the Delete() method to
+  // safely queue CordzHandle instances for deletion.
+  bool SafeToDelete() const;
+
+  // Deletes the provided instance, or puts it on the delete queue to be deleted
+  // once there are no more sample tokens (snapshot) instances potentially
+  // referencing the instance. `handle` should not be null.
+  static void Delete(CordzHandle* handle);
+
+  // Returns the current entries in the delete queue in LIFO order.
+  static std::vector<const CordzHandle*> DiagnosticsGetDeleteQueue();
+
+  // Returns true if the provided handle is nullptr or guarded by this handle.
+  // Since the CordzSnapshot token is itself a CordzHandle, this method will
+  // allow tests to check if that token is keeping an arbitrary CordzHandle
+  // alive.
+  bool DiagnosticsHandleIsSafeToInspect(const CordzHandle* handle) const;
+
+  // Returns the current entries in the delete queue, in LIFO order, that are
+  // protected by this. CordzHandle objects are only placed on the delete queue
+  // after CordzHandle::Delete is called with them as an argument. Only
+  // CordzHandle objects that are not also CordzSnapshot objects will be
+  // included in the return vector. For each of the handles in the return
+  // vector, the earliest that their memory can be freed is when this
+  // CordzSnapshot object is deleted.
+  std::vector<const CordzHandle*> DiagnosticsGetSafeToInspectDeletedHandles();
+
+ protected:
+  explicit CordzHandle(bool is_snapshot);
+  virtual ~CordzHandle();
+
+ private:
+  const bool is_snapshot_;
+
+  // dq_prev_ and dq_next_ require the global queue mutex to be held.
+  // Unfortunately we can't use thread annotations such that the thread safety
+  // analysis understands that queue_ and global_queue_ are one and the same.
+  CordzHandle* dq_prev_  = nullptr;
+  CordzHandle* dq_next_ = nullptr;
+};
+
+class CordzSnapshot : public CordzHandle {
+ public:
+  CordzSnapshot() : CordzHandle(true) {}
+};
+
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STRINGS_INTERNAL_CORDZ_HANDLE_H_
diff --git a/absl/strings/internal/cordz_handle_test.cc b/absl/strings/internal/cordz_handle_test.cc
new file mode 100644
index 00000000000..f2deea2b05a
--- /dev/null
+++ b/absl/strings/internal/cordz_handle_test.cc
@@ -0,0 +1,267 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+#include "absl/strings/internal/cordz_handle.h"
+
+#include <atomic>
+#include <random>
+#include <vector>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/memory/memory.h"
+#include "absl/synchronization/internal/thread_pool.h"
+#include "absl/synchronization/notification.h"
+#include "absl/time/clock.h"
+#include "absl/time/time.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+namespace {
+
+using ::testing::ElementsAre;
+using ::testing::Gt;
+using ::testing::IsEmpty;
+using ::testing::SizeIs;
+
+// Local less verbose helper
+std::vector<const CordzHandle*> DeleteQueue() {
+  return CordzHandle::DiagnosticsGetDeleteQueue();
+}
+
+struct CordzHandleDeleteTracker : public CordzHandle {
+  bool* deleted;
+  explicit CordzHandleDeleteTracker(bool* deleted) : deleted(deleted) {}
+  ~CordzHandleDeleteTracker() override { *deleted = true; }
+};
+
+TEST(CordzHandleTest, DeleteQueueIsEmpty) {
+  EXPECT_THAT(DeleteQueue(), SizeIs(0));
+}
+
+TEST(CordzHandleTest, CordzHandleCreateDelete) {
+  bool deleted = false;
+  auto* handle = new CordzHandleDeleteTracker(&deleted);
+  EXPECT_FALSE(handle->is_snapshot());
+  EXPECT_TRUE(handle->SafeToDelete());
+  EXPECT_THAT(DeleteQueue(), SizeIs(0));
+
+  CordzHandle::Delete(handle);
+  EXPECT_THAT(DeleteQueue(), SizeIs(0));
+  EXPECT_TRUE(deleted);
+}
+
+TEST(CordzHandleTest, CordzSnapshotCreateDelete) {
+  auto* snapshot = new CordzSnapshot();
+  EXPECT_TRUE(snapshot->is_snapshot());
+  EXPECT_TRUE(snapshot->SafeToDelete());
+  EXPECT_THAT(DeleteQueue(), ElementsAre(snapshot));
+  delete snapshot;
+  EXPECT_THAT(DeleteQueue(), SizeIs(0));
+}
+
+TEST(CordzHandleTest, CordzHandleCreateDeleteWithSnapshot) {
+  bool deleted = false;
+  auto* snapshot = new CordzSnapshot();
+  auto* handle = new CordzHandleDeleteTracker(&deleted);
+  EXPECT_FALSE(handle->SafeToDelete());
+
+  CordzHandle::Delete(handle);
+  EXPECT_THAT(DeleteQueue(), ElementsAre(handle, snapshot));
+  EXPECT_FALSE(deleted);
+  EXPECT_FALSE(handle->SafeToDelete());
+
+  delete snapshot;
+  EXPECT_THAT(DeleteQueue(), SizeIs(0));
+  EXPECT_TRUE(deleted);
+}
+
+TEST(CordzHandleTest, MultiSnapshot) {
+  bool deleted[3] = {false, false, false};
+
+  CordzSnapshot* snapshot[3];
+  CordzHandleDeleteTracker* handle[3];
+  for (int i = 0; i < 3; ++i) {
+    snapshot[i] = new CordzSnapshot();
+    handle[i] = new CordzHandleDeleteTracker(&deleted[i]);
+    CordzHandle::Delete(handle[i]);
+  }
+
+  EXPECT_THAT(DeleteQueue(), ElementsAre(handle[2], snapshot[2], handle[1],
+                                         snapshot[1], handle[0], snapshot[0]));
+  EXPECT_THAT(deleted, ElementsAre(false, false, false));
+
+  delete snapshot[1];
+  EXPECT_THAT(DeleteQueue(), ElementsAre(handle[2], snapshot[2], handle[1],
+                                         handle[0], snapshot[0]));
+  EXPECT_THAT(deleted, ElementsAre(false, false, false));
+
+  delete snapshot[0];
+  EXPECT_THAT(DeleteQueue(), ElementsAre(handle[2], snapshot[2]));
+  EXPECT_THAT(deleted, ElementsAre(true, true, false));
+
+  delete snapshot[2];
+  EXPECT_THAT(DeleteQueue(), SizeIs(0));
+  EXPECT_THAT(deleted, ElementsAre(true, true, deleted));
+}
+
+TEST(CordzHandleTest, DiagnosticsHandleIsSafeToInspect) {
+  CordzSnapshot snapshot1;
+  EXPECT_TRUE(snapshot1.DiagnosticsHandleIsSafeToInspect(nullptr));
+
+  auto* handle1 = new CordzHandle();
+  EXPECT_TRUE(snapshot1.DiagnosticsHandleIsSafeToInspect(handle1));
+
+  CordzHandle::Delete(handle1);
+  EXPECT_TRUE(snapshot1.DiagnosticsHandleIsSafeToInspect(handle1));
+
+  CordzSnapshot snapshot2;
+  auto* handle2 = new CordzHandle();
+  EXPECT_TRUE(snapshot1.DiagnosticsHandleIsSafeToInspect(handle1));
+  EXPECT_TRUE(snapshot1.DiagnosticsHandleIsSafeToInspect(handle2));
+  EXPECT_FALSE(snapshot2.DiagnosticsHandleIsSafeToInspect(handle1));
+  EXPECT_TRUE(snapshot2.DiagnosticsHandleIsSafeToInspect(handle2));
+
+  CordzHandle::Delete(handle2);
+  EXPECT_TRUE(snapshot1.DiagnosticsHandleIsSafeToInspect(handle1));
+}
+
+TEST(CordzHandleTest, DiagnosticsGetSafeToInspectDeletedHandles) {
+  EXPECT_THAT(DeleteQueue(), IsEmpty());
+
+  auto* handle = new CordzHandle();
+  auto* snapshot1 = new CordzSnapshot();
+
+  // snapshot1 should be able to see handle.
+  EXPECT_THAT(DeleteQueue(), ElementsAre(snapshot1));
+  EXPECT_TRUE(snapshot1->DiagnosticsHandleIsSafeToInspect(handle));
+  EXPECT_THAT(snapshot1->DiagnosticsGetSafeToInspectDeletedHandles(),
+              IsEmpty());
+
+  // This handle will be safe to inspect as long as snapshot1 is alive. However,
+  // since only snapshot1 can prove that it's alive, it will be hidden from
+  // snapshot2.
+  CordzHandle::Delete(handle);
+
+  // This snapshot shouldn't be able to see handle because handle was already
+  // sent to Delete.
+  auto* snapshot2 = new CordzSnapshot();
+
+  // DeleteQueue elements are LIFO order.
+  EXPECT_THAT(DeleteQueue(), ElementsAre(snapshot2, handle, snapshot1));
+
+  EXPECT_TRUE(snapshot1->DiagnosticsHandleIsSafeToInspect(handle));
+  EXPECT_FALSE(snapshot2->DiagnosticsHandleIsSafeToInspect(handle));
+
+  EXPECT_THAT(snapshot1->DiagnosticsGetSafeToInspectDeletedHandles(),
+              ElementsAre(handle));
+  EXPECT_THAT(snapshot2->DiagnosticsGetSafeToInspectDeletedHandles(),
+              IsEmpty());
+
+  CordzHandle::Delete(snapshot1);
+  EXPECT_THAT(DeleteQueue(), ElementsAre(snapshot2));
+
+  CordzHandle::Delete(snapshot2);
+  EXPECT_THAT(DeleteQueue(), IsEmpty());
+}
+
+// Create and delete CordzHandle and CordzSnapshot objects in multiple threads
+// so that tsan has some time to chew on it and look for memory problems.
+TEST(CordzHandleTest, MultiThreaded) {
+  Notification stop;
+  static constexpr int kNumThreads = 4;
+  // Keep the number of handles relatively small so that the test will naturally
+  // transition to an empty delete queue during the test. If there are, say, 100
+  // handles, that will virtually never happen. With 10 handles and around 50k
+  // iterations in each of 4 threads, the delete queue appears to become empty
+  // around 200 times.
+  static constexpr int kNumHandles = 10;
+
+  // Each thread is going to pick a random index and atomically swap its
+  // CordzHandle with one in handles. This way, each thread can avoid
+  // manipulating a CordzHandle that might be operated upon in another thread.
+  std::vector<std::atomic<CordzHandle*>> handles(kNumHandles);
+
+  // global bool which is set when any thread did get some 'safe to inspect'
+  // handles. On some platforms and OSS tests, we might risk that some pool
+  // threads are starved, stalled, or just got a few unlikely random 'handle'
+  // coin tosses, so we satisfy this test with simply observing 'some' thread
+  // did something meaningful, which should minimize the potential for flakes.
+  std::atomic<bool> found_safe_to_inspect(false);
+
+  {
+    absl::synchronization_internal::ThreadPool pool(kNumThreads);
+    for (int i = 0; i < kNumThreads; ++i) {
+      pool.Schedule([&stop, &handles, &found_safe_to_inspect]() {
+        std::minstd_rand gen;
+        std::uniform_int_distribution<int> dist_type(0, 2);
+        std::uniform_int_distribution<int> dist_handle(0, kNumHandles - 1);
+
+        while (!stop.HasBeenNotified()) {
+          CordzHandle* handle;
+          switch (dist_type(gen)) {
+            case 0:
+              handle = new CordzHandle();
+              break;
+            case 1:
+              handle = new CordzSnapshot();
+              break;
+            default:
+              handle = nullptr;
+              break;
+          }
+          CordzHandle* old_handle = handles[dist_handle(gen)].exchange(handle);
+          if (old_handle != nullptr) {
+            std::vector<const CordzHandle*> safe_to_inspect =
+                old_handle->DiagnosticsGetSafeToInspectDeletedHandles();
+            for (const CordzHandle* handle : safe_to_inspect) {
+              // We're in a tight loop, so don't generate too many error
+              // messages.
+              ASSERT_FALSE(handle->is_snapshot());
+            }
+            if (!safe_to_inspect.empty()) {
+              found_safe_to_inspect.store(true);
+            }
+            CordzHandle::Delete(old_handle);
+          }
+        }
+
+        // Have each thread attempt to clean up everything. Some thread will be
+        // the last to reach this cleanup code, and it will be guaranteed to
+        // clean up everything because nothing remains to create new handles.
+        for (auto& h : handles) {
+          if (CordzHandle* handle = h.exchange(nullptr)) {
+            CordzHandle::Delete(handle);
+          }
+        }
+      });
+    }
+
+    // The threads will hammer away.  Give it a little bit of time for tsan to
+    // spot errors.
+    absl::SleepFor(absl::Seconds(3));
+    stop.Notify();
+  }
+
+  // Confirm that the test did *something*. This check will be satisfied as
+  // long as any thread has deleted a CordzSnapshot object and a non-snapshot
+  // CordzHandle was deleted after the CordzSnapshot was created.
+  // See also comments on `found_safe_to_inspect`
+  EXPECT_TRUE(found_safe_to_inspect.load());
+}
+
+}  // namespace
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/strings/internal/cordz_info.cc b/absl/strings/internal/cordz_info.cc
new file mode 100644
index 00000000000..321d15ddf07
--- /dev/null
+++ b/absl/strings/internal/cordz_info.cc
@@ -0,0 +1,431 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/strings/internal/cordz_info.h"
+
+#include <atomic>
+#include <cassert>
+#include <cstddef>
+#include <cstdint>
+#include <cstring>
+
+#include "absl/base/config.h"
+#include "absl/base/const_init.h"
+#include "absl/base/macros.h"
+#include "absl/base/no_destructor.h"
+#include "absl/base/optimization.h"
+#include "absl/base/thread_annotations.h"
+#include "absl/container/inlined_vector.h"
+#include "absl/debugging/stacktrace.h"
+#include "absl/strings/internal/cord_internal.h"
+#include "absl/strings/internal/cord_rep_btree.h"
+#include "absl/strings/internal/cord_rep_crc.h"
+#include "absl/strings/internal/cordz_handle.h"
+#include "absl/strings/internal/cordz_statistics.h"
+#include "absl/strings/internal/cordz_update_tracker.h"
+#include "absl/synchronization/mutex.h"
+#include "absl/time/clock.h"
+#include "absl/types/span.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+
+namespace {
+
+// CordRepAnalyzer performs the analysis of a cord.
+//
+// It computes absolute node counts and total memory usage, and an 'estimated
+// fair share memory usage` statistic.
+// Conceptually, it divides the 'memory usage' at each location in the 'cord
+// graph' by the cumulative reference count of that location. The cumulative
+// reference count is the factored total of all edges leading into that node.
+//
+// The top level node is treated specially: we assume the current thread
+// (typically called from the CordzHandler) to hold a reference purely to
+// perform a safe analysis, and not being part of the application. So we
+// subtract 1 from the reference count of the top node to compute the
+// 'application fair share' excluding the reference of the current thread.
+//
+// An example of fair sharing, and why we multiply reference counts:
+// Assume we have 2 CordReps, both being a Substring referencing a Flat:
+//   CordSubstring A (refcount = 5) --> child Flat C (refcount = 2)
+//   CordSubstring B (refcount = 9) --> child Flat C (refcount = 2)
+//
+// Flat C has 2 incoming edges from the 2 substrings (refcount = 2) and is not
+// referenced directly anywhere else. Translated into a 'fair share', we then
+// attribute 50% of the memory (memory / refcount = 2) to each incoming edge.
+// Rep A has a refcount of 5, so we attribute each incoming edge 1 / 5th of the
+// memory cost below it, i.e.: the fair share of Rep A of the memory used by C
+// is then 'memory C / (refcount C * refcount A) + (memory A / refcount A)'.
+// It is also easy to see how all incoming edges add up to 100%.
+class CordRepAnalyzer {
+ public:
+  // Creates an analyzer instance binding to `statistics`.
+  explicit CordRepAnalyzer(CordzStatistics& statistics)
+      : statistics_(statistics) {}
+
+  // Analyzes the memory statistics and node counts for the provided `rep`, and
+  // adds the results to `statistics`. Note that node counts and memory sizes
+  // are not initialized, computed values are added to any existing values.
+  void AnalyzeCordRep(const CordRep* rep) {
+    ABSL_ASSERT(rep != nullptr);
+
+    // Process all linear nodes.
+    // As per the class comments, use refcout - 1 on the top level node, as the
+    // top level node is assumed to be referenced only for analysis purposes.
+    size_t refcount = rep->refcount.Get();
+    RepRef repref{rep, (refcount > 1) ? refcount - 1 : 1};
+
+    // Process the top level CRC node, if present.
+    if (repref.tag() == CRC) {
+      statistics_.node_count++;
+      statistics_.node_counts.crc++;
+      memory_usage_.Add(sizeof(CordRepCrc), repref.refcount);
+      repref = repref.Child(repref.rep->crc()->child);
+    }
+
+    // Process all top level linear nodes (substrings and flats).
+    repref = CountLinearReps(repref, memory_usage_);
+
+    switch (repref.tag()) {
+      case CordRepKind::BTREE:
+        AnalyzeBtree(repref);
+        break;
+      default:
+        // We should have a btree node if not null.
+        ABSL_ASSERT(repref.tag() == CordRepKind::UNUSED_0);
+        break;
+    }
+
+    // Adds values to output
+    statistics_.estimated_memory_usage += memory_usage_.total;
+    statistics_.estimated_fair_share_memory_usage +=
+        static_cast<size_t>(memory_usage_.fair_share);
+  }
+
+ private:
+  // RepRef identifies a CordRep* inside the Cord tree with its cumulative
+  // refcount including itself. For example, a tree consisting of a substring
+  // with a refcount of 3 and a child flat with a refcount of 4 will have RepRef
+  // refcounts of 3 and 12 respectively.
+  struct RepRef {
+    const CordRep* rep;
+    size_t refcount;
+
+    // Returns a 'child' RepRef which contains the cumulative reference count
+    // of this instance multiplied by the child's reference count. Returns a
+    // nullptr RepRef value with a refcount of 0 if `child` is nullptr.
+    RepRef Child(const CordRep* child) const {
+      if (child == nullptr) return RepRef{nullptr, 0};
+      return RepRef{child, refcount * child->refcount.Get()};
+    }
+
+    // Returns the tag of this rep, or UNUSED_0 if this instance is null
+    constexpr CordRepKind tag() const {
+      ABSL_ASSERT(rep == nullptr || rep->tag != CordRepKind::UNUSED_0);
+      return rep ? static_cast<CordRepKind>(rep->tag) : CordRepKind::UNUSED_0;
+    }
+  };
+
+  // Memory usage values
+  struct MemoryUsage {
+    size_t total = 0;
+    double fair_share = 0.0;
+
+    // Adds 'size` memory usage to this class, with a cumulative (recursive)
+    // reference count of `refcount`
+    void Add(size_t size, size_t refcount) {
+      total += size;
+      fair_share += static_cast<double>(size) / refcount;
+    }
+  };
+
+  // Counts a flat of the provide allocated size
+  void CountFlat(size_t size) {
+    statistics_.node_count++;
+    statistics_.node_counts.flat++;
+    if (size <= 64) {
+      statistics_.node_counts.flat_64++;
+    } else if (size <= 128) {
+      statistics_.node_counts.flat_128++;
+    } else if (size <= 256) {
+      statistics_.node_counts.flat_256++;
+    } else if (size <= 512) {
+      statistics_.node_counts.flat_512++;
+    } else if (size <= 1024) {
+      statistics_.node_counts.flat_1k++;
+    }
+  }
+
+  // Processes 'linear' reps (substring, flat, external) not requiring iteration
+  // or recursion. Returns RefRep{null} if all reps were processed, else returns
+  // the top-most non-linear concat or ring cordrep.
+  // Node counts are updated into `statistics_`, memory usage is update into
+  // `memory_usage`, which typically references `memory_usage_` except for ring
+  // buffers where we count children unrounded.
+  RepRef CountLinearReps(RepRef rep, MemoryUsage& memory_usage) {
+    // Consume all substrings
+    while (rep.tag() == SUBSTRING) {
+      statistics_.node_count++;
+      statistics_.node_counts.substring++;
+      memory_usage.Add(sizeof(CordRepSubstring), rep.refcount);
+      rep = rep.Child(rep.rep->substring()->child);
+    }
+
+    // Consume possible FLAT
+    if (rep.tag() >= FLAT) {
+      size_t size = rep.rep->flat()->AllocatedSize();
+      CountFlat(size);
+      memory_usage.Add(size, rep.refcount);
+      return RepRef{nullptr, 0};
+    }
+
+    // Consume possible external
+    if (rep.tag() == EXTERNAL) {
+      statistics_.node_count++;
+      statistics_.node_counts.external++;
+      size_t size = rep.rep->length + sizeof(CordRepExternalImpl<intptr_t>);
+      memory_usage.Add(size, rep.refcount);
+      return RepRef{nullptr, 0};
+    }
+
+    return rep;
+  }
+
+  // Analyzes the provided btree.
+  void AnalyzeBtree(RepRef rep) {
+    statistics_.node_count++;
+    statistics_.node_counts.btree++;
+    memory_usage_.Add(sizeof(CordRepBtree), rep.refcount);
+    const CordRepBtree* tree = rep.rep->btree();
+    if (tree->height() > 0) {
+      for (CordRep* edge : tree->Edges()) {
+        AnalyzeBtree(rep.Child(edge));
+      }
+    } else {
+      for (CordRep* edge : tree->Edges()) {
+        CountLinearReps(rep.Child(edge), memory_usage_);
+      }
+    }
+  }
+
+  CordzStatistics& statistics_;
+  MemoryUsage memory_usage_;
+};
+
+}  // namespace
+
+CordzInfo::List* CordzInfo::GlobalList() {
+  static absl::NoDestructor<CordzInfo::List> list;
+  return list.get();
+}
+
+CordzInfo* CordzInfo::Head(const CordzSnapshot& snapshot) {
+  ABSL_ASSERT(snapshot.is_snapshot());
+
+  // We obtain the lock here as we must synchronize the first call into the list
+  // with any concurrent 'Untrack()` operation to avoid any read in the list to
+  // reorder before the observation of the thread 'untracking a cord' of the
+  // delete queue being empty or not. After this all next observations are safe
+  // as we have established all subsequent untracks will be queued for delete.
+  // We do enforce in DEBUG builds that the 'head' value is present in the
+  // delete queue: ODR violations may lead to 'snapshot' and 'global_list'
+  // being in different libraries / modules.
+  auto global_list = GlobalList();
+  absl::MutexLock l(global_list->mutex);
+  ABSL_ASSERT(snapshot.DiagnosticsHandleIsSafeToInspect(global_list->head));
+  return global_list->head;
+}
+
+CordzInfo* CordzInfo::Next(const CordzSnapshot& snapshot) const {
+  ABSL_ASSERT(snapshot.is_snapshot());
+
+  // We do not need a lock here. See also comments in Head().
+  CordzInfo* next = ci_next_.load(std::memory_order_acquire);
+  ABSL_ASSERT(snapshot.DiagnosticsHandleIsSafeToInspect(this));
+  ABSL_ASSERT(snapshot.DiagnosticsHandleIsSafeToInspect(next));
+  return next;
+}
+
+void CordzInfo::TrackCord(InlineData& cord, MethodIdentifier method,
+                          int64_t sampling_stride) {
+  assert(cord.is_tree());
+  assert(!cord.is_profiled());
+  CordzInfo* cordz_info =
+      new CordzInfo(cord.as_tree(), nullptr, method, sampling_stride);
+  cord.set_cordz_info(cordz_info);
+  cordz_info->Track();
+}
+
+void CordzInfo::TrackCord(InlineData& cord, const InlineData& src,
+                          MethodIdentifier method) {
+  assert(cord.is_tree());
+  assert(src.is_tree());
+
+  // Unsample current as we the current cord is being replaced with 'src',
+  // so any method history is no longer relevant.
+  CordzInfo* cordz_info = cord.cordz_info();
+  if (cordz_info != nullptr) cordz_info->Untrack();
+
+  // Start new cord sample
+  cordz_info = new CordzInfo(cord.as_tree(), src.cordz_info(), method,
+                             src.cordz_info()->sampling_stride());
+  cord.set_cordz_info(cordz_info);
+  cordz_info->Track();
+}
+
+void CordzInfo::MaybeTrackCordImpl(InlineData& cord, const InlineData& src,
+                                   MethodIdentifier method) {
+  if (src.is_profiled()) {
+    TrackCord(cord, src, method);
+  } else if (cord.is_profiled()) {
+    cord.cordz_info()->Untrack();
+    cord.clear_cordz_info();
+  }
+}
+
+CordzInfo::MethodIdentifier CordzInfo::GetParentMethod(const CordzInfo* src) {
+  if (src == nullptr) return MethodIdentifier::kUnknown;
+  return src->parent_method_ != MethodIdentifier::kUnknown ? src->parent_method_
+                                                           : src->method_;
+}
+
+size_t CordzInfo::FillParentStack(const CordzInfo* src, void** stack) {
+  assert(stack);
+  if (src == nullptr) return 0;
+  if (src->parent_stack_depth_) {
+    memcpy(stack, src->parent_stack_, src->parent_stack_depth_ * sizeof(void*));
+    return src->parent_stack_depth_;
+  }
+  memcpy(stack, src->stack_, src->stack_depth_ * sizeof(void*));
+  return src->stack_depth_;
+}
+
+CordzInfo::CordzInfo(CordRep* rep, const CordzInfo* src,
+                     MethodIdentifier method, int64_t sampling_stride)
+    : rep_(rep),
+      stack_depth_(
+          static_cast<size_t>(absl::GetStackTrace(stack_,
+                                                  /*max_depth=*/kMaxStackDepth,
+                                                  /*skip_count=*/1))),
+      parent_stack_depth_(FillParentStack(src, parent_stack_)),
+      method_(method),
+      parent_method_(GetParentMethod(src)),
+      create_time_(absl::Now()),
+      sampling_stride_(sampling_stride) {
+  update_tracker_.LossyAdd(method);
+  if (src) {
+    // Copy parent counters.
+    update_tracker_.LossyAdd(src->update_tracker_);
+  }
+}
+
+CordzInfo::~CordzInfo() {
+  // `rep_` is potentially kept alive if CordzInfo is included
+  // in a collection snapshot (which should be rare).
+  if (ABSL_PREDICT_FALSE(rep_)) {
+    CordRep::Unref(rep_);
+  }
+}
+
+void CordzInfo::Track() {
+  absl::MutexLock l(list_->mutex);
+  CordzInfo* const head = list_->head;
+  if (head != nullptr) {
+    head->ci_prev_.store(this, std::memory_order_release);
+  }
+  ci_next_.store(head, std::memory_order_release);
+  list_->head = this;
+}
+
+void CordzInfo::Untrack() {
+  ODRCheck();
+  {
+    absl::MutexLock l(list_->mutex);
+
+    CordzInfo* const head = list_->head;
+    CordzInfo* const next = ci_next_.load(std::memory_order_acquire);
+    CordzInfo* const prev = ci_prev_.load(std::memory_order_acquire);
+
+    if (next) {
+      ABSL_ASSERT(next->ci_prev_.load(std::memory_order_acquire) == this);
+      next->ci_prev_.store(prev, std::memory_order_release);
+    }
+    if (prev) {
+      ABSL_ASSERT(head != this);
+      ABSL_ASSERT(prev->ci_next_.load(std::memory_order_acquire) == this);
+      prev->ci_next_.store(next, std::memory_order_release);
+    } else {
+      ABSL_ASSERT(head == this);
+      list_->head = next;
+    }
+  }
+
+  // We can no longer be discovered: perform a fast path check if we are not
+  // listed on any delete queue, so we can directly delete this instance.
+  if (SafeToDelete()) {
+    UnsafeSetCordRep(nullptr);
+    delete this;
+    return;
+  }
+
+  // We are likely part of a snapshot, extend the life of the CordRep
+  {
+    absl::MutexLock lock(mutex_);
+    if (rep_) CordRep::Ref(rep_);
+  }
+  CordzHandle::Delete(this);
+}
+
+void CordzInfo::Lock(MethodIdentifier method)
+    ABSL_EXCLUSIVE_LOCK_FUNCTION(mutex_) {
+  mutex_.lock();
+  update_tracker_.LossyAdd(method);
+  assert(rep_);
+}
+
+void CordzInfo::Unlock() ABSL_UNLOCK_FUNCTION(mutex_) {
+  bool tracked = rep_ != nullptr;
+  mutex_.unlock();
+  if (!tracked) {
+    Untrack();
+  }
+}
+
+absl::Span<void* const> CordzInfo::GetStack() const {
+  return absl::MakeConstSpan(stack_, stack_depth_);
+}
+
+absl::Span<void* const> CordzInfo::GetParentStack() const {
+  return absl::MakeConstSpan(parent_stack_, parent_stack_depth_);
+}
+
+CordzStatistics CordzInfo::GetCordzStatistics() const {
+  CordzStatistics stats;
+  stats.method = method_;
+  stats.parent_method = parent_method_;
+  stats.update_tracker = update_tracker_;
+  if (CordRep* rep = RefCordRep()) {
+    stats.size = rep->length;
+    CordRepAnalyzer analyzer(stats);
+    analyzer.AnalyzeCordRep(rep);
+    CordRep::Unref(rep);
+  }
+  return stats;
+}
+
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/strings/internal/cordz_info.h b/absl/strings/internal/cordz_info.h
new file mode 100644
index 00000000000..04609cd3092
--- /dev/null
+++ b/absl/strings/internal/cordz_info.h
@@ -0,0 +1,299 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_STRINGS_INTERNAL_CORDZ_INFO_H_
+#define ABSL_STRINGS_INTERNAL_CORDZ_INFO_H_
+
+#include <atomic>
+#include <cstddef>
+#include <cstdint>
+#include <functional>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/const_init.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/base/optimization.h"
+#include "absl/base/thread_annotations.h"
+#include "absl/strings/internal/cord_internal.h"
+#include "absl/strings/internal/cordz_functions.h"
+#include "absl/strings/internal/cordz_handle.h"
+#include "absl/strings/internal/cordz_statistics.h"
+#include "absl/strings/internal/cordz_update_tracker.h"
+#include "absl/synchronization/mutex.h"
+#include "absl/time/time.h"
+#include "absl/types/span.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+
+// CordzInfo tracks a profiled Cord. Each of these objects can be in two places.
+// If a Cord is alive, the CordzInfo will be in the global_cordz_infos map, and
+// can also be retrieved via the linked list starting with
+// global_cordz_infos_head and continued via the cordz_info_next() method. When
+// a Cord has reached the end of its lifespan, the CordzInfo object will be
+// migrated out of the global_cordz_infos list and the global_cordz_infos_map,
+// and will either be deleted or appended to the global_delete_queue. If it is
+// placed on the global_delete_queue, the CordzInfo object will be cleaned in
+// the destructor of a CordzSampleToken object.
+class ABSL_LOCKABLE CordzInfo : public CordzHandle {
+ public:
+  using MethodIdentifier = CordzUpdateTracker::MethodIdentifier;
+
+  // TrackCord creates a CordzInfo instance which tracks important metrics of
+  // a sampled cord, and stores the created CordzInfo instance into `cord'. All
+  // CordzInfo instances are placed in a global list which is used to discover
+  // and snapshot all actively tracked cords. Callers are responsible for
+  // calling UntrackCord() before the tracked Cord instance is deleted, or to
+  // stop tracking the sampled Cord. Callers are also responsible for guarding
+  // changes to the 'tree' value of a Cord (InlineData.tree) through the Lock()
+  // and Unlock() calls. Any change resulting in a new tree value for the cord
+  // requires a call to SetCordRep() before the old tree has been unreffed
+  // and/or deleted. `method` identifies the Cord public API method initiating
+  // the cord to be sampled.
+  // Requires `cord` to hold a tree, and `cord.cordz_info()` to be null.
+  static void TrackCord(InlineData& cord, MethodIdentifier method,
+                        int64_t sampling_stride);
+
+  // Identical to TrackCord(), except that this function fills the
+  // `parent_stack` and `parent_method` properties of the returned CordzInfo
+  // instance from the provided `src` instance if `src` is sampled.
+  // This function should be used for sampling 'copy constructed' and 'copy
+  // assigned' cords. This function allows 'cord` to be already sampled, in
+  // which case the CordzInfo will be newly created from `src`.
+  static void TrackCord(InlineData& cord, const InlineData& src,
+                        MethodIdentifier method);
+
+  // Maybe sample the cord identified by 'cord' for method 'method'.
+  // Uses `cordz_should_profile` to randomly pick cords to be sampled, and if
+  // so, invokes `TrackCord` to start sampling `cord`.
+  static void MaybeTrackCord(InlineData& cord, MethodIdentifier method);
+
+  // Maybe sample the cord identified by 'cord' for method 'method'.
+  // `src` identifies a 'parent' cord which is assigned to `cord`, typically the
+  // input cord for a copy constructor, or an assign method such as `operator=`
+  // `cord` will be sampled if (and only if) `src` is sampled.
+  // If `cord` is currently being sampled and `src` is not being sampled, then
+  // this function will stop sampling the cord and reset the cord's cordz_info.
+  //
+  // Previously this function defined that `cord` will be sampled if either
+  // `src` is sampled, or if `cord` is randomly picked for sampling. However,
+  // this can cause issues, as there may be paths where some cord is assigned an
+  // indirect copy of it's own value. As such a 'string of copies' would then
+  // remain sampled (`src.is_profiled`), then assigning such a cord back to
+  // 'itself' creates a cycle where the cord will converge to 'always sampled`.
+  //
+  // For example:
+  //
+  //   Cord x;
+  //   for (...) {
+  //     // Copy ctor --> y.is_profiled := x.is_profiled | random(...)
+  //     Cord y = x;
+  //     ...
+  //     // Assign x = y --> x.is_profiled = y.is_profiled | random(...)
+  //     //              ==> x.is_profiled |= random(...)
+  //     //              ==> x converges to 'always profiled'
+  //     x = y;
+  //   }
+  static void MaybeTrackCord(InlineData& cord, const InlineData& src,
+                             MethodIdentifier method);
+
+  // Stops tracking changes for a sampled cord, and deletes the provided info.
+  // This function must be called before the sampled cord instance is deleted,
+  // and before the root cordrep of the sampled cord is unreffed.
+  // This function may extend the lifetime of the cordrep in cases where the
+  // CordInfo instance is being held by a concurrent collection thread.
+  void Untrack();
+
+  // Invokes UntrackCord() on `info` if `info` is not null.
+  static void MaybeUntrackCord(CordzInfo* info);
+
+  CordzInfo() = delete;
+  CordzInfo(const CordzInfo&) = delete;
+  CordzInfo& operator=(const CordzInfo&) = delete;
+
+  // Retrieves the oldest existing CordzInfo.
+  static CordzInfo* Head(const CordzSnapshot& snapshot);
+
+  // Retrieves the next oldest existing CordzInfo older than 'this' instance.
+  CordzInfo* Next(const CordzSnapshot& snapshot) const;
+
+  // Locks this instance for the update identified by `method`.
+  // Increases the count for `method` in `update_tracker`.
+  void Lock(MethodIdentifier method) ABSL_EXCLUSIVE_LOCK_FUNCTION(mutex_);
+
+  // Unlocks this instance. If the contained `rep` has been set to null
+  // indicating the Cord has been cleared or is otherwise no longer sampled,
+  // then this method will delete this CordzInfo instance.
+  void Unlock() ABSL_UNLOCK_FUNCTION(mutex_);
+
+  // Asserts that this CordzInfo instance is locked.
+  void AssertHeld() ABSL_ASSERT_EXCLUSIVE_LOCK(mutex_);
+
+  // Updates the `rep` property of this instance. This methods is invoked by
+  // Cord logic each time the root node of a sampled Cord changes, and before
+  // the old root reference count is deleted. This guarantees that collection
+  // code can always safely take a reference on the tracked cord.
+  // Requires a lock to be held through the `Lock()` method.
+  // TODO(b/117940323): annotate with ABSL_EXCLUSIVE_LOCKS_REQUIRED once all
+  // Cord code is in a state where this can be proven true by the compiler.
+  void SetCordRep(CordRep* rep);
+
+  // Returns the current `rep` property of this instance with a reference
+  // added, or null if this instance represents a cord that has since been
+  // deleted or untracked.
+  CordRep* RefCordRep() const ABSL_LOCKS_EXCLUDED(mutex_);
+
+  // Returns the current value of `rep_` for testing purposes only.
+  CordRep* GetCordRepForTesting() const ABSL_NO_THREAD_SAFETY_ANALYSIS {
+    return rep_;
+  }
+
+  // Sets the current value of `rep_` for testing purposes only.
+  void SetCordRepForTesting(CordRep* rep) ABSL_NO_THREAD_SAFETY_ANALYSIS {
+    rep_ = rep;
+  }
+
+  // Returns the stack trace for where the cord was first sampled. Cords are
+  // potentially sampled when they promote from an inlined cord to a tree or
+  // ring representation, which is not necessarily the location where the cord
+  // was first created. Some cords are created as inlined cords, and only as
+  // data is added do they become a non-inlined cord. However, typically the
+  // location represents reasonably well where the cord is 'created'.
+  absl::Span<void* const> GetStack() const;
+
+  // Returns the stack trace for a sampled cord's 'parent stack trace'. This
+  // value may be set if the cord is sampled (promoted) after being created
+  // from, or being assigned the value of an existing (sampled) cord.
+  absl::Span<void* const> GetParentStack() const;
+
+  // Retrieves the CordzStatistics associated with this Cord. The statistics
+  // are only updated when a Cord goes through a mutation, such as an Append
+  // or RemovePrefix.
+  CordzStatistics GetCordzStatistics() const;
+
+  int64_t sampling_stride() const { return sampling_stride_; }
+
+ private:
+  // Global cordz info list. CordzInfo stores a pointer to the global list
+  // instance to harden against ODR violations.
+  struct List {
+    absl::Mutex mutex;
+    CordzInfo* head ABSL_GUARDED_BY(mutex){nullptr};
+  };
+
+  static List* GlobalList();
+
+  static constexpr size_t kMaxStackDepth = 64;
+
+  explicit CordzInfo(CordRep* rep, const CordzInfo* src,
+                     MethodIdentifier method, int64_t weight);
+  ~CordzInfo() override;
+
+  // Sets `rep_` without holding a lock.
+  void UnsafeSetCordRep(CordRep* rep) ABSL_NO_THREAD_SAFETY_ANALYSIS;
+
+  void Track();
+
+  // Returns the parent method from `src`, which is either `parent_method_` or
+  // `method_` depending on `parent_method_` being kUnknown.
+  // Returns kUnknown if `src` is null.
+  static MethodIdentifier GetParentMethod(const CordzInfo* src);
+
+  // Fills the provided stack from `src`, copying either `parent_stack_` or
+  // `stack_` depending on `parent_stack_` being empty, returning the size of
+  // the parent stack.
+  // Returns 0 if `src` is null.
+  static size_t FillParentStack(const CordzInfo* src, void** stack);
+
+  void ODRCheck() const {
+#ifndef NDEBUG
+    ABSL_RAW_CHECK(list_ == GlobalList(), "ODR violation in Cord");
+#endif
+  }
+
+  // Non-inlined implementation of `MaybeTrackCord`, which is executed if
+  // either `src` is sampled or `cord` is sampled, and either untracks or
+  // tracks `cord` as documented per `MaybeTrackCord`.
+  static void MaybeTrackCordImpl(InlineData& cord, const InlineData& src,
+                                 MethodIdentifier method);
+
+  List* const list_ = GlobalList();
+
+  // ci_prev_ and ci_next_ require the global list mutex to be held.
+  // Unfortunately we can't use thread annotations such that the thread safety
+  // analysis understands that list_ and GlobalList() are one and the same.
+  std::atomic<CordzInfo*> ci_prev_{nullptr};
+  std::atomic<CordzInfo*> ci_next_{nullptr};
+
+  mutable absl::Mutex mutex_;
+  CordRep* rep_ ABSL_GUARDED_BY(mutex_);
+
+  void* stack_[kMaxStackDepth];
+  void* parent_stack_[kMaxStackDepth];
+  const size_t stack_depth_;
+  const size_t parent_stack_depth_;
+  const MethodIdentifier method_;
+  const MethodIdentifier parent_method_;
+  CordzUpdateTracker update_tracker_;
+  const absl::Time create_time_;
+  const int64_t sampling_stride_;
+};
+
+inline ABSL_ATTRIBUTE_ALWAYS_INLINE void CordzInfo::MaybeTrackCord(
+    InlineData& cord, MethodIdentifier method) {
+  auto stride = cordz_should_profile();
+  if (ABSL_PREDICT_FALSE(stride > 0)) {
+    TrackCord(cord, method, stride);
+  }
+}
+
+inline ABSL_ATTRIBUTE_ALWAYS_INLINE void CordzInfo::MaybeTrackCord(
+    InlineData& cord, const InlineData& src, MethodIdentifier method) {
+  if (ABSL_PREDICT_FALSE(InlineData::is_either_profiled(cord, src))) {
+    MaybeTrackCordImpl(cord, src, method);
+  }
+}
+
+inline ABSL_ATTRIBUTE_ALWAYS_INLINE void CordzInfo::MaybeUntrackCord(
+    CordzInfo* info) {
+  if (ABSL_PREDICT_FALSE(info)) {
+    info->Untrack();
+  }
+}
+
+inline void CordzInfo::AssertHeld() ABSL_ASSERT_EXCLUSIVE_LOCK(mutex_) {
+#ifndef NDEBUG
+  mutex_.AssertHeld();
+#endif
+}
+
+inline void CordzInfo::SetCordRep(CordRep* rep) {
+  AssertHeld();
+  rep_ = rep;
+}
+
+inline void CordzInfo::UnsafeSetCordRep(CordRep* rep) { rep_ = rep; }
+
+inline CordRep* CordzInfo::RefCordRep() const ABSL_LOCKS_EXCLUDED(mutex_) {
+  MutexLock lock(mutex_);
+  return rep_ ? CordRep::Ref(rep_) : nullptr;
+}
+
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STRINGS_INTERNAL_CORDZ_INFO_H_
diff --git a/absl/strings/internal/cordz_info_statistics_test.cc b/absl/strings/internal/cordz_info_statistics_test.cc
new file mode 100644
index 00000000000..859bb20b360
--- /dev/null
+++ b/absl/strings/internal/cordz_info_statistics_test.cc
@@ -0,0 +1,516 @@
+// Copyright 2021 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <cassert>
+#include <cstddef>
+#include <cstdint>
+#include <iostream>
+#include <random>
+#include <vector>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+#include "absl/crc/internal/crc_cord_state.h"
+#include "absl/strings/cord.h"
+#include "absl/strings/internal/cord_internal.h"
+#include "absl/strings/internal/cord_rep_btree.h"
+#include "absl/strings/internal/cord_rep_crc.h"
+#include "absl/strings/internal/cord_rep_flat.h"
+#include "absl/strings/internal/cordz_info.h"
+#include "absl/strings/internal/cordz_sample_token.h"
+#include "absl/strings/internal/cordz_statistics.h"
+#include "absl/strings/internal/cordz_update_scope.h"
+#include "absl/strings/internal/cordz_update_tracker.h"
+#include "absl/strings/string_view.h"
+#include "absl/synchronization/internal/thread_pool.h"
+#include "absl/synchronization/notification.h"
+#include "absl/time/clock.h"
+#include "absl/time/time.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+
+// Do not print statistics contents, the matcher prints them as needed.
+inline void PrintTo(const CordzStatistics& stats, std::ostream* s) {
+  if (s) *s << "CordzStatistics{...}";
+}
+
+namespace {
+
+using ::testing::Ge;
+
+// Creates a flat of the specified allocated size
+CordRepFlat* Flat(size_t size) {
+  // Round up to a tag size, as we are going to poke an exact tag size back into
+  // the allocated flat. 'size returning allocators' could grant us more than we
+  // wanted, but we are ok to poke the 'requested' size in the tag, even in the
+  // presence of sized deletes, so we need to make sure the size rounds
+  // perfectly to a tag value.
+  assert(size >= kMinFlatSize);
+  size = RoundUpForTag(size);
+  CordRepFlat* flat = CordRepFlat::New(size - kFlatOverhead);
+  flat->tag = AllocatedSizeToTag(size);
+  flat->length = size - kFlatOverhead;
+  return flat;
+}
+
+// Creates an external of the specified length
+CordRepExternal* External(size_t length = 512) {
+  return static_cast<CordRepExternal*>(
+      NewExternalRep(absl::string_view("", length), [](absl::string_view) {}));
+}
+
+// Creates a substring on the provided rep of length - 1
+CordRepSubstring* Substring(CordRep* rep) {
+  auto* substring = new CordRepSubstring;
+  substring->length = rep->length - 1;
+  substring->tag = SUBSTRING;
+  substring->child = rep;
+  return substring;
+}
+
+// Reference count helper
+struct RefHelper {
+  std::vector<CordRep*> refs;
+
+  ~RefHelper() {
+    for (CordRep* rep : refs) {
+      CordRep::Unref(rep);
+    }
+  }
+
+  // Invokes CordRep::Unref() on `rep` when this instance is destroyed.
+  template <typename T>
+  T* NeedsUnref(T* rep) {
+    refs.push_back(rep);
+    return rep;
+  }
+
+  // Adds `n` reference counts to `rep` which will be unreffed when this
+  // instance is destroyed.
+  template <typename T>
+  T* Ref(T* rep, size_t n = 1) {
+    while (n--) {
+      NeedsUnref(CordRep::Ref(rep));
+    }
+    return rep;
+  }
+};
+
+// Sizeof helper. Returns the allocated size of `p`, excluding any child
+// elements for substring, concat and ring cord reps.
+template <typename T>
+size_t SizeOf(const T* rep) {
+  return sizeof(T);
+}
+
+template <>
+size_t SizeOf(const CordRepFlat* rep) {
+  return rep->AllocatedSize();
+}
+
+template <>
+size_t SizeOf(const CordRepExternal* rep) {
+  // See cord.cc
+  return sizeof(CordRepExternalImpl<intptr_t>) + rep->length;
+}
+
+// Computes fair share memory used in a naive 'we dare to recurse' way.
+double FairShareImpl(CordRep* rep, size_t ref) {
+  double self = 0.0, children = 0.0;
+  ref *= rep->refcount.Get();
+  if (rep->tag >= FLAT) {
+    self = SizeOf(rep->flat());
+  } else if (rep->tag == EXTERNAL) {
+    self = SizeOf(rep->external());
+  } else if (rep->tag == SUBSTRING) {
+    self = SizeOf(rep->substring());
+    children = FairShareImpl(rep->substring()->child, ref);
+  } else if (rep->tag == BTREE) {
+    self = SizeOf(rep->btree());
+    for (CordRep*edge : rep->btree()->Edges()) {
+      children += FairShareImpl(edge, ref);
+    }
+  } else {
+    assert(false);
+  }
+  return self / ref + children;
+}
+
+// Returns the fair share memory size from `ShareFhareImpl()` as a size_t.
+size_t FairShare(CordRep* rep, size_t ref = 1) {
+  return static_cast<size_t>(FairShareImpl(rep, ref));
+}
+
+// Samples the cord and returns CordzInfo::GetStatistics()
+CordzStatistics SampleCord(CordRep* rep) {
+  InlineData cord(rep);
+  CordzInfo::TrackCord(cord, CordzUpdateTracker::kUnknown, 1);
+  CordzStatistics stats = cord.cordz_info()->GetCordzStatistics();
+  cord.cordz_info()->Untrack();
+  return stats;
+}
+
+MATCHER_P(EqStatistics, stats, "Statistics equal expected values") {
+  bool ok = true;
+
+#define STATS_MATCHER_EXPECT_EQ(member)                              \
+  if (stats.member != arg.member) {                                  \
+    *result_listener << "\n    stats." << #member                    \
+                     << ": actual = " << arg.member << ", expected " \
+                     << stats.member;                                \
+    ok = false;                                                      \
+  }
+
+  STATS_MATCHER_EXPECT_EQ(size);
+  STATS_MATCHER_EXPECT_EQ(node_count);
+  STATS_MATCHER_EXPECT_EQ(node_counts.flat);
+  STATS_MATCHER_EXPECT_EQ(node_counts.flat_64);
+  STATS_MATCHER_EXPECT_EQ(node_counts.flat_128);
+  STATS_MATCHER_EXPECT_EQ(node_counts.flat_256);
+  STATS_MATCHER_EXPECT_EQ(node_counts.flat_512);
+  STATS_MATCHER_EXPECT_EQ(node_counts.flat_1k);
+  STATS_MATCHER_EXPECT_EQ(node_counts.external);
+  STATS_MATCHER_EXPECT_EQ(node_counts.concat);
+  STATS_MATCHER_EXPECT_EQ(node_counts.substring);
+  STATS_MATCHER_EXPECT_EQ(node_counts.ring);
+  STATS_MATCHER_EXPECT_EQ(node_counts.btree);
+  STATS_MATCHER_EXPECT_EQ(estimated_memory_usage);
+  STATS_MATCHER_EXPECT_EQ(estimated_fair_share_memory_usage);
+
+#undef STATS_MATCHER_EXPECT_EQ
+
+  return ok;
+}
+
+TEST(CordzInfoStatisticsTest, Flat) {
+  RefHelper ref;
+  auto* flat = ref.NeedsUnref(Flat(512));
+
+  CordzStatistics expected;
+  expected.size = flat->length;
+  expected.estimated_memory_usage = SizeOf(flat);
+  expected.estimated_fair_share_memory_usage = expected.estimated_memory_usage;
+  expected.node_count = 1;
+  expected.node_counts.flat = 1;
+  expected.node_counts.flat_512 = 1;
+
+  EXPECT_THAT(SampleCord(flat), EqStatistics(expected));
+}
+
+TEST(CordzInfoStatisticsTest, SharedFlat) {
+  RefHelper ref;
+  auto* flat = ref.Ref(ref.NeedsUnref(Flat(64)));
+
+  CordzStatistics expected;
+  expected.size = flat->length;
+  expected.estimated_memory_usage = SizeOf(flat);
+  expected.estimated_fair_share_memory_usage = SizeOf(flat) / 2;
+  expected.node_count = 1;
+  expected.node_counts.flat = 1;
+  expected.node_counts.flat_64 = 1;
+
+  EXPECT_THAT(SampleCord(flat), EqStatistics(expected));
+}
+
+TEST(CordzInfoStatisticsTest, External) {
+  RefHelper ref;
+  auto* external = ref.NeedsUnref(External());
+
+  CordzStatistics expected;
+  expected.size = external->length;
+  expected.estimated_memory_usage = SizeOf(external);
+  expected.estimated_fair_share_memory_usage = SizeOf(external);
+  expected.node_count = 1;
+  expected.node_counts.external = 1;
+
+  EXPECT_THAT(SampleCord(external), EqStatistics(expected));
+}
+
+TEST(CordzInfoStatisticsTest, SharedExternal) {
+  RefHelper ref;
+  auto* external = ref.Ref(ref.NeedsUnref(External()));
+
+  CordzStatistics expected;
+  expected.size = external->length;
+  expected.estimated_memory_usage = SizeOf(external);
+  expected.estimated_fair_share_memory_usage = SizeOf(external) / 2;
+  expected.node_count = 1;
+  expected.node_counts.external = 1;
+
+  EXPECT_THAT(SampleCord(external), EqStatistics(expected));
+}
+
+TEST(CordzInfoStatisticsTest, Substring) {
+  RefHelper ref;
+  auto* flat = Flat(1024);
+  auto* substring = ref.NeedsUnref(Substring(flat));
+
+  CordzStatistics expected;
+  expected.size = substring->length;
+  expected.estimated_memory_usage = SizeOf(substring) + SizeOf(flat);
+  expected.estimated_fair_share_memory_usage = expected.estimated_memory_usage;
+  expected.node_count = 2;
+  expected.node_counts.flat = 1;
+  expected.node_counts.flat_1k = 1;
+  expected.node_counts.substring = 1;
+
+  EXPECT_THAT(SampleCord(substring), EqStatistics(expected));
+}
+
+TEST(CordzInfoStatisticsTest, SharedSubstring) {
+  RefHelper ref;
+  auto* flat = ref.Ref(Flat(511), 2);
+  auto* substring = ref.Ref(ref.NeedsUnref(Substring(flat)));
+
+  CordzStatistics expected;
+  expected.size = substring->length;
+  expected.estimated_memory_usage = SizeOf(flat) + SizeOf(substring);
+  expected.estimated_fair_share_memory_usage =
+      SizeOf(substring) / 2 + SizeOf(flat) / 6;
+  expected.node_count = 2;
+  expected.node_counts.flat = 1;
+  expected.node_counts.flat_512 = 1;
+  expected.node_counts.substring = 1;
+
+  EXPECT_THAT(SampleCord(substring), EqStatistics(expected));
+}
+
+TEST(CordzInfoStatisticsTest, BtreeLeaf) {
+  ASSERT_THAT(CordRepBtree::kMaxCapacity, Ge(3u));
+  RefHelper ref;
+  auto* flat1 = Flat(2000);
+  auto* flat2 = Flat(200);
+  auto* substr = Substring(flat2);
+  auto* external = External(3000);
+
+  CordRepBtree* tree = CordRepBtree::Create(flat1);
+  tree = CordRepBtree::Append(tree, substr);
+  tree = CordRepBtree::Append(tree, external);
+  size_t flat3_count = CordRepBtree::kMaxCapacity - 3;
+  size_t flat3_size = 0;
+  for (size_t i = 0; i < flat3_count; ++i) {
+    auto* flat3 = Flat(70);
+    flat3_size += SizeOf(flat3);
+    tree = CordRepBtree::Append(tree, flat3);
+  }
+  ref.NeedsUnref(tree);
+
+  CordzStatistics expected;
+  expected.size = tree->length;
+  expected.estimated_memory_usage = SizeOf(tree) + SizeOf(flat1) +
+                                    SizeOf(flat2) + SizeOf(substr) +
+                                    flat3_size + SizeOf(external);
+  expected.estimated_fair_share_memory_usage = expected.estimated_memory_usage;
+  expected.node_count = 1 + 3 + 1 + flat3_count;
+  expected.node_counts.flat = 2 + flat3_count;
+  expected.node_counts.flat_128 = flat3_count;
+  expected.node_counts.flat_256 = 1;
+  expected.node_counts.external = 1;
+  expected.node_counts.substring = 1;
+  expected.node_counts.btree = 1;
+
+  EXPECT_THAT(SampleCord(tree), EqStatistics(expected));
+}
+
+TEST(CordzInfoStatisticsTest, BtreeNodeShared) {
+  RefHelper ref;
+  static constexpr int leaf_count = 3;
+  const size_t flat3_count = CordRepBtree::kMaxCapacity - 3;
+  ASSERT_THAT(flat3_count, Ge(0u));
+
+  CordRepBtree* tree = nullptr;
+  size_t mem_size = 0;
+  for (int i = 0; i < leaf_count; ++i) {
+    auto* flat1 = ref.Ref(Flat(2000), 9);
+    mem_size += SizeOf(flat1);
+    if (i == 0) {
+      tree = CordRepBtree::Create(flat1);
+    } else {
+      tree = CordRepBtree::Append(tree, flat1);
+    }
+
+    auto* flat2 = Flat(200);
+    auto* substr = Substring(flat2);
+    mem_size += SizeOf(flat2) + SizeOf(substr);
+    tree = CordRepBtree::Append(tree, substr);
+
+    auto* external = External(30);
+    mem_size += SizeOf(external);
+    tree = CordRepBtree::Append(tree, external);
+
+    for (size_t i = 0; i < flat3_count; ++i) {
+      auto* flat3 = Flat(70);
+      mem_size += SizeOf(flat3);
+      tree = CordRepBtree::Append(tree, flat3);
+    }
+
+    if (i == 0) {
+      mem_size += SizeOf(tree);
+    } else {
+      mem_size += SizeOf(tree->Edges().back()->btree());
+    }
+  }
+  ref.NeedsUnref(tree);
+
+  // Ref count: 2 for top (add 1), 5 for leaf 0 (add 4).
+  ref.Ref(tree, 1);
+  ref.Ref(tree->Edges().front(), 4);
+
+  CordzStatistics expected;
+  expected.size = tree->length;
+  expected.estimated_memory_usage = SizeOf(tree) + mem_size;
+  expected.estimated_fair_share_memory_usage = FairShare(tree);
+
+  expected.node_count = 1 + leaf_count * (1 + 3 + 1 + flat3_count);
+  expected.node_counts.flat = leaf_count * (2 + flat3_count);
+  expected.node_counts.flat_128 = leaf_count * flat3_count;
+  expected.node_counts.flat_256 = leaf_count;
+  expected.node_counts.external = leaf_count;
+  expected.node_counts.substring = leaf_count;
+  expected.node_counts.btree = 1 + leaf_count;
+
+  EXPECT_THAT(SampleCord(tree), EqStatistics(expected));
+}
+
+TEST(CordzInfoStatisticsTest, Crc) {
+  RefHelper ref;
+  auto* left = Flat(1000);
+  auto* crc = ref.NeedsUnref(CordRepCrc::New(left, {}));
+
+  CordzStatistics expected;
+  expected.size = left->length;
+  expected.estimated_memory_usage = SizeOf(crc) + SizeOf(left);
+  expected.estimated_fair_share_memory_usage = expected.estimated_memory_usage;
+  expected.node_count = 2;
+  expected.node_counts.flat = 1;
+  expected.node_counts.flat_1k = 1;
+  expected.node_counts.crc = 1;
+
+  EXPECT_THAT(SampleCord(crc), EqStatistics(expected));
+}
+
+TEST(CordzInfoStatisticsTest, EmptyCrc) {
+  RefHelper ref;
+  auto* crc = ref.NeedsUnref(CordRepCrc::New(nullptr, {}));
+
+  CordzStatistics expected;
+  expected.size = 0;
+  expected.estimated_memory_usage = SizeOf(crc);
+  expected.estimated_fair_share_memory_usage = expected.estimated_memory_usage;
+  expected.node_count = 1;
+  expected.node_counts.crc = 1;
+
+  EXPECT_THAT(SampleCord(crc), EqStatistics(expected));
+}
+
+TEST(CordzInfoStatisticsTest, ThreadSafety) {
+  Notification stop;
+  static constexpr int kNumThreads = 8;
+  int64_t sampled_node_count = 0;
+
+  {
+    absl::synchronization_internal::ThreadPool pool(kNumThreads);
+
+    // Run analyzer thread emulating a CordzHandler collection.
+    pool.Schedule([&]() {
+      while (!stop.HasBeenNotified()) {
+        // Run every 10us (about 100K total collections).
+        absl::SleepFor(absl::Microseconds(10));
+        CordzSampleToken token;
+        for (const CordzInfo& cord_info : token) {
+          CordzStatistics stats = cord_info.GetCordzStatistics();
+          sampled_node_count += stats.node_count;
+        }
+      }
+    });
+
+    // Run 'application threads'
+    for (int i = 0; i < kNumThreads; ++i) {
+      pool.Schedule([&]() {
+        // Track 0 - 2 cordz infos at a time, providing permutations of 0, 1
+        // and 2 CordzHandle and CordzInfo queues being active, with plenty of
+        // 'empty to non empty' transitions.
+        InlineData cords[2];
+        std::minstd_rand gen;
+        std::uniform_int_distribution<int> coin_toss(0, 1);
+        std::uniform_int_distribution<int> dice_roll(1, 6);
+
+        while (!stop.HasBeenNotified()) {
+          for (InlineData& cord : cords) {
+            // 50/50 flip the state of the cord
+            if (coin_toss(gen) != 0) {
+              if (cord.is_tree()) {
+                // 50/50 simulate delete (untrack) or 'edit to empty'
+                if (coin_toss(gen) != 0) {
+                  CordzInfo::MaybeUntrackCord(cord.cordz_info());
+                } else {
+                  CordzUpdateScope scope(cord.cordz_info(),
+                                         CordzUpdateTracker::kUnknown);
+                  scope.SetCordRep(nullptr);
+                }
+                CordRep::Unref(cord.as_tree());
+                cord.set_inline_size(0);
+              } else {
+                // Coin toss to 50% btree, and 50% flat.
+                CordRep* rep = Flat(256);
+                if (coin_toss(gen) != 0) {
+                  rep = CordRepBtree::Create(rep);
+                }
+
+                // Maybe CRC this cord
+                if (dice_roll(gen) == 6) {
+                  if (dice_roll(gen) == 6) {
+                    // Empty CRC rep
+                    CordRep::Unref(rep);
+                    rep = CordRepCrc::New(nullptr, {});
+                  } else {
+                    // Regular CRC rep
+                    rep = CordRepCrc::New(rep, {});
+                  }
+                }
+                cord.make_tree(rep);
+
+                // 50/50 sample
+                if (coin_toss(gen) != 0) {
+                  CordzInfo::TrackCord(cord, CordzUpdateTracker::kUnknown, 1);
+                }
+              }
+            }
+          }
+        }
+        for (InlineData& cord : cords) {
+          if (cord.is_tree()) {
+            CordzInfo::MaybeUntrackCord(cord.cordz_info());
+            CordRep::Unref(cord.as_tree());
+          }
+        }
+      });
+    }
+
+    // Run for 1 second to give memory and thread safety analyzers plenty of
+    // time to detect any mishaps or undefined behaviors.
+    absl::SleepFor(absl::Seconds(1));
+    stop.Notify();
+  }
+
+  std::cout << "Sampled " << sampled_node_count << " nodes\n";
+}
+
+}  // namespace
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/strings/internal/cordz_info_test.cc b/absl/strings/internal/cordz_info_test.cc
new file mode 100644
index 00000000000..10de80bccb8
--- /dev/null
+++ b/absl/strings/internal/cordz_info_test.cc
@@ -0,0 +1,346 @@
+// Copyright 2019 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/strings/internal/cordz_info.h"
+
+#include <cstddef>
+#include <memory>
+#include <string>
+#include <vector>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+#include "absl/debugging/stacktrace.h"
+#include "absl/debugging/symbolize.h"
+#include "absl/strings/cordz_test_helpers.h"
+#include "absl/strings/internal/cord_internal.h"
+#include "absl/strings/internal/cord_rep_flat.h"
+#include "absl/strings/internal/cordz_handle.h"
+#include "absl/strings/internal/cordz_statistics.h"
+#include "absl/strings/internal/cordz_update_tracker.h"
+#include "absl/strings/str_cat.h"
+#include "absl/types/span.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+namespace {
+
+using ::testing::ElementsAre;
+using ::testing::Eq;
+using ::testing::HasSubstr;
+using ::testing::Ne;
+using ::testing::SizeIs;
+
+// Used test values
+auto constexpr kUnknownMethod = CordzUpdateTracker::kUnknown;
+auto constexpr kTrackCordMethod = CordzUpdateTracker::kConstructorString;
+auto constexpr kChildMethod = CordzUpdateTracker::kConstructorCord;
+auto constexpr kUpdateMethod = CordzUpdateTracker::kAppendString;
+
+// Local less verbose helper
+std::vector<const CordzHandle*> DeleteQueue() {
+  return CordzHandle::DiagnosticsGetDeleteQueue();
+}
+
+std::string FormatStack(absl::Span<void* const> raw_stack) {
+  static constexpr size_t buf_size = 1 << 14;
+  std::unique_ptr<char[]> buf(new char[buf_size]);
+  std::string output;
+  for (void* stackp : raw_stack) {
+    if (absl::Symbolize(stackp, buf.get(), buf_size)) {
+      absl::StrAppend(&output, "    ", buf.get(), "\n");
+    }
+  }
+  return output;
+}
+
+TEST(CordzInfoTest, TrackCord) {
+  TestCordData data;
+  CordzInfo::TrackCord(data.data, kTrackCordMethod, 1);
+  CordzInfo* info = data.data.cordz_info();
+  ASSERT_THAT(info, Ne(nullptr));
+  EXPECT_FALSE(info->is_snapshot());
+  EXPECT_THAT(CordzInfo::Head(CordzSnapshot()), Eq(info));
+  EXPECT_THAT(info->GetCordRepForTesting(), Eq(data.rep.rep));
+  info->Untrack();
+}
+
+TEST(CordzInfoTest, MaybeTrackChildCordWithoutSampling) {
+  CordzSamplingIntervalHelper sample_none(99999);
+  TestCordData parent, child;
+  CordzInfo::MaybeTrackCord(child.data, parent.data, kTrackCordMethod);
+  EXPECT_THAT(child.data.cordz_info(), Eq(nullptr));
+}
+
+TEST(CordzInfoTest, MaybeTrackChildCordWithSampling) {
+  CordzSamplingIntervalHelper sample_all(1);
+  TestCordData parent, child;
+  CordzInfo::MaybeTrackCord(child.data, parent.data, kTrackCordMethod);
+  EXPECT_THAT(child.data.cordz_info(), Eq(nullptr));
+}
+
+TEST(CordzInfoTest, MaybeTrackChildCordWithoutSamplingParentSampled) {
+  CordzSamplingIntervalHelper sample_none(99999);
+  TestCordData parent, child;
+  CordzInfo::TrackCord(parent.data, kTrackCordMethod, 1);
+  CordzInfo::MaybeTrackCord(child.data, parent.data, kTrackCordMethod);
+  CordzInfo* parent_info = parent.data.cordz_info();
+  CordzInfo* child_info = child.data.cordz_info();
+  ASSERT_THAT(child_info, Ne(nullptr));
+  EXPECT_THAT(child_info->GetCordRepForTesting(), Eq(child.rep.rep));
+  EXPECT_THAT(child_info->GetParentStack(), parent_info->GetStack());
+  parent_info->Untrack();
+  child_info->Untrack();
+}
+
+TEST(CordzInfoTest, MaybeTrackChildCordWithoutSamplingChildSampled) {
+  CordzSamplingIntervalHelper sample_none(99999);
+  TestCordData parent, child;
+  CordzInfo::TrackCord(child.data, kTrackCordMethod, 1);
+  CordzInfo::MaybeTrackCord(child.data, parent.data, kTrackCordMethod);
+  EXPECT_THAT(child.data.cordz_info(), Eq(nullptr));
+}
+
+TEST(CordzInfoTest, MaybeTrackChildCordWithSamplingChildSampled) {
+  CordzSamplingIntervalHelper sample_all(1);
+  TestCordData parent, child;
+  CordzInfo::TrackCord(child.data, kTrackCordMethod, 1);
+  CordzInfo::MaybeTrackCord(child.data, parent.data, kTrackCordMethod);
+  EXPECT_THAT(child.data.cordz_info(), Eq(nullptr));
+}
+
+TEST(CordzInfoTest, UntrackCord) {
+  TestCordData data;
+  CordzInfo::TrackCord(data.data, kTrackCordMethod, 1);
+  CordzInfo* info = data.data.cordz_info();
+
+  info->Untrack();
+  EXPECT_THAT(DeleteQueue(), SizeIs(0u));
+}
+
+TEST(CordzInfoTest, UntrackCordWithSnapshot) {
+  TestCordData data;
+  CordzInfo::TrackCord(data.data, kTrackCordMethod, 1);
+  CordzInfo* info = data.data.cordz_info();
+
+  CordzSnapshot snapshot;
+  info->Untrack();
+  EXPECT_THAT(CordzInfo::Head(CordzSnapshot()), Eq(nullptr));
+  EXPECT_THAT(info->GetCordRepForTesting(), Eq(data.rep.rep));
+  EXPECT_THAT(DeleteQueue(), ElementsAre(info, &snapshot));
+}
+
+TEST(CordzInfoTest, SetCordRep) {
+  TestCordData data;
+  CordzInfo::TrackCord(data.data, kTrackCordMethod, 1);
+  CordzInfo* info = data.data.cordz_info();
+
+  TestCordRep rep;
+  info->Lock(CordzUpdateTracker::kAppendCord);
+  info->SetCordRep(rep.rep);
+  info->Unlock();
+  EXPECT_THAT(info->GetCordRepForTesting(), Eq(rep.rep));
+
+  info->Untrack();
+}
+
+TEST(CordzInfoTest, SetCordRepNullUntracksCordOnUnlock) {
+  TestCordData data;
+  CordzInfo::TrackCord(data.data, kTrackCordMethod, 1);
+  CordzInfo* info = data.data.cordz_info();
+
+  info->Lock(CordzUpdateTracker::kAppendString);
+  info->SetCordRep(nullptr);
+  EXPECT_THAT(info->GetCordRepForTesting(), Eq(nullptr));
+  EXPECT_THAT(CordzInfo::Head(CordzSnapshot()), Eq(info));
+
+  info->Unlock();
+  EXPECT_THAT(CordzInfo::Head(CordzSnapshot()), Eq(nullptr));
+}
+
+TEST(CordzInfoTest, RefCordRep) {
+  TestCordData data;
+  CordzInfo::TrackCord(data.data, kTrackCordMethod, 1);
+  CordzInfo* info = data.data.cordz_info();
+
+  size_t refcount = data.rep.rep->refcount.Get();
+  EXPECT_THAT(info->RefCordRep(), Eq(data.rep.rep));
+  EXPECT_THAT(data.rep.rep->refcount.Get(), Eq(refcount + 1));
+  CordRep::Unref(data.rep.rep);
+  info->Untrack();
+}
+
+#if GTEST_HAS_DEATH_TEST
+
+TEST(CordzInfoTest, SetCordRepRequiresMutex) {
+  TestCordData data;
+  CordzInfo::TrackCord(data.data, kTrackCordMethod, 1);
+  CordzInfo* info = data.data.cordz_info();
+  TestCordRep rep;
+  EXPECT_DEBUG_DEATH(info->SetCordRep(rep.rep), ".*");
+  info->Untrack();
+}
+
+#endif  // GTEST_HAS_DEATH_TEST
+
+TEST(CordzInfoTest, TrackUntrackHeadFirstV2) {
+  CordzSnapshot snapshot;
+  EXPECT_THAT(CordzInfo::Head(snapshot), Eq(nullptr));
+
+  TestCordData data;
+  CordzInfo::TrackCord(data.data, kTrackCordMethod, 1);
+  CordzInfo* info1 = data.data.cordz_info();
+  ASSERT_THAT(CordzInfo::Head(snapshot), Eq(info1));
+  EXPECT_THAT(info1->Next(snapshot), Eq(nullptr));
+
+  TestCordData data2;
+  CordzInfo::TrackCord(data2.data, kTrackCordMethod, 1);
+  CordzInfo* info2 = data2.data.cordz_info();
+  ASSERT_THAT(CordzInfo::Head(snapshot), Eq(info2));
+  EXPECT_THAT(info2->Next(snapshot), Eq(info1));
+  EXPECT_THAT(info1->Next(snapshot), Eq(nullptr));
+
+  info2->Untrack();
+  ASSERT_THAT(CordzInfo::Head(snapshot), Eq(info1));
+  EXPECT_THAT(info1->Next(snapshot), Eq(nullptr));
+
+  info1->Untrack();
+  ASSERT_THAT(CordzInfo::Head(snapshot), Eq(nullptr));
+}
+
+TEST(CordzInfoTest, TrackUntrackTailFirstV2) {
+  CordzSnapshot snapshot;
+  EXPECT_THAT(CordzInfo::Head(snapshot), Eq(nullptr));
+
+  TestCordData data;
+  CordzInfo::TrackCord(data.data, kTrackCordMethod, 1);
+  CordzInfo* info1 = data.data.cordz_info();
+  ASSERT_THAT(CordzInfo::Head(snapshot), Eq(info1));
+  EXPECT_THAT(info1->Next(snapshot), Eq(nullptr));
+
+  TestCordData data2;
+  CordzInfo::TrackCord(data2.data, kTrackCordMethod, 1);
+  CordzInfo* info2 = data2.data.cordz_info();
+  ASSERT_THAT(CordzInfo::Head(snapshot), Eq(info2));
+  EXPECT_THAT(info2->Next(snapshot), Eq(info1));
+  EXPECT_THAT(info1->Next(snapshot), Eq(nullptr));
+
+  info1->Untrack();
+  ASSERT_THAT(CordzInfo::Head(snapshot), Eq(info2));
+  EXPECT_THAT(info2->Next(snapshot), Eq(nullptr));
+
+  info2->Untrack();
+  ASSERT_THAT(CordzInfo::Head(snapshot), Eq(nullptr));
+}
+
+TEST(CordzInfoTest, StackV2) {
+  TestCordData data;
+  // kMaxStackDepth is intentionally less than 64 (which is the max depth that
+  // Cordz will record) because if the actual stack depth is over 64
+  // (which it is on Apple platforms) then the expected_stack will end up
+  // catching a few frames at the end that the actual_stack didn't get and
+  // it will no longer be subset. At the time of this writing 58 is the max
+  // that will allow this test to pass (with a minimum os version of iOS 9), so
+  // rounded down to 50 to hopefully not run into this in the future if Apple
+  // makes small modifications to its testing stack. 50 is sufficient to prove
+  // that we got a decent stack.
+  static constexpr int kMaxStackDepth = 50;
+  CordzInfo::TrackCord(data.data, kTrackCordMethod, 1);
+  CordzInfo* info = data.data.cordz_info();
+  std::vector<void*> local_stack;
+  local_stack.resize(kMaxStackDepth);
+  // In some environments we don't get stack traces. For example in Android
+  // absl::GetStackTrace will return 0 indicating it didn't find any stack. The
+  // resultant formatted stack will be "", but that still equals the stack
+  // recorded in CordzInfo, which is also empty. The skip_count is 1 so that the
+  // line number of the current stack isn't included in the HasSubstr check.
+  local_stack.resize(static_cast<size_t>(
+      absl::GetStackTrace(local_stack.data(), kMaxStackDepth,
+                          /*skip_count=*/1)));
+
+  std::string got_stack = FormatStack(info->GetStack());
+  std::string expected_stack = FormatStack(local_stack);
+  // If TrackCord is inlined, got_stack should match expected_stack. If it isn't
+  // inlined, got_stack should include an additional frame not present in
+  // expected_stack. Either way, expected_stack should be a substring of
+  // got_stack.
+  EXPECT_THAT(got_stack, HasSubstr(expected_stack));
+
+  info->Untrack();
+}
+
+// Local helper functions to get different stacks for child and parent.
+CordzInfo* TrackChildCord(InlineData& data, const InlineData& parent) {
+  CordzInfo::TrackCord(data, parent, kChildMethod);
+  return data.cordz_info();
+}
+CordzInfo* TrackParentCord(InlineData& data) {
+  CordzInfo::TrackCord(data, kTrackCordMethod, 1);
+  return data.cordz_info();
+}
+
+TEST(CordzInfoTest, GetStatistics) {
+  TestCordData data;
+  CordzInfo* info = TrackParentCord(data.data);
+
+  CordzStatistics statistics = info->GetCordzStatistics();
+  EXPECT_THAT(statistics.size, Eq(data.rep.rep->length));
+  EXPECT_THAT(statistics.method, Eq(kTrackCordMethod));
+  EXPECT_THAT(statistics.parent_method, Eq(kUnknownMethod));
+  EXPECT_THAT(statistics.update_tracker.Value(kTrackCordMethod), Eq(1));
+
+  info->Untrack();
+}
+
+TEST(CordzInfoTest, LockCountsMethod) {
+  TestCordData data;
+  CordzInfo* info = TrackParentCord(data.data);
+
+  info->Lock(kUpdateMethod);
+  info->Unlock();
+  info->Lock(kUpdateMethod);
+  info->Unlock();
+
+  CordzStatistics statistics = info->GetCordzStatistics();
+  EXPECT_THAT(statistics.update_tracker.Value(kUpdateMethod), Eq(2));
+
+  info->Untrack();
+}
+
+TEST(CordzInfoTest, FromParent) {
+  TestCordData parent;
+  TestCordData child;
+  CordzInfo* info_parent = TrackParentCord(parent.data);
+  CordzInfo* info_child = TrackChildCord(child.data, parent.data);
+
+  std::string stack = FormatStack(info_parent->GetStack());
+  std::string parent_stack = FormatStack(info_child->GetParentStack());
+  EXPECT_THAT(stack, Eq(parent_stack));
+
+  CordzStatistics statistics = info_child->GetCordzStatistics();
+  EXPECT_THAT(statistics.size, Eq(child.rep.rep->length));
+  EXPECT_THAT(statistics.method, Eq(kChildMethod));
+  EXPECT_THAT(statistics.parent_method, Eq(kTrackCordMethod));
+  EXPECT_THAT(statistics.update_tracker.Value(kChildMethod), Eq(1));
+
+  info_parent->Untrack();
+  info_child->Untrack();
+}
+
+}  // namespace
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/strings/internal/cordz_sample_token.cc b/absl/strings/internal/cordz_sample_token.cc
new file mode 100644
index 00000000000..ba1270d8f01
--- /dev/null
+++ b/absl/strings/internal/cordz_sample_token.cc
@@ -0,0 +1,64 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/strings/internal/cordz_sample_token.h"
+
+#include "absl/base/config.h"
+#include "absl/strings/internal/cordz_handle.h"
+#include "absl/strings/internal/cordz_info.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+
+CordzSampleToken::Iterator& CordzSampleToken::Iterator::operator++() {
+  if (current_) {
+    current_ = current_->Next(*token_);
+  }
+  return *this;
+}
+
+CordzSampleToken::Iterator CordzSampleToken::Iterator::operator++(int) {
+  Iterator it(*this);
+  operator++();
+  return it;
+}
+
+bool operator==(const CordzSampleToken::Iterator& lhs,
+                const CordzSampleToken::Iterator& rhs) {
+  return lhs.current_ == rhs.current_ &&
+         (lhs.current_ == nullptr || lhs.token_ == rhs.token_);
+}
+
+bool operator!=(const CordzSampleToken::Iterator& lhs,
+                const CordzSampleToken::Iterator& rhs) {
+  return !(lhs == rhs);
+}
+
+CordzSampleToken::Iterator::reference CordzSampleToken::Iterator::operator*()
+    const {
+  return *current_;
+}
+
+CordzSampleToken::Iterator::pointer CordzSampleToken::Iterator::operator->()
+    const {
+  return current_;
+}
+
+CordzSampleToken::Iterator::Iterator(const CordzSampleToken* token)
+    : token_(token), current_(CordzInfo::Head(*token)) {}
+
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/strings/internal/cordz_sample_token.h b/absl/strings/internal/cordz_sample_token.h
new file mode 100644
index 00000000000..aaf260e7ed6
--- /dev/null
+++ b/absl/strings/internal/cordz_sample_token.h
@@ -0,0 +1,100 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <cstddef>
+#include <iterator>
+
+#include "absl/base/config.h"
+#include "absl/strings/internal/cordz_handle.h"
+#include "absl/strings/internal/cordz_info.h"
+
+#ifndef ABSL_STRINGS_INTERNAL_CORDZ_SAMPLE_TOKEN_H_
+#define ABSL_STRINGS_INTERNAL_CORDZ_SAMPLE_TOKEN_H_
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+
+// The existence of a CordzSampleToken guarantees that a reader can traverse the
+// global_cordz_infos_head linked-list without needing to hold a mutex. When a
+// CordzSampleToken exists, all CordzInfo objects that would be destroyed are
+// instead appended to a deletion queue. When the CordzSampleToken is destroyed,
+// it will also clean up any of these CordzInfo objects.
+//
+// E.g., ST are CordzSampleToken objects and CH are CordzHandle objects.
+//   ST1 <- CH1 <- CH2 <- ST2 <- CH3 <- global_delete_queue_tail
+//
+// This list tracks that CH1 and CH2 were created after ST1, so the thread
+// holding ST1 might have a reference to CH1, CH2, ST2, and CH3. However, ST2
+// was created later, so the thread holding the ST2 token cannot have a
+// reference to ST1, CH1, or CH2. If ST1 is cleaned up first, that thread will
+// delete ST1, CH1, and CH2. If instead ST2 is cleaned up first, that thread
+// will only delete ST2.
+//
+// If ST1 is cleaned up first, the new list will be:
+//   ST2 <- CH3 <- global_delete_queue_tail
+//
+// If ST2 is cleaned up first, the new list will be:
+//   ST1 <- CH1 <- CH2 <- CH3 <- global_delete_queue_tail
+//
+// All new CordzHandle objects are appended to the list, so if a new thread
+// comes along before either ST1 or ST2 are cleaned up, the new list will be:
+//   ST1 <- CH1 <- CH2 <- ST2 <- CH3 <- ST3 <- global_delete_queue_tail
+//
+// A thread must hold the global_delete_queue_mu mutex whenever it's altering
+// this list.
+//
+// It is safe for thread that holds a CordzSampleToken to read
+// global_cordz_infos at any time since the objects it is able to retrieve will
+// not be deleted while the CordzSampleToken exists.
+class CordzSampleToken : public CordzSnapshot {
+ public:
+  class Iterator {
+   public:
+    using iterator_category = std::input_iterator_tag;
+    using value_type = const CordzInfo&;
+    using difference_type = ptrdiff_t;
+    using pointer = const CordzInfo*;
+    using reference = value_type;
+
+    Iterator() = default;
+
+    Iterator& operator++();
+    Iterator operator++(int);
+    friend bool operator==(const Iterator& lhs, const Iterator& rhs);
+    friend bool operator!=(const Iterator& lhs, const Iterator& rhs);
+    reference operator*() const;
+    pointer operator->() const;
+
+   private:
+    friend class CordzSampleToken;
+    explicit Iterator(const CordzSampleToken* token);
+
+    const CordzSampleToken* token_ = nullptr;
+    pointer current_ = nullptr;
+  };
+
+  CordzSampleToken() = default;
+  CordzSampleToken(const CordzSampleToken&) = delete;
+  CordzSampleToken& operator=(const CordzSampleToken&) = delete;
+
+  Iterator begin() { return Iterator(this); }
+  Iterator end() { return Iterator(); }
+};
+
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STRINGS_INTERNAL_CORDZ_SAMPLE_TOKEN_H_
diff --git a/absl/strings/internal/cordz_sample_token_test.cc b/absl/strings/internal/cordz_sample_token_test.cc
new file mode 100644
index 00000000000..8386b2cf4da
--- /dev/null
+++ b/absl/strings/internal/cordz_sample_token_test.cc
@@ -0,0 +1,204 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/strings/internal/cordz_sample_token.h"
+
+#include <cstddef>
+#include <iterator>
+#include <memory>
+#include <type_traits>
+#include <vector>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/memory/memory.h"
+#include "absl/random/random.h"
+#include "absl/strings/cordz_test_helpers.h"
+#include "absl/strings/internal/cord_rep_flat.h"
+#include "absl/strings/internal/cordz_handle.h"
+#include "absl/strings/internal/cordz_info.h"
+#include "absl/strings/internal/cordz_update_tracker.h"
+#include "absl/synchronization/internal/thread_pool.h"
+#include "absl/synchronization/notification.h"
+#include "absl/time/clock.h"
+#include "absl/time/time.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+namespace {
+
+using ::testing::ElementsAre;
+using ::testing::Eq;
+using ::testing::Ne;
+
+// Used test values
+auto constexpr kTrackCordMethod = CordzUpdateTracker::kConstructorString;
+
+TEST(CordzSampleTokenTest, IteratorTraits) {
+  static_assert(std::is_copy_constructible_v<CordzSampleToken::Iterator>);
+  static_assert(std::is_copy_assignable_v<CordzSampleToken::Iterator>);
+  static_assert(std::is_move_constructible_v<CordzSampleToken::Iterator>);
+  static_assert(std::is_move_assignable_v<CordzSampleToken::Iterator>);
+  static_assert(
+      std::is_same_v<
+          std::iterator_traits<CordzSampleToken::Iterator>::iterator_category,
+          std::input_iterator_tag>);
+  static_assert(std::is_same_v<
+                std::iterator_traits<CordzSampleToken::Iterator>::value_type,
+                const CordzInfo&>);
+  static_assert(
+      std::is_same_v<
+          std::iterator_traits<CordzSampleToken::Iterator>::difference_type,
+          ptrdiff_t>);
+  static_assert(
+      std::is_same_v<std::iterator_traits<CordzSampleToken::Iterator>::pointer,
+                     const CordzInfo*>);
+  static_assert(std::is_same_v<
+                std::iterator_traits<CordzSampleToken::Iterator>::reference,
+                const CordzInfo&>);
+}
+
+TEST(CordzSampleTokenTest, IteratorEmpty) {
+  CordzSampleToken token;
+  EXPECT_THAT(token.begin(), Eq(token.end()));
+}
+
+TEST(CordzSampleTokenTest, Iterator) {
+  TestCordData cord1, cord2, cord3;
+  CordzInfo::TrackCord(cord1.data, kTrackCordMethod, 1);
+  CordzInfo* info1 = cord1.data.cordz_info();
+  CordzInfo::TrackCord(cord2.data, kTrackCordMethod, 1);
+  CordzInfo* info2 = cord2.data.cordz_info();
+  CordzInfo::TrackCord(cord3.data, kTrackCordMethod, 1);
+  CordzInfo* info3 = cord3.data.cordz_info();
+
+  CordzSampleToken token;
+  std::vector<const CordzInfo*> found;
+  for (const CordzInfo& cord_info : token) {
+    found.push_back(&cord_info);
+  }
+
+  EXPECT_THAT(found, ElementsAre(info3, info2, info1));
+
+  info1->Untrack();
+  info2->Untrack();
+  info3->Untrack();
+}
+
+TEST(CordzSampleTokenTest, IteratorEquality) {
+  TestCordData cord1;
+  TestCordData cord2;
+  TestCordData cord3;
+  CordzInfo::TrackCord(cord1.data, kTrackCordMethod, 1);
+  CordzInfo* info1 = cord1.data.cordz_info();
+
+  CordzSampleToken token1;
+  // lhs starts with the CordzInfo corresponding to cord1 at the head.
+  CordzSampleToken::Iterator lhs = token1.begin();
+
+  CordzInfo::TrackCord(cord2.data, kTrackCordMethod, 1);
+  CordzInfo* info2 = cord2.data.cordz_info();
+
+  CordzSampleToken token2;
+  // rhs starts with the CordzInfo corresponding to cord2 at the head.
+  CordzSampleToken::Iterator rhs = token2.begin();
+
+  CordzInfo::TrackCord(cord3.data, kTrackCordMethod, 1);
+  CordzInfo* info3 = cord3.data.cordz_info();
+
+  // lhs is on cord1 while rhs is on cord2.
+  EXPECT_THAT(lhs, Ne(rhs));
+
+  rhs++;
+  // lhs and rhs are both on cord1, but they didn't come from the same
+  // CordzSampleToken.
+  EXPECT_THAT(lhs, Ne(rhs));
+
+  lhs++;
+  rhs++;
+  // Both lhs and rhs are done, so they are on nullptr.
+  EXPECT_THAT(lhs, Eq(rhs));
+
+  info1->Untrack();
+  info2->Untrack();
+  info3->Untrack();
+}
+
+TEST(CordzSampleTokenTest, MultiThreaded) {
+  Notification stop;
+  static constexpr int kNumThreads = 4;
+  static constexpr int kNumCords = 3;
+  static constexpr int kNumTokens = 3;
+  absl::synchronization_internal::ThreadPool pool(kNumThreads);
+
+  for (int i = 0; i < kNumThreads; ++i) {
+    pool.Schedule([&stop]() {
+      absl::BitGen gen;
+      TestCordData cords[kNumCords];
+      std::unique_ptr<CordzSampleToken> tokens[kNumTokens];
+
+      while (!stop.HasBeenNotified()) {
+        // Randomly perform one of five actions:
+        //   1) Untrack
+        //   2) Track
+        //   3) Iterate over Cords visible to a token.
+        //   4) Unsample
+        //   5) Sample
+        int index = absl::Uniform(gen, 0, kNumCords);
+        if (absl::Bernoulli(gen, 0.5)) {
+          TestCordData& cord = cords[index];
+          // Track/untrack.
+          if (cord.data.is_profiled()) {
+            // 1) Untrack
+            cord.data.cordz_info()->Untrack();
+            cord.data.clear_cordz_info();
+          } else {
+            // 2) Track
+            CordzInfo::TrackCord(cord.data, kTrackCordMethod, 1);
+          }
+        } else {
+          std::unique_ptr<CordzSampleToken>& token = tokens[index];
+          if (token) {
+            if (absl::Bernoulli(gen, 0.5)) {
+              // 3) Iterate over Cords visible to a token.
+              for (const CordzInfo& info : *token) {
+                // This is trivial work to allow us to compile the loop.
+                EXPECT_THAT(info.Next(*token), Ne(&info));
+              }
+            } else {
+              // 4) Unsample
+              token = nullptr;
+            }
+          } else {
+            // 5) Sample
+            token = std::make_unique<CordzSampleToken>();
+          }
+        }
+      }
+      for (TestCordData& cord : cords) {
+        CordzInfo::MaybeUntrackCord(cord.data.cordz_info());
+      }
+    });
+  }
+  // The threads will hammer away.  Give it a little bit of time for tsan to
+  // spot errors.
+  absl::SleepFor(absl::Seconds(3));
+  stop.Notify();
+}
+
+}  // namespace
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/strings/internal/cordz_statistics.h b/absl/strings/internal/cordz_statistics.h
new file mode 100644
index 00000000000..c024d11b9e4
--- /dev/null
+++ b/absl/strings/internal/cordz_statistics.h
@@ -0,0 +1,89 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_STRINGS_INTERNAL_CORDZ_STATISTICS_H_
+#define ABSL_STRINGS_INTERNAL_CORDZ_STATISTICS_H_
+
+#include <cstddef>
+#include <cstdint>
+
+#include "absl/base/config.h"
+#include "absl/strings/internal/cordz_update_tracker.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+
+// CordzStatistics captures some meta information about a Cord's shape.
+struct CordzStatistics {
+  using MethodIdentifier = CordzUpdateTracker::MethodIdentifier;
+
+  // Node counts information
+  struct NodeCounts {
+    size_t flat = 0;       // #flats
+    size_t flat_64 = 0;    // #flats up to 64 bytes
+    size_t flat_128 = 0;   // #flats up to 128 bytes
+    size_t flat_256 = 0;   // #flats up to 256 bytes
+    size_t flat_512 = 0;   // #flats up to 512 bytes
+    size_t flat_1k = 0;    // #flats up to 1K bytes
+    size_t external = 0;   // #external reps
+    size_t substring = 0;  // #substring reps
+    size_t concat = 0;     // #concat reps
+    size_t ring = 0;       // #ring buffer reps
+    size_t btree = 0;      // #btree reps
+    size_t crc = 0;        // #crc reps
+  };
+
+  // The size of the cord in bytes. This matches the result of Cord::size().
+  size_t size = 0;
+
+  // The estimated memory used by the sampled cord. This value matches the
+  // value as reported by Cord::EstimatedMemoryUsage().
+  // A value of 0 implies the property has not been recorded.
+  size_t estimated_memory_usage = 0;
+
+  // The effective memory used by the sampled cord, inversely weighted by the
+  // effective indegree of each allocated node. This is a representation of the
+  // fair share of memory usage that should be attributed to the sampled cord.
+  // This value is more useful for cases where one or more nodes are referenced
+  // by multiple Cord instances, and for cases where a Cord includes the same
+  // node multiple times (either directly or indirectly).
+  // A value of 0 implies the property has not been recorded.
+  size_t estimated_fair_share_memory_usage = 0;
+
+  // The total number of nodes referenced by this cord.
+  // For ring buffer Cords, this includes the 'ring buffer' node.
+  // For btree Cords, this includes all 'CordRepBtree' tree nodes as well as all
+  // the substring, flat and external nodes referenced by the tree.
+  // A value of 0 implies the property has not been recorded.
+  size_t node_count = 0;
+
+  // Detailed node counts per type
+  NodeCounts node_counts;
+
+  // The cord method responsible for sampling the cord.
+  MethodIdentifier method = MethodIdentifier::kUnknown;
+
+  // The cord method responsible for sampling the parent cord if applicable.
+  MethodIdentifier parent_method = MethodIdentifier::kUnknown;
+
+  // Update tracker tracking invocation count per cord method.
+  CordzUpdateTracker update_tracker;
+};
+
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STRINGS_INTERNAL_CORDZ_STATISTICS_H_
diff --git a/absl/strings/internal/cordz_update_scope.h b/absl/strings/internal/cordz_update_scope.h
new file mode 100644
index 00000000000..57ba75de93c
--- /dev/null
+++ b/absl/strings/internal/cordz_update_scope.h
@@ -0,0 +1,71 @@
+// Copyright 2021 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_STRINGS_INTERNAL_CORDZ_UPDATE_SCOPE_H_
+#define ABSL_STRINGS_INTERNAL_CORDZ_UPDATE_SCOPE_H_
+
+#include "absl/base/config.h"
+#include "absl/base/optimization.h"
+#include "absl/base/thread_annotations.h"
+#include "absl/strings/internal/cord_internal.h"
+#include "absl/strings/internal/cordz_info.h"
+#include "absl/strings/internal/cordz_update_tracker.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+
+// CordzUpdateScope scopes an update to the provided CordzInfo.
+// The class invokes `info->Lock(method)` and `info->Unlock()` to guard
+// cordrep updates. This class does nothing if `info` is null.
+// See also the 'Lock`, `Unlock` and `SetCordRep` methods in `CordzInfo`.
+class ABSL_SCOPED_LOCKABLE CordzUpdateScope {
+ public:
+  CordzUpdateScope(CordzInfo* info, CordzUpdateTracker::MethodIdentifier method)
+      ABSL_EXCLUSIVE_LOCK_FUNCTION(info)
+      : info_(info) {
+    if (ABSL_PREDICT_FALSE(info_)) {
+      info->Lock(method);
+    }
+  }
+
+  // CordzUpdateScope can not be copied or assigned to.
+  CordzUpdateScope(CordzUpdateScope&& rhs) = delete;
+  CordzUpdateScope(const CordzUpdateScope&) = delete;
+  CordzUpdateScope& operator=(CordzUpdateScope&& rhs) = delete;
+  CordzUpdateScope& operator=(const CordzUpdateScope&) = delete;
+
+  ~CordzUpdateScope() ABSL_UNLOCK_FUNCTION() {
+    if (ABSL_PREDICT_FALSE(info_)) {
+      info_->Unlock();
+    }
+  }
+
+  void SetCordRep(CordRep* rep) const {
+    if (ABSL_PREDICT_FALSE(info_)) {
+      info_->SetCordRep(rep);
+    }
+  }
+
+  CordzInfo* info() const { return info_; }
+
+ private:
+  CordzInfo* info_;
+};
+
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STRINGS_INTERNAL_CORDZ_UPDATE_SCOPE_H_
diff --git a/absl/strings/internal/cordz_update_scope_test.cc b/absl/strings/internal/cordz_update_scope_test.cc
new file mode 100644
index 00000000000..1b4701f13af
--- /dev/null
+++ b/absl/strings/internal/cordz_update_scope_test.cc
@@ -0,0 +1,49 @@
+// Copyright 2021 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/strings/internal/cordz_update_scope.h"
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+#include "absl/strings/cordz_test_helpers.h"
+#include "absl/strings/internal/cord_rep_flat.h"
+#include "absl/strings/internal/cordz_info.h"
+#include "absl/strings/internal/cordz_update_tracker.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+
+namespace {
+
+// Used test values
+auto constexpr kTrackCordMethod = CordzUpdateTracker::kConstructorString;
+
+TEST(CordzUpdateScopeTest, ScopeNullptr) {
+  CordzUpdateScope scope(nullptr, kTrackCordMethod);
+}
+
+TEST(CordzUpdateScopeTest, ScopeSampledCord) {
+  TestCordData cord;
+  CordzInfo::TrackCord(cord.data, kTrackCordMethod, 1);
+  CordzUpdateScope scope(cord.data.cordz_info(), kTrackCordMethod);
+  cord.data.cordz_info()->SetCordRep(nullptr);
+}
+
+}  // namespace
+ABSL_NAMESPACE_END
+}  // namespace cord_internal
+
+}  // namespace absl
diff --git a/absl/strings/internal/cordz_update_tracker.h b/absl/strings/internal/cordz_update_tracker.h
new file mode 100644
index 00000000000..c5170662bf4
--- /dev/null
+++ b/absl/strings/internal/cordz_update_tracker.h
@@ -0,0 +1,123 @@
+// Copyright 2021 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_STRINGS_INTERNAL_CORDZ_UPDATE_TRACKER_H_
+#define ABSL_STRINGS_INTERNAL_CORDZ_UPDATE_TRACKER_H_
+
+#include <atomic>
+#include <cstdint>
+
+#include "absl/base/config.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+
+// CordzUpdateTracker tracks counters for Cord update methods.
+//
+// The purpose of CordzUpdateTracker is to track the number of calls to methods
+// updating Cord data for sampled cords. The class internally uses 'lossy'
+// atomic operations: Cord is thread-compatible, so there is no need to
+// synchronize updates. However, Cordz collection threads may call 'Value()' at
+// any point, so the class needs to provide thread safe access.
+//
+// This class is thread-safe. But as per above comments, all non-const methods
+// should be used single-threaded only: updates are thread-safe but lossy.
+class CordzUpdateTracker {
+ public:
+  // Tracked update methods.
+  enum MethodIdentifier {
+    kUnknown,
+    kAppendCord,
+    kAppendCordBuffer,
+    kAppendExternalMemory,
+    kAppendString,
+    kAssignCord,
+    kAssignString,
+    kClear,
+    kConstructorCord,
+    kConstructorString,
+    kCordReader,
+    kFlatten,
+    kGetAppendBuffer,
+    kGetAppendRegion,
+    kMakeCordFromExternal,
+    kMoveAppendCord,
+    kMoveAssignCord,
+    kMovePrependCord,
+    kPrependCord,
+    kPrependCordBuffer,
+    kPrependString,
+    kRemovePrefix,
+    kRemoveSuffix,
+    kSetExpectedChecksum,
+    kSubCord,
+
+    // kNumMethods defines the number of entries: must be the last entry.
+    kNumMethods,
+  };
+
+  // Constructs a new instance. All counters are zero-initialized.
+  constexpr CordzUpdateTracker() noexcept : values_{} {}
+
+  // Copy constructs a new instance.
+  CordzUpdateTracker(const CordzUpdateTracker& rhs) noexcept { *this = rhs; }
+
+  // Assigns the provided value to this instance.
+  CordzUpdateTracker& operator=(const CordzUpdateTracker& rhs) noexcept {
+    for (int i = 0; i < kNumMethods; ++i) {
+      values_[i].store(rhs.values_[i].load(std::memory_order_relaxed),
+                       std::memory_order_relaxed);
+    }
+    return *this;
+  }
+
+  // Returns the value for the specified method.
+  int64_t Value(MethodIdentifier method) const {
+    return values_[method].load(std::memory_order_relaxed);
+  }
+
+  // Increases the value for the specified method by `n`
+  void LossyAdd(MethodIdentifier method, int64_t n = 1) {
+    auto& value = values_[method];
+    value.store(value.load(std::memory_order_relaxed) + n,
+                std::memory_order_relaxed);
+  }
+
+  // Adds all the values from `src` to this instance
+  void LossyAdd(const CordzUpdateTracker& src) {
+    for (int i = 0; i < kNumMethods; ++i) {
+      MethodIdentifier method = static_cast<MethodIdentifier>(i);
+      if (int64_t value = src.Value(method)) {
+        LossyAdd(method, value);
+      }
+    }
+  }
+
+ private:
+  // Until C++20 std::atomic is not constexpr default-constructible, so we need
+  // a wrapper for this class to be constexpr constructible.
+  class Counter : public std::atomic<int64_t> {
+   public:
+    constexpr Counter() noexcept : std::atomic<int64_t>(0) {}
+  };
+
+  Counter values_[kNumMethods];
+};
+
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STRINGS_INTERNAL_CORDZ_UPDATE_TRACKER_H_
diff --git a/absl/strings/internal/cordz_update_tracker_test.cc b/absl/strings/internal/cordz_update_tracker_test.cc
new file mode 100644
index 00000000000..fb89a37385a
--- /dev/null
+++ b/absl/strings/internal/cordz_update_tracker_test.cc
@@ -0,0 +1,148 @@
+// Copyright 2021 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/strings/internal/cordz_update_tracker.h"
+
+#include <array>
+#include <cstdint>
+#include <thread>  // NOLINT
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/synchronization/notification.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+namespace {
+
+using ::testing::AnyOf;
+using ::testing::Eq;
+
+using Method = CordzUpdateTracker::MethodIdentifier;
+using Methods = std::array<Method, Method::kNumMethods>;
+
+// Returns an array of all methods defined in `MethodIdentifier`
+Methods AllMethods() {
+  return Methods{Method::kUnknown,
+                 Method::kAppendCord,
+                 Method::kAppendCordBuffer,
+                 Method::kAppendExternalMemory,
+                 Method::kAppendString,
+                 Method::kAssignCord,
+                 Method::kAssignString,
+                 Method::kClear,
+                 Method::kConstructorCord,
+                 Method::kConstructorString,
+                 Method::kCordReader,
+                 Method::kFlatten,
+                 Method::kGetAppendBuffer,
+                 Method::kGetAppendRegion,
+                 Method::kMakeCordFromExternal,
+                 Method::kMoveAppendCord,
+                 Method::kMoveAssignCord,
+                 Method::kMovePrependCord,
+                 Method::kPrependCord,
+                 Method::kPrependCordBuffer,
+                 Method::kPrependString,
+                 Method::kRemovePrefix,
+                 Method::kRemoveSuffix,
+                 Method::kSetExpectedChecksum,
+                 Method::kSubCord};
+}
+
+TEST(CordzUpdateTracker, IsConstExprAndInitializesToZero) {
+  constexpr CordzUpdateTracker tracker;
+  for (Method method : AllMethods()) {
+    ASSERT_THAT(tracker.Value(method), Eq(0));
+  }
+}
+
+TEST(CordzUpdateTracker, LossyAdd) {
+  int64_t n = 1;
+  CordzUpdateTracker tracker;
+  for (Method method : AllMethods()) {
+    tracker.LossyAdd(method, n);
+    EXPECT_THAT(tracker.Value(method), Eq(n));
+    n += 2;
+  }
+}
+
+TEST(CordzUpdateTracker, CopyConstructor) {
+  int64_t n = 1;
+  CordzUpdateTracker src;
+  for (Method method : AllMethods()) {
+    src.LossyAdd(method, n);
+    n += 2;
+  }
+
+  n = 1;
+  CordzUpdateTracker tracker(src);
+  for (Method method : AllMethods()) {
+    EXPECT_THAT(tracker.Value(method), Eq(n));
+    n += 2;
+  }
+}
+
+TEST(CordzUpdateTracker, OperatorAssign) {
+  int64_t n = 1;
+  CordzUpdateTracker src;
+  CordzUpdateTracker tracker;
+  for (Method method : AllMethods()) {
+    src.LossyAdd(method, n);
+    n += 2;
+  }
+
+  n = 1;
+  tracker = src;
+  for (Method method : AllMethods()) {
+    EXPECT_THAT(tracker.Value(method), Eq(n));
+    n += 2;
+  }
+}
+
+TEST(CordzUpdateTracker, ThreadSanitizedValueCheck) {
+  absl::Notification done;
+  CordzUpdateTracker tracker;
+
+  std::thread reader([&done, &tracker] {
+    while (!done.HasBeenNotified()) {
+      int n = 1;
+      for (Method method : AllMethods()) {
+        EXPECT_THAT(tracker.Value(method), AnyOf(Eq(n), Eq(0)));
+        n += 2;
+      }
+    }
+    int n = 1;
+    for (Method method : AllMethods()) {
+      EXPECT_THAT(tracker.Value(method), Eq(n));
+      n += 2;
+    }
+  });
+
+  int64_t n = 1;
+  for (Method method : AllMethods()) {
+    tracker.LossyAdd(method, n);
+    n += 2;
+  }
+  done.Notify();
+  reader.join();
+}
+
+}  // namespace
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/strings/internal/damerau_levenshtein_distance.cc b/absl/strings/internal/damerau_levenshtein_distance.cc
new file mode 100644
index 00000000000..70d1c0afa8b
--- /dev/null
+++ b/absl/strings/internal/damerau_levenshtein_distance.cc
@@ -0,0 +1,99 @@
+// Copyright 2022 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/strings/internal/damerau_levenshtein_distance.h"
+
+#include <algorithm>
+#include <array>
+#include <cstddef>
+#include <cstdint>
+#include <numeric>
+#include <utility>
+
+#include "absl/base/config.h"
+#include "absl/strings/match.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace strings_internal {
+// Calculate DamerauLevenshtein (adjacent transpositions) distance
+// between two strings,
+// https://en.wikipedia.org/wiki/Damerau%E2%80%93Levenshtein_distance. The
+// algorithm follows the condition that no substring is edited more than once.
+// While this can reduce is larger distance, it's a) a much simpler algorithm
+// and b) more realistic for the case that typographic mistakes should be
+// detected.
+// When the distance is larger than cutoff, or one of the strings has more
+// than MAX_SIZE=100 characters, the code returns min(MAX_SIZE, cutoff) + 1.
+uint8_t CappedDamerauLevenshteinDistance(absl::string_view s1,
+                                         absl::string_view s2, uint8_t cutoff) {
+  const uint8_t MAX_SIZE = 100;
+  const uint8_t _cutoff = std::min(MAX_SIZE, cutoff);
+  const uint8_t cutoff_plus_1 = static_cast<uint8_t>(_cutoff + 1);
+
+  if (s1.size() > s2.size()) std::swap(s1, s2);
+  if (s1.size() + _cutoff < s2.size() || s2.size() > MAX_SIZE)
+    return cutoff_plus_1;
+
+  if (s1.empty())
+    return static_cast<uint8_t>(s2.size());
+
+  // Lower diagonal bound: y = x - lower_diag
+  const uint8_t lower_diag =
+      _cutoff - static_cast<uint8_t>(s2.size() - s1.size());
+  // Upper diagonal bound: y = x + upper_diag
+  const uint8_t upper_diag = _cutoff;
+
+  // d[i][j] is the number of edits required to convert s1[0, i] to s2[0, j]
+  std::array<std::array<uint8_t, MAX_SIZE + 2>, MAX_SIZE + 2> d;
+  std::iota(d[0].begin(), d[0].begin() + upper_diag + 1, 0);
+  d[0][cutoff_plus_1] = cutoff_plus_1;
+  for (size_t i = 1; i <= s1.size(); ++i) {
+    // Deduce begin of relevant window.
+    size_t j_begin = 1;
+    if (i > lower_diag) {
+      j_begin = i - lower_diag;
+      d[i][j_begin - 1] = cutoff_plus_1;
+    } else {
+      d[i][0] = static_cast<uint8_t>(i);
+    }
+
+    // Deduce end of relevant window.
+    size_t j_end = i + upper_diag;
+    if (j_end > s2.size()) {
+      j_end = s2.size();
+    } else {
+      d[i][j_end + 1] = cutoff_plus_1;
+    }
+
+    for (size_t j = j_begin; j <= j_end; ++j) {
+      const uint8_t deletion_distance = d[i - 1][j] + 1;
+      const uint8_t insertion_distance = d[i][j - 1] + 1;
+      const uint8_t mismatched_tail_cost = s1[i - 1] == s2[j - 1] ? 0 : 1;
+      const uint8_t mismatch_distance = d[i - 1][j - 1] + mismatched_tail_cost;
+      uint8_t transposition_distance = _cutoff + 1;
+      if (i > 1 && j > 1 && s1[i - 1] == s2[j - 2] && s1[i - 2] == s2[j - 1])
+        transposition_distance = d[i - 2][j - 2] + 1;
+      d[i][j] = std::min({cutoff_plus_1, deletion_distance, insertion_distance,
+                          mismatch_distance, transposition_distance});
+    }
+  }
+  return d[s1.size()][s2.size()];
+}
+
+}  // namespace strings_internal
+
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/strings/internal/damerau_levenshtein_distance.h b/absl/strings/internal/damerau_levenshtein_distance.h
new file mode 100644
index 00000000000..600386a18b4
--- /dev/null
+++ b/absl/strings/internal/damerau_levenshtein_distance.h
@@ -0,0 +1,35 @@
+// Copyright 2022 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_STRINGS_INTERNAL_DAMERAU_LEVENSHTEIN_DISTANCE_H_
+#define ABSL_STRINGS_INTERNAL_DAMERAU_LEVENSHTEIN_DISTANCE_H_
+
+#include <cstdint>
+
+#include "absl/base/config.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace strings_internal {
+// Calculate DamerauLevenshtein distance between two strings.
+// When the distance is larger than cutoff, the code just returns cutoff + 1.
+uint8_t CappedDamerauLevenshteinDistance(absl::string_view s1,
+                                         absl::string_view s2, uint8_t cutoff);
+
+}  // namespace strings_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STRINGS_INTERNAL_DAMERAU_LEVENSHTEIN_DISTANCE_H_
diff --git a/absl/strings/internal/damerau_levenshtein_distance_benchmark.cc b/absl/strings/internal/damerau_levenshtein_distance_benchmark.cc
new file mode 100644
index 00000000000..e10c05590ac
--- /dev/null
+++ b/absl/strings/internal/damerau_levenshtein_distance_benchmark.cc
@@ -0,0 +1,57 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <cstddef>
+#include <string>
+
+#include "absl/strings/internal/damerau_levenshtein_distance.h"
+#include "benchmark/benchmark.h"
+
+namespace {
+
+std::string MakeTestString(int desired_length, int num_edits) {
+  std::string test(desired_length, 'x');
+  for (int i = 0; (i < num_edits) && (i * 8 < desired_length); ++i) {
+    test[i * 8] = 'y';
+  }
+  return test;
+}
+
+void BenchmarkArgs(benchmark::internal::Benchmark* benchmark) {
+  // Each column is 8 bytes.
+  const auto string_size = {1, 8, 64, 100};
+  const auto num_edits = {1, 2, 16, 16, 64, 80};
+  const auto distance_cap = {1, 2, 3, 16, 64, 80};
+  for (const int s : string_size) {
+    for (const int n : num_edits) {
+      for (const int d : distance_cap) {
+        if (n > s) continue;
+        benchmark->Args({s, n, d});
+      }
+    }
+  }
+}
+
+using absl::strings_internal::CappedDamerauLevenshteinDistance;
+void BM_Distance(benchmark::State& state) {
+  std::string s1 = MakeTestString(state.range(0), 0);
+  std::string s2 = MakeTestString(state.range(0), state.range(1));
+  const size_t cap = state.range(2);
+  for (auto _ : state) {
+    CappedDamerauLevenshteinDistance(s1, s2, cap);
+  }
+}
+BENCHMARK(BM_Distance)->Apply(BenchmarkArgs);
+
+}  // namespace
diff --git a/absl/strings/internal/damerau_levenshtein_distance_test.cc b/absl/strings/internal/damerau_levenshtein_distance_test.cc
new file mode 100644
index 00000000000..4d6674d1ef3
--- /dev/null
+++ b/absl/strings/internal/damerau_levenshtein_distance_test.cc
@@ -0,0 +1,100 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/strings/internal/damerau_levenshtein_distance.h"
+
+#include <cstdint>
+#include <string>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+
+namespace {
+
+using absl::strings_internal::CappedDamerauLevenshteinDistance;
+
+TEST(Distance, TestDistances) {
+  EXPECT_THAT(CappedDamerauLevenshteinDistance("ab", "ab", 6), uint8_t{0});
+  EXPECT_THAT(CappedDamerauLevenshteinDistance("a", "b", 6), uint8_t{1});
+  EXPECT_THAT(CappedDamerauLevenshteinDistance("ca", "abc", 6), uint8_t{3});
+  EXPECT_THAT(CappedDamerauLevenshteinDistance("abcd", "ad", 6), uint8_t{2});
+  EXPECT_THAT(CappedDamerauLevenshteinDistance("abcd", "cadb", 6), uint8_t{4});
+  EXPECT_THAT(CappedDamerauLevenshteinDistance("abcd", "bdac", 6), uint8_t{4});
+  EXPECT_THAT(CappedDamerauLevenshteinDistance("ab", "ab", 0), uint8_t{0});
+  EXPECT_THAT(CappedDamerauLevenshteinDistance("", "", 0), uint8_t{0});
+  // combinations for 3-character strings:
+  // 1, 2, 3 removals, insertions or replacements and transpositions
+  EXPECT_THAT(CappedDamerauLevenshteinDistance("abc", "abc", 6), uint8_t{0});
+  for (auto res :
+       {"", "ca", "efg", "ea", "ce", "ceb", "eca", "cae", "cea", "bea"}) {
+    EXPECT_THAT(CappedDamerauLevenshteinDistance("abc", res, 6), uint8_t{3});
+    EXPECT_THAT(CappedDamerauLevenshteinDistance(res, "abc", 6), uint8_t{3});
+  }
+  for (auto res :
+       {"a",   "b",   "c",   "ba",  "cb",  "bca", "cab", "cba", "ace",
+        "efc", "ebf", "aef", "ae",  "be",  "eb",  "ec",  "ecb", "bec",
+        "bce", "cbe", "ace", "eac", "aeb", "bae", "eab", "eba"}) {
+    EXPECT_THAT(CappedDamerauLevenshteinDistance("abc", res, 6), uint8_t{2});
+    EXPECT_THAT(CappedDamerauLevenshteinDistance(res, "abc", 6), uint8_t{2});
+  }
+  for (auto res : {"ab", "ac", "bc", "acb", "bac", "ebc", "aec", "abe"}) {
+    EXPECT_THAT(CappedDamerauLevenshteinDistance("abc", res, 6), uint8_t{1});
+    EXPECT_THAT(CappedDamerauLevenshteinDistance(res, "abc", 6), uint8_t{1});
+  }
+}
+
+TEST(Distance, TestCutoff) {
+  // Returning cutoff + 1 if the value is larger than cutoff or string longer
+  // than MAX_SIZE.
+  EXPECT_THAT(CappedDamerauLevenshteinDistance("abcd", "a", 3), uint8_t{3});
+  EXPECT_THAT(CappedDamerauLevenshteinDistance("abcd", "a", 2), uint8_t{3});
+  EXPECT_THAT(CappedDamerauLevenshteinDistance("abcd", "a", 1), uint8_t{2});
+  EXPECT_THAT(CappedDamerauLevenshteinDistance("abcdefg", "a", 2), uint8_t{3});
+  EXPECT_THAT(CappedDamerauLevenshteinDistance("a", "abcde", 2), uint8_t{3});
+  EXPECT_THAT(CappedDamerauLevenshteinDistance(std::string(102, 'a'),
+                                               std::string(102, 'a'), 105),
+              uint8_t{101});
+  EXPECT_THAT(CappedDamerauLevenshteinDistance(std::string(100, 'a'),
+                                               std::string(100, 'a'), 100),
+              uint8_t{0});
+  EXPECT_THAT(CappedDamerauLevenshteinDistance(std::string(100, 'a'),
+                                               std::string(100, 'b'), 100),
+              uint8_t{100});
+  EXPECT_THAT(CappedDamerauLevenshteinDistance(std::string(100, 'a'),
+                                               std::string(99, 'a'), 2),
+              uint8_t{1});
+  EXPECT_THAT(CappedDamerauLevenshteinDistance(std::string(100, 'a'),
+                                               std::string(101, 'a'), 2),
+              uint8_t{3});
+  EXPECT_THAT(CappedDamerauLevenshteinDistance(std::string(100, 'a'),
+                                               std::string(101, 'a'), 2),
+              uint8_t{3});
+  EXPECT_THAT(CappedDamerauLevenshteinDistance(std::string(UINT8_MAX + 1, 'a'),
+                                               std::string(UINT8_MAX + 1, 'b'),
+                                               UINT8_MAX),
+              uint8_t{101});
+  EXPECT_THAT(CappedDamerauLevenshteinDistance(std::string(UINT8_MAX - 1, 'a'),
+                                               std::string(UINT8_MAX - 1, 'b'),
+                                               UINT8_MAX),
+              uint8_t{101});
+  EXPECT_THAT(
+      CappedDamerauLevenshteinDistance(std::string(UINT8_MAX, 'a'),
+                                       std::string(UINT8_MAX, 'b'), UINT8_MAX),
+      uint8_t{101});
+  EXPECT_THAT(CappedDamerauLevenshteinDistance(std::string(UINT8_MAX - 1, 'a'),
+                                               std::string(UINT8_MAX - 1, 'a'),
+                                               UINT8_MAX),
+              uint8_t{101});
+}
+}  // namespace
diff --git a/absl/strings/internal/escaping.cc b/absl/strings/internal/escaping.cc
new file mode 100644
index 00000000000..3c206b40beb
--- /dev/null
+++ b/absl/strings/internal/escaping.cc
@@ -0,0 +1,69 @@
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/strings/internal/escaping.h"
+
+#include <cstddef>
+#include <limits>
+
+#include "absl/base/internal/endian.h"
+#include "absl/base/internal/raw_logging.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace strings_internal {
+
+size_t CalculateBase64EscapedLenInternal(size_t input_len, bool do_padding) {
+  // Base64 encodes three bytes of input at a time. If the input is not
+  // divisible by three, we pad as appropriate.
+  //
+  // Base64 encodes each three bytes of input into four bytes of output.
+  constexpr size_t kMaxSize = (std::numeric_limits<size_t>::max() - 1) / 4 * 3;
+  ABSL_INTERNAL_CHECK(input_len <= kMaxSize,
+                      "CalculateBase64EscapedLenInternal() overflow");
+  size_t len = (input_len / 3) * 4;
+
+  // Since all base 64 input is an integral number of octets, only the following
+  // cases can arise:
+  if (input_len % 3 == 0) {
+    // (from https://tools.ietf.org/html/rfc3548)
+    // (1) the final quantum of encoding input is an integral multiple of 24
+    // bits; here, the final unit of encoded output will be an integral
+    // multiple of 4 characters with no "=" padding,
+  } else if (input_len % 3 == 1) {
+    // (from https://tools.ietf.org/html/rfc3548)
+    // (2) the final quantum of encoding input is exactly 8 bits; here, the
+    // final unit of encoded output will be two characters followed by two
+    // "=" padding characters, or
+    len += 2;
+    if (do_padding) {
+      len += 2;
+    }
+  } else {  // (input_len % 3 == 2)
+    // (from https://tools.ietf.org/html/rfc3548)
+    // (3) the final quantum of encoding input is exactly 16 bits; here, the
+    // final unit of encoded output will be three characters followed by one
+    // "=" padding character.
+    len += 3;
+    if (do_padding) {
+      len += 1;
+    }
+  }
+
+  return len;
+}
+
+}  // namespace strings_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/strings/internal/escaping.h b/absl/strings/internal/escaping.h
new file mode 100644
index 00000000000..d936db6f3e7
--- /dev/null
+++ b/absl/strings/internal/escaping.h
@@ -0,0 +1,35 @@
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_STRINGS_INTERNAL_ESCAPING_H_
+#define ABSL_STRINGS_INTERNAL_ESCAPING_H_
+
+#include <cstddef>
+
+#include "absl/base/config.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace strings_internal {
+
+// Calculates the length of a Base64 encoding (RFC 4648) of a string of length
+// `input_len`, with or without padding per `do_padding`. Note that 'web-safe'
+// encoding (section 5 of the RFC) does not change this length.
+size_t CalculateBase64EscapedLenInternal(size_t input_len, bool do_padding);
+
+}  // namespace strings_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STRINGS_INTERNAL_ESCAPING_H_
diff --git a/absl/strings/internal/escaping_test_common.h b/absl/strings/internal/escaping_test_common.h
index bd803031817..7b18017a089 100644
--- a/absl/strings/internal/escaping_test_common.h
+++ b/absl/strings/internal/escaping_test_common.h
@@ -22,6 +22,7 @@
 #include "absl/strings/string_view.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace strings_internal {
 
 struct base64_testcase {
@@ -126,6 +127,7 @@ inline const std::array<base64_testcase, 5>& base64_strings() {
 }
 
 }  // namespace strings_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_INTERNAL_ESCAPING_TEST_COMMON_H_
diff --git a/absl/strings/internal/generic_printer.cc b/absl/strings/internal/generic_printer.cc
new file mode 100644
index 00000000000..267213a7533
--- /dev/null
+++ b/absl/strings/internal/generic_printer.cc
@@ -0,0 +1,120 @@
+// Copyright 2025 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/strings/internal/generic_printer.h"
+
+#include <cstddef>
+#include <cstdlib>
+#include <limits>
+#include <ostream>
+#include <string>
+
+#include "absl/base/config.h"
+#include "absl/strings/ascii.h"
+#include "absl/strings/escaping.h"
+#include "absl/strings/numbers.h"
+#include "absl/strings/str_format.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace internal_generic_printer {
+
+// Out-of-line helper for PrintAsStringWithEscaping.
+std::ostream& PrintEscapedString(std::ostream& os, absl::string_view v) {
+  return os << "\"" << absl::CHexEscape(v) << "\"";
+}
+
+// Retuns a string representation of 'v', shortened if possible.
+template <class T, class F>
+std::string TryShorten(T v, F strtox) {
+  std::string printed =
+      absl::StrFormat("%.*g", std::numeric_limits<T>::max_digits10 / 2, v);
+  T parsed = strtox(printed.data());
+  if (parsed != v) {
+    printed =
+        absl::StrFormat("%.*g", std::numeric_limits<T>::max_digits10 + 1, v);
+  }
+  return printed;
+}
+
+// Out-of-line helpers for floating point values. These don't necessarily
+// ensure that values are precise, but rather that they are wide enough to
+// represent distinct values. go/c++17std/numeric.limits.members.html
+std::ostream& PrintPreciseFP(std::ostream& os, float v) {
+  // TryShorten formats with absl::StrFormat(), which is locale-independent and
+  // always emits a '.' radix. Use absl::SimpleAtof() for locale-independent
+  // parsing.
+  return os << TryShorten(v, [](const char* buf) {
+           float out = 0;
+           static_cast<void>(absl::SimpleAtof(buf, &out));
+           return out;
+         }) << "f";
+}
+std::ostream& PrintPreciseFP(std::ostream& os, double v) {
+  // TryShorten formats with absl::StrFormat(), which is locale-independent and
+  // always emits a '.' radix. Use absl::SimpleAtod() for locale-independent
+  // parsing.
+  return os << TryShorten(v, [](const char* buf) {
+           double out = 0;
+           static_cast<void>(absl::SimpleAtod(buf, &out));
+           return out;
+         });
+}
+std::ostream& PrintPreciseFP(std::ostream& os, long double v) {
+  // No locale-independent long double parser is available, so this path keeps
+  // std::strtold and remains locale-sensitive.
+  return os << TryShorten(v, [](const char* buf) {
+           char* unused;
+           return std::strtold(buf, &unused);
+         }) << "L";
+}
+
+// Prints a nibble of 'v' in hexadecimal.
+inline char hexnib(int v) {
+  return static_cast<char>((v < 10 ? '0' : ('a' - 10)) + v);
+}
+
+template <typename T>
+static std::ostream& PrintCharImpl(std::ostream& os, T v) {
+  // Specialization for chars: print as 'c' if printable, otherwise
+  // hex-escaped.
+  return (absl::ascii_isprint(static_cast<unsigned char>(v))
+              ? (os << (v == '\'' ? "'\\" : "'") << v)
+              : (os << "'\\x" << hexnib((v >> 4) & 0xf) << hexnib(v & 0xf)))
+         << "' (0x" << hexnib((v >> 4) & 0xf) << hexnib(v & 0xf) << " "
+         << static_cast<int>(v) << ")";
+}
+
+std::ostream& PrintChar(std::ostream& os, char c) {
+  return PrintCharImpl(os, c);
+}
+
+std::ostream& PrintChar(std::ostream& os, signed char c) {
+  return PrintCharImpl(os, c);
+}
+
+std::ostream& PrintChar(std::ostream& os, unsigned char c) {
+  return PrintCharImpl(os, c);
+}
+
+std::ostream& PrintByte(std::ostream& os, std::byte b) {
+  auto v = std::to_integer<int>(b);
+  os << "0x" << hexnib((v >> 4) & 0xf) << hexnib(v & 0xf);
+  return os;
+}
+
+}  // namespace internal_generic_printer
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/strings/internal/generic_printer.h b/absl/strings/internal/generic_printer.h
new file mode 100644
index 00000000000..ed60155ba04
--- /dev/null
+++ b/absl/strings/internal/generic_printer.h
@@ -0,0 +1,115 @@
+// Copyright 2025 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_STRINGS_INTERNAL_GENERIC_PRINTER_H_
+#define ABSL_STRINGS_INTERNAL_GENERIC_PRINTER_H_
+
+#include "absl/strings/internal/generic_printer_internal.h"  // IWYU pragma: export
+
+// Helpers for printing objects in generic code.
+//
+// These functions help with streaming in generic code. It may be desirable, for
+// example, to log values from generic functions; however, operator<< may not be
+// overloaded for all types.
+//
+// The helpers in this library use, in order of precedence:
+//
+//  1. std::string and std::string_view are quoted and escaped. (The specific
+//     format is not guaranteed to be stable.)
+//  2. A defined AbslStringify() method.
+//  3. absl::log_internal::LogContainer, if the object is a STL container.
+//  4. For std::tuple, std::pair, and std::optional, recursively calls
+//     GenericPrint for each element.
+//  5. Floating point values are printed with enough precision for round-trip.
+//  6. char values are quoted, with non-printable values escaped, and the
+//     char's numeric value is included.
+//  7. A defined operator<< overload (which should be found by ADL).
+//  8. A defined DebugString() const method.
+//  9. A fallback value with basic information otherwise.
+//
+// Note that the fallback value means that if no formatting conversion is
+// defined, you will not see a compile-time error. This also means that
+// GenericPrint() can safely be used in generic template contexts, and can
+// format any types needed (even though the output will vary).
+//
+// Example usage:
+//
+//   // All values after GenericPrint() are formatted:
+//   LOG(INFO) << GenericPrint()
+//             << int_var         // <- printed normally
+//             << float_var       // <- sufficient precision for round-trip
+//             << " unchanged literal text ";
+//
+//   // Just one value is formatted:
+//   LOG(INFO) << GenericPrint(string("this is quoted and escaped\t\n"))
+//             << GenericPrint("but not this, ");
+//             << string("and not this.");
+//
+// To make a type loggable with GenericPrint, prefer defining operator<< as a
+// friend function. For example:
+//
+//   class TypeToLog {
+//    public:
+//     string ToString() const;  // Many types already implement this.
+//                               // Define out-of-line if it is complex.
+//     friend std::ostream& operator<<(std::ostream& os, const TypeToLog& v) {
+//       return os << v.ToString();  // OK to define in-line instead, if simple.
+//     }
+//   };
+//
+// (Defining operator<< as an inline friend free function allows it to be found
+// by Argument-Dependent Lookup, or ADL, which is the mechanism typically used
+// for operator overload resolution. An inline friend function is the tightest
+// scope possible for overloading the left-hand side of an operator.)
+
+#include <ostream>
+#include <utility>
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace strings_internal {
+
+// Helper for logging values in generic code.
+//
+// Example usage:
+//
+//   template <typename T>
+//   void GenericFunction() {
+//     T v1, v2;
+//     VLOG(1) << GenericPrint() << v1 << v2;  // GenericPrint everything
+//     VLOG(1) << GenericPrint(v1) << v2;  // GenericPrint just v1
+//   }
+//
+inline constexpr internal_generic_printer::GenericPrintAdapterFactory
+    GenericPrint{};
+
+// Generic printer type: this class can be used, for example, as a template
+// argument to allow users to provide alternative printing strategies.
+//
+// For example, to allow callers to provide a custom strategy:
+//
+//   template <typename T, typename PrinterT = GenericPrinter<T>>
+//   void GenericFunction() {
+//     T value;
+//     VLOG(1) << PrinterT{value};
+//   }
+//
+template <typename T>
+using GenericPrinter = internal_generic_printer::GenericPrinter<T>;
+
+}  // namespace strings_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STRINGS_INTERNAL_GENERIC_PRINTER_H_
diff --git a/absl/strings/internal/generic_printer_internal.h b/absl/strings/internal/generic_printer_internal.h
new file mode 100644
index 00000000000..32a93739322
--- /dev/null
+++ b/absl/strings/internal/generic_printer_internal.h
@@ -0,0 +1,444 @@
+// Copyright 2025 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_STRINGS_INTERNAL_GENERIC_PRINTER_INTERNAL_H_
+#define ABSL_STRINGS_INTERNAL_GENERIC_PRINTER_INTERNAL_H_
+
+#include <cstddef>
+#include <cstdint>
+#include <memory>
+#include <optional>
+#include <ostream>
+#include <string>
+#include <tuple>
+#include <type_traits>
+#include <utility>
+#include <variant>
+#include <vector>
+
+#include "absl/base/config.h"
+#include "absl/log/internal/container.h"
+#include "absl/meta/internal/requires.h"
+#include "absl/strings/has_absl_stringify.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/string_view.h"
+
+// NOTE: we do not want to expand the dependencies of this library. All
+// compatibility detection must be done in a generic way, without having to
+// include the headers of other libraries.
+
+// Internal implementation details: see generic_printer.h.
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace internal_generic_printer {
+
+template <typename T>
+std::ostream& GenericPrintImpl(std::ostream& os, const T& v);
+
+// Scope blocker: we must always use ADL in our predicates below.
+struct Anonymous;
+std::ostream& operator<<(const Anonymous&, const Anonymous&) = delete;
+
+// Logging policy for LogContainer. Our SFINAE overload will not fail
+// if the contained type cannot be printed, so make sure to circle back to
+// GenericPrinter.
+struct ContainerLogPolicy {
+  void LogOpening(std::ostream& os) const { os << "["; }
+  void LogClosing(std::ostream& os) const { os << "]"; }
+  void LogFirstSeparator(std::ostream& os) const { os << ""; }
+  void LogSeparator(std::ostream& os) const { os << ", "; }
+  int64_t MaxElements() const { return 15; }
+  template <typename T>
+  void Log(std::ostream& os, const T& element) const {
+    internal_generic_printer::GenericPrintImpl(os, element);
+  }
+  void LogEllipsis(std::ostream& os) const { os << "..."; }
+};
+
+// Out-of-line helper for PrintAsStringWithEscaping.
+std::ostream& PrintEscapedString(std::ostream& os, absl::string_view v);
+
+// Trait to recognize a string, possibly with a custom allocator.
+template <class T>
+inline constexpr bool is_any_string = false;
+template <class A>
+inline constexpr bool
+    is_any_string<std::basic_string<char, std::char_traits<char>, A>> = true;
+
+// Trait to recognize a supported pointer. Below are documented reasons why
+// raw pointers and std::shared_ptr are not currently supported.
+// See also http://b/239459272#comment9 and the discussion in the comment
+// threads of cl/485200996.
+//
+// The tl;dr:
+// 1. Pointers that logically represent an object (or nullptr) are safe to print
+//    out.
+// 2. Pointers that may represent some other concept (delineating memory bounds,
+//    overridden managed-memory deleters to mimic RAII, ...) may be unsafe to
+//    print out.
+//
+// raw pointers:
+//  - A pointer one-past the last element of an array
+//  - Non-null but non-dereferenceable: https://gcc.godbolt.org/z/sqsqGvKbP
+//
+// `std::shared_ptr`:
+//  - "Aliasing" / similar to raw pointers: https://gcc.godbolt.org/z/YbWPzvhae
+template <class T, class = void>
+inline constexpr bool is_supported_ptr = false;
+// `std::unique_ptr` has the same theoretical risks as raw pointers, but those
+// risks are far less likely (typically requiring a custom deleter), and the
+// benefits of supporting unique_ptr outweigh the costs.
+//  - Note: `std::unique_ptr<T[]>` cannot safely be and is not dereferenced.
+template <class A, class... Deleter>
+inline constexpr bool is_supported_ptr<std::unique_ptr<A, Deleter...>> = true;
+// `ArenaSafeUniquePtr` is at least as safe as `std::unique_ptr`.
+template <class T>
+inline constexpr bool is_supported_ptr<
+    T,
+    // Check for `ArenaSafeUniquePtr` without having to include its header here.
+    // This does match any type named `ArenaSafeUniquePtr`, regardless of the
+    // namespace it is defined in, but it's pretty plausible that any such type
+    // would be a fork.
+    decltype(T().~ArenaSafeUniquePtr())> = true;
+
+// `proto2::Arena::UniquePtr` is at least as safe as `std::unique_ptr`.
+template <class T>
+inline constexpr bool is_supported_ptr<
+    T,
+    // Check for `proto2::Arena::UniquePtr` without having to include its
+    // header here. This does match any type named `UniquePtr`, regardless
+    // of the scope it is defined in, but we try to restrict by probing some
+    // methods.
+    std::void_t<decltype(
+        // Check the class name using the destructor.
+        T().~UniquePtr(),
+        // Check some other members, to try to duck type into the right class.
+        T().get(), T().reset(), T().try_heap_release(),
+        T().GetOwningArena()
+            ->template MakeUnique<int>(nullptr)
+            .~UniquePtr())>> = true;
+
+// Specialization for floats: print floating point types using their
+// max_digits10 precision. This ensures each distinct underlying values
+// can be represented uniquely, even though it's not (strictly speaking)
+// the most precise representation.
+std::ostream& PrintPreciseFP(std::ostream& os, float v);
+std::ostream& PrintPreciseFP(std::ostream& os, double v);
+std::ostream& PrintPreciseFP(std::ostream& os, long double v);
+
+std::ostream& PrintChar(std::ostream& os, char c);
+std::ostream& PrintChar(std::ostream& os, signed char c);
+std::ostream& PrintChar(std::ostream& os, unsigned char c);
+
+std::ostream& PrintByte(std::ostream& os, std::byte b);
+
+template <class... Ts>
+std::ostream& PrintTuple(std::ostream& os, const std::tuple<Ts...>& tuple) {
+  absl::string_view sep = "";
+  const auto print_one = [&](const auto& v) {
+    os << sep;
+    (GenericPrintImpl)(os, v);
+    sep = ", ";
+  };
+  os << "<";
+  std::apply([&](const auto&... v) { (print_one(v), ...); }, tuple);
+  os << ">";
+  return os;
+}
+
+template <typename T, typename U>
+std::ostream& PrintPair(std::ostream& os, const std::pair<T, U>& p) {
+  os << "<";
+  (GenericPrintImpl)(os, p.first);
+  os << ", ";
+  (GenericPrintImpl)(os, p.second);
+  os << ">";
+  return os;
+}
+
+template <typename T>
+std::ostream& PrintOptionalLike(std::ostream& os, const T& v) {
+  if (v.has_value()) {
+    os << "<";
+    (GenericPrintImpl)(os, *v);
+    os << ">";
+  } else {
+    (GenericPrintImpl)(os, std::nullopt);
+  }
+  return os;
+}
+
+template <typename... Ts>
+std::ostream& PrintVariant(std::ostream& os, const std::variant<Ts...>& v) {
+  os << "(";
+  os << "'(index = " << v.index() << ")' ";
+
+  // NOTE(derekbailey): This may throw a std::bad_variant_access if the variant
+  // is "valueless", which only occurs if exceptions are thrown. This is
+  // non-relevant when not using exceptions, but it is worth mentioning if that
+  // invariant is ever changed.
+  std::visit([&](const auto& arg) { (GenericPrintImpl)(os, arg); }, v);
+  os << ")";
+  return os;
+}
+
+template <typename StatusOrLike>
+std::ostream& PrintStatusOrLike(std::ostream& os, const StatusOrLike& v) {
+  os << "<";
+  if (v.ok()) {
+    os << "OK: ";
+    (GenericPrintImpl)(os, *v);
+  } else {
+    (GenericPrintImpl)(os, v.status());
+  }
+  os << ">";
+  return os;
+}
+
+template <typename SmartPointer>
+std::ostream& PrintSmartPointerContents(std::ostream& os,
+                                        const SmartPointer& v) {
+  os << "<";
+  if (v == nullptr) {
+    (GenericPrintImpl)(os, nullptr);
+  } else {
+    // Cast to void* so that every type (e.g. `char*`) is printed as an address.
+    os << absl::implicit_cast<const void*>(v.get()) << " pointing to ";
+
+    if constexpr (meta_internal::Requires<SmartPointer>(
+                      [](auto&& p) -> decltype(p[0]) {})) {
+      // e.g. std::unique_ptr<int[]>, which only has operator[]
+      os << "an array";
+    } else if constexpr (std::is_object_v<
+                             typename SmartPointer::element_type>) {
+      (GenericPrintImpl)(os, *v);
+    } else {
+      // e.g. std::unique_ptr<void, MyCustomDeleter>
+      os << "a non-object type";
+    }
+  }
+  os << ">";
+  return os;
+}
+
+template <typename T>
+std::ostream& GenericPrintImpl(std::ostream& os, const T& v) {
+  if constexpr (is_any_string<T> || std::is_same_v<T, absl::string_view>) {
+    // Specialization for strings: prints with plausible quoting and escaping.
+    return PrintEscapedString(os, v);
+  } else if constexpr (is_supported_ptr<T>) {
+    return (PrintSmartPointerContents)(os, v);
+  } else if constexpr (absl::HasAbslStringify<T>::value) {
+    // If someone has specified `AbslStringify`, we should prefer that.
+    return os << absl::StrCat(v);
+  } else if constexpr (meta_internal::Requires<const T>(
+                           [&](auto&& w)
+                               -> decltype((
+                                   PrintTuple)(std::declval<std::ostream&>(),
+                                               w)) {})) {
+    // For tuples, use `< elem0, ..., elemN >`.
+    return (PrintTuple)(os, v);
+
+  } else if constexpr (meta_internal::Requires<const T>(
+                           [&](auto&& w)
+                               -> decltype((
+                                   PrintPair)(std::declval<std::ostream&>(),
+                                              w)) {})) {
+    // For pairs, use `< first, second >`.
+    return (PrintPair)(os, v);
+
+  } else if constexpr (meta_internal::Requires<const T>(
+                           [&](auto&& w)
+                               -> decltype((
+                                   PrintVariant)(std::declval<std::ostream&>(),
+                                                 w)) {})) {
+    // For std::variant, use `std::visit(v)`
+    return (PrintVariant)(os, v);
+  } else if constexpr (meta_internal::Requires<const T>(
+                           [&](auto&& w) -> decltype(w.ok(), w.status(), *w) {
+                           })) {
+    return (PrintStatusOrLike)(os, v);
+  } else if constexpr (meta_internal::Requires<const T>(
+                           [&](auto&& w) -> decltype(w.has_value(), *w) {})) {
+    return (PrintOptionalLike)(os, v);
+  } else if constexpr (std::is_same_v<T, std::nullopt_t>) {
+    // Specialization for nullopt.
+    return os << "nullopt";
+
+  } else if constexpr (std::is_same_v<T, std::nullptr_t>) {
+    // Specialization for nullptr.
+    return os << "nullptr";
+
+  } else if constexpr (std::is_same_v<T, std::monostate>) {
+    // Specialization for `std::monostate`.
+    return os << "monostate";
+
+  } else if constexpr (std::is_floating_point_v<T>) {
+    // For floating point print with enough precision for a roundtrip.
+    return PrintPreciseFP(os, v);
+
+  } else if constexpr (std::is_same_v<T, char> ||
+                       std::is_same_v<T, signed char> ||
+                       std::is_same_v<T, unsigned char>) {
+    // Chars are printed as the char (if a printable char) and the integral
+    // representation in hex and decimal.
+    return PrintChar(os, v);
+
+  } else if constexpr (std::is_same_v<T, std::byte>) {
+    return PrintByte(os, v);
+
+  } else if constexpr (std::is_same_v<T, bool> ||
+                       std::is_same_v<T,
+                                      typename std::vector<bool>::reference> ||
+                       std::is_same_v<
+                           T, typename std::vector<bool>::const_reference>) {
+    return os << (v ? "true" : "false");
+
+  } else if constexpr (meta_internal::Requires<const T>(
+                           [&](auto&& w)
+                               -> decltype(ProtobufInternalGetEnumDescriptor(
+                                   w)) {})) {
+    os << static_cast<std::underlying_type_t<T>>(v);
+    if (auto* desc =
+            ProtobufInternalGetEnumDescriptor(T{})->FindValueByNumber(v)) {
+      os << "(" << desc->name() << ")";
+    }
+    return os;
+  } else if constexpr (!std::is_enum_v<T> &&
+                       meta_internal::Requires<const T>(
+                           [&](auto&& w) -> decltype(absl::StrCat(w)) {})) {
+    return os << absl::StrCat(v);
+
+  } else if constexpr (meta_internal::Requires<const T>(
+                           [&](auto&& w)
+                               -> decltype(std::declval<std::ostream&>()
+                                           << log_internal::LogContainer(w)) {
+                           })) {
+    // For containers, use `[ elem0, ..., elemN ]` with a max of 15.
+    return os << log_internal::LogContainer(v, ContainerLogPolicy());
+
+  } else if constexpr (meta_internal::Requires<const T>(
+                           [&](auto&& w)
+                               -> decltype(std::declval<std::ostream&>() << w) {
+                           })) {
+    // Streaming
+    return os << v;
+
+  } else if constexpr (meta_internal::Requires<const T>(
+                           [&](auto&& w)
+                               -> decltype(std::declval<std::ostream&>()
+                                           << w.DebugString()) {})) {
+    // DebugString
+    return os << v.DebugString();
+
+  } else if constexpr (std::is_enum_v<T>) {
+    // In case the underlying type is some kind of char, we have to recurse.
+    return GenericPrintImpl(os, static_cast<std::underlying_type_t<T>>(v));
+
+  } else {
+    // Default: we don't have anything to stream the value.
+    return os << "[unprintable value of size " << sizeof(T) << " @" << &v
+              << "]";
+  }
+}
+
+// GenericPrinter always has a valid operator<<. It defers to the disjuction
+// above.
+template <typename T>
+class GenericPrinter {
+ public:
+  explicit GenericPrinter(const T& value) : value_(value) {}
+
+ private:
+  friend std::ostream& operator<<(std::ostream& os, const GenericPrinter& gp) {
+    return internal_generic_printer::GenericPrintImpl(os, gp.value_);
+  }
+  const T& value_;
+};
+
+struct GenericPrintStreamAdapter {
+  template <class StreamT>
+  struct Impl {
+    // Stream operator: this object will adapt to the underlying stream type,
+    // but only if the Impl is an rvalue. For example, this works:
+    //   std::cout << GenericPrint() << foo;
+    // but not:
+    //   auto adapter = (std::cout << GenericPrint());
+    //   adapter << foo;
+    template <typename T>
+    Impl&& operator<<(const T& value) && {
+      os << internal_generic_printer::GenericPrinter<T>(value);
+      return std::move(*this);
+    }
+
+    // Inhibit using a stack variable for the adapter:
+    template <typename T>
+    Impl& operator<<(const T& value) & = delete;
+
+    // Detects a Flush() method, for LogMessage compatibility.
+    template <typename T>
+    class HasFlushMethod {
+     private:
+      template <typename C>
+      static std::true_type Test(decltype(&C::Flush));
+      template <typename C>
+      static std::false_type Test(...);
+
+     public:
+      static constexpr bool value = decltype(Test<T>(nullptr))::value;
+    };
+
+    // LogMessage compatibility requires a Flush() method.
+    void Flush() {
+      if constexpr (HasFlushMethod<StreamT>::value) {
+        os.Flush();
+      }
+    }
+
+    StreamT& os;
+  };
+
+  // If Impl is evaluated on the RHS of an 'operator&&', and 'lhs && Impl.os'
+  // implicitly converts to void, then it's fine for Impl to do so, too. This
+  // will create precisely as many objects as 'lhs && Impl.os', so we should
+  // both observe any side effects, and avoid observing multiple side
+  // effects. (See absl::log_internal::Voidify for an example of why this might
+  // be useful.)
+  template <typename LHS, typename RHS>
+  friend auto operator&&(LHS&& lhs, Impl<RHS>&& rhs)
+      -> decltype(lhs && rhs.os) {
+    return lhs && rhs.os;
+  }
+
+  template <class StreamT>
+  friend Impl<StreamT> operator<<(StreamT& os, GenericPrintStreamAdapter&&) {
+    return Impl<StreamT>{os};
+  }
+};
+
+struct GenericPrintAdapterFactory {
+  internal_generic_printer::GenericPrintStreamAdapter operator()() const {
+    return internal_generic_printer::GenericPrintStreamAdapter{};
+  }
+  template <typename T>
+  internal_generic_printer::GenericPrinter<T> operator()(const T& value) const {
+    return internal_generic_printer::GenericPrinter<T>{value};
+  }
+};
+
+}  // namespace internal_generic_printer
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STRINGS_INTERNAL_GENERIC_PRINTER_INTERNAL_H_
diff --git a/absl/strings/internal/generic_printer_test.cc b/absl/strings/internal/generic_printer_test.cc
new file mode 100644
index 00000000000..e7ecc09d8a4
--- /dev/null
+++ b/absl/strings/internal/generic_printer_test.cc
@@ -0,0 +1,727 @@
+// Copyright 2025 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/strings/internal/generic_printer.h"
+
+#include <array>
+#include <cinttypes>
+#include <clocale>
+#include <cmath>
+#include <cstddef>
+#include <cstdint>
+#include <limits>
+#include <map>
+#include <memory>
+#include <optional>
+#include <ostream>
+#include <sstream>
+#include <string>
+#include <tuple>
+#include <type_traits>
+#include <utility>
+#include <variant>
+#include <vector>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/cleanup/cleanup.h"
+#include "absl/container/flat_hash_map.h"
+#include "absl/status/status.h"
+#include "absl/status/statusor.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/str_format.h"
+#include "absl/strings/string_view.h"
+#include "absl/strings/substitute.h"
+
+namespace generic_logging_test {
+struct NotStreamable {};
+}  // namespace generic_logging_test
+
+static std::ostream& operator<<(std::ostream& os,
+                                const generic_logging_test::NotStreamable&) {
+  return os << "This overload should NOT be found by GenericPrint.";
+}
+
+// Types to test selection logic for streamable and non-streamable types.
+namespace generic_logging_test {
+struct Streamable {
+  int x;
+  friend std::ostream& operator<<(std::ostream& os, const Streamable& l) {
+    return os << "Streamable{" << l.x << "}";
+  }
+};
+}  // namespace generic_logging_test
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace strings_internal {
+namespace {
+
+using ::testing::AllOf;
+using ::testing::AnyOf;
+using ::testing::ContainsRegex;
+using ::testing::EndsWith;
+using ::testing::Eq;
+using ::testing::HasSubstr;
+using ::testing::MatchesRegex;
+
+struct AbslStringifiable {
+  template <typename S>
+  friend void AbslStringify(S& sink, const AbslStringifiable&) {
+    sink.Append("AbslStringifiable!");
+  }
+};
+
+auto IsUnprintable() {
+#ifdef GTEST_USES_SIMPLE_RE
+  return HasSubstr("unprintable value of size");
+#else
+  return ContainsRegex(
+      "\\[unprintable value of size [0-9]+ @(0x)?[0-9a-fA-F]+\\]");
+#endif
+}
+
+auto HasExactlyNInstancesOf(int n, absl::string_view me) {
+#ifdef GTEST_USES_SIMPLE_RE
+  (void)n;
+  return HasSubstr(me);
+#else
+  absl::string_view value_m_times = "(.*$0){$1}.*";
+
+  return AllOf(MatchesRegex(absl::Substitute(value_m_times, me, n)),
+               Not(MatchesRegex(absl::Substitute(value_m_times, me, n + 1))));
+#endif
+}
+
+template <typename T>
+std::string GenericPrintToString(const T& v) {
+  std::stringstream ss;
+  ss << GenericPrint(v);
+  {
+    std::stringstream ss2;
+    ss2 << GenericPrint() << v;
+    EXPECT_EQ(ss.str(), ss2.str());
+  }
+  return ss.str();
+}
+
+TEST(GenericPrinterTest, Bool) {
+  EXPECT_EQ("true", GenericPrintToString(true));
+  EXPECT_EQ("false", GenericPrintToString(false));
+}
+
+TEST(GenericPrinterTest, VectorOfBool) {
+  std::vector<bool> v{true, false, true};
+  const auto& cv = v;
+  EXPECT_EQ("[true, false, true]", GenericPrintToString(v));
+  EXPECT_EQ("true", GenericPrintToString(v[0]));
+  EXPECT_EQ("true", GenericPrintToString(cv[0]));
+}
+
+TEST(GenericPrinterTest, CharLiterals) {
+  EXPECT_EQ(R"(a"\b)", GenericPrintToString(R"(a"\b)"));
+}
+
+TEST(GenericPrinterTest, Builtin) {
+  EXPECT_EQ("123", GenericPrintToString(123));
+}
+
+TEST(GenericPrinterTest, AbslStringifiable) {
+  EXPECT_EQ("AbslStringifiable!", GenericPrintToString(AbslStringifiable{}));
+}
+
+TEST(GenericPrinterTest, Nullptr) {
+  EXPECT_EQ("nullptr", GenericPrintToString(nullptr));
+}
+
+TEST(GenericPrinterTest, Chars) {
+  EXPECT_EQ(R"('\x0a' (0x0a 10))", GenericPrintToString('\x0a'));
+  EXPECT_EQ(R"(' ' (0x20 32))", GenericPrintToString(' '));
+  EXPECT_EQ(R"('~' (0x7e 126))", GenericPrintToString('~'));
+  EXPECT_EQ(R"('\'' (0x27 39))", GenericPrintToString('\''));
+}
+
+TEST(GenericPrinterTest, SignedChars) {
+  EXPECT_EQ(R"('\x0a' (0x0a 10))",
+            GenericPrintToString(static_cast<signed char>('\x0a')));
+  EXPECT_EQ(R"(' ' (0x20 32))",
+            GenericPrintToString(static_cast<signed char>(' ')));
+  EXPECT_EQ(R"('~' (0x7e 126))",
+            GenericPrintToString(static_cast<signed char>('~')));
+  EXPECT_EQ(R"('\'' (0x27 39))",
+            GenericPrintToString(static_cast<signed char>('\'')));
+}
+
+TEST(GenericPrinterTest, UnsignedChars) {
+  EXPECT_EQ(R"('\x0a' (0x0a 10))",
+            GenericPrintToString(static_cast<unsigned char>('\x0a')));
+  EXPECT_EQ(R"(' ' (0x20 32))",
+            GenericPrintToString(static_cast<unsigned char>(' ')));
+  EXPECT_EQ(R"('~' (0x7e 126))",
+            GenericPrintToString(static_cast<unsigned char>('~')));
+  EXPECT_EQ(R"('\'' (0x27 39))",
+            GenericPrintToString(static_cast<unsigned char>('\'')));
+}
+
+TEST(GenericPrinterTest, Bytes) {
+  EXPECT_EQ("0x00", GenericPrintToString(static_cast<std::byte>(0)));
+  EXPECT_EQ("0x7f", GenericPrintToString(static_cast<std::byte>(0x7F)));
+  EXPECT_EQ("0xff", GenericPrintToString(static_cast<std::byte>(0xFF)));
+}
+
+TEST(GenericPrinterTest, Strings) {
+  const std::string expected_quotes = R"("a\"\\b")";
+  EXPECT_EQ(expected_quotes, GenericPrintToString(std::string(R"(a"\b)")));
+  const std::string expected_nonprintable = R"("\x00\xcd\n\xab")";
+  EXPECT_EQ(expected_nonprintable,
+            GenericPrintToString(absl::string_view("\0\315\n\xAB", 4)));
+}
+
+TEST(GenericPrinterTest, PreciseFloat) {
+  // Instead of testing exactly how the values are formatted, just check that
+  // they are distinct.
+
+  // Ensure concise output for exact values:
+  EXPECT_EQ("1f", GenericPrintToString(1.f));
+  EXPECT_EQ("1.1f", GenericPrintToString(1.1f));
+
+  // Plausible real-world values:
+  float f = 10.0000095f;
+  EXPECT_NE(GenericPrintToString(f), GenericPrintToString(10.0000105f));
+  // Smallest increment for a real-world value:
+  EXPECT_NE(GenericPrintToString(f),
+            GenericPrintToString(std::nextafter(f, 11)));
+  // The two smallest (finite) values possible:
+  EXPECT_NE(GenericPrintToString(std::numeric_limits<float>::lowest()),
+            GenericPrintToString(
+                std::nextafter(std::numeric_limits<float>::lowest(), 1)));
+  // Ensure the value has the correct type suffix:
+  EXPECT_THAT(GenericPrintToString(0.f), EndsWith("f"));
+}
+
+TEST(GenericPrinterTest, PreciseDouble) {
+  EXPECT_EQ("1", GenericPrintToString(1.));
+  EXPECT_EQ("1.1", GenericPrintToString(1.1));
+  double d = 10.000000000000002;
+  EXPECT_NE(GenericPrintToString(d), GenericPrintToString(10.000000000000004));
+  EXPECT_NE(GenericPrintToString(d),
+            GenericPrintToString(std::nextafter(d, 11)));
+  EXPECT_NE(GenericPrintToString(std::numeric_limits<double>::lowest()),
+            GenericPrintToString(
+                std::nextafter(std::numeric_limits<double>::lowest(), 1)));
+  EXPECT_THAT(GenericPrintToString(0.), EndsWith("0"));
+}
+
+TEST(GenericPrinterTest, PreciseLongDouble) {
+  EXPECT_EQ("1L", GenericPrintToString(1.L));
+  EXPECT_EQ("1.1L", GenericPrintToString(1.1L));
+  long double ld = 10.0000000000000000000000000000002;
+  EXPECT_NE(GenericPrintToString(ld),
+            GenericPrintToString(10.0000000000000000000000000000004));
+  EXPECT_NE(GenericPrintToString(ld),
+            GenericPrintToString(std::nextafter(ld, 11)));
+  EXPECT_NE(GenericPrintToString(std::numeric_limits<long double>::lowest()),
+            GenericPrintToString(
+                std::nextafter(std::numeric_limits<long double>::lowest(), 1)));
+  EXPECT_THAT(GenericPrintToString(0.L), EndsWith("L"));
+}
+
+TEST(GenericPrinterTest, PreciseFPUnderCommaRadixLocale) {
+  // The values are formatted with locale-independent absl::StrFormat (a '.'
+  // radix), so the round-trip shortening must not depend on LC_NUMERIC. Under a
+  // comma-radix locale a locale-sensitive reader stops at the '.', which used
+  // to defeat the shortened output.
+  const char* saved = std::setlocale(LC_NUMERIC, nullptr);
+  std::string saved_locale = saved ? saved : "C";
+  absl::Cleanup restore = [&] {
+    std::setlocale(LC_NUMERIC, saved_locale.c_str());
+  };
+
+  bool set = false;
+  for (const char* name : {"de_DE.UTF-8", "fr_FR.UTF-8", "de_DE", "fr_FR"}) {
+    if (std::setlocale(LC_NUMERIC, name) != nullptr) {
+      set = true;
+      break;
+    }
+  }
+  if (!set) {
+    GTEST_SKIP() << "No comma-radix locale available on this system.";
+  }
+
+  EXPECT_EQ("1.1f", GenericPrintToString(1.1f));
+  EXPECT_EQ("1.1", GenericPrintToString(1.1));
+}
+
+TEST(GenericPrinterTest, StreamableLvalue) {
+  generic_logging_test::Streamable x{234};
+  EXPECT_EQ("Streamable{234}", GenericPrintToString(x));
+}
+
+TEST(GenericPrinterTest, StreamableXvalue) {
+  EXPECT_EQ("Streamable{345}",
+            GenericPrintToString(generic_logging_test::Streamable{345}));
+}
+
+TEST(GenericPrinterTest, NotStreamableWithoutGenericPrint) {
+  ::generic_logging_test::NotStreamable x;
+  std::stringstream ss;
+  ::operator<<(ss, x);
+  EXPECT_EQ(ss.str(), "This overload should NOT be found by GenericPrint.");
+}
+
+TEST(GenericPrinterTest, NotStreamableLvalue) {
+  generic_logging_test::NotStreamable x;
+  EXPECT_THAT(GenericPrintToString(x), IsUnprintable());
+}
+
+TEST(GenericPrinterTest, NotStreamableXvalue) {
+  EXPECT_THAT(GenericPrintToString(generic_logging_test::NotStreamable{}),
+              IsUnprintable());
+}
+
+TEST(GenericPrinterTest, DebugString) {
+  struct WithDebugString {
+    std::string val;
+    std::string DebugString() const {
+      return absl::StrCat("WithDebugString{", val, "}");
+    }
+  };
+  EXPECT_EQ("WithDebugString{foo}",
+            GenericPrintToString(WithDebugString{"foo"}));
+}
+
+TEST(GenericPrinterTest, Vector) {
+  std::vector<int> v = {4, 5, 6};
+  EXPECT_THAT(GenericPrintToString(v), MatchesRegex(".*4,? 5,? 6.*"));
+}
+
+TEST(GenericPrinterTest, StreamableVector) {
+  std::vector<generic_logging_test::Streamable> v = {{7}, {8}, {9}};
+  EXPECT_THAT(GenericPrintToString(v),
+              MatchesRegex(".*Streamable.7.,? Streamable.8.,? Streamable.9.*"));
+}
+
+TEST(GenericPrinterTest, Map) {
+  absl::flat_hash_map<
+      std::string, absl::flat_hash_map<std::string, std::pair<double, double>>>
+      v = {{"A", {{"B", {.5, .25}}}}};
+
+  EXPECT_THAT(GenericPrintToString(v), R"([<"A", [<"B", <0.5, 0.25>>]>])");
+
+  std::map<std::string, std::map<std::string, std::pair<double, double>>> v2 = {
+      {"A", {{"B", {.5, .25}}}}};
+
+  EXPECT_THAT(GenericPrintToString(v2), R"([<"A", [<"B", <0.5, 0.25>>]>])");
+}
+
+TEST(GenericPrinterTest, StreamAdapter) {
+  std::stringstream ss;
+  static_assert(
+      std::is_same<std::remove_reference_t<decltype(ss << GenericPrint())>,
+                   internal_generic_printer::GenericPrintStreamAdapter::Impl<
+                       std::stringstream>>::value,
+      "expected ostream << GenericPrint() to yield adapter impl");
+
+  ss << GenericPrint() << "again, " << "back-up, " << "cue, "
+     << "double-u, " << "eye, "
+     << "four: " << generic_logging_test::NotStreamable{};
+  EXPECT_THAT(
+      ss.str(),
+      MatchesRegex(
+          "again, back-up, cue, double-u, eye, four: .unprintable value.*"));
+}
+
+TEST(GenericPrinterTest, NotStreamableVector) {
+  std::vector<generic_logging_test::NotStreamable> v = {{}, {}, {}};
+#ifdef GTEST_USES_SIMPLE_RE
+  EXPECT_THAT(GenericPrintToString(v), HasSubstr("unprintable"));
+#else
+  EXPECT_THAT(GenericPrintToString(v), MatchesRegex(".*(unprintable.*){3}.*"));
+#endif
+}
+
+struct CustomContainer : public std::array<int, 4> {
+  template <typename Sink>
+  friend void AbslStringify(Sink& sink, const CustomContainer& c) {
+    absl::Format(&sink, "%d %d", c[0], c[1]);
+  }
+};
+
+// Checks that AbslStringify (go/totw/215) is respected for container-like
+// types.
+TEST(GenericPrinterTest, ContainerLikeCustomLogging) {
+  CustomContainer c = {1, 2, 3, 4};
+  EXPECT_EQ(GenericPrintToString(c), "1 2");
+}
+
+// Test helper: this function demonstrates customizable printing logic:
+// 'GenericPrinter<T>' can be nominated as a default template argument.
+template <typename T, typename Printer = GenericPrinter<T>>
+std::string SpecializablePrint(const T& v) {
+  std::stringstream ss;
+  ss << Printer{v};
+  return ss.str();
+}
+
+TEST(GenericPrinterTest, DefaultPrinter) {
+  EXPECT_EQ("123", SpecializablePrint(123));
+}
+
+// Example of custom printing logic. This doesn't actually test anything in
+// GenericPrinter, but it's a working example of customizing printing logic (as
+// opposed to the comments in generic_printer.h).
+struct CustomPrinter {
+  explicit CustomPrinter(int) {}
+  friend std::ostream& operator<<(std::ostream& os, CustomPrinter&&) {
+    return os << "custom printer";
+  }
+};
+
+TEST(GenericPrinterTest, CustomPrinter) {
+  EXPECT_EQ("custom printer", (SpecializablePrint<int, CustomPrinter>(123)));
+}
+
+TEST(GenricPrinterTest, Nullopt) {
+  EXPECT_EQ("nullopt", GenericPrintToString(std::nullopt));
+}
+
+TEST(GenericPrinterTest, Optional) {
+  EXPECT_EQ("nullopt", GenericPrintToString(std::optional<int>()));
+  EXPECT_EQ("nullopt", GenericPrintToString(std::optional<int>(std::nullopt)));
+  EXPECT_EQ("<3>", GenericPrintToString(std::make_optional(3)));
+  EXPECT_EQ("<Streamable{3}>", GenericPrintToString(std::make_optional(
+                                   generic_logging_test::Streamable{3})));
+}
+
+TEST(GenericPrinterTest, Monostate) {
+  EXPECT_EQ("monostate", GenericPrintToString(std::monostate{}));
+}
+
+TEST(GenericPrinterTest, Tuple) {
+  EXPECT_EQ("<1, two, 3>", GenericPrintToString(std::make_tuple(1, "two", 3)));
+}
+
+TEST(GenericPrinterTest, EmptyTuple) {
+  EXPECT_EQ("<>", GenericPrintToString(std::make_tuple()));
+}
+
+TEST(GenericPrinterTest, TupleWithStreamableMember) {
+  EXPECT_EQ("<1, two, Streamable{3}>",
+            GenericPrintToString(std::make_tuple(
+                1, "two", generic_logging_test::Streamable{3})));
+}
+
+TEST(GenericPrinterTest, Variant) {
+  EXPECT_EQ(R"(('(index = 0)' "cow"))",
+            GenericPrintToString(std::variant<std::string, float>("cow")));
+
+  EXPECT_EQ("('(index = 1)' 1.1f)",
+            GenericPrintToString(std::variant<std::string, float>(1.1F)));
+}
+
+TEST(GenericPrinterTest, VariantMonostate) {
+  EXPECT_EQ("('(index = 0)' monostate)",
+            GenericPrintToString(std::variant<std::monostate, std::string>()));
+}
+
+TEST(GenericPrinterTest, VariantNonStreamable) {
+  EXPECT_EQ(R"(('(index = 0)' "cow"))",
+            GenericPrintToString(
+                std::variant<std::string, generic_logging_test::NotStreamable>(
+                    "cow")));
+
+  EXPECT_THAT(
+      GenericPrintToString(
+          std::variant<std::string, generic_logging_test::NotStreamable>(
+              generic_logging_test::NotStreamable{})),
+      IsUnprintable());
+}
+
+TEST(GenericPrinterTest, VariantNestedVariant) {
+  EXPECT_EQ(
+      "('(index = 1)' ('(index = 1)' 1.1f))",
+      GenericPrintToString(std::variant<std::string, std::variant<int, float>>(
+          std::variant<int, float>(1.1F))));
+}
+
+TEST(GenericPrinterTest, VariantInPlace) {
+  EXPECT_EQ("('(index = 0)' 17)", GenericPrintToString(std::variant<int, int>(
+                                      std::in_place_index<0>, 17)));
+
+  EXPECT_EQ("('(index = 1)' 17)", GenericPrintToString(std::variant<int, int>(
+                                      std::in_place_index<1>, 17)));
+}
+
+TEST(GenericPrinterTest, StatusOrLikeOkPrintsValue) {
+  EXPECT_EQ(R"(<OK: "cow">)",
+            GenericPrintToString(absl::StatusOr<std::string>("cow")));
+
+  EXPECT_EQ(R"(<OK: 1.1f>)", GenericPrintToString(absl::StatusOr<float>(1.1F)));
+}
+
+TEST(GenericPrinterTest, StatusOrLikeNonOkPrintsStatus) {
+  EXPECT_THAT(
+      GenericPrintToString(absl::StatusOr<float>(
+          absl::InvalidArgumentError("my error message"))),
+      AllOf(HasSubstr("my error message"), HasSubstr("INVALID_ARGUMENT")));
+
+  EXPECT_THAT(GenericPrintToString(
+                  absl::StatusOr<int>(absl::AbortedError("other message"))),
+              AllOf(HasSubstr("other message"), HasSubstr("ABORTED")));
+}
+
+TEST(GenericPrinterTest, StatusOrLikeNonStreamableValueUnprintable) {
+  EXPECT_THAT(
+      GenericPrintToString(absl::StatusOr<generic_logging_test::NotStreamable>(
+          generic_logging_test::NotStreamable{})),
+      IsUnprintable());
+}
+
+TEST(GenericPrinterTest, StatusOrLikeNonStreamableErrorStillPrintable) {
+  EXPECT_THAT(
+      GenericPrintToString(absl::StatusOr<generic_logging_test::NotStreamable>(
+          absl::AbortedError("other message"))),
+      AllOf(HasSubstr("other message"), HasSubstr("ABORTED")));
+}
+
+TEST(GenericPrinterTest, IsSupportedPointer) {
+  using internal_generic_printer::is_supported_ptr;
+
+  EXPECT_TRUE(is_supported_ptr<std::unique_ptr<std::string>>);
+  EXPECT_TRUE(is_supported_ptr<std::unique_ptr<int[]>>);
+  EXPECT_TRUE((is_supported_ptr<std::unique_ptr<void, void (*)(void*)>>));
+
+  EXPECT_FALSE(is_supported_ptr<int*>);
+  EXPECT_FALSE(is_supported_ptr<std::shared_ptr<int>>);
+  EXPECT_FALSE(is_supported_ptr<std::weak_ptr<int>>);
+}
+
+TEST(GenericPrinterTest, SmartPointerPrintsNullptrForAllNullptrs) {
+  std::unique_ptr<std::string> up;
+
+  EXPECT_EQ("<nullptr>", GenericPrintToString(up));
+}
+
+TEST(GenericPrinterTest, SmartPointerPrintsValueIfNonNull) {
+  EXPECT_THAT(GenericPrintToString(std::make_unique<int>(5)),
+              HasSubstr("pointing to 5"));
+}
+
+TEST(GenericPrinterTest, SmartPointerPrintsAddressOfPointee) {
+  auto i = std::make_unique<int>(5);
+  auto c = std::make_unique<char>('z');
+  char memory[] = "abcdefg";
+  auto cp = std::make_unique<char*>(memory);
+
+  EXPECT_THAT(GenericPrintToString(i),
+              AnyOf(Eq(absl::StrFormat("<%0*" PRIXPTR " pointing to 5>",
+                                       sizeof(void*) * 2,
+                                       reinterpret_cast<uintptr_t>(&*i))),
+                    Eq(absl::StrFormat("<%#x pointing to 5>",
+                                       reinterpret_cast<uintptr_t>(&*i)))));
+
+  EXPECT_THAT(
+      GenericPrintToString(c),
+      AnyOf(HasSubstr(absl::StrFormat("<%0*" PRIXPTR " pointing to 'z'",
+                                      sizeof(void*) * 2,
+                                      reinterpret_cast<uintptr_t>(&*c))),
+            HasSubstr(absl::StrFormat("<%#x pointing to 'z'",
+                                      reinterpret_cast<uintptr_t>(&*c)))));
+
+  EXPECT_THAT(GenericPrintToString(cp),
+              AnyOf(Eq(absl::StrFormat("<%0*" PRIXPTR " pointing to abcdefg>",
+                                       sizeof(void*) * 2,
+                                       reinterpret_cast<uintptr_t>(&*cp))),
+                    Eq(absl::StrFormat("<%#x pointing to abcdefg>",
+                                       reinterpret_cast<uintptr_t>(&*cp)))));
+}
+
+TEST(GenericPrinterTest, SmartPointerToArrayOnlyPrintsAddressAndHelpText) {
+  auto empty = std::make_unique<int[]>(0);
+  auto nonempty = std::make_unique<int[]>(5);
+  nonempty[0] = 12345;
+  nonempty[4] = 54321;
+  // NOTE: ArenaSafeUniquePtr is not meant to support array-type template
+  // parameters, so we skip testing that here.
+  // http://g/c-users/J-AEFrFHssY/UMMFzCkdBAAJ, b/265984185.
+
+  EXPECT_THAT(
+      GenericPrintToString(nonempty),
+      AllOf(AnyOf(HasSubstr(absl::StrFormat(
+                      "%0*" PRIXPTR, sizeof(void*) * 2,
+                      reinterpret_cast<uintptr_t>(nonempty.get()))),
+                  HasSubstr(absl::StrFormat(
+                      "%#x", reinterpret_cast<uintptr_t>(nonempty.get())))),
+            HasSubstr("array"), Not(HasSubstr("to 54321")),
+            Not(HasSubstr("to 12345"))));
+
+  EXPECT_THAT(
+      GenericPrintToString(empty),
+      AllOf(AnyOf(HasSubstr(absl::StrFormat(
+                      "%0*" PRIXPTR, sizeof(void*) * 2,
+                      reinterpret_cast<uintptr_t>(empty.get()))),
+                  HasSubstr(absl::StrFormat(
+                      "%#x", reinterpret_cast<uintptr_t>(empty.get())))),
+            HasSubstr("array")));
+}
+
+TEST(GenericPrinterTest, SmartPointerToNonObjectType) {
+  auto int_ptr_deleter = [](void* data) {
+    int* p = static_cast<int*>(data);
+    delete p;
+  };
+
+  std::unique_ptr<void, decltype(int_ptr_deleter)> void_ptr(new int(959),
+                                                            int_ptr_deleter);
+
+  EXPECT_THAT(GenericPrintToString(void_ptr),
+              HasSubstr("pointing to a non-object type"));
+}
+
+TEST(GenericPrinterTest, PrintsCustomDeleterSmartPointer) {
+  // Delete `p` (if not nullptr) only on the 4th time the deleter is used.
+  auto four_deleter = [](std::string* p) {
+    static int counter = 0;
+    if (p == nullptr) return;  // skip calls to moved-from destructors.
+    if (++counter >= 4) delete p;
+  };
+
+  // Have four `unique_ptr`s "manage" the same string-pointer, with only the
+  // final (4th) call to the deleter deleting the string pointer.
+  auto* unique_string = new std::string("unique string");
+  std::vector<std::unique_ptr<std::string, decltype(four_deleter)>> test_ptrs;
+  for (int i = 0; i < 4; ++i) {
+    test_ptrs.emplace_back(unique_string, four_deleter);
+  }
+
+  EXPECT_THAT(GenericPrintToString(test_ptrs),
+              HasExactlyNInstancesOf(4, "unique string"));
+}
+
+// Ensure that GenericPrint is robust to recursion when a type's operator<<
+// calls into GenericPrint internally.
+struct CustomRecursive {
+  std::unique_ptr<CustomRecursive> next;
+  int val = 0;
+
+  friend std::ostream& operator<<(std::ostream& os, const CustomRecursive& cr) {
+    return os << "custom print: next = " << GenericPrintToString(cr.next);
+  }
+};
+
+TEST(GenericPrinterTest, DISABLED_CustomPrintOverloadRecursionDetected) {
+  auto r1 = std::make_unique<CustomRecursive>();
+  r1->val = 1;
+  auto& r2 = r1->next = std::make_unique<CustomRecursive>();
+  r2->val = 2;
+  r2->next = std::move(r1);
+
+  EXPECT_THAT(GenericPrintToString(*r2),
+              AllOf(HasExactlyNInstancesOf(2, "custom print"),
+                    HasExactlyNInstancesOf(1, "<recursive>")));
+
+  r2->next = nullptr;  // break the cycle
+}
+// <end DISABLED_ test section>
+
+enum CStyleEnum { kValue0, kValue1 };
+TEST(GenericPrinterTest, Enum) {
+  EXPECT_EQ("1", GenericPrintToString(kValue1));
+}
+
+enum class CppStyleEnum { kValue0, kValue1, kValue2 };
+TEST(GenericPrinterTest, EnumClass) {
+  EXPECT_EQ("2", GenericPrintToString(CppStyleEnum::kValue2));
+}
+
+enum class CharBasedEnum : char { kValueA = 'A', kValue1 = '\x01' };
+TEST(GenericPrinterTest, CharBasedEnum) {
+  EXPECT_EQ("'A' (0x41 65)", GenericPrintToString(CharBasedEnum::kValueA));
+  EXPECT_EQ("'\\x01' (0x01 1)", GenericPrintToString(CharBasedEnum::kValue1));
+}
+
+enum class WideBasedEnum : uint64_t {
+  kValue = std::numeric_limits<uint64_t>::max()
+};
+TEST(GenericPrinterTest, WideBasedEnum) {
+  EXPECT_EQ(absl::StrCat(std::numeric_limits<uint64_t>::max()),
+            GenericPrintToString(WideBasedEnum::kValue));
+}
+
+enum CStyleEnumWithStringify { kValueA = 0, kValueB = 2 };
+template <typename Sink>
+void AbslStringify(Sink& sink, CStyleEnumWithStringify e) {
+  switch (e) {
+    case CStyleEnumWithStringify::kValueA:
+      sink.Append("A");
+      return;
+    case CStyleEnumWithStringify::kValueB:
+      sink.Append("B");
+      return;
+  }
+  sink.Append("??");
+}
+TEST(GenericPrinterTest, CStyleEnumWithStringify) {
+  EXPECT_EQ("A", GenericPrintToString(CStyleEnumWithStringify::kValueA));
+  EXPECT_EQ("??",
+            GenericPrintToString(static_cast<CStyleEnumWithStringify>(1)));
+}
+
+enum class CppStyleEnumWithStringify { kValueA, kValueB, kValueC };
+template <typename Sink>
+void AbslStringify(Sink& sink, CppStyleEnumWithStringify e) {
+  switch (e) {
+    case CppStyleEnumWithStringify::kValueA:
+      sink.Append("A");
+      return;
+    case CppStyleEnumWithStringify::kValueB:
+      sink.Append("B");
+      return;
+    case CppStyleEnumWithStringify::kValueC:
+      sink.Append("C");
+      return;
+  }
+  sink.Append("??");
+}
+TEST(GenericPrinterTest, CppStyleEnumWithStringify) {
+  EXPECT_EQ("A", GenericPrintToString(CppStyleEnumWithStringify::kValueA));
+  EXPECT_EQ("??",
+            GenericPrintToString(static_cast<CppStyleEnumWithStringify>(17)));
+}
+
+enum class CharBasedEnumWithStringify : char { kValueA = 'A', kValueB = 'B' };
+template <typename Sink>
+void AbslStringify(Sink& sink, CharBasedEnumWithStringify e) {
+  switch (e) {
+    case CharBasedEnumWithStringify::kValueA:
+      sink.Append("charA");
+      return;
+    case CharBasedEnumWithStringify::kValueB:
+      sink.Append("charB");
+      return;
+  }
+  sink.Append("??");
+}
+TEST(GenericPrinterTest, CharBasedEnumWithStringify) {
+  EXPECT_EQ("charA", GenericPrintToString(CharBasedEnumWithStringify::kValueA));
+  EXPECT_EQ("??",
+            GenericPrintToString(static_cast<CharBasedEnumWithStringify>('W')));
+}
+
+}  // namespace
+}  // namespace strings_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/strings/internal/memutil.cc b/absl/strings/internal/memutil.cc
index 77aa63c2c37..0bbd8aa1bd1 100644
--- a/absl/strings/internal/memutil.cc
+++ b/absl/strings/internal/memutil.cc
@@ -16,7 +16,10 @@
 
 #include <cstdlib>
 
+#include "absl/strings/ascii.h"
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace strings_internal {
 
 int memcasecmp(const char* s1, const char* s2, size_t len) {
@@ -24,87 +27,22 @@ int memcasecmp(const char* s1, const char* s2, size_t len) {
   const unsigned char* us2 = reinterpret_cast<const unsigned char*>(s2);
 
   for (size_t i = 0; i < len; i++) {
-    const int diff =
-        int{static_cast<unsigned char>(absl::ascii_tolower(us1[i]))} -
-        int{static_cast<unsigned char>(absl::ascii_tolower(us2[i]))};
-    if (diff != 0) return diff;
+    unsigned char c1 = us1[i];
+    unsigned char c2 = us2[i];
+    // If bytes are the same, they will be the same when converted to lower.
+    // So we only need to convert if bytes are not equal.
+    // NOTE(b/308193381): We do not use `absl::ascii_tolower` here in order
+    // to avoid its lookup table and improve performance.
+    if (c1 != c2) {
+      c1 = c1 >= 'A' && c1 <= 'Z' ? c1 - 'A' + 'a' : c1;
+      c2 = c2 >= 'A' && c2 <= 'Z' ? c2 - 'A' + 'a' : c2;
+      const int diff = int{c1} - int{c2};
+      if (diff != 0) return diff;
+    }
   }
   return 0;
 }
 
-char* memdup(const char* s, size_t slen) {
-  void* copy;
-  if ((copy = malloc(slen)) == nullptr) return nullptr;
-  memcpy(copy, s, slen);
-  return reinterpret_cast<char*>(copy);
-}
-
-char* memrchr(const char* s, int c, size_t slen) {
-  for (const char* e = s + slen - 1; e >= s; e--) {
-    if (*e == c) return const_cast<char*>(e);
-  }
-  return nullptr;
-}
-
-size_t memspn(const char* s, size_t slen, const char* accept) {
-  const char* p = s;
-  const char* spanp;
-  char c, sc;
-
-cont:
-  c = *p++;
-  if (slen-- == 0) return p - 1 - s;
-  for (spanp = accept; (sc = *spanp++) != '\0';)
-    if (sc == c) goto cont;
-  return p - 1 - s;
-}
-
-size_t memcspn(const char* s, size_t slen, const char* reject) {
-  const char* p = s;
-  const char* spanp;
-  char c, sc;
-
-  while (slen-- != 0) {
-    c = *p++;
-    for (spanp = reject; (sc = *spanp++) != '\0';)
-      if (sc == c) return p - 1 - s;
-  }
-  return p - s;
-}
-
-char* mempbrk(const char* s, size_t slen, const char* accept) {
-  const char* scanp;
-  int sc;
-
-  for (; slen; ++s, --slen) {
-    for (scanp = accept; (sc = *scanp++) != '\0';)
-      if (sc == *s) return const_cast<char*>(s);
-  }
-  return nullptr;
-}
-
-// This is significantly faster for case-sensitive matches with very
-// few possible matches.  See unit test for benchmarks.
-const char* memmatch(const char* phaystack, size_t haylen, const char* pneedle,
-                     size_t neelen) {
-  if (0 == neelen) {
-    return phaystack;  // even if haylen is 0
-  }
-  if (haylen < neelen) return nullptr;
-
-  const char* match;
-  const char* hayend = phaystack + haylen - neelen + 1;
-  // A static cast is used here to work around the fact that memchr returns
-  // a void* on Posix-compliant systems and const void* on Windows.
-  while ((match = static_cast<const char*>(
-              memchr(phaystack, pneedle[0], hayend - phaystack)))) {
-    if (memcmp(match, pneedle, neelen) == 0)
-      return match;
-    else
-      phaystack = match + 1;
-  }
-  return nullptr;
-}
-
 }  // namespace strings_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/internal/memutil.h b/absl/strings/internal/memutil.h
index 7c071a8201f..b5911a01cd8 100644
--- a/absl/strings/internal/memutil.h
+++ b/absl/strings/internal/memutil.h
@@ -14,51 +14,6 @@
 // limitations under the License.
 //
 
-// These routines provide mem versions of standard C string routines,
-// such as strpbrk.  They function exactly the same as the str versions,
-// so if you wonder what they are, replace the word "mem" by
-// "str" and check out the man page.  I could return void*, as the
-// strutil.h mem*() routines tend to do, but I return char* instead
-// since this is by far the most common way these functions are called.
-//
-// The difference between the mem and str versions is the mem version
-// takes a pointer and a length, rather than a '\0'-terminated string.
-// The memcase* routines defined here assume the locale is "C"
-// (they use absl::ascii_tolower instead of tolower).
-//
-// These routines are based on the BSD library.
-//
-// Here's a list of routines from string.h, and their mem analogues.
-// Functions in lowercase are defined in string.h; those in UPPERCASE
-// are defined here:
-//
-// strlen                  --
-// strcat strncat          MEMCAT
-// strcpy strncpy          memcpy
-// --                      memccpy   (very cool function, btw)
-// --                      memmove
-// --                      memset
-// strcmp strncmp          memcmp
-// strcasecmp strncasecmp  MEMCASECMP
-// strchr                  memchr
-// strcoll                 --
-// strxfrm                 --
-// strdup strndup          MEMDUP
-// strrchr                 MEMRCHR
-// strspn                  MEMSPN
-// strcspn                 MEMCSPN
-// strpbrk                 MEMPBRK
-// strstr                  MEMSTR MEMMEM
-// (g)strcasestr           MEMCASESTR MEMCASEMEM
-// strtok                  --
-// strprefix               MEMPREFIX      (strprefix is from strutil.h)
-// strcaseprefix           MEMCASEPREFIX  (strcaseprefix is from strutil.h)
-// strsuffix               MEMSUFFIX      (strsuffix is from strutil.h)
-// strcasesuffix           MEMCASESUFFIX  (strcasesuffix is from strutil.h)
-// --                      MEMIS
-// --                      MEMCASEIS
-// strcount                MEMCOUNT       (strcount is from strutil.h)
-
 #ifndef ABSL_STRINGS_INTERNAL_MEMUTIL_H_
 #define ABSL_STRINGS_INTERNAL_MEMUTIL_H_
 
@@ -69,78 +24,17 @@
 #include "absl/strings/ascii.h"  // for absl::ascii_tolower
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace strings_internal {
 
-inline char* memcat(char* dest, size_t destlen, const char* src,
-                    size_t srclen) {
-  return reinterpret_cast<char*>(memcpy(dest + destlen, src, srclen));
-}
-
+// Performs a byte-by-byte comparison of `len` bytes of the strings `s1` and
+// `s2`, ignoring the case of the characters. It returns an integer less than,
+// equal to, or greater than zero if `s1` is found, respectively, to be less
+// than, to match, or be greater than `s2`.
 int memcasecmp(const char* s1, const char* s2, size_t len);
-char* memdup(const char* s, size_t slen);
-char* memrchr(const char* s, int c, size_t slen);
-size_t memspn(const char* s, size_t slen, const char* accept);
-size_t memcspn(const char* s, size_t slen, const char* reject);
-char* mempbrk(const char* s, size_t slen, const char* accept);
-
-// This is for internal use only.  Don't call this directly
-template <bool case_sensitive>
-const char* int_memmatch(const char* haystack, size_t haylen,
-                         const char* needle, size_t neelen) {
-  if (0 == neelen) {
-    return haystack;  // even if haylen is 0
-  }
-  const char* hayend = haystack + haylen;
-  const char* needlestart = needle;
-  const char* needleend = needlestart + neelen;
-
-  for (; haystack < hayend; ++haystack) {
-    char hay = case_sensitive
-                   ? *haystack
-                   : absl::ascii_tolower(static_cast<unsigned char>(*haystack));
-    char nee = case_sensitive
-                   ? *needle
-                   : absl::ascii_tolower(static_cast<unsigned char>(*needle));
-    if (hay == nee) {
-      if (++needle == needleend) {
-        return haystack + 1 - neelen;
-      }
-    } else if (needle != needlestart) {
-      // must back up haystack in case a prefix matched (find "aab" in "aaab")
-      haystack -= needle - needlestart;  // for loop will advance one more
-      needle = needlestart;
-    }
-  }
-  return nullptr;
-}
-
-// These are the guys you can call directly
-inline const char* memstr(const char* phaystack, size_t haylen,
-                          const char* pneedle) {
-  return int_memmatch<true>(phaystack, haylen, pneedle, strlen(pneedle));
-}
-
-inline const char* memcasestr(const char* phaystack, size_t haylen,
-                              const char* pneedle) {
-  return int_memmatch<false>(phaystack, haylen, pneedle, strlen(pneedle));
-}
-
-inline const char* memmem(const char* phaystack, size_t haylen,
-                          const char* pneedle, size_t needlelen) {
-  return int_memmatch<true>(phaystack, haylen, pneedle, needlelen);
-}
-
-inline const char* memcasemem(const char* phaystack, size_t haylen,
-                              const char* pneedle, size_t needlelen) {
-  return int_memmatch<false>(phaystack, haylen, pneedle, needlelen);
-}
-
-// This is significantly faster for case-sensitive matches with very
-// few possible matches.  See unit test for benchmarks.
-const char* memmatch(const char* phaystack, size_t haylen, const char* pneedle,
-                     size_t neelen);
 
 }  // namespace strings_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_INTERNAL_MEMUTIL_H_
diff --git a/absl/strings/internal/memutil_benchmark.cc b/absl/strings/internal/memutil_benchmark.cc
index dc95c3e5e55..fb4f21f12b2 100644
--- a/absl/strings/internal/memutil_benchmark.cc
+++ b/absl/strings/internal/memutil_benchmark.cc
@@ -12,75 +12,19 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-#include "absl/strings/internal/memutil.h"
-
 #include <algorithm>
+#include <cstdint>
 #include <cstdlib>
 
-#include "benchmark/benchmark.h"
 #include "absl/strings/ascii.h"
+#include "absl/strings/internal/memutil.h"
+#include "benchmark/benchmark.h"
 
 // We fill the haystack with aaaaaaaaaaaaaaaaaa...aaaab.
 // That gives us:
 // - an easy search: 'b'
 // - a medium search: 'ab'.  That means every letter is a possible match.
 // - a pathological search: 'aaaaaa.......aaaaab' (half as many a's as haytack)
-// We benchmark case-sensitive and case-insensitive versions of
-// three memmem implementations:
-// - memmem() from memutil.h
-// - search() from STL
-// - memmatch(), a custom implementation using memchr and memcmp.
-// Here are sample results:
-//
-// Run on (12 X 3800 MHz CPU s)
-// CPU Caches:
-//   L1 Data 32K (x6)
-//   L1 Instruction 32K (x6)
-//   L2 Unified 256K (x6)
-//   L3 Unified 15360K (x1)
-// ----------------------------------------------------------------
-// Benchmark                           Time          CPU Iterations
-// ----------------------------------------------------------------
-// BM_Memmem                        3583 ns      3582 ns     196469  2.59966GB/s
-// BM_MemmemMedium                 13743 ns     13742 ns      50901  693.986MB/s
-// BM_MemmemPathological        13695030 ns  13693977 ns         51  713.133kB/s
-// BM_Memcasemem                    3299 ns      3299 ns     212942  2.82309GB/s
-// BM_MemcasememMedium             16407 ns     16406 ns      42170  581.309MB/s
-// BM_MemcasememPathological    17267745 ns  17266030 ns         41  565.598kB/s
-// BM_Search                        1610 ns      1609 ns     431321  5.78672GB/s
-// BM_SearchMedium                 11111 ns     11110 ns      63001  858.414MB/s
-// BM_SearchPathological        12117390 ns  12116397 ns         58  805.984kB/s
-// BM_Searchcase                    3081 ns      3081 ns     229949  3.02313GB/s
-// BM_SearchcaseMedium             16003 ns     16001 ns      44170  595.998MB/s
-// BM_SearchcasePathological    15823413 ns  15821909 ns         44  617.222kB/s
-// BM_Memmatch                       197 ns       197 ns    3584225  47.2951GB/s
-// BM_MemmatchMedium               52333 ns     52329 ns      13280  182.244MB/s
-// BM_MemmatchPathological        659799 ns    659727 ns       1058  14.4556MB/s
-// BM_Memcasematch                  5460 ns      5460 ns     127606  1.70586GB/s
-// BM_MemcasematchMedium           32861 ns     32857 ns      21258  290.248MB/s
-// BM_MemcasematchPathological  15154243 ns  15153089 ns         46  644.464kB/s
-// BM_MemmemStartup                    5 ns         5 ns  150821500
-// BM_SearchStartup                    5 ns         5 ns  150644203
-// BM_MemmatchStartup                  7 ns         7 ns   97068802
-//
-// Conclusions:
-//
-// The following recommendations are based on the sample results above. However,
-// we have found that the performance of STL search can vary significantly
-// depending on compiler and standard library implementation. We recommend you
-// run the benchmarks for yourself on relevant platforms.
-//
-// If you need case-insensitive, STL search is slightly better than memmem for
-// all cases.
-//
-// Case-sensitive is more subtle:
-// Custom memmatch is _very_ fast at scanning, so if you have very few possible
-// matches in your haystack, that's the way to go. Performance drops
-// significantly with more matches.
-//
-// STL search is slightly faster than memmem in the medium and pathological
-// benchmarks. However, the performance of memmem is currently more dependable
-// across platforms and build configurations.
 
 namespace {
 
@@ -94,96 +38,10 @@ const char* MakeHaystack() {
 }
 const char* const kHaystack = MakeHaystack();
 
-void BM_Memmem(benchmark::State& state) {
-  for (auto _ : state) {
-    benchmark::DoNotOptimize(
-        absl::strings_internal::memmem(kHaystack, kHaystackSize, "b", 1));
-  }
-  state.SetBytesProcessed(kHaystackSize64 * state.iterations());
-}
-BENCHMARK(BM_Memmem);
-
-void BM_MemmemMedium(benchmark::State& state) {
-  for (auto _ : state) {
-    benchmark::DoNotOptimize(
-        absl::strings_internal::memmem(kHaystack, kHaystackSize, "ab", 2));
-  }
-  state.SetBytesProcessed(kHaystackSize64 * state.iterations());
-}
-BENCHMARK(BM_MemmemMedium);
-
-void BM_MemmemPathological(benchmark::State& state) {
-  for (auto _ : state) {
-    benchmark::DoNotOptimize(absl::strings_internal::memmem(
-        kHaystack, kHaystackSize, kHaystack + kHaystackSize / 2,
-        kHaystackSize - kHaystackSize / 2));
-  }
-  state.SetBytesProcessed(kHaystackSize64 * state.iterations());
-}
-BENCHMARK(BM_MemmemPathological);
-
-void BM_Memcasemem(benchmark::State& state) {
-  for (auto _ : state) {
-    benchmark::DoNotOptimize(
-        absl::strings_internal::memcasemem(kHaystack, kHaystackSize, "b", 1));
-  }
-  state.SetBytesProcessed(kHaystackSize64 * state.iterations());
-}
-BENCHMARK(BM_Memcasemem);
-
-void BM_MemcasememMedium(benchmark::State& state) {
-  for (auto _ : state) {
-    benchmark::DoNotOptimize(
-        absl::strings_internal::memcasemem(kHaystack, kHaystackSize, "ab", 2));
-  }
-  state.SetBytesProcessed(kHaystackSize64 * state.iterations());
-}
-BENCHMARK(BM_MemcasememMedium);
-
-void BM_MemcasememPathological(benchmark::State& state) {
-  for (auto _ : state) {
-    benchmark::DoNotOptimize(absl::strings_internal::memcasemem(
-        kHaystack, kHaystackSize, kHaystack + kHaystackSize / 2,
-        kHaystackSize - kHaystackSize / 2));
-  }
-  state.SetBytesProcessed(kHaystackSize64 * state.iterations());
-}
-BENCHMARK(BM_MemcasememPathological);
-
 bool case_eq(const char a, const char b) {
   return absl::ascii_tolower(a) == absl::ascii_tolower(b);
 }
 
-void BM_Search(benchmark::State& state) {
-  for (auto _ : state) {
-    benchmark::DoNotOptimize(std::search(kHaystack, kHaystack + kHaystackSize,
-                                         kHaystack + kHaystackSize - 1,
-                                         kHaystack + kHaystackSize));
-  }
-  state.SetBytesProcessed(kHaystackSize64 * state.iterations());
-}
-BENCHMARK(BM_Search);
-
-void BM_SearchMedium(benchmark::State& state) {
-  for (auto _ : state) {
-    benchmark::DoNotOptimize(std::search(kHaystack, kHaystack + kHaystackSize,
-                                         kHaystack + kHaystackSize - 2,
-                                         kHaystack + kHaystackSize));
-  }
-  state.SetBytesProcessed(kHaystackSize64 * state.iterations());
-}
-BENCHMARK(BM_SearchMedium);
-
-void BM_SearchPathological(benchmark::State& state) {
-  for (auto _ : state) {
-    benchmark::DoNotOptimize(std::search(kHaystack, kHaystack + kHaystackSize,
-                                         kHaystack + kHaystackSize / 2,
-                                         kHaystack + kHaystackSize));
-  }
-  state.SetBytesProcessed(kHaystackSize64 * state.iterations());
-}
-BENCHMARK(BM_SearchPathological);
-
 void BM_Searchcase(benchmark::State& state) {
   for (auto _ : state) {
     benchmark::DoNotOptimize(std::search(kHaystack, kHaystack + kHaystackSize,
@@ -241,34 +99,6 @@ const char* memcasematch(const char* phaystack, size_t haylen,
   return nullptr;
 }
 
-void BM_Memmatch(benchmark::State& state) {
-  for (auto _ : state) {
-    benchmark::DoNotOptimize(
-        absl::strings_internal::memmatch(kHaystack, kHaystackSize, "b", 1));
-  }
-  state.SetBytesProcessed(kHaystackSize64 * state.iterations());
-}
-BENCHMARK(BM_Memmatch);
-
-void BM_MemmatchMedium(benchmark::State& state) {
-  for (auto _ : state) {
-    benchmark::DoNotOptimize(
-        absl::strings_internal::memmatch(kHaystack, kHaystackSize, "ab", 2));
-  }
-  state.SetBytesProcessed(kHaystackSize64 * state.iterations());
-}
-BENCHMARK(BM_MemmatchMedium);
-
-void BM_MemmatchPathological(benchmark::State& state) {
-  for (auto _ : state) {
-    benchmark::DoNotOptimize(absl::strings_internal::memmatch(
-        kHaystack, kHaystackSize, kHaystack + kHaystackSize / 2,
-        kHaystackSize - kHaystackSize / 2));
-  }
-  state.SetBytesProcessed(kHaystackSize64 * state.iterations());
-}
-BENCHMARK(BM_MemmatchPathological);
-
 void BM_Memcasematch(benchmark::State& state) {
   for (auto _ : state) {
     benchmark::DoNotOptimize(memcasematch(kHaystack, kHaystackSize, "b", 1));
@@ -295,29 +125,4 @@ void BM_MemcasematchPathological(benchmark::State& state) {
 }
 BENCHMARK(BM_MemcasematchPathological);
 
-void BM_MemmemStartup(benchmark::State& state) {
-  for (auto _ : state) {
-    benchmark::DoNotOptimize(absl::strings_internal::memmem(
-        kHaystack + kHaystackSize - 10, 10, kHaystack + kHaystackSize - 1, 1));
-  }
-}
-BENCHMARK(BM_MemmemStartup);
-
-void BM_SearchStartup(benchmark::State& state) {
-  for (auto _ : state) {
-    benchmark::DoNotOptimize(
-        std::search(kHaystack + kHaystackSize - 10, kHaystack + kHaystackSize,
-                    kHaystack + kHaystackSize - 1, kHaystack + kHaystackSize));
-  }
-}
-BENCHMARK(BM_SearchStartup);
-
-void BM_MemmatchStartup(benchmark::State& state) {
-  for (auto _ : state) {
-    benchmark::DoNotOptimize(absl::strings_internal::memmatch(
-        kHaystack + kHaystackSize - 10, 10, kHaystack + kHaystackSize - 1, 1));
-  }
-}
-BENCHMARK(BM_MemmatchStartup);
-
 }  // namespace
diff --git a/absl/strings/internal/memutil_test.cc b/absl/strings/internal/memutil_test.cc
index d8681ddf4e3..277be2c4916 100644
--- a/absl/strings/internal/memutil_test.cc
+++ b/absl/strings/internal/memutil_test.cc
@@ -19,42 +19,12 @@
 #include <cstdlib>
 
 #include "gtest/gtest.h"
-#include "absl/strings/ascii.h"
 
 namespace {
 
-static char* memcasechr(const char* s, int c, size_t slen) {
-  c = absl::ascii_tolower(c);
-  for (; slen; ++s, --slen) {
-    if (absl::ascii_tolower(*s) == c) return const_cast<char*>(s);
-  }
-  return nullptr;
-}
-
-static const char* memcasematch(const char* phaystack, size_t haylen,
-                                const char* pneedle, size_t neelen) {
-  if (0 == neelen) {
-    return phaystack;  // even if haylen is 0
-  }
-  if (haylen < neelen) return nullptr;
-
-  const char* match;
-  const char* hayend = phaystack + haylen - neelen + 1;
-  while ((match = static_cast<char*>(
-              memcasechr(phaystack, pneedle[0], hayend - phaystack)))) {
-    if (absl::strings_internal::memcasecmp(match, pneedle, neelen) == 0)
-      return match;
-    else
-      phaystack = match + 1;
-  }
-  return nullptr;
-}
-
-TEST(MemUtilTest, AllTests) {
+TEST(MemUtil, memcasecmp) {
   // check memutil functions
-  char a[1000];
-  absl::strings_internal::memcat(a, 0, "hello", sizeof("hello") - 1);
-  absl::strings_internal::memcat(a, 5, " there", sizeof(" there") - 1);
+  const char a[] = "hello there";
 
   EXPECT_EQ(absl::strings_internal::memcasecmp(a, "heLLO there",
                                                sizeof("hello there") - 1),
@@ -66,114 +36,6 @@ TEST(MemUtilTest, AllTests) {
                                                sizeof("hello there") - 2),
             0);
   EXPECT_EQ(absl::strings_internal::memcasecmp(a, "whatever", 0), 0);
-
-  char* p = absl::strings_internal::memdup("hello", 5);
-  free(p);
-
-  p = absl::strings_internal::memrchr("hello there", 'e',
-                                      sizeof("hello there") - 1);
-  EXPECT_TRUE(p && p[-1] == 'r');
-  p = absl::strings_internal::memrchr("hello there", 'e',
-                                      sizeof("hello there") - 2);
-  EXPECT_TRUE(p && p[-1] == 'h');
-  p = absl::strings_internal::memrchr("hello there", 'u',
-                                      sizeof("hello there") - 1);
-  EXPECT_TRUE(p == nullptr);
-
-  int len = absl::strings_internal::memspn("hello there",
-                                           sizeof("hello there") - 1, "hole");
-  EXPECT_EQ(len, sizeof("hello") - 1);
-  len = absl::strings_internal::memspn("hello there", sizeof("hello there") - 1,
-                                       "u");
-  EXPECT_EQ(len, 0);
-  len = absl::strings_internal::memspn("hello there", sizeof("hello there") - 1,
-                                       "");
-  EXPECT_EQ(len, 0);
-  len = absl::strings_internal::memspn("hello there", sizeof("hello there") - 1,
-                                       "trole h");
-  EXPECT_EQ(len, sizeof("hello there") - 1);
-  len = absl::strings_internal::memspn("hello there!",
-                                       sizeof("hello there!") - 1, "trole h");
-  EXPECT_EQ(len, sizeof("hello there") - 1);
-  len = absl::strings_internal::memspn("hello there!",
-                                       sizeof("hello there!") - 2, "trole h!");
-  EXPECT_EQ(len, sizeof("hello there!") - 2);
-
-  len = absl::strings_internal::memcspn("hello there",
-                                        sizeof("hello there") - 1, "leho");
-  EXPECT_EQ(len, 0);
-  len = absl::strings_internal::memcspn("hello there",
-                                        sizeof("hello there") - 1, "u");
-  EXPECT_EQ(len, sizeof("hello there") - 1);
-  len = absl::strings_internal::memcspn("hello there",
-                                        sizeof("hello there") - 1, "");
-  EXPECT_EQ(len, sizeof("hello there") - 1);
-  len = absl::strings_internal::memcspn("hello there",
-                                        sizeof("hello there") - 1, " ");
-  EXPECT_EQ(len, 5);
-
-  p = absl::strings_internal::mempbrk("hello there", sizeof("hello there") - 1,
-                                      "leho");
-  EXPECT_TRUE(p && p[1] == 'e' && p[2] == 'l');
-  p = absl::strings_internal::mempbrk("hello there", sizeof("hello there") - 1,
-                                      "nu");
-  EXPECT_TRUE(p == nullptr);
-  p = absl::strings_internal::mempbrk("hello there!",
-                                      sizeof("hello there!") - 2, "!");
-  EXPECT_TRUE(p == nullptr);
-  p = absl::strings_internal::mempbrk("hello there", sizeof("hello there") - 1,
-                                      " t ");
-  EXPECT_TRUE(p && p[-1] == 'o' && p[1] == 't');
-
-  {
-    const char kHaystack[] = "0123456789";
-    EXPECT_EQ(absl::strings_internal::memmem(kHaystack, 0, "", 0), kHaystack);
-    EXPECT_EQ(absl::strings_internal::memmem(kHaystack, 10, "012", 3),
-              kHaystack);
-    EXPECT_EQ(absl::strings_internal::memmem(kHaystack, 10, "0xx", 1),
-              kHaystack);
-    EXPECT_EQ(absl::strings_internal::memmem(kHaystack, 10, "789", 3),
-              kHaystack + 7);
-    EXPECT_EQ(absl::strings_internal::memmem(kHaystack, 10, "9xx", 1),
-              kHaystack + 9);
-    EXPECT_TRUE(absl::strings_internal::memmem(kHaystack, 10, "9xx", 3) ==
-                nullptr);
-    EXPECT_TRUE(absl::strings_internal::memmem(kHaystack, 10, "xxx", 1) ==
-                nullptr);
-  }
-  {
-    const char kHaystack[] = "aBcDeFgHiJ";
-    EXPECT_EQ(absl::strings_internal::memcasemem(kHaystack, 0, "", 0),
-              kHaystack);
-    EXPECT_EQ(absl::strings_internal::memcasemem(kHaystack, 10, "Abc", 3),
-              kHaystack);
-    EXPECT_EQ(absl::strings_internal::memcasemem(kHaystack, 10, "Axx", 1),
-              kHaystack);
-    EXPECT_EQ(absl::strings_internal::memcasemem(kHaystack, 10, "hIj", 3),
-              kHaystack + 7);
-    EXPECT_EQ(absl::strings_internal::memcasemem(kHaystack, 10, "jxx", 1),
-              kHaystack + 9);
-    EXPECT_TRUE(absl::strings_internal::memcasemem(kHaystack, 10, "jxx", 3) ==
-                nullptr);
-    EXPECT_TRUE(absl::strings_internal::memcasemem(kHaystack, 10, "xxx", 1) ==
-                nullptr);
-  }
-  {
-    const char kHaystack[] = "0123456789";
-    EXPECT_EQ(absl::strings_internal::memmatch(kHaystack, 0, "", 0), kHaystack);
-    EXPECT_EQ(absl::strings_internal::memmatch(kHaystack, 10, "012", 3),
-              kHaystack);
-    EXPECT_EQ(absl::strings_internal::memmatch(kHaystack, 10, "0xx", 1),
-              kHaystack);
-    EXPECT_EQ(absl::strings_internal::memmatch(kHaystack, 10, "789", 3),
-              kHaystack + 7);
-    EXPECT_EQ(absl::strings_internal::memmatch(kHaystack, 10, "9xx", 1),
-              kHaystack + 9);
-    EXPECT_TRUE(absl::strings_internal::memmatch(kHaystack, 10, "9xx", 3) ==
-                nullptr);
-    EXPECT_TRUE(absl::strings_internal::memmatch(kHaystack, 10, "xxx", 1) ==
-                nullptr);
-  }
 }
 
 }  // namespace
diff --git a/absl/strings/internal/numbers_test_common.h b/absl/strings/internal/numbers_test_common.h
index 28247205af4..eaa88a88975 100644
--- a/absl/strings/internal/numbers_test_common.h
+++ b/absl/strings/internal/numbers_test_common.h
@@ -23,7 +23,10 @@
 #include <limits>
 #include <string>
 
+#include "absl/base/config.h"
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace strings_internal {
 
 template <typename IntType>
@@ -42,8 +45,9 @@ inline bool Itoa(IntType value, int base, std::string* destination) {
   while (value != 0) {
     const IntType next_value = value / base;
     // Can't use std::abs here because of problems when IntType is unsigned.
-    int remainder = value > next_value * base ? value - next_value * base
-                                              : next_value * base - value;
+    int remainder =
+        static_cast<int>(value > next_value * base ? value - next_value * base
+                                                   : next_value * base - value);
     char c = remainder < 10 ? '0' + remainder : 'A' + remainder - 10;
     destination->insert(0, 1, c);
     value = next_value;
@@ -166,7 +170,7 @@ inline const std::array<uint64_test_case, 34>& strtouint64_test_cases() {
 
       {"0x1234", true, 16, 0x1234},
 
-      // Base-10 std::string version.
+      // Base-10 string version.
       {"1234", true, 0, 1234},
       {nullptr, false, 0, 0},
   }};
@@ -174,6 +178,7 @@ inline const std::array<uint64_test_case, 34>& strtouint64_test_cases() {
 }
 
 }  // namespace strings_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_INTERNAL_NUMBERS_TEST_COMMON_H_
diff --git a/absl/strings/internal/ostringstream.cc b/absl/strings/internal/ostringstream.cc
index d0f0f84b6ae..a0e5ec08c2a 100644
--- a/absl/strings/internal/ostringstream.cc
+++ b/absl/strings/internal/ostringstream.cc
@@ -14,21 +14,30 @@
 
 #include "absl/strings/internal/ostringstream.h"
 
+#include <cassert>
+#include <cstddef>
+#include <ios>
+#include <streambuf>
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace strings_internal {
 
-OStringStream::Buf::int_type OStringStream::overflow(int c) {
-  assert(s_);
-  if (!Buf::traits_type::eq_int_type(c, Buf::traits_type::eof()))
-    s_->push_back(static_cast<char>(c));
+OStringStream::Streambuf::int_type OStringStream::Streambuf::overflow(int c) {
+  assert(str_);
+  if (!std::streambuf::traits_type::eq_int_type(
+          c, std::streambuf::traits_type::eof()))
+    str_->push_back(static_cast<char>(c));
   return 1;
 }
 
-std::streamsize OStringStream::xsputn(const char* s, std::streamsize n) {
-  assert(s_);
-  s_->append(s, n);
+std::streamsize OStringStream::Streambuf::xsputn(const char* s,
+                                                 std::streamsize n) {
+  assert(str_);
+  str_->append(s, static_cast<size_t>(n));
   return n;
 }
 
 }  // namespace strings_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/internal/ostringstream.h b/absl/strings/internal/ostringstream.h
index 207920151b7..c0e237dbe8f 100644
--- a/absl/strings/internal/ostringstream.h
+++ b/absl/strings/internal/ostringstream.h
@@ -16,13 +16,16 @@
 #define ABSL_STRINGS_INTERNAL_OSTRINGSTREAM_H_
 
 #include <cassert>
+#include <ios>
 #include <ostream>
 #include <streambuf>
 #include <string>
+#include <utility>
 
-#include "absl/base/port.h"
+#include "absl/base/config.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace strings_internal {
 
 // The same as std::ostringstream but appends to a user-specified std::string,
@@ -59,29 +62,53 @@ namespace strings_internal {
 //   strm << 3.14;
 //
 // Note: flush() has no effect. No reason to call it.
-class OStringStream : private std::basic_streambuf<char>, public std::ostream {
+class OStringStream final : public std::ostream {
  public:
   // The argument can be null, in which case you'll need to call str(p) with a
   // non-null argument before you can write to the stream.
   //
   // The destructor of OStringStream doesn't use the std::string. It's OK to
   // destroy the std::string before the stream.
-  explicit OStringStream(std::string* s) : std::ostream(this), s_(s) {}
+  explicit OStringStream(std::string* str)
+      : std::ostream(&buf_), buf_(str) {}
+  OStringStream(OStringStream&& that)
+      : std::ostream(std::move(static_cast<std::ostream&>(that))),
+        buf_(that.buf_) {
+    rdbuf(&buf_);
+  }
+  OStringStream& operator=(OStringStream&& that) {
+    std::ostream::operator=(std::move(static_cast<std::ostream&>(that)));
+    buf_ = that.buf_;
+    rdbuf(&buf_);
+    return *this;
+  }
 
-  std::string* str() { return s_; }
-  const std::string* str() const { return s_; }
-  void str(std::string* s) { s_ = s; }
+  std::string* str() { return buf_.str(); }
+  const std::string* str() const { return buf_.str(); }
+  void str(std::string* str) { buf_.str(str); }
 
  private:
-  using Buf = std::basic_streambuf<char>;
+  class Streambuf final : public std::streambuf {
+   public:
+    explicit Streambuf(std::string* str) : str_(str) {}
+    Streambuf(const Streambuf&) = default;
+    Streambuf& operator=(const Streambuf&) = default;
 
-  Buf::int_type overflow(int c) override;
-  std::streamsize xsputn(const char* s, std::streamsize n) override;
+    std::string* str() { return str_; }
+    const std::string* str() const { return str_; }
+    void str(std::string* str) { str_ = str; }
 
-  std::string* s_;
+   protected:
+    int_type overflow(int c) override;
+    std::streamsize xsputn(const char* s, std::streamsize n) override;
+
+   private:
+    std::string* str_;
+  } buf_;
 };
 
 }  // namespace strings_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_INTERNAL_OSTRINGSTREAM_H_
diff --git a/absl/strings/internal/ostringstream_benchmark.cc b/absl/strings/internal/ostringstream_benchmark.cc
index 5979f18236e..a95cf4d82ae 100644
--- a/absl/strings/internal/ostringstream_benchmark.cc
+++ b/absl/strings/internal/ostringstream_benchmark.cc
@@ -12,11 +12,10 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-#include "absl/strings/internal/ostringstream.h"
-
 #include <sstream>
 #include <string>
 
+#include "absl/strings/internal/ostringstream.h"
 #include "benchmark/benchmark.h"
 
 namespace {
diff --git a/absl/strings/internal/ostringstream_test.cc b/absl/strings/internal/ostringstream_test.cc
index 2879e50eb38..c0f88c1de70 100644
--- a/absl/strings/internal/ostringstream_test.cc
+++ b/absl/strings/internal/ostringstream_test.cc
@@ -14,10 +14,12 @@
 
 #include "absl/strings/internal/ostringstream.h"
 
+#include <ios>
 #include <memory>
 #include <ostream>
 #include <string>
 #include <type_traits>
+#include <utility>
 
 #include "gtest/gtest.h"
 
@@ -25,28 +27,54 @@ namespace {
 
 TEST(OStringStream, IsOStream) {
   static_assert(
-      std::is_base_of<std::ostream, absl::strings_internal::OStringStream>(),
-      "");
+      std::is_base_of<std::ostream, absl::strings_internal::OStringStream>());
 }
 
-TEST(OStringStream, ConstructDestroy) {
+TEST(OStringStream, ConstructNullptr) {
+  absl::strings_internal::OStringStream strm(nullptr);
+  EXPECT_EQ(nullptr, strm.str());
+}
+
+TEST(OStringStream, ConstructStr) {
+  std::string s = "abc";
   {
-    absl::strings_internal::OStringStream strm(nullptr);
-    EXPECT_EQ(nullptr, strm.str());
+    absl::strings_internal::OStringStream strm(&s);
+    EXPECT_EQ(&s, strm.str());
   }
+  EXPECT_EQ("abc", s);
+}
+
+TEST(OStringStream, Destroy) {
+  std::unique_ptr<std::string> s(new std::string);
+  absl::strings_internal::OStringStream strm(s.get());
+  s.reset();
+}
+
+TEST(OStringStream, MoveConstruct) {
+  std::string s = "abc";
   {
-    std::string s = "abc";
-    {
-      absl::strings_internal::OStringStream strm(&s);
-      EXPECT_EQ(&s, strm.str());
-    }
-    EXPECT_EQ("abc", s);
+    absl::strings_internal::OStringStream strm1(&s);
+    strm1 << std::hex << 16;
+    EXPECT_EQ(&s, strm1.str());
+    absl::strings_internal::OStringStream strm2(std::move(strm1));
+    strm2 << 16;  // We should still be in base 16.
+    EXPECT_EQ(&s, strm2.str());
   }
+  EXPECT_EQ("abc1010", s);
+}
+
+TEST(OStringStream, MoveAssign) {
+  std::string s = "abc";
   {
-    std::unique_ptr<std::string> s(new std::string);
-    absl::strings_internal::OStringStream strm(s.get());
-    s.reset();
+    absl::strings_internal::OStringStream strm1(&s);
+    strm1 << std::hex << 16;
+    EXPECT_EQ(&s, strm1.str());
+    absl::strings_internal::OStringStream strm2(nullptr);
+    strm2 = std::move(strm1);
+    strm2 << 16;  // We should still be in base 16.
+    EXPECT_EQ(&s, strm2.str());
   }
+  EXPECT_EQ("abc1010", s);
 }
 
 TEST(OStringStream, Str) {
@@ -54,8 +82,8 @@ TEST(OStringStream, Str) {
   absl::strings_internal::OStringStream strm(&s1);
   const absl::strings_internal::OStringStream& c_strm(strm);
 
-  static_assert(std::is_same<decltype(strm.str()), std::string*>(), "");
-  static_assert(std::is_same<decltype(c_strm.str()), const std::string*>(), "");
+  static_assert(std::is_same<decltype(strm.str()), std::string*>());
+  static_assert(std::is_same<decltype(c_strm.str()), const std::string*>());
 
   EXPECT_EQ(&s1, strm.str());
   EXPECT_EQ(&s1, c_strm.str());
diff --git a/absl/strings/internal/pow10_helper.cc b/absl/strings/internal/pow10_helper.cc
index 03ed8d07cd4..42e96c3425d 100644
--- a/absl/strings/internal/pow10_helper.cc
+++ b/absl/strings/internal/pow10_helper.cc
@@ -17,6 +17,7 @@
 #include <cmath>
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace strings_internal {
 
 namespace {
@@ -117,4 +118,5 @@ double Pow10(int exp) {
 }
 
 }  // namespace strings_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/internal/pow10_helper.h b/absl/strings/internal/pow10_helper.h
index 9d1aa710db5..c37c2c3ffee 100644
--- a/absl/strings/internal/pow10_helper.h
+++ b/absl/strings/internal/pow10_helper.h
@@ -22,7 +22,10 @@
 
 #include <vector>
 
+#include "absl/base/config.h"
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace strings_internal {
 
 // Computes the precise value of 10^exp. (I.e. the nearest representable
@@ -31,6 +34,7 @@ namespace strings_internal {
 double Pow10(int exp);
 
 }  // namespace strings_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_INTERNAL_POW10_HELPER_H_
diff --git a/absl/strings/internal/pow10_helper_test.cc b/absl/strings/internal/pow10_helper_test.cc
index a4a68b5dab3..4e1f34b3dcc 100644
--- a/absl/strings/internal/pow10_helper_test.cc
+++ b/absl/strings/internal/pow10_helper_test.cc
@@ -15,11 +15,13 @@
 #include "absl/strings/internal/pow10_helper.h"
 
 #include <cmath>
+#include <cstdint>
 
 #include "gtest/gtest.h"
 #include "absl/strings/str_format.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace strings_internal {
 
 namespace {
@@ -117,4 +119,5 @@ TEST(Pow10HelperTest, Works) {
 
 }  // namespace
 }  // namespace strings_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/internal/resize_uninitialized.h b/absl/strings/internal/resize_uninitialized.h
index 469962b2749..a2fcab1a790 100644
--- a/absl/strings/internal/resize_uninitialized.h
+++ b/absl/strings/internal/resize_uninitialized.h
@@ -17,30 +17,34 @@
 #ifndef ABSL_STRINGS_INTERNAL_RESIZE_UNINITIALIZED_H_
 #define ABSL_STRINGS_INTERNAL_RESIZE_UNINITIALIZED_H_
 
+#include <algorithm>
+#include <cstddef>
 #include <string>
 #include <type_traits>
 #include <utility>
 
+#include "absl/base/optimization.h"
 #include "absl/base/port.h"
-#include "absl/meta/type_traits.h"  //  for void_t
+#include "absl/strings/resize_and_overwrite.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace strings_internal {
 
-// Is a subclass of true_type or false_type, depending on whether or not
-// T has a __resize_default_init member.
+// In this type trait, we look for a __resize_default_init member function, and
+// we use it if available, otherwise, we use resize. We provide HasMember to
+// indicate whether __resize_default_init is present.
 template <typename string_type, typename = void>
 struct ResizeUninitializedTraits {
   using HasMember = std::false_type;
   static void Resize(string_type* s, size_t new_size) { s->resize(new_size); }
 };
 
-// __resize_default_init is provided by libc++ >= 8.0 and by Google's internal
-// ::string implementation.
+// __resize_default_init is provided by libc++ >= 8.0
 template <typename string_type>
 struct ResizeUninitializedTraits<
-    string_type, absl::void_t<decltype(std::declval<string_type&>()
-                                           .__resize_default_init(237))> > {
+    string_type, std::void_t<decltype(std::declval<string_type&>()
+                                          .__resize_default_init(237))> > {
   using HasMember = std::true_type;
   static void Resize(string_type* s, size_t new_size) {
     s->__resize_default_init(new_size);
@@ -66,7 +70,46 @@ inline void STLStringResizeUninitialized(string_type* s, size_t new_size) {
   ResizeUninitializedTraits<string_type>::Resize(s, new_size);
 }
 
+// Like STLStringResizeUninitialized(str, new_size), except guaranteed to use
+// exponential growth so that the amortized complexity of increasing the string
+// size by a small amount is O(1), in contrast to O(str->size()) in the case of
+// precise growth.
+//
+// TODO: b/446221957 - Delete this function.
+template <typename string_type>
+[[deprecated]]
+void STLStringResizeUninitializedAmortized(string_type* s, size_t new_size) {
+  if (new_size > s->size()) {
+    if (new_size > s->capacity()) {
+      // Make sure to always grow by at least a factor of 2x. Change min_growth
+      // if you want to experiment with other growth strategies.
+      const auto min_growth = s->capacity();
+      if (ABSL_PREDICT_FALSE(s->capacity() > s->max_size() - min_growth)) {
+        s->reserve(s->max_size());
+      } else if (new_size < s->capacity() + min_growth) {
+        s->reserve(s->capacity() + min_growth);
+      }
+    }
+    // This calls absl::strings_internal::StringResizeAndOverwriteImpl() because
+    // the public API absl::StringResizeAndOverwrite() verifies that the
+    // required range has been initialized. No other code should be calling
+    // absl::strings_internal::StringResizeAndOverwriteImpl(). Instead it should
+    // be implemented correctly with absl::StringResizeAndOverwrite().
+    absl::strings_internal::StringResizeAndOverwriteImpl(
+        *s, new_size, [](typename string_type::value_type*, size_t buf_size) {
+          // TODO: b/446221957 - It is undefined behavior if any character in
+          // the range [0, return_value) is uninitialized, but we rely on this
+          // here to implement the old STLStringResizeUninitializedAmortized()
+          // API.
+          return buf_size;
+        });
+  } else {
+    s->erase(new_size);
+  }
+}
+
 }  // namespace strings_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_INTERNAL_RESIZE_UNINITIALIZED_H_
diff --git a/absl/strings/internal/resize_uninitialized_test.cc b/absl/strings/internal/resize_uninitialized_test.cc
index c5be0b12290..d7d22ca2dfd 100644
--- a/absl/strings/internal/resize_uninitialized_test.cc
+++ b/absl/strings/internal/resize_uninitialized_test.cc
@@ -14,21 +14,49 @@
 
 #include "absl/strings/internal/resize_uninitialized.h"
 
+#include <cstddef>
+#include <limits>
+#include <string>
+
 #include "gtest/gtest.h"
 
 namespace {
 
 int resize_call_count = 0;
 
+// A mock string class whose only purpose is to track how many times its
+// resize() method has been called.
 struct resizable_string {
+  using value_type = char;
+  using size_type = size_t;
+  size_t size() const { return 0; }
+  size_t capacity() const { return 0; }
+  char* data() { return buffer; }
+  char& operator[](size_t) { return buffer[0]; }
   void resize(size_t) { resize_call_count += 1; }
+  void reserve(size_t) {}
+  resizable_string& erase(size_t = 0, size_t = 0) { return *this; }
+  size_t max_size() const { return std::numeric_limits<size_t>::max(); }
+  char buffer[1] = {};
 };
 
 int resize_default_init_call_count = 0;
 
-struct resize_default_init_string {
+// A mock string class whose only purpose is to track how many times its
+// resize()/__resize_default_init() methods have been called.
+struct default_init_string {
+  using value_type = char;
+  using size_type = size_t;
+  size_t size() const { return 0; }
+  size_t capacity() const { return 0; }
+  char* data() { return buffer; }
+  char& operator[](size_t) { return buffer[0]; }
   void resize(size_t) { resize_call_count += 1; }
   void __resize_default_init(size_t) { resize_default_init_call_count += 1; }
+  void reserve(size_t) {}
+  default_init_string& erase(size_t = 0, size_t = 0) { return *this; }
+  size_t max_size() const { return std::numeric_limits<size_t>::max(); }
+  char buffer[1];
 };
 
 TEST(ResizeUninit, WithAndWithout) {
@@ -51,7 +79,7 @@ TEST(ResizeUninit, WithAndWithout) {
   resize_call_count = 0;
   resize_default_init_call_count = 0;
   {
-    resize_default_init_string rus;
+    default_init_string rus;
 
     EXPECT_EQ(resize_call_count, 0);
     EXPECT_EQ(resize_default_init_call_count, 0);
@@ -65,4 +93,17 @@ TEST(ResizeUninit, WithAndWithout) {
   }
 }
 
+TEST(ResizeUninit, Amortized) {
+  std::string str;
+  size_t prev_cap = str.capacity();
+  int cap_increase_count = 0;
+  for (int i = 0; i < 1000; ++i) {
+    absl::strings_internal::STLStringResizeUninitializedAmortized(&str, i);
+    size_t new_cap = str.capacity();
+    if (new_cap > prev_cap) ++cap_increase_count;
+    prev_cap = new_cap;
+  }
+  EXPECT_LT(cap_increase_count, 50);
+}
+
 }  // namespace
diff --git a/absl/strings/internal/stl_type_traits.h b/absl/strings/internal/stl_type_traits.h
deleted file mode 100644
index 202ab374a83..00000000000
--- a/absl/strings/internal/stl_type_traits.h
+++ /dev/null
@@ -1,246 +0,0 @@
-// Copyright 2017 The Abseil Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-//
-
-// Thie file provides the IsStrictlyBaseOfAndConvertibleToSTLContainer type
-// trait metafunction to assist in working with the _GLIBCXX_DEBUG debug
-// wrappers of STL containers.
-//
-// DO NOT INCLUDE THIS FILE DIRECTLY. Use this file by including
-// absl/strings/str_split.h.
-//
-// IWYU pragma: private, include "absl/strings/str_split.h"
-
-#ifndef ABSL_STRINGS_INTERNAL_STL_TYPE_TRAITS_H_
-#define ABSL_STRINGS_INTERNAL_STL_TYPE_TRAITS_H_
-
-#include <array>
-#include <bitset>
-#include <deque>
-#include <forward_list>
-#include <list>
-#include <map>
-#include <set>
-#include <type_traits>
-#include <unordered_map>
-#include <unordered_set>
-#include <vector>
-
-#include "absl/meta/type_traits.h"
-
-namespace absl {
-namespace strings_internal {
-
-template <typename C, template <typename...> class T>
-struct IsSpecializationImpl : std::false_type {};
-template <template <typename...> class T, typename... Args>
-struct IsSpecializationImpl<T<Args...>, T> : std::true_type {};
-template <typename C, template <typename...> class T>
-using IsSpecialization = IsSpecializationImpl<absl::decay_t<C>, T>;
-
-template <typename C>
-struct IsArrayImpl : std::false_type {};
-template <template <typename, size_t> class A, typename T, size_t N>
-struct IsArrayImpl<A<T, N>> : std::is_same<A<T, N>, std::array<T, N>> {};
-template <typename C>
-using IsArray = IsArrayImpl<absl::decay_t<C>>;
-
-template <typename C>
-struct IsBitsetImpl : std::false_type {};
-template <template <size_t> class B, size_t N>
-struct IsBitsetImpl<B<N>> : std::is_same<B<N>, std::bitset<N>> {};
-template <typename C>
-using IsBitset = IsBitsetImpl<absl::decay_t<C>>;
-
-template <typename C>
-struct IsSTLContainer
-    : absl::disjunction<
-          IsArray<C>, IsBitset<C>, IsSpecialization<C, std::deque>,
-          IsSpecialization<C, std::forward_list>,
-          IsSpecialization<C, std::list>, IsSpecialization<C, std::map>,
-          IsSpecialization<C, std::multimap>, IsSpecialization<C, std::set>,
-          IsSpecialization<C, std::multiset>,
-          IsSpecialization<C, std::unordered_map>,
-          IsSpecialization<C, std::unordered_multimap>,
-          IsSpecialization<C, std::unordered_set>,
-          IsSpecialization<C, std::unordered_multiset>,
-          IsSpecialization<C, std::vector>> {};
-
-template <typename C, template <typename...> class T, typename = void>
-struct IsBaseOfSpecializationImpl : std::false_type {};
-// IsBaseOfSpecializationImpl needs multiple partial specializations to SFINAE
-// on the existence of container dependent types and plug them into the STL
-// template.
-template <typename C, template <typename, typename> class T>
-struct IsBaseOfSpecializationImpl<
-    C, T, absl::void_t<typename C::value_type, typename C::allocator_type>>
-    : std::is_base_of<C,
-                      T<typename C::value_type, typename C::allocator_type>> {};
-template <typename C, template <typename, typename, typename> class T>
-struct IsBaseOfSpecializationImpl<
-    C, T,
-    absl::void_t<typename C::key_type, typename C::key_compare,
-                 typename C::allocator_type>>
-    : std::is_base_of<C, T<typename C::key_type, typename C::key_compare,
-                           typename C::allocator_type>> {};
-template <typename C, template <typename, typename, typename, typename> class T>
-struct IsBaseOfSpecializationImpl<
-    C, T,
-    absl::void_t<typename C::key_type, typename C::mapped_type,
-                 typename C::key_compare, typename C::allocator_type>>
-    : std::is_base_of<C,
-                      T<typename C::key_type, typename C::mapped_type,
-                        typename C::key_compare, typename C::allocator_type>> {
-};
-template <typename C, template <typename, typename, typename, typename> class T>
-struct IsBaseOfSpecializationImpl<
-    C, T,
-    absl::void_t<typename C::key_type, typename C::hasher,
-                 typename C::key_equal, typename C::allocator_type>>
-    : std::is_base_of<C, T<typename C::key_type, typename C::hasher,
-                           typename C::key_equal, typename C::allocator_type>> {
-};
-template <typename C,
-          template <typename, typename, typename, typename, typename> class T>
-struct IsBaseOfSpecializationImpl<
-    C, T,
-    absl::void_t<typename C::key_type, typename C::mapped_type,
-                 typename C::hasher, typename C::key_equal,
-                 typename C::allocator_type>>
-    : std::is_base_of<C, T<typename C::key_type, typename C::mapped_type,
-                           typename C::hasher, typename C::key_equal,
-                           typename C::allocator_type>> {};
-template <typename C, template <typename...> class T>
-using IsBaseOfSpecialization = IsBaseOfSpecializationImpl<absl::decay_t<C>, T>;
-
-template <typename C>
-struct IsBaseOfArrayImpl : std::false_type {};
-template <template <typename, size_t> class A, typename T, size_t N>
-struct IsBaseOfArrayImpl<A<T, N>> : std::is_base_of<A<T, N>, std::array<T, N>> {
-};
-template <typename C>
-using IsBaseOfArray = IsBaseOfArrayImpl<absl::decay_t<C>>;
-
-template <typename C>
-struct IsBaseOfBitsetImpl : std::false_type {};
-template <template <size_t> class B, size_t N>
-struct IsBaseOfBitsetImpl<B<N>> : std::is_base_of<B<N>, std::bitset<N>> {};
-template <typename C>
-using IsBaseOfBitset = IsBaseOfBitsetImpl<absl::decay_t<C>>;
-
-template <typename C>
-struct IsBaseOfSTLContainer
-    : absl::disjunction<IsBaseOfArray<C>, IsBaseOfBitset<C>,
-                        IsBaseOfSpecialization<C, std::deque>,
-                        IsBaseOfSpecialization<C, std::forward_list>,
-                        IsBaseOfSpecialization<C, std::list>,
-                        IsBaseOfSpecialization<C, std::map>,
-                        IsBaseOfSpecialization<C, std::multimap>,
-                        IsBaseOfSpecialization<C, std::set>,
-                        IsBaseOfSpecialization<C, std::multiset>,
-                        IsBaseOfSpecialization<C, std::unordered_map>,
-                        IsBaseOfSpecialization<C, std::unordered_multimap>,
-                        IsBaseOfSpecialization<C, std::unordered_set>,
-                        IsBaseOfSpecialization<C, std::unordered_multiset>,
-                        IsBaseOfSpecialization<C, std::vector>> {};
-
-template <typename C, template <typename...> class T, typename = void>
-struct IsConvertibleToSpecializationImpl : std::false_type {};
-// IsConvertibleToSpecializationImpl needs multiple partial specializations to
-// SFINAE on the existence of container dependent types and plug them into the
-// STL template.
-template <typename C, template <typename, typename> class T>
-struct IsConvertibleToSpecializationImpl<
-    C, T, absl::void_t<typename C::value_type, typename C::allocator_type>>
-    : std::is_convertible<
-          C, T<typename C::value_type, typename C::allocator_type>> {};
-template <typename C, template <typename, typename, typename> class T>
-struct IsConvertibleToSpecializationImpl<
-    C, T,
-    absl::void_t<typename C::key_type, typename C::key_compare,
-                 typename C::allocator_type>>
-    : std::is_convertible<C, T<typename C::key_type, typename C::key_compare,
-                               typename C::allocator_type>> {};
-template <typename C, template <typename, typename, typename, typename> class T>
-struct IsConvertibleToSpecializationImpl<
-    C, T,
-    absl::void_t<typename C::key_type, typename C::mapped_type,
-                 typename C::key_compare, typename C::allocator_type>>
-    : std::is_convertible<
-          C, T<typename C::key_type, typename C::mapped_type,
-               typename C::key_compare, typename C::allocator_type>> {};
-template <typename C, template <typename, typename, typename, typename> class T>
-struct IsConvertibleToSpecializationImpl<
-    C, T,
-    absl::void_t<typename C::key_type, typename C::hasher,
-                 typename C::key_equal, typename C::allocator_type>>
-    : std::is_convertible<
-          C, T<typename C::key_type, typename C::hasher, typename C::key_equal,
-               typename C::allocator_type>> {};
-template <typename C,
-          template <typename, typename, typename, typename, typename> class T>
-struct IsConvertibleToSpecializationImpl<
-    C, T,
-    absl::void_t<typename C::key_type, typename C::mapped_type,
-                 typename C::hasher, typename C::key_equal,
-                 typename C::allocator_type>>
-    : std::is_convertible<C, T<typename C::key_type, typename C::mapped_type,
-                               typename C::hasher, typename C::key_equal,
-                               typename C::allocator_type>> {};
-template <typename C, template <typename...> class T>
-using IsConvertibleToSpecialization =
-    IsConvertibleToSpecializationImpl<absl::decay_t<C>, T>;
-
-template <typename C>
-struct IsConvertibleToArrayImpl : std::false_type {};
-template <template <typename, size_t> class A, typename T, size_t N>
-struct IsConvertibleToArrayImpl<A<T, N>>
-    : std::is_convertible<A<T, N>, std::array<T, N>> {};
-template <typename C>
-using IsConvertibleToArray = IsConvertibleToArrayImpl<absl::decay_t<C>>;
-
-template <typename C>
-struct IsConvertibleToBitsetImpl : std::false_type {};
-template <template <size_t> class B, size_t N>
-struct IsConvertibleToBitsetImpl<B<N>>
-    : std::is_convertible<B<N>, std::bitset<N>> {};
-template <typename C>
-using IsConvertibleToBitset = IsConvertibleToBitsetImpl<absl::decay_t<C>>;
-
-template <typename C>
-struct IsConvertibleToSTLContainer
-    : absl::disjunction<
-          IsConvertibleToArray<C>, IsConvertibleToBitset<C>,
-          IsConvertibleToSpecialization<C, std::deque>,
-          IsConvertibleToSpecialization<C, std::forward_list>,
-          IsConvertibleToSpecialization<C, std::list>,
-          IsConvertibleToSpecialization<C, std::map>,
-          IsConvertibleToSpecialization<C, std::multimap>,
-          IsConvertibleToSpecialization<C, std::set>,
-          IsConvertibleToSpecialization<C, std::multiset>,
-          IsConvertibleToSpecialization<C, std::unordered_map>,
-          IsConvertibleToSpecialization<C, std::unordered_multimap>,
-          IsConvertibleToSpecialization<C, std::unordered_set>,
-          IsConvertibleToSpecialization<C, std::unordered_multiset>,
-          IsConvertibleToSpecialization<C, std::vector>> {};
-
-template <typename C>
-struct IsStrictlyBaseOfAndConvertibleToSTLContainer
-    : absl::conjunction<absl::negation<IsSTLContainer<C>>,
-                        IsBaseOfSTLContainer<C>,
-                        IsConvertibleToSTLContainer<C>> {};
-
-}  // namespace strings_internal
-}  // namespace absl
-#endif  // ABSL_STRINGS_INTERNAL_STL_TYPE_TRAITS_H_
diff --git a/absl/strings/internal/str_format/arg.cc b/absl/strings/internal/str_format/arg.cc
index b40be8ff382..2125be31c6b 100644
--- a/absl/strings/internal/str_format/arg.cc
+++ b/absl/strings/internal/str_format/arg.cc
@@ -1,24 +1,50 @@
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
 //
 // POSIX spec:
 //   http://pubs.opengroup.org/onlinepubs/009695399/functions/fprintf.html
 //
 #include "absl/strings/internal/str_format/arg.h"
 
+#include <algorithm>
 #include <cassert>
-#include <cerrno>
+#include <cstddef>
+#include <cstdint>
 #include <cstdlib>
+#include <cstring>
+#include <cwchar>
+#include <limits>
 #include <string>
+#include <string_view>
 #include <type_traits>
 
-#include "absl/base/port.h"
+#include "absl/base/config.h"
+#include "absl/base/optimization.h"
+#include "absl/container/fixed_array.h"
+#include "absl/numeric/int128.h"
+#include "absl/strings/internal/str_format/extension.h"
 #include "absl/strings/internal/str_format/float_conversion.h"
+#include "absl/strings/internal/utf8.h"
+#include "absl/strings/numbers.h"
+#include "absl/strings/string_view.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace str_format_internal {
 namespace {
 
-const char kDigit[2][32] = { "0123456789abcdef", "0123456789ABCDEF" };
-
 // Reduce *capacity by s.size(), clipped to a 0 minimum.
 void ReducePadding(string_view s, size_t *capacity) {
   *capacity = Excess(s.size(), *capacity);
@@ -32,6 +58,10 @@ void ReducePadding(size_t n, size_t *capacity) {
 template <typename T>
 struct MakeUnsigned : std::make_unsigned<T> {};
 template <>
+struct MakeUnsigned<absl::int128> {
+  using type = absl::uint128;
+};
+template <>
 struct MakeUnsigned<absl::uint128> {
   using type = absl::uint128;
 };
@@ -39,141 +69,200 @@ struct MakeUnsigned<absl::uint128> {
 template <typename T>
 struct IsSigned : std::is_signed<T> {};
 template <>
+struct IsSigned<absl::int128> : std::true_type {};
+template <>
 struct IsSigned<absl::uint128> : std::false_type {};
 
-class ConvertedIntInfo {
+// Integral digit printer.
+// Call one of the PrintAs* routines after construction once.
+// Use with_neg_and_zero/without_neg_or_zero/is_negative to access the results.
+class IntDigits {
  public:
+  // Print the unsigned integer as octal.
+  // Supports unsigned integral types and uint128.
+  template <typename T>
+  void PrintAsOct(T v) {
+    static_assert(!IsSigned<T>::value);
+    char *p = storage_ + sizeof(storage_);
+    do {
+      *--p = static_cast<char>('0' + (static_cast<size_t>(v) & 7));
+      v >>= 3;
+    } while (v);
+    start_ = p;
+    size_ = static_cast<size_t>(storage_ + sizeof(storage_) - p);
+  }
+
+  // Print the signed or unsigned integer as decimal.
+  // Supports all integral types.
   template <typename T>
-  ConvertedIntInfo(T v, ConversionChar conv) {
-    using Unsigned = typename MakeUnsigned<T>::type;
-    auto u = static_cast<Unsigned>(v);
-    if (IsNeg(v)) {
-      is_neg_ = true;
-      u = Unsigned{} - u;
-    } else {
-      is_neg_ = false;
+  void PrintAsDec(T v) {
+    static_assert(std::is_integral_v<T>);
+    start_ = storage_;
+    size_ = static_cast<size_t>(numbers_internal::FastIntToBuffer(v, storage_) -
+                                storage_);
+  }
+
+  void PrintAsDec(int128 v) {
+    auto u = static_cast<uint128>(v);
+    bool add_neg = false;
+    if (v < 0) {
+      add_neg = true;
+      u = uint128{} - u;
     }
-    UnsignedToStringRight(u, conv);
+    PrintAsDec(u, add_neg);
   }
 
-  string_view digits() const {
-    return {end() - size_, static_cast<size_t>(size_)};
+  void PrintAsDec(uint128 v, bool add_neg = false) {
+    // This function can be sped up if needed. We can call FastIntToBuffer
+    // twice, or fix FastIntToBuffer to support uint128.
+    char *p = storage_ + sizeof(storage_);
+    do {
+      p -= 2;
+      numbers_internal::PutTwoDigits(static_cast<uint32_t>(v % 100), p);
+      v /= 100;
+    } while (v);
+    if (p[0] == '0') {
+      // We printed one too many hexits.
+      ++p;
+    }
+    if (add_neg) {
+      *--p = '-';
+    }
+    size_ = static_cast<size_t>(storage_ + sizeof(storage_) - p);
+    start_ = p;
   }
-  bool is_neg() const { return is_neg_; }
 
- private:
-  template <typename T, bool IsSigned>
-  struct IsNegImpl {
-    static bool Eval(T v) { return v < 0; }
-  };
+  // Print the unsigned integer as hex using lowercase.
+  // Supports unsigned integral types and uint128.
   template <typename T>
-  struct IsNegImpl<T, false> {
-    static bool Eval(T) {
-      return false;
+  void PrintAsHexLower(T v) {
+    static_assert(!IsSigned<T>::value);
+    char *p = storage_ + sizeof(storage_);
+
+    do {
+      p -= 2;
+      constexpr const char* table = numbers_internal::kHexTable;
+      std::memcpy(p, table + 2 * (static_cast<size_t>(v) & 0xFF), 2);
+      if (sizeof(T) == 1) break;
+      v >>= 8;
+    } while (v);
+    if (p[0] == '0') {
+      // We printed one too many digits.
+      ++p;
     }
-  };
+    start_ = p;
+    size_ = static_cast<size_t>(storage_ + sizeof(storage_) - p);
+  }
 
+  // Print the unsigned integer as hex using uppercase.
+  // Supports unsigned integral types and uint128.
   template <typename T>
-  bool IsNeg(T v) {
-    return IsNegImpl<T, IsSigned<T>::value>::Eval(v);
+  void PrintAsHexUpper(T v) {
+    static_assert(!IsSigned<T>::value);
+    char *p = storage_ + sizeof(storage_);
+
+    // kHexTable is only lowercase, so do it manually for uppercase.
+    do {
+      *--p = "0123456789ABCDEF"[static_cast<size_t>(v) & 15];
+      v >>= 4;
+    } while (v);
+    start_ = p;
+    size_ = static_cast<size_t>(storage_ + sizeof(storage_) - p);
   }
 
-  template <typename T>
-  void UnsignedToStringRight(T u, ConversionChar conv) {
-    char *p = end();
-    switch (conv.radix()) {
-      default:
-      case 10:
-        for (; u; u /= 10)
-          *--p = static_cast<char>('0' + static_cast<size_t>(u % 10));
-        break;
-      case 8:
-        for (; u; u /= 8)
-          *--p = static_cast<char>('0' + static_cast<size_t>(u % 8));
-        break;
-      case 16: {
-        const char *digits = kDigit[conv.upper() ? 1 : 0];
-        for (; u; u /= 16) *--p = digits[static_cast<size_t>(u % 16)];
-        break;
-      }
-    }
-    size_ = static_cast<int>(end() - p);
+  // The printed value including the '-' sign if available.
+  // For inputs of value `0`, this will return "0"
+  string_view with_neg_and_zero() const { return {start_, size_}; }
+
+  // The printed value not including the '-' sign.
+  // For inputs of value `0`, this will return "".
+  string_view without_neg_or_zero() const {
+    static_assert('-' < '0', "The check below verifies both.");
+    size_t advance = start_[0] <= '0' ? 1 : 0;
+    return {start_ + advance, size_ - advance};
   }
 
-  const char *end() const { return storage_ + sizeof(storage_); }
-  char *end() { return storage_ + sizeof(storage_); }
+  bool is_negative() const { return start_[0] == '-'; }
 
-  bool is_neg_;
-  int size_;
-  // Max size: 128 bit value as octal -> 43 digits
-  char storage_[128 / 3 + 1];
+ private:
+  const char *start_;
+  size_t size_;
+  // Max size: 128 bit value as octal -> 43 digits, plus sign char
+  char storage_[128 / 3 + 1 + 1];
 };
 
 // Note: 'o' conversions do not have a base indicator, it's just that
 // the '#' flag is specified to modify the precision for 'o' conversions.
-string_view BaseIndicator(const ConvertedIntInfo &info,
-                          const ConversionSpec conv) {
-  bool alt = conv.flags().alt;
-  int radix = conv.conv().radix();
-  if (conv.conv().id() == ConversionChar::p)
-    alt = true;  // always show 0x for %p.
+string_view BaseIndicator(const IntDigits &as_digits,
+                          const FormatConversionSpecImpl conv) {
+  // always show 0x for %p.
+  bool alt = conv.has_alt_flag() ||
+             conv.conversion_char() == FormatConversionCharInternal::p;
+  bool hex = (conv.conversion_char() == FormatConversionCharInternal::x ||
+              conv.conversion_char() == FormatConversionCharInternal::X ||
+              conv.conversion_char() == FormatConversionCharInternal::p);
   // From the POSIX description of '#' flag:
   //   "For x or X conversion specifiers, a non-zero result shall have
   //   0x (or 0X) prefixed to it."
-  if (alt && radix == 16 && !info.digits().empty()) {
-    if (conv.conv().upper()) return "0X";
-    return "0x";
+  if (alt && hex && !as_digits.without_neg_or_zero().empty()) {
+    return conv.conversion_char() == FormatConversionCharInternal::X ? "0X"
+                                                                     : "0x";
   }
   return {};
 }
 
-string_view SignColumn(bool neg, const ConversionSpec conv) {
-  if (conv.conv().is_signed()) {
+string_view SignColumn(bool neg, const FormatConversionSpecImpl conv) {
+  if (conv.conversion_char() == FormatConversionCharInternal::d ||
+      conv.conversion_char() == FormatConversionCharInternal::i) {
     if (neg) return "-";
-    if (conv.flags().show_pos) return "+";
-    if (conv.flags().sign_col) return " ";
+    if (conv.has_show_pos_flag()) return "+";
+    if (conv.has_sign_col_flag()) return " ";
   }
   return {};
 }
 
-bool ConvertCharImpl(unsigned char v, const ConversionSpec conv,
-                     FormatSinkImpl *sink) {
+bool ConvertCharImpl(char v,
+                     const FormatConversionSpecImpl conv,
+                     FormatSinkImpl* sink) {
   size_t fill = 0;
-  if (conv.width() >= 0) fill = conv.width();
+  if (conv.width() >= 0)
+    fill = static_cast<size_t>(conv.width());
   ReducePadding(1, &fill);
-  if (!conv.flags().left) sink->Append(fill, ' ');
+  if (!conv.has_left_flag()) sink->Append(fill, ' ');
   sink->Append(1, v);
-  if (conv.flags().left) sink->Append(fill, ' ');
+  if (conv.has_left_flag()) sink->Append(fill, ' ');
   return true;
 }
 
-bool ConvertIntImplInner(const ConvertedIntInfo &info,
-                         const ConversionSpec conv, FormatSinkImpl *sink) {
+bool ConvertIntImplInnerSlow(const IntDigits &as_digits,
+                             const FormatConversionSpecImpl conv,
+                             FormatSinkImpl *sink) {
   // Print as a sequence of Substrings:
   //   [left_spaces][sign][base_indicator][zeroes][formatted][right_spaces]
   size_t fill = 0;
-  if (conv.width() >= 0) fill = conv.width();
+  if (conv.width() >= 0)
+    fill = static_cast<size_t>(conv.width());
 
-  string_view formatted = info.digits();
+  string_view formatted = as_digits.without_neg_or_zero();
   ReducePadding(formatted, &fill);
 
-  string_view sign = SignColumn(info.is_neg(), conv);
+  string_view sign = SignColumn(as_digits.is_negative(), conv);
   ReducePadding(sign, &fill);
 
-  string_view base_indicator = BaseIndicator(info, conv);
+  string_view base_indicator = BaseIndicator(as_digits, conv);
   ReducePadding(base_indicator, &fill);
 
-  int precision = conv.precision();
-  bool precision_specified = precision >= 0;
-  if (!precision_specified)
-    precision = 1;
+  bool precision_specified = conv.precision() >= 0;
+  size_t precision =
+      precision_specified ? static_cast<size_t>(conv.precision()) : size_t{1};
 
-  if (conv.flags().alt && conv.conv().id() == ConversionChar::o) {
+  if (conv.has_alt_flag() &&
+      conv.conversion_char() == FormatConversionCharInternal::o) {
     // From POSIX description of the '#' (alt) flag:
     //   "For o conversion, it increases the precision (if necessary) to
     //   force the first digit of the result to be zero."
     if (formatted.empty() || *formatted.begin() != '0') {
-      int needed = static_cast<int>(formatted.size()) + 1;
+      size_t needed = formatted.size() + 1;
       precision = std::max(precision, needed);
     }
   }
@@ -181,13 +270,13 @@ bool ConvertIntImplInner(const ConvertedIntInfo &info,
   size_t num_zeroes = Excess(formatted.size(), precision);
   ReducePadding(num_zeroes, &fill);
 
-  size_t num_left_spaces = !conv.flags().left ? fill : 0;
-  size_t num_right_spaces = conv.flags().left ? fill : 0;
+  size_t num_left_spaces = !conv.has_left_flag() ? fill : 0;
+  size_t num_right_spaces = conv.has_left_flag() ? fill : 0;
 
   // From POSIX description of the '0' (zero) flag:
   //   "For d, i, o, u, x, and X conversion specifiers, if a precision
   //   is specified, the '0' flag is ignored."
-  if (!precision_specified && conv.flags().zero) {
+  if (!precision_specified && conv.has_zero_flag()) {
     num_zeroes += num_left_spaces;
     num_left_spaces = 0;
   }
@@ -202,72 +291,200 @@ bool ConvertIntImplInner(const ConvertedIntInfo &info,
 }
 
 template <typename T>
-bool ConvertIntImplInner(T v, const ConversionSpec conv, FormatSinkImpl *sink) {
-  ConvertedIntInfo info(v, conv.conv());
-  if (conv.flags().basic && conv.conv().id() != ConversionChar::p) {
-    if (info.is_neg()) sink->Append(1, '-');
-    if (info.digits().empty()) {
-      sink->Append(1, '0');
-    } else {
-      sink->Append(info.digits());
-    }
-    return true;
+bool ConvertFloatArg(T v, FormatConversionSpecImpl conv, FormatSinkImpl *sink) {
+  if (conv.conversion_char() == FormatConversionCharInternal::v) {
+    conv.set_conversion_char(FormatConversionCharInternal::g);
   }
-  return ConvertIntImplInner(info, conv, sink);
+
+  return FormatConversionCharIsFloat(conv.conversion_char()) &&
+         ConvertFloatImpl(v, conv, sink);
 }
 
-template <typename T>
-bool ConvertIntArg(T v, const ConversionSpec conv, FormatSinkImpl *sink) {
-  if (conv.conv().is_float()) {
-    return FormatConvertImpl(static_cast<double>(v), conv, sink).value;
+inline bool ConvertStringArg(string_view v, const FormatConversionSpecImpl conv,
+                             FormatSinkImpl *sink) {
+  if (conv.is_basic()) {
+    sink->Append(v);
+    return true;
   }
-  if (conv.conv().id() == ConversionChar::c)
-    return ConvertCharImpl(static_cast<unsigned char>(v), conv, sink);
-  if (!conv.conv().is_integral())
+  return sink->PutPaddedString(v, conv.width(), conv.precision(),
+                               conv.has_left_flag());
+}
+
+inline bool IsLowSurrogate(uint32_t c) { return c >= 0xDC00 && c <= 0xDFFF; }
+
+inline bool ConvertStringArg(const wchar_t *v,
+                             size_t len,
+                             const FormatConversionSpecImpl conv,
+                             FormatSinkImpl *sink) {
+  // Each wide character may result in up to 4 bytes (UTF-8 code units).
+  constexpr size_t kMaxUtf8CodeUnitsPerWideChar = 4;
+  if (len > (std::numeric_limits<decltype(len)>::max)() /
+                kMaxUtf8CodeUnitsPerWideChar) {
+    // Size too large; we can't handle this.
     return false;
-  if (!conv.conv().is_signed() && IsSigned<T>::value) {
-    using U = typename MakeUnsigned<T>::type;
-    return FormatConvertImpl(static_cast<U>(v), conv, sink).value;
   }
-  return ConvertIntImplInner(v, conv, sink);
+  FixedArray<char> mb(len * kMaxUtf8CodeUnitsPerWideChar);
+  strings_internal::ShiftState s;
+  size_t chars_written = 0;
+  for (size_t i = 0; i < len; ++i) {
+    // A high surrogate must be immediately followed by a low surrogate. If it
+    // isn't, the UTF-16 input is malformed and WideToUtf8() would otherwise
+    // leave a partial sequence in the buffer. The single wchar_t path already
+    // rejects an unpaired surrogate, so reject it here too.
+    if (s.saw_high_surrogate) {
+      const uint32_t cu = static_cast<uint32_t>(v[i]);
+      if (!IsLowSurrogate(cu)) return false;
+    }
+    const size_t chars =
+        strings_internal::WideToUtf8(v[i], &mb[chars_written], s);
+    if (chars == static_cast<size_t>(-1)) { return false; }
+    chars_written += chars;
+  }
+  // A trailing high surrogate has no low surrogate to complete it.
+  if (s.saw_high_surrogate) return false;
+  return ConvertStringArg(string_view(mb.data(), chars_written), conv, sink);
 }
 
-template <typename T>
-bool ConvertFloatArg(T v, const ConversionSpec conv, FormatSinkImpl *sink) {
-  return conv.conv().is_float() && ConvertFloatImpl(v, conv, sink);
+bool ConvertWCharTImpl(wchar_t v, const FormatConversionSpecImpl conv,
+                       FormatSinkImpl *sink) {
+  char mb[4];
+  strings_internal::ShiftState s;
+  const size_t chars_written = strings_internal::WideToUtf8(v, mb, s);
+  return chars_written != static_cast<size_t>(-1) && !s.saw_high_surrogate &&
+         ConvertStringArg(string_view(mb, chars_written), conv, sink);
 }
 
-inline bool ConvertStringArg(string_view v, const ConversionSpec conv,
-                             FormatSinkImpl *sink) {
-  if (conv.conv().id() != ConversionChar::s)
-    return false;
-  if (conv.flags().basic) {
-    sink->Append(v);
+}  // namespace
+
+bool ConvertBoolArg(bool v, FormatSinkImpl *sink) {
+  if (v) {
+    sink->Append("true");
+  } else {
+    sink->Append("false");
+  }
+  return true;
+}
+
+template <typename T>
+bool ConvertIntArg(T v, FormatConversionSpecImpl conv, FormatSinkImpl *sink) {
+  using U = typename MakeUnsigned<T>::type;
+  IntDigits as_digits;
+
+  // This odd casting is due to a bug in -Wswitch behavior in gcc49 which causes
+  // it to complain about a switch/case type mismatch, even though both are
+  // FormatConversionChar.  Likely this is because at this point
+  // FormatConversionChar is declared, but not defined.
+  switch (static_cast<uint8_t>(conv.conversion_char())) {
+    case static_cast<uint8_t>(FormatConversionCharInternal::c):
+      return (std::is_same_v<T, wchar_t> || (conv.length_mod() == LengthMod::l))
+                 ? ConvertWCharTImpl(static_cast<wchar_t>(v), conv, sink)
+                 : ConvertCharImpl(static_cast<char>(v), conv, sink);
+
+    case static_cast<uint8_t>(FormatConversionCharInternal::o):
+      as_digits.PrintAsOct(static_cast<U>(v));
+      break;
+
+    case static_cast<uint8_t>(FormatConversionCharInternal::x):
+      as_digits.PrintAsHexLower(static_cast<U>(v));
+      break;
+    case static_cast<uint8_t>(FormatConversionCharInternal::X):
+      as_digits.PrintAsHexUpper(static_cast<U>(v));
+      break;
+
+    case static_cast<uint8_t>(FormatConversionCharInternal::u):
+      as_digits.PrintAsDec(static_cast<U>(v));
+      break;
+
+    case static_cast<uint8_t>(FormatConversionCharInternal::d):
+    case static_cast<uint8_t>(FormatConversionCharInternal::i):
+    case static_cast<uint8_t>(FormatConversionCharInternal::v):
+      as_digits.PrintAsDec(v);
+      break;
+
+    case static_cast<uint8_t>(FormatConversionCharInternal::a):
+    case static_cast<uint8_t>(FormatConversionCharInternal::e):
+    case static_cast<uint8_t>(FormatConversionCharInternal::f):
+    case static_cast<uint8_t>(FormatConversionCharInternal::g):
+    case static_cast<uint8_t>(FormatConversionCharInternal::A):
+    case static_cast<uint8_t>(FormatConversionCharInternal::E):
+    case static_cast<uint8_t>(FormatConversionCharInternal::F):
+    case static_cast<uint8_t>(FormatConversionCharInternal::G):
+      return ConvertFloatImpl(static_cast<double>(v), conv, sink);
+
+    default:
+      ABSL_ASSUME(false);
+  }
+
+  if (conv.is_basic()) {
+    sink->Append(as_digits.with_neg_and_zero());
     return true;
   }
-  return sink->PutPaddedString(v, conv.width(), conv.precision(),
-                               conv.flags().left);
+  return ConvertIntImplInnerSlow(as_digits, conv, sink);
 }
 
-}  // namespace
+template bool ConvertIntArg<char>(char v, FormatConversionSpecImpl conv,
+                                  FormatSinkImpl *sink);
+template bool ConvertIntArg<signed char>(signed char v,
+                                         FormatConversionSpecImpl conv,
+                                         FormatSinkImpl *sink);
+template bool ConvertIntArg<unsigned char>(unsigned char v,
+                                           FormatConversionSpecImpl conv,
+                                           FormatSinkImpl *sink);
+template bool ConvertIntArg<wchar_t>(wchar_t v, FormatConversionSpecImpl conv,
+                                     FormatSinkImpl *sink);
+template bool ConvertIntArg<short>(short v,  // NOLINT
+                                   FormatConversionSpecImpl conv,
+                                   FormatSinkImpl *sink);
+template bool ConvertIntArg<unsigned short>(unsigned short v,  // NOLINT
+                                            FormatConversionSpecImpl conv,
+                                            FormatSinkImpl *sink);
+template bool ConvertIntArg<int>(int v, FormatConversionSpecImpl conv,
+                                 FormatSinkImpl *sink);
+template bool ConvertIntArg<unsigned int>(unsigned int v,
+                                          FormatConversionSpecImpl conv,
+                                          FormatSinkImpl *sink);
+template bool ConvertIntArg<long>(long v,  // NOLINT
+                                  FormatConversionSpecImpl conv,
+                                  FormatSinkImpl *sink);
+template bool ConvertIntArg<unsigned long>(unsigned long v,  // NOLINT
+                                           FormatConversionSpecImpl conv,
+                                           FormatSinkImpl *sink);
+template bool ConvertIntArg<long long>(long long v,  // NOLINT
+                                       FormatConversionSpecImpl conv,
+                                       FormatSinkImpl *sink);
+template bool ConvertIntArg<unsigned long long>(unsigned long long v,  // NOLINT
+                                                FormatConversionSpecImpl conv,
+                                                FormatSinkImpl *sink);
 
 // ==================== Strings ====================
-ConvertResult<Conv::s> FormatConvertImpl(const std::string &v,
-                                         const ConversionSpec conv,
-                                         FormatSinkImpl *sink) {
+StringConvertResult FormatConvertImpl(const std::string &v,
+                                      const FormatConversionSpecImpl conv,
+                                      FormatSinkImpl *sink) {
   return {ConvertStringArg(v, conv, sink)};
 }
 
-ConvertResult<Conv::s> FormatConvertImpl(string_view v,
-                                         const ConversionSpec conv,
-                                         FormatSinkImpl *sink) {
+StringConvertResult FormatConvertImpl(const std::wstring &v,
+                                      const FormatConversionSpecImpl conv,
+                                      FormatSinkImpl *sink) {
+  return {ConvertStringArg(v.data(), v.size(), conv, sink)};
+}
+
+StringConvertResult FormatConvertImpl(string_view v,
+                                      const FormatConversionSpecImpl conv,
+                                      FormatSinkImpl *sink) {
   return {ConvertStringArg(v, conv, sink)};
 }
 
-ConvertResult<Conv::s | Conv::p> FormatConvertImpl(const char *v,
-                                                   const ConversionSpec conv,
-                                                   FormatSinkImpl *sink) {
-  if (conv.conv().id() == ConversionChar::p)
+StringConvertResult FormatConvertImpl(std::wstring_view v,
+                                      const FormatConversionSpecImpl conv,
+                                      FormatSinkImpl* sink) {
+  return {ConvertStringArg(v.data(), v.size(), conv, sink)};
+}
+
+StringPtrConvertResult FormatConvertImpl(const char* v,
+                                         const FormatConversionSpecImpl conv,
+                                         FormatSinkImpl* sink) {
+  if (conv.conversion_char() == FormatConversionCharInternal::p)
     return {FormatConvertImpl(VoidPtr(v), conv, sink).value};
   size_t len;
   if (v == nullptr) {
@@ -275,96 +492,134 @@ ConvertResult<Conv::s | Conv::p> FormatConvertImpl(const char *v,
   } else if (conv.precision() < 0) {
     len = std::strlen(v);
   } else {
-    // If precision is set, we look for the null terminator on the valid range.
-    len = std::find(v, v + conv.precision(), '\0') - v;
+    // If precision is set, we look for the NUL-terminator on the valid range.
+    len = static_cast<size_t>(std::find(v, v + conv.precision(), '\0') - v);
   }
   return {ConvertStringArg(string_view(v, len), conv, sink)};
 }
 
+StringPtrConvertResult FormatConvertImpl(const wchar_t* v,
+                                         const FormatConversionSpecImpl conv,
+                                         FormatSinkImpl* sink) {
+  if (conv.conversion_char() == FormatConversionCharInternal::p) {
+    return {FormatConvertImpl(VoidPtr(v), conv, sink).value};
+  }
+  size_t len;
+  if (v == nullptr) {
+    len = 0;
+  } else if (conv.precision() < 0) {
+    len = std::wcslen(v);
+  } else {
+    // If precision is set, we look for the NUL-terminator on the valid range.
+    len = static_cast<size_t>(std::find(v, v + conv.precision(), L'\0') - v);
+  }
+  return {ConvertStringArg(v, len, conv, sink)};
+}
+
+StringPtrConvertResult FormatConvertImpl(std::nullptr_t,
+                                         const FormatConversionSpecImpl conv,
+                                         FormatSinkImpl* sink) {
+  return FormatConvertImpl(static_cast<const char*>(nullptr), conv, sink);
+}
+
 // ==================== Raw pointers ====================
-ConvertResult<Conv::p> FormatConvertImpl(VoidPtr v, const ConversionSpec conv,
-                                         FormatSinkImpl *sink) {
-  if (conv.conv().id() != ConversionChar::p)
-    return {false};
+ArgConvertResult<FormatConversionCharSetInternal::p> FormatConvertImpl(
+    VoidPtr v, const FormatConversionSpecImpl conv, FormatSinkImpl *sink) {
   if (!v.value) {
     sink->Append("(nil)");
     return {true};
   }
-  return {ConvertIntImplInner(v.value, conv, sink)};
+  IntDigits as_digits;
+  as_digits.PrintAsHexLower(v.value);
+  return {ConvertIntImplInnerSlow(as_digits, conv, sink)};
 }
 
 // ==================== Floats ====================
-FloatingConvertResult FormatConvertImpl(float v, const ConversionSpec conv,
+FloatingConvertResult FormatConvertImpl(float v,
+                                        const FormatConversionSpecImpl conv,
                                         FormatSinkImpl *sink) {
   return {ConvertFloatArg(v, conv, sink)};
 }
-FloatingConvertResult FormatConvertImpl(double v, const ConversionSpec conv,
+FloatingConvertResult FormatConvertImpl(double v,
+                                        const FormatConversionSpecImpl conv,
                                         FormatSinkImpl *sink) {
   return {ConvertFloatArg(v, conv, sink)};
 }
 FloatingConvertResult FormatConvertImpl(long double v,
-                                        const ConversionSpec conv,
+                                        const FormatConversionSpecImpl conv,
                                         FormatSinkImpl *sink) {
   return {ConvertFloatArg(v, conv, sink)};
 }
 
 // ==================== Chars ====================
-IntegralConvertResult FormatConvertImpl(char v, const ConversionSpec conv,
-                                        FormatSinkImpl *sink) {
+CharConvertResult FormatConvertImpl(char v, const FormatConversionSpecImpl conv,
+                                    FormatSinkImpl *sink) {
   return {ConvertIntArg(v, conv, sink)};
 }
+CharConvertResult FormatConvertImpl(wchar_t v,
+                                    const FormatConversionSpecImpl conv,
+                                    FormatSinkImpl* sink) {
+  return {ConvertIntArg(v, conv, sink)};
+}
+
+// ==================== Ints ====================
 IntegralConvertResult FormatConvertImpl(signed char v,
-                                        const ConversionSpec conv,
+                                        const FormatConversionSpecImpl conv,
                                         FormatSinkImpl *sink) {
   return {ConvertIntArg(v, conv, sink)};
 }
 IntegralConvertResult FormatConvertImpl(unsigned char v,
-                                        const ConversionSpec conv,
+                                        const FormatConversionSpecImpl conv,
                                         FormatSinkImpl *sink) {
   return {ConvertIntArg(v, conv, sink)};
 }
-
-// ==================== Ints ====================
 IntegralConvertResult FormatConvertImpl(short v,  // NOLINT
-                                        const ConversionSpec conv,
+                                        const FormatConversionSpecImpl conv,
                                         FormatSinkImpl *sink) {
   return {ConvertIntArg(v, conv, sink)};
 }
 IntegralConvertResult FormatConvertImpl(unsigned short v,  // NOLINT
-                                        const ConversionSpec conv,
+                                        const FormatConversionSpecImpl conv,
                                         FormatSinkImpl *sink) {
   return {ConvertIntArg(v, conv, sink)};
 }
-IntegralConvertResult FormatConvertImpl(int v, const ConversionSpec conv,
+IntegralConvertResult FormatConvertImpl(int v,
+                                        const FormatConversionSpecImpl conv,
                                         FormatSinkImpl *sink) {
   return {ConvertIntArg(v, conv, sink)};
 }
-IntegralConvertResult FormatConvertImpl(unsigned v, const ConversionSpec conv,
+IntegralConvertResult FormatConvertImpl(unsigned v,
+                                        const FormatConversionSpecImpl conv,
                                         FormatSinkImpl *sink) {
   return {ConvertIntArg(v, conv, sink)};
 }
 IntegralConvertResult FormatConvertImpl(long v,  // NOLINT
-                                        const ConversionSpec conv,
+                                        const FormatConversionSpecImpl conv,
                                         FormatSinkImpl *sink) {
   return {ConvertIntArg(v, conv, sink)};
 }
 IntegralConvertResult FormatConvertImpl(unsigned long v,  // NOLINT
-                                        const ConversionSpec conv,
+                                        const FormatConversionSpecImpl conv,
                                         FormatSinkImpl *sink) {
   return {ConvertIntArg(v, conv, sink)};
 }
 IntegralConvertResult FormatConvertImpl(long long v,  // NOLINT
-                                        const ConversionSpec conv,
+                                        const FormatConversionSpecImpl conv,
                                         FormatSinkImpl *sink) {
   return {ConvertIntArg(v, conv, sink)};
 }
 IntegralConvertResult FormatConvertImpl(unsigned long long v,  // NOLINT
-                                        const ConversionSpec conv,
+                                        const FormatConversionSpecImpl conv,
+                                        FormatSinkImpl *sink) {
+  return {ConvertIntArg(v, conv, sink)};
+}
+IntegralConvertResult FormatConvertImpl(absl::int128 v,
+                                        const FormatConversionSpecImpl conv,
                                         FormatSinkImpl *sink) {
   return {ConvertIntArg(v, conv, sink)};
 }
 IntegralConvertResult FormatConvertImpl(absl::uint128 v,
-                                        const ConversionSpec conv,
+                                        const FormatConversionSpecImpl conv,
                                         FormatSinkImpl *sink) {
   return {ConvertIntArg(v, conv, sink)};
 }
@@ -372,6 +627,8 @@ IntegralConvertResult FormatConvertImpl(absl::uint128 v,
 ABSL_INTERNAL_FORMAT_DISPATCH_OVERLOADS_EXPAND_();
 
 
+
 }  // namespace str_format_internal
 
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/internal/str_format/arg.h b/absl/strings/internal/str_format/arg.h
index 5cb3a14d683..d8fef301d49 100644
--- a/absl/strings/internal/str_format/arg.h
+++ b/absl/strings/internal/str_format/arg.h
@@ -1,41 +1,176 @@
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
 #ifndef ABSL_STRINGS_INTERNAL_STR_FORMAT_ARG_H_
 #define ABSL_STRINGS_INTERNAL_STR_FORMAT_ARG_H_
 
 #include <string.h>
 #include <wchar.h>
 
+#include <algorithm>
+#include <cstddef>
+#include <cstdint>
 #include <cstdio>
-#include <iomanip>
 #include <limits>
 #include <memory>
 #include <sstream>
 #include <string>
+#include <string_view>
 #include <type_traits>
+#include <utility>
 
-#include "absl/base/port.h"
+#include "absl/base/config.h"
+#include "absl/base/optimization.h"
 #include "absl/meta/type_traits.h"
 #include "absl/numeric/int128.h"
+#include "absl/strings/has_absl_stringify.h"
 #include "absl/strings/internal/str_format/extension.h"
 #include "absl/strings/string_view.h"
 
-class Cord;
-class CordReader;
-
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
+class Cord;
 class FormatCountCapture;
 class FormatSink;
 
+template <absl::FormatConversionCharSet C>
+struct FormatConvertResult;
+class FormatConversionSpec;
+
 namespace str_format_internal {
 
+template <FormatConversionCharSet C>
+struct ArgConvertResult {
+  bool value;
+};
+
+using IntegralConvertResult = ArgConvertResult<FormatConversionCharSetUnion(
+    FormatConversionCharSetInternal::c,
+    FormatConversionCharSetInternal::kNumeric,
+    FormatConversionCharSetInternal::kStar,
+    FormatConversionCharSetInternal::v)>;
+using FloatingConvertResult = ArgConvertResult<FormatConversionCharSetUnion(
+    FormatConversionCharSetInternal::kFloating,
+    FormatConversionCharSetInternal::v)>;
+using CharConvertResult = ArgConvertResult<FormatConversionCharSetUnion(
+    FormatConversionCharSetInternal::c,
+    FormatConversionCharSetInternal::kNumeric,
+    FormatConversionCharSetInternal::kStar)>;
+
 template <typename T, typename = void>
 struct HasUserDefinedConvert : std::false_type {};
 
 template <typename T>
-struct HasUserDefinedConvert<
-    T, void_t<decltype(AbslFormatConvert(
-           std::declval<const T&>(), std::declval<ConversionSpec>(),
-           std::declval<FormatSink*>()))>> : std::true_type {};
+struct HasUserDefinedConvert<T, std::void_t<decltype(AbslFormatConvert(
+                                    std::declval<const T&>(),
+                                    std::declval<const FormatConversionSpec&>(),
+                                    std::declval<FormatSink*>()))>>
+    : std::true_type {};
+
+// These declarations prevent ADL lookup from continuing in absl namespaces,
+// we are deliberately using these as ADL hooks and want them to consider
+// non-absl namespaces only.
+void AbslFormatConvert();
+void AbslStringify();
+
+template <typename T>
+bool ConvertIntArg(T v, FormatConversionSpecImpl conv, FormatSinkImpl* sink);
+
+// Forward declarations of internal `ConvertIntArg` function template
+// instantiations are here to avoid including the template body in the headers
+// and instantiating it in large numbers of translation units. Explicit
+// instantiations can be found in "absl/strings/internal/str_format/arg.cc"
+extern template bool ConvertIntArg<char>(char v, FormatConversionSpecImpl conv,
+                                         FormatSinkImpl* sink);
+extern template bool ConvertIntArg<signed char>(signed char v,
+                                                FormatConversionSpecImpl conv,
+                                                FormatSinkImpl* sink);
+extern template bool ConvertIntArg<unsigned char>(unsigned char v,
+                                                  FormatConversionSpecImpl conv,
+                                                  FormatSinkImpl* sink);
+extern template bool ConvertIntArg<wchar_t>(wchar_t v,
+                                            FormatConversionSpecImpl conv,
+                                            FormatSinkImpl* sink);
+extern template bool ConvertIntArg<short>(short v,  // NOLINT
+                                          FormatConversionSpecImpl conv,
+                                          FormatSinkImpl* sink);
+extern template bool ConvertIntArg<unsigned short>(   // NOLINT
+    unsigned short v, FormatConversionSpecImpl conv,  // NOLINT
+    FormatSinkImpl* sink);
+extern template bool ConvertIntArg<int>(int v, FormatConversionSpecImpl conv,
+                                        FormatSinkImpl* sink);
+extern template bool ConvertIntArg<unsigned int>(unsigned int v,
+                                                 FormatConversionSpecImpl conv,
+                                                 FormatSinkImpl* sink);
+extern template bool ConvertIntArg<long>(                           // NOLINT
+    long v, FormatConversionSpecImpl conv, FormatSinkImpl* sink);   // NOLINT
+extern template bool ConvertIntArg<unsigned long>(unsigned long v,  // NOLINT
+                                                  FormatConversionSpecImpl conv,
+                                                  FormatSinkImpl* sink);
+extern template bool ConvertIntArg<long long>(long long v,  // NOLINT
+                                              FormatConversionSpecImpl conv,
+                                              FormatSinkImpl* sink);
+extern template bool ConvertIntArg<unsigned long long>(   // NOLINT
+    unsigned long long v, FormatConversionSpecImpl conv,  // NOLINT
+    FormatSinkImpl* sink);
+
+template <typename T>
+auto FormatConvertImpl(const T& v, FormatConversionSpecImpl conv,
+                       FormatSinkImpl* sink)
+    -> decltype(AbslFormatConvert(v,
+                                  std::declval<const FormatConversionSpec&>(),
+                                  std::declval<FormatSink*>())) {
+  using FormatConversionSpecT =
+      std::enable_if_t<sizeof(const T& (*)()) != 0, FormatConversionSpec>;
+  using FormatSinkT = std::enable_if_t<sizeof(const T& (*)()) != 0, FormatSink>;
+  auto fcs = conv.Wrap<FormatConversionSpecT>();
+  auto fs = sink->Wrap<FormatSinkT>();
+  return AbslFormatConvert(v, fcs, &fs);
+}
+
+template <typename T>
+auto FormatConvertImpl(const T& v, FormatConversionSpecImpl conv,
+                       FormatSinkImpl* sink)
+    -> std::enable_if_t<std::is_enum_v<T> &&
+                            std::is_void_v<decltype(AbslStringify(
+                                std::declval<FormatSink&>(), v))>,
+                        IntegralConvertResult> {
+  if (conv.conversion_char() == FormatConversionCharInternal::v) {
+    using FormatSinkT =
+        std::enable_if_t<sizeof(const T& (*)()) != 0, FormatSink>;
+    auto fs = sink->Wrap<FormatSinkT>();
+    AbslStringify(fs, v);
+    return {true};
+  } else {
+    return {
+        ConvertIntArg(static_cast<std::underlying_type_t<T>>(v), conv, sink)};
+  }
+}
+
+template <typename T>
+auto FormatConvertImpl(const T& v, FormatConversionSpecImpl,
+                       FormatSinkImpl* sink)
+    -> std::enable_if_t<!std::is_enum_v<T> && !std::is_same_v<T, absl::Cord> &&
+                            std::is_void_v<decltype(AbslStringify(
+                                std::declval<FormatSink&>(), v))>,
+                        ArgConvertResult<FormatConversionCharSetInternal::v>> {
+  using FormatSinkT = std::enable_if_t<sizeof(const T& (*)()) != 0, FormatSink>;
+  auto fs = sink->Wrap<FormatSinkT>();
+  AbslStringify(fs, v);
+  return {true};
+}
 
 template <typename T>
 class StreamedWrapper;
@@ -44,6 +179,13 @@ class StreamedWrapper;
 // then convert it, appending to `sink` and return `true`.
 // Otherwise fail and return `false`.
 
+// AbslFormatConvert(v, conv, sink) is intended to be found by ADL on 'v'
+// as an extension mechanism. These FormatConvertImpl functions are the default
+// implementations.
+// The ADL search is augmented via the 'Sink*' parameter, which also
+// serves as a disambiguator to reject possible unintended 'AbslFormatConvert'
+// functions in the namespaces associated with 'v'.
+
 // Raw pointers.
 struct VoidPtr {
   VoidPtr() = default;
@@ -53,32 +195,67 @@ struct VoidPtr {
       : value(ptr ? reinterpret_cast<uintptr_t>(ptr) : 0) {}
   uintptr_t value;
 };
-ConvertResult<Conv::p> FormatConvertImpl(VoidPtr v, ConversionSpec conv,
-                                         FormatSinkImpl* sink);
+
+template <FormatConversionCharSet C>
+constexpr FormatConversionCharSet ExtractCharSet(FormatConvertResult<C>) {
+  return C;
+}
+
+template <FormatConversionCharSet C>
+constexpr FormatConversionCharSet ExtractCharSet(ArgConvertResult<C>) {
+  return C;
+}
+
+ArgConvertResult<FormatConversionCharSetInternal::p> FormatConvertImpl(
+    VoidPtr v, FormatConversionSpecImpl conv, FormatSinkImpl* sink);
 
 // Strings.
-ConvertResult<Conv::s> FormatConvertImpl(const std::string& v,
-                                         ConversionSpec conv,
+using StringConvertResult = ArgConvertResult<FormatConversionCharSetUnion(
+    FormatConversionCharSetInternal::s, FormatConversionCharSetInternal::v)>;
+StringConvertResult FormatConvertImpl(const std::string& v,
+                                      FormatConversionSpecImpl conv,
+                                      FormatSinkImpl* sink);
+StringConvertResult FormatConvertImpl(const std::wstring& v,
+                                      FormatConversionSpecImpl conv,
+                                      FormatSinkImpl* sink);
+StringConvertResult FormatConvertImpl(string_view v,
+                                      FormatConversionSpecImpl conv,
+                                      FormatSinkImpl* sink);
+StringConvertResult FormatConvertImpl(std::wstring_view v,
+                                      FormatConversionSpecImpl conv,
+                                      FormatSinkImpl* sink);
+#if !defined(ABSL_USES_STD_STRING_VIEW)
+inline StringConvertResult FormatConvertImpl(std::string_view v,
+                                             FormatConversionSpecImpl conv,
+                                             FormatSinkImpl* sink) {
+  return FormatConvertImpl(absl::string_view(v.data(), v.size()), conv, sink);
+}
+#endif  // !ABSL_USES_STD_STRING_VIEW
+
+using StringPtrConvertResult = ArgConvertResult<FormatConversionCharSetUnion(
+    FormatConversionCharSetInternal::s, FormatConversionCharSetInternal::p)>;
+StringPtrConvertResult FormatConvertImpl(const char* v,
+                                         FormatConversionSpecImpl conv,
                                          FormatSinkImpl* sink);
-ConvertResult<Conv::s> FormatConvertImpl(string_view v, ConversionSpec conv,
+StringPtrConvertResult FormatConvertImpl(const wchar_t* v,
+                                         FormatConversionSpecImpl conv,
                                          FormatSinkImpl* sink);
-ConvertResult<Conv::s | Conv::p> FormatConvertImpl(const char* v,
-                                                   ConversionSpec conv,
-                                                   FormatSinkImpl* sink);
+// This overload is needed to disambiguate, since `nullptr` could match either
+// of the other overloads equally well.
+StringPtrConvertResult FormatConvertImpl(std::nullptr_t,
+                                         FormatConversionSpecImpl conv,
+                                         FormatSinkImpl* sink);
+
 template <class AbslCord,
-          typename std::enable_if<
-              std::is_same<AbslCord, ::Cord>::value>::type* = nullptr,
-          class AbslCordReader = ::CordReader>
-ConvertResult<Conv::s> FormatConvertImpl(const AbslCord& value,
-                                         ConversionSpec conv,
-                                         FormatSinkImpl* sink) {
-  if (conv.conv().id() != ConversionChar::s) return {false};
-
-  bool is_left = conv.flags().left;
+          std::enable_if_t<std::is_same_v<AbslCord, absl::Cord>>* = nullptr>
+StringConvertResult FormatConvertImpl(const AbslCord& value,
+                                      FormatConversionSpecImpl conv,
+                                      FormatSinkImpl* sink) {
+  bool is_left = conv.has_left_flag();
   size_t space_remaining = 0;
 
   int width = conv.width();
-  if (width >= 0) space_remaining = width;
+  if (width >= 0) space_remaining = static_cast<size_t>(width);
 
   size_t to_write = value.size();
 
@@ -90,80 +267,100 @@ ConvertResult<Conv::s> FormatConvertImpl(const AbslCord& value,
 
   if (space_remaining > 0 && !is_left) sink->Append(space_remaining, ' ');
 
-  string_view piece;
-  for (AbslCordReader reader(value);
-       to_write > 0 && reader.ReadFragment(&piece); to_write -= piece.size()) {
-    if (piece.size() > to_write) piece.remove_suffix(piece.size() - to_write);
+  for (string_view piece : value.Chunks()) {
+    if (piece.size() > to_write) {
+      piece.remove_suffix(piece.size() - to_write);
+      to_write = 0;
+    } else {
+      to_write -= piece.size();
+    }
     sink->Append(piece);
+    if (to_write == 0) {
+      break;
+    }
   }
 
   if (space_remaining > 0 && is_left) sink->Append(space_remaining, ' ');
   return {true};
 }
 
-using IntegralConvertResult =
-    ConvertResult<Conv::c | Conv::numeric | Conv::star>;
-using FloatingConvertResult = ConvertResult<Conv::floating>;
+bool ConvertBoolArg(bool v, FormatSinkImpl* sink);
 
 // Floats.
-FloatingConvertResult FormatConvertImpl(float v, ConversionSpec conv,
+FloatingConvertResult FormatConvertImpl(float v, FormatConversionSpecImpl conv,
                                         FormatSinkImpl* sink);
-FloatingConvertResult FormatConvertImpl(double v, ConversionSpec conv,
+FloatingConvertResult FormatConvertImpl(double v, FormatConversionSpecImpl conv,
                                         FormatSinkImpl* sink);
-FloatingConvertResult FormatConvertImpl(long double v, ConversionSpec conv,
+FloatingConvertResult FormatConvertImpl(long double v,
+                                        FormatConversionSpecImpl conv,
                                         FormatSinkImpl* sink);
 
 // Chars.
-IntegralConvertResult FormatConvertImpl(char v, ConversionSpec conv,
-                                        FormatSinkImpl* sink);
-IntegralConvertResult FormatConvertImpl(signed char v, ConversionSpec conv,
-                                        FormatSinkImpl* sink);
-IntegralConvertResult FormatConvertImpl(unsigned char v, ConversionSpec conv,
-                                        FormatSinkImpl* sink);
+CharConvertResult FormatConvertImpl(char v, FormatConversionSpecImpl conv,
+                                    FormatSinkImpl* sink);
+CharConvertResult FormatConvertImpl(wchar_t v, FormatConversionSpecImpl conv,
+                                    FormatSinkImpl* sink);
 
 // Ints.
+IntegralConvertResult FormatConvertImpl(signed char v,
+                                        FormatConversionSpecImpl conv,
+                                        FormatSinkImpl* sink);
+IntegralConvertResult FormatConvertImpl(unsigned char v,
+                                        FormatConversionSpecImpl conv,
+                                        FormatSinkImpl* sink);
 IntegralConvertResult FormatConvertImpl(short v,  // NOLINT
-                                        ConversionSpec conv,
+                                        FormatConversionSpecImpl conv,
                                         FormatSinkImpl* sink);
 IntegralConvertResult FormatConvertImpl(unsigned short v,  // NOLINT
-                                        ConversionSpec conv,
+                                        FormatConversionSpecImpl conv,
                                         FormatSinkImpl* sink);
-IntegralConvertResult FormatConvertImpl(int v, ConversionSpec conv,
+IntegralConvertResult FormatConvertImpl(int v, FormatConversionSpecImpl conv,
                                         FormatSinkImpl* sink);
-IntegralConvertResult FormatConvertImpl(unsigned v, ConversionSpec conv,
+IntegralConvertResult FormatConvertImpl(unsigned v,
+                                        FormatConversionSpecImpl conv,
                                         FormatSinkImpl* sink);
 IntegralConvertResult FormatConvertImpl(long v,  // NOLINT
-                                        ConversionSpec conv,
+                                        FormatConversionSpecImpl conv,
                                         FormatSinkImpl* sink);
 IntegralConvertResult FormatConvertImpl(unsigned long v,  // NOLINT
-                                        ConversionSpec conv,
+                                        FormatConversionSpecImpl conv,
                                         FormatSinkImpl* sink);
 IntegralConvertResult FormatConvertImpl(long long v,  // NOLINT
-                                        ConversionSpec conv,
+                                        FormatConversionSpecImpl conv,
                                         FormatSinkImpl* sink);
 IntegralConvertResult FormatConvertImpl(unsigned long long v,  // NOLINT
-                                        ConversionSpec conv,
+                                        FormatConversionSpecImpl conv,
+                                        FormatSinkImpl* sink);
+IntegralConvertResult FormatConvertImpl(int128 v, FormatConversionSpecImpl conv,
                                         FormatSinkImpl* sink);
-IntegralConvertResult FormatConvertImpl(uint128 v, ConversionSpec conv,
+IntegralConvertResult FormatConvertImpl(uint128 v,
+                                        FormatConversionSpecImpl conv,
                                         FormatSinkImpl* sink);
-template <typename T, enable_if_t<std::is_same<T, bool>::value, int> = 0>
-IntegralConvertResult FormatConvertImpl(T v, ConversionSpec conv,
+
+// This function needs to be a template due to ambiguity regarding type
+// conversions.
+template <typename T, std::enable_if_t<std::is_same_v<T, bool>, int> = 0>
+IntegralConvertResult FormatConvertImpl(T v, FormatConversionSpecImpl conv,
                                         FormatSinkImpl* sink) {
+  if (conv.conversion_char() == FormatConversionCharInternal::v) {
+    return {ConvertBoolArg(v, sink)};
+  }
+
   return FormatConvertImpl(static_cast<int>(v), conv, sink);
 }
 
 // We provide this function to help the checker, but it is never defined.
 // FormatArgImpl will use the underlying Convert functions instead.
 template <typename T>
-typename std::enable_if<std::is_enum<T>::value &&
-                            !HasUserDefinedConvert<T>::value,
-                        IntegralConvertResult>::type
-FormatConvertImpl(T v, ConversionSpec conv, FormatSinkImpl* sink);
+std::enable_if_t<std::is_enum_v<T> && !HasUserDefinedConvert<T>::value &&
+                     !HasAbslStringify<T>::value,
+                 IntegralConvertResult>
+FormatConvertImpl(T v, FormatConversionSpecImpl conv, FormatSinkImpl* sink);
 
 template <typename T>
-ConvertResult<Conv::s> FormatConvertImpl(const StreamedWrapper<T>& v,
-                                         ConversionSpec conv,
-                                         FormatSinkImpl* out) {
+StringConvertResult FormatConvertImpl(const StreamedWrapper<T>& v,
+                                      FormatConversionSpecImpl conv,
+                                      FormatSinkImpl* out) {
   std::ostringstream oss;
   oss << v.v_;
   if (!oss) return {false};
@@ -174,22 +371,24 @@ ConvertResult<Conv::s> FormatConvertImpl(const StreamedWrapper<T>& v,
 // until after FormatCountCapture is fully defined.
 struct FormatCountCaptureHelper {
   template <class T = int>
-  static ConvertResult<Conv::n> ConvertHelper(const FormatCountCapture& v,
-                                              ConversionSpec conv,
-                                              FormatSinkImpl* sink) {
-    const absl::enable_if_t<sizeof(T) != 0, FormatCountCapture>& v2 = v;
+  static ArgConvertResult<FormatConversionCharSetInternal::n> ConvertHelper(
+      const FormatCountCapture& v, FormatConversionSpecImpl conv,
+      FormatSinkImpl* sink) {
+    const std::enable_if_t<sizeof(T) != 0, FormatCountCapture>& v2 = v;
 
-    if (conv.conv().id() != str_format_internal::ConversionChar::n)
+    if (conv.conversion_char() !=
+        str_format_internal::FormatConversionCharInternal::n) {
       return {false};
+    }
     *v2.p_ = static_cast<int>(sink->size());
     return {true};
   }
 };
 
 template <class T = int>
-ConvertResult<Conv::n> FormatConvertImpl(const FormatCountCapture& v,
-                                         ConversionSpec conv,
-                                         FormatSinkImpl* sink) {
+ArgConvertResult<FormatConversionCharSetInternal::n> FormatConvertImpl(
+    const FormatCountCapture& v, FormatConversionSpecImpl conv,
+    FormatSinkImpl* sink) {
   return FormatCountCaptureHelper::ConvertHelper(v, conv, sink);
 }
 
@@ -198,13 +397,13 @@ ConvertResult<Conv::n> FormatConvertImpl(const FormatCountCapture& v,
 struct FormatArgImplFriend {
   template <typename Arg>
   static bool ToInt(Arg arg, int* out) {
-    // A value initialized ConversionSpec has a `none` conv, which tells the
-    // dispatcher to run the `int` conversion.
+    // A value initialized FormatConversionSpecImpl has a `none` conv, which
+    // tells the dispatcher to run the `int` conversion.
     return arg.dispatcher_(arg.data_, {}, out);
   }
 
   template <typename Arg>
-  static bool Convert(Arg arg, str_format_internal::ConversionSpec conv,
+  static bool Convert(Arg arg, FormatConversionSpecImpl conv,
                       FormatSinkImpl* out) {
     return arg.dispatcher_(arg.data_, conv, out);
   }
@@ -215,6 +414,15 @@ struct FormatArgImplFriend {
   }
 };
 
+template <typename Arg>
+constexpr FormatConversionCharSet ArgumentToConv() {
+  using ConvResult = decltype(str_format_internal::FormatConvertImpl(
+      std::declval<const Arg&>(),
+      std::declval<const FormatConversionSpecImpl&>(),
+      std::declval<FormatSinkImpl*>()));
+  return absl::str_format_internal::ExtractCharSet(ConvResult{});
+}
+
 // A type-erased handle to a format argument.
 class FormatArgImpl {
  private:
@@ -228,21 +436,20 @@ class FormatArgImpl {
     char buf[kInlinedSpace];
   };
 
-  using Dispatcher = bool (*)(Data, ConversionSpec, void* out);
+  using Dispatcher = bool (*)(Data, FormatConversionSpecImpl, void* out);
 
   template <typename T>
   struct store_by_value
-      : std::integral_constant<bool, (sizeof(T) <= kInlinedSpace) &&
-                                         (std::is_integral<T>::value ||
-                                          std::is_floating_point<T>::value ||
-                                          std::is_pointer<T>::value ||
-                                          std::is_same<VoidPtr, T>::value)> {};
+      : std::bool_constant<
+            (sizeof(T) <= kInlinedSpace) &&
+            (std::is_integral_v<T> || std::is_floating_point_v<T> ||
+             std::is_pointer_v<T> || std::is_same_v<VoidPtr, T>)> {};
 
   enum StoragePolicy { ByPointer, ByVolatilePointer, ByValue };
   template <typename T>
   struct storage_policy
       : std::integral_constant<StoragePolicy,
-                               (std::is_volatile<T>::value
+                               (std::is_volatile_v<T>
                                     ? ByVolatilePointer
                                     : (store_by_value<T>::value ? ByValue
                                                                 : ByPointer))> {
@@ -252,26 +459,30 @@ class FormatArgImpl {
   // Anything with a user-defined Convert will get its own vtable.
   // For everything else:
   //   - Decay char* and char arrays into `const char*`
+  //   - Decay wchar_t* and wchar_t arrays into `const wchar_t*`
   //   - Decay any other pointer to `const void*`
-  //   - Decay all enums to their underlying type.
+  //   - Decay all enums to the integral promotion of their underlying type.
   //   - Decay function pointers to void*.
   template <typename T, typename = void>
   struct DecayType {
     static constexpr bool kHasUserDefined =
-        str_format_internal::HasUserDefinedConvert<T>::value;
-    using type = typename std::conditional<
-        !kHasUserDefined && std::is_convertible<T, const char*>::value,
-        const char*,
-        typename std::conditional<!kHasUserDefined &&
-                                      std::is_convertible<T, VoidPtr>::value,
-                                  VoidPtr, const T&>::type>::type;
+        str_format_internal::HasUserDefinedConvert<T>::value ||
+        HasAbslStringify<T>::value;
+    using type = std::conditional_t<
+        !kHasUserDefined && std::is_convertible_v<T, const char*>, const char*,
+        std::conditional_t<
+            !kHasUserDefined && std::is_convertible_v<T, const wchar_t*>,
+            const wchar_t*,
+            std::conditional_t<!kHasUserDefined &&
+                                   std::is_convertible_v<T, VoidPtr>,
+                               VoidPtr, const T&>>>;
   };
   template <typename T>
-  struct DecayType<T,
-                   typename std::enable_if<
-                       !str_format_internal::HasUserDefinedConvert<T>::value &&
-                       std::is_enum<T>::value>::type> {
-    using type = typename std::underlying_type<T>::type;
+  struct DecayType<
+      T,
+      std::enable_if_t<!str_format_internal::HasUserDefinedConvert<T>::value &&
+                       !HasAbslStringify<T>::value && std::is_enum_v<T>>> {
+    using type = decltype(+std::underlying_type_t<T>());
   };
 
  public:
@@ -279,7 +490,7 @@ class FormatArgImpl {
   explicit FormatArgImpl(const T& value) {
     using D = typename DecayType<T>::type;
     static_assert(
-        std::is_same<D, const T&>::value || storage_policy<D>::value == ByValue,
+        std::is_same_v<D, const T&> || storage_policy<D>::value == ByValue,
         "Decayed types must be stored by value");
     Init(static_cast<D>(value));
   }
@@ -336,12 +547,12 @@ class FormatArgImpl {
 
   template <typename T>
   static int ToIntVal(const T& val) {
-    using CommonType = typename std::conditional<std::is_signed<T>::value,
-                                                 int64_t, uint64_t>::type;
+    using CommonType =
+        std::conditional_t<std::is_signed_v<T>, int64_t, uint64_t>;
     if (static_cast<CommonType>(val) >
         static_cast<CommonType>((std::numeric_limits<int>::max)())) {
       return (std::numeric_limits<int>::max)();
-    } else if (std::is_signed<T>::value &&
+    } else if (std::is_signed_v<T> &&
                static_cast<CommonType>(val) <
                    static_cast<CommonType>((std::numeric_limits<int>::min)())) {
       return (std::numeric_limits<int>::min)();
@@ -359,8 +570,8 @@ class FormatArgImpl {
   template <typename T>
   static bool ToInt(Data arg, int* out, std::false_type,
                     std::true_type /* is_enum */) {
-    *out = ToIntVal(static_cast<typename std::underlying_type<T>::type>(
-        Manager<T>::Value(arg)));
+    *out = ToIntVal(
+        static_cast<std::underlying_type_t<T>>(Manager<T>::Value(arg)));
     return true;
   }
 
@@ -370,13 +581,17 @@ class FormatArgImpl {
   }
 
   template <typename T>
-  static bool Dispatch(Data arg, ConversionSpec spec, void* out) {
+  static bool Dispatch(Data arg, FormatConversionSpecImpl spec, void* out) {
     // A `none` conv indicates that we want the `int` conversion.
-    if (ABSL_PREDICT_FALSE(spec.conv().id() == ConversionChar::none)) {
+    if (ABSL_PREDICT_FALSE(spec.conversion_char() ==
+                           FormatConversionCharInternal::kNone)) {
       return ToInt<T>(arg, static_cast<int*>(out), std::is_integral<T>(),
                       std::is_enum<T>());
     }
-
+    if (ABSL_PREDICT_FALSE(
+            !Contains(ArgumentToConv<T>(), spec.conversion_char()))) {
+      return false;
+    }
     return str_format_internal::FormatConvertImpl(
                Manager<T>::Value(arg), spec, static_cast<FormatSinkImpl*>(out))
         .value;
@@ -386,10 +601,11 @@ class FormatArgImpl {
   Dispatcher dispatcher_;
 };
 
-#define ABSL_INTERNAL_FORMAT_DISPATCH_INSTANTIATE_(T, E) \
-  E template bool FormatArgImpl::Dispatch<T>(Data, ConversionSpec, void*)
+#define ABSL_INTERNAL_FORMAT_DISPATCH_INSTANTIATE_(T, E)                     \
+  E template bool FormatArgImpl::Dispatch<T>(Data, FormatConversionSpecImpl, \
+                                             void*)
 
-#define ABSL_INTERNAL_FORMAT_DISPATCH_OVERLOADS_EXPAND_(...)                   \
+#define ABSL_INTERNAL_FORMAT_DISPATCH_OVERLOADS_EXPAND_NO_WSTRING_VIEW_(...)   \
   ABSL_INTERNAL_FORMAT_DISPATCH_INSTANTIATE_(str_format_internal::VoidPtr,     \
                                              __VA_ARGS__);                     \
   ABSL_INTERNAL_FORMAT_DISPATCH_INSTANTIATE_(bool, __VA_ARGS__);               \
@@ -408,17 +624,26 @@ class FormatArgImpl {
                                              __VA_ARGS__);                     \
   ABSL_INTERNAL_FORMAT_DISPATCH_INSTANTIATE_(unsigned long long, /* NOLINT */  \
                                              __VA_ARGS__);                     \
+  ABSL_INTERNAL_FORMAT_DISPATCH_INSTANTIATE_(int128, __VA_ARGS__);             \
   ABSL_INTERNAL_FORMAT_DISPATCH_INSTANTIATE_(uint128, __VA_ARGS__);            \
   ABSL_INTERNAL_FORMAT_DISPATCH_INSTANTIATE_(float, __VA_ARGS__);              \
   ABSL_INTERNAL_FORMAT_DISPATCH_INSTANTIATE_(double, __VA_ARGS__);             \
   ABSL_INTERNAL_FORMAT_DISPATCH_INSTANTIATE_(long double, __VA_ARGS__);        \
   ABSL_INTERNAL_FORMAT_DISPATCH_INSTANTIATE_(const char*, __VA_ARGS__);        \
   ABSL_INTERNAL_FORMAT_DISPATCH_INSTANTIATE_(std::string, __VA_ARGS__);        \
-  ABSL_INTERNAL_FORMAT_DISPATCH_INSTANTIATE_(string_view, __VA_ARGS__)
+  ABSL_INTERNAL_FORMAT_DISPATCH_INSTANTIATE_(string_view, __VA_ARGS__);        \
+  ABSL_INTERNAL_FORMAT_DISPATCH_INSTANTIATE_(const wchar_t*, __VA_ARGS__);     \
+  ABSL_INTERNAL_FORMAT_DISPATCH_INSTANTIATE_(std::wstring, __VA_ARGS__)
+
+#define ABSL_INTERNAL_FORMAT_DISPATCH_OVERLOADS_EXPAND_(...)       \
+  ABSL_INTERNAL_FORMAT_DISPATCH_OVERLOADS_EXPAND_NO_WSTRING_VIEW_( \
+      __VA_ARGS__);                                                \
+  ABSL_INTERNAL_FORMAT_DISPATCH_INSTANTIATE_(std::wstring_view, __VA_ARGS__)
 
 ABSL_INTERNAL_FORMAT_DISPATCH_OVERLOADS_EXPAND_(extern);
 
 }  // namespace str_format_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_INTERNAL_STR_FORMAT_ARG_H_
diff --git a/absl/strings/internal/str_format/arg_test.cc b/absl/strings/internal/str_format/arg_test.cc
index 3421fac11bf..f663d7c5c85 100644
--- a/absl/strings/internal/str_format/arg_test.cc
+++ b/absl/strings/internal/str_format/arg_test.cc
@@ -6,14 +6,22 @@
 //
 //      https://www.apache.org/licenses/LICENSE-2.0
 //
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
 #include "absl/strings/internal/str_format/arg.h"
 
-#include <ostream>
+#include <limits>
 #include <string>
 #include "gtest/gtest.h"
+#include "absl/base/config.h"
 #include "absl/strings/str_format.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace str_format_internal {
 namespace {
 
@@ -22,8 +30,17 @@ class FormatArgImplTest : public ::testing::Test {
   enum Color { kRed, kGreen, kBlue };
 
   static const char *hi() { return "hi"; }
+
+  struct X {};
+
+  X x_;
 };
 
+inline FormatConvertResult<FormatConversionCharSet{}> AbslFormatConvert(
+    const FormatArgImplTest::X &, const FormatConversionSpec &, FormatSink *) {
+  return {false};
+}
+
 TEST_F(FormatArgImplTest, ToInt) {
   int out = 0;
   EXPECT_TRUE(FormatArgImplFriend::ToInt(FormatArgImpl(1), &out));
@@ -58,6 +75,7 @@ TEST_F(FormatArgImplTest, ToInt) {
       FormatArgImpl(static_cast<int *>(nullptr)), &out));
   EXPECT_FALSE(FormatArgImplFriend::ToInt(FormatArgImpl(hi()), &out));
   EXPECT_FALSE(FormatArgImplFriend::ToInt(FormatArgImpl("hi"), &out));
+  EXPECT_FALSE(FormatArgImplFriend::ToInt(FormatArgImpl(x_), &out));
   EXPECT_TRUE(FormatArgImplFriend::ToInt(FormatArgImpl(kBlue), &out));
   EXPECT_EQ(2, out);
 }
@@ -76,6 +94,21 @@ TEST_F(FormatArgImplTest, CharArraysDecayToCharPtr) {
             FormatArgImplFriend::GetVTablePtrForTest(FormatArgImpl(kMyArray)));
 }
 
+extern const wchar_t kMyWCharTArray[];
+
+TEST_F(FormatArgImplTest, WCharTArraysDecayToWCharTPtr) {
+  const wchar_t* a = L"";
+  EXPECT_EQ(FormatArgImplFriend::GetVTablePtrForTest(FormatArgImpl(a)),
+            FormatArgImplFriend::GetVTablePtrForTest(FormatArgImpl(L"")));
+  EXPECT_EQ(FormatArgImplFriend::GetVTablePtrForTest(FormatArgImpl(a)),
+            FormatArgImplFriend::GetVTablePtrForTest(FormatArgImpl(L"A")));
+  EXPECT_EQ(FormatArgImplFriend::GetVTablePtrForTest(FormatArgImpl(a)),
+            FormatArgImplFriend::GetVTablePtrForTest(FormatArgImpl(L"ABC")));
+  EXPECT_EQ(
+      FormatArgImplFriend::GetVTablePtrForTest(FormatArgImpl(a)),
+      FormatArgImplFriend::GetVTablePtrForTest(FormatArgImpl(kMyWCharTArray)));
+}
+
 TEST_F(FormatArgImplTest, OtherPtrDecayToVoidPtr) {
   auto expected = FormatArgImplFriend::GetVTablePtrForTest(
       FormatArgImpl(static_cast<void *>(nullptr)));
@@ -94,11 +127,12 @@ TEST_F(FormatArgImplTest, OtherPtrDecayToVoidPtr) {
 TEST_F(FormatArgImplTest, WorksWithCharArraysOfUnknownSize) {
   std::string s;
   FormatSinkImpl sink(&s);
-  ConversionSpec conv;
-  conv.set_conv(ConversionChar::FromChar('s'));
-  conv.set_flags(Flags());
-  conv.set_width(-1);
-  conv.set_precision(-1);
+  FormatConversionSpecImpl conv;
+  FormatConversionSpecImplFriend::SetConversionChar(
+      FormatConversionCharInternal::s, &conv);
+  FormatConversionSpecImplFriend::SetFlags(Flags(), &conv);
+  FormatConversionSpecImplFriend::SetWidth(-1, &conv);
+  FormatConversionSpecImplFriend::SetPrecision(-1, &conv);
   EXPECT_TRUE(
       FormatArgImplFriend::Convert(FormatArgImpl(kMyArray), conv, &sink));
   sink.Flush();
@@ -106,6 +140,23 @@ TEST_F(FormatArgImplTest, WorksWithCharArraysOfUnknownSize) {
 }
 const char kMyArray[] = "ABCDE";
 
+TEST_F(FormatArgImplTest, WorksWithWCharTArraysOfUnknownSize) {
+  std::string s;
+  FormatSinkImpl sink(&s);
+  FormatConversionSpecImpl conv;
+  FormatConversionSpecImplFriend::SetConversionChar(
+      FormatConversionCharInternal::s, &conv);
+  FormatConversionSpecImplFriend::SetFlags(Flags(), &conv);
+  FormatConversionSpecImplFriend::SetWidth(-1, &conv);
+  FormatConversionSpecImplFriend::SetPrecision(-1, &conv);
+  EXPECT_TRUE(
+      FormatArgImplFriend::Convert(FormatArgImpl(kMyWCharTArray), conv, &sink));
+  sink.Flush();
+  EXPECT_EQ("ABCDE", s);
+}
+const wchar_t kMyWCharTArray[] = L"ABCDE";
+
 }  // namespace
 }  // namespace str_format_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/internal/str_format/bind.cc b/absl/strings/internal/str_format/bind.cc
index a31859ece2b..689fa5ca4c2 100644
--- a/absl/strings/internal/str_format/bind.cc
+++ b/absl/strings/internal/str_format/bind.cc
@@ -1,11 +1,40 @@
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
 #include "absl/strings/internal/str_format/bind.h"
 
+#include <algorithm>
+#include <cassert>
 #include <cerrno>
+#include <cstddef>
+#include <cstdio>
+#include <ios>
 #include <limits>
+#include <ostream>
 #include <sstream>
 #include <string>
+#include "absl/base/config.h"
+#include "absl/base/optimization.h"
+#include "absl/strings/internal/str_format/arg.h"
+#include "absl/strings/internal/str_format/constexpr_parser.h"
+#include "absl/strings/internal/str_format/extension.h"
+#include "absl/strings/internal/str_format/output.h"
+#include "absl/strings/string_view.h"
+#include "absl/types/span.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace str_format_internal {
 
 namespace {
@@ -17,7 +46,8 @@ inline bool BindFromPosition(int position, int* value,
     return false;
   }
   // -1 because positions are 1-based
-  return FormatArgImplFriend::ToInt(pack[position - 1], value);
+  return FormatArgImplFriend::ToInt(pack[static_cast<size_t>(position) - 1],
+                                    value);
 }
 
 class ArgContext {
@@ -41,9 +71,9 @@ inline bool ArgContext::Bind(const UnboundConversion* unbound,
   const FormatArgImpl* arg = nullptr;
   int arg_position = unbound->arg_position;
   if (static_cast<size_t>(arg_position - 1) >= pack_.size()) return false;
-  arg = &pack_[arg_position - 1];  // 1-based
+  arg = &pack_[static_cast<size_t>(arg_position - 1)];  // 1-based
 
-  if (!unbound->flags.basic) {
+  if (unbound->flags != Flags::kBasic) {
     int width = unbound->width.value();
     bool force_left = false;
     if (unbound->width.is_from_arg()) {
@@ -65,19 +95,23 @@ inline bool ArgContext::Bind(const UnboundConversion* unbound,
         return false;
     }
 
-    bound->set_width(width);
-    bound->set_precision(precision);
-    bound->set_flags(unbound->flags);
-    if (force_left)
-      bound->set_left(true);
+    FormatConversionSpecImplFriend::SetWidth(width, bound);
+    FormatConversionSpecImplFriend::SetPrecision(precision, bound);
+
+    if (force_left) {
+      FormatConversionSpecImplFriend::SetFlags(unbound->flags | Flags::kLeft,
+                                               bound);
+    } else {
+      FormatConversionSpecImplFriend::SetFlags(unbound->flags, bound);
+    }
+
+    FormatConversionSpecImplFriend::SetLengthMod(unbound->length_mod, bound);
   } else {
-    bound->set_flags(unbound->flags);
-    bound->set_width(-1);
-    bound->set_precision(-1);
+    FormatConversionSpecImplFriend::SetFlags(unbound->flags, bound);
+    FormatConversionSpecImplFriend::SetWidth(-1, bound);
+    FormatConversionSpecImplFriend::SetPrecision(-1, bound);
   }
-
-  bound->set_length_mod(unbound->length_mod);
-  bound->set_conv(unbound->conv);
+  FormatConversionSpecImplFriend::SetConversionChar(unbound->conv, bound);
   bound->set_arg(arg);
   return true;
 }
@@ -139,10 +173,11 @@ class SummarizingConverter {
     UntypedFormatSpecImpl spec("%d");
 
     std::ostringstream ss;
-    ss << "{" << Streamable(spec, {*bound.arg()}) << ":" << bound.flags();
+    ss << "{" << Streamable(spec, {*bound.arg()}) << ":"
+       << FormatConversionSpecImplFriend::FlagsToString(bound);
     if (bound.width() >= 0) ss << bound.width();
     if (bound.precision() >= 0) ss << "." << bound.precision();
-    ss << bound.length_mod() << bound.conv() << "}";
+    ss << bound.conversion_char() << "}";
     Append(ss.str());
     return true;
   }
@@ -196,6 +231,15 @@ std::string& AppendPack(std::string* out, const UntypedFormatSpecImpl format,
   return *out;
 }
 
+std::string FormatPack(UntypedFormatSpecImpl format,
+                       absl::Span<const FormatArgImpl> args) {
+  std::string out;
+  if (ABSL_PREDICT_FALSE(!FormatUntyped(&out, format, args))) {
+    out.clear();
+  }
+  return out;
+}
+
 int FprintF(std::FILE* output, const UntypedFormatSpecImpl format,
             absl::Span<const FormatArgImpl> args) {
   FILERawSink sink(output);
@@ -207,7 +251,7 @@ int FprintF(std::FILE* output, const UntypedFormatSpecImpl format,
     errno = sink.error();
     return -1;
   }
-  if (sink.count() > std::numeric_limits<int>::max()) {
+  if (sink.count() > size_t{std::numeric_limits<int>::max()}) {
     errno = EFBIG;
     return -1;
   }
@@ -223,8 +267,13 @@ int SnprintF(char* output, size_t size, const UntypedFormatSpecImpl format,
   }
   size_t total = sink.total_written();
   if (size) output[std::min(total, size - 1)] = 0;
+  if (total > size_t{std::numeric_limits<int>::max()}) {
+    errno = EFBIG;
+    return -1;
+  }
   return static_cast<int>(total);
 }
 
 }  // namespace str_format_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/internal/str_format/bind.h b/absl/strings/internal/str_format/bind.h
index 7df140a4363..1c87e1edaa0 100644
--- a/absl/strings/internal/str_format/bind.h
+++ b/absl/strings/internal/str_format/bind.h
@@ -1,24 +1,45 @@
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
 #ifndef ABSL_STRINGS_INTERNAL_STR_FORMAT_BIND_H_
 #define ABSL_STRINGS_INTERNAL_STR_FORMAT_BIND_H_
 
-#include <array>
+#include <cassert>
 #include <cstdio>
-#include <sstream>
+#include <ostream>
 #include <string>
+#include <utility>
 
-#include "absl/base/port.h"
+#include "absl/base/config.h"
+#include "absl/container/inlined_vector.h"
 #include "absl/strings/internal/str_format/arg.h"
 #include "absl/strings/internal/str_format/checker.h"
+#include "absl/strings/internal/str_format/constexpr_parser.h"
+#include "absl/strings/internal/str_format/extension.h"
 #include "absl/strings/internal/str_format/parser.h"
+#include "absl/strings/string_view.h"
 #include "absl/types/span.h"
+#include "absl/utility/utility.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 class UntypedFormatSpec;
 
 namespace str_format_internal {
 
-class BoundConversion : public ConversionSpec {
+class BoundConversion : public FormatConversionSpecImpl {
  public:
   const FormatArgImpl* arg() const { return arg_; }
   void set_arg(const FormatArgImpl* a) { arg_ = a; }
@@ -59,7 +80,7 @@ class UntypedFormatSpecImpl {
   size_t size_;
 };
 
-template <typename T, typename...>
+template <typename T, FormatConversionCharSet...>
 struct MakeDependent {
   using type = T;
 };
@@ -67,25 +88,55 @@ struct MakeDependent {
 // Implicitly convertible from `const char*`, `string_view`, and the
 // `ExtendedParsedFormat` type. This abstraction allows all format functions to
 // operate on any without providing too many overloads.
-template <typename... Args>
+template <FormatConversionCharSet... Args>
 class FormatSpecTemplate
     : public MakeDependent<UntypedFormatSpec, Args...>::type {
   using Base = typename MakeDependent<UntypedFormatSpec, Args...>::type;
 
+  template <bool res>
+  struct ErrorMaker {
+    constexpr bool operator()(int) const { return res; }
+  };
+
+  template <int i, int j>
+  static constexpr bool CheckArity(ErrorMaker<true> SpecifierCount = {},
+                                   ErrorMaker<i == j> ParametersPassed = {}) {
+    static_assert(SpecifierCount(i) == ParametersPassed(j),
+                  "Number of arguments passed must match the number of "
+                  "conversion specifiers.");
+    return true;
+  }
+
+  template <FormatConversionCharSet specified, FormatConversionCharSet passed,
+            int arg>
+  static constexpr bool CheckMatch(
+      ErrorMaker<Contains(specified, passed)> MismatchedArgumentNumber = {}) {
+    static_assert(MismatchedArgumentNumber(arg),
+                  "Passed argument must match specified format.");
+    return true;
+  }
+
+  template <FormatConversionCharSet... C, size_t... I>
+  static bool CheckMatches(std::index_sequence<I...>) {
+    bool res[] = {true, CheckMatch<Args, C, I + 1>()...};
+    (void)res;
+    return true;
+  }
+
  public:
-#if ABSL_INTERNAL_ENABLE_FORMAT_CHECKER
+#ifdef ABSL_INTERNAL_ENABLE_FORMAT_CHECKER
 
-  // Honeypot overload for when the std::string is not constexpr.
+  // Honeypot overload for when the string is not constexpr.
   // We use the 'unavailable' attribute to give a better compiler error than
   // just 'method is deleted'.
   FormatSpecTemplate(...)  // NOLINT
-      __attribute__((unavailable("Format std::string is not constexpr.")));
+      __attribute__((unavailable("Format string is not constexpr.")));
 
   // Honeypot overload for when the format is constexpr and invalid.
   // We use the 'unavailable' attribute to give a better compiler error than
   // just 'method is deleted'.
   // To avoid checking the format twice, we just check that the format is
-  // constexpr. If is it valid, then the overload below will kick in.
+  // constexpr. If it is valid, then the overload below will kick in.
   // We add the template here to make this overload have lower priority.
   template <typename = void>
   FormatSpecTemplate(const char* s)  // NOLINT
@@ -97,20 +148,19 @@ class FormatSpecTemplate
   template <typename T = void>
   FormatSpecTemplate(string_view s)  // NOLINT
       __attribute__((enable_if(str_format_internal::EnsureConstexpr(s),
-                               "constexpr trap"))) {
+                               "constexpr trap")))
+      : Base("to avoid noise in the compiler error") {
     static_assert(sizeof(T*) == 0,
                   "Format specified does not match the arguments passed.");
   }
 
   // Good format overload.
   FormatSpecTemplate(const char* s)  // NOLINT
-      __attribute__((enable_if(ValidFormatImpl<ArgumentToConv<Args>()...>(s),
-                               "bad format trap")))
+      __attribute__((enable_if(ValidFormatImpl<Args...>(s), "bad format trap")))
       : Base(s) {}
 
   FormatSpecTemplate(string_view s)  // NOLINT
-      __attribute__((enable_if(ValidFormatImpl<ArgumentToConv<Args>()...>(s),
-                               "bad format trap")))
+      __attribute__((enable_if(ValidFormatImpl<Args...>(s), "bad format trap")))
       : Base(s) {}
 
 #else  // ABSL_INTERNAL_ENABLE_FORMAT_CHECKER
@@ -120,34 +170,19 @@ class FormatSpecTemplate
 
 #endif  // ABSL_INTERNAL_ENABLE_FORMAT_CHECKER
 
-  template <Conv... C, typename = typename std::enable_if<
-                           sizeof...(C) == sizeof...(Args) &&
-                           AllOf(Contains(ArgumentToConv<Args>(),
-                                          C)...)>::type>
+  template <FormatConversionCharSet... C>
   FormatSpecTemplate(const ExtendedParsedFormat<C...>& pc)  // NOLINT
-      : Base(&pc) {}
-};
-
-template <typename... Args>
-struct FormatSpecDeductionBarrier {
-  using type = FormatSpecTemplate<Args...>;
+      : Base(&pc) {
+    CheckArity<sizeof...(C), sizeof...(Args)>();
+    CheckMatches<C...>(std::make_index_sequence<sizeof...(C)>{});
+  }
 };
 
 class Streamable {
  public:
   Streamable(const UntypedFormatSpecImpl& format,
              absl::Span<const FormatArgImpl> args)
-      : format_(format) {
-    if (args.size() <= ABSL_ARRAYSIZE(few_args_)) {
-      for (size_t i = 0; i < args.size(); ++i) {
-        few_args_[i] = args[i];
-      }
-      args_ = absl::MakeSpan(few_args_, args.size());
-    } else {
-      many_args_.assign(args.begin(), args.end());
-      args_ = many_args_;
-    }
-  }
+      : format_(format), args_(args.begin(), args.end()) {}
 
   std::ostream& Print(std::ostream& os) const;
 
@@ -157,12 +192,7 @@ class Streamable {
 
  private:
   const UntypedFormatSpecImpl& format_;
-  absl::Span<const FormatArgImpl> args_;
-  // if args_.size() is 4 or less:
-  FormatArgImpl few_args_[4] = {FormatArgImpl(0), FormatArgImpl(0),
-                                FormatArgImpl(0), FormatArgImpl(0)};
-  // if args_.size() is more than 4:
-  std::vector<FormatArgImpl> many_args_;
+  absl::InlinedVector<FormatArgImpl, 4> args_;
 };
 
 // for testing
@@ -171,19 +201,14 @@ std::string Summarize(UntypedFormatSpecImpl format,
 bool BindWithPack(const UnboundConversion* props,
                   absl::Span<const FormatArgImpl> pack, BoundConversion* bound);
 
-bool FormatUntyped(FormatRawSinkImpl raw_sink,
-                   UntypedFormatSpecImpl format,
+bool FormatUntyped(FormatRawSinkImpl raw_sink, UntypedFormatSpecImpl format,
                    absl::Span<const FormatArgImpl> args);
 
 std::string& AppendPack(std::string* out, UntypedFormatSpecImpl format,
                         absl::Span<const FormatArgImpl> args);
 
-inline std::string FormatPack(const UntypedFormatSpecImpl format,
-                              absl::Span<const FormatArgImpl> args) {
-  std::string out;
-  AppendPack(&out, format, args);
-  return out;
-}
+std::string FormatPack(UntypedFormatSpecImpl format,
+                       absl::Span<const FormatArgImpl> args);
 
 int FprintF(std::FILE* output, UntypedFormatSpecImpl format,
             absl::Span<const FormatArgImpl> args);
@@ -195,17 +220,19 @@ int SnprintF(char* output, size_t size, UntypedFormatSpecImpl format,
 template <typename T>
 class StreamedWrapper {
  public:
-  explicit StreamedWrapper(const T& v) : v_(v) { }
+  explicit StreamedWrapper(const T& v) : v_(v) {}
 
  private:
   template <typename S>
-  friend ConvertResult<Conv::s> FormatConvertImpl(const StreamedWrapper<S>& v,
-                                                  ConversionSpec conv,
-                                                  FormatSinkImpl* out);
+  friend ArgConvertResult<FormatConversionCharSetUnion(
+      FormatConversionCharSetInternal::s, FormatConversionCharSetInternal::v)>
+  FormatConvertImpl(const StreamedWrapper<S>& v, FormatConversionSpecImpl conv,
+                    FormatSinkImpl* out);
   const T& v_;
 };
 
 }  // namespace str_format_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_INTERNAL_STR_FORMAT_BIND_H_
diff --git a/absl/strings/internal/str_format/bind_test.cc b/absl/strings/internal/str_format/bind_test.cc
index 2574801a5c3..c4fb200ca4a 100644
--- a/absl/strings/internal/str_format/bind_test.cc
+++ b/absl/strings/internal/str_format/bind_test.cc
@@ -1,11 +1,31 @@
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
 #include "absl/strings/internal/str_format/bind.h"
 
 #include <string.h>
+
 #include <limits>
 
 #include "gtest/gtest.h"
+#include "absl/strings/internal/str_format/arg.h"
+#include "absl/strings/internal/str_format/constexpr_parser.h"
+#include "absl/strings/string_view.h"
+#include "absl/types/span.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace str_format_internal {
 namespace {
 
@@ -112,18 +132,18 @@ TEST_F(FormatBindTest, FormatPack) {
                                 FormatArgImpl(ia[2]), FormatArgImpl(ia[3]),
                                 FormatArgImpl(ia[4])};
   const Expectation kExpect[] = {
-    {__LINE__, "a%4db%dc", "a{10:4d}b{20:d}c"},
-    {__LINE__, "a%.4db%dc", "a{10:.4d}b{20:d}c"},
-    {__LINE__, "a%4.5db%dc", "a{10:4.5d}b{20:d}c"},
-    {__LINE__, "a%db%4.5dc", "a{10:d}b{20:4.5d}c"},
-    {__LINE__, "a%db%*.*dc", "a{10:d}b{40:20.30d}c"},
-    {__LINE__, "a%.*fb", "a{20:.10f}b"},
-    {__LINE__, "a%1$db%2$*3$.*4$dc", "a{10:d}b{20:30.40d}c"},
-    {__LINE__, "a%4$db%3$*2$.*1$dc", "a{40:d}b{30:20.10d}c"},
-    {__LINE__, "a%04ldb", "a{10:04ld}b"},
-    {__LINE__, "a%-#04lldb", "a{10:-#04lld}b"},
-    {__LINE__, "a%1$*5$db", "a{10:-10d}b"},
-    {__LINE__, "a%1$.*5$db", "a{10:d}b"},
+      {__LINE__, "a%4db%dc", "a{10:4d}b{20:d}c"},
+      {__LINE__, "a%.4db%dc", "a{10:.4d}b{20:d}c"},
+      {__LINE__, "a%4.5db%dc", "a{10:4.5d}b{20:d}c"},
+      {__LINE__, "a%db%4.5dc", "a{10:d}b{20:4.5d}c"},
+      {__LINE__, "a%db%*.*dc", "a{10:d}b{40:20.30d}c"},
+      {__LINE__, "a%.*fb", "a{20:.10f}b"},
+      {__LINE__, "a%1$db%2$*3$.*4$dc", "a{10:d}b{20:30.40d}c"},
+      {__LINE__, "a%4$db%3$*2$.*1$dc", "a{40:d}b{30:20.10d}c"},
+      {__LINE__, "a%04ldb", "a{10:04d}b"},
+      {__LINE__, "a%-#04lldb", "a{10:-#04d}b"},
+      {__LINE__, "a%1$*5$db", "a{10:-10d}b"},
+      {__LINE__, "a%1$.*5$db", "a{10:d}b"},
   };
   for (const Expectation &e : kExpect) {
     absl::string_view fmt = e.fmt;
@@ -138,4 +158,5 @@ TEST_F(FormatBindTest, FormatPack) {
 
 }  // namespace
 }  // namespace str_format_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/internal/str_format/checker.h b/absl/strings/internal/str_format/checker.h
index 8b594f2d5cc..14e9a237c2d 100644
--- a/absl/strings/internal/str_format/checker.h
+++ b/absl/strings/internal/str_format/checker.h
@@ -1,325 +1,101 @@
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
 #ifndef ABSL_STRINGS_INTERNAL_STR_FORMAT_CHECKER_H_
 #define ABSL_STRINGS_INTERNAL_STR_FORMAT_CHECKER_H_
 
+#include <algorithm>
+#include <cstddef>
+
+#include "absl/base/attributes.h"
+#include "absl/base/const_init.h"
 #include "absl/strings/internal/str_format/arg.h"
+#include "absl/strings/internal/str_format/constexpr_parser.h"
 #include "absl/strings/internal/str_format/extension.h"
+#include "absl/strings/string_view.h"
 
 // Compile time check support for entry points.
 
 #ifndef ABSL_INTERNAL_ENABLE_FORMAT_CHECKER
-#if defined(__clang__) && !defined(__native_client__)
-#if __has_attribute(enable_if)
+// We disable format checker under vscode intellisense compilation.
+// See https://github.com/microsoft/vscode-cpptools/issues/3683 for
+// more details.
+#if ABSL_HAVE_ATTRIBUTE(enable_if) && !defined(__INTELLISENSE__)
 #define ABSL_INTERNAL_ENABLE_FORMAT_CHECKER 1
-#endif  // __has_attribute(enable_if)
-#endif  // defined(__clang__) && !defined(__native_client__)
+#endif  // ABSL_HAVE_ATTRIBUTE(enable_if) && !defined(__INTELLISENSE__)
 #endif  // ABSL_INTERNAL_ENABLE_FORMAT_CHECKER
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace str_format_internal {
 
-constexpr bool AllOf() { return true; }
-
-template <typename... T>
-constexpr bool AllOf(bool b, T... t) {
-  return b && AllOf(t...);
-}
-
-template <typename Arg>
-constexpr Conv ArgumentToConv() {
-  return decltype(str_format_internal::FormatConvertImpl(
-      std::declval<const Arg&>(), std::declval<const ConversionSpec&>(),
-      std::declval<FormatSinkImpl*>()))::kConv;
-}
-
-#if ABSL_INTERNAL_ENABLE_FORMAT_CHECKER
-
-constexpr bool ContainsChar(const char* chars, char c) {
-  return *chars == c || (*chars && ContainsChar(chars + 1, c));
-}
-
-// A constexpr compatible list of Convs.
-struct ConvList {
-  const Conv* array;
-  int count;
-
-  // We do the bound check here to avoid having to do it on the callers.
-  // Returning an empty Conv has the same effect as short circuiting because it
-  // will never match any conversion.
-  constexpr Conv operator[](int i) const {
-    return i < count ? array[i] : Conv{};
-  }
-
-  constexpr ConvList without_front() const {
-    return count != 0 ? ConvList{array + 1, count - 1} : *this;
-  }
-};
-
-template <size_t count>
-struct ConvListT {
-  // Make sure the array has size > 0.
-  Conv list[count ? count : 1];
-};
-
-constexpr char GetChar(string_view str, size_t index) {
-  return index < str.size() ? str[index] : char{};
-}
-
-constexpr string_view ConsumeFront(string_view str, size_t len = 1) {
-  return len <= str.size() ? string_view(str.data() + len, str.size() - len)
-                           : string_view();
-}
-
-constexpr string_view ConsumeAnyOf(string_view format, const char* chars) {
-  return ContainsChar(chars, GetChar(format, 0))
-             ? ConsumeAnyOf(ConsumeFront(format), chars)
-             : format;
-}
-
-constexpr bool IsDigit(char c) { return c >= '0' && c <= '9'; }
-
-// Helper class for the ParseDigits function.
-// It encapsulates the two return values we need there.
-struct Integer {
-  string_view format;
-  int value;
-
-  // If the next character is a '$', consume it.
-  // Otherwise, make `this` an invalid positional argument.
-  constexpr Integer ConsumePositionalDollar() const {
-    return GetChar(format, 0) == '$' ? Integer{ConsumeFront(format), value}
-                                     : Integer{format, 0};
-  }
-};
-
-constexpr Integer ParseDigits(string_view format, int value = 0) {
-  return IsDigit(GetChar(format, 0))
-             ? ParseDigits(ConsumeFront(format),
-                           10 * value + GetChar(format, 0) - '0')
-             : Integer{format, value};
-}
-
-// Parse digits for a positional argument.
-// The parsing also consumes the '$'.
-constexpr Integer ParsePositional(string_view format) {
-  return ParseDigits(format).ConsumePositionalDollar();
-}
-
-// Parses a single conversion specifier.
-// See ConvParser::Run() for post conditions.
-class ConvParser {
-  constexpr ConvParser SetFormat(string_view format) const {
-    return ConvParser(format, args_, error_, arg_position_, is_positional_);
-  }
-
-  constexpr ConvParser SetArgs(ConvList args) const {
-    return ConvParser(format_, args, error_, arg_position_, is_positional_);
-  }
-
-  constexpr ConvParser SetError(bool error) const {
-    return ConvParser(format_, args_, error_ || error, arg_position_,
-                      is_positional_);
-  }
-
-  constexpr ConvParser SetArgPosition(int arg_position) const {
-    return ConvParser(format_, args_, error_, arg_position, is_positional_);
-  }
-
-  // Consumes the next arg and verifies that it matches `conv`.
-  // `error_` is set if there is no next arg or if it doesn't match `conv`.
-  constexpr ConvParser ConsumeNextArg(char conv) const {
-    return SetArgs(args_.without_front()).SetError(!Contains(args_[0], conv));
-  }
-
-  // Verify that positional argument `i.value` matches `conv`.
-  // `error_` is set if `i.value` is not a valid argument or if it doesn't
-  // match.
-  constexpr ConvParser VerifyPositional(Integer i, char conv) const {
-    return SetFormat(i.format).SetError(!Contains(args_[i.value - 1], conv));
-  }
-
-  // Parse the position of the arg and store it in `arg_position_`.
-  constexpr ConvParser ParseArgPosition(Integer arg) const {
-    return SetFormat(arg.format).SetArgPosition(arg.value);
-  }
-
-  // Consume the flags.
-  constexpr ConvParser ParseFlags() const {
-    return SetFormat(ConsumeAnyOf(format_, "-+ #0"));
-  }
-
-  // Consume the width.
-  // If it is '*', we verify that it matches `args_`. `error_` is set if it
-  // doesn't match.
-  constexpr ConvParser ParseWidth() const {
-    return IsDigit(GetChar(format_, 0))
-               ? SetFormat(ParseDigits(format_).format)
-               : GetChar(format_, 0) == '*'
-                     ? is_positional_
-                           ? VerifyPositional(
-                                 ParsePositional(ConsumeFront(format_)), '*')
-                           : SetFormat(ConsumeFront(format_))
-                                 .ConsumeNextArg('*')
-                     : *this;
-  }
-
-  // Consume the precision.
-  // If it is '*', we verify that it matches `args_`. `error_` is set if it
-  // doesn't match.
-  constexpr ConvParser ParsePrecision() const {
-    return GetChar(format_, 0) != '.'
-               ? *this
-               : GetChar(format_, 1) == '*'
-                     ? is_positional_
-                           ? VerifyPositional(
-                                 ParsePositional(ConsumeFront(format_, 2)), '*')
-                           : SetFormat(ConsumeFront(format_, 2))
-                                 .ConsumeNextArg('*')
-                     : SetFormat(ParseDigits(ConsumeFront(format_)).format);
-  }
-
-  // Consume the length characters.
-  constexpr ConvParser ParseLength() const {
-    return SetFormat(ConsumeAnyOf(format_, "lLhjztq"));
-  }
-
-  // Consume the conversion character and verify that it matches `args_`.
-  // `error_` is set if it doesn't match.
-  constexpr ConvParser ParseConversion() const {
-    return is_positional_
-               ? VerifyPositional({ConsumeFront(format_), arg_position_},
-                                  GetChar(format_, 0))
-               : ConsumeNextArg(GetChar(format_, 0))
-                     .SetFormat(ConsumeFront(format_));
-  }
-
-  constexpr ConvParser(string_view format, ConvList args, bool error,
-                       int arg_position, bool is_positional)
-      : format_(format),
-        args_(args),
-        error_(error),
-        arg_position_(arg_position),
-        is_positional_(is_positional) {}
-
- public:
-  constexpr ConvParser(string_view format, ConvList args, bool is_positional)
-      : format_(format),
-        args_(args),
-        error_(false),
-        arg_position_(0),
-        is_positional_(is_positional) {}
-
-  // Consume the whole conversion specifier.
-  // `format()` will be set to the character after the conversion character.
-  // `error()` will be set if any of the arguments do not match.
-  constexpr ConvParser Run() const {
-    return (is_positional_ ? ParseArgPosition(ParsePositional(format_)) : *this)
-        .ParseFlags()
-        .ParseWidth()
-        .ParsePrecision()
-        .ParseLength()
-        .ParseConversion();
-  }
-
-  constexpr string_view format() const { return format_; }
-  constexpr ConvList args() const { return args_; }
-  constexpr bool error() const { return error_; }
-  constexpr bool is_positional() const { return is_positional_; }
-
- private:
-  string_view format_;
-  // Current list of arguments. If we are not in positional mode we will consume
-  // from the front.
-  ConvList args_;
-  bool error_;
-  // Holds the argument position of the conversion character, if we are in
-  // positional mode. Otherwise, it is unspecified.
-  int arg_position_;
-  // Whether we are in positional mode.
-  // It changes the behavior of '*' and where to find the converted argument.
-  bool is_positional_;
-};
-
-// Parses a whole format expression.
-// See FormatParser::Run().
-class FormatParser {
-  static constexpr bool FoundPercent(string_view format) {
-    return format.empty() ||
-           (GetChar(format, 0) == '%' && GetChar(format, 1) != '%');
-  }
-
-  // We use an inner function to increase the recursion limit.
-  // The inner function consumes up to `limit` characters on every run.
-  // This increases the limit from 512 to ~512*limit.
-  static constexpr string_view ConsumeNonPercentInner(string_view format,
-                                                      int limit = 20) {
-    return FoundPercent(format) || !limit
-               ? format
-               : ConsumeNonPercentInner(
-                     ConsumeFront(format, GetChar(format, 0) == '%' &&
-                                                  GetChar(format, 1) == '%'
-                                              ? 2
-                                              : 1),
-                     limit - 1);
-  }
-
-  // Consume characters until the next conversion spec %.
-  // It skips %%.
-  static constexpr string_view ConsumeNonPercent(string_view format) {
-    return FoundPercent(format)
-               ? format
-               : ConsumeNonPercent(ConsumeNonPercentInner(format));
-  }
-
-  static constexpr bool IsPositional(string_view format) {
-    return IsDigit(GetChar(format, 0)) ? IsPositional(ConsumeFront(format))
-                                       : GetChar(format, 0) == '$';
-  }
-
-  constexpr bool RunImpl(bool is_positional) const {
-    // In non-positional mode we require all arguments to be consumed.
-    // In positional mode just reaching the end of the format without errors is
-    // enough.
-    return (format_.empty() && (is_positional || args_.count == 0)) ||
-           (!format_.empty() &&
-            ValidateArg(
-                ConvParser(ConsumeFront(format_), args_, is_positional).Run()));
-  }
-
-  constexpr bool ValidateArg(ConvParser conv) const {
-    return !conv.error() && FormatParser(conv.format(), conv.args())
-                                .RunImpl(conv.is_positional());
-  }
-
- public:
-  constexpr FormatParser(string_view format, ConvList args)
-      : format_(ConsumeNonPercent(format)), args_(args) {}
-
-  // Runs the parser for `format` and `args`.
-  // It verifies that the format is valid and that all conversion specifiers
-  // match the arguments passed.
-  // In non-positional mode it also verfies that all arguments are consumed.
-  constexpr bool Run() const {
-    return RunImpl(!format_.empty() && IsPositional(ConsumeFront(format_)));
-  }
-
- private:
-  string_view format_;
-  // Current list of arguments.
-  // If we are not in positional mode we will consume from the front and will
-  // have to be empty in the end.
-  ConvList args_;
-};
+#ifdef ABSL_INTERNAL_ENABLE_FORMAT_CHECKER
 
-template <Conv... C>
+template <FormatConversionCharSet... C>
 constexpr bool ValidFormatImpl(string_view format) {
-  return FormatParser(format,
-                      {ConvListT<sizeof...(C)>{{C...}}.list, sizeof...(C)})
-      .Run();
+  int next_arg = 0;
+  const char* p = format.data();
+  const char* const end = p + format.size();
+  constexpr FormatConversionCharSet
+      kAllowedConvs[(std::max)(sizeof...(C), size_t{1})] = {C...};
+  bool used[(std::max)(sizeof...(C), size_t{1})]{};
+  constexpr int kNumArgs = sizeof...(C);
+  while (p != end) {
+    while (p != end && *p != '%') ++p;
+    if (p == end) {
+      break;
+    }
+    if (p + 1 >= end) return false;
+    if (p[1] == '%') {
+      // %%
+      p += 2;
+      continue;
+    }
+
+    UnboundConversion conv(absl::kConstInit);
+    p = ConsumeUnboundConversion(p + 1, end, &conv, &next_arg);
+    if (p == nullptr) return false;
+    if (conv.arg_position <= 0 || conv.arg_position > kNumArgs) {
+      return false;
+    }
+    if (!Contains(kAllowedConvs[conv.arg_position - 1], conv.conv)) {
+      return false;
+    }
+    used[conv.arg_position - 1] = true;
+    for (auto extra : {conv.width, conv.precision}) {
+      if (extra.is_from_arg()) {
+        int pos = extra.get_from_arg();
+        if (pos <= 0 || pos > kNumArgs) return false;
+        used[pos - 1] = true;
+        if (!Contains(kAllowedConvs[pos - 1], '*')) {
+          return false;
+        }
+      }
+    }
+  }
+  if (sizeof...(C) != 0) {
+    for (bool b : used) {
+      if (!b) return false;
+    }
+  }
+  return true;
 }
 
 #endif  // ABSL_INTERNAL_ENABLE_FORMAT_CHECKER
 
 }  // namespace str_format_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_INTERNAL_STR_FORMAT_CHECKER_H_
diff --git a/absl/strings/internal/str_format/checker_test.cc b/absl/strings/internal/str_format/checker_test.cc
index c1d8c769ef7..21f1a7f4e61 100644
--- a/absl/strings/internal/str_format/checker_test.cc
+++ b/absl/strings/internal/str_format/checker_test.cc
@@ -1,41 +1,61 @@
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
 #include <string>
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
 #include "absl/strings/str_format.h"
+#include "absl/strings/string_view.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace str_format_internal {
 namespace {
 
-std::string ConvToString(Conv conv) {
+std::string ConvToString(FormatConversionCharSet conv) {
   std::string out;
-#define CONV_SET_CASE(c) \
-  if (Contains(conv, Conv::c)) out += #c;
-  ABSL_CONVERSION_CHARS_EXPAND_(CONV_SET_CASE, )
+#define CONV_SET_CASE(c)                                    \
+  if (Contains(conv, FormatConversionCharSetInternal::c)) { \
+    out += #c;                                              \
+  }
+  ABSL_INTERNAL_CONVERSION_CHARS_EXPAND_(CONV_SET_CASE, )
 #undef CONV_SET_CASE
-  if (Contains(conv, Conv::star)) out += "*";
+  if (Contains(conv, FormatConversionCharSetInternal::kStar)) {
+    out += "*";
+  }
   return out;
 }
 
 TEST(StrFormatChecker, ArgumentToConv) {
-  Conv conv = ArgumentToConv<std::string>();
-  EXPECT_EQ(ConvToString(conv), "s");
+  FormatConversionCharSet conv = ArgumentToConv<std::string>();
+  EXPECT_EQ(ConvToString(conv), "sv");
 
   conv = ArgumentToConv<const char*>();
   EXPECT_EQ(ConvToString(conv), "sp");
 
   conv = ArgumentToConv<double>();
-  EXPECT_EQ(ConvToString(conv), "fFeEgGaA");
+  EXPECT_EQ(ConvToString(conv), "fFeEgGaAv");
 
   conv = ArgumentToConv<int>();
-  EXPECT_EQ(ConvToString(conv), "cdiouxXfFeEgGaA*");
+  EXPECT_EQ(ConvToString(conv), "cdiouxXfFeEgGaAv*");
 
   conv = ArgumentToConv<std::string*>();
   EXPECT_EQ(ConvToString(conv), "p");
 }
 
-#if ABSL_INTERNAL_ENABLE_FORMAT_CHECKER
+#ifdef ABSL_INTERNAL_ENABLE_FORMAT_CHECKER
 
 struct Case {
   bool result;
@@ -74,6 +94,7 @@ TEST(StrFormatChecker, ValidFormat) {
       ValidFormat<void (*)(), volatile int*>("%p %p"),  //
       ValidFormat<string_view, const char*, double, void*>(
           "string_view=%s const char*=%s double=%f void*=%p)"),
+      ValidFormat<int>("%v"),  //
 
       ValidFormat<int>("%% %1$d"),               //
       ValidFormat<int>("%1$ld"),                 //
@@ -90,7 +111,9 @@ TEST(StrFormatChecker, ValidFormat) {
       ValidFormat<int, double>("%2$.*1$f"),      //
       ValidFormat<void*, string_view, const char*, double>(
           "string_view=%2$s const char*=%3$s double=%4$f void*=%1$p "
-          "repeat=%3$s)")};
+          "repeat=%3$s)"),
+      ValidFormat<std::string>("%1$v"),
+  };
 
   for (Case c : trues) {
     EXPECT_TRUE(c.result) << c.format;
@@ -111,6 +134,8 @@ TEST(StrFormatChecker, ValidFormat) {
       ValidFormat<int>("%*d"),               //
       ValidFormat<std::string>("%p"),        //
       ValidFormat<int (*)(int)>("%d"),       //
+      ValidFormat<int>("%1v"),               //
+      ValidFormat<int>("%.1v"),              //
 
       ValidFormat<>("%3$d"),                     //
       ValidFormat<>("%1$r"),                     //
@@ -119,13 +144,14 @@ TEST(StrFormatChecker, ValidFormat) {
       ValidFormat<int>("%1$*2$1d"),              //
       ValidFormat<int>("%1$1-d"),                //
       ValidFormat<std::string, int>("%2$*1$s"),  //
-      ValidFormat<std::string>("%1$p"),
+      ValidFormat<std::string>("%1$p"),          //
+      ValidFormat<int>("%1$*2$v"),               //
 
       ValidFormat<int, int>("%d %2$d"),  //
   };
 
   for (Case c : falses) {
-    EXPECT_FALSE(c.result) << c.format;
+    EXPECT_FALSE(c.result) << "format<" << c.format << ">";
   }
 }
 
@@ -147,4 +173,5 @@ TEST(StrFormatChecker, LongFormat) {
 
 }  // namespace
 }  // namespace str_format_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/internal/str_format/constexpr_parser.h b/absl/strings/internal/str_format/constexpr_parser.h
new file mode 100644
index 00000000000..8f593870474
--- /dev/null
+++ b/absl/strings/internal/str_format/constexpr_parser.h
@@ -0,0 +1,357 @@
+// Copyright 2022 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_STRINGS_INTERNAL_STR_FORMAT_CONSTEXPR_PARSER_H_
+#define ABSL_STRINGS_INTERNAL_STR_FORMAT_CONSTEXPR_PARSER_H_
+
+#include <cassert>
+#include <cstdint>
+#include <cstdio>
+#include <limits>
+
+#include "absl/base/config.h"
+#include "absl/base/const_init.h"
+#include "absl/base/optimization.h"
+#include "absl/strings/internal/str_format/extension.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace str_format_internal {
+
+// The analyzed properties of a single specified conversion.
+struct UnboundConversion {
+  // This is a user defined default constructor on purpose to skip the
+  // initialization of parts of the object that are not necessary.
+  UnboundConversion() {}  // NOLINT
+
+  // This constructor is provided for the static checker. We don't want to do
+  // the unnecessary initialization in the normal case.
+  explicit constexpr UnboundConversion(absl::ConstInitType)
+      : arg_position{}, width{}, precision{} {}
+
+  class InputValue {
+   public:
+    constexpr void set_value(int value) {
+      assert(value >= 0);
+      value_ = value;
+    }
+    constexpr int value() const { return value_; }
+
+    // Marks the value as "from arg". aka the '*' format.
+    // Requires `value >= 1`.
+    // When set, is_from_arg() return true and get_from_arg() returns the
+    // original value.
+    // `value()`'s return value is unspecified in this state.
+    constexpr void set_from_arg(int value) {
+      assert(value > 0);
+      value_ = -value - 1;
+    }
+    constexpr bool is_from_arg() const { return value_ < -1; }
+    constexpr int get_from_arg() const {
+      assert(is_from_arg());
+      return -value_ - 1;
+    }
+
+   private:
+    int value_ = -1;
+  };
+
+  // No need to initialize. It will always be set in the parser.
+  int arg_position;
+
+  InputValue width;
+  InputValue precision;
+
+  Flags flags = Flags::kBasic;
+  LengthMod length_mod = LengthMod::none;
+  FormatConversionChar conv = FormatConversionCharInternal::kNone;
+};
+
+// Helper tag class for the table below.
+// It allows fast `char -> ConversionChar/LengthMod/Flags` checking and
+// conversions.
+class ConvTag {
+ public:
+  constexpr ConvTag(FormatConversionChar conversion_char)  // NOLINT
+      : tag_(static_cast<uint8_t>(conversion_char)) {}
+  constexpr ConvTag(LengthMod length_mod)  // NOLINT
+      : tag_(0x80 | static_cast<uint8_t>(length_mod)) {}
+  constexpr ConvTag(Flags flags)  // NOLINT
+      : tag_(0xc0 | static_cast<uint8_t>(flags)) {}
+  constexpr ConvTag() : tag_(0xFF) {}
+
+  constexpr bool is_conv() const { return (tag_ & 0x80) == 0; }
+  constexpr bool is_length() const { return (tag_ & 0xC0) == 0x80; }
+  constexpr bool is_flags() const { return (tag_ & 0xE0) == 0xC0; }
+
+  constexpr FormatConversionChar as_conv() const {
+    assert(is_conv());
+    assert(!is_length());
+    assert(!is_flags());
+    return static_cast<FormatConversionChar>(tag_);
+  }
+  constexpr LengthMod as_length() const {
+    assert(!is_conv());
+    assert(is_length());
+    assert(!is_flags());
+    return static_cast<LengthMod>(tag_ & 0x3F);
+  }
+  constexpr Flags as_flags() const {
+    assert(!is_conv());
+    assert(!is_length());
+    assert(is_flags());
+    return static_cast<Flags>(tag_ & 0x1F);
+  }
+
+ private:
+  uint8_t tag_;
+};
+
+struct ConvTagHolder {
+  using CC = FormatConversionCharInternal;
+  using LM = LengthMod;
+
+  // Abbreviations to fit in the table below.
+  static constexpr auto kFSign = Flags::kSignCol;
+  static constexpr auto kFAlt = Flags::kAlt;
+  static constexpr auto kFPos = Flags::kShowPos;
+  static constexpr auto kFLeft = Flags::kLeft;
+  static constexpr auto kFZero = Flags::kZero;
+
+  static constexpr ConvTag value[256] = {
+      {},     {},    {},    {},    {},    {},     {},    {},     // 00-07
+      {},     {},    {},    {},    {},    {},     {},    {},     // 08-0f
+      {},     {},    {},    {},    {},    {},     {},    {},     // 10-17
+      {},     {},    {},    {},    {},    {},     {},    {},     // 18-1f
+      kFSign, {},    {},    kFAlt, {},    {},     {},    {},     //  !"#$%&'
+      {},     {},    {},    kFPos, {},    kFLeft, {},    {},     // ()*+,-./
+      kFZero, {},    {},    {},    {},    {},     {},    {},     // 01234567
+      {},     {},    {},    {},    {},    {},     {},    {},     // 89:;<=>?
+      {},     CC::A, {},    {},    {},    CC::E,  CC::F, CC::G,  // @ABCDEFG
+      {},     {},    {},    {},    LM::L, {},     {},    {},     // HIJKLMNO
+      {},     {},    {},    {},    {},    {},     {},    {},     // PQRSTUVW
+      CC::X,  {},    {},    {},    {},    {},     {},    {},     // XYZ[\]^_
+      {},     CC::a, {},    CC::c, CC::d, CC::e,  CC::f, CC::g,  // `abcdefg
+      LM::h,  CC::i, LM::j, {},    LM::l, {},     CC::n, CC::o,  // hijklmno
+      CC::p,  LM::q, {},    CC::s, LM::t, CC::u,  CC::v, {},     // pqrstuvw
+      CC::x,  {},    LM::z, {},    {},    {},     {},    {},     // xyz{|}!
+      {},     {},    {},    {},    {},    {},     {},    {},     // 80-87
+      {},     {},    {},    {},    {},    {},     {},    {},     // 88-8f
+      {},     {},    {},    {},    {},    {},     {},    {},     // 90-97
+      {},     {},    {},    {},    {},    {},     {},    {},     // 98-9f
+      {},     {},    {},    {},    {},    {},     {},    {},     // a0-a7
+      {},     {},    {},    {},    {},    {},     {},    {},     // a8-af
+      {},     {},    {},    {},    {},    {},     {},    {},     // b0-b7
+      {},     {},    {},    {},    {},    {},     {},    {},     // b8-bf
+      {},     {},    {},    {},    {},    {},     {},    {},     // c0-c7
+      {},     {},    {},    {},    {},    {},     {},    {},     // c8-cf
+      {},     {},    {},    {},    {},    {},     {},    {},     // d0-d7
+      {},     {},    {},    {},    {},    {},     {},    {},     // d8-df
+      {},     {},    {},    {},    {},    {},     {},    {},     // e0-e7
+      {},     {},    {},    {},    {},    {},     {},    {},     // e8-ef
+      {},     {},    {},    {},    {},    {},     {},    {},     // f0-f7
+      {},     {},    {},    {},    {},    {},     {},    {},     // f8-ff
+  };
+};
+
+// Keep a single table for all the conversion chars and length modifiers.
+constexpr ConvTag GetTagForChar(char c) {
+  return ConvTagHolder::value[static_cast<unsigned char>(c)];
+}
+
+constexpr bool CheckFastPathSetting(const UnboundConversion& conv) {
+  bool width_precision_needed =
+      conv.width.value() >= 0 || conv.precision.value() >= 0;
+  if (width_precision_needed && conv.flags == Flags::kBasic) {
+#if defined(__clang__)
+    // Some compilers complain about this in constexpr even when not executed,
+    // so only enable the error dump in clang.
+    fprintf(stderr,
+            "basic=%d left=%d show_pos=%d sign_col=%d alt=%d zero=%d "
+            "width=%d precision=%d\n",
+            conv.flags == Flags::kBasic ? 1 : 0,
+            FlagsContains(conv.flags, Flags::kLeft) ? 1 : 0,
+            FlagsContains(conv.flags, Flags::kShowPos) ? 1 : 0,
+            FlagsContains(conv.flags, Flags::kSignCol) ? 1 : 0,
+            FlagsContains(conv.flags, Flags::kAlt) ? 1 : 0,
+            FlagsContains(conv.flags, Flags::kZero) ? 1 : 0, conv.width.value(),
+            conv.precision.value());
+#endif  // defined(__clang__)
+    return false;
+  }
+  return true;
+}
+
+constexpr int ParseDigits(char& c, const char*& pos, const char* const end) {
+  int digits = c - '0';
+  // We do not want to overflow `digits` so we consume at most digits10
+  // digits. If there are more digits the parsing will fail later on when the
+  // digit doesn't match the expected characters.
+  int num_digits = std::numeric_limits<int>::digits10;
+  for (;;) {
+    if (ABSL_PREDICT_FALSE(pos == end)) break;
+    c = *pos++;
+    if ('0' > c || c > '9') break;
+    --num_digits;
+    if (ABSL_PREDICT_FALSE(!num_digits)) break;
+    digits = 10 * digits + c - '0';
+  }
+  return digits;
+}
+
+template <bool is_positional>
+constexpr const char* ConsumeConversion(const char* pos, const char* const end,
+                                        UnboundConversion* conv,
+                                        int* next_arg) {
+  const char* const original_pos = pos;
+  char c = 0;
+  // Read the next char into `c` and update `pos`. Returns false if there are
+  // no more chars to read.
+#define ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR()          \
+  do {                                                  \
+    if (ABSL_PREDICT_FALSE(pos == end)) return nullptr; \
+    c = *pos++;                                         \
+  } while (0)
+
+  if (is_positional) {
+    ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
+    if (ABSL_PREDICT_FALSE(c < '1' || c > '9')) return nullptr;
+    conv->arg_position = ParseDigits(c, pos, end);
+    assert(conv->arg_position > 0);
+    if (ABSL_PREDICT_FALSE(c != '$')) return nullptr;
+  }
+
+  ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
+
+  // We should start with the basic flag on.
+  assert(conv->flags == Flags::kBasic);
+
+  // Any non alpha character makes this conversion not basic.
+  // This includes flags (-+ #0), width (1-9, *) or precision (.).
+  // All conversion characters and length modifiers are alpha characters.
+  if (c < 'A') {
+    while (c <= '0') {
+      auto tag = GetTagForChar(c);
+      if (tag.is_flags()) {
+        conv->flags = conv->flags | tag.as_flags();
+        ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
+      } else {
+        break;
+      }
+    }
+
+    if (c <= '9') {
+      if (c >= '0') {
+        int maybe_width = ParseDigits(c, pos, end);
+        if (!is_positional && c == '$') {
+          if (ABSL_PREDICT_FALSE(*next_arg != 0)) return nullptr;
+          // Positional conversion.
+          *next_arg = -1;
+          return ConsumeConversion<true>(original_pos, end, conv, next_arg);
+        }
+        conv->flags = conv->flags | Flags::kNonBasic;
+        conv->width.set_value(maybe_width);
+      } else if (c == '*') {
+        conv->flags = conv->flags | Flags::kNonBasic;
+        ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
+        if (is_positional) {
+          if (ABSL_PREDICT_FALSE(c < '1' || c > '9')) return nullptr;
+          conv->width.set_from_arg(ParseDigits(c, pos, end));
+          if (ABSL_PREDICT_FALSE(c != '$')) return nullptr;
+          ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
+        } else {
+          conv->width.set_from_arg(++*next_arg);
+        }
+      }
+    }
+
+    if (c == '.') {
+      conv->flags = conv->flags | Flags::kNonBasic;
+      ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
+      if ('0' <= c && c <= '9') {
+        conv->precision.set_value(ParseDigits(c, pos, end));
+      } else if (c == '*') {
+        ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
+        if (is_positional) {
+          if (ABSL_PREDICT_FALSE(c < '1' || c > '9')) return nullptr;
+          conv->precision.set_from_arg(ParseDigits(c, pos, end));
+          if (c != '$') return nullptr;
+          ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
+        } else {
+          conv->precision.set_from_arg(++*next_arg);
+        }
+      } else {
+        conv->precision.set_value(0);
+      }
+    }
+  }
+
+  auto tag = GetTagForChar(c);
+
+  if (ABSL_PREDICT_FALSE(c == 'v' && conv->flags != Flags::kBasic)) {
+    return nullptr;
+  }
+
+  if (ABSL_PREDICT_FALSE(!tag.is_conv())) {
+    if (ABSL_PREDICT_FALSE(!tag.is_length())) return nullptr;
+
+    // It is a length modifier.
+    LengthMod length_mod = tag.as_length();
+    ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
+    if (c == 'h' && length_mod == LengthMod::h) {
+      conv->length_mod = LengthMod::hh;
+      ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
+    } else if (c == 'l' && length_mod == LengthMod::l) {
+      conv->length_mod = LengthMod::ll;
+      ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
+    } else {
+      conv->length_mod = length_mod;
+    }
+    tag = GetTagForChar(c);
+
+    if (ABSL_PREDICT_FALSE(c == 'v')) return nullptr;
+    if (ABSL_PREDICT_FALSE(!tag.is_conv())) return nullptr;
+
+    // `wchar_t` args are marked non-basic so `Bind()` will copy the length mod.
+    if (conv->length_mod == LengthMod::l && c == 'c') {
+      conv->flags = conv->flags | Flags::kNonBasic;
+    }
+  }
+#undef ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR
+
+  assert(CheckFastPathSetting(*conv));
+  (void)(&CheckFastPathSetting);
+
+  conv->conv = tag.as_conv();
+  if (!is_positional) conv->arg_position = ++*next_arg;
+  return pos;
+}
+
+// Consume conversion spec prefix (not including '%') of [p, end) if valid.
+// Examples of valid specs would be e.g.: "s", "d", "-12.6f".
+// If valid, it returns the first character following the conversion spec,
+// and the spec part is broken down and returned in 'conv'.
+// If invalid, returns nullptr.
+constexpr const char* ConsumeUnboundConversion(const char* p, const char* end,
+                                               UnboundConversion* conv,
+                                               int* next_arg) {
+  if (*next_arg < 0) return ConsumeConversion<true>(p, end, conv, next_arg);
+  return ConsumeConversion<false>(p, end, conv, next_arg);
+}
+
+}  // namespace str_format_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STRINGS_INTERNAL_STR_FORMAT_CONSTEXPR_PARSER_H_
diff --git a/absl/strings/internal/str_format/convert_test.cc b/absl/strings/internal/str_format/convert_test.cc
index 814ccf4c65e..46b8588045e 100644
--- a/absl/strings/internal/str_format/convert_test.cc
+++ b/absl/strings/internal/str_format/convert_test.cc
@@ -1,52 +1,168 @@
-#include <errno.h>
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <assert.h>
+#include <locale.h>
 #include <stdarg.h>
 #include <stdio.h>
-#include <cctype>
+
+#include <algorithm>
+#include <climits>
 #include <cmath>
+#include <cstdint>
+#include <cstdlib>
+#include <cstring>
+#include <cwctype>
+#include <limits>
+#include <optional>
+#include <set>
+#include <sstream>
 #include <string>
+#include <string_view>
+#include <thread>  // NOLINT
+#include <type_traits>
+#include <vector>
 
+#include "gmock/gmock.h"
 #include "gtest/gtest.h"
+#include "absl/base/attributes.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/log/absl_check.h"
+#include "absl/log/log.h"
+#include "absl/numeric/int128.h"
+#include "absl/strings/ascii.h"
+#include "absl/strings/internal/str_format/arg.h"
 #include "absl/strings/internal/str_format/bind.h"
+#include "absl/strings/match.h"
+#include "absl/strings/str_format.h"
+#include "absl/strings/string_view.h"
+#include "absl/types/span.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace str_format_internal {
 namespace {
 
+struct NativePrintfTraits {
+  bool hex_float_has_glibc_rounding;
+  bool hex_float_prefers_denormal_repr;
+  bool hex_float_uses_minimal_precision_when_not_specified;
+  bool hex_float_optimizes_leading_digit_bit_count;
+};
+
 template <typename T, size_t N>
 size_t ArraySize(T (&)[N]) {
   return N;
 }
 
-std::string LengthModFor(float) { return ""; }
-std::string LengthModFor(double) { return ""; }
-std::string LengthModFor(long double) { return "L"; }
-std::string LengthModFor(char) { return "hh"; }
-std::string LengthModFor(signed char) { return "hh"; }
-std::string LengthModFor(unsigned char) { return "hh"; }
-std::string LengthModFor(short) { return "h"; }           // NOLINT
-std::string LengthModFor(unsigned short) { return "h"; }  // NOLINT
-std::string LengthModFor(int) { return ""; }
-std::string LengthModFor(unsigned) { return ""; }
-std::string LengthModFor(long) { return "l"; }                 // NOLINT
-std::string LengthModFor(unsigned long) { return "l"; }        // NOLINT
-std::string LengthModFor(long long) { return "ll"; }           // NOLINT
-std::string LengthModFor(unsigned long long) { return "ll"; }  // NOLINT
+template <typename T>
+struct AlwaysFalse : std::false_type {};
+
+template <typename T>
+std::string LengthModFor() {
+  static_assert(AlwaysFalse<T>::value, "Unsupported type");
+  return "";
+}
+template <>
+std::string LengthModFor<char>() {
+  return "hh";
+}
+template <>
+std::string LengthModFor<signed char>() {
+  return "hh";
+}
+template <>
+std::string LengthModFor<unsigned char>() {
+  return "hh";
+}
+template <>
+std::string LengthModFor<short>() {  // NOLINT
+  return "h";
+}
+template <>
+std::string LengthModFor<unsigned short>() {  // NOLINT
+  return "h";
+}
+template <>
+std::string LengthModFor<int>() {
+  return "";
+}
+template <>
+std::string LengthModFor<unsigned>() {
+  return "";
+}
+template <>
+std::string LengthModFor<long>() {  // NOLINT
+  return "l";
+}
+template <>
+std::string LengthModFor<unsigned long>() {  // NOLINT
+  return "l";
+}
+template <>
+std::string LengthModFor<long long>() {  // NOLINT
+  return "ll";
+}
+template <>
+std::string LengthModFor<unsigned long long>() {  // NOLINT
+  return "ll";
+}
+
+// An integral type of the same rank and signedness as `wchar_t`, that isn't
+// `wchar_t`.
+using IntegralTypeForWCharT =
+    std::conditional_t<std::is_signed_v<wchar_t>,
+                       // Some STLs are broken and return `wchar_t` from
+                       // `std::make_[un]signed_t<wchar_t>` when the signedness
+                       // matches. Work around by round-tripping through the
+                       // opposite signedness.
+                       std::make_signed_t<std::make_unsigned_t<wchar_t>>,
+                       std::make_unsigned_t<std::make_signed_t<wchar_t>>>;
+
+// Given an integral type `T`, returns a type of the same rank and signedness
+// that is guaranteed to not be `wchar_t`.
+template <typename T>
+using MatchingIntegralType =
+    std::conditional_t<std::is_same_v<T, wchar_t>, IntegralTypeForWCharT, T>;
 
 std::string EscCharImpl(int v) {
-  if (std::isprint(static_cast<unsigned char>(v))) {
-    return std::string(1, static_cast<char>(v));
-  }
   char buf[64];
-  int n = snprintf(buf, sizeof(buf), "\\%#.2x",
-                   static_cast<unsigned>(v & 0xff));
-  assert(n > 0 && n < sizeof(buf));
-  return std::string(buf, n);
+  int n = absl::ascii_isprint(static_cast<unsigned char>(v))
+              ? snprintf(buf, sizeof(buf), "'%c'", v)
+              : snprintf(buf, sizeof(buf), "'\\x%.*x'", CHAR_BIT / 4,
+                         static_cast<unsigned>(
+                             static_cast<std::make_unsigned_t<char>>(v)));
+  assert(n > 0 && static_cast<size_t>(n) < sizeof(buf));
+  return std::string(buf, static_cast<size_t>(n));
 }
 
 std::string Esc(char v) { return EscCharImpl(v); }
 std::string Esc(signed char v) { return EscCharImpl(v); }
 std::string Esc(unsigned char v) { return EscCharImpl(v); }
 
+std::string Esc(wchar_t v) {
+  char buf[64];
+  int n = std::iswprint(static_cast<wint_t>(v))
+              ? snprintf(buf, sizeof(buf), "L'%lc'", static_cast<wint_t>(v))
+              : snprintf(buf, sizeof(buf), "L'\\x%.*llx'",
+                         static_cast<int>(sizeof(wchar_t) * CHAR_BIT / 4),
+                         static_cast<unsigned long long>(
+                             static_cast<std::make_unsigned_t<wchar_t>>(v)));
+  assert(n > 0 && static_cast<size_t>(n) < sizeof(buf));
+  return std::string(buf, static_cast<size_t>(n));
+}
+
 template <typename T>
 std::string Esc(const T &v) {
   std::ostringstream oss;
@@ -54,7 +170,7 @@ std::string Esc(const T &v) {
   return oss.str();
 }
 
-void StrAppend(std::string *dst, const char *format, va_list ap) {
+void StrAppendV(std::string *dst, const char *format, va_list ap) {
   // First try with a small fixed size buffer
   static const int kSpaceLength = 1024;
   char space[kSpaceLength];
@@ -69,7 +185,7 @@ void StrAppend(std::string *dst, const char *format, va_list ap) {
   if (result < kSpaceLength) {
     if (result >= 0) {
       // Normal case -- everything fit.
-      dst->append(space, result);
+      dst->append(space, static_cast<size_t>(result));
       return;
     }
     if (result < 0) {
@@ -80,7 +196,7 @@ void StrAppend(std::string *dst, const char *format, va_list ap) {
 
   // Increase the buffer size to the size requested by vsnprintf,
   // plus one for the closing \0.
-  int length = result + 1;
+  size_t length = static_cast<size_t>(result) + 1;
   char *buf = new char[length];
 
   // Restore the va_list before we use it again
@@ -88,22 +204,90 @@ void StrAppend(std::string *dst, const char *format, va_list ap) {
   result = vsnprintf(buf, length, format, backup_ap);
   va_end(backup_ap);
 
-  if (result >= 0 && result < length) {
+  if (result >= 0 && static_cast<size_t>(result) < length) {
     // It fit
-    dst->append(buf, result);
+    dst->append(buf, static_cast<size_t>(result));
   }
   delete[] buf;
 }
 
+void StrAppend(std::string *, const char *, ...) ABSL_PRINTF_ATTRIBUTE(2, 3);
+void StrAppend(std::string *out, const char *format, ...) {
+  va_list ap;
+  va_start(ap, format);
+  StrAppendV(out, format, ap);
+  va_end(ap);
+}
+
+std::string StrPrint(const char *, ...) ABSL_PRINTF_ATTRIBUTE(1, 2);
 std::string StrPrint(const char *format, ...) {
   va_list ap;
   va_start(ap, format);
   std::string result;
-  StrAppend(&result, format, ap);
+  StrAppendV(&result, format, ap);
   va_end(ap);
   return result;
 }
 
+NativePrintfTraits VerifyNativeImplementationImpl() {
+  NativePrintfTraits result;
+
+  // >>> hex_float_has_glibc_rounding. To have glibc's rounding behavior we need
+  // to meet three requirements:
+  //
+  //   - The threshold for rounding up is 8 (for e.g. MSVC uses 9).
+  //   - If the digits lower than than the 8 are non-zero then we round up.
+  //   - If the digits lower than the 8 are all zero then we round toward even.
+  //
+  // The numbers below represent all the cases covering {below,at,above} the
+  // threshold (8) with both {zero,non-zero} lower bits and both {even,odd}
+  // preceding digits.
+  const double d0079 = 65657.0;  // 0x1.0079p+16
+  const double d0179 = 65913.0;  // 0x1.0179p+16
+  const double d0080 = 65664.0;  // 0x1.0080p+16
+  const double d0180 = 65920.0;  // 0x1.0180p+16
+  const double d0081 = 65665.0;  // 0x1.0081p+16
+  const double d0181 = 65921.0;  // 0x1.0181p+16
+  result.hex_float_has_glibc_rounding =
+      StartsWith(StrPrint("%.2a", d0079), "0x1.00") &&
+      StartsWith(StrPrint("%.2a", d0179), "0x1.01") &&
+      StartsWith(StrPrint("%.2a", d0080), "0x1.00") &&
+      StartsWith(StrPrint("%.2a", d0180), "0x1.02") &&
+      StartsWith(StrPrint("%.2a", d0081), "0x1.01") &&
+      StartsWith(StrPrint("%.2a", d0181), "0x1.02");
+
+  // >>> hex_float_prefers_denormal_repr. Formatting `denormal` on glibc yields
+  // "0x0.0000000000001p-1022", whereas on std libs that don't use denormal
+  // representation it would either be 0x1p-1074 or 0x1.0000000000000-1074.
+  const double denormal = std::numeric_limits<double>::denorm_min();
+  result.hex_float_prefers_denormal_repr =
+      StartsWith(StrPrint("%a", denormal), "0x0.0000000000001");
+
+  // >>> hex_float_uses_minimal_precision_when_not_specified. Some (non-glibc)
+  // libs will format the following as "0x1.0079000000000p+16".
+  result.hex_float_uses_minimal_precision_when_not_specified =
+      (StrPrint("%a", d0079) == "0x1.0079p+16");
+
+  // >>> hex_float_optimizes_leading_digit_bit_count. The number 1.5, when
+  // formatted by glibc should yield "0x1.8p+0" for `double` and "0xcp-3" for
+  // `long double`, i.e., number of bits in the leading digit is adapted to the
+  // number of bits in the mantissa.
+  const double d_15 = 1.5;
+  const long double ld_15 = 1.5;
+  result.hex_float_optimizes_leading_digit_bit_count =
+      StartsWith(StrPrint("%a", d_15), "0x1.8") &&
+      StartsWith(StrPrint("%La", ld_15), "0xc");
+
+  return result;
+}
+
+const NativePrintfTraits &VerifyNativeImplementation() {
+  static NativePrintfTraits native_traits = VerifyNativeImplementationImpl();
+  return native_traits;
+}
+
+bool IsNativeHexFloatConversion(char f) { return f == 'a' || f == 'A'; }
+
 class FormatConvertTest : public ::testing::Test { };
 
 template <typename T>
@@ -136,12 +320,26 @@ TEST_F(FormatConvertTest, BasicString) {
   TestStringConvert(static_cast<const char*>("hello"));
   TestStringConvert(std::string("hello"));
   TestStringConvert(string_view("hello"));
+  TestStringConvert(std::string_view("hello"));
+
+#if GTEST_HAS_STD_WSTRING
+  TestStringConvert(L"hello");
+  TestStringConvert(static_cast<const wchar_t*>(L"hello"));
+  TestStringConvert(std::wstring(L"hello"));
+  TestStringConvert(std::wstring_view(L"hello"));
+#endif  // GTEST_HAS_STD_WSTRING
 }
 
 TEST_F(FormatConvertTest, NullString) {
   const char* p = nullptr;
   UntypedFormatSpecImpl format("%s");
   EXPECT_EQ("", FormatPack(format, {FormatArgImpl(p)}));
+
+#if GTEST_HAS_STD_WSTRING
+  const wchar_t* wp = nullptr;
+  UntypedFormatSpecImpl wformat("%ls");
+  EXPECT_EQ("", FormatPack(wformat, {FormatArgImpl(wp)}));
+#endif  // GTEST_HAS_STD_WSTRING
 }
 
 TEST_F(FormatConvertTest, StringPrecision) {
@@ -151,70 +349,185 @@ TEST_F(FormatConvertTest, StringPrecision) {
   UntypedFormatSpecImpl format("%.1s");
   EXPECT_EQ("a", FormatPack(format, {FormatArgImpl(p)}));
 
-  // We cap at the nul terminator.
+  // We cap at the NUL-terminator.
   p = "ABC";
   UntypedFormatSpecImpl format2("%.10s");
   EXPECT_EQ("ABC", FormatPack(format2, {FormatArgImpl(p)}));
+
+#if GTEST_HAS_STD_WSTRING
+  // We cap at the precision.
+  wchar_t wc = L'a';
+  const wchar_t* wp = &wc;
+  UntypedFormatSpecImpl wformat("%.1ls");
+  EXPECT_EQ("a", FormatPack(wformat, {FormatArgImpl(wp)}));
+
+  // We cap at the NUL-terminator.
+  wp = L"ABC";
+  UntypedFormatSpecImpl wformat2("%.10ls");
+  EXPECT_EQ("ABC", FormatPack(wformat2, {FormatArgImpl(wp)}));
+#endif  // GTEST_HAS_STD_WSTRING
+}
+
+TEST_F(FormatConvertTest, WideStringUnpairedSurrogate) {
+  // The single wchar_t ("%lc") path rejects an unpaired surrogate. The wide
+  // string ("%ls") path should reject it too rather than emitting a partial
+  // UTF-8 sequence. A failed conversion yields an empty result.
+  auto format_ls = [](const std::wstring& ws) {
+    UntypedFormatSpecImpl format("%ls");
+    return FormatPack(format, {FormatArgImpl(ws)});
+  };
+
+  // A well-formed surrogate pair (U+10000) still converts.
+  std::wstring pair;
+  pair.push_back(static_cast<wchar_t>(0xD800));
+  pair.push_back(static_cast<wchar_t>(0xDC00));
+  EXPECT_EQ("\xF0\x90\x80\x80", format_ls(pair));
+
+  // Trailing high surrogate with no low surrogate to complete it.
+  std::wstring trailing_high;
+  trailing_high.push_back(static_cast<wchar_t>(0xD800));
+  EXPECT_EQ("", format_ls(trailing_high));
+
+  // High surrogate followed by a non-surrogate.
+  std::wstring high_then_ascii;
+  high_then_ascii.push_back(static_cast<wchar_t>(0xD800));
+  high_then_ascii.push_back(L'A');
+  EXPECT_EQ("", format_ls(high_then_ascii));
+
+  // High surrogate followed by another high surrogate.
+  std::wstring high_then_high;
+  high_then_high.push_back(static_cast<wchar_t>(0xD800));
+  high_then_high.push_back(static_cast<wchar_t>(0xD800));
+  EXPECT_EQ("", format_ls(high_then_high));
+
+  // Isolated low surrogate.
+  std::wstring lone_low;
+  lone_low.push_back(static_cast<wchar_t>(0xDC00));
+  EXPECT_EQ("", format_ls(lone_low));
+}
+
+// Pointer formatting is implementation defined. This checks that the argument
+// can be matched to `ptr`.
+MATCHER_P(MatchesPointerString, ptr, "") {
+  if (ptr == nullptr && arg == "(nil)") {
+    return true;
+  }
+  void* parsed = nullptr;
+  if (sscanf(arg.c_str(), "%p", &parsed) != 1) {
+    LOG(FATAL) << "Could not parse " << arg;
+  }
+  return ptr == parsed;
 }
 
 TEST_F(FormatConvertTest, Pointer) {
-#ifdef _MSC_VER
-  // MSVC's printf implementation prints pointers differently. We can't easily
-  // compare our implementation to theirs.
-  return;
-#endif
   static int x = 0;
   const int *xp = &x;
+  const int* inil = nullptr;
+
+  using VoidF = void (*)();
+  VoidF fp = [] {};
+  VoidF fnil = nullptr;
+
   char c = 'h';
   char *mcp = &c;
   const char *cp = "hi";
   const char *cnil = nullptr;
-  const int *inil = nullptr;
-  using VoidF = void (*)();
-  VoidF fp = [] {}, fnil = nullptr;
   volatile char vc;
   volatile char* vcp = &vc;
   volatile char* vcnil = nullptr;
-  const FormatArgImpl args[] = {
-      FormatArgImpl(xp),   FormatArgImpl(cp),  FormatArgImpl(inil),
-      FormatArgImpl(cnil), FormatArgImpl(mcp), FormatArgImpl(fp),
-      FormatArgImpl(fnil), FormatArgImpl(vcp), FormatArgImpl(vcnil),
-  };
-  struct Expectation {
-    std::string out;
-    const char *fmt;
-  };
-  const Expectation kExpect[] = {
-      {StrPrint("%p", &x), "%p"},
-      {StrPrint("%20p", &x), "%20p"},
-      {StrPrint("%.1p", &x), "%.1p"},
-      {StrPrint("%.20p", &x), "%.20p"},
-      {StrPrint("%30.20p", &x), "%30.20p"},
-
-      {StrPrint("%-p", &x), "%-p"},
-      {StrPrint("%-20p", &x), "%-20p"},
-      {StrPrint("%-.1p", &x), "%-.1p"},
-      {StrPrint("%.20p", &x), "%.20p"},
-      {StrPrint("%-30.20p", &x), "%-30.20p"},
-
-      {StrPrint("%p", cp), "%2$p"},   // const char*
-      {"(nil)", "%3$p"},              // null const char *
-      {"(nil)", "%4$p"},              // null const int *
-      {StrPrint("%p", mcp), "%5$p"},  // nonconst char*
-
-      {StrPrint("%p", fp), "%6$p"},   // function pointer
-      {StrPrint("%p", vcp), "%8$p"},  // function pointer
-
-#ifndef __APPLE__
-      // Apple's printf differs here (0x0 vs. nil)
-      {StrPrint("%p", fnil), "%7$p"},   // null function pointer
-      {StrPrint("%p", vcnil), "%9$p"},  // null function pointer
-#endif
+
+#if GTEST_HAS_STD_WSTRING
+  wchar_t wc = L'h';
+  wchar_t *mwcp = &wc;
+  const wchar_t *wcp = L"hi";
+  const wchar_t *wcnil = nullptr;
+  volatile wchar_t vwc;
+  volatile wchar_t *vwcp = &vwc;
+  volatile wchar_t *vwcnil = nullptr;
+#endif  // GTEST_HAS_STD_WSTRING
+
+  const FormatArgImpl args_array[] = {
+      FormatArgImpl(xp),   FormatArgImpl(inil),   FormatArgImpl(fp),
+      FormatArgImpl(fnil), FormatArgImpl(mcp),     FormatArgImpl(cp),
+      FormatArgImpl(cnil),  FormatArgImpl(vcp),    FormatArgImpl(vcnil),
+#if GTEST_HAS_STD_WSTRING
+      FormatArgImpl(mwcp),  FormatArgImpl(wcp),   FormatArgImpl(wcnil),
+      FormatArgImpl(vwcp), FormatArgImpl(vwcnil),
+#endif  // GTEST_HAS_STD_WSTRING
   };
-  for (const Expectation &e : kExpect) {
-    UntypedFormatSpecImpl format(e.fmt);
-    EXPECT_EQ(e.out, FormatPack(format, absl::MakeSpan(args))) << e.fmt;
-  }
+  auto args = absl::MakeConstSpan(args_array);
+
+  EXPECT_THAT(FormatPack(UntypedFormatSpecImpl("%p"), args),
+              MatchesPointerString(&x));
+  EXPECT_THAT(FormatPack(UntypedFormatSpecImpl("%20p"), args),
+              MatchesPointerString(&x));
+  EXPECT_THAT(FormatPack(UntypedFormatSpecImpl("%.1p"), args),
+              MatchesPointerString(&x));
+  EXPECT_THAT(FormatPack(UntypedFormatSpecImpl("%.20p"), args),
+              MatchesPointerString(&x));
+  EXPECT_THAT(FormatPack(UntypedFormatSpecImpl("%30.20p"), args),
+              MatchesPointerString(&x));
+
+  EXPECT_THAT(FormatPack(UntypedFormatSpecImpl("%-p"), args),
+              MatchesPointerString(&x));
+  EXPECT_THAT(FormatPack(UntypedFormatSpecImpl("%-20p"), args),
+              MatchesPointerString(&x));
+  EXPECT_THAT(FormatPack(UntypedFormatSpecImpl("%-.1p"), args),
+              MatchesPointerString(&x));
+  EXPECT_THAT(FormatPack(UntypedFormatSpecImpl("%.20p"), args),
+              MatchesPointerString(&x));
+  EXPECT_THAT(FormatPack(UntypedFormatSpecImpl("%-30.20p"), args),
+              MatchesPointerString(&x));
+
+  // const int*
+  EXPECT_THAT(FormatPack(UntypedFormatSpecImpl("%1$p"), args),
+              MatchesPointerString(xp));
+  // null const int*
+  EXPECT_THAT(FormatPack(UntypedFormatSpecImpl("%2$p"), args),
+              MatchesPointerString(nullptr));
+
+  // function pointer
+  EXPECT_THAT(FormatPack(UntypedFormatSpecImpl("%3$p"), args),
+              MatchesPointerString(reinterpret_cast<const void*>(fp)));
+  // null function pointer
+  EXPECT_THAT(FormatPack(UntypedFormatSpecImpl("%4$p"), args),
+              MatchesPointerString(nullptr));
+
+  // nonconst char*
+  EXPECT_THAT(FormatPack(UntypedFormatSpecImpl("%5$p"), args),
+              MatchesPointerString(mcp));
+  // const char*
+  EXPECT_THAT(FormatPack(UntypedFormatSpecImpl("%6$p"), args),
+              MatchesPointerString(cp));
+  // null const char*
+  EXPECT_THAT(FormatPack(UntypedFormatSpecImpl("%7$p"), args),
+              MatchesPointerString(nullptr));
+  // volatile char*
+  EXPECT_THAT(
+      FormatPack(UntypedFormatSpecImpl("%8$p"), args),
+      MatchesPointerString(reinterpret_cast<volatile const void*>(vcp)));
+  // null volatile char*
+  EXPECT_THAT(FormatPack(UntypedFormatSpecImpl("%9$p"), args),
+              MatchesPointerString(nullptr));
+
+#if GTEST_HAS_STD_WSTRING
+  // nonconst wchar_t*
+  EXPECT_THAT(FormatPack(UntypedFormatSpecImpl("%10$p"), args),
+              MatchesPointerString(mwcp));
+  // const wchar_t*
+  EXPECT_THAT(FormatPack(UntypedFormatSpecImpl("%11$p"), args),
+              MatchesPointerString(wcp));
+  // null const wchar_t*
+  EXPECT_THAT(FormatPack(UntypedFormatSpecImpl("%12$p"), args),
+              MatchesPointerString(nullptr));
+  // volatile wchar_t*
+  EXPECT_THAT(
+      FormatPack(UntypedFormatSpecImpl("%13$p"), args),
+      MatchesPointerString(reinterpret_cast<volatile const void *>(vwcp)));
+  // null volatile wchar_t*
+  EXPECT_THAT(FormatPack(UntypedFormatSpecImpl("%14$p"), args),
+              MatchesPointerString(nullptr));
+#endif  // GTEST_HAS_STD_WSTRING
 }
 
 struct Cardinal {
@@ -252,8 +565,8 @@ std::vector<std::string> AllFlagCombinations() {
 
 TYPED_TEST_P(TypedFormatConvertTest, AllIntsWithFlags) {
   typedef TypeParam T;
-  typedef typename std::make_unsigned<T>::type UnsignedT;
-  using remove_volatile_t = typename std::remove_volatile<T>::type;
+  typedef std::make_unsigned_t<T> UnsignedT;
+  using remove_volatile_t = std::remove_volatile_t<T>;
   const T kMin = std::numeric_limits<remove_volatile_t>::min();
   const T kMax = std::numeric_limits<remove_volatile_t>::max();
   const T kVals[] = {
@@ -292,7 +605,7 @@ TYPED_TEST_P(TypedFormatConvertTest, AllIntsWithFlags) {
 
             const bool is_signed_conv = (conv_char == 'd' || conv_char == 'i');
             const bool is_unsigned_to_signed =
-                !std::is_signed<T>::value && is_signed_conv;
+                !std::is_signed_v<T> && is_signed_conv;
             // Don't consider sign-related flags '+' and ' ' when doing
             // unsigned to signed conversions.
             if (is_unsigned_to_signed &&
@@ -313,12 +626,15 @@ TYPED_TEST_P(TypedFormatConvertTest, AllIntsWithFlags) {
               // as printf can't do that conversion properly. For those
               // cases, we do expect agreement with printf with a "%u"
               // and the unsigned equivalent of 'val'.
-              UnsignedT uval = val;
-              old_fmt += LengthModFor(uval);
+              UnsignedT uval =
+                  static_cast<std::remove_volatile_t<UnsignedT>>(val);
+              old_fmt += LengthModFor<
+                  MatchingIntegralType<std::remove_cv_t<decltype(uval)>>>();
               old_fmt += "u";
               old_result = StrPrint(old_fmt.c_str(), uval);
             } else {
-              old_fmt += LengthModFor(val);
+              old_fmt += LengthModFor<
+                  MatchingIntegralType<std::remove_cv_t<decltype(val)>>>();
               old_fmt += conv_char;
               old_result = StrPrint(old_fmt.c_str(), val);
             }
@@ -336,37 +652,109 @@ TYPED_TEST_P(TypedFormatConvertTest, AllIntsWithFlags) {
   }
 }
 
+template <typename T>
+std::optional<std::string> StrPrintChar(T c) {
+  return StrPrint("%c", static_cast<int>(c));
+}
+template <>
+std::optional<std::string> StrPrintChar(wchar_t c) {
+  // musl libc has a bug where ("%lc", 0) writes no characters, and Android
+  // doesn't support forcing UTF-8 via setlocale(). Hardcode the expected
+  // answers for ASCII inputs to maximize test coverage on these platforms.
+  if (static_cast<std::make_unsigned_t<wchar_t>>(c) < 0x80) {
+    return std::string(1, static_cast<char>(c));
+  }
+
+  // Force a UTF-8 locale to match the expected `StrFormat()` behavior.
+  // It's important to copy the string returned by `old_locale` here, because
+  // its contents are not guaranteed to be valid after the next `setlocale()`
+  // call.
+  std::string old_locale = setlocale(LC_CTYPE, nullptr);
+  if (!setlocale(LC_CTYPE, "en_US.UTF-8")) {
+    return std::nullopt;
+  }
+  const std::string output = StrPrint("%lc", static_cast<wint_t>(c));
+  setlocale(LC_CTYPE, old_locale.c_str());
+  return output;
+}
+
+template <typename T>
+std::remove_volatile_t<T> GetMaxForConversion() {
+  return static_cast<std::remove_volatile_t<T>>(
+      std::numeric_limits<int>::max());
+}
+
+template <>
+wchar_t GetMaxForConversion<wchar_t>() {
+  // Don't return values that aren't legal Unicode. For wchar_t conversions in a
+  // UTF-8 locale, conversion behavior for such values is unspecified, and we
+  // don't care about matching it.
+  return (sizeof(wchar_t) * CHAR_BIT <= 16) ? wchar_t{0xffff}
+                                            : static_cast<wchar_t>(0x10ffff);
+}
+
 TYPED_TEST_P(TypedFormatConvertTest, Char) {
+  // Pass a bunch of values of type TypeParam to both FormatPack and libc's
+  // vsnprintf("%c", ...) (wrapped in StrPrint) to make sure we get the same
+  // value.
   typedef TypeParam T;
-  using remove_volatile_t = typename std::remove_volatile<T>::type;
-  static const T kMin = std::numeric_limits<remove_volatile_t>::min();
-  static const T kMax = std::numeric_limits<remove_volatile_t>::max();
-  T kVals[] = {
-    remove_volatile_t(1), remove_volatile_t(2), remove_volatile_t(10),
-    remove_volatile_t(-1), remove_volatile_t(-2), remove_volatile_t(-10),
-    remove_volatile_t(0),
-    kMin + remove_volatile_t(1), kMin,
-    kMax - remove_volatile_t(1), kMax
+  using remove_volatile_t = std::remove_volatile_t<T>;
+  std::vector<remove_volatile_t> vals = {
+      remove_volatile_t(1),  remove_volatile_t(2),  remove_volatile_t(10),   //
+      remove_volatile_t(-1), remove_volatile_t(-2), remove_volatile_t(-10),  //
+      remove_volatile_t(0),
   };
-  for (const T &c : kVals) {
+
+  // We'd like to test values near std::numeric_limits::min() and
+  // std::numeric_limits::max(), too, but vsnprintf("%c", ...) can't handle
+  // anything larger than an int. Add in the most extreme values we can without
+  // exceeding that range.
+  // Special case: Formatting a wchar_t should behave like vsnprintf("%lc").
+  // Technically vsnprintf can accept a wint_t in this case, but since we must
+  // pass a wchar_t to FormatPack, the largest type we can use here is wchar_t.
+  using ArgType = std::conditional_t<std::is_same_v<T, wchar_t>, wchar_t, int>;
+  static const T kMin =
+      static_cast<remove_volatile_t>(std::numeric_limits<ArgType>::min());
+  static const T kMax = GetMaxForConversion<T>();
+  vals.insert(vals.end(), {static_cast<remove_volatile_t>(kMin + 1), kMin,
+                           static_cast<remove_volatile_t>(kMax - 1), kMax});
+
+  static const auto kMaxWCharT =
+      static_cast<remove_volatile_t>(GetMaxForConversion<wchar_t>());
+  for (const T c : vals) {
+    SCOPED_TRACE(Esc(c));
     const FormatArgImpl args[] = {FormatArgImpl(c)};
     UntypedFormatSpecImpl format("%c");
-    EXPECT_EQ(StrPrint("%c", c), FormatPack(format, absl::MakeSpan(args)));
+    std::optional<std::string> result = StrPrintChar(c);
+    if (result.has_value()) {
+      EXPECT_EQ(result.value(), FormatPack(format, absl::MakeSpan(args)));
+    }
+
+    // Also test that if the format specifier is "%lc", the argument is treated
+    // as if it's a `wchar_t`.
+    const T wc =
+        std::max(remove_volatile_t{0},
+                 std::min(static_cast<remove_volatile_t>(c), kMaxWCharT));
+    SCOPED_TRACE(Esc(wc));
+    const FormatArgImpl wide_args[] = {FormatArgImpl(wc)};
+    UntypedFormatSpecImpl wide_format("%lc");
+    result = StrPrintChar(static_cast<wchar_t>(wc));
+    if (result.has_value()) {
+      EXPECT_EQ(result.value(),
+                FormatPack(wide_format, absl::MakeSpan(wide_args)));
+    }
   }
 }
 
-REGISTER_TYPED_TEST_CASE_P(TypedFormatConvertTest, AllIntsWithFlags, Char);
+REGISTER_TYPED_TEST_SUITE_P(TypedFormatConvertTest, AllIntsWithFlags, Char);
 
-typedef ::testing::Types<
-    int, unsigned, volatile int,
-    short, unsigned short,
-    long, unsigned long,
-    long long, unsigned long long,
-    signed char, unsigned char, char>
+typedef ::testing::Types<int, unsigned, volatile int, short,   // NOLINT
+                         unsigned short, long, unsigned long,  // NOLINT
+                         long long, unsigned long long,        // NOLINT
+                         signed char, unsigned char, char, wchar_t>
     AllIntTypes;
-INSTANTIATE_TYPED_TEST_CASE_P(TypedFormatConvertTestWithAllIntTypes,
-                              TypedFormatConvertTest, AllIntTypes);
-
+INSTANTIATE_TYPED_TEST_SUITE_P(TypedFormatConvertTestWithAllIntTypes,
+                               TypedFormatConvertTest, AllIntTypes);
 TEST_F(FormatConvertTest, VectorBool) {
   // Make sure vector<bool>'s values behave as bools.
   std::vector<bool> v = {true, false};
@@ -378,6 +766,60 @@ TEST_F(FormatConvertTest, VectorBool) {
                             FormatArgImpl(cv[0]), FormatArgImpl(cv[1])})));
 }
 
+#if GTEST_HAS_STD_WSTRING
+TEST_F(FormatConvertTest, UnicodeWideString) {
+  // StrFormat() should be able to convert wide strings containing Unicode
+  // characters (to UTF-8).
+  const FormatArgImpl args[] = {FormatArgImpl(L"\u47e3 \U00011112")};
+  // `u8""` forces UTF-8 encoding; MSVC will default to e.g. CP1252 (and warn)
+  // without it. However, the resulting character type differs between pre-C++20
+  // (`char`) and C++20 (`char8_t`). So deduce the right character type for all
+  // C++ versions, init it with UTF-8, then `memcpy()` to get the result as a
+  // `char*`.
+  using ConstChar8T = std::remove_reference_t<decltype(*u8"a")>;
+  ConstChar8T kOutputUtf8[] = u8"\u47e3 \U00011112";
+  char output[sizeof kOutputUtf8];
+  std::memcpy(output, kOutputUtf8, sizeof kOutputUtf8);
+  EXPECT_EQ(output,
+            FormatPack(UntypedFormatSpecImpl("%ls"), absl::MakeSpan(args)));
+}
+#endif  // GTEST_HAS_STD_WSTRING
+
+TEST_F(FormatConvertTest, Int128) {
+  absl::int128 positive = static_cast<absl::int128>(0x1234567890abcdef) * 1979;
+  absl::int128 negative = -positive;
+  absl::int128 max = absl::Int128Max(), min = absl::Int128Min();
+  const FormatArgImpl args[] = {FormatArgImpl(positive),
+                                FormatArgImpl(negative), FormatArgImpl(max),
+                                FormatArgImpl(min)};
+
+  struct Case {
+    const char* format;
+    const char* expected;
+  } cases[] = {
+      {"%1$d", "2595989796776606496405"},
+      {"%1$30d", "        2595989796776606496405"},
+      {"%1$-30d", "2595989796776606496405        "},
+      {"%1$u", "2595989796776606496405"},
+      {"%1$x", "8cba9876066020f695"},
+      {"%2$d", "-2595989796776606496405"},
+      {"%2$30d", "       -2595989796776606496405"},
+      {"%2$-30d", "-2595989796776606496405       "},
+      {"%2$u", "340282366920938460867384810655161715051"},
+      {"%2$x", "ffffffffffffff73456789f99fdf096b"},
+      {"%3$d", "170141183460469231731687303715884105727"},
+      {"%3$u", "170141183460469231731687303715884105727"},
+      {"%3$x", "7fffffffffffffffffffffffffffffff"},
+      {"%4$d", "-170141183460469231731687303715884105728"},
+      {"%4$x", "80000000000000000000000000000000"},
+  };
+
+  for (auto c : cases) {
+    UntypedFormatSpecImpl format(c.format);
+    EXPECT_EQ(c.expected, FormatPack(format, absl::MakeSpan(args)));
+  }
+}
+
 TEST_F(FormatConvertTest, Uint128) {
   absl::uint128 v = static_cast<absl::uint128>(0x1234567890abcdef) * 1979;
   absl::uint128 max = absl::Uint128Max();
@@ -403,17 +845,124 @@ TEST_F(FormatConvertTest, Uint128) {
   }
 }
 
-TEST_F(FormatConvertTest, Float) {
-#ifdef _MSC_VER
-  // MSVC has a different rounding policy than us so we can't test our
-  // implementation against the native one there.
-  return;
-#endif  // _MSC_VER
+template <typename Floating>
+void TestWithMultipleFormatsHelper(Floating tested_float) {
+  const NativePrintfTraits &native_traits = VerifyNativeImplementation();
+  // Reserve the space to ensure we don't allocate memory in the output itself.
+  std::string str_format_result;
+  str_format_result.reserve(1 << 20);
+  std::string string_printf_result;
+  string_printf_result.reserve(1 << 20);
 
   const char *const kFormats[] = {
-      "%",  "%.3",  "%8.5",   "%9",   "%.60", "%.30",   "%03",    "%+",
-      "% ", "%-10", "%#15.3", "%#.0", "%.0",  "%1$*2$", "%1$.*2$"};
+      "%",  "%.3", "%8.5", "%500",   "%.5000", "%.60", "%.30",   "%03",
+      "%+", "% ",  "%-10", "%#15.3", "%#.0",   "%.0",  "%1$*2$", "%1$.*2$"};
+
+  for (const char *fmt : kFormats) {
+    for (char f : {'f', 'F',  //
+                   'g', 'G',  //
+                   'a', 'A',  //
+                   'e', 'E'}) {
+      std::string fmt_str = std::string(fmt) + f;
+
+      if (IsNativeHexFloatConversion(f) &&
+          !native_traits.hex_float_has_glibc_rounding) {
+        continue;
+      }
+
+      if (!native_traits.hex_float_prefers_denormal_repr &&
+          IsNativeHexFloatConversion(f) &&
+          std::fpclassify(tested_float) == FP_SUBNORMAL) {
+        continue;
+      }
+        int i = -10;
+        FormatArgImpl args[2] = {FormatArgImpl(tested_float), FormatArgImpl(i)};
+        UntypedFormatSpecImpl format(fmt_str);
+
+        string_printf_result.clear();
+        StrAppend(&string_printf_result, fmt_str.c_str(), tested_float, i);
+        str_format_result.clear();
+
+        {
+          AppendPack(&str_format_result, format, absl::MakeSpan(args));
+        }
+
+        // For values that we know won't match the standard library
+        // implementation we skip verification, but still run the algorithm to
+        // catch asserts/sanitizer bugs.
+#ifdef _MSC_VER
+        // MSVC has a different rounding policy than us so we can't test our
+        // implementation against the native one there.
+        continue;
+#elif defined(__APPLE__)
+        // Apple formats NaN differently (+nan) vs. (nan)
+        if (std::isnan(tested_float)) continue;
+#endif
+        // We use ASSERT_EQ here because failures are usually correlated and a
+        // bug would print way too many failed expectations causing the test
+        // to time out.
+        ASSERT_EQ(string_printf_result, str_format_result)
+            << fmt_str << " " << StrPrint("%.18g", tested_float) << " "
+            << StrPrint("%a", tested_float) << " "
+            << StrPrint("%.50f", tested_float);
+    }
+  }
+}
+
+TEST_F(FormatConvertTest, Float) {
+  std::vector<float> floats = {0.0f,
+                               -0.0f,
+                               .9999999f,
+                               9999999.f,
+                               std::numeric_limits<float>::max(),
+                               -std::numeric_limits<float>::max(),
+                               std::numeric_limits<float>::min(),
+                               -std::numeric_limits<float>::min(),
+                               std::numeric_limits<float>::lowest(),
+                               -std::numeric_limits<float>::lowest(),
+                               std::numeric_limits<float>::epsilon(),
+                               std::numeric_limits<float>::epsilon() + 1.0f,
+                               std::numeric_limits<float>::infinity(),
+                               -std::numeric_limits<float>::infinity(),
+                               std::nanf("")};
+
+  // Some regression tests.
+  floats.push_back(0.999999989f);
+
+  if (std::numeric_limits<float>::has_denorm != std::denorm_absent) {
+    floats.push_back(std::numeric_limits<float>::denorm_min());
+    floats.push_back(-std::numeric_limits<float>::denorm_min());
+  }
+
+  for (float base :
+       {1.f, 12.f, 123.f, 1234.f, 12345.f, 123456.f, 1234567.f, 12345678.f,
+        123456789.f, 1234567890.f, 12345678901.f, 12345678.f, 12345678.f}) {
+    for (int exp = -123; exp <= 123; ++exp) {
+      for (int sign : {1, -1}) {
+        floats.push_back(sign * std::ldexp(base, exp));
+      }
+    }
+  }
+
+  for (int exp = -300; exp <= 300; ++exp) {
+    const float all_ones_mantissa = 0xffffff;
+    floats.push_back(std::ldexp(all_ones_mantissa, exp));
+  }
+
+  // Remove duplicates to speed up the logic below.
+  std::sort(floats.begin(), floats.end(), [](const float a, const float b) {
+    if (std::isnan(a)) return false;
+    if (std::isnan(b)) return true;
+    return a < b;
+  });
+  floats.erase(std::unique(floats.begin(), floats.end()), floats.end());
 
+  for (float f : floats) {
+    TestWithMultipleFormatsHelper(f);
+  }
+}
+
+TEST_F(FormatConvertTest, Double) {
   std::vector<double> doubles = {0.0,
                                  -0.0,
                                  .99999999999999,
@@ -427,12 +976,8 @@ TEST_F(FormatConvertTest, Float) {
                                  std::numeric_limits<double>::epsilon(),
                                  std::numeric_limits<double>::epsilon() + 1,
                                  std::numeric_limits<double>::infinity(),
-                                 -std::numeric_limits<double>::infinity()};
-
-#ifndef __APPLE__
-  // Apple formats NaN differently (+nan) vs. (nan)
-  doubles.push_back(std::nan(""));
-#endif
+                                 -std::numeric_limits<double>::infinity(),
+                                 std::nan("")};
 
   // Some regression tests.
   doubles.push_back(0.99999999999999989);
@@ -452,43 +997,367 @@ TEST_F(FormatConvertTest, Float) {
     }
   }
 
-  for (const char *fmt : kFormats) {
-    for (char f : {'f', 'F',  //
-                   'g', 'G',  //
-                   'a', 'A',  //
-                   'e', 'E'}) {
-      std::string fmt_str = std::string(fmt) + f;
-      for (double d : doubles) {
-        int i = -10;
-        FormatArgImpl args[2] = {FormatArgImpl(d), FormatArgImpl(i)};
-        UntypedFormatSpecImpl format(fmt_str);
-        // We use ASSERT_EQ here because failures are usually correlated and a
-        // bug would print way too many failed expectations causing the test to
-        // time out.
-        ASSERT_EQ(StrPrint(fmt_str.c_str(), d, i),
-                  FormatPack(format, absl::MakeSpan(args)))
-            << fmt_str << " " << StrPrint("%.18g", d) << " "
-            << StrPrint("%.999f", d);
-      }
+  for (int exp = -300; exp <= 300; ++exp) {
+    const double all_ones_mantissa = 0x1fffffffffffff;
+    doubles.push_back(std::ldexp(all_ones_mantissa, exp));
+  }
+
+  // Remove duplicates to speed up the logic below.
+  std::sort(doubles.begin(), doubles.end(), [](const double a, const double b) {
+    if (std::isnan(a)) return false;
+    if (std::isnan(b)) return true;
+    return a < b;
+  });
+  doubles.erase(std::unique(doubles.begin(), doubles.end()), doubles.end());
+
+  for (double d : doubles) {
+    TestWithMultipleFormatsHelper(d);
+  }
+}
+
+TEST_F(FormatConvertTest, DoubleRound) {
+  std::string s;
+  const auto format = [&](const char *fmt, double d) -> std::string & {
+    s.clear();
+    FormatArgImpl args[1] = {FormatArgImpl(d)};
+    AppendPack(&s, UntypedFormatSpecImpl(fmt), absl::MakeSpan(args));
+#if !defined(_MSC_VER)
+    // MSVC has a different rounding policy than us so we can't test our
+    // implementation against the native one there.
+    EXPECT_EQ(StrPrint(fmt, d), s);
+#endif  // _MSC_VER
+
+    return s;
+  };
+  // All of these values have to be exactly represented.
+  // Otherwise we might not be testing what we think we are testing.
+
+  // These values can fit in a 64bit "fast" representation.
+  const double exact_value = 0.00000000000005684341886080801486968994140625;
+  assert(exact_value == std::pow(2, -44));
+  // Round up at a 5xx.
+  EXPECT_EQ(format("%.13f", exact_value), "0.0000000000001");
+  // Round up at a >5
+  EXPECT_EQ(format("%.14f", exact_value), "0.00000000000006");
+  // Round down at a <5
+  EXPECT_EQ(format("%.16f", exact_value), "0.0000000000000568");
+  // Nine handling
+  EXPECT_EQ(format("%.35f", exact_value),
+            "0.00000000000005684341886080801486969");
+  EXPECT_EQ(format("%.36f", exact_value),
+            "0.000000000000056843418860808014869690");
+  // Round down the last nine.
+  EXPECT_EQ(format("%.37f", exact_value),
+            "0.0000000000000568434188608080148696899");
+  EXPECT_EQ(format("%.10f", 0.000003814697265625), "0.0000038147");
+  // Round up the last nine
+  EXPECT_EQ(format("%.11f", 0.000003814697265625), "0.00000381470");
+  EXPECT_EQ(format("%.12f", 0.000003814697265625), "0.000003814697");
+
+  // Round to even (down)
+  EXPECT_EQ(format("%.43f", exact_value),
+            "0.0000000000000568434188608080148696899414062");
+  // Exact
+  EXPECT_EQ(format("%.44f", exact_value),
+            "0.00000000000005684341886080801486968994140625");
+  // Round to even (up), let make the last digits 75 instead of 25
+  EXPECT_EQ(format("%.43f", exact_value + std::pow(2, -43)),
+            "0.0000000000001705302565824240446090698242188");
+  // Exact, just to check.
+  EXPECT_EQ(format("%.44f", exact_value + std::pow(2, -43)),
+            "0.00000000000017053025658242404460906982421875");
+
+  // This value has to be small enough that it won't fit in the uint128
+  // representation for printing.
+  const double small_exact_value =
+      0.000000000000000000000000000000000000752316384526264005099991383822237233803945956334136013765601092018187046051025390625;  // NOLINT
+  assert(small_exact_value == std::pow(2, -120));
+  // Round up at a 5xx.
+  EXPECT_EQ(format("%.37f", small_exact_value),
+            "0.0000000000000000000000000000000000008");
+  // Round down at a <5
+  EXPECT_EQ(format("%.38f", small_exact_value),
+            "0.00000000000000000000000000000000000075");
+  // Round up at a >5
+  EXPECT_EQ(format("%.41f", small_exact_value),
+            "0.00000000000000000000000000000000000075232");
+  // Nine handling
+  EXPECT_EQ(format("%.55f", small_exact_value),
+            "0.0000000000000000000000000000000000007523163845262640051");
+  EXPECT_EQ(format("%.56f", small_exact_value),
+            "0.00000000000000000000000000000000000075231638452626400510");
+  EXPECT_EQ(format("%.57f", small_exact_value),
+            "0.000000000000000000000000000000000000752316384526264005100");
+  EXPECT_EQ(format("%.58f", small_exact_value),
+            "0.0000000000000000000000000000000000007523163845262640051000");
+  // Round down the last nine
+  EXPECT_EQ(format("%.59f", small_exact_value),
+            "0.00000000000000000000000000000000000075231638452626400509999");
+  // Round up the last nine
+  EXPECT_EQ(format("%.79f", small_exact_value),
+            "0.000000000000000000000000000000000000"
+            "7523163845262640050999913838222372338039460");
+
+  // Round to even (down)
+  EXPECT_EQ(format("%.119f", small_exact_value),
+            "0.000000000000000000000000000000000000"
+            "75231638452626400509999138382223723380"
+            "394595633413601376560109201818704605102539062");
+  // Exact
+  EXPECT_EQ(format("%.120f", small_exact_value),
+            "0.000000000000000000000000000000000000"
+            "75231638452626400509999138382223723380"
+            "3945956334136013765601092018187046051025390625");
+  // Round to even (up), let make the last digits 75 instead of 25
+  EXPECT_EQ(format("%.119f", small_exact_value + std::pow(2, -119)),
+            "0.000000000000000000000000000000000002"
+            "25694915357879201529997415146671170141"
+            "183786900240804129680327605456113815307617188");
+  // Exact, just to check.
+  EXPECT_EQ(format("%.120f", small_exact_value + std::pow(2, -119)),
+            "0.000000000000000000000000000000000002"
+            "25694915357879201529997415146671170141"
+            "1837869002408041296803276054561138153076171875");
+
+  // Scientific exponent cases
+  // Round to even with all zeros after round digit
+  EXPECT_EQ(format("%0.13e", 1671075773261250), "1.6710757732612e+15");
+
+  // Rounding where precision is in first digit run
+  EXPECT_EQ(format("%0.1e", -1.93437090148818698e-297), "-1.9e-297");
+  EXPECT_EQ(format("%0.1e", -9.92255780642280927e-298), "-9.9e-298");
+
+  // Rounding large negative exponent first digit
+  EXPECT_EQ(format("%0.1e", -8.956e-294), "-9.0e-294");
+
+  // FormatGNegativeExpSlow: 2^(-77) has decomposed.exponent < -128.
+  EXPECT_EQ(format("%.10g", std::ldexp(1.0, -77)), "6.6174449e-24");
+
+  // FormatGPositiveExpSlow: 2^130 has total_bits > 128.
+  EXPECT_EQ(format("%.10g", std::ldexp(1.0, 130)),
+            "1.361129468e+39");
+}
+
+TEST_F(FormatConvertTest, DoubleRoundA) {
+  const NativePrintfTraits &native_traits = VerifyNativeImplementation();
+  std::string s;
+  const auto format = [&](const char *fmt, double d) -> std::string & {
+    s.clear();
+    FormatArgImpl args[1] = {FormatArgImpl(d)};
+    AppendPack(&s, UntypedFormatSpecImpl(fmt), absl::MakeSpan(args));
+    if (native_traits.hex_float_has_glibc_rounding) {
+      EXPECT_EQ(StrPrint(fmt, d), s);
+    }
+    return s;
+  };
+
+  // 0x1.00018000p+100
+  const double on_boundary_odd = 1267679614447900152596896153600.0;
+  EXPECT_EQ(format("%.0a", on_boundary_odd), "0x1p+100");
+  EXPECT_EQ(format("%.1a", on_boundary_odd), "0x1.0p+100");
+  EXPECT_EQ(format("%.2a", on_boundary_odd), "0x1.00p+100");
+  EXPECT_EQ(format("%.3a", on_boundary_odd), "0x1.000p+100");
+  EXPECT_EQ(format("%.4a", on_boundary_odd), "0x1.0002p+100");  // round
+  EXPECT_EQ(format("%.5a", on_boundary_odd), "0x1.00018p+100");
+  EXPECT_EQ(format("%.6a", on_boundary_odd), "0x1.000180p+100");
+
+  // 0x1.00028000p-2
+  const double on_boundary_even = 0.250009536743164062500;
+  EXPECT_EQ(format("%.0a", on_boundary_even), "0x1p-2");
+  EXPECT_EQ(format("%.1a", on_boundary_even), "0x1.0p-2");
+  EXPECT_EQ(format("%.2a", on_boundary_even), "0x1.00p-2");
+  EXPECT_EQ(format("%.3a", on_boundary_even), "0x1.000p-2");
+  EXPECT_EQ(format("%.4a", on_boundary_even), "0x1.0002p-2");  // no round
+  EXPECT_EQ(format("%.5a", on_boundary_even), "0x1.00028p-2");
+  EXPECT_EQ(format("%.6a", on_boundary_even), "0x1.000280p-2");
+
+  // 0x1.00018001p+1
+  const double slightly_over = 2.00004577683284878730773925781250;
+  EXPECT_EQ(format("%.0a", slightly_over), "0x1p+1");
+  EXPECT_EQ(format("%.1a", slightly_over), "0x1.0p+1");
+  EXPECT_EQ(format("%.2a", slightly_over), "0x1.00p+1");
+  EXPECT_EQ(format("%.3a", slightly_over), "0x1.000p+1");
+  EXPECT_EQ(format("%.4a", slightly_over), "0x1.0002p+1");
+  EXPECT_EQ(format("%.5a", slightly_over), "0x1.00018p+1");
+  EXPECT_EQ(format("%.6a", slightly_over), "0x1.000180p+1");
+
+  // 0x1.00017fffp+0
+  const double slightly_under = 1.000022887950763106346130371093750;
+  EXPECT_EQ(format("%.0a", slightly_under), "0x1p+0");
+  EXPECT_EQ(format("%.1a", slightly_under), "0x1.0p+0");
+  EXPECT_EQ(format("%.2a", slightly_under), "0x1.00p+0");
+  EXPECT_EQ(format("%.3a", slightly_under), "0x1.000p+0");
+  EXPECT_EQ(format("%.4a", slightly_under), "0x1.0001p+0");
+  EXPECT_EQ(format("%.5a", slightly_under), "0x1.00018p+0");
+  EXPECT_EQ(format("%.6a", slightly_under), "0x1.000180p+0");
+  EXPECT_EQ(format("%.7a", slightly_under), "0x1.0001800p+0");
+
+  // 0x1.1b3829ac28058p+3
+  const double hex_value = 8.85060580848964661981881363317370414733886718750;
+  EXPECT_EQ(format("%.0a", hex_value), "0x1p+3");
+  EXPECT_EQ(format("%.1a", hex_value), "0x1.2p+3");
+  EXPECT_EQ(format("%.2a", hex_value), "0x1.1bp+3");
+  EXPECT_EQ(format("%.3a", hex_value), "0x1.1b4p+3");
+  EXPECT_EQ(format("%.4a", hex_value), "0x1.1b38p+3");
+  EXPECT_EQ(format("%.5a", hex_value), "0x1.1b383p+3");
+  EXPECT_EQ(format("%.6a", hex_value), "0x1.1b382ap+3");
+  EXPECT_EQ(format("%.7a", hex_value), "0x1.1b3829bp+3");
+  EXPECT_EQ(format("%.8a", hex_value), "0x1.1b3829acp+3");
+  EXPECT_EQ(format("%.9a", hex_value), "0x1.1b3829ac3p+3");
+  EXPECT_EQ(format("%.10a", hex_value), "0x1.1b3829ac28p+3");
+  EXPECT_EQ(format("%.11a", hex_value), "0x1.1b3829ac280p+3");
+  EXPECT_EQ(format("%.12a", hex_value), "0x1.1b3829ac2806p+3");
+  EXPECT_EQ(format("%.13a", hex_value), "0x1.1b3829ac28058p+3");
+  EXPECT_EQ(format("%.14a", hex_value), "0x1.1b3829ac280580p+3");
+  EXPECT_EQ(format("%.15a", hex_value), "0x1.1b3829ac2805800p+3");
+  EXPECT_EQ(format("%.16a", hex_value), "0x1.1b3829ac28058000p+3");
+  EXPECT_EQ(format("%.17a", hex_value), "0x1.1b3829ac280580000p+3");
+  EXPECT_EQ(format("%.18a", hex_value), "0x1.1b3829ac2805800000p+3");
+  EXPECT_EQ(format("%.19a", hex_value), "0x1.1b3829ac28058000000p+3");
+  EXPECT_EQ(format("%.20a", hex_value), "0x1.1b3829ac280580000000p+3");
+  EXPECT_EQ(format("%.21a", hex_value), "0x1.1b3829ac2805800000000p+3");
+
+  // 0x1.0818283848586p+3
+  const double hex_value2 = 8.2529488658208371987257123691961169242858886718750;
+  EXPECT_EQ(format("%.0a", hex_value2), "0x1p+3");
+  EXPECT_EQ(format("%.1a", hex_value2), "0x1.1p+3");
+  EXPECT_EQ(format("%.2a", hex_value2), "0x1.08p+3");
+  EXPECT_EQ(format("%.3a", hex_value2), "0x1.082p+3");
+  EXPECT_EQ(format("%.4a", hex_value2), "0x1.0818p+3");
+  EXPECT_EQ(format("%.5a", hex_value2), "0x1.08183p+3");
+  EXPECT_EQ(format("%.6a", hex_value2), "0x1.081828p+3");
+  EXPECT_EQ(format("%.7a", hex_value2), "0x1.0818284p+3");
+  EXPECT_EQ(format("%.8a", hex_value2), "0x1.08182838p+3");
+  EXPECT_EQ(format("%.9a", hex_value2), "0x1.081828385p+3");
+  EXPECT_EQ(format("%.10a", hex_value2), "0x1.0818283848p+3");
+  EXPECT_EQ(format("%.11a", hex_value2), "0x1.08182838486p+3");
+  EXPECT_EQ(format("%.12a", hex_value2), "0x1.081828384858p+3");
+  EXPECT_EQ(format("%.13a", hex_value2), "0x1.0818283848586p+3");
+  EXPECT_EQ(format("%.14a", hex_value2), "0x1.08182838485860p+3");
+  EXPECT_EQ(format("%.15a", hex_value2), "0x1.081828384858600p+3");
+  EXPECT_EQ(format("%.16a", hex_value2), "0x1.0818283848586000p+3");
+  EXPECT_EQ(format("%.17a", hex_value2), "0x1.08182838485860000p+3");
+  EXPECT_EQ(format("%.18a", hex_value2), "0x1.081828384858600000p+3");
+  EXPECT_EQ(format("%.19a", hex_value2), "0x1.0818283848586000000p+3");
+  EXPECT_EQ(format("%.20a", hex_value2), "0x1.08182838485860000000p+3");
+  EXPECT_EQ(format("%.21a", hex_value2), "0x1.081828384858600000000p+3");
+}
+
+TEST_F(FormatConvertTest, LongDoubleRoundA) {
+  if (std::numeric_limits<long double>::digits % 4 != 0) {
+    // This test doesn't really make sense to run on platforms where a long
+    // double has a different mantissa size (mod 4) than Prod, since then the
+    // leading digit will be formatted differently.
+    return;
+  }
+  const NativePrintfTraits &native_traits = VerifyNativeImplementation();
+  std::string s;
+  const auto format = [&](const char *fmt, long double d) -> std::string & {
+    s.clear();
+    FormatArgImpl args[1] = {FormatArgImpl(d)};
+    AppendPack(&s, UntypedFormatSpecImpl(fmt), absl::MakeSpan(args));
+    if (native_traits.hex_float_has_glibc_rounding &&
+        native_traits.hex_float_optimizes_leading_digit_bit_count) {
+      EXPECT_EQ(StrPrint(fmt, d), s);
+    }
+    return s;
+  };
+
+  // 0x8.8p+4
+  const long double on_boundary_even = 136.0;
+  EXPECT_EQ(format("%.0La", on_boundary_even), "0x8p+4");
+  EXPECT_EQ(format("%.1La", on_boundary_even), "0x8.8p+4");
+  EXPECT_EQ(format("%.2La", on_boundary_even), "0x8.80p+4");
+  EXPECT_EQ(format("%.3La", on_boundary_even), "0x8.800p+4");
+  EXPECT_EQ(format("%.4La", on_boundary_even), "0x8.8000p+4");
+  EXPECT_EQ(format("%.5La", on_boundary_even), "0x8.80000p+4");
+  EXPECT_EQ(format("%.6La", on_boundary_even), "0x8.800000p+4");
+
+  // 0x9.8p+4
+  const long double on_boundary_odd = 152.0;
+  EXPECT_EQ(format("%.0La", on_boundary_odd), "0xap+4");
+  EXPECT_EQ(format("%.1La", on_boundary_odd), "0x9.8p+4");
+  EXPECT_EQ(format("%.2La", on_boundary_odd), "0x9.80p+4");
+  EXPECT_EQ(format("%.3La", on_boundary_odd), "0x9.800p+4");
+  EXPECT_EQ(format("%.4La", on_boundary_odd), "0x9.8000p+4");
+  EXPECT_EQ(format("%.5La", on_boundary_odd), "0x9.80000p+4");
+  EXPECT_EQ(format("%.6La", on_boundary_odd), "0x9.800000p+4");
+
+  // 0x8.80001p+24
+  const long double slightly_over = 142606352.0;
+  EXPECT_EQ(format("%.0La", slightly_over), "0x9p+24");
+  EXPECT_EQ(format("%.1La", slightly_over), "0x8.8p+24");
+  EXPECT_EQ(format("%.2La", slightly_over), "0x8.80p+24");
+  EXPECT_EQ(format("%.3La", slightly_over), "0x8.800p+24");
+  EXPECT_EQ(format("%.4La", slightly_over), "0x8.8000p+24");
+  EXPECT_EQ(format("%.5La", slightly_over), "0x8.80001p+24");
+  EXPECT_EQ(format("%.6La", slightly_over), "0x8.800010p+24");
+
+  // 0x8.7ffffp+24
+  const long double slightly_under = 142606320.0;
+  EXPECT_EQ(format("%.0La", slightly_under), "0x8p+24");
+  EXPECT_EQ(format("%.1La", slightly_under), "0x8.8p+24");
+  EXPECT_EQ(format("%.2La", slightly_under), "0x8.80p+24");
+  EXPECT_EQ(format("%.3La", slightly_under), "0x8.800p+24");
+  EXPECT_EQ(format("%.4La", slightly_under), "0x8.8000p+24");
+  EXPECT_EQ(format("%.5La", slightly_under), "0x8.7ffffp+24");
+  EXPECT_EQ(format("%.6La", slightly_under), "0x8.7ffff0p+24");
+  EXPECT_EQ(format("%.7La", slightly_under), "0x8.7ffff00p+24");
+
+  // 0xc.0828384858688000p+128
+  const long double eights = 4094231060438608800781871108094404067328.0;
+  EXPECT_EQ(format("%.0La", eights), "0xcp+128");
+  EXPECT_EQ(format("%.1La", eights), "0xc.1p+128");
+  EXPECT_EQ(format("%.2La", eights), "0xc.08p+128");
+  EXPECT_EQ(format("%.3La", eights), "0xc.083p+128");
+  EXPECT_EQ(format("%.4La", eights), "0xc.0828p+128");
+  EXPECT_EQ(format("%.5La", eights), "0xc.08284p+128");
+  EXPECT_EQ(format("%.6La", eights), "0xc.082838p+128");
+  EXPECT_EQ(format("%.7La", eights), "0xc.0828385p+128");
+  EXPECT_EQ(format("%.8La", eights), "0xc.08283848p+128");
+  EXPECT_EQ(format("%.9La", eights), "0xc.082838486p+128");
+  EXPECT_EQ(format("%.10La", eights), "0xc.0828384858p+128");
+  EXPECT_EQ(format("%.11La", eights), "0xc.08283848587p+128");
+  EXPECT_EQ(format("%.12La", eights), "0xc.082838485868p+128");
+  EXPECT_EQ(format("%.13La", eights), "0xc.0828384858688p+128");
+  EXPECT_EQ(format("%.14La", eights), "0xc.08283848586880p+128");
+  EXPECT_EQ(format("%.15La", eights), "0xc.082838485868800p+128");
+  EXPECT_EQ(format("%.16La", eights), "0xc.0828384858688000p+128");
+}
+
+// We don't actually store the results. This is just to exercise the rest of the
+// machinery.
+struct NullSink {
+  friend void AbslFormatFlush(NullSink *, string_view) {}
+};
+
+template <typename... T>
+bool FormatWithNullSink(absl::string_view fmt, const T &... a) {
+  NullSink sink;
+  FormatArgImpl args[] = {FormatArgImpl(a)...};
+  return FormatUntyped(&sink, UntypedFormatSpecImpl(fmt), absl::MakeSpan(args));
+}
+
+TEST_F(FormatConvertTest, ExtremeWidthPrecision) {
+  for (const char *fmt : {"f"}) {
+    for (double d : {1e-100, 1.0, 1e100}) {
+      constexpr int max = std::numeric_limits<int>::max();
+      EXPECT_TRUE(FormatWithNullSink(std::string("%.*") + fmt, max, d));
+      EXPECT_TRUE(FormatWithNullSink(std::string("%1.*") + fmt, max, d));
+      EXPECT_TRUE(FormatWithNullSink(std::string("%*") + fmt, max, d));
+      EXPECT_TRUE(FormatWithNullSink(std::string("%*.*") + fmt, max, max, d));
     }
   }
 }
 
 TEST_F(FormatConvertTest, LongDouble) {
-  const char *const kFormats[] = {"%",    "%.3", "%8.5", "%9",
+  const NativePrintfTraits &native_traits = VerifyNativeImplementation();
+  const char *const kFormats[] = {"%",    "%.3", "%8.5", "%9",  "%.5000",
                                   "%.60", "%+",  "% ",   "%-10"};
 
-  // This value is not representable in double, but it is in long double that
-  // uses the extended format.
-  // This is to verify that we are not truncating the value mistakenly through a
-  // double.
-  long double very_precise = 10000000000000000.25L;
-
   std::vector<long double> doubles = {
       0.0,
       -0.0,
-      very_precise,
-      1 / very_precise,
       std::numeric_limits<long double>::max(),
       -std::numeric_limits<long double>::max(),
       std::numeric_limits<long double>::min(),
@@ -496,28 +1365,66 @@ TEST_F(FormatConvertTest, LongDouble) {
       std::numeric_limits<long double>::infinity(),
       -std::numeric_limits<long double>::infinity()};
 
+  for (long double base : {1.L, 12.L, 123.L, 1234.L, 12345.L, 123456.L,
+                           1234567.L, 12345678.L, 123456789.L, 1234567890.L,
+                           12345678901.L, 123456789012.L, 1234567890123.L,
+                           // This value is not representable in double, but it
+                           // is in long double that uses the extended format.
+                           // This is to verify that we are not truncating the
+                           // value mistakenly through a double.
+                           10000000000000000.25L}) {
+    for (int exp : {-1000, -500, 0, 500, 1000}) {
+      for (int sign : {1, -1}) {
+        doubles.push_back(sign * std::ldexp(base, exp));
+        doubles.push_back(sign / std::ldexp(base, exp));
+      }
+    }
+  }
+
+  // Regression tests
+  //
+  // Using a string literal because not all platforms support hex literals or it
+  // might be out of range.
+  doubles.push_back(std::strtold("-0xf.ffffffb5feafffbp-16324L", nullptr));
+
   for (const char *fmt : kFormats) {
     for (char f : {'f', 'F',  //
                    'g', 'G',  //
                    'a', 'A',  //
                    'e', 'E'}) {
       std::string fmt_str = std::string(fmt) + 'L' + f;
+
+      if (IsNativeHexFloatConversion(f)) {
+        if (!native_traits.hex_float_has_glibc_rounding ||
+            !native_traits.hex_float_optimizes_leading_digit_bit_count) {
+          continue;
+        }
+      }
+
       for (auto d : doubles) {
         FormatArgImpl arg(d);
         UntypedFormatSpecImpl format(fmt_str);
+        std::string result = FormatPack(format, {&arg, 1});
+
+#ifdef _MSC_VER
+        // MSVC has a different rounding policy than us so we can't test our
+        // implementation against the native one there.
+        continue;
+#endif  // _MSC_VER
+
         // We use ASSERT_EQ here because failures are usually correlated and a
         // bug would print way too many failed expectations causing the test to
         // time out.
-        ASSERT_EQ(StrPrint(fmt_str.c_str(), d),
-                  FormatPack(format, {&arg, 1}))
+        ASSERT_EQ(StrPrint(fmt_str.c_str(), d), result)
             << fmt_str << " " << StrPrint("%.18Lg", d) << " "
-            << StrPrint("%.999Lf", d);
+            << StrPrint("%La", d) << " " << StrPrint("%.1080Lf", d);
       }
     }
   }
 }
 
-TEST_F(FormatConvertTest, IntAsFloat) {
+TEST_F(FormatConvertTest, IntAsDouble) {
+  const NativePrintfTraits &native_traits = VerifyNativeImplementation();
   const int kMin = std::numeric_limits<int>::min();
   const int kMax = std::numeric_limits<int>::max();
   const int ia[] = {
@@ -533,14 +1440,17 @@ TEST_F(FormatConvertTest, IntAsFloat) {
       const char *fmt;
     };
     const double dx = static_cast<double>(fx);
-    const Expectation kExpect[] = {
-      { __LINE__, StrPrint("%f", dx), "%f" },
-      { __LINE__, StrPrint("%12f", dx), "%12f" },
-      { __LINE__, StrPrint("%.12f", dx), "%.12f" },
-      { __LINE__, StrPrint("%12a", dx), "%12a" },
-      { __LINE__, StrPrint("%.12a", dx), "%.12a" },
+    std::vector<Expectation> expect = {
+        {__LINE__, StrPrint("%f", dx), "%f"},
+        {__LINE__, StrPrint("%12f", dx), "%12f"},
+        {__LINE__, StrPrint("%.12f", dx), "%.12f"},
+        {__LINE__, StrPrint("%.12a", dx), "%.12a"},
     };
-    for (const Expectation &e : kExpect) {
+    if (native_traits.hex_float_uses_minimal_precision_when_not_specified) {
+      Expectation ex = {__LINE__, StrPrint("%12a", dx), "%12a"};
+      expect.push_back(ex);
+    }
+    for (const Expectation &e : expect) {
       SCOPED_TRACE(e.line);
       SCOPED_TRACE(e.fmt);
       UntypedFormatSpecImpl format(e.fmt);
@@ -585,6 +1495,33 @@ TEST_F(FormatConvertTest, ExpectedFailures) {
   EXPECT_TRUE(FormatFails("%*d", ""));
 }
 
+// Sanity check to make sure that we are testing what we think we're testing on
+// e.g. the x86_64+glibc platform.
+TEST_F(FormatConvertTest, GlibcHasCorrectTraits) {
+#if defined(__GLIBC__) && defined(__x86_64__)
+  constexpr bool kIsSupportedGlibc = true;
+#else
+  constexpr bool kIsSupportedGlibc = false;
+#endif
+
+  if (!kIsSupportedGlibc) {
+    GTEST_SKIP() << "Test does not support this platform";
+  }
+
+  const NativePrintfTraits &native_traits = VerifyNativeImplementation();
+  // If one of the following tests break then it is either because the above PP
+  // macro guards failed to exclude a new platform (likely) or because something
+  // has changed in the implementation of glibc sprintf float formatting
+  // behavior.  If the latter, then the code that computes these flags needs to
+  // be revisited and/or possibly the StrFormat implementation.
+  EXPECT_TRUE(native_traits.hex_float_has_glibc_rounding);
+  EXPECT_TRUE(native_traits.hex_float_prefers_denormal_repr);
+  EXPECT_TRUE(
+      native_traits.hex_float_uses_minimal_precision_when_not_specified);
+  EXPECT_TRUE(native_traits.hex_float_optimizes_leading_digit_bit_count);
+}
+
 }  // namespace
 }  // namespace str_format_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/internal/str_format/extension.cc b/absl/strings/internal/str_format/extension.cc
index d7f58159d5f..5b16a862e97 100644
--- a/absl/strings/internal/str_format/extension.cc
+++ b/absl/strings/internal/str_format/extension.cc
@@ -16,69 +16,42 @@
 #include "absl/strings/internal/str_format/extension.h"
 
 #include <errno.h>
+
 #include <algorithm>
+#include <cstddef>
 #include <string>
 
+#include "absl/strings/string_view.h"
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace str_format_internal {
-namespace {
-// clang-format off
-#define ABSL_LENGTH_MODS_EXPAND_ \
-  X_VAL(h) X_SEP \
-  X_VAL(hh) X_SEP \
-  X_VAL(l) X_SEP \
-  X_VAL(ll) X_SEP \
-  X_VAL(L) X_SEP \
-  X_VAL(j) X_SEP \
-  X_VAL(z) X_SEP \
-  X_VAL(t) X_SEP \
-  X_VAL(q)
-// clang-format on
-}  // namespace
-
-const LengthMod::Spec LengthMod::kSpecs[] = {
-#define X_VAL(id) { LengthMod::id, #id, strlen(#id) }
-#define X_SEP ,
-    ABSL_LENGTH_MODS_EXPAND_, {LengthMod::none, "", 0}
-#undef X_VAL
-#undef X_SEP
-};
 
-const ConversionChar::Spec ConversionChar::kSpecs[] = {
-#define X_VAL(id) { ConversionChar::id, #id[0] }
-#define X_SEP ,
-    ABSL_CONVERSION_CHARS_EXPAND_(X_VAL, X_SEP),
-    {ConversionChar::none, '\0'},
-#undef X_VAL
-#undef X_SEP
-};
-
-std::string Flags::ToString() const {
+std::string FlagsToString(Flags v) {
   std::string s;
-  s.append(left     ? "-" : "");
-  s.append(show_pos ? "+" : "");
-  s.append(sign_col ? " " : "");
-  s.append(alt      ? "#" : "");
-  s.append(zero     ? "0" : "");
+  s.append(FlagsContains(v, Flags::kLeft) ? "-" : "");
+  s.append(FlagsContains(v, Flags::kShowPos) ? "+" : "");
+  s.append(FlagsContains(v, Flags::kSignCol) ? " " : "");
+  s.append(FlagsContains(v, Flags::kAlt) ? "#" : "");
+  s.append(FlagsContains(v, Flags::kZero) ? "0" : "");
   return s;
 }
 
-const size_t LengthMod::kNumValues;
-
-const size_t ConversionChar::kNumValues;
-
-bool FormatSinkImpl::PutPaddedString(string_view v, int w, int p, bool l) {
+bool FormatSinkImpl::PutPaddedString(string_view value, int width,
+                                     int precision, bool left) {
   size_t space_remaining = 0;
-  if (w >= 0) space_remaining = w;
-  size_t n = v.size();
-  if (p >= 0) n = std::min(n, static_cast<size_t>(p));
-  string_view shown(v.data(), n);
+  if (width >= 0)
+    space_remaining = static_cast<size_t>(width);
+  size_t n = value.size();
+  if (precision >= 0) n = std::min(n, static_cast<size_t>(precision));
+  string_view shown(value.data(), n);
   space_remaining = Excess(shown.size(), space_remaining);
-  if (!l) Append(space_remaining, ' ');
+  if (!left) Append(space_remaining, ' ');
   Append(shown);
-  if (l) Append(space_remaining, ' ');
+  if (left) Append(space_remaining, ' ');
   return true;
 }
 
 }  // namespace str_format_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/internal/str_format/extension.h b/absl/strings/internal/str_format/extension.h
index eb81f8a17ca..0e0b262faa8 100644
--- a/absl/strings/internal/str_format/extension.h
+++ b/absl/strings/internal/str_format/extension.h
@@ -16,18 +16,23 @@
 #ifndef ABSL_STRINGS_INTERNAL_STR_FORMAT_EXTENSION_H_
 #define ABSL_STRINGS_INTERNAL_STR_FORMAT_EXTENSION_H_
 
-#include <limits.h>
+
 #include <cstddef>
+#include <cstdint>
 #include <cstring>
 #include <ostream>
+#include <string>
 
-#include "absl/base/port.h"
+#include "absl/base/config.h"
 #include "absl/strings/internal/str_format/output.h"
 #include "absl/strings/string_view.h"
 
-class Cord;
-
 namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+enum class FormatConversionChar : uint8_t;
+enum class FormatConversionCharSet : uint64_t;
+enum class LengthMod : std::uint8_t { h, hh, l, ll, L, j, z, t, q, none };
 
 namespace str_format_internal {
 
@@ -65,7 +70,7 @@ class FormatSinkImpl {
   ~FormatSinkImpl() { Flush(); }
 
   void Flush() {
-    raw_.Write(string_view(buf_, pos_ - buf_));
+    raw_.Write(string_view(buf_, static_cast<size_t>(pos_ - buf_)));
     pos_ = buf_;
   }
 
@@ -102,7 +107,7 @@ class FormatSinkImpl {
   size_t size() const { return size_; }
 
   // Put 'v' to 'sink' with specified width, precision, and left flag.
-  bool PutPaddedString(string_view v, int w, int p, bool l);
+  bool PutPaddedString(string_view v, int width, int precision, bool left);
 
   template <typename T>
   T Wrap() {
@@ -115,7 +120,9 @@ class FormatSinkImpl {
   }
 
  private:
-  size_t Avail() const { return buf_ + sizeof(buf_) - pos_; }
+  size_t Avail() const {
+    return static_cast<size_t>(buf_ + sizeof(buf_) - pos_);
+  }
 
   FormatRawSinkImpl raw_;
   size_t size_ = 0;
@@ -123,71 +130,38 @@ class FormatSinkImpl {
   char buf_[1024];
 };
 
-struct Flags {
-  bool basic : 1;     // fastest conversion: no flags, width, or precision
-  bool left : 1;      // "-"
-  bool show_pos : 1;  // "+"
-  bool sign_col : 1;  // " "
-  bool alt : 1;       // "#"
-  bool zero : 1;      // "0"
-  std::string ToString() const;
-  friend std::ostream& operator<<(std::ostream& os, const Flags& v) {
-    return os << v.ToString();
-  }
+enum class Flags : uint8_t {
+  kBasic = 0,
+  kLeft = 1 << 0,
+  kShowPos = 1 << 1,
+  kSignCol = 1 << 2,
+  kAlt = 1 << 3,
+  kZero = 1 << 4,
+  // This is not a real flag. It just exists to turn off kBasic when no other
+  // flags are set. This is for when width/precision are specified, or a length
+  // modifier affects the behavior ("%lc").
+  kNonBasic = 1 << 5,
 };
 
-struct LengthMod {
- public:
-  enum Id : uint8_t {
-    h, hh, l, ll, L, j, z, t, q, none
-  };
-  static const size_t kNumValues = none + 1;
-
-  LengthMod() : id_(none) {}
-
-  // Index into the opaque array of LengthMod enums.
-  // Requires: i < kNumValues
-  static LengthMod FromIndex(size_t i) {
-    return LengthMod(kSpecs[i].value);
-  }
-
-  static LengthMod FromId(Id id) { return LengthMod(id); }
-
-  // The length modifier std::string associated with a specified LengthMod.
-  string_view name() const {
-    const Spec& spec = kSpecs[id_];
-    return {spec.name, spec.name_length};
-  }
-
-  Id id() const { return id_; }
-
-  friend bool operator==(const LengthMod& a, const LengthMod& b) {
-    return a.id() == b.id();
-  }
-  friend bool operator!=(const LengthMod& a, const LengthMod& b) {
-    return !(a == b);
-  }
-  friend std::ostream& operator<<(std::ostream& os, const LengthMod& v) {
-    return os << v.name();
-  }
+constexpr Flags operator|(Flags a, Flags b) {
+  return static_cast<Flags>(static_cast<uint8_t>(a) | static_cast<uint8_t>(b));
+}
 
- private:
-  struct Spec {
-    Id value;
-    const char *name;
-    size_t name_length;
-  };
-  static const Spec kSpecs[];
+constexpr bool FlagsContains(Flags haystack, Flags needle) {
+  return (static_cast<uint8_t>(haystack) & static_cast<uint8_t>(needle)) ==
+         static_cast<uint8_t>(needle);
+}
 
-  explicit LengthMod(Id id) : id_(id) {}
+std::string FlagsToString(Flags v);
 
-  Id id_;
-};
+inline std::ostream& operator<<(std::ostream& os, Flags v) {
+  return os << FlagsToString(v);
+}
 
 // clang-format off
-#define ABSL_CONVERSION_CHARS_EXPAND_(X_VAL, X_SEP) \
+#define ABSL_INTERNAL_CONVERSION_CHARS_EXPAND_(X_VAL, X_SEP) \
   /* text */ \
-  X_VAL(c) X_SEP X_VAL(C) X_SEP X_VAL(s) X_SEP X_VAL(S) X_SEP \
+  X_VAL(c) X_SEP X_VAL(s) X_SEP \
   /* ints */ \
   X_VAL(d) X_SEP X_VAL(i) X_SEP X_VAL(o) X_SEP \
   X_VAL(u) X_SEP X_VAL(x) X_SEP X_VAL(X) X_SEP \
@@ -195,125 +169,133 @@ struct LengthMod {
   X_VAL(f) X_SEP X_VAL(F) X_SEP X_VAL(e) X_SEP X_VAL(E) X_SEP \
   X_VAL(g) X_SEP X_VAL(G) X_SEP X_VAL(a) X_SEP X_VAL(A) X_SEP \
   /* misc */ \
-  X_VAL(n) X_SEP X_VAL(p)
+  X_VAL(n) X_SEP X_VAL(p) X_SEP X_VAL(v)
 // clang-format on
 
-struct ConversionChar {
- public:
-  enum Id : uint8_t {
-    c, C, s, S,              // text
+// This type should not be referenced, it exists only to provide labels
+// internally that match the values declared in FormatConversionChar in
+// str_format.h. This is meant to allow internal libraries to use the same
+// declared interface type as the public interface
+// (absl::StrFormatConversionChar) while keeping the definition in a public
+// header.
+// Internal libraries should use the form
+// `FormatConversionCharInternal::c`, `FormatConversionCharInternal::kNone` for
+// comparisons.  Use in switch statements is not recommended due to a bug in how
+// gcc 4.9 -Wswitch handles declared but undefined enums.
+struct FormatConversionCharInternal {
+  FormatConversionCharInternal() = delete;
+
+ private:
+  // clang-format off
+  enum class Enum : uint8_t {
+    c, s,                    // text
     d, i, o, u, x, X,        // int
     f, F, e, E, g, G, a, A,  // float
-    n, p,                    // misc
-    none
+    n, p, v,                    // misc
+    kNone
   };
-  static const size_t kNumValues = none + 1;
-
-  ConversionChar() : id_(none) {}
-
+  // clang-format on
  public:
-  // Index into the opaque array of ConversionChar enums.
-  // Requires: i < kNumValues
-  static ConversionChar FromIndex(size_t i) {
-    return ConversionChar(kSpecs[i].value);
-  }
-
-  static ConversionChar FromChar(char c) {
-    ConversionChar::Id out_id = ConversionChar::none;
-    switch (c) {
-#define X_VAL(id)                \
-  case #id[0]:                   \
-    out_id = ConversionChar::id; \
-    break;
-      ABSL_CONVERSION_CHARS_EXPAND_(X_VAL, )
-#undef X_VAL
-      default:
-        break;
-    }
-    return ConversionChar(out_id);
-  }
-
-  static ConversionChar FromId(Id id) { return ConversionChar(id); }
-  Id id() const { return id_; }
+#define ABSL_INTERNAL_X_VAL(id)              \
+  static constexpr FormatConversionChar id = \
+      static_cast<FormatConversionChar>(Enum::id);
+  ABSL_INTERNAL_CONVERSION_CHARS_EXPAND_(ABSL_INTERNAL_X_VAL, )
+#undef ABSL_INTERNAL_X_VAL
+  static constexpr FormatConversionChar kNone =
+      static_cast<FormatConversionChar>(Enum::kNone);
+};
+// clang-format on
 
-  int radix() const {
-    switch (id()) {
-      case x: case X: case a: case A: case p: return 16;
-      case o: return 8;
-      default: return 10;
-    }
+inline FormatConversionChar FormatConversionCharFromChar(char c) {
+  switch (c) {
+#define ABSL_INTERNAL_X_VAL(id) \
+  case #id[0]:                  \
+    return FormatConversionCharInternal::id;
+    ABSL_INTERNAL_CONVERSION_CHARS_EXPAND_(ABSL_INTERNAL_X_VAL, )
+#undef ABSL_INTERNAL_X_VAL
   }
+  return FormatConversionCharInternal::kNone;
+}
 
-  bool upper() const {
-    switch (id()) {
-      case X: case F: case E: case G: case A: return true;
-      default: return false;
-    }
+inline bool FormatConversionCharIsUpper(FormatConversionChar c) {
+  if (c == FormatConversionCharInternal::X ||
+      c == FormatConversionCharInternal::F ||
+      c == FormatConversionCharInternal::E ||
+      c == FormatConversionCharInternal::G ||
+      c == FormatConversionCharInternal::A) {
+    return true;
+  } else {
+    return false;
   }
+}
 
-  bool is_signed() const {
-    switch (id()) {
-      case d: case i: return true;
-      default: return false;
-    }
+inline bool FormatConversionCharIsFloat(FormatConversionChar c) {
+  if (c == FormatConversionCharInternal::a ||
+      c == FormatConversionCharInternal::e ||
+      c == FormatConversionCharInternal::f ||
+      c == FormatConversionCharInternal::g ||
+      c == FormatConversionCharInternal::A ||
+      c == FormatConversionCharInternal::E ||
+      c == FormatConversionCharInternal::F ||
+      c == FormatConversionCharInternal::G) {
+    return true;
+  } else {
+    return false;
   }
+}
 
-  bool is_integral() const {
-    switch (id()) {
-      case d: case i: case u: case o: case x: case X:
-        return true;
-      default: return false;
-    }
+inline char FormatConversionCharToChar(FormatConversionChar c) {
+  if (c == FormatConversionCharInternal::kNone) {
+    return '\0';
+
+#define ABSL_INTERNAL_X_VAL(e)                       \
+  } else if (c == FormatConversionCharInternal::e) { \
+    return #e[0];
+#define ABSL_INTERNAL_X_SEP
+  ABSL_INTERNAL_CONVERSION_CHARS_EXPAND_(ABSL_INTERNAL_X_VAL,
+                                         ABSL_INTERNAL_X_SEP)
+  } else {
+    return '\0';
   }
 
-  bool is_float() const {
-    switch (id()) {
-      case a: case e: case f: case g: case A: case E: case F: case G:
-        return true;
-      default: return false;
-    }
-  }
+#undef ABSL_INTERNAL_X_VAL
+#undef ABSL_INTERNAL_X_SEP
+}
 
-  bool IsValid() const { return id() != none; }
+// The associated char.
+inline std::ostream& operator<<(std::ostream& os, FormatConversionChar v) {
+  char c = FormatConversionCharToChar(v);
+  if (!c) c = '?';
+  return os << c;
+}
 
-  // The associated char.
-  char Char() const { return kSpecs[id_].name; }
+struct FormatConversionSpecImplFriend;
 
-  friend bool operator==(const ConversionChar& a, const ConversionChar& b) {
-    return a.id() == b.id();
-  }
-  friend bool operator!=(const ConversionChar& a, const ConversionChar& b) {
-    return !(a == b);
+class FormatConversionSpecImpl {
+ public:
+  // Width and precision are not specified, no flags are set.
+  bool is_basic() const { return flags_ == Flags::kBasic; }
+  bool has_left_flag() const { return FlagsContains(flags_, Flags::kLeft); }
+  bool has_show_pos_flag() const {
+    return FlagsContains(flags_, Flags::kShowPos);
   }
-  friend std::ostream& operator<<(std::ostream& os, const ConversionChar& v) {
-    char c = v.Char();
-    if (!c) c = '?';
-    return os << c;
+  bool has_sign_col_flag() const {
+    return FlagsContains(flags_, Flags::kSignCol);
   }
+  bool has_alt_flag() const { return FlagsContains(flags_, Flags::kAlt); }
+  bool has_zero_flag() const { return FlagsContains(flags_, Flags::kZero); }
 
- private:
-  struct Spec {
-    Id value;
-    char name;
-  };
-  static const Spec kSpecs[];
-
-  explicit ConversionChar(Id id) : id_(id) {}
-
-  Id id_;
-};
-
-class ConversionSpec {
- public:
-  Flags flags() const { return flags_; }
   LengthMod length_mod() const { return length_mod_; }
-  ConversionChar conv() const {
+
+  FormatConversionChar conversion_char() const {
     // Keep this field first in the struct . It generates better code when
     // accessing it when ConversionSpec is passed by value in registers.
-    static_assert(offsetof(ConversionSpec, conv_) == 0, "");
+    static_assert(offsetof(FormatConversionSpecImpl, conv_) == 0);
     return conv_;
   }
 
+  void set_conversion_char(FormatConversionChar c) { conv_ = c; }
+
   // Returns the specified width. If width is unspecfied, it returns a negative
   // value.
   int width() const { return width_; }
@@ -321,46 +303,100 @@ class ConversionSpec {
   // negative value.
   int precision() const { return precision_; }
 
-  void set_flags(Flags f) { flags_ = f; }
-  void set_length_mod(LengthMod lm) { length_mod_ = lm; }
-  void set_conv(ConversionChar c) { conv_ = c; }
-  void set_width(int w) { width_ = w; }
-  void set_precision(int p) { precision_ = p; }
-  void set_left(bool b) { flags_.left = b; }
+  template <typename T>
+  T Wrap() {
+    return T(*this);
+  }
 
  private:
-  ConversionChar conv_;
+  friend struct str_format_internal::FormatConversionSpecImplFriend;
+  FormatConversionChar conv_ = FormatConversionCharInternal::kNone;
   Flags flags_;
-  LengthMod length_mod_;
+  LengthMod length_mod_ = LengthMod::none;
   int width_;
   int precision_;
 };
 
-constexpr uint64_t ConversionCharToConvValue(char conv) {
+struct FormatConversionSpecImplFriend final {
+  static void SetFlags(Flags f, FormatConversionSpecImpl* conv) {
+    conv->flags_ = f;
+  }
+  static void SetLengthMod(LengthMod l, FormatConversionSpecImpl* conv) {
+    conv->length_mod_ = l;
+  }
+  static void SetConversionChar(FormatConversionChar c,
+                                FormatConversionSpecImpl* conv) {
+    conv->conv_ = c;
+  }
+  static void SetWidth(int w, FormatConversionSpecImpl* conv) {
+    conv->width_ = w;
+  }
+  static void SetPrecision(int p, FormatConversionSpecImpl* conv) {
+    conv->precision_ = p;
+  }
+  static std::string FlagsToString(const FormatConversionSpecImpl& spec) {
+    return str_format_internal::FlagsToString(spec.flags_);
+  }
+};
+
+// Type safe OR operator.
+// We need this for two reasons:
+//  1. operator| on enums makes them decay to integers and the result is an
+//     integer. We need the result to stay as an enum.
+//  2. We use "enum class" which would not work even if we accepted the decay.
+constexpr FormatConversionCharSet FormatConversionCharSetUnion(
+    FormatConversionCharSet a) {
+  return a;
+}
+
+template <typename... CharSet>
+constexpr FormatConversionCharSet FormatConversionCharSetUnion(
+    FormatConversionCharSet a, CharSet... rest) {
+  return static_cast<FormatConversionCharSet>(
+      static_cast<uint64_t>(a) |
+      static_cast<uint64_t>(FormatConversionCharSetUnion(rest...)));
+}
+
+constexpr uint64_t FormatConversionCharToConvInt(FormatConversionChar c) {
+  return uint64_t{1} << (1 + static_cast<uint8_t>(c));
+}
+
+constexpr uint64_t FormatConversionCharToConvInt(char conv) {
   return
-#define CONV_SET_CASE(c) \
-  conv == #c[0] ? (uint64_t{1} << (1 + ConversionChar::Id::c)):
-      ABSL_CONVERSION_CHARS_EXPAND_(CONV_SET_CASE, )
-#undef CONV_SET_CASE
+#define ABSL_INTERNAL_CHAR_SET_CASE(c)                                 \
+  conv == #c[0]                                                        \
+      ? FormatConversionCharToConvInt(FormatConversionCharInternal::c) \
+      :
+      ABSL_INTERNAL_CONVERSION_CHARS_EXPAND_(ABSL_INTERNAL_CHAR_SET_CASE, )
+#undef ABSL_INTERNAL_CHAR_SET_CASE
                   conv == '*'
           ? 1
           : 0;
 }
 
-enum class Conv : uint64_t {
-#define CONV_SET_CASE(c) c = ConversionCharToConvValue(#c[0]),
-  ABSL_CONVERSION_CHARS_EXPAND_(CONV_SET_CASE, )
-#undef CONV_SET_CASE
+constexpr FormatConversionCharSet FormatConversionCharToConvValue(char conv) {
+  return static_cast<FormatConversionCharSet>(
+      FormatConversionCharToConvInt(conv));
+}
+
+struct FormatConversionCharSetInternal {
+#define ABSL_INTERNAL_CHAR_SET_CASE(c)         \
+  static constexpr FormatConversionCharSet c = \
+      FormatConversionCharToConvValue(#c[0]);
+  ABSL_INTERNAL_CONVERSION_CHARS_EXPAND_(ABSL_INTERNAL_CHAR_SET_CASE, )
+#undef ABSL_INTERNAL_CHAR_SET_CASE
 
   // Used for width/precision '*' specification.
-  star = ConversionCharToConvValue('*'),
-
-  // Some predefined values:
-  integral = d | i | u | o | x | X,
-  floating = a | e | f | g | A | E | F | G,
-  numeric = integral | floating,
-  string = s,
-  pointer = p
+  static constexpr FormatConversionCharSet kStar =
+      FormatConversionCharToConvValue('*');
+
+  static constexpr FormatConversionCharSet kIntegral =
+      FormatConversionCharSetUnion(d, i, u, o, x, X);
+  static constexpr FormatConversionCharSet kFloating =
+      FormatConversionCharSetUnion(a, e, f, g, A, E, F, G);
+  static constexpr FormatConversionCharSet kNumeric =
+      FormatConversionCharSetUnion(kIntegral, kFloating);
+  static constexpr FormatConversionCharSet kPointer = p;
 };
 
 // Type safe OR operator.
@@ -368,36 +404,44 @@ enum class Conv : uint64_t {
 //  1. operator| on enums makes them decay to integers and the result is an
 //     integer. We need the result to stay as an enum.
 //  2. We use "enum class" which would not work even if we accepted the decay.
-constexpr Conv operator|(Conv a, Conv b) {
-  return Conv(static_cast<uint64_t>(a) | static_cast<uint64_t>(b));
+constexpr FormatConversionCharSet operator|(FormatConversionCharSet a,
+                                            FormatConversionCharSet b) {
+  return FormatConversionCharSetUnion(a, b);
 }
 
+// Overloaded conversion functions to support absl::ParsedFormat.
 // Get a conversion with a single character in it.
-constexpr Conv ConversionCharToConv(char c) {
-  return Conv(ConversionCharToConvValue(c));
+constexpr FormatConversionCharSet ToFormatConversionCharSet(char c) {
+  return static_cast<FormatConversionCharSet>(
+      FormatConversionCharToConvValue(c));
 }
 
+// Get a conversion with a single character in it.
+constexpr FormatConversionCharSet ToFormatConversionCharSet(
+    FormatConversionCharSet c) {
+  return c;
+}
+
+template <typename T>
+void ToFormatConversionCharSet(T) = delete;
+
 // Checks whether `c` exists in `set`.
-constexpr bool Contains(Conv set, char c) {
-  return (static_cast<uint64_t>(set) & ConversionCharToConvValue(c)) != 0;
+constexpr bool Contains(FormatConversionCharSet set, char c) {
+  return (static_cast<uint64_t>(set) &
+          static_cast<uint64_t>(FormatConversionCharToConvValue(c))) != 0;
 }
 
 // Checks whether all the characters in `c` are contained in `set`
-constexpr bool Contains(Conv set, Conv c) {
+constexpr bool Contains(FormatConversionCharSet set,
+                        FormatConversionCharSet c) {
   return (static_cast<uint64_t>(set) & static_cast<uint64_t>(c)) ==
          static_cast<uint64_t>(c);
 }
 
-// Return type of the AbslFormatConvert() functions.
-// The Conv template parameter is used to inform the framework of what
-// conversion characters are supported by that AbslFormatConvert routine.
-template <Conv C>
-struct ConvertResult {
-  static constexpr Conv kConv = C;
-  bool value;
-};
-template <Conv C>
-constexpr Conv ConvertResult<C>::kConv;
+// Checks whether all the characters in `c` are contained in `set`
+constexpr bool Contains(FormatConversionCharSet set, FormatConversionChar c) {
+  return (static_cast<uint64_t>(set) & FormatConversionCharToConvInt(c)) != 0;
+}
 
 // Return capacity - used, clipped to a minimum of 0.
 inline size_t Excess(size_t used, size_t capacity) {
@@ -406,6 +450,7 @@ inline size_t Excess(size_t used, size_t capacity) {
 
 }  // namespace str_format_internal
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_INTERNAL_STR_FORMAT_EXTENSION_H_
diff --git a/absl/strings/internal/str_format/extension_test.cc b/absl/strings/internal/str_format/extension_test.cc
index 4e23fefbd5b..fc4dc4f53bc 100644
--- a/absl/strings/internal/str_format/extension_test.cc
+++ b/absl/strings/internal/str_format/extension_test.cc
@@ -16,18 +16,37 @@
 
 #include "absl/strings/internal/str_format/extension.h"
 
+#include <cstddef>
 #include <random>
 #include <string>
 
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/random/random.h"
 #include "absl/strings/str_format.h"
+#include "absl/strings/string_view.h"
 
-#include "gtest/gtest.h"
+namespace my_namespace {
+class UserDefinedType {
+ public:
+  UserDefinedType() = default;
+
+  void Append(absl::string_view str) { value_.append(str.data(), str.size()); }
+  const std::string& Value() const { return value_; }
+
+  friend void AbslFormatFlush(UserDefinedType* x, absl::string_view str) {
+    x->Append(str);
+  }
+
+ private:
+  std::string value_;
+};
+}  // namespace my_namespace
 
 namespace {
 
 std::string MakeRandomString(size_t len) {
-  std::random_device rd;
-  std::mt19937 gen(rd());
+  absl::InsecureBitGen gen;
   std::uniform_int_distribution<> dis('a', 'z');
   std::string s(len, '0');
   for (char& c : s) {
@@ -63,4 +82,29 @@ TEST(FormatExtensionTest, SinkAppendChars) {
     EXPECT_EQ(actual, expected);
   }
 }
+
+TEST(FormatExtensionTest, VerifyEnumEquality) {
+#define X_VAL(id)                           \
+  EXPECT_EQ(absl::FormatConversionChar::id, \
+            absl::str_format_internal::FormatConversionCharInternal::id);
+  ABSL_INTERNAL_CONVERSION_CHARS_EXPAND_(X_VAL, );
+#undef X_VAL
+
+#define X_VAL(id)                              \
+  EXPECT_EQ(absl::FormatConversionCharSet::id, \
+            absl::str_format_internal::FormatConversionCharSetInternal::id);
+  ABSL_INTERNAL_CONVERSION_CHARS_EXPAND_(X_VAL, );
+#undef X_VAL
+}
+
+TEST(FormatExtensionTest, SetConversionChar) {
+  absl::str_format_internal::FormatConversionSpecImpl spec;
+  EXPECT_EQ(spec.conversion_char(),
+            absl::str_format_internal::FormatConversionCharInternal::kNone);
+  spec.set_conversion_char(
+      absl::str_format_internal::FormatConversionCharInternal::d);
+  EXPECT_EQ(spec.conversion_char(),
+            absl::str_format_internal::FormatConversionCharInternal::d);
+}
+
 }  // namespace
diff --git a/absl/strings/internal/str_format/float_conversion.cc b/absl/strings/internal/str_format/float_conversion.cc
index 9236acdc74c..137e159d710 100644
--- a/absl/strings/internal/str_format/float_conversion.cc
+++ b/absl/strings/internal/str_format/float_conversion.cc
@@ -1,25 +1,1046 @@
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
 #include "absl/strings/internal/str_format/float_conversion.h"
 
 #include <string.h>
+
 #include <algorithm>
+#include <array>
 #include <cassert>
 #include <cmath>
+#include <cstdint>
+#include <cstring>
+#include <limits>
+#include <optional>
 #include <string>
+#include <type_traits>
 
+#include "absl/base/attributes.h"
 #include "absl/base/config.h"
+#include "absl/base/optimization.h"
+#include "absl/functional/function_ref.h"
+#include "absl/meta/type_traits.h"
+#include "absl/numeric/bits.h"
+#include "absl/numeric/int128.h"
+#include "absl/numeric/internal/representation.h"
+#include "absl/strings/internal/str_format/extension.h"
+#include "absl/strings/numbers.h"
+#include "absl/strings/string_view.h"
+#include "absl/types/span.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace str_format_internal {
 
 namespace {
 
-char *CopyStringTo(string_view v, char *out) {
+using ::absl::numeric_internal::IsDoubleDouble;
+
+// The code below wants to avoid heap allocations.
+// To do so it needs to allocate memory on the stack.
+// `StackArray` will allocate memory on the stack in the form of a uint32_t
+// array and call the provided callback with said memory.
+// It will allocate memory in increments of 512 bytes. We could allocate the
+// largest needed unconditionally, but that is more than we need in most of
+// cases. This way we use less stack in the common cases.
+class StackArray {
+  using Func = absl::FunctionRef<void(absl::Span<uint32_t>)>;
+  static constexpr size_t kStep = 512 / sizeof(uint32_t);
+  // 5 steps is 2560 bytes, which is enough to hold a long double with the
+  // largest/smallest exponents.
+  // The operations below will static_assert their particular maximum.
+  static constexpr size_t kNumSteps = 5;
+
+  // We do not want this function to be inlined.
+  // Otherwise the caller will allocate the stack space unnecessarily for all
+  // the variants even though it only calls one.
+  template <size_t steps>
+  ABSL_ATTRIBUTE_NOINLINE static void RunWithCapacityImpl(Func f) {
+    uint32_t values[steps * kStep]{};
+    f(absl::MakeSpan(values));
+  }
+
+ public:
+  static constexpr size_t kMaxCapacity = kStep * kNumSteps;
+
+  static void RunWithCapacity(size_t capacity, Func f) {
+    assert(capacity <= kMaxCapacity);
+    const size_t step = (capacity + kStep - 1) / kStep;
+    assert(step <= kNumSteps);
+    switch (step) {
+      case 1:
+        return RunWithCapacityImpl<1>(f);
+      case 2:
+        return RunWithCapacityImpl<2>(f);
+      case 3:
+        return RunWithCapacityImpl<3>(f);
+      case 4:
+        return RunWithCapacityImpl<4>(f);
+      case 5:
+        return RunWithCapacityImpl<5>(f);
+    }
+
+    assert(false && "Invalid capacity");
+  }
+};
+
+// Calculates `10 * (*v) + carry` and stores the result in `*v` and returns
+// the carry.
+// Requires: `0 <= carry <= 9`
+template <typename Int>
+inline char MultiplyBy10WithCarry(Int* v, char carry) {
+  using BiggerInt = std::conditional_t<sizeof(Int) == 4, uint64_t, uint128>;
+  BiggerInt tmp =
+      10 * static_cast<BiggerInt>(*v) + static_cast<BiggerInt>(carry);
+  *v = static_cast<Int>(tmp);
+  return static_cast<char>(tmp >> (sizeof(Int) * 8));
+}
+
+// Calculates `(2^64 * carry + *v) / 10`.
+// Stores the quotient in `*v` and returns the remainder.
+// Requires: `0 <= carry <= 9`
+inline char DivideBy10WithCarry(uint64_t* v, char carry) {
+  constexpr uint64_t divisor = 10;
+  // 2^64 / divisor = chunk_quotient + chunk_remainder / divisor
+  constexpr uint64_t chunk_quotient = (uint64_t{1} << 63) / (divisor / 2);
+  constexpr uint64_t chunk_remainder = uint64_t{} - chunk_quotient * divisor;
+
+  const uint64_t carry_u64 = static_cast<uint64_t>(carry);
+  const uint64_t mod = *v % divisor;
+  const uint64_t next_carry = chunk_remainder * carry_u64 + mod;
+  *v = *v / divisor + carry_u64 * chunk_quotient + next_carry / divisor;
+  return static_cast<char>(next_carry % divisor);
+}
+
+using MaxFloatType = std::conditional_t<IsDoubleDouble(), double, long double>;
+
+// Generates the decimal representation for an integer of the form `v * 2^exp`,
+// where `v` and `exp` are both positive integers.
+// It generates the digits from the left (ie the most significant digit first)
+// to allow for direct printing into the sink.
+//
+// Requires `0 <= exp` and `exp <= numeric_limits<MaxFloatType>::max_exponent`.
+class BinaryToDecimal {
+  static constexpr size_t ChunksNeeded(int exp) {
+    // We will left shift a uint128 by `exp` bits, so we need `128+exp` total
+    // bits. Round up to 32.
+    // See constructor for details about adding `10%` to the value.
+    return static_cast<size_t>(((128 + exp + 31) / 32 * 11 + 9) / 10);
+  }
+
+ public:
+  // Run the conversion for `v * 2^exp` and call `f(binary_to_decimal)`.
+  // This function will allocate enough stack space to perform the conversion.
+  static void RunConversion(uint128 v, int exp,
+                            absl::FunctionRef<void(BinaryToDecimal)> f) {
+    assert(exp > 0);
+    assert(exp <= std::numeric_limits<MaxFloatType>::max_exponent);
+    static_assert(
+        StackArray::kMaxCapacity >=
+            ChunksNeeded(std::numeric_limits<MaxFloatType>::max_exponent));
+
+    StackArray::RunWithCapacity(
+        ChunksNeeded(exp),
+        [=](absl::Span<uint32_t> input) { f(BinaryToDecimal(input, v, exp)); });
+  }
+
+  size_t TotalDigits() const {
+    return (decimal_end_ - decimal_start_) * kDigitsPerChunk +
+           CurrentDigits().size();
+  }
+
+  // See the current block of digits.
+  absl::string_view CurrentDigits() const {
+    return absl::string_view(&digits_[kDigitsPerChunk - size_], size_);
+  }
+
+  // Advance the current view of digits.
+  // Returns `false` when no more digits are available.
+  bool AdvanceDigits() {
+    if (decimal_start_ >= decimal_end_) return false;
+
+    uint32_t w = data_[decimal_start_++];
+    for (size_ = 0; size_ < kDigitsPerChunk; w /= 10) {
+      digits_[kDigitsPerChunk - ++size_] = w % 10 + '0';
+    }
+    return true;
+  }
+
+ private:
+  BinaryToDecimal(absl::Span<uint32_t> data, uint128 v, int exp) : data_(data) {
+    // We need to print the digits directly into the sink object without
+    // buffering them all first. To do this we need two things:
+    // - to know the total number of digits to do padding when necessary
+    // - to generate the decimal digits from the left.
+    //
+    // In order to do this, we do a two pass conversion.
+    // On the first pass we convert the binary representation of the value into
+    // a decimal representation in which each uint32_t chunk holds up to 9
+    // decimal digits.  In the second pass we take each decimal-holding-uint32_t
+    // value and generate the ascii decimal digits into `digits_`.
+    //
+    // The binary and decimal representations actually share the same memory
+    // region. As we go converting the chunks from binary to decimal we free
+    // them up and reuse them for the decimal representation. One caveat is that
+    // the decimal representation is around 7% less efficient in space than the
+    // binary one. We allocate an extra 10% memory to account for this. See
+    // ChunksNeeded for this calculation.
+    size_t after_chunk_index = static_cast<size_t>(exp / 32 + 1);
+    decimal_start_ = decimal_end_ = ChunksNeeded(exp);
+    const int offset = exp % 32;
+    // Left shift v by exp bits.
+    data_[after_chunk_index - 1] = static_cast<uint32_t>(v << offset);
+    for (v >>= (32 - offset); v; v >>= 32)
+      data_[++after_chunk_index - 1] = static_cast<uint32_t>(v);
+
+    while (after_chunk_index > 0) {
+      // While we have more than one chunk available, go in steps of 1e9.
+      // `data_[after_chunk_index - 1]` holds the highest non-zero binary chunk,
+      // so keep the variable updated.
+      uint32_t carry = 0;
+      for (size_t i = after_chunk_index; i > 0; --i) {
+        uint64_t tmp = uint64_t{data_[i - 1]} + (uint64_t{carry} << 32);
+        data_[i - 1] = static_cast<uint32_t>(tmp / uint64_t{1000000000});
+        carry = static_cast<uint32_t>(tmp % uint64_t{1000000000});
+      }
+
+      // If the highest chunk is now empty, remove it from view.
+      if (data_[after_chunk_index - 1] == 0)
+        --after_chunk_index;
+
+      --decimal_start_;
+      assert(decimal_start_ != after_chunk_index - 1);
+      data_[decimal_start_] = carry;
+    }
+
+    // Fill the first set of digits. The first chunk might not be complete, so
+    // handle differently.
+    for (uint32_t first = data_[decimal_start_++]; first != 0; first /= 10) {
+      digits_[kDigitsPerChunk - ++size_] = first % 10 + '0';
+    }
+  }
+
+ private:
+  static constexpr size_t kDigitsPerChunk = 9;
+
+  size_t decimal_start_;
+  size_t decimal_end_;
+
+  std::array<char, kDigitsPerChunk> digits_;
+  size_t size_ = 0;
+
+  absl::Span<uint32_t> data_;
+};
+
+// Converts a value of the form `x * 2^-exp` into a sequence of decimal digits.
+// Requires `-exp < 0` and
+// `-exp >= limits<MaxFloatType>::min_exponent - limits<MaxFloatType>::digits`.
+class FractionalDigitGenerator {
+ private:
+  static constexpr size_t ChunksNeeded(int exp) {
+    // We need 128 bits for mantissa and `exp` bits for exponent.
+    return static_cast<size_t>((128 + exp + 31) / 32);
+  }
+
+ public:
+  // Run the conversion for `v * 2^exp` and call `f(generator)`.
+  // This function will allocate enough stack space to perform the conversion.
+  static void RunConversion(
+      uint128 v, int exp, absl::FunctionRef<void(FractionalDigitGenerator)> f) {
+    using Limits = std::numeric_limits<MaxFloatType>;
+    assert(-exp < 0);
+    // We need enough precision to cover all digits of MaxFloatType, and we add
+    // 128 bits of headroom for fractional digit generation.
+    const int margin = Limits::digits + 128;
+    assert(-exp >= Limits::min_exponent - margin);
+    static_assert(StackArray::kMaxCapacity >=
+                      ChunksNeeded(margin - Limits::min_exponent));
+    StackArray::RunWithCapacity(
+        ChunksNeeded(exp), [=](absl::Span<uint32_t> input) {
+          f(FractionalDigitGenerator(input, v, exp));
+        });
+  }
+
+  // Returns true if there are any more non-zero digits left.
+  bool HasMoreDigits() const { return next_digit_ != 0 || after_chunk_index_; }
+
+  // Returns true if the remainder digits are greater than 5000...
+  bool IsGreaterThanHalf() const {
+    return next_digit_ > 5 || (next_digit_ == 5 && after_chunk_index_);
+  }
+  // Returns true if the remainder digits are exactly 5000...
+  bool IsExactlyHalf() const { return next_digit_ == 5 && !after_chunk_index_; }
+
+  struct Digits {
+    char digit_before_nine;
+    size_t num_nines;
+  };
+
+  // Get the next set of digits.
+  // They are composed by a non-9 digit followed by a runs of zero or more 9s.
+  Digits GetDigits() {
+    Digits digits{next_digit_, 0};
+
+    next_digit_ = GetOneDigit();
+    while (next_digit_ == 9) {
+      ++digits.num_nines;
+      next_digit_ = GetOneDigit();
+    }
+
+    return digits;
+  }
+
+ private:
+  // Return the next digit.
+  char GetOneDigit() {
+    if (!after_chunk_index_)
+      return 0;
+
+    char carry = 0;
+    for (size_t i = after_chunk_index_; i > 0; --i) {
+      carry = MultiplyBy10WithCarry(&data_[i - 1], carry);
+    }
+    // If the lowest chunk is now empty, remove it from view.
+    if (data_[after_chunk_index_ - 1] == 0)
+      --after_chunk_index_;
+    return carry;
+  }
+
+  FractionalDigitGenerator(absl::Span<uint32_t> data, uint128 v, int exp)
+      : after_chunk_index_(static_cast<size_t>(exp / 32 + 1)), data_(data) {
+    const int offset = exp % 32;
+    // Right shift `v` by `exp` bits.
+    data_[after_chunk_index_ - 1] = static_cast<uint32_t>(v << (32 - offset));
+    v >>= offset;
+    // Make sure we don't overflow the data. We already calculated that
+    // non-zero bits fit, so we might not have space for leading zero bits.
+    for (size_t pos = after_chunk_index_ - 1; v; v >>= 32)
+      data_[--pos] = static_cast<uint32_t>(v);
+
+    // Fill next_digit_, as GetDigits expects it to be populated always.
+    next_digit_ = GetOneDigit();
+  }
+
+  char next_digit_;
+  size_t after_chunk_index_;
+  absl::Span<uint32_t> data_;
+};
+
+// Count the number of leading zero bits.
+int LeadingZeros(uint64_t v) { return countl_zero(v); }
+int LeadingZeros(uint128 v) {
+  auto high = static_cast<uint64_t>(v >> 64);
+  auto low = static_cast<uint64_t>(v);
+  return high != 0 ? countl_zero(high) : 64 + countl_zero(low);
+}
+
+// Round up the text digits starting at `p`.
+// The buffer must have an extra digit that is known to not need rounding.
+// This is done below by having an extra '0' digit on the left.
+void RoundUp(char *p) {
+  while (*p == '9' || *p == '.') {
+    if (*p == '9') *p = '0';
+    --p;
+  }
+  ++*p;
+}
+
+// Check the previous digit and round up or down to follow the round-to-even
+// policy.
+void RoundToEven(char *p) {
+  if (*p == '.') --p;
+  if (*p % 2 == 1) RoundUp(p);
+}
+
+// Simple integral decimal digit printing for values that fit in 64-bits.
+// Returns the pointer to the last written digit.
+char *PrintIntegralDigitsFromRightFast(uint64_t v, char *p) {
+  do {
+    *--p = DivideBy10WithCarry(&v, 0) + '0';
+  } while (v != 0);
+  return p;
+}
+
+// Simple integral decimal digit printing for values that fit in 128-bits.
+// Returns the pointer to the last written digit.
+char *PrintIntegralDigitsFromRightFast(uint128 v, char *p) {
+  auto high = static_cast<uint64_t>(v >> 64);
+  auto low = static_cast<uint64_t>(v);
+
+  while (high != 0) {
+    char carry = DivideBy10WithCarry(&high, 0);
+    carry = DivideBy10WithCarry(&low, carry);
+    *--p = carry + '0';
+  }
+  return PrintIntegralDigitsFromRightFast(low, p);
+}
+
+// Simple fractional decimal digit printing for values that fir in 64-bits after
+// shifting.
+// Performs rounding if necessary to fit within `precision`.
+// Returns the pointer to one after the last character written.
+char* PrintFractionalDigitsFast(uint64_t v,
+                                char* start,
+                                int exp,
+                                size_t precision) {
+  char *p = start;
+  v <<= (64 - exp);
+  while (precision > 0) {
+    if (!v) return p;
+    *p++ = MultiplyBy10WithCarry(&v, 0) + '0';
+    --precision;
+  }
+
+  // We need to round.
+  if (v < 0x8000000000000000) {
+    // We round down, so nothing to do.
+  } else if (v > 0x8000000000000000) {
+    // We round up.
+    RoundUp(p - 1);
+  } else {
+    RoundToEven(p - 1);
+  }
+
+  return p;
+}
+
+// Simple fractional decimal digit printing for values that fir in 128-bits
+// after shifting.
+// Performs rounding if necessary to fit within `precision`.
+// Returns the pointer to one after the last character written.
+char* PrintFractionalDigitsFast(uint128 v,
+                                char* start,
+                                int exp,
+                                size_t precision) {
+  char *p = start;
+  v <<= (128 - exp);
+  auto high = static_cast<uint64_t>(v >> 64);
+  auto low = static_cast<uint64_t>(v);
+
+  // While we have digits to print and `low` is not empty, do the long
+  // multiplication.
+  while (precision > 0 && low != 0) {
+    char carry = MultiplyBy10WithCarry(&low, 0);
+    carry = MultiplyBy10WithCarry(&high, carry);
+
+    *p++ = carry + '0';
+    --precision;
+  }
+
+  // Now `low` is empty, so use a faster approach for the rest of the digits.
+  // This block is pretty much the same as the main loop for the 64-bit case
+  // above.
+  while (precision > 0) {
+    if (!high) return p;
+    *p++ = MultiplyBy10WithCarry(&high, 0) + '0';
+    --precision;
+  }
+
+  // We need to round.
+  if (high < 0x8000000000000000) {
+    // We round down, so nothing to do.
+  } else if (high > 0x8000000000000000 || low != 0) {
+    // We round up.
+    RoundUp(p - 1);
+  } else {
+    RoundToEven(p - 1);
+  }
+
+  return p;
+}
+
+struct FractionalDigitPrinterResult {
+  char* end;
+  size_t skipped_zeros;
+  bool nonzero_remainder;
+};
+
+FractionalDigitPrinterResult PrintFractionalDigitsScientific(
+    uint64_t v, char* start, int exp, size_t precision, bool skip_zeros) {
+  char* p = start;
+  v <<= (64 - exp);
+
+  size_t skipped_zeros = 0;
+  while (v != 0 && precision > 0) {
+    char carry = MultiplyBy10WithCarry(&v, 0);
+    if (skip_zeros) {
+      if (carry == 0) {
+        ++skipped_zeros;
+        continue;
+      }
+      skip_zeros = false;
+    }
+    *p++ = carry + '0';
+    --precision;
+  }
+  return {p, skipped_zeros, v != 0};
+}
+
+FractionalDigitPrinterResult PrintFractionalDigitsScientific(
+    uint128 v, char* start, int exp, size_t precision, bool skip_zeros) {
+  char* p = start;
+  v <<= (128 - exp);
+  auto high = static_cast<uint64_t>(v >> 64);
+  auto low = static_cast<uint64_t>(v);
+
+  size_t skipped_zeros = 0;
+  while (precision > 0 && low != 0) {
+    char carry = MultiplyBy10WithCarry(&low, 0);
+    carry = MultiplyBy10WithCarry(&high, carry);
+    if (skip_zeros) {
+      if (carry == 0) {
+        ++skipped_zeros;
+        continue;
+      }
+      skip_zeros = false;
+    }
+    *p++ = carry + '0';
+    --precision;
+  }
+
+  while (precision > 0 && high != 0) {
+    char carry = MultiplyBy10WithCarry(&high, 0);
+    if (skip_zeros) {
+      if (carry == 0) {
+        ++skipped_zeros;
+        continue;
+      }
+      skip_zeros = false;
+    }
+    *p++ = carry + '0';
+    --precision;
+  }
+
+  return {p, skipped_zeros, high != 0 || low != 0};
+}
+
+struct FormatState {
+  char sign_char;
+  size_t precision;
+  const FormatConversionSpecImpl &conv;
+  FormatSinkImpl *sink;
+
+  // In `alt` mode (flag #) we keep the `.` even if there are no fractional
+  // digits. In non-alt mode, we strip it.
+  bool ShouldPrintDot() const { return precision != 0 || conv.has_alt_flag(); }
+};
+
+struct Padding {
+  size_t left_spaces;
+  size_t zeros;
+  size_t right_spaces;
+};
+
+Padding ExtraWidthToPadding(size_t total_size, const FormatState &state) {
+  if (state.conv.width() < 0 ||
+      static_cast<size_t>(state.conv.width()) <= total_size) {
+    return {0, 0, 0};
+  }
+  size_t missing_chars = static_cast<size_t>(state.conv.width()) - total_size;
+  if (state.conv.has_left_flag()) {
+    return {0, 0, missing_chars};
+  } else if (state.conv.has_zero_flag()) {
+    return {0, missing_chars, 0};
+  } else {
+    return {missing_chars, 0, 0};
+  }
+}
+
+void FinalPrint(const FormatState& state,
+                absl::string_view data,
+                size_t padding_offset,
+                size_t trailing_zeros,
+                absl::string_view data_postfix) {
+  if (state.conv.width() < 0) {
+    // No width specified. Fast-path.
+    if (state.sign_char != '\0') state.sink->Append(1, state.sign_char);
+    state.sink->Append(data);
+    state.sink->Append(trailing_zeros, '0');
+    state.sink->Append(data_postfix);
+    return;
+  }
+
+  auto padding =
+      ExtraWidthToPadding((state.sign_char != '\0' ? 1 : 0) + data.size() +
+                              data_postfix.size() + trailing_zeros,
+                          state);
+
+  state.sink->Append(padding.left_spaces, ' ');
+  if (state.sign_char != '\0') state.sink->Append(1, state.sign_char);
+  // Padding in general needs to be inserted somewhere in the middle of `data`.
+  state.sink->Append(data.substr(0, padding_offset));
+  state.sink->Append(padding.zeros, '0');
+  state.sink->Append(data.substr(padding_offset));
+  state.sink->Append(trailing_zeros, '0');
+  state.sink->Append(data_postfix);
+  state.sink->Append(padding.right_spaces, ' ');
+}
+
+// Fastpath %f formatter for when the shifted value fits in a simple integral
+// type.
+// Prints `v*2^exp` with the options from `state`.
+template <typename Int>
+void FormatFFast(Int v, int exp, const FormatState &state) {
+  constexpr int input_bits = sizeof(Int) * 8;
+
+  static constexpr size_t integral_size =
+      /* in case we need to round up an extra digit */ 1 +
+      /* decimal digits for uint128 */ 40 + 1;
+  char buffer[integral_size + /* . */ 1 + /* max digits uint128 */ 128];
+  buffer[integral_size] = '.';
+  char *const integral_digits_end = buffer + integral_size;
+  char *integral_digits_start;
+  char *const fractional_digits_start = buffer + integral_size + 1;
+  char *fractional_digits_end = fractional_digits_start;
+
+  if (exp >= 0) {
+    const int total_bits = input_bits - LeadingZeros(v) + exp;
+    integral_digits_start =
+        total_bits <= 64
+            ? PrintIntegralDigitsFromRightFast(static_cast<uint64_t>(v) << exp,
+                                               integral_digits_end)
+            : PrintIntegralDigitsFromRightFast(static_cast<uint128>(v) << exp,
+                                               integral_digits_end);
+  } else {
+    exp = -exp;
+
+    integral_digits_start = PrintIntegralDigitsFromRightFast(
+        exp < input_bits ? v >> exp : 0, integral_digits_end);
+    // PrintFractionalDigits may pull a carried 1 all the way up through the
+    // integral portion.
+    integral_digits_start[-1] = '0';
+
+    fractional_digits_end =
+        exp <= 64 ? PrintFractionalDigitsFast(v, fractional_digits_start, exp,
+                                              state.precision)
+                  : PrintFractionalDigitsFast(static_cast<uint128>(v),
+                                              fractional_digits_start, exp,
+                                              state.precision);
+    // There was a carry, so include the first digit too.
+    if (integral_digits_start[-1] != '0') --integral_digits_start;
+  }
+
+  size_t size =
+      static_cast<size_t>(fractional_digits_end - integral_digits_start);
+
+  // In `alt` mode (flag #) we keep the `.` even if there are no fractional
+  // digits. In non-alt mode, we strip it.
+  if (!state.ShouldPrintDot()) --size;
+  FinalPrint(state, absl::string_view(integral_digits_start, size),
+             /*padding_offset=*/0,
+             state.precision - static_cast<size_t>(fractional_digits_end -
+                                                   fractional_digits_start),
+             /*data_postfix=*/"");
+}
+
+// Slow %f formatter for when the shifted value does not fit in a uint128, and
+// `exp > 0`.
+// Prints `v*2^exp` with the options from `state`.
+// This one is guaranteed to not have fractional digits, so we don't have to
+// worry about anything after the `.`.
+void FormatFPositiveExpSlow(uint128 v, int exp, const FormatState& state,
+                            bool strip_trailing_zeros = false) {
+  BinaryToDecimal::RunConversion(v, exp, [&](BinaryToDecimal btd) {
+    const size_t total_digits =
+        btd.TotalDigits() + (state.ShouldPrintDot() ? state.precision + 1 : 0);
+
+    const auto padding = ExtraWidthToPadding(
+        total_digits + (state.sign_char != '\0' ? 1 : 0), state);
+
+    state.sink->Append(padding.left_spaces, ' ');
+    if (state.sign_char != '\0')
+      state.sink->Append(1, state.sign_char);
+    state.sink->Append(padding.zeros, '0');
+
+    do {
+      state.sink->Append(btd.CurrentDigits());
+    } while (btd.AdvanceDigits());
+
+    if (state.ShouldPrintDot() && !strip_trailing_zeros) {
+      state.sink->Append(1, '.');
+    }
+    if (!strip_trailing_zeros) {
+      state.sink->Append(state.precision, '0');
+    }
+    state.sink->Append(padding.right_spaces, ' ');
+  });
+}
+
+// Slow %f formatter for when the shifted value does not fit in a uint128, and
+// `exp < 0`.
+// Prints `v*2^exp` with the options from `state`.
+// This one is guaranteed to be < 1.0, so we don't have to worry about integral
+// digits.
+void FormatFNegativeExpSlow(uint128 v, int exp, const FormatState& state,
+                            size_t digits_to_trim = 0) {
+  const bool print_dot =
+      (state.precision > digits_to_trim) || state.conv.has_alt_flag();
+  const size_t total_digits =
+      /* 0 */ 1 + (print_dot ? (state.precision - digits_to_trim) + 1 : 0);
+  auto padding =
+      ExtraWidthToPadding(total_digits + (state.sign_char ? 1 : 0), state);
+  padding.zeros += 1;
+  state.sink->Append(padding.left_spaces, ' ');
+  if (state.sign_char != '\0') state.sink->Append(1, state.sign_char);
+  state.sink->Append(padding.zeros, '0');
+  if (print_dot) state.sink->Append(1, '.');
+  // Print digits
+  size_t digits_to_go = state.precision - digits_to_trim;
+
+  FractionalDigitGenerator::RunConversion(
+      v, exp, [&](FractionalDigitGenerator digit_gen) {
+        // There are no digits to print here.
+        if (state.precision == 0) return;
+
+        // We go one digit at a time, while keeping track of runs of nines.
+        // The runs of nines are used to perform rounding when necessary.
+
+        while (digits_to_go > 0 && digit_gen.HasMoreDigits()) {
+          auto digits = digit_gen.GetDigits();
+
+          // Now we have a digit and a run of nines.
+          // See if we can print them all.
+          if (digits.num_nines + 1 < digits_to_go) {
+            // We don't have to round yet, so print them.
+            state.sink->Append(1, digits.digit_before_nine + '0');
+            state.sink->Append(digits.num_nines, '9');
+            digits_to_go -= digits.num_nines + 1;
+
+          } else {
+            // We can't print all the nines, see where we have to truncate.
+
+            bool round_up = false;
+            if (digits.num_nines + 1 > digits_to_go) {
+              // We round up at a nine. No need to print them.
+              round_up = true;
+            } else {
+              // We can fit all the nines, but truncate just after it.
+              if (digit_gen.IsGreaterThanHalf()) {
+                round_up = true;
+              } else if (digit_gen.IsExactlyHalf()) {
+                // Round to even
+                round_up =
+                    digits.num_nines != 0 || digits.digit_before_nine % 2 == 1;
+              }
+            }
+
+            if (round_up) {
+              state.sink->Append(1, digits.digit_before_nine + '1');
+              --digits_to_go;
+              // The rest will be zeros.
+            } else {
+              state.sink->Append(1, digits.digit_before_nine + '0');
+              state.sink->Append(digits_to_go - 1, '9');
+              digits_to_go = 0;
+            }
+            return;
+          }
+        }
+      });
+
+  state.sink->Append(digits_to_go, '0');
+  state.sink->Append(padding.right_spaces, ' ');
+}
+
+template <typename Int>
+void FormatF(Int mantissa, int exp, const FormatState &state) {
+  if (exp >= 0) {
+    const int total_bits =
+        static_cast<int>(sizeof(Int) * 8) - LeadingZeros(mantissa) + exp;
+
+    // Fallback to the slow stack-based approach if we can't do it in a 64 or
+    // 128 bit state.
+    if (ABSL_PREDICT_FALSE(total_bits > 128)) {
+      return FormatFPositiveExpSlow(mantissa, exp, state);
+    }
+  } else {
+    // Fallback to the slow stack-based approach if we can't do it in a 64 or
+    // 128 bit state.
+    if (ABSL_PREDICT_FALSE(exp < -128)) {
+      return FormatFNegativeExpSlow(mantissa, -exp, state);
+    }
+  }
+  return FormatFFast(mantissa, exp, state);
+}
+
+// Grab the group of four bits (nibble) from `n`. E.g., nibble 1 corresponds to
+// bits 4-7.
+template <typename Int>
+uint8_t GetNibble(Int n, size_t nibble_index) {
+  constexpr Int mask_low_nibble = Int{0xf};
+  int shift = static_cast<int>(nibble_index * 4);
+  n &= mask_low_nibble << shift;
+  return static_cast<uint8_t>((n >> shift) & 0xf);
+}
+
+// Add one to the given nibble, applying carry to higher nibbles. Returns true
+// if overflow, false otherwise.
+template <typename Int>
+bool IncrementNibble(size_t nibble_index, Int* n) {
+  constexpr size_t kShift = sizeof(Int) * 8 - 1;
+  constexpr size_t kNumNibbles = sizeof(Int) * 8 / 4;
+  Int before = *n >> kShift;
+  // Here we essentially want to take the number 1 and move it into the
+  // requested nibble, then add it to *n to effectively increment the nibble.
+  // However, ASan will complain if we try to shift the 1 beyond the limits of
+  // the Int, i.e., if the nibble_index is out of range. So therefore we check
+  // for this and if we are out of range we just add 0 which leaves *n
+  // unchanged, which seems like the reasonable thing to do in that case.
+  *n += ((nibble_index >= kNumNibbles)
+             ? 0
+             : (Int{1} << static_cast<int>(nibble_index * 4)));
+  Int after = *n >> kShift;
+  return (before && !after) || (nibble_index >= kNumNibbles);
+}
+
+// Return a mask with 1's in the given nibble and all lower nibbles.
+template <typename Int>
+Int MaskUpToNibbleInclusive(size_t nibble_index) {
+  constexpr size_t kNumNibbles = sizeof(Int) * 8 / 4;
+  static const Int ones = ~Int{0};
+  ++nibble_index;
+  return ones >> static_cast<int>(
+                     4 * (std::max(kNumNibbles, nibble_index) - nibble_index));
+}
+
+// Return a mask with 1's below the given nibble.
+template <typename Int>
+Int MaskUpToNibbleExclusive(size_t nibble_index) {
+  return nibble_index == 0 ? 0 : MaskUpToNibbleInclusive<Int>(nibble_index - 1);
+}
+
+template <typename Int>
+Int MoveToNibble(uint8_t nibble, size_t nibble_index) {
+  return Int{nibble} << static_cast<int>(4 * nibble_index);
+}
+
+// Given mantissa size, find optimal # of mantissa bits to put in initial digit.
+//
+// In the hex representation we keep a single hex digit to the left of the dot.
+// However, the question as to how many bits of the mantissa should be put into
+// that hex digit in theory is arbitrary, but in practice it is optimal to
+// choose based on the size of the mantissa. E.g., for a `double`, there are 53
+// mantissa bits, so that means that we should put 1 bit to the left of the dot,
+// thereby leaving 52 bits to the right, which is evenly divisible by four and
+// thus all fractional digits represent actual precision. For a `long double`,
+// on the other hand, there are 64 bits of mantissa, thus we can use all four
+// bits for the initial hex digit and still have a number left over (60) that is
+// a multiple of four. Once again, the goal is to have all fractional digits
+// represent real precision.
+template <typename Float>
+constexpr size_t HexFloatLeadingDigitSizeInBits() {
+  return std::numeric_limits<Float>::digits % 4 > 0
+             ? static_cast<size_t>(std::numeric_limits<Float>::digits % 4)
+             : size_t{4};
+}
+
+// This function captures the rounding behavior of glibc for hex float
+// representations. E.g. when rounding 0x1.ab800000 to a precision of .2
+// ("%.2a") glibc will round up because it rounds toward the even number (since
+// 0xb is an odd number, it will round up to 0xc). However, when rounding at a
+// point that is not followed by 800000..., it disregards the parity and rounds
+// up if > 8 and rounds down if < 8.
+template <typename Int>
+bool HexFloatNeedsRoundUp(Int mantissa,
+                          size_t final_nibble_displayed,
+                          uint8_t leading) {
+  // If the last nibble (hex digit) to be displayed is the lowest on in the
+  // mantissa then that means that we don't have any further nibbles to inform
+  // rounding, so don't round.
+  if (final_nibble_displayed == 0) {
+    return false;
+  }
+  size_t rounding_nibble_idx = final_nibble_displayed - 1;
+  constexpr size_t kTotalNibbles = sizeof(Int) * 8 / 4;
+  assert(final_nibble_displayed <= kTotalNibbles);
+  Int mantissa_up_to_rounding_nibble_inclusive =
+      mantissa & MaskUpToNibbleInclusive<Int>(rounding_nibble_idx);
+  Int eight = MoveToNibble<Int>(8, rounding_nibble_idx);
+  if (mantissa_up_to_rounding_nibble_inclusive != eight) {
+    return mantissa_up_to_rounding_nibble_inclusive > eight;
+  }
+  // Nibble in question == 8.
+  uint8_t round_if_odd = (final_nibble_displayed == kTotalNibbles)
+                             ? leading
+                             : GetNibble(mantissa, final_nibble_displayed);
+  return round_if_odd % 2 == 1;
+}
+
+// Stores values associated with a Float type needed by the FormatA
+// implementation in order to avoid templatizing that function by the Float
+// type.
+struct HexFloatTypeParams {
+  template <typename Float>
+  explicit HexFloatTypeParams(Float)
+      : min_exponent(std::numeric_limits<Float>::min_exponent - 1),
+        leading_digit_size_bits(HexFloatLeadingDigitSizeInBits<Float>()) {
+    assert(leading_digit_size_bits >= 1 && leading_digit_size_bits <= 4);
+  }
+
+  int min_exponent;
+  size_t leading_digit_size_bits;
+};
+
+// Hex Float Rounding. First check if we need to round; if so, then we do that
+// by manipulating (incrementing) the mantissa, that way we can later print the
+// mantissa digits by iterating through them in the same way regardless of
+// whether a rounding happened.
+template <typename Int>
+void FormatARound(bool precision_specified, const FormatState &state,
+                  uint8_t *leading, Int *mantissa, int *exp) {
+  constexpr size_t kTotalNibbles = sizeof(Int) * 8 / 4;
+  // Index of the last nibble that we could display given precision.
+  size_t final_nibble_displayed =
+      precision_specified
+          ? (std::max(kTotalNibbles, state.precision) - state.precision)
+          : 0;
+  if (HexFloatNeedsRoundUp(*mantissa, final_nibble_displayed, *leading)) {
+    // Need to round up.
+    bool overflow = IncrementNibble(final_nibble_displayed, mantissa);
+    *leading += (overflow ? 1 : 0);
+    if (ABSL_PREDICT_FALSE(*leading > 15)) {
+      // We have overflowed the leading digit. This would mean that we would
+      // need two hex digits to the left of the dot, which is not allowed. So
+      // adjust the mantissa and exponent so that the result is always 1.0eXXX.
+      *leading = 1;
+      *mantissa = 0;
+      *exp += 4;
+    }
+  }
+  // Now that we have handled a possible round-up we can go ahead and zero out
+  // all the nibbles of the mantissa that we won't need.
+  if (precision_specified) {
+    *mantissa &= ~MaskUpToNibbleExclusive<Int>(final_nibble_displayed);
+  }
+}
+
+template <typename Int>
+void FormatANormalize(const HexFloatTypeParams float_traits, uint8_t *leading,
+                      Int *mantissa, int *exp) {
+  constexpr size_t kIntBits = sizeof(Int) * 8;
+  static const Int kHighIntBit = Int{1} << (kIntBits - 1);
+  const size_t kLeadDigitBitsCount = float_traits.leading_digit_size_bits;
+  // Normalize mantissa so that highest bit set is in MSB position, unless we
+  // get interrupted by the exponent threshold.
+  while (*mantissa && !(*mantissa & kHighIntBit)) {
+    if (ABSL_PREDICT_FALSE(*exp - 1 < float_traits.min_exponent)) {
+      *mantissa >>= (float_traits.min_exponent - *exp);
+      *exp = float_traits.min_exponent;
+      return;
+    }
+    *mantissa <<= 1;
+    --*exp;
+  }
+  // Extract bits for leading digit then shift them away leaving the
+  // fractional part.
+  *leading = static_cast<uint8_t>(
+      *mantissa >> static_cast<int>(kIntBits - kLeadDigitBitsCount));
+  *exp -= (*mantissa != 0) ? static_cast<int>(kLeadDigitBitsCount) : *exp;
+  *mantissa <<= static_cast<int>(kLeadDigitBitsCount);
+}
+
+template <typename Int>
+void FormatA(const HexFloatTypeParams float_traits, Int mantissa, int exp,
+             bool uppercase, const FormatState &state) {
+  // Int properties.
+  constexpr size_t kIntBits = sizeof(Int) * 8;
+  constexpr size_t kTotalNibbles = sizeof(Int) * 8 / 4;
+  // Did the user specify a precision explicitly?
+  const bool precision_specified = state.conv.precision() >= 0;
+
+  // ========== Normalize/Denormalize ==========
+  exp += kIntBits;  // make all digits fractional digits.
+  // This holds the (up to four) bits of leading digit, i.e., the '1' in the
+  // number 0x1.e6fp+2. It's always > 0 unless number is zero or denormal.
+  uint8_t leading = 0;
+  FormatANormalize(float_traits, &leading, &mantissa, &exp);
+
+  // =============== Rounding ==================
+  // Check if we need to round; if so, then we do that by manipulating
+  // (incrementing) the mantissa before beginning to print characters.
+  FormatARound(precision_specified, state, &leading, &mantissa, &exp);
+
+  // ============= Format Result ===============
+  // This buffer holds the "0x1.ab1de3" portion of "0x1.ab1de3pe+2". Compute the
+  // size with long double which is the largest of the floats.
+  constexpr size_t kBufSizeForHexFloatRepr =
+      2                                                // 0x
+      + std::numeric_limits<MaxFloatType>::digits / 4  // number of hex digits
+      + 1                                              // round up
+      + 1;                                             // "." (dot)
+  char digits_buffer[kBufSizeForHexFloatRepr];
+  char *digits_iter = digits_buffer;
+  const char *const digits =
+      static_cast<const char *>("0123456789ABCDEF0123456789abcdef") +
+      (uppercase ? 0 : 16);
+
+  // =============== Hex Prefix ================
+  *digits_iter++ = '0';
+  *digits_iter++ = uppercase ? 'X' : 'x';
+
+  // ========== Non-Fractional Digit ===========
+  *digits_iter++ = digits[leading];
+
+  // ================== Dot ====================
+  // There are three reasons we might need a dot. Keep in mind that, at this
+  // point, the mantissa holds only the fractional part.
+  if ((precision_specified && state.precision > 0) ||
+      (!precision_specified && mantissa > 0) || state.conv.has_alt_flag()) {
+    *digits_iter++ = '.';
+  }
+
+  // ============ Fractional Digits ============
+  size_t digits_emitted = 0;
+  while (mantissa > 0) {
+    *digits_iter++ = digits[GetNibble(mantissa, kTotalNibbles - 1)];
+    mantissa <<= 4;
+    ++digits_emitted;
+  }
+  size_t trailing_zeros = 0;
+  if (precision_specified) {
+    assert(state.precision >= digits_emitted);
+    trailing_zeros = state.precision - digits_emitted;
+  }
+  auto digits_result = string_view(
+      digits_buffer, static_cast<size_t>(digits_iter - digits_buffer));
+
+  // =============== Exponent ==================
+  constexpr size_t kBufSizeForExpDecRepr =
+      numbers_internal::kFastToBufferSize  // required for FastIntToBuffer
+      + 1                                  // 'p' or 'P'
+      + 1;                                 // '+' or '-'
+  char exp_buffer[kBufSizeForExpDecRepr];
+  exp_buffer[0] = uppercase ? 'P' : 'p';
+  exp_buffer[1] = exp >= 0 ? '+' : '-';
+  numbers_internal::FastIntToBuffer(exp < 0 ? -exp : exp, exp_buffer + 2);
+
+  // ============ Assemble Result ==============
+  FinalPrint(state,
+             digits_result,                        // 0xN.NNN...
+             2,                                    // offset of any padding
+             static_cast<size_t>(trailing_zeros),  // remaining mantissa padding
+             exp_buffer);                          // exponent
+}
+
+char *CopyStringTo(absl::string_view v, char *out) {
   std::memcpy(out, v.data(), v.size());
   return out + v.size();
 }
 
 template <typename Float>
-bool FallbackToSnprintf(const Float v, const ConversionSpec &conv,
+bool FallbackToSnprintf(const Float v, const FormatConversionSpecImpl &conv,
                         FormatSinkImpl *sink) {
   int w = conv.width() >= 0 ? conv.width() : 0;
   int p = conv.precision() >= 0 ? conv.precision() : -1;
@@ -27,25 +1048,25 @@ bool FallbackToSnprintf(const Float v, const ConversionSpec &conv,
   {
     char *fp = fmt;
     *fp++ = '%';
-    fp = CopyStringTo(conv.flags().ToString(), fp);
+    fp = CopyStringTo(FormatConversionSpecImplFriend::FlagsToString(conv), fp);
     fp = CopyStringTo("*.*", fp);
     if (std::is_same<long double, Float>()) {
       *fp++ = 'L';
     }
-    *fp++ = conv.conv().Char();
+    *fp++ = FormatConversionCharToChar(conv.conversion_char());
     *fp = 0;
     assert(fp < fmt + sizeof(fmt));
   }
   std::string space(512, '\0');
-  string_view result;
+  absl::string_view result;
   while (true) {
     int n = snprintf(&space[0], space.size(), fmt, w, p, v);
     if (n < 0) return false;
     if (static_cast<size_t>(n) < space.size()) {
-      result = string_view(space.data(), n);
+      result = absl::string_view(space.data(), static_cast<size_t>(n));
       break;
     }
-    space.resize(n + 1);
+    space.resize(static_cast<size_t>(n) + 1);
   }
   sink->Append(result);
   return true;
@@ -53,13 +1074,13 @@ bool FallbackToSnprintf(const Float v, const ConversionSpec &conv,
 
 // 128-bits in decimal: ceil(128*log(2)/log(10))
 //   or std::numeric_limits<__uint128_t>::digits10
-constexpr int kMaxFixedPrecision = 39;
+constexpr size_t kMaxFixedPrecision = 39;
 
-constexpr int kBufferLength = /*sign*/ 1 +
-                              /*integer*/ kMaxFixedPrecision +
-                              /*point*/ 1 +
-                              /*fraction*/ kMaxFixedPrecision +
-                              /*exponent e+123*/ 5;
+constexpr size_t kBufferLength = /*sign*/ 1 +
+                                 /*integer*/ kMaxFixedPrecision +
+                                 /*point*/ 1 +
+                                 /*fraction*/ kMaxFixedPrecision +
+                                 /*exponent e+123*/ 5;
 
 struct Buffer {
   void push_front(char c) {
@@ -75,14 +1096,14 @@ struct Buffer {
     --end;
   }
 
-  char &back() {
+  char &back() const {
     assert(begin < end);
     return end[-1];
   }
 
   char last_digit() const { return end[-1] == '.' ? end[-2] : end[-1]; }
 
-  int size() const { return static_cast<int>(end - begin); }
+  size_t size() const { return static_cast<size_t>(end - begin); }
 
   char data[kBufferLength];
   char *begin;
@@ -95,19 +1116,25 @@ enum class FormatStyle { Fixed, Precision };
 // Otherwise, return false.
 template <typename Float>
 bool ConvertNonNumericFloats(char sign_char, Float v,
-                             const ConversionSpec &conv, FormatSinkImpl *sink) {
+                             const FormatConversionSpecImpl &conv,
+                             FormatSinkImpl *sink) {
   char text[4], *ptr = text;
-  if (sign_char) *ptr++ = sign_char;
+  if (sign_char != '\0') *ptr++ = sign_char;
   if (std::isnan(v)) {
-    ptr = std::copy_n(conv.conv().upper() ? "NAN" : "nan", 3, ptr);
+    ptr = std::copy_n(
+        FormatConversionCharIsUpper(conv.conversion_char()) ? "NAN" : "nan", 3,
+        ptr);
   } else if (std::isinf(v)) {
-    ptr = std::copy_n(conv.conv().upper() ? "INF" : "inf", 3, ptr);
+    ptr = std::copy_n(
+        FormatConversionCharIsUpper(conv.conversion_char()) ? "INF" : "inf", 3,
+        ptr);
   } else {
     return false;
   }
 
-  return sink->PutPaddedString(string_view(text, ptr - text), conv.width(), -1,
-                               conv.flags().left);
+  return sink->PutPaddedString(
+      string_view(text, static_cast<size_t>(ptr - text)), conv.width(), -1,
+      conv.has_left_flag());
 }
 
 // Round up the last digit of the value.
@@ -134,40 +1161,23 @@ void RoundUp(Buffer *buffer, int *exp) {
   }
 }
 
-void PrintExponent(int exp, char e, Buffer *out) {
-  out->push_back(e);
-  if (exp < 0) {
-    out->push_back('-');
-    exp = -exp;
-  } else {
-    out->push_back('+');
-  }
-  // Exponent digits.
-  if (exp > 99) {
-    out->push_back(exp / 100 + '0');
-    out->push_back(exp / 10 % 10 + '0');
-    out->push_back(exp % 10 + '0');
-  } else {
-    out->push_back(exp / 10 + '0');
-    out->push_back(exp % 10 + '0');
-  }
-}
-
 template <typename Float, typename Int>
 constexpr bool CanFitMantissa() {
   return
-#if defined(__clang__) && !defined(__SSE3__)
+#if defined(__clang__) && (__clang_major__ < 9) && !defined(__SSE3__)
       // Workaround for clang bug: https://bugs.llvm.org/show_bug.cgi?id=38289
       // Casting from long double to uint64_t is miscompiled and drops bits.
-      (!std::is_same<Float, long double>::value ||
-       !std::is_same<Int, uint64_t>::value) &&
+      (!std::is_same_v<Float, long double> || !std::is_same_v<Int, uint64_t>) &&
 #endif
       std::numeric_limits<Float>::digits <= std::numeric_limits<Int>::digits;
 }
 
 template <typename Float>
 struct Decomposed {
-  Float mantissa;
+  using MantissaType =
+      std::conditional_t<std::is_same_v<long double, Float>, uint128, uint64_t>;
+  static_assert(std::numeric_limits<Float>::digits <= sizeof(MantissaType) * 8);
+  MantissaType mantissa;
   int exponent;
 };
 
@@ -178,15 +1188,16 @@ Decomposed<Float> Decompose(Float v) {
   Float m = std::frexp(v, &exp);
   m = std::ldexp(m, std::numeric_limits<Float>::digits);
   exp -= std::numeric_limits<Float>::digits;
-  return {m, exp};
+
+  return {static_cast<typename Decomposed<Float>::MantissaType>(m), exp};
 }
 
 // Print 'digits' as decimal.
 // In Fixed mode, we add a '.' at the end.
 // In Precision mode, we add a '.' after the first digit.
 template <FormatStyle mode, typename Int>
-int PrintIntegralDigits(Int digits, Buffer *out) {
-  int printed = 0;
+size_t PrintIntegralDigits(Int digits, Buffer* out) {
+  size_t printed = 0;
   if (digits) {
     for (; digits; digits /= 10) out->push_front(digits % 10 + '0');
     printed = out->size();
@@ -204,174 +1215,835 @@ int PrintIntegralDigits(Int digits, Buffer *out) {
   return printed;
 }
 
-// Back out 'extra_digits' digits and round up if necessary.
-bool RemoveExtraPrecision(int extra_digits, bool has_leftover_value,
-                          Buffer *out, int *exp_out) {
-  if (extra_digits <= 0) return false;
+std::optional<int> GetOneDigit(BinaryToDecimal& btd,
+                               absl::string_view& digits_view) {
+  if (digits_view.empty() && !btd.AdvanceDigits()) {
+    return std::nullopt;
+  }
+  char d = digits_view.front();
+  digits_view.remove_prefix(1);
+  return d - '0';
+}
 
-  // Back out the extra digits
-  out->end -= extra_digits;
+struct DigitRun {
+  std::optional<int> digit;
+  size_t nines;
+};
 
-  bool needs_to_round_up = [&] {
-    // We look at the digit just past the end.
-    // There must be 'extra_digits' extra valid digits after end.
-    if (*out->end > '5') return true;
-    if (*out->end < '5') return false;
-    if (has_leftover_value || std::any_of(out->end + 1, out->end + extra_digits,
-                                          [](char c) { return c != '0'; }))
-      return true;
+DigitRun GetDigits(BinaryToDecimal& btd, absl::string_view& digits_view) {
+  auto peek_digit = [&]() -> std::optional<int> {
+    if (digits_view.empty()) {
+      if (!btd.AdvanceDigits()) return std::nullopt;
+      digits_view = btd.CurrentDigits();
+    }
+    return digits_view.front() - '0';
+  };
 
-    // Ends in ...50*, round to even.
-    return out->last_digit() % 2 == 1;
-  }();
+  auto digit_before_nines = GetOneDigit(btd, digits_view);
+  if (!digit_before_nines.has_value()) return {std::nullopt, 0};
 
-  if (needs_to_round_up) {
-    RoundUp<FormatStyle::Precision>(out, exp_out);
+  auto next_digit = peek_digit();
+  size_t num_nines = 0;
+  while (next_digit == 9) {
+    // consume the 9
+    GetOneDigit(btd, digits_view);
+    ++num_nines;
+    next_digit = peek_digit();
   }
-  return true;
+  return digit_before_nines == 9 ? DigitRun{std::nullopt, num_nines + 1}
+                                 : DigitRun{digit_before_nines, num_nines};
 }
 
-// Print the value into the buffer.
-// This will not include the exponent, which will be returned in 'exp_out' for
-// Precision mode.
-template <typename Int, typename Float, FormatStyle mode>
-bool FloatToBufferImpl(Int int_mantissa, int exp, int precision, Buffer *out,
-                       int *exp_out) {
-  assert((CanFitMantissa<Float, Int>()));
+template <typename Int>
+void FormatE(Int mantissa, int exp, bool uppercase, const FormatState& state) {
+  if (exp > 0) {
+    const int total_bits =
+        static_cast<int>(sizeof(Int) * 8) - LeadingZeros(mantissa) + exp;
+    if (total_bits > 128) {
+      FormatEPositiveExpSlow(mantissa, exp, uppercase, state);
+      return;
+    }
+  } else {
+    if (ABSL_PREDICT_FALSE(exp < -128)) {
+      FormatENegativeExpSlow(mantissa, exp, uppercase, state);
+      return;
+    }
+  }
+  FormatEFast(mantissa, exp, uppercase, state);
+}
 
-  const int int_bits = std::numeric_limits<Int>::digits;
+// Guaranteed to fit into 128 bits at this point
+template <typename Int>
+void FormatEFast(Int v, int exp, bool uppercase, const FormatState& state) {
+  if (!v) {
+    absl::string_view mantissa_str = state.ShouldPrintDot() ? "0." : "0";
+    FinalPrint(state, mantissa_str, 0, state.precision,
+               uppercase ? "E+00" : "e+00");
+    return;
+  }
+  constexpr int kInputBits = sizeof(Int) * 8;
+  constexpr int kMaxFractionalDigits = 128;
+  constexpr int kBufferSize = 2 +                    // '.' + rounding
+                              kMaxFixedPrecision +   // Integral
+                              kMaxFractionalDigits;  // Fractional
+  const int total_bits = kInputBits - LeadingZeros(v) + exp;
+  char buffer[kBufferSize];
+  char* integral_start = buffer + 2;
+  char* integral_end = buffer + 2 + kMaxFixedPrecision;
+  char* final_start;
+  char* final_end;
+  bool zero_integral = false;
+  int scientific_exp = 0;
+  size_t digits_printed = 0;
+  size_t trailing_zeros = 0;
+  bool has_more_non_zero = false;
 
-  // In precision mode, we start printing one char to the right because it will
-  // also include the '.'
-  // In fixed mode we put the dot afterwards on the right.
-  out->begin = out->end =
-      out->data + 1 + kMaxFixedPrecision + (mode == FormatStyle::Precision);
+  auto check_integral_zeros =
+      [](char* const begin, char* const end,
+         const size_t precision, size_t digits_processed) -> bool {
+    // When considering rounding to even, we care about the digits after the
+    // round digit which means the total digits to move from the start is
+    // precision + 2 since the first digit we print before the decimal point
+    // is not a part of precision.
+    size_t digit_upper_bound = precision + 2;
+    if (digits_processed > digit_upper_bound) {
+      return std::any_of(begin + digit_upper_bound, end,
+                         [](char c) { return c != '0'; });
+    }
+    return false;
+  };
 
   if (exp >= 0) {
-    if (std::numeric_limits<Float>::digits + exp > int_bits) {
-      // The value will overflow the Int
-      return false;
+    integral_end = total_bits <= 64 ? numbers_internal::FastIntToBuffer(
+                               static_cast<uint64_t>(v) << exp, integral_start)
+                         : numbers_internal::FastIntToBuffer(
+                               static_cast<uint128>(v) << exp, integral_start);
+    *integral_end = '0';
+    final_start = integral_start;
+    // Integral is guaranteed to be non-zero at this point.
+    scientific_exp = static_cast<int>(integral_end - integral_start) - 1;
+    digits_printed = static_cast<size_t>(integral_end - integral_start);
+    final_end = integral_end;
+    has_more_non_zero = check_integral_zeros(integral_start, integral_end,
+                                             state.precision, digits_printed);
+  } else {
+    exp = -exp;
+    if (exp < kInputBits) {
+      integral_end =
+          numbers_internal::FastIntToBuffer(v >> exp, integral_start);
     }
-    int digits_printed = PrintIntegralDigits<mode>(int_mantissa << exp, out);
-    int digits_to_zero_pad = precision;
-    if (mode == FormatStyle::Precision) {
-      *exp_out = digits_printed - 1;
-      digits_to_zero_pad -= digits_printed - 1;
-      if (RemoveExtraPrecision(-digits_to_zero_pad, false, out, exp_out)) {
-        return true;
+    *integral_end = '0';
+    // We didn't move integral_start and it gets set to 0 in
+    zero_integral = exp >= kInputBits || v >> exp == 0;
+    if (!zero_integral) {
+      digits_printed = static_cast<size_t>(integral_end - integral_start);
+      has_more_non_zero = check_integral_zeros(integral_start, integral_end,
+                                               state.precision, digits_printed);
+      final_end = integral_end;
+    }
+    // Print fractional digits
+    char* fractional_start = integral_end;
+
+    size_t digits_to_print = (state.precision + 1) >= digits_printed
+                                 ? state.precision + 1 - digits_printed
+                                 : 0;
+    bool print_extra = digits_printed <= state.precision + 1;
+    auto [fractional_end, skipped_zeros, has_nonzero_rem] =
+        exp <= 64 ? PrintFractionalDigitsScientific(
+                        v, fractional_start, exp, digits_to_print + print_extra,
+                        zero_integral)
+                  : PrintFractionalDigitsScientific(
+                        static_cast<uint128>(v), fractional_start, exp,
+                        digits_to_print + print_extra, zero_integral);
+    final_end = fractional_end;
+    *fractional_end = '0';
+    has_more_non_zero |= has_nonzero_rem;
+    digits_printed += static_cast<size_t>(fractional_end - fractional_start);
+    if (zero_integral) {
+      scientific_exp = -1 * static_cast<int>(skipped_zeros + 1);
+    } else {
+      scientific_exp = static_cast<int>(integral_end - integral_start) - 1;
+    }
+    // Don't do any rounding here, we will do it ourselves.
+    final_start = zero_integral ? fractional_start : integral_start;
+  }
+
+  // For rounding
+  if (digits_printed >= state.precision + 1) {
+    final_start[-1] = '0';
+    char* round_digit_ptr = final_start + 1 + state.precision;
+    if (*round_digit_ptr > '5') {
+      RoundUp(round_digit_ptr - 1);
+    } else if (*round_digit_ptr == '5') {
+      if (has_more_non_zero) {
+        RoundUp(round_digit_ptr - 1);
+      } else {
+        RoundToEven(round_digit_ptr - 1);
       }
     }
-    for (; digits_to_zero_pad-- > 0;) out->push_back('0');
-    return true;
+    final_end = round_digit_ptr;
+    if (final_start[-1] == '1') {
+      --final_start;
+      ++scientific_exp;
+      --final_end;
+    }
+  } else {
+    // Need to pad with zeros.
+    trailing_zeros = state.precision - (digits_printed - 1);
   }
 
-  exp = -exp;
-  // We need at least 4 empty bits for the next decimal digit.
-  // We will multiply by 10.
-  if (exp > int_bits - 4) return false;
+  if (state.precision > 0 || state.ShouldPrintDot()) {
+    final_start[-1] = *final_start;
+    *final_start = '.';
+    --final_start;
+  }
+
+  // We need to add 2 to the buffer size for the +/- sign and the e
+  constexpr size_t kExpBufferSize = numbers_internal::kFastToBufferSize + 2;
+  char exp_buffer[kExpBufferSize];
+  char* exp_ptr_start = exp_buffer;
+  char* exp_ptr = exp_ptr_start;
+  *exp_ptr++ = uppercase ? 'E' : 'e';
+  if (scientific_exp >= 0) {
+    *exp_ptr++ = '+';
+  } else {
+    *exp_ptr++ = '-';
+    scientific_exp = -scientific_exp;
+  }
 
-  const Int mask = (Int{1} << exp) - 1;
+  if (scientific_exp < 10) {
+    *exp_ptr++ = '0';
+  }
+  exp_ptr = numbers_internal::FastIntToBuffer(scientific_exp, exp_ptr);
+  FinalPrint(state,
+             absl::string_view(final_start,
+                               static_cast<size_t>(final_end - final_start)),
+             0, trailing_zeros,
+             absl::string_view(exp_ptr_start,
+                               static_cast<size_t>(exp_ptr - exp_ptr_start)));
+}
 
-  // Print the integral part first.
-  int digits_printed = PrintIntegralDigits<mode>(int_mantissa >> exp, out);
-  int_mantissa &= mask;
+void FormatENegativeExpSlow(uint128 mantissa, int exp, bool uppercase,
+                            const FormatState& state,
+                            size_t digits_to_trim = 0) {
+  assert(exp < 0);
 
-  int fractional_count = precision;
-  if (mode == FormatStyle::Precision) {
-    if (digits_printed == 0) {
-      // Find the first non-zero digit, when in Precision mode.
-      *exp_out = 0;
-      if (int_mantissa) {
-        while (int_mantissa <= mask) {
-          int_mantissa *= 10;
-          --*exp_out;
+  FractionalDigitGenerator::RunConversion(
+      mantissa, -exp,
+      [&](FractionalDigitGenerator digit_gen) {
+        int first_digit = 0;
+        size_t nines = 0;
+        int num_leading_zeros = 0;
+        while (digit_gen.HasMoreDigits()) {
+          auto digits = digit_gen.GetDigits();
+          if (digits.digit_before_nine != 0) {
+            first_digit = digits.digit_before_nine;
+            nines = digits.num_nines;
+            break;
+          } else if (digits.num_nines > 0) {
+            // This also means the first digit is 0
+            first_digit = 9;
+            nines = digits.num_nines - 1;
+            num_leading_zeros++;
+            break;
+          }
+          num_leading_zeros++;
         }
-      }
-      out->push_front(static_cast<char>(int_mantissa >> exp) + '0');
-      out->push_back('.');
-      int_mantissa &= mask;
-    } else {
-      // We already have a digit, and a '.'
-      *exp_out = digits_printed - 1;
-      fractional_count -= *exp_out;
-      if (RemoveExtraPrecision(-fractional_count, int_mantissa != 0, out,
-                               exp_out)) {
-        // If we had enough digits, return right away.
-        // The code below will try to round again otherwise.
-        return true;
-      }
-    }
+        size_t precision = state.precision;
+        if (precision > digits_to_trim) {
+          precision -= digits_to_trim;
+        } else {
+          precision = 0;
+        }
+        bool change_to_zeros = false;
+        if (nines >= precision || state.precision == 0) {
+          bool round_up = false;
+          if (nines == precision) {
+            round_up = digit_gen.IsGreaterThanHalf();
+          } else {
+            round_up = nines > 0 || digit_gen.IsGreaterThanHalf();
+          }
+          if (round_up) {
+            first_digit = (first_digit == 9 ? 1 : first_digit + 1);
+            num_leading_zeros -= (first_digit == 1);
+            change_to_zeros = true;
+          }
+        }
+        int scientific_exp = -(num_leading_zeros + 1);
+        assert(scientific_exp < 0);
+        char exp_buffer[numbers_internal::kFastToBufferSize];
+        char* exp_start = exp_buffer;
+        *exp_start++ = '-';
+        if (scientific_exp > -10) {
+          *exp_start++ = '0';
+        }
+        scientific_exp *= -1;
+        char* exp_end =
+            numbers_internal::FastIntToBuffer(scientific_exp, exp_start);
+        const size_t total_digits =
+            1  // First digit
+            +
+            ((precision > 0 || state.conv.has_alt_flag()) ? 1
+                                                          : 0)  // Decimal point
+            + precision  // Digits after decimal
+            + 1          // 'e' or 'E'
+            + static_cast<size_t>(exp_end - exp_buffer);  // Exponent digits
+
+        const auto padding = ExtraWidthToPadding(
+            total_digits + (state.sign_char != '\0' ? 1 : 0), state);
+        state.sink->Append(padding.left_spaces, ' ');
+
+        if (state.sign_char != '\0') {
+          state.sink->Append(1, state.sign_char);
+        }
+
+        state.sink->Append(1, static_cast<char>(first_digit + '0'));
+        if (precision > 0 || state.conv.has_alt_flag()) {
+          state.sink->Append(1, '.');
+        }
+        size_t digits_to_go = precision;
+        size_t nines_to_print = std::min(nines, digits_to_go);
+        state.sink->Append(nines_to_print, change_to_zeros ? '0' : '9');
+        digits_to_go -= nines_to_print;
+        while (digits_to_go > 0 && digit_gen.HasMoreDigits()) {
+          auto digits = digit_gen.GetDigits();
+
+          if (digits.num_nines + 1 < digits_to_go) {
+            state.sink->Append(1, digits.digit_before_nine + '0');
+            state.sink->Append(digits.num_nines, '9');
+            digits_to_go -= digits.num_nines + 1;
+          } else {
+            bool round_up = false;
+            if (digits.num_nines + 1 > digits_to_go) {
+              round_up = true;
+            } else if (digit_gen.IsGreaterThanHalf()) {
+              round_up = true;
+            } else if (digit_gen.IsExactlyHalf()) {
+              round_up =
+                  digits.num_nines != 0 || digits.digit_before_nine % 2 == 1;
+            }
+            if (round_up) {
+              state.sink->Append(1, digits.digit_before_nine + '1');
+              --digits_to_go;
+            } else {
+              state.sink->Append(1, digits.digit_before_nine + '0');
+              state.sink->Append(digits_to_go - 1, '9');
+              digits_to_go = 0;
+            }
+            break;
+          }
+        }
+        state.sink->Append(digits_to_go, '0');
+        state.sink->Append(1, uppercase ? 'E' : 'e');
+        state.sink->Append(absl::string_view(
+            exp_buffer, static_cast<size_t>(exp_end - exp_buffer)));
+        state.sink->Append(padding.right_spaces, ' ');
+      });
+}
+
+void FormatEPositiveExpSlow(uint128 mantissa, int exp, bool uppercase,
+                            const FormatState& state,
+                            size_t digits_to_trim = 0) {
+  BinaryToDecimal::RunConversion(
+      mantissa, exp, [&](BinaryToDecimal btd) {
+        int scientific_exp = static_cast<int>(btd.TotalDigits() - 1);
+        absl::string_view digits_view = btd.CurrentDigits();
+
+        size_t digits_to_go = state.precision + 1;
+        auto [first_digit_opt, nines] = GetDigits(btd, digits_view);
+        if (!first_digit_opt.has_value() && nines == 0) {
+          return;
+        }
+
+        int first_digit = first_digit_opt.value_or(9);
+        if (!first_digit_opt) {
+          --nines;
+        }
+
+        // At this point we are guaranteed to have some sort of first digit
+        bool change_to_zeros = false;
+        if (nines + 1 >= digits_to_go) {
+          // Everything we need to print is in the first DigitRun
+          auto next_digit_opt = GetDigits(btd, digits_view).digit;
+          if (nines == state.precision) {
+            change_to_zeros = next_digit_opt.value_or(0) > 4;
+          } else {
+            change_to_zeros = true;
+          }
+          if (change_to_zeros) {
+            if (first_digit != 9) {
+              first_digit = first_digit + 1;
+            } else {
+              first_digit = 1;
+              ++scientific_exp;
+            }
+          }
+        }
+
+        char exp_buffer[numbers_internal::kFastToBufferSize];
+        char* exp_buffer_end =
+            numbers_internal::FastIntToBuffer(scientific_exp, exp_buffer);
+        const bool print_dot =
+            (state.precision > digits_to_trim) || state.conv.has_alt_flag();
+        const size_t exp_size =
+            static_cast<size_t>(exp_buffer_end - exp_buffer) + 2 +
+            (scientific_exp < 10 ? 1 : 0);
+        const size_t total_digits_out = 1 + (print_dot ? 1 : 0) +
+                                        (state.precision - digits_to_trim) +
+                                        exp_size;
+
+        const auto padding = ExtraWidthToPadding(
+            total_digits_out + (state.sign_char != '\0' ? 1 : 0), state);
+
+        state.sink->Append(padding.left_spaces, ' ');
+        if (state.sign_char != '\0') {
+          state.sink->Append(1, state.sign_char);
+        }
+        state.sink->Append(1, static_cast<char>(first_digit + '0'));
+        --digits_to_go;
+        if (print_dot) {
+          state.sink->Append(1, '.');
+        }
+
+        size_t remaining_to_print = state.precision - digits_to_trim;
+        auto append_with_trim = [&](size_t count, char c) {
+          size_t to_append = std::min(count, remaining_to_print);
+          if (to_append > 0) {
+            state.sink->Append(to_append, c);
+            remaining_to_print -= to_append;
+          }
+        };
+
+        size_t nines_to_append = std::min(digits_to_go, nines);
+        append_with_trim(nines_to_append, change_to_zeros ? '0' : '9');
+        digits_to_go -= nines_to_append;
+
+        while (digits_to_go > 0) {
+          auto [digit_opt, curr_nines] = GetDigits(btd, digits_view);
+          if (!digit_opt.has_value()) break;
+          int digit = *digit_opt;
+          if (curr_nines + 1 < digits_to_go) {
+            append_with_trim(1, static_cast<char>(digit + '0'));
+            append_with_trim(curr_nines, '9');
+            digits_to_go -= curr_nines + 1;
+          } else {
+            bool need_round_up = false;
+            auto next_digit_opt = GetDigits(btd, digits_view).digit;
+            if (digits_to_go == 1) {
+              need_round_up = curr_nines > 0 || next_digit_opt > 4;
+            } else if (digits_to_go == curr_nines + 1) {
+              // Only round if next digit is > 4
+              need_round_up = next_digit_opt.value_or(0) > 4;
+            } else {
+              // we know we need to round since nine is after precision ends
+              need_round_up = true;
+            }
+            append_with_trim(1, static_cast<char>(digit + need_round_up + '0'));
+            append_with_trim(digits_to_go - 1, need_round_up ? '0' : '9');
+            digits_to_go = 0;
+          }
+        }
+
+        if (digits_to_go > 0) {
+          append_with_trim(digits_to_go, '0');
+        }
+
+        state.sink->Append(1, uppercase ? 'E' : 'e');
+        state.sink->Append(1, scientific_exp >= 0 ? '+' : '-');
+        if (scientific_exp < 10) {
+          state.sink->Append(1, '0');
+        }
+        state.sink->Append(absl::string_view(
+            exp_buffer, static_cast<size_t>(exp_buffer_end - exp_buffer)));
+        state.sink->Append(padding.right_spaces, ' ');
+      });
+}
+
+//
+template <typename Int>
+void FormatGFast(Int v, int exp, bool uppercase, const FormatState& state) {
+  if (!v) {
+    absl::string_view mantissa_str =
+        state.ShouldPrintDot() && state.conv.has_alt_flag() ? "0." : "0";
+    FinalPrint(state, mantissa_str, 0,
+               state.conv.has_alt_flag() * state.precision, "");
+    return;
   }
+  constexpr int kInputBits = sizeof(Int) * 8;
+  constexpr int kMaxFractionalDigits = 128;
+  // We need enough headroom to the left of our starting pointer to support
+  // a potential prefix shift for values between 1e-1 and 1e-4.
+  // The prefix "0.000" is 5 chars, plus potential rounding carry (1 char).
+  constexpr int kHeadroom = 32;
+  constexpr int kBufferSize = kHeadroom +           // headroom + rounding + '.'
+                              kMaxFixedPrecision +  // Integral
+                              kMaxFractionalDigits;  // Fractional
+  const int total_bits = kInputBits - LeadingZeros(v) + exp;
+  char buffer[kBufferSize];
+  char* integral_start = buffer + kHeadroom;
+  char* integral_end = buffer + kHeadroom + kMaxFixedPrecision;
+  char* final_start;
+  char* final_end;
+  bool zero_integral = false;
+  int scientific_exp = 0;
+  size_t digits_printed = 0;
+  size_t trailing_zeros = 0;
+  bool has_more_non_zero = false;
 
-  auto get_next_digit = [&] {
-    int_mantissa *= 10;
-    int digit = static_cast<int>(int_mantissa >> exp);
-    int_mantissa &= mask;
-    return digit;
+  auto check_integral_zeros = [](char* const begin, char* const end,
+                                 const size_t precision,
+                                 size_t digits_processed) -> bool {
+    // When considering rounding to even, we care about the digits after the
+    // round digit which means the total digits to move from the start is
+    // precision + 2 since the first digit we print before the decimal point
+    // is not a part of precision.
+    size_t digit_upper_bound = precision + 2;
+    if (digits_processed > digit_upper_bound) {
+      return std::any_of(begin + digit_upper_bound, end,
+                         [](char c) { return c != '0'; });
+    }
+    return false;
   };
 
-  // Print fractional_count more digits, if available.
-  for (; fractional_count > 0; --fractional_count) {
-    out->push_back(get_next_digit() + '0');
-  }
+  if (exp >= 0) {
+    integral_end = total_bits <= 64
+                       ? numbers_internal::FastIntToBuffer(
+                             static_cast<uint64_t>(v) << exp, integral_start)
+                       : numbers_internal::FastIntToBuffer(
+                             static_cast<uint128>(v) << exp, integral_start);
+    *integral_end = '0';
+    final_start = integral_start;
+    // Integral is guaranteed to be non-zero at this point.
+    scientific_exp = static_cast<int>(integral_end - integral_start) - 1;
+    digits_printed = static_cast<size_t>(integral_end - integral_start);
+    final_end = integral_end;
+    has_more_non_zero = check_integral_zeros(integral_start, integral_end,
+                                             state.precision, digits_printed);
+  } else {
+    exp = -exp;
+    if (exp < kInputBits) {
+      integral_end =
+          numbers_internal::FastIntToBuffer(v >> exp, integral_start);
+    }
+    *integral_end = '0';
+    // We didn't move integral_start and it gets set to 0 in
+    zero_integral = exp >= kInputBits || v >> exp == 0;
+    if (!zero_integral) {
+      digits_printed = static_cast<size_t>(integral_end - integral_start);
+      has_more_non_zero = check_integral_zeros(integral_start, integral_end,
+                                               state.precision, digits_printed);
+      final_end = integral_end;
+    }
+    // Print fractional digits
+    char* fractional_start = integral_end;
 
-  int next_digit = get_next_digit();
-  if (next_digit > 5 ||
-      (next_digit == 5 && (int_mantissa || out->last_digit() % 2 == 1))) {
-    RoundUp<mode>(out, exp_out);
+    size_t digits_to_print = (state.precision + 1) >= digits_printed
+                                 ? state.precision + 1 - digits_printed
+                                 : 0;
+    bool print_extra = digits_printed <= state.precision + 1;
+    auto [fractional_end, skipped_zeros, has_nonzero_rem] =
+        exp <= 64 ? PrintFractionalDigitsScientific(
+                        v, fractional_start, exp, digits_to_print + print_extra,
+                        zero_integral)
+                  : PrintFractionalDigitsScientific(
+                        static_cast<uint128>(v), fractional_start, exp,
+                        digits_to_print + print_extra, zero_integral);
+    final_end = fractional_end;
+    *fractional_end = '0';
+    has_more_non_zero |= has_nonzero_rem;
+    digits_printed += static_cast<size_t>(fractional_end - fractional_start);
+    if (zero_integral) {
+      scientific_exp = -1 * static_cast<int>(skipped_zeros + 1);
+    } else {
+      scientific_exp = static_cast<int>(integral_end - integral_start) - 1;
+    }
+    // Don't do any rounding here, we will do it ourselves.
+    final_start = zero_integral ? fractional_start : integral_start;
   }
 
-  return true;
-}
-
-template <FormatStyle mode, typename Float>
-bool FloatToBuffer(Decomposed<Float> decomposed, int precision, Buffer *out,
-                   int *exp) {
-  if (precision > kMaxFixedPrecision) return false;
+  // For rounding
+  if (digits_printed >= state.precision + 1) {
+    final_start[-1] = '0';
+    char* round_digit_ptr = final_start + 1 + state.precision;
+    if (*round_digit_ptr > '5') {
+      RoundUp(round_digit_ptr - 1);
+    } else if (*round_digit_ptr == '5') {
+      if (has_more_non_zero) {
+        RoundUp(round_digit_ptr - 1);
+      } else {
+        RoundToEven(round_digit_ptr - 1);
+      }
+    }
+    final_end = round_digit_ptr;
+    if (final_start[-1] == '1') {
+      --final_start;
+      ++scientific_exp;
+      --final_end;
+    }
+  } else {
+    // Need to pad with zeros.
+    trailing_zeros = state.precision - (digits_printed - 1);
+  }
 
-  // Try with uint64_t.
-  if (CanFitMantissa<Float, std::uint64_t>() &&
-      FloatToBufferImpl<std::uint64_t, Float, mode>(
-          static_cast<std::uint64_t>(decomposed.mantissa),
-          static_cast<std::uint64_t>(decomposed.exponent), precision, out, exp))
-    return true;
+  if (state.precision > 0 || state.ShouldPrintDot()) {
+    final_start[-1] = *final_start;
+    *final_start = '.';
+    --final_start;
+  }
+  // We have scientific exp at this point
+  if ((scientific_exp < 0 ||
+       state.precision + 1 > static_cast<size_t>(scientific_exp)) &&
+      scientific_exp >= -4) {
+    if (scientific_exp < 0) {
+      // Have 1.23456, needs 0.00123456
+      // Move the first digit
+      final_start[1] = *final_start;
+      if (!state.ShouldPrintDot()) {
+        ++final_end;
+      }
+      // Add some zeros
+      for (; scientific_exp < -1; ++scientific_exp) {
+        *final_start = '0';
+        --final_start;
+      }
+      *final_start-- = '.';
+      *final_start = '0';
+    } else if (scientific_exp > 0) {
+      // Have 1.23456, needs 1234.56
+      // Move the '.' scientific_exp positions to the right.
+      std::rotate(final_start + 1, final_start + 2,
+                  final_start + scientific_exp + 2);
+    }
+    scientific_exp = 0;
+  }
+  auto const& conv = state.conv;
+  if (!conv.has_alt_flag()) {
+    trailing_zeros = 0;
+    while (final_end[-1] == '0') {
+      --final_end;
+    }
+    if (final_end[-1] == '.') --final_end;
+  }
+  if (scientific_exp) {
+    // We need to add 2 to the buffer size for the +/- sign and the e
+    constexpr size_t kExpBufferSize = numbers_internal::kFastToBufferSize + 2;
+    char exp_buffer[kExpBufferSize];
+    char* exp_ptr_start = exp_buffer;
+    char* exp_ptr = exp_ptr_start;
+    *exp_ptr++ = uppercase ? 'E' : 'e';
+    if (scientific_exp >= 0) {
+      *exp_ptr++ = '+';
+    } else {
+      *exp_ptr++ = '-';
+      scientific_exp = -scientific_exp;
+    }
 
-#if defined(ABSL_HAVE_INTRINSIC_INT128)
-  // If that is not enough, try with __uint128_t.
-  return CanFitMantissa<Float, __uint128_t>() &&
-         FloatToBufferImpl<__uint128_t, Float, mode>(
-             static_cast<__uint128_t>(decomposed.mantissa),
-             static_cast<__uint128_t>(decomposed.exponent), precision, out,
-             exp);
-#endif
-  return false;
-}
-
-void WriteBufferToSink(char sign_char, string_view str,
-                       const ConversionSpec &conv, FormatSinkImpl *sink) {
-  int left_spaces = 0, zeros = 0, right_spaces = 0;
-  int missing_chars =
-      conv.width() >= 0 ? std::max(conv.width() - static_cast<int>(str.size()) -
-                                       static_cast<int>(sign_char != 0),
-                                   0)
-                        : 0;
-  if (conv.flags().left) {
-    right_spaces = missing_chars;
-  } else if (conv.flags().zero) {
-    zeros = missing_chars;
+    if (scientific_exp < 10) {
+      *exp_ptr++ = '0';
+    }
+    exp_ptr = numbers_internal::FastIntToBuffer(scientific_exp, exp_ptr);
+    FinalPrint(state,
+               absl::string_view(
+                   final_start, static_cast<size_t>((final_end - final_start))),
+               0, trailing_zeros,
+               absl::string_view(exp_ptr_start,
+                                 static_cast<size_t>(exp_ptr - exp_ptr_start)));
   } else {
-    left_spaces = missing_chars;
+    FinalPrint(state,
+               absl::string_view(
+                   final_start, static_cast<size_t>((final_end - final_start))),
+               0, trailing_zeros, "");
   }
+}
 
-  sink->Append(left_spaces, ' ');
-  if (sign_char) sink->Append(1, sign_char);
-  sink->Append(zeros, '0');
-  sink->Append(str);
-  sink->Append(right_spaces, ' ');
+template <typename Int>
+void FormatGNegativeExpSlow(Int mantissa, int exp, bool uppercase,
+                            const FormatState& state) {
+  // Most of the code here is to decide whether to use E-style or F-style
+  // formatting, with the actual formatting done in FormatENegativeExpSlow and
+  // FormatFNegativeExpSlow.
+  FractionalDigitGenerator::RunConversion(
+      mantissa, -exp, [&](FractionalDigitGenerator digit_gen) {
+        int first_digit = 0;
+        size_t nines = 0;
+        int num_leading_zeros = 0;
+        size_t num_trailing_zeros = 0;
+        while (digit_gen.HasMoreDigits()) {
+          auto digits = digit_gen.GetDigits();
+          if (digits.digit_before_nine != 0) {
+            first_digit = digits.digit_before_nine;
+            nines = digits.num_nines;
+            break;
+          } else if (digits.num_nines > 0) {
+            // This also means the first digit is 0
+            first_digit = 9;
+            nines = digits.num_nines - 1;
+            num_leading_zeros++;
+            break;
+          }
+          num_leading_zeros++;
+        }
+        if (nines >= state.precision || state.precision == 0) {
+          bool round_up = false;
+          if (nines == state.precision) {
+            round_up = digit_gen.IsGreaterThanHalf();
+          } else {
+            round_up = nines > 0 || digit_gen.IsGreaterThanHalf();
+          }
+          if (round_up) {
+            first_digit = (first_digit == 9 ? 1 : first_digit + 1);
+            num_leading_zeros -= (first_digit == 1);
+            num_trailing_zeros = state.precision;
+          }
+        }
+        int scientific_exp = -(num_leading_zeros + 1);
+        assert(scientific_exp < 0);
+        size_t digits_to_go = state.precision + 1;
+        if (state.conv.has_alt_flag()) {
+          num_trailing_zeros = 0;
+        }
+        if (!state.conv.has_alt_flag() && !num_trailing_zeros) {
+          num_trailing_zeros = (first_digit == 0);
+          digits_to_go -= std::min(digits_to_go, nines + 1);
+          while (digits_to_go > 0 && digit_gen.HasMoreDigits()) {
+            auto digits = digit_gen.GetDigits();
+            if (digits.num_nines + 1 < digits_to_go) {
+              if (digits.digit_before_nine == 0 && digits.num_nines == 0) {
+                ++num_trailing_zeros;
+              } else {
+                num_trailing_zeros = 0;
+              }
+              digits_to_go -= digits.num_nines + 1;
+            } else {
+              bool round_up = false;
+              if (digits.num_nines + 1 > digits_to_go) {
+                round_up = true;
+              } else if (digit_gen.IsGreaterThanHalf()) {
+                round_up = true;
+              } else if (digit_gen.IsExactlyHalf()) {
+                round_up =
+                    digits.num_nines != 0 || digits.digit_before_nine % 2 == 1;
+              }
+
+              if (digits_to_go == 1) {
+                if (digits.digit_before_nine + (round_up ? 1 : 0) == 0) {
+                  ++num_trailing_zeros;
+                } else {
+                  num_trailing_zeros = 0;
+                }
+              } else {
+                num_trailing_zeros = round_up ? digits_to_go - 1 : 0;
+              }
+              digits_to_go = 0;
+            }
+          }
+        }
+        if (!num_trailing_zeros) {
+          num_trailing_zeros = !state.conv.has_alt_flag() * digits_to_go;
+        }
+        if (scientific_exp <= -4) {
+          FormatENegativeExpSlow(static_cast<uint128>(mantissa), exp, uppercase,
+                                 state, num_trailing_zeros);
+        } else {
+          FormatState f_state = state;
+          f_state.precision = static_cast<size_t>(
+              static_cast<int>(state.precision) - scientific_exp);
+          FormatFNegativeExpSlow(static_cast<uint128>(mantissa), -exp, f_state,
+                                 num_trailing_zeros);
+        }
+      });
+}
+template <typename Int>
+void FormatGPositiveExpSlow(Int mantissa, int exp, bool uppercase,
+                            const FormatState& state) {
+  BinaryToDecimal::RunConversion(mantissa, exp, [&](BinaryToDecimal btd) {
+    int scientific_exp = static_cast<int>(btd.TotalDigits()) - 1;
+    absl::string_view digits = btd.CurrentDigits();
+    size_t digits_to_go = state.precision + 1;
+    auto [first_digit_opt, nines] = GetDigits(btd, digits);
+    int first_digit = first_digit_opt.value_or(9);
+    if (!first_digit_opt) {
+      --nines;
+    }
+    // At this point we are guaranteed to have some sort of first digit
+    bool change_to_zeros = false;
+    size_t num_trailing_zeros = 0;
+    if (nines + 1 >= digits_to_go) {
+      // Everything we need to print is in the first DigitRun
+      auto next_digit_opt = GetDigits(btd, digits).digit;
+      if (nines == state.precision) {
+        change_to_zeros = next_digit_opt.value_or(0) > 4;
+      } else {
+        change_to_zeros = true;
+      }
+      if (change_to_zeros) {
+        if (first_digit != 9) {
+          first_digit = first_digit + 1;
+        } else {
+          first_digit = 1;
+          ++scientific_exp;
+        }
+        num_trailing_zeros = state.precision;
+      }
+    }
+    if (state.conv.has_alt_flag()) {
+      num_trailing_zeros = 0;
+    }
+    // At this point the number of trailing zeros is not covered by the first
+    // DigitRun
+    if (!state.conv.has_alt_flag() && !num_trailing_zeros) {
+      num_trailing_zeros = first_digit == 0;
+      digits_to_go -= std::min(digits_to_go, nines + 1);
+      while (digits_to_go > 0) {
+        auto [digit_opt, curr_nines] = GetDigits(btd, digits);
+        if (!digit_opt.has_value()) {
+          break;
+        }
+        if (curr_nines + 1 < digits_to_go) {
+          int digit = *digit_opt;
+          // If the previous one was a 0 we are too
+          if (digit == 0 && curr_nines == 0) {
+            ++num_trailing_zeros;
+            --digits_to_go;
+          } else {
+            num_trailing_zeros = 0;
+            --digits_to_go;
+            digits_to_go -= std::min(digits_to_go, curr_nines);
+          }
+        } else {
+          auto next_digit_opt = GetDigits(btd, digits).digit;
+          if (digits_to_go == 1) {
+            if (*digit_opt == 0) {
+              if (curr_nines || next_digit_opt > 4) {
+                num_trailing_zeros = 0;
+              } else {
+                ++num_trailing_zeros;
+              }
+            } else {
+              num_trailing_zeros = 0;
+            }
+          } else if (digits_to_go == curr_nines + 1) {
+            num_trailing_zeros = next_digit_opt > 4 ? digits_to_go - 1 : 0;
+          } else {
+            num_trailing_zeros = digits_to_go - 1;
+          }
+          digits_to_go = 0;
+        }
+      }
+    }
+    assert(scientific_exp >= 0);
+    // By this point the exponent is accurate
+    if (static_cast<size_t>(scientific_exp) > state.precision) {
+      FormatEPositiveExpSlow(mantissa, exp, uppercase, state,
+                             num_trailing_zeros);
+    } else {
+      FormatFPositiveExpSlow(mantissa, exp, state, !state.conv.has_alt_flag());
+    }
+  });
 }
 
 template <typename Float>
-bool FloatToSink(const Float v, const ConversionSpec &conv,
+bool FloatToSink(const Float v, const FormatConversionSpecImpl &conv,
                  FormatSinkImpl *sink) {
   // Print the sign or the sign column.
   Float abs_v = v;
@@ -379,9 +2051,9 @@ bool FloatToSink(const Float v, const ConversionSpec &conv,
   if (std::signbit(abs_v)) {
     sign_char = '-';
     abs_v = -abs_v;
-  } else if (conv.flags().show_pos) {
+  } else if (conv.has_show_pos_flag()) {
     sign_char = '+';
-  } else if (conv.flags().sign_col) {
+  } else if (conv.has_sign_col_flag()) {
     sign_char = ' ';
   }
 
@@ -390,96 +2062,81 @@ bool FloatToSink(const Float v, const ConversionSpec &conv,
     return true;
   }
 
-  int precision = conv.precision() < 0 ? 6 : conv.precision();
-
-  int exp = 0;
+  size_t precision =
+      conv.precision() < 0 ? 6 : static_cast<size_t>(conv.precision());
 
   auto decomposed = Decompose(abs_v);
 
-  Buffer buffer;
-
-  switch (conv.conv().id()) {
-    case ConversionChar::f:
-    case ConversionChar::F:
-      if (!FloatToBuffer<FormatStyle::Fixed>(decomposed, precision, &buffer,
-                                             nullptr)) {
-        return FallbackToSnprintf(v, conv, sink);
-      }
-      if (!conv.flags().alt && buffer.back() == '.') buffer.pop_back();
-      break;
+  FormatConversionChar c = conv.conversion_char();
 
-    case ConversionChar::e:
-    case ConversionChar::E:
-      if (!FloatToBuffer<FormatStyle::Precision>(decomposed, precision, &buffer,
-                                                 &exp)) {
-        return FallbackToSnprintf(v, conv, sink);
-      }
-      if (!conv.flags().alt && buffer.back() == '.') buffer.pop_back();
-      PrintExponent(exp, conv.conv().upper() ? 'E' : 'e', &buffer);
-      break;
-
-    case ConversionChar::g:
-    case ConversionChar::G:
-      precision = std::max(0, precision - 1);
-      if (!FloatToBuffer<FormatStyle::Precision>(decomposed, precision, &buffer,
-                                                 &exp)) {
-        return FallbackToSnprintf(v, conv, sink);
-      }
-      if (precision + 1 > exp && exp >= -4) {
-        if (exp < 0) {
-          // Have 1.23456, needs 0.00123456
-          // Move the first digit
-          buffer.begin[1] = *buffer.begin;
-          // Add some zeros
-          for (; exp < -1; ++exp) *buffer.begin-- = '0';
-          *buffer.begin-- = '.';
-          *buffer.begin = '0';
-        } else if (exp > 0) {
-          // Have 1.23456, needs 1234.56
-          // Move the '.' exp positions to the right.
-          std::rotate(buffer.begin + 1, buffer.begin + 2,
-                      buffer.begin + exp + 2);
-        }
-        exp = 0;
-      }
-      if (!conv.flags().alt) {
-        while (buffer.back() == '0') buffer.pop_back();
-        if (buffer.back() == '.') buffer.pop_back();
-      }
-      if (exp) PrintExponent(exp, conv.conv().upper() ? 'E' : 'e', &buffer);
-      break;
-
-    case ConversionChar::a:
-    case ConversionChar::A:
-      return FallbackToSnprintf(v, conv, sink);
-
-    default:
-      return false;
+  if (c == FormatConversionCharInternal::f ||
+      c == FormatConversionCharInternal::F) {
+    FormatF(decomposed.mantissa, decomposed.exponent,
+            {sign_char, precision, conv, sink});
+    return true;
+  } else if (c == FormatConversionCharInternal::e ||
+             c == FormatConversionCharInternal::E) {
+    FormatE(decomposed.mantissa, decomposed.exponent,
+            FormatConversionCharIsUpper(conv.conversion_char()),
+            {sign_char, precision, conv, sink});
+    return true;
+  } else if (c == FormatConversionCharInternal::g ||
+             c == FormatConversionCharInternal::G) {
+    precision = std::max(precision, size_t{1}) - 1;
+    constexpr int input_bits = sizeof(decomposed.mantissa) * 8;
+    const int total_bits =
+        input_bits - LeadingZeros(decomposed.mantissa) + decomposed.exponent;
+    if (decomposed.exponent >= 0 && total_bits > 128) {
+      FormatGPositiveExpSlow(
+          decomposed.mantissa, decomposed.exponent,
+          FormatConversionCharIsUpper(conv.conversion_char()),
+          {sign_char, precision, conv, sink});
+      return true;
+    } else if (decomposed.exponent < -128) {
+      FormatGNegativeExpSlow(
+          decomposed.mantissa, decomposed.exponent,
+          FormatConversionCharIsUpper(conv.conversion_char()),
+          {sign_char, precision, conv, sink});
+      return true;
+    }
+    FormatGFast(decomposed.mantissa, decomposed.exponent,
+                FormatConversionCharIsUpper(conv.conversion_char()),
+                {sign_char, precision, conv, sink});
+    return true;
+  } else if (c == FormatConversionCharInternal::a ||
+             c == FormatConversionCharInternal::A) {
+    bool uppercase = (c == FormatConversionCharInternal::A);
+    FormatA(HexFloatTypeParams(Float{}), decomposed.mantissa,
+            decomposed.exponent, uppercase, {sign_char, precision, conv, sink});
+    return true;
+  } else {
+    return false;
   }
-
-  WriteBufferToSink(sign_char,
-                    string_view(buffer.begin, buffer.end - buffer.begin), conv,
-                    sink);
-
-  return true;
 }
 
 }  // namespace
 
-bool ConvertFloatImpl(long double v, const ConversionSpec &conv,
+bool ConvertFloatImpl(long double v, const FormatConversionSpecImpl &conv,
                       FormatSinkImpl *sink) {
+  if (IsDoubleDouble()) {
+    // This is the `double-double` representation of `long double`. We do not
+    // handle it natively. Fallback to snprintf.
+    return FallbackToSnprintf(v, conv, sink);
+  }
+
   return FloatToSink(v, conv, sink);
 }
 
-bool ConvertFloatImpl(float v, const ConversionSpec &conv,
+bool ConvertFloatImpl(float v, const FormatConversionSpecImpl &conv,
                       FormatSinkImpl *sink) {
-  return FloatToSink(v, conv, sink);
+  return FloatToSink(static_cast<double>(v), conv, sink);
 }
 
-bool ConvertFloatImpl(double v, const ConversionSpec &conv,
+bool ConvertFloatImpl(double v, const FormatConversionSpecImpl &conv,
                       FormatSinkImpl *sink) {
   return FloatToSink(v, conv, sink);
 }
 
 }  // namespace str_format_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/internal/str_format/float_conversion.h b/absl/strings/internal/str_format/float_conversion.h
index 8ba5566d3ee..71100e71425 100644
--- a/absl/strings/internal/str_format/float_conversion.h
+++ b/absl/strings/internal/str_format/float_conversion.h
@@ -1,21 +1,37 @@
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
 #ifndef ABSL_STRINGS_INTERNAL_STR_FORMAT_FLOAT_CONVERSION_H_
 #define ABSL_STRINGS_INTERNAL_STR_FORMAT_FLOAT_CONVERSION_H_
 
 #include "absl/strings/internal/str_format/extension.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace str_format_internal {
 
-bool ConvertFloatImpl(float v, const ConversionSpec &conv,
+bool ConvertFloatImpl(float v, const FormatConversionSpecImpl &conv,
                       FormatSinkImpl *sink);
 
-bool ConvertFloatImpl(double v, const ConversionSpec &conv,
+bool ConvertFloatImpl(double v, const FormatConversionSpecImpl &conv,
                       FormatSinkImpl *sink);
 
-bool ConvertFloatImpl(long double v, const ConversionSpec &conv,
+bool ConvertFloatImpl(long double v, const FormatConversionSpecImpl &conv,
                       FormatSinkImpl *sink);
 
 }  // namespace str_format_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_INTERNAL_STR_FORMAT_FLOAT_CONVERSION_H_
diff --git a/absl/strings/internal/str_format/output.cc b/absl/strings/internal/str_format/output.cc
index 38987b6385d..1e61d194bef 100644
--- a/absl/strings/internal/str_format/output.cc
+++ b/absl/strings/internal/str_format/output.cc
@@ -15,9 +15,15 @@
 #include "absl/strings/internal/str_format/output.h"
 
 #include <errno.h>
+
+#include <algorithm>
+#include <cstdio>
 #include <cstring>
 
+#include "absl/strings/string_view.h"
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace str_format_internal {
 
 namespace {
@@ -32,9 +38,11 @@ struct ClearErrnoGuard {
 
 void BufferRawSink::Write(string_view v) {
   size_t to_write = std::min(v.size(), size_);
-  std::memcpy(buffer_, v.data(), to_write);
-  buffer_ += to_write;
-  size_ -= to_write;
+  if (to_write > 0) {
+    std::memcpy(buffer_, v.data(), to_write);
+    buffer_ += to_write;
+    size_ -= to_write;
+  }
   total_written_ += v.size();
 }
 
@@ -67,4 +75,5 @@ void FILERawSink::Write(string_view v) {
 }
 
 }  // namespace str_format_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/internal/str_format/output.h b/absl/strings/internal/str_format/output.h
index 42da6417be0..15e751ab6f0 100644
--- a/absl/strings/internal/str_format/output.h
+++ b/absl/strings/internal/str_format/output.h
@@ -22,15 +22,15 @@
 #define ABSL_STRINGS_INTERNAL_STR_FORMAT_OUTPUT_H_
 
 #include <cstdio>
+#include <ios>
 #include <ostream>
 #include <string>
 
 #include "absl/base/port.h"
 #include "absl/strings/string_view.h"
 
-class Cord;
-
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace str_format_internal {
 
 // RawSink implementation that writes into a char* buffer.
@@ -72,13 +72,7 @@ inline void AbslFormatFlush(std::string* out, string_view s) {
   out->append(s.data(), s.size());
 }
 inline void AbslFormatFlush(std::ostream* out, string_view s) {
-  out->write(s.data(), s.size());
-}
-
-template <class AbslCord, typename = typename std::enable_if<
-                              std::is_same<AbslCord, ::Cord>::value>::type>
-inline void AbslFormatFlush(AbslCord* out, string_view s) {
-  out->Append(s);
+  out->write(s.data(), static_cast<std::streamsize>(s.size()));
 }
 
 inline void AbslFormatFlush(FILERawSink* sink, string_view v) {
@@ -89,13 +83,15 @@ inline void AbslFormatFlush(BufferRawSink* sink, string_view v) {
   sink->Write(v);
 }
 
+// This is a SFINAE to get a better compiler error message when the type
+// is not supported.
 template <typename T>
-auto InvokeFlush(T* out, string_view s)
-    -> decltype(str_format_internal::AbslFormatFlush(out, s)) {
-  str_format_internal::AbslFormatFlush(out, s);
+auto InvokeFlush(T* out, string_view s) -> decltype(AbslFormatFlush(out, s)) {
+  AbslFormatFlush(out, s);
 }
 
 }  // namespace str_format_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_INTERNAL_STR_FORMAT_OUTPUT_H_
diff --git a/absl/strings/internal/str_format/output_test.cc b/absl/strings/internal/str_format/output_test.cc
index 6e04abef4d9..83c9589d571 100644
--- a/absl/strings/internal/str_format/output_test.cc
+++ b/absl/strings/internal/str_format/output_test.cc
@@ -14,13 +14,16 @@
 
 #include "absl/strings/internal/str_format/output.h"
 
+#include <algorithm>
 #include <sstream>
 #include <string>
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
+#include "absl/strings/cord.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace {
 
 TEST(InvokeFlush, String) {
@@ -36,6 +39,12 @@ TEST(InvokeFlush, Stream) {
   EXPECT_EQ(str.str(), "ABCDEF");
 }
 
+TEST(InvokeFlush, Cord) {
+  absl::Cord str("ABC");
+  str_format_internal::InvokeFlush(&str, "DEF");
+  EXPECT_EQ(str, "ABCDEF");
+}
+
 TEST(BufferRawSink, Limits) {
   char buf[16];
   {
@@ -67,5 +76,5 @@ TEST(BufferRawSink, Limits) {
 }
 
 }  // namespace
+ABSL_NAMESPACE_END
 }  // namespace absl
-
diff --git a/absl/strings/internal/str_format/parser.cc b/absl/strings/internal/str_format/parser.cc
index 9ef5615cc9e..475d22132d4 100644
--- a/absl/strings/internal/str_format/parser.cc
+++ b/absl/strings/internal/str_format/parser.cc
@@ -1,236 +1,73 @@
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
 #include "absl/strings/internal/str_format/parser.h"
 
 #include <assert.h>
 #include <string.h>
 #include <wchar.h>
-#include <cctype>
-#include <cstdint>
 
 #include <algorithm>
+#include <cctype>
+#include <cstdint>
 #include <initializer_list>
 #include <limits>
 #include <ostream>
 #include <string>
 #include <unordered_set>
 
+#include "absl/base/attributes.h"
+#include "absl/strings/internal/str_format/constexpr_parser.h"
+#include "absl/strings/internal/str_format/extension.h"
+#include "absl/strings/string_view.h"
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace str_format_internal {
 
-using CC = ConversionChar::Id;
-using LM = LengthMod::Id;
-ABSL_CONST_INIT const ConvTag kTags[256] = {
-    {},    {},    {},    {},    {},    {},    {},    {},     // 00-07
-    {},    {},    {},    {},    {},    {},    {},    {},     // 08-0f
-    {},    {},    {},    {},    {},    {},    {},    {},     // 10-17
-    {},    {},    {},    {},    {},    {},    {},    {},     // 18-1f
-    {},    {},    {},    {},    {},    {},    {},    {},     // 20-27
-    {},    {},    {},    {},    {},    {},    {},    {},     // 28-2f
-    {},    {},    {},    {},    {},    {},    {},    {},     // 30-37
-    {},    {},    {},    {},    {},    {},    {},    {},     // 38-3f
-    {},    CC::A, {},    CC::C, {},    CC::E, CC::F, CC::G,  // @ABCDEFG
-    {},    {},    {},    {},    LM::L, {},    {},    {},     // HIJKLMNO
-    {},    {},    {},    CC::S, {},    {},    {},    {},     // PQRSTUVW
-    CC::X, {},    {},    {},    {},    {},    {},    {},     // XYZ[\]^_
-    {},    CC::a, {},    CC::c, CC::d, CC::e, CC::f, CC::g,  // `abcdefg
-    LM::h, CC::i, LM::j, {},    LM::l, {},    CC::n, CC::o,  // hijklmno
-    CC::p, LM::q, {},    CC::s, LM::t, CC::u, {},    {},     // pqrstuvw
-    CC::x, {},    LM::z, {},    {},    {},    {},    {},     // xyz{|}!
-    {},    {},    {},    {},    {},    {},    {},    {},     // 80-87
-    {},    {},    {},    {},    {},    {},    {},    {},     // 88-8f
-    {},    {},    {},    {},    {},    {},    {},    {},     // 90-97
-    {},    {},    {},    {},    {},    {},    {},    {},     // 98-9f
-    {},    {},    {},    {},    {},    {},    {},    {},     // a0-a7
-    {},    {},    {},    {},    {},    {},    {},    {},     // a8-af
-    {},    {},    {},    {},    {},    {},    {},    {},     // b0-b7
-    {},    {},    {},    {},    {},    {},    {},    {},     // b8-bf
-    {},    {},    {},    {},    {},    {},    {},    {},     // c0-c7
-    {},    {},    {},    {},    {},    {},    {},    {},     // c8-cf
-    {},    {},    {},    {},    {},    {},    {},    {},     // d0-d7
-    {},    {},    {},    {},    {},    {},    {},    {},     // d8-df
-    {},    {},    {},    {},    {},    {},    {},    {},     // e0-e7
-    {},    {},    {},    {},    {},    {},    {},    {},     // e8-ef
-    {},    {},    {},    {},    {},    {},    {},    {},     // f0-f7
-    {},    {},    {},    {},    {},    {},    {},    {},     // f8-ff
-};
-
-namespace {
+// Define the array for non-constexpr uses.
+constexpr ConvTag ConvTagHolder::value[256];
 
-bool CheckFastPathSetting(const UnboundConversion& conv) {
-  bool should_be_basic = !conv.flags.left &&      //
-                         !conv.flags.show_pos &&  //
-                         !conv.flags.sign_col &&  //
-                         !conv.flags.alt &&       //
-                         !conv.flags.zero &&      //
-                         (conv.width.value() == -1) &&
-                         (conv.precision.value() == -1);
-  if (should_be_basic != conv.flags.basic) {
-    fprintf(stderr,
-            "basic=%d left=%d show_pos=%d sign_col=%d alt=%d zero=%d "
-            "width=%d precision=%d\n",
-            conv.flags.basic, conv.flags.left, conv.flags.show_pos,
-            conv.flags.sign_col, conv.flags.alt, conv.flags.zero,
-            conv.width.value(), conv.precision.value());
-  }
-  return should_be_basic == conv.flags.basic;
+ABSL_ATTRIBUTE_NOINLINE const char* ConsumeUnboundConversionNoInline(
+    const char* p, const char* end, UnboundConversion* conv, int* next_arg) {
+  return ConsumeUnboundConversion(p, end, conv, next_arg);
 }
 
-template <bool is_positional>
-const char *ConsumeConversion(const char *pos, const char *const end,
-                              UnboundConversion *conv, int *next_arg) {
-  const char* const original_pos = pos;
-  char c;
-  // Read the next char into `c` and update `pos`. Returns false if there are
-  // no more chars to read.
-#define ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR()          \
-  do {                                                  \
-    if (ABSL_PREDICT_FALSE(pos == end)) return nullptr; \
-    c = *pos++;                                         \
-  } while (0)
-
-  const auto parse_digits = [&] {
-    int digits = c - '0';
-    // We do not want to overflow `digits` so we consume at most digits10
-    // digits. If there are more digits the parsing will fail later on when the
-    // digit doesn't match the expected characters.
-    int num_digits = std::numeric_limits<int>::digits10;
-    for (;;) {
-      if (ABSL_PREDICT_FALSE(pos == end)) break;
-      c = *pos++;
-      if (!std::isdigit(c)) break;
-      --num_digits;
-      if (ABSL_PREDICT_FALSE(!num_digits)) break;
-      digits = 10 * digits + c - '0';
-    }
-    return digits;
-  };
-
-  if (is_positional) {
-    ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
-    if (ABSL_PREDICT_FALSE(c < '1' || c > '9')) return nullptr;
-    conv->arg_position = parse_digits();
-    assert(conv->arg_position > 0);
-    if (ABSL_PREDICT_FALSE(c != '$')) return nullptr;
+std::string LengthModToString(LengthMod v) {
+  switch (v) {
+    case LengthMod::h:
+      return "h";
+    case LengthMod::hh:
+      return "hh";
+    case LengthMod::l:
+      return "l";
+    case LengthMod::ll:
+      return "ll";
+    case LengthMod::L:
+      return "L";
+    case LengthMod::j:
+      return "j";
+    case LengthMod::z:
+      return "z";
+    case LengthMod::t:
+      return "t";
+    case LengthMod::q:
+      return "q";
+    case LengthMod::none:
+      return "";
   }
-
-  ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
-
-  // We should start with the basic flag on.
-  assert(conv->flags.basic);
-
-  // Any non alpha character makes this conversion not basic.
-  // This includes flags (-+ #0), width (1-9, *) or precision (.).
-  // All conversion characters and length modifiers are alpha characters.
-  if (c < 'A') {
-    conv->flags.basic = false;
-
-    for (; c <= '0';) {
-      // FIXME: We might be able to speed this up reusing the lookup table from
-      // above. It might require changing Flags to be a plain integer where we
-      // can |= a value.
-      switch (c) {
-        case '-':
-          conv->flags.left = true;
-          break;
-        case '+':
-          conv->flags.show_pos = true;
-          break;
-        case ' ':
-          conv->flags.sign_col = true;
-          break;
-        case '#':
-          conv->flags.alt = true;
-          break;
-        case '0':
-          conv->flags.zero = true;
-          break;
-        default:
-          goto flags_done;
-      }
-      ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
-    }
-flags_done:
-
-    if (c <= '9') {
-      if (c >= '0') {
-        int maybe_width = parse_digits();
-        if (!is_positional && c == '$') {
-          if (ABSL_PREDICT_FALSE(*next_arg != 0)) return nullptr;
-          // Positional conversion.
-          *next_arg = -1;
-          conv->flags = Flags();
-          conv->flags.basic = true;
-          return ConsumeConversion<true>(original_pos, end, conv, next_arg);
-        }
-        conv->width.set_value(maybe_width);
-      } else if (c == '*') {
-        ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
-        if (is_positional) {
-          if (ABSL_PREDICT_FALSE(c < '1' || c > '9')) return nullptr;
-          conv->width.set_from_arg(parse_digits());
-          if (ABSL_PREDICT_FALSE(c != '$')) return nullptr;
-          ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
-        } else {
-          conv->width.set_from_arg(++*next_arg);
-        }
-      }
-    }
-
-    if (c == '.') {
-      ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
-      if (std::isdigit(c)) {
-        conv->precision.set_value(parse_digits());
-      } else if (c == '*') {
-        ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
-        if (is_positional) {
-          if (ABSL_PREDICT_FALSE(c < '1' || c > '9')) return nullptr;
-          conv->precision.set_from_arg(parse_digits());
-          if (c != '$') return nullptr;
-          ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
-        } else {
-          conv->precision.set_from_arg(++*next_arg);
-        }
-      } else {
-        conv->precision.set_value(0);
-      }
-    }
-  }
-
-  auto tag = GetTagForChar(c);
-
-  if (ABSL_PREDICT_FALSE(!tag.is_conv())) {
-    if (ABSL_PREDICT_FALSE(!tag.is_length())) return nullptr;
-
-    // It is a length modifier.
-    using str_format_internal::LengthMod;
-    LengthMod length_mod = tag.as_length();
-    ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
-    if (c == 'h' && length_mod.id() == LengthMod::h) {
-      conv->length_mod = LengthMod::FromId(LengthMod::hh);
-      ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
-    } else if (c == 'l' && length_mod.id() == LengthMod::l) {
-      conv->length_mod = LengthMod::FromId(LengthMod::ll);
-      ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
-    } else {
-      conv->length_mod = length_mod;
-    }
-    tag = GetTagForChar(c);
-    if (ABSL_PREDICT_FALSE(!tag.is_conv())) return nullptr;
-  }
-
-  assert(CheckFastPathSetting(*conv));
-  (void)(&CheckFastPathSetting);
-
-  conv->conv = tag.as_conv();
-  if (!is_positional) conv->arg_position = ++*next_arg;
-  return pos;
-}
-
-}  // namespace
-
-const char *ConsumeUnboundConversion(const char *p, const char *end,
-                                     UnboundConversion *conv, int *next_arg) {
-  if (*next_arg < 0) return ConsumeConversion<true>(p, end, conv, next_arg);
-  return ConsumeConversion<false>(p, end, conv, next_arg);
+  return "";
 }
 
 struct ParsedFormatBase::ParsedFormatConsumer {
@@ -268,22 +105,24 @@ struct ParsedFormatBase::ParsedFormatConsumer {
   char* data_pos;
 };
 
-ParsedFormatBase::ParsedFormatBase(string_view format, bool allow_ignored,
-                                   std::initializer_list<Conv> convs)
+ParsedFormatBase::ParsedFormatBase(
+    string_view format, bool allow_ignored,
+    std::initializer_list<FormatConversionCharSet> convs)
     : data_(format.empty() ? nullptr : new char[format.size()]) {
   has_error_ = !ParseFormatString(format, ParsedFormatConsumer(this)) ||
                !MatchesConversions(allow_ignored, convs);
 }
 
 bool ParsedFormatBase::MatchesConversions(
-    bool allow_ignored, std::initializer_list<Conv> convs) const {
+    bool allow_ignored,
+    std::initializer_list<FormatConversionCharSet> convs) const {
   std::unordered_set<int> used;
   auto add_if_valid_conv = [&](int pos, char c) {
-      if (static_cast<size_t>(pos) > convs.size() ||
-          !Contains(convs.begin()[pos - 1], c))
-        return false;
-      used.insert(pos);
-      return true;
+    if (static_cast<size_t>(pos) > convs.size() ||
+        !Contains(convs.begin()[pos - 1], c))
+      return false;
+    used.insert(pos);
+    return true;
   };
   for (const ConversionItem &item : items_) {
     if (!item.is_conversion) continue;
@@ -294,10 +133,13 @@ bool ParsedFormatBase::MatchesConversions(
     if (conv.width.is_from_arg() &&
         !add_if_valid_conv(conv.width.get_from_arg(), '*'))
       return false;
-    if (!add_if_valid_conv(conv.arg_position, conv.conv.Char())) return false;
+    if (!add_if_valid_conv(conv.arg_position,
+                           FormatConversionCharToChar(conv.conv)))
+      return false;
   }
   return used.size() == convs.size() || allow_ignored;
 }
 
 }  // namespace str_format_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/internal/str_format/parser.h b/absl/strings/internal/str_format/parser.h
index b7614a09fa4..329b060a891 100644
--- a/absl/strings/internal/str_format/parser.h
+++ b/absl/strings/internal/str_format/parser.h
@@ -1,107 +1,47 @@
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
 #ifndef ABSL_STRINGS_INTERNAL_STR_FORMAT_PARSER_H_
 #define ABSL_STRINGS_INTERNAL_STR_FORMAT_PARSER_H_
 
-#include <limits.h>
 #include <stddef.h>
 #include <stdlib.h>
 
 #include <cassert>
+#include <cstring>
 #include <initializer_list>
-#include <iosfwd>
-#include <iterator>
 #include <memory>
+#include <string>
+#include <utility>
 #include <vector>
 
+#include "absl/base/config.h"
+#include "absl/base/optimization.h"
 #include "absl/strings/internal/str_format/checker.h"
+#include "absl/strings/internal/str_format/constexpr_parser.h"
 #include "absl/strings/internal/str_format/extension.h"
+#include "absl/strings/string_view.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace str_format_internal {
 
-// The analyzed properties of a single specified conversion.
-struct UnboundConversion {
-  UnboundConversion()
-      : flags() /* This is required to zero all the fields of flags. */ {
-    flags.basic = true;
-  }
-
-  class InputValue {
-   public:
-    void set_value(int value) {
-      assert(value >= 0);
-      value_ = value;
-    }
-    int value() const { return value_; }
-
-    // Marks the value as "from arg". aka the '*' format.
-    // Requires `value >= 1`.
-    // When set, is_from_arg() return true and get_from_arg() returns the
-    // original value.
-    // `value()`'s return value is unspecfied in this state.
-    void set_from_arg(int value) {
-      assert(value > 0);
-      value_ = -value - 1;
-    }
-    bool is_from_arg() const { return value_ < -1; }
-    int get_from_arg() const {
-      assert(is_from_arg());
-      return -value_ - 1;
-    }
+std::string LengthModToString(LengthMod v);
 
-   private:
-    int value_ = -1;
-  };
-
-  // No need to initialize. It will always be set in the parser.
-  int arg_position;
-
-  InputValue width;
-  InputValue precision;
-
-  Flags flags;
-  LengthMod length_mod;
-  ConversionChar conv;
-};
-
-// Consume conversion spec prefix (not including '%') of [p, end) if valid.
-// Examples of valid specs would be e.g.: "s", "d", "-12.6f".
-// If valid, it returns the first character following the conversion spec,
-// and the spec part is broken down and returned in 'conv'.
-// If invalid, returns nullptr.
-const char* ConsumeUnboundConversion(const char* p, const char* end,
-                                     UnboundConversion* conv, int* next_arg);
-
-// Helper tag class for the table below.
-// It allows fast `char -> ConversionChar/LengthMod` checking and conversions.
-class ConvTag {
- public:
-  constexpr ConvTag(ConversionChar::Id id) : tag_(id) {}  // NOLINT
-  // We invert the length modifiers to make them negative so that we can easily
-  // test for them.
-  constexpr ConvTag(LengthMod::Id id) : tag_(~id) {}  // NOLINT
-  // Everything else is -128, which is negative to make is_conv() simpler.
-  constexpr ConvTag() : tag_(-128) {}
-
-  bool is_conv() const { return tag_ >= 0; }
-  bool is_length() const { return tag_ < 0 && tag_ != -128; }
-  ConversionChar as_conv() const {
-    assert(is_conv());
-    return ConversionChar::FromId(static_cast<ConversionChar::Id>(tag_));
-  }
-  LengthMod as_length() const {
-    assert(is_length());
-    return LengthMod::FromId(static_cast<LengthMod::Id>(~tag_));
-  }
-
- private:
-  std::int8_t tag_;
-};
-
-extern const ConvTag kTags[256];
-// Keep a single table for all the conversion chars and length modifiers.
-inline ConvTag GetTagForChar(char c) {
-  return kTags[static_cast<unsigned char>(c)];
-}
+const char* ConsumeUnboundConversionNoInline(const char* p, const char* end,
+                                             UnboundConversion* conv,
+                                             int* next_arg);
 
 // Parse the format string provided in 'src' and pass the identified items into
 // 'consumer'.
@@ -119,13 +59,15 @@ bool ParseFormatString(string_view src, Consumer consumer) {
   const char* p = src.data();
   const char* const end = p + src.size();
   while (p != end) {
-    const char* percent = static_cast<const char*>(memchr(p, '%', end - p));
+    const char* percent =
+        static_cast<const char*>(memchr(p, '%', static_cast<size_t>(end - p)));
     if (!percent) {
       // We found the last substring.
-      return consumer.Append(string_view(p, end - p));
+      return consumer.Append(string_view(p, static_cast<size_t>(end - p)));
     }
     // We found a percent, so push the text run then process the percent.
-    if (ABSL_PREDICT_FALSE(!consumer.Append(string_view(p, percent - p)))) {
+    if (ABSL_PREDICT_FALSE(!consumer.Append(
+            string_view(p, static_cast<size_t>(percent - p))))) {
       return false;
     }
     if (ABSL_PREDICT_FALSE(percent + 1 >= end)) return false;
@@ -133,7 +75,7 @@ bool ParseFormatString(string_view src, Consumer consumer) {
     auto tag = GetTagForChar(percent[1]);
     if (tag.is_conv()) {
       if (ABSL_PREDICT_FALSE(next_arg < 0)) {
-        // This indicates an error in the format std::string.
+        // This indicates an error in the format string.
         // The only way to get `next_arg < 0` here is to have a positional
         // argument first which sets next_arg to -1 and then a non-positional
         // argument.
@@ -153,10 +95,11 @@ bool ParseFormatString(string_view src, Consumer consumer) {
       }
     } else if (percent[1] != '%') {
       UnboundConversion conv;
-      p = ConsumeUnboundConversion(percent + 1, end, &conv, &next_arg);
+      p = ConsumeUnboundConversionNoInline(percent + 1, end, &conv, &next_arg);
       if (ABSL_PREDICT_FALSE(p == nullptr)) return false;
       if (ABSL_PREDICT_FALSE(!consumer.ConvertOne(
-          conv, string_view(percent + 1, p - (percent + 1))))) {
+              conv, string_view(percent + 1,
+                                static_cast<size_t>(p - (percent + 1)))))) {
         return false;
       }
     } else {
@@ -176,8 +119,9 @@ constexpr bool EnsureConstexpr(string_view s) {
 
 class ParsedFormatBase {
  public:
-  explicit ParsedFormatBase(string_view format, bool allow_ignored,
-                            std::initializer_list<Conv> convs);
+  explicit ParsedFormatBase(
+      string_view format, bool allow_ignored,
+      std::initializer_list<FormatConversionCharSet> convs);
 
   ParsedFormatBase(const ParsedFormatBase& other) { *this = other; }
 
@@ -188,8 +132,10 @@ class ParsedFormatBase {
     has_error_ = other.has_error_;
     items_ = other.items_;
     size_t text_size = items_.empty() ? 0 : items_.back().text_end;
-    data_.reset(new char[text_size]);
-    memcpy(data_.get(), other.data_.get(), text_size);
+    data_ = std::make_unique<char[]>(text_size);
+    if (text_size > 0) {
+      memcpy(data_.get(), other.data_.get(), text_size);
+    }
     return *this;
   }
 
@@ -209,7 +155,8 @@ class ParsedFormatBase {
     string_view text(base, 0);
     for (const auto& item : items_) {
       const char* const end = text.data() + text.size();
-      text = string_view(end, (base + item.text_end) - end);
+      text =
+          string_view(end, static_cast<size_t>((base + item.text_end) - end));
       if (item.is_conversion) {
         if (!consumer.ConvertOne(item.conv, text)) return false;
       } else {
@@ -224,8 +171,9 @@ class ParsedFormatBase {
  private:
   // Returns whether the conversions match and if !allow_ignored it verifies
   // that all conversions are used by the format.
-  bool MatchesConversions(bool allow_ignored,
-                          std::initializer_list<Conv> convs) const;
+  bool MatchesConversions(
+      bool allow_ignored,
+      std::initializer_list<FormatConversionCharSet> convs) const;
 
   struct ParsedFormatConsumer;
 
@@ -270,14 +218,14 @@ class ParsedFormatBase {
 // This is the only API that allows the user to pass a runtime specified format
 // string. These factory functions will return NULL if the format does not match
 // the conversions requested by the user.
-template <str_format_internal::Conv... C>
+template <FormatConversionCharSet... C>
 class ExtendedParsedFormat : public str_format_internal::ParsedFormatBase {
  public:
   explicit ExtendedParsedFormat(string_view format)
-#if ABSL_INTERNAL_ENABLE_FORMAT_CHECKER
+#ifdef ABSL_INTERNAL_ENABLE_FORMAT_CHECKER
       __attribute__((
           enable_if(str_format_internal::EnsureConstexpr(format),
-                    "Format std::string is not constexpr."),
+                    "Format string is not constexpr."),
           enable_if(str_format_internal::ValidFormatImpl<C...>(format),
                     "Format specified does not match the template arguments.")))
 #endif  // ABSL_INTERNAL_ENABLE_FORMAT_CHECKER
@@ -317,6 +265,7 @@ class ExtendedParsedFormat : public str_format_internal::ParsedFormatBase {
       : ParsedFormatBase(s, allow_ignored, {C...}) {}
 };
 }  // namespace str_format_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_INTERNAL_STR_FORMAT_PARSER_H_
diff --git a/absl/strings/internal/str_format/parser_test.cc b/absl/strings/internal/str_format/parser_test.cc
index 6d356093ce6..b62742efe4e 100644
--- a/absl/strings/internal/str_format/parser_test.cc
+++ b/absl/strings/internal/str_format/parser_test.cc
@@ -1,12 +1,36 @@
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
 #include "absl/strings/internal/str_format/parser.h"
 
 #include <string.h>
 
+#include <algorithm>
+#include <initializer_list>
+#include <iterator>
+#include <string>
+#include <utility>
+
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
-#include "absl/base/macros.h"
+#include "absl/base/config.h"
+#include "absl/strings/internal/str_format/constexpr_parser.h"
+#include "absl/strings/internal/str_format/extension.h"
+#include "absl/strings/string_view.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace str_format_internal {
 
 namespace {
@@ -16,7 +40,7 @@ using testing::Pair;
 TEST(LengthModTest, Names) {
   struct Expectation {
     int line;
-    LengthMod::Id id;
+    LengthMod mod;
     const char *name;
   };
   const Expectation kExpect[] = {
@@ -31,37 +55,33 @@ TEST(LengthModTest, Names) {
     {__LINE__, LengthMod::t,    "t" },
     {__LINE__, LengthMod::q,    "q" },
   };
-  EXPECT_EQ(ABSL_ARRAYSIZE(kExpect), LengthMod::kNumValues);
+  EXPECT_EQ(std::size(kExpect), 10);
   for (auto e : kExpect) {
     SCOPED_TRACE(e.line);
-    LengthMod mod = LengthMod::FromId(e.id);
-    EXPECT_EQ(e.id, mod.id());
-    EXPECT_EQ(e.name, mod.name());
+    EXPECT_EQ(e.name, LengthModToString(e.mod));
   }
 }
 
 TEST(ConversionCharTest, Names) {
   struct Expectation {
-    ConversionChar::Id id;
+    FormatConversionChar id;
     char name;
   };
   // clang-format off
   const Expectation kExpect[] = {
-#define X(c) {ConversionChar::c, #c[0]}
-    X(c), X(C), X(s), X(S),                          // text
+#define X(c) {FormatConversionCharInternal::c, #c[0]}
+    X(c), X(s),                                      // text
     X(d), X(i), X(o), X(u), X(x), X(X),              // int
     X(f), X(F), X(e), X(E), X(g), X(G), X(a), X(A),  // float
     X(n), X(p),                                      // misc
 #undef X
-    {ConversionChar::none, '\0'},
+    {FormatConversionCharInternal::kNone, '\0'},
   };
   // clang-format on
-  EXPECT_EQ(ABSL_ARRAYSIZE(kExpect), ConversionChar::kNumValues);
   for (auto e : kExpect) {
     SCOPED_TRACE(e.name);
-    ConversionChar v = ConversionChar::FromId(e.id);
-    EXPECT_EQ(e.id, v.id());
-    EXPECT_EQ(e.name, v.Char());
+    FormatConversionChar v = e.id;
+    EXPECT_EQ(e.name, FormatConversionCharToChar(v));
   }
 }
 
@@ -99,10 +119,14 @@ TEST_F(ConsumeUnboundConversionTest, ConsumeSpecification) {
     {__LINE__, "ba",   "",     "ba"},  // 'b' is invalid
     {__LINE__, "l",    "",     "l" },  // just length mod isn't okay
     {__LINE__, "d",    "d",    ""  },  // basic
+    {__LINE__, "v",    "v",    ""  },  // basic
     {__LINE__, "d ",   "d",    " " },  // leave suffix
     {__LINE__, "dd",   "d",    "d" },  // don't be greedy
     {__LINE__, "d9",   "d",    "9" },  // leave non-space suffix
     {__LINE__, "dzz",  "d",    "zz"},  // length mod as suffix
+    {__LINE__, "3v",   "",     "3v"},  // 'v' cannot have modifiers
+    {__LINE__, "hv",   "",     "hv"},  // 'v' cannot have modifiers
+    {__LINE__, "1$v",   "1$v",     ""},  // 'v' can have use posix syntax
     {__LINE__, "1$*2$d", "1$*2$d", ""  },  // arg indexing and * allowed.
     {__LINE__, "0-14.3hhd", "0-14.3hhd", ""},  // precision, width
     {__LINE__, " 0-+#14.3hhd", " 0-+#14.3hhd", ""},  // flags
@@ -120,13 +144,12 @@ TEST_F(ConsumeUnboundConversionTest, BasicConversion) {
   EXPECT_FALSE(Run("dd"));  // no excess allowed
 
   EXPECT_TRUE(Run("d"));
-  EXPECT_EQ('d', o.conv.Char());
+  EXPECT_EQ('d', FormatConversionCharToChar(o.conv));
   EXPECT_FALSE(o.width.is_from_arg());
   EXPECT_LT(o.width.value(), 0);
   EXPECT_FALSE(o.precision.is_from_arg());
   EXPECT_LT(o.precision.value(), 0);
   EXPECT_EQ(1, o.arg_position);
-  EXPECT_EQ(LengthMod::none, o.length_mod.id());
 }
 
 TEST_F(ConsumeUnboundConversionTest, ArgPosition) {
@@ -162,7 +185,7 @@ TEST_F(ConsumeUnboundConversionTest, ArgPosition) {
 
 TEST_F(ConsumeUnboundConversionTest, WidthAndPrecision) {
   EXPECT_TRUE(Run("14d"));
-  EXPECT_EQ('d', o.conv.Char());
+  EXPECT_EQ('d', FormatConversionCharToChar(o.conv));
   EXPECT_FALSE(o.width.is_from_arg());
   EXPECT_EQ(14, o.width.value());
   EXPECT_FALSE(o.precision.is_from_arg());
@@ -253,22 +276,29 @@ TEST_F(ConsumeUnboundConversionTest, WidthAndPrecision) {
 
 TEST_F(ConsumeUnboundConversionTest, Flags) {
   static const char kAllFlags[] = "-+ #0";
-  static const int kNumFlags = ABSL_ARRAYSIZE(kAllFlags) - 1;
+  static const int kNumFlags = std::size(kAllFlags) - 1;
   for (int rev = 0; rev < 2; ++rev) {
     for (int i = 0; i < 1 << kNumFlags; ++i) {
       std::string fmt;
       for (int k = 0; k < kNumFlags; ++k)
         if ((i >> k) & 1) fmt += kAllFlags[k];
       // flag order shouldn't matter
-      if (rev == 1) { std::reverse(fmt.begin(), fmt.end()); }
+      if (rev == 1) {
+        std::reverse(fmt.begin(), fmt.end());
+      }
       fmt += 'd';
       SCOPED_TRACE(fmt);
       EXPECT_TRUE(Run(fmt.c_str()));
-      EXPECT_EQ(fmt.find('-') == std::string::npos, !o.flags.left);
-      EXPECT_EQ(fmt.find('+') == std::string::npos, !o.flags.show_pos);
-      EXPECT_EQ(fmt.find(' ') == std::string::npos, !o.flags.sign_col);
-      EXPECT_EQ(fmt.find('#') == std::string::npos, !o.flags.alt);
-      EXPECT_EQ(fmt.find('0') == std::string::npos, !o.flags.zero);
+      EXPECT_EQ(fmt.find('-') == std::string::npos,
+                !FlagsContains(o.flags, Flags::kLeft));
+      EXPECT_EQ(fmt.find('+') == std::string::npos,
+                !FlagsContains(o.flags, Flags::kShowPos));
+      EXPECT_EQ(fmt.find(' ') == std::string::npos,
+                !FlagsContains(o.flags, Flags::kSignCol));
+      EXPECT_EQ(fmt.find('#') == std::string::npos,
+                !FlagsContains(o.flags, Flags::kAlt));
+      EXPECT_EQ(fmt.find('0') == std::string::npos,
+                !FlagsContains(o.flags, Flags::kZero));
     }
   }
 }
@@ -278,17 +308,40 @@ TEST_F(ConsumeUnboundConversionTest, BasicFlag) {
   for (const char* fmt : {"d", "llx", "G", "1$X"}) {
     SCOPED_TRACE(fmt);
     EXPECT_TRUE(Run(fmt));
-    EXPECT_TRUE(o.flags.basic);
+    EXPECT_EQ(o.flags, Flags::kBasic);
   }
 
   // Flag is off
-  for (const char* fmt : {"3d", ".llx", "-G", "1$#X"}) {
+  for (const char* fmt : {"3d", ".llx", "-G", "1$#X", "lc"}) {
     SCOPED_TRACE(fmt);
     EXPECT_TRUE(Run(fmt));
-    EXPECT_FALSE(o.flags.basic);
+    EXPECT_NE(o.flags, Flags::kBasic);
   }
 }
 
+TEST_F(ConsumeUnboundConversionTest, LengthMod) {
+  EXPECT_TRUE(Run("d"));
+  EXPECT_EQ(LengthMod::none, o.length_mod);
+  EXPECT_TRUE(Run("hd"));
+  EXPECT_EQ(LengthMod::h, o.length_mod);
+  EXPECT_TRUE(Run("hhd"));
+  EXPECT_EQ(LengthMod::hh, o.length_mod);
+  EXPECT_TRUE(Run("ld"));
+  EXPECT_EQ(LengthMod::l, o.length_mod);
+  EXPECT_TRUE(Run("lld"));
+  EXPECT_EQ(LengthMod::ll, o.length_mod);
+  EXPECT_TRUE(Run("Lf"));
+  EXPECT_EQ(LengthMod::L, o.length_mod);
+  EXPECT_TRUE(Run("qf"));
+  EXPECT_EQ(LengthMod::q, o.length_mod);
+  EXPECT_TRUE(Run("jd"));
+  EXPECT_EQ(LengthMod::j, o.length_mod);
+  EXPECT_TRUE(Run("zd"));
+  EXPECT_EQ(LengthMod::z, o.length_mod);
+  EXPECT_TRUE(Run("td"));
+  EXPECT_EQ(LengthMod::t, o.length_mod);
+}
+
 struct SummarizeConsumer {
   std::string* out;
   explicit SummarizeConsumer(std::string* out) : out(out) {}
@@ -309,7 +362,7 @@ struct SummarizeConsumer {
     if (conv.precision.is_from_arg()) {
       *out += "." + std::to_string(conv.precision.get_from_arg()) + "$*";
     }
-    *out += conv.conv.Char();
+    *out += FormatConversionCharToChar(conv.conv);
     *out += "}";
     return true;
   }
@@ -330,7 +383,8 @@ TEST_F(ParsedFormatTest, ValueSemantics) {
   ParsedFormatBase p2 = p1;  // copy construct (empty)
   EXPECT_EQ(SummarizeParsedFormat(p1), SummarizeParsedFormat(p2));
 
-  p1 = ParsedFormatBase("hello%s", true, {Conv::s});  // move assign
+  p1 = ParsedFormatBase("hello%s", true,
+                        {FormatConversionCharSetInternal::s});  // move assign
   EXPECT_EQ("[hello]{s:1$s}", SummarizeParsedFormat(p1));
 
   ParsedFormatBase p3 = p1;  // copy construct (nonempty)
@@ -348,7 +402,7 @@ TEST_F(ParsedFormatTest, ValueSemantics) {
 
 struct ExpectParse {
   const char* in;
-  std::initializer_list<Conv> conv_set;
+  std::initializer_list<FormatConversionCharSet> conv_set;
   const char* out;
 };
 
@@ -358,9 +412,9 @@ TEST_F(ParsedFormatTest, Parsing) {
   const ExpectParse kExpect[] = {
       {"", {}, ""},
       {"ab", {}, "[ab]"},
-      {"a%d", {Conv::d}, "[a]{d:1$d}"},
-      {"a%+d", {Conv::d}, "[a]{+d:1$d}"},
-      {"a% d", {Conv::d}, "[a]{ d:1$d}"},
+      {"a%d", {FormatConversionCharSetInternal::d}, "[a]{d:1$d}"},
+      {"a%+d", {FormatConversionCharSetInternal::d}, "[a]{+d:1$d}"},
+      {"a% d", {FormatConversionCharSetInternal::d}, "[a]{ d:1$d}"},
       {"a%b %d", {}, "[a]!"},  // stop after error
   };
   for (const auto& e : kExpect) {
@@ -372,13 +426,13 @@ TEST_F(ParsedFormatTest, Parsing) {
 
 TEST_F(ParsedFormatTest, ParsingFlagOrder) {
   const ExpectParse kExpect[] = {
-      {"a%+ 0d", {Conv::d}, "[a]{+ 0d:1$d}"},
-      {"a%+0 d", {Conv::d}, "[a]{+0 d:1$d}"},
-      {"a%0+ d", {Conv::d}, "[a]{0+ d:1$d}"},
-      {"a% +0d", {Conv::d}, "[a]{ +0d:1$d}"},
-      {"a%0 +d", {Conv::d}, "[a]{0 +d:1$d}"},
-      {"a% 0+d", {Conv::d}, "[a]{ 0+d:1$d}"},
-      {"a%+   0+d", {Conv::d}, "[a]{+   0+d:1$d}"},
+      {"a%+ 0d", {FormatConversionCharSetInternal::d}, "[a]{+ 0d:1$d}"},
+      {"a%+0 d", {FormatConversionCharSetInternal::d}, "[a]{+0 d:1$d}"},
+      {"a%0+ d", {FormatConversionCharSetInternal::d}, "[a]{0+ d:1$d}"},
+      {"a% +0d", {FormatConversionCharSetInternal::d}, "[a]{ +0d:1$d}"},
+      {"a%0 +d", {FormatConversionCharSetInternal::d}, "[a]{0 +d:1$d}"},
+      {"a% 0+d", {FormatConversionCharSetInternal::d}, "[a]{ 0+d:1$d}"},
+      {"a%+   0+d", {FormatConversionCharSetInternal::d}, "[a]{+   0+d:1$d}"},
   };
   for (const auto& e : kExpect) {
     SCOPED_TRACE(e.in);
@@ -389,4 +443,5 @@ TEST_F(ParsedFormatTest, ParsingFlagOrder) {
 
 }  // namespace
 }  // namespace str_format_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/internal/str_join_internal.h b/absl/strings/internal/str_join_internal.h
index 7c35f4de05e..7698cfaf2fc 100644
--- a/absl/strings/internal/str_join_internal.h
+++ b/absl/strings/internal/str_join_internal.h
@@ -31,18 +31,27 @@
 #ifndef ABSL_STRINGS_INTERNAL_STR_JOIN_INTERNAL_H_
 #define ABSL_STRINGS_INTERNAL_STR_JOIN_INTERNAL_H_
 
+#include <cstdint>
 #include <cstring>
+#include <initializer_list>
 #include <iterator>
+#include <limits>
 #include <memory>
 #include <string>
+#include <tuple>
 #include <type_traits>
 #include <utility>
 
+#include "absl/base/config.h"
+#include "absl/base/internal/iterator_traits.h"
+#include "absl/base/internal/raw_logging.h"
 #include "absl/strings/internal/ostringstream.h"
-#include "absl/strings/internal/resize_uninitialized.h"
+#include "absl/strings/resize_and_overwrite.h"
 #include "absl/strings/str_cat.h"
+#include "absl/strings/string_view.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace strings_internal {
 
 //
@@ -220,36 +229,45 @@ std::string JoinAlgorithm(Iterator start, Iterator end, absl::string_view s,
 // range will be traversed twice: once to calculate the total needed size, and
 // then again to copy the elements and delimiters to the output string.
 template <typename Iterator,
-          typename = typename std::enable_if<std::is_convertible<
-              typename std::iterator_traits<Iterator>::iterator_category,
-              std::forward_iterator_tag>::value>::type>
+          typename = std::enable_if_t<
+              base_internal::IsAtLeastForwardIterator<Iterator>::value>>
 std::string JoinAlgorithm(Iterator start, Iterator end, absl::string_view s,
                           NoFormatter) {
   std::string result;
   if (start != end) {
     // Sums size
-    size_t result_size = start->size();
+    auto&& start_value = *start;
+    // Use uint64_t to prevent size_t overflow. We assume it is not possible for
+    // in memory strings to overflow a uint64_t.
+    uint64_t result_size = start_value.size();
     for (Iterator it = start; ++it != end;) {
       result_size += s.size();
-      result_size += it->size();
+      result_size += (*it).size();
     }
 
     if (result_size > 0) {
-      STLStringResizeUninitialized(&result, result_size);
-
-      // Joins strings
-      char* result_buf = &*result.begin();
-      memcpy(result_buf, start->data(), start->size());
-      result_buf += start->size();
-      for (Iterator it = start; ++it != end;) {
-        memcpy(result_buf, s.data(), s.size());
-        result_buf += s.size();
-        memcpy(result_buf, it->data(), it->size());
-        result_buf += it->size();
-      }
+      constexpr uint64_t kMaxSize =
+          uint64_t{(std::numeric_limits<size_t>::max)()};
+      ABSL_INTERNAL_CHECK(result_size <= kMaxSize, "size_t overflow");
+
+      StringResizeAndOverwrite(
+          result, static_cast<size_t>(result_size),
+          [&start, &end, &start_value, s](char* result_buf,
+                                          size_t result_buf_size) {
+            // Joins strings
+            memcpy(result_buf, start_value.data(), start_value.size());
+            result_buf += start_value.size();
+            for (Iterator it = start; ++it != end;) {
+              memcpy(result_buf, s.data(), s.size());
+              result_buf += s.size();
+              auto&& value = *it;
+              memcpy(result_buf, value.data(), value.size());
+              result_buf += value.size();
+            }
+            return result_buf_size;
+          });
     }
   }
-
   return result;
 }
 
@@ -306,7 +324,17 @@ std::string JoinRange(const Range& range, absl::string_view separator) {
   return JoinRange(begin(range), end(range), separator);
 }
 
+template <typename Tuple, std::size_t... I>
+std::string JoinTuple(const Tuple& value, absl::string_view separator,
+                      std::index_sequence<I...>) {
+  return JoinRange(
+      std::initializer_list<absl::string_view>{
+          static_cast<const AlphaNum&>(std::get<I>(value)).Piece()...},
+      separator);
+}
+
 }  // namespace strings_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_INTERNAL_STR_JOIN_INTERNAL_H_
diff --git a/absl/strings/internal/str_split_internal.h b/absl/strings/internal/str_split_internal.h
index 52f6222615c..b36eac1ced5 100644
--- a/absl/strings/internal/str_split_internal.h
+++ b/absl/strings/internal/str_split_internal.h
@@ -30,75 +30,51 @@
 #define ABSL_STRINGS_INTERNAL_STR_SPLIT_INTERNAL_H_
 
 #include <array>
+#include <cassert>
+#include <cstddef>
 #include <initializer_list>
 #include <iterator>
-#include <map>
+#include <string>
+#include <tuple>
 #include <type_traits>
 #include <utility>
 #include <vector>
 
+#include "absl/base/attributes.h"
 #include "absl/base/macros.h"
 #include "absl/base/port.h"
 #include "absl/meta/type_traits.h"
 #include "absl/strings/string_view.h"
 
-#ifdef _GLIBCXX_DEBUG
-#include "absl/strings/internal/stl_type_traits.h"
-#endif  // _GLIBCXX_DEBUG
-
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace strings_internal {
 
 // This class is implicitly constructible from everything that absl::string_view
-// is implicitly constructible from. If it's constructed from a temporary
-// string, the data is moved into a data member so its lifetime matches that of
-// the ConvertibleToStringView instance.
+// is implicitly constructible from, except for rvalue strings.  This means it
+// can be used as a function parameter in places where passing a temporary
+// string might cause memory lifetime issues.
 class ConvertibleToStringView {
  public:
   ConvertibleToStringView(const char* s)  // NOLINT(runtime/explicit)
-      : value_(s) {}
-  ConvertibleToStringView(char* s) : value_(s) {}  // NOLINT(runtime/explicit)
+      : value_(s) {
+    assert(s != nullptr);
+  }
+  ConvertibleToStringView(char* s) : value_(s) {  // NOLINT(runtime/explicit)
+    assert(s != nullptr);
+  }
   ConvertibleToStringView(absl::string_view s)     // NOLINT(runtime/explicit)
       : value_(s) {}
   ConvertibleToStringView(const std::string& s)  // NOLINT(runtime/explicit)
       : value_(s) {}
 
-  // Matches rvalue strings and moves their data to a member.
-ConvertibleToStringView(std::string&& s)  // NOLINT(runtime/explicit)
-    : copy_(std::move(s)), value_(copy_) {}
-
-  ConvertibleToStringView(const ConvertibleToStringView& other)
-      : copy_(other.copy_),
-        value_(other.IsSelfReferential() ? copy_ : other.value_) {}
-
-  ConvertibleToStringView(ConvertibleToStringView&& other) {
-    StealMembers(std::move(other));
-  }
-
-  ConvertibleToStringView& operator=(ConvertibleToStringView other) {
-    StealMembers(std::move(other));
-    return *this;
-  }
+  // Disable conversion from rvalue strings.
+  ConvertibleToStringView(std::string&& s) = delete;
+  ConvertibleToStringView(const std::string&& s) = delete;
 
   absl::string_view value() const { return value_; }
 
  private:
-  // Returns true if ctsp's value refers to its internal copy_ member.
-  bool IsSelfReferential() const { return value_.data() == copy_.data(); }
-
-  void StealMembers(ConvertibleToStringView&& other) {
-    if (other.IsSelfReferential()) {
-      copy_ = std::move(other.copy_);
-      value_ = copy_;
-      other.value_ = other.copy_;
-    } else {
-      value_ = other.value_;
-    }
-  }
-
-  // Holds the data moved from temporary std::string arguments. Declared first
-  // so that 'value' can refer to 'copy_'.
-  std::string copy_;
   absl::string_view value_;
 };
 
@@ -160,7 +136,8 @@ class SplitIterator {
       const absl::string_view text = splitter_->text();
       const absl::string_view d = delimiter_.Find(text, pos_);
       if (d.data() == text.data() + text.size()) state_ = kLastState;
-      curr_ = text.substr(pos_, d.data() - (text.data() + pos_));
+      curr_ = text.substr(pos_,
+                          static_cast<size_t>(d.data() - (text.data() + pos_)));
       pos_ += curr_.size() + d.size();
     } while (!predicate_(curr_));
     return *this;
@@ -193,21 +170,27 @@ class SplitIterator {
 template <typename T, typename = void>
 struct HasMappedType : std::false_type {};
 template <typename T>
-struct HasMappedType<T, absl::void_t<typename T::mapped_type>>
-    : std::true_type {};
+struct HasMappedType<T, std::void_t<typename T::mapped_type>> : std::true_type {
+};
 
 // HasValueType<T>::value is true iff there exists a type T::value_type.
 template <typename T, typename = void>
 struct HasValueType : std::false_type {};
 template <typename T>
-struct HasValueType<T, absl::void_t<typename T::value_type>> : std::true_type {
-};
+struct HasValueType<T, std::void_t<typename T::value_type>> : std::true_type {};
 
 // HasConstIterator<T>::value is true iff there exists a type T::const_iterator.
 template <typename T, typename = void>
 struct HasConstIterator : std::false_type {};
 template <typename T>
-struct HasConstIterator<T, absl::void_t<typename T::const_iterator>>
+struct HasConstIterator<T, std::void_t<typename T::const_iterator>>
+    : std::true_type {};
+
+// HasEmplace<T>::value is true iff there exists a method T::emplace().
+template <typename T, typename = void>
+struct HasEmplace : std::false_type {};
+template <typename T>
+struct HasEmplace<T, std::void_t<decltype(std::declval<T>().emplace())>>
     : std::true_type {};
 
 // IsInitializerList<T>::value is true iff T is an std::initializer_list. More
@@ -217,7 +200,7 @@ template <typename T>
 std::true_type IsInitializerListDispatch(std::initializer_list<T>*);
 template <typename T>
 struct IsInitializerList
-    : decltype(IsInitializerListDispatch(static_cast<T*>(nullptr))) {};
+    : decltype(IsInitializerListDispatch(static_cast<T*>(nullptr))){};
 
 // A SplitterIsConvertibleTo<C>::type alias exists iff the specified condition
 // is true for type 'C'.
@@ -238,7 +221,7 @@ struct SplitterIsConvertibleToImpl<C, true, false>
 
 template <typename C>
 struct SplitterIsConvertibleToImpl<C, true, true>
-    : absl::conjunction<
+    : std::conjunction<
           std::is_constructible<typename C::key_type, absl::string_view>,
           std::is_constructible<typename C::mapped_type, absl::string_view>> {};
 
@@ -246,14 +229,31 @@ template <typename C>
 struct SplitterIsConvertibleTo
     : SplitterIsConvertibleToImpl<
           C,
-#ifdef _GLIBCXX_DEBUG
-          !IsStrictlyBaseOfAndConvertibleToSTLContainer<C>::value &&
-#endif  // _GLIBCXX_DEBUG
-              !IsInitializerList<
-                  typename std::remove_reference<C>::type>::value &&
+          !IsInitializerList<std::remove_reference_t<C>>::value &&
               HasValueType<C>::value && HasConstIterator<C>::value,
-          HasMappedType<C>::value> {
-};
+          HasMappedType<C>::value> {};
+
+template <typename StringType, typename Container, typename = void>
+struct ShouldUseLifetimeBound : std::false_type {};
+
+template <typename StringType, typename Container>
+struct ShouldUseLifetimeBound<
+    StringType, Container,
+    std::enable_if_t<
+        std::is_same_v<StringType, std::string> &&
+        std::is_same_v<typename Container::value_type, absl::string_view>>>
+    : std::true_type {};
+
+template <typename StringType, typename First, typename Second>
+using ShouldUseLifetimeBoundForPair =
+    std::bool_constant<std::is_same_v<StringType, std::string> &&
+                       (std::is_same_v<First, absl::string_view> ||
+                        std::is_same_v<Second, absl::string_view>)>;
+
+template <typename StringType, typename ElementType, std::size_t Size>
+using ShouldUseLifetimeBoundForArray =
+    std::bool_constant<std::is_same_v<StringType, std::string> &&
+                       std::is_same_v<ElementType, absl::string_view>>;
 
 // This class implements the range that is returned by absl::StrSplit(). This
 // class has templated conversion operators that allow it to be implicitly
@@ -272,7 +272,11 @@ struct SplitterIsConvertibleTo
 // the split strings: only strings for which the predicate returns true will be
 // kept. A Predicate object is any unary functor that takes an absl::string_view
 // and returns bool.
-template <typename Delimiter, typename Predicate>
+//
+// The StringType parameter can be either string_view or string, depending on
+// whether the Splitter refers to a string stored elsewhere, or if the string
+// resides inside the Splitter itself.
+template <typename Delimiter, typename Predicate, typename StringType>
 class Splitter {
  public:
   using DelimiterType = Delimiter;
@@ -280,12 +284,12 @@ class Splitter {
   using const_iterator = strings_internal::SplitIterator<Splitter>;
   using value_type = typename std::iterator_traits<const_iterator>::value_type;
 
-  Splitter(ConvertibleToStringView input_text, Delimiter d, Predicate p)
+  Splitter(StringType input_text, Delimiter d, Predicate p)
       : text_(std::move(input_text)),
         delimiter_(std::move(d)),
         predicate_(std::move(p)) {}
 
-  absl::string_view text() const { return text_.value(); }
+  absl::string_view text() const { return text_; }
   const Delimiter& delimiter() const { return delimiter_; }
   const Predicate& predicate() const { return predicate_; }
 
@@ -297,10 +301,24 @@ class Splitter {
 
   // An implicit conversion operator that is restricted to only those containers
   // that the splitter is convertible to.
-  template <typename Container,
-            typename = typename std::enable_if<
-                SplitterIsConvertibleTo<Container>::value>::type>
-  operator Container() const {  // NOLINT(runtime/explicit)
+  template <
+      typename Container,
+      std::enable_if_t<ShouldUseLifetimeBound<StringType, Container>::value &&
+                           SplitterIsConvertibleTo<Container>::value,
+                       std::nullptr_t> = nullptr>
+  // NOLINTNEXTLINE(google-explicit-constructor)
+  operator Container() const ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return ConvertToContainer<Container, typename Container::value_type,
+                              HasMappedType<Container>::value>()(*this);
+  }
+
+  template <
+      typename Container,
+      std::enable_if_t<!ShouldUseLifetimeBound<StringType, Container>::value &&
+                           SplitterIsConvertibleTo<Container>::value,
+                       std::nullptr_t> = nullptr>
+  // NOLINTNEXTLINE(google-explicit-constructor)
+  operator Container() const {
     return ConvertToContainer<Container, typename Container::value_type,
                               HasMappedType<Container>::value>()(*this);
   }
@@ -309,8 +327,58 @@ class Splitter {
   // strings returned by the begin() iterator. Either/both of .first and .second
   // will be constructed with empty strings if the iterator doesn't have a
   // corresponding value.
+  template <typename First, typename Second,
+            std::enable_if_t<
+                ShouldUseLifetimeBoundForPair<StringType, First, Second>::value,
+                std::nullptr_t> = nullptr>
+  // NOLINTNEXTLINE(google-explicit-constructor)
+  operator std::pair<First, Second>() const ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return ConvertToPair<First, Second>();
+  }
+
+  template <typename First, typename Second,
+            std::enable_if_t<!ShouldUseLifetimeBoundForPair<StringType, First,
+                                                            Second>::value,
+                             std::nullptr_t> = nullptr>
+  // NOLINTNEXTLINE(google-explicit-constructor)
+  operator std::pair<First, Second>() const {
+    return ConvertToPair<First, Second>();
+  }
+
+  // Returns an array with its elements set to the first few strings returned by
+  // the begin() iterator.  If there is not a corresponding value the empty
+  // string is used.
+  template <typename ElementType, std::size_t Size,
+            std::enable_if_t<ShouldUseLifetimeBoundForArray<
+                                 StringType, ElementType, Size>::value,
+                             std::nullptr_t> = nullptr>
+  // NOLINTNEXTLINE(google-explicit-constructor)
+  operator std::array<ElementType, Size>() const ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return ConvertToArray<ElementType, Size>();
+  }
+
+  template <typename ElementType, std::size_t Size,
+            std::enable_if_t<!ShouldUseLifetimeBoundForArray<
+                                 StringType, ElementType, Size>::value,
+                             std::nullptr_t> = nullptr>
+  // NOLINTNEXTLINE(google-explicit-constructor)
+  operator std::array<ElementType, Size>() const {
+    return ConvertToArray<ElementType, Size>();
+  }
+
+ private:
+  template <typename ElementType, std::size_t Size>
+  std::array<ElementType, Size> ConvertToArray() const {
+    std::array<ElementType, Size> a;
+    auto it = begin();
+    for (std::size_t i = 0; i < Size && it != end(); ++i, ++it) {
+      a[i] = ElementType(*it);
+    }
+    return a;
+  }
+
   template <typename First, typename Second>
-  operator std::pair<First, Second>() const {  // NOLINT(runtime/explicit)
+  std::pair<First, Second> ConvertToPair() const {
     absl::string_view first, second;
     auto it = begin();
     if (it != end()) {
@@ -322,7 +390,6 @@ class Splitter {
     return {First(first), Second(second)};
   }
 
- private:
   // ConvertToContainer is a functor converting a Splitter to the requested
   // Container of ValueType. It is specialized below to optimize splitting to
   // certain combinations of Container and ValueType.
@@ -335,7 +402,7 @@ class Splitter {
     Container operator()(const Splitter& splitter) const {
       Container c;
       auto it = std::inserter(c, c.end());
-      for (const auto sp : splitter) {
+      for (const auto& sp : splitter) {
         *it++ = ValueType(sp);
       }
       return c;
@@ -368,7 +435,10 @@ class Splitter {
           ar[index].size = it->size();
           ++it;
         } while (++index != ar.size() && !it.at_end());
-        v.insert(v.end(), ar.begin(), ar.begin() + index);
+        // We static_cast index to a signed type to work around overzealous
+        // compiler warnings about signedness.
+        v.insert(v.end(), ar.begin(),
+                 ar.begin() + static_cast<ptrdiff_t>(index));
       }
       return v;
     }
@@ -396,58 +466,52 @@ class Splitter {
   // value.
   template <typename Container, typename First, typename Second>
   struct ConvertToContainer<Container, std::pair<const First, Second>, true> {
+    using iterator = typename Container::iterator;
+
     Container operator()(const Splitter& splitter) const {
       Container m;
-      typename Container::iterator it;
+      iterator it;
       bool insert = true;
-      for (const auto sp : splitter) {
+      for (const absl::string_view sv : splitter) {
         if (insert) {
-          it = Inserter<Container>::Insert(&m, First(sp), Second());
+          it = InsertOrEmplace(&m, sv);
         } else {
-          it->second = Second(sp);
+          it->second = Second(sv);
         }
         insert = !insert;
       }
       return m;
     }
 
-    // Inserts the key and value into the given map, returning an iterator to
-    // the inserted item. Specialized for std::map and std::multimap to use
-    // emplace() and adapt emplace()'s return value.
-    template <typename Map>
-    struct Inserter {
-      using M = Map;
-      template <typename... Args>
-      static typename M::iterator Insert(M* m, Args&&... args) {
-        return m->insert(std::make_pair(std::forward<Args>(args)...)).first;
-      }
-    };
-
-    template <typename... Ts>
-    struct Inserter<std::map<Ts...>> {
-      using M = std::map<Ts...>;
-      template <typename... Args>
-      static typename M::iterator Insert(M* m, Args&&... args) {
-        return m->emplace(std::make_pair(std::forward<Args>(args)...)).first;
-      }
-    };
-
-    template <typename... Ts>
-    struct Inserter<std::multimap<Ts...>> {
-      using M = std::multimap<Ts...>;
-      template <typename... Args>
-      static typename M::iterator Insert(M* m, Args&&... args) {
-        return m->emplace(std::make_pair(std::forward<Args>(args)...));
-      }
-    };
+    // Inserts the key and an empty value into the map, returning an iterator to
+    // the inserted item. We use emplace() if available, otherwise insert().
+    template <typename M>
+    static std::enable_if_t<HasEmplace<M>::value, iterator> InsertOrEmplace(
+        M* m, absl::string_view key) {
+      // Use piecewise_construct to support old versions of gcc in which pair
+      // constructor can't otherwise construct string from string_view.
+      return ToIter(m->emplace(std::piecewise_construct, std::make_tuple(key),
+                               std::tuple<>()));
+    }
+    template <typename M>
+    static std::enable_if_t<!HasEmplace<M>::value, iterator> InsertOrEmplace(
+        M* m, absl::string_view key) {
+      return ToIter(m->insert(std::make_pair(First(key), Second(""))));
+    }
+
+    static iterator ToIter(std::pair<iterator, bool> pair) {
+      return pair.first;
+    }
+    static iterator ToIter(iterator iter) { return iter; }
   };
 
-  ConvertibleToStringView text_;
+  StringType text_;
   Delimiter delimiter_;
   Predicate predicate_;
 };
 
 }  // namespace strings_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_INTERNAL_STR_SPLIT_INTERNAL_H_
diff --git a/absl/strings/internal/string_constant.h b/absl/strings/internal/string_constant.h
new file mode 100644
index 00000000000..d52c3308462
--- /dev/null
+++ b/absl/strings/internal/string_constant.h
@@ -0,0 +1,67 @@
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_STRINGS_INTERNAL_STRING_CONSTANT_H_
+#define ABSL_STRINGS_INTERNAL_STRING_CONSTANT_H_
+
+#include "absl/meta/type_traits.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace strings_internal {
+
+// StringConstant<T> represents a compile time string constant.
+// It can be accessed via its `absl::string_view value` static member.
+// It is guaranteed that the `string_view` returned has constant `.data()`,
+// constant `.size()` and constant `value[i]` for all `0 <= i < .size()`
+//
+// The `T` is an opaque type. It is guaranteed that different string constants
+// will have different values of `T`. This allows users to associate the string
+// constant with other static state at compile time.
+//
+// Instances should be made using the `MakeStringConstant()` factory function
+// below.
+template <typename T>
+struct StringConstant {
+ private:
+  static constexpr bool TryConstexprEval(absl::string_view view) {
+    return view.empty() || 2 * view[0] != 1;
+  }
+
+ public:
+  static constexpr absl::string_view value = T{}();
+  constexpr absl::string_view operator()() const { return value; }
+
+  // Check to be sure `view` points to constant data.
+  // Otherwise, it can't be constant evaluated.
+  static_assert(TryConstexprEval(value),
+                "The input string_view must point to constant data.");
+};
+
+// Factory function for `StringConstant` instances.
+// It supports callables that have a constexpr default constructor and a
+// constexpr operator().
+// It must return an `absl::string_view` or `const char*` pointing to constant
+// data. This is validated at compile time.
+template <typename T>
+constexpr StringConstant<T> MakeStringConstant(T) {
+  return {};
+}
+
+}  // namespace strings_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STRINGS_INTERNAL_STRING_CONSTANT_H_
diff --git a/absl/strings/internal/string_constant_test.cc b/absl/strings/internal/string_constant_test.cc
new file mode 100644
index 00000000000..9743c52735f
--- /dev/null
+++ b/absl/strings/internal/string_constant_test.cc
@@ -0,0 +1,62 @@
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/strings/internal/string_constant.h"
+
+#include <type_traits>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/strings/string_view.h"
+
+namespace {
+
+using absl::strings_internal::MakeStringConstant;
+
+struct Callable {
+  constexpr absl::string_view operator()() const {
+    return absl::string_view("Callable", 8);
+  }
+};
+
+TEST(StringConstant, Traits) {
+  constexpr auto str = MakeStringConstant(Callable{});
+  using T = decltype(str);
+
+  EXPECT_TRUE(std::is_empty_v<T>);
+  EXPECT_TRUE(std::is_trivial_v<T>);
+  EXPECT_TRUE(std::is_trivially_default_constructible_v<T>);
+  EXPECT_TRUE(std::is_trivially_copy_constructible_v<T>);
+  EXPECT_TRUE(std::is_trivially_move_constructible_v<T>);
+  EXPECT_TRUE(std::is_trivially_destructible_v<T>);
+}
+
+TEST(StringConstant, MakeFromCallable) {
+  constexpr auto str = MakeStringConstant(Callable{});
+  using T = decltype(str);
+  EXPECT_EQ(Callable{}(), T::value);
+  EXPECT_EQ(Callable{}(), str());
+}
+
+TEST(StringConstant, MakeFromStringConstant) {
+  // We want to make sure the StringConstant itself is a valid input to the
+  // factory function.
+  constexpr auto str = MakeStringConstant(Callable{});
+  constexpr auto str2 = MakeStringConstant(str);
+  using T = decltype(str2);
+  EXPECT_EQ(Callable{}(), T::value);
+  EXPECT_EQ(Callable{}(), str2());
+}
+
+}  // namespace
diff --git a/absl/strings/internal/stringify_sink.cc b/absl/strings/internal/stringify_sink.cc
new file mode 100644
index 00000000000..4efe80f8f85
--- /dev/null
+++ b/absl/strings/internal/stringify_sink.cc
@@ -0,0 +1,32 @@
+// Copyright 2022 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/strings/internal/stringify_sink.h"
+
+#include <cstddef>
+
+#include "absl/strings/string_view.h"
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace strings_internal {
+
+void StringifySink::Append(size_t count, char ch) { buffer_.append(count, ch); }
+
+void StringifySink::Append(string_view v) {
+  buffer_.append(v.data(), v.size());
+}
+
+}  // namespace strings_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/strings/internal/stringify_sink.h b/absl/strings/internal/stringify_sink.h
new file mode 100644
index 00000000000..b2e75d92856
--- /dev/null
+++ b/absl/strings/internal/stringify_sink.h
@@ -0,0 +1,70 @@
+// Copyright 2022 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_STRINGS_INTERNAL_STRINGIFY_SINK_H_
+#define ABSL_STRINGS_INTERNAL_STRINGIFY_SINK_H_
+
+#include <array>
+#include <cstddef>
+#include <string>
+#include <type_traits>
+#include <utility>
+
+#include "absl/strings/numbers.h"
+#include "absl/strings/string_view.h"
+#include "absl/types/source_location.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+namespace strings_internal {
+class StringifySink {
+ public:
+  void Append(size_t count, char ch);
+
+  void Append(string_view v);
+
+  // Support `absl::Format(&sink, format, args...)`.
+  friend void AbslFormatFlush(StringifySink* sink, absl::string_view v) {
+    sink->Append(v);
+  }
+
+ private:
+  template <typename T>
+  friend string_view ExtractStringification(StringifySink& sink, const T& v);
+
+  std::string buffer_;
+};
+
+template <typename T>
+string_view ExtractStringification(StringifySink& sink, const T& v) {
+  AbslStringify(sink, v);
+  return sink.buffer_;
+}
+
+}  // namespace strings_internal
+
+template <typename Sink>
+void AbslStringify(Sink& sink, SourceLocation l) {
+  sink.Append(l.file_name());
+  sink.Append(":");
+  std::array<char, numbers_internal::kFastToBufferSize> buffer;
+  numbers_internal::FastIntToBuffer(l.line(), buffer.data());
+  sink.Append(buffer.data());
+}
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STRINGS_INTERNAL_STRINGIFY_SINK_H_
diff --git a/absl/strings/internal/stringify_stream.h b/absl/strings/internal/stringify_stream.h
new file mode 100644
index 00000000000..6110b338596
--- /dev/null
+++ b/absl/strings/internal/stringify_stream.h
@@ -0,0 +1,119 @@
+// Copyright 2026 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: stringify_stream.h
+// -----------------------------------------------------------------------------
+
+#ifndef ABSL_STRINGS_INTERNAL_STRINGIFY_STREAM_H_
+#define ABSL_STRINGS_INTERNAL_STRINGIFY_STREAM_H_
+
+// StringifyStream is an adaptor for any std::ostream, that provides a
+// stream insertion (<<) operator with the following behavior when inserting
+// some value of type T:
+//
+//   - If there is a suitable overload of operator<< already defined for T, it
+//     will be used.
+//
+//   - If there is no operator<< overload, but there is an AbslStringify defined
+//     for T, it will be used as a fallback.
+//
+//   - Otherwise it is a compilation error.
+//
+// For reference, AbslStringify typically has the form:
+//
+//   struct Foo {
+//     template <typename Sink>
+//     friend void AbslStringify(Sink& sink, const Foo& foo) { ... }
+//   };
+//
+// This permits the following usage, for example:
+//
+//   StringifyStream(std::cout) << Foo();
+//
+
+#include <cstddef>
+#include <ostream>
+#include <string>
+#include <type_traits>
+#include <utility>
+
+#include "absl/base/config.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+namespace strings_internal {
+
+class StringifyStream {
+ public:
+  // Constructor: adapts (but does not take ownership of) some underlying
+  // std::ostream instance.
+  explicit StringifyStream(std::ostream& os) : sink_{os} {}
+
+  // Stream insertion: delegate to an overload of operator<< if defined for type
+  // T, otherwise fall back to AbslStringify for T.
+  template <typename T>
+  friend StringifyStream& operator<<(StringifyStream& stream, const T& t) {
+    if constexpr (HasStreamInsertion<T>::value) {
+      stream.sink_.os << t;
+    } else {
+      AbslStringify(stream.sink_, t);
+    }
+    return stream;
+  }
+
+  // Rvalue-ref overload, required when the StringifyStream parameter hasn't
+  // been bound to a variable.
+  template <typename T>
+  friend StringifyStream& operator<<(StringifyStream&& stream, const T& t) {
+    return stream << t;
+  }
+
+  // Overload for things like << std::endl which need an explicit type in order
+  // to resolve to the appropriate overload or template instantiation.
+  StringifyStream& operator<<(std::ostream& (*func)(std::ostream&)) {
+    sink_.os << func;
+    return *this;
+  }
+
+ private:
+  // Abseil "stringify sink" concept (stringify_sink.h)
+  struct Sink {
+    std::ostream& os;
+    void Append(size_t count, char ch) { os << std::string(count, ch); }
+    void Append(absl::string_view v) { os << v; }
+    friend void AbslFormatFlush(Sink* sink, absl::string_view v) {
+      sink->Append(v);
+    }
+  } sink_;
+
+  // SFINAE helper to identify types with a defined operator<< overload.
+  template <typename T, typename = void>
+  struct HasStreamInsertion : std::false_type {};
+
+  template <typename T>
+  struct HasStreamInsertion<T,
+                            std::void_t<decltype(std::declval<std::ostream&>()
+                                                 << std::declval<const T&>())>>
+      : std::true_type {};
+};
+
+}  // namespace strings_internal
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STRINGS_INTERNAL_STRINGIFY_STREAM_H_
diff --git a/absl/strings/internal/stringify_stream_test.cc b/absl/strings/internal/stringify_stream_test.cc
new file mode 100644
index 00000000000..71a769189dd
--- /dev/null
+++ b/absl/strings/internal/stringify_stream_test.cc
@@ -0,0 +1,111 @@
+// Copyright 2026 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/strings/internal/stringify_stream.h"
+
+#include <cstddef>
+#include <iomanip>
+#include <ostream>
+#include <sstream>
+
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+#include "absl/strings/str_format.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace strings_internal {
+namespace {
+
+// Exercises the Append(size_t, char) overload
+struct AppendNCharsTest {
+  size_t count;
+  char ch;
+
+  template <typename Sink>
+  friend void AbslStringify(Sink& sink, const AppendNCharsTest& t) {
+    sink.Append(t.count, t.ch);
+  }
+};
+TEST(StringifyStreamTest, AppendNChars) {
+  std::ostringstream os;
+  StringifyStream(os) << AppendNCharsTest{5, 'a'};
+  EXPECT_EQ(os.str(), "aaaaa");
+}
+
+// Exercises the Append(absl::string_view) overload
+struct AppendStringViewTest {
+  absl::string_view v;
+
+  template <typename Sink>
+  friend void AbslStringify(Sink& sink, const AppendStringViewTest& t) {
+    sink.Append(t.v);
+  }
+};
+TEST(StringifyStreamTest, AppendStringView) {
+  std::ostringstream os;
+  StringifyStream(os) << AppendStringViewTest{"abc"};
+  EXPECT_EQ(os.str(), "abc");
+}
+
+// Exercises AbslFormatFlush(OStringStreamSink*, absl::string_view v)
+struct AbslFormatFlushTest {
+  absl::string_view a, b, c;
+
+  template <typename Sink>
+  friend void AbslStringify(Sink& sink, const AbslFormatFlushTest& t) {
+    absl::Format(&sink, "%s, %s, %s", t.a, t.b, t.c);
+  }
+};
+TEST(StringifyStreamTest, AbslFormatFlush) {
+  std::ostringstream os;
+  StringifyStream(os) << AbslFormatFlushTest{"a", "b", "c"};
+  EXPECT_EQ(os.str(), "a, b, c");
+}
+
+// If overloads of both AbslStringify and operator<< are defined for the type,
+// the operator<< overload should take precedence.
+struct PreferStreamInsertionOverAbslStringifyTest {
+  friend std::ostream& operator<<(  // NOLINT(clang-diagnostic-unused-function)
+      std::ostream& os, const PreferStreamInsertionOverAbslStringifyTest&) {
+    return os << "good";
+  }
+
+  template <typename Sink>
+  friend void AbslStringify  // NOLINT(clang-diagnostic-unused-function)
+      (Sink& sink, const PreferStreamInsertionOverAbslStringifyTest&) {
+    sink.Append("bad");
+  }
+};
+TEST(StringifyStreamTest, PreferStreamInsertionOverAbslStringify) {
+  std::ostringstream os;
+  StringifyStream(os) << PreferStreamInsertionOverAbslStringifyTest{};
+  EXPECT_EQ(os.str(), "good");
+}
+TEST(StringifyStreamTest, SupportEndl) {
+  std::ostringstream os;
+  StringifyStream(os) << std::endl;
+  EXPECT_EQ(os.str(), "\n");
+}
+TEST(StringifyStreamTest, SupportSetbase) {
+  std::ostringstream os;
+  StringifyStream(os) << std::setbase(16) << 255;
+  EXPECT_EQ(os.str(), "ff");
+}
+
+}  // namespace
+}  // namespace strings_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/strings/internal/utf8.cc b/absl/strings/internal/utf8.cc
index 82d36c24b34..61945f5869b 100644
--- a/absl/strings/internal/utf8.cc
+++ b/absl/strings/internal/utf8.cc
@@ -16,36 +16,133 @@
 
 #include "absl/strings/internal/utf8.h"
 
+#include <cstddef>
+#include <cstdint>
+#include <limits>
+
+#include "absl/base/config.h"
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace strings_internal {
 
-size_t EncodeUTF8Char(char *buffer, char32_t utf8_char) {
+size_t EncodeUTF8Char(char* buffer, char32_t utf8_char) {
   if (utf8_char <= 0x7F) {
     *buffer = static_cast<char>(utf8_char);
     return 1;
   } else if (utf8_char <= 0x7FF) {
-    buffer[1] = 0x80 | (utf8_char & 0x3F);
+    buffer[1] = static_cast<char>(0x80 | (utf8_char & 0x3F));
     utf8_char >>= 6;
-    buffer[0] = 0xC0 | utf8_char;
+    buffer[0] = static_cast<char>(0xC0 | utf8_char);
     return 2;
   } else if (utf8_char <= 0xFFFF) {
-    buffer[2] = 0x80 | (utf8_char & 0x3F);
+    buffer[2] = static_cast<char>(0x80 | (utf8_char & 0x3F));
     utf8_char >>= 6;
-    buffer[1] = 0x80 | (utf8_char & 0x3F);
+    buffer[1] = static_cast<char>(0x80 | (utf8_char & 0x3F));
     utf8_char >>= 6;
-    buffer[0] = 0xE0 | utf8_char;
+    buffer[0] = static_cast<char>(0xE0 | utf8_char);
     return 3;
   } else {
-    buffer[3] = 0x80 | (utf8_char & 0x3F);
+    buffer[3] = static_cast<char>(0x80 | (utf8_char & 0x3F));
     utf8_char >>= 6;
-    buffer[2] = 0x80 | (utf8_char & 0x3F);
+    buffer[2] = static_cast<char>(0x80 | (utf8_char & 0x3F));
     utf8_char >>= 6;
-    buffer[1] = 0x80 | (utf8_char & 0x3F);
+    buffer[1] = static_cast<char>(0x80 | (utf8_char & 0x3F));
     utf8_char >>= 6;
-    buffer[0] = 0xF0 | utf8_char;
+    buffer[0] = static_cast<char>(0xF0 | utf8_char);
     return 4;
   }
 }
 
+size_t WideToUtf8(wchar_t wc, char* buf, ShiftState& s) {
+  // Reinterpret the output buffer `buf` as `unsigned char*` for subsequent
+  // bitwise operations. This ensures well-defined behavior for bit
+  // manipulations (avoiding issues with signed `char`) and is safe under C++
+  // aliasing rules, as `unsigned char` can alias any type.
+  auto* ubuf = reinterpret_cast<unsigned char*>(buf);
+  const uint32_t v = static_cast<uint32_t>(wc);
+  constexpr size_t kError = static_cast<size_t>(-1);
+
+  if (v <= 0x007F) {
+    // 1-byte sequence (U+0000 to U+007F).
+    // 0xxxxxxx.
+    ubuf[0] = (0b0111'1111 & v);
+    s = {};  // Reset surrogate state.
+    return 1;
+  } else if (0x0080 <= v && v <= 0x07FF) {
+    // 2-byte sequence (U+0080 to U+07FF).
+    // 110xxxxx 10xxxxxx.
+    ubuf[0] = 0b1100'0000 | (0b0001'1111 & (v >> 6));
+    ubuf[1] = 0b1000'0000 | (0b0011'1111 & v);
+    s = {};  // Reset surrogate state.
+    return 2;
+  } else if ((0x0800 <= v && v <= 0xD7FF) || (0xE000 <= v && v <= 0xFFFF)) {
+    // 3-byte sequence (U+0800 to U+D7FF or U+E000 to U+FFFF).
+    // Excludes surrogate code points U+D800-U+DFFF.
+    // 1110xxxx 10xxxxxx 10xxxxxx.
+    ubuf[0] = 0b1110'0000 | (0b0000'1111 & (v >> 12));
+    ubuf[1] = 0b1000'0000 | (0b0011'1111 & (v >> 6));
+    ubuf[2] = 0b1000'0000 | (0b0011'1111 & v);
+    s = {};  // Reset surrogate state.
+    return 3;
+  } else if (0xD800 <= v && v <= 0xDBFF) {
+    // High Surrogate (U+D800 to U+DBFF).
+    // This part forms the first two bytes of an eventual 4-byte UTF-8 sequence.
+    const unsigned char high_bits_val = (0b0000'1111 & (v >> 6)) + 1;
+
+    // First byte of the 4-byte UTF-8 sequence (11110xxx).
+    ubuf[0] = 0b1111'0000 | (0b0000'0111 & (high_bits_val >> 2));
+    // Second byte of the 4-byte UTF-8 sequence (10xxxxxx).
+    ubuf[1] = 0b1000'0000 |                           //
+              (0b0011'0000 & (high_bits_val << 4)) |  //
+              (0b0000'1111 & (v >> 2));
+    // Set state for high surrogate after writing to buffer.
+    s = {true, static_cast<unsigned char>(0b0000'0011 & v)};
+    return 2;  // Wrote 2 bytes, expecting 2 more from a low surrogate.
+  } else if (0xDC00 <= v && v <= 0xDFFF) {
+    // Low Surrogate (U+DC00 to U+DFFF).
+    // This part forms the last two bytes of a 4-byte UTF-8 sequence,
+    // using state from a preceding high surrogate.
+    if (!s.saw_high_surrogate) {
+      // Error: Isolated low surrogate without a preceding high surrogate.
+      // s remains in its current (problematic) state.
+      // Caller should handle error.
+      return kError;
+    }
+
+    // Third byte of the 4-byte UTF-8 sequence (10xxxxxx).
+    ubuf[0] = 0b1000'0000 |                    //
+              (0b0011'0000 & (s.bits << 4)) |  //
+              (0b0000'1111 & (v >> 6));
+    // Fourth byte of the 4-byte UTF-8 sequence (10xxxxxx).
+    ubuf[1] = 0b1000'0000 | (0b0011'1111 & v);
+
+    s = {};    // Reset surrogate state, pair complete.
+    return 2;  // Wrote 2 more bytes, completing the 4-byte sequence.
+  } else if constexpr (0xFFFF < std::numeric_limits<wchar_t>::max()) {
+    // Conditionally compile the 4-byte direct conversion branch.
+    // This block is compiled only if wchar_t can represent values > 0xFFFF.
+    // It's placed after surrogate checks to ensure surrogates are handled by
+    // their specific logic. This inner 'if' is the runtime check for the 4-byte
+    // range. At this point, v is known not to be in the 1, 2, or 3-byte BMP
+    // ranges, nor is it a surrogate code point.
+    if (0x10000 <= v && v <= 0x10FFFF) {
+      // 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx.
+      ubuf[0] = 0b1111'0000 | (0b0000'0111 & (v >> 18));
+      ubuf[1] = 0b1000'0000 | (0b0011'1111 & (v >> 12));
+      ubuf[2] = 0b1000'0000 | (0b0011'1111 & (v >> 6));
+      ubuf[3] = 0b1000'0000 | (0b0011'1111 & v);
+      s = {};  // Reset surrogate state.
+      return 4;
+    }
+  }
+
+  // Invalid wchar_t value (e.g., out of Unicode range, or unhandled after all
+  // checks).
+  s = {};  // Reset surrogate state.
+  return kError;
+}
+
 }  // namespace strings_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/internal/utf8.h b/absl/strings/internal/utf8.h
index 0423630473b..ed1db110a7b 100644
--- a/absl/strings/internal/utf8.h
+++ b/absl/strings/internal/utf8.h
@@ -20,7 +20,10 @@
 #include <cstddef>
 #include <cstdint>
 
+#include "absl/base/config.h"
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace strings_internal {
 
 // For Unicode code points 0 through 0x10FFFF, EncodeUTF8Char writes
@@ -38,9 +41,24 @@ namespace strings_internal {
 // characters into buffer, however never will more than kMaxEncodedUTF8Size
 // bytes be written, regardless of the value of utf8_char.
 enum { kMaxEncodedUTF8Size = 4 };
-size_t EncodeUTF8Char(char *buffer, char32_t utf8_char);
+size_t EncodeUTF8Char(char* buffer, char32_t utf8_char);
+
+struct ShiftState {
+  bool saw_high_surrogate = false;
+  unsigned char bits = 0;
+};
+
+// Converts `wc` from UTF-16 or UTF-32 to UTF-8 and writes to `buf`. `buf` is
+// assumed to have enough space for the output. `s` is used to carry state
+// between successive calls with a UTF-16 surrogate pair. Returns the number of
+// chars written, or `static_cast<size_t>(-1)` on failure.
+//
+// This is basically std::wcrtomb(), but always outputting UTF-8 instead of
+// respecting the current locale.
+size_t WideToUtf8(wchar_t wc, char* buf, ShiftState& s);
 
 }  // namespace strings_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_INTERNAL_UTF8_H_
diff --git a/absl/strings/internal/utf8_test.cc b/absl/strings/internal/utf8_test.cc
index 88dd5036e3d..b88d7bb88a4 100644
--- a/absl/strings/internal/utf8_test.cc
+++ b/absl/strings/internal/utf8_test.cc
@@ -14,14 +14,29 @@
 
 #include "absl/strings/internal/utf8.h"
 
+#include <cstddef>
 #include <cstdint>
+#include <cstring>
+#include <string>
+#include <type_traits>
 #include <utility>
+#include <vector>
 
+#include "gmock/gmock.h"
 #include "gtest/gtest.h"
 #include "absl/base/port.h"
+#include "absl/strings/string_view.h"
 
 namespace {
 
+using ::absl::strings_internal::kMaxEncodedUTF8Size;
+using ::absl::strings_internal::ShiftState;
+using ::absl::strings_internal::WideToUtf8;
+using ::testing::StartsWith;
+using ::testing::TestParamInfo;
+using ::testing::TestWithParam;
+using ::testing::ValuesIn;
+
 #if !defined(__cpp_char8_t)
 #if defined(__clang__)
 #pragma clang diagnostic push
@@ -33,12 +48,12 @@ TEST(EncodeUTF8Char, BasicFunction) {
                                               {0x00010000, u8"\U00010000"},
                                               {0x0000FFFF, u8"\U0000FFFF"},
                                               {0x0010FFFD, u8"\U0010FFFD"}};
-  for (auto &test : tests) {
+  for (auto& test : tests) {
     char buf0[7] = {'\x00', '\x00', '\x00', '\x00', '\x00', '\x00', '\x00'};
     char buf1[7] = {'\xFF', '\xFF', '\xFF', '\xFF', '\xFF', '\xFF', '\xFF'};
-    char *buf0_written =
+    char* buf0_written =
         &buf0[absl::strings_internal::EncodeUTF8Char(buf0, test.first)];
-    char *buf1_written =
+    char* buf1_written =
         &buf1[absl::strings_internal::EncodeUTF8Char(buf1, test.first)];
     int apparent_length = 7;
     while (buf0[apparent_length - 1] == '\x00' &&
@@ -63,4 +78,182 @@ TEST(EncodeUTF8Char, BasicFunction) {
 #endif
 #endif  // !defined(__cpp_char8_t)
 
+struct WideToUtf8TestCase {
+  std::string description;
+  wchar_t input;
+  std::string expected_utf8_str;
+  size_t expected_bytes_written;
+  ShiftState initial_state = {false, 0};
+  ShiftState expected_state = {false, 0};
+};
+
+std::vector<WideToUtf8TestCase> GetWideToUtf8TestCases() {
+  constexpr size_t kError = static_cast<size_t>(-1);
+  std::vector<WideToUtf8TestCase> cases = {
+      {"ASCII_A", L'A', "A", 1},
+      {"NullChar", L'\0', std::string("\0", 1), 1},
+      {"ASCII_Max_7F", L'\x7F', "\x7F", 1},
+
+      {"TwoByte_Min_80", L'\u0080', "\xC2\x80", 2},
+      {"PoundSign_A3", L'\u00A3', "\xC2\xA3", 2},
+      {"TwoByte_Max_7FF", L'\u07FF', "\xDF\xBF", 2},
+
+      {"ThreeByte_Min_800", L'\u0800', "\xE0\xA0\x80", 3},
+      {"EuroSign_20AC", L'\u20AC', "\xE2\x82\xAC", 3},
+      {"BMP_MaxBeforeSurrogates_D7FF", L'\uD7FF', "\xED\x9F\xBF", 3},
+      {"BMP_FFFF", L'\uFFFF', "\xEF\xBF\xBF", 3},
+
+      {"IsolatedHighSurr_D800", L'\xD800', "\xF0\x90", 2, {}, {true, 0}},
+      {"IsolatedHighSurr_DBFF", L'\xDBFF', "\xF4\x8F", 2, {}, {true, 3}},
+
+      {"HighSurr_D800_after_HighD800",
+       L'\xD800',
+       "\xF0\x90",
+       2,
+       {true, 0},
+       {true, 0}},
+      {"HighSurr_DBFF_after_HighDBFF",
+       L'\xDBFF',
+       "\xF4\x8F",
+       2,
+       {true, 3},
+       {true, 3}},
+
+      {"LowSurr_DC00_after_HighD800", L'\xDC00', "\x80\x80", 2, {true, 0}, {}},
+      {"LowSurr_DFFD_after_HighDBFF", L'\xDFFD', "\xBF\xBD", 2, {true, 3}, {}},
+      {"LowSurr_DC00_with_InitialState_saw_high_bits_1",
+       L'\xDC00',
+       "\x90\x80",
+       2,
+       {true, 1},
+       {}},
+
+      // Final state = initial on error.
+      {"Error_IsolatedLowSurr_DC00_NoPriorHigh", L'\xDC00', "", kError, {}, {}},
+      {"Error_IsolatedLowSurr_DFFF_NoPriorHigh", L'\xDFFF', "", kError, {}, {}},
+
+#if (defined(WCHAR_MAX) && WCHAR_MAX > 0xFFFF)
+      {"DirectSupplementaryChars_U10000", static_cast<wchar_t>(0x10000),
+       "\xF0\x90\x80\x80", 4},
+      {"DirectSupplementaryChars_U10FFFD", static_cast<wchar_t>(0x10FFFD),
+       "\xF4\x8F\xBF\xBD", 4},
+#endif
+  };
+
+  wchar_t minus_one = static_cast<wchar_t>(-1);
+  if constexpr (sizeof(wchar_t) == 2) {
+    cases.push_back({"WChar_MinusOne_as_FFFF", minus_one, "\xEF\xBF\xBF", 3});
+  } else {
+    cases.push_back(
+        {"Error_WChar_MinusOne_as_FFFFFFFF", minus_one, "", kError, {}, {}});
+  }
+
+  if constexpr (sizeof(wchar_t) >= 4) {
+#ifdef WCHAR_MAX
+    if (static_cast<uintmax_t>(WCHAR_MAX) >= 0x110000UL) {
+      cases.push_back({"Error_OutOfRange_110000",
+                       static_cast<wchar_t>(0x110000UL),
+                       "",
+                       kError,
+                       {},
+                       {}});
+    }
+#else
+    cases.push_back({"Error_OutOfRange_110000_fallback",
+                     static_cast<wchar_t>(0x110000UL),
+                     "",
+                     kError,
+                     {},
+                     {}});
+#endif
+  }
+  return cases;
+}
+
+class WideToUtf8ParamTest : public TestWithParam<WideToUtf8TestCase> {};
+
+TEST_P(WideToUtf8ParamTest, SingleCharConversion) {
+  const auto& test_case = GetParam();
+  ShiftState state = test_case.initial_state;
+  constexpr char kFillChar = '\xAB';
+  std::string buffer(32, kFillChar);
+
+  size_t bytes_written = WideToUtf8(test_case.input, buffer.data(), state);
+
+  EXPECT_EQ(bytes_written, test_case.expected_bytes_written);
+  EXPECT_THAT(buffer, StartsWith(test_case.expected_utf8_str));
+
+  // The remaining bytes should be unchanged.
+  ASSERT_LT(test_case.expected_utf8_str.length(), buffer.size());
+  EXPECT_EQ(buffer[test_case.expected_utf8_str.length()], kFillChar);
+
+  EXPECT_EQ(state.saw_high_surrogate,
+            test_case.expected_state.saw_high_surrogate);
+  EXPECT_EQ(state.bits, test_case.expected_state.bits);
+}
+
+INSTANTIATE_TEST_SUITE_P(WideCharToUtf8Conversion, WideToUtf8ParamTest,
+                         ValuesIn(GetWideToUtf8TestCases()),
+                         [](auto info) { return info.param.description; });
+
+// Comprehensive test string for validating wchar_t to UTF-8 conversion.
+// This string is designed to cover a variety of Unicode character types and
+// sequences:
+// 1. Basic ASCII characters (within names, numbers, and spacing).
+// 2. Common 2-byte UTF-8 sequences:
+//    - Accented Latin characters (e.g., 'á' in "Holá").
+//    - Hebrew text with combining vowel points (e.g., "שָׁלוֹם").
+// 3. Common 3-byte UTF-8 sequences:
+//    - Currency symbols (e.g., '€').
+//    - CJK characters (e.g., "你好", "中").
+//    - Components of complex emojis like the Zero Width Joiner (ZWJ) and
+//      Heart symbol.
+// 4. Various 4-byte UTF-8 sequences (representing Supplementary Plane
+// characters):
+//    - An emoji with a skin tone modifier ("👍🏻").
+//    - A flag emoji composed of regional indicators ("🇺🇸").
+//    - A complex ZWJ emoji sequence ("👩‍❤️‍💋‍👨") combining
+//      SP characters (👩, 💋, 👨) with BMP characters (ZWJ and ❤️).
+//    - These are critical for testing the correct handling of surrogate pairs
+//      when wchar_t is 2 bytes (e.g., on Windows).
+// The goal is to ensure accurate conversion across a diverse set of
+// characters.
+//
+// clang-format off
+#define WIDE_STRING_LITERAL L"Holá €1 你好 שָׁלוֹם 👍🏻🇺🇸👩‍❤️‍💋‍👨 中"
+#define UTF8_STRING_LITERAL u8"Holá €1 你好 שָׁלוֹם 👍🏻🇺🇸👩‍❤️‍💋‍👨 中"
+// clang-format on
+
+absl::string_view GetUtf8TestString() {
+  // `u8""` forces UTF-8 encoding; MSVC will default to e.g. CP1252 (and warn)
+  // without it. However, the resulting character type differs between pre-C++20
+  // (`char`) and C++20 (`char8_t`). So deduce the right character type for all
+  // C++ versions, init it with UTF-8, then `memcpy()` to get the result as a
+  // `char*`
+  static absl::string_view kUtf8TestString = [] {
+    using ConstChar8T = std::remove_reference_t<decltype(*u8"a")>;
+    constexpr ConstChar8T kOutputUtf8[] = UTF8_STRING_LITERAL;
+    static char output[sizeof kOutputUtf8];
+    std::memcpy(output, kOutputUtf8, sizeof kOutputUtf8);
+    return output;
+  }();
+
+  return kUtf8TestString;
+}
+
+TEST(WideToUtf8, FullString) {
+  std::string buffer(kMaxEncodedUTF8Size * sizeof(WIDE_STRING_LITERAL), '\0');
+  char* buffer_ptr = buffer.data();
+
+  ShiftState state;
+  for (const wchar_t wc : WIDE_STRING_LITERAL) {
+    buffer_ptr += WideToUtf8(wc, buffer_ptr, state);
+  }
+
+  EXPECT_THAT(buffer, StartsWith(GetUtf8TestString()));
+}
+
+#undef WIDE_STRING_LITERAL
+#undef UTF8_STRING_LITERAL
+
 }  // namespace
diff --git a/absl/strings/match.cc b/absl/strings/match.cc
index 7b24241adf5..ac7cf0d8249 100644
--- a/absl/strings/match.cc
+++ b/absl/strings/match.cc
@@ -14,25 +14,215 @@
 
 #include "absl/strings/match.h"
 
+#include <algorithm>
+#include <cstddef>
+#include <cstdint>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/endian.h"
+#include "absl/base/optimization.h"
+#include "absl/numeric/bits.h"
+#include "absl/strings/ascii.h"
 #include "absl/strings/internal/memutil.h"
+#include "absl/strings/string_view.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
-bool EqualsIgnoreCase(absl::string_view piece1, absl::string_view piece2) {
+bool EqualsIgnoreCase(absl::string_view piece1,
+                      absl::string_view piece2) noexcept {
   return (piece1.size() == piece2.size() &&
           0 == absl::strings_internal::memcasecmp(piece1.data(), piece2.data(),
                                                   piece1.size()));
   // memcasecmp uses absl::ascii_tolower().
 }
 
-bool StartsWithIgnoreCase(absl::string_view text, absl::string_view prefix) {
+namespace {
+
+// For larger haystacks (n >= 256), Case-Insensitive Boyer-Moore-Horspool
+// provides sub-linear O(N / M) average-case performance, although it still has
+// O(N * M) theoretical worst-case scaling.
+// Scans the haystack from left to right using a search window of size `m`.
+// For each window offset, Horspool inspects the rightmost character of the
+// window (`haystack[pos + m - 1]`) first, enabling multi-byte shifts when
+// mismatches occur and reducing average search time to O(N / M).
+ABSL_ATTRIBUTE_NOINLINE bool StrContainsIgnoreCaseBMH(
+    absl::string_view haystack, absl::string_view needle) noexcept {
+  const size_t n = haystack.size();
+  const size_t m = needle.size();
+
+  // Step 1: Initialize the 256-entry shift table.
+  // Unknown characters default to full window shift of `m` bytes.
+  size_t shift[256];
+  for (size_t i = 0; i < 256; ++i) {
+    shift[i] = m;
+  }
+
+  // Populate shift distances for needle[0..m-2]. Dual assignment for
+  // ascii_tolower and ascii_toupper stores distance from rightmost occurrence
+  // to end of needle.
+  for (size_t i = 0; i < m - 1; ++i) {
+    const unsigned char c = static_cast<unsigned char>(needle[i]);
+    shift[static_cast<unsigned char>(absl::ascii_tolower(c))] = m - 1 - i;
+    shift[static_cast<unsigned char>(absl::ascii_toupper(c))] = m - 1 - i;
+  }
+
+  // Step 2: Search loop across candidate window offsets.
+  size_t pos = 0;
+  while (pos <= n - m) {
+    const unsigned char last_hay =
+        static_cast<unsigned char>(haystack[pos + m - 1]);
+    const unsigned char last_needle = static_cast<unsigned char>(needle[m - 1]);
+
+    // Check 1: Inspect right-most character of window first.
+    if (last_hay == last_needle ||
+        absl::ascii_tolower(last_hay) == absl::ascii_tolower(last_needle)) {
+      // Check 2: Pre-filter on first character of window.
+      const unsigned char first_hay = static_cast<unsigned char>(haystack[pos]);
+      const unsigned char first_needle = static_cast<unsigned char>(needle[0]);
+      if (first_hay == first_needle ||
+          absl::ascii_tolower(first_hay) == absl::ascii_tolower(first_needle)) {
+        // Check 3: Compare interior (m - 2) bytes.
+        if (EqualsIgnoreCase(haystack.substr(pos + 1, m - 2),
+                             needle.substr(1, m - 2))) {
+          return true;
+        }
+      }
+    }
+
+    // Advance window by shift distance determined by right-most haystack byte.
+    pos += shift[last_hay];
+  }
+  return false;
+}
+
+}  // namespace
+
+bool StrContainsIgnoreCase(absl::string_view haystack,
+                           absl::string_view needle) noexcept {
+  const size_t n = haystack.size();
+  const size_t m = needle.size();
+  if (m == 0) return true;
+  if (n < m) return false;
+  if (m == 1) return StrContainsIgnoreCase(haystack, needle[0]);
+
+  // For short haystacks (n < 256) or small needles (m == 2), avoid the
+  // initialization overhead of a 256-entry shift table. Instead, use a fast
+  // first-and-last character prefilter before inspecting interior bytes.
+  if (n < 256 || m == 2) {
+    const char first_needle =
+        absl::ascii_tolower(static_cast<unsigned char>(needle[0]));
+    const char last_needle =
+        absl::ascii_tolower(static_cast<unsigned char>(needle[m - 1]));
+
+    for (size_t pos = 0; pos <= n - m; ++pos) {
+      const unsigned char first_hay = static_cast<unsigned char>(haystack[pos]);
+      if (absl::ascii_tolower(first_hay) != first_needle) continue;
+
+      const unsigned char last_hay =
+          static_cast<unsigned char>(haystack[pos + m - 1]);
+      if (absl::ascii_tolower(last_hay) != last_needle) continue;
+
+      if (m == 2 || EqualsIgnoreCase(haystack.substr(pos + 1, m - 2),
+                                     needle.substr(1, m - 2))) {
+        return true;
+      }
+    }
+    return false;
+  }
+
+  return StrContainsIgnoreCaseBMH(haystack, needle);
+}
+
+bool StrContainsIgnoreCase(absl::string_view haystack,
+                           char needle) noexcept {
+  char upper_needle = absl::ascii_toupper(static_cast<unsigned char>(needle));
+  char lower_needle = absl::ascii_tolower(static_cast<unsigned char>(needle));
+  if (upper_needle == lower_needle) {
+    return StrContains(haystack, needle);
+  }
+  if (haystack.size() < 64) {
+    for (char c : haystack) {
+      if (c == lower_needle || c == upper_needle) return true;
+    }
+    return false;
+  }
+  const char both_cstr[3] = {lower_needle, upper_needle, '\0'};
+  return haystack.find_first_of(both_cstr) != absl::string_view::npos;
+}
+
+bool StartsWithIgnoreCase(absl::string_view text,
+                          absl::string_view prefix) noexcept {
   return (text.size() >= prefix.size()) &&
          EqualsIgnoreCase(text.substr(0, prefix.size()), prefix);
 }
 
-bool EndsWithIgnoreCase(absl::string_view text, absl::string_view suffix) {
+bool EndsWithIgnoreCase(absl::string_view text,
+                        absl::string_view suffix) noexcept {
   return (text.size() >= suffix.size()) &&
          EqualsIgnoreCase(text.substr(text.size() - suffix.size()), suffix);
 }
 
+absl::string_view FindLongestCommonPrefix(absl::string_view a,
+                                          absl::string_view b) {
+  const absl::string_view::size_type limit = std::min(a.size(), b.size());
+  const char* const pa = a.data();
+  const char* const pb = b.data();
+  absl::string_view::size_type count = (unsigned) 0;
+
+  if (ABSL_PREDICT_FALSE(limit < 8)) {
+    while (ABSL_PREDICT_TRUE(count + 2 <= limit)) {
+      uint16_t xor_bytes = absl::little_endian::Load16(pa + count) ^
+                           absl::little_endian::Load16(pb + count);
+      if (ABSL_PREDICT_FALSE(xor_bytes != 0)) {
+        if (ABSL_PREDICT_TRUE((xor_bytes & 0xff) == 0)) ++count;
+        return absl::string_view(pa, count);
+      }
+      count += 2;
+    }
+    if (ABSL_PREDICT_TRUE(count != limit)) {
+      if (ABSL_PREDICT_TRUE(pa[count] == pb[count])) ++count;
+    }
+    return absl::string_view(pa, count);
+  }
+
+  do {
+    uint64_t xor_bytes = absl::little_endian::Load64(pa + count) ^
+                         absl::little_endian::Load64(pb + count);
+    if (ABSL_PREDICT_FALSE(xor_bytes != 0)) {
+      count += static_cast<uint64_t>(absl::countr_zero(xor_bytes) >> 3);
+      return absl::string_view(pa, count);
+    }
+    count += 8;
+  } while (ABSL_PREDICT_TRUE(count + 8 < limit));
+
+  count = limit - 8;
+  uint64_t xor_bytes = absl::little_endian::Load64(pa + count) ^
+                       absl::little_endian::Load64(pb + count);
+  if (ABSL_PREDICT_TRUE(xor_bytes != 0)) {
+    count += static_cast<uint64_t>(absl::countr_zero(xor_bytes) >> 3);
+    return absl::string_view(pa, count);
+  }
+  return absl::string_view(pa, limit);
+}
+
+absl::string_view FindLongestCommonSuffix(absl::string_view a,
+                                          absl::string_view b) {
+  const absl::string_view::size_type limit = std::min(a.size(), b.size());
+  if (limit == 0) return absl::string_view();
+
+  const char* pa = a.data() + a.size() - 1;
+  const char* pb = b.data() + b.size() - 1;
+  absl::string_view::size_type count = (unsigned) 0;
+  while (count < limit && *pa == *pb) {
+    --pa;
+    --pb;
+    ++count;
+  }
+
+  return absl::string_view(++pa, count);
+}
+
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/match.h b/absl/strings/match.h
index 762f359f796..ce4fe784737 100644
--- a/absl/strings/match.h
+++ b/absl/strings/match.h
@@ -20,7 +20,7 @@
 // This file contains simple utilities for performing string matching checks.
 // All of these function parameters are specified as `absl::string_view`,
 // meaning that these functions can accept `std::string`, `absl::string_view` or
-// nul-terminated C-style strings.
+// NUL-terminated C-style strings.
 //
 // Examples:
 //   std::string s = "foo";
@@ -38,51 +38,92 @@
 #include "absl/strings/string_view.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 // StrContains()
 //
 // Returns whether a given string `haystack` contains the substring `needle`.
-inline bool StrContains(absl::string_view haystack, absl::string_view needle) {
+inline bool StrContains(absl::string_view haystack,
+                        absl::string_view needle) noexcept {
   return haystack.find(needle, 0) != haystack.npos;
 }
 
+inline bool StrContains(absl::string_view haystack, char needle) noexcept {
+  return haystack.find(needle) != haystack.npos;
+}
+
 // StartsWith()
 //
 // Returns whether a given string `text` begins with `prefix`.
-inline bool StartsWith(absl::string_view text, absl::string_view prefix) {
-  return prefix.empty() ||
-         (text.size() >= prefix.size() &&
-          memcmp(text.data(), prefix.data(), prefix.size()) == 0);
+inline constexpr bool StartsWith(absl::string_view text,
+                                 absl::string_view prefix) noexcept {
+  if (prefix.empty()) {
+    return true;
+  }
+  if (text.size() < prefix.size()) {
+    return false;
+  }
+  absl::string_view possible_match = text.substr(0, prefix.size());
+
+  return possible_match == prefix;
 }
 
 // EndsWith()
 //
 // Returns whether a given string `text` ends with `suffix`.
-inline bool EndsWith(absl::string_view text, absl::string_view suffix) {
-  return suffix.empty() ||
-         (text.size() >= suffix.size() &&
-          memcmp(text.data() + (text.size() - suffix.size()), suffix.data(),
-                 suffix.size()) == 0);
+inline constexpr bool EndsWith(absl::string_view text,
+                               absl::string_view suffix) noexcept {
+  if (suffix.empty()) {
+    return true;
+  }
+  if (text.size() < suffix.size()) {
+    return false;
+  }
+  absl::string_view possible_match = text.substr(text.size() - suffix.size());
+  return possible_match == suffix;
 }
+// StrContainsIgnoreCase()
+//
+// Returns whether a given ASCII string `haystack` contains the ASCII substring
+// `needle`, ignoring case in the comparison.
+bool StrContainsIgnoreCase(absl::string_view haystack,
+                           absl::string_view needle) noexcept;
+
+bool StrContainsIgnoreCase(absl::string_view haystack,
+                           char needle) noexcept;
 
 // EqualsIgnoreCase()
 //
 // Returns whether given ASCII strings `piece1` and `piece2` are equal, ignoring
 // case in the comparison.
-bool EqualsIgnoreCase(absl::string_view piece1, absl::string_view piece2);
+bool EqualsIgnoreCase(absl::string_view piece1,
+                      absl::string_view piece2) noexcept;
 
 // StartsWithIgnoreCase()
 //
 // Returns whether a given ASCII string `text` starts with `prefix`,
 // ignoring case in the comparison.
-bool StartsWithIgnoreCase(absl::string_view text, absl::string_view prefix);
+bool StartsWithIgnoreCase(absl::string_view text,
+                          absl::string_view prefix) noexcept;
 
 // EndsWithIgnoreCase()
 //
 // Returns whether a given ASCII string `text` ends with `suffix`, ignoring
 // case in the comparison.
-bool EndsWithIgnoreCase(absl::string_view text, absl::string_view suffix);
+bool EndsWithIgnoreCase(absl::string_view text,
+                        absl::string_view suffix) noexcept;
+
+// Yields the longest prefix in common between both input strings.
+// Pointer-wise, the returned result is a subset of input "a".
+absl::string_view FindLongestCommonPrefix(absl::string_view a,
+                                          absl::string_view b);
+
+// Yields the longest suffix in common between both input strings.
+// Pointer-wise, the returned result is a subset of input "a".
+absl::string_view FindLongestCommonSuffix(absl::string_view a,
+                                          absl::string_view b);
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_MATCH_H_
diff --git a/absl/strings/match_benchmark.cc b/absl/strings/match_benchmark.cc
new file mode 100644
index 00000000000..d2c32c064c0
--- /dev/null
+++ b/absl/strings/match_benchmark.cc
@@ -0,0 +1,97 @@
+// Copyright 2026 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <cstddef>
+#include <cstdint>
+#include <string>
+
+#include "absl/base/internal/raw_logging.h"
+#include "absl/strings/match.h"
+#include "benchmark/benchmark.h"
+
+namespace {
+
+void BM_ContainsIgnoreCase_Match(benchmark::State& state) {
+  const size_t haystack_size = static_cast<size_t>(state.range(0));
+  const size_t needle_size = static_cast<size_t>(state.range(1));
+  std::string haystack(haystack_size, 'a');
+  std::string needle(needle_size, 'b');
+
+  // Place a case-insensitive match at the very end of the haystack
+  haystack.replace(haystack_size - needle_size, needle_size, needle_size, 'B');
+
+  ABSL_RAW_CHECK(absl::StrContainsIgnoreCase(haystack, needle),
+                 "BM_ContainsIgnoreCase_Match must return true");
+
+  for (auto _ : state) {
+    benchmark::DoNotOptimize(absl::StrContainsIgnoreCase(haystack, needle));
+  }
+  state.SetBytesProcessed(static_cast<int64_t>(state.iterations()) *
+                          static_cast<int64_t>(haystack_size));
+}
+BENCHMARK(BM_ContainsIgnoreCase_Match)
+    ->Args({4, 1})
+    ->Args({64, 1})
+    ->Args({64, 8})
+    ->Args({1024, 16})
+    ->Args({65536, 1})
+    ->Args({65536, 64})
+    ->Args({65536, 1024})
+    ->Args({65536, 16384});
+
+void BM_ContainsIgnoreCase_Mismatch(benchmark::State& state) {
+  const size_t haystack_size = static_cast<size_t>(state.range(0));
+  const size_t needle_size = static_cast<size_t>(state.range(1));
+  const std::string haystack(haystack_size, 'a');
+  const std::string needle(needle_size, 'b');
+
+  ABSL_RAW_CHECK(!absl::StrContainsIgnoreCase(haystack, needle),
+                 "BM_ContainsIgnoreCase_Mismatch must return false");
+
+  for (auto _ : state) {
+    benchmark::DoNotOptimize(absl::StrContainsIgnoreCase(haystack, needle));
+  }
+  state.SetBytesProcessed(static_cast<int64_t>(state.iterations()) *
+                          static_cast<int64_t>(haystack_size));
+}
+BENCHMARK(BM_ContainsIgnoreCase_Mismatch)
+    ->Args({4, 1})
+    ->Args({64, 1})
+    ->Args({64, 8})
+    ->Args({1024, 16})
+    ->Args({65536, 1})
+    ->Args({65536, 64})
+    ->Args({65536, 1024})
+    ->Args({65536, 16384});
+
+void BM_ContainsIgnoreCase_Adversarial(benchmark::State& state) {
+  const size_t haystack_size = static_cast<size_t>(state.range(0));
+  const std::string haystack(haystack_size, 'a');
+  std::string needle(haystack_size / 2, 'a');
+
+  // Make the needle not match the haystack due to just the last character.
+  needle.push_back('b');
+
+  ABSL_RAW_CHECK(!absl::StrContainsIgnoreCase(haystack, needle),
+                 "BM_ContainsIgnoreCase_Adversarial must return false");
+
+  for (auto _ : state) {
+    benchmark::DoNotOptimize(absl::StrContainsIgnoreCase(haystack, needle));
+  }
+  state.SetBytesProcessed(static_cast<int64_t>(state.iterations()) *
+                          static_cast<int64_t>(haystack_size));
+}
+BENCHMARK(BM_ContainsIgnoreCase_Adversarial)->Range(16 << 10, 128 << 10);
+
+}  // namespace
diff --git a/absl/strings/match_test.cc b/absl/strings/match_test.cc
index 4c313dda14e..76bad73614b 100644
--- a/absl/strings/match_test.cc
+++ b/absl/strings/match_test.cc
@@ -14,7 +14,12 @@
 
 #include "absl/strings/match.h"
 
+#include <cstddef>
+#include <string>
+
 #include "gtest/gtest.h"
+#include "absl/strings/ascii.h"
+#include "absl/strings/string_view.h"
 
 namespace {
 
@@ -66,6 +71,23 @@ TEST(MatchTest, Contains) {
   EXPECT_FALSE(absl::StrContains("", "a"));
 }
 
+TEST(MatchTest, ContainsChar) {
+  absl::string_view a("abcdefg");
+  absl::string_view b("abcd");
+  EXPECT_TRUE(absl::StrContains(a, 'a'));
+  EXPECT_TRUE(absl::StrContains(a, 'b'));
+  EXPECT_TRUE(absl::StrContains(a, 'e'));
+  EXPECT_FALSE(absl::StrContains(a, 'h'));
+
+  EXPECT_TRUE(absl::StrContains(b, 'a'));
+  EXPECT_TRUE(absl::StrContains(b, 'b'));
+  EXPECT_FALSE(absl::StrContains(b, 'e'));
+  EXPECT_FALSE(absl::StrContains(b, 'h'));
+
+  EXPECT_FALSE(absl::StrContains("", 'a'));
+  EXPECT_FALSE(absl::StrContains("", 'a'));
+}
+
 TEST(MatchTest, ContainsNull) {
   const std::string s = "foo";
   const char* cs = "foo";
@@ -107,4 +129,420 @@ TEST(MatchTest, EndsWithIgnoreCase) {
   EXPECT_FALSE(absl::EndsWithIgnoreCase("", "fo"));
 }
 
+TEST(MatchTest, ContainsIgnoreCase) {
+  EXPECT_TRUE(absl::StrContainsIgnoreCase("foo", "foo"));
+  EXPECT_TRUE(absl::StrContainsIgnoreCase("FOO", "Foo"));
+  EXPECT_TRUE(absl::StrContainsIgnoreCase("--FOO", "Foo"));
+  EXPECT_TRUE(absl::StrContainsIgnoreCase("FOO--", "Foo"));
+  EXPECT_FALSE(absl::StrContainsIgnoreCase("BAR", "Foo"));
+  EXPECT_FALSE(absl::StrContainsIgnoreCase("BAR", "Foo"));
+  EXPECT_TRUE(absl::StrContainsIgnoreCase("123456", "123456"));
+  EXPECT_TRUE(absl::StrContainsIgnoreCase("123456", "234"));
+  EXPECT_TRUE(absl::StrContainsIgnoreCase("", ""));
+  EXPECT_TRUE(absl::StrContainsIgnoreCase("abc", ""));
+  EXPECT_FALSE(absl::StrContainsIgnoreCase("", "a"));
+}
+
+TEST(MatchTest, ContainsCharIgnoreCase) {
+  absl::string_view a("AaBCdefg!");
+  absl::string_view b("AaBCd!");
+  EXPECT_TRUE(absl::StrContainsIgnoreCase(a, 'a'));
+  EXPECT_TRUE(absl::StrContainsIgnoreCase(a, 'A'));
+  EXPECT_TRUE(absl::StrContainsIgnoreCase(a, 'b'));
+  EXPECT_TRUE(absl::StrContainsIgnoreCase(a, 'B'));
+  EXPECT_TRUE(absl::StrContainsIgnoreCase(a, 'e'));
+  EXPECT_TRUE(absl::StrContainsIgnoreCase(a, 'E'));
+  EXPECT_FALSE(absl::StrContainsIgnoreCase(a, 'h'));
+  EXPECT_FALSE(absl::StrContainsIgnoreCase(a, 'H'));
+  EXPECT_TRUE(absl::StrContainsIgnoreCase(a, '!'));
+  EXPECT_FALSE(absl::StrContainsIgnoreCase(a, '?'));
+
+  EXPECT_TRUE(absl::StrContainsIgnoreCase(b, 'a'));
+  EXPECT_TRUE(absl::StrContainsIgnoreCase(b, 'A'));
+  EXPECT_TRUE(absl::StrContainsIgnoreCase(b, 'b'));
+  EXPECT_TRUE(absl::StrContainsIgnoreCase(b, 'B'));
+  EXPECT_FALSE(absl::StrContainsIgnoreCase(b, 'e'));
+  EXPECT_FALSE(absl::StrContainsIgnoreCase(b, 'E'));
+  EXPECT_FALSE(absl::StrContainsIgnoreCase(b, 'h'));
+  EXPECT_FALSE(absl::StrContainsIgnoreCase(b, 'H'));
+  EXPECT_TRUE(absl::StrContainsIgnoreCase(b, '!'));
+  EXPECT_FALSE(absl::StrContainsIgnoreCase(b, '?'));
+
+  EXPECT_FALSE(absl::StrContainsIgnoreCase("", 'a'));
+  EXPECT_FALSE(absl::StrContainsIgnoreCase("", 'A'));
+  EXPECT_FALSE(absl::StrContainsIgnoreCase("", '0'));
+}
+
+TEST(MatchTest, ContainsIgnoreCaseLinearScaling) {
+  // Adversarial case where naive shift-by-one substring scan exhibits
+  // quadratic O(N * M) time: haystack = 'a' * N, needle = 'a' * (M - 1) + 'b'.
+  //
+  // Boyer-Moore-Horspool inspects the last byte first, immediately detecting
+  // the mismatch ('a' != 'b') at each window offset and shifting by 1,
+  // completing in linear O(N) time.
+  const std::string haystack(1 << 17, 'a');  // 128 KiB
+  std::string needle(1 << 16, 'a');          // 64 KiB
+  needle.push_back('b');
+
+  EXPECT_FALSE(absl::StrContainsIgnoreCase(haystack, needle));
+
+  std::string matching_needle(1 << 16, 'A');
+  EXPECT_TRUE(absl::StrContainsIgnoreCase(haystack, matching_needle));
+}
+
+std::string MakeBMHNeedle(size_t m) {
+  std::string needle(m, 'y');
+  for (size_t i = 0; i < m; i += 2) {
+    needle[i] = static_cast<char>('A' + (i % 26));
+    needle[i + 1] = static_cast<char>('a' + ((i + 1) % 26));
+  }
+  return needle;
+}
+
+TEST(MatchTest, ContainsIgnoreCaseBMHMatchAtStart) {
+  // Test BMH search (N >= 256, M >= 256) when match occurs at pos == 0.
+  constexpr size_t n = 1000;
+  constexpr size_t m = 300;
+  std::string needle = MakeBMHNeedle(m);
+  std::string haystack(n, 'x');
+  haystack.replace(0, m, needle);
+
+  EXPECT_TRUE(absl::StrContainsIgnoreCase(haystack, needle));
+}
+
+TEST(MatchTest, ContainsIgnoreCaseBMHMatchInMiddle) {
+  // Test BMH search (N >= 256, M >= 256) when match occurs in the middle of
+  // haystack.
+  constexpr size_t n = 1000;
+  constexpr size_t m = 300;
+  std::string needle = MakeBMHNeedle(m);
+  std::string haystack(n, 'x');
+  haystack.replace(200, m, needle);
+
+  EXPECT_TRUE(absl::StrContainsIgnoreCase(haystack, needle));
+}
+
+TEST(MatchTest, ContainsIgnoreCaseBMHMatchAtEnd) {
+  // Test BMH search (N >= 256, M >= 256) when match occurs at the very end (pos
+  // == n - m). Verifies that while (pos <= n - m) boundary condition includes
+  // the final window.
+  constexpr size_t n = 1000;
+  constexpr size_t m = 300;
+  std::string needle = MakeBMHNeedle(m);
+  std::string haystack(n, 'x');
+  haystack.replace(n - m, m, needle);
+
+  EXPECT_TRUE(absl::StrContainsIgnoreCase(haystack, needle));
+}
+
+TEST(MatchTest, ContainsIgnoreCaseBMHMismatchAtEnd) {
+  // Test BMH search (N >= 256, M >= 256) when needle is placed at pos == n - m
+  // but fails on the last character.
+  constexpr size_t n = 1000;
+  constexpr size_t m = 300;
+  std::string needle = MakeBMHNeedle(m);
+  std::string haystack(n, 'x');
+  haystack.replace(n - m, m, needle);
+  haystack.back() = 'Z';
+
+  EXPECT_FALSE(absl::StrContainsIgnoreCase(haystack, needle));
+}
+
+TEST(MatchTest, ContainsIgnoreCaseBMHInteriorMismatch) {
+  // Test BMH search (N >= 256, M >= 256) when the first and last characters
+  // of a window match, but interior bytes mismatch.
+  // This exercises the false branch of EqualsIgnoreCase(...) inside BMH,
+  // falling through line 91 to advance the window.
+  constexpr size_t n = 1000;
+  constexpr size_t m = 300;
+  std::string needle = MakeBMHNeedle(m);
+
+  // 1. First and last chars match at pos == 100, but interior byte 10
+  // mismatches.
+  std::string haystack(n, 'x');
+  haystack.replace(100, m, needle);
+  haystack[100 + 10] = (needle[10] == 'Q') ? 'W' : 'Q';
+
+  EXPECT_FALSE(absl::StrContainsIgnoreCase(haystack, needle));
+
+  // 2. Add a valid match at the end to verify BMH continues scanning after
+  // falling through an interior mismatch.
+  haystack.replace(n - m, m, needle);
+  EXPECT_TRUE(absl::StrContainsIgnoreCase(haystack, needle));
+}
+
+TEST(MatchTest, ContainsIgnoreCaseBMHMixedCaseAndShifts) {
+  // Test large needle (M >= 256) with mixed casing and partial matches
+  // triggering multi-byte skip shifts during BMH scanning.
+  std::string needle;
+  needle.reserve(300);
+  for (size_t i = 0; i < 300; ++i) {
+    needle.push_back(static_cast<char>('a' + (i % 10)));
+  }
+
+  std::string haystack;
+  haystack.reserve(2000);
+  for (int i = 0; i < 5; ++i) {
+    haystack.append(needle.substr(0, 250));
+    haystack.append("XYZ");
+  }
+
+  std::string needle_upper = needle;
+  for (char& c : needle_upper) {
+    c = static_cast<char>(absl::ascii_toupper(static_cast<unsigned char>(c)));
+  }
+
+  EXPECT_FALSE(absl::StrContainsIgnoreCase(haystack, needle_upper));
+
+  haystack.append(needle_upper);
+  EXPECT_TRUE(absl::StrContainsIgnoreCase(haystack, needle));
+}
+
+TEST(MatchTest, ContainsIgnoreCaseHaystackThresholdBoundary) {
+  // Test haystack threshold boundary N = 255 (prefilter branch) vs N = 256 (BMH
+  // branch).
+  std::string needle = "AbC";
+
+  // N = 255 (prefilter branch)
+  std::string h255(255, '.');
+  h255.replace(100, 3, "aBc");
+  EXPECT_TRUE(absl::StrContainsIgnoreCase(h255, needle));
+  EXPECT_TRUE(absl::StrContainsIgnoreCase(h255, "abc"));
+  EXPECT_FALSE(absl::StrContainsIgnoreCase(std::string(255, '.'), needle));
+
+  // N = 256 (BMH branch)
+  std::string h256(256, '.');
+  h256.replace(100, 3, "aBc");
+  EXPECT_TRUE(absl::StrContainsIgnoreCase(h256, needle));
+  EXPECT_TRUE(absl::StrContainsIgnoreCase(h256, "abc"));
+  EXPECT_FALSE(absl::StrContainsIgnoreCase(std::string(256, '.'), needle));
+}
+
+TEST(MatchTest, ContainsIgnoreCaseSmallNeedleFastPath) {
+  // Test small needle (M = 2) fast path, which uses prefilter regardless of
+  // haystack size (N >= 256).
+  std::string needle = "xY";
+  std::string h1000(1000, 'a');
+
+  // Match at start
+  h1000.replace(0, 2, "Xy");
+  EXPECT_TRUE(absl::StrContainsIgnoreCase(h1000, needle));
+
+  // Match at end (pos == 998)
+  h1000 = std::string(1000, 'a');
+  h1000.replace(998, 2, "Xy");
+  EXPECT_TRUE(absl::StrContainsIgnoreCase(h1000, needle));
+
+  // Mismatch
+  h1000 = std::string(1000, 'a');
+  EXPECT_FALSE(absl::StrContainsIgnoreCase(h1000, needle));
+}
+
+TEST(MatchTest, ContainsIgnoreCaseNeedleThresholdBoundary) {
+  // Test needle length boundaries M = 255, M = 256, and M = 257 in N = 300
+  // haystack.
+  std::string h300(300, 'a');
+  std::string n255(255, 'A');
+  std::string n256(256, 'A');
+  std::string n257(257, 'A');
+
+  EXPECT_TRUE(absl::StrContainsIgnoreCase(h300, n255));
+  EXPECT_TRUE(absl::StrContainsIgnoreCase(h300, n256));
+  EXPECT_TRUE(absl::StrContainsIgnoreCase(h300, n257));
+
+  n255.back() = 'B';
+  n256.back() = 'B';
+  n257.back() = 'B';
+
+  EXPECT_FALSE(absl::StrContainsIgnoreCase(h300, n255));
+  EXPECT_FALSE(absl::StrContainsIgnoreCase(h300, n256));
+  EXPECT_FALSE(absl::StrContainsIgnoreCase(h300, n257));
+}
+
+TEST(MatchTest, ContainsIgnoreCaseBMHNonAsciiBytes) {
+  // Test BMH algorithm handling of high-bit non-ASCII bytes (values > 127) in
+  // needle and haystack.
+  std::string needle(300, '\0');
+  for (size_t i = 0; i < 300; ++i) {
+    needle[i] = static_cast<char>(128 + (i % 128));
+  }
+  std::string haystack(1000, '\x7F');
+  haystack.replace(500, 300, needle);
+  EXPECT_TRUE(absl::StrContainsIgnoreCase(haystack, needle));
+
+  haystack[799] = '\x00';
+  EXPECT_FALSE(absl::StrContainsIgnoreCase(haystack, needle));
+}
+
+TEST(MatchTest, ContainsIgnoreCaseBMHExactLengthMatch) {
+  // Test BMH search when haystack size equals needle size (N == M == 300).
+  std::string needle(300, 'X');
+  std::string haystack(300, 'x');
+  EXPECT_TRUE(absl::StrContainsIgnoreCase(haystack, needle));
+
+  haystack.back() = 'y';
+  EXPECT_FALSE(absl::StrContainsIgnoreCase(haystack, needle));
+}
+
+TEST(MatchTest, ContainsIgnoreCaseBMHAllIdenticalCharNeedle) {
+  // Test BMH search with needle consisting of all identical characters (M >=
+  // 256).
+  std::string needle(300, 'K');
+  std::string haystack(1000, 'k');
+  EXPECT_TRUE(absl::StrContainsIgnoreCase(haystack, needle));
+
+  std::string short_haystack(299, 'k');
+  EXPECT_FALSE(absl::StrContainsIgnoreCase(short_haystack, needle));
+}
+
+TEST(MatchTest, ContainsCharIgnoreCaseLargeHaystack) {
+  // Test StrContainsIgnoreCase(haystack, char) for N >= 64, exercising the
+  // find_first_of fallback branch.
+  constexpr size_t sizes[] = {64, 100, 1000};
+  for (size_t n : sizes) {
+    std::string haystack(n, '.');
+
+    // 1. Alphabetic character (exercises find_first_of fallback branch)
+    haystack[0] = 'z';
+    EXPECT_TRUE(absl::StrContainsIgnoreCase(haystack, 'Z'));
+    EXPECT_TRUE(absl::StrContainsIgnoreCase(haystack, 'z'));
+
+    haystack = std::string(n, '.');
+    haystack[n / 2] = 'Z';
+    EXPECT_TRUE(absl::StrContainsIgnoreCase(haystack, 'z'));
+    EXPECT_TRUE(absl::StrContainsIgnoreCase(haystack, 'Z'));
+
+    haystack = std::string(n, '.');
+    haystack[n - 1] = 'z';
+    EXPECT_TRUE(absl::StrContainsIgnoreCase(haystack, 'Z'));
+    EXPECT_TRUE(absl::StrContainsIgnoreCase(haystack, 'z'));
+
+    haystack = std::string(n, '.');
+    EXPECT_FALSE(absl::StrContainsIgnoreCase(haystack, 'z'));
+    EXPECT_FALSE(absl::StrContainsIgnoreCase(haystack, 'Z'));
+
+    // 2. Non-alphabetic character (upper_needle == lower_needle)
+    haystack[n / 2] = '9';
+    EXPECT_TRUE(absl::StrContainsIgnoreCase(haystack, '9'));
+    EXPECT_FALSE(absl::StrContainsIgnoreCase(haystack, '8'));
+  }
+}
+
+TEST(MatchTest, FindLongestCommonPrefix) {
+  EXPECT_EQ(absl::FindLongestCommonPrefix("", ""), "");
+  EXPECT_EQ(absl::FindLongestCommonPrefix("", "abc"), "");
+  EXPECT_EQ(absl::FindLongestCommonPrefix("abc", ""), "");
+  EXPECT_EQ(absl::FindLongestCommonPrefix("ab", "abc"), "ab");
+  EXPECT_EQ(absl::FindLongestCommonPrefix("abc", "ab"), "ab");
+  EXPECT_EQ(absl::FindLongestCommonPrefix("abc", "abd"), "ab");
+  EXPECT_EQ(absl::FindLongestCommonPrefix("abc", "abcd"), "abc");
+  EXPECT_EQ(absl::FindLongestCommonPrefix("abcd", "abcd"), "abcd");
+  EXPECT_EQ(absl::FindLongestCommonPrefix("abcd", "efgh"), "");
+
+  // "abcde" v. "abc" but in the middle of other data
+  EXPECT_EQ(absl::FindLongestCommonPrefix(
+                absl::string_view("1234 abcdef").substr(5, 5),
+                absl::string_view("5678 abcdef").substr(5, 3)),
+            "abc");
+}
+
+// Since the little-endian implementation involves a bit of if-else and various
+// return paths, the following tests aims to provide full test coverage of the
+// implementation.
+TEST(MatchTest, FindLongestCommonPrefixLoad16Mismatch) {
+  const std::string x1 = "abcdefgh";
+  const std::string x2 = "abcde_";
+  EXPECT_EQ(absl::FindLongestCommonPrefix(x1, x2), "abcde");
+  EXPECT_EQ(absl::FindLongestCommonPrefix(x2, x1), "abcde");
+}
+
+TEST(MatchTest, FindLongestCommonPrefixLoad16MatchesNoLast) {
+  const std::string x1 = "abcdef";
+  const std::string x2 = "abcdef";
+  EXPECT_EQ(absl::FindLongestCommonPrefix(x1, x2), "abcdef");
+  EXPECT_EQ(absl::FindLongestCommonPrefix(x2, x1), "abcdef");
+}
+
+TEST(MatchTest, FindLongestCommonPrefixLoad16MatchesLastCharMismatches) {
+  const std::string x1 = "abcdefg";
+  const std::string x2 = "abcdef_h";
+  EXPECT_EQ(absl::FindLongestCommonPrefix(x1, x2), "abcdef");
+  EXPECT_EQ(absl::FindLongestCommonPrefix(x2, x1), "abcdef");
+}
+
+TEST(MatchTest, FindLongestCommonPrefixLoad16MatchesLastMatches) {
+  const std::string x1 = "abcde";
+  const std::string x2 = "abcdefgh";
+  EXPECT_EQ(absl::FindLongestCommonPrefix(x1, x2), "abcde");
+  EXPECT_EQ(absl::FindLongestCommonPrefix(x2, x1), "abcde");
+}
+
+TEST(MatchTest, FindLongestCommonPrefixSize8Load64Mismatches) {
+  const std::string x1 = "abcdefghijk";
+  const std::string x2 = "abcde_g_";
+  EXPECT_EQ(absl::FindLongestCommonPrefix(x1, x2), "abcde");
+  EXPECT_EQ(absl::FindLongestCommonPrefix(x2, x1), "abcde");
+}
+
+TEST(MatchTest, FindLongestCommonPrefixSize8Load64Matches) {
+  const std::string x1 = "abcdefgh";
+  const std::string x2 = "abcdefgh";
+  EXPECT_EQ(absl::FindLongestCommonPrefix(x1, x2), "abcdefgh");
+  EXPECT_EQ(absl::FindLongestCommonPrefix(x2, x1), "abcdefgh");
+}
+
+TEST(MatchTest, FindLongestCommonPrefixSize15Load64Mismatches) {
+  const std::string x1 = "012345670123456";
+  const std::string x2 = "0123456701_34_6";
+  EXPECT_EQ(absl::FindLongestCommonPrefix(x1, x2), "0123456701");
+  EXPECT_EQ(absl::FindLongestCommonPrefix(x2, x1), "0123456701");
+}
+
+TEST(MatchTest, FindLongestCommonPrefixSize15Load64Matches) {
+  const std::string x1 = "012345670123456";
+  const std::string x2 = "0123456701234567";
+  EXPECT_EQ(absl::FindLongestCommonPrefix(x1, x2), "012345670123456");
+  EXPECT_EQ(absl::FindLongestCommonPrefix(x2, x1), "012345670123456");
+}
+
+TEST(MatchTest, FindLongestCommonPrefixSizeFirstByteOfLast8BytesMismatch) {
+  const std::string x1 = "012345670123456701234567";
+  const std::string x2 = "0123456701234567_1234567";
+  EXPECT_EQ(absl::FindLongestCommonPrefix(x1, x2), "0123456701234567");
+  EXPECT_EQ(absl::FindLongestCommonPrefix(x2, x1), "0123456701234567");
+}
+
+TEST(MatchTest, FindLongestCommonPrefixLargeLastCharMismatches) {
+  const std::string x1(300, 'x');
+  std::string x2 = x1;
+  x2.back() = '#';
+  EXPECT_EQ(absl::FindLongestCommonPrefix(x1, x2), std::string(299, 'x'));
+  EXPECT_EQ(absl::FindLongestCommonPrefix(x2, x1), std::string(299, 'x'));
+}
+
+TEST(MatchTest, FindLongestCommonPrefixLargeFullMatch) {
+  const std::string x1(300, 'x');
+  const std::string x2 = x1;
+  EXPECT_EQ(absl::FindLongestCommonPrefix(x1, x2), std::string(300, 'x'));
+  EXPECT_EQ(absl::FindLongestCommonPrefix(x2, x1), std::string(300, 'x'));
+}
+
+TEST(MatchTest, FindLongestCommonSuffix) {
+  EXPECT_EQ(absl::FindLongestCommonSuffix("", ""), "");
+  EXPECT_EQ(absl::FindLongestCommonSuffix("", "abc"), "");
+  EXPECT_EQ(absl::FindLongestCommonSuffix("abc", ""), "");
+  EXPECT_EQ(absl::FindLongestCommonSuffix("bc", "abc"), "bc");
+  EXPECT_EQ(absl::FindLongestCommonSuffix("abc", "bc"), "bc");
+  EXPECT_EQ(absl::FindLongestCommonSuffix("abc", "dbc"), "bc");
+  EXPECT_EQ(absl::FindLongestCommonSuffix("bcd", "abcd"), "bcd");
+  EXPECT_EQ(absl::FindLongestCommonSuffix("abcd", "abcd"), "abcd");
+  EXPECT_EQ(absl::FindLongestCommonSuffix("abcd", "efgh"), "");
+
+  // "abcde" v. "cde" but in the middle of other data
+  EXPECT_EQ(absl::FindLongestCommonSuffix(
+                absl::string_view("1234 abcdef").substr(5, 5),
+                absl::string_view("5678 abcdef").substr(7, 3)),
+            "cde");
+}
+
 }  // namespace
diff --git a/absl/strings/numbers.cc b/absl/strings/numbers.cc
index 38d148690e2..924c9122f99 100644
--- a/absl/strings/numbers.cc
+++ b/absl/strings/numbers.cc
@@ -18,33 +18,51 @@
 #include "absl/strings/numbers.h"
 
 #include <algorithm>
+#include <array>
 #include <cassert>
-#include <cfloat>          // for DBL_DIG and FLT_DIG
-#include <cmath>           // for HUGE_VAL
+#include <cfloat>  // for DBL_DIG and FLT_DIG
+#include <clocale>  // for localeconv
+#include <cmath>   // for HUGE_VAL
 #include <cstdint>
 #include <cstdio>
 #include <cstdlib>
 #include <cstring>
 #include <iterator>
 #include <limits>
-#include <memory>
+#include <system_error>  // NOLINT(build/c++11)
 #include <utility>
 
-#include "absl/base/internal/bits.h"
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/endian.h"
 #include "absl/base/internal/raw_logging.h"
+#include "absl/base/macros.h"
+#include "absl/base/nullability.h"
+#include "absl/base/optimization.h"
+#include "absl/numeric/bits.h"
+#include "absl/numeric/int128.h"
 #include "absl/strings/ascii.h"
 #include "absl/strings/charconv.h"
-#include "absl/strings/internal/memutil.h"
 #include "absl/strings/match.h"
-#include "absl/strings/str_cat.h"
+#include "absl/strings/string_view.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
-bool SimpleAtof(absl::string_view str, float* out) {
+bool SimpleAtof(absl::string_view str, float* absl_nonnull out) {
   *out = 0.0;
   str = StripAsciiWhitespace(str);
-  if (!str.empty() && str[0] == '+') {
+  if (str.empty()) {
+    // absl::from_chars doesn't accept empty strings.
+    return false;
+  }
+  // std::from_chars doesn't accept an initial +, but SimpleAtof does, so if one
+  // is present, skip it, while avoiding accepting "+-0" as valid.
+  if (str[0] == '+') {
     str.remove_prefix(1);
+    if (str.empty() || str[0] == '-') {
+      return false;
+    }
   }
   auto result = absl::from_chars(str.data(), str.data() + str.size(), *out);
   if (result.ec == std::errc::invalid_argument) {
@@ -66,11 +84,20 @@ bool SimpleAtof(absl::string_view str, float* out) {
   return true;
 }
 
-bool SimpleAtod(absl::string_view str, double* out) {
+bool SimpleAtod(absl::string_view str, double* absl_nonnull out) {
   *out = 0.0;
   str = StripAsciiWhitespace(str);
-  if (!str.empty() && str[0] == '+') {
+  if (str.empty()) {
+    // absl::from_chars doesn't accept empty strings.
+    return false;
+  }
+  // std::from_chars doesn't accept an initial +, but SimpleAtod does, so if one
+  // is present, skip it, while avoiding accepting "+-0" as valid.
+  if (str[0] == '+') {
     str.remove_prefix(1);
+    if (str.empty() || str[0] == '-') {
+      return false;
+    }
   }
   auto result = absl::from_chars(str.data(), str.data() + str.size(), *out);
   if (result.ec == std::errc::invalid_argument) {
@@ -92,44 +119,7 @@ bool SimpleAtod(absl::string_view str, double* out) {
   return true;
 }
 
-namespace {
-
-// Writes a two-character representation of 'i' to 'buf'. 'i' must be in the
-// range 0 <= i < 100, and buf must have space for two characters. Example:
-//   char buf[2];
-//   PutTwoDigits(42, buf);
-//   // buf[0] == '4'
-//   // buf[1] == '2'
-inline void PutTwoDigits(size_t i, char* buf) {
-  static const char two_ASCII_digits[100][2] = {
-    {'0', '0'}, {'0', '1'}, {'0', '2'}, {'0', '3'}, {'0', '4'},
-    {'0', '5'}, {'0', '6'}, {'0', '7'}, {'0', '8'}, {'0', '9'},
-    {'1', '0'}, {'1', '1'}, {'1', '2'}, {'1', '3'}, {'1', '4'},
-    {'1', '5'}, {'1', '6'}, {'1', '7'}, {'1', '8'}, {'1', '9'},
-    {'2', '0'}, {'2', '1'}, {'2', '2'}, {'2', '3'}, {'2', '4'},
-    {'2', '5'}, {'2', '6'}, {'2', '7'}, {'2', '8'}, {'2', '9'},
-    {'3', '0'}, {'3', '1'}, {'3', '2'}, {'3', '3'}, {'3', '4'},
-    {'3', '5'}, {'3', '6'}, {'3', '7'}, {'3', '8'}, {'3', '9'},
-    {'4', '0'}, {'4', '1'}, {'4', '2'}, {'4', '3'}, {'4', '4'},
-    {'4', '5'}, {'4', '6'}, {'4', '7'}, {'4', '8'}, {'4', '9'},
-    {'5', '0'}, {'5', '1'}, {'5', '2'}, {'5', '3'}, {'5', '4'},
-    {'5', '5'}, {'5', '6'}, {'5', '7'}, {'5', '8'}, {'5', '9'},
-    {'6', '0'}, {'6', '1'}, {'6', '2'}, {'6', '3'}, {'6', '4'},
-    {'6', '5'}, {'6', '6'}, {'6', '7'}, {'6', '8'}, {'6', '9'},
-    {'7', '0'}, {'7', '1'}, {'7', '2'}, {'7', '3'}, {'7', '4'},
-    {'7', '5'}, {'7', '6'}, {'7', '7'}, {'7', '8'}, {'7', '9'},
-    {'8', '0'}, {'8', '1'}, {'8', '2'}, {'8', '3'}, {'8', '4'},
-    {'8', '5'}, {'8', '6'}, {'8', '7'}, {'8', '8'}, {'8', '9'},
-    {'9', '0'}, {'9', '1'}, {'9', '2'}, {'9', '3'}, {'9', '4'},
-    {'9', '5'}, {'9', '6'}, {'9', '7'}, {'9', '8'}, {'9', '9'}
-  };
-  assert(i < 100);
-  memcpy(buf, two_ASCII_digits[i], 2);
-}
-
-}  // namespace
-
-bool SimpleAtob(absl::string_view str, bool* out) {
+bool SimpleAtob(absl::string_view str, bool* absl_nonnull out) {
   ABSL_RAW_CHECK(out != nullptr, "Output pointer must not be nullptr.");
   if (EqualsIgnoreCase(str, "true") || EqualsIgnoreCase(str, "t") ||
       EqualsIgnoreCase(str, "yes") || EqualsIgnoreCase(str, "y") ||
@@ -160,144 +150,685 @@ bool SimpleAtob(absl::string_view str, bool* out) {
 
 namespace {
 
-// Used to optimize printing a decimal number's final digit.
-const char one_ASCII_final_digits[10][2] {
-  {'0', 0}, {'1', 0}, {'2', 0}, {'3', 0}, {'4', 0},
-  {'5', 0}, {'6', 0}, {'7', 0}, {'8', 0}, {'9', 0},
-};
+// Various routines to encode integers to strings.
+
+// We split data encodings into a group of 2 digits, 4 digits, 8 digits as
+// it's easier to combine powers of two into scalar arithmetic.
+
+// Previous implementation used a lookup table of 200 bytes for every 2 bytes
+// and it was memory bound, any L1 cache miss would result in a much slower
+// result. When benchmarking with a cache eviction rate of several percent,
+// this implementation proved to be better.
+
+// These constants represent '00', '0000' and '00000000' as ascii strings in
+// integers. We can add these numbers if we encode to bytes from 0 to 9. as
+// 'i' = '0' + i for 0 <= i <= 9.
+constexpr uint32_t kTwoZeroBytes = 0x0101 * '0';
+constexpr uint64_t kFourZeroBytes = 0x01010101 * '0';
+constexpr uint64_t kEightZeroBytes = 0x0101010101010101ull * '0';
+
+// * 103 / 1024 is a division by 10 for values from 0 to 99. It's also a
+// division of a structure [k takes 2 bytes][m takes 2 bytes], then * 103 / 1024
+// will be [k / 10][m / 10]. It allows parallel division.
+constexpr uint64_t kDivisionBy10Mul = 103u;
+constexpr uint64_t kDivisionBy10Div = 1 << 10;
+
+// * 10486 / 1048576 is a division by 100 for values from 0 to 9999.
+constexpr uint64_t kDivisionBy100Mul = 10486u;
+constexpr uint64_t kDivisionBy100Div = 1 << 20;
+
+// Encode functions write the ASCII output of input `n` to `out_str`.
+inline char* EncodeHundred(uint32_t n, char* absl_nonnull out_str) {
+  int num_digits = static_cast<int>(n - 10) >> 8;
+  uint32_t div10 = (n * kDivisionBy10Mul) / kDivisionBy10Div;
+  uint32_t mod10 = n - 10u * div10;
+  uint32_t base = kTwoZeroBytes + div10 + (mod10 << 8);
+  base >>= num_digits & 8;
+  little_endian::Store16(out_str, static_cast<uint16_t>(base));
+  return out_str + 2 + num_digits;
+}
 
-}  // namespace
+inline char* EncodeTenThousand(uint32_t n, char* absl_nonnull out_str) {
+  // We split lower 2 digits and upper 2 digits of n into 2 byte consecutive
+  // blocks. 123 ->  [\0\1][\0\23]. We divide by 10 both blocks
+  // (it's 1 division + zeroing upper bits), and compute modulo 10 as well "in
+  // parallel". Then we combine both results to have both ASCII digits,
+  // strip trailing zeros, add ASCII '0000' and return.
+  uint32_t div100 = (n * kDivisionBy100Mul) / kDivisionBy100Div;
+  uint32_t mod100 = n - 100ull * div100;
+  uint32_t hundreds = (mod100 << 16) + div100;
+  uint32_t tens = (hundreds * kDivisionBy10Mul) / kDivisionBy10Div;
+  tens &= (0xFull << 16) | 0xFull;
+  tens += (hundreds - 10ull * tens) << 8;
+  ABSL_ASSUME(tens != 0);
+  // The result can contain trailing zero bits, we need to strip them to a first
+  // significant byte in a final representation. For example, for n = 123, we
+  // have tens to have representation \0\1\2\3. We do `& -8` to round
+  // to a multiple to 8 to strip zero bytes, not all zero bits.
+  // countr_zero to help.
+  // 0 minus 8 to make MSVC happy.
+  uint32_t zeroes = static_cast<uint32_t>(absl::countr_zero(tens)) & (0 - 8u);
+  tens += kFourZeroBytes;
+  tens >>= zeroes;
+  little_endian::Store32(out_str, tens);
+  return out_str + sizeof(tens) - zeroes / 8;
+}
 
-char* numbers_internal::FastIntToBuffer(uint32_t i, char* buffer) {
-  uint32_t digits;
-  // The idea of this implementation is to trim the number of divides to as few
-  // as possible, and also reducing memory stores and branches, by going in
-  // steps of two digits at a time rather than one whenever possible.
-  // The huge-number case is first, in the hopes that the compiler will output
-  // that case in one branch-free block of code, and only output conditional
-  // branches into it from below.
-  if (i >= 1000000000) {     // >= 1,000,000,000
-    digits = i / 100000000;  //      100,000,000
-    i -= digits * 100000000;
-    PutTwoDigits(digits, buffer);
-    buffer += 2;
-  lt100_000_000:
-    digits = i / 1000000;  // 1,000,000
-    i -= digits * 1000000;
-    PutTwoDigits(digits, buffer);
-    buffer += 2;
-  lt1_000_000:
-    digits = i / 10000;  // 10,000
-    i -= digits * 10000;
-    PutTwoDigits(digits, buffer);
-    buffer += 2;
-  lt10_000:
-    digits = i / 100;
-    i -= digits * 100;
-    PutTwoDigits(digits, buffer);
-    buffer += 2;
- lt100:
-    digits = i;
-    PutTwoDigits(digits, buffer);
-    buffer += 2;
-    *buffer = 0;
-    return buffer;
+// Helper function to produce an ASCII representation of `i`.
+//
+// Function returns an 8-byte integer which when summed with `kEightZeroBytes`,
+// can be treated as a printable buffer with ascii representation of `i`,
+// possibly with leading zeros.
+//
+// Example:
+//
+//  uint64_t buffer = PrepareEightDigits(102030) + kEightZeroBytes;
+//  char* ascii = reinterpret_cast<char*>(&buffer);
+//  // Note two leading zeros:
+//  EXPECT_EQ(absl::string_view(ascii, 8), "00102030");
+//
+// Pre-condition: `i` must be less than 100000000.
+inline uint64_t PrepareEightDigits(uint32_t i) {
+  ABSL_ASSUME(i < 10000'0000);
+  // Prepare 2 blocks of 4 digits "in parallel".
+  uint32_t hi = i / 10000;
+  uint32_t lo = i % 10000;
+  uint64_t merged = hi | (uint64_t{lo} << 32);
+  uint64_t div100 = ((merged * kDivisionBy100Mul) / kDivisionBy100Div) &
+                    ((0x7Full << 32) | 0x7Full);
+  uint64_t mod100 = merged - 100ull * div100;
+  uint64_t hundreds = (mod100 << 16) + div100;
+  uint64_t tens = (hundreds * kDivisionBy10Mul) / kDivisionBy10Div;
+  tens &= (0xFull << 48) | (0xFull << 32) | (0xFull << 16) | 0xFull;
+  tens += (hundreds - 10ull * tens) << 8;
+  return tens;
+}
+
+
+// Encodes v to buffer as 16 digits padded with leading zeros.
+// Pre-condition: v must be < 10^16.
+inline char* EncodePadded16(uint64_t v, char* absl_nonnull buffer) {
+  constexpr uint64_t k1e8 = 100000000;
+  uint32_t hi = static_cast<uint32_t>(v / k1e8);
+  uint32_t lo = static_cast<uint32_t>(v % k1e8);
+  little_endian::Store64(buffer, PrepareEightDigits(hi) + kEightZeroBytes);
+  little_endian::Store64(buffer + 8, PrepareEightDigits(lo) + kEightZeroBytes);
+  return buffer + 16;
+}
+
+inline ABSL_ATTRIBUTE_ALWAYS_INLINE char* absl_nonnull EncodeFullU32(
+    uint32_t n, char* absl_nonnull out_str) {
+  if (n < 10) {
+    *out_str = static_cast<char>('0' + n);
+    return out_str + 1;
+  }
+  if (n < 100'000'000) {
+    uint64_t bottom = PrepareEightDigits(n);
+    ABSL_ASSUME(bottom != 0);
+    // 0 minus 8 to make MSVC happy.
+    uint32_t zeroes =
+        static_cast<uint32_t>(absl::countr_zero(bottom)) & (0 - 8u);
+    little_endian::Store64(out_str, (bottom + kEightZeroBytes) >> zeroes);
+    return out_str + sizeof(bottom) - zeroes / 8;
   }
+  uint32_t div08 = n / 100'000'000;
+  uint32_t mod08 = n % 100'000'000;
+  uint64_t bottom = PrepareEightDigits(mod08) + kEightZeroBytes;
+  out_str = EncodeHundred(div08, out_str);
+  little_endian::Store64(out_str, bottom);
+  return out_str + sizeof(bottom);
+}
 
-  if (i < 100) {
-    digits = i;
-    if (i >= 10) goto lt100;
-    memcpy(buffer, one_ASCII_final_digits[i], 2);
-    return buffer + 1;
-  }
-  if (i < 10000) {  //    10,000
-    if (i >= 1000) goto lt10_000;
-    digits = i / 100;
-    i -= digits * 100;
-    *buffer++ = '0' + digits;
-    goto lt100;
-  }
-  if (i < 1000000) {  //    1,000,000
-    if (i >= 100000) goto lt1_000_000;
-    digits = i / 10000;  //    10,000
-    i -= digits * 10000;
-    *buffer++ = '0' + digits;
-    goto lt10_000;
-  }
-  if (i < 100000000) {  //    100,000,000
-    if (i >= 10000000) goto lt100_000_000;
-    digits = i / 1000000;  //   1,000,000
-    i -= digits * 1000000;
-    *buffer++ = '0' + digits;
-    goto lt1_000_000;
-  }
-  // we already know that i < 1,000,000,000
-  digits = i / 100000000;  //   100,000,000
-  i -= digits * 100000000;
-  *buffer++ = '0' + digits;
-  goto lt100_000_000;
-}
-
-char* numbers_internal::FastIntToBuffer(int32_t i, char* buffer) {
-  uint32_t u = i;
+inline ABSL_ATTRIBUTE_ALWAYS_INLINE char* absl_nonnull EncodeFullU64(
+    uint64_t i, char* absl_nonnull buffer) {
+  if (i <= std::numeric_limits<uint32_t>::max()) {
+    return EncodeFullU32(static_cast<uint32_t>(i), buffer);
+  }
+  uint32_t mod08;
+  if (i < 1'0000'0000'0000'0000ull) {
+    uint32_t div08 = static_cast<uint32_t>(i / 100'000'000ull);
+    mod08 = static_cast<uint32_t>(i % 100'000'000ull);
+    buffer = EncodeFullU32(div08, buffer);
+  } else {
+    uint64_t div08 = i / 100'000'000ull;
+    mod08 = static_cast<uint32_t>(i % 100'000'000ull);
+    uint32_t div016 = static_cast<uint32_t>(div08 / 100'000'000ull);
+    uint32_t div08mod08 = static_cast<uint32_t>(div08 % 100'000'000ull);
+    uint64_t mid_result = PrepareEightDigits(div08mod08) + kEightZeroBytes;
+    buffer = EncodeTenThousand(div016, buffer);
+    little_endian::Store64(buffer, mid_result);
+    buffer += sizeof(mid_result);
+  }
+  uint64_t mod_result = PrepareEightDigits(mod08) + kEightZeroBytes;
+  little_endian::Store64(buffer, mod_result);
+  return buffer + sizeof(mod_result);
+}
+
+inline ABSL_ATTRIBUTE_ALWAYS_INLINE char* absl_nonnull EncodeFullU128(
+    uint128 i, char* absl_nonnull buffer) {
+  if (absl::Uint128High64(i) == 0) {
+    return EncodeFullU64(absl::Uint128Low64(i), buffer);
+  }
+  // We divide the number into 16-digit chunks because `EncodePadded16` is
+  // optimized to handle 16 digits at a time (as two 8-digit chunks).
+  constexpr uint64_t k1e16 = uint64_t{10'000'000'000'000'000};
+  uint128 high = i / k1e16;
+  uint64_t low = absl::Uint128Low64(i % k1e16);
+  uint64_t mid = absl::Uint128Low64(high % k1e16);
+  high /= k1e16;
+
+  if (high == 0) {
+    buffer = EncodeFullU64(mid, buffer);
+    buffer = EncodePadded16(low, buffer);
+  } else {
+    buffer = EncodeFullU64(absl::Uint128Low64(high), buffer);
+    buffer = EncodePadded16(mid, buffer);
+    buffer = EncodePadded16(low, buffer);
+  }
+  return buffer;
+}
+
+}  // namespace
+
+void numbers_internal::PutTwoDigits(uint32_t i, char* absl_nonnull buf) {
+  assert(i < 100);
+  uint32_t base = kTwoZeroBytes;
+  uint32_t div10 = (i * kDivisionBy10Mul) / kDivisionBy10Div;
+  uint32_t mod10 = i - 10u * div10;
+  base += div10 + (mod10 << 8);
+  little_endian::Store16(buf, static_cast<uint16_t>(base));
+}
+
+char* absl_nonnull numbers_internal::FastIntToBuffer(
+    uint32_t n, char* absl_nonnull out_str) {
+  out_str = EncodeFullU32(n, out_str);
+  *out_str = '\0';
+  return out_str;
+}
+
+char* absl_nonnull numbers_internal::FastIntToBuffer(
+    int32_t i, char* absl_nonnull buffer) {
+  uint32_t u = static_cast<uint32_t>(i);
   if (i < 0) {
     *buffer++ = '-';
     // We need to do the negation in modular (i.e., "unsigned")
-    // arithmetic; MSVC++ apprently warns for plain "-u", so
+    // arithmetic; MSVC++ apparently warns for plain "-u", so
     // we write the equivalent expression "0 - u" instead.
     u = 0 - u;
   }
-  return numbers_internal::FastIntToBuffer(u, buffer);
+  buffer = EncodeFullU32(u, buffer);
+  *buffer = '\0';
+  return buffer;
 }
 
-char* numbers_internal::FastIntToBuffer(uint64_t i, char* buffer) {
-  uint32_t u32 = static_cast<uint32_t>(i);
-  if (u32 == i) return numbers_internal::FastIntToBuffer(u32, buffer);
-
-  // Here we know i has at least 10 decimal digits.
-  uint64_t top_1to11 = i / 1000000000;
-  u32 = static_cast<uint32_t>(i - top_1to11 * 1000000000);
-  uint32_t top_1to11_32 = static_cast<uint32_t>(top_1to11);
+char* absl_nonnull numbers_internal::FastIntToBuffer(
+    uint64_t i, char* absl_nonnull buffer) {
+  buffer = EncodeFullU64(i, buffer);
+  *buffer = '\0';
+  return buffer;
+}
 
-  if (top_1to11_32 == top_1to11) {
-    buffer = numbers_internal::FastIntToBuffer(top_1to11_32, buffer);
-  } else {
-    // top_1to11 has more than 32 bits too; print it in two steps.
-    uint32_t top_8to9 = static_cast<uint32_t>(top_1to11 / 100);
-    uint32_t mid_2 = static_cast<uint32_t>(top_1to11 - top_8to9 * 100);
-    buffer = numbers_internal::FastIntToBuffer(top_8to9, buffer);
-    PutTwoDigits(mid_2, buffer);
-    buffer += 2;
-  }
-
-  // We have only 9 digits now, again the maximum uint32_t can handle fully.
-  uint32_t digits = u32 / 10000000;  // 10,000,000
-  u32 -= digits * 10000000;
-  PutTwoDigits(digits, buffer);
-  buffer += 2;
-  digits = u32 / 100000;  // 100,000
-  u32 -= digits * 100000;
-  PutTwoDigits(digits, buffer);
-  buffer += 2;
-  digits = u32 / 1000;  // 1,000
-  u32 -= digits * 1000;
-  PutTwoDigits(digits, buffer);
-  buffer += 2;
-  digits = u32 / 10;
-  u32 -= digits * 10;
-  PutTwoDigits(digits, buffer);
-  buffer += 2;
-  memcpy(buffer, one_ASCII_final_digits[u32], 2);
-  return buffer + 1;
-}
-
-char* numbers_internal::FastIntToBuffer(int64_t i, char* buffer) {
-  uint64_t u = i;
+char* absl_nonnull numbers_internal::FastIntToBuffer(
+    int64_t i, char* absl_nonnull buffer) {
+  uint64_t u = static_cast<uint64_t>(i);
   if (i < 0) {
     *buffer++ = '-';
+    // We need to do the negation in modular (i.e., "unsigned")
+    // arithmetic; MSVC++ apparently warns for plain "-u", so
+    // we write the equivalent expression "0 - u" instead.
     u = 0 - u;
   }
-  return numbers_internal::FastIntToBuffer(u, buffer);
+  buffer = EncodeFullU64(u, buffer);
+  *buffer = '\0';
+  return buffer;
+}
+
+char* absl_nonnull numbers_internal::FastIntToBuffer(
+    uint128 i, char* absl_nonnull buffer) {
+  buffer = EncodeFullU128(i, buffer);
+  *buffer = '\0';
+  return buffer;
+}
+
+char* absl_nonnull numbers_internal::FastIntToBuffer(
+    int128 i, char* absl_nonnull buffer) {
+  uint128 u = static_cast<uint128>(i);
+  if (i < 0) {
+    *buffer++ = '-';
+    u = -u;
+  }
+  buffer = EncodeFullU128(u, buffer);
+  *buffer = '\0';
+  return buffer;
+}
+
+// Although DBL_DIG is typically 15, DBL_MAX is normally represented with 17
+// digits of precision. When converted to a string value with fewer digits
+// of precision using strtod(), the result can be bigger than DBL_MAX due to
+// a rounding error. Converting this value back to a double will produce an
+// Inf which will trigger a SIGFPE if FP exceptions are enabled. We skip
+// the precision check for sufficiently large values to avoid the SIGFPE.
+static constexpr double kDoublePrecisionCheckMax =
+    std::numeric_limits<double>::max() / 1.000000000000001;
+
+char* absl_nonnull numbers_internal::RoundTripDoubleToBuffer(
+    double d, char* absl_nonnull buffer) {
+  // DBL_DIG is 15 for IEEE-754 doubles, which are used on almost all
+  // platforms these days.  Just in case some system exists where DBL_DIG
+  // is significantly larger -- and risks overflowing our buffer -- we have
+  // this assert.
+  static_assert(std::numeric_limits<double>::digits10 < 20,
+                "std::numeric_limits<double>::digits10 is too big");
+
+  // We avoid snprintf for NaNs because it inconsistently outputs "-nan"
+  // or "nan" when given a nan with the sign bit set.
+  if (std::isnan(d)) {
+    strcpy(buffer, "nan");  // NOLINT(runtime/printf)
+    return buffer;
+  }
+  bool full_precision_needed = true;
+  if (std::abs(d) <= kDoublePrecisionCheckMax) {
+    int snprintf_result =
+        snprintf(buffer, numbers_internal::kFastToBufferSize, "%.*g",
+                 std::numeric_limits<double>::digits10, d);
+
+    // The snprintf should never overflow because the buffer is significantly
+    // larger than the precision we asked for.
+    ABSL_ASSERT(snprintf_result > 0 &&
+                snprintf_result < numbers_internal::kFastToBufferSize);
+
+    full_precision_needed = strtod(buffer, nullptr) != d;
+  }
+
+  if (full_precision_needed) {
+    int snprintf_result =
+        snprintf(buffer, numbers_internal::kFastToBufferSize, "%.*g",
+                 std::numeric_limits<double>::digits10 + 2, d);
+
+    // Should never overflow; see above.
+    ABSL_ASSERT(snprintf_result > 0 &&
+                snprintf_result < numbers_internal::kFastToBufferSize);
+  }
+
+  // snprintf() writes the radix character chosen by the global C locale's
+  // LC_NUMERIC category, so a process that has called setlocale() can end up
+  // with a separator other than '.' here. The rest of Abseil's float formatting
+  // (RoundTripFloatToBuffer, SixDigitsToBuffer) is locale- independent and
+  // SimpleAtod() only accepts '.', so rewrite the radix back to '.' to keep
+  // absl::HighPrecision(double) locale-independent and round-trippable through
+  // SimpleAtod().
+  // TODO: b/526633099 - Once all supported compilers ship std::to_chars with
+  // floating-point support, use it here for inherent locale independence.
+  const char* radix = localeconv()->decimal_point;
+  // Skip an empty decimal_point (some minimal environments leave it ""), which
+  // would otherwise match the beginning of the buffer and corrupt it.
+  if (radix[0] != '\0' && std::strcmp(radix, ".") != 0) {
+    if (char* p = std::strstr(buffer, radix)) {
+      const size_t radix_len = std::strlen(radix);
+      *p = '.';
+      // A multibyte radix (rare, but possible in some locales) leaves trailing
+      // bytes behind; collapse them so the output is a single '.'.
+      if (radix_len > 1) {
+        std::memmove(p + 1, p + radix_len, std::strlen(p + radix_len) + 1);
+      }
+    }
+  }
+  return buffer;
+}
+
+namespace {
+
+// This table is used to quickly calculate the base-ten exponent of a given
+// float, and then to provide a multiplier to bring that number into the
+// range 1-999,999,999, that is, into uint32_t range.  Finally, the exp
+// string is made available so there is one less int-to-string conversion
+// to be done.
+struct Spec {
+  double min_range;
+  double multiplier;
+  const char expstr[5];
+};
+
+// clang-format off
+constexpr Spec kNegExpTable[] = {
+    Spec{1.4e-45f, 1e+55, "e-45"},
+    Spec{1e-44f, 1e+54, "e-44"},
+    Spec{1e-43f, 1e+53, "e-43"},
+    Spec{1e-42f, 1e+52, "e-42"},
+    Spec{1e-41f, 1e+51, "e-41"},
+    Spec{1e-40f, 1e+50, "e-40"},
+    Spec{1e-39f, 1e+49, "e-39"},
+    Spec{1e-38f, 1e+48, "e-38"},
+    Spec{1e-37f, 1e+47, "e-37"},
+    Spec{1e-36f, 1e+46, "e-36"},
+    Spec{1e-35f, 1e+45, "e-35"},
+    Spec{1e-34f, 1e+44, "e-34"},
+    Spec{1e-33f, 1e+43, "e-33"},
+    Spec{1e-32f, 1e+42, "e-32"},
+    Spec{1e-31f, 1e+41, "e-31"},
+    Spec{1e-30f, 1e+40, "e-30"},
+    Spec{1e-29f, 1e+39, "e-29"},
+    Spec{1e-28f, 1e+38, "e-28"},
+    Spec{1e-27f, 1e+37, "e-27"},
+    Spec{1e-26f, 1e+36, "e-26"},
+    Spec{1e-25f, 1e+35, "e-25"},
+    Spec{1e-24f, 1e+34, "e-24"},
+    Spec{1e-23f, 1e+33, "e-23"},
+    Spec{1e-22f, 1e+32, "e-22"},
+    Spec{1e-21f, 1e+31, "e-21"},
+    Spec{1e-20f, 1e+30, "e-20"},
+    Spec{1e-19f, 1e+29, "e-19"},
+    Spec{1e-18f, 1e+28, "e-18"},
+    Spec{1e-17f, 1e+27, "e-17"},
+    Spec{1e-16f, 1e+26, "e-16"},
+    Spec{1e-15f, 1e+25, "e-15"},
+    Spec{1e-14f, 1e+24, "e-14"},
+    Spec{1e-13f, 1e+23, "e-13"},
+    Spec{1e-12f, 1e+22, "e-12"},
+    Spec{1e-11f, 1e+21, "e-11"},
+    Spec{1e-10f, 1e+20, "e-10"},
+    Spec{1e-09f, 1e+19, "e-09"},
+    Spec{1e-08f, 1e+18, "e-08"},
+    Spec{1e-07f, 1e+17, "e-07"},
+    Spec{1e-06f, 1e+16, "e-06"},
+    Spec{1e-05f, 1e+15, "e-05"},
+    Spec{1e-04f, 1e+14, "e-04"},
+};
+// clang-format on
+
+// clang-format off
+constexpr Spec kPosExpTable[] = {
+    Spec{1e+08f, 1e+02, "e+08"},
+    Spec{1e+09f, 1e+01, "e+09"},
+    Spec{1e+10f, 1e+00, "e+10"},
+    Spec{1e+11f, 1e-01, "e+11"},
+    Spec{1e+12f, 1e-02, "e+12"},
+    Spec{1e+13f, 1e-03, "e+13"},
+    Spec{1e+14f, 1e-04, "e+14"},
+    Spec{1e+15f, 1e-05, "e+15"},
+    Spec{1e+16f, 1e-06, "e+16"},
+    Spec{1e+17f, 1e-07, "e+17"},
+    Spec{1e+18f, 1e-08, "e+18"},
+    Spec{1e+19f, 1e-09, "e+19"},
+    Spec{1e+20f, 1e-10, "e+20"},
+    Spec{1e+21f, 1e-11, "e+21"},
+    Spec{1e+22f, 1e-12, "e+22"},
+    Spec{1e+23f, 1e-13, "e+23"},
+    Spec{1e+24f, 1e-14, "e+24"},
+    Spec{1e+25f, 1e-15, "e+25"},
+    Spec{1e+26f, 1e-16, "e+26"},
+    Spec{1e+27f, 1e-17, "e+27"},
+    Spec{1e+28f, 1e-18, "e+28"},
+    Spec{1e+29f, 1e-19, "e+29"},
+    Spec{1e+30f, 1e-20, "e+30"},
+    Spec{1e+31f, 1e-21, "e+31"},
+    Spec{1e+32f, 1e-22, "e+32"},
+    Spec{1e+33f, 1e-23, "e+33"},
+    Spec{1e+34f, 1e-24, "e+34"},
+    Spec{1e+35f, 1e-25, "e+35"},
+    Spec{1e+36f, 1e-26, "e+36"},
+    Spec{1e+37f, 1e-27, "e+37"},
+    Spec{1e+38f, 1e-28, "e+38"},
+    Spec{1e+39,  1e-29, "e+39"},
+};
+// clang-format on
+
+struct ExpCompare {
+  bool operator()(const Spec& spec, double d) const {
+    return spec.min_range < d;
+  }
+};
+
+}  // namespace
+
+// Utility routine(s) for RoundTripFloatToBuffer:
+// OutputNecessaryDigits takes two 11-digit numbers, whose integer portion
+// represents the fractional part of a floating-point number, and outputs a
+// number that is in-between them, with the fewest digits possible. For
+// instance, given 12345678900 and 12345876900, it would output "0123457".
+// When there are multiple final digits that would satisfy this requirement,
+// this routine attempts to use a digit that would represent the average of
+// lower_double and upper_double.
+//
+// Although the routine works using integers, all callers use doubles, so
+// for their convenience this routine accepts doubles.
+static char* absl_nonnull OutputNecessaryDigits(double lower_double,
+                                                double upper_double,
+                                                char* absl_nonnull out) {
+  assert(lower_double > 0);
+  assert(lower_double < upper_double - 10);
+  assert(upper_double < 100000000000.0);
+
+  // Narrow the range a bit; without this bias, an input of lower=87654320010.0
+  // and upper=87654320100.0 would produce an output of 876543201
+  //
+  // We do this in three steps: first, we lower the upper bound and truncate it
+  // to an integer.  Then, we increase the lower bound by exactly the amount we
+  // just decreased the upper bound by - at that point, the midpoint is exactly
+  // where it used to be.  Then we truncate the lower bound.
+
+  uint64_t upper64 = static_cast<uint64_t>(upper_double - (1.0 / 1024));
+  double shrink = upper_double - upper64;
+  uint64_t lower64 = static_cast<uint64_t>(lower_double + shrink);
+
+  // Theory of operation: we convert the lower number to ascii representation,
+  // two digits at a time.  As we go, we remove the same digits from the upper
+  // number.  When we see the upper number does not share those same digits, we
+  // know we can stop converting. When we stop, the last digit we output is
+  // taken from the average of upper and lower values, rounded up.
+  char buf[2];
+  uint32_t lodigits =
+      static_cast<uint32_t>(lower64 / 1000000000);  // 1,000,000,000
+  uint64_t mul64 = lodigits * uint64_t{1000000000};
+
+  numbers_internal::PutTwoDigits(lodigits, out);
+  out += 2;
+  if (upper64 - mul64 >= 1000000000) {  // digit mismatch!
+    numbers_internal::PutTwoDigits(static_cast<uint32_t>(upper64 / 1000000000),
+                                   buf);
+    if (out[-2] != buf[0]) {
+      out[-2] = static_cast<char>('0' + (upper64 + lower64 + 10000000000) /
+                                            20000000000);
+      --out;
+    } else {
+      numbers_internal::PutTwoDigits(
+          static_cast<uint32_t>((upper64 + lower64 + 1000000000) / 2000000000),
+          out - 2);
+    }
+    *out = '\0';
+    return out;
+  }
+  uint32_t lower = static_cast<uint32_t>(lower64 - mul64);
+  uint32_t upper = static_cast<uint32_t>(upper64 - mul64);
+
+  lodigits = lower / 10000000;  // 10,000,000
+  uint32_t mul = lodigits * 10000000;
+  numbers_internal::PutTwoDigits(lodigits, out);
+  out += 2;
+  if (upper - mul >= 10000000) {  // digit mismatch!
+    numbers_internal::PutTwoDigits(upper / 10000000, buf);
+    if (out[-2] != buf[0]) {
+      out[-2] = '0' + (upper + lower + 100000000) / 200000000;
+      --out;
+    } else {
+      numbers_internal::PutTwoDigits((upper + lower + 10000000) / 20000000,
+                                      out - 2);
+    }
+    *out = '\0';
+    return out;
+  }
+  lower -= mul;
+  upper -= mul;
+
+  lodigits = lower / 100000;  // 100,000
+  mul = lodigits * 100000;
+  numbers_internal::PutTwoDigits(lodigits, out);
+  out += 2;
+  if (upper - mul >= 100000) {  // digit mismatch!
+    numbers_internal::PutTwoDigits(upper / 100000, buf);
+    if (out[-2] != buf[0]) {
+      out[-2] = static_cast<char>('0' + (upper + lower + 1000000) / 2000000);
+      --out;
+    } else {
+      numbers_internal::PutTwoDigits((upper + lower + 100000) / 200000,
+                                      out - 2);
+    }
+    *out = '\0';
+    return out;
+  }
+  lower -= mul;
+  upper -= mul;
+
+  lodigits = lower / 1000;
+  mul = lodigits * 1000;
+  numbers_internal::PutTwoDigits(lodigits, out);
+  out += 2;
+  if (upper - mul >= 1000) {  // digit mismatch!
+    numbers_internal::PutTwoDigits(upper / 1000, buf);
+    if (out[-2] != buf[0]) {
+      out[-2] = static_cast<char>('0' + (upper + lower + 10000) / 20000);
+      --out;
+    } else {
+      numbers_internal::PutTwoDigits((upper + lower + 1000) / 2000, out - 2);
+    }
+    *out = '\0';
+    return out;
+  }
+  lower -= mul;
+  upper -= mul;
+
+  numbers_internal::PutTwoDigits(lower / 10, out);
+  out += 2;
+  numbers_internal::PutTwoDigits(upper / 10, buf);
+  if (out[-2] != buf[0]) {
+    out[-2] = static_cast<char>('0' + (upper + lower + 100) / 200);
+    --out;
+  } else {
+    numbers_internal::PutTwoDigits((upper + lower + 10) / 20, out - 2);
+  }
+  *out = '\0';
+  return out;
+}
+
+// RoundTripFloatToBuffer converts the given float into a string which, if
+// passed to strtof, will produce the exact same original float.  It does this
+// by computing the range of possible doubles which map to the given float, and
+// then examining the digits of the doubles in that range.  If all the doubles
+// in the range start with "2.37", then clearly our float does, too.  As soon as
+// they diverge, only one more digit is needed.
+char* absl_nonnull numbers_internal::RoundTripFloatToBuffer(
+    float f, char* absl_nonnull buffer) {
+  static_assert(std::numeric_limits<float>::is_iec559,
+                "IEEE-754/IEC-559 support only");
+
+  // We write data to out, incrementing as we go, but FloatToBuffer always
+  // returns the address of the buffer passed in.
+  char* out = buffer;
+
+  if (std::isnan(f)) {
+    strcpy(out, "nan");  // NOLINT(runtime/printf)
+    return buffer;
+  }
+  if (f == 0) {  // +0 and -0 are handled here
+    if (std::signbit(f)) {
+      strcpy(out, "-0");  // NOLINT(runtime/printf)
+    } else {
+      strcpy(out, "0");  // NOLINT(runtime/printf)
+    }
+    return buffer;
+  }
+  if (f < 0) {
+    *out++ = '-';
+    f = -f;
+  }
+  if (f > std::numeric_limits<float>::max()) {
+    strcpy(out, "inf");  // NOLINT(runtime/printf)
+    return buffer;
+  }
+
+  double next_lower = nextafterf(f, 0.0f);
+  // For all doubles in the range lower_bound < f < upper_bound, the
+  // nearest float is f.
+  double lower_bound = (f + next_lower) * 0.5;
+  double upper_bound = f + (f - lower_bound);
+  // Note: because std::nextafter is slow, we calculate upper_bound
+  // assuming that it is the same distance from f as lower_bound is.
+  // For exact powers of two, upper_bound is actually twice as far
+  // from f as lower_bound is, but this turns out not to matter.
+
+  // Most callers pass floats that are either 0 or within the
+  // range 0.0001 through 100,000,000, so handle those first,
+  // since they don't need exponential notation.
+  const Spec* spec = nullptr;
+  if (f < 1.0) {
+    if (f >= 0.0001f) {
+      // For fractional values, we set up the multiplier at the same
+      // time as we output the leading "0." / "0.0" / "0.00" / "0.000"
+      double multiplier = 1e+11;
+      *out++ = '0';
+      *out++ = '.';
+      if (f < 0.1f) {
+        multiplier = 1e+12;
+        *out++ = '0';
+        if (f < 0.01f) {
+          multiplier = 1e+13;
+          *out++ = '0';
+          if (f < 0.001f) {
+            multiplier = 1e+14;
+            *out++ = '0';
+          }
+        }
+      }
+      OutputNecessaryDigits(lower_bound * multiplier, upper_bound * multiplier,
+                            out);
+      return buffer;
+    }
+    spec = std::lower_bound(std::begin(kNegExpTable), std::end(kNegExpTable),
+                            double{f}, ExpCompare());
+    if (spec == std::end(kNegExpTable)) --spec;
+  } else if (f < 1e8) {
+    // Handling non-exponential format greater than 1.0 is similar to the above,
+    // but instead of 0.0 / 0.00 / 0.000, the prefix is simply the truncated
+    // integer part of f.
+    int32_t as_int = static_cast<int32_t>(f);
+    out = numbers_internal::FastIntToBuffer(as_int, out);
+    // Easy: if the integer part is within (lower_bound, upper_bound), then we
+    // are already done.
+    if (as_int > lower_bound && as_int < upper_bound) {
+      return buffer;
+    }
+    *out++ = '.';
+    OutputNecessaryDigits((lower_bound - as_int) * 1e11,
+                          (upper_bound - as_int) * 1e11, out);
+    return buffer;
+  } else {
+    spec = std::lower_bound(std::begin(kPosExpTable), std::end(kPosExpTable),
+                            double{f}, ExpCompare());
+    if (spec == std::end(kPosExpTable)) --spec;
+  }
+  // Exponential notation from here on.  "spec" was computed using lower_bound,
+  // which means it's the first spec from the table where min_range is greater
+  // or equal to f.
+  // Unfortunately that's not quite what we want; we want a min_range that is
+  // less or equal.  So first thing, if it was greater, back up one entry.
+  if (spec->min_range > f) --spec;
+
+  // The digits might be "237000123", but we want "2.37000123",
+  // so we output the digits one character later, and then move the first
+  // digit back so we can stick the "." in.
+  char* start = out;
+  out = OutputNecessaryDigits(lower_bound * spec->multiplier,
+                              upper_bound * spec->multiplier, start + 1);
+  start[0] = start[1];
+  start[1] = '.';
+
+  // If it turns out there was only one digit output, then back up over the '.'
+  if (out == &start[2]) --out;
+
+  // Now add the "e+NN" part.
+  memcpy(out, spec->expstr, 4);
+  out[4] = '\0';
+  return buffer;
 }
 
 // Given a 128-bit number expressed as a pair of uint64_t, high half first,
@@ -337,7 +868,7 @@ static std::pair<uint64_t, uint64_t> Mul32(std::pair<uint64_t, uint64_t> num,
   uint64_t bits128_up = (bits96_127 >> 32) + (bits64_127 < bits64_95);
   if (bits128_up == 0) return {bits64_127, bits0_63};
 
-  int shift = 64 - base_internal::CountLeadingZeros64(bits128_up);
+  auto shift = static_cast<unsigned>(bit_width(bits128_up));
   uint64_t lo = (bits0_63 >> shift) + (bits64_127 << (64 - shift));
   uint64_t hi = (bits64_127 >> shift) + (bits128_up << (64 - shift));
   return {hi, lo};
@@ -353,7 +884,7 @@ static std::pair<uint64_t, uint64_t> PowFive(uint64_t num, int expfive) {
     result = Mul32(result, 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5);
     expfive -= 13;
   }
-  constexpr int powers_of_five[13] = {
+  constexpr uint32_t powers_of_five[13] = {
       1,
       5,
       5 * 5,
@@ -368,7 +899,7 @@ static std::pair<uint64_t, uint64_t> PowFive(uint64_t num, int expfive) {
       5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5,
       5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5};
   result = Mul32(result, powers_of_five[expfive & 15]);
-  int shift = base_internal::CountLeadingZeros64(result.first);
+  int shift = countl_zero(result.first);
   if (shift != 0) {
     result.first = (result.first << shift) + (result.second >> (64 - shift));
     result.second = (result.second << shift);
@@ -427,21 +958,22 @@ static ExpDigits SplitToSix(const double value) {
   // Since we'd like to know if the fractional part of d is close to a half,
   // we multiply it by 65536 and see if the fractional part is close to 32768.
   // (The number doesn't have to be a power of two,but powers of two are faster)
-  uint64_t d64k = d * 65536;
-  int dddddd;  // A 6-digit decimal integer.
+  uint64_t d64k = static_cast<uint64_t>(d * 65536);
+  uint32_t dddddd;  // A 6-digit decimal integer.
   if ((d64k % 65536) == 32767 || (d64k % 65536) == 32768) {
     // OK, it's fairly likely that precision was lost above, which is
     // not a surprise given only 52 mantissa bits are available.  Therefore
     // redo the calculation using 128-bit numbers.  (64 bits are not enough).
 
     // Start out with digits rounded down; maybe add one below.
-    dddddd = static_cast<int>(d64k / 65536);
+    dddddd = static_cast<uint32_t>(d64k / 65536);
 
     // mantissa is a 64-bit integer representing M.mmm... * 2^63.  The actual
     // value we're representing, of course, is M.mmm... * 2^exp2.
     int exp2;
     double m = std::frexp(value, &exp2);
-    uint64_t mantissa = m * (32768.0 * 65536.0 * 65536.0 * 65536.0);
+    uint64_t mantissa =
+        static_cast<uint64_t>(m * (32768.0 * 65536.0 * 65536.0 * 65536.0));
     // std::frexp returns an m value in the range [0.5, 1.0), however we
     // can't multiply it by 2^64 and convert to an integer because some FPUs
     // throw an exception when converting an number higher than 2^63 into an
@@ -485,7 +1017,7 @@ static ExpDigits SplitToSix(const double value) {
     }
   } else {
     // Here, we are not close to the edge.
-    dddddd = static_cast<int>((d64k + 32768) / 65536);
+    dddddd = static_cast<uint32_t>((d64k + 32768) / 65536);
   }
   if (dddddd == 1000000) {
     dddddd = 100000;
@@ -493,21 +1025,22 @@ static ExpDigits SplitToSix(const double value) {
   }
   exp_dig.exponent = exp;
 
-  int two_digits = dddddd / 10000;
+  uint32_t two_digits = dddddd / 10000;
   dddddd -= two_digits * 10000;
-  PutTwoDigits(two_digits, &exp_dig.digits[0]);
+  numbers_internal::PutTwoDigits(two_digits, &exp_dig.digits[0]);
 
   two_digits = dddddd / 100;
   dddddd -= two_digits * 100;
-  PutTwoDigits(two_digits, &exp_dig.digits[2]);
+  numbers_internal::PutTwoDigits(two_digits, &exp_dig.digits[2]);
 
-  PutTwoDigits(dddddd, &exp_dig.digits[4]);
+  numbers_internal::PutTwoDigits(dddddd, &exp_dig.digits[4]);
   return exp_dig;
 }
 
 // Helper function for fast formatting of floating-point.
 // The result is the same as "%g", a.k.a. "%.6g".
-size_t numbers_internal::SixDigitsToBuffer(double d, char* const buffer) {
+size_t numbers_internal::SixDigitsToBuffer(double d,
+                                           char* absl_nonnull const buffer) {
   static_assert(std::numeric_limits<float>::is_iec559,
                 "IEEE-754/IEC-559 support only");
 
@@ -523,15 +1056,15 @@ size_t numbers_internal::SixDigitsToBuffer(double d, char* const buffer) {
     if (std::signbit(d)) *out++ = '-';
     *out++ = '0';
     *out = 0;
-    return out - buffer;
+    return static_cast<size_t>(out - buffer);
   }
   if (d < 0) {
     *out++ = '-';
     d = -d;
   }
-  if (std::isinf(d)) {
+  if (d > std::numeric_limits<double>::max()) {
     strcpy(out, "inf");  // NOLINT(runtime/printf)
-    return out + 3 - buffer;
+    return static_cast<size_t>(out + 3 - buffer);
   }
 
   auto exp_dig = SplitToSix(d);
@@ -543,7 +1076,7 @@ size_t numbers_internal::SixDigitsToBuffer(double d, char* const buffer) {
     case 5:
       memcpy(out, &digits[0], 6), out += 6;
       *out = 0;
-      return out - buffer;
+      return static_cast<size_t>(out - buffer);
     case 4:
       memcpy(out, &digits[0], 5), out += 5;
       if (digits[5] != '0') {
@@ -551,7 +1084,7 @@ size_t numbers_internal::SixDigitsToBuffer(double d, char* const buffer) {
         *out++ = digits[5];
       }
       *out = 0;
-      return out - buffer;
+      return static_cast<size_t>(out - buffer);
     case 3:
       memcpy(out, &digits[0], 4), out += 4;
       if ((digits[5] | digits[4]) != '0') {
@@ -560,7 +1093,7 @@ size_t numbers_internal::SixDigitsToBuffer(double d, char* const buffer) {
         if (digits[5] != '0') *out++ = digits[5];
       }
       *out = 0;
-      return out - buffer;
+      return static_cast<size_t>(out - buffer);
     case 2:
       memcpy(out, &digits[0], 3), out += 3;
       *out++ = '.';
@@ -569,7 +1102,7 @@ size_t numbers_internal::SixDigitsToBuffer(double d, char* const buffer) {
       while (out[-1] == '0') --out;
       if (out[-1] == '.') --out;
       *out = 0;
-      return out - buffer;
+      return static_cast<size_t>(out - buffer);
     case 1:
       memcpy(out, &digits[0], 2), out += 2;
       *out++ = '.';
@@ -578,7 +1111,7 @@ size_t numbers_internal::SixDigitsToBuffer(double d, char* const buffer) {
       while (out[-1] == '0') --out;
       if (out[-1] == '.') --out;
       *out = 0;
-      return out - buffer;
+      return static_cast<size_t>(out - buffer);
     case 0:
       memcpy(out, &digits[0], 1), out += 1;
       *out++ = '.';
@@ -587,26 +1120,26 @@ size_t numbers_internal::SixDigitsToBuffer(double d, char* const buffer) {
       while (out[-1] == '0') --out;
       if (out[-1] == '.') --out;
       *out = 0;
-      return out - buffer;
+      return static_cast<size_t>(out - buffer);
     case -4:
       out[2] = '0';
       ++out;
-      ABSL_FALLTHROUGH_INTENDED;
+      [[fallthrough]];
     case -3:
       out[2] = '0';
       ++out;
-      ABSL_FALLTHROUGH_INTENDED;
+      [[fallthrough]];
     case -2:
       out[2] = '0';
       ++out;
-      ABSL_FALLTHROUGH_INTENDED;
+      [[fallthrough]];
     case -1:
       out += 2;
       memcpy(out, &digits[0], 6);
       out += 6;
       while (out[-1] == '0') --out;
       *out = 0;
-      return out - buffer;
+      return static_cast<size_t>(out - buffer);
   }
   assert(exp < -4 || exp >= 6);
   out[0] = digits[0];
@@ -625,19 +1158,19 @@ size_t numbers_internal::SixDigitsToBuffer(double d, char* const buffer) {
   if (exp > 99) {
     int dig1 = exp / 100;
     exp -= dig1 * 100;
-    *out++ = '0' + dig1;
+    *out++ = '0' + static_cast<char>(dig1);
   }
-  PutTwoDigits(exp, out);
+  PutTwoDigits(static_cast<uint32_t>(exp), out);
   out += 2;
   *out = 0;
-  return out - buffer;
+  return static_cast<size_t>(out - buffer);
 }
 
 namespace {
 // Represents integer values of digits.
 // Uses 36 to indicate an invalid character since we support
 // bases up to 36.
-static const int8_t kAsciiToInt[256] = {
+static constexpr std::array<int8_t, 256> kAsciiToInt = {
     36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36,  // 16 36s.
     36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36,
     36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 0,  1,  2,  3,  4,  5,
@@ -654,9 +1187,10 @@ static const int8_t kAsciiToInt[256] = {
     36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36};
 
 // Parse the sign and optional hex or oct prefix in text.
-inline bool safe_parse_sign_and_base(absl::string_view* text /*inout*/,
-                                     int* base_ptr /*inout*/,
-                                     bool* negative_ptr /*output*/) {
+inline bool safe_parse_sign_and_base(
+    absl::string_view* absl_nonnull text /*inout*/,
+    int* absl_nonnull base_ptr /*inout*/,
+    bool* absl_nonnull negative_ptr /*output*/) {
   if (text->data() == nullptr) {
     return false;
   }
@@ -666,10 +1200,12 @@ inline bool safe_parse_sign_and_base(absl::string_view* text /*inout*/,
   int base = *base_ptr;
 
   // Consume whitespace.
-  while (start < end && absl::ascii_isspace(start[0])) {
+  while (start < end &&
+         absl::ascii_isspace(static_cast<unsigned char>(start[0]))) {
     ++start;
   }
-  while (start < end && absl::ascii_isspace(end[-1])) {
+  while (start < end &&
+         absl::ascii_isspace(static_cast<unsigned char>(end[-1]))) {
     --end;
   }
   if (start >= end) {
@@ -718,7 +1254,7 @@ inline bool safe_parse_sign_and_base(absl::string_view* text /*inout*/,
   } else {
     return false;
   }
-  *text = absl::string_view(start, end - start);
+  *text = absl::string_view(start, static_cast<size_t>(end - start));
   *base_ptr = base;
   return true;
 }
@@ -754,8 +1290,8 @@ inline bool safe_parse_sign_and_base(absl::string_view* text /*inout*/,
 // commonly used bases.
 template <typename IntType>
 struct LookupTables {
-  static const IntType kVmaxOverBase[];
-  static const IntType kVminOverBase[];
+  ABSL_CONST_INIT static const IntType kVmaxOverBase[];
+  ABSL_CONST_INIT static const IntType kVminOverBase[];
 };
 
 // An array initializer macro for X/base where base in [0, 36].
@@ -770,32 +1306,192 @@ struct LookupTables {
         X / 35, X / 36,                                                   \
   }
 
+// This kVmaxOverBase is generated with
+//  for (int base = 2; base < 37; ++base) {
+//    absl::uint128 max = std::numeric_limits<absl::uint128>::max();
+//    auto result = max / base;
+//    std::cout << "    MakeUint128(" << absl::Uint128High64(result) << "u, "
+//              << absl::Uint128Low64(result) << "u),\n";
+//  }
+// See https://godbolt.org/z/aneYsb
+//
+// uint128& operator/=(uint128) is not constexpr, so hardcode the resulting
+// array to avoid a static initializer.
+template <>
+ABSL_CONST_INIT const uint128 LookupTables<uint128>::kVmaxOverBase[] = {
+    0,
+    0,
+    MakeUint128(9223372036854775807u, 18446744073709551615u),
+    MakeUint128(6148914691236517205u, 6148914691236517205u),
+    MakeUint128(4611686018427387903u, 18446744073709551615u),
+    MakeUint128(3689348814741910323u, 3689348814741910323u),
+    MakeUint128(3074457345618258602u, 12297829382473034410u),
+    MakeUint128(2635249153387078802u, 5270498306774157604u),
+    MakeUint128(2305843009213693951u, 18446744073709551615u),
+    MakeUint128(2049638230412172401u, 14347467612885206812u),
+    MakeUint128(1844674407370955161u, 11068046444225730969u),
+    MakeUint128(1676976733973595601u, 8384883669867978007u),
+    MakeUint128(1537228672809129301u, 6148914691236517205u),
+    MakeUint128(1418980313362273201u, 4256940940086819603u),
+    MakeUint128(1317624576693539401u, 2635249153387078802u),
+    MakeUint128(1229782938247303441u, 1229782938247303441u),
+    MakeUint128(1152921504606846975u, 18446744073709551615u),
+    MakeUint128(1085102592571150095u, 1085102592571150095u),
+    MakeUint128(1024819115206086200u, 16397105843297379214u),
+    MakeUint128(970881267037344821u, 16504981539634861972u),
+    MakeUint128(922337203685477580u, 14757395258967641292u),
+    MakeUint128(878416384462359600u, 14054662151397753612u),
+    MakeUint128(838488366986797800u, 13415813871788764811u),
+    MakeUint128(802032351030850070u, 4812194106185100421u),
+    MakeUint128(768614336404564650u, 12297829382473034410u),
+    MakeUint128(737869762948382064u, 11805916207174113034u),
+    MakeUint128(709490156681136600u, 11351842506898185609u),
+    MakeUint128(683212743470724133u, 17080318586768103348u),
+    MakeUint128(658812288346769700u, 10540996613548315209u),
+    MakeUint128(636094623231363848u, 15266270957552732371u),
+    MakeUint128(614891469123651720u, 9838263505978427528u),
+    MakeUint128(595056260442243600u, 9520900167075897608u),
+    MakeUint128(576460752303423487u, 18446744073709551615u),
+    MakeUint128(558992244657865200u, 8943875914525843207u),
+    MakeUint128(542551296285575047u, 9765923333140350855u),
+    MakeUint128(527049830677415760u, 8432797290838652167u),
+    MakeUint128(512409557603043100u, 8198552921648689607u),
+};
+
+// This kVmaxOverBase generated with
+//   for (int base = 2; base < 37; ++base) {
+//    absl::int128 max = std::numeric_limits<absl::int128>::max();
+//    auto result = max / base;
+//    std::cout << "\tMakeInt128(" << absl::Int128High64(result) << ", "
+//              << absl::Int128Low64(result) << "u),\n";
+//  }
+// See https://godbolt.org/z/7djYWz
+//
+// int128& operator/=(int128) is not constexpr, so hardcode the resulting array
+// to avoid a static initializer.
+template <>
+ABSL_CONST_INIT const int128 LookupTables<int128>::kVmaxOverBase[] = {
+    0,
+    0,
+    MakeInt128(4611686018427387903, 18446744073709551615u),
+    MakeInt128(3074457345618258602, 12297829382473034410u),
+    MakeInt128(2305843009213693951, 18446744073709551615u),
+    MakeInt128(1844674407370955161, 11068046444225730969u),
+    MakeInt128(1537228672809129301, 6148914691236517205u),
+    MakeInt128(1317624576693539401, 2635249153387078802u),
+    MakeInt128(1152921504606846975, 18446744073709551615u),
+    MakeInt128(1024819115206086200, 16397105843297379214u),
+    MakeInt128(922337203685477580, 14757395258967641292u),
+    MakeInt128(838488366986797800, 13415813871788764811u),
+    MakeInt128(768614336404564650, 12297829382473034410u),
+    MakeInt128(709490156681136600, 11351842506898185609u),
+    MakeInt128(658812288346769700, 10540996613548315209u),
+    MakeInt128(614891469123651720, 9838263505978427528u),
+    MakeInt128(576460752303423487, 18446744073709551615u),
+    MakeInt128(542551296285575047, 9765923333140350855u),
+    MakeInt128(512409557603043100, 8198552921648689607u),
+    MakeInt128(485440633518672410, 17475862806672206794u),
+    MakeInt128(461168601842738790, 7378697629483820646u),
+    MakeInt128(439208192231179800, 7027331075698876806u),
+    MakeInt128(419244183493398900, 6707906935894382405u),
+    MakeInt128(401016175515425035, 2406097053092550210u),
+    MakeInt128(384307168202282325, 6148914691236517205u),
+    MakeInt128(368934881474191032, 5902958103587056517u),
+    MakeInt128(354745078340568300, 5675921253449092804u),
+    MakeInt128(341606371735362066, 17763531330238827482u),
+    MakeInt128(329406144173384850, 5270498306774157604u),
+    MakeInt128(318047311615681924, 7633135478776366185u),
+    MakeInt128(307445734561825860, 4919131752989213764u),
+    MakeInt128(297528130221121800, 4760450083537948804u),
+    MakeInt128(288230376151711743, 18446744073709551615u),
+    MakeInt128(279496122328932600, 4471937957262921603u),
+    MakeInt128(271275648142787523, 14106333703424951235u),
+    MakeInt128(263524915338707880, 4216398645419326083u),
+    MakeInt128(256204778801521550, 4099276460824344803u),
+};
+
+// This kVminOverBase generated with
+//  for (int base = 2; base < 37; ++base) {
+//    absl::int128 min = std::numeric_limits<absl::int128>::min();
+//    auto result = min / base;
+//    std::cout << "\tMakeInt128(" << absl::Int128High64(result) << ", "
+//              << absl::Int128Low64(result) << "u),\n";
+//  }
+//
+// See https://godbolt.org/z/7djYWz
+//
+// int128& operator/=(int128) is not constexpr, so hardcode the resulting array
+// to avoid a static initializer.
+template <>
+ABSL_CONST_INIT const int128 LookupTables<int128>::kVminOverBase[] = {
+    0,
+    0,
+    MakeInt128(-4611686018427387904, 0u),
+    MakeInt128(-3074457345618258603, 6148914691236517206u),
+    MakeInt128(-2305843009213693952, 0u),
+    MakeInt128(-1844674407370955162, 7378697629483820647u),
+    MakeInt128(-1537228672809129302, 12297829382473034411u),
+    MakeInt128(-1317624576693539402, 15811494920322472814u),
+    MakeInt128(-1152921504606846976, 0u),
+    MakeInt128(-1024819115206086201, 2049638230412172402u),
+    MakeInt128(-922337203685477581, 3689348814741910324u),
+    MakeInt128(-838488366986797801, 5030930201920786805u),
+    MakeInt128(-768614336404564651, 6148914691236517206u),
+    MakeInt128(-709490156681136601, 7094901566811366007u),
+    MakeInt128(-658812288346769701, 7905747460161236407u),
+    MakeInt128(-614891469123651721, 8608480567731124088u),
+    MakeInt128(-576460752303423488, 0u),
+    MakeInt128(-542551296285575048, 8680820740569200761u),
+    MakeInt128(-512409557603043101, 10248191152060862009u),
+    MakeInt128(-485440633518672411, 970881267037344822u),
+    MakeInt128(-461168601842738791, 11068046444225730970u),
+    MakeInt128(-439208192231179801, 11419412998010674810u),
+    MakeInt128(-419244183493398901, 11738837137815169211u),
+    MakeInt128(-401016175515425036, 16040647020617001406u),
+    MakeInt128(-384307168202282326, 12297829382473034411u),
+    MakeInt128(-368934881474191033, 12543785970122495099u),
+    MakeInt128(-354745078340568301, 12770822820260458812u),
+    MakeInt128(-341606371735362067, 683212743470724134u),
+    MakeInt128(-329406144173384851, 13176245766935394012u),
+    MakeInt128(-318047311615681925, 10813608594933185431u),
+    MakeInt128(-307445734561825861, 13527612320720337852u),
+    MakeInt128(-297528130221121801, 13686293990171602812u),
+    MakeInt128(-288230376151711744, 0u),
+    MakeInt128(-279496122328932601, 13974806116446630013u),
+    MakeInt128(-271275648142787524, 4340410370284600381u),
+    MakeInt128(-263524915338707881, 14230345428290225533u),
+    MakeInt128(-256204778801521551, 14347467612885206813u),
+};
+
 template <typename IntType>
-const IntType LookupTables<IntType>::kVmaxOverBase[] =
+ABSL_CONST_INIT const IntType LookupTables<IntType>::kVmaxOverBase[] =
     X_OVER_BASE_INITIALIZER(std::numeric_limits<IntType>::max());
 
 template <typename IntType>
-const IntType LookupTables<IntType>::kVminOverBase[] =
+ABSL_CONST_INIT const IntType LookupTables<IntType>::kVminOverBase[] =
     X_OVER_BASE_INITIALIZER(std::numeric_limits<IntType>::min());
 
 #undef X_OVER_BASE_INITIALIZER
 
 template <typename IntType>
 inline bool safe_parse_positive_int(absl::string_view text, int base,
-                                    IntType* value_p) {
+                                    IntType* absl_nonnull value_p) {
   IntType value = 0;
   const IntType vmax = std::numeric_limits<IntType>::max();
   assert(vmax > 0);
   assert(base >= 0);
-  assert(vmax >= static_cast<IntType>(base));
+  const IntType base_inttype = static_cast<IntType>(base);
+  assert(vmax >= base_inttype);
   const IntType vmax_over_base = LookupTables<IntType>::kVmaxOverBase[base];
+  assert(base < 2 ||
+         std::numeric_limits<IntType>::max() / base_inttype == vmax_over_base);
   const char* start = text.data();
   const char* end = start + text.size();
   // loop over digits
   for (; start < end; ++start) {
     unsigned char c = static_cast<unsigned char>(start[0]);
-    int digit = kAsciiToInt[c];
-    if (digit >= base) {
+    IntType digit = static_cast<IntType>(kAsciiToInt[c]);
+    if (digit >= base_inttype) {
       *value_p = value;
       return false;
     }
@@ -803,7 +1499,7 @@ inline bool safe_parse_positive_int(absl::string_view text, int base,
       *value_p = vmax;
       return false;
     }
-    value *= base;
+    value *= base_inttype;
     if (value > vmax - digit) {
       *value_p = vmax;
       return false;
@@ -816,12 +1512,14 @@ inline bool safe_parse_positive_int(absl::string_view text, int base,
 
 template <typename IntType>
 inline bool safe_parse_negative_int(absl::string_view text, int base,
-                                    IntType* value_p) {
+                                    IntType* absl_nonnull value_p) {
   IntType value = 0;
   const IntType vmin = std::numeric_limits<IntType>::min();
   assert(vmin < 0);
   assert(vmin <= 0 - base);
   IntType vmin_over_base = LookupTables<IntType>::kVminOverBase[base];
+  assert(base < 2 ||
+         std::numeric_limits<IntType>::min() / base == vmin_over_base);
   // 2003 c++ standard [expr.mul]
   // "... the sign of the remainder is implementation-defined."
   // Although (vmin/base)*base + vmin%base is always vmin.
@@ -858,8 +1556,8 @@ inline bool safe_parse_negative_int(absl::string_view text, int base,
 // Input format based on POSIX.1-2008 strtol
 // http://pubs.opengroup.org/onlinepubs/9699919799/functions/strtol.html
 template <typename IntType>
-inline bool safe_int_internal(absl::string_view text, IntType* value_p,
-                              int base) {
+inline bool safe_int_internal(absl::string_view text,
+                              IntType* absl_nonnull value_p, int base) {
   *value_p = 0;
   bool negative;
   if (!safe_parse_sign_and_base(&text, &base, &negative)) {
@@ -873,8 +1571,8 @@ inline bool safe_int_internal(absl::string_view text, IntType* value_p,
 }
 
 template <typename IntType>
-inline bool safe_uint_internal(absl::string_view text, IntType* value_p,
-                               int base) {
+inline bool safe_uint_internal(absl::string_view text,
+                               IntType* absl_nonnull value_p, int base) {
   *value_p = 0;
   bool negative;
   if (!safe_parse_sign_and_base(&text, &base, &negative) || negative) {
@@ -885,21 +1583,79 @@ inline bool safe_uint_internal(absl::string_view text, IntType* value_p,
 }  // anonymous namespace
 
 namespace numbers_internal {
-bool safe_strto32_base(absl::string_view text, int32_t* value, int base) {
+
+// Digit conversion.
+ABSL_CONST_INIT ABSL_DLL const char kHexChar[] =
+    "0123456789abcdef";
+
+ABSL_CONST_INIT ABSL_DLL const char kHexTable[513] =
+    "000102030405060708090a0b0c0d0e0f"
+    "101112131415161718191a1b1c1d1e1f"
+    "202122232425262728292a2b2c2d2e2f"
+    "303132333435363738393a3b3c3d3e3f"
+    "404142434445464748494a4b4c4d4e4f"
+    "505152535455565758595a5b5c5d5e5f"
+    "606162636465666768696a6b6c6d6e6f"
+    "707172737475767778797a7b7c7d7e7f"
+    "808182838485868788898a8b8c8d8e8f"
+    "909192939495969798999a9b9c9d9e9f"
+    "a0a1a2a3a4a5a6a7a8a9aaabacadaeaf"
+    "b0b1b2b3b4b5b6b7b8b9babbbcbdbebf"
+    "c0c1c2c3c4c5c6c7c8c9cacbcccdcecf"
+    "d0d1d2d3d4d5d6d7d8d9dadbdcdddedf"
+    "e0e1e2e3e4e5e6e7e8e9eaebecedeeef"
+    "f0f1f2f3f4f5f6f7f8f9fafbfcfdfeff";
+
+bool safe_strto8_base(absl::string_view text, int8_t* absl_nonnull value,
+                      int base) {
+  return safe_int_internal<int8_t>(text, value, base);
+}
+
+bool safe_strto16_base(absl::string_view text, int16_t* absl_nonnull value,
+                       int base) {
+  return safe_int_internal<int16_t>(text, value, base);
+}
+
+bool safe_strto32_base(absl::string_view text, int32_t* absl_nonnull value,
+                       int base) {
   return safe_int_internal<int32_t>(text, value, base);
 }
 
-bool safe_strto64_base(absl::string_view text, int64_t* value, int base) {
+bool safe_strto64_base(absl::string_view text, int64_t* absl_nonnull value,
+                       int base) {
   return safe_int_internal<int64_t>(text, value, base);
 }
 
-bool safe_strtou32_base(absl::string_view text, uint32_t* value, int base) {
+bool safe_strto128_base(absl::string_view text, int128* absl_nonnull value,
+                        int base) {
+  return safe_int_internal<absl::int128>(text, value, base);
+}
+
+bool safe_strtou8_base(absl::string_view text, uint8_t* absl_nonnull value,
+                       int base) {
+  return safe_uint_internal<uint8_t>(text, value, base);
+}
+
+bool safe_strtou16_base(absl::string_view text, uint16_t* absl_nonnull value,
+                        int base) {
+  return safe_uint_internal<uint16_t>(text, value, base);
+}
+
+bool safe_strtou32_base(absl::string_view text, uint32_t* absl_nonnull value,
+                        int base) {
   return safe_uint_internal<uint32_t>(text, value, base);
 }
 
-bool safe_strtou64_base(absl::string_view text, uint64_t* value, int base) {
+bool safe_strtou64_base(absl::string_view text, uint64_t* absl_nonnull value,
+                        int base) {
   return safe_uint_internal<uint64_t>(text, value, base);
 }
-}  // namespace numbers_internal
 
+bool safe_strtou128_base(absl::string_view text, uint128* absl_nonnull value,
+                         int base) {
+  return safe_uint_internal<absl::uint128>(text, value, base);
+}
+
+}  // namespace numbers_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/numbers.h b/absl/strings/numbers.h
index 100839b0462..87d146b309e 100644
--- a/absl/strings/numbers.h
+++ b/absl/strings/numbers.h
@@ -1,4 +1,3 @@
-//
 // Copyright 2017 The Abseil Authors.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
@@ -25,6 +24,7 @@
 #define ABSL_STRINGS_NUMBERS_H_
 
 #include <cstddef>
+#include <cstdint>
 #include <cstdlib>
 #include <cstring>
 #include <ctime>
@@ -32,40 +32,60 @@
 #include <string>
 #include <type_traits>
 
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/endian.h"
 #include "absl/base/macros.h"
+#include "absl/base/nullability.h"
 #include "absl/base/port.h"
+#include "absl/numeric/bits.h"
 #include "absl/numeric/int128.h"
 #include "absl/strings/string_view.h"
 
+#ifdef __SSSE3__
+#include <tmmintrin.h>
+#endif
+
+#ifdef _MSC_VER
+#include <intrin.h>
+#endif
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 // SimpleAtoi()
 //
-// Converts the given string into an integer value, returning `true` if
-// successful. The string must reflect a base-10 integer (optionally followed or
-// preceded by ASCII whitespace) whose value falls within the range of the
-// integer type. If any errors are encountered, this function returns `false`,
-// leaving `out` in an unspecified state.
+// Converts the given string (optionally followed or preceded by ASCII
+// whitespace) into an integer value, returning `true` if successful. The string
+// must reflect a base-10 integer whose value falls within the range of the
+// integer type (optionally preceded by a `+` or `-`). If any errors are
+// encountered, this function returns `false`, leaving `out` in an unspecified
+// state.
 template <typename int_type>
-ABSL_MUST_USE_RESULT bool SimpleAtoi(absl::string_view str, int_type* out);
+[[nodiscard]] bool SimpleAtoi(absl::string_view str,
+                              int_type* absl_nonnull out);
 
 // SimpleAtof()
 //
 // Converts the given string (optionally followed or preceded by ASCII
-// whitespace) into a float, which may be rounded on overflow or underflow.
+// whitespace) into a float, which may be rounded on overflow or underflow,
+// returning `true` if successful.
 // See https://en.cppreference.com/w/c/string/byte/strtof for details about the
-// allowed formats for `str`. If any errors are encountered, this function
+// allowed formats for `str`, except SimpleAtof() is locale-independent and will
+// always use the "C" locale. If any errors are encountered, this function
 // returns `false`, leaving `out` in an unspecified state.
-ABSL_MUST_USE_RESULT bool SimpleAtof(absl::string_view str, float* out);
+[[nodiscard]] bool SimpleAtof(absl::string_view str, float* absl_nonnull out);
 
 // SimpleAtod()
 //
 // Converts the given string (optionally followed or preceded by ASCII
-// whitespace) into a double, which may be rounded on overflow or underflow.
+// whitespace) into a double, which may be rounded on overflow or underflow,
+// returning `true` if successful.
 // See https://en.cppreference.com/w/c/string/byte/strtof for details about the
-// allowed formats for `str`. If any errors are encountered, this function
+// allowed formats for `str`, except SimpleAtod is locale-independent and will
+// always use the "C" locale. If any errors are encountered, this function
 // returns `false`, leaving `out` in an unspecified state.
-ABSL_MUST_USE_RESULT bool SimpleAtod(absl::string_view str, double* out);
+[[nodiscard]] bool SimpleAtod(absl::string_view str, double* absl_nonnull out);
 
 // SimpleAtob()
 //
@@ -75,23 +95,92 @@ ABSL_MUST_USE_RESULT bool SimpleAtod(absl::string_view str, double* out);
 // are interpreted as boolean `false`: "false", "f", "no", "n", "0". If any
 // errors are encountered, this function returns `false`, leaving `out` in an
 // unspecified state.
-ABSL_MUST_USE_RESULT bool SimpleAtob(absl::string_view str, bool* out);
+[[nodiscard]] bool SimpleAtob(absl::string_view str, bool* absl_nonnull out);
+
+// SimpleHexAtoi()
+//
+// Converts a hexadecimal string (optionally followed or preceded by ASCII
+// whitespace) to an integer, returning `true` if successful. Only valid base-16
+// hexadecimal integers whose value falls within the range of the integer type
+// (optionally preceded by a `+` or `-`) can be converted. A valid hexadecimal
+// value may include both upper and lowercase character symbols, and may
+// optionally include a leading "0x" (or "0X") number prefix, which is ignored
+// by this function. If any errors are encountered, this function returns
+// `false`, leaving `out` in an unspecified state.
+template <typename int_type>
+[[nodiscard]] bool SimpleHexAtoi(absl::string_view str,
+                                 int_type* absl_nonnull out);
+
+// Overloads of SimpleHexAtoi() for 128 bit integers.
+[[nodiscard]] inline bool SimpleHexAtoi(absl::string_view str,
+                                        absl::int128* absl_nonnull out);
+[[nodiscard]] inline bool SimpleHexAtoi(absl::string_view str,
+                                        absl::uint128* absl_nonnull out);
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 // End of public API.  Implementation details follow.
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace numbers_internal {
 
+template <typename int_type>
+constexpr bool is_signed() {
+  if constexpr (std::is_arithmetic_v<int_type>) {
+    // Use std::numeric_limits<T>::is_signed where it's defined to work.
+    return std::numeric_limits<int_type>::is_signed;
+  }
+  // TODO(jorg): This signed-ness check is used because it works correctly
+  // with enums, and it also serves to check that int_type is not a pointer.
+  // If one day something like std::is_signed<enum E> works, switch to it.
+  return static_cast<int_type>(1) - 2 < 0;
+}
+
+// Digit conversion.
+ABSL_DLL extern const char kHexChar[17];  // 0123456789abcdef
+ABSL_DLL extern const char
+    kHexTable[513];  // 000102030405060708090a0b0c0d0e0f1011...
+
+// Writes a two-character representation of 'i' to 'buf'. 'i' must be in the
+// range 0 <= i < 100, and buf must have space for two characters. Example:
+//   char buf[2];
+//   PutTwoDigits(42, buf);
+//   // buf[0] == '4'
+//   // buf[1] == '2'
+void PutTwoDigits(uint32_t i, char* absl_nonnull buf);
+
 // safe_strto?() functions for implementing SimpleAtoi()
-bool safe_strto32_base(absl::string_view text, int32_t* value, int base);
-bool safe_strto64_base(absl::string_view text, int64_t* value, int base);
-bool safe_strtou32_base(absl::string_view text, uint32_t* value, int base);
-bool safe_strtou64_base(absl::string_view text, uint64_t* value, int base);
 
-static const int kFastToBufferSize = 32;
-static const int kSixDigitsToBufferSize = 16;
+bool safe_strto8_base(absl::string_view text, int8_t* absl_nonnull value,
+                      int base);
+bool safe_strto16_base(absl::string_view text, int16_t* absl_nonnull value,
+                       int base);
+bool safe_strto32_base(absl::string_view text, int32_t* absl_nonnull value,
+                       int base);
+bool safe_strto64_base(absl::string_view text, int64_t* absl_nonnull value,
+                       int base);
+bool safe_strto128_base(absl::string_view text,
+                        absl::int128* absl_nonnull value, int base);
+bool safe_strtou8_base(absl::string_view text, uint8_t* absl_nonnull value,
+                       int base);
+bool safe_strtou16_base(absl::string_view text, uint16_t* absl_nonnull value,
+                        int base);
+bool safe_strtou32_base(absl::string_view text, uint32_t* absl_nonnull value,
+                        int base);
+bool safe_strtou64_base(absl::string_view text, uint64_t* absl_nonnull value,
+                        int base);
+bool safe_strtou128_base(absl::string_view text,
+                         absl::uint128* absl_nonnull value, int base);
+
+inline constexpr int kFastToBuffer128Size = 41;
+inline constexpr int kFastToBufferSize = 32;
+inline constexpr int kSixDigitsToBufferSize = 16;
+
+// Helper function used to implement absl::HighPrecision().
+char* absl_nonnull RoundTripDoubleToBuffer(double d, char* absl_nonnull buffer);
+char* absl_nonnull RoundTripFloatToBuffer(float f, char* absl_nonnull buffer);
 
 // Helper function for fast formatting of floating-point values.
 // The result is the same as printf's "%g", a.k.a. "%.6g"; that is, six
@@ -99,35 +188,53 @@ static const int kSixDigitsToBufferSize = 16;
 // outside the range 0.0001-999999 are output using scientific notation
 // (1.23456e+06). This routine is heavily optimized.
 // Required buffer size is `kSixDigitsToBufferSize`.
-size_t SixDigitsToBuffer(double d, char* buffer);
+size_t SixDigitsToBuffer(double d, char* absl_nonnull buffer);
 
-// These functions are intended for speed. All functions take an output buffer
+// WARNING: These functions may write more characters than necessary, because
+// they are intended for speed. All functions take an output buffer
 // as an argument and return a pointer to the last byte they wrote, which is the
-// terminating '\0'. At most `kFastToBufferSize` bytes are written.
-char* FastIntToBuffer(int32_t, char*);
-char* FastIntToBuffer(uint32_t, char*);
-char* FastIntToBuffer(int64_t, char*);
-char* FastIntToBuffer(uint64_t, char*);
-
-// For enums and integer types that are not an exact match for the types above,
-// use templates to call the appropriate one of the four overloads above.
+// terminating '\0'. The maximum size written is `kFastToBufferSize` for 64-bit
+// integers or less, and `kFastToBuffer128Size` for 128-bit integers.
+char* absl_nonnull FastIntToBuffer(int32_t i, char* absl_nonnull buffer)
+    ABSL_INTERNAL_NEED_MIN_SIZE(buffer, kFastToBufferSize);
+char* absl_nonnull FastIntToBuffer(uint32_t n, char* absl_nonnull out_str)
+    ABSL_INTERNAL_NEED_MIN_SIZE(out_str, kFastToBufferSize);
+char* absl_nonnull FastIntToBuffer(int64_t i, char* absl_nonnull buffer)
+    ABSL_INTERNAL_NEED_MIN_SIZE(buffer, kFastToBufferSize);
+char* absl_nonnull FastIntToBuffer(uint64_t i, char* absl_nonnull buffer)
+    ABSL_INTERNAL_NEED_MIN_SIZE(buffer, kFastToBufferSize);
+char* absl_nonnull FastIntToBuffer(int128 i, char* absl_nonnull buffer)
+    ABSL_INTERNAL_NEED_MIN_SIZE(buffer, kFastToBuffer128Size);
+char* absl_nonnull FastIntToBuffer(uint128 i, char* absl_nonnull buffer)
+    ABSL_INTERNAL_NEED_MIN_SIZE(buffer, kFastToBuffer128Size);
+
+// For enums and integer types that are up to 128 bits and are not an exact
+// match for the types above, use templates to call the appropriate one of the
+// four overloads above.
 template <typename int_type>
-char* FastIntToBuffer(int_type i, char* buffer) {
-  static_assert(sizeof(i) <= 64 / 8,
-                "FastIntToBuffer works only with 64-bit-or-less integers.");
-  // TODO(jorg): This signed-ness check is used because it works correctly
-  // with enums, and it also serves to check that int_type is not a pointer.
-  // If one day something like std::is_signed<enum E> works, switch to it.
-  if (static_cast<int_type>(1) - 2 < 0) {  // Signed
-    if (sizeof(i) > 32 / 8) {           // 33-bit to 64-bit
+char* absl_nonnull FastIntToBuffer(int_type i,
+                                          char* absl_nonnull buffer)
+    ABSL_INTERNAL_NEED_MIN_SIZE(
+        buffer, (sizeof(int_type) > 8 ? kFastToBuffer128Size
+                                      : kFastToBufferSize)) {
+  // These conditions are constexpr bools to suppress MSVC warning C4127.
+  constexpr bool kIsSigned = is_signed<int_type>();
+  constexpr bool kUse64Bit = sizeof(i) > 32 / 8;
+  constexpr bool kUse128Bit = sizeof(i) > 64 / 8;
+  if (kIsSigned) {
+    if constexpr (kUse128Bit) {
+      return FastIntToBuffer(static_cast<int128>(i), buffer);
+    } else if constexpr (kUse64Bit) {
       return FastIntToBuffer(static_cast<int64_t>(i), buffer);
-    } else {  // 32-bit or less
+    } else {
       return FastIntToBuffer(static_cast<int32_t>(i), buffer);
     }
-  } else {                     // Unsigned
-    if (sizeof(i) > 32 / 8) {  // 33-bit to 64-bit
+  } else {
+    if constexpr (kUse128Bit) {
+      return FastIntToBuffer(static_cast<uint128>(i), buffer);
+    } else if constexpr (kUse64Bit) {
       return FastIntToBuffer(static_cast<uint64_t>(i), buffer);
-    } else {  // 32-bit or less
+    } else {
       return FastIntToBuffer(static_cast<uint32_t>(i), buffer);
     }
   }
@@ -136,54 +243,121 @@ char* FastIntToBuffer(int_type i, char* buffer) {
 // Implementation of SimpleAtoi, generalized to support arbitrary base (used
 // with base different from 10 elsewhere in Abseil implementation).
 template <typename int_type>
-ABSL_MUST_USE_RESULT bool safe_strtoi_base(absl::string_view s, int_type* out,
-                                           int base) {
-  static_assert(sizeof(*out) == 4 || sizeof(*out) == 8,
-                "SimpleAtoi works only with 32-bit or 64-bit integers.");
-  static_assert(!std::is_floating_point<int_type>::value,
+[[nodiscard]] bool safe_strtoi_base(absl::string_view s,
+                                    int_type* absl_nonnull out, int base) {
+  static_assert(sizeof(*out) == 1 || sizeof(*out) == 2 || sizeof(*out) == 4 ||
+                    sizeof(*out) == 8,
+                "SimpleAtoi works only with 8, 16, 32, or 64-bit integers.");
+  static_assert(!std::is_floating_point_v<int_type>,
                 "Use SimpleAtof or SimpleAtod instead.");
   bool parsed;
-  // TODO(jorg): This signed-ness check is used because it works correctly
-  // with enums, and it also serves to check that int_type is not a pointer.
-  // If one day something like std::is_signed<enum E> works, switch to it.
-  if (static_cast<int_type>(1) - 2 < 0) {  // Signed
-    if (sizeof(*out) == 64 / 8) {       // 64-bit
+  // These conditions are constexpr bools to suppress MSVC warning C4127.
+  constexpr bool kIsSigned = is_signed<int_type>();
+  constexpr int kIntTypeSize = sizeof(*out) * 8;
+  if (kIsSigned) {
+    if (kIntTypeSize == 64) {
       int64_t val;
       parsed = numbers_internal::safe_strto64_base(s, &val, base);
       *out = static_cast<int_type>(val);
-    } else {  // 32-bit
+    } else if (kIntTypeSize == 32) {
       int32_t val;
       parsed = numbers_internal::safe_strto32_base(s, &val, base);
       *out = static_cast<int_type>(val);
+    } else if (kIntTypeSize == 16) {
+      int16_t val;
+      parsed = numbers_internal::safe_strto16_base(s, &val, base);
+      *out = static_cast<int_type>(val);
+    } else if (kIntTypeSize == 8) {
+      int8_t val;
+      parsed = numbers_internal::safe_strto8_base(s, &val, base);
+      *out = static_cast<int_type>(val);
     }
-  } else {                         // Unsigned
-    if (sizeof(*out) == 64 / 8) {  // 64-bit
+  } else {
+    if (kIntTypeSize == 64) {
       uint64_t val;
       parsed = numbers_internal::safe_strtou64_base(s, &val, base);
       *out = static_cast<int_type>(val);
-    } else {  // 32-bit
+    } else if (kIntTypeSize == 32) {
       uint32_t val;
       parsed = numbers_internal::safe_strtou32_base(s, &val, base);
       *out = static_cast<int_type>(val);
+    } else if (kIntTypeSize == 16) {
+      uint16_t val;
+      parsed = numbers_internal::safe_strtou16_base(s, &val, base);
+      *out = static_cast<int_type>(val);
+    } else if (kIntTypeSize == 8) {
+      uint8_t val;
+      parsed = numbers_internal::safe_strtou8_base(s, &val, base);
+      *out = static_cast<int_type>(val);
     }
   }
   return parsed;
 }
 
+// FastHexToBufferZeroPad16()
+//
+// Outputs `val` into `out` as if by `snprintf(out, 17, "%016x", val)` but
+// without the terminating null character. Thus `out` must be of length >= 16.
+// Returns the number of non-pad digits of the output (it can never be zero
+// since 0 has one digit).
+inline size_t FastHexToBufferZeroPad16(uint64_t val, char* absl_nonnull out) {
+#ifdef ABSL_INTERNAL_HAVE_SSSE3
+  uint64_t be = absl::big_endian::FromHost64(val);
+  const auto kNibbleMask = _mm_set1_epi8(0xf);
+  const auto kHexDigits = _mm_setr_epi8('0', '1', '2', '3', '4', '5', '6', '7',
+                                        '8', '9', 'a', 'b', 'c', 'd', 'e', 'f');
+  auto v = _mm_loadl_epi64(reinterpret_cast<__m128i*>(&be));  // load lo dword
+  auto v4 = _mm_srli_epi64(v, 4);                            // shift 4 right
+  auto il = _mm_unpacklo_epi8(v4, v);                        // interleave bytes
+  auto m = _mm_and_si128(il, kNibbleMask);                   // mask out nibbles
+  auto hexchars = _mm_shuffle_epi8(kHexDigits, m);           // hex chars
+  _mm_storeu_si128(reinterpret_cast<__m128i*>(out), hexchars);
+#else
+  for (int i = 0; i < 8; ++i) {
+    auto byte = (val >> (56 - 8 * i)) & 0xFF;
+    auto* hex = &absl::numbers_internal::kHexTable[byte * 2];
+    std::memcpy(out + 2 * i, hex, 2);
+  }
+#endif
+  // | 0x1 so that even 0 has 1 digit.
+  return 16 - static_cast<size_t>(countl_zero(val | 0x1) / 4);
+}
+
 }  // namespace numbers_internal
 
-// SimpleAtoi()
-//
-// Converts a string to an integer, using `safe_strto?()` functions for actual
-// parsing, returning `true` if successful. The `safe_strto?()` functions apply
-// strict checking; the string must be a base-10 integer, optionally followed or
-// preceded by ASCII whitespace, with a value in the range of the corresponding
-// integer type.
 template <typename int_type>
-ABSL_MUST_USE_RESULT bool SimpleAtoi(absl::string_view str, int_type* out) {
+[[nodiscard]] bool SimpleAtoi(absl::string_view str,
+                              int_type* absl_nonnull out) {
   return numbers_internal::safe_strtoi_base(str, out, 10);
 }
 
+[[nodiscard]] inline bool SimpleAtoi(absl::string_view str,
+                                     absl::int128* absl_nonnull out) {
+  return numbers_internal::safe_strto128_base(str, out, 10);
+}
+
+[[nodiscard]] inline bool SimpleAtoi(absl::string_view str,
+                                     absl::uint128* absl_nonnull out) {
+  return numbers_internal::safe_strtou128_base(str, out, 10);
+}
+
+template <typename int_type>
+[[nodiscard]] bool SimpleHexAtoi(absl::string_view str,
+                                 int_type* absl_nonnull out) {
+  return numbers_internal::safe_strtoi_base(str, out, 16);
+}
+
+[[nodiscard]] inline bool SimpleHexAtoi(absl::string_view str,
+                                        absl::int128* absl_nonnull out) {
+  return numbers_internal::safe_strto128_base(str, out, 16);
+}
+
+[[nodiscard]] inline bool SimpleHexAtoi(absl::string_view str,
+                                        absl::uint128* absl_nonnull out) {
+  return numbers_internal::safe_strtou128_base(str, out, 16);
+}
+
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_NUMBERS_H_
diff --git a/absl/strings/numbers_benchmark.cc b/absl/strings/numbers_benchmark.cc
index 54dbedd3535..9511024ba40 100644
--- a/absl/strings/numbers_benchmark.cc
+++ b/absl/strings/numbers_benchmark.cc
@@ -13,14 +13,18 @@
 // limitations under the License.
 
 #include <cstdint>
+#include <limits>
 #include <random>
 #include <string>
 #include <type_traits>
 #include <vector>
 
-#include "benchmark/benchmark.h"
 #include "absl/base/internal/raw_logging.h"
+#include "absl/random/distributions.h"
+#include "absl/random/random.h"
 #include "absl/strings/numbers.h"
+#include "absl/strings/string_view.h"
+#include "benchmark/benchmark.h"
 
 namespace {
 
@@ -30,7 +34,7 @@ void BM_FastIntToBuffer(benchmark::State& state) {
   char buf[absl::numbers_internal::kFastToBufferSize];
   // Use the unsigned type to increment to take advantage of well-defined
   // modular arithmetic.
-  typename std::make_unsigned<T>::type x = 0;
+  std::make_unsigned_t<T> x = 0;
   for (auto _ : state) {
     absl::numbers_internal::FastIntToBuffer(static_cast<T>(x), buf);
     x += inc;
@@ -260,4 +264,25 @@ BENCHMARK_TEMPLATE(BM_SimpleAtod, std::string)
     ->ArgPair(10, 4)
     ->ArgPair(10, 8);
 
+void BM_FastHexToBufferZeroPad16(benchmark::State& state) {
+  absl::BitGen rng;
+  std::vector<uint64_t> nums;
+  nums.resize(1000);
+  auto min = std::numeric_limits<uint64_t>::min();
+  auto max = std::numeric_limits<uint64_t>::max();
+  for (auto& num : nums) {
+    num = absl::LogUniform(rng, min, max);
+  }
+
+  char buf[16];
+  while (state.KeepRunningBatch(nums.size())) {
+    for (auto num : nums) {
+      auto digits = absl::numbers_internal::FastHexToBufferZeroPad16(num, buf);
+      benchmark::DoNotOptimize(digits);
+      benchmark::DoNotOptimize(buf);
+    }
+  }
+}
+BENCHMARK(BM_FastHexToBufferZeroPad16);
+
 }  // namespace
diff --git a/absl/strings/numbers_test.cc b/absl/strings/numbers_test.cc
index 77d7e7835b8..6f0cde84649 100644
--- a/absl/strings/numbers_test.cc
+++ b/absl/strings/numbers_test.cc
@@ -17,44 +17,59 @@
 #include "absl/strings/numbers.h"
 
 #include <sys/types.h>
+
 #include <cfenv>  // NOLINT(build/c++11)
+#include <cfloat>
 #include <cinttypes>
 #include <climits>
+#include <clocale>
 #include <cmath>
 #include <cstddef>
 #include <cstdint>
 #include <cstdio>
 #include <cstdlib>
 #include <cstring>
+#include <ios>
 #include <limits>
 #include <numeric>
 #include <random>
 #include <set>
 #include <string>
+#include <type_traits>
 #include <vector>
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
-#include "absl/base/internal/raw_logging.h"
-#include "absl/strings/str_cat.h"
-
+#include "absl/cleanup/cleanup.h"
+#include "absl/log/log.h"
+#include "absl/numeric/int128.h"
+#include "absl/random/random.h"
 #include "absl/strings/internal/numbers_test_common.h"
+#include "absl/strings/internal/ostringstream.h"
 #include "absl/strings/internal/pow10_helper.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/string_view.h"
 
 namespace {
 
+using absl::SimpleAtoi;
+using absl::SimpleHexAtoi;
 using absl::numbers_internal::kSixDigitsToBufferSize;
+using absl::numbers_internal::safe_strto16_base;
 using absl::numbers_internal::safe_strto32_base;
 using absl::numbers_internal::safe_strto64_base;
+using absl::numbers_internal::safe_strto8_base;
+using absl::numbers_internal::safe_strtou16_base;
 using absl::numbers_internal::safe_strtou32_base;
 using absl::numbers_internal::safe_strtou64_base;
+using absl::numbers_internal::safe_strtou8_base;
 using absl::numbers_internal::SixDigitsToBuffer;
 using absl::strings_internal::Itoa;
 using absl::strings_internal::strtouint32_test_cases;
 using absl::strings_internal::strtouint64_test_cases;
-using absl::SimpleAtoi;
 using testing::Eq;
 using testing::MatchesRegex;
+using testing::Pointee;
 
 // Number of floats to test with.
 // 5,000,000 is a reasonable default for a test that only takes a few seconds.
@@ -146,6 +161,8 @@ struct MyInteger {
 
 typedef MyInteger<int64_t> MyInt64;
 typedef MyInteger<uint64_t> MyUInt64;
+typedef MyInteger<absl::uint128> MyUInt128;
+typedef MyInteger<absl::int128> MyInt128;
 
 void CheckInt32(int32_t x) {
   char buffer[absl::numbers_internal::kFastToBufferSize];
@@ -199,11 +216,40 @@ void CheckUInt64(uint64_t x) {
   EXPECT_EQ(expected, std::string(&buffer[1], my_actual)) << " Input " << x;
 }
 
+void CheckUInt128(absl::uint128 x) {
+  char buffer[absl::numbers_internal::kFastToBuffer128Size];
+  char* actual = absl::numbers_internal::FastIntToBuffer(x, buffer);
+  std::string s;
+  absl::strings_internal::OStringStream strm(&s);
+  strm << x;
+  EXPECT_EQ(s, std::string(buffer, actual)) << " Input " << s;
+
+  char* my_actual =
+      absl::numbers_internal::FastIntToBuffer(MyUInt128(x), buffer);
+  EXPECT_EQ(s, std::string(buffer, my_actual)) << " Input " << s;
+}
+
+void CheckInt128(absl::int128 x) {
+  char buffer[absl::numbers_internal::kFastToBuffer128Size];
+  char* actual = absl::numbers_internal::FastIntToBuffer(x, buffer);
+  std::string s;
+  absl::strings_internal::OStringStream strm(&s);
+  strm << x;
+  EXPECT_EQ(s, std::string(buffer, actual)) << " Input " << s;
+
+  char* my_actual =
+      absl::numbers_internal::FastIntToBuffer(MyInt128(x), buffer);
+  EXPECT_EQ(s, std::string(buffer, my_actual)) << " Input " << s;
+}
+
 void CheckHex64(uint64_t v) {
   char expected[16 + 1];
   std::string actual = absl::StrCat(absl::Hex(v, absl::kZeroPad16));
   snprintf(expected, sizeof(expected), "%016" PRIx64, static_cast<uint64_t>(v));
   EXPECT_EQ(expected, actual) << " Input " << v;
+  actual = absl::StrCat(absl::Hex(v, absl::kSpacePad16));
+  snprintf(expected, sizeof(expected), "%16" PRIx64, static_cast<uint64_t>(v));
+  EXPECT_EQ(expected, actual) << " Input " << v;
 }
 
 TEST(Numbers, TestFastPrints) {
@@ -235,6 +281,34 @@ TEST(Numbers, TestFastPrints) {
   CheckUInt64(uint64_t{1000000000000000000});
   CheckUInt64(uint64_t{1199999999999999999});
   CheckUInt64(std::numeric_limits<uint64_t>::max());
+  CheckUInt128(0);
+  CheckUInt128(1);
+  CheckUInt128(9);
+  CheckUInt128(10);
+  CheckUInt128(99);
+  CheckUInt128(100);
+  CheckUInt128(std::numeric_limits<uint64_t>::max());
+  CheckUInt128(absl::uint128(std::numeric_limits<uint64_t>::max()) + 1);
+  CheckUInt128(absl::MakeUint128(1, 0));
+  absl::uint128 k1e16 = 10000000000000000ULL;
+  CheckUInt128(k1e16 - 1);
+  CheckUInt128(k1e16);
+  CheckUInt128(k1e16 + 1);
+  CheckUInt128(k1e16 * k1e16 - 1);
+  CheckUInt128(k1e16 * k1e16);
+  CheckUInt128(k1e16 * k1e16 + 1);
+  CheckUInt128(absl::Uint128Max() - 1);
+  CheckUInt128(absl::Uint128Max());
+
+  CheckInt128(0);
+  CheckInt128(1);
+  CheckInt128(-1);
+  CheckInt128(10);
+  CheckInt128(-10);
+  CheckInt128(absl::Int128Max());
+  CheckInt128(absl::Int128Min());
+  CheckInt128(absl::MakeInt128(-1, 1));
+  CheckInt128(absl::MakeInt128(-1, std::numeric_limits<uint64_t>::max()));
 
   for (int i = 0; i < 10000; i++) {
     CheckHex64(i);
@@ -263,6 +337,64 @@ void VerifySimpleAtoiBad(in_val_type in_value) {
 }
 
 TEST(NumbersTest, Atoi) {
+  // SimpleAtoi(absl::string_view, int8_t)
+  VerifySimpleAtoiGood<int8_t>(0, 0);
+  VerifySimpleAtoiGood<int8_t>(42, 42);
+  VerifySimpleAtoiGood<int8_t>(-42, -42);
+
+  VerifySimpleAtoiGood<int8_t>(std::numeric_limits<int8_t>::min(),
+                               std::numeric_limits<int8_t>::min());
+  VerifySimpleAtoiGood<int8_t>(std::numeric_limits<int8_t>::max(),
+                               std::numeric_limits<int8_t>::max());
+
+  VerifySimpleAtoiBad<int8_t>(std::numeric_limits<uint8_t>::max());
+  VerifySimpleAtoiBad<int8_t>(std::numeric_limits<int16_t>::min());
+  VerifySimpleAtoiBad<int8_t>(std::numeric_limits<int16_t>::max());
+
+  // SimpleAtoi(absl::string_view, uint8_t)
+  VerifySimpleAtoiGood<uint8_t>(0, 0);
+  VerifySimpleAtoiGood<uint8_t>(42, 42);
+  VerifySimpleAtoiBad<uint8_t>(-42);
+
+  VerifySimpleAtoiBad<uint8_t>(std::numeric_limits<int8_t>::min());
+  VerifySimpleAtoiGood<uint8_t>(std::numeric_limits<int8_t>::max(),
+                                std::numeric_limits<int8_t>::max());
+  VerifySimpleAtoiGood<uint8_t>(std::numeric_limits<uint8_t>::max(),
+                                std::numeric_limits<uint8_t>::max());
+
+  VerifySimpleAtoiBad<uint8_t>(std::numeric_limits<int16_t>::min());
+  VerifySimpleAtoiBad<uint8_t>(std::numeric_limits<int16_t>::max());
+  VerifySimpleAtoiBad<uint8_t>(std::numeric_limits<uint16_t>::max());
+
+  // SimpleAtoi(absl::string_view, uint16_t)
+  VerifySimpleAtoiGood<int16_t>(0, 0);
+  VerifySimpleAtoiGood<int16_t>(42, 42);
+  VerifySimpleAtoiGood<int16_t>(-42, -42);
+
+  VerifySimpleAtoiGood<int16_t>(std::numeric_limits<int16_t>::min(),
+                                std::numeric_limits<int16_t>::min());
+  VerifySimpleAtoiGood<int16_t>(std::numeric_limits<int16_t>::max(),
+                                std::numeric_limits<int16_t>::max());
+
+  VerifySimpleAtoiBad<int16_t>(std::numeric_limits<uint16_t>::max());
+  VerifySimpleAtoiBad<int16_t>(std::numeric_limits<int32_t>::min());
+  VerifySimpleAtoiBad<int16_t>(std::numeric_limits<int32_t>::max());
+
+  // SimpleAtoi(absl::string_view, uint16_t)
+  VerifySimpleAtoiGood<uint16_t>(0, 0);
+  VerifySimpleAtoiGood<uint16_t>(42, 42);
+  VerifySimpleAtoiBad<uint16_t>(-42);
+
+  VerifySimpleAtoiBad<uint16_t>(std::numeric_limits<int16_t>::min());
+  VerifySimpleAtoiGood<uint16_t>(std::numeric_limits<int16_t>::max(),
+                                 std::numeric_limits<int16_t>::max());
+  VerifySimpleAtoiGood<uint16_t>(std::numeric_limits<uint16_t>::max(),
+                                 std::numeric_limits<uint16_t>::max());
+
+  VerifySimpleAtoiBad<uint16_t>(std::numeric_limits<int16_t>::min());
+  VerifySimpleAtoiBad<uint16_t>(std::numeric_limits<int32_t>::max());
+  VerifySimpleAtoiBad<uint16_t>(std::numeric_limits<uint32_t>::max());
+
   // SimpleAtoi(absl::string_view, int32_t)
   VerifySimpleAtoiGood<int32_t>(0, 0);
   VerifySimpleAtoiGood<int32_t>(42, 42);
@@ -320,18 +452,305 @@ TEST(NumbersTest, Atoi) {
   VerifySimpleAtoiGood<uint64_t>(std::numeric_limits<uint64_t>::max(),
                                  std::numeric_limits<uint64_t>::max());
 
+  // SimpleAtoi(absl::string_view, absl::uint128)
+  VerifySimpleAtoiGood<absl::uint128>(0, 0);
+  VerifySimpleAtoiGood<absl::uint128>(42, 42);
+  VerifySimpleAtoiBad<absl::uint128>(-42);
+
+  VerifySimpleAtoiBad<absl::uint128>(std::numeric_limits<int32_t>::min());
+  VerifySimpleAtoiGood<absl::uint128>(std::numeric_limits<int32_t>::max(),
+                                      std::numeric_limits<int32_t>::max());
+  VerifySimpleAtoiGood<absl::uint128>(std::numeric_limits<uint32_t>::max(),
+                                      std::numeric_limits<uint32_t>::max());
+  VerifySimpleAtoiBad<absl::uint128>(std::numeric_limits<int64_t>::min());
+  VerifySimpleAtoiGood<absl::uint128>(std::numeric_limits<int64_t>::max(),
+                                      std::numeric_limits<int64_t>::max());
+  VerifySimpleAtoiGood<absl::uint128>(std::numeric_limits<uint64_t>::max(),
+                                      std::numeric_limits<uint64_t>::max());
+  VerifySimpleAtoiGood<absl::uint128>(
+      std::numeric_limits<absl::uint128>::max(),
+      std::numeric_limits<absl::uint128>::max());
+
+  // SimpleAtoi(absl::string_view, absl::int128)
+  VerifySimpleAtoiGood<absl::int128>(0, 0);
+  VerifySimpleAtoiGood<absl::int128>(42, 42);
+  VerifySimpleAtoiGood<absl::int128>(-42, -42);
+
+  VerifySimpleAtoiGood<absl::int128>(std::numeric_limits<int32_t>::min(),
+                                      std::numeric_limits<int32_t>::min());
+  VerifySimpleAtoiGood<absl::int128>(std::numeric_limits<int32_t>::max(),
+                                      std::numeric_limits<int32_t>::max());
+  VerifySimpleAtoiGood<absl::int128>(std::numeric_limits<uint32_t>::max(),
+                                      std::numeric_limits<uint32_t>::max());
+  VerifySimpleAtoiGood<absl::int128>(std::numeric_limits<int64_t>::min(),
+                                      std::numeric_limits<int64_t>::min());
+  VerifySimpleAtoiGood<absl::int128>(std::numeric_limits<int64_t>::max(),
+                                      std::numeric_limits<int64_t>::max());
+  VerifySimpleAtoiGood<absl::int128>(std::numeric_limits<uint64_t>::max(),
+                                      std::numeric_limits<uint64_t>::max());
+  VerifySimpleAtoiGood<absl::int128>(
+      std::numeric_limits<absl::int128>::min(),
+      std::numeric_limits<absl::int128>::min());
+  VerifySimpleAtoiGood<absl::int128>(
+      std::numeric_limits<absl::int128>::max(),
+      std::numeric_limits<absl::int128>::max());
+  VerifySimpleAtoiBad<absl::int128>(std::numeric_limits<absl::uint128>::max());
+
   // Some other types
+  VerifySimpleAtoiGood<short>(-42, -42);  // NOLINT: runtime-int
   VerifySimpleAtoiGood<int>(-42, -42);
   VerifySimpleAtoiGood<int32_t>(-42, -42);
   VerifySimpleAtoiGood<uint32_t>(42, 42);
   VerifySimpleAtoiGood<unsigned int>(42, 42);
   VerifySimpleAtoiGood<int64_t>(-42, -42);
-  VerifySimpleAtoiGood<long>(-42, -42);  // NOLINT(runtime/int)
+  VerifySimpleAtoiGood<long>(-42, -42);  // NOLINT: runtime-int
   VerifySimpleAtoiGood<uint64_t>(42, 42);
   VerifySimpleAtoiGood<size_t>(42, 42);
   VerifySimpleAtoiGood<std::string::size_type>(42, 42);
 }
 
+TEST(NumbersTest, AtodEmpty) {
+  double d;
+  EXPECT_FALSE(absl::SimpleAtod("", &d));
+  // Empty string_view takes a different code path from "".
+  EXPECT_FALSE(absl::SimpleAtod({}, &d));
+}
+
+TEST(NumbersTest, AtofEmpty) {
+  float f;
+  EXPECT_FALSE(absl::SimpleAtof("", &f));
+  // Empty string_view takes a different code path from "".
+  EXPECT_FALSE(absl::SimpleAtof({}, &f));
+}
+
+TEST(NumbersTest, Atod) {
+  // DBL_TRUE_MIN and FLT_TRUE_MIN were not mandated in <cfloat> before C++17.
+#if !defined(DBL_TRUE_MIN)
+  static constexpr double DBL_TRUE_MIN =
+      4.940656458412465441765687928682213723650598026143247644255856825e-324;
+#endif
+#if !defined(FLT_TRUE_MIN)
+  static constexpr float FLT_TRUE_MIN =
+      1.401298464324817070923729583289916131280261941876515771757068284e-45f;
+#endif
+
+  double d;
+  float f;
+
+  // NaN can be spelled in multiple ways.
+  EXPECT_TRUE(absl::SimpleAtod("NaN", &d));
+  EXPECT_TRUE(std::isnan(d));
+  EXPECT_TRUE(absl::SimpleAtod("nAN", &d));
+  EXPECT_TRUE(std::isnan(d));
+  EXPECT_TRUE(absl::SimpleAtod("-nan", &d));
+  EXPECT_TRUE(std::isnan(d));
+
+  // Likewise for Infinity.
+  EXPECT_TRUE(absl::SimpleAtod("inf", &d));
+  EXPECT_TRUE(std::isinf(d) && (d > 0));
+  EXPECT_TRUE(absl::SimpleAtod("+Infinity", &d));
+  EXPECT_TRUE(std::isinf(d) && (d > 0));
+  EXPECT_TRUE(absl::SimpleAtod("-INF", &d));
+  EXPECT_TRUE(std::isinf(d) && (d < 0));
+
+  // Parse DBL_MAX. Parsing something more than twice as big should also
+  // produce infinity.
+  EXPECT_TRUE(absl::SimpleAtod("1.7976931348623157e+308", &d));
+  EXPECT_EQ(d, 1.7976931348623157e+308);
+  EXPECT_TRUE(absl::SimpleAtod("5e308", &d));
+  EXPECT_TRUE(std::isinf(d) && (d > 0));
+  // Ditto, but for FLT_MAX.
+  EXPECT_TRUE(absl::SimpleAtof("3.4028234663852886e+38", &f));
+  EXPECT_EQ(f, 3.4028234663852886e+38f);
+  EXPECT_TRUE(absl::SimpleAtof("7e38", &f));
+  EXPECT_TRUE(std::isinf(f) && (f > 0));
+
+  // Parse the largest N such that parsing 1eN produces a finite value and the
+  // smallest M = N + 1 such that parsing 1eM produces infinity.
+  //
+  // The 309 exponent (and 39) confirms the "definition of
+  // kEiselLemireMaxExclExp10" comment in charconv.cc.
+  EXPECT_TRUE(absl::SimpleAtod("1e308", &d));
+  EXPECT_EQ(d, 1e308);
+  EXPECT_FALSE(std::isinf(d));
+  EXPECT_TRUE(absl::SimpleAtod("1e309", &d));
+  EXPECT_TRUE(std::isinf(d));
+  // Ditto, but for Atof instead of Atod.
+  EXPECT_TRUE(absl::SimpleAtof("1e38", &f));
+  EXPECT_EQ(f, 1e38f);
+  EXPECT_FALSE(std::isinf(f));
+  EXPECT_TRUE(absl::SimpleAtof("1e39", &f));
+  EXPECT_TRUE(std::isinf(f));
+
+  // Parse the largest N such that parsing 9.999999999999999999eN, with 19
+  // nines, produces a finite value.
+  //
+  // 9999999999999999999, with 19 nines but no decimal point, is the largest
+  // "repeated nines" integer that fits in a uint64_t.
+  EXPECT_TRUE(absl::SimpleAtod("9.999999999999999999e307", &d));
+  EXPECT_EQ(d, 9.999999999999999999e307);
+  EXPECT_FALSE(std::isinf(d));
+  EXPECT_TRUE(absl::SimpleAtod("9.999999999999999999e308", &d));
+  EXPECT_TRUE(std::isinf(d));
+  // Ditto, but for Atof instead of Atod.
+  EXPECT_TRUE(absl::SimpleAtof("9.999999999999999999e37", &f));
+  EXPECT_EQ(f, 9.999999999999999999e37f);
+  EXPECT_FALSE(std::isinf(f));
+  EXPECT_TRUE(absl::SimpleAtof("9.999999999999999999e38", &f));
+  EXPECT_TRUE(std::isinf(f));
+
+  // Parse DBL_MIN (normal), DBL_TRUE_MIN (subnormal) and (DBL_TRUE_MIN / 10)
+  // (effectively zero).
+  EXPECT_TRUE(absl::SimpleAtod("2.2250738585072014e-308", &d));
+  EXPECT_EQ(d, 2.2250738585072014e-308);
+  EXPECT_TRUE(absl::SimpleAtod("4.9406564584124654e-324", &d));
+  EXPECT_EQ(d, 4.9406564584124654e-324);
+  EXPECT_TRUE(absl::SimpleAtod("4.9406564584124654e-325", &d));
+  EXPECT_EQ(d, 0);
+  // Ditto, but for FLT_MIN, FLT_TRUE_MIN and (FLT_TRUE_MIN / 10).
+  EXPECT_TRUE(absl::SimpleAtof("1.1754943508222875e-38", &f));
+  EXPECT_EQ(f, 1.1754943508222875e-38f);
+  EXPECT_TRUE(absl::SimpleAtof("1.4012984643248171e-45", &f));
+  EXPECT_EQ(f, 1.4012984643248171e-45f);
+  EXPECT_TRUE(absl::SimpleAtof("1.4012984643248171e-46", &f));
+  EXPECT_EQ(f, 0);
+
+  // Parse the largest N (the most negative -N) such that parsing 1e-N produces
+  // a normal or subnormal (but still positive) or zero value.
+  EXPECT_TRUE(absl::SimpleAtod("1e-307", &d));
+  EXPECT_EQ(d, 1e-307);
+  EXPECT_GE(d, DBL_MIN);
+  EXPECT_LT(d, DBL_MIN * 10);
+  EXPECT_TRUE(absl::SimpleAtod("1e-323", &d));
+  EXPECT_EQ(d, 1e-323);
+  EXPECT_GE(d, DBL_TRUE_MIN);
+  EXPECT_LT(d, DBL_TRUE_MIN * 10);
+  EXPECT_TRUE(absl::SimpleAtod("1e-324", &d));
+  EXPECT_EQ(d, 0);
+  // Ditto, but for Atof instead of Atod.
+  EXPECT_TRUE(absl::SimpleAtof("1e-37", &f));
+  EXPECT_EQ(f, 1e-37f);
+  EXPECT_GE(f, FLT_MIN);
+  EXPECT_LT(f, FLT_MIN * 10);
+  EXPECT_TRUE(absl::SimpleAtof("1e-45", &f));
+  EXPECT_EQ(f, 1e-45f);
+  EXPECT_GE(f, FLT_TRUE_MIN);
+  EXPECT_LT(f, FLT_TRUE_MIN * 10);
+  EXPECT_TRUE(absl::SimpleAtof("1e-46", &f));
+  EXPECT_EQ(f, 0);
+
+  // Parse the largest N (the most negative -N) such that parsing
+  // 9.999999999999999999e-N, with 19 nines, produces a normal or subnormal
+  // (but still positive) or zero value.
+  //
+  // 9999999999999999999, with 19 nines but no decimal point, is the largest
+  // "repeated nines" integer that fits in a uint64_t.
+  //
+  // The -324/-325 exponents (and -46/-47) confirms the "definition of
+  // kEiselLemireMinInclExp10" comment in charconv.cc.
+  EXPECT_TRUE(absl::SimpleAtod("9.999999999999999999e-308", &d));
+  EXPECT_EQ(d, 9.999999999999999999e-308);
+  EXPECT_GE(d, DBL_MIN);
+  EXPECT_LT(d, DBL_MIN * 10);
+  EXPECT_TRUE(absl::SimpleAtod("9.999999999999999999e-324", &d));
+  EXPECT_EQ(d, 9.999999999999999999e-324);
+  EXPECT_GE(d, DBL_TRUE_MIN);
+  EXPECT_LT(d, DBL_TRUE_MIN * 10);
+  EXPECT_TRUE(absl::SimpleAtod("9.999999999999999999e-325", &d));
+  EXPECT_EQ(d, 0);
+  // Ditto, but for Atof instead of Atod.
+  EXPECT_TRUE(absl::SimpleAtof("9.999999999999999999e-38", &f));
+  EXPECT_EQ(f, 9.999999999999999999e-38f);
+  EXPECT_GE(f, FLT_MIN);
+  EXPECT_LT(f, FLT_MIN * 10);
+  EXPECT_TRUE(absl::SimpleAtof("9.999999999999999999e-46", &f));
+  EXPECT_EQ(f, 9.999999999999999999e-46f);
+  EXPECT_GE(f, FLT_TRUE_MIN);
+  EXPECT_LT(f, FLT_TRUE_MIN * 10);
+  EXPECT_TRUE(absl::SimpleAtof("9.999999999999999999e-47", &f));
+  EXPECT_EQ(f, 0);
+
+  // Leading and/or trailing whitespace is OK.
+  EXPECT_TRUE(absl::SimpleAtod("  \t\r\n  2.718", &d));
+  EXPECT_EQ(d, 2.718);
+  EXPECT_TRUE(absl::SimpleAtod("  3.141  ", &d));
+  EXPECT_EQ(d, 3.141);
+
+  // Leading or trailing not-whitespace is not OK.
+  EXPECT_FALSE(absl::SimpleAtod("n 0", &d));
+  EXPECT_FALSE(absl::SimpleAtod("0n ", &d));
+
+  // Multiple leading 0s are OK.
+  EXPECT_TRUE(absl::SimpleAtod("000123", &d));
+  EXPECT_EQ(d, 123);
+  EXPECT_TRUE(absl::SimpleAtod("000.456", &d));
+  EXPECT_EQ(d, 0.456);
+
+  // An absent leading 0 (for a fraction < 1) is OK.
+  EXPECT_TRUE(absl::SimpleAtod(".5", &d));
+  EXPECT_EQ(d, 0.5);
+  EXPECT_TRUE(absl::SimpleAtod("-.707", &d));
+  EXPECT_EQ(d, -0.707);
+
+  // Unary + is OK.
+  EXPECT_TRUE(absl::SimpleAtod("+6.0221408e+23", &d));
+  EXPECT_EQ(d, 6.0221408e+23);
+
+  // Underscores are not OK.
+  EXPECT_FALSE(absl::SimpleAtod("123_456", &d));
+
+  // The decimal separator must be '.' and is never ','.
+  EXPECT_TRUE(absl::SimpleAtod("8.9", &d));
+  EXPECT_FALSE(absl::SimpleAtod("8,9", &d));
+
+  // These examples are called out in the EiselLemire function's comments.
+  EXPECT_TRUE(absl::SimpleAtod("4503599627370497.5", &d));
+  EXPECT_EQ(d, 4503599627370497.5);
+  EXPECT_TRUE(absl::SimpleAtod("1e+23", &d));
+  EXPECT_EQ(d, 1e+23);
+  EXPECT_TRUE(absl::SimpleAtod("9223372036854775807", &d));
+  EXPECT_EQ(d, 9223372036854775807);
+  // Ditto, but for Atof instead of Atod.
+  EXPECT_TRUE(absl::SimpleAtof("0.0625", &f));
+  EXPECT_EQ(f, 0.0625f);
+  EXPECT_TRUE(absl::SimpleAtof("20040229.0", &f));
+  EXPECT_EQ(f, 20040229.0f);
+  EXPECT_TRUE(absl::SimpleAtof("2147483647.0", &f));
+  EXPECT_EQ(f, 2147483647.0f);
+
+  // Some parsing algorithms don't always round correctly (but absl::SimpleAtod
+  // should). This test case comes from
+  // https://github.com/serde-rs/json/issues/707
+  //
+  // See also atod_manual_test.cc for running many more test cases.
+  EXPECT_TRUE(absl::SimpleAtod("122.416294033786585", &d));
+  EXPECT_EQ(d, 122.416294033786585);
+  EXPECT_TRUE(absl::SimpleAtof("122.416294033786585", &f));
+  EXPECT_EQ(f, 122.416294033786585f);
+}
+
+TEST(NumbersTest, Prefixes) {
+  double d;
+  EXPECT_FALSE(absl::SimpleAtod("++1", &d));
+  EXPECT_FALSE(absl::SimpleAtod("+-1", &d));
+  EXPECT_FALSE(absl::SimpleAtod("-+1", &d));
+  EXPECT_FALSE(absl::SimpleAtod("--1", &d));
+  EXPECT_TRUE(absl::SimpleAtod("-1", &d));
+  EXPECT_EQ(d, -1.);
+  EXPECT_TRUE(absl::SimpleAtod("+1", &d));
+  EXPECT_EQ(d, +1.);
+
+  float f;
+  EXPECT_FALSE(absl::SimpleAtof("++1", &f));
+  EXPECT_FALSE(absl::SimpleAtof("+-1", &f));
+  EXPECT_FALSE(absl::SimpleAtof("-+1", &f));
+  EXPECT_FALSE(absl::SimpleAtof("--1", &f));
+  EXPECT_TRUE(absl::SimpleAtof("-1", &f));
+  EXPECT_EQ(f, -1.f);
+  EXPECT_TRUE(absl::SimpleAtof("+1", &f));
+  EXPECT_EQ(f, +1.f);
+}
+
 TEST(NumbersTest, Atoenum) {
   enum E01 {
     E01_zero = 0,
@@ -386,6 +805,353 @@ TEST(NumbersTest, Atoenum) {
   VerifySimpleAtoiGood<E_biguint>(E_biguint_max32, E_biguint_max32);
 }
 
+template <typename int_type, typename in_val_type>
+void VerifySimpleHexAtoiGood(in_val_type in_value, int_type exp_value) {
+  std::string s;
+  absl::strings_internal::OStringStream strm(&s);
+  if (in_value >= 0) {
+    if constexpr (std::is_arithmetic_v<in_val_type>) {
+      absl::StrAppend(&s, absl::Hex(in_value));
+    } else {
+      // absl::Hex doesn't work with absl::(u)int128.
+      strm << std::hex << in_value;
+    }
+  } else {
+    // Inefficient for small integers, but works with all integral types.
+    strm << "-" << std::hex << -absl::uint128(in_value);
+  }
+  int_type x = static_cast<int_type>(~exp_value);
+  EXPECT_TRUE(SimpleHexAtoi(s, &x))
+      << "in_value=" << std::hex << in_value << " s=" << s << " x=" << x;
+  EXPECT_EQ(exp_value, x);
+  x = static_cast<int_type>(~exp_value);
+  EXPECT_TRUE(SimpleHexAtoi(
+      s.c_str(), &x));  // NOLINT: readability-redundant-string-conversions
+  EXPECT_EQ(exp_value, x);
+}
+
+template <typename int_type, typename in_val_type>
+void VerifySimpleHexAtoiBad(in_val_type in_value) {
+  std::string s;
+  absl::strings_internal::OStringStream strm(&s);
+  if (in_value >= 0) {
+    if constexpr (std::is_arithmetic_v<in_val_type>) {
+      absl::StrAppend(&s, absl::Hex(in_value));
+    } else {
+      // absl::Hex doesn't work with absl::(u)int128.
+      strm << std::hex << in_value;
+    }
+  } else {
+    // Inefficient for small integers, but works with all integral types.
+    strm << "-" << std::hex << -absl::uint128(in_value);
+  }
+  int_type x;
+  EXPECT_FALSE(SimpleHexAtoi(s, &x));
+  EXPECT_FALSE(SimpleHexAtoi(
+      s.c_str(), &x));  // NOLINT: readability-redundant-string-conversions
+}
+
+TEST(NumbersTest, HexAtoi) {
+  // SimpleHexAtoi(absl::string_view, int8_t)
+  VerifySimpleHexAtoiGood<int8_t>(0, 0);
+  VerifySimpleHexAtoiGood<int8_t>(0x42, 0x42);
+  VerifySimpleHexAtoiGood<int8_t>(-0x42, -0x42);
+
+  VerifySimpleHexAtoiGood<int8_t>(std::numeric_limits<int8_t>::min(),
+                                  std::numeric_limits<int8_t>::min());
+  VerifySimpleHexAtoiGood<int8_t>(std::numeric_limits<int8_t>::max(),
+                                  std::numeric_limits<int8_t>::max());
+
+  // SimpleHexAtoi(absl::string_view, uint8_t)
+  VerifySimpleHexAtoiGood<uint8_t>(0, 0);
+  VerifySimpleHexAtoiGood<uint8_t>(0x42, 0x42);
+  VerifySimpleHexAtoiBad<uint8_t>(-0x42);
+
+  VerifySimpleHexAtoiBad<uint8_t>(std::numeric_limits<int8_t>::min());
+  VerifySimpleHexAtoiGood<uint8_t>(std::numeric_limits<int8_t>::max(),
+                                   std::numeric_limits<int8_t>::max());
+  VerifySimpleHexAtoiGood<uint8_t>(std::numeric_limits<uint8_t>::max(),
+                                   std::numeric_limits<uint8_t>::max());
+  VerifySimpleHexAtoiBad<uint8_t>(std::numeric_limits<int16_t>::min());
+  VerifySimpleHexAtoiBad<uint8_t>(std::numeric_limits<int16_t>::max());
+  VerifySimpleHexAtoiBad<uint8_t>(std::numeric_limits<uint16_t>::max());
+
+  // SimpleHexAtoi(absl::string_view, int16_t)
+  VerifySimpleHexAtoiGood<int16_t>(0, 0);
+  VerifySimpleHexAtoiGood<int16_t>(0x42, 0x42);
+  VerifySimpleHexAtoiGood<int16_t>(-0x42, -0x42);
+
+  VerifySimpleHexAtoiGood<int16_t>(std::numeric_limits<int16_t>::min(),
+                                   std::numeric_limits<int16_t>::min());
+  VerifySimpleHexAtoiGood<int16_t>(std::numeric_limits<int16_t>::max(),
+                                   std::numeric_limits<int16_t>::max());
+
+  // SimpleHexAtoi(absl::string_view, uint16_t)
+  VerifySimpleHexAtoiGood<uint16_t>(0, 0);
+  VerifySimpleHexAtoiGood<uint16_t>(0x42, 0x42);
+  VerifySimpleHexAtoiBad<uint16_t>(-0x42);
+
+  VerifySimpleHexAtoiBad<uint16_t>(std::numeric_limits<int16_t>::min());
+  VerifySimpleHexAtoiGood<uint16_t>(std::numeric_limits<int16_t>::max(),
+                                    std::numeric_limits<int16_t>::max());
+  VerifySimpleHexAtoiGood<uint16_t>(std::numeric_limits<uint16_t>::max(),
+                                    std::numeric_limits<uint16_t>::max());
+  VerifySimpleHexAtoiBad<uint16_t>(std::numeric_limits<int32_t>::min());
+  VerifySimpleHexAtoiBad<uint16_t>(std::numeric_limits<int32_t>::max());
+  VerifySimpleHexAtoiBad<uint16_t>(std::numeric_limits<uint32_t>::max());
+
+  // SimpleHexAtoi(absl::string_view, int32_t)
+  VerifySimpleHexAtoiGood<int32_t>(0, 0);
+  VerifySimpleHexAtoiGood<int32_t>(0x42, 0x42);
+  VerifySimpleHexAtoiGood<int32_t>(-0x42, -0x42);
+
+  VerifySimpleHexAtoiGood<int32_t>(std::numeric_limits<int32_t>::min(),
+                                   std::numeric_limits<int32_t>::min());
+  VerifySimpleHexAtoiGood<int32_t>(std::numeric_limits<int32_t>::max(),
+                                   std::numeric_limits<int32_t>::max());
+
+  // SimpleHexAtoi(absl::string_view, uint32_t)
+  VerifySimpleHexAtoiGood<uint32_t>(0, 0);
+  VerifySimpleHexAtoiGood<uint32_t>(0x42, 0x42);
+  VerifySimpleHexAtoiBad<uint32_t>(-0x42);
+
+  VerifySimpleHexAtoiBad<uint32_t>(std::numeric_limits<int32_t>::min());
+  VerifySimpleHexAtoiGood<uint32_t>(std::numeric_limits<int32_t>::max(),
+                                    std::numeric_limits<int32_t>::max());
+  VerifySimpleHexAtoiGood<uint32_t>(std::numeric_limits<uint32_t>::max(),
+                                    std::numeric_limits<uint32_t>::max());
+  VerifySimpleHexAtoiBad<uint32_t>(std::numeric_limits<int64_t>::min());
+  VerifySimpleHexAtoiBad<uint32_t>(std::numeric_limits<int64_t>::max());
+  VerifySimpleHexAtoiBad<uint32_t>(std::numeric_limits<uint64_t>::max());
+
+  // SimpleHexAtoi(absl::string_view, int64_t)
+  VerifySimpleHexAtoiGood<int64_t>(0, 0);
+  VerifySimpleHexAtoiGood<int64_t>(0x42, 0x42);
+  VerifySimpleHexAtoiGood<int64_t>(-0x42, -0x42);
+
+  VerifySimpleHexAtoiGood<int64_t>(std::numeric_limits<int32_t>::min(),
+                                   std::numeric_limits<int32_t>::min());
+  VerifySimpleHexAtoiGood<int64_t>(std::numeric_limits<int32_t>::max(),
+                                   std::numeric_limits<int32_t>::max());
+  VerifySimpleHexAtoiGood<int64_t>(std::numeric_limits<uint32_t>::max(),
+                                   std::numeric_limits<uint32_t>::max());
+  VerifySimpleHexAtoiGood<int64_t>(std::numeric_limits<int64_t>::min(),
+                                   std::numeric_limits<int64_t>::min());
+  VerifySimpleHexAtoiGood<int64_t>(std::numeric_limits<int64_t>::max(),
+                                   std::numeric_limits<int64_t>::max());
+  VerifySimpleHexAtoiBad<int64_t>(std::numeric_limits<uint64_t>::max());
+
+  // SimpleHexAtoi(absl::string_view, uint64_t)
+  VerifySimpleHexAtoiGood<uint64_t>(0, 0);
+  VerifySimpleHexAtoiGood<uint64_t>(0x42, 0x42);
+  VerifySimpleHexAtoiBad<uint64_t>(-0x42);
+
+  VerifySimpleHexAtoiBad<uint64_t>(std::numeric_limits<int32_t>::min());
+  VerifySimpleHexAtoiGood<uint64_t>(std::numeric_limits<int32_t>::max(),
+                                    std::numeric_limits<int32_t>::max());
+  VerifySimpleHexAtoiGood<uint64_t>(std::numeric_limits<uint32_t>::max(),
+                                    std::numeric_limits<uint32_t>::max());
+  VerifySimpleHexAtoiBad<uint64_t>(std::numeric_limits<int64_t>::min());
+  VerifySimpleHexAtoiGood<uint64_t>(std::numeric_limits<int64_t>::max(),
+                                    std::numeric_limits<int64_t>::max());
+  VerifySimpleHexAtoiGood<uint64_t>(std::numeric_limits<uint64_t>::max(),
+                                    std::numeric_limits<uint64_t>::max());
+
+  // SimpleHexAtoi(absl::string_view, absl::uint128)
+  VerifySimpleHexAtoiGood<absl::uint128>(0, 0);
+  VerifySimpleHexAtoiGood<absl::uint128>(0x42, 0x42);
+  VerifySimpleHexAtoiBad<absl::uint128>(-0x42);
+
+  VerifySimpleHexAtoiBad<absl::uint128>(std::numeric_limits<int32_t>::min());
+  VerifySimpleHexAtoiGood<absl::uint128>(std::numeric_limits<int32_t>::max(),
+                                         std::numeric_limits<int32_t>::max());
+  VerifySimpleHexAtoiGood<absl::uint128>(std::numeric_limits<uint32_t>::max(),
+                                         std::numeric_limits<uint32_t>::max());
+  VerifySimpleHexAtoiBad<absl::uint128>(std::numeric_limits<int64_t>::min());
+  VerifySimpleHexAtoiGood<absl::uint128>(std::numeric_limits<int64_t>::max(),
+                                         std::numeric_limits<int64_t>::max());
+  VerifySimpleHexAtoiGood<absl::uint128>(std::numeric_limits<uint64_t>::max(),
+                                         std::numeric_limits<uint64_t>::max());
+  VerifySimpleHexAtoiGood<absl::uint128>(
+      std::numeric_limits<absl::uint128>::max(),
+      std::numeric_limits<absl::uint128>::max());
+
+  // Some other types
+  VerifySimpleHexAtoiGood<short>(-0x42, -0x42);  // NOLINT: runtime-int
+  VerifySimpleHexAtoiGood<int>(-0x42, -0x42);
+  VerifySimpleHexAtoiGood<int32_t>(-0x42, -0x42);
+  VerifySimpleHexAtoiGood<uint32_t>(0x42, 0x42);
+  VerifySimpleHexAtoiGood<unsigned int>(0x42, 0x42);
+  VerifySimpleHexAtoiGood<int64_t>(-0x42, -0x42);
+  VerifySimpleHexAtoiGood<long>(-0x42, -0x42);  // NOLINT: runtime-int
+  VerifySimpleHexAtoiGood<uint64_t>(0x42, 0x42);
+  VerifySimpleHexAtoiGood<size_t>(0x42, 0x42);
+  VerifySimpleHexAtoiGood<std::string::size_type>(0x42, 0x42);
+
+  // Number prefix
+  int32_t value;
+  EXPECT_TRUE(safe_strto32_base("0x34234324", &value, 16));
+  EXPECT_EQ(0x34234324, value);
+
+  EXPECT_TRUE(safe_strto32_base("0X34234324", &value, 16));
+  EXPECT_EQ(0x34234324, value);
+
+  // ASCII whitespace
+  EXPECT_TRUE(safe_strto32_base(" \t\n 34234324", &value, 16));
+  EXPECT_EQ(0x34234324, value);
+
+  EXPECT_TRUE(safe_strto32_base("34234324 \t\n ", &value, 16));
+  EXPECT_EQ(0x34234324, value);
+}
+
+TEST(stringtest, safe_strto8_base) {
+  int8_t value;
+  EXPECT_TRUE(safe_strto8_base("0x34", &value, 16));
+  EXPECT_EQ(0x34, value);
+
+  EXPECT_TRUE(safe_strto8_base("0X34", &value, 16));
+  EXPECT_EQ(0x34, value);
+
+  EXPECT_TRUE(safe_strto8_base("34", &value, 16));
+  EXPECT_EQ(0x34, value);
+
+  EXPECT_TRUE(safe_strto8_base("0", &value, 16));
+  EXPECT_EQ(0, value);
+
+  EXPECT_TRUE(safe_strto8_base(" \t\n -0x34", &value, 16));
+  EXPECT_EQ(-0x34, value);
+
+  EXPECT_TRUE(safe_strto8_base(" \t\n -34", &value, 16));
+  EXPECT_EQ(-0x34, value);
+
+  EXPECT_TRUE(safe_strto8_base("76", &value, 8));
+  EXPECT_EQ(076, value);
+
+  EXPECT_TRUE(safe_strto8_base("-0123", &value, 8));
+  EXPECT_EQ(-0123, value);
+
+  EXPECT_FALSE(safe_strto8_base("183", &value, 8));
+
+  // Autodetect base.
+  EXPECT_TRUE(safe_strto8_base("0", &value, 0));
+  EXPECT_EQ(0, value);
+
+  EXPECT_TRUE(safe_strto8_base("077", &value, 0));
+  EXPECT_EQ(077, value);  // Octal interpretation
+
+  // Leading zero indicates octal, but then followed by invalid digit.
+  EXPECT_FALSE(safe_strto8_base("088", &value, 0));
+
+  // Leading 0x indicated hex, but then followed by invalid digit.
+  EXPECT_FALSE(safe_strto8_base("0xG", &value, 0));
+
+  // Base-10 version.
+  EXPECT_TRUE(safe_strto8_base("124", &value, 10));
+  EXPECT_EQ(124, value);
+
+  EXPECT_TRUE(safe_strto8_base("0", &value, 10));
+  EXPECT_EQ(0, value);
+
+  EXPECT_TRUE(safe_strto8_base(" \t\n -124", &value, 10));
+  EXPECT_EQ(-124, value);
+
+  EXPECT_TRUE(safe_strto8_base("124 \n\t ", &value, 10));
+  EXPECT_EQ(124, value);
+
+  // Invalid ints.
+  EXPECT_FALSE(safe_strto8_base("", &value, 10));
+  EXPECT_FALSE(safe_strto8_base("  ", &value, 10));
+  EXPECT_FALSE(safe_strto8_base("abc", &value, 10));
+  EXPECT_FALSE(safe_strto8_base("34a", &value, 10));
+  EXPECT_FALSE(safe_strto8_base("34.3", &value, 10));
+
+  // Out of bounds.
+  EXPECT_FALSE(safe_strto8_base("128", &value, 10));
+  EXPECT_FALSE(safe_strto8_base("-129", &value, 10));
+
+  // String version.
+  EXPECT_TRUE(safe_strto8_base(std::string("0x12"), &value, 16));
+  EXPECT_EQ(0x12, value);
+
+  // Base-10 string version.
+  EXPECT_TRUE(safe_strto8_base("123", &value, 10));
+  EXPECT_EQ(123, value);
+}
+
+TEST(stringtest, safe_strto16_base) {
+  int16_t value;
+  EXPECT_TRUE(safe_strto16_base("0x3423", &value, 16));
+  EXPECT_EQ(0x3423, value);
+
+  EXPECT_TRUE(safe_strto16_base("0X3423", &value, 16));
+  EXPECT_EQ(0x3423, value);
+
+  EXPECT_TRUE(safe_strto16_base("3423", &value, 16));
+  EXPECT_EQ(0x3423, value);
+
+  EXPECT_TRUE(safe_strto16_base("0", &value, 16));
+  EXPECT_EQ(0, value);
+
+  EXPECT_TRUE(safe_strto16_base(" \t\n -0x3423", &value, 16));
+  EXPECT_EQ(-0x3423, value);
+
+  EXPECT_TRUE(safe_strto16_base(" \t\n -3423", &value, 16));
+  EXPECT_EQ(-0x3423, value);
+
+  EXPECT_TRUE(safe_strto16_base("34567", &value, 8));
+  EXPECT_EQ(034567, value);
+
+  EXPECT_TRUE(safe_strto16_base("-01234", &value, 8));
+  EXPECT_EQ(-01234, value);
+
+  EXPECT_FALSE(safe_strto16_base("1834", &value, 8));
+
+  // Autodetect base.
+  EXPECT_TRUE(safe_strto16_base("0", &value, 0));
+  EXPECT_EQ(0, value);
+
+  EXPECT_TRUE(safe_strto16_base("077", &value, 0));
+  EXPECT_EQ(077, value);  // Octal interpretation
+
+  // Leading zero indicates octal, but then followed by invalid digit.
+  EXPECT_FALSE(safe_strto16_base("088", &value, 0));
+
+  // Leading 0x indicated hex, but then followed by invalid digit.
+  EXPECT_FALSE(safe_strto16_base("0xG", &value, 0));
+
+  // Base-10 version.
+  EXPECT_TRUE(safe_strto16_base("3423", &value, 10));
+  EXPECT_EQ(3423, value);
+
+  EXPECT_TRUE(safe_strto16_base("0", &value, 10));
+  EXPECT_EQ(0, value);
+
+  EXPECT_TRUE(safe_strto16_base(" \t\n -3423", &value, 10));
+  EXPECT_EQ(-3423, value);
+
+  EXPECT_TRUE(safe_strto16_base("3423 \n\t ", &value, 10));
+  EXPECT_EQ(3423, value);
+
+  // Invalid ints.
+  EXPECT_FALSE(safe_strto16_base("", &value, 10));
+  EXPECT_FALSE(safe_strto16_base("  ", &value, 10));
+  EXPECT_FALSE(safe_strto16_base("abc", &value, 10));
+  EXPECT_FALSE(safe_strto16_base("324a", &value, 10));
+  EXPECT_FALSE(safe_strto16_base("4234.3", &value, 10));
+
+  // Out of bounds.
+  EXPECT_FALSE(safe_strto16_base("32768", &value, 10));
+  EXPECT_FALSE(safe_strto16_base("-32769", &value, 10));
+
+  // String version.
+  EXPECT_TRUE(safe_strto16_base(std::string("0x1234"), &value, 16));
+  EXPECT_EQ(0x1234, value);
+
+  // Base-10 string version.
+  EXPECT_TRUE(safe_strto16_base("1234", &value, 10));
+  EXPECT_EQ(1234, value);
+}
+
 TEST(stringtest, safe_strto32_base) {
   int32_t value;
   EXPECT_TRUE(safe_strto32_base("0x34234324", &value, 16));
@@ -455,81 +1221,11 @@ TEST(stringtest, safe_strto32_base) {
   EXPECT_TRUE(safe_strto32_base(std::string("0x1234"), &value, 16));
   EXPECT_EQ(0x1234, value);
 
-  // Base-10 std::string version.
+  // Base-10 string version.
   EXPECT_TRUE(safe_strto32_base("1234", &value, 10));
   EXPECT_EQ(1234, value);
 }
 
-TEST(stringtest, safe_strto32_range) {
-  // These tests verify underflow/overflow behaviour.
-  int32_t value;
-  EXPECT_FALSE(safe_strto32_base("2147483648", &value, 10));
-  EXPECT_EQ(std::numeric_limits<int32_t>::max(), value);
-
-  EXPECT_TRUE(safe_strto32_base("-2147483648", &value, 10));
-  EXPECT_EQ(std::numeric_limits<int32_t>::min(), value);
-
-  EXPECT_FALSE(safe_strto32_base("-2147483649", &value, 10));
-  EXPECT_EQ(std::numeric_limits<int32_t>::min(), value);
-}
-
-TEST(stringtest, safe_strto64_range) {
-  // These tests verify underflow/overflow behaviour.
-  int64_t value;
-  EXPECT_FALSE(safe_strto64_base("9223372036854775808", &value, 10));
-  EXPECT_EQ(std::numeric_limits<int64_t>::max(), value);
-
-  EXPECT_TRUE(safe_strto64_base("-9223372036854775808", &value, 10));
-  EXPECT_EQ(std::numeric_limits<int64_t>::min(), value);
-
-  EXPECT_FALSE(safe_strto64_base("-9223372036854775809", &value, 10));
-  EXPECT_EQ(std::numeric_limits<int64_t>::min(), value);
-}
-
-TEST(stringtest, safe_strto32_leading_substring) {
-  // These tests verify this comment in numbers.h:
-  // On error, returns false, and sets *value to: [...]
-  //   conversion of leading substring if available ("123@@@" -> 123)
-  //   0 if no leading substring available
-  int32_t value;
-  EXPECT_FALSE(safe_strto32_base("04069@@@", &value, 10));
-  EXPECT_EQ(4069, value);
-
-  EXPECT_FALSE(safe_strto32_base("04069@@@", &value, 8));
-  EXPECT_EQ(0406, value);
-
-  EXPECT_FALSE(safe_strto32_base("04069balloons", &value, 10));
-  EXPECT_EQ(4069, value);
-
-  EXPECT_FALSE(safe_strto32_base("04069balloons", &value, 16));
-  EXPECT_EQ(0x4069ba, value);
-
-  EXPECT_FALSE(safe_strto32_base("@@@", &value, 10));
-  EXPECT_EQ(0, value);  // there was no leading substring
-}
-
-TEST(stringtest, safe_strto64_leading_substring) {
-  // These tests verify this comment in numbers.h:
-  // On error, returns false, and sets *value to: [...]
-  //   conversion of leading substring if available ("123@@@" -> 123)
-  //   0 if no leading substring available
-  int64_t value;
-  EXPECT_FALSE(safe_strto64_base("04069@@@", &value, 10));
-  EXPECT_EQ(4069, value);
-
-  EXPECT_FALSE(safe_strto64_base("04069@@@", &value, 8));
-  EXPECT_EQ(0406, value);
-
-  EXPECT_FALSE(safe_strto64_base("04069balloons", &value, 10));
-  EXPECT_EQ(4069, value);
-
-  EXPECT_FALSE(safe_strto64_base("04069balloons", &value, 16));
-  EXPECT_EQ(0x4069ba, value);
-
-  EXPECT_FALSE(safe_strto64_base("@@@", &value, 10));
-  EXPECT_EQ(0, value);  // there was no leading substring
-}
-
 TEST(stringtest, safe_strto64_base) {
   int64_t value;
   EXPECT_TRUE(safe_strto64_base("0x3423432448783446", &value, 16));
@@ -596,23 +1292,160 @@ TEST(stringtest, safe_strto64_base) {
   EXPECT_TRUE(safe_strto64_base(std::string("0x1234"), &value, 16));
   EXPECT_EQ(0x1234, value);
 
-  // Base-10 std::string version.
+  // Base-10 string version.
   EXPECT_TRUE(safe_strto64_base("1234", &value, 10));
   EXPECT_EQ(1234, value);
 }
 
+TEST(stringtest, safe_strto8_range) {
+  // These tests verify underflow/overflow behaviour.
+  int8_t value;
+  EXPECT_FALSE(safe_strto8_base("128", &value, 10));
+  EXPECT_EQ(std::numeric_limits<int8_t>::max(), value);
+
+  EXPECT_TRUE(safe_strto8_base("-128", &value, 10));
+  EXPECT_EQ(std::numeric_limits<int8_t>::min(), value);
+
+  EXPECT_FALSE(safe_strto8_base("-129", &value, 10));
+  EXPECT_EQ(std::numeric_limits<int8_t>::min(), value);
+}
+
+TEST(stringtest, safe_strto16_range) {
+  // These tests verify underflow/overflow behaviour.
+  int16_t value;
+  EXPECT_FALSE(safe_strto16_base("32768", &value, 10));
+  EXPECT_EQ(std::numeric_limits<int16_t>::max(), value);
+
+  EXPECT_TRUE(safe_strto16_base("-32768", &value, 10));
+  EXPECT_EQ(std::numeric_limits<int16_t>::min(), value);
+
+  EXPECT_FALSE(safe_strto16_base("-32769", &value, 10));
+  EXPECT_EQ(std::numeric_limits<int16_t>::min(), value);
+}
+
+TEST(stringtest, safe_strto32_range) {
+  // These tests verify underflow/overflow behaviour.
+  int32_t value;
+  EXPECT_FALSE(safe_strto32_base("2147483648", &value, 10));
+  EXPECT_EQ(std::numeric_limits<int32_t>::max(), value);
+
+  EXPECT_TRUE(safe_strto32_base("-2147483648", &value, 10));
+  EXPECT_EQ(std::numeric_limits<int32_t>::min(), value);
+
+  EXPECT_FALSE(safe_strto32_base("-2147483649", &value, 10));
+  EXPECT_EQ(std::numeric_limits<int32_t>::min(), value);
+}
+
+TEST(stringtest, safe_strto64_range) {
+  // These tests verify underflow/overflow behaviour.
+  int64_t value;
+  EXPECT_FALSE(safe_strto64_base("9223372036854775808", &value, 10));
+  EXPECT_EQ(std::numeric_limits<int64_t>::max(), value);
+
+  EXPECT_TRUE(safe_strto64_base("-9223372036854775808", &value, 10));
+  EXPECT_EQ(std::numeric_limits<int64_t>::min(), value);
+
+  EXPECT_FALSE(safe_strto64_base("-9223372036854775809", &value, 10));
+  EXPECT_EQ(std::numeric_limits<int64_t>::min(), value);
+}
+
+TEST(stringtest, safe_strto8_leading_substring) {
+  // These tests verify this comment in numbers.h:
+  // On error, returns false, and sets *value to: [...]
+  //   conversion of leading substring if available ("123@@@" -> 123)
+  //   0 if no leading substring available
+  int8_t value;
+  EXPECT_FALSE(safe_strto8_base("069@@@", &value, 10));
+  EXPECT_EQ(69, value);
+
+  EXPECT_FALSE(safe_strto8_base("01769@@@", &value, 8));
+  EXPECT_EQ(0176, value);
+
+  EXPECT_FALSE(safe_strto8_base("069balloons", &value, 10));
+  EXPECT_EQ(69, value);
+
+  EXPECT_FALSE(safe_strto8_base("07bland", &value, 16));
+  EXPECT_EQ(0x7b, value);
+
+  EXPECT_FALSE(safe_strto8_base("@@@", &value, 10));
+  EXPECT_EQ(0, value);  // there was no leading substring
+}
+
+TEST(stringtest, safe_strto16_leading_substring) {
+  // These tests verify this comment in numbers.h:
+  // On error, returns false, and sets *value to: [...]
+  //   conversion of leading substring if available ("123@@@" -> 123)
+  //   0 if no leading substring available
+  int16_t value;
+  EXPECT_FALSE(safe_strto16_base("04069@@@", &value, 10));
+  EXPECT_EQ(4069, value);
+
+  EXPECT_FALSE(safe_strto16_base("04069@@@", &value, 8));
+  EXPECT_EQ(0406, value);
+
+  EXPECT_FALSE(safe_strto16_base("04069balloons", &value, 10));
+  EXPECT_EQ(4069, value);
+
+  EXPECT_FALSE(safe_strto16_base("069balloons", &value, 16));
+  EXPECT_EQ(0x69ba, value);
+
+  EXPECT_FALSE(safe_strto16_base("@@@", &value, 10));
+  EXPECT_EQ(0, value);  // there was no leading substring
+}
+
+TEST(stringtest, safe_strto32_leading_substring) {
+  // These tests verify this comment in numbers.h:
+  // On error, returns false, and sets *value to: [...]
+  //   conversion of leading substring if available ("123@@@" -> 123)
+  //   0 if no leading substring available
+  int32_t value;
+  EXPECT_FALSE(safe_strto32_base("04069@@@", &value, 10));
+  EXPECT_EQ(4069, value);
+
+  EXPECT_FALSE(safe_strto32_base("04069@@@", &value, 8));
+  EXPECT_EQ(0406, value);
+
+  EXPECT_FALSE(safe_strto32_base("04069balloons", &value, 10));
+  EXPECT_EQ(4069, value);
+
+  EXPECT_FALSE(safe_strto32_base("04069balloons", &value, 16));
+  EXPECT_EQ(0x4069ba, value);
+
+  EXPECT_FALSE(safe_strto32_base("@@@", &value, 10));
+  EXPECT_EQ(0, value);  // there was no leading substring
+}
+
+TEST(stringtest, safe_strto64_leading_substring) {
+  // These tests verify this comment in numbers.h:
+  // On error, returns false, and sets *value to: [...]
+  //   conversion of leading substring if available ("123@@@" -> 123)
+  //   0 if no leading substring available
+  int64_t value;
+  EXPECT_FALSE(safe_strto64_base("04069@@@", &value, 10));
+  EXPECT_EQ(4069, value);
+
+  EXPECT_FALSE(safe_strto64_base("04069@@@", &value, 8));
+  EXPECT_EQ(0406, value);
+
+  EXPECT_FALSE(safe_strto64_base("04069balloons", &value, 10));
+  EXPECT_EQ(4069, value);
+
+  EXPECT_FALSE(safe_strto64_base("04069balloons", &value, 16));
+  EXPECT_EQ(0x4069ba, value);
+
+  EXPECT_FALSE(safe_strto64_base("@@@", &value, 10));
+  EXPECT_EQ(0, value);  // there was no leading substring
+}
+
 const size_t kNumRandomTests = 10000;
 
-template <typename IntType>
-void test_random_integer_parse_base(bool (*parse_func)(absl::string_view,
-                                                       IntType* value,
-                                                       int base)) {
-  using RandomEngine = std::minstd_rand0;
-  std::random_device rd;
-  RandomEngine rng(rd());
+template <typename IntType,
+          bool parse_func(absl::string_view, IntType* value, int base)>
+void test_random_integer_parse_base() {
+  absl::InsecureBitGen rng;
   std::uniform_int_distribution<IntType> random_int(
       std::numeric_limits<IntType>::min());
-  std::uniform_int_distribution<int> random_base(2, 35);
+  std::uniform_int_distribution<int> random_base(2, 36);
   for (size_t i = 0; i < kNumRandomTests; i++) {
     IntType value = random_int(rng);
     int base = random_base(rng);
@@ -640,17 +1473,153 @@ void test_random_integer_parse_base(bool (*parse_func)(absl::string_view,
   }
 }
 
+TEST(stringtest, safe_strto16_random) {
+  test_random_integer_parse_base<int16_t, safe_strto16_base>();
+}
 TEST(stringtest, safe_strto32_random) {
-  test_random_integer_parse_base<int32_t>(&safe_strto32_base);
+  test_random_integer_parse_base<int32_t, safe_strto32_base>();
 }
 TEST(stringtest, safe_strto64_random) {
-  test_random_integer_parse_base<int64_t>(&safe_strto64_base);
+  test_random_integer_parse_base<int64_t, safe_strto64_base>();
+}
+TEST(stringtest, safe_strtou16_random) {
+  test_random_integer_parse_base<uint16_t, safe_strtou16_base>();
 }
 TEST(stringtest, safe_strtou32_random) {
-  test_random_integer_parse_base<uint32_t>(&safe_strtou32_base);
+  test_random_integer_parse_base<uint32_t, safe_strtou32_base>();
 }
 TEST(stringtest, safe_strtou64_random) {
-  test_random_integer_parse_base<uint64_t>(&safe_strtou64_base);
+  test_random_integer_parse_base<uint64_t, safe_strtou64_base>();
+}
+TEST(stringtest, safe_strtou128_random) {
+  // random number generators don't work for uint128 so this code must be custom
+  // implemented for uint128, but is generally the same as what's above.
+  // test_random_integer_parse_base<absl::uint128>(
+  //     &absl::numbers_internal::safe_strtou128_base);
+  using IntType = absl::uint128;
+  constexpr auto parse_func = &absl::numbers_internal::safe_strtou128_base;
+
+  absl::InsecureBitGen rng;
+  std::uniform_int_distribution<uint64_t> random_uint64(
+      std::numeric_limits<uint64_t>::min());
+  std::uniform_int_distribution<int> random_base(2, 36);
+
+  for (size_t i = 0; i < kNumRandomTests; i++) {
+    IntType value = random_uint64(rng);
+    value = (value << 64) + random_uint64(rng);
+    int base = random_base(rng);
+    std::string str_value;
+    EXPECT_TRUE(Itoa<IntType>(value, base, &str_value));
+    IntType parsed_value;
+
+    // Test successful parse
+    EXPECT_TRUE(parse_func(str_value, &parsed_value, base));
+    EXPECT_EQ(parsed_value, value);
+
+    // Test overflow
+    EXPECT_FALSE(
+        parse_func(absl::StrCat(std::numeric_limits<IntType>::max(), value),
+                   &parsed_value, base));
+
+    // Test underflow
+    EXPECT_FALSE(parse_func(absl::StrCat("-", value), &parsed_value, base));
+  }
+}
+TEST(stringtest, safe_strto128_random) {
+  // random number generators don't work for int128 so this code must be custom
+  // implemented for int128, but is generally the same as what's above.
+  // test_random_integer_parse_base<absl::int128>(
+  //     &absl::numbers_internal::safe_strto128_base);
+  using IntType = absl::int128;
+  constexpr auto parse_func = &absl::numbers_internal::safe_strto128_base;
+
+  absl::InsecureBitGen rng;
+  std::uniform_int_distribution<int64_t> random_int64(
+      std::numeric_limits<int64_t>::min());
+  std::uniform_int_distribution<uint64_t> random_uint64(
+      std::numeric_limits<uint64_t>::min());
+  std::uniform_int_distribution<int> random_base(2, 36);
+
+  for (size_t i = 0; i < kNumRandomTests; ++i) {
+    int64_t high = random_int64(rng);
+    uint64_t low = random_uint64(rng);
+    IntType value = absl::MakeInt128(high, low);
+
+    int base = random_base(rng);
+    std::string str_value;
+    EXPECT_TRUE(Itoa<IntType>(value, base, &str_value));
+    IntType parsed_value;
+
+    // Test successful parse
+    EXPECT_TRUE(parse_func(str_value, &parsed_value, base));
+    EXPECT_EQ(parsed_value, value);
+
+    // Test overflow
+    EXPECT_FALSE(
+        parse_func(absl::StrCat(std::numeric_limits<IntType>::max(), value),
+                   &parsed_value, base));
+
+    // Test underflow
+    EXPECT_FALSE(
+        parse_func(absl::StrCat(std::numeric_limits<IntType>::min(), value),
+                   &parsed_value, base));
+  }
+}
+
+TEST(stringtest, safe_strtou8_exhaustive) {
+  // Testing the entire space for uint8_t since it is small.
+  using IntType = uint8_t;
+  constexpr auto parse_func = &absl::numbers_internal::safe_strtou8_base;
+
+  for (int i = std::numeric_limits<IntType>::min();
+       i <= std::numeric_limits<IntType>::max(); i++) {
+    IntType value = static_cast<IntType>(i);
+    for (int base = 2; base <= 36; base++) {
+      std::string str_value;
+      EXPECT_TRUE(Itoa<IntType>(value, base, &str_value));
+      IntType parsed_value;
+
+      // Test successful parse
+      EXPECT_TRUE(parse_func(str_value, &parsed_value, base));
+      EXPECT_EQ(parsed_value, value);
+
+      // Test overflow
+      EXPECT_FALSE(
+          parse_func(absl::StrCat(std::numeric_limits<IntType>::max(), value),
+                     &parsed_value, base));
+      // Test underflow
+      EXPECT_FALSE(parse_func(absl::StrCat("-", value), &parsed_value, base));
+    }
+  }
+}
+
+TEST(stringtest, safe_strto8_exhaustive) {
+  // Testing the entire space for int8_t since it is small.
+  using IntType = int8_t;
+  constexpr auto parse_func = &absl::numbers_internal::safe_strto8_base;
+
+  for (int i = std::numeric_limits<IntType>::min();
+       i <= std::numeric_limits<IntType>::max(); i++) {
+    IntType value = static_cast<IntType>(i);
+    for (int base = 2; base <= 36; base++) {
+      std::string str_value;
+      EXPECT_TRUE(Itoa<IntType>(value, base, &str_value));
+      IntType parsed_value;
+
+      // Test successful parse
+      EXPECT_TRUE(parse_func(str_value, &parsed_value, base));
+      EXPECT_EQ(parsed_value, value);
+
+      // Test overflow
+      EXPECT_FALSE(
+          parse_func(absl::StrCat(std::numeric_limits<IntType>::max(), value),
+                     &parsed_value, base));
+      // Test underflow
+      EXPECT_FALSE(
+          parse_func(absl::StrCat(std::numeric_limits<IntType>::min(), value),
+                     &parsed_value, base));
+    }
+  }
 }
 
 TEST(stringtest, safe_strtou32_base) {
@@ -713,11 +1682,11 @@ TEST(stringtest, safe_strtou64_base_length_delimited) {
   }
 }
 
-// feenableexcept() and fedisableexcept() are missing on macOS, MSVC,
-// and WebAssembly.
-#if defined(_MSC_VER) || defined(__APPLE__) || defined(__EMSCRIPTEN__)
-#define ABSL_MISSING_FEENABLEEXCEPT 1
-#define ABSL_MISSING_FEDISABLEEXCEPT 1
+// feenableexcept() and fedisableexcept() are extensions supported by some libc
+// implementations.
+#if defined(__GLIBC__) || defined(__BIONIC__)
+#define ABSL_HAVE_FEENABLEEXCEPT 1
+#define ABSL_HAVE_FEDISABLEEXCEPT 1
 #endif
 
 class SimpleDtoaTest : public testing::Test {
@@ -725,7 +1694,7 @@ class SimpleDtoaTest : public testing::Test {
   void SetUp() override {
     // Store the current floating point env & clear away any pending exceptions.
     feholdexcept(&fp_env_);
-#ifndef ABSL_MISSING_FEENABLEEXCEPT
+#ifdef ABSL_HAVE_FEENABLEEXCEPT
     // Turn on floating point exceptions.
     feenableexcept(FE_DIVBYZERO | FE_INVALID | FE_OVERFLOW);
 #endif
@@ -735,7 +1704,7 @@ class SimpleDtoaTest : public testing::Test {
     // Restore the floating point environment to the original state.
     // In theory fedisableexcept is unnecessary; fesetenv will also do it.
     // In practice, our toolchains have subtle bugs.
-#ifndef ABSL_MISSING_FEDISABLEEXCEPT
+#ifdef ABSL_HAVE_FEDISABLEEXCEPT
     fedisableexcept(FE_DIVBYZERO | FE_INVALID | FE_OVERFLOW);
 #endif
     fesetenv(&fp_env_);
@@ -750,6 +1719,37 @@ class SimpleDtoaTest : public testing::Test {
   fenv_t fp_env_;
 };
 
+TEST(SimpleDtoa, HighPrecisionIsLocaleIndependent) {
+  // absl::HighPrecision(double) routes through RoundTripDoubleToBuffer(), which
+  // used to leak the global C locale's radix character (e.g. ',' under de_DE)
+  // into its output.  HighPrecision() promises a value that SimpleAtod() reads
+  // back exactly, and SimpleAtod() only accepts '.', so the radix must stay '.'
+  // regardless of the active locale.
+  std::string old_locale = setlocale(LC_NUMERIC, nullptr);
+  auto restore_locale =
+      absl::MakeCleanup([&] { setlocale(LC_NUMERIC, old_locale.c_str()); });
+  const char* comma_locales[] = {"de_DE.UTF-8", "de_DE", "fr_FR.UTF-8", "fr_FR",
+                                 "nl_NL.UTF-8"};
+  bool changed = false;
+  for (const char* loc : comma_locales) {
+    if (setlocale(LC_NUMERIC, loc) != nullptr) {
+      changed = true;
+      break;
+    }
+  }
+  if (!changed) {
+    GTEST_SKIP() << "No comma-radix locale available on this system.";
+  }
+  EXPECT_EQ(absl::StrCat(absl::HighPrecision(0.5)), "0.5");
+  EXPECT_EQ(absl::StrCat(absl::HighPrecision(-1.25)), "-1.25");
+  EXPECT_EQ(absl::StrCat(absl::HighPrecision(3.14159265358979)),
+            "3.14159265358979");
+  double parsed = 0;
+  EXPECT_TRUE(
+      absl::SimpleAtod(absl::StrCat(absl::HighPrecision(0.1)), &parsed));
+  EXPECT_EQ(parsed, 0.1);
+}
+
 // Run the given runnable functor for "cases" test cases, chosen over the
 // available range of float.  pi and e and 1/e are seeded, and then all
 // available integer powers of 2 and 10 are multiplied against them.  In
@@ -813,6 +1813,39 @@ void ExhaustiveFloat(uint32_t cases, R&& runnable) {
   }
 }
 
+TEST_F(SimpleDtoaTest, ExhaustiveFloatToBuffer) {
+  uint64_t test_count = 0;
+  std::vector<float> mismatches;
+  ExhaustiveFloat(kFloatNumCases, [&](float f) {
+    if (f != f) return;  // rule out NaNs
+    ++test_count;
+    char fastbuf[absl::numbers_internal::kFastToBufferSize];
+    absl::numbers_internal::RoundTripFloatToBuffer(f, fastbuf);
+    float round_trip = strtof(fastbuf, nullptr);
+    if (f != round_trip) {
+      mismatches.push_back(f);
+      if (mismatches.size() < 10) {
+        LOG(ERROR) << "Round-trip failure with float.  f=" << f << "="
+                   << ToNineDigits(f) << " fast=" << fastbuf
+                   << " strtof=" << ToNineDigits(round_trip);
+      }
+    }
+  });
+  if (!mismatches.empty()) {
+    EXPECT_EQ(mismatches.size(), 0);
+    for (size_t i = 0; i < mismatches.size(); ++i) {
+      if (i > 100) i = mismatches.size() - 1;
+      float f = mismatches[i];
+      char buf[absl::numbers_internal::kFastToBufferSize];
+      float rt = strtof(buf, nullptr);
+      LOG(ERROR) << "Mismatch #" << i << "  f=" << f << " (" << ToNineDigits(f)
+                 << ") fast='"
+                 << absl::numbers_internal::RoundTripFloatToBuffer(f, buf)
+                 << "' rt=" << ToNineDigits(rt);
+    }
+  }
+}
+
 TEST_F(SimpleDtoaTest, ExhaustiveDoubleToSixDigits) {
   uint64_t test_count = 0;
   std::vector<double> mismatches;
@@ -826,11 +1859,9 @@ TEST_F(SimpleDtoaTest, ExhaustiveDoubleToSixDigits) {
     if (strcmp(sixdigitsbuf, snprintfbuf) != 0) {
       mismatches.push_back(d);
       if (mismatches.size() < 10) {
-        ABSL_RAW_LOG(ERROR, "%s",
-                     absl::StrCat("Six-digit failure with double.  ", "d=", d,
-                                  "=", d, " sixdigits=", sixdigitsbuf,
-                                  " printf(%g)=", snprintfbuf)
-                         .c_str());
+        LOG(ERROR) << "Six-digit failure with double.  d=" << d
+                   << " sixdigits=" << sixdigitsbuf
+                   << " printf(%g)=" << snprintfbuf;
       }
     }
   };
@@ -878,12 +1909,10 @@ TEST_F(SimpleDtoaTest, ExhaustiveDoubleToSixDigits) {
       if (kFloatNumCases >= 1e9) {
         // The exhaustive test takes a very long time, so log progress.
         char buf[kSixDigitsToBufferSize];
-        ABSL_RAW_LOG(
-            INFO, "%s",
-            absl::StrCat("Exp ", exponent, " powten=", powten, "(", powten,
-                         ") (",
-                         std::string(buf, SixDigitsToBuffer(powten, buf)), ")")
-                .c_str());
+        LOG(INFO) << "Exp " << exponent << " powten=" << powten << "(" << powten
+                  << ") ("
+                  << absl::string_view(buf, SixDigitsToBuffer(powten, buf))
+                  << ")";
       }
       for (int digits : digit_testcases) {
         if (exponent == 308 && digits >= 179769) break;  // don't overflow!
@@ -908,24 +1937,149 @@ TEST_F(SimpleDtoaTest, ExhaustiveDoubleToSixDigits) {
       double before = nextafter(d, 0.0);
       double after = nextafter(d, 1.7976931348623157e308);
       char b1[32], b2[kSixDigitsToBufferSize];
-      ABSL_RAW_LOG(
-          ERROR, "%s",
-          absl::StrCat(
-              "Mismatch #", i, "  d=", d, " (", ToNineDigits(d), ")",
-              " sixdigits='", sixdigitsbuf, "'", " snprintf='", snprintfbuf,
-              "'", " Before.=", PerfectDtoa(before), " ",
-              (SixDigitsToBuffer(before, b2), b2),
-              " vs snprintf=", (snprintf(b1, sizeof(b1), "%g", before), b1),
-              " Perfect=", PerfectDtoa(d), " ", (SixDigitsToBuffer(d, b2), b2),
-              " vs snprintf=", (snprintf(b1, sizeof(b1), "%g", d), b1),
-              " After.=.", PerfectDtoa(after), " ",
-              (SixDigitsToBuffer(after, b2), b2),
-              " vs snprintf=", (snprintf(b1, sizeof(b1), "%g", after), b1))
-              .c_str());
+      LOG(ERROR) << "Mismatch #" << i << "  d=" << d << " (" << ToNineDigits(d)
+                 << ") sixdigits='" << sixdigitsbuf << "' snprintf='"
+                 << snprintfbuf << "' Before.=" << PerfectDtoa(before) << " "
+                 << (SixDigitsToBuffer(before, b2), b2) << " vs snprintf="
+                 << (snprintf(b1, sizeof(b1), "%g", before), b1)
+                 << " Perfect=" << PerfectDtoa(d) << " "
+                 << (SixDigitsToBuffer(d, b2), b2)
+                 << " vs snprintf=" << (snprintf(b1, sizeof(b1), "%g", d), b1)
+                 << " After.=." << PerfectDtoa(after) << " "
+                 << (SixDigitsToBuffer(after, b2), b2) << " vs snprintf="
+                 << (snprintf(b1, sizeof(b1), "%g", after), b1);
     }
   }
 }
 
+TEST(StrToInt8, Partial) {
+  struct Int8TestLine {
+    std::string input;
+    bool status;
+    int8_t value;
+  };
+  const int8_t int8_min = std::numeric_limits<int8_t>::min();
+  const int8_t int8_max = std::numeric_limits<int8_t>::max();
+  Int8TestLine int8_test_line[] = {
+      {"", false, 0},
+      {" ", false, 0},
+      {"-", false, 0},
+      {"123@@@", false, 123},
+      {absl::StrCat(int8_min, int8_max), false, int8_min},
+      {absl::StrCat(int8_max, int8_max), false, int8_max},
+  };
+
+  for (const Int8TestLine& test_line : int8_test_line) {
+    int8_t value = -2;
+    bool status = safe_strto8_base(test_line.input, &value, 10);
+    EXPECT_EQ(test_line.status, status) << test_line.input;
+    EXPECT_EQ(test_line.value, value) << test_line.input;
+    value = -2;
+    status = safe_strto8_base(test_line.input, &value, 10);
+    EXPECT_EQ(test_line.status, status) << test_line.input;
+    EXPECT_EQ(test_line.value, value) << test_line.input;
+    value = -2;
+    status = safe_strto8_base(absl::string_view(test_line.input), &value, 10);
+    EXPECT_EQ(test_line.status, status) << test_line.input;
+    EXPECT_EQ(test_line.value, value) << test_line.input;
+  }
+}
+
+TEST(StrToUint8, Partial) {
+  struct Uint8TestLine {
+    std::string input;
+    bool status;
+    uint8_t value;
+  };
+  const uint8_t uint8_max = std::numeric_limits<uint8_t>::max();
+  Uint8TestLine uint8_test_line[] = {
+      {"", false, 0},
+      {" ", false, 0},
+      {"-", false, 0},
+      {"123@@@", false, 123},
+      {absl::StrCat(uint8_max, uint8_max), false, uint8_max},
+  };
+
+  for (const Uint8TestLine& test_line : uint8_test_line) {
+    uint8_t value = 2;
+    bool status = safe_strtou8_base(test_line.input, &value, 10);
+    EXPECT_EQ(test_line.status, status) << test_line.input;
+    EXPECT_EQ(test_line.value, value) << test_line.input;
+    value = 2;
+    status = safe_strtou8_base(test_line.input, &value, 10);
+    EXPECT_EQ(test_line.status, status) << test_line.input;
+    EXPECT_EQ(test_line.value, value) << test_line.input;
+    value = 2;
+    status = safe_strtou8_base(absl::string_view(test_line.input), &value, 10);
+    EXPECT_EQ(test_line.status, status) << test_line.input;
+    EXPECT_EQ(test_line.value, value) << test_line.input;
+  }
+}
+
+TEST(StrToInt16, Partial) {
+  struct Int16TestLine {
+    std::string input;
+    bool status;
+    int16_t value;
+  };
+  const int16_t int16_min = std::numeric_limits<int16_t>::min();
+  const int16_t int16_max = std::numeric_limits<int16_t>::max();
+  Int16TestLine int16_test_line[] = {
+      {"", false, 0},
+      {" ", false, 0},
+      {"-", false, 0},
+      {"123@@@", false, 123},
+      {absl::StrCat(int16_min, int16_max), false, int16_min},
+      {absl::StrCat(int16_max, int16_max), false, int16_max},
+  };
+
+  for (const Int16TestLine& test_line : int16_test_line) {
+    int16_t value = -2;
+    bool status = safe_strto16_base(test_line.input, &value, 10);
+    EXPECT_EQ(test_line.status, status) << test_line.input;
+    EXPECT_EQ(test_line.value, value) << test_line.input;
+    value = -2;
+    status = safe_strto16_base(test_line.input, &value, 10);
+    EXPECT_EQ(test_line.status, status) << test_line.input;
+    EXPECT_EQ(test_line.value, value) << test_line.input;
+    value = -2;
+    status = safe_strto16_base(absl::string_view(test_line.input), &value, 10);
+    EXPECT_EQ(test_line.status, status) << test_line.input;
+    EXPECT_EQ(test_line.value, value) << test_line.input;
+  }
+}
+
+TEST(StrToUint16, Partial) {
+  struct Uint16TestLine {
+    std::string input;
+    bool status;
+    uint16_t value;
+  };
+  const uint16_t uint16_max = std::numeric_limits<uint16_t>::max();
+  Uint16TestLine uint16_test_line[] = {
+      {"", false, 0},
+      {" ", false, 0},
+      {"-", false, 0},
+      {"123@@@", false, 123},
+      {absl::StrCat(uint16_max, uint16_max), false, uint16_max},
+  };
+
+  for (const Uint16TestLine& test_line : uint16_test_line) {
+    uint16_t value = 2;
+    bool status = safe_strtou16_base(test_line.input, &value, 10);
+    EXPECT_EQ(test_line.status, status) << test_line.input;
+    EXPECT_EQ(test_line.value, value) << test_line.input;
+    value = 2;
+    status = safe_strtou16_base(test_line.input, &value, 10);
+    EXPECT_EQ(test_line.status, status) << test_line.input;
+    EXPECT_EQ(test_line.value, value) << test_line.input;
+    value = 2;
+    status = safe_strtou16_base(absl::string_view(test_line.input), &value, 10);
+    EXPECT_EQ(test_line.status, status) << test_line.input;
+    EXPECT_EQ(test_line.value, value) << test_line.input;
+  }
+}
+
 TEST(StrToInt32, Partial) {
   struct Int32TestLine {
     std::string input;
@@ -1184,4 +2338,53 @@ TEST(StrToUint64Base, PrefixOnly) {
   }
 }
 
+void TestFastHexToBufferZeroPad16(uint64_t v) {
+  char buf[16];
+  auto digits = absl::numbers_internal::FastHexToBufferZeroPad16(v, buf);
+  absl::string_view res(buf, 16);
+  char buf2[17];
+  snprintf(buf2, sizeof(buf2), "%016" PRIx64, v);
+  EXPECT_EQ(res, buf2) << v;
+  size_t expected_digits = snprintf(buf2, sizeof(buf2), "%" PRIx64, v);
+  EXPECT_EQ(digits, expected_digits) << v;
+}
+
+TEST(FastHexToBufferZeroPad16, Smoke) {
+  TestFastHexToBufferZeroPad16(std::numeric_limits<uint64_t>::min());
+  TestFastHexToBufferZeroPad16(std::numeric_limits<uint64_t>::max());
+  TestFastHexToBufferZeroPad16(std::numeric_limits<int64_t>::min());
+  TestFastHexToBufferZeroPad16(std::numeric_limits<int64_t>::max());
+  absl::BitGen rng;
+  for (int i = 0; i < 100000; ++i) {
+    TestFastHexToBufferZeroPad16(
+        absl::LogUniform(rng, std::numeric_limits<uint64_t>::min(),
+                         std::numeric_limits<uint64_t>::max()));
+  }
+}
+
+template <typename Int>
+void ExpectWritesNull() {
+  {
+    char buf[absl::numbers_internal::kFastToBufferSize];
+    Int x = std::numeric_limits<Int>::min();
+    EXPECT_THAT(absl::numbers_internal::FastIntToBuffer(x, buf), Pointee('\0'));
+  }
+  {
+    char buf[absl::numbers_internal::kFastToBufferSize];
+    Int x = std::numeric_limits<Int>::max();
+    EXPECT_THAT(absl::numbers_internal::FastIntToBuffer(x, buf), Pointee('\0'));
+  }
+}
+
+TEST(FastIntToBuffer, WritesNull) {
+  ExpectWritesNull<int8_t>();
+  ExpectWritesNull<uint8_t>();
+  ExpectWritesNull<int16_t>();
+  ExpectWritesNull<uint16_t>();
+  ExpectWritesNull<int32_t>();
+  ExpectWritesNull<uint32_t>();
+  ExpectWritesNull<int64_t>();
+  ExpectWritesNull<uint32_t>();
+}
+
 }  // namespace
diff --git a/absl/strings/resize_and_overwrite.h b/absl/strings/resize_and_overwrite.h
new file mode 100644
index 00000000000..4d2e2b88869
--- /dev/null
+++ b/absl/strings/resize_and_overwrite.h
@@ -0,0 +1,198 @@
+// Copyright 2025 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: resize_and_overwrite.h
+// -----------------------------------------------------------------------------
+//
+// This file contains a polyfill for C++23's
+// std::basic_string<CharT,Traits,Allocator>::resize_and_overwrite
+//
+// The polyfill takes the form of a free function:
+
+// template<typename T, typename Op>
+// void StringResizeAndOverwrite(T& str, typename T::size_type count, Op op);
+//
+// This avoids the cost of initializing a suitably-sized std::string when it is
+// intended to be used as a char array, for example, to be populated by a
+// C-style API.
+//
+// Example usage:
+//
+// std::string IntToString(int n) {
+//   std::string result;
+//   constexpr size_t kMaxIntChars = 10;
+//   absl::StringResizeAndOverwrite(
+//       result, kMaxIntChars, [n](char* buffer, size_t buffer_size) {
+//         return snprintf(buffer, buffer_size, "%d", n);
+//       });
+//   return result;
+//  }
+//
+// https://en.cppreference.com/w/cpp/string/basic_string/resize_and_overwrite.html
+// https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p1072r10.html
+
+#ifndef ABSL_STRINGS_RESIZE_AND_OVERWRITE_H_
+#define ABSL_STRINGS_RESIZE_AND_OVERWRITE_H_
+
+#include <cstddef>
+#include <string>  // IWYU pragma: keep
+#include <type_traits>
+#include <utility>
+
+#include "absl/base/config.h"
+#include "absl/base/dynamic_annotations.h"
+#include "absl/base/internal/hardening.h"
+#include "absl/base/macros.h"
+#include "absl/base/optimization.h"
+#include "absl/base/throw_delegate.h"
+
+#if defined(__cpp_lib_string_resize_and_overwrite) && \
+    __cpp_lib_string_resize_and_overwrite >= 202110L
+#define ABSL_INTERNAL_HAS_RESIZE_AND_OVERWRITE 1
+#endif
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+namespace strings_internal {
+
+#ifndef ABSL_INTERNAL_HAS_RESIZE_AND_OVERWRITE
+
+inline size_t ProbeResizeAndOverwriteOp(char*, size_t) { return 0; }
+
+// Prior to C++23, Google's libc++ backports resize_and_overwrite as
+// __google_nonstandard_backport_resize_and_overwrite
+template <typename T, typename = void>
+struct has__google_nonstandard_backport_resize_and_overwrite : std::false_type {
+};
+
+template <typename T>
+struct has__google_nonstandard_backport_resize_and_overwrite<
+    T,
+    std::void_t<
+        decltype(std::declval<T&>()
+                     .__google_nonstandard_backport_resize_and_overwrite(
+                         std::declval<size_t>(), ProbeResizeAndOverwriteOp))>>
+    : std::true_type {};
+
+// Prior to C++23, the version of libstdc++ that shipped with GCC >= 14
+// has __resize_and_overwrite.
+template <typename T, typename = void>
+struct has__resize_and_overwrite : std::false_type {};
+
+template <typename T>
+struct has__resize_and_overwrite<
+    T, std::void_t<decltype(std::declval<T&>().__resize_and_overwrite(
+           std::declval<size_t>(), ProbeResizeAndOverwriteOp))>>
+    : std::true_type {};
+
+// libc++ used  __resize_default_init to achieve uninitialized string resizes
+// before removing it September 2025, in favor of resize_and_overwrite.
+// https://github.com/llvm/llvm-project/commit/92f5d8df361bb1bb6dea88f86faeedfd295ab970
+template <typename T, typename = void>
+struct has__resize_default_init : std::false_type {};
+
+template <typename T>
+struct has__resize_default_init<
+    T, std::void_t<decltype(std::declval<T&>().__resize_default_init(42))>>
+    : std::true_type {};
+
+// Prior to C++23, some versions of MSVC have _Resize_and_overwrite.
+template <typename T, typename = void>
+struct has_Resize_and_overwrite : std::false_type {};
+
+template <typename T>
+struct has_Resize_and_overwrite<
+    T, std::void_t<decltype(std::declval<T&>()._Resize_and_overwrite(
+           std::declval<size_t>(), ProbeResizeAndOverwriteOp))>>
+    : std::true_type {};
+
+#endif  // ifndef ABSL_INTERNAL_HAS_RESIZE_AND_OVERWRITE
+
+// A less-efficient fallback implementation that uses resize().
+template <typename T, typename Op>
+void StringResizeAndOverwriteFallback(T& str, typename T::size_type n, Op op) {
+  if (ABSL_PREDICT_FALSE(n > str.max_size())) {
+    ThrowStdLengthError("absl::StringResizeAndOverwrite");
+  }
+#ifdef ABSL_HAVE_MEMORY_SANITIZER
+  auto old_size = str.size();
+#endif
+  str.resize(n);
+#ifdef ABSL_HAVE_MEMORY_SANITIZER
+  if (old_size < n) {
+    ABSL_ANNOTATE_MEMORY_IS_UNINITIALIZED(str.data() + old_size, n - old_size);
+  }
+#endif
+  typename T::size_type new_size =
+      static_cast<typename T::size_type>(std::move(op)(str.data(), n));
+  absl::base_internal::HardeningAssertGE(new_size, typename T::size_type{0});
+  absl::base_internal::HardeningAssertLE(new_size, n);
+  absl::base_internal::HardeningAssert(str.data()[n] ==
+                                       typename T::value_type{});
+  str.erase(new_size);
+}
+
+template <typename T, typename Op>
+void StringResizeAndOverwriteImpl(T& str, typename T::size_type n, Op op) {
+#ifdef ABSL_INTERNAL_HAS_RESIZE_AND_OVERWRITE
+  str.resize_and_overwrite(n, std::move(op));
+#else
+  if constexpr (strings_internal::
+                    has__google_nonstandard_backport_resize_and_overwrite<
+                        T>::value) {
+    str.__google_nonstandard_backport_resize_and_overwrite(n, std::move(op));
+  } else if constexpr (strings_internal::has__resize_and_overwrite<T>::value) {
+    str.__resize_and_overwrite(n, std::move(op));
+  } else if constexpr (strings_internal::has__resize_default_init<T>::value) {
+    str.__resize_default_init(n);
+    str.__resize_default_init(
+        static_cast<typename T::size_type>(std::move(op)(str.data(), n)));
+  } else if constexpr (strings_internal::has_Resize_and_overwrite<T>::value) {
+    str._Resize_and_overwrite(n, std::move(op));
+  } else {
+    strings_internal::StringResizeAndOverwriteFallback(str, n, std::move(op));
+  }
+#endif
+}
+
+}  // namespace strings_internal
+
+// Resizes `str` to contain at most `n` characters, using the user-provided
+// operation `op` to modify the possibly indeterminate contents. `op` must
+// return the finalized length of `str`.
+//
+// Invalidates all iterators, pointers, and references into `str`, regardless
+// of whether reallocation occurs.
+//
+// `op(value_type* buf, size_t buf_size)` is allowed to write `value_type{}` to
+// `buf[buf_size]`, which facilitiates interoperation with functions that write
+// a trailing NUL. Please note that this requirement is more strict than
+// `basic_string::resize_and_overwrite()`, which allows writing an abitrary
+// value to `buf[buf_size]`.
+template <typename T, typename Op>
+void StringResizeAndOverwrite(T& str, typename T::size_type n, Op op) {
+  strings_internal::StringResizeAndOverwriteImpl(str, n, std::move(op));
+#if defined(ABSL_HAVE_MEMORY_SANITIZER)
+  __msan_check_mem_is_initialized(str.data(), str.size());
+#endif
+}
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#undef ABSL_INTERNAL_HAS_RESIZE_AND_OVERWRITE
+
+#endif  // ABSL_STRINGS_RESIZE_AND_OVERWRITE_H_
diff --git a/absl/strings/resize_and_overwrite_test.cc b/absl/strings/resize_and_overwrite_test.cc
new file mode 100644
index 00000000000..1070decd31f
--- /dev/null
+++ b/absl/strings/resize_and_overwrite_test.cc
@@ -0,0 +1,154 @@
+// Copyright 2025 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/strings/resize_and_overwrite.h"
+
+#include <algorithm>
+#include <cstddef>
+#include <string>
+
+#include "gtest/gtest.h"
+#include "absl/base/dynamic_annotations.h"
+#include "absl/log/absl_check.h"
+
+namespace {
+
+struct ResizeAndOverwriteParam {
+  size_t initial_size;
+  size_t requested_capacity;
+  size_t final_size;
+};
+
+using StringResizeAndOverwriteTest =
+    ::testing::TestWithParam<ResizeAndOverwriteParam>;
+
+TEST_P(StringResizeAndOverwriteTest, StringResizeAndOverwrite) {
+  const auto& param = GetParam();
+  std::string s(param.initial_size, 'a');
+  absl::StringResizeAndOverwrite(
+      s, param.requested_capacity, [&](char* p, size_t n) {
+        ABSL_CHECK_EQ(n, param.requested_capacity);
+        if (param.final_size >= param.initial_size) {
+          // Append case.
+          std::fill(p + param.initial_size, p + param.final_size, 'b');
+        } else if (param.final_size > 0) {
+          // Truncate case.
+          p[param.final_size - 1] = 'b';
+        }
+        p[param.final_size] = '\0';
+        return param.final_size;
+      });
+
+  std::string expected;
+  if (param.final_size >= param.initial_size) {
+    // Append case.
+    expected = std::string(param.initial_size, 'a') +
+               std::string(param.final_size - param.initial_size, 'b');
+  } else if (param.final_size > 0) {
+    // Truncate case.
+    expected = std::string(param.final_size - 1, 'a') + std::string("b");
+  }
+
+  EXPECT_EQ(s, expected);
+  EXPECT_EQ(s.c_str()[param.final_size], '\0');
+}
+
+TEST_P(StringResizeAndOverwriteTest, StringResizeAndOverwriteFallback) {
+  const auto& param = GetParam();
+  std::string s(param.initial_size, 'a');
+  absl::strings_internal::StringResizeAndOverwriteFallback(
+      s, param.requested_capacity, [&](char* p, size_t n) {
+        ABSL_CHECK_EQ(n, param.requested_capacity);
+        if (param.final_size >= param.initial_size) {
+          // Append case.
+          std::fill(p + param.initial_size, p + param.final_size, 'b');
+        } else if (param.final_size > 0) {
+          // Truncate case.
+          p[param.final_size - 1] = 'b';
+        }
+        p[param.final_size] = '\0';
+        return param.final_size;
+      });
+
+  std::string expected;
+  if (param.final_size >= param.initial_size) {
+    // Append case.
+    expected = std::string(param.initial_size, 'a') +
+               std::string(param.final_size - param.initial_size, 'b');
+  } else if (param.final_size > 0) {
+    // Truncate case.
+    expected = std::string(param.final_size - 1, 'a') + std::string("b");
+  }
+
+  EXPECT_EQ(s, expected);
+  EXPECT_EQ(s.c_str()[param.final_size], '\0');
+}
+
+#ifdef ABSL_HAVE_MEMORY_SANITIZER
+constexpr bool kMSan = true;
+#else
+constexpr bool kMSan = false;
+#endif
+
+TEST_P(StringResizeAndOverwriteTest, Initialized) {
+  if (!kMSan) {
+    GTEST_SKIP() << "Skipping test without MSan.";
+  }
+
+  const auto& param = GetParam();
+  std::string s(param.initial_size, 'a');
+
+  auto op = [&]() {
+    absl::StringResizeAndOverwrite(s, param.requested_capacity,
+                                   [&](char*, size_t) {
+                                     // Fail to initialize the buffer in full.
+                                     return param.final_size;
+                                   });
+  };
+
+  if (param.initial_size < param.final_size) {
+#ifndef NDEBUG
+    EXPECT_DEATH_IF_SUPPORTED(op(),
+                              "MemorySanitizer: use-of-uninitialized-value");
+#endif
+  } else {
+    // The string is fully initialized from the initial constructor, or we skip
+    // the check in optimized builds.
+    op();
+  }
+}
+
+// clang-format off
+INSTANTIATE_TEST_SUITE_P(StringResizeAndOverwriteTestSuite,
+                         StringResizeAndOverwriteTest,
+                         ::testing::ValuesIn<ResizeAndOverwriteParam>({
+                             // Append cases.
+                             {0,  10,  5},
+                             {10, 10, 10},
+                             {10, 15, 15},
+                             {10, 20, 15},
+                             {10, 40, 40},
+                             {10, 50, 40},
+                             {30, 35, 35},
+                             {30, 45, 35},
+                             {10, 30, 15},
+                             // Truncate cases.
+                             {15, 15, 10},
+                             {40, 40, 35},
+                             {40, 30, 10},
+                             {10, 15, 0},
+                         }));
+// clang-format on
+
+}  // namespace
diff --git a/absl/strings/str_cat.cc b/absl/strings/str_cat.cc
index 996194457fd..b79078d65a2 100644
--- a/absl/strings/str_cat.cc
+++ b/absl/strings/str_cat.cc
@@ -15,66 +15,25 @@
 #include "absl/strings/str_cat.h"
 
 #include <assert.h>
-#include <algorithm>
+
+#include <cstddef>
 #include <cstdint>
 #include <cstring>
+#include <initializer_list>
+#include <limits>
+#include <string>
 
-#include "absl/strings/ascii.h"
-#include "absl/strings/internal/resize_uninitialized.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/base/nullability.h"
+#include "absl/base/optimization.h"
+#include "absl/base/throw_delegate.h"
+#include "absl/strings/internal/append_and_overwrite.h"
+#include "absl/strings/resize_and_overwrite.h"
+#include "absl/strings/string_view.h"
 
 namespace absl {
-
-AlphaNum::AlphaNum(Hex hex) {
-  char* const end = &digits_[numbers_internal::kFastToBufferSize];
-  char* writer = end;
-  uint64_t value = hex.value;
-  static const char hexdigits[] = "0123456789abcdef";
-  do {
-    *--writer = hexdigits[value & 0xF];
-    value >>= 4;
-  } while (value != 0);
-
-  char* beg;
-  if (end - writer < hex.width) {
-    beg = end - hex.width;
-    std::fill_n(beg, writer - beg, hex.fill);
-  } else {
-    beg = writer;
-  }
-
-  piece_ = absl::string_view(beg, end - beg);
-}
-
-AlphaNum::AlphaNum(Dec dec) {
-  assert(dec.width <= numbers_internal::kFastToBufferSize);
-  char* const end = &digits_[numbers_internal::kFastToBufferSize];
-  char* const minfill = end - dec.width;
-  char* writer = end;
-  uint64_t value = dec.value;
-  bool neg = dec.neg;
-  while (value > 9) {
-    *--writer = '0' + (value % 10);
-    value /= 10;
-  }
-  *--writer = '0' + value;
-  if (neg) *--writer = '-';
-
-  ptrdiff_t fillers = writer - minfill;
-  if (fillers > 0) {
-    // Tricky: if the fill character is ' ', then it's <fill><+/-><digits>
-    // But...: if the fill character is '0', then it's <+/-><fill><digits>
-    bool add_sign_again = false;
-    if (neg && dec.fill == '0') {  // If filling with '0',
-      ++writer;                    // ignore the sign we just added
-      add_sign_again = true;       // and re-add the sign later.
-    }
-    writer -= fillers;
-    std::fill_n(writer, fillers, dec.fill);
-    if (add_sign_again) *--writer = '-';
-  }
-
-  piece_ = absl::string_view(writer, end - writer);
-}
+ABSL_NAMESPACE_BEGIN
 
 // ----------------------------------------------------------------------
 // StrCat()
@@ -83,9 +42,10 @@ AlphaNum::AlphaNum(Dec dec) {
 //    of a mix of raw C strings, string_views, strings, and integer values.
 // ----------------------------------------------------------------------
 
+namespace {
 // Append is merely a version of memcpy that returns the address of the byte
 // after the area just overwritten.
-static char* Append(char* out, const AlphaNum& x) {
+inline char* absl_nonnull Append(char* absl_nonnull out, const AlphaNum& x) {
   // memcpy is allowed to overwrite arbitrary memory, so doing this after the
   // call would force an extra fetch of x.size().
   char* after = out + x.size();
@@ -95,43 +55,90 @@ static char* Append(char* out, const AlphaNum& x) {
   return after;
 }
 
+// Safely adds size_t values, throwing std::length_error if overflow occurs.
+inline size_t SafeAdd(size_t a, size_t b) {
+  const uint64_t sum = static_cast<uint64_t>(a) + b;
+  if (ABSL_PREDICT_FALSE(sum > (std::numeric_limits<size_t>::max)())) {
+    ThrowStdLengthError("absl string append length overflow");
+  }
+  return static_cast<size_t>(sum);
+}
+
+inline size_t SafeAdd(std::initializer_list<size_t> sizes) {
+  uint64_t sum = 0;
+  for (size_t size : sizes) {
+    sum += size;
+  }
+  if (ABSL_PREDICT_FALSE(sum > (std::numeric_limits<size_t>::max)())) {
+    ThrowStdLengthError("absl string append length overflow");
+  }
+  return static_cast<size_t>(sum);
+}
+
+}  // namespace
+
 std::string StrCat(const AlphaNum& a, const AlphaNum& b) {
   std::string result;
-  absl::strings_internal::STLStringResizeUninitialized(&result,
-                                                       a.size() + b.size());
-  char* const begin = &result[0];
-  char* out = begin;
-  out = Append(out, a);
-  out = Append(out, b);
-  assert(out == begin + result.size());
+  // Use uint64_t to prevent size_t overflow. We assume it is not possible for
+  // in memory strings to overflow a uint64_t.
+  constexpr uint64_t kMaxSize = uint64_t{std::numeric_limits<size_t>::max()};
+  const uint64_t result_size =
+      static_cast<uint64_t>(a.size()) + static_cast<uint64_t>(b.size());
+  ABSL_INTERNAL_CHECK(result_size <= kMaxSize, "size_t overflow");
+  absl::StringResizeAndOverwrite(result, static_cast<size_t>(result_size),
+                                 [&a, &b](char* const begin, size_t buf_size) {
+                                   char* out = begin;
+                                   out = Append(out, a);
+                                   out = Append(out, b);
+                                   assert(out == begin + buf_size);
+                                   return buf_size;
+                                 });
   return result;
 }
 
 std::string StrCat(const AlphaNum& a, const AlphaNum& b, const AlphaNum& c) {
   std::string result;
-  strings_internal::STLStringResizeUninitialized(
-      &result, a.size() + b.size() + c.size());
-  char* const begin = &result[0];
-  char* out = begin;
-  out = Append(out, a);
-  out = Append(out, b);
-  out = Append(out, c);
-  assert(out == begin + result.size());
+  // Use uint64_t to prevent size_t overflow. We assume it is not possible for
+  // in memory strings to overflow a uint64_t.
+  constexpr uint64_t kMaxSize = uint64_t{std::numeric_limits<size_t>::max()};
+  const uint64_t result_size = static_cast<uint64_t>(a.size()) +
+                               static_cast<uint64_t>(b.size()) +
+                               static_cast<uint64_t>(c.size());
+  ABSL_INTERNAL_CHECK(result_size <= kMaxSize, "size_t overflow");
+  absl::StringResizeAndOverwrite(
+      result, static_cast<size_t>(result_size),
+      [&a, &b, &c](char* const begin, size_t buf_size) {
+        char* out = begin;
+        out = Append(out, a);
+        out = Append(out, b);
+        out = Append(out, c);
+        assert(out == begin + buf_size);
+        return buf_size;
+      });
   return result;
 }
 
 std::string StrCat(const AlphaNum& a, const AlphaNum& b, const AlphaNum& c,
                    const AlphaNum& d) {
   std::string result;
-  strings_internal::STLStringResizeUninitialized(
-      &result, a.size() + b.size() + c.size() + d.size());
-  char* const begin = &result[0];
-  char* out = begin;
-  out = Append(out, a);
-  out = Append(out, b);
-  out = Append(out, c);
-  out = Append(out, d);
-  assert(out == begin + result.size());
+  // Use uint64_t to prevent size_t overflow. We assume it is not possible for
+  // in memory strings to overflow a uint64_t.
+  constexpr uint64_t kMaxSize = uint64_t{std::numeric_limits<size_t>::max()};
+  const uint64_t result_size =
+      static_cast<uint64_t>(a.size()) + static_cast<uint64_t>(b.size()) +
+      static_cast<uint64_t>(c.size()) + static_cast<uint64_t>(d.size());
+  ABSL_INTERNAL_CHECK(result_size <= kMaxSize, "size_t overflow");
+  absl::StringResizeAndOverwrite(
+      result, static_cast<size_t>(result_size),
+      [&a, &b, &c, &d](char* const begin, size_t buf_size) {
+        char* out = begin;
+        out = Append(out, a);
+        out = Append(out, b);
+        out = Append(out, c);
+        out = Append(out, d);
+        assert(out == begin + buf_size);
+        return buf_size;
+      });
   return result;
 }
 
@@ -140,20 +147,27 @@ namespace strings_internal {
 // Do not call directly - these are not part of the public API.
 std::string CatPieces(std::initializer_list<absl::string_view> pieces) {
   std::string result;
-  size_t total_size = 0;
-  for (const absl::string_view piece : pieces) total_size += piece.size();
-  strings_internal::STLStringResizeUninitialized(&result, total_size);
-
-  char* const begin = &result[0];
-  char* out = begin;
-  for (const absl::string_view piece : pieces) {
-    const size_t this_size = piece.size();
-    if (this_size != 0) {
-      memcpy(out, piece.data(), this_size);
-      out += this_size;
-    }
+  // Use uint64_t to prevent size_t overflow. We assume it is not possible for
+  // in memory strings to overflow a uint64_t.
+  constexpr uint64_t kMaxSize = uint64_t{std::numeric_limits<size_t>::max()};
+  uint64_t total_size = 0;
+  for (absl::string_view piece : pieces) {
+    total_size += piece.size();
   }
-  assert(out == begin + result.size());
+  ABSL_INTERNAL_CHECK(total_size <= kMaxSize, "size_t overflow");
+  absl::StringResizeAndOverwrite(result, static_cast<size_t>(total_size),
+                                 [&pieces](char* const begin, size_t buf_size) {
+                                   char* out = begin;
+                                   for (absl::string_view piece : pieces) {
+                                     const size_t this_size = piece.size();
+                                     if (this_size != 0) {
+                                       memcpy(out, piece.data(), this_size);
+                                       out += this_size;
+                                     }
+                                   }
+                                   assert(out == begin + buf_size);
+                                   return buf_size;
+                                 });
   return result;
 }
 
@@ -166,80 +180,91 @@ std::string CatPieces(std::initializer_list<absl::string_view> pieces) {
   assert(((src).size() == 0) ||      \
          (uintptr_t((src).data() - (dest).data()) > uintptr_t((dest).size())))
 
-void AppendPieces(std::string* dest,
+void AppendPieces(std::string* absl_nonnull dest,
                   std::initializer_list<absl::string_view> pieces) {
-  size_t old_size = dest->size();
-  size_t total_size = old_size;
-  for (const absl::string_view piece : pieces) {
+  size_t to_append = 0;
+  for (absl::string_view piece : pieces) {
     ASSERT_NO_OVERLAP(*dest, piece);
-    total_size += piece.size();
-  }
-  strings_internal::STLStringResizeUninitialized(dest, total_size);
-
-  char* const begin = &(*dest)[0];
-  char* out = begin + old_size;
-  for (const absl::string_view piece : pieces) {
-    const size_t this_size = piece.size();
-    if (this_size != 0) {
-      memcpy(out, piece.data(), this_size);
-      out += this_size;
-    }
+    to_append = SafeAdd(to_append, piece.size());
   }
-  assert(out == begin + dest->size());
+  StringAppendAndOverwrite(*dest, to_append,
+                           [&pieces](char* const buf, size_t buf_size) {
+                             char* out = buf;
+                             for (absl::string_view piece : pieces) {
+                               const size_t this_size = piece.size();
+                               if (this_size != 0) {
+                                 memcpy(out, piece.data(), this_size);
+                                 out += this_size;
+                               }
+                             }
+                             assert(out == buf + buf_size);
+                             return buf_size;
+                           });
 }
 
 }  // namespace strings_internal
 
-void StrAppend(std::string* dest, const AlphaNum& a) {
+void StrAppend(std::string* absl_nonnull dest, const AlphaNum& a) {
   ASSERT_NO_OVERLAP(*dest, a);
-  dest->append(a.data(), a.size());
+  strings_internal::StringAppendAndOverwrite(
+      *dest, a.size(), [&a](char* const buf, size_t buf_size) {
+        char* out = buf;
+        out = Append(out, a);
+        assert(out == buf + buf_size);
+        return buf_size;
+      });
 }
 
-void StrAppend(std::string* dest, const AlphaNum& a, const AlphaNum& b) {
+void StrAppend(std::string* absl_nonnull dest, const AlphaNum& a,
+               const AlphaNum& b) {
   ASSERT_NO_OVERLAP(*dest, a);
   ASSERT_NO_OVERLAP(*dest, b);
-  std::string::size_type old_size = dest->size();
-  strings_internal::STLStringResizeUninitialized(
-      dest, old_size + a.size() + b.size());
-  char* const begin = &(*dest)[0];
-  char* out = begin + old_size;
-  out = Append(out, a);
-  out = Append(out, b);
-  assert(out == begin + dest->size());
+  strings_internal::StringAppendAndOverwrite(
+      *dest, SafeAdd(a.size(), b.size()),
+      [&a, &b](char* const buf, size_t buf_size) {
+        char* out = buf;
+        out = Append(out, a);
+        out = Append(out, b);
+        assert(out == buf + buf_size);
+        return buf_size;
+      });
 }
 
-void StrAppend(std::string* dest, const AlphaNum& a, const AlphaNum& b,
-               const AlphaNum& c) {
+void StrAppend(std::string* absl_nonnull dest, const AlphaNum& a,
+               const AlphaNum& b, const AlphaNum& c) {
   ASSERT_NO_OVERLAP(*dest, a);
   ASSERT_NO_OVERLAP(*dest, b);
   ASSERT_NO_OVERLAP(*dest, c);
-  std::string::size_type old_size = dest->size();
-  strings_internal::STLStringResizeUninitialized(
-      dest, old_size + a.size() + b.size() + c.size());
-  char* const begin = &(*dest)[0];
-  char* out = begin + old_size;
-  out = Append(out, a);
-  out = Append(out, b);
-  out = Append(out, c);
-  assert(out == begin + dest->size());
+  strings_internal::StringAppendAndOverwrite(
+      *dest, SafeAdd({a.size(), b.size(), c.size()}),
+      [&a, &b, &c](char* const buf, size_t buf_size) {
+        char* out = buf;
+        out = Append(out, a);
+        out = Append(out, b);
+        out = Append(out, c);
+        assert(out == buf + buf_size);
+        return buf_size;
+      });
 }
 
-void StrAppend(std::string* dest, const AlphaNum& a, const AlphaNum& b,
-               const AlphaNum& c, const AlphaNum& d) {
+void StrAppend(std::string* absl_nonnull dest, const AlphaNum& a,
+               const AlphaNum& b, const AlphaNum& c, const AlphaNum& d) {
   ASSERT_NO_OVERLAP(*dest, a);
   ASSERT_NO_OVERLAP(*dest, b);
   ASSERT_NO_OVERLAP(*dest, c);
   ASSERT_NO_OVERLAP(*dest, d);
-  std::string::size_type old_size = dest->size();
-  strings_internal::STLStringResizeUninitialized(
-      dest, old_size + a.size() + b.size() + c.size() + d.size());
-  char* const begin = &(*dest)[0];
-  char* out = begin + old_size;
-  out = Append(out, a);
-  out = Append(out, b);
-  out = Append(out, c);
-  out = Append(out, d);
-  assert(out == begin + dest->size());
+  strings_internal::StringAppendAndOverwrite(
+      *dest, SafeAdd({a.size(), b.size(), c.size(), d.size()}),
+      [&a, &b, &c, &d](char* const buf, size_t buf_size) {
+        char* out = buf;
+        out = Append(out, a);
+        out = Append(out, b);
+        out = Append(out, c);
+        out = Append(out, d);
+        assert(out == buf + buf_size);
+        return buf_size;
+      });
 }
 
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/str_cat.h b/absl/strings/str_cat.h
index a99aac01ebb..3ae1197d75b 100644
--- a/absl/strings/str_cat.h
+++ b/absl/strings/str_cat.h
@@ -43,27 +43,88 @@
 // Floating point numbers are formatted with six-digit precision, which is
 // the default for "std::cout <<" or printf "%g" (the same as "%.6g").
 //
+// Floating point values can also be converted to a string which, if passed to
+// `strtod()`, would produce the exact same original double (except in case of
+// NaN; all NaNs are considered the same value) by passing the number to
+// absl::HighPrecision. HighPrecision tries to keep the string short but
+// it's not guaranteed to be as short as possible.
+// See http://go/faster-double-strcat
+//
 // You can convert to hexadecimal output rather than decimal output using the
 // `Hex` type contained here. To do so, pass `Hex(my_int)` as a parameter to
 // `StrCat()` or `StrAppend()`. You may specify a minimum hex field width using
 // a `PadSpec` enum.
 //
+// User-defined types can be formatted with the `AbslStringify()` customization
+// point. The API relies on detecting an overload in the user-defined type's
+// namespace of a free (non-member) `AbslStringify()` function as a definition
+// (typically declared as a friend and implemented in-line.
+// with the following signature:
+//
+// class MyClass { ... };
+//
+// template <typename Sink>
+// void AbslStringify(Sink& sink, const MyClass& value);
+//
+// An `AbslStringify()` overload for a type should only be declared in the same
+// file and namespace as said type.
+//
+// Note that `AbslStringify()` also supports use with `absl::StrFormat()` and
+// `absl::Substitute()`.
+//
+// Example:
+//
+// struct Point {
+//   // To add formatting support to `Point`, we simply need to add a free
+//   // (non-member) function `AbslStringify()`. This method specifies how
+//   // Point should be printed when absl::StrCat() is called on it. You can add
+//   // such a free function using a friend declaration within the body of the
+//   // class. The sink parameter is a templated type to avoid requiring
+//   // dependencies.
+//   template <typename Sink> friend void AbslStringify(Sink&
+//   sink, const Point& p) {
+//     absl::Format(&sink, "(%v, %v)", p.x, p.y);
+//   }
+//
+//   int x;
+//   int y;
+// };
 // -----------------------------------------------------------------------------
 
 #ifndef ABSL_STRINGS_STR_CAT_H_
 #define ABSL_STRINGS_STR_CAT_H_
 
+#include <algorithm>
 #include <array>
+#include <cassert>
+#include <cstddef>
 #include <cstdint>
+#include <cstring>
+#include <initializer_list>
+#include <limits>
 #include <string>
 #include <type_traits>
+#include <utility>
 #include <vector>
 
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/macros.h"
+#include "absl/base/nullability.h"
 #include "absl/base/port.h"
+#include "absl/meta/type_traits.h"
+#include "absl/strings/has_absl_stringify.h"
+#include "absl/strings/internal/stringify_sink.h"
 #include "absl/strings/numbers.h"
+#include "absl/strings/resize_and_overwrite.h"
 #include "absl/strings/string_view.h"
 
+#if !defined(ABSL_USES_STD_STRING_VIEW)
+#include <string_view>
+#endif
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 namespace strings_internal {
 // AlphaNumBuffer allows a way to pass a string to StrCat without having to do
@@ -136,33 +197,50 @@ struct Hex {
   char fill;
 
   template <typename Int>
-  explicit Hex(
-      Int v, PadSpec spec = absl::kNoPad,
-      typename std::enable_if<sizeof(Int) == 1 &&
-                              !std::is_pointer<Int>::value>::type* = nullptr)
+  explicit Hex(Int v, PadSpec spec = absl::kNoPad,
+               std::enable_if_t<sizeof(Int) == 1 && !std::is_pointer_v<Int>,
+                                bool> = true)
       : Hex(spec, static_cast<uint8_t>(v)) {}
   template <typename Int>
-  explicit Hex(
-      Int v, PadSpec spec = absl::kNoPad,
-      typename std::enable_if<sizeof(Int) == 2 &&
-                              !std::is_pointer<Int>::value>::type* = nullptr)
+  explicit Hex(Int v, PadSpec spec = absl::kNoPad,
+               std::enable_if_t<sizeof(Int) == 2 && !std::is_pointer_v<Int>,
+                                bool> = true)
       : Hex(spec, static_cast<uint16_t>(v)) {}
   template <typename Int>
-  explicit Hex(
-      Int v, PadSpec spec = absl::kNoPad,
-      typename std::enable_if<sizeof(Int) == 4 &&
-                              !std::is_pointer<Int>::value>::type* = nullptr)
+  explicit Hex(Int v, PadSpec spec = absl::kNoPad,
+               std::enable_if_t<sizeof(Int) == 4 && !std::is_pointer_v<Int>,
+                                bool> = true)
       : Hex(spec, static_cast<uint32_t>(v)) {}
   template <typename Int>
-  explicit Hex(
-      Int v, PadSpec spec = absl::kNoPad,
-      typename std::enable_if<sizeof(Int) == 8 &&
-                              !std::is_pointer<Int>::value>::type* = nullptr)
+  explicit Hex(Int v, PadSpec spec = absl::kNoPad,
+               std::enable_if_t<sizeof(Int) == 8 && !std::is_pointer_v<Int>,
+                                bool> = true)
       : Hex(spec, static_cast<uint64_t>(v)) {}
   template <typename Pointee>
-  explicit Hex(Pointee* v, PadSpec spec = absl::kNoPad)
+  explicit Hex(Pointee* absl_nullable v, PadSpec spec = absl::kNoPad)
       : Hex(spec, reinterpret_cast<uintptr_t>(v)) {}
 
+  template <typename S>
+  friend void AbslStringify(S& sink, Hex hex) {
+    static_assert(
+        numbers_internal::kFastToBufferSize >= 32,
+        "This function only works when output buffer >= 32 bytes long");
+    char buffer[numbers_internal::kFastToBufferSize];
+    char* const end = &buffer[numbers_internal::kFastToBufferSize];
+    auto real_width =
+        absl::numbers_internal::FastHexToBufferZeroPad16(hex.value, end - 16);
+    if (real_width >= hex.width) {
+      sink.Append(absl::string_view(end - real_width, real_width));
+    } else {
+      // Pad first 16 chars because FastHexToBufferZeroPad16 pads only to 16 and
+      // max pad width can be up to 20.
+      std::memset(end - 32, hex.fill, 16);
+      // Patch up everything else up to the real_width.
+      std::memset(end - real_width - 16, hex.fill, 16);
+      sink.Append(absl::string_view(end - hex.width, hex.width));
+    }
+  }
+
  private:
   Hex(PadSpec spec, uint64_t v)
       : value(v),
@@ -188,97 +266,193 @@ struct Dec {
 
   template <typename Int>
   explicit Dec(Int v, PadSpec spec = absl::kNoPad,
-               typename std::enable_if<(sizeof(Int) <= 8)>::type* = nullptr)
+               std::enable_if_t<sizeof(Int) <= 8, bool> = true)
       : value(v >= 0 ? static_cast<uint64_t>(v)
                      : uint64_t{0} - static_cast<uint64_t>(v)),
-        width(spec == absl::kNoPad
-                  ? 1
-                  : spec >= absl::kSpacePad2 ? spec - absl::kSpacePad2 + 2
-                                             : spec - absl::kZeroPad2 + 2),
+        width(spec == absl::kNoPad       ? 1
+              : spec >= absl::kSpacePad2 ? spec - absl::kSpacePad2 + 2
+                                         : spec - absl::kZeroPad2 + 2),
         fill(spec >= absl::kSpacePad2 ? ' ' : '0'),
         neg(v < 0) {}
+
+  template <typename S>
+  friend void AbslStringify(S& sink, Dec dec) {
+    assert(dec.width <= numbers_internal::kFastToBufferSize);
+    char buffer[numbers_internal::kFastToBufferSize];
+    char* const end = &buffer[numbers_internal::kFastToBufferSize];
+    char* const minfill = end - dec.width;
+    char* writer = end;
+    uint64_t val = dec.value;
+    while (val > 9) {
+      *--writer = '0' + (val % 10);
+      val /= 10;
+    }
+    *--writer = '0' + static_cast<char>(val);
+    if (dec.neg) *--writer = '-';
+
+    ptrdiff_t fillers = writer - minfill;
+    if (fillers > 0) {
+      // Tricky: if the fill character is ' ', then it's <fill><+/-><digits>
+      // But...: if the fill character is '0', then it's <+/-><fill><digits>
+      bool add_sign_again = false;
+      if (dec.neg && dec.fill == '0') {  // If filling with '0',
+        ++writer;                    // ignore the sign we just added
+        add_sign_again = true;       // and re-add the sign later.
+      }
+      writer -= fillers;
+      std::fill_n(writer, fillers, dec.fill);
+      if (add_sign_again) *--writer = '-';
+    }
+
+    sink.Append(absl::string_view(writer, static_cast<size_t>(end - writer)));
+  }
 };
 
+// -----------------------------------------------------------------------------
+// HighPrecision
+// -----------------------------------------------------------------------------
+//
+// Converts floating point values to a string which, if passed to
+// `absl::SimpleAtof`/`absl::SimpleAtod`, would produce the exact same original
+// floating point value (except in case of NaN; all NaNs are considered the same
+// value). Tries to keep the string short but it's not guaranteed to be as short
+// as possible.
+//
+// HighPrecision is conisderably slower than the default formatting, so only use
+// it if you need the string to convert back to the same floating-point value.
+
+inline strings_internal::AlphaNumBuffer<numbers_internal::kFastToBufferSize>
+HighPrecision(float f) {
+  strings_internal::AlphaNumBuffer<numbers_internal::kFastToBufferSize> result;
+  result.size =
+      strlen(numbers_internal::RoundTripFloatToBuffer(f, &result.data[0]));
+  return result;
+}
+
+inline strings_internal::AlphaNumBuffer<numbers_internal::kFastToBufferSize>
+HighPrecision(double d) {
+  strings_internal::AlphaNumBuffer<numbers_internal::kFastToBufferSize> result;
+  result.size =
+      strlen(numbers_internal::RoundTripDoubleToBuffer(d, &result.data[0]));
+  return result;
+}
+
 // -----------------------------------------------------------------------------
 // AlphaNum
 // -----------------------------------------------------------------------------
 //
 // The `AlphaNum` class acts as the main parameter type for `StrCat()` and
-// `StrAppend()`, providing efficient conversion of numeric, boolean, and
-// hexadecimal values (through the `Hex` type) into strings.
+// `StrAppend()`, providing efficient conversion of numeric, boolean, decimal,
+// and hexadecimal values (through the `Dec` and `Hex` types) into strings.
+// `AlphaNum` should only be used as a function parameter. Do not instantiate
+//  `AlphaNum` directly as a stack variable.
 
 class AlphaNum {
  public:
   // No bool ctor -- bools convert to an integral type.
   // A bool ctor would also convert incoming pointers (bletch).
 
+  // Prevent brace initialization
+  template <typename T>
+  AlphaNum(std::initializer_list<T>) = delete;  // NOLINT(runtime/explicit)
+
   AlphaNum(int x)  // NOLINT(runtime/explicit)
-      : piece_(digits_,
-               numbers_internal::FastIntToBuffer(x, digits_) - &digits_[0]) {}
+      : piece_(digits_, static_cast<size_t>(
+                            numbers_internal::FastIntToBuffer(x, digits_) -
+                            &digits_[0])) {}
   AlphaNum(unsigned int x)  // NOLINT(runtime/explicit)
-      : piece_(digits_,
-               numbers_internal::FastIntToBuffer(x, digits_) - &digits_[0]) {}
+      : piece_(digits_, static_cast<size_t>(
+                            numbers_internal::FastIntToBuffer(x, digits_) -
+                            &digits_[0])) {}
   AlphaNum(long x)  // NOLINT(*)
-      : piece_(digits_,
-               numbers_internal::FastIntToBuffer(x, digits_) - &digits_[0]) {}
+      : piece_(digits_, static_cast<size_t>(
+                            numbers_internal::FastIntToBuffer(x, digits_) -
+                            &digits_[0])) {}
   AlphaNum(unsigned long x)  // NOLINT(*)
-      : piece_(digits_,
-               numbers_internal::FastIntToBuffer(x, digits_) - &digits_[0]) {}
+      : piece_(digits_, static_cast<size_t>(
+                            numbers_internal::FastIntToBuffer(x, digits_) -
+                            &digits_[0])) {}
   AlphaNum(long long x)  // NOLINT(*)
-      : piece_(digits_,
-               numbers_internal::FastIntToBuffer(x, digits_) - &digits_[0]) {}
+      : piece_(digits_, static_cast<size_t>(
+                            numbers_internal::FastIntToBuffer(x, digits_) -
+                            &digits_[0])) {}
   AlphaNum(unsigned long long x)  // NOLINT(*)
-      : piece_(digits_,
-               numbers_internal::FastIntToBuffer(x, digits_) - &digits_[0]) {}
+      : piece_(digits_, static_cast<size_t>(
+                            numbers_internal::FastIntToBuffer(x, digits_) -
+                            &digits_[0])) {}
 
   AlphaNum(float f)  // NOLINT(runtime/explicit)
       : piece_(digits_, numbers_internal::SixDigitsToBuffer(f, digits_)) {}
   AlphaNum(double f)  // NOLINT(runtime/explicit)
       : piece_(digits_, numbers_internal::SixDigitsToBuffer(f, digits_)) {}
 
-  AlphaNum(Hex hex);  // NOLINT(runtime/explicit)
-  AlphaNum(Dec dec);  // NOLINT(runtime/explicit)
-
   template <size_t size>
   AlphaNum(  // NOLINT(runtime/explicit)
-      const strings_internal::AlphaNumBuffer<size>& buf)
+      const strings_internal::AlphaNumBuffer<size>& buf
+          ABSL_ATTRIBUTE_LIFETIME_BOUND)
       : piece_(&buf.data[0], buf.size) {}
 
-  AlphaNum(const char* c_str) : piece_(c_str) {}  // NOLINT(runtime/explicit)
-  AlphaNum(absl::string_view pc) : piece_(pc) {}  // NOLINT(runtime/explicit)
+  AlphaNum(const char* absl_nullable c_str  // NOLINT(runtime/explicit)
+               ABSL_ATTRIBUTE_LIFETIME_BOUND)
+      : piece_(NullSafeStringView(c_str)) {}
+  AlphaNum(absl::string_view pc  // NOLINT(runtime/explicit)
+               ABSL_ATTRIBUTE_LIFETIME_BOUND)
+      : piece_(pc) {}
+
+#if !defined(ABSL_USES_STD_STRING_VIEW)
+  AlphaNum(std::string_view pc  // NOLINT(runtime/explicit)
+               ABSL_ATTRIBUTE_LIFETIME_BOUND)
+      : piece_(pc.data(), pc.size()) {}
+#endif  // !ABSL_USES_STD_STRING_VIEW
+
+  template <typename T, typename = std::enable_if_t<HasAbslStringify<T>::value>>
+  AlphaNum(  // NOLINT(runtime/explicit)
+      const T& v ABSL_ATTRIBUTE_LIFETIME_BOUND,
+      strings_internal::StringifySink&& sink ABSL_ATTRIBUTE_LIFETIME_BOUND = {})
+      : piece_(strings_internal::ExtractStringification(sink, v)) {}
 
   template <typename Allocator>
   AlphaNum(  // NOLINT(runtime/explicit)
-      const std::basic_string<char, std::char_traits<char>, Allocator>& str)
+      const std::basic_string<char, std::char_traits<char>, Allocator>& str
+          ABSL_ATTRIBUTE_LIFETIME_BOUND)
       : piece_(str) {}
 
-  // Use std::string literals ":" instead of character literals ':'.
+  // Use string literals ":" instead of character literals ':'.
   AlphaNum(char c) = delete;  // NOLINT(runtime/explicit)
 
   AlphaNum(const AlphaNum&) = delete;
   AlphaNum& operator=(const AlphaNum&) = delete;
 
   absl::string_view::size_type size() const { return piece_.size(); }
-  const char* data() const { return piece_.data(); }
+  const char* absl_nullable data() const { return piece_.data(); }
   absl::string_view Piece() const { return piece_; }
 
-  // Normal enums are already handled by the integer formatters.
-  // This overload matches only scoped enums.
+  // Match unscoped enums.  Use integral promotion so that a `char`-backed
+  // enum becomes a wider integral type AlphaNum will accept.
   template <typename T,
-            typename = typename std::enable_if<
-                std::is_enum<T>{} && !std::is_convertible<T, int>{}>::type>
+            typename = std::enable_if_t<std::is_enum<T>{} &&
+                                        std::is_convertible<T, int>{} &&
+                                        !HasAbslStringify<T>::value>>
   AlphaNum(T e)  // NOLINT(runtime/explicit)
-      : AlphaNum(static_cast<typename std::underlying_type<T>::type>(e)) {}
+      : AlphaNum(+e) {}
+
+  // This overload matches scoped enums.  We must explicitly cast to the
+  // underlying type, but use integral promotion for the same reason as above.
+  template <typename T, std::enable_if_t<std::is_enum<T>{} &&
+                                             !std::is_convertible<T, int>{} &&
+                                             !HasAbslStringify<T>::value,
+                                         char*> = nullptr>
+  AlphaNum(T e)  // NOLINT(runtime/explicit)
+      : AlphaNum(+static_cast<std::underlying_type_t<T>>(e)) {}
 
   // vector<bool>::reference and const_reference require special help to
   // convert to `AlphaNum` because it requires two user defined conversions.
   template <
       typename T,
-      typename std::enable_if<
-          std::is_class<T>::value &&
-          (std::is_same<T, std::vector<bool>::reference>::value ||
-           std::is_same<T, std::vector<bool>::const_reference>::value)>::type* =
-          nullptr>
+      std::enable_if_t<
+          std::is_class_v<T> &&
+          (std::is_same_v<T, std::vector<bool>::reference> ||
+           std::is_same_v<T, std::vector<bool>::const_reference>)>* = nullptr>
   AlphaNum(T e) : AlphaNum(static_cast<bool>(e)) {}  // NOLINT(runtime/explicit)
 
  private:
@@ -290,7 +464,8 @@ class AlphaNum {
 // StrCat()
 // -----------------------------------------------------------------------------
 //
-// Merges given strings or numbers, using no delimiter(s).
+// Merges given strings or numbers, using no delimiter(s), returning the merged
+// result as a string.
 //
 // `StrCat()` is designed to be the fastest possible way to construct a string
 // out of a mix of raw C strings, string_views, strings, bool values,
@@ -317,28 +492,108 @@ namespace strings_internal {
 
 // Do not call directly - this is not part of the public API.
 std::string CatPieces(std::initializer_list<absl::string_view> pieces);
-void AppendPieces(std::string* dest,
+void AppendPieces(std::string* absl_nonnull dest,
                   std::initializer_list<absl::string_view> pieces);
 
+template <typename Integer>
+std::string IntegerToString(Integer i) {
+  // Any integer (signed/unsigned) up to 64 bits can be formatted into a buffer
+  // with 22 bytes (including NULL at the end).
+  constexpr size_t kMaxDigits10 = 22;
+  std::string result;
+  StringResizeAndOverwrite(
+      result, kMaxDigits10, [i](char* start, size_t buf_size) {
+        // Note: This can be optimized to not write last zero.
+        char* end = numbers_internal::FastIntToBuffer(i, start);
+        auto size = static_cast<size_t>(end - start);
+        ABSL_ASSERT(size < buf_size);
+        return size;
+      });
+  return result;
+}
+
+template <typename Float>
+std::string FloatToString(Float f) {
+  std::string result;
+  StringResizeAndOverwrite(result, numbers_internal::kSixDigitsToBufferSize,
+                           [f](char* start, size_t buf_size) {
+                             size_t size =
+                                 numbers_internal::SixDigitsToBuffer(f, start);
+                             ABSL_ASSERT(size < buf_size);
+                             return size;
+                           });
+  return result;
+}
+
+// `SingleArgStrCat` overloads take built-in `int`, `long` and `long long` types
+// (signed / unsigned) to avoid ambiguity on the call side. If we used int32_t
+// and int64_t, then at least one of the three (`int` / `long` / `long long`)
+// would have been ambiguous when passed to `SingleArgStrCat`.
+inline std::string SingleArgStrCat(int x) { return IntegerToString(x); }
+inline std::string SingleArgStrCat(unsigned int x) {
+  return IntegerToString(x);
+}
+// NOLINTNEXTLINE
+inline std::string SingleArgStrCat(long x) { return IntegerToString(x); }
+// NOLINTNEXTLINE
+inline std::string SingleArgStrCat(unsigned long x) {
+  return IntegerToString(x);
+}
+// NOLINTNEXTLINE
+inline std::string SingleArgStrCat(long long x) { return IntegerToString(x); }
+// NOLINTNEXTLINE
+inline std::string SingleArgStrCat(unsigned long long x) {
+  return IntegerToString(x);
+}
+inline std::string SingleArgStrCat(float x) { return FloatToString(x); }
+inline std::string SingleArgStrCat(double x) { return FloatToString(x); }
+
+// As of September 2023, the SingleArgStrCat() optimization is only enabled for
+// libc++. The reasons for this are:
+// 1) The SSO size for libc++ is 23, while libstdc++ and MSSTL have an SSO size
+// of 15. Since IntegerToString unconditionally resizes the string to 22 bytes,
+// this causes both libstdc++ and MSSTL to allocate.
+// 2) strings_internal::STLStringResizeUninitialized() only has an
+// implementation that avoids initialization when using libc++. This isn't as
+// relevant as (1), and the cost should be benchmarked if (1) ever changes on
+// libstc++ or MSSTL.
+#ifdef _LIBCPP_VERSION
+#define ABSL_INTERNAL_STRCAT_ENABLE_FAST_CASE true
+#else
+#define ABSL_INTERNAL_STRCAT_ENABLE_FAST_CASE false
+#endif
+
+template <typename T, typename = std::enable_if_t<
+                          ABSL_INTERNAL_STRCAT_ENABLE_FAST_CASE &&
+                          std::is_arithmetic<T>{} && !std::is_same<T, char>{}>>
+using EnableIfFastCase = T;
+
+#undef ABSL_INTERNAL_STRCAT_ENABLE_FAST_CASE
+
 }  // namespace strings_internal
 
-ABSL_MUST_USE_RESULT inline std::string StrCat() { return std::string(); }
+[[nodiscard]] inline std::string StrCat() { return std::string(); }
 
-ABSL_MUST_USE_RESULT inline std::string StrCat(const AlphaNum& a) {
+template <typename T>
+[[nodiscard]] inline std::string StrCat(
+    strings_internal::EnableIfFastCase<T> a) {
+  return strings_internal::SingleArgStrCat(a);
+}
+[[nodiscard]] inline std::string StrCat(const AlphaNum& a) {
   return std::string(a.data(), a.size());
 }
 
-ABSL_MUST_USE_RESULT std::string StrCat(const AlphaNum& a, const AlphaNum& b);
-ABSL_MUST_USE_RESULT std::string StrCat(const AlphaNum& a, const AlphaNum& b,
-                                        const AlphaNum& c);
-ABSL_MUST_USE_RESULT std::string StrCat(const AlphaNum& a, const AlphaNum& b,
-                                        const AlphaNum& c, const AlphaNum& d);
+[[nodiscard]] std::string StrCat(const AlphaNum& a, const AlphaNum& b);
+[[nodiscard]] std::string StrCat(const AlphaNum& a, const AlphaNum& b,
+                                 const AlphaNum& c);
+[[nodiscard]] std::string StrCat(const AlphaNum& a, const AlphaNum& b,
+                                 const AlphaNum& c, const AlphaNum& d);
 
 // Support 5 or more arguments
 template <typename... AV>
-ABSL_MUST_USE_RESULT inline std::string StrCat(
-    const AlphaNum& a, const AlphaNum& b, const AlphaNum& c, const AlphaNum& d,
-    const AlphaNum& e, const AV&... args) {
+[[nodiscard]] inline std::string StrCat(const AlphaNum& a, const AlphaNum& b,
+                                        const AlphaNum& c, const AlphaNum& d,
+                                        const AlphaNum& e, const AV&... args) {
   return strings_internal::CatPieces(
       {a.Piece(), b.Piece(), c.Piece(), d.Piece(), e.Piece(),
        static_cast<const AlphaNum&>(args).Piece()...});
@@ -371,19 +626,20 @@ ABSL_MUST_USE_RESULT inline std::string StrCat(
 //   absl::string_view p = s;
 //   StrAppend(&s, p);
 
-inline void StrAppend(std::string*) {}
-void StrAppend(std::string* dest, const AlphaNum& a);
-void StrAppend(std::string* dest, const AlphaNum& a, const AlphaNum& b);
-void StrAppend(std::string* dest, const AlphaNum& a, const AlphaNum& b,
-               const AlphaNum& c);
-void StrAppend(std::string* dest, const AlphaNum& a, const AlphaNum& b,
-               const AlphaNum& c, const AlphaNum& d);
+inline void StrAppend(std::string* absl_nonnull) {}
+void StrAppend(std::string* absl_nonnull dest, const AlphaNum& a);
+void StrAppend(std::string* absl_nonnull dest, const AlphaNum& a,
+               const AlphaNum& b);
+void StrAppend(std::string* absl_nonnull dest, const AlphaNum& a,
+               const AlphaNum& b, const AlphaNum& c);
+void StrAppend(std::string* absl_nonnull dest, const AlphaNum& a,
+               const AlphaNum& b, const AlphaNum& c, const AlphaNum& d);
 
 // Support 5 or more arguments
 template <typename... AV>
-inline void StrAppend(std::string* dest, const AlphaNum& a, const AlphaNum& b,
-                      const AlphaNum& c, const AlphaNum& d, const AlphaNum& e,
-                      const AV&... args) {
+inline void StrAppend(std::string* absl_nonnull dest, const AlphaNum& a,
+                      const AlphaNum& b, const AlphaNum& c, const AlphaNum& d,
+                      const AlphaNum& e, const AV&... args) {
   strings_internal::AppendPieces(
       dest, {a.Piece(), b.Piece(), c.Piece(), d.Piece(), e.Piece(),
              static_cast<const AlphaNum&>(args).Piece()...});
@@ -400,6 +656,7 @@ SixDigits(double d) {
   return result;
 }
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_STR_CAT_H_
diff --git a/absl/strings/str_cat_benchmark.cc b/absl/strings/str_cat_benchmark.cc
index 14c63b3fc9f..f92c64212de 100644
--- a/absl/strings/str_cat_benchmark.cc
+++ b/absl/strings/str_cat_benchmark.cc
@@ -12,18 +12,27 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-#include "absl/strings/str_cat.h"
-
+#include <array>
 #include <cstdint>
+#include <cstdio>
+#include <cstdlib>
+#include <cstring>
 #include <string>
+#include <tuple>
+#include <utility>
 
-#include "benchmark/benchmark.h"
+#include "absl/random/log_uniform_int_distribution.h"
+#include "absl/random/random.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/str_format.h"
+#include "absl/strings/string_view.h"
 #include "absl/strings/substitute.h"
+#include "benchmark/benchmark.h"
 
 namespace {
 
 const char kStringOne[] = "Once Upon A Time, ";
-const char kStringTwo[] = "There was a std::string benchmark";
+const char kStringTwo[] = "There was a string benchmark";
 
 // We want to include negative numbers in the benchmark, so this function
 // is used to count 0, 1, -1, 2, -2, 3, -3, ...
@@ -103,6 +112,17 @@ void BM_HexCat_By_StrCat(benchmark::State& state) {
 }
 BENCHMARK(BM_HexCat_By_StrCat);
 
+void BM_HexCat_By_StrFormat(benchmark::State& state) {
+  int i = 0;
+  for (auto _ : state) {
+    std::string result =
+        absl::StrFormat("%s %x", kStringOne, int64_t{i} + 0x10000000);
+    benchmark::DoNotOptimize(result);
+    i = IncrementAlternatingSign(i);
+  }
+}
+BENCHMARK(BM_HexCat_By_StrFormat);
+
 void BM_HexCat_By_Substitute(benchmark::State& state) {
   int i = 0;
   for (auto _ : state) {
@@ -137,4 +157,132 @@ void BM_DoubleToString_By_SixDigits(benchmark::State& state) {
 }
 BENCHMARK(BM_DoubleToString_By_SixDigits);
 
+void BM_FloatToString_By_StrFormat(benchmark::State& state) {
+  int i = 0;
+  float foo = 0.0f;
+  for (auto _ : state) {
+    std::string result =
+        absl::StrFormat("%f != %lld", foo += 1.001f, int64_t{i});
+    benchmark::DoNotOptimize(result);
+    i = IncrementAlternatingSign(i);
+  }
+}
+BENCHMARK(BM_FloatToString_By_StrFormat);
+
+template <typename Table, size_t... Index>
+void BM_StrAppendImpl(benchmark::State& state, Table table, size_t total_bytes,
+                      std::index_sequence<Index...>) {
+  for (auto s : state) {
+    const size_t table_size = table.size();
+    size_t i = 0;
+    std::string result;
+    while (result.size() < total_bytes) {
+      absl::StrAppend(&result, std::get<Index>(table[i])...);
+      benchmark::DoNotOptimize(result);
+      ++i;
+      i -= i >= table_size ? table_size : 0;
+    }
+  }
+}
+
+template <typename Array>
+void BM_StrAppend(benchmark::State& state, Array&& table) {
+  const size_t total_bytes = state.range(0);
+  const int chunks_at_a_time = state.range(1);
+
+  switch (chunks_at_a_time) {
+    case 1:
+      return BM_StrAppendImpl(state, std::forward<Array>(table), total_bytes,
+                              std::make_index_sequence<1>());
+    case 2:
+      return BM_StrAppendImpl(state, std::forward<Array>(table), total_bytes,
+                              std::make_index_sequence<2>());
+    case 4:
+      return BM_StrAppendImpl(state, std::forward<Array>(table), total_bytes,
+                              std::make_index_sequence<4>());
+    case 8:
+      return BM_StrAppendImpl(state, std::forward<Array>(table), total_bytes,
+                              std::make_index_sequence<8>());
+    default:
+      std::abort();
+  }
+}
+
+void BM_StrAppendStr(benchmark::State& state) {
+  using T = absl::string_view;
+  using Row = std::tuple<T, T, T, T, T, T, T, T>;
+  constexpr absl::string_view kChunk = "0123456789";
+  Row row = {kChunk, kChunk, kChunk, kChunk, kChunk, kChunk, kChunk, kChunk};
+  return BM_StrAppend(state, std::array<Row, 1>({row}));
+}
+
+template <typename T>
+void BM_StrAppendInt(benchmark::State& state) {
+  absl::BitGen rng;
+  absl::log_uniform_int_distribution<T> dist;
+  std::array<std::tuple<T, T, T, T, T, T, T, T>, (1 << 7)> table;
+  for (size_t i = 0; i < table.size(); ++i) {
+    table[i] = {dist(rng), dist(rng), dist(rng), dist(rng),
+                dist(rng), dist(rng), dist(rng), dist(rng)};
+  }
+  return BM_StrAppend(state, table);
+}
+
+void StrAppendConfig(::benchmark::Benchmark* benchmark) {
+  for (int bytes : {10, 100, 1000, 10000}) {
+    for (int chunks : {1, 2, 4, 8}) {
+      // Only add the ones that divide properly. Otherwise we are over counting.
+      if (bytes % (10 * chunks) == 0) {
+        benchmark->Args({bytes, chunks});
+      }
+    }
+  }
+}
+
+BENCHMARK(BM_StrAppendStr)->Apply(StrAppendConfig);
+BENCHMARK(BM_StrAppendInt<int64_t>)->Apply(StrAppendConfig);
+BENCHMARK(BM_StrAppendInt<uint64_t>)->Apply(StrAppendConfig);
+BENCHMARK(BM_StrAppendInt<int32_t>)->Apply(StrAppendConfig);
+BENCHMARK(BM_StrAppendInt<uint32_t>)->Apply(StrAppendConfig);
+
+template <typename... Chunks>
+void BM_StrCatImpl(benchmark::State& state,
+                      Chunks... chunks) {
+  for (auto s : state) {
+    std::string result = absl::StrCat(chunks...);
+    benchmark::DoNotOptimize(result);
+  }
+}
+
+void BM_StrCat(benchmark::State& state) {
+  const int chunks_at_a_time = state.range(0);
+  const absl::string_view kChunk = "0123456789";
+
+  switch (chunks_at_a_time) {
+    case 1:
+      return BM_StrCatImpl(state, kChunk);
+    case 2:
+      return BM_StrCatImpl(state, kChunk, kChunk);
+    case 3:
+      return BM_StrCatImpl(state, kChunk, kChunk, kChunk);
+    case 4:
+      return BM_StrCatImpl(state, kChunk, kChunk, kChunk, kChunk);
+    default:
+      std::abort();
+  }
+}
+
+BENCHMARK(BM_StrCat)->Arg(1)->Arg(2)->Arg(3)->Arg(4);
+
+void BM_StrCat_int(benchmark::State& state) {
+  int i = 0;
+  for (auto s : state) {
+    std::string result = absl::StrCat(i);
+    benchmark::DoNotOptimize(result);
+    i = IncrementAlternatingSign(i);
+  }
+}
+
+BENCHMARK(BM_StrCat_int);
+
 }  // namespace
diff --git a/absl/strings/str_cat_test.cc b/absl/strings/str_cat_test.cc
index be39880be78..7b21c9cc4d7 100644
--- a/absl/strings/str_cat_test.cc
+++ b/absl/strings/str_cat_test.cc
@@ -16,12 +16,18 @@
 
 #include "absl/strings/str_cat.h"
 
+#include <cstddef>
 #include <cstdint>
+#include <cstdlib>
+#include <limits>
 #include <string>
+#include <string_view>
 #include <vector>
 
 #include "gtest/gtest.h"
-#include "absl/strings/substitute.h"
+#include "absl/base/config.h"
+#include "absl/strings/str_format.h"
+#include "absl/strings/string_view.h"
 
 #ifdef __ANDROID__
 // Android assert messages only go to system log, so death tests cannot inspect
@@ -69,7 +75,7 @@ TEST(StrCat, Ints) {
 TEST(StrCat, Enums) {
   enum SmallNumbers { One = 1, Ten = 10 } e = Ten;
   EXPECT_EQ("10", absl::StrCat(e));
-  EXPECT_EQ("-5", absl::StrCat(SmallNumbers(-5)));
+  EXPECT_EQ("1", absl::StrCat(One));
 
   enum class Option { Boxers = 1, Briefs = -1 };
 
@@ -134,6 +140,9 @@ TEST(StrCat, Basics) {
   result = absl::StrCat(-1);
   EXPECT_EQ(result, "-1");
 
+  result = absl::StrCat(absl::HighPrecision(0.5));
+  EXPECT_EQ(result, "0.5");
+
   result = absl::StrCat(absl::SixDigits(0.5));
   EXPECT_EQ(result, "0.5");
 
@@ -162,7 +171,7 @@ TEST(StrCat, Basics) {
   EXPECT_EQ(result, "12345678910, 10987654321!");
 
   std::string one =
-      "1";  // Actually, it's the size of this std::string that we want; a
+      "1";  // Actually, it's the size of this string that we want; a
             // 64-bit build distinguishes between size_t and uint64_t,
             // even though they're both unsigned 64-bit values.
   result = absl::StrCat("And a ", one.size(), " and a ",
@@ -176,16 +185,27 @@ TEST(StrCat, Basics) {
   EXPECT_EQ(result, "To output a char by ASCII/numeric value, use +: 33");
 
   float f = 100000.5;
+  result = absl::StrCat("A hundred K and a half is ", absl::HighPrecision(f));
+  EXPECT_EQ(result, "A hundred K and a half is 100000.5");
+
   result = absl::StrCat("A hundred K and a half is ", absl::SixDigits(f));
   EXPECT_EQ(result, "A hundred K and a half is 100000");
 
   f = 100001.5;
+  result = absl::StrCat("A hundred K and one and a half is ",
+                        absl::HighPrecision(f));
+  EXPECT_EQ(result, "A hundred K and one and a half is 100001.5");
+
   result =
       absl::StrCat("A hundred K and one and a half is ", absl::SixDigits(f));
   EXPECT_EQ(result, "A hundred K and one and a half is 100002");
 
   double d = 100000.5;
   d *= d;
+  result = absl::StrCat("A hundred K and a half squared is ",
+                        absl::HighPrecision(d));
+  EXPECT_EQ(result, "A hundred K and a half squared is 10000100000.25");
+
   result =
       absl::StrCat("A hundred K and a half squared is ", absl::SixDigits(d));
   EXPECT_EQ(result, "A hundred K and a half squared is 1.00001e+10");
@@ -210,6 +230,17 @@ TEST(StrCat, CornerCases) {
   EXPECT_EQ(result, "");
 }
 
+TEST(StrCat, StdStringView) {
+  std::string_view pieces[] = {"Hello", ", ", "World", "!"};
+  EXPECT_EQ(absl::StrCat(pieces[0], pieces[1], pieces[2], pieces[3]),
+                         "Hello, World!");
+}
+
+TEST(StrCat, NullConstCharPtr) {
+  const char* null = nullptr;
+  EXPECT_EQ(absl::StrCat("mon", null, "key"), "monkey");
+}
+
 // A minimal allocator that uses malloc().
 template <typename T>
 struct Mallocator {
@@ -375,7 +406,7 @@ TEST(StrAppend, Basics) {
   EXPECT_EQ(result.substr(old_size), "12345678910, 10987654321!");
 
   std::string one =
-      "1";  // Actually, it's the size of this std::string that we want; a
+      "1";  // Actually, it's the size of this string that we want; a
             // 64-bit build distinguishes between size_t and uint64_t,
             // even though they're both unsigned 64-bit values.
   old_size = result.size();
@@ -391,6 +422,20 @@ TEST(StrAppend, Basics) {
   EXPECT_EQ(result.substr(old_size),
             "To output a char by ASCII/numeric value, use +: 33");
 
+  float f = 100000.5;
+  old_size = result.size();
+  absl::StrAppend(&result, "A hundred K and a half is ",
+                  absl::HighPrecision(f));
+  EXPECT_EQ(result.substr(old_size), "A hundred K and a half is 100000.5");
+
+  double d = f;
+  d *= d;
+  old_size = result.size();
+  absl::StrAppend(&result, "A hundred K and a half squared is ",
+                  absl::HighPrecision(d));
+  EXPECT_EQ(result.substr(old_size),
+            "A hundred K and a half squared is 10000100000.25");
+
   // Test 9 arguments, the old maximum
   old_size = result.size();
   absl::StrAppend(&result, 1, 22, 333, 4444, 55555, 666666, 7777777, 88888888,
@@ -437,7 +482,7 @@ TEST(StrCat, AvoidsMemcpyWithNullptr) {
   EXPECT_EQ(result, "12345");
 }
 
-#ifdef GTEST_HAS_DEATH_TEST
+#if GTEST_HAS_DEATH_TEST
 TEST(StrAppend, Death) {
   std::string s = "self";
   // on linux it's "assertion", on mac it's "Assertion",
@@ -463,7 +508,7 @@ TEST(StrAppend, CornerCases) {
 }
 
 TEST(StrAppend, CornerCasesNonEmptyAppend) {
-  for (std::string result : {"hello", "a std::string too long to fit in the SSO"}) {
+  for (std::string result : {"hello", "a string too long to fit in the SSO"}) {
     const std::string expected = result;
     absl::StrAppend(&result, "");
     EXPECT_EQ(result, expected);
@@ -607,4 +652,69 @@ TEST(Numbers, TestFunctionsMovedOverFromNumbersMain) {
   TestFastPrints();
 }
 
+struct PointStringify {
+  template <typename FormatSink>
+  friend void AbslStringify(FormatSink& sink, const PointStringify& p) {
+    sink.Append("(");
+    sink.Append(absl::StrCat(p.x));
+    sink.Append(", ");
+    sink.Append(absl::StrCat(p.y));
+    sink.Append(")");
+  }
+
+  double x = 10.0;
+  double y = 20.0;
+};
+
+TEST(StrCat, AbslStringifyExample) {
+  PointStringify p;
+  EXPECT_EQ(absl::StrCat(p), "(10, 20)");
+  EXPECT_EQ(absl::StrCat("a ", p, " z"), "a (10, 20) z");
+}
+
+struct PointStringifyUsingFormat {
+  template <typename FormatSink>
+  friend void AbslStringify(FormatSink& sink,
+                            const PointStringifyUsingFormat& p) {
+    absl::Format(&sink, "(%g, %g)", p.x, p.y);
+  }
+
+  double x = 10.0;
+  double y = 20.0;
+};
+
+TEST(StrCat, AbslStringifyExampleUsingFormat) {
+  PointStringifyUsingFormat p;
+  EXPECT_EQ(absl::StrCat(p), "(10, 20)");
+  EXPECT_EQ(absl::StrCat("a ", p, " z"), "a (10, 20) z");
+}
+
+enum class EnumWithStringify { Many = 0, Choices = 1 };
+
+template <typename Sink>
+void AbslStringify(Sink& sink, EnumWithStringify e) {
+  absl::Format(&sink, "%s", e == EnumWithStringify::Many ? "Many" : "Choices");
+}
+
+TEST(StrCat, AbslStringifyWithEnum) {
+  const auto e = EnumWithStringify::Choices;
+  EXPECT_EQ(absl::StrCat(e), "Choices");
+}
+
+template <typename Integer>
+void CheckSingleArgumentIntegerLimits() {
+  Integer max = std::numeric_limits<Integer>::max();
+  Integer min = std::numeric_limits<Integer>::min();
+
+  EXPECT_EQ(absl::StrCat(max), std::to_string(max));
+  EXPECT_EQ(absl::StrCat(min), std::to_string(min));
+}
+
+TEST(StrCat, SingleArgumentLimits) {
+  CheckSingleArgumentIntegerLimits<int32_t>();
+  CheckSingleArgumentIntegerLimits<uint32_t>();
+  CheckSingleArgumentIntegerLimits<int64_t>();
+  CheckSingleArgumentIntegerLimits<uint64_t>();
+}
+
 }  // namespace
diff --git a/absl/strings/str_format.h b/absl/strings/str_format.h
index 607e2bcaae1..6ec289ebaf7 100644
--- a/absl/strings/str_format.h
+++ b/absl/strings/str_format.h
@@ -19,7 +19,7 @@
 //
 // The `str_format` library is a typesafe replacement for the family of
 // `printf()` string formatting routines within the `<cstdio>` standard library
-// header. Like the `printf` family, the `str_format` uses a "format string" to
+// header. Like the `printf` family, `str_format` uses a "format string" to
 // perform argument substitutions based on types. See the `FormatSpec` section
 // below for format string documentation.
 //
@@ -36,10 +36,12 @@
 //   * `absl::StreamFormat()` to more efficiently write a format string to a
 //     stream, such as`std::cout`.
 //   * `absl::PrintF()`, `absl::FPrintF()` and `absl::SNPrintF()` as
-//     replacements for `std::printf()`, `std::fprintf()` and `std::snprintf()`.
+//     drop-in replacements for `std::printf()`, `std::fprintf()` and
+//     `std::snprintf()`.
 //
-//     Note: a version of `std::sprintf()` is not supported as it is
-//     generally unsafe due to buffer overflows.
+//     Note: An `absl::SPrintF()` drop-in replacement is not supported as it
+//     is generally unsafe due to buffer overflows. Use `absl::StrFormat` which
+//     returns the string as output instead of expecting a pre-allocated buffer.
 //
 // Additionally, you can provide a format string (and its associated arguments)
 // using one of the following abstractions:
@@ -47,7 +49,8 @@
 //   * A `FormatSpec` class template fully encapsulates a format string and its
 //     type arguments and is usually provided to `str_format` functions as a
 //     variadic argument of type `FormatSpec<Arg...>`. The `FormatSpec<Args...>`
-//     template is evaluated at compile-time, providing type safety.
+//     template is evaluated at compile-time, providing type safety (supported
+//     on GCC and Clang; on MSVC, these checks are deferred to runtime).
 //   * A `ParsedFormat` instance, which encapsulates a specific, pre-compiled
 //     format string for a specific set of type(s), and which can be passed
 //     between API boundaries. (The `FormatSpec` type should not be used
@@ -57,8 +60,7 @@
 // arbitrary sink types:
 //
 //   * A generic `Format()` function to write outputs to arbitrary sink types,
-//     which must implement a `RawSinkFormat` interface. (See
-//     `str_format_sink.h` for more information.)
+//     which must implement a `FormatRawSink` interface.
 //
 //   * A `FormatUntyped()` function that is similar to `Format()` except it is
 //     loosely typed. `FormatUntyped()` is not a template and does not perform
@@ -66,22 +68,29 @@
 //     boolean from a runtime check.
 //
 // In addition, the `str_format` library provides extension points for
-// augmenting formatting to new types. These extensions are fully documented
-// within the `str_format_extension.h` header file.
+// augmenting formatting to new types.  See "StrFormat Extensions" below.
 
 #ifndef ABSL_STRINGS_STR_FORMAT_H_
 #define ABSL_STRINGS_STR_FORMAT_H_
 
+#include <cstdint>
 #include <cstdio>
 #include <string>
+#include <type_traits>
 
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/nullability.h"
 #include "absl/strings/internal/str_format/arg.h"  // IWYU pragma: export
 #include "absl/strings/internal/str_format/bind.h"  // IWYU pragma: export
 #include "absl/strings/internal/str_format/checker.h"  // IWYU pragma: export
 #include "absl/strings/internal/str_format/extension.h"  // IWYU pragma: export
 #include "absl/strings/internal/str_format/parser.h"  // IWYU pragma: export
+#include "absl/strings/string_view.h"
+#include "absl/types/span.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 // UntypedFormatSpec
 //
@@ -103,7 +112,8 @@ class UntypedFormatSpec {
   explicit UntypedFormatSpec(string_view s) : spec_(s) {}
 
  protected:
-  explicit UntypedFormatSpec(const str_format_internal::ParsedFormatBase* pc)
+  explicit UntypedFormatSpec(
+      const str_format_internal::ParsedFormatBase* absl_nonnull pc)
       : spec_(pc) {}
 
  private:
@@ -143,7 +153,7 @@ str_format_internal::StreamedWrapper<T> FormatStreamed(const T& v) {
 //   EXPECT_EQ(8, n);
 class FormatCountCapture {
  public:
-  explicit FormatCountCapture(int* p) : p_(p) {}
+  explicit FormatCountCapture(int* absl_nonnull p) : p_(p) {}
 
  private:
   // FormatCountCaptureHelper is used to define FormatConvertImpl() for this
@@ -152,8 +162,8 @@ class FormatCountCapture {
   // Unused() is here because of the false positive from -Wunused-private-field
   // p_ is used in the templated function of the friend FormatCountCaptureHelper
   // class.
-  int* Unused() { return p_; }
-  int* p_;
+  int* absl_nonnull Unused() { return p_; }
+  int* absl_nonnull p_;
 };
 
 // FormatSpec
@@ -172,7 +182,7 @@ class FormatCountCapture {
 // For a `FormatSpec` to be valid at compile-time, it must be provided as
 // either:
 //
-// * A `constexpr` literal or `absl::string_view`, which is how it most often
+// * A `constexpr` literal or `absl::string_view`, which is how it is most often
 //   used.
 // * A `ParsedFormat` instantiation, which ensures the format string is
 //   valid before use. (See below.)
@@ -192,9 +202,9 @@ class FormatCountCapture {
 //   absl::StrFormat(formatString, "TheVillage", 6);
 //
 // A format string generally follows the POSIX syntax as used within the POSIX
-// `printf` specification.
+// `printf` specification. (Exceptions are noted below.)
 //
-// (See http://pubs.opengroup.org/onlinepubs/9699919799/functions/fprintf.html.)
+// (See http://pubs.opengroup.org/onlinepubs/9699919799/functions/fprintf.html)
 //
 // In specific, the `FormatSpec` supports the following type specifiers:
 //   * `c` for characters
@@ -212,6 +222,10 @@ class FormatCountCapture {
 //   * `n` for the special case of writing out the number of characters
 //     written to this point. The resulting value must be captured within an
 //     `absl::FormatCountCapture` type.
+//   * `v` for values using the default format for a deduced type. These deduced
+//     types include many of the primitive types denoted here as well as
+//     user-defined types containing the proper extensions. (See below for more
+//     information.)
 //
 // Implementation-defined behavior:
 //   * A null pointer provided to "%s" or "%p" is output as "(nil)".
@@ -240,22 +254,34 @@ class FormatCountCapture {
 //         "%s%d%n", "hello", 123, absl::FormatCountCapture(&n));
 //     EXPECT_EQ(8, n);
 //
+// NOTE: the `v` specifier (for "value") is a type specifier not present in the
+// POSIX specification. %v will format values according to their deduced type.
+// `v` uses `d` for signed integer values, `u` for unsigned integer values, `g`
+// for floating point values, and formats boolean values as "true"/"false"
+// (instead of 1 or 0 for booleans formatted using d). `const char*` is not
+// supported; please use `std::string` and `string_view`. `char` is also not
+// supported due to ambiguity of the type. This specifier does not support
+// modifiers.
+//
 // The `FormatSpec` intrinsically supports all of these fundamental C++ types:
 //
-// *   Characters: `char`, `signed char`, `unsigned char`
+// *   Characters: `char`, `signed char`, `unsigned char`, `wchar_t`
 // *   Integers: `int`, `short`, `unsigned short`, `unsigned`, `long`,
 //         `unsigned long`, `long long`, `unsigned long long`
+// *   Enums: printed as their underlying integral value
 // *   Floating-point: `float`, `double`, `long double`
 //
 // However, in the `str_format` library, a format conversion specifies a broader
 // C++ conceptual category instead of an exact type. For example, `%s` binds to
-// any string-like argument, so `std::string`, `absl::string_view`, and
-// `const char*` are all accepted. Likewise, `%d` accepts any integer-like
-// argument, etc.
-
+// any string-like argument, so `std::string`, `std::wstring`,
+// `absl::string_view`, `const char*`, and `const wchar_t*` are all accepted.
+// Likewise, `%d` accepts any integer-like argument, etc.
+//
+// Note: Compile-time format string checking is supported on GCC and
+// Clang. On MSVC, these checks are performed at runtime instead.
 template <typename... Args>
-using FormatSpec =
-    typename str_format_internal::FormatSpecDeductionBarrier<Args...>::type;
+using FormatSpec = str_format_internal::FormatSpecTemplate<
+    str_format_internal::ArgumentToConv<Args>()...>;
 
 // ParsedFormat
 //
@@ -272,8 +298,8 @@ using FormatSpec =
 // Example:
 //
 //   // Verified at compile time.
-//   absl::ParsedFormat<'s', 'd'> formatString("Welcome to %s, Number %d!");
-//   absl::StrFormat(formatString, "TheVillage", 6);
+//   absl::ParsedFormat<'s', 'd'> format_string("Welcome to %s, Number %d!");
+//   absl::StrFormat(format_string, "TheVillage", 6);
 //
 //   // Verified at runtime.
 //   auto format_runtime = absl::ParsedFormat<'d'>::New(format_string);
@@ -282,9 +308,30 @@ using FormatSpec =
 //   } else {
 //     ... error case ...
 //   }
-template <char... Conv>
-using ParsedFormat = str_format_internal::ExtendedParsedFormat<
-    str_format_internal::ConversionCharToConv(Conv)...>;
+
+// An 'extended' format is also allowed that can specify multiple conversion
+// characters per format argument, using a combination of
+// `absl::FormatConversionCharSet` enum values (logically a set union)
+//  via the `|` operator. (Single character-based arguments are still accepted,
+// but cannot be combined). Some common conversions also have predefined enum
+// values, such as `absl::FormatConversionCharSet::kIntegral`.
+//
+// Example:
+//   // Extended format supports multiple conversion characters per argument,
+//   // specified via a combination of `FormatConversionCharSet` enums.
+//   using MyFormat = absl::ParsedFormat<absl::FormatConversionCharSet::d |
+//                                       absl::FormatConversionCharSet::x>;
+//   MyFormat GetFormat(bool use_hex) {
+//     if (use_hex) return MyFormat("foo %x bar");
+//     return MyFormat("foo %d bar");
+//   }
+//   // `format` can be used with any value that supports 'd' and 'x',
+//   // like `int`.
+//   auto format = GetFormat(use_hex);
+//   value = StringF(format, i);
+template <auto... Conv>
+using ParsedFormat = absl::str_format_internal::ExtendedParsedFormat<
+    absl::str_format_internal::ToFormatConversionCharSet(Conv)...>;
 
 // StrFormat()
 //
@@ -309,8 +356,8 @@ using ParsedFormat = str_format_internal::ExtendedParsedFormat<
 //
 // Returns an empty string in case of error.
 template <typename... Args>
-ABSL_MUST_USE_RESULT std::string StrFormat(const FormatSpec<Args...>& format,
-                                           const Args&... args) {
+[[nodiscard]] std::string StrFormat(const FormatSpec<Args...>& format,
+                                    const Args&... args) {
   return str_format_internal::FormatPack(
       str_format_internal::UntypedFormatSpecImpl::Extract(format),
       {str_format_internal::FormatArgImpl(args)...});
@@ -327,7 +374,7 @@ ABSL_MUST_USE_RESULT std::string StrFormat(const FormatSpec<Args...>& format,
 //   std::string orig("For example PI is approximately ");
 //   std::cout << StrAppendFormat(&orig, "%12.6f", 3.14);
 template <typename... Args>
-std::string& StrAppendFormat(std::string* dst,
+std::string& StrAppendFormat(std::string* absl_nonnull dst,
                              const FormatSpec<Args...>& format,
                              const Args&... args) {
   return str_format_internal::AppendPack(
@@ -339,14 +386,14 @@ std::string& StrAppendFormat(std::string* dst,
 //
 // Writes to an output stream given a format string and zero or more arguments,
 // generally in a manner that is more efficient than streaming the result of
-// `absl:: StrFormat()`. The returned object must be streamed before the full
+// `absl::StrFormat()`. The returned object must be streamed before the full
 // expression ends.
 //
 // Example:
 //
 //   std::cout << StreamFormat("%12.6f", 3.14);
 template <typename... Args>
-ABSL_MUST_USE_RESULT str_format_internal::Streamable StreamFormat(
+[[nodiscard]] str_format_internal::Streamable StreamFormat(
     const FormatSpec<Args...>& format, const Args&... args) {
   return str_format_internal::Streamable(
       str_format_internal::UntypedFormatSpecImpl::Extract(format),
@@ -387,7 +434,7 @@ int PrintF(const FormatSpec<Args...>& format, const Args&... args) {
 //   Outputs: "The capital of Mongolia is Ulaanbaatar"
 //
 template <typename... Args>
-int FPrintF(std::FILE* output, const FormatSpec<Args...>& format,
+int FPrintF(std::FILE* absl_nonnull output, const FormatSpec<Args...>& format,
             const Args&... args) {
   return str_format_internal::FprintF(
       output, str_format_internal::UntypedFormatSpecImpl::Extract(format),
@@ -400,6 +447,12 @@ int FPrintF(std::FILE* output, const FormatSpec<Args...>& format,
 // This function is functionally equivalent to `std::snprintf()` (and
 // type-safe); prefer `absl::SNPrintF()` over `std::snprintf()`.
 //
+// In particular, a successful call to `absl::SNPrintF()` writes at most `size`
+// bytes of the formatted output to `output`, including a NUL-terminator, and
+// returns the number of bytes that would have been written if truncation did
+// not occur. In the event of an error, a negative value is returned and `errno`
+// is set.
+//
 // Example:
 //
 //   std::string_view s = "Ulaanbaatar";
@@ -410,8 +463,8 @@ int FPrintF(std::FILE* output, const FormatSpec<Args...>& format,
 //   Post-condition: output == "The capital of Mongolia is Ulaanbaatar"
 //
 template <typename... Args>
-int SNPrintF(char* output, std::size_t size, const FormatSpec<Args...>& format,
-             const Args&... args) {
+int SNPrintF(char* absl_nonnull output, std::size_t size,
+             const FormatSpec<Args...>& format, const Args&... args) {
   return str_format_internal::SnprintF(
       output, size, str_format_internal::UntypedFormatSpecImpl::Extract(format),
       {str_format_internal::FormatArgImpl(args)...});
@@ -425,16 +478,25 @@ int SNPrintF(char* output, std::size_t size, const FormatSpec<Args...>& format,
 //
 // FormatRawSink is a type erased wrapper around arbitrary sink objects
 // specifically used as an argument to `Format()`.
+//
+// All the object has to do define an overload of `AbslFormatFlush()` for the
+// sink, usually by adding a ADL-based free function in the same namespace as
+// the sink:
+//
+//   void AbslFormatFlush(MySink* dest, absl::string_view part);
+//
+// where `dest` is the pointer passed to `absl::Format()`. The function should
+// append `part` to `dest`.
+//
 // FormatRawSink does not own the passed sink object. The passed object must
 // outlive the FormatRawSink.
 class FormatRawSink {
  public:
   // Implicitly convert from any type that provides the hook function as
   // described above.
-  template <typename T,
-            typename = typename std::enable_if<std::is_constructible<
-                str_format_internal::FormatRawSinkImpl, T*>::value>::type>
-  FormatRawSink(T* raw)  // NOLINT
+  template <typename T, typename = std::enable_if_t<std::is_constructible_v<
+                            str_format_internal::FormatRawSinkImpl, T*>>>
+  FormatRawSink(T* absl_nonnull raw)  // NOLINT
       : sink_(raw) {}
 
  private:
@@ -448,12 +510,13 @@ class FormatRawSink {
 // `absl::FormatRawSink` interface), using a format string and zero or more
 // additional arguments.
 //
-// By default, `std::string` and `std::ostream` are supported as destination
-// objects. If a `std::string` is used the formatted string is appended to it.
+// By default, `std::string`, `std::ostream`, and `absl::Cord` are supported as
+// destination objects. If a `std::string` is used the formatted string is
+// appended to it.
 //
-// `absl::Format()` is a generic version of `absl::StrFormat(), for custom
-// sinks. The format string, like format strings for `StrFormat()`, is checked
-// at compile-time.
+// `absl::Format()` is a generic version of `absl::StrAppendFormat()`, for
+// custom sinks. The format string, like format strings for `StrFormat()`, is
+// checked at compile-time.
 //
 // On failure, this function returns `false` and the state of the sink is
 // unspecified.
@@ -493,8 +556,7 @@ using FormatArg = str_format_internal::FormatArgImpl;
 // The arguments are provided in an `absl::Span<const absl::FormatArg>`.
 // Each `absl::FormatArg` object binds to a single argument and keeps a
 // reference to it. The values used to create the `FormatArg` objects must
-// outlive this function call. (See `str_format_arg.h` for information on
-// the `FormatArg` class.)_
+// outlive this function call.
 //
 // Example:
 //
@@ -516,14 +578,306 @@ using FormatArg = str_format_internal::FormatArgImpl;
 //     return std::move(out);
 //   }
 //
-ABSL_MUST_USE_RESULT inline bool FormatUntyped(
-    FormatRawSink raw_sink, const UntypedFormatSpec& format,
-    absl::Span<const FormatArg> args) {
+[[nodiscard]] inline bool FormatUntyped(FormatRawSink raw_sink,
+                                        const UntypedFormatSpec& format,
+                                        absl::Span<const FormatArg> args) {
   return str_format_internal::FormatUntyped(
       str_format_internal::FormatRawSinkImpl::Extract(raw_sink),
       str_format_internal::UntypedFormatSpecImpl::Extract(format), args);
 }
 
+//------------------------------------------------------------------------------
+// StrFormat Extensions
+//------------------------------------------------------------------------------
+//
+// AbslStringify()
+//
+// A simpler customization API for formatting user-defined types using
+// absl::StrFormat(). The API relies on detecting an overload in the
+// user-defined type's namespace of a free (non-member) `AbslStringify()`
+// function as a friend definition with the following signature:
+//
+// template <typename Sink>
+// void AbslStringify(Sink& sink, const X& value);
+//
+// An `AbslStringify()` overload for a type should only be declared in the same
+// file and namespace as said type.
+//
+// Note that unlike with AbslFormatConvert(), AbslStringify() does not allow
+// customization of allowed conversion characters. AbslStringify() uses `%v` as
+// the underlying conversion specifier. Additionally, AbslStringify() supports
+// use with absl::StrCat while AbslFormatConvert() does not.
+//
+// Example:
+//
+// struct Point {
+//   // To add formatting support to `Point`, we simply need to add a free
+//   // (non-member) function `AbslStringify()`. This method prints in the
+//   // request format using the underlying `%v` specifier. You can add such a
+//   // free function using a friend declaration within the body of the class.
+//   // The sink parameter is a templated type to avoid requiring dependencies.
+//   template <typename Sink>
+//   friend void AbslStringify(Sink& sink, const Point& p) {
+//     absl::Format(&sink, "(%v, %v)", p.x, p.y);
+//   }
+//
+//   int x;
+//   int y;
+// };
+//
+// AbslFormatConvert()
+//
+// The StrFormat library provides a customization API for formatting
+// user-defined types using absl::StrFormat(). The API relies on detecting an
+// overload in the user-defined type's namespace of a free (non-member)
+// `AbslFormatConvert()` function, usually as a friend definition with the
+// following signature:
+//
+// absl::FormatConvertResult<...> AbslFormatConvert(
+//     const X& value,
+//     const absl::FormatConversionSpec& spec,
+//     absl::FormatSink *sink);
+//
+// An `AbslFormatConvert()` overload for a type should only be declared in the
+// same file and namespace as said type.
+//
+// The abstractions within this definition include:
+//
+// * An `absl::FormatConversionSpec` to specify the fields to pull from a
+//   user-defined type's format string
+// * An `absl::FormatSink` to hold the converted string data during the
+//   conversion process.
+// * An `absl::FormatConvertResult` to hold the status of the returned
+//   formatting operation
+//
+// The return type encodes all the conversion characters that your
+// AbslFormatConvert() routine accepts.  The return value should be {true}.
+// A return value of {false} will result in `StrFormat()` returning
+// an empty string.  This result will be propagated to the result of
+// `FormatUntyped`.
+//
+// Example:
+//
+// struct Point {
+//   // To add formatting support to `Point`, we simply need to add a free
+//   // (non-member) function `AbslFormatConvert()`.  This method interprets
+//   // `spec` to print in the request format. The allowed conversion characters
+//   // can be restricted via the type of the result, in this example
+//   // string and integral formatting are allowed (but not, for instance
+//   // floating point characters like "%f").  You can add such a free function
+//   // using a friend declaration within the body of the class:
+//   friend absl::FormatConvertResult<absl::FormatConversionCharSet::kString |
+//                                    absl::FormatConversionCharSet::kIntegral>
+//   AbslFormatConvert(const Point& p, const absl::FormatConversionSpec& spec,
+//                     absl::FormatSink* s) {
+//     if (spec.conversion_char() == absl::FormatConversionChar::s) {
+//       absl::Format(s, "x=%vy=%v", p.x, p.y);
+//     } else {
+//       absl::Format(s, "%v,%v", p.x, p.y);
+//     }
+//     return {true};
+//   }
+//
+//   int x;
+//   int y;
+// };
+
+// clang-format off
+
+// FormatConversionChar
+//
+// Specifies the formatting character provided in the format string
+// passed to `StrFormat()`.
+enum class FormatConversionChar : uint8_t {
+  c, s,                    // text
+  d, i, o, u, x, X,        // int
+  f, F, e, E, g, G, a, A,  // float
+  n, p, v                  // misc
+};
+// clang-format on
+
+// FormatConversionSpec
+//
+// Specifies modifications to the conversion of the format string, through use
+// of one or more format flags in the source format string.
+class FormatConversionSpec {
+ public:
+  // FormatConversionSpec::is_basic()
+  //
+  // Indicates that width and precision are not specified, and no additional
+  // flags are set for this conversion character in the format string.
+  bool is_basic() const { return impl_.is_basic(); }
+
+  // FormatConversionSpec::has_left_flag()
+  //
+  // Indicates whether the result should be left justified for this conversion
+  // character in the format string. This flag is set through use of a '-'
+  // character in the format string. E.g. "%-s"
+  bool has_left_flag() const { return impl_.has_left_flag(); }
+
+  // FormatConversionSpec::has_show_pos_flag()
+  //
+  // Indicates whether a sign column is prepended to the result for this
+  // conversion character in the format string, even if the result is positive.
+  // This flag is set through use of a '+' character in the format string.
+  // E.g. "%+d"
+  bool has_show_pos_flag() const { return impl_.has_show_pos_flag(); }
+
+  // FormatConversionSpec::has_sign_col_flag()
+  //
+  // Indicates whether a mandatory sign column is added to the result for this
+  // conversion character. This flag is set through use of a space character
+  // (' ') in the format string. E.g. "% i"
+  bool has_sign_col_flag() const { return impl_.has_sign_col_flag(); }
+
+  // FormatConversionSpec::has_alt_flag()
+  //
+  // Indicates whether an "alternate" format is applied to the result for this
+  // conversion character. Alternative forms depend on the type of conversion
+  // character, and unallowed alternatives are undefined. This flag is set
+  // through use of a '#' character in the format string. E.g. "%#h"
+  bool has_alt_flag() const { return impl_.has_alt_flag(); }
+
+  // FormatConversionSpec::has_zero_flag()
+  //
+  // Indicates whether zeroes should be prepended to the result for this
+  // conversion character instead of spaces. This flag is set through use of the
+  // '0' character in the format string. E.g. "%0f"
+  bool has_zero_flag() const { return impl_.has_zero_flag(); }
+
+  // FormatConversionSpec::conversion_char()
+  //
+  // Returns the underlying conversion character.
+  FormatConversionChar conversion_char() const {
+    return impl_.conversion_char();
+  }
+
+  // FormatConversionSpec::width()
+  //
+  // Returns the specified width (indicated through use of a non-zero integer
+  // value or '*' character) of the conversion character. If width is
+  // unspecified, it returns a negative value.
+  int width() const { return impl_.width(); }
+
+  // FormatConversionSpec::precision()
+  //
+  // Returns the specified precision (through use of the '.' character followed
+  // by a non-zero integer value or '*' character) of the conversion character.
+  // If precision is unspecified, it returns a negative value.
+  int precision() const { return impl_.precision(); }
+
+ private:
+  explicit FormatConversionSpec(
+      str_format_internal::FormatConversionSpecImpl impl)
+      : impl_(impl) {}
+
+  friend str_format_internal::FormatConversionSpecImpl;
+
+  absl::str_format_internal::FormatConversionSpecImpl impl_;
+};
+
+// Type safe OR operator for FormatConversionCharSet to allow accepting multiple
+// conversion chars in custom format converters.
+constexpr FormatConversionCharSet operator|(FormatConversionCharSet a,
+                                            FormatConversionCharSet b) {
+  return static_cast<FormatConversionCharSet>(static_cast<uint64_t>(a) |
+                                              static_cast<uint64_t>(b));
+}
+
+// FormatConversionCharSet
+//
+// Specifies the _accepted_ conversion types as a template parameter to
+// FormatConvertResult for custom implementations of `AbslFormatConvert`.
+// Note the helper predefined alias definitions (kIntegral, etc.) below.
+enum class FormatConversionCharSet : uint64_t {
+  // text
+  c = str_format_internal::FormatConversionCharToConvInt('c'),
+  s = str_format_internal::FormatConversionCharToConvInt('s'),
+  // integer
+  d = str_format_internal::FormatConversionCharToConvInt('d'),
+  i = str_format_internal::FormatConversionCharToConvInt('i'),
+  o = str_format_internal::FormatConversionCharToConvInt('o'),
+  u = str_format_internal::FormatConversionCharToConvInt('u'),
+  x = str_format_internal::FormatConversionCharToConvInt('x'),
+  X = str_format_internal::FormatConversionCharToConvInt('X'),
+  // Float
+  f = str_format_internal::FormatConversionCharToConvInt('f'),
+  F = str_format_internal::FormatConversionCharToConvInt('F'),
+  e = str_format_internal::FormatConversionCharToConvInt('e'),
+  E = str_format_internal::FormatConversionCharToConvInt('E'),
+  g = str_format_internal::FormatConversionCharToConvInt('g'),
+  G = str_format_internal::FormatConversionCharToConvInt('G'),
+  a = str_format_internal::FormatConversionCharToConvInt('a'),
+  A = str_format_internal::FormatConversionCharToConvInt('A'),
+  // misc
+  n = str_format_internal::FormatConversionCharToConvInt('n'),
+  p = str_format_internal::FormatConversionCharToConvInt('p'),
+  v = str_format_internal::FormatConversionCharToConvInt('v'),
+
+  // Used for width/precision '*' specification.
+  kStar = static_cast<uint64_t>(
+      absl::str_format_internal::FormatConversionCharSetInternal::kStar),
+  // Some predefined values:
+  kIntegral = d | i | u | o | x | X,
+  kFloating = a | e | f | g | A | E | F | G,
+  kNumeric = kIntegral | kFloating,
+  kString = s,
+  kPointer = p,
+};
+
+// FormatSink
+//
+// A format sink is a generic abstraction to which conversions may write their
+// formatted string data. `absl::FormatConvert()` uses this sink to write its
+// formatted string.
+//
+class FormatSink {
+ public:
+  // FormatSink::Append()
+  //
+  // Appends `count` copies of `ch` to the format sink.
+  void Append(size_t count, char ch) { sink_->Append(count, ch); }
+
+  // Overload of FormatSink::Append() for appending the characters of a string
+  // view to a format sink.
+  void Append(string_view v) { sink_->Append(v); }
+
+  // FormatSink::PutPaddedString()
+  //
+  // Appends `precision` number of bytes of `v` to the format sink. If this is
+  // less than `width`, spaces will be appended first (if `left` is false), or
+  // after (if `left` is true) to ensure the total amount appended is
+  // at least `width`.
+  bool PutPaddedString(string_view v, int width, int precision, bool left) {
+    return sink_->PutPaddedString(v, width, precision, left);
+  }
+
+  // Support `absl::Format(&sink, format, args...)`.
+  friend void AbslFormatFlush(FormatSink* absl_nonnull sink,
+                              absl::string_view v) {
+    sink->Append(v);
+  }
+
+ private:
+  friend str_format_internal::FormatSinkImpl;
+  explicit FormatSink(str_format_internal::FormatSinkImpl* absl_nonnull s)
+      : sink_(s) {}
+  str_format_internal::FormatSinkImpl* absl_nonnull sink_;
+};
+
+// FormatConvertResult
+//
+// Indicates whether a call to AbslFormatConvert() was successful.
+// This return type informs the StrFormat extension framework (through
+// ADL but using the return type) of what conversion characters are supported.
+// It is strongly discouraged to return {false}, as this will result in an
+// empty string in StrFormat.
+template <FormatConversionCharSet C>
+struct FormatConvertResult {
+  bool value;
+};
+
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_STR_FORMAT_H_
diff --git a/absl/strings/str_format_test.cc b/absl/strings/str_format_test.cc
index cfd81bb34be..cbe5ff9a653 100644
--- a/absl/strings/str_format_test.cc
+++ b/absl/strings/str_format_test.cc
@@ -1,15 +1,40 @@
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
 
+#include "absl/strings/str_format.h"
+
+#include <cerrno>
 #include <cstdarg>
 #include <cstdint>
 #include <cstdio>
+#include <iterator>
+#include <ostream>
+#include <sstream>
 #include <string>
+#include <type_traits>
 
-#include "gmock/gmock.h"
 #include "gtest/gtest.h"
-#include "absl/strings/str_format.h"
+#include "absl/base/config.h"
+#include "absl/base/macros.h"
+#include "absl/strings/cord.h"
+#include "absl/strings/internal/str_format/constexpr_parser.h"
+#include "absl/strings/str_cat.h"
 #include "absl/strings/string_view.h"
+#include "absl/types/span.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace {
 using str_format_internal::FormatArgImpl;
 
@@ -25,13 +50,25 @@ TEST_F(FormatEntryPointTest, Format) {
   EXPECT_TRUE(Format(&sink, pc, 123));
   EXPECT_EQ("A format 123", sink);
 }
+
+TEST_F(FormatEntryPointTest, FormatWithV) {
+  std::string sink;
+  EXPECT_TRUE(Format(&sink, "A format %v", 123));
+  EXPECT_EQ("A format 123", sink);
+  sink.clear();
+
+  ParsedFormat<'v'> pc("A format %v");
+  EXPECT_TRUE(Format(&sink, pc, 123));
+  EXPECT_EQ("A format 123", sink);
+}
+
 TEST_F(FormatEntryPointTest, UntypedFormat) {
   constexpr const char* formats[] = {
     "",
     "a",
     "%80d",
-#if !defined(_MSC_VER) && !defined(__ANDROID__) && !defined(__native_client__)
-    // MSVC, NaCL and Android don't support positional syntax.
+#if !defined(_MSC_VER) && !defined(__ANDROID__)
+    // MSVC and Android don't support positional syntax.
     "complicated multipart %% %1$d format %1$0999d",
 #endif  // _MSC_VER
   };
@@ -67,6 +104,14 @@ TEST_F(FormatEntryPointTest, StringFormat) {
   EXPECT_EQ("=123=", StrFormat(view, 123));
 }
 
+TEST_F(FormatEntryPointTest, StringFormatV) {
+  std::string hello = "hello";
+  EXPECT_EQ("hello", StrFormat("%v", hello));
+  EXPECT_EQ("123", StrFormat("%v", 123));
+  constexpr absl::string_view view("=%v=", 4);
+  EXPECT_EQ("=123=", StrFormat(view, 123));
+}
+
 TEST_F(FormatEntryPointTest, AppendFormat) {
   std::string s;
   std::string& r = StrAppendFormat(&s, "%d", 123);
@@ -74,6 +119,13 @@ TEST_F(FormatEntryPointTest, AppendFormat) {
   EXPECT_EQ("123", r);
 }
 
+TEST_F(FormatEntryPointTest, AppendFormatWithV) {
+  std::string s;
+  std::string& r = StrAppendFormat(&s, "%v", 123);
+  EXPECT_EQ(&s, &r);  // should be same object
+  EXPECT_EQ("123", r);
+}
+
 TEST_F(FormatEntryPointTest, AppendFormatFail) {
   std::string s = "orig";
 
@@ -86,15 +138,33 @@ TEST_F(FormatEntryPointTest, AppendFormatFail) {
                 {&arg, 1}));
 }
 
+TEST_F(FormatEntryPointTest, AppendFormatFailWithV) {
+  std::string s = "orig";
+
+  UntypedFormatSpec format(" more %v");
+  FormatArgImpl arg("not an int");
+
+  EXPECT_EQ("orig",
+            str_format_internal::AppendPack(
+                &s, str_format_internal::UntypedFormatSpecImpl::Extract(format),
+                {&arg, 1}));
+}
 
 TEST_F(FormatEntryPointTest, ManyArgs) {
-  EXPECT_EQ("24", StrFormat("%24$d", 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13,
-                            14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24));
-  EXPECT_EQ("60", StrFormat("%60$d", 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13,
-                            14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26,
-                            27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39,
-                            40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52,
-                            53, 54, 55, 56, 57, 58, 59, 60));
+  EXPECT_EQ(
+      "60 59 58 57 56 55 54 53 52 51 50 49 48 47 46 45 44 43 42 41 40 39 38 37 "
+      "36 35 34 33 32 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 "
+      "12 11 10 9 8 7 6 5 4 3 2 1",
+      StrFormat("%60$d %59$d %58$d %57$d %56$d %55$d %54$d %53$d %52$d %51$d "
+                "%50$d %49$d %48$d %47$d %46$d %45$d %44$d %43$d %42$d %41$d "
+                "%40$d %39$d %38$d %37$d %36$d %35$d %34$d %33$d %32$d %31$d "
+                "%30$d %29$d %28$d %27$d %26$d %25$d %24$d %23$d %22$d %21$d "
+                "%20$d %19$d %18$d %17$d %16$d %15$d %14$d %13$d %12$d %11$d "
+                "%10$d %9$d %8$d %7$d %6$d %5$d %4$d %3$d %2$d %1$d",
+                1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18,
+                19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34,
+                35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50,
+                51, 52, 53, 54, 55, 56, 57, 58, 59, 60));
 }
 
 TEST_F(FormatEntryPointTest, Preparsed) {
@@ -106,6 +176,15 @@ TEST_F(FormatEntryPointTest, Preparsed) {
   EXPECT_EQ("=123=", StrFormat(ParsedFormat<'d'>(view), 123));
 }
 
+TEST_F(FormatEntryPointTest, PreparsedWithV) {
+  ParsedFormat<'v'> pc("%v");
+  EXPECT_EQ("123", StrFormat(pc, 123));
+  // rvalue ok?
+  EXPECT_EQ("123", StrFormat(ParsedFormat<'v'>("%v"), 123));
+  constexpr absl::string_view view("=%v=", 4);
+  EXPECT_EQ("=123=", StrFormat(ParsedFormat<'v'>(view), 123));
+}
+
 TEST_F(FormatEntryPointTest, FormatCountCapture) {
   int n = 0;
   EXPECT_EQ("", StrFormat("%n", FormatCountCapture(&n)));
@@ -114,6 +193,14 @@ TEST_F(FormatEntryPointTest, FormatCountCapture) {
   EXPECT_EQ(3, n);
 }
 
+TEST_F(FormatEntryPointTest, FormatCountCaptureWithV) {
+  int n = 0;
+  EXPECT_EQ("", StrFormat("%n", FormatCountCapture(&n)));
+  EXPECT_EQ(0, n);
+  EXPECT_EQ("123", StrFormat("%v%n", 123, FormatCountCapture(&n)));
+  EXPECT_EQ(3, n);
+}
+
 TEST_F(FormatEntryPointTest, FormatCountCaptureWrongType) {
   // Should reject int*.
   int n = 0;
@@ -126,6 +213,18 @@ TEST_F(FormatEntryPointTest, FormatCountCaptureWrongType) {
                     absl::MakeSpan(args)));
 }
 
+TEST_F(FormatEntryPointTest, FormatCountCaptureWrongTypeWithV) {
+  // Should reject int*.
+  int n = 0;
+  UntypedFormatSpec format("%v%n");
+  int i = 123, *ip = &n;
+  FormatArgImpl args[2] = {FormatArgImpl(i), FormatArgImpl(ip)};
+
+  EXPECT_EQ("", str_format_internal::FormatPack(
+                    str_format_internal::UntypedFormatSpecImpl::Extract(format),
+                    absl::MakeSpan(args)));
+}
+
 TEST_F(FormatEntryPointTest, FormatCountCaptureMultiple) {
   int n1 = 0;
   int n2 = 0;
@@ -148,14 +247,29 @@ TEST_F(FormatEntryPointTest, FormatCountCaptureExample) {
       s);
 }
 
+TEST_F(FormatEntryPointTest, FormatCountCaptureExampleWithV) {
+  int n;
+  std::string s;
+  std::string a1 = "(1,1)";
+  std::string a2 = "(1,2)";
+  std::string a3 = "(2,2)";
+  StrAppendFormat(&s, "%v: %n%v\n", a1, FormatCountCapture(&n), a2);
+  StrAppendFormat(&s, "%*s%v\n", n, "", a3);
+  EXPECT_EQ(7, n);
+  EXPECT_EQ(
+      "(1,1): (1,2)\n"
+      "       (2,2)\n",
+      s);
+}
+
 TEST_F(FormatEntryPointTest, Stream) {
   const std::string formats[] = {
     "",
     "a",
     "%80d",
     "%d %u %c %s %f %g",
-#if !defined(_MSC_VER) && !defined(__ANDROID__) && !defined(__native_client__)
-    // MSVC, NaCL and Android don't support positional syntax.
+#if !defined(_MSC_VER) && !defined(__ANDROID__)
+    // MSVC and Android don't support positional syntax.
     "complicated multipart %% %1$d format %1$080d",
 #endif  // _MSC_VER
   };
@@ -166,7 +280,7 @@ TEST_F(FormatEntryPointTest, Stream) {
     std::ostringstream oss;
     oss << StreamFormat(*parsed, 123, 3, 49, "multistreaming!!!", 1.01, 1.01);
     int fmt_result = snprintf(&*buf.begin(), buf.size(), fmt.c_str(),  //
-                                 123, 3, 49, "multistreaming!!!", 1.01, 1.01);
+                              123, 3, 49, "multistreaming!!!", 1.01, 1.01);
     ASSERT_TRUE(oss) << fmt;
     ASSERT_TRUE(fmt_result >= 0 && static_cast<size_t>(fmt_result) < buf.size())
         << fmt_result;
@@ -174,6 +288,36 @@ TEST_F(FormatEntryPointTest, Stream) {
   }
 }
 
+TEST_F(FormatEntryPointTest, StreamWithV) {
+  const std::string formats[] = {
+      "",
+      "a",
+      "%v %u %c %v %f %v",
+  };
+
+  const std::string formats_for_buf[] = {
+      "",
+      "a",
+      "%d %u %c %s %f %g",
+  };
+
+  std::string buf(4096, '\0');
+  for (auto i = 0; i < std::size(formats); ++i) {
+    const auto parsed =
+        ParsedFormat<'v', 'u', 'c', 'v', 'f', 'v'>::NewAllowIgnored(formats[i]);
+    std::ostringstream oss;
+    oss << StreamFormat(*parsed, 123, 3, 49,
+                        absl::string_view("multistreaming!!!"), 1.01, 1.01);
+    int fmt_result =
+        snprintf(&*buf.begin(), buf.size(), formats_for_buf[i].c_str(),  //
+                 123, 3, 49, "multistreaming!!!", 1.01, 1.01);
+    ASSERT_TRUE(oss) << formats[i];
+    ASSERT_TRUE(fmt_result >= 0 && static_cast<size_t>(fmt_result) < buf.size())
+        << fmt_result;
+    EXPECT_EQ(buf.c_str(), oss.str());
+  }
+}
+
 TEST_F(FormatEntryPointTest, StreamOk) {
   std::ostringstream oss;
   oss << StreamFormat("hello %d", 123);
@@ -181,6 +325,13 @@ TEST_F(FormatEntryPointTest, StreamOk) {
   EXPECT_TRUE(oss.good());
 }
 
+TEST_F(FormatEntryPointTest, StreamOkWithV) {
+  std::ostringstream oss;
+  oss << StreamFormat("hello %v", 123);
+  EXPECT_EQ("hello 123", oss.str());
+  EXPECT_TRUE(oss.good());
+}
+
 TEST_F(FormatEntryPointTest, StreamFail) {
   std::ostringstream oss;
   UntypedFormatSpec format("hello %d");
@@ -191,6 +342,16 @@ TEST_F(FormatEntryPointTest, StreamFail) {
   EXPECT_TRUE(oss.fail());
 }
 
+TEST_F(FormatEntryPointTest, StreamFailWithV) {
+  std::ostringstream oss;
+  UntypedFormatSpec format("hello %v");
+  FormatArgImpl arg("non-numeric");
+  oss << str_format_internal::Streamable(
+      str_format_internal::UntypedFormatSpecImpl::Extract(format), {&arg, 1});
+  EXPECT_EQ("hello ", oss.str());  // partial write
+  EXPECT_TRUE(oss.fail());
+}
+
 std::string WithSnprintf(const char* fmt, ...) {
   std::string buf;
   buf.resize(128);
@@ -232,6 +393,12 @@ TEST_F(FormatEntryPointTest, FormatStreamed) {
   EXPECT_EQ("123", StrFormat("%s", FormatStreamed(StreamFormat("%d", 123))));
 }
 
+TEST_F(FormatEntryPointTest, FormatStreamedWithV) {
+  EXPECT_EQ("123", StrFormat("%v", FormatStreamed(123)));
+  EXPECT_EQ("X", StrFormat("%v", FormatStreamed(streamed_test::X())));
+  EXPECT_EQ("123", StrFormat("%v", FormatStreamed(StreamFormat("%d", 123))));
+}
+
 // Helper class that creates a temporary file and exposes a FILE* to it.
 // It will close the file on destruction.
 class TempFile {
@@ -241,7 +408,7 @@ class TempFile {
 
   std::FILE* file() const { return file_; }
 
-  // Read the file into a std::string.
+  // Read the file into a string.
   std::string ReadFile() {
     std::fseek(file_, 0, SEEK_END);
     int size = std::ftell(file_);
@@ -267,6 +434,14 @@ TEST_F(FormatEntryPointTest, FPrintF) {
   EXPECT_EQ(tmp.ReadFile(), "STRING: ABC NUMBER: -000000019");
 }
 
+TEST_F(FormatEntryPointTest, FPrintFWithV) {
+  TempFile tmp;
+  int result =
+      FPrintF(tmp.file(), "STRING: %v NUMBER: %010d", std::string("ABC"), -19);
+  EXPECT_EQ(result, 30);
+  EXPECT_EQ(tmp.ReadFile(), "STRING: ABC NUMBER: -000000019");
+}
+
 TEST_F(FormatEntryPointTest, FPrintFError) {
   errno = 0;
   int result = FPrintF(stdin, "ABC");
@@ -301,6 +476,23 @@ TEST_F(FormatEntryPointTest, PrintF) {
   EXPECT_EQ(result, 30);
   EXPECT_EQ(tmp.ReadFile(), "STRING: ABC NUMBER: -000000019");
 }
+
+TEST_F(FormatEntryPointTest, PrintFWithV) {
+  int stdout_tmp = dup(STDOUT_FILENO);
+
+  TempFile tmp;
+  std::fflush(stdout);
+  dup2(fileno(tmp.file()), STDOUT_FILENO);
+
+  int result = PrintF("STRING: %v NUMBER: %010d", std::string("ABC"), -19);
+
+  std::fflush(stdout);
+  dup2(stdout_tmp, STDOUT_FILENO);
+  close(stdout_tmp);
+
+  EXPECT_EQ(result, 30);
+  EXPECT_EQ(tmp.ReadFile(), "STRING: ABC NUMBER: -000000019");
+}
 #endif  // __GLIBC__
 
 TEST_F(FormatEntryPointTest, SNPrintF) {
@@ -326,13 +518,64 @@ TEST_F(FormatEntryPointTest, SNPrintF) {
   EXPECT_EQ(result, 17);
   EXPECT_EQ(std::string(buffer), "NUMBER: 1234567");
 
-  result = SNPrintF(nullptr, 0, "Just checking the %s of the output.", "size");
+  // Test that the buffer is never written to if the size is zero.
+  buffer[0] = '\0';
+  result = SNPrintF(buffer, 0, "Just checking the %s of the output.", "size");
+  EXPECT_EQ(result, 37);
+  EXPECT_EQ(buffer[0], '\0');
+}
+
+TEST_F(FormatEntryPointTest, SNPrintFTooLarge) {
+  // Formatting more than INT_MAX bytes cannot be represented in the int return
+  // value, so the call reports an error via errno rather than returning a
+  // truncated (and possibly negative) count, matching FPrintF. A zero size
+  // means nothing is written to the buffer.
+  char buffer[16];
+  int width = 2000000000;
+  errno = 0;
+  int result = SNPrintF(buffer, 0, "%*d %*d", width, 0, width, 0);
+  EXPECT_LT(result, 0);
+  EXPECT_EQ(errno, EFBIG);
+}
+
+TEST_F(FormatEntryPointTest, SNPrintFWithV) {
+  char buffer[16];
+  int result =
+      SNPrintF(buffer, sizeof(buffer), "STRING: %v", std::string("ABC"));
+  EXPECT_EQ(result, 11);
+  EXPECT_EQ(std::string(buffer), "STRING: ABC");
+
+  result = SNPrintF(buffer, sizeof(buffer), "NUMBER: %v", 123456);
+  EXPECT_EQ(result, 14);
+  EXPECT_EQ(std::string(buffer), "NUMBER: 123456");
+
+  result = SNPrintF(buffer, sizeof(buffer), "NUMBER: %v", 1234567);
+  EXPECT_EQ(result, 15);
+  EXPECT_EQ(std::string(buffer), "NUMBER: 1234567");
+
+  result = SNPrintF(buffer, sizeof(buffer), "NUMBER: %v", 12345678);
+  EXPECT_EQ(result, 16);
+  EXPECT_EQ(std::string(buffer), "NUMBER: 1234567");
+
+  result = SNPrintF(buffer, sizeof(buffer), "NUMBER: %v", 123456789);
+  EXPECT_EQ(result, 17);
+  EXPECT_EQ(std::string(buffer), "NUMBER: 1234567");
+
+  std::string size = "size";
+
+  // Test that the buffer is never written to if the size is zero.
+  buffer[0] = '\0';
+  result = SNPrintF(buffer, 0, "Just checking the %v of the output.", size);
   EXPECT_EQ(result, 37);
+  EXPECT_EQ(buffer[0], '\0');
 }
 
 TEST(StrFormat, BehavesAsDocumented) {
   std::string s = absl::StrFormat("%s, %d!", "Hello", 123);
   EXPECT_EQ("Hello, 123!", s);
+  std::string hello = "Hello";
+  std::string s2 = absl::StrFormat("%v, %v!", hello, 123);
+  EXPECT_EQ("Hello, 123!", s2);
   // The format of a replacement is
   // '%'[position][flags][width['.'precision]][length_modifier][format]
   EXPECT_EQ(absl::StrFormat("%1$+3.2Lf", 1.1), "+1.10");
@@ -344,24 +587,34 @@ TEST(StrFormat, BehavesAsDocumented) {
   EXPECT_EQ(StrFormat("%c", int{'a'}), "a");
   EXPECT_EQ(StrFormat("%c", long{'a'}), "a");  // NOLINT
   EXPECT_EQ(StrFormat("%c", uint64_t{'a'}), "a");
-  //     "s" - std::string       Eg: "C" -> "C", std::string("C++") -> "C++"
+  //     "s" - string       Eg: "C" -> "C", std::string("C++") -> "C++"
   //           Formats std::string, char*, string_view, and Cord.
   EXPECT_EQ(StrFormat("%s", "C"), "C");
+  EXPECT_EQ(StrFormat("%v", std::string("C")), "C");
   EXPECT_EQ(StrFormat("%s", std::string("C++")), "C++");
+  EXPECT_EQ(StrFormat("%v", std::string("C++")), "C++");
   EXPECT_EQ(StrFormat("%s", string_view("view")), "view");
+  EXPECT_EQ(StrFormat("%v", string_view("view")), "view");
+  EXPECT_EQ(StrFormat("%s", absl::Cord("cord")), "cord");
+  EXPECT_EQ(StrFormat("%v", absl::Cord("cord")), "cord");
   // Integral Conversion
   //     These format integral types: char, int, long, uint64_t, etc.
   EXPECT_EQ(StrFormat("%d", char{10}), "10");
   EXPECT_EQ(StrFormat("%d", int{10}), "10");
   EXPECT_EQ(StrFormat("%d", long{10}), "10");  // NOLINT
   EXPECT_EQ(StrFormat("%d", uint64_t{10}), "10");
+  EXPECT_EQ(StrFormat("%v", int{10}), "10");
+  EXPECT_EQ(StrFormat("%v", long{10}), "10");  // NOLINT
+  EXPECT_EQ(StrFormat("%v", uint64_t{10}), "10");
   //     d,i - signed decimal          Eg: -10 -> "-10"
   EXPECT_EQ(StrFormat("%d", -10), "-10");
   EXPECT_EQ(StrFormat("%i", -10), "-10");
+  EXPECT_EQ(StrFormat("%v", -10), "-10");
   //      o  - octal                   Eg:  10 -> "12"
   EXPECT_EQ(StrFormat("%o", 10), "12");
   //      u  - unsigned decimal        Eg:  10 -> "10"
   EXPECT_EQ(StrFormat("%u", 10), "10");
+  EXPECT_EQ(StrFormat("%v", 10), "10");
   //     x/X - lower,upper case hex    Eg:  10 -> "a"/"A"
   EXPECT_EQ(StrFormat("%x", 10), "a");
   EXPECT_EQ(StrFormat("%X", 10), "A");
@@ -386,6 +639,8 @@ TEST(StrFormat, BehavesAsDocumented) {
   EXPECT_EQ(StrFormat("%g", .01), "0.01");
   EXPECT_EQ(StrFormat("%g", 1e10), "1e+10");
   EXPECT_EQ(StrFormat("%G", 1e10), "1E+10");
+  EXPECT_EQ(StrFormat("%v", .01), "0.01");
+  EXPECT_EQ(StrFormat("%v", 1e10), "1e+10");
   //     a/A - lower,upper case hex    Eg: -3.0 -> "-0x1.8p+1"/"-0X1.8P+1"
 
 // On Android platform <=21, there is a regression in hexfloat formatting.
@@ -400,6 +655,10 @@ TEST(StrFormat, BehavesAsDocumented) {
   const int& something = *reinterpret_cast<const int*>(ptr_value);
   EXPECT_EQ(StrFormat("%p", &something), StrFormat("0x%x", ptr_value));
 
+  // The output of formatting a null pointer is not documented as being a
+  // specific thing, but the attempt should at least compile.
+  (void)StrFormat("%p", nullptr);
+
   // Output widths are supported, with optional flags.
   EXPECT_EQ(StrFormat("%3d", 1), "  1");
   EXPECT_EQ(StrFormat("%3d", 123456), "123456");
@@ -410,6 +669,8 @@ TEST(StrFormat, BehavesAsDocumented) {
   EXPECT_EQ(StrFormat("%#o", 10), "012");
   EXPECT_EQ(StrFormat("%#x", 15), "0xf");
   EXPECT_EQ(StrFormat("%04d", 8), "0008");
+  EXPECT_EQ(StrFormat("%#04x", 0), "0000");
+  EXPECT_EQ(StrFormat("%#04x", 1), "0x01");
   // Posix positional substitution.
   EXPECT_EQ(absl::StrFormat("%2$s, %3$s, %1$s!", "vici", "veni", "vidi"),
             "veni, vidi, vici!");
@@ -423,6 +684,11 @@ TEST(StrFormat, BehavesAsDocumented) {
   EXPECT_EQ(StrFormat("%zd", int{1}), "1");
   EXPECT_EQ(StrFormat("%td", int{1}), "1");
   EXPECT_EQ(StrFormat("%qd", int{1}), "1");
+
+  // Bool is handled correctly depending on whether %v is used
+  EXPECT_EQ(StrFormat("%v", true), "true");
+  EXPECT_EQ(StrFormat("%v", false), "false");
+  EXPECT_EQ(StrFormat("%d", true), "1");
 }
 
 using str_format_internal::ExtendedParsedFormat;
@@ -449,7 +715,7 @@ struct SummarizeConsumer {
     if (conv.precision.is_from_arg()) {
       *out += "." + std::to_string(conv.precision.get_from_arg()) + "$*";
     }
-    *out += conv.conv.Char();
+    *out += str_format_internal::FormatConversionCharToChar(conv.conv);
     *out += "}";
     return true;
   }
@@ -472,6 +738,15 @@ TEST_F(ParsedFormatTest, SimpleChecked) {
             SummarizeParsedFormat(ParsedFormat<'s', '*', 'd'>("%s %.*d")));
 }
 
+TEST_F(ParsedFormatTest, SimpleCheckedWithV) {
+  EXPECT_EQ("[ABC]{v:1$v}[DEF]",
+            SummarizeParsedFormat(ParsedFormat<'v'>("ABC%vDEF")));
+  EXPECT_EQ("{v:1$v}[FFF]{v:2$v}[ZZZ]{f:3$f}",
+            SummarizeParsedFormat(ParsedFormat<'v', 'v', 'f'>("%vFFF%vZZZ%f")));
+  EXPECT_EQ("{v:1$v}[ ]{.*d:3$.2$*d}",
+            SummarizeParsedFormat(ParsedFormat<'v', '*', 'd'>("%v %.*d")));
+}
+
 TEST_F(ParsedFormatTest, SimpleUncheckedCorrect) {
   auto f = ParsedFormat<'d'>::New("ABC%dDEF");
   ASSERT_TRUE(f);
@@ -502,6 +777,23 @@ TEST_F(ParsedFormatTest, SimpleUncheckedCorrect) {
             SummarizeParsedFormat(*dollar));
 }
 
+TEST_F(ParsedFormatTest, SimpleUncheckedCorrectWithV) {
+  auto f = ParsedFormat<'v'>::New("ABC%vDEF");
+  ASSERT_TRUE(f);
+  EXPECT_EQ("[ABC]{v:1$v}[DEF]", SummarizeParsedFormat(*f));
+
+  std::string format = "%vFFF%vZZZ%f";
+  auto f2 = ParsedFormat<'v', 'v', 'f'>::New(format);
+
+  ASSERT_TRUE(f2);
+  EXPECT_EQ("{v:1$v}[FFF]{v:2$v}[ZZZ]{f:3$f}", SummarizeParsedFormat(*f2));
+
+  f2 = ParsedFormat<'v', 'v', 'f'>::New("%v %v %f");
+
+  ASSERT_TRUE(f2);
+  EXPECT_EQ("{v:1$v}[ ]{v:2$v}[ ]{f:3$f}", SummarizeParsedFormat(*f2));
+}
+
 TEST_F(ParsedFormatTest, SimpleUncheckedIgnoredArgs) {
   EXPECT_FALSE((ParsedFormat<'d', 's'>::New("ABC")));
   EXPECT_FALSE((ParsedFormat<'d', 's'>::New("%dABC")));
@@ -517,6 +809,18 @@ TEST_F(ParsedFormatTest, SimpleUncheckedIgnoredArgs) {
   EXPECT_EQ("[ABC]{2$s:2$s}", SummarizeParsedFormat(*f));
 }
 
+TEST_F(ParsedFormatTest, SimpleUncheckedIgnoredArgsWithV) {
+  EXPECT_FALSE((ParsedFormat<'v', 'v'>::New("ABC")));
+  EXPECT_FALSE((ParsedFormat<'v', 'v'>::New("%vABC")));
+  EXPECT_FALSE((ParsedFormat<'v', 's'>::New("ABC%2$s")));
+  auto f = ParsedFormat<'v', 'v'>::NewAllowIgnored("ABC");
+  ASSERT_TRUE(f);
+  EXPECT_EQ("[ABC]", SummarizeParsedFormat(*f));
+  f = ParsedFormat<'v', 'v'>::NewAllowIgnored("%vABC");
+  ASSERT_TRUE(f);
+  EXPECT_EQ("{v:1$v}[ABC]", SummarizeParsedFormat(*f));
+}
+
 TEST_F(ParsedFormatTest, SimpleUncheckedUnsupported) {
   EXPECT_FALSE(ParsedFormat<'d'>::New("%1$d %1$x"));
   EXPECT_FALSE(ParsedFormat<'x'>::New("%1$d %1$x"));
@@ -531,76 +835,245 @@ TEST_F(ParsedFormatTest, SimpleUncheckedIncorrect) {
   EXPECT_FALSE((ParsedFormat<'s', 'd', 'g'>::New(format)));
 }
 
-using str_format_internal::Conv;
+TEST_F(ParsedFormatTest, SimpleUncheckedIncorrectWithV) {
+  EXPECT_FALSE(ParsedFormat<'v'>::New(""));
+
+  EXPECT_FALSE(ParsedFormat<'v'>::New("ABC%vDEF%v"));
+
+  std::string format = "%vFFF%vZZZ%f";
+  EXPECT_FALSE((ParsedFormat<'v', 'v', 'g'>::New(format)));
+}
+
+#if defined(__cpp_nontype_template_parameter_auto)
+
+template <auto T>
+std::true_type IsValidParsedFormatArgTest(ParsedFormat<T>*);
+
+template <auto T>
+std::false_type IsValidParsedFormatArgTest(...);
+
+template <auto T>
+using IsValidParsedFormatArg = decltype(IsValidParsedFormatArgTest<T>(nullptr));
+
+TEST_F(ParsedFormatTest, OnlyValidTypesAllowed) {
+  ASSERT_TRUE(IsValidParsedFormatArg<'c'>::value);
+
+  ASSERT_TRUE(IsValidParsedFormatArg<FormatConversionCharSet::d>::value);
+
+  ASSERT_TRUE(IsValidParsedFormatArg<absl::FormatConversionCharSet::d |
+                                     absl::FormatConversionCharSet::x>::value);
+  ASSERT_TRUE(
+      IsValidParsedFormatArg<absl::FormatConversionCharSet::kIntegral>::value);
+
+  // This is an easy mistake to make, however, this will reduce to an integer
+  // which has no meaning, so we need to ensure it doesn't compile.
+  ASSERT_FALSE(IsValidParsedFormatArg<'x' | 'd'>::value);
+
+  // For now, we disallow construction based on ConversionChar (rather than
+  // CharSet)
+  ASSERT_FALSE(IsValidParsedFormatArg<absl::FormatConversionChar::d>::value);
+}
+
+TEST_F(ParsedFormatTest, ExtendedTyping) {
+  EXPECT_FALSE(ParsedFormat<FormatConversionCharSet::d>::New(""));
+  ASSERT_TRUE(ParsedFormat<absl::FormatConversionCharSet::d>::New("%d"));
+  auto v1 = ParsedFormat<'d', absl::FormatConversionCharSet::s>::New("%d%s");
+  ASSERT_TRUE(v1);
+  auto v2 = ParsedFormat<absl::FormatConversionCharSet::d, 's'>::New("%d%s");
+  ASSERT_TRUE(v2);
+  auto v3 = ParsedFormat<absl::FormatConversionCharSet::d |
+                             absl::FormatConversionCharSet::s,
+                         's'>::New("%d%s");
+  ASSERT_TRUE(v3);
+  auto v4 = ParsedFormat<absl::FormatConversionCharSet::d |
+                             absl::FormatConversionCharSet::s,
+                         's'>::New("%s%s");
+  ASSERT_TRUE(v4);
+}
+
+TEST_F(ParsedFormatTest, ExtendedTypingWithV) {
+  EXPECT_FALSE(ParsedFormat<FormatConversionCharSet::v>::New(""));
+  ASSERT_TRUE(ParsedFormat<absl::FormatConversionCharSet::v>::New("%v"));
+  auto v1 = ParsedFormat<'v', absl::FormatConversionCharSet::v>::New("%v%v");
+  ASSERT_TRUE(v1);
+  auto v2 = ParsedFormat<absl::FormatConversionCharSet::v, 'v'>::New("%v%v");
+  ASSERT_TRUE(v2);
+  auto v3 = ParsedFormat<absl::FormatConversionCharSet::v |
+                             absl::FormatConversionCharSet::v,
+                         'v'>::New("%v%v");
+  ASSERT_TRUE(v3);
+  auto v4 = ParsedFormat<absl::FormatConversionCharSet::v |
+                             absl::FormatConversionCharSet::v,
+                         'v'>::New("%v%v");
+  ASSERT_TRUE(v4);
+}
+#endif
 
 TEST_F(ParsedFormatTest, UncheckedCorrect) {
-  auto f = ExtendedParsedFormat<Conv::d>::New("ABC%dDEF");
+  auto f =
+      ExtendedParsedFormat<absl::FormatConversionCharSet::d>::New("ABC%dDEF");
   ASSERT_TRUE(f);
   EXPECT_EQ("[ABC]{d:1$d}[DEF]", SummarizeParsedFormat(*f));
 
   std::string format = "%sFFF%dZZZ%f";
-  auto f2 =
-      ExtendedParsedFormat<Conv::string, Conv::d, Conv::floating>::New(format);
+  auto f2 = ExtendedParsedFormat<
+      absl::FormatConversionCharSet::kString, absl::FormatConversionCharSet::d,
+      absl::FormatConversionCharSet::kFloating>::New(format);
 
   ASSERT_TRUE(f2);
   EXPECT_EQ("{s:1$s}[FFF]{d:2$d}[ZZZ]{f:3$f}", SummarizeParsedFormat(*f2));
 
-  f2 = ExtendedParsedFormat<Conv::string, Conv::d, Conv::floating>::New(
-      "%s %d %f");
+  f2 = ExtendedParsedFormat<
+      absl::FormatConversionCharSet::kString, absl::FormatConversionCharSet::d,
+      absl::FormatConversionCharSet::kFloating>::New("%s %d %f");
 
   ASSERT_TRUE(f2);
   EXPECT_EQ("{s:1$s}[ ]{d:2$d}[ ]{f:3$f}", SummarizeParsedFormat(*f2));
 
-  auto star = ExtendedParsedFormat<Conv::star, Conv::d>::New("%*d");
+  auto star =
+      ExtendedParsedFormat<absl::FormatConversionCharSet::kStar,
+                           absl::FormatConversionCharSet::d>::New("%*d");
   ASSERT_TRUE(star);
   EXPECT_EQ("{*d:2$1$*d}", SummarizeParsedFormat(*star));
 
-  auto dollar = ExtendedParsedFormat<Conv::d, Conv::s>::New("%2$s %1$d");
+  auto dollar =
+      ExtendedParsedFormat<absl::FormatConversionCharSet::d,
+                           absl::FormatConversionCharSet::s>::New("%2$s %1$d");
   ASSERT_TRUE(dollar);
   EXPECT_EQ("{2$s:2$s}[ ]{1$d:1$d}", SummarizeParsedFormat(*dollar));
   // with reuse
-  dollar = ExtendedParsedFormat<Conv::d, Conv::s>::New("%2$s %1$d %1$d");
+  dollar = ExtendedParsedFormat<
+      absl::FormatConversionCharSet::d,
+      absl::FormatConversionCharSet::s>::New("%2$s %1$d %1$d");
   ASSERT_TRUE(dollar);
   EXPECT_EQ("{2$s:2$s}[ ]{1$d:1$d}[ ]{1$d:1$d}",
             SummarizeParsedFormat(*dollar));
 }
 
+TEST_F(ParsedFormatTest, UncheckedCorrectWithV) {
+  auto f =
+      ExtendedParsedFormat<absl::FormatConversionCharSet::v>::New("ABC%vDEF");
+  ASSERT_TRUE(f);
+  EXPECT_EQ("[ABC]{v:1$v}[DEF]", SummarizeParsedFormat(*f));
+
+  std::string format = "%vFFF%vZZZ%f";
+  auto f2 = ExtendedParsedFormat<
+      absl::FormatConversionCharSet::v, absl::FormatConversionCharSet::v,
+      absl::FormatConversionCharSet::kFloating>::New(format);
+
+  ASSERT_TRUE(f2);
+  EXPECT_EQ("{v:1$v}[FFF]{v:2$v}[ZZZ]{f:3$f}", SummarizeParsedFormat(*f2));
+
+  f2 = ExtendedParsedFormat<
+      absl::FormatConversionCharSet::v, absl::FormatConversionCharSet::v,
+      absl::FormatConversionCharSet::kFloating>::New("%v %v %f");
+
+  ASSERT_TRUE(f2);
+  EXPECT_EQ("{v:1$v}[ ]{v:2$v}[ ]{f:3$f}", SummarizeParsedFormat(*f2));
+}
+
 TEST_F(ParsedFormatTest, UncheckedIgnoredArgs) {
-  EXPECT_FALSE((ExtendedParsedFormat<Conv::d, Conv::s>::New("ABC")));
-  EXPECT_FALSE((ExtendedParsedFormat<Conv::d, Conv::s>::New("%dABC")));
-  EXPECT_FALSE((ExtendedParsedFormat<Conv::d, Conv::s>::New("ABC%2$s")));
-  auto f = ExtendedParsedFormat<Conv::d, Conv::s>::NewAllowIgnored("ABC");
+  EXPECT_FALSE(
+      (ExtendedParsedFormat<absl::FormatConversionCharSet::d,
+                            absl::FormatConversionCharSet::s>::New("ABC")));
+  EXPECT_FALSE(
+      (ExtendedParsedFormat<absl::FormatConversionCharSet::d,
+                            absl::FormatConversionCharSet::s>::New("%dABC")));
+  EXPECT_FALSE(
+      (ExtendedParsedFormat<absl::FormatConversionCharSet::d,
+                            absl::FormatConversionCharSet::s>::New("ABC%2$s")));
+  auto f = ExtendedParsedFormat<
+      absl::FormatConversionCharSet::d,
+      absl::FormatConversionCharSet::s>::NewAllowIgnored("ABC");
   ASSERT_TRUE(f);
   EXPECT_EQ("[ABC]", SummarizeParsedFormat(*f));
-  f = ExtendedParsedFormat<Conv::d, Conv::s>::NewAllowIgnored("%dABC");
+  f = ExtendedParsedFormat<
+      absl::FormatConversionCharSet::d,
+      absl::FormatConversionCharSet::s>::NewAllowIgnored("%dABC");
   ASSERT_TRUE(f);
   EXPECT_EQ("{d:1$d}[ABC]", SummarizeParsedFormat(*f));
-  f = ExtendedParsedFormat<Conv::d, Conv::s>::NewAllowIgnored("ABC%2$s");
+  f = ExtendedParsedFormat<
+      absl::FormatConversionCharSet::d,
+      absl::FormatConversionCharSet::s>::NewAllowIgnored("ABC%2$s");
   ASSERT_TRUE(f);
   EXPECT_EQ("[ABC]{2$s:2$s}", SummarizeParsedFormat(*f));
 }
 
+TEST_F(ParsedFormatTest, UncheckedIgnoredArgsWithV) {
+  EXPECT_FALSE(
+      (ExtendedParsedFormat<absl::FormatConversionCharSet::v,
+                            absl::FormatConversionCharSet::v>::New("ABC")));
+  EXPECT_FALSE(
+      (ExtendedParsedFormat<absl::FormatConversionCharSet::v,
+                            absl::FormatConversionCharSet::v>::New("%vABC")));
+  EXPECT_FALSE((ExtendedParsedFormat<absl::FormatConversionCharSet::v,
+                                     absl::FormatConversionCharSet::s>::
+                    New("ABC%2$s")));
+  auto f = ExtendedParsedFormat<
+      absl::FormatConversionCharSet::v,
+      absl::FormatConversionCharSet::v>::NewAllowIgnored("ABC");
+  ASSERT_TRUE(f);
+  EXPECT_EQ("[ABC]", SummarizeParsedFormat(*f));
+  f = ExtendedParsedFormat<
+      absl::FormatConversionCharSet::v,
+      absl::FormatConversionCharSet::v>::NewAllowIgnored("%vABC");
+  ASSERT_TRUE(f);
+  EXPECT_EQ("{v:1$v}[ABC]", SummarizeParsedFormat(*f));
+}
+
 TEST_F(ParsedFormatTest, UncheckedMultipleTypes) {
-  auto dx = ExtendedParsedFormat<Conv::d | Conv::x>::New("%1$d %1$x");
+  auto dx =
+      ExtendedParsedFormat<absl::FormatConversionCharSet::d |
+                           absl::FormatConversionCharSet::x>::New("%1$d %1$x");
   EXPECT_TRUE(dx);
   EXPECT_EQ("{1$d:1$d}[ ]{1$x:1$x}", SummarizeParsedFormat(*dx));
 
-  dx = ExtendedParsedFormat<Conv::d | Conv::x>::New("%1$d");
+  dx = ExtendedParsedFormat<absl::FormatConversionCharSet::d |
+                            absl::FormatConversionCharSet::x>::New("%1$d");
   EXPECT_TRUE(dx);
   EXPECT_EQ("{1$d:1$d}", SummarizeParsedFormat(*dx));
 }
 
 TEST_F(ParsedFormatTest, UncheckedIncorrect) {
-  EXPECT_FALSE(ExtendedParsedFormat<Conv::d>::New(""));
+  EXPECT_FALSE(ExtendedParsedFormat<absl::FormatConversionCharSet::d>::New(""));
 
-  EXPECT_FALSE(ExtendedParsedFormat<Conv::d>::New("ABC%dDEF%d"));
+  EXPECT_FALSE(ExtendedParsedFormat<absl::FormatConversionCharSet::d>::New(
+      "ABC%dDEF%d"));
 
   std::string format = "%sFFF%dZZZ%f";
-  EXPECT_FALSE((ExtendedParsedFormat<Conv::s, Conv::d, Conv::g>::New(format)));
+  EXPECT_FALSE(
+      (ExtendedParsedFormat<absl::FormatConversionCharSet::s,
+                            absl::FormatConversionCharSet::d,
+                            absl::FormatConversionCharSet::g>::New(format)));
+}
+
+TEST_F(ParsedFormatTest, UncheckedIncorrectWithV) {
+  EXPECT_FALSE(ExtendedParsedFormat<absl::FormatConversionCharSet::v>::New(""));
+
+  EXPECT_FALSE(ExtendedParsedFormat<absl::FormatConversionCharSet::v>::New(
+      "ABC%vDEF%v"));
+
+  std::string format = "%vFFF%vZZZ%f";
+  EXPECT_FALSE(
+      (ExtendedParsedFormat<absl::FormatConversionCharSet::v,
+                            absl::FormatConversionCharSet::g>::New(format)));
 }
 
 TEST_F(ParsedFormatTest, RegressionMixPositional) {
-  EXPECT_FALSE((ExtendedParsedFormat<Conv::d, Conv::o>::New("%1$d %o")));
+  EXPECT_FALSE(
+      (ExtendedParsedFormat<absl::FormatConversionCharSet::d,
+                            absl::FormatConversionCharSet::o>::New("%1$d %o")));
+}
+
+TEST_F(ParsedFormatTest, DisallowModifiersWithV) {
+  auto f = ParsedFormat<'v'>::New("ABC%80vDEF");
+  EXPECT_EQ(f, nullptr);
+
+  f = ParsedFormat<'v'>::New("ABC%0vDEF");
+  EXPECT_EQ(f, nullptr);
+
+  f = ParsedFormat<'v'>::New("ABC%.1vDEF");
+  EXPECT_EQ(f, nullptr);
 }
 
 using FormatWrapperTest = ::testing::Test;
@@ -616,14 +1089,139 @@ TEST_F(FormatWrapperTest, ConstexprStringFormat) {
   EXPECT_EQ(WrappedFormat("%s there", "hello"), "hello there");
 }
 
+TEST_F(FormatWrapperTest, ConstexprStringFormatWithV) {
+  std::string hello = "hello";
+  EXPECT_EQ(WrappedFormat("%v there", hello), "hello there");
+}
+
 TEST_F(FormatWrapperTest, ParsedFormat) {
   ParsedFormat<'s'> format("%s there");
   EXPECT_EQ(WrappedFormat(format, "hello"), "hello there");
 }
 
+TEST_F(FormatWrapperTest, ParsedFormatWithV) {
+  std::string hello = "hello";
+  ParsedFormat<'v'> format("%v there");
+  EXPECT_EQ(WrappedFormat(format, hello), "hello there");
+}
+
 }  // namespace
+ABSL_NAMESPACE_END
 }  // namespace absl
 
+namespace {
+using FormatExtensionTest = ::testing::Test;
+
+struct Point {
+  friend absl::FormatConvertResult<absl::FormatConversionCharSet::kString |
+                                   absl::FormatConversionCharSet::kIntegral |
+                                   absl::FormatConversionCharSet::v>
+  AbslFormatConvert(const Point& p, const absl::FormatConversionSpec& spec,
+                    absl::FormatSink* s) {
+    if (spec.conversion_char() == absl::FormatConversionChar::s) {
+      s->Append(absl::StrCat("x=", p.x, " y=", p.y));
+    } else {
+      s->Append(absl::StrCat(p.x, ",", p.y));
+    }
+    return {true};
+  }
+
+  int x = 10;
+  int y = 20;
+};
+
+TEST_F(FormatExtensionTest, AbslFormatConvertExample) {
+  Point p;
+  EXPECT_EQ(absl::StrFormat("a %s z", p), "a x=10 y=20 z");
+  EXPECT_EQ(absl::StrFormat("a %d z", p), "a 10,20 z");
+  EXPECT_EQ(absl::StrFormat("a %v z", p), "a 10,20 z");
+
+  // Typed formatting will fail to compile an invalid format.
+  // StrFormat("%f", p);  // Does not compile.
+  std::string actual;
+  absl::UntypedFormatSpec f1("%f");
+  // FormatUntyped will return false for bad character.
+  EXPECT_FALSE(absl::FormatUntyped(&actual, f1, {absl::FormatArg(p)}));
+}
+
+struct PointStringify {
+  template <typename FormatSink>
+  friend void AbslStringify(FormatSink& sink, const PointStringify& p) {
+    sink.Append(absl::StrCat("(", p.x, ", ", p.y, ")"));
+  }
+
+  double x = 10.0;
+  double y = 20.0;
+};
+
+TEST_F(FormatExtensionTest, AbslStringifyExample) {
+  PointStringify p;
+  EXPECT_EQ(absl::StrFormat("a %v z", p), "a (10, 20) z");
+}
+
+struct PointStringifyUsingFormat {
+  template <typename FormatSink>
+  friend void AbslStringify(FormatSink& sink,
+                            const PointStringifyUsingFormat& p) {
+    absl::Format(&sink, "(%g, %g)", p.x, p.y);
+  }
+
+  double x = 10.0;
+  double y = 20.0;
+};
+
+TEST_F(FormatExtensionTest, AbslStringifyExampleUsingFormat) {
+  PointStringifyUsingFormat p;
+  EXPECT_EQ(absl::StrFormat("a %v z", p), "a (10, 20) z");
+}
+
+enum class EnumClassWithStringify { Many = 0, Choices = 1 };
+
+template <typename Sink>
+void AbslStringify(Sink& sink, EnumClassWithStringify e) {
+  absl::Format(&sink, "%s",
+               e == EnumClassWithStringify::Many ? "Many" : "Choices");
+}
+
+enum EnumWithStringify { Many, Choices };
+
+template <typename Sink>
+void AbslStringify(Sink& sink, EnumWithStringify e) {
+  absl::Format(&sink, "%s", e == EnumWithStringify::Many ? "Many" : "Choices");
+}
+
+TEST_F(FormatExtensionTest, AbslStringifyWithEnumWithV) {
+  const auto e_class = EnumClassWithStringify::Choices;
+  EXPECT_EQ(absl::StrFormat("My choice is %v", e_class),
+            "My choice is Choices");
+
+  const auto e = EnumWithStringify::Choices;
+  EXPECT_EQ(absl::StrFormat("My choice is %v", e), "My choice is Choices");
+}
+
+TEST_F(FormatExtensionTest, AbslStringifyEnumWithD) {
+  const auto e_class = EnumClassWithStringify::Many;
+  EXPECT_EQ(absl::StrFormat("My choice is %d", e_class), "My choice is 0");
+
+  const auto e = EnumWithStringify::Choices;
+  EXPECT_EQ(absl::StrFormat("My choice is %d", e), "My choice is 1");
+}
+
+enum class EnumWithLargerValue { x = 32 };
+
+template <typename Sink>
+void AbslStringify(Sink& sink, EnumWithLargerValue e) {
+  absl::Format(&sink, "%s", "Many");
+}
+
+TEST_F(FormatExtensionTest, AbslStringifyEnumOtherSpecifiers) {
+  const auto e = EnumWithLargerValue::x;
+  EXPECT_EQ(absl::StrFormat("My choice is %g", e), "My choice is 32");
+  EXPECT_EQ(absl::StrFormat("My choice is %x", e), "My choice is 20");
+}
+
+}  // namespace
+
 // Some codegen thunks that we can use to easily dump the generated assembly for
 // different StrFormat calls.
 
diff --git a/absl/strings/str_join.h b/absl/strings/str_join.h
index 4772f5d17c8..dd9f7f44f35 100644
--- a/absl/strings/str_join.h
+++ b/absl/strings/str_join.h
@@ -60,6 +60,7 @@
 #include "absl/strings/string_view.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 // -----------------------------------------------------------------------------
 // Concept: Formatter
@@ -71,21 +72,15 @@ namespace absl {
 // functions. You may provide your own Formatter to enable `absl::StrJoin()` to
 // work with arbitrary types.
 //
-// The following is an example of a custom Formatter that simply uses
-// `std::to_string()` to format an integer as a std::string.
+// The following is an example of a custom Formatter that uses
+// `absl::FormatDuration` to join a list of `absl::Duration`s.
 //
-//   struct MyFormatter {
-//     void operator()(std::string* out, int i) const {
-//       out->append(std::to_string(i));
-//     }
-//   };
-//
-// You would use the above formatter by passing an instance of it as the final
-// argument to `absl::StrJoin()`:
-//
-//   std::vector<int> v = {1, 2, 3, 4};
-//   std::string s = absl::StrJoin(v, "-", MyFormatter());
-//   EXPECT_EQ("1-2-3-4", s);
+//   std::vector<absl::Duration> v = {absl::Seconds(1), absl::Milliseconds(10)};
+//   std::string s =
+//       absl::StrJoin(v, ", ", [](std::string* out, absl::Duration dur) {
+//         absl::StrAppend(out, absl::FormatDuration(dur));
+//       });
+//   EXPECT_EQ(s, "1s, 10ms");
 //
 // The following standard formatters are provided within this file:
 //
@@ -143,7 +138,7 @@ strings_internal::DereferenceFormatterImpl<Formatter> DereferenceFormatter(
       std::forward<Formatter>(f));
 }
 
-// Function overload of `DererefenceFormatter()` for using a default
+// Function overload of `DereferenceFormatter()` for using a default
 // `AlphaNumFormatter()`.
 inline strings_internal::DereferenceFormatterImpl<
     strings_internal::AlphaNumFormatterImpl>
@@ -169,21 +164,21 @@ DereferenceFormatter() {
 //   // of `absl::string_view` or even `const char*`.
 //   std::vector<std::string> v = {"foo", "bar", "baz"};
 //   std::string s = absl::StrJoin(v, "-");
-//   EXPECT_EQ("foo-bar-baz", s);
+//   EXPECT_EQ(s, "foo-bar-baz");
 //
 // Example 2:
 //   // Joins the values in the given `std::initializer_list<>` specified using
 //   // brace initialization. This pattern also works with an initializer_list
 //   // of ints or `absl::string_view` -- any `AlphaNum`-compatible type.
 //   std::string s = absl::StrJoin({"foo", "bar", "baz"}, "-");
-//   EXPECT_EQ("foo-bar-baz", s);
+//   EXPECT_EQs, "foo-bar-baz");
 //
 // Example 3:
 //   // Joins a collection of ints. This pattern also works with floats,
 //   // doubles, int64s -- any `StrCat()`-compatible type.
 //   std::vector<int> v = {1, 2, 3, -4};
 //   std::string s = absl::StrJoin(v, "-");
-//   EXPECT_EQ("1-2-3--4", s);
+//   EXPECT_EQ(s, "1-2-3--4");
 //
 // Example 4:
 //   // Joins a collection of pointer-to-int. By default, pointers are
@@ -194,7 +189,7 @@ DereferenceFormatter() {
 //   int x = 1, y = 2, z = 3;
 //   std::vector<int*> v = {&x, &y, &z};
 //   std::string s = absl::StrJoin(v, "-");
-//   EXPECT_EQ("1-2-3", s);
+//   EXPECT_EQ(s, "1-2-3");
 //
 // Example 5:
 //   // Dereferencing of `std::unique_ptr<>` is also supported:
@@ -203,42 +198,42 @@ DereferenceFormatter() {
 //   v.emplace_back(new int(2));
 //   v.emplace_back(new int(3));
 //   std::string s = absl::StrJoin(v, "-");
-//   EXPECT_EQ("1-2-3", s);
+//   EXPECT_EQ(s, "1-2-3");
 //
 // Example 6:
 //   // Joins a `std::map`, with each key-value pair separated by an equals
 //   // sign. This pattern would also work with, say, a
 //   // `std::vector<std::pair<>>`.
 //   std::map<std::string, int> m = {
-//       std::make_pair("a", 1),
-//       std::make_pair("b", 2),
-//       std::make_pair("c", 3)};
+//       {"a", 1},
+//       {"b", 2},
+//       {"c", 3}};
 //   std::string s = absl::StrJoin(m, ",", absl::PairFormatter("="));
-//   EXPECT_EQ("a=1,b=2,c=3", s);
+//   EXPECT_EQ(s, "a=1,b=2,c=3");
 //
 // Example 7:
 //   // These examples show how `absl::StrJoin()` handles a few common edge
 //   // cases:
 //   std::vector<std::string> v_empty;
-//   EXPECT_EQ("", absl::StrJoin(v_empty, "-"));
+//   EXPECT_EQ(absl::StrJoin(v_empty, "-"), "");
 //
 //   std::vector<std::string> v_one_item = {"foo"};
-//   EXPECT_EQ("foo", absl::StrJoin(v_one_item, "-"));
+//   EXPECT_EQ(absl::StrJoin(v_one_item, "-"), "foo");
 //
 //   std::vector<std::string> v_empty_string = {""};
-//   EXPECT_EQ("", absl::StrJoin(v_empty_string, "-"));
+//   EXPECT_EQ(absl::StrJoin(v_empty_string, "-"), "");
 //
 //   std::vector<std::string> v_one_item_empty_string = {"a", ""};
-//   EXPECT_EQ("a-", absl::StrJoin(v_one_item_empty_string, "-"));
+//   EXPECT_EQ(absl::StrJoin(v_one_item_empty_string, "-"), "a-");
 //
 //   std::vector<std::string> v_two_empty_string = {"", ""};
-//   EXPECT_EQ("-", absl::StrJoin(v_two_empty_string, "-"));
+//   EXPECT_EQ(absl::StrJoin(v_two_empty_string, "-"), "-");
 //
 // Example 8:
 //   // Joins a `std::tuple<T...>` of heterogeneous types, converting each to
 //   // a std::string using the `absl::AlphaNum` class.
 //   std::string s = absl::StrJoin(std::make_tuple(123, "abc", 0.456), "-");
-//   EXPECT_EQ("123-abc-0.456", s);
+//   EXPECT_EQ(s, "123-abc-0.456");
 
 template <typename Iterator, typename Formatter>
 std::string StrJoin(Iterator start, Iterator end, absl::string_view sep,
@@ -252,12 +247,20 @@ std::string StrJoin(const Range& range, absl::string_view separator,
   return strings_internal::JoinRange(range, separator, fmt);
 }
 
-template <typename T, typename Formatter>
+template <
+    typename T, typename Formatter,
+    typename = std::enable_if_t<!std::is_convertible_v<T, absl::string_view>>>
 std::string StrJoin(std::initializer_list<T> il, absl::string_view separator,
                     Formatter&& fmt) {
   return strings_internal::JoinRange(il, separator, fmt);
 }
 
+template <typename Formatter>
+inline std::string StrJoin(std::initializer_list<absl::string_view> il,
+                           absl::string_view separator, Formatter&& fmt) {
+  return strings_internal::JoinRange(il, separator, fmt);
+}
+
 template <typename... T, typename Formatter>
 std::string StrJoin(const std::tuple<T...>& value, absl::string_view separator,
                     Formatter&& fmt) {
@@ -274,18 +277,25 @@ std::string StrJoin(const Range& range, absl::string_view separator) {
   return strings_internal::JoinRange(range, separator);
 }
 
-template <typename T>
-std::string StrJoin(std::initializer_list<T> il,
-                    absl::string_view separator) {
+template <typename T, typename = std::enable_if_t<
+                          !std::is_convertible_v<T, absl::string_view>>>
+std::string StrJoin(std::initializer_list<T> il, absl::string_view separator) {
+  return strings_internal::JoinRange(il, separator);
+}
+
+inline std::string StrJoin(std::initializer_list<absl::string_view> il,
+                           absl::string_view separator) {
   return strings_internal::JoinRange(il, separator);
 }
 
 template <typename... T>
 std::string StrJoin(const std::tuple<T...>& value,
                     absl::string_view separator) {
-  return strings_internal::JoinAlgorithm(value, separator, AlphaNumFormatter());
+  return strings_internal::JoinTuple(value, separator,
+                                     std::index_sequence_for<T...>{});
 }
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_STR_JOIN_H_
diff --git a/absl/strings/str_join_benchmark.cc b/absl/strings/str_join_benchmark.cc
index d6f689ff300..ffe6696a509 100644
--- a/absl/strings/str_join_benchmark.cc
+++ b/absl/strings/str_join_benchmark.cc
@@ -13,12 +13,12 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-#include "absl/strings/str_join.h"
-
 #include <string>
-#include <vector>
+#include <tuple>
 #include <utility>
+#include <vector>
 
+#include "absl/strings/str_join.h"
 #include "benchmark/benchmark.h"
 
 namespace {
@@ -94,4 +94,13 @@ BENCHMARK(BM_JoinStreamable)
     ->ArgPair(16, 256)
     ->ArgPair(256, 256);
 
+void BM_JoinTuple(benchmark::State& state) {
+  for (auto _ : state) {
+    std::string s =
+        absl::StrJoin(std::make_tuple(123456789, 987654321, 24680, 13579), "/");
+    benchmark::DoNotOptimize(s);
+  }
+}
+BENCHMARK(BM_JoinTuple);
+
 }  // namespace
diff --git a/absl/strings/str_join_test.cc b/absl/strings/str_join_test.cc
index 921d9c2bfa7..66572489b29 100644
--- a/absl/strings/str_join_test.cc
+++ b/absl/strings/str_join_test.cc
@@ -21,11 +21,13 @@
 #include <cstdio>
 #include <functional>
 #include <initializer_list>
+#include <iterator>
 #include <map>
 #include <memory>
 #include <ostream>
+#include <string>
 #include <tuple>
-#include <type_traits>
+#include <utility>
 #include <vector>
 
 #include "gtest/gtest.h"
@@ -33,6 +35,7 @@
 #include "absl/memory/memory.h"
 #include "absl/strings/str_cat.h"
 #include "absl/strings/str_split.h"
+#include "absl/strings/string_view.h"
 
 namespace {
 
@@ -91,7 +94,7 @@ TEST(StrJoin, APIExamples) {
   {
     // Array of ints
     const int a[] = {1, 2, 3, -4};
-    EXPECT_EQ("1-2-3--4", absl::StrJoin(a, a + ABSL_ARRAYSIZE(a), "-"));
+    EXPECT_EQ("1-2-3--4", absl::StrJoin(a, a + std::size(a), "-"));
   }
 
   {
@@ -134,26 +137,26 @@ TEST(StrJoin, APIExamples) {
   //
 
   {
-    // Empty range yields an empty std::string.
+    // Empty range yields an empty string.
     std::vector<std::string> v;
     EXPECT_EQ("", absl::StrJoin(v, "-"));
   }
 
   {
-    // A range of 1 element gives a std::string with that element but no
+    // A range of 1 element gives a string with that element but no
     // separator.
     std::vector<std::string> v = {"foo"};
     EXPECT_EQ("foo", absl::StrJoin(v, "-"));
   }
 
   {
-    // A range with a single empty std::string element
+    // A range with a single empty string element
     std::vector<std::string> v = {""};
     EXPECT_EQ("", absl::StrJoin(v, "-"));
   }
 
   {
-    // A range with 2 elements, one of which is an empty std::string
+    // A range with 2 elements, one of which is an empty string
     std::vector<std::string> v = {"a", ""};
     EXPECT_EQ("a-", absl::StrJoin(v, "-"));
   }
@@ -425,6 +428,42 @@ TEST(StrJoin, InitializerList) {
   }
 }
 
+TEST(StrJoin, StringViewInitializerList) {
+  {
+    // Tests initializer_list of string_views
+    std::string b = "b";
+    EXPECT_EQ("a-b-c", absl::StrJoin({"a", b, "c"}, "-"));
+  }
+  {
+    // Tests initializer_list of string_views with a non-default formatter
+    TestingParenFormatter f;
+    std::string b = "b";
+    EXPECT_EQ("(a)-(b)-(c)", absl::StrJoin({"a", b, "c"}, "-", f));
+  }
+
+  class NoCopy {
+   public:
+    explicit NoCopy(absl::string_view view) : view_(view) {}
+    NoCopy(const NoCopy&) = delete;
+    operator absl::string_view() { return view_; }  // NOLINT
+   private:
+    absl::string_view view_;
+  };
+  {
+    // Tests initializer_list of string_views preferred over initializer_list<T>
+    // for T that is implicitly convertible to string_view
+    EXPECT_EQ("a-b-c",
+              absl::StrJoin({NoCopy("a"), NoCopy("b"), NoCopy("c")}, "-"));
+  }
+  {
+    // Tests initializer_list of string_views preferred over initializer_list<T>
+    // for T that is implicitly convertible to string_view
+    TestingParenFormatter f;
+    EXPECT_EQ("(a)-(b)-(c)",
+              absl::StrJoin({NoCopy("a"), NoCopy("b"), NoCopy("c")}, "-", f));
+  }
+}
+
 TEST(StrJoin, Tuple) {
   EXPECT_EQ("", absl::StrJoin(std::make_tuple(), "-"));
   EXPECT_EQ("hello", absl::StrJoin(std::make_tuple("hello"), "-"));
@@ -462,13 +501,145 @@ TEST(StrJoin, Tuple) {
             absl::StrJoin(std::make_tuple(&x, &y, &z), "-",
                           absl::DereferenceFormatter(TestFormatter())));
   EXPECT_EQ("0x0000000a-hell-3.",
-            absl::StrJoin(std::make_tuple(absl::make_unique<int>(x),
-                                          absl::make_unique<std::string>(y),
-                                          absl::make_unique<double>(z)),
+            absl::StrJoin(std::make_tuple(std::make_unique<int>(x),
+                                          std::make_unique<std::string>(y),
+                                          std::make_unique<double>(z)),
                           "-", absl::DereferenceFormatter(TestFormatter())));
   EXPECT_EQ("0x0000000a-hell-3.",
-            absl::StrJoin(std::make_tuple(absl::make_unique<int>(x), &y, &z),
+            absl::StrJoin(std::make_tuple(std::make_unique<int>(x), &y, &z),
                           "-", absl::DereferenceFormatter(TestFormatter())));
 }
 
+// A minimal value type for `StrJoin` inputs.
+// Used to ensure we do not excessively require more a specific type, such as a
+// `string_view`.
+//
+// Anything that can be  `data()` and `size()` is OK.
+class TestValue {
+ public:
+  TestValue(const char* data, size_t size) : data_(data), size_(size) {}
+  const char* data() const { return data_; }
+  size_t size() const { return size_; }
+
+ private:
+  const char* data_;
+  size_t size_;
+};
+
+// A minimal C++20 forward iterator, used to test that we do not impose
+// excessive requirements on StrJoin inputs.
+//
+// The 2 main differences between pre-C++20 LegacyForwardIterator and the
+// C++20 ForwardIterator are:
+// 1. `operator->` is not required in C++20.
+// 2. `operator*` result does not need to be an lvalue (a reference).
+//
+// The `operator->` requirement was removed on page 17 in:
+// http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p1037r0.pdf
+//
+// See the `[iterator.requirements]` section of the C++ standard.
+//
+// The value type is a template parameter so that we can test the behaviour
+// of `StrJoin` specializations, e.g. the NoFormatter specialization for
+// `string_view`.
+template <typename ValueT>
+class TestIterator {
+ public:
+  using iterator_category = std::forward_iterator_tag;
+  using value_type = ValueT;
+  using pointer = void;
+  using reference = const value_type&;
+  using difference_type = int;
+
+  // `data` must outlive the result.
+  static TestIterator begin(const std::vector<absl::string_view>& data) {
+    return TestIterator(&data, 0);
+  }
+
+  static TestIterator end(const std::vector<absl::string_view>& data) {
+    return TestIterator(nullptr, data.size());
+  }
+
+  bool operator==(const TestIterator& other) const {
+    return pos_ == other.pos_;
+  }
+  bool operator!=(const TestIterator& other) const {
+    return pos_ != other.pos_;
+  }
+
+  // This deliberately returns a `prvalue`.
+  // The requirement to return a reference was removed in C++20.
+  value_type operator*() const {
+    return ValueT((*data_)[pos_].data(), (*data_)[pos_].size());
+  }
+
+  // `operator->()` is deliberately omitted.
+  // The requirement to provide it was removed in C++20.
+
+  TestIterator& operator++() {
+    ++pos_;
+    return *this;
+  }
+
+  TestIterator operator++(int) {
+    TestIterator result = *this;
+    ++(*this);
+    return result;
+  }
+
+  TestIterator& operator--() {
+    --pos_;
+    return *this;
+  }
+
+  TestIterator operator--(int) {
+    TestIterator result = *this;
+    --(*this);
+    return result;
+  }
+
+ private:
+  TestIterator(const std::vector<absl::string_view>* data, size_t pos)
+      : data_(data), pos_(pos) {}
+
+  const std::vector<absl::string_view>* data_;
+  size_t pos_;
+};
+
+template <typename ValueT>
+class TestIteratorRange {
+ public:
+  // `data` must be non-null and must outlive the result.
+  explicit TestIteratorRange(const std::vector<absl::string_view>& data)
+      : begin_(TestIterator<ValueT>::begin(data)),
+        end_(TestIterator<ValueT>::end(data)) {}
+
+  const TestIterator<ValueT>& begin() const { return begin_; }
+  const TestIterator<ValueT>& end() const { return end_; }
+
+ private:
+  TestIterator<ValueT> begin_;
+  TestIterator<ValueT> end_;
+};
+
+TEST(StrJoin, TestIteratorRequirementsNoFormatter) {
+  const std::vector<absl::string_view> a = {"a", "b", "c"};
+
+  // When the value type is string-like (`std::string` or `string_view`),
+  // the NoFormatter template specialization is used internally.
+  EXPECT_EQ("a-b-c",
+            absl::StrJoin(TestIteratorRange<absl::string_view>(a), "-"));
+}
+
+TEST(StrJoin, TestIteratorRequirementsCustomFormatter) {
+  const std::vector<absl::string_view> a = {"a", "b", "c"};
+  EXPECT_EQ("a-b-c",
+            absl::StrJoin(TestIteratorRange<TestValue>(a), "-",
+                          [](std::string* out, const TestValue& value) {
+                            absl::StrAppend(
+                                out,
+                                absl::string_view(value.data(), value.size()));
+                          }));
+}
+
 }  // namespace
diff --git a/absl/strings/str_replace.cc b/absl/strings/str_replace.cc
index 280f63d32a9..377e30cfd73 100644
--- a/absl/strings/str_replace.cc
+++ b/absl/strings/str_replace.cc
@@ -14,9 +14,19 @@
 
 #include "absl/strings/str_replace.h"
 
+#include <cstddef>
+#include <initializer_list>
+#include <string>
+#include <utility>
+#include <vector>
+
+#include "absl/base/config.h"
+#include "absl/base/nullability.h"
 #include "absl/strings/str_cat.h"
+#include "absl/strings/string_view.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace strings_internal {
 
 using FixedMapping =
@@ -27,8 +37,8 @@ using FixedMapping =
 // occurred.
 int ApplySubstitutions(
     absl::string_view s,
-    std::vector<strings_internal::ViableSubstitution>* subs_ptr,
-    std::string* result_ptr) {
+    std::vector<strings_internal::ViableSubstitution>* absl_nonnull subs_ptr,
+    std::string* absl_nonnull result_ptr) {
   auto& subs = *subs_ptr;
   int substitutions = 0;
   size_t pos = 0;
@@ -73,8 +83,9 @@ std::string StrReplaceAll(absl::string_view s,
 }
 
 int StrReplaceAll(strings_internal::FixedMapping replacements,
-                  std::string* target) {
+                  std::string* absl_nonnull target) {
   return StrReplaceAll<strings_internal::FixedMapping>(replacements, target);
 }
 
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/str_replace.h b/absl/strings/str_replace.h
index 30540d02cdf..ac1dcac0948 100644
--- a/absl/strings/str_replace.h
+++ b/absl/strings/str_replace.h
@@ -38,14 +38,18 @@
 #ifndef ABSL_STRINGS_STR_REPLACE_H_
 #define ABSL_STRINGS_STR_REPLACE_H_
 
+#include <cstddef>
+#include <initializer_list>
 #include <string>
 #include <utility>
 #include <vector>
 
 #include "absl/base/attributes.h"
+#include "absl/base/nullability.h"
 #include "absl/strings/string_view.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 // StrReplaceAll()
 //
@@ -64,7 +68,7 @@ namespace absl {
 //        {"$who", "Bob"},
 //        {"#Noun", "Apples"}});
 //   EXPECT_EQ("Bob bought 5 Apples. Thanks Bob!", s);
-ABSL_MUST_USE_RESULT std::string StrReplaceAll(
+[[nodiscard]] std::string StrReplaceAll(
     absl::string_view s,
     std::initializer_list<std::pair<absl::string_view, absl::string_view>>
         replacements);
@@ -112,7 +116,7 @@ std::string StrReplaceAll(absl::string_view s,
 int StrReplaceAll(
     std::initializer_list<std::pair<absl::string_view, absl::string_view>>
         replacements,
-    std::string* target);
+    std::string* absl_nonnull target);
 
 // Overload of `StrReplaceAll()` to replace patterns within a given output
 // string *in place* with replacements provided within a container of key/value
@@ -127,7 +131,8 @@ int StrReplaceAll(
 //  EXPECT_EQ(count, 2);
 //  EXPECT_EQ("if (ptr &lt; &amp;foo)", s);
 template <typename StrToStrMapping>
-int StrReplaceAll(const StrToStrMapping& replacements, std::string* target);
+int StrReplaceAll(const StrToStrMapping& replacements,
+                  std::string* absl_nonnull target);
 
 // Implementation details only, past this point.
 namespace strings_internal {
@@ -157,7 +162,7 @@ template <typename StrToStrMapping>
 std::vector<ViableSubstitution> FindSubstitutions(
     absl::string_view s, const StrToStrMapping& replacements) {
   std::vector<ViableSubstitution> subs;
-  subs.reserve(replacements.size());
+  subs.reserve(std::size(replacements));
 
   for (const auto& rep : replacements) {
     using std::get;
@@ -184,8 +189,8 @@ std::vector<ViableSubstitution> FindSubstitutions(
 }
 
 int ApplySubstitutions(absl::string_view s,
-                       std::vector<ViableSubstitution>* subs_ptr,
-                       std::string* result_ptr);
+                       std::vector<ViableSubstitution>* absl_nonnull subs_ptr,
+                       std::string* absl_nonnull result_ptr);
 
 }  // namespace strings_internal
 
@@ -200,7 +205,8 @@ std::string StrReplaceAll(absl::string_view s,
 }
 
 template <typename StrToStrMapping>
-int StrReplaceAll(const StrToStrMapping& replacements, std::string* target) {
+int StrReplaceAll(const StrToStrMapping& replacements,
+                  std::string* absl_nonnull target) {
   auto subs = strings_internal::FindSubstitutions(*target, replacements);
   if (subs.empty()) return 0;
 
@@ -212,6 +218,7 @@ int StrReplaceAll(const StrToStrMapping& replacements, std::string* target) {
   return substitutions;
 }
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_STR_REPLACE_H_
diff --git a/absl/strings/str_replace_benchmark.cc b/absl/strings/str_replace_benchmark.cc
index 95b2dc105e2..cbe757210f2 100644
--- a/absl/strings/str_replace_benchmark.cc
+++ b/absl/strings/str_replace_benchmark.cc
@@ -12,13 +12,12 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-#include "absl/strings/str_replace.h"
-
 #include <cstring>
 #include <string>
 
-#include "benchmark/benchmark.h"
 #include "absl/base/internal/raw_logging.h"
+#include "absl/strings/str_replace.h"
+#include "benchmark/benchmark.h"
 
 namespace {
 
@@ -62,7 +61,7 @@ void SetUpStrings() {
       }
     }
     // big_string->resize(50);
-    // OK, we've set up the std::string, now let's set up expectations - first by
+    // OK, we've set up the string, now let's set up expectations - first by
     // just replacing "the" with "box"
     after_replacing_the = new std::string(*big_string);
     for (size_t pos = 0;
diff --git a/absl/strings/str_replace_test.cc b/absl/strings/str_replace_test.cc
index 1ca23aff554..8922856735f 100644
--- a/absl/strings/str_replace_test.cc
+++ b/absl/strings/str_replace_test.cc
@@ -16,16 +16,20 @@
 
 #include <list>
 #include <map>
+#include <string>
 #include <tuple>
+#include <utility>
+#include <vector>
 
 #include "gtest/gtest.h"
 #include "absl/strings/str_cat.h"
 #include "absl/strings/str_split.h"
+#include "absl/strings/string_view.h"
 
 TEST(StrReplaceAll, OneReplacement) {
   std::string s;
 
-  // Empty std::string.
+  // Empty string.
   s = absl::StrReplaceAll(s, {{"", ""}});
   EXPECT_EQ(s, "");
   s = absl::StrReplaceAll(s, {{"x", ""}});
@@ -47,7 +51,7 @@ TEST(StrReplaceAll, OneReplacement) {
   s = absl::StrReplaceAll("abc", {{"xyz", "123"}});
   EXPECT_EQ(s, "abc");
 
-  // Replace entire std::string.
+  // Replace entire string.
   s = absl::StrReplaceAll("abc", {{"abc", "xyz"}});
   EXPECT_EQ(s, "xyz");
 
@@ -88,7 +92,7 @@ TEST(StrReplaceAll, OneReplacement) {
 TEST(StrReplaceAll, ManyReplacements) {
   std::string s;
 
-  // Empty std::string.
+  // Empty string.
   s = absl::StrReplaceAll("", {{"", ""}, {"x", ""}, {"", "y"}, {"x", "y"}});
   EXPECT_EQ(s, "");
 
@@ -96,7 +100,7 @@ TEST(StrReplaceAll, ManyReplacements) {
   s = absl::StrReplaceAll("abc", {{"", ""}, {"", "y"}, {"x", ""}});
   EXPECT_EQ(s, "abc");
 
-  // Replace entire std::string, one char at a time
+  // Replace entire string, one char at a time
   s = absl::StrReplaceAll("abc", {{"a", "x"}, {"b", "y"}, {"c", "z"}});
   EXPECT_EQ(s, "xyz");
   s = absl::StrReplaceAll("zxy", {{"z", "x"}, {"x", "y"}, {"y", "z"}});
@@ -152,6 +156,14 @@ TEST(StrReplaceAll, ManyReplacementsInMap) {
   EXPECT_EQ("Bob bought 5 Apples. Thanks Bob!", s);
 }
 
+TEST(StrReplaceAll, ManyReplacementsInArray) {
+  std::pair<std::string, std::string> replacements[] = {
+      {"$who", "Bob"}, {"$count", "5"}, {"#Noun", "Apples"}};
+  std::string s = absl::StrReplaceAll("$who bought $count #Noun. Thanks $who!",
+                                      replacements);
+  EXPECT_EQ("Bob bought 5 Apples. Thanks Bob!", s);
+}
+
 TEST(StrReplaceAll, ReplacementsInPlace) {
   std::string s = std::string("$who bought $count #Noun. Thanks $who!");
   int count;
@@ -174,8 +186,18 @@ TEST(StrReplaceAll, ReplacementsInPlaceInMap) {
   EXPECT_EQ("Bob bought 5 Apples. Thanks Bob!", s);
 }
 
+TEST(StrReplaceAll, ReplacementsInPlaceInArray) {
+  std::string s = std::string("$who bought $count #Noun. Thanks $who!");
+  std::pair<std::string, std::string> replacements[] = {
+      {"$who", "Bob"}, {"$count", "5"}, {"#Noun", "Apples"}};
+  int count;
+  count = absl::StrReplaceAll(replacements, &s);
+  EXPECT_EQ(count, 4);
+  EXPECT_EQ("Bob bought 5 Apples. Thanks Bob!", s);
+}
+
 struct Cont {
-  Cont() {}
+  Cont() = default;
   explicit Cont(absl::string_view src) : data(src) {}
 
   absl::string_view data;
@@ -264,7 +286,7 @@ TEST(StrReplaceAll, Inplace) {
   std::string s;
   int reps;
 
-  // Empty std::string.
+  // Empty string.
   s = "";
   reps = absl::StrReplaceAll({{"", ""}, {"x", ""}, {"", "y"}, {"x", "y"}}, &s);
   EXPECT_EQ(reps, 0);
@@ -276,7 +298,7 @@ TEST(StrReplaceAll, Inplace) {
   EXPECT_EQ(reps, 0);
   EXPECT_EQ(s, "abc");
 
-  // Replace entire std::string, one char at a time
+  // Replace entire string, one char at a time
   s = "abc";
   reps = absl::StrReplaceAll({{"a", "x"}, {"b", "y"}, {"c", "z"}}, &s);
   EXPECT_EQ(reps, 3);
diff --git a/absl/strings/str_split.cc b/absl/strings/str_split.cc
index 25931307f4e..abe486b915e 100644
--- a/absl/strings/str_split.cc
+++ b/absl/strings/str_split.cc
@@ -15,18 +15,16 @@
 #include "absl/strings/str_split.h"
 
 #include <algorithm>
-#include <cassert>
-#include <cstdint>
+#include <cstddef>
 #include <cstdlib>
 #include <cstring>
-#include <iterator>
-#include <limits>
-#include <memory>
 
+#include "absl/base/config.h"
 #include "absl/base/internal/raw_logging.h"
-#include "absl/strings/ascii.h"
+#include "absl/strings/string_view.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 namespace {
 
@@ -41,7 +39,7 @@ absl::string_view GenericFind(absl::string_view text,
                               absl::string_view delimiter, size_t pos,
                               FindPolicy find_policy) {
   if (delimiter.empty() && text.length() > 0) {
-    // Special case for empty std::string delimiters: always return a zero-length
+    // Special case for empty string delimiters: always return a zero-length
     // absl::string_view referring to the item at position 1 past pos.
     return absl::string_view(text.data() + pos + 1, 0);
   }
@@ -59,19 +57,23 @@ absl::string_view GenericFind(absl::string_view text,
 // Finds using absl::string_view::find(), therefore the length of the found
 // delimiter is delimiter.length().
 struct LiteralPolicy {
-  size_t Find(absl::string_view text, absl::string_view delimiter, size_t pos) {
+  static size_t Find(absl::string_view text, absl::string_view delimiter,
+                     size_t pos) {
     return text.find(delimiter, pos);
   }
-  size_t Length(absl::string_view delimiter) { return delimiter.length(); }
+  static size_t Length(absl::string_view delimiter) {
+    return delimiter.length();
+  }
 };
 
 // Finds using absl::string_view::find_first_of(), therefore the length of the
 // found delimiter is 1.
 struct AnyOfPolicy {
-  size_t Find(absl::string_view text, absl::string_view delimiter, size_t pos) {
+  static size_t Find(absl::string_view text, absl::string_view delimiter,
+                     size_t pos) {
     return text.find_first_of(delimiter, pos);
   }
-  size_t Length(absl::string_view /* delimiter */) { return 1; }
+  static size_t Length(absl::string_view /* delimiter */) { return 1; }
 };
 
 }  // namespace
@@ -94,6 +96,11 @@ absl::string_view ByString::Find(absl::string_view text, size_t pos) const {
   return GenericFind(text, delimiter_, pos, LiteralPolicy());
 }
 
+absl::string_view ByAsciiWhitespace::Find(absl::string_view text,
+                                          size_t pos) const {
+  return GenericFind(text, " \t\v\f\r\n", pos, AnyOfPolicy());
+}
+
 //
 // ByChar
 //
@@ -122,11 +129,10 @@ ByLength::ByLength(ptrdiff_t length) : length_(length) {
   ABSL_RAW_CHECK(length > 0, "");
 }
 
-absl::string_view ByLength::Find(absl::string_view text,
-                                      size_t pos) const {
+absl::string_view ByLength::Find(absl::string_view text, size_t pos) const {
   pos = std::min(pos, text.size());  // truncate `pos`
   absl::string_view substr = text.substr(pos);
-  // If the std::string is shorter than the chunk size we say we
+  // If the string is shorter than the chunk size we say we
   // "can't find the delimiter" so this will be the last chunk.
   if (substr.length() <= static_cast<size_t>(length_))
     return absl::string_view(text.data() + text.size(), 0);
@@ -134,4 +140,5 @@ absl::string_view ByLength::Find(absl::string_view text,
   return absl::string_view(substr.data() + length_, 0);
 }
 
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/str_split.h b/absl/strings/str_split.h
index 7333078938b..b72b44634b5 100644
--- a/absl/strings/str_split.h
+++ b/absl/strings/str_split.h
@@ -40,15 +40,19 @@
 #include <map>
 #include <set>
 #include <string>
+#include <type_traits>
 #include <utility>
 #include <vector>
 
 #include "absl/base/internal/raw_logging.h"
+#include "absl/base/macros.h"
+#include "absl/strings/ascii.h"
 #include "absl/strings/internal/str_split_internal.h"
 #include "absl/strings/string_view.h"
 #include "absl/strings/strip.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 //------------------------------------------------------------------------------
 // Delimiters
@@ -125,7 +129,25 @@ class ByString {
   absl::string_view Find(absl::string_view text, size_t pos) const;
 
  private:
-  const std::string delimiter_;
+  std::string delimiter_;
+};
+
+// ByAsciiWhitespace
+//
+// A sub-string delimiter that splits by ASCII whitespace
+// (space, tab, vertical tab, formfeed, linefeed, or carriage return).
+// Note: you probably want to use absl::SkipEmpty() as well!
+//
+// This class is equivalent to ByAnyChar with ASCII whitespace chars.
+//
+// Example:
+//
+//   std::vector<std::string> v = absl::StrSplit(
+//       "a b\tc\n  d  \n", absl::ByAsciiWhitespace(), absl::SkipEmpty());
+//   // v[0] == "a", v[1] == "b", v[2] == "c", v[3] == "d"
+class ByAsciiWhitespace {
+ public:
+  absl::string_view Find(absl::string_view text, size_t pos) const;
 };
 
 // ByChar
@@ -257,7 +279,7 @@ template <typename Delimiter>
 class MaxSplitsImpl {
  public:
   MaxSplitsImpl(Delimiter delimiter, int limit)
-      : delimiter_(delimiter), limit_(limit), count_(0) {}
+      : delimiter_(std::move(delimiter)), limit_(limit), count_(0) {}
   absl::string_view Find(absl::string_view text, size_t pos) {
     if (count_++ == limit_) {
       return absl::string_view(text.data() + text.size(),
@@ -362,22 +384,33 @@ struct SkipEmpty {
 //   // v[0] == " a ", v[1] == " ", v[2] == "b"
 struct SkipWhitespace {
   bool operator()(absl::string_view sp) const {
-    sp = absl::StripAsciiWhitespace(sp);
+    sp = absl::StripLeadingAsciiWhitespace(sp);
     return !sp.empty();
   }
 };
 
+template <typename T>
+using EnableSplitIfString =
+    std::enable_if_t<std::is_same_v<T, std::string> ||
+                         std::is_same_v<T, const std::string>,
+                     int>;
+
 //------------------------------------------------------------------------------
 //                                  StrSplit()
 //------------------------------------------------------------------------------
 
 // StrSplit()
 //
-// Splits a given string based on the provided `Delimiter` object, returning the
-// elements within the type specified by the caller. Optionally, you may pass a
-// `Predicate` to `StrSplit()` indicating whether to include or exclude the
-// resulting element within the final result set. (See the overviews for
-// Delimiters and Predicates above.)
+// Splits a string into a sequence of substrings identified by `Delimiter`. The
+// input is processed sequentially from beginning to end, and each resulting
+// substring is filtered by an optional `Predicate` before inclusion in the
+// result set. `StrSplit()` returns a lazy range that preserves the substrings
+// original order and is convertible to the collection type specified by the
+// caller.
+//
+// Optionally, you may pass a `Predicate` to `StrSplit()` indicating whether to
+// include or exclude the resulting element within the final result set. (See
+// the overviews for Delimiters and Predicates above.)
 //
 // Example:
 //
@@ -418,8 +451,10 @@ struct SkipWhitespace {
 // behavior works for:
 //
 // 1) All standard STL containers including `std::vector`, `std::list`,
-//    `std::deque`, `std::set`,`std::multiset`, 'std::map`, and `std::multimap`
+//    `std::deque`, `std::set`,`std::multiset`, 'std::map`, and `std::multimap`.
 // 2) `std::pair` (which is not actually a container). See below.
+// 3) `std::array`, which is a container but has different behavior due to its
+//    fixed size. See below.
 //
 // Example:
 //
@@ -430,7 +465,7 @@ struct SkipWhitespace {
 //   // Stores results in a std::set<std::string>, which also performs
 //   // de-duplication and orders the elements in ascending order.
 //   std::set<std::string> a = absl::StrSplit("b,a,c,a,b", ',');
-//   // v[0] == "a", v[1] == "b", v[2] = "c"
+//   // a[0] == "a", a[1] == "b", a[2] == "c"
 //
 //   // `StrSplit()` can be used within a range-based for loop, in which case
 //   // each element will be of type `absl::string_view`.
@@ -453,8 +488,7 @@ struct SkipWhitespace {
 // first two split strings become the `std::pair` `.first` and `.second`
 // members, respectively. The remaining split substrings are discarded. If there
 // are less than two split substrings, the empty string is used for the
-// corresponding
-// `std::pair` member.
+// corresponding `std::pair` member.
 //
 // Example:
 //
@@ -462,6 +496,21 @@ struct SkipWhitespace {
 //   std::pair<std::string, std::string> p = absl::StrSplit("a,b,c", ',');
 //   // p.first == "a", p.second == "b"       // "c" is omitted.
 //
+//
+// Splitting to `std::array` is similar to splitting to `std::pair`, but for
+// N elements instead of two; missing elements are filled with the empty string
+// and extra elements are discarded.
+//
+// Examples:
+//
+//   // Stores first two split strings as the elements in a std::array.
+//   std::array<std::string, 2> a = absl::StrSplit("a,b,c", ',');
+//   // a[0] == "a", a[1] == "b"   // "c" is omitted.
+//
+//   // The second element is empty.
+//   std::array<std::string, 2> a = absl::StrSplit("a,", ',');
+//   // a[0] == "a", a[1] == ""
+//
 // The `StrSplit()` function can be used multiple times to perform more
 // complicated splitting logic, such as intelligently parsing key-value pairs.
 //
@@ -487,25 +536,54 @@ struct SkipWhitespace {
 // Try not to depend on this distinction because the bug may one day be fixed.
 template <typename Delimiter>
 strings_internal::Splitter<
-    typename strings_internal::SelectDelimiter<Delimiter>::type, AllowEmpty>
+    typename strings_internal::SelectDelimiter<Delimiter>::type, AllowEmpty,
+    absl::string_view>
 StrSplit(strings_internal::ConvertibleToStringView text, Delimiter d) {
   using DelimiterType =
       typename strings_internal::SelectDelimiter<Delimiter>::type;
-  return strings_internal::Splitter<DelimiterType, AllowEmpty>(
+  return strings_internal::Splitter<DelimiterType, AllowEmpty,
+                                    absl::string_view>(
+      text.value(), DelimiterType(d), AllowEmpty());
+}
+
+template <typename Delimiter, typename StringType,
+          EnableSplitIfString<StringType> = 0>
+strings_internal::Splitter<
+    typename strings_internal::SelectDelimiter<Delimiter>::type, AllowEmpty,
+    std::string>
+StrSplit(StringType&& text, Delimiter d) {
+  using DelimiterType =
+      typename strings_internal::SelectDelimiter<Delimiter>::type;
+  return strings_internal::Splitter<DelimiterType, AllowEmpty, std::string>(
       std::move(text), DelimiterType(d), AllowEmpty());
 }
 
 template <typename Delimiter, typename Predicate>
 strings_internal::Splitter<
-    typename strings_internal::SelectDelimiter<Delimiter>::type, Predicate>
+    typename strings_internal::SelectDelimiter<Delimiter>::type, Predicate,
+    absl::string_view>
 StrSplit(strings_internal::ConvertibleToStringView text, Delimiter d,
          Predicate p) {
   using DelimiterType =
       typename strings_internal::SelectDelimiter<Delimiter>::type;
-  return strings_internal::Splitter<DelimiterType, Predicate>(
+  return strings_internal::Splitter<DelimiterType, Predicate,
+                                    absl::string_view>(
+      text.value(), DelimiterType(std::move(d)), std::move(p));
+}
+
+template <typename Delimiter, typename Predicate, typename StringType,
+          EnableSplitIfString<StringType> = 0>
+strings_internal::Splitter<
+    typename strings_internal::SelectDelimiter<Delimiter>::type, Predicate,
+    std::string>
+StrSplit(StringType&& text, Delimiter d, Predicate p) {
+  using DelimiterType =
+      typename strings_internal::SelectDelimiter<Delimiter>::type;
+  return strings_internal::Splitter<DelimiterType, Predicate, std::string>(
       std::move(text), DelimiterType(d), std::move(p));
 }
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_STR_SPLIT_H_
diff --git a/absl/strings/str_split_benchmark.cc b/absl/strings/str_split_benchmark.cc
index f38dfcfe5af..5a2e95308e5 100644
--- a/absl/strings/str_split_benchmark.cc
+++ b/absl/strings/str_split_benchmark.cc
@@ -12,17 +12,17 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-#include "absl/strings/str_split.h"
-
+#include <cstddef>
 #include <iterator>
 #include <string>
 #include <unordered_map>
 #include <unordered_set>
 #include <vector>
 
-#include "benchmark/benchmark.h"
 #include "absl/base/internal/raw_logging.h"
+#include "absl/strings/str_split.h"
 #include "absl/strings/string_view.h"
+#include "benchmark/benchmark.h"
 
 namespace {
 
diff --git a/absl/strings/str_split_test.cc b/absl/strings/str_split_test.cc
index 02f27bc4149..81553112957 100644
--- a/absl/strings/str_split_test.cc
+++ b/absl/strings/str_split_test.cc
@@ -14,55 +14,57 @@
 
 #include "absl/strings/str_split.h"
 
+#include <array>
+#include <cstddef>
+#include <cstdint>
 #include <deque>
 #include <initializer_list>
+#include <iterator>
 #include <list>
 #include <map>
 #include <memory>
+#include <set>
 #include <string>
-#include <type_traits>
 #include <unordered_map>
 #include <unordered_set>
+#include <utility>
 #include <vector>
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
-#include "absl/base/dynamic_annotations.h"  // for RunningOnValgrind
 #include "absl/base/macros.h"
-#include "absl/strings/numbers.h"
+#include "absl/container/btree_map.h"
+#include "absl/container/btree_set.h"
+#include "absl/container/flat_hash_map.h"
+#include "absl/container/node_hash_map.h"
+#include "absl/hash/hash.h"
+#include "absl/strings/string_view.h"
 
 namespace {
 
 using ::testing::ElementsAre;
+using ::testing::IsEmpty;
 using ::testing::Pair;
 using ::testing::UnorderedElementsAre;
 
 TEST(Split, TraitsTest) {
-  static_assert(!absl::strings_internal::SplitterIsConvertibleTo<int>::value,
-                "");
+  static_assert(!absl::strings_internal::SplitterIsConvertibleTo<int>::value);
   static_assert(
-      !absl::strings_internal::SplitterIsConvertibleTo<std::string>::value, "");
+      !absl::strings_internal::SplitterIsConvertibleTo<std::string>::value);
   static_assert(absl::strings_internal::SplitterIsConvertibleTo<
-                    std::vector<std::string>>::value,
-                "");
-  static_assert(
-      !absl::strings_internal::SplitterIsConvertibleTo<std::vector<int>>::value,
-      "");
+                std::vector<std::string>>::value);
+  static_assert(!absl::strings_internal::SplitterIsConvertibleTo<
+                std::vector<int>>::value);
   static_assert(absl::strings_internal::SplitterIsConvertibleTo<
-                    std::vector<absl::string_view>>::value,
-                "");
+                std::vector<absl::string_view>>::value);
   static_assert(absl::strings_internal::SplitterIsConvertibleTo<
-                    std::map<std::string, std::string>>::value,
-                "");
+                std::map<std::string, std::string>>::value);
   static_assert(absl::strings_internal::SplitterIsConvertibleTo<
-                    std::map<absl::string_view, absl::string_view>>::value,
-                "");
+                std::map<absl::string_view, absl::string_view>>::value);
   static_assert(!absl::strings_internal::SplitterIsConvertibleTo<
-                    std::map<int, std::string>>::value,
-                "");
+                std::map<int, std::string>>::value);
   static_assert(!absl::strings_internal::SplitterIsConvertibleTo<
-                    std::map<std::string, int>>::value,
-                "");
+                std::map<std::string, int>>::value);
 }
 
 // This tests the overall split API, which is made up of the absl::StrSplit()
@@ -71,7 +73,7 @@ TEST(Split, TraitsTest) {
 // namespaces just like callers will need to use.
 TEST(Split, APIExamples) {
   {
-    // Passes std::string delimiter. Assumes the default of ByString.
+    // Passes string delimiter. Assumes the default of ByString.
     std::vector<std::string> v = absl::StrSplit("a,b,c", ",");  // NOLINT
     EXPECT_THAT(v, ElementsAre("a", "b", "c"));
 
@@ -97,7 +99,7 @@ TEST(Split, APIExamples) {
   }
 
   {
-    // Uses the Literal std::string "=>" as the delimiter.
+    // Uses the Literal string "=>" as the delimiter.
     const std::vector<std::string> v = absl::StrSplit("a=>b=>c", "=>");
     EXPECT_THAT(v, ElementsAre("a", "b", "c"));
   }
@@ -121,17 +123,17 @@ TEST(Split, APIExamples) {
   }
 
   {
-    // Splits the input std::string into individual characters by using an empty
-    // std::string as the delimiter.
+    // Splits the input string into individual characters by using an empty
+    // string as the delimiter.
     std::vector<std::string> v = absl::StrSplit("abc", "");
     EXPECT_THAT(v, ElementsAre("a", "b", "c"));
   }
 
   {
-    // Splits std::string data with embedded NUL characters, using NUL as the
+    // Splits string data with embedded NUL characters, using NUL as the
     // delimiter. A simple delimiter of "\0" doesn't work because strlen() will
-    // say that's the empty std::string when constructing the absl::string_view
-    // delimiter. Instead, a non-empty std::string containing NUL can be used as the
+    // say that's the empty string when constructing the absl::string_view
+    // delimiter. Instead, a non-empty string containing NUL can be used as the
     // delimiter.
     std::string embedded_nulls("a\0b\0c", 5);
     std::string null_delim("\0", 1);
@@ -208,7 +210,7 @@ TEST(Split, APIExamples) {
     std::multimap<std::string, std::string> m =
         absl::StrSplit("a,1,b,2,a,3", ',');
     EXPECT_EQ(3, m.size());
-    auto it = m.find("a");
+    auto it = m.lower_bound("a");
     EXPECT_EQ("1", it->second);
     ++it;
     EXPECT_EQ("3", it->second);
@@ -365,7 +367,7 @@ TEST(SplitIterator, EqualityAsEndCondition) {
 TEST(Splitter, RangeIterators) {
   auto splitter = absl::StrSplit("a,b,c", ',');
   std::vector<absl::string_view> output;
-  for (const absl::string_view p : splitter) {
+  for (absl::string_view p : splitter) {
     output.push_back(p);
   }
   EXPECT_THAT(output, ElementsAre("a", "b", "c"));
@@ -390,6 +392,12 @@ void TestPairConversionOperator(const Splitter& splitter) {
   EXPECT_EQ(p, (std::pair<FirstType, SecondType>("a", "b")));
 }
 
+template <typename StringType, typename Splitter>
+void TestArrayConversionOperator(const Splitter& splitter) {
+  std::array<StringType, 2> a = splitter;
+  EXPECT_THAT(a, ElementsAre("a", "b"));
+}
+
 TEST(Splitter, ConversionOperator) {
   auto splitter = absl::StrSplit("a,b,c,d", ',');
 
@@ -403,7 +411,14 @@ TEST(Splitter, ConversionOperator) {
   TestConversionOperator<std::set<std::string>>(splitter);
   TestConversionOperator<std::multiset<absl::string_view>>(splitter);
   TestConversionOperator<std::multiset<std::string>>(splitter);
+  TestConversionOperator<absl::btree_set<absl::string_view>>(splitter);
+  TestConversionOperator<absl::btree_set<std::string>>(splitter);
+  TestConversionOperator<absl::btree_multiset<absl::string_view>>(splitter);
+  TestConversionOperator<absl::btree_multiset<std::string>>(splitter);
   TestConversionOperator<std::unordered_set<std::string>>(splitter);
+  TestConversionOperator<
+      std::unordered_set<absl::string_view, absl::Hash<absl::string_view>>>(
+      splitter);
 
   // Tests conversion to map-like objects.
 
@@ -419,8 +434,45 @@ TEST(Splitter, ConversionOperator) {
   TestMapConversionOperator<std::multimap<std::string, absl::string_view>>(
       splitter);
   TestMapConversionOperator<std::multimap<std::string, std::string>>(splitter);
+  TestMapConversionOperator<
+      absl::btree_map<absl::string_view, absl::string_view>>(splitter);
+  TestMapConversionOperator<absl::btree_map<absl::string_view, std::string>>(
+      splitter);
+  TestMapConversionOperator<absl::btree_map<std::string, absl::string_view>>(
+      splitter);
+  TestMapConversionOperator<absl::btree_map<std::string, std::string>>(
+      splitter);
+  TestMapConversionOperator<
+      absl::btree_multimap<absl::string_view, absl::string_view>>(splitter);
+  TestMapConversionOperator<
+      absl::btree_multimap<absl::string_view, std::string>>(splitter);
+  TestMapConversionOperator<
+      absl::btree_multimap<std::string, absl::string_view>>(splitter);
+  TestMapConversionOperator<absl::btree_multimap<std::string, std::string>>(
+      splitter);
   TestMapConversionOperator<std::unordered_map<std::string, std::string>>(
       splitter);
+  TestMapConversionOperator<std::unordered_map<
+      absl::string_view, absl::string_view, absl::Hash<absl::string_view>>>(
+      splitter);
+  TestMapConversionOperator<std::unordered_map<absl::string_view, std::string,
+                                               absl::Hash<absl::string_view>>>(
+      splitter);
+  TestMapConversionOperator<std::unordered_map<std::string, absl::string_view,
+                                               absl::Hash<absl::string_view>>>(
+      splitter);
+  TestMapConversionOperator<
+      absl::node_hash_map<absl::string_view, absl::string_view>>(splitter);
+  TestMapConversionOperator<
+      absl::node_hash_map<absl::string_view, std::string>>(splitter);
+  TestMapConversionOperator<
+      absl::node_hash_map<std::string, absl::string_view>>(splitter);
+  TestMapConversionOperator<
+      absl::flat_hash_map<absl::string_view, absl::string_view>>(splitter);
+  TestMapConversionOperator<
+      absl::flat_hash_map<absl::string_view, std::string>>(splitter);
+  TestMapConversionOperator<
+      absl::flat_hash_map<std::string, absl::string_view>>(splitter);
 
   // Tests conversion to std::pair
 
@@ -428,6 +480,10 @@ TEST(Splitter, ConversionOperator) {
   TestPairConversionOperator<absl::string_view, std::string>(splitter);
   TestPairConversionOperator<std::string, absl::string_view>(splitter);
   TestPairConversionOperator<std::string, std::string>(splitter);
+
+  // Tests conversion to std::array
+  TestArrayConversionOperator<std::string>(splitter);
+  TestArrayConversionOperator<absl::string_view>(splitter);
 }
 
 // A few additional tests for conversion to std::pair. This conversion is
@@ -436,7 +492,7 @@ TEST(Splitter, ConversionOperator) {
 // less-than, equal-to, and more-than 2 strings.
 TEST(Splitter, ToPair) {
   {
-    // Empty std::string
+    // Empty string
     std::pair<std::string, std::string> p = absl::StrSplit("", ',');
     EXPECT_EQ("", p.first);
     EXPECT_EQ("", p.second);
@@ -472,6 +528,41 @@ TEST(Splitter, ToPair) {
   }
 }
 
+// std::array tests similar to std::pair tests above, testing fewer, exactly,
+// or more elements than the array size.
+TEST(Splitter, ToArray) {
+  {
+    // Empty string
+    std::array<std::string, 2> p = absl::StrSplit("", ',');
+    EXPECT_THAT(p, ElementsAre("", ""));
+  }
+
+  {
+    // Only first
+    std::array<std::string, 2> p = absl::StrSplit("a", ',');
+    EXPECT_THAT(p, ElementsAre("a", ""));
+  }
+
+  {
+    // Only second
+    std::array<std::string, 2> p = absl::StrSplit(",b", ',');
+    EXPECT_THAT(p, ElementsAre("", "b"));
+  }
+
+  {
+    // First and second.
+    std::array<std::string, 2> p = absl::StrSplit("a,b", ',');
+    EXPECT_THAT(p, ElementsAre("a", "b"));
+  }
+
+  {
+    // First and second and then more stuff that will be ignored.
+    std::array<std::string, 2> p = absl::StrSplit("a,b,c", ',');
+    EXPECT_THAT(p, ElementsAre("a", "b"));
+    // "c" is omitted.
+  }
+}
+
 TEST(Splitter, Predicates) {
   static const char kTestChars[] = ",a, ,b,";
   using absl::AllowEmpty;
@@ -565,10 +656,10 @@ TEST(Split, AcceptsCertainTemporaries) {
 
 TEST(Split, Temporary) {
   // Use a std::string longer than the SSO length, so that when the temporary is
-  // destroyed, if the splitter keeps a reference to the std::string's contents,
+  // destroyed, if the splitter keeps a reference to the string's contents,
   // it'll reference freed memory instead of just dead on-stack memory.
   const char input[] = "a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u";
-  EXPECT_LT(sizeof(std::string), ABSL_ARRAYSIZE(input))
+  EXPECT_LT(sizeof(std::string), std::size(input))
       << "Input should be larger than fits on the stack.";
 
   // This happens more often in C++11 as part of a range-based for loop.
@@ -651,14 +742,14 @@ TEST(Split, UTF8) {
   // Tests splitting utf8 strings and utf8 delimiters.
   std::string utf8_string = u8"\u03BA\u1F79\u03C3\u03BC\u03B5";
   {
-    // A utf8 input std::string with an ascii delimiter.
+    // A utf8 input string with an ascii delimiter.
     std::string to_split = "a," + utf8_string;
     std::vector<absl::string_view> v = absl::StrSplit(to_split, ',');
     EXPECT_THAT(v, ElementsAre("a", utf8_string));
   }
 
   {
-    // A utf8 input std::string and a utf8 delimiter.
+    // A utf8 input string and a utf8 delimiter.
     std::string to_split = "a," + utf8_string + ",b";
     std::string unicode_delimiter = "," + utf8_string + ",";
     std::vector<absl::string_view> v =
@@ -667,7 +758,7 @@ TEST(Split, UTF8) {
   }
 
   {
-    // A utf8 input std::string and ByAnyChar with ascii chars.
+    // A utf8 input string and ByAnyChar with ascii chars.
     std::vector<absl::string_view> v =
         absl::StrSplit(u8"Foo h\u00E4llo th\u4E1Ere", absl::ByAnyChar(" \t"));
     EXPECT_THAT(v, ElementsAre("Foo", u8"h\u00E4llo", u8"th\u4E1Ere"));
@@ -814,10 +905,10 @@ TEST(Delimiter, ByString) {
   ByString comma_string(",");
   TestComma(comma_string);
 
-  // The first occurrence of empty std::string ("") in a std::string is at position 0.
+  // The first occurrence of empty string ("") in a string is at position 0.
   // There is a test below that demonstrates this for absl::string_view::find().
   // If the ByString delimiter returned position 0 for this, there would
-  // be an infinite loop in the SplitIterator code. To avoid this, empty std::string
+  // be an infinite loop in the SplitIterator code. To avoid this, empty string
   // is a special case in that it always returns the item at position 1.
   absl::string_view abc("abc");
   EXPECT_EQ(0, abc.find(""));  // "" is found at position 0
@@ -876,7 +967,7 @@ TEST(Delimiter, ByAnyChar) {
   EXPECT_FALSE(IsFoundAt("=", two_delims, -1));
 
   // ByAnyChar behaves just like ByString when given a delimiter of empty
-  // std::string. That is, it always returns a zero-length absl::string_view
+  // string. That is, it always returns a zero-length absl::string_view
   // referring to the item at position 1, not position 0.
   ByAnyChar empty("");
   EXPECT_FALSE(IsFoundAt("", empty, 0));
@@ -885,6 +976,45 @@ TEST(Delimiter, ByAnyChar) {
   EXPECT_TRUE(IsFoundAt("abc", empty, 1));
 }
 
+//
+// Tests for ByAsciiWhitespace
+//
+TEST(Split, ByAsciiWhitespace) {
+  using absl::ByAsciiWhitespace;
+  using absl::SkipEmpty;
+  std::vector<absl::string_view> results;
+
+  results = absl::StrSplit("aaaa\n", ByAsciiWhitespace());
+  EXPECT_THAT(results, ElementsAre("aaaa", ""));
+
+  results = absl::StrSplit("aaaa\n", ByAsciiWhitespace(), SkipEmpty());
+  EXPECT_THAT(results, ElementsAre("aaaa"));
+
+  results = absl::StrSplit(" ", ByAsciiWhitespace());
+  EXPECT_THAT(results, ElementsAre("", ""));
+
+  results = absl::StrSplit(" ", ByAsciiWhitespace(), SkipEmpty());
+  EXPECT_THAT(results, IsEmpty());
+
+  results = absl::StrSplit("a", ByAsciiWhitespace());
+  EXPECT_THAT(results, ElementsAre("a"));
+
+  results = absl::StrSplit("", ByAsciiWhitespace());
+  EXPECT_THAT(results, ElementsAre(""));
+
+  results = absl::StrSplit("", ByAsciiWhitespace(), SkipEmpty());
+  EXPECT_THAT(results, IsEmpty());
+
+  results = absl::StrSplit("a b\tc\n  d\n", ByAsciiWhitespace());
+  EXPECT_THAT(results, ElementsAre("a", "b", "c", "", "", "d", ""));
+
+  results = absl::StrSplit("a b\tc\n  d  \n", ByAsciiWhitespace(), SkipEmpty());
+  EXPECT_THAT(results, ElementsAre("a", "b", "c", "d"));
+
+  results = absl::StrSplit("a\t\n\v\f\r b", ByAsciiWhitespace(), SkipEmpty());
+  EXPECT_THAT(results, ElementsAre("a", "b"));
+}
+
 //
 // Tests for ByLength
 //
@@ -907,13 +1037,19 @@ TEST(Delimiter, ByLength) {
 }
 
 TEST(Split, WorksWithLargeStrings) {
+#if defined(ABSL_HAVE_ADDRESS_SANITIZER) || \
+    defined(ABSL_HAVE_MEMORY_SANITIZER) || defined(ABSL_HAVE_THREAD_SANITIZER)
+  constexpr size_t kSize = (uint32_t{1} << 26) + 1;  // 64M + 1 byte
+#else
+  constexpr size_t kSize = (uint32_t{1} << 31) + 1;  // 2G + 1 byte
+#endif
   if (sizeof(size_t) > 4) {
-    std::string s((uint32_t{1} << 31) + 1, 'x');  // 2G + 1 byte
+    std::string s(kSize, 'x');
     s.back() = '-';
     std::vector<absl::string_view> v = absl::StrSplit(s, '-');
     EXPECT_EQ(2, v.size());
     // The first element will contain 2G of 'x's.
-    // testing::StartsWith is too slow with a 2G std::string.
+    // testing::StartsWith is too slow with a 2G string.
     EXPECT_EQ('x', v[0][0]);
     EXPECT_EQ('x', v[0][1]);
     EXPECT_EQ('x', v[0][3]);
diff --git a/absl/strings/string_view.cc b/absl/strings/string_view.cc
deleted file mode 100644
index dc034a83eea..00000000000
--- a/absl/strings/string_view.cc
+++ /dev/null
@@ -1,233 +0,0 @@
-// Copyright 2017 The Abseil Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-#include "absl/strings/string_view.h"
-
-#ifndef ABSL_HAVE_STD_STRING_VIEW
-
-#include <algorithm>
-#include <climits>
-#include <cstring>
-#include <ostream>
-
-#include "absl/strings/internal/memutil.h"
-
-namespace absl {
-
-namespace {
-void WritePadding(std::ostream& o, size_t pad) {
-  char fill_buf[32];
-  memset(fill_buf, o.fill(), sizeof(fill_buf));
-  while (pad) {
-    size_t n = std::min(pad, sizeof(fill_buf));
-    o.write(fill_buf, n);
-    pad -= n;
-  }
-}
-
-class LookupTable {
- public:
-  // For each character in wanted, sets the index corresponding
-  // to the ASCII code of that character. This is used by
-  // the find_.*_of methods below to tell whether or not a character is in
-  // the lookup table in constant time.
-  explicit LookupTable(string_view wanted) {
-    for (char c : wanted) {
-      table_[Index(c)] = true;
-    }
-  }
-  bool operator[](char c) const { return table_[Index(c)]; }
-
- private:
-  static unsigned char Index(char c) { return static_cast<unsigned char>(c); }
-  bool table_[UCHAR_MAX + 1] = {};
-};
-
-}  // namespace
-
-std::ostream& operator<<(std::ostream& o, string_view piece) {
-  std::ostream::sentry sentry(o);
-  if (sentry) {
-    size_t lpad = 0;
-    size_t rpad = 0;
-    if (static_cast<size_t>(o.width()) > piece.size()) {
-      size_t pad = o.width() - piece.size();
-      if ((o.flags() & o.adjustfield) == o.left) {
-        rpad = pad;
-      } else {
-        lpad = pad;
-      }
-    }
-    if (lpad) WritePadding(o, lpad);
-    o.write(piece.data(), piece.size());
-    if (rpad) WritePadding(o, rpad);
-    o.width(0);
-  }
-  return o;
-}
-
-string_view::size_type string_view::find(string_view s, size_type pos) const
-    noexcept {
-  if (empty() || pos > length_) {
-    if (empty() && pos == 0 && s.empty()) return 0;
-    return npos;
-  }
-  const char* result =
-      strings_internal::memmatch(ptr_ + pos, length_ - pos, s.ptr_, s.length_);
-  return result ? result - ptr_ : npos;
-}
-
-string_view::size_type string_view::find(char c, size_type pos) const noexcept {
-  if (empty() || pos >= length_) {
-    return npos;
-  }
-  const char* result =
-      static_cast<const char*>(memchr(ptr_ + pos, c, length_ - pos));
-  return result != nullptr ? result - ptr_ : npos;
-}
-
-string_view::size_type string_view::rfind(string_view s, size_type pos) const
-    noexcept {
-  if (length_ < s.length_) return npos;
-  if (s.empty()) return std::min(length_, pos);
-  const char* last = ptr_ + std::min(length_ - s.length_, pos) + s.length_;
-  const char* result = std::find_end(ptr_, last, s.ptr_, s.ptr_ + s.length_);
-  return result != last ? result - ptr_ : npos;
-}
-
-// Search range is [0..pos] inclusive.  If pos == npos, search everything.
-string_view::size_type string_view::rfind(char c, size_type pos) const
-    noexcept {
-  // Note: memrchr() is not available on Windows.
-  if (empty()) return npos;
-  for (size_type i = std::min(pos, length_ - 1);; --i) {
-    if (ptr_[i] == c) {
-      return i;
-    }
-    if (i == 0) break;
-  }
-  return npos;
-}
-
-string_view::size_type string_view::find_first_of(string_view s,
-                                                  size_type pos) const
-    noexcept {
-  if (empty() || s.empty()) {
-    return npos;
-  }
-  // Avoid the cost of LookupTable() for a single-character search.
-  if (s.length_ == 1) return find_first_of(s.ptr_[0], pos);
-  LookupTable tbl(s);
-  for (size_type i = pos; i < length_; ++i) {
-    if (tbl[ptr_[i]]) {
-      return i;
-    }
-  }
-  return npos;
-}
-
-string_view::size_type string_view::find_first_not_of(string_view s,
-                                                      size_type pos) const
-    noexcept {
-  if (empty()) return npos;
-  // Avoid the cost of LookupTable() for a single-character search.
-  if (s.length_ == 1) return find_first_not_of(s.ptr_[0], pos);
-  LookupTable tbl(s);
-  for (size_type i = pos; i < length_; ++i) {
-    if (!tbl[ptr_[i]]) {
-      return i;
-    }
-  }
-  return npos;
-}
-
-string_view::size_type string_view::find_first_not_of(char c,
-                                                      size_type pos) const
-    noexcept {
-  if (empty()) return npos;
-  for (; pos < length_; ++pos) {
-    if (ptr_[pos] != c) {
-      return pos;
-    }
-  }
-  return npos;
-}
-
-string_view::size_type string_view::find_last_of(string_view s,
-                                                 size_type pos) const noexcept {
-  if (empty() || s.empty()) return npos;
-  // Avoid the cost of LookupTable() for a single-character search.
-  if (s.length_ == 1) return find_last_of(s.ptr_[0], pos);
-  LookupTable tbl(s);
-  for (size_type i = std::min(pos, length_ - 1);; --i) {
-    if (tbl[ptr_[i]]) {
-      return i;
-    }
-    if (i == 0) break;
-  }
-  return npos;
-}
-
-string_view::size_type string_view::find_last_not_of(string_view s,
-                                                     size_type pos) const
-    noexcept {
-  if (empty()) return npos;
-  size_type i = std::min(pos, length_ - 1);
-  if (s.empty()) return i;
-  // Avoid the cost of LookupTable() for a single-character search.
-  if (s.length_ == 1) return find_last_not_of(s.ptr_[0], pos);
-  LookupTable tbl(s);
-  for (;; --i) {
-    if (!tbl[ptr_[i]]) {
-      return i;
-    }
-    if (i == 0) break;
-  }
-  return npos;
-}
-
-string_view::size_type string_view::find_last_not_of(char c,
-                                                     size_type pos) const
-    noexcept {
-  if (empty()) return npos;
-  size_type i = std::min(pos, length_ - 1);
-  for (;; --i) {
-    if (ptr_[i] != c) {
-      return i;
-    }
-    if (i == 0) break;
-  }
-  return npos;
-}
-
-// MSVC has non-standard behavior that implicitly creates definitions for static
-// const members. These implicit definitions conflict with explicit out-of-class
-// member definitions that are required by the C++ standard, resulting in
-// LNK1169 "multiply defined" errors at link time. __declspec(selectany) asks
-// MSVC to choose only one definition for the symbol it decorates. See details
-// at https://msdn.microsoft.com/en-us/library/34h23df8(v=vs.100).aspx
-#ifdef _MSC_VER
-#define ABSL_STRING_VIEW_SELECTANY __declspec(selectany)
-#else
-#define ABSL_STRING_VIEW_SELECTANY
-#endif
-
-ABSL_STRING_VIEW_SELECTANY
-constexpr string_view::size_type string_view::npos;
-ABSL_STRING_VIEW_SELECTANY
-constexpr string_view::size_type string_view::kMaxSize;
-
-}  // namespace absl
-
-#endif  // ABSL_HAVE_STD_STRING_VIEW
diff --git a/absl/strings/string_view.h b/absl/strings/string_view.h
index 82344eb246b..9daa1492ec7 100644
--- a/absl/strings/string_view.h
+++ b/absl/strings/string_view.h
@@ -1,4 +1,3 @@
-//
 // Copyright 2017 The Abseil Authors.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
@@ -17,569 +16,31 @@
 // File: string_view.h
 // -----------------------------------------------------------------------------
 //
-// This file contains the definition of the `absl::string_view` class. A
-// `string_view` points to a contiguous span of characters, often part or all of
-// another `std::string`, double-quoted string literal, character array, or even
-// another `string_view`.
-//
-// This `absl::string_view` abstraction is designed to be a drop-in
-// replacement for the C++17 `std::string_view` abstraction.
+// Historical note: Abseil once provided an implementation of
+// `absl::string_view` as a polyfill for `std::string_view` prior to C++17. Now
+// that C++17 is required, `absl::string_view` is an alias for
+// `std::string_view`
+
 #ifndef ABSL_STRINGS_STRING_VIEW_H_
 #define ABSL_STRINGS_STRING_VIEW_H_
 
-#include <algorithm>
-#include "absl/base/config.h"
-
-#ifdef ABSL_HAVE_STD_STRING_VIEW
-
-#include <string_view>  // IWYU pragma: export
-
-namespace absl {
-using std::string_view;
-}  // namespace absl
-
-#else  // ABSL_HAVE_STD_STRING_VIEW
+#include <string_view>
 
-#include <cassert>
-#include <cstddef>
-#include <cstring>
-#include <iosfwd>
-#include <iterator>
-#include <limits>
-#include <string>
-
-#include "absl/base/internal/throw_delegate.h"
-#include "absl/base/macros.h"
-#include "absl/base/optimization.h"
-#include "absl/base/port.h"
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/nullability.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
-// absl::string_view
-//
-// A `string_view` provides a lightweight view into the string data provided by
-// a `std::string`, double-quoted string literal, character array, or even
-// another `string_view`. A `string_view` does *not* own the string to which it
-// points, and that data cannot be modified through the view.
-//
-// You can use `string_view` as a function or method parameter anywhere a
-// parameter can receive a double-quoted string literal, `const char*`,
-// `std::string`, or another `absl::string_view` argument with no need to copy
-// the string data. Systematic use of `string_view` within function arguments
-// reduces data copies and `strlen()` calls.
-//
-// Because of its small size, prefer passing `string_view` by value:
-//
-//   void MyFunction(absl::string_view arg);
-//
-// If circumstances require, you may also pass one by const reference:
-//
-//   void MyFunction(const absl::string_view& arg);  // not preferred
-//
-// Passing by value generates slightly smaller code for many architectures.
-//
-// In either case, the source data of the `string_view` must outlive the
-// `string_view` itself.
-//
-// A `string_view` is also suitable for local variables if you know that the
-// lifetime of the underlying object is longer than the lifetime of your
-// `string_view` variable. However, beware of binding a `string_view` to a
-// temporary value:
-//
-//   // BAD use of string_view: lifetime problem
-//   absl::string_view sv = obj.ReturnAString();
-//
-//   // GOOD use of string_view: str outlives sv
-//   std::string str = obj.ReturnAString();
-//   absl::string_view sv = str;
-//
-// Due to lifetime issues, a `string_view` is sometimes a poor choice for a
-// return value and usually a poor choice for a data member. If you do use a
-// `string_view` this way, it is your responsibility to ensure that the object
-// pointed to by the `string_view` outlives the `string_view`.
-//
-// A `string_view` may represent a whole string or just part of a string. For
-// example, when splitting a string, `std::vector<absl::string_view>` is a
-// natural data type for the output.
-//
-// When constructed from a source which is nul-terminated, the `string_view`
-// itself will not include the nul-terminator unless a specific size (including
-// the nul) is passed to the constructor. As a result, common idioms that work
-// on nul-terminated strings do not work on `string_view` objects. If you write
-// code that scans a `string_view`, you must check its length rather than test
-// for nul, for example. Note, however, that nuls may still be embedded within
-// a `string_view` explicitly.
-//
-// You may create a null `string_view` in two ways:
-//
-//   absl::string_view sv();
-//   absl::string_view sv(nullptr, 0);
-//
-// For the above, `sv.data() == nullptr`, `sv.length() == 0`, and
-// `sv.empty() == true`. Also, if you create a `string_view` with a non-null
-// pointer then `sv.data() != nullptr`. Thus, you can use `string_view()` to
-// signal an undefined value that is different from other `string_view` values
-// in a similar fashion to how `const char* p1 = nullptr;` is different from
-// `const char* p2 = "";`. However, in practice, it is not recommended to rely
-// on this behavior.
-//
-// Be careful not to confuse a null `string_view` with an empty one. A null
-// `string_view` is an empty `string_view`, but some empty `string_view`s are
-// not null. Prefer checking for emptiness over checking for null.
-//
-// There are many ways to create an empty string_view:
-//
-//   const char* nullcp = nullptr;
-//   // string_view.size() will return 0 in all cases.
-//   absl::string_view();
-//   absl::string_view(nullcp, 0);
-//   absl::string_view("");
-//   absl::string_view("", 0);
-//   absl::string_view("abcdef", 0);
-//   absl::string_view("abcdef" + 6, 0);
-//
-// All empty `string_view` objects whether null or not, are equal:
-//
-//   absl::string_view() == absl::string_view("", 0)
-//   absl::string_view(nullptr, 0) == absl::string_view("abcdef"+6, 0)
-class string_view {
- public:
-  using traits_type = std::char_traits<char>;
-  using value_type = char;
-  using pointer = char*;
-  using const_pointer = const char*;
-  using reference = char&;
-  using const_reference = const char&;
-  using const_iterator = const char*;
-  using iterator = const_iterator;
-  using const_reverse_iterator = std::reverse_iterator<const_iterator>;
-  using reverse_iterator = const_reverse_iterator;
-  using size_type = size_t;
-  using difference_type = std::ptrdiff_t;
-
-  static constexpr size_type npos = static_cast<size_type>(-1);
-
-  // Null `string_view` constructor
-  constexpr string_view() noexcept : ptr_(nullptr), length_(0) {}
-
-  // Implicit constructors
-
-  template <typename Allocator>
-  string_view(  // NOLINT(runtime/explicit)
-      const std::basic_string<char, std::char_traits<char>, Allocator>&
-          str) noexcept
-      // This is implemented in terms of `string_view(p, n)` so `str.size()`
-      // doesn't need to be reevaluated after `ptr_` is set.
-      : string_view(str.data(), str.size()) {}
-
-  // Implicit constructor of a `string_view` from nul-terminated `str`. When
-  // accepting possibly null strings, use `absl::NullSafeStringView(str)`
-  // instead (see below).
-  constexpr string_view(const char* str)  // NOLINT(runtime/explicit)
-      : ptr_(str),
-        length_(str ? CheckLengthInternal(StrlenInternal(str)) : 0) {}
-
-  // Implicit constructor of a `string_view` from a `const char*` and length.
-  constexpr string_view(const char* data, size_type len)
-      : ptr_(data), length_(CheckLengthInternal(len)) {}
-
-  // NOTE: Harmlessly omitted to work around gdb bug.
-  //   constexpr string_view(const string_view&) noexcept = default;
-  //   string_view& operator=(const string_view&) noexcept = default;
-
-  // Iterators
-
-  // string_view::begin()
-  //
-  // Returns an iterator pointing to the first character at the beginning of the
-  // `string_view`, or `end()` if the `string_view` is empty.
-  constexpr const_iterator begin() const noexcept { return ptr_; }
-
-  // string_view::end()
-  //
-  // Returns an iterator pointing just beyond the last character at the end of
-  // the `string_view`. This iterator acts as a placeholder; attempting to
-  // access it results in undefined behavior.
-  constexpr const_iterator end() const noexcept { return ptr_ + length_; }
-
-  // string_view::cbegin()
-  //
-  // Returns a const iterator pointing to the first character at the beginning
-  // of the `string_view`, or `end()` if the `string_view` is empty.
-  constexpr const_iterator cbegin() const noexcept { return begin(); }
-
-  // string_view::cend()
-  //
-  // Returns a const iterator pointing just beyond the last character at the end
-  // of the `string_view`. This pointer acts as a placeholder; attempting to
-  // access its element results in undefined behavior.
-  constexpr const_iterator cend() const noexcept { return end(); }
-
-  // string_view::rbegin()
-  //
-  // Returns a reverse iterator pointing to the last character at the end of the
-  // `string_view`, or `rend()` if the `string_view` is empty.
-  const_reverse_iterator rbegin() const noexcept {
-    return const_reverse_iterator(end());
-  }
-
-  // string_view::rend()
-  //
-  // Returns a reverse iterator pointing just before the first character at the
-  // beginning of the `string_view`. This pointer acts as a placeholder;
-  // attempting to access its element results in undefined behavior.
-  const_reverse_iterator rend() const noexcept {
-    return const_reverse_iterator(begin());
-  }
-
-  // string_view::crbegin()
-  //
-  // Returns a const reverse iterator pointing to the last character at the end
-  // of the `string_view`, or `crend()` if the `string_view` is empty.
-  const_reverse_iterator crbegin() const noexcept { return rbegin(); }
-
-  // string_view::crend()
-  //
-  // Returns a const reverse iterator pointing just before the first character
-  // at the beginning of the `string_view`. This pointer acts as a placeholder;
-  // attempting to access its element results in undefined behavior.
-  const_reverse_iterator crend() const noexcept { return rend(); }
-
-  // Capacity Utilities
-
-  // string_view::size()
-  //
-  // Returns the number of characters in the `string_view`.
-  constexpr size_type size() const noexcept {
-    return length_;
-  }
-
-  // string_view::length()
-  //
-  // Returns the number of characters in the `string_view`. Alias for `size()`.
-  constexpr size_type length() const noexcept { return size(); }
-
-  // string_view::max_size()
-  //
-  // Returns the maximum number of characters the `string_view` can hold.
-  constexpr size_type max_size() const noexcept { return kMaxSize; }
-
-  // string_view::empty()
-  //
-  // Checks if the `string_view` is empty (refers to no characters).
-  constexpr bool empty() const noexcept { return length_ == 0; }
-
-  // string_view::operator[]
-  //
-  // Returns the ith element of the `string_view` using the array operator.
-  // Note that this operator does not perform any bounds checking.
-  constexpr const_reference operator[](size_type i) const { return ptr_[i]; }
-
-  // string_view::at()
-  //
-  // Returns the ith element of the `string_view`. Bounds checking is performed,
-  // and an exception of type `std::out_of_range` will be thrown on invalid
-  // access.
-  constexpr const_reference at(size_type i) const {
-    return ABSL_PREDICT_TRUE(i < size())
-               ? ptr_[i]
-               : (base_internal::ThrowStdOutOfRange("absl::string_view::at"),
-                  ptr_[i]);
-  }
-
-  // string_view::front()
-  //
-  // Returns the first element of a `string_view`.
-  constexpr const_reference front() const { return ptr_[0]; }
-
-  // string_view::back()
-  //
-  // Returns the last element of a `string_view`.
-  constexpr const_reference back() const { return ptr_[size() - 1]; }
-
-  // string_view::data()
-  //
-  // Returns a pointer to the underlying character array (which is of course
-  // stored elsewhere). Note that `string_view::data()` may contain embedded nul
-  // characters, but the returned buffer may or may not be nul-terminated;
-  // therefore, do not pass `data()` to a routine that expects a nul-terminated
-  // std::string.
-  constexpr const_pointer data() const noexcept { return ptr_; }
-
-  // Modifiers
-
-  // string_view::remove_prefix()
-  //
-  // Removes the first `n` characters from the `string_view`. Note that the
-  // underlying std::string is not changed, only the view.
-  void remove_prefix(size_type n) {
-    assert(n <= length_);
-    ptr_ += n;
-    length_ -= n;
-  }
-
-  // string_view::remove_suffix()
-  //
-  // Removes the last `n` characters from the `string_view`. Note that the
-  // underlying std::string is not changed, only the view.
-  void remove_suffix(size_type n) {
-    assert(n <= length_);
-    length_ -= n;
-  }
-
-  // string_view::swap()
-  //
-  // Swaps this `string_view` with another `string_view`.
-  void swap(string_view& s) noexcept {
-    auto t = *this;
-    *this = s;
-    s = t;
-  }
-
-  // Explicit conversion operators
-
-  // Converts to `std::basic_string`.
-  template <typename A>
-  explicit operator std::basic_string<char, traits_type, A>() const {
-    if (!data()) return {};
-    return std::basic_string<char, traits_type, A>(data(), size());
-  }
-
-  // string_view::copy()
-  //
-  // Copies the contents of the `string_view` at offset `pos` and length `n`
-  // into `buf`.
-  size_type copy(char* buf, size_type n, size_type pos = 0) const {
-    if (ABSL_PREDICT_FALSE(pos > length_)) {
-      base_internal::ThrowStdOutOfRange("absl::string_view::copy");
-    }
-    size_type rlen = (std::min)(length_ - pos, n);
-    if (rlen > 0) {
-      const char* start = ptr_ + pos;
-      traits_type::copy(buf, start, rlen);
-    }
-    return rlen;
-  }
-
-  // string_view::substr()
-  //
-  // Returns a "substring" of the `string_view` (at offset `pos` and length
-  // `n`) as another string_view. This function throws `std::out_of_bounds` if
-  // `pos > size`.
-  string_view substr(size_type pos, size_type n = npos) const {
-    if (ABSL_PREDICT_FALSE(pos > length_))
-      base_internal::ThrowStdOutOfRange("absl::string_view::substr");
-    n = (std::min)(n, length_ - pos);
-    return string_view(ptr_ + pos, n);
-  }
-
-  // string_view::compare()
-  //
-  // Performs a lexicographical comparison between the `string_view` and
-  // another `absl::string_view`, returning -1 if `this` is less than, 0 if
-  // `this` is equal to, and 1 if `this` is greater than the passed std::string
-  // view. Note that in the case of data equality, a further comparison is made
-  // on the respective sizes of the two `string_view`s to determine which is
-  // smaller, equal, or greater.
-  int compare(string_view x) const noexcept {
-    auto min_length = (std::min)(length_, x.length_);
-    if (min_length > 0) {
-      int r = memcmp(ptr_, x.ptr_, min_length);
-      if (r < 0) return -1;
-      if (r > 0) return 1;
-    }
-    if (length_ < x.length_) return -1;
-    if (length_ > x.length_) return 1;
-    return 0;
-  }
-
-  // Overload of `string_view::compare()` for comparing a substring of the
-  // 'string_view` and another `absl::string_view`.
-  int compare(size_type pos1, size_type count1, string_view v) const {
-    return substr(pos1, count1).compare(v);
-  }
-
-  // Overload of `string_view::compare()` for comparing a substring of the
-  // `string_view` and a substring of another `absl::string_view`.
-  int compare(size_type pos1, size_type count1, string_view v, size_type pos2,
-              size_type count2) const {
-    return substr(pos1, count1).compare(v.substr(pos2, count2));
-  }
-
-  // Overload of `string_view::compare()` for comparing a `string_view` and a
-  // a different  C-style std::string `s`.
-  int compare(const char* s) const { return compare(string_view(s)); }
-
-  // Overload of `string_view::compare()` for comparing a substring of the
-  // `string_view` and a different std::string C-style std::string `s`.
-  int compare(size_type pos1, size_type count1, const char* s) const {
-    return substr(pos1, count1).compare(string_view(s));
-  }
-
-  // Overload of `string_view::compare()` for comparing a substring of the
-  // `string_view` and a substring of a different C-style std::string `s`.
-  int compare(size_type pos1, size_type count1, const char* s,
-              size_type count2) const {
-    return substr(pos1, count1).compare(string_view(s, count2));
-  }
-
-  // Find Utilities
-
-  // string_view::find()
-  //
-  // Finds the first occurrence of the substring `s` within the `string_view`,
-  // returning the position of the first character's match, or `npos` if no
-  // match was found.
-  size_type find(string_view s, size_type pos = 0) const noexcept;
-
-  // Overload of `string_view::find()` for finding the given character `c`
-  // within the `string_view`.
-  size_type find(char c, size_type pos = 0) const noexcept;
-
-  // string_view::rfind()
-  //
-  // Finds the last occurrence of a substring `s` within the `string_view`,
-  // returning the position of the first character's match, or `npos` if no
-  // match was found.
-  size_type rfind(string_view s, size_type pos = npos) const
-      noexcept;
-
-  // Overload of `string_view::rfind()` for finding the last given character `c`
-  // within the `string_view`.
-  size_type rfind(char c, size_type pos = npos) const noexcept;
-
-  // string_view::find_first_of()
-  //
-  // Finds the first occurrence of any of the characters in `s` within the
-  // `string_view`, returning the start position of the match, or `npos` if no
-  // match was found.
-  size_type find_first_of(string_view s, size_type pos = 0) const
-      noexcept;
-
-  // Overload of `string_view::find_first_of()` for finding a character `c`
-  // within the `string_view`.
-  size_type find_first_of(char c, size_type pos = 0) const
-      noexcept {
-    return find(c, pos);
-  }
-
-  // string_view::find_last_of()
-  //
-  // Finds the last occurrence of any of the characters in `s` within the
-  // `string_view`, returning the start position of the match, or `npos` if no
-  // match was found.
-  size_type find_last_of(string_view s, size_type pos = npos) const
-      noexcept;
-
-  // Overload of `string_view::find_last_of()` for finding a character `c`
-  // within the `string_view`.
-  size_type find_last_of(char c, size_type pos = npos) const
-      noexcept {
-    return rfind(c, pos);
-  }
-
-  // string_view::find_first_not_of()
-  //
-  // Finds the first occurrence of any of the characters not in `s` within the
-  // `string_view`, returning the start position of the first non-match, or
-  // `npos` if no non-match was found.
-  size_type find_first_not_of(string_view s, size_type pos = 0) const noexcept;
-
-  // Overload of `string_view::find_first_not_of()` for finding a character
-  // that is not `c` within the `string_view`.
-  size_type find_first_not_of(char c, size_type pos = 0) const noexcept;
-
-  // string_view::find_last_not_of()
-  //
-  // Finds the last occurrence of any of the characters not in `s` within the
-  // `string_view`, returning the start position of the last non-match, or
-  // `npos` if no non-match was found.
-  size_type find_last_not_of(string_view s,
-                                          size_type pos = npos) const noexcept;
-
-  // Overload of `string_view::find_last_not_of()` for finding a character
-  // that is not `c` within the `string_view`.
-  size_type find_last_not_of(char c, size_type pos = npos) const
-      noexcept;
-
- private:
-  static constexpr size_type kMaxSize =
-      (std::numeric_limits<difference_type>::max)();
-
-  static constexpr size_type CheckLengthInternal(size_type len) {
-    return ABSL_ASSERT(len <= kMaxSize), len;
-  }
-
-  static constexpr size_type StrlenInternal(const char* str) {
-#if defined(_MSC_VER) && _MSC_VER >= 1910 && !defined(__clang__)
-    // MSVC 2017+ can evaluate this at compile-time.
-    const char* begin = str;
-    while (*str != '\0') ++str;
-    return str - begin;
-#elif ABSL_HAVE_BUILTIN(__builtin_strlen) || \
-    (defined(__GNUC__) && !defined(__clang__))
-    // GCC has __builtin_strlen according to
-    // https://gcc.gnu.org/onlinedocs/gcc-4.7.0/gcc/Other-Builtins.html, but
-    // ABSL_HAVE_BUILTIN doesn't detect that, so we use the extra checks above.
-    // __builtin_strlen is constexpr.
-    return __builtin_strlen(str);
-#else
-    return str ? strlen(str) : 0;
-#endif
-  }
-
-  const char* ptr_;
-  size_type length_;
-};
-
-// This large function is defined inline so that in a fairly common case where
-// one of the arguments is a literal, the compiler can elide a lot of the
-// following comparisons.
-inline bool operator==(string_view x, string_view y) noexcept {
-  auto len = x.size();
-  if (len != y.size()) {
-    return false;
-  }
-
-  return x.data() == y.data() || len <= 0 ||
-         memcmp(x.data(), y.data(), len) == 0;
-}
-
-inline bool operator!=(string_view x, string_view y) noexcept {
-  return !(x == y);
-}
-
-inline bool operator<(string_view x, string_view y) noexcept {
-  auto min_size = (std::min)(x.size(), y.size());
-  const int r = min_size == 0 ? 0 : memcmp(x.data(), y.data(), min_size);
-  return (r < 0) || (r == 0 && x.size() < y.size());
-}
-
-inline bool operator>(string_view x, string_view y) noexcept { return y < x; }
-
-inline bool operator<=(string_view x, string_view y) noexcept {
-  return !(y < x);
-}
-
-inline bool operator>=(string_view x, string_view y) noexcept {
-  return !(x < y);
-}
-
-// IO Insertion Operator
-std::ostream& operator<<(std::ostream& o, string_view piece);
-
-}  // namespace absl
-
-#endif  // ABSL_HAVE_STD_STRING_VIEW
-
-namespace absl {
+using std::string_view;
 
 // ClippedSubstr()
 //
 // Like `s.substr(pos, n)`, but clips `pos` to an upper bound of `s.size()`.
 // Provided because std::string_view::substr throws if `pos > size()`
-inline string_view ClippedSubstr(string_view s, size_t pos,
-                                 size_t n = string_view::npos) {
+inline string_view ClippedSubstr(string_view s ABSL_ATTRIBUTE_LIFETIME_BOUND,
+                                 size_t pos, size_t n = string_view::npos) {
   pos = (std::min)(pos, static_cast<size_t>(s.size()));
   return s.substr(pos, n);
 }
@@ -589,10 +50,11 @@ inline string_view ClippedSubstr(string_view s, size_t pos,
 // Creates an `absl::string_view` from a pointer `p` even if it's null-valued.
 // This function should be used where an `absl::string_view` can be created from
 // a possibly-null pointer.
-inline string_view NullSafeStringView(const char* p) {
+constexpr string_view NullSafeStringView(const char* absl_nullable p) {
   return p ? string_view(p) : string_view();
 }
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_STRING_VIEW_H_
diff --git a/absl/strings/string_view_benchmark.cc b/absl/strings/string_view_benchmark.cc
deleted file mode 100644
index f2e4214c511..00000000000
--- a/absl/strings/string_view_benchmark.cc
+++ /dev/null
@@ -1,347 +0,0 @@
-// Copyright 2018 The Abseil Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-#include "absl/strings/string_view.h"
-
-#include <algorithm>
-#include <cstdint>
-#include <map>
-#include <random>
-#include <string>
-#include <unordered_set>
-#include <vector>
-
-#include "benchmark/benchmark.h"
-#include "absl/base/attributes.h"
-#include "absl/base/internal/raw_logging.h"
-#include "absl/base/macros.h"
-#include "absl/strings/str_cat.h"
-
-namespace {
-
-void BM_StringViewFromString(benchmark::State& state) {
-  std::string s(state.range(0), 'x');
-  std::string* ps = &s;
-  struct SV {
-    SV() = default;
-    explicit SV(const std::string& s) : sv(s) {}
-    absl::string_view sv;
-  } sv;
-  SV* psv = &sv;
-  benchmark::DoNotOptimize(ps);
-  benchmark::DoNotOptimize(psv);
-  for (auto _ : state) {
-    new (psv) SV(*ps);
-    benchmark::DoNotOptimize(sv);
-  }
-}
-BENCHMARK(BM_StringViewFromString)->Arg(12)->Arg(128);
-
-// Provide a forcibly out-of-line wrapper for operator== that can be used in
-// benchmarks to measure the impact of inlining.
-ABSL_ATTRIBUTE_NOINLINE
-bool NonInlinedEq(absl::string_view a, absl::string_view b) { return a == b; }
-
-// We use functions that cannot be inlined to perform the comparison loops so
-// that inlining of the operator== can't optimize away *everything*.
-ABSL_ATTRIBUTE_NOINLINE
-void DoEqualityComparisons(benchmark::State& state, absl::string_view a,
-                           absl::string_view b) {
-  for (auto _ : state) {
-    benchmark::DoNotOptimize(a == b);
-  }
-}
-
-void BM_EqualIdentical(benchmark::State& state) {
-  std::string x(state.range(0), 'a');
-  DoEqualityComparisons(state, x, x);
-}
-BENCHMARK(BM_EqualIdentical)->DenseRange(0, 3)->Range(4, 1 << 10);
-
-void BM_EqualSame(benchmark::State& state) {
-  std::string x(state.range(0), 'a');
-  std::string y = x;
-  DoEqualityComparisons(state, x, y);
-}
-BENCHMARK(BM_EqualSame)
-    ->DenseRange(0, 10)
-    ->Arg(20)
-    ->Arg(40)
-    ->Arg(70)
-    ->Arg(110)
-    ->Range(160, 4096);
-
-void BM_EqualDifferent(benchmark::State& state) {
-  const int len = state.range(0);
-  std::string x(len, 'a');
-  std::string y = x;
-  if (len > 0) {
-    y[len - 1] = 'b';
-  }
-  DoEqualityComparisons(state, x, y);
-}
-BENCHMARK(BM_EqualDifferent)->DenseRange(0, 3)->Range(4, 1 << 10);
-
-// This benchmark is intended to check that important simplifications can be
-// made with absl::string_view comparisons against constant strings. The idea is
-// that if constant strings cause redundant components of the comparison, the
-// compiler should detect and eliminate them. Here we use 8 different strings,
-// each with the same size. Provided our comparison makes the implementation
-// inline-able by the compiler, it should fold all of these away into a single
-// size check once per loop iteration.
-ABSL_ATTRIBUTE_NOINLINE
-void DoConstantSizeInlinedEqualityComparisons(benchmark::State& state,
-                                              absl::string_view a) {
-  for (auto _ : state) {
-    benchmark::DoNotOptimize(a == "aaa");
-    benchmark::DoNotOptimize(a == "bbb");
-    benchmark::DoNotOptimize(a == "ccc");
-    benchmark::DoNotOptimize(a == "ddd");
-    benchmark::DoNotOptimize(a == "eee");
-    benchmark::DoNotOptimize(a == "fff");
-    benchmark::DoNotOptimize(a == "ggg");
-    benchmark::DoNotOptimize(a == "hhh");
-  }
-}
-void BM_EqualConstantSizeInlined(benchmark::State& state) {
-  std::string x(state.range(0), 'a');
-  DoConstantSizeInlinedEqualityComparisons(state, x);
-}
-// We only need to check for size of 3, and <> 3 as this benchmark only has to
-// do with size differences.
-BENCHMARK(BM_EqualConstantSizeInlined)->DenseRange(2, 4);
-
-// This benchmark exists purely to give context to the above timings: this is
-// what they would look like if the compiler is completely unable to simplify
-// between two comparisons when they are comparing against constant strings.
-ABSL_ATTRIBUTE_NOINLINE
-void DoConstantSizeNonInlinedEqualityComparisons(benchmark::State& state,
-                                                 absl::string_view a) {
-  for (auto _ : state) {
-    // Force these out-of-line to compare with the above function.
-    benchmark::DoNotOptimize(NonInlinedEq(a, "aaa"));
-    benchmark::DoNotOptimize(NonInlinedEq(a, "bbb"));
-    benchmark::DoNotOptimize(NonInlinedEq(a, "ccc"));
-    benchmark::DoNotOptimize(NonInlinedEq(a, "ddd"));
-    benchmark::DoNotOptimize(NonInlinedEq(a, "eee"));
-    benchmark::DoNotOptimize(NonInlinedEq(a, "fff"));
-    benchmark::DoNotOptimize(NonInlinedEq(a, "ggg"));
-    benchmark::DoNotOptimize(NonInlinedEq(a, "hhh"));
-  }
-}
-
-void BM_EqualConstantSizeNonInlined(benchmark::State& state) {
-  std::string x(state.range(0), 'a');
-  DoConstantSizeNonInlinedEqualityComparisons(state, x);
-}
-// We only need to check for size of 3, and <> 3 as this benchmark only has to
-// do with size differences.
-BENCHMARK(BM_EqualConstantSizeNonInlined)->DenseRange(2, 4);
-
-void BM_CompareSame(benchmark::State& state) {
-  const int len = state.range(0);
-  std::string x;
-  for (int i = 0; i < len; i++) {
-    x += 'a';
-  }
-  std::string y = x;
-  absl::string_view a = x;
-  absl::string_view b = y;
-
-  for (auto _ : state) {
-    benchmark::DoNotOptimize(a.compare(b));
-  }
-}
-BENCHMARK(BM_CompareSame)->DenseRange(0, 3)->Range(4, 1 << 10);
-
-void BM_find_string_view_len_one(benchmark::State& state) {
-  std::string haystack(state.range(0), '0');
-  absl::string_view s(haystack);
-  for (auto _ : state) {
-    benchmark::DoNotOptimize(s.find("x"));  // not present; length 1
-  }
-}
-BENCHMARK(BM_find_string_view_len_one)->Range(1, 1 << 20);
-
-void BM_find_string_view_len_two(benchmark::State& state) {
-  std::string haystack(state.range(0), '0');
-  absl::string_view s(haystack);
-  for (auto _ : state) {
-    benchmark::DoNotOptimize(s.find("xx"));  // not present; length 2
-  }
-}
-BENCHMARK(BM_find_string_view_len_two)->Range(1, 1 << 20);
-
-void BM_find_one_char(benchmark::State& state) {
-  std::string haystack(state.range(0), '0');
-  absl::string_view s(haystack);
-  for (auto _ : state) {
-    benchmark::DoNotOptimize(s.find('x'));  // not present
-  }
-}
-BENCHMARK(BM_find_one_char)->Range(1, 1 << 20);
-
-void BM_rfind_one_char(benchmark::State& state) {
-  std::string haystack(state.range(0), '0');
-  absl::string_view s(haystack);
-  for (auto _ : state) {
-    benchmark::DoNotOptimize(s.rfind('x'));  // not present
-  }
-}
-BENCHMARK(BM_rfind_one_char)->Range(1, 1 << 20);
-
-void BM_worst_case_find_first_of(benchmark::State& state, int haystack_len) {
-  const int needle_len = state.range(0);
-  std::string needle;
-  for (int i = 0; i < needle_len; ++i) {
-    needle += 'a' + i;
-  }
-  std::string haystack(haystack_len, '0');  // 1000 zeros.
-
-  absl::string_view s(haystack);
-  for (auto _ : state) {
-    benchmark::DoNotOptimize(s.find_first_of(needle));
-  }
-}
-
-void BM_find_first_of_short(benchmark::State& state) {
-  BM_worst_case_find_first_of(state, 10);
-}
-
-void BM_find_first_of_medium(benchmark::State& state) {
-  BM_worst_case_find_first_of(state, 100);
-}
-
-void BM_find_first_of_long(benchmark::State& state) {
-  BM_worst_case_find_first_of(state, 1000);
-}
-
-BENCHMARK(BM_find_first_of_short)->DenseRange(0, 4)->Arg(8)->Arg(16)->Arg(32);
-BENCHMARK(BM_find_first_of_medium)->DenseRange(0, 4)->Arg(8)->Arg(16)->Arg(32);
-BENCHMARK(BM_find_first_of_long)->DenseRange(0, 4)->Arg(8)->Arg(16)->Arg(32);
-
-struct EasyMap : public std::map<absl::string_view, uint64_t> {
-  explicit EasyMap(size_t) {}
-};
-
-// This templated benchmark helper function is intended to stress operator== or
-// operator< in a realistic test.  It surely isn't entirely realistic, but it's
-// a start.  The test creates a map of type Map, a template arg, and populates
-// it with table_size key/value pairs. Each key has WordsPerKey words.  After
-// creating the map, a number of lookups are done in random order.  Some keys
-// are used much more frequently than others in this phase of the test.
-template <typename Map, int WordsPerKey>
-void StringViewMapBenchmark(benchmark::State& state) {
-  const int table_size = state.range(0);
-  const double kFractionOfKeysThatAreHot = 0.2;
-  const int kNumLookupsOfHotKeys = 20;
-  const int kNumLookupsOfColdKeys = 1;
-  const char* words[] = {"the",   "quick",  "brown",    "fox",      "jumped",
-                         "over",  "the",    "lazy",     "dog",      "and",
-                         "found", "a",      "large",    "mushroom", "and",
-                         "a",     "couple", "crickets", "eating",   "pie"};
-  // Create some keys that consist of words in random order.
-  std::random_device r;
-  std::seed_seq seed({r(), r(), r(), r(), r(), r(), r(), r()});
-  std::mt19937 rng(seed);
-  std::vector<std::string> keys(table_size);
-  std::vector<int> all_indices;
-  const int kBlockSize = 1 << 12;
-  std::unordered_set<std::string> t(kBlockSize);
-  std::uniform_int_distribution<int> uniform(0, ABSL_ARRAYSIZE(words) - 1);
-  for (int i = 0; i < table_size; i++) {
-    all_indices.push_back(i);
-    do {
-      keys[i].clear();
-      for (int j = 0; j < WordsPerKey; j++) {
-        absl::StrAppend(&keys[i], j > 0 ? " " : "", words[uniform(rng)]);
-      }
-    } while (!t.insert(keys[i]).second);
-  }
-
-  // Create a list of strings to lookup: a permutation of the array of
-  // keys we just created, with repeats.  "Hot" keys get repeated more.
-  std::shuffle(all_indices.begin(), all_indices.end(), rng);
-  const int num_hot = table_size * kFractionOfKeysThatAreHot;
-  const int num_cold = table_size - num_hot;
-  std::vector<int> hot_indices(all_indices.begin(),
-                               all_indices.begin() + num_hot);
-  std::vector<int> indices;
-  for (int i = 0; i < kNumLookupsOfColdKeys; i++) {
-    indices.insert(indices.end(), all_indices.begin(), all_indices.end());
-  }
-  for (int i = 0; i < kNumLookupsOfHotKeys - kNumLookupsOfColdKeys; i++) {
-    indices.insert(indices.end(), hot_indices.begin(), hot_indices.end());
-  }
-  std::shuffle(indices.begin(), indices.end(), rng);
-  ABSL_RAW_CHECK(
-      num_cold * kNumLookupsOfColdKeys + num_hot * kNumLookupsOfHotKeys ==
-          indices.size(),
-      "");
-  // After constructing the array we probe it with absl::string_views built from
-  // test_strings.  This means operator== won't see equal pointers, so
-  // it'll have to check for equal lengths and equal characters.
-  std::vector<std::string> test_strings(indices.size());
-  for (int i = 0; i < indices.size(); i++) {
-    test_strings[i] = keys[indices[i]];
-  }
-
-  // Run the benchmark. It includes map construction but is mostly
-  // map lookups.
-  for (auto _ : state) {
-    Map h(table_size);
-    for (int i = 0; i < table_size; i++) {
-      h[keys[i]] = i * 2;
-    }
-    ABSL_RAW_CHECK(h.size() == table_size, "");
-    uint64_t sum = 0;
-    for (int i = 0; i < indices.size(); i++) {
-      sum += h[test_strings[i]];
-    }
-    benchmark::DoNotOptimize(sum);
-  }
-}
-
-void BM_StdMap_4(benchmark::State& state) {
-  StringViewMapBenchmark<EasyMap, 4>(state);
-}
-BENCHMARK(BM_StdMap_4)->Range(1 << 10, 1 << 16);
-
-void BM_StdMap_8(benchmark::State& state) {
-  StringViewMapBenchmark<EasyMap, 8>(state);
-}
-BENCHMARK(BM_StdMap_8)->Range(1 << 10, 1 << 16);
-
-void BM_CopyToStringNative(benchmark::State& state) {
-  std::string src(state.range(0), 'x');
-  absl::string_view sv(src);
-  std::string dst;
-  for (auto _ : state) {
-    dst.assign(sv.begin(), sv.end());
-  }
-}
-BENCHMARK(BM_CopyToStringNative)->Range(1 << 3, 1 << 12);
-
-void BM_AppendToStringNative(benchmark::State& state) {
-  std::string src(state.range(0), 'x');
-  absl::string_view sv(src);
-  std::string dst;
-  for (auto _ : state) {
-    dst.clear();
-    dst.insert(dst.end(), sv.begin(), sv.end());
-  }
-}
-BENCHMARK(BM_AppendToStringNative)->Range(1 << 3, 1 << 12);
-
-}  // namespace
diff --git a/absl/strings/string_view_test.cc b/absl/strings/string_view_test.cc
index eb8b170bbbd..d6cec6faf33 100644
--- a/absl/strings/string_view_test.cc
+++ b/absl/strings/string_view_test.cc
@@ -14,684 +14,27 @@
 
 #include "absl/strings/string_view.h"
 
-#include <stdlib.h>
+#include <array>
+#include <cstddef>
+#include <cstdlib>
+#include <cstring>
 #include <iomanip>
+#include <ios>
 #include <iterator>
 #include <limits>
 #include <map>
+#include <memory>
 #include <sstream>
-#include <stdexcept>
 #include <string>
 #include <type_traits>
 #include <utility>
 
 #include "gtest/gtest.h"
 #include "absl/base/config.h"
-#include "absl/base/dynamic_annotations.h"
-
-#if defined(ABSL_HAVE_STD_STRING_VIEW) || defined(__ANDROID__)
-// We don't control the death messaging when using std::string_view.
-// Android assert messages only go to system log, so death tests cannot inspect
-// the message for matching.
-#define ABSL_EXPECT_DEATH_IF_SUPPORTED(statement, regex) \
-  EXPECT_DEATH_IF_SUPPORTED(statement, ".*")
-#else
-#define ABSL_EXPECT_DEATH_IF_SUPPORTED(statement, regex) \
-  EXPECT_DEATH_IF_SUPPORTED(statement, regex)
-#endif
+#include "absl/meta/type_traits.h"
 
 namespace {
 
-// A minimal allocator that uses malloc().
-template <typename T>
-struct Mallocator {
-  typedef T value_type;
-  typedef size_t size_type;
-  typedef ptrdiff_t difference_type;
-  typedef T* pointer;
-  typedef const T* const_pointer;
-  typedef T& reference;
-  typedef const T& const_reference;
-
-  size_type max_size() const {
-    return size_t(std::numeric_limits<size_type>::max()) / sizeof(value_type);
-  }
-  template <typename U>
-  struct rebind {
-    typedef Mallocator<U> other;
-  };
-  Mallocator() = default;
-  template <class U>
-  Mallocator(const Mallocator<U>&) {}  // NOLINT(runtime/explicit)
-
-  T* allocate(size_t n) { return static_cast<T*>(std::malloc(n * sizeof(T))); }
-  void deallocate(T* p, size_t) { std::free(p); }
-};
-template <typename T, typename U>
-bool operator==(const Mallocator<T>&, const Mallocator<U>&) {
-  return true;
-}
-template <typename T, typename U>
-bool operator!=(const Mallocator<T>&, const Mallocator<U>&) {
-  return false;
-}
-
-TEST(StringViewTest, Ctor) {
-  {
-    // Null.
-    absl::string_view s10;
-    EXPECT_TRUE(s10.data() == nullptr);
-    EXPECT_EQ(0, s10.length());
-  }
-
-  {
-    // const char* without length.
-    const char* hello = "hello";
-    absl::string_view s20(hello);
-    EXPECT_TRUE(s20.data() == hello);
-    EXPECT_EQ(5, s20.length());
-
-    // const char* with length.
-    absl::string_view s21(hello, 4);
-    EXPECT_TRUE(s21.data() == hello);
-    EXPECT_EQ(4, s21.length());
-
-    // Not recommended, but valid C++
-    absl::string_view s22(hello, 6);
-    EXPECT_TRUE(s22.data() == hello);
-    EXPECT_EQ(6, s22.length());
-  }
-
-  {
-    // std::string.
-    std::string hola = "hola";
-    absl::string_view s30(hola);
-    EXPECT_TRUE(s30.data() == hola.data());
-    EXPECT_EQ(4, s30.length());
-
-    // std::string with embedded '\0'.
-    hola.push_back('\0');
-    hola.append("h2");
-    hola.push_back('\0');
-    absl::string_view s31(hola);
-    EXPECT_TRUE(s31.data() == hola.data());
-    EXPECT_EQ(8, s31.length());
-  }
-
-  {
-    using mstring =
-        std::basic_string<char, std::char_traits<char>, Mallocator<char>>;
-    mstring str1("BUNGIE-JUMPING!");
-    const mstring str2("SLEEPING!");
-
-    absl::string_view s1(str1);
-    s1.remove_prefix(strlen("BUNGIE-JUM"));
-
-    absl::string_view s2(str2);
-    s2.remove_prefix(strlen("SLEE"));
-
-    EXPECT_EQ(s1, s2);
-    EXPECT_EQ(s1, "PING!");
-  }
-
-  // TODO(mec): absl::string_view(const absl::string_view&);
-}
-
-TEST(StringViewTest, Swap) {
-  absl::string_view a("a");
-  absl::string_view b("bbb");
-  EXPECT_TRUE(noexcept(a.swap(b)));
-  a.swap(b);
-  EXPECT_EQ(a, "bbb");
-  EXPECT_EQ(b, "a");
-  a.swap(b);
-  EXPECT_EQ(a, "a");
-  EXPECT_EQ(b, "bbb");
-}
-
-TEST(StringViewTest, STLComparator) {
-  std::string s1("foo");
-  std::string s2("bar");
-  std::string s3("baz");
-
-  absl::string_view p1(s1);
-  absl::string_view p2(s2);
-  absl::string_view p3(s3);
-
-  typedef std::map<absl::string_view, int> TestMap;
-  TestMap map;
-
-  map.insert(std::make_pair(p1, 0));
-  map.insert(std::make_pair(p2, 1));
-  map.insert(std::make_pair(p3, 2));
-  EXPECT_EQ(map.size(), 3);
-
-  TestMap::const_iterator iter = map.begin();
-  EXPECT_EQ(iter->second, 1);
-  ++iter;
-  EXPECT_EQ(iter->second, 2);
-  ++iter;
-  EXPECT_EQ(iter->second, 0);
-  ++iter;
-  EXPECT_TRUE(iter == map.end());
-
-  TestMap::iterator new_iter = map.find("zot");
-  EXPECT_TRUE(new_iter == map.end());
-
-  new_iter = map.find("bar");
-  EXPECT_TRUE(new_iter != map.end());
-
-  map.erase(new_iter);
-  EXPECT_EQ(map.size(), 2);
-
-  iter = map.begin();
-  EXPECT_EQ(iter->second, 2);
-  ++iter;
-  EXPECT_EQ(iter->second, 0);
-  ++iter;
-  EXPECT_TRUE(iter == map.end());
-}
-
-#define COMPARE(result, op, x, y)                                      \
-  EXPECT_EQ(result, absl::string_view((x)) op absl::string_view((y))); \
-  EXPECT_EQ(result, absl::string_view((x)).compare(absl::string_view((y))) op 0)
-
-TEST(StringViewTest, ComparisonOperators) {
-  COMPARE(true, ==, "",   "");
-  COMPARE(true, ==, "", absl::string_view());
-  COMPARE(true, ==, absl::string_view(), "");
-  COMPARE(true, ==, "a",  "a");
-  COMPARE(true, ==, "aa", "aa");
-  COMPARE(false, ==, "a",  "");
-  COMPARE(false, ==, "",   "a");
-  COMPARE(false, ==, "a",  "b");
-  COMPARE(false, ==, "a",  "aa");
-  COMPARE(false, ==, "aa", "a");
-
-  COMPARE(false, !=, "",   "");
-  COMPARE(false, !=, "a",  "a");
-  COMPARE(false, !=, "aa", "aa");
-  COMPARE(true, !=, "a",  "");
-  COMPARE(true, !=, "",   "a");
-  COMPARE(true, !=, "a",  "b");
-  COMPARE(true, !=, "a",  "aa");
-  COMPARE(true, !=, "aa", "a");
-
-  COMPARE(true, <, "a",  "b");
-  COMPARE(true, <, "a",  "aa");
-  COMPARE(true, <, "aa", "b");
-  COMPARE(true, <, "aa", "bb");
-  COMPARE(false, <, "a",  "a");
-  COMPARE(false, <, "b",  "a");
-  COMPARE(false, <, "aa", "a");
-  COMPARE(false, <, "b",  "aa");
-  COMPARE(false, <, "bb", "aa");
-
-  COMPARE(true, <=, "a",  "a");
-  COMPARE(true, <=, "a",  "b");
-  COMPARE(true, <=, "a",  "aa");
-  COMPARE(true, <=, "aa", "b");
-  COMPARE(true, <=, "aa", "bb");
-  COMPARE(false, <=, "b",  "a");
-  COMPARE(false, <=, "aa", "a");
-  COMPARE(false, <=, "b",  "aa");
-  COMPARE(false, <=, "bb", "aa");
-
-  COMPARE(false, >=, "a",  "b");
-  COMPARE(false, >=, "a",  "aa");
-  COMPARE(false, >=, "aa", "b");
-  COMPARE(false, >=, "aa", "bb");
-  COMPARE(true, >=, "a",  "a");
-  COMPARE(true, >=, "b",  "a");
-  COMPARE(true, >=, "aa", "a");
-  COMPARE(true, >=, "b",  "aa");
-  COMPARE(true, >=, "bb", "aa");
-
-  COMPARE(false, >, "a",  "a");
-  COMPARE(false, >, "a",  "b");
-  COMPARE(false, >, "a",  "aa");
-  COMPARE(false, >, "aa", "b");
-  COMPARE(false, >, "aa", "bb");
-  COMPARE(true, >, "b",  "a");
-  COMPARE(true, >, "aa", "a");
-  COMPARE(true, >, "b",  "aa");
-  COMPARE(true, >, "bb", "aa");
-}
-
-TEST(StringViewTest, ComparisonOperatorsByCharacterPosition) {
-  std::string x;
-  for (int i = 0; i < 256; i++) {
-    x += 'a';
-    std::string y = x;
-    COMPARE(true, ==, x, y);
-    for (int j = 0; j < i; j++) {
-      std::string z = x;
-      z[j] = 'b';       // Differs in position 'j'
-      COMPARE(false, ==, x, z);
-      COMPARE(true, <, x, z);
-      COMPARE(true, >, z, x);
-      if (j + 1 < i) {
-        z[j + 1] = 'A';  // Differs in position 'j+1' as well
-        COMPARE(false, ==, x, z);
-        COMPARE(true, <, x, z);
-        COMPARE(true, >, z, x);
-        z[j + 1] = 'z';  // Differs in position 'j+1' as well
-        COMPARE(false, ==, x, z);
-        COMPARE(true, <, x, z);
-        COMPARE(true, >, z, x);
-      }
-    }
-  }
-}
-#undef COMPARE
-
-// Sadly, our users often confuse std::string::npos with
-// absl::string_view::npos; So much so that we test here that they are the same.
-// They need to both be unsigned, and both be the maximum-valued integer of
-// their type.
-
-template <typename T>
-struct is_type {
-  template <typename U>
-  static bool same(U) {
-    return false;
-  }
-  static bool same(T) { return true; }
-};
-
-TEST(StringViewTest, NposMatchesStdStringView) {
-  EXPECT_EQ(absl::string_view::npos, std::string::npos);
-
-  EXPECT_TRUE(is_type<size_t>::same(absl::string_view::npos));
-  EXPECT_FALSE(is_type<size_t>::same(""));
-
-  // Make sure absl::string_view::npos continues to be a header constant.
-  char test[absl::string_view::npos & 1] = {0};
-  EXPECT_EQ(0, test[0]);
-}
-
-TEST(StringViewTest, STL1) {
-  const absl::string_view a("abcdefghijklmnopqrstuvwxyz");
-  const absl::string_view b("abc");
-  const absl::string_view c("xyz");
-  const absl::string_view d("foobar");
-  const absl::string_view e;
-  std::string temp("123");
-  temp += '\0';
-  temp += "456";
-  const absl::string_view f(temp);
-
-  EXPECT_EQ(a[6], 'g');
-  EXPECT_EQ(b[0], 'a');
-  EXPECT_EQ(c[2], 'z');
-  EXPECT_EQ(f[3], '\0');
-  EXPECT_EQ(f[5], '5');
-
-  EXPECT_EQ(*d.data(), 'f');
-  EXPECT_EQ(d.data()[5], 'r');
-  EXPECT_TRUE(e.data() == nullptr);
-
-  EXPECT_EQ(*a.begin(), 'a');
-  EXPECT_EQ(*(b.begin() + 2), 'c');
-  EXPECT_EQ(*(c.end() - 1), 'z');
-
-  EXPECT_EQ(*a.rbegin(), 'z');
-  EXPECT_EQ(*(b.rbegin() + 2), 'a');
-  EXPECT_EQ(*(c.rend() - 1), 'x');
-  EXPECT_TRUE(a.rbegin() + 26 == a.rend());
-
-  EXPECT_EQ(a.size(), 26);
-  EXPECT_EQ(b.size(), 3);
-  EXPECT_EQ(c.size(), 3);
-  EXPECT_EQ(d.size(), 6);
-  EXPECT_EQ(e.size(), 0);
-  EXPECT_EQ(f.size(), 7);
-
-  EXPECT_TRUE(!d.empty());
-  EXPECT_TRUE(d.begin() != d.end());
-  EXPECT_TRUE(d.begin() + 6 == d.end());
-
-  EXPECT_TRUE(e.empty());
-  EXPECT_TRUE(e.begin() == e.end());
-
-  char buf[4] = { '%', '%', '%', '%' };
-  EXPECT_EQ(a.copy(buf, 4), 4);
-  EXPECT_EQ(buf[0], a[0]);
-  EXPECT_EQ(buf[1], a[1]);
-  EXPECT_EQ(buf[2], a[2]);
-  EXPECT_EQ(buf[3], a[3]);
-  EXPECT_EQ(a.copy(buf, 3, 7), 3);
-  EXPECT_EQ(buf[0], a[7]);
-  EXPECT_EQ(buf[1], a[8]);
-  EXPECT_EQ(buf[2], a[9]);
-  EXPECT_EQ(buf[3], a[3]);
-  EXPECT_EQ(c.copy(buf, 99), 3);
-  EXPECT_EQ(buf[0], c[0]);
-  EXPECT_EQ(buf[1], c[1]);
-  EXPECT_EQ(buf[2], c[2]);
-  EXPECT_EQ(buf[3], a[3]);
-#ifdef ABSL_HAVE_EXCEPTIONS
-  EXPECT_THROW(a.copy(buf, 1, 27), std::out_of_range);
-#else
-  ABSL_EXPECT_DEATH_IF_SUPPORTED(a.copy(buf, 1, 27), "absl::string_view::copy");
-#endif
-}
-
-// Separated from STL1() because some compilers produce an overly
-// large stack frame for the combined function.
-TEST(StringViewTest, STL2) {
-  const absl::string_view a("abcdefghijklmnopqrstuvwxyz");
-  const absl::string_view b("abc");
-  const absl::string_view c("xyz");
-  absl::string_view d("foobar");
-  const absl::string_view e;
-  const absl::string_view f(
-      "123"
-      "\0"
-      "456",
-      7);
-
-  d = absl::string_view();
-  EXPECT_EQ(d.size(), 0);
-  EXPECT_TRUE(d.empty());
-  EXPECT_TRUE(d.data() == nullptr);
-  EXPECT_TRUE(d.begin() == d.end());
-
-  EXPECT_EQ(a.find(b), 0);
-  EXPECT_EQ(a.find(b, 1), absl::string_view::npos);
-  EXPECT_EQ(a.find(c), 23);
-  EXPECT_EQ(a.find(c, 9), 23);
-  EXPECT_EQ(a.find(c, absl::string_view::npos), absl::string_view::npos);
-  EXPECT_EQ(b.find(c), absl::string_view::npos);
-  EXPECT_EQ(b.find(c, absl::string_view::npos), absl::string_view::npos);
-  EXPECT_EQ(a.find(d), 0);
-  EXPECT_EQ(a.find(e), 0);
-  EXPECT_EQ(a.find(d, 12), 12);
-  EXPECT_EQ(a.find(e, 17), 17);
-  absl::string_view g("xx not found bb");
-  EXPECT_EQ(a.find(g), absl::string_view::npos);
-  // empty std::string nonsense
-  EXPECT_EQ(d.find(b), absl::string_view::npos);
-  EXPECT_EQ(e.find(b), absl::string_view::npos);
-  EXPECT_EQ(d.find(b, 4), absl::string_view::npos);
-  EXPECT_EQ(e.find(b, 7), absl::string_view::npos);
-
-  size_t empty_search_pos = std::string().find(std::string());
-  EXPECT_EQ(d.find(d), empty_search_pos);
-  EXPECT_EQ(d.find(e), empty_search_pos);
-  EXPECT_EQ(e.find(d), empty_search_pos);
-  EXPECT_EQ(e.find(e), empty_search_pos);
-  EXPECT_EQ(d.find(d, 4), std::string().find(std::string(), 4));
-  EXPECT_EQ(d.find(e, 4), std::string().find(std::string(), 4));
-  EXPECT_EQ(e.find(d, 4), std::string().find(std::string(), 4));
-  EXPECT_EQ(e.find(e, 4), std::string().find(std::string(), 4));
-
-  EXPECT_EQ(a.find('a'), 0);
-  EXPECT_EQ(a.find('c'), 2);
-  EXPECT_EQ(a.find('z'), 25);
-  EXPECT_EQ(a.find('$'), absl::string_view::npos);
-  EXPECT_EQ(a.find('\0'), absl::string_view::npos);
-  EXPECT_EQ(f.find('\0'), 3);
-  EXPECT_EQ(f.find('3'), 2);
-  EXPECT_EQ(f.find('5'), 5);
-  EXPECT_EQ(g.find('o'), 4);
-  EXPECT_EQ(g.find('o', 4), 4);
-  EXPECT_EQ(g.find('o', 5), 8);
-  EXPECT_EQ(a.find('b', 5), absl::string_view::npos);
-  // empty std::string nonsense
-  EXPECT_EQ(d.find('\0'), absl::string_view::npos);
-  EXPECT_EQ(e.find('\0'), absl::string_view::npos);
-  EXPECT_EQ(d.find('\0', 4), absl::string_view::npos);
-  EXPECT_EQ(e.find('\0', 7), absl::string_view::npos);
-  EXPECT_EQ(d.find('x'), absl::string_view::npos);
-  EXPECT_EQ(e.find('x'), absl::string_view::npos);
-  EXPECT_EQ(d.find('x', 4), absl::string_view::npos);
-  EXPECT_EQ(e.find('x', 7), absl::string_view::npos);
-
-  EXPECT_EQ(a.rfind(b), 0);
-  EXPECT_EQ(a.rfind(b, 1), 0);
-  EXPECT_EQ(a.rfind(c), 23);
-  EXPECT_EQ(a.rfind(c, 22), absl::string_view::npos);
-  EXPECT_EQ(a.rfind(c, 1), absl::string_view::npos);
-  EXPECT_EQ(a.rfind(c, 0), absl::string_view::npos);
-  EXPECT_EQ(b.rfind(c), absl::string_view::npos);
-  EXPECT_EQ(b.rfind(c, 0), absl::string_view::npos);
-  EXPECT_EQ(a.rfind(d), std::string(a).rfind(std::string()));
-  EXPECT_EQ(a.rfind(e), std::string(a).rfind(std::string()));
-  EXPECT_EQ(a.rfind(d, 12), 12);
-  EXPECT_EQ(a.rfind(e, 17), 17);
-  EXPECT_EQ(a.rfind(g), absl::string_view::npos);
-  EXPECT_EQ(d.rfind(b), absl::string_view::npos);
-  EXPECT_EQ(e.rfind(b), absl::string_view::npos);
-  EXPECT_EQ(d.rfind(b, 4), absl::string_view::npos);
-  EXPECT_EQ(e.rfind(b, 7), absl::string_view::npos);
-  // empty std::string nonsense
-  EXPECT_EQ(d.rfind(d, 4), std::string().rfind(std::string()));
-  EXPECT_EQ(e.rfind(d, 7), std::string().rfind(std::string()));
-  EXPECT_EQ(d.rfind(e, 4), std::string().rfind(std::string()));
-  EXPECT_EQ(e.rfind(e, 7), std::string().rfind(std::string()));
-  EXPECT_EQ(d.rfind(d), std::string().rfind(std::string()));
-  EXPECT_EQ(e.rfind(d), std::string().rfind(std::string()));
-  EXPECT_EQ(d.rfind(e), std::string().rfind(std::string()));
-  EXPECT_EQ(e.rfind(e), std::string().rfind(std::string()));
-
-  EXPECT_EQ(g.rfind('o'), 8);
-  EXPECT_EQ(g.rfind('q'), absl::string_view::npos);
-  EXPECT_EQ(g.rfind('o', 8), 8);
-  EXPECT_EQ(g.rfind('o', 7), 4);
-  EXPECT_EQ(g.rfind('o', 3), absl::string_view::npos);
-  EXPECT_EQ(f.rfind('\0'), 3);
-  EXPECT_EQ(f.rfind('\0', 12), 3);
-  EXPECT_EQ(f.rfind('3'), 2);
-  EXPECT_EQ(f.rfind('5'), 5);
-  // empty std::string nonsense
-  EXPECT_EQ(d.rfind('o'), absl::string_view::npos);
-  EXPECT_EQ(e.rfind('o'), absl::string_view::npos);
-  EXPECT_EQ(d.rfind('o', 4), absl::string_view::npos);
-  EXPECT_EQ(e.rfind('o', 7), absl::string_view::npos);
-}
-
-// Continued from STL2
-TEST(StringViewTest, STL2FindFirst) {
-  const absl::string_view a("abcdefghijklmnopqrstuvwxyz");
-  const absl::string_view b("abc");
-  const absl::string_view c("xyz");
-  absl::string_view d("foobar");
-  const absl::string_view e;
-  const absl::string_view f(
-      "123"
-      "\0"
-      "456",
-      7);
-  absl::string_view g("xx not found bb");
-
-  d = absl::string_view();
-  EXPECT_EQ(a.find_first_of(b), 0);
-  EXPECT_EQ(a.find_first_of(b, 0), 0);
-  EXPECT_EQ(a.find_first_of(b, 1), 1);
-  EXPECT_EQ(a.find_first_of(b, 2), 2);
-  EXPECT_EQ(a.find_first_of(b, 3), absl::string_view::npos);
-  EXPECT_EQ(a.find_first_of(c), 23);
-  EXPECT_EQ(a.find_first_of(c, 23), 23);
-  EXPECT_EQ(a.find_first_of(c, 24), 24);
-  EXPECT_EQ(a.find_first_of(c, 25), 25);
-  EXPECT_EQ(a.find_first_of(c, 26), absl::string_view::npos);
-  EXPECT_EQ(g.find_first_of(b), 13);
-  EXPECT_EQ(g.find_first_of(c), 0);
-  EXPECT_EQ(a.find_first_of(f), absl::string_view::npos);
-  EXPECT_EQ(f.find_first_of(a), absl::string_view::npos);
-  // empty std::string nonsense
-  EXPECT_EQ(a.find_first_of(d), absl::string_view::npos);
-  EXPECT_EQ(a.find_first_of(e), absl::string_view::npos);
-  EXPECT_EQ(d.find_first_of(b), absl::string_view::npos);
-  EXPECT_EQ(e.find_first_of(b), absl::string_view::npos);
-  EXPECT_EQ(d.find_first_of(d), absl::string_view::npos);
-  EXPECT_EQ(e.find_first_of(d), absl::string_view::npos);
-  EXPECT_EQ(d.find_first_of(e), absl::string_view::npos);
-  EXPECT_EQ(e.find_first_of(e), absl::string_view::npos);
-
-  EXPECT_EQ(a.find_first_not_of(b), 3);
-  EXPECT_EQ(a.find_first_not_of(c), 0);
-  EXPECT_EQ(b.find_first_not_of(a), absl::string_view::npos);
-  EXPECT_EQ(c.find_first_not_of(a), absl::string_view::npos);
-  EXPECT_EQ(f.find_first_not_of(a), 0);
-  EXPECT_EQ(a.find_first_not_of(f), 0);
-  EXPECT_EQ(a.find_first_not_of(d), 0);
-  EXPECT_EQ(a.find_first_not_of(e), 0);
-  // empty std::string nonsense
-  EXPECT_EQ(a.find_first_not_of(d), 0);
-  EXPECT_EQ(a.find_first_not_of(e), 0);
-  EXPECT_EQ(a.find_first_not_of(d, 1), 1);
-  EXPECT_EQ(a.find_first_not_of(e, 1), 1);
-  EXPECT_EQ(a.find_first_not_of(d, a.size() - 1), a.size() - 1);
-  EXPECT_EQ(a.find_first_not_of(e, a.size() - 1), a.size() - 1);
-  EXPECT_EQ(a.find_first_not_of(d, a.size()), absl::string_view::npos);
-  EXPECT_EQ(a.find_first_not_of(e, a.size()), absl::string_view::npos);
-  EXPECT_EQ(a.find_first_not_of(d, absl::string_view::npos),
-            absl::string_view::npos);
-  EXPECT_EQ(a.find_first_not_of(e, absl::string_view::npos),
-            absl::string_view::npos);
-  EXPECT_EQ(d.find_first_not_of(a), absl::string_view::npos);
-  EXPECT_EQ(e.find_first_not_of(a), absl::string_view::npos);
-  EXPECT_EQ(d.find_first_not_of(d), absl::string_view::npos);
-  EXPECT_EQ(e.find_first_not_of(d), absl::string_view::npos);
-  EXPECT_EQ(d.find_first_not_of(e), absl::string_view::npos);
-  EXPECT_EQ(e.find_first_not_of(e), absl::string_view::npos);
-
-  absl::string_view h("====");
-  EXPECT_EQ(h.find_first_not_of('='), absl::string_view::npos);
-  EXPECT_EQ(h.find_first_not_of('=', 3), absl::string_view::npos);
-  EXPECT_EQ(h.find_first_not_of('\0'), 0);
-  EXPECT_EQ(g.find_first_not_of('x'), 2);
-  EXPECT_EQ(f.find_first_not_of('\0'), 0);
-  EXPECT_EQ(f.find_first_not_of('\0', 3), 4);
-  EXPECT_EQ(f.find_first_not_of('\0', 2), 2);
-  // empty std::string nonsense
-  EXPECT_EQ(d.find_first_not_of('x'), absl::string_view::npos);
-  EXPECT_EQ(e.find_first_not_of('x'), absl::string_view::npos);
-  EXPECT_EQ(d.find_first_not_of('\0'), absl::string_view::npos);
-  EXPECT_EQ(e.find_first_not_of('\0'), absl::string_view::npos);
-}
-
-// Continued from STL2
-TEST(StringViewTest, STL2FindLast) {
-  const absl::string_view a("abcdefghijklmnopqrstuvwxyz");
-  const absl::string_view b("abc");
-  const absl::string_view c("xyz");
-  absl::string_view d("foobar");
-  const absl::string_view e;
-  const absl::string_view f(
-      "123"
-      "\0"
-      "456",
-      7);
-  absl::string_view g("xx not found bb");
-  absl::string_view h("====");
-  absl::string_view i("56");
-
-  d = absl::string_view();
-  EXPECT_EQ(h.find_last_of(a), absl::string_view::npos);
-  EXPECT_EQ(g.find_last_of(a), g.size()-1);
-  EXPECT_EQ(a.find_last_of(b), 2);
-  EXPECT_EQ(a.find_last_of(c), a.size()-1);
-  EXPECT_EQ(f.find_last_of(i), 6);
-  EXPECT_EQ(a.find_last_of('a'), 0);
-  EXPECT_EQ(a.find_last_of('b'), 1);
-  EXPECT_EQ(a.find_last_of('z'), 25);
-  EXPECT_EQ(a.find_last_of('a', 5), 0);
-  EXPECT_EQ(a.find_last_of('b', 5), 1);
-  EXPECT_EQ(a.find_last_of('b', 0), absl::string_view::npos);
-  EXPECT_EQ(a.find_last_of('z', 25), 25);
-  EXPECT_EQ(a.find_last_of('z', 24), absl::string_view::npos);
-  EXPECT_EQ(f.find_last_of(i, 5), 5);
-  EXPECT_EQ(f.find_last_of(i, 6), 6);
-  EXPECT_EQ(f.find_last_of(a, 4), absl::string_view::npos);
-  // empty std::string nonsense
-  EXPECT_EQ(f.find_last_of(d), absl::string_view::npos);
-  EXPECT_EQ(f.find_last_of(e), absl::string_view::npos);
-  EXPECT_EQ(f.find_last_of(d, 4), absl::string_view::npos);
-  EXPECT_EQ(f.find_last_of(e, 4), absl::string_view::npos);
-  EXPECT_EQ(d.find_last_of(d), absl::string_view::npos);
-  EXPECT_EQ(d.find_last_of(e), absl::string_view::npos);
-  EXPECT_EQ(e.find_last_of(d), absl::string_view::npos);
-  EXPECT_EQ(e.find_last_of(e), absl::string_view::npos);
-  EXPECT_EQ(d.find_last_of(f), absl::string_view::npos);
-  EXPECT_EQ(e.find_last_of(f), absl::string_view::npos);
-  EXPECT_EQ(d.find_last_of(d, 4), absl::string_view::npos);
-  EXPECT_EQ(d.find_last_of(e, 4), absl::string_view::npos);
-  EXPECT_EQ(e.find_last_of(d, 4), absl::string_view::npos);
-  EXPECT_EQ(e.find_last_of(e, 4), absl::string_view::npos);
-  EXPECT_EQ(d.find_last_of(f, 4), absl::string_view::npos);
-  EXPECT_EQ(e.find_last_of(f, 4), absl::string_view::npos);
-
-  EXPECT_EQ(a.find_last_not_of(b), a.size()-1);
-  EXPECT_EQ(a.find_last_not_of(c), 22);
-  EXPECT_EQ(b.find_last_not_of(a), absl::string_view::npos);
-  EXPECT_EQ(b.find_last_not_of(b), absl::string_view::npos);
-  EXPECT_EQ(f.find_last_not_of(i), 4);
-  EXPECT_EQ(a.find_last_not_of(c, 24), 22);
-  EXPECT_EQ(a.find_last_not_of(b, 3), 3);
-  EXPECT_EQ(a.find_last_not_of(b, 2), absl::string_view::npos);
-  // empty std::string nonsense
-  EXPECT_EQ(f.find_last_not_of(d), f.size()-1);
-  EXPECT_EQ(f.find_last_not_of(e), f.size()-1);
-  EXPECT_EQ(f.find_last_not_of(d, 4), 4);
-  EXPECT_EQ(f.find_last_not_of(e, 4), 4);
-  EXPECT_EQ(d.find_last_not_of(d), absl::string_view::npos);
-  EXPECT_EQ(d.find_last_not_of(e), absl::string_view::npos);
-  EXPECT_EQ(e.find_last_not_of(d), absl::string_view::npos);
-  EXPECT_EQ(e.find_last_not_of(e), absl::string_view::npos);
-  EXPECT_EQ(d.find_last_not_of(f), absl::string_view::npos);
-  EXPECT_EQ(e.find_last_not_of(f), absl::string_view::npos);
-  EXPECT_EQ(d.find_last_not_of(d, 4), absl::string_view::npos);
-  EXPECT_EQ(d.find_last_not_of(e, 4), absl::string_view::npos);
-  EXPECT_EQ(e.find_last_not_of(d, 4), absl::string_view::npos);
-  EXPECT_EQ(e.find_last_not_of(e, 4), absl::string_view::npos);
-  EXPECT_EQ(d.find_last_not_of(f, 4), absl::string_view::npos);
-  EXPECT_EQ(e.find_last_not_of(f, 4), absl::string_view::npos);
-
-  EXPECT_EQ(h.find_last_not_of('x'), h.size() - 1);
-  EXPECT_EQ(h.find_last_not_of('='), absl::string_view::npos);
-  EXPECT_EQ(b.find_last_not_of('c'), 1);
-  EXPECT_EQ(h.find_last_not_of('x', 2), 2);
-  EXPECT_EQ(h.find_last_not_of('=', 2), absl::string_view::npos);
-  EXPECT_EQ(b.find_last_not_of('b', 1), 0);
-  // empty std::string nonsense
-  EXPECT_EQ(d.find_last_not_of('x'), absl::string_view::npos);
-  EXPECT_EQ(e.find_last_not_of('x'), absl::string_view::npos);
-  EXPECT_EQ(d.find_last_not_of('\0'), absl::string_view::npos);
-  EXPECT_EQ(e.find_last_not_of('\0'), absl::string_view::npos);
-}
-
-// Continued from STL2
-TEST(StringViewTest, STL2Substr) {
-  const absl::string_view a("abcdefghijklmnopqrstuvwxyz");
-  const absl::string_view b("abc");
-  const absl::string_view c("xyz");
-  absl::string_view d("foobar");
-  const absl::string_view e;
-
-  d = absl::string_view();
-  EXPECT_EQ(a.substr(0, 3), b);
-  EXPECT_EQ(a.substr(23), c);
-  EXPECT_EQ(a.substr(23, 3), c);
-  EXPECT_EQ(a.substr(23, 99), c);
-  EXPECT_EQ(a.substr(0), a);
-  EXPECT_EQ(a.substr(3, 2), "de");
-  // empty std::string nonsense
-  EXPECT_EQ(d.substr(0, 99), e);
-  // use of npos
-  EXPECT_EQ(a.substr(0, absl::string_view::npos), a);
-  EXPECT_EQ(a.substr(23, absl::string_view::npos), c);
-  // throw exception
-#ifdef ABSL_HAVE_EXCEPTIONS
-  EXPECT_THROW((void)a.substr(99, 2), std::out_of_range);
-#else
-  ABSL_EXPECT_DEATH_IF_SUPPORTED((void)a.substr(99, 2),
-                                 "absl::string_view::substr");
-#endif
-}
-
 TEST(StringViewTest, TruncSubstr) {
   const absl::string_view hi("hi");
   EXPECT_EQ("", absl::ClippedSubstr(hi, 0, 0));
@@ -703,223 +46,11 @@ TEST(StringViewTest, TruncSubstr) {
   EXPECT_EQ("", absl::ClippedSubstr(hi, 3, 2));  // truncation
 }
 
-TEST(StringViewTest, UTF8) {
-  std::string utf8 = "\u00E1";
-  std::string utf8_twice = utf8 + " " + utf8;
-  int utf8_len = strlen(utf8.data());
-  EXPECT_EQ(utf8_len, absl::string_view(utf8_twice).find_first_of(" "));
-  EXPECT_EQ(utf8_len, absl::string_view(utf8_twice).find_first_of(" \t"));
-}
-
-TEST(StringViewTest, FindConformance) {
-  struct {
-    std::string haystack;
-    std::string needle;
-  } specs[] = {
-    {"", ""},
-    {"", "a"},
-    {"a", ""},
-    {"a", "a"},
-    {"a", "b"},
-    {"aa", ""},
-    {"aa", "a"},
-    {"aa", "b"},
-    {"ab", "a"},
-    {"ab", "b"},
-    {"abcd", ""},
-    {"abcd", "a"},
-    {"abcd", "d"},
-    {"abcd", "ab"},
-    {"abcd", "bc"},
-    {"abcd", "cd"},
-    {"abcd", "abcd"},
-  };
-  for (const auto& s : specs) {
-    SCOPED_TRACE(s.haystack);
-    SCOPED_TRACE(s.needle);
-    std::string st = s.haystack;
-    absl::string_view sp = s.haystack;
-    for (size_t i = 0; i <= sp.size(); ++i) {
-      size_t pos = (i == sp.size()) ? absl::string_view::npos : i;
-      SCOPED_TRACE(pos);
-      EXPECT_EQ(sp.find(s.needle, pos),
-                st.find(s.needle, pos));
-      EXPECT_EQ(sp.rfind(s.needle, pos),
-                st.rfind(s.needle, pos));
-      EXPECT_EQ(sp.find_first_of(s.needle, pos),
-                st.find_first_of(s.needle, pos));
-      EXPECT_EQ(sp.find_first_not_of(s.needle, pos),
-                st.find_first_not_of(s.needle, pos));
-      EXPECT_EQ(sp.find_last_of(s.needle, pos),
-                st.find_last_of(s.needle, pos));
-      EXPECT_EQ(sp.find_last_not_of(s.needle, pos),
-                st.find_last_not_of(s.needle, pos));
-    }
-  }
-}
-
-TEST(StringViewTest, Remove) {
-  absl::string_view a("foobar");
-  std::string s1("123");
-  s1 += '\0';
-  s1 += "456";
-  absl::string_view e;
-  std::string s2;
-
-  // remove_prefix
-  absl::string_view c(a);
-  c.remove_prefix(3);
-  EXPECT_EQ(c, "bar");
-  c = a;
-  c.remove_prefix(0);
-  EXPECT_EQ(c, a);
-  c.remove_prefix(c.size());
-  EXPECT_EQ(c, e);
-
-  // remove_suffix
-  c = a;
-  c.remove_suffix(3);
-  EXPECT_EQ(c, "foo");
-  c = a;
-  c.remove_suffix(0);
-  EXPECT_EQ(c, a);
-  c.remove_suffix(c.size());
-  EXPECT_EQ(c, e);
-}
-
-TEST(StringViewTest, Set) {
-  absl::string_view a("foobar");
-  absl::string_view empty;
-  absl::string_view b;
-
-  // set
-  b = absl::string_view("foobar", 6);
-  EXPECT_EQ(b, a);
-  b = absl::string_view("foobar", 0);
-  EXPECT_EQ(b, empty);
-  b = absl::string_view("foobar", 7);
-  EXPECT_NE(b, a);
-
-  b = absl::string_view("foobar");
-  EXPECT_EQ(b, a);
-}
-
-TEST(StringViewTest, FrontBack) {
-  static const char arr[] = "abcd";
-  const absl::string_view csp(arr, 4);
-  EXPECT_EQ(&arr[0], &csp.front());
-  EXPECT_EQ(&arr[3], &csp.back());
-}
-
-TEST(StringViewTest, FrontBackSingleChar) {
-  static const char c = 'a';
-  const absl::string_view csp(&c, 1);
-  EXPECT_EQ(&c, &csp.front());
-  EXPECT_EQ(&c, &csp.back());
-}
-
-// `std::string_view::string_view(const char*)` calls
-// `std::char_traits<char>::length(const char*)` to get the string length. In
-// libc++, it doesn't allow `nullptr` in the constexpr context, with the error
-// "read of dereferenced null pointer is not allowed in a constant expression".
-// At run time, the behavior of `std::char_traits::length()` on `nullptr` is
-// undefined by the standard and usually results in crash with libc++.
-// GCC also started rejected this in libstdc++ starting in GCC9.
-// In MSVC, creating a constexpr string_view from nullptr also triggers an
-// "unevaluable pointer value" error. This compiler implementation conforms
-// to the standard, but `absl::string_view` implements a different
-// behavior for historical reasons. We work around tests that construct
-// `string_view` from `nullptr` when using libc++.
-#if !defined(ABSL_HAVE_STD_STRING_VIEW) ||                    \
-    (!(defined(_GLIBCXX_RELEASE) && _GLIBCXX_RELEASE >= 9) && \
-     !defined(_LIBCPP_VERSION) && !defined(_MSC_VER))
-#define ABSL_HAVE_STRING_VIEW_FROM_NULLPTR 1
-#endif
-
-TEST(StringViewTest, NULLInput) {
-  absl::string_view s;
-  EXPECT_EQ(s.data(), nullptr);
-  EXPECT_EQ(s.size(), 0);
-
-#ifdef ABSL_HAVE_STRING_VIEW_FROM_NULLPTR
-  s = absl::string_view(nullptr);
-  EXPECT_EQ(s.data(), nullptr);
-  EXPECT_EQ(s.size(), 0);
-
-  // .ToString() on a absl::string_view with nullptr should produce the empty
-  // std::string.
-  EXPECT_EQ("", std::string(s));
-#endif  // ABSL_HAVE_STRING_VIEW_FROM_NULLPTR
-}
-
-TEST(StringViewTest, Comparisons2) {
-  // The `compare` member has 6 overloads (v: string_view, s: const char*):
-  //  (1) compare(v)
-  //  (2) compare(pos1, count1, v)
-  //  (3) compare(pos1, count1, v, pos2, count2)
-  //  (4) compare(s)
-  //  (5) compare(pos1, count1, s)
-  //  (6) compare(pos1, count1, s, count2)
-
-  absl::string_view abc("abcdefghijklmnopqrstuvwxyz");
-
-  // check comparison operations on strings longer than 4 bytes.
-  EXPECT_EQ(abc, absl::string_view("abcdefghijklmnopqrstuvwxyz"));
-  EXPECT_EQ(abc.compare(absl::string_view("abcdefghijklmnopqrstuvwxyz")), 0);
-
-  EXPECT_LT(abc, absl::string_view("abcdefghijklmnopqrstuvwxzz"));
-  EXPECT_LT(abc.compare(absl::string_view("abcdefghijklmnopqrstuvwxzz")), 0);
-
-  EXPECT_GT(abc, absl::string_view("abcdefghijklmnopqrstuvwxyy"));
-  EXPECT_GT(abc.compare(absl::string_view("abcdefghijklmnopqrstuvwxyy")), 0);
-
-  // The "substr" variants of `compare`.
-  absl::string_view digits("0123456789");
-  auto npos = absl::string_view::npos;
-
-  // Taking string_view
-  EXPECT_EQ(digits.compare(3, npos, absl::string_view("3456789")), 0);  // 2
-  EXPECT_EQ(digits.compare(3, 4, absl::string_view("3456")), 0);        // 2
-  EXPECT_EQ(digits.compare(10, 0, absl::string_view()), 0);             // 2
-  EXPECT_EQ(digits.compare(3, 4, absl::string_view("0123456789"), 3, 4),
-            0);  // 3
-  EXPECT_LT(digits.compare(3, 4, absl::string_view("0123456789"), 3, 5),
-            0);  // 3
-  EXPECT_LT(digits.compare(0, npos, absl::string_view("0123456789"), 3, 5),
-            0);  // 3
-  // Taking const char*
-  EXPECT_EQ(digits.compare(3, 4, "3456"), 0);                 // 5
-  EXPECT_EQ(digits.compare(3, npos, "3456789"), 0);           // 5
-  EXPECT_EQ(digits.compare(10, 0, ""), 0);                    // 5
-  EXPECT_EQ(digits.compare(3, 4, "0123456789", 3, 4), 0);     // 6
-  EXPECT_LT(digits.compare(3, 4, "0123456789", 3, 5), 0);     // 6
-  EXPECT_LT(digits.compare(0, npos, "0123456789", 3, 5), 0);  // 6
-}
-
-TEST(StringViewTest, At) {
-  absl::string_view abc = "abc";
-  EXPECT_EQ(abc.at(0), 'a');
-  EXPECT_EQ(abc.at(1), 'b');
-  EXPECT_EQ(abc.at(2), 'c');
-#ifdef ABSL_HAVE_EXCEPTIONS
-  EXPECT_THROW(abc.at(3), std::out_of_range);
-#else
-  ABSL_EXPECT_DEATH_IF_SUPPORTED(abc.at(3), "absl::string_view::at");
-#endif
-}
-
-struct MyCharAlloc : std::allocator<char> {};
-
-TEST(StringViewTest, ExplicitConversionOperator) {
-  absl::string_view sp = "hi";
-  EXPECT_EQ(sp, std::string(sp));
-}
-
 TEST(StringViewTest, NullSafeStringView) {
   {
     absl::string_view s = absl::NullSafeStringView(nullptr);
     EXPECT_EQ(nullptr, s.data());
-    EXPECT_EQ(0, s.size());
+    EXPECT_EQ(0u, s.size());
     EXPECT_EQ(absl::string_view(), s);
   }
   {
@@ -931,253 +62,24 @@ TEST(StringViewTest, NullSafeStringView) {
   }
 }
 
-TEST(StringViewTest, ConstexprCompiles) {
-  constexpr absl::string_view sp;
-#ifdef ABSL_HAVE_STRING_VIEW_FROM_NULLPTR
-  constexpr absl::string_view cstr(nullptr);
-#endif
-  constexpr absl::string_view cstr_len("cstr", 4);
-
-#if defined(ABSL_HAVE_STD_STRING_VIEW)
-  // In libstdc++ (as of 7.2), `std::string_view::string_view(const char*)`
-  // calls `std::char_traits<char>::length(const char*)` to get the std::string
-  // length, but it is not marked constexpr yet. See GCC bug:
-  // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78156
-  // Also, there is a LWG issue that adds constexpr to length() which was just
-  // resolved 2017-06-02. See
-  // http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#2232
-  // TODO(zhangxy): Update the condition when libstdc++ adopts the constexpr
-  // length().
-#if !defined(__GLIBCXX__)
-#define ABSL_HAVE_CONSTEXPR_STRING_VIEW_FROM_CSTR 1
-#endif  // !__GLIBCXX__
-
-#else  // ABSL_HAVE_STD_STRING_VIEW
-
-// This duplicates the check for __builtin_strlen in the header.
-#if ABSL_HAVE_BUILTIN(__builtin_strlen) || \
-    (defined(__GNUC__) && !defined(__clang__))
-#define ABSL_HAVE_CONSTEXPR_STRING_VIEW_FROM_CSTR 1
-#elif defined(__GNUC__)  // GCC or clang
-#error GCC/clang should have constexpr string_view.
-#endif
-
-// MSVC 2017+ should be able to construct a constexpr string_view from a cstr.
-#if defined(_MSC_VER) && _MSC_VER >= 1910
-#define ABSL_HAVE_CONSTEXPR_STRING_VIEW_FROM_CSTR 1
-#endif
-
-#endif  // ABSL_HAVE_STD_STRING_VIEW
-
-#ifdef ABSL_HAVE_CONSTEXPR_STRING_VIEW_FROM_CSTR
-  constexpr absl::string_view cstr_strlen("foo");
-  EXPECT_EQ(cstr_strlen.length(), 3);
-  constexpr absl::string_view cstr_strlen2 = "bar";
-  EXPECT_EQ(cstr_strlen2, "bar");
-#endif
-
-#if !defined(__clang__) || 3 < __clang_major__ || \
-  (3 == __clang_major__ && 4 < __clang_minor__)
-  // older clang versions (< 3.5) complain that:
-  //   "cannot perform pointer arithmetic on null pointer"
-  constexpr absl::string_view::iterator const_begin_empty = sp.begin();
-  constexpr absl::string_view::iterator const_end_empty = sp.end();
-  EXPECT_EQ(const_begin_empty, const_end_empty);
-
-#ifdef ABSL_HAVE_STRING_VIEW_FROM_NULLPTR
-  constexpr absl::string_view::iterator const_begin_nullptr = cstr.begin();
-  constexpr absl::string_view::iterator const_end_nullptr = cstr.end();
-  EXPECT_EQ(const_begin_nullptr, const_end_nullptr);
-#endif  // ABSL_HAVE_STRING_VIEW_FROM_NULLPTR
-#endif  // !defined(__clang__) || ...
-
-  constexpr absl::string_view::iterator const_begin = cstr_len.begin();
-  constexpr absl::string_view::iterator const_end = cstr_len.end();
-  constexpr absl::string_view::size_type const_size = cstr_len.size();
-  constexpr absl::string_view::size_type const_length = cstr_len.length();
-  static_assert(const_begin + const_size == const_end,
-                "pointer arithmetic check");
-  static_assert(const_begin + const_length == const_end,
-                "pointer arithmetic check");
-#ifndef _MSC_VER
-  // MSVC has bugs doing constexpr pointer arithmetic.
-  // https://developercommunity.visualstudio.com/content/problem/482192/bad-pointer-arithmetic-in-constepxr-2019-rc1-svc1.html
-  EXPECT_EQ(const_begin + const_size, const_end);
-  EXPECT_EQ(const_begin + const_length, const_end);
-#endif
-
-  constexpr bool isempty = sp.empty();
-  EXPECT_TRUE(isempty);
-
-  constexpr const char c = cstr_len[2];
-  EXPECT_EQ(c, 't');
-
-  constexpr const char cfront = cstr_len.front();
-  constexpr const char cback = cstr_len.back();
-  EXPECT_EQ(cfront, 'c');
-  EXPECT_EQ(cback, 'r');
-
-  constexpr const char* np = sp.data();
-  constexpr const char* cstr_ptr = cstr_len.data();
-  EXPECT_EQ(np, nullptr);
-  EXPECT_NE(cstr_ptr, nullptr);
-
-  constexpr size_t sp_npos = sp.npos;
-  EXPECT_EQ(sp_npos, -1);
-}
-
-TEST(StringViewTest, Noexcept) {
-  EXPECT_TRUE((std::is_nothrow_constructible<absl::string_view,
-                                             const std::string&>::value));
-  EXPECT_TRUE((std::is_nothrow_constructible<absl::string_view,
-                                             const std::string&>::value));
-  EXPECT_TRUE(std::is_nothrow_constructible<absl::string_view>::value);
-  constexpr absl::string_view sp;
-  EXPECT_TRUE(noexcept(sp.begin()));
-  EXPECT_TRUE(noexcept(sp.end()));
-  EXPECT_TRUE(noexcept(sp.cbegin()));
-  EXPECT_TRUE(noexcept(sp.cend()));
-  EXPECT_TRUE(noexcept(sp.rbegin()));
-  EXPECT_TRUE(noexcept(sp.rend()));
-  EXPECT_TRUE(noexcept(sp.crbegin()));
-  EXPECT_TRUE(noexcept(sp.crend()));
-  EXPECT_TRUE(noexcept(sp.size()));
-  EXPECT_TRUE(noexcept(sp.length()));
-  EXPECT_TRUE(noexcept(sp.empty()));
-  EXPECT_TRUE(noexcept(sp.data()));
-  EXPECT_TRUE(noexcept(sp.compare(sp)));
-  EXPECT_TRUE(noexcept(sp.find(sp)));
-  EXPECT_TRUE(noexcept(sp.find('f')));
-  EXPECT_TRUE(noexcept(sp.rfind(sp)));
-  EXPECT_TRUE(noexcept(sp.rfind('f')));
-  EXPECT_TRUE(noexcept(sp.find_first_of(sp)));
-  EXPECT_TRUE(noexcept(sp.find_first_of('f')));
-  EXPECT_TRUE(noexcept(sp.find_last_of(sp)));
-  EXPECT_TRUE(noexcept(sp.find_last_of('f')));
-  EXPECT_TRUE(noexcept(sp.find_first_not_of(sp)));
-  EXPECT_TRUE(noexcept(sp.find_first_not_of('f')));
-  EXPECT_TRUE(noexcept(sp.find_last_not_of(sp)));
-  EXPECT_TRUE(noexcept(sp.find_last_not_of('f')));
-}
-
-TEST(ComparisonOpsTest, StringCompareNotAmbiguous) {
-  EXPECT_EQ("hello", std::string("hello"));
-  EXPECT_LT("hello", std::string("world"));
-}
-
-TEST(ComparisonOpsTest, HeterogenousStringViewEquals) {
-  EXPECT_EQ(absl::string_view("hello"), std::string("hello"));
-  EXPECT_EQ("hello", absl::string_view("hello"));
-}
-
-TEST(FindOneCharTest, EdgeCases) {
-  absl::string_view a("xxyyyxx");
-
-  // Set a = "xyyyx".
-  a.remove_prefix(1);
-  a.remove_suffix(1);
-
-  EXPECT_EQ(0, a.find('x'));
-  EXPECT_EQ(0, a.find('x', 0));
-  EXPECT_EQ(4, a.find('x', 1));
-  EXPECT_EQ(4, a.find('x', 4));
-  EXPECT_EQ(absl::string_view::npos, a.find('x', 5));
-
-  EXPECT_EQ(4, a.rfind('x'));
-  EXPECT_EQ(4, a.rfind('x', 5));
-  EXPECT_EQ(4, a.rfind('x', 4));
-  EXPECT_EQ(0, a.rfind('x', 3));
-  EXPECT_EQ(0, a.rfind('x', 0));
-
-  // Set a = "yyy".
-  a.remove_prefix(1);
-  a.remove_suffix(1);
-
-  EXPECT_EQ(absl::string_view::npos, a.find('x'));
-  EXPECT_EQ(absl::string_view::npos, a.rfind('x'));
-}
-
-#ifndef THREAD_SANITIZER  // Allocates too much memory for tsan.
-TEST(HugeStringView, TwoPointTwoGB) {
-  if (sizeof(size_t) <= 4 || RunningOnValgrind())
-    return;
-  // Try a huge std::string piece.
-  const size_t size = size_t{2200} * 1000 * 1000;
-  std::string s(size, 'a');
-  absl::string_view sp(s);
-  EXPECT_EQ(size, sp.length());
-  sp.remove_prefix(1);
-  EXPECT_EQ(size - 1, sp.length());
-  sp.remove_suffix(2);
-  EXPECT_EQ(size - 1 - 2, sp.length());
-}
-#endif  // THREAD_SANITIZER
-
-#if !defined(NDEBUG) && !defined(ABSL_HAVE_STD_STRING_VIEW)
-TEST(NonNegativeLenTest, NonNegativeLen) {
-  ABSL_EXPECT_DEATH_IF_SUPPORTED(absl::string_view("xyz", -1),
-                                 "len <= kMaxSize");
-}
-
-TEST(LenExceedsMaxSizeTest, LenExceedsMaxSize) {
-  auto max_size = absl::string_view().max_size();
-
-  // This should construct ok (although the view itself is obviously invalid).
-  absl::string_view ok_view("", max_size);
-
-  // Adding one to the max should trigger an assertion.
-  ABSL_EXPECT_DEATH_IF_SUPPORTED(absl::string_view("", max_size + 1),
-                                 "len <= kMaxSize");
-}
-#endif  // !defined(NDEBUG) && !defined(ABSL_HAVE_STD_STRING_VIEW)
-
-class StringViewStreamTest : public ::testing::Test {
- public:
-  // Set negative 'width' for right justification.
-  template <typename T>
-  std::string Pad(const T& s, int width, char fill = 0) {
-    std::ostringstream oss;
-    if (fill != 0) {
-      oss << std::setfill(fill);
-    }
-    if (width < 0) {
-      width = -width;
-      oss << std::right;
-    }
-    oss << std::setw(width) << s;
-    return oss.str();
-  }
-};
-
-TEST_F(StringViewStreamTest, Padding) {
-  std::string s("hello");
-  absl::string_view sp(s);
-  for (int w = -64; w < 64; ++w) {
-    SCOPED_TRACE(w);
-    EXPECT_EQ(Pad(s, w), Pad(sp, w));
-  }
-  for (int w = -64; w < 64; ++w) {
-    SCOPED_TRACE(w);
-    EXPECT_EQ(Pad(s, w, '#'), Pad(sp, w, '#'));
+TEST(StringViewTest, ConstexprNullSafeStringView) {
+  {
+    constexpr absl::string_view s = absl::NullSafeStringView(nullptr);
+    EXPECT_EQ(nullptr, s.data());
+    EXPECT_EQ(0u, s.size());
+    EXPECT_EQ(absl::string_view(), s);
   }
-}
-
-TEST_F(StringViewStreamTest, ResetsWidth) {
-  // Width should reset after one formatted write.
-  // If we weren't resetting width after formatting the string_view,
-  // we'd have width=5 carrying over to the printing of the "]",
-  // creating "[###hi####]".
-  std::string s = "hi";
-  absl::string_view sp = s;
   {
-    std::ostringstream oss;
-    oss << "[" << std::setfill('#') << std::setw(5) << s << "]";
-    ASSERT_EQ("[###hi]", oss.str());
+    static constexpr char kHi[] = "hi";
+    absl::string_view s = absl::NullSafeStringView(kHi);
+    EXPECT_EQ(kHi, s.data());
+    EXPECT_EQ(strlen(kHi), s.size());
+    EXPECT_EQ(absl::string_view("hi"), s);
   }
   {
-    std::ostringstream oss;
-    oss << "[" << std::setfill('#') << std::setw(5) << sp << "]";
-    EXPECT_EQ("[###hi]", oss.str());
+    constexpr absl::string_view s = absl::NullSafeStringView("hello");
+    EXPECT_EQ(s.size(), 5u);
+    EXPECT_EQ("hello", s);
   }
 }
 
diff --git a/absl/strings/strip.h b/absl/strings/strip.h
index e1341e08ea7..55398ffe889 100644
--- a/absl/strings/strip.h
+++ b/absl/strings/strip.h
@@ -24,39 +24,46 @@
 #include <cstddef>
 #include <string>
 
+#include "absl/base/attributes.h"
 #include "absl/base/macros.h"
+#include "absl/base/nullability.h"
 #include "absl/strings/ascii.h"
 #include "absl/strings/match.h"
 #include "absl/strings/string_view.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 // ConsumePrefix()
 //
-// Strips the `expected` prefix from the start of the given string, returning
-// `true` if the strip operation succeeded or false otherwise.
+// Strips the `expected` prefix, if found, from the start of `str`.
+// If the operation succeeded, `true` is returned.  If not, `false`
+// is returned and `str` is not modified.
 //
 // Example:
 //
 //   absl::string_view input("abc");
 //   EXPECT_TRUE(absl::ConsumePrefix(&input, "a"));
 //   EXPECT_EQ(input, "bc");
-inline bool ConsumePrefix(absl::string_view* str, absl::string_view expected) {
+inline constexpr bool ConsumePrefix(absl::string_view* absl_nonnull str,
+                                    absl::string_view expected) {
   if (!absl::StartsWith(*str, expected)) return false;
   str->remove_prefix(expected.size());
   return true;
 }
 // ConsumeSuffix()
 //
-// Strips the `expected` suffix from the end of the given string, returning
-// `true` if the strip operation succeeded or false otherwise.
+// Strips the `expected` suffix, if found, from the end of `str`.
+// If the operation succeeded, `true` is returned.  If not, `false`
+// is returned and `str` is not modified.
 //
 // Example:
 //
 //   absl::string_view input("abcdef");
 //   EXPECT_TRUE(absl::ConsumeSuffix(&input, "def"));
 //   EXPECT_EQ(input, "abc");
-inline bool ConsumeSuffix(absl::string_view* str, absl::string_view expected) {
+inline constexpr bool ConsumeSuffix(absl::string_view* absl_nonnull str,
+                                    absl::string_view expected) {
   if (!absl::EndsWith(*str, expected)) return false;
   str->remove_suffix(expected.size());
   return true;
@@ -64,26 +71,29 @@ inline bool ConsumeSuffix(absl::string_view* str, absl::string_view expected) {
 
 // StripPrefix()
 //
-// Returns a view into the input string 'str' with the given 'prefix' removed,
+// Returns a view into the input string `str` with the given `prefix` removed,
 // but leaving the original string intact. If the prefix does not match at the
 // start of the string, returns the original string instead.
-ABSL_MUST_USE_RESULT inline absl::string_view StripPrefix(
-    absl::string_view str, absl::string_view prefix) {
+[[nodiscard]] inline constexpr absl::string_view StripPrefix(
+    absl::string_view str ABSL_ATTRIBUTE_LIFETIME_BOUND,
+    absl::string_view prefix) {
   if (absl::StartsWith(str, prefix)) str.remove_prefix(prefix.size());
   return str;
 }
 
 // StripSuffix()
 //
-// Returns a view into the input string 'str' with the given 'suffix' removed,
+// Returns a view into the input string `str` with the given `suffix` removed,
 // but leaving the original string intact. If the suffix does not match at the
 // end of the string, returns the original string instead.
-ABSL_MUST_USE_RESULT inline absl::string_view StripSuffix(
-    absl::string_view str, absl::string_view suffix) {
+[[nodiscard]] inline constexpr absl::string_view StripSuffix(
+    absl::string_view str ABSL_ATTRIBUTE_LIFETIME_BOUND,
+    absl::string_view suffix) {
   if (absl::EndsWith(str, suffix)) str.remove_suffix(suffix.size());
   return str;
 }
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_STRIP_H_
diff --git a/absl/strings/strip_test.cc b/absl/strings/strip_test.cc
index e4e00cb66eb..b5cda6c5058 100644
--- a/absl/strings/strip_test.cc
+++ b/absl/strings/strip_test.cc
@@ -20,9 +20,11 @@
 #include <cassert>
 #include <cstdio>
 #include <cstring>
+#include <string>
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
+#include "absl/strings/ascii.h"
 #include "absl/strings/string_view.h"
 
 namespace {
diff --git a/absl/strings/substitute.cc b/absl/strings/substitute.cc
index bc176950e45..244c8a55e4a 100644
--- a/absl/strings/substitute.cc
+++ b/absl/strings/substitute.cc
@@ -15,18 +15,43 @@
 #include "absl/strings/substitute.h"
 
 #include <algorithm>
-
+#include <cassert>
+#include <cstddef>
+#include <cstdint>
+#include <limits>
+#include <string>
+#include <type_traits>
+
+#include "absl/base/config.h"
 #include "absl/base/internal/raw_logging.h"
+#include "absl/base/nullability.h"
 #include "absl/strings/ascii.h"
 #include "absl/strings/escaping.h"
-#include "absl/strings/internal/resize_uninitialized.h"
+#include "absl/strings/internal/append_and_overwrite.h"
+#include "absl/strings/numbers.h"
+#include "absl/strings/str_cat.h"
 #include "absl/strings/string_view.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace substitute_internal {
 
-void SubstituteAndAppendArray(std::string* output, absl::string_view format,
-                              const absl::string_view* args_array,
+// Checked addition to avoid overflow on 32-bit. (In 64-bit the callers wouldn't
+// get anywhere close to the limit given that 2^64 bytes of memory is beyond
+// anything physically possible.)
+// Note that we don't declare the parameters as size_t in order to avoid
+// accidental implicit conversions.
+template <int&..., typename T>
+[[nodiscard]] std::enable_if_t<std::is_same_v<T, size_t>, T> CheckedAdd(T a,
+                                                                        T b) {
+  ABSL_INTERNAL_CHECK(b <= (std::numeric_limits<T>::max)() - a,
+                      "unsigned integer overflow");
+  return a + b;
+}
+
+void SubstituteAndAppendArray(std::string* absl_nonnull output,
+                              absl::string_view format,
+                              const absl::string_view* absl_nullable args_array,
                               size_t num_args) {
   // Determine total size needed.
   size_t size = 0;
@@ -35,67 +60,71 @@ void SubstituteAndAppendArray(std::string* output, absl::string_view format,
       if (i + 1 >= format.size()) {
 #ifndef NDEBUG
         ABSL_RAW_LOG(FATAL,
-                     "Invalid strings::Substitute() format std::string: \"%s\".",
+                     "Invalid absl::Substitute() format string: \"%s\".",
                      absl::CEscape(format).c_str());
 #endif
         return;
-      } else if (absl::ascii_isdigit(format[i + 1])) {
+      } else if (absl::ascii_isdigit(
+                     static_cast<unsigned char>(format[i + 1]))) {
         int index = format[i + 1] - '0';
         if (static_cast<size_t>(index) >= num_args) {
 #ifndef NDEBUG
           ABSL_RAW_LOG(
               FATAL,
-              "Invalid strings::Substitute() format std::string: asked for \"$"
-              "%d\", but only %d args were given.  Full format std::string was: "
+              "Invalid absl::Substitute() format string: asked for \"$"
+              "%d\", but only %d args were given.  Full format string was: "
               "\"%s\".",
               index, static_cast<int>(num_args), absl::CEscape(format).c_str());
 #endif
           return;
         }
-        size += args_array[index].size();
+        size = CheckedAdd(size, args_array[index].size());
         ++i;  // Skip next char.
       } else if (format[i + 1] == '$') {
-        ++size;
+        size = CheckedAdd(size, size_t{1});
         ++i;  // Skip next char.
       } else {
 #ifndef NDEBUG
         ABSL_RAW_LOG(FATAL,
-                     "Invalid strings::Substitute() format std::string: \"%s\".",
+                     "Invalid absl::Substitute() format string: \"%s\".",
                      absl::CEscape(format).c_str());
 #endif
         return;
       }
     } else {
-      ++size;
+      size = CheckedAdd(size, size_t{1});
     }
   }
 
   if (size == 0) return;
 
-  // Build the std::string.
-  size_t original_size = output->size();
-  strings_internal::STLStringResizeUninitialized(output, original_size + size);
-  char* target = &(*output)[original_size];
-  for (size_t i = 0; i < format.size(); i++) {
-    if (format[i] == '$') {
-      if (absl::ascii_isdigit(format[i + 1])) {
-        const absl::string_view src = args_array[format[i + 1] - '0'];
-        target = std::copy(src.begin(), src.end(), target);
-        ++i;  // Skip next char.
-      } else if (format[i + 1] == '$') {
-        *target++ = '$';
-        ++i;  // Skip next char.
-      }
-    } else {
-      *target++ = format[i];
-    }
-  }
-
-  assert(target == output->data() + output->size());
+  // Build the string.
+  ABSL_INTERNAL_CHECK(size <= output->max_size() - output->size(),
+                      "Exceeds std::string::max_size()");
+  strings_internal::StringAppendAndOverwrite(
+      *output, size, [format, args_array](char* const buf, size_t buf_size) {
+        char* target = buf;
+        for (size_t i = 0; i < format.size(); i++) {
+          if (format[i] == '$') {
+            if (absl::ascii_isdigit(
+                    static_cast<unsigned char>(format[i + 1]))) {
+              const absl::string_view src = args_array[format[i + 1] - '0'];
+              target = std::copy(src.begin(), src.end(), target);
+              ++i;  // Skip next char.
+            } else if (format[i + 1] == '$') {
+              *target++ = '$';
+              ++i;  // Skip next char.
+            }
+          } else {
+            *target++ = format[i];
+          }
+        }
+        assert(target == buf + buf_size);
+        return buf_size;
+      });
 }
 
-static const char kHexDigits[] = "0123456789abcdef";
-Arg::Arg(const void* value) {
+Arg::Arg(const void* absl_nullable value) {
   static_assert(sizeof(scratch_) >= sizeof(value) * 2 + 2,
                 "fix sizeof(scratch_)");
   if (value == nullptr) {
@@ -104,12 +133,13 @@ Arg::Arg(const void* value) {
     char* ptr = scratch_ + sizeof(scratch_);
     uintptr_t num = reinterpret_cast<uintptr_t>(value);
     do {
-      *--ptr = kHexDigits[num & 0xf];
+      *--ptr = absl::numbers_internal::kHexChar[num & 0xf];
       num >>= 4;
     } while (num != 0);
     *--ptr = 'x';
     *--ptr = '0';
-    piece_ = absl::string_view(ptr, scratch_ + sizeof(scratch_) - ptr);
+    piece_ = absl::string_view(
+        ptr, static_cast<size_t>(scratch_ + sizeof(scratch_) - ptr));
   }
 }
 
@@ -119,7 +149,7 @@ Arg::Arg(Hex hex) {
   char* writer = end;
   uint64_t value = hex.value;
   do {
-    *--writer = kHexDigits[value & 0xF];
+    *--writer = absl::numbers_internal::kHexChar[value & 0xF];
     value >>= 4;
   } while (value != 0);
 
@@ -131,7 +161,7 @@ Arg::Arg(Hex hex) {
     beg = writer;
   }
 
-  piece_ = absl::string_view(beg, end - beg);
+  piece_ = absl::string_view(beg, static_cast<size_t>(end - beg));
 }
 
 // TODO(jorg): Don't duplicate so much code between here and str_cat.cc
@@ -146,7 +176,7 @@ Arg::Arg(Dec dec) {
     *--writer = '0' + (value % 10);
     value /= 10;
   }
-  *--writer = '0' + value;
+  *--writer = '0' + static_cast<char>(value);
   if (neg) *--writer = '-';
 
   ptrdiff_t fillers = writer - minfill;
@@ -163,8 +193,9 @@ Arg::Arg(Dec dec) {
     if (add_sign_again) *--writer = '-';
   }
 
-  piece_ = absl::string_view(writer, end - writer);
+  piece_ = absl::string_view(writer, static_cast<size_t>(end - writer));
 }
 
 }  // namespace substitute_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/substitute.h b/absl/strings/substitute.h
index 233e9dcf938..accf26ec020 100644
--- a/absl/strings/substitute.h
+++ b/absl/strings/substitute.h
@@ -50,11 +50,13 @@
 //
 // Supported types:
 //   * absl::string_view, std::string, const char* (null is equivalent to "")
-//   * int32_t, int64_t, uint32_t, uint64
+//   * int32_t, int64_t, uint32_t, uint64_t
 //   * float, double
 //   * bool (Printed as "true" or "false")
 //   * pointer types other than char* (Printed as "0x<lower case hex string>",
 //     except that null is printed as "NULL")
+//   * user-defined types via the `AbslStringify()` customization point. See the
+//     documentation for `absl::StrCat` for an explanation on how to use this.
 //
 // If an invalid format string is provided, Substitute returns an empty string
 // and SubstituteAndAppend does not change the provided output string.
@@ -71,14 +73,18 @@
 #define ABSL_STRINGS_SUBSTITUTE_H_
 
 #include <cstring>
+#include <iterator>
 #include <string>
 #include <type_traits>
 #include <vector>
 
 #include "absl/base/macros.h"
+#include "absl/base/nullability.h"
 #include "absl/base/port.h"
 #include "absl/strings/ascii.h"
 #include "absl/strings/escaping.h"
+#include "absl/strings/has_absl_stringify.h"
+#include "absl/strings/internal/stringify_sink.h"
 #include "absl/strings/numbers.h"
 #include "absl/strings/str_cat.h"
 #include "absl/strings/str_split.h"
@@ -86,6 +92,7 @@
 #include "absl/strings/strip.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace substitute_internal {
 
 // Arg
@@ -98,17 +105,17 @@ namespace substitute_internal {
 // This class has implicit constructors.
 class Arg {
  public:
-  // Overloads for std::string-y things
+  // Overloads for string-y things
   //
   // Explicitly overload `const char*` so the compiler doesn't cast to `bool`.
-  Arg(const char* value)  // NOLINT(runtime/explicit)
+  Arg(const char* absl_nullable value)  // NOLINT(google-explicit-constructor)
       : piece_(absl::NullSafeStringView(value)) {}
   template <typename Allocator>
   Arg(  // NOLINT
       const std::basic_string<char, std::char_traits<char>, Allocator>&
           value) noexcept
       : piece_(value) {}
-  Arg(absl::string_view value)  // NOLINT(runtime/explicit)
+  Arg(absl::string_view value)  // NOLINT(google-explicit-constructor)
       : piece_(value) {}
 
   // Overloads for primitives
@@ -118,58 +125,91 @@ class Arg {
   // probably using them as 8-bit integers and would probably prefer an integer
   // representation. However, we can't really know, so we make the caller decide
   // what to do.
-  Arg(char value)  // NOLINT(runtime/explicit)
-      : piece_(scratch_, 1) { scratch_[0] = value; }
+  Arg(char value) {  // NOLINT(google-explicit-constructor)
+    scratch_[0] = value;
+    piece_ = absl::string_view(scratch_, 1);
+  }
   Arg(short value)  // NOLINT(*)
       : piece_(scratch_,
-               numbers_internal::FastIntToBuffer(value, scratch_) - scratch_) {}
+               static_cast<size_t>(
+                   numbers_internal::FastIntToBuffer(value, scratch_) -
+                   scratch_)) {}
   Arg(unsigned short value)  // NOLINT(*)
       : piece_(scratch_,
-               numbers_internal::FastIntToBuffer(value, scratch_) - scratch_) {}
-  Arg(int value)  // NOLINT(runtime/explicit)
+               static_cast<size_t>(
+                   numbers_internal::FastIntToBuffer(value, scratch_) -
+                   scratch_)) {}
+  Arg(int value)  // NOLINT(google-explicit-constructor)
       : piece_(scratch_,
-               numbers_internal::FastIntToBuffer(value, scratch_) - scratch_) {}
-  Arg(unsigned int value)  // NOLINT(runtime/explicit)
+               static_cast<size_t>(
+                   numbers_internal::FastIntToBuffer(value, scratch_) -
+                   scratch_)) {}
+  Arg(unsigned int value)  // NOLINT(google-explicit-constructor)
       : piece_(scratch_,
-               numbers_internal::FastIntToBuffer(value, scratch_) - scratch_) {}
+               static_cast<size_t>(
+                   numbers_internal::FastIntToBuffer(value, scratch_) -
+                   scratch_)) {}
   Arg(long value)  // NOLINT(*)
       : piece_(scratch_,
-               numbers_internal::FastIntToBuffer(value, scratch_) - scratch_) {}
+               static_cast<size_t>(
+                   numbers_internal::FastIntToBuffer(value, scratch_) -
+                   scratch_)) {}
   Arg(unsigned long value)  // NOLINT(*)
       : piece_(scratch_,
-               numbers_internal::FastIntToBuffer(value, scratch_) - scratch_) {}
+               static_cast<size_t>(
+                   numbers_internal::FastIntToBuffer(value, scratch_) -
+                   scratch_)) {}
   Arg(long long value)  // NOLINT(*)
       : piece_(scratch_,
-               numbers_internal::FastIntToBuffer(value, scratch_) - scratch_) {}
+               static_cast<size_t>(
+                   numbers_internal::FastIntToBuffer(value, scratch_) -
+                   scratch_)) {}
   Arg(unsigned long long value)  // NOLINT(*)
       : piece_(scratch_,
-               numbers_internal::FastIntToBuffer(value, scratch_) - scratch_) {}
-  Arg(float value)  // NOLINT(runtime/explicit)
+               static_cast<size_t>(
+                   numbers_internal::FastIntToBuffer(value, scratch_) -
+                   scratch_)) {}
+  Arg(float value)  // NOLINT(google-explicit-constructor)
       : piece_(scratch_, numbers_internal::SixDigitsToBuffer(value, scratch_)) {
   }
-  Arg(double value)  // NOLINT(runtime/explicit)
+  Arg(double value)  // NOLINT(google-explicit-constructor)
       : piece_(scratch_, numbers_internal::SixDigitsToBuffer(value, scratch_)) {
   }
-  Arg(bool value)  // NOLINT(runtime/explicit)
+  Arg(bool value)  // NOLINT(google-explicit-constructor)
       : piece_(value ? "true" : "false") {}
 
-  Arg(Hex hex);  // NOLINT(runtime/explicit)
-  Arg(Dec dec);  // NOLINT(runtime/explicit)
+  template <typename T, typename = std::enable_if_t<HasAbslStringify<T>::value>>
+  Arg(  // NOLINT(google-explicit-constructor)
+      const T& v, strings_internal::StringifySink&& sink = {})
+      : piece_(strings_internal::ExtractStringification(sink, v)) {}
+
+  Arg(Hex hex);  // NOLINT(google-explicit-constructor)
+  Arg(Dec dec);  // NOLINT(google-explicit-constructor)
 
-  // vector<bool>::reference and const_reference require special help to
-  // convert to `AlphaNum` because it requires two user defined conversions.
+  // vector<bool>::reference and const_reference require special help to convert
+  // to `Arg` because it requires two user defined conversions.
   template <typename T,
-            absl::enable_if_t<
-                std::is_class<T>::value &&
-                (std::is_same<T, std::vector<bool>::reference>::value ||
-                 std::is_same<T, std::vector<bool>::const_reference>::value)>* =
-                nullptr>
+            std::enable_if_t<
+                std::is_class_v<T> &&
+                    (std::is_same_v<T, std::vector<bool>::reference> ||
+                     std::is_same_v<T, std::vector<bool>::const_reference>),
+                bool> = true>
   Arg(T value)  // NOLINT(google-explicit-constructor)
       : Arg(static_cast<bool>(value)) {}
 
   // `void*` values, with the exception of `char*`, are printed as
   // "0x<hex value>". However, in the case of `nullptr`, "NULL" is printed.
-  Arg(const void* value);  // NOLINT(runtime/explicit)
+  Arg(  // NOLINT(google-explicit-constructor)
+      const void* absl_nullable value);
+
+  // Normal enums are already handled by the integer formatters.
+  // This overload matches only scoped enums.
+  template <typename T,
+            typename = std::enable_if_t<std::is_enum<T>{} &&
+                                        !std::is_convertible<T, int>{} &&
+                                        !HasAbslStringify<T>::value>>
+  Arg(T value)  // NOLINT(google-explicit-constructor)
+      : Arg(static_cast<std::underlying_type_t<T>>(value)) {}
 
   Arg(const Arg&) = delete;
   Arg& operator=(const Arg&) = delete;
@@ -183,24 +223,31 @@ class Arg {
 
 // Internal helper function. Don't call this from outside this implementation.
 // This interface may change without notice.
-void SubstituteAndAppendArray(std::string* output, absl::string_view format,
-                              const absl::string_view* args_array,
+void SubstituteAndAppendArray(std::string* absl_nonnull output,
+                              absl::string_view format,
+                              const absl::string_view* absl_nullable args_array,
                               size_t num_args);
 
 #if defined(ABSL_BAD_CALL_IF)
-constexpr int CalculateOneBit(const char* format) {
-  return (*format < '0' || *format > '9') ? 0 : (1 << (*format - '0'));
+constexpr int CalculateOneBit(const char* absl_nonnull format) {
+  // Returns:
+  // * 2^N for '$N' when N is in [0-9]
+  // * 0 for correct '$' escaping: '$$'.
+  // * -1 otherwise.
+  return (*format < '0' || *format > '9') ? (*format == '$' ? 0 : -1)
+                                          : (1 << (*format - '0'));
 }
 
-constexpr const char* SkipNumber(const char* format) {
+constexpr const char* absl_nonnull SkipNumber(const char* absl_nonnull format) {
   return !*format ? format : (format + 1);
 }
 
-constexpr int PlaceholderBitmask(const char* format) {
-  return !*format ? 0 : *format != '$'
-                             ? PlaceholderBitmask(format + 1)
-                             : (CalculateOneBit(format + 1) |
-                                   PlaceholderBitmask(SkipNumber(format + 1)));
+constexpr int PlaceholderBitmask(const char* absl_nonnull format) {
+  return !*format
+             ? 0
+             : *format != '$' ? PlaceholderBitmask(format + 1)
+                              : (CalculateOneBit(format + 1) |
+                                 PlaceholderBitmask(SkipNumber(format + 1)));
 }
 #endif  // ABSL_BAD_CALL_IF
 
@@ -228,35 +275,40 @@ constexpr int PlaceholderBitmask(const char* format) {
 //    absl::SubstituteAndAppend(boilerplate, format, args...);
 //  }
 //
-inline void SubstituteAndAppend(std::string* output, absl::string_view format) {
+inline void SubstituteAndAppend(std::string* absl_nonnull output,
+                                absl::string_view format) {
   substitute_internal::SubstituteAndAppendArray(output, format, nullptr, 0);
 }
 
-inline void SubstituteAndAppend(std::string* output, absl::string_view format,
+inline void SubstituteAndAppend(std::string* absl_nonnull output,
+                                absl::string_view format,
                                 const substitute_internal::Arg& a0) {
   const absl::string_view args[] = {a0.piece()};
   substitute_internal::SubstituteAndAppendArray(output, format, args,
-                                                ABSL_ARRAYSIZE(args));
+                                                std::size(args));
 }
 
-inline void SubstituteAndAppend(std::string* output, absl::string_view format,
+inline void SubstituteAndAppend(std::string* absl_nonnull output,
+                                absl::string_view format,
                                 const substitute_internal::Arg& a0,
                                 const substitute_internal::Arg& a1) {
   const absl::string_view args[] = {a0.piece(), a1.piece()};
   substitute_internal::SubstituteAndAppendArray(output, format, args,
-                                                ABSL_ARRAYSIZE(args));
+                                                std::size(args));
 }
 
-inline void SubstituteAndAppend(std::string* output, absl::string_view format,
+inline void SubstituteAndAppend(std::string* absl_nonnull output,
+                                absl::string_view format,
                                 const substitute_internal::Arg& a0,
                                 const substitute_internal::Arg& a1,
                                 const substitute_internal::Arg& a2) {
   const absl::string_view args[] = {a0.piece(), a1.piece(), a2.piece()};
   substitute_internal::SubstituteAndAppendArray(output, format, args,
-                                                ABSL_ARRAYSIZE(args));
+                                                std::size(args));
 }
 
-inline void SubstituteAndAppend(std::string* output, absl::string_view format,
+inline void SubstituteAndAppend(std::string* absl_nonnull output,
+                                absl::string_view format,
                                 const substitute_internal::Arg& a0,
                                 const substitute_internal::Arg& a1,
                                 const substitute_internal::Arg& a2,
@@ -264,10 +316,11 @@ inline void SubstituteAndAppend(std::string* output, absl::string_view format,
   const absl::string_view args[] = {a0.piece(), a1.piece(), a2.piece(),
                                     a3.piece()};
   substitute_internal::SubstituteAndAppendArray(output, format, args,
-                                                ABSL_ARRAYSIZE(args));
+                                                std::size(args));
 }
 
-inline void SubstituteAndAppend(std::string* output, absl::string_view format,
+inline void SubstituteAndAppend(std::string* absl_nonnull output,
+                                absl::string_view format,
                                 const substitute_internal::Arg& a0,
                                 const substitute_internal::Arg& a1,
                                 const substitute_internal::Arg& a2,
@@ -276,39 +329,35 @@ inline void SubstituteAndAppend(std::string* output, absl::string_view format,
   const absl::string_view args[] = {a0.piece(), a1.piece(), a2.piece(),
                                     a3.piece(), a4.piece()};
   substitute_internal::SubstituteAndAppendArray(output, format, args,
-                                                ABSL_ARRAYSIZE(args));
+                                                std::size(args));
 }
 
-inline void SubstituteAndAppend(std::string* output, absl::string_view format,
-                                const substitute_internal::Arg& a0,
-                                const substitute_internal::Arg& a1,
-                                const substitute_internal::Arg& a2,
-                                const substitute_internal::Arg& a3,
-                                const substitute_internal::Arg& a4,
-                                const substitute_internal::Arg& a5) {
+inline void SubstituteAndAppend(
+    std::string* absl_nonnull output, absl::string_view format,
+    const substitute_internal::Arg& a0, const substitute_internal::Arg& a1,
+    const substitute_internal::Arg& a2, const substitute_internal::Arg& a3,
+    const substitute_internal::Arg& a4, const substitute_internal::Arg& a5) {
   const absl::string_view args[] = {a0.piece(), a1.piece(), a2.piece(),
                                     a3.piece(), a4.piece(), a5.piece()};
   substitute_internal::SubstituteAndAppendArray(output, format, args,
-                                                ABSL_ARRAYSIZE(args));
+                                                std::size(args));
 }
 
-inline void SubstituteAndAppend(std::string* output, absl::string_view format,
-                                const substitute_internal::Arg& a0,
-                                const substitute_internal::Arg& a1,
-                                const substitute_internal::Arg& a2,
-                                const substitute_internal::Arg& a3,
-                                const substitute_internal::Arg& a4,
-                                const substitute_internal::Arg& a5,
-                                const substitute_internal::Arg& a6) {
+inline void SubstituteAndAppend(
+    std::string* absl_nonnull output, absl::string_view format,
+    const substitute_internal::Arg& a0, const substitute_internal::Arg& a1,
+    const substitute_internal::Arg& a2, const substitute_internal::Arg& a3,
+    const substitute_internal::Arg& a4, const substitute_internal::Arg& a5,
+    const substitute_internal::Arg& a6) {
   const absl::string_view args[] = {a0.piece(), a1.piece(), a2.piece(),
                                     a3.piece(), a4.piece(), a5.piece(),
                                     a6.piece()};
   substitute_internal::SubstituteAndAppendArray(output, format, args,
-                                                ABSL_ARRAYSIZE(args));
+                                                std::size(args));
 }
 
 inline void SubstituteAndAppend(
-    std::string* output, absl::string_view format,
+    std::string* absl_nonnull output, absl::string_view format,
     const substitute_internal::Arg& a0, const substitute_internal::Arg& a1,
     const substitute_internal::Arg& a2, const substitute_internal::Arg& a3,
     const substitute_internal::Arg& a4, const substitute_internal::Arg& a5,
@@ -317,11 +366,11 @@ inline void SubstituteAndAppend(
                                     a3.piece(), a4.piece(), a5.piece(),
                                     a6.piece(), a7.piece()};
   substitute_internal::SubstituteAndAppendArray(output, format, args,
-                                                ABSL_ARRAYSIZE(args));
+                                                std::size(args));
 }
 
 inline void SubstituteAndAppend(
-    std::string* output, absl::string_view format,
+    std::string* absl_nonnull output, absl::string_view format,
     const substitute_internal::Arg& a0, const substitute_internal::Arg& a1,
     const substitute_internal::Arg& a2, const substitute_internal::Arg& a3,
     const substitute_internal::Arg& a4, const substitute_internal::Arg& a5,
@@ -331,11 +380,11 @@ inline void SubstituteAndAppend(
                                     a3.piece(), a4.piece(), a5.piece(),
                                     a6.piece(), a7.piece(), a8.piece()};
   substitute_internal::SubstituteAndAppendArray(output, format, args,
-                                                ABSL_ARRAYSIZE(args));
+                                                std::size(args));
 }
 
 inline void SubstituteAndAppend(
-    std::string* output, absl::string_view format,
+    std::string* absl_nonnull output, absl::string_view format,
     const substitute_internal::Arg& a0, const substitute_internal::Arg& a1,
     const substitute_internal::Arg& a2, const substitute_internal::Arg& a3,
     const substitute_internal::Arg& a4, const substitute_internal::Arg& a5,
@@ -345,116 +394,135 @@ inline void SubstituteAndAppend(
       a0.piece(), a1.piece(), a2.piece(), a3.piece(), a4.piece(),
       a5.piece(), a6.piece(), a7.piece(), a8.piece(), a9.piece()};
   substitute_internal::SubstituteAndAppendArray(output, format, args,
-                                                ABSL_ARRAYSIZE(args));
+                                                std::size(args));
 }
 
 #if defined(ABSL_BAD_CALL_IF)
 // This body of functions catches cases where the number of placeholders
 // doesn't match the number of data arguments.
-void SubstituteAndAppend(std::string* output, const char* format)
-    ABSL_BAD_CALL_IF(substitute_internal::PlaceholderBitmask(format) != 0,
-                     "There were no substitution arguments "
-                     "but this format std::string has a $[0-9] in it");
+void SubstituteAndAppend(std::string* absl_nonnull output,
+                         const char* absl_nonnull format)
+    ABSL_BAD_CALL_IF(
+        substitute_internal::PlaceholderBitmask(format) != 0,
+        "There were no substitution arguments "
+        "but this format string either has a $[0-9] in it or contains "
+        "an unescaped $ character (use $$ instead)");
 
-void SubstituteAndAppend(std::string* output, const char* format,
+void SubstituteAndAppend(std::string* absl_nonnull output,
+                         const char* absl_nonnull format,
                          const substitute_internal::Arg& a0)
     ABSL_BAD_CALL_IF(substitute_internal::PlaceholderBitmask(format) != 1,
                      "There was 1 substitution argument given, but "
-                     "this format std::string is either missing its $0, or "
-                     "contains one of $1-$9");
+                     "this format string is missing its $0, contains "
+                     "one of $1-$9, or contains an unescaped $ character (use "
+                     "$$ instead)");
 
-void SubstituteAndAppend(std::string* output, const char* format,
+void SubstituteAndAppend(std::string* absl_nonnull output,
+                         const char* absl_nonnull format,
                          const substitute_internal::Arg& a0,
                          const substitute_internal::Arg& a1)
-    ABSL_BAD_CALL_IF(substitute_internal::PlaceholderBitmask(format) != 3,
-                     "There were 2 substitution arguments given, but "
-                     "this format std::string is either missing its $0/$1, or "
-                     "contains one of $2-$9");
+    ABSL_BAD_CALL_IF(
+        substitute_internal::PlaceholderBitmask(format) != 3,
+        "There were 2 substitution arguments given, but this format string is "
+        "missing its $0/$1, contains one of $2-$9, or contains an "
+        "unescaped $ character (use $$ instead)");
 
-void SubstituteAndAppend(std::string* output, const char* format,
+void SubstituteAndAppend(std::string* absl_nonnull output,
+                         const char* absl_nonnull format,
                          const substitute_internal::Arg& a0,
                          const substitute_internal::Arg& a1,
                          const substitute_internal::Arg& a2)
-    ABSL_BAD_CALL_IF(substitute_internal::PlaceholderBitmask(format) != 7,
-                     "There were 3 substitution arguments given, but "
-                     "this format std::string is either missing its $0/$1/$2, or "
-                     "contains one of $3-$9");
+    ABSL_BAD_CALL_IF(
+        substitute_internal::PlaceholderBitmask(format) != 7,
+        "There were 3 substitution arguments given, but "
+        "this format string is missing its $0/$1/$2, contains one of "
+        "$3-$9, or contains an unescaped $ character (use $$ instead)");
 
-void SubstituteAndAppend(std::string* output, const char* format,
+void SubstituteAndAppend(std::string* absl_nonnull output,
+                         const char* absl_nonnull format,
                          const substitute_internal::Arg& a0,
                          const substitute_internal::Arg& a1,
                          const substitute_internal::Arg& a2,
                          const substitute_internal::Arg& a3)
-    ABSL_BAD_CALL_IF(substitute_internal::PlaceholderBitmask(format) != 15,
-                     "There were 4 substitution arguments given, but "
-                     "this format std::string is either missing its $0-$3, or "
-                     "contains one of $4-$9");
+    ABSL_BAD_CALL_IF(
+        substitute_internal::PlaceholderBitmask(format) != 15,
+        "There were 4 substitution arguments given, but "
+        "this format string is missing its $0-$3, contains one of "
+        "$4-$9, or contains an unescaped $ character (use $$ instead)");
 
-void SubstituteAndAppend(std::string* output, const char* format,
+void SubstituteAndAppend(std::string* absl_nonnull output,
+                         const char* absl_nonnull format,
                          const substitute_internal::Arg& a0,
                          const substitute_internal::Arg& a1,
                          const substitute_internal::Arg& a2,
                          const substitute_internal::Arg& a3,
                          const substitute_internal::Arg& a4)
-    ABSL_BAD_CALL_IF(substitute_internal::PlaceholderBitmask(format) != 31,
-                     "There were 5 substitution arguments given, but "
-                     "this format std::string is either missing its $0-$4, or "
-                     "contains one of $5-$9");
+    ABSL_BAD_CALL_IF(
+        substitute_internal::PlaceholderBitmask(format) != 31,
+        "There were 5 substitution arguments given, but "
+        "this format string is missing its $0-$4, contains one of "
+        "$5-$9, or contains an unescaped $ character (use $$ instead)");
 
-void SubstituteAndAppend(std::string* output, const char* format,
-                         const substitute_internal::Arg& a0,
-                         const substitute_internal::Arg& a1,
-                         const substitute_internal::Arg& a2,
-                         const substitute_internal::Arg& a3,
-                         const substitute_internal::Arg& a4,
-                         const substitute_internal::Arg& a5)
-    ABSL_BAD_CALL_IF(substitute_internal::PlaceholderBitmask(format) != 63,
-                     "There were 6 substitution arguments given, but "
-                     "this format std::string is either missing its $0-$5, or "
-                     "contains one of $6-$9");
+void SubstituteAndAppend(
+    std::string* absl_nonnull output, const char* absl_nonnull format,
+    const substitute_internal::Arg& a0, const substitute_internal::Arg& a1,
+    const substitute_internal::Arg& a2, const substitute_internal::Arg& a3,
+    const substitute_internal::Arg& a4, const substitute_internal::Arg& a5)
+    ABSL_BAD_CALL_IF(
+        substitute_internal::PlaceholderBitmask(format) != 63,
+        "There were 6 substitution arguments given, but "
+        "this format string is missing its $0-$5, contains one of "
+        "$6-$9, or contains an unescaped $ character (use $$ instead)");
 
 void SubstituteAndAppend(
-    std::string* output, const char* format, const substitute_internal::Arg& a0,
-    const substitute_internal::Arg& a1, const substitute_internal::Arg& a2,
-    const substitute_internal::Arg& a3, const substitute_internal::Arg& a4,
-    const substitute_internal::Arg& a5, const substitute_internal::Arg& a6)
-    ABSL_BAD_CALL_IF(substitute_internal::PlaceholderBitmask(format) != 127,
-                     "There were 7 substitution arguments given, but "
-                     "this format std::string is either missing its $0-$6, or "
-                     "contains one of $7-$9");
+    std::string* absl_nonnull output, const char* absl_nonnull format,
+    const substitute_internal::Arg& a0, const substitute_internal::Arg& a1,
+    const substitute_internal::Arg& a2, const substitute_internal::Arg& a3,
+    const substitute_internal::Arg& a4, const substitute_internal::Arg& a5,
+    const substitute_internal::Arg& a6)
+    ABSL_BAD_CALL_IF(
+        substitute_internal::PlaceholderBitmask(format) != 127,
+        "There were 7 substitution arguments given, but "
+        "this format string is missing its $0-$6, contains one of "
+        "$7-$9, or contains an unescaped $ character (use $$ instead)");
 
 void SubstituteAndAppend(
-    std::string* output, const char* format, const substitute_internal::Arg& a0,
-    const substitute_internal::Arg& a1, const substitute_internal::Arg& a2,
-    const substitute_internal::Arg& a3, const substitute_internal::Arg& a4,
-    const substitute_internal::Arg& a5, const substitute_internal::Arg& a6,
-    const substitute_internal::Arg& a7)
-    ABSL_BAD_CALL_IF(substitute_internal::PlaceholderBitmask(format) != 255,
-                     "There were 8 substitution arguments given, but "
-                     "this format std::string is either missing its $0-$7, or "
-                     "contains one of $8-$9");
+    std::string* absl_nonnull output, const char* absl_nonnull format,
+    const substitute_internal::Arg& a0, const substitute_internal::Arg& a1,
+    const substitute_internal::Arg& a2, const substitute_internal::Arg& a3,
+    const substitute_internal::Arg& a4, const substitute_internal::Arg& a5,
+    const substitute_internal::Arg& a6, const substitute_internal::Arg& a7)
+    ABSL_BAD_CALL_IF(
+        substitute_internal::PlaceholderBitmask(format) != 255,
+        "There were 8 substitution arguments given, but "
+        "this format string is missing its $0-$7, contains one of "
+        "$8-$9, or contains an unescaped $ character (use $$ instead)");
 
 void SubstituteAndAppend(
-    std::string* output, const char* format, const substitute_internal::Arg& a0,
-    const substitute_internal::Arg& a1, const substitute_internal::Arg& a2,
-    const substitute_internal::Arg& a3, const substitute_internal::Arg& a4,
-    const substitute_internal::Arg& a5, const substitute_internal::Arg& a6,
-    const substitute_internal::Arg& a7, const substitute_internal::Arg& a8)
+    std::string* absl_nonnull output, const char* absl_nonnull format,
+    const substitute_internal::Arg& a0, const substitute_internal::Arg& a1,
+    const substitute_internal::Arg& a2, const substitute_internal::Arg& a3,
+    const substitute_internal::Arg& a4, const substitute_internal::Arg& a5,
+    const substitute_internal::Arg& a6, const substitute_internal::Arg& a7,
+    const substitute_internal::Arg& a8)
     ABSL_BAD_CALL_IF(
         substitute_internal::PlaceholderBitmask(format) != 511,
         "There were 9 substitution arguments given, but "
-        "this format std::string is either missing its $0-$8, or contains a $9");
+        "this format string is missing its $0-$8, contains a $9, or "
+        "contains an unescaped $ character (use $$ instead)");
 
 void SubstituteAndAppend(
-    std::string* output, const char* format, const substitute_internal::Arg& a0,
-    const substitute_internal::Arg& a1, const substitute_internal::Arg& a2,
-    const substitute_internal::Arg& a3, const substitute_internal::Arg& a4,
-    const substitute_internal::Arg& a5, const substitute_internal::Arg& a6,
-    const substitute_internal::Arg& a7, const substitute_internal::Arg& a8,
-    const substitute_internal::Arg& a9)
-    ABSL_BAD_CALL_IF(substitute_internal::PlaceholderBitmask(format) != 1023,
-                     "There were 10 substitution arguments given, but this "
-                     "format std::string doesn't contain all of $0 through $9");
+    std::string* absl_nonnull output, const char* absl_nonnull format,
+    const substitute_internal::Arg& a0, const substitute_internal::Arg& a1,
+    const substitute_internal::Arg& a2, const substitute_internal::Arg& a3,
+    const substitute_internal::Arg& a4, const substitute_internal::Arg& a5,
+    const substitute_internal::Arg& a6, const substitute_internal::Arg& a7,
+    const substitute_internal::Arg& a8, const substitute_internal::Arg& a9)
+    ABSL_BAD_CALL_IF(
+        substitute_internal::PlaceholderBitmask(format) != 1023,
+        "There were 10 substitution arguments given, but this "
+        "format string either doesn't contain all of $0 through $9 or "
+        "contains an unescaped $ character (use $$ instead)");
 #endif  // ABSL_BAD_CALL_IF
 
 // Substitute()
@@ -473,20 +541,20 @@ void SubstituteAndAppend(
 //  void VarMsg(absl::string_view format, const Args&... args) {
 //    std::string s = absl::Substitute(format, args...);
 
-ABSL_MUST_USE_RESULT inline std::string Substitute(absl::string_view format) {
+[[nodiscard]] inline std::string Substitute(absl::string_view format) {
   std::string result;
   SubstituteAndAppend(&result, format);
   return result;
 }
 
-ABSL_MUST_USE_RESULT inline std::string Substitute(
+[[nodiscard]] inline std::string Substitute(
     absl::string_view format, const substitute_internal::Arg& a0) {
   std::string result;
   SubstituteAndAppend(&result, format, a0);
   return result;
 }
 
-ABSL_MUST_USE_RESULT inline std::string Substitute(
+[[nodiscard]] inline std::string Substitute(
     absl::string_view format, const substitute_internal::Arg& a0,
     const substitute_internal::Arg& a1) {
   std::string result;
@@ -494,7 +562,7 @@ ABSL_MUST_USE_RESULT inline std::string Substitute(
   return result;
 }
 
-ABSL_MUST_USE_RESULT inline std::string Substitute(
+[[nodiscard]] inline std::string Substitute(
     absl::string_view format, const substitute_internal::Arg& a0,
     const substitute_internal::Arg& a1, const substitute_internal::Arg& a2) {
   std::string result;
@@ -502,7 +570,7 @@ ABSL_MUST_USE_RESULT inline std::string Substitute(
   return result;
 }
 
-ABSL_MUST_USE_RESULT inline std::string Substitute(
+[[nodiscard]] inline std::string Substitute(
     absl::string_view format, const substitute_internal::Arg& a0,
     const substitute_internal::Arg& a1, const substitute_internal::Arg& a2,
     const substitute_internal::Arg& a3) {
@@ -511,7 +579,7 @@ ABSL_MUST_USE_RESULT inline std::string Substitute(
   return result;
 }
 
-ABSL_MUST_USE_RESULT inline std::string Substitute(
+[[nodiscard]] inline std::string Substitute(
     absl::string_view format, const substitute_internal::Arg& a0,
     const substitute_internal::Arg& a1, const substitute_internal::Arg& a2,
     const substitute_internal::Arg& a3, const substitute_internal::Arg& a4) {
@@ -520,7 +588,7 @@ ABSL_MUST_USE_RESULT inline std::string Substitute(
   return result;
 }
 
-ABSL_MUST_USE_RESULT inline std::string Substitute(
+[[nodiscard]] inline std::string Substitute(
     absl::string_view format, const substitute_internal::Arg& a0,
     const substitute_internal::Arg& a1, const substitute_internal::Arg& a2,
     const substitute_internal::Arg& a3, const substitute_internal::Arg& a4,
@@ -530,7 +598,7 @@ ABSL_MUST_USE_RESULT inline std::string Substitute(
   return result;
 }
 
-ABSL_MUST_USE_RESULT inline std::string Substitute(
+[[nodiscard]] inline std::string Substitute(
     absl::string_view format, const substitute_internal::Arg& a0,
     const substitute_internal::Arg& a1, const substitute_internal::Arg& a2,
     const substitute_internal::Arg& a3, const substitute_internal::Arg& a4,
@@ -540,7 +608,7 @@ ABSL_MUST_USE_RESULT inline std::string Substitute(
   return result;
 }
 
-ABSL_MUST_USE_RESULT inline std::string Substitute(
+[[nodiscard]] inline std::string Substitute(
     absl::string_view format, const substitute_internal::Arg& a0,
     const substitute_internal::Arg& a1, const substitute_internal::Arg& a2,
     const substitute_internal::Arg& a3, const substitute_internal::Arg& a4,
@@ -551,7 +619,7 @@ ABSL_MUST_USE_RESULT inline std::string Substitute(
   return result;
 }
 
-ABSL_MUST_USE_RESULT inline std::string Substitute(
+[[nodiscard]] inline std::string Substitute(
     absl::string_view format, const substitute_internal::Arg& a0,
     const substitute_internal::Arg& a1, const substitute_internal::Arg& a2,
     const substitute_internal::Arg& a3, const substitute_internal::Arg& a4,
@@ -562,7 +630,7 @@ ABSL_MUST_USE_RESULT inline std::string Substitute(
   return result;
 }
 
-ABSL_MUST_USE_RESULT inline std::string Substitute(
+[[nodiscard]] inline std::string Substitute(
     absl::string_view format, const substitute_internal::Arg& a0,
     const substitute_internal::Arg& a1, const substitute_internal::Arg& a2,
     const substitute_internal::Arg& a3, const substitute_internal::Arg& a4,
@@ -577,89 +645,100 @@ ABSL_MUST_USE_RESULT inline std::string Substitute(
 #if defined(ABSL_BAD_CALL_IF)
 // This body of functions catches cases where the number of placeholders
 // doesn't match the number of data arguments.
-std::string Substitute(const char* format)
+std::string Substitute(const char* absl_nonnull format)
     ABSL_BAD_CALL_IF(substitute_internal::PlaceholderBitmask(format) != 0,
                      "There were no substitution arguments "
-                     "but this format std::string has a $[0-9] in it");
+                     "but this format string either has a $[0-9] in it or "
+                     "contains an unescaped $ character (use $$ instead)");
 
-std::string Substitute(const char* format, const substitute_internal::Arg& a0)
-    ABSL_BAD_CALL_IF(substitute_internal::PlaceholderBitmask(format) != 1,
-                     "There was 1 substitution argument given, but "
-                     "this format std::string is either missing its $0, or "
-                     "contains one of $1-$9");
+std::string Substitute(const char* absl_nonnull format,
+                       const substitute_internal::Arg& a0)
+    ABSL_BAD_CALL_IF(
+        substitute_internal::PlaceholderBitmask(format) != 1,
+        "There was 1 substitution argument given, but "
+        "this format string is missing its $0, contains one of $1-$9, "
+        "or contains an unescaped $ character (use $$ instead)");
 
-std::string Substitute(const char* format, const substitute_internal::Arg& a0,
+std::string Substitute(const char* absl_nonnull format,
+                       const substitute_internal::Arg& a0,
                        const substitute_internal::Arg& a1)
-    ABSL_BAD_CALL_IF(substitute_internal::PlaceholderBitmask(format) != 3,
-                     "There were 2 substitution arguments given, but "
-                     "this format std::string is either missing its $0/$1, or "
-                     "contains one of $2-$9");
+    ABSL_BAD_CALL_IF(
+        substitute_internal::PlaceholderBitmask(format) != 3,
+        "There were 2 substitution arguments given, but "
+        "this format string is missing its $0/$1, contains one of "
+        "$2-$9, or contains an unescaped $ character (use $$ instead)");
 
-std::string Substitute(const char* format, const substitute_internal::Arg& a0,
+std::string Substitute(const char* absl_nonnull format,
+                       const substitute_internal::Arg& a0,
                        const substitute_internal::Arg& a1,
                        const substitute_internal::Arg& a2)
-    ABSL_BAD_CALL_IF(substitute_internal::PlaceholderBitmask(format) != 7,
-                     "There were 3 substitution arguments given, but "
-                     "this format std::string is either missing its $0/$1/$2, or "
-                     "contains one of $3-$9");
+    ABSL_BAD_CALL_IF(
+        substitute_internal::PlaceholderBitmask(format) != 7,
+        "There were 3 substitution arguments given, but "
+        "this format string is missing its $0/$1/$2, contains one of "
+        "$3-$9, or contains an unescaped $ character (use $$ instead)");
 
-std::string Substitute(const char* format, const substitute_internal::Arg& a0,
+std::string Substitute(const char* absl_nonnull format,
+                       const substitute_internal::Arg& a0,
                        const substitute_internal::Arg& a1,
                        const substitute_internal::Arg& a2,
                        const substitute_internal::Arg& a3)
-    ABSL_BAD_CALL_IF(substitute_internal::PlaceholderBitmask(format) != 15,
-                     "There were 4 substitution arguments given, but "
-                     "this format std::string is either missing its $0-$3, or "
-                     "contains one of $4-$9");
+    ABSL_BAD_CALL_IF(
+        substitute_internal::PlaceholderBitmask(format) != 15,
+        "There were 4 substitution arguments given, but "
+        "this format string is missing its $0-$3, contains one of "
+        "$4-$9, or contains an unescaped $ character (use $$ instead)");
 
-std::string Substitute(const char* format, const substitute_internal::Arg& a0,
+std::string Substitute(const char* absl_nonnull format,
+                       const substitute_internal::Arg& a0,
                        const substitute_internal::Arg& a1,
                        const substitute_internal::Arg& a2,
                        const substitute_internal::Arg& a3,
                        const substitute_internal::Arg& a4)
-    ABSL_BAD_CALL_IF(substitute_internal::PlaceholderBitmask(format) != 31,
-                     "There were 5 substitution arguments given, but "
-                     "this format std::string is either missing its $0-$4, or "
-                     "contains one of $5-$9");
+    ABSL_BAD_CALL_IF(
+        substitute_internal::PlaceholderBitmask(format) != 31,
+        "There were 5 substitution arguments given, but "
+        "this format string is missing its $0-$4, contains one of "
+        "$5-$9, or contains an unescaped $ character (use $$ instead)");
 
-std::string Substitute(const char* format, const substitute_internal::Arg& a0,
+std::string Substitute(const char* absl_nonnull format,
+                       const substitute_internal::Arg& a0,
                        const substitute_internal::Arg& a1,
                        const substitute_internal::Arg& a2,
                        const substitute_internal::Arg& a3,
                        const substitute_internal::Arg& a4,
                        const substitute_internal::Arg& a5)
-    ABSL_BAD_CALL_IF(substitute_internal::PlaceholderBitmask(format) != 63,
-                     "There were 6 substitution arguments given, but "
-                     "this format std::string is either missing its $0-$5, or "
-                     "contains one of $6-$9");
+    ABSL_BAD_CALL_IF(
+        substitute_internal::PlaceholderBitmask(format) != 63,
+        "There were 6 substitution arguments given, but "
+        "this format string is missing its $0-$5, contains one of "
+        "$6-$9, or contains an unescaped $ character (use $$ instead)");
 
-std::string Substitute(const char* format, const substitute_internal::Arg& a0,
-                       const substitute_internal::Arg& a1,
-                       const substitute_internal::Arg& a2,
-                       const substitute_internal::Arg& a3,
-                       const substitute_internal::Arg& a4,
-                       const substitute_internal::Arg& a5,
-                       const substitute_internal::Arg& a6)
-    ABSL_BAD_CALL_IF(substitute_internal::PlaceholderBitmask(format) != 127,
-                     "There were 7 substitution arguments given, but "
-                     "this format std::string is either missing its $0-$6, or "
-                     "contains one of $7-$9");
-
-std::string Substitute(const char* format, const substitute_internal::Arg& a0,
-                       const substitute_internal::Arg& a1,
-                       const substitute_internal::Arg& a2,
-                       const substitute_internal::Arg& a3,
-                       const substitute_internal::Arg& a4,
-                       const substitute_internal::Arg& a5,
-                       const substitute_internal::Arg& a6,
-                       const substitute_internal::Arg& a7)
-    ABSL_BAD_CALL_IF(substitute_internal::PlaceholderBitmask(format) != 255,
-                     "There were 8 substitution arguments given, but "
-                     "this format std::string is either missing its $0-$7, or "
-                     "contains one of $8-$9");
+std::string Substitute(
+    const char* absl_nonnull format, const substitute_internal::Arg& a0,
+    const substitute_internal::Arg& a1, const substitute_internal::Arg& a2,
+    const substitute_internal::Arg& a3, const substitute_internal::Arg& a4,
+    const substitute_internal::Arg& a5, const substitute_internal::Arg& a6)
+    ABSL_BAD_CALL_IF(
+        substitute_internal::PlaceholderBitmask(format) != 127,
+        "There were 7 substitution arguments given, but "
+        "this format string is missing its $0-$6, contains one of "
+        "$7-$9, or contains an unescaped $ character (use $$ instead)");
+
+std::string Substitute(
+    const char* absl_nonnull format, const substitute_internal::Arg& a0,
+    const substitute_internal::Arg& a1, const substitute_internal::Arg& a2,
+    const substitute_internal::Arg& a3, const substitute_internal::Arg& a4,
+    const substitute_internal::Arg& a5, const substitute_internal::Arg& a6,
+    const substitute_internal::Arg& a7)
+    ABSL_BAD_CALL_IF(
+        substitute_internal::PlaceholderBitmask(format) != 255,
+        "There were 8 substitution arguments given, but "
+        "this format string is missing its $0-$7, contains one of "
+        "$8-$9, or contains an unescaped $ character (use $$ instead)");
 
 std::string Substitute(
-    const char* format, const substitute_internal::Arg& a0,
+    const char* absl_nonnull format, const substitute_internal::Arg& a0,
     const substitute_internal::Arg& a1, const substitute_internal::Arg& a2,
     const substitute_internal::Arg& a3, const substitute_internal::Arg& a4,
     const substitute_internal::Arg& a5, const substitute_internal::Arg& a6,
@@ -667,20 +746,24 @@ std::string Substitute(
     ABSL_BAD_CALL_IF(
         substitute_internal::PlaceholderBitmask(format) != 511,
         "There were 9 substitution arguments given, but "
-        "this format std::string is either missing its $0-$8, or contains a $9");
+        "this format string is missing its $0-$8, contains a $9, or "
+        "contains an unescaped $ character (use $$ instead)");
 
 std::string Substitute(
-    const char* format, const substitute_internal::Arg& a0,
+    const char* absl_nonnull format, const substitute_internal::Arg& a0,
     const substitute_internal::Arg& a1, const substitute_internal::Arg& a2,
     const substitute_internal::Arg& a3, const substitute_internal::Arg& a4,
     const substitute_internal::Arg& a5, const substitute_internal::Arg& a6,
     const substitute_internal::Arg& a7, const substitute_internal::Arg& a8,
     const substitute_internal::Arg& a9)
-    ABSL_BAD_CALL_IF(substitute_internal::PlaceholderBitmask(format) != 1023,
-                     "There were 10 substitution arguments given, but this "
-                     "format std::string doesn't contain all of $0 through $9");
+    ABSL_BAD_CALL_IF(
+        substitute_internal::PlaceholderBitmask(format) != 1023,
+        "There were 10 substitution arguments given, but this "
+        "format string either doesn't contain all of $0 through $9 or "
+        "contains an unescaped $ character (use $$ instead)");
 #endif  // ABSL_BAD_CALL_IF
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_SUBSTITUTE_H_
diff --git a/absl/strings/substitute_benchmark.cc b/absl/strings/substitute_benchmark.cc
new file mode 100644
index 00000000000..e58ec867e4d
--- /dev/null
+++ b/absl/strings/substitute_benchmark.cc
@@ -0,0 +1,158 @@
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <cstddef>
+#include <cstdint>
+#include <string>
+
+#include "absl/strings/str_cat.h"
+#include "absl/strings/substitute.h"
+#include "benchmark/benchmark.h"
+
+namespace {
+
+void BM_Substitute(benchmark::State& state) {
+  std::string s(state.range(0), 'x');
+  int64_t bytes = 0;
+  for (auto _ : state) {
+    std::string result = absl::Substitute("$0 $1 $2 $3 $4 $5 $6 $7 $8 $9 $$", s,
+                                          s, s, s, s, s, s, s, s, s);
+    bytes += result.size();
+  }
+  state.SetBytesProcessed(bytes);
+}
+BENCHMARK(BM_Substitute)->Range(0, 1024);
+
+// Like BM_Substitute, but use char* strings (which must then be copied
+// to STL strings) for all parameters.  This demonstrates that it is faster
+// to use absl::Substitute() even if your inputs are char* strings.
+void BM_SubstituteCstr(benchmark::State& state) {
+  std::string s(state.range(0), 'x');
+  int64_t bytes = 0;
+  for (auto _ : state) {
+    std::string result =
+        absl::Substitute("$0 $1 $2 $3 $4 $5 $6 $7 $8 $9 $$", s.c_str(),
+                         s.c_str(), s.c_str(), s.c_str(), s.c_str(), s.c_str(),
+                         s.c_str(), s.c_str(), s.c_str(), s.c_str());
+    bytes += result.size();
+  }
+  state.SetBytesProcessed(bytes);
+}
+BENCHMARK(BM_SubstituteCstr)->Range(0, 1024);
+
+// For comparison with BM_Substitute.
+void BM_StringPrintf(benchmark::State& state) {
+  std::string s(state.range(0), 'x');
+  int64_t bytes = 0;
+  for (auto _ : state) {
+    std::string result = absl::StrCat(s, " ", s, " ", s, " ", s, " ", s, " ", s,
+                                      " ", s, " ", s, " ", s, " ", s);
+    bytes += result.size();
+  }
+  state.SetBytesProcessed(bytes);
+}
+BENCHMARK(BM_StringPrintf)->Range(0, 1024);
+
+// Benchmark using absl::Substitute() together with SimpleItoa() to print
+// numbers.  This demonstrates that absl::Substitute() is faster than
+// StringPrintf() even when the inputs are numbers.
+void BM_SubstituteNumber(benchmark::State& state) {
+  const int n = state.range(0);
+  int64_t bytes = 0;
+  for (auto _ : state) {
+    std::string result =
+        absl::Substitute("$0 $1 $2 $3 $4 $5 $6 $7 $8 $9 $$", n, n + 1, n + 2,
+                         n + 3, n + 4, n + 5, n + 6, n + 7, n + 8, n + 9);
+    bytes += result.size();
+  }
+  state.SetBytesProcessed(bytes);
+}
+BENCHMARK(BM_SubstituteNumber)->Arg(0)->Arg(1 << 20);
+
+// For comparison with BM_SubstituteNumber.
+void BM_StrCatNumber(benchmark::State& state) {
+  const int n = state.range(0);
+  int64_t bytes = 0;
+  for (auto _ : state) {
+    std::string result =
+        absl::StrCat(n, " ", n + 1, " ", n + 2, " ", n + 3, " ", n + 4, " ",
+                     n + 5, " ", n + 6, " ", n + 7, " ", n + 8, " ", n + 9);
+    bytes += result.size();
+  }
+  state.SetBytesProcessed(bytes);
+}
+BENCHMARK(BM_StrCatNumber)->Arg(0)->Arg(1 << 20);
+
+// Benchmark using absl::Substitute() with a single substitution, to test the
+// speed at which it copies simple text.  Even in this case, it's faster
+// that StringPrintf().
+void BM_SubstituteSimpleText(benchmark::State& state) {
+  std::string s(state.range(0), 'x');
+  int64_t bytes = 0;
+  for (auto _ : state) {
+    std::string result = absl::Substitute("$0", s.c_str());
+    bytes += result.size();
+  }
+  state.SetBytesProcessed(bytes);
+}
+BENCHMARK(BM_SubstituteSimpleText)->Range(0, 1024);
+
+// For comparison with BM_SubstituteSimpleText.
+void BM_StrCatSimpleText(benchmark::State& state) {
+  std::string s(state.range(0), 'x');
+  int64_t bytes = 0;
+  for (auto _ : state) {
+    std::string result = absl::StrCat("", s);
+    bytes += result.size();
+  }
+  state.SetBytesProcessed(bytes);
+}
+BENCHMARK(BM_StrCatSimpleText)->Range(0, 1024);
+
+std::string MakeFormatByDensity(int density, bool subs_mode) {
+  std::string format((2 + density) * 10, 'x');
+  for (size_t i = 0; i < 10; ++i) {
+    format[density * i] = subs_mode ? '$' : '%';
+    format[density * i + 1] = subs_mode ? ('0' + i) : 's';
+  }
+  return format;
+}
+
+void BM_SubstituteDensity(benchmark::State& state) {
+  const std::string s(10, 'x');
+  const std::string format = MakeFormatByDensity(state.range(0), true);
+  int64_t bytes = 0;
+  for (auto _ : state) {
+    std::string result = absl::Substitute(format, s, s, s, s, s, s, s, s, s, s);
+    bytes += result.size();
+  }
+  state.SetBytesProcessed(bytes);
+}
+BENCHMARK(BM_SubstituteDensity)->Range(0, 256);
+
+void BM_StrCatDensity(benchmark::State& state) {
+  const std::string s(10, 'x');
+  const std::string format(state.range(0), 'x');
+  int64_t bytes = 0;
+  for (auto _ : state) {
+    std::string result =
+        absl::StrCat(s, format, s, format, s, format, s, format, s, format, s,
+                     format, s, format, s, format, s, format, s, format);
+    bytes += result.size();
+  }
+  state.SetBytesProcessed(bytes);
+}
+BENCHMARK(BM_StrCatDensity)->Range(0, 256);
+
+}  // namespace
diff --git a/absl/strings/substitute_test.cc b/absl/strings/substitute_test.cc
index e27abb17727..e62a89a3db9 100644
--- a/absl/strings/substitute_test.cc
+++ b/absl/strings/substitute_test.cc
@@ -15,13 +15,27 @@
 #include "absl/strings/substitute.h"
 
 #include <cstdint>
+#include <cstring>
+#include <limits>
+#include <string>
 #include <vector>
 
 #include "gtest/gtest.h"
 #include "absl/strings/str_cat.h"
+#include "absl/strings/string_view.h"
 
 namespace {
 
+struct MyStruct {
+  template <typename Sink>
+  friend void AbslStringify(Sink& sink, const MyStruct& s) {
+    sink.Append("MyStruct{.value = ");
+    sink.Append(absl::StrCat(s.value));
+    sink.Append("}");
+  }
+  int value;
+};
+
 TEST(SubstituteTest, Substitute) {
   // Basic.
   EXPECT_EQ("Hello, world!", absl::Substitute("$0, $1!", "Hello", "world"));
@@ -70,7 +84,7 @@ TEST(SubstituteTest, Substitute) {
   // Volatile Pointer.
   // Like C++ streamed I/O, such pointers implicitly become bool
   volatile int vol = 237;
-  volatile int *volatile volptr = &vol;
+  volatile int* volatile volptr = &vol;
   str = absl::Substitute("$0", volptr);
   EXPECT_EQ("true", str);
 
@@ -89,7 +103,7 @@ TEST(SubstituteTest, Substitute) {
   str = absl::Substitute("$0", char_buf);
   EXPECT_EQ("print me too", str);
 
-  // null char* is "doubly" special. Represented as the empty std::string.
+  // null char* is "doubly" special. Represented as the empty string.
   char_p = nullptr;
   str = absl::Substitute("$0", char_p);
   EXPECT_EQ("", str);
@@ -128,6 +142,11 @@ TEST(SubstituteTest, Substitute) {
 
   const char* null_cstring = nullptr;
   EXPECT_EQ("Text: ''", absl::Substitute("Text: '$0'", null_cstring));
+
+  MyStruct s1 = MyStruct{17};
+  MyStruct s2 = MyStruct{1043};
+  EXPECT_EQ("MyStruct{.value = 17}, MyStruct{.value = 1043}",
+            absl::Substitute("$0, $1", s1, s2));
 }
 
 TEST(SubstituteTest, SubstituteAndAppend) {
@@ -171,6 +190,12 @@ TEST(SubstituteTest, SubstituteAndAppend) {
   absl::SubstituteAndAppend(&str, "$0 $1 $2 $3 $4 $5 $6 $7 $8 $9", "a", "b",
                             "c", "d", "e", "f", "g", "h", "i", "j");
   EXPECT_EQ("a b c d e f g h i j", str);
+
+  str.clear();
+  MyStruct s1 = MyStruct{17};
+  MyStruct s2 = MyStruct{1043};
+  absl::SubstituteAndAppend(&str, "$0, $1", s1, s2);
+  EXPECT_EQ("MyStruct{.value = 17}, MyStruct{.value = 1043}", str);
 }
 
 TEST(SubstituteTest, VectorBoolRef) {
@@ -184,19 +209,90 @@ TEST(SubstituteTest, VectorBoolRef) {
   EXPECT_EQ("Logic be like: true false true false", str);
 }
 
-#ifdef GTEST_HAS_DEATH_TEST
+TEST(SubstituteTest, Enums) {
+  enum UnscopedEnum { kEnum0 = 0, kEnum1 = 1 };
+  EXPECT_EQ("0 1", absl::Substitute("$0 $1", UnscopedEnum::kEnum0,
+                                    UnscopedEnum::kEnum1));
+
+  enum class ScopedEnum { kEnum0 = 0, kEnum1 = 1 };
+  EXPECT_EQ("0 1",
+            absl::Substitute("$0 $1", ScopedEnum::kEnum0, ScopedEnum::kEnum1));
+
+  enum class ScopedEnumInt32 : int32_t { kEnum0 = 989, kEnum1 = INT32_MIN };
+  EXPECT_EQ("989 -2147483648",
+            absl::Substitute("$0 $1", ScopedEnumInt32::kEnum0,
+                             ScopedEnumInt32::kEnum1));
+
+  enum class ScopedEnumUInt32 : uint32_t { kEnum0 = 1, kEnum1 = UINT32_MAX };
+  EXPECT_EQ("1 4294967295", absl::Substitute("$0 $1", ScopedEnumUInt32::kEnum0,
+                                             ScopedEnumUInt32::kEnum1));
+
+  enum class ScopedEnumInt64 : int64_t { kEnum0 = -1, kEnum1 = 42949672950 };
+  EXPECT_EQ("-1 42949672950", absl::Substitute("$0 $1", ScopedEnumInt64::kEnum0,
+                                               ScopedEnumInt64::kEnum1));
+
+  enum class ScopedEnumUInt64 : uint64_t { kEnum0 = 1, kEnum1 = 42949672950 };
+  EXPECT_EQ("1 42949672950", absl::Substitute("$0 $1", ScopedEnumUInt64::kEnum0,
+                                              ScopedEnumUInt64::kEnum1));
+
+  enum class ScopedEnumChar : signed char { kEnum0 = -1, kEnum1 = 1 };
+  EXPECT_EQ("-1 1", absl::Substitute("$0 $1", ScopedEnumChar::kEnum0,
+                                     ScopedEnumChar::kEnum1));
+
+  enum class ScopedEnumUChar : unsigned char {
+    kEnum0 = 0,
+    kEnum1 = 1,
+    kEnumMax = 255
+  };
+  EXPECT_EQ("0 1 255", absl::Substitute("$0 $1 $2", ScopedEnumUChar::kEnum0,
+                                        ScopedEnumUChar::kEnum1,
+                                        ScopedEnumUChar::kEnumMax));
+
+  enum class ScopedEnumInt16 : int16_t { kEnum0 = -100, kEnum1 = 10000 };
+  EXPECT_EQ("-100 10000", absl::Substitute("$0 $1", ScopedEnumInt16::kEnum0,
+                                           ScopedEnumInt16::kEnum1));
+
+  enum class ScopedEnumUInt16 : uint16_t { kEnum0 = 0, kEnum1 = 10000 };
+  EXPECT_EQ("0 10000", absl::Substitute("$0 $1", ScopedEnumUInt16::kEnum0,
+                                        ScopedEnumUInt16::kEnum1));
+}
+
+enum class EnumWithStringify { Many = 0, Choices = 1 };
+
+template <typename Sink>
+void AbslStringify(Sink& sink, EnumWithStringify e) {
+  sink.Append(e == EnumWithStringify::Many ? "Many" : "Choices");
+}
+
+TEST(SubstituteTest, AbslStringifyWithEnum) {
+  const auto e = EnumWithStringify::Choices;
+  EXPECT_EQ(absl::Substitute("$0", e), "Choices");
+}
+
+#if GTEST_HAS_DEATH_TEST
 
 TEST(SubstituteDeathTest, SubstituteDeath) {
   EXPECT_DEBUG_DEATH(
       static_cast<void>(absl::Substitute(absl::string_view("-$2"), "a", "b")),
-      "Invalid strings::Substitute\\(\\) format std::string: asked for \"\\$2\", "
+      "Invalid absl::Substitute\\(\\) format string: asked for \"\\$2\", "
       "but only 2 args were given.");
   EXPECT_DEBUG_DEATH(
-      static_cast<void>(absl::Substitute("-$z-")),
-      "Invalid strings::Substitute\\(\\) format std::string: \"-\\$z-\"");
+      static_cast<void>(absl::Substitute(absl::string_view("-$z-"))),
+      "Invalid absl::Substitute\\(\\) format string: \"-\\$z-\"");
   EXPECT_DEBUG_DEATH(
-      static_cast<void>(absl::Substitute("-$")),
-      "Invalid strings::Substitute\\(\\) format std::string: \"-\\$\"");
+      static_cast<void>(absl::Substitute(absl::string_view("-$"))),
+      "Invalid absl::Substitute\\(\\) format string: \"-\\$\"");
+}
+
+TEST(SubstituteDeathTest, OverflowDeath) {
+  // HACK: We pretend the string_view is extremely long, in order to test an
+  // overflow condition that only occurs in 32-bit without using an impractical
+  // amount of memory.
+  EXPECT_DEATH(static_cast<void>(absl::Substitute(
+                   "$0$0$0",
+                   absl::string_view(
+                       "abc", (std::numeric_limits<size_t>::max() / 3) + 100))),
+               "overflow");
 }
 
 #endif  // GTEST_HAS_DEATH_TEST
diff --git a/absl/strings/testdata/getline-1.txt b/absl/strings/testdata/getline-1.txt
deleted file mode 100644
index 19b909733ae..00000000000
--- a/absl/strings/testdata/getline-1.txt
+++ /dev/null
@@ -1,3 +0,0 @@
-alpha
-
-beta gamma
diff --git a/absl/strings/testdata/getline-2.txt b/absl/strings/testdata/getline-2.txt
deleted file mode 100644
index d6842d8ee36..00000000000
--- a/absl/strings/testdata/getline-2.txt
+++ /dev/null
@@ -1 +0,0 @@
-one.two.three
diff --git a/absl/synchronization/BUILD.bazel b/absl/synchronization/BUILD.bazel
index 36dc98f39e7..57740efa5ef 100644
--- a/absl/synchronization/BUILD.bazel
+++ b/absl/synchronization/BUILD.bazel
@@ -14,7 +14,9 @@
 # limitations under the License.
 #
 
-load("@rules_cc//cc:defs.bzl", "cc_binary", "cc_library", "cc_test")
+load("@rules_cc//cc:cc_binary.bzl", "cc_binary")
+load("@rules_cc//cc:cc_library.bzl", "cc_library")
+load("@rules_cc//cc:cc_test.bzl", "cc_test")
 load(
     "//absl:copts/configure_copts.bzl",
     "ABSL_DEFAULT_COPTS",
@@ -22,9 +24,16 @@ load(
     "ABSL_TEST_COPTS",
 )
 
-package(default_visibility = ["//visibility:public"])
+package(
+    default_visibility = ["//visibility:private"],
+    features = [
+        "header_modules",
+        "layering_check",
+        "parse_headers",
+    ],
+)
 
-licenses(["notice"])  # Apache 2.0
+licenses(["notice"])
 
 # Internal data structure for efficiently detecting mutex dependency cycles
 cc_library(
@@ -35,14 +44,15 @@ cc_library(
     hdrs = [
         "internal/graphcycles.h",
     ],
-    copts = ABSL_DEFAULT_COPTS,
+    copts = ABSL_DEFAULT_COPTS + select({
+        "//conditions:default": [],
+    }),
     linkopts = ABSL_DEFAULT_LINKOPTS,
-    visibility = [
-        "//absl:__subpackages__",
-    ],
     deps = [
+        "//absl/algorithm:container",
         "//absl/base",
         "//absl/base:base_internal",
+        "//absl/base:config",
         "//absl/base:core_headers",
         "//absl/base:malloc_internal",
         "//absl/base:raw_logging_internal",
@@ -51,46 +61,75 @@ cc_library(
 
 cc_library(
     name = "kernel_timeout_internal",
+    srcs = ["internal/kernel_timeout.cc"],
     hdrs = ["internal/kernel_timeout.h"],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     visibility = [
-        "//absl/synchronization:__pkg__",
+        "@do_not_use_for_gloop_visibility_only//gloop/base:__subpackages__",
+        "@do_not_use_for_gloop_visibility_only//gloop/thread:__subpackages__",
     ],
     deps = [
-        "//absl/base:core_headers",
+        "//absl/base:config",
         "//absl/base:raw_logging_internal",
         "//absl/time",
     ],
 )
 
+cc_test(
+    name = "kernel_timeout_internal_test",
+    srcs = ["internal/kernel_timeout_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    flaky = 1,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":kernel_timeout_internal",
+        "//absl/time",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
 cc_library(
     name = "synchronization",
     srcs = [
         "barrier.cc",
         "blocking_counter.cc",
         "internal/create_thread_identity.cc",
+        "internal/futex_waiter.cc",
         "internal/per_thread_sem.cc",
-        "internal/waiter.cc",
+        "internal/pthread_waiter.cc",
+        "internal/sem_waiter.cc",
+        "internal/stdcpp_waiter.cc",
+        "internal/waiter_base.cc",
+        "internal/win32_waiter.cc",
+        "mutex.cc",
         "notification.cc",
-    ] + select({
-        "//conditions:default": ["mutex.cc"],
-    }),
+    ],
     hdrs = [
         "barrier.h",
         "blocking_counter.h",
         "internal/create_thread_identity.h",
-        "internal/mutex_nonprod.inc",
+        "internal/futex.h",
+        "internal/futex_waiter.h",
         "internal/per_thread_sem.h",
+        "internal/pthread_waiter.h",
+        "internal/sem_waiter.h",
+        "internal/stdcpp_waiter.h",
         "internal/waiter.h",
+        "internal/waiter_base.h",
+        "internal/win32_waiter.h",
         "mutex.h",
         "notification.h",
     ],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = select({
-        "//absl:windows": [],
+        "@rules_cc//cc/compiler:msvc-cl": [],
+        "@rules_cc//cc/compiler:clang-cl": [],
+        "@rules_cc//cc/compiler:emscripten": [],
         "//conditions:default": ["-pthread"],
     }) + ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
     deps = [
         ":graphcycles_internal",
         ":kernel_timeout_internal",
@@ -101,11 +140,16 @@ cc_library(
         "//absl/base:core_headers",
         "//absl/base:dynamic_annotations",
         "//absl/base:malloc_internal",
+        "//absl/base:nullability",
         "//absl/base:raw_logging_internal",
+        "//absl/base:tracing_internal",
         "//absl/debugging:stacktrace",
         "//absl/debugging:symbolize",
+        "//absl/meta:type_traits",
         "//absl/time",
-    ],
+    ] + select({
+        "//conditions:default": [],
+    }),
 )
 
 cc_test(
@@ -114,10 +158,14 @@ cc_test(
     srcs = ["barrier_test.cc"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    tags = [
+        "no_test_wasm",  # b/122473323
+    ],
     deps = [
         ":synchronization",
         "//absl/time",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -127,10 +175,32 @@ cc_test(
     srcs = ["blocking_counter_test.cc"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    tags = [
+        "no_test_wasm",  # b/122473323
+    ],
     deps = [
         ":synchronization",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:tracing_internal",
         "//absl/time",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_binary(
+    name = "blocking_counter_benchmark",
+    testonly = True,
+    srcs = ["blocking_counter_benchmark.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    tags = ["benchmark"],
+    deps = [
+        ":synchronization",
+        ":thread_pool",
+        "//absl/base:no_destructor",
+        "@google_benchmark//:benchmark_main",
     ],
 )
 
@@ -142,9 +212,13 @@ cc_test(
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":graphcycles_internal",
+        "//absl/base:config",
         "//absl/base:core_headers",
-        "//absl/base:raw_logging_internal",
-        "@com_google_googletest//:gtest_main",
+        "//absl/container:flat_hash_set",
+        "//absl/log",
+        "//absl/log:check",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -159,13 +233,13 @@ cc_test(
     deps = [
         ":graphcycles_internal",
         "//absl/base:raw_logging_internal",
-        "@com_github_google_benchmark//:benchmark_main",
+        "@google_benchmark//:benchmark_main",
     ],
 )
 
 cc_library(
     name = "thread_pool",
-    testonly = 1,
+    testonly = True,
     hdrs = ["internal/thread_pool.h"],
     linkopts = ABSL_DEFAULT_LINKOPTS,
     visibility = [
@@ -174,6 +248,7 @@ cc_library(
     deps = [
         ":synchronization",
         "//absl/base:core_headers",
+        "//absl/functional:any_invocable",
     ],
 )
 
@@ -182,44 +257,59 @@ cc_test(
     size = "large",
     srcs = ["mutex_test.cc"],
     copts = ABSL_TEST_COPTS,
+    flaky = 1,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     shard_count = 25,
     deps = [
         ":synchronization",
         ":thread_pool",
-        "//absl/base",
+        "//absl/base:config",
         "//absl/base:core_headers",
-        "//absl/base:raw_logging_internal",
-        "//absl/memory",
+        "//absl/log",
+        "//absl/log:check",
+        "//absl/random",
         "//absl/time",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "mutex_method_pointer_test",
+    srcs = ["mutex_method_pointer_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":synchronization",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
 cc_library(
     name = "mutex_benchmark_common",
-    testonly = 1,
+    testonly = True,
     srcs = ["mutex_benchmark.cc"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     visibility = [
-        "//absl/synchronization:__pkg__",
     ],
     deps = [
         ":synchronization",
         ":thread_pool",
         "//absl/base",
-        "@com_github_google_benchmark//:benchmark_main",
+        "//absl/base:no_destructor",
+        "//absl/base:raw_logging_internal",
+        "@google_benchmark//:benchmark_main",
     ],
     alwayslink = 1,
 )
 
 cc_binary(
     name = "mutex_benchmark",
-    testonly = 1,
+    testonly = True,
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
-    visibility = ["//visibility:private"],
     deps = [
         ":mutex_benchmark_common",
     ],
@@ -230,41 +320,74 @@ cc_test(
     size = "small",
     srcs = ["notification_test.cc"],
     copts = ABSL_TEST_COPTS,
+    flaky = 1,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    tags = ["no_test_lexan"],
     deps = [
         ":synchronization",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:tracing_internal",
         "//absl/time",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
 cc_library(
     name = "per_thread_sem_test_common",
-    testonly = 1,
+    testonly = True,
     srcs = ["internal/per_thread_sem_test.cc"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "@do_not_use_for_gloop_visibility_only//gloop/base:__subpackages__",
+    ],
     deps = [
+        ":kernel_timeout_internal",
         ":synchronization",
         "//absl/base",
+        "//absl/base:config",
+        "//absl/base:core_headers",
         "//absl/strings",
         "//absl/time",
-        "@com_google_googletest//:gtest",
+        "@googletest//:gtest",
     ],
     alwayslink = 1,
 )
 
 cc_test(
     name = "per_thread_sem_test",
-    size = "medium",
+    size = "large",
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    tags = [
+        "no_test_wasm",
+        "noubsan",  # TODO(b/417700722):  timeouts under UBSAN.
+    ],
     deps = [
         ":per_thread_sem_test_common",
         ":synchronization",
         "//absl/strings",
         "//absl/time",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "waiter_test",
+    srcs = ["internal/waiter_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    flaky = 1,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":kernel_timeout_internal",
+        ":synchronization",
+        ":thread_pool",
+        "//absl/random",
+        "//absl/time",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -275,10 +398,13 @@ cc_test(
     ],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
-    tags = ["no_test_ios_x86_64"],
+    tags = [
+        "no_test_ios_x86_64",
+        "no_test_wasm",
+    ],
     deps = [
         ":synchronization",
         "//absl/base:core_headers",
-        "//absl/base:raw_logging_internal",
+        "//absl/log:check",
     ],
 )
diff --git a/absl/synchronization/CMakeLists.txt b/absl/synchronization/CMakeLists.txt
index 3c47a1bca58..2805000d6e9 100644
--- a/absl/synchronization/CMakeLists.txt
+++ b/absl/synchronization/CMakeLists.txt
@@ -14,6 +14,7 @@
 # limitations under the License.
 #
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     graphcycles_internal
@@ -26,24 +27,45 @@ absl_cc_library(
   DEPS
     absl::base
     absl::base_internal
+    absl::config
     absl::core_headers
     absl::malloc_internal
     absl::raw_logging_internal
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     kernel_timeout_internal
   HDRS
     "internal/kernel_timeout.h"
+  SRCS
+    "internal/kernel_timeout.cc"
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
+    absl::base
+    absl::config
     absl::core_headers
     absl::raw_logging_internal
     absl::time
 )
 
+absl_cc_test(
+  NAME
+    kernel_timeout_internal_test
+  SRCS
+    "internal/kernel_timeout_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::kernel_timeout_internal
+    absl::config
+    absl::random_random
+    absl::time
+    GTest::gmock_main
+)
+
 absl_cc_library(
   NAME
     synchronization
@@ -51,17 +73,28 @@ absl_cc_library(
     "barrier.h"
     "blocking_counter.h"
     "internal/create_thread_identity.h"
-    "internal/mutex_nonprod.inc"
+    "internal/futex.h"
+    "internal/futex_waiter.h"
     "internal/per_thread_sem.h"
+    "internal/pthread_waiter.h"
+    "internal/sem_waiter.h"
+    "internal/stdcpp_waiter.h"
     "internal/waiter.h"
+    "internal/waiter_base.h"
+    "internal/win32_waiter.h"
     "mutex.h"
     "notification.h"
   SRCS
     "barrier.cc"
     "blocking_counter.cc"
     "internal/create_thread_identity.cc"
+    "internal/futex_waiter.cc"
     "internal/per_thread_sem.cc"
-    "internal/waiter.cc"
+    "internal/pthread_waiter.cc"
+    "internal/sem_waiter.cc"
+    "internal/stdcpp_waiter.cc"
+    "internal/waiter_base.cc"
+    "internal/win32_waiter.cc"
     "notification.cc"
     "mutex.cc"
   COPTS
@@ -76,10 +109,13 @@ absl_cc_library(
     absl::core_headers
     absl::dynamic_annotations
     absl::malloc_internal
+    absl::meta
+    absl::nullability
     absl::raw_logging_internal
     absl::stacktrace
     absl::symbolize
     absl::time
+    absl::tracing_internal
     Threads::Threads
   PUBLIC
 )
@@ -94,7 +130,7 @@ absl_cc_test(
   DEPS
     absl::synchronization
     absl::time
-    gmock_main
+    GTest::gmock_main
 )
 
 absl_cc_test(
@@ -107,7 +143,8 @@ absl_cc_test(
   DEPS
     absl::synchronization
     absl::time
-    gmock_main
+    absl::tracing_internal
+    GTest::gmock_main
 )
 
 absl_cc_test(
@@ -118,12 +155,15 @@ absl_cc_test(
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
-    absl::graphcycles_internal
+    absl::check
     absl::core_headers
-    absl::raw_logging_internal
-    gmock_main
+    absl::flat_hash_map
+    absl::graphcycles_internal
+    absl::log
+    GTest::gmock_main
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     thread_pool
@@ -132,8 +172,9 @@ absl_cc_library(
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
-    absl::synchronization
+    absl::any_invocable
     absl::core_headers
+    absl::synchronization
   TESTONLY
 )
 
@@ -148,11 +189,28 @@ absl_cc_test(
     absl::synchronization
     absl::thread_pool
     absl::base
+    absl::check
+    absl::config
     absl::core_headers
+    absl::log
     absl::memory
-    absl::raw_logging_internal
+    absl::random_random
     absl::time
-    gmock_main
+    GTest::gmock_main
+)
+
+absl_cc_test(
+  NAME
+    mutex_method_pointer_test
+  SRCS
+    "mutex_method_pointer_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::base
+    absl::config
+    absl::synchronization
+    GTest::gmock_main
 )
 
 absl_cc_test(
@@ -163,11 +221,16 @@ absl_cc_test(
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
+    absl::base
+    absl::config
+    absl::core_headers
     absl::synchronization
     absl::time
-    gmock_main
+    absl::tracing_internal
+    GTest::gmock_main
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     per_thread_sem_test_common
@@ -178,9 +241,10 @@ absl_cc_library(
   DEPS
     absl::synchronization
     absl::base
+    absl::config
     absl::strings
     absl::time
-    gmock
+    GTest::gmock
   TESTONLY
 )
 
@@ -196,7 +260,24 @@ absl_cc_test(
     absl::synchronization
     absl::strings
     absl::time
-    gmock_main
+    GTest::gmock_main
+)
+
+absl_cc_test(
+  NAME
+    waiter_test
+  SRCS
+    "internal/waiter_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::config
+    absl::kernel_timeout_internal
+    absl::random_random
+    absl::synchronization
+    absl::thread_pool
+    absl::time
+    GTest::gmock_main
 )
 
 absl_cc_test(
@@ -209,5 +290,5 @@ absl_cc_test(
   DEPS
     absl::synchronization
     absl::core_headers
-    absl::raw_logging_internal
+    absl::check
 )
diff --git a/absl/synchronization/barrier.cc b/absl/synchronization/barrier.cc
index c2c539ac4ac..e5cbdc60b86 100644
--- a/absl/synchronization/barrier.cc
+++ b/absl/synchronization/barrier.cc
@@ -14,10 +14,12 @@
 
 #include "absl/synchronization/barrier.h"
 
+#include "absl/base/config.h"
 #include "absl/base/internal/raw_logging.h"
 #include "absl/synchronization/mutex.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 // Return whether int *arg is zero.
 static bool IsZero(void *arg) {
@@ -25,7 +27,7 @@ static bool IsZero(void *arg) {
 }
 
 bool Barrier::Block() {
-  MutexLock l(&this->lock_);
+  MutexLock l(this->lock_);
 
   this->num_to_block_--;
   if (this->num_to_block_ < 0) {
@@ -47,4 +49,5 @@ bool Barrier::Block() {
   return this->num_to_exit_ == 0;
 }
 
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/synchronization/barrier.h b/absl/synchronization/barrier.h
index cb5d821a219..d8e754406f4 100644
--- a/absl/synchronization/barrier.h
+++ b/absl/synchronization/barrier.h
@@ -23,6 +23,7 @@
 #include "absl/synchronization/mutex.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 // Barrier
 //
@@ -73,5 +74,6 @@ class Barrier {
   int num_to_exit_ ABSL_GUARDED_BY(lock_);
 };
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 #endif  // ABSL_SYNCHRONIZATION_BARRIER_H_
diff --git a/absl/synchronization/barrier_test.cc b/absl/synchronization/barrier_test.cc
index bfc6cb1883b..2cfb57c4f6d 100644
--- a/absl/synchronization/barrier_test.cc
+++ b/absl/synchronization/barrier_test.cc
@@ -20,7 +20,7 @@
 #include "gtest/gtest.h"
 #include "absl/synchronization/mutex.h"
 #include "absl/time/clock.h"
-
+#include "absl/time/time.h"
 
 TEST(Barrier, SanityTest) {
   constexpr int kNumThreads = 10;
@@ -37,7 +37,7 @@ TEST(Barrier, SanityTest) {
     }
 
     // Increment the counter.
-    absl::MutexLock lock(&mutex);
+    absl::MutexLock lock(mutex);
     ++counter;
   };
 
@@ -57,7 +57,7 @@ TEST(Barrier, SanityTest) {
   // The counter should still be zero since no thread should have
   // been able to pass the barrier yet.
   {
-    absl::MutexLock lock(&mutex);
+    absl::MutexLock lock(mutex);
     EXPECT_EQ(counter, 0);
   }
 
@@ -70,6 +70,6 @@ TEST(Barrier, SanityTest) {
   }
 
   // All threads should now have incremented the counter.
-  absl::MutexLock lock(&mutex);
+  absl::MutexLock lock(mutex);
   EXPECT_EQ(counter, kNumThreads);
 }
diff --git a/absl/synchronization/blocking_counter.cc b/absl/synchronization/blocking_counter.cc
index 481a06b2d24..d38b260928b 100644
--- a/absl/synchronization/blocking_counter.cc
+++ b/absl/synchronization/blocking_counter.cc
@@ -14,42 +14,62 @@
 
 #include "absl/synchronization/blocking_counter.h"
 
+#include <atomic>
+
+#include "absl/base/config.h"
 #include "absl/base/internal/raw_logging.h"
+#include "absl/base/internal/tracing.h"
+#include "absl/synchronization/mutex.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+namespace {
+
+// Return whether int *arg is true.
+bool IsDone(void *arg) { return *reinterpret_cast<bool *>(arg); }
 
-// Return whether int *arg is zero.
-static bool IsZero(void *arg) {
-  return 0 == *reinterpret_cast<int *>(arg);
+}  // namespace
+
+BlockingCounter::BlockingCounter(int initial_count)
+    : count_(initial_count),
+      num_waiting_(0),
+      done_{initial_count == 0 ? true : false} {
+  ABSL_RAW_CHECK(initial_count >= 0, "BlockingCounter initial_count negative");
 }
 
 bool BlockingCounter::DecrementCount() {
-  MutexLock l(&lock_);
-  count_--;
-  if (count_ < 0) {
-    ABSL_RAW_LOG(
-        FATAL,
-        "BlockingCounter::DecrementCount() called too many times.  count=%d",
-        count_);
+  int count = count_.fetch_sub(1, std::memory_order_acq_rel) - 1;
+  ABSL_RAW_CHECK(count >= 0,
+                 "BlockingCounter::DecrementCount() called too many times");
+  if (count == 0) {
+    base_internal::TraceSignal(this, TraceObjectKind());
+    MutexLock l(lock_);
+    done_ = true;
+    return true;
   }
-  return count_ == 0;
+  return false;
 }
 
 void BlockingCounter::Wait() {
-  MutexLock l(&this->lock_);
-  ABSL_RAW_CHECK(count_ >= 0, "BlockingCounter underflow");
+  base_internal::TraceWait(this, TraceObjectKind());
+  {
+    MutexLock l(this->lock_);
 
-  // only one thread may call Wait(). To support more than one thread,
-  // implement a counter num_to_exit, like in the Barrier class.
-  ABSL_RAW_CHECK(num_waiting_ == 0, "multiple threads called Wait()");
-  num_waiting_++;
+    // only one thread may call Wait(). To support more than one thread,
+    // implement a counter num_to_exit, like in the Barrier class.
+    ABSL_RAW_CHECK(num_waiting_ == 0, "multiple threads called Wait()");
+    num_waiting_++;
 
-  this->lock_.Await(Condition(IsZero, &this->count_));
+    this->lock_.Await(Condition(IsDone, &this->done_));
 
-  // At this point, We know that all threads executing DecrementCount have
-  // released the lock, and so will not touch this object again.
-  // Therefore, the thread calling this method is free to delete the object
-  // after we return from this method.
+    // At this point, we know that all threads executing DecrementCount
+    // will not touch this object again.
+    // Therefore, the thread calling this method is free to delete the object
+    // after we return from this method.
+  }
+  base_internal::TraceContinue(this, TraceObjectKind());
 }
 
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/synchronization/blocking_counter.h b/absl/synchronization/blocking_counter.h
index 77560fc0565..46c010bb971 100644
--- a/absl/synchronization/blocking_counter.h
+++ b/absl/synchronization/blocking_counter.h
@@ -20,10 +20,14 @@
 #ifndef ABSL_SYNCHRONIZATION_BLOCKING_COUNTER_H_
 #define ABSL_SYNCHRONIZATION_BLOCKING_COUNTER_H_
 
+#include <atomic>
+
+#include "absl/base/internal/tracing.h"
 #include "absl/base/thread_annotations.h"
 #include "absl/synchronization/mutex.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 // BlockingCounter
 //
@@ -59,8 +63,7 @@ namespace absl {
 //
 class BlockingCounter {
  public:
-  explicit BlockingCounter(int initial_count)
-      : count_(initial_count), num_waiting_(0) {}
+  explicit BlockingCounter(int initial_count);
 
   BlockingCounter(const BlockingCounter&) = delete;
   BlockingCounter& operator=(const BlockingCounter&) = delete;
@@ -87,11 +90,18 @@ class BlockingCounter {
   void Wait();
 
  private:
+  // Convenience helper to reduce verbosity at call sites.
+  static constexpr base_internal::ObjectKind TraceObjectKind() {
+    return base_internal::ObjectKind::kBlockingCounter;
+  }
+
   Mutex lock_;
-  int count_ ABSL_GUARDED_BY(lock_);
+  std::atomic<int> count_;
   int num_waiting_ ABSL_GUARDED_BY(lock_);
+  bool done_ ABSL_GUARDED_BY(lock_);
 };
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_SYNCHRONIZATION_BLOCKING_COUNTER_H_
diff --git a/absl/synchronization/blocking_counter_benchmark.cc b/absl/synchronization/blocking_counter_benchmark.cc
new file mode 100644
index 00000000000..ea2bf9f9d7c
--- /dev/null
+++ b/absl/synchronization/blocking_counter_benchmark.cc
@@ -0,0 +1,84 @@
+// Copyright 2021 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <limits>
+
+#include "absl/base/no_destructor.h"
+#include "absl/synchronization/blocking_counter.h"
+#include "absl/synchronization/internal/thread_pool.h"
+#include "benchmark/benchmark.h"
+
+namespace {
+
+void BM_BlockingCounter_SingleThread(benchmark::State& state) {
+  for (auto _ : state) {
+    int iterations = state.range(0);
+    absl::BlockingCounter counter{iterations};
+    for (int i = 0; i < iterations; ++i) {
+      counter.DecrementCount();
+    }
+    counter.Wait();
+  }
+}
+BENCHMARK(BM_BlockingCounter_SingleThread)
+    ->ArgName("iterations")
+    ->Arg(2)
+    ->Arg(4)
+    ->Arg(16)
+    ->Arg(64)
+    ->Arg(256);
+
+void BM_BlockingCounter_DecrementCount(benchmark::State& state) {
+  static absl::NoDestructor<absl::BlockingCounter> counter(
+      std::numeric_limits<int>::max());
+  for (auto _ : state) {
+    counter->DecrementCount();
+  }
+}
+BENCHMARK(BM_BlockingCounter_DecrementCount)
+    ->Threads(2)
+    ->Threads(4)
+    ->Threads(6)
+    ->Threads(8)
+    ->Threads(10)
+    ->Threads(12)
+    ->Threads(16)
+    ->Threads(32)
+    ->Threads(64)
+    ->Threads(128);
+
+void BM_BlockingCounter_Wait(benchmark::State& state) {
+  int num_threads = state.range(0);
+  absl::synchronization_internal::ThreadPool pool(num_threads);
+  for (auto _ : state) {
+    absl::BlockingCounter counter{num_threads};
+    pool.Schedule([num_threads, &counter, &pool]() {
+      for (int i = 0; i < num_threads; ++i) {
+        pool.Schedule([&counter]() { counter.DecrementCount(); });
+      }
+    });
+    counter.Wait();
+  }
+}
+BENCHMARK(BM_BlockingCounter_Wait)
+    ->ArgName("threads")
+    ->Arg(2)
+    ->Arg(4)
+    ->Arg(8)
+    ->Arg(16)
+    ->Arg(32)
+    ->Arg(64)
+    ->Arg(128);
+
+}  // namespace
diff --git a/absl/synchronization/blocking_counter_test.cc b/absl/synchronization/blocking_counter_test.cc
index c63e3392578..0c42b56a060 100644
--- a/absl/synchronization/blocking_counter_test.cc
+++ b/absl/synchronization/blocking_counter_test.cc
@@ -15,13 +15,18 @@
 #include "absl/synchronization/blocking_counter.h"
 
 #include <thread>  // NOLINT(build/c++11)
+#include <tuple>
 #include <vector>
 
 #include "gtest/gtest.h"
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/tracing.h"
 #include "absl/time/clock.h"
 #include "absl/time/time.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace {
 
 void PauseAndDecreaseCounter(BlockingCounter* counter, int* done) {
@@ -62,5 +67,80 @@ TEST(BlockingCounterTest, BasicFunctionality) {
   }
 }
 
+TEST(BlockingCounterTest, WaitZeroInitialCount) {
+  BlockingCounter counter(0);
+  counter.Wait();
+}
+
+#if GTEST_HAS_DEATH_TEST
+TEST(BlockingCounterTest, WaitNegativeInitialCount) {
+  EXPECT_DEATH(BlockingCounter counter(-1),
+               "BlockingCounter initial_count negative");
+}
+#endif
+
+}  // namespace
+
+#if ABSL_HAVE_ATTRIBUTE_WEAK
+
+namespace base_internal {
+
+namespace {
+
+using TraceRecord = std::tuple<const void*, ObjectKind>;
+
+thread_local TraceRecord tls_signal;
+thread_local TraceRecord tls_wait;
+thread_local TraceRecord tls_continue;
+
 }  // namespace
+
+// Strong extern "C" implementation.
+extern "C" {
+
+void ABSL_INTERNAL_C_SYMBOL(AbslInternalTraceWait)(const void* object,
+                                                   ObjectKind kind) {
+  tls_wait = {object, kind};
+}
+
+void ABSL_INTERNAL_C_SYMBOL(AbslInternalTraceContinue)(const void* object,
+                                                       ObjectKind kind) {
+  tls_continue = {object, kind};
+}
+
+void ABSL_INTERNAL_C_SYMBOL(AbslInternalTraceSignal)(const void* object,
+                                                     ObjectKind kind) {
+  tls_signal = {object, kind};
+}
+
+}  // extern "C"
+
+TEST(BlockingCounterTest, TracesSignal) {
+  BlockingCounter counter(2);
+
+  tls_signal = {};
+  counter.DecrementCount();
+  EXPECT_EQ(tls_signal, TraceRecord(nullptr, ObjectKind::kUnknown));
+
+  tls_signal = {};
+  counter.DecrementCount();
+  EXPECT_EQ(tls_signal, TraceRecord(&counter, ObjectKind::kBlockingCounter));
+}
+
+TEST(BlockingCounterTest, TracesWaitContinue) {
+  BlockingCounter counter(1);
+  counter.DecrementCount();
+
+  tls_wait = {};
+  tls_continue = {};
+  counter.Wait();
+  EXPECT_EQ(tls_wait, TraceRecord(&counter, ObjectKind::kBlockingCounter));
+  EXPECT_EQ(tls_continue, TraceRecord(&counter, ObjectKind::kBlockingCounter));
+}
+
+}  // namespace base_internal
+
+#endif  // ABSL_HAVE_ATTRIBUTE_WEAK
+
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/synchronization/internal/create_thread_identity.cc b/absl/synchronization/internal/create_thread_identity.cc
index ce3676a8848..d7168da55c1 100644
--- a/absl/synchronization/internal/create_thread_identity.cc
+++ b/absl/synchronization/internal/create_thread_identity.cc
@@ -12,31 +12,35 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-#include <stdint.h>
-#include <new>
+#include "absl/base/internal/low_level_alloc.h"  // IWYU pragma: keep
 
 // This file is a no-op if the required LowLevelAlloc support is missing.
-#include "absl/base/internal/low_level_alloc.h"
 #ifndef ABSL_LOW_LEVEL_ALLOC_MISSING
 
-#include <string.h>
+#include <atomic>
+#include <cstdint>
+#include <new>
 
 #include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/scheduling_mode.h"
 #include "absl/base/internal/spinlock.h"
 #include "absl/base/internal/thread_identity.h"
 #include "absl/synchronization/internal/per_thread_sem.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace synchronization_internal {
 
 // ThreadIdentity storage is persistent, we maintain a free-list of previously
 // released ThreadIdentity objects.
-static base_internal::SpinLock freelist_lock(
-    base_internal::kLinkerInitialized);
-static base_internal::ThreadIdentity* thread_identity_freelist;
+ABSL_CONST_INIT static base_internal::SpinLock freelist_lock(
+    base_internal::SCHEDULE_KERNEL_ONLY);
+ABSL_CONST_INIT static base_internal::ThreadIdentity* thread_identity_freelist;
 
 // A per-thread destructor for reclaiming associated ThreadIdentity objects.
-// Since we must preserve their storage we cache them for re-use.
+// Since we must preserve their storage, we cache them for re-use instead of
+// truly destructing the object.
 static void ReclaimThreadIdentity(void* v) {
   base_internal::ThreadIdentity* identity =
       static_cast<base_internal::ThreadIdentity*>(v);
@@ -56,7 +60,7 @@ static void ReclaimThreadIdentity(void* v) {
   //     association state in this case.
   base_internal::ClearCurrentThreadIdentity();
   {
-    base_internal::SpinLockHolder l(&freelist_lock);
+    base_internal::SpinLockHolder l(freelist_lock);
     identity->next = thread_identity_freelist;
     thread_identity_freelist = identity;
   }
@@ -68,7 +72,25 @@ static intptr_t RoundUp(intptr_t addr, intptr_t align) {
   return (addr + align - 1) & ~(align - 1);
 }
 
-static void ResetThreadIdentity(base_internal::ThreadIdentity* identity) {
+void OneTimeInitThreadIdentity(base_internal::ThreadIdentity* identity) {
+  PerThreadSem::Init(identity);
+  identity->ticker.store(0, std::memory_order_relaxed);
+  identity->wait_start.store(0, std::memory_order_relaxed);
+  identity->is_idle.store(false, std::memory_order_relaxed);
+  // To avoid a circular dependency we declare only the storage in the header
+  // and use placement new to construct the SpinLock.
+  static_assert(
+      sizeof(base_internal::ThreadIdentity::SchedulerState::
+                 association_lock_word) == sizeof(base_internal::SpinLock),
+      "Wrong size for SpinLock");
+  // Protects the association between this identity and its schedulable.  Should
+  // never be cooperative.
+  new (&identity->scheduler_state.association_lock_word)
+      base_internal::SpinLock(base_internal::SCHEDULE_KERNEL_ONLY);
+}
+
+static void ResetThreadIdentityBetweenReuse(
+    base_internal::ThreadIdentity* identity) {
   base_internal::PerThreadSynch* pts = &identity->per_thread_synch;
   pts->next = nullptr;
   pts->skip = nullptr;
@@ -84,7 +106,17 @@ static void ResetThreadIdentity(base_internal::ThreadIdentity* identity) {
   pts->wake = false;
   pts->cond_waiter = false;
   pts->all_locks = nullptr;
-  identity->waiter_state = {};
+  base_internal::ThreadIdentity::SchedulerState* ss =
+      &identity->scheduler_state;
+  ss->bound_schedulable.store(nullptr, std::memory_order_relaxed);
+  ss->association_lock_word = 0;
+  ss->scheduling_disabled_depth.store(0, std::memory_order_relaxed);
+  ss->potentially_blocking_depth = 0;
+  ss->schedule_next_state = 0;
+  ss->waking_designated_waker = false;
+  identity->static_initialization_depth = 0;
+  identity->wait_state.store(base_internal::ThreadIdentity::WaitState::kActive,
+                             std::memory_order_relaxed);
   identity->blocked_count_ptr = nullptr;
   identity->ticker.store(0, std::memory_order_relaxed);
   identity->wait_start.store(0, std::memory_order_relaxed);
@@ -97,7 +129,7 @@ static base_internal::ThreadIdentity* NewThreadIdentity() {
 
   {
     // Re-use a previously released object if possible.
-    base_internal::SpinLockHolder l(&freelist_lock);
+    base_internal::SpinLockHolder l(freelist_lock);
     if (thread_identity_freelist) {
       identity = thread_identity_freelist;  // Take list-head.
       thread_identity_freelist = thread_identity_freelist->next;
@@ -114,8 +146,12 @@ static base_internal::ThreadIdentity* NewThreadIdentity() {
     identity = reinterpret_cast<base_internal::ThreadIdentity*>(
         RoundUp(reinterpret_cast<intptr_t>(allocation),
                 base_internal::PerThreadSynch::kAlignment));
+    // Note that *identity is never constructed.
+    // TODO(b/357097463): change this "one time init" to be a proper
+    // constructor.
+    OneTimeInitThreadIdentity(identity);
   }
-  ResetThreadIdentity(identity);
+  ResetThreadIdentityBetweenReuse(identity);
 
   return identity;
 }
@@ -125,13 +161,13 @@ static base_internal::ThreadIdentity* NewThreadIdentity() {
 // REQUIRES: CurrentThreadIdentity(false) == nullptr
 base_internal::ThreadIdentity* CreateThreadIdentity() {
   base_internal::ThreadIdentity* identity = NewThreadIdentity();
-  PerThreadSem::Init(identity);
   // Associate the value with the current thread, and attach our destructor.
   base_internal::SetCurrentThreadIdentity(identity, ReclaimThreadIdentity);
   return identity;
 }
 
 }  // namespace synchronization_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_LOW_LEVEL_ALLOC_MISSING
diff --git a/absl/synchronization/internal/create_thread_identity.h b/absl/synchronization/internal/create_thread_identity.h
index ebb16c56a7c..f8bfbe986a9 100644
--- a/absl/synchronization/internal/create_thread_identity.h
+++ b/absl/synchronization/internal/create_thread_identity.h
@@ -26,9 +26,11 @@
 #define ABSL_SYNCHRONIZATION_INTERNAL_CREATE_THREAD_IDENTITY_H_
 
 #include "absl/base/internal/thread_identity.h"
+#include "absl/base/optimization.h"
 #include "absl/base/port.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace synchronization_internal {
 
 // Allocates and attaches a ThreadIdentity object for the calling thread.
@@ -49,6 +51,7 @@ inline base_internal::ThreadIdentity* GetOrCreateCurrentThreadIdentity() {
 }
 
 }  // namespace synchronization_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_SYNCHRONIZATION_INTERNAL_CREATE_THREAD_IDENTITY_H_
diff --git a/absl/synchronization/internal/futex.h b/absl/synchronization/internal/futex.h
new file mode 100644
index 00000000000..b4a0cf41dfd
--- /dev/null
+++ b/absl/synchronization/internal/futex.h
@@ -0,0 +1,176 @@
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+#ifndef ABSL_SYNCHRONIZATION_INTERNAL_FUTEX_H_
+#define ABSL_SYNCHRONIZATION_INTERNAL_FUTEX_H_
+
+#include <errno.h>
+#include <stdio.h>
+#include <time.h>
+
+#include <atomic>
+#include <cstdint>
+#include <limits>
+
+#include "absl/base/config.h"
+#include "absl/base/optimization.h"
+#include "absl/synchronization/internal/kernel_timeout.h"
+
+#ifndef _WIN32
+#include <sys/time.h>
+#include <unistd.h>
+#endif
+
+#ifdef __linux__
+#include <linux/futex.h>
+#include <sys/syscall.h>
+#endif
+
+#ifdef ABSL_INTERNAL_HAVE_FUTEX
+#error ABSL_INTERNAL_HAVE_FUTEX may not be set on the command line
+#elif defined(__BIONIC__)
+// Bionic supports all the futex operations we need even when some of the futex
+// definitions are missing.
+#define ABSL_INTERNAL_HAVE_FUTEX
+#elif defined(__linux__) && defined(FUTEX_CLOCK_REALTIME)
+// FUTEX_CLOCK_REALTIME requires Linux >= 2.6.28.
+#define ABSL_INTERNAL_HAVE_FUTEX
+#endif
+
+#ifdef ABSL_INTERNAL_HAVE_FUTEX
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace synchronization_internal {
+
+// Some Android headers are missing these definitions even though they
+// support these futex operations.
+#ifdef __BIONIC__
+#ifndef SYS_futex
+#define SYS_futex __NR_futex
+#endif
+#ifndef FUTEX_WAIT_BITSET
+#define FUTEX_WAIT_BITSET 9
+#endif
+#ifndef FUTEX_PRIVATE_FLAG
+#define FUTEX_PRIVATE_FLAG 128
+#endif
+#ifndef FUTEX_CLOCK_REALTIME
+#define FUTEX_CLOCK_REALTIME 256
+#endif
+#ifndef FUTEX_BITSET_MATCH_ANY
+#define FUTEX_BITSET_MATCH_ANY 0xFFFFFFFF
+#endif
+#endif
+
+#if defined(__NR_futex_time64) && !defined(SYS_futex_time64)
+#define SYS_futex_time64 __NR_futex_time64
+#endif
+
+#if defined(SYS_futex_time64) && !defined(SYS_futex)
+#define SYS_futex SYS_futex_time64
+using FutexTimespec = struct timespec;
+#else
+// Some libc implementations have switched to an unconditional 64-bit `time_t`
+// definition. This means that `struct timespec` may not match the layout
+// expected by the kernel ABI on 32-bit platforms. So we define the
+// FutexTimespec that matches the kernel timespec definition. It should be safe
+// to use this struct for 64-bit userspace builds too, since it will use another
+// SYS_futex kernel call with 64-bit tv_sec inside timespec.
+struct FutexTimespec {
+  long tv_sec;   // NOLINT
+  long tv_nsec;  // NOLINT
+};
+#endif
+
+class FutexImpl {
+ public:
+  // Atomically check that `*v == val`, and if it is, then sleep until the until
+  // woken by `Wake()`.
+  static int Wait(std::atomic<int32_t>* v, int32_t val) {
+    return WaitAbsoluteTimeout(v, val, nullptr);
+  }
+
+  // Atomically check that `*v == val`, and if it is, then sleep until
+  // CLOCK_REALTIME reaches `*abs_timeout`, or until woken by `Wake()`.
+  static int WaitAbsoluteTimeout(std::atomic<int32_t>* v, int32_t val,
+                                 const struct timespec* abs_timeout) {
+    FutexTimespec ts;
+    // https://locklessinc.com/articles/futex_cheat_sheet/
+    // Unlike FUTEX_WAIT, FUTEX_WAIT_BITSET uses absolute time.
+    auto err = syscall(
+        SYS_futex, reinterpret_cast<int32_t*>(v),
+        FUTEX_WAIT_BITSET | FUTEX_PRIVATE_FLAG | FUTEX_CLOCK_REALTIME, val,
+        ToFutexTimespec(abs_timeout, &ts), nullptr, FUTEX_BITSET_MATCH_ANY);
+    if (err != 0) {
+      return -errno;
+    }
+    return 0;
+  }
+
+  // Atomically check that `*v == val`, and if it is, then sleep until
+  // `*rel_timeout` has elapsed, or until woken by `Wake()`.
+  static int WaitRelativeTimeout(std::atomic<int32_t>* v, int32_t val,
+                                 const struct timespec* rel_timeout) {
+    FutexTimespec ts;
+    // Atomically check that the futex value is still 0, and if it
+    // is, sleep until abs_timeout or until woken by FUTEX_WAKE.
+    auto err =
+        syscall(SYS_futex, reinterpret_cast<int32_t*>(v), FUTEX_PRIVATE_FLAG,
+                val, ToFutexTimespec(rel_timeout, &ts));
+    if (err != 0) {
+      return -errno;
+    }
+    return 0;
+  }
+
+  // Wakes at most `count` waiters that have entered the sleep state on `v`.
+  static int Wake(std::atomic<int32_t>* v, int32_t count) {
+    auto err = syscall(SYS_futex, reinterpret_cast<int32_t*>(v),
+                       FUTEX_WAKE | FUTEX_PRIVATE_FLAG, count);
+    if (ABSL_PREDICT_FALSE(err < 0)) {
+      return -errno;
+    }
+    return 0;
+  }
+
+ private:
+  static FutexTimespec* ToFutexTimespec(const struct timespec* userspace_ts,
+                                        FutexTimespec* futex_ts) {
+    if (userspace_ts == nullptr) {
+      return nullptr;
+    }
+
+    using FutexSeconds = decltype(futex_ts->tv_sec);
+    using FutexNanoseconds = decltype(futex_ts->tv_nsec);
+
+    constexpr auto kMaxSeconds{(std::numeric_limits<FutexSeconds>::max)()};
+    if (userspace_ts->tv_sec > kMaxSeconds) {
+      futex_ts->tv_sec = kMaxSeconds;
+    } else {
+      futex_ts->tv_sec = static_cast<FutexSeconds>(userspace_ts->tv_sec);
+    }
+    futex_ts->tv_nsec = static_cast<FutexNanoseconds>(userspace_ts->tv_nsec);
+    return futex_ts;
+  }
+};
+
+class Futex : public FutexImpl {};
+
+}  // namespace synchronization_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_INTERNAL_HAVE_FUTEX
+
+#endif  // ABSL_SYNCHRONIZATION_INTERNAL_FUTEX_H_
diff --git a/absl/synchronization/internal/futex_waiter.cc b/absl/synchronization/internal/futex_waiter.cc
new file mode 100644
index 00000000000..d7666d9db40
--- /dev/null
+++ b/absl/synchronization/internal/futex_waiter.cc
@@ -0,0 +1,106 @@
+// Copyright 2023 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/synchronization/internal/futex_waiter.h"
+
+#ifdef ABSL_INTERNAL_HAVE_FUTEX_WAITER
+
+#include <atomic>
+#include <cerrno>
+#include <cstdint>
+
+#include "absl/base/config.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/base/optimization.h"
+#include "absl/synchronization/internal/futex.h"
+#include "absl/synchronization/internal/kernel_timeout.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace synchronization_internal {
+
+int FutexWaiter::WaitUntil(std::atomic<int32_t>* v, int32_t val,
+                           KernelTimeout t) {
+#ifdef CLOCK_MONOTONIC
+  constexpr bool kHasClockMonotonic = true;
+#else
+  constexpr bool kHasClockMonotonic = false;
+#endif
+
+  // We can't call Futex::WaitUntil() here because the prodkernel implementation
+  // does not know about KernelTimeout::SupportsSteadyClock().
+  if (!t.has_timeout()) {
+    return Futex::Wait(v, val);
+  } else if (kHasClockMonotonic && KernelTimeout::SupportsSteadyClock() &&
+             t.is_relative_timeout()) {
+    auto rel_timespec = t.MakeRelativeTimespec();
+    return Futex::WaitRelativeTimeout(v, val, &rel_timespec);
+  } else {
+    auto abs_timespec = t.MakeAbsTimespec();
+    return Futex::WaitAbsoluteTimeout(v, val, &abs_timespec);
+  }
+}
+
+bool FutexWaiter::Wait(KernelTimeout t) {
+  // Loop until we can atomically decrement futex from a positive
+  // value, waiting on a futex while we believe it is zero.
+  // Note that, since the thread ticker is just reset, we don't need to check
+  // whether the thread is idle on the very first pass of the loop.
+  bool first_pass = true;
+  while (true) {
+    int32_t x = futex_.load(std::memory_order_relaxed);
+    while (x != 0) {
+      if (!futex_.compare_exchange_weak(x, x - 1,
+                                        std::memory_order_acquire,
+                                        std::memory_order_relaxed)) {
+        continue;  // Raced with someone, retry.
+      }
+      return true;  // Consumed a wakeup, we are done.
+    }
+
+    if (!first_pass) MaybeBecomeIdle();
+    const int err = WaitUntil(&futex_, 0, t);
+    if (err != 0) {
+      if (err == -EINTR || err == -EWOULDBLOCK) {
+        // Do nothing, the loop will retry.
+      } else if (err == -ETIMEDOUT) {
+        return false;
+      } else {
+        ABSL_RAW_LOG(FATAL, "Futex operation failed with error %d\n", err);
+      }
+    }
+    first_pass = false;
+  }
+}
+
+void FutexWaiter::Post() {
+  if (futex_.fetch_add(1, std::memory_order_release) == 0) {
+    // We incremented from 0, need to wake a potential waiter.
+    Poke();
+  }
+}
+
+void FutexWaiter::Poke() {
+  // Wake one thread waiting on the futex.
+  const int err = Futex::Wake(&futex_, 1);
+  if (ABSL_PREDICT_FALSE(err < 0)) {
+    ABSL_RAW_LOG(FATAL, "Futex operation failed with error %d\n", err);
+  }
+}
+
+}  // namespace synchronization_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_INTERNAL_HAVE_FUTEX_WAITER
diff --git a/absl/synchronization/internal/futex_waiter.h b/absl/synchronization/internal/futex_waiter.h
new file mode 100644
index 00000000000..11dfa93b081
--- /dev/null
+++ b/absl/synchronization/internal/futex_waiter.h
@@ -0,0 +1,63 @@
+// Copyright 2023 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+#ifndef ABSL_SYNCHRONIZATION_INTERNAL_FUTEX_WAITER_H_
+#define ABSL_SYNCHRONIZATION_INTERNAL_FUTEX_WAITER_H_
+
+#include <atomic>
+#include <cstdint>
+
+#include "absl/base/config.h"
+#include "absl/synchronization/internal/kernel_timeout.h"
+#include "absl/synchronization/internal/futex.h"
+#include "absl/synchronization/internal/waiter_base.h"
+
+#ifdef ABSL_INTERNAL_HAVE_FUTEX
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace synchronization_internal {
+
+#define ABSL_INTERNAL_HAVE_FUTEX_WAITER 1
+
+class FutexWaiter : public WaiterCrtp<FutexWaiter> {
+ public:
+  FutexWaiter() : futex_(0) {}
+
+  bool Wait(KernelTimeout t);
+  void Post();
+  void Poke();
+
+  static constexpr char kName[] = "FutexWaiter";
+
+ private:
+  // Atomically check that `*v == val`, and if it is, then sleep until the
+  // timeout `t` has been reached, or until woken by `Wake()`.
+  static int WaitUntil(std::atomic<int32_t>* v, int32_t val,
+                       KernelTimeout t);
+
+  // Futexes are defined by specification to be 32-bits.
+  // Thus std::atomic<int32_t> must be just an int32_t with lockfree methods.
+  std::atomic<int32_t> futex_;
+  static_assert(sizeof(int32_t) == sizeof(futex_), "Wrong size for futex");
+};
+
+}  // namespace synchronization_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_INTERNAL_HAVE_FUTEX
+
+#endif  // ABSL_SYNCHRONIZATION_INTERNAL_FUTEX_WAITER_H_
diff --git a/absl/synchronization/internal/graphcycles.cc b/absl/synchronization/internal/graphcycles.cc
index 0c8c7564489..25b51ffe0b8 100644
--- a/absl/synchronization/internal/graphcycles.cc
+++ b/absl/synchronization/internal/graphcycles.cc
@@ -28,38 +28,46 @@
 // (2) When a new edge (x->y) is inserted, do nothing if rank[x] < rank[y].
 // (3) Otherwise: adjust ranks in the neighborhood of x and y.
 
-#include "absl/base/attributes.h"
+#include "absl/base/internal/low_level_alloc.h"  // IWYU pragma: keep
 // This file is a no-op if the required LowLevelAlloc support is missing.
-#include "absl/base/internal/low_level_alloc.h"
 #ifndef ABSL_LOW_LEVEL_ALLOC_MISSING
 
-#include "absl/synchronization/internal/graphcycles.h"
-
 #include <algorithm>
 #include <array>
+#include <cinttypes>
+#include <cstddef>
+#include <cstdint>
+#include <limits>
+
+#include "absl/algorithm/container.h"
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
 #include "absl/base/internal/hide_ptr.h"
 #include "absl/base/internal/raw_logging.h"
+#include "absl/base/internal/scheduling_mode.h"
 #include "absl/base/internal/spinlock.h"
+#include "absl/base/macros.h"
+#include "absl/synchronization/internal/graphcycles.h"
 
 // Do not use STL.   This module does not use standard memory allocation.
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace synchronization_internal {
 
 namespace {
 
 // Avoid LowLevelAlloc's default arena since it calls malloc hooks in
 // which people are doing things like acquiring Mutexes.
-static absl::base_internal::SpinLock arena_mu(
-    absl::base_internal::kLinkerInitialized);
-static base_internal::LowLevelAlloc::Arena* arena;
+ABSL_CONST_INIT static absl::base_internal::SpinLock arena_mu(
+    base_internal::SCHEDULE_KERNEL_ONLY);
+ABSL_CONST_INIT static base_internal::LowLevelAlloc::Arena* arena;
 
 static void InitArenaIfNecessary() {
-  arena_mu.Lock();
+  base_internal::SpinLockHolder l(arena_mu);
   if (arena == nullptr) {
     arena = base_internal::LowLevelAlloc::NewArena(0);
   }
-  arena_mu.Unlock();
 }
 
 // Number of inlined elements in Vec.  Hash table implementation
@@ -86,7 +94,7 @@ class Vec {
   T* end() { return ptr_ + size_; }
   const T& operator[](uint32_t i) const { return ptr_[i]; }
   T& operator[](uint32_t i) { return ptr_[i]; }
-  const T& back() const { return ptr_[size_-1]; }
+  const T& back() const { return ptr_[size_ - 1]; }
   void pop_back() { size_--; }
 
   void push_back(const T& v) {
@@ -112,7 +120,7 @@ class Vec {
     if (src->ptr_ == src->space_) {
       // Need to actually copy
       resize(src->size_);
-      std::copy(src->ptr_, src->ptr_ + src->size_, ptr_);
+      std::copy_n(src->ptr_, src->size_, ptr_);
       src->size_ = 0;
     } else {
       Discard();
@@ -146,7 +154,7 @@ class Vec {
     size_t request = static_cast<size_t>(capacity_) * sizeof(T);
     T* copy = static_cast<T*>(
         base_internal::LowLevelAlloc::AllocWithArena(request, arena));
-    std::copy(ptr_, ptr_ + size_, copy);
+    std::copy_n(ptr_, size_, copy);
     Discard();
     ptr_ = copy;
   }
@@ -175,13 +183,13 @@ class NodeSet {
     }
     table_[i] = v;
     // Double when 75% full.
-    if (occupied_ >= table_.size() - table_.size()/4) Grow();
+    if (occupied_ >= table_.size() - table_.size() / 4) Grow();
     return true;
   }
 
-  void erase(uint32_t v) {
+  void erase(int32_t v) {
     uint32_t i = FindIndex(v);
-    if (static_cast<uint32_t>(table_[i]) == v) {
+    if (table_[i] == v) {
       table_[i] = kDel;
     }
   }
@@ -190,10 +198,10 @@ class NodeSet {
   // Example:
   //    HASH_FOR_EACH(elem, node->out) { ... }
 #define HASH_FOR_EACH(elem, eset) \
-  for (int32_t elem, _cursor = 0; (eset).Next(&_cursor, &elem); )
+  for (int32_t elem, _cursor = 0; (eset).Next(&_cursor, &elem);)
   bool Next(int32_t* cursor, int32_t* elem) {
     while (static_cast<uint32_t>(*cursor) < table_.size()) {
-      int32_t v = table_[*cursor];
+      int32_t v = table_[static_cast<uint32_t>(*cursor)];
       (*cursor)++;
       if (v >= 0) {
         *elem = v;
@@ -206,26 +214,28 @@ class NodeSet {
  private:
   enum : int32_t { kEmpty = -1, kDel = -2 };
   Vec<int32_t> table_;
-  uint32_t occupied_;     // Count of non-empty slots (includes deleted slots)
+  uint32_t occupied_;  // Count of non-empty slots (includes deleted slots)
 
-  static uint32_t Hash(uint32_t a) { return a * 41; }
+  static uint32_t Hash(int32_t a) { return static_cast<uint32_t>(a) * 41; }
 
   // Return index for storing v.  May return an empty index or deleted index
-  int FindIndex(int32_t v) const {
+  uint32_t FindIndex(int32_t v) const {
     // Search starting at hash index.
     const uint32_t mask = table_.size() - 1;
     uint32_t i = Hash(v) & mask;
-    int deleted_index = -1;  // If >= 0, index of first deleted element we see
+    uint32_t deleted_index = 0;  // index of first deleted element we see
+    bool seen_deleted_element = false;
     while (true) {
       int32_t e = table_[i];
       if (v == e) {
         return i;
       } else if (e == kEmpty) {
         // Return any previously encountered deleted slot.
-        return (deleted_index >= 0) ? deleted_index : i;
-      } else if (e == kDel && deleted_index < 0) {
+        return seen_deleted_element ? deleted_index : i;
+      } else if (e == kDel && !seen_deleted_element) {
         // Keep searching since v might be present later.
         deleted_index = i;
+        seen_deleted_element = true;
       }
       i = (i + 1) & mask;  // Linear probing; quadratic is slightly slower.
     }
@@ -265,25 +275,23 @@ inline GraphId MakeId(int32_t index, uint32_t version) {
   return g;
 }
 
-inline int32_t NodeIndex(GraphId id) {
-  return static_cast<uint32_t>(id.handle & 0xfffffffful);
-}
+inline int32_t NodeIndex(GraphId id) { return static_cast<int32_t>(id.handle); }
 
 inline uint32_t NodeVersion(GraphId id) {
   return static_cast<uint32_t>(id.handle >> 32);
 }
 
 struct Node {
-  int32_t rank;               // rank number assigned by Pearce-Kelly algorithm
-  uint32_t version;           // Current version number
-  int32_t next_hash;          // Next entry in hash table
-  bool visited;               // Temporary marker used by depth-first-search
-  uintptr_t masked_ptr;       // User-supplied pointer
-  NodeSet in;                 // List of immediate predecessor nodes in graph
-  NodeSet out;                // List of immediate successor nodes in graph
-  int priority;               // Priority of recorded stack trace.
-  int nstack;                 // Depth of recorded stack trace.
-  void* stack[40];            // stack[0,nstack-1] holds stack trace for node.
+  int32_t rank;          // rank number assigned by Pearce-Kelly algorithm
+  uint32_t version;      // Current version number
+  int32_t next_hash;     // Next entry in hash table
+  bool visited;          // Temporary marker used by depth-first-search
+  uintptr_t masked_ptr;  // User-supplied pointer
+  NodeSet in;            // List of immediate predecessor nodes in graph
+  NodeSet out;           // List of immediate successor nodes in graph
+  int priority;          // Priority of recorded stack trace.
+  int nstack;            // Depth of recorded stack trace.
+  void* stack[40];       // stack[0,nstack-1] holds stack trace for node.
 };
 
 // Hash table for pointer to node index lookups.
@@ -296,7 +304,7 @@ class PointerMap {
   int32_t Find(void* ptr) {
     auto masked = base_internal::HidePtr(ptr);
     for (int32_t i = table_[Hash(ptr)]; i != -1;) {
-      Node* n = (*nodes_)[i];
+      Node* n = (*nodes_)[static_cast<uint32_t>(i)];
       if (n->masked_ptr == masked) return i;
       i = n->next_hash;
     }
@@ -305,7 +313,7 @@ class PointerMap {
 
   void Add(void* ptr, int32_t i) {
     int32_t* head = &table_[Hash(ptr)];
-    (*nodes_)[i]->next_hash = *head;
+    (*nodes_)[static_cast<uint32_t>(i)]->next_hash = *head;
     *head = i;
   }
 
@@ -313,9 +321,9 @@ class PointerMap {
     // Advance through linked list while keeping track of the
     // predecessor slot that points to the current entry.
     auto masked = base_internal::HidePtr(ptr);
-    for (int32_t* slot = &table_[Hash(ptr)]; *slot != -1; ) {
+    for (int32_t* slot = &table_[Hash(ptr)]; *slot != -1;) {
       int32_t index = *slot;
-      Node* n = (*nodes_)[index];
+      Node* n = (*nodes_)[static_cast<uint32_t>(index)];
       if (n->masked_ptr == masked) {
         *slot = n->next_hash;  // Remove n from linked list
         n->next_hash = -1;
@@ -328,7 +336,7 @@ class PointerMap {
 
  private:
   // Number of buckets in hash table for pointer lookups.
-  static constexpr uint32_t kHashTableSize = 8171;  // should be prime
+  static constexpr uint32_t kHashTableSize = 262139;  // should be prime
 
   const Vec<Node*>* nodes_;
   std::array<int32_t, kHashTableSize> table_;
@@ -356,10 +364,18 @@ struct GraphCycles::Rep {
 };
 
 static Node* FindNode(GraphCycles::Rep* rep, GraphId id) {
-  Node* n = rep->nodes_[NodeIndex(id)];
+  Node* n = rep->nodes_[static_cast<uint32_t>(NodeIndex(id))];
   return (n->version == NodeVersion(id)) ? n : nullptr;
 }
 
+void GraphCycles::TestOnlyAddNodes(uint32_t n) {
+  uint32_t old_size = rep_->nodes_.size();
+  rep_->nodes_.resize(n);
+  for (auto i = old_size; i < n; ++i) {
+    rep_->nodes_[i] = nullptr;
+  }
+}
+
 GraphCycles::GraphCycles() {
   InitArenaIfNecessary();
   rep_ = new (base_internal::LowLevelAlloc::AllocWithArena(sizeof(Rep), arena))
@@ -368,6 +384,9 @@ GraphCycles::GraphCycles() {
 
 GraphCycles::~GraphCycles() {
   for (auto* node : rep_->nodes_) {
+    if (node == nullptr) {
+      continue;
+    }
     node->Node::~Node();
     base_internal::LowLevelAlloc::Free(node);
   }
@@ -382,19 +401,22 @@ bool GraphCycles::CheckInvariants() const {
     Node* nx = r->nodes_[x];
     void* ptr = base_internal::UnhidePtr<void>(nx->masked_ptr);
     if (ptr != nullptr && static_cast<uint32_t>(r->ptrmap_.Find(ptr)) != x) {
-      ABSL_RAW_LOG(FATAL, "Did not find live node in hash table %u %p", x, ptr);
+      ABSL_RAW_LOG(FATAL, "Did not find live node in hash table %" PRIu32 " %p",
+                   x, ptr);
     }
     if (nx->visited) {
-      ABSL_RAW_LOG(FATAL, "Did not clear visited marker on node %u", x);
+      ABSL_RAW_LOG(FATAL, "Did not clear visited marker on node %" PRIu32, x);
     }
     if (!ranks.insert(nx->rank)) {
-      ABSL_RAW_LOG(FATAL, "Duplicate occurrence of rank %d", nx->rank);
+      ABSL_RAW_LOG(FATAL, "Duplicate occurrence of rank %" PRId32, nx->rank);
     }
     HASH_FOR_EACH(y, nx->out) {
-      Node* ny = r->nodes_[y];
+      Node* ny = r->nodes_[static_cast<uint32_t>(y)];
       if (nx->rank >= ny->rank) {
-        ABSL_RAW_LOG(FATAL, "Edge %u->%d has bad rank assignment %d->%d", x, y,
-                     nx->rank, ny->rank);
+        ABSL_RAW_LOG(FATAL,
+                     "Edge %" PRIu32 " ->%" PRId32
+                     " has bad rank assignment %" PRId32 "->%" PRId32,
+                     x, y, nx->rank, ny->rank);
       }
     }
   }
@@ -404,14 +426,14 @@ bool GraphCycles::CheckInvariants() const {
 GraphId GraphCycles::GetId(void* ptr) {
   int32_t i = rep_->ptrmap_.Find(ptr);
   if (i != -1) {
-    return MakeId(i, rep_->nodes_[i]->version);
+    return MakeId(i, rep_->nodes_[static_cast<uint32_t>(i)]->version);
   } else if (rep_->free_nodes_.empty()) {
     Node* n =
         new (base_internal::LowLevelAlloc::AllocWithArena(sizeof(Node), arena))
             Node;
     n->version = 1;  // Avoid 0 since it is used by InvalidGraphId()
     n->visited = false;
-    n->rank = rep_->nodes_.size();
+    n->rank = static_cast<int32_t>(rep_->nodes_.size());
     n->masked_ptr = base_internal::HidePtr(ptr);
     n->nstack = 0;
     n->priority = 0;
@@ -423,7 +445,7 @@ GraphId GraphCycles::GetId(void* ptr) {
     // a permutation of [0,rep_->nodes_.size()-1].
     int32_t r = rep_->free_nodes_.back();
     rep_->free_nodes_.pop_back();
-    Node* n = rep_->nodes_[r];
+    Node* n = rep_->nodes_[static_cast<uint32_t>(r)];
     n->masked_ptr = base_internal::HidePtr(ptr);
     n->nstack = 0;
     n->priority = 0;
@@ -437,12 +459,12 @@ void GraphCycles::RemoveNode(void* ptr) {
   if (i == -1) {
     return;
   }
-  Node* x = rep_->nodes_[i];
+  Node* x = rep_->nodes_[static_cast<uint32_t>(i)];
   HASH_FOR_EACH(y, x->out) {
-    rep_->nodes_[y]->in.erase(i);
+    rep_->nodes_[static_cast<uint32_t>(y)]->in.erase(i);
   }
   HASH_FOR_EACH(y, x->in) {
-    rep_->nodes_[y]->out.erase(i);
+    rep_->nodes_[static_cast<uint32_t>(y)]->out.erase(i);
   }
   x->in.clear();
   x->out.clear();
@@ -457,8 +479,7 @@ void GraphCycles::RemoveNode(void* ptr) {
 
 void* GraphCycles::Ptr(GraphId id) {
   Node* n = FindNode(rep_, id);
-  return n == nullptr ? nullptr
-                      : base_internal::UnhidePtr<void>(n->masked_ptr);
+  return n == nullptr ? nullptr : base_internal::UnhidePtr<void>(n->masked_ptr);
 }
 
 bool GraphCycles::HasNode(GraphId node) {
@@ -485,8 +506,8 @@ static bool ForwardDFS(GraphCycles::Rep* r, int32_t n, int32_t upper_bound);
 static void BackwardDFS(GraphCycles::Rep* r, int32_t n, int32_t lower_bound);
 static void Reorder(GraphCycles::Rep* r);
 static void Sort(const Vec<Node*>&, Vec<int32_t>* delta);
-static void MoveToList(
-    GraphCycles::Rep* r, Vec<int32_t>* src, Vec<int32_t>* dst);
+static void MoveToList(GraphCycles::Rep* r, Vec<int32_t>* src,
+                       Vec<int32_t>* dst);
 
 bool GraphCycles::InsertEdge(GraphId idx, GraphId idy) {
   Rep* r = rep_;
@@ -518,7 +539,7 @@ bool GraphCycles::InsertEdge(GraphId idx, GraphId idy) {
     // Since we do not call Reorder() on this path, clear any visited
     // markers left by ForwardDFS.
     for (const auto& d : r->deltaf_) {
-      r->nodes_[d]->visited = false;
+      r->nodes_[static_cast<uint32_t>(d)]->visited = false;
     }
     return false;
   }
@@ -536,14 +557,14 @@ static bool ForwardDFS(GraphCycles::Rep* r, int32_t n, int32_t upper_bound) {
   while (!r->stack_.empty()) {
     n = r->stack_.back();
     r->stack_.pop_back();
-    Node* nn = r->nodes_[n];
+    Node* nn = r->nodes_[static_cast<uint32_t>(n)];
     if (nn->visited) continue;
 
     nn->visited = true;
     r->deltaf_.push_back(n);
 
     HASH_FOR_EACH(w, nn->out) {
-      Node* nw = r->nodes_[w];
+      Node* nw = r->nodes_[static_cast<uint32_t>(w)];
       if (nw->rank == upper_bound) {
         return false;  // Cycle
       }
@@ -562,14 +583,14 @@ static void BackwardDFS(GraphCycles::Rep* r, int32_t n, int32_t lower_bound) {
   while (!r->stack_.empty()) {
     n = r->stack_.back();
     r->stack_.pop_back();
-    Node* nn = r->nodes_[n];
+    Node* nn = r->nodes_[static_cast<uint32_t>(n)];
     if (nn->visited) continue;
 
     nn->visited = true;
     r->deltab_.push_back(n);
 
     HASH_FOR_EACH(w, nn->in) {
-      Node* nw = r->nodes_[w];
+      Node* nw = r->nodes_[static_cast<uint32_t>(w)];
       if (!nw->visited && lower_bound < nw->rank) {
         r->stack_.push_back(w);
       }
@@ -588,13 +609,12 @@ static void Reorder(GraphCycles::Rep* r) {
 
   // Produce sorted list of all ranks that will be reassigned.
   r->merged_.resize(r->deltab_.size() + r->deltaf_.size());
-  std::merge(r->deltab_.begin(), r->deltab_.end(),
-             r->deltaf_.begin(), r->deltaf_.end(),
-             r->merged_.begin());
+  std::merge(r->deltab_.begin(), r->deltab_.end(), r->deltaf_.begin(),
+             r->deltaf_.end(), r->merged_.begin());
 
   // Assign the ranks in order to the collected list.
   for (uint32_t i = 0; i < r->list_.size(); i++) {
-    r->nodes_[r->list_[i]]->rank = r->merged_[i];
+    r->nodes_[static_cast<uint32_t>(r->list_[i])]->rank = r->merged_[i];
   }
 }
 
@@ -602,20 +622,23 @@ static void Sort(const Vec<Node*>& nodes, Vec<int32_t>* delta) {
   struct ByRank {
     const Vec<Node*>* nodes;
     bool operator()(int32_t a, int32_t b) const {
-      return (*nodes)[a]->rank < (*nodes)[b]->rank;
+      return (*nodes)[static_cast<uint32_t>(a)]->rank <
+             (*nodes)[static_cast<uint32_t>(b)]->rank;
     }
   };
   ByRank cmp;
   cmp.nodes = &nodes;
-  std::sort(delta->begin(), delta->end(), cmp);
+  absl::c_sort(*delta, cmp);
 }
 
-static void MoveToList(
-    GraphCycles::Rep* r, Vec<int32_t>* src, Vec<int32_t>* dst) {
+static void MoveToList(GraphCycles::Rep* r, Vec<int32_t>* src,
+                       Vec<int32_t>* dst) {
   for (auto& v : *src) {
     int32_t w = v;
-    v = r->nodes_[w]->rank;         // Replace v entry with its rank
-    r->nodes_[w]->visited = false;  // Prepare for future DFS calls
+    // Replace v entry with its rank
+    v = r->nodes_[static_cast<uint32_t>(w)]->rank;
+    // Prepare for future DFS calls
+    r->nodes_[static_cast<uint32_t>(w)]->visited = false;
     dst->push_back(w);
   }
 }
@@ -645,7 +668,8 @@ int GraphCycles::FindPath(GraphId idx, GraphId idy, int max_path_len,
     }
 
     if (path_len < max_path_len) {
-      path[path_len] = MakeId(n, rep_->nodes_[n]->version);
+      path[path_len] =
+          MakeId(n, rep_->nodes_[static_cast<uint32_t>(n)]->version);
     }
     path_len++;
     r->stack_.push_back(-1);  // Will remove tentative path entry
@@ -654,7 +678,7 @@ int GraphCycles::FindPath(GraphId idx, GraphId idy, int max_path_len,
       return path_len;
     }
 
-    HASH_FOR_EACH(w, r->nodes_[n]->out) {
+    HASH_FOR_EACH(w, r->nodes_[static_cast<uint32_t>(n)]->out) {
       if (seen.insert(w)) {
         r->stack_.push_back(w);
       }
@@ -674,7 +698,8 @@ void GraphCycles::UpdateStackTrace(GraphId id, int priority,
   if (n == nullptr || n->priority >= priority) {
     return;
   }
-  n->nstack = (*get_stack_trace)(n->stack, ABSL_ARRAYSIZE(n->stack));
+  n->nstack =
+      (*get_stack_trace)(n->stack, static_cast<int>(std::size(n->stack)));
   n->priority = priority;
 }
 
@@ -690,6 +715,7 @@ int GraphCycles::GetStackTrace(GraphId id, void*** ptr) {
 }
 
 }  // namespace synchronization_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_LOW_LEVEL_ALLOC_MISSING
diff --git a/absl/synchronization/internal/graphcycles.h b/absl/synchronization/internal/graphcycles.h
index e08dc09df27..da065040530 100644
--- a/absl/synchronization/internal/graphcycles.h
+++ b/absl/synchronization/internal/graphcycles.h
@@ -40,7 +40,10 @@
 
 #include <cstdint>
 
+#include "absl/base/config.h"
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace synchronization_internal {
 
 // Opaque identifier for a graph node.
@@ -77,20 +80,20 @@ class GraphCycles {
   // Attempt to insert an edge from source_node to dest_node.  If the
   // edge would introduce a cycle, return false without making any
   // changes. Otherwise add the edge and return true.
-  bool InsertEdge(GraphId source_node, GraphId dest_node);
+  bool InsertEdge(GraphId idx, GraphId idy);
 
   // Remove any edge that exists from source_node to dest_node.
-  void RemoveEdge(GraphId source_node, GraphId dest_node);
+  void RemoveEdge(GraphId x, GraphId y);
 
   // Return whether node exists in the graph.
   bool HasNode(GraphId node);
 
   // Return whether there is an edge directly from source_node to dest_node.
-  bool HasEdge(GraphId source_node, GraphId dest_node) const;
+  bool HasEdge(GraphId x, GraphId y) const;
 
   // Return whether dest_node is reachable from source_node
   // by following edges.
-  bool IsReachable(GraphId source_node, GraphId dest_node) const;
+  bool IsReachable(GraphId x, GraphId y) const;
 
   // Find a path from "source" to "dest".  If such a path exists,
   // place the nodes on the path in the array path[], and return
@@ -104,7 +107,7 @@ class GraphCycles {
   // source and destination node if they are identical; therefore, the
   // return value is at most one greater than the number of nodes in
   // the graph.
-  int FindPath(GraphId source, GraphId dest, int max_path_len,
+  int FindPath(GraphId idx, GraphId idy, int max_path_len,
                GraphId path[]) const;
 
   // Update the stack trace recorded for id with the current stack
@@ -123,6 +126,11 @@ class GraphCycles {
   // Expensive: should only be called from graphcycles_test.cc.
   bool CheckInvariants() const;
 
+  // Test-only method to add more nodes. The nodes will not be valid, and this
+  // method should only be used to test the behavior of the graph when it is
+  // very full.
+  void TestOnlyAddNodes(uint32_t n);
+
   // ----------------------------------------------------
   struct Rep;
  private:
@@ -132,6 +140,7 @@ class GraphCycles {
 };
 
 }  // namespace synchronization_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
-#endif
+#endif  // ABSL_SYNCHRONIZATION_INTERNAL_GRAPHCYCLES_H_
diff --git a/absl/synchronization/internal/graphcycles_benchmark.cc b/absl/synchronization/internal/graphcycles_benchmark.cc
index 54823e0ba51..9012801f780 100644
--- a/absl/synchronization/internal/graphcycles_benchmark.cc
+++ b/absl/synchronization/internal/graphcycles_benchmark.cc
@@ -12,14 +12,13 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-#include "absl/synchronization/internal/graphcycles.h"
-
 #include <algorithm>
 #include <cstdint>
 #include <vector>
 
-#include "benchmark/benchmark.h"
 #include "absl/base/internal/raw_logging.h"
+#include "absl/synchronization/internal/graphcycles.h"
+#include "benchmark/benchmark.h"
 
 namespace {
 
diff --git a/absl/synchronization/internal/graphcycles_test.cc b/absl/synchronization/internal/graphcycles_test.cc
index 58e8477b6d1..d4ba0f6edd8 100644
--- a/absl/synchronization/internal/graphcycles_test.cc
+++ b/absl/synchronization/internal/graphcycles_test.cc
@@ -14,17 +14,26 @@
 
 #include "absl/synchronization/internal/graphcycles.h"
 
+#include <climits>
+#include <cstdint>
+#include <cstdio>
+#include <iterator>
 #include <map>
 #include <random>
+#include <string>
 #include <unordered_set>
 #include <utility>
 #include <vector>
 
 #include "gtest/gtest.h"
-#include "absl/base/internal/raw_logging.h"
+#include "absl/base/config.h"
 #include "absl/base/macros.h"
+#include "absl/container/flat_hash_set.h"
+#include "absl/log/check.h"
+#include "absl/log/log.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace synchronization_internal {
 
 // We emulate a GraphCycles object with a node vector and an edge vector.
@@ -46,8 +55,8 @@ static GraphId Get(const IdMap& id, int num) {
 }
 
 // Return whether "to" is reachable from "from".
-static bool IsReachable(Edges *edges, int from, int to,
-                        std::unordered_set<int> *seen) {
+static bool IsReachable(Edges* edges, int from, int to,
+                        absl::flat_hash_set<int>* seen) {
   seen->insert(from);     // we are investigating "from"; don't do it again
   if (from == to) return true;
   for (const auto &edge : *edges) {
@@ -64,56 +73,56 @@ static bool IsReachable(Edges *edges, int from, int to,
 }
 
 static void PrintEdges(Edges *edges) {
-  ABSL_RAW_LOG(INFO, "EDGES (%zu)", edges->size());
+  LOG(INFO) << "EDGES (" << edges->size() << ")";
   for (const auto &edge : *edges) {
     int a = edge.from;
     int b = edge.to;
-    ABSL_RAW_LOG(INFO, "%d %d", a, b);
+    LOG(INFO) << a << " " << b;
   }
-  ABSL_RAW_LOG(INFO, "---");
+  LOG(INFO) << "---";
 }
 
 static void PrintGCEdges(Nodes *nodes, const IdMap &id, GraphCycles *gc) {
-  ABSL_RAW_LOG(INFO, "GC EDGES");
+  LOG(INFO) << "GC EDGES";
   for (int a : *nodes) {
     for (int b : *nodes) {
       if (gc->HasEdge(Get(id, a), Get(id, b))) {
-        ABSL_RAW_LOG(INFO, "%d %d", a, b);
+        LOG(INFO) << a << " " << b;
       }
     }
   }
-  ABSL_RAW_LOG(INFO, "---");
+  LOG(INFO) << "---";
 }
 
 static void PrintTransitiveClosure(Nodes *nodes, Edges *edges) {
-  ABSL_RAW_LOG(INFO, "Transitive closure");
+  LOG(INFO) << "Transitive closure";
   for (int a : *nodes) {
     for (int b : *nodes) {
-      std::unordered_set<int> seen;
+      absl::flat_hash_set<int> seen;
       if (IsReachable(edges, a, b, &seen)) {
-        ABSL_RAW_LOG(INFO, "%d %d", a, b);
+        LOG(INFO) << a << " " << b;
       }
     }
   }
-  ABSL_RAW_LOG(INFO, "---");
+  LOG(INFO) << "---";
 }
 
 static void PrintGCTransitiveClosure(Nodes *nodes, const IdMap &id,
                                      GraphCycles *gc) {
-  ABSL_RAW_LOG(INFO, "GC Transitive closure");
+  LOG(INFO) << "GC Transitive closure";
   for (int a : *nodes) {
     for (int b : *nodes) {
       if (gc->IsReachable(Get(id, a), Get(id, b))) {
-        ABSL_RAW_LOG(INFO, "%d %d", a, b);
+        LOG(INFO) << a << " " << b;
       }
     }
   }
-  ABSL_RAW_LOG(INFO, "---");
+  LOG(INFO) << "---";
 }
 
 static void CheckTransitiveClosure(Nodes *nodes, Edges *edges, const IdMap &id,
                                    GraphCycles *gc) {
-  std::unordered_set<int> seen;
+  absl::flat_hash_set<int> seen;
   for (const auto &a : *nodes) {
     for (const auto &b : *nodes) {
       seen.clear();
@@ -124,9 +133,8 @@ static void CheckTransitiveClosure(Nodes *nodes, Edges *edges, const IdMap &id,
         PrintGCEdges(nodes, id, gc);
         PrintTransitiveClosure(nodes, edges);
         PrintGCTransitiveClosure(nodes, id, gc);
-        ABSL_RAW_LOG(FATAL, "gc_reachable %s reachable %s a %d b %d",
-                     gc_reachable ? "true" : "false",
-                     reachable ? "true" : "false", a, b);
+        LOG(FATAL) << "gc_reachable " << gc_reachable << " reachable "
+                   << reachable << " a " << a << " b " << b;
       }
     }
   }
@@ -141,7 +149,7 @@ static void CheckEdges(Nodes *nodes, Edges *edges, const IdMap &id,
     if (!gc->HasEdge(Get(id, a), Get(id, b))) {
       PrintEdges(edges);
       PrintGCEdges(nodes, id, gc);
-      ABSL_RAW_LOG(FATAL, "!gc->HasEdge(%d, %d)", a, b);
+      LOG(FATAL) << "!gc->HasEdge(" << a << ", " << b << ")";
     }
   }
   for (const auto &a : *nodes) {
@@ -154,13 +162,12 @@ static void CheckEdges(Nodes *nodes, Edges *edges, const IdMap &id,
   if (count != edges->size()) {
     PrintEdges(edges);
     PrintGCEdges(nodes, id, gc);
-    ABSL_RAW_LOG(FATAL, "edges->size() %zu  count %d", edges->size(), count);
+    LOG(FATAL) << "edges->size() " << edges->size() << "  count " << count;
   }
 }
 
 static void CheckInvariants(const GraphCycles &gc) {
-  if (ABSL_PREDICT_FALSE(!gc.CheckInvariants()))
-    ABSL_RAW_LOG(FATAL, "CheckInvariants");
+  CHECK(gc.CheckInvariants()) << "CheckInvariants";
 }
 
 // Returns the index of a randomly chosen node in *nodes.
@@ -224,7 +231,7 @@ TEST(GraphCycles, RandomizedTest) {
       break;
 
     case 1:    // Remove a node
-      if (nodes.size() > 0) {
+      if (!nodes.empty()) {
         int node_index = RandomNode(&rng, &nodes);
         int node = nodes[node_index];
         nodes[node_index] = nodes.back();
@@ -245,7 +252,7 @@ TEST(GraphCycles, RandomizedTest) {
       break;
 
     case 2:   // Add an edge
-      if (nodes.size() > 0) {
+      if (!nodes.empty()) {
         int from = RandomNode(&rng, &nodes);
         int to = RandomNode(&rng, &nodes);
         if (EdgeIndex(&edges, nodes[from], nodes[to]) == -1) {
@@ -255,7 +262,7 @@ TEST(GraphCycles, RandomizedTest) {
             new_edge.to = nodes[to];
             edges.push_back(new_edge);
           } else {
-            std::unordered_set<int> seen;
+            absl::flat_hash_set<int> seen;
             ASSERT_TRUE(IsReachable(&edges, nodes[to], nodes[from], &seen))
                 << "Edge " << nodes[to] << "->" << nodes[from];
           }
@@ -264,7 +271,7 @@ TEST(GraphCycles, RandomizedTest) {
       break;
 
     case 3:    // Remove an edge
-      if (edges.size() > 0) {
+      if (!edges.empty()) {
         int i = RandomEdge(&rng, &edges);
         int from = edges[i].from;
         int to = edges[i].to;
@@ -277,13 +284,13 @@ TEST(GraphCycles, RandomizedTest) {
       break;
 
     case 4:   // Check a path
-      if (nodes.size() > 0) {
+      if (!nodes.empty()) {
         int from = RandomNode(&rng, &nodes);
         int to = RandomNode(&rng, &nodes);
         GraphId path[2*kMaxNodes];
         int path_len = graph_cycles.FindPath(id[nodes[from]], id[nodes[to]],
-                                             ABSL_ARRAYSIZE(path), path);
-        std::unordered_set<int> seen;
+                                             std::size(path), path);
+        absl::flat_hash_set<int> seen;
         bool reachable = IsReachable(&edges, nodes[from], nodes[to], &seen);
         bool gc_reachable =
             graph_cycles.IsReachable(Get(id, nodes[from]), Get(id, nodes[to]));
@@ -308,7 +315,7 @@ TEST(GraphCycles, RandomizedTest) {
       break;
 
     default:
-      ABSL_RAW_LOG(FATAL, "op %d", op);
+      LOG(FATAL) << "op " << op;
     }
 
     // Very rarely, test graph expansion by adding then removing many nodes.
@@ -386,10 +393,10 @@ class GraphCyclesTest : public ::testing::Test {
 
   std::string Path(int x, int y) {
     GraphId path[5];
-    int np = g_.FindPath(Get(id_, x), Get(id_, y), ABSL_ARRAYSIZE(path), path);
+    int np = g_.FindPath(Get(id_, x), Get(id_, y), std::size(path), path);
     std::string result;
     for (int i = 0; i < np; i++) {
-      if (i >= ABSL_ARRAYSIZE(path)) {
+      if (i >= int{std::size(path)}) {
         result += " ...";
         break;
       }
@@ -458,5 +465,24 @@ TEST_F(GraphCyclesTest, ManyEdges) {
   CheckInvariants(g_);
 }
 
+TEST(GraphCycles, IntegerOverflow) {
+  GraphCycles graph_cycles;
+  uintptr_t buf = 0;
+  GraphId prev_id = graph_cycles.GetId(reinterpret_cast<void*>(buf));
+  buf += 1;
+  GraphId id = graph_cycles.GetId(reinterpret_cast<void*>(buf));
+  ASSERT_TRUE(graph_cycles.InsertEdge(prev_id, id));
+
+  // INT_MAX / 40 is enough to cause an overflow when multiplied by 41.
+  graph_cycles.TestOnlyAddNodes(INT_MAX / 40);
+
+  buf += 1;
+  GraphId newid = graph_cycles.GetId(reinterpret_cast<void*>(buf));
+  graph_cycles.HasEdge(prev_id, newid);
+
+  graph_cycles.RemoveNode(reinterpret_cast<void*>(buf));
+}
+
 }  // namespace synchronization_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/synchronization/internal/kernel_timeout.cc b/absl/synchronization/internal/kernel_timeout.cc
new file mode 100644
index 00000000000..e359cfcb084
--- /dev/null
+++ b/absl/synchronization/internal/kernel_timeout.cc
@@ -0,0 +1,218 @@
+// Copyright 2023 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/synchronization/internal/kernel_timeout.h"
+
+#include <algorithm>
+#include <chrono>  // NOLINT(build/c++11)
+#include <cstdint>
+#include <ctime>
+#include <limits>
+
+#include "absl/base/config.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/time/clock.h"
+#include "absl/time/time.h"
+
+#ifndef _WIN32
+#include <sys/types.h>
+#endif
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace synchronization_internal {
+
+int64_t KernelTimeout::SteadyClockNow() {
+  if (!SupportsSteadyClock()) {
+    return absl::GetCurrentTimeNanos();
+  }
+  return std::chrono::duration_cast<std::chrono::nanoseconds>(
+             std::chrono::steady_clock::now().time_since_epoch())
+      .count();
+}
+
+KernelTimeout::KernelTimeout(absl::Time t) {
+  // `absl::InfiniteFuture()` is a common "no timeout" value and cheaper to
+  // compare than convert.
+  if (t == absl::InfiniteFuture()) {
+    rep_ = kNoTimeout;
+    return;
+  }
+
+  int64_t unix_nanos = absl::ToUnixNanos(t);
+
+  // A timeout that lands before the unix epoch is converted to 0.
+  // In theory implementations should expire these timeouts immediately.
+  if (unix_nanos < 0) {
+    unix_nanos = 0;
+  }
+
+  // Values greater than or equal to kMaxNanos are converted to infinite.
+  if (unix_nanos >= kMaxNanos) {
+    rep_ = kNoTimeout;
+    return;
+  }
+
+  rep_ = static_cast<uint64_t>(unix_nanos) << 1;
+}
+
+KernelTimeout::KernelTimeout(absl::Duration d) {
+  // `absl::InfiniteDuration()` is a common "no timeout" value and cheaper to
+  // compare than convert.
+  if (d == absl::InfiniteDuration()) {
+    rep_ = kNoTimeout;
+    return;
+  }
+
+  int64_t nanos = absl::ToInt64Nanoseconds(d);
+
+  // Negative durations are normalized to 0.
+  // In theory implementations should expire these timeouts immediately.
+  if (nanos < 0) {
+    nanos = 0;
+  }
+
+  int64_t now = SteadyClockNow();
+  if (nanos > kMaxNanos - now) {
+    // Durations that would be greater than kMaxNanos are converted to infinite.
+    rep_ = kNoTimeout;
+    return;
+  }
+
+  nanos += now;
+  rep_ = (static_cast<uint64_t>(nanos) << 1) | uint64_t{1};
+}
+
+int64_t KernelTimeout::MakeAbsNanos() const {
+  if (!has_timeout()) {
+    return kMaxNanos;
+  }
+
+  int64_t nanos = RawAbsNanos();
+
+  if (is_relative_timeout()) {
+    // We need to change epochs, because the relative timeout might be
+    // represented by an absolute timestamp from another clock.
+    nanos = std::max<int64_t>(nanos - SteadyClockNow(), 0);
+    int64_t now = absl::GetCurrentTimeNanos();
+    if (nanos > kMaxNanos - now) {
+      // Overflow.
+      nanos = kMaxNanos;
+    } else {
+      nanos += now;
+    }
+  } else if (nanos == 0) {
+    // Some callers have assumed that 0 means no timeout, so instead we return a
+    // time of 1 nanosecond after the epoch.
+    nanos = 1;
+  }
+
+  return nanos;
+}
+
+int64_t KernelTimeout::InNanosecondsFromNow() const {
+  if (!has_timeout()) {
+    return kMaxNanos;
+  }
+
+  int64_t nanos = RawAbsNanos();
+  if (is_absolute_timeout()) {
+    return std::max<int64_t>(nanos - absl::GetCurrentTimeNanos(), 0);
+  }
+  return std::max<int64_t>(nanos - SteadyClockNow(), 0);
+}
+
+struct timespec KernelTimeout::MakeAbsTimespec() const {
+  return absl::ToTimespec(absl::Nanoseconds(MakeAbsNanos()));
+}
+
+struct timespec KernelTimeout::MakeRelativeTimespec() const {
+  return absl::ToTimespec(absl::Nanoseconds(InNanosecondsFromNow()));
+}
+
+#ifndef _WIN32
+struct timespec KernelTimeout::MakeClockAbsoluteTimespec(clockid_t c) const {
+  if (!has_timeout()) {
+    return absl::ToTimespec(absl::Nanoseconds(kMaxNanos));
+  }
+
+  int64_t nanos = RawAbsNanos();
+  if (is_absolute_timeout()) {
+    nanos -= absl::GetCurrentTimeNanos();
+  } else {
+    nanos -= SteadyClockNow();
+  }
+
+  struct timespec now;
+  ABSL_RAW_CHECK(clock_gettime(c, &now) == 0, "clock_gettime() failed");
+  absl::Duration from_clock_epoch =
+      absl::DurationFromTimespec(now) + absl::Nanoseconds(nanos);
+  if (from_clock_epoch <= absl::ZeroDuration()) {
+    // Some callers have assumed that 0 means no timeout, so instead we return a
+    // time of 1 nanosecond after the epoch. For safety we also do not return
+    // negative values.
+    return absl::ToTimespec(absl::Nanoseconds(1));
+  }
+  return absl::ToTimespec(from_clock_epoch);
+}
+#endif
+
+KernelTimeout::DWord KernelTimeout::InMillisecondsFromNow() const {
+  constexpr DWord kInfinite = std::numeric_limits<DWord>::max();
+
+  if (!has_timeout()) {
+    return kInfinite;
+  }
+
+  constexpr uint64_t kNanosInMillis = uint64_t{1'000'000};
+  constexpr uint64_t kMaxValueNanos =
+      std::numeric_limits<int64_t>::max() - kNanosInMillis + 1;
+
+  uint64_t ns_from_now = static_cast<uint64_t>(InNanosecondsFromNow());
+  if (ns_from_now >= kMaxValueNanos) {
+    // Rounding up would overflow.
+    return kInfinite;
+  }
+  // Convert to milliseconds, always rounding up.
+  uint64_t ms_from_now = (ns_from_now + kNanosInMillis - 1) / kNanosInMillis;
+  if (ms_from_now > kInfinite) {
+    return kInfinite;
+  }
+  return static_cast<DWord>(ms_from_now);
+}
+
+std::chrono::time_point<std::chrono::system_clock>
+KernelTimeout::ToChronoTimePoint() const {
+  if (!has_timeout()) {
+    return std::chrono::time_point<std::chrono::system_clock>::max();
+  }
+
+  // The cast to std::microseconds is because (on some platforms) the
+  // std::ratio used by std::chrono::steady_clock doesn't convert to
+  // std::nanoseconds, so it doesn't compile.
+  auto micros = std::chrono::duration_cast<std::chrono::microseconds>(
+      std::chrono::nanoseconds(MakeAbsNanos()));
+  return std::chrono::system_clock::from_time_t(0) + micros;
+}
+
+std::chrono::nanoseconds KernelTimeout::ToChronoDuration() const {
+  if (!has_timeout()) {
+    return std::chrono::nanoseconds::max();
+  }
+  return std::chrono::nanoseconds(InNanosecondsFromNow());
+}
+
+}  // namespace synchronization_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/synchronization/internal/kernel_timeout.h b/absl/synchronization/internal/kernel_timeout.h
index 61c72e75d13..7fc42915881 100644
--- a/absl/synchronization/internal/kernel_timeout.h
+++ b/absl/synchronization/internal/kernel_timeout.h
@@ -11,105 +11,96 @@
 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 // See the License for the specific language governing permissions and
 // limitations under the License.
-//
-
-// An optional absolute timeout, with nanosecond granularity,
-// compatible with absl::Time. Suitable for in-register
-// parameter-passing (e.g. syscalls.)
-// Constructible from a absl::Time (for a timeout to be respected) or {}
-// (for "no timeout".)
-// This is a private low-level API for use by a handful of low-level
-// components that are friends of this class. Higher-level components
-// should build APIs based on absl::Time and absl::Duration.
 
 #ifndef ABSL_SYNCHRONIZATION_INTERNAL_KERNEL_TIMEOUT_H_
 #define ABSL_SYNCHRONIZATION_INTERNAL_KERNEL_TIMEOUT_H_
 
-#include <time.h>
 #include <algorithm>
+#include <chrono>  // NOLINT(build/c++11)
+#include <cstdint>
+#include <ctime>
 #include <limits>
 
+#include "absl/base/config.h"
 #include "absl/base/internal/raw_logging.h"
 #include "absl/time/clock.h"
 #include "absl/time/time.h"
 
+#ifndef _WIN32
+#include <sys/types.h>
+#endif
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace synchronization_internal {
 
-class Futex;
-class Waiter;
-
+// An optional timeout, with nanosecond granularity.
+//
+// This is a private low-level API for use by a handful of low-level
+// components. Higher-level components should build APIs based on
+// absl::Time and absl::Duration.
 class KernelTimeout {
  public:
-  // A timeout that should expire at <t>.  Any value, in the full
-  // InfinitePast() to InfiniteFuture() range, is valid here and will be
-  // respected.
-  explicit KernelTimeout(absl::Time t) : ns_(MakeNs(t)) {}
-  // No timeout.
-  KernelTimeout() : ns_(0) {}
-
-  // A more explicit factory for those who prefer it.  Equivalent to {}.
-  static KernelTimeout Never() { return {}; }
-
-  // We explicitly do not support other custom formats: timespec, int64_t nanos.
-  // Unify on this and absl::Time, please.
+  // Construct an absolute timeout that should expire at `t`.
+  explicit KernelTimeout(absl::Time t);
+
+  // Construct a relative timeout that should expire after `d`.
+  explicit KernelTimeout(absl::Duration d);
+
+  // Infinite timeout.
+  constexpr KernelTimeout() : rep_(kNoTimeout) {}
+
+  // A more explicit factory for those who prefer it.
+  // Equivalent to `KernelTimeout()`.
+  static constexpr KernelTimeout Never() { return KernelTimeout(); }
+
+  // Returns true if there is a timeout that will eventually expire.
+  // Returns false if the timeout is infinite.
+  bool has_timeout() const { return rep_ != kNoTimeout; }
+
+  // If `has_timeout()` is true, returns true if the timeout was provided as an
+  // `absl::Time`. The return value is undefined if `has_timeout()` is false
+  // because all indefinite timeouts are equivalent.
+  bool is_absolute_timeout() const { return (rep_ & 1) == 0; }
+
+  // If `has_timeout()` is true, returns true if the timeout was provided as an
+  // `absl::Duration`. The return value is undefined if `has_timeout()` is false
+  // because all indefinite timeouts are equivalent.
+  bool is_relative_timeout() const { return (rep_ & 1) == 1; }
+
+  // Convert to `struct timespec` for interfaces that expect an absolute
+  // timeout. If !has_timeout() or is_relative_timeout(), attempts to convert to
+  // a reasonable absolute timeout, but callers should to test has_timeout() and
+  // is_relative_timeout() and prefer to use a more appropriate interface.
+  struct timespec MakeAbsTimespec() const;
+
+  // Convert to `struct timespec` for interfaces that expect a relative
+  // timeout. If !has_timeout() or is_absolute_timeout(), attempts to convert to
+  // a reasonable relative timeout, but callers should to test has_timeout() and
+  // is_absolute_timeout() and prefer to use a more appropriate interface. Since
+  // the return value is a relative duration, it should be recomputed by calling
+  // this method in the case of a spurious wakeup.
+  struct timespec MakeRelativeTimespec() const;
+
+#ifndef _WIN32
+  // Convert to `struct timespec` for interfaces that expect an absolute timeout
+  // on a specific clock `c`. This is similar to `MakeAbsTimespec()`, but
+  // callers usually want to use this method with `CLOCK_MONOTONIC` when
+  // relative timeouts are requested, and when the appropriate interface expects
+  // an absolute timeout relative to a specific clock (for example,
+  // pthread_cond_clockwait() or sem_clockwait()). If !has_timeout(), attempts
+  // to convert to a reasonable absolute timeout, but callers should to test
+  // has_timeout() prefer to use a more appropriate interface.
+  struct timespec MakeClockAbsoluteTimespec(clockid_t c) const;
+#endif
 
-  bool has_timeout() const { return ns_ != 0; }
+  // Convert to unix epoch nanos for interfaces that expect an absolute timeout
+  // in nanoseconds. If !has_timeout() or is_relative_timeout(), attempts to
+  // convert to a reasonable absolute timeout, but callers should to test
+  // has_timeout() and is_relative_timeout() and prefer to use a more
+  // appropriate interface.
+  int64_t MakeAbsNanos() const;
 
- private:
-  // internal rep, not user visible: ns after unix epoch.
-  // zero = no timeout.
-  // Negative we treat as an unlikely (and certainly expired!) but valid
-  // timeout.
-  int64_t ns_;
-
-  static int64_t MakeNs(absl::Time t) {
-    // optimization--InfiniteFuture is common "no timeout" value
-    // and cheaper to compare than convert.
-    if (t == absl::InfiniteFuture()) return 0;
-    int64_t x = ToUnixNanos(t);
-
-    // A timeout that lands exactly on the epoch (x=0) needs to be respected,
-    // so we alter it unnoticably to 1.  Negative timeouts are in
-    // theory supported, but handled poorly by the kernel (long
-    // delays) so push them forward too; since all such times have
-    // already passed, it's indistinguishable.
-    if (x <= 0) x = 1;
-    // A time larger than what can be represented to the kernel is treated
-    // as no timeout.
-    if (x == (std::numeric_limits<int64_t>::max)()) x = 0;
-    return x;
-  }
-
-  // Convert to parameter for sem_timedwait/futex/similar.  Only for approved
-  // users.  Do not call if !has_timeout.
-  struct timespec MakeAbsTimespec() {
-    int64_t n = ns_;
-    static const int64_t kNanosPerSecond = 1000 * 1000 * 1000;
-    if (n == 0) {
-      ABSL_RAW_LOG(
-          ERROR,
-          "Tried to create a timespec from a non-timeout; never do this.");
-      // But we'll try to continue sanely.  no-timeout ~= saturated timeout.
-      n = (std::numeric_limits<int64_t>::max)();
-    }
-
-    // Kernel APIs validate timespecs as being at or after the epoch,
-    // despite the kernel time type being signed.  However, no one can
-    // tell the difference between a timeout at or before the epoch (since
-    // all such timeouts have expired!)
-    if (n < 0) n = 0;
-
-    struct timespec abstime;
-    int64_t seconds = (std::min)(n / kNanosPerSecond,
-                               int64_t{(std::numeric_limits<time_t>::max)()});
-    abstime.tv_sec = static_cast<time_t>(seconds);
-    abstime.tv_nsec =
-        static_cast<decltype(abstime.tv_nsec)>(n % kNanosPerSecond);
-    return abstime;
-  }
-
-#ifdef _WIN32
   // Converts to milliseconds from now, or INFINITE when
   // !has_timeout(). For use by SleepConditionVariableSRW on
   // Windows. Callers should recognize that the return value is a
@@ -119,35 +110,69 @@ class KernelTimeout {
   // so we define our own DWORD and INFINITE instead of getting them from
   // <intsafe.h> and <WinBase.h>.
   typedef unsigned long DWord;  // NOLINT
-  DWord InMillisecondsFromNow() const {
-    constexpr DWord kInfinite = (std::numeric_limits<DWord>::max)();
-    if (!has_timeout()) {
-      return kInfinite;
-    }
-    // The use of absl::Now() to convert from absolute time to
-    // relative time means that absl::Now() cannot use anything that
-    // depends on KernelTimeout (for example, Mutex) on Windows.
-    int64_t now = ToUnixNanos(absl::Now());
-    if (ns_ >= now) {
-      // Round up so that Now() + ms_from_now >= ns_.
-      constexpr uint64_t max_nanos =
-          (std::numeric_limits<int64_t>::max)() - 999999u;
-      uint64_t ms_from_now =
-          (std::min<uint64_t>(max_nanos, ns_ - now) + 999999u) / 1000000u;
-      if (ms_from_now > kInfinite) {
-        return kInfinite;
-      }
-      return static_cast<DWord>(ms_from_now);
-    }
-    return 0;
-  }
-#endif
+  DWord InMillisecondsFromNow() const;
+
+  // Convert to std::chrono::time_point for interfaces that expect an absolute
+  // timeout, like std::condition_variable::wait_until(). If !has_timeout() or
+  // is_relative_timeout(), attempts to convert to a reasonable absolute
+  // timeout, but callers should test has_timeout() and is_relative_timeout()
+  // and prefer to use a more appropriate interface.
+  std::chrono::time_point<std::chrono::system_clock> ToChronoTimePoint() const;
+
+  // Convert to std::chrono::time_point for interfaces that expect a relative
+  // timeout, like std::condition_variable::wait_for(). If !has_timeout() or
+  // is_absolute_timeout(), attempts to convert to a reasonable relative
+  // timeout, but callers should test has_timeout() and is_absolute_timeout()
+  // and prefer to use a more appropriate interface. Since the return value is a
+  // relative duration, it should be recomputed by calling this method in the
+  // case of a spurious wakeup.
+  std::chrono::nanoseconds ToChronoDuration() const;
+
+  // Returns true if steady (aka monotonic) clocks are supported by the system.
+  // This currently returns true on all platforms, but we have encountered
+  // platforms that once lacked steady clock support.
+  static constexpr bool SupportsSteadyClock() { return true; }
 
-  friend class Futex;
-  friend class Waiter;
+ private:
+  // Returns the current time, expressed as a count of nanoseconds since the
+  // epoch used by an arbitrary clock. The implementation tries to use a steady
+  // (monotonic) clock if one is available.
+  static int64_t SteadyClockNow();
+
+  // Internal representation.
+  //   - If the value is kNoTimeout, then the timeout is infinite, and
+  //     has_timeout() will return true.
+  //   - If the low bit is 0, then the high 63 bits is the number of nanoseconds
+  //     after the unix epoch.
+  //   - If the low bit is 1, then the high 63 bits is the number of nanoseconds
+  //     after the epoch used by SteadyClockNow().
+  //
+  // In all cases the time is stored as an absolute time, the only difference is
+  // the clock epoch. The use of absolute times is important since in the case
+  // of a relative timeout with a spurious wakeup, the program would have to
+  // restart the wait, and thus needs a way of recomputing the remaining time.
+  uint64_t rep_;
+
+  // Returns the number of nanoseconds stored in the internal representation.
+  // When combined with the clock epoch indicated by the low bit (which is
+  // accessed through is_absolute_timeout() and is_relative_timeout()), the
+  // return value is used to compute when the timeout should occur.
+  int64_t RawAbsNanos() const { return static_cast<int64_t>(rep_ >> 1); }
+
+  // Converts to nanoseconds from now. Since the return value is a relative
+  // duration, it should be recomputed by calling this method in the case of a
+  // spurious wakeup.
+  int64_t InNanosecondsFromNow() const;
+
+  // A value that represents no timeout (or an infinite timeout).
+  static constexpr uint64_t kNoTimeout = (std::numeric_limits<uint64_t>::max)();
+
+  // The maximum value that can be stored in the high 63 bits.
+  static constexpr int64_t kMaxNanos = (std::numeric_limits<int64_t>::max)();
 };
 
 }  // namespace synchronization_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_SYNCHRONIZATION_INTERNAL_KERNEL_TIMEOUT_H_
diff --git a/absl/synchronization/internal/kernel_timeout_test.cc b/absl/synchronization/internal/kernel_timeout_test.cc
new file mode 100644
index 00000000000..dc5bfc925f4
--- /dev/null
+++ b/absl/synchronization/internal/kernel_timeout_test.cc
@@ -0,0 +1,407 @@
+// Copyright 2023 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/synchronization/internal/kernel_timeout.h"
+
+#include <algorithm>
+#include <chrono>  // NOLINT(build/c++11)
+#include <cstdint>
+#include <ctime>
+#include <limits>
+
+#include "gtest/gtest.h"
+#include "absl/time/civil_time.h"
+#include "absl/time/clock.h"
+#include "absl/time/time.h"
+
+#if 0  // All supported platforms currently have steady clocks.
+#define ABSL_INTERNAL_KERNEL_TIMEOUT_SUPPORTS_STEADY_CLOCK 0
+#else
+#define ABSL_INTERNAL_KERNEL_TIMEOUT_SUPPORTS_STEADY_CLOCK 1
+#endif
+
+static_assert(
+    absl::synchronization_internal::KernelTimeout::SupportsSteadyClock() ==
+    static_cast<bool>(ABSL_INTERNAL_KERNEL_TIMEOUT_SUPPORTS_STEADY_CLOCK));
+
+// Randomizing the value of clock_gettime() for CLOCK_MONOTONIC.
+// This works by overriding a weak symbol in glibc.
+// We should be resistant to this randomization when !SupportsSteadyClock().
+#if !ABSL_INTERNAL_KERNEL_TIMEOUT_SUPPORTS_STEADY_CLOCK && \
+    !defined(ABSL_HAVE_ADDRESS_SANITIZER) &&               \
+    !defined(ABSL_HAVE_MEMORY_SANITIZER) &&                \
+    !defined(ABSL_HAVE_THREAD_SANITIZER)
+extern "C" int __clock_gettime(clockid_t c, struct timespec* ts);
+
+extern "C" int clock_gettime(clockid_t c, struct timespec* ts) {
+  if (c == CLOCK_MONOTONIC &&
+      !absl::synchronization_internal::KernelTimeout::SupportsSteadyClock()) {
+    thread_local absl::BitGen gen;  // NOLINT
+    ts->tv_sec = absl::Uniform(gen, 0, 1'000'000'000);
+    ts->tv_nsec = absl::Uniform(gen, 0, 1'000'000'000);
+    return 0;
+  }
+  return __clock_gettime(c, ts);
+}
+#endif
+
+namespace {
+
+#if defined(ABSL_HAVE_ADDRESS_SANITIZER) ||                        \
+    defined(ABSL_HAVE_MEMORY_SANITIZER) ||                         \
+    defined(ABSL_HAVE_THREAD_SANITIZER) || defined(__ANDROID__) || \
+    defined(__APPLE__) || defined(_WIN32) || defined(_WIN64)
+constexpr absl::Duration kTimingBound = absl::Milliseconds(5);
+#else
+constexpr absl::Duration kTimingBound = absl::Microseconds(250);
+#endif
+
+using absl::synchronization_internal::KernelTimeout;
+
+// Returns a time far in the future.
+// On 32-bit time_t systems, the maximum representable time is in January 2038.
+// On 64-bit systems, the maximum representable time in KernelTimeout is in
+// April 2262 (int64_t max nanoseconds).
+absl::Time FarFuture() {
+  static const absl::Time far_future =
+      sizeof(time_t) <= 4
+          ? absl::FromCivil(absl::CivilSecond(2038, 1, 1, 0, 0, 0),
+                            absl::UTCTimeZone())
+          : absl::FromCivil(absl::CivilSecond(2262, 1, 1, 0, 0, 0),
+                            absl::UTCTimeZone());
+  return far_future;
+}
+
+// TODO(b/348224897): re-enabled when the flakiness is fixed.
+TEST(KernelTimeout, DISABLED_FiniteTimes) {
+  constexpr absl::Duration kDurationsToTest[] = {
+    absl::ZeroDuration(),
+    absl::Nanoseconds(1),
+    absl::Microseconds(1),
+    absl::Milliseconds(1),
+    absl::Seconds(1),
+    absl::Minutes(1),
+    absl::Hours(1),
+    absl::Hours(1000),
+    -absl::Nanoseconds(1),
+    -absl::Microseconds(1),
+    -absl::Milliseconds(1),
+    -absl::Seconds(1),
+    -absl::Minutes(1),
+    -absl::Hours(1),
+    -absl::Hours(1000),
+  };
+
+  for (auto duration : kDurationsToTest) {
+    const absl::Time now = absl::Now();
+    const absl::Time when = now + duration;
+    SCOPED_TRACE(duration);
+    KernelTimeout t(when);
+    EXPECT_TRUE(t.has_timeout());
+    EXPECT_TRUE(t.is_absolute_timeout());
+    EXPECT_FALSE(t.is_relative_timeout());
+    EXPECT_EQ(absl::TimeFromTimespec(t.MakeAbsTimespec()), when);
+#ifndef _WIN32
+    EXPECT_LE(
+        absl::AbsDuration(absl::Now() + duration -
+                          absl::TimeFromTimespec(
+                              t.MakeClockAbsoluteTimespec(CLOCK_REALTIME))),
+        absl::Milliseconds(10));
+#endif
+    EXPECT_LE(
+        absl::AbsDuration(absl::DurationFromTimespec(t.MakeRelativeTimespec()) -
+                          std::max(duration, absl::ZeroDuration())),
+        kTimingBound);
+    EXPECT_EQ(absl::FromUnixNanos(t.MakeAbsNanos()), when);
+    EXPECT_LE(absl::AbsDuration(absl::Milliseconds(t.InMillisecondsFromNow()) -
+                                std::max(duration, absl::ZeroDuration())),
+              absl::Milliseconds(5));
+    EXPECT_LE(absl::AbsDuration(absl::FromChrono(t.ToChronoTimePoint()) - when),
+              absl::Microseconds(1));
+    EXPECT_LE(absl::AbsDuration(absl::FromChrono(t.ToChronoDuration()) -
+                                std::max(duration, absl::ZeroDuration())),
+              kTimingBound);
+  }
+}
+
+TEST(KernelTimeout, InfiniteFuture) {
+  KernelTimeout t(absl::InfiniteFuture());
+  EXPECT_FALSE(t.has_timeout());
+  // Callers are expected to check has_timeout() instead of using the methods
+  // below, but we do try to do something reasonable if they don't. We may not
+  // be able to round-trip back to absl::InfiniteDuration() or
+  // absl::InfiniteFuture(), but we should return a very large value.
+  EXPECT_GT(absl::TimeFromTimespec(t.MakeAbsTimespec()), FarFuture());
+#ifndef _WIN32
+  EXPECT_GT(absl::TimeFromTimespec(t.MakeClockAbsoluteTimespec(CLOCK_REALTIME)),
+            FarFuture());
+#endif
+  EXPECT_GT(absl::DurationFromTimespec(t.MakeRelativeTimespec()),
+            absl::Hours(100000));
+  EXPECT_GT(absl::FromUnixNanos(t.MakeAbsNanos()), FarFuture());
+  EXPECT_EQ(t.InMillisecondsFromNow(),
+            std::numeric_limits<KernelTimeout::DWord>::max());
+  EXPECT_EQ(t.ToChronoTimePoint(),
+            std::chrono::time_point<std::chrono::system_clock>::max());
+  EXPECT_GE(t.ToChronoDuration(), std::chrono::nanoseconds::max());
+}
+
+TEST(KernelTimeout, DefaultConstructor) {
+  // The default constructor is equivalent to absl::InfiniteFuture().
+  KernelTimeout t;
+  EXPECT_FALSE(t.has_timeout());
+  // Callers are expected to check has_timeout() instead of using the methods
+  // below, but we do try to do something reasonable if they don't. We may not
+  // be able to round-trip back to absl::InfiniteDuration() or
+  // absl::InfiniteFuture(), but we should return a very large value.
+  EXPECT_GT(absl::TimeFromTimespec(t.MakeAbsTimespec()), FarFuture());
+#ifndef _WIN32
+  EXPECT_GT(absl::TimeFromTimespec(t.MakeClockAbsoluteTimespec(CLOCK_REALTIME)),
+            FarFuture());
+#endif
+  EXPECT_GT(absl::DurationFromTimespec(t.MakeRelativeTimespec()),
+            absl::Hours(100000));
+  EXPECT_GT(absl::FromUnixNanos(t.MakeAbsNanos()), FarFuture());
+  EXPECT_EQ(t.InMillisecondsFromNow(),
+            std::numeric_limits<KernelTimeout::DWord>::max());
+  EXPECT_EQ(t.ToChronoTimePoint(),
+            std::chrono::time_point<std::chrono::system_clock>::max());
+  EXPECT_GE(t.ToChronoDuration(), std::chrono::nanoseconds::max());
+}
+
+TEST(KernelTimeout, TimeMaxNanos) {
+  // Time >= kMaxNanos should behave as no timeout.
+  KernelTimeout t(absl::FromUnixNanos(std::numeric_limits<int64_t>::max()));
+  EXPECT_FALSE(t.has_timeout());
+  // Callers are expected to check has_timeout() instead of using the methods
+  // below, but we do try to do something reasonable if they don't. We may not
+  // be able to round-trip back to absl::InfiniteDuration() or
+  // absl::InfiniteFuture(), but we should return a very large value.
+  EXPECT_GT(absl::TimeFromTimespec(t.MakeAbsTimespec()), FarFuture());
+#ifndef _WIN32
+  EXPECT_GT(absl::TimeFromTimespec(t.MakeClockAbsoluteTimespec(CLOCK_REALTIME)),
+            FarFuture());
+#endif
+  EXPECT_GT(absl::DurationFromTimespec(t.MakeRelativeTimespec()),
+            absl::Hours(100000));
+  EXPECT_GT(absl::FromUnixNanos(t.MakeAbsNanos()), FarFuture());
+  EXPECT_EQ(t.InMillisecondsFromNow(),
+            std::numeric_limits<KernelTimeout::DWord>::max());
+  EXPECT_EQ(t.ToChronoTimePoint(),
+            std::chrono::time_point<std::chrono::system_clock>::max());
+  EXPECT_GE(t.ToChronoDuration(), std::chrono::nanoseconds::max());
+}
+
+TEST(KernelTimeout, Never) {
+  // KernelTimeout::Never() is equivalent to absl::InfiniteFuture().
+  KernelTimeout t = KernelTimeout::Never();
+  EXPECT_FALSE(t.has_timeout());
+  // Callers are expected to check has_timeout() instead of using the methods
+  // below, but we do try to do something reasonable if they don't. We may not
+  // be able to round-trip back to absl::InfiniteDuration() or
+  // absl::InfiniteFuture(), but we should return a very large value.
+  EXPECT_GT(absl::TimeFromTimespec(t.MakeAbsTimespec()), FarFuture());
+#ifndef _WIN32
+  EXPECT_GT(absl::TimeFromTimespec(t.MakeClockAbsoluteTimespec(CLOCK_REALTIME)),
+            FarFuture());
+#endif
+  EXPECT_GT(absl::DurationFromTimespec(t.MakeRelativeTimespec()),
+            absl::Hours(100000));
+  EXPECT_GT(absl::FromUnixNanos(t.MakeAbsNanos()), FarFuture());
+  EXPECT_EQ(t.InMillisecondsFromNow(),
+            std::numeric_limits<KernelTimeout::DWord>::max());
+  EXPECT_EQ(t.ToChronoTimePoint(),
+            std::chrono::time_point<std::chrono::system_clock>::max());
+  EXPECT_GE(t.ToChronoDuration(), std::chrono::nanoseconds::max());
+}
+
+TEST(KernelTimeout, InfinitePast) {
+  KernelTimeout t(absl::InfinitePast());
+  EXPECT_TRUE(t.has_timeout());
+  EXPECT_TRUE(t.is_absolute_timeout());
+  EXPECT_FALSE(t.is_relative_timeout());
+  EXPECT_LE(absl::TimeFromTimespec(t.MakeAbsTimespec()),
+            absl::FromUnixNanos(1));
+#ifndef _WIN32
+  EXPECT_LE(absl::TimeFromTimespec(t.MakeClockAbsoluteTimespec(CLOCK_REALTIME)),
+            absl::FromUnixSeconds(1));
+#endif
+  EXPECT_EQ(absl::DurationFromTimespec(t.MakeRelativeTimespec()),
+            absl::ZeroDuration());
+  EXPECT_LE(absl::FromUnixNanos(t.MakeAbsNanos()), absl::FromUnixNanos(1));
+  EXPECT_EQ(t.InMillisecondsFromNow(), KernelTimeout::DWord{0});
+  EXPECT_LT(t.ToChronoTimePoint(), std::chrono::system_clock::from_time_t(0) +
+                                       std::chrono::seconds(1));
+  EXPECT_EQ(t.ToChronoDuration(), std::chrono::nanoseconds(0));
+}
+
+// TODO(b/348224897): re-enabled when the flakiness is fixed.
+TEST(KernelTimeout, DISABLED_FiniteDurations) {
+  constexpr absl::Duration kDurationsToTest[] = {
+    absl::ZeroDuration(),
+    absl::Nanoseconds(1),
+    absl::Microseconds(1),
+    absl::Milliseconds(1),
+    absl::Seconds(1),
+    absl::Minutes(1),
+    absl::Hours(1),
+    absl::Hours(1000),
+  };
+
+  for (auto duration : kDurationsToTest) {
+    SCOPED_TRACE(duration);
+    KernelTimeout t(duration);
+    EXPECT_TRUE(t.has_timeout());
+    EXPECT_FALSE(t.is_absolute_timeout());
+    EXPECT_TRUE(t.is_relative_timeout());
+    EXPECT_LE(absl::AbsDuration(absl::Now() + duration -
+                                absl::TimeFromTimespec(t.MakeAbsTimespec())),
+              absl::Milliseconds(5));
+#ifndef _WIN32
+    EXPECT_LE(
+        absl::AbsDuration(absl::Now() + duration -
+                          absl::TimeFromTimespec(
+                              t.MakeClockAbsoluteTimespec(CLOCK_REALTIME))),
+        absl::Milliseconds(5));
+#endif
+    EXPECT_LE(
+        absl::AbsDuration(absl::DurationFromTimespec(t.MakeRelativeTimespec()) -
+                          duration),
+        kTimingBound);
+    EXPECT_LE(absl::AbsDuration(absl::Now() + duration -
+                                absl::FromUnixNanos(t.MakeAbsNanos())),
+              absl::Milliseconds(5));
+    EXPECT_LE(absl::Milliseconds(t.InMillisecondsFromNow()) - duration,
+              absl::Milliseconds(5));
+    EXPECT_LE(absl::AbsDuration(absl::Now() + duration -
+                                absl::FromChrono(t.ToChronoTimePoint())),
+              kTimingBound);
+    EXPECT_LE(
+        absl::AbsDuration(absl::FromChrono(t.ToChronoDuration()) - duration),
+        kTimingBound);
+  }
+}
+
+// TODO(b/348224897): re-enabled when the flakiness is fixed.
+TEST(KernelTimeout, DISABLED_NegativeDurations) {
+  constexpr absl::Duration kDurationsToTest[] = {
+    -absl::ZeroDuration(),
+    -absl::Nanoseconds(1),
+    -absl::Microseconds(1),
+    -absl::Milliseconds(1),
+    -absl::Seconds(1),
+    -absl::Minutes(1),
+    -absl::Hours(1),
+    -absl::Hours(1000),
+    -absl::InfiniteDuration(),
+  };
+
+  for (auto duration : kDurationsToTest) {
+    // Negative durations should all be converted to zero durations or "now".
+    SCOPED_TRACE(duration);
+    KernelTimeout t(duration);
+    EXPECT_TRUE(t.has_timeout());
+    EXPECT_FALSE(t.is_absolute_timeout());
+    EXPECT_TRUE(t.is_relative_timeout());
+    EXPECT_LE(absl::AbsDuration(absl::Now() -
+                                absl::TimeFromTimespec(t.MakeAbsTimespec())),
+              absl::Milliseconds(5));
+#ifndef _WIN32
+    EXPECT_LE(absl::AbsDuration(absl::Now() - absl::TimeFromTimespec(
+                                                  t.MakeClockAbsoluteTimespec(
+                                                      CLOCK_REALTIME))),
+              absl::Milliseconds(5));
+#endif
+    EXPECT_EQ(absl::DurationFromTimespec(t.MakeRelativeTimespec()),
+              absl::ZeroDuration());
+    EXPECT_LE(
+        absl::AbsDuration(absl::Now() - absl::FromUnixNanos(t.MakeAbsNanos())),
+        absl::Milliseconds(5));
+    EXPECT_EQ(t.InMillisecondsFromNow(), KernelTimeout::DWord{0});
+    EXPECT_LE(absl::AbsDuration(absl::Now() -
+                                absl::FromChrono(t.ToChronoTimePoint())),
+              absl::Milliseconds(5));
+    EXPECT_EQ(t.ToChronoDuration(), std::chrono::nanoseconds(0));
+  }
+}
+
+TEST(KernelTimeout, InfiniteDuration) {
+  KernelTimeout t(absl::InfiniteDuration());
+  EXPECT_FALSE(t.has_timeout());
+  // Callers are expected to check has_timeout() instead of using the methods
+  // below, but we do try to do something reasonable if they don't. We may not
+  // be able to round-trip back to absl::InfiniteDuration() or
+  // absl::InfiniteFuture(), but we should return a very large value.
+  EXPECT_GT(absl::TimeFromTimespec(t.MakeAbsTimespec()), FarFuture());
+#ifndef _WIN32
+  EXPECT_GT(absl::TimeFromTimespec(t.MakeClockAbsoluteTimespec(CLOCK_REALTIME)),
+            FarFuture());
+#endif
+  EXPECT_GT(absl::DurationFromTimespec(t.MakeRelativeTimespec()),
+            absl::Hours(100000));
+  EXPECT_GT(absl::FromUnixNanos(t.MakeAbsNanos()), FarFuture());
+  EXPECT_EQ(t.InMillisecondsFromNow(),
+            std::numeric_limits<KernelTimeout::DWord>::max());
+  EXPECT_EQ(t.ToChronoTimePoint(),
+            std::chrono::time_point<std::chrono::system_clock>::max());
+  EXPECT_GE(t.ToChronoDuration(), std::chrono::nanoseconds::max());
+}
+
+TEST(KernelTimeout, DurationMaxNanos) {
+  // Duration >= kMaxNanos should behave as no timeout.
+  KernelTimeout t(absl::Nanoseconds(std::numeric_limits<int64_t>::max()));
+  EXPECT_FALSE(t.has_timeout());
+  // Callers are expected to check has_timeout() instead of using the methods
+  // below, but we do try to do something reasonable if they don't. We may not
+  // be able to round-trip back to absl::InfiniteDuration() or
+  // absl::InfiniteFuture(), but we should return a very large value.
+  EXPECT_GT(absl::TimeFromTimespec(t.MakeAbsTimespec()), FarFuture());
+#ifndef _WIN32
+  EXPECT_GT(absl::TimeFromTimespec(t.MakeClockAbsoluteTimespec(CLOCK_REALTIME)),
+            FarFuture());
+#endif
+  EXPECT_GT(absl::DurationFromTimespec(t.MakeRelativeTimespec()),
+            absl::Hours(100000));
+  EXPECT_GT(absl::FromUnixNanos(t.MakeAbsNanos()), FarFuture());
+  EXPECT_EQ(t.InMillisecondsFromNow(),
+            std::numeric_limits<KernelTimeout::DWord>::max());
+  EXPECT_EQ(t.ToChronoTimePoint(),
+            std::chrono::time_point<std::chrono::system_clock>::max());
+  EXPECT_GE(t.ToChronoDuration(), std::chrono::nanoseconds::max());
+}
+
+TEST(KernelTimeout, OverflowNanos) {
+  // Test what happens when KernelTimeout is constructed with an absl::Duration
+  // that would overflow now_nanos + duration.
+  int64_t now_nanos = absl::ToUnixNanos(absl::Now());
+  int64_t limit = std::numeric_limits<int64_t>::max() - now_nanos;
+  absl::Duration duration = absl::Nanoseconds(limit) + absl::Seconds(1);
+  KernelTimeout t(duration);
+  // Timeouts should still be far in the future.
+  EXPECT_GT(absl::TimeFromTimespec(t.MakeAbsTimespec()), FarFuture());
+#ifndef _WIN32
+  EXPECT_GT(absl::TimeFromTimespec(t.MakeClockAbsoluteTimespec(CLOCK_REALTIME)),
+            FarFuture());
+#endif
+  EXPECT_GT(absl::DurationFromTimespec(t.MakeRelativeTimespec()),
+            absl::Hours(100000));
+  EXPECT_GT(absl::FromUnixNanos(t.MakeAbsNanos()), FarFuture());
+  EXPECT_LE(absl::Milliseconds(t.InMillisecondsFromNow()) - duration,
+            absl::Milliseconds(5));
+  EXPECT_GT(t.ToChronoTimePoint(),
+            std::chrono::system_clock::now() + std::chrono::hours(100000));
+  EXPECT_GT(t.ToChronoDuration(), std::chrono::hours(100000));
+}
+
+}  // namespace
diff --git a/absl/synchronization/internal/mutex_nonprod.cc b/absl/synchronization/internal/mutex_nonprod.cc
deleted file mode 100644
index 267deaffa35..00000000000
--- a/absl/synchronization/internal/mutex_nonprod.cc
+++ /dev/null
@@ -1,318 +0,0 @@
-// Copyright 2017 The Abseil Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-// Implementation of a small subset of Mutex and CondVar functionality
-// for platforms where the production implementation hasn't been fully
-// ported yet.
-
-#include "absl/synchronization/mutex.h"
-
-#if defined(_WIN32)
-#include <chrono>  // NOLINT(build/c++11)
-#else
-#include <sys/time.h>
-#include <time.h>
-#endif
-
-#include <algorithm>
-
-#include "absl/base/internal/raw_logging.h"
-#include "absl/time/time.h"
-
-namespace absl {
-namespace synchronization_internal {
-
-namespace {
-
-// Return the current time plus the timeout.
-absl::Time DeadlineFromTimeout(absl::Duration timeout) {
-  return absl::Now() + timeout;
-}
-
-// Limit the deadline to a positive, 32-bit time_t value to accommodate
-// implementation restrictions.  This also deals with InfinitePast and
-// InfiniteFuture.
-absl::Time LimitedDeadline(absl::Time deadline) {
-  deadline = std::max(absl::FromTimeT(0), deadline);
-  deadline = std::min(deadline, absl::FromTimeT(0x7fffffff));
-  return deadline;
-}
-
-}  // namespace
-
-#if defined(_WIN32)
-
-MutexImpl::MutexImpl() {}
-
-MutexImpl::~MutexImpl() {
-  if (locked_) {
-    std_mutex_.unlock();
-  }
-}
-
-void MutexImpl::Lock() {
-  std_mutex_.lock();
-  locked_ = true;
-}
-
-bool MutexImpl::TryLock() {
-  bool locked = std_mutex_.try_lock();
-  if (locked) locked_ = true;
-  return locked;
-}
-
-void MutexImpl::Unlock() {
-  locked_ = false;
-  released_.SignalAll();
-  std_mutex_.unlock();
-}
-
-CondVarImpl::CondVarImpl() {}
-
-CondVarImpl::~CondVarImpl() {}
-
-void CondVarImpl::Signal() { std_cv_.notify_one(); }
-
-void CondVarImpl::SignalAll() { std_cv_.notify_all(); }
-
-void CondVarImpl::Wait(MutexImpl* mu) {
-  mu->released_.SignalAll();
-  std_cv_.wait(mu->std_mutex_);
-}
-
-bool CondVarImpl::WaitWithDeadline(MutexImpl* mu, absl::Time deadline) {
-  mu->released_.SignalAll();
-  time_t when = ToTimeT(deadline);
-  int64_t nanos = ToInt64Nanoseconds(deadline - absl::FromTimeT(when));
-  std::chrono::system_clock::time_point deadline_tp =
-      std::chrono::system_clock::from_time_t(when) +
-      std::chrono::duration_cast<std::chrono::system_clock::duration>(
-          std::chrono::nanoseconds(nanos));
-  auto deadline_since_epoch =
-      std::chrono::duration_cast<std::chrono::duration<double>>(
-          deadline_tp - std::chrono::system_clock::from_time_t(0));
-  return std_cv_.wait_until(mu->std_mutex_, deadline_tp) ==
-         std::cv_status::timeout;
-}
-
-#else  // ! _WIN32
-
-MutexImpl::MutexImpl() {
-  ABSL_RAW_CHECK(pthread_mutex_init(&pthread_mutex_, nullptr) == 0,
-                 "pthread error");
-}
-
-MutexImpl::~MutexImpl() {
-  if (locked_) {
-    ABSL_RAW_CHECK(pthread_mutex_unlock(&pthread_mutex_) == 0, "pthread error");
-  }
-  ABSL_RAW_CHECK(pthread_mutex_destroy(&pthread_mutex_) == 0, "pthread error");
-}
-
-void MutexImpl::Lock() {
-  ABSL_RAW_CHECK(pthread_mutex_lock(&pthread_mutex_) == 0, "pthread error");
-  locked_ = true;
-}
-
-bool MutexImpl::TryLock() {
-  bool locked = (0 == pthread_mutex_trylock(&pthread_mutex_));
-  if (locked) locked_ = true;
-  return locked;
-}
-
-void MutexImpl::Unlock() {
-  locked_ = false;
-  released_.SignalAll();
-  ABSL_RAW_CHECK(pthread_mutex_unlock(&pthread_mutex_) == 0, "pthread error");
-}
-
-CondVarImpl::CondVarImpl() {
-  ABSL_RAW_CHECK(pthread_cond_init(&pthread_cv_, nullptr) == 0,
-                 "pthread error");
-}
-
-CondVarImpl::~CondVarImpl() {
-  ABSL_RAW_CHECK(pthread_cond_destroy(&pthread_cv_) == 0, "pthread error");
-}
-
-void CondVarImpl::Signal() {
-  ABSL_RAW_CHECK(pthread_cond_signal(&pthread_cv_) == 0, "pthread error");
-}
-
-void CondVarImpl::SignalAll() {
-  ABSL_RAW_CHECK(pthread_cond_broadcast(&pthread_cv_) == 0, "pthread error");
-}
-
-void CondVarImpl::Wait(MutexImpl* mu) {
-  mu->released_.SignalAll();
-  ABSL_RAW_CHECK(pthread_cond_wait(&pthread_cv_, &mu->pthread_mutex_) == 0,
-                 "pthread error");
-}
-
-bool CondVarImpl::WaitWithDeadline(MutexImpl* mu, absl::Time deadline) {
-  mu->released_.SignalAll();
-  struct timespec ts = ToTimespec(deadline);
-  int rc = pthread_cond_timedwait(&pthread_cv_, &mu->pthread_mutex_, &ts);
-  if (rc == ETIMEDOUT) return true;
-  ABSL_RAW_CHECK(rc == 0, "pthread error");
-  return false;
-}
-
-#endif  // ! _WIN32
-
-void MutexImpl::Await(const Condition& cond) {
-  if (cond.Eval()) return;
-  released_.SignalAll();
-  do {
-    released_.Wait(this);
-  } while (!cond.Eval());
-}
-
-bool MutexImpl::AwaitWithDeadline(const Condition& cond, absl::Time deadline) {
-  if (cond.Eval()) return true;
-  released_.SignalAll();
-  while (true) {
-    if (released_.WaitWithDeadline(this, deadline)) return false;
-    if (cond.Eval()) return true;
-  }
-}
-
-}  // namespace synchronization_internal
-
-Mutex::Mutex() {}
-
-Mutex::~Mutex() {}
-
-void Mutex::Lock() { impl()->Lock(); }
-
-void Mutex::Unlock() { impl()->Unlock(); }
-
-bool Mutex::TryLock() { return impl()->TryLock(); }
-
-void Mutex::ReaderLock() { Lock(); }
-
-void Mutex::ReaderUnlock() { Unlock(); }
-
-void Mutex::Await(const Condition& cond) { impl()->Await(cond); }
-
-void Mutex::LockWhen(const Condition& cond) {
-  Lock();
-  Await(cond);
-}
-
-bool Mutex::AwaitWithDeadline(const Condition& cond, absl::Time deadline) {
-  return impl()->AwaitWithDeadline(
-      cond, synchronization_internal::LimitedDeadline(deadline));
-}
-
-bool Mutex::AwaitWithTimeout(const Condition& cond, absl::Duration timeout) {
-  return AwaitWithDeadline(
-      cond, synchronization_internal::DeadlineFromTimeout(timeout));
-}
-
-bool Mutex::LockWhenWithDeadline(const Condition& cond, absl::Time deadline) {
-  Lock();
-  return AwaitWithDeadline(cond, deadline);
-}
-
-bool Mutex::LockWhenWithTimeout(const Condition& cond, absl::Duration timeout) {
-  return LockWhenWithDeadline(
-      cond, synchronization_internal::DeadlineFromTimeout(timeout));
-}
-
-void Mutex::ReaderLockWhen(const Condition& cond) {
-  ReaderLock();
-  Await(cond);
-}
-
-bool Mutex::ReaderLockWhenWithTimeout(const Condition& cond,
-                                      absl::Duration timeout) {
-  return LockWhenWithTimeout(cond, timeout);
-}
-bool Mutex::ReaderLockWhenWithDeadline(const Condition& cond,
-                                       absl::Time deadline) {
-  return LockWhenWithDeadline(cond, deadline);
-}
-
-void Mutex::EnableDebugLog(const char*) {}
-void Mutex::EnableInvariantDebugging(void (*)(void*), void*) {}
-void Mutex::ForgetDeadlockInfo() {}
-void Mutex::AssertHeld() const {}
-void Mutex::AssertReaderHeld() const {}
-void Mutex::AssertNotHeld() const {}
-
-CondVar::CondVar() {}
-
-CondVar::~CondVar() {}
-
-void CondVar::Signal() { impl()->Signal(); }
-
-void CondVar::SignalAll() { impl()->SignalAll(); }
-
-void CondVar::Wait(Mutex* mu) { return impl()->Wait(mu->impl()); }
-
-bool CondVar::WaitWithDeadline(Mutex* mu, absl::Time deadline) {
-  return impl()->WaitWithDeadline(
-      mu->impl(), synchronization_internal::LimitedDeadline(deadline));
-}
-
-bool CondVar::WaitWithTimeout(Mutex* mu, absl::Duration timeout) {
-  return WaitWithDeadline(mu, absl::Now() + timeout);
-}
-
-void CondVar::EnableDebugLog(const char*) {}
-
-#ifdef THREAD_SANITIZER
-extern "C" void __tsan_read1(void *addr);
-#else
-#define __tsan_read1(addr)  // do nothing if TSan not enabled
-#endif
-
-// A function that just returns its argument, dereferenced
-static bool Dereference(void *arg) {
-  // ThreadSanitizer does not instrument this file for memory accesses.
-  // This function dereferences a user variable that can participate
-  // in a data race, so we need to manually tell TSan about this memory access.
-  __tsan_read1(arg);
-  return *(static_cast<bool *>(arg));
-}
-
-Condition::Condition() {}   // null constructor, used for kTrue only
-const Condition Condition::kTrue;
-
-Condition::Condition(bool (*func)(void *), void *arg)
-    : eval_(&CallVoidPtrFunction),
-      function_(func),
-      method_(nullptr),
-      arg_(arg) {}
-
-bool Condition::CallVoidPtrFunction(const Condition *c) {
-  return (*c->function_)(c->arg_);
-}
-
-Condition::Condition(const bool *cond)
-    : eval_(CallVoidPtrFunction),
-      function_(Dereference),
-      method_(nullptr),
-      // const_cast is safe since Dereference does not modify arg
-      arg_(const_cast<bool *>(cond)) {}
-
-bool Condition::Eval() const {
-  // eval_ == null for kTrue
-  return (this->eval_ == nullptr) || (*this->eval_)(this);
-}
-
-void RegisterSymbolizer(bool (*)(const void*, char*, int)) {}
-
-}  // namespace absl
diff --git a/absl/synchronization/internal/mutex_nonprod.inc b/absl/synchronization/internal/mutex_nonprod.inc
deleted file mode 100644
index b8d5af79ad3..00000000000
--- a/absl/synchronization/internal/mutex_nonprod.inc
+++ /dev/null
@@ -1,259 +0,0 @@
-// Do not include.  This is an implementation detail of base/mutex.h.
-//
-// Declares three classes:
-//
-// base::internal::MutexImpl - implementation helper for Mutex
-// base::internal::CondVarImpl - implementation helper for CondVar
-// base::internal::SynchronizationStorage<T> - implementation helper for
-//                                             Mutex, CondVar
-
-#include <type_traits>
-
-#if defined(_WIN32)
-#include <condition_variable>
-#include <mutex>
-#else
-#include <pthread.h>
-#endif
-
-#include "absl/base/call_once.h"
-#include "absl/time/time.h"
-
-// Declare that Mutex::ReaderLock is actually Lock().  Intended primarily
-// for tests, and even then as a last resort.
-#ifdef ABSL_MUTEX_READER_LOCK_IS_EXCLUSIVE
-#error ABSL_MUTEX_READER_LOCK_IS_EXCLUSIVE cannot be directly set
-#else
-#define ABSL_MUTEX_READER_LOCK_IS_EXCLUSIVE 1
-#endif
-
-// Declare that Mutex::EnableInvariantDebugging is not implemented.
-// Intended primarily for tests, and even then as a last resort.
-#ifdef ABSL_MUTEX_ENABLE_INVARIANT_DEBUGGING_NOT_IMPLEMENTED
-#error ABSL_MUTEX_ENABLE_INVARIANT_DEBUGGING_NOT_IMPLEMENTED cannot be directly set
-#else
-#define ABSL_MUTEX_ENABLE_INVARIANT_DEBUGGING_NOT_IMPLEMENTED 1
-#endif
-
-namespace absl {
-class Condition;
-
-namespace synchronization_internal {
-
-class MutexImpl;
-
-// Do not use this implementation detail of CondVar. Provides most of the
-// implementation, but should not be placed directly in static storage
-// because it will not linker initialize properly. See
-// SynchronizationStorage<T> below for what we mean by linker
-// initialization.
-class CondVarImpl {
- public:
-  CondVarImpl();
-  CondVarImpl(const CondVarImpl&) = delete;
-  CondVarImpl& operator=(const CondVarImpl&) = delete;
-  ~CondVarImpl();
-
-  void Signal();
-  void SignalAll();
-  void Wait(MutexImpl* mutex);
-  bool WaitWithDeadline(MutexImpl* mutex, absl::Time deadline);
-
- private:
-#if defined(_WIN32)
-  std::condition_variable_any std_cv_;
-#else
-  pthread_cond_t pthread_cv_;
-#endif
-};
-
-// Do not use this implementation detail of Mutex. Provides most of the
-// implementation, but should not be placed directly in static storage
-// because it will not linker initialize properly. See
-// SynchronizationStorage<T> below for what we mean by linker
-// initialization.
-class MutexImpl {
- public:
-  MutexImpl();
-  MutexImpl(const MutexImpl&) = delete;
-  MutexImpl& operator=(const MutexImpl&) = delete;
-  ~MutexImpl();
-
-  void Lock();
-  bool TryLock();
-  void Unlock();
-  void Await(const Condition& cond);
-  bool AwaitWithDeadline(const Condition& cond, absl::Time deadline);
-
- private:
-  friend class CondVarImpl;
-
-#if defined(_WIN32)
-  std::mutex std_mutex_;
-#else
-  pthread_mutex_t pthread_mutex_;
-#endif
-
-  // True if the underlying mutex is locked.  If the destructor is entered
-  // while locked_, the underlying mutex is unlocked.  Mutex supports
-  // destruction while locked, but the same is undefined behavior for both
-  // pthread_mutex_t and std::mutex.
-  bool locked_ = false;
-
-  // Signaled before releasing the lock, in support of Await.
-  CondVarImpl released_;
-};
-
-// Do not use this implementation detail of CondVar and Mutex.  A storage
-// space for T that supports a LinkerInitialized constructor. T must
-// have a default constructor, which is called by the first call to
-// get(). T's destructor is never called if the LinkerInitialized
-// constructor is called.
-//
-// Objects constructed with the default constructor are constructed and
-// destructed like any other object, and should never be allocated in
-// static storage.
-//
-// Objects constructed with the LinkerInitialized constructor should
-// always be in static storage. For such objects, calls to get() are always
-// valid, except from signal handlers.
-//
-// Note that this implementation relies on undefined language behavior that
-// are known to hold for the set of supported compilers. An analysis
-// follows.
-//
-// From the C++11 standard:
-//
-// [basic.life] says an object has non-trivial initialization if it is of
-// class type and it is initialized by a constructor other than a trivial
-// default constructor.  (the LinkerInitialized constructor is
-// non-trivial)
-//
-// [basic.life] says the lifetime of an object with a non-trivial
-// constructor begins when the call to the constructor is complete.
-//
-// [basic.life] says the lifetime of an object with non-trivial destructor
-// ends when the call to the destructor begins.
-//
-// [basic.life] p5 specifies undefined behavior when accessing non-static
-// members of an instance outside its
-// lifetime. (SynchronizationStorage::get() access non-static members)
-//
-// So, LinkerInitialized object of SynchronizationStorage uses a
-// non-trivial constructor, which is called at some point during dynamic
-// initialization, and is therefore subject to order of dynamic
-// initialization bugs, where get() is called before the object's
-// constructor is, resulting in undefined behavior.
-//
-// Similarly, a LinkerInitialized SynchronizationStorage object has a
-// non-trivial destructor, and so its lifetime ends at some point during
-// destruction of objects with static storage duration [basic.start.term]
-// p4. There is a window where other exit code could call get() after this
-// occurs, resulting in undefined behavior.
-//
-// Combined, these statements imply that LinkerInitialized instances
-// of SynchronizationStorage<T> rely on undefined behavior.
-//
-// However, in practice, the implementation works on all supported
-// compilers. Specifically, we rely on:
-//
-// a) zero-initialization being sufficient to initialize
-// LinkerInitialized instances for the purposes of calling
-// get(), regardless of when the constructor is called. This is
-// because the is_dynamic_ boolean is correctly zero-initialized to
-// false.
-//
-// b) the LinkerInitialized constructor is a NOP, and immaterial to
-// even to concurrent calls to get().
-//
-// c) the destructor being a NOP for LinkerInitialized objects
-// (guaranteed by a check for !is_dynamic_), and so any concurrent and
-// subsequent calls to get() functioning as if the destructor were not
-// called, by virtue of the instances' storage remaining valid after the
-// destructor runs.
-//
-// d) That a-c apply transitively when SynchronizationStorage<T> is the
-// only member of a class allocated in static storage.
-//
-// Nothing in the language standard guarantees that a-d hold.  In practice,
-// these hold in all supported compilers.
-//
-// Future direction:
-//
-// Ideally, we would simply use std::mutex or a similar class, which when
-// allocated statically would support use immediately after static
-// initialization up until static storage is reclaimed (i.e. the properties
-// we require of all "linker initialized" instances).
-//
-// Regarding construction in static storage, std::mutex is required to
-// provide a constexpr default constructor [thread.mutex.class], which
-// ensures the instance's lifetime begins with static initialization
-// [basic.start.init], and so is immune to any problems caused by the order
-// of dynamic initialization. However, as of this writing Microsoft's
-// Visual Studio does not provide a constexpr constructor for std::mutex.
-// See
-// https://blogs.msdn.microsoft.com/vcblog/2015/06/02/constexpr-complete-for-vs-2015-rtm-c11-compiler-c17-stl/
-//
-// Regarding destruction of instances in static storage, [basic.life] does
-// say an object ends when storage in which the occupies is released, in
-// the case of non-trivial destructor. However, std::mutex is not specified
-// to have a trivial destructor.
-//
-// So, we would need a class with a constexpr default constructor and a
-// trivial destructor. Today, we can achieve neither desired property using
-// std::mutex directly.
-template <typename T>
-class SynchronizationStorage {
- public:
-  // Instances allocated on the heap or on the stack should use the default
-  // constructor.
-  SynchronizationStorage()
-      : is_dynamic_(true), once_() {}
-
-  // Instances allocated in static storage (not on the heap, not on the
-  // stack) should use this constructor.
-  explicit SynchronizationStorage(base_internal::LinkerInitialized) {}
-
-  constexpr explicit SynchronizationStorage(absl::ConstInitType)
-      : is_dynamic_(false), once_(), space_{{0}} {}
-
-  SynchronizationStorage(SynchronizationStorage&) = delete;
-  SynchronizationStorage& operator=(SynchronizationStorage&) = delete;
-
-  ~SynchronizationStorage() {
-    if (is_dynamic_) {
-      get()->~T();
-    }
-  }
-
-  // Retrieve the object in storage. This is fast and thread safe, but does
-  // incur the cost of absl::call_once().
-  //
-  // For instances in static storage constructed with the
-  // LinkerInitialized constructor, may be called at any time without
-  // regard for order of dynamic initialization or destruction of objects
-  // in static storage. See the class comment for caveats.
-  T* get() {
-    absl::call_once(once_, SynchronizationStorage::Construct, this);
-    return reinterpret_cast<T*>(&space_);
-  }
-
- private:
-  static void Construct(SynchronizationStorage<T>* self) {
-    new (&self->space_) T();
-  }
-
-  // When true, T's destructor is run when this is destructed.
-  //
-  // The LinkerInitialized constructor assumes this value will be set
-  // false by static initialization.
-  bool is_dynamic_;
-
-  absl::once_flag once_;
-
-  // An aligned space for T.
-  typename std::aligned_storage<sizeof(T), alignof(T)>::type space_;
-};
-
-}  // namespace synchronization_internal
-}  // namespace absl
diff --git a/absl/synchronization/internal/per_thread_sem.cc b/absl/synchronization/internal/per_thread_sem.cc
index b7014fb2b02..8ba2a20e983 100644
--- a/absl/synchronization/internal/per_thread_sem.cc
+++ b/absl/synchronization/internal/per_thread_sem.cc
@@ -12,19 +12,23 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+#include "absl/base/internal/low_level_alloc.h"  // IWYU pragma: keep
+
 // This file is a no-op if the required LowLevelAlloc support is missing.
-#include "absl/base/internal/low_level_alloc.h"
 #ifndef ABSL_LOW_LEVEL_ALLOC_MISSING
 
-#include "absl/synchronization/internal/per_thread_sem.h"
-
 #include <atomic>
 
 #include "absl/base/attributes.h"
+#include "absl/base/config.h"
 #include "absl/base/internal/thread_identity.h"
+#include "absl/synchronization/internal/create_thread_identity.h"
+#include "absl/synchronization/internal/kernel_timeout.h"
+#include "absl/synchronization/internal/per_thread_sem.h"
 #include "absl/synchronization/internal/waiter.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace synchronization_internal {
 
 void PerThreadSem::SetThreadBlockedCounter(std::atomic<int> *counter) {
@@ -39,13 +43,6 @@ std::atomic<int> *PerThreadSem::GetThreadBlockedCounter() {
   return identity->blocked_count_ptr;
 }
 
-void PerThreadSem::Init(base_internal::ThreadIdentity *identity) {
-  Waiter::GetWaiter(identity)->Init();
-  identity->ticker.store(0, std::memory_order_relaxed);
-  identity->wait_start.store(0, std::memory_order_relaxed);
-  identity->is_idle.store(false, std::memory_order_relaxed);
-}
-
 void PerThreadSem::Tick(base_internal::ThreadIdentity *identity) {
   const int ticker =
       identity->ticker.fetch_add(1, std::memory_order_relaxed) + 1;
@@ -53,21 +50,33 @@ void PerThreadSem::Tick(base_internal::ThreadIdentity *identity) {
   const bool is_idle = identity->is_idle.load(std::memory_order_relaxed);
   if (wait_start && (ticker - wait_start > Waiter::kIdlePeriods) && !is_idle) {
     // Wakeup the waiting thread since it is time for it to become idle.
-    Waiter::GetWaiter(identity)->Poke();
+    ABSL_INTERNAL_C_SYMBOL(AbslInternalPerThreadSemPoke)(identity);
   }
 }
 
 }  // namespace synchronization_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 extern "C" {
 
-ABSL_ATTRIBUTE_WEAK void AbslInternalPerThreadSemPost(
+ABSL_ATTRIBUTE_WEAK void ABSL_INTERNAL_C_SYMBOL(AbslInternalPerThreadSemInit)(
+    absl::base_internal::ThreadIdentity *identity) {
+  new (absl::synchronization_internal::Waiter::GetWaiter(identity))
+      absl::synchronization_internal::Waiter();
+}
+
+ABSL_ATTRIBUTE_WEAK void ABSL_INTERNAL_C_SYMBOL(AbslInternalPerThreadSemPost)(
     absl::base_internal::ThreadIdentity *identity) {
   absl::synchronization_internal::Waiter::GetWaiter(identity)->Post();
 }
 
-ABSL_ATTRIBUTE_WEAK bool AbslInternalPerThreadSemWait(
+ABSL_ATTRIBUTE_WEAK void ABSL_INTERNAL_C_SYMBOL(AbslInternalPerThreadSemPoke)(
+    absl::base_internal::ThreadIdentity *identity) {
+  absl::synchronization_internal::Waiter::GetWaiter(identity)->Poke();
+}
+
+ABSL_ATTRIBUTE_WEAK bool ABSL_INTERNAL_C_SYMBOL(AbslInternalPerThreadSemWait)(
     absl::synchronization_internal::KernelTimeout t) {
   bool timeout = false;
   absl::base_internal::ThreadIdentity *identity;
diff --git a/absl/synchronization/internal/per_thread_sem.h b/absl/synchronization/internal/per_thread_sem.h
index e7da070ba72..7d1099efc6e 100644
--- a/absl/synchronization/internal/per_thread_sem.h
+++ b/absl/synchronization/internal/per_thread_sem.h
@@ -30,8 +30,14 @@
 #include "absl/base/internal/thread_identity.h"
 #include "absl/synchronization/internal/create_thread_identity.h"
 #include "absl/synchronization/internal/kernel_timeout.h"
+#include "absl/time/time.h"
+
+namespace gloop_do_not_use {
+struct SynchronizationBenchmarkPeer;
+}  // namespace gloop_do_not_use
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 class Mutex;
 
@@ -63,7 +69,7 @@ class PerThreadSem {
  private:
   // Create the PerThreadSem associated with "identity".  Initializes count=0.
   // REQUIRES: May only be called by ThreadIdentity.
-  static void Init(base_internal::ThreadIdentity* identity);
+  static inline void Init(base_internal::ThreadIdentity* identity);
 
   // Increments "identity"'s count.
   static inline void Post(base_internal::ThreadIdentity* identity);
@@ -73,13 +79,19 @@ class PerThreadSem {
   // !t.has_timeout() => Wait(t) will return true.
   static inline bool Wait(KernelTimeout t);
 
-  // White-listed callers.
+  // Waits until either our count > 0 or the absolute time t has passed.
+  // If count > 0, decrements count and returns true.  Otherwise returns false.
+  static inline bool WaitAbsolute(absl::Time t);
+
+  // Permitted callers.
   friend class PerThreadSemTest;
   friend class absl::Mutex;
-  friend absl::base_internal::ThreadIdentity* CreateThreadIdentity();
+  friend struct ::gloop_do_not_use::SynchronizationBenchmarkPeer;
+  friend void OneTimeInitThreadIdentity(absl::base_internal::ThreadIdentity*);
 };
 
 }  // namespace synchronization_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 // In some build configurations we pass --detect-odr-violations to the
@@ -89,20 +101,33 @@ class PerThreadSem {
 // By changing our extension points to be extern "C", we dodge this
 // check.
 extern "C" {
-void AbslInternalPerThreadSemPost(
+void ABSL_INTERNAL_C_SYMBOL(AbslInternalPerThreadSemInit)(
     absl::base_internal::ThreadIdentity* identity);
-bool AbslInternalPerThreadSemWait(
+void ABSL_INTERNAL_C_SYMBOL(AbslInternalPerThreadSemPost)(
+    absl::base_internal::ThreadIdentity* identity);
+bool ABSL_INTERNAL_C_SYMBOL(AbslInternalPerThreadSemWait)(
     absl::synchronization_internal::KernelTimeout t);
+void ABSL_INTERNAL_C_SYMBOL(AbslInternalPerThreadSemPoke)(
+    absl::base_internal::ThreadIdentity* identity);
 }  // extern "C"
 
+void absl::synchronization_internal::PerThreadSem::Init(
+    absl::base_internal::ThreadIdentity* identity) {
+  ABSL_INTERNAL_C_SYMBOL(AbslInternalPerThreadSemInit)(identity);
+}
+
 void absl::synchronization_internal::PerThreadSem::Post(
     absl::base_internal::ThreadIdentity* identity) {
-  AbslInternalPerThreadSemPost(identity);
+  ABSL_INTERNAL_C_SYMBOL(AbslInternalPerThreadSemPost)(identity);
 }
 
 bool absl::synchronization_internal::PerThreadSem::Wait(
     absl::synchronization_internal::KernelTimeout t) {
-  return AbslInternalPerThreadSemWait(t);
+  return ABSL_INTERNAL_C_SYMBOL(AbslInternalPerThreadSemWait)(t);
+}
+
+bool absl::synchronization_internal::PerThreadSem::WaitAbsolute(absl::Time t) {
+  return Wait(KernelTimeout(t));
 }
 
 #endif  // ABSL_SYNCHRONIZATION_INTERNAL_PER_THREAD_SEM_H_
diff --git a/absl/synchronization/internal/per_thread_sem_test.cc b/absl/synchronization/internal/per_thread_sem_test.cc
index dba72390d3d..4c5ca694607 100644
--- a/absl/synchronization/internal/per_thread_sem_test.cc
+++ b/absl/synchronization/internal/per_thread_sem_test.cc
@@ -14,18 +14,25 @@
 
 #include "absl/synchronization/internal/per_thread_sem.h"
 
+#include <algorithm>
 #include <atomic>
 #include <condition_variable>  // NOLINT(build/c++11)
+#include <cstdint>
+#include <cstdio>
 #include <functional>
 #include <limits>
-#include <mutex>               // NOLINT(build/c++11)
+#include <mutex>  // NOLINT(build/c++11)
 #include <string>
-#include <thread>              // NOLINT(build/c++11)
+#include <thread>  // NOLINT(build/c++11)
 
 #include "gtest/gtest.h"
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
 #include "absl/base/internal/cycleclock.h"
 #include "absl/base/internal/thread_identity.h"
 #include "absl/strings/str_cat.h"
+#include "absl/synchronization/internal/create_thread_identity.h"
+#include "absl/synchronization/internal/kernel_timeout.h"
 #include "absl/time/clock.h"
 #include "absl/time/time.h"
 
@@ -33,6 +40,7 @@
 // primitives which might use PerThreadSem, most notably absl::Mutex.
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace synchronization_internal {
 
 class SimpleSemaphore {
@@ -130,9 +138,9 @@ class PerThreadSemTest : public testing::Test {
     return PerThreadSem::Wait(t);
   }
 
-  // convenience overload
+  // absl::Time overload, absolute expiry
   static bool Wait(absl::Time t) {
-    return Wait(KernelTimeout(t));
+    return PerThreadSem::WaitAbsolute(t);
   }
 
   static void Tick(base_internal::ThreadIdentity *identity) {
@@ -157,7 +165,11 @@ TEST_F(PerThreadSemTest, Timeouts) {
   const absl::Duration elapsed = absl::Now() - start;
   // Allow for a slight early return, to account for quality of implementation
   // issues on various platforms.
-  const absl::Duration slop = absl::Microseconds(200);
+  absl::Duration slop = absl::Milliseconds(1);
+#ifdef _MSC_VER
+  // Use higher slop on MSVC due to flaky test failures.
+  slop = absl::Milliseconds(16);
+#endif
   EXPECT_LE(delay - slop, elapsed)
       << "Wait returned " << delay - elapsed
       << " early (with " << slop << " slop), start time was " << start;
@@ -172,7 +184,17 @@ TEST_F(PerThreadSemTest, Timeouts) {
   EXPECT_TRUE(Wait(negative_timeout));
 }
 
+TEST_F(PerThreadSemTest, ThreadIdentityReuse) {
+  // Create a base_internal::ThreadIdentity object and keep reusing it. There
+  // should be no memory or resource leaks.
+  for (int i = 0; i < 10000; i++) {
+    std::thread t([]() { GetOrCreateCurrentThreadIdentity(); });
+    t.join();
+  }
+}
+
 }  // namespace
 
 }  // namespace synchronization_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/synchronization/internal/pthread_waiter.cc b/absl/synchronization/internal/pthread_waiter.cc
new file mode 100644
index 00000000000..5b27d0fa7f8
--- /dev/null
+++ b/absl/synchronization/internal/pthread_waiter.cc
@@ -0,0 +1,162 @@
+// Copyright 2023 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/synchronization/internal/pthread_waiter.h"
+
+#ifdef ABSL_INTERNAL_HAVE_PTHREAD_WAITER
+
+#include <pthread.h>
+#include <sys/time.h>
+#include <unistd.h>
+
+#include <cassert>
+#include <cerrno>
+
+#include "absl/base/config.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/base/optimization.h"
+#include "absl/synchronization/internal/kernel_timeout.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace synchronization_internal {
+
+namespace {
+class PthreadMutexHolder {
+ public:
+  explicit PthreadMutexHolder(pthread_mutex_t *mu) : mu_(mu) {
+    const int err = pthread_mutex_lock(mu_);
+    if (err != 0) {
+      ABSL_RAW_LOG(FATAL, "pthread_mutex_lock failed: %d", err);
+    }
+  }
+
+  PthreadMutexHolder(const PthreadMutexHolder &rhs) = delete;
+  PthreadMutexHolder &operator=(const PthreadMutexHolder &rhs) = delete;
+
+  ~PthreadMutexHolder() {
+    const int err = pthread_mutex_unlock(mu_);
+    if (err != 0) {
+      ABSL_RAW_LOG(FATAL, "pthread_mutex_unlock failed: %d", err);
+    }
+  }
+
+ private:
+  pthread_mutex_t *mu_;
+};
+}  // namespace
+
+PthreadWaiter::PthreadWaiter() : waiter_count_(0), wakeup_count_(0) {
+  const int err = pthread_mutex_init(&mu_, nullptr);
+  if (err != 0) {
+    ABSL_RAW_LOG(FATAL, "pthread_mutex_init failed: %d", err);
+  }
+
+  const int err2 = pthread_cond_init(&cv_, nullptr);
+  if (err2 != 0) {
+    ABSL_RAW_LOG(FATAL, "pthread_cond_init failed: %d", err2);
+  }
+}
+
+#ifdef __APPLE__
+#define ABSL_INTERNAL_HAS_PTHREAD_COND_TIMEDWAIT_RELATIVE_NP 1
+#endif
+
+#if defined(__GLIBC__) && \
+    (__GLIBC__ > 2 || (__GLIBC__ == 2 && __GLIBC_MINOR__ >= 30))
+#define ABSL_INTERNAL_HAVE_PTHREAD_COND_CLOCKWAIT 1
+#elif defined(__ANDROID_API__) && __ANDROID_API__ >= 30
+#define ABSL_INTERNAL_HAVE_PTHREAD_COND_CLOCKWAIT 1
+#endif
+
+// Calls pthread_cond_timedwait() or possibly something else like
+// pthread_cond_timedwait_relative_np() depending on the platform and
+// KernelTimeout requested. The return value is the same as the return
+// value of pthread_cond_timedwait().
+int PthreadWaiter::TimedWait(KernelTimeout t) {
+  assert(t.has_timeout());
+  if (KernelTimeout::SupportsSteadyClock() && t.is_relative_timeout()) {
+#ifdef ABSL_INTERNAL_HAS_PTHREAD_COND_TIMEDWAIT_RELATIVE_NP
+    const auto rel_timeout = t.MakeRelativeTimespec();
+    return pthread_cond_timedwait_relative_np(&cv_, &mu_, &rel_timeout);
+#elif defined(ABSL_INTERNAL_HAVE_PTHREAD_COND_CLOCKWAIT) && \
+    defined(CLOCK_MONOTONIC)
+    const auto abs_clock_timeout = t.MakeClockAbsoluteTimespec(CLOCK_MONOTONIC);
+    return pthread_cond_clockwait(&cv_, &mu_, CLOCK_MONOTONIC,
+                                  &abs_clock_timeout);
+#endif
+  }
+
+  const auto abs_timeout = t.MakeAbsTimespec();
+  return pthread_cond_timedwait(&cv_, &mu_, &abs_timeout);
+}
+
+bool PthreadWaiter::Wait(KernelTimeout t) {
+  PthreadMutexHolder h(&mu_);
+  ++waiter_count_;
+  // Loop until we find a wakeup to consume or timeout.
+  // Note that, since the thread ticker is just reset, we don't need to check
+  // whether the thread is idle on the very first pass of the loop.
+  bool first_pass = true;
+  while (wakeup_count_ == 0) {
+    if (!first_pass) MaybeBecomeIdle();
+    // No wakeups available, time to wait.
+    if (!t.has_timeout()) {
+      const int err = pthread_cond_wait(&cv_, &mu_);
+      if (err != 0) {
+        ABSL_RAW_LOG(FATAL, "pthread_cond_wait failed: %d", err);
+      }
+    } else {
+      const int err = TimedWait(t);
+      if (err == ETIMEDOUT) {
+        --waiter_count_;
+        return false;
+      }
+      if (err != 0) {
+        ABSL_RAW_LOG(FATAL, "PthreadWaiter::TimedWait() failed: %d", err);
+      }
+    }
+    first_pass = false;
+  }
+  // Consume a wakeup and we're done.
+  --wakeup_count_;
+  --waiter_count_;
+  return true;
+}
+
+void PthreadWaiter::Post() {
+  PthreadMutexHolder h(&mu_);
+  ++wakeup_count_;
+  InternalCondVarPoke();
+}
+
+void PthreadWaiter::Poke() {
+  PthreadMutexHolder h(&mu_);
+  InternalCondVarPoke();
+}
+
+void PthreadWaiter::InternalCondVarPoke() {
+  if (waiter_count_ != 0) {
+    const int err = pthread_cond_signal(&cv_);
+    if (ABSL_PREDICT_FALSE(err != 0)) {
+      ABSL_RAW_LOG(FATAL, "pthread_cond_signal failed: %d", err);
+    }
+  }
+}
+
+}  // namespace synchronization_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_INTERNAL_HAVE_PTHREAD_WAITER
diff --git a/absl/synchronization/internal/pthread_waiter.h b/absl/synchronization/internal/pthread_waiter.h
new file mode 100644
index 00000000000..23db76ad442
--- /dev/null
+++ b/absl/synchronization/internal/pthread_waiter.h
@@ -0,0 +1,60 @@
+// Copyright 2023 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+#ifndef ABSL_SYNCHRONIZATION_INTERNAL_PTHREAD_WAITER_H_
+#define ABSL_SYNCHRONIZATION_INTERNAL_PTHREAD_WAITER_H_
+
+#if !defined(_WIN32) && !defined(__MINGW32__)
+#include <pthread.h>
+
+#include "absl/base/config.h"
+#include "absl/synchronization/internal/kernel_timeout.h"
+#include "absl/synchronization/internal/waiter_base.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace synchronization_internal {
+
+#define ABSL_INTERNAL_HAVE_PTHREAD_WAITER 1
+
+class PthreadWaiter : public WaiterCrtp<PthreadWaiter> {
+ public:
+  PthreadWaiter();
+
+  bool Wait(KernelTimeout t);
+  void Post();
+  void Poke();
+
+  static constexpr char kName[] = "PthreadWaiter";
+
+ private:
+  int TimedWait(KernelTimeout t);
+
+  // REQUIRES: mu_ must be held.
+  void InternalCondVarPoke();
+
+  pthread_mutex_t mu_;
+  pthread_cond_t cv_;
+  int waiter_count_;
+  int wakeup_count_;  // Unclaimed wakeups.
+};
+
+}  // namespace synchronization_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // !defined(_WIN32) && !defined(__MINGW32__)
+
+#endif  // ABSL_SYNCHRONIZATION_INTERNAL_PTHREAD_WAITER_H_
diff --git a/absl/synchronization/internal/sem_waiter.cc b/absl/synchronization/internal/sem_waiter.cc
new file mode 100644
index 00000000000..50f5960d7b7
--- /dev/null
+++ b/absl/synchronization/internal/sem_waiter.cc
@@ -0,0 +1,115 @@
+// Copyright 2023 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/synchronization/internal/sem_waiter.h"
+
+#ifdef ABSL_INTERNAL_HAVE_SEM_WAITER
+
+#include <semaphore.h>
+
+#include <atomic>
+#include <cassert>
+#include <cerrno>
+
+#include "absl/base/config.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/synchronization/internal/kernel_timeout.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace synchronization_internal {
+
+SemWaiter::SemWaiter() : wakeups_(0) {
+  if (sem_init(&sem_, 0, 0) != 0) {
+    ABSL_RAW_LOG(FATAL, "sem_init failed with errno %d\n", errno);
+  }
+}
+
+#if defined(__GLIBC__) && \
+    (__GLIBC__ > 2 || (__GLIBC__ == 2 && __GLIBC_MINOR__ >= 30))
+#define ABSL_INTERNAL_HAVE_SEM_CLOCKWAIT 1
+#elif defined(__ANDROID_API__) && __ANDROID_API__ >= 30
+#define ABSL_INTERNAL_HAVE_SEM_CLOCKWAIT 1
+#endif
+
+// Calls sem_timedwait() or possibly something else like
+// sem_clockwait() depending on the platform and
+// KernelTimeout requested. The return value is the same as a call to the return
+// value to a call to sem_timedwait().
+int SemWaiter::TimedWait(KernelTimeout t) {
+  if (KernelTimeout::SupportsSteadyClock() && t.is_relative_timeout()) {
+#if defined(ABSL_INTERNAL_HAVE_SEM_CLOCKWAIT) && defined(CLOCK_MONOTONIC)
+    const auto abs_clock_timeout = t.MakeClockAbsoluteTimespec(CLOCK_MONOTONIC);
+    return sem_clockwait(&sem_, CLOCK_MONOTONIC, &abs_clock_timeout);
+#endif
+  }
+
+  const auto abs_timeout = t.MakeAbsTimespec();
+  return sem_timedwait(&sem_, &abs_timeout);
+}
+
+bool SemWaiter::Wait(KernelTimeout t) {
+  // Loop until we timeout or consume a wakeup.
+  // Note that, since the thread ticker is just reset, we don't need to check
+  // whether the thread is idle on the very first pass of the loop.
+  bool first_pass = true;
+  while (true) {
+    int x = wakeups_.load(std::memory_order_relaxed);
+    while (x != 0) {
+      if (!wakeups_.compare_exchange_weak(x, x - 1,
+                                          std::memory_order_acquire,
+                                          std::memory_order_relaxed)) {
+        continue;  // Raced with someone, retry.
+      }
+      // Successfully consumed a wakeup, we're done.
+      return true;
+    }
+
+    if (!first_pass) MaybeBecomeIdle();
+    // Nothing to consume, wait (looping on EINTR).
+    while (true) {
+      if (!t.has_timeout()) {
+        if (sem_wait(&sem_) == 0) break;
+        if (errno == EINTR) continue;
+        ABSL_RAW_LOG(FATAL, "sem_wait failed: %d", errno);
+      } else {
+        if (TimedWait(t) == 0) break;
+        if (errno == EINTR) continue;
+        if (errno == ETIMEDOUT) return false;
+        ABSL_RAW_LOG(FATAL, "SemWaiter::TimedWait() failed: %d", errno);
+      }
+    }
+    first_pass = false;
+  }
+}
+
+void SemWaiter::Post() {
+  // Post a wakeup.
+  if (wakeups_.fetch_add(1, std::memory_order_release) == 0) {
+    // We incremented from 0, need to wake a potential waiter.
+    Poke();
+  }
+}
+
+void SemWaiter::Poke() {
+  if (sem_post(&sem_) != 0) {  // Wake any semaphore waiter.
+    ABSL_RAW_LOG(FATAL, "sem_post failed with errno %d\n", errno);
+  }
+}
+
+}  // namespace synchronization_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_INTERNAL_HAVE_SEM_WAITER
diff --git a/absl/synchronization/internal/sem_waiter.h b/absl/synchronization/internal/sem_waiter.h
new file mode 100644
index 00000000000..c22746f903f
--- /dev/null
+++ b/absl/synchronization/internal/sem_waiter.h
@@ -0,0 +1,65 @@
+// Copyright 2023 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+#ifndef ABSL_SYNCHRONIZATION_INTERNAL_SEM_WAITER_H_
+#define ABSL_SYNCHRONIZATION_INTERNAL_SEM_WAITER_H_
+
+#include "absl/base/config.h"
+
+#ifdef ABSL_HAVE_SEMAPHORE_H
+#include <semaphore.h>
+
+#include <atomic>
+#include <cstdint>
+
+#include "absl/base/internal/thread_identity.h"
+#include "absl/synchronization/internal/futex.h"
+#include "absl/synchronization/internal/kernel_timeout.h"
+#include "absl/synchronization/internal/waiter_base.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace synchronization_internal {
+
+#define ABSL_INTERNAL_HAVE_SEM_WAITER 1
+
+class SemWaiter : public WaiterCrtp<SemWaiter> {
+ public:
+  SemWaiter();
+
+  bool Wait(KernelTimeout t);
+  void Post();
+  void Poke();
+
+  static constexpr char kName[] = "SemWaiter";
+
+ private:
+  int TimedWait(KernelTimeout t);
+
+  sem_t sem_;
+
+  // This seems superfluous, but for Poke() we need to cause spurious
+  // wakeups on the semaphore. Hence we can't actually use the
+  // semaphore's count.
+  std::atomic<int> wakeups_;
+};
+
+}  // namespace synchronization_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_HAVE_SEMAPHORE_H
+
+#endif  // ABSL_SYNCHRONIZATION_INTERNAL_SEM_WAITER_H_
diff --git a/absl/synchronization/internal/stdcpp_waiter.cc b/absl/synchronization/internal/stdcpp_waiter.cc
new file mode 100644
index 00000000000..c9368d5dc2a
--- /dev/null
+++ b/absl/synchronization/internal/stdcpp_waiter.cc
@@ -0,0 +1,83 @@
+// Copyright 2023 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/synchronization/internal/stdcpp_waiter.h"
+
+#ifdef ABSL_INTERNAL_HAVE_STDCPP_WAITER
+
+#include <condition_variable>  // NOLINT(build/c++11)
+#include <mutex>  // NOLINT(build/c++11)
+
+#include "absl/base/config.h"
+#include "absl/synchronization/internal/kernel_timeout.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace synchronization_internal {
+
+StdcppWaiter::StdcppWaiter() : waiter_count_(0), wakeup_count_(0) {}
+
+bool StdcppWaiter::Wait(KernelTimeout t) {
+  std::unique_lock<std::mutex> lock(mu_);
+  ++waiter_count_;
+
+  // Loop until we find a wakeup to consume or timeout.
+  // Note that, since the thread ticker is just reset, we don't need to check
+  // whether the thread is idle on the very first pass of the loop.
+  bool first_pass = true;
+  while (wakeup_count_ == 0) {
+    if (!first_pass) MaybeBecomeIdle();
+    // No wakeups available, time to wait.
+    if (!t.has_timeout()) {
+      cv_.wait(lock);
+    } else {
+      auto wait_result = t.SupportsSteadyClock() && t.is_relative_timeout()
+                             ? cv_.wait_for(lock, t.ToChronoDuration())
+                             : cv_.wait_until(lock, t.ToChronoTimePoint());
+      if (wait_result == std::cv_status::timeout) {
+        --waiter_count_;
+        return false;
+      }
+    }
+    first_pass = false;
+  }
+
+  // Consume a wakeup and we're done.
+  --wakeup_count_;
+  --waiter_count_;
+  return true;
+}
+
+void StdcppWaiter::Post() {
+  std::lock_guard<std::mutex> lock(mu_);
+  ++wakeup_count_;
+  InternalCondVarPoke();
+}
+
+void StdcppWaiter::Poke() {
+  std::lock_guard<std::mutex> lock(mu_);
+  InternalCondVarPoke();
+}
+
+void StdcppWaiter::InternalCondVarPoke() {
+  if (waiter_count_ != 0) {
+    cv_.notify_one();
+  }
+}
+
+}  // namespace synchronization_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_INTERNAL_HAVE_STDCPP_WAITER
diff --git a/absl/synchronization/internal/stdcpp_waiter.h b/absl/synchronization/internal/stdcpp_waiter.h
new file mode 100644
index 00000000000..e592a27bd55
--- /dev/null
+++ b/absl/synchronization/internal/stdcpp_waiter.h
@@ -0,0 +1,56 @@
+// Copyright 2023 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+#ifndef ABSL_SYNCHRONIZATION_INTERNAL_STDCPP_WAITER_H_
+#define ABSL_SYNCHRONIZATION_INTERNAL_STDCPP_WAITER_H_
+
+#include <condition_variable>  // NOLINT(build/c++11)
+#include <mutex>  // NOLINT(build/c++11)
+
+#include "absl/base/config.h"
+#include "absl/synchronization/internal/kernel_timeout.h"
+#include "absl/synchronization/internal/waiter_base.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace synchronization_internal {
+
+#define ABSL_INTERNAL_HAVE_STDCPP_WAITER 1
+
+class StdcppWaiter : public WaiterCrtp<StdcppWaiter> {
+ public:
+  StdcppWaiter();
+
+  bool Wait(KernelTimeout t);
+  void Post();
+  void Poke();
+
+  static constexpr char kName[] = "StdcppWaiter";
+
+ private:
+  // REQUIRES: mu_ must be held.
+  void InternalCondVarPoke();
+
+  std::mutex mu_;
+  std::condition_variable cv_;
+  int waiter_count_;
+  int wakeup_count_;  // Unclaimed wakeups.
+};
+
+}  // namespace synchronization_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_SYNCHRONIZATION_INTERNAL_STDCPP_WAITER_H_
diff --git a/absl/synchronization/internal/thread_pool.h b/absl/synchronization/internal/thread_pool.h
index a00f2be8e89..f87adf635d3 100644
--- a/absl/synchronization/internal/thread_pool.h
+++ b/absl/synchronization/internal/thread_pool.h
@@ -20,18 +20,22 @@
 #include <functional>
 #include <queue>
 #include <thread>  // NOLINT(build/c++11)
+#include <utility>
 #include <vector>
 
 #include "absl/base/thread_annotations.h"
+#include "absl/functional/any_invocable.h"
 #include "absl/synchronization/mutex.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace synchronization_internal {
 
 // A simple ThreadPool implementation for tests.
 class ThreadPool {
  public:
   explicit ThreadPool(int num_threads) {
+    threads_.reserve(num_threads);
     for (int i = 0; i < num_threads; ++i) {
       threads_.push_back(std::thread(&ThreadPool::WorkLoop, this));
     }
@@ -42,7 +46,7 @@ class ThreadPool {
 
   ~ThreadPool() {
     {
-      absl::MutexLock l(&mu_);
+      absl::MutexLock l(mu_);
       for (size_t i = 0; i < threads_.size(); i++) {
         queue_.push(nullptr);  // Shutdown signal.
       }
@@ -53,9 +57,9 @@ class ThreadPool {
   }
 
   // Schedule a function to be run on a ThreadPool thread immediately.
-  void Schedule(std::function<void()> func) {
+  void Schedule(absl::AnyInvocable<void()> func) {
     assert(func != nullptr);
-    absl::MutexLock l(&mu_);
+    absl::MutexLock l(mu_);
     queue_.push(std::move(func));
   }
 
@@ -66,9 +70,9 @@ class ThreadPool {
 
   void WorkLoop() {
     while (true) {
-      std::function<void()> func;
+      absl::AnyInvocable<void()> func;
       {
-        absl::MutexLock l(&mu_);
+        absl::MutexLock l(mu_);
         mu_.Await(absl::Condition(this, &ThreadPool::WorkAvailable));
         func = std::move(queue_.front());
         queue_.pop();
@@ -81,11 +85,12 @@ class ThreadPool {
   }
 
   absl::Mutex mu_;
-  std::queue<std::function<void()>> queue_ ABSL_GUARDED_BY(mu_);
+  std::queue<absl::AnyInvocable<void()>> queue_ ABSL_GUARDED_BY(mu_);
   std::vector<std::thread> threads_;
 };
 
 }  // namespace synchronization_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_SYNCHRONIZATION_INTERNAL_THREAD_POOL_H_
diff --git a/absl/synchronization/internal/waiter.cc b/absl/synchronization/internal/waiter.cc
deleted file mode 100644
index 961457809f1..00000000000
--- a/absl/synchronization/internal/waiter.cc
+++ /dev/null
@@ -1,457 +0,0 @@
-// Copyright 2017 The Abseil Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-#include "absl/synchronization/internal/waiter.h"
-
-#include "absl/base/config.h"
-
-#ifdef _WIN32
-#include <windows.h>
-#else
-#include <pthread.h>
-#include <sys/time.h>
-#include <unistd.h>
-#endif
-
-#ifdef __linux__
-#include <linux/futex.h>
-#include <sys/syscall.h>
-#endif
-
-#ifdef ABSL_HAVE_SEMAPHORE_H
-#include <semaphore.h>
-#endif
-
-#include <errno.h>
-#include <stdio.h>
-#include <time.h>
-
-#include <atomic>
-#include <cassert>
-#include <cstdint>
-#include <new>
-#include <type_traits>
-
-#include "absl/base/internal/raw_logging.h"
-#include "absl/base/internal/thread_identity.h"
-#include "absl/base/optimization.h"
-#include "absl/synchronization/internal/kernel_timeout.h"
-
-namespace absl {
-namespace synchronization_internal {
-
-static void MaybeBecomeIdle() {
-  base_internal::ThreadIdentity *identity =
-      base_internal::CurrentThreadIdentityIfPresent();
-  assert(identity != nullptr);
-  const bool is_idle = identity->is_idle.load(std::memory_order_relaxed);
-  const int ticker = identity->ticker.load(std::memory_order_relaxed);
-  const int wait_start = identity->wait_start.load(std::memory_order_relaxed);
-  if (!is_idle && ticker - wait_start > Waiter::kIdlePeriods) {
-    identity->is_idle.store(true, std::memory_order_relaxed);
-  }
-}
-
-#if ABSL_WAITER_MODE == ABSL_WAITER_MODE_FUTEX
-
-// Some Android headers are missing these definitions even though they
-// support these futex operations.
-#ifdef __BIONIC__
-#ifndef SYS_futex
-#define SYS_futex __NR_futex
-#endif
-#ifndef FUTEX_WAIT_BITSET
-#define FUTEX_WAIT_BITSET 9
-#endif
-#ifndef FUTEX_PRIVATE_FLAG
-#define FUTEX_PRIVATE_FLAG 128
-#endif
-#ifndef FUTEX_CLOCK_REALTIME
-#define FUTEX_CLOCK_REALTIME 256
-#endif
-#ifndef FUTEX_BITSET_MATCH_ANY
-#define FUTEX_BITSET_MATCH_ANY 0xFFFFFFFF
-#endif
-#endif
-
-class Futex {
- public:
-  static int WaitUntil(std::atomic<int32_t> *v, int32_t val,
-                       KernelTimeout t) {
-    int err = 0;
-    if (t.has_timeout()) {
-      // https://locklessinc.com/articles/futex_cheat_sheet/
-      // Unlike FUTEX_WAIT, FUTEX_WAIT_BITSET uses absolute time.
-      struct timespec abs_timeout = t.MakeAbsTimespec();
-      // Atomically check that the futex value is still 0, and if it
-      // is, sleep until abs_timeout or until woken by FUTEX_WAKE.
-      err = syscall(
-          SYS_futex, reinterpret_cast<int32_t *>(v),
-          FUTEX_WAIT_BITSET | FUTEX_PRIVATE_FLAG | FUTEX_CLOCK_REALTIME, val,
-          &abs_timeout, nullptr, FUTEX_BITSET_MATCH_ANY);
-    } else {
-      // Atomically check that the futex value is still 0, and if it
-      // is, sleep until woken by FUTEX_WAKE.
-      err = syscall(SYS_futex, reinterpret_cast<int32_t *>(v),
-                    FUTEX_WAIT | FUTEX_PRIVATE_FLAG, val, nullptr);
-    }
-    if (err != 0) {
-      err = -errno;
-    }
-    return err;
-  }
-
-  static int Wake(std::atomic<int32_t> *v, int32_t count) {
-    int err = syscall(SYS_futex, reinterpret_cast<int32_t *>(v),
-                      FUTEX_WAKE | FUTEX_PRIVATE_FLAG, count);
-    if (ABSL_PREDICT_FALSE(err < 0)) {
-      err = -errno;
-    }
-    return err;
-  }
-};
-
-void Waiter::Init() {
-  futex_.store(0, std::memory_order_relaxed);
-}
-
-bool Waiter::Wait(KernelTimeout t) {
-  // Loop until we can atomically decrement futex from a positive
-  // value, waiting on a futex while we believe it is zero.
-  while (true) {
-    int32_t x = futex_.load(std::memory_order_relaxed);
-    if (x != 0) {
-      if (!futex_.compare_exchange_weak(x, x - 1,
-                                        std::memory_order_acquire,
-                                        std::memory_order_relaxed)) {
-        continue;  // Raced with someone, retry.
-      }
-      return true;  // Consumed a wakeup, we are done.
-    }
-
-    const int err = Futex::WaitUntil(&futex_, 0, t);
-    if (err != 0) {
-      if (err == -EINTR || err == -EWOULDBLOCK) {
-        // Do nothing, the loop will retry.
-      } else if (err == -ETIMEDOUT) {
-        return false;
-      } else {
-        ABSL_RAW_LOG(FATAL, "Futex operation failed with error %d\n", err);
-      }
-    }
-
-    MaybeBecomeIdle();
-  }
-}
-
-void Waiter::Post() {
-  if (futex_.fetch_add(1, std::memory_order_release) == 0) {
-    // We incremented from 0, need to wake a potential waker.
-    Poke();
-  }
-}
-
-void Waiter::Poke() {
-  // Wake one thread waiting on the futex.
-  const int err = Futex::Wake(&futex_, 1);
-  if (ABSL_PREDICT_FALSE(err < 0)) {
-    ABSL_RAW_LOG(FATAL, "Futex operation failed with error %d\n", err);
-  }
-}
-
-#elif ABSL_WAITER_MODE == ABSL_WAITER_MODE_CONDVAR
-
-class PthreadMutexHolder {
- public:
-  explicit PthreadMutexHolder(pthread_mutex_t *mu) : mu_(mu) {
-    const int err = pthread_mutex_lock(mu_);
-    if (err != 0) {
-      ABSL_RAW_LOG(FATAL, "pthread_mutex_lock failed: %d", err);
-    }
-  }
-
-  PthreadMutexHolder(const PthreadMutexHolder &rhs) = delete;
-  PthreadMutexHolder &operator=(const PthreadMutexHolder &rhs) = delete;
-
-  ~PthreadMutexHolder() {
-    const int err = pthread_mutex_unlock(mu_);
-    if (err != 0) {
-      ABSL_RAW_LOG(FATAL, "pthread_mutex_unlock failed: %d", err);
-    }
-  }
-
- private:
-  pthread_mutex_t *mu_;
-};
-
-void Waiter::Init() {
-  const int err = pthread_mutex_init(&mu_, 0);
-  if (err != 0) {
-    ABSL_RAW_LOG(FATAL, "pthread_mutex_init failed: %d", err);
-  }
-
-  const int err2 = pthread_cond_init(&cv_, 0);
-  if (err2 != 0) {
-    ABSL_RAW_LOG(FATAL, "pthread_cond_init failed: %d", err2);
-  }
-
-  waiter_count_.store(0, std::memory_order_relaxed);
-  wakeup_count_.store(0, std::memory_order_relaxed);
-}
-
-bool Waiter::Wait(KernelTimeout t) {
-  struct timespec abs_timeout;
-  if (t.has_timeout()) {
-    abs_timeout = t.MakeAbsTimespec();
-  }
-
-  PthreadMutexHolder h(&mu_);
-  waiter_count_.fetch_add(1, std::memory_order_relaxed);
-  // Loop until we find a wakeup to consume or timeout.
-  while (true) {
-    int x = wakeup_count_.load(std::memory_order_relaxed);
-    if (x != 0) {
-      if (!wakeup_count_.compare_exchange_weak(x, x - 1,
-                                               std::memory_order_acquire,
-                                               std::memory_order_relaxed)) {
-        continue;  // Raced with someone, retry.
-      }
-      // Successfully consumed a wakeup, we're done.
-      waiter_count_.fetch_sub(1, std::memory_order_relaxed);
-      return true;
-    }
-
-    // No wakeups available, time to wait.
-    if (!t.has_timeout()) {
-      const int err = pthread_cond_wait(&cv_, &mu_);
-      if (err != 0) {
-        ABSL_RAW_LOG(FATAL, "pthread_cond_wait failed: %d", err);
-      }
-    } else {
-      const int err = pthread_cond_timedwait(&cv_, &mu_, &abs_timeout);
-      if (err == ETIMEDOUT) {
-        waiter_count_.fetch_sub(1, std::memory_order_relaxed);
-        return false;
-      }
-      if (err != 0) {
-        ABSL_RAW_LOG(FATAL, "pthread_cond_wait failed: %d", err);
-      }
-    }
-    MaybeBecomeIdle();
-  }
-}
-
-void Waiter::Post() {
-  wakeup_count_.fetch_add(1, std::memory_order_release);
-  Poke();
-}
-
-void Waiter::Poke() {
-  if (waiter_count_.load(std::memory_order_relaxed) == 0) {
-    return;
-  }
-  // Potentially a waker. Take the lock and check again.
-  PthreadMutexHolder h(&mu_);
-  if (waiter_count_.load(std::memory_order_relaxed) == 0) {
-    return;
-  }
-  const int err = pthread_cond_signal(&cv_);
-  if (err != 0) {
-    ABSL_RAW_LOG(FATAL, "pthread_cond_signal failed: %d", err);
-  }
-}
-
-#elif ABSL_WAITER_MODE == ABSL_WAITER_MODE_SEM
-
-void Waiter::Init() {
-  if (sem_init(&sem_, 0, 0) != 0) {
-    ABSL_RAW_LOG(FATAL, "sem_init failed with errno %d\n", errno);
-  }
-  wakeups_.store(0, std::memory_order_relaxed);
-}
-
-bool Waiter::Wait(KernelTimeout t) {
-  struct timespec abs_timeout;
-  if (t.has_timeout()) {
-    abs_timeout = t.MakeAbsTimespec();
-  }
-
-  // Loop until we timeout or consume a wakeup.
-  while (true) {
-    int x = wakeups_.load(std::memory_order_relaxed);
-    if (x != 0) {
-      if (!wakeups_.compare_exchange_weak(x, x - 1,
-                                          std::memory_order_acquire,
-                                          std::memory_order_relaxed)) {
-        continue;  // Raced with someone, retry.
-      }
-      // Successfully consumed a wakeup, we're done.
-      return true;
-    }
-
-    // Nothing to consume, wait (looping on EINTR).
-    while (true) {
-      if (!t.has_timeout()) {
-        if (sem_wait(&sem_) == 0) break;
-        if (errno == EINTR) continue;
-        ABSL_RAW_LOG(FATAL, "sem_wait failed: %d", errno);
-      } else {
-        if (sem_timedwait(&sem_, &abs_timeout) == 0) break;
-        if (errno == EINTR) continue;
-        if (errno == ETIMEDOUT) return false;
-        ABSL_RAW_LOG(FATAL, "sem_timedwait failed: %d", errno);
-      }
-    }
-    MaybeBecomeIdle();
-  }
-}
-
-void Waiter::Post() {
-  wakeups_.fetch_add(1, std::memory_order_release);  // Post a wakeup.
-  Poke();
-}
-
-void Waiter::Poke() {
-  if (sem_post(&sem_) != 0) {  // Wake any semaphore waiter.
-    ABSL_RAW_LOG(FATAL, "sem_post failed with errno %d\n", errno);
-  }
-}
-
-#elif ABSL_WAITER_MODE == ABSL_WAITER_MODE_WIN32
-
-class Waiter::WinHelper {
- public:
-  static SRWLOCK *GetLock(Waiter *w) {
-    return reinterpret_cast<SRWLOCK *>(&w->mu_storage_);
-  }
-
-  static CONDITION_VARIABLE *GetCond(Waiter *w) {
-    return reinterpret_cast<CONDITION_VARIABLE *>(&w->cv_storage_);
-  }
-
-  static_assert(sizeof(SRWLOCK) == sizeof(Waiter::SRWLockStorage),
-                "SRWLockStorage does not have the same size as SRWLOCK");
-  static_assert(
-      alignof(SRWLOCK) == alignof(Waiter::SRWLockStorage),
-      "SRWLockStorage does not have the same alignment as SRWLOCK");
-
-  static_assert(sizeof(CONDITION_VARIABLE) ==
-                    sizeof(Waiter::ConditionVariableStorage),
-                "ABSL_CONDITION_VARIABLE_STORAGE does not have the same size "
-                "as CONDITION_VARIABLE");
-  static_assert(alignof(CONDITION_VARIABLE) ==
-                    alignof(Waiter::ConditionVariableStorage),
-                "ConditionVariableStorage does not have the same "
-                "alignment as CONDITION_VARIABLE");
-
-  // The SRWLOCK and CONDITION_VARIABLE types must be trivially constructible
-  // and destructible because we never call their constructors or destructors.
-  static_assert(std::is_trivially_constructible<SRWLOCK>::value,
-                "The SRWLOCK type must be trivially constructible");
-  static_assert(std::is_trivially_constructible<CONDITION_VARIABLE>::value,
-                "The CONDITION_VARIABLE type must be trivially constructible");
-  static_assert(std::is_trivially_destructible<SRWLOCK>::value,
-                "The SRWLOCK type must be trivially destructible");
-  static_assert(std::is_trivially_destructible<CONDITION_VARIABLE>::value,
-                "The CONDITION_VARIABLE type must be trivially destructible");
-};
-
-class LockHolder {
- public:
-  explicit LockHolder(SRWLOCK* mu) : mu_(mu) {
-    AcquireSRWLockExclusive(mu_);
-  }
-
-  LockHolder(const LockHolder&) = delete;
-  LockHolder& operator=(const LockHolder&) = delete;
-
-  ~LockHolder() {
-    ReleaseSRWLockExclusive(mu_);
-  }
-
- private:
-  SRWLOCK* mu_;
-};
-
-void Waiter::Init() {
-  auto *mu = ::new (static_cast<void *>(&mu_storage_)) SRWLOCK;
-  auto *cv = ::new (static_cast<void *>(&cv_storage_)) CONDITION_VARIABLE;
-  InitializeSRWLock(mu);
-  InitializeConditionVariable(cv);
-  waiter_count_.store(0, std::memory_order_relaxed);
-  wakeup_count_.store(0, std::memory_order_relaxed);
-}
-
-bool Waiter::Wait(KernelTimeout t) {
-  SRWLOCK *mu = WinHelper::GetLock(this);
-  CONDITION_VARIABLE *cv = WinHelper::GetCond(this);
-
-  LockHolder h(mu);
-  waiter_count_.fetch_add(1, std::memory_order_relaxed);
-
-  // Loop until we find a wakeup to consume or timeout.
-  while (true) {
-    int x = wakeup_count_.load(std::memory_order_relaxed);
-    if (x != 0) {
-      if (!wakeup_count_.compare_exchange_weak(x, x - 1,
-                                               std::memory_order_acquire,
-                                               std::memory_order_relaxed)) {
-        continue;  // Raced with someone, retry.
-      }
-      // Successfully consumed a wakeup, we're done.
-      waiter_count_.fetch_sub(1, std::memory_order_relaxed);
-      return true;
-    }
-
-    // No wakeups available, time to wait.
-    if (!SleepConditionVariableSRW(cv, mu, t.InMillisecondsFromNow(), 0)) {
-      // GetLastError() returns a Win32 DWORD, but we assign to
-      // unsigned long to simplify the ABSL_RAW_LOG case below.  The uniform
-      // initialization guarantees this is not a narrowing conversion.
-      const unsigned long err{GetLastError()};  // NOLINT(runtime/int)
-      if (err == ERROR_TIMEOUT) {
-        waiter_count_.fetch_sub(1, std::memory_order_relaxed);
-        return false;
-      } else {
-        ABSL_RAW_LOG(FATAL, "SleepConditionVariableSRW failed: %lu", err);
-      }
-    }
-
-    MaybeBecomeIdle();
-  }
-}
-
-void Waiter::Post() {
-  wakeup_count_.fetch_add(1, std::memory_order_release);
-  Poke();
-}
-
-void Waiter::Poke() {
-  if (waiter_count_.load(std::memory_order_relaxed) == 0) {
-    return;
-  }
-  // Potentially a waker. Take the lock and check again.
-  LockHolder h(WinHelper::GetLock(this));
-  if (waiter_count_.load(std::memory_order_relaxed) == 0) {
-    return;
-  }
-  WakeConditionVariable(WinHelper::GetCond(this));
-}
-
-#else
-#error Unknown ABSL_WAITER_MODE
-#endif
-
-}  // namespace synchronization_internal
-}  // namespace absl
diff --git a/absl/synchronization/internal/waiter.h b/absl/synchronization/internal/waiter.h
index a3e3124e441..6ba204bea0a 100644
--- a/absl/synchronization/internal/waiter.h
+++ b/absl/synchronization/internal/waiter.h
@@ -17,133 +17,53 @@
 #define ABSL_SYNCHRONIZATION_INTERNAL_WAITER_H_
 
 #include "absl/base/config.h"
-
-#ifndef _WIN32
-#include <pthread.h>
-#endif
-
-#ifdef ABSL_HAVE_SEMAPHORE_H
-#include <semaphore.h>
-#endif
-
-#include <atomic>
-#include <cstdint>
-
-#include "absl/base/internal/thread_identity.h"
-#include "absl/synchronization/internal/kernel_timeout.h"
+#include "absl/synchronization/internal/futex_waiter.h"
+#include "absl/synchronization/internal/pthread_waiter.h"
+#include "absl/synchronization/internal/sem_waiter.h"
+#include "absl/synchronization/internal/stdcpp_waiter.h"
+#include "absl/synchronization/internal/win32_waiter.h"
 
 // May be chosen at compile time via -DABSL_FORCE_WAITER_MODE=<index>
 #define ABSL_WAITER_MODE_FUTEX 0
 #define ABSL_WAITER_MODE_SEM 1
 #define ABSL_WAITER_MODE_CONDVAR 2
 #define ABSL_WAITER_MODE_WIN32 3
+#define ABSL_WAITER_MODE_STDCPP 4
 
 #if defined(ABSL_FORCE_WAITER_MODE)
 #define ABSL_WAITER_MODE ABSL_FORCE_WAITER_MODE
-#elif defined(_WIN32)
+#elif defined(ABSL_INTERNAL_HAVE_WIN32_WAITER)
 #define ABSL_WAITER_MODE ABSL_WAITER_MODE_WIN32
-#elif defined(__linux__)
+#elif defined(ABSL_INTERNAL_HAVE_FUTEX_WAITER)
 #define ABSL_WAITER_MODE ABSL_WAITER_MODE_FUTEX
-#elif defined(ABSL_HAVE_SEMAPHORE_H)
+#elif defined(ABSL_INTERNAL_HAVE_SEM_WAITER)
 #define ABSL_WAITER_MODE ABSL_WAITER_MODE_SEM
-#else
+#elif defined(ABSL_INTERNAL_HAVE_PTHREAD_WAITER)
 #define ABSL_WAITER_MODE ABSL_WAITER_MODE_CONDVAR
+#elif defined(ABSL_INTERNAL_HAVE_STDCPP_WAITER)
+#define ABSL_WAITER_MODE ABSL_WAITER_MODE_STDCPP
+#else
+#error ABSL_WAITER_MODE is undefined
 #endif
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace synchronization_internal {
 
-// Waiter is an OS-specific semaphore.
-class Waiter {
- public:
-  // No constructor, instances use the reserved space in ThreadIdentity.
-  // All initialization logic belongs in `Init()`.
-  Waiter() = delete;
-  Waiter(const Waiter&) = delete;
-  Waiter& operator=(const Waiter&) = delete;
-
-  // Prepare any data to track waits.
-  void Init();
-
-  // Blocks the calling thread until a matching call to `Post()` or
-  // `t` has passed. Returns `true` if woken (`Post()` called),
-  // `false` on timeout.
-  bool Wait(KernelTimeout t);
-
-  // Restart the caller of `Wait()` as with a normal semaphore.
-  void Post();
-
-  // If anyone is waiting, wake them up temporarily and cause them to
-  // call `MaybeBecomeIdle()`. They will then return to waiting for a
-  // `Post()` or timeout.
-  void Poke();
-
-  // Returns the Waiter associated with the identity.
-  static Waiter* GetWaiter(base_internal::ThreadIdentity* identity) {
-    static_assert(
-        sizeof(Waiter) <= sizeof(base_internal::ThreadIdentity::WaiterState),
-        "Insufficient space for Waiter");
-    return reinterpret_cast<Waiter*>(identity->waiter_state.data);
-  }
-
-  // How many periods to remain idle before releasing resources
-#ifndef THREAD_SANITIZER
-  static const int kIdlePeriods = 60;
-#else
-  // Memory consumption under ThreadSanitizer is a serious concern,
-  // so we release resources sooner. The value of 1 leads to 1 to 2 second
-  // delay before marking a thread as idle.
-  static const int kIdlePeriods = 1;
-#endif
-
- private:
 #if ABSL_WAITER_MODE == ABSL_WAITER_MODE_FUTEX
-  // Futexes are defined by specification to be 32-bits.
-  // Thus std::atomic<int32_t> must be just an int32_t with lockfree methods.
-  std::atomic<int32_t> futex_;
-  static_assert(sizeof(int32_t) == sizeof(futex_), "Wrong size for futex");
-
-#elif ABSL_WAITER_MODE == ABSL_WAITER_MODE_CONDVAR
-  pthread_mutex_t mu_;
-  pthread_cond_t cv_;
-  std::atomic<int> waiter_count_;
-  std::atomic<int> wakeup_count_;  // Unclaimed wakeups, written under lock.
-
+using Waiter = FutexWaiter;
 #elif ABSL_WAITER_MODE == ABSL_WAITER_MODE_SEM
-  sem_t sem_;
-  // This seems superfluous, but for Poke() we need to cause spurious
-  // wakeups on the semaphore. Hence we can't actually use the
-  // semaphore's count.
-  std::atomic<int> wakeups_;
-
+using Waiter = SemWaiter;
+#elif ABSL_WAITER_MODE == ABSL_WAITER_MODE_CONDVAR
+using Waiter = PthreadWaiter;
 #elif ABSL_WAITER_MODE == ABSL_WAITER_MODE_WIN32
-  // The Windows API has lots of choices for synchronization
-  // primivitives.  We are using SRWLOCK and CONDITION_VARIABLE
-  // because they don't require a destructor to release system
-  // resources.
-  //
-  // However, we can't include Windows.h in our headers, so we use aligned
-  // storage buffers to define the storage.
-  using SRWLockStorage =
-      typename std::aligned_storage<sizeof(void*), alignof(void*)>::type;
-  using ConditionVariableStorage =
-      typename std::aligned_storage<sizeof(void*), alignof(void*)>::type;
-
-  // WinHelper - Used to define utilities for accessing the lock and
-  // condition variable storage once the types are complete.
-  class WinHelper;
-
-  SRWLockStorage mu_storage_;
-  ConditionVariableStorage cv_storage_;
-  std::atomic<int> waiter_count_;
-  std::atomic<int> wakeup_count_;
-
-#else
-  #error Unknown ABSL_WAITER_MODE
+using Waiter = Win32Waiter;
+#elif ABSL_WAITER_MODE == ABSL_WAITER_MODE_STDCPP
+using Waiter = StdcppWaiter;
 #endif
-};
 
 }  // namespace synchronization_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_SYNCHRONIZATION_INTERNAL_WAITER_H_
diff --git a/absl/synchronization/internal/waiter_base.cc b/absl/synchronization/internal/waiter_base.cc
new file mode 100644
index 00000000000..011de18a8ff
--- /dev/null
+++ b/absl/synchronization/internal/waiter_base.cc
@@ -0,0 +1,41 @@
+// Copyright 2023 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/synchronization/internal/waiter_base.h"
+
+#include <atomic>
+#include <cassert>
+
+#include "absl/base/config.h"
+#include "absl/base/internal/thread_identity.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace synchronization_internal {
+
+void WaiterBase::MaybeBecomeIdle() {
+  base_internal::ThreadIdentity *identity =
+      base_internal::CurrentThreadIdentityIfPresent();
+  assert(identity != nullptr);
+  const bool is_idle = identity->is_idle.load(std::memory_order_relaxed);
+  const int ticker = identity->ticker.load(std::memory_order_relaxed);
+  const int wait_start = identity->wait_start.load(std::memory_order_relaxed);
+  if (!is_idle && ticker - wait_start > kIdlePeriods) {
+    identity->is_idle.store(true, std::memory_order_relaxed);
+  }
+}
+
+}  // namespace synchronization_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/synchronization/internal/waiter_base.h b/absl/synchronization/internal/waiter_base.h
new file mode 100644
index 00000000000..cf175481937
--- /dev/null
+++ b/absl/synchronization/internal/waiter_base.h
@@ -0,0 +1,90 @@
+// Copyright 2023 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+#ifndef ABSL_SYNCHRONIZATION_INTERNAL_WAITER_BASE_H_
+#define ABSL_SYNCHRONIZATION_INTERNAL_WAITER_BASE_H_
+
+#include "absl/base/config.h"
+#include "absl/base/internal/thread_identity.h"
+#include "absl/synchronization/internal/kernel_timeout.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace synchronization_internal {
+
+// `Waiter` is a platform specific semaphore implementation that `PerThreadSem`
+// waits on to implement blocking in `absl::Mutex`.  Implementations should
+// inherit from `WaiterCrtp` and must implement `Wait()`, `Post()`, and `Poke()`
+// as described in `WaiterBase`.  `waiter.h` selects the implementation and uses
+// static-dispatch for performance.
+class WaiterBase {
+ public:
+  WaiterBase() = default;
+
+  // Not copyable or movable
+  WaiterBase(const WaiterBase&) = delete;
+  WaiterBase& operator=(const WaiterBase&) = delete;
+
+  // Blocks the calling thread until a matching call to `Post()` or
+  // `t` has passed. Returns `true` if woken (`Post()` called),
+  // `false` on timeout.
+  //
+  // bool Wait(KernelTimeout t);
+
+  // Restart the caller of `Wait()` as with a normal semaphore.
+  //
+  // void Post();
+
+  // If anyone is waiting, wake them up temporarily and cause them to
+  // call `MaybeBecomeIdle()`. They will then return to waiting for a
+  // `Post()` or timeout.
+  //
+  // void Poke();
+
+  // Returns the name of this implementation. Used only for debugging.
+  //
+  // static constexpr char kName[];
+
+  // How many periods to remain idle before releasing resources
+#ifndef ABSL_HAVE_THREAD_SANITIZER
+  static constexpr int kIdlePeriods = 60;
+#else
+  // Memory consumption under ThreadSanitizer is a serious concern,
+  // so we release resources sooner. The value of 1 leads to 1 to 2 second
+  // delay before marking a thread as idle.
+  static constexpr int kIdlePeriods = 1;
+#endif
+
+ protected:
+  static void MaybeBecomeIdle();
+};
+
+template <typename T>
+class WaiterCrtp : public WaiterBase {
+ public:
+  // Returns the Waiter associated with the identity.
+  static T* GetWaiter(base_internal::ThreadIdentity* identity) {
+    static_assert(
+        sizeof(T) <= sizeof(base_internal::ThreadIdentity::WaiterState),
+        "Insufficient space for Waiter");
+    return reinterpret_cast<T*>(identity->waiter_state.data);
+  }
+};
+
+}  // namespace synchronization_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_SYNCHRONIZATION_INTERNAL_WAITER_BASE_H_
diff --git a/absl/synchronization/internal/waiter_test.cc b/absl/synchronization/internal/waiter_test.cc
new file mode 100644
index 00000000000..44f0d6fa89c
--- /dev/null
+++ b/absl/synchronization/internal/waiter_test.cc
@@ -0,0 +1,190 @@
+// Copyright 2023 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/synchronization/internal/waiter.h"
+
+#include <ctime>
+#include <iostream>
+#include <ostream>
+
+#include "gtest/gtest.h"
+#include "absl/random/random.h"
+#include "absl/synchronization/internal/create_thread_identity.h"
+#include "absl/synchronization/internal/futex_waiter.h"
+#include "absl/synchronization/internal/kernel_timeout.h"
+#include "absl/synchronization/internal/pthread_waiter.h"
+#include "absl/synchronization/internal/sem_waiter.h"
+#include "absl/synchronization/internal/stdcpp_waiter.h"
+#include "absl/synchronization/internal/thread_pool.h"
+#include "absl/time/clock.h"
+#include "absl/time/time.h"
+
+// Test go/btm support by randomizing the value of clock_gettime() for
+// CLOCK_MONOTONIC. This works by overriding a weak symbol in glibc.
+// We should be resistant to this randomization when !SupportsSteadyClock().
+#if defined(__GOOGLE_GRTE_VERSION__) &&      \
+    !defined(ABSL_HAVE_ADDRESS_SANITIZER) && \
+    !defined(ABSL_HAVE_MEMORY_SANITIZER) &&  \
+    !defined(ABSL_HAVE_THREAD_SANITIZER)
+extern "C" int __clock_gettime(clockid_t c, struct timespec* ts);
+
+extern "C" int clock_gettime(clockid_t c, struct timespec* ts) {
+  if (c == CLOCK_MONOTONIC &&
+      !absl::synchronization_internal::KernelTimeout::SupportsSteadyClock()) {
+    thread_local absl::BitGen gen;  // NOLINT
+    ts->tv_sec = absl::Uniform(gen, 0, 1'000'000'000);
+    ts->tv_nsec = absl::Uniform(gen, 0, 1'000'000'000);
+    return 0;
+  }
+  return __clock_gettime(c, ts);
+}
+#endif
+
+namespace {
+
+TEST(Waiter, PrintPlatformImplementation) {
+  // Allows us to verify that the platform is using the expected implementation.
+  std::cout << absl::synchronization_internal::Waiter::kName << std::endl;
+}
+
+template <typename T>
+class WaiterTest : public ::testing::Test {
+ public:
+  // Waiter implementations assume that a ThreadIdentity has already been
+  // created.
+  WaiterTest() {
+    absl::synchronization_internal::GetOrCreateCurrentThreadIdentity();
+  }
+};
+
+TYPED_TEST_SUITE_P(WaiterTest);
+
+absl::Duration WithTolerance(absl::Duration d) { return d * 0.95; }
+
+TYPED_TEST_P(WaiterTest, WaitNoTimeout) {
+  absl::synchronization_internal::ThreadPool tp(1);
+  TypeParam waiter;
+  tp.Schedule([&]() {
+    // Include some `Poke()` calls to ensure they don't cause `waiter` to return
+    // from `Wait()`.
+    waiter.Poke();
+    absl::SleepFor(absl::Seconds(1));
+    waiter.Poke();
+    absl::SleepFor(absl::Seconds(1));
+    waiter.Post();
+  });
+  absl::Time start = absl::Now();
+  EXPECT_TRUE(
+      waiter.Wait(absl::synchronization_internal::KernelTimeout::Never()));
+  absl::Duration waited = absl::Now() - start;
+  EXPECT_GE(waited, WithTolerance(absl::Seconds(2)));
+}
+
+TYPED_TEST_P(WaiterTest, WaitDurationWoken) {
+  absl::synchronization_internal::ThreadPool tp(1);
+  TypeParam waiter;
+  tp.Schedule([&]() {
+    // Include some `Poke()` calls to ensure they don't cause `waiter` to return
+    // from `Wait()`.
+    waiter.Poke();
+    absl::SleepFor(absl::Milliseconds(500));
+    waiter.Post();
+  });
+  absl::Time start = absl::Now();
+  EXPECT_TRUE(waiter.Wait(
+      absl::synchronization_internal::KernelTimeout(absl::Seconds(10))));
+  absl::Duration waited = absl::Now() - start;
+  EXPECT_GE(waited, WithTolerance(absl::Milliseconds(500)));
+#ifndef _MSC_VER
+  // Skip on MSVC due to flakiness.
+  EXPECT_LT(waited, absl::Seconds(2));
+#endif
+}
+
+TYPED_TEST_P(WaiterTest, WaitTimeWoken) {
+  absl::synchronization_internal::ThreadPool tp(1);
+  TypeParam waiter;
+  tp.Schedule([&]() {
+    // Include some `Poke()` calls to ensure they don't cause `waiter` to return
+    // from `Wait()`.
+    waiter.Poke();
+    absl::SleepFor(absl::Milliseconds(500));
+    waiter.Post();
+  });
+  absl::Time start = absl::Now();
+  EXPECT_TRUE(waiter.Wait(absl::synchronization_internal::KernelTimeout(
+      start + absl::Seconds(10))));
+  absl::Duration waited = absl::Now() - start;
+  EXPECT_GE(waited, WithTolerance(absl::Milliseconds(500)));
+  #ifndef _MSC_VER
+    // Skip on MSVC due to flakiness.
+    EXPECT_LT(waited, absl::Seconds(2));
+  #endif
+}
+
+TYPED_TEST_P(WaiterTest, WaitDurationReached) {
+  TypeParam waiter;
+  absl::Time start = absl::Now();
+  EXPECT_FALSE(waiter.Wait(
+      absl::synchronization_internal::KernelTimeout(absl::Milliseconds(500))));
+  absl::Duration waited = absl::Now() - start;
+  EXPECT_GE(waited, WithTolerance(absl::Milliseconds(500)));
+  #ifndef _MSC_VER
+    // Skip on MSVC due to flakiness.
+    EXPECT_LT(waited, absl::Seconds(1));
+  #endif
+}
+
+TYPED_TEST_P(WaiterTest, WaitTimeReached) {
+  TypeParam waiter;
+  absl::Time start = absl::Now();
+  EXPECT_FALSE(waiter.Wait(absl::synchronization_internal::KernelTimeout(
+      start + absl::Milliseconds(500))));
+  absl::Duration waited = absl::Now() - start;
+  EXPECT_GE(waited, WithTolerance(absl::Milliseconds(500)));
+  #ifndef _MSC_VER
+    // Skip on MSVC due to flakiness.
+    EXPECT_LT(waited, absl::Seconds(1));
+  #endif
+}
+
+REGISTER_TYPED_TEST_SUITE_P(WaiterTest,
+                            WaitNoTimeout,
+                            WaitDurationWoken,
+                            WaitTimeWoken,
+                            WaitDurationReached,
+                            WaitTimeReached);
+
+#ifdef ABSL_INTERNAL_HAVE_FUTEX_WAITER
+INSTANTIATE_TYPED_TEST_SUITE_P(Futex, WaiterTest,
+                               absl::synchronization_internal::FutexWaiter);
+#endif
+#ifdef ABSL_INTERNAL_HAVE_PTHREAD_WAITER
+INSTANTIATE_TYPED_TEST_SUITE_P(Pthread, WaiterTest,
+                               absl::synchronization_internal::PthreadWaiter);
+#endif
+#ifdef ABSL_INTERNAL_HAVE_SEM_WAITER
+INSTANTIATE_TYPED_TEST_SUITE_P(Sem, WaiterTest,
+                               absl::synchronization_internal::SemWaiter);
+#endif
+#ifdef ABSL_INTERNAL_HAVE_WIN32_WAITER
+INSTANTIATE_TYPED_TEST_SUITE_P(Win32, WaiterTest,
+                               absl::synchronization_internal::Win32Waiter);
+#endif
+#ifdef ABSL_INTERNAL_HAVE_STDCPP_WAITER
+INSTANTIATE_TYPED_TEST_SUITE_P(Stdcpp, WaiterTest,
+                               absl::synchronization_internal::StdcppWaiter);
+#endif
+
+}  // namespace
diff --git a/absl/synchronization/internal/win32_waiter.cc b/absl/synchronization/internal/win32_waiter.cc
new file mode 100644
index 00000000000..eade1ca5cbf
--- /dev/null
+++ b/absl/synchronization/internal/win32_waiter.cc
@@ -0,0 +1,147 @@
+// Copyright 2023 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/synchronization/internal/win32_waiter.h"
+
+#ifdef ABSL_INTERNAL_HAVE_WIN32_WAITER
+
+#include <windows.h>
+
+#include "absl/base/config.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/base/internal/thread_identity.h"
+#include "absl/base/optimization.h"
+#include "absl/synchronization/internal/kernel_timeout.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace synchronization_internal {
+
+class Win32Waiter::WinHelper {
+ public:
+  static SRWLOCK *GetLock(Win32Waiter *w) {
+    return reinterpret_cast<SRWLOCK *>(&w->mu_storage_);
+  }
+
+  static CONDITION_VARIABLE *GetCond(Win32Waiter *w) {
+    return reinterpret_cast<CONDITION_VARIABLE *>(&w->cv_storage_);
+  }
+
+  static_assert(sizeof(SRWLOCK) == sizeof(void *),
+                "`mu_storage_` does not have the same size as SRWLOCK");
+  static_assert(alignof(SRWLOCK) == alignof(void *),
+                "`mu_storage_` does not have the same alignment as SRWLOCK");
+
+  static_assert(sizeof(CONDITION_VARIABLE) == sizeof(void *),
+                "`ABSL_CONDITION_VARIABLE_STORAGE` does not have the same size "
+                "as `CONDITION_VARIABLE`");
+  static_assert(
+      alignof(CONDITION_VARIABLE) == alignof(void *),
+      "`cv_storage_` does not have the same alignment as `CONDITION_VARIABLE`");
+
+  // The SRWLOCK and CONDITION_VARIABLE types must be trivially constructible
+  // and destructible because we never call their constructors or destructors.
+  static_assert(std::is_trivially_constructible_v<SRWLOCK>,
+                "The `SRWLOCK` type must be trivially constructible");
+  static_assert(
+      std::is_trivially_constructible_v<CONDITION_VARIABLE>,
+      "The `CONDITION_VARIABLE` type must be trivially constructible");
+  static_assert(std::is_trivially_destructible_v<SRWLOCK>,
+                "The `SRWLOCK` type must be trivially destructible");
+  static_assert(std::is_trivially_destructible_v<CONDITION_VARIABLE>,
+                "The `CONDITION_VARIABLE` type must be trivially destructible");
+};
+
+class LockHolder {
+ public:
+  explicit LockHolder(SRWLOCK* mu) : mu_(mu) {
+    AcquireSRWLockExclusive(mu_);
+  }
+
+  LockHolder(const LockHolder&) = delete;
+  LockHolder& operator=(const LockHolder&) = delete;
+
+  ~LockHolder() {
+    ReleaseSRWLockExclusive(mu_);
+  }
+
+ private:
+  SRWLOCK* mu_;
+};
+
+Win32Waiter::Win32Waiter() {
+  auto *mu = ::new (static_cast<void *>(&mu_storage_)) SRWLOCK;
+  auto *cv = ::new (static_cast<void *>(&cv_storage_)) CONDITION_VARIABLE;
+  InitializeSRWLock(mu);
+  InitializeConditionVariable(cv);
+  waiter_count_ = 0;
+  wakeup_count_ = 0;
+}
+
+bool Win32Waiter::Wait(KernelTimeout t) {
+  SRWLOCK *mu = WinHelper::GetLock(this);
+  CONDITION_VARIABLE *cv = WinHelper::GetCond(this);
+
+  LockHolder h(mu);
+  ++waiter_count_;
+
+  // Loop until we find a wakeup to consume or timeout.
+  // Note that, since the thread ticker is just reset, we don't need to check
+  // whether the thread is idle on the very first pass of the loop.
+  bool first_pass = true;
+  while (wakeup_count_ == 0) {
+    if (!first_pass) MaybeBecomeIdle();
+    // No wakeups available, time to wait.
+    if (!SleepConditionVariableSRW(cv, mu, t.InMillisecondsFromNow(), 0)) {
+      // GetLastError() returns a Win32 DWORD, but we assign to
+      // unsigned long to simplify the ABSL_RAW_LOG case below.  The uniform
+      // initialization guarantees this is not a narrowing conversion.
+      const unsigned long err{GetLastError()};  // NOLINT(runtime/int)
+      if (err == ERROR_TIMEOUT) {
+        --waiter_count_;
+        return false;
+      } else {
+        ABSL_RAW_LOG(FATAL, "SleepConditionVariableSRW failed: %lu", err);
+      }
+    }
+    first_pass = false;
+  }
+  // Consume a wakeup and we're done.
+  --wakeup_count_;
+  --waiter_count_;
+  return true;
+}
+
+void Win32Waiter::Post() {
+  LockHolder h(WinHelper::GetLock(this));
+  ++wakeup_count_;
+  InternalCondVarPoke();
+}
+
+void Win32Waiter::Poke() {
+  LockHolder h(WinHelper::GetLock(this));
+  InternalCondVarPoke();
+}
+
+void Win32Waiter::InternalCondVarPoke() {
+  if (waiter_count_ != 0) {
+    WakeConditionVariable(WinHelper::GetCond(this));
+  }
+}
+
+}  // namespace synchronization_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_INTERNAL_HAVE_WIN32_WAITER
diff --git a/absl/synchronization/internal/win32_waiter.h b/absl/synchronization/internal/win32_waiter.h
new file mode 100644
index 00000000000..fdab264e8cb
--- /dev/null
+++ b/absl/synchronization/internal/win32_waiter.h
@@ -0,0 +1,72 @@
+// Copyright 2023 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+#ifndef ABSL_SYNCHRONIZATION_INTERNAL_WIN32_WAITER_H_
+#define ABSL_SYNCHRONIZATION_INTERNAL_WIN32_WAITER_H_
+
+#ifdef _WIN32
+#include <sdkddkver.h>
+#endif
+
+#if defined(_WIN32) && !defined(__MINGW32__) && \
+    _WIN32_WINNT >= _WIN32_WINNT_VISTA
+
+#include "absl/base/config.h"
+#include "absl/synchronization/internal/kernel_timeout.h"
+#include "absl/synchronization/internal/waiter_base.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace synchronization_internal {
+
+#define ABSL_INTERNAL_HAVE_WIN32_WAITER 1
+
+class Win32Waiter : public WaiterCrtp<Win32Waiter> {
+ public:
+  Win32Waiter();
+
+  bool Wait(KernelTimeout t);
+  void Post();
+  void Poke();
+
+  static constexpr char kName[] = "Win32Waiter";
+
+ private:
+  // WinHelper - Used to define utilities for accessing the lock and
+  // condition variable storage once the types are complete.
+  class WinHelper;
+
+  // REQUIRES: WinHelper::GetLock(this) must be held.
+  void InternalCondVarPoke();
+
+  // We can't include Windows.h in our headers, so we use aligned character
+  // buffers to define the storage of SRWLOCK and CONDITION_VARIABLE.
+  // SRW locks and condition variables do not need to be explicitly destroyed.
+  // https://docs.microsoft.com/en-us/windows/win32/api/synchapi/nf-synchapi-initializesrwlock
+  // https://stackoverflow.com/questions/28975958/why-does-windows-have-no-deleteconditionvariable-function-to-go-together-with
+  alignas(void*) unsigned char mu_storage_[sizeof(void*)];
+  alignas(void*) unsigned char cv_storage_[sizeof(void*)];
+  int waiter_count_;
+  int wakeup_count_;
+};
+
+}  // namespace synchronization_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // defined(_WIN32) && !defined(__MINGW32__) &&
+        // _WIN32_WINNT >= _WIN32_WINNT_VISTA
+
+#endif  // ABSL_SYNCHRONIZATION_INTERNAL_WIN32_WAITER_H_
diff --git a/absl/synchronization/lifetime_test.cc b/absl/synchronization/lifetime_test.cc
index 34b8875b4b7..a659bcfbba6 100644
--- a/absl/synchronization/lifetime_test.cc
+++ b/absl/synchronization/lifetime_test.cc
@@ -14,12 +14,11 @@
 
 #include <cstdlib>
 #include <thread>  // NOLINT(build/c++11), Abseil test
-#include <type_traits>
 
 #include "absl/base/attributes.h"
 #include "absl/base/const_init.h"
-#include "absl/base/internal/raw_logging.h"
 #include "absl/base/thread_annotations.h"
+#include "absl/log/check.h"
 #include "absl/synchronization/mutex.h"
 #include "absl/synchronization/notification.h"
 
@@ -35,20 +34,20 @@ namespace {
 // Thread two waits on 'notification', then sets 'state' inside the 'mutex',
 // signalling the change via 'condvar'.
 //
-// These tests use ABSL_RAW_CHECK to validate invariants, rather than EXPECT or
-// ASSERT from gUnit, because we need to invoke them during global destructors,
-// when gUnit teardown would have already begun.
+// These tests use CHECK to validate invariants, rather than EXPECT or ASSERT
+// from gUnit, because we need to invoke them during global destructors, when
+// gUnit teardown would have already begun.
 void ThreadOne(absl::Mutex* mutex, absl::CondVar* condvar,
                absl::Notification* notification, bool* state) {
   // Test that the notification is in a valid initial state.
-  ABSL_RAW_CHECK(!notification->HasBeenNotified(), "invalid Notification");
-  ABSL_RAW_CHECK(*state == false, "*state not initialized");
+  CHECK(!notification->HasBeenNotified()) << "invalid Notification";
+  CHECK(!*state) << "*state not initialized";
 
   {
-    absl::MutexLock lock(mutex);
+    absl::MutexLock lock(*mutex);
 
     notification->Notify();
-    ABSL_RAW_CHECK(notification->HasBeenNotified(), "invalid Notification");
+    CHECK(notification->HasBeenNotified()) << "invalid Notification";
 
     while (*state == false) {
       condvar->Wait(mutex);
@@ -58,13 +57,13 @@ void ThreadOne(absl::Mutex* mutex, absl::CondVar* condvar,
 
 void ThreadTwo(absl::Mutex* mutex, absl::CondVar* condvar,
                absl::Notification* notification, bool* state) {
-  ABSL_RAW_CHECK(*state == false, "*state not initialized");
+  CHECK(!*state) << "*state not initialized";
 
   // Wake thread one
   notification->WaitForNotification();
-  ABSL_RAW_CHECK(notification->HasBeenNotified(), "invalid Notification");
+  CHECK(notification->HasBeenNotified()) << "invalid Notification";
   {
-    absl::MutexLock lock(mutex);
+    absl::MutexLock lock(*mutex);
     *state = true;
     condvar->Signal();
   }
@@ -122,6 +121,10 @@ class OnDestruction {
   Function fn_;
 };
 
+// These tests require that the compiler correctly supports C++11 constant
+// initialization... but MSVC has a known regression (since v19.10) till v19.25:
+// https://developercommunity.visualstudio.com/content/problem/336946/class-with-constexpr-constructor-not-using-static.html
+#if defined(__clang__) || !(defined(_MSC_VER) && _MSC_VER < 1925)
 // kConstInit
 // Test early usage.  (Declaration comes first; definitions must appear after
 // the test runner.)
@@ -144,13 +147,14 @@ ABSL_CONST_INIT absl::Mutex early_const_init_mutex(absl::kConstInit);
 // before the constructors of either grab_lock or check_still_locked are run.)
 extern absl::Mutex const_init_sanity_mutex;
 OnConstruction grab_lock([]() ABSL_NO_THREAD_SAFETY_ANALYSIS {
-  const_init_sanity_mutex.Lock();
+  const_init_sanity_mutex.lock();
 });
 ABSL_CONST_INIT absl::Mutex const_init_sanity_mutex(absl::kConstInit);
 OnConstruction check_still_locked([]() ABSL_NO_THREAD_SAFETY_ANALYSIS {
   const_init_sanity_mutex.AssertHeld();
-  const_init_sanity_mutex.Unlock();
+  const_init_sanity_mutex.unlock();
 });
+#endif  // defined(__clang__) || !(defined(_MSC_VER) && _MSC_VER > 1900)
 
 // Test shutdown usage.  (Declarations come first; definitions must appear after
 // the test runner.)
diff --git a/absl/synchronization/mutex.cc b/absl/synchronization/mutex.cc
index 100def2dd83..68782564443 100644
--- a/absl/synchronization/mutex.cc
+++ b/absl/synchronization/mutex.cc
@@ -14,20 +14,7 @@
 
 #include "absl/synchronization/mutex.h"
 
-#ifdef _WIN32
-#include <windows.h>
-#ifdef ERROR
-#undef ERROR
-#endif
-#else
-#include <fcntl.h>
-#include <pthread.h>
-#include <sched.h>
-#include <sys/time.h>
-#endif
-
 #include <assert.h>
-#include <errno.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -35,29 +22,56 @@
 
 #include <algorithm>
 #include <atomic>
-#include <cinttypes>
+#include <cstddef>
+#include <cstdint>
+#include <cstdlib>
+#include <cstring>
+#include <iterator>
 #include <thread>  // NOLINT(build/c++11)
 
 #include "absl/base/attributes.h"
+#include "absl/base/call_once.h"
 #include "absl/base/config.h"
 #include "absl/base/dynamic_annotations.h"
 #include "absl/base/internal/atomic_hook.h"
 #include "absl/base/internal/cycleclock.h"
 #include "absl/base/internal/hide_ptr.h"
 #include "absl/base/internal/low_level_alloc.h"
+#include "absl/base/internal/low_level_scheduling.h"
 #include "absl/base/internal/raw_logging.h"
+#include "absl/base/internal/scheduling_mode.h"
 #include "absl/base/internal/spinlock.h"
 #include "absl/base/internal/sysinfo.h"
 #include "absl/base/internal/thread_identity.h"
-#include "absl/base/port.h"
+#include "absl/base/internal/tsan_mutex_interface.h"
+#include "absl/base/macros.h"
+#include "absl/base/optimization.h"
+#include "absl/base/thread_annotations.h"
 #include "absl/debugging/stacktrace.h"
 #include "absl/debugging/symbolize.h"
+#include "absl/synchronization/internal/create_thread_identity.h"
 #include "absl/synchronization/internal/graphcycles.h"
+#include "absl/synchronization/internal/kernel_timeout.h"
 #include "absl/synchronization/internal/per_thread_sem.h"
+#include "absl/time/clock.h"
 #include "absl/time/time.h"
 
+#ifdef _WIN32
+#include <windows.h>
+#ifdef ERROR
+#undef ERROR
+#endif
+#else
+#include <fcntl.h>
+#include <pthread.h>
+#include <sched.h>
+#include <sys/time.h>
+#endif
+
 using absl::base_internal::CurrentThreadIdentityIfPresent;
+using absl::base_internal::CycleClock;
 using absl::base_internal::PerThreadSynch;
+using absl::base_internal::SchedulingGuard;
 using absl::base_internal::ThreadIdentity;
 using absl::synchronization_internal::GetOrCreateCurrentThreadIdentity;
 using absl::synchronization_internal::GraphCycles;
@@ -67,14 +81,17 @@ using absl::synchronization_internal::KernelTimeout;
 using absl::synchronization_internal::PerThreadSem;
 
 extern "C" {
-ABSL_ATTRIBUTE_WEAK void AbslInternalMutexYield() { std::this_thread::yield(); }
+ABSL_ATTRIBUTE_WEAK void ABSL_INTERNAL_C_SYMBOL(AbslInternalMutexYield)() {
+  std::this_thread::yield();
+}
 }  // extern "C"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 namespace {
 
-#if defined(THREAD_SANITIZER)
+#if defined(ABSL_HAVE_THREAD_SANITIZER)
 constexpr OnDeadlockCycle kDeadlockDetectionDefault = OnDeadlockCycle::kIgnore;
 #else
 constexpr OnDeadlockCycle kDeadlockDetectionDefault = OnDeadlockCycle::kAbort;
@@ -84,131 +101,144 @@ ABSL_CONST_INIT std::atomic<OnDeadlockCycle> synch_deadlock_detection(
     kDeadlockDetectionDefault);
 ABSL_CONST_INIT std::atomic<bool> synch_check_invariants(false);
 
-// ------------------------------------------ spinlock support
-
-// Make sure read-only globals used in the Mutex code are contained on the
-// same cacheline and cacheline aligned to eliminate any false sharing with
-// other globals from this and other modules.
-static struct MutexGlobals {
-  MutexGlobals() {
-    // Find machine-specific data needed for Delay() and
-    // TryAcquireWithSpinning(). This runs in the global constructor
-    // sequence, and before that zeros are safe values.
-    num_cpus = absl::base_internal::NumCPUs();
-    spinloop_iterations = num_cpus > 1 ? 1500 : 0;
-  }
-  int num_cpus;
-  int spinloop_iterations;
-  // Pad this struct to a full cacheline to prevent false sharing.
-  char padding[ABSL_CACHELINE_SIZE - 2 * sizeof(int)];
-} ABSL_CACHELINE_ALIGNED mutex_globals;
-static_assert(
-    sizeof(MutexGlobals) == ABSL_CACHELINE_SIZE,
-    "MutexGlobals must occupy an entire cacheline to prevent false sharing");
-
-ABSL_CONST_INIT absl::base_internal::AtomicHook<void (*)(int64_t wait_cycles)>
+ABSL_INTERNAL_ATOMIC_HOOK_ATTRIBUTES
+absl::base_internal::AtomicHook<void (*)(int64_t wait_cycles)>
     submit_profile_data;
-ABSL_CONST_INIT absl::base_internal::AtomicHook<
-    void (*)(const char *msg, const void *obj, int64_t wait_cycles)> mutex_tracer;
-ABSL_CONST_INIT absl::base_internal::AtomicHook<
-    void (*)(const char *msg, const void *cv)> cond_var_tracer;
-ABSL_CONST_INIT absl::base_internal::AtomicHook<
-    bool (*)(const void *pc, char *out, int out_size)>
-    symbolizer(absl::Symbolize);
+ABSL_INTERNAL_ATOMIC_HOOK_ATTRIBUTES absl::base_internal::AtomicHook<void (*)(
+    const char* msg, const void* obj, int64_t wait_cycles)>
+    mutex_tracer;
+ABSL_INTERNAL_ATOMIC_HOOK_ATTRIBUTES
+absl::base_internal::AtomicHook<void (*)(const char* msg, const void* cv)>
+    cond_var_tracer;
 
 }  // namespace
 
-static inline bool EvalConditionAnnotated(const Condition *cond, Mutex *mu,
+static inline bool EvalConditionAnnotated(const Condition* cond, Mutex* mu,
                                           bool locking, bool trylock,
                                           bool read_lock);
 
-void RegisterMutexProfiler(void (*fn)(int64_t wait_timestamp)) {
+void RegisterMutexProfiler(void (*fn)(int64_t wait_cycles)) {
   submit_profile_data.Store(fn);
 }
 
-void RegisterMutexTracer(void (*fn)(const char *msg, const void *obj,
+void RegisterMutexTracer(void (*fn)(const char* msg, const void* obj,
                                     int64_t wait_cycles)) {
   mutex_tracer.Store(fn);
 }
 
-void RegisterCondVarTracer(void (*fn)(const char *msg, const void *cv)) {
+void RegisterCondVarTracer(void (*fn)(const char* msg, const void* cv)) {
   cond_var_tracer.Store(fn);
 }
 
-void RegisterSymbolizer(bool (*fn)(const void *pc, char *out, int out_size)) {
-  symbolizer.Store(fn);
-}
-
-// spinlock delay on iteration c.  Returns new c.
 namespace {
-  enum DelayMode { AGGRESSIVE, GENTLE };
+// Represents the strategy for spin and yield.
+// See the comment in GetMutexGlobals() for more information.
+enum DelayMode { AGGRESSIVE, GENTLE };
+
+struct ABSL_CACHELINE_ALIGNED MutexGlobals {
+  absl::once_flag once;
+  // Note: this variable is initialized separately in Mutex::LockSlow,
+  // so that Mutex::Lock does not have a stack frame in optimized build.
+  std::atomic<int> spinloop_iterations{0};
+  int32_t mutex_sleep_spins[2] = {};
+  absl::Duration mutex_sleep_time;
 };
-static int Delay(int32_t c, DelayMode mode) {
-  // If this a uniprocessor, only yield/sleep.  Otherwise, if the mode is
-  // aggressive then spin many times before yielding.  If the mode is
-  // gentle then spin only a few times before yielding.  Aggressive spinning is
-  // used to ensure that an Unlock() call, which  must get the spin lock for
-  // any thread to make progress gets it without undue delay.
-  int32_t limit = (mutex_globals.num_cpus > 1) ?
-      ((mode == AGGRESSIVE) ? 5000 : 250) : 0;
+
+ABSL_CONST_INIT static MutexGlobals globals;
+
+absl::Duration MeasureTimeToYield() {
+  absl::Time before = absl::Now();
+  ABSL_INTERNAL_C_SYMBOL(AbslInternalMutexYield)();
+  return absl::Now() - before;
+}
+
+const MutexGlobals& GetMutexGlobals() {
+  absl::base_internal::LowLevelCallOnce(&globals.once, [&]() {
+    if (absl::base_internal::NumCPUs() > 1) {
+      // If the mode is aggressive then spin many times before yielding.
+      // If the mode is gentle then spin only a few times before yielding.
+      // Aggressive spinning is used to ensure that an Unlock() call,
+      // which must get the spin lock for any thread to make progress gets it
+      // without undue delay.
+      globals.mutex_sleep_spins[AGGRESSIVE] = 5000;
+      globals.mutex_sleep_spins[GENTLE] = 250;
+      globals.mutex_sleep_time = absl::Microseconds(10);
+    } else {
+      // If this a uniprocessor, only yield/sleep. Real-time threads are often
+      // unable to yield, so the sleep time needs to be long enough to keep
+      // the calling thread asleep until scheduling happens.
+      globals.mutex_sleep_spins[AGGRESSIVE] = 0;
+      globals.mutex_sleep_spins[GENTLE] = 0;
+      globals.mutex_sleep_time = MeasureTimeToYield() * 5;
+      globals.mutex_sleep_time =
+          std::min(globals.mutex_sleep_time, absl::Milliseconds(1));
+      globals.mutex_sleep_time =
+          std::max(globals.mutex_sleep_time, absl::Microseconds(10));
+    }
+  });
+  return globals;
+}
+}  // namespace
+
+namespace synchronization_internal {
+// Returns the Mutex delay on iteration `c` depending on the given `mode`.
+// The returned value should be used as `c` for the next call to `MutexDelay`.
+int MutexDelay(int32_t c, int mode) {
+  const int32_t limit = GetMutexGlobals().mutex_sleep_spins[mode];
+  const absl::Duration sleep_time = GetMutexGlobals().mutex_sleep_time;
   if (c < limit) {
-    c++;               // spin
+    // Spin.
+    c++;
   } else {
+    SchedulingGuard::ScopedEnable enable_rescheduling;
     ABSL_TSAN_MUTEX_PRE_DIVERT(nullptr, 0);
-    if (c == limit) {  // yield once
-      AbslInternalMutexYield();
+    if (c == limit) {
+      // Yield once.
+      ABSL_INTERNAL_C_SYMBOL(AbslInternalMutexYield)();
       c++;
-    } else {           // then wait
-      absl::SleepFor(absl::Microseconds(10));
+    } else {
+      // Then wait.
+      absl::SleepFor(sleep_time);
       c = 0;
     }
     ABSL_TSAN_MUTEX_POST_DIVERT(nullptr, 0);
   }
-  return (c);
+  return c;
 }
+}  // namespace synchronization_internal
 
 // --------------------------Generic atomic ops
 // Ensure that "(*pv & bits) == bits" by doing an atomic update of "*pv" to
 // "*pv | bits" if necessary.  Wait until (*pv & wait_until_clear)==0
 // before making any change.
+// Returns true if bits were previously unset and set by the call.
 // This is used to set flags in mutex and condition variable words.
-static void AtomicSetBits(std::atomic<intptr_t>* pv, intptr_t bits,
+static bool AtomicSetBits(std::atomic<intptr_t>* pv, intptr_t bits,
                           intptr_t wait_until_clear) {
-  intptr_t v;
-  do {
-    v = pv->load(std::memory_order_relaxed);
-  } while ((v & bits) != bits &&
-           ((v & wait_until_clear) != 0 ||
-            !pv->compare_exchange_weak(v, v | bits,
-                                       std::memory_order_release,
-                                       std::memory_order_relaxed)));
-}
-
-// Ensure that "(*pv & bits) == 0" by doing an atomic update of "*pv" to
-// "*pv & ~bits" if necessary.  Wait until (*pv & wait_until_clear)==0
-// before making any change.
-// This is used to unset flags in mutex and condition variable words.
-static void AtomicClearBits(std::atomic<intptr_t>* pv, intptr_t bits,
-                            intptr_t wait_until_clear) {
-  intptr_t v;
-  do {
-    v = pv->load(std::memory_order_relaxed);
-  } while ((v & bits) != 0 &&
-           ((v & wait_until_clear) != 0 ||
-            !pv->compare_exchange_weak(v, v & ~bits,
-                                       std::memory_order_release,
-                                       std::memory_order_relaxed)));
+  for (;;) {
+    intptr_t v = pv->load(std::memory_order_relaxed);
+    if ((v & bits) == bits) {
+      return false;
+    }
+    if ((v & wait_until_clear) != 0) {
+      continue;
+    }
+    if (pv->compare_exchange_weak(v, v | bits, std::memory_order_release,
+                                  std::memory_order_relaxed)) {
+      return true;
+    }
+  }
 }
 
 //------------------------------------------------------------------
 
 // Data for doing deadlock detection.
-static absl::base_internal::SpinLock deadlock_graph_mu(
-    absl::base_internal::kLinkerInitialized);
+ABSL_CONST_INIT static absl::base_internal::SpinLock deadlock_graph_mu(
+    base_internal::SCHEDULE_KERNEL_ONLY);
 
-// graph used to detect deadlocks.
-static GraphCycles *deadlock_graph ABSL_GUARDED_BY(deadlock_graph_mu)
-    ABSL_PT_GUARDED_BY(deadlock_graph_mu);
+// Graph used to detect deadlocks.
+ABSL_CONST_INIT static GraphCycles* deadlock_graph
+    ABSL_GUARDED_BY(deadlock_graph_mu) ABSL_PT_GUARDED_BY(deadlock_graph_mu);
 
 //------------------------------------------------------------------
 // An event mechanism for debugging mutex use.
@@ -251,7 +281,7 @@ enum {                    // Event flags
 // Properties of the events.
 static const struct {
   int flags;
-  const char *msg;
+  const char* msg;
 } event_properties[] = {
     {SYNCH_F_LCK_W | SYNCH_F_TRY, "TryLock succeeded "},
     {0, "TryLock failed "},
@@ -269,20 +299,19 @@ static const struct {
     {0, "SignalAll on "},
 };
 
-static absl::base_internal::SpinLock synch_event_mu(
-    absl::base_internal::kLinkerInitialized);
-// protects synch_event
+ABSL_CONST_INIT static absl::base_internal::SpinLock synch_event_mu(
+    base_internal::SCHEDULE_KERNEL_ONLY);
 
 // Hash table size; should be prime > 2.
 // Can't be too small, as it's used for deadlock detection information.
-static const uint32_t kNSynchEvent = 1031;
+static constexpr uint32_t kNSynchEvent = 1031;
 
-static struct SynchEvent {     // this is a trivial hash table for the events
+static struct SynchEvent {  // this is a trivial hash table for the events
   // struct is freed when refcount reaches 0
   int refcount ABSL_GUARDED_BY(synch_event_mu);
 
   // buckets have linear, 0-terminated  chains
-  SynchEvent *next ABSL_GUARDED_BY(synch_event_mu);
+  SynchEvent* next ABSL_GUARDED_BY(synch_event_mu);
 
   // Constant after initialization
   uintptr_t masked_addr;  // object at this address is called "name"
@@ -290,13 +319,13 @@ static struct SynchEvent {     // this is a trivial hash table for the events
   // No explicit synchronization used.  Instead we assume that the
   // client who enables/disables invariants/logging on a Mutex does so
   // while the Mutex is not being concurrently accessed by others.
-  void (*invariant)(void *arg);  // called on each event
-  void *arg;            // first arg to (*invariant)()
-  bool log;             // logging turned on
+  void (*invariant)(void* arg);  // called on each event
+  void* arg;                     // first arg to (*invariant)()
+  bool log;                      // logging turned on
 
   // Constant after initialization
-  char name[1];         // actually longer---null-terminated std::string
-} * synch_event[kNSynchEvent] ABSL_GUARDED_BY(synch_event_mu);
+  char name[1];  // actually longer---NUL-terminated string
+}* synch_event[kNSynchEvent] ABSL_GUARDED_BY(synch_event_mu);
 
 // Ensure that the object at "addr" has a SynchEvent struct associated with it,
 // set "bits" in the word there (waiting until lockbit is clear before doing
@@ -305,89 +334,95 @@ static struct SynchEvent {     // this is a trivial hash table for the events
 // the string name is copied into it.
 // When used with a mutex, the caller should also ensure that kMuEvent
 // is set in the mutex word, and similarly for condition variables and kCVEvent.
-static SynchEvent *EnsureSynchEvent(std::atomic<intptr_t> *addr,
-                                    const char *name, intptr_t bits,
+static SynchEvent* EnsureSynchEvent(std::atomic<intptr_t>* addr,
+                                    const char* name, intptr_t bits,
                                     intptr_t lockbit) {
-  uint32_t h = reinterpret_cast<intptr_t>(addr) % kNSynchEvent;
-  SynchEvent *e;
-  // first look for existing SynchEvent struct..
-  synch_event_mu.Lock();
-  for (e = synch_event[h];
-       e != nullptr && e->masked_addr != base_internal::HidePtr(addr);
-       e = e->next) {
+  uint32_t h = reinterpret_cast<uintptr_t>(addr) % kNSynchEvent;
+  synch_event_mu.lock();
+  // When a Mutex/CondVar is destroyed, we don't remove the associated
+  // SynchEvent to keep destructors empty in release builds for performance
+  // reasons. If the current call is the first to set bits (kMuEvent/kCVEvent),
+  // we don't look up the existing even because (if it exists, it must be for
+  // the previous Mutex/CondVar that existed at the same address).
+  // The leaking events must not be a problem for tests, which should create
+  // bounded amount of events. And debug logging is not supposed to be enabled
+  // in production. However, if it's accidentally enabled, or briefly enabled
+  // for some debugging, we don't want to crash the program. Instead we drop
+  // all events, if we accumulated too many of them. Size of a single event
+  // is ~48 bytes, so 100K events is ~5 MB.
+  // Additionally we could delete the old event for the same address,
+  // but it would require a better hashmap (if we accumulate too many events,
+  // linked lists will grow and traversing them will be very slow).
+  constexpr size_t kMaxSynchEventCount = 100 << 10;
+  // Total number of live synch events.
+  static size_t synch_event_count ABSL_GUARDED_BY(synch_event_mu);
+  if (++synch_event_count > kMaxSynchEventCount) {
+    synch_event_count = 0;
+    ABSL_RAW_LOG(ERROR,
+                 "Accumulated %zu Mutex debug objects. If you see this"
+                 " in production, it may mean that the production code"
+                 " accidentally calls "
+                 "Mutex/CondVar::EnableDebugLog/EnableInvariantDebugging.",
+                 kMaxSynchEventCount);
+    for (auto*& head : synch_event) {
+      for (auto* e = head; e != nullptr;) {
+        SynchEvent* next = e->next;
+        if (--(e->refcount) == 0) {
+          base_internal::LowLevelAlloc::Free(e);
+        }
+        e = next;
+      }
+      head = nullptr;
+    }
+  }
+  SynchEvent* e = nullptr;
+  if (!AtomicSetBits(addr, bits, lockbit)) {
+    for (e = synch_event[h];
+         e != nullptr && e->masked_addr != base_internal::HidePtr(addr);
+         e = e->next) {
+    }
   }
   if (e == nullptr) {  // no SynchEvent struct found; make one.
     if (name == nullptr) {
       name = "";
     }
     size_t l = strlen(name);
-    e = reinterpret_cast<SynchEvent *>(
+    e = reinterpret_cast<SynchEvent*>(
         base_internal::LowLevelAlloc::Alloc(sizeof(*e) + l));
-    e->refcount = 2;    // one for return value, one for linked list
+    e->refcount = 2;  // one for return value, one for linked list
     e->masked_addr = base_internal::HidePtr(addr);
     e->invariant = nullptr;
     e->arg = nullptr;
     e->log = false;
     strcpy(e->name, name);  // NOLINT(runtime/printf)
     e->next = synch_event[h];
-    AtomicSetBits(addr, bits, lockbit);
     synch_event[h] = e;
   } else {
-    e->refcount++;      // for return value
+    e->refcount++;  // for return value
   }
-  synch_event_mu.Unlock();
+  synch_event_mu.unlock();
   return e;
 }
 
-// Deallocate the SynchEvent *e, whose refcount has fallen to zero.
-static void DeleteSynchEvent(SynchEvent *e) {
-  base_internal::LowLevelAlloc::Free(e);
-}
-
 // Decrement the reference count of *e, or do nothing if e==null.
-static void UnrefSynchEvent(SynchEvent *e) {
+static void UnrefSynchEvent(SynchEvent* e) {
   if (e != nullptr) {
-    synch_event_mu.Lock();
+    synch_event_mu.lock();
     bool del = (--(e->refcount) == 0);
-    synch_event_mu.Unlock();
+    synch_event_mu.unlock();
     if (del) {
-      DeleteSynchEvent(e);
+      base_internal::LowLevelAlloc::Free(e);
     }
   }
 }
 
-// Forget the mapping from the object (Mutex or CondVar) at address addr
-// to SynchEvent object, and clear "bits" in its word (waiting until lockbit
-// is clear before doing so).
-static void ForgetSynchEvent(std::atomic<intptr_t> *addr, intptr_t bits,
-                             intptr_t lockbit) {
-  uint32_t h = reinterpret_cast<intptr_t>(addr) % kNSynchEvent;
-  SynchEvent **pe;
-  SynchEvent *e;
-  synch_event_mu.Lock();
-  for (pe = &synch_event[h];
-       (e = *pe) != nullptr && e->masked_addr != base_internal::HidePtr(addr);
-       pe = &e->next) {
-  }
-  bool del = false;
-  if (e != nullptr) {
-    *pe = e->next;
-    del = (--(e->refcount) == 0);
-  }
-  AtomicClearBits(addr, bits, lockbit);
-  synch_event_mu.Unlock();
-  if (del) {
-    DeleteSynchEvent(e);
-  }
-}
-
 // Return a refcounted reference to the SynchEvent of the object at address
 // "addr", if any.  The pointer returned is valid until the UnrefSynchEvent() is
 // called.
-static SynchEvent *GetSynchEvent(const void *addr) {
-  uint32_t h = reinterpret_cast<intptr_t>(addr) % kNSynchEvent;
-  SynchEvent *e;
-  synch_event_mu.Lock();
+static SynchEvent* GetSynchEvent(const void* addr) {
+  uint32_t h = reinterpret_cast<uintptr_t>(addr) % kNSynchEvent;
+  SynchEvent* e;
+  synch_event_mu.lock();
   for (e = synch_event[h];
        e != nullptr && e->masked_addr != base_internal::HidePtr(addr);
        e = e->next) {
@@ -395,25 +430,31 @@ static SynchEvent *GetSynchEvent(const void *addr) {
   if (e != nullptr) {
     e->refcount++;
   }
-  synch_event_mu.Unlock();
+  synch_event_mu.unlock();
   return e;
 }
 
 // Called when an event "ev" occurs on a Mutex of CondVar "obj"
 // if event recording is on
-static void PostSynchEvent(void *obj, int ev) {
-  SynchEvent *e = GetSynchEvent(obj);
+static void PostSynchEvent(void* obj, int ev) {
+  SynchEvent* e = GetSynchEvent(obj);
   // logging is on if event recording is on and either there's no event struct,
   // or it explicitly says to log
   if (e == nullptr || e->log) {
-    void *pcs[40];
-    int n = absl::GetStackTrace(pcs, ABSL_ARRAYSIZE(pcs), 1);
+    void* pcs[40];
+    int n = absl::GetStackTrace(pcs, std::size(pcs), 1);
     // A buffer with enough space for the ASCII for all the PCs, even on a
     // 64-bit machine.
-    char buffer[ABSL_ARRAYSIZE(pcs) * 24];
-    int pos = snprintf(buffer, sizeof (buffer), " @");
+    char buffer[std::size(pcs) * 24];
+    int pos = snprintf(buffer, sizeof(buffer), " @");
     for (int i = 0; i != n; i++) {
-      pos += snprintf(&buffer[pos], sizeof (buffer) - pos, " %p", pcs[i]);
+      int b = snprintf(&buffer[pos], sizeof(buffer) - static_cast<size_t>(pos),
+                       " %p", pcs[i]);
+      if (b < 0 ||
+          static_cast<size_t>(b) >= sizeof(buffer) - static_cast<size_t>(pos)) {
+        break;
+      }
+      pos += b;
     }
     ABSL_RAW_LOG(INFO, "%s%p %s %s", event_properties[ev].msg, obj,
                  (e == nullptr ? "" : e->name), buffer);
@@ -427,13 +468,13 @@ static void PostSynchEvent(void *obj, int ev) {
     // get false positive race reports later.
     // Reuse EvalConditionAnnotated to properly call into user code.
     struct local {
-      static bool pred(SynchEvent *ev) {
+      static bool pred(SynchEvent* ev) {
         (*ev->invariant)(ev->arg);
         return false;
       }
     };
     Condition cond(&local::pred, e);
-    Mutex *mu = static_cast<Mutex *>(obj);
+    Mutex* mu = static_cast<Mutex*>(obj);
     const bool locking = (flags & SYNCH_F_UNLOCK) == 0;
     const bool trylock = (flags & SYNCH_F_TRY) != 0;
     const bool read_lock = (flags & SYNCH_F_R) != 0;
@@ -459,43 +500,45 @@ static void PostSynchEvent(void *obj, int ev) {
 // PerThreadSynch struct points at the most recent SynchWaitParams struct when
 // the thread is on a Mutex's waiter queue.
 struct SynchWaitParams {
-  SynchWaitParams(Mutex::MuHow how_arg, const Condition *cond_arg,
-                  KernelTimeout timeout_arg, Mutex *cvmu_arg,
-                  PerThreadSynch *thread_arg,
-                  std::atomic<intptr_t> *cv_word_arg)
+  SynchWaitParams(Mutex::MuHow how_arg, const Condition* cond_arg,
+                  KernelTimeout timeout_arg, Mutex* cvmu_arg,
+                  PerThreadSynch* thread_arg,
+                  std::atomic<intptr_t>* cv_word_arg)
       : how(how_arg),
         cond(cond_arg),
         timeout(timeout_arg),
         cvmu(cvmu_arg),
         thread(thread_arg),
         cv_word(cv_word_arg),
-        contention_start_cycles(base_internal::CycleClock::Now()) {}
+        contention_start_cycles(CycleClock::Now()),
+        should_submit_contention_data(false) {}
 
   const Mutex::MuHow how;  // How this thread needs to wait.
-  const Condition *cond;  // The condition that this thread is waiting for.
-                          // In Mutex, this field is set to zero if a timeout
-                          // expires.
-  KernelTimeout timeout;  // timeout expiry---absolute time
-                          // In Mutex, this field is set to zero if a timeout
-                          // expires.
-  Mutex *const cvmu;      // used for transfer from cond var to mutex
-  PerThreadSynch *const thread;  // thread that is waiting
+  const Condition* cond;   // The condition that this thread is waiting for.
+                           // In Mutex, this field is set to zero if a timeout
+                           // expires.
+  KernelTimeout timeout;   // timeout expiry---absolute time
+                           // In Mutex, this field is set to zero if a timeout
+                           // expires.
+  Mutex* const cvmu;       // used for transfer from cond var to mutex
+  PerThreadSynch* const thread;  // thread that is waiting
 
   // If not null, thread should be enqueued on the CondVar whose state
   // word is cv_word instead of queueing normally on the Mutex.
-  std::atomic<intptr_t> *cv_word;
+  std::atomic<intptr_t>* cv_word;
 
   int64_t contention_start_cycles;  // Time (in cycles) when this thread started
-                                  // to contend for the mutex.
+                                    // to contend for the mutex.
+  bool should_submit_contention_data;
 };
 
 struct SynchLocksHeld {
-  int n;              // number of valid entries in locks[]
-  bool overflow;      // true iff we overflowed the array at some point
+  int n;          // number of valid entries in locks[]
+  bool overflow;  // true iff we overflowed the array at some point
   struct {
-    Mutex *mu;        // lock acquired
-    int32_t count;      // times acquired
-    GraphId id;       // deadlock_graph id of acquired lock
+    Mutex* mu;      // lock acquired
+    int32_t count;  // times acquired
+    GraphId id;     // deadlock_graph id of acquired lock
   } locks[40];
   // If a thread overfills the array during deadlock detection, we
   // continue, discarding information as needed.  If no overflow has
@@ -505,11 +548,11 @@ struct SynchLocksHeld {
 
 // A sentinel value in lists that is not 0.
 // A 0 value is used to mean "not on a list".
-static PerThreadSynch *const kPerThreadSynchNull =
-  reinterpret_cast<PerThreadSynch *>(1);
+static PerThreadSynch* const kPerThreadSynchNull =
+    reinterpret_cast<PerThreadSynch*>(1);
 
-static SynchLocksHeld *LocksHeldAlloc() {
-  SynchLocksHeld *ret = reinterpret_cast<SynchLocksHeld *>(
+static SynchLocksHeld* LocksHeldAlloc() {
+  SynchLocksHeld* ret = reinterpret_cast<SynchLocksHeld*>(
       base_internal::LowLevelAlloc::Alloc(sizeof(SynchLocksHeld)));
   ret->n = 0;
   ret->overflow = false;
@@ -517,24 +560,24 @@ static SynchLocksHeld *LocksHeldAlloc() {
 }
 
 // Return the PerThreadSynch-struct for this thread.
-static PerThreadSynch *Synch_GetPerThread() {
-  ThreadIdentity *identity = GetOrCreateCurrentThreadIdentity();
+static PerThreadSynch* Synch_GetPerThread() {
+  ThreadIdentity* identity = GetOrCreateCurrentThreadIdentity();
   return &identity->per_thread_synch;
 }
 
-static PerThreadSynch *Synch_GetPerThreadAnnotated(Mutex *mu) {
+static PerThreadSynch* Synch_GetPerThreadAnnotated(Mutex* mu) {
   if (mu) {
     ABSL_TSAN_MUTEX_PRE_DIVERT(mu, 0);
   }
-  PerThreadSynch *w = Synch_GetPerThread();
+  PerThreadSynch* w = Synch_GetPerThread();
   if (mu) {
     ABSL_TSAN_MUTEX_POST_DIVERT(mu, 0);
   }
   return w;
 }
 
-static SynchLocksHeld *Synch_GetAllLocks() {
-  PerThreadSynch *s = Synch_GetPerThread();
+static SynchLocksHeld* Synch_GetAllLocks() {
+  PerThreadSynch* s = Synch_GetPerThread();
   if (s->all_locks == nullptr) {
     s->all_locks = LocksHeldAlloc();  // Freed by ReclaimThreadIdentity.
   }
@@ -542,27 +585,26 @@ static SynchLocksHeld *Synch_GetAllLocks() {
 }
 
 // Post on "w"'s associated PerThreadSem.
-inline void Mutex::IncrementSynchSem(Mutex *mu, PerThreadSynch *w) {
-  if (mu) {
-    ABSL_TSAN_MUTEX_PRE_DIVERT(mu, 0);
-  }
+void Mutex::IncrementSynchSem(Mutex* mu, PerThreadSynch* w) {
+  static_cast<void>(mu);  // Prevent unused param warning in non-TSAN builds.
+  ABSL_TSAN_MUTEX_PRE_DIVERT(mu, 0);
+  // We miss synchronization around passing PerThreadSynch between threads
+  // since it happens inside of the Mutex code, so we need to ignore all
+  // accesses to the object.
+  ABSL_ANNOTATE_IGNORE_READS_AND_WRITES_BEGIN();
   PerThreadSem::Post(w->thread_identity());
-  if (mu) {
-    ABSL_TSAN_MUTEX_POST_DIVERT(mu, 0);
-  }
+  ABSL_ANNOTATE_IGNORE_READS_AND_WRITES_END();
+  ABSL_TSAN_MUTEX_POST_DIVERT(mu, 0);
 }
 
 // Wait on "w"'s associated PerThreadSem; returns false if timeout expired.
-bool Mutex::DecrementSynchSem(Mutex *mu, PerThreadSynch *w, KernelTimeout t) {
-  if (mu) {
-    ABSL_TSAN_MUTEX_PRE_DIVERT(mu, 0);
-  }
+bool Mutex::DecrementSynchSem(Mutex* mu, PerThreadSynch* w, KernelTimeout t) {
+  static_cast<void>(mu);  // Prevent unused param warning in non-TSAN builds.
+  ABSL_TSAN_MUTEX_PRE_DIVERT(mu, 0);
   assert(w == Synch_GetPerThread());
   static_cast<void>(w);
   bool res = PerThreadSem::Wait(t);
-  if (mu) {
-    ABSL_TSAN_MUTEX_POST_DIVERT(mu, 0);
-  }
+  ABSL_TSAN_MUTEX_POST_DIVERT(mu, 0);
   return res;
 }
 
@@ -574,7 +616,7 @@ bool Mutex::DecrementSynchSem(Mutex *mu, PerThreadSynch *w, KernelTimeout t) {
 // Mutex code checking that the "waitp" field has not been reused.
 void Mutex::InternalAttemptToUseMutexInFatalSignalHandler() {
   // Fix the per-thread state only if it exists.
-  ThreadIdentity *identity = CurrentThreadIdentityIfPresent();
+  ThreadIdentity* identity = CurrentThreadIdentityIfPresent();
   if (identity != nullptr) {
     identity->per_thread_synch.suppress_fatal_errors = true;
   }
@@ -583,21 +625,6 @@ void Mutex::InternalAttemptToUseMutexInFatalSignalHandler() {
                                  std::memory_order_release);
 }
 
-// --------------------------time support
-
-// Return the current time plus the timeout.  Use the same clock as
-// PerThreadSem::Wait() for consistency.  Unfortunately, we don't have
-// such a choice when a deadline is given directly.
-static absl::Time DeadlineFromTimeout(absl::Duration timeout) {
-#ifndef _WIN32
-  struct timeval tv;
-  gettimeofday(&tv, nullptr);
-  return absl::TimeFromTimeval(tv) + timeout;
-#else
-  return absl::Now() + timeout;
-#endif
-}
-
 // --------------------------Mutexes
 
 // In the layout below, the msb of the bottom byte is currently unused.  Also,
@@ -608,24 +635,36 @@ static absl::Time DeadlineFromTimeout(absl::Duration timeout) {
 //    bit-twiddling trick in Mutex::Unlock().
 //  o kMuWriter / kMuReader == kMuWrWait / kMuWait,
 //    to enable the bit-twiddling trick in CheckForMutexCorruption().
-static const intptr_t kMuReader      = 0x0001L;  // a reader holds the lock
-static const intptr_t kMuDesig       = 0x0002L;  // there's a designated waker
-static const intptr_t kMuWait        = 0x0004L;  // threads are waiting
-static const intptr_t kMuWriter      = 0x0008L;  // a writer holds the lock
-static const intptr_t kMuEvent       = 0x0010L;  // record this mutex's events
+static const intptr_t kMuReader = 0x0001L;  // a reader holds the lock
+// There's a designated waker.
 // INVARIANT1:  there's a thread that was blocked on the mutex, is
 // no longer, yet has not yet acquired the mutex.  If there's a
 // designated waker, all threads can avoid taking the slow path in
 // unlock because the designated waker will subsequently acquire
 // the lock and wake someone.  To maintain INVARIANT1 the bit is
 // set when a thread is unblocked(INV1a), and threads that were
-// unblocked reset the bit when they either acquire or re-block
-// (INV1b).
-static const intptr_t kMuWrWait      = 0x0020L;  // runnable writer is waiting
-                                                 // for a reader
-static const intptr_t kMuSpin        = 0x0040L;  // spinlock protects wait list
-static const intptr_t kMuLow         = 0x00ffL;  // mask all mutex bits
-static const intptr_t kMuHigh        = ~kMuLow;  // mask pointer/reader count
+// unblocked reset the bit when they either acquire or re-block (INV1b).
+static const intptr_t kMuDesig = 0x0002L;
+static const intptr_t kMuWait = 0x0004L;    // threads are waiting
+static const intptr_t kMuWriter = 0x0008L;  // a writer holds the lock
+static const intptr_t kMuEvent = 0x0010L;   // record this mutex's events
+// Runnable writer is waiting for a reader.
+// If set, new readers will not lock the mutex to avoid writer starvation.
+// Note: if a reader has higher priority than the writer, it will still lock
+// the mutex ahead of the waiting writer, but in a very inefficient manner:
+// the reader will first queue itself and block, but then the last unlocking
+// reader will wake it.
+static const intptr_t kMuWrWait = 0x0020L;
+static const intptr_t kMuSpin = 0x0040L;  // spinlock protects wait list
+static const intptr_t kMuLow = 0x00ffL;   // mask all mutex bits
+static const intptr_t kMuHigh = ~kMuLow;  // mask pointer/reader count
+
+static_assert((0xab & (kMuWriter | kMuReader)) == (kMuWriter | kMuReader),
+              "The debug allocator's uninitialized pattern (0xab) must be an "
+              "invalid mutex state");
+static_assert((0xcd & (kMuWriter | kMuReader)) == (kMuWriter | kMuReader),
+              "The debug allocator's freed pattern (0xcd) must be an invalid "
+              "mutex state");
 
 // Hack to make constant values available to gdb pretty printer
 enum {
@@ -651,6 +690,7 @@ static const intptr_t kMuOne = 0x0100;  // a count of one reader
 // flags passed to Enqueue and LockSlow{,WithTimeout,Loop}
 static const int kMuHasBlocked = 0x01;  // already blocked (MUST == 1)
 static const int kMuIsCond = 0x02;      // conditional waiter (CV or Condition)
+static const int kMuIsFer = 0x04;       // wait morphing from a CondVar
 
 static_assert(PerThreadSynch::kAlignment > kMuLow,
               "PerThreadSynch::kAlignment must be greater than kMuLow");
@@ -700,64 +740,86 @@ static constexpr bool kDebugMode = false;
 static constexpr bool kDebugMode = true;
 #endif
 
-#ifdef THREAD_SANITIZER
+#ifdef ABSL_INTERNAL_HAVE_TSAN_INTERFACE
 static unsigned TsanFlags(Mutex::MuHow how) {
   return how == kShared ? __tsan_mutex_read_lock : 0;
 }
 #endif
 
-static bool DebugOnlyIsExiting() {
-  return false;
-}
+#if defined(__APPLE__) || defined(ABSL_BUILD_DLL)
+// When building a dll symbol export lists may reference the destructor
+// and want it to be an exported symbol rather than an inline function.
+// Some apple builds also do dynamic library build but don't say it explicitly.
+Mutex::~Mutex() { Dtor(); }
+#endif
 
-Mutex::~Mutex() {
-  intptr_t v = mu_.load(std::memory_order_relaxed);
-  if ((v & kMuEvent) != 0 && !DebugOnlyIsExiting()) {
-    ForgetSynchEvent(&this->mu_, kMuEvent, kMuSpin);
-  }
+#if !defined(NDEBUG) || defined(ABSL_HAVE_THREAD_SANITIZER) || \
+    defined(ABSL_BUILD_DLL)
+void Mutex::Dtor() {
   if (kDebugMode) {
     this->ForgetDeadlockInfo();
   }
   ABSL_TSAN_MUTEX_DESTROY(this, __tsan_mutex_not_static);
 }
+#endif
 
-void Mutex::EnableDebugLog(const char *name) {
-  SynchEvent *e = EnsureSynchEvent(&this->mu_, name, kMuEvent, kMuSpin);
+void Mutex::EnableDebugLog(const char* name) {
+  // Need to disable writes here and in EnableInvariantDebugging to prevent
+  // false race reports on SynchEvent objects. TSan ignores synchronization
+  // on synch_event_mu in Lock/Unlock/etc methods due to mutex annotations,
+  // but it sees few accesses to SynchEvent in EvalConditionAnnotated.
+  // If we don't ignore accesses here, it can result in false races
+  // between EvalConditionAnnotated and SynchEvent reuse in EnsureSynchEvent.
+  ABSL_ANNOTATE_IGNORE_WRITES_BEGIN();
+  SynchEvent* e = EnsureSynchEvent(&this->mu_, name, kMuEvent, kMuSpin);
   e->log = true;
   UnrefSynchEvent(e);
+  // This prevents "error: undefined symbol: absl::Mutex::~Mutex()"
+  // in a release build (NDEBUG defined) when a test does "#undef NDEBUG"
+  // to use assert macro. In such case, the test does not get the dtor
+  // definition because it's supposed to be outline when NDEBUG is not defined,
+  // and this source file does not define one either because NDEBUG is defined.
+  // Since it's not possible to take address of a destructor, we move the
+  // actual destructor code into the separate Dtor function and force the
+  // compiler to emit this function even if it's inline by taking its address.
+  ABSL_ATTRIBUTE_UNUSED volatile auto dtor = &Mutex::Dtor;
+  ABSL_ANNOTATE_IGNORE_WRITES_END();
 }
 
 void EnableMutexInvariantDebugging(bool enabled) {
   synch_check_invariants.store(enabled, std::memory_order_release);
 }
 
-void Mutex::EnableInvariantDebugging(void (*invariant)(void *),
-                                     void *arg) {
+void Mutex::EnableInvariantDebugging(void (*invariant)(void*), void* arg) {
+  ABSL_ANNOTATE_IGNORE_WRITES_BEGIN();
   if (synch_check_invariants.load(std::memory_order_acquire) &&
       invariant != nullptr) {
-    SynchEvent *e = EnsureSynchEvent(&this->mu_, nullptr, kMuEvent, kMuSpin);
+    SynchEvent* e = EnsureSynchEvent(&this->mu_, nullptr, kMuEvent, kMuSpin);
     e->invariant = invariant;
     e->arg = arg;
     UnrefSynchEvent(e);
   }
+  ABSL_ANNOTATE_IGNORE_WRITES_END();
 }
 
 void SetMutexDeadlockDetectionMode(OnDeadlockCycle mode) {
   synch_deadlock_detection.store(mode, std::memory_order_release);
 }
 
-// Return true iff threads x and y are waiting on the same condition for the
-// same type of lock.  Requires that x and y be waiting on the same Mutex
-// queue.
-static bool MuSameCondition(PerThreadSynch *x, PerThreadSynch *y) {
-  return x->waitp->how == y->waitp->how &&
+// Return true iff threads x and y are part of the same equivalence
+// class of waiters. An equivalence class is defined as the set of
+// waiters with the same condition, type of lock, and thread priority.
+//
+// Requires that x and y be waiting on the same Mutex queue.
+static bool MuEquivalentWaiter(PerThreadSynch* x, PerThreadSynch* y) {
+  return x->waitp->how == y->waitp->how && x->priority == y->priority &&
          Condition::GuaranteedEqual(x->waitp->cond, y->waitp->cond);
 }
 
 // Given the contents of a mutex word containing a PerThreadSynch pointer,
 // return the pointer.
-static inline PerThreadSynch *GetPerThreadSynch(intptr_t v) {
-  return reinterpret_cast<PerThreadSynch *>(v & kMuHigh);
+static inline PerThreadSynch* GetPerThreadSynch(intptr_t v) {
+  return reinterpret_cast<PerThreadSynch*>(v & kMuHigh);
 }
 
 // The next several routines maintain the per-thread next and skip fields
@@ -769,18 +831,19 @@ static inline PerThreadSynch *GetPerThreadSynch(intptr_t v) {
 //     - invalid (iff x is not in a Mutex wait queue),
 //     - null, or
 //     - a pointer to a distinct thread waiting later in the same Mutex queue
-//       such that all threads in [x, x->skip] have the same condition and
-//       lock type (MuSameCondition() is true for all pairs in [x, x->skip]).
+//       such that all threads in [x, x->skip] have the same condition, priority
+//       and lock type (MuEquivalentWaiter() is true for all pairs in [x,
+//       x->skip]).
 // In addition, if x->skip is  valid, (x->may_skip || x->skip == null)
 //
-// By the spec of MuSameCondition(), it is not necessary when removing the
+// By the spec of MuEquivalentWaiter(), it is not necessary when removing the
 // first runnable thread y from the front a Mutex queue to adjust the skip
 // field of another thread x because if x->skip==y, x->skip must (have) become
 // invalid before y is removed.  The function TryRemove can remove a specified
 // thread from an arbitrary position in the queue whether runnable or not, so
 // it fixes up skip fields that would otherwise be left dangling.
 // The statement
-//     if (x->may_skip && MuSameCondition(x, x->next)) { x->skip = x->next; }
+//     if (x->may_skip && MuEquivalentWaiter(x, x->next)) { x->skip = x->next; }
 // maintains the invariant provided x is not the last waiter in a Mutex queue
 // The statement
 //          if (x->skip != null) { x->skip = x->skip->skip; }
@@ -814,17 +877,17 @@ static inline PerThreadSynch *GetPerThreadSynch(intptr_t v) {
 // except those in the added node and the former "head" node.  This implies
 // that the new node is added after head, and so must be the new head or the
 // new front of the queue.
-static PerThreadSynch *Skip(PerThreadSynch *x) {
-  PerThreadSynch *x0 = nullptr;
-  PerThreadSynch *x1 = x;
-  PerThreadSynch *x2 = x->skip;
+static PerThreadSynch* Skip(PerThreadSynch* x) {
+  PerThreadSynch* x0 = nullptr;
+  PerThreadSynch* x1 = x;
+  PerThreadSynch* x2 = x->skip;
   if (x2 != nullptr) {
     // Each iteration attempts to advance sequence (x0,x1,x2) to next sequence
     // such that   x1 == x0->skip && x2 == x1->skip
     while ((x0 = x1, x1 = x2, x2 = x2->skip) != nullptr) {
-      x0->skip = x2;      // short-circuit skip from x0 to x2
+      x0->skip = x2;  // short-circuit skip from x0 to x2
     }
-    x->skip = x1;         // short-circuit skip from x to result
+    x->skip = x1;  // short-circuit skip from x to result
   }
   return x1;
 }
@@ -833,7 +896,7 @@ static PerThreadSynch *Skip(PerThreadSynch *x) {
 // The latter is going to be removed out of order, because of a timeout.
 // Check whether "ancestor" has a skip field pointing to "to_be_removed",
 // and fix it if it does.
-static void FixSkip(PerThreadSynch *ancestor, PerThreadSynch *to_be_removed) {
+static void FixSkip(PerThreadSynch* ancestor, PerThreadSynch* to_be_removed) {
   if (ancestor->skip == to_be_removed) {  // ancestor->skip left dangling
     if (to_be_removed->skip != nullptr) {
       ancestor->skip = to_be_removed->skip;  // can skip past to_be_removed
@@ -845,7 +908,7 @@ static void FixSkip(PerThreadSynch *ancestor, PerThreadSynch *to_be_removed) {
   }
 }
 
-static void CondVarEnqueue(SynchWaitParams *waitp);
+static void CondVarEnqueue(SynchWaitParams* waitp);
 
 // Enqueue thread "waitp->thread" on a waiter queue.
 // Called with mutex spinlock held if head != nullptr
@@ -866,8 +929,8 @@ static void CondVarEnqueue(SynchWaitParams *waitp);
 // returned. This mechanism is used by CondVar to queue a thread on the
 // condition variable queue instead of the mutex queue in implementing Wait().
 // In this case, Enqueue() can return nullptr (if head==nullptr).
-static PerThreadSynch *Enqueue(PerThreadSynch *head,
-                               SynchWaitParams *waitp, intptr_t mu, int flags) {
+static PerThreadSynch* Enqueue(PerThreadSynch* head, SynchWaitParams* waitp,
+                               intptr_t mu, int flags) {
   // If we have been given a cv_word, call CondVarEnqueue() and return
   // the previous head of the Mutex waiter queue.
   if (waitp->cv_word != nullptr) {
@@ -875,30 +938,25 @@ static PerThreadSynch *Enqueue(PerThreadSynch *head,
     return head;
   }
 
-  PerThreadSynch *s = waitp->thread;
+  PerThreadSynch* s = waitp->thread;
   ABSL_RAW_CHECK(
       s->waitp == nullptr ||    // normal case
           s->waitp == waitp ||  // Fer()---transfer from condition variable
           s->suppress_fatal_errors,
       "detected illegal recursion into Mutex code");
   s->waitp = waitp;
-  s->skip = nullptr;             // maintain skip invariant (see above)
-  s->may_skip = true;            // always true on entering queue
-  s->wake = false;               // not being woken
+  s->skip = nullptr;   // maintain skip invariant (see above)
+  s->may_skip = true;  // always true on entering queue
+  s->wake = false;     // not being woken
   s->cond_waiter = ((flags & kMuIsCond) != 0);
-  if (head == nullptr) {         // s is the only waiter
-    s->next = s;                 // it's the only entry in the cycle
-    s->readers = mu;             // reader count is from mu word
-    s->maybe_unlocking = false;  // no one is searching an empty list
-    head = s;                    // s is new head
-  } else {
-    PerThreadSynch *enqueue_after = nullptr;  // we'll put s after this element
 #ifdef ABSL_HAVE_PTHREAD_GETSCHEDPARAM
-    int64_t now_cycles = base_internal::CycleClock::Now();
+  if ((flags & kMuIsFer) == 0) {
+    assert(s == Synch_GetPerThread());
+    int64_t now_cycles = CycleClock::Now();
     if (s->next_priority_read_cycles < now_cycles) {
       // Every so often, update our idea of the thread's priority.
       // pthread_getschedparam() is 5% of the block/wakeup time;
-      // base_internal::CycleClock::Now() is 0.5%.
+      // CycleClock::Now() is 0.5%.
       int policy;
       struct sched_param param;
       const int err = pthread_getschedparam(pthread_self(), &policy, &param);
@@ -907,40 +965,41 @@ static PerThreadSynch *Enqueue(PerThreadSynch *head,
       } else {
         s->priority = param.sched_priority;
         s->next_priority_read_cycles =
-            now_cycles +
-            static_cast<int64_t>(base_internal::CycleClock::Frequency());
+            now_cycles + static_cast<int64_t>(CycleClock::Frequency());
       }
     }
+  }
+#endif
+  if (head == nullptr) {         // s is the only waiter
+    s->next = s;                 // it's the only entry in the cycle
+    s->readers = mu;             // reader count is from mu word
+    s->maybe_unlocking = false;  // no one is searching an empty list
+    head = s;                    // s is new head
+  } else {
+    PerThreadSynch* enqueue_after = nullptr;  // we'll put s after this element
+#ifdef ABSL_HAVE_PTHREAD_GETSCHEDPARAM
     if (s->priority > head->priority) {  // s's priority is above head's
       // try to put s in priority-fifo order, or failing that at the front.
       if (!head->maybe_unlocking) {
-        // No unlocker can be scanning the queue, so we can insert between
-        // skip-chains, and within a skip-chain if it has the same condition as
-        // s.  We insert in priority-fifo order, examining the end of every
-        // skip-chain, plus every element with the same condition as s.
-        PerThreadSynch *advance_to = head;    // next value of enqueue_after
-        PerThreadSynch *cur;                  // successor of enqueue_after
+        // No unlocker can be scanning the queue, so we can insert into the
+        // middle of the queue.
+        //
+        // Within a skip chain, all waiters have the same priority, so we can
+        // skip forward through the chains until we find one with a lower
+        // priority than the waiter to be enqueued.
+        PerThreadSynch* advance_to = head;  // next value of enqueue_after
         do {
           enqueue_after = advance_to;
-          cur = enqueue_after->next;  // this advance ensures progress
-          advance_to = Skip(cur);   // normally, advance to end of skip chain
-                                    // (side-effect: optimizes skip chain)
-          if (advance_to != cur && s->priority > advance_to->priority &&
-              MuSameCondition(s, cur)) {
-            // but this skip chain is not a singleton, s has higher priority
-            // than its tail and has the same condition as the chain,
-            // so we can insert within the skip-chain
-            advance_to = cur;         // advance by just one
-          }
+          // (side-effect: optimizes skip chain)
+          advance_to = Skip(enqueue_after->next);
         } while (s->priority <= advance_to->priority);
-              // termination guaranteed because s->priority > head->priority
-              // and head is the end of a skip chain
-      } else if (waitp->how == kExclusive &&
-                 Condition::GuaranteedEqual(waitp->cond, nullptr)) {
+        // termination guaranteed because s->priority > head->priority
+        // and head is the end of a skip chain
+      } else if (waitp->how == kExclusive && waitp->cond == nullptr) {
         // An unlocker could be scanning the queue, but we know it will recheck
         // the queue front for writers that have no condition, which is what s
         // is, so an insert at front is safe.
-        enqueue_after = head;       // add after head, at front
+        enqueue_after = head;  // add after head, at front
       }
     }
 #endif
@@ -950,31 +1009,49 @@ static PerThreadSynch *Enqueue(PerThreadSynch *head,
 
       // enqueue_after can be: head, Skip(...), or cur.
       // The first two imply enqueue_after->skip == nullptr, and
-      // the last is used only if MuSameCondition(s, cur).
+      // the last is used only if MuEquivalentWaiter(s, cur).
       // We require this because clearing enqueue_after->skip
       // is impossible; enqueue_after's predecessors might also
       // incorrectly skip over s if we were to allow other
       // insertion points.
-      ABSL_RAW_CHECK(
-          enqueue_after->skip == nullptr || MuSameCondition(enqueue_after, s),
-          "Mutex Enqueue failure");
+      ABSL_RAW_CHECK(enqueue_after->skip == nullptr ||
+                         MuEquivalentWaiter(enqueue_after, s),
+                     "Mutex Enqueue failure");
 
       if (enqueue_after != head && enqueue_after->may_skip &&
-          MuSameCondition(enqueue_after, enqueue_after->next)) {
+          MuEquivalentWaiter(enqueue_after, enqueue_after->next)) {
         // enqueue_after can skip to its new successor, s
         enqueue_after->skip = enqueue_after->next;
       }
-      if (MuSameCondition(s, s->next)) {  // s->may_skip is known to be true
-        s->skip = s->next;                // s may skip to its successor
+      if (MuEquivalentWaiter(s, s->next)) {  // s->may_skip is known to be true
+        s->skip = s->next;                   // s may skip to its successor
+      }
+    } else if ((flags & kMuHasBlocked) &&
+               (s->priority >= head->next->priority) &&
+               (!head->maybe_unlocking ||
+                (waitp->how == kExclusive &&
+                 Condition::GuaranteedEqual(waitp->cond, nullptr)))) {
+      // This thread has already waited, then was woken, then failed to acquire
+      // the mutex and now tries to requeue. Try to requeue it at head,
+      // otherwise it can suffer bad latency (wait whole queue several times).
+      // However, we need to be conservative. First, we need to ensure that we
+      // respect priorities. Then, we need to be careful to not break wait
+      // queue invariants: we require either that unlocker is not scanning
+      // the queue or that the current thread is a writer with no condition
+      // (unlocker will recheck the queue for such waiters).
+      s->next = head->next;
+      head->next = s;
+      if (MuEquivalentWaiter(s, s->next)) {  // s->may_skip is known to be true
+        s->skip = s->next;                   // s may skip to its successor
       }
-    } else {   // enqueue not done any other way, so
-               // we're inserting s at the back
+    } else {  // enqueue not done any other way, so
+              // we're inserting s at the back
       // s will become new head; copy data from head into it
-      s->next = head->next;        // add s after head
+      s->next = head->next;  // add s after head
       head->next = s;
       s->readers = head->readers;  // reader count is from previous head
       s->maybe_unlocking = head->maybe_unlocking;  // same for unlock hint
-      if (head->may_skip && MuSameCondition(head, s)) {
+      if (head->may_skip && MuEquivalentWaiter(head, s)) {
         // head now has successor; may skip
         head->skip = s;
       }
@@ -989,17 +1066,17 @@ static PerThreadSynch *Enqueue(PerThreadSynch *head,
 // whose last element is head.  The new head element is returned, or null
 // if the list is made empty.
 // Dequeue is called with both spinlock and Mutex held.
-static PerThreadSynch *Dequeue(PerThreadSynch *head, PerThreadSynch *pw) {
-  PerThreadSynch *w = pw->next;
-  pw->next = w->next;         // snip w out of list
-  if (head == w) {            // we removed the head
+static PerThreadSynch* Dequeue(PerThreadSynch* head, PerThreadSynch* pw) {
+  PerThreadSynch* w = pw->next;
+  pw->next = w->next;                 // snip w out of list
+  if (head == w) {                    // we removed the head
     head = (pw == w) ? nullptr : pw;  // either emptied list, or pw is new head
-  } else if (pw != head && MuSameCondition(pw, pw->next)) {
+  } else if (pw != head && MuEquivalentWaiter(pw, pw->next)) {
     // pw can skip to its new successor
     if (pw->next->skip !=
         nullptr) {  // either skip to its successors skip target
       pw->skip = pw->next->skip;
-    } else {                   // or to pw's successor
+    } else {  // or to pw's successor
       pw->skip = pw->next;
     }
   }
@@ -1012,27 +1089,27 @@ static PerThreadSynch *Dequeue(PerThreadSynch *head, PerThreadSynch *pw) {
 // singly-linked list wake_list in the order found.   Assumes that
 // there is only one such element if the element has how == kExclusive.
 // Return the new head.
-static PerThreadSynch *DequeueAllWakeable(PerThreadSynch *head,
-                                          PerThreadSynch *pw,
-                                          PerThreadSynch **wake_tail) {
-  PerThreadSynch *orig_h = head;
-  PerThreadSynch *w = pw->next;
+static PerThreadSynch* DequeueAllWakeable(PerThreadSynch* head,
+                                          PerThreadSynch* pw,
+                                          PerThreadSynch** wake_tail) {
+  PerThreadSynch* orig_h = head;
+  PerThreadSynch* w = pw->next;
   bool skipped = false;
   do {
-    if (w->wake) {                    // remove this element
+    if (w->wake) {  // remove this element
       ABSL_RAW_CHECK(pw->skip == nullptr, "bad skip in DequeueAllWakeable");
       // we're removing pw's successor so either pw->skip is zero or we should
       // already have removed pw since if pw->skip!=null, pw has the same
       // condition as w.
       head = Dequeue(head, pw);
-      w->next = *wake_tail;           // keep list terminated
-      *wake_tail = w;                 // add w to wake_list;
-      wake_tail = &w->next;           // next addition to end
+      w->next = *wake_tail;               // keep list terminated
+      *wake_tail = w;                     // add w to wake_list;
+      wake_tail = &w->next;               // next addition to end
       if (w->waitp->how == kExclusive) {  // wake at most 1 writer
         break;
       }
-    } else {                // not waking this one; skip
-      pw = Skip(w);       // skip as much as possible
+    } else {         // not waking this one; skip
+      pw = Skip(w);  // skip as much as possible
       skipped = true;
     }
     w = pw->next;
@@ -1050,24 +1127,27 @@ static PerThreadSynch *DequeueAllWakeable(PerThreadSynch *head,
 
 // Try to remove thread s from the list of waiters on this mutex.
 // Does nothing if s is not on the waiter list.
-void Mutex::TryRemove(PerThreadSynch *s) {
+void Mutex::TryRemove(PerThreadSynch* s) {
+  SchedulingGuard::ScopedDisable disable_rescheduling;
   intptr_t v = mu_.load(std::memory_order_relaxed);
   // acquire spinlock & lock
   if ((v & (kMuWait | kMuSpin | kMuWriter | kMuReader)) == kMuWait &&
       mu_.compare_exchange_strong(v, v | kMuSpin | kMuWriter,
                                   std::memory_order_acquire,
                                   std::memory_order_relaxed)) {
-    PerThreadSynch *h = GetPerThreadSynch(v);
+    PerThreadSynch* h = GetPerThreadSynch(v);
     if (h != nullptr) {
-      PerThreadSynch *pw = h;   // pw is w's predecessor
-      PerThreadSynch *w;
+      PerThreadSynch* pw = h;  // pw is w's predecessor
+      PerThreadSynch* w;
       if ((w = pw->next) != s) {  // search for thread,
         do {                      // processing at least one element
-          if (!MuSameCondition(s, w)) {  // seeking different condition
-            pw = Skip(w);                // so skip all that won't match
+          // If the current element isn't equivalent to the waiter to be
+          // removed, we can skip the entire chain.
+          if (!MuEquivalentWaiter(s, w)) {
+            pw = Skip(w);  // so skip all that won't match
             // we don't have to worry about dangling skip fields
             // in the threads we skipped; none can point to s
-            // because their condition differs from s
+            // because they are in a different equivalence class.
           } else {          // seeking same condition
             FixSkip(w, s);  // fix up any skip pointer from w to s
             pw = w;
@@ -1076,7 +1156,7 @@ void Mutex::TryRemove(PerThreadSynch *s) {
           // process the first thread again.
         } while ((w = pw->next) != s && pw != h);
       }
-      if (w == s) {                 // found thread; remove it
+      if (w == s) {  // found thread; remove it
         // pw->skip may be non-zero here; the loop above ensured that
         // no ancestor of s can skip to s, so removal is safe anyway.
         h = Dequeue(h, pw);
@@ -1085,16 +1165,15 @@ void Mutex::TryRemove(PerThreadSynch *s) {
       }
     }
     intptr_t nv;
-    do {                        // release spinlock and lock
+    do {  // release spinlock and lock
       v = mu_.load(std::memory_order_relaxed);
       nv = v & (kMuDesig | kMuEvent);
       if (h != nullptr) {
         nv |= kMuWait | reinterpret_cast<intptr_t>(h);
-        h->readers = 0;            // we hold writer lock
+        h->readers = 0;              // we hold writer lock
         h->maybe_unlocking = false;  // finished unlocking
       }
-    } while (!mu_.compare_exchange_weak(v, nv,
-                                        std::memory_order_release,
+    } while (!mu_.compare_exchange_weak(v, nv, std::memory_order_release,
                                         std::memory_order_relaxed));
   }
 }
@@ -1104,7 +1183,7 @@ void Mutex::TryRemove(PerThreadSynch *s) {
 // if the wait extends past the absolute time specified, even if "s" is still
 // on the mutex queue.  In this case, remove "s" from the queue and return
 // true, otherwise return false.
-ABSL_XRAY_LOG_ARGS(1) void Mutex::Block(PerThreadSynch *s) {
+void Mutex::Block(PerThreadSynch* s) {
   while (s->state.load(std::memory_order_acquire) == PerThreadSynch::kQueued) {
     if (!DecrementSynchSem(this, s, s->waitp->timeout)) {
       // After a timeout, we go into a spin loop until we remove ourselves
@@ -1115,7 +1194,7 @@ ABSL_XRAY_LOG_ARGS(1) void Mutex::Block(PerThreadSynch *s) {
       this->TryRemove(s);
       int c = 0;
       while (s->next != nullptr) {
-        c = Delay(c, GENTLE);
+        c = synchronization_internal::MutexDelay(c, GENTLE);
         this->TryRemove(s);
       }
       if (kDebugMode) {
@@ -1123,7 +1202,7 @@ ABSL_XRAY_LOG_ARGS(1) void Mutex::Block(PerThreadSynch *s) {
         // is not on the queue.
         this->TryRemove(s);
       }
-      s->waitp->timeout = KernelTimeout::Never();      // timeout is satisfied
+      s->waitp->timeout = KernelTimeout::Never();  // timeout is satisfied
       s->waitp->cond = nullptr;  // condition no longer relevant for wakeups
     }
   }
@@ -1133,8 +1212,8 @@ ABSL_XRAY_LOG_ARGS(1) void Mutex::Block(PerThreadSynch *s) {
 }
 
 // Wake thread w, and return the next thread in the list.
-PerThreadSynch *Mutex::Wakeup(PerThreadSynch *w) {
-  PerThreadSynch *next = w->next;
+PerThreadSynch* Mutex::Wakeup(PerThreadSynch* w) {
+  PerThreadSynch* next = w->next;
   w->next = nullptr;
   w->state.store(PerThreadSynch::kAvailable, std::memory_order_release);
   IncrementSynchSem(this, w);
@@ -1142,7 +1221,7 @@ PerThreadSynch *Mutex::Wakeup(PerThreadSynch *w) {
   return next;
 }
 
-static GraphId GetGraphIdLocked(Mutex *mu)
+static GraphId GetGraphIdLocked(Mutex* mu)
     ABSL_EXCLUSIVE_LOCKS_REQUIRED(deadlock_graph_mu) {
   if (!deadlock_graph) {  // (re)create the deadlock graph.
     deadlock_graph =
@@ -1152,24 +1231,23 @@ static GraphId GetGraphIdLocked(Mutex *mu)
   return deadlock_graph->GetId(mu);
 }
 
-static GraphId GetGraphId(Mutex *mu) ABSL_LOCKS_EXCLUDED(deadlock_graph_mu) {
-  deadlock_graph_mu.Lock();
+static GraphId GetGraphId(Mutex* mu) ABSL_LOCKS_EXCLUDED(deadlock_graph_mu) {
+  base_internal::SpinLockHolder l(deadlock_graph_mu);
   GraphId id = GetGraphIdLocked(mu);
-  deadlock_graph_mu.Unlock();
   return id;
 }
 
 // Record a lock acquisition.  This is used in debug mode for deadlock
 // detection.  The held_locks pointer points to the relevant data
 // structure for each case.
-static void LockEnter(Mutex* mu, GraphId id, SynchLocksHeld *held_locks) {
+static void LockEnter(Mutex* mu, GraphId id, SynchLocksHeld* held_locks) {
   int n = held_locks->n;
   int i = 0;
   while (i != n && held_locks->locks[i].id != id) {
     i++;
   }
   if (i == n) {
-    if (n == ABSL_ARRAYSIZE(held_locks->locks)) {
+    if (n == static_cast<int>(std::size(held_locks->locks))) {
       held_locks->overflow = true;  // lost some data
     } else {                        // we have room for lock
       held_locks->locks[i].mu = mu;
@@ -1186,7 +1264,7 @@ static void LockEnter(Mutex* mu, GraphId id, SynchLocksHeld *held_locks) {
 // eventually followed by a call to LockLeave(mu, id, x) by the same thread.
 // It does not process the event if is not needed when deadlock detection is
 // disabled.
-static void LockLeave(Mutex* mu, GraphId id, SynchLocksHeld *held_locks) {
+static void LockLeave(Mutex* mu, GraphId id, SynchLocksHeld* held_locks) {
   int n = held_locks->n;
   int i = 0;
   while (i != n && held_locks->locks[i].id != id) {
@@ -1201,11 +1279,11 @@ static void LockLeave(Mutex* mu, GraphId id, SynchLocksHeld *held_locks) {
         i++;
       }
       if (i == n) {  // mu missing means releasing unheld lock
-        SynchEvent *mu_events = GetSynchEvent(mu);
+        SynchEvent* mu_events = GetSynchEvent(mu);
         ABSL_RAW_LOG(FATAL,
                      "thread releasing lock it does not hold: %p %s; "
                      ,
-                     static_cast<void *>(mu),
+                     static_cast<void*>(mu),
                      mu_events == nullptr ? "" : mu_events->name);
       }
     }
@@ -1222,7 +1300,7 @@ static void LockLeave(Mutex* mu, GraphId id, SynchLocksHeld *held_locks) {
 }
 
 // Call LockEnter() if in debug mode and deadlock detection is enabled.
-static inline void DebugOnlyLockEnter(Mutex *mu) {
+static inline void DebugOnlyLockEnter(Mutex* mu) {
   if (kDebugMode) {
     if (synch_deadlock_detection.load(std::memory_order_acquire) !=
         OnDeadlockCycle::kIgnore) {
@@ -1232,7 +1310,7 @@ static inline void DebugOnlyLockEnter(Mutex *mu) {
 }
 
 // Call LockEnter() if in debug mode and deadlock detection is enabled.
-static inline void DebugOnlyLockEnter(Mutex *mu, GraphId id) {
+static inline void DebugOnlyLockEnter(Mutex* mu, GraphId id) {
   if (kDebugMode) {
     if (synch_deadlock_detection.load(std::memory_order_acquire) !=
         OnDeadlockCycle::kIgnore) {
@@ -1242,7 +1320,7 @@ static inline void DebugOnlyLockEnter(Mutex *mu, GraphId id) {
 }
 
 // Call LockLeave() if in debug mode and deadlock detection is enabled.
-static inline void DebugOnlyLockLeave(Mutex *mu) {
+static inline void DebugOnlyLockLeave(Mutex* mu) {
   if (kDebugMode) {
     if (synch_deadlock_detection.load(std::memory_order_acquire) !=
         OnDeadlockCycle::kIgnore) {
@@ -1251,36 +1329,39 @@ static inline void DebugOnlyLockLeave(Mutex *mu) {
   }
 }
 
-static char *StackString(void **pcs, int n, char *buf, int maxlen,
+static char* StackString(void** pcs, int n, char* buf, int maxlen,
                          bool symbolize) {
-  static const int kSymLen = 200;
+  static constexpr int kSymLen = 200;
   char sym[kSymLen];
   int len = 0;
   for (int i = 0; i != n; i++) {
+    if (len >= maxlen) return buf;
+    size_t count = static_cast<size_t>(maxlen - len);
     if (symbolize) {
-      if (!symbolizer(pcs[i], sym, kSymLen)) {
+      if (!absl::Symbolize(pcs[i], sym, kSymLen)) {
         sym[0] = '\0';
       }
-      snprintf(buf + len, maxlen - len, "%s\t@ %p %s\n",
-               (i == 0 ? "\n" : ""),
-               pcs[i], sym);
+      snprintf(buf + len, count, "%s\t@ %p %s\n", (i == 0 ? "\n" : ""), pcs[i],
+               sym);
     } else {
-      snprintf(buf + len, maxlen - len, " %p", pcs[i]);
+      snprintf(buf + len, count, " %p", pcs[i]);
     }
-    len += strlen(&buf[len]);
+    len += static_cast<int>(strlen(&buf[len]));
   }
   return buf;
 }
 
-static char *CurrentStackString(char *buf, int maxlen, bool symbolize) {
-  void *pcs[40];
-  return StackString(pcs, absl::GetStackTrace(pcs, ABSL_ARRAYSIZE(pcs), 2), buf,
+static char* CurrentStackString(char* buf, int maxlen, bool symbolize) {
+  void* pcs[40];
+  return StackString(pcs, absl::GetStackTrace(pcs, std::size(pcs), 2), buf,
                      maxlen, symbolize);
 }
 
 namespace {
-enum { kMaxDeadlockPathLen = 10 };  // maximum length of a deadlock cycle;
-                                    // a path this long would be remarkable
+enum {
+  kMaxDeadlockPathLen = 10
+};  // maximum length of a deadlock cycle;
+    // a path this long would be remarkable
 // Buffers required to report a deadlock.
 // We do not allocate them on stack to avoid large stack frame.
 struct DeadlockReportBuffers {
@@ -1290,11 +1371,11 @@ struct DeadlockReportBuffers {
 
 struct ScopedDeadlockReportBuffers {
   ScopedDeadlockReportBuffers() {
-    b = reinterpret_cast<DeadlockReportBuffers *>(
+    b = reinterpret_cast<DeadlockReportBuffers*>(
         base_internal::LowLevelAlloc::Alloc(sizeof(*b)));
   }
   ~ScopedDeadlockReportBuffers() { base_internal::LowLevelAlloc::Free(b); }
-  DeadlockReportBuffers *b;
+  DeadlockReportBuffers* b;
 };
 
 // Helper to pass to GraphCycles::UpdateStackTrace.
@@ -1305,15 +1386,15 @@ int GetStack(void** stack, int max_depth) {
 
 // Called in debug mode when a thread is about to acquire a lock in a way that
 // may block.
-static GraphId DeadlockCheck(Mutex *mu) {
+static GraphId DeadlockCheck(Mutex* mu) {
   if (synch_deadlock_detection.load(std::memory_order_acquire) ==
       OnDeadlockCycle::kIgnore) {
     return InvalidGraphId();
   }
 
-  SynchLocksHeld *all_locks = Synch_GetAllLocks();
+  SynchLocksHeld* all_locks = Synch_GetAllLocks();
 
-  absl::base_internal::SpinLockHolder lock(&deadlock_graph_mu);
+  absl::base_internal::SpinLockHolder lock(deadlock_graph_mu);
   const GraphId mu_id = GetGraphIdLocked(mu);
 
   if (all_locks->n == 0) {
@@ -1333,8 +1414,8 @@ static GraphId DeadlockCheck(Mutex *mu) {
   // For each other mutex already held by this thread:
   for (int i = 0; i != all_locks->n; i++) {
     const GraphId other_node_id = all_locks->locks[i].id;
-    const Mutex *other =
-        static_cast<const Mutex *>(deadlock_graph->Ptr(other_node_id));
+    const Mutex* other =
+        static_cast<const Mutex*>(deadlock_graph->Ptr(other_node_id));
     if (other == nullptr) {
       // Ignore stale lock
       continue;
@@ -1343,46 +1424,52 @@ static GraphId DeadlockCheck(Mutex *mu) {
     // Add the acquired-before edge to the graph.
     if (!deadlock_graph->InsertEdge(other_node_id, mu_id)) {
       ScopedDeadlockReportBuffers scoped_buffers;
-      DeadlockReportBuffers *b = scoped_buffers.b;
+      DeadlockReportBuffers* b = scoped_buffers.b;
       static int number_of_reported_deadlocks = 0;
       number_of_reported_deadlocks++;
       // Symbolize only 2 first deadlock report to avoid huge slowdowns.
       bool symbolize = number_of_reported_deadlocks <= 2;
       ABSL_RAW_LOG(ERROR, "Potential Mutex deadlock: %s",
                    CurrentStackString(b->buf, sizeof (b->buf), symbolize));
-      int len = 0;
+      size_t len = 0;
       for (int j = 0; j != all_locks->n; j++) {
         void* pr = deadlock_graph->Ptr(all_locks->locks[j].id);
         if (pr != nullptr) {
-          snprintf(b->buf + len, sizeof (b->buf) - len, " %p", pr);
-          len += static_cast<int>(strlen(&b->buf[len]));
+          snprintf(b->buf + len, sizeof(b->buf) - len, " %p", pr);
+          len += strlen(&b->buf[len]);
         }
       }
-      ABSL_RAW_LOG(ERROR, "Acquiring %p    Mutexes held: %s",
-                   static_cast<void *>(mu), b->buf);
+      ABSL_RAW_LOG(ERROR,
+                   "Acquiring absl::Mutex %p while holding %s; a cycle in the "
+                   "historical lock ordering graph has been observed",
+                   static_cast<void*>(mu), b->buf);
       ABSL_RAW_LOG(ERROR, "Cycle: ");
       int path_len = deadlock_graph->FindPath(
-          mu_id, other_node_id, ABSL_ARRAYSIZE(b->path), b->path);
-      for (int j = 0; j != path_len; j++) {
+          mu_id, other_node_id, static_cast<int>(std::size(b->path)), b->path);
+      for (int j = 0;
+           j != path_len && j != static_cast<int>(std::size(b->path)); j++) {
         GraphId id = b->path[j];
-        Mutex *path_mu = static_cast<Mutex *>(deadlock_graph->Ptr(id));
+        Mutex* path_mu = static_cast<Mutex*>(deadlock_graph->Ptr(id));
         if (path_mu == nullptr) continue;
         void** stack;
         int depth = deadlock_graph->GetStackTrace(id, &stack);
         snprintf(b->buf, sizeof(b->buf),
-                 "mutex@%p stack: ", static_cast<void *>(path_mu));
+                 "mutex@%p stack: ", static_cast<void*>(path_mu));
         StackString(stack, depth, b->buf + strlen(b->buf),
                     static_cast<int>(sizeof(b->buf) - strlen(b->buf)),
                     symbolize);
         ABSL_RAW_LOG(ERROR, "%s", b->buf);
       }
+      if (path_len > static_cast<int>(std::size(b->path))) {
+        ABSL_RAW_LOG(ERROR, "(long cycle; list truncated)");
+      }
       if (synch_deadlock_detection.load(std::memory_order_acquire) ==
           OnDeadlockCycle::kAbort) {
-        deadlock_graph_mu.Unlock();  // avoid deadlock in fatal sighandler
+        deadlock_graph_mu.unlock();  // avoid deadlock in fatal sighandler
         ABSL_RAW_LOG(FATAL, "dying due to potential deadlock");
         return mu_id;
       }
-      break;   // report at most one potential deadlock per acquisition
+      break;  // report at most one potential deadlock per acquisition
     }
   }
 
@@ -1391,7 +1478,7 @@ static GraphId DeadlockCheck(Mutex *mu) {
 
 // Invoke DeadlockCheck() iff we're in debug mode and
 // deadlock checking has been enabled.
-static inline GraphId DebugOnlyDeadlockCheck(Mutex *mu) {
+static inline GraphId DebugOnlyDeadlockCheck(Mutex* mu) {
   if (kDebugMode && synch_deadlock_detection.load(std::memory_order_acquire) !=
                         OnDeadlockCycle::kIgnore) {
     return DeadlockCheck(mu);
@@ -1403,11 +1490,11 @@ static inline GraphId DebugOnlyDeadlockCheck(Mutex *mu) {
 void Mutex::ForgetDeadlockInfo() {
   if (kDebugMode && synch_deadlock_detection.load(std::memory_order_acquire) !=
                         OnDeadlockCycle::kIgnore) {
-    deadlock_graph_mu.Lock();
+    deadlock_graph_mu.lock();
     if (deadlock_graph != nullptr) {
       deadlock_graph->RemoveNode(this);
     }
-    deadlock_graph_mu.Unlock();
+    deadlock_graph_mu.unlock();
   }
 }
 
@@ -1418,13 +1505,13 @@ void Mutex::AssertNotHeld() const {
       (mu_.load(std::memory_order_relaxed) & (kMuWriter | kMuReader)) != 0 &&
       synch_deadlock_detection.load(std::memory_order_acquire) !=
           OnDeadlockCycle::kIgnore) {
-    GraphId id = GetGraphId(const_cast<Mutex *>(this));
-    SynchLocksHeld *locks = Synch_GetAllLocks();
+    GraphId id = GetGraphId(const_cast<Mutex*>(this));
+    SynchLocksHeld* locks = Synch_GetAllLocks();
     for (int i = 0; i != locks->n; i++) {
       if (locks->locks[i].id == id) {
-        SynchEvent *mu_events = GetSynchEvent(this);
+        SynchEvent* mu_events = GetSynchEvent(this);
         ABSL_RAW_LOG(FATAL, "thread should not hold mutex %p %s",
-                     static_cast<const void *>(this),
+                     static_cast<const void*>(this),
                      (mu_events == nullptr ? "" : mu_events->name));
       }
     }
@@ -1434,34 +1521,32 @@ void Mutex::AssertNotHeld() const {
 // Attempt to acquire *mu, and return whether successful.  The implementation
 // may spin for a short while if the lock cannot be acquired immediately.
 static bool TryAcquireWithSpinning(std::atomic<intptr_t>* mu) {
-  int c = mutex_globals.spinloop_iterations;
-  int result = -1;  // result of operation:  0=false, 1=true, -1=unknown
-
+  int c = globals.spinloop_iterations.load(std::memory_order_relaxed);
   do {  // do/while somewhat faster on AMD
     intptr_t v = mu->load(std::memory_order_relaxed);
-    if ((v & (kMuReader|kMuEvent)) != 0) {  // a reader or tracing -> give up
-      result = 0;
+    if ((v & (kMuReader | kMuEvent)) != 0) {
+      return false;                       // a reader or tracing -> give up
     } else if (((v & kMuWriter) == 0) &&  // no holder -> try to acquire
                mu->compare_exchange_strong(v, kMuWriter | v,
                                            std::memory_order_acquire,
                                            std::memory_order_relaxed)) {
-      result = 1;
+      return true;
     }
-  } while (result == -1 && --c > 0);
-  return result == 1;
+  } while (--c > 0);
+  return false;
 }
 
-ABSL_XRAY_LOG_ARGS(1) void Mutex::Lock() {
+void Mutex::lock() {
   ABSL_TSAN_MUTEX_PRE_LOCK(this, 0);
   GraphId id = DebugOnlyDeadlockCheck(this);
   intptr_t v = mu_.load(std::memory_order_relaxed);
   // try fast acquire, then spin loop
-  if ((v & (kMuWriter | kMuReader | kMuEvent)) != 0 ||
-      !mu_.compare_exchange_strong(v, kMuWriter | v,
-                                   std::memory_order_acquire,
-                                   std::memory_order_relaxed)) {
+  if (ABSL_PREDICT_FALSE((v & (kMuWriter | kMuReader | kMuEvent)) != 0) ||
+      ABSL_PREDICT_FALSE(!mu_.compare_exchange_strong(
+          v, kMuWriter | v, std::memory_order_acquire,
+          std::memory_order_relaxed))) {
     // try spin acquire, then slow loop
-    if (!TryAcquireWithSpinning(&this->mu_)) {
+    if (ABSL_PREDICT_FALSE(!TryAcquireWithSpinning(&this->mu_))) {
       this->LockSlow(kExclusive, nullptr, 0);
     }
   }
@@ -1469,187 +1554,159 @@ ABSL_XRAY_LOG_ARGS(1) void Mutex::Lock() {
   ABSL_TSAN_MUTEX_POST_LOCK(this, 0, 0);
 }
 
-ABSL_XRAY_LOG_ARGS(1) void Mutex::ReaderLock() {
+void Mutex::lock_shared() {
   ABSL_TSAN_MUTEX_PRE_LOCK(this, __tsan_mutex_read_lock);
   GraphId id = DebugOnlyDeadlockCheck(this);
   intptr_t v = mu_.load(std::memory_order_relaxed);
-  // try fast acquire, then slow loop
-  if ((v & (kMuWriter | kMuWait | kMuEvent)) != 0 ||
-      !mu_.compare_exchange_strong(v, (kMuReader | v) + kMuOne,
-                                   std::memory_order_acquire,
-                                   std::memory_order_relaxed)) {
-    this->LockSlow(kShared, nullptr, 0);
+  for (;;) {
+    // If there are non-readers holding the lock, use the slow loop.
+    if (ABSL_PREDICT_FALSE(v & (kMuWriter | kMuWait | kMuEvent)) != 0) {
+      this->LockSlow(kShared, nullptr, 0);
+      break;
+    }
+    // We can avoid the loop and only use the CAS when the lock is free or
+    // only held by readers.
+    if (ABSL_PREDICT_TRUE(mu_.compare_exchange_weak(
+            v, (kMuReader | v) + kMuOne, std::memory_order_acquire,
+            std::memory_order_relaxed))) {
+      break;
+    }
   }
   DebugOnlyLockEnter(this, id);
   ABSL_TSAN_MUTEX_POST_LOCK(this, __tsan_mutex_read_lock, 0);
 }
 
-void Mutex::LockWhen(const Condition &cond) {
-  ABSL_TSAN_MUTEX_PRE_LOCK(this, 0);
-  GraphId id = DebugOnlyDeadlockCheck(this);
-  this->LockSlow(kExclusive, &cond, 0);
-  DebugOnlyLockEnter(this, id);
-  ABSL_TSAN_MUTEX_POST_LOCK(this, 0, 0);
-}
-
-bool Mutex::LockWhenWithTimeout(const Condition &cond, absl::Duration timeout) {
-  return LockWhenWithDeadline(cond, DeadlineFromTimeout(timeout));
-}
-
-bool Mutex::LockWhenWithDeadline(const Condition &cond, absl::Time deadline) {
-  ABSL_TSAN_MUTEX_PRE_LOCK(this, 0);
-  GraphId id = DebugOnlyDeadlockCheck(this);
-  bool res = LockSlowWithDeadline(kExclusive, &cond,
-                                  KernelTimeout(deadline), 0);
-  DebugOnlyLockEnter(this, id);
-  ABSL_TSAN_MUTEX_POST_LOCK(this, 0, 0);
-  return res;
-}
-
-void Mutex::ReaderLockWhen(const Condition &cond) {
-  ABSL_TSAN_MUTEX_PRE_LOCK(this, __tsan_mutex_read_lock);
-  GraphId id = DebugOnlyDeadlockCheck(this);
-  this->LockSlow(kShared, &cond, 0);
-  DebugOnlyLockEnter(this, id);
-  ABSL_TSAN_MUTEX_POST_LOCK(this, __tsan_mutex_read_lock, 0);
-}
-
-bool Mutex::ReaderLockWhenWithTimeout(const Condition &cond,
-                                      absl::Duration timeout) {
-  return ReaderLockWhenWithDeadline(cond, DeadlineFromTimeout(timeout));
-}
-
-bool Mutex::ReaderLockWhenWithDeadline(const Condition &cond,
-                                       absl::Time deadline) {
-  ABSL_TSAN_MUTEX_PRE_LOCK(this, __tsan_mutex_read_lock);
+bool Mutex::LockWhenCommon(const Condition& cond,
+                           synchronization_internal::KernelTimeout t,
+                           bool write) {
+  MuHow how = write ? kExclusive : kShared;
+  ABSL_TSAN_MUTEX_PRE_LOCK(this, TsanFlags(how));
   GraphId id = DebugOnlyDeadlockCheck(this);
-  bool res = LockSlowWithDeadline(kShared, &cond, KernelTimeout(deadline), 0);
+  bool res = LockSlowWithDeadline(how, &cond, t, 0);
   DebugOnlyLockEnter(this, id);
-  ABSL_TSAN_MUTEX_POST_LOCK(this, __tsan_mutex_read_lock, 0);
+  ABSL_TSAN_MUTEX_POST_LOCK(this, TsanFlags(how), 0);
   return res;
 }
 
-void Mutex::Await(const Condition &cond) {
-  if (cond.Eval()) {    // condition already true; nothing to do
-    if (kDebugMode) {
-      this->AssertReaderHeld();
-    }
-  } else {              // normal case
-    ABSL_RAW_CHECK(this->AwaitCommon(cond, KernelTimeout::Never()),
-                   "condition untrue on return from Await");
+bool Mutex::AwaitCommon(const Condition& cond, KernelTimeout t) {
+  if (kDebugMode) {
+    this->AssertReaderHeld();
   }
-}
-
-bool Mutex::AwaitWithTimeout(const Condition &cond, absl::Duration timeout) {
-  return AwaitWithDeadline(cond, DeadlineFromTimeout(timeout));
-}
-
-bool Mutex::AwaitWithDeadline(const Condition &cond, absl::Time deadline) {
-  if (cond.Eval()) {      // condition already true; nothing to do
-    if (kDebugMode) {
-      this->AssertReaderHeld();
-    }
+  if (cond.Eval()) {  // condition already true; nothing to do
     return true;
   }
-
-  KernelTimeout t{deadline};
-  bool res = this->AwaitCommon(cond, t);
-  ABSL_RAW_CHECK(res || t.has_timeout(),
-                 "condition untrue on return from Await");
-  return res;
-}
-
-bool Mutex::AwaitCommon(const Condition &cond, KernelTimeout t) {
-  this->AssertReaderHeld();
   MuHow how =
       (mu_.load(std::memory_order_relaxed) & kMuWriter) ? kExclusive : kShared;
   ABSL_TSAN_MUTEX_PRE_UNLOCK(this, TsanFlags(how));
-  SynchWaitParams waitp(
-      how, &cond, t, nullptr /*no cvmu*/, Synch_GetPerThreadAnnotated(this),
-      nullptr /*no cv_word*/);
-  int flags = kMuHasBlocked;
-  if (!Condition::GuaranteedEqual(&cond, nullptr)) {
-    flags |= kMuIsCond;
-  }
+  SynchWaitParams waitp(how, &cond, t, nullptr /*no cvmu*/,
+                        Synch_GetPerThreadAnnotated(this),
+                        nullptr /*no cv_word*/);
   this->UnlockSlow(&waitp);
   this->Block(waitp.thread);
   ABSL_TSAN_MUTEX_POST_UNLOCK(this, TsanFlags(how));
   ABSL_TSAN_MUTEX_PRE_LOCK(this, TsanFlags(how));
-  this->LockSlowLoop(&waitp, flags);
+  this->LockSlowLoop(&waitp, kMuHasBlocked | kMuIsCond);
   bool res = waitp.cond != nullptr ||  // => cond known true from LockSlowLoop
              EvalConditionAnnotated(&cond, this, true, false, how == kShared);
   ABSL_TSAN_MUTEX_POST_LOCK(this, TsanFlags(how), 0);
+  ABSL_RAW_CHECK(res || t.has_timeout(),
+                 "condition untrue on return from Await");
   return res;
 }
 
-ABSL_XRAY_LOG_ARGS(1) bool Mutex::TryLock() {
+bool Mutex::try_lock() {
   ABSL_TSAN_MUTEX_PRE_LOCK(this, __tsan_mutex_try_lock);
   intptr_t v = mu_.load(std::memory_order_relaxed);
-  if ((v & (kMuWriter | kMuReader | kMuEvent)) == 0 &&  // try fast acquire
-      mu_.compare_exchange_strong(v, kMuWriter | v,
-                                  std::memory_order_acquire,
-                                  std::memory_order_relaxed)) {
-    DebugOnlyLockEnter(this);
-    ABSL_TSAN_MUTEX_POST_LOCK(this, __tsan_mutex_try_lock, 0);
-    return true;
-  }
-  if ((v & kMuEvent) != 0) {              // we're recording events
-    if ((v & kExclusive->slow_need_zero) == 0 &&  // try fast acquire
-        mu_.compare_exchange_strong(
-            v, (kExclusive->fast_or | v) + kExclusive->fast_add,
-            std::memory_order_acquire, std::memory_order_relaxed)) {
+  // Try fast acquire.
+  if (ABSL_PREDICT_TRUE((v & (kMuWriter | kMuReader | kMuEvent)) == 0)) {
+    if (ABSL_PREDICT_TRUE(mu_.compare_exchange_strong(
+            v, kMuWriter | v, std::memory_order_acquire,
+            std::memory_order_relaxed))) {
       DebugOnlyLockEnter(this);
-      PostSynchEvent(this, SYNCH_EV_TRYLOCK_SUCCESS);
       ABSL_TSAN_MUTEX_POST_LOCK(this, __tsan_mutex_try_lock, 0);
       return true;
-    } else {
-      PostSynchEvent(this, SYNCH_EV_TRYLOCK_FAILED);
     }
+  } else if (ABSL_PREDICT_FALSE((v & kMuEvent) != 0)) {
+    // We're recording events.
+    return TryLockSlow();
   }
   ABSL_TSAN_MUTEX_POST_LOCK(
       this, __tsan_mutex_try_lock | __tsan_mutex_try_lock_failed, 0);
   return false;
 }
 
-ABSL_XRAY_LOG_ARGS(1) bool Mutex::ReaderTryLock() {
+ABSL_ATTRIBUTE_NOINLINE bool Mutex::TryLockSlow() {
+  intptr_t v = mu_.load(std::memory_order_relaxed);
+  if ((v & kExclusive->slow_need_zero) == 0 &&  // try fast acquire
+      mu_.compare_exchange_strong(
+          v, (kExclusive->fast_or | v) + kExclusive->fast_add,
+          std::memory_order_acquire, std::memory_order_relaxed)) {
+    DebugOnlyLockEnter(this);
+    PostSynchEvent(this, SYNCH_EV_TRYLOCK_SUCCESS);
+    ABSL_TSAN_MUTEX_POST_LOCK(this, __tsan_mutex_try_lock, 0);
+    return true;
+  }
+  PostSynchEvent(this, SYNCH_EV_TRYLOCK_FAILED);
+  ABSL_TSAN_MUTEX_POST_LOCK(
+      this, __tsan_mutex_try_lock | __tsan_mutex_try_lock_failed, 0);
+  return false;
+}
+
+bool Mutex::try_lock_shared() {
   ABSL_TSAN_MUTEX_PRE_LOCK(this,
                            __tsan_mutex_read_lock | __tsan_mutex_try_lock);
   intptr_t v = mu_.load(std::memory_order_relaxed);
+  // Clang tends to unroll the loop when compiling with optimization.
+  // But in this case it just unnecessary increases code size.
+  // If CAS is failing due to contention, the jump cost is negligible.
+#if defined(__clang__)
+#pragma nounroll
+#endif
   // The while-loops (here and below) iterate only if the mutex word keeps
-  // changing (typically because the reader count changes) under the CAS.  We
-  // limit the number of attempts to avoid having to think about livelock.
-  int loop_limit = 5;
-  while ((v & (kMuWriter|kMuWait|kMuEvent)) == 0 && loop_limit != 0) {
-    if (mu_.compare_exchange_strong(v, (kMuReader | v) + kMuOne,
-                                    std::memory_order_acquire,
-                                    std::memory_order_relaxed)) {
+  // changing (typically because the reader count changes) under the CAS.
+  // We limit the number of attempts to avoid having to think about livelock.
+  for (int loop_limit = 5; loop_limit != 0; loop_limit--) {
+    if (ABSL_PREDICT_FALSE((v & (kMuWriter | kMuWait | kMuEvent)) != 0)) {
+      break;
+    }
+    if (ABSL_PREDICT_TRUE(mu_.compare_exchange_strong(
+            v, (kMuReader | v) + kMuOne, std::memory_order_acquire,
+            std::memory_order_relaxed))) {
       DebugOnlyLockEnter(this);
       ABSL_TSAN_MUTEX_POST_LOCK(
           this, __tsan_mutex_read_lock | __tsan_mutex_try_lock, 0);
       return true;
     }
-    loop_limit--;
-    v = mu_.load(std::memory_order_relaxed);
   }
-  if ((v & kMuEvent) != 0) {   // we're recording events
-    loop_limit = 5;
-    while ((v & kShared->slow_need_zero) == 0 && loop_limit != 0) {
-      if (mu_.compare_exchange_strong(v, (kMuReader | v) + kMuOne,
-                                      std::memory_order_acquire,
-                                      std::memory_order_relaxed)) {
-        DebugOnlyLockEnter(this);
-        PostSynchEvent(this, SYNCH_EV_READERTRYLOCK_SUCCESS);
-        ABSL_TSAN_MUTEX_POST_LOCK(
-            this, __tsan_mutex_read_lock | __tsan_mutex_try_lock, 0);
-        return true;
-      }
-      loop_limit--;
-      v = mu_.load(std::memory_order_relaxed);
-    }
-    if ((v & kMuEvent) != 0) {
-      PostSynchEvent(this, SYNCH_EV_READERTRYLOCK_FAILED);
+  if (ABSL_PREDICT_TRUE((v & kMuEvent) == 0)) {
+    ABSL_TSAN_MUTEX_POST_LOCK(this,
+                              __tsan_mutex_read_lock | __tsan_mutex_try_lock |
+                                  __tsan_mutex_try_lock_failed,
+                              0);
+    return false;
+  }
+  // we're recording events
+  return ReaderTryLockSlow();
+}
+
+ABSL_ATTRIBUTE_NOINLINE bool Mutex::ReaderTryLockSlow() {
+  intptr_t v = mu_.load(std::memory_order_relaxed);
+#if defined(__clang__)
+#pragma nounroll
+#endif
+  for (int loop_limit = 5; loop_limit != 0; loop_limit--) {
+    if ((v & kShared->slow_need_zero) == 0 &&
+        mu_.compare_exchange_strong(v, (kMuReader | v) + kMuOne,
+                                    std::memory_order_acquire,
+                                    std::memory_order_relaxed)) {
+      DebugOnlyLockEnter(this);
+      PostSynchEvent(this, SYNCH_EV_READERTRYLOCK_SUCCESS);
+      ABSL_TSAN_MUTEX_POST_LOCK(
+          this, __tsan_mutex_read_lock | __tsan_mutex_try_lock, 0);
+      return true;
     }
   }
+  PostSynchEvent(this, SYNCH_EV_READERTRYLOCK_FAILED);
   ABSL_TSAN_MUTEX_POST_LOCK(this,
                             __tsan_mutex_read_lock | __tsan_mutex_try_lock |
                                 __tsan_mutex_try_lock_failed,
@@ -1657,7 +1714,7 @@ ABSL_XRAY_LOG_ARGS(1) bool Mutex::ReaderTryLock() {
   return false;
 }
 
-ABSL_XRAY_LOG_ARGS(1) void Mutex::Unlock() {
+void Mutex::unlock() {
   ABSL_TSAN_MUTEX_PRE_UNLOCK(this, 0);
   DebugOnlyLockLeave(this);
   intptr_t v = mu_.load(std::memory_order_relaxed);
@@ -1670,24 +1727,42 @@ ABSL_XRAY_LOG_ARGS(1) void Mutex::Unlock() {
   // should_try_cas is whether we'll try a compare-and-swap immediately.
   // NOTE: optimized out when kDebugMode is false.
   bool should_try_cas = ((v & (kMuEvent | kMuWriter)) == kMuWriter &&
-                          (v & (kMuWait | kMuDesig)) != kMuWait);
+                         (v & (kMuWait | kMuDesig)) != kMuWait);
+
   // But, we can use an alternate computation of it, that compilers
   // currently don't find on their own.  When that changes, this function
   // can be simplified.
-  intptr_t x = (v ^ (kMuWriter | kMuWait)) & (kMuWriter | kMuEvent);
-  intptr_t y = (v ^ (kMuWriter | kMuWait)) & (kMuWait | kMuDesig);
-  // Claim: "x == 0 && y > 0" is equal to should_try_cas.
-  // Also, because kMuWriter and kMuEvent exceed kMuDesig and kMuWait,
-  // all possible non-zero values for x exceed all possible values for y.
-  // Therefore, (x == 0 && y > 0) == (x < y).
-  if (kDebugMode && should_try_cas != (x < y)) {
+  //
+  // should_try_cas is true iff the bits satisfy the following conditions:
+  //
+  //                   Ev Wr Wa De
+  // equal to           0  1
+  // and not equal to         1  0
+  //
+  // after xoring by    0  1  0  1,  this is equivalent to:
+  //
+  // equal to           0  0
+  // and not equal to         1  1,  which is the same as:
+  //
+  // smaller than       0  0  1  1
+  static_assert(kMuEvent > kMuWait, "Needed for should_try_cas_fast");
+  static_assert(kMuEvent > kMuDesig, "Needed for should_try_cas_fast");
+  static_assert(kMuWriter > kMuWait, "Needed for should_try_cas_fast");
+  static_assert(kMuWriter > kMuDesig, "Needed for should_try_cas_fast");
+
+  bool should_try_cas_fast =
+      ((v ^ (kMuWriter | kMuDesig)) &
+       (kMuEvent | kMuWriter | kMuWait | kMuDesig)) < (kMuWait | kMuDesig);
+
+  if (kDebugMode && should_try_cas != should_try_cas_fast) {
     // We would usually use PRIdPTR here, but is not correctly implemented
     // within the android toolchain.
     ABSL_RAW_LOG(FATAL, "internal logic error %llx %llx %llx\n",
-                 static_cast<long long>(v), static_cast<long long>(x),
-                 static_cast<long long>(y));
+                 static_cast<long long>(v),
+                 static_cast<long long>(should_try_cas),
+                 static_cast<long long>(should_try_cas_fast));
   }
-  if (x < y &&
+  if (should_try_cas_fast &&
       mu_.compare_exchange_strong(v, v & ~(kMuWrWait | kMuWriter),
                                   std::memory_order_release,
                                   std::memory_order_relaxed)) {
@@ -1700,7 +1775,7 @@ ABSL_XRAY_LOG_ARGS(1) void Mutex::Unlock() {
 
 // Requires v to represent a reader-locked state.
 static bool ExactlyOneReader(intptr_t v) {
-  assert((v & (kMuWriter|kMuReader)) == kMuReader);
+  assert((v & (kMuWriter | kMuReader)) == kMuReader);
   assert((v & kMuHigh) != 0);
   // The more straightforward "(v & kMuHigh) == kMuOne" also works, but
   // on some architectures the following generates slightly smaller code.
@@ -1709,52 +1784,82 @@ static bool ExactlyOneReader(intptr_t v) {
   return (v & kMuMultipleWaitersMask) == 0;
 }
 
-ABSL_XRAY_LOG_ARGS(1) void Mutex::ReaderUnlock() {
+void Mutex::unlock_shared() {
   ABSL_TSAN_MUTEX_PRE_UNLOCK(this, __tsan_mutex_read_lock);
   DebugOnlyLockLeave(this);
   intptr_t v = mu_.load(std::memory_order_relaxed);
-  assert((v & (kMuWriter|kMuReader)) == kMuReader);
-  if ((v & (kMuReader|kMuWait|kMuEvent)) == kMuReader) {
+  assert((v & (kMuWriter | kMuReader)) == kMuReader);
+  for (;;) {
+    if (ABSL_PREDICT_FALSE((v & (kMuReader | kMuWait | kMuEvent)) !=
+                           kMuReader)) {
+      this->UnlockSlow(nullptr /*no waitp*/);  // take slow path
+      break;
+    }
     // fast reader release (reader with no waiters)
-    intptr_t clear = ExactlyOneReader(v) ? kMuReader|kMuOne : kMuOne;
-    if (mu_.compare_exchange_strong(v, v - clear,
-                                    std::memory_order_release,
-                                    std::memory_order_relaxed)) {
-      ABSL_TSAN_MUTEX_POST_UNLOCK(this, __tsan_mutex_read_lock);
-      return;
+    intptr_t clear = ExactlyOneReader(v) ? kMuReader | kMuOne : kMuOne;
+    if (ABSL_PREDICT_TRUE(
+            mu_.compare_exchange_strong(v, v - clear, std::memory_order_release,
+                                        std::memory_order_relaxed))) {
+      break;
     }
   }
-  this->UnlockSlow(nullptr /*no waitp*/);  // take slow path
   ABSL_TSAN_MUTEX_POST_UNLOCK(this, __tsan_mutex_read_lock);
 }
 
-// The zap_desig_waker bitmask is used to clear the designated waker flag in
-// the mutex if this thread has blocked, and therefore may be the designated
-// waker.
-static const intptr_t zap_desig_waker[] = {
-    ~static_cast<intptr_t>(0),  // not blocked
-    ~static_cast<intptr_t>(
-        kMuDesig)  // blocked; turn off the designated waker bit
-};
+// Clears the designated waker flag in the mutex if this thread has blocked, and
+// therefore may be the designated waker.
+static intptr_t ClearDesignatedWakerMask(int flag) {
+  assert(flag >= 0);
+  assert(flag <= 1);
+  switch (flag) {
+    case 0:  // not blocked
+      return ~static_cast<intptr_t>(0);
+    case 1:  // blocked; turn off the designated waker bit
+      return ~static_cast<intptr_t>(kMuDesig);
+  }
+  ABSL_UNREACHABLE();
+}
 
-// The ignore_waiting_writers bitmask is used to ignore the existence
-// of waiting writers if a reader that has already blocked once
-// wakes up.
-static const intptr_t ignore_waiting_writers[] = {
-    ~static_cast<intptr_t>(0),  // not blocked
-    ~static_cast<intptr_t>(
-        kMuWrWait)  // blocked; pretend there are no waiting writers
-};
+// Conditionally ignores the existence of waiting writers if a reader that has
+// already blocked once wakes up.
+static intptr_t IgnoreWaitingWritersMask(int flag) {
+  assert(flag >= 0);
+  assert(flag <= 1);
+  switch (flag) {
+    case 0:  // not blocked
+      return ~static_cast<intptr_t>(0);
+    case 1:  // blocked; pretend there are no waiting writers
+      return ~static_cast<intptr_t>(kMuWrWait);
+  }
+  ABSL_UNREACHABLE();
+}
 
 // Internal version of LockWhen().  See LockSlowWithDeadline()
-void Mutex::LockSlow(MuHow how, const Condition *cond, int flags) {
+ABSL_ATTRIBUTE_NOINLINE void Mutex::LockSlow(MuHow how, const Condition* cond,
+                                             int flags) {
+  // Note: we specifically initialize spinloop_iterations after the first use
+  // in TryAcquireWithSpinning so that Lock function does not have any non-tail
+  // calls and consequently a stack frame. It's fine to have spinloop_iterations
+  // uninitialized (meaning no spinning) in all initial uncontended Lock calls
+  // and in the first contended call. After that we will have
+  // spinloop_iterations properly initialized.
+  if (ABSL_PREDICT_FALSE(
+          globals.spinloop_iterations.load(std::memory_order_relaxed) == 0)) {
+    if (absl::base_internal::NumCPUs() > 1) {
+      // If this is multiprocessor, allow spinning.
+      globals.spinloop_iterations.store(1500, std::memory_order_relaxed);
+    } else {
+      // If this a uniprocessor, only yield/sleep.
+      globals.spinloop_iterations.store(-1, std::memory_order_relaxed);
+    }
+  }
   ABSL_RAW_CHECK(
       this->LockSlowWithDeadline(how, cond, KernelTimeout::Never(), flags),
       "condition untrue on return from LockSlow");
 }
 
 // Compute cond->Eval() and tell race detectors that we do it under mutex mu.
-static inline bool EvalConditionAnnotated(const Condition *cond, Mutex *mu,
+static inline bool EvalConditionAnnotated(const Condition* cond, Mutex* mu,
                                           bool locking, bool trylock,
                                           bool read_lock) {
   // Delicate annotation dance.
@@ -1762,9 +1867,9 @@ static inline bool EvalConditionAnnotated(const Condition *cond, Mutex *mu,
   // All memory accesses are ignored inside of mutex operations + for unlock
   // operation tsan considers that we've already released the mutex.
   bool res = false;
-#ifdef THREAD_SANITIZER
-  const int flags = read_lock ? __tsan_mutex_read_lock : 0;
-  const int tryflags = flags | (trylock ? __tsan_mutex_try_lock : 0);
+#ifdef ABSL_INTERNAL_HAVE_TSAN_INTERFACE
+  const uint32_t flags = read_lock ? __tsan_mutex_read_lock : 0;
+  const uint32_t tryflags = flags | (trylock ? __tsan_mutex_try_lock : 0);
 #endif
   if (locking) {
     // For lock we pretend that we have finished the operation,
@@ -1804,7 +1909,7 @@ static inline bool EvalConditionAnnotated(const Condition *cond, Mutex *mu,
 // tsan). As the result there is no tsan-visible synchronization between the
 // addition and this thread. So if we would enable race detection here,
 // it would race with the predicate initialization.
-static inline bool EvalConditionIgnored(Mutex *mu, const Condition *cond) {
+static inline bool EvalConditionIgnored(Mutex* mu, const Condition* cond) {
   // Memory accesses are already ignored inside of lock/unlock operations,
   // but synchronization operations are also ignored. When we evaluate the
   // predicate we must ignore only memory accesses but not synchronization,
@@ -1812,9 +1917,9 @@ static inline bool EvalConditionIgnored(Mutex *mu, const Condition *cond) {
   // So we "divert" (which un-ignores both memory accesses and synchronization)
   // and then separately turn on ignores of memory accesses.
   ABSL_TSAN_MUTEX_PRE_DIVERT(mu, 0);
-  ANNOTATE_IGNORE_READS_AND_WRITES_BEGIN();
+  ABSL_ANNOTATE_IGNORE_READS_AND_WRITES_BEGIN();
   bool res = cond->Eval();
-  ANNOTATE_IGNORE_READS_AND_WRITES_END();
+  ABSL_ANNOTATE_IGNORE_READS_AND_WRITES_END();
   ABSL_TSAN_MUTEX_POST_DIVERT(mu, 0);
   static_cast<void>(mu);  // Prevent unused param warning in non-TSAN builds.
   return res;
@@ -1829,14 +1934,16 @@ static inline bool EvalConditionIgnored(Mutex *mu, const Condition *cond) {
 //   obstruct this call
 // - kMuIsCond indicates that this is a conditional acquire (condition variable,
 //   Await,  LockWhen) so contention profiling should be suppressed.
-bool Mutex::LockSlowWithDeadline(MuHow how, const Condition *cond,
+bool Mutex::LockSlowWithDeadline(MuHow how, const Condition* cond,
                                  KernelTimeout t, int flags) {
   intptr_t v = mu_.load(std::memory_order_relaxed);
   bool unlock = false;
   if ((v & how->fast_need_zero) == 0 &&  // try fast acquire
       mu_.compare_exchange_strong(
-          v, (how->fast_or | (v & zap_desig_waker[flags & kMuHasBlocked])) +
-                 how->fast_add,
+          v,
+          (how->fast_or |
+           (v & ClearDesignatedWakerMask(flags & kMuHasBlocked))) +
+              how->fast_add,
           std::memory_order_acquire, std::memory_order_relaxed)) {
     if (cond == nullptr ||
         EvalConditionAnnotated(cond, this, true, false, how == kShared)) {
@@ -1844,10 +1951,10 @@ bool Mutex::LockSlowWithDeadline(MuHow how, const Condition *cond,
     }
     unlock = true;
   }
-  SynchWaitParams waitp(
-      how, cond, t, nullptr /*no cvmu*/, Synch_GetPerThreadAnnotated(this),
-      nullptr /*no cv_word*/);
-  if (!Condition::GuaranteedEqual(cond, nullptr)) {
+  SynchWaitParams waitp(how, cond, t, nullptr /*no cvmu*/,
+                        Synch_GetPerThreadAnnotated(this),
+                        nullptr /*no cv_word*/);
+  if (cond != nullptr) {
     flags |= kMuIsCond;
   }
   if (unlock) {
@@ -1875,7 +1982,7 @@ static void CheckForMutexCorruption(intptr_t v, const char* label) {
   // Test for either of two situations that should not occur in v:
   //   kMuWriter and kMuReader
   //   kMuWrWait and !kMuWait
-  const uintptr_t w = v ^ kMuWait;
+  const uintptr_t w = static_cast<uintptr_t>(v ^ kMuWait);
   // By flipping that bit, we can now test for:
   //   kMuWriter and kMuReader in w
   //   kMuWrWait and kMuWait in w
@@ -1887,19 +1994,20 @@ static void CheckForMutexCorruption(intptr_t v, const char* label) {
   if (ABSL_PREDICT_TRUE((w & (w << 3) & (kMuWriter | kMuWrWait)) == 0)) return;
   RAW_CHECK_FMT((v & (kMuWriter | kMuReader)) != (kMuWriter | kMuReader),
                 "%s: Mutex corrupt: both reader and writer lock held: %p",
-                label, reinterpret_cast<void *>(v));
+                label, reinterpret_cast<void*>(v));
   RAW_CHECK_FMT((v & (kMuWait | kMuWrWait)) != kMuWrWait,
-                "%s: Mutex corrupt: waiting writer with no waiters: %p",
-                label, reinterpret_cast<void *>(v));
+                "%s: Mutex corrupt: waiting writer with no waiters: %p", label,
+                reinterpret_cast<void*>(v));
   assert(false);
 }
 
-void Mutex::LockSlowLoop(SynchWaitParams *waitp, int flags) {
+void Mutex::LockSlowLoop(SynchWaitParams* waitp, int flags) {
+  SchedulingGuard::ScopedDisable disable_rescheduling;
   int c = 0;
   intptr_t v = mu_.load(std::memory_order_relaxed);
   if ((v & kMuEvent) != 0) {
-    PostSynchEvent(this,
-         waitp->how == kExclusive?  SYNCH_EV_LOCK: SYNCH_EV_READERLOCK);
+    PostSynchEvent(
+        this, waitp->how == kExclusive ? SYNCH_EV_LOCK : SYNCH_EV_READERLOCK);
   }
   ABSL_RAW_CHECK(
       waitp->thread->waitp == nullptr || waitp->thread->suppress_fatal_errors,
@@ -1909,9 +2017,10 @@ void Mutex::LockSlowLoop(SynchWaitParams *waitp, int flags) {
     CheckForMutexCorruption(v, "Lock");
     if ((v & waitp->how->slow_need_zero) == 0) {
       if (mu_.compare_exchange_strong(
-              v, (waitp->how->fast_or |
-                  (v & zap_desig_waker[flags & kMuHasBlocked])) +
-                     waitp->how->fast_add,
+              v,
+              (waitp->how->fast_or |
+               (v & ClearDesignatedWakerMask(flags & kMuHasBlocked))) +
+                  waitp->how->fast_add,
               std::memory_order_acquire, std::memory_order_relaxed)) {
         if (waitp->cond == nullptr ||
             EvalConditionAnnotated(waitp->cond, this, true, false,
@@ -1923,13 +2032,14 @@ void Mutex::LockSlowLoop(SynchWaitParams *waitp, int flags) {
         flags |= kMuHasBlocked;
         c = 0;
       }
-    } else {                      // need to access waiter list
+    } else {  // need to access waiter list
       bool dowait = false;
-      if ((v & (kMuSpin|kMuWait)) == 0) {   // no waiters
+      if ((v & (kMuSpin | kMuWait)) == 0) {  // no waiters
         // This thread tries to become the one and only waiter.
-        PerThreadSynch *new_h = Enqueue(nullptr, waitp, v, flags);
-        intptr_t nv = (v & zap_desig_waker[flags & kMuHasBlocked] & kMuLow) |
-                      kMuWait;
+        PerThreadSynch* new_h = Enqueue(nullptr, waitp, v, flags);
+        intptr_t nv =
+            (v & ClearDesignatedWakerMask(flags & kMuHasBlocked) & kMuLow) |
+            kMuWait;
         ABSL_RAW_CHECK(new_h != nullptr, "Enqueue to empty list failed");
         if (waitp->how == kExclusive && (v & kMuReader) != 0) {
           nv |= kMuWrWait;
@@ -1938,21 +2048,22 @@ void Mutex::LockSlowLoop(SynchWaitParams *waitp, int flags) {
                 v, reinterpret_cast<intptr_t>(new_h) | nv,
                 std::memory_order_release, std::memory_order_relaxed)) {
           dowait = true;
-        } else {            // attempted Enqueue() failed
+        } else {  // attempted Enqueue() failed
           // zero out the waitp field set by Enqueue()
           waitp->thread->waitp = nullptr;
         }
       } else if ((v & waitp->how->slow_inc_need_zero &
-                  ignore_waiting_writers[flags & kMuHasBlocked]) == 0) {
+                  IgnoreWaitingWritersMask(flags & kMuHasBlocked)) == 0) {
         // This is a reader that needs to increment the reader count,
         // but the count is currently held in the last waiter.
         if (mu_.compare_exchange_strong(
-                v, (v & zap_desig_waker[flags & kMuHasBlocked]) | kMuSpin |
-                       kMuReader,
+                v,
+                (v & ClearDesignatedWakerMask(flags & kMuHasBlocked)) |
+                    kMuSpin | kMuReader,
                 std::memory_order_acquire, std::memory_order_relaxed)) {
-          PerThreadSynch *h = GetPerThreadSynch(v);
-          h->readers += kMuOne;       // inc reader count in waiter
-          do {                        // release spinlock
+          PerThreadSynch* h = GetPerThreadSynch(v);
+          h->readers += kMuOne;  // inc reader count in waiter
+          do {                   // release spinlock
             v = mu_.load(std::memory_order_relaxed);
           } while (!mu_.compare_exchange_weak(v, (v & ~kMuSpin) | kMuReader,
                                               std::memory_order_release,
@@ -1962,28 +2073,30 @@ void Mutex::LockSlowLoop(SynchWaitParams *waitp, int flags) {
                                      waitp->how == kShared)) {
             break;  // we timed out, or condition true, so return
           }
-          this->UnlockSlow(waitp);           // got lock but condition false
+          this->UnlockSlow(waitp);  // got lock but condition false
           this->Block(waitp->thread);
           flags |= kMuHasBlocked;
           c = 0;
         }
       } else if ((v & kMuSpin) == 0 &&  // attempt to queue ourselves
                  mu_.compare_exchange_strong(
-                     v, (v & zap_desig_waker[flags & kMuHasBlocked]) | kMuSpin |
-                            kMuWait,
+                     v,
+                     (v & ClearDesignatedWakerMask(flags & kMuHasBlocked)) |
+                         kMuSpin | kMuWait,
                      std::memory_order_acquire, std::memory_order_relaxed)) {
-        PerThreadSynch *h = GetPerThreadSynch(v);
-        PerThreadSynch *new_h = Enqueue(h, waitp, v, flags);
+        PerThreadSynch* h = GetPerThreadSynch(v);
+        PerThreadSynch* new_h = Enqueue(h, waitp, v, flags);
         intptr_t wr_wait = 0;
         ABSL_RAW_CHECK(new_h != nullptr, "Enqueue to list failed");
         if (waitp->how == kExclusive && (v & kMuReader) != 0) {
-          wr_wait = kMuWrWait;      // give priority to a waiting writer
+          wr_wait = kMuWrWait;  // give priority to a waiting writer
         }
-        do {                        // release spinlock
+        do {  // release spinlock
           v = mu_.load(std::memory_order_relaxed);
         } while (!mu_.compare_exchange_weak(
-            v, (v & (kMuLow & ~kMuSpin)) | kMuWait | wr_wait |
-            reinterpret_cast<intptr_t>(new_h),
+            v,
+            (v & (kMuLow & ~kMuSpin)) | kMuWait | wr_wait |
+                reinterpret_cast<intptr_t>(new_h),
             std::memory_order_release, std::memory_order_relaxed));
         dowait = true;
       }
@@ -1996,15 +2109,16 @@ void Mutex::LockSlowLoop(SynchWaitParams *waitp, int flags) {
     ABSL_RAW_CHECK(
         waitp->thread->waitp == nullptr || waitp->thread->suppress_fatal_errors,
         "detected illegal recursion into Mutex code");
-    c = Delay(c, GENTLE);          // delay, then try again
+    // delay, then try again
+    c = synchronization_internal::MutexDelay(c, GENTLE);
   }
   ABSL_RAW_CHECK(
       waitp->thread->waitp == nullptr || waitp->thread->suppress_fatal_errors,
       "detected illegal recursion into Mutex code");
   if ((v & kMuEvent) != 0) {
-    PostSynchEvent(this,
-                   waitp->how == kExclusive? SYNCH_EV_LOCK_RETURNING :
-                                      SYNCH_EV_READERLOCK_RETURNING);
+    PostSynchEvent(this, waitp->how == kExclusive
+                             ? SYNCH_EV_LOCK_RETURNING
+                             : SYNCH_EV_READERLOCK_RETURNING);
   }
 }
 
@@ -2013,27 +2127,29 @@ void Mutex::LockSlowLoop(SynchWaitParams *waitp, int flags) {
 // which holds the lock but is not runnable because its condition is false
 // or it is in the process of blocking on a condition variable; it must requeue
 // itself on the mutex/condvar to wait for its condition to become true.
-void Mutex::UnlockSlow(SynchWaitParams *waitp) {
+ABSL_ATTRIBUTE_NOINLINE void Mutex::UnlockSlow(SynchWaitParams* waitp) {
+  SchedulingGuard::ScopedDisable disable_rescheduling;
   intptr_t v = mu_.load(std::memory_order_relaxed);
   this->AssertReaderHeld();
   CheckForMutexCorruption(v, "Unlock");
   if ((v & kMuEvent) != 0) {
-    PostSynchEvent(this,
-                (v & kMuWriter) != 0? SYNCH_EV_UNLOCK: SYNCH_EV_READERUNLOCK);
+    PostSynchEvent(
+        this, (v & kMuWriter) != 0 ? SYNCH_EV_UNLOCK : SYNCH_EV_READERUNLOCK);
   }
   int c = 0;
   // the waiter under consideration to wake, or zero
-  PerThreadSynch *w = nullptr;
+  PerThreadSynch* w = nullptr;
   // the predecessor to w or zero
-  PerThreadSynch *pw = nullptr;
+  PerThreadSynch* pw = nullptr;
   // head of the list searched previously, or zero
-  PerThreadSynch *old_h = nullptr;
+  PerThreadSynch* old_h = nullptr;
   // a condition that's known to be false.
-  const Condition *known_false = nullptr;
-  PerThreadSynch *wake_list = kPerThreadSynchNull;   // list of threads to wake
-  intptr_t wr_wait = 0;        // set to kMuWrWait if we wake a reader and a
-                               // later writer could have acquired the lock
-                               // (starvation avoidance)
+  PerThreadSynch* wake_list = kPerThreadSynchNull;  // list of threads to wake
+  intptr_t wr_wait = 0;  // set to kMuWrWait if we wake a reader and a
+                         // later writer could have acquired the lock
+                         // (starvation avoidance)
+  // When non-null, clear its "woken_has_waiters" field before returning.
+  absl::base_internal::ThreadIdentity* clear_waking_des_waker = nullptr;
   ABSL_RAW_CHECK(waitp == nullptr || waitp->thread->waitp == nullptr ||
                      waitp->thread->suppress_fatal_errors,
                  "detected illegal recursion into Mutex code");
@@ -2053,8 +2169,7 @@ void Mutex::UnlockSlow(SynchWaitParams *waitp) {
     } else if ((v & (kMuReader | kMuWait)) == kMuReader && waitp == nullptr) {
       // fast reader release (reader with no waiters)
       intptr_t clear = ExactlyOneReader(v) ? kMuReader | kMuOne : kMuOne;
-      if (mu_.compare_exchange_strong(v, v - clear,
-                                      std::memory_order_release,
+      if (mu_.compare_exchange_strong(v, v - clear, std::memory_order_release,
                                       std::memory_order_relaxed)) {
         return;
       }
@@ -2062,16 +2177,16 @@ void Mutex::UnlockSlow(SynchWaitParams *waitp) {
                mu_.compare_exchange_strong(v, v | kMuSpin,
                                            std::memory_order_acquire,
                                            std::memory_order_relaxed)) {
-      if ((v & kMuWait) == 0) {       // no one to wake
+      if ((v & kMuWait) == 0) {  // no one to wake
         intptr_t nv;
         bool do_enqueue = true;  // always Enqueue() the first time
         ABSL_RAW_CHECK(waitp != nullptr,
                        "UnlockSlow is confused");  // about to sleep
-        do {    // must loop to release spinlock as reader count may change
+        do {  // must loop to release spinlock as reader count may change
           v = mu_.load(std::memory_order_relaxed);
           // decrement reader count if there are readers
-          intptr_t new_readers = (v >= kMuOne)?  v - kMuOne : v;
-          PerThreadSynch *new_h = nullptr;
+          intptr_t new_readers = (v >= kMuOne) ? v - kMuOne : v;
+          PerThreadSynch* new_h = nullptr;
           if (do_enqueue) {
             // If we are enqueuing on a CondVar (waitp->cv_word != nullptr) then
             // we must not retry here.  The initial attempt will always have
@@ -2095,21 +2210,20 @@ void Mutex::UnlockSlow(SynchWaitParams *waitp) {
           }
           // release spinlock & our lock; retry if reader-count changed
           // (writer count cannot change since we hold lock)
-        } while (!mu_.compare_exchange_weak(v, nv,
-                                            std::memory_order_release,
+        } while (!mu_.compare_exchange_weak(v, nv, std::memory_order_release,
                                             std::memory_order_relaxed));
         break;
       }
 
       // There are waiters.
       // Set h to the head of the circular waiter list.
-      PerThreadSynch *h = GetPerThreadSynch(v);
+      PerThreadSynch* h = GetPerThreadSynch(v);
       if ((v & kMuReader) != 0 && (h->readers & kMuHigh) > kMuOne) {
         // a reader but not the last
-        h->readers -= kMuOne;  // release our lock
-        intptr_t nv = v;       // normally just release spinlock
+        h->readers -= kMuOne;    // release our lock
+        intptr_t nv = v;         // normally just release spinlock
         if (waitp != nullptr) {  // but waitp!=nullptr => must queue ourselves
-          PerThreadSynch *new_h = Enqueue(h, waitp, v, kMuIsCond);
+          PerThreadSynch* new_h = Enqueue(h, waitp, v, kMuIsCond);
           ABSL_RAW_CHECK(new_h != nullptr,
                          "waiters disappeared during Enqueue()!");
           nv &= kMuLow;
@@ -2127,17 +2241,17 @@ void Mutex::UnlockSlow(SynchWaitParams *waitp) {
 
       // The lock is becoming free, and there's a waiter
       if (old_h != nullptr &&
-          !old_h->may_skip) {                  // we used old_h as a terminator
-        old_h->may_skip = true;                // allow old_h to skip once more
+          !old_h->may_skip) {    // we used old_h as a terminator
+        old_h->may_skip = true;  // allow old_h to skip once more
         ABSL_RAW_CHECK(old_h->skip == nullptr, "illegal skip from head");
-        if (h != old_h && MuSameCondition(old_h, old_h->next)) {
+        if (h != old_h && MuEquivalentWaiter(old_h, old_h->next)) {
           old_h->skip = old_h->next;  // old_h not head & can skip to successor
         }
       }
       if (h->next->waitp->how == kExclusive &&
-          Condition::GuaranteedEqual(h->next->waitp->cond, nullptr)) {
+          h->next->waitp->cond == nullptr) {
         // easy case: writer with no condition; no need to search
-        pw = h;                       // wake w, the successor of h (=pw)
+        pw = h;  // wake w, the successor of h (=pw)
         w = h->next;
         w->wake = true;
         // We are waking up a writer.  This writer may be racing against
@@ -2160,13 +2274,13 @@ void Mutex::UnlockSlow(SynchWaitParams *waitp) {
         // waiter has a condition or is a reader.  We avoid searching over
         // waiters we've searched on previous iterations by starting at
         // old_h if it's set.  If old_h==h, there's no one to wakeup at all.
-        if (old_h == h) {      // we've searched before, and nothing's new
-                               // so there's no one to wake.
-          intptr_t nv = (v & ~(kMuReader|kMuWriter|kMuWrWait));
+        if (old_h == h) {  // we've searched before, and nothing's new
+                           // so there's no one to wake.
+          intptr_t nv = (v & ~(kMuReader | kMuWriter | kMuWrWait));
           h->readers = 0;
-          h->maybe_unlocking = false;   // finished unlocking
-          if (waitp != nullptr) {       // we must queue ourselves and sleep
-            PerThreadSynch *new_h = Enqueue(h, waitp, v, kMuIsCond);
+          h->maybe_unlocking = false;  // finished unlocking
+          if (waitp != nullptr) {      // we must queue ourselves and sleep
+            PerThreadSynch* new_h = Enqueue(h, waitp, v, kMuIsCond);
             nv &= kMuLow;
             if (new_h != nullptr) {
               nv |= kMuWait | reinterpret_cast<intptr_t>(new_h);
@@ -2180,12 +2294,12 @@ void Mutex::UnlockSlow(SynchWaitParams *waitp) {
         }
 
         // set up to walk the list
-        PerThreadSynch *w_walk;   // current waiter during list walk
-        PerThreadSynch *pw_walk;  // previous waiter during list walk
-        if (old_h != nullptr) {  // we've searched up to old_h before
+        PerThreadSynch* w_walk;   // current waiter during list walk
+        PerThreadSynch* pw_walk;  // previous waiter during list walk
+        if (old_h != nullptr) {   // we've searched up to old_h before
           pw_walk = old_h;
           w_walk = old_h->next;
-        } else {            // no prior search, start at beginning
+        } else {  // no prior search, start at beginning
           pw_walk =
               nullptr;  // h->next's predecessor may change; don't record it
           w_walk = h->next;
@@ -2211,36 +2325,32 @@ void Mutex::UnlockSlow(SynchWaitParams *waitp) {
         // to walk the path from w_walk to h inclusive. (TryRemove() can remove
         // a waiter anywhere, but it acquires both the spinlock and the Mutex)
 
-        old_h = h;        // remember we searched to here
+        old_h = h;  // remember we searched to here
 
         // Walk the path upto and including h looking for waiters we can wake.
         while (pw_walk != h) {
           w_walk->wake = false;
           if (w_walk->waitp->cond ==
                   nullptr ||  // no condition => vacuously true OR
-              (w_walk->waitp->cond != known_false &&
-               // this thread's condition is not known false, AND
-               //  is in fact true
-               EvalConditionIgnored(this, w_walk->waitp->cond))) {
+                              // this thread's condition is true
+              EvalConditionIgnored(this, w_walk->waitp->cond)) {
             if (w == nullptr) {
-              w_walk->wake = true;    // can wake this waiter
+              w_walk->wake = true;  // can wake this waiter
               w = w_walk;
               pw = pw_walk;
               if (w_walk->waitp->how == kExclusive) {
                 wr_wait = kMuWrWait;
-                break;                // bail if waking this writer
+                break;  // bail if waking this writer
               }
             } else if (w_walk->waitp->how == kShared) {  // wake if a reader
               w_walk->wake = true;
-            } else {   // writer with true condition
+            } else {  // writer with true condition
               wr_wait = kMuWrWait;
             }
-          } else {                  // can't wake; condition false
-            known_false = w_walk->waitp->cond;  // remember last false condition
           }
-          if (w_walk->wake) {   // we're waking reader w_walk
-            pw_walk = w_walk;   // don't skip similar waiters
-          } else {              // not waking; skip as much as possible
+          if (w_walk->wake) {  // we're waking reader w_walk
+            pw_walk = w_walk;  // don't skip similar waiters
+          } else {             // not waking; skip as much as possible
             pw_walk = Skip(w_walk);
           }
           // If pw_walk == h, then load of pw_walk->next can race with
@@ -2267,8 +2377,8 @@ void Mutex::UnlockSlow(SynchWaitParams *waitp) {
       h = DequeueAllWakeable(h, pw, &wake_list);
 
       intptr_t nv = (v & kMuEvent) | kMuDesig;
-                                             // assume no waiters left,
-                                             // set kMuDesig for INV1a
+      // assume no waiters left,
+      // set kMuDesig for INV1a
 
       if (waitp != nullptr) {  // we must queue ourselves and sleep
         h = Enqueue(h, waitp, v, kMuIsCond);
@@ -2281,8 +2391,15 @@ void Mutex::UnlockSlow(SynchWaitParams *waitp) {
 
       if (h != nullptr) {  // there are waiters left
         h->readers = 0;
-        h->maybe_unlocking = false;     // finished unlocking
+        h->maybe_unlocking = false;  // finished unlocking
         nv |= wr_wait | kMuWait | reinterpret_cast<intptr_t>(h);
+
+        // Signal to any Scheduler that we are waking from Mutex Unlock
+        // and there are more waiters left, signaling possible contention.
+        ABSL_TSAN_MUTEX_PRE_DIVERT(this, 0);
+        clear_waking_des_waker = GetOrCreateCurrentThreadIdentity();
+        ABSL_TSAN_MUTEX_POST_DIVERT(this, 0);
+        clear_waking_des_waker->scheduler_state.waking_designated_waker = true;
       }
 
       // release both spinlock & lock
@@ -2290,25 +2407,38 @@ void Mutex::UnlockSlow(SynchWaitParams *waitp) {
       mu_.store(nv, std::memory_order_release);
       break;  // out of for(;;)-loop
     }
-    c = Delay(c, AGGRESSIVE);  // aggressive here; no one can proceed till we do
-  }                            // end of for(;;)-loop
+    // aggressive here; no one can proceed till we do
+    c = synchronization_internal::MutexDelay(c, AGGRESSIVE);
+  }  // end of for(;;)-loop
 
   if (wake_list != kPerThreadSynchNull) {
-    int64_t enqueue_timestamp = wake_list->waitp->contention_start_cycles;
-    bool cond_waiter = wake_list->cond_waiter;
+    int64_t total_wait_cycles = 0;
+    int64_t max_wait_cycles = 0;
+    int64_t now = CycleClock::Now();
     do {
-      wake_list = Wakeup(wake_list);              // wake waiters
+      // Profile lock contention events only if the waiter was trying to acquire
+      // the lock, not waiting on a condition variable or Condition.
+      if (!wake_list->cond_waiter) {
+        int64_t cycles_waited =
+            (now - wake_list->waitp->contention_start_cycles);
+        total_wait_cycles += cycles_waited;
+        if (max_wait_cycles == 0) max_wait_cycles = cycles_waited;
+        wake_list->waitp->contention_start_cycles = now;
+        wake_list->waitp->should_submit_contention_data = true;
+      }
+      wake_list = Wakeup(wake_list);  // wake waiters
     } while (wake_list != kPerThreadSynchNull);
-    if (!cond_waiter) {
-      // Sample lock contention events only if the (first) waiter was trying to
-      // acquire the lock, not waiting on a condition variable or Condition.
-      int64_t wait_cycles = base_internal::CycleClock::Now() - enqueue_timestamp;
-      mutex_tracer("slow release", this, wait_cycles);
+    if (total_wait_cycles > 0) {
+      mutex_tracer("slow release", this, total_wait_cycles);
       ABSL_TSAN_MUTEX_PRE_DIVERT(this, 0);
-      submit_profile_data(enqueue_timestamp);
+      submit_profile_data(total_wait_cycles);
       ABSL_TSAN_MUTEX_POST_DIVERT(this, 0);
     }
   }
+
+  if (clear_waking_des_waker) {
+    clear_waking_des_waker->scheduler_state.waking_designated_waker = false;
+  }
 }
 
 // Used by CondVar implementation to reacquire mutex after waking from
@@ -2328,14 +2458,15 @@ void Mutex::Trans(MuHow how) {
 // condition variable.  If this mutex is free, we simply wake the thread.
 // It will later acquire the mutex with high probability.  Otherwise, we
 // enqueue thread w on this mutex.
-void Mutex::Fer(PerThreadSynch *w) {
+void Mutex::Fer(PerThreadSynch* w) {
+  SchedulingGuard::ScopedDisable disable_rescheduling;
   int c = 0;
   ABSL_RAW_CHECK(w->waitp->cond == nullptr,
                  "Mutex::Fer while waiting on Condition");
-  ABSL_RAW_CHECK(!w->waitp->timeout.has_timeout(),
-                 "Mutex::Fer while in timed wait");
   ABSL_RAW_CHECK(w->waitp->cv_word == nullptr,
                  "Mutex::Fer with pending CondVar queueing");
+  // The CondVar timeout is not relevant for the Mutex wait.
+  w->waitp->timeout = {};
   for (;;) {
     intptr_t v = mu_.load(std::memory_order_relaxed);
     // Note: must not queue if the mutex is unlocked (nobody will wake it).
@@ -2352,9 +2483,10 @@ void Mutex::Fer(PerThreadSynch *w) {
       IncrementSynchSem(this, w);
       return;
     } else {
-      if ((v & (kMuSpin|kMuWait)) == 0) {       // no waiters
+      if ((v & (kMuSpin | kMuWait)) == 0) {  // no waiters
         // This thread tries to become the one and only waiter.
-        PerThreadSynch *new_h = Enqueue(nullptr, w->waitp, v, kMuIsCond);
+        PerThreadSynch* new_h =
+            Enqueue(nullptr, w->waitp, v, kMuIsCond | kMuIsFer);
         ABSL_RAW_CHECK(new_h != nullptr,
                        "Enqueue failed");  // we must queue ourselves
         if (mu_.compare_exchange_strong(
@@ -2364,8 +2496,8 @@ void Mutex::Fer(PerThreadSynch *w) {
         }
       } else if ((v & kMuSpin) == 0 &&
                  mu_.compare_exchange_strong(v, v | kMuSpin | kMuWait)) {
-        PerThreadSynch *h = GetPerThreadSynch(v);
-        PerThreadSynch *new_h = Enqueue(h, w->waitp, v, kMuIsCond);
+        PerThreadSynch* h = GetPerThreadSynch(v);
+        PerThreadSynch* new_h = Enqueue(h, w->waitp, v, kMuIsCond | kMuIsFer);
         ABSL_RAW_CHECK(new_h != nullptr,
                        "Enqueue failed");  // we must queue ourselves
         do {
@@ -2378,25 +2510,24 @@ void Mutex::Fer(PerThreadSynch *w) {
         return;
       }
     }
-    c = Delay(c, GENTLE);
+    c = synchronization_internal::MutexDelay(c, GENTLE);
   }
 }
 
 void Mutex::AssertHeld() const {
   if ((mu_.load(std::memory_order_relaxed) & kMuWriter) == 0) {
-    SynchEvent *e = GetSynchEvent(this);
+    SynchEvent* e = GetSynchEvent(this);
     ABSL_RAW_LOG(FATAL, "thread should hold write lock on Mutex %p %s",
-                 static_cast<const void *>(this),
-                 (e == nullptr ? "" : e->name));
+                 static_cast<const void*>(this), (e == nullptr ? "" : e->name));
   }
 }
 
 void Mutex::AssertReaderHeld() const {
   if ((mu_.load(std::memory_order_relaxed) & (kMuReader | kMuWriter)) == 0) {
-    SynchEvent *e = GetSynchEvent(this);
-    ABSL_RAW_LOG(
-        FATAL, "thread should hold at least a read lock on Mutex %p %s",
-        static_cast<const void *>(this), (e == nullptr ? "" : e->name));
+    SynchEvent* e = GetSynchEvent(this);
+    ABSL_RAW_LOG(FATAL,
+                 "thread should hold at least a read lock on Mutex %p %s",
+                 static_cast<const void*>(this), (e == nullptr ? "" : e->name));
   }
 }
 
@@ -2407,41 +2538,38 @@ static const intptr_t kCvEvent = 0x0002L;  // record events
 static const intptr_t kCvLow = 0x0003L;  // low order bits of CV
 
 // Hack to make constant values available to gdb pretty printer
-enum { kGdbCvSpin = kCvSpin, kGdbCvEvent = kCvEvent, kGdbCvLow = kCvLow, };
+enum {
+  kGdbCvSpin = kCvSpin,
+  kGdbCvEvent = kCvEvent,
+  kGdbCvLow = kCvLow,
+};
 
 static_assert(PerThreadSynch::kAlignment > kCvLow,
               "PerThreadSynch::kAlignment must be greater than kCvLow");
 
-void CondVar::EnableDebugLog(const char *name) {
-  SynchEvent *e = EnsureSynchEvent(&this->cv_, name, kCvEvent, kCvSpin);
+void CondVar::EnableDebugLog(const char* name) {
+  SynchEvent* e = EnsureSynchEvent(&this->cv_, name, kCvEvent, kCvSpin);
   e->log = true;
   UnrefSynchEvent(e);
 }
 
-CondVar::~CondVar() {
-  if ((cv_.load(std::memory_order_relaxed) & kCvEvent) != 0) {
-    ForgetSynchEvent(&this->cv_, kCvEvent, kCvSpin);
-  }
-}
-
-
 // Remove thread s from the list of waiters on this condition variable.
-void CondVar::Remove(PerThreadSynch *s) {
+void CondVar::Remove(PerThreadSynch* s) {
+  SchedulingGuard::ScopedDisable disable_rescheduling;
   intptr_t v;
   int c = 0;
   for (v = cv_.load(std::memory_order_relaxed);;
        v = cv_.load(std::memory_order_relaxed)) {
     if ((v & kCvSpin) == 0 &&  // attempt to acquire spinlock
-        cv_.compare_exchange_strong(v, v | kCvSpin,
-                                    std::memory_order_acquire,
+        cv_.compare_exchange_strong(v, v | kCvSpin, std::memory_order_acquire,
                                     std::memory_order_relaxed)) {
-      PerThreadSynch *h = reinterpret_cast<PerThreadSynch *>(v & ~kCvLow);
+      PerThreadSynch* h = reinterpret_cast<PerThreadSynch*>(v & ~kCvLow);
       if (h != nullptr) {
-        PerThreadSynch *w = h;
+        PerThreadSynch* w = h;
         while (w->next != s && w->next != h) {  // search for thread
           w = w->next;
         }
-        if (w->next == s) {           // found thread; remove it
+        if (w->next == s) {  // found thread; remove it
           w->next = s->next;
           if (h == s) {
             h = (w == s) ? nullptr : w;
@@ -2450,12 +2578,13 @@ void CondVar::Remove(PerThreadSynch *s) {
           s->state.store(PerThreadSynch::kAvailable, std::memory_order_release);
         }
       }
-                                      // release spinlock
+      // release spinlock
       cv_.store((v & kCvEvent) | reinterpret_cast<intptr_t>(h),
                 std::memory_order_release);
       return;
     } else {
-      c = Delay(c, GENTLE);            // try again after a delay
+      // try again after a delay
+      c = synchronization_internal::MutexDelay(c, GENTLE);
     }
   }
 }
@@ -2467,19 +2596,19 @@ void CondVar::Remove(PerThreadSynch *s) {
 // before calling Mutex::UnlockSlow(), the Mutex code might be re-entered (via
 // the logging code, or via a Condition function) and might potentially attempt
 // to block this thread.  That would be a problem if the thread were already on
-// a the condition variable waiter queue.  Thus, we use the waitp->cv_word
-// to tell the unlock code to call CondVarEnqueue() to queue the thread on the
-// condition variable queue just before the mutex is to be unlocked, and (most
+// a condition variable waiter queue.  Thus, we use the waitp->cv_word to tell
+// the unlock code to call CondVarEnqueue() to queue the thread on the condition
+// variable queue just before the mutex is to be unlocked, and (most
 // importantly) after any call to an external routine that might re-enter the
 // mutex code.
-static void CondVarEnqueue(SynchWaitParams *waitp) {
+static void CondVarEnqueue(SynchWaitParams* waitp) {
   // This thread might be transferred to the Mutex queue by Fer() when
   // we are woken.  To make sure that is what happens, Enqueue() doesn't
   // call CondVarEnqueue() again but instead uses its normal code.  We
   // must do this before we queue ourselves so that cv_word will be null
   // when seen by the dequeuer, who may wish immediately to requeue
   // this thread on another queue.
-  std::atomic<intptr_t> *cv_word = waitp->cv_word;
+  std::atomic<intptr_t>* cv_word = waitp->cv_word;
   waitp->cv_word = nullptr;
 
   intptr_t v = cv_word->load(std::memory_order_relaxed);
@@ -2488,12 +2617,12 @@ static void CondVarEnqueue(SynchWaitParams *waitp) {
          !cv_word->compare_exchange_weak(v, v | kCvSpin,
                                          std::memory_order_acquire,
                                          std::memory_order_relaxed)) {
-    c = Delay(c, GENTLE);
+    c = synchronization_internal::MutexDelay(c, GENTLE);
     v = cv_word->load(std::memory_order_relaxed);
   }
   ABSL_RAW_CHECK(waitp->thread->waitp == nullptr, "waiting when shouldn't be");
-  waitp->thread->waitp = waitp;      // prepare ourselves for waiting
-  PerThreadSynch *h = reinterpret_cast<PerThreadSynch *>(v & ~kCvLow);
+  waitp->thread->waitp = waitp;  // prepare ourselves for waiting
+  PerThreadSynch* h = reinterpret_cast<PerThreadSynch*>(v & ~kCvLow);
   if (h == nullptr) {  // add this thread to waiter list
     waitp->thread->next = waitp->thread;
   } else {
@@ -2506,8 +2635,8 @@ static void CondVarEnqueue(SynchWaitParams *waitp) {
                  std::memory_order_release);
 }
 
-bool CondVar::WaitCommon(Mutex *mutex, KernelTimeout t) {
-  bool rc = false;          // return value; true iff we timed-out
+bool CondVar::WaitCommon(Mutex* mutex, KernelTimeout t) {
+  bool rc = false;  // return value; true iff we timed-out
 
   intptr_t mutex_v = mutex->mu_.load(std::memory_order_relaxed);
   Mutex::MuHow mutex_how = ((mutex_v & kMuWriter) != 0) ? kExclusive : kShared;
@@ -2532,6 +2661,23 @@ bool CondVar::WaitCommon(Mutex *mutex, KernelTimeout t) {
   while (waitp.thread->state.load(std::memory_order_acquire) ==
          PerThreadSynch::kQueued) {
     if (!Mutex::DecrementSynchSem(mutex, waitp.thread, t)) {
+      // DecrementSynchSem returned due to timeout.
+      // Now we will either (1) remove ourselves from the wait list in Remove
+      // below, in which case Remove will set thread.state = kAvailable and
+      // we will not call DecrementSynchSem again; or (2) Signal/SignalAll
+      // has removed us concurrently and is calling Wakeup, which will set
+      // thread.state = kAvailable and post to the semaphore.
+      // It's important to reset the timeout for the case (2) because otherwise
+      // we can live-lock in this loop since DecrementSynchSem will always
+      // return immediately due to timeout, but Signal/SignalAll is not
+      // necessary set thread.state = kAvailable yet (and is not scheduled
+      // due to thread priorities or other scheduler artifacts).
+      // Note this could also be resolved if Signal/SignalAll would set
+      // thread.state = kAvailable while holding the wait list spin lock.
+      // But this can't be easily done for SignalAll since it grabs the whole
+      // wait list with a single compare-exchange and does not really grab
+      // the spin lock.
+      t = KernelTimeout::Never();
       this->Remove(waitp.thread);
       rc = true;
     }
@@ -2557,47 +2703,18 @@ bool CondVar::WaitCommon(Mutex *mutex, KernelTimeout t) {
   return rc;
 }
 
-bool CondVar::WaitWithTimeout(Mutex *mu, absl::Duration timeout) {
-  return WaitWithDeadline(mu, DeadlineFromTimeout(timeout));
-}
-
-bool CondVar::WaitWithDeadline(Mutex *mu, absl::Time deadline) {
-  return WaitCommon(mu, KernelTimeout(deadline));
-}
-
-void CondVar::Wait(Mutex *mu) {
-  WaitCommon(mu, KernelTimeout::Never());
-}
-
-// Wake thread w
-// If it was a timed wait, w will be waiting on w->cv
-// Otherwise, if it was not a Mutex mutex, w will be waiting on w->sem
-// Otherwise, w is transferred to the Mutex mutex via Mutex::Fer().
-void CondVar::Wakeup(PerThreadSynch *w) {
-  if (w->waitp->timeout.has_timeout() || w->waitp->cvmu == nullptr) {
-    // The waiting thread only needs to observe "w->state == kAvailable" to be
-    // released, we must cache "cvmu" before clearing "next".
-    Mutex *mu = w->waitp->cvmu;
-    w->next = nullptr;
-    w->state.store(PerThreadSynch::kAvailable, std::memory_order_release);
-    Mutex::IncrementSynchSem(mu, w);
-  } else {
-    w->waitp->cvmu->Fer(w);
-  }
-}
-
 void CondVar::Signal() {
+  SchedulingGuard::ScopedDisable disable_rescheduling;
   ABSL_TSAN_MUTEX_PRE_SIGNAL(nullptr, 0);
   intptr_t v;
   int c = 0;
   for (v = cv_.load(std::memory_order_relaxed); v != 0;
        v = cv_.load(std::memory_order_relaxed)) {
     if ((v & kCvSpin) == 0 &&  // attempt to acquire spinlock
-        cv_.compare_exchange_strong(v, v | kCvSpin,
-                                    std::memory_order_acquire,
+        cv_.compare_exchange_strong(v, v | kCvSpin, std::memory_order_acquire,
                                     std::memory_order_relaxed)) {
-      PerThreadSynch *h = reinterpret_cast<PerThreadSynch *>(v & ~kCvLow);
-      PerThreadSynch *w = nullptr;
+      PerThreadSynch* h = reinterpret_cast<PerThreadSynch*>(v & ~kCvLow);
+      PerThreadSynch* w = nullptr;
       if (h != nullptr) {  // remove first waiter
         w = h->next;
         if (w == h) {
@@ -2606,11 +2723,11 @@ void CondVar::Signal() {
           h->next = w->next;
         }
       }
-                                      // release spinlock
+      // release spinlock
       cv_.store((v & kCvEvent) | reinterpret_cast<intptr_t>(h),
                 std::memory_order_release);
       if (w != nullptr) {
-        CondVar::Wakeup(w);                // wake waiter, if there was one
+        w->waitp->cvmu->Fer(w);  // wake waiter, if there was one
         cond_var_tracer("Signal wakeup", this);
       }
       if ((v & kCvEvent) != 0) {
@@ -2619,13 +2736,13 @@ void CondVar::Signal() {
       ABSL_TSAN_MUTEX_POST_SIGNAL(nullptr, 0);
       return;
     } else {
-      c = Delay(c, GENTLE);
+      c = synchronization_internal::MutexDelay(c, GENTLE);
     }
   }
   ABSL_TSAN_MUTEX_POST_SIGNAL(nullptr, 0);
 }
 
-void CondVar::SignalAll () {
+void CondVar::SignalAll() {
   ABSL_TSAN_MUTEX_PRE_SIGNAL(nullptr, 0);
   intptr_t v;
   int c = 0;
@@ -2639,14 +2756,14 @@ void CondVar::SignalAll () {
     if ((v & kCvSpin) == 0 &&
         cv_.compare_exchange_strong(v, v & kCvEvent, std::memory_order_acquire,
                                     std::memory_order_relaxed)) {
-      PerThreadSynch *h = reinterpret_cast<PerThreadSynch *>(v & ~kCvLow);
+      PerThreadSynch* h = reinterpret_cast<PerThreadSynch*>(v & ~kCvLow);
       if (h != nullptr) {
-        PerThreadSynch *w;
-        PerThreadSynch *n = h->next;
-        do {                          // for every thread, wake it up
+        PerThreadSynch* w;
+        PerThreadSynch* n = h->next;
+        do {  // for every thread, wake it up
           w = n;
           n = n->next;
-          CondVar::Wakeup(w);
+          w->waitp->cvmu->Fer(w);
         } while (w != h);
         cond_var_tracer("SignalAll wakeup", this);
       }
@@ -2656,7 +2773,8 @@ void CondVar::SignalAll () {
       ABSL_TSAN_MUTEX_POST_SIGNAL(nullptr, 0);
       return;
     } else {
-      c = Delay(c, GENTLE);           // try again after a delay
+      // try again after a delay
+      c = synchronization_internal::MutexDelay(c, GENTLE);
     }
   }
   ABSL_TSAN_MUTEX_POST_SIGNAL(nullptr, 0);
@@ -2665,59 +2783,60 @@ void CondVar::SignalAll () {
 void ReleasableMutexLock::Release() {
   ABSL_RAW_CHECK(this->mu_ != nullptr,
                  "ReleasableMutexLock::Release may only be called once");
-  this->mu_->Unlock();
+  this->mu_->unlock();
   this->mu_ = nullptr;
 }
 
-#ifdef THREAD_SANITIZER
-extern "C" void __tsan_read1(void *addr);
+#ifdef ABSL_HAVE_THREAD_SANITIZER
+extern "C" void __tsan_read1(void* addr);
 #else
 #define __tsan_read1(addr)  // do nothing if TSan not enabled
 #endif
 
 // A function that just returns its argument, dereferenced
-static bool Dereference(void *arg) {
+static bool Dereference(void* arg) {
   // ThreadSanitizer does not instrument this file for memory accesses.
   // This function dereferences a user variable that can participate
   // in a data race, so we need to manually tell TSan about this memory access.
   __tsan_read1(arg);
-  return *(static_cast<bool *>(arg));
+  return *(static_cast<bool*>(arg));
 }
 
-Condition::Condition() {}   // null constructor, used for kTrue only
-const Condition Condition::kTrue;
+ABSL_CONST_INIT const Condition Condition::kTrue;
 
-Condition::Condition(bool (*func)(void *), void *arg)
-    : eval_(&CallVoidPtrFunction),
-      function_(func),
-      method_(nullptr),
-      arg_(arg) {}
+Condition::Condition(bool (*func)(void*), void* arg)
+    : eval_(&CallVoidPtrFunction), arg_(arg) {
+  static_assert(sizeof(&func) <= sizeof(callback_),
+                "An overlarge function pointer passed to Condition.");
+  StoreCallback(func);
+}
 
-bool Condition::CallVoidPtrFunction(const Condition *c) {
-  return (*c->function_)(c->arg_);
+bool Condition::CallVoidPtrFunction(const Condition* c) {
+  using FunctionPointer = bool (*)(void*);
+  FunctionPointer function_pointer;
+  std::memcpy(&function_pointer, c->callback_, sizeof(function_pointer));
+  return (*function_pointer)(c->arg_);
 }
 
-Condition::Condition(const bool *cond)
+Condition::Condition(const bool* cond)
     : eval_(CallVoidPtrFunction),
-      function_(Dereference),
-      method_(nullptr),
       // const_cast is safe since Dereference does not modify arg
-      arg_(const_cast<bool *>(cond)) {}
-
-bool Condition::Eval() const {
-  // eval_ == null for kTrue
-  return (this->eval_ == nullptr) || (*this->eval_)(this);
+      arg_(const_cast<bool*>(cond)) {
+  using FunctionPointer = bool (*)(void*);
+  const FunctionPointer dereference = Dereference;
+  StoreCallback(dereference);
 }
 
-bool Condition::GuaranteedEqual(const Condition *a, const Condition *b) {
-  if (a == nullptr) {
-    return b == nullptr || b->eval_ == nullptr;
-  }
-  if (b == nullptr || b->eval_ == nullptr) {
-    return a->eval_ == nullptr;
+bool Condition::Eval() const { return (*this->eval_)(this); }
+
+bool Condition::GuaranteedEqual(const Condition* a, const Condition* b) {
+  if (a == nullptr || b == nullptr) {
+    return a == b;
   }
-  return a->eval_ == b->eval_ && a->function_ == b->function_ &&
-         a->arg_ == b->arg_ && a->method_ == b->method_;
+  // Check equality of the representative fields.
+  return a->eval_ == b->eval_ && a->arg_ == b->arg_ &&
+         !memcmp(a->callback_, b->callback_, sizeof(a->callback_));
 }
 
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/synchronization/mutex.h b/absl/synchronization/mutex.h
index d33318d347d..b81c59509c4 100644
--- a/absl/synchronization/mutex.h
+++ b/absl/synchronization/mutex.h
@@ -31,22 +31,23 @@
 //
 //  MutexLock - An RAII wrapper to acquire and release a `Mutex` for exclusive/
 //              write access within the current scope.
+//
 //  ReaderMutexLock
 //            - An RAII wrapper to acquire and release a `Mutex` for shared/read
 //              access within the current scope.
 //
 //  WriterMutexLock
-//            - Alias for `MutexLock` above, designed for use in distinguishing
-//              reader and writer locks within code.
+//            - Effectively an alias for `MutexLock` above, designed for use in
+//              distinguishing reader and writer locks within code.
 //
 // In addition to simple mutex locks, this file also defines ways to perform
 // locking under certain conditions.
 //
-//  Condition   - (Preferred) Used to wait for a particular predicate that
-//                depends on state protected by the `Mutex` to become true.
-//  CondVar     - A lower-level variant of `Condition` that relies on
-//                application code to explicitly signal the `CondVar` when
-//                a condition has been met.
+//  Condition - (Preferred) Used to wait for a particular predicate that
+//              depends on state protected by the `Mutex` to become true.
+//  CondVar   - A lower-level variant of `Condition` that relies on
+//              application code to explicitly signal the `CondVar` when
+//              a condition has been met.
 //
 // See below for more information on using `Condition` or `CondVar`.
 //
@@ -59,33 +60,40 @@
 
 #include <atomic>
 #include <cstdint>
-#include <string>
+#include <cstring>
+#include <type_traits>
 
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
 #include "absl/base/const_init.h"
-#include "absl/base/internal/identity.h"
-#include "absl/base/internal/low_level_alloc.h"
 #include "absl/base/internal/thread_identity.h"
 #include "absl/base/internal/tsan_mutex_interface.h"
-#include "absl/base/port.h"
+#include "absl/base/macros.h"
+#include "absl/base/nullability.h"
 #include "absl/base/thread_annotations.h"
+#include "absl/meta/type_traits.h"
 #include "absl/synchronization/internal/kernel_timeout.h"
 #include "absl/synchronization/internal/per_thread_sem.h"
 #include "absl/time/time.h"
 
-// Decide if we should use the non-production implementation because
-// the production implementation hasn't been fully ported yet.
-#ifdef ABSL_INTERNAL_USE_NONPROD_MUTEX
-#error ABSL_INTERNAL_USE_NONPROD_MUTEX cannot be directly set
-#elif defined(ABSL_LOW_LEVEL_ALLOC_MISSING)
-#define ABSL_INTERNAL_USE_NONPROD_MUTEX 1
-#include "absl/synchronization/internal/mutex_nonprod.inc"
-#endif
-
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 class Condition;
 struct SynchWaitParams;
 
+namespace synchronization_internal {
+
+template <typename T, typename = void>
+struct HasConstMemberCallOperator : std::false_type {};
+
+template <typename T>
+struct HasConstMemberCallOperator<
+    T, std::void_t<decltype(static_cast<bool (T::*)() const>(&T::operator()))>>
+    : std::true_type {};
+
+}  // namespace synchronization_internal
+
 // -----------------------------------------------------------------------------
 // Mutex
 // -----------------------------------------------------------------------------
@@ -95,28 +103,44 @@ struct SynchWaitParams;
 // invariants. Proper usage of mutexes prevents concurrent access by different
 // threads to the same resource.
 //
-// A `Mutex` has two basic operations: `Mutex::Lock()` and `Mutex::Unlock()`.
-// The `Lock()` operation *acquires* a `Mutex` (in a state known as an
-// *exclusive* -- or write -- lock), while the `Unlock()` operation *releases* a
-// Mutex. During the span of time between the Lock() and Unlock() operations,
-// a mutex is said to be *held*. By design all mutexes support exclusive/write
+// A `Mutex` has two basic operations: `Mutex::lock()` and `Mutex::unlock()`.
+// The `lock()` operation *acquires* a `Mutex` (in a state known as an
+// *exclusive* -- or *write* -- lock), and the `unlock()` operation *releases* a
+// Mutex. During the span of time between the lock() and unlock() operations,
+// a mutex is said to be *held*. By design, all mutexes support exclusive/write
 // locks, as this is the most common way to use a mutex.
 //
+// Mutex operations are only allowed under certain conditions; otherwise an
+// operation is "invalid", and disallowed by the API. The conditions concern
+// both the current state of the mutex and the identity of the threads that
+// are performing the operations.
+//
 // The `Mutex` state machine for basic lock/unlock operations is quite simple:
 //
-// |                | Lock()     | Unlock() |
-// |----------------+------------+----------|
-// | Free           | Exclusive  | invalid  |
-// | Exclusive      | blocks     | Free     |
+// |                | lock()                 | unlock() |
+// |----------------+------------------------+----------|
+// | Free           | Exclusive              | invalid  |
+// | Exclusive      | blocks, then exclusive | Free     |
+//
+// The full conditions are as follows.
+//
+// * Calls to `unlock()` require that the mutex be held, and must be made in the
+//   same thread that performed the corresponding `lock()` operation which
+//   acquired the mutex; otherwise the call is invalid.
+//
+// * The mutex being non-reentrant (or non-recursive) means that a call to
+//   `lock()` or `try_lock()` must not be made in a thread that already holds
+//   the mutex; such a call is invalid.
 //
-// Attempts to `Unlock()` must originate from the thread that performed the
-// corresponding `Lock()` operation.
+// * In other words, the state of being "held" has both a temporal component
+//   (from `lock()` until `unlock()`) as well as a thread identity component:
+//   the mutex is held *by a particular thread*.
 //
-// An "invalid" operation is disallowed by the API. The `Mutex` implementation
-// is allowed to do anything on an invalid call, including but not limited to
+// An "invalid" operation has undefined behavior. The `Mutex` implementation
+// is allowed to do anything on an invalid call, including, but not limited to,
 // crashing with a useful error message, silently succeeding, or corrupting
-// data structures. In debug mode, the implementation attempts to crash with a
-// useful error message.
+// data structures. In debug mode, the implementation may crash with a useful
+// error message.
 //
 // `Mutex` is not guaranteed to be "fair" in prioritizing waiting threads; it
 // is, however, approximately fair over long periods, and starvation-free for
@@ -130,12 +154,13 @@ struct SynchWaitParams;
 // issues that could potentially result in race conditions and deadlocks.
 //
 // For more information about the lock annotations, please see
-// [Thread Safety Analysis](http://clang.llvm.org/docs/ThreadSafetyAnalysis.html)
-// in the Clang documentation.
+// [Thread Safety
+// Analysis](http://clang.llvm.org/docs/ThreadSafetyAnalysis.html) in the Clang
+// documentation.
 //
 // See also `MutexLock`, below, for scoped `Mutex` acquisition.
 
-class ABSL_LOCKABLE Mutex {
+class ABSL_LOCKABLE ABSL_ATTRIBUTE_WARN_UNUSED Mutex {
  public:
   // Creates a `Mutex` that is not held by anyone. This constructor is
   // typically used for Mutexes allocated on the heap or the stack.
@@ -154,36 +179,50 @@ class ABSL_LOCKABLE Mutex {
   //
   // Example usage:
   //   namespace foo {
-  //   ABSL_CONST_INIT Mutex mu(absl::kConstInit);
+  //   ABSL_CONST_INIT absl::Mutex mu(absl::kConstInit);
   //   }
   explicit constexpr Mutex(absl::ConstInitType);
 
   ~Mutex();
 
-  // Mutex::Lock()
+  // Mutex::lock()
   //
   // Blocks the calling thread, if necessary, until this `Mutex` is free, and
   // then acquires it exclusively. (This lock is also known as a "write lock.")
-  void Lock() ABSL_EXCLUSIVE_LOCK_FUNCTION();
+  void lock() ABSL_EXCLUSIVE_LOCK_FUNCTION();
 
-  // Mutex::Unlock()
+  ABSL_DEPRECATE_AND_INLINE()
+  inline void Lock() ABSL_EXCLUSIVE_LOCK_FUNCTION() { lock(); }
+
+  // Mutex::unlock()
   //
   // Releases this `Mutex` and returns it from the exclusive/write state to the
-  // free state. Caller must hold the `Mutex` exclusively.
-  void Unlock() ABSL_UNLOCK_FUNCTION();
+  // free state. Calling thread must hold the `Mutex` exclusively.
+  void unlock() ABSL_UNLOCK_FUNCTION();
+
+  ABSL_DEPRECATE_AND_INLINE()
+  inline void Unlock() ABSL_UNLOCK_FUNCTION() { unlock(); }
 
-  // Mutex::TryLock()
+  // Mutex::try_lock()
   //
   // If the mutex can be acquired without blocking, does so exclusively and
   // returns `true`. Otherwise, returns `false`. Returns `true` with high
   // probability if the `Mutex` was free.
-  bool TryLock() ABSL_EXCLUSIVE_TRYLOCK_FUNCTION(true);
+  [[nodiscard]] bool try_lock() ABSL_EXCLUSIVE_TRYLOCK_FUNCTION(true);
+
+  ABSL_DEPRECATE_AND_INLINE()
+  [[nodiscard]] bool TryLock() ABSL_EXCLUSIVE_TRYLOCK_FUNCTION(true) {
+    return try_lock();
+  }
 
   // Mutex::AssertHeld()
   //
-  // Return immediately if this thread holds the `Mutex` exclusively (in write
-  // mode). Otherwise, may report an error (typically by crashing with a
-  // diagnostic), or may return immediately.
+  // Require that the mutex be held exclusively (write mode) by this thread.
+  //
+  // If the mutex is not currently held by this thread, this function may report
+  // an error (typically by crashing with a diagnostic) or it may do nothing.
+  // This function is intended only as a tool to assist debugging; it doesn't
+  // guarantee correctness.
   void AssertHeld() const ABSL_ASSERT_EXCLUSIVE_LOCK();
 
   // ---------------------------------------------------------------------------
@@ -194,19 +233,19 @@ class ABSL_LOCKABLE Mutex {
   // Neither read-locks nor write-locks are reentrant/recursive to avoid
   // potential client programming errors.
   //
-  // The Mutex API provides `Writer*()` aliases for the existing `Lock()`,
-  // `Unlock()` and `TryLock()` methods for use within applications mixing
-  // reader/writer locks. Using `Reader*()` and `Writer*()` operations in this
+  // The Mutex API provides `Writer*()` aliases for the existing `lock()`,
+  // `unlock()` and `try_lock()` methods for use within applications mixing
+  // reader/writer locks. Using `*_shared()` and `Writer*()` operations in this
   // manner can make locking behavior clearer when mixing read and write modes.
   //
   // Introducing reader locks necessarily complicates the `Mutex` state
   // machine somewhat. The table below illustrates the allowed state transitions
-  // of a mutex in such cases. Note that ReaderLock() may block even if the lock
-  // is held in shared mode; this occurs when another thread is blocked on a
-  // call to WriterLock().
+  // of a mutex in such cases. Note that lock_shared() may block even if the
+  // lock is held in shared mode; this occurs when another thread is blocked on
+  // a call to lock().
   //
   // ---------------------------------------------------------------------------
-  //     Operation: WriterLock() Unlock()  ReaderLock()           ReaderUnlock()
+  //     Operation: lock()       unlock()  lock_shared() unlock_shared()
   // ---------------------------------------------------------------------------
   // State
   // ---------------------------------------------------------------------------
@@ -218,34 +257,48 @@ class ABSL_LOCKABLE Mutex {
   //
   // In comments below, "shared" refers to a state of Shared(n) for any n > 0.
 
-  // Mutex::ReaderLock()
+  // Mutex::lock_shared()
   //
   // Blocks the calling thread, if necessary, until this `Mutex` is either free,
   // or in shared mode, and then acquires a share of it. Note that
-  // `ReaderLock()` will block if some other thread has an exclusive/writer lock
-  // on the mutex.
+  // `lock_shared()` will block if some other thread has an exclusive/writer
+  // lock on the mutex.
+  void lock_shared() ABSL_SHARED_LOCK_FUNCTION();
 
-  void ReaderLock() ABSL_SHARED_LOCK_FUNCTION();
+  ABSL_DEPRECATE_AND_INLINE()
+  void ReaderLock() ABSL_SHARED_LOCK_FUNCTION() { lock_shared(); }
 
-  // Mutex::ReaderUnlock()
+  // Mutex::unlock_shared()
   //
-  // Releases a read share of this `Mutex`. `ReaderUnlock` may return a mutex to
-  // the free state if this thread holds the last reader lock on the mutex. Note
-  // that you cannot call `ReaderUnlock()` on a mutex held in write mode.
-  void ReaderUnlock() ABSL_UNLOCK_FUNCTION();
+  // Releases a read share of this `Mutex`. `unlock_shared` may return a mutex
+  // to the free state if this thread holds the last reader lock on the mutex.
+  // Note that you cannot call `unlock_shared()` on a mutex held in write mode.
+  void unlock_shared() ABSL_UNLOCK_FUNCTION();
+
+  ABSL_DEPRECATE_AND_INLINE()
+  void ReaderUnlock() ABSL_UNLOCK_FUNCTION() { unlock_shared(); }
 
-  // Mutex::ReaderTryLock()
+  // Mutex::try_lock_shared()
   //
   // If the mutex can be acquired without blocking, acquires this mutex for
   // shared access and returns `true`. Otherwise, returns `false`. Returns
   // `true` with high probability if the `Mutex` was free or shared.
-  bool ReaderTryLock() ABSL_SHARED_TRYLOCK_FUNCTION(true);
+  [[nodiscard]] bool try_lock_shared() ABSL_SHARED_TRYLOCK_FUNCTION(true);
+
+  ABSL_DEPRECATE_AND_INLINE()
+  [[nodiscard]] bool ReaderTryLock() ABSL_SHARED_TRYLOCK_FUNCTION(true) {
+    return try_lock_shared();
+  }
 
   // Mutex::AssertReaderHeld()
   //
-  // Returns immediately if this thread holds the `Mutex` in at least shared
-  // mode (read mode). Otherwise, may report an error (typically by
-  // crashing with a diagnostic), or may return immediately.
+  // Require that the mutex be held at least in shared mode (read mode) by this
+  // thread.
+  //
+  // If the mutex is not currently held by this thread, this function may report
+  // an error (typically by crashing with a diagnostic) or it may do nothing.
+  // This function is intended only as a tool to assist debugging; it doesn't
+  // guarantee correctness.
   void AssertReaderHeld() const ABSL_ASSERT_SHARED_LOCK();
 
   // Mutex::WriterLock()
@@ -255,14 +308,17 @@ class ABSL_LOCKABLE Mutex {
   // Aliases for `Mutex::Lock()`, `Mutex::Unlock()`, and `Mutex::TryLock()`.
   //
   // These methods may be used (along with the complementary `Reader*()`
-  // methods) to distingish simple exclusive `Mutex` usage (`Lock()`,
+  // methods) to distinguish simple exclusive `Mutex` usage (`Lock()`,
   // etc.) from reader/writer lock usage.
-  void WriterLock() ABSL_EXCLUSIVE_LOCK_FUNCTION() { this->Lock(); }
+  ABSL_DEPRECATE_AND_INLINE()
+  void WriterLock() ABSL_EXCLUSIVE_LOCK_FUNCTION() { lock(); }
 
-  void WriterUnlock() ABSL_UNLOCK_FUNCTION() { this->Unlock(); }
+  ABSL_DEPRECATE_AND_INLINE()
+  void WriterUnlock() ABSL_UNLOCK_FUNCTION() { unlock(); }
 
-  bool WriterTryLock() ABSL_EXCLUSIVE_TRYLOCK_FUNCTION(true) {
-    return this->TryLock();
+  ABSL_DEPRECATE_AND_INLINE()
+  [[nodiscard]] bool WriterTryLock() ABSL_EXCLUSIVE_TRYLOCK_FUNCTION(true) {
+    return try_lock();
   }
 
   // ---------------------------------------------------------------------------
@@ -305,7 +361,9 @@ class ABSL_LOCKABLE Mutex {
   // `true`, `Await()` *may* skip the release/re-acquire step.
   //
   // `Await()` requires that this thread holds this `Mutex` in some mode.
-  void Await(const Condition &cond);
+  void Await(const Condition& cond) {
+    AwaitCommon(cond, synchronization_internal::KernelTimeout::Never());
+  }
 
   // Mutex::LockWhen()
   // Mutex::ReaderLockWhen()
@@ -315,11 +373,17 @@ class ABSL_LOCKABLE Mutex {
   // be acquired, then atomically acquires this `Mutex`. `LockWhen()` is
   // logically equivalent to `*Lock(); Await();` though they may have different
   // performance characteristics.
-  void LockWhen(const Condition &cond) ABSL_EXCLUSIVE_LOCK_FUNCTION();
+  void LockWhen(const Condition& cond) ABSL_EXCLUSIVE_LOCK_FUNCTION() {
+    LockWhenCommon(cond, synchronization_internal::KernelTimeout::Never(),
+                   true);
+  }
 
-  void ReaderLockWhen(const Condition &cond) ABSL_SHARED_LOCK_FUNCTION();
+  void ReaderLockWhen(const Condition& cond) ABSL_SHARED_LOCK_FUNCTION() {
+    LockWhenCommon(cond, synchronization_internal::KernelTimeout::Never(),
+                   false);
+  }
 
-  void WriterLockWhen(const Condition &cond) ABSL_EXCLUSIVE_LOCK_FUNCTION() {
+  void WriterLockWhen(const Condition& cond) ABSL_EXCLUSIVE_LOCK_FUNCTION() {
     this->LockWhen(cond);
   }
 
@@ -330,24 +394,27 @@ class ABSL_LOCKABLE Mutex {
   // Mutex::AwaitWithTimeout()
   // Mutex::AwaitWithDeadline()
   //
-  // If `cond` is initially true, do nothing, or act as though `cond` is
-  // initially false.
-  //
-  // If `cond` is initially false, unlock this `Mutex` and block until
-  // simultaneously:
+  // Unlocks this `Mutex` and blocks until simultaneously:
   //   - either `cond` is true or the {timeout has expired, deadline has passed}
   //     and
   //   - this `Mutex` can be reacquired,
   // then reacquire this `Mutex` in the same mode in which it was previously
   // held, returning `true` iff `cond` is `true` on return.
   //
+  // If the condition is initially `true`, the implementation *may* skip the
+  // release/re-acquire step and return immediately.
+  //
   // Deadlines in the past are equivalent to an immediate deadline.
   // Negative timeouts are equivalent to a zero timeout.
   //
   // This method requires that this thread holds this `Mutex` in some mode.
-  bool AwaitWithTimeout(const Condition &cond, absl::Duration timeout);
+  bool AwaitWithTimeout(const Condition& cond, absl::Duration timeout) {
+    return AwaitCommon(cond, synchronization_internal::KernelTimeout{timeout});
+  }
 
-  bool AwaitWithDeadline(const Condition &cond, absl::Time deadline);
+  bool AwaitWithDeadline(const Condition& cond, absl::Time deadline) {
+    return AwaitCommon(cond, synchronization_internal::KernelTimeout{deadline});
+  }
 
   // Mutex::LockWhenWithTimeout()
   // Mutex::ReaderLockWhenWithTimeout()
@@ -360,11 +427,17 @@ class ABSL_LOCKABLE Mutex {
   // `true` on return.
   //
   // Negative timeouts are equivalent to a zero timeout.
-  bool LockWhenWithTimeout(const Condition &cond, absl::Duration timeout)
-      ABSL_EXCLUSIVE_LOCK_FUNCTION();
-  bool ReaderLockWhenWithTimeout(const Condition &cond, absl::Duration timeout)
-      ABSL_SHARED_LOCK_FUNCTION();
-  bool WriterLockWhenWithTimeout(const Condition &cond, absl::Duration timeout)
+  bool LockWhenWithTimeout(const Condition& cond, absl::Duration timeout)
+      ABSL_EXCLUSIVE_LOCK_FUNCTION() {
+    return LockWhenCommon(
+        cond, synchronization_internal::KernelTimeout{timeout}, true);
+  }
+  bool ReaderLockWhenWithTimeout(const Condition& cond, absl::Duration timeout)
+      ABSL_SHARED_LOCK_FUNCTION() {
+    return LockWhenCommon(
+        cond, synchronization_internal::KernelTimeout{timeout}, false);
+  }
+  bool WriterLockWhenWithTimeout(const Condition& cond, absl::Duration timeout)
       ABSL_EXCLUSIVE_LOCK_FUNCTION() {
     return this->LockWhenWithTimeout(cond, timeout);
   }
@@ -380,11 +453,17 @@ class ABSL_LOCKABLE Mutex {
   // on return.
   //
   // Deadlines in the past are equivalent to an immediate deadline.
-  bool LockWhenWithDeadline(const Condition &cond, absl::Time deadline)
-      ABSL_EXCLUSIVE_LOCK_FUNCTION();
-  bool ReaderLockWhenWithDeadline(const Condition &cond, absl::Time deadline)
-      ABSL_SHARED_LOCK_FUNCTION();
-  bool WriterLockWhenWithDeadline(const Condition &cond, absl::Time deadline)
+  bool LockWhenWithDeadline(const Condition& cond, absl::Time deadline)
+      ABSL_EXCLUSIVE_LOCK_FUNCTION() {
+    return LockWhenCommon(
+        cond, synchronization_internal::KernelTimeout{deadline}, true);
+  }
+  bool ReaderLockWhenWithDeadline(const Condition& cond, absl::Time deadline)
+      ABSL_SHARED_LOCK_FUNCTION() {
+    return LockWhenCommon(
+        cond, synchronization_internal::KernelTimeout{deadline}, false);
+  }
+  bool WriterLockWhenWithDeadline(const Condition& cond, absl::Time deadline)
       ABSL_EXCLUSIVE_LOCK_FUNCTION() {
     return this->LockWhenWithDeadline(cond, deadline);
   }
@@ -406,7 +485,9 @@ class ABSL_LOCKABLE Mutex {
   // substantially reduce `Mutex` performance; it should be set only for
   // non-production runs.  Optimization options may also disable invariant
   // checks.
-  void EnableInvariantDebugging(void (*invariant)(void *), void *arg);
+  void EnableInvariantDebugging(
+      void (*absl_nullable invariant)(void* absl_nullability_unknown),
+      void* absl_nullability_unknown arg);
 
   // Mutex::EnableDebugLog()
   //
@@ -415,7 +496,7 @@ class ABSL_LOCKABLE Mutex {
   // call to `EnableInvariantDebugging()` or `EnableDebugLog()` has been made.
   //
   // Note: This method substantially reduces `Mutex` performance.
-  void EnableDebugLog(const char *name);
+  void EnableDebugLog(const char* absl_nullable name);
 
   // Deadlock detection
 
@@ -443,7 +524,7 @@ class ABSL_LOCKABLE Mutex {
 
   // A `MuHow` is a constant that indicates how a lock should be acquired.
   // Internal implementation detail.  Clients should ignore.
-  typedef const struct MuHowS *MuHow;
+  typedef const struct MuHowS* MuHow;
 
   // Mutex::InternalAttemptToUseMutexInFatalSignalHandler()
   //
@@ -461,53 +542,52 @@ class ABSL_LOCKABLE Mutex {
   static void InternalAttemptToUseMutexInFatalSignalHandler();
 
  private:
-#ifdef ABSL_INTERNAL_USE_NONPROD_MUTEX
-  friend class CondVar;
-
-  synchronization_internal::MutexImpl *impl() { return impl_.get(); }
-
-  synchronization_internal::SynchronizationStorage<
-      synchronization_internal::MutexImpl>
-      impl_;
-#else
   std::atomic<intptr_t> mu_;  // The Mutex state.
 
   // Post()/Wait() versus associated PerThreadSem; in class for required
   // friendship with PerThreadSem.
-  static inline void IncrementSynchSem(Mutex *mu,
-                                       base_internal::PerThreadSynch *w);
-  static inline bool DecrementSynchSem(
-      Mutex *mu, base_internal::PerThreadSynch *w,
-      synchronization_internal::KernelTimeout t);
+  static void IncrementSynchSem(Mutex* absl_nonnull mu,
+                                base_internal::PerThreadSynch* absl_nonnull w);
+  static bool DecrementSynchSem(Mutex* absl_nonnull mu,
+                                base_internal::PerThreadSynch* absl_nonnull w,
+                                synchronization_internal::KernelTimeout t);
 
   // slow path acquire
-  void LockSlowLoop(SynchWaitParams *waitp, int flags);
+  void LockSlowLoop(SynchWaitParams* absl_nonnull waitp, int flags);
   // wrappers around LockSlowLoop()
-  bool LockSlowWithDeadline(MuHow how, const Condition *cond,
+  bool LockSlowWithDeadline(MuHow absl_nonnull how,
+                            const Condition* absl_nullable cond,
                             synchronization_internal::KernelTimeout t,
                             int flags);
-  void LockSlow(MuHow how, const Condition *cond,
+  void LockSlow(MuHow absl_nonnull how, const Condition* absl_nullable cond,
                 int flags) ABSL_ATTRIBUTE_COLD;
   // slow path release
-  void UnlockSlow(SynchWaitParams *waitp) ABSL_ATTRIBUTE_COLD;
+  void UnlockSlow(SynchWaitParams* absl_nullable waitp) ABSL_ATTRIBUTE_COLD;
+  // TryLock slow path.
+  bool TryLockSlow();
+  // ReaderTryLock slow path.
+  bool ReaderTryLockSlow();
   // Common code between Await() and AwaitWithTimeout/Deadline()
-  bool AwaitCommon(const Condition &cond,
+  bool AwaitCommon(const Condition& cond,
                    synchronization_internal::KernelTimeout t);
+  bool LockWhenCommon(const Condition& cond,
+                      synchronization_internal::KernelTimeout t, bool write);
   // Attempt to remove thread s from queue.
-  void TryRemove(base_internal::PerThreadSynch *s);
+  void TryRemove(base_internal::PerThreadSynch* absl_nonnull s);
   // Block a thread on mutex.
-  void Block(base_internal::PerThreadSynch *s);
+  void Block(base_internal::PerThreadSynch* absl_nonnull s);
   // Wake a thread; return successor.
-  base_internal::PerThreadSynch *Wakeup(base_internal::PerThreadSynch *w);
+  base_internal::PerThreadSynch* absl_nullable Wakeup(
+      base_internal::PerThreadSynch* absl_nonnull w);
+  void Dtor();
 
-  friend class CondVar;   // for access to Trans()/Fer().
-  void Trans(MuHow how);  // used for CondVar->Mutex transfer
-  void Fer(
-      base_internal::PerThreadSynch *w);  // used for CondVar->Mutex transfer
-#endif
+  friend class CondVar;                // for access to Trans()/Fer().
+  void Trans(MuHow absl_nonnull how);  // used for CondVar->Mutex transfer
+  void Fer(base_internal::PerThreadSynch* absl_nonnull
+               w);  // used for CondVar->Mutex transfer
 
   // Catch the error of writing Mutex when intending MutexLock.
-  Mutex(const volatile Mutex * /*ignored*/) {}  // NOLINT(runtime/explicit)
+  explicit Mutex(const volatile Mutex* absl_nullable /*ignored*/) {}
 
   Mutex(const Mutex&) = delete;
   Mutex& operator=(const Mutex&) = delete;
@@ -525,31 +605,60 @@ class ABSL_LOCKABLE Mutex {
 // Example:
 //
 // Class Foo {
-//
+//  public:
 //   Foo::Bar* Baz() {
-//     MutexLock l(&lock_);
+//     MutexLock lock(mu_);
 //     ...
 //     return bar;
 //   }
 //
 // private:
-//   Mutex lock_;
+//   Mutex mu_;
 // };
 class ABSL_SCOPED_LOCKABLE MutexLock {
  public:
-  explicit MutexLock(Mutex *mu) ABSL_EXCLUSIVE_LOCK_FUNCTION(mu) : mu_(mu) {
-    this->mu_->Lock();
+  // Constructors
+
+  // Calls `mu.lock()` and returns when that call returns. That is, `mu` is
+  // guaranteed to be locked when this object is constructed.
+  explicit MutexLock(Mutex& mu ABSL_INTERNAL_ATTRIBUTE_CAPTURED_BY_THIS)
+      ABSL_EXCLUSIVE_LOCK_FUNCTION(mu)
+      : mu_(mu) {
+    this->mu_.lock();
   }
 
-  MutexLock(const MutexLock &) = delete;  // NOLINT(runtime/mutex)
-  MutexLock(MutexLock&&) = delete;  // NOLINT(runtime/mutex)
+  // Calls `mu->lock()` and returns when that call returns. That is, `*mu` is
+  // guaranteed to be locked when this object is constructed. Requires that
+  // `mu` be dereferenceable.
+  [[deprecated("Use the constructor that takes a reference instead")]]
+  ABSL_REFACTOR_INLINE
+  explicit MutexLock(Mutex* absl_nonnull mu) ABSL_EXCLUSIVE_LOCK_FUNCTION(mu)
+      : MutexLock(*mu) {}
+
+  // Like above, but calls `mu.LockWhen(cond)` instead. That is, in addition to
+  // the above, the condition given by `cond` is also guaranteed to hold when
+  // this object is constructed.
+  explicit MutexLock(Mutex& mu ABSL_INTERNAL_ATTRIBUTE_CAPTURED_BY_THIS,
+                     const Condition& cond) ABSL_EXCLUSIVE_LOCK_FUNCTION(mu)
+      : mu_(mu) {
+    this->mu_.LockWhen(cond);
+  }
+
+  [[deprecated("Use the constructor that takes a reference instead")]]
+  ABSL_REFACTOR_INLINE
+  explicit MutexLock(Mutex* absl_nonnull mu, const Condition& cond)
+      ABSL_EXCLUSIVE_LOCK_FUNCTION(mu)
+      : MutexLock(*mu, cond) {}
+
+  MutexLock(const MutexLock&) = delete;  // NOLINT(runtime/mutex)
+  MutexLock(MutexLock&&) = delete;       // NOLINT(runtime/mutex)
   MutexLock& operator=(const MutexLock&) = delete;
   MutexLock& operator=(MutexLock&&) = delete;
 
-  ~MutexLock() ABSL_UNLOCK_FUNCTION() { this->mu_->Unlock(); }
+  ~MutexLock() ABSL_UNLOCK_FUNCTION() { this->mu_.unlock(); }
 
  private:
-  Mutex *const mu_;
+  Mutex& mu_;
 };
 
 // ReaderMutexLock
@@ -558,19 +667,38 @@ class ABSL_SCOPED_LOCKABLE MutexLock {
 // releases a shared lock on a `Mutex` via RAII.
 class ABSL_SCOPED_LOCKABLE ReaderMutexLock {
  public:
-  explicit ReaderMutexLock(Mutex *mu) ABSL_SHARED_LOCK_FUNCTION(mu) : mu_(mu) {
-    mu->ReaderLock();
+  explicit ReaderMutexLock(Mutex& mu ABSL_INTERNAL_ATTRIBUTE_CAPTURED_BY_THIS)
+      ABSL_SHARED_LOCK_FUNCTION(mu)
+      : mu_(mu) {
+    mu.lock_shared();
+  }
+
+  [[deprecated("Use the constructor that takes a reference instead")]]
+  ABSL_REFACTOR_INLINE
+  explicit ReaderMutexLock(Mutex* absl_nonnull mu) ABSL_SHARED_LOCK_FUNCTION(mu)
+      : ReaderMutexLock(*mu) {}
+
+  explicit ReaderMutexLock(Mutex& mu ABSL_INTERNAL_ATTRIBUTE_CAPTURED_BY_THIS,
+                           const Condition& cond) ABSL_SHARED_LOCK_FUNCTION(mu)
+      : mu_(mu) {
+    mu.ReaderLockWhen(cond);
   }
 
+  [[deprecated("Use the constructor that takes a reference instead")]]
+  ABSL_REFACTOR_INLINE
+  explicit ReaderMutexLock(Mutex* absl_nonnull mu, const Condition& cond)
+      ABSL_SHARED_LOCK_FUNCTION(mu)
+      : ReaderMutexLock(*mu, cond) {}
+
   ReaderMutexLock(const ReaderMutexLock&) = delete;
   ReaderMutexLock(ReaderMutexLock&&) = delete;
   ReaderMutexLock& operator=(const ReaderMutexLock&) = delete;
   ReaderMutexLock& operator=(ReaderMutexLock&&) = delete;
 
-  ~ReaderMutexLock() ABSL_UNLOCK_FUNCTION() { this->mu_->ReaderUnlock(); }
+  ~ReaderMutexLock() ABSL_UNLOCK_FUNCTION() { this->mu_.unlock_shared(); }
 
  private:
-  Mutex *const mu_;
+  Mutex& mu_;
 };
 
 // WriterMutexLock
@@ -579,32 +707,52 @@ class ABSL_SCOPED_LOCKABLE ReaderMutexLock {
 // releases a write (exclusive) lock on a `Mutex` via RAII.
 class ABSL_SCOPED_LOCKABLE WriterMutexLock {
  public:
-  explicit WriterMutexLock(Mutex *mu) ABSL_EXCLUSIVE_LOCK_FUNCTION(mu)
+  explicit WriterMutexLock(Mutex& mu ABSL_INTERNAL_ATTRIBUTE_CAPTURED_BY_THIS)
+      ABSL_EXCLUSIVE_LOCK_FUNCTION(mu)
       : mu_(mu) {
-    mu->WriterLock();
+    mu.lock();
   }
 
+  [[deprecated("Use the constructor that takes a reference instead")]]
+  ABSL_REFACTOR_INLINE
+  explicit WriterMutexLock(Mutex* absl_nonnull mu)
+      ABSL_EXCLUSIVE_LOCK_FUNCTION(mu)
+      : WriterMutexLock(*mu) {}
+
+  explicit WriterMutexLock(Mutex& mu ABSL_INTERNAL_ATTRIBUTE_CAPTURED_BY_THIS,
+                           const Condition& cond)
+      ABSL_EXCLUSIVE_LOCK_FUNCTION(mu)
+      : mu_(mu) {
+    mu.WriterLockWhen(cond);
+  }
+
+  [[deprecated("Use the constructor that takes a reference instead")]]
+  ABSL_REFACTOR_INLINE
+  explicit WriterMutexLock(Mutex* absl_nonnull mu, const Condition& cond)
+      ABSL_EXCLUSIVE_LOCK_FUNCTION(mu)
+      : WriterMutexLock(*mu, cond) {}
+
   WriterMutexLock(const WriterMutexLock&) = delete;
   WriterMutexLock(WriterMutexLock&&) = delete;
   WriterMutexLock& operator=(const WriterMutexLock&) = delete;
   WriterMutexLock& operator=(WriterMutexLock&&) = delete;
 
-  ~WriterMutexLock() ABSL_UNLOCK_FUNCTION() { this->mu_->WriterUnlock(); }
+  ~WriterMutexLock() ABSL_UNLOCK_FUNCTION() { this->mu_.unlock(); }
 
  private:
-  Mutex *const mu_;
+  Mutex& mu_;
 };
 
 // -----------------------------------------------------------------------------
 // Condition
 // -----------------------------------------------------------------------------
 //
-// As noted above, `Mutex` contains a number of member functions which take a
-// `Condition` as an argument; clients can wait for conditions to become `true`
-// before attempting to acquire the mutex. These sections are known as
-// "condition critical" sections. To use a `Condition`, you simply need to
-// construct it, and use within an appropriate `Mutex` member function;
-// everything else in the `Condition` class is an implementation detail.
+// `Mutex` contains a number of member functions which take a `Condition` as an
+// argument; clients can wait for conditions to become `true` before attempting
+// to acquire the mutex. These sections are known as "condition critical"
+// sections. To use a `Condition`, you simply need to construct it, and use
+// within an appropriate `Mutex` member function; everything else in the
+// `Condition` class is an implementation detail.
 //
 // A `Condition` is specified as a function pointer which returns a boolean.
 // `Condition` functions should be pure functions -- their results should depend
@@ -622,16 +770,26 @@ class ABSL_SCOPED_LOCKABLE WriterMutexLock {
 // `noexcept`; until then this requirement cannot be enforced in the
 // type system.)
 //
-// Note: to use a `Condition`, you need only construct it and pass it within the
-// appropriate `Mutex' member function, such as `Mutex::Await()`.
+// Note: to use a `Condition`, you need only construct it and pass it to a
+// suitable `Mutex' member function, such as `Mutex::Await()`, or to the
+// constructor of one of the scope guard classes.
 //
-// Example:
+// Example using LockWhen/Unlock:
 //
 //   // assume count_ is not internal reference count
-//   int count_ GUARDED_BY(mu_);
+//   int count_ ABSL_GUARDED_BY(mu_);
+//   Condition count_is_zero(+[](int *count) { return *count == 0; }, &count_);
 //
-//   mu_.LockWhen(Condition(+[](int* count) { return *count == 0; },
-//         &count_));
+//   mu_.LockWhen(count_is_zero);
+//   // ...
+//   mu_.Unlock();
+//
+// Example using a scope guard:
+//
+//   {
+//     MutexLock lock(mu_, count_is_zero);
+//     // ...
+//   }
 //
 // When multiple threads are waiting on exactly the same condition, make sure
 // that they are constructed with the same parameters (same pointer to function
@@ -640,7 +798,8 @@ class ABSL_SCOPED_LOCKABLE WriterMutexLock {
 class Condition {
  public:
   // A Condition that returns the result of "(*func)(arg)"
-  Condition(bool (*func)(void *), void *arg);
+  Condition(bool (*absl_nonnull func)(void* absl_nullability_unknown),
+            void* absl_nullability_unknown arg);
 
   // Templated version for people who are averse to casts.
   //
@@ -651,27 +810,49 @@ class Condition {
   // Note: lambdas in this case must contain no bound variables.
   //
   // See class comment for performance advice.
-  template<typename T>
-  Condition(bool (*func)(T *), T *arg);
+  template <typename T>
+  Condition(bool (*absl_nonnull func)(T* absl_nullability_unknown),
+            T* absl_nullability_unknown arg);
+
+  // Same as above, but allows for cases where `arg` comes from a pointer that
+  // is convertible to the function parameter type `T*` but not an exact match.
+  //
+  // For example, the argument might be `X*` but the function takes `const X*`,
+  // or the argument might be `Derived*` while the function takes `Base*`, and
+  // so on for cases where the argument pointer can be implicitly converted.
+  //
+  // Implementation notes: This constructor overload is required in addition to
+  // the one above to allow deduction of `T` from `arg` for cases such as where
+  // a function template is passed as `func`. Also, the dummy `typename = void`
+  // template parameter exists just to work around a MSVC mangling bug.
+  template <typename T, typename = void>
+  Condition(
+      bool (*absl_nonnull func)(T* absl_nullability_unknown),
+      typename absl::type_identity<T>::type* absl_nullability_unknown
+          arg);
 
   // Templated version for invoking a method that returns a `bool`.
   //
   // `Condition(object, &Class::Method)` constructs a `Condition` that evaluates
   // `object->Method()`.
   //
-  // Implementation Note: `absl::internal::identity` is used to allow methods to
-  // come from base classes. A simpler signature like
+  // Implementation Note: `absl::type_identity` is used to allow
+  // methods to come from base classes. A simpler signature like
   // `Condition(T*, bool (T::*)())` does not suffice.
-  template<typename T>
-  Condition(T *object, bool (absl::internal::identity<T>::type::* method)());
+  template <typename T>
+  Condition(
+      T* absl_nonnull object,
+      bool (absl::type_identity<T>::type::* absl_nonnull method)());
 
   // Same as above, for const members
-  template<typename T>
-  Condition(const T *object,
-            bool (absl::internal::identity<T>::type::* method)() const);
+  template <typename T>
+  Condition(
+      const T* absl_nonnull object,
+      bool (absl::type_identity<T>::type::* absl_nonnull method)()
+          const);
 
   // A Condition that returns the value of `*cond`
-  explicit Condition(const bool *cond);
+  explicit Condition(const bool* absl_nonnull cond);
 
   // Templated version for invoking a functor that returns a `bool`.
   // This approach accepts pointers to non-mutable lambdas, `std::function`,
@@ -685,6 +866,11 @@ class Condition {
   //     return processed_ >= current;
   //   };
   //   mu_.Await(Condition(&reached));
+  //
+  // NOTE: never use "mu_.AssertHeld()" instead of "mu_.AssertReaderHeld()" in
+  // the lambda as it may be called when the mutex is being unlocked from a
+  // scope holding only a reader lock, which will make the assertion not
+  // fulfilled and crash the binary.
 
   // See class comment for performance advice. In particular, if there
   // might be more than one waiter for the same condition, make sure
@@ -693,13 +879,35 @@ class Condition {
   // Implementation note: The second template parameter ensures that this
   // constructor doesn't participate in overload resolution if T doesn't have
   // `bool operator() const`.
-  template <typename T, typename E = decltype(
-      static_cast<bool (T::*)() const>(&T::operator()))>
-  explicit Condition(const T *obj)
+  template <typename T,
+            std::enable_if_t<
+                synchronization_internal::HasConstMemberCallOperator<T>::value,
+                int> = 0>
+  explicit Condition(const T* absl_nonnull obj)
       : Condition(obj, static_cast<bool (T::*)() const>(&T::operator())) {}
 
+  // Constructor for functors that do not match the `bool operator()() const`
+  // signature, such as those using C++23 "deducing this" or static operator().
+  template <
+      typename T,
+      typename = std::enable_if_t<
+          !synchronization_internal::HasConstMemberCallOperator<T>::value &&
+          sizeof(static_cast<bool (*)(const T&)>(&T::operator())) != 0>>
+  explicit Condition(const T* absl_nonnull obj)
+      : Condition(&CallByRef<T>, obj) {}
+
   // A Condition that always returns `true`.
-  static const Condition kTrue;
+  // kTrue is only useful in a narrow set of circumstances, mostly when
+  // it's passed conditionally. For example:
+  //
+  //   mu.LockWhen(some_flag ? kTrue : SomeOtherCondition);
+  //
+  // Note: {LockWhen,Await}With{Deadline,Timeout} methods with kTrue condition
+  // don't return immediately when the timeout happens, they still block until
+  // the Mutex becomes available. The return value of these methods does
+  // not indicate if the timeout was reached; rather it indicates whether or
+  // not the condition is true.
+  ABSL_CONST_INIT static const Condition kTrue;
 
   // Evaluates the condition.
   bool Eval() const;
@@ -711,25 +919,67 @@ class Condition {
   // Two `Condition` values are guaranteed equal if both their `func` and `arg`
   // components are the same. A null pointer is equivalent to a `true`
   // condition.
-  static bool GuaranteedEqual(const Condition *a, const Condition *b);
+  static bool GuaranteedEqual(const Condition* absl_nullable a,
+                              const Condition* absl_nullable b);
 
  private:
-  typedef bool (*InternalFunctionType)(void * arg);
-  typedef bool (Condition::*InternalMethodType)();
-  typedef bool (*InternalMethodCallerType)(void * arg,
-                                           InternalMethodType internal_method);
+  // Sizing an allocation for a method pointer can be subtle. In the Itanium
+  // specifications, a method pointer has a predictable, uniform size. On the
+  // other hand, MSVC ABI, method pointer sizes vary based on the
+  // inheritance of the class. Specifically, method pointers from classes with
+  // multiple inheritance are bigger than those of classes with single
+  // inheritance. Other variations also exist.
+
+#ifndef _MSC_VER
+  // Allocation for a function pointer or method pointer.
+  // The {0} initializer ensures that all unused bytes of this buffer are
+  // always zeroed out.  This is necessary, because GuaranteedEqual() compares
+  // all of the bytes, unaware of which bytes are relevant to a given `eval_`.
+  using MethodPtr = bool (Condition::*)();
+  char callback_[sizeof(MethodPtr)] = {0};
+#else
+  // It is well known that the larget MSVC pointer-to-member is 24 bytes. This
+  // may be the largest known pointer-to-member of any platform. For this
+  // reason we will allocate 24 bytes for MSVC platform toolchains.
+  char callback_[24] = {0};
+#endif
 
-  bool (*eval_)(const Condition*);  // Actual evaluator
-  InternalFunctionType function_;   // function taking pointer returning bool
-  InternalMethodType method_;       // method returning bool
-  void *arg_;                       // arg of function_ or object of method_
+  // Function with which to evaluate callbacks and/or arguments.
+  bool (*absl_nullable eval_)(const Condition* absl_nonnull) = nullptr;
 
-  Condition();        // null constructor used only to create kTrue
+  // Either an argument for a function call or an object for a method call.
+  void* absl_nullable arg_ = nullptr;
 
   // Various functions eval_ can point to:
-  static bool CallVoidPtrFunction(const Condition*);
-  template <typename T> static bool CastAndCallFunction(const Condition* c);
-  template <typename T> static bool CastAndCallMethod(const Condition* c);
+  static bool CallVoidPtrFunction(const Condition* absl_nonnull c);
+  template <typename T>
+  static bool CastAndCallFunction(const Condition* absl_nonnull c);
+  template <typename T, typename ConditionMethodPtr>
+  static bool CastAndCallMethod(const Condition* absl_nonnull c);
+
+  template <typename T>
+  static bool CallByRef(const T* absl_nonnull self) {
+    return (*self)();
+  }
+
+  // Helper methods for storing, validating, and reading callback arguments.
+  template <typename T>
+  void StoreCallback(T callback) {
+    static_assert(
+        sizeof(callback) <= sizeof(callback_),
+        "An overlarge pointer was passed as a callback to Condition.");
+    std::memcpy(callback_, &callback, sizeof(callback));
+  }
+
+  template <typename T>
+  void ReadCallback(T* absl_nonnull callback) const {
+    std::memcpy(callback, callback_, sizeof(*callback));
+  }
+
+  static bool AlwaysTrue(const Condition* absl_nullable) { return true; }
+
+  // Used only to create kTrue.
+  constexpr Condition() : eval_(AlwaysTrue), arg_(nullptr) {}
 };
 
 // -----------------------------------------------------------------------------
@@ -757,9 +1007,9 @@ class Condition {
 //
 // Usage to wake T is:
 //       mu.Lock();
-//      // process data, possibly establishing C
-//      if (C) { cv->Signal(); }
-//      mu.Unlock();
+//       // process data, possibly establishing C
+//       if (C) { cv->Signal(); }
+//       mu.Unlock();
 //
 // If C may be useful to more than one waiter, use `SignalAll()` instead of
 // `Signal()`.
@@ -769,8 +1019,9 @@ class Condition {
 //
 class CondVar {
  public:
+  // A `CondVar` allocated on the heap or on the stack can use the this
+  // constructor.
   CondVar();
-  ~CondVar();
 
   // CondVar::Wait()
   //
@@ -779,7 +1030,9 @@ class CondVar {
   // spurious wakeup), then reacquires the `Mutex` and returns.
   //
   // Requires and ensures that the current thread holds the `Mutex`.
-  void Wait(Mutex *mu);
+  void Wait(Mutex* absl_nonnull mu) {
+    WaitCommon(mu, synchronization_internal::KernelTimeout::Never());
+  }
 
   // CondVar::WaitWithTimeout()
   //
@@ -794,7 +1047,9 @@ class CondVar {
   // to return `true` or `false`.
   //
   // Requires and ensures that the current thread holds the `Mutex`.
-  bool WaitWithTimeout(Mutex *mu, absl::Duration timeout);
+  bool WaitWithTimeout(Mutex* absl_nonnull mu, absl::Duration timeout) {
+    return WaitCommon(mu, synchronization_internal::KernelTimeout(timeout));
+  }
 
   // CondVar::WaitWithDeadline()
   //
@@ -811,7 +1066,9 @@ class CondVar {
   // to return `true` or `false`.
   //
   // Requires and ensures that the current thread holds the `Mutex`.
-  bool WaitWithDeadline(Mutex *mu, absl::Time deadline);
+  bool WaitWithDeadline(Mutex* absl_nonnull mu, absl::Time deadline) {
+    return WaitCommon(mu, synchronization_internal::KernelTimeout(deadline));
+  }
 
   // CondVar::Signal()
   //
@@ -828,25 +1085,17 @@ class CondVar {
   // Causes all subsequent uses of this `CondVar` to be logged via
   // `ABSL_RAW_LOG(INFO)`. Log entries are tagged with `name` if `name != 0`.
   // Note: this method substantially reduces `CondVar` performance.
-  void EnableDebugLog(const char *name);
+  void EnableDebugLog(const char* absl_nullable name);
 
  private:
-#ifdef ABSL_INTERNAL_USE_NONPROD_MUTEX
-  synchronization_internal::CondVarImpl *impl() { return impl_.get(); }
-  synchronization_internal::SynchronizationStorage<
-      synchronization_internal::CondVarImpl>
-      impl_;
-#else
-  bool WaitCommon(Mutex *mutex, synchronization_internal::KernelTimeout t);
-  void Remove(base_internal::PerThreadSynch *s);
-  void Wakeup(base_internal::PerThreadSynch *w);
+  bool WaitCommon(Mutex* absl_nonnull mutex,
+                  synchronization_internal::KernelTimeout t);
+  void Remove(base_internal::PerThreadSynch* absl_nonnull s);
   std::atomic<intptr_t> cv_;  // Condition variable state.
-#endif
   CondVar(const CondVar&) = delete;
   CondVar& operator=(const CondVar&) = delete;
 };
 
-
 // Variants of MutexLock.
 //
 // If you find yourself using one of these, consider instead using
@@ -857,18 +1106,30 @@ class CondVar {
 // MutexLockMaybe is like MutexLock, but is a no-op when mu is null.
 class ABSL_SCOPED_LOCKABLE MutexLockMaybe {
  public:
-  explicit MutexLockMaybe(Mutex *mu) ABSL_EXCLUSIVE_LOCK_FUNCTION(mu)
+  explicit MutexLockMaybe(Mutex* absl_nullable mu)
+      ABSL_EXCLUSIVE_LOCK_FUNCTION(mu)
+      : mu_(mu) {
+    if (this->mu_ != nullptr) {
+      this->mu_->lock();
+    }
+  }
+
+  explicit MutexLockMaybe(Mutex* absl_nullable mu, const Condition& cond)
+      ABSL_EXCLUSIVE_LOCK_FUNCTION(mu)
       : mu_(mu) {
     if (this->mu_ != nullptr) {
-      this->mu_->Lock();
+      this->mu_->LockWhen(cond);
     }
   }
+
   ~MutexLockMaybe() ABSL_UNLOCK_FUNCTION() {
-    if (this->mu_ != nullptr) { this->mu_->Unlock(); }
+    if (this->mu_ != nullptr) {
+      this->mu_->unlock();
+    }
   }
 
  private:
-  Mutex *const mu_;
+  Mutex* absl_nullable const mu_;
   MutexLockMaybe(const MutexLockMaybe&) = delete;
   MutexLockMaybe(MutexLockMaybe&&) = delete;
   MutexLockMaybe& operator=(const MutexLockMaybe&) = delete;
@@ -881,90 +1142,142 @@ class ABSL_SCOPED_LOCKABLE MutexLockMaybe {
 // mutex before destruction. `Release()` may be called at most once.
 class ABSL_SCOPED_LOCKABLE ReleasableMutexLock {
  public:
-  explicit ReleasableMutexLock(Mutex *mu) ABSL_EXCLUSIVE_LOCK_FUNCTION(mu)
-      : mu_(mu) {
-    this->mu_->Lock();
+  explicit ReleasableMutexLock(
+      Mutex& mu ABSL_INTERNAL_ATTRIBUTE_CAPTURED_BY_THIS)
+      ABSL_EXCLUSIVE_LOCK_FUNCTION(mu)
+      : mu_(&mu) {
+    this->mu_->lock();
+  }
+
+  [[deprecated("Use the constructor that takes a reference instead")]]
+  ABSL_REFACTOR_INLINE
+  explicit ReleasableMutexLock(Mutex* absl_nonnull mu)
+      ABSL_EXCLUSIVE_LOCK_FUNCTION(mu)
+      : ReleasableMutexLock(*mu) {}
+
+  explicit ReleasableMutexLock(
+      Mutex& mu ABSL_INTERNAL_ATTRIBUTE_CAPTURED_BY_THIS,
+      const Condition& cond) ABSL_EXCLUSIVE_LOCK_FUNCTION(mu)
+      : mu_(&mu) {
+    this->mu_->LockWhen(cond);
   }
+
+  [[deprecated("Use the constructor that takes a reference instead")]]
+  ABSL_REFACTOR_INLINE
+  explicit ReleasableMutexLock(Mutex* absl_nonnull mu, const Condition& cond)
+      ABSL_EXCLUSIVE_LOCK_FUNCTION(mu)
+      : ReleasableMutexLock(*mu, cond) {}
+
   ~ReleasableMutexLock() ABSL_UNLOCK_FUNCTION() {
-    if (this->mu_ != nullptr) { this->mu_->Unlock(); }
+    if (this->mu_ != nullptr) {
+      this->mu_->unlock();
+    }
   }
 
   void Release() ABSL_UNLOCK_FUNCTION();
 
  private:
-  Mutex *mu_;
+  Mutex* absl_nullable mu_;
   ReleasableMutexLock(const ReleasableMutexLock&) = delete;
   ReleasableMutexLock(ReleasableMutexLock&&) = delete;
   ReleasableMutexLock& operator=(const ReleasableMutexLock&) = delete;
   ReleasableMutexLock& operator=(ReleasableMutexLock&&) = delete;
 };
 
-#ifdef ABSL_INTERNAL_USE_NONPROD_MUTEX
-inline constexpr Mutex::Mutex(absl::ConstInitType) : impl_(absl::kConstInit) {}
-
-#else
 inline Mutex::Mutex() : mu_(0) {
   ABSL_TSAN_MUTEX_CREATE(this, __tsan_mutex_not_static);
 }
 
-inline constexpr Mutex::Mutex(absl::ConstInitType) : mu_(0) {}
+constexpr Mutex::Mutex(absl::ConstInitType) : mu_(0) {}
 
-inline CondVar::CondVar() : cv_(0) {}
+#if !defined(__APPLE__) && !defined(ABSL_BUILD_DLL)
+ABSL_ATTRIBUTE_ALWAYS_INLINE
+inline Mutex::~Mutex() { Dtor(); }
 #endif
 
+#if defined(NDEBUG) && !defined(ABSL_HAVE_THREAD_SANITIZER) && \
+    !defined(ABSL_BUILD_DLL)
+// Under NDEBUG and without TSAN, Dtor is normally fully inlined for
+// performance. However, when building Abseil as a shared library
+// (ABSL_BUILD_DLL), we must provide an out-of-line definition. This ensures the
+// Mutex::Dtor symbol is exported from the DLL, maintaining ABI compatibility
+// with clients that might be built in debug mode and thus expect the symbol.
+ABSL_ATTRIBUTE_ALWAYS_INLINE
+inline void Mutex::Dtor() {}
+#endif
+
+inline CondVar::CondVar() : cv_(0) {}
+
 // static
-template <typename T>
-bool Condition::CastAndCallMethod(const Condition *c) {
-  typedef bool (T::*MemberType)();
-  MemberType rm = reinterpret_cast<MemberType>(c->method_);
-  T *x = static_cast<T *>(c->arg_);
-  return (x->*rm)();
+template <typename T, typename ConditionMethodPtr>
+bool Condition::CastAndCallMethod(const Condition* absl_nonnull c) {
+  T* object = static_cast<T*>(c->arg_);
+  ConditionMethodPtr condition_method_pointer;
+  c->ReadCallback(&condition_method_pointer);
+  return (object->*condition_method_pointer)();
 }
 
 // static
 template <typename T>
-bool Condition::CastAndCallFunction(const Condition *c) {
-  typedef bool (*FuncType)(T *);
-  FuncType fn = reinterpret_cast<FuncType>(c->function_);
-  T *x = static_cast<T *>(c->arg_);
-  return (*fn)(x);
+bool Condition::CastAndCallFunction(const Condition* absl_nonnull c) {
+  bool (*function)(T*);
+  c->ReadCallback(&function);
+  T* argument = static_cast<T*>(c->arg_);
+  return (*function)(argument);
 }
 
 template <typename T>
-inline Condition::Condition(bool (*func)(T *), T *arg)
+inline Condition::Condition(
+    bool (*absl_nonnull func)(T* absl_nullability_unknown),
+    T* absl_nullability_unknown arg)
     : eval_(&CastAndCallFunction<T>),
-      function_(reinterpret_cast<InternalFunctionType>(func)),
-      method_(nullptr),
-      arg_(const_cast<void *>(static_cast<const void *>(arg))) {}
+      arg_(const_cast<void*>(static_cast<const void*>(arg))) {
+  static_assert(sizeof(&func) <= sizeof(callback_),
+                "An overlarge function pointer was passed to Condition.");
+  StoreCallback(func);
+}
+
+template <typename T, typename>
+inline Condition::Condition(
+    bool (*absl_nonnull func)(T* absl_nullability_unknown),
+    typename absl::type_identity<T>::type* absl_nullability_unknown
+        arg)
+    // Just delegate to the overload above.
+    : Condition(func, arg) {}
 
 template <typename T>
-inline Condition::Condition(T *object,
-                            bool (absl::internal::identity<T>::type::*method)())
-    : eval_(&CastAndCallMethod<T>),
-      function_(nullptr),
-      method_(reinterpret_cast<InternalMethodType>(method)),
-      arg_(object) {}
+inline Condition::Condition(
+    T* absl_nonnull object,
+    bool (absl::type_identity<T>::type::* absl_nonnull method)())
+    : eval_(&CastAndCallMethod<T, decltype(method)>), arg_(object) {
+  static_assert(sizeof(&method) <= sizeof(callback_),
+                "An overlarge method pointer was passed to Condition.");
+  StoreCallback(method);
+}
 
 template <typename T>
-inline Condition::Condition(const T *object,
-                            bool (absl::internal::identity<T>::type::*method)()
-                                const)
-    : eval_(&CastAndCallMethod<T>),
-      function_(nullptr),
-      method_(reinterpret_cast<InternalMethodType>(method)),
-      arg_(reinterpret_cast<void *>(const_cast<T *>(object))) {}
+inline Condition::Condition(
+    const T* absl_nonnull object,
+    bool (absl::type_identity<T>::type::* absl_nonnull method)()
+        const)
+    : eval_(&CastAndCallMethod<const T, decltype(method)>),
+      arg_(reinterpret_cast<void*>(const_cast<T*>(object))) {
+  StoreCallback(method);
+}
 
-// Register a hook for profiling support.
+// Register hooks for profiling support.
 //
 // The function pointer registered here will be called whenever a mutex is
-// contended.  The callback is given the absl/base/cycleclock.h timestamp when
-// waiting began.
+// contended.  The callback is given the cycles for which waiting happened (as
+// measured by //absl/base/internal/cycleclock.h, and which may not
+// be real "cycle" counts.)
 //
-// Calls to this function do not race or block, but there is no ordering
-// guaranteed between calls to this function and call to the provided hook.
-// In particular, the previously registered hook may still be called for some
-// time after this function returns.
-void RegisterMutexProfiler(void (*fn)(int64_t wait_timestamp));
+// There is no ordering guarantee between when the hook is registered and when
+// callbacks will begin.  Only a single profiler can be installed in a running
+// binary; if this function is called a second time with a different function
+// pointer, the value is ignored (and will cause an assertion failure in debug
+// mode.)
+void RegisterMutexProfiler(void (*absl_nonnull fn)(int64_t wait_cycles));
 
 // Register a hook for Mutex tracing.
 //
@@ -976,12 +1289,11 @@ void RegisterMutexProfiler(void (*fn)(int64_t wait_timestamp));
 //
 // The only event name currently sent is "slow release".
 //
-// This has the same memory ordering concerns as RegisterMutexProfiler() above.
-void RegisterMutexTracer(void (*fn)(const char *msg, const void *obj,
-                              int64_t wait_cycles));
-
-// TODO(gfalcon): Combine RegisterMutexProfiler() and RegisterMutexTracer()
-// into a single interface, since they are only ever called in pairs.
+// This has the same ordering and single-use limitations as
+// RegisterMutexProfiler() above.
+void RegisterMutexTracer(void (*absl_nonnull fn)(const char* absl_nonnull msg,
+                                                 const void* absl_nonnull obj,
+                                                 int64_t wait_cycles));
 
 // Register a hook for CondVar tracing.
 //
@@ -993,24 +1305,10 @@ void RegisterMutexTracer(void (*fn)(const char *msg, const void *obj,
 // Events that can be sent are "Wait", "Unwait", "Signal wakeup", and
 // "SignalAll wakeup".
 //
-// This has the same memory ordering concerns as RegisterMutexProfiler() above.
-void RegisterCondVarTracer(void (*fn)(const char *msg, const void *cv));
-
-// Register a hook for symbolizing stack traces in deadlock detector reports.
-//
-// 'pc' is the program counter being symbolized, 'out' is the buffer to write
-// into, and 'out_size' is the size of the buffer.  This function can return
-// false if symbolizing failed, or true if a null-terminated symbol was written
-// to 'out.'
-//
-// This has the same memory ordering concerns as RegisterMutexProfiler() above.
-//
-// DEPRECATED: The default symbolizer function is absl::Symbolize() and the
-// ability to register a different hook for symbolizing stack traces will be
-// removed on or after 2023-05-01.
-ABSL_DEPRECATED("absl::RegisterSymbolizer() is deprecated and will be removed "
-                "on or after 2023-05-01")
-void RegisterSymbolizer(bool (*fn)(const void *pc, char *out, int out_size));
+// This has the same ordering and single-use limitations as
+// RegisterMutexProfiler() above.
+void RegisterCondVarTracer(void (*absl_nonnull fn)(
+    const char* absl_nonnull msg, const void* absl_nonnull cv));
 
 // EnableMutexInvariantDebugging()
 //
@@ -1027,7 +1325,7 @@ void EnableMutexInvariantDebugging(bool enabled);
 enum class OnDeadlockCycle {
   kIgnore,  // Neither report on nor attempt to track cycles in lock ordering
   kReport,  // Report lock cycles to stderr when detected
-  kAbort,  // Report lock cycles to stderr when detected, then abort
+  kAbort,   // Report lock cycles to stderr when detected, then abort
 };
 
 // SetMutexDeadlockDetectionMode()
@@ -1039,6 +1337,7 @@ enum class OnDeadlockCycle {
 // the manner chosen here.
 void SetMutexDeadlockDetectionMode(OnDeadlockCycle mode);
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 // In some build configurations we pass --detect-odr-violations to the
@@ -1048,7 +1347,7 @@ void SetMutexDeadlockDetectionMode(OnDeadlockCycle mode);
 // By changing our extension points to be extern "C", we dodge this
 // check.
 extern "C" {
-void AbslInternalMutexYield();
+void ABSL_INTERNAL_C_SYMBOL(AbslInternalMutexYield)();
 }  // extern "C"
 
 #endif  // ABSL_SYNCHRONIZATION_MUTEX_H_
diff --git a/absl/synchronization/mutex_benchmark.cc b/absl/synchronization/mutex_benchmark.cc
index ab1880012a0..2df1d678aa4 100644
--- a/absl/synchronization/mutex_benchmark.cc
+++ b/absl/synchronization/mutex_benchmark.cc
@@ -12,13 +12,20 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+#include <atomic>
 #include <cstdint>
+#include <limits>
 #include <mutex>  // NOLINT(build/c++11)
 #include <vector>
 
 #include "absl/base/internal/cycleclock.h"
+#include "absl/base/internal/raw_logging.h"
 #include "absl/base/internal/spinlock.h"
+#include "absl/base/internal/thread_identity.h"
+#include "absl/base/no_destructor.h"
 #include "absl/synchronization/blocking_counter.h"
+#include "absl/synchronization/internal/create_thread_identity.h"
+#include "absl/synchronization/internal/per_thread_sem.h"
 #include "absl/synchronization/internal/thread_pool.h"
 #include "absl/synchronization/mutex.h"
 #include "benchmark/benchmark.h"
@@ -26,13 +33,41 @@
 namespace {
 
 void BM_Mutex(benchmark::State& state) {
-  static absl::Mutex* mu = new absl::Mutex;
+  static absl::NoDestructor<absl::Mutex> mu;
   for (auto _ : state) {
-    absl::MutexLock lock(mu);
+    absl::MutexLock lock(*mu);
   }
 }
 BENCHMARK(BM_Mutex)->UseRealTime()->Threads(1)->ThreadPerCpu();
 
+void BM_ReaderLock(benchmark::State& state) {
+  static absl::NoDestructor<absl::Mutex> mu;
+  for (auto _ : state) {
+    absl::ReaderMutexLock lock(*mu);
+  }
+}
+BENCHMARK(BM_ReaderLock)->UseRealTime()->Threads(1)->ThreadPerCpu();
+
+void BM_TryLock(benchmark::State& state) {
+  absl::Mutex mu;
+  for (auto _ : state) {
+    if (mu.try_lock()) {
+      mu.unlock();
+    }
+  }
+}
+BENCHMARK(BM_TryLock);
+
+void BM_ReaderTryLock(benchmark::State& state) {
+  static absl::NoDestructor<absl::Mutex> mu;
+  for (auto _ : state) {
+    if (mu->try_lock_shared()) {
+      mu->unlock_shared();
+    }
+  }
+}
+BENCHMARK(BM_ReaderTryLock)->UseRealTime()->Threads(1)->ThreadPerCpu();
+
 static void DelayNs(int64_t ns, int* data) {
   int64_t end = absl::base_internal::CycleClock::Now() +
                 ns * absl::base_internal::CycleClock::Frequency() / 1e9;
@@ -42,31 +77,129 @@ static void DelayNs(int64_t ns, int* data) {
   }
 }
 
-template <typename MutexType>
-class RaiiLocker {
+// RAII object to change the Mutex priority of the running thread.
+class ScopedThreadMutexPriority {
  public:
-  explicit RaiiLocker(MutexType* mu) : mu_(mu) { mu_->Lock(); }
-  ~RaiiLocker() { mu_->Unlock(); }
- private:
-  MutexType* mu_;
+  explicit ScopedThreadMutexPriority(int priority) {
+    absl::base_internal::ThreadIdentity* identity =
+        absl::synchronization_internal::GetOrCreateCurrentThreadIdentity();
+    identity->per_thread_synch.priority = priority;
+    // Bump next_priority_read_cycles to the infinite future so that the
+    // implementation doesn't re-read the thread's actual scheduler priority
+    // and replace our temporary scoped priority.
+    identity->per_thread_synch.next_priority_read_cycles =
+        std::numeric_limits<int64_t>::max();
+  }
+  ~ScopedThreadMutexPriority() {
+    // Reset the "next priority read time" back to the infinite past so that
+    // the next time the Mutex implementation wants to know this thread's
+    // priority, it re-reads it from the OS instead of using our overridden
+    // priority.
+    absl::synchronization_internal::GetOrCreateCurrentThreadIdentity()
+        ->per_thread_synch.next_priority_read_cycles =
+        std::numeric_limits<int64_t>::min();
+  }
 };
 
-template <>
-class RaiiLocker<std::mutex> {
- public:
-  explicit RaiiLocker(std::mutex* mu) : mu_(mu) { mu_->lock(); }
-  ~RaiiLocker() { mu_->unlock(); }
- private:
-  std::mutex* mu_;
-};
+void BM_MutexEnqueue(benchmark::State& state) {
+  // In the "multiple priorities" variant of the benchmark, one of the
+  // threads runs with Mutex priority 0 while the rest run at elevated priority.
+  // This benchmarks the performance impact of the presence of a low priority
+  // waiter when a higher priority waiter adds itself of the queue
+  // (b/175224064).
+  //
+  // NOTE: The actual scheduler priority is not modified in this benchmark:
+  // all of the threads get CPU slices with the same priority. Only the
+  // Mutex queueing behavior is modified.
+  const bool multiple_priorities = state.range(0);
+  ScopedThreadMutexPriority priority_setter(
+      (multiple_priorities && state.thread_index() != 0) ? 1 : 0);
+
+  struct Shared {
+    absl::Mutex mu;
+    std::atomic<int> looping_threads{0};
+    std::atomic<int> blocked_threads{0};
+    std::atomic<bool> thread_has_mutex{false};
+  };
+  static absl::NoDestructor<Shared> shared;
+
+  // Set up 'blocked_threads' to count how many threads are currently blocked
+  // in Abseil synchronization code.
+  //
+  // NOTE: Blocking done within the Google Benchmark library itself (e.g.
+  // the barrier which synchronizes threads entering and exiting the benchmark
+  // loop) does _not_ get registered in this counter. This is because Google
+  // Benchmark uses its own synchronization primitives based on std::mutex, not
+  // Abseil synchronization primitives. If at some point the benchmark library
+  // merges into Abseil, this code may break.
+  absl::synchronization_internal::PerThreadSem::SetThreadBlockedCounter(
+      &shared->blocked_threads);
+
+  // The benchmark framework may run several iterations in the same process,
+  // reusing the same static-initialized 'shared' object. Given the semantics
+  // of the members, here, we expect everything to be reset to zero by the
+  // end of any iteration. Assert that's the case, just to be sure.
+  ABSL_RAW_CHECK(
+      shared->looping_threads.load(std::memory_order_relaxed) == 0 &&
+          shared->blocked_threads.load(std::memory_order_relaxed) == 0 &&
+          !shared->thread_has_mutex.load(std::memory_order_relaxed),
+      "Shared state isn't zeroed at start of benchmark iteration");
+
+  static constexpr int kBatchSize = 1000;
+  while (state.KeepRunningBatch(kBatchSize)) {
+    shared->looping_threads.fetch_add(1);
+    for (int i = 0; i < kBatchSize; i++) {
+      {
+        absl::MutexLock l(shared->mu);
+        shared->thread_has_mutex.store(true, std::memory_order_relaxed);
+        // Spin until all other threads are either out of the benchmark loop
+        // or blocked on the mutex. This ensures that the mutex queue is kept
+        // at its maximal length to benchmark the performance of queueing on
+        // a highly contended mutex.
+        while (shared->looping_threads.load(std::memory_order_relaxed) -
+                   shared->blocked_threads.load(std::memory_order_relaxed) !=
+               1) {
+        }
+        shared->thread_has_mutex.store(false);
+      }
+      // Spin until some other thread has acquired the mutex before we block
+      // again. This ensures that we always go through the slow (queueing)
+      // acquisition path rather than reacquiring the mutex we just released.
+      while (!shared->thread_has_mutex.load(std::memory_order_relaxed) &&
+             shared->looping_threads.load(std::memory_order_relaxed) > 1) {
+      }
+    }
+    // The benchmark framework uses a barrier to ensure that all of the threads
+    // complete their benchmark loop together before any of the threads exit
+    // the loop. So, we need to remove ourselves from the "looping threads"
+    // counter here before potentially blocking on that barrier. Otherwise,
+    // another thread spinning above might wait forever for this thread to
+    // block on the mutex while we in fact are waiting to exit.
+    shared->looping_threads.fetch_add(-1);
+  }
+  absl::synchronization_internal::PerThreadSem::SetThreadBlockedCounter(
+      nullptr);
+}
+
+BENCHMARK(BM_MutexEnqueue)
+    ->Threads(4)
+    ->Threads(64)
+    ->Threads(128)
+    ->Threads(512)
+    ->ArgName("multiple_priorities")
+    ->Arg(false)
+    ->Arg(true);
 
 template <typename MutexType>
 void BM_Contended(benchmark::State& state) {
+  int priority = state.thread_index() % state.range(1);
+  ScopedThreadMutexPriority priority_setter(priority);
+
   struct Shared {
     MutexType mu;
     int data = 0;
   };
-  static auto* shared = new Shared;
+  static absl::NoDestructor<Shared> shared;
   int local = 0;
   for (auto _ : state) {
     // Here we model both local work outside of the critical section as well as
@@ -79,86 +212,56 @@ void BM_Contended(benchmark::State& state) {
     // To achieve this amount of local work is multiplied by number of threads
     // to keep ratio between local work and critical section approximately
     // equal regardless of number of threads.
-    DelayNs(100 * state.threads, &local);
-    RaiiLocker<MutexType> locker(&shared->mu);
+    DelayNs(100 * state.threads(), &local);
+    std::scoped_lock locker(shared->mu);
     DelayNs(state.range(0), &shared->data);
   }
 }
+void SetupBenchmarkArgs(benchmark::internal::Benchmark* bm,
+                        bool do_test_priorities) {
+  const int max_num_priorities = do_test_priorities ? 2 : 1;
+  bm->UseRealTime()
+      // ThreadPerCpu poorly handles non-power-of-two CPU counts.
+      ->Threads(1)
+      ->Threads(2)
+      ->Threads(4)
+      ->Threads(6)
+      ->Threads(8)
+      ->Threads(12)
+      ->Threads(16)
+      ->Threads(24)
+      ->Threads(32)
+      ->Threads(48)
+      ->Threads(64)
+      ->Threads(96)
+      ->Threads(128)
+      ->Threads(192)
+      ->Threads(256)
+      ->ArgNames({"cs_ns", "num_prios"});
+  // Some empirically chosen amounts of work in critical section.
+  // 1 is low contention, 2000 is high contention and few values in between.
+  for (int critical_section_ns : {1, 20, 50, 200, 2000}) {
+    for (int num_priorities = 1; num_priorities <= max_num_priorities;
+         num_priorities++) {
+      bm->ArgPair(critical_section_ns, num_priorities);
+    }
+  }
+}
 
 BENCHMARK_TEMPLATE(BM_Contended, absl::Mutex)
-    ->UseRealTime()
-    // ThreadPerCpu poorly handles non-power-of-two CPU counts.
-    ->Threads(1)
-    ->Threads(2)
-    ->Threads(4)
-    ->Threads(6)
-    ->Threads(8)
-    ->Threads(12)
-    ->Threads(16)
-    ->Threads(24)
-    ->Threads(32)
-    ->Threads(48)
-    ->Threads(64)
-    ->Threads(96)
-    ->Threads(128)
-    ->Threads(192)
-    ->Threads(256)
-    // Some empirically chosen amounts of work in critical section.
-    // 1 is low contention, 200 is high contention and few values in between.
-    ->Arg(1)
-    ->Arg(20)
-    ->Arg(50)
-    ->Arg(200);
+    ->Apply([](benchmark::internal::Benchmark* bm) {
+      SetupBenchmarkArgs(bm, /*do_test_priorities=*/true);
+    });
 
 BENCHMARK_TEMPLATE(BM_Contended, absl::base_internal::SpinLock)
-    ->UseRealTime()
-    // ThreadPerCpu poorly handles non-power-of-two CPU counts.
-    ->Threads(1)
-    ->Threads(2)
-    ->Threads(4)
-    ->Threads(6)
-    ->Threads(8)
-    ->Threads(12)
-    ->Threads(16)
-    ->Threads(24)
-    ->Threads(32)
-    ->Threads(48)
-    ->Threads(64)
-    ->Threads(96)
-    ->Threads(128)
-    ->Threads(192)
-    ->Threads(256)
-    // Some empirically chosen amounts of work in critical section.
-    // 1 is low contention, 200 is high contention and few values in between.
-    ->Arg(1)
-    ->Arg(20)
-    ->Arg(50)
-    ->Arg(200);
+    ->Apply([](benchmark::internal::Benchmark* bm) {
+      SetupBenchmarkArgs(bm, /*do_test_priorities=*/false);
+    });
 
 BENCHMARK_TEMPLATE(BM_Contended, std::mutex)
-    ->UseRealTime()
-    // ThreadPerCpu poorly handles non-power-of-two CPU counts.
-    ->Threads(1)
-    ->Threads(2)
-    ->Threads(4)
-    ->Threads(6)
-    ->Threads(8)
-    ->Threads(12)
-    ->Threads(16)
-    ->Threads(24)
-    ->Threads(32)
-    ->Threads(48)
-    ->Threads(64)
-    ->Threads(96)
-    ->Threads(128)
-    ->Threads(192)
-    ->Threads(256)
-    // Some empirically chosen amounts of work in critical section.
-    // 1 is low contention, 200 is high contention and few values in between.
-    ->Arg(1)
-    ->Arg(20)
-    ->Arg(50)
-    ->Arg(200);
+    ->Apply([](benchmark::internal::Benchmark* bm) {
+      SetupBenchmarkArgs(bm, /*do_test_priorities=*/false);
+    });
 
 // Measure the overhead of conditions on mutex release (when they must be
 // evaluated).  Mutex has (some) support for equivalence classes allowing
@@ -175,7 +278,7 @@ void BM_ConditionWaiters(benchmark::State& state) {
       init->DecrementCount();
       m->LockWhen(absl::Condition(
           static_cast<bool (*)(int*)>([](int* v) { return *v == 0; }), p));
-      m->Unlock();
+      m->unlock();
     }
   };
 
@@ -201,19 +304,19 @@ void BM_ConditionWaiters(benchmark::State& state) {
   init.Wait();
 
   for (auto _ : state) {
-    mu.Lock();
-    mu.Unlock();  // Each unlock requires Condition evaluation for our waiters.
+    mu.lock();
+    mu.unlock();  // Each unlock requires Condition evaluation for our waiters.
   }
 
-  mu.Lock();
+  mu.lock();
   for (int i = 0; i < num_classes; i++) {
     equivalence_classes[i] = 0;
   }
-  mu.Unlock();
+  mu.unlock();
 }
 
 // Some configurations have higher thread limits than others.
-#if defined(__linux__) && !defined(THREAD_SANITIZER)
+#if defined(__linux__) && !defined(ABSL_HAVE_THREAD_SANITIZER)
 constexpr int kMaxConditionWaiters = 8192;
 #else
 constexpr int kMaxConditionWaiters = 1024;
diff --git a/absl/synchronization/mutex_method_pointer_test.cc b/absl/synchronization/mutex_method_pointer_test.cc
new file mode 100644
index 00000000000..420750b6c3e
--- /dev/null
+++ b/absl/synchronization/mutex_method_pointer_test.cc
@@ -0,0 +1,138 @@
+// Copyright 2017 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <string>
+
+#include "gtest/gtest.h"
+
+#ifdef _MSC_VER
+#include "absl/synchronization/mutex.h"
+#endif
+
+namespace {
+
+class IncompleteClass;
+
+#ifdef _MSC_VER
+// These tests verify expectations about sizes of MSVC pointers to methods.
+// Pointers to methods are distinguished by whether their class hierarchies
+// contain single inheritance, multiple inheritance, or virtual inheritance.
+
+// Declare classes of the various MSVC inheritance types.
+class __single_inheritance SingleInheritance{};
+class __multiple_inheritance MultipleInheritance;
+class __virtual_inheritance VirtualInheritance;
+
+TEST(MutexMethodPointerTest, MicrosoftMethodPointerSize) {
+  void (SingleInheritance::*single_inheritance_method_pointer)();
+  void (MultipleInheritance::*multiple_inheritance_method_pointer)();
+  void (VirtualInheritance::*virtual_inheritance_method_pointer)();
+
+#if defined(_M_IX86) || defined(_M_ARM)
+  static_assert(sizeof(single_inheritance_method_pointer) == 4,
+                "Unexpected sizeof(single_inheritance_method_pointer).");
+  static_assert(sizeof(multiple_inheritance_method_pointer) == 8,
+                "Unexpected sizeof(multiple_inheritance_method_pointer).");
+  static_assert(sizeof(virtual_inheritance_method_pointer) == 12,
+                "Unexpected sizeof(virtual_inheritance_method_pointer).");
+#elif defined(_M_X64) || defined(__aarch64__)
+  static_assert(sizeof(single_inheritance_method_pointer) == 8,
+                "Unexpected sizeof(single_inheritance_method_pointer).");
+  static_assert(sizeof(multiple_inheritance_method_pointer) == 16,
+                "Unexpected sizeof(multiple_inheritance_method_pointer).");
+  static_assert(sizeof(virtual_inheritance_method_pointer) == 16,
+                "Unexpected sizeof(virtual_inheritance_method_pointer).");
+#endif
+  void (IncompleteClass::*incomplete_class_method_pointer)();
+  static_assert(sizeof(incomplete_class_method_pointer) >=
+                    sizeof(virtual_inheritance_method_pointer),
+                "Failed invariant: sizeof(incomplete_class_method_pointer) >= "
+                "sizeof(virtual_inheritance_method_pointer)!");
+}
+
+class Callback {
+  bool x = true;
+
+ public:
+  Callback() {}
+  bool method() {
+    x = !x;
+    return x;
+  }
+};
+
+class M2 {
+  bool x = true;
+
+ public:
+  M2() {}
+  bool method2() {
+    x = !x;
+    return x;
+  }
+};
+
+class MultipleInheritance : public Callback, public M2 {};
+
+TEST(MutexMethodPointerTest, ConditionWithMultipleInheritanceMethod) {
+  // This test ensures that Condition can deal with method pointers from classes
+  // with multiple inheritance.
+  MultipleInheritance object = MultipleInheritance();
+  absl::Condition condition(&object, &MultipleInheritance::method);
+  EXPECT_FALSE(condition.Eval());
+  EXPECT_TRUE(condition.Eval());
+}
+
+class __virtual_inheritance VirtualInheritance : virtual public Callback {
+  bool x = false;
+
+ public:
+  VirtualInheritance() {}
+  bool method() {
+    x = !x;
+    return x;
+  }
+};
+
+TEST(MutexMethodPointerTest, ConditionWithVirtualInheritanceMethod) {
+  // This test ensures that Condition can deal with method pointers from classes
+  // with virtual inheritance.
+  VirtualInheritance object = VirtualInheritance();
+  absl::Condition condition(&object, &VirtualInheritance::method);
+  EXPECT_TRUE(condition.Eval());
+  EXPECT_FALSE(condition.Eval());
+}
+#endif  // #ifdef _MSC_VER
+
+TEST(MutexMethodPointerTest, ConditionWithIncompleteClassMethod) {
+  using IncompleteClassMethodPointer = void (IncompleteClass::*)();
+
+  union CallbackSlot {
+    void (*anonymous_function_pointer)();
+    IncompleteClassMethodPointer incomplete_class_method_pointer;
+  };
+
+  static_assert(sizeof(CallbackSlot) >= sizeof(IncompleteClassMethodPointer),
+                "The callback slot is not big enough for method pointers.");
+  static_assert(
+      sizeof(CallbackSlot) == sizeof(IncompleteClassMethodPointer),
+      "The callback slot is not big enough for anonymous function pointers.");
+
+#if defined(_MSC_VER)
+  static_assert(sizeof(IncompleteClassMethodPointer) <= 24,
+                "The pointer to a method of an incomplete class is too big.");
+#endif
+}
+
+}  // namespace
diff --git a/absl/synchronization/mutex_test.cc b/absl/synchronization/mutex_test.cc
index afb363af612..a96bcff7ebf 100644
--- a/absl/synchronization/mutex_test.cc
+++ b/absl/synchronization/mutex_test.cc
@@ -14,29 +14,40 @@
 
 #include "absl/synchronization/mutex.h"
 
-#ifdef _WIN32
-#include <windows.h>
-#endif
-
 #include <algorithm>
 #include <atomic>
 #include <cstdlib>
 #include <functional>
 #include <memory>
+#include <mutex>  // NOLINT(build/c++11)
+#include <ostream>
 #include <random>
+#include <shared_mutex>  // NOLINT(build/c++14)
 #include <string>
 #include <thread>  // NOLINT(build/c++11)
+#include <type_traits>
 #include <vector>
 
 #include "gtest/gtest.h"
-#include "absl/base/attributes.h"
-#include "absl/base/internal/raw_logging.h"
-#include "absl/base/internal/sysinfo.h"
-#include "absl/memory/memory.h"
+#include "absl/base/config.h"
+#include "absl/base/thread_annotations.h"
+#include "absl/log/check.h"
+#include "absl/log/log.h"
+#include "absl/random/random.h"
+#include "absl/synchronization/internal/create_thread_identity.h"
 #include "absl/synchronization/internal/thread_pool.h"
 #include "absl/time/clock.h"
 #include "absl/time/time.h"
 
+#ifdef _WIN32
+#include <windows.h>
+#endif
+
+#ifdef ABSL_HAVE_PTHREAD_GETSCHEDPARAM
+#include <pthread.h>
+#include <string.h>
+#endif
+
 namespace {
 
 // TODO(dmauro): Replace with a commandline flag.
@@ -44,7 +55,7 @@ static constexpr bool kExtendedTest = false;
 
 std::unique_ptr<absl::synchronization_internal::ThreadPool> CreatePool(
     int threads) {
-  return absl::make_unique<absl::synchronization_internal::ThreadPool>(threads);
+  return std::make_unique<absl::synchronization_internal::ThreadPool>(threads);
 }
 
 std::unique_ptr<absl::synchronization_internal::ThreadPool>
@@ -63,6 +74,11 @@ static void ScheduleAfter(absl::synchronization_internal::ThreadPool *tp,
   });
 }
 
+struct ScopedInvariantDebugging {
+  ScopedInvariantDebugging() { absl::EnableMutexInvariantDebugging(true); }
+  ~ScopedInvariantDebugging() { absl::EnableMutexInvariantDebugging(false); }
+};
+
 struct TestContext {
   int iterations;
   int threads;
@@ -85,13 +101,13 @@ static void SetInvariantChecked(bool new_value) {
 
 static void CheckSumG0G1(void *v) {
   TestContext *cxt = static_cast<TestContext *>(v);
-  ABSL_RAW_CHECK(cxt->g0 == -cxt->g1, "Error in CheckSumG0G1");
+  CHECK_EQ(cxt->g0, -cxt->g1) << "Error in CheckSumG0G1";
   SetInvariantChecked(true);
 }
 
 static void TestMu(TestContext *cxt, int c) {
   for (int i = 0; i != cxt->iterations; i++) {
-    absl::MutexLock l(&cxt->mu);
+    absl::MutexLock l(cxt->mu);
     int a = cxt->g0 + 1;
     cxt->g0 = a;
     cxt->g1--;
@@ -102,17 +118,17 @@ static void TestTry(TestContext *cxt, int c) {
   for (int i = 0; i != cxt->iterations; i++) {
     do {
       std::this_thread::yield();
-    } while (!cxt->mu.TryLock());
+    } while (!cxt->mu.try_lock());
     int a = cxt->g0 + 1;
     cxt->g0 = a;
     cxt->g1--;
-    cxt->mu.Unlock();
+    cxt->mu.unlock();
   }
 }
 
 static void TestR20ms(TestContext *cxt, int c) {
   for (int i = 0; i != cxt->iterations; i++) {
-    absl::ReaderMutexLock l(&cxt->mu);
+    absl::ReaderMutexLock l(cxt->mu);
     absl::SleepFor(absl::Milliseconds(20));
     cxt->mu.AssertReaderHeld();
   }
@@ -121,7 +137,7 @@ static void TestR20ms(TestContext *cxt, int c) {
 static void TestRW(TestContext *cxt, int c) {
   if ((c & 1) == 0) {
     for (int i = 0; i != cxt->iterations; i++) {
-      absl::WriterMutexLock l(&cxt->mu);
+      absl::WriterMutexLock l(cxt->mu);
       cxt->g0++;
       cxt->g1--;
       cxt->mu.AssertHeld();
@@ -129,8 +145,8 @@ static void TestRW(TestContext *cxt, int c) {
     }
   } else {
     for (int i = 0; i != cxt->iterations; i++) {
-      absl::ReaderMutexLock l(&cxt->mu);
-      ABSL_RAW_CHECK(cxt->g0 == -cxt->g1, "Error in TestRW");
+      absl::ReaderMutexLock l(cxt->mu);
+      CHECK_EQ(cxt->g0, -cxt->g1) << "Error in TestRW";
       cxt->mu.AssertReaderHeld();
     }
   }
@@ -151,11 +167,11 @@ static void TestAwait(TestContext *cxt, int c) {
   MyContext mc;
   mc.target = c;
   mc.cxt = cxt;
-  absl::MutexLock l(&cxt->mu);
+  absl::MutexLock l(cxt->mu);
   cxt->mu.AssertHeld();
   while (cxt->g0 < cxt->iterations) {
     cxt->mu.Await(absl::Condition(&mc, &MyContext::MyTurn));
-    ABSL_RAW_CHECK(mc.MyTurn(), "Error in TestAwait");
+    CHECK(mc.MyTurn()) << "Error in TestAwait";
     cxt->mu.AssertHeld();
     if (cxt->g0 < cxt->iterations) {
       int a = cxt->g0 + 1;
@@ -167,7 +183,7 @@ static void TestAwait(TestContext *cxt, int c) {
 
 static void TestSignalAll(TestContext *cxt, int c) {
   int target = c;
-  absl::MutexLock l(&cxt->mu);
+  absl::MutexLock l(cxt->mu);
   cxt->mu.AssertHeld();
   while (cxt->g0 < cxt->iterations) {
     while (cxt->g0 != target && cxt->g0 != cxt->iterations) {
@@ -183,9 +199,9 @@ static void TestSignalAll(TestContext *cxt, int c) {
 }
 
 static void TestSignal(TestContext *cxt, int c) {
-  ABSL_RAW_CHECK(cxt->threads == 2, "TestSignal should use 2 threads");
+  CHECK_EQ(cxt->threads, 2) << "TestSignal should use 2 threads";
   int target = c;
-  absl::MutexLock l(&cxt->mu);
+  absl::MutexLock l(cxt->mu);
   cxt->mu.AssertHeld();
   while (cxt->g0 < cxt->iterations) {
     while (cxt->g0 != target && cxt->g0 != cxt->iterations) {
@@ -202,7 +218,7 @@ static void TestSignal(TestContext *cxt, int c) {
 
 static void TestCVTimeout(TestContext *cxt, int c) {
   int target = c;
-  absl::MutexLock l(&cxt->mu);
+  absl::MutexLock l(cxt->mu);
   cxt->mu.AssertHeld();
   while (cxt->g0 < cxt->iterations) {
     while (cxt->g0 != target && cxt->g0 != cxt->iterations) {
@@ -220,38 +236,36 @@ static void TestCVTimeout(TestContext *cxt, int c) {
 static bool G0GE2(TestContext *cxt) { return cxt->g0 >= 2; }
 
 static void TestTime(TestContext *cxt, int c, bool use_cv) {
-  ABSL_RAW_CHECK(cxt->iterations == 1, "TestTime should only use 1 iteration");
-  ABSL_RAW_CHECK(cxt->threads > 2, "TestTime should use more than 2 threads");
+  CHECK_EQ(cxt->iterations, 1) << "TestTime should only use 1 iteration";
+  CHECK_GT(cxt->threads, 2) << "TestTime should use more than 2 threads";
   const bool kFalse = false;
   absl::Condition false_cond(&kFalse);
   absl::Condition g0ge2(G0GE2, cxt);
   if (c == 0) {
-    absl::MutexLock l(&cxt->mu);
+    absl::MutexLock l(cxt->mu);
 
     absl::Time start = absl::Now();
     if (use_cv) {
       cxt->cv.WaitWithTimeout(&cxt->mu, absl::Seconds(1));
     } else {
-      ABSL_RAW_CHECK(!cxt->mu.AwaitWithTimeout(false_cond, absl::Seconds(1)),
-                     "TestTime failed");
+      CHECK(!cxt->mu.AwaitWithTimeout(false_cond, absl::Seconds(1)))
+          << "TestTime failed";
     }
     absl::Duration elapsed = absl::Now() - start;
-    ABSL_RAW_CHECK(
-        absl::Seconds(0.9) <= elapsed && elapsed <= absl::Seconds(2.0),
-        "TestTime failed");
-    ABSL_RAW_CHECK(cxt->g0 == 1, "TestTime failed");
+    CHECK(absl::Seconds(0.9) <= elapsed && elapsed <= absl::Seconds(2.0))
+        << "TestTime failed";
+    CHECK_EQ(cxt->g0, 1) << "TestTime failed";
 
     start = absl::Now();
     if (use_cv) {
       cxt->cv.WaitWithTimeout(&cxt->mu, absl::Seconds(1));
     } else {
-      ABSL_RAW_CHECK(!cxt->mu.AwaitWithTimeout(false_cond, absl::Seconds(1)),
-                     "TestTime failed");
+      CHECK(!cxt->mu.AwaitWithTimeout(false_cond, absl::Seconds(1)))
+          << "TestTime failed";
     }
     elapsed = absl::Now() - start;
-    ABSL_RAW_CHECK(
-        absl::Seconds(0.9) <= elapsed && elapsed <= absl::Seconds(2.0),
-        "TestTime failed");
+    CHECK(absl::Seconds(0.9) <= elapsed && elapsed <= absl::Seconds(2.0))
+        << "TestTime failed";
     cxt->g0++;
     if (use_cv) {
       cxt->cv.Signal();
@@ -261,26 +275,24 @@ static void TestTime(TestContext *cxt, int c, bool use_cv) {
     if (use_cv) {
       cxt->cv.WaitWithTimeout(&cxt->mu, absl::Seconds(4));
     } else {
-      ABSL_RAW_CHECK(!cxt->mu.AwaitWithTimeout(false_cond, absl::Seconds(4)),
-                     "TestTime failed");
+      CHECK(!cxt->mu.AwaitWithTimeout(false_cond, absl::Seconds(4)))
+          << "TestTime failed";
     }
     elapsed = absl::Now() - start;
-    ABSL_RAW_CHECK(
-        absl::Seconds(3.9) <= elapsed && elapsed <= absl::Seconds(6.0),
-        "TestTime failed");
-    ABSL_RAW_CHECK(cxt->g0 >= 3, "TestTime failed");
+    CHECK(absl::Seconds(3.9) <= elapsed && elapsed <= absl::Seconds(6.0))
+        << "TestTime failed";
+    CHECK_GE(cxt->g0, 3) << "TestTime failed";
 
     start = absl::Now();
     if (use_cv) {
       cxt->cv.WaitWithTimeout(&cxt->mu, absl::Seconds(1));
     } else {
-      ABSL_RAW_CHECK(!cxt->mu.AwaitWithTimeout(false_cond, absl::Seconds(1)),
-                     "TestTime failed");
+      CHECK(!cxt->mu.AwaitWithTimeout(false_cond, absl::Seconds(1)))
+          << "TestTime failed";
     }
     elapsed = absl::Now() - start;
-    ABSL_RAW_CHECK(
-        absl::Seconds(0.9) <= elapsed && elapsed <= absl::Seconds(2.0),
-        "TestTime failed");
+    CHECK(absl::Seconds(0.9) <= elapsed && elapsed <= absl::Seconds(2.0))
+        << "TestTime failed";
     if (use_cv) {
       cxt->cv.SignalAll();
     }
@@ -289,42 +301,40 @@ static void TestTime(TestContext *cxt, int c, bool use_cv) {
     if (use_cv) {
       cxt->cv.WaitWithTimeout(&cxt->mu, absl::Seconds(1));
     } else {
-      ABSL_RAW_CHECK(!cxt->mu.AwaitWithTimeout(false_cond, absl::Seconds(1)),
-                     "TestTime failed");
+      CHECK(!cxt->mu.AwaitWithTimeout(false_cond, absl::Seconds(1)))
+          << "TestTime failed";
     }
     elapsed = absl::Now() - start;
-    ABSL_RAW_CHECK(absl::Seconds(0.9) <= elapsed &&
-                   elapsed <= absl::Seconds(2.0), "TestTime failed");
-    ABSL_RAW_CHECK(cxt->g0 == cxt->threads, "TestTime failed");
+    CHECK(absl::Seconds(0.9) <= elapsed && elapsed <= absl::Seconds(2.0))
+        << "TestTime failed";
+    CHECK_EQ(cxt->g0, cxt->threads) << "TestTime failed";
 
   } else if (c == 1) {
-    absl::MutexLock l(&cxt->mu);
+    absl::MutexLock l(cxt->mu);
     const absl::Time start = absl::Now();
     if (use_cv) {
       cxt->cv.WaitWithTimeout(&cxt->mu, absl::Milliseconds(500));
     } else {
-      ABSL_RAW_CHECK(
-          !cxt->mu.AwaitWithTimeout(false_cond, absl::Milliseconds(500)),
-          "TestTime failed");
+      CHECK(!cxt->mu.AwaitWithTimeout(false_cond, absl::Milliseconds(500)))
+          << "TestTime failed";
     }
     const absl::Duration elapsed = absl::Now() - start;
-    ABSL_RAW_CHECK(
-        absl::Seconds(0.4) <= elapsed && elapsed <= absl::Seconds(0.9),
-        "TestTime failed");
+    CHECK(absl::Seconds(0.4) <= elapsed && elapsed <= absl::Seconds(0.9))
+        << "TestTime failed";
     cxt->g0++;
   } else if (c == 2) {
-    absl::MutexLock l(&cxt->mu);
+    absl::MutexLock l(cxt->mu);
     if (use_cv) {
       while (cxt->g0 < 2) {
         cxt->cv.WaitWithTimeout(&cxt->mu, absl::Seconds(100));
       }
     } else {
-      ABSL_RAW_CHECK(cxt->mu.AwaitWithTimeout(g0ge2, absl::Seconds(100)),
-                     "TestTime failed");
+      CHECK(cxt->mu.AwaitWithTimeout(g0ge2, absl::Seconds(100)))
+          << "TestTime failed";
     }
     cxt->g0++;
   } else {
-    absl::MutexLock l(&cxt->mu);
+    absl::MutexLock l(cxt->mu);
     if (use_cv) {
       while (cxt->g0 < 2) {
         cxt->cv.Wait(&cxt->mu);
@@ -341,12 +351,12 @@ static void TestMuTime(TestContext *cxt, int c) { TestTime(cxt, c, false); }
 static void TestCVTime(TestContext *cxt, int c) { TestTime(cxt, c, true); }
 
 static void EndTest(int *c0, int *c1, absl::Mutex *mu, absl::CondVar *cv,
-                    const std::function<void(int)>& cb) {
-  mu->Lock();
+                    const std::function<void(int)> &cb) {
+  mu->lock();
   int c = (*c0)++;
-  mu->Unlock();
+  mu->unlock();
   cb(c);
-  absl::MutexLock l(mu);
+  absl::MutexLock l(*mu);
   (*c1)++;
   cv->Signal();
 }
@@ -364,15 +374,15 @@ static int RunTestCommon(TestContext *cxt, void (*test)(TestContext *cxt, int),
   cxt->threads = threads;
   absl::synchronization_internal::ThreadPool tp(threads);
   for (int i = 0; i != threads; i++) {
-    tp.Schedule(std::bind(&EndTest, &c0, &c1, &mu2, &cv2,
-                          std::function<void(int)>(
-                              std::bind(test, cxt, std::placeholders::_1))));
+    tp.Schedule(std::bind(
+        &EndTest, &c0, &c1, &mu2, &cv2,
+        std::function<void(int)>(std::bind(test, cxt, std::placeholders::_1))));
   }
-  mu2.Lock();
+  mu2.lock();
   while (c1 != threads) {
     cv2.Wait(&mu2);
   }
-  mu2.Unlock();
+  mu2.unlock();
   return cxt->g0;
 }
 
@@ -392,13 +402,12 @@ static int RunTestWithInvariantDebugging(void (*test)(TestContext *cxt, int),
                                          int threads, int iterations,
                                          int operations,
                                          void (*invariant)(void *)) {
-  absl::EnableMutexInvariantDebugging(true);
+  ScopedInvariantDebugging scoped_debugging;
   SetInvariantChecked(false);
   TestContext cxt;
   cxt.mu.EnableInvariantDebugging(invariant, &cxt);
   int ret = RunTestCommon(&cxt, test, threads, iterations, operations);
-  ABSL_RAW_CHECK(GetInvariantChecked(), "Invariant not checked");
-  absl::EnableMutexInvariantDebugging(false);  // Restore.
+  CHECK(GetInvariantChecked()) << "Invariant not checked";
   return ret;
 }
 #endif
@@ -414,7 +423,7 @@ struct TimeoutBugStruct {
 static void WaitForA(TimeoutBugStruct *x) {
   x->mu.LockWhen(absl::Condition(&x->a));
   x->a_waiter_count--;
-  x->mu.Unlock();
+  x->mu.unlock();
 }
 
 static bool NoAWaiters(TimeoutBugStruct *x) { return x->a_waiter_count == 0; }
@@ -437,27 +446,27 @@ TEST(Mutex, CondVarWaitSignalsAwait) {
   // Thread A.  Sets barrier, waits for release using Mutex::Await, then
   // signals released_cv.
   pool->Schedule([&state] {
-    state.release_mu.Lock();
+    state.release_mu.lock();
 
-    state.barrier_mu.Lock();
+    state.barrier_mu.lock();
     state.barrier = true;
-    state.barrier_mu.Unlock();
+    state.barrier_mu.unlock();
 
     state.release_mu.Await(absl::Condition(&state.release));
     state.released_cv.Signal();
-    state.release_mu.Unlock();
+    state.release_mu.unlock();
   });
 
   state.barrier_mu.LockWhen(absl::Condition(&state.barrier));
-  state.barrier_mu.Unlock();
-  state.release_mu.Lock();
+  state.barrier_mu.unlock();
+  state.release_mu.lock();
   // Thread A is now blocked on release by way of Mutex::Await().
 
   // Set release.  Calling released_cv.Wait() should un-block thread A,
   // which will signal released_cv.  If not, the test will hang.
   state.release = true;
   state.released_cv.Wait(&state.release_mu);
-  state.release_mu.Unlock();
+  state.release_mu.unlock();
 }
 
 // Test that a CondVar.WaitWithTimeout(&mutex) can un-block a call to
@@ -478,20 +487,20 @@ TEST(Mutex, CondVarWaitWithTimeoutSignalsAwait) {
   // Thread A.  Sets barrier, waits for release using Mutex::Await, then
   // signals released_cv.
   pool->Schedule([&state] {
-    state.release_mu.Lock();
+    state.release_mu.lock();
 
-    state.barrier_mu.Lock();
+    state.barrier_mu.lock();
     state.barrier = true;
-    state.barrier_mu.Unlock();
+    state.barrier_mu.unlock();
 
     state.release_mu.Await(absl::Condition(&state.release));
     state.released_cv.Signal();
-    state.release_mu.Unlock();
+    state.release_mu.unlock();
   });
 
   state.barrier_mu.LockWhen(absl::Condition(&state.barrier));
-  state.barrier_mu.Unlock();
-  state.release_mu.Lock();
+  state.barrier_mu.unlock();
+  state.release_mu.lock();
   // Thread A is now blocked on release by way of Mutex::Await().
 
   // Set release.  Calling released_cv.Wait() should un-block thread A,
@@ -502,7 +511,7 @@ TEST(Mutex, CondVarWaitWithTimeoutSignalsAwait) {
       << "; Unrecoverable test failure: CondVar::WaitWithTimeout did not "
          "unblock the absl::Mutex::Await call in another thread.";
 
-  state.release_mu.Unlock();
+  state.release_mu.unlock();
 }
 
 // Test for regression of a bug in loop of TryRemove()
@@ -528,7 +537,7 @@ TEST(Mutex, MutexTimeoutBug) {
 
   x.a = true;                                    // wakeup the two waiters on A
   x.mu.Await(absl::Condition(&NoAWaiters, &x));  // wait for them to exit
-  x.mu.Unlock();
+  x.mu.unlock();
 }
 
 struct CondVarWaitDeadlock : testing::TestWithParam<int> {
@@ -548,27 +557,27 @@ struct CondVarWaitDeadlock : testing::TestWithParam<int> {
 
   void Waiter1() {
     if (read_lock1) {
-      mu.ReaderLock();
+      mu.lock_shared();
       while (!cond1) {
         cv.Wait(&mu);
       }
-      mu.ReaderUnlock();
+      mu.unlock_shared();
     } else {
-      mu.Lock();
+      mu.lock();
       while (!cond1) {
         cv.Wait(&mu);
       }
-      mu.Unlock();
+      mu.unlock();
     }
   }
 
   void Waiter2() {
     if (read_lock2) {
       mu.ReaderLockWhen(absl::Condition(&cond2));
-      mu.ReaderUnlock();
+      mu.unlock_shared();
     } else {
       mu.LockWhen(absl::Condition(&cond2));
-      mu.Unlock();
+      mu.unlock();
     }
   }
 };
@@ -592,21 +601,21 @@ TEST_P(CondVarWaitDeadlock, Test) {
   absl::SleepFor(absl::Milliseconds(100));
 
   // Wake condwaiter.
-  mu.Lock();
+  mu.lock();
   cond1 = true;
   if (signal_unlocked) {
-    mu.Unlock();
+    mu.unlock();
     cv.Signal();
   } else {
     cv.Signal();
-    mu.Unlock();
+    mu.unlock();
   }
   waiter1.reset();  // "join" waiter1
 
   // Wake waiter.
-  mu.Lock();
+  mu.lock();
   cond2 = true;
-  mu.Unlock();
+  mu.unlock();
   waiter2.reset();  // "join" waiter2
 }
 
@@ -631,19 +640,19 @@ struct DequeueAllWakeableBugStruct {
 
 // Test for regression of a bug in loop of DequeueAllWakeable()
 static void AcquireAsReader(DequeueAllWakeableBugStruct *x) {
-  x->mu.ReaderLock();
-  x->mu2.Lock();
+  x->mu.lock_shared();
+  x->mu2.lock();
   x->unfinished_count--;
   x->done1 = (x->unfinished_count == 0);
-  x->mu2.Unlock();
+  x->mu2.unlock();
   // make sure that both readers acquired mu before we release it.
   absl::SleepFor(absl::Seconds(2));
-  x->mu.ReaderUnlock();
+  x->mu.unlock_shared();
 
-  x->mu2.Lock();
+  x->mu2.lock();
   x->finished_count--;
   x->done2 = (x->finished_count == 0);
-  x->mu2.Unlock();
+  x->mu2.unlock();
 }
 
 // Test for regression of a bug in loop of DequeueAllWakeable()
@@ -655,21 +664,21 @@ TEST(Mutex, MutexReaderWakeupBug) {
   x.done1 = false;
   x.finished_count = 2;
   x.done2 = false;
-  x.mu.Lock();  // acquire mu exclusively
+  x.mu.lock();  // acquire mu exclusively
   // queue two thread that will block on reader locks on x.mu
   tp->Schedule(std::bind(&AcquireAsReader, &x));
   tp->Schedule(std::bind(&AcquireAsReader, &x));
   absl::SleepFor(absl::Seconds(1));  // give time for reader threads to block
-  x.mu.Unlock();                     // wake them up
+  x.mu.unlock();                     // wake them up
 
   // both readers should finish promptly
   EXPECT_TRUE(
       x.mu2.LockWhenWithTimeout(absl::Condition(&x.done1), absl::Seconds(10)));
-  x.mu2.Unlock();
+  x.mu2.unlock();
 
   EXPECT_TRUE(
       x.mu2.LockWhenWithTimeout(absl::Condition(&x.done2), absl::Seconds(10)));
-  x.mu2.Unlock();
+  x.mu2.unlock();
 }
 
 struct LockWhenTestStruct {
@@ -680,16 +689,16 @@ struct LockWhenTestStruct {
   bool waiting = false;
 };
 
-static bool LockWhenTestIsCond(LockWhenTestStruct* s) {
-  s->mu2.Lock();
+static bool LockWhenTestIsCond(LockWhenTestStruct *s) {
+  s->mu2.lock();
   s->waiting = true;
-  s->mu2.Unlock();
+  s->mu2.unlock();
   return s->cond;
 }
 
-static void LockWhenTestWaitForIsCond(LockWhenTestStruct* s) {
+static void LockWhenTestWaitForIsCond(LockWhenTestStruct *s) {
   s->mu1.LockWhen(absl::Condition(&LockWhenTestIsCond, s));
-  s->mu1.Unlock();
+  s->mu1.unlock();
 }
 
 TEST(Mutex, LockWhen) {
@@ -697,17 +706,51 @@ TEST(Mutex, LockWhen) {
 
   std::thread t(LockWhenTestWaitForIsCond, &s);
   s.mu2.LockWhen(absl::Condition(&s.waiting));
-  s.mu2.Unlock();
+  s.mu2.unlock();
 
-  s.mu1.Lock();
+  s.mu1.lock();
   s.cond = true;
-  s.mu1.Unlock();
+  s.mu1.unlock();
 
   t.join();
 }
 
+TEST(Mutex, LockWhenGuard) {
+  absl::Mutex mu;
+  int n = 30;
+  bool done = false;
+
+  // We don't inline the lambda because the conversion is ambiguous in MSVC.
+  bool (*cond_eq_10)(int *) = [](int *p) { return *p == 10; };
+  bool (*cond_lt_10)(int *) = [](int *p) { return *p < 10; };
+
+  std::thread t1([&mu, &n, &done, cond_eq_10]() {
+    absl::ReaderMutexLock lock(mu, absl::Condition(cond_eq_10, &n));
+    done = true;
+  });
+
+  std::thread t2[10];
+  for (std::thread &t : t2) {
+    t = std::thread([&mu, &n, cond_lt_10]() {
+      absl::WriterMutexLock lock(mu, absl::Condition(cond_lt_10, &n));
+      ++n;
+    });
+  }
+
+  {
+    absl::MutexLock lock(mu);
+    n = 0;
+  }
+
+  for (std::thread &t : t2) t.join();
+  t1.join();
+
+  EXPECT_TRUE(done);
+  EXPECT_EQ(n, 10);
+}
+
 // --------------------------------------------------------
-// The following test requires Mutex::ReaderLock to be a real shared
+// The following test requires Mutex::lock_shared to be a real shared
 // lock, which is not the case in all builds.
 #if !defined(ABSL_MUTEX_READER_LOCK_IS_EXCLUSIVE)
 
@@ -734,9 +777,9 @@ struct ReaderDecrementBugStruct {
 // L >= mu, L < mu_waiting_on_cond
 static bool IsCond(void *v) {
   ReaderDecrementBugStruct *x = reinterpret_cast<ReaderDecrementBugStruct *>(v);
-  x->mu2.Lock();
+  x->mu2.lock();
   x->waiting_on_cond = true;
-  x->mu2.Unlock();
+  x->mu2.unlock();
   return x->cond;
 }
 
@@ -749,23 +792,23 @@ static bool AllDone(void *v) {
 // L={}
 static void WaitForCond(ReaderDecrementBugStruct *x) {
   absl::Mutex dummy;
-  absl::MutexLock l(&dummy);
+  absl::MutexLock l(dummy);
   x->mu.LockWhen(absl::Condition(&IsCond, x));
   x->done--;
-  x->mu.Unlock();
+  x->mu.unlock();
 }
 
 // L={}
 static void GetReadLock(ReaderDecrementBugStruct *x) {
-  x->mu.ReaderLock();
-  x->mu2.Lock();
+  x->mu.lock_shared();
+  x->mu2.lock();
   x->have_reader_lock = true;
   x->mu2.Await(absl::Condition(&x->complete));
-  x->mu2.Unlock();
-  x->mu.ReaderUnlock();
-  x->mu.Lock();
+  x->mu2.unlock();
+  x->mu.unlock_shared();
+  x->mu.lock();
   x->done--;
-  x->mu.Unlock();
+  x->mu.unlock();
 }
 
 // Test for reader counter being decremented incorrectly by waiter
@@ -781,32 +824,32 @@ TEST(Mutex, MutexReaderDecrementBug) ABSL_NO_THREAD_SAFETY_ANALYSIS {
   // Run WaitForCond() and wait for it to sleep
   std::thread thread1(WaitForCond, &x);
   x.mu2.LockWhen(absl::Condition(&x.waiting_on_cond));
-  x.mu2.Unlock();
+  x.mu2.unlock();
 
   // Run GetReadLock(), and wait for it to get the read lock
   std::thread thread2(GetReadLock, &x);
   x.mu2.LockWhen(absl::Condition(&x.have_reader_lock));
-  x.mu2.Unlock();
+  x.mu2.unlock();
 
   // Get the reader lock ourselves, and release it.
-  x.mu.ReaderLock();
-  x.mu.ReaderUnlock();
+  x.mu.lock_shared();
+  x.mu.unlock_shared();
 
   // The lock should be held in read mode by GetReadLock().
   // If we have the bug, the lock will be free.
   x.mu.AssertReaderHeld();
 
   // Wake up all the threads.
-  x.mu2.Lock();
+  x.mu2.lock();
   x.complete = true;
-  x.mu2.Unlock();
+  x.mu2.unlock();
 
   // TODO(delesley): turn on analysis once lock upgrading is supported.
   // (This call upgrades the lock from shared to exclusive.)
-  x.mu.Lock();
+  x.mu.lock();
   x.cond = true;
   x.mu.Await(absl::Condition(&AllDone, &x));
-  x.mu.Unlock();
+  x.mu.unlock();
 
   thread1.join();
   thread2.join();
@@ -815,52 +858,153 @@ TEST(Mutex, MutexReaderDecrementBug) ABSL_NO_THREAD_SAFETY_ANALYSIS {
 
 // Test that we correctly handle the situation when a lock is
 // held and then destroyed (w/o unlocking).
-#ifdef THREAD_SANITIZER
+#ifdef ABSL_HAVE_THREAD_SANITIZER
 // TSAN reports errors when locked Mutexes are destroyed.
-TEST(Mutex, DISABLED_LockedMutexDestructionBug) NO_THREAD_SAFETY_ANALYSIS {
+TEST(Mutex, DISABLED_LockedMutexDestructionBug) ABSL_NO_THREAD_SAFETY_ANALYSIS {
 #else
 TEST(Mutex, LockedMutexDestructionBug) ABSL_NO_THREAD_SAFETY_ANALYSIS {
 #endif
   for (int i = 0; i != 10; i++) {
     // Create, lock and destroy 10 locks.
     const int kNumLocks = 10;
-    auto mu = absl::make_unique<absl::Mutex[]>(kNumLocks);
+    auto mu = std::make_unique<absl::Mutex[]>(kNumLocks);
     for (int j = 0; j != kNumLocks; j++) {
       if ((j % 2) == 0) {
-        mu[j].WriterLock();
+        mu[j].lock();
       } else {
-        mu[j].ReaderLock();
+        mu[j].lock_shared();
       }
     }
   }
 }
 
-// --------------------------------------------------------
-// Test for bug with pattern of readers using a condvar.  The bug was that if a
-// reader went to sleep on a condition variable while one or more other readers
-// held the lock, but there were no waiters, the reader count (held in the
-// mutex word) would be lost.  (This is because Enqueue() had at one time
-// always placed the thread on the Mutex queue.  Later (CL 4075610), to
-// tolerate re-entry into Mutex from a Condition predicate, Enqueue() was
-// changed so that it could also place a thread on a condition-variable.  This
-// introduced the case where Enqueue() returned with an empty queue, and this
-// case was handled incorrectly in one place.)
+// Some functions taking pointers to non-const.
+bool Equals42(int *p) { return *p == 42; }
+bool Equals43(int *p) { return *p == 43; }
 
-static void ReaderForReaderOnCondVar(absl::Mutex *mu, absl::CondVar *cv,
-                                     int *running) {
-  std::random_device dev;
-  std::mt19937 gen(dev());
-  std::uniform_int_distribution<int> random_millis(0, 15);
-  mu->ReaderLock();
-  while (*running == 3) {
-    absl::SleepFor(absl::Milliseconds(random_millis(gen)));
-    cv->WaitWithTimeout(mu, absl::Milliseconds(random_millis(gen)));
+// Some functions taking pointers to const.
+bool ConstEquals42(const int *p) { return *p == 42; }
+bool ConstEquals43(const int *p) { return *p == 43; }
+
+// Some function templates taking pointers. Note it's possible for `T` to be
+// deduced as non-const or const, which creates the potential for ambiguity,
+// but which the implementation is careful to avoid.
+template <typename T>
+bool TemplateEquals42(T *p) {
+  return *p == 42;
+}
+template <typename T>
+bool TemplateEquals43(T *p) {
+  return *p == 43;
+}
+
+TEST(Mutex, FunctionPointerCondition) {
+  // Some arguments.
+  int x = 42;
+  const int const_x = 42;
+
+  // Parameter non-const, argument non-const.
+  EXPECT_TRUE(absl::Condition(Equals42, &x).Eval());
+  EXPECT_FALSE(absl::Condition(Equals43, &x).Eval());
+
+  // Parameter const, argument non-const.
+  EXPECT_TRUE(absl::Condition(ConstEquals42, &x).Eval());
+  EXPECT_FALSE(absl::Condition(ConstEquals43, &x).Eval());
+
+  // Parameter const, argument const.
+  EXPECT_TRUE(absl::Condition(ConstEquals42, &const_x).Eval());
+  EXPECT_FALSE(absl::Condition(ConstEquals43, &const_x).Eval());
+
+  // Parameter type deduced, argument non-const.
+  EXPECT_TRUE(absl::Condition(TemplateEquals42, &x).Eval());
+  EXPECT_FALSE(absl::Condition(TemplateEquals43, &x).Eval());
+
+  // Parameter type deduced, argument const.
+  EXPECT_TRUE(absl::Condition(TemplateEquals42, &const_x).Eval());
+  EXPECT_FALSE(absl::Condition(TemplateEquals43, &const_x).Eval());
+
+  // Parameter non-const, argument const is not well-formed.
+  EXPECT_FALSE((std::is_constructible_v<absl::Condition, decltype(Equals42),
+                                        decltype(&const_x)>));
+  // Validate use of is_constructible by contrasting to a well-formed case.
+  EXPECT_TRUE((std::is_constructible_v<absl::Condition, decltype(ConstEquals42),
+                                       decltype(&const_x)>));
+}
+
+// Example base and derived class for use in predicates and test below. Not a
+// particularly realistic example, but it suffices for testing purposes.
+struct Base {
+  explicit Base(int v) : value(v) {}
+  int value;
+};
+struct Derived : Base {
+  explicit Derived(int v) : Base(v) {}
+};
+
+// Some functions taking pointer to non-const `Base`.
+bool BaseEquals42(Base *p) { return p->value == 42; }
+bool BaseEquals43(Base *p) { return p->value == 43; }
+
+// Some functions taking pointer to const `Base`.
+bool ConstBaseEquals42(const Base *p) { return p->value == 42; }
+bool ConstBaseEquals43(const Base *p) { return p->value == 43; }
+
+TEST(Mutex, FunctionPointerConditionWithDerivedToBaseConversion) {
+  // Some arguments.
+  Derived derived(42);
+  const Derived const_derived(42);
+
+  // Parameter non-const base, argument derived non-const.
+  EXPECT_TRUE(absl::Condition(BaseEquals42, &derived).Eval());
+  EXPECT_FALSE(absl::Condition(BaseEquals43, &derived).Eval());
+
+  // Parameter const base, argument derived non-const.
+  EXPECT_TRUE(absl::Condition(ConstBaseEquals42, &derived).Eval());
+  EXPECT_FALSE(absl::Condition(ConstBaseEquals43, &derived).Eval());
+
+  // Parameter const base, argument derived const.
+  EXPECT_TRUE(absl::Condition(ConstBaseEquals42, &const_derived).Eval());
+  EXPECT_FALSE(absl::Condition(ConstBaseEquals43, &const_derived).Eval());
+
+  // Parameter const base, argument derived const.
+  EXPECT_TRUE(absl::Condition(ConstBaseEquals42, &const_derived).Eval());
+  EXPECT_FALSE(absl::Condition(ConstBaseEquals43, &const_derived).Eval());
+
+  // Parameter derived, argument base is not well-formed.
+  bool (*derived_pred)(const Derived *) = [](const Derived *) { return true; };
+  EXPECT_FALSE((
+      std::is_constructible_v<absl::Condition, decltype(derived_pred), Base*>));
+  EXPECT_FALSE((std::is_constructible_v<absl::Condition, decltype(derived_pred),
+                                        const Base*>));
+  // Validate use of is_constructible by contrasting to well-formed cases.
+  EXPECT_TRUE((std::is_constructible_v<absl::Condition, decltype(derived_pred),
+                                       Derived*>));
+  EXPECT_TRUE((std::is_constructible_v<absl::Condition, decltype(derived_pred),
+                                       const Derived*>));
+}
+
+struct Constable {
+  bool WotsAllThisThen() const { return true; }
+};
+
+TEST(Mutex, FunctionPointerConditionWithConstMethod) {
+  const Constable chapman;
+  EXPECT_TRUE(absl::Condition(&chapman, &Constable::WotsAllThisThen).Eval());
+}
+
+#ifdef __cpp_explicit_this_parameter
+struct TrueViaDeducingThis {
+  template <class This, class... Args>
+  bool operator()(this const This&, Args...) {
+    return true;
   }
-  mu->ReaderUnlock();
-  mu->Lock();
-  (*running)--;
-  mu->Unlock();
+};
+
+TEST(Mutex, FunctorConditionDeducingThis) {
+  TrueViaDeducingThis f;
+  EXPECT_TRUE(absl::Condition(&f).Eval());
 }
+#endif
 
 struct True {
   template <class... Args>
@@ -910,6 +1054,45 @@ TEST(Mutex, FunctorCondition) {
   }
 }
 
+TEST(Mutex, ConditionSwap) {
+  // Ensure that Conditions can be swap'ed.
+  bool b1 = true;
+  absl::Condition c1(&b1);
+  bool b2 = false;
+  absl::Condition c2(&b2);
+  EXPECT_TRUE(c1.Eval());
+  EXPECT_FALSE(c2.Eval());
+  std::swap(c1, c2);
+  EXPECT_FALSE(c1.Eval());
+  EXPECT_TRUE(c2.Eval());
+}
+
+// --------------------------------------------------------
+// Test for bug with pattern of readers using a condvar.  The bug was that if a
+// reader went to sleep on a condition variable while one or more other readers
+// held the lock, but there were no waiters, the reader count (held in the
+// mutex word) would be lost.  (This is because Enqueue() had at one time
+// always placed the thread on the Mutex queue.  Later (CL 4075610), to
+// tolerate re-entry into Mutex from a Condition predicate, Enqueue() was
+// changed so that it could also place a thread on a condition-variable.  This
+// introduced the case where Enqueue() returned with an empty queue, and this
+// case was handled incorrectly in one place.)
+
+static void ReaderForReaderOnCondVar(absl::Mutex *mu, absl::CondVar *cv,
+                                     int *running) {
+  absl::InsecureBitGen gen;
+  std::uniform_int_distribution<int> random_millis(0, 15);
+  mu->lock_shared();
+  while (*running == 3) {
+    absl::SleepFor(absl::Milliseconds(random_millis(gen)));
+    cv->WaitWithTimeout(mu, absl::Milliseconds(random_millis(gen)));
+  }
+  mu->unlock_shared();
+  mu->lock();
+  (*running)--;
+  mu->unlock();
+}
+
 static bool IntIsZero(int *x) { return *x == 0; }
 
 // Test for reader waiting condition variable when there are other readers
@@ -922,10 +1105,10 @@ TEST(Mutex, TestReaderOnCondVar) {
   tp->Schedule(std::bind(&ReaderForReaderOnCondVar, &mu, &cv, &running));
   tp->Schedule(std::bind(&ReaderForReaderOnCondVar, &mu, &cv, &running));
   absl::SleepFor(absl::Seconds(2));
-  mu.Lock();
+  mu.lock();
   running--;
   mu.Await(absl::Condition(&IntIsZero, &running));
-  mu.Unlock();
+  mu.unlock();
 }
 
 // --------------------------------------------------------
@@ -949,9 +1132,9 @@ static bool ConditionWithAcquire(AcquireFromConditionStruct *x) {
     bool always_false = false;
     x->mu1.LockWhenWithTimeout(absl::Condition(&always_false),
                                absl::Milliseconds(100));
-    x->mu1.Unlock();
+    x->mu1.unlock();
   }
-  ABSL_RAW_CHECK(x->value < 4, "should not be invoked a fourth time");
+  CHECK_LT(x->value, 4) << "should not be invoked a fourth time";
 
   // We arrange for the condition to return true on only the 2nd and 3rd calls.
   return x->value == 2 || x->value == 3;
@@ -961,7 +1144,7 @@ static void WaitForCond2(AcquireFromConditionStruct *x) {
   // wait for cond0 to become true
   x->mu0.LockWhen(absl::Condition(&ConditionWithAcquire, x));
   x->done = true;
-  x->mu0.Unlock();
+  x->mu0.unlock();
 }
 
 // Test for Condition whose function acquires other Mutexes
@@ -977,12 +1160,12 @@ TEST(Mutex, AcquireFromCondition) {
   // return false.
   absl::SleepFor(absl::Milliseconds(500));  // allow T time to hang
 
-  x.mu0.Lock();
+  x.mu0.lock();
   x.cv.WaitWithTimeout(&x.mu0, absl::Milliseconds(500));  // wake T
   // T will be woken because the Wait() will call ConditionWithAcquire()
   // for the second time, and it will return true.
 
-  x.mu0.Unlock();
+  x.mu0.unlock();
 
   // T will then acquire the lock and recheck its own condition.
   // It will find the condition true, as this is the third invocation,
@@ -998,12 +1181,9 @@ TEST(Mutex, AcquireFromCondition) {
   // is conceptually waiting both on the condition variable, and on mu2.
 
   x.mu0.LockWhen(absl::Condition(&x.done));
-  x.mu0.Unlock();
+  x.mu0.unlock();
 }
 
-// The deadlock detector is not part of non-prod builds, so do not test it.
-#if !defined(ABSL_INTERNAL_USE_NONPROD_MUTEX)
-
 TEST(Mutex, DeadlockDetector) {
   absl::SetMutexDeadlockDetectionMode(absl::OnDeadlockCycle::kAbort);
 
@@ -1013,20 +1193,20 @@ TEST(Mutex, DeadlockDetector) {
   absl::Mutex m3;
   absl::Mutex m4;
 
-  m1.Lock();  // m1 gets ID1
-  m2.Lock();  // m2 gets ID2
-  m3.Lock();  // m3 gets ID3
-  m3.Unlock();
-  m2.Unlock();
+  m1.lock();  // m1 gets ID1
+  m2.lock();  // m2 gets ID2
+  m3.lock();  // m3 gets ID3
+  m3.unlock();
+  m2.unlock();
   // m1 still held
   m1.ForgetDeadlockInfo();  // m1 loses ID
-  m2.Lock();                // m2 gets ID2
-  m3.Lock();                // m3 gets ID3
-  m4.Lock();                // m4 gets ID4
-  m3.Unlock();
-  m2.Unlock();
-  m4.Unlock();
-  m1.Unlock();
+  m2.lock();                // m2 gets ID2
+  m3.lock();                // m3 gets ID3
+  m4.lock();                // m4 gets ID4
+  m3.unlock();
+  m2.unlock();
+  m4.unlock();
+  m1.unlock();
 }
 
 // Bazel has a test "warning" file that programs can write to if the
@@ -1067,7 +1247,7 @@ class ScopedDisableBazelTestWarnings {
 const char ScopedDisableBazelTestWarnings::kVarName[] =
     "TEST_WARNINGS_OUTPUT_FILE";
 
-#ifdef THREAD_SANITIZER
+#ifdef ABSL_HAVE_THREAD_SANITIZER
 // This test intentionally creates deadlocks to test the deadlock detector.
 TEST(Mutex, DISABLED_DeadlockDetectorBazelWarning) {
 #else
@@ -1081,18 +1261,37 @@ TEST(Mutex, DeadlockDetectorBazelWarning) {
 
   absl::Mutex mu0;
   absl::Mutex mu1;
-  bool got_mu0 = mu0.TryLock();
-  mu1.Lock();  // acquire mu1 while holding mu0
+  bool got_mu0 = mu0.try_lock();
+  mu1.lock();  // acquire mu1 while holding mu0
   if (got_mu0) {
-    mu0.Unlock();
+    mu0.unlock();
   }
-  if (mu0.TryLock()) {  // try lock shouldn't cause deadlock detector to fire
-    mu0.Unlock();
+  if (mu0.try_lock()) {  // try lock shouldn't cause deadlock detector to fire
+    mu0.unlock();
   }
-  mu0.Lock();  // acquire mu0 while holding mu1; should get one deadlock
+  mu0.lock();  // acquire mu0 while holding mu1; should get one deadlock
                // report here
-  mu0.Unlock();
-  mu1.Unlock();
+  mu0.unlock();
+  mu1.unlock();
+
+  absl::SetMutexDeadlockDetectionMode(absl::OnDeadlockCycle::kAbort);
+}
+
+TEST(Mutex, DeadlockDetectorLongCycle) {
+  absl::SetMutexDeadlockDetectionMode(absl::OnDeadlockCycle::kReport);
+
+  // This test generates a warning if it passes, and crashes otherwise.
+  // Cause bazel to ignore the warning.
+  ScopedDisableBazelTestWarnings disable_bazel_test_warnings;
+
+  // Check that we survive a deadlock with a lock cycle.
+  std::vector<absl::Mutex> mutex(100);
+  for (size_t i = 0; i != mutex.size(); i++) {
+    mutex[i].lock();
+    mutex[(i + 1) % mutex.size()].lock();
+    mutex[i].unlock();
+    mutex[(i + 1) % mutex.size()].unlock();
+  }
 
   absl::SetMutexDeadlockDetectionMode(absl::OnDeadlockCycle::kAbort);
 }
@@ -1101,27 +1300,27 @@ TEST(Mutex, DeadlockDetectorBazelWarning) {
 // annotation-based static thread-safety analysis is not currently
 // predicate-aware and cannot tell if the two for-loops that acquire and
 // release the locks have the same predicates.
-TEST(Mutex, DeadlockDetectorStessTest) ABSL_NO_THREAD_SAFETY_ANALYSIS {
+TEST(Mutex, DeadlockDetectorStressTest) ABSL_NO_THREAD_SAFETY_ANALYSIS {
   // Stress test: Here we create a large number of locks and use all of them.
   // If a deadlock detector keeps a full graph of lock acquisition order,
   // it will likely be too slow for this test to pass.
   const int n_locks = 1 << 17;
-  auto array_of_locks = absl::make_unique<absl::Mutex[]>(n_locks);
+  auto array_of_locks = std::make_unique<absl::Mutex[]>(n_locks);
   for (int i = 0; i < n_locks; i++) {
     int end = std::min(n_locks, i + 5);
     // acquire and then release locks i, i+1, ..., i+4
     for (int j = i; j < end; j++) {
-      array_of_locks[j].Lock();
+      array_of_locks[j].lock();
     }
     for (int j = i; j < end; j++) {
-      array_of_locks[j].Unlock();
+      array_of_locks[j].unlock();
     }
   }
 }
 
-#ifdef THREAD_SANITIZER
+#ifdef ABSL_HAVE_THREAD_SANITIZER
 // TSAN reports errors when locked Mutexes are destroyed.
-TEST(Mutex, DISABLED_DeadlockIdBug) NO_THREAD_SAFETY_ANALYSIS {
+TEST(Mutex, DISABLED_DeadlockIdBug) ABSL_NO_THREAD_SAFETY_ANALYSIS {
 #else
 TEST(Mutex, DeadlockIdBug) ABSL_NO_THREAD_SAFETY_ANALYSIS {
 #endif
@@ -1135,11 +1334,11 @@ TEST(Mutex, DeadlockIdBug) ABSL_NO_THREAD_SAFETY_ANALYSIS {
   absl::Mutex b, c;
 
   // Hold mutex.
-  a->Lock();
+  a->lock();
 
   // Force deadlock id assignment by acquiring another lock.
-  b.Lock();
-  b.Unlock();
+  b.lock();
+  b.unlock();
 
   // Delete the mutex. The Mutex destructor tries to remove held locks,
   // but the attempt isn't foolproof.  It can fail if:
@@ -1154,10 +1353,9 @@ TEST(Mutex, DeadlockIdBug) ABSL_NO_THREAD_SAFETY_ANALYSIS {
   // We should end up getting assigned the same deadlock id that was
   // freed up when "a" was deleted, which will cause a spurious deadlock
   // report if the held lock entry for "a" was not invalidated.
-  c.Lock();
-  c.Unlock();
+  c.lock();
+  c.unlock();
 }
-#endif  // !defined(ABSL_INTERNAL_USE_NONPROD_MUTEX)
 
 // --------------------------------------------------------
 // Test for timeouts/deadlines on condition waits that are specified using
@@ -1174,7 +1372,7 @@ static absl::Duration TimeoutTestAllowedSchedulingDelay() {
 
 // Returns true if `actual_delay` is close enough to `expected_delay` to pass
 // the timeouts/deadlines test.  Otherwise, logs warnings and returns false.
-ABSL_MUST_USE_RESULT
+[[nodiscard]]
 static bool DelayIsWithinBounds(absl::Duration expected_delay,
                                 absl::Duration actual_delay) {
   bool pass = true;
@@ -1183,11 +1381,9 @@ static bool DelayIsWithinBounds(absl::Duration expected_delay,
   // different clock than absl::Now(), but these cases should be handled by the
   // the retry mechanism in each TimeoutTest.
   if (actual_delay < expected_delay) {
-    ABSL_RAW_LOG(WARNING,
-                 "Actual delay %s was too short, expected %s (difference %s)",
-                 absl::FormatDuration(actual_delay).c_str(),
-                 absl::FormatDuration(expected_delay).c_str(),
-                 absl::FormatDuration(actual_delay - expected_delay).c_str());
+    LOG(WARNING) << "Actual delay " << actual_delay
+                 << " was too short, expected " << expected_delay
+                 << " (difference " << actual_delay - expected_delay << ")";
     pass = false;
   }
   // If the expected delay is <= zero then allow a small error tolerance, since
@@ -1198,11 +1394,9 @@ static bool DelayIsWithinBounds(absl::Duration expected_delay,
                                  ? absl::Milliseconds(10)
                                  : TimeoutTestAllowedSchedulingDelay();
   if (actual_delay > expected_delay + tolerance) {
-    ABSL_RAW_LOG(WARNING,
-                 "Actual delay %s was too long, expected %s (difference %s)",
-                 absl::FormatDuration(actual_delay).c_str(),
-                 absl::FormatDuration(expected_delay).c_str(),
-                 absl::FormatDuration(actual_delay - expected_delay).c_str());
+    LOG(WARNING) << "Actual delay " << actual_delay
+                 << " was too long, expected " << expected_delay
+                 << " (difference " << actual_delay - expected_delay << ")";
     pass = false;
   }
   return pass;
@@ -1252,12 +1446,6 @@ std::ostream &operator<<(std::ostream &os, const TimeoutTestParam &param) {
             << " expected_delay: " << param.expected_delay;
 }
 
-std::string FormatString(const TimeoutTestParam &param) {
-  std::ostringstream os;
-  os << param;
-  return os.str();
-}
-
 // Like `thread::Executor::ScheduleAt` except:
 // a) Delays zero or negative are executed immediately in the current thread.
 // b) Infinite delays are never scheduled.
@@ -1387,13 +1575,13 @@ INSTANTIATE_TEST_SUITE_P(All, TimeoutTest,
 
 TEST_P(TimeoutTest, Await) {
   const TimeoutTestParam params = GetParam();
-  ABSL_RAW_LOG(INFO, "Params: %s", FormatString(params).c_str());
+  LOG(INFO) << "Params: " << params;
 
   // Because this test asserts bounds on scheduling delays it is flaky.  To
   // compensate it loops forever until it passes.  Failures express as test
   // timeouts, in which case the test log can be used to diagnose the issue.
   for (int attempt = 1;; ++attempt) {
-    ABSL_RAW_LOG(INFO, "Attempt %d", attempt);
+    LOG(INFO) << "Attempt " << attempt;
 
     absl::Mutex mu;
     bool value = false;  // condition value (under mu)
@@ -1401,11 +1589,11 @@ TEST_P(TimeoutTest, Await) {
     std::unique_ptr<absl::synchronization_internal::ThreadPool> pool =
         CreateDefaultPool();
     RunAfterDelay(params.satisfy_condition_delay, pool.get(), [&] {
-      absl::MutexLock l(&mu);
+      absl::MutexLock l(mu);
       value = true;
     });
 
-    absl::MutexLock lock(&mu);
+    absl::MutexLock lock(mu);
     absl::Time start_time = absl::Now();
     absl::Condition cond(&value);
     bool result =
@@ -1421,13 +1609,13 @@ TEST_P(TimeoutTest, Await) {
 
 TEST_P(TimeoutTest, LockWhen) {
   const TimeoutTestParam params = GetParam();
-  ABSL_RAW_LOG(INFO, "Params: %s", FormatString(params).c_str());
+  LOG(INFO) << "Params: " << params;
 
   // Because this test asserts bounds on scheduling delays it is flaky.  To
   // compensate it loops forever until it passes.  Failures express as test
   // timeouts, in which case the test log can be used to diagnose the issue.
   for (int attempt = 1;; ++attempt) {
-    ABSL_RAW_LOG(INFO, "Attempt %d", attempt);
+    LOG(INFO) << "Attempt " << attempt;
 
     absl::Mutex mu;
     bool value = false;  // condition value (under mu)
@@ -1435,7 +1623,7 @@ TEST_P(TimeoutTest, LockWhen) {
     std::unique_ptr<absl::synchronization_internal::ThreadPool> pool =
         CreateDefaultPool();
     RunAfterDelay(params.satisfy_condition_delay, pool.get(), [&] {
-      absl::MutexLock l(&mu);
+      absl::MutexLock l(mu);
       value = true;
     });
 
@@ -1445,7 +1633,7 @@ TEST_P(TimeoutTest, LockWhen) {
         params.use_absolute_deadline
             ? mu.LockWhenWithDeadline(cond, start_time + params.wait_timeout)
             : mu.LockWhenWithTimeout(cond, params.wait_timeout);
-    mu.Unlock();
+    mu.unlock();
 
     if (DelayIsWithinBounds(params.expected_delay, absl::Now() - start_time)) {
       EXPECT_EQ(params.expected_result, result);
@@ -1456,13 +1644,13 @@ TEST_P(TimeoutTest, LockWhen) {
 
 TEST_P(TimeoutTest, ReaderLockWhen) {
   const TimeoutTestParam params = GetParam();
-  ABSL_RAW_LOG(INFO, "Params: %s", FormatString(params).c_str());
+  LOG(INFO) << "Params: " << params;
 
   // Because this test asserts bounds on scheduling delays it is flaky.  To
   // compensate it loops forever until it passes.  Failures express as test
   // timeouts, in which case the test log can be used to diagnose the issue.
   for (int attempt = 0;; ++attempt) {
-    ABSL_RAW_LOG(INFO, "Attempt %d", attempt);
+    LOG(INFO) << "Attempt " << attempt;
 
     absl::Mutex mu;
     bool value = false;  // condition value (under mu)
@@ -1470,7 +1658,7 @@ TEST_P(TimeoutTest, ReaderLockWhen) {
     std::unique_ptr<absl::synchronization_internal::ThreadPool> pool =
         CreateDefaultPool();
     RunAfterDelay(params.satisfy_condition_delay, pool.get(), [&] {
-      absl::MutexLock l(&mu);
+      absl::MutexLock l(mu);
       value = true;
     });
 
@@ -1481,7 +1669,7 @@ TEST_P(TimeoutTest, ReaderLockWhen) {
                                             start_time + params.wait_timeout)
             : mu.ReaderLockWhenWithTimeout(absl::Condition(&value),
                                            params.wait_timeout);
-    mu.ReaderUnlock();
+    mu.unlock_shared();
 
     if (DelayIsWithinBounds(params.expected_delay, absl::Now() - start_time)) {
       EXPECT_EQ(params.expected_result, result);
@@ -1492,13 +1680,13 @@ TEST_P(TimeoutTest, ReaderLockWhen) {
 
 TEST_P(TimeoutTest, Wait) {
   const TimeoutTestParam params = GetParam();
-  ABSL_RAW_LOG(INFO, "Params: %s", FormatString(params).c_str());
+  LOG(INFO) << "Params: " << params;
 
   // Because this test asserts bounds on scheduling delays it is flaky.  To
   // compensate it loops forever until it passes.  Failures express as test
   // timeouts, in which case the test log can be used to diagnose the issue.
   for (int attempt = 0;; ++attempt) {
-    ABSL_RAW_LOG(INFO, "Attempt %d", attempt);
+    LOG(INFO) << "Attempt " << attempt;
 
     absl::Mutex mu;
     bool value = false;  // condition value (under mu)
@@ -1507,12 +1695,12 @@ TEST_P(TimeoutTest, Wait) {
     std::unique_ptr<absl::synchronization_internal::ThreadPool> pool =
         CreateDefaultPool();
     RunAfterDelay(params.satisfy_condition_delay, pool.get(), [&] {
-      absl::MutexLock l(&mu);
+      absl::MutexLock l(mu);
       value = true;
       cv.Signal();
     });
 
-    absl::MutexLock lock(&mu);
+    absl::MutexLock lock(mu);
     absl::Time start_time = absl::Now();
     absl::Duration timeout = params.wait_timeout;
     absl::Time deadline = start_time + timeout;
@@ -1538,17 +1726,72 @@ TEST(Mutex, Logging) {
   logged_mutex.EnableDebugLog("fido_mutex");
   absl::CondVar logged_cv;
   logged_cv.EnableDebugLog("rover_cv");
-  logged_mutex.Lock();
+  logged_mutex.lock();
   logged_cv.WaitWithTimeout(&logged_mutex, absl::Milliseconds(20));
-  logged_mutex.Unlock();
-  logged_mutex.ReaderLock();
-  logged_mutex.ReaderUnlock();
-  logged_mutex.Lock();
-  logged_mutex.Unlock();
+  logged_mutex.unlock();
+  logged_mutex.lock_shared();
+  logged_mutex.unlock_shared();
+  logged_mutex.lock();
+  logged_mutex.unlock();
   logged_cv.Signal();
   logged_cv.SignalAll();
 }
 
+TEST(Mutex, LoggingAddressReuse) {
+  // Repeatedly re-create a Mutex with debug logging at the same address.
+  ScopedInvariantDebugging scoped_debugging;
+  alignas(absl::Mutex) unsigned char storage[sizeof(absl::Mutex)];
+  auto invariant =
+      +[](void *alive) { EXPECT_TRUE(*static_cast<bool *>(alive)); };
+  constexpr size_t kIters = 10;
+  bool alive[kIters] = {};
+  for (size_t i = 0; i < kIters; ++i) {
+    absl::Mutex *mu = new (storage) absl::Mutex;
+    alive[i] = true;
+    mu->EnableDebugLog("Mutex");
+    mu->EnableInvariantDebugging(invariant, &alive[i]);
+    mu->lock();
+    mu->unlock();
+    mu->~Mutex();
+    alive[i] = false;
+  }
+}
+
+TEST(Mutex, LoggingBankrupcy) {
+  // Test the case with too many live Mutexes with debug logging.
+  ScopedInvariantDebugging scoped_debugging;
+  std::vector<absl::Mutex> mus(1 << 20);
+  for (auto &mu : mus) {
+    mu.EnableDebugLog("Mutex");
+  }
+}
+
+TEST(Mutex, SynchEventRace) {
+  // Regression test for a false TSan race report in
+  // EnableInvariantDebugging/EnableDebugLog related to SynchEvent reuse.
+  ScopedInvariantDebugging scoped_debugging;
+  std::vector<std::thread> threads;
+  for (size_t i = 0; i < 5; i++) {
+    threads.emplace_back([&] {
+      for (size_t j = 0; j < (1 << 17); j++) {
+        {
+          absl::Mutex mu;
+          mu.EnableInvariantDebugging([](void *) {}, nullptr);
+          mu.lock();
+          mu.unlock();
+        }
+        {
+          absl::Mutex mu;
+          mu.EnableDebugLog("Mutex");
+        }
+      }
+    });
+  }
+  for (auto &thread : threads) {
+    thread.join();
+  }
+}
+
 // --------------------------------------------------------
 
 // Generate the vector of thread counts for tests parameterized on thread count.
@@ -1662,8 +1905,7 @@ TEST(Mutex, Timed) {
 TEST(Mutex, CVTime) {
   int threads = 10;  // Use a fixed thread count of 10
   int iterations = 1;
-  EXPECT_EQ(RunTest(&TestCVTime, threads, iterations, 1),
-            threads * iterations);
+  EXPECT_EQ(RunTest(&TestCVTime, threads, iterations, 1), threads * iterations);
 }
 
 TEST(Mutex, MuTime) {
@@ -1672,4 +1914,151 @@ TEST(Mutex, MuTime) {
   EXPECT_EQ(RunTest(&TestMuTime, threads, iterations, 1), threads * iterations);
 }
 
+TEST(Mutex, SignalExitedThread) {
+  // The test may expose a race when Mutex::unlock signals a thread
+  // that has already exited.
+#if defined(__wasm__) || defined(__asmjs__)
+  constexpr int kThreads = 1;  // OOMs under WASM
+#else
+  constexpr int kThreads = 100;
+#endif
+  std::vector<std::thread> top;
+  for (unsigned i = 0; i < 2 * std::thread::hardware_concurrency(); i++) {
+    top.emplace_back([&]() {
+      for (int i = 0; i < kThreads; i++) {
+        absl::Mutex mu;
+        std::thread t([&]() {
+          mu.lock();
+          mu.unlock();
+        });
+        mu.lock();
+        mu.unlock();
+        t.join();
+      }
+    });
+  }
+  for (auto &th : top) th.join();
+}
+
+TEST(Mutex, WriterPriority) {
+  absl::Mutex mu;
+  bool wrote = false;
+  std::atomic<bool> saw_wrote{false};
+  auto readfunc = [&]() {
+    for (size_t i = 0; i < 10; ++i) {
+      absl::ReaderMutexLock lock(mu);
+      if (wrote) {
+        saw_wrote = true;
+        break;
+      }
+      absl::SleepFor(absl::Seconds(1));
+    }
+  };
+  std::thread t1(readfunc);
+  absl::SleepFor(absl::Milliseconds(500));
+  std::thread t2(readfunc);
+  // Note: this test guards against a bug that was related to an uninit
+  // PerThreadSynch::priority, so the writer intentionally runs on a new thread.
+  std::thread t3([&]() {
+    // The writer should be able squeeze between the two alternating readers.
+    absl::MutexLock lock(mu);
+    wrote = true;
+  });
+  t1.join();
+  t2.join();
+  t3.join();
+  EXPECT_TRUE(saw_wrote.load());
+}
+
+#ifdef ABSL_HAVE_PTHREAD_GETSCHEDPARAM
+TEST(Mutex, CondVarPriority) {
+  // A regression test for a bug in condition variable wait morphing,
+  // which resulted in the waiting thread getting priority of the waking thread.
+  int err = 0;
+  sched_param param;
+  param.sched_priority = 7;
+  std::thread test([&]() {
+    err = pthread_setschedparam(pthread_self(), SCHED_FIFO, &param);
+  });
+  test.join();
+  if (err) {
+    // Setting priority usually requires special privileges.
+    GTEST_SKIP() << "failed to set priority: " << strerror(err);
+  }
+  absl::Mutex mu;
+  absl::CondVar cv;
+  bool locked = false;
+  bool notified = false;
+  bool waiting = false;
+  bool morph = false;
+  std::thread th([&]() {
+    EXPECT_EQ(0, pthread_setschedparam(pthread_self(), SCHED_FIFO, &param));
+    mu.lock();
+    locked = true;
+    mu.Await(absl::Condition(&notified));
+    mu.unlock();
+    EXPECT_EQ(absl::synchronization_internal::GetOrCreateCurrentThreadIdentity()
+                  ->per_thread_synch.priority,
+              param.sched_priority);
+    mu.lock();
+    mu.Await(absl::Condition(&waiting));
+    morph = true;
+    absl::SleepFor(absl::Seconds(1));
+    cv.Signal();
+    mu.unlock();
+  });
+  mu.lock();
+  mu.Await(absl::Condition(&locked));
+  notified = true;
+  mu.unlock();
+  mu.lock();
+  waiting = true;
+  while (!morph) {
+    cv.Wait(&mu);
+  }
+  mu.unlock();
+  th.join();
+  EXPECT_NE(absl::synchronization_internal::GetOrCreateCurrentThreadIdentity()
+                ->per_thread_synch.priority,
+            param.sched_priority);
+}
+#endif
+
+TEST(Mutex, LockWhenWithTimeoutResult) {
+  // Check various corner cases for Await/LockWhen return value
+  // with always true/always false conditions.
+  absl::Mutex mu;
+  const bool kAlwaysTrue = true, kAlwaysFalse = false;
+  const absl::Condition kTrueCond(&kAlwaysTrue), kFalseCond(&kAlwaysFalse);
+  EXPECT_TRUE(mu.LockWhenWithTimeout(kTrueCond, absl::Milliseconds(1)));
+  mu.unlock();
+  EXPECT_FALSE(mu.LockWhenWithTimeout(kFalseCond, absl::Milliseconds(1)));
+  EXPECT_TRUE(mu.AwaitWithTimeout(kTrueCond, absl::Milliseconds(1)));
+  EXPECT_FALSE(mu.AwaitWithTimeout(kFalseCond, absl::Milliseconds(1)));
+  std::thread th1([&]() {
+    EXPECT_TRUE(mu.LockWhenWithTimeout(kTrueCond, absl::Milliseconds(1)));
+    mu.unlock();
+  });
+  std::thread th2([&]() {
+    EXPECT_FALSE(mu.LockWhenWithTimeout(kFalseCond, absl::Milliseconds(1)));
+    mu.unlock();
+  });
+  absl::SleepFor(absl::Milliseconds(100));
+  mu.unlock();
+  th1.join();
+  th2.join();
+}
+
+TEST(Mutex, ScopedLock) {
+  absl::Mutex mu;
+  {
+    std::scoped_lock l(mu);
+  }
+
+  {
+    std::shared_lock l(mu);
+    EXPECT_TRUE(l.owns_lock());
+  }
+}
+
 }  // namespace
diff --git a/absl/synchronization/notification.cc b/absl/synchronization/notification.cc
index 53ace00870c..63da068cbbd 100644
--- a/absl/synchronization/notification.cc
+++ b/absl/synchronization/notification.cc
@@ -16,15 +16,19 @@
 
 #include <atomic>
 
-#include "absl/base/attributes.h"
+#include "absl/base/config.h"
 #include "absl/base/internal/raw_logging.h"
+#include "absl/base/internal/tracing.h"
+#include "absl/base/optimization.h"
 #include "absl/synchronization/mutex.h"
 #include "absl/time/time.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 void Notification::Notify() {
-  MutexLock l(&this->mutex_);
+  base_internal::TraceSignal(this, TraceObjectKind());
+  MutexLock l(this->mutex_);
 
 #ifndef NDEBUG
   if (ABSL_PREDICT_FALSE(notified_yet_.load(std::memory_order_relaxed))) {
@@ -41,45 +45,43 @@ void Notification::Notify() {
 Notification::~Notification() {
   // Make sure that the thread running Notify() exits before the object is
   // destructed.
-  MutexLock l(&this->mutex_);
-}
-
-static inline bool HasBeenNotifiedInternal(
-    const std::atomic<bool> *notified_yet) {
-  return notified_yet->load(std::memory_order_acquire);
-}
-
-bool Notification::HasBeenNotified() const {
-  return HasBeenNotifiedInternal(&this->notified_yet_);
+  MutexLock l(this->mutex_);
 }
 
 void Notification::WaitForNotification() const {
+  base_internal::TraceWait(this, TraceObjectKind());
   if (!HasBeenNotifiedInternal(&this->notified_yet_)) {
-    this->mutex_.LockWhen(Condition(&HasBeenNotifiedInternal,
-                                    &this->notified_yet_));
-    this->mutex_.Unlock();
+    this->mutex_.LockWhen(
+        Condition(&HasBeenNotifiedInternal, &this->notified_yet_));
+    this->mutex_.unlock();
   }
+  base_internal::TraceContinue(this, TraceObjectKind());
 }
 
 bool Notification::WaitForNotificationWithTimeout(
     absl::Duration timeout) const {
+  base_internal::TraceWait(this, TraceObjectKind());
   bool notified = HasBeenNotifiedInternal(&this->notified_yet_);
   if (!notified) {
     notified = this->mutex_.LockWhenWithTimeout(
         Condition(&HasBeenNotifiedInternal, &this->notified_yet_), timeout);
-    this->mutex_.Unlock();
+    this->mutex_.unlock();
   }
+  base_internal::TraceContinue(notified ? this : nullptr, TraceObjectKind());
   return notified;
 }
 
 bool Notification::WaitForNotificationWithDeadline(absl::Time deadline) const {
+  base_internal::TraceWait(this, TraceObjectKind());
   bool notified = HasBeenNotifiedInternal(&this->notified_yet_);
   if (!notified) {
     notified = this->mutex_.LockWhenWithDeadline(
         Condition(&HasBeenNotifiedInternal, &this->notified_yet_), deadline);
-    this->mutex_.Unlock();
+    this->mutex_.unlock();
   }
+  base_internal::TraceContinue(notified ? this : nullptr, TraceObjectKind());
   return notified;
 }
 
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/synchronization/notification.h b/absl/synchronization/notification.h
index 82d111a5492..420b82003fd 100644
--- a/absl/synchronization/notification.h
+++ b/absl/synchronization/notification.h
@@ -22,7 +22,7 @@
 // The `Notification` object maintains a private boolean "notified" state that
 // transitions to `true` at most once. The `Notification` class provides the
 // following primary member functions:
-//   * `HasBeenNotified() `to query its state
+//   * `HasBeenNotified()` to query its state
 //   * `WaitForNotification*()` to have threads wait until the "notified" state
 //      is `true`.
 //   * `Notify()` to set the notification's "notified" state to `true` and
@@ -52,11 +52,13 @@
 
 #include <atomic>
 
-#include "absl/base/macros.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/tracing.h"
 #include "absl/synchronization/mutex.h"
 #include "absl/time/time.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 // -----------------------------------------------------------------------------
 // Notification
@@ -73,7 +75,13 @@ class Notification {
   // Notification::HasBeenNotified()
   //
   // Returns the value of the notification's internal "notified" state.
-  bool HasBeenNotified() const;
+  [[nodiscard]] bool HasBeenNotified() const {
+    if (HasBeenNotifiedInternal(&this->notified_yet_)) {
+      base_internal::TraceObserved(this, TraceObjectKind());
+      return true;
+    }
+    return false;
+  }
 
   // Notification::WaitForNotification()
   //
@@ -105,10 +113,20 @@ class Notification {
   void Notify();
 
  private:
+  // Convenience helper to reduce verbosity at call sites.
+  static constexpr base_internal::ObjectKind TraceObjectKind() {
+    return base_internal::ObjectKind::kNotification;
+  }
+
+  static bool HasBeenNotifiedInternal(const std::atomic<bool>* notified_yet) {
+    return notified_yet->load(std::memory_order_acquire);
+  }
+
   mutable Mutex mutex_;
   std::atomic<bool> notified_yet_;  // written under mutex_
 };
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_SYNCHRONIZATION_NOTIFICATION_H_
diff --git a/absl/synchronization/notification_test.cc b/absl/synchronization/notification_test.cc
index 059d4cd2521..ded80a5f667 100644
--- a/absl/synchronization/notification_test.cc
+++ b/absl/synchronization/notification_test.cc
@@ -15,12 +15,19 @@
 #include "absl/synchronization/notification.h"
 
 #include <thread>  // NOLINT(build/c++11)
+#include <tuple>
 #include <vector>
 
 #include "gtest/gtest.h"
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/tracing.h"
 #include "absl/synchronization/mutex.h"
+#include "absl/time/clock.h"
+#include "absl/time/time.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 // A thread-safe class that holds a counter.
 class ThreadSafeCounter {
@@ -28,17 +35,17 @@ class ThreadSafeCounter {
   ThreadSafeCounter() : count_(0) {}
 
   void Increment() {
-    MutexLock lock(&mutex_);
+    MutexLock lock(mutex_);
     ++count_;
   }
 
   int Get() const {
-    MutexLock lock(&mutex_);
+    MutexLock lock(mutex_);
     return count_;
   }
 
   void WaitUntilGreaterOrEqual(int n) {
-    MutexLock lock(&mutex_);
+    MutexLock lock(mutex_);
     auto cond = [this, n]() { return count_ >= n; };
     mutex_.Await(Condition(&cond));
   }
@@ -78,7 +85,11 @@ static void BasicTests(bool notify_before_waiting, Notification* notification) {
 
   // Allow for a slight early return, to account for quality of implementation
   // issues on various platforms.
-  const absl::Duration slop = absl::Microseconds(200);
+  absl::Duration slop = absl::Milliseconds(5);
+#ifdef _MSC_VER
+  // Avoid flakiness on MSVC.
+  slop = absl::Milliseconds(15);
+#endif
   EXPECT_LE(delay - slop, elapsed)
       << "WaitForNotificationWithTimeout returned " << delay - elapsed
       << " early (with " << slop << " slop), start time was " << start;
@@ -128,4 +139,93 @@ TEST(NotificationTest, SanityTest) {
   BasicTests(true, &local_notification2);
 }
 
+#if ABSL_HAVE_ATTRIBUTE_WEAK
+
+namespace base_internal {
+
+namespace {
+
+using TraceRecord = std::tuple<const void*, ObjectKind>;
+
+thread_local TraceRecord tls_signal;
+thread_local TraceRecord tls_wait;
+thread_local TraceRecord tls_continue;
+thread_local TraceRecord tls_observed;
+
+}  // namespace
+
+// Strong extern "C" implementation.
+extern "C" {
+
+void ABSL_INTERNAL_C_SYMBOL(AbslInternalTraceWait)(const void* object,
+                                                   ObjectKind kind) {
+  tls_wait = {object, kind};
+}
+
+void ABSL_INTERNAL_C_SYMBOL(AbslInternalTraceContinue)(const void* object,
+                                                       ObjectKind kind) {
+  tls_continue = {object, kind};
+}
+
+void ABSL_INTERNAL_C_SYMBOL(AbslInternalTraceSignal)(const void* object,
+                                                     ObjectKind kind) {
+  tls_signal = {object, kind};
+}
+
+void ABSL_INTERNAL_C_SYMBOL(AbslInternalTraceObserved)(const void* object,
+                                                       ObjectKind kind) {
+  tls_observed = {object, kind};
+}
+
+}  // extern "C"
+
+TEST(NotificationTest, TracesNotify) {
+  Notification n;
+  tls_signal = {};
+  n.Notify();
+  EXPECT_EQ(tls_signal, TraceRecord(&n, ObjectKind::kNotification));
+}
+
+TEST(NotificationTest, TracesWaitForNotification) {
+  Notification n;
+  n.Notify();
+  tls_wait = tls_continue = {};
+  n.WaitForNotification();
+  EXPECT_EQ(tls_wait, TraceRecord(&n, ObjectKind::kNotification));
+  EXPECT_EQ(tls_continue, TraceRecord(&n, ObjectKind::kNotification));
+}
+
+TEST(NotificationTest, TracesWaitForNotificationWithTimeout) {
+  Notification n;
+
+  tls_wait = tls_continue = {};
+  n.WaitForNotificationWithTimeout(absl::Milliseconds(1));
+  EXPECT_EQ(tls_wait, TraceRecord(&n, ObjectKind::kNotification));
+  EXPECT_EQ(tls_continue, TraceRecord(nullptr, ObjectKind::kNotification));
+
+  n.Notify();
+  tls_wait = tls_continue = {};
+  n.WaitForNotificationWithTimeout(absl::Milliseconds(1));
+  EXPECT_EQ(tls_wait, TraceRecord(&n, ObjectKind::kNotification));
+  EXPECT_EQ(tls_continue, TraceRecord(&n, ObjectKind::kNotification));
+}
+
+TEST(NotificationTest, TracesHasBeenNotified) {
+  Notification n;
+
+  tls_observed = {};
+  ASSERT_FALSE(n.HasBeenNotified());
+  EXPECT_EQ(tls_observed, TraceRecord(nullptr, ObjectKind::kUnknown));
+
+  n.Notify();
+  tls_observed = {};
+  ASSERT_TRUE(n.HasBeenNotified());
+  EXPECT_EQ(tls_observed, TraceRecord(&n, ObjectKind::kNotification));
+}
+
+}  // namespace base_internal
+
+#endif  // ABSL_HAVE_ATTRIBUTE_WEAK
+
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/time/BUILD.bazel b/absl/time/BUILD.bazel
index a615152f01b..073c229d480 100644
--- a/absl/time/BUILD.bazel
+++ b/absl/time/BUILD.bazel
@@ -14,7 +14,9 @@
 # limitations under the License.
 #
 
-load("@rules_cc//cc:defs.bzl", "cc_library", "cc_test")
+load("@rules_cc//cc:cc_binary.bzl", "cc_binary")
+load("@rules_cc//cc:cc_library.bzl", "cc_library")
+load("@rules_cc//cc:cc_test.bzl", "cc_test")
 load(
     "//absl:copts/configure_copts.bzl",
     "ABSL_DEFAULT_COPTS",
@@ -22,9 +24,16 @@ load(
     "ABSL_TEST_COPTS",
 )
 
-package(default_visibility = ["//visibility:public"])
+package(
+    default_visibility = ["//visibility:private"],
+    features = [
+        "header_modules",
+        "layering_check",
+        "parse_headers",
+    ],
+)
 
-licenses(["notice"])  # Apache 2.0
+licenses(["notice"])
 
 cc_library(
     name = "time",
@@ -44,35 +53,77 @@ cc_library(
     ],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
     deps = [
         "//absl/base",
+        "//absl/base:config",
         "//absl/base:core_headers",
         "//absl/base:raw_logging_internal",
         "//absl/numeric:int128",
         "//absl/strings",
+        "//absl/strings:string_view",
         "//absl/time/internal/cctz:civil_time",
         "//absl/time/internal/cctz:time_zone",
+        "//absl/types:optional",
     ],
 )
 
 cc_library(
-    name = "test_util",
-    testonly = 1,
+    name = "clock_interface",
     srcs = [
-        "internal/test_util.cc",
-        "internal/zoneinfo.inc",
+        "clock_interface.cc",
+    ],
+    hdrs = [
+        "clock_interface.h",
     ],
-    hdrs = ["internal/test_util.h"],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
-    visibility = [
-        "//absl/time:__pkg__",
+    visibility = ["//visibility:public"],
+    deps = [
+        ":time",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:no_destructor",
+        "//absl/base:nullability",
+        "//absl/base:raw_logging_internal",
+        "//absl/synchronization",
     ],
+)
+
+cc_library(
+    name = "simulated_clock",
+    testonly = True,
+    srcs = [
+        "simulated_clock.cc",
+    ],
+    hdrs = [
+        "simulated_clock.h",
+    ],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
     deps = [
+        ":clock_interface",
         ":time",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:nullability",
+        "//absl/synchronization",
+    ],
+)
+
+cc_library(
+    name = "test_util",
+    testonly = True,
+    srcs = ["internal/test_util.cc"],
+    hdrs = ["internal/test_util.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:private"],
+    deps = [
+        ":time",
+        "//absl/base:config",
         "//absl/base:raw_logging_internal",
-        "//absl/time/internal/cctz:time_zone",
-        "@com_google_googletest//:gtest",
     ],
 )
 
@@ -87,19 +138,91 @@ cc_test(
         "time_zone_test.cc",
     ],
     copts = ABSL_TEST_COPTS,
+    data = ["//absl/time/internal/cctz:zoneinfo"],
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":test_util",
         ":time",
         "//absl/base:config",
         "//absl/base:core_headers",
+        "//absl/hash:hash_testing",
+        "//absl/numeric:int128",
+        "//absl/random",
+        "//absl/strings",
+        "//absl/strings:str_format",
         "//absl/time/internal/cctz:time_zone",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "clock_interface_test",
+    srcs = [
+        "clock_interface_test.cc",
+    ],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":clock_interface",
+        ":time",
+        "//absl/functional:bind_front",
+        "//absl/synchronization",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
 cc_test(
+    name = "simulated_clock_test",
+    srcs = [
+        "simulated_clock_test.cc",
+    ],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":clock_interface",
+        ":simulated_clock",
+        ":time",
+        "//absl/base:core_headers",
+        "//absl/base:raw_logging_internal",
+        "//absl/functional:bind_front",
+        "//absl/random",
+        "//absl/synchronization",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "flag_test",
+    srcs = [
+        "flag_test.cc",
+    ],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    tags = [
+        "no_test_android_arm",
+        "no_test_android_arm64",
+        "no_test_android_x86",
+        "no_test_ios_sim_arm64",
+        "no_test_ios_x86_64",
+        "no_test_lexan",
+        "no_test_loonix",
+        "no_test_wasm",
+    ],
+    deps = [
+        ":time",
+        "//absl/flags:flag",
+        "//absl/flags:reflection",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_binary(
     name = "time_benchmark",
+    testonly = True,
     srcs = [
         "civil_time_benchmark.cc",
         "clock_benchmark.cc",
@@ -108,6 +231,7 @@ cc_test(
         "time_benchmark.cc",
     ],
     copts = ABSL_TEST_COPTS,
+    data = ["//absl/time/internal/cctz:zoneinfo"],
     linkopts = ABSL_DEFAULT_LINKOPTS,
     tags = [
         "benchmark",
@@ -117,7 +241,8 @@ cc_test(
         ":time",
         "//absl/base",
         "//absl/base:core_headers",
+        "//absl/flags:flag",
         "//absl/hash",
-        "@com_github_google_benchmark//:benchmark_main",
+        "@google_benchmark//:benchmark_main",
     ],
 )
diff --git a/absl/time/CMakeLists.txt b/absl/time/CMakeLists.txt
index 853563e875c..c3ae98c13c9 100644
--- a/absl/time/CMakeLists.txt
+++ b/absl/time/CMakeLists.txt
@@ -49,15 +49,11 @@ absl_cc_library(
     "internal/cctz/include/cctz/civil_time.h"
     "internal/cctz/include/cctz/civil_time_detail.h"
   SRCS
-  "internal/cctz/src/civil_time_detail.cc"
+    "internal/cctz/src/civil_time_detail.cc"
   COPTS
     ${ABSL_DEFAULT_COPTS}
 )
 
-if(APPLE)
-  find_library(CoreFoundation CoreFoundation)
-endif()
-
 absl_cc_library(
   NAME
     time_zone
@@ -81,12 +77,18 @@ absl_cc_library(
     "internal/cctz/src/time_zone_posix.h"
     "internal/cctz/src/tzfile.h"
     "internal/cctz/src/zone_info_source.cc"
+    $<$<PLATFORM_ID:Windows>:internal/cctz/src/time_zone_name_win.cc>
+    $<$<PLATFORM_ID:Windows>:internal/cctz/src/time_zone_name_win.h>
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
-    $<$<PLATFORM_ID:Darwin>:${CoreFoundation}>
+    Threads::Threads
+    # TODO(#1495): Use $<LINK_LIBRARY:FRAMEWORK,CoreFoundation> once our
+    # minimum CMake version >= 3.24
+    $<$<PLATFORM_ID:Darwin,iOS,tvOS,visionOS,watchOS>:-Wl,-framework,CoreFoundation>
 )
 
+# Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
     time_internal_test_util
@@ -94,14 +96,13 @@ absl_cc_library(
     "internal/test_util.h"
   SRCS
     "internal/test_util.cc"
-    "internal/zoneinfo.inc"
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
     absl::time
+    absl::config
     absl::raw_logging_internal
     absl::time_zone
-    gmock
   TESTONLY
 )
 
@@ -122,6 +123,96 @@ absl_cc_test(
     absl::time
     absl::config
     absl::core_headers
+    absl::hash_testing
+    absl::random_random
+    absl::strings
+    absl::str_format
     absl::time_zone
-    gmock_main
+    GTest::gmock_main
+)
+
+absl_cc_library(
+  NAME
+    clock_interface
+  HDRS
+    "clock_interface.h"
+  SRCS
+    "clock_interface.cc"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::time
+    absl::config
+    absl::core_headers
+    absl::no_destructor
+    absl::nullability
+    absl::raw_logging_internal
+    absl::synchronization
+)
+
+absl_cc_test(
+  NAME
+    clock_interface_test
+  SRCS
+    "clock_interface_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::clock_interface
+    absl::time
+    absl::config
+    absl::bind_front
+    absl::synchronization
+    GTest::gmock_main
+)
+
+absl_cc_library(
+  NAME
+    simulated_clock
+  HDRS
+    "simulated_clock.h"
+  SRCS
+    "simulated_clock.cc"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::clock_interface
+    absl::time
+    absl::config
+    absl::core_headers
+    absl::nullability
+    absl::synchronization
+  TESTONLY
+)
+
+absl_cc_test(
+  NAME
+    simulated_clock_test
+  SRCS
+    "simulated_clock_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::clock_interface
+    absl::simulated_clock
+    absl::time
+    absl::config
+    absl::bind_front
+    absl::random_random
+    absl::raw_logging_internal
+    absl::synchronization
+    GTest::gmock_main
+)
+
+absl_cc_test(
+  NAME
+    flag_test
+  SRCS
+    "flag_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::flags
+    absl::flags_reflection
+    GTest::gmock_main
 )
diff --git a/absl/time/civil_time.cc b/absl/time/civil_time.cc
index 7b86fa8bb28..eae7e7f48f8 100644
--- a/absl/time/civil_time.cc
+++ b/absl/time/civil_time.cc
@@ -14,13 +14,16 @@
 
 #include "absl/time/civil_time.h"
 
+#include <cerrno>
 #include <cstdlib>
+#include <ostream>
 #include <string>
 
 #include "absl/strings/str_cat.h"
 #include "absl/time/time.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 namespace {
 
@@ -37,16 +40,14 @@ std::string FormatYearAnd(string_view fmt, CivilSecond cs) {
   const CivilSecond ncs(NormalizeYear(cs.year()), cs.month(), cs.day(),
                         cs.hour(), cs.minute(), cs.second());
   const TimeZone utc = UTCTimeZone();
-  // TODO(absl-team): Avoid conversion of fmt std::string.
-  return StrCat(cs.year(),
-                FormatTime(std::string(fmt), FromCivil(ncs, utc), utc));
+  return StrCat(cs.year(), FormatTime(fmt, FromCivil(ncs, utc), utc));
 }
 
 template <typename CivilT>
 bool ParseYearAnd(string_view fmt, string_view s, CivilT* c) {
   // Civil times support a larger year range than absl::Time, so we need to
   // parse the year separately, normalize it, then use absl::ParseTime on the
-  // normalized std::string.
+  // normalized string.
   const std::string ss = std::string(s);  // TODO(absl-team): Avoid conversion.
   const char* const np = ss.c_str();
   char* endp;
@@ -54,13 +55,20 @@ bool ParseYearAnd(string_view fmt, string_view s, CivilT* c) {
   const civil_year_t y =
       std::strtoll(np, &endp, 10);  // NOLINT(runtime/deprecated_fn)
   if (endp == np || errno == ERANGE) return false;
-  const std::string norm = StrCat(NormalizeYear(y), endp);
+  const civil_year_t normalized_year = NormalizeYear(y);
+  const std::string norm = StrCat(normalized_year, endp);
 
   const TimeZone utc = UTCTimeZone();
   Time t;
   if (ParseTime(StrCat("%Y", fmt), norm, utc, &t, nullptr)) {
     const auto cs = ToCivilSecond(t, utc);
-    *c = CivilT(y, cs.month(), cs.day(), cs.hour(), cs.minute(), cs.second());
+    // Field normalization while parsing (e.g. a ":60" leap second or an
+    // end-of-year rollover) can carry into the year. The other fields are taken
+    // from `cs`, so the same carry must be applied to the original year;
+    // otherwise the reconstructed value would use the wrong (un-carried) year.
+    const civil_year_t year = y + (cs.year() - normalized_year);
+    *c =
+        CivilT(year, cs.month(), cs.day(), cs.hour(), cs.minute(), cs.second());
     return true;
   }
 
@@ -81,7 +89,7 @@ bool ParseAs(string_view s, CivilT2* c) {
 
 template <typename CivilT>
 bool ParseLenient(string_view s, CivilT* c) {
-  // A fastpath for when the given std::string data parses exactly into the given
+  // A fastpath for when the given string data parses exactly into the given
   // type T (e.g., s="YYYY-MM-DD" and CivilT=CivilDay).
   if (ParseCivilTime(s, c)) return true;
   // Try parsing as each of the 6 types, trying the most common types first
@@ -97,26 +105,26 @@ bool ParseLenient(string_view s, CivilT* c) {
 }  // namespace
 
 std::string FormatCivilTime(CivilSecond c) {
-  return FormatYearAnd("-%m-%dT%H:%M:%S", c);
+  return FormatYearAnd("-%m-%d%ET%H:%M:%S", c);
 }
 std::string FormatCivilTime(CivilMinute c) {
-  return FormatYearAnd("-%m-%dT%H:%M", c);
+  return FormatYearAnd("-%m-%d%ET%H:%M", c);
 }
 std::string FormatCivilTime(CivilHour c) {
-  return FormatYearAnd("-%m-%dT%H", c);
+  return FormatYearAnd("-%m-%d%ET%H", c);
 }
 std::string FormatCivilTime(CivilDay c) { return FormatYearAnd("-%m-%d", c); }
 std::string FormatCivilTime(CivilMonth c) { return FormatYearAnd("-%m", c); }
 std::string FormatCivilTime(CivilYear c) { return FormatYearAnd("", c); }
 
 bool ParseCivilTime(string_view s, CivilSecond* c) {
-  return ParseYearAnd("-%m-%dT%H:%M:%S", s, c);
+  return ParseYearAnd("-%m-%d%ET%H:%M:%S", s, c);
 }
 bool ParseCivilTime(string_view s, CivilMinute* c) {
-  return ParseYearAnd("-%m-%dT%H:%M", s, c);
+  return ParseYearAnd("-%m-%d%ET%H:%M", s, c);
 }
 bool ParseCivilTime(string_view s, CivilHour* c) {
-  return ParseYearAnd("-%m-%dT%H", s, c);
+  return ParseYearAnd("-%m-%d%ET%H", s, c);
 }
 bool ParseCivilTime(string_view s, CivilDay* c) {
   return ParseYearAnd("-%m-%d", s, c);
@@ -168,6 +176,32 @@ std::ostream& operator<<(std::ostream& os, CivilSecond s) {
   return os << FormatCivilTime(s);
 }
 
+bool AbslParseFlag(string_view s, CivilSecond* c, std::string*) {
+  return ParseLenientCivilTime(s, c);
+}
+bool AbslParseFlag(string_view s, CivilMinute* c, std::string*) {
+  return ParseLenientCivilTime(s, c);
+}
+bool AbslParseFlag(string_view s, CivilHour* c, std::string*) {
+  return ParseLenientCivilTime(s, c);
+}
+bool AbslParseFlag(string_view s, CivilDay* c, std::string*) {
+  return ParseLenientCivilTime(s, c);
+}
+bool AbslParseFlag(string_view s, CivilMonth* c, std::string*) {
+  return ParseLenientCivilTime(s, c);
+}
+bool AbslParseFlag(string_view s, CivilYear* c, std::string*) {
+  return ParseLenientCivilTime(s, c);
+}
+std::string AbslUnparseFlag(CivilSecond c) { return FormatCivilTime(c); }
+std::string AbslUnparseFlag(CivilMinute c) { return FormatCivilTime(c); }
+std::string AbslUnparseFlag(CivilHour c) { return FormatCivilTime(c); }
+std::string AbslUnparseFlag(CivilDay c) { return FormatCivilTime(c); }
+std::string AbslUnparseFlag(CivilMonth c) { return FormatCivilTime(c); }
+std::string AbslUnparseFlag(CivilYear c) { return FormatCivilTime(c); }
+
 }  // namespace time_internal
 
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/time/civil_time.h b/absl/time/civil_time.h
index c8166d07c94..865a99bf591 100644
--- a/absl/time/civil_time.h
+++ b/absl/time/civil_time.h
@@ -55,7 +55,7 @@
 // Example:
 //
 //   // Construct a civil-time object for a specific day
-//   const absl::CivilDay cd(1969, 07, 20);
+//   const absl::CivilDay cd(1969, 7, 20);
 //
 //   // Construct a civil-time object for a specific second
 //   const absl::CivilSecond cd(2018, 8, 1, 12, 0, 1);
@@ -65,17 +65,20 @@
 // Example:
 //
 //   // Valid in C++14
-//   constexpr absl::CivilDay cd(1969, 07, 20);
+//   constexpr absl::CivilDay cd(1969, 7, 20);
 
 #ifndef ABSL_TIME_CIVIL_TIME_H_
 #define ABSL_TIME_CIVIL_TIME_H_
 
+#include <iosfwd>
 #include <string>
 
+#include "absl/base/config.h"
 #include "absl/strings/string_view.h"
 #include "absl/time/internal/cctz/include/cctz/civil_time.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 namespace time_internal {
 struct second_tag : cctz::detail::second_tag {};
@@ -480,6 +483,13 @@ std::string FormatCivilTime(CivilYear c);
 //   absl::CivilDay d;
 //   bool ok = absl::ParseCivilTime("2018-01-02", &d); // OK
 //
+// Parsing tolerates the following variations from the standard format:
+// * Leading and trailing whitespace is ignored.
+// * The year component may be negative (prefixed with '-') and may contain
+//   an arbitrary number of digits.
+// * Sub-year components (month, day, hour, minute, second) may consist of
+//   either one or two digits.
+//
 // Note that parsing will fail if the string's format does not match the
 // expected type exactly. `ParseLenientCivilTime()` below is more lenient.
 //
@@ -492,9 +502,12 @@ bool ParseCivilTime(absl::string_view s, CivilYear* c);
 
 // ParseLenientCivilTime()
 //
-// Parses any of the formats accepted by `absl::ParseCivilTime()`, but is more
-// lenient if the format of the string does not exactly match the associated
-// type.
+// Parses any of the formats accepted by `absl::ParseCivilTime()`. Unlike
+// `ParseCivilTime()`, the input string format does not need to match the
+// target civil-time type. Discrepancies are resolved as follows:
+// * Extra components in the input string are ignored.
+// * Missing components are defaulted to their minimum valid values.
+// This behavior is consistent with civil-time converting constructors.
 //
 // Example:
 //
@@ -512,6 +525,32 @@ bool ParseLenientCivilTime(absl::string_view s, CivilYear* c);
 
 namespace time_internal {  // For functions found via ADL on civil-time tags.
 
+// Support for StrFormat(), StrCat(), etc
+template <typename Sink>
+void AbslStringify(Sink& sink, CivilSecond c) {
+  sink.Append(FormatCivilTime(c));
+}
+template <typename Sink>
+void AbslStringify(Sink& sink, CivilMinute c) {
+  sink.Append(FormatCivilTime(c));
+}
+template <typename Sink>
+void AbslStringify(Sink& sink, CivilHour c) {
+  sink.Append(FormatCivilTime(c));
+}
+template <typename Sink>
+void AbslStringify(Sink& sink, CivilDay c) {
+  sink.Append(FormatCivilTime(c));
+}
+template <typename Sink>
+void AbslStringify(Sink& sink, CivilMonth c) {
+  sink.Append(FormatCivilTime(c));
+}
+template <typename Sink>
+void AbslStringify(Sink& sink, CivilYear c) {
+  sink.Append(FormatCivilTime(c));
+}
+
 // Streaming Operators
 //
 // Each civil-time type may be sent to an output stream using operator<<().
@@ -519,7 +558,7 @@ namespace time_internal {  // For functions found via ADL on civil-time tags.
 //
 // Example:
 //
-//   absl::CivilDay d = absl::CivilDay("1969-07-20");
+//   absl::CivilDay d = absl::CivilDay(1969, 7, 20);
 //   std::cout << "Date is: " << d << "\n";
 //
 std::ostream& operator<<(std::ostream& os, CivilYear y);
@@ -529,8 +568,32 @@ std::ostream& operator<<(std::ostream& os, CivilHour h);
 std::ostream& operator<<(std::ostream& os, CivilMinute m);
 std::ostream& operator<<(std::ostream& os, CivilSecond s);
 
+// AbslParseFlag()
+//
+// Parses the command-line flag string representation `s` into a civil-time
+// value. Flags must be specified in a format that is valid for
+// `absl::ParseLenientCivilTime()`.
+bool AbslParseFlag(absl::string_view s, CivilSecond* c, std::string* error);
+bool AbslParseFlag(absl::string_view s, CivilMinute* c, std::string* error);
+bool AbslParseFlag(absl::string_view s, CivilHour* c, std::string* error);
+bool AbslParseFlag(absl::string_view s, CivilDay* c, std::string* error);
+bool AbslParseFlag(absl::string_view s, CivilMonth* c, std::string* error);
+bool AbslParseFlag(absl::string_view s, CivilYear* c, std::string* error);
+
+// AbslUnparseFlag()
+//
+// Unparses a civil-time value into a command-line string representation using
+// the format specified by `absl::ParseCivilTime()`.
+std::string AbslUnparseFlag(CivilSecond c);
+std::string AbslUnparseFlag(CivilMinute c);
+std::string AbslUnparseFlag(CivilHour c);
+std::string AbslUnparseFlag(CivilDay c);
+std::string AbslUnparseFlag(CivilMonth c);
+std::string AbslUnparseFlag(CivilYear c);
+
 }  // namespace time_internal
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_TIME_CIVIL_TIME_H_
diff --git a/absl/time/civil_time_benchmark.cc b/absl/time/civil_time_benchmark.cc
index f04dbe200ed..2818a90e532 100644
--- a/absl/time/civil_time_benchmark.cc
+++ b/absl/time/civil_time_benchmark.cc
@@ -12,12 +12,13 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-#include "absl/time/civil_time.h"
-
+#include <cstddef>
 #include <numeric>
+#include <string>
 #include <vector>
 
 #include "absl/hash/hash.h"
+#include "absl/time/civil_time.h"
 #include "benchmark/benchmark.h"
 
 namespace {
@@ -42,7 +43,7 @@ void BM_Difference_Days(benchmark::State& state) {
   const absl::CivilDay c(2014, 8, 22);
   const absl::CivilDay epoch(1970, 1, 1);
   while (state.KeepRunning()) {
-    const absl::civil_diff_t n = c - epoch;
+    absl::civil_diff_t n = c - epoch;
     benchmark::DoNotOptimize(n);
   }
 }
@@ -60,7 +61,7 @@ BENCHMARK(BM_Step_Days);
 void BM_Format(benchmark::State& state) {
   const absl::CivilSecond c(2014, 1, 2, 3, 4, 5);
   while (state.KeepRunning()) {
-    const std::string s = absl::FormatCivilTime(c);
+    std::string s = absl::FormatCivilTime(c);
     benchmark::DoNotOptimize(s);
   }
 }
@@ -70,7 +71,7 @@ void BM_Parse(benchmark::State& state) {
   const std::string f = "2014-01-02T03:04:05";
   absl::CivilSecond c;
   while (state.KeepRunning()) {
-    const bool b = absl::ParseCivilTime(f, &c);
+    bool b = absl::ParseCivilTime(f, &c);
     benchmark::DoNotOptimize(b);
   }
 }
@@ -80,7 +81,7 @@ void BM_RoundTripFormatParse(benchmark::State& state) {
   const absl::CivilSecond c(2014, 1, 2, 3, 4, 5);
   absl::CivilSecond out;
   while (state.KeepRunning()) {
-    const bool b = absl::ParseCivilTime(absl::FormatCivilTime(c), &out);
+    bool b = absl::ParseCivilTime(absl::FormatCivilTime(c), &out);
     benchmark::DoNotOptimize(b);
   }
 }
@@ -95,7 +96,8 @@ void BM_CivilTimeAbslHash(benchmark::State& state) {
   absl::Hash<T> absl_hasher;
   while (state.KeepRunningBatch(kSize)) {
     for (const T civil_time : civil_times) {
-      benchmark::DoNotOptimize(absl_hasher(civil_time));
+      size_t hash = absl_hasher(civil_time);
+      benchmark::DoNotOptimize(hash);
     }
   }
 }
diff --git a/absl/time/civil_time_test.cc b/absl/time/civil_time_test.cc
index 0ebd97adbc5..fed0c30b25b 100644
--- a/absl/time/civil_time_test.cc
+++ b/absl/time/civil_time_test.cc
@@ -14,12 +14,18 @@
 
 #include "absl/time/civil_time.h"
 
+#include <cstddef>
+#include <iomanip>
+#include <iterator>
 #include <limits>
 #include <sstream>
 #include <type_traits>
 
-#include "absl/base/macros.h"
 #include "gtest/gtest.h"
+#include "absl/hash/hash_testing.h"
+#include "absl/strings/has_absl_stringify.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/str_format.h"
 
 namespace {
 
@@ -75,8 +81,7 @@ TEST(CivilTime, FieldsConstruction) {
             absl::FormatCivilTime(absl::CivilMinute(2015, 1, 2)));
   EXPECT_EQ("2015-01-01T00:00",
             absl::FormatCivilTime(absl::CivilMinute(2015, 1)));
-  EXPECT_EQ("2015-01-01T00:00",
-            absl::FormatCivilTime(absl::CivilMinute(2015)));
+  EXPECT_EQ("2015-01-01T00:00", absl::FormatCivilTime(absl::CivilMinute(2015)));
 
   EXPECT_EQ("2015-01-02T03",
             absl::FormatCivilTime(absl::CivilHour(2015, 1, 2, 3, 4, 5)));
@@ -86,82 +91,63 @@ TEST(CivilTime, FieldsConstruction) {
             absl::FormatCivilTime(absl::CivilHour(2015, 1, 2, 3)));
   EXPECT_EQ("2015-01-02T00",
             absl::FormatCivilTime(absl::CivilHour(2015, 1, 2)));
-  EXPECT_EQ("2015-01-01T00",
-            absl::FormatCivilTime(absl::CivilHour(2015, 1)));
-  EXPECT_EQ("2015-01-01T00",
-            absl::FormatCivilTime(absl::CivilHour(2015)));
+  EXPECT_EQ("2015-01-01T00", absl::FormatCivilTime(absl::CivilHour(2015, 1)));
+  EXPECT_EQ("2015-01-01T00", absl::FormatCivilTime(absl::CivilHour(2015)));
 
   EXPECT_EQ("2015-01-02",
             absl::FormatCivilTime(absl::CivilDay(2015, 1, 2, 3, 4, 5)));
   EXPECT_EQ("2015-01-02",
             absl::FormatCivilTime(absl::CivilDay(2015, 1, 2, 3, 4)));
-  EXPECT_EQ("2015-01-02",
-            absl::FormatCivilTime(absl::CivilDay(2015, 1, 2, 3)));
-  EXPECT_EQ("2015-01-02",
-            absl::FormatCivilTime(absl::CivilDay(2015, 1, 2)));
-  EXPECT_EQ("2015-01-01",
-            absl::FormatCivilTime(absl::CivilDay(2015, 1)));
-  EXPECT_EQ("2015-01-01",
-            absl::FormatCivilTime(absl::CivilDay(2015)));
+  EXPECT_EQ("2015-01-02", absl::FormatCivilTime(absl::CivilDay(2015, 1, 2, 3)));
+  EXPECT_EQ("2015-01-02", absl::FormatCivilTime(absl::CivilDay(2015, 1, 2)));
+  EXPECT_EQ("2015-01-01", absl::FormatCivilTime(absl::CivilDay(2015, 1)));
+  EXPECT_EQ("2015-01-01", absl::FormatCivilTime(absl::CivilDay(2015)));
 
   EXPECT_EQ("2015-01",
             absl::FormatCivilTime(absl::CivilMonth(2015, 1, 2, 3, 4, 5)));
   EXPECT_EQ("2015-01",
             absl::FormatCivilTime(absl::CivilMonth(2015, 1, 2, 3, 4)));
-  EXPECT_EQ("2015-01",
-            absl::FormatCivilTime(absl::CivilMonth(2015, 1, 2, 3)));
-  EXPECT_EQ("2015-01",
-            absl::FormatCivilTime(absl::CivilMonth(2015, 1, 2)));
-  EXPECT_EQ("2015-01",
-            absl::FormatCivilTime(absl::CivilMonth(2015, 1)));
-  EXPECT_EQ("2015-01",
-            absl::FormatCivilTime(absl::CivilMonth(2015)));
+  EXPECT_EQ("2015-01", absl::FormatCivilTime(absl::CivilMonth(2015, 1, 2, 3)));
+  EXPECT_EQ("2015-01", absl::FormatCivilTime(absl::CivilMonth(2015, 1, 2)));
+  EXPECT_EQ("2015-01", absl::FormatCivilTime(absl::CivilMonth(2015, 1)));
+  EXPECT_EQ("2015-01", absl::FormatCivilTime(absl::CivilMonth(2015)));
 
   EXPECT_EQ("2015",
             absl::FormatCivilTime(absl::CivilYear(2015, 1, 2, 3, 4, 5)));
-  EXPECT_EQ("2015",
-            absl::FormatCivilTime(absl::CivilYear(2015, 1, 2, 3, 4)));
-  EXPECT_EQ("2015",
-            absl::FormatCivilTime(absl::CivilYear(2015, 1, 2, 3)));
-  EXPECT_EQ("2015",
-            absl::FormatCivilTime(absl::CivilYear(2015, 1, 2)));
-  EXPECT_EQ("2015",
-            absl::FormatCivilTime(absl::CivilYear(2015, 1)));
-  EXPECT_EQ("2015",
-            absl::FormatCivilTime(absl::CivilYear(2015)));
+  EXPECT_EQ("2015", absl::FormatCivilTime(absl::CivilYear(2015, 1, 2, 3, 4)));
+  EXPECT_EQ("2015", absl::FormatCivilTime(absl::CivilYear(2015, 1, 2, 3)));
+  EXPECT_EQ("2015", absl::FormatCivilTime(absl::CivilYear(2015, 1, 2)));
+  EXPECT_EQ("2015", absl::FormatCivilTime(absl::CivilYear(2015, 1)));
+  EXPECT_EQ("2015", absl::FormatCivilTime(absl::CivilYear(2015)));
 }
 
 TEST(CivilTime, FieldsConstructionLimits) {
   const int kIntMax = std::numeric_limits<int>::max();
-  EXPECT_EQ("2038-01-19T03:14:07",
-            absl::FormatCivilTime(absl::CivilSecond(
-                1970, 1, 1, 0, 0, kIntMax)));
-  EXPECT_EQ("6121-02-11T05:21:07",
-            absl::FormatCivilTime(absl::CivilSecond(
-                1970, 1, 1, 0, kIntMax, kIntMax)));
+  EXPECT_EQ("2038-01-19T03:14:07", absl::FormatCivilTime(absl::CivilSecond(
+                                       1970, 1, 1, 0, 0, kIntMax)));
+  EXPECT_EQ("6121-02-11T05:21:07", absl::FormatCivilTime(absl::CivilSecond(
+                                       1970, 1, 1, 0, kIntMax, kIntMax)));
   EXPECT_EQ("251104-11-20T12:21:07",
-            absl::FormatCivilTime(absl::CivilSecond(
-                1970, 1, 1, kIntMax, kIntMax, kIntMax)));
+            absl::FormatCivilTime(
+                absl::CivilSecond(1970, 1, 1, kIntMax, kIntMax, kIntMax)));
   EXPECT_EQ("6130715-05-30T12:21:07",
-            absl::FormatCivilTime(absl::CivilSecond(
-                1970, 1, kIntMax, kIntMax, kIntMax, kIntMax)));
+            absl::FormatCivilTime(absl::CivilSecond(1970, 1, kIntMax, kIntMax,
+                                                    kIntMax, kIntMax)));
   EXPECT_EQ("185087685-11-26T12:21:07",
             absl::FormatCivilTime(absl::CivilSecond(
                 1970, kIntMax, kIntMax, kIntMax, kIntMax, kIntMax)));
 
   const int kIntMin = std::numeric_limits<int>::min();
-  EXPECT_EQ("1901-12-13T20:45:52",
-            absl::FormatCivilTime(absl::CivilSecond(
-                1970, 1, 1, 0, 0, kIntMin)));
-  EXPECT_EQ("-2182-11-20T18:37:52",
-            absl::FormatCivilTime(absl::CivilSecond(
-                1970, 1, 1, 0, kIntMin, kIntMin)));
+  EXPECT_EQ("1901-12-13T20:45:52", absl::FormatCivilTime(absl::CivilSecond(
+                                       1970, 1, 1, 0, 0, kIntMin)));
+  EXPECT_EQ("-2182-11-20T18:37:52", absl::FormatCivilTime(absl::CivilSecond(
+                                        1970, 1, 1, 0, kIntMin, kIntMin)));
   EXPECT_EQ("-247165-02-11T10:37:52",
-            absl::FormatCivilTime(absl::CivilSecond(
-                1970, 1, 1, kIntMin, kIntMin, kIntMin)));
+            absl::FormatCivilTime(
+                absl::CivilSecond(1970, 1, 1, kIntMin, kIntMin, kIntMin)));
   EXPECT_EQ("-6126776-08-01T10:37:52",
-            absl::FormatCivilTime(absl::CivilSecond(
-                1970, 1, kIntMin, kIntMin, kIntMin, kIntMin)));
+            absl::FormatCivilTime(absl::CivilSecond(1970, 1, kIntMin, kIntMin,
+                                                    kIntMin, kIntMin)));
   EXPECT_EQ("-185083747-10-31T10:37:52",
             absl::FormatCivilTime(absl::CivilSecond(
                 1970, kIntMin, kIntMin, kIntMin, kIntMin, kIntMin)));
@@ -170,14 +156,10 @@ TEST(CivilTime, FieldsConstructionLimits) {
 TEST(CivilTime, RangeLimits) {
   const absl::civil_year_t kYearMax =
       std::numeric_limits<absl::civil_year_t>::max();
-  EXPECT_EQ(absl::CivilYear(kYearMax),
-            absl::CivilYear::max());
-  EXPECT_EQ(absl::CivilMonth(kYearMax, 12),
-            absl::CivilMonth::max());
-  EXPECT_EQ(absl::CivilDay(kYearMax, 12, 31),
-            absl::CivilDay::max());
-  EXPECT_EQ(absl::CivilHour(kYearMax, 12, 31, 23),
-            absl::CivilHour::max());
+  EXPECT_EQ(absl::CivilYear(kYearMax), absl::CivilYear::max());
+  EXPECT_EQ(absl::CivilMonth(kYearMax, 12), absl::CivilMonth::max());
+  EXPECT_EQ(absl::CivilDay(kYearMax, 12, 31), absl::CivilDay::max());
+  EXPECT_EQ(absl::CivilHour(kYearMax, 12, 31, 23), absl::CivilHour::max());
   EXPECT_EQ(absl::CivilMinute(kYearMax, 12, 31, 23, 59),
             absl::CivilMinute::max());
   EXPECT_EQ(absl::CivilSecond(kYearMax, 12, 31, 23, 59, 59),
@@ -185,16 +167,11 @@ TEST(CivilTime, RangeLimits) {
 
   const absl::civil_year_t kYearMin =
       std::numeric_limits<absl::civil_year_t>::min();
-  EXPECT_EQ(absl::CivilYear(kYearMin),
-            absl::CivilYear::min());
-  EXPECT_EQ(absl::CivilMonth(kYearMin, 1),
-            absl::CivilMonth::min());
-  EXPECT_EQ(absl::CivilDay(kYearMin, 1, 1),
-            absl::CivilDay::min());
-  EXPECT_EQ(absl::CivilHour(kYearMin, 1, 1, 0),
-            absl::CivilHour::min());
-  EXPECT_EQ(absl::CivilMinute(kYearMin, 1, 1, 0, 0),
-            absl::CivilMinute::min());
+  EXPECT_EQ(absl::CivilYear(kYearMin), absl::CivilYear::min());
+  EXPECT_EQ(absl::CivilMonth(kYearMin, 1), absl::CivilMonth::min());
+  EXPECT_EQ(absl::CivilDay(kYearMin, 1, 1), absl::CivilDay::min());
+  EXPECT_EQ(absl::CivilHour(kYearMin, 1, 1, 0), absl::CivilHour::min());
+  EXPECT_EQ(absl::CivilMinute(kYearMin, 1, 1, 0, 0), absl::CivilMinute::min());
   EXPECT_EQ(absl::CivilSecond(kYearMin, 1, 1, 0, 0, 0),
             absl::CivilSecond::min());
 }
@@ -243,40 +220,25 @@ TEST(CivilTime, ImplicitCrossAlignment) {
   EXPECT_EQ(month, year);
 
   // Ensures unsafe conversions are not allowed.
-  EXPECT_FALSE(
-      (std::is_convertible<absl::CivilSecond, absl::CivilMinute>::value));
-  EXPECT_FALSE(
-      (std::is_convertible<absl::CivilSecond, absl::CivilHour>::value));
-  EXPECT_FALSE(
-      (std::is_convertible<absl::CivilSecond, absl::CivilDay>::value));
-  EXPECT_FALSE(
-      (std::is_convertible<absl::CivilSecond, absl::CivilMonth>::value));
-  EXPECT_FALSE(
-      (std::is_convertible<absl::CivilSecond, absl::CivilYear>::value));
-
-  EXPECT_FALSE(
-      (std::is_convertible<absl::CivilMinute, absl::CivilHour>::value));
-  EXPECT_FALSE(
-      (std::is_convertible<absl::CivilMinute, absl::CivilDay>::value));
-  EXPECT_FALSE(
-      (std::is_convertible<absl::CivilMinute, absl::CivilMonth>::value));
-  EXPECT_FALSE(
-      (std::is_convertible<absl::CivilMinute, absl::CivilYear>::value));
-
-  EXPECT_FALSE(
-      (std::is_convertible<absl::CivilHour, absl::CivilDay>::value));
-  EXPECT_FALSE(
-      (std::is_convertible<absl::CivilHour, absl::CivilMonth>::value));
-  EXPECT_FALSE(
-      (std::is_convertible<absl::CivilHour, absl::CivilYear>::value));
-
-  EXPECT_FALSE(
-      (std::is_convertible<absl::CivilDay, absl::CivilMonth>::value));
-  EXPECT_FALSE(
-      (std::is_convertible<absl::CivilDay, absl::CivilYear>::value));
-
-  EXPECT_FALSE(
-      (std::is_convertible<absl::CivilMonth, absl::CivilYear>::value));
+  EXPECT_FALSE((std::is_convertible_v<absl::CivilSecond, absl::CivilMinute>));
+  EXPECT_FALSE((std::is_convertible_v<absl::CivilSecond, absl::CivilHour>));
+  EXPECT_FALSE((std::is_convertible_v<absl::CivilSecond, absl::CivilDay>));
+  EXPECT_FALSE((std::is_convertible_v<absl::CivilSecond, absl::CivilMonth>));
+  EXPECT_FALSE((std::is_convertible_v<absl::CivilSecond, absl::CivilYear>));
+
+  EXPECT_FALSE((std::is_convertible_v<absl::CivilMinute, absl::CivilHour>));
+  EXPECT_FALSE((std::is_convertible_v<absl::CivilMinute, absl::CivilDay>));
+  EXPECT_FALSE((std::is_convertible_v<absl::CivilMinute, absl::CivilMonth>));
+  EXPECT_FALSE((std::is_convertible_v<absl::CivilMinute, absl::CivilYear>));
+
+  EXPECT_FALSE((std::is_convertible_v<absl::CivilHour, absl::CivilDay>));
+  EXPECT_FALSE((std::is_convertible_v<absl::CivilHour, absl::CivilMonth>));
+  EXPECT_FALSE((std::is_convertible_v<absl::CivilHour, absl::CivilYear>));
+
+  EXPECT_FALSE((std::is_convertible_v<absl::CivilDay, absl::CivilMonth>));
+  EXPECT_FALSE((std::is_convertible_v<absl::CivilDay, absl::CivilYear>));
+
+  EXPECT_FALSE((std::is_convertible_v<absl::CivilMonth, absl::CivilYear>));
 }
 
 TEST(CivilTime, ExplicitCrossAlignment) {
@@ -334,33 +296,33 @@ struct HasDiff {
 
 TEST(CivilTime, DisallowCrossAlignedDifference) {
   // Difference is allowed between types with the same alignment.
-  static_assert(HasDiff<absl::CivilSecond, absl::CivilSecond>::value, "");
-  static_assert(HasDiff<absl::CivilMinute, absl::CivilMinute>::value, "");
-  static_assert(HasDiff<absl::CivilHour, absl::CivilHour>::value, "");
-  static_assert(HasDiff<absl::CivilDay, absl::CivilDay>::value, "");
-  static_assert(HasDiff<absl::CivilMonth, absl::CivilMonth>::value, "");
-  static_assert(HasDiff<absl::CivilYear, absl::CivilYear>::value, "");
+  static_assert(HasDiff<absl::CivilSecond, absl::CivilSecond>::value);
+  static_assert(HasDiff<absl::CivilMinute, absl::CivilMinute>::value);
+  static_assert(HasDiff<absl::CivilHour, absl::CivilHour>::value);
+  static_assert(HasDiff<absl::CivilDay, absl::CivilDay>::value);
+  static_assert(HasDiff<absl::CivilMonth, absl::CivilMonth>::value);
+  static_assert(HasDiff<absl::CivilYear, absl::CivilYear>::value);
 
   // Difference is disallowed between types with different alignments.
-  static_assert(!HasDiff<absl::CivilSecond, absl::CivilMinute>::value, "");
-  static_assert(!HasDiff<absl::CivilSecond, absl::CivilHour>::value, "");
-  static_assert(!HasDiff<absl::CivilSecond, absl::CivilDay>::value, "");
-  static_assert(!HasDiff<absl::CivilSecond, absl::CivilMonth>::value, "");
-  static_assert(!HasDiff<absl::CivilSecond, absl::CivilYear>::value, "");
+  static_assert(!HasDiff<absl::CivilSecond, absl::CivilMinute>::value);
+  static_assert(!HasDiff<absl::CivilSecond, absl::CivilHour>::value);
+  static_assert(!HasDiff<absl::CivilSecond, absl::CivilDay>::value);
+  static_assert(!HasDiff<absl::CivilSecond, absl::CivilMonth>::value);
+  static_assert(!HasDiff<absl::CivilSecond, absl::CivilYear>::value);
 
-  static_assert(!HasDiff<absl::CivilMinute, absl::CivilHour>::value, "");
-  static_assert(!HasDiff<absl::CivilMinute, absl::CivilDay>::value, "");
-  static_assert(!HasDiff<absl::CivilMinute, absl::CivilMonth>::value, "");
-  static_assert(!HasDiff<absl::CivilMinute, absl::CivilYear>::value, "");
+  static_assert(!HasDiff<absl::CivilMinute, absl::CivilHour>::value);
+  static_assert(!HasDiff<absl::CivilMinute, absl::CivilDay>::value);
+  static_assert(!HasDiff<absl::CivilMinute, absl::CivilMonth>::value);
+  static_assert(!HasDiff<absl::CivilMinute, absl::CivilYear>::value);
 
-  static_assert(!HasDiff<absl::CivilHour, absl::CivilDay>::value, "");
-  static_assert(!HasDiff<absl::CivilHour, absl::CivilMonth>::value, "");
-  static_assert(!HasDiff<absl::CivilHour, absl::CivilYear>::value, "");
+  static_assert(!HasDiff<absl::CivilHour, absl::CivilDay>::value);
+  static_assert(!HasDiff<absl::CivilHour, absl::CivilMonth>::value);
+  static_assert(!HasDiff<absl::CivilHour, absl::CivilYear>::value);
 
-  static_assert(!HasDiff<absl::CivilDay, absl::CivilMonth>::value, "");
-  static_assert(!HasDiff<absl::CivilDay, absl::CivilYear>::value, "");
+  static_assert(!HasDiff<absl::CivilDay, absl::CivilMonth>::value);
+  static_assert(!HasDiff<absl::CivilDay, absl::CivilYear>::value);
 
-  static_assert(!HasDiff<absl::CivilMonth, absl::CivilYear>::value, "");
+  static_assert(!HasDiff<absl::CivilMonth, absl::CivilYear>::value);
 }
 
 TEST(CivilTime, ValueSemantics) {
@@ -414,8 +376,7 @@ TEST(CivilTime, Relational) {
   // Tests the relational operators of two different civil-time types.
   TEST_RELATIONAL(absl::CivilDay(2014, 1, 1),
                   absl::CivilMinute(2014, 1, 1, 1, 1));
-  TEST_RELATIONAL(absl::CivilDay(2014, 1, 1),
-                  absl::CivilMonth(2014, 2));
+  TEST_RELATIONAL(absl::CivilDay(2014, 1, 1), absl::CivilMonth(2014, 2));
 
 #undef TEST_RELATIONAL
 }
@@ -791,7 +752,7 @@ TEST(CivilTime, FormatAndParseLenient) {
   EXPECT_EQ("2015", absl::FormatCivilTime(y));
 }
 
-TEST(CivilTime, ParseEdgeCases) {
+TEST(CivilTime, ParseLenientEdgeCases) {
   absl::CivilSecond ss;
   EXPECT_TRUE(
       absl::ParseLenientCivilTime("9223372036854775807-12-31T23:59:59", &ss));
@@ -809,8 +770,7 @@ TEST(CivilTime, ParseEdgeCases) {
   EXPECT_EQ("-9223372036854775808-01-01T00:00", absl::FormatCivilTime(mm));
 
   absl::CivilHour hh;
-  EXPECT_TRUE(
-      absl::ParseLenientCivilTime("9223372036854775807-12-31T23", &hh));
+  EXPECT_TRUE(absl::ParseLenientCivilTime("9223372036854775807-12-31T23", &hh));
   EXPECT_EQ("9223372036854775807-12-31T23", absl::FormatCivilTime(hh));
   EXPECT_TRUE(
       absl::ParseLenientCivilTime("-9223372036854775808-01-01T00", &hh));
@@ -868,6 +828,82 @@ TEST(CivilTime, ParseEdgeCases) {
   EXPECT_FALSE(absl::ParseLenientCivilTime("9223372036854775808", &y)) << y;
 }
 
+TEST(CivilTime, ParseEdgeCases) {
+  absl::CivilYear y;
+  absl::CivilMonth m;
+  absl::CivilDay d;
+  absl::CivilSecond ss;
+  EXPECT_TRUE(absl::ParseCivilTime("0", &y)) << y;
+  EXPECT_EQ(absl::CivilYear(0), y);
+  EXPECT_TRUE(absl::ParseCivilTime("0-1", &m)) << m;
+  EXPECT_EQ(absl::CivilMonth(0, 1), m);
+  EXPECT_TRUE(absl::ParseCivilTime(" 2015 ", &y)) << y;
+  EXPECT_EQ(absl::CivilYear(2015), y);
+  EXPECT_TRUE(absl::ParseCivilTime(
+      "000000000000000000000000000000000000000000000000000000000000002015", &y))
+      << y;
+  EXPECT_EQ(absl::CivilYear(2015), y);
+  EXPECT_TRUE(absl::ParseCivilTime(" 2015-6 ", &m)) << m;
+  EXPECT_EQ(absl::CivilMonth(2015, 6), m);
+  EXPECT_TRUE(absl::ParseCivilTime("0002015-6-7", &d)) << d;
+  EXPECT_EQ(absl::CivilDay(2015, 6, 7), d);
+  EXPECT_TRUE(absl::ParseCivilTime("2015-06-07T10:11:12 ", &ss)) << ss;
+  EXPECT_EQ(absl::CivilSecond(2015, 6, 7, 10, 11, 12), ss);
+  EXPECT_TRUE(absl::ParseCivilTime(" 2015-06-07T10:11:1 ", &ss)) << ss;
+  EXPECT_EQ(absl::CivilSecond(2015, 6, 7, 10, 11, 1), ss);
+  EXPECT_TRUE(absl::ParseCivilTime("-01-01", &m)) << m;
+  EXPECT_EQ(absl::CivilMonth(-1, 1), m);
+}
+
+TEST(CivilTime, ParseFieldNormalizationCarriesYear) {
+  // When a field normalizes past the end of the year (e.g. a ":60" leap
+  // second on the last second of December), the carry must be reflected in
+  // the parsed year, so parsing agrees with direct field construction.
+  absl::CivilSecond ss;
+  EXPECT_TRUE(absl::ParseCivilTime("2020-12-31T23:59:60", &ss)) << ss;
+  EXPECT_EQ(absl::CivilSecond(2020, 12, 31, 23, 59, 60), ss);
+  EXPECT_EQ(absl::CivilSecond(2021, 1, 1, 0, 0, 0), ss);
+
+  EXPECT_TRUE(absl::ParseLenientCivilTime("2020-12-31T23:59:60", &ss)) << ss;
+  EXPECT_EQ(absl::CivilSecond(2021, 1, 1, 0, 0, 0), ss);
+
+  // The carry also works for negative years crossing zero.
+  EXPECT_TRUE(absl::ParseCivilTime("-1-12-31T23:59:60", &ss)) << ss;
+  EXPECT_EQ(absl::CivilSecond(0, 1, 1, 0, 0, 0), ss);
+}
+
+TEST(CivilTime, AbslStringify) {
+  static_assert(absl::HasAbslStringify<absl::CivilSecond>::value);
+  static_assert(absl::HasAbslStringify<absl::CivilMinute>::value);
+  static_assert(absl::HasAbslStringify<absl::CivilHour>::value);
+  static_assert(absl::HasAbslStringify<absl::CivilDay>::value);
+  static_assert(absl::HasAbslStringify<absl::CivilMonth>::value);
+  static_assert(absl::HasAbslStringify<absl::CivilYear>::value);
+
+  EXPECT_EQ("2015-01-02T03:04:05",
+            absl::StrCat(absl::CivilSecond(2015, 1, 2, 3, 4, 5)));
+  EXPECT_EQ("2015-01-02T03:04:05",
+            absl::StrFormat("%v", absl::CivilSecond(2015, 1, 2, 3, 4, 5)));
+
+  EXPECT_EQ("2015-01-02T03:04",
+            absl::StrCat(absl::CivilMinute(2015, 1, 2, 3, 4)));
+  EXPECT_EQ("2015-01-02T03:04",
+            absl::StrFormat("%v", absl::CivilMinute(2015, 1, 2, 3, 4)));
+
+  EXPECT_EQ("2015-01-02T03", absl::StrCat(absl::CivilHour(2015, 1, 2, 3)));
+  EXPECT_EQ("2015-01-02T03",
+            absl::StrFormat("%v", absl::CivilHour(2015, 1, 2, 3)));
+
+  EXPECT_EQ("2015-01-02", absl::StrCat(absl::CivilDay(2015, 1, 2)));
+  EXPECT_EQ("2015-01-02", absl::StrFormat("%v", absl::CivilDay(2015, 1, 2)));
+
+  EXPECT_EQ("2015-01", absl::StrCat(absl::CivilMonth(2015, 1)));
+  EXPECT_EQ("2015-01", absl::StrFormat("%v", absl::CivilMonth(2015, 1)));
+
+  EXPECT_EQ("2015", absl::StrCat(absl::CivilYear(2015)));
+  EXPECT_EQ("2015", absl::StrFormat("%v", absl::CivilYear(2015)));
+}
+
 TEST(CivilTime, OutputStream) {
   absl::CivilSecond cs(2016, 2, 3, 4, 5, 6);
   {
@@ -1161,22 +1197,16 @@ TEST(CivilTime, LeapYears) {
       int day;
     } leap_day;  // The date of the day after Feb 28.
   } kLeapYearTable[]{
-      {1900, 365, {3, 1}},
-      {1999, 365, {3, 1}},
+      {1900, 365, {3, 1}},  {1999, 365, {3, 1}},
       {2000, 366, {2, 29}},  // leap year
-      {2001, 365, {3, 1}},
-      {2002, 365, {3, 1}},
-      {2003, 365, {3, 1}},
-      {2004, 366, {2, 29}},  // leap year
-      {2005, 365, {3, 1}},
-      {2006, 365, {3, 1}},
-      {2007, 365, {3, 1}},
-      {2008, 366, {2, 29}},  // leap year
-      {2009, 365, {3, 1}},
-      {2100, 365, {3, 1}},
+      {2001, 365, {3, 1}},  {2002, 365, {3, 1}},
+      {2003, 365, {3, 1}},  {2004, 366, {2, 29}},  // leap year
+      {2005, 365, {3, 1}},  {2006, 365, {3, 1}},
+      {2007, 365, {3, 1}},  {2008, 366, {2, 29}},  // leap year
+      {2009, 365, {3, 1}},  {2100, 365, {3, 1}},
   };
 
-  for (int i = 0; i < ABSL_ARRAYSIZE(kLeapYearTable); ++i) {
+  for (size_t i = 0; i < std::size(kLeapYearTable); ++i) {
     const int y = kLeapYearTable[i].year;
     const int m = kLeapYearTable[i].leap_day.month;
     const int d = kLeapYearTable[i].leap_day.day;
@@ -1203,7 +1233,7 @@ TEST(CivilTime, FirstThursdayInMonth) {
 
   // Bonus: Date of Thanksgiving in the United States
   // Rule: Fourth Thursday of November
-  const absl::CivilDay thanksgiving = thursday +  7 * 3;
+  const absl::CivilDay thanksgiving = thursday + 7 * 3;
   EXPECT_EQ("2014-11-27", absl::FormatCivilTime(thanksgiving));
 }
 
@@ -1228,7 +1258,7 @@ TEST(CivilTime, DocumentationExample) {
   EXPECT_EQ(0, day_floor.hour());  // 09:09:09 is floored
   EXPECT_EQ(absl::CivilDay(2015, 1, 2), day_floor);
 
-  // Unspecified fields default to their minium value
+  // Unspecified fields default to their minimum value
   absl::CivilDay day_default(2015);  // Defaults to Jan 1
   EXPECT_EQ(absl::CivilDay(2015, 1, 1), day_default);
 
@@ -1240,4 +1270,137 @@ TEST(CivilTime, DocumentationExample) {
   }
 }
 
+TEST(CivilTime, SupportsHash) {
+  // To check that every field is used by the hash implementation, test
+  // with values that have all fields the same bar one, for each field.
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly({
+      absl::CivilYear(2017),
+      absl::CivilYear(2018),
+  }));
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly({
+      absl::CivilMonth(2017, 1),
+      absl::CivilMonth(2017, 2),
+      absl::CivilMonth(2018, 1),
+      absl::CivilMonth(2018, 2),
+  }));
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly({
+      absl::CivilDay(2017, 1, 1),
+      absl::CivilDay(2017, 1, 2),
+      absl::CivilDay(2017, 2, 1),
+      absl::CivilDay(2017, 2, 2),
+      absl::CivilDay(2018, 1, 1),
+      absl::CivilDay(2018, 1, 2),
+      absl::CivilDay(2018, 2, 1),
+      absl::CivilDay(2018, 2, 2),
+  }));
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly({
+      absl::CivilHour(2017, 1, 1, 0),
+      absl::CivilHour(2017, 1, 1, 1),
+      absl::CivilHour(2017, 1, 2, 0),
+      absl::CivilHour(2017, 1, 2, 1),
+      absl::CivilHour(2017, 2, 1, 0),
+      absl::CivilHour(2017, 2, 1, 1),
+      absl::CivilHour(2017, 2, 2, 0),
+      absl::CivilHour(2017, 2, 2, 1),
+      absl::CivilHour(2018, 1, 1, 0),
+      absl::CivilHour(2018, 1, 1, 1),
+      absl::CivilHour(2018, 1, 2, 0),
+      absl::CivilHour(2018, 1, 2, 1),
+      absl::CivilHour(2018, 2, 1, 0),
+      absl::CivilHour(2018, 2, 1, 1),
+      absl::CivilHour(2018, 2, 2, 0),
+      absl::CivilHour(2018, 2, 2, 1),
+  }));
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly({
+      absl::CivilMinute(2017, 1, 1, 0, 0), absl::CivilMinute(2017, 1, 1, 0, 1),
+      absl::CivilMinute(2017, 1, 1, 1, 0), absl::CivilMinute(2017, 1, 1, 1, 1),
+      absl::CivilMinute(2017, 1, 2, 0, 0), absl::CivilMinute(2017, 1, 2, 0, 1),
+      absl::CivilMinute(2017, 1, 2, 1, 0), absl::CivilMinute(2017, 1, 2, 1, 1),
+      absl::CivilMinute(2017, 2, 1, 0, 0), absl::CivilMinute(2017, 2, 1, 0, 1),
+      absl::CivilMinute(2017, 2, 1, 1, 0), absl::CivilMinute(2017, 2, 1, 1, 1),
+      absl::CivilMinute(2017, 2, 2, 0, 0), absl::CivilMinute(2017, 2, 2, 0, 1),
+      absl::CivilMinute(2017, 2, 2, 1, 0), absl::CivilMinute(2017, 2, 2, 1, 1),
+      absl::CivilMinute(2018, 1, 1, 0, 0), absl::CivilMinute(2018, 1, 1, 0, 1),
+      absl::CivilMinute(2018, 1, 1, 1, 0), absl::CivilMinute(2018, 1, 1, 1, 1),
+      absl::CivilMinute(2018, 1, 2, 0, 0), absl::CivilMinute(2018, 1, 2, 0, 1),
+      absl::CivilMinute(2018, 1, 2, 1, 0), absl::CivilMinute(2018, 1, 2, 1, 1),
+      absl::CivilMinute(2018, 2, 1, 0, 0), absl::CivilMinute(2018, 2, 1, 0, 1),
+      absl::CivilMinute(2018, 2, 1, 1, 0), absl::CivilMinute(2018, 2, 1, 1, 1),
+      absl::CivilMinute(2018, 2, 2, 0, 0), absl::CivilMinute(2018, 2, 2, 0, 1),
+      absl::CivilMinute(2018, 2, 2, 1, 0), absl::CivilMinute(2018, 2, 2, 1, 1),
+  }));
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly({
+      absl::CivilSecond(2017, 1, 1, 0, 0, 0),
+      absl::CivilSecond(2017, 1, 1, 0, 0, 1),
+      absl::CivilSecond(2017, 1, 1, 0, 1, 0),
+      absl::CivilSecond(2017, 1, 1, 0, 1, 1),
+      absl::CivilSecond(2017, 1, 1, 1, 0, 0),
+      absl::CivilSecond(2017, 1, 1, 1, 0, 1),
+      absl::CivilSecond(2017, 1, 1, 1, 1, 0),
+      absl::CivilSecond(2017, 1, 1, 1, 1, 1),
+      absl::CivilSecond(2017, 1, 2, 0, 0, 0),
+      absl::CivilSecond(2017, 1, 2, 0, 0, 1),
+      absl::CivilSecond(2017, 1, 2, 0, 1, 0),
+      absl::CivilSecond(2017, 1, 2, 0, 1, 1),
+      absl::CivilSecond(2017, 1, 2, 1, 0, 0),
+      absl::CivilSecond(2017, 1, 2, 1, 0, 1),
+      absl::CivilSecond(2017, 1, 2, 1, 1, 0),
+      absl::CivilSecond(2017, 1, 2, 1, 1, 1),
+      absl::CivilSecond(2017, 2, 1, 0, 0, 0),
+      absl::CivilSecond(2017, 2, 1, 0, 0, 1),
+      absl::CivilSecond(2017, 2, 1, 0, 1, 0),
+      absl::CivilSecond(2017, 2, 1, 0, 1, 1),
+      absl::CivilSecond(2017, 2, 1, 1, 0, 0),
+      absl::CivilSecond(2017, 2, 1, 1, 0, 1),
+      absl::CivilSecond(2017, 2, 1, 1, 1, 0),
+      absl::CivilSecond(2017, 2, 1, 1, 1, 1),
+      absl::CivilSecond(2017, 2, 2, 0, 0, 0),
+      absl::CivilSecond(2017, 2, 2, 0, 0, 1),
+      absl::CivilSecond(2017, 2, 2, 0, 1, 0),
+      absl::CivilSecond(2017, 2, 2, 0, 1, 1),
+      absl::CivilSecond(2017, 2, 2, 1, 0, 0),
+      absl::CivilSecond(2017, 2, 2, 1, 0, 1),
+      absl::CivilSecond(2017, 2, 2, 1, 1, 0),
+      absl::CivilSecond(2017, 2, 2, 1, 1, 1),
+      absl::CivilSecond(2018, 1, 1, 0, 0, 0),
+      absl::CivilSecond(2018, 1, 1, 0, 0, 1),
+      absl::CivilSecond(2018, 1, 1, 0, 1, 0),
+      absl::CivilSecond(2018, 1, 1, 0, 1, 1),
+      absl::CivilSecond(2018, 1, 1, 1, 0, 0),
+      absl::CivilSecond(2018, 1, 1, 1, 0, 1),
+      absl::CivilSecond(2018, 1, 1, 1, 1, 0),
+      absl::CivilSecond(2018, 1, 1, 1, 1, 1),
+      absl::CivilSecond(2018, 1, 2, 0, 0, 0),
+      absl::CivilSecond(2018, 1, 2, 0, 0, 1),
+      absl::CivilSecond(2018, 1, 2, 0, 1, 0),
+      absl::CivilSecond(2018, 1, 2, 0, 1, 1),
+      absl::CivilSecond(2018, 1, 2, 1, 0, 0),
+      absl::CivilSecond(2018, 1, 2, 1, 0, 1),
+      absl::CivilSecond(2018, 1, 2, 1, 1, 0),
+      absl::CivilSecond(2018, 1, 2, 1, 1, 1),
+      absl::CivilSecond(2018, 2, 1, 0, 0, 0),
+      absl::CivilSecond(2018, 2, 1, 0, 0, 1),
+      absl::CivilSecond(2018, 2, 1, 0, 1, 0),
+      absl::CivilSecond(2018, 2, 1, 0, 1, 1),
+      absl::CivilSecond(2018, 2, 1, 1, 0, 0),
+      absl::CivilSecond(2018, 2, 1, 1, 0, 1),
+      absl::CivilSecond(2018, 2, 1, 1, 1, 0),
+      absl::CivilSecond(2018, 2, 1, 1, 1, 1),
+      absl::CivilSecond(2018, 2, 2, 0, 0, 0),
+      absl::CivilSecond(2018, 2, 2, 0, 0, 1),
+      absl::CivilSecond(2018, 2, 2, 0, 1, 0),
+      absl::CivilSecond(2018, 2, 2, 0, 1, 1),
+      absl::CivilSecond(2018, 2, 2, 1, 0, 0),
+      absl::CivilSecond(2018, 2, 2, 1, 0, 1),
+      absl::CivilSecond(2018, 2, 2, 1, 1, 0),
+      absl::CivilSecond(2018, 2, 2, 1, 1, 1),
+  }));
+}
+
 }  // namespace
diff --git a/absl/time/clock.cc b/absl/time/clock.cc
index bc6f5ccf276..b87611aca8c 100644
--- a/absl/time/clock.cc
+++ b/absl/time/clock.cc
@@ -14,12 +14,6 @@
 
 #include "absl/time/clock.h"
 
-#include "absl/base/attributes.h"
-
-#ifdef _WIN32
-#include <windows.h>
-#endif
-
 #include <algorithm>
 #include <atomic>
 #include <cerrno>
@@ -27,13 +21,20 @@
 #include <ctime>
 #include <limits>
 
+#include "absl/base/attributes.h"
 #include "absl/base/internal/spinlock.h"
 #include "absl/base/internal/unscaledcycleclock.h"
 #include "absl/base/macros.h"
+#include "absl/base/optimization.h"
 #include "absl/base/port.h"
 #include "absl/base/thread_annotations.h"
 
+#ifdef _WIN32
+#include <windows.h>
+#endif
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 Time Now() {
   // TODO(bww): Get a timespec instead so we don't have to divide.
   int64_t n = absl::GetCurrentTimeNanos();
@@ -43,19 +44,19 @@ Time Now() {
   }
   return time_internal::FromUnixDuration(absl::Nanoseconds(n));
 }
+ABSL_NAMESPACE_END
 }  // namespace absl
 
-// Decide if we should use the fast GetCurrentTimeNanos() algorithm
-// based on the cyclecounter, otherwise just get the time directly
-// from the OS on every call. This can be chosen at compile-time via
+// Decide if we should use the fast GetCurrentTimeNanos() algorithm based on the
+// cyclecounter, otherwise just get the time directly from the OS on every call.
+// By default, the fast algorithm based on the cyclecount is disabled because in
+// certain situations, for example, if the OS enters a "sleep" mode, it may
+// produce incorrect values immediately upon waking.
+// This can be chosen at compile-time via
 // -DABSL_USE_CYCLECLOCK_FOR_GET_CURRENT_TIME_NANOS=[0|1]
 #ifndef ABSL_USE_CYCLECLOCK_FOR_GET_CURRENT_TIME_NANOS
-#if ABSL_USE_UNSCALED_CYCLECLOCK
-#define ABSL_USE_CYCLECLOCK_FOR_GET_CURRENT_TIME_NANOS 1
-#else
 #define ABSL_USE_CYCLECLOCK_FOR_GET_CURRENT_TIME_NANOS 0
 #endif
-#endif
 
 #if defined(__APPLE__) || defined(_WIN32)
 #include "absl/time/internal/get_current_time_chrono.inc"
@@ -71,9 +72,9 @@ Time Now() {
 
 #if !ABSL_USE_CYCLECLOCK_FOR_GET_CURRENT_TIME_NANOS
 namespace absl {
-int64_t GetCurrentTimeNanos() {
-  return GET_CURRENT_TIME_NANOS_FROM_SYSTEM();
-}
+ABSL_NAMESPACE_BEGIN
+int64_t GetCurrentTimeNanos() { return GET_CURRENT_TIME_NANOS_FROM_SYSTEM(); }
+ABSL_NAMESPACE_END
 }  // namespace absl
 #else  // Use the cyclecounter-based implementation below.
 
@@ -83,91 +84,33 @@ int64_t GetCurrentTimeNanos() {
   ::absl::time_internal::UnscaledCycleClockWrapperForGetCurrentTime::Now()
 #endif
 
-// The following counters are used only by the test code.
-static int64_t stats_initializations;
-static int64_t stats_reinitializations;
-static int64_t stats_calibrations;
-static int64_t stats_slow_paths;
-static int64_t stats_fast_slow_paths;
-
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
+
+// On some processors, consecutive reads of the cycle counter may yield the
+// same value (weakly-increasing). In debug mode, clear the least significant
+// bits to discourage depending on a strictly-increasing Now() value.
+// In x86-64's debug mode, discourage depending on a strictly-increasing Now()
+// value.
+#if !defined(NDEBUG) && defined(__x86_64__)
+constexpr int64_t kCycleClockNowMask = ~int64_t{0xff};
+#else
+constexpr int64_t kCycleClockNowMask = ~int64_t{0};
+#endif
+
 // This is a friend wrapper around UnscaledCycleClock::Now()
 // (needed to access UnscaledCycleClock).
 class UnscaledCycleClockWrapperForGetCurrentTime {
  public:
-  static int64_t Now() { return base_internal::UnscaledCycleClock::Now(); }
+  static int64_t Now() {
+    return base_internal::UnscaledCycleClock::Now() & kCycleClockNowMask;
+  }
 };
 }  // namespace time_internal
 
 // uint64_t is used in this module to provide an extra bit in multiplications
 
-// Return the time in ns as told by the kernel interface.  Place in *cycleclock
-// the value of the cycleclock at about the time of the syscall.
-// This call represents the time base that this module synchronizes to.
-// Ensures that *cycleclock does not step back by up to (1 << 16) from
-// last_cycleclock, to discard small backward counter steps.  (Larger steps are
-// assumed to be complete resyncs, which shouldn't happen.  If they do, a full
-// reinitialization of the outer algorithm should occur.)
-static int64_t GetCurrentTimeNanosFromKernel(uint64_t last_cycleclock,
-                                             uint64_t *cycleclock) {
-  // We try to read clock values at about the same time as the kernel clock.
-  // This value gets adjusted up or down as estimate of how long that should
-  // take, so we can reject attempts that take unusually long.
-  static std::atomic<uint64_t> approx_syscall_time_in_cycles{10 * 1000};
-
-  uint64_t local_approx_syscall_time_in_cycles =  // local copy
-      approx_syscall_time_in_cycles.load(std::memory_order_relaxed);
-
-  int64_t current_time_nanos_from_system;
-  uint64_t before_cycles;
-  uint64_t after_cycles;
-  uint64_t elapsed_cycles;
-  int loops = 0;
-  do {
-    before_cycles = GET_CURRENT_TIME_NANOS_CYCLECLOCK_NOW();
-    current_time_nanos_from_system = GET_CURRENT_TIME_NANOS_FROM_SYSTEM();
-    after_cycles = GET_CURRENT_TIME_NANOS_CYCLECLOCK_NOW();
-    // elapsed_cycles is unsigned, so is large on overflow
-    elapsed_cycles = after_cycles - before_cycles;
-    if (elapsed_cycles >= local_approx_syscall_time_in_cycles &&
-        ++loops == 20) {  // clock changed frequencies?  Back off.
-      loops = 0;
-      if (local_approx_syscall_time_in_cycles < 1000 * 1000) {
-        local_approx_syscall_time_in_cycles =
-            (local_approx_syscall_time_in_cycles + 1) << 1;
-      }
-      approx_syscall_time_in_cycles.store(
-          local_approx_syscall_time_in_cycles,
-          std::memory_order_relaxed);
-    }
-  } while (elapsed_cycles >= local_approx_syscall_time_in_cycles ||
-           last_cycleclock - after_cycles < (static_cast<uint64_t>(1) << 16));
-
-  // Number of times in a row we've seen a kernel time call take substantially
-  // less than approx_syscall_time_in_cycles.
-  static std::atomic<uint32_t> seen_smaller{ 0 };
-
-  // Adjust approx_syscall_time_in_cycles to be within a factor of 2
-  // of the typical time to execute one iteration of the loop above.
-  if ((local_approx_syscall_time_in_cycles >> 1) < elapsed_cycles) {
-    // measured time is no smaller than half current approximation
-    seen_smaller.store(0, std::memory_order_relaxed);
-  } else if (seen_smaller.fetch_add(1, std::memory_order_relaxed) >= 3) {
-    // smaller delays several times in a row; reduce approximation by 12.5%
-    const uint64_t new_approximation =
-        local_approx_syscall_time_in_cycles -
-        (local_approx_syscall_time_in_cycles >> 3);
-    approx_syscall_time_in_cycles.store(new_approximation,
-                                        std::memory_order_relaxed);
-    seen_smaller.store(0, std::memory_order_relaxed);
-  }
-
-  *cycleclock = after_cycles;
-  return current_time_nanos_from_system;
-}
-
-
 // ---------------------------------------------------------------------
 // An implementation of reader-write locks that use no atomic ops in the read
 // case.  This is a generalization of Lamport's method for reading a multiword
@@ -181,7 +124,7 @@ static int64_t GetCurrentTimeNanosFromKernel(uint64_t last_cycleclock,
 // spin-delay tuning.
 
 // Acquire seqlock (*seq) and return the value to be written to unlock.
-static inline uint64_t SeqAcquire(std::atomic<uint64_t> *seq) {
+static inline uint64_t SeqAcquire(std::atomic<uint64_t>* seq) {
   uint64_t x = seq->fetch_add(1, std::memory_order_relaxed);
 
   // We put a release fence between update to *seq and writes to shared data.
@@ -191,12 +134,12 @@ static inline uint64_t SeqAcquire(std::atomic<uint64_t> *seq) {
   // fetch_add would be before it, not after.
   std::atomic_thread_fence(std::memory_order_release);
 
-  return x + 2;   // original word plus 2
+  return x + 2;  // original word plus 2
 }
 
 // Release seqlock (*seq) by writing x to it---a value previously returned by
 // SeqAcquire.
-static inline void SeqRelease(std::atomic<uint64_t> *seq, uint64_t x) {
+static inline void SeqRelease(std::atomic<uint64_t>* seq, uint64_t x) {
   // The unlock store to *seq must have release ordering so that all
   // updates to shared data must finish before this store.
   seq->store(x, std::memory_order_release);  // release lock for readers
@@ -216,43 +159,122 @@ static const uint64_t kMinNSBetweenSamples = 2000 << 20;
 // We require that kMinNSBetweenSamples shifted by kScale
 // have at least a bit left over for 64-bit calculations.
 static_assert(((kMinNSBetweenSamples << (kScale + 1)) >> (kScale + 1)) ==
-               kMinNSBetweenSamples,
-               "cannot represent kMaxBetweenSamplesNSScaled");
-
-// A reader-writer lock protecting the static locations below.
-// See SeqAcquire() and SeqRelease() above.
-static absl::base_internal::SpinLock lock(
-    absl::base_internal::kLinkerInitialized);
-static std::atomic<uint64_t> seq(0);
+                  kMinNSBetweenSamples,
+              "cannot represent kMaxBetweenSamplesNSScaled");
 
 // data from a sample of the kernel's time value
 struct TimeSampleAtomic {
-  std::atomic<uint64_t> raw_ns;              // raw kernel time
-  std::atomic<uint64_t> base_ns;             // our estimate of time
-  std::atomic<uint64_t> base_cycles;         // cycle counter reading
-  std::atomic<uint64_t> nsscaled_per_cycle;  // cycle period
+  std::atomic<uint64_t> raw_ns{0};              // raw kernel time
+  std::atomic<uint64_t> base_ns{0};             // our estimate of time
+  std::atomic<uint64_t> base_cycles{0};         // cycle counter reading
+  std::atomic<uint64_t> nsscaled_per_cycle{0};  // cycle period
   // cycles before we'll sample again (a scaled reciprocal of the period,
   // to avoid a division on the fast path).
-  std::atomic<uint64_t> min_cycles_per_sample;
+  std::atomic<uint64_t> min_cycles_per_sample{0};
 };
 // Same again, but with non-atomic types
 struct TimeSample {
-  uint64_t raw_ns;                 // raw kernel time
-  uint64_t base_ns;                // our estimate of time
-  uint64_t base_cycles;            // cycle counter reading
-  uint64_t nsscaled_per_cycle;     // cycle period
-  uint64_t min_cycles_per_sample;  // approx cycles before next sample
+  uint64_t raw_ns = 0;                 // raw kernel time
+  uint64_t base_ns = 0;                // our estimate of time
+  uint64_t base_cycles = 0;            // cycle counter reading
+  uint64_t nsscaled_per_cycle = 0;     // cycle period
+  uint64_t min_cycles_per_sample = 0;  // approx cycles before next sample
 };
 
-static struct TimeSampleAtomic last_sample;   // the last sample; under seq
+struct ABSL_CACHELINE_ALIGNED TimeState {
+  std::atomic<uint64_t> seq{0};
+  TimeSampleAtomic last_sample;  // the last sample; under seq
+
+  // The following counters are used only by the test code.
+  int64_t stats_initializations{0};
+  int64_t stats_reinitializations{0};
+  int64_t stats_calibrations{0};
+  int64_t stats_slow_paths{0};
+  int64_t stats_fast_slow_paths{0};
+
+  uint64_t last_now_cycles ABSL_GUARDED_BY(lock){0};
+
+  // Used by GetCurrentTimeNanosFromKernel().
+  // We try to read clock values at about the same time as the kernel clock.
+  // This value gets adjusted up or down as estimate of how long that should
+  // take, so we can reject attempts that take unusually long.
+  std::atomic<uint64_t> approx_syscall_time_in_cycles{10 * 1000};
+  // Number of times in a row we've seen a kernel time call take substantially
+  // less than approx_syscall_time_in_cycles.
+  std::atomic<uint32_t> kernel_time_seen_smaller{0};
+
+  // A reader-writer lock protecting the static locations below.
+  // See SeqAcquire() and SeqRelease() above.
+  absl::base_internal::SpinLock lock{base_internal::SCHEDULE_KERNEL_ONLY};
+};
+ABSL_CONST_INIT static TimeState time_state;
+
+// Return the time in ns as told by the kernel interface.  Place in *cycleclock
+// the value of the cycleclock at about the time of the syscall.
+// This call represents the time base that this module synchronizes to.
+// Ensures that *cycleclock does not step back by up to (1 << 16) from
+// last_cycleclock, to discard small backward counter steps.  (Larger steps are
+// assumed to be complete resyncs, which shouldn't happen.  If they do, a full
+// reinitialization of the outer algorithm should occur.)
+static int64_t GetCurrentTimeNanosFromKernel(uint64_t last_cycleclock,
+                                             uint64_t* cycleclock)
+    ABSL_EXCLUSIVE_LOCKS_REQUIRED(time_state.lock) {
+  uint64_t local_approx_syscall_time_in_cycles =  // local copy
+      time_state.approx_syscall_time_in_cycles.load(std::memory_order_relaxed);
+
+  int64_t current_time_nanos_from_system;
+  uint64_t before_cycles;
+  uint64_t after_cycles;
+  uint64_t elapsed_cycles;
+  int loops = 0;
+  do {
+    before_cycles =
+        static_cast<uint64_t>(GET_CURRENT_TIME_NANOS_CYCLECLOCK_NOW());
+    current_time_nanos_from_system = GET_CURRENT_TIME_NANOS_FROM_SYSTEM();
+    after_cycles =
+        static_cast<uint64_t>(GET_CURRENT_TIME_NANOS_CYCLECLOCK_NOW());
+    // elapsed_cycles is unsigned, so is large on overflow
+    elapsed_cycles = after_cycles - before_cycles;
+    if (elapsed_cycles >= local_approx_syscall_time_in_cycles &&
+        ++loops == 20) {  // clock changed frequencies?  Back off.
+      loops = 0;
+      if (local_approx_syscall_time_in_cycles < 1000 * 1000) {
+        local_approx_syscall_time_in_cycles =
+            (local_approx_syscall_time_in_cycles + 1) << 1;
+      }
+      time_state.approx_syscall_time_in_cycles.store(
+          local_approx_syscall_time_in_cycles, std::memory_order_relaxed);
+    }
+  } while (elapsed_cycles >= local_approx_syscall_time_in_cycles ||
+           last_cycleclock - after_cycles < (static_cast<uint64_t>(1) << 16));
+
+  // Adjust approx_syscall_time_in_cycles to be within a factor of 2
+  // of the typical time to execute one iteration of the loop above.
+  if ((local_approx_syscall_time_in_cycles >> 1) < elapsed_cycles) {
+    // measured time is no smaller than half current approximation
+    time_state.kernel_time_seen_smaller.store(0, std::memory_order_relaxed);
+  } else if (time_state.kernel_time_seen_smaller.fetch_add(
+                 1, std::memory_order_relaxed) >= 3) {
+    // smaller delays several times in a row; reduce approximation by 12.5%
+    const uint64_t new_approximation =
+        local_approx_syscall_time_in_cycles -
+        (local_approx_syscall_time_in_cycles >> 3);
+    time_state.approx_syscall_time_in_cycles.store(new_approximation,
+                                                   std::memory_order_relaxed);
+    time_state.kernel_time_seen_smaller.store(0, std::memory_order_relaxed);
+  }
+
+  *cycleclock = after_cycles;
+  return current_time_nanos_from_system;
+}
 
 static int64_t GetCurrentTimeNanosSlowPath() ABSL_ATTRIBUTE_COLD;
 
 // Read the contents of *atomic into *sample.
 // Each field is read atomically, but to maintain atomicity between fields,
 // the access must be done under a lock.
-static void ReadTimeSampleAtomic(const struct TimeSampleAtomic *atomic,
-                                 struct TimeSample *sample) {
+static void ReadTimeSampleAtomic(const struct TimeSampleAtomic* atomic,
+                                 struct TimeSample* sample) {
   sample->base_ns = atomic->base_ns.load(std::memory_order_relaxed);
   sample->base_cycles = atomic->base_cycles.load(std::memory_order_relaxed);
   sample->nsscaled_per_cycle =
@@ -307,30 +329,34 @@ int64_t GetCurrentTimeNanos() {
   // contribute to register pressure - reading it early before initializing
   // the other pieces of the calculation minimizes spill/restore instructions,
   // minimizing icache cost.
-  uint64_t now_cycles = GET_CURRENT_TIME_NANOS_CYCLECLOCK_NOW();
+  uint64_t now_cycles =
+      static_cast<uint64_t>(GET_CURRENT_TIME_NANOS_CYCLECLOCK_NOW());
 
   // Acquire pairs with the barrier in SeqRelease - if this load sees that
   // store, the shared-data reads necessarily see that SeqRelease's updates
   // to the same shared data.
-  seq_read0 = seq.load(std::memory_order_acquire);
-
-  base_ns = last_sample.base_ns.load(std::memory_order_relaxed);
-  base_cycles = last_sample.base_cycles.load(std::memory_order_relaxed);
+  seq_read0 = time_state.seq.load(std::memory_order_acquire);
+
+  // The algorithm does not require that the following four loads be ordered
+  // with respect to one another; it requires only that they precede the load of
+  // time_state.seq below them. Nevertheless, we mark each of them as an
+  // acquire-load, rather than using a barrier immediately before the
+  // time_state.seq load, because the former is likely faster on most CPUs of
+  // interest. Architectures that may see a regression because of this approach
+  // include PowerPC and MIPS.
+  base_ns = time_state.last_sample.base_ns.load(std::memory_order_acquire);
+  base_cycles =
+      time_state.last_sample.base_cycles.load(std::memory_order_acquire);
   nsscaled_per_cycle =
-      last_sample.nsscaled_per_cycle.load(std::memory_order_relaxed);
-  min_cycles_per_sample =
-      last_sample.min_cycles_per_sample.load(std::memory_order_relaxed);
-
-  // This acquire fence pairs with the release fence in SeqAcquire.  Since it
-  // is sequenced between reads of shared data and seq_read1, the reads of
-  // shared data are effectively acquiring.
-  std::atomic_thread_fence(std::memory_order_acquire);
+      time_state.last_sample.nsscaled_per_cycle.load(std::memory_order_acquire);
+  min_cycles_per_sample = time_state.last_sample.min_cycles_per_sample.load(
+      std::memory_order_acquire);
 
   // The shared-data reads are effectively acquire ordered, and the
   // shared-data writes are effectively release ordered. Therefore if our
   // shared-data reads see any of a particular update's shared-data writes,
   // seq_read1 is guaranteed to see that update's SeqAcquire.
-  seq_read1 = seq.load(std::memory_order_relaxed);
+  seq_read1 = time_state.seq.load(std::memory_order_relaxed);
 
   // Fast path.  Return if min_cycles_per_sample has not yet elapsed since the
   // last sample, and we read a consistent sample.  The fast path activates
@@ -343,10 +369,11 @@ int64_t GetCurrentTimeNanos() {
   // last_sample was updated). This is harmless, because delta_cycles will wrap
   // and report a time much much bigger than min_cycles_per_sample. In that case
   // we will take the slow path.
-  uint64_t delta_cycles = now_cycles - base_cycles;
+  uint64_t delta_cycles;
   if (seq_read0 == seq_read1 && (seq_read0 & 1) == 0 &&
-      delta_cycles < min_cycles_per_sample) {
-    return base_ns + ((delta_cycles * nsscaled_per_cycle) >> kScale);
+      (delta_cycles = now_cycles - base_cycles) < min_cycles_per_sample) {
+    return static_cast<int64_t>(
+        base_ns + ((delta_cycles * nsscaled_per_cycle) >> kScale));
   }
   return GetCurrentTimeNanosSlowPath();
 }
@@ -371,7 +398,7 @@ static uint64_t SafeDivideAndScale(uint64_t a, uint64_t b) {
 
 static uint64_t UpdateLastSample(
     uint64_t now_cycles, uint64_t now_ns, uint64_t delta_cycles,
-    const struct TimeSample *sample) ABSL_ATTRIBUTE_COLD;
+    const struct TimeSample* sample) ABSL_ATTRIBUTE_COLD;
 
 // The slow path of GetCurrentTimeNanos().  This is taken while gathering
 // initial samples, when enough time has elapsed since the last sample, and if
@@ -385,24 +412,25 @@ static uint64_t UpdateLastSample(
 // TODO(absl-team): Remove this attribute when our compiler is smart enough
 // to do the right thing.
 ABSL_ATTRIBUTE_NOINLINE
-static int64_t GetCurrentTimeNanosSlowPath() LOCKS_EXCLUDED(lock) {
+static int64_t GetCurrentTimeNanosSlowPath()
+    ABSL_LOCKS_EXCLUDED(time_state.lock) {
   // Serialize access to slow-path.  Fast-path readers are not blocked yet, and
   // code below must not modify last_sample until the seqlock is acquired.
-  lock.Lock();
+  base_internal::SpinLockHolder l(time_state.lock);
 
   // Sample the kernel time base.  This is the definition of
   // "now" if we take the slow path.
-  static uint64_t last_now_cycles;  // protected by lock
   uint64_t now_cycles;
-  uint64_t now_ns = GetCurrentTimeNanosFromKernel(last_now_cycles, &now_cycles);
-  last_now_cycles = now_cycles;
+  uint64_t now_ns = static_cast<uint64_t>(
+      GetCurrentTimeNanosFromKernel(time_state.last_now_cycles, &now_cycles));
+  time_state.last_now_cycles = now_cycles;
 
   uint64_t estimated_base_ns;
 
   // ----------
   // Read the "last_sample" values again; this time holding the write lock.
   struct TimeSample sample;
-  ReadTimeSampleAtomic(&last_sample, &sample);
+  ReadTimeSampleAtomic(&time_state.last_sample, &sample);
 
   // ----------
   // Try running the fast path again; another thread may have updated the
@@ -411,17 +439,15 @@ static int64_t GetCurrentTimeNanosSlowPath() LOCKS_EXCLUDED(lock) {
   if (delta_cycles < sample.min_cycles_per_sample) {
     // Another thread updated the sample.  This path does not take the seqlock
     // so that blocked readers can make progress without blocking new readers.
-    estimated_base_ns = sample.base_ns +
-        ((delta_cycles * sample.nsscaled_per_cycle) >> kScale);
-    stats_fast_slow_paths++;
+    estimated_base_ns =
+        sample.base_ns + ((delta_cycles * sample.nsscaled_per_cycle) >> kScale);
+    time_state.stats_fast_slow_paths++;
   } else {
     estimated_base_ns =
         UpdateLastSample(now_cycles, now_ns, delta_cycles, &sample);
   }
 
-  lock.Unlock();
-
-  return estimated_base_ns;
+  return static_cast<int64_t>(estimated_base_ns);
 }
 
 // Main part of the algorithm.  Locks out readers, updates the approximation
@@ -429,10 +455,11 @@ static int64_t GetCurrentTimeNanosSlowPath() LOCKS_EXCLUDED(lock) {
 // for readers.  Returns the new estimated time.
 static uint64_t UpdateLastSample(uint64_t now_cycles, uint64_t now_ns,
                                  uint64_t delta_cycles,
-                                 const struct TimeSample *sample)
-    EXCLUSIVE_LOCKS_REQUIRED(lock) {
+                                 const struct TimeSample* sample)
+    ABSL_EXCLUSIVE_LOCKS_REQUIRED(time_state.lock) {
   uint64_t estimated_base_ns = now_ns;
-  uint64_t lock_value = SeqAcquire(&seq);  // acquire seqlock to block readers
+  uint64_t lock_value =
+      SeqAcquire(&time_state.seq);  // acquire seqlock to block readers
 
   // The 5s in the next if-statement limits the time for which we will trust
   // the cycle counter and our last sample to give a reasonable result.
@@ -442,12 +469,16 @@ static uint64_t UpdateLastSample(uint64_t now_cycles, uint64_t now_ns,
       sample->raw_ns + static_cast<uint64_t>(5) * 1000 * 1000 * 1000 < now_ns ||
       now_ns < sample->raw_ns || now_cycles < sample->base_cycles) {
     // record this sample, and forget any previously known slope.
-    last_sample.raw_ns.store(now_ns, std::memory_order_relaxed);
-    last_sample.base_ns.store(estimated_base_ns, std::memory_order_relaxed);
-    last_sample.base_cycles.store(now_cycles, std::memory_order_relaxed);
-    last_sample.nsscaled_per_cycle.store(0, std::memory_order_relaxed);
-    last_sample.min_cycles_per_sample.store(0, std::memory_order_relaxed);
-    stats_initializations++;
+    time_state.last_sample.raw_ns.store(now_ns, std::memory_order_relaxed);
+    time_state.last_sample.base_ns.store(estimated_base_ns,
+                                         std::memory_order_relaxed);
+    time_state.last_sample.base_cycles.store(now_cycles,
+                                             std::memory_order_relaxed);
+    time_state.last_sample.nsscaled_per_cycle.store(0,
+                                                    std::memory_order_relaxed);
+    time_state.last_sample.min_cycles_per_sample.store(
+        0, std::memory_order_relaxed);
+    time_state.stats_initializations++;
   } else if (sample->raw_ns + 500 * 1000 * 1000 < now_ns &&
              sample->base_cycles + 50 < now_cycles) {
     // Enough time has passed to compute the cycle time.
@@ -461,8 +492,8 @@ static uint64_t UpdateLastSample(uint64_t now_cycles, uint64_t now_ns,
         estimated_scaled_ns = (delta_cycles >> s) * sample->nsscaled_per_cycle;
       } while (estimated_scaled_ns / sample->nsscaled_per_cycle !=
                (delta_cycles >> s));
-      estimated_base_ns = sample->base_ns +
-                          (estimated_scaled_ns >> (kScale - s));
+      estimated_base_ns =
+          sample->base_ns + (estimated_scaled_ns >> (kScale - s));
     }
 
     // Compute the assumed cycle time kMinNSBetweenSamples ns into the future
@@ -473,7 +504,8 @@ static uint64_t UpdateLastSample(uint64_t now_cycles, uint64_t now_ns,
     uint64_t assumed_next_sample_delta_cycles =
         SafeDivideAndScale(kMinNSBetweenSamples, measured_nsscaled_per_cycle);
 
-    int64_t diff_ns = now_ns - estimated_base_ns;  // estimate low by this much
+    // Estimate low by this much.
+    int64_t diff_ns = static_cast<int64_t>(now_ns - estimated_base_ns);
 
     // We want to set nsscaled_per_cycle so that our estimate of the ns time
     // at the assumed cycle time is the assumed ns time.
@@ -484,41 +516,48 @@ static uint64_t UpdateLastSample(uint64_t now_cycles, uint64_t now_ns,
     // of our current error, by solving:
     //  kMinNSBetweenSamples + diff_ns - (diff_ns / 16) ==
     //  (assumed_next_sample_delta_cycles * nsscaled_per_cycle) >> kScale
-    ns = kMinNSBetweenSamples + diff_ns - (diff_ns / 16);
+    ns = static_cast<uint64_t>(static_cast<int64_t>(kMinNSBetweenSamples) +
+                               diff_ns - (diff_ns / 16));
     uint64_t new_nsscaled_per_cycle =
         SafeDivideAndScale(ns, assumed_next_sample_delta_cycles);
-    if (new_nsscaled_per_cycle != 0 &&
-        diff_ns < 100 * 1000 * 1000 && -diff_ns < 100 * 1000 * 1000) {
+    if (new_nsscaled_per_cycle != 0 && diff_ns < 100 * 1000 * 1000 &&
+        -diff_ns < 100 * 1000 * 1000) {
       // record the cycle time measurement
-      last_sample.nsscaled_per_cycle.store(
+      time_state.last_sample.nsscaled_per_cycle.store(
           new_nsscaled_per_cycle, std::memory_order_relaxed);
       uint64_t new_min_cycles_per_sample =
           SafeDivideAndScale(kMinNSBetweenSamples, new_nsscaled_per_cycle);
-      last_sample.min_cycles_per_sample.store(
+      time_state.last_sample.min_cycles_per_sample.store(
           new_min_cycles_per_sample, std::memory_order_relaxed);
-      stats_calibrations++;
+      time_state.stats_calibrations++;
     } else {  // something went wrong; forget the slope
-      last_sample.nsscaled_per_cycle.store(0, std::memory_order_relaxed);
-      last_sample.min_cycles_per_sample.store(0, std::memory_order_relaxed);
+      time_state.last_sample.nsscaled_per_cycle.store(
+          0, std::memory_order_relaxed);
+      time_state.last_sample.min_cycles_per_sample.store(
+          0, std::memory_order_relaxed);
       estimated_base_ns = now_ns;
-      stats_reinitializations++;
+      time_state.stats_reinitializations++;
     }
-    last_sample.raw_ns.store(now_ns, std::memory_order_relaxed);
-    last_sample.base_ns.store(estimated_base_ns, std::memory_order_relaxed);
-    last_sample.base_cycles.store(now_cycles, std::memory_order_relaxed);
+    time_state.last_sample.raw_ns.store(now_ns, std::memory_order_relaxed);
+    time_state.last_sample.base_ns.store(estimated_base_ns,
+                                         std::memory_order_relaxed);
+    time_state.last_sample.base_cycles.store(now_cycles,
+                                             std::memory_order_relaxed);
   } else {
     // have a sample, but no slope; waiting for enough time for a calibration
-    stats_slow_paths++;
+    time_state.stats_slow_paths++;
   }
 
-  SeqRelease(&seq, lock_value);  // release the readers
+  SeqRelease(&time_state.seq, lock_value);  // release the readers
 
   return estimated_base_ns;
 }
+ABSL_NAMESPACE_END
 }  // namespace absl
 #endif  // ABSL_USE_CYCLECLOCK_FOR_GET_CURRENT_TIME_NANOS
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace {
 
 // Returns the maximum duration that SleepOnce() can sleep for.
@@ -536,7 +575,7 @@ constexpr absl::Duration MaxSleep() {
 // REQUIRES: to_sleep <= MaxSleep().
 void SleepOnce(absl::Duration to_sleep) {
 #ifdef _WIN32
-  Sleep(to_sleep / absl::Milliseconds(1));
+  Sleep(static_cast<DWORD>(to_sleep / absl::Milliseconds(1)));
 #else
   struct timespec sleep_time = absl::ToTimespec(to_sleep);
   while (nanosleep(&sleep_time, &sleep_time) != 0 && errno == EINTR) {
@@ -546,11 +585,13 @@ void SleepOnce(absl::Duration to_sleep) {
 }
 
 }  // namespace
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 extern "C" {
 
-ABSL_ATTRIBUTE_WEAK void AbslInternalSleepFor(absl::Duration duration) {
+ABSL_ATTRIBUTE_WEAK void ABSL_INTERNAL_C_SYMBOL(AbslInternalSleepFor)(
+    absl::Duration duration) {
   while (duration > absl::ZeroDuration()) {
     absl::Duration to_sleep = std::min(duration, absl::MaxSleep());
     absl::SleepOnce(to_sleep);
diff --git a/absl/time/clock.h b/absl/time/clock.h
index bb52e4f6ca9..2be89a2598a 100644
--- a/absl/time/clock.h
+++ b/absl/time/clock.h
@@ -22,14 +22,21 @@
 #ifndef ABSL_TIME_CLOCK_H_
 #define ABSL_TIME_CLOCK_H_
 
+#include <cstdint>
+
+#include "absl/base/config.h"
 #include "absl/base/macros.h"
 #include "absl/time/time.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 // Now()
 //
 // Returns the current time, expressed as an `absl::Time` absolute time value.
+//
+// To improve testability, consider injecting an absl::Clock and using
+// absl::Clock::TimeNow() instead.
 absl::Time Now();
 
 // GetCurrentTimeNanos()
@@ -49,6 +56,7 @@ int64_t GetCurrentTimeNanos();
 // * Returns immediately when passed a nonpositive duration.
 void SleepFor(absl::Duration duration);
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 // -----------------------------------------------------------------------------
@@ -62,11 +70,12 @@ void SleepFor(absl::Duration duration);
 // By changing our extension points to be extern "C", we dodge this
 // check.
 extern "C" {
-void AbslInternalSleepFor(absl::Duration duration);
+ABSL_DLL void ABSL_INTERNAL_C_SYMBOL(AbslInternalSleepFor)(
+    absl::Duration duration);
 }  // extern "C"
 
 inline void absl::SleepFor(absl::Duration duration) {
-  AbslInternalSleepFor(duration);
+  ABSL_INTERNAL_C_SYMBOL(AbslInternalSleepFor)(duration);
 }
 
 #endif  // ABSL_TIME_CLOCK_H_
diff --git a/absl/time/clock_benchmark.cc b/absl/time/clock_benchmark.cc
index c5c795ecbd2..df14ce6ff5d 100644
--- a/absl/time/clock_benchmark.cc
+++ b/absl/time/clock_benchmark.cc
@@ -13,15 +13,16 @@
 
 #include "absl/time/clock.h"
 
+#include <cstdio>
+
+#include "absl/base/internal/cycleclock.h"
+#include "benchmark/benchmark.h"
+
 #if !defined(_WIN32)
 #include <sys/time.h>
 #else
 #include <winsock2.h>
 #endif  // _WIN32
-#include <cstdio>
-
-#include "absl/base/internal/cycleclock.h"
-#include "benchmark/benchmark.h"
 
 namespace {
 
diff --git a/absl/time/clock_interface.cc b/absl/time/clock_interface.cc
new file mode 100644
index 00000000000..800d0b1d9ec
--- /dev/null
+++ b/absl/time/clock_interface.cc
@@ -0,0 +1,71 @@
+// Copyright 2026 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/time/clock_interface.h"
+
+#include "absl/base/config.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/base/no_destructor.h"
+#include "absl/synchronization/mutex.h"
+#include "absl/time/clock.h"
+#include "absl/time/time.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+namespace {
+
+class RealTimeClock final : public Clock {
+ private:
+#ifdef _MSC_VER
+  // Disable MSVC warning "destructor never returns, potential memory leak"
+#pragma warning(push)
+#pragma warning(disable : 4722)
+#endif
+  ~RealTimeClock() override {
+    ABSL_RAW_LOG(FATAL, "RealTimeClock should never be destroyed");
+  }
+#ifdef _MSC_VER
+#pragma warning(pop)
+#endif
+
+ public:
+  absl::Time TimeNow() override { return absl::Now(); }
+
+  void Sleep(absl::Duration d) override { absl::SleepFor(d); }
+
+  void SleepUntil(absl::Time wakeup_time) override {
+    absl::Duration d = wakeup_time - TimeNow();
+    if (d > absl::ZeroDuration()) {
+      Sleep(d);
+    }
+  }
+
+  bool AwaitWithDeadline(absl::Mutex* mu, const absl::Condition& cond,
+                         absl::Time deadline) override {
+    return mu->AwaitWithDeadline(cond, deadline);
+  }
+};
+
+}  // namespace
+
+Clock::~Clock() = default;  // go/key-method
+
+Clock& Clock::GetRealClock() {
+  static absl::NoDestructor<RealTimeClock> rtclock;
+  return *rtclock;
+}
+
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/time/clock_interface.h b/absl/time/clock_interface.h
new file mode 100644
index 00000000000..a14d85291a2
--- /dev/null
+++ b/absl/time/clock_interface.h
@@ -0,0 +1,93 @@
+// Copyright 2026 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: clock_interface.h
+// -----------------------------------------------------------------------------
+
+#ifndef ABSL_TIME_CLOCK_INTERFACE_H_
+#define ABSL_TIME_CLOCK_INTERFACE_H_
+
+#include "absl/base/config.h"
+#include "absl/base/macros.h"
+#include "absl/base/nullability.h"
+#include "absl/synchronization/mutex.h"
+#include "absl/time/time.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+// An abstract interface representing a Clock, which is an object that can
+// tell you the current time, sleep, and wait for a condition variable.
+//
+// This interface allows decoupling code that uses time from the code that
+// creates a point in time.  You can use this to your advantage by injecting
+// Clocks into interfaces rather than having implementations call absl::Now()
+// directly.
+//
+// Implementations of this interface must be thread-safe.
+//
+// The Clock::GetRealClock() function returns a reference to the global realtime
+// clock.
+//
+// Example:
+//
+//   bool IsWeekend(Clock& clock) {
+//     absl::Time now = clock.TimeNow();
+//     // ... code to check if 'now' is a weekend.
+//   }
+//
+//   // Production code.
+//   IsWeekend(Clock::GetRealClock());
+//
+//   // Test code:
+//   MyTestClock test_clock(SATURDAY);
+//   IsWeekend(test_clock);
+//
+class Clock {
+ public:
+  // Returns a reference to the global realtime clock.
+  // The returned clock is thread-safe.
+  static Clock& GetRealClock();
+
+  virtual ~Clock();
+
+  // Returns the current time.
+  virtual absl::Time TimeNow() = 0;
+
+  // Sleeps for the specified duration.
+  virtual void Sleep(absl::Duration d) = 0;
+
+  // Sleeps until the specified time.
+  virtual void SleepUntil(absl::Time wakeup_time) = 0;
+
+  // Returns when cond is true or the deadline has passed.  Returns true iff
+  // cond holds when returning.
+  //
+  // Requires *mu to be held at least in shared mode.  It will be held when
+  // evaluating cond, and upon return, but it may be released and reacquired
+  // in the meantime.
+  //
+  // This method is similar to mu->AwaitWithDeadline() except that the
+  // latter only works with real-time deadlines.  This call works properly
+  // with simulated time if invoked on a simulated clock.
+  virtual bool AwaitWithDeadline(absl::Mutex* absl_nonnull mu,
+                                 const absl::Condition& cond,
+                                 absl::Time deadline) = 0;
+};
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_TIME_CLOCK_INTERFACE_H_
diff --git a/absl/time/clock_interface_test.cc b/absl/time/clock_interface_test.cc
new file mode 100644
index 00000000000..6434bc075b7
--- /dev/null
+++ b/absl/time/clock_interface_test.cc
@@ -0,0 +1,128 @@
+// Copyright 2026 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/time/clock_interface.h"
+
+#include <cstdint>
+#include <functional>
+#include <thread>  // NOLINT(build/c++11)
+#include <vector>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/functional/bind_front.h"
+#include "absl/synchronization/mutex.h"
+#include "absl/synchronization/notification.h"
+#include "absl/time/clock.h"
+#include "absl/time/time.h"
+
+namespace {
+
+constexpr absl::Duration kLongPause = absl::Milliseconds(150);
+
+#ifdef _MSC_VER
+// As of 2026-01-29, multithreaded tests on MSVC are too flaky.
+const char* kSkipFlakyReason =
+    "Skipping this timing test because it is too flaky";
+#else
+const char* kSkipFlakyReason = nullptr;
+#endif
+
+
+void AwaitWithDeadlineAndNotify(absl::Clock* clock, absl::Mutex* mu,
+                                absl::Condition* cond, absl::Time wakeup_time,
+                                absl::Notification* note, bool* return_val) {
+  mu->lock_shared();
+  *return_val = clock->AwaitWithDeadline(mu, *cond, wakeup_time);
+  mu->unlock_shared();
+  note->Notify();
+}
+
+TEST(RealClockTest, AwaitWithVeryLargeDeadline) {
+  if (kSkipFlakyReason != nullptr) {
+    GTEST_SKIP() << kSkipFlakyReason;
+  }
+
+  absl::Clock& clock = absl::Clock::GetRealClock();
+  absl::Mutex mu;
+  bool f = false;
+  absl::Condition cond(&f);
+  absl::Notification note;
+  bool return_val;
+
+  std::thread thread(AwaitWithDeadlineAndNotify, &clock, &mu, &cond,
+                     absl::InfiniteFuture(), &note, &return_val);
+
+  EXPECT_FALSE(note.HasBeenNotified());
+
+  mu.lock();
+  f = true;
+  mu.unlock();
+
+  absl::SleepFor(kLongPause);
+  EXPECT_TRUE(note.HasBeenNotified());
+  EXPECT_TRUE(return_val);
+  note.WaitForNotification();  // In case the expectation fails.
+  thread.join();
+}
+
+TEST(RealClockTest, AwaitWithPastDeadline) {
+  if (kSkipFlakyReason != nullptr) {
+    GTEST_SKIP() << kSkipFlakyReason;
+  }
+
+  absl::Clock& clock = absl::Clock::GetRealClock();
+  absl::Mutex mu;
+  bool f = false;
+  absl::Condition cond(&f);
+  absl::Notification note;
+  bool return_val;
+
+  std::thread thread(AwaitWithDeadlineAndNotify, &clock, &mu, &cond,
+                     clock.TimeNow() - absl::Seconds(10), &note, &return_val);
+  absl::Time start = absl::Now();
+  note.WaitForNotification();  // AwaitWithDeadline() should ping note.
+  EXPECT_GE(kLongPause, absl::Now() - start);
+  EXPECT_FALSE(return_val);
+  thread.join();
+}
+
+TEST(RealClockTest, AwaitWithSmallDeadline) {
+  if (kSkipFlakyReason != nullptr) {
+    GTEST_SKIP() << kSkipFlakyReason;
+  }
+
+  absl::Clock& clock = absl::Clock::GetRealClock();
+  absl::Mutex mu;
+  bool f = false;
+  absl::Condition cond(&f);
+
+  for (int i = 0; i < 5; ++i) {
+    absl::Duration wait_time = absl::Milliseconds(i);
+    absl::Duration elapsed_time;
+    {
+      absl::MutexLock lock(mu);
+      absl::Time start = clock.TimeNow();
+      absl::Time wakeup_time = start + wait_time;
+      EXPECT_FALSE(clock.AwaitWithDeadline(&mu, cond, wakeup_time));
+      elapsed_time = clock.TimeNow() - start;
+    }
+    if (elapsed_time >= absl::ZeroDuration()) {  // ignore jumps backwards
+      // 60 microseconds of slop to allow for non-monotonic clocks.
+      EXPECT_GE(elapsed_time, wait_time - absl::Microseconds(60));
+    }
+  }
+}
+
+}  // namespace
diff --git a/absl/time/clock_test.cc b/absl/time/clock_test.cc
index 4bcfc6bc727..9370a88270d 100644
--- a/absl/time/clock_test.cc
+++ b/absl/time/clock_test.cc
@@ -14,17 +14,21 @@
 
 #include "absl/time/clock.h"
 
+#include "gtest/gtest.h"
 #include "absl/base/config.h"
+#include "absl/time/time.h"
+
 #if defined(ABSL_HAVE_ALARM)
 #include <signal.h>
 #include <unistd.h>
+#ifdef _AIX
+// sig_t is not defined in AIX.
+typedef void (*sig_t)(int);
+#endif
 #elif defined(__linux__) || defined(__APPLE__)
 #error all known Linux and Apple targets have alarm
 #endif
 
-#include "gtest/gtest.h"
-#include "absl/time/time.h"
-
 namespace {
 
 TEST(Time, Now) {
@@ -110,8 +114,8 @@ TEST(SleepFor, Bounded) {
   EXPECT_TRUE(AssertSleepForBounded(d, early, late, timeout,
                                     AlarmPolicy::kWithoutAlarm));
 #if defined(ABSL_HAVE_ALARM)
-  EXPECT_TRUE(AssertSleepForBounded(d, early, late, timeout,
-                                    AlarmPolicy::kWithAlarm));
+  EXPECT_TRUE(
+      AssertSleepForBounded(d, early, late, timeout, AlarmPolicy::kWithAlarm));
 #endif
 }
 
diff --git a/absl/time/duration.cc b/absl/time/duration.cc
index a3ac61a9189..487ccdd87b2 100644
--- a/absl/time/duration.cc
+++ b/absl/time/duration.cc
@@ -49,14 +49,9 @@
 //
 // Arithmetic overflows/underflows to +/- infinity and saturates.
 
-#if defined(_MSC_VER)
-#include <winsock2.h>  // for timeval
-#endif
-
 #include <algorithm>
 #include <cassert>
-#include <cctype>
-#include <cerrno>
+#include <chrono>  // NOLINT(build/c++11)
 #include <cmath>
 #include <cstdint>
 #include <cstdlib>
@@ -66,11 +61,20 @@
 #include <limits>
 #include <string>
 
+#include "absl/base/attributes.h"
 #include "absl/base/casts.h"
+#include "absl/base/config.h"
 #include "absl/numeric/int128.h"
+#include "absl/strings/string_view.h"
+#include "absl/strings/strip.h"
 #include "absl/time/time.h"
 
+#if defined(_MSC_VER)
+#include <winsock2.h>  // for timeval
+#endif
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 namespace {
 
@@ -92,13 +96,6 @@ inline bool IsValidDivisor(double d) {
   return d != 0.0;
 }
 
-// Can't use std::round() because it is only available in C++11.
-// Note that we ignore the possibility of floating-point over/underflow.
-template <typename Double>
-inline double Round(Double d) {
-  return d < 0 ? std::ceil(d - 0.5) : std::floor(d + 0.5);
-}
-
 // *sec may be positive or negative.  *ticks must be in the range
 // -kTicksPerSecond < *ticks < kTicksPerSecond.  If *ticks is negative it
 // will be normalized to a positive value by adjusting *sec accordingly.
@@ -197,15 +194,16 @@ inline int64_t DecodeTwosComp(uint64_t v) { return absl::bit_cast<int64_t>(v); }
 // double as overflow cases.
 inline bool SafeAddRepHi(double a_hi, double b_hi, Duration* d) {
   double c = a_hi + b_hi;
-  if (c >= kint64max) {
+  if (c >= static_cast<double>(kint64max)) {
     *d = InfiniteDuration();
     return false;
   }
-  if (c <= kint64min) {
+  if (c <= static_cast<double>(kint64min)) {
     *d = -InfiniteDuration();
     return false;
   }
-  *d = time_internal::MakeDuration(c, time_internal::GetRepLo(*d));
+  *d = time_internal::MakeDuration(static_cast<int64_t>(c),
+                                   time_internal::GetRepLo(*d));
   return true;
 }
 
@@ -222,7 +220,7 @@ struct SafeMultiply {
                  ? static_cast<uint128>(Uint128Low64(a) * Uint128Low64(b))
                  : a * b;
     }
-    return b == 0 ? b : (a > kuint128max / b) ? kuint128max : a * b;
+    return b == 0 ? b : (a > Uint128Max() / b) ? Uint128Max() : a * b;
   }
 };
 
@@ -242,8 +240,8 @@ inline Duration ScaleFixed(Duration d, int64_t r) {
 template <template <typename> class Operation>
 inline Duration ScaleDouble(Duration d, double r) {
   Operation<double> op;
-  double hi_doub = op(time_internal::GetRepHi(d), r);
-  double lo_doub = op(time_internal::GetRepLo(d), r);
+  double hi_doub = op(static_cast<double>(time_internal::GetRepHi(d)), r);
+  double lo_doub = op(static_cast<double>(time_internal::GetRepLo(d)), r);
 
   double hi_int = 0;
   double hi_frac = std::modf(hi_doub, &hi_int);
@@ -256,12 +254,15 @@ inline Duration ScaleDouble(Duration d, double r) {
   double lo_frac = std::modf(lo_doub, &lo_int);
 
   // Rolls lo into hi if necessary.
-  int64_t lo64 = Round(lo_frac * kTicksPerSecond);
+  int64_t lo64 = static_cast<int64_t>(std::round(lo_frac * kTicksPerSecond));
 
   Duration ans;
   if (!SafeAddRepHi(hi_int, lo_int, &ans)) return ans;
   int64_t hi64 = time_internal::GetRepHi(ans);
-  if (!SafeAddRepHi(hi64, lo64 / kTicksPerSecond, &ans)) return ans;
+  if (!SafeAddRepHi(static_cast<double>(hi64),
+                    static_cast<double>(lo64 / kTicksPerSecond), &ans)) {
+    return ans;
+  }
   hi64 = time_internal::GetRepHi(ans);
   lo64 %= kTicksPerSecond;
   NormalizeTicks(&hi64, &lo64);
@@ -283,33 +284,35 @@ inline bool IDivFastPath(const Duration num, const Duration den, int64_t* q,
   int64_t den_hi = time_internal::GetRepHi(den);
   uint32_t den_lo = time_internal::GetRepLo(den);
 
-  if (den_hi == 0 && den_lo == kTicksPerNanosecond) {
-    // Dividing by 1ns
-    if (num_hi >= 0 && num_hi < (kint64max - kTicksPerSecond) / 1000000000) {
-      *q = num_hi * 1000000000 + num_lo / kTicksPerNanosecond;
-      *rem = time_internal::MakeDuration(0, num_lo % den_lo);
-      return true;
-    }
-  } else if (den_hi == 0 && den_lo == 100 * kTicksPerNanosecond) {
-    // Dividing by 100ns (common when converting to Universal time)
-    if (num_hi >= 0 && num_hi < (kint64max - kTicksPerSecond) / 10000000) {
-      *q = num_hi * 10000000 + num_lo / (100 * kTicksPerNanosecond);
-      *rem = time_internal::MakeDuration(0, num_lo % den_lo);
-      return true;
-    }
-  } else if (den_hi == 0 && den_lo == 1000 * kTicksPerNanosecond) {
-    // Dividing by 1us
-    if (num_hi >= 0 && num_hi < (kint64max - kTicksPerSecond) / 1000000) {
-      *q = num_hi * 1000000 + num_lo / (1000 * kTicksPerNanosecond);
-      *rem = time_internal::MakeDuration(0, num_lo % den_lo);
-      return true;
-    }
-  } else if (den_hi == 0 && den_lo == 1000000 * kTicksPerNanosecond) {
-    // Dividing by 1ms
-    if (num_hi >= 0 && num_hi < (kint64max - kTicksPerSecond) / 1000) {
-      *q = num_hi * 1000 + num_lo / (1000000 * kTicksPerNanosecond);
-      *rem = time_internal::MakeDuration(0, num_lo % den_lo);
-      return true;
+  if (den_hi == 0) {
+    if (den_lo == kTicksPerNanosecond) {
+      // Dividing by 1ns
+      if (num_hi >= 0 && num_hi < (kint64max - kTicksPerSecond) / 1000000000) {
+        *q = num_hi * 1000000000 + num_lo / kTicksPerNanosecond;
+        *rem = time_internal::MakeDuration(0, num_lo % den_lo);
+        return true;
+      }
+    } else if (den_lo == 100 * kTicksPerNanosecond) {
+      // Dividing by 100ns (common when converting to Universal time)
+      if (num_hi >= 0 && num_hi < (kint64max - kTicksPerSecond) / 10000000) {
+        *q = num_hi * 10000000 + num_lo / (100 * kTicksPerNanosecond);
+        *rem = time_internal::MakeDuration(0, num_lo % den_lo);
+        return true;
+      }
+    } else if (den_lo == 1000 * kTicksPerNanosecond) {
+      // Dividing by 1us
+      if (num_hi >= 0 && num_hi < (kint64max - kTicksPerSecond) / 1000000) {
+        *q = num_hi * 1000000 + num_lo / (1000 * kTicksPerNanosecond);
+        *rem = time_internal::MakeDuration(0, num_lo % den_lo);
+        return true;
+      }
+    } else if (den_lo == 1000000 * kTicksPerNanosecond) {
+      // Dividing by 1ms
+      if (num_hi >= 0 && num_hi < (kint64max - kTicksPerSecond) / 1000) {
+        *q = num_hi * 1000 + num_lo / (1000000 * kTicksPerNanosecond);
+        *rem = time_internal::MakeDuration(0, num_lo % den_lo);
+        return true;
+      }
     }
   } else if (den_hi > 0 && den_lo == 0) {
     // Dividing by positive multiple of 1s
@@ -345,19 +348,10 @@ inline bool IDivFastPath(const Duration num, const Duration den, int64_t* q,
 
 }  // namespace
 
-namespace time_internal {
-
-// The 'satq' argument indicates whether the quotient should saturate at the
-// bounds of int64_t.  If it does saturate, the difference will spill over to
-// the remainder.  If it does not saturate, the remainder remain accurate,
-// but the returned quotient will over/underflow int64_t and should not be used.
-int64_t IDivDuration(bool satq, const Duration num, const Duration den,
-                   Duration* rem) {
-  int64_t q = 0;
-  if (IDivFastPath(num, den, &q, rem)) {
-    return q;
-  }
+namespace {
 
+int64_t IDivSlowPath(bool satq, const Duration num, const Duration den,
+                     Duration* rem) {
   const bool num_neg = num < ZeroDuration();
   const bool den_neg = den < ZeroDuration();
   const bool quotient_neg = num_neg != den_neg;
@@ -394,7 +388,27 @@ int64_t IDivDuration(bool satq, const Duration num, const Duration den,
   return -static_cast<int64_t>(Uint128Low64(quotient128 - 1) & kint64max) - 1;
 }
 
-}  // namespace time_internal
+// The 'satq' argument indicates whether the quotient should saturate at the
+// bounds of int64_t.  If it does saturate, the difference will spill over to
+// the remainder.  If it does not saturate, the remainder remain accurate,
+// but the returned quotient will over/underflow int64_t and should not be used.
+ABSL_ATTRIBUTE_ALWAYS_INLINE inline int64_t IDivDurationImpl(bool satq,
+                                                             const Duration num,
+                                                             const Duration den,
+                                                             Duration* rem) {
+  int64_t q = 0;
+  if (IDivFastPath(num, den, &q, rem)) {
+    return q;
+  }
+  return IDivSlowPath(satq, num, den, rem);
+}
+
+}  // namespace
+
+int64_t IDivDuration(Duration num, Duration den, Duration* rem) {
+  return IDivDurationImpl(true, num, den,
+                          rem);  // trunc towards zero
+}
 
 //
 // Additive operators.
@@ -403,16 +417,18 @@ int64_t IDivDuration(bool satq, const Duration num, const Duration den,
 Duration& Duration::operator+=(Duration rhs) {
   if (time_internal::IsInfiniteDuration(*this)) return *this;
   if (time_internal::IsInfiniteDuration(rhs)) return *this = rhs;
-  const int64_t orig_rep_hi = rep_hi_;
-  rep_hi_ =
-      DecodeTwosComp(EncodeTwosComp(rep_hi_) + EncodeTwosComp(rhs.rep_hi_));
+  const int64_t orig_rep_hi = rep_hi_.Get();
+  rep_hi_ = DecodeTwosComp(EncodeTwosComp(rep_hi_.Get()) +
+                           EncodeTwosComp(rhs.rep_hi_.Get()));
   if (rep_lo_ >= kTicksPerSecond - rhs.rep_lo_) {
-    rep_hi_ = DecodeTwosComp(EncodeTwosComp(rep_hi_) + 1);
+    rep_hi_ = DecodeTwosComp(EncodeTwosComp(rep_hi_.Get()) + 1);
     rep_lo_ -= kTicksPerSecond;
   }
   rep_lo_ += rhs.rep_lo_;
-  if (rhs.rep_hi_ < 0 ? rep_hi_ > orig_rep_hi : rep_hi_ < orig_rep_hi) {
-    return *this = rhs.rep_hi_ < 0 ? -InfiniteDuration() : InfiniteDuration();
+  if (rhs.rep_hi_.Get() < 0 ? rep_hi_.Get() > orig_rep_hi
+                            : rep_hi_.Get() < orig_rep_hi) {
+    return *this =
+               rhs.rep_hi_.Get() < 0 ? -InfiniteDuration() : InfiniteDuration();
   }
   return *this;
 }
@@ -420,18 +436,21 @@ Duration& Duration::operator+=(Duration rhs) {
 Duration& Duration::operator-=(Duration rhs) {
   if (time_internal::IsInfiniteDuration(*this)) return *this;
   if (time_internal::IsInfiniteDuration(rhs)) {
-    return *this = rhs.rep_hi_ >= 0 ? -InfiniteDuration() : InfiniteDuration();
+    return *this = rhs.rep_hi_.Get() >= 0 ? -InfiniteDuration()
+                                          : InfiniteDuration();
   }
-  const int64_t orig_rep_hi = rep_hi_;
-  rep_hi_ =
-      DecodeTwosComp(EncodeTwosComp(rep_hi_) - EncodeTwosComp(rhs.rep_hi_));
+  const int64_t orig_rep_hi = rep_hi_.Get();
+  rep_hi_ = DecodeTwosComp(EncodeTwosComp(rep_hi_.Get()) -
+                           EncodeTwosComp(rhs.rep_hi_.Get()));
   if (rep_lo_ < rhs.rep_lo_) {
-    rep_hi_ = DecodeTwosComp(EncodeTwosComp(rep_hi_) - 1);
+    rep_hi_ = DecodeTwosComp(EncodeTwosComp(rep_hi_.Get()) - 1);
     rep_lo_ += kTicksPerSecond;
   }
   rep_lo_ -= rhs.rep_lo_;
-  if (rhs.rep_hi_ < 0 ? rep_hi_ < orig_rep_hi : rep_hi_ > orig_rep_hi) {
-    return *this = rhs.rep_hi_ >= 0 ? -InfiniteDuration() : InfiniteDuration();
+  if (rhs.rep_hi_.Get() < 0 ? rep_hi_.Get() < orig_rep_hi
+                            : rep_hi_.Get() > orig_rep_hi) {
+    return *this = rhs.rep_hi_.Get() >= 0 ? -InfiniteDuration()
+                                          : InfiniteDuration();
   }
   return *this;
 }
@@ -442,7 +461,7 @@ Duration& Duration::operator-=(Duration rhs) {
 
 Duration& Duration::operator*=(int64_t r) {
   if (time_internal::IsInfiniteDuration(*this)) {
-    const bool is_neg = (r < 0) != (rep_hi_ < 0);
+    const bool is_neg = (r < 0) != (rep_hi_.Get() < 0);
     return *this = is_neg ? -InfiniteDuration() : InfiniteDuration();
   }
   return *this = ScaleFixed<SafeMultiply>(*this, r);
@@ -450,7 +469,7 @@ Duration& Duration::operator*=(int64_t r) {
 
 Duration& Duration::operator*=(double r) {
   if (time_internal::IsInfiniteDuration(*this) || !IsFinite(r)) {
-    const bool is_neg = (std::signbit(r) != 0) != (rep_hi_ < 0);
+    const bool is_neg = std::isnan(r) || std::signbit(r) != (rep_hi_.Get() < 0);
     return *this = is_neg ? -InfiniteDuration() : InfiniteDuration();
   }
   return *this = ScaleDouble<std::multiplies>(*this, r);
@@ -458,7 +477,7 @@ Duration& Duration::operator*=(double r) {
 
 Duration& Duration::operator/=(int64_t r) {
   if (time_internal::IsInfiniteDuration(*this) || r == 0) {
-    const bool is_neg = (r < 0) != (rep_hi_ < 0);
+    const bool is_neg = (r < 0) != (rep_hi_.Get() < 0);
     return *this = is_neg ? -InfiniteDuration() : InfiniteDuration();
   }
   return *this = ScaleFixed<std::divides>(*this, r);
@@ -466,14 +485,14 @@ Duration& Duration::operator/=(int64_t r) {
 
 Duration& Duration::operator/=(double r) {
   if (time_internal::IsInfiniteDuration(*this) || !IsValidDivisor(r)) {
-    const bool is_neg = (std::signbit(r) != 0) != (rep_hi_ < 0);
+    const bool is_neg = std::isnan(r) || std::signbit(r) != (rep_hi_.Get() < 0);
     return *this = is_neg ? -InfiniteDuration() : InfiniteDuration();
   }
   return *this = ScaleDouble<std::divides>(*this, r);
 }
 
 Duration& Duration::operator%=(Duration rhs) {
-  time_internal::IDivDuration(false, *this, rhs, this);
+  IDivDurationImpl(false, *this, rhs, this);
   return *this;
 }
 
@@ -499,9 +518,7 @@ double FDivDuration(Duration num, Duration den) {
 // Trunc/Floor/Ceil.
 //
 
-Duration Trunc(Duration d, Duration unit) {
-  return d - (d % unit);
-}
+Duration Trunc(Duration d, Duration unit) { return d - (d % unit); }
 
 Duration Floor(const Duration d, const Duration unit) {
   const absl::Duration td = Trunc(d, unit);
@@ -536,50 +553,6 @@ Duration DurationFromTimeval(timeval tv) {
 //
 // Conversion to other duration types.
 //
-
-int64_t ToInt64Nanoseconds(Duration d) {
-  if (time_internal::GetRepHi(d) >= 0 &&
-      time_internal::GetRepHi(d) >> 33 == 0) {
-    return (time_internal::GetRepHi(d) * 1000 * 1000 * 1000) +
-           (time_internal::GetRepLo(d) / kTicksPerNanosecond);
-  }
-  return d / Nanoseconds(1);
-}
-int64_t ToInt64Microseconds(Duration d) {
-  if (time_internal::GetRepHi(d) >= 0 &&
-      time_internal::GetRepHi(d) >> 43 == 0) {
-    return (time_internal::GetRepHi(d) * 1000 * 1000) +
-           (time_internal::GetRepLo(d) / (kTicksPerNanosecond * 1000));
-  }
-  return d / Microseconds(1);
-}
-int64_t ToInt64Milliseconds(Duration d) {
-  if (time_internal::GetRepHi(d) >= 0 &&
-      time_internal::GetRepHi(d) >> 53 == 0) {
-    return (time_internal::GetRepHi(d) * 1000) +
-           (time_internal::GetRepLo(d) / (kTicksPerNanosecond * 1000 * 1000));
-  }
-  return d / Milliseconds(1);
-}
-int64_t ToInt64Seconds(Duration d) {
-  int64_t hi = time_internal::GetRepHi(d);
-  if (time_internal::IsInfiniteDuration(d)) return hi;
-  if (hi < 0 && time_internal::GetRepLo(d) != 0) ++hi;
-  return hi;
-}
-int64_t ToInt64Minutes(Duration d) {
-  int64_t hi = time_internal::GetRepHi(d);
-  if (time_internal::IsInfiniteDuration(d)) return hi;
-  if (hi < 0 && time_internal::GetRepLo(d) != 0) ++hi;
-  return hi / 60;
-}
-int64_t ToInt64Hours(Duration d) {
-  int64_t hi = time_internal::GetRepHi(d);
-  if (time_internal::IsInfiniteDuration(d)) return hi;
-  if (hi < 0 && time_internal::GetRepLo(d) != 0) ++hi;
-  return hi / (60 * 60);
-}
-
 double ToDoubleNanoseconds(Duration d) {
   return FDivDuration(d, Nanoseconds(1));
 }
@@ -589,15 +562,9 @@ double ToDoubleMicroseconds(Duration d) {
 double ToDoubleMilliseconds(Duration d) {
   return FDivDuration(d, Milliseconds(1));
 }
-double ToDoubleSeconds(Duration d) {
-  return FDivDuration(d, Seconds(1));
-}
-double ToDoubleMinutes(Duration d) {
-  return FDivDuration(d, Minutes(1));
-}
-double ToDoubleHours(Duration d) {
-  return FDivDuration(d, Hours(1));
-}
+double ToDoubleSeconds(Duration d) { return FDivDuration(d, Seconds(1)); }
+double ToDoubleMinutes(Duration d) { return FDivDuration(d, Minutes(1)); }
+double ToDoubleHours(Duration d) { return FDivDuration(d, Hours(1)); }
 
 timespec ToTimespec(Duration d) {
   timespec ts;
@@ -613,7 +580,7 @@ timespec ToTimespec(Duration d) {
         rep_lo -= kTicksPerSecond;
       }
     }
-    ts.tv_sec = rep_hi;
+    ts.tv_sec = static_cast<decltype(ts.tv_sec)>(rep_hi);
     if (ts.tv_sec == rep_hi) {  // no time_t narrowing
       ts.tv_nsec = rep_lo / kTicksPerNanosecond;
       return ts;
@@ -641,7 +608,7 @@ timeval ToTimeval(Duration d) {
       ts.tv_nsec -= 1000 * 1000 * 1000;
     }
   }
-  tv.tv_sec = ts.tv_sec;
+  tv.tv_sec = static_cast<decltype(tv.tv_sec)>(ts.tv_sec);
   if (tv.tv_sec != ts.tv_sec) {  // narrowing
     if (ts.tv_sec < 0) {
       tv.tv_sec = std::numeric_limits<decltype(tv.tv_sec)>::min();
@@ -687,7 +654,7 @@ namespace {
 char* Format64(char* ep, int width, int64_t v) {
   do {
     --width;
-    *--ep = '0' + (v % 10);  // contiguous digits
+    *--ep = static_cast<char>('0' + (v % 10));  // contiguous digits
   } while (v /= 10);
   while (--width >= 0) *--ep = '0';  // zero pad
   return ep;
@@ -707,47 +674,48 @@ char* Format64(char* ep, int width, int64_t v) {
 // fractional digits, because it is in the noise of what a Duration can
 // represent.
 struct DisplayUnit {
-  const char* abbr;
+  absl::string_view abbr;
   int prec;
   double pow10;
 };
-const DisplayUnit kDisplayNano = {"ns", 2, 1e2};
-const DisplayUnit kDisplayMicro = {"us", 5, 1e5};
-const DisplayUnit kDisplayMilli = {"ms", 8, 1e8};
-const DisplayUnit kDisplaySec = {"s", 11, 1e11};
-const DisplayUnit kDisplayMin = {"m", -1, 0.0};   // prec ignored
-const DisplayUnit kDisplayHour = {"h", -1, 0.0};  // prec ignored
+constexpr DisplayUnit kDisplayNano = {"ns", 2, 1e2};
+constexpr DisplayUnit kDisplayMicro = {"us", 5, 1e5};
+constexpr DisplayUnit kDisplayMilli = {"ms", 8, 1e8};
+constexpr DisplayUnit kDisplaySec = {"s", 11, 1e11};
+constexpr DisplayUnit kDisplayMin = {"m", -1, 0.0};   // prec ignored
+constexpr DisplayUnit kDisplayHour = {"h", -1, 0.0};  // prec ignored
 
 void AppendNumberUnit(std::string* out, int64_t n, DisplayUnit unit) {
   char buf[sizeof("2562047788015216")];  // hours in max duration
   char* const ep = buf + sizeof(buf);
   char* bp = Format64(ep, 0, n);
   if (*bp != '0' || bp + 1 != ep) {
-    out->append(bp, ep - bp);
-    out->append(unit.abbr);
+    out->append(bp, static_cast<size_t>(ep - bp));
+    out->append(unit.abbr.data(), unit.abbr.size());
   }
 }
 
 // Note: unit.prec is limited to double's digits10 value (typically 15) so it
 // always fits in buf[].
 void AppendNumberUnit(std::string* out, double n, DisplayUnit unit) {
-  const int buf_size = std::numeric_limits<double>::digits10;
-  const int prec = std::min(buf_size, unit.prec);
-  char buf[buf_size];  // also large enough to hold integer part
+  constexpr int kBufferSize = std::numeric_limits<double>::digits10;
+  const int prec = std::min(kBufferSize, unit.prec);
+  char buf[kBufferSize];  // also large enough to hold integer part
   char* ep = buf + sizeof(buf);
   double d = 0;
-  int64_t frac_part = Round(std::modf(n, &d) * unit.pow10);
-  int64_t int_part = d;
+  int64_t frac_part =
+      static_cast<int64_t>(std::round(std::modf(n, &d) * unit.pow10));
+  int64_t int_part = static_cast<int64_t>(d);
   if (int_part != 0 || frac_part != 0) {
     char* bp = Format64(ep, 0, int_part);  // always < 1000
-    out->append(bp, ep - bp);
+    out->append(bp, static_cast<size_t>(ep - bp));
     if (frac_part != 0) {
       out->push_back('.');
       bp = Format64(ep, prec, frac_part);
       while (ep[-1] == '0') --ep;
-      out->append(bp, ep - bp);
+      out->append(bp, static_cast<size_t>(ep - bp));
     }
-    out->append(unit.abbr);
+    out->append(unit.abbr.data(), unit.abbr.size());
   }
 }
 
@@ -758,15 +726,17 @@ void AppendNumberUnit(std::string* out, double n, DisplayUnit unit) {
 //   form "72h3m0.5s". Leading zero units are omitted.  As a special
 //   case, durations less than one second format use a smaller unit
 //   (milli-, micro-, or nanoseconds) to ensure that the leading digit
-//   is non-zero.  The zero duration formats as 0, with no unit.
+//   is non-zero.
+// Unlike Go, we format the zero duration as 0, with no unit.
 std::string FormatDuration(Duration d) {
-  const Duration min_duration = Seconds(kint64min);
-  if (d == min_duration) {
+  constexpr Duration kMinDuration = Seconds(kint64min);
+  std::string s;
+  if (d == kMinDuration) {
     // Avoid needing to negate kint64min by directly returning what the
     // following code should produce in that case.
-    return "-2562047788015215h30m8s";
+    s = "-2562047788015215h30m8s";
+    return s;
   }
-  std::string s;
   if (d < ZeroDuration()) {
     s.append("-");
     d = -d;
@@ -799,23 +769,27 @@ namespace {
 // A helper for ParseDuration() that parses a leading number from the given
 // string and stores the result in *int_part/*frac_part/*frac_scale.  The
 // given string pointer is modified to point to the first unconsumed char.
-bool ConsumeDurationNumber(const char** dpp, int64_t* int_part,
+bool ConsumeDurationNumber(const char** dpp, const char* ep, int64_t* int_part,
                            int64_t* frac_part, int64_t* frac_scale) {
   *int_part = 0;
   *frac_part = 0;
   *frac_scale = 1;  // invariant: *frac_part < *frac_scale
   const char* start = *dpp;
-  for (; std::isdigit(**dpp); *dpp += 1) {
+  for (; *dpp != ep; *dpp += 1) {
     const int d = **dpp - '0';  // contiguous digits
+    if (d < 0 || 10 <= d) break;
+
     if (*int_part > kint64max / 10) return false;
     *int_part *= 10;
     if (*int_part > kint64max - d) return false;
     *int_part += d;
   }
   const bool int_part_empty = (*dpp == start);
-  if (**dpp != '.') return !int_part_empty;
-  for (*dpp += 1; std::isdigit(**dpp); *dpp += 1) {
+  if (*dpp == ep || **dpp != '.') return !int_part_empty;
+
+  for (*dpp += 1; *dpp != ep; *dpp += 1) {
     const int d = **dpp - '0';  // contiguous digits
+    if (d < 0 || 10 <= d) break;
     if (*frac_scale <= kint64max / 10) {
       *frac_part *= 10;
       *frac_part += d;
@@ -829,32 +803,56 @@ bool ConsumeDurationNumber(const char** dpp, int64_t* int_part,
 // ns, us, ms, s, m, h) from the given string and stores the resulting unit
 // in "*unit".  The given string pointer is modified to point to the first
 // unconsumed char.
-bool ConsumeDurationUnit(const char** start, Duration* unit) {
-  const char *s = *start;
-  bool ok = true;
-  if (strncmp(s, "ns", 2) == 0) {
-    s += 2;
-    *unit = Nanoseconds(1);
-  } else if (strncmp(s, "us", 2) == 0) {
-    s += 2;
-    *unit = Microseconds(1);
-  } else if (strncmp(s, "ms", 2) == 0) {
-    s += 2;
-    *unit = Milliseconds(1);
-  } else if (strncmp(s, "s", 1) == 0) {
-    s += 1;
-    *unit = Seconds(1);
-  } else if (strncmp(s, "m", 1) == 0) {
-    s += 1;
-    *unit = Minutes(1);
-  } else if (strncmp(s, "h", 1) == 0) {
-    s += 1;
-    *unit = Hours(1);
-  } else {
-    ok = false;
+bool ConsumeDurationUnit(const char** start, const char* end, Duration* unit) {
+  size_t size = static_cast<size_t>(end - *start);
+  switch (size) {
+    case 0:
+      return false;
+    default:
+      switch (**start) {
+        case 'n':
+          if (*(*start + 1) == 's') {
+            *start += 2;
+            *unit = Nanoseconds(1);
+            return true;
+          }
+          break;
+        case 'u':
+          if (*(*start + 1) == 's') {
+            *start += 2;
+            *unit = Microseconds(1);
+            return true;
+          }
+          break;
+        case 'm':
+          if (*(*start + 1) == 's') {
+            *start += 2;
+            *unit = Milliseconds(1);
+            return true;
+          }
+          break;
+        default:
+          break;
+      }
+      [[fallthrough]];
+    case 1:
+      switch (**start) {
+        case 's':
+          *unit = Seconds(1);
+          *start += 1;
+          return true;
+        case 'm':
+          *unit = Minutes(1);
+          *start += 1;
+          return true;
+        case 'h':
+          *unit = Hours(1);
+          *start += 1;
+          return true;
+        default:
+          return false;
+      }
   }
-  *start = s;
-  return ok;
 }
 
 }  // namespace
@@ -864,39 +862,38 @@ bool ConsumeDurationUnit(const char** start, Duration* unit) {
 //   a possibly signed sequence of decimal numbers, each with optional
 //   fraction and a unit suffix, such as "300ms", "-1.5h" or "2h45m".
 //   Valid time units are "ns", "us" "ms", "s", "m", "h".
-bool ParseDuration(const std::string& dur_string, Duration* d) {
-  const char* start = dur_string.c_str();
+bool ParseDuration(absl::string_view dur_sv, Duration* d) {
   int sign = 1;
-
-  if (*start == '-' || *start == '+') {
-    sign = *start == '-' ? -1 : 1;
-    ++start;
-  }
-
-  // Can't parse a duration from an empty std::string.
-  if (*start == '\0') {
-    return false;
+  if (absl::ConsumePrefix(&dur_sv, "-")) {
+    sign = -1;
+  } else {
+    absl::ConsumePrefix(&dur_sv, "+");
   }
+  if (dur_sv.empty()) return false;
 
-  // Special case for a std::string of "0".
-  if (*start == '0' && *(start + 1) == '\0') {
+  // Special case for a string of "0".
+  if (dur_sv == "0") {
     *d = ZeroDuration();
     return true;
   }
 
-  if (strcmp(start, "inf") == 0) {
+  if (dur_sv == "inf") {
     *d = sign * InfiniteDuration();
     return true;
   }
 
+  const char* start = dur_sv.data();
+  const char* end = start + dur_sv.size();
+
   Duration dur;
-  while (*start != '\0') {
+  while (start != end) {
     int64_t int_part;
     int64_t frac_part;
     int64_t frac_scale;
     Duration unit;
-    if (!ConsumeDurationNumber(&start, &int_part, &frac_part, &frac_scale) ||
-        !ConsumeDurationUnit(&start, &unit)) {
+    if (!ConsumeDurationNumber(&start, end, &int_part, &frac_part,
+                               &frac_scale) ||
+        !ConsumeDurationUnit(&start, end, &unit)) {
       return false;
     }
     if (int_part != 0) dur += sign * int_part * unit;
@@ -906,10 +903,16 @@ bool ParseDuration(const std::string& dur_string, Duration* d) {
   return true;
 }
 
+bool AbslParseFlag(absl::string_view text, Duration* dst, std::string*) {
+  return ParseDuration(text, dst);
+}
+
+std::string AbslUnparseFlag(Duration d) { return FormatDuration(d); }
 bool ParseFlag(const std::string& text, Duration* dst, std::string* ) {
   return ParseDuration(text, dst);
 }
 
 std::string UnparseFlag(Duration d) { return FormatDuration(d); }
 
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/time/duration_benchmark.cc b/absl/time/duration_benchmark.cc
index 83a836c8c8a..5a33c65850f 100644
--- a/absl/time/duration_benchmark.cc
+++ b/absl/time/duration_benchmark.cc
@@ -15,12 +15,18 @@
 #include <cstddef>
 #include <cstdint>
 #include <ctime>
+#include <iterator>
 #include <string>
 
 #include "absl/base/attributes.h"
+#include "absl/flags/flag.h"
 #include "absl/time/time.h"
 #include "benchmark/benchmark.h"
 
+ABSL_FLAG(absl::Duration, absl_duration_flag_for_benchmark,
+          absl::Milliseconds(1),
+          "Flag to use for benchmarking duration flag access speed.");
+
 namespace {
 
 //
@@ -285,6 +291,26 @@ void BM_Duration_IDivDuration_Hours(benchmark::State& state) {
 }
 BENCHMARK(BM_Duration_IDivDuration_Hours);
 
+void BM_Duration_Modulo(benchmark::State& state) {
+  int i = 0;
+  while (state.KeepRunning()) {
+    auto mod = absl::Seconds(i) % absl::Nanoseconds(12345);
+    benchmark::DoNotOptimize(mod);
+    ++i;
+  }
+}
+BENCHMARK(BM_Duration_Modulo);
+
+void BM_Duration_Modulo_FastPath(benchmark::State& state) {
+  int i = 0;
+  while (state.KeepRunning()) {
+    auto mod = absl::Seconds(i) % absl::Milliseconds(1);
+    benchmark::DoNotOptimize(mod);
+    ++i;
+  }
+}
+BENCHMARK(BM_Duration_Modulo_FastPath);
+
 void BM_Duration_ToInt64Nanoseconds(benchmark::State& state) {
   absl::Duration d = absl::Seconds(100000);
   while (state.KeepRunning()) {
@@ -333,6 +359,150 @@ void BM_Duration_ToInt64Hours(benchmark::State& state) {
 }
 BENCHMARK(BM_Duration_ToInt64Hours);
 
+//
+// ToDoubleXYZ
+//
+void BM_Duration_ToDoubleNanoseconds(benchmark::State& state) {
+  absl::Duration d = absl::Seconds(100000);
+  while (state.KeepRunning()) {
+    benchmark::DoNotOptimize(d);
+    double result = absl::ToDoubleNanoseconds(d);
+    benchmark::DoNotOptimize(result);
+  }
+}
+BENCHMARK(BM_Duration_ToDoubleNanoseconds);
+
+void BM_Duration_ToDoubleMicroseconds(benchmark::State& state) {
+  absl::Duration d = absl::Seconds(100000);
+  while (state.KeepRunning()) {
+    benchmark::DoNotOptimize(d);
+    double result = absl::ToDoubleMicroseconds(d);
+    benchmark::DoNotOptimize(result);
+  }
+}
+BENCHMARK(BM_Duration_ToDoubleMicroseconds);
+
+void BM_Duration_ToDoubleMilliseconds(benchmark::State& state) {
+  absl::Duration d = absl::Seconds(100000);
+  while (state.KeepRunning()) {
+    benchmark::DoNotOptimize(d);
+    double result = absl::ToDoubleMilliseconds(d);
+    benchmark::DoNotOptimize(result);
+  }
+}
+BENCHMARK(BM_Duration_ToDoubleMilliseconds);
+
+void BM_Duration_ToDoubleSeconds(benchmark::State& state) {
+  absl::Duration d = absl::Seconds(100000);
+  while (state.KeepRunning()) {
+    benchmark::DoNotOptimize(d);
+    double result = absl::ToDoubleSeconds(d);
+    benchmark::DoNotOptimize(result);
+  }
+}
+BENCHMARK(BM_Duration_ToDoubleSeconds);
+
+void BM_Duration_ToDoubleMinutes(benchmark::State& state) {
+  absl::Duration d = absl::Seconds(100000);
+  while (state.KeepRunning()) {
+    benchmark::DoNotOptimize(d);
+    double result = absl::ToDoubleMinutes(d);
+    benchmark::DoNotOptimize(result);
+  }
+}
+BENCHMARK(BM_Duration_ToDoubleMinutes);
+
+void BM_Duration_ToDoubleHours(benchmark::State& state) {
+  absl::Duration d = absl::Seconds(100000);
+  while (state.KeepRunning()) {
+    benchmark::DoNotOptimize(d);
+    double result = absl::ToDoubleHours(d);
+    benchmark::DoNotOptimize(result);
+  }
+}
+BENCHMARK(BM_Duration_ToDoubleHours);
+
+//
+// ToDoubleXYZ Latency
+//
+void BM_Duration_ToDoubleNanoseconds_Latency(benchmark::State& state) {
+  absl::Duration d1 = absl::Seconds(100000);
+  absl::Duration d2 = absl::Seconds(100000);
+  double result = 1;
+  while (state.KeepRunning()) {
+    benchmark::DoNotOptimize(d1);
+    benchmark::DoNotOptimize(d2);
+    benchmark::DoNotOptimize(result);
+    result = absl::ToDoubleNanoseconds(result < 0 ? d1 : d2);
+  }
+}
+BENCHMARK(BM_Duration_ToDoubleNanoseconds_Latency);
+
+void BM_Duration_ToDoubleMicroseconds_Latency(benchmark::State& state) {
+  absl::Duration d1 = absl::Seconds(100000);
+  absl::Duration d2 = absl::Seconds(100000);
+  double result = 1;
+  while (state.KeepRunning()) {
+    benchmark::DoNotOptimize(d1);
+    benchmark::DoNotOptimize(d2);
+    benchmark::DoNotOptimize(result);
+    result = absl::ToDoubleMicroseconds(result < 0 ? d1 : d2);
+  }
+}
+BENCHMARK(BM_Duration_ToDoubleMicroseconds_Latency);
+
+void BM_Duration_ToDoubleMilliseconds_Latency(benchmark::State& state) {
+  absl::Duration d1 = absl::Seconds(100000);
+  absl::Duration d2 = absl::Seconds(100000);
+  double result = 1;
+  while (state.KeepRunning()) {
+    benchmark::DoNotOptimize(d1);
+    benchmark::DoNotOptimize(d2);
+    benchmark::DoNotOptimize(result);
+    result = absl::ToDoubleMilliseconds(result < 0 ? d1 : d2);
+  }
+}
+BENCHMARK(BM_Duration_ToDoubleMilliseconds_Latency);
+
+void BM_Duration_ToDoubleSeconds_Latency(benchmark::State& state) {
+  absl::Duration d1 = absl::Seconds(100000);
+  absl::Duration d2 = absl::Seconds(100000);
+  double result = 1;
+  while (state.KeepRunning()) {
+    benchmark::DoNotOptimize(d1);
+    benchmark::DoNotOptimize(d2);
+    benchmark::DoNotOptimize(result);
+    result = absl::ToDoubleSeconds(result < 0 ? d1 : d2);
+  }
+}
+BENCHMARK(BM_Duration_ToDoubleSeconds_Latency);
+
+void BM_Duration_ToDoubleMinutes_Latency(benchmark::State& state) {
+  absl::Duration d1 = absl::Seconds(100000);
+  absl::Duration d2 = absl::Seconds(100000);
+  double result = 1;
+  while (state.KeepRunning()) {
+    benchmark::DoNotOptimize(d1);
+    benchmark::DoNotOptimize(d2);
+    benchmark::DoNotOptimize(result);
+    result = absl::ToDoubleMinutes(result < 0 ? d1 : d2);
+  }
+}
+BENCHMARK(BM_Duration_ToDoubleMinutes_Latency);
+
+void BM_Duration_ToDoubleHours_Latency(benchmark::State& state) {
+  absl::Duration d1 = absl::Seconds(100000);
+  absl::Duration d2 = absl::Seconds(100000);
+  double result = 1;
+  while (state.KeepRunning()) {
+    benchmark::DoNotOptimize(d1);
+    benchmark::DoNotOptimize(d2);
+    benchmark::DoNotOptimize(result);
+    result = absl::ToDoubleHours(result < 0 ? d1 : d2);
+  }
+}
+BENCHMARK(BM_Duration_ToDoubleHours_Latency);
+
 //
 // To/FromTimespec
 //
@@ -402,7 +572,7 @@ const char* const kDurations[] = {
     "-2h3m4.005006007s",                   // 3
     "2562047788015215h30m7.99999999975s",  // 4
 };
-const int kNumDurations = sizeof(kDurations) / sizeof(kDurations[0]);
+const int kNumDurations = std::size(kDurations);
 
 void BM_Duration_FormatDuration(benchmark::State& state) {
   const std::string s = kDurations[state.range(0)];
@@ -425,4 +595,15 @@ void BM_Duration_ParseDuration(benchmark::State& state) {
 }
 BENCHMARK(BM_Duration_ParseDuration)->DenseRange(0, kNumDurations - 1);
 
+//
+// Flag access
+//
+void BM_Duration_GetFlag(benchmark::State& state) {
+  while (state.KeepRunning()) {
+    benchmark::DoNotOptimize(
+        absl::GetFlag(FLAGS_absl_duration_flag_for_benchmark));
+  }
+}
+BENCHMARK(BM_Duration_GetFlag);
+
 }  // namespace
diff --git a/absl/time/duration_test.cc b/absl/time/duration_test.cc
index 5dce9ac8a55..8fffa954f1c 100644
--- a/absl/time/duration_test.cc
+++ b/absl/time/duration_test.cc
@@ -12,11 +12,19 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-#if defined(_MSC_VER)
-#include <winsock2.h>  // for timeval
+#include "absl/base/config.h"
+
+// For feature testing and determining which headers can be included.
+#if ABSL_INTERNAL_CPLUSPLUS_LANG >= 202002L
+#include <version>
 #endif
 
+#include <array>
+#include <cfloat>
 #include <chrono>  // NOLINT(build/c++11)
+#ifdef __cpp_lib_three_way_comparison
+#include <compare>
+#endif  // __cpp_lib_three_way_comparison
 #include <cmath>
 #include <cstdint>
 #include <ctime>
@@ -24,11 +32,18 @@
 #include <limits>
 #include <random>
 #include <string>
+#include <type_traits>
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
+#include "absl/random/random.h"
+#include "absl/strings/str_format.h"
 #include "absl/time/time.h"
 
+#if defined(_MSC_VER)
+#include <winsock2.h>  // for timeval
+#endif
+
 namespace {
 
 constexpr int64_t kint64max = std::numeric_limits<int64_t>::max();
@@ -42,8 +57,7 @@ absl::Duration ApproxYears(int64_t n) { return absl::Hours(n) * 365 * 24; }
 // timespec ts1, ts2;
 // EXPECT_THAT(ts1, TimespecMatcher(ts2));
 MATCHER_P(TimespecMatcher, ts, "") {
-  if (ts.tv_sec == arg.tv_sec && ts.tv_nsec == arg.tv_nsec)
-    return true;
+  if (ts.tv_sec == arg.tv_sec && ts.tv_nsec == arg.tv_nsec) return true;
   *result_listener << "expected: {" << ts.tv_sec << ", " << ts.tv_nsec << "} ";
   *result_listener << "actual: {" << arg.tv_sec << ", " << arg.tv_nsec << "}";
   return false;
@@ -53,14 +67,15 @@ MATCHER_P(TimespecMatcher, ts, "") {
 // timeval tv1, tv2;
 // EXPECT_THAT(tv1, TimevalMatcher(tv2));
 MATCHER_P(TimevalMatcher, tv, "") {
-  if (tv.tv_sec == arg.tv_sec && tv.tv_usec == arg.tv_usec)
-    return true;
+  if (tv.tv_sec == arg.tv_sec && tv.tv_usec == arg.tv_usec) return true;
   *result_listener << "expected: {" << tv.tv_sec << ", " << tv.tv_usec << "} ";
   *result_listener << "actual: {" << arg.tv_sec << ", " << arg.tv_usec << "}";
   return false;
 }
 
 TEST(Duration, ConstExpr) {
+  static_assert(std::is_trivially_destructible_v<absl::Duration>,
+                "Duration is documented as being trivially destructible");
   constexpr absl::Duration d0 = absl::ZeroDuration();
   static_assert(d0 == absl::ZeroDuration(), "ZeroDuration()");
   constexpr absl::Duration d1 = absl::Seconds(1);
@@ -206,19 +221,19 @@ TEST(Duration, ToConversionDeprecated) {
 
 template <int64_t N>
 void TestFromChronoBasicEquality() {
-  using std::chrono::nanoseconds;
+  using std::chrono::hours;
   using std::chrono::microseconds;
   using std::chrono::milliseconds;
-  using std::chrono::seconds;
   using std::chrono::minutes;
-  using std::chrono::hours;
+  using std::chrono::nanoseconds;
+  using std::chrono::seconds;
 
-  static_assert(absl::Nanoseconds(N) == absl::FromChrono(nanoseconds(N)), "");
-  static_assert(absl::Microseconds(N) == absl::FromChrono(microseconds(N)), "");
-  static_assert(absl::Milliseconds(N) == absl::FromChrono(milliseconds(N)), "");
-  static_assert(absl::Seconds(N) == absl::FromChrono(seconds(N)), "");
-  static_assert(absl::Minutes(N) == absl::FromChrono(minutes(N)), "");
-  static_assert(absl::Hours(N) == absl::FromChrono(hours(N)), "");
+  static_assert(absl::Nanoseconds(N) == absl::FromChrono(nanoseconds(N)));
+  static_assert(absl::Microseconds(N) == absl::FromChrono(microseconds(N)));
+  static_assert(absl::Milliseconds(N) == absl::FromChrono(milliseconds(N)));
+  static_assert(absl::Seconds(N) == absl::FromChrono(seconds(N)));
+  static_assert(absl::Minutes(N) == absl::FromChrono(minutes(N)));
+  static_assert(absl::Hours(N) == absl::FromChrono(hours(N)));
 }
 
 TEST(Duration, FromChrono) {
@@ -271,12 +286,12 @@ TEST(Duration, FromChrono) {
 
 template <int64_t N>
 void TestToChrono() {
-  using std::chrono::nanoseconds;
+  using std::chrono::hours;
   using std::chrono::microseconds;
   using std::chrono::milliseconds;
-  using std::chrono::seconds;
   using std::chrono::minutes;
-  using std::chrono::hours;
+  using std::chrono::nanoseconds;
+  using std::chrono::seconds;
 
   EXPECT_EQ(nanoseconds(N), absl::ToChronoNanoseconds(absl::Nanoseconds(N)));
   EXPECT_EQ(microseconds(N), absl::ToChronoMicroseconds(absl::Microseconds(N)));
@@ -303,12 +318,12 @@ void TestToChrono() {
 }
 
 TEST(Duration, ToChrono) {
-  using std::chrono::nanoseconds;
+  using std::chrono::hours;
   using std::chrono::microseconds;
   using std::chrono::milliseconds;
-  using std::chrono::seconds;
   using std::chrono::minutes;
-  using std::chrono::hours;
+  using std::chrono::nanoseconds;
+  using std::chrono::seconds;
 
   TestToChrono<kint64min>();
   TestToChrono<-1>();
@@ -428,6 +443,15 @@ TEST(Duration, InfinityComparison) {
   EXPECT_LT(-inf, any_dur);
   EXPECT_LT(-inf, inf);
   EXPECT_GT(inf, -inf);
+
+#ifdef ABSL_INTERNAL_TIME_HAS_THREE_WAY_COMPARISON
+  EXPECT_EQ(inf <=> inf, std::strong_ordering::equal);
+  EXPECT_EQ(-inf <=> -inf, std::strong_ordering::equal);
+  EXPECT_EQ(-inf <=> inf, std::strong_ordering::less);
+  EXPECT_EQ(inf <=> -inf, std::strong_ordering::greater);
+  EXPECT_EQ(any_dur <=> inf, std::strong_ordering::less);
+  EXPECT_EQ(any_dur <=> -inf, std::strong_ordering::greater);
+#endif  // ABSL_INTERNAL_TIME_HAS_THREE_WAY_COMPARISON
 }
 
 TEST(Duration, InfinityAddition) {
@@ -493,9 +517,20 @@ TEST(Duration, InfinitySubtraction) {
   // Interesting case
   absl::Duration almost_neg_inf = sec_min;
   EXPECT_LT(-inf, almost_neg_inf);
+
+#ifdef ABSL_INTERNAL_TIME_HAS_THREE_WAY_COMPARISON
+  EXPECT_EQ(-inf <=> almost_neg_inf, std::strong_ordering::less);
+  EXPECT_EQ(almost_neg_inf <=> -inf, std::strong_ordering::greater);
+#endif  // ABSL_INTERNAL_TIME_HAS_THREE_WAY_COMPARISON
+
   almost_neg_inf -= -absl::Nanoseconds(1);
   EXPECT_LT(-inf, almost_neg_inf);
 
+#ifdef ABSL_INTERNAL_TIME_HAS_THREE_WAY_COMPARISON
+  EXPECT_EQ(-inf <=> almost_neg_inf, std::strong_ordering::less);
+  EXPECT_EQ(almost_neg_inf <=> -inf, std::strong_ordering::greater);
+#endif  // ABSL_INTERNAL_TIME_HAS_THREE_WAY_COMPARISON
+
   // For reference: IEEE 754 behavior
   const double dbl_inf = std::numeric_limits<double>::infinity();
   EXPECT_TRUE(std::isnan(dbl_inf - dbl_inf));  // We return inf
@@ -509,18 +544,18 @@ TEST(Duration, InfinityMultiplication) {
   const absl::Duration sec_min = absl::Seconds(kint64min);
   const absl::Duration inf = absl::InfiniteDuration();
 
-#define TEST_INF_MUL_WITH_TYPE(T)                                     \
-  EXPECT_EQ(inf, inf * static_cast<T>(2));                            \
-  EXPECT_EQ(-inf, inf * static_cast<T>(-2));                          \
-  EXPECT_EQ(-inf, -inf * static_cast<T>(2));                          \
-  EXPECT_EQ(inf, -inf * static_cast<T>(-2));                          \
-  EXPECT_EQ(inf, inf * static_cast<T>(0));                            \
-  EXPECT_EQ(-inf, -inf * static_cast<T>(0));                          \
-  EXPECT_EQ(inf, sec_max * static_cast<T>(2));                        \
-  EXPECT_EQ(inf, sec_min * static_cast<T>(-2));                       \
-  EXPECT_EQ(inf, (sec_max / static_cast<T>(2)) * static_cast<T>(3));  \
-  EXPECT_EQ(-inf, sec_max * static_cast<T>(-2));                      \
-  EXPECT_EQ(-inf, sec_min * static_cast<T>(2));                       \
+#define TEST_INF_MUL_WITH_TYPE(T)                                    \
+  EXPECT_EQ(inf, inf* static_cast<T>(2));                            \
+  EXPECT_EQ(-inf, inf* static_cast<T>(-2));                          \
+  EXPECT_EQ(-inf, -inf* static_cast<T>(2));                          \
+  EXPECT_EQ(inf, -inf* static_cast<T>(-2));                          \
+  EXPECT_EQ(inf, inf* static_cast<T>(0));                            \
+  EXPECT_EQ(-inf, -inf* static_cast<T>(0));                          \
+  EXPECT_EQ(inf, sec_max* static_cast<T>(2));                        \
+  EXPECT_EQ(inf, sec_min* static_cast<T>(-2));                       \
+  EXPECT_EQ(inf, (sec_max / static_cast<T>(2)) * static_cast<T>(3)); \
+  EXPECT_EQ(-inf, sec_max* static_cast<T>(-2));                      \
+  EXPECT_EQ(-inf, sec_min* static_cast<T>(2));                       \
   EXPECT_EQ(-inf, (sec_min / static_cast<T>(2)) * static_cast<T>(3));
 
   TEST_INF_MUL_WITH_TYPE(int64_t);  // NOLINT(readability/function)
@@ -762,11 +797,6 @@ TEST(Duration, DivisionByZero) {
   const double dbl_inf = std::numeric_limits<double>::infinity();
   const double dbl_denorm = std::numeric_limits<double>::denorm_min();
 
-  // IEEE 754 behavior
-  double z = 0.0, two = 2.0;
-  EXPECT_TRUE(std::isinf(two / z));
-  EXPECT_TRUE(std::isnan(z / z));  // We'll return inf
-
   // Operator/(Duration, double)
   EXPECT_EQ(inf, zero / 0.0);
   EXPECT_EQ(-inf, zero / -0.0);
@@ -809,18 +839,18 @@ TEST(Duration, DivisionByZero) {
 
 TEST(Duration, NaN) {
   // Note that IEEE 754 does not define the behavior of a nan's sign when it is
-  // copied, so the code below allows for either + or - InfiniteDuration.
+  // copied. We return -InfiniteDuration in either case.
 #define TEST_NAN_HANDLING(NAME, NAN)           \
   do {                                         \
     const auto inf = absl::InfiniteDuration(); \
     auto x = NAME(NAN);                        \
-    EXPECT_TRUE(x == inf || x == -inf);        \
+    EXPECT_TRUE(x == -inf);                    \
     auto y = NAME(42);                         \
     y *= NAN;                                  \
-    EXPECT_TRUE(y == inf || y == -inf);        \
+    EXPECT_TRUE(y == -inf);                    \
     auto z = NAME(42);                         \
     z /= NAN;                                  \
-    EXPECT_TRUE(z == inf || z == -inf);        \
+    EXPECT_TRUE(z == -inf);                    \
   } while (0)
 
   const double nan = std::numeric_limits<double>::quiet_NaN();
@@ -859,18 +889,33 @@ TEST(Duration, Range) {
 
   EXPECT_LT(neg_full_range, full_range);
   EXPECT_EQ(neg_full_range, -full_range);
+
+#ifdef ABSL_INTERNAL_TIME_HAS_THREE_WAY_COMPARISON
+  EXPECT_EQ(range_future <=> absl::InfiniteDuration(),
+            std::strong_ordering::less);
+  EXPECT_EQ(range_past <=> -absl::InfiniteDuration(),
+            std::strong_ordering::greater);
+  EXPECT_EQ(full_range <=> absl::ZeroDuration(),  //
+            std::strong_ordering::greater);
+  EXPECT_EQ(full_range <=> -absl::InfiniteDuration(),
+            std::strong_ordering::greater);
+  EXPECT_EQ(neg_full_range <=> -absl::InfiniteDuration(),
+            std::strong_ordering::greater);
+  EXPECT_EQ(neg_full_range <=> full_range, std::strong_ordering::less);
+  EXPECT_EQ(neg_full_range <=> -full_range, std::strong_ordering::equal);
+#endif  // ABSL_INTERNAL_TIME_HAS_THREE_WAY_COMPARISON
 }
 
 TEST(Duration, RelationalOperators) {
-#define TEST_REL_OPS(UNIT)               \
-  static_assert(UNIT(2) == UNIT(2), ""); \
-  static_assert(UNIT(1) != UNIT(2), ""); \
-  static_assert(UNIT(1) < UNIT(2), "");  \
-  static_assert(UNIT(3) > UNIT(2), "");  \
-  static_assert(UNIT(1) <= UNIT(2), ""); \
-  static_assert(UNIT(2) <= UNIT(2), ""); \
-  static_assert(UNIT(3) >= UNIT(2), ""); \
-  static_assert(UNIT(2) >= UNIT(2), "");
+#define TEST_REL_OPS(UNIT)           \
+  static_assert(UNIT(2) == UNIT(2)); \
+  static_assert(UNIT(1) != UNIT(2)); \
+  static_assert(UNIT(1) < UNIT(2));  \
+  static_assert(UNIT(3) > UNIT(2));  \
+  static_assert(UNIT(1) <= UNIT(2)); \
+  static_assert(UNIT(2) <= UNIT(2)); \
+  static_assert(UNIT(3) >= UNIT(2)); \
+  static_assert(UNIT(2) >= UNIT(2));
 
   TEST_REL_OPS(absl::Nanoseconds);
   TEST_REL_OPS(absl::Microseconds);
@@ -882,6 +927,26 @@ TEST(Duration, RelationalOperators) {
 #undef TEST_REL_OPS
 }
 
+#ifdef ABSL_INTERNAL_TIME_HAS_THREE_WAY_COMPARISON
+
+TEST(Duration, SpaceshipOperators) {
+#define TEST_REL_OPS(UNIT)                                           \
+  static_assert(UNIT(2) <=> UNIT(2) == std::strong_ordering::equal); \
+  static_assert(UNIT(1) <=> UNIT(2) == std::strong_ordering::less);  \
+  static_assert(UNIT(3) <=> UNIT(2) == std::strong_ordering::greater);
+
+  TEST_REL_OPS(absl::Nanoseconds);
+  TEST_REL_OPS(absl::Microseconds);
+  TEST_REL_OPS(absl::Milliseconds);
+  TEST_REL_OPS(absl::Seconds);
+  TEST_REL_OPS(absl::Minutes);
+  TEST_REL_OPS(absl::Hours);
+
+#undef TEST_REL_OPS
+}
+
+#endif  // ABSL_INTERNAL_TIME_HAS_THREE_WAY_COMPARISON
+
 TEST(Duration, Addition) {
 #define TEST_ADD_OPS(UNIT)                  \
   do {                                      \
@@ -1050,7 +1115,7 @@ TEST(Duration, Multiplication) {
   EXPECT_EQ(absl::Seconds(666666666) + absl::Nanoseconds(666666667) +
                 absl::Nanoseconds(1) / 2,
             sigfigs / 3);
-  sigfigs = absl::Seconds(7000000000LL);
+  sigfigs = absl::Seconds(int64_t{7000000000});
   EXPECT_EQ(absl::Seconds(2333333333) + absl::Nanoseconds(333333333) +
                 absl::Nanoseconds(1) / 4,
             sigfigs / 3);
@@ -1092,8 +1157,7 @@ TEST(Duration, Multiplication) {
   EXPECT_EQ(0, absl::Nanoseconds(-1) / absl::Seconds(1));  // Actual -1e-9
 
   // Tests identity a = (a/b)*b + a%b
-#define TEST_MOD_IDENTITY(a, b) \
-  EXPECT_EQ((a), ((a) / (b))*(b) + ((a)%(b)))
+#define TEST_MOD_IDENTITY(a, b) EXPECT_EQ((a), ((a) / (b)) * (b) + ((a) % (b)))
 
   TEST_MOD_IDENTITY(absl::Seconds(0), absl::Seconds(2));
   TEST_MOD_IDENTITY(absl::Seconds(1), absl::Seconds(1));
@@ -1325,7 +1389,7 @@ TEST(Duration, SmallConversions) {
 
   EXPECT_EQ(absl::ZeroDuration(), absl::Seconds(0));
   // TODO(bww): Is the next one OK?
-  EXPECT_EQ(absl::ZeroDuration(), absl::Seconds(0.124999999e-9));
+  EXPECT_EQ(absl::ZeroDuration(), absl::Seconds(std::nextafter(0.125e-9, 0)));
   EXPECT_EQ(absl::Nanoseconds(1) / 4, absl::Seconds(0.125e-9));
   EXPECT_EQ(absl::Nanoseconds(1) / 4, absl::Seconds(0.250e-9));
   EXPECT_EQ(absl::Nanoseconds(1) / 2, absl::Seconds(0.375e-9));
@@ -1335,7 +1399,7 @@ TEST(Duration, SmallConversions) {
   EXPECT_EQ(absl::Nanoseconds(1), absl::Seconds(0.875e-9));
   EXPECT_EQ(absl::Nanoseconds(1), absl::Seconds(1.000e-9));
 
-  EXPECT_EQ(absl::ZeroDuration(), absl::Seconds(-0.124999999e-9));
+  EXPECT_EQ(absl::ZeroDuration(), absl::Seconds(std::nextafter(-0.125e-9, 0)));
   EXPECT_EQ(-absl::Nanoseconds(1) / 4, absl::Seconds(-0.125e-9));
   EXPECT_EQ(-absl::Nanoseconds(1) / 4, absl::Seconds(-0.250e-9));
   EXPECT_EQ(-absl::Nanoseconds(1) / 2, absl::Seconds(-0.375e-9));
@@ -1374,10 +1438,13 @@ TEST(Duration, SmallConversions) {
   EXPECT_THAT(ToTimeval(absl::Nanoseconds(2000)), TimevalMatcher(tv));
 }
 
-void VerifySameAsMul(double time_as_seconds, int* const misses) {
+void VerifyApproxSameAsMul(double time_as_seconds, int* const misses) {
   auto direct_seconds = absl::Seconds(time_as_seconds);
   auto mul_by_one_second = time_as_seconds * absl::Seconds(1);
-  if (direct_seconds != mul_by_one_second) {
+  // These are expected to differ by up to one tick due to fused multiply/add
+  // contraction.
+  if (absl::AbsDuration(direct_seconds - mul_by_one_second) >
+      absl::time_internal::MakeDuration(0, 1u)) {
     if (*misses > 10) return;
     ASSERT_LE(++(*misses), 10) << "Too many errors, not reporting more.";
     EXPECT_EQ(direct_seconds, mul_by_one_second)
@@ -1389,8 +1456,17 @@ void VerifySameAsMul(double time_as_seconds, int* const misses) {
 // For a variety of interesting durations, we find the exact point
 // where one double converts to that duration, and the very next double
 // converts to the next duration.  For both of those points, verify that
-// Seconds(point) returns the same duration as point * Seconds(1.0)
+// Seconds(point) returns a duration near point * Seconds(1.0). (They may
+// not be exactly equal due to fused multiply/add contraction.)
 TEST(Duration, ToDoubleSecondsCheckEdgeCases) {
+#if (defined(__i386__) || defined(_M_IX86)) && FLT_EVAL_METHOD != 0
+  // We're using an x87-compatible FPU, and intermediate operations can be
+  // performed with 80-bit floats. This means the edge cases are different than
+  // what we expect here, so just skip this test.
+  GTEST_SKIP()
+      << "Skipping the test because we detected x87 floating-point semantics";
+#endif
+
   constexpr uint32_t kTicksPerSecond = absl::time_internal::kTicksPerSecond;
   constexpr auto duration_tick = absl::time_internal::MakeDuration(0, 1u);
   int misses = 0;
@@ -1428,17 +1504,15 @@ TEST(Duration, ToDoubleSecondsCheckEdgeCases) {
         }
         // Now low_edge is the highest double that converts to Duration d,
         // and high_edge is the lowest double that converts to Duration after_d.
-        VerifySameAsMul(low_edge, &misses);
-        VerifySameAsMul(high_edge, &misses);
+        VerifyApproxSameAsMul(low_edge, &misses);
+        VerifyApproxSameAsMul(high_edge, &misses);
       }
     }
   }
 }
 
 TEST(Duration, ToDoubleSecondsCheckRandom) {
-  std::random_device rd;
-  std::seed_seq seed({rd(), rd(), rd(), rd(), rd(), rd(), rd(), rd()});
-  std::mt19937_64 gen(seed);
+  absl::InsecureBitGen gen;
   // We want doubles distributed from 1/8ns up to 2^63, where
   // as many values are tested from 1ns to 2ns as from 1sec to 2sec,
   // so even distribute along a log-scale of those values, and
@@ -1449,8 +1523,8 @@ TEST(Duration, ToDoubleSecondsCheckRandom) {
   int misses = 0;
   for (int i = 0; i < 1000000; ++i) {
     double d = std::exp(uniform(gen));
-    VerifySameAsMul(d, &misses);
-    VerifySameAsMul(-d, &misses);
+    VerifyApproxSameAsMul(d, &misses);
+    VerifyApproxSameAsMul(-d, &misses);
   }
 }
 
@@ -1810,4 +1884,18 @@ TEST(Duration, FormatParseRoundTrip) {
 #undef TEST_PARSE_ROUNDTRIP
 }
 
+TEST(Duration, AbslStringify) {
+  // FormatDuration is already well tested, so just use one test case here to
+  // verify that StrFormat("%v", d) works as expected.
+  absl::Duration d = absl::Seconds(1);
+  EXPECT_EQ(absl::StrFormat("%v", d), absl::FormatDuration(d));
+}
+
+TEST(Duration, NoPadding) {
+  // Should match the size of a struct with uint32_t alignment and no padding.
+  using NoPadding = std::array<uint32_t, 3>;
+  EXPECT_EQ(sizeof(NoPadding), sizeof(absl::Duration));
+  EXPECT_EQ(alignof(NoPadding), alignof(absl::Duration));
+}
+
 }  // namespace
diff --git a/absl/time/flag_test.cc b/absl/time/flag_test.cc
new file mode 100644
index 00000000000..8f8532b7eb7
--- /dev/null
+++ b/absl/time/flag_test.cc
@@ -0,0 +1,147 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/flags/flag.h"
+
+#include <string>
+
+#include "gtest/gtest.h"
+#include "absl/flags/reflection.h"
+#include "absl/time/civil_time.h"
+#include "absl/time/time.h"
+
+ABSL_FLAG(absl::CivilSecond, test_flag_civil_second,
+          absl::CivilSecond(2015, 1, 2, 3, 4, 5), "");
+ABSL_FLAG(absl::CivilMinute, test_flag_civil_minute,
+          absl::CivilMinute(2015, 1, 2, 3, 4), "");
+ABSL_FLAG(absl::CivilHour, test_flag_civil_hour, absl::CivilHour(2015, 1, 2, 3),
+          "");
+ABSL_FLAG(absl::CivilDay, test_flag_civil_day, absl::CivilDay(2015, 1, 2), "");
+ABSL_FLAG(absl::CivilMonth, test_flag_civil_month, absl::CivilMonth(2015, 1),
+          "");
+ABSL_FLAG(absl::CivilYear, test_flag_civil_year, absl::CivilYear(2015), "");
+
+ABSL_FLAG(absl::Duration, test_duration_flag, absl::Seconds(5),
+          "For testing support for Duration flags");
+ABSL_FLAG(absl::Time, test_time_flag, absl::InfinitePast(),
+          "For testing support for Time flags");
+
+namespace {
+
+bool SetFlagValue(absl::string_view flag_name, absl::string_view value) {
+  auto* flag = absl::FindCommandLineFlag(flag_name);
+  if (!flag) return false;
+  std::string err;
+  return flag->ParseFrom(value, &err);
+}
+
+bool GetFlagValue(absl::string_view flag_name, std::string& value) {
+  auto* flag = absl::FindCommandLineFlag(flag_name);
+  if (!flag) return false;
+  value = flag->CurrentValue();
+  return true;
+}
+
+TEST(CivilTime, FlagSupport) {
+  // Tests the default setting of the flags.
+  const absl::CivilSecond kDefaultSec(2015, 1, 2, 3, 4, 5);
+  EXPECT_EQ(absl::CivilSecond(kDefaultSec),
+            absl::GetFlag(FLAGS_test_flag_civil_second));
+  EXPECT_EQ(absl::CivilMinute(kDefaultSec),
+            absl::GetFlag(FLAGS_test_flag_civil_minute));
+  EXPECT_EQ(absl::CivilHour(kDefaultSec),
+            absl::GetFlag(FLAGS_test_flag_civil_hour));
+  EXPECT_EQ(absl::CivilDay(kDefaultSec),
+            absl::GetFlag(FLAGS_test_flag_civil_day));
+  EXPECT_EQ(absl::CivilMonth(kDefaultSec),
+            absl::GetFlag(FLAGS_test_flag_civil_month));
+  EXPECT_EQ(absl::CivilYear(kDefaultSec),
+            absl::GetFlag(FLAGS_test_flag_civil_year));
+
+  // Sets flags to a new value.
+  const absl::CivilSecond kNewSec(2016, 6, 7, 8, 9, 10);
+  absl::SetFlag(&FLAGS_test_flag_civil_second, absl::CivilSecond(kNewSec));
+  absl::SetFlag(&FLAGS_test_flag_civil_minute, absl::CivilMinute(kNewSec));
+  absl::SetFlag(&FLAGS_test_flag_civil_hour, absl::CivilHour(kNewSec));
+  absl::SetFlag(&FLAGS_test_flag_civil_day, absl::CivilDay(kNewSec));
+  absl::SetFlag(&FLAGS_test_flag_civil_month, absl::CivilMonth(kNewSec));
+  absl::SetFlag(&FLAGS_test_flag_civil_year, absl::CivilYear(kNewSec));
+
+  EXPECT_EQ(absl::CivilSecond(kNewSec),
+            absl::GetFlag(FLAGS_test_flag_civil_second));
+  EXPECT_EQ(absl::CivilMinute(kNewSec),
+            absl::GetFlag(FLAGS_test_flag_civil_minute));
+  EXPECT_EQ(absl::CivilHour(kNewSec),
+            absl::GetFlag(FLAGS_test_flag_civil_hour));
+  EXPECT_EQ(absl::CivilDay(kNewSec), absl::GetFlag(FLAGS_test_flag_civil_day));
+  EXPECT_EQ(absl::CivilMonth(kNewSec),
+            absl::GetFlag(FLAGS_test_flag_civil_month));
+  EXPECT_EQ(absl::CivilYear(kNewSec),
+            absl::GetFlag(FLAGS_test_flag_civil_year));
+}
+
+TEST(Duration, FlagSupport) {
+  EXPECT_EQ(absl::Seconds(5), absl::GetFlag(FLAGS_test_duration_flag));
+
+  absl::SetFlag(&FLAGS_test_duration_flag, absl::Seconds(10));
+  EXPECT_EQ(absl::Seconds(10), absl::GetFlag(FLAGS_test_duration_flag));
+
+  EXPECT_TRUE(SetFlagValue("test_duration_flag", "20s"));
+  EXPECT_EQ(absl::Seconds(20), absl::GetFlag(FLAGS_test_duration_flag));
+
+  std::string current_flag_value;
+  EXPECT_TRUE(GetFlagValue("test_duration_flag", current_flag_value));
+  EXPECT_EQ("20s", current_flag_value);
+}
+
+TEST(Time, FlagSupport) {
+  EXPECT_EQ(absl::InfinitePast(), absl::GetFlag(FLAGS_test_time_flag));
+
+  const absl::Time t = absl::FromCivil(absl::CivilSecond(2016, 1, 2, 3, 4, 5),
+                                       absl::UTCTimeZone());
+  absl::SetFlag(&FLAGS_test_time_flag, t);
+  EXPECT_EQ(t, absl::GetFlag(FLAGS_test_time_flag));
+
+  // Successful parse
+  EXPECT_TRUE(SetFlagValue("test_time_flag", "2016-01-02T03:04:06Z"));
+  EXPECT_EQ(t + absl::Seconds(1), absl::GetFlag(FLAGS_test_time_flag));
+  EXPECT_TRUE(SetFlagValue("test_time_flag", "2016-01-02T03:04:07.0Z"));
+  EXPECT_EQ(t + absl::Seconds(2), absl::GetFlag(FLAGS_test_time_flag));
+  EXPECT_TRUE(SetFlagValue("test_time_flag", "2016-01-02T03:04:08.000Z"));
+  EXPECT_EQ(t + absl::Seconds(3), absl::GetFlag(FLAGS_test_time_flag));
+  EXPECT_TRUE(SetFlagValue("test_time_flag", "2016-01-02T03:04:09+00:00"));
+  EXPECT_EQ(t + absl::Seconds(4), absl::GetFlag(FLAGS_test_time_flag));
+  EXPECT_TRUE(SetFlagValue("test_time_flag", "2016-01-02T03:04:05.123+00:00"));
+  EXPECT_EQ(t + absl::Milliseconds(123), absl::GetFlag(FLAGS_test_time_flag));
+  EXPECT_TRUE(SetFlagValue("test_time_flag", "2016-01-02T03:04:05.123+08:00"));
+  EXPECT_EQ(t + absl::Milliseconds(123) - absl::Hours(8),
+            absl::GetFlag(FLAGS_test_time_flag));
+  EXPECT_TRUE(SetFlagValue("test_time_flag", "infinite-future"));
+  EXPECT_EQ(absl::InfiniteFuture(), absl::GetFlag(FLAGS_test_time_flag));
+  EXPECT_TRUE(SetFlagValue("test_time_flag", "infinite-past"));
+  EXPECT_EQ(absl::InfinitePast(), absl::GetFlag(FLAGS_test_time_flag));
+
+  EXPECT_FALSE(SetFlagValue("test_time_flag", "2016-01-02T03:04:06"));
+  EXPECT_FALSE(SetFlagValue("test_time_flag", "2016-01-02"));
+  EXPECT_FALSE(SetFlagValue("test_time_flag", "2016-01-02Z"));
+  EXPECT_FALSE(SetFlagValue("test_time_flag", "2016-01-02+00:00"));
+  EXPECT_FALSE(SetFlagValue("test_time_flag", "2016-99-99T03:04:06Z"));
+
+  EXPECT_TRUE(SetFlagValue("test_time_flag", "2016-01-02T03:04:05Z"));
+  std::string current_flag_value;
+  EXPECT_TRUE(GetFlagValue("test_time_flag", current_flag_value));
+  EXPECT_EQ("2016-01-02T03:04:05+00:00", current_flag_value);
+}
+
+}  // namespace
diff --git a/absl/time/format.cc b/absl/time/format.cc
index d6ca860038d..aa2ae2c5493 100644
--- a/absl/time/format.cc
+++ b/absl/time/format.cc
@@ -13,26 +13,32 @@
 // limitations under the License.
 
 #include <string.h>
+
 #include <cctype>
 #include <cstdint>
+#include <utility>
 
+#include "absl/strings/ascii.h"
+#include "absl/strings/match.h"
+#include "absl/strings/string_view.h"
 #include "absl/time/internal/cctz/include/cctz/time_zone.h"
 #include "absl/time/time.h"
 
 namespace cctz = absl::time_internal::cctz;
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
-extern const char RFC3339_full[] = "%Y-%m-%dT%H:%M:%E*S%Ez";
-extern const char RFC3339_sec[] =  "%Y-%m-%dT%H:%M:%S%Ez";
+ABSL_DLL extern const char RFC3339_full[] = "%Y-%m-%d%ET%H:%M:%E*S%Ez";
+ABSL_DLL extern const char RFC3339_sec[] = "%Y-%m-%d%ET%H:%M:%S%Ez";
 
-extern const char RFC1123_full[] = "%a, %d %b %E4Y %H:%M:%S %z";
-extern const char RFC1123_no_wday[] =  "%d %b %E4Y %H:%M:%S %z";
+ABSL_DLL extern const char RFC1123_full[] = "%a, %d %b %E4Y %H:%M:%S %z";
+ABSL_DLL extern const char RFC1123_no_wday[] = "%d %b %E4Y %H:%M:%S %z";
 
 namespace {
 
-const char kInfiniteFutureStr[] = "infinite-future";
-const char kInfinitePastStr[] = "infinite-past";
+constexpr absl::string_view kInfiniteFutureStr = "infinite-future";
+constexpr absl::string_view kInfinitePastStr = "infinite-past";
 
 struct cctz_parts {
   cctz::time_point<cctz::seconds> sec;
@@ -60,19 +66,20 @@ cctz_parts Split(absl::Time t) {
 // details about rep_hi and rep_lo.
 absl::Time Join(const cctz_parts& parts) {
   const int64_t rep_hi = (parts.sec - unix_epoch()).count();
-  const uint32_t rep_lo = parts.fem.count() / (1000 * 1000 / 4);
+  const uint32_t rep_lo =
+      static_cast<uint32_t>(parts.fem.count() / (1000 * 1000 / 4));
   const auto d = time_internal::MakeDuration(rep_hi, rep_lo);
   return time_internal::FromUnixDuration(d);
 }
 
 }  // namespace
 
-std::string FormatTime(const std::string& format, absl::Time t,
+std::string FormatTime(absl::string_view format, absl::Time t,
                        absl::TimeZone tz) {
-  if (t == absl::InfiniteFuture()) return kInfiniteFutureStr;
-  if (t == absl::InfinitePast()) return kInfinitePastStr;
+  if (t == absl::InfiniteFuture()) return std::string(kInfiniteFutureStr);
+  if (t == absl::InfinitePast()) return std::string(kInfinitePastStr);
   const auto parts = Split(t);
-  return cctz::detail::format(format, parts.sec, parts.fem,
+  return cctz::detail::format(std::string(format), parts.sec, parts.fem,
                               cctz::time_zone(tz));
 }
 
@@ -84,57 +91,55 @@ std::string FormatTime(absl::Time t) {
   return absl::FormatTime(RFC3339_full, t, absl::LocalTimeZone());
 }
 
-bool ParseTime(const std::string& format, const std::string& input,
+bool ParseTime(absl::string_view format, absl::string_view input,
                absl::Time* time, std::string* err) {
   return absl::ParseTime(format, input, absl::UTCTimeZone(), time, err);
 }
 
 // If the input string does not contain an explicit UTC offset, interpret
 // the fields with respect to the given TimeZone.
-bool ParseTime(const std::string& format, const std::string& input,
+bool ParseTime(absl::string_view format, absl::string_view input,
                absl::TimeZone tz, absl::Time* time, std::string* err) {
-  const char* data = input.c_str();
-  while (std::isspace(*data)) ++data;
-
-  size_t inf_size = strlen(kInfiniteFutureStr);
-  if (strncmp(data, kInfiniteFutureStr, inf_size) == 0) {
-    const char* new_data = data + inf_size;
-    while (std::isspace(*new_data)) ++new_data;
-    if (*new_data == '\0') {
-      *time = InfiniteFuture();
-      return true;
-    }
-  }
-
-  inf_size = strlen(kInfinitePastStr);
-  if (strncmp(data, kInfinitePastStr, inf_size) == 0) {
-    const char* new_data = data + inf_size;
-    while (std::isspace(*new_data)) ++new_data;
-    if (*new_data == '\0') {
-      *time = InfinitePast();
-      return true;
+  static constexpr struct Literal {
+    absl::string_view name;
+    absl::Time value;
+  } kLiterals[] = {
+      {kInfiniteFutureStr, InfiniteFuture()},
+      {kInfinitePastStr, InfinitePast()},
+  };
+  input = StripLeadingAsciiWhitespace(input);
+  for (const auto& lit : kLiterals) {
+    if (absl::StartsWith(input, lit.name)) {
+      absl::string_view tail = input.substr(lit.name.size());
+      // The trailing portion must be empty or whitespace.
+      if (StripLeadingAsciiWhitespace(tail).empty()) {
+        *time = lit.value;
+        return true;
+      }
     }
   }
 
   std::string error;
   cctz_parts parts;
-  const bool b = cctz::detail::parse(format, input, cctz::time_zone(tz),
-                                     &parts.sec, &parts.fem, &error);
+  const bool b =
+      cctz::detail::parse(std::string(format), std::string(input),
+                          cctz::time_zone(tz), &parts.sec, &parts.fem, &error);
   if (b) {
     *time = Join(parts);
   } else if (err != nullptr) {
-    *err = error;
+    *err = std::move(error);
   }
   return b;
 }
 
 // Functions required to support absl::Time flags.
-bool ParseFlag(const std::string& text, absl::Time* t, std::string* error) {
+bool AbslParseFlag(absl::string_view text, absl::Time* t, std::string* error) {
   return absl::ParseTime(RFC3339_full, text, absl::UTCTimeZone(), t, error);
 }
 
-std::string UnparseFlag(absl::Time t) {
+std::string AbslUnparseFlag(absl::Time t) {
   return absl::FormatTime(RFC3339_full, t, absl::UTCTimeZone());
 }
 
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/time/format_benchmark.cc b/absl/time/format_benchmark.cc
index 249c51d8758..45109faf674 100644
--- a/absl/time/format_benchmark.cc
+++ b/absl/time/format_benchmark.cc
@@ -12,6 +12,7 @@
 // limitations under the License.
 
 #include <cstddef>
+#include <iterator>
 #include <string>
 
 #include "absl/time/internal/test_util.h"
@@ -26,10 +27,10 @@ const char* const kFormats[] = {
     absl::RFC1123_no_wday,  // 1
     absl::RFC3339_full,     // 2
     absl::RFC3339_sec,      // 3
-    "%Y-%m-%dT%H:%M:%S",    // 4
+    "%Y-%m-%d%ET%H:%M:%S",  // 4
     "%Y-%m-%d",             // 5
 };
-const int kNumFormats = sizeof(kFormats) / sizeof(kFormats[0]);
+const int kNumFormats = std::size(kFormats);
 }  // namespace
 
 void BM_Format_FormatTime(benchmark::State& state) {
diff --git a/absl/time/format_test.cc b/absl/time/format_test.cc
index ab1f30594ea..38c0e3d89db 100644
--- a/absl/time/format_test.cc
+++ b/absl/time/format_test.cc
@@ -173,7 +173,7 @@ TEST(ParseTime, WithTimeZone) {
   absl::Time t;
   std::string e;
 
-  // We can parse a std::string without a UTC offset if we supply a timezone.
+  // We can parse a string without a UTC offset if we supply a timezone.
   EXPECT_TRUE(
       absl::ParseTime("%Y-%m-%d %H:%M:%S", "2013-06-28 19:08:09", tz, &t, &e))
       << e;
@@ -245,6 +245,18 @@ TEST(ParseTime, ErrorCases) {
   EXPECT_THAT(err, HasSubstr("Illegal trailing data"));
   EXPECT_FALSE(absl::ParseTime("%Ez", "-00:-0", &t, &err)) << err;
   EXPECT_THAT(err, HasSubstr("Illegal trailing data"));
+
+  // Check that we do not accept strings with embedded NULs.
+  EXPECT_FALSE(
+      absl::ParseTime("%Y", std::string("2026\0payload", 12), &t, &err));
+  EXPECT_THAT(err, HasSubstr("Illegal trailing data"));
+
+  // High-bit character test for sign-extension bugs.
+  for (int i = 128; i < 256; ++i) {
+    char c = static_cast<char>(i);
+    std::string input = std::string(1, c) + "2015-01-02";
+    EXPECT_FALSE(absl::ParseTime("%Y-%m-%d", input, &t, &err));
+  }
 }
 
 TEST(ParseTime, ExtendedSeconds) {
@@ -327,7 +339,7 @@ TEST(ParseTime, InfiniteTime) {
   EXPECT_TRUE(absl::ParseTime("%H:%M blah", "  infinite-past  ", &t, &err));
   EXPECT_EQ(absl::InfinitePast(), t);
 
-  // "infinite-future" as literal std::string
+  // "infinite-future" as literal string
   absl::TimeZone tz = absl::UTCTimeZone();
   EXPECT_TRUE(absl::ParseTime("infinite-future %H:%M", "infinite-future 03:04",
                               &t, &err));
@@ -335,7 +347,7 @@ TEST(ParseTime, InfiniteTime) {
   EXPECT_EQ(3, tz.At(t).cs.hour());
   EXPECT_EQ(4, tz.At(t).cs.minute());
 
-  // "infinite-past" as literal std::string
+  // "infinite-past" as literal string
   EXPECT_TRUE(
       absl::ParseTime("infinite-past %H:%M", "infinite-past 03:04", &t, &err));
   EXPECT_NE(absl::InfinitePast(), t);
diff --git a/absl/time/internal/cctz/BUILD.bazel b/absl/time/internal/cctz/BUILD.bazel
index 9fceffe2b76..edec2820737 100644
--- a/absl/time/internal/cctz/BUILD.bazel
+++ b/absl/time/internal/cctz/BUILD.bazel
@@ -12,25 +12,18 @@
 #   See the License for the specific language governing permissions and
 #   limitations under the License.
 
-load("@rules_cc//cc:defs.bzl", "cc_library", "cc_test")
+load("@rules_cc//cc:cc_binary.bzl", "cc_binary")
+load("@rules_cc//cc:cc_library.bzl", "cc_library")
+load("@rules_cc//cc:cc_test.bzl", "cc_test")
+load("//absl:copts/configure_copts.bzl", "ABSL_DEFAULT_COPTS", "ABSL_DEFAULT_LINKOPTS", "ABSL_TEST_COPTS")
 
-package(features = ["-parse_headers"])
+package(features = [
+    "header_modules",
+    "layering_check",
+    "parse_headers",
+])
 
-licenses(["notice"])  # Apache License
-
-config_setting(
-    name = "osx",
-    constraint_values = [
-        "@bazel_tools//platforms:osx",
-    ],
-)
-
-config_setting(
-    name = "ios",
-    constraint_values = [
-        "@bazel_tools//platforms:ios",
-    ],
-)
+licenses(["notice"])
 
 ### libraries
 
@@ -40,8 +33,11 @@ cc_library(
     hdrs = [
         "include/cctz/civil_time.h",
     ],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
     textual_hdrs = ["include/cctz/civil_time_detail.h"],
     visibility = ["//visibility:public"],
+    deps = ["//absl/base:config"],
 )
 
 cc_library(
@@ -63,33 +59,58 @@ cc_library(
         "src/time_zone_posix.h",
         "src/tzfile.h",
         "src/zone_info_source.cc",
-    ],
+    ] + select({
+        "@platforms//os:windows": [
+            "src/time_zone_name_win.cc",
+            "src/time_zone_name_win.h",
+        ],
+        "//conditions:default": [],
+    }),
     hdrs = [
         "include/cctz/time_zone.h",
         "include/cctz/zone_info_source.h",
     ],
-    linkopts = select({
-        ":osx": [
-            "-framework Foundation",
-        ],
-        ":ios": [
-            "-framework Foundation",
-        ],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS + select({
+        "@platforms//os:osx": ["-Wl,-framework,CoreFoundation"],
+        "@platforms//os:ios": ["-Wl,-framework,CoreFoundation"],
         "//conditions:default": [],
     }),
     visibility = ["//visibility:public"],
-    deps = [":civil_time"],
+    deps = [
+        ":civil_time",
+        "//absl/base:config",
+    ] + select(
+        {
+            "//conditions:default": [],
+        },
+    ),
 )
 
 ### tests
 
+cc_library(
+    name = "test_time_zone_names",
+    testonly = True,
+    srcs = ["src/test_time_zone_names.cc"],
+    hdrs = ["src/test_time_zone_names.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:private"],
+    deps = ["//absl/base:config"],
+)
+
 cc_test(
     name = "civil_time_test",
     size = "small",
     srcs = ["src/civil_time_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":civil_time",
-        "@com_google_googletest//:gtest_main",
+        "//absl/base:config",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -97,16 +118,21 @@ cc_test(
     name = "time_zone_format_test",
     size = "small",
     srcs = ["src/time_zone_format_test.cc"],
+    copts = ABSL_TEST_COPTS,
     data = [":zoneinfo"],
+    linkopts = ABSL_DEFAULT_LINKOPTS,
     tags = [
         "no_test_android_arm",
         "no_test_android_arm64",
         "no_test_android_x86",
+        "no_test_wasm",
     ],
     deps = [
         ":civil_time",
         ":time_zone",
-        "@com_google_googletest//:gtest_main",
+        "//absl/base:config",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -114,17 +140,61 @@ cc_test(
     name = "time_zone_lookup_test",
     size = "small",
     timeout = "moderate",
-    srcs = ["src/time_zone_lookup_test.cc"],
+    srcs = [
+        "src/time_zone_lookup_test.cc",
+        "src/tzfile.h",
+    ],
+    copts = ABSL_TEST_COPTS,
     data = [":zoneinfo"],
+    linkopts = ABSL_DEFAULT_LINKOPTS,
     tags = [
         "no_test_android_arm",
         "no_test_android_arm64",
         "no_test_android_x86",
+        "no_test_wasm",
+    ],
+    deps = [
+        ":civil_time",
+        ":test_time_zone_names",
+        ":time_zone",
+        "//absl/base:config",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "time_zone_name_win_test",
+    size = "small",
+    srcs = select({
+        "@platforms//os:windows": ["src/time_zone_name_win_test.cc"],
+        "//conditions:default": [],
+    }),
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":time_zone",
+        "//absl/base:config",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "time_zone_posix_test",
+    size = "small",
+    srcs = [
+        "src/time_zone_posix.h",
+        "src/time_zone_posix_test.cc",
     ],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":civil_time",
         ":time_zone",
-        "@com_google_googletest//:gtest_main",
+        "//absl/base:config",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -139,12 +209,16 @@ cc_test(
         "src/time_zone_info.h",
         "src/tzfile.h",
     ],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
     linkstatic = 1,
     tags = ["benchmark"],
     deps = [
         ":civil_time",
+        ":test_time_zone_names",
         ":time_zone",
-        "@com_github_google_benchmark//:benchmark_main",
+        "//absl/base:config",
+        "@google_benchmark//:benchmark_main",
     ],
 )
 
@@ -155,4 +229,5 @@ cc_test(
 filegroup(
     name = "zoneinfo",
     srcs = glob(["testdata/zoneinfo/**"]),
+    visibility = ["//absl/time:__subpackages__"],
 )
diff --git a/absl/time/internal/cctz/include/cctz/civil_time.h b/absl/time/internal/cctz/include/cctz/civil_time.h
index 19b311bbdba..d47ff86fe43 100644
--- a/absl/time/internal/cctz/include/cctz/civil_time.h
+++ b/absl/time/internal/cctz/include/cctz/civil_time.h
@@ -15,9 +15,11 @@
 #ifndef ABSL_TIME_INTERNAL_CCTZ_CIVIL_TIME_H_
 #define ABSL_TIME_INTERNAL_CCTZ_CIVIL_TIME_H_
 
+#include "absl/base/config.h"
 #include "absl/time/internal/cctz/include/cctz/civil_time_detail.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 namespace cctz {
 
@@ -324,6 +326,7 @@ using detail::get_yearday;
 
 }  // namespace cctz
 }  // namespace time_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_TIME_INTERNAL_CCTZ_CIVIL_TIME_H_
diff --git a/absl/time/internal/cctz/include/cctz/civil_time_detail.h b/absl/time/internal/cctz/include/cctz/civil_time_detail.h
index 433078a7309..fe3b8bdf822 100644
--- a/absl/time/internal/cctz/include/cctz/civil_time_detail.h
+++ b/absl/time/internal/cctz/include/cctz/civil_time_detail.h
@@ -20,6 +20,8 @@
 #include <ostream>
 #include <type_traits>
 
+#include "absl/base/config.h"
+
 // Disable constexpr support unless we are in C++14 mode.
 #if __cpp_constexpr >= 201304 || (defined(_MSC_VER) && _MSC_VER >= 1910)
 #define CONSTEXPR_D constexpr  // data
@@ -32,6 +34,7 @@
 #endif
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 namespace cctz {
 
@@ -53,8 +56,8 @@ using second_t = std::int_fast8_t;  // [0:59]
 
 // Normalized civil-time fields: Y-M-D HH:MM:SS.
 struct fields {
-  CONSTEXPR_M fields(year_t year, month_t month, day_t day,
-                     hour_t hour, minute_t minute, second_t second)
+  CONSTEXPR_M fields(year_t year, month_t month, day_t day, hour_t hour,
+                     minute_t minute, second_t second)
       : y(year), m(month), d(day), hh(hour), mm(minute), ss(second) {}
   std::int_least64_t y;
   std::int_least8_t m;
@@ -81,19 +84,30 @@ CONSTEXPR_F bool is_leap_year(year_t y) noexcept {
   return y % 4 == 0 && (y % 100 != 0 || y % 400 == 0);
 }
 CONSTEXPR_F int year_index(year_t y, month_t m) noexcept {
-  return (static_cast<int>((y + (m > 2)) % 400) + 400) % 400;
+  const int yi = static_cast<int>((y + (m > 2)) % 400);
+  return yi < 0 ? yi + 400 : yi;
 }
-CONSTEXPR_F int days_per_century(year_t y, month_t m) noexcept {
-  const int yi = year_index(y, m);
+CONSTEXPR_F int days_per_century(int yi) noexcept {
   return 36524 + (yi == 0 || yi > 300);
 }
-CONSTEXPR_F int days_per_4years(year_t y, month_t m) noexcept {
-  const int yi = year_index(y, m);
+CONSTEXPR_F int days_per_4years(int yi) noexcept {
   return 1460 + (yi == 0 || yi > 300 || (yi - 1) % 100 < 96);
 }
 CONSTEXPR_F int days_per_year(year_t y, month_t m) noexcept {
   return is_leap_year(y + (m > 2)) ? 366 : 365;
 }
+// The compiler cannot optimize away the check if we use
+// -fsanitize=array-bounds.
+// m is guaranteed to be in [1:12] in the caller, but the compiler cannot
+// optimize away the check even when this function is inlined into BreakTime.
+// To reduce the overhead, we use no_sanitize to skip the unnecessary
+// -fsanitize=array-bounds check. Remove no_sanitize once the missed
+// optimization is fixed.
+#if defined(__clang__) && defined(__has_cpp_attribute)
+#if __has_cpp_attribute(clang::no_sanitize)
+[[clang::no_sanitize("array-bounds")]]
+#endif
+#endif
 CONSTEXPR_F int days_per_month(year_t y, month_t m) noexcept {
   CONSTEXPR_D int k_days_per_month[1 + 12] = {
       -1, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31  // non leap year
@@ -101,59 +115,74 @@ CONSTEXPR_F int days_per_month(year_t y, month_t m) noexcept {
   return k_days_per_month[m] + (m == 2 && is_leap_year(y));
 }
 
-CONSTEXPR_F fields n_day(year_t y, month_t m, diff_t d, diff_t cd,
-                         hour_t hh, minute_t mm, second_t ss) noexcept {
-  y += (cd / 146097) * 400;
+CONSTEXPR_F fields n_day(year_t y, month_t m, diff_t d, diff_t cd, hour_t hh,
+                         minute_t mm, second_t ss) noexcept {
+  year_t ey = y % 400;
+  const year_t oey = ey;
+  ey += (cd / 146097) * 400;
   cd %= 146097;
   if (cd < 0) {
-    y -= 400;
+    ey -= 400;
     cd += 146097;
   }
-  y += (d / 146097) * 400;
+  ey += (d / 146097) * 400;
   d = d % 146097 + cd;
   if (d > 0) {
     if (d > 146097) {
-      y += 400;
+      ey += 400;
       d -= 146097;
     }
   } else {
     if (d > -365) {
       // We often hit the previous year when stepping a civil time backwards,
       // so special case it to avoid counting up by 100/4/1-year chunks.
-      y -= 1;
-      d += days_per_year(y, m);
+      ey -= 1;
+      d += days_per_year(ey, m);
     } else {
-      y -= 400;
+      ey -= 400;
       d += 146097;
     }
   }
   if (d > 365) {
-    for (int n = days_per_century(y, m); d > n; n = days_per_century(y, m)) {
+    int yi = year_index(ey, m);  // Index into Gregorian 400 year cycle.
+    for (;;) {
+      int n = days_per_century(yi);
+      if (d <= n) break;
       d -= n;
-      y += 100;
+      ey += 100;
+      yi += 100;
+      if (yi >= 400) yi -= 400;
     }
-    for (int n = days_per_4years(y, m); d > n; n = days_per_4years(y, m)) {
+    for (;;) {
+      int n = days_per_4years(yi);
+      if (d <= n) break;
       d -= n;
-      y += 4;
+      ey += 4;
+      yi += 4;
+      if (yi >= 400) yi -= 400;
     }
-    for (int n = days_per_year(y, m); d > n; n = days_per_year(y, m)) {
+    for (;;) {
+      int n = days_per_year(ey, m);
+      if (d <= n) break;
       d -= n;
-      ++y;
+      ++ey;
     }
   }
   if (d > 28) {
-    for (int n = days_per_month(y, m); d > n; n = days_per_month(y, m)) {
+    for (;;) {
+      int n = days_per_month(ey, m);
+      if (d <= n) break;
       d -= n;
       if (++m > 12) {
-        ++y;
+        ++ey;
         m = 1;
       }
     }
   }
-  return fields(y, m, static_cast<day_t>(d), hh, mm, ss);
+  return fields(y + (ey - oey), m, static_cast<day_t>(d), hh, mm, ss);
 }
-CONSTEXPR_F fields n_mon(year_t y, diff_t m, diff_t d, diff_t cd,
-                         hour_t hh, minute_t mm, second_t ss) noexcept {
+CONSTEXPR_F fields n_mon(year_t y, diff_t m, diff_t d, diff_t cd, hour_t hh,
+                         minute_t mm, second_t ss) noexcept {
   if (m != 12) {
     y += m / 12;
     m %= 12;
@@ -164,8 +193,8 @@ CONSTEXPR_F fields n_mon(year_t y, diff_t m, diff_t d, diff_t cd,
   }
   return n_day(y, static_cast<month_t>(m), d, cd, hh, mm, ss);
 }
-CONSTEXPR_F fields n_hour(year_t y, diff_t m, diff_t d, diff_t cd,
-                          diff_t hh, minute_t mm, second_t ss) noexcept {
+CONSTEXPR_F fields n_hour(year_t y, diff_t m, diff_t d, diff_t cd, diff_t hh,
+                          minute_t mm, second_t ss) noexcept {
   cd += hh / 24;
   hh %= 24;
   if (hh < 0) {
@@ -264,8 +293,8 @@ CONSTEXPR_F diff_t ymd_ord(year_t y, month_t m, day_t d) noexcept {
 // yet the difference between two such extreme values may actually be
 // small, so we take a little care to avoid overflow when possible by
 // exploiting the 146097-day cycle.
-CONSTEXPR_F diff_t day_difference(year_t y1, month_t m1, day_t d1,
-                                  year_t y2, month_t m2, day_t d2) noexcept {
+CONSTEXPR_F diff_t day_difference(year_t y1, month_t m1, day_t d1, year_t y2,
+                                  month_t m2, day_t d2) noexcept {
   const diff_t a_c4_off = y1 % 400;
   const diff_t b_c4_off = y2 % 400;
   diff_t c4_diff = (y1 - a_c4_off) - (y2 - b_c4_off);
@@ -305,9 +334,7 @@ CONSTEXPR_F diff_t difference(second_tag, fields f1, fields f2) noexcept {
 ////////////////////////////////////////////////////////////////////////
 
 // Aligns the (normalized) fields struct to the indicated field.
-CONSTEXPR_F fields align(second_tag, fields f) noexcept {
-  return f;
-}
+CONSTEXPR_F fields align(second_tag, fields f) noexcept { return f; }
 CONSTEXPR_F fields align(minute_tag, fields f) noexcept {
   return fields{f.y, f.m, f.d, f.hh, f.mm, 0};
 }
@@ -386,11 +413,11 @@ class civil_time {
       : civil_time(ct.f_) {}
 
   // Factories for the maximum/minimum representable civil_time.
-  static CONSTEXPR_F civil_time (max)() {
+  static CONSTEXPR_F auto(max)() -> civil_time {
     const auto max_year = (std::numeric_limits<std::int_least64_t>::max)();
     return civil_time(max_year, 12, 31, 23, 59, 59);
   }
-  static CONSTEXPR_F civil_time (min)() {
+  static CONSTEXPR_F auto(min)() -> civil_time {
     const auto min_year = (std::numeric_limits<std::int_least64_t>::min)();
     return civil_time(min_year, 1, 1, 0, 0, 0);
   }
@@ -405,28 +432,18 @@ class civil_time {
 
   // Assigning arithmetic.
   CONSTEXPR_M civil_time& operator+=(diff_t n) noexcept {
-    f_ = step(T{}, f_, n);
-    return *this;
+    return *this = *this + n;
   }
   CONSTEXPR_M civil_time& operator-=(diff_t n) noexcept {
-    if (n != (std::numeric_limits<diff_t>::min)()) {
-      f_ = step(T{}, f_, -n);
-    } else {
-      f_ = step(T{}, step(T{}, f_, -(n + 1)), 1);
-    }
-    return *this;
-  }
-  CONSTEXPR_M civil_time& operator++() noexcept {
-    return *this += 1;
+    return *this = *this - n;
   }
+  CONSTEXPR_M civil_time& operator++() noexcept { return *this += 1; }
   CONSTEXPR_M civil_time operator++(int) noexcept {
     const civil_time a = *this;
     ++*this;
     return a;
   }
-  CONSTEXPR_M civil_time& operator--() noexcept {
-    return *this -= 1;
-  }
+  CONSTEXPR_M civil_time& operator--() noexcept { return *this -= 1; }
   CONSTEXPR_M civil_time operator--(int) noexcept {
     const civil_time a = *this;
     --*this;
@@ -435,13 +452,15 @@ class civil_time {
 
   // Binary arithmetic operators.
   friend CONSTEXPR_F civil_time operator+(civil_time a, diff_t n) noexcept {
-    return a += n;
+    return civil_time(step(T{}, a.f_, n));
   }
   friend CONSTEXPR_F civil_time operator+(diff_t n, civil_time a) noexcept {
-    return a += n;
+    return a + n;
   }
   friend CONSTEXPR_F civil_time operator-(civil_time a, diff_t n) noexcept {
-    return a -= n;
+    return n != (std::numeric_limits<diff_t>::min)()
+               ? civil_time(step(T{}, a.f_, -n))
+               : civil_time(step(T{}, step(T{}, a.f_, -(n + 1)), 1));
   }
   friend CONSTEXPR_F diff_t operator-(civil_time lhs, civil_time rhs) noexcept {
     return difference(T{}, lhs.f_, rhs.f_);
@@ -483,17 +502,17 @@ using civil_second = civil_time<second_tag>;
 template <typename T1, typename T2>
 CONSTEXPR_F bool operator<(const civil_time<T1>& lhs,
                            const civil_time<T2>& rhs) noexcept {
-  return (lhs.year() < rhs.year() ||
-          (lhs.year() == rhs.year() &&
-           (lhs.month() < rhs.month() ||
-            (lhs.month() == rhs.month() &&
-             (lhs.day() < rhs.day() ||
-              (lhs.day() == rhs.day() &&
-               (lhs.hour() < rhs.hour() ||
-                (lhs.hour() == rhs.hour() &&
-                 (lhs.minute() < rhs.minute() ||
-                  (lhs.minute() == rhs.minute() &&
-                   (lhs.second() < rhs.second())))))))))));
+  return (
+      lhs.year() < rhs.year() ||
+      (lhs.year() == rhs.year() &&
+       (lhs.month() < rhs.month() ||
+        (lhs.month() == rhs.month() &&
+         (lhs.day() < rhs.day() || (lhs.day() == rhs.day() &&
+                                    (lhs.hour() < rhs.hour() ||
+                                     (lhs.hour() == rhs.hour() &&
+                                      (lhs.minute() < rhs.minute() ||
+                                       (lhs.minute() == rhs.minute() &&
+                                        (lhs.second() < rhs.second())))))))))));
 }
 template <typename T1, typename T2>
 CONSTEXPR_F bool operator<=(const civil_time<T1>& lhs,
@@ -615,6 +634,7 @@ std::ostream& operator<<(std::ostream& os, weekday wd);
 }  // namespace detail
 }  // namespace cctz
 }  // namespace time_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #undef CONSTEXPR_M
diff --git a/absl/time/internal/cctz/include/cctz/time_zone.h b/absl/time/internal/cctz/include/cctz/time_zone.h
index ef6c4ba1095..5b232f51b83 100644
--- a/absl/time/internal/cctz/include/cctz/time_zone.h
+++ b/absl/time/internal/cctz/include/cctz/time_zone.h
@@ -22,12 +22,16 @@
 
 #include <chrono>
 #include <cstdint>
+#include <limits>
+#include <ratio>  // NOLINT: We use std::ratio in this header
 #include <string>
 #include <utility>
 
+#include "absl/base/config.h"
 #include "absl/time/internal/cctz/include/cctz/civil_time.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 namespace cctz {
 
@@ -39,20 +43,9 @@ using sys_seconds = seconds;  // Deprecated.  Use cctz::seconds instead.
 
 namespace detail {
 template <typename D>
-inline std::pair<time_point<seconds>, D>
-split_seconds(const time_point<D>& tp) {
-  auto sec = std::chrono::time_point_cast<seconds>(tp);
-  auto sub = tp - sec;
-  if (sub.count() < 0) {
-    sec -= seconds(1);
-    sub += seconds(1);
-  }
-  return {sec, std::chrono::duration_cast<D>(sub)};
-}
-inline std::pair<time_point<seconds>, seconds>
-split_seconds(const time_point<seconds>& tp) {
-  return {tp, seconds::zero()};
-}
+std::pair<time_point<seconds>, D> split_seconds(const time_point<D>& tp);
+std::pair<time_point<seconds>, seconds> split_seconds(
+    const time_point<seconds>& tp);
 }  // namespace detail
 
 // cctz::time_zone is an opaque, small, value-type class representing a
@@ -194,22 +187,20 @@ class time_zone {
   bool next_transition(const time_point<seconds>& tp,
                        civil_transition* trans) const;
   template <typename D>
-  bool next_transition(const time_point<D>& tp,
-                       civil_transition* trans) const {
+  bool next_transition(const time_point<D>& tp, civil_transition* trans) const {
     return next_transition(detail::split_seconds(tp).first, trans);
   }
   bool prev_transition(const time_point<seconds>& tp,
                        civil_transition* trans) const;
   template <typename D>
-  bool prev_transition(const time_point<D>& tp,
-                       civil_transition* trans) const {
+  bool prev_transition(const time_point<D>& tp, civil_transition* trans) const {
     return prev_transition(detail::split_seconds(tp).first, trans);
   }
 
   // version() and description() provide additional information about the
   // time zone. The content of each of the returned strings is unspecified,
   // however, when the IANA Time Zone Database is the underlying data source
-  // the version() std::string will be in the familar form (e.g, "2018e") or
+  // the version() string will be in the familiar form (e.g, "2018e") or
   // empty when unavailable.
   //
   // Note: These functions are for informational or testing purposes only.
@@ -220,9 +211,7 @@ class time_zone {
   friend bool operator==(time_zone lhs, time_zone rhs) {
     return &lhs.effective_impl() == &rhs.effective_impl();
   }
-  friend bool operator!=(time_zone lhs, time_zone rhs) {
-    return !(lhs == rhs);
-  }
+  friend bool operator!=(time_zone lhs, time_zone rhs) { return !(lhs == rhs); }
 
   template <typename H>
   friend H AbslHashValue(H h, time_zone tz) {
@@ -281,6 +270,20 @@ std::string format(const std::string&, const time_point<seconds>&,
                    const femtoseconds&, const time_zone&);
 bool parse(const std::string&, const std::string&, const time_zone&,
            time_point<seconds>*, femtoseconds*, std::string* err = nullptr);
+template <typename Rep, std::intmax_t Denom>
+bool join_seconds(
+    const time_point<seconds>& sec, const femtoseconds& fs,
+    time_point<std::chrono::duration<Rep, std::ratio<1, Denom>>>* tpp);
+template <typename Rep, std::intmax_t Num>
+bool join_seconds(
+    const time_point<seconds>& sec, const femtoseconds& fs,
+    time_point<std::chrono::duration<Rep, std::ratio<Num, 1>>>* tpp);
+template <typename Rep>
+bool join_seconds(
+    const time_point<seconds>& sec, const femtoseconds& fs,
+    time_point<std::chrono::duration<Rep, std::ratio<1, 1>>>* tpp);
+bool join_seconds(const time_point<seconds>& sec, const femtoseconds&,
+                  time_point<seconds>* tpp);
 }  // namespace detail
 
 // Formats the given time_point in the given cctz::time_zone according to
@@ -294,6 +297,7 @@ bool parse(const std::string&, const std::string&, const time_zone&,
 //   - %E#f - Fractional seconds with # digits of precision
 //   - %E*f - Fractional seconds with full precision (a literal '*')
 //   - %E4Y - Four-character years (-999 ... -001, 0000, 0001 ... 9999)
+//   - %ET  - The RFC3339 "date-time" separator "T"
 //
 // Note that %E0S behaves like %S, and %E0f produces no characters. In
 // contrast %E*f always produces at least one digit, which may be '0'.
@@ -323,7 +327,8 @@ inline std::string format(const std::string& fmt, const time_point<D>& tp,
 // returns the corresponding time_point. Uses strftime()-like formatting
 // options, with the same extensions as cctz::format(), but with the
 // exceptions that %E#S is interpreted as %E*S, and %E#f as %E*f. %Ez
-// and %E*z also accept the same inputs.
+// and %E*z also accept the same inputs, which (along with %z) includes
+// 'z' and 'Z' as synonyms for +00:00.  %ET accepts either 'T' or 't'.
 //
 // %Y consumes as many numeric characters as it can, so the matching data
 // should always be terminated with a non-numeric. %E4Y always consumes
@@ -369,17 +374,87 @@ inline bool parse(const std::string& fmt, const std::string& input,
                   const time_zone& tz, time_point<D>* tpp) {
   time_point<seconds> sec;
   detail::femtoseconds fs;
-  const bool b = detail::parse(fmt, input, tz, &sec, &fs);
-  if (b) {
-    // TODO: Return false if unrepresentable as a time_point<D>.
-    *tpp = std::chrono::time_point_cast<D>(sec);
-    *tpp += std::chrono::duration_cast<D>(fs);
+  return detail::parse(fmt, input, tz, &sec, &fs) &&
+         detail::join_seconds(sec, fs, tpp);
+}
+
+namespace detail {
+
+// Split a time_point<D> into a time_point<seconds> and a D subseconds.
+// Undefined behavior if time_point<seconds> is not of sufficient range.
+// Note that this means it is UB to call cctz::time_zone::lookup(tp) or
+// cctz::format(fmt, tp, tz) with a time_point that is outside the range
+// of a 64-bit std::time_t.
+template <typename D>
+std::pair<time_point<seconds>, D> split_seconds(const time_point<D>& tp) {
+  auto sec = std::chrono::time_point_cast<seconds>(tp);
+  auto sub = tp - sec;
+  if (sub.count() < 0) {
+    sec -= seconds(1);
+    sub += seconds(1);
   }
-  return b;
+  return {sec, std::chrono::duration_cast<D>(sub)};
+}
+
+inline std::pair<time_point<seconds>, seconds> split_seconds(
+    const time_point<seconds>& tp) {
+  return {tp, seconds::zero()};
+}
+
+// Join a time_point<seconds> and femto subseconds into a time_point<D>.
+// Floors to the resolution of time_point<D>. Returns false if time_point<D>
+// is not of sufficient range.
+template <typename Rep, std::intmax_t Denom>
+bool join_seconds(
+    const time_point<seconds>& sec, const femtoseconds& fs,
+    time_point<std::chrono::duration<Rep, std::ratio<1, Denom>>>* tpp) {
+  using D = std::chrono::duration<Rep, std::ratio<1, Denom>>;
+  // TODO(#199): Return false if result unrepresentable as a time_point<D>.
+  *tpp = std::chrono::time_point_cast<D>(sec);
+  *tpp += std::chrono::duration_cast<D>(fs);
+  return true;
 }
 
+template <typename Rep, std::intmax_t Num>
+bool join_seconds(
+    const time_point<seconds>& sec, const femtoseconds&,
+    time_point<std::chrono::duration<Rep, std::ratio<Num, 1>>>* tpp) {
+  using D = std::chrono::duration<Rep, std::ratio<Num, 1>>;
+  auto count = sec.time_since_epoch().count();
+  if (count >= 0 || count % Num == 0) {
+    count /= Num;
+  } else {
+    count /= Num;
+    count -= 1;
+  }
+  if (count > (std::numeric_limits<Rep>::max)()) return false;
+  if (count < (std::numeric_limits<Rep>::min)()) return false;
+  *tpp = time_point<D>() + D{static_cast<Rep>(count)};
+  return true;
+}
+
+template <typename Rep>
+bool join_seconds(
+    const time_point<seconds>& sec, const femtoseconds&,
+    time_point<std::chrono::duration<Rep, std::ratio<1, 1>>>* tpp) {
+  using D = std::chrono::duration<Rep, std::ratio<1, 1>>;
+  auto count = sec.time_since_epoch().count();
+  if (count > (std::numeric_limits<Rep>::max)()) return false;
+  if (count < (std::numeric_limits<Rep>::min)()) return false;
+  *tpp = time_point<D>() + D{static_cast<Rep>(count)};
+  return true;
+}
+
+inline bool join_seconds(const time_point<seconds>& sec, const femtoseconds&,
+                         time_point<seconds>* tpp) {
+  *tpp = sec;
+  return true;
+}
+
+}  // namespace detail
 }  // namespace cctz
 }  // namespace time_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_TIME_INTERNAL_CCTZ_TIME_ZONE_H_
diff --git a/absl/time/internal/cctz/include/cctz/zone_info_source.h b/absl/time/internal/cctz/include/cctz/zone_info_source.h
index 2b898d182ca..012eb4ec30e 100644
--- a/absl/time/internal/cctz/include/cctz/zone_info_source.h
+++ b/absl/time/internal/cctz/include/cctz/zone_info_source.h
@@ -20,7 +20,10 @@
 #include <memory>
 #include <string>
 
+#include "absl/base/config.h"
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 namespace cctz {
 
@@ -30,19 +33,21 @@ class ZoneInfoSource {
   virtual ~ZoneInfoSource();
 
   virtual std::size_t Read(void* ptr, std::size_t size) = 0;  // like fread()
-  virtual int Skip(std::size_t offset) = 0;  // like fseek()
+  virtual int Skip(std::size_t offset) = 0;                   // like fseek()
 
   // Until the zoneinfo data supports versioning information, we provide
   // a way for a ZoneInfoSource to indicate it out-of-band.  The default
-  // implementation returns an empty std::string.
+  // implementation returns an empty string.
   virtual std::string Version() const;
 };
 
 }  // namespace cctz
 }  // namespace time_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 namespace cctz_extension {
 
@@ -52,8 +57,8 @@ namespace cctz_extension {
 using ZoneInfoSourceFactory =
     std::unique_ptr<absl::time_internal::cctz::ZoneInfoSource> (*)(
         const std::string&,
-        const std::function<std::unique_ptr<absl::time_internal::cctz::ZoneInfoSource>(
-            const std::string&)>&);
+        const std::function<std::unique_ptr<
+            absl::time_internal::cctz::ZoneInfoSource>(const std::string&)>&);
 
 // The user can control the mapping of zone names to zoneinfo data by
 // providing a definition for cctz_extension::zone_info_source_factory.
@@ -91,6 +96,7 @@ extern ZoneInfoSourceFactory zone_info_source_factory;
 
 }  // namespace cctz_extension
 }  // namespace time_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_TIME_INTERNAL_CCTZ_ZONE_INFO_SOURCE_H_
diff --git a/absl/time/internal/cctz/src/cctz_benchmark.cc b/absl/time/internal/cctz/src/cctz_benchmark.cc
index a40f504e063..17ff576f79e 100644
--- a/absl/time/internal/cctz/src/cctz_benchmark.cc
+++ b/absl/time/internal/cctz/src/cctz_benchmark.cc
@@ -23,10 +23,12 @@
 #include "benchmark/benchmark.h"
 #include "absl/time/internal/cctz/include/cctz/civil_time.h"
 #include "absl/time/internal/cctz/include/cctz/time_zone.h"
-#include "time_zone_impl.h"
+#include "absl/time/internal/cctz/src/test_time_zone_names.h"
+#include "absl/time/internal/cctz/src/time_zone_impl.h"
 
 namespace {
 
+// SKIP_ABSL_INLINE_NAMESPACE_CHECK
 namespace cctz = absl::time_internal::cctz;
 
 void BM_Difference_Days(benchmark::State& state) {
@@ -97,613 +99,16 @@ void BM_PrevWeekday(benchmark::State& state) {
 }
 BENCHMARK(BM_PrevWeekday);
 
-const char RFC3339_full[] = "%Y-%m-%dT%H:%M:%E*S%Ez";
-const char RFC3339_sec[] = "%Y-%m-%dT%H:%M:%S%Ez";
+const char RFC3339_full[] = "%Y-%m-%d%ET%H:%M:%E*S%Ez";
+const char RFC3339_sec[] = "%Y-%m-%d%ET%H:%M:%S%Ez";
 
 const char RFC1123_full[] = "%a, %d %b %Y %H:%M:%S %z";
 const char RFC1123_no_wday[] = "%d %b %Y %H:%M:%S %z";
 
-// A list of known time-zone names.
-// TODO: Refactor with src/time_zone_lookup_test.cc.
-const char* const kTimeZoneNames[] = {
-  "Africa/Abidjan",
-  "Africa/Accra",
-  "Africa/Addis_Ababa",
-  "Africa/Algiers",
-  "Africa/Asmara",
-  "Africa/Asmera",
-  "Africa/Bamako",
-  "Africa/Bangui",
-  "Africa/Banjul",
-  "Africa/Bissau",
-  "Africa/Blantyre",
-  "Africa/Brazzaville",
-  "Africa/Bujumbura",
-  "Africa/Cairo",
-  "Africa/Casablanca",
-  "Africa/Ceuta",
-  "Africa/Conakry",
-  "Africa/Dakar",
-  "Africa/Dar_es_Salaam",
-  "Africa/Djibouti",
-  "Africa/Douala",
-  "Africa/El_Aaiun",
-  "Africa/Freetown",
-  "Africa/Gaborone",
-  "Africa/Harare",
-  "Africa/Johannesburg",
-  "Africa/Juba",
-  "Africa/Kampala",
-  "Africa/Khartoum",
-  "Africa/Kigali",
-  "Africa/Kinshasa",
-  "Africa/Lagos",
-  "Africa/Libreville",
-  "Africa/Lome",
-  "Africa/Luanda",
-  "Africa/Lubumbashi",
-  "Africa/Lusaka",
-  "Africa/Malabo",
-  "Africa/Maputo",
-  "Africa/Maseru",
-  "Africa/Mbabane",
-  "Africa/Mogadishu",
-  "Africa/Monrovia",
-  "Africa/Nairobi",
-  "Africa/Ndjamena",
-  "Africa/Niamey",
-  "Africa/Nouakchott",
-  "Africa/Ouagadougou",
-  "Africa/Porto-Novo",
-  "Africa/Sao_Tome",
-  "Africa/Timbuktu",
-  "Africa/Tripoli",
-  "Africa/Tunis",
-  "Africa/Windhoek",
-  "America/Adak",
-  "America/Anchorage",
-  "America/Anguilla",
-  "America/Antigua",
-  "America/Araguaina",
-  "America/Argentina/Buenos_Aires",
-  "America/Argentina/Catamarca",
-  "America/Argentina/ComodRivadavia",
-  "America/Argentina/Cordoba",
-  "America/Argentina/Jujuy",
-  "America/Argentina/La_Rioja",
-  "America/Argentina/Mendoza",
-  "America/Argentina/Rio_Gallegos",
-  "America/Argentina/Salta",
-  "America/Argentina/San_Juan",
-  "America/Argentina/San_Luis",
-  "America/Argentina/Tucuman",
-  "America/Argentina/Ushuaia",
-  "America/Aruba",
-  "America/Asuncion",
-  "America/Atikokan",
-  "America/Atka",
-  "America/Bahia",
-  "America/Bahia_Banderas",
-  "America/Barbados",
-  "America/Belem",
-  "America/Belize",
-  "America/Blanc-Sablon",
-  "America/Boa_Vista",
-  "America/Bogota",
-  "America/Boise",
-  "America/Buenos_Aires",
-  "America/Cambridge_Bay",
-  "America/Campo_Grande",
-  "America/Cancun",
-  "America/Caracas",
-  "America/Catamarca",
-  "America/Cayenne",
-  "America/Cayman",
-  "America/Chicago",
-  "America/Chihuahua",
-  "America/Coral_Harbour",
-  "America/Cordoba",
-  "America/Costa_Rica",
-  "America/Creston",
-  "America/Cuiaba",
-  "America/Curacao",
-  "America/Danmarkshavn",
-  "America/Dawson",
-  "America/Dawson_Creek",
-  "America/Denver",
-  "America/Detroit",
-  "America/Dominica",
-  "America/Edmonton",
-  "America/Eirunepe",
-  "America/El_Salvador",
-  "America/Ensenada",
-  "America/Fort_Nelson",
-  "America/Fort_Wayne",
-  "America/Fortaleza",
-  "America/Glace_Bay",
-  "America/Godthab",
-  "America/Goose_Bay",
-  "America/Grand_Turk",
-  "America/Grenada",
-  "America/Guadeloupe",
-  "America/Guatemala",
-  "America/Guayaquil",
-  "America/Guyana",
-  "America/Halifax",
-  "America/Havana",
-  "America/Hermosillo",
-  "America/Indiana/Indianapolis",
-  "America/Indiana/Knox",
-  "America/Indiana/Marengo",
-  "America/Indiana/Petersburg",
-  "America/Indiana/Tell_City",
-  "America/Indiana/Vevay",
-  "America/Indiana/Vincennes",
-  "America/Indiana/Winamac",
-  "America/Indianapolis",
-  "America/Inuvik",
-  "America/Iqaluit",
-  "America/Jamaica",
-  "America/Jujuy",
-  "America/Juneau",
-  "America/Kentucky/Louisville",
-  "America/Kentucky/Monticello",
-  "America/Knox_IN",
-  "America/Kralendijk",
-  "America/La_Paz",
-  "America/Lima",
-  "America/Los_Angeles",
-  "America/Louisville",
-  "America/Lower_Princes",
-  "America/Maceio",
-  "America/Managua",
-  "America/Manaus",
-  "America/Marigot",
-  "America/Martinique",
-  "America/Matamoros",
-  "America/Mazatlan",
-  "America/Mendoza",
-  "America/Menominee",
-  "America/Merida",
-  "America/Metlakatla",
-  "America/Mexico_City",
-  "America/Miquelon",
-  "America/Moncton",
-  "America/Monterrey",
-  "America/Montevideo",
-  "America/Montreal",
-  "America/Montserrat",
-  "America/Nassau",
-  "America/New_York",
-  "America/Nipigon",
-  "America/Nome",
-  "America/Noronha",
-  "America/North_Dakota/Beulah",
-  "America/North_Dakota/Center",
-  "America/North_Dakota/New_Salem",
-  "America/Ojinaga",
-  "America/Panama",
-  "America/Pangnirtung",
-  "America/Paramaribo",
-  "America/Phoenix",
-  "America/Port-au-Prince",
-  "America/Port_of_Spain",
-  "America/Porto_Acre",
-  "America/Porto_Velho",
-  "America/Puerto_Rico",
-  "America/Punta_Arenas",
-  "America/Rainy_River",
-  "America/Rankin_Inlet",
-  "America/Recife",
-  "America/Regina",
-  "America/Resolute",
-  "America/Rio_Branco",
-  "America/Rosario",
-  "America/Santa_Isabel",
-  "America/Santarem",
-  "America/Santiago",
-  "America/Santo_Domingo",
-  "America/Sao_Paulo",
-  "America/Scoresbysund",
-  "America/Shiprock",
-  "America/Sitka",
-  "America/St_Barthelemy",
-  "America/St_Johns",
-  "America/St_Kitts",
-  "America/St_Lucia",
-  "America/St_Thomas",
-  "America/St_Vincent",
-  "America/Swift_Current",
-  "America/Tegucigalpa",
-  "America/Thule",
-  "America/Thunder_Bay",
-  "America/Tijuana",
-  "America/Toronto",
-  "America/Tortola",
-  "America/Vancouver",
-  "America/Virgin",
-  "America/Whitehorse",
-  "America/Winnipeg",
-  "America/Yakutat",
-  "America/Yellowknife",
-  "Antarctica/Casey",
-  "Antarctica/Davis",
-  "Antarctica/DumontDUrville",
-  "Antarctica/Macquarie",
-  "Antarctica/Mawson",
-  "Antarctica/McMurdo",
-  "Antarctica/Palmer",
-  "Antarctica/Rothera",
-  "Antarctica/South_Pole",
-  "Antarctica/Syowa",
-  "Antarctica/Troll",
-  "Antarctica/Vostok",
-  "Arctic/Longyearbyen",
-  "Asia/Aden",
-  "Asia/Almaty",
-  "Asia/Amman",
-  "Asia/Anadyr",
-  "Asia/Aqtau",
-  "Asia/Aqtobe",
-  "Asia/Ashgabat",
-  "Asia/Ashkhabad",
-  "Asia/Atyrau",
-  "Asia/Baghdad",
-  "Asia/Bahrain",
-  "Asia/Baku",
-  "Asia/Bangkok",
-  "Asia/Barnaul",
-  "Asia/Beirut",
-  "Asia/Bishkek",
-  "Asia/Brunei",
-  "Asia/Calcutta",
-  "Asia/Chita",
-  "Asia/Choibalsan",
-  "Asia/Chongqing",
-  "Asia/Chungking",
-  "Asia/Colombo",
-  "Asia/Dacca",
-  "Asia/Damascus",
-  "Asia/Dhaka",
-  "Asia/Dili",
-  "Asia/Dubai",
-  "Asia/Dushanbe",
-  "Asia/Famagusta",
-  "Asia/Gaza",
-  "Asia/Harbin",
-  "Asia/Hebron",
-  "Asia/Ho_Chi_Minh",
-  "Asia/Hong_Kong",
-  "Asia/Hovd",
-  "Asia/Irkutsk",
-  "Asia/Istanbul",
-  "Asia/Jakarta",
-  "Asia/Jayapura",
-  "Asia/Jerusalem",
-  "Asia/Kabul",
-  "Asia/Kamchatka",
-  "Asia/Karachi",
-  "Asia/Kashgar",
-  "Asia/Kathmandu",
-  "Asia/Katmandu",
-  "Asia/Khandyga",
-  "Asia/Kolkata",
-  "Asia/Krasnoyarsk",
-  "Asia/Kuala_Lumpur",
-  "Asia/Kuching",
-  "Asia/Kuwait",
-  "Asia/Macao",
-  "Asia/Macau",
-  "Asia/Magadan",
-  "Asia/Makassar",
-  "Asia/Manila",
-  "Asia/Muscat",
-  "Asia/Nicosia",
-  "Asia/Novokuznetsk",
-  "Asia/Novosibirsk",
-  "Asia/Omsk",
-  "Asia/Oral",
-  "Asia/Phnom_Penh",
-  "Asia/Pontianak",
-  "Asia/Pyongyang",
-  "Asia/Qatar",
-  "Asia/Qostanay",
-  "Asia/Qyzylorda",
-  "Asia/Rangoon",
-  "Asia/Riyadh",
-  "Asia/Saigon",
-  "Asia/Sakhalin",
-  "Asia/Samarkand",
-  "Asia/Seoul",
-  "Asia/Shanghai",
-  "Asia/Singapore",
-  "Asia/Srednekolymsk",
-  "Asia/Taipei",
-  "Asia/Tashkent",
-  "Asia/Tbilisi",
-  "Asia/Tehran",
-  "Asia/Tel_Aviv",
-  "Asia/Thimbu",
-  "Asia/Thimphu",
-  "Asia/Tokyo",
-  "Asia/Tomsk",
-  "Asia/Ujung_Pandang",
-  "Asia/Ulaanbaatar",
-  "Asia/Ulan_Bator",
-  "Asia/Urumqi",
-  "Asia/Ust-Nera",
-  "Asia/Vientiane",
-  "Asia/Vladivostok",
-  "Asia/Yakutsk",
-  "Asia/Yangon",
-  "Asia/Yekaterinburg",
-  "Asia/Yerevan",
-  "Atlantic/Azores",
-  "Atlantic/Bermuda",
-  "Atlantic/Canary",
-  "Atlantic/Cape_Verde",
-  "Atlantic/Faeroe",
-  "Atlantic/Faroe",
-  "Atlantic/Jan_Mayen",
-  "Atlantic/Madeira",
-  "Atlantic/Reykjavik",
-  "Atlantic/South_Georgia",
-  "Atlantic/St_Helena",
-  "Atlantic/Stanley",
-  "Australia/ACT",
-  "Australia/Adelaide",
-  "Australia/Brisbane",
-  "Australia/Broken_Hill",
-  "Australia/Canberra",
-  "Australia/Currie",
-  "Australia/Darwin",
-  "Australia/Eucla",
-  "Australia/Hobart",
-  "Australia/LHI",
-  "Australia/Lindeman",
-  "Australia/Lord_Howe",
-  "Australia/Melbourne",
-  "Australia/NSW",
-  "Australia/North",
-  "Australia/Perth",
-  "Australia/Queensland",
-  "Australia/South",
-  "Australia/Sydney",
-  "Australia/Tasmania",
-  "Australia/Victoria",
-  "Australia/West",
-  "Australia/Yancowinna",
-  "Brazil/Acre",
-  "Brazil/DeNoronha",
-  "Brazil/East",
-  "Brazil/West",
-  "CET",
-  "CST6CDT",
-  "Canada/Atlantic",
-  "Canada/Central",
-  "Canada/Eastern",
-  "Canada/Mountain",
-  "Canada/Newfoundland",
-  "Canada/Pacific",
-  "Canada/Saskatchewan",
-  "Canada/Yukon",
-  "Chile/Continental",
-  "Chile/EasterIsland",
-  "Cuba",
-  "EET",
-  "EST",
-  "EST5EDT",
-  "Egypt",
-  "Eire",
-  "Etc/GMT",
-  "Etc/GMT+0",
-  "Etc/GMT+1",
-  "Etc/GMT+10",
-  "Etc/GMT+11",
-  "Etc/GMT+12",
-  "Etc/GMT+2",
-  "Etc/GMT+3",
-  "Etc/GMT+4",
-  "Etc/GMT+5",
-  "Etc/GMT+6",
-  "Etc/GMT+7",
-  "Etc/GMT+8",
-  "Etc/GMT+9",
-  "Etc/GMT-0",
-  "Etc/GMT-1",
-  "Etc/GMT-10",
-  "Etc/GMT-11",
-  "Etc/GMT-12",
-  "Etc/GMT-13",
-  "Etc/GMT-14",
-  "Etc/GMT-2",
-  "Etc/GMT-3",
-  "Etc/GMT-4",
-  "Etc/GMT-5",
-  "Etc/GMT-6",
-  "Etc/GMT-7",
-  "Etc/GMT-8",
-  "Etc/GMT-9",
-  "Etc/GMT0",
-  "Etc/Greenwich",
-  "Etc/UCT",
-  "Etc/UTC",
-  "Etc/Universal",
-  "Etc/Zulu",
-  "Europe/Amsterdam",
-  "Europe/Andorra",
-  "Europe/Astrakhan",
-  "Europe/Athens",
-  "Europe/Belfast",
-  "Europe/Belgrade",
-  "Europe/Berlin",
-  "Europe/Bratislava",
-  "Europe/Brussels",
-  "Europe/Bucharest",
-  "Europe/Budapest",
-  "Europe/Busingen",
-  "Europe/Chisinau",
-  "Europe/Copenhagen",
-  "Europe/Dublin",
-  "Europe/Gibraltar",
-  "Europe/Guernsey",
-  "Europe/Helsinki",
-  "Europe/Isle_of_Man",
-  "Europe/Istanbul",
-  "Europe/Jersey",
-  "Europe/Kaliningrad",
-  "Europe/Kiev",
-  "Europe/Kirov",
-  "Europe/Lisbon",
-  "Europe/Ljubljana",
-  "Europe/London",
-  "Europe/Luxembourg",
-  "Europe/Madrid",
-  "Europe/Malta",
-  "Europe/Mariehamn",
-  "Europe/Minsk",
-  "Europe/Monaco",
-  "Europe/Moscow",
-  "Europe/Nicosia",
-  "Europe/Oslo",
-  "Europe/Paris",
-  "Europe/Podgorica",
-  "Europe/Prague",
-  "Europe/Riga",
-  "Europe/Rome",
-  "Europe/Samara",
-  "Europe/San_Marino",
-  "Europe/Sarajevo",
-  "Europe/Saratov",
-  "Europe/Simferopol",
-  "Europe/Skopje",
-  "Europe/Sofia",
-  "Europe/Stockholm",
-  "Europe/Tallinn",
-  "Europe/Tirane",
-  "Europe/Tiraspol",
-  "Europe/Ulyanovsk",
-  "Europe/Uzhgorod",
-  "Europe/Vaduz",
-  "Europe/Vatican",
-  "Europe/Vienna",
-  "Europe/Vilnius",
-  "Europe/Volgograd",
-  "Europe/Warsaw",
-  "Europe/Zagreb",
-  "Europe/Zaporozhye",
-  "Europe/Zurich",
-  "GB",
-  "GB-Eire",
-  "GMT",
-  "GMT+0",
-  "GMT-0",
-  "GMT0",
-  "Greenwich",
-  "HST",
-  "Hongkong",
-  "Iceland",
-  "Indian/Antananarivo",
-  "Indian/Chagos",
-  "Indian/Christmas",
-  "Indian/Cocos",
-  "Indian/Comoro",
-  "Indian/Kerguelen",
-  "Indian/Mahe",
-  "Indian/Maldives",
-  "Indian/Mauritius",
-  "Indian/Mayotte",
-  "Indian/Reunion",
-  "Iran",
-  "Israel",
-  "Jamaica",
-  "Japan",
-  "Kwajalein",
-  "Libya",
-  "MET",
-  "MST",
-  "MST7MDT",
-  "Mexico/BajaNorte",
-  "Mexico/BajaSur",
-  "Mexico/General",
-  "NZ",
-  "NZ-CHAT",
-  "Navajo",
-  "PRC",
-  "PST8PDT",
-  "Pacific/Apia",
-  "Pacific/Auckland",
-  "Pacific/Bougainville",
-  "Pacific/Chatham",
-  "Pacific/Chuuk",
-  "Pacific/Easter",
-  "Pacific/Efate",
-  "Pacific/Enderbury",
-  "Pacific/Fakaofo",
-  "Pacific/Fiji",
-  "Pacific/Funafuti",
-  "Pacific/Galapagos",
-  "Pacific/Gambier",
-  "Pacific/Guadalcanal",
-  "Pacific/Guam",
-  "Pacific/Honolulu",
-  "Pacific/Johnston",
-  "Pacific/Kiritimati",
-  "Pacific/Kosrae",
-  "Pacific/Kwajalein",
-  "Pacific/Majuro",
-  "Pacific/Marquesas",
-  "Pacific/Midway",
-  "Pacific/Nauru",
-  "Pacific/Niue",
-  "Pacific/Norfolk",
-  "Pacific/Noumea",
-  "Pacific/Pago_Pago",
-  "Pacific/Palau",
-  "Pacific/Pitcairn",
-  "Pacific/Pohnpei",
-  "Pacific/Ponape",
-  "Pacific/Port_Moresby",
-  "Pacific/Rarotonga",
-  "Pacific/Saipan",
-  "Pacific/Samoa",
-  "Pacific/Tahiti",
-  "Pacific/Tarawa",
-  "Pacific/Tongatapu",
-  "Pacific/Truk",
-  "Pacific/Wake",
-  "Pacific/Wallis",
-  "Pacific/Yap",
-  "Poland",
-  "Portugal",
-  "ROC",
-  "ROK",
-  "Singapore",
-  "Turkey",
-  "UCT",
-  "US/Alaska",
-  "US/Aleutian",
-  "US/Arizona",
-  "US/Central",
-  "US/East-Indiana",
-  "US/Eastern",
-  "US/Hawaii",
-  "US/Indiana-Starke",
-  "US/Michigan",
-  "US/Mountain",
-  "US/Pacific",
-  "US/Samoa",
-  "UTC",
-  "Universal",
-  "W-SU",
-  "WET",
-  "Zulu",
-  nullptr
-};
-
 std::vector<std::string> AllTimeZoneNames() {
   std::vector<std::string> names;
-  for (const char* const* namep = kTimeZoneNames; *namep != nullptr; ++namep) {
+  for (const char* const* namep = cctz::kTimeZoneNames; *namep != nullptr;
+       ++namep) {
     names.push_back(std::string("file:") + *namep);
   }
   assert(!names.empty());
@@ -992,12 +397,13 @@ void BM_Time_FromCivilDay0_Libc(benchmark::State& state) {
 BENCHMARK(BM_Time_FromCivilDay0_Libc);
 
 const char* const kFormats[] = {
-    RFC1123_full,         // 0
-    RFC1123_no_wday,      // 1
-    RFC3339_full,         // 2
-    RFC3339_sec,          // 3
-    "%Y-%m-%dT%H:%M:%S",  // 4
-    "%Y-%m-%d",           // 5
+    RFC1123_full,           // 0
+    RFC1123_no_wday,        // 1
+    RFC3339_full,           // 2
+    RFC3339_sec,            // 3
+    "%Y-%m-%d%ET%H:%M:%S",  // 4
+    "%Y-%m-%d",             // 5
+    "%F%ET%T",              // 6
 };
 const int kNumFormats = sizeof(kFormats) / sizeof(kFormats[0]);
 
diff --git a/absl/time/internal/cctz/src/civil_time_detail.cc b/absl/time/internal/cctz/src/civil_time_detail.cc
index cb40b6bc53c..0b07e397e56 100644
--- a/absl/time/internal/cctz/src/civil_time_detail.cc
+++ b/absl/time/internal/cctz/src/civil_time_detail.cc
@@ -18,7 +18,10 @@
 #include <ostream>
 #include <sstream>
 
+#include "absl/base/config.h"
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 namespace cctz {
 namespace detail {
@@ -87,4 +90,5 @@ std::ostream& operator<<(std::ostream& os, weekday wd) {
 }  // namespace detail
 }  // namespace cctz
 }  // namespace time_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/time/internal/cctz/src/civil_time_test.cc b/absl/time/internal/cctz/src/civil_time_test.cc
index 10a5ffe5873..a5a71230419 100644
--- a/absl/time/internal/cctz/src/civil_time_test.cc
+++ b/absl/time/internal/cctz/src/civil_time_test.cc
@@ -21,8 +21,10 @@
 #include <type_traits>
 
 #include "gtest/gtest.h"
+#include "absl/base/config.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 namespace cctz {
 
@@ -232,6 +234,16 @@ TEST(CivilTime, Difference) {
   static_assert(diff == 365, "Difference");
 }
 
+// NOTE: Run this with --copt=-ftrapv to detect overflow problems.
+TEST(CivilTime, ConstructionWithHugeYear) {
+  constexpr civil_hour h(-9223372036854775807, 1, 1, -1);
+  static_assert(h.year() == -9223372036854775807 - 1,
+                "ConstructionWithHugeYear");
+  static_assert(h.month() == 12, "ConstructionWithHugeYear");
+  static_assert(h.day() == 31, "ConstructionWithHugeYear");
+  static_assert(h.hour() == 23, "ConstructionWithHugeYear");
+}
+
 // NOTE: Run this with --copt=-ftrapv to detect overflow problems.
 TEST(CivilTime, DifferenceWithHugeYear) {
   {
@@ -1014,19 +1026,13 @@ TEST(CivilTime, LeapYears) {
       int day;
     } leap_day;  // The date of the day after Feb 28.
   } kLeapYearTable[]{
-      {1900, 365, {3, 1}},
-      {1999, 365, {3, 1}},
+      {1900, 365, {3, 1}},  {1999, 365, {3, 1}},
       {2000, 366, {2, 29}},  // leap year
-      {2001, 365, {3, 1}},
-      {2002, 365, {3, 1}},
-      {2003, 365, {3, 1}},
-      {2004, 366, {2, 29}},  // leap year
-      {2005, 365, {3, 1}},
-      {2006, 365, {3, 1}},
-      {2007, 365, {3, 1}},
-      {2008, 366, {2, 29}},  // leap year
-      {2009, 365, {3, 1}},
-      {2100, 365, {3, 1}},
+      {2001, 365, {3, 1}},  {2002, 365, {3, 1}},
+      {2003, 365, {3, 1}},  {2004, 366, {2, 29}},  // leap year
+      {2005, 365, {3, 1}},  {2006, 365, {3, 1}},
+      {2007, 365, {3, 1}},  {2008, 366, {2, 29}},  // leap year
+      {2009, 365, {3, 1}},  {2100, 365, {3, 1}},
   };
 
   for (const auto& e : kLeapYearTable) {
@@ -1056,4 +1062,5 @@ TEST(CivilTime, FirstThursdayInMonth) {
 
 }  // namespace cctz
 }  // namespace time_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/time/internal/cctz/src/test_time_zone_names.cc b/absl/time/internal/cctz/src/test_time_zone_names.cc
new file mode 100644
index 00000000000..ab54c9ae70a
--- /dev/null
+++ b/absl/time/internal/cctz/src/test_time_zone_names.cc
@@ -0,0 +1,515 @@
+// Copyright 2025 Google Inc. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//   https://www.apache.org/licenses/LICENSE-2.0
+//
+//   Unless required by applicable law or agreed to in writing, software
+//   distributed under the License is distributed on an "AS IS" BASIS,
+//   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+//   See the License for the specific language governing permissions and
+//   limitations under the License.
+
+#include "absl/time/internal/cctz/src/test_time_zone_names.h"
+
+#include "absl/base/config.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace time_internal {
+namespace cctz {
+
+// A list of known time-zone names.
+const char* const kTimeZoneNames[] = {"Africa/Abidjan",
+                                      "Africa/Accra",
+                                      "Africa/Addis_Ababa",
+                                      "Africa/Algiers",
+                                      "Africa/Asmara",
+                                      "Africa/Bamako",
+                                      "Africa/Bangui",
+                                      "Africa/Banjul",
+                                      "Africa/Bissau",
+                                      "Africa/Blantyre",
+                                      "Africa/Brazzaville",
+                                      "Africa/Bujumbura",
+                                      "Africa/Cairo",
+                                      "Africa/Casablanca",
+                                      "Africa/Ceuta",
+                                      "Africa/Conakry",
+                                      "Africa/Dakar",
+                                      "Africa/Dar_es_Salaam",
+                                      "Africa/Djibouti",
+                                      "Africa/Douala",
+                                      "Africa/El_Aaiun",
+                                      "Africa/Freetown",
+                                      "Africa/Gaborone",
+                                      "Africa/Harare",
+                                      "Africa/Johannesburg",
+                                      "Africa/Juba",
+                                      "Africa/Kampala",
+                                      "Africa/Khartoum",
+                                      "Africa/Kigali",
+                                      "Africa/Kinshasa",
+                                      "Africa/Lagos",
+                                      "Africa/Libreville",
+                                      "Africa/Lome",
+                                      "Africa/Luanda",
+                                      "Africa/Lubumbashi",
+                                      "Africa/Lusaka",
+                                      "Africa/Malabo",
+                                      "Africa/Maputo",
+                                      "Africa/Maseru",
+                                      "Africa/Mbabane",
+                                      "Africa/Mogadishu",
+                                      "Africa/Monrovia",
+                                      "Africa/Nairobi",
+                                      "Africa/Ndjamena",
+                                      "Africa/Niamey",
+                                      "Africa/Nouakchott",
+                                      "Africa/Ouagadougou",
+                                      "Africa/Porto-Novo",
+                                      "Africa/Sao_Tome",
+                                      "Africa/Timbuktu",
+                                      "Africa/Tripoli",
+                                      "Africa/Tunis",
+                                      "Africa/Windhoek",
+                                      "America/Adak",
+                                      "America/Anchorage",
+                                      "America/Anguilla",
+                                      "America/Antigua",
+                                      "America/Araguaina",
+                                      "America/Argentina/Buenos_Aires",
+                                      "America/Argentina/Catamarca",
+                                      "America/Argentina/Cordoba",
+                                      "America/Argentina/Jujuy",
+                                      "America/Argentina/La_Rioja",
+                                      "America/Argentina/Mendoza",
+                                      "America/Argentina/Rio_Gallegos",
+                                      "America/Argentina/Salta",
+                                      "America/Argentina/San_Juan",
+                                      "America/Argentina/San_Luis",
+                                      "America/Argentina/Tucuman",
+                                      "America/Argentina/Ushuaia",
+                                      "America/Aruba",
+                                      "America/Asuncion",
+                                      "America/Atikokan",
+                                      "America/Atka",
+                                      "America/Bahia",
+                                      "America/Bahia_Banderas",
+                                      "America/Barbados",
+                                      "America/Belem",
+                                      "America/Belize",
+                                      "America/Blanc-Sablon",
+                                      "America/Boa_Vista",
+                                      "America/Bogota",
+                                      "America/Boise",
+                                      "America/Cambridge_Bay",
+                                      "America/Campo_Grande",
+                                      "America/Cancun",
+                                      "America/Caracas",
+                                      "America/Cayenne",
+                                      "America/Cayman",
+                                      "America/Chicago",
+                                      "America/Chihuahua",
+                                      "America/Ciudad_Juarez",
+                                      "America/Coral_Harbour",
+                                      "America/Costa_Rica",
+                                      "America/Coyhaique",
+                                      "America/Creston",
+                                      "America/Cuiaba",
+                                      "America/Curacao",
+                                      "America/Danmarkshavn",
+                                      "America/Dawson",
+                                      "America/Dawson_Creek",
+                                      "America/Denver",
+                                      "America/Detroit",
+                                      "America/Dominica",
+                                      "America/Edmonton",
+                                      "America/Eirunepe",
+                                      "America/El_Salvador",
+                                      "America/Ensenada",
+                                      "America/Fort_Nelson",
+                                      "America/Fortaleza",
+                                      "America/Glace_Bay",
+                                      "America/Goose_Bay",
+                                      "America/Grand_Turk",
+                                      "America/Grenada",
+                                      "America/Guadeloupe",
+                                      "America/Guatemala",
+                                      "America/Guayaquil",
+                                      "America/Guyana",
+                                      "America/Halifax",
+                                      "America/Havana",
+                                      "America/Hermosillo",
+                                      "America/Indiana/Indianapolis",
+                                      "America/Indiana/Knox",
+                                      "America/Indiana/Marengo",
+                                      "America/Indiana/Petersburg",
+                                      "America/Indiana/Tell_City",
+                                      "America/Indiana/Vevay",
+                                      "America/Indiana/Vincennes",
+                                      "America/Indiana/Winamac",
+                                      "America/Inuvik",
+                                      "America/Iqaluit",
+                                      "America/Jamaica",
+                                      "America/Juneau",
+                                      "America/Kentucky/Louisville",
+                                      "America/Kentucky/Monticello",
+                                      "America/Kralendijk",
+                                      "America/La_Paz",
+                                      "America/Lima",
+                                      "America/Los_Angeles",
+                                      "America/Lower_Princes",
+                                      "America/Maceio",
+                                      "America/Managua",
+                                      "America/Manaus",
+                                      "America/Marigot",
+                                      "America/Martinique",
+                                      "America/Matamoros",
+                                      "America/Mazatlan",
+                                      "America/Menominee",
+                                      "America/Merida",
+                                      "America/Metlakatla",
+                                      "America/Mexico_City",
+                                      "America/Miquelon",
+                                      "America/Moncton",
+                                      "America/Monterrey",
+                                      "America/Montevideo",
+                                      "America/Montreal",
+                                      "America/Montserrat",
+                                      "America/Nassau",
+                                      "America/New_York",
+                                      "America/Nipigon",
+                                      "America/Nome",
+                                      "America/Noronha",
+                                      "America/North_Dakota/Beulah",
+                                      "America/North_Dakota/Center",
+                                      "America/North_Dakota/New_Salem",
+                                      "America/Nuuk",
+                                      "America/Ojinaga",
+                                      "America/Panama",
+                                      "America/Pangnirtung",
+                                      "America/Paramaribo",
+                                      "America/Phoenix",
+                                      "America/Port-au-Prince",
+                                      "America/Port_of_Spain",
+                                      "America/Porto_Acre",
+                                      "America/Porto_Velho",
+                                      "America/Puerto_Rico",
+                                      "America/Punta_Arenas",
+                                      "America/Rainy_River",
+                                      "America/Rankin_Inlet",
+                                      "America/Recife",
+                                      "America/Regina",
+                                      "America/Resolute",
+                                      "America/Rio_Branco",
+                                      "America/Santa_Isabel",
+                                      "America/Santarem",
+                                      "America/Santiago",
+                                      "America/Santo_Domingo",
+                                      "America/Sao_Paulo",
+                                      "America/Scoresbysund",
+                                      "America/Shiprock",
+                                      "America/Sitka",
+                                      "America/St_Barthelemy",
+                                      "America/St_Johns",
+                                      "America/St_Kitts",
+                                      "America/St_Lucia",
+                                      "America/St_Thomas",
+                                      "America/St_Vincent",
+                                      "America/Swift_Current",
+                                      "America/Tegucigalpa",
+                                      "America/Thule",
+                                      "America/Thunder_Bay",
+                                      "America/Tijuana",
+                                      "America/Toronto",
+                                      "America/Tortola",
+                                      "America/Vancouver",
+                                      "America/Virgin",
+                                      "America/Whitehorse",
+                                      "America/Winnipeg",
+                                      "America/Yakutat",
+                                      "America/Yellowknife",
+                                      "Antarctica/Casey",
+                                      "Antarctica/Davis",
+                                      "Antarctica/DumontDUrville",
+                                      "Antarctica/Macquarie",
+                                      "Antarctica/Mawson",
+                                      "Antarctica/McMurdo",
+                                      "Antarctica/Palmer",
+                                      "Antarctica/Rothera",
+                                      "Antarctica/Syowa",
+                                      "Antarctica/Troll",
+                                      "Antarctica/Vostok",
+                                      "Arctic/Longyearbyen",
+                                      "Asia/Aden",
+                                      "Asia/Almaty",
+                                      "Asia/Amman",
+                                      "Asia/Anadyr",
+                                      "Asia/Aqtau",
+                                      "Asia/Aqtobe",
+                                      "Asia/Ashgabat",
+                                      "Asia/Atyrau",
+                                      "Asia/Baghdad",
+                                      "Asia/Bahrain",
+                                      "Asia/Baku",
+                                      "Asia/Bangkok",
+                                      "Asia/Barnaul",
+                                      "Asia/Beirut",
+                                      "Asia/Bishkek",
+                                      "Asia/Brunei",
+                                      "Asia/Chita",
+                                      "Asia/Choibalsan",
+                                      "Asia/Chongqing",
+                                      "Asia/Colombo",
+                                      "Asia/Damascus",
+                                      "Asia/Dhaka",
+                                      "Asia/Dili",
+                                      "Asia/Dubai",
+                                      "Asia/Dushanbe",
+                                      "Asia/Famagusta",
+                                      "Asia/Gaza",
+                                      "Asia/Harbin",
+                                      "Asia/Hebron",
+                                      "Asia/Ho_Chi_Minh",
+                                      "Asia/Hong_Kong",
+                                      "Asia/Hovd",
+                                      "Asia/Irkutsk",
+                                      "Asia/Istanbul",
+                                      "Asia/Jakarta",
+                                      "Asia/Jayapura",
+                                      "Asia/Jerusalem",
+                                      "Asia/Kabul",
+                                      "Asia/Kamchatka",
+                                      "Asia/Karachi",
+                                      "Asia/Kashgar",
+                                      "Asia/Kathmandu",
+                                      "Asia/Khandyga",
+                                      "Asia/Kolkata",
+                                      "Asia/Krasnoyarsk",
+                                      "Asia/Kuala_Lumpur",
+                                      "Asia/Kuching",
+                                      "Asia/Kuwait",
+                                      "Asia/Macau",
+                                      "Asia/Magadan",
+                                      "Asia/Makassar",
+                                      "Asia/Manila",
+                                      "Asia/Muscat",
+                                      "Asia/Nicosia",
+                                      "Asia/Novokuznetsk",
+                                      "Asia/Novosibirsk",
+                                      "Asia/Omsk",
+                                      "Asia/Oral",
+                                      "Asia/Phnom_Penh",
+                                      "Asia/Pontianak",
+                                      "Asia/Pyongyang",
+                                      "Asia/Qatar",
+                                      "Asia/Qostanay",
+                                      "Asia/Qyzylorda",
+                                      "Asia/Riyadh",
+                                      "Asia/Sakhalin",
+                                      "Asia/Samarkand",
+                                      "Asia/Seoul",
+                                      "Asia/Shanghai",
+                                      "Asia/Singapore",
+                                      "Asia/Srednekolymsk",
+                                      "Asia/Taipei",
+                                      "Asia/Tashkent",
+                                      "Asia/Tbilisi",
+                                      "Asia/Tehran",
+                                      "Asia/Tel_Aviv",
+                                      "Asia/Thimphu",
+                                      "Asia/Tokyo",
+                                      "Asia/Tomsk",
+                                      "Asia/Ulaanbaatar",
+                                      "Asia/Urumqi",
+                                      "Asia/Ust-Nera",
+                                      "Asia/Vientiane",
+                                      "Asia/Vladivostok",
+                                      "Asia/Yakutsk",
+                                      "Asia/Yangon",
+                                      "Asia/Yekaterinburg",
+                                      "Asia/Yerevan",
+                                      "Atlantic/Azores",
+                                      "Atlantic/Bermuda",
+                                      "Atlantic/Canary",
+                                      "Atlantic/Cape_Verde",
+                                      "Atlantic/Faroe",
+                                      "Atlantic/Jan_Mayen",
+                                      "Atlantic/Madeira",
+                                      "Atlantic/Reykjavik",
+                                      "Atlantic/South_Georgia",
+                                      "Atlantic/St_Helena",
+                                      "Atlantic/Stanley",
+                                      "Australia/Adelaide",
+                                      "Australia/Brisbane",
+                                      "Australia/Broken_Hill",
+                                      "Australia/Canberra",
+                                      "Australia/Currie",
+                                      "Australia/Darwin",
+                                      "Australia/Eucla",
+                                      "Australia/Hobart",
+                                      "Australia/Lindeman",
+                                      "Australia/Lord_Howe",
+                                      "Australia/Melbourne",
+                                      "Australia/Perth",
+                                      "Australia/Sydney",
+                                      "Australia/Yancowinna",
+                                      "Etc/GMT",
+                                      "Etc/GMT+0",
+                                      "Etc/GMT+1",
+                                      "Etc/GMT+10",
+                                      "Etc/GMT+11",
+                                      "Etc/GMT+12",
+                                      "Etc/GMT+2",
+                                      "Etc/GMT+3",
+                                      "Etc/GMT+4",
+                                      "Etc/GMT+5",
+                                      "Etc/GMT+6",
+                                      "Etc/GMT+7",
+                                      "Etc/GMT+8",
+                                      "Etc/GMT+9",
+                                      "Etc/GMT-0",
+                                      "Etc/GMT-1",
+                                      "Etc/GMT-10",
+                                      "Etc/GMT-11",
+                                      "Etc/GMT-12",
+                                      "Etc/GMT-13",
+                                      "Etc/GMT-14",
+                                      "Etc/GMT-2",
+                                      "Etc/GMT-3",
+                                      "Etc/GMT-4",
+                                      "Etc/GMT-5",
+                                      "Etc/GMT-6",
+                                      "Etc/GMT-7",
+                                      "Etc/GMT-8",
+                                      "Etc/GMT-9",
+                                      "Etc/GMT0",
+                                      "Etc/Greenwich",
+                                      "Etc/UCT",
+                                      "Etc/UTC",
+                                      "Etc/Universal",
+                                      "Etc/Zulu",
+                                      "Europe/Amsterdam",
+                                      "Europe/Andorra",
+                                      "Europe/Astrakhan",
+                                      "Europe/Athens",
+                                      "Europe/Belfast",
+                                      "Europe/Belgrade",
+                                      "Europe/Berlin",
+                                      "Europe/Bratislava",
+                                      "Europe/Brussels",
+                                      "Europe/Bucharest",
+                                      "Europe/Budapest",
+                                      "Europe/Busingen",
+                                      "Europe/Chisinau",
+                                      "Europe/Copenhagen",
+                                      "Europe/Dublin",
+                                      "Europe/Gibraltar",
+                                      "Europe/Guernsey",
+                                      "Europe/Helsinki",
+                                      "Europe/Isle_of_Man",
+                                      "Europe/Istanbul",
+                                      "Europe/Jersey",
+                                      "Europe/Kaliningrad",
+                                      "Europe/Kirov",
+                                      "Europe/Kyiv",
+                                      "Europe/Lisbon",
+                                      "Europe/Ljubljana",
+                                      "Europe/London",
+                                      "Europe/Luxembourg",
+                                      "Europe/Madrid",
+                                      "Europe/Malta",
+                                      "Europe/Mariehamn",
+                                      "Europe/Minsk",
+                                      "Europe/Monaco",
+                                      "Europe/Moscow",
+                                      "Europe/Nicosia",
+                                      "Europe/Oslo",
+                                      "Europe/Paris",
+                                      "Europe/Podgorica",
+                                      "Europe/Prague",
+                                      "Europe/Riga",
+                                      "Europe/Rome",
+                                      "Europe/Samara",
+                                      "Europe/San_Marino",
+                                      "Europe/Sarajevo",
+                                      "Europe/Saratov",
+                                      "Europe/Simferopol",
+                                      "Europe/Skopje",
+                                      "Europe/Sofia",
+                                      "Europe/Stockholm",
+                                      "Europe/Tallinn",
+                                      "Europe/Tirane",
+                                      "Europe/Tiraspol",
+                                      "Europe/Ulyanovsk",
+                                      "Europe/Vaduz",
+                                      "Europe/Vatican",
+                                      "Europe/Vienna",
+                                      "Europe/Vilnius",
+                                      "Europe/Volgograd",
+                                      "Europe/Warsaw",
+                                      "Europe/Zagreb",
+                                      "Europe/Zurich",
+                                      "Factory",
+                                      "Indian/Antananarivo",
+                                      "Indian/Chagos",
+                                      "Indian/Christmas",
+                                      "Indian/Cocos",
+                                      "Indian/Comoro",
+                                      "Indian/Kerguelen",
+                                      "Indian/Mahe",
+                                      "Indian/Maldives",
+                                      "Indian/Mauritius",
+                                      "Indian/Mayotte",
+                                      "Indian/Reunion",
+                                      "Pacific/Apia",
+                                      "Pacific/Auckland",
+                                      "Pacific/Bougainville",
+                                      "Pacific/Chatham",
+                                      "Pacific/Chuuk",
+                                      "Pacific/Easter",
+                                      "Pacific/Efate",
+                                      "Pacific/Fakaofo",
+                                      "Pacific/Fiji",
+                                      "Pacific/Funafuti",
+                                      "Pacific/Galapagos",
+                                      "Pacific/Gambier",
+                                      "Pacific/Guadalcanal",
+                                      "Pacific/Guam",
+                                      "Pacific/Honolulu",
+                                      "Pacific/Johnston",
+                                      "Pacific/Kanton",
+                                      "Pacific/Kiritimati",
+                                      "Pacific/Kosrae",
+                                      "Pacific/Kwajalein",
+                                      "Pacific/Majuro",
+                                      "Pacific/Marquesas",
+                                      "Pacific/Midway",
+                                      "Pacific/Nauru",
+                                      "Pacific/Niue",
+                                      "Pacific/Norfolk",
+                                      "Pacific/Noumea",
+                                      "Pacific/Pago_Pago",
+                                      "Pacific/Palau",
+                                      "Pacific/Pitcairn",
+                                      "Pacific/Pohnpei",
+                                      "Pacific/Port_Moresby",
+                                      "Pacific/Rarotonga",
+                                      "Pacific/Saipan",
+                                      "Pacific/Samoa",
+                                      "Pacific/Tahiti",
+                                      "Pacific/Tarawa",
+                                      "Pacific/Tongatapu",
+                                      "Pacific/Wake",
+                                      "Pacific/Wallis",
+                                      "Pacific/Yap",
+                                      "UTC",
+                                      nullptr};
+
+}  // namespace cctz
+}  // namespace time_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/time/internal/cctz/src/test_time_zone_names.h b/absl/time/internal/cctz/src/test_time_zone_names.h
new file mode 100644
index 00000000000..1993994a1d2
--- /dev/null
+++ b/absl/time/internal/cctz/src/test_time_zone_names.h
@@ -0,0 +1,33 @@
+// Copyright 2025 Google Inc. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//   https://www.apache.org/licenses/LICENSE-2.0
+//
+//   Unless required by applicable law or agreed to in writing, software
+//   distributed under the License is distributed on an "AS IS" BASIS,
+//   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+//   See the License for the specific language governing permissions and
+//   limitations under the License.
+
+#ifndef ABSL_TIME_INTERNAL_CCTZ_TEST_TIME_ZONE_NAMES_H_
+#define ABSL_TIME_INTERNAL_CCTZ_TEST_TIME_ZONE_NAMES_H_
+
+#include "absl/base/config.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace time_internal {
+namespace cctz {
+
+// A list of known time-zone names.
+extern const char* const kTimeZoneNames[];
+
+}  // namespace cctz
+}  // namespace time_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_TIME_INTERNAL_CCTZ_TEST_TIME_ZONE_NAMES_H_
diff --git a/absl/time/internal/cctz/src/time_zone_fixed.cc b/absl/time/internal/cctz/src/time_zone_fixed.cc
index b0d159a2e52..b33410d2ca2 100644
--- a/absl/time/internal/cctz/src/time_zone_fixed.cc
+++ b/absl/time/internal/cctz/src/time_zone_fixed.cc
@@ -12,7 +12,7 @@
 //   See the License for the specific language governing permissions and
 //   limitations under the License.
 
-#include "time_zone_fixed.h"
+#include "absl/time/internal/cctz/src/time_zone_fixed.h"
 
 #include <algorithm>
 #include <cassert>
@@ -20,7 +20,10 @@
 #include <cstring>
 #include <string>
 
+#include "absl/base/config.h"
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 namespace cctz {
 
@@ -37,12 +40,19 @@ char* Format02d(char* p, int v) {
   return p;
 }
 
+// Returns the value of the decimal digit ch, or -1 if ch is not a digit.
+// Note that std::strchr() also matches kDigits' terminating '\0', which
+// would otherwise be taken for a tenth digit.
+int ParseDigit(char ch) {
+  const char* const dp = std::strchr(kDigits, ch);
+  return (dp == nullptr || *dp == '\0') ? -1 : static_cast<int>(dp - kDigits);
+}
+
 int Parse02d(const char* p) {
-  if (const char* ap = std::strchr(kDigits, *p)) {
-    int v = static_cast<int>(ap - kDigits);
-    if (const char* bp = std::strchr(kDigits, *++p)) {
-      return (v * 10) + static_cast<int>(bp - kDigits);
-    }
+  const int hi = ParseDigit(p[0]);
+  if (hi >= 0) {
+    const int lo = ParseDigit(p[1]);
+    if (lo >= 0) return (hi * 10) + lo;
   }
   return -1;
 }
@@ -50,7 +60,7 @@ int Parse02d(const char* p) {
 }  // namespace
 
 bool FixedOffsetFromName(const std::string& name, seconds* offset) {
-  if (name.compare(0, std::string::npos, "UTC", 3) == 0) {
+  if (name == "UTC" || name == "UTC0") {
     *offset = seconds::zero();
     return true;
   }
@@ -59,11 +69,9 @@ bool FixedOffsetFromName(const std::string& name, seconds* offset) {
   const char* const ep = kFixedZonePrefix + prefix_len;
   if (name.size() != prefix_len + 9)  // <prefix>+99:99:99
     return false;
-  if (!std::equal(kFixedZonePrefix, ep, name.begin()))
-    return false;
+  if (!std::equal(kFixedZonePrefix, ep, name.begin())) return false;
   const char* np = name.data() + prefix_len;
-  if (np[0] != '+' && np[0] != '-')
-    return false;
+  if (np[0] != '+' && np[0] != '-') return false;
   if (np[3] != ':' || np[6] != ':')  // see note below about large offsets
     return false;
 
@@ -88,29 +96,29 @@ std::string FixedOffsetToName(const seconds& offset) {
     // offsets and to (somewhat) limit the total number of zones.
     return "UTC";
   }
-  int seconds = static_cast<int>(offset.count());
-  const char sign = (seconds < 0 ? '-' : '+');
-  int minutes = seconds / 60;
-  seconds %= 60;
+  int offset_seconds = static_cast<int>(offset.count());
+  const char sign = (offset_seconds < 0 ? '-' : '+');
+  int offset_minutes = offset_seconds / 60;
+  offset_seconds %= 60;
   if (sign == '-') {
-    if (seconds > 0) {
-      seconds -= 60;
-      minutes += 1;
+    if (offset_seconds > 0) {
+      offset_seconds -= 60;
+      offset_minutes += 1;
     }
-    seconds = -seconds;
-    minutes = -minutes;
+    offset_seconds = -offset_seconds;
+    offset_minutes = -offset_minutes;
   }
-  int hours = minutes / 60;
-  minutes %= 60;
+  int offset_hours = offset_minutes / 60;
+  offset_minutes %= 60;
   const std::size_t prefix_len = sizeof(kFixedZonePrefix) - 1;
   char buf[prefix_len + sizeof("-24:00:00")];
-  char* ep = std::copy(kFixedZonePrefix, kFixedZonePrefix + prefix_len, buf);
+  char* ep = std::copy_n(kFixedZonePrefix, prefix_len, buf);
   *ep++ = sign;
-  ep = Format02d(ep, hours);
+  ep = Format02d(ep, offset_hours);
   *ep++ = ':';
-  ep = Format02d(ep, minutes);
+  ep = Format02d(ep, offset_minutes);
   *ep++ = ':';
-  ep = Format02d(ep, seconds);
+  ep = Format02d(ep, offset_seconds);
   *ep++ = '\0';
   assert(ep == buf + sizeof(buf));
   return buf;
@@ -135,4 +143,5 @@ std::string FixedOffsetToAbbr(const seconds& offset) {
 
 }  // namespace cctz
 }  // namespace time_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/time/internal/cctz/src/time_zone_fixed.h b/absl/time/internal/cctz/src/time_zone_fixed.h
index 9c1f5e7d7d7..e74a0bbd9dd 100644
--- a/absl/time/internal/cctz/src/time_zone_fixed.h
+++ b/absl/time/internal/cctz/src/time_zone_fixed.h
@@ -17,9 +17,11 @@
 
 #include <string>
 
+#include "absl/base/config.h"
 #include "absl/time/internal/cctz/include/cctz/time_zone.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 namespace cctz {
 
@@ -44,6 +46,7 @@ std::string FixedOffsetToAbbr(const seconds& offset);
 
 }  // namespace cctz
 }  // namespace time_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_TIME_INTERNAL_CCTZ_TIME_ZONE_FIXED_H_
diff --git a/absl/time/internal/cctz/src/time_zone_format.cc b/absl/time/internal/cctz/src/time_zone_format.cc
index 84e280b1397..f97de2f9592 100644
--- a/absl/time/internal/cctz/src/time_zone_format.cc
+++ b/absl/time/internal/cctz/src/time_zone_format.cc
@@ -13,17 +13,21 @@
 //   limitations under the License.
 
 #if !defined(HAS_STRPTIME)
-# if !defined(_MSC_VER) && !defined(__MINGW32__)
-#  define HAS_STRPTIME 1  // assume everyone has strptime() except windows
-# endif
+#if defined(_MSC_VER) || defined(__MINGW32__) || defined(__VXWORKS__)
+#define HAS_STRPTIME 0
+#else
+#define HAS_STRPTIME 1  // Assume everyone else has strptime().
+#endif
 #endif
 
-#if defined(HAS_STRPTIME) && HAS_STRPTIME
-# if !defined(_XOPEN_SOURCE)
-#  define _XOPEN_SOURCE  // Definedness suffices for strptime.
-# endif
+#if HAS_STRPTIME
+#if !defined(_XOPEN_SOURCE) && !defined(__FreeBSD__) && \
+    !defined(__OpenBSD__) && !defined(__APPLE__)
+#define _XOPEN_SOURCE 500  // Exposes definitions for SUSv2 (UNIX 98).
+#endif
 #endif
 
+#include "absl/base/config.h"
 #include "absl/time/internal/cctz/include/cctz/time_zone.h"
 
 // Include time.h directly since, by C++ standards, ctime doesn't have to
@@ -38,22 +42,32 @@
 #include <ctime>
 #include <limits>
 #include <string>
-#include <vector>
 #if !HAS_STRPTIME
 #include <iomanip>
 #include <sstream>
 #endif
 
 #include "absl/time/internal/cctz/include/cctz/civil_time.h"
-#include "time_zone_if.h"
+#include "absl/time/internal/cctz/src/time_zone_if.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 namespace cctz {
 namespace detail {
 
 namespace {
 
+// The ctype functions have undefined behavior for negative char values,
+// so these helpers ensure the argument is always in the unsigned-char domain.
+bool isdigit(char ch) {
+  return std::isdigit(static_cast<unsigned char>(ch)) != 0;
+}
+
+bool isspace(char ch) {
+  return std::isspace(static_cast<unsigned char>(ch)) != 0;
+}
+
 #if !HAS_STRPTIME
 // Build a strptime() using C++11's std::get_time().
 char* strptime(const char* s, const char* fmt, std::tm* tm) {
@@ -65,6 +79,48 @@ char* strptime(const char* s, const char* fmt, std::tm* tm) {
 }
 #endif
 
+// Convert a cctz::weekday to a tm_wday value (0-6, Sunday = 0).
+int ToTmWday(weekday wd) {
+  switch (wd) {
+    case weekday::sunday:
+      return 0;
+    case weekday::monday:
+      return 1;
+    case weekday::tuesday:
+      return 2;
+    case weekday::wednesday:
+      return 3;
+    case weekday::thursday:
+      return 4;
+    case weekday::friday:
+      return 5;
+    case weekday::saturday:
+      return 6;
+  }
+  return 0; /*NOTREACHED*/
+}
+
+// Convert a tm_wday value (0-6, Sunday = 0) to a cctz::weekday.
+weekday FromTmWday(int tm_wday) {
+  switch (tm_wday) {
+    case 0:
+      return weekday::sunday;
+    case 1:
+      return weekday::monday;
+    case 2:
+      return weekday::tuesday;
+    case 3:
+      return weekday::wednesday;
+    case 4:
+      return weekday::thursday;
+    case 5:
+      return weekday::friday;
+    case 6:
+      return weekday::saturday;
+  }
+  return weekday::sunday; /*NOTREACHED*/
+}
+
 std::tm ToTM(const time_zone::absolute_lookup& al) {
   std::tm tm{};
   tm.tm_sec = al.cs.second();
@@ -73,7 +129,7 @@ std::tm ToTM(const time_zone::absolute_lookup& al) {
   tm.tm_mday = al.cs.day();
   tm.tm_mon = al.cs.month() - 1;
 
-  // Saturate tm.tm_year is cases of over/underflow.
+  // Saturate tm.tm_year in cases of over/underflow.
   if (al.cs.year() < std::numeric_limits<int>::min() + 1900) {
     tm.tm_year = std::numeric_limits<int>::min();
   } else if (al.cs.year() - 1900 > std::numeric_limits<int>::max()) {
@@ -82,34 +138,19 @@ std::tm ToTM(const time_zone::absolute_lookup& al) {
     tm.tm_year = static_cast<int>(al.cs.year() - 1900);
   }
 
-  switch (get_weekday(al.cs)) {
-    case weekday::sunday:
-      tm.tm_wday = 0;
-      break;
-    case weekday::monday:
-      tm.tm_wday = 1;
-      break;
-    case weekday::tuesday:
-      tm.tm_wday = 2;
-      break;
-    case weekday::wednesday:
-      tm.tm_wday = 3;
-      break;
-    case weekday::thursday:
-      tm.tm_wday = 4;
-      break;
-    case weekday::friday:
-      tm.tm_wday = 5;
-      break;
-    case weekday::saturday:
-      tm.tm_wday = 6;
-      break;
-  }
+  tm.tm_wday = ToTmWday(get_weekday(al.cs));
   tm.tm_yday = get_yearday(al.cs) - 1;
   tm.tm_isdst = al.is_dst ? 1 : 0;
   return tm;
 }
 
+// Returns the week of the year [0:53] given a civil day and the day on
+// which weeks are defined to start.
+int ToWeek(const civil_day& cd, weekday week_start) {
+  const civil_day d(cd.year() % 400, cd.month(), cd.day());
+  return static_cast<int>((d - prev_weekday(civil_year(d), week_start)) / 7);
+}
+
 const char kDigits[] = "0123456789";
 
 // Formats a 64-bit integer in the given field width.  Note that it is up
@@ -184,19 +225,15 @@ char* FormatOffset(char* ep, int offset, const char* mode) {
 
 // Formats a std::tm using strftime(3).
 void FormatTM(std::string* out, const std::string& fmt, const std::tm& tm) {
-  // strftime(3) returns the number of characters placed in the output
-  // array (which may be 0 characters).  It also returns 0 to indicate
-  // an error, like the array wasn't large enough.  To accommodate this,
-  // the following code grows the buffer size from 2x the format std::string
-  // length up to 32x.
-  for (std::size_t i = 2; i != 32; i *= 2) {
-    std::size_t buf_size = fmt.size() * i;
-    std::vector<char> buf(buf_size);
-    if (std::size_t len = strftime(&buf[0], buf_size, fmt.c_str(), &tm)) {
-      out->append(&buf[0], len);
-      return;
-    }
-  }
+  // We assume that 16 times the length of the format string will
+  // be sufficient to store the result.  The extreme case appears
+  // to be "%c" (2 chars), which, in the POSIX locale, produces
+  // "Thu Jan 1 00:00:00 1970" (24 chars).
+  auto out_size = out->size();
+  auto buf_size = (16 * fmt.size()) + 1;
+  out->resize(out_size + buf_size);
+  auto len = strftime(&(*out)[out_size], buf_size, fmt.c_str(), &tm);
+  out->resize(out_size + len);
 }
 
 // Used for %E#S/%E#f specifiers and for data values in parse().
@@ -288,6 +325,7 @@ const std::int_fast64_t kExp10[kDigits10_64 + 1] = {
 //   - %E#S - Seconds with # digits of fractional precision
 //   - %E*S - Seconds with full fractional precision (a literal '*')
 //   - %E4Y - Four-character years (-999 ... -001, 0000, 0001 ... 9999)
+//   - %ET  - The RFC3339 "date-time" separator "T"
 //
 // The standard specifiers from RFC3339_* (%Y, %m, %d, %H, %M, and %S) are
 // handled internally for performance reasons.  strftime(3) is slow due to
@@ -303,12 +341,12 @@ const std::int_fast64_t kExp10[kDigits10_64 + 1] = {
 std::string format(const std::string& format, const time_point<seconds>& tp,
                    const detail::femtoseconds& fs, const time_zone& tz) {
   std::string result;
-  result.reserve(format.size());  // A reasonable guess for the result size.
+  result.reserve(2 * format.size());  // A guess for the result size.
   const time_zone::absolute_lookup al = tz.lookup(tp);
   const std::tm tm = ToTM(al);
 
   // Scratch buffer for internal conversions.
-  char buf[3 + kDigits10_64];  // enough for longest conversion
+  char buf[6 + (kDigits10_64 + 2)];  // enough for longest conversion (%F)
   char* const ep = buf + sizeof(buf);
   char* bp;  // works back from ep
 
@@ -317,23 +355,29 @@ std::string format(const std::string& format, const time_point<seconds>& tp,
   //   [pending ... cur) : formatting pending, but no special cases
   //   [cur ... format.end()) : unexamined
   // Initially, everything is in the unexamined part.
-  const char* pending = format.c_str();  // NUL terminated
+  const char* pending = format.data();
   const char* cur = pending;
-  const char* end = pending + format.length();
+  const char* const end = pending + format.size();
 
   while (cur != end) {  // while something is unexamined
     // Moves cur to the next percent sign.
     const char* start = cur;
-    while (cur != end && *cur != '%') ++cur;
+    while (cur != end && *cur != '%') {
+      if (*cur == '\0' && pending != start) {
+        FormatTM(&result, std::string(pending, cur), tm);
+        pending = start = cur;
+      }
+      ++cur;
+    }
 
     // If the new pending text is all ordinary, copy it out.
     if (cur != start && pending == start) {
-      result.append(pending, static_cast<std::size_t>(cur - pending));
+      result.append(pending, cur);
       pending = start = cur;
     }
 
     // Span the sequential percent signs.
-    const char* percent = cur;
+    const char* const percent = cur;
     while (cur != end && *cur == '%') ++cur;
 
     // If the new pending text is all percents, copy out one
@@ -352,49 +396,87 @@ std::string format(const std::string& format, const time_point<seconds>& tp,
     if (cur == end || (cur - percent) % 2 == 0) continue;
 
     // Simple specifiers that we handle ourselves.
-    if (strchr("YmdeHMSzZs%", *cur)) {
-      if (cur - 1 != pending) {
-        FormatTM(&result, std::string(pending, cur - 1), tm);
-      }
+    if (*cur == '\0' || strchr("YmdeFUuWwHMSTzZs%", *cur)) {
+      FormatTM(&result, std::string(pending, cur - 1), tm);
       switch (*cur) {
+        case '\0':
+          // Because we allow NULs in the format string, we must give
+          // some meaning to the "%\0" specifier.  We choose the common
+          // (but undefined) strftime() behavior of echoing unknown
+          // specifiers.
+          result.push_back('%');
+          result.push_back('\0');
+          break;
         case 'Y':
           // This avoids the tm.tm_year overflow problem for %Y, however
           // tm.tm_year will still be used by other specifiers like %D.
           bp = Format64(ep, 0, al.cs.year());
-          result.append(bp, static_cast<std::size_t>(ep - bp));
+          result.append(bp, ep);
           break;
         case 'm':
           bp = Format02d(ep, al.cs.month());
-          result.append(bp, static_cast<std::size_t>(ep - bp));
+          result.append(bp, ep);
           break;
         case 'd':
         case 'e':
           bp = Format02d(ep, al.cs.day());
           if (*cur == 'e' && *bp == '0') *bp = ' ';  // for Windows
-          result.append(bp, static_cast<std::size_t>(ep - bp));
+          result.append(bp, ep);
+          break;
+        case 'F':
+          bp = Format02d(ep, al.cs.day());
+          *--bp = '-';
+          bp = Format02d(bp, al.cs.month());
+          *--bp = '-';
+          bp = Format64(bp, 0, al.cs.year());
+          result.append(bp, ep);
+          break;
+        case 'U':
+          bp = Format02d(ep, ToWeek(civil_day(al.cs), weekday::sunday));
+          result.append(bp, ep);
+          break;
+        case 'u':
+          bp = Format64(ep, 0, tm.tm_wday ? tm.tm_wday : 7);
+          result.append(bp, ep);
+          break;
+        case 'W':
+          bp = Format02d(ep, ToWeek(civil_day(al.cs), weekday::monday));
+          result.append(bp, ep);
+          break;
+        case 'w':
+          bp = Format64(ep, 0, tm.tm_wday);
+          result.append(bp, ep);
           break;
         case 'H':
           bp = Format02d(ep, al.cs.hour());
-          result.append(bp, static_cast<std::size_t>(ep - bp));
+          result.append(bp, ep);
           break;
         case 'M':
           bp = Format02d(ep, al.cs.minute());
-          result.append(bp, static_cast<std::size_t>(ep - bp));
+          result.append(bp, ep);
           break;
         case 'S':
           bp = Format02d(ep, al.cs.second());
-          result.append(bp, static_cast<std::size_t>(ep - bp));
+          result.append(bp, ep);
+          break;
+        case 'T':
+          bp = Format02d(ep, al.cs.second());
+          *--bp = ':';
+          bp = Format02d(bp, al.cs.minute());
+          *--bp = ':';
+          bp = Format02d(bp, al.cs.hour());
+          result.append(bp, ep);
           break;
         case 'z':
           bp = FormatOffset(ep, al.offset, "");
-          result.append(bp, static_cast<std::size_t>(ep - bp));
+          result.append(bp, ep);
           break;
         case 'Z':
           result.append(al.abbr);
           break;
         case 's':
           bp = Format64(ep, 0, ToUnixSeconds(tp));
-          result.append(bp, static_cast<std::size_t>(ep - bp));
+          result.append(bp, ep);
           break;
         case '%':
           result.push_back('%');
@@ -408,33 +490,27 @@ std::string format(const std::string& format, const time_point<seconds>& tp,
     if (*cur == ':' && cur + 1 != end) {
       if (*(cur + 1) == 'z') {
         // Formats %:z.
-        if (cur - 1 != pending) {
-          FormatTM(&result, std::string(pending, cur - 1), tm);
-        }
+        FormatTM(&result, std::string(pending, cur - 1), tm);
         bp = FormatOffset(ep, al.offset, ":");
-        result.append(bp, static_cast<std::size_t>(ep - bp));
+        result.append(bp, ep);
         pending = cur += 2;
         continue;
       }
       if (*(cur + 1) == ':' && cur + 2 != end) {
         if (*(cur + 2) == 'z') {
           // Formats %::z.
-          if (cur - 1 != pending) {
-            FormatTM(&result, std::string(pending, cur - 1), tm);
-          }
+          FormatTM(&result, std::string(pending, cur - 1), tm);
           bp = FormatOffset(ep, al.offset, ":*");
-          result.append(bp, static_cast<std::size_t>(ep - bp));
+          result.append(bp, ep);
           pending = cur += 3;
           continue;
         }
         if (*(cur + 2) == ':' && cur + 3 != end) {
           if (*(cur + 3) == 'z') {
             // Formats %:::z.
-            if (cur - 1 != pending) {
-              FormatTM(&result, std::string(pending, cur - 1), tm);
-            }
+            FormatTM(&result, std::string(pending, cur - 1), tm);
             bp = FormatOffset(ep, al.offset, ":*:");
-            result.append(bp, static_cast<std::size_t>(ep - bp));
+            result.append(bp, ep);
             pending = cur += 4;
             continue;
           }
@@ -446,28 +522,27 @@ std::string format(const std::string& format, const time_point<seconds>& tp,
     if (*cur != 'E' || ++cur == end) continue;
 
     // Format our extensions.
-    if (*cur == 'z') {
+    if (*cur == 'T') {
+      // Formats %ET.
+      FormatTM(&result, std::string(pending, cur - 2), tm);
+      result.append("T");
+      pending = ++cur;
+    } else if (*cur == 'z') {
       // Formats %Ez.
-      if (cur - 2 != pending) {
-        FormatTM(&result, std::string(pending, cur - 2), tm);
-      }
+      FormatTM(&result, std::string(pending, cur - 2), tm);
       bp = FormatOffset(ep, al.offset, ":");
-      result.append(bp, static_cast<std::size_t>(ep - bp));
+      result.append(bp, ep);
       pending = ++cur;
     } else if (*cur == '*' && cur + 1 != end && *(cur + 1) == 'z') {
       // Formats %E*z.
-      if (cur - 2 != pending) {
-        FormatTM(&result, std::string(pending, cur - 2), tm);
-      }
+      FormatTM(&result, std::string(pending, cur - 2), tm);
       bp = FormatOffset(ep, al.offset, ":*");
-      result.append(bp, static_cast<std::size_t>(ep - bp));
+      result.append(bp, ep);
       pending = cur += 2;
     } else if (*cur == '*' && cur + 1 != end &&
                (*(cur + 1) == 'S' || *(cur + 1) == 'f')) {
       // Formats %E*S or %E*F.
-      if (cur - 2 != pending) {
-        FormatTM(&result, std::string(pending, cur - 2), tm);
-      }
+      FormatTM(&result, std::string(pending, cur - 2), tm);
       char* cp = ep;
       bp = Format64(cp, 15, fs.count());
       while (cp != bp && cp[-1] == '0') --cp;
@@ -480,34 +555,31 @@ std::string format(const std::string& format, const time_point<seconds>& tp,
           if (cp == bp) *--bp = '0';
           break;
       }
-      result.append(bp, static_cast<std::size_t>(cp - bp));
+      result.append(bp, cp);
       pending = cur += 2;
     } else if (*cur == '4' && cur + 1 != end && *(cur + 1) == 'Y') {
       // Formats %E4Y.
-      if (cur - 2 != pending) {
-        FormatTM(&result, std::string(pending, cur - 2), tm);
-      }
+      FormatTM(&result, std::string(pending, cur - 2), tm);
       bp = Format64(ep, 4, al.cs.year());
-      result.append(bp, static_cast<std::size_t>(ep - bp));
+      result.append(bp, ep);
       pending = cur += 2;
-    } else if (std::isdigit(*cur)) {
+    } else if (isdigit(*cur)) {
       // Possibly found %E#S or %E#f.
       int n = 0;
       if (const char* np = ParseInt(cur, 0, 0, 1024, &n)) {
         if (*np == 'S' || *np == 'f') {
           // Formats %E#S or %E#f.
-          if (cur - 2 != pending) {
-            FormatTM(&result, std::string(pending, cur - 2), tm);
-          }
+          FormatTM(&result, std::string(pending, cur - 2), tm);
           bp = ep;
           if (n > 0) {
             if (n > kDigits10_64) n = kDigits10_64;
-            bp = Format64(bp, n, (n > 15) ? fs.count() * kExp10[n - 15]
-                                          : fs.count() / kExp10[15 - n]);
+            bp = Format64(bp, n,
+                          (n > 15) ? fs.count() * kExp10[n - 15]
+                                   : fs.count() / kExp10[15 - n]);
             if (*np == 'S') *--bp = '.';
           }
           if (*np == 'S') bp = Format02d(bp, al.cs.second());
-          result.append(bp, static_cast<std::size_t>(ep - bp));
+          result.append(bp, ep);
           pending = cur = ++np;
         }
       }
@@ -515,9 +587,7 @@ std::string format(const std::string& format, const time_point<seconds>& tp,
   }
 
   // Formats any remaining data.
-  if (end != pending) {
-    FormatTM(&result, std::string(pending, end), tm);
-  }
+  FormatTM(&result, std::string(pending, end), tm);
 
   return result;
 }
@@ -548,7 +618,7 @@ const char* ParseOffset(const char* dp, const char* mode, int* offset) {
       } else {
         dp = nullptr;
       }
-    } else if (first == 'Z') {  // Zulu
+    } else if (first == 'Z' || first == 'z') {  // Zulu
       *offset = 0;
     } else {
       dp = nullptr;
@@ -560,7 +630,7 @@ const char* ParseOffset(const char* dp, const char* mode, int* offset) {
 const char* ParseZone(const char* dp, std::string* zone) {
   zone->clear();
   if (dp != nullptr) {
-    while (*dp != '\0' && !std::isspace(*dp)) zone->push_back(*dp++);
+    while (*dp != '\0' && !isspace(*dp)) zone->push_back(*dp++);
     if (zone->empty()) dp = nullptr;
   }
   return dp;
@@ -599,12 +669,32 @@ const char* ParseTM(const char* dp, const char* fmt, std::tm* tm) {
   return dp;
 }
 
+// Sets year, tm_mon and tm_mday given the year, week_num, and tm_wday,
+// and the day on which weeks are defined to start.  Returns false if year
+// would need to move outside its bounds.
+bool FromWeek(int week_num, weekday week_start, year_t* year, std::tm* tm) {
+  const civil_year y(*year % 400);
+  civil_day cd = prev_weekday(y, week_start);  // week 0
+  cd = next_weekday(cd - 1, FromTmWday(tm->tm_wday)) + (week_num * 7);
+  if (const year_t shift = cd.year() - y.year()) {
+    if (shift > 0) {
+      if (*year > std::numeric_limits<year_t>::max() - shift) return false;
+    } else {
+      if (*year < std::numeric_limits<year_t>::min() - shift) return false;
+    }
+    *year += shift;
+  }
+  tm->tm_mon = cd.month() - 1;
+  tm->tm_mday = cd.day();
+  return true;
+}
+
 }  // namespace
 
 // Uses strptime(3) to parse the given input.  Supports the same extended
 // format specifiers as format(), although %E#S and %E*S are treated
 // identically (and similarly for %E#f and %E*f).  %Ez and %E*z also accept
-// the same inputs.
+// the same inputs. %ET accepts either 'T' or 't'.
 //
 // The standard specifiers from RFC3339_* (%Y, %m, %d, %H, %M, and %S) are
 // handled internally so that we can normally avoid strptime() altogether
@@ -618,11 +708,15 @@ const char* ParseTM(const char* dp, const char* fmt, std::tm* tm) {
 bool parse(const std::string& format, const std::string& input,
            const time_zone& tz, time_point<seconds>* sec,
            detail::femtoseconds* fs, std::string* err) {
-  // The unparsed input.
+  // The unparsed input.  Even though we allow NULs in input, and
+  // match them against corresponding NULs in format, we depend on
+  // *edata being a NUL so that we can call strptime().  This also
+  // makes our handling of input easier.
   const char* data = input.c_str();  // NUL terminated
+  const char* const edata = data + input.size();
 
   // Skips leading whitespace.
-  while (std::isspace(*data)) ++data;
+  while (isspace(*data)) ++data;
 
   const year_t kyearmax = std::numeric_limits<year_t>::max();
   const year_t kyearmin = std::numeric_limits<year_t>::min();
@@ -645,23 +739,29 @@ bool parse(const std::string& format, const std::string& input,
   int offset = 0;  // No offset from passed tz.
   std::string zone = "UTC";
 
+  // Even though we allow NULs in format, and match them against
+  // corresponding NULs in input, we simplify its handling by also
+  // ensuring that *efmt is a NUL.
   const char* fmt = format.c_str();  // NUL terminated
+  const char* const efmt = fmt + format.size();
   bool twelve_hour = false;
   bool afternoon = false;
+  int week_num = -1;
+  weekday week_start = weekday::sunday;
 
   bool saw_percent_s = false;
   std::int_fast64_t percent_s = 0;
 
   // Steps through format, one specifier at a time.
-  while (data != nullptr && *fmt != '\0') {
-    if (std::isspace(*fmt)) {
-      while (std::isspace(*data)) ++data;
-      while (std::isspace(*++fmt)) continue;
+  while (data != nullptr && fmt != efmt) {
+    if (isspace(*fmt)) {
+      while (isspace(*data)) ++data;
+      while (isspace(*++fmt)) continue;
       continue;
     }
 
     if (*fmt != '%') {
-      if (*data == *fmt) {
+      if (data != edata && *data == *fmt) {
         ++data;
         ++fmt;
       } else {
@@ -670,14 +770,21 @@ bool parse(const std::string& format, const std::string& input,
       continue;
     }
 
-    const char* percent = fmt;
-    if (*++fmt == '\0') {
+    const char* const percent = fmt;
+    if (++fmt == efmt) {
       data = nullptr;
       continue;
     }
     switch (*fmt++) {
+      case '\0':
+        // Because we allow NULs in the format string, we must give
+        // some meaning to the "%\0" specifier.  We choose the common
+        // (but undefined) strptime() behavior of failing on unknown
+        // specifiers.
+        data = nullptr;
+        continue;
       case 'Y':
-        // Symmetrically with FormatTime(), directly handing %Y avoids the
+        // Symmetrically with format(), directly handing %Y avoids the
         // tm.tm_year overflow problem.  However, tm.tm_year will still be
         // used by other specifiers like %D.
         data = ParseInt(data, 0, kyearmin, kyearmax, &year);
@@ -686,10 +793,41 @@ bool parse(const std::string& format, const std::string& input,
       case 'm':
         data = ParseInt(data, 2, 1, 12, &tm.tm_mon);
         if (data != nullptr) tm.tm_mon -= 1;
+        week_num = -1;
         continue;
       case 'd':
       case 'e':
         data = ParseInt(data, 2, 1, 31, &tm.tm_mday);
+        week_num = -1;
+        continue;
+      case 'F':
+        data = ParseInt(data, 0, kyearmin, kyearmax, &year);
+        if (data != nullptr) {
+          saw_year = true;
+          data = (*data == '-' ? data + 1 : nullptr);
+        }
+        data = ParseInt(data, 2, 1, 12, &tm.tm_mon);
+        if (data != nullptr) {
+          tm.tm_mon -= 1;
+          data = (*data == '-' ? data + 1 : nullptr);
+        }
+        data = ParseInt(data, 2, 1, 31, &tm.tm_mday);
+        week_num = -1;
+        continue;
+      case 'U':
+        data = ParseInt(data, 0, 0, 53, &week_num);
+        week_start = weekday::sunday;
+        continue;
+      case 'W':
+        data = ParseInt(data, 0, 0, 53, &week_num);
+        week_start = weekday::monday;
+        continue;
+      case 'u':
+        data = ParseInt(data, 0, 1, 7, &tm.tm_wday);
+        if (data != nullptr) tm.tm_wday %= 7;
+        continue;
+      case 'w':
+        data = ParseInt(data, 0, 0, 6, &tm.tm_wday);
         continue;
       case 'H':
         data = ParseInt(data, 2, 0, 23, &tm.tm_hour);
@@ -701,13 +839,20 @@ bool parse(const std::string& format, const std::string& input,
       case 'S':
         data = ParseInt(data, 2, 0, 60, &tm.tm_sec);
         continue;
+      case 'T':
+        data = ParseInt(data, 2, 0, 23, &tm.tm_hour);
+        twelve_hour = false;
+        data = (data != nullptr && *data == ':' ? data + 1 : nullptr);
+        data = ParseInt(data, 2, 0, 59, &tm.tm_min);
+        data = (data != nullptr && *data == ':' ? data + 1 : nullptr);
+        data = ParseInt(data, 2, 0, 60, &tm.tm_sec);
+        continue;
       case 'I':
       case 'l':
       case 'r':  // probably uses %I
         twelve_hour = true;
         break;
       case 'R':  // uses %H
-      case 'T':  // uses %H
       case 'c':  // probably uses %H
       case 'X':  // probably uses %H
         twelve_hour = false;
@@ -720,10 +865,9 @@ bool parse(const std::string& format, const std::string& input,
         data = ParseZone(data, &zone);
         continue;
       case 's':
-        data = ParseInt(data, 0,
-                        std::numeric_limits<std::int_fast64_t>::min(),
-                        std::numeric_limits<std::int_fast64_t>::max(),
-                        &percent_s);
+        data =
+            ParseInt(data, 0, std::numeric_limits<std::int_fast64_t>::min(),
+                     std::numeric_limits<std::int_fast64_t>::max(), &percent_s);
         if (data != nullptr) saw_percent_s = true;
         continue;
       case ':':
@@ -740,6 +884,15 @@ bool parse(const std::string& format, const std::string& input,
         data = (*data == '%' ? data + 1 : nullptr);
         continue;
       case 'E':
+        if (fmt[0] == 'T') {
+          if (*data == 'T' || *data == 't') {
+            ++data;
+            ++fmt;
+          } else {
+            data = nullptr;
+          }
+          continue;
+        }
         if (fmt[0] == 'z' || (fmt[0] == '*' && fmt[1] == 'z')) {
           data = ParseOffset(data, ":", &offset);
           if (data != nullptr) saw_offset = true;
@@ -755,7 +908,7 @@ bool parse(const std::string& format, const std::string& input,
           continue;
         }
         if (fmt[0] == '*' && fmt[1] == 'f') {
-          if (data != nullptr && std::isdigit(*data)) {
+          if (data != nullptr && isdigit(*data)) {
             data = ParseSubSeconds(data, &subseconds);
           }
           fmt += 2;
@@ -774,7 +927,7 @@ bool parse(const std::string& format, const std::string& input,
           fmt += 2;
           continue;
         }
-        if (std::isdigit(*fmt)) {
+        if (isdigit(*fmt)) {
           int n = 0;  // value ignored
           if (const char* np = ParseInt(fmt, 0, 0, 1024, &n)) {
             if (*np == 'S') {
@@ -786,7 +939,7 @@ bool parse(const std::string& format, const std::string& input,
               continue;
             }
             if (*np == 'f') {
-              if (data != nullptr && std::isdigit(*data)) {
+              if (data != nullptr && isdigit(*data)) {
                 data = ParseSubSeconds(data, &subseconds);
               }
               fmt = ++np;
@@ -806,8 +959,8 @@ bool parse(const std::string& format, const std::string& input,
     }
 
     // Parses the current specifier.
-    const char* orig_data = data;
-    std::string spec(percent, static_cast<std::size_t>(fmt - percent));
+    const char* const orig_data = data;
+    std::string spec(percent, fmt);
     data = ParseTM(data, spec.c_str(), &tm);
 
     // If we successfully parsed %p we need to remember whether the result
@@ -816,10 +969,9 @@ bool parse(const std::string& format, const std::string& input,
     // to a PM hour.
     if (spec == "%p" && data != nullptr) {
       std::string test_input = "1";
-      test_input.append(orig_data, static_cast<std::size_t>(data - orig_data));
-      const char* test_data = test_input.c_str();
+      test_input.append(orig_data, data);
       std::tm tmp{};
-      ParseTM(test_data, "%I%p", &tmp);
+      ParseTM(test_input.c_str(), "%I%p", &tmp);
       afternoon = (tmp.tm_hour == 13);
     }
   }
@@ -835,10 +987,10 @@ bool parse(const std::string& format, const std::string& input,
   }
 
   // Skip any remaining whitespace.
-  while (std::isspace(*data)) ++data;
+  while (isspace(*data)) ++data;
 
-  // parse() must consume the entire input std::string.
-  if (*data != '\0') {
+  // parse() must consume the entire input string.
+  if (data != edata) {
     if (err != nullptr) *err = "Illegal trailing data in input string";
     return false;
   }
@@ -872,6 +1024,14 @@ bool parse(const std::string& format, const std::string& input,
     year += 1900;
   }
 
+  // Compute year, tm.tm_mon and tm.tm_mday if we parsed a week number.
+  if (week_num != -1) {
+    if (!FromWeek(week_num, week_start, &year, &tm)) {
+      if (err != nullptr) *err = "Out-of-range field";
+      return false;
+    }
+  }
+
   const int month = tm.tm_mon + 1;
   civil_second cs(year, month, tm.tm_mday, tm.tm_hour, tm.tm_min, tm.tm_sec);
 
@@ -916,4 +1076,5 @@ bool parse(const std::string& format, const std::string& input,
 }  // namespace detail
 }  // namespace cctz
 }  // namespace time_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/time/internal/cctz/src/time_zone_format_test.cc b/absl/time/internal/cctz/src/time_zone_format_test.cc
index 705ccdcdcc4..8793a805c66 100644
--- a/absl/time/internal/cctz/src/time_zone_format_test.cc
+++ b/absl/time/internal/cctz/src/time_zone_format_test.cc
@@ -12,20 +12,26 @@
 //   See the License for the specific language governing permissions and
 //   limitations under the License.
 
-#include "absl/time/internal/cctz/include/cctz/time_zone.h"
-
 #include <chrono>
+#include <cstdint>
 #include <iomanip>
 #include <sstream>
 #include <string>
 
-#include "absl/time/internal/cctz/include/cctz/civil_time.h"
+#include "absl/base/config.h"
+#include "absl/time/internal/cctz/include/cctz/time_zone.h"
+#if defined(__linux__)
+#include <features.h>
+#endif
+
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
+#include "absl/time/internal/cctz/include/cctz/civil_time.h"
 
 namespace chrono = std::chrono;
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 namespace cctz {
 
@@ -47,21 +53,24 @@ namespace {
     EXPECT_STREQ(zone, al.abbr);                                  \
   } while (0)
 
-const char RFC3339_full[] = "%Y-%m-%dT%H:%M:%E*S%Ez";
-const char RFC3339_sec[] =  "%Y-%m-%dT%H:%M:%S%Ez";
+const char RFC3339_full[] = "%Y-%m-%d%ET%H:%M:%E*S%Ez";
+const char RFC3339_sec[] = "%Y-%m-%d%ET%H:%M:%S%Ez";
 
 const char RFC1123_full[] = "%a, %d %b %Y %H:%M:%S %z";
-const char RFC1123_no_wday[] =  "%d %b %Y %H:%M:%S %z";
+const char RFC1123_no_wday[] = "%d %b %Y %H:%M:%S %z";
 
 // A helper that tests the given format specifier by itself, and with leading
 // and trailing characters.  For example: TestFormatSpecifier(tp, "%a", "Thu").
 template <typename D>
 void TestFormatSpecifier(time_point<D> tp, time_zone tz, const std::string& fmt,
                          const std::string& ans) {
-  EXPECT_EQ(ans, format(fmt, tp, tz)) << fmt;
-  EXPECT_EQ("xxx " + ans, format("xxx " + fmt, tp, tz));
-  EXPECT_EQ(ans + " yyy", format(fmt + " yyy", tp, tz));
-  EXPECT_EQ("xxx " + ans + " yyy", format("xxx " + fmt + " yyy", tp, tz));
+  EXPECT_EQ(ans, absl::time_internal::cctz::format(fmt, tp, tz)) << fmt;
+  EXPECT_EQ("xxx " + ans,
+            absl::time_internal::cctz::format("xxx " + fmt, tp, tz));
+  EXPECT_EQ(ans + " yyy",
+            absl::time_internal::cctz::format(fmt + " yyy", tp, tz));
+  EXPECT_EQ("xxx " + ans + " yyy",
+            absl::time_internal::cctz::format("xxx " + fmt + " yyy", tp, tz));
 }
 
 }  // namespace
@@ -77,23 +86,29 @@ TEST(Format, TimePointResolution) {
       chrono::system_clock::from_time_t(1420167845) +
       chrono::milliseconds(123) + chrono::microseconds(456) +
       chrono::nanoseconds(789);
-  EXPECT_EQ(
-      "03:04:05.123456789",
-      format(kFmt, chrono::time_point_cast<chrono::nanoseconds>(t0), utc));
-  EXPECT_EQ(
-      "03:04:05.123456",
-      format(kFmt, chrono::time_point_cast<chrono::microseconds>(t0), utc));
-  EXPECT_EQ(
-      "03:04:05.123",
-      format(kFmt, chrono::time_point_cast<chrono::milliseconds>(t0), utc));
-  EXPECT_EQ("03:04:05",
-            format(kFmt, chrono::time_point_cast<chrono::seconds>(t0), utc));
+  EXPECT_EQ("03:04:05.123456789",
+            absl::time_internal::cctz::format(
+                kFmt, chrono::time_point_cast<chrono::nanoseconds>(t0), utc));
+  EXPECT_EQ("03:04:05.123456",
+            absl::time_internal::cctz::format(
+                kFmt, chrono::time_point_cast<chrono::microseconds>(t0), utc));
+  EXPECT_EQ("03:04:05.123",
+            absl::time_internal::cctz::format(
+                kFmt, chrono::time_point_cast<chrono::milliseconds>(t0), utc));
   EXPECT_EQ("03:04:05",
-            format(kFmt, chrono::time_point_cast<absl::time_internal::cctz::seconds>(t0), utc));
+            absl::time_internal::cctz::format(
+                kFmt, chrono::time_point_cast<chrono::seconds>(t0), utc));
+  EXPECT_EQ(
+      "03:04:05",
+      absl::time_internal::cctz::format(
+          kFmt, chrono::time_point_cast<absl::time_internal::cctz::seconds>(t0),
+          utc));
   EXPECT_EQ("03:04:00",
-            format(kFmt, chrono::time_point_cast<chrono::minutes>(t0), utc));
+            absl::time_internal::cctz::format(
+                kFmt, chrono::time_point_cast<chrono::minutes>(t0), utc));
   EXPECT_EQ("03:00:00",
-            format(kFmt, chrono::time_point_cast<chrono::hours>(t0), utc));
+            absl::time_internal::cctz::format(
+                kFmt, chrono::time_point_cast<chrono::hours>(t0), utc));
 }
 
 TEST(Format, TimePointExtendedResolution) {
@@ -110,12 +125,10 @@ TEST(Format, TimePointExtendedResolution) {
   EXPECT_EQ(
       "12:34:56.012345678901234",
       detail::format(kFmt, tp, detail::femtoseconds(12345678901234), utc));
-  EXPECT_EQ(
-      "12:34:56.001234567890123",
-      detail::format(kFmt, tp, detail::femtoseconds(1234567890123), utc));
-  EXPECT_EQ(
-      "12:34:56.000123456789012",
-      detail::format(kFmt, tp, detail::femtoseconds(123456789012), utc));
+  EXPECT_EQ("12:34:56.001234567890123",
+            detail::format(kFmt, tp, detail::femtoseconds(1234567890123), utc));
+  EXPECT_EQ("12:34:56.000123456789012",
+            detail::format(kFmt, tp, detail::femtoseconds(123456789012), utc));
 
   EXPECT_EQ("12:34:56.000000000000123",
             detail::format(kFmt, tp, detail::femtoseconds(123), utc));
@@ -129,46 +142,49 @@ TEST(Format, Basics) {
   time_zone tz = utc_time_zone();
   time_point<chrono::nanoseconds> tp = chrono::system_clock::from_time_t(0);
 
-  // Starts with a couple basic edge cases.
-  EXPECT_EQ("", format("", tp, tz));
-  EXPECT_EQ(" ", format(" ", tp, tz));
-  EXPECT_EQ("  ", format("  ", tp, tz));
-  EXPECT_EQ("xxx", format("xxx", tp, tz));
+  // Start with basic edge cases.
+  EXPECT_EQ("", absl::time_internal::cctz::format("", tp, tz));
+  EXPECT_EQ(" ", absl::time_internal::cctz::format(" ", tp, tz));
+  EXPECT_EQ("  ", absl::time_internal::cctz::format("  ", tp, tz));
+  EXPECT_EQ("xxx", absl::time_internal::cctz::format("xxx", tp, tz));
   std::string big(128, 'x');
-  EXPECT_EQ(big, format(big, tp, tz));
+  EXPECT_EQ(big, absl::time_internal::cctz::format(big, tp, tz));
   // Cause the 1024-byte buffer to grow.
   std::string bigger(100000, 'x');
-  EXPECT_EQ(bigger, format(bigger, tp, tz));
+  EXPECT_EQ(bigger, absl::time_internal::cctz::format(bigger, tp, tz));
 
   tp += chrono::hours(13) + chrono::minutes(4) + chrono::seconds(5);
   tp += chrono::milliseconds(6) + chrono::microseconds(7) +
         chrono::nanoseconds(8);
-  EXPECT_EQ("1970-01-01", format("%Y-%m-%d", tp, tz));
-  EXPECT_EQ("13:04:05", format("%H:%M:%S", tp, tz));
-  EXPECT_EQ("13:04:05.006", format("%H:%M:%E3S", tp, tz));
-  EXPECT_EQ("13:04:05.006007", format("%H:%M:%E6S", tp, tz));
-  EXPECT_EQ("13:04:05.006007008", format("%H:%M:%E9S", tp, tz));
+  EXPECT_EQ("1970-01-01",
+            absl::time_internal::cctz::format("%Y-%m-%d", tp, tz));
+  EXPECT_EQ("13:04:05", absl::time_internal::cctz::format("%H:%M:%S", tp, tz));
+  EXPECT_EQ("13:04:05.006",
+            absl::time_internal::cctz::format("%H:%M:%E3S", tp, tz));
+  EXPECT_EQ("13:04:05.006007",
+            absl::time_internal::cctz::format("%H:%M:%E6S", tp, tz));
+  EXPECT_EQ("13:04:05.006007008",
+            absl::time_internal::cctz::format("%H:%M:%E9S", tp, tz));
 }
 
 TEST(Format, PosixConversions) {
   const time_zone tz = utc_time_zone();
-  auto tp = chrono::system_clock::from_time_t(0);
+  auto tp =
+      chrono::system_clock::from_time_t(308189482);  // 1979-10-08T00:11:22Z
 
-  TestFormatSpecifier(tp, tz, "%d", "01");
-  TestFormatSpecifier(tp, tz, "%e", " 1");  // extension but internal support
+  TestFormatSpecifier(tp, tz, "%d", "08");
+  TestFormatSpecifier(tp, tz, "%e", " 8");  // extension but internal support
   TestFormatSpecifier(tp, tz, "%H", "00");
   TestFormatSpecifier(tp, tz, "%I", "12");
-  TestFormatSpecifier(tp, tz, "%j", "001");
-  TestFormatSpecifier(tp, tz, "%m", "01");
-  TestFormatSpecifier(tp, tz, "%M", "00");
-  TestFormatSpecifier(tp, tz, "%S", "00");
-  TestFormatSpecifier(tp, tz, "%U", "00");
-#if !defined(__EMSCRIPTEN__)
-  TestFormatSpecifier(tp, tz, "%w", "4");  // 4=Thursday
-#endif
-  TestFormatSpecifier(tp, tz, "%W", "00");
-  TestFormatSpecifier(tp, tz, "%y", "70");
-  TestFormatSpecifier(tp, tz, "%Y", "1970");
+  TestFormatSpecifier(tp, tz, "%j", "281");
+  TestFormatSpecifier(tp, tz, "%m", "10");
+  TestFormatSpecifier(tp, tz, "%M", "11");
+  TestFormatSpecifier(tp, tz, "%S", "22");
+  TestFormatSpecifier(tp, tz, "%U", "40");
+  TestFormatSpecifier(tp, tz, "%w", "1");  // 1=Monday
+  TestFormatSpecifier(tp, tz, "%W", "41");
+  TestFormatSpecifier(tp, tz, "%y", "79");
+  TestFormatSpecifier(tp, tz, "%Y", "1979");
   TestFormatSpecifier(tp, tz, "%z", "+0000");
   TestFormatSpecifier(tp, tz, "%Z", "UTC");
   TestFormatSpecifier(tp, tz, "%%", "%");
@@ -176,19 +192,21 @@ TEST(Format, PosixConversions) {
 #if defined(__linux__)
   // SU/C99/TZ extensions
   TestFormatSpecifier(tp, tz, "%C", "19");
-  TestFormatSpecifier(tp, tz, "%D", "01/01/70");
-  TestFormatSpecifier(tp, tz, "%F", "1970-01-01");
-  TestFormatSpecifier(tp, tz, "%g", "70");
-  TestFormatSpecifier(tp, tz, "%G", "1970");
+  TestFormatSpecifier(tp, tz, "%D", "10/08/79");
+  TestFormatSpecifier(tp, tz, "%F", "1979-10-08");
+  TestFormatSpecifier(tp, tz, "%g", "79");
+  TestFormatSpecifier(tp, tz, "%G", "1979");
+#if defined(__GLIBC__)
   TestFormatSpecifier(tp, tz, "%k", " 0");
   TestFormatSpecifier(tp, tz, "%l", "12");
+#endif
   TestFormatSpecifier(tp, tz, "%n", "\n");
-  TestFormatSpecifier(tp, tz, "%R", "00:00");
+  TestFormatSpecifier(tp, tz, "%R", "00:11");
   TestFormatSpecifier(tp, tz, "%t", "\t");
-  TestFormatSpecifier(tp, tz, "%T", "00:00:00");
-  TestFormatSpecifier(tp, tz, "%u", "4");  // 4=Thursday
-  TestFormatSpecifier(tp, tz, "%V", "01");
-  TestFormatSpecifier(tp, tz, "%s", "0");
+  TestFormatSpecifier(tp, tz, "%T", "00:11:22");
+  TestFormatSpecifier(tp, tz, "%u", "1");  // 1=Monday
+  TestFormatSpecifier(tp, tz, "%V", "41");
+  TestFormatSpecifier(tp, tz, "%s", "308189482");
 #endif
 }
 
@@ -202,7 +220,8 @@ TEST(Format, LocaleSpecific) {
   TestFormatSpecifier(tp, tz, "%B", "January");
 
   // %c should at least produce the numeric year and time-of-day.
-  const std::string s = format("%c", tp, utc_time_zone());
+  const std::string s =
+      absl::time_internal::cctz::format("%c", tp, utc_time_zone());
   EXPECT_THAT(s, testing::HasSubstr("1970"));
   EXPECT_THAT(s, testing::HasSubstr("00:00:00"));
 
@@ -213,7 +232,9 @@ TEST(Format, LocaleSpecific) {
 #if defined(__linux__)
   // SU/C99/TZ extensions
   TestFormatSpecifier(tp, tz, "%h", "Jan");  // Same as %b
+#if defined(__GLIBC__)
   TestFormatSpecifier(tp, tz, "%P", "am");
+#endif
   TestFormatSpecifier(tp, tz, "%r", "12:00:00 AM");
 
   // Modified conversion specifiers %E_
@@ -266,49 +287,61 @@ TEST(Format, ExtendedSeconds) {
   // No subseconds.
   time_point<chrono::nanoseconds> tp = chrono::system_clock::from_time_t(0);
   tp += chrono::seconds(5);
-  EXPECT_EQ("05", format("%E*S", tp, tz));
-  EXPECT_EQ("05", format("%E0S", tp, tz));
-  EXPECT_EQ("05.0", format("%E1S", tp, tz));
-  EXPECT_EQ("05.00", format("%E2S", tp, tz));
-  EXPECT_EQ("05.000", format("%E3S", tp, tz));
-  EXPECT_EQ("05.0000", format("%E4S", tp, tz));
-  EXPECT_EQ("05.00000", format("%E5S", tp, tz));
-  EXPECT_EQ("05.000000", format("%E6S", tp, tz));
-  EXPECT_EQ("05.0000000", format("%E7S", tp, tz));
-  EXPECT_EQ("05.00000000", format("%E8S", tp, tz));
-  EXPECT_EQ("05.000000000", format("%E9S", tp, tz));
-  EXPECT_EQ("05.0000000000", format("%E10S", tp, tz));
-  EXPECT_EQ("05.00000000000", format("%E11S", tp, tz));
-  EXPECT_EQ("05.000000000000", format("%E12S", tp, tz));
-  EXPECT_EQ("05.0000000000000", format("%E13S", tp, tz));
-  EXPECT_EQ("05.00000000000000", format("%E14S", tp, tz));
-  EXPECT_EQ("05.000000000000000", format("%E15S", tp, tz));
+  EXPECT_EQ("05", absl::time_internal::cctz::format("%E*S", tp, tz));
+  EXPECT_EQ("05", absl::time_internal::cctz::format("%E0S", tp, tz));
+  EXPECT_EQ("05.0", absl::time_internal::cctz::format("%E1S", tp, tz));
+  EXPECT_EQ("05.00", absl::time_internal::cctz::format("%E2S", tp, tz));
+  EXPECT_EQ("05.000", absl::time_internal::cctz::format("%E3S", tp, tz));
+  EXPECT_EQ("05.0000", absl::time_internal::cctz::format("%E4S", tp, tz));
+  EXPECT_EQ("05.00000", absl::time_internal::cctz::format("%E5S", tp, tz));
+  EXPECT_EQ("05.000000", absl::time_internal::cctz::format("%E6S", tp, tz));
+  EXPECT_EQ("05.0000000", absl::time_internal::cctz::format("%E7S", tp, tz));
+  EXPECT_EQ("05.00000000", absl::time_internal::cctz::format("%E8S", tp, tz));
+  EXPECT_EQ("05.000000000", absl::time_internal::cctz::format("%E9S", tp, tz));
+  EXPECT_EQ("05.0000000000",
+            absl::time_internal::cctz::format("%E10S", tp, tz));
+  EXPECT_EQ("05.00000000000",
+            absl::time_internal::cctz::format("%E11S", tp, tz));
+  EXPECT_EQ("05.000000000000",
+            absl::time_internal::cctz::format("%E12S", tp, tz));
+  EXPECT_EQ("05.0000000000000",
+            absl::time_internal::cctz::format("%E13S", tp, tz));
+  EXPECT_EQ("05.00000000000000",
+            absl::time_internal::cctz::format("%E14S", tp, tz));
+  EXPECT_EQ("05.000000000000000",
+            absl::time_internal::cctz::format("%E15S", tp, tz));
 
   // With subseconds.
   tp += chrono::milliseconds(6) + chrono::microseconds(7) +
         chrono::nanoseconds(8);
-  EXPECT_EQ("05.006007008", format("%E*S", tp, tz));
-  EXPECT_EQ("05", format("%E0S", tp, tz));
-  EXPECT_EQ("05.0", format("%E1S", tp, tz));
-  EXPECT_EQ("05.00", format("%E2S", tp, tz));
-  EXPECT_EQ("05.006", format("%E3S", tp, tz));
-  EXPECT_EQ("05.0060", format("%E4S", tp, tz));
-  EXPECT_EQ("05.00600", format("%E5S", tp, tz));
-  EXPECT_EQ("05.006007", format("%E6S", tp, tz));
-  EXPECT_EQ("05.0060070", format("%E7S", tp, tz));
-  EXPECT_EQ("05.00600700", format("%E8S", tp, tz));
-  EXPECT_EQ("05.006007008", format("%E9S", tp, tz));
-  EXPECT_EQ("05.0060070080", format("%E10S", tp, tz));
-  EXPECT_EQ("05.00600700800", format("%E11S", tp, tz));
-  EXPECT_EQ("05.006007008000", format("%E12S", tp, tz));
-  EXPECT_EQ("05.0060070080000", format("%E13S", tp, tz));
-  EXPECT_EQ("05.00600700800000", format("%E14S", tp, tz));
-  EXPECT_EQ("05.006007008000000", format("%E15S", tp, tz));
+  EXPECT_EQ("05.006007008", absl::time_internal::cctz::format("%E*S", tp, tz));
+  EXPECT_EQ("05", absl::time_internal::cctz::format("%E0S", tp, tz));
+  EXPECT_EQ("05.0", absl::time_internal::cctz::format("%E1S", tp, tz));
+  EXPECT_EQ("05.00", absl::time_internal::cctz::format("%E2S", tp, tz));
+  EXPECT_EQ("05.006", absl::time_internal::cctz::format("%E3S", tp, tz));
+  EXPECT_EQ("05.0060", absl::time_internal::cctz::format("%E4S", tp, tz));
+  EXPECT_EQ("05.00600", absl::time_internal::cctz::format("%E5S", tp, tz));
+  EXPECT_EQ("05.006007", absl::time_internal::cctz::format("%E6S", tp, tz));
+  EXPECT_EQ("05.0060070", absl::time_internal::cctz::format("%E7S", tp, tz));
+  EXPECT_EQ("05.00600700", absl::time_internal::cctz::format("%E8S", tp, tz));
+  EXPECT_EQ("05.006007008", absl::time_internal::cctz::format("%E9S", tp, tz));
+  EXPECT_EQ("05.0060070080",
+            absl::time_internal::cctz::format("%E10S", tp, tz));
+  EXPECT_EQ("05.00600700800",
+            absl::time_internal::cctz::format("%E11S", tp, tz));
+  EXPECT_EQ("05.006007008000",
+            absl::time_internal::cctz::format("%E12S", tp, tz));
+  EXPECT_EQ("05.0060070080000",
+            absl::time_internal::cctz::format("%E13S", tp, tz));
+  EXPECT_EQ("05.00600700800000",
+            absl::time_internal::cctz::format("%E14S", tp, tz));
+  EXPECT_EQ("05.006007008000000",
+            absl::time_internal::cctz::format("%E15S", tp, tz));
 
   // Times before the Unix epoch.
   tp = chrono::system_clock::from_time_t(0) + chrono::microseconds(-1);
   EXPECT_EQ("1969-12-31 23:59:59.999999",
-            format("%Y-%m-%d %H:%M:%E*S", tp, tz));
+            absl::time_internal::cctz::format("%Y-%m-%d %H:%M:%E*S", tp, tz));
 
   // Here is a "%E*S" case we got wrong for a while.  While the first
   // instant below is correctly rendered as "...:07.333304", the second
@@ -316,10 +349,10 @@ TEST(Format, ExtendedSeconds) {
   tp = chrono::system_clock::from_time_t(0) +
        chrono::microseconds(1395024427333304);
   EXPECT_EQ("2014-03-17 02:47:07.333304",
-            format("%Y-%m-%d %H:%M:%E*S", tp, tz));
+            absl::time_internal::cctz::format("%Y-%m-%d %H:%M:%E*S", tp, tz));
   tp += chrono::microseconds(1);
   EXPECT_EQ("2014-03-17 02:47:07.333305",
-            format("%Y-%m-%d %H:%M:%E*S", tp, tz));
+            absl::time_internal::cctz::format("%Y-%m-%d %H:%M:%E*S", tp, tz));
 }
 
 TEST(Format, ExtendedSubeconds) {
@@ -328,60 +361,69 @@ TEST(Format, ExtendedSubeconds) {
   // No subseconds.
   time_point<chrono::nanoseconds> tp = chrono::system_clock::from_time_t(0);
   tp += chrono::seconds(5);
-  EXPECT_EQ("0", format("%E*f", tp, tz));
-  EXPECT_EQ("", format("%E0f", tp, tz));
-  EXPECT_EQ("0", format("%E1f", tp, tz));
-  EXPECT_EQ("00", format("%E2f", tp, tz));
-  EXPECT_EQ("000", format("%E3f", tp, tz));
-  EXPECT_EQ("0000", format("%E4f", tp, tz));
-  EXPECT_EQ("00000", format("%E5f", tp, tz));
-  EXPECT_EQ("000000", format("%E6f", tp, tz));
-  EXPECT_EQ("0000000", format("%E7f", tp, tz));
-  EXPECT_EQ("00000000", format("%E8f", tp, tz));
-  EXPECT_EQ("000000000", format("%E9f", tp, tz));
-  EXPECT_EQ("0000000000", format("%E10f", tp, tz));
-  EXPECT_EQ("00000000000", format("%E11f", tp, tz));
-  EXPECT_EQ("000000000000", format("%E12f", tp, tz));
-  EXPECT_EQ("0000000000000", format("%E13f", tp, tz));
-  EXPECT_EQ("00000000000000", format("%E14f", tp, tz));
-  EXPECT_EQ("000000000000000", format("%E15f", tp, tz));
+  EXPECT_EQ("0", absl::time_internal::cctz::format("%E*f", tp, tz));
+  EXPECT_EQ("", absl::time_internal::cctz::format("%E0f", tp, tz));
+  EXPECT_EQ("0", absl::time_internal::cctz::format("%E1f", tp, tz));
+  EXPECT_EQ("00", absl::time_internal::cctz::format("%E2f", tp, tz));
+  EXPECT_EQ("000", absl::time_internal::cctz::format("%E3f", tp, tz));
+  EXPECT_EQ("0000", absl::time_internal::cctz::format("%E4f", tp, tz));
+  EXPECT_EQ("00000", absl::time_internal::cctz::format("%E5f", tp, tz));
+  EXPECT_EQ("000000", absl::time_internal::cctz::format("%E6f", tp, tz));
+  EXPECT_EQ("0000000", absl::time_internal::cctz::format("%E7f", tp, tz));
+  EXPECT_EQ("00000000", absl::time_internal::cctz::format("%E8f", tp, tz));
+  EXPECT_EQ("000000000", absl::time_internal::cctz::format("%E9f", tp, tz));
+  EXPECT_EQ("0000000000", absl::time_internal::cctz::format("%E10f", tp, tz));
+  EXPECT_EQ("00000000000", absl::time_internal::cctz::format("%E11f", tp, tz));
+  EXPECT_EQ("000000000000", absl::time_internal::cctz::format("%E12f", tp, tz));
+  EXPECT_EQ("0000000000000",
+            absl::time_internal::cctz::format("%E13f", tp, tz));
+  EXPECT_EQ("00000000000000",
+            absl::time_internal::cctz::format("%E14f", tp, tz));
+  EXPECT_EQ("000000000000000",
+            absl::time_internal::cctz::format("%E15f", tp, tz));
 
   // With subseconds.
   tp += chrono::milliseconds(6) + chrono::microseconds(7) +
         chrono::nanoseconds(8);
-  EXPECT_EQ("006007008", format("%E*f", tp, tz));
-  EXPECT_EQ("", format("%E0f", tp, tz));
-  EXPECT_EQ("0", format("%E1f", tp, tz));
-  EXPECT_EQ("00", format("%E2f", tp, tz));
-  EXPECT_EQ("006", format("%E3f", tp, tz));
-  EXPECT_EQ("0060", format("%E4f", tp, tz));
-  EXPECT_EQ("00600", format("%E5f", tp, tz));
-  EXPECT_EQ("006007", format("%E6f", tp, tz));
-  EXPECT_EQ("0060070", format("%E7f", tp, tz));
-  EXPECT_EQ("00600700", format("%E8f", tp, tz));
-  EXPECT_EQ("006007008", format("%E9f", tp, tz));
-  EXPECT_EQ("0060070080", format("%E10f", tp, tz));
-  EXPECT_EQ("00600700800", format("%E11f", tp, tz));
-  EXPECT_EQ("006007008000", format("%E12f", tp, tz));
-  EXPECT_EQ("0060070080000", format("%E13f", tp, tz));
-  EXPECT_EQ("00600700800000", format("%E14f", tp, tz));
-  EXPECT_EQ("006007008000000", format("%E15f", tp, tz));
+  EXPECT_EQ("006007008", absl::time_internal::cctz::format("%E*f", tp, tz));
+  EXPECT_EQ("", absl::time_internal::cctz::format("%E0f", tp, tz));
+  EXPECT_EQ("0", absl::time_internal::cctz::format("%E1f", tp, tz));
+  EXPECT_EQ("00", absl::time_internal::cctz::format("%E2f", tp, tz));
+  EXPECT_EQ("006", absl::time_internal::cctz::format("%E3f", tp, tz));
+  EXPECT_EQ("0060", absl::time_internal::cctz::format("%E4f", tp, tz));
+  EXPECT_EQ("00600", absl::time_internal::cctz::format("%E5f", tp, tz));
+  EXPECT_EQ("006007", absl::time_internal::cctz::format("%E6f", tp, tz));
+  EXPECT_EQ("0060070", absl::time_internal::cctz::format("%E7f", tp, tz));
+  EXPECT_EQ("00600700", absl::time_internal::cctz::format("%E8f", tp, tz));
+  EXPECT_EQ("006007008", absl::time_internal::cctz::format("%E9f", tp, tz));
+  EXPECT_EQ("0060070080", absl::time_internal::cctz::format("%E10f", tp, tz));
+  EXPECT_EQ("00600700800", absl::time_internal::cctz::format("%E11f", tp, tz));
+  EXPECT_EQ("006007008000", absl::time_internal::cctz::format("%E12f", tp, tz));
+  EXPECT_EQ("0060070080000",
+            absl::time_internal::cctz::format("%E13f", tp, tz));
+  EXPECT_EQ("00600700800000",
+            absl::time_internal::cctz::format("%E14f", tp, tz));
+  EXPECT_EQ("006007008000000",
+            absl::time_internal::cctz::format("%E15f", tp, tz));
 
   // Times before the Unix epoch.
   tp = chrono::system_clock::from_time_t(0) + chrono::microseconds(-1);
-  EXPECT_EQ("1969-12-31 23:59:59.999999",
-            format("%Y-%m-%d %H:%M:%S.%E*f", tp, tz));
+  EXPECT_EQ(
+      "1969-12-31 23:59:59.999999",
+      absl::time_internal::cctz::format("%Y-%m-%d %H:%M:%S.%E*f", tp, tz));
 
   // Here is a "%E*S" case we got wrong for a while.  While the first
   // instant below is correctly rendered as "...:07.333304", the second
   // one used to appear as "...:07.33330499999999999".
   tp = chrono::system_clock::from_time_t(0) +
        chrono::microseconds(1395024427333304);
-  EXPECT_EQ("2014-03-17 02:47:07.333304",
-            format("%Y-%m-%d %H:%M:%S.%E*f", tp, tz));
+  EXPECT_EQ(
+      "2014-03-17 02:47:07.333304",
+      absl::time_internal::cctz::format("%Y-%m-%d %H:%M:%S.%E*f", tp, tz));
   tp += chrono::microseconds(1);
-  EXPECT_EQ("2014-03-17 02:47:07.333305",
-            format("%Y-%m-%d %H:%M:%S.%E*f", tp, tz));
+  EXPECT_EQ(
+      "2014-03-17 02:47:07.333305",
+      absl::time_internal::cctz::format("%Y-%m-%d %H:%M:%S.%E*f", tp, tz));
 }
 
 TEST(Format, CompareExtendSecondsVsSubseconds) {
@@ -397,15 +439,17 @@ TEST(Format, CompareExtendSecondsVsSubseconds) {
   time_point<chrono::nanoseconds> tp = chrono::system_clock::from_time_t(0);
   tp += chrono::seconds(5);
   // ... %E*S and %S.%E*f are different.
-  EXPECT_EQ("05", format(fmt_A("*"), tp, tz));
-  EXPECT_EQ("05.0", format(fmt_B("*"), tp, tz));
+  EXPECT_EQ("05", absl::time_internal::cctz::format(fmt_A("*"), tp, tz));
+  EXPECT_EQ("05.0", absl::time_internal::cctz::format(fmt_B("*"), tp, tz));
   // ... %E0S and %S.%E0f are different.
-  EXPECT_EQ("05", format(fmt_A("0"), tp, tz));
-  EXPECT_EQ("05.", format(fmt_B("0"), tp, tz));
+  EXPECT_EQ("05", absl::time_internal::cctz::format(fmt_A("0"), tp, tz));
+  EXPECT_EQ("05.", absl::time_internal::cctz::format(fmt_B("0"), tp, tz));
   // ... %E<prec>S and %S.%E<prec>f are the same for prec in [1:15].
   for (int prec = 1; prec <= 15; ++prec) {
-    const std::string a = format(fmt_A(std::to_string(prec)), tp, tz);
-    const std::string b = format(fmt_B(std::to_string(prec)), tp, tz);
+    const std::string a =
+        absl::time_internal::cctz::format(fmt_A(std::to_string(prec)), tp, tz);
+    const std::string b =
+        absl::time_internal::cctz::format(fmt_B(std::to_string(prec)), tp, tz);
     EXPECT_EQ(a, b) << "prec=" << prec;
   }
 
@@ -413,15 +457,19 @@ TEST(Format, CompareExtendSecondsVsSubseconds) {
   // ... %E*S and %S.%E*f are the same.
   tp += chrono::milliseconds(6) + chrono::microseconds(7) +
         chrono::nanoseconds(8);
-  EXPECT_EQ("05.006007008", format(fmt_A("*"), tp, tz));
-  EXPECT_EQ("05.006007008", format(fmt_B("*"), tp, tz));
+  EXPECT_EQ("05.006007008",
+            absl::time_internal::cctz::format(fmt_A("*"), tp, tz));
+  EXPECT_EQ("05.006007008",
+            absl::time_internal::cctz::format(fmt_B("*"), tp, tz));
   // ... %E0S and %S.%E0f are different.
-  EXPECT_EQ("05", format(fmt_A("0"), tp, tz));
-  EXPECT_EQ("05.", format(fmt_B("0"), tp, tz));
+  EXPECT_EQ("05", absl::time_internal::cctz::format(fmt_A("0"), tp, tz));
+  EXPECT_EQ("05.", absl::time_internal::cctz::format(fmt_B("0"), tp, tz));
   // ... %E<prec>S and %S.%E<prec>f are the same for prec in [1:15].
   for (int prec = 1; prec <= 15; ++prec) {
-    const std::string a = format(fmt_A(std::to_string(prec)), tp, tz);
-    const std::string b = format(fmt_B(std::to_string(prec)), tp, tz);
+    const std::string a =
+        absl::time_internal::cctz::format(fmt_A(std::to_string(prec)), tp, tz);
+    const std::string b =
+        absl::time_internal::cctz::format(fmt_B(std::to_string(prec)), tp, tz);
     EXPECT_EQ(a, b) << "prec=" << prec;
   }
 }
@@ -594,31 +642,31 @@ TEST(Format, ExtendedYears) {
 
   // %E4Y zero-pads the year to produce at least 4 chars, including the sign.
   auto tp = convert(civil_second(-999, 11, 27, 0, 0, 0), utc);
-  EXPECT_EQ("-9991127", format(e4y_fmt, tp, utc));
+  EXPECT_EQ("-9991127", absl::time_internal::cctz::format(e4y_fmt, tp, utc));
   tp = convert(civil_second(-99, 11, 27, 0, 0, 0), utc);
-  EXPECT_EQ("-0991127", format(e4y_fmt, tp, utc));
+  EXPECT_EQ("-0991127", absl::time_internal::cctz::format(e4y_fmt, tp, utc));
   tp = convert(civil_second(-9, 11, 27, 0, 0, 0), utc);
-  EXPECT_EQ("-0091127", format(e4y_fmt, tp, utc));
+  EXPECT_EQ("-0091127", absl::time_internal::cctz::format(e4y_fmt, tp, utc));
   tp = convert(civil_second(-1, 11, 27, 0, 0, 0), utc);
-  EXPECT_EQ("-0011127", format(e4y_fmt, tp, utc));
+  EXPECT_EQ("-0011127", absl::time_internal::cctz::format(e4y_fmt, tp, utc));
   tp = convert(civil_second(0, 11, 27, 0, 0, 0), utc);
-  EXPECT_EQ("00001127", format(e4y_fmt, tp, utc));
+  EXPECT_EQ("00001127", absl::time_internal::cctz::format(e4y_fmt, tp, utc));
   tp = convert(civil_second(1, 11, 27, 0, 0, 0), utc);
-  EXPECT_EQ("00011127", format(e4y_fmt, tp, utc));
+  EXPECT_EQ("00011127", absl::time_internal::cctz::format(e4y_fmt, tp, utc));
   tp = convert(civil_second(9, 11, 27, 0, 0, 0), utc);
-  EXPECT_EQ("00091127", format(e4y_fmt, tp, utc));
+  EXPECT_EQ("00091127", absl::time_internal::cctz::format(e4y_fmt, tp, utc));
   tp = convert(civil_second(99, 11, 27, 0, 0, 0), utc);
-  EXPECT_EQ("00991127", format(e4y_fmt, tp, utc));
+  EXPECT_EQ("00991127", absl::time_internal::cctz::format(e4y_fmt, tp, utc));
   tp = convert(civil_second(999, 11, 27, 0, 0, 0), utc);
-  EXPECT_EQ("09991127", format(e4y_fmt, tp, utc));
+  EXPECT_EQ("09991127", absl::time_internal::cctz::format(e4y_fmt, tp, utc));
   tp = convert(civil_second(9999, 11, 27, 0, 0, 0), utc);
-  EXPECT_EQ("99991127", format(e4y_fmt, tp, utc));
+  EXPECT_EQ("99991127", absl::time_internal::cctz::format(e4y_fmt, tp, utc));
 
   // When the year is outside [-999:9999], more than 4 chars are produced.
   tp = convert(civil_second(-1000, 11, 27, 0, 0, 0), utc);
-  EXPECT_EQ("-10001127", format(e4y_fmt, tp, utc));
+  EXPECT_EQ("-10001127", absl::time_internal::cctz::format(e4y_fmt, tp, utc));
   tp = convert(civil_second(10000, 11, 27, 0, 0, 0), utc);
-  EXPECT_EQ("100001127", format(e4y_fmt, tp, utc));
+  EXPECT_EQ("100001127", absl::time_internal::cctz::format(e4y_fmt, tp, utc));
 }
 
 TEST(Format, RFC3339Format) {
@@ -627,45 +675,64 @@ TEST(Format, RFC3339Format) {
 
   time_point<chrono::nanoseconds> tp =
       convert(civil_second(1977, 6, 28, 9, 8, 7), tz);
-  EXPECT_EQ("1977-06-28T09:08:07-07:00", format(RFC3339_full, tp, tz));
-  EXPECT_EQ("1977-06-28T09:08:07-07:00", format(RFC3339_sec, tp, tz));
+  EXPECT_EQ("1977-06-28T09:08:07-07:00",
+            absl::time_internal::cctz::format(RFC3339_full, tp, tz));
+  EXPECT_EQ("1977-06-28T09:08:07-07:00",
+            absl::time_internal::cctz::format(RFC3339_sec, tp, tz));
 
   tp += chrono::milliseconds(100);
-  EXPECT_EQ("1977-06-28T09:08:07.1-07:00", format(RFC3339_full, tp, tz));
-  EXPECT_EQ("1977-06-28T09:08:07-07:00", format(RFC3339_sec, tp, tz));
+  EXPECT_EQ("1977-06-28T09:08:07.1-07:00",
+            absl::time_internal::cctz::format(RFC3339_full, tp, tz));
+  EXPECT_EQ("1977-06-28T09:08:07-07:00",
+            absl::time_internal::cctz::format(RFC3339_sec, tp, tz));
 
   tp += chrono::milliseconds(20);
-  EXPECT_EQ("1977-06-28T09:08:07.12-07:00", format(RFC3339_full, tp, tz));
-  EXPECT_EQ("1977-06-28T09:08:07-07:00", format(RFC3339_sec, tp, tz));
+  EXPECT_EQ("1977-06-28T09:08:07.12-07:00",
+            absl::time_internal::cctz::format(RFC3339_full, tp, tz));
+  EXPECT_EQ("1977-06-28T09:08:07-07:00",
+            absl::time_internal::cctz::format(RFC3339_sec, tp, tz));
 
   tp += chrono::milliseconds(3);
-  EXPECT_EQ("1977-06-28T09:08:07.123-07:00", format(RFC3339_full, tp, tz));
-  EXPECT_EQ("1977-06-28T09:08:07-07:00", format(RFC3339_sec, tp, tz));
+  EXPECT_EQ("1977-06-28T09:08:07.123-07:00",
+            absl::time_internal::cctz::format(RFC3339_full, tp, tz));
+  EXPECT_EQ("1977-06-28T09:08:07-07:00",
+            absl::time_internal::cctz::format(RFC3339_sec, tp, tz));
 
   tp += chrono::microseconds(400);
-  EXPECT_EQ("1977-06-28T09:08:07.1234-07:00", format(RFC3339_full, tp, tz));
-  EXPECT_EQ("1977-06-28T09:08:07-07:00", format(RFC3339_sec, tp, tz));
+  EXPECT_EQ("1977-06-28T09:08:07.1234-07:00",
+            absl::time_internal::cctz::format(RFC3339_full, tp, tz));
+  EXPECT_EQ("1977-06-28T09:08:07-07:00",
+            absl::time_internal::cctz::format(RFC3339_sec, tp, tz));
 
   tp += chrono::microseconds(50);
-  EXPECT_EQ("1977-06-28T09:08:07.12345-07:00", format(RFC3339_full, tp, tz));
-  EXPECT_EQ("1977-06-28T09:08:07-07:00", format(RFC3339_sec, tp, tz));
+  EXPECT_EQ("1977-06-28T09:08:07.12345-07:00",
+            absl::time_internal::cctz::format(RFC3339_full, tp, tz));
+  EXPECT_EQ("1977-06-28T09:08:07-07:00",
+            absl::time_internal::cctz::format(RFC3339_sec, tp, tz));
 
   tp += chrono::microseconds(6);
-  EXPECT_EQ("1977-06-28T09:08:07.123456-07:00", format(RFC3339_full, tp, tz));
-  EXPECT_EQ("1977-06-28T09:08:07-07:00", format(RFC3339_sec, tp, tz));
+  EXPECT_EQ("1977-06-28T09:08:07.123456-07:00",
+            absl::time_internal::cctz::format(RFC3339_full, tp, tz));
+  EXPECT_EQ("1977-06-28T09:08:07-07:00",
+            absl::time_internal::cctz::format(RFC3339_sec, tp, tz));
 
   tp += chrono::nanoseconds(700);
-  EXPECT_EQ("1977-06-28T09:08:07.1234567-07:00", format(RFC3339_full, tp, tz));
-  EXPECT_EQ("1977-06-28T09:08:07-07:00", format(RFC3339_sec, tp, tz));
+  EXPECT_EQ("1977-06-28T09:08:07.1234567-07:00",
+            absl::time_internal::cctz::format(RFC3339_full, tp, tz));
+  EXPECT_EQ("1977-06-28T09:08:07-07:00",
+            absl::time_internal::cctz::format(RFC3339_sec, tp, tz));
 
   tp += chrono::nanoseconds(80);
-  EXPECT_EQ("1977-06-28T09:08:07.12345678-07:00", format(RFC3339_full, tp, tz));
-  EXPECT_EQ("1977-06-28T09:08:07-07:00", format(RFC3339_sec, tp, tz));
+  EXPECT_EQ("1977-06-28T09:08:07.12345678-07:00",
+            absl::time_internal::cctz::format(RFC3339_full, tp, tz));
+  EXPECT_EQ("1977-06-28T09:08:07-07:00",
+            absl::time_internal::cctz::format(RFC3339_sec, tp, tz));
 
   tp += chrono::nanoseconds(9);
   EXPECT_EQ("1977-06-28T09:08:07.123456789-07:00",
-            format(RFC3339_full, tp, tz));
-  EXPECT_EQ("1977-06-28T09:08:07-07:00", format(RFC3339_sec, tp, tz));
+            absl::time_internal::cctz::format(RFC3339_full, tp, tz));
+  EXPECT_EQ("1977-06-28T09:08:07-07:00",
+            absl::time_internal::cctz::format(RFC3339_sec, tp, tz));
 }
 
 TEST(Format, RFC1123Format) {  // locale specific
@@ -673,8 +740,80 @@ TEST(Format, RFC1123Format) {  // locale specific
   EXPECT_TRUE(load_time_zone("America/Los_Angeles", &tz));
 
   auto tp = convert(civil_second(1977, 6, 28, 9, 8, 7), tz);
-  EXPECT_EQ("Tue, 28 Jun 1977 09:08:07 -0700", format(RFC1123_full, tp, tz));
-  EXPECT_EQ("28 Jun 1977 09:08:07 -0700", format(RFC1123_no_wday, tp, tz));
+  EXPECT_EQ("Tue, 28 Jun 1977 09:08:07 -0700",
+            absl::time_internal::cctz::format(RFC1123_full, tp, tz));
+  EXPECT_EQ("28 Jun 1977 09:08:07 -0700",
+            absl::time_internal::cctz::format(RFC1123_no_wday, tp, tz));
+}
+
+TEST(Format, Week) {
+  const time_zone utc = utc_time_zone();
+
+  auto tp = convert(civil_second(2017, 1, 1, 0, 0, 0), utc);
+  EXPECT_EQ("2017-01-7",
+            absl::time_internal::cctz::format("%Y-%U-%u", tp, utc));
+  EXPECT_EQ("2017-00-0",
+            absl::time_internal::cctz::format("%Y-%W-%w", tp, utc));
+
+  tp = convert(civil_second(2017, 12, 31, 0, 0, 0), utc);
+  EXPECT_EQ("2017-53-7",
+            absl::time_internal::cctz::format("%Y-%U-%u", tp, utc));
+  EXPECT_EQ("2017-52-0",
+            absl::time_internal::cctz::format("%Y-%W-%w", tp, utc));
+
+  tp = convert(civil_second(2018, 1, 1, 0, 0, 0), utc);
+  EXPECT_EQ("2018-00-1",
+            absl::time_internal::cctz::format("%Y-%U-%u", tp, utc));
+  EXPECT_EQ("2018-01-1",
+            absl::time_internal::cctz::format("%Y-%W-%w", tp, utc));
+
+  tp = convert(civil_second(2018, 12, 31, 0, 0, 0), utc);
+  EXPECT_EQ("2018-52-1",
+            absl::time_internal::cctz::format("%Y-%U-%u", tp, utc));
+  EXPECT_EQ("2018-53-1",
+            absl::time_internal::cctz::format("%Y-%W-%w", tp, utc));
+
+  tp = convert(civil_second(2019, 1, 1, 0, 0, 0), utc);
+  EXPECT_EQ("2019-00-2",
+            absl::time_internal::cctz::format("%Y-%U-%u", tp, utc));
+  EXPECT_EQ("2019-00-2",
+            absl::time_internal::cctz::format("%Y-%W-%w", tp, utc));
+
+  tp = convert(civil_second(2019, 12, 31, 0, 0, 0), utc);
+  EXPECT_EQ("2019-52-2",
+            absl::time_internal::cctz::format("%Y-%U-%u", tp, utc));
+  EXPECT_EQ("2019-52-2",
+            absl::time_internal::cctz::format("%Y-%W-%w", tp, utc));
+}
+
+TEST(Format, NULsInFormatString) {
+  time_zone tz = utc_time_zone();
+  time_point<chrono::nanoseconds> tp = chrono::system_clock::from_time_t(0);
+
+  // All NULs.
+  std::string nuls(32, '\0');
+  EXPECT_EQ(nuls, absl::time_internal::cctz::format(nuls, tp, tz));
+
+  // NULs amongst ordinary text plus the internal "%\0" specifier.
+  std::string percent_nul("\0X\0\0X%\0\0X\0\0X\0", 13);
+  EXPECT_EQ(percent_nul,
+            absl::time_internal::cctz::format(percent_nul, tp, tz));
+
+  // NULs amongst ordinary text plus a non-internal specifier (%I).
+  EXPECT_EQ(std::string("X\0\0\0X12\0X\0\0\0X", 13),
+            absl::time_internal::cctz::format(
+                std::string("X\0\0\0X%I\0X\0\0\0X", 13), tp, tz));
+
+#if defined(__linux__) && defined(__GLIBC__)
+  // Depending upon strftime() behavior on "%E"
+  // and "%O", "%E\0" and "%0\0" produce themselves.
+  std::string percent_E_nul("abc%E\0xyz", 9);
+  EXPECT_EQ(percent_E_nul,
+            absl::time_internal::cctz::format(percent_E_nul, tp, tz));
+  std::string percent_O_nul("abc%O\0xyz", 9);
+  EXPECT_EQ(percent_O_nul,
+            absl::time_internal::cctz::format(percent_O_nul, tp, tz));
+#endif
 }
 
 //
@@ -687,39 +826,46 @@ TEST(Parse, TimePointResolution) {
 
   time_point<chrono::nanoseconds> tp_ns;
   EXPECT_TRUE(parse(kFmt, "03:04:05.123456789", utc, &tp_ns));
-  EXPECT_EQ("03:04:05.123456789", format(kFmt, tp_ns, utc));
+  EXPECT_EQ("03:04:05.123456789",
+            absl::time_internal::cctz::format(kFmt, tp_ns, utc));
   EXPECT_TRUE(parse(kFmt, "03:04:05.123456", utc, &tp_ns));
-  EXPECT_EQ("03:04:05.123456", format(kFmt, tp_ns, utc));
+  EXPECT_EQ("03:04:05.123456",
+            absl::time_internal::cctz::format(kFmt, tp_ns, utc));
 
   time_point<chrono::microseconds> tp_us;
   EXPECT_TRUE(parse(kFmt, "03:04:05.123456789", utc, &tp_us));
-  EXPECT_EQ("03:04:05.123456", format(kFmt, tp_us, utc));
+  EXPECT_EQ("03:04:05.123456",
+            absl::time_internal::cctz::format(kFmt, tp_us, utc));
   EXPECT_TRUE(parse(kFmt, "03:04:05.123456", utc, &tp_us));
-  EXPECT_EQ("03:04:05.123456", format(kFmt, tp_us, utc));
+  EXPECT_EQ("03:04:05.123456",
+            absl::time_internal::cctz::format(kFmt, tp_us, utc));
   EXPECT_TRUE(parse(kFmt, "03:04:05.123", utc, &tp_us));
-  EXPECT_EQ("03:04:05.123", format(kFmt, tp_us, utc));
+  EXPECT_EQ("03:04:05.123",
+            absl::time_internal::cctz::format(kFmt, tp_us, utc));
 
   time_point<chrono::milliseconds> tp_ms;
   EXPECT_TRUE(parse(kFmt, "03:04:05.123456", utc, &tp_ms));
-  EXPECT_EQ("03:04:05.123", format(kFmt, tp_ms, utc));
+  EXPECT_EQ("03:04:05.123",
+            absl::time_internal::cctz::format(kFmt, tp_ms, utc));
   EXPECT_TRUE(parse(kFmt, "03:04:05.123", utc, &tp_ms));
-  EXPECT_EQ("03:04:05.123", format(kFmt, tp_ms, utc));
+  EXPECT_EQ("03:04:05.123",
+            absl::time_internal::cctz::format(kFmt, tp_ms, utc));
   EXPECT_TRUE(parse(kFmt, "03:04:05", utc, &tp_ms));
-  EXPECT_EQ("03:04:05", format(kFmt, tp_ms, utc));
+  EXPECT_EQ("03:04:05", absl::time_internal::cctz::format(kFmt, tp_ms, utc));
 
   time_point<chrono::seconds> tp_s;
   EXPECT_TRUE(parse(kFmt, "03:04:05.123", utc, &tp_s));
-  EXPECT_EQ("03:04:05", format(kFmt, tp_s, utc));
+  EXPECT_EQ("03:04:05", absl::time_internal::cctz::format(kFmt, tp_s, utc));
   EXPECT_TRUE(parse(kFmt, "03:04:05", utc, &tp_s));
-  EXPECT_EQ("03:04:05", format(kFmt, tp_s, utc));
+  EXPECT_EQ("03:04:05", absl::time_internal::cctz::format(kFmt, tp_s, utc));
 
   time_point<chrono::minutes> tp_m;
   EXPECT_TRUE(parse(kFmt, "03:04:05", utc, &tp_m));
-  EXPECT_EQ("03:04:00", format(kFmt, tp_m, utc));
+  EXPECT_EQ("03:04:00", absl::time_internal::cctz::format(kFmt, tp_m, utc));
 
   time_point<chrono::hours> tp_h;
   EXPECT_TRUE(parse(kFmt, "03:04:05", utc, &tp_h));
-  EXPECT_EQ("03:00:00", format(kFmt, tp_h, utc));
+  EXPECT_EQ("03:00:00", absl::time_internal::cctz::format(kFmt, tp_h, utc));
 }
 
 TEST(Parse, TimePointExtendedResolution) {
@@ -765,7 +911,7 @@ TEST(Parse, WithTimeZone) {
   EXPECT_TRUE(load_time_zone("America/Los_Angeles", &tz));
   time_point<chrono::nanoseconds> tp;
 
-  // We can parse a std::string without a UTC offset if we supply a timezone.
+  // We can parse a string without a UTC offset if we supply a timezone.
   EXPECT_TRUE(parse("%Y-%m-%d %H:%M:%S", "2013-06-28 19:08:09", tz, &tp));
   ExpectTime(tp, tz, 2013, 6, 28, 19, 8, 9, -7 * 60 * 60, true, "PDT");
 
@@ -814,6 +960,9 @@ TEST(Parse, ErrorCases) {
   const time_zone tz = utc_time_zone();
   auto tp = chrono::system_clock::from_time_t(0);
 
+  // No data.
+  EXPECT_FALSE(parse("X", "", tz, &tp));
+
   // Illegal trailing data.
   EXPECT_FALSE(parse("%S", "123", tz, &tp));
 
@@ -853,6 +1002,27 @@ TEST(Parse, ErrorCases) {
   EXPECT_FALSE(parse("%Ez", "-00:-0", tz, &tp));
 }
 
+TEST(Parse, NonAsciiInput) {
+  const time_zone tz = utc_time_zone();
+  auto tp = chrono::system_clock::from_time_t(0);
+
+  // High-bit-set bytes reach the ctype functions during parsing. 0xA0 is not
+  // ASCII whitespace, so the leading-whitespace skip must not consume it and
+  // the parse must fail rather than pass a negative char to std::isspace().
+  EXPECT_FALSE(parse("%Y-%m-%d",
+                     "\xA0"
+                     "2016-01-02",
+                     tz, &tp));
+  EXPECT_FALSE(parse("%Y",
+                     "\xA0"
+                     "2016",
+                     tz, &tp));
+
+  // A leading ASCII space is still skipped as before.
+  EXPECT_TRUE(parse("%Y-%m-%d", " 2016-01-02", tz, &tp));
+  EXPECT_EQ(2016, convert(tp, utc_time_zone()).year());
+}
+
 TEST(Parse, PosixConversions) {
   time_zone tz = utc_time_zone();
   auto tp = chrono::system_clock::from_time_t(0);
@@ -1014,9 +1184,11 @@ TEST(Parse, LocaleSpecific) {
   EXPECT_TRUE(parse("%h", "Feb", tz, &tp));
   EXPECT_EQ(2, convert(tp, tz).month());  // Equivalent to %b
 
+#if defined(__GLIBC__)
   tp = reset;
   EXPECT_TRUE(parse("%l %p", "5 PM", tz, &tp));
   EXPECT_EQ(17, convert(tp, tz).hour());
+#endif
 
   tp = reset;
   EXPECT_TRUE(parse("%r", "03:44:55 PM", tz, &tp));
@@ -1024,6 +1196,7 @@ TEST(Parse, LocaleSpecific) {
   EXPECT_EQ(44, convert(tp, tz).minute());
   EXPECT_EQ(55, convert(tp, tz).second());
 
+#if defined(__GLIBC__)
   tp = reset;
   EXPECT_TRUE(parse("%Ec", "Tue Nov 19 05:06:07 2013", tz, &tp));
   EXPECT_EQ(convert(civil_second(2013, 11, 19, 5, 6, 7), tz), tp);
@@ -1095,6 +1268,7 @@ TEST(Parse, LocaleSpecific) {
   EXPECT_TRUE(parse("%Oy", "04", tz, &tp));
   EXPECT_EQ(2004, convert(tp, tz).year());
 #endif
+#endif
 }
 
 TEST(Parse, ExtendedSeconds) {
@@ -1105,7 +1279,7 @@ TEST(Parse, ExtendedSeconds) {
   // All %E<prec>S cases are treated the same as %E*S on input.
   auto precisions = {"*", "0", "1",  "2",  "3",  "4",  "5",  "6", "7",
                      "8", "9", "10", "11", "12", "13", "14", "15"};
-  for (const std::string& prec : precisions) {
+  for (const std::string prec : precisions) {
     const std::string fmt = "%E" + prec + "S";
     SCOPED_TRACE(fmt);
     time_point<chrono::nanoseconds> tp = unix_epoch;
@@ -1187,7 +1361,7 @@ TEST(Parse, ExtendedSubeconds) {
   // All %E<prec>f cases are treated the same as %E*f on input.
   auto precisions = {"*", "0", "1",  "2",  "3",  "4",  "5",  "6", "7",
                      "8", "9", "10", "11", "12", "13", "14", "15"};
-  for (const std::string& prec : precisions) {
+  for (const std::string prec : precisions) {
     const std::string fmt = "%E" + prec + "f";
     SCOPED_TRACE(fmt);
     time_point<chrono::nanoseconds> tp = unix_epoch - chrono::seconds(1);
@@ -1250,9 +1424,9 @@ TEST(Parse, ExtendedSubecondsScan) {
         const auto expected = chrono::system_clock::from_time_t(0) +
                               chrono::nanoseconds(micros * 1000 + ns);
         for (int ps = 0; ps < 1000; ps += 250) {
-          std::ostringstream oss;
+          std::ostringstream ps_oss;
           oss << std::setfill('0') << std::setw(3) << ps;
-          const std::string input = nanos + oss.str() + "999";
+          const std::string input = nanos + ps_oss.str() + "999";
           EXPECT_TRUE(parse("%E*f", input, tz, &tp));
           EXPECT_EQ(expected + chrono::nanoseconds(ps) / 1000, tp) << input;
         }
@@ -1377,10 +1551,120 @@ TEST(Parse, RFC3339Format) {
   EXPECT_TRUE(parse(RFC3339_sec, "2014-02-12T20:21:00+00:00", tz, &tp));
   ExpectTime(tp, tz, 2014, 2, 12, 20, 21, 0, 0, false, "UTC");
 
-  // Check that %Ez also accepts "Z" as a synonym for "+00:00".
+  // Check that %ET also accepts "t".
   time_point<chrono::nanoseconds> tp2;
-  EXPECT_TRUE(parse(RFC3339_sec, "2014-02-12T20:21:00Z", tz, &tp2));
+  EXPECT_TRUE(parse(RFC3339_sec, "2014-02-12t20:21:00+00:00", tz, &tp2));
   EXPECT_EQ(tp, tp2);
+
+  // Check that %Ez also accepts "Z" as a synonym for "+00:00".
+  time_point<chrono::nanoseconds> tp3;
+  EXPECT_TRUE(parse(RFC3339_sec, "2014-02-12T20:21:00Z", tz, &tp3));
+  EXPECT_EQ(tp, tp3);
+
+  // Check that %Ez also accepts "z" as a synonym for "+00:00".
+  time_point<chrono::nanoseconds> tp4;
+  EXPECT_TRUE(parse(RFC3339_sec, "2014-02-12T20:21:00z", tz, &tp4));
+  EXPECT_EQ(tp, tp4);
+}
+
+TEST(Parse, Week) {
+  const time_zone utc = utc_time_zone();
+  time_point<absl::time_internal::cctz::seconds> tp;
+
+  auto exp = convert(civil_second(2017, 1, 1, 0, 0, 0), utc);
+  EXPECT_TRUE(parse("%Y-%U-%u", "2017-01-7", utc, &tp));
+  EXPECT_EQ(exp, tp);
+  EXPECT_TRUE(parse("%Y-%W-%w", "2017-00-0", utc, &tp));
+  EXPECT_EQ(exp, tp);
+
+  exp = convert(civil_second(2017, 12, 31, 0, 0, 0), utc);
+  EXPECT_TRUE(parse("%Y-%U-%u", "2017-53-7", utc, &tp));
+  EXPECT_EQ(exp, tp);
+  EXPECT_TRUE(parse("%Y-%W-%w", "2017-52-0", utc, &tp));
+  EXPECT_EQ(exp, tp);
+
+  exp = convert(civil_second(2018, 1, 1, 0, 0, 0), utc);
+  EXPECT_TRUE(parse("%Y-%U-%u", "2018-00-1", utc, &tp));
+  EXPECT_EQ(exp, tp);
+  EXPECT_TRUE(parse("%Y-%W-%w", "2018-01-1", utc, &tp));
+  EXPECT_EQ(exp, tp);
+
+  exp = convert(civil_second(2018, 12, 31, 0, 0, 0), utc);
+  EXPECT_TRUE(parse("%Y-%U-%u", "2018-52-1", utc, &tp));
+  EXPECT_EQ(exp, tp);
+  EXPECT_TRUE(parse("%Y-%W-%w", "2018-53-1", utc, &tp));
+  EXPECT_EQ(exp, tp);
+
+  exp = convert(civil_second(2019, 1, 1, 0, 0, 0), utc);
+  EXPECT_TRUE(parse("%Y-%U-%u", "2019-00-2", utc, &tp));
+  EXPECT_EQ(exp, tp);
+  EXPECT_TRUE(parse("%Y-%W-%w", "2019-00-2", utc, &tp));
+  EXPECT_EQ(exp, tp);
+
+  exp = convert(civil_second(2019, 12, 31, 0, 0, 0), utc);
+  EXPECT_TRUE(parse("%Y-%U-%u", "2019-52-2", utc, &tp));
+  EXPECT_EQ(exp, tp);
+  EXPECT_TRUE(parse("%Y-%W-%w", "2019-52-2", utc, &tp));
+  EXPECT_EQ(exp, tp);
+}
+
+TEST(Parse, WeekYearShift) {
+  // %U/%W conversions with week values in {0, 52, 53} can slip
+  // into the previous/following calendar years.
+  const time_zone utc = utc_time_zone();
+  time_point<absl::time_internal::cctz::seconds> tp;
+
+  auto exp = convert(civil_second(2019, 12, 31, 0, 0, 0), utc);
+  EXPECT_TRUE(parse("%Y-%U-%u", "2020-00-2", utc, &tp));
+  EXPECT_EQ(exp, tp);
+  EXPECT_TRUE(parse("%Y-%W-%w", "2020-00-2", utc, &tp));
+  EXPECT_EQ(exp, tp);
+
+  exp = convert(civil_second(2021, 1, 1, 0, 0, 0), utc);
+  EXPECT_TRUE(parse("%Y-%U-%u", "2020-52-5", utc, &tp));
+  EXPECT_EQ(exp, tp);
+  EXPECT_TRUE(parse("%Y-%W-%w", "2020-52-5", utc, &tp));
+  EXPECT_EQ(exp, tp);
+
+  // Slipping into the previous/following calendar years should fail when
+  // we're already at the extremes.
+  EXPECT_FALSE(parse("%Y-%U-%u", "-9223372036854775808-0-7", utc, &tp));
+  EXPECT_FALSE(parse("%Y-%U-%u", "9223372036854775807-53-7", utc, &tp));
+}
+
+TEST(Parse, NULsInFormatAndInputStrings) {
+  const time_zone utc = utc_time_zone();
+  time_point<absl::time_internal::cctz::seconds> tp;
+
+  // Check that NULs are parsed just like any other chars.
+  EXPECT_FALSE(
+      parse(std::string("abc\0def", 7), std::string("abc\0xyz", 7), utc, &tp));
+  EXPECT_FALSE(
+      parse(std::string("%Y\0def", 6), std::string("2026\0xyz", 8), utc, &tp));
+  EXPECT_TRUE(
+      parse(std::string("%Y\0xyz", 6), std::string("2026\0xyz", 8), utc, &tp));
+  ExpectTime(tp, utc, 2026, 1, 1, 0, 0, 0, 0, false, "UTC");
+
+  // All NULs.
+  std::string nuls(32, '\0');
+  EXPECT_TRUE(parse(nuls, nuls, utc, &tp));
+  ExpectTime(tp, utc, 1970, 1, 1, 0, 0, 0, 0, false, "UTC");
+
+  // The "%\0" specifier never matches, even on the empty string and itself.
+  std::string percent_nul("%\0", 2);
+  EXPECT_FALSE(parse(percent_nul, "", utc, &tp));
+  EXPECT_FALSE(parse(percent_nul, percent_nul, utc, &tp));
+
+#if defined(__linux__) && defined(__GLIBC__)
+  // Depending upon strptime() behavior on "%E"
+  // and "%O", "%E\0" and "%0\0" also never match.
+  std::string percent_E_nul("%E\0", 3);
+  EXPECT_FALSE(parse(percent_E_nul, "", utc, &tp));
+  EXPECT_FALSE(parse(percent_E_nul, percent_E_nul, utc, &tp));
+  std::string percent_O_nul("%O\0", 3);
+  EXPECT_FALSE(parse(percent_O_nul, "", utc, &tp));
+  EXPECT_FALSE(parse(percent_O_nul, percent_O_nul, utc, &tp));
+#endif
 }
 
 TEST(Parse, MaxRange) {
@@ -1399,7 +1683,7 @@ TEST(Parse, MaxRange) {
       parse(RFC3339_sec, "292277026596-12-04T14:30:07-01:00", utc, &tp));
   EXPECT_EQ(tp, time_point<absl::time_internal::cctz::seconds>::max());
   EXPECT_FALSE(
-      parse(RFC3339_sec, "292277026596-12-04T15:30:07-01:00", utc, &tp));
+      parse(RFC3339_sec, "292277026596-12-04T14:30:08-01:00", utc, &tp));
 
   // tests the lower limit using +00:00 offset
   EXPECT_TRUE(
@@ -1416,14 +1700,90 @@ TEST(Parse, MaxRange) {
       parse(RFC3339_sec, "-292277022657-01-27T08:29:51+01:00", utc, &tp));
 
   // tests max/min civil-second overflow
-  EXPECT_FALSE(parse(RFC3339_sec, "9223372036854775807-12-31T23:59:59-00:01",
-                     utc, &tp));
+  EXPECT_FALSE(
+      parse(RFC3339_sec, "9223372036854775807-12-31T23:59:59-00:01", utc, &tp));
   EXPECT_FALSE(parse(RFC3339_sec, "-9223372036854775808-01-01T00:00:00+00:01",
                      utc, &tp));
+}
+
+TEST(Parse, TimePointOverflow) {
+  const time_zone utc = utc_time_zone();
 
-  // TODO: Add tests that parsing times with fractional seconds overflow
-  // appropriately. This can't be done until cctz::parse() properly detects
-  // overflow when combining the chrono seconds and femto.
+  using D = chrono::duration<std::int64_t, std::nano>;
+  time_point<D> tp;
+
+  EXPECT_TRUE(
+      parse(RFC3339_full, "2262-04-11T23:47:16.8547758079+00:00", utc, &tp));
+  EXPECT_EQ(tp, time_point<D>::max());
+  EXPECT_EQ("2262-04-11T23:47:16.854775807+00:00",
+            absl::time_internal::cctz::format(RFC3339_full, tp, utc));
+#if 0
+  // TODO(#199): Will fail until cctz::parse() properly detects overflow.
+  EXPECT_FALSE(
+      parse(RFC3339_full, "2262-04-11T23:47:16.8547758080+00:00", utc, &tp));
+  EXPECT_TRUE(
+      parse(RFC3339_full, "1677-09-21T00:12:43.1452241920+00:00", utc, &tp));
+  EXPECT_EQ(tp, time_point<D>::min());
+  EXPECT_EQ("1677-09-21T00:12:43.145224192+00:00",
+            absl::time_internal::cctz::format(RFC3339_full, tp, utc));
+  EXPECT_FALSE(
+      parse(RFC3339_full, "1677-09-21T00:12:43.1452241919+00:00", utc, &tp));
+#endif
+
+  using DS = chrono::duration<std::int8_t, chrono::seconds::period>;
+  time_point<DS> stp;
+
+  EXPECT_TRUE(parse(RFC3339_full, "1970-01-01T00:02:07.9+00:00", utc, &stp));
+  EXPECT_EQ(stp, time_point<DS>::max());
+  EXPECT_EQ("1970-01-01T00:02:07+00:00",
+            absl::time_internal::cctz::format(RFC3339_full, stp, utc));
+  EXPECT_FALSE(parse(RFC3339_full, "1970-01-01T00:02:08+00:00", utc, &stp));
+
+  EXPECT_TRUE(parse(RFC3339_full, "1969-12-31T23:57:52+00:00", utc, &stp));
+  EXPECT_EQ(stp, time_point<DS>::min());
+  EXPECT_EQ("1969-12-31T23:57:52+00:00",
+            absl::time_internal::cctz::format(RFC3339_full, stp, utc));
+  EXPECT_FALSE(parse(RFC3339_full, "1969-12-31T23:57:51.9+00:00", utc, &stp));
+
+  using DM = chrono::duration<std::int8_t, chrono::minutes::period>;
+  time_point<DM> mtp;
+
+  EXPECT_TRUE(parse(RFC3339_full, "1970-01-01T02:07:59+00:00", utc, &mtp));
+  EXPECT_EQ(mtp, time_point<DM>::max());
+  EXPECT_EQ("1970-01-01T02:07:00+00:00",
+            absl::time_internal::cctz::format(RFC3339_full, mtp, utc));
+  EXPECT_FALSE(parse(RFC3339_full, "1970-01-01T02:08:00+00:00", utc, &mtp));
+
+  EXPECT_TRUE(parse(RFC3339_full, "1969-12-31T21:52:00+00:00", utc, &mtp));
+  EXPECT_EQ(mtp, time_point<DM>::min());
+  EXPECT_EQ("1969-12-31T21:52:00+00:00",
+            absl::time_internal::cctz::format(RFC3339_full, mtp, utc));
+  EXPECT_FALSE(parse(RFC3339_full, "1969-12-31T21:51:59+00:00", utc, &mtp));
+}
+
+TEST(Parse, TimePointOverflowFloor) {
+  const time_zone utc = utc_time_zone();
+
+  using D = chrono::duration<std::int64_t, std::micro>;
+  time_point<D> tp;
+
+  EXPECT_TRUE(
+      parse(RFC3339_full, "294247-01-10T04:00:54.7758079+00:00", utc, &tp));
+  EXPECT_EQ(tp, time_point<D>::max());
+  EXPECT_EQ("294247-01-10T04:00:54.775807+00:00",
+            absl::time_internal::cctz::format(RFC3339_full, tp, utc));
+#if 0
+  // TODO(#199): Will fail until cctz::parse() properly detects overflow.
+  EXPECT_FALSE(
+      parse(RFC3339_full, "294247-01-10T04:00:54.7758080+00:00", utc, &tp));
+  EXPECT_TRUE(
+      parse(RFC3339_full, "-290308-12-21T19:59:05.2241920+00:00", utc, &tp));
+  EXPECT_EQ(tp, time_point<D>::min());
+  EXPECT_EQ("-290308-12-21T19:59:05.224192+00:00",
+            absl::time_internal::cctz::format(RFC3339_full, tp, utc));
+  EXPECT_FALSE(
+      parse(RFC3339_full, "-290308-12-21T19:59:05.2241919+00:00", utc, &tp));
+#endif
 }
 
 //
@@ -1436,10 +1796,19 @@ TEST(FormatParse, RoundTrip) {
   const auto in = convert(civil_second(1977, 6, 28, 9, 8, 7), lax);
   const auto subseconds = chrono::nanoseconds(654321);
 
+  // No format specifiers, but include NUL.
+  {
+    time_point<chrono::nanoseconds> out;
+    const auto fmt = std::string("\0\1\2\3\4\5\6\7abcdefgh01234567");
+    const auto s = absl::time_internal::cctz::format(fmt, in, lax);
+    EXPECT_TRUE(parse(fmt, s, lax, &out)) << s;
+  }
+
   // RFC3339, which renders subseconds.
   {
     time_point<chrono::nanoseconds> out;
-    const std::string s = format(RFC3339_full, in + subseconds, lax);
+    const auto s =
+        absl::time_internal::cctz::format(RFC3339_full, in + subseconds, lax);
     EXPECT_TRUE(parse(RFC3339_full, s, lax, &out)) << s;
     EXPECT_EQ(in + subseconds, out);  // RFC3339_full includes %Ez
   }
@@ -1447,7 +1816,7 @@ TEST(FormatParse, RoundTrip) {
   // RFC1123, which only does whole seconds.
   {
     time_point<chrono::nanoseconds> out;
-    const std::string s = format(RFC1123_full, in, lax);
+    const auto s = absl::time_internal::cctz::format(RFC1123_full, in, lax);
     EXPECT_TRUE(parse(RFC1123_full, s, lax, &out)) << s;
     EXPECT_EQ(in, out);  // RFC1123_full includes %z
   }
@@ -1465,7 +1834,7 @@ TEST(FormatParse, RoundTrip) {
   {
     time_point<chrono::nanoseconds> out;
     time_zone utc = utc_time_zone();
-    const std::string s = format("%c", in, utc);
+    const auto s = absl::time_internal::cctz::format("%c", in, utc);
     EXPECT_TRUE(parse("%c", s, utc, &out)) << s;
     EXPECT_EQ(in, out);
   }
@@ -1474,8 +1843,10 @@ TEST(FormatParse, RoundTrip) {
 
 TEST(FormatParse, RoundTripDistantFuture) {
   const time_zone utc = utc_time_zone();
-  const time_point<absl::time_internal::cctz::seconds> in = time_point<absl::time_internal::cctz::seconds>::max();
-  const std::string s = format(RFC3339_full, in, utc);
+  const time_point<absl::time_internal::cctz::seconds> in =
+      time_point<absl::time_internal::cctz::seconds>::max();
+  const std::string s =
+      absl::time_internal::cctz::format(RFC3339_full, in, utc);
   time_point<absl::time_internal::cctz::seconds> out;
   EXPECT_TRUE(parse(RFC3339_full, s, utc, &out)) << s;
   EXPECT_EQ(in, out);
@@ -1483,8 +1854,10 @@ TEST(FormatParse, RoundTripDistantFuture) {
 
 TEST(FormatParse, RoundTripDistantPast) {
   const time_zone utc = utc_time_zone();
-  const time_point<absl::time_internal::cctz::seconds> in = time_point<absl::time_internal::cctz::seconds>::min();
-  const std::string s = format(RFC3339_full, in, utc);
+  const time_point<absl::time_internal::cctz::seconds> in =
+      time_point<absl::time_internal::cctz::seconds>::min();
+  const std::string s =
+      absl::time_internal::cctz::format(RFC3339_full, in, utc);
   time_point<absl::time_internal::cctz::seconds> out;
   EXPECT_TRUE(parse(RFC3339_full, s, utc, &out)) << s;
   EXPECT_EQ(in, out);
@@ -1492,4 +1865,5 @@ TEST(FormatParse, RoundTripDistantPast) {
 
 }  // namespace cctz
 }  // namespace time_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/time/internal/cctz/src/time_zone_if.cc b/absl/time/internal/cctz/src/time_zone_if.cc
index 09aaee5fb43..09f83e82f77 100644
--- a/absl/time/internal/cctz/src/time_zone_if.cc
+++ b/absl/time/internal/cctz/src/time_zone_if.cc
@@ -12,25 +12,30 @@
 //   See the License for the specific language governing permissions and
 //   limitations under the License.
 
-#include "time_zone_if.h"
-#include "time_zone_info.h"
-#include "time_zone_libc.h"
+#include "absl/time/internal/cctz/src/time_zone_if.h"
+
+#include "absl/base/config.h"
+#include "absl/time/internal/cctz/src/time_zone_info.h"
+#include "absl/time/internal/cctz/src/time_zone_libc.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 namespace cctz {
 
-std::unique_ptr<TimeZoneIf> TimeZoneIf::Load(const std::string& name) {
+std::unique_ptr<TimeZoneIf> TimeZoneIf::UTC() { return TimeZoneInfo::UTC(); }
+
+std::unique_ptr<TimeZoneIf> TimeZoneIf::Make(const std::string& name) {
   // Support "libc:localtime" and "libc:*" to access the legacy
   // localtime and UTC support respectively from the C library.
+  // NOTE: The "libc:*" zones are internal, test-only interfaces, and
+  // are subject to change/removal without notice. Do not use them.
   if (name.compare(0, 5, "libc:") == 0) {
-    return std::unique_ptr<TimeZoneIf>(new TimeZoneLibC(name.substr(5)));
+    return TimeZoneLibC::Make(name.substr(5));
   }
 
-  // Otherwise use the "zoneinfo" implementation by default.
-  std::unique_ptr<TimeZoneInfo> tz(new TimeZoneInfo);
-  if (!tz->Load(name)) tz.reset();
-  return std::unique_ptr<TimeZoneIf>(tz.release());
+  // Otherwise use the "zoneinfo" implementation.
+  return TimeZoneInfo::Make(name);
 }
 
 // Defined out-of-line to avoid emitting a weak vtable in all TUs.
@@ -38,4 +43,5 @@ TimeZoneIf::~TimeZoneIf() {}
 
 }  // namespace cctz
 }  // namespace time_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/time/internal/cctz/src/time_zone_if.h b/absl/time/internal/cctz/src/time_zone_if.h
index d000b7a5f78..bec9beb5cb7 100644
--- a/absl/time/internal/cctz/src/time_zone_if.h
+++ b/absl/time/internal/cctz/src/time_zone_if.h
@@ -20,10 +20,12 @@
 #include <memory>
 #include <string>
 
+#include "absl/base/config.h"
 #include "absl/time/internal/cctz/include/cctz/civil_time.h"
 #include "absl/time/internal/cctz/include/cctz/time_zone.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 namespace cctz {
 
@@ -31,15 +33,15 @@ namespace cctz {
 // Subclasses implement the functions for civil-time conversions in the zone.
 class TimeZoneIf {
  public:
-  // A factory function for TimeZoneIf implementations.
-  static std::unique_ptr<TimeZoneIf> Load(const std::string& name);
+  // Factory functions for TimeZoneIf implementations.
+  static std::unique_ptr<TimeZoneIf> UTC();  // never fails
+  static std::unique_ptr<TimeZoneIf> Make(const std::string& name);
 
   virtual ~TimeZoneIf();
 
   virtual time_zone::absolute_lookup BreakTime(
       const time_point<seconds>& tp) const = 0;
-  virtual time_zone::civil_lookup MakeTime(
-      const civil_second& cs) const = 0;
+  virtual time_zone::civil_lookup MakeTime(const civil_second& cs) const = 0;
 
   virtual bool NextTransition(const time_point<seconds>& tp,
                               time_zone::civil_transition* trans) const = 0;
@@ -50,23 +52,29 @@ class TimeZoneIf {
   virtual std::string Description() const = 0;
 
  protected:
-  TimeZoneIf() {}
+  TimeZoneIf() = default;
+  TimeZoneIf(const TimeZoneIf&) = delete;
+  TimeZoneIf& operator=(const TimeZoneIf&) = delete;
 };
 
 // Convert between time_point<seconds> and a count of seconds since the
 // Unix epoch.  We assume that the std::chrono::system_clock and the
-// Unix clock are second aligned, but not that they share an epoch.
+// Unix clock are second aligned, and that the results are representable.
+// (That is, that they share an epoch, which is required since C++20.)
 inline std::int_fast64_t ToUnixSeconds(const time_point<seconds>& tp) {
   return (tp - std::chrono::time_point_cast<seconds>(
-                   std::chrono::system_clock::from_time_t(0))).count();
+                   std::chrono::system_clock::from_time_t(0)))
+      .count();
 }
 inline time_point<seconds> FromUnixSeconds(std::int_fast64_t t) {
   return std::chrono::time_point_cast<seconds>(
-             std::chrono::system_clock::from_time_t(0)) + seconds(t);
+             std::chrono::system_clock::from_time_t(0)) +
+         seconds(t);
 }
 
 }  // namespace cctz
 }  // namespace time_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_TIME_INTERNAL_CCTZ_TIME_ZONE_IF_H_
diff --git a/absl/time/internal/cctz/src/time_zone_impl.cc b/absl/time/internal/cctz/src/time_zone_impl.cc
index a26151d3e91..5cf38e780a2 100644
--- a/absl/time/internal/cctz/src/time_zone_impl.cc
+++ b/absl/time/internal/cctz/src/time_zone_impl.cc
@@ -12,16 +12,20 @@
 //   See the License for the specific language governing permissions and
 //   limitations under the License.
 
-#include "time_zone_impl.h"
+#include "absl/time/internal/cctz/src/time_zone_impl.h"
 
+#include <deque>
+#include <memory>
 #include <mutex>
 #include <string>
 #include <unordered_map>
 #include <utility>
 
-#include "time_zone_fixed.h"
+#include "absl/base/config.h"
+#include "absl/time/internal/cctz/src/time_zone_fixed.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 namespace cctz {
 
@@ -42,22 +46,19 @@ std::mutex& TimeZoneMutex() {
 
 }  // namespace
 
-time_zone time_zone::Impl::UTC() {
-  return time_zone(UTCImpl());
-}
+time_zone time_zone::Impl::UTC() { return time_zone(UTCImpl()); }
 
 bool time_zone::Impl::LoadTimeZone(const std::string& name, time_zone* tz) {
-  const time_zone::Impl* const utc_impl = UTCImpl();
+  const Impl* const utc_impl = UTCImpl();
 
-  // First check for UTC (which is never a key in time_zone_map).
+  // Check for UTC (which is never a key in time_zone_map).
   auto offset = seconds::zero();
   if (FixedOffsetFromName(name, &offset) && offset == seconds::zero()) {
     *tz = time_zone(utc_impl);
     return true;
   }
 
-  // Then check, under a shared lock, whether the time zone has already
-  // been loaded. This is the common path. TODO: Move to shared_mutex.
+  // Check whether the time zone has already been loaded.
   {
     std::lock_guard<std::mutex> lock(TimeZoneMutex());
     if (time_zone_map != nullptr) {
@@ -69,20 +70,24 @@ bool time_zone::Impl::LoadTimeZone(const std::string& name, time_zone* tz) {
     }
   }
 
-  // Now check again, under an exclusive lock.
+  // Load the new time zone (outside the lock).
+  std::unique_ptr<const Impl> new_impl(new Impl(name));
+
+  // Add the new time zone to the map.
   std::lock_guard<std::mutex> lock(TimeZoneMutex());
   if (time_zone_map == nullptr) time_zone_map = new TimeZoneImplByName;
+  if (!new_impl->zone_) {
+    // Load failed, but a successful insertion may have happened concurrently.
+    // Check it now that we have the lock. Otherwise, avoid caching negative
+    // entries to avoid unbounded growth and DoS attacks.
+    auto itr = time_zone_map->find(name);
+    const Impl* impl = (itr != time_zone_map->end()) ? itr->second : utc_impl;
+    *tz = time_zone(impl);
+    return impl != utc_impl;
+  }
   const Impl*& impl = (*time_zone_map)[name];
-  if (impl == nullptr) {
-    // The first thread in loads the new time zone.
-    Impl* new_impl = new Impl(name);
-    new_impl->zone_ = TimeZoneIf::Load(new_impl->name_);
-    if (new_impl->zone_ == nullptr) {
-      delete new_impl;  // free the nascent Impl
-      impl = utc_impl;  // and fallback to UTC
-    } else {
-      impl = new_impl;  // install new time zone
-    }
+  if (impl == nullptr) {  // this thread won any load race
+    impl = new_impl.release();
   }
   *tz = time_zone(impl);
   return impl != utc_impl;
@@ -91,23 +96,29 @@ bool time_zone::Impl::LoadTimeZone(const std::string& name, time_zone* tz) {
 void time_zone::Impl::ClearTimeZoneMapTestOnly() {
   std::lock_guard<std::mutex> lock(TimeZoneMutex());
   if (time_zone_map != nullptr) {
-    // Existing time_zone::Impl* entries are in the wild, so we simply
-    // leak them.  Future requests will result in reloading the data.
+    // Existing time_zone::Impl* entries are in the wild, so we can't delete
+    // them. Instead, we move them to a private container, where they are
+    // logically unreachable but not "leaked".  Future requests will result
+    // in reloading the data.
+    static auto* cleared = new std::deque<const time_zone::Impl*>;
+    for (const auto& element : *time_zone_map) {
+      cleared->push_back(element.second);
+    }
     time_zone_map->clear();
   }
 }
 
-time_zone::Impl::Impl(const std::string& name) : name_(name) {}
+time_zone::Impl::Impl() : name_("UTC"), zone_(TimeZoneIf::UTC()) {}
+
+time_zone::Impl::Impl(const std::string& name)
+    : name_(name), zone_(TimeZoneIf::Make(name_)) {}
 
 const time_zone::Impl* time_zone::Impl::UTCImpl() {
-  static Impl* utc_impl = [] {
-    Impl* impl = new Impl("UTC");
-    impl->zone_ = TimeZoneIf::Load(impl->name_);  // never fails
-    return impl;
-  }();
+  static const Impl* utc_impl = new Impl;
   return utc_impl;
 }
 
 }  // namespace cctz
 }  // namespace time_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/time/internal/cctz/src/time_zone_impl.h b/absl/time/internal/cctz/src/time_zone_impl.h
index b73fad9bb88..da3aec525c4 100644
--- a/absl/time/internal/cctz/src/time_zone_impl.h
+++ b/absl/time/internal/cctz/src/time_zone_impl.h
@@ -18,12 +18,14 @@
 #include <memory>
 #include <string>
 
+#include "absl/base/config.h"
 #include "absl/time/internal/cctz/include/cctz/civil_time.h"
 #include "absl/time/internal/cctz/include/cctz/time_zone.h"
-#include "time_zone_if.h"
-#include "time_zone_info.h"
+#include "absl/time/internal/cctz/src/time_zone_if.h"
+#include "absl/time/internal/cctz/src/time_zone_info.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 namespace cctz {
 
@@ -69,14 +71,18 @@ class time_zone::Impl {
     return zone_->PrevTransition(tp, trans);
   }
 
-  // Returns an implementation-defined version std::string for this time zone.
+  // Returns an implementation-defined version string for this time zone.
   std::string Version() const { return zone_->Version(); }
 
   // Returns an implementation-defined description of this time zone.
   std::string Description() const { return zone_->Description(); }
 
  private:
+  Impl();
   explicit Impl(const std::string& name);
+  Impl(const Impl&) = delete;
+  Impl& operator=(const Impl&) = delete;
+
   static const Impl* UTCImpl();
 
   const std::string name_;
@@ -85,6 +91,7 @@ class time_zone::Impl {
 
 }  // namespace cctz
 }  // namespace time_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_TIME_INTERNAL_CCTZ_TIME_ZONE_IMPL_H_
diff --git a/absl/time/internal/cctz/src/time_zone_info.cc b/absl/time/internal/cctz/src/time_zone_info.cc
index d3e1ae29bb5..9abbde72d62 100644
--- a/absl/time/internal/cctz/src/time_zone_info.cc
+++ b/absl/time/internal/cctz/src/time_zone_info.cc
@@ -30,7 +30,17 @@
 // Note that we assume the proleptic Gregorian calendar and 60-second
 // minutes throughout.
 
-#include "time_zone_info.h"
+#include "absl/time/internal/cctz/src/time_zone_info.h"
+
+#include "absl/base/config.h"
+
+#if !defined(_MSC_VER)
+#include <fcntl.h>
+#if !defined(__APPLE__)
+#include <sys/stat.h>
+#endif
+#include <unistd.h>
+#endif
 
 #include <algorithm>
 #include <cassert>
@@ -39,17 +49,22 @@
 #include <cstdio>
 #include <cstdlib>
 #include <cstring>
+#include <fstream>
 #include <functional>
-#include <iostream>
+#include <limits>
 #include <memory>
 #include <sstream>
 #include <string>
+#include <utility>
+#include <vector>
 
 #include "absl/time/internal/cctz/include/cctz/civil_time.h"
-#include "time_zone_fixed.h"
-#include "time_zone_posix.h"
+#include "absl/time/internal/cctz/src/time_zone_fixed.h"
+#include "absl/time/internal/cctz/src/time_zone_posix.h"
+#include "absl/time/internal/cctz/src/tzfile.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 namespace cctz {
 
@@ -65,8 +80,8 @@ const std::int_least32_t kDaysPerYear[2] = {365, 366};
 // The day offsets of the beginning of each (1-based) month in non-leap and
 // leap years respectively (e.g., 335 days before December in a leap year).
 const std::int_least16_t kMonthOffsets[2][1 + 12 + 1] = {
-  {-1, 0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334, 365},
-  {-1, 0, 31, 60, 91, 121, 152, 182, 213, 244, 274, 305, 335, 366},
+    {-1, 0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334, 365},
+    {-1, 0, 31, 60, 91, 121, 152, 182, 213, 244, 274, 305, 335, 366},
 };
 
 // We reject leap-second encoded zoneinfo and so assume 60-second minutes.
@@ -77,10 +92,31 @@ const std::int_least64_t kSecsPer400Years = 146097LL * kSecsPerDay;
 
 // Like kDaysPerYear[] but scaled up by a factor of kSecsPerDay.
 const std::int_least32_t kSecsPerYear[2] = {
-  365 * kSecsPerDay,
-  366 * kSecsPerDay,
+    365 * kSecsPerDay,
+    366 * kSecsPerDay,
 };
 
+// Convert a cctz::weekday to a POSIX TZ weekday number (0==Sun, ..., 6=Sat).
+inline int ToPosixWeekday(weekday wd) {
+  switch (wd) {
+    case weekday::sunday:
+      return 0;
+    case weekday::monday:
+      return 1;
+    case weekday::tuesday:
+      return 2;
+    case weekday::wednesday:
+      return 3;
+    case weekday::thursday:
+      return 4;
+    case weekday::friday:
+      return 5;
+    case weekday::saturday:
+      return 6;
+  }
+  return 0; /*NOTREACHED*/
+}
+
 // Single-byte, unsigned numeric values are encoded directly.
 inline std::uint_fast8_t Decode8(const char* cp) {
   return static_cast<std::uint_fast8_t>(*cp) & 0xff;
@@ -110,6 +146,64 @@ std::int_fast64_t Decode64(const char* cp) {
   return static_cast<std::int_fast64_t>(v - s64maxU - 1) - s64max - 1;
 }
 
+struct Header {            // counts of:
+  std::size_t timecnt;     // transition times
+  std::size_t typecnt;     // transition types
+  std::size_t charcnt;     // zone abbreviation characters
+  std::size_t leapcnt;     // leap seconds (we expect none)
+  std::size_t ttisstdcnt;  // UTC/local indicators (unused)
+  std::size_t ttisutcnt;   // standard/wall indicators (unused)
+
+  bool Build(const tzhead& tzh);
+  std::size_t DataLength(std::size_t time_len) const;
+};
+
+// Builds the in-memory header using the raw bytes from the file.
+bool Header::Build(const tzhead& tzh) {
+  std::int_fast32_t v;
+  if ((v = Decode32(tzh.tzh_timecnt)) < 0) return false;
+  timecnt = static_cast<std::size_t>(v);
+  if ((v = Decode32(tzh.tzh_typecnt)) < 0) return false;
+  typecnt = static_cast<std::size_t>(v);
+  if ((v = Decode32(tzh.tzh_charcnt)) < 0) return false;
+  charcnt = static_cast<std::size_t>(v);
+  if ((v = Decode32(tzh.tzh_leapcnt)) < 0) return false;
+  leapcnt = static_cast<std::size_t>(v);
+  if ((v = Decode32(tzh.tzh_ttisstdcnt)) < 0) return false;
+  ttisstdcnt = static_cast<std::size_t>(v);
+  if ((v = Decode32(tzh.tzh_ttisutcnt)) < 0) return false;
+  ttisutcnt = static_cast<std::size_t>(v);
+  return true;
+}
+
+// How many bytes of data are associated with this header. The result
+// depends upon whether this is a section with 4-byte or 8-byte times.
+std::size_t Header::DataLength(std::size_t time_len) const {
+  std::size_t len = 0;
+  len += (time_len + 1) * timecnt;  // unix_time + type_index
+  len += (4 + 1 + 1) * typecnt;     // utc_offset + is_dst + abbr_index
+  len += 1 * charcnt;               // abbreviations
+  len += (time_len + 4) * leapcnt;  // leap-time + TAI-UTC
+  len += 1 * ttisstdcnt;            // UTC/local indicators
+  len += 1 * ttisutcnt;             // standard/wall indicators
+  return len;
+}
+
+// Does the rule for future transitions call for year-round daylight time?
+// See tz/zic.c:stringzone() for the details on how such rules are encoded.
+bool AllYearDST(const PosixTimeZone& posix) {
+  if (posix.dst_start.date.fmt != PosixTransition::N) return false;
+  if (posix.dst_start.date.n.day != 0) return false;
+  if (posix.dst_start.time.offset != 0) return false;
+
+  if (posix.dst_end.date.fmt != PosixTransition::J) return false;
+  if (posix.dst_end.date.j.day != kDaysPerYear[0]) return false;
+  const auto offset = posix.std_offset - posix.dst_offset;
+  if (posix.dst_end.time.offset + offset != kSecsPerDay) return false;
+
+  return true;
+}
+
 // Generate a year-relative offset for a PosixTransition.
 std::int_fast64_t TransOffset(bool leap_year, int jan1_weekday,
                               const PosixTransition& pt) {
@@ -172,8 +266,405 @@ inline time_zone::civil_lookup MakeRepeated(const Transition& tr,
 }
 
 inline civil_second YearShift(const civil_second& cs, year_t shift) {
-  return civil_second(cs.year() + shift, cs.month(), cs.day(),
-                      cs.hour(), cs.minute(), cs.second());
+  return civil_second(cs.year() + shift, cs.month(), cs.day(), cs.hour(),
+                      cs.minute(), cs.second());
+}
+
+}  // namespace
+
+// Find/make a transition type with these attributes.
+bool TimeZoneInfo::GetTransitionType(std::int_fast32_t utc_offset, bool is_dst,
+                                     const std::string& abbr,
+                                     std::uint_least8_t* index) {
+  std::size_t type_index = 0;
+  std::size_t abbr_index = abbreviations_.size();
+  for (; type_index != transition_types_.size(); ++type_index) {
+    const TransitionType& tt(transition_types_[type_index]);
+    const char* tt_abbr = &abbreviations_[tt.abbr_index];
+    if (tt_abbr == abbr) abbr_index = tt.abbr_index;
+    if (tt.utc_offset == utc_offset && tt.is_dst == is_dst) {
+      if (abbr_index == tt.abbr_index) break;  // reuse
+    }
+  }
+  if (type_index > 255 || abbr_index > 255) {
+    // No index space (8 bits) available for a new type or abbreviation.
+    return false;
+  }
+  if (type_index == transition_types_.size()) {
+    TransitionType& tt(*transition_types_.emplace(transition_types_.end()));
+    tt.utc_offset = static_cast<std::int_least32_t>(utc_offset);
+    tt.is_dst = is_dst;
+    if (abbr_index == abbreviations_.size()) {
+      abbreviations_.append(abbr);
+      abbreviations_.append(1, '\0');
+    }
+    tt.abbr_index = static_cast<std::uint_least8_t>(abbr_index);
+  }
+  *index = static_cast<std::uint_least8_t>(type_index);
+  return true;
+}
+
+// zic(8) can generate no-op transitions when a zone changes rules at an
+// instant when there is actually no discontinuity.  So we check whether
+// two transitions have equivalent types (same offset/is_dst/abbr).
+bool TimeZoneInfo::EquivTransitions(std::uint_fast8_t tt1_index,
+                                    std::uint_fast8_t tt2_index) const {
+  if (tt1_index == tt2_index) return true;
+  const TransitionType& tt1(transition_types_[tt1_index]);
+  const TransitionType& tt2(transition_types_[tt2_index]);
+  if (tt1.utc_offset != tt2.utc_offset) return false;
+  if (tt1.is_dst != tt2.is_dst) return false;
+  if (tt1.abbr_index != tt2.abbr_index) return false;
+  return true;
+}
+
+// Use the POSIX-TZ-environment-variable-style string to handle times
+// in years after the last transition stored in the zoneinfo data.
+bool TimeZoneInfo::ExtendTransitions() {
+  extended_ = false;
+  if (future_spec_.empty()) return true;  // last transition prevails
+
+  PosixTimeZone posix;
+  if (!ParsePosixSpec(future_spec_, &posix)) return false;
+
+  // Find transition type for the future std specification.
+  std::uint_least8_t std_ti;
+  if (!GetTransitionType(posix.std_offset, false, posix.std_abbr, &std_ti))
+    return false;
+
+  if (posix.dst_abbr.empty()) {  // std only
+    // The future specification should match the last transition, and
+    // that means that handling the future will fall out naturally.
+    return EquivTransitions(transitions_.back().type_index, std_ti);
+  }
+
+  // Find transition type for the future dst specification.
+  std::uint_least8_t dst_ti;
+  if (!GetTransitionType(posix.dst_offset, true, posix.dst_abbr, &dst_ti))
+    return false;
+
+  if (AllYearDST(posix)) {  // dst only
+    // The future specification should match the last transition, and
+    // that means that handling the future will fall out naturally.
+    return EquivTransitions(transitions_.back().type_index, dst_ti);
+  }
+
+  // We require that zoneinfo data with a rule for future transitions
+  // ends with a non-negative transition.  This removes the need to add
+  // any "second-half" transition to ensure differences between adjacent
+  // transitions are always representable, while also guaranteeing that
+  // the arithmetic used to shift between 400-year cycles never overflows.
+  // All valid zones easily meet this requirement.
+  if (transitions_.back().unix_time < 0) return false;
+
+  // Extend the transitions for an additional 401 years using the future
+  // specification. Years beyond those can be handled by mapping back to
+  // a cycle-equivalent year within that range. Note that we need 401
+  // (well, at least the first transition in the 401st year) so that the
+  // end of the 400th year is mapped back to an extended year. And first
+  // we may also need two additional transitions for the current year.
+  transitions_.reserve(transitions_.size() + 2 + 401 * 2);
+  extended_ = true;
+
+  const Transition& last(transitions_.back());
+  const std::int_fast64_t last_time = last.unix_time;
+  const TransitionType& last_tt(transition_types_[last.type_index]);
+  last_year_ = LocalTime(last_time, last_tt).cs.year();
+  bool leap_year = IsLeap(last_year_);
+  const civil_second jan1(last_year_);
+  std::int_fast64_t jan1_time = jan1 - civil_second();
+  int jan1_weekday = ToPosixWeekday(get_weekday(jan1));
+
+  Transition dst = {0, dst_ti, civil_second(), civil_second()};
+  Transition std = {0, std_ti, civil_second(), civil_second()};
+  for (const year_t limit = last_year_ + 401;; ++last_year_) {
+    auto dst_trans_off = TransOffset(leap_year, jan1_weekday, posix.dst_start);
+    auto std_trans_off = TransOffset(leap_year, jan1_weekday, posix.dst_end);
+    dst.unix_time = jan1_time + dst_trans_off - posix.std_offset;
+    std.unix_time = jan1_time + std_trans_off - posix.dst_offset;
+    const auto* ta = dst.unix_time < std.unix_time ? &dst : &std;
+    const auto* tb = dst.unix_time < std.unix_time ? &std : &dst;
+    if (last_time < tb->unix_time) {
+      if (last_time < ta->unix_time) transitions_.push_back(*ta);
+      transitions_.push_back(*tb);
+    }
+    if (last_year_ == limit) break;
+    jan1_time += kSecsPerYear[leap_year];
+    jan1_weekday = (jan1_weekday + kDaysPerYear[leap_year]) % 7;
+    leap_year = !leap_year && IsLeap(last_year_ + 1);
+  }
+
+  return true;
+}
+
+namespace {
+
+using FilePtr = std::unique_ptr<FILE, int (*)(FILE*)>;
+
+// fopen(3) adaptor for reading zoneinfo files (read-only binary mode).
+inline FilePtr FOpen(const char* path) {
+#if defined(_MSC_VER)
+  FILE* fp;
+  if (fopen_s(&fp, path, "rb") != 0) fp = nullptr;
+  return FilePtr(fp, fclose);
+#else
+  // Open non-blocking and verify the target is a regular file before handing it
+  // to stdio. Zone names are potentially attacker-controlled, and a plain
+  // fopen() on a FIFO or device node (reachable via the "file:" prefix or an
+  // absolute path) would block indefinitely or read unbounded data.
+#ifndef O_CLOEXEC
+#define O_CLOEXEC 0
+#endif
+  const int fd = open(path, O_RDONLY | O_NONBLOCK | O_CLOEXEC);
+  if (fd >= 0) {
+#if defined(__APPLE__)
+    // On Apple platforms, avoid fstat() to prevent triggering Apple's Required
+    // Reason API (NSPrivacyAccessedAPICategoryFileTimestamp) scanner.
+    // Security implications of skipping this check on Apple platforms:
+    //   - FIFO hangs are mitigated by O_NONBLOCK
+    //   - Path traversal is handled by UnsafePath()
+    //   - /usr/share/zoneinfo is a read-only system volume on iOS
+    // On general POSIX operating systems, the check remains as
+    // defense-in-depth.
+    auto is_regular = [](int) { return true; };
+#else
+    auto is_regular = [](int stat_fd) {
+      struct stat st;
+      return fstat(stat_fd, &st) == 0 && S_ISREG(st.st_mode);
+    };
+#endif
+    if (is_regular(fd)) {
+      FILE* fp = fdopen(fd, "rb");
+      if (fp != nullptr) return FilePtr(fp, fclose);
+    }
+    close(fd);
+  }
+  return FilePtr(nullptr, fclose);
+#endif
+}
+
+// Returns true if c separates path components. Windows accepts either
+// form, so a "..\" component walks up a directory just like a "../" one.
+inline bool IsPathSeparator(char c) {
+#if defined(_WIN32)
+  return c == '/' || c == '\\';
+#else
+  return c == '/';
+#endif
+}
+
+// Returns true if the zone name starting at pos contains an unsafe path.
+bool UnsafePath(const std::string& name, std::size_t pos) {
+  // Path traversal: a ".." component that is at the beginning or preceded
+  // by a separator, and at the end or followed by a separator.
+  for (auto i = pos; (i = name.find("..", i)) != std::string::npos; i += 2) {
+    if ((i == pos || IsPathSeparator(name[i - 1])) &&
+        (i == name.size() - 2 || IsPathSeparator(name[i + 2]))) {
+      return true;
+    }
+  }
+  return false;
+}
+
+// A stdio(3)-backed implementation of ZoneInfoSource.
+class FileZoneInfoSource : public ZoneInfoSource {
+ public:
+  static std::unique_ptr<ZoneInfoSource> Open(const std::string& name);
+
+  std::size_t Read(void* ptr, std::size_t size) override {
+    size = std::min(size, len_);
+    std::size_t nread = fread(ptr, 1, size, fp_.get());
+    len_ -= nread;
+    return nread;
+  }
+  int Skip(std::size_t offset) override {
+    offset = std::min(offset, len_);
+    int rc = fseek(fp_.get(), static_cast<long>(offset), SEEK_CUR);
+    if (rc == 0) len_ -= offset;
+    return rc;
+  }
+  std::string Version() const override {
+    // TODO: It would nice if the zoneinfo data included the tzdb version.
+    return std::string();
+  }
+
+ protected:
+  explicit FileZoneInfoSource(
+      FilePtr fp, std::size_t len = std::numeric_limits<std::size_t>::max())
+      : fp_(std::move(fp)), len_(len) {}
+
+ private:
+  FilePtr fp_;
+  std::size_t len_;
+};
+
+std::unique_ptr<ZoneInfoSource> FileZoneInfoSource::Open(
+    const std::string& name) {
+  // Use of the "file:" prefix is intended for testing purposes only.
+  const std::size_t pos = (name.compare(0, 5, "file:") == 0) ? 5 : 0;
+
+  // Reject unsafe paths (e.g., "../../etc/passwd").
+  if (UnsafePath(name, pos)) {
+    return nullptr;
+  }
+
+  // Map the time-zone name to a path name.
+  std::string path;
+  if (pos == name.size() || name[pos] != '/') {
+    const char* tzdir = "/usr/share/zoneinfo";
+    char* tzdir_env = nullptr;
+#if defined(_MSC_VER)
+    _dupenv_s(&tzdir_env, nullptr, "TZDIR");
+#else
+    tzdir_env = std::getenv("TZDIR");
+#endif
+    if (tzdir_env && *tzdir_env) tzdir = tzdir_env;
+    path += tzdir;
+    path += '/';
+#if defined(_MSC_VER)
+    free(tzdir_env);
+#endif
+  }
+  path.append(name, pos, std::string::npos);
+
+  // Open the zoneinfo file.
+  auto fp = FOpen(path.c_str());
+  if (fp == nullptr) return nullptr;
+  return std::unique_ptr<ZoneInfoSource>(new FileZoneInfoSource(std::move(fp)));
+}
+
+class AndroidZoneInfoSource : public FileZoneInfoSource {
+ public:
+  static std::unique_ptr<ZoneInfoSource> Open(const std::string& name);
+  std::string Version() const override { return version_; }
+
+ private:
+  explicit AndroidZoneInfoSource(FilePtr fp, std::size_t len,
+                                 std::string version)
+      : FileZoneInfoSource(std::move(fp), len), version_(std::move(version)) {}
+  std::string version_;
+};
+
+std::unique_ptr<ZoneInfoSource> AndroidZoneInfoSource::Open(
+    const std::string& name) {
+  // Use of the "file:" prefix is intended for testing purposes only.
+  const std::size_t pos = (name.compare(0, 5, "file:") == 0) ? 5 : 0;
+
+  // See Android's libc/tzcode/bionic.cpp for additional information.
+  for (const char* tzdata : {"/apex/com.android.tzdata/etc/tz/tzdata",
+                             "/data/misc/zoneinfo/current/tzdata",
+                             "/system/usr/share/zoneinfo/tzdata"}) {
+    auto fp = FOpen(tzdata);
+    if (fp == nullptr) continue;
+
+    char hbuf[24];  // covers header.zonetab_offset too
+    if (fread(hbuf, 1, sizeof(hbuf), fp.get()) != sizeof(hbuf)) continue;
+    if (strncmp(hbuf, "tzdata", 6) != 0) continue;
+    const char* vers = (hbuf[11] == '\0') ? hbuf + 6 : "";
+    const std::int_fast32_t index_offset = Decode32(hbuf + 12);
+    const std::int_fast32_t data_offset = Decode32(hbuf + 16);
+    if (index_offset < 0 || data_offset < index_offset) continue;
+    if (fseek(fp.get(), static_cast<long>(index_offset), SEEK_SET) != 0)
+      continue;
+
+    char ebuf[52];  // covers entry.unused too
+    const std::size_t index_size =
+        static_cast<std::size_t>(data_offset - index_offset);
+    const std::size_t zonecnt = index_size / sizeof(ebuf);
+    if (zonecnt * sizeof(ebuf) != index_size) continue;
+    for (std::size_t i = 0; i != zonecnt; ++i) {
+      if (fread(ebuf, 1, sizeof(ebuf), fp.get()) != sizeof(ebuf)) break;
+      const std::int_fast64_t start =
+          std::int_fast64_t{data_offset} + Decode32(ebuf + 40);
+      const std::int_fast32_t length = Decode32(ebuf + 44);
+      if (start < 0 || length < 0) break;
+      // fseek() takes a long
+      if (start > std::numeric_limits<long>::max()) break;
+      ebuf[40] = '\0';  // ensure zone name is NUL terminated
+      if (strcmp(name.c_str() + pos, ebuf) == 0) {
+        if (fseek(fp.get(), static_cast<long>(start), SEEK_SET) != 0) break;
+        return std::unique_ptr<ZoneInfoSource>(new AndroidZoneInfoSource(
+            std::move(fp), static_cast<std::size_t>(length), vers));
+      }
+    }
+  }
+
+  return nullptr;
+}
+
+// A zoneinfo source for use inside Fuchsia components. This attempts to
+// read zoneinfo files from one of several known paths in a component's
+// incoming namespace. [Config data][1] is preferred, but package-specific
+// resources are also supported.
+//
+// Fuchsia's implementation supports `FileZoneInfoSource::Version()`.
+//
+// [1]:
+// https://fuchsia.dev/fuchsia-src/development/components/data#using_config_data_in_your_component
+class FuchsiaZoneInfoSource : public FileZoneInfoSource {
+ public:
+  static std::unique_ptr<ZoneInfoSource> Open(const std::string& name);
+  std::string Version() const override { return version_; }
+
+ private:
+  explicit FuchsiaZoneInfoSource(FilePtr fp, std::string version)
+      : FileZoneInfoSource(std::move(fp)), version_(std::move(version)) {}
+  std::string version_;
+};
+
+std::unique_ptr<ZoneInfoSource> FuchsiaZoneInfoSource::Open(
+    const std::string& name) {
+  // Use of the "file:" prefix is intended for testing purposes only.
+  const std::size_t pos = (name.compare(0, 5, "file:") == 0) ? 5 : 0;
+
+  // Reject unsafe paths (e.g., "../../etc/passwd").
+  if (UnsafePath(name, pos)) {
+    return nullptr;
+  }
+
+  // Prefixes where a Fuchsia component might find zoneinfo files,
+  // in descending order of preference.
+  const auto kTzdataPrefixes = {
+      // The tzdata from `config-data`.
+      "/config/data/tzdata/",
+      // The tzdata bundled in the component's package.
+      "/pkg/data/tzdata/",
+      // General data storage.
+      "/data/tzdata/",
+      // The recommended path for routed-in tzdata files.
+      // See for details:
+      // https://fuchsia.dev/fuchsia-src/concepts/process/namespaces?hl=en#typical_directory_structure
+      "/config/tzdata/",
+  };
+  const auto kEmptyPrefix = {""};
+  const bool name_absolute = (pos != name.size() && name[pos] == '/');
+  const auto prefixes = name_absolute ? kEmptyPrefix : kTzdataPrefixes;
+
+  // Fuchsia builds place zoneinfo files at "<prefix><format><name>".
+  for (const std::string prefix : prefixes) {
+    std::string path = prefix;
+    if (!prefix.empty()) path += "zoneinfo/tzif2/";  // format
+    path.append(name, pos, std::string::npos);
+
+    auto fp = FOpen(path.c_str());
+    if (fp == nullptr) continue;
+
+    std::string version;
+    if (!prefix.empty()) {
+      // Fuchsia builds place the version in "<prefix>revision.txt".
+      std::ifstream version_stream(prefix + "revision.txt");
+      if (version_stream.is_open()) {
+        // revision.txt should contain no newlines, but to be
+        // defensive we read just the first line.
+        std::getline(version_stream, version);
+      }
+    }
+
+    return std::unique_ptr<ZoneInfoSource>(
+        new FuchsiaZoneInfoSource(std::move(fp), std::move(version)));
+  }
+
+  return nullptr;
 }
 
 }  // namespace
@@ -186,15 +677,13 @@ bool TimeZoneInfo::ResetToBuiltinUTC(const seconds& offset) {
   tt.is_dst = false;
   tt.abbr_index = 0;
 
-  // We temporarily add some redundant, contemporary (2013 through 2023)
+  // We temporarily add some redundant, contemporary (2015 through 2025)
   // transitions for performance reasons.  See TimeZoneInfo::LocalTime().
   // TODO: Fix the performance issue and remove the extra transitions.
   transitions_.clear();
   transitions_.reserve(12);
   for (const std::int_fast64_t unix_time : {
-           -(1LL << 59),  // BIG_BANG
-           1356998400LL,  // 2013-01-01T00:00:00+00:00
-           1388534400LL,  // 2014-01-01T00:00:00+00:00
+           -(1LL << 59),  // a "first half" transition
            1420070400LL,  // 2015-01-01T00:00:00+00:00
            1451606400LL,  // 2016-01-01T00:00:00+00:00
            1483228800LL,  // 2017-01-01T00:00:00+00:00
@@ -204,7 +693,8 @@ bool TimeZoneInfo::ResetToBuiltinUTC(const seconds& offset) {
            1609459200LL,  // 2021-01-01T00:00:00+00:00
            1640995200LL,  // 2022-01-01T00:00:00+00:00
            1672531200LL,  // 2023-01-01T00:00:00+00:00
-           2147483647LL,  // 2^31 - 1
+           1704067200LL,  // 2024-01-01T00:00:00+00:00
+           1735689600LL,  // 2025-01-01T00:00:00+00:00
        }) {
     Transition& tr(*transitions_.emplace(transitions_.end()));
     tr.unix_time = unix_time;
@@ -215,7 +705,7 @@ bool TimeZoneInfo::ResetToBuiltinUTC(const seconds& offset) {
 
   default_transition_type_ = 0;
   abbreviations_ = FixedOffsetToAbbr(offset);
-  abbreviations_.append(1, '\0');  // add NUL
+  abbreviations_.append(1, '\0');
   future_spec_.clear();  // never needed for a fixed-offset zone
   extended_ = false;
 
@@ -226,198 +716,27 @@ bool TimeZoneInfo::ResetToBuiltinUTC(const seconds& offset) {
   return true;
 }
 
-// Builds the in-memory header using the raw bytes from the file.
-bool TimeZoneInfo::Header::Build(const tzhead& tzh) {
-  std::int_fast32_t v;
-  if ((v = Decode32(tzh.tzh_timecnt)) < 0) return false;
-  timecnt = static_cast<std::size_t>(v);
-  if ((v = Decode32(tzh.tzh_typecnt)) < 0) return false;
-  typecnt = static_cast<std::size_t>(v);
-  if ((v = Decode32(tzh.tzh_charcnt)) < 0) return false;
-  charcnt = static_cast<std::size_t>(v);
-  if ((v = Decode32(tzh.tzh_leapcnt)) < 0) return false;
-  leapcnt = static_cast<std::size_t>(v);
-  if ((v = Decode32(tzh.tzh_ttisstdcnt)) < 0) return false;
-  ttisstdcnt = static_cast<std::size_t>(v);
-  if ((v = Decode32(tzh.tzh_ttisutcnt)) < 0) return false;
-  ttisutcnt = static_cast<std::size_t>(v);
-  return true;
-}
-
-// How many bytes of data are associated with this header. The result
-// depends upon whether this is a section with 4-byte or 8-byte times.
-std::size_t TimeZoneInfo::Header::DataLength(std::size_t time_len) const {
-  std::size_t len = 0;
-  len += (time_len + 1) * timecnt;  // unix_time + type_index
-  len += (4 + 1 + 1) * typecnt;     // utc_offset + is_dst + abbr_index
-  len += 1 * charcnt;               // abbreviations
-  len += (time_len + 4) * leapcnt;  // leap-time + TAI-UTC
-  len += 1 * ttisstdcnt;            // UTC/local indicators
-  len += 1 * ttisutcnt;             // standard/wall indicators
-  return len;
-}
-
-// Check that the TransitionType has the expected offset/is_dst/abbreviation.
-void TimeZoneInfo::CheckTransition(const std::string& name,
-                                   const TransitionType& tt,
-                                   std::int_fast32_t offset, bool is_dst,
-                                   const std::string& abbr) const {
-  if (tt.utc_offset != offset || tt.is_dst != is_dst ||
-      &abbreviations_[tt.abbr_index] != abbr) {
-    std::clog << name << ": Transition"
-              << " offset=" << tt.utc_offset << "/"
-              << (tt.is_dst ? "DST" : "STD")
-              << "/abbr=" << &abbreviations_[tt.abbr_index]
-              << " does not match POSIX spec '" << future_spec_ << "'\n";
-  }
-}
-
-// zic(8) can generate no-op transitions when a zone changes rules at an
-// instant when there is actually no discontinuity.  So we check whether
-// two transitions have equivalent types (same offset/is_dst/abbr).
-bool TimeZoneInfo::EquivTransitions(std::uint_fast8_t tt1_index,
-                                    std::uint_fast8_t tt2_index) const {
-  if (tt1_index == tt2_index) return true;
-  const TransitionType& tt1(transition_types_[tt1_index]);
-  const TransitionType& tt2(transition_types_[tt2_index]);
-  if (tt1.is_dst != tt2.is_dst) return false;
-  if (tt1.utc_offset != tt2.utc_offset) return false;
-  if (tt1.abbr_index != tt2.abbr_index) return false;
-  return true;
-}
-
-// Use the POSIX-TZ-environment-variable-style string to handle times
-// in years after the last transition stored in the zoneinfo data.
-void TimeZoneInfo::ExtendTransitions(const std::string& name,
-                                     const Header& hdr) {
-  extended_ = false;
-  bool extending = !future_spec_.empty();
-
-  PosixTimeZone posix;
-  if (extending && !ParsePosixSpec(future_spec_, &posix)) {
-    std::clog << name << ": Failed to parse '" << future_spec_ << "'\n";
-    extending = false;
-  }
-
-  if (extending && posix.dst_abbr.empty()) {  // std only
-    // The future specification should match the last/default transition,
-    // and that means that handling the future will fall out naturally.
-    std::uint_fast8_t index = default_transition_type_;
-    if (hdr.timecnt != 0) index = transitions_[hdr.timecnt - 1].type_index;
-    const TransitionType& tt(transition_types_[index]);
-    CheckTransition(name, tt, posix.std_offset, false, posix.std_abbr);
-    extending = false;
-  }
-
-  if (extending && hdr.timecnt < 2) {
-    std::clog << name << ": Too few transitions for POSIX spec\n";
-    extending = false;
-  }
-
-  if (!extending) {
-    // Ensure that there is always a transition in the second half of the
-    // time line (the BIG_BANG transition is in the first half) so that the
-    // signed difference between a civil_second and the civil_second of its
-    // previous transition is always representable, without overflow.
-    const Transition& last(transitions_.back());
-    if (last.unix_time < 0) {
-      const std::uint_fast8_t type_index = last.type_index;
-      Transition& tr(*transitions_.emplace(transitions_.end()));
-      tr.unix_time = 2147483647;  // 2038-01-19T03:14:07+00:00
-      tr.type_index = type_index;
-    }
-    return;  // last transition wins
-  }
-
-  // Extend the transitions for an additional 400 years using the
-  // future specification. Years beyond those can be handled by
-  // mapping back to a cycle-equivalent year within that range.
-  // zic(8) should probably do this so that we don't have to.
-  // TODO: Reduce the extension by the number of compatible
-  // transitions already in place.
-  transitions_.reserve(hdr.timecnt + 400 * 2 + 1);
-  transitions_.resize(hdr.timecnt + 400 * 2);
-  extended_ = true;
-
-  // The future specification should match the last two transitions,
-  // and those transitions should have different is_dst flags.  Note
-  // that nothing says the UTC offset used by the is_dst transition
-  // must be greater than that used by the !is_dst transition.  (See
-  // Europe/Dublin, for example.)
-  const Transition* tr0 = &transitions_[hdr.timecnt - 1];
-  const Transition* tr1 = &transitions_[hdr.timecnt - 2];
-  const TransitionType* tt0 = &transition_types_[tr0->type_index];
-  const TransitionType* tt1 = &transition_types_[tr1->type_index];
-  const TransitionType& dst(tt0->is_dst ? *tt0 : *tt1);
-  const TransitionType& std(tt0->is_dst ? *tt1 : *tt0);
-  CheckTransition(name, dst, posix.dst_offset, true, posix.dst_abbr);
-  CheckTransition(name, std, posix.std_offset, false, posix.std_abbr);
-
-  // Add the transitions to tr1 and back to tr0 for each extra year.
-  last_year_ = LocalTime(tr0->unix_time, *tt0).cs.year();
-  bool leap_year = IsLeap(last_year_);
-  const civil_day jan1(last_year_, 1, 1);
-  std::int_fast64_t jan1_time = civil_second(jan1) - civil_second();
-  int jan1_weekday = (static_cast<int>(get_weekday(jan1)) + 1) % 7;
-  Transition* tr = &transitions_[hdr.timecnt];  // next trans to fill
-  if (LocalTime(tr1->unix_time, *tt1).cs.year() != last_year_) {
-    // Add a single extra transition to align to a calendar year.
-    transitions_.resize(transitions_.size() + 1);
-    assert(tr == &transitions_[hdr.timecnt]);  // no reallocation
-    const PosixTransition& pt1(tt0->is_dst ? posix.dst_end : posix.dst_start);
-    std::int_fast64_t tr1_offset = TransOffset(leap_year, jan1_weekday, pt1);
-    tr->unix_time = jan1_time + tr1_offset - tt0->utc_offset;
-    tr++->type_index = tr1->type_index;
-    tr0 = &transitions_[hdr.timecnt];
-    tr1 = &transitions_[hdr.timecnt - 1];
-    tt0 = &transition_types_[tr0->type_index];
-    tt1 = &transition_types_[tr1->type_index];
-  }
-  const PosixTransition& pt1(tt0->is_dst ? posix.dst_end : posix.dst_start);
-  const PosixTransition& pt0(tt0->is_dst ? posix.dst_start : posix.dst_end);
-  for (const year_t limit = last_year_ + 400; last_year_ < limit;) {
-    last_year_ += 1;  // an additional year of generated transitions
-    jan1_time += kSecsPerYear[leap_year];
-    jan1_weekday = (jan1_weekday + kDaysPerYear[leap_year]) % 7;
-    leap_year = !leap_year && IsLeap(last_year_);
-    std::int_fast64_t tr1_offset = TransOffset(leap_year, jan1_weekday, pt1);
-    tr->unix_time = jan1_time + tr1_offset - tt0->utc_offset;
-    tr++->type_index = tr1->type_index;
-    std::int_fast64_t tr0_offset = TransOffset(leap_year, jan1_weekday, pt0);
-    tr->unix_time = jan1_time + tr0_offset - tt1->utc_offset;
-    tr++->type_index = tr0->type_index;
-  }
-  assert(tr == &transitions_[0] + transitions_.size());
-}
-
-bool TimeZoneInfo::Load(const std::string& name, ZoneInfoSource* zip) {
+bool TimeZoneInfo::Load(ZoneInfoSource* zip) {
   // Read and validate the header.
   tzhead tzh;
-  if (zip->Read(&tzh, sizeof(tzh)) != sizeof(tzh))
-    return false;
+  if (zip->Read(&tzh, sizeof(tzh)) != sizeof(tzh)) return false;
   if (strncmp(tzh.tzh_magic, TZ_MAGIC, sizeof(tzh.tzh_magic)) != 0)
     return false;
   Header hdr;
-  if (!hdr.Build(tzh))
-    return false;
+  if (!hdr.Build(tzh)) return false;
   std::size_t time_len = 4;
   if (tzh.tzh_version[0] != '\0') {
     // Skip the 4-byte data.
-    if (zip->Skip(hdr.DataLength(time_len)) != 0)
-      return false;
+    if (zip->Skip(hdr.DataLength(time_len)) != 0) return false;
     // Read and validate the header for the 8-byte data.
-    if (zip->Read(&tzh, sizeof(tzh)) != sizeof(tzh))
-      return false;
+    if (zip->Read(&tzh, sizeof(tzh)) != sizeof(tzh)) return false;
     if (strncmp(tzh.tzh_magic, TZ_MAGIC, sizeof(tzh.tzh_magic)) != 0)
       return false;
-    if (tzh.tzh_version[0] == '\0')
-      return false;
-    if (!hdr.Build(tzh))
-      return false;
+    if (tzh.tzh_version[0] == '\0') return false;
+    if (!hdr.Build(tzh)) return false;
     time_len = 8;
   }
-  if (hdr.typecnt == 0)
-    return false;
+  if (hdr.typecnt == 0) return false;
   if (hdr.leapcnt != 0) {
     // This code assumes 60-second minutes so we do not want
     // the leap-second encoded zoneinfo. We could reverse the
@@ -425,40 +744,45 @@ bool TimeZoneInfo::Load(const std::string& name, ZoneInfoSource* zip) {
     // so currently we simply reject such data.
     return false;
   }
-  if (hdr.ttisstdcnt != 0 && hdr.ttisstdcnt != hdr.typecnt)
-    return false;
-  if (hdr.ttisutcnt != 0 && hdr.ttisutcnt != hdr.typecnt)
-    return false;
+  if (hdr.ttisstdcnt != 0 && hdr.ttisstdcnt != hdr.typecnt) return false;
+  if (hdr.ttisutcnt != 0 && hdr.ttisutcnt != hdr.typecnt) return false;
+
+  // Bound the header counts before sizing tbuf so that a hostile TZif blob
+  // cannot force a very large zero-filled allocation from a tiny input.
+  if (hdr.timecnt > TZ_MAX_TIMES) return false;
+  if (hdr.typecnt > TZ_MAX_TYPES) return false;
+  if (hdr.charcnt > TZ_MAX_CHARS) return false;
 
   // Read the data into a local buffer.
   std::size_t len = hdr.DataLength(time_len);
   std::vector<char> tbuf(len);
-  if (zip->Read(tbuf.data(), len) != len)
-    return false;
+  if (zip->Read(tbuf.data(), len) != len) return false;
   const char* bp = tbuf.data();
 
   // Decode and validate the transitions.
-  transitions_.reserve(hdr.timecnt + 2);  // We might add a couple.
+  transitions_.reserve(hdr.timecnt + 2);
   transitions_.resize(hdr.timecnt);
   for (std::size_t i = 0; i != hdr.timecnt; ++i) {
     transitions_[i].unix_time = (time_len == 4) ? Decode32(bp) : Decode64(bp);
     bp += time_len;
-    if (i != 0) {
-      // Check that the transitions are ordered by time (as zic guarantees).
-      if (!Transition::ByUnixTime()(transitions_[i - 1], transitions_[i]))
-        return false;  // out of order
-    }
+    // A valid zoneinfo file keeps transition times far from the int64 limits.
+    // A hostile one can place them at the extremes, where the
+    // reverse-conversion arithmetic in MakeTime() (tr.unix_time +/- a sub-day
+    // civil delta, see MakeSkipped()/MakeRepeated()) overflows. Bound them to
+    // +/-(1<<59), the times used by the no-op transitions added below.
+    if (transitions_[i].unix_time < -(1LL << 59) ||
+        transitions_[i].unix_time > (1LL << 59))
+      return false;  // out of range
   }
   bool seen_type_0 = false;
   for (std::size_t i = 0; i != hdr.timecnt; ++i) {
     transitions_[i].type_index = Decode8(bp++);
-    if (transitions_[i].type_index >= hdr.typecnt)
-      return false;
-    if (transitions_[i].type_index == 0)
-      seen_type_0 = true;
+    if (transitions_[i].type_index >= hdr.typecnt) return false;
+    if (transitions_[i].type_index == 0) seen_type_0 = true;
   }
 
   // Decode and validate the transition types.
+  transition_types_.reserve(hdr.typecnt + 2);
   transition_types_.resize(hdr.typecnt);
   for (std::size_t i = 0; i != hdr.typecnt; ++i) {
     transition_types_[i].utc_offset =
@@ -469,26 +793,29 @@ bool TimeZoneInfo::Load(const std::string& name, ZoneInfoSource* zip) {
     bp += 4;
     transition_types_[i].is_dst = (Decode8(bp++) != 0);
     transition_types_[i].abbr_index = Decode8(bp++);
-    if (transition_types_[i].abbr_index >= hdr.charcnt)
-      return false;
+    if (transition_types_[i].abbr_index >= hdr.charcnt) return false;
   }
 
   // Determine the before-first-transition type.
   default_transition_type_ = 0;
   if (seen_type_0 && hdr.timecnt != 0) {
-    std::uint_fast8_t index = 0;
+    std::size_t index = 0;
     if (transition_types_[0].is_dst) {
       index = transitions_[0].type_index;
-      while (index != 0 && transition_types_[index].is_dst)
-        --index;
+      while (index != 0 && transition_types_[index].is_dst) --index;
     }
-    while (index != hdr.typecnt && transition_types_[index].is_dst)
-      ++index;
+    while (index != hdr.typecnt && transition_types_[index].is_dst) ++index;
     if (index != hdr.typecnt)
-      default_transition_type_ = index;
+      default_transition_type_ = static_cast<std::uint_fast8_t>(index);
   }
 
-  // Copy all the abbreviations.
+  // Copy all the abbreviations. The area holds NUL-terminated strings, and
+  // LocalTime() hands out a pointer into it, so the final abbreviation has
+  // to be terminated within the area itself. Otherwise an abbreviation runs
+  // on into whatever ExtendTransitions() later appends. (hdr.charcnt != 0
+  // because every abbr_index was validated to be less than it.)
+  if (bp[hdr.charcnt - 1] != '\0') return false;
+  abbreviations_.reserve(hdr.charcnt + 10);
   abbreviations_.assign(bp, hdr.charcnt);
   bp += hdr.charcnt;
 
@@ -496,9 +823,9 @@ bool TimeZoneInfo::Load(const std::string& name, ZoneInfoSource* zip) {
   // encoded zoneinfo. The ttisstd/ttisgmt indicators only apply when
   // interpreting a POSIX spec that does not include start/end rules, and
   // that isn't the case here (see "zic -p").
-  bp += (8 + 4) * hdr.leapcnt;  // leap-time + TAI-UTC
-  bp += 1 * hdr.ttisstdcnt;     // UTC/local indicators
-  bp += 1 * hdr.ttisutcnt;      // standard/wall indicators
+  bp += (time_len + 4) * hdr.leapcnt;  // leap-time + TAI-UTC
+  bp += 1 * hdr.ttisstdcnt;            // UTC/local indicators
+  bp += 1 * hdr.ttisutcnt;             // standard/wall indicators
   assert(bp == tbuf.data() + tbuf.size());
 
   future_spec_.clear();
@@ -509,11 +836,9 @@ bool TimeZoneInfo::Load(const std::string& name, ZoneInfoSource* zip) {
       unsigned char ch;  // all non-EOF results are positive
       return (azip->Read(&ch, 1) == 1) ? ch : EOF;
     };
-    if (get_char(zip) != '\n')
-      return false;
+    if (get_char(zip) != '\n') return false;
     for (int c = get_char(zip); c != '\n'; c = get_char(zip)) {
-      if (c == EOF)
-        return false;
+      if (c == EOF) return false;
       future_spec_.push_back(static_cast<char>(c));
     }
   }
@@ -522,38 +847,36 @@ bool TimeZoneInfo::Load(const std::string& name, ZoneInfoSource* zip) {
 
   // If we did not find version information during the standard loading
   // process (as of tzh_version '3' that is unsupported), then ask the
-  // ZoneInfoSource for any out-of-bound version std::string it may be privy to.
+  // ZoneInfoSource for any out-of-bound version string it may be privy to.
   if (version_.empty()) {
     version_ = zip->Version();
   }
 
-  // Trim redundant transitions. zic may have added these to work around
-  // differences between the glibc and reference implementations (see
-  // zic.c:dontmerge) and the Qt library (see zic.c:WORK_AROUND_QTBUG_53071).
-  // For us, they just get in the way when we do future_spec_ extension.
-  while (hdr.timecnt > 1) {
-    if (!EquivTransitions(transitions_[hdr.timecnt - 1].type_index,
-                          transitions_[hdr.timecnt - 2].type_index)) {
-      break;
-    }
-    hdr.timecnt -= 1;
-  }
-  transitions_.resize(hdr.timecnt);
-
   // Ensure that there is always a transition in the first half of the
-  // time line (the second half is handled in ExtendTransitions()) so that
-  // the signed difference between a civil_second and the civil_second of
-  // its previous transition is always representable, without overflow.
-  // A contemporary zic will usually have already done this for us.
+  // time line (the second half is handled below) so that the signed
+  // difference between a civil_second and the civil_second of its
+  // previous transition is always representable, without overflow.
   if (transitions_.empty() || transitions_.front().unix_time >= 0) {
     Transition& tr(*transitions_.emplace(transitions_.begin()));
-    tr.unix_time = -(1LL << 59);  // see tz/zic.c "BIG_BANG"
+    tr.unix_time = -(1LL << 59);  // -18267312070-10-26T17:01:52+00:00
     tr.type_index = default_transition_type_;
-    hdr.timecnt += 1;
   }
 
   // Extend the transitions using the future specification.
-  ExtendTransitions(name, hdr);
+  if (!ExtendTransitions()) return false;
+
+  // Ensure that there is always a transition in the second half of the
+  // time line (the first half is handled above) so that the signed
+  // difference between a civil_second and the civil_second of its
+  // previous transition is always representable, without overflow.
+  const Transition& last(transitions_.back());
+  if (last.unix_time < 0) {
+    assert(!extended_);
+    const std::uint_fast8_t type_index = last.type_index;
+    Transition& tr(*transitions_.emplace(transitions_.end()));
+    tr.unix_time = 2147483647;  // 2038-01-19T03:14:07+00:00
+    tr.type_index = type_index;
+  }
 
   // Compute the local civil time for each transition and the preceding
   // second. These will be used for reverse conversions in MakeTime().
@@ -564,11 +887,13 @@ bool TimeZoneInfo::Load(const std::string& name, ZoneInfoSource* zip) {
     ttp = &transition_types_[tr.type_index];
     tr.civil_sec = LocalTime(tr.unix_time, *ttp).cs;
     if (i != 0) {
-      // Check that the transitions are ordered by civil time. Essentially
-      // this means that an offset change cannot cross another such change.
-      // No one does this in practice, and we depend on it in MakeTime().
-      if (!Transition::ByCivilTime()(transitions_[i - 1], tr))
+      // Check that offset changes don't cross each other. No one
+      // does this in practice, and we depend on increasing absolute
+      // and civil times in BreakTime() and MakeTime() respectively.
+      if (!Transition::ByUnixTime()(transitions_[i - 1], tr) ||
+          !Transition::ByCivilTime()(transitions_[i - 1], tr)) {
         return false;  // out of order
+      }
     }
   }
 
@@ -583,145 +908,6 @@ bool TimeZoneInfo::Load(const std::string& name, ZoneInfoSource* zip) {
   return true;
 }
 
-namespace {
-
-// fopen(3) adaptor.
-inline FILE* FOpen(const char* path, const char* mode) {
-#if defined(_MSC_VER)
-  FILE* fp;
-  if (fopen_s(&fp, path, mode) != 0) fp = nullptr;
-  return fp;
-#else
-  return fopen(path, mode);  // TODO: Enable the close-on-exec flag.
-#endif
-}
-
-// A stdio(3)-backed implementation of ZoneInfoSource.
-class FileZoneInfoSource : public ZoneInfoSource {
- public:
-  static std::unique_ptr<ZoneInfoSource> Open(const std::string& name);
-
-  std::size_t Read(void* ptr, std::size_t size) override {
-    size = std::min(size, len_);
-    std::size_t nread = fread(ptr, 1, size, fp_.get());
-    len_ -= nread;
-    return nread;
-  }
-  int Skip(std::size_t offset) override {
-    offset = std::min(offset, len_);
-    int rc = fseek(fp_.get(), static_cast<long>(offset), SEEK_CUR);
-    if (rc == 0) len_ -= offset;
-    return rc;
-  }
-  std::string Version() const override {
-    // TODO: It would nice if the zoneinfo data included the tzdb version.
-    return std::string();
-  }
-
- protected:
-  explicit FileZoneInfoSource(
-      FILE* fp, std::size_t len = std::numeric_limits<std::size_t>::max())
-      : fp_(fp, fclose), len_(len) {}
-
- private:
-  std::unique_ptr<FILE, int(*)(FILE*)> fp_;
-  std::size_t len_;
-};
-
-std::unique_ptr<ZoneInfoSource> FileZoneInfoSource::Open(
-    const std::string& name) {
-  // Use of the "file:" prefix is intended for testing purposes only.
-  const std::size_t pos = (name.compare(0, 5, "file:") == 0) ? 5 : 0;
-
-  // Map the time-zone name to a path name.
-  std::string path;
-  if (pos == name.size() || name[pos] != '/') {
-    const char* tzdir = "/usr/share/zoneinfo";
-    char* tzdir_env = nullptr;
-#if defined(_MSC_VER)
-    _dupenv_s(&tzdir_env, nullptr, "TZDIR");
-#else
-    tzdir_env = std::getenv("TZDIR");
-#endif
-    if (tzdir_env && *tzdir_env) tzdir = tzdir_env;
-    path += tzdir;
-    path += '/';
-#if defined(_MSC_VER)
-    free(tzdir_env);
-#endif
-  }
-  path.append(name, pos, std::string::npos);
-
-  // Open the zoneinfo file.
-  FILE* fp = FOpen(path.c_str(), "rb");
-  if (fp == nullptr) return nullptr;
-  std::size_t length = 0;
-  if (fseek(fp, 0, SEEK_END) == 0) {
-    long pos = ftell(fp);
-    if (pos >= 0) {
-      length = static_cast<std::size_t>(pos);
-    }
-    rewind(fp);
-  }
-  return std::unique_ptr<ZoneInfoSource>(new FileZoneInfoSource(fp, length));
-}
-
-class AndroidZoneInfoSource : public FileZoneInfoSource {
- public:
-  static std::unique_ptr<ZoneInfoSource> Open(const std::string& name);
-  std::string Version() const override { return version_; }
-
- private:
-  explicit AndroidZoneInfoSource(FILE* fp, std::size_t len, const char* vers)
-      : FileZoneInfoSource(fp, len), version_(vers) {}
-  std::string version_;
-};
-
-std::unique_ptr<ZoneInfoSource> AndroidZoneInfoSource::Open(
-    const std::string& name) {
-  // Use of the "file:" prefix is intended for testing purposes only.
-  const std::size_t pos = (name.compare(0, 5, "file:") == 0) ? 5 : 0;
-
-  // See Android's libc/tzcode/bionic.cpp for additional information.
-  for (const char* tzdata : {"/data/misc/zoneinfo/current/tzdata",
-                             "/system/usr/share/zoneinfo/tzdata"}) {
-    std::unique_ptr<FILE, int (*)(FILE*)> fp(FOpen(tzdata, "rb"), fclose);
-    if (fp.get() == nullptr) continue;
-
-    char hbuf[24];  // covers header.zonetab_offset too
-    if (fread(hbuf, 1, sizeof(hbuf), fp.get()) != sizeof(hbuf)) continue;
-    if (strncmp(hbuf, "tzdata", 6) != 0) continue;
-    const char* vers = (hbuf[11] == '\0') ? hbuf + 6 : "";
-    const std::int_fast32_t index_offset = Decode32(hbuf + 12);
-    const std::int_fast32_t data_offset = Decode32(hbuf + 16);
-    if (index_offset < 0 || data_offset < index_offset) continue;
-    if (fseek(fp.get(), static_cast<long>(index_offset), SEEK_SET) != 0)
-      continue;
-
-    char ebuf[52];  // covers entry.unused too
-    const std::size_t index_size =
-        static_cast<std::size_t>(data_offset - index_offset);
-    const std::size_t zonecnt = index_size / sizeof(ebuf);
-    if (zonecnt * sizeof(ebuf) != index_size) continue;
-    for (std::size_t i = 0; i != zonecnt; ++i) {
-      if (fread(ebuf, 1, sizeof(ebuf), fp.get()) != sizeof(ebuf)) break;
-      const std::int_fast32_t start = data_offset + Decode32(ebuf + 40);
-      const std::int_fast32_t length = Decode32(ebuf + 44);
-      if (start < 0 || length < 0) break;
-      ebuf[40] = '\0';  // ensure zone name is NUL terminated
-      if (strcmp(name.c_str() + pos, ebuf) == 0) {
-        if (fseek(fp.get(), static_cast<long>(start), SEEK_SET) != 0) break;
-        return std::unique_ptr<ZoneInfoSource>(new AndroidZoneInfoSource(
-            fp.release(), static_cast<std::size_t>(length), vers));
-      }
-    }
-  }
-
-  return nullptr;
-}
-
-}  // namespace
-
 bool TimeZoneInfo::Load(const std::string& name) {
   // We can ensure that the loading of UTC or any other fixed-offset
   // zone never fails because the simple, fixed-offset state can be
@@ -734,12 +920,25 @@ bool TimeZoneInfo::Load(const std::string& name) {
 
   // Find and use a ZoneInfoSource to load the named zone.
   auto zip = cctz_extension::zone_info_source_factory(
-      name, [](const std::string& name) -> std::unique_ptr<ZoneInfoSource> {
-        if (auto zip = FileZoneInfoSource::Open(name)) return zip;
-        if (auto zip = AndroidZoneInfoSource::Open(name)) return zip;
+      name, [](const std::string& n) -> std::unique_ptr<ZoneInfoSource> {
+        if (auto z = FileZoneInfoSource::Open(n)) return z;
+        if (auto z = AndroidZoneInfoSource::Open(n)) return z;
+        if (auto z = FuchsiaZoneInfoSource::Open(n)) return z;
         return nullptr;
       });
-  return zip != nullptr && Load(name, zip.get());
+  return zip != nullptr && Load(zip.get());
+}
+
+std::unique_ptr<TimeZoneInfo> TimeZoneInfo::UTC() {
+  auto tz = std::unique_ptr<TimeZoneInfo>(new TimeZoneInfo);
+  tz->ResetToBuiltinUTC(seconds::zero());
+  return tz;
+}
+
+std::unique_ptr<TimeZoneInfo> TimeZoneInfo::Make(const std::string& name) {
+  auto tz = std::unique_ptr<TimeZoneInfo>(new TimeZoneInfo);
+  if (!tz->Load(name)) tz.reset();  // fallback to UTC
+  return tz;
 }
 
 // BreakTime() translation for a particular transition type.
@@ -748,13 +947,13 @@ time_zone::absolute_lookup TimeZoneInfo::LocalTime(
   // A civil time in "+offset" looks like (time+offset) in UTC.
   // Note: We perform two additions in the civil_second domain to
   // sidestep the chance of overflow in (unix_time + tt.utc_offset).
-  return {(civil_second() + unix_time) + tt.utc_offset,
-          tt.utc_offset, tt.is_dst, &abbreviations_[tt.abbr_index]};
+  return {(civil_second() + unix_time) + tt.utc_offset, tt.utc_offset,
+          tt.is_dst, &abbreviations_[tt.abbr_index]};
 }
 
 // BreakTime() translation for a particular transition.
-time_zone::absolute_lookup TimeZoneInfo::LocalTime(
-    std::int_fast64_t unix_time, const Transition& tr) const {
+time_zone::absolute_lookup TimeZoneInfo::LocalTime(std::int_fast64_t unix_time,
+                                                   const Transition& tr) const {
   const TransitionType& tt = transition_types_[tr.type_index];
   // Note: (unix_time - tr.unix_time) will never overflow as we
   // have ensured that there is always a "nearby" transition.
@@ -897,9 +1096,7 @@ time_zone::civil_lookup TimeZoneInfo::MakeTime(const civil_second& cs) const {
   return MakeUnique(tr->unix_time + (cs - tr->civil_sec));
 }
 
-std::string TimeZoneInfo::Version() const {
-  return version_;
-}
+std::string TimeZoneInfo::Version() const { return version_; }
 
 std::string TimeZoneInfo::Description() const {
   std::ostringstream oss;
@@ -915,14 +1112,14 @@ bool TimeZoneInfo::NextTransition(const time_point<seconds>& tp,
   const Transition* begin = &transitions_[0];
   const Transition* end = begin + transitions_.size();
   if (begin->unix_time <= -(1LL << 59)) {
-    // Do not report the BIG_BANG found in recent zoneinfo data as it is
-    // really a sentinel, not a transition.  See tz/zic.c.
+    // Do not report the BIG_BANG found in some zoneinfo data as it is
+    // really a sentinel, not a transition.  See pre-2018f tz/zic.c.
     ++begin;
   }
   std::int_fast64_t unix_time = ToUnixSeconds(tp);
   const Transition target = {unix_time, 0, civil_second(), civil_second()};
-  const Transition* tr = std::upper_bound(begin, end, target,
-                                          Transition::ByUnixTime());
+  const Transition* tr =
+      std::upper_bound(begin, end, target, Transition::ByUnixTime());
   for (; tr != end; ++tr) {  // skip no-op transitions
     std::uint_fast8_t prev_type_index =
         (tr == begin) ? default_transition_type_ : tr[-1].type_index;
@@ -941,8 +1138,8 @@ bool TimeZoneInfo::PrevTransition(const time_point<seconds>& tp,
   const Transition* begin = &transitions_[0];
   const Transition* end = begin + transitions_.size();
   if (begin->unix_time <= -(1LL << 59)) {
-    // Do not report the BIG_BANG found in recent zoneinfo data as it is
-    // really a sentinel, not a transition.  See tz/zic.c.
+    // Do not report the BIG_BANG found in some zoneinfo data as it is
+    // really a sentinel, not a transition.  See pre-2018f tz/zic.c.
     ++begin;
   }
   std::int_fast64_t unix_time = ToUnixSeconds(tp);
@@ -956,8 +1153,8 @@ bool TimeZoneInfo::PrevTransition(const time_point<seconds>& tp,
     unix_time += 1;  // ceils
   }
   const Transition target = {unix_time, 0, civil_second(), civil_second()};
-  const Transition* tr = std::lower_bound(begin, end, target,
-                                          Transition::ByUnixTime());
+  const Transition* tr =
+      std::lower_bound(begin, end, target, Transition::ByUnixTime());
   for (; tr != begin; --tr) {  // skip no-op transitions
     std::uint_fast8_t prev_type_index =
         (tr - 1 == begin) ? default_transition_type_ : tr[-2].type_index;
@@ -972,4 +1169,5 @@ bool TimeZoneInfo::PrevTransition(const time_point<seconds>& tp,
 
 }  // namespace cctz
 }  // namespace time_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/time/internal/cctz/src/time_zone_info.h b/absl/time/internal/cctz/src/time_zone_info.h
index 81cd402f972..efe2f9cbfb6 100644
--- a/absl/time/internal/cctz/src/time_zone_info.h
+++ b/absl/time/internal/cctz/src/time_zone_info.h
@@ -18,16 +18,19 @@
 #include <atomic>
 #include <cstddef>
 #include <cstdint>
+#include <memory>
 #include <string>
 #include <vector>
 
+#include "absl/base/config.h"
 #include "absl/time/internal/cctz/include/cctz/civil_time.h"
 #include "absl/time/internal/cctz/include/cctz/time_zone.h"
 #include "absl/time/internal/cctz/include/cctz/zone_info_source.h"
-#include "time_zone_if.h"
-#include "tzfile.h"
+#include "absl/time/internal/cctz/src/time_zone_if.h"
+#include "absl/time/internal/cctz/src/tzfile.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 namespace cctz {
 
@@ -62,18 +65,14 @@ struct TransitionType {
 // A time zone backed by the IANA Time Zone Database (zoneinfo).
 class TimeZoneInfo : public TimeZoneIf {
  public:
-  TimeZoneInfo() = default;
-  TimeZoneInfo(const TimeZoneInfo&) = delete;
-  TimeZoneInfo& operator=(const TimeZoneInfo&) = delete;
-
-  // Loads the zoneinfo for the given name, returning true if successful.
-  bool Load(const std::string& name);
+  // Factories.
+  static std::unique_ptr<TimeZoneInfo> UTC();  // never fails
+  static std::unique_ptr<TimeZoneInfo> Make(const std::string& name);
 
   // TimeZoneIf implementations.
   time_zone::absolute_lookup BreakTime(
       const time_point<seconds>& tp) const override;
-  time_zone::civil_lookup MakeTime(
-      const civil_second& cs) const override;
+  time_zone::civil_lookup MakeTime(const civil_second& cs) const override;
   bool NextTransition(const time_point<seconds>& tp,
                       time_zone::civil_transition* trans) const override;
   bool PrevTransition(const time_point<seconds>& tp,
@@ -82,27 +81,19 @@ class TimeZoneInfo : public TimeZoneIf {
   std::string Description() const override;
 
  private:
-  struct Header {  // counts of:
-    std::size_t timecnt;     // transition times
-    std::size_t typecnt;     // transition types
-    std::size_t charcnt;     // zone abbreviation characters
-    std::size_t leapcnt;     // leap seconds (we expect none)
-    std::size_t ttisstdcnt;  // UTC/local indicators (unused)
-    std::size_t ttisutcnt;   // standard/wall indicators (unused)
-
-    bool Build(const tzhead& tzh);
-    std::size_t DataLength(std::size_t time_len) const;
-  };
+  TimeZoneInfo() = default;
+  TimeZoneInfo(const TimeZoneInfo&) = delete;
+  TimeZoneInfo& operator=(const TimeZoneInfo&) = delete;
 
-  void CheckTransition(const std::string& name, const TransitionType& tt,
-                       std::int_fast32_t offset, bool is_dst,
-                       const std::string& abbr) const;
+  bool GetTransitionType(std::int_fast32_t utc_offset, bool is_dst,
+                         const std::string& abbr, std::uint_least8_t* index);
   bool EquivTransitions(std::uint_fast8_t tt1_index,
                         std::uint_fast8_t tt2_index) const;
-  void ExtendTransitions(const std::string& name, const Header& hdr);
+  bool ExtendTransitions();
 
   bool ResetToBuiltinUTC(const seconds& offset);
-  bool Load(const std::string& name, ZoneInfoSource* zip);
+  bool Load(const std::string& name);
+  bool Load(ZoneInfoSource* zip);
 
   // Helpers for BreakTime() and MakeTime().
   time_zone::absolute_lookup LocalTime(std::int_fast64_t unix_time,
@@ -114,7 +105,7 @@ class TimeZoneInfo : public TimeZoneIf {
 
   std::vector<Transition> transitions_;  // ordered by unix_time and civil_sec
   std::vector<TransitionType> transition_types_;  // distinct transition types
-  std::uint_fast8_t default_transition_type_;  // for before first transition
+  std::uint_fast8_t default_transition_type_;     // for before first transition
   std::string abbreviations_;  // all the NUL-terminated abbreviations
 
   std::string version_;      // the tzdata version if available
@@ -131,6 +122,7 @@ class TimeZoneInfo : public TimeZoneIf {
 
 }  // namespace cctz
 }  // namespace time_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_TIME_INTERNAL_CCTZ_TIME_ZONE_INFO_H_
diff --git a/absl/time/internal/cctz/src/time_zone_libc.cc b/absl/time/internal/cctz/src/time_zone_libc.cc
index 6095e764df0..362da1ab58b 100644
--- a/absl/time/internal/cctz/src/time_zone_libc.cc
+++ b/absl/time/internal/cctz/src/time_zone_libc.cc
@@ -12,21 +12,29 @@
 //   See the License for the specific language governing permissions and
 //   limitations under the License.
 
-#if defined(_WIN32) || defined(_WIN64)
+#if !defined(_CRT_SECURE_NO_WARNINGS) && defined(_WIN32)
 #define _CRT_SECURE_NO_WARNINGS 1
 #endif
 
-#include "time_zone_libc.h"
+#include "absl/time/internal/cctz/src/time_zone_libc.h"
 
 #include <chrono>
 #include <ctime>
 #include <limits>
 #include <utility>
 
+#include "absl/base/config.h"
 #include "absl/time/internal/cctz/include/cctz/civil_time.h"
 #include "absl/time/internal/cctz/include/cctz/time_zone.h"
 
+#if defined(_AIX)
+extern "C" {
+extern long altzone;
+}
+#endif
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 namespace cctz {
 
@@ -42,7 +50,7 @@ auto tm_zone(const std::tm& tm) -> decltype(_tzname[0]) {
   const bool is_dst = tm.tm_isdst > 0;
   return _tzname[is_dst];
 }
-#elif defined(__sun)
+#elif defined(__sun) || defined(_AIX)
 // Uses the globals: 'timezone', 'altzone' and 'tzname'.
 auto tm_gmtoff(const std::tm& tm) -> decltype(timezone) {
   const bool is_dst = tm.tm_isdst > 0;
@@ -54,7 +62,7 @@ auto tm_zone(const std::tm& tm) -> decltype(tzname[0]) {
 }
 #elif defined(__native_client__) || defined(__myriad2__) || \
     defined(__EMSCRIPTEN__)
-// Uses the globals: 'timezone' and 'tzname'.
+// Uses the globals: '_timezone' and 'tzname'.
 auto tm_gmtoff(const std::tm& tm) -> decltype(_timezone + 0) {
   const bool is_dst = tm.tm_isdst > 0;
   return _timezone + (is_dst ? 60 * 60 : 0);
@@ -63,6 +71,16 @@ auto tm_zone(const std::tm& tm) -> decltype(tzname[0]) {
   const bool is_dst = tm.tm_isdst > 0;
   return tzname[is_dst];
 }
+#elif defined(__VXWORKS__)
+// Uses the globals: 'timezone' and 'tzname'.
+auto tm_gmtoff(const std::tm& tm) -> decltype(timezone + 0) {
+  const bool is_dst = tm.tm_isdst > 0;
+  return timezone + (is_dst ? 60 * 60 : 0);
+}
+auto tm_zone(const std::tm& tm) -> decltype(tzname[0]) {
+  const bool is_dst = tm.tm_isdst > 0;
+  return tzname[is_dst];
+}
 #else
 // Adapt to different spellings of the struct std::tm extension fields.
 #if defined(tm_gmtoff)
@@ -84,9 +102,7 @@ auto tm_gmtoff(const T& tm) -> decltype(tm.__tm_gmtoff) {
 }
 #endif  // tm_gmtoff
 #if defined(tm_zone)
-auto tm_zone(const std::tm& tm) -> decltype(tm.tm_zone) {
-  return tm.tm_zone;
-}
+auto tm_zone(const std::tm& tm) -> decltype(tm.tm_zone) { return tm.tm_zone; }
 #elif defined(__tm_zone)
 auto tm_zone(const std::tm& tm) -> decltype(tm.__tm_zone) {
   return tm.__tm_zone;
@@ -102,58 +118,59 @@ auto tm_zone(const T& tm) -> decltype(tm.__tm_zone) {
 }
 #endif  // tm_zone
 #endif
+using tm_gmtoff_t = decltype(tm_gmtoff(std::tm{}));
 
-inline std::tm* gm_time(const std::time_t *timep, std::tm *result) {
+inline std::tm* gm_time(const std::time_t* timep, std::tm* result) {
 #if defined(_WIN32) || defined(_WIN64)
-    return gmtime_s(result, timep) ? nullptr : result;
+  return gmtime_s(result, timep) ? nullptr : result;
 #else
-    return gmtime_r(timep, result);
+  return gmtime_r(timep, result);
 #endif
 }
 
-inline std::tm* local_time(const std::time_t *timep, std::tm *result) {
+inline std::tm* local_time(const std::time_t* timep, std::tm* result) {
 #if defined(_WIN32) || defined(_WIN64)
-    return localtime_s(result, timep) ? nullptr : result;
+  return localtime_s(result, timep) ? nullptr : result;
 #else
-    return localtime_r(timep, result);
+  return localtime_r(timep, result);
 #endif
 }
 
-// Converts a civil second and "dst" flag into a time_t and UTC offset.
+// Converts a civil second and "dst" flag into a time_t and a struct tm.
 // Returns false if time_t cannot represent the requested civil second.
 // Caller must have already checked that cs.year() will fit into a tm_year.
-bool make_time(const civil_second& cs, int is_dst, std::time_t* t, int* off) {
-  std::tm tm;
-  tm.tm_year = static_cast<int>(cs.year() - year_t{1900});
-  tm.tm_mon = cs.month() - 1;
-  tm.tm_mday = cs.day();
-  tm.tm_hour = cs.hour();
-  tm.tm_min = cs.minute();
-  tm.tm_sec = cs.second();
-  tm.tm_isdst = is_dst;
-  *t = std::mktime(&tm);
+bool make_time(const civil_second& cs, int is_dst, std::time_t* t,
+               std::tm* tm) {
+  tm->tm_year = static_cast<int>(cs.year() - year_t{1900});
+  tm->tm_mon = cs.month() - 1;
+  tm->tm_mday = cs.day();
+  tm->tm_hour = cs.hour();
+  tm->tm_min = cs.minute();
+  tm->tm_sec = cs.second();
+  tm->tm_isdst = is_dst;
+  *t = std::mktime(tm);
   if (*t == std::time_t{-1}) {
     std::tm tm2;
     const std::tm* tmp = local_time(t, &tm2);
-    if (tmp == nullptr || tmp->tm_year != tm.tm_year ||
-        tmp->tm_mon != tm.tm_mon || tmp->tm_mday != tm.tm_mday ||
-        tmp->tm_hour != tm.tm_hour || tmp->tm_min != tm.tm_min ||
-        tmp->tm_sec != tm.tm_sec) {
+    if (tmp == nullptr || tmp->tm_year != tm->tm_year ||
+        tmp->tm_mon != tm->tm_mon || tmp->tm_mday != tm->tm_mday ||
+        tmp->tm_hour != tm->tm_hour || tmp->tm_min != tm->tm_min ||
+        tmp->tm_sec != tm->tm_sec) {
       // A true error (not just one second before the epoch).
       return false;
     }
   }
-  *off = static_cast<int>(tm_gmtoff(tm));
   return true;
 }
 
 // Find the least time_t in [lo:hi] where local time matches offset, given:
 // (1) lo doesn't match, (2) hi does, and (3) there is only one transition.
-std::time_t find_trans(std::time_t lo, std::time_t hi, int offset) {
+std::time_t find_trans(std::time_t lo, std::time_t hi, tm_gmtoff_t offset) {
   std::tm tm;
   while (lo + 1 != hi) {
     const std::time_t mid = lo + (hi - lo) / 2;
-    if (std::tm* tmp = local_time(&mid, &tm)) {
+    std::tm* tmp = local_time(&mid, &tm);
+    if (tmp != nullptr) {
       if (tm_gmtoff(*tmp) == offset) {
         hi = mid;
       } else {
@@ -163,7 +180,8 @@ std::time_t find_trans(std::time_t lo, std::time_t hi, int offset) {
       // If std::tm cannot hold some result we resort to a linear search,
       // ignoring all failed conversions.  Slow, but never really happens.
       while (++lo != hi) {
-        if (std::tm* tmp = local_time(&lo, &tm)) {
+        tmp = local_time(&lo, &tm);
+        if (tmp != nullptr) {
           if (tm_gmtoff(*tmp) == offset) break;
         }
       }
@@ -175,8 +193,9 @@ std::time_t find_trans(std::time_t lo, std::time_t hi, int offset) {
 
 }  // namespace
 
-TimeZoneLibC::TimeZoneLibC(const std::string& name)
-    : local_(name == "localtime") {}
+std::unique_ptr<TimeZoneLibC> TimeZoneLibC::Make(const std::string& name) {
+  return std::unique_ptr<TimeZoneLibC>(new TimeZoneLibC(name));
+}
 
 time_zone::absolute_lookup TimeZoneLibC::BreakTime(
     const time_point<seconds>& tp) const {
@@ -208,8 +227,8 @@ time_zone::absolute_lookup TimeZoneLibC::BreakTime(
   }
 
   const year_t year = tmp->tm_year + year_t{1900};
-  al.cs = civil_second(year, tmp->tm_mon + 1, tmp->tm_mday,
-                       tmp->tm_hour, tmp->tm_min, tmp->tm_sec);
+  al.cs = civil_second(year, tmp->tm_mon + 1, tmp->tm_mday, tmp->tm_hour,
+                       tmp->tm_min, tmp->tm_sec);
   al.offset = static_cast<int>(tm_gmtoff(*tmp));
   al.abbr = local_ ? tm_zone(*tmp) : "UTC";  // as expected by cctz
   al.is_dst = tmp->tm_isdst > 0;
@@ -223,11 +242,10 @@ time_zone::civil_lookup TimeZoneLibC::MakeTime(const civil_second& cs) const {
         civil_second() + ToUnixSeconds(time_point<seconds>::min());
     static const civil_second max_tp_cs =
         civil_second() + ToUnixSeconds(time_point<seconds>::max());
-    const time_point<seconds> tp =
-        (cs < min_tp_cs)
-            ? time_point<seconds>::min()
-            : (cs > max_tp_cs) ? time_point<seconds>::max()
-                               : FromUnixSeconds(cs - civil_second());
+    const time_point<seconds> tp = (cs < min_tp_cs) ? time_point<seconds>::min()
+                                   : (cs > max_tp_cs)
+                                       ? time_point<seconds>::max()
+                                       : FromUnixSeconds(cs - civil_second());
     return {time_zone::civil_lookup::UNIQUE, tp, tp, tp};
   }
 
@@ -247,33 +265,37 @@ time_zone::civil_lookup TimeZoneLibC::MakeTime(const civil_second& cs) const {
   // We probe with "is_dst" values of 0 and 1 to try to distinguish unique
   // civil seconds from skipped or repeated ones.  This is not always possible
   // however, as the "dst" flag does not change over some offset transitions.
-  // We are also subject to the vagaries of mktime() implementations.
+  // We are also subject to the vagaries of mktime() implementations. For
+  // example, some implementations treat "tm_isdst" as a demand (useless),
+  // and some as a disambiguator (useful).
   std::time_t t0, t1;
-  int offset0, offset1;
-  if (make_time(cs, 0, &t0, &offset0) && make_time(cs, 1, &t1, &offset1)) {
-    if (t0 == t1) {
+  std::tm tm0, tm1;
+  if (make_time(cs, 0, &t0, &tm0) && make_time(cs, 1, &t1, &tm1)) {
+    if (tm0.tm_isdst == tm1.tm_isdst) {
       // The civil time was singular (pre == trans == post).
-      const time_point<seconds> tp = FromUnixSeconds(t0);
+      const time_point<seconds> tp = FromUnixSeconds(tm0.tm_isdst ? t1 : t0);
       return {time_zone::civil_lookup::UNIQUE, tp, tp, tp};
     }
 
-    if (t0 > t1) {
+    tm_gmtoff_t offset = tm_gmtoff(tm0);
+    if (t0 < t1) {  // negative DST
       std::swap(t0, t1);
-      std::swap(offset0, offset1);
+      offset = tm_gmtoff(tm1);
     }
-    const std::time_t tt = find_trans(t0, t1, offset1);
+
+    const std::time_t tt = find_trans(t1, t0, offset);
     const time_point<seconds> trans = FromUnixSeconds(tt);
 
-    if (offset0 < offset1) {
+    if (tm0.tm_isdst) {
       // The civil time did not exist (pre >= trans > post).
-      const time_point<seconds> pre = FromUnixSeconds(t1);
-      const time_point<seconds> post = FromUnixSeconds(t0);
+      const time_point<seconds> pre = FromUnixSeconds(t0);
+      const time_point<seconds> post = FromUnixSeconds(t1);
       return {time_zone::civil_lookup::SKIPPED, pre, trans, post};
     }
 
     // The civil time was ambiguous (pre < trans <= post).
-    const time_point<seconds> pre = FromUnixSeconds(t0);
-    const time_point<seconds> post = FromUnixSeconds(t1);
+    const time_point<seconds> pre = FromUnixSeconds(t1);
+    const time_point<seconds> post = FromUnixSeconds(t0);
     return {time_zone::civil_lookup::REPEATED, pre, trans, post};
   }
 
@@ -302,6 +324,10 @@ std::string TimeZoneLibC::Description() const {
   return local_ ? "localtime" : "UTC";
 }
 
+TimeZoneLibC::TimeZoneLibC(const std::string& name)
+    : local_(name == "localtime") {}
+
 }  // namespace cctz
 }  // namespace time_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/time/internal/cctz/src/time_zone_libc.h b/absl/time/internal/cctz/src/time_zone_libc.h
index 0d18e9a7f3d..f1ed5cd08f1 100644
--- a/absl/time/internal/cctz/src/time_zone_libc.h
+++ b/absl/time/internal/cctz/src/time_zone_libc.h
@@ -15,26 +15,28 @@
 #ifndef ABSL_TIME_INTERNAL_CCTZ_TIME_ZONE_LIBC_H_
 #define ABSL_TIME_INTERNAL_CCTZ_TIME_ZONE_LIBC_H_
 
+#include <memory>
 #include <string>
 
-#include "time_zone_if.h"
+#include "absl/base/config.h"
+#include "absl/time/internal/cctz/src/time_zone_if.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 namespace cctz {
 
 // A time zone backed by gmtime_r(3), localtime_r(3), and mktime(3),
 // and which therefore only supports UTC and the local time zone.
-// TODO: Add support for fixed offsets from UTC.
 class TimeZoneLibC : public TimeZoneIf {
  public:
-  explicit TimeZoneLibC(const std::string& name);
+  // Factory.
+  static std::unique_ptr<TimeZoneLibC> Make(const std::string& name);
 
   // TimeZoneIf implementations.
   time_zone::absolute_lookup BreakTime(
       const time_point<seconds>& tp) const override;
-  time_zone::civil_lookup MakeTime(
-      const civil_second& cs) const override;
+  time_zone::civil_lookup MakeTime(const civil_second& cs) const override;
   bool NextTransition(const time_point<seconds>& tp,
                       time_zone::civil_transition* trans) const override;
   bool PrevTransition(const time_point<seconds>& tp,
@@ -43,11 +45,16 @@ class TimeZoneLibC : public TimeZoneIf {
   std::string Description() const override;
 
  private:
+  explicit TimeZoneLibC(const std::string& name);
+  TimeZoneLibC(const TimeZoneLibC&) = delete;
+  TimeZoneLibC& operator=(const TimeZoneLibC&) = delete;
+
   const bool local_;  // localtime or UTC
 };
 
 }  // namespace cctz
 }  // namespace time_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_TIME_INTERNAL_CCTZ_TIME_ZONE_LIBC_H_
diff --git a/absl/time/internal/cctz/src/time_zone_lookup.cc b/absl/time/internal/cctz/src/time_zone_lookup.cc
index 3c53dd1ade9..d1078de433e 100644
--- a/absl/time/internal/cctz/src/time_zone_lookup.cc
+++ b/absl/time/internal/cctz/src/time_zone_lookup.cc
@@ -12,63 +12,45 @@
 //   See the License for the specific language governing permissions and
 //   limitations under the License.
 
+#include "absl/base/config.h"
 #include "absl/time/internal/cctz/include/cctz/time_zone.h"
 
 #if defined(__ANDROID__)
 #include <sys/system_properties.h>
-#if defined(__ANDROID_API__) && __ANDROID_API__ >= 21
-#include <dlfcn.h>
-#endif
 #endif
 
 #if defined(__APPLE__)
 #include <CoreFoundation/CFTimeZone.h>
+
 #include <vector>
 #endif
 
+#if defined(__Fuchsia__)
+#include <fuchsia/intl/cpp/fidl.h>
+#include <lib/async-loop/cpp/loop.h>
+#include <lib/fdio/directory.h>
+#include <zircon/types.h>
+#endif
+
+#include <array>
+#include <cstdint>
 #include <cstdlib>
 #include <cstring>
 #include <string>
 
-#include "time_zone_fixed.h"
-#include "time_zone_impl.h"
+#include "absl/time/internal/cctz/src/time_zone_fixed.h"
+#include "absl/time/internal/cctz/src/time_zone_impl.h"
+
+#if defined(_WIN32)
+#include "absl/time/internal/cctz/src/time_zone_name_win.h"
+#endif  // _WIN32
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 namespace cctz {
 
-#if defined(__ANDROID__) && defined(__ANDROID_API__) && __ANDROID_API__ >= 21
-namespace {
-// Android 'L' removes __system_property_get() from the NDK, however
-// it is still a hidden symbol in libc so we use dlsym() to access it.
-// See Chromium's base/sys_info_android.cc for a similar example.
-
-using property_get_func = int (*)(const char*, char*);
-
-property_get_func LoadSystemPropertyGet() {
-  int flag = RTLD_LAZY | RTLD_GLOBAL;
-#if defined(RTLD_NOLOAD)
-  flag |= RTLD_NOLOAD;  // libc.so should already be resident
-#endif
-  if (void* handle = dlopen("libc.so", flag)) {
-    void* sym = dlsym(handle, "__system_property_get");
-    dlclose(handle);
-    return reinterpret_cast<property_get_func>(sym);
-  }
-  return nullptr;
-}
-
-int __system_property_get(const char* name, char* value) {
-  static property_get_func system_property_get = LoadSystemPropertyGet();
-  return system_property_get ? system_property_get(name, value) : -1;
-}
-
-}  // namespace
-#endif
-
-std::string time_zone::name() const {
-  return effective_impl().Name();
-}
+std::string time_zone::name() const { return effective_impl().Name(); }
 
 time_zone::absolute_lookup time_zone::lookup(
     const time_point<seconds>& tp) const {
@@ -89,9 +71,7 @@ bool time_zone::prev_transition(const time_point<seconds>& tp,
   return effective_impl().PrevTransition(tp, trans);
 }
 
-std::string time_zone::version() const {
-  return effective_impl().Version();
-}
+std::string time_zone::version() const { return effective_impl().Version(); }
 
 std::string time_zone::description() const {
   return effective_impl().Description();
@@ -134,13 +114,62 @@ time_zone local_time_zone() {
   if (CFStringRef tz_name = CFTimeZoneGetName(tz_default)) {
     CFStringEncoding encoding = kCFStringEncodingUTF8;
     CFIndex length = CFStringGetLength(tz_name);
-    buffer.resize(CFStringGetMaximumSizeForEncoding(length, encoding) + 1);
-    if (CFStringGetCString(tz_name, &buffer[0], buffer.size(), encoding)) {
+    CFIndex max_size = CFStringGetMaximumSizeForEncoding(length, encoding) + 1;
+    buffer.resize(static_cast<size_t>(max_size));
+    if (CFStringGetCString(tz_name, &buffer[0], max_size, encoding)) {
       zone = &buffer[0];
     }
   }
   CFRelease(tz_default);
 #endif
+#if defined(__Fuchsia__)
+  std::string primary_tz;
+  [&]() {
+    // Note: We can't use the synchronous FIDL API here because it doesn't
+    // allow timeouts; if the FIDL call failed, local_time_zone() would never
+    // return.
+
+    const zx::duration kTimeout = zx::msec(500);
+
+    // Don't attach to the thread because otherwise the thread's dispatcher
+    // would be set to null when the loop is destroyed, causing any other FIDL
+    // code running on the same thread to crash.
+    async::Loop loop(&kAsyncLoopConfigNeverAttachToThread);
+
+    fuchsia::intl::PropertyProviderHandle handle;
+    zx_status_t status = fdio_service_connect_by_name(
+        fuchsia::intl::PropertyProvider::Name_,
+        handle.NewRequest().TakeChannel().release());
+    if (status != ZX_OK) {
+      return;
+    }
+
+    fuchsia::intl::PropertyProviderPtr intl_provider;
+    status = intl_provider.Bind(std::move(handle), loop.dispatcher());
+    if (status != ZX_OK) {
+      return;
+    }
+
+    intl_provider->GetProfile(
+        [&loop, &primary_tz](fuchsia::intl::Profile profile) {
+          if (!profile.time_zones().empty()) {
+            primary_tz = profile.time_zones()[0].id;
+          }
+          loop.Quit();
+        });
+    loop.Run(zx::deadline_after(kTimeout));
+  }();
+
+  if (!primary_tz.empty()) {
+    zone = primary_tz.c_str();
+  }
+#endif
+#if defined(_WIN32)
+  std::string win32_tz = GetWindowsLocalTimeZone();
+  if (!win32_tz.empty()) {
+    zone = win32_tz.c_str();
+  }
+#endif
 
   // Allow ${TZ} to override to default zone.
   char* tz_env = nullptr;
@@ -184,4 +213,5 @@ time_zone local_time_zone() {
 
 }  // namespace cctz
 }  // namespace time_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/time/internal/cctz/src/time_zone_lookup_test.cc b/absl/time/internal/cctz/src/time_zone_lookup_test.cc
index 42dd6d5512f..f53ac03eaf8 100644
--- a/absl/time/internal/cctz/src/time_zone_lookup_test.cc
+++ b/absl/time/internal/cctz/src/time_zone_lookup_test.cc
@@ -12,625 +12,40 @@
 //   See the License for the specific language governing permissions and
 //   limitations under the License.
 
-#include "absl/time/internal/cctz/include/cctz/time_zone.h"
-
 #include <chrono>
 #include <cstddef>
 #include <cstdlib>
+#include <cstring>
 #include <future>
 #include <limits>
+#include <memory>
 #include <string>
 #include <thread>
+#include <utility>
 #include <vector>
 
-#include "absl/time/internal/cctz/include/cctz/civil_time.h"
+#include "absl/base/config.h"
+#include "absl/time/internal/cctz/include/cctz/time_zone.h"
+
+#if defined(__linux__)
+#include <features.h>
+#endif
+
 #include "gtest/gtest.h"
+#include "absl/time/internal/cctz/include/cctz/civil_time.h"
+#include "absl/time/internal/cctz/include/cctz/zone_info_source.h"
+#include "absl/time/internal/cctz/src/test_time_zone_names.h"
+#include "absl/time/internal/cctz/src/tzfile.h"
 
 namespace chrono = std::chrono;
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 namespace cctz {
 
 namespace {
 
-// A list of known time-zone names.
-const char* const kTimeZoneNames[] = {
-  "Africa/Abidjan",
-  "Africa/Accra",
-  "Africa/Addis_Ababa",
-  "Africa/Algiers",
-  "Africa/Asmara",
-  "Africa/Asmera",
-  "Africa/Bamako",
-  "Africa/Bangui",
-  "Africa/Banjul",
-  "Africa/Bissau",
-  "Africa/Blantyre",
-  "Africa/Brazzaville",
-  "Africa/Bujumbura",
-  "Africa/Cairo",
-  "Africa/Casablanca",
-  "Africa/Ceuta",
-  "Africa/Conakry",
-  "Africa/Dakar",
-  "Africa/Dar_es_Salaam",
-  "Africa/Djibouti",
-  "Africa/Douala",
-  "Africa/El_Aaiun",
-  "Africa/Freetown",
-  "Africa/Gaborone",
-  "Africa/Harare",
-  "Africa/Johannesburg",
-  "Africa/Juba",
-  "Africa/Kampala",
-  "Africa/Khartoum",
-  "Africa/Kigali",
-  "Africa/Kinshasa",
-  "Africa/Lagos",
-  "Africa/Libreville",
-  "Africa/Lome",
-  "Africa/Luanda",
-  "Africa/Lubumbashi",
-  "Africa/Lusaka",
-  "Africa/Malabo",
-  "Africa/Maputo",
-  "Africa/Maseru",
-  "Africa/Mbabane",
-  "Africa/Mogadishu",
-  "Africa/Monrovia",
-  "Africa/Nairobi",
-  "Africa/Ndjamena",
-  "Africa/Niamey",
-  "Africa/Nouakchott",
-  "Africa/Ouagadougou",
-  "Africa/Porto-Novo",
-  "Africa/Sao_Tome",
-  "Africa/Timbuktu",
-  "Africa/Tripoli",
-  "Africa/Tunis",
-  "Africa/Windhoek",
-  "America/Adak",
-  "America/Anchorage",
-  "America/Anguilla",
-  "America/Antigua",
-  "America/Araguaina",
-  "America/Argentina/Buenos_Aires",
-  "America/Argentina/Catamarca",
-  "America/Argentina/ComodRivadavia",
-  "America/Argentina/Cordoba",
-  "America/Argentina/Jujuy",
-  "America/Argentina/La_Rioja",
-  "America/Argentina/Mendoza",
-  "America/Argentina/Rio_Gallegos",
-  "America/Argentina/Salta",
-  "America/Argentina/San_Juan",
-  "America/Argentina/San_Luis",
-  "America/Argentina/Tucuman",
-  "America/Argentina/Ushuaia",
-  "America/Aruba",
-  "America/Asuncion",
-  "America/Atikokan",
-  "America/Atka",
-  "America/Bahia",
-  "America/Bahia_Banderas",
-  "America/Barbados",
-  "America/Belem",
-  "America/Belize",
-  "America/Blanc-Sablon",
-  "America/Boa_Vista",
-  "America/Bogota",
-  "America/Boise",
-  "America/Buenos_Aires",
-  "America/Cambridge_Bay",
-  "America/Campo_Grande",
-  "America/Cancun",
-  "America/Caracas",
-  "America/Catamarca",
-  "America/Cayenne",
-  "America/Cayman",
-  "America/Chicago",
-  "America/Chihuahua",
-  "America/Coral_Harbour",
-  "America/Cordoba",
-  "America/Costa_Rica",
-  "America/Creston",
-  "America/Cuiaba",
-  "America/Curacao",
-  "America/Danmarkshavn",
-  "America/Dawson",
-  "America/Dawson_Creek",
-  "America/Denver",
-  "America/Detroit",
-  "America/Dominica",
-  "America/Edmonton",
-  "America/Eirunepe",
-  "America/El_Salvador",
-  "America/Ensenada",
-  "America/Fort_Nelson",
-  "America/Fort_Wayne",
-  "America/Fortaleza",
-  "America/Glace_Bay",
-  "America/Godthab",
-  "America/Goose_Bay",
-  "America/Grand_Turk",
-  "America/Grenada",
-  "America/Guadeloupe",
-  "America/Guatemala",
-  "America/Guayaquil",
-  "America/Guyana",
-  "America/Halifax",
-  "America/Havana",
-  "America/Hermosillo",
-  "America/Indiana/Indianapolis",
-  "America/Indiana/Knox",
-  "America/Indiana/Marengo",
-  "America/Indiana/Petersburg",
-  "America/Indiana/Tell_City",
-  "America/Indiana/Vevay",
-  "America/Indiana/Vincennes",
-  "America/Indiana/Winamac",
-  "America/Indianapolis",
-  "America/Inuvik",
-  "America/Iqaluit",
-  "America/Jamaica",
-  "America/Jujuy",
-  "America/Juneau",
-  "America/Kentucky/Louisville",
-  "America/Kentucky/Monticello",
-  "America/Knox_IN",
-  "America/Kralendijk",
-  "America/La_Paz",
-  "America/Lima",
-  "America/Los_Angeles",
-  "America/Louisville",
-  "America/Lower_Princes",
-  "America/Maceio",
-  "America/Managua",
-  "America/Manaus",
-  "America/Marigot",
-  "America/Martinique",
-  "America/Matamoros",
-  "America/Mazatlan",
-  "America/Mendoza",
-  "America/Menominee",
-  "America/Merida",
-  "America/Metlakatla",
-  "America/Mexico_City",
-  "America/Miquelon",
-  "America/Moncton",
-  "America/Monterrey",
-  "America/Montevideo",
-  "America/Montreal",
-  "America/Montserrat",
-  "America/Nassau",
-  "America/New_York",
-  "America/Nipigon",
-  "America/Nome",
-  "America/Noronha",
-  "America/North_Dakota/Beulah",
-  "America/North_Dakota/Center",
-  "America/North_Dakota/New_Salem",
-  "America/Ojinaga",
-  "America/Panama",
-  "America/Pangnirtung",
-  "America/Paramaribo",
-  "America/Phoenix",
-  "America/Port-au-Prince",
-  "America/Port_of_Spain",
-  "America/Porto_Acre",
-  "America/Porto_Velho",
-  "America/Puerto_Rico",
-  "America/Punta_Arenas",
-  "America/Rainy_River",
-  "America/Rankin_Inlet",
-  "America/Recife",
-  "America/Regina",
-  "America/Resolute",
-  "America/Rio_Branco",
-  "America/Rosario",
-  "America/Santa_Isabel",
-  "America/Santarem",
-  "America/Santiago",
-  "America/Santo_Domingo",
-  "America/Sao_Paulo",
-  "America/Scoresbysund",
-  "America/Shiprock",
-  "America/Sitka",
-  "America/St_Barthelemy",
-  "America/St_Johns",
-  "America/St_Kitts",
-  "America/St_Lucia",
-  "America/St_Thomas",
-  "America/St_Vincent",
-  "America/Swift_Current",
-  "America/Tegucigalpa",
-  "America/Thule",
-  "America/Thunder_Bay",
-  "America/Tijuana",
-  "America/Toronto",
-  "America/Tortola",
-  "America/Vancouver",
-  "America/Virgin",
-  "America/Whitehorse",
-  "America/Winnipeg",
-  "America/Yakutat",
-  "America/Yellowknife",
-  "Antarctica/Casey",
-  "Antarctica/Davis",
-  "Antarctica/DumontDUrville",
-  "Antarctica/Macquarie",
-  "Antarctica/Mawson",
-  "Antarctica/McMurdo",
-  "Antarctica/Palmer",
-  "Antarctica/Rothera",
-  "Antarctica/South_Pole",
-  "Antarctica/Syowa",
-  "Antarctica/Troll",
-  "Antarctica/Vostok",
-  "Arctic/Longyearbyen",
-  "Asia/Aden",
-  "Asia/Almaty",
-  "Asia/Amman",
-  "Asia/Anadyr",
-  "Asia/Aqtau",
-  "Asia/Aqtobe",
-  "Asia/Ashgabat",
-  "Asia/Ashkhabad",
-  "Asia/Atyrau",
-  "Asia/Baghdad",
-  "Asia/Bahrain",
-  "Asia/Baku",
-  "Asia/Bangkok",
-  "Asia/Barnaul",
-  "Asia/Beirut",
-  "Asia/Bishkek",
-  "Asia/Brunei",
-  "Asia/Calcutta",
-  "Asia/Chita",
-  "Asia/Choibalsan",
-  "Asia/Chongqing",
-  "Asia/Chungking",
-  "Asia/Colombo",
-  "Asia/Dacca",
-  "Asia/Damascus",
-  "Asia/Dhaka",
-  "Asia/Dili",
-  "Asia/Dubai",
-  "Asia/Dushanbe",
-  "Asia/Famagusta",
-  "Asia/Gaza",
-  "Asia/Harbin",
-  "Asia/Hebron",
-  "Asia/Ho_Chi_Minh",
-  "Asia/Hong_Kong",
-  "Asia/Hovd",
-  "Asia/Irkutsk",
-  "Asia/Istanbul",
-  "Asia/Jakarta",
-  "Asia/Jayapura",
-  "Asia/Jerusalem",
-  "Asia/Kabul",
-  "Asia/Kamchatka",
-  "Asia/Karachi",
-  "Asia/Kashgar",
-  "Asia/Kathmandu",
-  "Asia/Katmandu",
-  "Asia/Khandyga",
-  "Asia/Kolkata",
-  "Asia/Krasnoyarsk",
-  "Asia/Kuala_Lumpur",
-  "Asia/Kuching",
-  "Asia/Kuwait",
-  "Asia/Macao",
-  "Asia/Macau",
-  "Asia/Magadan",
-  "Asia/Makassar",
-  "Asia/Manila",
-  "Asia/Muscat",
-  "Asia/Nicosia",
-  "Asia/Novokuznetsk",
-  "Asia/Novosibirsk",
-  "Asia/Omsk",
-  "Asia/Oral",
-  "Asia/Phnom_Penh",
-  "Asia/Pontianak",
-  "Asia/Pyongyang",
-  "Asia/Qatar",
-  "Asia/Qostanay",
-  "Asia/Qyzylorda",
-  "Asia/Rangoon",
-  "Asia/Riyadh",
-  "Asia/Saigon",
-  "Asia/Sakhalin",
-  "Asia/Samarkand",
-  "Asia/Seoul",
-  "Asia/Shanghai",
-  "Asia/Singapore",
-  "Asia/Srednekolymsk",
-  "Asia/Taipei",
-  "Asia/Tashkent",
-  "Asia/Tbilisi",
-  "Asia/Tehran",
-  "Asia/Tel_Aviv",
-  "Asia/Thimbu",
-  "Asia/Thimphu",
-  "Asia/Tokyo",
-  "Asia/Tomsk",
-  "Asia/Ujung_Pandang",
-  "Asia/Ulaanbaatar",
-  "Asia/Ulan_Bator",
-  "Asia/Urumqi",
-  "Asia/Ust-Nera",
-  "Asia/Vientiane",
-  "Asia/Vladivostok",
-  "Asia/Yakutsk",
-  "Asia/Yangon",
-  "Asia/Yekaterinburg",
-  "Asia/Yerevan",
-  "Atlantic/Azores",
-  "Atlantic/Bermuda",
-  "Atlantic/Canary",
-  "Atlantic/Cape_Verde",
-  "Atlantic/Faeroe",
-  "Atlantic/Faroe",
-  "Atlantic/Jan_Mayen",
-  "Atlantic/Madeira",
-  "Atlantic/Reykjavik",
-  "Atlantic/South_Georgia",
-  "Atlantic/St_Helena",
-  "Atlantic/Stanley",
-  "Australia/ACT",
-  "Australia/Adelaide",
-  "Australia/Brisbane",
-  "Australia/Broken_Hill",
-  "Australia/Canberra",
-  "Australia/Currie",
-  "Australia/Darwin",
-  "Australia/Eucla",
-  "Australia/Hobart",
-  "Australia/LHI",
-  "Australia/Lindeman",
-  "Australia/Lord_Howe",
-  "Australia/Melbourne",
-  "Australia/NSW",
-  "Australia/North",
-  "Australia/Perth",
-  "Australia/Queensland",
-  "Australia/South",
-  "Australia/Sydney",
-  "Australia/Tasmania",
-  "Australia/Victoria",
-  "Australia/West",
-  "Australia/Yancowinna",
-  "Brazil/Acre",
-  "Brazil/DeNoronha",
-  "Brazil/East",
-  "Brazil/West",
-  "CET",
-  "CST6CDT",
-  "Canada/Atlantic",
-  "Canada/Central",
-  "Canada/Eastern",
-  "Canada/Mountain",
-  "Canada/Newfoundland",
-  "Canada/Pacific",
-  "Canada/Saskatchewan",
-  "Canada/Yukon",
-  "Chile/Continental",
-  "Chile/EasterIsland",
-  "Cuba",
-  "EET",
-  "EST",
-  "EST5EDT",
-  "Egypt",
-  "Eire",
-  "Etc/GMT",
-  "Etc/GMT+0",
-  "Etc/GMT+1",
-  "Etc/GMT+10",
-  "Etc/GMT+11",
-  "Etc/GMT+12",
-  "Etc/GMT+2",
-  "Etc/GMT+3",
-  "Etc/GMT+4",
-  "Etc/GMT+5",
-  "Etc/GMT+6",
-  "Etc/GMT+7",
-  "Etc/GMT+8",
-  "Etc/GMT+9",
-  "Etc/GMT-0",
-  "Etc/GMT-1",
-  "Etc/GMT-10",
-  "Etc/GMT-11",
-  "Etc/GMT-12",
-  "Etc/GMT-13",
-  "Etc/GMT-14",
-  "Etc/GMT-2",
-  "Etc/GMT-3",
-  "Etc/GMT-4",
-  "Etc/GMT-5",
-  "Etc/GMT-6",
-  "Etc/GMT-7",
-  "Etc/GMT-8",
-  "Etc/GMT-9",
-  "Etc/GMT0",
-  "Etc/Greenwich",
-  "Etc/UCT",
-  "Etc/UTC",
-  "Etc/Universal",
-  "Etc/Zulu",
-  "Europe/Amsterdam",
-  "Europe/Andorra",
-  "Europe/Astrakhan",
-  "Europe/Athens",
-  "Europe/Belfast",
-  "Europe/Belgrade",
-  "Europe/Berlin",
-  "Europe/Bratislava",
-  "Europe/Brussels",
-  "Europe/Bucharest",
-  "Europe/Budapest",
-  "Europe/Busingen",
-  "Europe/Chisinau",
-  "Europe/Copenhagen",
-  "Europe/Dublin",
-  "Europe/Gibraltar",
-  "Europe/Guernsey",
-  "Europe/Helsinki",
-  "Europe/Isle_of_Man",
-  "Europe/Istanbul",
-  "Europe/Jersey",
-  "Europe/Kaliningrad",
-  "Europe/Kiev",
-  "Europe/Kirov",
-  "Europe/Lisbon",
-  "Europe/Ljubljana",
-  "Europe/London",
-  "Europe/Luxembourg",
-  "Europe/Madrid",
-  "Europe/Malta",
-  "Europe/Mariehamn",
-  "Europe/Minsk",
-  "Europe/Monaco",
-  "Europe/Moscow",
-  "Europe/Nicosia",
-  "Europe/Oslo",
-  "Europe/Paris",
-  "Europe/Podgorica",
-  "Europe/Prague",
-  "Europe/Riga",
-  "Europe/Rome",
-  "Europe/Samara",
-  "Europe/San_Marino",
-  "Europe/Sarajevo",
-  "Europe/Saratov",
-  "Europe/Simferopol",
-  "Europe/Skopje",
-  "Europe/Sofia",
-  "Europe/Stockholm",
-  "Europe/Tallinn",
-  "Europe/Tirane",
-  "Europe/Tiraspol",
-  "Europe/Ulyanovsk",
-  "Europe/Uzhgorod",
-  "Europe/Vaduz",
-  "Europe/Vatican",
-  "Europe/Vienna",
-  "Europe/Vilnius",
-  "Europe/Volgograd",
-  "Europe/Warsaw",
-  "Europe/Zagreb",
-  "Europe/Zaporozhye",
-  "Europe/Zurich",
-  "GB",
-  "GB-Eire",
-  "GMT",
-  "GMT+0",
-  "GMT-0",
-  "GMT0",
-  "Greenwich",
-  "HST",
-  "Hongkong",
-  "Iceland",
-  "Indian/Antananarivo",
-  "Indian/Chagos",
-  "Indian/Christmas",
-  "Indian/Cocos",
-  "Indian/Comoro",
-  "Indian/Kerguelen",
-  "Indian/Mahe",
-  "Indian/Maldives",
-  "Indian/Mauritius",
-  "Indian/Mayotte",
-  "Indian/Reunion",
-  "Iran",
-  "Israel",
-  "Jamaica",
-  "Japan",
-  "Kwajalein",
-  "Libya",
-  "MET",
-  "MST",
-  "MST7MDT",
-  "Mexico/BajaNorte",
-  "Mexico/BajaSur",
-  "Mexico/General",
-  "NZ",
-  "NZ-CHAT",
-  "Navajo",
-  "PRC",
-  "PST8PDT",
-  "Pacific/Apia",
-  "Pacific/Auckland",
-  "Pacific/Bougainville",
-  "Pacific/Chatham",
-  "Pacific/Chuuk",
-  "Pacific/Easter",
-  "Pacific/Efate",
-  "Pacific/Enderbury",
-  "Pacific/Fakaofo",
-  "Pacific/Fiji",
-  "Pacific/Funafuti",
-  "Pacific/Galapagos",
-  "Pacific/Gambier",
-  "Pacific/Guadalcanal",
-  "Pacific/Guam",
-  "Pacific/Honolulu",
-  "Pacific/Johnston",
-  "Pacific/Kiritimati",
-  "Pacific/Kosrae",
-  "Pacific/Kwajalein",
-  "Pacific/Majuro",
-  "Pacific/Marquesas",
-  "Pacific/Midway",
-  "Pacific/Nauru",
-  "Pacific/Niue",
-  "Pacific/Norfolk",
-  "Pacific/Noumea",
-  "Pacific/Pago_Pago",
-  "Pacific/Palau",
-  "Pacific/Pitcairn",
-  "Pacific/Pohnpei",
-  "Pacific/Ponape",
-  "Pacific/Port_Moresby",
-  "Pacific/Rarotonga",
-  "Pacific/Saipan",
-  "Pacific/Samoa",
-  "Pacific/Tahiti",
-  "Pacific/Tarawa",
-  "Pacific/Tongatapu",
-  "Pacific/Truk",
-  "Pacific/Wake",
-  "Pacific/Wallis",
-  "Pacific/Yap",
-  "Poland",
-  "Portugal",
-  "ROC",
-  "ROK",
-  "Singapore",
-  "Turkey",
-  "UCT",
-  "US/Alaska",
-  "US/Aleutian",
-  "US/Arizona",
-  "US/Central",
-  "US/East-Indiana",
-  "US/Eastern",
-  "US/Hawaii",
-  "US/Indiana-Starke",
-  "US/Michigan",
-  "US/Mountain",
-  "US/Pacific",
-  "US/Samoa",
-  "UTC",
-  "Universal",
-  "W-SU",
-  "WET",
-  "Zulu",
-  nullptr
-};
-
 // Helper to return a loaded time zone by value (UTC on error).
 time_zone LoadZone(const std::string& name) {
   time_zone tz;
@@ -717,6 +132,22 @@ TEST(TimeZones, LoadZonesConcurrently) {
 }
 #endif
 
+TEST(TimeZone, UTC) {
+  const time_zone utc = utc_time_zone();
+
+  time_zone loaded_utc;
+  EXPECT_TRUE(load_time_zone("UTC", &loaded_utc));
+  EXPECT_EQ(loaded_utc, utc);
+
+  time_zone loaded_utc0;
+  EXPECT_TRUE(load_time_zone("UTC0", &loaded_utc0));
+  EXPECT_EQ(loaded_utc0, utc);
+
+  time_zone loaded_bad;
+  EXPECT_FALSE(load_time_zone("Invalid/TimeZone", &loaded_bad));
+  EXPECT_EQ(loaded_bad, utc);
+}
+
 TEST(TimeZone, NamedTimeZones) {
   const time_zone utc = utc_time_zone();
   EXPECT_EQ("UTC", utc.name());
@@ -724,7 +155,8 @@ TEST(TimeZone, NamedTimeZones) {
   EXPECT_EQ("America/New_York", nyc.name());
   const time_zone syd = LoadZone("Australia/Sydney");
   EXPECT_EQ("Australia/Sydney", syd.name());
-  const time_zone fixed0 = fixed_time_zone(absl::time_internal::cctz::seconds::zero());
+  const time_zone fixed0 =
+      fixed_time_zone(absl::time_internal::cctz::seconds::zero());
   EXPECT_EQ("UTC", fixed0.name());
   const time_zone fixed_pos = fixed_time_zone(
       chrono::hours(3) + chrono::minutes(25) + chrono::seconds(45));
@@ -749,11 +181,41 @@ TEST(TimeZone, Failures) {
   EXPECT_EQ(chrono::system_clock::from_time_t(0),
             convert(civil_second(1970, 1, 1, 0, 0, 0), tz));  // UTC
 
-  // Loading an empty std::string timezone should fail.
+  // Loading an empty string timezone should fail.
   tz = LoadZone("America/Los_Angeles");
   EXPECT_FALSE(load_time_zone("", &tz));
   EXPECT_EQ(chrono::system_clock::from_time_t(0),
             convert(civil_second(1970, 1, 1, 0, 0, 0), tz));  // UTC
+
+  // Reject a fixed-offset name with a NUL where a digit belongs.
+  for (const int i : {10, 11, 13, 14, 16, 17}) {
+    std::string name = "Fixed/UTC+00:00:00";
+    name[static_cast<std::size_t>(i)] = '\0';
+    EXPECT_FALSE(load_time_zone(name, &tz)) << "NUL at offset " << i;
+  }
+
+  // Reject path-traversal components.
+  EXPECT_FALSE(load_time_zone("file:../etc/passwd", &tz));
+  EXPECT_FALSE(load_time_zone("file:../../etc/passwd", &tz));
+  EXPECT_FALSE(load_time_zone("file:/../etc/passwd", &tz));
+  EXPECT_FALSE(load_time_zone("file:America/../America/Los_Angeles", &tz));
+
+#if defined(_WIN32)
+  // Windows accepts '\' as a path separator, so these escape as well.
+  // If they were admitted, the second would resolve back into the zoneinfo
+  // directory and load, failing the test. Elsewhere '\' is an ordinary
+  // filename character, so these would only fail as nonexistent names.
+  EXPECT_FALSE(load_time_zone("file:..\\etc\\passwd", &tz));
+  EXPECT_FALSE(load_time_zone("file:America\\..\\America/Los_Angeles", &tz));
+#endif
+
+#if !defined(_MSC_VER)
+  // Reject non-regular files and directories. The check lives in the
+  // non-MSVC FOpen(), so only expect it there.
+  EXPECT_FALSE(load_time_zone("file:/dev/null", &tz));
+  EXPECT_FALSE(load_time_zone("file:/dev/stdin", &tz));
+  EXPECT_FALSE(load_time_zone("file:/tmp", &tz));
+#endif
 }
 
 TEST(TimeZone, Equality) {
@@ -767,7 +229,8 @@ TEST(TimeZone, Equality) {
   EXPECT_EQ(implicit_utc, explicit_utc);
   EXPECT_EQ(implicit_utc.name(), explicit_utc.name());
 
-  const time_zone fixed_zero = fixed_time_zone(absl::time_internal::cctz::seconds::zero());
+  const time_zone fixed_zero =
+      fixed_time_zone(absl::time_internal::cctz::seconds::zero());
   EXPECT_EQ(fixed_zero, LoadZone(fixed_zero.name()));
   EXPECT_EQ(fixed_zero, explicit_utc);
 
@@ -806,8 +269,8 @@ TEST(TimeZone, Equality) {
 TEST(StdChronoTimePoint, TimeTAlignment) {
   // Ensures that the Unix epoch and the system clock epoch are an integral
   // number of seconds apart. This simplifies conversions to/from time_t.
-  auto diff = chrono::system_clock::time_point() -
-              chrono::system_clock::from_time_t(0);
+  auto diff =
+      chrono::system_clock::time_point() - chrono::system_clock::from_time_t(0);
   EXPECT_EQ(chrono::system_clock::time_point::duration::zero(),
             diff % chrono::seconds(1));
 }
@@ -816,20 +279,20 @@ TEST(BreakTime, TimePointResolution) {
   const time_zone utc = utc_time_zone();
   const auto t0 = chrono::system_clock::from_time_t(0);
 
-  ExpectTime(chrono::time_point_cast<chrono::nanoseconds>(t0), utc,
-             1970, 1, 1, 0, 0, 0, 0, false, "UTC");
-  ExpectTime(chrono::time_point_cast<chrono::microseconds>(t0), utc,
-             1970, 1, 1, 0, 0, 0, 0, false, "UTC");
-  ExpectTime(chrono::time_point_cast<chrono::milliseconds>(t0), utc,
-             1970, 1, 1, 0, 0, 0, 0, false, "UTC");
-  ExpectTime(chrono::time_point_cast<chrono::seconds>(t0), utc,
-             1970, 1, 1, 0, 0, 0, 0, false, "UTC");
-  ExpectTime(chrono::time_point_cast<absl::time_internal::cctz::seconds>(t0), utc,
-             1970, 1, 1, 0, 0, 0, 0, false, "UTC");
-  ExpectTime(chrono::time_point_cast<chrono::minutes>(t0), utc,
-             1970, 1, 1, 0, 0, 0, 0, false, "UTC");
-  ExpectTime(chrono::time_point_cast<chrono::hours>(t0), utc,
-             1970, 1, 1, 0, 0, 0, 0, false, "UTC");
+  ExpectTime(chrono::time_point_cast<chrono::nanoseconds>(t0), utc, 1970, 1, 1,
+             0, 0, 0, 0, false, "UTC");
+  ExpectTime(chrono::time_point_cast<chrono::microseconds>(t0), utc, 1970, 1, 1,
+             0, 0, 0, 0, false, "UTC");
+  ExpectTime(chrono::time_point_cast<chrono::milliseconds>(t0), utc, 1970, 1, 1,
+             0, 0, 0, 0, false, "UTC");
+  ExpectTime(chrono::time_point_cast<chrono::seconds>(t0), utc, 1970, 1, 1, 0,
+             0, 0, 0, false, "UTC");
+  ExpectTime(chrono::time_point_cast<absl::time_internal::cctz::seconds>(t0),
+             utc, 1970, 1, 1, 0, 0, 0, 0, false, "UTC");
+  ExpectTime(chrono::time_point_cast<chrono::minutes>(t0), utc, 1970, 1, 1, 0,
+             0, 0, 0, false, "UTC");
+  ExpectTime(chrono::time_point_cast<chrono::hours>(t0), utc, 1970, 1, 1, 0, 0,
+             0, 0, false, "UTC");
 }
 
 TEST(BreakTime, LocalTimeInUTC) {
@@ -890,19 +353,19 @@ TEST(MakeTime, TimePointResolution) {
   const time_zone utc = utc_time_zone();
   const time_point<chrono::nanoseconds> tp_ns =
       convert(civil_second(2015, 1, 2, 3, 4, 5), utc);
-  EXPECT_EQ("04:05", format("%M:%E*S", tp_ns, utc));
+  EXPECT_EQ("04:05", absl::time_internal::cctz::format("%M:%E*S", tp_ns, utc));
   const time_point<chrono::microseconds> tp_us =
       convert(civil_second(2015, 1, 2, 3, 4, 5), utc);
-  EXPECT_EQ("04:05", format("%M:%E*S", tp_us, utc));
+  EXPECT_EQ("04:05", absl::time_internal::cctz::format("%M:%E*S", tp_us, utc));
   const time_point<chrono::milliseconds> tp_ms =
       convert(civil_second(2015, 1, 2, 3, 4, 5), utc);
-  EXPECT_EQ("04:05", format("%M:%E*S", tp_ms, utc));
+  EXPECT_EQ("04:05", absl::time_internal::cctz::format("%M:%E*S", tp_ms, utc));
   const time_point<chrono::seconds> tp_s =
       convert(civil_second(2015, 1, 2, 3, 4, 5), utc);
-  EXPECT_EQ("04:05", format("%M:%E*S", tp_s, utc));
+  EXPECT_EQ("04:05", absl::time_internal::cctz::format("%M:%E*S", tp_s, utc));
   const time_point<absl::time_internal::cctz::seconds> tp_s64 =
       convert(civil_second(2015, 1, 2, 3, 4, 5), utc);
-  EXPECT_EQ("04:05", format("%M:%E*S", tp_s64, utc));
+  EXPECT_EQ("04:05", absl::time_internal::cctz::format("%M:%E*S", tp_s64, utc));
 
   // These next two require chrono::time_point_cast because the conversion
   // from a resolution of seconds (the return value of convert()) to a
@@ -910,11 +373,10 @@ TEST(MakeTime, TimePointResolution) {
   const time_point<chrono::minutes> tp_m =
       chrono::time_point_cast<chrono::minutes>(
           convert(civil_second(2015, 1, 2, 3, 4, 5), utc));
-  EXPECT_EQ("04:00", format("%M:%E*S", tp_m, utc));
-  const time_point<chrono::hours> tp_h =
-      chrono::time_point_cast<chrono::hours>(
-          convert(civil_second(2015, 1, 2, 3, 4, 5), utc));
-  EXPECT_EQ("00:00", format("%M:%E*S", tp_h, utc));
+  EXPECT_EQ("04:00", absl::time_internal::cctz::format("%M:%E*S", tp_m, utc));
+  const time_point<chrono::hours> tp_h = chrono::time_point_cast<chrono::hours>(
+      convert(civil_second(2015, 1, 2, 3, 4, 5), utc));
+  EXPECT_EQ("00:00", absl::time_internal::cctz::format("%M:%E*S", tp_h, utc));
 }
 
 TEST(MakeTime, Normalization) {
@@ -932,7 +394,7 @@ TEST(MakeTime, Normalization) {
 
 // NOTE: Run this with -ftrapv to detect overflow problems.
 TEST(MakeTime, SysSecondsLimits) {
-  const char RFC3339[] =  "%Y-%m-%dT%H:%M:%S%Ez";
+  const char RFC3339[] = "%Y-%m-%d%ET%H:%M:%S%Ez";
   const time_zone utc = utc_time_zone();
   const time_zone east = fixed_time_zone(chrono::hours(14));
   const time_zone west = fixed_time_zone(-chrono::hours(14));
@@ -940,9 +402,11 @@ TEST(MakeTime, SysSecondsLimits) {
 
   // Approach the maximal time_point<cctz::seconds> value from below.
   tp = convert(civil_second(292277026596, 12, 4, 15, 30, 6), utc);
-  EXPECT_EQ("292277026596-12-04T15:30:06+00:00", format(RFC3339, tp, utc));
+  EXPECT_EQ("292277026596-12-04T15:30:06+00:00",
+            absl::time_internal::cctz::format(RFC3339, tp, utc));
   tp = convert(civil_second(292277026596, 12, 4, 15, 30, 7), utc);
-  EXPECT_EQ("292277026596-12-04T15:30:07+00:00", format(RFC3339, tp, utc));
+  EXPECT_EQ("292277026596-12-04T15:30:07+00:00",
+            absl::time_internal::cctz::format(RFC3339, tp, utc));
   EXPECT_EQ(time_point<absl::time_internal::cctz::seconds>::max(), tp);
   tp = convert(civil_second(292277026596, 12, 4, 15, 30, 8), utc);
   EXPECT_EQ(time_point<absl::time_internal::cctz::seconds>::max(), tp);
@@ -951,7 +415,8 @@ TEST(MakeTime, SysSecondsLimits) {
 
   // Checks that we can also get the maximal value for a far-east zone.
   tp = convert(civil_second(292277026596, 12, 5, 5, 30, 7), east);
-  EXPECT_EQ("292277026596-12-05T05:30:07+14:00", format(RFC3339, tp, east));
+  EXPECT_EQ("292277026596-12-05T05:30:07+14:00",
+            absl::time_internal::cctz::format(RFC3339, tp, east));
   EXPECT_EQ(time_point<absl::time_internal::cctz::seconds>::max(), tp);
   tp = convert(civil_second(292277026596, 12, 5, 5, 30, 8), east);
   EXPECT_EQ(time_point<absl::time_internal::cctz::seconds>::max(), tp);
@@ -960,7 +425,8 @@ TEST(MakeTime, SysSecondsLimits) {
 
   // Checks that we can also get the maximal value for a far-west zone.
   tp = convert(civil_second(292277026596, 12, 4, 1, 30, 7), west);
-  EXPECT_EQ("292277026596-12-04T01:30:07-14:00", format(RFC3339, tp, west));
+  EXPECT_EQ("292277026596-12-04T01:30:07-14:00",
+            absl::time_internal::cctz::format(RFC3339, tp, west));
   EXPECT_EQ(time_point<absl::time_internal::cctz::seconds>::max(), tp);
   tp = convert(civil_second(292277026596, 12, 4, 7, 30, 8), west);
   EXPECT_EQ(time_point<absl::time_internal::cctz::seconds>::max(), tp);
@@ -969,9 +435,11 @@ TEST(MakeTime, SysSecondsLimits) {
 
   // Approach the minimal time_point<cctz::seconds> value from above.
   tp = convert(civil_second(-292277022657, 1, 27, 8, 29, 53), utc);
-  EXPECT_EQ("-292277022657-01-27T08:29:53+00:00", format(RFC3339, tp, utc));
+  EXPECT_EQ("-292277022657-01-27T08:29:53+00:00",
+            absl::time_internal::cctz::format(RFC3339, tp, utc));
   tp = convert(civil_second(-292277022657, 1, 27, 8, 29, 52), utc);
-  EXPECT_EQ("-292277022657-01-27T08:29:52+00:00", format(RFC3339, tp, utc));
+  EXPECT_EQ("-292277022657-01-27T08:29:52+00:00",
+            absl::time_internal::cctz::format(RFC3339, tp, utc));
   EXPECT_EQ(time_point<absl::time_internal::cctz::seconds>::min(), tp);
   tp = convert(civil_second(-292277022657, 1, 27, 8, 29, 51), utc);
   EXPECT_EQ(time_point<absl::time_internal::cctz::seconds>::min(), tp);
@@ -980,7 +448,8 @@ TEST(MakeTime, SysSecondsLimits) {
 
   // Checks that we can also get the minimal value for a far-east zone.
   tp = convert(civil_second(-292277022657, 1, 27, 22, 29, 52), east);
-  EXPECT_EQ("-292277022657-01-27T22:29:52+14:00", format(RFC3339, tp, east));
+  EXPECT_EQ("-292277022657-01-27T22:29:52+14:00",
+            absl::time_internal::cctz::format(RFC3339, tp, east));
   EXPECT_EQ(time_point<absl::time_internal::cctz::seconds>::min(), tp);
   tp = convert(civil_second(-292277022657, 1, 27, 22, 29, 51), east);
   EXPECT_EQ(time_point<absl::time_internal::cctz::seconds>::min(), tp);
@@ -989,7 +458,8 @@ TEST(MakeTime, SysSecondsLimits) {
 
   // Checks that we can also get the minimal value for a far-west zone.
   tp = convert(civil_second(-292277022657, 1, 26, 18, 29, 52), west);
-  EXPECT_EQ("-292277022657-01-26T18:29:52-14:00", format(RFC3339, tp, west));
+  EXPECT_EQ("-292277022657-01-26T18:29:52-14:00",
+            absl::time_internal::cctz::format(RFC3339, tp, west));
   EXPECT_EQ(time_point<absl::time_internal::cctz::seconds>::min(), tp);
   tp = convert(civil_second(-292277022657, 1, 26, 18, 29, 51), west);
   EXPECT_EQ(time_point<absl::time_internal::cctz::seconds>::min(), tp);
@@ -1003,13 +473,23 @@ TEST(MakeTime, SysSecondsLimits) {
 #if defined(_WIN32) || defined(_WIN64)
     // localtime_s() and gmtime_s() don't believe in years outside [1970:3000].
 #else
-    const time_zone utc = LoadZone("libc:UTC");
+    const time_zone cut = LoadZone("libc:UTC");
     const year_t max_tm_year = year_t{std::numeric_limits<int>::max()} + 1900;
-    tp = convert(civil_second(max_tm_year, 12, 31, 23, 59, 59), utc);
-    EXPECT_EQ("2147485547-12-31T23:59:59+00:00", format(RFC3339, tp, utc));
+    tp = convert(civil_second(max_tm_year, 12, 31, 23, 59, 59), cut);
+#if defined(__FreeBSD__) || defined(__OpenBSD__) || defined(__EMSCRIPTEN__)
+    // Some gmtime_r() impls fail on extreme positive values.
+#else
+    EXPECT_EQ("2147485547-12-31T23:59:59+00:00",
+              absl::time_internal::cctz::format(RFC3339, tp, cut));
+#endif
     const year_t min_tm_year = year_t{std::numeric_limits<int>::min()} + 1900;
-    tp = convert(civil_second(min_tm_year, 1, 1, 0, 0, 0), utc);
-    EXPECT_EQ("-2147481748-01-01T00:00:00+00:00", format(RFC3339, tp, utc));
+    tp = convert(civil_second(min_tm_year, 1, 1, 0, 0, 0), cut);
+#if defined(__Fuchsia__) || defined(__EMSCRIPTEN__)
+    // Some gmtime_r() impls fail on extreme negative values (fxbug.dev/78527).
+#else
+    EXPECT_EQ("-2147481748-01-01T00:00:00+00:00",
+              absl::time_internal::cctz::format(RFC3339, tp, cut));
+#endif
 #endif
   }
 }
@@ -1021,24 +501,24 @@ TEST(MakeTime, LocalTimeLibC) {
   //  1) we know how to change the time zone used by localtime()/mktime(),
   //  2) cctz and localtime()/mktime() will use similar-enough tzdata, and
   //  3) we have some idea about how mktime() behaves during transitions.
-#if defined(__linux__) && !defined(__ANDROID__)
+#if defined(__linux__) && defined(__GLIBC__) && !defined(__ANDROID__)
   const char* const ep = getenv("TZ");
   std::string tz_name = (ep != nullptr) ? ep : "";
   for (const char* const* np = kTimeZoneNames; *np != nullptr; ++np) {
     ASSERT_EQ(0, setenv("TZ", *np, 1));  // change what "localtime" means
     const auto zi = local_time_zone();
     const auto lc = LoadZone("libc:localtime");
-    time_zone::civil_transition trans;
+    time_zone::civil_transition transition;
     for (auto tp = zi.lookup(civil_second()).trans;
-         zi.next_transition(tp, &trans);
-         tp = zi.lookup(trans.to).trans) {
-      const auto fcl = zi.lookup(trans.from);
-      const auto tcl = zi.lookup(trans.to);
-      civil_second cs;  // compare cs in zi and lc
+         zi.next_transition(tp, &transition);
+         tp = zi.lookup(transition.to).trans) {
+      const auto fcl = zi.lookup(transition.from);
+      const auto tcl = zi.lookup(transition.to);
+      civil_second cs, us;  // compare cs and us in zi and lc
       if (fcl.kind == time_zone::civil_lookup::UNIQUE) {
         if (tcl.kind == time_zone::civil_lookup::UNIQUE) {
           // Both unique; must be an is_dst or abbr change.
-          ASSERT_EQ(trans.from, trans.to);
+          ASSERT_EQ(transition.from, transition.to);
           const auto trans = fcl.trans;
           const auto tal = zi.lookup(trans);
           const auto tprev = trans - absl::time_internal::cctz::seconds(1);
@@ -1049,13 +529,15 @@ TEST(MakeTime, LocalTimeLibC) {
           continue;
         }
         ASSERT_EQ(time_zone::civil_lookup::REPEATED, tcl.kind);
-        cs = trans.to;
+        cs = transition.to;
+        us = transition.from;
       } else {
         ASSERT_EQ(time_zone::civil_lookup::UNIQUE, tcl.kind);
         ASSERT_EQ(time_zone::civil_lookup::SKIPPED, fcl.kind);
-        cs = trans.from;
+        cs = transition.from;
+        us = transition.to;
       }
-      if (cs.year() > 2037) break;  // limit test time (and to 32-bit time_t)
+      if (us.year() > 2037) break;  // limit test time (and to 32-bit time_t)
       const auto cl_zi = zi.lookup(cs);
       if (zi.lookup(cl_zi.pre).is_dst == zi.lookup(cl_zi.post).is_dst) {
         // The "libc" implementation cannot correctly classify transitions
@@ -1087,6 +569,13 @@ TEST(MakeTime, LocalTimeLibC) {
       EXPECT_EQ(cl_zi.pre, cl_lc.pre);
       EXPECT_EQ(cl_zi.trans, cl_lc.trans);
       EXPECT_EQ(cl_zi.post, cl_lc.post);
+      const auto ucl_zi = zi.lookup(us);
+      const auto ucl_lc = lc.lookup(us);
+      SCOPED_TRACE(testing::Message() << "For " << us << " in " << *np);
+      EXPECT_EQ(ucl_zi.kind, ucl_lc.kind);
+      EXPECT_EQ(ucl_zi.pre, ucl_lc.pre);
+      EXPECT_EQ(ucl_zi.trans, ucl_lc.trans);
+      EXPECT_EQ(ucl_zi.post, ucl_lc.post);
     }
   }
   if (ep == nullptr) {
@@ -1160,6 +649,45 @@ TEST(PrevTransition, AmericaNewYork) {
   // We have a transition but we don't know which one.
 }
 
+TEST(NextTransition, Scan) {
+  for (const char* const* np = kTimeZoneNames; *np != nullptr; ++np) {
+    SCOPED_TRACE(testing::Message() << "In " << *np);
+    time_zone tz;
+    // EXPECT_TRUE(load_time_zone(*np, &tz));
+    if (!load_time_zone(*np, &tz)) {
+      continue;  // tolerate kTimeZoneNames/zoneinfo skew
+    }
+
+    auto tp = time_point<absl::time_internal::cctz::seconds>::min();
+    time_zone::civil_transition trans;
+    while (tz.next_transition(tp, &trans)) {
+      time_zone::civil_lookup from_cl = tz.lookup(trans.from);
+      EXPECT_NE(from_cl.kind, time_zone::civil_lookup::REPEATED);
+      time_zone::civil_lookup to_cl = tz.lookup(trans.to);
+      EXPECT_NE(to_cl.kind, time_zone::civil_lookup::SKIPPED);
+
+      auto trans_tp = to_cl.trans;
+      time_zone::absolute_lookup trans_al = tz.lookup(trans_tp);
+      EXPECT_EQ(trans_al.cs, trans.to);
+      auto pre_trans_tp = trans_tp - absl::time_internal::cctz::seconds(1);
+      time_zone::absolute_lookup pre_trans_al = tz.lookup(pre_trans_tp);
+      EXPECT_EQ(pre_trans_al.cs + 1, trans.from);
+
+      auto offset_delta = trans_al.offset - pre_trans_al.offset;
+      EXPECT_EQ(offset_delta, trans.to - trans.from);
+      if (offset_delta == 0) {
+        // This "transition" is only an is_dst or abbr change.
+        EXPECT_EQ(to_cl.kind, time_zone::civil_lookup::UNIQUE);
+        if (trans_al.is_dst == pre_trans_al.is_dst) {
+          EXPECT_STRNE(trans_al.abbr, pre_trans_al.abbr);
+        }
+      }
+
+      tp = trans_tp;  // continue scan from transition
+    }
+  }
+}
+
 TEST(TimeZoneEdgeCase, AmericaNewYork) {
   const time_zone tz = LoadZone("America/New_York");
 
@@ -1351,35 +879,24 @@ TEST(TimeZoneEdgeCase, AmericaJamaica) {
   ExpectTime(tp, tz, 1983, 12, 31, 23, 59, 59, -5 * 3600, false, "EST");
 }
 
-TEST(TimeZoneEdgeCase, WET) {
-  // Cover some non-existent times within forward transitions.
-  const time_zone tz = LoadZone("WET");
+TEST(TimeZoneEdgeCase, EuropeLisbon) {
+  // Cover a non-existent time within a forward transition.
+  const time_zone tz = LoadZone("Europe/Lisbon");
 
-  // Before the first transition.
-  auto tp = convert(civil_second(1977, 1, 1, 0, 0, 0), tz);
-  ExpectTime(tp, tz, 1977, 1, 1, 0, 0, 0, 0, false, "WET");
-
-  // Over the first transition.
-  //     228877199 == Sun,  3 Apr 1977 00:59:59 +0000 (WET)
-  //     228877200 == Sun,  3 Apr 1977 02:00:00 +0100 (WEST)
-  tp = convert(civil_second(1977, 4, 3, 0, 59, 59), tz);
-  ExpectTime(tp, tz, 1977, 4, 3, 0, 59, 59, 0, false, "WET");
+  // Over a forward transition.
+  //     354671999 == Sat, 28 Mar 1981 23:59:59 +0000 (WET)
+  //     354672000 == Sun, 29 Mar 1981 01:00:00 +0100 (WEST)
+  auto tp = convert(civil_second(1981, 3, 28, 23, 59, 59), tz);
+  ExpectTime(tp, tz, 1981, 3, 28, 23, 59, 59, 0, false, "WET");
   tp += absl::time_internal::cctz::seconds(1);
-  ExpectTime(tp, tz, 1977, 4, 3, 2, 0, 0, 1 * 3600, true, "WEST");
+  ExpectTime(tp, tz, 1981, 3, 29, 1, 0, 0, 1 * 3600, true, "WEST");
 
-  // A non-existent time within the first transition.
-  time_zone::civil_lookup cl1 = tz.lookup(civil_second(1977, 4, 3, 1, 15, 0));
+  // A non-existent time within the transition.
+  time_zone::civil_lookup cl1 = tz.lookup(civil_second(1981, 3, 29, 0, 15, 0));
   EXPECT_EQ(time_zone::civil_lookup::SKIPPED, cl1.kind);
-  ExpectTime(cl1.pre, tz, 1977, 4, 3, 2, 15, 0, 1 * 3600, true, "WEST");
-  ExpectTime(cl1.trans, tz, 1977, 4, 3, 2, 0, 0, 1 * 3600, true, "WEST");
-  ExpectTime(cl1.post, tz, 1977, 4, 3, 0, 15, 0, 0 * 3600, false, "WET");
-
-  // A non-existent time within the second forward transition.
-  time_zone::civil_lookup cl2 = tz.lookup(civil_second(1978, 4, 2, 1, 15, 0));
-  EXPECT_EQ(time_zone::civil_lookup::SKIPPED, cl2.kind);
-  ExpectTime(cl2.pre, tz, 1978, 4, 2, 2, 15, 0, 1 * 3600, true, "WEST");
-  ExpectTime(cl2.trans, tz, 1978, 4, 2, 2, 0, 0, 1 * 3600, true, "WEST");
-  ExpectTime(cl2.post, tz, 1978, 4, 2, 0, 15, 0, 0 * 3600, false, "WET");
+  ExpectTime(cl1.pre, tz, 1981, 3, 29, 1, 15, 0, 1 * 3600, true, "WEST");
+  ExpectTime(cl1.trans, tz, 1981, 3, 29, 1, 0, 0, 1 * 3600, true, "WEST");
+  ExpectTime(cl1.post, tz, 1981, 3, 28, 23, 15, 0, 0 * 3600, false, "WET");
 }
 
 TEST(TimeZoneEdgeCase, FixedOffsets) {
@@ -1431,6 +948,205 @@ TEST(TimeZoneEdgeCase, UTC5DigitYear) {
   ExpectTime(tp, tz, 10000, 1, 1, 0, 0, 0, 0 * 3600, false, "UTC");
 }
 
+// A ZoneInfoSource implementation backed by an in-memory string buffer.
+class StringZoneInfoSource : public ZoneInfoSource {
+ public:
+  explicit StringZoneInfoSource(std::string data)
+      : data_(std::move(data)), offset_(0) {}
+
+  std::size_t Read(void* ptr, std::size_t size) override {
+    std::size_t n = (std::min)(size, data_.size() - offset_);
+    std::memcpy(ptr, data_.data() + offset_, n);
+    offset_ += n;
+    return n;
+  }
+
+  int Skip(std::size_t offset) override {
+    if (offset > data_.size() - offset_) return -1;
+    offset_ += offset;
+    return 0;
+  }
+
+ private:
+  std::string data_;
+  std::size_t offset_;
+};
+
+// Constructs a minimal TZif2 string with a single 64-bit transition
+// at the given transition time and a future POSIX rule. The abbreviation
+// area holds abbr verbatim, so a valid file's abbr must include the
+// trailing '\0' (e.g., std::string{"EST", 4}).
+std::string MakeExtendedTzif(std::int_fast64_t unix_time,
+                             std::int_fast32_t utc_offset,
+                             const std::string& abbr,
+                             const std::string& future_spec) {
+  std::string s;
+  auto append32 = [&s](std::int_fast32_t v) {
+    const std::int_fast32_t s32max = 0x7fffffff;
+    const auto s32maxU = static_cast<std::uint_fast32_t>(s32max);
+    std::uint_fast32_t uv;
+    if (v >= 0) {
+      uv = static_cast<std::uint_fast32_t>(v);
+    } else {
+      uv = static_cast<std::uint_fast32_t>(v + s32max + 1) + s32maxU + 1;
+    }
+    for (int i = 3; i >= 0; --i) {
+      s.push_back(static_cast<char>((uv >> (i * 8)) & 0xff));
+    }
+  };
+  auto append64 = [&s](std::int_fast64_t v) {
+    const std::int_fast64_t s64max = 0x7fffffffffffffff;
+    const auto s64maxU = static_cast<std::uint_fast64_t>(s64max);
+    std::uint_fast64_t uv;
+    if (v >= 0) {
+      uv = static_cast<std::uint_fast64_t>(v);
+    } else {
+      uv = static_cast<std::uint_fast64_t>(v + s64max + 1) + s64maxU + 1;
+    }
+    for (int i = 7; i >= 0; --i) {
+      s.push_back(static_cast<char>((uv >> (i * 8)) & 0xff));
+    }
+  };
+
+  const std::size_t charcnt = abbr.size();
+
+  // 32-bit header
+  s.append(TZ_MAGIC, 4);
+  s.push_back('2');    // tzh_version
+  s.append(15, '\0');  // tzh_reserved
+  append32(0);         // tzh_ttisutcnt
+  append32(0);         // tzh_ttisstdcnt
+  append32(0);         // tzh_leapcnt
+  append32(0);         // tzh_timecnt (0 32-bit transitions)
+  append32(1);         // tzh_typecnt (1 ttinfo record)
+  append32(static_cast<std::int_fast32_t>(charcnt));  // tzh_charcnt
+
+  // 32-bit data block
+  append32(utc_offset);  // tt_utoff
+  s.push_back(0);        // tt_isdst (standard time)
+  s.push_back(0);        // tt_desigidx
+  s.append(abbr);        // abbreviation table
+
+  // 64-bit header
+  s.append(TZ_MAGIC, 4);
+  s.push_back('2');    // tzh_version
+  s.append(15, '\0');  // tzh_reserved
+  append32(0);         // tzh_ttisutcnt
+  append32(0);         // tzh_ttisstdcnt
+  append32(0);         // tzh_leapcnt
+  append32(1);         // tzh_timecnt (1 64-bit transition)
+  append32(1);         // tzh_typecnt (1 ttinfo record)
+  append32(static_cast<std::int_fast32_t>(charcnt));  // tzh_charcnt
+
+  // 64-bit data block
+  append64(unix_time);   // transition time
+  s.push_back(0);        // type index for transition
+  append32(utc_offset);  // tt_utoff
+  s.push_back(0);        // tt_isdst (standard time)
+  s.push_back(0);        // tt_desigidx
+  s.append(abbr);        // abbreviation table
+
+  // POSIX footer
+  s.push_back('\n');
+  s.append(future_spec);
+  s.push_back('\n');
+  return s;
+}
+
+std::unique_ptr<ZoneInfoSource> ExtendedTestFactory(
+    const std::string& name,
+    const std::function<std::unique_ptr<ZoneInfoSource>(const std::string&)>&
+        fallback) {
+  if (name == "test:ExtendedBeforeEpoch") {
+    // -1 (1969-12-31T23:59:59Z) is the latest final transition before the
+    // epoch, so the zone is rejected despite the future specification.
+    return std::unique_ptr<ZoneInfoSource>(
+        new StringZoneInfoSource(MakeExtendedTzif(
+            -1, -5 * 3600, std::string{"EST", 4}, "EST5EDT,M3.2.0,M11.1.0")));
+  }
+  if (name == "test:ExtendedFarFuture") {
+    // 0 (1970-01-01T00:00:00Z) is the earliest final transition an extended
+    // zone may have, which maximizes the 400-year shift that BreakTime()
+    // needs for a lookup at the maximum time.
+    return std::unique_ptr<ZoneInfoSource>(
+        new StringZoneInfoSource(MakeExtendedTzif(
+            0, -5 * 3600, std::string{"EST", 4}, "EST5EDT,M3.2.0,M11.1.0")));
+  }
+  if (name == "test:ExtendedOverlappingRules") {
+    // The daylight time of one year starts more than four days after its
+    // nominal date, while the next year's ends more than five days before
+    // its own, so the two years' generated transitions overlap.
+    return std::unique_ptr<ZoneInfoSource>(
+        new StringZoneInfoSource(MakeExtendedTzif(
+            0, -5 * 3600, std::string{"STD", 4},
+            "STD-12:00:00DST12:00:00,358/100:00:00,1/-139:00:00")));
+  }
+  if (name == "test:UnterminatedAbbreviation") {
+    // The abbreviation area is missing its final NUL, so the abbreviation
+    // would run into whatever ExtendTransitions() appends behind it.
+    return std::unique_ptr<ZoneInfoSource>(new StringZoneInfoSource(
+        MakeExtendedTzif(0, -5 * 3600, "EST", "EST5EDT,M3.2.0,M11.1.0")));
+  }
+  return fallback(name);
+}
+
+// Tests that a TZif file whose abbreviation area is not NUL-terminated
+// is rejected.
+TEST(TimeZoneEdgeCase, UnterminatedAbbreviation) {
+  auto prev_factory = cctz_extension::zone_info_source_factory;
+  cctz_extension::zone_info_source_factory = ExtendedTestFactory;
+
+  time_zone tz;
+  EXPECT_FALSE(load_time_zone("test:UnterminatedAbbreviation", &tz));
+
+  cctz_extension::zone_info_source_factory = prev_factory;
+}
+
+// Tests that a TZif file whose explicit transitions end before epoch
+// is rejected when it has a POSIX DST footer string.
+TEST(TimeZoneEdgeCase, ExtendedBeforeEpoch) {
+  auto prev_factory = cctz_extension::zone_info_source_factory;
+  cctz_extension::zone_info_source_factory = ExtendedTestFactory;
+
+  // Extended zones must end with a non-negative explicit transition.
+  time_zone tz;
+  EXPECT_FALSE(load_time_zone("test:ExtendedBeforeEpoch", &tz));
+
+  cctz_extension::zone_info_source_factory = prev_factory;
+}
+
+// A transition time may include a day offset of up to +/-167 hours, so the
+// rules for consecutive years can overlap, producing transitions that go
+// backward in unix time. BreakTime() binary searches the transitions by
+// unix time, so such a zone must be rejected.
+TEST(TimeZoneEdgeCase, ExtendedOverlappingRules) {
+  auto prev_factory = cctz_extension::zone_info_source_factory;
+  cctz_extension::zone_info_source_factory = ExtendedTestFactory;
+
+  time_zone tz;
+  EXPECT_FALSE(load_time_zone("test:ExtendedOverlappingRules", &tz));
+
+  cctz_extension::zone_info_source_factory = prev_factory;
+}
+
+// Looking up the maximum time in an extended zone must fold back through the
+// 400-year cycle without overflowing when BreakTime() computes the shift.
+TEST(TimeZoneEdgeCase, ExtendedFarFuture) {
+  auto prev_factory = cctz_extension::zone_info_source_factory;
+  cctz_extension::zone_info_source_factory = ExtendedTestFactory;
+
+  time_zone tz;
+  ASSERT_TRUE(load_time_zone("test:ExtendedFarFuture", &tz));
+
+  auto tp_max = time_point<absl::time_internal::cctz::seconds>::max();
+  ExpectTime(tp_max, tz, 292277026596, 12, 4, 10, 30, 7, -5 * 3600, false,
+             "EST");
+  EXPECT_STREQ("EST", tz.lookup(tp_max).abbr);
+
+  cctz_extension::zone_info_source_factory = prev_factory;
+}
+
 }  // namespace cctz
 }  // namespace time_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/time/internal/cctz/src/time_zone_name_win.cc b/absl/time/internal/cctz/src/time_zone_name_win.cc
new file mode 100644
index 00000000000..7a1236fb602
--- /dev/null
+++ b/absl/time/internal/cctz/src/time_zone_name_win.cc
@@ -0,0 +1,185 @@
+// Copyright 2025 Google Inc. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//   https://www.apache.org/licenses/LICENSE-2.0
+//
+//   Unless required by applicable law or agreed to in writing, software
+//   distributed under the License is distributed on an "AS IS" BASIS,
+//   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+//   See the License for the specific language governing permissions and
+//   limitations under the License.
+
+#include "absl/time/internal/cctz/src/time_zone_name_win.h"
+
+#include "absl/base/config.h"
+
+#if !defined(NOMINMAX)
+#define NOMINMAX
+#endif  // !defined(NOMINMAX)
+#include <windows.h>
+
+#include <algorithm>
+#include <atomic>
+#include <cstddef>
+#include <cstdint>
+#include <limits>
+#include <string>
+#include <type_traits>
+#include <utility>
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace time_internal {
+namespace cctz {
+namespace {
+
+// Define UChar as wchar_t here because Win32 APIs receive UTF-16 strings as
+// wchar_t* instead of char16_t*. Using char16_t would require additional casts.
+using UChar = wchar_t;
+
+enum UErrorCode : std::int32_t {
+  U_ZERO_ERROR = 0,
+  U_BUFFER_OVERFLOW_ERROR = 15,
+};
+
+bool U_SUCCESS(UErrorCode error) { return error <= U_ZERO_ERROR; }
+
+using ucal_getTimeZoneIDForWindowsID_func = std::int32_t(__cdecl*)(
+    const UChar* winid, std::int32_t len, const char* region, UChar* id,
+    std::int32_t id_capacity, UErrorCode* status);
+
+std::atomic<bool> g_unavailable;
+std::atomic<ucal_getTimeZoneIDForWindowsID_func>
+    g_ucal_getTimeZoneIDForWindowsID;
+
+template <typename T>
+static T AsProcAddress(HMODULE module, const char* name) {
+  static_assert(
+      std::is_pointer<T>::value &&
+          std::is_function<typename std::remove_pointer<T>::type>::value,
+      "T must be a function pointer type");
+  const auto proc_address = ::GetProcAddress(module, name);
+  return reinterpret_cast<T>(reinterpret_cast<void*>(proc_address));
+}
+
+std::wstring GetSystem32Dir() {
+  std::wstring result;
+  std::uint32_t len = std::max<std::uint32_t>(
+      static_cast<std::uint32_t>(std::min<size_t>(
+          result.capacity(), std::numeric_limits<std::uint32_t>::max())),
+      1);
+  do {
+    result.resize(len);
+    len = ::GetSystemDirectoryW(&result[0], len);
+  } while (len > result.size());
+  result.resize(len);
+  return result;
+}
+
+ucal_getTimeZoneIDForWindowsID_func LoadIcuGetTimeZoneIDForWindowsID() {
+  // This function is intended to be lock free to avoid potential deadlocks
+  // with loader-lock taken inside LoadLibraryW. As LoadLibraryW and
+  // GetProcAddress are idempotent unless the DLL is unloaded, we just need to
+  // make sure global variables are read/written atomically, where
+  // memory_order_relaxed is also acceptable.
+
+  if (g_unavailable.load(std::memory_order_relaxed)) {
+    return nullptr;
+  }
+
+  {
+    const auto ucal_getTimeZoneIDForWindowsIDRef =
+        g_ucal_getTimeZoneIDForWindowsID.load(std::memory_order_relaxed);
+    if (ucal_getTimeZoneIDForWindowsIDRef != nullptr) {
+      return ucal_getTimeZoneIDForWindowsIDRef;
+    }
+  }
+
+  const std::wstring system32_dir = GetSystem32Dir();
+  if (system32_dir.empty()) {
+    g_unavailable.store(true, std::memory_order_relaxed);
+    return nullptr;
+  }
+
+  // Here LoadLibraryExW(L"icu.dll", nullptr, LOAD_LIBRARY_SEARCH_SYSTEM32) does
+  // not work if "icu.dll" is already loaded from somewhere other than the
+  // system32 directory. Specifying the full path with LoadLibraryW is more
+  // reliable.
+  const std::wstring icu_dll_path = system32_dir + L"\\icu.dll";
+  const HMODULE icu_dll = ::LoadLibraryW(icu_dll_path.c_str());
+  if (icu_dll == nullptr) {
+    g_unavailable.store(true, std::memory_order_relaxed);
+    return nullptr;
+  }
+
+  const auto ucal_getTimeZoneIDForWindowsIDRef =
+      AsProcAddress<ucal_getTimeZoneIDForWindowsID_func>(
+          icu_dll, "ucal_getTimeZoneIDForWindowsID");
+  if (ucal_getTimeZoneIDForWindowsIDRef == nullptr) {
+    g_unavailable.store(true, std::memory_order_relaxed);
+    return nullptr;
+  }
+
+  g_ucal_getTimeZoneIDForWindowsID.store(ucal_getTimeZoneIDForWindowsIDRef,
+                                         std::memory_order_relaxed);
+
+  return ucal_getTimeZoneIDForWindowsIDRef;
+}
+
+// Convert wchar_t array (UTF-16) to UTF-8 string
+std::string Utf16ToUtf8(const wchar_t* ptr, size_t size) {
+  if (size > static_cast<size_t>(std::numeric_limits<int>::max())) {
+    return std::string();
+  }
+  const int chars_len = static_cast<int>(size);
+  std::string result;
+  std::size_t len = std::max<std::size_t>(
+      std::min<size_t>(result.capacity(), std::numeric_limits<int>::max()), 1);
+  do {
+    result.resize(len);
+    len = static_cast<std::size_t>(::WideCharToMultiByte(
+        CP_UTF8, WC_ERR_INVALID_CHARS, ptr, chars_len, &result[0],
+        static_cast<int>(len), nullptr, nullptr));
+  } while (len > result.size());
+  result.resize(len);
+  return result;
+}
+
+}  // namespace
+
+std::string GetWindowsLocalTimeZone() {
+  const auto getTimeZoneIDForWindowsID = LoadIcuGetTimeZoneIDForWindowsID();
+  if (getTimeZoneIDForWindowsID == nullptr) {
+    return std::string();
+  }
+
+  DYNAMIC_TIME_ZONE_INFORMATION info = {};
+  if (::GetDynamicTimeZoneInformation(&info) == TIME_ZONE_ID_INVALID) {
+    return std::string();
+  }
+
+  std::wstring result;
+  std::size_t len = std::max<std::size_t>(
+      std::min<size_t>(result.capacity(), std::numeric_limits<int>::max()), 1);
+  for (;;) {
+    UErrorCode status = U_ZERO_ERROR;
+    result.resize(len);
+    len = static_cast<std::size_t>(
+        getTimeZoneIDForWindowsID(info.TimeZoneKeyName, -1, nullptr, &result[0],
+                                  static_cast<int>(len), &status));
+    if (U_SUCCESS(status)) {
+      return Utf16ToUtf8(result.data(), len);
+    }
+    if (status != U_BUFFER_OVERFLOW_ERROR) {
+      return std::string();
+    }
+  }
+}
+
+}  // namespace cctz
+}  // namespace time_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/time/internal/cctz/src/time_zone_name_win.h b/absl/time/internal/cctz/src/time_zone_name_win.h
new file mode 100644
index 00000000000..f53b5a9d00f
--- /dev/null
+++ b/absl/time/internal/cctz/src/time_zone_name_win.h
@@ -0,0 +1,37 @@
+// Copyright 2025 Google Inc. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//   https://www.apache.org/licenses/LICENSE-2.0
+//
+//   Unless required by applicable law or agreed to in writing, software
+//   distributed under the License is distributed on an "AS IS" BASIS,
+//   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+//   See the License for the specific language governing permissions and
+//   limitations under the License.
+
+#ifndef ABSL_TIME_INTERNAL_CCTZ_TIME_ZONE_NAME_WIN_H_
+#define ABSL_TIME_INTERNAL_CCTZ_TIME_ZONE_NAME_WIN_H_
+
+#include <string>
+
+#include "absl/base/config.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace time_internal {
+namespace cctz {
+
+// Returns the local time zone ID in IANA format (e.g. "America/Los_Angeles"),
+// or the empty string on failure. Not supported on Windows 10 1809 and earlier,
+// where "icu.dll" is not available in the System32 directory.
+std::string GetWindowsLocalTimeZone();
+
+}  // namespace cctz
+}  // namespace time_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_TIME_INTERNAL_CCTZ_TIME_ZONE_NAME_WIN_H_
diff --git a/absl/time/internal/cctz/src/time_zone_name_win_test.cc b/absl/time/internal/cctz/src/time_zone_name_win_test.cc
new file mode 100644
index 00000000000..260c9727c3f
--- /dev/null
+++ b/absl/time/internal/cctz/src/time_zone_name_win_test.cc
@@ -0,0 +1,49 @@
+// Copyright 2026 Google Inc. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//   https://www.apache.org/licenses/LICENSE-2.0
+//
+//   Unless required by applicable law or agreed to in writing, software
+//   distributed under the License is distributed on an "AS IS" BASIS,
+//   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+//   See the License for the specific language governing permissions and
+//   limitations under the License.
+
+#include "absl/time/internal/cctz/src/time_zone_name_win.h"
+
+#include <windows.h>
+
+#include <string>
+
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+#include "absl/time/internal/cctz/include/cctz/time_zone.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace time_internal {
+namespace cctz {
+
+TEST(TimeZoneNameWin, GetWindowsLocalTimeZone) {
+  // On Windows 10 1809+ (where icu.dll is available in System32),
+  // GetWindowsLocalTimeZone() should return a valid IANA time zone name.
+  // Note that LOAD_LIBRARY_SEARCH_SYSTEM32 is not sufficient to reliably load
+  // "icu.dll" in the production code, but it should be OK for testing purposes.
+  HMODULE icu_dll =
+      ::LoadLibraryExW(L"icu.dll", nullptr, LOAD_LIBRARY_SEARCH_SYSTEM32);
+  const std::string tz = GetWindowsLocalTimeZone();
+  if (icu_dll != nullptr) {
+    EXPECT_FALSE(tz.empty());
+    ::FreeLibrary(icu_dll);
+  } else {
+    EXPECT_TRUE(tz.empty());
+  }
+}
+
+}  // namespace cctz
+}  // namespace time_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/time/internal/cctz/src/time_zone_posix.cc b/absl/time/internal/cctz/src/time_zone_posix.cc
index 038740e1d67..62c5e870dd1 100644
--- a/absl/time/internal/cctz/src/time_zone_posix.cc
+++ b/absl/time/internal/cctz/src/time_zone_posix.cc
@@ -12,14 +12,17 @@
 //   See the License for the specific language governing permissions and
 //   limitations under the License.
 
-#include "time_zone_posix.h"
+#include "absl/time/internal/cctz/src/time_zone_posix.h"
 
 #include <cstddef>
 #include <cstring>
 #include <limits>
 #include <string>
 
+#include "absl/base/config.h"
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 namespace cctz {
 
@@ -44,10 +47,11 @@ const char* ParseInt(const char* p, int min, int max, int* vp) {
   return p;
 }
 
-// abbr = <.*?> | [^-+,\d]{3,}
+// abbr = <.+?> | [^-+,\d]{3,}
 const char* ParseAbbr(const char* p, std::string* abbr) {
   const char* op = p;
   if (*p == '<') {  // special zoneinfo <...> form
+    if (*++p == '>' || *p == '\0') return nullptr;  // no "<>"
     while (*++p != '>') {
       if (*p == '\0') return nullptr;
     }
@@ -89,14 +93,17 @@ const char* ParseOffset(const char* p, int min_hour, int max_hour, int sign,
   return p;
 }
 
-// datetime = ( Jn | n | Mm.w.d ) [ / offset ]
+// datetime = , ( Jn | n | Mm.w.d ) [ / offset ]
 const char* ParseDateTime(const char* p, PosixTransition* res) {
-  if (p != nullptr && *p == ',') {
+  if (p != nullptr) {
+    if (*p != ',') return nullptr;
     if (*++p == 'M') {
       int month = 0;
-      if ((p = ParseInt(p + 1, 1, 12, &month)) != nullptr && *p == '.') {
+      if ((p = ParseInt(p + 1, 1, 12, &month)) != nullptr) {
+        if (*p != '.') return nullptr;
         int week = 0;
-        if ((p = ParseInt(p + 1, 1, 5, &week)) != nullptr && *p == '.') {
+        if ((p = ParseInt(p + 1, 1, 5, &week)) != nullptr) {
+          if (*p != '.') return nullptr;
           int weekday = 0;
           if ((p = ParseInt(p + 1, 0, 6, &weekday)) != nullptr) {
             res->date.fmt = PosixTransition::M;
@@ -119,17 +126,17 @@ const char* ParseDateTime(const char* p, PosixTransition* res) {
         res->date.n.day = static_cast<std::int_fast16_t>(day);
       }
     }
-  }
-  if (p != nullptr) {
-    res->time.offset = 2 * 60 * 60;  // default offset is 02:00:00
-    if (*p == '/') p = ParseOffset(p + 1, -167, 167, 1, &res->time.offset);
+    if (p != nullptr) {
+      res->time.offset = 2 * 60 * 60;  // default offset is 02:00:00
+      if (*p == '/') p = ParseOffset(p + 1, -167, 167, 1, &res->time.offset);
+    }
   }
   return p;
 }
 
 }  // namespace
 
-// spec = std offset [ dst [ offset ] , datetime , datetime ]
+// spec = std offset [ dst [ offset ] datetime datetime ]
 bool ParsePosixSpec(const std::string& spec, PosixTimeZone* res) {
   const char* p = spec.c_str();
   if (*p == ':') return false;
@@ -152,4 +159,5 @@ bool ParsePosixSpec(const std::string& spec, PosixTimeZone* res) {
 
 }  // namespace cctz
 }  // namespace time_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/time/internal/cctz/src/time_zone_posix.h b/absl/time/internal/cctz/src/time_zone_posix.h
index 6a60022fa7d..23f569945be 100644
--- a/absl/time/internal/cctz/src/time_zone_posix.h
+++ b/absl/time/internal/cctz/src/time_zone_posix.h
@@ -13,7 +13,7 @@
 //   limitations under the License.
 
 // Parsing of a POSIX zone spec as described in the TZ part of section 8.3 in
-// http://pubs.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap08.html.
+// https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/V1_chap08.html.
 //
 // The current POSIX spec for America/Los_Angeles is "PST8PDT,M3.2.0,M11.1.0",
 // which would be broken down as ...
@@ -55,7 +55,10 @@
 #include <cstdint>
 #include <string>
 
+#include "absl/base/config.h"
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 namespace cctz {
 
@@ -101,7 +104,7 @@ struct PosixTransition {
 
 // The entirety of a POSIX-string specified time-zone rule. The standard
 // abbreviation and offset are always given. If the time zone includes
-// daylight saving, then the daylight abbrevation is non-empty and the
+// daylight saving, then the daylight abbreviation is non-empty and the
 // remaining fields are also valid. Note that the start/end transitions
 // are not ordered---in the southern hemisphere the transition to end
 // daylight time occurs first in any particular year.
@@ -123,6 +126,7 @@ bool ParsePosixSpec(const std::string& spec, PosixTimeZone* res);
 
 }  // namespace cctz
 }  // namespace time_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_TIME_INTERNAL_CCTZ_TIME_ZONE_POSIX_H_
diff --git a/absl/time/internal/cctz/src/time_zone_posix_test.cc b/absl/time/internal/cctz/src/time_zone_posix_test.cc
new file mode 100644
index 00000000000..83b4b9d2124
--- /dev/null
+++ b/absl/time/internal/cctz/src/time_zone_posix_test.cc
@@ -0,0 +1,198 @@
+// Copyright 2026 Google Inc. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//   https://www.apache.org/licenses/LICENSE-2.0
+//
+//   Unless required by applicable law or agreed to in writing, software
+//   distributed under the License is distributed on an "AS IS" BASIS,
+//   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+//   See the License for the specific language governing permissions and
+//   limitations under the License.
+
+#include "absl/time/internal/cctz/src/time_zone_posix.h"
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace time_internal {
+namespace cctz {
+
+using ::testing::Eq;
+using ::testing::IsEmpty;
+
+// We only support the second POSIX format (that is, neither the
+// "first character is a <colon>" format, nor the "geographical
+// or a special timezone" format).  We also require DST start/end
+// rules whenever a DST abbreviation is given (zic always provides
+// them).  So, ...
+//
+//   spec = abbr offset [ abbr [ offset ] datetime datetime ]
+//   abbr = <.+?> | [^-+,\d]{3,}
+//   offset = [+|-]hh[:mm[:ss]]
+//   datetime = , ( Jn | n | Mm.w.d ) [ / offset ]
+
+TEST(ParsePosixSpec, UnsupportedFormats) {
+  PosixTimeZone zone;
+  EXPECT_FALSE(ParsePosixSpec(":characters", &zone));
+  EXPECT_FALSE(ParsePosixSpec("Area/Location", &zone));
+}
+
+TEST(ParsePosixSpec, StdOnly) {
+  PosixTimeZone zone;
+
+  // America/Cancun
+  EXPECT_TRUE(ParsePosixSpec("EST5", &zone));
+  EXPECT_THAT(zone.std_abbr, Eq("EST"));
+  EXPECT_THAT(zone.std_offset, Eq(-5 * 60 * 60));
+  EXPECT_THAT(zone.dst_abbr, IsEmpty());
+
+  // Australia/Darwin
+  EXPECT_TRUE(ParsePosixSpec("ACST-9:30", &zone));
+  EXPECT_THAT(zone.std_abbr, Eq("ACST"));
+  EXPECT_THAT(zone.std_offset, Eq((9 * 60 + 30) * 60));
+  EXPECT_THAT(zone.dst_abbr, IsEmpty());
+
+  // Australia/Eucla
+  EXPECT_TRUE(ParsePosixSpec("<+0845>-8:45", &zone));
+  EXPECT_THAT(zone.std_abbr, Eq("+0845"));
+  EXPECT_THAT(zone.std_offset, Eq((8 * 60 + 45) * 60));
+  EXPECT_THAT(zone.dst_abbr, IsEmpty());
+}
+
+TEST(ParsePosixSpec, WithDst) {
+  PosixTimeZone zone;
+
+  // America/New_York
+  EXPECT_TRUE(ParsePosixSpec("EST5EDT,M3.2.0,M11.1.0", &zone));
+  EXPECT_THAT(zone.std_abbr, Eq("EST"));
+  EXPECT_THAT(zone.std_offset, Eq(-5 * 60 * 60));
+  EXPECT_THAT(zone.dst_abbr, Eq("EDT"));
+  EXPECT_THAT(zone.dst_offset, Eq(-4 * 60 * 60));
+  EXPECT_THAT(zone.dst_start.date.fmt, Eq(PosixTransition::M));
+  EXPECT_THAT(zone.dst_start.date.m.month, Eq(3));
+  EXPECT_THAT(zone.dst_start.date.m.week, Eq(2));
+  EXPECT_THAT(zone.dst_start.date.m.weekday, Eq(0));
+  EXPECT_THAT(zone.dst_start.time.offset, Eq(2 * 60 * 60));
+  EXPECT_THAT(zone.dst_end.date.fmt, Eq(PosixTransition::M));
+  EXPECT_THAT(zone.dst_end.date.m.month, Eq(11));
+  EXPECT_THAT(zone.dst_end.date.m.week, Eq(1));
+  EXPECT_THAT(zone.dst_end.date.m.weekday, Eq(0));
+  EXPECT_THAT(zone.dst_end.time.offset, Eq(2 * 60 * 60));
+
+  // Australia/Adelaide
+  EXPECT_TRUE(ParsePosixSpec("ACST-9:30ACDT,M10.1.0,M4.1.0/3", &zone));
+  EXPECT_THAT(zone.std_abbr, Eq("ACST"));
+  EXPECT_THAT(zone.std_offset, Eq((9 * 60 + 30) * 60));
+  EXPECT_THAT(zone.dst_abbr, Eq("ACDT"));
+  EXPECT_THAT(zone.dst_offset, Eq((10 * 60 + 30) * 60));
+  EXPECT_THAT(zone.dst_start.date.fmt, Eq(PosixTransition::M));
+  EXPECT_THAT(zone.dst_start.date.m.month, Eq(10));
+  EXPECT_THAT(zone.dst_start.date.m.week, Eq(1));
+  EXPECT_THAT(zone.dst_start.date.m.weekday, Eq(0));
+  EXPECT_THAT(zone.dst_start.time.offset, Eq(2 * 60 * 60));
+  EXPECT_THAT(zone.dst_end.date.fmt, Eq(PosixTransition::M));
+  EXPECT_THAT(zone.dst_end.date.m.month, Eq(4));
+  EXPECT_THAT(zone.dst_end.date.m.week, Eq(1));
+  EXPECT_THAT(zone.dst_end.date.m.weekday, Eq(0));
+  EXPECT_THAT(zone.dst_end.time.offset, Eq(3 * 60 * 60));
+
+  // Australia/Lord_Howe
+  EXPECT_TRUE(ParsePosixSpec("<+1030>-10:30<+11>-11,M10.1.0,M4.1.0", &zone));
+  EXPECT_THAT(zone.std_abbr, Eq("+1030"));
+  EXPECT_THAT(zone.std_offset, Eq((10 * 60 + 30) * 60));
+  EXPECT_THAT(zone.dst_abbr, Eq("+11"));
+  EXPECT_THAT(zone.dst_offset, Eq(11 * 60 * 60));
+  EXPECT_THAT(zone.dst_start.date.fmt, Eq(PosixTransition::M));
+  EXPECT_THAT(zone.dst_start.date.m.month, Eq(10));
+  EXPECT_THAT(zone.dst_start.date.m.week, Eq(1));
+  EXPECT_THAT(zone.dst_start.date.m.weekday, Eq(0));
+  EXPECT_THAT(zone.dst_start.time.offset, Eq(2 * 60 * 60));
+  EXPECT_THAT(zone.dst_end.date.fmt, Eq(PosixTransition::M));
+  EXPECT_THAT(zone.dst_end.date.m.month, Eq(4));
+  EXPECT_THAT(zone.dst_end.date.m.week, Eq(1));
+  EXPECT_THAT(zone.dst_end.date.m.weekday, Eq(0));
+  EXPECT_THAT(zone.dst_end.time.offset, Eq(2 * 60 * 60));
+
+  // Africa/Casablanca (year-round DST)
+  EXPECT_TRUE(ParsePosixSpec("<+00>0<+01>,0/0,J365/25", &zone));
+  EXPECT_THAT(zone.std_abbr, Eq("+00"));
+  EXPECT_THAT(zone.std_offset, Eq(0));
+  EXPECT_THAT(zone.dst_abbr, Eq("+01"));
+  EXPECT_THAT(zone.dst_offset, Eq(1 * 60 * 60));
+  EXPECT_THAT(zone.dst_start.date.fmt, Eq(PosixTransition::N));
+  EXPECT_THAT(zone.dst_start.date.n.day, Eq(0));
+  EXPECT_THAT(zone.dst_start.time.offset, Eq(0));
+  EXPECT_THAT(zone.dst_end.date.fmt, Eq(PosixTransition::J));
+  EXPECT_THAT(zone.dst_end.date.n.day, Eq(365));
+  EXPECT_THAT(zone.dst_end.time.offset, Eq(25 * 60 * 60));
+}
+
+TEST(TimeZonePosix, ParseErrors) {
+  PosixTimeZone zone;
+
+  // STD abbreviation errors.
+  EXPECT_FALSE(ParsePosixSpec("ET5", &zone));
+  EXPECT_FALSE(ParsePosixSpec("ET+", &zone));
+  EXPECT_FALSE(ParsePosixSpec("ET-", &zone));
+  EXPECT_FALSE(ParsePosixSpec("ET,", &zone));
+  EXPECT_FALSE(ParsePosixSpec("<", &zone));
+  EXPECT_FALSE(ParsePosixSpec("<00", &zone));
+  EXPECT_FALSE(ParsePosixSpec("<>0", &zone));
+
+  // STD offset errors.
+  EXPECT_FALSE(ParsePosixSpec("<00>", &zone));
+  EXPECT_FALSE(ParsePosixSpec("<00>+", &zone));
+  EXPECT_FALSE(ParsePosixSpec("<00>-", &zone));
+  EXPECT_FALSE(ParsePosixSpec("<00>?", &zone));
+
+  // DST abbreviation errors.
+  EXPECT_FALSE(ParsePosixSpec("EST5DT,M3.2.0,M11.1.0", &zone));
+  EXPECT_FALSE(ParsePosixSpec("EST5EDT+,M3.2.0,M11.1.0", &zone));
+  EXPECT_FALSE(ParsePosixSpec("EST5EDT-,M3.2.0,M11.1.0", &zone));
+  EXPECT_FALSE(ParsePosixSpec("<00>0<-01", &zone));
+  EXPECT_FALSE(ParsePosixSpec("EST5<>,M3.2.0,M11.1.0", &zone));
+
+  // DST offset errors.
+  EXPECT_FALSE(ParsePosixSpec("<01>1<00>?,0,0", &zone));
+  EXPECT_FALSE(ParsePosixSpec("<01>1<00>+?,0,0", &zone));
+  EXPECT_FALSE(ParsePosixSpec("<01>1<00>-?,0,0", &zone));
+
+  // Malformed DST start date/time.
+  EXPECT_FALSE(ParsePosixSpec("EST5EDT", &zone));
+  EXPECT_FALSE(ParsePosixSpec("EST5EDT,", &zone));
+  EXPECT_FALSE(ParsePosixSpec("EST5EDT,M13.2.0,M11.1.0", &zone));
+  EXPECT_FALSE(ParsePosixSpec("EST5EDT,M3.6.0,M11.1.0", &zone));
+  EXPECT_FALSE(ParsePosixSpec("EST5EDT,M3.2.7,M11.1.0", &zone));
+  EXPECT_FALSE(ParsePosixSpec("EST5EDT,J0,M11.1.0", &zone));
+  EXPECT_FALSE(ParsePosixSpec("EST5EDT,366,M11.1.0", &zone));
+  EXPECT_FALSE(ParsePosixSpec("EST5EDT,M3.2.0/,M11.1.0", &zone));
+  EXPECT_FALSE(ParsePosixSpec("EST5EDT,M3.2.0/?,M11.1.0", &zone));
+  EXPECT_FALSE(ParsePosixSpec("EST5EDT,M3.2.0/1:?,M11.1.0", &zone));
+  EXPECT_FALSE(ParsePosixSpec("EST5EDT,M3.2.0/1:2:?,M11.1.0", &zone));
+
+  // Malformed DST end date/time.
+  EXPECT_FALSE(ParsePosixSpec("EST5EDT,M3.2.0", &zone));
+  EXPECT_FALSE(ParsePosixSpec("EST5EDT,M3.2.0,M?.1.0", &zone));
+  EXPECT_FALSE(ParsePosixSpec("EST5EDT,M3.2.0,M11.?.0", &zone));
+  EXPECT_FALSE(ParsePosixSpec("EST5EDT,M3.2.0,M11.1.?", &zone));
+  EXPECT_FALSE(ParsePosixSpec("EST5EDT,M3.2.0,J?", &zone));
+  EXPECT_FALSE(ParsePosixSpec("EST5EDT,M3.2.0,?", &zone));
+  EXPECT_FALSE(ParsePosixSpec("EST5EDT,M3.2.0,M11.1.0/", &zone));
+  EXPECT_FALSE(ParsePosixSpec("EST5EDT,M3.2.0,M11.1.0/168", &zone));
+  EXPECT_FALSE(ParsePosixSpec("EST5EDT,M3.2.0,M11.1.0/167:60", &zone));
+  EXPECT_FALSE(ParsePosixSpec("EST5EDT,M3.2.0,M11.1.0/167:59:60", &zone));
+
+  // Trailing data.
+  EXPECT_FALSE(ParsePosixSpec("EST5EDT,M3.2.0,M11.1.0junk", &zone));
+}
+
+}  // namespace cctz
+}  // namespace time_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/time/internal/cctz/src/tzfile.h b/absl/time/internal/cctz/src/tzfile.h
index 51b1f1f3d83..00e75dea5d8 100644
--- a/absl/time/internal/cctz/src/tzfile.h
+++ b/absl/time/internal/cctz/src/tzfile.h
@@ -17,41 +17,25 @@
 ** Thank you!
 */
 
-/*
-** Information about time zone files.
-*/
-
-#ifndef TZDIR
-#define TZDIR	"/usr/share/zoneinfo" /* Time zone object file directory */
-#endif /* !defined TZDIR */
-
-#ifndef TZDEFAULT
-#define TZDEFAULT	"/etc/localtime"
-#endif /* !defined TZDEFAULT */
-
-#ifndef TZDEFRULES
-#define TZDEFRULES	"posixrules"
-#endif /* !defined TZDEFRULES */
-
-
-/* See Internet RFC 8536 for more details about the following format.  */
+/* Information about time zone files.
+   See Internet RFC 9636 for more details about the following format.  */
 
 /*
 ** Each file begins with. . .
 */
 
-#define	TZ_MAGIC	"TZif"
+#define TZ_MAGIC "TZif"
 
 struct tzhead {
-	char	tzh_magic[4];		/* TZ_MAGIC */
-	char	tzh_version[1];		/* '\0' or '2' or '3' as of 2013 */
-	char	tzh_reserved[15];	/* reserved; must be zero */
-	char	tzh_ttisutcnt[4];	/* coded number of trans. time flags */
-	char	tzh_ttisstdcnt[4];	/* coded number of trans. time flags */
-	char	tzh_leapcnt[4];		/* coded number of leap seconds */
-	char	tzh_timecnt[4];		/* coded number of transition times */
-	char	tzh_typecnt[4];		/* coded number of local time types */
-	char	tzh_charcnt[4];		/* coded number of abbr. chars */
+  char tzh_magic[4];      /* TZ_MAGIC */
+  char tzh_version[1];    /* '\0' or '2'-'4' as of 2021 */
+  char tzh_reserved[15];  /* reserved; must be zero */
+  char tzh_ttisutcnt[4];  /* coded number of trans. time flags */
+  char tzh_ttisstdcnt[4]; /* coded number of trans. time flags */
+  char tzh_leapcnt[4];    /* coded number of leap seconds */
+  char tzh_timecnt[4];    /* coded number of transition times */
+  char tzh_typecnt[4];    /* coded number of local time types */
+  char tzh_charcnt[4];    /* coded number of abbr. chars */
 };
 
 /*
@@ -84,14 +68,16 @@ struct tzhead {
 ** If tzh_version is '2' or greater, the above is followed by a second instance
 ** of tzhead and a second instance of the data in which each coded transition
 ** time uses 8 rather than 4 chars,
-** then a POSIX-TZ-environment-variable-style std::string for use in handling
+** then a POSIX.1-2017 proleptic TZ string for use in handling
 ** instants after the last transition time stored in the file
-** (with nothing between the newlines if there is no POSIX representation for
-** such instants).
+** (with nothing between the newlines if there is no POSIX.1-2017
+** representation for such instants).
 **
-** If tz_version is '3' or greater, the above is extended as follows.
-** First, the POSIX TZ std::string's hour offset may range from -167
-** through 167 as compared to the POSIX-required 0 through 24.
+** If tz_version is '3' or greater, the TZ string can be any POSIX.1-2024
+** proleptic TZ string, which means the above is extended as follows.
+** First, the TZ string's hour offset may range from -167
+** through 167 as compared to the range 0 through 24 required
+** by POSIX.1-2017 and earlier.
 ** Second, its DST start time may be January 1 at 00:00 and its stop
 ** time December 31 at 24:00 plus the difference between DST and
 ** standard time, indicating DST all year.
@@ -103,21 +89,27 @@ struct tzhead {
 */
 
 #ifndef TZ_MAX_TIMES
-#define TZ_MAX_TIMES	2000
+/* The following limit applies to localtime.c; zic has no such limit.
+   The limit must be at least 310 for Asia/Hebron with 'zic -b fat'.  */
+#define TZ_MAX_TIMES 2000
 #endif /* !defined TZ_MAX_TIMES */
 
 #ifndef TZ_MAX_TYPES
-/* This must be at least 17 for Europe/Samara and Europe/Vilnius.  */
-#define TZ_MAX_TYPES	256 /* Limited by what (unsigned char)'s can hold */
-#endif /* !defined TZ_MAX_TYPES */
+/* This must be at least 18 for Europe/Vilnius with 'zic -b fat'.  */
+#define TZ_MAX_TYPES 256 /* Limited to 256 by Internet RFC 9636.  */
+#endif                   /* !defined TZ_MAX_TYPES */
 
 #ifndef TZ_MAX_CHARS
-#define TZ_MAX_CHARS	50	/* Maximum number of abbreviation characters */
-				/* (limited by what unsigned chars can hold) */
-#endif /* !defined TZ_MAX_CHARS */
+/* This must be at least 40 for America/Anchorage.  */
+#define TZ_MAX_CHARS 256 /* Maximum number of abbreviation characters */
+                         /* (limited to 256 by Internet RFC 9636) */
+#endif                   /* !defined TZ_MAX_CHARS */
 
 #ifndef TZ_MAX_LEAPS
-#define TZ_MAX_LEAPS	50	/* Maximum number of leap second corrections */
-#endif /* !defined TZ_MAX_LEAPS */
+/* The following limit applies to localtime.c; zic has no such limit.
+   The limit must be at least 27 for leap seconds from 1972 through mid-2023.
+   There's a plan to discontinue leap seconds by 2035.  */
+#define TZ_MAX_LEAPS 50 /* Maximum number of leap second corrections */
+#endif                  /* !defined TZ_MAX_LEAPS */
 
 #endif /* !defined TZFILE_H */
diff --git a/absl/time/internal/cctz/src/zone_info_source.cc b/absl/time/internal/cctz/src/zone_info_source.cc
index 42f50c5e8fb..9bc8197bab1 100644
--- a/absl/time/internal/cctz/src/zone_info_source.cc
+++ b/absl/time/internal/cctz/src/zone_info_source.cc
@@ -14,7 +14,10 @@
 
 #include "absl/time/internal/cctz/include/cctz/zone_info_source.h"
 
+#include "absl/base/config.h"
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 namespace cctz {
 
@@ -24,9 +27,11 @@ std::string ZoneInfoSource::Version() const { return std::string(); }
 
 }  // namespace cctz
 }  // namespace time_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 namespace cctz_extension {
 
@@ -36,8 +41,9 @@ namespace {
 // defers to the fallback factory.
 std::unique_ptr<absl::time_internal::cctz::ZoneInfoSource> DefaultFactory(
     const std::string& name,
-    const std::function<std::unique_ptr<absl::time_internal::cctz::ZoneInfoSource>(
-        const std::string& name)>& fallback_factory) {
+    const std::function<
+        std::unique_ptr<absl::time_internal::cctz::ZoneInfoSource>(
+            const std::string& name)>& fallback_factory) {
   return fallback_factory(name);
 }
 
@@ -49,21 +55,53 @@ std::unique_ptr<absl::time_internal::cctz::ZoneInfoSource> DefaultFactory(
 #if !defined(__has_attribute)
 #define __has_attribute(x) 0
 #endif
-#if __has_attribute(weak) || defined(__GNUC__)
-ZoneInfoSourceFactory zone_info_source_factory
-    __attribute__((weak)) = DefaultFactory;
-#elif defined(_MSC_VER) && !defined(_LIBCPP_VERSION)
+// MinGW is GCC on Windows, so while it asserts __has_attribute(weak), the
+// Windows linker cannot handle that. Nor does the MinGW compiler know how to
+// pass "#pragma comment(linker, ...)" to the Windows linker.
+#if (__has_attribute(weak) || defined(__GNUC__)) && !defined(__MINGW32__) && \
+    !defined(__CYGWIN__)
+ZoneInfoSourceFactory zone_info_source_factory __attribute__((weak)) =
+    DefaultFactory;
+#elif defined(_MSC_VER) && !defined(__MINGW32__) && !defined(_LIBCPP_VERSION)
 extern ZoneInfoSourceFactory zone_info_source_factory;
 extern ZoneInfoSourceFactory default_factory;
 ZoneInfoSourceFactory default_factory = DefaultFactory;
-#if defined(_M_IX86)
-#pragma comment( \
-    linker,      \
-    "/alternatename:?zone_info_source_factory@cctz_extension@time_internal@absl@@3P6A?AV?$unique_ptr@VZoneInfoSource@cctz@time_internal@absl@@U?$default_delete@VZoneInfoSource@cctz@time_internal@absl@@@std@@@std@@ABV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@5@ABV?$function@$$A6A?AV?$unique_ptr@VZoneInfoSource@cctz@time_internal@absl@@U?$default_delete@VZoneInfoSource@cctz@time_internal@absl@@@std@@@std@@ABV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@2@@Z@5@@ZA=?default_factory@cctz_extension@time_internal@absl@@3P6A?AV?$unique_ptr@VZoneInfoSource@cctz@time_internal@absl@@U?$default_delete@VZoneInfoSource@cctz@time_internal@absl@@@std@@@std@@ABV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@5@ABV?$function@$$A6A?AV?$unique_ptr@VZoneInfoSource@cctz@time_internal@absl@@U?$default_delete@VZoneInfoSource@cctz@time_internal@absl@@@std@@@std@@ABV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@2@@Z@5@@ZA")
+#if defined(_M_IX86) || defined(_M_ARM)
+#pragma comment(                                                                                                         \
+    linker,                                                                                                              \
+    "/alternatename:?zone_info_source_factory@cctz_extension@time_internal@" ABSL_INTERNAL_MANGLED_NS                    \
+    "@@3P6A?AV?$unique_ptr@VZoneInfoSource@cctz@time_internal@" ABSL_INTERNAL_MANGLED_NS                                 \
+    "@@U?$default_delete@VZoneInfoSource@cctz@time_internal@" ABSL_INTERNAL_MANGLED_NS                                   \
+    "@@@std@@@std@@ABV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@" ABSL_INTERNAL_MANGLED_BACKREFERENCE      \
+    "@ABV?$function@$$A6A?AV?$unique_ptr@VZoneInfoSource@cctz@time_internal@" ABSL_INTERNAL_MANGLED_NS                   \
+    "@@U?$default_delete@VZoneInfoSource@cctz@time_internal@" ABSL_INTERNAL_MANGLED_NS                                   \
+    "@@@std@@@std@@ABV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@2@@Z@" ABSL_INTERNAL_MANGLED_BACKREFERENCE \
+    "@@ZA=?default_factory@cctz_extension@time_internal@" ABSL_INTERNAL_MANGLED_NS                                       \
+    "@@3P6A?AV?$unique_ptr@VZoneInfoSource@cctz@time_internal@" ABSL_INTERNAL_MANGLED_NS                                 \
+    "@@U?$default_delete@VZoneInfoSource@cctz@time_internal@" ABSL_INTERNAL_MANGLED_NS                                   \
+    "@@@std@@@std@@ABV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@" ABSL_INTERNAL_MANGLED_BACKREFERENCE      \
+    "@ABV?$function@$$A6A?AV?$unique_ptr@VZoneInfoSource@cctz@time_internal@" ABSL_INTERNAL_MANGLED_NS                   \
+    "@@U?$default_delete@VZoneInfoSource@cctz@time_internal@" ABSL_INTERNAL_MANGLED_NS                                   \
+    "@@@std@@@std@@ABV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@2@@Z@" ABSL_INTERNAL_MANGLED_BACKREFERENCE \
+    "@@ZA")
 #elif defined(_M_IA_64) || defined(_M_AMD64) || defined(_M_ARM64)
-#pragma comment( \
-    linker,      \
-    "/alternatename:?zone_info_source_factory@cctz_extension@time_internal@absl@@3P6A?AV?$unique_ptr@VZoneInfoSource@cctz@time_internal@absl@@U?$default_delete@VZoneInfoSource@cctz@time_internal@absl@@@std@@@std@@AEBV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@5@AEBV?$function@$$A6A?AV?$unique_ptr@VZoneInfoSource@cctz@time_internal@absl@@U?$default_delete@VZoneInfoSource@cctz@time_internal@absl@@@std@@@std@@AEBV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@2@@Z@5@@ZEA=?default_factory@cctz_extension@time_internal@absl@@3P6A?AV?$unique_ptr@VZoneInfoSource@cctz@time_internal@absl@@U?$default_delete@VZoneInfoSource@cctz@time_internal@absl@@@std@@@std@@AEBV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@5@AEBV?$function@$$A6A?AV?$unique_ptr@VZoneInfoSource@cctz@time_internal@absl@@U?$default_delete@VZoneInfoSource@cctz@time_internal@absl@@@std@@@std@@AEBV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@2@@Z@5@@ZEA")
+#pragma comment(                                                                                                          \
+    linker,                                                                                                               \
+    "/alternatename:?zone_info_source_factory@cctz_extension@time_internal@" ABSL_INTERNAL_MANGLED_NS                     \
+    "@@3P6A?AV?$unique_ptr@VZoneInfoSource@cctz@time_internal@" ABSL_INTERNAL_MANGLED_NS                                  \
+    "@@U?$default_delete@VZoneInfoSource@cctz@time_internal@" ABSL_INTERNAL_MANGLED_NS                                    \
+    "@@@std@@@std@@AEBV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@" ABSL_INTERNAL_MANGLED_BACKREFERENCE      \
+    "@AEBV?$function@$$A6A?AV?$unique_ptr@VZoneInfoSource@cctz@time_internal@" ABSL_INTERNAL_MANGLED_NS                   \
+    "@@U?$default_delete@VZoneInfoSource@cctz@time_internal@" ABSL_INTERNAL_MANGLED_NS                                    \
+    "@@@std@@@std@@AEBV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@2@@Z@" ABSL_INTERNAL_MANGLED_BACKREFERENCE \
+    "@@ZEA=?default_factory@cctz_extension@time_internal@" ABSL_INTERNAL_MANGLED_NS                                       \
+    "@@3P6A?AV?$unique_ptr@VZoneInfoSource@cctz@time_internal@" ABSL_INTERNAL_MANGLED_NS                                  \
+    "@@U?$default_delete@VZoneInfoSource@cctz@time_internal@" ABSL_INTERNAL_MANGLED_NS                                    \
+    "@@@std@@@std@@AEBV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@" ABSL_INTERNAL_MANGLED_BACKREFERENCE      \
+    "@AEBV?$function@$$A6A?AV?$unique_ptr@VZoneInfoSource@cctz@time_internal@" ABSL_INTERNAL_MANGLED_NS                   \
+    "@@U?$default_delete@VZoneInfoSource@cctz@time_internal@" ABSL_INTERNAL_MANGLED_NS                                    \
+    "@@@std@@@std@@AEBV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@2@@Z@" ABSL_INTERNAL_MANGLED_BACKREFERENCE \
+    "@@ZEA")
 #else
 #error Unsupported MSVC platform
 #endif  // _M_<PLATFORM>
@@ -74,4 +112,5 @@ ZoneInfoSourceFactory zone_info_source_factory = DefaultFactory;
 
 }  // namespace cctz_extension
 }  // namespace time_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/time/internal/cctz/testdata/README.zoneinfo b/absl/time/internal/cctz/testdata/README.zoneinfo
index 95fb4a91d17..0fe558585a5 100644
--- a/absl/time/internal/cctz/testdata/README.zoneinfo
+++ b/absl/time/internal/cctz/testdata/README.zoneinfo
@@ -21,7 +21,8 @@ New versions can be generated using the following shell script.
                 REDO=posix_only \
                 LOCALTIME=Factory \
                 TZDATA_TEXT= \
-                ZONETABLES=zone1970.tab
+                PACKRATDATA=backzone PACKRATLIST=zone.tab \
+                ZONETABLES=zone1970.tab\ zonenow.tab
     tar --create --dereference --hard-dereference --file tzfile.tar \
         --directory=tz tzfile.h
     tar --create --dereference --hard-dereference --file zoneinfo.tar \
diff --git a/absl/time/internal/cctz/testdata/version b/absl/time/internal/cctz/testdata/version
index db18f8311da..9217a2da674 100644
--- a/absl/time/internal/cctz/testdata/version
+++ b/absl/time/internal/cctz/testdata/version
@@ -1 +1 @@
-2019c
+2026c
diff --git a/absl/time/internal/cctz/testdata/zoneinfo/Africa/Abidjan b/absl/time/internal/cctz/testdata/zoneinfo/Africa/Abidjan
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diff --git a/absl/time/internal/cctz/testdata/zoneinfo/Africa/Accra b/absl/time/internal/cctz/testdata/zoneinfo/Africa/Accra
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diff --git a/absl/time/internal/cctz/testdata/zoneinfo/Africa/Addis_Ababa b/absl/time/internal/cctz/testdata/zoneinfo/Africa/Addis_Ababa
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diff --git a/absl/time/internal/cctz/testdata/zoneinfo/Africa/Bangui b/absl/time/internal/cctz/testdata/zoneinfo/Africa/Bangui
index 0c80137c748..0021d2d7fd6 100644
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index 82ea5aaf0c6..0da1d1e211b 100644
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diff --git a/absl/time/internal/cctz/testdata/zoneinfo/Africa/Blantyre b/absl/time/internal/cctz/testdata/zoneinfo/Africa/Blantyre
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diff --git a/absl/time/internal/cctz/testdata/zoneinfo/Africa/Brazzaville b/absl/time/internal/cctz/testdata/zoneinfo/Africa/Brazzaville
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diff --git a/absl/time/internal/cctz/testdata/zoneinfo/Africa/Cairo b/absl/time/internal/cctz/testdata/zoneinfo/Africa/Cairo
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diff --git a/absl/time/internal/cctz/testdata/zoneinfo/Africa/Casablanca b/absl/time/internal/cctz/testdata/zoneinfo/Africa/Casablanca
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diff --git a/absl/time/internal/cctz/testdata/zoneinfo/Africa/Ceuta b/absl/time/internal/cctz/testdata/zoneinfo/Africa/Ceuta
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diff --git a/absl/time/internal/cctz/testdata/zoneinfo/Africa/Conakry b/absl/time/internal/cctz/testdata/zoneinfo/Africa/Conakry
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diff --git a/absl/time/internal/cctz/testdata/zoneinfo/Africa/Dakar b/absl/time/internal/cctz/testdata/zoneinfo/Africa/Dakar
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diff --git a/absl/time/internal/cctz/testdata/zoneinfo/Africa/Gaborone b/absl/time/internal/cctz/testdata/zoneinfo/Africa/Gaborone
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diff --git a/absl/time/internal/cctz/testdata/zoneinfo/WET b/absl/time/internal/cctz/testdata/zoneinfo/WET
index c27390b5b63..7e9aae727b2 100644
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diff --git a/absl/time/internal/cctz/testdata/zoneinfo/Zulu b/absl/time/internal/cctz/testdata/zoneinfo/Zulu
index 91558be0c2b..00841a62213 100644
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diff --git a/absl/time/internal/cctz/testdata/zoneinfo/iso3166.tab b/absl/time/internal/cctz/testdata/zoneinfo/iso3166.tab
index a4ff61a4d32..4ae35234b97 100644
--- a/absl/time/internal/cctz/testdata/zoneinfo/iso3166.tab
+++ b/absl/time/internal/cctz/testdata/zoneinfo/iso3166.tab
@@ -3,17 +3,22 @@
 # This file is in the public domain, so clarified as of
 # 2009-05-17 by Arthur David Olson.
 #
-# From Paul Eggert (2015-05-02):
+# From Paul Eggert (2025-07-01):
 # This file contains a table of two-letter country codes.  Columns are
-# separated by a single tab.  Lines beginning with '#' are comments.
+# separated by a single tab.  Lines beginning with ‘#’ are comments.
 # All text uses UTF-8 encoding.  The columns of the table are as follows:
 #
 # 1.  ISO 3166-1 alpha-2 country code, current as of
-#     ISO 3166-1 N976 (2018-11-06).  See: Updates on ISO 3166-1
-#     https://isotc.iso.org/livelink/livelink/Open/16944257
-# 2.  The usual English name for the coded region,
-#     chosen so that alphabetic sorting of subsets produces helpful lists.
-#     This is not the same as the English name in the ISO 3166 tables.
+#     ISO/TC 46 N1127 (2024-02-29).  See: ISO/TC 46 Documents
+#     https://www.iso.org/committee/48750.html?view=documents
+# 2.  The usual English name for the coded region.  This sometimes
+#     departs from ISO-listed names, sometimes so that sorted subsets
+#     of names are useful (e.g., “Samoa (American)” and “Samoa
+#     (western)” rather than “American Samoa” and “Samoa”),
+#     sometimes to avoid confusion among non-experts (e.g.,
+#     “Czech Republic” and “Turkey” rather than “Czechia” and “Türkiye”),
+#     and sometimes to omit needless detail or churn (e.g., “Netherlands”
+#     rather than “Netherlands (the)” or “Netherlands (Kingdom of the)”).
 #
 # The table is sorted by country code.
 #
@@ -66,7 +71,7 @@ CD	Congo (Dem. Rep.)
 CF	Central African Rep.
 CG	Congo (Rep.)
 CH	Switzerland
-CI	Côte d'Ivoire
+CI	Côte d’Ivoire
 CK	Cook Islands
 CL	Chile
 CM	Cameroon
@@ -238,7 +243,7 @@ SY	Syria
 SZ	Eswatini (Swaziland)
 TC	Turks & Caicos Is
 TD	Chad
-TF	French Southern & Antarctic Lands
+TF	French S. Terr.
 TG	Togo
 TH	Thailand
 TJ	Tajikistan
diff --git a/absl/time/internal/cctz/testdata/zoneinfo/localtime b/absl/time/internal/cctz/testdata/zoneinfo/localtime
deleted file mode 100644
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diff --git a/absl/time/internal/cctz/testdata/zoneinfo/zone1970.tab b/absl/time/internal/cctz/testdata/zoneinfo/zone1970.tab
index 822ffa1f1af..635eabcbf2d 100644
--- a/absl/time/internal/cctz/testdata/zoneinfo/zone1970.tab
+++ b/absl/time/internal/cctz/testdata/zoneinfo/zone1970.tab
@@ -2,15 +2,15 @@
 #
 # This file is in the public domain.
 #
-# From Paul Eggert (2018-06-27):
+# From Paul Eggert (2025-05-15):
 # This file contains a table where each row stands for a timezone where
 # civil timestamps have agreed since 1970.  Columns are separated by
-# a single tab.  Lines beginning with '#' are comments.  All text uses
+# a single tab.  Lines beginning with ‘#’ are comments.  All text uses
 # UTF-8 encoding.  The columns of the table are as follows:
 #
 # 1.  The countries that overlap the timezone, as a comma-separated list
-#     of ISO 3166 2-character country codes.  See the file 'iso3166.tab'.
-# 2.  Latitude and longitude of the timezone's principal location
+#     of ISO 3166 2-character country codes.
+# 2.  Latitude and longitude of the timezone’s principal location
 #     in ISO 6709 sign-degrees-minutes-seconds format,
 #     either ±DDMM±DDDMM or ±DDMMSS±DDDMMSS,
 #     first latitude (+ is north), then longitude (+ is east).
@@ -18,7 +18,10 @@
 #     Please see the theory.html file for how these names are chosen.
 #     If multiple timezones overlap a country, each has a row in the
 #     table, with each column 1 containing the country code.
-# 4.  Comments; present if and only if a country has multiple timezones.
+# 4.  Comments; present if and only if countries have multiple timezones,
+#     and useful only for those countries.  For example, the comments
+#     for the row with countries CH,DE,LI and name Europe/Zurich
+#     are useful only for DE, since CH and LI have no other timezones.
 #
 # If a timezone covers multiple countries, the most-populous city is used,
 # and that country is listed first in column 1; any other countries
@@ -34,37 +37,34 @@
 #country-
 #codes	coordinates	TZ	comments
 AD	+4230+00131	Europe/Andorra
-AE,OM	+2518+05518	Asia/Dubai
+AE,OM,RE,SC,TF	+2518+05518	Asia/Dubai	Crozet
 AF	+3431+06912	Asia/Kabul
 AL	+4120+01950	Europe/Tirane
 AM	+4011+04430	Asia/Yerevan
 AQ	-6617+11031	Antarctica/Casey	Casey
 AQ	-6835+07758	Antarctica/Davis	Davis
-AQ	-6640+14001	Antarctica/DumontDUrville	Dumont-d'Urville
 AQ	-6736+06253	Antarctica/Mawson	Mawson
 AQ	-6448-06406	Antarctica/Palmer	Palmer
 AQ	-6734-06808	Antarctica/Rothera	Rothera
-AQ	-690022+0393524	Antarctica/Syowa	Syowa
 AQ	-720041+0023206	Antarctica/Troll	Troll
 AQ	-7824+10654	Antarctica/Vostok	Vostok
 AR	-3436-05827	America/Argentina/Buenos_Aires	Buenos Aires (BA, CF)
-AR	-3124-06411	America/Argentina/Cordoba	Argentina (most areas: CB, CC, CN, ER, FM, MN, SE, SF)
+AR	-3124-06411	America/Argentina/Cordoba	most areas: CB, CC, CN, ER, FM, MN, SE, SF
 AR	-2447-06525	America/Argentina/Salta	Salta (SA, LP, NQ, RN)
 AR	-2411-06518	America/Argentina/Jujuy	Jujuy (JY)
 AR	-2649-06513	America/Argentina/Tucuman	Tucumán (TM)
-AR	-2828-06547	America/Argentina/Catamarca	Catamarca (CT); Chubut (CH)
+AR	-2828-06547	America/Argentina/Catamarca	Catamarca (CT), Chubut (CH)
 AR	-2926-06651	America/Argentina/La_Rioja	La Rioja (LR)
 AR	-3132-06831	America/Argentina/San_Juan	San Juan (SJ)
 AR	-3253-06849	America/Argentina/Mendoza	Mendoza (MZ)
 AR	-3319-06621	America/Argentina/San_Luis	San Luis (SL)
 AR	-5138-06913	America/Argentina/Rio_Gallegos	Santa Cruz (SC)
 AR	-5448-06818	America/Argentina/Ushuaia	Tierra del Fuego (TF)
-AS,UM	-1416-17042	Pacific/Pago_Pago	Samoa, Midway
+AS,UM	-1416-17042	Pacific/Pago_Pago	Midway
 AT	+4813+01620	Europe/Vienna
 AU	-3133+15905	Australia/Lord_Howe	Lord Howe Island
 AU	-5430+15857	Antarctica/Macquarie	Macquarie Island
-AU	-4253+14719	Australia/Hobart	Tasmania (most areas)
-AU	-3956+14352	Australia/Currie	Tasmania (King Island)
+AU	-4253+14719	Australia/Hobart	Tasmania
 AU	-3749+14458	Australia/Melbourne	Victoria
 AU	-3352+15113	Australia/Sydney	New South Wales (most areas)
 AU	-3157+14127	Australia/Broken_Hill	New South Wales (Yancowinna)
@@ -77,13 +77,12 @@ AU	-3143+12852	Australia/Eucla	Western Australia (Eucla)
 AZ	+4023+04951	Asia/Baku
 BB	+1306-05937	America/Barbados
 BD	+2343+09025	Asia/Dhaka
-BE	+5050+00420	Europe/Brussels
+BE,LU,NL	+5050+00420	Europe/Brussels
 BG	+4241+02319	Europe/Sofia
 BM	+3217-06446	Atlantic/Bermuda
-BN	+0456+11455	Asia/Brunei
 BO	-1630-06809	America/La_Paz
 BR	-0351-03225	America/Noronha	Atlantic islands
-BR	-0127-04829	America/Belem	Pará (east); Amapá
+BR	-0127-04829	America/Belem	Pará (east), Amapá
 BR	-0343-03830	America/Fortaleza	Brazil (northeast: MA, PI, CE, RN, PB)
 BR	-0803-03454	America/Recife	Pernambuco
 BR	-0712-04812	America/Araguaina	Tocantins
@@ -98,44 +97,35 @@ BR	+0249-06040	America/Boa_Vista	Roraima
 BR	-0308-06001	America/Manaus	Amazonas (east)
 BR	-0640-06952	America/Eirunepe	Amazonas (west)
 BR	-0958-06748	America/Rio_Branco	Acre
-BS	+2505-07721	America/Nassau
 BT	+2728+08939	Asia/Thimphu
 BY	+5354+02734	Europe/Minsk
 BZ	+1730-08812	America/Belize
-CA	+4734-05243	America/St_Johns	Newfoundland; Labrador (southeast)
-CA	+4439-06336	America/Halifax	Atlantic - NS (most areas); PE
+CA	+4734-05243	America/St_Johns	Newfoundland, Labrador (SE)
+CA	+4439-06336	America/Halifax	Atlantic - NS (most areas), PE
 CA	+4612-05957	America/Glace_Bay	Atlantic - NS (Cape Breton)
 CA	+4606-06447	America/Moncton	Atlantic - New Brunswick
 CA	+5320-06025	America/Goose_Bay	Atlantic - Labrador (most areas)
-CA	+5125-05707	America/Blanc-Sablon	AST - QC (Lower North Shore)
-CA	+4339-07923	America/Toronto	Eastern - ON, QC (most areas)
-CA	+4901-08816	America/Nipigon	Eastern - ON, QC (no DST 1967-73)
-CA	+4823-08915	America/Thunder_Bay	Eastern - ON (Thunder Bay)
-CA	+6344-06828	America/Iqaluit	Eastern - NU (most east areas)
-CA	+6608-06544	America/Pangnirtung	Eastern - NU (Pangnirtung)
-CA	+484531-0913718	America/Atikokan	EST - ON (Atikokan); NU (Coral H)
-CA	+4953-09709	America/Winnipeg	Central - ON (west); Manitoba
-CA	+4843-09434	America/Rainy_River	Central - ON (Rainy R, Ft Frances)
+CA,BS	+4339-07923	America/Toronto	Eastern - ON & QC (most areas)
+CA	+6344-06828	America/Iqaluit	Eastern - NU (most areas)
+CA	+4953-09709	America/Winnipeg	Central - ON (west), Manitoba
 CA	+744144-0944945	America/Resolute	Central - NU (Resolute)
 CA	+624900-0920459	America/Rankin_Inlet	Central - NU (central)
 CA	+5024-10439	America/Regina	CST - SK (most areas)
 CA	+5017-10750	America/Swift_Current	CST - SK (midwest)
-CA	+5333-11328	America/Edmonton	Mountain - AB; BC (E); SK (W)
+CA	+5333-11328	America/Edmonton	CST - AB, BC(E), NT(E), SK(W)
 CA	+690650-1050310	America/Cambridge_Bay	Mountain - NU (west)
-CA	+6227-11421	America/Yellowknife	Mountain - NT (central)
 CA	+682059-1334300	America/Inuvik	Mountain - NT (west)
-CA	+4906-11631	America/Creston	MST - BC (Creston)
-CA	+5946-12014	America/Dawson_Creek	MST - BC (Dawson Cr, Ft St John)
+CA	+4916-12307	America/Vancouver	MST - BC (most areas)
+CA	+5546-12014	America/Dawson_Creek	MST - BC (Dawson Cr, Ft St John)
 CA	+5848-12242	America/Fort_Nelson	MST - BC (Ft Nelson)
-CA	+4916-12307	America/Vancouver	Pacific - BC (most areas)
-CA	+6043-13503	America/Whitehorse	Pacific - Yukon (south)
-CA	+6404-13925	America/Dawson	Pacific - Yukon (north)
-CC	-1210+09655	Indian/Cocos
-CH,DE,LI	+4723+00832	Europe/Zurich	Swiss time
-CI,BF,GM,GN,ML,MR,SH,SL,SN,TG	+0519-00402	Africa/Abidjan
+CA	+6043-13503	America/Whitehorse	MST - Yukon (east)
+CA	+6404-13925	America/Dawson	MST - Yukon (west)
+CH,DE,LI	+4723+00832	Europe/Zurich	Büsingen
+CI,BF,GH,GM,GN,IS,ML,MR,SH,SL,SN,TG	+0519-00402	Africa/Abidjan
 CK	-2114-15946	Pacific/Rarotonga
-CL	-3327-07040	America/Santiago	Chile (most areas)
-CL	-5309-07055	America/Punta_Arenas	Region of Magallanes
+CL	-3327-07040	America/Santiago	most of Chile
+CL	-4534-07204	America/Coyhaique	Aysén Region
+CL	-5309-07055	America/Punta_Arenas	Magallanes Region
 CL	-2709-10926	Pacific/Easter	Easter Island
 CN	+3114+12128	Asia/Shanghai	Beijing Time
 CN	+4348+08735	Asia/Urumqi	Xinjiang Time
@@ -143,13 +133,10 @@ CO	+0436-07405	America/Bogota
 CR	+0956-08405	America/Costa_Rica
 CU	+2308-08222	America/Havana
 CV	+1455-02331	Atlantic/Cape_Verde
-CW,AW,BQ,SX	+1211-06900	America/Curacao
-CX	-1025+10543	Indian/Christmas
-CY	+3510+03322	Asia/Nicosia	Cyprus (most areas)
+CY	+3510+03322	Asia/Nicosia	most of Cyprus
 CY	+3507+03357	Asia/Famagusta	Northern Cyprus
 CZ,SK	+5005+01426	Europe/Prague
-DE	+5230+01322	Europe/Berlin	Germany (most areas)
-DK	+5540+01235	Europe/Copenhagen
+DE,DK,NO,SE,SJ	+5230+01322	Europe/Berlin	most of Germany
 DO	+1828-06954	America/Santo_Domingo
 DZ	+3647+00303	Africa/Algiers
 EC	-0210-07950	America/Guayaquil	Ecuador (mainland)
@@ -163,17 +150,14 @@ ES	+2806-01524	Atlantic/Canary	Canary Islands
 FI,AX	+6010+02458	Europe/Helsinki
 FJ	-1808+17825	Pacific/Fiji
 FK	-5142-05751	Atlantic/Stanley
-FM	+0725+15147	Pacific/Chuuk	Chuuk/Truk, Yap
-FM	+0658+15813	Pacific/Pohnpei	Pohnpei/Ponape
 FM	+0519+16259	Pacific/Kosrae	Kosrae
 FO	+6201-00646	Atlantic/Faroe
-FR	+4852+00220	Europe/Paris
+FR,MC	+4852+00220	Europe/Paris
 GB,GG,IM,JE	+513030-0000731	Europe/London
 GE	+4143+04449	Asia/Tbilisi
 GF	+0456-05220	America/Cayenne
-GH	+0533-00013	Africa/Accra
 GI	+3608-00521	Europe/Gibraltar
-GL	+6411-05144	America/Godthab	Greenland (most areas)
+GL	+6411-05144	America/Nuuk	most of Greenland
 GL	+7646-01840	America/Danmarkshavn	National Park (east coast)
 GL	+7029-02158	America/Scoresbysund	Scoresbysund/Ittoqqortoormiit
 GL	+7634-06847	America/Thule	Thule/Pituffik
@@ -189,93 +173,86 @@ HT	+1832-07220	America/Port-au-Prince
 HU	+4730+01905	Europe/Budapest
 ID	-0610+10648	Asia/Jakarta	Java, Sumatra
 ID	-0002+10920	Asia/Pontianak	Borneo (west, central)
-ID	-0507+11924	Asia/Makassar	Borneo (east, south); Sulawesi/Celebes, Bali, Nusa Tengarra; Timor (west)
-ID	-0232+14042	Asia/Jayapura	New Guinea (West Papua / Irian Jaya); Malukus/Moluccas
+ID	-0507+11924	Asia/Makassar	Borneo (east, south), Sulawesi/Celebes, Bali, Nusa Tengarra, Timor (west)
+ID	-0232+14042	Asia/Jayapura	New Guinea (West Papua / Irian Jaya), Malukus/Moluccas
 IE	+5320-00615	Europe/Dublin
 IL	+314650+0351326	Asia/Jerusalem
 IN	+2232+08822	Asia/Kolkata
 IO	-0720+07225	Indian/Chagos
 IQ	+3321+04425	Asia/Baghdad
 IR	+3540+05126	Asia/Tehran
-IS	+6409-02151	Atlantic/Reykjavik
 IT,SM,VA	+4154+01229	Europe/Rome
 JM	+175805-0764736	America/Jamaica
 JO	+3157+03556	Asia/Amman
-JP	+353916+1394441	Asia/Tokyo
+JP,AU	+353916+1394441	Asia/Tokyo	Eyre Bird Observatory
 KE,DJ,ER,ET,KM,MG,SO,TZ,UG,YT	-0117+03649	Africa/Nairobi
 KG	+4254+07436	Asia/Bishkek
-KI	+0125+17300	Pacific/Tarawa	Gilbert Islands
-KI	-0308-17105	Pacific/Enderbury	Phoenix Islands
+KI,MH,TV,UM,WF	+0125+17300	Pacific/Tarawa	Gilberts, Marshalls, Wake
+KI	-0247-17143	Pacific/Kanton	Phoenix Islands
 KI	+0152-15720	Pacific/Kiritimati	Line Islands
 KP	+3901+12545	Asia/Pyongyang
 KR	+3733+12658	Asia/Seoul
-KZ	+4315+07657	Asia/Almaty	Kazakhstan (most areas)
+KZ	+4315+07657	Asia/Almaty	most of Kazakhstan
 KZ	+4448+06528	Asia/Qyzylorda	Qyzylorda/Kyzylorda/Kzyl-Orda
 KZ	+5312+06337	Asia/Qostanay	Qostanay/Kostanay/Kustanay
 KZ	+5017+05710	Asia/Aqtobe	Aqtöbe/Aktobe
 KZ	+4431+05016	Asia/Aqtau	Mangghystaū/Mankistau
-KZ	+4707+05156	Asia/Atyrau	Atyraū/Atirau/Gur'yev
+KZ	+4707+05156	Asia/Atyrau	Atyraū/Atirau/Gur’yev
 KZ	+5113+05121	Asia/Oral	West Kazakhstan
 LB	+3353+03530	Asia/Beirut
 LK	+0656+07951	Asia/Colombo
 LR	+0618-01047	Africa/Monrovia
 LT	+5441+02519	Europe/Vilnius
-LU	+4936+00609	Europe/Luxembourg
 LV	+5657+02406	Europe/Riga
 LY	+3254+01311	Africa/Tripoli
 MA	+3339-00735	Africa/Casablanca
-MC	+4342+00723	Europe/Monaco
 MD	+4700+02850	Europe/Chisinau
-MH	+0709+17112	Pacific/Majuro	Marshall Islands (most areas)
 MH	+0905+16720	Pacific/Kwajalein	Kwajalein
-MM	+1647+09610	Asia/Yangon
-MN	+4755+10653	Asia/Ulaanbaatar	Mongolia (most areas)
-MN	+4801+09139	Asia/Hovd	Bayan-Ölgii, Govi-Altai, Hovd, Uvs, Zavkhan
-MN	+4804+11430	Asia/Choibalsan	Dornod, Sükhbaatar
+MM,CC	+1647+09610	Asia/Yangon
+MN	+4755+10653	Asia/Ulaanbaatar	most of Mongolia
+MN	+4801+09139	Asia/Hovd	Bayan-Ölgii, Hovd, Uvs
 MO	+221150+1133230	Asia/Macau
 MQ	+1436-06105	America/Martinique
 MT	+3554+01431	Europe/Malta
 MU	-2010+05730	Indian/Mauritius
-MV	+0410+07330	Indian/Maldives
-MX	+1924-09909	America/Mexico_City	Central Time
-MX	+2105-08646	America/Cancun	Eastern Standard Time - Quintana Roo
-MX	+2058-08937	America/Merida	Central Time - Campeche, Yucatán
-MX	+2540-10019	America/Monterrey	Central Time - Durango; Coahuila, Nuevo León, Tamaulipas (most areas)
-MX	+2550-09730	America/Matamoros	Central Time US - Coahuila, Nuevo León, Tamaulipas (US border)
-MX	+2313-10625	America/Mazatlan	Mountain Time - Baja California Sur, Nayarit, Sinaloa
-MX	+2838-10605	America/Chihuahua	Mountain Time - Chihuahua (most areas)
-MX	+2934-10425	America/Ojinaga	Mountain Time US - Chihuahua (US border)
-MX	+2904-11058	America/Hermosillo	Mountain Standard Time - Sonora
-MX	+3232-11701	America/Tijuana	Pacific Time US - Baja California
-MX	+2048-10515	America/Bahia_Banderas	Central Time - Bahía de Banderas
-MY	+0310+10142	Asia/Kuala_Lumpur	Malaysia (peninsula)
-MY	+0133+11020	Asia/Kuching	Sabah, Sarawak
+MV,TF	+0410+07330	Indian/Maldives	Kerguelen, St Paul I, Amsterdam I
+MX	+1924-09909	America/Mexico_City	Central Mexico
+MX	+2105-08646	America/Cancun	Quintana Roo
+MX	+2058-08937	America/Merida	Campeche, Yucatán
+MX	+2540-10019	America/Monterrey	Durango; Coahuila, Nuevo León, Tamaulipas (most areas)
+MX	+2550-09730	America/Matamoros	Coahuila, Nuevo León, Tamaulipas (US border)
+MX	+2838-10605	America/Chihuahua	Chihuahua (most areas)
+MX	+3144-10629	America/Ciudad_Juarez	Chihuahua (US border - west)
+MX	+2934-10425	America/Ojinaga	Chihuahua (US border - east)
+MX	+2313-10625	America/Mazatlan	Baja California Sur, Nayarit (most areas), Sinaloa
+MX	+2048-10515	America/Bahia_Banderas	Bahía de Banderas
+MX	+2904-11058	America/Hermosillo	Sonora
+MX	+3232-11701	America/Tijuana	Baja California
+MY,BN	+0133+11020	Asia/Kuching	Sabah, Sarawak
 MZ,BI,BW,CD,MW,RW,ZM,ZW	-2558+03235	Africa/Maputo	Central Africa Time
 NA	-2234+01706	Africa/Windhoek
 NC	-2216+16627	Pacific/Noumea
 NF	-2903+16758	Pacific/Norfolk
 NG,AO,BJ,CD,CF,CG,CM,GA,GQ,NE	+0627+00324	Africa/Lagos	West Africa Time
 NI	+1209-08617	America/Managua
-NL	+5222+00454	Europe/Amsterdam
-NO,SJ	+5955+01045	Europe/Oslo
 NP	+2743+08519	Asia/Kathmandu
 NR	-0031+16655	Pacific/Nauru
 NU	-1901-16955	Pacific/Niue
 NZ,AQ	-3652+17446	Pacific/Auckland	New Zealand time
 NZ	-4357-17633	Pacific/Chatham	Chatham Islands
-PA,KY	+0858-07932	America/Panama
+PA,CA,KY	+0858-07932	America/Panama	EST - ON (Atikokan), NU (Coral H)
 PE	-1203-07703	America/Lima
 PF	-1732-14934	Pacific/Tahiti	Society Islands
 PF	-0900-13930	Pacific/Marquesas	Marquesas Islands
 PF	-2308-13457	Pacific/Gambier	Gambier Islands
-PG	-0930+14710	Pacific/Port_Moresby	Papua New Guinea (most areas)
+PG,AQ,FM	-0930+14710	Pacific/Port_Moresby	Papua New Guinea (most areas), Chuuk, Yap, Dumont d’Urville
 PG	-0613+15534	Pacific/Bougainville	Bougainville
-PH	+1435+12100	Asia/Manila
+PH	+143512+1205804	Asia/Manila
 PK	+2452+06703	Asia/Karachi
 PL	+5215+02100	Europe/Warsaw
 PM	+4703-05620	America/Miquelon
 PN	-2504-13005	Pacific/Pitcairn
-PR	+182806-0660622	America/Puerto_Rico
+PR,AG,CA,AI,AW,BL,BQ,CW,DM,GD,GP,KN,LC,MF,MS,SX,TT,VC,VG,VI	+182806-0660622	America/Puerto_Rico	AST - QC (Lower North Shore)
 PS	+3130+03428	Asia/Gaza	Gaza Strip
 PS	+313200+0350542	Asia/Hebron	West Bank
 PT	+3843-00908	Europe/Lisbon	Portugal (mainland)
@@ -284,16 +261,15 @@ PT	+3744-02540	Atlantic/Azores	Azores
 PW	+0720+13429	Pacific/Palau
 PY	-2516-05740	America/Asuncion
 QA,BH	+2517+05132	Asia/Qatar
-RE,TF	-2052+05528	Indian/Reunion	Réunion, Crozet, Scattered Islands
 RO	+4426+02606	Europe/Bucharest
 RS,BA,HR,ME,MK,SI	+4450+02030	Europe/Belgrade
 RU	+5443+02030	Europe/Kaliningrad	MSK-01 - Kaliningrad
 RU	+554521+0373704	Europe/Moscow	MSK+00 - Moscow area
-# Mention RU and UA alphabetically.  See "territorial claims" above.
-RU,UA	+4457+03406	Europe/Simferopol	MSK+00 - Crimea
+# Mention RU and UA alphabetically.  See “territorial claims” above.
+RU,UA	+4457+03406	Europe/Simferopol	Crimea
 RU	+5836+04939	Europe/Kirov	MSK+00 - Kirov
+RU	+4844+04425	Europe/Volgograd	MSK+00 - Volgograd
 RU	+4621+04803	Europe/Astrakhan	MSK+01 - Astrakhan
-RU	+4844+04425	Europe/Volgograd	MSK+01 - Volgograd
 RU	+5134+04602	Europe/Saratov	MSK+01 - Saratov
 RU	+5420+04824	Europe/Ulyanovsk	MSK+01 - Ulyanovsk
 RU	+5312+05009	Europe/Samara	MSK+01 - Samara, Udmurtia
@@ -312,15 +288,13 @@ RU	+4310+13156	Asia/Vladivostok	MSK+07 - Amur River
 RU	+643337+1431336	Asia/Ust-Nera	MSK+07 - Oymyakonsky
 RU	+5934+15048	Asia/Magadan	MSK+08 - Magadan
 RU	+4658+14242	Asia/Sakhalin	MSK+08 - Sakhalin Island
-RU	+6728+15343	Asia/Srednekolymsk	MSK+08 - Sakha (E); North Kuril Is
+RU	+6728+15343	Asia/Srednekolymsk	MSK+08 - Sakha (E), N Kuril Is
 RU	+5301+15839	Asia/Kamchatka	MSK+09 - Kamchatka
 RU	+6445+17729	Asia/Anadyr	MSK+09 - Bering Sea
-SA,KW,YE	+2438+04643	Asia/Riyadh
-SB	-0932+16012	Pacific/Guadalcanal
-SC	-0440+05528	Indian/Mahe
+SA,AQ,KW,YE	+2438+04643	Asia/Riyadh	Syowa
+SB,FM	-0932+16012	Pacific/Guadalcanal	Pohnpei
 SD	+1536+03232	Africa/Khartoum
-SE	+5920+01803	Europe/Stockholm
-SG	+0117+10351	Asia/Singapore
+SG,AQ,MY	+0117+10351	Asia/Singapore	peninsular Malaysia, Concordia
 SR	+0550-05510	America/Paramaribo
 SS	+0451+03137	Africa/Juba
 ST	+0020+00644	Africa/Sao_Tome
@@ -328,22 +302,16 @@ SV	+1342-08912	America/El_Salvador
 SY	+3330+03618	Asia/Damascus
 TC	+2128-07108	America/Grand_Turk
 TD	+1207+01503	Africa/Ndjamena
-TF	-492110+0701303	Indian/Kerguelen	Kerguelen, St Paul Island, Amsterdam Island
-TH,KH,LA,VN	+1345+10031	Asia/Bangkok	Indochina (most areas)
+TH,CX,KH,LA,VN	+1345+10031	Asia/Bangkok	north Vietnam
 TJ	+3835+06848	Asia/Dushanbe
 TK	-0922-17114	Pacific/Fakaofo
 TL	-0833+12535	Asia/Dili
 TM	+3757+05823	Asia/Ashgabat
 TN	+3648+01011	Africa/Tunis
-TO	-2110-17510	Pacific/Tongatapu
+TO	-210800-1751200	Pacific/Tongatapu
 TR	+4101+02858	Europe/Istanbul
-TT,AG,AI,BL,DM,GD,GP,KN,LC,MF,MS,VC,VG,VI	+1039-06131	America/Port_of_Spain
-TV	-0831+17913	Pacific/Funafuti
 TW	+2503+12130	Asia/Taipei
-UA	+5026+03031	Europe/Kiev	Ukraine (most areas)
-UA	+4837+02218	Europe/Uzhgorod	Ruthenia
-UA	+4750+03510	Europe/Zaporozhye	Zaporozh'ye/Zaporizhia; Lugansk/Luhansk (east)
-UM	+1917+16637	Pacific/Wake	Wake Island
+UA	+5026+03031	Europe/Kyiv	most of Ukraine
 US	+404251-0740023	America/New_York	Eastern (most areas)
 US	+421953-0830245	America/Detroit	Eastern - MI (most areas)
 US	+381515-0854534	America/Kentucky/Louisville	Eastern - KY (Louisville area)
@@ -362,8 +330,8 @@ US	+470659-1011757	America/North_Dakota/Center	Central - ND (Oliver)
 US	+465042-1012439	America/North_Dakota/New_Salem	Central - ND (Morton rural)
 US	+471551-1014640	America/North_Dakota/Beulah	Central - ND (Mercer)
 US	+394421-1045903	America/Denver	Mountain (most areas)
-US	+433649-1161209	America/Boise	Mountain - ID (south); OR (east)
-US	+332654-1120424	America/Phoenix	MST - Arizona (except Navajo)
+US	+433649-1161209	America/Boise	Mountain - ID (south), OR (east)
+US,CA	+332654-1120424	America/Phoenix	MST - AZ (most areas), Creston BC
 US	+340308-1181434	America/Los_Angeles	Pacific
 US	+611305-1495401	America/Anchorage	Alaska (most areas)
 US	+581807-1342511	America/Juneau	Alaska - Juneau area
@@ -371,14 +339,37 @@ US	+571035-1351807	America/Sitka	Alaska - Sitka area
 US	+550737-1313435	America/Metlakatla	Alaska - Annette Island
 US	+593249-1394338	America/Yakutat	Alaska - Yakutat
 US	+643004-1652423	America/Nome	Alaska (west)
-US	+515248-1763929	America/Adak	Aleutian Islands
-US,UM	+211825-1575130	Pacific/Honolulu	Hawaii
+US	+515248-1763929	America/Adak	Alaska - western Aleutians
+US	+211825-1575130	Pacific/Honolulu	Hawaii
 UY	-345433-0561245	America/Montevideo
 UZ	+3940+06648	Asia/Samarkand	Uzbekistan (west)
 UZ	+4120+06918	Asia/Tashkent	Uzbekistan (east)
 VE	+1030-06656	America/Caracas
-VN	+1045+10640	Asia/Ho_Chi_Minh	Vietnam (south)
+VN	+1045+10640	Asia/Ho_Chi_Minh	south Vietnam
 VU	-1740+16825	Pacific/Efate
-WF	-1318-17610	Pacific/Wallis
 WS	-1350-17144	Pacific/Apia
 ZA,LS,SZ	-2615+02800	Africa/Johannesburg
+#
+# The next section contains experimental tab-separated comments for
+# use by user agents like tzselect that identify continents and oceans.
+#
+# For example, the comment ‘#@AQ<tab>Antarctica/’ means the country code
+# AQ is in the continent Antarctica regardless of the Zone name,
+# so Pacific/Auckland should be listed under Antarctica as well as
+# under the Pacific because its line’s country codes include AQ.
+#
+# If more than one country code is affected each is listed separated
+# by commas, e.g., ‘#@IS,SH<tab>Atlantic/’.  If a country code is in
+# more than one continent or ocean, each is listed separated by
+# commas, e.g., the second column of ‘#@CY,TR<tab>Asia/,Europe/’.
+#
+# These experimental comments are present only for country codes where
+# the continent or ocean is not already obvious from the Zone name.
+# For example, there is no such comment for RU since it already
+# corresponds to Zone names starting with both ‘Europe/’ and ‘Asia/’.
+#
+#@AQ	Antarctica/
+#@IS,SH	Atlantic/
+#@CY,TR	Asia/,Europe/
+#@SJ	Arctic/
+#@CC,CX,KM,MG,YT	Indian/
diff --git a/absl/time/internal/cctz/testdata/zoneinfo/zonenow.tab b/absl/time/internal/cctz/testdata/zoneinfo/zonenow.tab
new file mode 100644
index 00000000000..9c3a8cf3c1b
--- /dev/null
+++ b/absl/time/internal/cctz/testdata/zoneinfo/zonenow.tab
@@ -0,0 +1,299 @@
+# tzdb timezone descriptions, for users who do not care about old timestamps
+#
+# This file is in the public domain.
+#
+# From Paul Eggert (2023-12-18):
+# This file contains a table where each row stands for a timezone
+# where civil timestamps are predicted to agree from now on.
+# This file is like zone1970.tab (see zone1970.tab’s comments),
+# but with the following changes:
+#
+# 1.  Each timezone corresponds to a set of clocks that are planned
+#     to agree from now on.  This is a larger set of clocks than in
+#     zone1970.tab, where each timezone’s clocks must agree from 1970 on.
+# 2.  The first column is irrelevant and ignored.
+# 3.  The table is sorted in a different way:
+#     first by standard time UTC offset;
+#     then, if DST is used, by daylight saving UTC offset;
+#     then by time zone abbreviation.
+# 4.  Every timezone has a nonempty comments column, with wording
+#     distinguishing the timezone only from other timezones with the
+#     same UTC offset at some point during the year.
+#
+# The format of this table is experimental, and may change in future versions.
+#
+# This table is intended as an aid for users, to help them select timezones
+# appropriate for their practical needs.  It is not intended to take or
+# endorse any position on legal or territorial claims.
+#
+#XX	coordinates	TZ	comments
+#
+# -11 - SST
+XX	-1416-17042	Pacific/Pago_Pago	Midway; Samoa (SST)
+#
+# -11
+XX	-1901-16955	Pacific/Niue	Niue
+#
+# -10 - HST
+XX	+211825-1575130	Pacific/Honolulu	Hawaii (HST)
+#
+# -10
+XX	-1732-14934	Pacific/Tahiti	Tahiti; Cook Islands
+#
+# -10/-09 - HST / HDT (North America DST)
+XX	+515248-1763929	America/Adak	western Aleutians in Alaska (HST/HDT)
+#
+# -09:30
+XX	-0900-13930	Pacific/Marquesas	Marquesas
+#
+# -09
+XX	-2308-13457	Pacific/Gambier	Gambier
+#
+# -09/-08 - AKST/AKDT (North America DST)
+XX	+611305-1495401	America/Anchorage	most of Alaska (AKST/AKDT)
+#
+# -08
+XX	-2504-13005	Pacific/Pitcairn	Pitcairn
+#
+# -08/-07 - PST/PDT (North America DST)
+XX	+340308-1181434	America/Los_Angeles	Pacific (PST/PDT) - US; Mexico near US border
+#
+# -08/-07 - PST/PDT (North America DST) until 2026-11-01 02:00; then MST
+XX	+4916-12307	America/Vancouver	Mountain Standard (MST) - British Columbia (most areas)
+#
+# -07 - MST
+XX	+332654-1120424	America/Phoenix	Mountain Standard (MST) - Arizona; western Mexico; Yukon
+#
+# -07/-06 - MST/MDT (North America DST)
+XX	+394421-1045903	America/Denver	Mountain (MST/MDT) - US; Mexico near US border; northern Canada
+#
+# -07/-06 - MST/MDT (North America DST) until 2026-11-01 02:00; then CST
+XX	+5333-11328	America/Edmonton	Central Standard (CST) - Alberta and some neighbors
+#
+# -06
+XX	-0054-08936	Pacific/Galapagos	Galápagos
+#
+# -06 - CST
+XX	+1924-09909	America/Mexico_City	Central Standard (CST) - Saskatchewan; central Mexico; Central America
+#
+# -06/-05 (Chile DST)
+XX	-2709-10926	Pacific/Easter	Easter Island
+#
+# -06/-05 - CST/CDT (North America DST)
+XX	+415100-0873900	America/Chicago	Central (CST/CDT) - US & Canada; Mexico near US border
+#
+# -05
+XX	-1203-07703	America/Lima	eastern South America
+#
+# -05 - EST
+XX	+175805-0764736	America/Jamaica	Eastern Standard (EST) - Caymans; Jamaica; eastern Mexico; Panama
+#
+# -05/-04 - CST/CDT (Cuba DST)
+XX	+2308-08222	America/Havana	Cuba
+#
+# -05/-04 - EST/EDT (North America DST)
+XX	+404251-0740023	America/New_York	Eastern (EST/EDT) - US & Canada
+#
+# -04
+XX	+1030-06656	America/Caracas	western South America
+#
+# -04 - AST
+XX	+1828-06954	America/Santo_Domingo	Atlantic Standard (AST) - eastern Caribbean
+#
+# -04/-03 (Chile DST)
+XX	-3327-07040	America/Santiago	most of Chile
+#
+# -04/-03 - AST/ADT (North America DST)
+XX	+4439-06336	America/Halifax	Atlantic (AST/ADT) - Canada; Bermuda
+#
+# -03:30/-02:30 - NST/NDT (North America DST)
+XX	+4734-05243	America/St_Johns	Newfoundland (NST/NDT)
+#
+# -03
+XX	-2332-04637	America/Sao_Paulo	eastern and southern South America
+#
+# -03/-02 (North America DST)
+XX	+4703-05620	America/Miquelon	St Pierre & Miquelon
+#
+# -02
+XX	-0351-03225	America/Noronha	Fernando de Noronha; South Georgia
+#
+# -02/-01 (EU DST)
+XX	+6411-05144	America/Nuuk	most of Greenland
+#
+# -01
+XX	+1455-02331	Atlantic/Cape_Verde	Cape Verde
+#
+# -01/+00 (EU DST)
+XX	+3744-02540	Atlantic/Azores	Azores
+#
+# +00 - GMT
+XX	+0519-00402	Africa/Abidjan	far western Africa; Iceland (GMT)
+#
+# +00/+01 - GMT/BST (EU DST)
+XX	+513030-0000731	Europe/London	United Kingdom (GMT/BST)
+#
+# +00/+01 - WET/WEST (EU DST)
+XX	+3843-00908	Europe/Lisbon	western Europe (WET/WEST)
+#
+# +00/+02 - Troll DST
+XX	-720041+0023206	Antarctica/Troll	Troll Station in Antarctica
+#
+# +01 - CET
+XX	+3647+00303	Africa/Algiers	Algeria, Tunisia (CET)
+#
+# +01 - WAT
+XX	+0627+00324	Africa/Lagos	western Africa (WAT)
+#
+# +01/+00 - IST/GMT (EU DST in reverse)
+XX	+5320-00615	Europe/Dublin	Ireland (IST/GMT)
+#
+# +01/+00 - (Morocco DST)
+XX	+3339-00735	Africa/Casablanca	Morocco
+#
+# +01/+02 - CET/CEST (EU DST)
+XX	+4852+00220	Europe/Paris	central Europe (CET/CEST)
+#
+# +02 - CAT
+XX	-2558+03235	Africa/Maputo	central Africa (CAT)
+#
+# +02 - EET
+XX	+3254+01311	Africa/Tripoli	Libya; Kaliningrad (EET)
+#
+# +02 - SAST
+XX	-2615+02800	Africa/Johannesburg	southern Africa (SAST)
+#
+# +02/+03 - EET/EEST (EU DST)
+XX	+3758+02343	Europe/Athens	eastern Europe (EET/EEST)
+#
+# +02/+03 - EET/EEST (Egypt DST)
+XX	+3003+03115	Africa/Cairo	Egypt
+#
+# +02/+03 - EET/EEST (Lebanon DST)
+XX	+3353+03530	Asia/Beirut	Lebanon
+#
+# +02/+03 - EET/EEST (Palestine DST)
+XX	+3130+03428	Asia/Gaza	Palestine
+#
+# +02/+03 - IST/IDT (Israel DST)
+XX	+314650+0351326	Asia/Jerusalem	Israel
+#
+# +03
+XX	+4101+02858	Europe/Istanbul	Near East; Belarus
+#
+# +03 - EAT
+XX	-0117+03649	Africa/Nairobi	eastern Africa (EAT)
+#
+# +03 - MSK
+XX	+554521+0373704	Europe/Moscow	Moscow (MSK)
+#
+# +03:30
+XX	+3540+05126	Asia/Tehran	Iran
+#
+# +04
+XX	+2518+05518	Asia/Dubai	Russia; Caucasus; Persian Gulf; Seychelles; Réunion
+#
+# +04:30
+XX	+3431+06912	Asia/Kabul	Afghanistan
+#
+# +05
+XX	+4120+06918	Asia/Tashkent	Russia; Kazakhstan; Tajikistan; Turkmenistan; Uzbekistan; Maldives
+#
+# +05 - PKT
+XX	+2452+06703	Asia/Karachi	Pakistan (PKT)
+#
+# +05:30
+XX	+0656+07951	Asia/Colombo	Sri Lanka
+#
+# +05:30 - IST
+XX	+2232+08822	Asia/Kolkata	India (IST)
+#
+# +05:45
+XX	+2743+08519	Asia/Kathmandu	Nepal
+#
+# +06
+XX	+2343+09025	Asia/Dhaka	Russia; Kyrgyzstan; Bhutan; Bangladesh; Chagos
+#
+# +06:30
+XX	+1647+09610	Asia/Yangon	Myanmar; Cocos
+#
+# +07
+XX	+1345+10031	Asia/Bangkok	Russia; Indochina; Christmas Island
+#
+# +07 - WIB
+XX	-0610+10648	Asia/Jakarta	Indonesia (WIB)
+#
+# +08
+XX	+0117+10351	Asia/Singapore	Russia; Brunei; Malaysia; Singapore; Concordia
+#
+# +08 - AWST
+XX	-3157+11551	Australia/Perth	Western Australia (AWST)
+#
+# +08 - CST
+XX	+3114+12128	Asia/Shanghai	China (CST)
+#
+# +08 - HKT
+XX	+2217+11409	Asia/Hong_Kong	Hong Kong (HKT)
+#
+# +08 - PHT
+XX	+143512+1205804	Asia/Manila	Philippines (PHT)
+#
+# +08 - WITA
+XX	-0507+11924	Asia/Makassar	Indonesia (WITA)
+#
+# +08:45
+XX	-3143+12852	Australia/Eucla	Eucla
+#
+# +09
+XX	+5203+11328	Asia/Chita	Russia; Palau; East Timor
+#
+# +09 - JST
+XX	+353916+1394441	Asia/Tokyo	Japan (JST); Eyre Bird Observatory
+#
+# +09 - KST
+XX	+3733+12658	Asia/Seoul	Korea (KST)
+#
+# +09 - WIT
+XX	-0232+14042	Asia/Jayapura	Indonesia (WIT)
+#
+# +09:30 - ACST
+XX	-1228+13050	Australia/Darwin	Northern Territory (ACST)
+#
+# +09:30/+10:30 - ACST/ACDT (Australia DST)
+XX	-3455+13835	Australia/Adelaide	South Australia (ACST/ACDT)
+#
+# +10
+XX	+4310+13156	Asia/Vladivostok	Russia; Yap; Chuuk; Papua New Guinea; Dumont d’Urville
+#
+# +10 - AEST
+XX	-2728+15302	Australia/Brisbane	Queensland (AEST)
+#
+# +10 - ChST
+XX	+1328+14445	Pacific/Guam	Mariana Islands (ChST)
+#
+# +10/+11 - AEST/AEDT (Australia DST)
+XX	-3352+15113	Australia/Sydney	southeast Australia (AEST/AEDT)
+#
+# +10:30/+11
+XX	-3133+15905	Australia/Lord_Howe	Lord Howe Island
+#
+# +11
+XX	-0613+15534	Pacific/Bougainville	Russia; Kosrae; Bougainville; Solomons
+#
+# +11/+12 (Australia DST)
+XX	-2903+16758	Pacific/Norfolk	Norfolk Island
+#
+# +12
+XX	+5301+15839	Asia/Kamchatka	Russia; Tuvalu; Fiji; etc.
+#
+# +12/+13 (New Zealand DST)
+XX	-3652+17446	Pacific/Auckland	New Zealand (NZST/NZDT)
+#
+# +12:45/+13:45 (Chatham DST)
+XX	-4357-17633	Pacific/Chatham	Chatham Islands
+#
+# +13
+XX	-210800-1751200	Pacific/Tongatapu	Kanton; Tokelau; Samoa (western); Tonga
+#
+# +14
+XX	+0152-15720	Pacific/Kiritimati	Kiritimati
diff --git a/absl/time/internal/get_current_time_chrono.inc b/absl/time/internal/get_current_time_chrono.inc
index 5180230dee5..5eeb6406aaf 100644
--- a/absl/time/internal/get_current_time_chrono.inc
+++ b/absl/time/internal/get_current_time_chrono.inc
@@ -16,6 +16,7 @@
 #include <cstdint>
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 
 static int64_t GetCurrentTimeNanosFromSystem() {
@@ -26,4 +27,5 @@ static int64_t GetCurrentTimeNanosFromSystem() {
 }
 
 }  // namespace time_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/time/internal/get_current_time_posix.inc b/absl/time/internal/get_current_time_posix.inc
index 65474ca6da1..42072000ae3 100644
--- a/absl/time/internal/get_current_time_posix.inc
+++ b/absl/time/internal/get_current_time_posix.inc
@@ -7,6 +7,7 @@
 #include "absl/base/internal/raw_logging.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 
 static int64_t GetCurrentTimeNanosFromSystem() {
@@ -19,4 +20,5 @@ static int64_t GetCurrentTimeNanosFromSystem() {
 }
 
 }  // namespace time_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/time/internal/test_util.cc b/absl/time/internal/test_util.cc
index 59166a7c82d..3e2452e9c33 100644
--- a/absl/time/internal/test_util.cc
+++ b/absl/time/internal/test_util.cc
@@ -14,16 +14,11 @@
 
 #include "absl/time/internal/test_util.h"
 
-#include <algorithm>
-#include <cstddef>
-#include <cstring>
-
+#include "absl/base/config.h"
 #include "absl/base/internal/raw_logging.h"
-#include "absl/time/internal/cctz/include/cctz/zone_info_source.h"
-
-namespace cctz = absl::time_internal::cctz;
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 
 TimeZone LoadTimeZone(const std::string& name) {
@@ -33,91 +28,5 @@ TimeZone LoadTimeZone(const std::string& name) {
 }
 
 }  // namespace time_internal
-}  // namespace absl
-
-namespace absl {
-namespace time_internal {
-namespace cctz_extension {
-namespace {
-
-// Embed the zoneinfo data for time zones used during tests and benchmarks.
-// The data was generated using "xxd -i zoneinfo-file".  There is no need
-// to update the data as long as the tests do not depend on recent changes
-// (and the past rules remain the same).
-#include "absl/time/internal/zoneinfo.inc"
-
-const struct ZoneInfo {
-  const char* name;
-  const char* data;
-  std::size_t length;
-} kZoneInfo[] = {
-    // The three real time zones used by :time_test and :time_benchmark.
-    {"America/Los_Angeles",  //
-     reinterpret_cast<char*>(America_Los_Angeles), America_Los_Angeles_len},
-    {"America/New_York",  //
-     reinterpret_cast<char*>(America_New_York), America_New_York_len},
-    {"Australia/Sydney",  //
-     reinterpret_cast<char*>(Australia_Sydney), Australia_Sydney_len},
-
-    // Other zones named in tests but which should fail to load.
-    {"Invalid/TimeZone", nullptr, 0},
-    {"", nullptr, 0},
-
-    // Also allow for loading the local time zone under TZ=US/Pacific.
-    {"US/Pacific",  //
-     reinterpret_cast<char*>(America_Los_Angeles), America_Los_Angeles_len},
-
-    // Allows use of the local time zone from a system-specific location.
-#ifdef _MSC_VER
-    {"localtime",  //
-     reinterpret_cast<char*>(America_Los_Angeles), America_Los_Angeles_len},
-#else
-    {"/etc/localtime",  //
-     reinterpret_cast<char*>(America_Los_Angeles), America_Los_Angeles_len},
-#endif
-};
-
-class TestZoneInfoSource : public cctz::ZoneInfoSource {
- public:
-  TestZoneInfoSource(const char* data, std::size_t size)
-      : data_(data), end_(data + size) {}
-
-  std::size_t Read(void* ptr, std::size_t size) override {
-    const std::size_t len = std::min<std::size_t>(size, end_ - data_);
-    memcpy(ptr, data_, len);
-    data_ += len;
-    return len;
-  }
-
-  int Skip(std::size_t offset) override {
-    data_ += std::min<std::size_t>(offset, end_ - data_);
-    return 0;
-  }
-
- private:
-  const char* data_;
-  const char* const end_;
-};
-
-std::unique_ptr<cctz::ZoneInfoSource> TestFactory(
-    const std::string& name,
-    const std::function<std::unique_ptr<cctz::ZoneInfoSource>(
-        const std::string& name)>& /*fallback_factory*/) {
-  for (const ZoneInfo& zoneinfo : kZoneInfo) {
-    if (name == zoneinfo.name) {
-      if (zoneinfo.data == nullptr) return nullptr;
-      return std::unique_ptr<cctz::ZoneInfoSource>(
-          new TestZoneInfoSource(zoneinfo.data, zoneinfo.length));
-    }
-  }
-  ABSL_RAW_LOG(FATAL, "Unexpected time zone \"%s\" in test", name.c_str());
-  return nullptr;
-}
-
-}  // namespace
-
-ZoneInfoSourceFactory zone_info_source_factory = TestFactory;
-
-}  // namespace cctz_extension
-}  // namespace time_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/time/internal/test_util.h b/absl/time/internal/test_util.h
index d7319ea8f5b..5c4bf1f680e 100644
--- a/absl/time/internal/test_util.h
+++ b/absl/time/internal/test_util.h
@@ -20,12 +20,14 @@
 #include "absl/time/time.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 
 // Loads the named timezone, but dies on any failure.
 absl::TimeZone LoadTimeZone(const std::string& name);
 
 }  // namespace time_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_TIME_INTERNAL_TEST_UTIL_H_
diff --git a/absl/time/internal/zoneinfo.inc b/absl/time/internal/zoneinfo.inc
deleted file mode 100644
index bfed82990dd..00000000000
--- a/absl/time/internal/zoneinfo.inc
+++ /dev/null
@@ -1,729 +0,0 @@
-unsigned char America_Los_Angeles[] = {
-  0x54, 0x5a, 0x69, 0x66, 0x32, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
-  0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x05,
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-  0x60, 0x90, 0x00, 0x00, 0x00, 0x00, 0x2f, 0x7e, 0x75, 0xa0, 0x00, 0x00,
-  0x00, 0x00, 0x30, 0x93, 0x42, 0x90, 0x00, 0x00, 0x00, 0x00, 0x31, 0x67,
-  0x92, 0x20, 0x00, 0x00, 0x00, 0x00, 0x32, 0x73, 0x24, 0x90, 0x00, 0x00,
-  0x00, 0x00, 0x33, 0x47, 0x74, 0x20, 0x00, 0x00, 0x00, 0x00, 0x34, 0x53,
-  0x06, 0x90, 0x00, 0x00, 0x00, 0x00, 0x35, 0x27, 0x56, 0x20, 0x00, 0x00,
-  0x00, 0x00, 0x36, 0x32, 0xe8, 0x90, 0x00, 0x00, 0x00, 0x00, 0x37, 0x07,
-  0x38, 0x20, 0x00, 0x00, 0x00, 0x00, 0x38, 0x1c, 0x05, 0x10, 0x00, 0x00,
-  0x00, 0x00, 0x38, 0xe7, 0x1a, 0x20, 0x00, 0x00, 0x00, 0x00, 0x39, 0xfb,
-  0xe7, 0x10, 0x00, 0x00, 0x00, 0x00, 0x3a, 0xc6, 0xfc, 0x20, 0x00, 0x00,
-  0x00, 0x00, 0x3b, 0xdb, 0xc9, 0x10, 0x00, 0x00, 0x00, 0x00, 0x3c, 0xb0,
-  0x18, 0xa0, 0x00, 0x00, 0x00, 0x00, 0x3d, 0xbb, 0xab, 0x10, 0x00, 0x00,
-  0x00, 0x00, 0x3e, 0x8f, 0xfa, 0xa0, 0x00, 0x00, 0x00, 0x00, 0x3f, 0x9b,
-  0x8d, 0x10, 0x00, 0x00, 0x00, 0x00, 0x40, 0x6f, 0xdc, 0xa0, 0x00, 0x00,
-  0x00, 0x00, 0x41, 0x84, 0xa9, 0x90, 0x00, 0x00, 0x00, 0x00, 0x42, 0x4f,
-  0xbe, 0xa0, 0x00, 0x00, 0x00, 0x00, 0x43, 0x64, 0x8b, 0x90, 0x00, 0x00,
-  0x00, 0x00, 0x44, 0x2f, 0xa0, 0xa0, 0x00, 0x00, 0x00, 0x00, 0x45, 0x44,
-  0x6d, 0x90, 0x00, 0x00, 0x00, 0x00, 0x45, 0xf3, 0xd3, 0x20, 0x00, 0x00,
-  0x00, 0x00, 0x47, 0x2d, 0x8a, 0x10, 0x00, 0x00, 0x00, 0x00, 0x47, 0xd3,
-  0xb5, 0x20, 0x00, 0x00, 0x00, 0x00, 0x49, 0x0d, 0x6c, 0x10, 0x00, 0x00,
-  0x00, 0x00, 0x49, 0xb3, 0x97, 0x20, 0x00, 0x00, 0x00, 0x00, 0x4a, 0xed,
-  0x4e, 0x10, 0x00, 0x00, 0x00, 0x00, 0x4b, 0x9c, 0xb3, 0xa0, 0x00, 0x00,
-  0x00, 0x00, 0x4c, 0xd6, 0x6a, 0x90, 0x00, 0x00, 0x00, 0x00, 0x4d, 0x7c,
-  0x95, 0xa0, 0x00, 0x00, 0x00, 0x00, 0x4e, 0xb6, 0x4c, 0x90, 0x00, 0x00,
-  0x00, 0x00, 0x4f, 0x5c, 0x77, 0xa0, 0x00, 0x00, 0x00, 0x00, 0x50, 0x96,
-  0x2e, 0x90, 0x00, 0x00, 0x00, 0x00, 0x51, 0x3c, 0x59, 0xa0, 0x00, 0x00,
-  0x00, 0x00, 0x52, 0x76, 0x10, 0x90, 0x00, 0x00, 0x00, 0x00, 0x53, 0x1c,
-  0x3b, 0xa0, 0x00, 0x00, 0x00, 0x00, 0x54, 0x55, 0xf2, 0x90, 0x00, 0x00,
-  0x00, 0x00, 0x54, 0xfc, 0x1d, 0xa0, 0x00, 0x00, 0x00, 0x00, 0x56, 0x35,
-  0xd4, 0x90, 0x00, 0x00, 0x00, 0x00, 0x56, 0xe5, 0x3a, 0x20, 0x00, 0x00,
-  0x00, 0x00, 0x58, 0x1e, 0xf1, 0x10, 0x00, 0x00, 0x00, 0x00, 0x58, 0xc5,
-  0x1c, 0x20, 0x00, 0x00, 0x00, 0x00, 0x59, 0xfe, 0xd3, 0x10, 0x00, 0x00,
-  0x00, 0x00, 0x5a, 0xa4, 0xfe, 0x20, 0x00, 0x00, 0x00, 0x00, 0x5b, 0xde,
-  0xb5, 0x10, 0x00, 0x00, 0x00, 0x00, 0x5c, 0x84, 0xe0, 0x20, 0x00, 0x00,
-  0x00, 0x00, 0x5d, 0xbe, 0x97, 0x10, 0x00, 0x00, 0x00, 0x00, 0x5e, 0x64,
-  0xc2, 0x20, 0x00, 0x00, 0x00, 0x00, 0x5f, 0x9e, 0x79, 0x10, 0x00, 0x00,
-  0x00, 0x00, 0x60, 0x4d, 0xde, 0xa0, 0x00, 0x00, 0x00, 0x00, 0x61, 0x87,
-  0x95, 0x90, 0x00, 0x00, 0x00, 0x00, 0x62, 0x2d, 0xc0, 0xa0, 0x00, 0x00,
-  0x00, 0x00, 0x63, 0x67, 0x77, 0x90, 0x00, 0x00, 0x00, 0x00, 0x64, 0x0d,
-  0xa2, 0xa0, 0x00, 0x00, 0x00, 0x00, 0x65, 0x47, 0x59, 0x90, 0x00, 0x00,
-  0x00, 0x00, 0x65, 0xed, 0x84, 0xa0, 0x00, 0x00, 0x00, 0x00, 0x67, 0x27,
-  0x3b, 0x90, 0x00, 0x00, 0x00, 0x00, 0x67, 0xcd, 0x66, 0xa0, 0x00, 0x00,
-  0x00, 0x00, 0x69, 0x07, 0x1d, 0x90, 0x00, 0x00, 0x00, 0x00, 0x69, 0xad,
-  0x48, 0xa0, 0x00, 0x00, 0x00, 0x00, 0x6a, 0xe6, 0xff, 0x90, 0x00, 0x00,
-  0x00, 0x00, 0x6b, 0x96, 0x65, 0x20, 0x00, 0x00, 0x00, 0x00, 0x6c, 0xd0,
-  0x1c, 0x10, 0x00, 0x00, 0x00, 0x00, 0x6d, 0x76, 0x47, 0x20, 0x00, 0x00,
-  0x00, 0x00, 0x6e, 0xaf, 0xfe, 0x10, 0x00, 0x00, 0x00, 0x00, 0x6f, 0x56,
-  0x29, 0x20, 0x00, 0x00, 0x00, 0x00, 0x70, 0x8f, 0xe0, 0x10, 0x00, 0x00,
-  0x00, 0x00, 0x71, 0x36, 0x0b, 0x20, 0x00, 0x00, 0x00, 0x00, 0x72, 0x6f,
-  0xc2, 0x10, 0x00, 0x00, 0x00, 0x00, 0x73, 0x15, 0xed, 0x20, 0x00, 0x00,
-  0x00, 0x00, 0x74, 0x4f, 0xa4, 0x10, 0x00, 0x00, 0x00, 0x00, 0x74, 0xff,
-  0x09, 0xa0, 0x00, 0x00, 0x00, 0x00, 0x76, 0x38, 0xc0, 0x90, 0x00, 0x00,
-  0x00, 0x00, 0x76, 0xde, 0xeb, 0xa0, 0x00, 0x00, 0x00, 0x00, 0x78, 0x18,
-  0xa2, 0x90, 0x00, 0x00, 0x00, 0x00, 0x78, 0xbe, 0xcd, 0xa0, 0x00, 0x00,
-  0x00, 0x00, 0x79, 0xf8, 0x84, 0x90, 0x00, 0x00, 0x00, 0x00, 0x7a, 0x9e,
-  0xaf, 0xa0, 0x00, 0x00, 0x00, 0x00, 0x7b, 0xd8, 0x66, 0x90, 0x00, 0x00,
-  0x00, 0x00, 0x7c, 0x7e, 0x91, 0xa0, 0x00, 0x00, 0x00, 0x00, 0x7d, 0xb8,
-  0x48, 0x90, 0x00, 0x00, 0x00, 0x00, 0x7e, 0x5e, 0x73, 0xa0, 0x00, 0x00,
-  0x00, 0x00, 0x7f, 0x98, 0x2a, 0x90, 0x00, 0x02, 0x01, 0x02, 0x01, 0x02,
-  0x03, 0x04, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01,
-  0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01,
-  0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01,
-  0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01,
-  0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01,
-  0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01,
-  0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01,
-  0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01,
-  0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01,
-  0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01,
-  0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01,
-  0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01,
-  0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01,
-  0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01,
-  0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01,
-  0x02, 0xff, 0xff, 0x91, 0x26, 0x00, 0x00, 0xff, 0xff, 0x9d, 0x90, 0x01,
-  0x04, 0xff, 0xff, 0x8f, 0x80, 0x00, 0x08, 0xff, 0xff, 0x9d, 0x90, 0x01,
-  0x0c, 0xff, 0xff, 0x9d, 0x90, 0x01, 0x10, 0x4c, 0x4d, 0x54, 0x00, 0x50,
-  0x44, 0x54, 0x00, 0x50, 0x53, 0x54, 0x00, 0x50, 0x57, 0x54, 0x00, 0x50,
-  0x50, 0x54, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x00,
-  0x01, 0x0a, 0x50, 0x53, 0x54, 0x38, 0x50, 0x44, 0x54, 0x2c, 0x4d, 0x33,
-  0x2e, 0x32, 0x2e, 0x30, 0x2c, 0x4d, 0x31, 0x31, 0x2e, 0x31, 0x2e, 0x30,
-  0x0a
-};
-unsigned int America_Los_Angeles_len = 2845;
-unsigned char America_New_York[] = {
-  0x54, 0x5a, 0x69, 0x66, 0x32, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
-  0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x05,
-  0x00, 0x00, 0x00, 0x05, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0xec,
-  0x00, 0x00, 0x00, 0x05, 0x00, 0x00, 0x00, 0x14, 0x80, 0x00, 0x00, 0x00,
-  0x9e, 0xa6, 0x1e, 0x70, 0x9f, 0xba, 0xeb, 0x60, 0xa0, 0x86, 0x00, 0x70,
-  0xa1, 0x9a, 0xcd, 0x60, 0xa2, 0x65, 0xe2, 0x70, 0xa3, 0x83, 0xe9, 0xe0,
-  0xa4, 0x6a, 0xae, 0x70, 0xa5, 0x35, 0xa7, 0x60, 0xa6, 0x53, 0xca, 0xf0,
-  0xa7, 0x15, 0x89, 0x60, 0xa8, 0x33, 0xac, 0xf0, 0xa8, 0xfe, 0xa5, 0xe0,
-  0xaa, 0x13, 0x8e, 0xf0, 0xaa, 0xde, 0x87, 0xe0, 0xab, 0xf3, 0x70, 0xf0,
-  0xac, 0xbe, 0x69, 0xe0, 0xad, 0xd3, 0x52, 0xf0, 0xae, 0x9e, 0x4b, 0xe0,
-  0xaf, 0xb3, 0x34, 0xf0, 0xb0, 0x7e, 0x2d, 0xe0, 0xb1, 0x9c, 0x51, 0x70,
-  0xb2, 0x67, 0x4a, 0x60, 0xb3, 0x7c, 0x33, 0x70, 0xb4, 0x47, 0x2c, 0x60,
-  0xb5, 0x5c, 0x15, 0x70, 0xb6, 0x27, 0x0e, 0x60, 0xb7, 0x3b, 0xf7, 0x70,
-  0xb8, 0x06, 0xf0, 0x60, 0xb9, 0x1b, 0xd9, 0x70, 0xb9, 0xe6, 0xd2, 0x60,
-  0xbb, 0x04, 0xf5, 0xf0, 0xbb, 0xc6, 0xb4, 0x60, 0xbc, 0xe4, 0xd7, 0xf0,
-  0xbd, 0xaf, 0xd0, 0xe0, 0xbe, 0xc4, 0xb9, 0xf0, 0xbf, 0x8f, 0xb2, 0xe0,
-  0xc0, 0xa4, 0x9b, 0xf0, 0xc1, 0x6f, 0x94, 0xe0, 0xc2, 0x84, 0x7d, 0xf0,
-  0xc3, 0x4f, 0x76, 0xe0, 0xc4, 0x64, 0x5f, 0xf0, 0xc5, 0x2f, 0x58, 0xe0,
-  0xc6, 0x4d, 0x7c, 0x70, 0xc7, 0x0f, 0x3a, 0xe0, 0xc8, 0x2d, 0x5e, 0x70,
-  0xc8, 0xf8, 0x57, 0x60, 0xca, 0x0d, 0x40, 0x70, 0xca, 0xd8, 0x39, 0x60,
-  0xcb, 0x88, 0xf0, 0x70, 0xd2, 0x23, 0xf4, 0x70, 0xd2, 0x60, 0xfb, 0xe0,
-  0xd3, 0x75, 0xe4, 0xf0, 0xd4, 0x40, 0xdd, 0xe0, 0xd5, 0x55, 0xc6, 0xf0,
-  0xd6, 0x20, 0xbf, 0xe0, 0xd7, 0x35, 0xa8, 0xf0, 0xd8, 0x00, 0xa1, 0xe0,
-  0xd9, 0x15, 0x8a, 0xf0, 0xd9, 0xe0, 0x83, 0xe0, 0xda, 0xfe, 0xa7, 0x70,
-  0xdb, 0xc0, 0x65, 0xe0, 0xdc, 0xde, 0x89, 0x70, 0xdd, 0xa9, 0x82, 0x60,
-  0xde, 0xbe, 0x6b, 0x70, 0xdf, 0x89, 0x64, 0x60, 0xe0, 0x9e, 0x4d, 0x70,
-  0xe1, 0x69, 0x46, 0x60, 0xe2, 0x7e, 0x2f, 0x70, 0xe3, 0x49, 0x28, 0x60,
-  0xe4, 0x5e, 0x11, 0x70, 0xe5, 0x57, 0x2e, 0xe0, 0xe6, 0x47, 0x2d, 0xf0,
-  0xe7, 0x37, 0x10, 0xe0, 0xe8, 0x27, 0x0f, 0xf0, 0xe9, 0x16, 0xf2, 0xe0,
-  0xea, 0x06, 0xf1, 0xf0, 0xea, 0xf6, 0xd4, 0xe0, 0xeb, 0xe6, 0xd3, 0xf0,
-  0xec, 0xd6, 0xb6, 0xe0, 0xed, 0xc6, 0xb5, 0xf0, 0xee, 0xbf, 0xd3, 0x60,
-  0xef, 0xaf, 0xd2, 0x70, 0xf0, 0x9f, 0xb5, 0x60, 0xf1, 0x8f, 0xb4, 0x70,
-  0xf2, 0x7f, 0x97, 0x60, 0xf3, 0x6f, 0x96, 0x70, 0xf4, 0x5f, 0x79, 0x60,
-  0xf5, 0x4f, 0x78, 0x70, 0xf6, 0x3f, 0x5b, 0x60, 0xf7, 0x2f, 0x5a, 0x70,
-  0xf8, 0x28, 0x77, 0xe0, 0xf9, 0x0f, 0x3c, 0x70, 0xfa, 0x08, 0x59, 0xe0,
-  0xfa, 0xf8, 0x58, 0xf0, 0xfb, 0xe8, 0x3b, 0xe0, 0xfc, 0xd8, 0x3a, 0xf0,
-  0xfd, 0xc8, 0x1d, 0xe0, 0xfe, 0xb8, 0x1c, 0xf0, 0xff, 0xa7, 0xff, 0xe0,
-  0x00, 0x97, 0xfe, 0xf0, 0x01, 0x87, 0xe1, 0xe0, 0x02, 0x77, 0xe0, 0xf0,
-  0x03, 0x70, 0xfe, 0x60, 0x04, 0x60, 0xfd, 0x70, 0x05, 0x50, 0xe0, 0x60,
-  0x06, 0x40, 0xdf, 0x70, 0x07, 0x30, 0xc2, 0x60, 0x07, 0x8d, 0x19, 0x70,
-  0x09, 0x10, 0xa4, 0x60, 0x09, 0xad, 0x94, 0xf0, 0x0a, 0xf0, 0x86, 0x60,
-  0x0b, 0xe0, 0x85, 0x70, 0x0c, 0xd9, 0xa2, 0xe0, 0x0d, 0xc0, 0x67, 0x70,
-  0x0e, 0xb9, 0x84, 0xe0, 0x0f, 0xa9, 0x83, 0xf0, 0x10, 0x99, 0x66, 0xe0,
-  0x11, 0x89, 0x65, 0xf0, 0x12, 0x79, 0x48, 0xe0, 0x13, 0x69, 0x47, 0xf0,
-  0x14, 0x59, 0x2a, 0xe0, 0x15, 0x49, 0x29, 0xf0, 0x16, 0x39, 0x0c, 0xe0,
-  0x17, 0x29, 0x0b, 0xf0, 0x18, 0x22, 0x29, 0x60, 0x19, 0x08, 0xed, 0xf0,
-  0x1a, 0x02, 0x0b, 0x60, 0x1a, 0xf2, 0x0a, 0x70, 0x1b, 0xe1, 0xed, 0x60,
-  0x1c, 0xd1, 0xec, 0x70, 0x1d, 0xc1, 0xcf, 0x60, 0x1e, 0xb1, 0xce, 0x70,
-  0x1f, 0xa1, 0xb1, 0x60, 0x20, 0x76, 0x00, 0xf0, 0x21, 0x81, 0x93, 0x60,
-  0x22, 0x55, 0xe2, 0xf0, 0x23, 0x6a, 0xaf, 0xe0, 0x24, 0x35, 0xc4, 0xf0,
-  0x25, 0x4a, 0x91, 0xe0, 0x26, 0x15, 0xa6, 0xf0, 0x27, 0x2a, 0x73, 0xe0,
-  0x27, 0xfe, 0xc3, 0x70, 0x29, 0x0a, 0x55, 0xe0, 0x29, 0xde, 0xa5, 0x70,
-  0x2a, 0xea, 0x37, 0xe0, 0x2b, 0xbe, 0x87, 0x70, 0x2c, 0xd3, 0x54, 0x60,
-  0x2d, 0x9e, 0x69, 0x70, 0x2e, 0xb3, 0x36, 0x60, 0x2f, 0x7e, 0x4b, 0x70,
-  0x30, 0x93, 0x18, 0x60, 0x31, 0x67, 0x67, 0xf0, 0x32, 0x72, 0xfa, 0x60,
-  0x33, 0x47, 0x49, 0xf0, 0x34, 0x52, 0xdc, 0x60, 0x35, 0x27, 0x2b, 0xf0,
-  0x36, 0x32, 0xbe, 0x60, 0x37, 0x07, 0x0d, 0xf0, 0x38, 0x1b, 0xda, 0xe0,
-  0x38, 0xe6, 0xef, 0xf0, 0x39, 0xfb, 0xbc, 0xe0, 0x3a, 0xc6, 0xd1, 0xf0,
-  0x3b, 0xdb, 0x9e, 0xe0, 0x3c, 0xaf, 0xee, 0x70, 0x3d, 0xbb, 0x80, 0xe0,
-  0x3e, 0x8f, 0xd0, 0x70, 0x3f, 0x9b, 0x62, 0xe0, 0x40, 0x6f, 0xb2, 0x70,
-  0x41, 0x84, 0x7f, 0x60, 0x42, 0x4f, 0x94, 0x70, 0x43, 0x64, 0x61, 0x60,
-  0x44, 0x2f, 0x76, 0x70, 0x45, 0x44, 0x43, 0x60, 0x45, 0xf3, 0xa8, 0xf0,
-  0x47, 0x2d, 0x5f, 0xe0, 0x47, 0xd3, 0x8a, 0xf0, 0x49, 0x0d, 0x41, 0xe0,
-  0x49, 0xb3, 0x6c, 0xf0, 0x4a, 0xed, 0x23, 0xe0, 0x4b, 0x9c, 0x89, 0x70,
-  0x4c, 0xd6, 0x40, 0x60, 0x4d, 0x7c, 0x6b, 0x70, 0x4e, 0xb6, 0x22, 0x60,
-  0x4f, 0x5c, 0x4d, 0x70, 0x50, 0x96, 0x04, 0x60, 0x51, 0x3c, 0x2f, 0x70,
-  0x52, 0x75, 0xe6, 0x60, 0x53, 0x1c, 0x11, 0x70, 0x54, 0x55, 0xc8, 0x60,
-  0x54, 0xfb, 0xf3, 0x70, 0x56, 0x35, 0xaa, 0x60, 0x56, 0xe5, 0x0f, 0xf0,
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-  0x00, 0x00, 0x00, 0x00, 0x49, 0xb3, 0x6c, 0xf0, 0x00, 0x00, 0x00, 0x00,
-  0x4a, 0xed, 0x23, 0xe0, 0x00, 0x00, 0x00, 0x00, 0x4b, 0x9c, 0x89, 0x70,
-  0x00, 0x00, 0x00, 0x00, 0x4c, 0xd6, 0x40, 0x60, 0x00, 0x00, 0x00, 0x00,
-  0x4d, 0x7c, 0x6b, 0x70, 0x00, 0x00, 0x00, 0x00, 0x4e, 0xb6, 0x22, 0x60,
-  0x00, 0x00, 0x00, 0x00, 0x4f, 0x5c, 0x4d, 0x70, 0x00, 0x00, 0x00, 0x00,
-  0x50, 0x96, 0x04, 0x60, 0x00, 0x00, 0x00, 0x00, 0x51, 0x3c, 0x2f, 0x70,
-  0x00, 0x00, 0x00, 0x00, 0x52, 0x75, 0xe6, 0x60, 0x00, 0x00, 0x00, 0x00,
-  0x53, 0x1c, 0x11, 0x70, 0x00, 0x00, 0x00, 0x00, 0x54, 0x55, 0xc8, 0x60,
-  0x00, 0x00, 0x00, 0x00, 0x54, 0xfb, 0xf3, 0x70, 0x00, 0x00, 0x00, 0x00,
-  0x56, 0x35, 0xaa, 0x60, 0x00, 0x00, 0x00, 0x00, 0x56, 0xe5, 0x0f, 0xf0,
-  0x00, 0x00, 0x00, 0x00, 0x58, 0x1e, 0xc6, 0xe0, 0x00, 0x00, 0x00, 0x00,
-  0x58, 0xc4, 0xf1, 0xf0, 0x00, 0x00, 0x00, 0x00, 0x59, 0xfe, 0xa8, 0xe0,
-  0x00, 0x00, 0x00, 0x00, 0x5a, 0xa4, 0xd3, 0xf0, 0x00, 0x00, 0x00, 0x00,
-  0x5b, 0xde, 0x8a, 0xe0, 0x00, 0x00, 0x00, 0x00, 0x5c, 0x84, 0xb5, 0xf0,
-  0x00, 0x00, 0x00, 0x00, 0x5d, 0xbe, 0x6c, 0xe0, 0x00, 0x00, 0x00, 0x00,
-  0x5e, 0x64, 0x97, 0xf0, 0x00, 0x00, 0x00, 0x00, 0x5f, 0x9e, 0x4e, 0xe0,
-  0x00, 0x00, 0x00, 0x00, 0x60, 0x4d, 0xb4, 0x70, 0x00, 0x00, 0x00, 0x00,
-  0x61, 0x87, 0x6b, 0x60, 0x00, 0x00, 0x00, 0x00, 0x62, 0x2d, 0x96, 0x70,
-  0x00, 0x00, 0x00, 0x00, 0x63, 0x67, 0x4d, 0x60, 0x00, 0x00, 0x00, 0x00,
-  0x64, 0x0d, 0x78, 0x70, 0x00, 0x00, 0x00, 0x00, 0x65, 0x47, 0x2f, 0x60,
-  0x00, 0x00, 0x00, 0x00, 0x65, 0xed, 0x5a, 0x70, 0x00, 0x00, 0x00, 0x00,
-  0x67, 0x27, 0x11, 0x60, 0x00, 0x00, 0x00, 0x00, 0x67, 0xcd, 0x3c, 0x70,
-  0x00, 0x00, 0x00, 0x00, 0x69, 0x06, 0xf3, 0x60, 0x00, 0x00, 0x00, 0x00,
-  0x69, 0xad, 0x1e, 0x70, 0x00, 0x00, 0x00, 0x00, 0x6a, 0xe6, 0xd5, 0x60,
-  0x00, 0x00, 0x00, 0x00, 0x6b, 0x96, 0x3a, 0xf0, 0x00, 0x00, 0x00, 0x00,
-  0x6c, 0xcf, 0xf1, 0xe0, 0x00, 0x00, 0x00, 0x00, 0x6d, 0x76, 0x1c, 0xf0,
-  0x00, 0x00, 0x00, 0x00, 0x6e, 0xaf, 0xd3, 0xe0, 0x00, 0x00, 0x00, 0x00,
-  0x6f, 0x55, 0xfe, 0xf0, 0x00, 0x00, 0x00, 0x00, 0x70, 0x8f, 0xb5, 0xe0,
-  0x00, 0x00, 0x00, 0x00, 0x71, 0x35, 0xe0, 0xf0, 0x00, 0x00, 0x00, 0x00,
-  0x72, 0x6f, 0x97, 0xe0, 0x00, 0x00, 0x00, 0x00, 0x73, 0x15, 0xc2, 0xf0,
-  0x00, 0x00, 0x00, 0x00, 0x74, 0x4f, 0x79, 0xe0, 0x00, 0x00, 0x00, 0x00,
-  0x74, 0xfe, 0xdf, 0x70, 0x00, 0x00, 0x00, 0x00, 0x76, 0x38, 0x96, 0x60,
-  0x00, 0x00, 0x00, 0x00, 0x76, 0xde, 0xc1, 0x70, 0x00, 0x00, 0x00, 0x00,
-  0x78, 0x18, 0x78, 0x60, 0x00, 0x00, 0x00, 0x00, 0x78, 0xbe, 0xa3, 0x70,
-  0x00, 0x00, 0x00, 0x00, 0x79, 0xf8, 0x5a, 0x60, 0x00, 0x00, 0x00, 0x00,
-  0x7a, 0x9e, 0x85, 0x70, 0x00, 0x00, 0x00, 0x00, 0x7b, 0xd8, 0x3c, 0x60,
-  0x00, 0x00, 0x00, 0x00, 0x7c, 0x7e, 0x67, 0x70, 0x00, 0x00, 0x00, 0x00,
-  0x7d, 0xb8, 0x1e, 0x60, 0x00, 0x00, 0x00, 0x00, 0x7e, 0x5e, 0x49, 0x70,
-  0x00, 0x00, 0x00, 0x00, 0x7f, 0x98, 0x00, 0x60, 0x00, 0x02, 0x01, 0x02,
-  0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02,
-  0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02,
-  0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02,
-  0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x03, 0x04,
-  0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01,
-  0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01,
-  0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01,
-  0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01,
-  0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01,
-  0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01,
-  0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01,
-  0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01,
-  0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01,
-  0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01,
-  0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01,
-  0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01,
-  0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01,
-  0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01,
-  0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01,
-  0x02, 0x01, 0x02, 0x01, 0x02, 0xff, 0xff, 0xba, 0x9e, 0x00, 0x00, 0xff,
-  0xff, 0xc7, 0xc0, 0x01, 0x04, 0xff, 0xff, 0xb9, 0xb0, 0x00, 0x08, 0xff,
-  0xff, 0xc7, 0xc0, 0x01, 0x0c, 0xff, 0xff, 0xc7, 0xc0, 0x01, 0x10, 0x4c,
-  0x4d, 0x54, 0x00, 0x45, 0x44, 0x54, 0x00, 0x45, 0x53, 0x54, 0x00, 0x45,
-  0x57, 0x54, 0x00, 0x45, 0x50, 0x54, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01,
-  0x00, 0x00, 0x00, 0x00, 0x01, 0x0a, 0x45, 0x53, 0x54, 0x35, 0x45, 0x44,
-  0x54, 0x2c, 0x4d, 0x33, 0x2e, 0x32, 0x2e, 0x30, 0x2c, 0x4d, 0x31, 0x31,
-  0x2e, 0x31, 0x2e, 0x30, 0x0a
-};
-unsigned int America_New_York_len = 3545;
-unsigned char Australia_Sydney[] = {
-  0x54, 0x5a, 0x69, 0x66, 0x32, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
-  0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x05,
-  0x00, 0x00, 0x00, 0x05, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x8e,
-  0x00, 0x00, 0x00, 0x05, 0x00, 0x00, 0x00, 0x0e, 0x80, 0x00, 0x00, 0x00,
-  0x9c, 0x4e, 0xa6, 0x9c, 0x9c, 0xbc, 0x20, 0xf0, 0xcb, 0x54, 0xb3, 0x00,
-  0xcb, 0xc7, 0x57, 0x70, 0xcc, 0xb7, 0x56, 0x80, 0xcd, 0xa7, 0x39, 0x70,
-  0xce, 0xa0, 0x73, 0x00, 0xcf, 0x87, 0x1b, 0x70, 0x03, 0x70, 0x39, 0x80,
-  0x04, 0x0d, 0x1c, 0x00, 0x05, 0x50, 0x1b, 0x80, 0x05, 0xf6, 0x38, 0x80,
-  0x07, 0x2f, 0xfd, 0x80, 0x07, 0xd6, 0x1a, 0x80, 0x09, 0x0f, 0xdf, 0x80,
-  0x09, 0xb5, 0xfc, 0x80, 0x0a, 0xef, 0xc1, 0x80, 0x0b, 0x9f, 0x19, 0x00,
-  0x0c, 0xd8, 0xde, 0x00, 0x0d, 0x7e, 0xfb, 0x00, 0x0e, 0xb8, 0xc0, 0x00,
-  0x0f, 0x5e, 0xdd, 0x00, 0x10, 0x98, 0xa2, 0x00, 0x11, 0x3e, 0xbf, 0x00,
-  0x12, 0x78, 0x84, 0x00, 0x13, 0x1e, 0xa1, 0x00, 0x14, 0x58, 0x66, 0x00,
-  0x14, 0xfe, 0x83, 0x00, 0x16, 0x38, 0x48, 0x00, 0x17, 0x0c, 0x89, 0x80,
-  0x18, 0x21, 0x64, 0x80, 0x18, 0xc7, 0x81, 0x80, 0x1a, 0x01, 0x46, 0x80,
-  0x1a, 0xa7, 0x63, 0x80, 0x1b, 0xe1, 0x28, 0x80, 0x1c, 0x87, 0x45, 0x80,
-  0x1d, 0xc1, 0x0a, 0x80, 0x1e, 0x79, 0x9c, 0x80, 0x1f, 0x97, 0xb2, 0x00,
-  0x20, 0x59, 0x7e, 0x80, 0x21, 0x80, 0xce, 0x80, 0x22, 0x42, 0x9b, 0x00,
-  0x23, 0x69, 0xeb, 0x00, 0x24, 0x22, 0x7d, 0x00, 0x25, 0x49, 0xcd, 0x00,
-  0x25, 0xef, 0xea, 0x00, 0x27, 0x29, 0xaf, 0x00, 0x27, 0xcf, 0xcc, 0x00,
-  0x29, 0x09, 0x91, 0x00, 0x29, 0xaf, 0xae, 0x00, 0x2a, 0xe9, 0x73, 0x00,
-  0x2b, 0x98, 0xca, 0x80, 0x2c, 0xd2, 0x8f, 0x80, 0x2d, 0x78, 0xac, 0x80,
-  0x2e, 0xb2, 0x71, 0x80, 0x2f, 0x58, 0x8e, 0x80, 0x30, 0x92, 0x53, 0x80,
-  0x31, 0x5d, 0x5a, 0x80, 0x32, 0x72, 0x35, 0x80, 0x33, 0x3d, 0x3c, 0x80,
-  0x34, 0x52, 0x17, 0x80, 0x35, 0x1d, 0x1e, 0x80, 0x36, 0x31, 0xf9, 0x80,
-  0x36, 0xfd, 0x00, 0x80, 0x38, 0x1b, 0x16, 0x00, 0x38, 0xdc, 0xe2, 0x80,
-  0x39, 0xa7, 0xe9, 0x80, 0x3a, 0xbc, 0xc4, 0x80, 0x3b, 0xda, 0xda, 0x00,
-  0x3c, 0xa5, 0xe1, 0x00, 0x3d, 0xba, 0xbc, 0x00, 0x3e, 0x85, 0xc3, 0x00,
-  0x3f, 0x9a, 0x9e, 0x00, 0x40, 0x65, 0xa5, 0x00, 0x41, 0x83, 0xba, 0x80,
-  0x42, 0x45, 0x87, 0x00, 0x43, 0x63, 0x9c, 0x80, 0x44, 0x2e, 0xa3, 0x80,
-  0x45, 0x43, 0x7e, 0x80, 0x46, 0x05, 0x4b, 0x00, 0x47, 0x23, 0x60, 0x80,
-  0x47, 0xf7, 0xa2, 0x00, 0x48, 0xe7, 0x93, 0x00, 0x49, 0xd7, 0x84, 0x00,
-  0x4a, 0xc7, 0x75, 0x00, 0x4b, 0xb7, 0x66, 0x00, 0x4c, 0xa7, 0x57, 0x00,
-  0x4d, 0x97, 0x48, 0x00, 0x4e, 0x87, 0x39, 0x00, 0x4f, 0x77, 0x2a, 0x00,
-  0x50, 0x70, 0x55, 0x80, 0x51, 0x60, 0x46, 0x80, 0x52, 0x50, 0x37, 0x80,
-  0x53, 0x40, 0x28, 0x80, 0x54, 0x30, 0x19, 0x80, 0x55, 0x20, 0x0a, 0x80,
-  0x56, 0x0f, 0xfb, 0x80, 0x56, 0xff, 0xec, 0x80, 0x57, 0xef, 0xdd, 0x80,
-  0x58, 0xdf, 0xce, 0x80, 0x59, 0xcf, 0xbf, 0x80, 0x5a, 0xbf, 0xb0, 0x80,
-  0x5b, 0xb8, 0xdc, 0x00, 0x5c, 0xa8, 0xcd, 0x00, 0x5d, 0x98, 0xbe, 0x00,
-  0x5e, 0x88, 0xaf, 0x00, 0x5f, 0x78, 0xa0, 0x00, 0x60, 0x68, 0x91, 0x00,
-  0x61, 0x58, 0x82, 0x00, 0x62, 0x48, 0x73, 0x00, 0x63, 0x38, 0x64, 0x00,
-  0x64, 0x28, 0x55, 0x00, 0x65, 0x18, 0x46, 0x00, 0x66, 0x11, 0x71, 0x80,
-  0x67, 0x01, 0x62, 0x80, 0x67, 0xf1, 0x53, 0x80, 0x68, 0xe1, 0x44, 0x80,
-  0x69, 0xd1, 0x35, 0x80, 0x6a, 0xc1, 0x26, 0x80, 0x6b, 0xb1, 0x17, 0x80,
-  0x6c, 0xa1, 0x08, 0x80, 0x6d, 0x90, 0xf9, 0x80, 0x6e, 0x80, 0xea, 0x80,
-  0x6f, 0x70, 0xdb, 0x80, 0x70, 0x6a, 0x07, 0x00, 0x71, 0x59, 0xf8, 0x00,
-  0x72, 0x49, 0xe9, 0x00, 0x73, 0x39, 0xda, 0x00, 0x74, 0x29, 0xcb, 0x00,
-  0x75, 0x19, 0xbc, 0x00, 0x76, 0x09, 0xad, 0x00, 0x76, 0xf9, 0x9e, 0x00,
-  0x77, 0xe9, 0x8f, 0x00, 0x78, 0xd9, 0x80, 0x00, 0x79, 0xc9, 0x71, 0x00,
-  0x7a, 0xb9, 0x62, 0x00, 0x7b, 0xb2, 0x8d, 0x80, 0x7c, 0xa2, 0x7e, 0x80,
-  0x7d, 0x92, 0x6f, 0x80, 0x7e, 0x82, 0x60, 0x80, 0x7f, 0x72, 0x51, 0x80,
-  0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02, 0x03, 0x04, 0x03,
-  0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03,
-  0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03,
-  0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03,
-  0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03,
-  0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03,
-  0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03,
-  0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03,
-  0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03,
-  0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03,
-  0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03,
-  0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x00, 0x00,
-  0x8d, 0xc4, 0x00, 0x00, 0x00, 0x00, 0x9a, 0xb0, 0x01, 0x04, 0x00, 0x00,
-  0x8c, 0xa0, 0x00, 0x09, 0x00, 0x00, 0x9a, 0xb0, 0x01, 0x04, 0x00, 0x00,
-  0x8c, 0xa0, 0x00, 0x09, 0x4c, 0x4d, 0x54, 0x00, 0x41, 0x45, 0x44, 0x54,
-  0x00, 0x41, 0x45, 0x53, 0x54, 0x00, 0x00, 0x00, 0x00, 0x01, 0x01, 0x00,
-  0x00, 0x00, 0x00, 0x00, 0x54, 0x5a, 0x69, 0x66, 0x32, 0x00, 0x00, 0x00,
-  0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
-  0x00, 0x00, 0x00, 0x05, 0x00, 0x00, 0x00, 0x05, 0x00, 0x00, 0x00, 0x00,
-  0x00, 0x00, 0x00, 0x8f, 0x00, 0x00, 0x00, 0x05, 0x00, 0x00, 0x00, 0x0e,
-  0xf8, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0xff, 0xff, 0xff, 0xff,
-  0x73, 0x16, 0x7f, 0x3c, 0xff, 0xff, 0xff, 0xff, 0x9c, 0x4e, 0xa6, 0x9c,
-  0xff, 0xff, 0xff, 0xff, 0x9c, 0xbc, 0x20, 0xf0, 0xff, 0xff, 0xff, 0xff,
-  0xcb, 0x54, 0xb3, 0x00, 0xff, 0xff, 0xff, 0xff, 0xcb, 0xc7, 0x57, 0x70,
-  0xff, 0xff, 0xff, 0xff, 0xcc, 0xb7, 0x56, 0x80, 0xff, 0xff, 0xff, 0xff,
-  0xcd, 0xa7, 0x39, 0x70, 0xff, 0xff, 0xff, 0xff, 0xce, 0xa0, 0x73, 0x00,
-  0xff, 0xff, 0xff, 0xff, 0xcf, 0x87, 0x1b, 0x70, 0x00, 0x00, 0x00, 0x00,
-  0x03, 0x70, 0x39, 0x80, 0x00, 0x00, 0x00, 0x00, 0x04, 0x0d, 0x1c, 0x00,
-  0x00, 0x00, 0x00, 0x00, 0x05, 0x50, 0x1b, 0x80, 0x00, 0x00, 0x00, 0x00,
-  0x05, 0xf6, 0x38, 0x80, 0x00, 0x00, 0x00, 0x00, 0x07, 0x2f, 0xfd, 0x80,
-  0x00, 0x00, 0x00, 0x00, 0x07, 0xd6, 0x1a, 0x80, 0x00, 0x00, 0x00, 0x00,
-  0x09, 0x0f, 0xdf, 0x80, 0x00, 0x00, 0x00, 0x00, 0x09, 0xb5, 0xfc, 0x80,
-  0x00, 0x00, 0x00, 0x00, 0x0a, 0xef, 0xc1, 0x80, 0x00, 0x00, 0x00, 0x00,
-  0x0b, 0x9f, 0x19, 0x00, 0x00, 0x00, 0x00, 0x00, 0x0c, 0xd8, 0xde, 0x00,
-  0x00, 0x00, 0x00, 0x00, 0x0d, 0x7e, 0xfb, 0x00, 0x00, 0x00, 0x00, 0x00,
-  0x0e, 0xb8, 0xc0, 0x00, 0x00, 0x00, 0x00, 0x00, 0x0f, 0x5e, 0xdd, 0x00,
-  0x00, 0x00, 0x00, 0x00, 0x10, 0x98, 0xa2, 0x00, 0x00, 0x00, 0x00, 0x00,
-  0x11, 0x3e, 0xbf, 0x00, 0x00, 0x00, 0x00, 0x00, 0x12, 0x78, 0x84, 0x00,
-  0x00, 0x00, 0x00, 0x00, 0x13, 0x1e, 0xa1, 0x00, 0x00, 0x00, 0x00, 0x00,
-  0x14, 0x58, 0x66, 0x00, 0x00, 0x00, 0x00, 0x00, 0x14, 0xfe, 0x83, 0x00,
-  0x00, 0x00, 0x00, 0x00, 0x16, 0x38, 0x48, 0x00, 0x00, 0x00, 0x00, 0x00,
-  0x17, 0x0c, 0x89, 0x80, 0x00, 0x00, 0x00, 0x00, 0x18, 0x21, 0x64, 0x80,
-  0x00, 0x00, 0x00, 0x00, 0x18, 0xc7, 0x81, 0x80, 0x00, 0x00, 0x00, 0x00,
-  0x1a, 0x01, 0x46, 0x80, 0x00, 0x00, 0x00, 0x00, 0x1a, 0xa7, 0x63, 0x80,
-  0x00, 0x00, 0x00, 0x00, 0x1b, 0xe1, 0x28, 0x80, 0x00, 0x00, 0x00, 0x00,
-  0x1c, 0x87, 0x45, 0x80, 0x00, 0x00, 0x00, 0x00, 0x1d, 0xc1, 0x0a, 0x80,
-  0x00, 0x00, 0x00, 0x00, 0x1e, 0x79, 0x9c, 0x80, 0x00, 0x00, 0x00, 0x00,
-  0x1f, 0x97, 0xb2, 0x00, 0x00, 0x00, 0x00, 0x00, 0x20, 0x59, 0x7e, 0x80,
-  0x00, 0x00, 0x00, 0x00, 0x21, 0x80, 0xce, 0x80, 0x00, 0x00, 0x00, 0x00,
-  0x22, 0x42, 0x9b, 0x00, 0x00, 0x00, 0x00, 0x00, 0x23, 0x69, 0xeb, 0x00,
-  0x00, 0x00, 0x00, 0x00, 0x24, 0x22, 0x7d, 0x00, 0x00, 0x00, 0x00, 0x00,
-  0x25, 0x49, 0xcd, 0x00, 0x00, 0x00, 0x00, 0x00, 0x25, 0xef, 0xea, 0x00,
-  0x00, 0x00, 0x00, 0x00, 0x27, 0x29, 0xaf, 0x00, 0x00, 0x00, 0x00, 0x00,
-  0x27, 0xcf, 0xcc, 0x00, 0x00, 0x00, 0x00, 0x00, 0x29, 0x09, 0x91, 0x00,
-  0x00, 0x00, 0x00, 0x00, 0x29, 0xaf, 0xae, 0x00, 0x00, 0x00, 0x00, 0x00,
-  0x2a, 0xe9, 0x73, 0x00, 0x00, 0x00, 0x00, 0x00, 0x2b, 0x98, 0xca, 0x80,
-  0x00, 0x00, 0x00, 0x00, 0x2c, 0xd2, 0x8f, 0x80, 0x00, 0x00, 0x00, 0x00,
-  0x2d, 0x78, 0xac, 0x80, 0x00, 0x00, 0x00, 0x00, 0x2e, 0xb2, 0x71, 0x80,
-  0x00, 0x00, 0x00, 0x00, 0x2f, 0x58, 0x8e, 0x80, 0x00, 0x00, 0x00, 0x00,
-  0x30, 0x92, 0x53, 0x80, 0x00, 0x00, 0x00, 0x00, 0x31, 0x5d, 0x5a, 0x80,
-  0x00, 0x00, 0x00, 0x00, 0x32, 0x72, 0x35, 0x80, 0x00, 0x00, 0x00, 0x00,
-  0x33, 0x3d, 0x3c, 0x80, 0x00, 0x00, 0x00, 0x00, 0x34, 0x52, 0x17, 0x80,
-  0x00, 0x00, 0x00, 0x00, 0x35, 0x1d, 0x1e, 0x80, 0x00, 0x00, 0x00, 0x00,
-  0x36, 0x31, 0xf9, 0x80, 0x00, 0x00, 0x00, 0x00, 0x36, 0xfd, 0x00, 0x80,
-  0x00, 0x00, 0x00, 0x00, 0x38, 0x1b, 0x16, 0x00, 0x00, 0x00, 0x00, 0x00,
-  0x38, 0xdc, 0xe2, 0x80, 0x00, 0x00, 0x00, 0x00, 0x39, 0xa7, 0xe9, 0x80,
-  0x00, 0x00, 0x00, 0x00, 0x3a, 0xbc, 0xc4, 0x80, 0x00, 0x00, 0x00, 0x00,
-  0x3b, 0xda, 0xda, 0x00, 0x00, 0x00, 0x00, 0x00, 0x3c, 0xa5, 0xe1, 0x00,
-  0x00, 0x00, 0x00, 0x00, 0x3d, 0xba, 0xbc, 0x00, 0x00, 0x00, 0x00, 0x00,
-  0x3e, 0x85, 0xc3, 0x00, 0x00, 0x00, 0x00, 0x00, 0x3f, 0x9a, 0x9e, 0x00,
-  0x00, 0x00, 0x00, 0x00, 0x40, 0x65, 0xa5, 0x00, 0x00, 0x00, 0x00, 0x00,
-  0x41, 0x83, 0xba, 0x80, 0x00, 0x00, 0x00, 0x00, 0x42, 0x45, 0x87, 0x00,
-  0x00, 0x00, 0x00, 0x00, 0x43, 0x63, 0x9c, 0x80, 0x00, 0x00, 0x00, 0x00,
-  0x44, 0x2e, 0xa3, 0x80, 0x00, 0x00, 0x00, 0x00, 0x45, 0x43, 0x7e, 0x80,
-  0x00, 0x00, 0x00, 0x00, 0x46, 0x05, 0x4b, 0x00, 0x00, 0x00, 0x00, 0x00,
-  0x47, 0x23, 0x60, 0x80, 0x00, 0x00, 0x00, 0x00, 0x47, 0xf7, 0xa2, 0x00,
-  0x00, 0x00, 0x00, 0x00, 0x48, 0xe7, 0x93, 0x00, 0x00, 0x00, 0x00, 0x00,
-  0x49, 0xd7, 0x84, 0x00, 0x00, 0x00, 0x00, 0x00, 0x4a, 0xc7, 0x75, 0x00,
-  0x00, 0x00, 0x00, 0x00, 0x4b, 0xb7, 0x66, 0x00, 0x00, 0x00, 0x00, 0x00,
-  0x4c, 0xa7, 0x57, 0x00, 0x00, 0x00, 0x00, 0x00, 0x4d, 0x97, 0x48, 0x00,
-  0x00, 0x00, 0x00, 0x00, 0x4e, 0x87, 0x39, 0x00, 0x00, 0x00, 0x00, 0x00,
-  0x4f, 0x77, 0x2a, 0x00, 0x00, 0x00, 0x00, 0x00, 0x50, 0x70, 0x55, 0x80,
-  0x00, 0x00, 0x00, 0x00, 0x51, 0x60, 0x46, 0x80, 0x00, 0x00, 0x00, 0x00,
-  0x52, 0x50, 0x37, 0x80, 0x00, 0x00, 0x00, 0x00, 0x53, 0x40, 0x28, 0x80,
-  0x00, 0x00, 0x00, 0x00, 0x54, 0x30, 0x19, 0x80, 0x00, 0x00, 0x00, 0x00,
-  0x55, 0x20, 0x0a, 0x80, 0x00, 0x00, 0x00, 0x00, 0x56, 0x0f, 0xfb, 0x80,
-  0x00, 0x00, 0x00, 0x00, 0x56, 0xff, 0xec, 0x80, 0x00, 0x00, 0x00, 0x00,
-  0x57, 0xef, 0xdd, 0x80, 0x00, 0x00, 0x00, 0x00, 0x58, 0xdf, 0xce, 0x80,
-  0x00, 0x00, 0x00, 0x00, 0x59, 0xcf, 0xbf, 0x80, 0x00, 0x00, 0x00, 0x00,
-  0x5a, 0xbf, 0xb0, 0x80, 0x00, 0x00, 0x00, 0x00, 0x5b, 0xb8, 0xdc, 0x00,
-  0x00, 0x00, 0x00, 0x00, 0x5c, 0xa8, 0xcd, 0x00, 0x00, 0x00, 0x00, 0x00,
-  0x5d, 0x98, 0xbe, 0x00, 0x00, 0x00, 0x00, 0x00, 0x5e, 0x88, 0xaf, 0x00,
-  0x00, 0x00, 0x00, 0x00, 0x5f, 0x78, 0xa0, 0x00, 0x00, 0x00, 0x00, 0x00,
-  0x60, 0x68, 0x91, 0x00, 0x00, 0x00, 0x00, 0x00, 0x61, 0x58, 0x82, 0x00,
-  0x00, 0x00, 0x00, 0x00, 0x62, 0x48, 0x73, 0x00, 0x00, 0x00, 0x00, 0x00,
-  0x63, 0x38, 0x64, 0x00, 0x00, 0x00, 0x00, 0x00, 0x64, 0x28, 0x55, 0x00,
-  0x00, 0x00, 0x00, 0x00, 0x65, 0x18, 0x46, 0x00, 0x00, 0x00, 0x00, 0x00,
-  0x66, 0x11, 0x71, 0x80, 0x00, 0x00, 0x00, 0x00, 0x67, 0x01, 0x62, 0x80,
-  0x00, 0x00, 0x00, 0x00, 0x67, 0xf1, 0x53, 0x80, 0x00, 0x00, 0x00, 0x00,
-  0x68, 0xe1, 0x44, 0x80, 0x00, 0x00, 0x00, 0x00, 0x69, 0xd1, 0x35, 0x80,
-  0x00, 0x00, 0x00, 0x00, 0x6a, 0xc1, 0x26, 0x80, 0x00, 0x00, 0x00, 0x00,
-  0x6b, 0xb1, 0x17, 0x80, 0x00, 0x00, 0x00, 0x00, 0x6c, 0xa1, 0x08, 0x80,
-  0x00, 0x00, 0x00, 0x00, 0x6d, 0x90, 0xf9, 0x80, 0x00, 0x00, 0x00, 0x00,
-  0x6e, 0x80, 0xea, 0x80, 0x00, 0x00, 0x00, 0x00, 0x6f, 0x70, 0xdb, 0x80,
-  0x00, 0x00, 0x00, 0x00, 0x70, 0x6a, 0x07, 0x00, 0x00, 0x00, 0x00, 0x00,
-  0x71, 0x59, 0xf8, 0x00, 0x00, 0x00, 0x00, 0x00, 0x72, 0x49, 0xe9, 0x00,
-  0x00, 0x00, 0x00, 0x00, 0x73, 0x39, 0xda, 0x00, 0x00, 0x00, 0x00, 0x00,
-  0x74, 0x29, 0xcb, 0x00, 0x00, 0x00, 0x00, 0x00, 0x75, 0x19, 0xbc, 0x00,
-  0x00, 0x00, 0x00, 0x00, 0x76, 0x09, 0xad, 0x00, 0x00, 0x00, 0x00, 0x00,
-  0x76, 0xf9, 0x9e, 0x00, 0x00, 0x00, 0x00, 0x00, 0x77, 0xe9, 0x8f, 0x00,
-  0x00, 0x00, 0x00, 0x00, 0x78, 0xd9, 0x80, 0x00, 0x00, 0x00, 0x00, 0x00,
-  0x79, 0xc9, 0x71, 0x00, 0x00, 0x00, 0x00, 0x00, 0x7a, 0xb9, 0x62, 0x00,
-  0x00, 0x00, 0x00, 0x00, 0x7b, 0xb2, 0x8d, 0x80, 0x00, 0x00, 0x00, 0x00,
-  0x7c, 0xa2, 0x7e, 0x80, 0x00, 0x00, 0x00, 0x00, 0x7d, 0x92, 0x6f, 0x80,
-  0x00, 0x00, 0x00, 0x00, 0x7e, 0x82, 0x60, 0x80, 0x00, 0x00, 0x00, 0x00,
-  0x7f, 0x72, 0x51, 0x80, 0x00, 0x02, 0x01, 0x02, 0x01, 0x02, 0x01, 0x02,
-  0x01, 0x02, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04,
-  0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04,
-  0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04,
-  0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04,
-  0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04,
-  0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04,
-  0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04,
-  0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04,
-  0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04,
-  0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04,
-  0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04, 0x03, 0x04,
-  0x03, 0x04, 0x03, 0x00, 0x00, 0x8d, 0xc4, 0x00, 0x00, 0x00, 0x00, 0x9a,
-  0xb0, 0x01, 0x04, 0x00, 0x00, 0x8c, 0xa0, 0x00, 0x09, 0x00, 0x00, 0x9a,
-  0xb0, 0x01, 0x04, 0x00, 0x00, 0x8c, 0xa0, 0x00, 0x09, 0x4c, 0x4d, 0x54,
-  0x00, 0x41, 0x45, 0x44, 0x54, 0x00, 0x41, 0x45, 0x53, 0x54, 0x00, 0x00,
-  0x00, 0x00, 0x01, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x0a, 0x41, 0x45,
-  0x53, 0x54, 0x2d, 0x31, 0x30, 0x41, 0x45, 0x44, 0x54, 0x2c, 0x4d, 0x31,
-  0x30, 0x2e, 0x31, 0x2e, 0x30, 0x2c, 0x4d, 0x34, 0x2e, 0x31, 0x2e, 0x30,
-  0x2f, 0x33, 0x0a
-};
-unsigned int Australia_Sydney_len = 2223;
diff --git a/absl/time/simulated_clock.cc b/absl/time/simulated_clock.cc
new file mode 100644
index 00000000000..6c98260a329
--- /dev/null
+++ b/absl/time/simulated_clock.cc
@@ -0,0 +1,225 @@
+// Copyright 2026 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/time/simulated_clock.h"
+
+#include <cstdint>
+#include <map>
+#include <memory>
+#include <optional>
+#include <utility>
+#include <vector>
+
+#include "absl/base/config.h"
+#include "absl/base/macros.h"
+#include "absl/base/nullability.h"
+#include "absl/base/thread_annotations.h"
+#include "absl/synchronization/mutex.h"
+#include "absl/time/clock.h"
+#include "absl/time/time.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+// There are a few tricky details in the implementation of SimulatedClock.
+//
+// The external mutex that is passed as a param of AwaitWithDeadline() is not
+// under the control of SimulatedClock; in particular, it can be destroyed any
+// time after AwaitWithDeadline() returns.  This requires the wakeup call from
+// AdvanceTime() to avoid grabbing the external mutex if AwaitWithDeadline()
+// has returned.  This is accomplished by allowing the waiter to be cancelled
+// via a bool guarded by a mutex in WakeUpInfo.
+//
+// Once AwaitWithDeadline() has released lock_, someone nefarious might call
+// the SimulatedClock destructor, so it isn't possible for AwaitWithDeadline()
+// to remove the wakeup call from waiters_ if the condition it is awaiting
+// becomes true; it cancels the waiter instead.  This means that,
+// theoretically, many obsolete entries could pile up in waiters_ if
+// AwaitWithDeadline() keeps being called but simulated time is not advanced.
+// This seems unlikely to happen in practice.
+//
+// The WakeUpInfo in waiters_ are always awoken via WakeUp() (and
+// removed) or cancelled before AwaitWithDeadline() returns.  If a
+// waiters_ value is cancelled then calling its WakeUp() method will
+// short-circuit before touching the external mutex.
+
+class SimulatedClock::WakeUpInfo {
+ public:
+  WakeUpInfo(absl::Mutex* mu, absl::Condition cond)
+      : mu_(mu),
+        cond_(cond),
+        wakeup_time_passed_(false),
+        cancelled_(false),
+        wakeup_called_(false) {}
+
+  void WakeUp() {
+    // If we are cancelled then AwaitWithDeadline may have returned, in which
+    // case we can't lock mu_.
+    {
+      absl::MutexLock lock(cancellation_mu_);
+      if (cancelled_) return;
+      wakeup_called_ = true;
+    }
+    absl::MutexLock lock(*mu_);
+    wakeup_time_passed_ = true;
+  }
+
+  void AwaitConditionOrWakeUp() {
+    mu_->Await(absl::Condition(this, &WakeUpInfo::Ready));
+  }
+
+  void CancelOrAwaitWakeUp() {
+    bool wakeup_called;
+    {
+      absl::MutexLock lock(cancellation_mu_);
+      cancelled_ = true;
+      wakeup_called = wakeup_called_;
+    }
+    if (wakeup_called && !wakeup_time_passed_) {
+      // Wait for WakeUp to complete.
+      //
+      // Note that this will unlock 'mu_'; this is actually necessary
+      // so that WakeUp() can unblock and complete.  This does allow
+      // for 'cond_' to potentially change from true to false; that is
+      // OK, since WakeUp() is being called, so the deadline must be
+      // past, and so this method is fulfilling its duties.  (Well, the
+      // destructor might be calling WakeUp(), but if you're deleting
+      // time itself while waiting for a deadline to pass, you deserve
+      // what you get.)
+      mu_->Await(absl::Condition(&wakeup_time_passed_));
+    }
+  }
+
+ private:
+  bool Ready() const { return wakeup_time_passed_ || cond_.Eval(); }
+
+  absl::Mutex* mu_;
+  absl::Condition cond_;
+  bool wakeup_time_passed_;
+  absl::Mutex cancellation_mu_;
+  bool cancelled_ ABSL_GUARDED_BY(cancellation_mu_);
+  bool wakeup_called_ ABSL_GUARDED_BY(cancellation_mu_);
+};
+
+SimulatedClock::SimulatedClock(absl::Time t) : now_(t) {}
+
+SimulatedClock::~SimulatedClock() {
+  // Wake up all existing waiters.
+  WaiterList waiters;
+  {
+    absl::MutexLock l(lock_);
+    waiters.swap(waiters_);
+  }
+  for (auto& iter : waiters) {
+    iter.second->WakeUp();
+  }
+}
+
+absl::Time SimulatedClock::TimeNow() {
+  absl::ReaderMutexLock l(lock_);
+  return now_;
+}
+
+void SimulatedClock::Sleep(absl::Duration d) { SleepUntil(TimeNow() + d); }
+
+int64_t SimulatedClock::SetTime(absl::Time t) ABSL_NO_THREAD_SAFETY_ANALYSIS {
+  return UpdateTime([this, t]()
+                        ABSL_EXCLUSIVE_LOCKS_REQUIRED(lock_) { now_ = t; });
+}
+
+int64_t SimulatedClock::AdvanceTime(absl::Duration d)
+    ABSL_NO_THREAD_SAFETY_ANALYSIS {
+  return UpdateTime([this, d]()
+                        ABSL_EXCLUSIVE_LOCKS_REQUIRED(lock_) { now_ += d; });
+}
+
+template <class T>
+int64_t SimulatedClock::UpdateTime(const T& now_updater) {
+  // Deadlock could occur if UpdateTime() were to hold lock_ while waking up
+  // waiters, since waking up requires acquiring the external mutex, and
+  // AwaitWithDeadline() acquires the mutexes in the opposite order.  So let's
+  // first grab all the wakeup callbacks, then release lock_, then call the
+  // callbacks.
+  std::vector<WaiterList::mapped_type> wakeup_calls;
+
+  lock_.lock();
+  now_updater();  // reset now_
+  WaiterList::iterator iter;
+  while (((iter = waiters_.begin()) != waiters_.end()) &&
+         (iter->first <= now_)) {
+    wakeup_calls.push_back(std::move(iter->second));
+    waiters_.erase(iter);
+  }
+  lock_.unlock();
+
+  for (const auto& wakeup_call : wakeup_calls) {
+    wakeup_call->WakeUp();
+  }
+
+  return static_cast<int64_t>(wakeup_calls.size());
+}
+
+void SimulatedClock::SleepUntil(absl::Time wakeup_time) {
+  absl::Mutex mu;
+  absl::MutexLock lock(mu);
+  bool f = false;
+  AwaitWithDeadline(&mu, absl::Condition(&f), wakeup_time);
+}
+
+bool SimulatedClock::AwaitWithDeadline(absl::Mutex* mu,
+                                       const absl::Condition& cond,
+                                       absl::Time deadline) {
+  mu->AssertReaderHeld();
+
+  // Evaluate cond outside our own lock to minimize contention.
+  const bool ready = cond.Eval();
+
+  lock_.lock();
+  num_await_calls_++;
+
+  // Return now if the deadline is already past, or if the condition is true.
+  // This avoids creating a WakeUpInfo that won't be deleted until an
+  // appropriate UpdateTime() call.
+  if (deadline <= now_ || ready) {
+    lock_.unlock();
+    return ready;
+  }
+
+  auto wakeup_info = std::make_shared<WakeUpInfo>(mu, cond);
+  waiters_.insert(std::make_pair(deadline, wakeup_info));
+
+  lock_.unlock();
+  // SimulatedClock may be destroyed any time after this, so we can't
+  // acquire lock_ again.
+
+  // Wait until either cond.Eval() becomes true, or the deadline has passed.
+  wakeup_info->AwaitConditionOrWakeUp();
+
+  // Cancel the wakeup call, or if it's already in progress, wait for it to
+  // finish, since we must ensure no one touches 'mu' or 'cond' after we return.
+  wakeup_info->CancelOrAwaitWakeUp();
+
+  return cond.Eval();
+}
+
+std::optional<absl::Time> SimulatedClock::GetEarliestWakeupTime() const {
+  absl::ReaderMutexLock l(lock_);
+  if (waiters_.empty()) {
+    return std::nullopt;
+  }
+  return waiters_.begin()->first;
+}
+
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/time/simulated_clock.h b/absl/time/simulated_clock.h
new file mode 100644
index 00000000000..b39475087ca
--- /dev/null
+++ b/absl/time/simulated_clock.h
@@ -0,0 +1,109 @@
+// Copyright 2026 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: simulated_clock.h
+// -----------------------------------------------------------------------------
+
+#ifndef ABSL_TIME_SIMULATED_CLOCK_H_
+#define ABSL_TIME_SIMULATED_CLOCK_H_
+
+#include <cstdint>
+#include <map>
+#include <memory>
+#include <optional>
+
+#include "absl/base/config.h"
+#include "absl/base/macros.h"
+#include "absl/base/nullability.h"
+#include "absl/base/thread_annotations.h"
+#include "absl/synchronization/mutex.h"
+#include "absl/time/clock.h"
+#include "absl/time/clock_interface.h"
+#include "absl/time/time.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+// A simulated clock is a concrete Clock implementation that does not "tick"
+// on its own.  Time is advanced by explicit calls to the AdvanceTime() or
+// SetTime() functions.
+//
+// Example:
+//   absl::SimulatedClock sim_clock;
+//   absl::Time now = sim_clock.TimeNow();
+//   // now == absl::UnixEpoch()
+//
+//   now = sim_clock.TimeNow();
+//   // now == absl::UnixEpoch() (still)
+//
+//   sim_clock.AdvanceTime(absl::Seconds(3));
+//   now = sim_clock.TimeNow();
+//   // now == absl::UnixEpoch() + absl::Seconds(3)
+//
+// This class is thread-safe.
+class SimulatedClock : public Clock {
+ public:
+  explicit SimulatedClock(absl::Time t);
+  SimulatedClock() : SimulatedClock(absl::UnixEpoch()) {}
+
+  // The destructor should be called only if all Sleep(), etc. and
+  // AdvanceTime() calls have completed. The code does its best to let
+  // any pending calls finish gracefully, but there are no guarantees.
+  ~SimulatedClock() override;
+
+  // Returns the simulated time.
+  absl::Time TimeNow() override;
+
+  // Sleeps until the specified duration has elapsed according to this clock.
+  void Sleep(absl::Duration d) override;
+
+  // Sleeps until the specified wakeup_time.
+  void SleepUntil(absl::Time wakeup_time) override;
+
+  // Sets the simulated time to the argument.  Wakes up any threads whose
+  // sleeps have now expired. Returns the number of woken threads.
+  int64_t SetTime(absl::Time t);
+
+  // Advances the simulated time by the specified duration.  Wakes up any
+  // threads whose sleeps have now expired. Returns the number of woken threads.
+  int64_t AdvanceTime(absl::Duration d);
+
+  // Blocks until the condition is true or until the simulated clock is
+  // advanced to or beyond the wakeup time (or both).
+  bool AwaitWithDeadline(absl::Mutex* absl_nonnull mu,
+                         const absl::Condition& cond,
+                         absl::Time deadline) override
+      ABSL_SHARED_LOCKS_REQUIRED(mu);
+
+  // Returns the earliest wakeup time.
+  std::optional<absl::Time> GetEarliestWakeupTime() const;
+
+ private:
+  template <class T>
+  int64_t UpdateTime(const T& now_updater) ABSL_LOCKS_EXCLUDED(lock_);
+
+  class WakeUpInfo;
+  using WaiterList = std::multimap<absl::Time, std::shared_ptr<WakeUpInfo>>;
+
+  mutable absl::Mutex lock_;
+  absl::Time now_ ABSL_GUARDED_BY(lock_);
+  WaiterList waiters_ ABSL_GUARDED_BY(lock_);
+  int64_t num_await_calls_ ABSL_GUARDED_BY(lock_) = 0;
+};
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_TIME_SIMULATED_CLOCK_H_
diff --git a/absl/time/simulated_clock_test.cc b/absl/time/simulated_clock_test.cc
new file mode 100644
index 00000000000..4618338e61d
--- /dev/null
+++ b/absl/time/simulated_clock_test.cc
@@ -0,0 +1,614 @@
+// Copyright 2026 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/time/simulated_clock.h"
+
+#include <cstddef>
+#include <cstdint>
+#include <memory>
+#include <thread>  // NOLINT(build/c++11)
+#include <vector>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/base/thread_annotations.h"
+#include "absl/random/random.h"
+#include "absl/synchronization/blocking_counter.h"
+#include "absl/synchronization/mutex.h"
+#include "absl/synchronization/notification.h"
+#include "absl/time/clock.h"
+#include "absl/time/clock_interface.h"
+#include "absl/time/time.h"
+
+namespace {
+
+constexpr absl::Duration kShortPause = absl::Milliseconds(20);
+constexpr absl::Duration kLongPause = absl::Milliseconds(1000);
+
+#ifdef _MSC_VER
+// As of 2026-01-29, multithreaded tests on MSVC are too flaky.
+const char* kSkipFlakyReason =
+    "Skipping this timing test because it is too flaky";
+#else
+const char* kSkipFlakyReason = nullptr;
+#endif
+
+TEST(SimulatedClock, TimeInitializedToZero) {
+  absl::SimulatedClock simclock;
+  EXPECT_EQ(absl::UnixEpoch(), simclock.TimeNow());
+}
+
+TEST(SimulatedClock, NowSetTime) {
+  absl::SimulatedClock simclock;
+  absl::Time now = simclock.TimeNow();
+
+  now += absl::Seconds(123);
+  simclock.SetTime(now);
+  EXPECT_EQ(now, simclock.TimeNow());
+
+  now += absl::Seconds(123);
+  simclock.SetTime(now);
+  EXPECT_EQ(now, simclock.TimeNow());
+
+  now += absl::ZeroDuration();
+  simclock.SetTime(now);
+  EXPECT_EQ(now, simclock.TimeNow());
+}
+
+TEST(SimulatedClock, NowAdvanceTime) {
+  absl::SimulatedClock simclock;
+  absl::Time now = simclock.TimeNow();
+
+  simclock.AdvanceTime(absl::Seconds(123));
+  now += absl::Seconds(123);
+  EXPECT_EQ(now, simclock.TimeNow());
+
+  simclock.AdvanceTime(absl::Seconds(123));
+  now += absl::Seconds(123);
+  EXPECT_EQ(now, simclock.TimeNow());
+
+  simclock.AdvanceTime(absl::ZeroDuration());
+  now += absl::ZeroDuration();
+  EXPECT_EQ(now, simclock.TimeNow());
+}
+
+void SleepAndNotify(absl::Clock* clock, absl::Duration sleep_secs,
+                    absl::Notification* note) {
+  clock->Sleep(sleep_secs);
+  note->Notify();
+}
+
+TEST(SimulatedClock, Sleep_SetToSleepTime) {
+  if (kSkipFlakyReason != nullptr) {
+    GTEST_SKIP() << kSkipFlakyReason;
+  }
+
+  absl::SimulatedClock simclock;
+  absl::Notification note;
+
+  std::thread tr(SleepAndNotify, &simclock, absl::Seconds(123), &note);
+
+  // wait for SleepAndNotify() to block
+  absl::SleepFor(kLongPause);
+  simclock.SetTime(absl::FromUnixSeconds(122));
+  // give Sleep() the opportunity to fail
+  absl::SleepFor(kShortPause);
+  EXPECT_FALSE(note.HasBeenNotified());
+  simclock.SetTime(absl::FromUnixSeconds(122 + 1));
+  // wait for Sleep() to return
+  absl::SleepFor(kLongPause);
+  EXPECT_TRUE(note.HasBeenNotified());
+  note.WaitForNotification();  // in case the expectation fails
+  tr.join();
+}
+
+TEST(SimulatedClock, SleepAdvanceToSleepTime) {
+  if (kSkipFlakyReason != nullptr) {
+    GTEST_SKIP() << kSkipFlakyReason;
+  }
+
+  absl::SimulatedClock simclock;
+  absl::Notification note;
+
+  std::thread tr(SleepAndNotify, &simclock, absl::Seconds(123), &note);
+  // wait for SleepAndNotify() to block
+  absl::SleepFor(kLongPause);
+  simclock.AdvanceTime(absl::Seconds(122));
+  // give Sleep() the opportunity to fail
+  absl::SleepFor(kShortPause);
+  EXPECT_FALSE(note.HasBeenNotified());
+  simclock.AdvanceTime(absl::Seconds(1));
+  // wait for Sleep() to return
+  absl::SleepFor(kLongPause);
+  EXPECT_TRUE(note.HasBeenNotified());
+  note.WaitForNotification();  // in case the expectation fails
+  tr.join();
+}
+
+TEST(SimulatedClock, SleepSetPastSleepTime) {
+  if (kSkipFlakyReason != nullptr) {
+    GTEST_SKIP() << kSkipFlakyReason;
+  }
+
+  absl::SimulatedClock simclock;
+  absl::Notification note;
+
+  std::thread tr(SleepAndNotify, &simclock, absl::Seconds(123), &note);
+  // wait for SleepAndNotify() to block
+  absl::SleepFor(kLongPause);
+  simclock.SetTime(absl::FromUnixSeconds(122));
+  // give Sleep() the opportunity to fail
+  absl::SleepFor(kShortPause);
+  EXPECT_FALSE(note.HasBeenNotified());
+  simclock.SetTime(absl::FromUnixSeconds(122 + 2));
+  // wait for Sleep() to return
+  absl::SleepFor(kLongPause);
+  EXPECT_TRUE(note.HasBeenNotified());
+  note.WaitForNotification();  // in case the expectation fails
+  tr.join();
+}
+
+TEST(SimulatedClock, SleepAdvancePastSleepTime) {
+  if (kSkipFlakyReason != nullptr) {
+    GTEST_SKIP() << kSkipFlakyReason;
+  }
+
+  absl::SimulatedClock simclock;
+  absl::Notification note;
+
+  std::thread tr(SleepAndNotify, &simclock, absl::Seconds(123), &note);
+  // wait for SleepAndNotify() to block
+  absl::SleepFor(kLongPause);
+  simclock.AdvanceTime(absl::Seconds(122));
+  // give Sleep() the opportunity to fail
+  absl::SleepFor(kShortPause);
+  EXPECT_FALSE(note.HasBeenNotified());
+  simclock.AdvanceTime(absl::Seconds(2));
+  // wait for Sleep() to return
+  absl::SleepFor(kLongPause);
+  EXPECT_TRUE(note.HasBeenNotified());
+  note.WaitForNotification();  // in case the expectation fails
+  tr.join();
+}
+
+TEST(SimulatedClock, SleepZeroSleepTime) {
+  if (kSkipFlakyReason != nullptr) {
+    GTEST_SKIP() << kSkipFlakyReason;
+  }
+
+  absl::SimulatedClock simclock;
+  absl::Notification note;
+
+  std::thread tr(SleepAndNotify, &simclock, absl::ZeroDuration(), &note);
+  // wait for SleepAndNotify() to ping note
+  absl::SleepFor(kLongPause);
+  EXPECT_TRUE(note.HasBeenNotified());
+  note.WaitForNotification();  // in case the expectation fails
+  tr.join();
+}
+
+void SleepUntilAndNotify(absl::Clock* clock, absl::Time wakeup_time,
+                         absl::Notification* note) {
+  clock->SleepUntil(wakeup_time);
+  note->Notify();
+}
+
+TEST(SimulatedClock, SleepUntilSetToSleepTime) {
+  if (kSkipFlakyReason != nullptr) {
+    GTEST_SKIP() << kSkipFlakyReason;
+  }
+
+  absl::SimulatedClock simclock;
+  absl::Notification note;
+
+  simclock.SetTime(absl::FromUnixSeconds(123));
+  std::thread tr(SleepUntilAndNotify, &simclock,
+                 absl::UnixEpoch() + absl::Seconds(246), &note);
+  // wait for SleepUntilAndNotify() to block
+  absl::SleepFor(kLongPause);
+  simclock.SetTime(absl::FromUnixSeconds(123 + 122));
+  // give SleepUntil() the opportunity to fail
+  absl::SleepFor(kShortPause);
+  EXPECT_FALSE(note.HasBeenNotified());
+  simclock.SetTime(absl::FromUnixSeconds(123 + 122 + 1));
+  absl::Time start = absl::Now();
+  note.WaitForNotification();  // SleepUntilAndNotify() should ping note
+  EXPECT_GE(absl::Milliseconds(50), absl::Now() - start);
+  tr.join();
+}
+
+TEST(SimulatedClock, SleepUntilAdvanceToSleepTime) {
+  if (kSkipFlakyReason != nullptr) {
+    GTEST_SKIP() << kSkipFlakyReason;
+  }
+
+  absl::SimulatedClock simclock;
+  absl::Notification note;
+
+  simclock.AdvanceTime(absl::Seconds(123));
+  std::thread tr(SleepUntilAndNotify, &simclock,
+                 absl::UnixEpoch() + absl::Seconds(246), &note);
+  // wait for SleepUntilAndNotify() to block
+  absl::SleepFor(kLongPause);
+  simclock.AdvanceTime(absl::Seconds(122));
+  // give SleepUntil() the opportunity to fail
+  absl::SleepFor(kShortPause);
+  EXPECT_FALSE(note.HasBeenNotified());
+  simclock.AdvanceTime(absl::Seconds(1));
+  absl::Time start = absl::Now();
+  note.WaitForNotification();  // SleepUntilAndNotify() should ping note
+  EXPECT_GE(absl::Milliseconds(70), absl::Now() - start);
+  tr.join();
+}
+
+TEST(SimulatedClock, SleepUntilSetPastSleepTime) {
+  if (kSkipFlakyReason != nullptr) {
+    GTEST_SKIP() << kSkipFlakyReason;
+  }
+
+  absl::SimulatedClock simclock;
+  absl::Notification note;
+
+  simclock.SetTime(absl::FromUnixSeconds(123));
+  std::thread tr(SleepUntilAndNotify, &simclock,
+                 absl::UnixEpoch() + absl::Seconds(246), &note);
+  // wait for SleepUntilAndNotify() to block
+  absl::SleepFor(kLongPause);
+  simclock.SetTime(absl::FromUnixSeconds(123 + 122));
+  // give SleepUntil() the opportunity to fail
+  absl::SleepFor(kShortPause);
+  EXPECT_FALSE(note.HasBeenNotified());
+  simclock.SetTime(absl::FromUnixSeconds(123 + 122 + 2));
+  // wait for SleepUntilAndNotify() to ping note
+  absl::SleepFor(kLongPause);
+  EXPECT_TRUE(note.HasBeenNotified());
+  note.WaitForNotification();  // in case the expectation fails
+  tr.join();
+}
+
+TEST(SimulatedClock, SleepUntilAdvancePastSleepTime) {
+  if (kSkipFlakyReason != nullptr) {
+    GTEST_SKIP() << kSkipFlakyReason;
+  }
+
+  absl::SimulatedClock simclock;
+  absl::Notification note;
+
+  simclock.AdvanceTime(absl::Seconds(123));
+  std::thread tr(SleepUntilAndNotify, &simclock,
+                 absl::UnixEpoch() + absl::Seconds(246), &note);
+  // wait for SleepUntilAndNotify() to block
+  absl::SleepFor(kLongPause);
+  simclock.AdvanceTime(absl::Seconds(122));
+  // give SleepUntil() the opportunity to fail
+  absl::SleepFor(kShortPause);
+  EXPECT_FALSE(note.HasBeenNotified());
+  simclock.AdvanceTime(absl::Seconds(2));
+  // wait for SleepUntilAndNotify() to ping note
+  absl::SleepFor(kLongPause);
+  EXPECT_TRUE(note.HasBeenNotified());
+  note.WaitForNotification();  // in case the expectation fails
+  tr.join();
+}
+
+TEST(SimulatedClock, SleepUntilTimeAlreadyPassed) {
+  if (kSkipFlakyReason != nullptr) {
+    GTEST_SKIP() << kSkipFlakyReason;
+  }
+
+  absl::SimulatedClock simclock;
+  absl::Notification note;
+
+  simclock.AdvanceTime(absl::Seconds(123));
+  std::thread tr(SleepUntilAndNotify, &simclock,
+                 absl::UnixEpoch() + absl::Seconds(123), &note);
+  // wait for SleepUntilAndNotify() to ping note
+  absl::SleepFor(kLongPause);
+  EXPECT_TRUE(note.HasBeenNotified());
+  note.WaitForNotification();  // in case the expectation fails
+  tr.join();
+}
+
+void AwaitWithDeadlineAndNotify(absl::Clock* clock, absl::Mutex* mu,
+                                absl::Condition* cond, absl::Time wakeup_time,
+                                absl::Notification* note, bool* return_val) {
+  mu->lock_shared();
+  *return_val = clock->AwaitWithDeadline(mu, *cond, wakeup_time);
+  mu->unlock_shared();
+  note->Notify();
+}
+
+TEST(SimulatedClock, AwaitWithDeadlineConditionInitiallyTrue) {
+  absl::SimulatedClock simclock;
+  absl::Mutex mu;
+  bool f = true;
+  absl::Condition cond(&f);
+  absl::MutexLock lock(mu);
+  ASSERT_TRUE(simclock.AwaitWithDeadline(&mu, cond, absl::InfiniteFuture()));
+}
+
+TEST(SimulatedClock, AwaitWithDeadlineConditionInitiallyFalse) {
+  if (kSkipFlakyReason != nullptr) {
+    GTEST_SKIP() << kSkipFlakyReason;
+  }
+
+  absl::SimulatedClock simclock;
+  absl::Mutex mu;
+  bool f = false;
+  absl::Condition cond(&f);
+  absl::Notification note;
+  bool return_val;
+
+  std::thread tr(AwaitWithDeadlineAndNotify, &simclock, &mu, &cond,
+                 absl::UnixEpoch() + absl::Seconds(123), &note, &return_val);
+  // wait for AwaitWithDeadline...() to block
+  absl::SleepFor(kShortPause);
+  EXPECT_FALSE(note.HasBeenNotified());
+  mu.lock();
+  f = true;
+  mu.unlock();
+  // wait for AwaitWithDeadline...() to ping note
+  absl::SleepFor(kLongPause);
+  EXPECT_TRUE(note.HasBeenNotified());
+  EXPECT_TRUE(return_val);
+  note.WaitForNotification();  // in case the expectation fails
+  tr.join();
+}
+
+TEST(SimulatedClock, AwaitWithDeadlineDeadlinePassed) {
+  if (kSkipFlakyReason != nullptr) {
+    GTEST_SKIP() << kSkipFlakyReason;
+  }
+
+  absl::SimulatedClock simclock;
+  absl::Mutex mu;
+  bool f = false;
+  absl::Condition cond(&f);
+  absl::Notification note;
+  bool return_val;
+
+  std::thread tr(AwaitWithDeadlineAndNotify, &simclock, &mu, &cond,
+                 absl::UnixEpoch() + absl::Seconds(123), &note, &return_val);
+  // wait for AwaitWithDeadline...() to block
+  absl::SleepFor(kLongPause);
+  EXPECT_FALSE(note.HasBeenNotified());
+  simclock.AdvanceTime(absl::Seconds(124));
+  // wait for AwaitWithDeadline...() to ping note
+  absl::SleepFor(kLongPause);
+  EXPECT_TRUE(note.HasBeenNotified());
+  EXPECT_FALSE(return_val);
+  note.WaitForNotification();  // in case the expectation fails
+  tr.join();
+}
+
+TEST(SimulatedClock, AwaitWithDeadlineDeadlineAlreadyPassed) {
+  if (kSkipFlakyReason != nullptr) {
+    GTEST_SKIP() << kSkipFlakyReason;
+  }
+
+  absl::SimulatedClock simclock;
+  absl::Mutex mu;
+  bool f = false;
+  absl::Condition cond(&f);
+  absl::Notification note;
+  bool return_val;
+
+  std::thread tr(AwaitWithDeadlineAndNotify, &simclock, &mu, &cond,
+                 absl::UnixEpoch(), &note, &return_val);
+  // wait for AwaitWithDeadline...() to ping note
+  absl::SleepFor(kLongPause);
+  EXPECT_TRUE(note.HasBeenNotified());
+  EXPECT_FALSE(return_val);
+  note.WaitForNotification();  // in case the expectation fails
+  tr.join();
+}
+
+void RacerMakesConditionTrue(absl::Notification* start_note, absl::Mutex* mu,
+                             bool* f, absl::BlockingCounter* threads_done) {
+  start_note->WaitForNotification();
+  absl::SleepFor(absl::Milliseconds(1));
+  mu->lock();
+  *f = true;
+  mu->unlock();
+  threads_done->DecrementCount();
+}
+
+void RacerAdvancesTime(absl::Notification* start_note,
+                       absl::SimulatedClock* simclock, absl::Duration d,
+                       absl::BlockingCounter* threads_done) {
+  start_note->WaitForNotification();
+  absl::SleepFor(absl::Milliseconds(1));
+  simclock->AdvanceTime(d);
+  threads_done->DecrementCount();
+}
+
+TEST(SimulatedClock, SimultaneousConditionTrueAndDeadline) {
+  absl::SimulatedClock simclock;
+  for (int iteration = 0; iteration < 100; ++iteration) {
+    auto mu = std::make_unique<absl::Mutex>();
+    bool f = false;
+    absl::Condition cond(&f);
+    absl::Notification note_start;
+    absl::BlockingCounter threads_done(2);
+    std::thread tr1(RacerMakesConditionTrue, &note_start, mu.get(), &f,
+                    &threads_done);
+    std::thread tr2(RacerAdvancesTime, &note_start, &simclock,
+                    absl::Seconds(20), &threads_done);
+    note_start.Notify();
+    mu->lock();
+    absl::Time deadline = simclock.TimeNow() + absl::Seconds(10);
+    simclock.AwaitWithDeadline(mu.get(), cond, deadline);
+    EXPECT_TRUE(f || (simclock.TimeNow() >= deadline));
+    if (f) {
+      // RacerMakesConditionTrue has unlocked mu and AwaitWithDeadline has
+      // returned, so it is safe to destruct mu. Do so while RacerAdvancesTime
+      // is possibly still running in an attempt to catch simclock holding on
+      // to a reference to mu and using it after AwaitWithDeadline returns.
+      mu->unlock();
+      mu = nullptr;
+    } else {
+      mu->unlock();
+    }
+    threads_done.Wait();
+    tr1.join();
+    tr2.join();
+  }
+}
+
+void RacerDeletesClock(absl::Mutex* mu, absl::Notification* start_note,
+                       absl::Clock* clock,
+                       absl::BlockingCounter* threads_done) {
+  start_note->WaitForNotification();
+  // mu is acquired temporarily to make sure that AwaitWithDeadline() in
+  // SimultaneousConditionTrueAndDestruction has blocked.
+  mu->lock();
+  mu->unlock();
+  absl::SleepFor(absl::Milliseconds(1));
+  delete clock;
+  threads_done->DecrementCount();
+}
+
+TEST(SimulatedClock, SimultaneousConditionTrueAndDestruction) {
+  for (int iteration = 0; iteration < 100; ++iteration) {
+    absl::Clock* clock = new absl::SimulatedClock();
+    absl::Mutex mu;
+    bool f = false;
+    absl::Condition cond(&f);
+    absl::Notification note_start;
+    absl::BlockingCounter threads_done(2);
+    std::thread tr1([&note_start, &mu, &f, &threads_done] {
+      RacerMakesConditionTrue(&note_start, &mu, &f, &threads_done);
+    });
+    std::thread tr2([&mu, &note_start, clock, &threads_done] {
+      RacerDeletesClock(&mu, &note_start, clock, &threads_done);
+    });
+    mu.lock();
+    note_start.Notify();
+    absl::Time deadline = absl::UnixEpoch() + absl::Seconds(100000);
+    clock->AwaitWithDeadline(&mu, cond, deadline);
+    mu.unlock();
+    threads_done.Wait();
+    tr1.join();
+    tr2.join();
+  }
+}
+
+class SimulatedClockTorturer {
+ public:
+  SimulatedClockTorturer(absl::SimulatedClock* simclock, int num_threads,
+                         int num_iterations)
+      : simclock_(simclock),
+        num_threads_(num_threads),
+        num_iterations_(num_iterations),
+        num_flags_(2 * num_threads),
+        mutex_and_flag_(static_cast<size_t>(num_flags_)) {}
+
+  // Implements a torture test.
+  //
+  // This method uses several groups of:
+  //   SimulatedClock
+  //   Mutex protected flag
+  // It starts several threads that call AwaitWithDeadline() and several
+  // threads that call AdvanceTime() or toggle flag values.
+  void DoTorture() {
+    // The threads calling AwaitWithDeadline() have a separate BlockingCounter
+    // than the threads calling AdvanceTime()/toggling flags, since the former
+    // would be deadlocked if all the threads that might unblock them had
+    // already finished.
+    absl::Notification go;
+    absl::BlockingCounter await_threads_done(num_threads_);
+    absl::Notification signal_threads_should_exit;
+    absl::BlockingCounter signal_threads_done(num_threads_);
+    std::vector<std::thread> trs;
+    for (int i = 0; i < num_threads_; ++i) {
+      trs.emplace_back(&SimulatedClockTorturer::AwaitRandomly, this, &go,
+                       &await_threads_done);
+    }
+    for (int i = 0; i < num_threads_; ++i) {
+      trs.emplace_back(&SimulatedClockTorturer::SignalRandomly, this, &go,
+                       &signal_threads_should_exit, &signal_threads_done);
+    }
+    go.Notify();
+    await_threads_done.Wait();
+    signal_threads_should_exit.Notify();
+    signal_threads_done.Wait();
+    for (auto& thread : trs) {
+      thread.join();
+    }
+  }
+
+ private:
+  // Randomly call AwaitWithDeadline() for num_iterations_ times.
+  void AwaitRandomly(absl::Notification* go,
+                     absl::BlockingCounter* threads_done) {
+    go->WaitForNotification();
+
+    absl::BitGen gen;
+    for (int i = 0; i < num_iterations_; ++i) {
+      auto& [mu, f] = mutex_and_flag_[absl::Uniform<size_t>(gen, size_t{0},
+                                         static_cast<size_t>(num_flags_))];
+      absl::MutexLock lock(mu);
+      absl::Time deadline = simclock_->TimeNow() + absl::Seconds(1);
+      simclock_->AwaitWithDeadline(&mu, absl::Condition(&f), deadline);
+      ABSL_RAW_CHECK(f || simclock_->TimeNow() >= deadline, "");
+    }
+
+    threads_done->DecrementCount();
+  }
+
+  // Randomly call AdvanceTime() or toggle a flag value until notified to
+  // stop.
+  void SignalRandomly(absl::Notification* go, absl::Notification* should_exit,
+                      absl::BlockingCounter* threads_done) {
+    go->WaitForNotification();
+
+    absl::BitGen gen;
+    while (!should_exit->HasBeenNotified()) {
+      int action = absl::Uniform<int>(gen, 0, num_flags_ + 1);
+      if (action < num_flags_) {
+        // Change a flag value.
+        auto& [mutex, flag] = mutex_and_flag_[static_cast<size_t>(action)];
+        absl::MutexLock lock(mutex);
+        flag = !flag;
+      } else {
+        // Advance time.
+        simclock_->AdvanceTime(absl::Seconds(1));
+      }
+    }
+
+    threads_done->DecrementCount();
+  }
+
+  absl::SimulatedClock* simclock_;
+  int num_threads_;
+  int num_iterations_;
+  int num_flags_;
+
+  struct MutexAndFlag {
+    absl::Mutex mutex;
+    bool flag ABSL_GUARDED_BY(mutex) = false;
+  };
+  std::vector<MutexAndFlag> mutex_and_flag_;
+};
+
+TEST(SimulatedClock, Torture) {
+  absl::SimulatedClock simclock;
+  constexpr int kNumThreads = 10;
+  constexpr int kNumIterations = 1000;
+  SimulatedClockTorturer torturer(&simclock, kNumThreads, kNumIterations);
+  torturer.DoTorture();
+}
+
+}  // namespace
diff --git a/absl/time/time.cc b/absl/time/time.cc
index 6a387bcee61..4be13cf5661 100644
--- a/absl/time/time.cc
+++ b/absl/time/time.cc
@@ -33,10 +33,6 @@
 
 #include "absl/time/time.h"
 
-#if defined(_MSC_VER)
-#include <winsock2.h>  // for timeval
-#endif
-
 #include <cstring>
 #include <ctime>
 #include <limits>
@@ -44,9 +40,14 @@
 #include "absl/time/internal/cctz/include/cctz/civil_time.h"
 #include "absl/time/internal/cctz/include/cctz/time_zone.h"
 
+#if defined(_MSC_VER)
+#include <winsock2.h>  // for timeval
+#endif
+
 namespace cctz = absl::time_internal::cctz;
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 namespace {
 
@@ -59,11 +60,13 @@ inline cctz::time_point<cctz::seconds> unix_epoch() {
 inline int64_t FloorToUnit(absl::Duration d, absl::Duration unit) {
   absl::Duration rem;
   int64_t q = absl::IDivDuration(d, unit, &rem);
-  return (q > 0 ||
-          rem >= ZeroDuration() ||
-          q == std::numeric_limits<int64_t>::min()) ? q : q - 1;
+  return (q > 0 || rem >= ZeroDuration() ||
+          q == std::numeric_limits<int64_t>::min())
+             ? q
+             : q - 1;
 }
 
+ABSL_INTERNAL_DISABLE_DEPRECATED_DECLARATION_WARNING
 inline absl::Time::Breakdown InfiniteFutureBreakdown() {
   absl::Time::Breakdown bd;
   bd.year = std::numeric_limits<int64_t>::max();
@@ -97,6 +100,7 @@ inline absl::Time::Breakdown InfinitePastBreakdown() {
   bd.zone_abbr = "-00";
   return bd;
 }
+ABSL_INTERNAL_RESTORE_DEPRECATED_DECLARATION_WARNING
 
 inline absl::TimeZone::CivilInfo InfiniteFutureCivilInfo() {
   TimeZone::CivilInfo ci;
@@ -118,6 +122,7 @@ inline absl::TimeZone::CivilInfo InfinitePastCivilInfo() {
   return ci;
 }
 
+ABSL_INTERNAL_DISABLE_DEPRECATED_DECLARATION_WARNING
 inline absl::TimeConversion InfiniteFutureTimeConversion() {
   absl::TimeConversion tc;
   tc.pre = tc.trans = tc.post = absl::InfiniteFuture();
@@ -133,9 +138,10 @@ inline TimeConversion InfinitePastTimeConversion() {
   tc.normalized = true;
   return tc;
 }
+ABSL_INTERNAL_RESTORE_DEPRECATED_DECLARATION_WARNING
 
 // Makes a Time from sec, overflowing to InfiniteFuture/InfinitePast as
-// necessary. If sec is min/max, then consult cs+tz to check for overlow.
+// necessary. If sec is min/max, then consult cs+tz to check for overflow.
 Time MakeTimeWithOverflow(const cctz::time_point<cctz::seconds>& sec,
                           const cctz::civil_second& cs,
                           const cctz::time_zone& tz,
@@ -201,6 +207,7 @@ bool FindTransition(const cctz::time_zone& tz,
 // Time
 //
 
+ABSL_INTERNAL_DISABLE_DEPRECATED_DECLARATION_WARNING
 absl::Time::Breakdown Time::In(absl::TimeZone tz) const {
   if (*this == absl::InfiniteFuture()) return InfiniteFutureBreakdown();
   if (*this == absl::InfinitePast()) return InfinitePastBreakdown();
@@ -225,6 +232,7 @@ absl::Time::Breakdown Time::In(absl::TimeZone tz) const {
   bd.zone_abbr = al.abbr;
   return bd;
 }
+ABSL_INTERNAL_RESTORE_DEPRECATED_DECLARATION_WARNING
 
 //
 // Conversions from/to other time types.
@@ -241,8 +249,8 @@ absl::Time FromUniversal(int64_t universal) {
 int64_t ToUnixNanos(Time t) {
   if (time_internal::GetRepHi(time_internal::ToUnixDuration(t)) >= 0 &&
       time_internal::GetRepHi(time_internal::ToUnixDuration(t)) >> 33 == 0) {
-    return (time_internal::GetRepHi(time_internal::ToUnixDuration(t)) *
-            1000 * 1000 * 1000) +
+    return (time_internal::GetRepHi(time_internal::ToUnixDuration(t)) * 1000 *
+            1000 * 1000) +
            (time_internal::GetRepLo(time_internal::ToUnixDuration(t)) / 4);
   }
   return FloorToUnit(time_internal::ToUnixDuration(t), absl::Nanoseconds(1));
@@ -251,8 +259,8 @@ int64_t ToUnixNanos(Time t) {
 int64_t ToUnixMicros(Time t) {
   if (time_internal::GetRepHi(time_internal::ToUnixDuration(t)) >= 0 &&
       time_internal::GetRepHi(time_internal::ToUnixDuration(t)) >> 43 == 0) {
-    return (time_internal::GetRepHi(time_internal::ToUnixDuration(t)) *
-            1000 * 1000) +
+    return (time_internal::GetRepHi(time_internal::ToUnixDuration(t)) * 1000 *
+            1000) +
            (time_internal::GetRepLo(time_internal::ToUnixDuration(t)) / 4000);
   }
   return FloorToUnit(time_internal::ToUnixDuration(t), absl::Microseconds(1));
@@ -295,7 +303,7 @@ timespec ToTimespec(Time t) {
   timespec ts;
   absl::Duration d = time_internal::ToUnixDuration(t);
   if (!time_internal::IsInfiniteDuration(d)) {
-    ts.tv_sec = time_internal::GetRepHi(d);
+    ts.tv_sec = static_cast<decltype(ts.tv_sec)>(time_internal::GetRepHi(d));
     if (ts.tv_sec == time_internal::GetRepHi(d)) {  // no time_t narrowing
       ts.tv_nsec = time_internal::GetRepLo(d) / 4;  // floor
       return ts;
@@ -314,7 +322,7 @@ timespec ToTimespec(Time t) {
 timeval ToTimeval(Time t) {
   timeval tv;
   timespec ts = absl::ToTimespec(t);
-  tv.tv_sec = ts.tv_sec;
+  tv.tv_sec = static_cast<decltype(tv.tv_sec)>(ts.tv_sec);
   if (tv.tv_sec != ts.tv_sec) {  // narrowing
     if (ts.tv_sec < 0) {
       tv.tv_sec = std::numeric_limits<decltype(tv.tv_sec)>::min();
@@ -396,7 +404,7 @@ bool TimeZone::PrevTransition(Time t, CivilTransition* trans) const {
 //
 // Conversions involving time zones.
 //
-
+ABSL_INTERNAL_DISABLE_DEPRECATED_DECLARATION_WARNING
 absl::TimeConversion ConvertDateTime(int64_t year, int mon, int day, int hour,
                                      int min, int sec, TimeZone tz) {
   // Avoids years that are too extreme for CivilSecond to normalize.
@@ -428,11 +436,20 @@ absl::TimeConversion ConvertDateTime(int64_t year, int mon, int day, int hour,
   }
   return tc;
 }
+ABSL_INTERNAL_RESTORE_DEPRECATED_DECLARATION_WARNING
 
 absl::Time FromTM(const struct tm& tm, absl::TimeZone tz) {
-  const CivilSecond cs(tm.tm_year + 1900, tm.tm_mon + 1, tm.tm_mday,
-                       tm.tm_hour, tm.tm_min, tm.tm_sec);
-  const auto ti = tz.At(cs);
+  civil_year_t tm_year = tm.tm_year;
+  // Avoids years that are too extreme for CivilSecond to normalize.
+  if (tm_year > 300000000000ll) return InfiniteFuture();
+  if (tm_year < -300000000000ll) return InfinitePast();
+  int tm_mon = tm.tm_mon;
+  if (tm_mon == std::numeric_limits<int>::max()) {
+    tm_mon -= 12;
+    tm_year += 1;
+  }
+  const auto ti = tz.At(CivilSecond(tm_year + 1900, tm_mon + 1, tm.tm_mday,
+                                    tm.tm_hour, tm.tm_min, tm.tm_sec));
   return tm.tm_isdst == 0 ? ti.post : ti.pre;
 }
 
@@ -486,4 +503,5 @@ struct tm ToTM(absl::Time t, absl::TimeZone tz) {
   return tm;
 }
 
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/time/time.h b/absl/time/time.h
index 9c8f31774ac..ea9be32062b 100644
--- a/absl/time/time.h
+++ b/absl/time/time.h
@@ -62,68 +62,93 @@
 #ifndef ABSL_TIME_TIME_H_
 #define ABSL_TIME_TIME_H_
 
-#if !defined(_MSC_VER)
-#include <sys/time.h>
-#else
-// We don't include `winsock2.h` because it drags in `windows.h` and friends,
-// and they define conflicting macros like OPAQUE, ERROR, and more. This has the
-// potential to break Abseil users.
-//
-// Instead we only forward declare `timeval` and require Windows users include
-// `winsock2.h` themselves. This is both inconsistent and troublesome, but so is
-// including 'windows.h' so we are picking the lesser of two evils here.
-struct timeval;
+#include "absl/base/config.h"
+
+// For feature testing and determining which headers can be included.
+#if ABSL_INTERNAL_CPLUSPLUS_LANG >= 202002L
+#include <version>
 #endif
+
 #include <chrono>  // NOLINT(build/c++11)
 #include <cmath>
+#ifdef __cpp_lib_three_way_comparison
+#include <compare>
+#endif  // __cpp_lib_three_way_comparison
 #include <cstdint>
 #include <ctime>
+#include <limits>
 #include <ostream>
+#include <ratio>  // NOLINT(build/c++11)
 #include <string>
 #include <type_traits>
 #include <utility>
 
+#include "absl/base/attributes.h"
+#include "absl/base/macros.h"
 #include "absl/strings/string_view.h"
 #include "absl/time/civil_time.h"
 #include "absl/time/internal/cctz/include/cctz/time_zone.h"
 
+#if !defined(_MSC_VER)
+#include <sys/time.h>
+#else
+// We don't include `winsock2.h` because it drags in `windows.h` and friends,
+// and they define conflicting macros like OPAQUE, ERROR, and more. This has the
+// potential to break Abseil users.
+//
+// Instead we only forward declare `timeval` and require Windows users include
+// `winsock2.h` themselves. This is both inconsistent and troublesome, but so is
+// including 'windows.h' so we are picking the lesser of two evils here.
+struct timeval;
+#endif
+
+#if defined(__cpp_impl_three_way_comparison) && \
+    defined(__cpp_lib_three_way_comparison)
+#define ABSL_INTERNAL_TIME_HAS_THREE_WAY_COMPARISON 1
+#endif
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 class Duration;  // Defined below
 class Time;      // Defined below
 class TimeZone;  // Defined below
 
 namespace time_internal {
-int64_t IDivDuration(bool satq, Duration num, Duration den, Duration* rem);
-constexpr Time FromUnixDuration(Duration d);
-constexpr Duration ToUnixDuration(Time t);
-constexpr int64_t GetRepHi(Duration d);
-constexpr uint32_t GetRepLo(Duration d);
-constexpr Duration MakeDuration(int64_t hi, uint32_t lo);
-constexpr Duration MakeDuration(int64_t hi, int64_t lo);
-inline Duration MakePosDoubleDuration(double n);
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr Time FromUnixDuration(Duration d);
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr Duration ToUnixDuration(Time t);
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr int64_t GetRepHi(Duration d);
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr uint32_t GetRepLo(Duration d);
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr Duration MakeDuration(int64_t hi,
+                                                              uint32_t lo);
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr Duration MakeDuration(int64_t hi,
+                                                              int64_t lo);
+ABSL_ATTRIBUTE_CONST_FUNCTION inline Duration MakePosDoubleDuration(double n);
 constexpr int64_t kTicksPerNanosecond = 4;
 constexpr int64_t kTicksPerSecond = 1000 * 1000 * 1000 * kTicksPerNanosecond;
 template <std::intmax_t N>
-constexpr Duration FromInt64(int64_t v, std::ratio<1, N>);
-constexpr Duration FromInt64(int64_t v, std::ratio<60>);
-constexpr Duration FromInt64(int64_t v, std::ratio<3600>);
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr Duration FromInt64(int64_t v,
+                                                           std::ratio<1, N>);
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr Duration FromInt64(int64_t v,
+                                                           std::ratio<60>);
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr Duration FromInt64(int64_t v,
+                                                           std::ratio<3600>);
 template <typename T>
-using EnableIfIntegral = typename std::enable_if<
-    std::is_integral<T>::value || std::is_enum<T>::value, int>::type;
+using EnableIfIntegral =
+    std::enable_if_t<std::is_integral_v<T> || std::is_enum_v<T>, int>;
 template <typename T>
-using EnableIfFloat =
-    typename std::enable_if<std::is_floating_point<T>::value, int>::type;
+using EnableIfFloat = std::enable_if_t<std::is_floating_point_v<T>, int>;
 }  // namespace time_internal
 
 // Duration
 //
-// The `absl::Duration` class represents a signed, fixed-length span of time.
+// The `absl::Duration` class represents a signed, fixed-length amount of time.
 // A `Duration` is generated using a unit-specific factory function, or is
 // the result of subtracting one `absl::Time` from another. Durations behave
 // like unit-safe integers and they support all the natural integer-like
 // arithmetic operations. Arithmetic overflows and saturates at +/- infinity.
-// `Duration` should be passed by value rather than const reference.
+// `Duration` is trivially destructible and should be passed by value rather
+// than const reference.
 //
 // Factory functions `Nanoseconds()`, `Microseconds()`, `Milliseconds()`,
 // `Seconds()`, `Minutes()`, `Hours()` and `InfiniteDuration()` allow for
@@ -160,7 +185,7 @@ class Duration {
   constexpr Duration() : rep_hi_(0), rep_lo_(0) {}  // zero-length duration
 
   // Copyable.
-#if !defined(__clang__) && defined(_MSC_VER) && _MSC_VER < 1910
+#if !defined(__clang__) && defined(_MSC_VER) && _MSC_VER < 1930
   // Explicitly defining the constexpr copy constructor avoids an MSVC bug.
   constexpr Duration(const Duration& d)
       : rep_hi_(d.rep_hi_), rep_lo_(d.rep_lo_) {}
@@ -179,23 +204,39 @@ class Duration {
   Duration& operator%=(Duration rhs);
 
   // Overloads that forward to either the int64_t or double overloads above.
-  // Integer operands must be representable as int64_t.
-  template <typename T>
+  // Integer operands must be representable as int64_t. Integer division is
+  // truncating, so values less than the resolution will be returned as zero.
+  // Floating-point multiplication and division is rounding (halfway cases
+  // rounding away from zero), so values less than the resolution may be
+  // returned as either the resolution or zero.  In particular, `d / 2.0`
+  // can produce `d` when it is the resolution and "even".
+  template <typename T, time_internal::EnableIfIntegral<T> = 0>
   Duration& operator*=(T r) {
     int64_t x = r;
     return *this *= x;
   }
-  template <typename T>
+
+  template <typename T, time_internal::EnableIfIntegral<T> = 0>
   Duration& operator/=(T r) {
     int64_t x = r;
     return *this /= x;
   }
-  Duration& operator*=(float r) { return *this *= static_cast<double>(r); }
-  Duration& operator/=(float r) { return *this /= static_cast<double>(r); }
+
+  template <typename T, time_internal::EnableIfFloat<T> = 0>
+  Duration& operator*=(T r) {
+    double x = r;
+    return *this *= x;
+  }
+
+  template <typename T, time_internal::EnableIfFloat<T> = 0>
+  Duration& operator/=(T r) {
+    double x = r;
+    return *this /= x;
+  }
 
   template <typename H>
   friend H AbslHashValue(H h, Duration d) {
-    return H::combine(std::move(h), d.rep_hi_, d.rep_lo_);
+    return H::combine(std::move(h), d.rep_hi_.Get(), d.rep_lo_);
   }
 
  private:
@@ -204,42 +245,122 @@ class Duration {
   friend constexpr Duration time_internal::MakeDuration(int64_t hi,
                                                         uint32_t lo);
   constexpr Duration(int64_t hi, uint32_t lo) : rep_hi_(hi), rep_lo_(lo) {}
-  int64_t rep_hi_;
+
+  // We store `rep_hi_` 4-byte rather than 8-byte aligned to avoid 4 bytes of
+  // tail padding.
+  class HiRep {
+   public:
+    // Default constructor default-initializes `hi_`, which has the same
+    // semantics as default-initializing an `int64_t` (undetermined value).
+    HiRep() = default;
+
+    HiRep(const HiRep&) = default;
+    HiRep& operator=(const HiRep&) = default;
+
+    explicit constexpr HiRep(const int64_t value)
+        :  // C++17 forbids default-initialization in constexpr contexts. We can
+           // remove this in C++20.
+#if defined(ABSL_IS_BIG_ENDIAN) && ABSL_IS_BIG_ENDIAN
+          hi_(0),
+          lo_(0)
+#else
+          lo_(0),
+          hi_(0)
+#endif
+    {
+      *this = value;
+    }
+
+    constexpr int64_t Get() const {
+      const uint64_t unsigned_value =
+          (static_cast<uint64_t>(hi_) << 32) | static_cast<uint64_t>(lo_);
+      // `static_cast<int64_t>(unsigned_value)` is implementation-defined
+      // before c++20. On all supported platforms the behaviour is that mandated
+      // by c++20, i.e. "If the destination type is signed, [...] the result is
+      // the unique value of the destination type equal to the source value
+      // modulo 2^n, where n is the number of bits used to represent the
+      // destination type."
+      static_assert(
+          (static_cast<int64_t>((std::numeric_limits<uint64_t>::max)()) ==
+           int64_t{-1}) &&
+              (static_cast<int64_t>(static_cast<uint64_t>(
+                                        (std::numeric_limits<int64_t>::max)()) +
+                                    1) ==
+               (std::numeric_limits<int64_t>::min)()),
+          "static_cast<int64_t>(uint64_t) does not have c++20 semantics");
+      return static_cast<int64_t>(unsigned_value);
+    }
+
+    constexpr HiRep& operator=(const int64_t value) {
+      // "If the destination type is unsigned, the resulting value is the
+      // smallest unsigned value equal to the source value modulo 2^n
+      // where `n` is the number of bits used to represent the destination
+      // type".
+      const auto unsigned_value = static_cast<uint64_t>(value);
+      hi_ = static_cast<uint32_t>(unsigned_value >> 32);
+      lo_ = static_cast<uint32_t>(unsigned_value);
+      return *this;
+    }
+
+   private:
+    // Notes:
+    //  - Ideally we would use a `char[]` and `std::bitcast`, but the latter
+    //    does not exist (and is not constexpr in `absl`) before c++20.
+    //  - Order is optimized depending on endianness so that the compiler can
+    //    turn `Get()` (resp. `operator=()`) into a single 8-byte load (resp.
+    //    store).
+#if defined(ABSL_IS_BIG_ENDIAN) && ABSL_IS_BIG_ENDIAN
+    uint32_t hi_;
+    uint32_t lo_;
+#else
+    uint32_t lo_;
+    uint32_t hi_;
+#endif
+  };
+  HiRep rep_hi_;
   uint32_t rep_lo_;
 };
 
 // Relational Operators
-constexpr bool operator<(Duration lhs, Duration rhs);
-constexpr bool operator>(Duration lhs, Duration rhs) { return rhs < lhs; }
-constexpr bool operator>=(Duration lhs, Duration rhs) { return !(lhs < rhs); }
-constexpr bool operator<=(Duration lhs, Duration rhs) { return !(rhs < lhs); }
-constexpr bool operator==(Duration lhs, Duration rhs);
-constexpr bool operator!=(Duration lhs, Duration rhs) { return !(lhs == rhs); }
 
-// Additive Operators
-constexpr Duration operator-(Duration d);
-inline Duration operator+(Duration lhs, Duration rhs) { return lhs += rhs; }
-inline Duration operator-(Duration lhs, Duration rhs) { return lhs -= rhs; }
+#ifdef ABSL_INTERNAL_TIME_HAS_THREE_WAY_COMPARISON
 
-// Multiplicative Operators
-// Integer operands must be representable as int64_t.
-template <typename T>
-Duration operator*(Duration lhs, T rhs) {
-  return lhs *= rhs;
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr std::strong_ordering operator<=>(
+    Duration lhs, Duration rhs);
+
+#endif  // ABSL_INTERNAL_TIME_HAS_THREE_WAY_COMPARISON
+
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr bool operator<(Duration lhs,
+                                                       Duration rhs);
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr bool operator>(Duration lhs,
+                                                       Duration rhs) {
+  return rhs < lhs;
 }
-template <typename T>
-Duration operator*(T lhs, Duration rhs) {
-  return rhs *= lhs;
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr bool operator>=(Duration lhs,
+                                                        Duration rhs) {
+  return !(lhs < rhs);
 }
-template <typename T>
-Duration operator/(Duration lhs, T rhs) {
-  return lhs /= rhs;
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr bool operator<=(Duration lhs,
+                                                        Duration rhs) {
+  return !(rhs < lhs);
+}
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr bool operator==(Duration lhs,
+                                                        Duration rhs);
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr bool operator!=(Duration lhs,
+                                                        Duration rhs) {
+  return !(lhs == rhs);
+}
+
+// Additive Operators
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr Duration operator-(Duration d);
+ABSL_ATTRIBUTE_CONST_FUNCTION inline Duration operator+(Duration lhs,
+                                                        Duration rhs) {
+  return lhs += rhs;
 }
-inline int64_t operator/(Duration lhs, Duration rhs) {
-  return time_internal::IDivDuration(true, lhs, rhs,
-                                     &lhs);  // trunc towards zero
+ABSL_ATTRIBUTE_CONST_FUNCTION inline Duration operator-(Duration lhs,
+                                                        Duration rhs) {
+  return lhs -= rhs;
 }
-inline Duration operator%(Duration lhs, Duration rhs) { return lhs %= rhs; }
 
 // IDivDuration()
 //
@@ -269,10 +390,7 @@ inline Duration operator%(Duration lhs, Duration rhs) { return lhs %= rhs; }
 //   // Here, q would overflow int64_t, so rem accounts for the difference.
 //   int64_t q = absl::IDivDuration(a, b, &rem);
 //   // q == std::numeric_limits<int64_t>::max(), rem == a - b * q
-inline int64_t IDivDuration(Duration num, Duration den, Duration* rem) {
-  return time_internal::IDivDuration(true, num, den,
-                                     rem);  // trunc towards zero
-}
+int64_t IDivDuration(Duration num, Duration den, Duration* rem);
 
 // FDivDuration()
 //
@@ -286,18 +404,44 @@ inline int64_t IDivDuration(Duration num, Duration den, Duration* rem) {
 //
 //   double d = absl::FDivDuration(absl::Milliseconds(1500), absl::Seconds(1));
 //   // d == 1.5
-double FDivDuration(Duration num, Duration den);
+ABSL_ATTRIBUTE_CONST_FUNCTION double FDivDuration(Duration num, Duration den);
+
+// Multiplicative Operators
+// Integer operands must be representable as int64_t.
+template <typename T>
+ABSL_ATTRIBUTE_CONST_FUNCTION Duration operator*(Duration lhs, T rhs) {
+  return lhs *= rhs;
+}
+template <typename T>
+ABSL_ATTRIBUTE_CONST_FUNCTION Duration operator*(T lhs, Duration rhs) {
+  return rhs *= lhs;
+}
+template <typename T>
+ABSL_ATTRIBUTE_CONST_FUNCTION Duration operator/(Duration lhs, T rhs) {
+  return lhs /= rhs;
+}
+ABSL_ATTRIBUTE_CONST_FUNCTION inline int64_t operator/(Duration lhs,
+                                                       Duration rhs) {
+  return IDivDuration(lhs, rhs,
+                      &lhs);  // trunc towards zero
+}
+ABSL_ATTRIBUTE_CONST_FUNCTION inline Duration operator%(Duration lhs,
+                                                        Duration rhs) {
+  return lhs %= rhs;
+}
 
 // ZeroDuration()
 //
 // Returns a zero-length duration. This function behaves just like the default
 // constructor, but the name helps make the semantics clear at call sites.
-constexpr Duration ZeroDuration() { return Duration(); }
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr Duration ZeroDuration() {
+  return Duration();
+}
 
 // AbsDuration()
 //
 // Returns the absolute value of a duration.
-inline Duration AbsDuration(Duration d) {
+ABSL_ATTRIBUTE_CONST_FUNCTION inline Duration AbsDuration(Duration d) {
   return (d < ZeroDuration()) ? -d : d;
 }
 
@@ -309,7 +453,7 @@ inline Duration AbsDuration(Duration d) {
 //
 //   absl::Duration d = absl::Nanoseconds(123456789);
 //   absl::Duration a = absl::Trunc(d, absl::Microseconds(1));  // 123456us
-Duration Trunc(Duration d, Duration unit);
+ABSL_ATTRIBUTE_CONST_FUNCTION Duration Trunc(Duration d, Duration unit);
 
 // Floor()
 //
@@ -320,7 +464,7 @@ Duration Trunc(Duration d, Duration unit);
 //
 //   absl::Duration d = absl::Nanoseconds(123456789);
 //   absl::Duration b = absl::Floor(d, absl::Microseconds(1));  // 123456us
-Duration Floor(Duration d, Duration unit);
+ABSL_ATTRIBUTE_CONST_FUNCTION Duration Floor(Duration d, Duration unit);
 
 // Ceil()
 //
@@ -331,7 +475,7 @@ Duration Floor(Duration d, Duration unit);
 //
 //   absl::Duration d = absl::Nanoseconds(123456789);
 //   absl::Duration c = absl::Ceil(d, absl::Microseconds(1));   // 123457us
-Duration Ceil(Duration d, Duration unit);
+ABSL_ATTRIBUTE_CONST_FUNCTION Duration Ceil(Duration d, Duration unit);
 
 // InfiniteDuration()
 //
@@ -367,7 +511,7 @@ Duration Ceil(Duration d, Duration unit);
 //
 // The examples involving the `/` operator above also apply to `IDivDuration()`
 // and `FDivDuration()`.
-constexpr Duration InfiniteDuration();
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr Duration InfiniteDuration();
 
 // Nanoseconds()
 // Microseconds()
@@ -390,12 +534,30 @@ constexpr Duration InfiniteDuration();
 //
 //   absl::Duration a = absl::Seconds(60);
 //   absl::Duration b = absl::Minutes(1);  // b == a
-constexpr Duration Nanoseconds(int64_t n);
-constexpr Duration Microseconds(int64_t n);
-constexpr Duration Milliseconds(int64_t n);
-constexpr Duration Seconds(int64_t n);
-constexpr Duration Minutes(int64_t n);
-constexpr Duration Hours(int64_t n);
+template <typename T, time_internal::EnableIfIntegral<T> = 0>
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr Duration Nanoseconds(T n) {
+  return time_internal::FromInt64(n, std::nano{});
+}
+template <typename T, time_internal::EnableIfIntegral<T> = 0>
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr Duration Microseconds(T n) {
+  return time_internal::FromInt64(n, std::micro{});
+}
+template <typename T, time_internal::EnableIfIntegral<T> = 0>
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr Duration Milliseconds(T n) {
+  return time_internal::FromInt64(n, std::milli{});
+}
+template <typename T, time_internal::EnableIfIntegral<T> = 0>
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr Duration Seconds(T n) {
+  return time_internal::FromInt64(n, std::ratio<1>{});
+}
+template <typename T, time_internal::EnableIfIntegral<T> = 0>
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr Duration Minutes(T n) {
+  return time_internal::FromInt64(n, std::ratio<60>{});
+}
+template <typename T, time_internal::EnableIfIntegral<T> = 0>
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr Duration Hours(T n) {
+  return time_internal::FromInt64(n, std::ratio<3600>{});
+}
 
 // Factory overloads for constructing `Duration` values from a floating-point
 // number of the unit indicated by the factory function's name. These functions
@@ -407,35 +569,38 @@ constexpr Duration Hours(int64_t n);
 //   auto a = absl::Seconds(1.5);        // OK
 //   auto b = absl::Milliseconds(1500);  // BETTER
 template <typename T, time_internal::EnableIfFloat<T> = 0>
-Duration Nanoseconds(T n) {
+ABSL_ATTRIBUTE_CONST_FUNCTION Duration Nanoseconds(T n) {
   return n * Nanoseconds(1);
 }
 template <typename T, time_internal::EnableIfFloat<T> = 0>
-Duration Microseconds(T n) {
+ABSL_ATTRIBUTE_CONST_FUNCTION Duration Microseconds(T n) {
   return n * Microseconds(1);
 }
 template <typename T, time_internal::EnableIfFloat<T> = 0>
-Duration Milliseconds(T n) {
+ABSL_ATTRIBUTE_CONST_FUNCTION Duration Milliseconds(T n) {
   return n * Milliseconds(1);
 }
 template <typename T, time_internal::EnableIfFloat<T> = 0>
-Duration Seconds(T n) {
+ABSL_ATTRIBUTE_CONST_FUNCTION Duration Seconds(T n) {
   if (n >= 0) {  // Note: `NaN >= 0` is false.
-    if (n >= (std::numeric_limits<int64_t>::max)()) return InfiniteDuration();
+    if (n >= static_cast<T>((std::numeric_limits<int64_t>::max)())) {
+      return InfiniteDuration();
+    }
     return time_internal::MakePosDoubleDuration(n);
   } else {
-    if (std::isnan(n))
-      return std::signbit(n) ? -InfiniteDuration() : InfiniteDuration();
-    if (n <= (std::numeric_limits<int64_t>::min)()) return -InfiniteDuration();
+    if (std::isnan(n)) return -InfiniteDuration();
+    if (n <= static_cast<T>((std::numeric_limits<int64_t>::min)())) {
+      return -InfiniteDuration();
+    }
     return -time_internal::MakePosDoubleDuration(-n);
   }
 }
 template <typename T, time_internal::EnableIfFloat<T> = 0>
-Duration Minutes(T n) {
+ABSL_ATTRIBUTE_CONST_FUNCTION Duration Minutes(T n) {
   return n * Minutes(1);
 }
 template <typename T, time_internal::EnableIfFloat<T> = 0>
-Duration Hours(T n) {
+ABSL_ATTRIBUTE_CONST_FUNCTION Duration Hours(T n) {
   return n * Hours(1);
 }
 
@@ -447,21 +612,22 @@ Duration Hours(T n) {
 // ToInt64Hours()
 //
 // Helper functions that convert a Duration to an integral count of the
-// indicated unit. These functions are shorthand for the `IDivDuration()`
-// function above; see its documentation for details about overflow, etc.
+// indicated unit. These return the same results as the `IDivDuration()`
+// function, though they usually do so more efficiently; see the
+// documentation of `IDivDuration()` for details about overflow, etc.
 //
 // Example:
 //
 //   absl::Duration d = absl::Milliseconds(1500);
 //   int64_t isec = absl::ToInt64Seconds(d);  // isec == 1
-int64_t ToInt64Nanoseconds(Duration d);
-int64_t ToInt64Microseconds(Duration d);
-int64_t ToInt64Milliseconds(Duration d);
-int64_t ToInt64Seconds(Duration d);
-int64_t ToInt64Minutes(Duration d);
-int64_t ToInt64Hours(Duration d);
-
-// ToDoubleNanoSeconds()
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr int64_t ToInt64Nanoseconds(Duration d);
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr int64_t ToInt64Microseconds(Duration d);
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr int64_t ToInt64Milliseconds(Duration d);
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr int64_t ToInt64Seconds(Duration d);
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr int64_t ToInt64Minutes(Duration d);
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr int64_t ToInt64Hours(Duration d);
+
+// ToDoubleNanoseconds()
 // ToDoubleMicroseconds()
 // ToDoubleMilliseconds()
 // ToDoubleSeconds()
@@ -476,12 +642,12 @@ int64_t ToInt64Hours(Duration d);
 //
 //   absl::Duration d = absl::Milliseconds(1500);
 //   double dsec = absl::ToDoubleSeconds(d);  // dsec == 1.5
-double ToDoubleNanoseconds(Duration d);
-double ToDoubleMicroseconds(Duration d);
-double ToDoubleMilliseconds(Duration d);
-double ToDoubleSeconds(Duration d);
-double ToDoubleMinutes(Duration d);
-double ToDoubleHours(Duration d);
+ABSL_ATTRIBUTE_CONST_FUNCTION double ToDoubleNanoseconds(Duration d);
+ABSL_ATTRIBUTE_CONST_FUNCTION double ToDoubleMicroseconds(Duration d);
+ABSL_ATTRIBUTE_CONST_FUNCTION double ToDoubleMilliseconds(Duration d);
+ABSL_ATTRIBUTE_CONST_FUNCTION double ToDoubleSeconds(Duration d);
+ABSL_ATTRIBUTE_CONST_FUNCTION double ToDoubleMinutes(Duration d);
+ABSL_ATTRIBUTE_CONST_FUNCTION double ToDoubleHours(Duration d);
 
 // FromChrono()
 //
@@ -491,12 +657,18 @@ double ToDoubleHours(Duration d);
 //
 //   std::chrono::milliseconds ms(123);
 //   absl::Duration d = absl::FromChrono(ms);
-constexpr Duration FromChrono(const std::chrono::nanoseconds& d);
-constexpr Duration FromChrono(const std::chrono::microseconds& d);
-constexpr Duration FromChrono(const std::chrono::milliseconds& d);
-constexpr Duration FromChrono(const std::chrono::seconds& d);
-constexpr Duration FromChrono(const std::chrono::minutes& d);
-constexpr Duration FromChrono(const std::chrono::hours& d);
+ABSL_ATTRIBUTE_PURE_FUNCTION constexpr Duration FromChrono(
+    const std::chrono::nanoseconds& d);
+ABSL_ATTRIBUTE_PURE_FUNCTION constexpr Duration FromChrono(
+    const std::chrono::microseconds& d);
+ABSL_ATTRIBUTE_PURE_FUNCTION constexpr Duration FromChrono(
+    const std::chrono::milliseconds& d);
+ABSL_ATTRIBUTE_PURE_FUNCTION constexpr Duration FromChrono(
+    const std::chrono::seconds& d);
+ABSL_ATTRIBUTE_PURE_FUNCTION constexpr Duration FromChrono(
+    const std::chrono::minutes& d);
+ABSL_ATTRIBUTE_PURE_FUNCTION constexpr Duration FromChrono(
+    const std::chrono::hours& d);
 
 // ToChronoNanoseconds()
 // ToChronoMicroseconds()
@@ -516,24 +688,33 @@ constexpr Duration FromChrono(const std::chrono::hours& d);
 //   auto y = absl::ToChronoNanoseconds(d);  // x == y
 //   auto z = absl::ToChronoSeconds(absl::InfiniteDuration());
 //   // z == std::chrono::seconds::max()
-std::chrono::nanoseconds ToChronoNanoseconds(Duration d);
-std::chrono::microseconds ToChronoMicroseconds(Duration d);
-std::chrono::milliseconds ToChronoMilliseconds(Duration d);
-std::chrono::seconds ToChronoSeconds(Duration d);
-std::chrono::minutes ToChronoMinutes(Duration d);
-std::chrono::hours ToChronoHours(Duration d);
+ABSL_ATTRIBUTE_CONST_FUNCTION std::chrono::nanoseconds ToChronoNanoseconds(
+    Duration d);
+ABSL_ATTRIBUTE_CONST_FUNCTION std::chrono::microseconds ToChronoMicroseconds(
+    Duration d);
+ABSL_ATTRIBUTE_CONST_FUNCTION std::chrono::milliseconds ToChronoMilliseconds(
+    Duration d);
+ABSL_ATTRIBUTE_CONST_FUNCTION std::chrono::seconds ToChronoSeconds(Duration d);
+ABSL_ATTRIBUTE_CONST_FUNCTION std::chrono::minutes ToChronoMinutes(Duration d);
+ABSL_ATTRIBUTE_CONST_FUNCTION std::chrono::hours ToChronoHours(Duration d);
 
 // FormatDuration()
 //
 // Returns a string representing the duration in the form "72h3m0.5s".
 // Returns "inf" or "-inf" for +/- `InfiniteDuration()`.
-std::string FormatDuration(Duration d);
+ABSL_ATTRIBUTE_CONST_FUNCTION std::string FormatDuration(Duration d);
 
 // Output stream operator.
 inline std::ostream& operator<<(std::ostream& os, Duration d) {
   return os << FormatDuration(d);
 }
 
+// Support for StrFormat(), StrCat() etc.
+template <typename Sink>
+void AbslStringify(Sink& sink, Duration d) {
+  sink.Append(FormatDuration(d));
+}
+
 // ParseDuration()
 //
 // Parses a duration string consisting of a possibly signed sequence of
@@ -541,11 +722,24 @@ inline std::ostream& operator<<(std::ostream& os, Duration d) {
 // suffix.  The valid suffixes are "ns", "us" "ms", "s", "m", and "h".
 // Simple examples include "300ms", "-1.5h", and "2h45m".  Parses "0" as
 // `ZeroDuration()`. Parses "inf" and "-inf" as +/- `InfiniteDuration()`.
-bool ParseDuration(const std::string& dur_string, Duration* d);
+bool ParseDuration(absl::string_view dur_string, Duration* d);
+
+// AbslParseFlag()
+//
+// Parses a command-line flag string representation `text` into a Duration
+// value. Duration flags must be specified in a format that is valid input for
+// `absl::ParseDuration()`.
+bool AbslParseFlag(absl::string_view text, Duration* dst, std::string* error);
 
-// Support for flag values of type Duration. Duration flags must be specified
-// in a format that is valid input for absl::ParseDuration().
+// AbslUnparseFlag()
+//
+// Unparses a Duration value into a command-line string representation using
+// the format specified by `absl::ParseDuration()`.
+std::string AbslUnparseFlag(Duration d);
+
+ABSL_DEPRECATED("Use AbslParseFlag() instead.")
 bool ParseFlag(const std::string& text, Duration* dst, std::string* error);
+ABSL_DEPRECATED("Use AbslUnparseFlag() instead.")
 std::string UnparseFlag(Duration d);
 
 // Time
@@ -554,8 +748,9 @@ std::string UnparseFlag(Duration d);
 // are provided for naturally expressing time calculations. Instances are
 // created using `absl::Now()` and the `absl::From*()` factory functions that
 // accept the gamut of other time representations. Formatting and parsing
-// functions are provided for conversion to and from strings.  `absl::Time`
-// should be passed by value rather than const reference.
+// functions are provided for conversion to and from strings. `absl::Time` is
+// trivially destructible and should be passed by value rather than const
+// reference.
 //
 // `absl::Time` assumes there are 60 seconds in a minute, which means the
 // underlying time scales must be "smeared" to eliminate leap seconds.
@@ -629,9 +824,8 @@ class Time {
   // `absl::TimeZone`.
   //
   // Deprecated. Use `absl::TimeZone::CivilInfo`.
-  struct
-      Breakdown {
-    int64_t year;          // year (e.g., 2013)
+  struct ABSL_DEPRECATED("Use `absl::TimeZone::CivilInfo`.") Breakdown {
+    int64_t year;        // year (e.g., 2013)
     int month;           // month of year [1:12]
     int day;             // day of month [1:31]
     int hour;            // hour of day [0:23]
@@ -656,7 +850,10 @@ class Time {
   // Returns the breakdown of this instant in the given TimeZone.
   //
   // Deprecated. Use `absl::TimeZone::At(Time)`.
+  ABSL_INTERNAL_DISABLE_DEPRECATED_DECLARATION_WARNING
+  ABSL_DEPRECATED("Use `absl::TimeZone::At(Time)`.")
   Breakdown In(TimeZone tz) const;
+  ABSL_INTERNAL_RESTORE_DEPRECATED_DECLARATION_WARNING
 
   template <typename H>
   friend H AbslHashValue(H h, Time t) {
@@ -666,6 +863,11 @@ class Time {
  private:
   friend constexpr Time time_internal::FromUnixDuration(Duration d);
   friend constexpr Duration time_internal::ToUnixDuration(Time t);
+
+#ifdef ABSL_INTERNAL_TIME_HAS_THREE_WAY_COMPARISON
+  friend constexpr std::strong_ordering operator<=>(Time lhs, Time rhs);
+#endif  // ABSL_INTERNAL_TIME_HAS_THREE_WAY_COMPARISON
+
   friend constexpr bool operator<(Time lhs, Time rhs);
   friend constexpr bool operator==(Time lhs, Time rhs);
   friend Duration operator-(Time lhs, Time rhs);
@@ -677,48 +879,78 @@ class Time {
 };
 
 // Relational Operators
-constexpr bool operator<(Time lhs, Time rhs) { return lhs.rep_ < rhs.rep_; }
-constexpr bool operator>(Time lhs, Time rhs) { return rhs < lhs; }
-constexpr bool operator>=(Time lhs, Time rhs) { return !(lhs < rhs); }
-constexpr bool operator<=(Time lhs, Time rhs) { return !(rhs < lhs); }
-constexpr bool operator==(Time lhs, Time rhs) { return lhs.rep_ == rhs.rep_; }
-constexpr bool operator!=(Time lhs, Time rhs) { return !(lhs == rhs); }
+#ifdef ABSL_INTERNAL_TIME_HAS_THREE_WAY_COMPARISON
+
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr std::strong_ordering operator<=>(
+    Time lhs, Time rhs) {
+  return lhs.rep_ <=> rhs.rep_;
+}
+
+#endif  // ABSL_INTERNAL_TIME_HAS_THREE_WAY_COMPARISON
+
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr bool operator<(Time lhs, Time rhs) {
+  return lhs.rep_ < rhs.rep_;
+}
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr bool operator>(Time lhs, Time rhs) {
+  return rhs < lhs;
+}
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr bool operator>=(Time lhs, Time rhs) {
+  return !(lhs < rhs);
+}
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr bool operator<=(Time lhs, Time rhs) {
+  return !(rhs < lhs);
+}
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr bool operator==(Time lhs, Time rhs) {
+  return lhs.rep_ == rhs.rep_;
+}
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr bool operator!=(Time lhs, Time rhs) {
+  return !(lhs == rhs);
+}
 
 // Additive Operators
-inline Time operator+(Time lhs, Duration rhs) { return lhs += rhs; }
-inline Time operator+(Duration lhs, Time rhs) { return rhs += lhs; }
-inline Time operator-(Time lhs, Duration rhs) { return lhs -= rhs; }
-inline Duration operator-(Time lhs, Time rhs) { return lhs.rep_ - rhs.rep_; }
+ABSL_ATTRIBUTE_CONST_FUNCTION inline Time operator+(Time lhs, Duration rhs) {
+  return lhs += rhs;
+}
+ABSL_ATTRIBUTE_CONST_FUNCTION inline Time operator+(Duration lhs, Time rhs) {
+  return rhs += lhs;
+}
+ABSL_ATTRIBUTE_CONST_FUNCTION inline Time operator-(Time lhs, Duration rhs) {
+  return lhs -= rhs;
+}
+ABSL_ATTRIBUTE_CONST_FUNCTION inline Duration operator-(Time lhs, Time rhs) {
+  return lhs.rep_ - rhs.rep_;
+}
 
 // UnixEpoch()
 //
 // Returns the `absl::Time` representing "1970-01-01 00:00:00.0 +0000".
-constexpr Time UnixEpoch() { return Time(); }
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr Time UnixEpoch() { return Time(); }
 
 // UniversalEpoch()
 //
 // Returns the `absl::Time` representing "0001-01-01 00:00:00.0 +0000", the
 // epoch of the ICU Universal Time Scale.
-constexpr Time UniversalEpoch() {
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr Time UniversalEpoch() {
   // 719162 is the number of days from 0001-01-01 to 1970-01-01,
   // assuming the Gregorian calendar.
-  return Time(time_internal::MakeDuration(-24 * 719162 * int64_t{3600}, 0U));
+  return Time(
+      time_internal::MakeDuration(-24 * 719162 * int64_t{3600}, uint32_t{0}));
 }
 
 // InfiniteFuture()
 //
 // Returns an `absl::Time` that is infinitely far in the future.
-constexpr Time InfiniteFuture() {
-  return Time(
-      time_internal::MakeDuration((std::numeric_limits<int64_t>::max)(), ~0U));
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr Time InfiniteFuture() {
+  return Time(time_internal::MakeDuration((std::numeric_limits<int64_t>::max)(),
+                                          ~uint32_t{0}));
 }
 
 // InfinitePast()
 //
 // Returns an `absl::Time` that is infinitely far in the past.
-constexpr Time InfinitePast() {
-  return Time(
-      time_internal::MakeDuration((std::numeric_limits<int64_t>::min)(), ~0U));
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr Time InfinitePast() {
+  return Time(time_internal::MakeDuration((std::numeric_limits<int64_t>::min)(),
+                                          ~uint32_t{0}));
 }
 
 // FromUnixNanos()
@@ -729,14 +961,15 @@ constexpr Time InfinitePast() {
 // FromUDate()
 // FromUniversal()
 //
-// Creates an `absl::Time` from a variety of other representations.
-constexpr Time FromUnixNanos(int64_t ns);
-constexpr Time FromUnixMicros(int64_t us);
-constexpr Time FromUnixMillis(int64_t ms);
-constexpr Time FromUnixSeconds(int64_t s);
-constexpr Time FromTimeT(time_t t);
-Time FromUDate(double udate);
-Time FromUniversal(int64_t universal);
+// Creates an `absl::Time` from a variety of other representations.  See
+// https://unicode-org.github.io/icu/userguide/datetime/universaltimescale.html
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr Time FromUnixNanos(int64_t ns);
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr Time FromUnixMicros(int64_t us);
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr Time FromUnixMillis(int64_t ms);
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr Time FromUnixSeconds(int64_t s);
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr Time FromTimeT(time_t t);
+ABSL_ATTRIBUTE_CONST_FUNCTION Time FromUDate(double udate);
+ABSL_ATTRIBUTE_CONST_FUNCTION Time FromUniversal(int64_t universal);
 
 // ToUnixNanos()
 // ToUnixMicros()
@@ -746,17 +979,19 @@ Time FromUniversal(int64_t universal);
 // ToUDate()
 // ToUniversal()
 //
-// Converts an `absl::Time` to a variety of other representations.  Note that
-// these operations round down toward negative infinity where necessary to
-// adjust to the resolution of the result type.  Beware of possible time_t
-// over/underflow in ToTime{T,val,spec}() on 32-bit platforms.
-int64_t ToUnixNanos(Time t);
-int64_t ToUnixMicros(Time t);
-int64_t ToUnixMillis(Time t);
-int64_t ToUnixSeconds(Time t);
-time_t ToTimeT(Time t);
-double ToUDate(Time t);
-int64_t ToUniversal(Time t);
+// Converts an `absl::Time` to a variety of other representations.  See
+// https://unicode-org.github.io/icu/userguide/datetime/universaltimescale.html
+//
+// Note that these operations round down toward negative infinity where
+// necessary to adjust to the resolution of the result type.  Beware of
+// possible time_t over/underflow in ToTime{T,val,spec}() on 32-bit platforms.
+ABSL_ATTRIBUTE_CONST_FUNCTION int64_t ToUnixNanos(Time t);
+ABSL_ATTRIBUTE_CONST_FUNCTION int64_t ToUnixMicros(Time t);
+ABSL_ATTRIBUTE_CONST_FUNCTION int64_t ToUnixMillis(Time t);
+ABSL_ATTRIBUTE_CONST_FUNCTION int64_t ToUnixSeconds(Time t);
+ABSL_ATTRIBUTE_CONST_FUNCTION time_t ToTimeT(Time t);
+ABSL_ATTRIBUTE_CONST_FUNCTION double ToUDate(Time t);
+ABSL_ATTRIBUTE_CONST_FUNCTION int64_t ToUniversal(Time t);
 
 // DurationFromTimespec()
 // DurationFromTimeval()
@@ -772,14 +1007,14 @@ int64_t ToUniversal(Time t);
 // and gettimeofday(2)), so conversion functions are provided for both cases.
 // The "to timespec/val" direction is easily handled via overloading, but
 // for "from timespec/val" the desired type is part of the function name.
-Duration DurationFromTimespec(timespec ts);
-Duration DurationFromTimeval(timeval tv);
-timespec ToTimespec(Duration d);
-timeval ToTimeval(Duration d);
-Time TimeFromTimespec(timespec ts);
-Time TimeFromTimeval(timeval tv);
-timespec ToTimespec(Time t);
-timeval ToTimeval(Time t);
+ABSL_ATTRIBUTE_CONST_FUNCTION Duration DurationFromTimespec(timespec ts);
+ABSL_ATTRIBUTE_CONST_FUNCTION Duration DurationFromTimeval(timeval tv);
+ABSL_ATTRIBUTE_CONST_FUNCTION timespec ToTimespec(Duration d);
+ABSL_ATTRIBUTE_CONST_FUNCTION timeval ToTimeval(Duration d);
+ABSL_ATTRIBUTE_CONST_FUNCTION Time TimeFromTimespec(timespec ts);
+ABSL_ATTRIBUTE_CONST_FUNCTION Time TimeFromTimeval(timeval tv);
+ABSL_ATTRIBUTE_CONST_FUNCTION timespec ToTimespec(Time t);
+ABSL_ATTRIBUTE_CONST_FUNCTION timeval ToTimeval(Time t);
 
 // FromChrono()
 //
@@ -790,7 +1025,8 @@ timeval ToTimeval(Time t);
 //   auto tp = std::chrono::system_clock::from_time_t(123);
 //   absl::Time t = absl::FromChrono(tp);
 //   // t == absl::FromTimeT(123)
-Time FromChrono(const std::chrono::system_clock::time_point& tp);
+ABSL_ATTRIBUTE_PURE_FUNCTION Time
+FromChrono(const std::chrono::system_clock::time_point& tp);
 
 // ToChronoTime()
 //
@@ -803,10 +1039,15 @@ Time FromChrono(const std::chrono::system_clock::time_point& tp);
 //   absl::Time t = absl::FromTimeT(123);
 //   auto tp = absl::ToChronoTime(t);
 //   // tp == std::chrono::system_clock::from_time_t(123);
-std::chrono::system_clock::time_point ToChronoTime(Time);
+ABSL_ATTRIBUTE_CONST_FUNCTION std::chrono::system_clock::time_point
+    ToChronoTime(Time);
 
-// Support for flag values of type Time. Time flags must be specified in a
-// format that matches absl::RFC3339_full. For example:
+// AbslParseFlag()
+//
+// Parses the command-line flag string representation `text` into a Time value.
+// Time flags must be specified in a format that matches absl::RFC3339_full.
+//
+// For example:
 //
 //   --start_time=2016-01-02T03:04:05.678+08:00
 //
@@ -815,8 +1056,13 @@ std::chrono::system_clock::time_point ToChronoTime(Time);
 // Additionally, if you'd like to specify a time as a count of
 // seconds/milliseconds/etc from the Unix epoch, use an absl::Duration flag
 // and add that duration to absl::UnixEpoch() to get an absl::Time.
-bool ParseFlag(const std::string& text, Time* t, std::string* error);
-std::string UnparseFlag(Time t);
+bool AbslParseFlag(absl::string_view text, Time* t, std::string* error);
+
+// AbslUnparseFlag()
+//
+// Unparses a Time value into a command-line string representation using
+// the format specified by `absl::ParseTime()`.
+std::string AbslUnparseFlag(Time t);
 
 // TimeZone
 //
@@ -1009,13 +1255,13 @@ class TimeZone {
 // Loads the named zone. May perform I/O on the initial load of the named
 // zone. If the name is invalid, or some other kind of error occurs, returns
 // `false` and `*tz` is set to the UTC time zone.
-inline bool LoadTimeZone(const std::string& name, TimeZone* tz) {
+inline bool LoadTimeZone(absl::string_view name, TimeZone* tz) {
   if (name == "localtime") {
     *tz = TimeZone(time_internal::cctz::local_time_zone());
     return true;
   }
   time_internal::cctz::time_zone cz;
-  const bool b = time_internal::cctz::load_time_zone(name, &cz);
+  const bool b = time_internal::cctz::load_time_zone(std::string(name), &cz);
   *tz = TimeZone(cz);
   return b;
 }
@@ -1061,22 +1307,25 @@ inline TimeZone LocalTimeZone() {
 //   absl::Time t = ...;
 //   absl::TimeZone tz = ...;
 //   const auto cd = absl::ToCivilDay(t, tz);
-inline CivilSecond ToCivilSecond(Time t, TimeZone tz) {
+ABSL_ATTRIBUTE_PURE_FUNCTION inline CivilSecond ToCivilSecond(Time t,
+                                                              TimeZone tz) {
   return tz.At(t).cs;  // already a CivilSecond
 }
-inline CivilMinute ToCivilMinute(Time t, TimeZone tz) {
+ABSL_ATTRIBUTE_PURE_FUNCTION inline CivilMinute ToCivilMinute(Time t,
+                                                              TimeZone tz) {
   return CivilMinute(tz.At(t).cs);
 }
-inline CivilHour ToCivilHour(Time t, TimeZone tz) {
+ABSL_ATTRIBUTE_PURE_FUNCTION inline CivilHour ToCivilHour(Time t, TimeZone tz) {
   return CivilHour(tz.At(t).cs);
 }
-inline CivilDay ToCivilDay(Time t, TimeZone tz) {
+ABSL_ATTRIBUTE_PURE_FUNCTION inline CivilDay ToCivilDay(Time t, TimeZone tz) {
   return CivilDay(tz.At(t).cs);
 }
-inline CivilMonth ToCivilMonth(Time t, TimeZone tz) {
+ABSL_ATTRIBUTE_PURE_FUNCTION inline CivilMonth ToCivilMonth(Time t,
+                                                            TimeZone tz) {
   return CivilMonth(tz.At(t).cs);
 }
-inline CivilYear ToCivilYear(Time t, TimeZone tz) {
+ABSL_ATTRIBUTE_PURE_FUNCTION inline CivilYear ToCivilYear(Time t, TimeZone tz) {
   return CivilYear(tz.At(t).cs);
 }
 
@@ -1092,7 +1341,8 @@ inline CivilYear ToCivilYear(Time t, TimeZone tz) {
 // being when two non-existent civil times map to the same transition time.
 //
 // Note: Accepts civil times of any alignment.
-inline Time FromCivil(CivilSecond ct, TimeZone tz) {
+ABSL_ATTRIBUTE_PURE_FUNCTION inline Time FromCivil(CivilSecond ct,
+                                                   TimeZone tz) {
   const auto ti = tz.At(ct);
   if (ti.kind == TimeZone::TimeInfo::SKIPPED) return ti.trans;
   return ti.pre;
@@ -1106,8 +1356,7 @@ inline Time FromCivil(CivilSecond ct, TimeZone tz) {
 // `absl::ConvertDateTime()`. Legacy version of `absl::TimeZone::TimeInfo`.
 //
 // Deprecated. Use `absl::TimeZone::TimeInfo`.
-struct
-    TimeConversion {
+struct ABSL_DEPRECATED("Use `absl::TimeZone::TimeInfo`.") TimeConversion {
   Time pre;    // time calculated using the pre-transition offset
   Time trans;  // when the civil-time discontinuity occurred
   Time post;   // time calculated using the post-transition offset
@@ -1141,8 +1390,11 @@ struct
 //   // absl::ToCivilDay(tc.pre, tz).day() == 1
 //
 // Deprecated. Use `absl::TimeZone::At(CivilSecond)`.
+ABSL_INTERNAL_DISABLE_DEPRECATED_DECLARATION_WARNING
+ABSL_DEPRECATED("Use `absl::TimeZone::At(CivilSecond)`.")
 TimeConversion ConvertDateTime(int64_t year, int mon, int day, int hour,
                                int min, int sec, TimeZone tz);
+ABSL_INTERNAL_RESTORE_DEPRECATED_DECLARATION_WARNING
 
 // FromDateTime()
 //
@@ -1159,27 +1411,34 @@ TimeConversion ConvertDateTime(int64_t year, int mon, int day, int hour,
 // Deprecated. Use `absl::FromCivil(CivilSecond, TimeZone)`. Note that the
 // behavior of `FromCivil()` differs from `FromDateTime()` for skipped civil
 // times. If you care about that see `absl::TimeZone::At(absl::CivilSecond)`.
-inline Time FromDateTime(int64_t year, int mon, int day, int hour,
-                         int min, int sec, TimeZone tz) {
+ABSL_DEPRECATED("Use `absl::FromCivil(CivilSecond, TimeZone)`.")
+inline Time FromDateTime(int64_t year, int mon, int day, int hour, int min,
+                         int sec, TimeZone tz) {
+  ABSL_INTERNAL_DISABLE_DEPRECATED_DECLARATION_WARNING
   return ConvertDateTime(year, mon, day, hour, min, sec, tz).pre;
+  ABSL_INTERNAL_RESTORE_DEPRECATED_DECLARATION_WARNING
 }
 
 // FromTM()
 //
 // Converts the `tm_year`, `tm_mon`, `tm_mday`, `tm_hour`, `tm_min`, and
 // `tm_sec` fields to an `absl::Time` using the given time zone. See ctime(3)
-// for a description of the expected values of the tm fields. If the indicated
-// time instant is not unique (see `absl::TimeZone::At(absl::CivilSecond)`
-// above), the `tm_isdst` field is consulted to select the desired instant
-// (`tm_isdst` > 0 means DST, `tm_isdst` == 0 means no DST, `tm_isdst` < 0
-// means use the post-transition offset).
-Time FromTM(const struct tm& tm, TimeZone tz);
+// for a description of the expected values of the tm fields. If the civil time
+// is unique (see `absl::TimeZone::At(absl::CivilSecond)` above), the matching
+// time instant is returned.  Otherwise, the `tm_isdst` field is consulted to
+// choose between the possible results.  For a repeated civil time, `tm_isdst !=
+// 0` returns the matching DST instant, while `tm_isdst == 0` returns the
+// matching non-DST instant.  For a skipped civil time there is no matching
+// instant, so `tm_isdst != 0` returns the DST instant, and `tm_isdst == 0`
+// returns the non-DST instant, that would have matched if the transition never
+// happened.
+ABSL_ATTRIBUTE_PURE_FUNCTION Time FromTM(const struct tm& tm, TimeZone tz);
 
 // ToTM()
 //
 // Converts the given `absl::Time` to a struct tm using the given time zone.
 // See ctime(3) for a description of the values of the tm fields.
-struct tm ToTM(Time t, TimeZone tz);
+ABSL_ATTRIBUTE_PURE_FUNCTION struct tm ToTM(Time t, TimeZone tz);
 
 // RFC3339_full
 // RFC3339_sec
@@ -1191,15 +1450,15 @@ struct tm ToTM(Time t, TimeZone tz);
 // time with UTC offset.  Also note the use of "%Y": RFC3339 mandates that
 // years have exactly four digits, but we allow them to take their natural
 // width.
-extern const char RFC3339_full[];  // %Y-%m-%dT%H:%M:%E*S%Ez
-extern const char RFC3339_sec[];   // %Y-%m-%dT%H:%M:%S%Ez
+ABSL_DLL extern const char RFC3339_full[];  // %Y-%m-%d%ET%H:%M:%E*S%Ez
+ABSL_DLL extern const char RFC3339_sec[];   // %Y-%m-%d%ET%H:%M:%S%Ez
 
 // RFC1123_full
 // RFC1123_no_wday
 //
 // FormatTime()/ParseTime() format specifiers for RFC1123 date/time strings.
-extern const char RFC1123_full[];     // %a, %d %b %E4Y %H:%M:%S %z
-extern const char RFC1123_no_wday[];  // %d %b %E4Y %H:%M:%S %z
+ABSL_DLL extern const char RFC1123_full[];     // %a, %d %b %E4Y %H:%M:%S %z
+ABSL_DLL extern const char RFC1123_no_wday[];  // %d %b %E4Y %H:%M:%S %z
 
 // FormatTime()
 //
@@ -1214,6 +1473,7 @@ extern const char RFC1123_no_wday[];  // %d %b %E4Y %H:%M:%S %z
 //   - %E#f - Fractional seconds with # digits of precision
 //   - %E*f - Fractional seconds with full precision (a literal '*')
 //   - %E4Y - Four-character years (-999 ... -001, 0000, 0001 ... 9999)
+//   - %ET  - The RFC3339 "date-time" separator "T"
 //
 // Note that %E0S behaves like %S, and %E0f produces no characters.  In
 // contrast %E*f always produces at least one digit, which may be '0'.
@@ -1237,26 +1497,34 @@ extern const char RFC1123_no_wday[];  // %d %b %E4Y %H:%M:%S %z
 // `absl::InfinitePast()`, the returned string will be exactly "infinite-past".
 // In both cases the given format string and `absl::TimeZone` are ignored.
 //
-std::string FormatTime(const std::string& format, Time t, TimeZone tz);
+ABSL_ATTRIBUTE_PURE_FUNCTION std::string FormatTime(absl::string_view format,
+                                                    Time t, TimeZone tz);
 
 // Convenience functions that format the given time using the RFC3339_full
 // format.  The first overload uses the provided TimeZone, while the second
 // uses LocalTimeZone().
-std::string FormatTime(Time t, TimeZone tz);
-std::string FormatTime(Time t);
+ABSL_ATTRIBUTE_PURE_FUNCTION std::string FormatTime(Time t, TimeZone tz);
+ABSL_ATTRIBUTE_PURE_FUNCTION std::string FormatTime(Time t);
 
 // Output stream operator.
 inline std::ostream& operator<<(std::ostream& os, Time t) {
   return os << FormatTime(t);
 }
 
+// Support for StrFormat(), StrCat() etc.
+template <typename Sink>
+void AbslStringify(Sink& sink, Time t) {
+  sink.Append(FormatTime(t));
+}
+
 // ParseTime()
 //
 // Parses an input string according to the provided format string and
 // returns the corresponding `absl::Time`. Uses strftime()-like formatting
 // options, with the same extensions as FormatTime(), but with the
 // exceptions that %E#S is interpreted as %E*S, and %E#f as %E*f.  %Ez
-// and %E*z also accept the same inputs.
+// and %E*z also accept the same inputs, which (along with %z) includes
+// 'z' and 'Z' as synonyms for +00:00.  %ET accepts either 'T' or 't'.
 //
 // %Y consumes as many numeric characters as it can, so the matching data
 // should always be terminated with a non-numeric.  %E4Y always consumes
@@ -1298,7 +1566,7 @@ inline std::ostream& operator<<(std::ostream& os, Time t) {
 // If the input string is "infinite-past", the returned `absl::Time` will be
 // `absl::InfinitePast()` and `true` will be returned.
 //
-bool ParseTime(const std::string& format, const std::string& input, Time* time,
+bool ParseTime(absl::string_view format, absl::string_view input, Time* time,
                std::string* err);
 
 // Like ParseTime() above, but if the format string does not contain a UTC
@@ -1308,7 +1576,7 @@ bool ParseTime(const std::string& format, const std::string& input, Time* time,
 // of ambiguity or non-existence, in which case the "pre" time (as defined
 // by TimeZone::TimeInfo) is returned.  For these reasons we recommend that
 // all date/time strings include a UTC offset so they're context independent.
-bool ParseTime(const std::string& format, const std::string& input, TimeZone tz,
+bool ParseTime(absl::string_view format, absl::string_view input, TimeZone tz,
                Time* time, std::string* err);
 
 // ============================================================================
@@ -1320,21 +1588,23 @@ namespace time_internal {
 // Creates a Duration with a given representation.
 // REQUIRES: hi,lo is a valid representation of a Duration as specified
 // in time/duration.cc.
-constexpr Duration MakeDuration(int64_t hi, uint32_t lo = 0) {
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr Duration MakeDuration(int64_t hi,
+                                                              uint32_t lo = 0) {
   return Duration(hi, lo);
 }
 
-constexpr Duration MakeDuration(int64_t hi, int64_t lo) {
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr Duration MakeDuration(int64_t hi,
+                                                              int64_t lo) {
   return MakeDuration(hi, static_cast<uint32_t>(lo));
 }
 
 // Make a Duration value from a floating-point number, as long as that number
 // is in the range [ 0 .. numeric_limits<int64_t>::max ), that is, as long as
 // it's positive and can be converted to int64_t without risk of UB.
-inline Duration MakePosDoubleDuration(double n) {
+ABSL_ATTRIBUTE_CONST_FUNCTION inline Duration MakePosDoubleDuration(double n) {
   const int64_t int_secs = static_cast<int64_t>(n);
-  const uint32_t ticks =
-      static_cast<uint32_t>((n - int_secs) * kTicksPerSecond + 0.5);
+  const uint32_t ticks = static_cast<uint32_t>(
+      std::round((n - static_cast<double>(int_secs)) * kTicksPerSecond));
   return ticks < kTicksPerSecond
              ? MakeDuration(int_secs, ticks)
              : MakeDuration(int_secs + 1, ticks - kTicksPerSecond);
@@ -1344,28 +1614,37 @@ inline Duration MakePosDoubleDuration(double n) {
 // pair. sec may be positive or negative.  ticks must be in the range
 // -kTicksPerSecond < *ticks < kTicksPerSecond.  If ticks is negative it
 // will be normalized to a positive value in the resulting Duration.
-constexpr Duration MakeNormalizedDuration(int64_t sec, int64_t ticks) {
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr Duration MakeNormalizedDuration(
+    int64_t sec, int64_t ticks) {
   return (ticks < 0) ? MakeDuration(sec - 1, ticks + kTicksPerSecond)
                      : MakeDuration(sec, ticks);
 }
 
 // Provide access to the Duration representation.
-constexpr int64_t GetRepHi(Duration d) { return d.rep_hi_; }
-constexpr uint32_t GetRepLo(Duration d) { return d.rep_lo_; }
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr int64_t GetRepHi(Duration d) {
+  return d.rep_hi_.Get();
+}
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr uint32_t GetRepLo(Duration d) {
+  return d.rep_lo_;
+}
 
 // Returns true iff d is positive or negative infinity.
-constexpr bool IsInfiniteDuration(Duration d) { return GetRepLo(d) == ~0U; }
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr bool IsInfiniteDuration(Duration d) {
+  return GetRepLo(d) == ~uint32_t{0};
+}
 
 // Returns an infinite Duration with the opposite sign.
 // REQUIRES: IsInfiniteDuration(d)
-constexpr Duration OppositeInfinity(Duration d) {
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr Duration OppositeInfinity(Duration d) {
   return GetRepHi(d) < 0
-             ? MakeDuration((std::numeric_limits<int64_t>::max)(), ~0U)
-             : MakeDuration((std::numeric_limits<int64_t>::min)(), ~0U);
+             ? MakeDuration((std::numeric_limits<int64_t>::max)(), ~uint32_t{0})
+             : MakeDuration((std::numeric_limits<int64_t>::min)(),
+                            ~uint32_t{0});
 }
 
 // Returns (-n)-1 (equivalently -(n+1)) without avoidable overflow.
-constexpr int64_t NegateAndSubtractOne(int64_t n) {
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr int64_t NegateAndSubtractOne(
+    int64_t n) {
   // Note: Good compilers will optimize this expression to ~n when using
   // a two's-complement representation (which is required for int64_t).
   return (n < 0) ? -(n + 1) : (-n) - 1;
@@ -1375,34 +1654,42 @@ constexpr int64_t NegateAndSubtractOne(int64_t n) {
 // functions depend on the above mentioned choice of the Unix epoch for the
 // Time representation (and both need to be Time friends).  Without this
 // knowledge, we would need to add-in/subtract-out UnixEpoch() respectively.
-constexpr Time FromUnixDuration(Duration d) { return Time(d); }
-constexpr Duration ToUnixDuration(Time t) { return t.rep_; }
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr Time FromUnixDuration(Duration d) {
+  return Time(d);
+}
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr Duration ToUnixDuration(Time t) {
+  return t.rep_;
+}
 
 template <std::intmax_t N>
-constexpr Duration FromInt64(int64_t v, std::ratio<1, N>) {
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr Duration FromInt64(int64_t v,
+                                                           std::ratio<1, N>) {
   static_assert(0 < N && N <= 1000 * 1000 * 1000, "Unsupported ratio");
   // Subsecond ratios cannot overflow.
   return MakeNormalizedDuration(
       v / N, v % N * kTicksPerNanosecond * 1000 * 1000 * 1000 / N);
 }
-constexpr Duration FromInt64(int64_t v, std::ratio<60>) {
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr Duration FromInt64(int64_t v,
+                                                           std::ratio<60>) {
   return (v <= (std::numeric_limits<int64_t>::max)() / 60 &&
           v >= (std::numeric_limits<int64_t>::min)() / 60)
              ? MakeDuration(v * 60)
-             : v > 0 ? InfiniteDuration() : -InfiniteDuration();
+         : v > 0 ? InfiniteDuration()
+                 : -InfiniteDuration();
 }
-constexpr Duration FromInt64(int64_t v, std::ratio<3600>) {
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr Duration FromInt64(int64_t v,
+                                                           std::ratio<3600>) {
   return (v <= (std::numeric_limits<int64_t>::max)() / 3600 &&
           v >= (std::numeric_limits<int64_t>::min)() / 3600)
              ? MakeDuration(v * 3600)
-             : v > 0 ? InfiniteDuration() : -InfiniteDuration();
+         : v > 0 ? InfiniteDuration()
+                 : -InfiniteDuration();
 }
 
 // IsValidRep64<T>(0) is true if the expression `int64_t{std::declval<T>()}` is
 // valid. That is, if a T can be assigned to an int64_t without narrowing.
 template <typename T>
-constexpr auto IsValidRep64(int)
-    -> decltype(int64_t{std::declval<T>()}, bool()) {
+constexpr auto IsValidRep64(int) -> decltype(int64_t{std::declval<T>()} == 0) {
   return true;
 }
 template <typename T>
@@ -1412,40 +1699,44 @@ constexpr auto IsValidRep64(char) -> bool {
 
 // Converts a std::chrono::duration to an absl::Duration.
 template <typename Rep, typename Period>
-constexpr Duration FromChrono(const std::chrono::duration<Rep, Period>& d) {
+ABSL_ATTRIBUTE_PURE_FUNCTION constexpr Duration FromChrono(
+    const std::chrono::duration<Rep, Period>& d) {
   static_assert(IsValidRep64<Rep>(0), "duration::rep is invalid");
   return FromInt64(int64_t{d.count()}, Period{});
 }
 
 template <typename Ratio>
-int64_t ToInt64(Duration d, Ratio) {
+ABSL_ATTRIBUTE_CONST_FUNCTION int64_t ToInt64(Duration d, Ratio) {
   // Note: This may be used on MSVC, which may have a system_clock period of
   // std::ratio<1, 10 * 1000 * 1000>
   return ToInt64Seconds(d * Ratio::den / Ratio::num);
 }
 // Fastpath implementations for the 6 common duration units.
-inline int64_t ToInt64(Duration d, std::nano) {
+ABSL_ATTRIBUTE_CONST_FUNCTION inline int64_t ToInt64(Duration d, std::nano) {
   return ToInt64Nanoseconds(d);
 }
-inline int64_t ToInt64(Duration d, std::micro) {
+ABSL_ATTRIBUTE_CONST_FUNCTION inline int64_t ToInt64(Duration d, std::micro) {
   return ToInt64Microseconds(d);
 }
-inline int64_t ToInt64(Duration d, std::milli) {
+ABSL_ATTRIBUTE_CONST_FUNCTION inline int64_t ToInt64(Duration d, std::milli) {
   return ToInt64Milliseconds(d);
 }
-inline int64_t ToInt64(Duration d, std::ratio<1>) {
+ABSL_ATTRIBUTE_CONST_FUNCTION inline int64_t ToInt64(Duration d,
+                                                     std::ratio<1>) {
   return ToInt64Seconds(d);
 }
-inline int64_t ToInt64(Duration d, std::ratio<60>) {
+ABSL_ATTRIBUTE_CONST_FUNCTION inline int64_t ToInt64(Duration d,
+                                                     std::ratio<60>) {
   return ToInt64Minutes(d);
 }
-inline int64_t ToInt64(Duration d, std::ratio<3600>) {
+ABSL_ATTRIBUTE_CONST_FUNCTION inline int64_t ToInt64(Duration d,
+                                                     std::ratio<3600>) {
   return ToInt64Hours(d);
 }
 
 // Converts an absl::Duration to a chrono duration of type T.
 template <typename T>
-T ToChronoDuration(Duration d) {
+ABSL_ATTRIBUTE_CONST_FUNCTION T ToChronoDuration(Duration d) {
   using Rep = typename T::rep;
   using Period = typename T::period;
   static_assert(IsValidRep64<Rep>(0), "duration::rep is invalid");
@@ -1459,42 +1750,41 @@ T ToChronoDuration(Duration d) {
 
 }  // namespace time_internal
 
-constexpr Duration Nanoseconds(int64_t n) {
-  return time_internal::FromInt64(n, std::nano{});
-}
-constexpr Duration Microseconds(int64_t n) {
-  return time_internal::FromInt64(n, std::micro{});
-}
-constexpr Duration Milliseconds(int64_t n) {
-  return time_internal::FromInt64(n, std::milli{});
-}
-constexpr Duration Seconds(int64_t n) {
-  return time_internal::FromInt64(n, std::ratio<1>{});
-}
-constexpr Duration Minutes(int64_t n) {
-  return time_internal::FromInt64(n, std::ratio<60>{});
-}
-constexpr Duration Hours(int64_t n) {
-  return time_internal::FromInt64(n, std::ratio<3600>{});
-}
-
-constexpr bool operator<(Duration lhs, Duration rhs) {
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr bool operator<(Duration lhs,
+                                                       Duration rhs) {
   return time_internal::GetRepHi(lhs) != time_internal::GetRepHi(rhs)
              ? time_internal::GetRepHi(lhs) < time_internal::GetRepHi(rhs)
-             : time_internal::GetRepHi(lhs) ==
-                       (std::numeric_limits<int64_t>::min)()
-                   ? time_internal::GetRepLo(lhs) + 1 <
-                         time_internal::GetRepLo(rhs) + 1
-                   : time_internal::GetRepLo(lhs) <
-                         time_internal::GetRepLo(rhs);
+         : time_internal::GetRepHi(lhs) == (std::numeric_limits<int64_t>::min)()
+             ? time_internal::GetRepLo(lhs) + 1 <
+                   time_internal::GetRepLo(rhs) + 1
+             : time_internal::GetRepLo(lhs) < time_internal::GetRepLo(rhs);
+}
+
+#ifdef ABSL_INTERNAL_TIME_HAS_THREE_WAY_COMPARISON
+
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr std::strong_ordering operator<=>(
+    Duration lhs, Duration rhs) {
+  const int64_t lhs_hi = time_internal::GetRepHi(lhs);
+  const int64_t rhs_hi = time_internal::GetRepHi(rhs);
+  if (auto c = lhs_hi <=> rhs_hi; c != std::strong_ordering::equal) {
+    return c;
+  }
+  const uint32_t lhs_lo = time_internal::GetRepLo(lhs);
+  const uint32_t rhs_lo = time_internal::GetRepLo(rhs);
+  return (lhs_hi == (std::numeric_limits<int64_t>::min)())
+             ? (lhs_lo + 1) <=> (rhs_lo + 1)
+             : lhs_lo <=> rhs_lo;
 }
 
-constexpr bool operator==(Duration lhs, Duration rhs) {
+#endif  // ABSL_INTERNAL_TIME_HAS_THREE_WAY_COMPARISON
+
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr bool operator==(Duration lhs,
+                                                        Duration rhs) {
   return time_internal::GetRepHi(lhs) == time_internal::GetRepHi(rhs) &&
          time_internal::GetRepLo(lhs) == time_internal::GetRepLo(rhs);
 }
 
-constexpr Duration operator-(Duration d) {
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr Duration operator-(Duration d) {
   // This is a little interesting because of the special cases.
   //
   // If rep_lo_ is zero, we have it easy; it's safe to negate rep_hi_, we're
@@ -1511,59 +1801,120 @@ constexpr Duration operator-(Duration d) {
                        (std::numeric_limits<int64_t>::min)()
                    ? InfiniteDuration()
                    : time_internal::MakeDuration(-time_internal::GetRepHi(d))
-             : time_internal::IsInfiniteDuration(d)
-                   ? time_internal::OppositeInfinity(d)
-                   : time_internal::MakeDuration(
-                         time_internal::NegateAndSubtractOne(
-                             time_internal::GetRepHi(d)),
-                         time_internal::kTicksPerSecond -
-                             time_internal::GetRepLo(d));
+         : time_internal::IsInfiniteDuration(d)
+             ? time_internal::OppositeInfinity(d)
+             : time_internal::MakeDuration(
+                   time_internal::NegateAndSubtractOne(
+                       time_internal::GetRepHi(d)),
+                   time_internal::kTicksPerSecond - time_internal::GetRepLo(d));
 }
 
-constexpr Duration InfiniteDuration() {
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr Duration InfiniteDuration() {
   return time_internal::MakeDuration((std::numeric_limits<int64_t>::max)(),
-                                     ~0U);
+                                     ~uint32_t{0});
 }
 
-constexpr Duration FromChrono(const std::chrono::nanoseconds& d) {
+ABSL_ATTRIBUTE_PURE_FUNCTION constexpr Duration FromChrono(
+    const std::chrono::nanoseconds& d) {
   return time_internal::FromChrono(d);
 }
-constexpr Duration FromChrono(const std::chrono::microseconds& d) {
+ABSL_ATTRIBUTE_PURE_FUNCTION constexpr Duration FromChrono(
+    const std::chrono::microseconds& d) {
   return time_internal::FromChrono(d);
 }
-constexpr Duration FromChrono(const std::chrono::milliseconds& d) {
+ABSL_ATTRIBUTE_PURE_FUNCTION constexpr Duration FromChrono(
+    const std::chrono::milliseconds& d) {
   return time_internal::FromChrono(d);
 }
-constexpr Duration FromChrono(const std::chrono::seconds& d) {
+ABSL_ATTRIBUTE_PURE_FUNCTION constexpr Duration FromChrono(
+    const std::chrono::seconds& d) {
   return time_internal::FromChrono(d);
 }
-constexpr Duration FromChrono(const std::chrono::minutes& d) {
+ABSL_ATTRIBUTE_PURE_FUNCTION constexpr Duration FromChrono(
+    const std::chrono::minutes& d) {
   return time_internal::FromChrono(d);
 }
-constexpr Duration FromChrono(const std::chrono::hours& d) {
+ABSL_ATTRIBUTE_PURE_FUNCTION constexpr Duration FromChrono(
+    const std::chrono::hours& d) {
   return time_internal::FromChrono(d);
 }
 
-constexpr Time FromUnixNanos(int64_t ns) {
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr Time FromUnixNanos(int64_t ns) {
   return time_internal::FromUnixDuration(Nanoseconds(ns));
 }
 
-constexpr Time FromUnixMicros(int64_t us) {
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr Time FromUnixMicros(int64_t us) {
   return time_internal::FromUnixDuration(Microseconds(us));
 }
 
-constexpr Time FromUnixMillis(int64_t ms) {
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr Time FromUnixMillis(int64_t ms) {
   return time_internal::FromUnixDuration(Milliseconds(ms));
 }
 
-constexpr Time FromUnixSeconds(int64_t s) {
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr Time FromUnixSeconds(int64_t s) {
   return time_internal::FromUnixDuration(Seconds(s));
 }
 
-constexpr Time FromTimeT(time_t t) {
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr Time FromTimeT(time_t t) {
   return time_internal::FromUnixDuration(Seconds(t));
 }
 
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr int64_t ToInt64Nanoseconds(Duration d) {
+  if (time_internal::GetRepHi(d) >= 0 &&
+      time_internal::GetRepHi(d) >> 33 == 0) {
+    return (time_internal::GetRepHi(d) * 1000 * 1000 * 1000) +
+           (time_internal::GetRepLo(d) / time_internal::kTicksPerNanosecond);
+  } else {
+    return d / Nanoseconds(1);
+  }
+}
+
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr int64_t ToInt64Microseconds(
+    Duration d) {
+  if (time_internal::GetRepHi(d) >= 0 &&
+      time_internal::GetRepHi(d) >> 43 == 0) {
+    return (time_internal::GetRepHi(d) * 1000 * 1000) +
+           (time_internal::GetRepLo(d) /
+            (time_internal::kTicksPerNanosecond * 1000));
+  } else {
+    return d / Microseconds(1);
+  }
+}
+
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr int64_t ToInt64Milliseconds(
+    Duration d) {
+  if (time_internal::GetRepHi(d) >= 0 &&
+      time_internal::GetRepHi(d) >> 53 == 0) {
+    return (time_internal::GetRepHi(d) * 1000) +
+           (time_internal::GetRepLo(d) /
+            (time_internal::kTicksPerNanosecond * 1000 * 1000));
+  } else {
+    return d / Milliseconds(1);
+  }
+}
+
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr int64_t ToInt64Seconds(Duration d) {
+  int64_t hi = time_internal::GetRepHi(d);
+  if (time_internal::IsInfiniteDuration(d)) return hi;
+  if (hi < 0 && time_internal::GetRepLo(d) != 0) ++hi;
+  return hi;
+}
+
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr int64_t ToInt64Minutes(Duration d) {
+  int64_t hi = time_internal::GetRepHi(d);
+  if (time_internal::IsInfiniteDuration(d)) return hi;
+  if (hi < 0 && time_internal::GetRepLo(d) != 0) ++hi;
+  return hi / 60;
+}
+
+ABSL_ATTRIBUTE_CONST_FUNCTION constexpr int64_t ToInt64Hours(Duration d) {
+  int64_t hi = time_internal::GetRepHi(d);
+  if (time_internal::IsInfiniteDuration(d)) return hi;
+  if (hi < 0 && time_internal::GetRepLo(d) != 0) ++hi;
+  return hi / (60 * 60);
+}
+
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_TIME_TIME_H_
diff --git a/absl/time/time_benchmark.cc b/absl/time/time_benchmark.cc
index 99e6279984e..db8090a7589 100644
--- a/absl/time/time_benchmark.cc
+++ b/absl/time/time_benchmark.cc
@@ -11,11 +11,6 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-#include "absl/time/time.h"
-
-#if !defined(_WIN32)
-#include <sys/time.h>
-#endif  // _WIN32
 #include <algorithm>
 #include <cmath>
 #include <cstddef>
@@ -26,8 +21,13 @@
 
 #include "absl/time/clock.h"
 #include "absl/time/internal/test_util.h"
+#include "absl/time/time.h"
 #include "benchmark/benchmark.h"
 
+#if !defined(_WIN32)
+#include <sys/time.h>
+#endif  // _WIN32
+
 namespace {
 
 //
@@ -185,9 +185,11 @@ void BM_Time_FromCivil_Absl(benchmark::State& state) {
   int i = 0;
   while (state.KeepRunning()) {
     if ((i & 1) == 0) {
-      absl::FromCivil(absl::CivilSecond(2014, 12, 18, 20, 16, 18), tz);
+      benchmark::DoNotOptimize(
+          absl::FromCivil(absl::CivilSecond(2014, 12, 18, 20, 16, 18), tz));
     } else {
-      absl::FromCivil(absl::CivilSecond(2013, 11, 15, 18, 30, 27), tz);
+      benchmark::DoNotOptimize(
+          absl::FromCivil(absl::CivilSecond(2013, 11, 15, 18, 30, 27), tz));
     }
     ++i;
   }
@@ -224,7 +226,8 @@ BENCHMARK(BM_Time_FromCivil_Libc);
 void BM_Time_FromCivilUTC_Absl(benchmark::State& state) {
   const absl::TimeZone tz = absl::UTCTimeZone();
   while (state.KeepRunning()) {
-    absl::FromCivil(absl::CivilSecond(2014, 12, 18, 20, 16, 18), tz);
+    benchmark::DoNotOptimize(
+        absl::FromCivil(absl::CivilSecond(2014, 12, 18, 20, 16, 18), tz));
   }
 }
 BENCHMARK(BM_Time_FromCivilUTC_Absl);
@@ -235,9 +238,11 @@ void BM_Time_FromCivilDay0_Absl(benchmark::State& state) {
   int i = 0;
   while (state.KeepRunning()) {
     if ((i & 1) == 0) {
-      absl::FromCivil(absl::CivilSecond(2014, 12, 0, 20, 16, 18), tz);
+      benchmark::DoNotOptimize(
+          absl::FromCivil(absl::CivilSecond(2014, 12, 0, 20, 16, 18), tz));
     } else {
-      absl::FromCivil(absl::CivilSecond(2013, 11, 0, 18, 30, 27), tz);
+      benchmark::DoNotOptimize(
+          absl::FromCivil(absl::CivilSecond(2013, 11, 0, 18, 30, 27), tz));
     }
     ++i;
   }
diff --git a/absl/time/time_test.cc b/absl/time/time_test.cc
index 37af39d9983..d53198a0acd 100644
--- a/absl/time/time_test.cc
+++ b/absl/time/time_test.cc
@@ -14,22 +14,39 @@
 
 #include "absl/time/time.h"
 
-#if defined(_MSC_VER)
-#include <winsock2.h>  // for timeval
+#include "absl/base/config.h"
+
+// For feature testing and determining which headers can be included.
+#if ABSL_INTERNAL_CPLUSPLUS_LANG >= 202002L
+#include <version>
 #endif
 
 #include <chrono>  // NOLINT(build/c++11)
+#ifdef __cpp_lib_three_way_comparison
+#include <compare>
+#endif  // __cpp_lib_three_way_comparison
+#include <cstdint>
 #include <cstring>
 #include <ctime>
 #include <iomanip>
+#include <ios>
 #include <limits>
 #include <string>
+#include <type_traits>
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
+#include "absl/hash/hash_testing.h"
+#include "absl/numeric/int128.h"
+#include "absl/strings/str_format.h"
+#include "absl/time/civil_time.h"
 #include "absl/time/clock.h"
 #include "absl/time/internal/test_util.h"
 
+#if defined(_MSC_VER)
+#include <winsock2.h>  // for timeval
+#endif
+
 namespace {
 
 #if defined(GTEST_USES_SIMPLE_RE) && GTEST_USES_SIMPLE_RE
@@ -57,8 +74,7 @@ const char kZoneAbbrRE[] = "[A-Za-z]{3,4}|[-+][0-9]{2}([0-9]{2})?";
 // timespec ts1, ts2;
 // EXPECT_THAT(ts1, TimespecMatcher(ts2));
 MATCHER_P(TimespecMatcher, ts, "") {
-  if (ts.tv_sec == arg.tv_sec && ts.tv_nsec == arg.tv_nsec)
-    return true;
+  if (ts.tv_sec == arg.tv_sec && ts.tv_nsec == arg.tv_nsec) return true;
   *result_listener << "expected: {" << ts.tv_sec << ", " << ts.tv_nsec << "} ";
   *result_listener << "actual: {" << arg.tv_sec << ", " << arg.tv_nsec << "}";
   return false;
@@ -68,30 +84,31 @@ MATCHER_P(TimespecMatcher, ts, "") {
 // timeval tv1, tv2;
 // EXPECT_THAT(tv1, TimevalMatcher(tv2));
 MATCHER_P(TimevalMatcher, tv, "") {
-  if (tv.tv_sec == arg.tv_sec && tv.tv_usec == arg.tv_usec)
-    return true;
+  if (tv.tv_sec == arg.tv_sec && tv.tv_usec == arg.tv_usec) return true;
   *result_listener << "expected: {" << tv.tv_sec << ", " << tv.tv_usec << "} ";
   *result_listener << "actual: {" << arg.tv_sec << ", " << arg.tv_usec << "}";
   return false;
 }
 
 TEST(Time, ConstExpr) {
+  static_assert(std::is_trivially_destructible_v<absl::Time>,
+                "Time is documented as being trivially destructible");
   constexpr absl::Time t0 = absl::UnixEpoch();
-  static_assert(t0 == absl::Time(), "UnixEpoch");
+  static_assert(t0 == absl::UnixEpoch(), "UnixEpoch");
   constexpr absl::Time t1 = absl::InfiniteFuture();
-  static_assert(t1 != absl::Time(), "InfiniteFuture");
+  static_assert(t1 != absl::UnixEpoch(), "InfiniteFuture");
   constexpr absl::Time t2 = absl::InfinitePast();
-  static_assert(t2 != absl::Time(), "InfinitePast");
+  static_assert(t2 != absl::UnixEpoch(), "InfinitePast");
   constexpr absl::Time t3 = absl::FromUnixNanos(0);
-  static_assert(t3 == absl::Time(), "FromUnixNanos");
+  static_assert(t3 == absl::UnixEpoch(), "FromUnixNanos");
   constexpr absl::Time t4 = absl::FromUnixMicros(0);
-  static_assert(t4 == absl::Time(), "FromUnixMicros");
+  static_assert(t4 == absl::UnixEpoch(), "FromUnixMicros");
   constexpr absl::Time t5 = absl::FromUnixMillis(0);
-  static_assert(t5 == absl::Time(), "FromUnixMillis");
+  static_assert(t5 == absl::UnixEpoch(), "FromUnixMillis");
   constexpr absl::Time t6 = absl::FromUnixSeconds(0);
-  static_assert(t6 == absl::Time(), "FromUnixSeconds");
+  static_assert(t6 == absl::UnixEpoch(), "FromUnixSeconds");
   constexpr absl::Time t7 = absl::FromTimeT(0);
-  static_assert(t7 == absl::Time(), "FromTimeT");
+  static_assert(t7 == absl::UnixEpoch(), "FromTimeT");
 }
 
 TEST(Time, ValueSemantics) {
@@ -102,7 +119,7 @@ TEST(Time, ValueSemantics) {
   EXPECT_EQ(a, b);
   EXPECT_EQ(a, c);
   EXPECT_EQ(b, c);
-  b = c;       // Assignment
+  b = c;  // Assignment
   EXPECT_EQ(a, b);
   EXPECT_EQ(a, c);
   EXPECT_EQ(b, c);
@@ -176,42 +193,67 @@ TEST(Time, RelationalOperators) {
   constexpr absl::Time t2 = absl::FromUnixNanos(1);
   constexpr absl::Time t3 = absl::FromUnixNanos(2);
 
-  static_assert(absl::Time() == t1, "");
-  static_assert(t1 == t1, "");
-  static_assert(t2 == t2, "");
-  static_assert(t3 == t3, "");
-
-  static_assert(t1 < t2, "");
-  static_assert(t2 < t3, "");
-  static_assert(t1 < t3, "");
-
-  static_assert(t1 <= t1, "");
-  static_assert(t1 <= t2, "");
-  static_assert(t2 <= t2, "");
-  static_assert(t2 <= t3, "");
-  static_assert(t3 <= t3, "");
-  static_assert(t1 <= t3, "");
-
-  static_assert(t2 > t1, "");
-  static_assert(t3 > t2, "");
-  static_assert(t3 > t1, "");
-
-  static_assert(t2 >= t2, "");
-  static_assert(t2 >= t1, "");
-  static_assert(t3 >= t3, "");
-  static_assert(t3 >= t2, "");
-  static_assert(t1 >= t1, "");
-  static_assert(t3 >= t1, "");
+  static_assert(absl::UnixEpoch() == t1);
+  static_assert(t1 == t1);
+  static_assert(t2 == t2);
+  static_assert(t3 == t3);
+
+  static_assert(t1 < t2);
+  static_assert(t2 < t3);
+  static_assert(t1 < t3);
+
+  static_assert(t1 <= t1);
+  static_assert(t1 <= t2);
+  static_assert(t2 <= t2);
+  static_assert(t2 <= t3);
+  static_assert(t3 <= t3);
+  static_assert(t1 <= t3);
+
+  static_assert(t2 > t1);
+  static_assert(t3 > t2);
+  static_assert(t3 > t1);
+
+  static_assert(t2 >= t2);
+  static_assert(t2 >= t1);
+  static_assert(t3 >= t3);
+  static_assert(t3 >= t2);
+  static_assert(t1 >= t1);
+  static_assert(t3 >= t1);
+
+#ifdef ABSL_INTERNAL_TIME_HAS_THREE_WAY_COMPARISON
+
+  static_assert((t1 <=> t1) == std::strong_ordering::equal);
+  static_assert((t2 <=> t2) == std::strong_ordering::equal);
+  static_assert((t3 <=> t3) == std::strong_ordering::equal);
+
+  static_assert((t1 <=> t2) == std::strong_ordering::less);
+  static_assert((t2 <=> t3) == std::strong_ordering::less);
+  static_assert((t1 <=> t3) == std::strong_ordering::less);
+
+  static_assert((t2 <=> t1) == std::strong_ordering::greater);
+  static_assert((t3 <=> t2) == std::strong_ordering::greater);
+  static_assert((t3 <=> t1) == std::strong_ordering::greater);
+
+#endif  // ABSL_INTERNAL_TIME_HAS_THREE_WAY_COMPARISON
 }
 
 TEST(Time, Infinity) {
   constexpr absl::Time ifuture = absl::InfiniteFuture();
   constexpr absl::Time ipast = absl::InfinitePast();
 
-  static_assert(ifuture == ifuture, "");
-  static_assert(ipast == ipast, "");
-  static_assert(ipast < ifuture, "");
-  static_assert(ifuture > ipast, "");
+  static_assert(ifuture == ifuture);
+  static_assert(ipast == ipast);
+  static_assert(ipast < ifuture);
+  static_assert(ifuture > ipast);
+
+#ifdef ABSL_INTERNAL_TIME_HAS_THREE_WAY_COMPARISON
+
+  static_assert((ifuture <=> ifuture) == std::strong_ordering::equal);
+  static_assert((ipast <=> ipast) == std::strong_ordering::equal);
+  static_assert((ipast <=> ifuture) == std::strong_ordering::less);
+  static_assert((ifuture <=> ipast) == std::strong_ordering::greater);
+
+#endif  // ABSL_INTERNAL_TIME_HAS_THREE_WAY_COMPARISON
 
   // Arithmetic saturates
   EXPECT_EQ(ifuture, ifuture + absl::Seconds(1));
@@ -225,8 +267,20 @@ TEST(Time, Infinity) {
   EXPECT_EQ(-absl::InfiniteDuration(), ipast - ipast);
 
   constexpr absl::Time t = absl::UnixEpoch();  // Any finite time.
-  static_assert(t < ifuture, "");
-  static_assert(t > ipast, "");
+  static_assert(t < ifuture);
+  static_assert(t > ipast);
+
+#ifdef ABSL_INTERNAL_TIME_HAS_THREE_WAY_COMPARISON
+
+  static_assert((t <=> ifuture) == std::strong_ordering::less);
+  static_assert((t <=> ipast) == std::strong_ordering::greater);
+  static_assert((ipast <=> t) == std::strong_ordering::less);
+  static_assert((ifuture <=> t) == std::strong_ordering::greater);
+
+#endif  // ABSL_INTERNAL_TIME_HAS_THREE_WAY_COMPARISON
+
+  EXPECT_EQ(ifuture, t + absl::InfiniteDuration());
+  EXPECT_EQ(ipast, t - absl::InfiniteDuration());
 }
 
 TEST(Time, FloorConversion) {
@@ -252,7 +306,7 @@ TEST(Time, FloorConversion) {
   EXPECT_EQ(1, absl::ToUnixNanos(absl::UnixEpoch() + absl::Nanoseconds(3) / 2));
   EXPECT_EQ(1, absl::ToUnixNanos(absl::UnixEpoch() + absl::Nanoseconds(1)));
   EXPECT_EQ(0, absl::ToUnixNanos(absl::UnixEpoch() + absl::Nanoseconds(1) / 2));
-  EXPECT_EQ(0, absl::ToUnixNanos(absl::UnixEpoch() + absl::Nanoseconds(0)));
+  EXPECT_EQ(0, absl::ToUnixNanos(absl::UnixEpoch() + absl::ZeroDuration()));
   EXPECT_EQ(-1,
             absl::ToUnixNanos(absl::UnixEpoch() - absl::Nanoseconds(1) / 2));
   EXPECT_EQ(-1, absl::ToUnixNanos(absl::UnixEpoch() - absl::Nanoseconds(1)));
@@ -269,7 +323,7 @@ TEST(Time, FloorConversion) {
   EXPECT_EQ(0,
             absl::ToUniversal(absl::UniversalEpoch() + absl::Nanoseconds(1)));
   EXPECT_EQ(0,
-            absl::ToUniversal(absl::UniversalEpoch() + absl::Nanoseconds(0)));
+            absl::ToUniversal(absl::UniversalEpoch() + absl::ZeroDuration()));
   EXPECT_EQ(-1,
             absl::ToUniversal(absl::UniversalEpoch() + absl::Nanoseconds(-1)));
   EXPECT_EQ(-1,
@@ -286,13 +340,13 @@ TEST(Time, FloorConversion) {
   } to_ts[] = {
       {absl::FromUnixSeconds(1) + absl::Nanoseconds(1), {1, 1}},
       {absl::FromUnixSeconds(1) + absl::Nanoseconds(1) / 2, {1, 0}},
-      {absl::FromUnixSeconds(1) + absl::Nanoseconds(0), {1, 0}},
-      {absl::FromUnixSeconds(0) + absl::Nanoseconds(0), {0, 0}},
+      {absl::FromUnixSeconds(1) + absl::ZeroDuration(), {1, 0}},
+      {absl::FromUnixSeconds(0) + absl::ZeroDuration(), {0, 0}},
       {absl::FromUnixSeconds(0) - absl::Nanoseconds(1) / 2, {-1, 999999999}},
       {absl::FromUnixSeconds(0) - absl::Nanoseconds(1), {-1, 999999999}},
       {absl::FromUnixSeconds(-1) + absl::Nanoseconds(1), {-1, 1}},
       {absl::FromUnixSeconds(-1) + absl::Nanoseconds(1) / 2, {-1, 0}},
-      {absl::FromUnixSeconds(-1) + absl::Nanoseconds(0), {-1, 0}},
+      {absl::FromUnixSeconds(-1) + absl::ZeroDuration(), {-1, 0}},
       {absl::FromUnixSeconds(-1) - absl::Nanoseconds(1) / 2, {-2, 999999999}},
   };
   for (const auto& test : to_ts) {
@@ -303,12 +357,12 @@ TEST(Time, FloorConversion) {
     absl::Time t;
   } from_ts[] = {
       {{1, 1}, absl::FromUnixSeconds(1) + absl::Nanoseconds(1)},
-      {{1, 0}, absl::FromUnixSeconds(1) + absl::Nanoseconds(0)},
-      {{0, 0}, absl::FromUnixSeconds(0) + absl::Nanoseconds(0)},
+      {{1, 0}, absl::FromUnixSeconds(1) + absl::ZeroDuration()},
+      {{0, 0}, absl::FromUnixSeconds(0) + absl::ZeroDuration()},
       {{0, -1}, absl::FromUnixSeconds(0) - absl::Nanoseconds(1)},
       {{-1, 999999999}, absl::FromUnixSeconds(0) - absl::Nanoseconds(1)},
       {{-1, 1}, absl::FromUnixSeconds(-1) + absl::Nanoseconds(1)},
-      {{-1, 0}, absl::FromUnixSeconds(-1) + absl::Nanoseconds(0)},
+      {{-1, 0}, absl::FromUnixSeconds(-1) + absl::ZeroDuration()},
       {{-1, -1}, absl::FromUnixSeconds(-1) - absl::Nanoseconds(1)},
       {{-2, 999999999}, absl::FromUnixSeconds(-1) - absl::Nanoseconds(1)},
   };
@@ -316,36 +370,36 @@ TEST(Time, FloorConversion) {
     EXPECT_EQ(test.t, absl::TimeFromTimespec(test.ts));
   }
 
-  // Tests ToTimeval()/TimeFromTimeval() (same as timespec above)
+  // Tests  absl::ToTimeval()/TimeFromTimeval() (same as timespec above)
   const struct {
     absl::Time t;
     timeval tv;
   } to_tv[] = {
       {absl::FromUnixSeconds(1) + absl::Microseconds(1), {1, 1}},
       {absl::FromUnixSeconds(1) + absl::Microseconds(1) / 2, {1, 0}},
-      {absl::FromUnixSeconds(1) + absl::Microseconds(0), {1, 0}},
-      {absl::FromUnixSeconds(0) + absl::Microseconds(0), {0, 0}},
+      {absl::FromUnixSeconds(1) + absl::ZeroDuration(), {1, 0}},
+      {absl::FromUnixSeconds(0) + absl::ZeroDuration(), {0, 0}},
       {absl::FromUnixSeconds(0) - absl::Microseconds(1) / 2, {-1, 999999}},
       {absl::FromUnixSeconds(0) - absl::Microseconds(1), {-1, 999999}},
       {absl::FromUnixSeconds(-1) + absl::Microseconds(1), {-1, 1}},
       {absl::FromUnixSeconds(-1) + absl::Microseconds(1) / 2, {-1, 0}},
-      {absl::FromUnixSeconds(-1) + absl::Microseconds(0), {-1, 0}},
+      {absl::FromUnixSeconds(-1) + absl::ZeroDuration(), {-1, 0}},
       {absl::FromUnixSeconds(-1) - absl::Microseconds(1) / 2, {-2, 999999}},
   };
   for (const auto& test : to_tv) {
-    EXPECT_THAT(ToTimeval(test.t), TimevalMatcher(test.tv));
+    EXPECT_THAT(absl::ToTimeval(test.t), TimevalMatcher(test.tv));
   }
   const struct {
     timeval tv;
     absl::Time t;
   } from_tv[] = {
       {{1, 1}, absl::FromUnixSeconds(1) + absl::Microseconds(1)},
-      {{1, 0}, absl::FromUnixSeconds(1) + absl::Microseconds(0)},
-      {{0, 0}, absl::FromUnixSeconds(0) + absl::Microseconds(0)},
+      {{1, 0}, absl::FromUnixSeconds(1) + absl::ZeroDuration()},
+      {{0, 0}, absl::FromUnixSeconds(0) + absl::ZeroDuration()},
       {{0, -1}, absl::FromUnixSeconds(0) - absl::Microseconds(1)},
       {{-1, 999999}, absl::FromUnixSeconds(0) - absl::Microseconds(1)},
       {{-1, 1}, absl::FromUnixSeconds(-1) + absl::Microseconds(1)},
-      {{-1, 0}, absl::FromUnixSeconds(-1) + absl::Microseconds(0)},
+      {{-1, 0}, absl::FromUnixSeconds(-1) + absl::ZeroDuration()},
       {{-1, -1}, absl::FromUnixSeconds(-1) - absl::Microseconds(1)},
       {{-2, 999999}, absl::FromUnixSeconds(-1) - absl::Microseconds(1)},
   };
@@ -357,19 +411,21 @@ TEST(Time, FloorConversion) {
   const int64_t min_plus_1 = std::numeric_limits<int64_t>::min() + 1;
   EXPECT_EQ(min_plus_1, absl::ToUnixSeconds(absl::FromUnixSeconds(min_plus_1)));
   EXPECT_EQ(std::numeric_limits<int64_t>::min(),
-            absl::ToUnixSeconds(
-                absl::FromUnixSeconds(min_plus_1) - absl::Nanoseconds(1) / 2));
+            absl::ToUnixSeconds(absl::FromUnixSeconds(min_plus_1) -
+                                absl::Nanoseconds(1) / 2));
 
   // Tests flooring near positive infinity.
   EXPECT_EQ(std::numeric_limits<int64_t>::max(),
-            absl::ToUnixSeconds(absl::FromUnixSeconds(
-                std::numeric_limits<int64_t>::max()) + absl::Nanoseconds(1) / 2));
+            absl::ToUnixSeconds(
+                absl::FromUnixSeconds(std::numeric_limits<int64_t>::max()) +
+                absl::Nanoseconds(1) / 2));
   EXPECT_EQ(std::numeric_limits<int64_t>::max(),
             absl::ToUnixSeconds(
                 absl::FromUnixSeconds(std::numeric_limits<int64_t>::max())));
   EXPECT_EQ(std::numeric_limits<int64_t>::max() - 1,
-            absl::ToUnixSeconds(absl::FromUnixSeconds(
-                std::numeric_limits<int64_t>::max()) - absl::Nanoseconds(1) / 2));
+            absl::ToUnixSeconds(
+                absl::FromUnixSeconds(std::numeric_limits<int64_t>::max()) -
+                absl::Nanoseconds(1) / 2));
 }
 
 TEST(Time, RoundtripConversion) {
@@ -433,7 +489,7 @@ TEST(Time, RoundtripConversion) {
                              testing::Eq)
       << now_time_t;
 
-  // TimeFromTimeval() and ToTimeval()
+  // TimeFromTimeval() and  absl::ToTimeval()
   timeval tv;
   tv.tv_sec = -1;
   tv.tv_usec = 0;
@@ -575,6 +631,50 @@ TEST(Time, ToChronoTime) {
             absl::ToChronoTime(absl::UnixEpoch() - tick));
 }
 
+// Check that absl::int128 works as a std::chrono::duration representation.
+TEST(Time, Chrono128) {
+  // Define a std::chrono::time_point type whose time[sic]_since_epoch() is
+  // a signed 128-bit count of attoseconds. This has a range and resolution
+  // (currently) beyond those of absl::Time, and undoubtedly also beyond those
+  // of std::chrono::system_clock::time_point.
+  //
+  // Note: The to/from-chrono support should probably be updated to handle
+  // such wide representations.
+  using Timestamp =
+      std::chrono::time_point<std::chrono::system_clock,
+                              std::chrono::duration<absl::int128, std::atto>>;
+
+  // Expect that we can round-trip the std::chrono::system_clock::time_point
+  // extremes through both absl::Time and Timestamp, and that Timestamp can
+  // handle the (current) absl::Time extremes.
+  //
+  // Note: We should use std::chrono::floor() instead of time_point_cast(),
+  // but floor() is only available since c++17.
+  for (const auto tp : {std::chrono::system_clock::time_point::min(),
+                        std::chrono::system_clock::time_point::max()}) {
+    EXPECT_EQ(tp, absl::ToChronoTime(absl::FromChrono(tp)));
+    EXPECT_EQ(tp, std::chrono::time_point_cast<
+                      std::chrono::system_clock::time_point::duration>(
+                      std::chrono::time_point_cast<Timestamp::duration>(tp)));
+  }
+  Timestamp::duration::rep v = std::numeric_limits<int64_t>::min();
+  v *= Timestamp::duration::period::den;
+  auto ts = Timestamp(Timestamp::duration(v));
+  ts += std::chrono::duration<int64_t, std::atto>(0);
+  EXPECT_EQ(std::numeric_limits<int64_t>::min(),
+            ts.time_since_epoch().count() / Timestamp::duration::period::den);
+  EXPECT_EQ(0,
+            ts.time_since_epoch().count() % Timestamp::duration::period::den);
+  v = std::numeric_limits<int64_t>::max();
+  v *= Timestamp::duration::period::den;
+  ts = Timestamp(Timestamp::duration(v));
+  ts += std::chrono::duration<int64_t, std::atto>(999999999750000000);
+  EXPECT_EQ(std::numeric_limits<int64_t>::max(),
+            ts.time_since_epoch().count() / Timestamp::duration::period::den);
+  EXPECT_EQ(999999999750000000,
+            ts.time_since_epoch().count() % Timestamp::duration::period::den);
+}
+
 TEST(Time, TimeZoneAt) {
   const absl::TimeZone nyc =
       absl::time_internal::LoadTimeZone("America/New_York");
@@ -674,14 +774,14 @@ TEST(Time, FromCivilUTC) {
 TEST(Time, ToTM) {
   const absl::TimeZone utc = absl::UTCTimeZone();
 
-  // Compares the results of ToTM() to gmtime_r() for lots of times over the
-  // course of a few days.
+  // Compares the results of absl::ToTM() to gmtime_r() for lots of times over
+  // the course of a few days.
   const absl::Time start =
       absl::FromCivil(absl::CivilSecond(2014, 1, 2, 3, 4, 5), utc);
   const absl::Time end =
       absl::FromCivil(absl::CivilSecond(2014, 1, 5, 3, 4, 5), utc);
   for (absl::Time t = start; t < end; t += absl::Seconds(30)) {
-    const struct tm tm_bt = ToTM(t, utc);
+    const struct tm tm_bt = absl::ToTM(t, utc);
     const time_t tt = absl::ToTimeT(t);
     struct tm tm_lc;
 #ifdef _WIN32
@@ -706,16 +806,16 @@ TEST(Time, ToTM) {
   const absl::TimeZone nyc =
       absl::time_internal::LoadTimeZone("America/New_York");
   absl::Time t = absl::FromCivil(absl::CivilSecond(2014, 3, 1, 0, 0, 0), nyc);
-  struct tm tm = ToTM(t, nyc);
+  struct tm tm = absl::ToTM(t, nyc);
   EXPECT_FALSE(tm.tm_isdst);
 
   // Checks that the tm_isdst field is correct when in daylight time.
   t = absl::FromCivil(absl::CivilSecond(2014, 4, 1, 0, 0, 0), nyc);
-  tm = ToTM(t, nyc);
+  tm = absl::ToTM(t, nyc);
   EXPECT_TRUE(tm.tm_isdst);
 
   // Checks overflow.
-  tm = ToTM(absl::InfiniteFuture(), nyc);
+  tm = absl::ToTM(absl::InfiniteFuture(), nyc);
   EXPECT_EQ(std::numeric_limits<int>::max() - 1900, tm.tm_year);
   EXPECT_EQ(11, tm.tm_mon);
   EXPECT_EQ(31, tm.tm_mday);
@@ -727,7 +827,7 @@ TEST(Time, ToTM) {
   EXPECT_FALSE(tm.tm_isdst);
 
   // Checks underflow.
-  tm = ToTM(absl::InfinitePast(), nyc);
+  tm = absl::ToTM(absl::InfinitePast(), nyc);
   EXPECT_EQ(std::numeric_limits<int>::min(), tm.tm_year);
   EXPECT_EQ(0, tm.tm_mon);
   EXPECT_EQ(1, tm.tm_mday);
@@ -753,13 +853,13 @@ TEST(Time, FromTM) {
   tm.tm_min = 2;
   tm.tm_sec = 3;
   tm.tm_isdst = -1;
-  absl::Time t = FromTM(tm, nyc);
+  absl::Time t = absl::FromTM(tm, nyc);
   EXPECT_EQ("2014-06-28T01:02:03-04:00", absl::FormatTime(t, nyc));  // DST
   tm.tm_isdst = 0;
-  t = FromTM(tm, nyc);
+  t = absl::FromTM(tm, nyc);
   EXPECT_EQ("2014-06-28T01:02:03-04:00", absl::FormatTime(t, nyc));  // DST
   tm.tm_isdst = 1;
-  t = FromTM(tm, nyc);
+  t = absl::FromTM(tm, nyc);
   EXPECT_EQ("2014-06-28T01:02:03-04:00", absl::FormatTime(t, nyc));  // DST
 
   // Adjusts tm to refer to an ambiguous time.
@@ -770,13 +870,13 @@ TEST(Time, FromTM) {
   tm.tm_min = 30;
   tm.tm_sec = 42;
   tm.tm_isdst = -1;
-  t = FromTM(tm, nyc);
+  t = absl::FromTM(tm, nyc);
   EXPECT_EQ("2014-11-02T01:30:42-04:00", absl::FormatTime(t, nyc));  // DST
   tm.tm_isdst = 0;
-  t = FromTM(tm, nyc);
+  t = absl::FromTM(tm, nyc);
   EXPECT_EQ("2014-11-02T01:30:42-05:00", absl::FormatTime(t, nyc));  // STD
   tm.tm_isdst = 1;
-  t = FromTM(tm, nyc);
+  t = absl::FromTM(tm, nyc);
   EXPECT_EQ("2014-11-02T01:30:42-04:00", absl::FormatTime(t, nyc));  // DST
 
   // Adjusts tm to refer to a skipped time.
@@ -787,14 +887,38 @@ TEST(Time, FromTM) {
   tm.tm_min = 30;
   tm.tm_sec = 42;
   tm.tm_isdst = -1;
-  t = FromTM(tm, nyc);
+  t = absl::FromTM(tm, nyc);
   EXPECT_EQ("2014-03-09T03:30:42-04:00", absl::FormatTime(t, nyc));  // DST
   tm.tm_isdst = 0;
-  t = FromTM(tm, nyc);
+  t = absl::FromTM(tm, nyc);
   EXPECT_EQ("2014-03-09T01:30:42-05:00", absl::FormatTime(t, nyc));  // STD
   tm.tm_isdst = 1;
-  t = FromTM(tm, nyc);
+  t = absl::FromTM(tm, nyc);
   EXPECT_EQ("2014-03-09T03:30:42-04:00", absl::FormatTime(t, nyc));  // DST
+
+  // Adjusts tm to refer to a time with a year larger than 2147483647.
+  tm.tm_year = 2147483647 - 1900 + 1;
+  tm.tm_mon = 6 - 1;
+  tm.tm_mday = 28;
+  tm.tm_hour = 1;
+  tm.tm_min = 2;
+  tm.tm_sec = 3;
+  tm.tm_isdst = -1;
+  t = absl::FromTM(tm, absl::UTCTimeZone());
+  EXPECT_EQ("2147483648-06-28T01:02:03+00:00",
+            absl::FormatTime(t, absl::UTCTimeZone()));
+
+  // Adjusts tm to refer to a time with a very large month.
+  tm.tm_year = 2019 - 1900;
+  tm.tm_mon = 2147483647;
+  tm.tm_mday = 28;
+  tm.tm_hour = 1;
+  tm.tm_min = 2;
+  tm.tm_sec = 3;
+  tm.tm_isdst = -1;
+  t = absl::FromTM(tm, absl::UTCTimeZone());
+  EXPECT_EQ("178958989-08-28T01:02:03+00:00",
+            absl::FormatTime(t, absl::UTCTimeZone()));
 }
 
 TEST(Time, TMRoundTrip) {
@@ -805,8 +929,8 @@ TEST(Time, TMRoundTrip) {
   absl::Time start = absl::FromCivil(absl::CivilHour(2014, 3, 9, 0), nyc);
   absl::Time end = absl::FromCivil(absl::CivilHour(2014, 3, 9, 4), nyc);
   for (absl::Time t = start; t < end; t += absl::Minutes(1)) {
-    struct tm tm = ToTM(t, nyc);
-    absl::Time rt = FromTM(tm, nyc);
+    struct tm tm = absl::ToTM(t, nyc);
+    absl::Time rt = absl::FromTM(tm, nyc);
     EXPECT_EQ(rt, t);
   }
 
@@ -814,8 +938,8 @@ TEST(Time, TMRoundTrip) {
   start = absl::FromCivil(absl::CivilHour(2014, 11, 2, 0), nyc);
   end = absl::FromCivil(absl::CivilHour(2014, 11, 2, 4), nyc);
   for (absl::Time t = start; t < end; t += absl::Minutes(1)) {
-    struct tm tm = ToTM(t, nyc);
-    absl::Time rt = FromTM(tm, nyc);
+    struct tm tm = absl::ToTM(t, nyc);
+    absl::Time rt = absl::FromTM(tm, nyc);
     EXPECT_EQ(rt, t);
   }
 
@@ -823,8 +947,8 @@ TEST(Time, TMRoundTrip) {
   start = absl::FromCivil(absl::CivilHour(2014, 6, 27, 22), nyc);
   end = absl::FromCivil(absl::CivilHour(2014, 6, 28, 4), nyc);
   for (absl::Time t = start; t < end; t += absl::Minutes(1)) {
-    struct tm tm = ToTM(t, nyc);
-    absl::Time rt = FromTM(tm, nyc);
+    struct tm tm = absl::ToTM(t, nyc);
+    absl::Time rt = absl::FromTM(tm, nyc);
     EXPECT_EQ(rt, t);
   }
 }
@@ -912,30 +1036,30 @@ TEST(Time, ConversionSaturation) {
   tv.tv_sec = max_timeval_sec;
   tv.tv_usec = 999998;
   t = absl::TimeFromTimeval(tv);
-  tv = ToTimeval(t);
+  tv = absl::ToTimeval(t);
   EXPECT_EQ(max_timeval_sec, tv.tv_sec);
   EXPECT_EQ(999998, tv.tv_usec);
   t += absl::Microseconds(1);
-  tv = ToTimeval(t);
+  tv = absl::ToTimeval(t);
   EXPECT_EQ(max_timeval_sec, tv.tv_sec);
   EXPECT_EQ(999999, tv.tv_usec);
   t += absl::Microseconds(1);  // no effect
-  tv = ToTimeval(t);
+  tv = absl::ToTimeval(t);
   EXPECT_EQ(max_timeval_sec, tv.tv_sec);
   EXPECT_EQ(999999, tv.tv_usec);
 
   tv.tv_sec = min_timeval_sec;
   tv.tv_usec = 1;
   t = absl::TimeFromTimeval(tv);
-  tv = ToTimeval(t);
+  tv = absl::ToTimeval(t);
   EXPECT_EQ(min_timeval_sec, tv.tv_sec);
   EXPECT_EQ(1, tv.tv_usec);
   t -= absl::Microseconds(1);
-  tv = ToTimeval(t);
+  tv = absl::ToTimeval(t);
   EXPECT_EQ(min_timeval_sec, tv.tv_sec);
   EXPECT_EQ(0, tv.tv_usec);
   t -= absl::Microseconds(1);  // no effect
-  tv = ToTimeval(t);
+  tv = absl::ToTimeval(t);
   EXPECT_EQ(min_timeval_sec, tv.tv_sec);
   EXPECT_EQ(0, tv.tv_usec);
 
@@ -976,15 +1100,15 @@ TEST(Time, ConversionSaturation) {
 
   // Checks how TimeZone::At() saturates on infinities.
   auto ci = utc.At(absl::InfiniteFuture());
-  EXPECT_CIVIL_INFO(ci, std::numeric_limits<int64_t>::max(), 12, 31, 23,
-                            59, 59, 0, false);
+  EXPECT_CIVIL_INFO(ci, std::numeric_limits<int64_t>::max(), 12, 31, 23, 59, 59,
+                    0, false);
   EXPECT_EQ(absl::InfiniteDuration(), ci.subsecond);
   EXPECT_EQ(absl::Weekday::thursday, absl::GetWeekday(ci.cs));
   EXPECT_EQ(365, absl::GetYearDay(ci.cs));
   EXPECT_STREQ("-00", ci.zone_abbr);  // artifact of TimeZone::At()
   ci = utc.At(absl::InfinitePast());
-  EXPECT_CIVIL_INFO(ci, std::numeric_limits<int64_t>::min(), 1, 1, 0, 0,
-                            0, 0, false);
+  EXPECT_CIVIL_INFO(ci, std::numeric_limits<int64_t>::min(), 1, 1, 0, 0, 0, 0,
+                    false);
   EXPECT_EQ(-absl::InfiniteDuration(), ci.subsecond);
   EXPECT_EQ(absl::Weekday::sunday, absl::GetWeekday(ci.cs));
   EXPECT_EQ(1, absl::GetYearDay(ci.cs));
@@ -998,7 +1122,8 @@ TEST(Time, ConversionSaturation) {
   EXPECT_EQ("292277026596-12-04T15:30:07+00:00",
             absl::FormatTime(absl::RFC3339_full, t, utc));
   EXPECT_EQ(
-      absl::UnixEpoch() + absl::Seconds(std::numeric_limits<int64_t>::max()), t);
+      absl::UnixEpoch() + absl::Seconds(std::numeric_limits<int64_t>::max()),
+      t);
 
   // Checks that we can also get the maximal Time value for a far-east zone.
   const absl::TimeZone plus14 = absl::FixedTimeZone(14 * 60 * 60);
@@ -1006,7 +1131,8 @@ TEST(Time, ConversionSaturation) {
   EXPECT_EQ("292277026596-12-05T05:30:07+14:00",
             absl::FormatTime(absl::RFC3339_full, t, plus14));
   EXPECT_EQ(
-      absl::UnixEpoch() + absl::Seconds(std::numeric_limits<int64_t>::max()), t);
+      absl::UnixEpoch() + absl::Seconds(std::numeric_limits<int64_t>::max()),
+      t);
 
   // One second later should push us to infinity.
   t = absl::FromCivil(absl::CivilSecond(292277026596, 12, 4, 15, 30, 8), utc);
@@ -1020,7 +1146,8 @@ TEST(Time, ConversionSaturation) {
   EXPECT_EQ("-292277022657-01-27T08:29:52+00:00",
             absl::FormatTime(absl::RFC3339_full, t, utc));
   EXPECT_EQ(
-      absl::UnixEpoch() + absl::Seconds(std::numeric_limits<int64_t>::min()), t);
+      absl::UnixEpoch() + absl::Seconds(std::numeric_limits<int64_t>::min()),
+      t);
 
   // Checks that we can also get the minimal Time value for a far-west zone.
   const absl::TimeZone minus12 = absl::FixedTimeZone(-12 * 60 * 60);
@@ -1029,7 +1156,8 @@ TEST(Time, ConversionSaturation) {
   EXPECT_EQ("-292277022657-01-26T20:29:52-12:00",
             absl::FormatTime(absl::RFC3339_full, t, minus12));
   EXPECT_EQ(
-      absl::UnixEpoch() + absl::Seconds(std::numeric_limits<int64_t>::min()), t);
+      absl::UnixEpoch() + absl::Seconds(std::numeric_limits<int64_t>::min()),
+      t);
 
   // One second before should push us to -infinity.
   t = absl::FromCivil(absl::CivilSecond(-292277022657, 1, 27, 8, 29, 51), utc);
@@ -1102,14 +1230,13 @@ TEST(Time, LegacyDateTime) {
   const int kMin = std::numeric_limits<int>::min();
   absl::Time t;
 
-  t = absl::FromDateTime(std::numeric_limits<absl::civil_year_t>::max(),
-                         kMax, kMax, kMax, kMax, kMax, utc);
+  t = absl::FromDateTime(std::numeric_limits<absl::civil_year_t>::max(), kMax,
+                         kMax, kMax, kMax, kMax, utc);
   EXPECT_EQ("infinite-future",
             absl::FormatTime(ymdhms, t, utc));  // no overflow
-  t = absl::FromDateTime(std::numeric_limits<absl::civil_year_t>::min(),
-                         kMin, kMin, kMin, kMin, kMin, utc);
-  EXPECT_EQ("infinite-past",
-            absl::FormatTime(ymdhms, t, utc));  // no overflow
+  t = absl::FromDateTime(std::numeric_limits<absl::civil_year_t>::min(), kMin,
+                         kMin, kMin, kMin, kMin, utc);
+  EXPECT_EQ("infinite-past", absl::FormatTime(ymdhms, t, utc));  // no overflow
 
   // Check normalization.
   EXPECT_TRUE(absl::ConvertDateTime(2013, 10, 32, 8, 30, 0, utc).normalized);
@@ -1202,4 +1329,36 @@ TEST(Time, PrevTransitionNYC) {
   // We have a transition but we don't know which one.
 }
 
+TEST(Time, AbslStringify) {
+  // FormatTime is already well tested, so just use one test case here to
+  // verify that StrFormat("%v", t) works as expected.
+  absl::Time t = absl::Now();
+  EXPECT_EQ(absl::StrFormat("%v", t), absl::FormatTime(t));
+}
+
+TEST(Time, SupportsHash) {
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly({
+      absl::UTCTimeZone(),
+      absl::FixedTimeZone(-8 * 60 * 60),
+      absl::UTCTimeZone(),
+  }));
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly({
+      absl::Now(),
+      absl::UnixEpoch(),
+      absl::UnixEpoch() + absl::Seconds(60),
+      absl::UnixEpoch() + absl::Minutes(1),
+      absl::InfiniteFuture(),
+      absl::InfinitePast(),
+  }));
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly({
+      absl::Seconds(1),
+      absl::Seconds(60),
+      absl::Minutes(1),
+      absl::InfiniteDuration(),
+      -absl::InfiniteDuration(),
+  }));
+}
+
 }  // namespace
diff --git a/absl/time/time_zone_test.cc b/absl/time/time_zone_test.cc
index 8f1e74aca54..229fcfccb0a 100644
--- a/absl/time/time_zone_test.cc
+++ b/absl/time/time_zone_test.cc
@@ -88,7 +88,7 @@ TEST(TimeZone, Failures) {
   EXPECT_FALSE(LoadTimeZone("Invalid/TimeZone", &tz));
   EXPECT_EQ(absl::UTCTimeZone(), tz);  // guaranteed fallback to UTC
 
-  // Loading an empty std::string timezone should fail.
+  // Loading an empty string timezone should fail.
   tz = absl::time_internal::LoadTimeZone("America/Los_Angeles");
   EXPECT_FALSE(LoadTimeZone("", &tz));
   EXPECT_EQ(absl::UTCTimeZone(), tz);  // guaranteed fallback to UTC
diff --git a/absl/types/BUILD.bazel b/absl/types/BUILD.bazel
index 236c24bc34d..3ccd28e608f 100644
--- a/absl/types/BUILD.bazel
+++ b/absl/types/BUILD.bazel
@@ -12,9 +12,10 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
-#
 
-load("@rules_cc//cc:defs.bzl", "cc_library", "cc_test")
+load("@rules_cc//cc:cc_binary.bzl", "cc_binary")
+load("@rules_cc//cc:cc_library.bzl", "cc_library")
+load("@rules_cc//cc:cc_test.bzl", "cc_test")
 load(
     "//absl:copts/configure_copts.bzl",
     "ABSL_DEFAULT_COPTS",
@@ -22,78 +23,56 @@ load(
     "ABSL_TEST_COPTS",
 )
 
-package(default_visibility = ["//visibility:public"])
+package(
+    default_visibility = ["//visibility:private"],
+    features = [
+        "header_modules",
+        "layering_check",
+        "parse_headers",
+    ],
+)
 
-licenses(["notice"])  # Apache 2.0
+licenses(["notice"])
 
 cc_library(
     name = "any",
     hdrs = ["any.h"],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
     deps = [
-        ":bad_any_cast",
         "//absl/base:config",
         "//absl/base:core_headers",
-        "//absl/meta:type_traits",
         "//absl/utility",
     ],
 )
 
 cc_library(
-    name = "bad_any_cast",
-    hdrs = ["bad_any_cast.h"],
+    name = "source_location",
+    srcs = ["source_location.cc"],
+    hdrs = ["source_location.h"],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
     deps = [
-        ":bad_any_cast_impl",
         "//absl/base:config",
-    ],
-)
-
-cc_library(
-    name = "bad_any_cast_impl",
-    srcs = [
-        "bad_any_cast.cc",
-        "bad_any_cast.h",
-    ],
-    copts = ABSL_DEFAULT_COPTS,
-    linkopts = ABSL_DEFAULT_LINKOPTS,
-    visibility = ["//visibility:private"],
-    deps = [
-        "//absl/base:config",
-        "//absl/base:raw_logging_internal",
+        "//absl/base:core_headers",
+        "//absl/base:nullability",
     ],
 )
 
 cc_test(
-    name = "any_test",
+    name = "source_location_test",
     size = "small",
-    srcs = [
-        "any_test.cc",
-    ],
-    copts = ABSL_TEST_COPTS,
-    linkopts = ABSL_DEFAULT_LINKOPTS,
-    deps = [
-        ":any",
-        "//absl/base:config",
-        "//absl/base:exception_testing",
-        "//absl/base:raw_logging_internal",
-        "//absl/container:test_instance_tracker",
-        "@com_google_googletest//:gtest_main",
-    ],
-)
-
-cc_test(
-    name = "any_exception_safety_test",
-    srcs = ["any_exception_safety_test.cc"],
+    srcs = ["source_location_test.cc"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
-        ":any",
+        ":source_location",
         "//absl/base:config",
-        "//absl/base:exception_safety_testing",
-        "@com_google_googletest//:gtest_main",
+        "//absl/strings:string_view",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
@@ -107,10 +86,15 @@ cc_library(
     ],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
     deps = [
         "//absl/algorithm",
+        "//absl/base:config",
         "//absl/base:core_headers",
+        "//absl/base:hardening",
+        "//absl/base:nullability",
         "//absl/base:throw_delegate",
+        "//absl/hash:weakly_mixed_integer",
         "//absl/meta:type_traits",
     ],
 )
@@ -128,98 +112,96 @@ cc_test(
         "//absl/base:exception_testing",
         "//absl/container:fixed_array",
         "//absl/container:inlined_vector",
+        "//absl/hash",
         "//absl/hash:hash_testing",
+        "//absl/meta:type_traits",
         "//absl/strings",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
 cc_library(
-    name = "optional",
-    srcs = ["internal/optional.h"],
-    hdrs = ["optional.h"],
-    copts = ABSL_DEFAULT_COPTS,
-    linkopts = ABSL_DEFAULT_LINKOPTS,
-    deps = [
-        ":bad_optional_access",
-        "//absl/base:base_internal",
-        "//absl/base:config",
-        "//absl/base:core_headers",
-        "//absl/memory",
-        "//absl/meta:type_traits",
-        "//absl/utility",
+    name = "any_span",
+    srcs = ["internal/any_span.h"],
+    hdrs = [
+        "any_span.h",
     ],
-)
-
-cc_library(
-    name = "bad_optional_access",
-    srcs = ["bad_optional_access.cc"],
-    hdrs = ["bad_optional_access.h"],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
     deps = [
         "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:hardening",
+        "//absl/base:nullability",
         "//absl/base:raw_logging_internal",
+        "//absl/base:throw_delegate",
+        "//absl/meta:type_traits",
     ],
 )
 
-cc_library(
-    name = "bad_variant_access",
-    srcs = ["bad_variant_access.cc"],
-    hdrs = ["bad_variant_access.h"],
-    copts = ABSL_DEFAULT_COPTS,
+cc_test(
+    name = "any_span_test",
+    size = "small",
+    srcs = ["any_span_test.cc"],
+    copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
+        ":any_span",
+        ":span",
         "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:exception_testing",
         "//absl/base:raw_logging_internal",
+        "//absl/hash:hash_testing",
+        "//absl/meta:type_traits",
+        "//absl/strings",
+        "//absl/strings:string_view",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
-cc_test(
-    name = "optional_test",
-    size = "small",
-    srcs = [
-        "optional_test.cc",
-    ],
-    copts = ABSL_TEST_COPTS,
-    linkopts = ABSL_DEFAULT_LINKOPTS,
+cc_binary(
+    name = "any_span_benchmark",
+    testonly = True,
+    srcs = ["any_span_benchmark.cc"],
+    tags = ["benchmark"],
     deps = [
-        ":optional",
+        ":any_span",
+        ":span",
         "//absl/base:config",
         "//absl/base:raw_logging_internal",
-        "//absl/meta:type_traits",
+        "//absl/flags:flag",
         "//absl/strings",
-        "@com_google_googletest//:gtest_main",
+        "//absl/strings:string_view",
+        "@google_benchmark//:benchmark_main",
     ],
 )
 
-cc_test(
-    name = "optional_exception_safety_test",
-    srcs = [
-        "optional_exception_safety_test.cc",
-    ],
-    copts = ABSL_TEST_COPTS,
+cc_library(
+    name = "optional",
+    hdrs = ["optional.h"],
+    copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
     deps = [
-        ":optional",
         "//absl/base:config",
-        "//absl/base:exception_safety_testing",
-        "@com_google_googletest//:gtest_main",
+        "//absl/base:core_headers",
+        "//absl/utility",
     ],
 )
 
 cc_library(
     name = "variant",
-    srcs = ["internal/variant.h"],
     hdrs = ["variant.h"],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
     deps = [
-        ":bad_variant_access",
-        "//absl/base:base_internal",
         "//absl/base:config",
         "//absl/base:core_headers",
-        "//absl/meta:type_traits",
         "//absl/utility",
     ],
 )
@@ -232,67 +214,63 @@ cc_test(
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":variant",
-        "//absl/base:config",
-        "//absl/base:core_headers",
-        "//absl/memory",
-        "//absl/meta:type_traits",
-        "//absl/strings",
-        "@com_google_googletest//:gtest_main",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
-cc_test(
-    name = "variant_benchmark",
-    srcs = [
-        "variant_benchmark.cc",
-    ],
-    copts = ABSL_TEST_COPTS,
+cc_library(
+    name = "compare",
+    hdrs = ["compare.h"],
+    copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
-    tags = ["benchmark"],
+    visibility = ["//visibility:public"],
     deps = [
-        ":variant",
-        "//absl/utility",
-        "@com_github_google_benchmark//:benchmark_main",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/meta:type_traits",
     ],
 )
 
 cc_test(
-    name = "variant_exception_safety_test",
+    name = "compare_test",
     size = "small",
     srcs = [
-        "variant_exception_safety_test.cc",
+        "compare_test.cc",
     ],
     copts = ABSL_TEST_COPTS,
-    linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
-        ":variant",
-        "//absl/base:config",
-        "//absl/base:exception_safety_testing",
-        "//absl/memory",
-        "@com_google_googletest//:gtest_main",
+        ":compare",
+        "//absl/base",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
 
 cc_library(
-    name = "compare",
-    hdrs = ["compare.h"],
+    name = "optional_ref",
+    hdrs = ["optional_ref.h"],
     copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
     deps = [
+        "//absl/base:config",
         "//absl/base:core_headers",
-        "//absl/meta:type_traits",
+        "//absl/base:hardening",
     ],
 )
 
 cc_test(
-    name = "compare_test",
+    name = "optional_ref_test",
     size = "small",
-    srcs = [
-        "compare_test.cc",
-    ],
+    srcs = ["optional_ref_test.cc"],
     copts = ABSL_TEST_COPTS,
     deps = [
-        ":compare",
-        "//absl/base",
-        "@com_google_googletest//:gtest_main",
+        ":optional_ref",
+        "//absl/base:config",
+        "//absl/log",
+        "//absl/strings",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
     ],
 )
diff --git a/absl/types/CMakeLists.txt b/absl/types/CMakeLists.txt
index 952efc34f63..66763976d65 100644
--- a/absl/types/CMakeLists.txt
+++ b/absl/types/CMakeLists.txt
@@ -21,84 +21,50 @@ absl_cc_library(
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
-    absl::bad_any_cast
     absl::config
     absl::core_headers
-    absl::type_traits
     absl::utility
   PUBLIC
 )
 
 absl_cc_library(
   NAME
-    bad_any_cast
+    any_span
   HDRS
-   "bad_any_cast.h"
-  COPTS
-    ${ABSL_DEFAULT_COPTS}
-  DEPS
-    absl::bad_any_cast_impl
-    absl::config
-  PUBLIC
-)
-
-absl_cc_library(
-  NAME
-    bad_any_cast_impl
+    "any_span.h"
   SRCS
-   "bad_any_cast.h"
-   "bad_any_cast.cc"
+    "internal/any_span.h"
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
     absl::config
+    absl::core_headers
+    absl::hardening
+    absl::nullability
     absl::raw_logging_internal
+    absl::throw_delegate
+    absl::type_traits
+  PUBLIC
 )
 
 absl_cc_test(
   NAME
-    any_test
-  SRCS
-    "any_test.cc"
-  COPTS
-    ${ABSL_TEST_COPTS}
-  DEPS
-    absl::any
-    absl::config
-    absl::exception_testing
-    absl::raw_logging_internal
-    absl::test_instance_tracker
-    gmock_main
-)
-
-absl_cc_test(
-  NAME
-    any_test_noexceptions
+    any_span_test
   SRCS
-    "any_test.cc"
+    "any_span_test.cc"
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
-    absl::any
     absl::config
+    absl::core_headers
     absl::exception_testing
+    absl::hash_testing
     absl::raw_logging_internal
-    absl::test_instance_tracker
-    gmock_main
-)
-
-absl_cc_test(
-  NAME
-    any_exception_safety_test
-  SRCS
-    "any_exception_safety_test.cc"
-  COPTS
-    ${ABSL_TEST_COPTS}
-  DEPS
-    absl::any
-    absl::config
-    absl::exception_safety_testing
-    gmock_main
+    absl::span
+    absl::string_view
+    absl::strings
+    absl::type_traits
+    GTest::gmock_main
 )
 
 absl_cc_library(
@@ -112,9 +78,13 @@ absl_cc_library(
     ${ABSL_DEFAULT_COPTS}
   DEPS
     absl::algorithm
+    absl::config
     absl::core_headers
+    absl::hardening
+    absl::nullability
     absl::throw_delegate
     absl::type_traits
+    absl::weakly_mixed_integer
   PUBLIC
 )
 
@@ -132,10 +102,12 @@ absl_cc_test(
     absl::core_headers
     absl::exception_testing
     absl::fixed_array
+    absl::hash
     absl::inlined_vector
     absl::hash_testing
     absl::strings
-    gmock_main
+    absl::type_traits
+    GTest::gmock_main
 )
 
 absl_cc_test(
@@ -155,7 +127,7 @@ absl_cc_test(
     absl::inlined_vector
     absl::hash_testing
     absl::strings
-    gmock_main
+    GTest::gmock_main
 )
 
 absl_cc_library(
@@ -163,158 +135,146 @@ absl_cc_library(
     optional
   HDRS
     "optional.h"
-  SRCS
-    "internal/optional.h"
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
-    absl::bad_optional_access
-    absl::base_internal
     absl::config
     absl::core_headers
-    absl::memory
-    absl::type_traits
     absl::utility
   PUBLIC
 )
 
 absl_cc_library(
   NAME
-    bad_optional_access
+    variant
   HDRS
-    "bad_optional_access.h"
-  SRCS
-    "bad_optional_access.cc"
+    "variant.h"
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
     absl::config
-    absl::raw_logging_internal
+    absl::core_headers
+    absl::utility
   PUBLIC
 )
 
-absl_cc_library(
+absl_cc_test(
   NAME
-    bad_variant_access
-  HDRS
-    "bad_variant_access.h"
+    variant_test
   SRCS
-    "bad_variant_access.cc"
+    "variant_test.cc"
   COPTS
-    ${ABSL_DEFAULT_COPTS}
+    ${ABSL_TEST_COPTS}
   DEPS
-    absl::config
-    absl::raw_logging_internal
-  PUBLIC
+    absl::variant
+    GTest::gmock_main
 )
 
-absl_cc_test(
+absl_cc_library(
   NAME
-    optional_test
+    source_location
+  HDRS
+    "source_location.h"
   SRCS
-    "optional_test.cc"
+    "source_location.cc"
   COPTS
-    ${ABSL_TEST_COPTS}
+    ${ABSL_DEFAULT_COPTS}
   DEPS
-    absl::optional
     absl::config
-    absl::raw_logging_internal
-    absl::strings
-    absl::type_traits
-    gmock_main
+    absl::core_headers
+    absl::nullability
+  PUBLIC
 )
 
 absl_cc_test(
   NAME
-    optional_exception_safety_test
+    source_location_test
   SRCS
-    "optional_exception_safety_test.cc"
+    "source_location_test.cc"
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
-    absl::optional
     absl::config
-    absl::exception_safety_testing
-    gmock_main
+    absl::source_location
+    GTest::gmock_main
 )
 
 absl_cc_library(
   NAME
-    variant
+    compare
   HDRS
-    "variant.h"
-  SRCS
-    "internal/variant.h"
+    "compare.h"
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
-    absl::bad_variant_access
-    absl::base_internal
     absl::config
     absl::core_headers
     absl::type_traits
-    absl::utility
   PUBLIC
 )
 
 absl_cc_test(
   NAME
-    variant_test
+    compare_test
   SRCS
-    "variant_test.cc"
+    "compare_test.cc"
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
-    absl::variant
-    absl::config
-    absl::core_headers
-    absl::memory
-    absl::type_traits
-    absl::strings
-    gmock_main
+    absl::base
+    absl::compare
+    GTest::gmock_main
 )
 
 absl_cc_library(
   NAME
-    compare
+    internal_optional_ref
   HDRS
-    "compare.h"
+    "optional_ref.h"
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
+    absl::config
     absl::core_headers
-    absl::type_traits
+    absl::hardening
   PUBLIC
 )
 
 absl_cc_test(
   NAME
-    compare_test
+    optional_ref_test
   SRCS
-    "compare_test.cc"
+    "optional_ref_test.cc"
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
     absl::base
-    absl::compare
-    gmock_main
+    absl::config
+    absl::internal_optional_ref
+    absl::strings
+    GTest::gmock_main
 )
 
-# TODO(cohenjon,zhangxy) Figure out why this test is failing on gcc 4.8
-if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU" AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.9)
-else()
-absl_cc_test(
+# Deprecated empty library.
+# Clients should remove this dependency.
+absl_cc_library(
   NAME
-    variant_exception_safety_test
-  SRCS
-    "variant_exception_safety_test.cc"
-  COPTS
-    ${ABSL_TEST_COPTS}
-  DEPS
-    absl::variant
-    absl::config
-    absl::exception_safety_testing
-    absl::memory
-    gmock_main
+    bad_any_cast
+  PUBLIC
+)
+
+# Deprecated empty library.
+# Clients should remove this dependency.
+absl_cc_library(
+  NAME
+    bad_optional_access
+  PUBLIC
+)
+
+# Deprecated empty library.
+# Clients should remove this dependency.
+absl_cc_library(
+  NAME
+    bad_variant_access
+  PUBLIC
 )
-endif()
diff --git a/absl/types/any.h b/absl/types/any.h
index 507c8b2cc4f..accb025b0eb 100644
--- a/absl/types/any.h
+++ b/absl/types/any.h
@@ -17,527 +17,50 @@
 // any.h
 // -----------------------------------------------------------------------------
 //
-// This header file define the `absl::any` type for holding a type-safe value
-// of any type. The 'absl::any` type is useful for providing a way to hold
-// something that is, as yet, unspecified. Such unspecified types
-// traditionally are passed between API boundaries until they are later cast to
-// their "destination" types. To cast to such a destination type, use
-// `absl::any_cast()`. Note that when casting an `absl::any`, you must cast it
-// to an explicit type; implicit conversions will throw.
-//
-// Example:
-//
-//   auto a = absl::any(65);
-//   absl::any_cast<int>(a);         // 65
-//   absl::any_cast<char>(a);        // throws absl::bad_any_cast
-//   absl::any_cast<std::string>(a); // throws absl::bad_any_cast
-//
-// `absl::any` is a C++11 compatible version of the C++17 `std::any` abstraction
-// and is designed to be a drop-in replacement for code compliant with C++17.
-//
-// Traditionally, the behavior of casting to a temporary unspecified type has
-// been accomplished with the `void *` paradigm, where the pointer was to some
-// other unspecified type. `absl::any` provides an "owning" version of `void *`
-// that avoids issues of pointer management.
-//
-// Note: just as in the case of `void *`, use of `absl::any` (and its C++17
-// version `std::any`) is a code smell indicating that your API might not be
-// constructed correctly. We have seen that most uses of `any` are unwarranted,
-// and `absl::any`, like `std::any`, is difficult to use properly. Before using
-// this abstraction, make sure that you should not instead be rewriting your
-// code to be more specific.
-//
-// Abseil expects to release an `absl::variant` type shortly (a C++11 compatible
-// version of the C++17 `std::variant), which is generally preferred for use
-// over `absl::any`.
+// Historical note: Abseil once provided an implementation of `absl::any` as a
+// polyfill for `std::any` prior to C++17. Now that C++17 is required,
+// `absl::any` is an alias for `std::any`.
+
 #ifndef ABSL_TYPES_ANY_H_
 #define ABSL_TYPES_ANY_H_
 
-#include "absl/base/config.h"
-#include "absl/utility/utility.h"
-
-#ifdef ABSL_HAVE_STD_ANY
-
 #include <any>  // IWYU pragma: export
-
-namespace absl {
-using std::any;
-using std::any_cast;
-using std::bad_any_cast;
-using std::make_any;
-}  // namespace absl
-
-#else  // ABSL_HAVE_STD_ANY
-
-#include <algorithm>
-#include <cstddef>
 #include <initializer_list>
-#include <memory>
-#include <stdexcept>
-#include <type_traits>
-#include <typeinfo>
-#include <utility>
 
-#include "absl/base/macros.h"
-#include "absl/meta/type_traits.h"
-#include "absl/types/bad_any_cast.h"
+#include "absl/base/config.h"
 
-// NOTE: This macro is an implementation detail that is undefined at the bottom
-// of the file. It is not intended for expansion directly from user code.
-#ifdef ABSL_ANY_DETAIL_HAS_RTTI
-#error ABSL_ANY_DETAIL_HAS_RTTI cannot be directly set
-#elif !defined(__GNUC__) || defined(__GXX_RTTI)
-#define ABSL_ANY_DETAIL_HAS_RTTI 1
-#endif  // !defined(__GNUC__) || defined(__GXX_RTTI)
+// Include-what-you-use cleanup required for these headers.
+#include "absl/base/attributes.h"
+#include "absl/base/macros.h"
+#include "absl/utility/utility.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
-namespace any_internal {
-
-template <typename Type>
-struct TypeTag {
-  constexpr static char dummy_var = 0;
-};
-
-template <typename Type>
-constexpr char TypeTag<Type>::dummy_var;
-
-// FastTypeId<Type>() evaluates at compile/link-time to a unique pointer for the
-// passed in type. These are meant to be good match for keys into maps or
-// straight up comparisons.
-template<typename Type>
-constexpr inline const void* FastTypeId() {
-  return &TypeTag<Type>::dummy_var;
-}
-
-}  // namespace any_internal
-
-class any;
-
-// swap()
-//
-// Swaps two `absl::any` values. Equivalent to `x.swap(y) where `x` and `y` are
-// `absl::any` types.
-void swap(any& x, any& y) noexcept;
+using any ABSL_DEPRECATE_AND_INLINE() = std::any;
 
-// make_any()
-//
-// Constructs an `absl::any` of type `T` with the given arguments.
 template <typename T, typename... Args>
-any make_any(Args&&... args);
-
-// Overload of `absl::make_any()` for constructing an `absl::any` type from an
-// initializer list.
-template <typename T, typename U, typename... Args>
-any make_any(std::initializer_list<U> il, Args&&... args);
-
-// any_cast()
-//
-// Statically casts the value of a `const absl::any` type to the given type.
-// This function will throw `absl::bad_any_cast` if the stored value type of the
-// `absl::any` does not match the cast.
-//
-// `any_cast()` can also be used to get a reference to the internal storage iff
-// a reference type is passed as its `ValueType`:
-//
-// Example:
-//
-//   absl::any my_any = std::vector<int>();
-//   absl::any_cast<std::vector<int>&>(my_any).push_back(42);
-template <typename ValueType>
-ValueType any_cast(const any& operand);
-
-// Overload of `any_cast()` to statically cast the value of a non-const
-// `absl::any` type to the given type. This function will throw
-// `absl::bad_any_cast` if the stored value type of the `absl::any` does not
-// match the cast.
-template <typename ValueType>
-ValueType any_cast(any& operand);  // NOLINT(runtime/references)
-
-// Overload of `any_cast()` to statically cast the rvalue of an `absl::any`
-// type. This function will throw `absl::bad_any_cast` if the stored value type
-// of the `absl::any` does not match the cast.
-template <typename ValueType>
-ValueType any_cast(any&& operand);
-
-// Overload of `any_cast()` to statically cast the value of a const pointer
-// `absl::any` type to the given pointer type, or `nullptr` if the stored value
-// type of the `absl::any` does not match the cast.
-template <typename ValueType>
-const ValueType* any_cast(const any* operand) noexcept;
-
-// Overload of `any_cast()` to statically cast the value of a pointer
-// `absl::any` type to the given pointer type, or `nullptr` if the stored value
-// type of the `absl::any` does not match the cast.
-template <typename ValueType>
-ValueType* any_cast(any* operand) noexcept;
-
-// -----------------------------------------------------------------------------
-// absl::any
-// -----------------------------------------------------------------------------
-//
-// An `absl::any` object provides the facility to either store an instance of a
-// type, known as the "contained object", or no value. An `absl::any` is used to
-// store values of types that are unknown at compile time. The `absl::any`
-// object, when containing a value, must contain a value type; storing a
-// reference type is neither desired nor supported.
-//
-// An `absl::any` can only store a type that is copy-constructible; move-only
-// types are not allowed within an `any` object.
-//
-// Example:
-//
-//   auto a = absl::any(65);                 // Literal, copyable
-//   auto b = absl::any(std::vector<int>()); // Default-initialized, copyable
-//   std::unique_ptr<Foo> my_foo;
-//   auto c = absl::any(std::move(my_foo));  // Error, not copy-constructible
-//
-// Note that `absl::any` makes use of decayed types (`absl::decay_t` in this
-// context) to remove const-volatile qualifiers (known as "cv qualifiers"),
-// decay functions to function pointers, etc. We essentially "decay" a given
-// type into its essential type.
-//
-// `absl::any` makes use of decayed types when determining the basic type `T` of
-// the value to store in the any's contained object. In the documentation below,
-// we explicitly denote this by using the phrase "a decayed type of `T`".
-//
-// Example:
-//
-//   const int a = 4;
-//   absl::any foo(a);  // Decay ensures we store an "int", not a "const int&".
-//
-//   void my_function() {}
-//   absl::any bar(my_function);  // Decay ensures we store a function pointer.
-//
-// `absl::any` is a C++11 compatible version of the C++17 `std::any` abstraction
-// and is designed to be a drop-in replacement for code compliant with C++17.
-class any {
- private:
-  template <typename T>
-  struct IsInPlaceType;
-
- public:
-  // Constructors
-
-  // Constructs an empty `absl::any` object (`any::has_value()` will return
-  // `false`).
-  constexpr any() noexcept;
-
-  // Copy constructs an `absl::any` object with a "contained object" of the
-  // passed type of `other` (or an empty `absl::any` if `other.has_value()` is
-  // `false`.
-  any(const any& other)
-      : obj_(other.has_value() ? other.obj_->Clone()
-                               : std::unique_ptr<ObjInterface>()) {}
-
-  // Move constructs an `absl::any` object with a "contained object" of the
-  // passed type of `other` (or an empty `absl::any` if `other.has_value()` is
-  // `false`).
-  any(any&& other) noexcept = default;
-
-  // Constructs an `absl::any` object with a "contained object" of the decayed
-  // type of `T`, which is initialized via `std::forward<T>(value)`.
-  //
-  // This constructor will not participate in overload resolution if the
-  // decayed type of `T` is not copy-constructible.
-  template <
-      typename T, typename VT = absl::decay_t<T>,
-      absl::enable_if_t<!absl::disjunction<
-          std::is_same<any, VT>, IsInPlaceType<VT>,
-          absl::negation<std::is_copy_constructible<VT> > >::value>* = nullptr>
-  any(T&& value) : obj_(new Obj<VT>(in_place, std::forward<T>(value))) {}
-
-  // Constructs an `absl::any` object with a "contained object" of the decayed
-  // type of `T`, which is initialized via `std::forward<T>(value)`.
-  template <typename T, typename... Args, typename VT = absl::decay_t<T>,
-            absl::enable_if_t<absl::conjunction<
-                std::is_copy_constructible<VT>,
-                std::is_constructible<VT, Args...>>::value>* = nullptr>
-  explicit any(in_place_type_t<T> /*tag*/, Args&&... args)
-      : obj_(new Obj<VT>(in_place, std::forward<Args>(args)...)) {}
-
-  // Constructs an `absl::any` object with a "contained object" of the passed
-  // type `VT` as a decayed type of `T`. `VT` is initialized as if
-  // direct-non-list-initializing an object of type `VT` with the arguments
-  // `initializer_list, std::forward<Args>(args)...`.
-  template <
-      typename T, typename U, typename... Args, typename VT = absl::decay_t<T>,
-      absl::enable_if_t<
-          absl::conjunction<std::is_copy_constructible<VT>,
-                            std::is_constructible<VT, std::initializer_list<U>&,
-                                                  Args...>>::value>* = nullptr>
-  explicit any(in_place_type_t<T> /*tag*/, std::initializer_list<U> ilist,
-               Args&&... args)
-      : obj_(new Obj<VT>(in_place, ilist, std::forward<Args>(args)...)) {}
-
-  // Assignment operators
-
-  // Copy assigns an `absl::any` object with a "contained object" of the
-  // passed type.
-  any& operator=(const any& rhs) {
-    any(rhs).swap(*this);
-    return *this;
-  }
-
-  // Move assigns an `absl::any` object with a "contained object" of the
-  // passed type. `rhs` is left in a valid but otherwise unspecified state.
-  any& operator=(any&& rhs) noexcept {
-    any(std::move(rhs)).swap(*this);
-    return *this;
-  }
-
-  // Assigns an `absl::any` object with a "contained object" of the passed type.
-  template <typename T, typename VT = absl::decay_t<T>,
-            absl::enable_if_t<absl::conjunction<
-                absl::negation<std::is_same<VT, any>>,
-                std::is_copy_constructible<VT>>::value>* = nullptr>
-  any& operator=(T&& rhs) {
-    any tmp(in_place_type_t<VT>(), std::forward<T>(rhs));
-    tmp.swap(*this);
-    return *this;
-  }
-
-  // Modifiers
-
-  // any::emplace()
-  //
-  // Emplaces a value within an `absl::any` object by calling `any::reset()`,
-  // initializing the contained value as if direct-non-list-initializing an
-  // object of type `VT` with the arguments `std::forward<Args>(args)...`, and
-  // returning a reference to the new contained value.
-  //
-  // Note: If an exception is thrown during the call to `VT`'s constructor,
-  // `*this` does not contain a value, and any previously contained value has
-  // been destroyed.
-  template <
-      typename T, typename... Args, typename VT = absl::decay_t<T>,
-      absl::enable_if_t<std::is_copy_constructible<VT>::value &&
-                        std::is_constructible<VT, Args...>::value>* = nullptr>
-  VT& emplace(Args&&... args) {
-    reset();  // NOTE: reset() is required here even in the world of exceptions.
-    Obj<VT>* const object_ptr =
-        new Obj<VT>(in_place, std::forward<Args>(args)...);
-    obj_ = std::unique_ptr<ObjInterface>(object_ptr);
-    return object_ptr->value;
-  }
-
-  // Overload of `any::emplace()` to emplace a value within an `absl::any`
-  // object by calling `any::reset()`, initializing the contained value as if
-  // direct-non-list-initializing an object of type `VT` with the arguments
-  // `initializer_list, std::forward<Args>(args)...`, and returning a reference
-  // to the new contained value.
-  //
-  // Note: If an exception is thrown during the call to `VT`'s constructor,
-  // `*this` does not contain a value, and any previously contained value has
-  // been destroyed. The function shall not participate in overload resolution
-  // unless `is_copy_constructible_v<VT>` is `true` and
-  // `is_constructible_v<VT, initializer_list<U>&, Args...>` is `true`.
-  template <
-      typename T, typename U, typename... Args, typename VT = absl::decay_t<T>,
-      absl::enable_if_t<std::is_copy_constructible<VT>::value &&
-                        std::is_constructible<VT, std::initializer_list<U>&,
-                                              Args...>::value>* = nullptr>
-  VT& emplace(std::initializer_list<U> ilist, Args&&... args) {
-    reset();  // NOTE: reset() is required here even in the world of exceptions.
-    Obj<VT>* const object_ptr =
-        new Obj<VT>(in_place, ilist, std::forward<Args>(args)...);
-    obj_ = std::unique_ptr<ObjInterface>(object_ptr);
-    return object_ptr->value;
-  }
-
-  // any::reset()
-  //
-  // Resets the state of the `absl::any` object, destroying the contained object
-  // if present.
-  void reset() noexcept { obj_ = nullptr; }
-
-  // any::swap()
-  //
-  // Swaps the passed value and the value of this `absl::any` object.
-  void swap(any& other) noexcept { obj_.swap(other.obj_); }
-
-  // Observers
-
-  // any::has_value()
-  //
-  // Returns `true` if the `any` object has a contained value, otherwise
-  // returns `false`.
-  bool has_value() const noexcept { return obj_ != nullptr; }
-
-#if ABSL_ANY_DETAIL_HAS_RTTI
-  // Returns: typeid(T) if *this has a contained object of type T, otherwise
-  // typeid(void).
-  const std::type_info& type() const noexcept {
-    if (has_value()) {
-      return obj_->Type();
-    }
-
-    return typeid(void);
-  }
-#endif  // ABSL_ANY_DETAIL_HAS_RTTI
-
- private:
-  // Tagged type-erased abstraction for holding a cloneable object.
-  class ObjInterface {
-   public:
-    virtual ~ObjInterface() = default;
-    virtual std::unique_ptr<ObjInterface> Clone() const = 0;
-    virtual const void* ObjTypeId() const noexcept = 0;
-#if ABSL_ANY_DETAIL_HAS_RTTI
-    virtual const std::type_info& Type() const noexcept = 0;
-#endif  // ABSL_ANY_DETAIL_HAS_RTTI
-  };
-
-  // Hold a value of some queryable type, with an ability to Clone it.
-  template <typename T>
-  class Obj : public ObjInterface {
-   public:
-    template <typename... Args>
-    explicit Obj(in_place_t /*tag*/, Args&&... args)
-        : value(std::forward<Args>(args)...) {}
-
-    std::unique_ptr<ObjInterface> Clone() const final {
-      return std::unique_ptr<ObjInterface>(new Obj(in_place, value));
-    }
-
-    const void* ObjTypeId() const noexcept final { return IdForType<T>(); }
-
-#if ABSL_ANY_DETAIL_HAS_RTTI
-    const std::type_info& Type() const noexcept final { return typeid(T); }
-#endif  // ABSL_ANY_DETAIL_HAS_RTTI
-
-    T value;
-  };
-
-  std::unique_ptr<ObjInterface> CloneObj() const {
-    if (!obj_) return nullptr;
-    return obj_->Clone();
-  }
-
-  template <typename T>
-  constexpr static const void* IdForType() {
-    // Note: This type dance is to make the behavior consistent with typeid.
-    using NormalizedType =
-        typename std::remove_cv<typename std::remove_reference<T>::type>::type;
-
-    return any_internal::FastTypeId<NormalizedType>();
-  }
-
-  const void* GetObjTypeId() const {
-    return obj_ ? obj_->ObjTypeId() : any_internal::FastTypeId<void>();
-  }
-
-  // `absl::any` nonmember functions //
-
-  // Description at the declaration site (top of file).
-  template <typename ValueType>
-  friend ValueType any_cast(const any& operand);
-
-  // Description at the declaration site (top of file).
-  template <typename ValueType>
-  friend ValueType any_cast(any& operand);  // NOLINT(runtime/references)
-
-  // Description at the declaration site (top of file).
-  template <typename T>
-  friend const T* any_cast(const any* operand) noexcept;
-
-  // Description at the declaration site (top of file).
-  template <typename T>
-  friend T* any_cast(any* operand) noexcept;
-
-  std::unique_ptr<ObjInterface> obj_;
-};
-
-// -----------------------------------------------------------------------------
-// Implementation Details
-// -----------------------------------------------------------------------------
-
-constexpr any::any() noexcept = default;
-
-template <typename T>
-struct any::IsInPlaceType : std::false_type {};
-
-template <typename T>
-struct any::IsInPlaceType<in_place_type_t<T>> : std::true_type {};
+[[deprecated]] constexpr decltype(std::any_cast<T>(std::declval<Args>()...))
+any_cast(Args&&... args) {
+  return std::any_cast<T>(std::forward<Args>(args)...);
+}
 
-inline void swap(any& x, any& y) noexcept { x.swap(y); }
+using bad_any_cast ABSL_DEPRECATE_AND_INLINE() = std::bad_any_cast;
 
-// Description at the declaration site (top of file).
 template <typename T, typename... Args>
-any make_any(Args&&... args) {
-  return any(in_place_type_t<T>(), std::forward<Args>(args)...);
+[[deprecated]] constexpr decltype(std::make_any<T>(std::declval<Args>()...))
+make_any(Args&&... args) {
+  return std::make_any<T>(std::forward<Args>(args)...);
 }
 
-// Description at the declaration site (top of file).
 template <typename T, typename U, typename... Args>
-any make_any(std::initializer_list<U> il, Args&&... args) {
-  return any(in_place_type_t<T>(), il, std::forward<Args>(args)...);
-}
-
-// Description at the declaration site (top of file).
-template <typename ValueType>
-ValueType any_cast(const any& operand) {
-  using U = typename std::remove_cv<
-      typename std::remove_reference<ValueType>::type>::type;
-  static_assert(std::is_constructible<ValueType, const U&>::value,
-                "Invalid ValueType");
-  auto* const result = (any_cast<U>)(&operand);
-  if (result == nullptr) {
-    any_internal::ThrowBadAnyCast();
-  }
-  return static_cast<ValueType>(*result);
-}
-
-// Description at the declaration site (top of file).
-template <typename ValueType>
-ValueType any_cast(any& operand) {  // NOLINT(runtime/references)
-  using U = typename std::remove_cv<
-      typename std::remove_reference<ValueType>::type>::type;
-  static_assert(std::is_constructible<ValueType, U&>::value,
-                "Invalid ValueType");
-  auto* result = (any_cast<U>)(&operand);
-  if (result == nullptr) {
-    any_internal::ThrowBadAnyCast();
-  }
-  return static_cast<ValueType>(*result);
-}
-
-// Description at the declaration site (top of file).
-template <typename ValueType>
-ValueType any_cast(any&& operand) {
-  using U = typename std::remove_cv<
-      typename std::remove_reference<ValueType>::type>::type;
-  static_assert(std::is_constructible<ValueType, U>::value,
-                "Invalid ValueType");
-  return static_cast<ValueType>(std::move((any_cast<U&>)(operand)));
-}
-
-// Description at the declaration site (top of file).
-template <typename T>
-const T* any_cast(const any* operand) noexcept {
-  using U =
-      typename std::remove_cv<typename std::remove_reference<T>::type>::type;
-  return operand && operand->GetObjTypeId() == any::IdForType<U>()
-             ? std::addressof(
-                   static_cast<const any::Obj<U>*>(operand->obj_.get())->value)
-             : nullptr;
-}
-
-// Description at the declaration site (top of file).
-template <typename T>
-T* any_cast(any* operand) noexcept {
-  using U =
-      typename std::remove_cv<typename std::remove_reference<T>::type>::type;
-  return operand && operand->GetObjTypeId() == any::IdForType<U>()
-             ? std::addressof(
-                   static_cast<any::Obj<U>*>(operand->obj_.get())->value)
-             : nullptr;
+[[deprecated]] constexpr decltype(std::make_any<T>(
+    std::declval<std::initializer_list<U>>(), std::declval<Args>()...))
+make_any(std::initializer_list<U> il, Args&&... args) {
+  return std::make_any<T>(il, std::forward<Args>(args)...);
 }
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
-#undef ABSL_ANY_DETAIL_HAS_RTTI
-
-#endif  // ABSL_HAVE_STD_ANY
-
 #endif  // ABSL_TYPES_ANY_H_
diff --git a/absl/types/any_exception_safety_test.cc b/absl/types/any_exception_safety_test.cc
deleted file mode 100644
index 17d7f5d7781..00000000000
--- a/absl/types/any_exception_safety_test.cc
+++ /dev/null
@@ -1,173 +0,0 @@
-// Copyright 2017 The Abseil Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-#include "absl/types/any.h"
-
-#include "absl/base/config.h"
-
-// This test is a no-op when absl::any is an alias for std::any and when
-// exceptions are not enabled.
-#if !defined(ABSL_HAVE_STD_ANY) && defined(ABSL_HAVE_EXCEPTIONS)
-
-#include <typeinfo>
-#include <vector>
-
-#include "gtest/gtest.h"
-#include "absl/base/internal/exception_safety_testing.h"
-
-using Thrower = testing::ThrowingValue<>;
-using NoThrowMoveThrower =
-    testing::ThrowingValue<testing::TypeSpec::kNoThrowMove>;
-using ThrowerList = std::initializer_list<Thrower>;
-using ThrowerVec = std::vector<Thrower>;
-using ThrowingAlloc = testing::ThrowingAllocator<Thrower>;
-using ThrowingThrowerVec = std::vector<Thrower, ThrowingAlloc>;
-
-namespace {
-
-testing::AssertionResult AnyInvariants(absl::any* a) {
-  using testing::AssertionFailure;
-  using testing::AssertionSuccess;
-
-  if (a->has_value()) {
-    if (a->type() == typeid(void)) {
-      return AssertionFailure()
-             << "A non-empty any should not have type `void`";
-    }
-  } else {
-    if (a->type() != typeid(void)) {
-      return AssertionFailure()
-             << "An empty any should have type void, but has type "
-             << a->type().name();
-    }
-  }
-
-  //  Make sure that reset() changes any to a valid state.
-  a->reset();
-  if (a->has_value()) {
-    return AssertionFailure() << "A reset `any` should be valueless";
-  }
-  if (a->type() != typeid(void)) {
-    return AssertionFailure() << "A reset `any` should have type() of `void`, "
-                                 "but instead has type "
-                              << a->type().name();
-  }
-  try {
-    auto unused = absl::any_cast<Thrower>(*a);
-    static_cast<void>(unused);
-    return AssertionFailure()
-           << "A reset `any` should not be able to be any_cast";
-  } catch (const absl::bad_any_cast&) {
-  } catch (...) {
-    return AssertionFailure()
-           << "Unexpected exception thrown from absl::any_cast";
-  }
-  return AssertionSuccess();
-}
-
-testing::AssertionResult AnyIsEmpty(absl::any* a) {
-  if (!a->has_value()) {
-    return testing::AssertionSuccess();
-  }
-  return testing::AssertionFailure()
-         << "a should be empty, but instead has value "
-         << absl::any_cast<Thrower>(*a).Get();
-}
-
-TEST(AnyExceptionSafety, Ctors) {
-  Thrower val(1);
-  testing::TestThrowingCtor<absl::any>(val);
-
-  Thrower copy(val);
-  testing::TestThrowingCtor<absl::any>(copy);
-
-  testing::TestThrowingCtor<absl::any>(absl::in_place_type_t<Thrower>(), 1);
-
-  testing::TestThrowingCtor<absl::any>(absl::in_place_type_t<ThrowerVec>(),
-                                       ThrowerList{val});
-
-  testing::TestThrowingCtor<absl::any,
-                            absl::in_place_type_t<ThrowingThrowerVec>,
-                            ThrowerList, ThrowingAlloc>(
-      absl::in_place_type_t<ThrowingThrowerVec>(), {val}, ThrowingAlloc());
-}
-
-TEST(AnyExceptionSafety, Assignment) {
-  auto original =
-      absl::any(absl::in_place_type_t<Thrower>(), 1, testing::nothrow_ctor);
-  auto any_is_strong = [original](absl::any* ap) {
-    return testing::AssertionResult(ap->has_value() &&
-                                    absl::any_cast<Thrower>(original) ==
-                                        absl::any_cast<Thrower>(*ap));
-  };
-  auto any_strong_tester = testing::MakeExceptionSafetyTester()
-                               .WithInitialValue(original)
-                               .WithContracts(AnyInvariants, any_is_strong);
-
-  Thrower val(2);
-  absl::any any_val(val);
-  NoThrowMoveThrower mv_val(2);
-
-  auto assign_any = [&any_val](absl::any* ap) { *ap = any_val; };
-  auto assign_val = [&val](absl::any* ap) { *ap = val; };
-  auto move = [&val](absl::any* ap) { *ap = std::move(val); };
-  auto move_movable = [&mv_val](absl::any* ap) { *ap = std::move(mv_val); };
-
-  EXPECT_TRUE(any_strong_tester.Test(assign_any));
-  EXPECT_TRUE(any_strong_tester.Test(assign_val));
-  EXPECT_TRUE(any_strong_tester.Test(move));
-  EXPECT_TRUE(any_strong_tester.Test(move_movable));
-
-  auto empty_any_is_strong = [](absl::any* ap) {
-    return testing::AssertionResult{!ap->has_value()};
-  };
-  auto strong_empty_any_tester =
-      testing::MakeExceptionSafetyTester()
-          .WithInitialValue(absl::any{})
-          .WithContracts(AnyInvariants, empty_any_is_strong);
-
-  EXPECT_TRUE(strong_empty_any_tester.Test(assign_any));
-  EXPECT_TRUE(strong_empty_any_tester.Test(assign_val));
-  EXPECT_TRUE(strong_empty_any_tester.Test(move));
-}
-
-TEST(AnyExceptionSafety, Emplace) {
-  auto initial_val =
-      absl::any{absl::in_place_type_t<Thrower>(), 1, testing::nothrow_ctor};
-  auto one_tester = testing::MakeExceptionSafetyTester()
-                        .WithInitialValue(initial_val)
-                        .WithContracts(AnyInvariants, AnyIsEmpty);
-
-  auto emp_thrower = [](absl::any* ap) { ap->emplace<Thrower>(2); };
-  auto emp_throwervec = [](absl::any* ap) {
-    std::initializer_list<Thrower> il{Thrower(2, testing::nothrow_ctor)};
-    ap->emplace<ThrowerVec>(il);
-  };
-  auto emp_movethrower = [](absl::any* ap) {
-    ap->emplace<NoThrowMoveThrower>(2);
-  };
-
-  EXPECT_TRUE(one_tester.Test(emp_thrower));
-  EXPECT_TRUE(one_tester.Test(emp_throwervec));
-  EXPECT_TRUE(one_tester.Test(emp_movethrower));
-
-  auto empty_tester = one_tester.WithInitialValue(absl::any{});
-
-  EXPECT_TRUE(empty_tester.Test(emp_thrower));
-  EXPECT_TRUE(empty_tester.Test(emp_throwervec));
-}
-
-}  // namespace
-
-#endif  // #if !defined(ABSL_HAVE_STD_ANY) && defined(ABSL_HAVE_EXCEPTIONS)
diff --git a/absl/types/any_span.h b/absl/types/any_span.h
new file mode 100644
index 00000000000..5c858926e73
--- /dev/null
+++ b/absl/types/any_span.h
@@ -0,0 +1,1057 @@
+// Copyright 2026 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: any_span.h
+// -----------------------------------------------------------------------------
+//
+// AnySpan provides a view of a random access container, much like absl::Span
+// (go/totw/93). See also go/totw/145#gtlanyspan for an introduction of AnySpan.
+//
+// The primary differences from absl::Span are:
+//  * AnySpan works with any random access container, whereas Span only works if
+//    elements are contiguous in memory -- both will work with std::vector, but
+//    only AnySpan will work with std::deque.
+//  * AnySpan performs a variety of transformations, such as dereferencing
+//    containers of pointers, or accessing specific members from a collection of
+//    structs, whereas Span does not offer such capability. For example,
+//    AnySpan<std::string> can handle both std::vector<std::string> and
+//    std::vector<std::string*>. Safe implicit conversions for a container's
+//    value type (such as up-casting from child classes, or converting
+//    reference_wrapper<T> to const T&) will happen implicitly.
+//  * AnySpan's generality has some small runtime cost, usually only a
+//    conditional branch per element access, or a function-pointer call in the
+//    worst case. Span may be preferable when the inputs are likely to be
+//    contiguous and performance is critical.
+//
+// AnySpan<T> is a mutable view to the elements and AnySpan<const T> is a
+// read-only view to the elements, similar to absl::Span.
+//
+// AnySpan only requires containers to provide a size() and an operator[] that
+// returns a reference. It will use data() if it returns a pointer to the type
+// returned by operator[], which allows it to perform some internal
+// optimizations (this should apply to many well behaved random access
+// containers that use arrays internally, but notably
+// RepeatedPtrField<T>::data() returns T** instead of T*).
+//
+// Using AnySpan as an input parameter:
+//
+// To write a function that can accept vector<MyMessage>,
+// vector<unique_ptr<MyMessage>>, or RepeatedPtrField<MyMessage> as inputs, you
+// can use AnySpan as the input to the function. AnySpan should be passed by
+// value and it is trivially copyable so it does not need to be moved:
+//
+//   void MyFunction(AnySpan<const MyMessage> messages);
+//
+// You can invoke MyFunction with a vector<MyMessage> or deque<MyMessage>:
+//
+//  std::vector<MyMessage> messages = ...;
+//  MyFunction(messages);
+//
+// Or a container of smart pointers:
+//
+//  std::deque<std::unique_ptr<MyMessage>> message_ptrs = ...;
+//  MyFunction(AnySpan<const MyMessage>(
+//      message_ptrs, any_span_transform::Deref()));
+//
+// Or, you can call the same function with a repeated proto field of type
+// MyMessage:
+//
+//  OtherMessage proto_message = ...;
+//  MyFunction(proto_message.repeated_field());
+//
+//
+// Using AnySpan as an output parameter:
+//
+// To write a function that allows mutation of a fixed-size container of
+// objects, you can use AnySpan with a non-const value type.
+//
+//   void MyMutatingFunction(AnySpan<MyMessage> messages);
+//
+// To bind a mutable AnySpan to a container, callers must construct it
+// explicitly around an lvalue:
+//
+//   std::vector<MyMessage> messages = ...;
+//   MyMutatingFunction(AnySpan<MyMessages>(messages));
+//
+// Or use one of the "Make" functions:
+//
+//   std::vector<MyMessage*> message_ptrs = ...;
+//   MyMutatingFunction(MakeDerefAnySpan(message_ptrs));
+//
+// Or, if you are already dealing with a mutable view-like object, construction
+// can usually be implicit:
+//
+//   absl::Span<MyMessage> mutable_span = ...;
+//   MyMutatingFunction(mutable_span);
+//
+// Transforming Spans:
+//
+// A set of useful transformation functors are provided (see the
+// any_span_transform namespace), but you can provide your own transforms as
+// well.
+//
+// Transforms work for both mutable and const values. When a transform is used
+// for a mutable AnySpan, it will usually have to accept its argument as a
+// mutable reference.
+//
+// Transforms can be any object supported by std::invoke, such as
+// callable objects, function pointers, member function pointers, and even data
+// members. Invoking a transform must return a reference to T or a reference to
+// a compatible object such as a std::reference_wrapper or a child class.
+// Transforms that return value types will not compile and would return
+// dangling references if they did.
+//
+//  struct MyStruct {
+//    int member;
+//  }
+//
+//  std::vector<MyStruct> structs = ...;
+//
+//  // Create an AnySpan<const int> that accesses the members of 'structs':
+//  auto mem_ptr = &MyStruct::member;
+//  AnySpan<const int> members(structs, mem_ptr);
+//
+//  // Or, using a lambda:
+//  auto get_member = [](const MyStruct& s) -> const int& {
+//    return s.member;
+//  };
+//  AnySpan<const int> members_from_lambda(structs, get_member);
+//
+// Transforms must outlive the spans that use them (even member/method pointers,
+// but not function pointer). Callable transforms must provide a const call
+// operator that takes a single argument and returns a reference. Transforms
+// will be executed every time an element is accessed, so complex transforms may
+// have significant performance consequences.
+//
+// Factory Functions:
+//
+// A set of useful functions for constructing common types of AnySpans are
+// provided. Factories with "Const" in the name produce AnySpans of const
+// elements. Factories with "Deref" in the name will dereference elements of the
+// container or array:
+//
+//  AnySpan<T> MakeAnySpan(Container& c);
+//  AnySpan<T> MakeDerefAnySpan(Container& c);
+//  AnySpan<T> MakeAnySpan(T* ptr, std::size_t size);
+//  AnySpan<const T> MakeConstAnySpan(const Container& c);
+//  AnySpan<const T> MakeConstDerefAnySpan(const Container& c);
+//  AnySpan<const T> MakeConstAnySpan(const T* ptr, std::size_t size);
+//
+// Lifetime Gotchas:
+//
+// Take care when constructing spans as named variables! AnySpan captures all
+// arguments by reference, even if it's a pointer:
+//
+//  AnySpan<T> span(v, &MyClass::SomeMethod);  // Dangling reference!
+//
+//  // Also bad! The lambda is destroyed before the span.
+//  AnySpan<T> span(v, [](U& u) { return SomeFunction(u); });
+//
+// Free functions are ok:
+//
+//  AnySpan<T> span(v, SomeFunction);  // This is OK.
+//  AnySpan<T> span(v, &SomeFunction);  // This is OK too.
+//
+// In all other cases, you must ensure that the object used as a transform
+// outlives the span, even if that object is a pointer type.
+//
+// AnySpan is also capable of capturing another AnySpan, so watch out for
+// implicit conversions between types of AnySpans:
+//
+//   // MakeDerefAnySpan() returns an AnySpan<Derived>, leaving 's' pointing to
+//   // a temporary!
+//   vector<Derived*> v;
+//   AnySpan<Base> s = MakeDerefAnySpan(v);
+//
+// Adapting Spans:
+//
+// Since AnySpan only expects operator[] and size(), it is relatively simple to
+// write light-weight adaptor classes that can behave like containers. See the
+// any_span_adaptor namespace for a utility class that does this for iterators
+// and views.
+//
+// Adapters are more powerful than transforms, since they allow you to change
+// the value type and element order of a container, but transforms will
+// generally perform better and leave code with fewer object lifetime concerns.
+//
+//
+// Note about RepeatedPtrField performance:
+//
+// AnySpan will use data() when it returns a pointer to the same type returned
+// by operator[], however RepeatedPtrField's operator[] returns T& and its
+// data() returns a T**. Because of this, AnySpan will fall back to a less
+// efficient version of type-erasure. If you have a performance critical use of
+// RepeatedPtrField, you might find this pattern to have better performance:
+//
+//  MyFunction(AnySpan<const MyMessage>(
+//      proto_message.repeated_field().data(),
+//      proto_message.repeated_field().size(),
+//      any_span_transform::Deref()));
+//
+#ifndef ABSL_TYPES_ANY_SPAN_H_
+#define ABSL_TYPES_ANY_SPAN_H_
+
+#include <algorithm>
+#include <cstddef>
+#include <functional>
+#include <initializer_list>
+#include <iterator>
+#include <type_traits>
+#include <utility>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/hardening.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/base/macros.h"
+#include "absl/base/nullability.h"
+#include "absl/base/optimization.h"
+#include "absl/base/throw_delegate.h"
+#include "absl/meta/type_traits.h"
+#include "absl/types/internal/any_span.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+// The accessors in the 'any_span_transform' namespace return references to
+// Transform functors that may be passed to AnySpan. Generally you should
+// prefer to use these functors whenever possible, as they may trigger internal
+// optimizations that are otherwise not possible, and they are valid for the
+// duration of the program, so you do not have to worry about their lifetime.
+namespace any_span_transform {
+
+//
+// Identity() returns a functor that returns whatever is passed to it. Generally
+// you should prefer to use AnySpan's implicit constructor directly, but this
+// may be useful if you are writing templates on top of AnySpan.
+//
+// Returns a const reference so that callers don't have to worry about
+// lifetime of the functor.
+//
+
+struct IdentityT {
+  template <typename T>
+  T& operator()(T& v) const {  // NOLINT(runtime/references)
+    return v;
+  }
+};
+
+inline const IdentityT& Identity() {
+  static const IdentityT f = {};
+  return f;
+}
+
+struct DerefT {
+  template <typename Ptr>
+  auto operator()(Ptr& ptr) const  // NOLINT(runtime/references)
+      -> decltype(*ptr) {
+    ABSL_RAW_DCHECK(ptr, "Cannot dereference null pointer");
+    return *ptr;
+  }
+};
+
+// Deref() returns a functor that dereferences whatever is passed to it. It
+// works for smart and raw pointers, as well as std::optional. Do not use this
+// with containers that may contain elements that cannot be dereferenced, such
+// as null pointers.
+//
+// Returns a const reference so that callers don't have to worry about lifetime
+// of the functor.
+inline const DerefT& Deref() {
+  static const DerefT f = {};
+  return f;
+}
+
+}  // namespace any_span_transform
+
+// Utilities for adapting things to look like the interface that AnySpan
+// expects. For the most part this is based on iterators and views, and is
+// intended to be composed with absl/types/iterator_adaptors.h.
+namespace any_span_adaptor {
+
+// Adapts a pair of iterators into a container-like object that AnySpan can
+// wrap. This is useful if you are faced with a range or view of random access
+// iterators. Iter must be a valid random access iterator.
+template <typename Iter>
+class Range {
+ public:
+  static_assert(
+      std::is_same_v<typename std::iterator_traits<Iter>::iterator_category,
+                     std::random_access_iterator_tag>,
+      "Iter must be a random access iterator.");
+
+  Range(Iter begin, Iter end) {
+    absl::base_internal::HardeningAssertLE(begin, end);
+    begin_ = begin;
+    end_ = end;
+  }
+
+  std::size_t size() const { return end_ - begin_; }
+
+  decltype(std::declval<Iter>()[0]) operator[](std::size_t i) const {
+    absl::base_internal::HardeningAssertLT(i, size());
+    return begin_[i];
+  }
+
+ private:
+  Iter begin_;
+  Iter end_;
+};
+
+// Returns a Range adaptor that wraps the given pair of iterators. The return
+// value of this function must outlive any spans that use it. Iter must be a
+// valid random access iterator.
+template <typename Iter>
+Range<Iter> MakeAdaptorFromRange(Iter begin, Iter end) {
+  return Range<Iter>(begin, end);
+}
+
+// Returns a Range adaptor that wraps the given view. The begin() and end()
+// functions of the given view must return valid random access iterators. The
+// return value of this function must outlive any spans that use it.
+template <typename View>
+auto MakeAdaptorFromView(View& view)  // NOLINT(runtime/references)
+    -> Range<decltype(view.begin())> {
+  return Range<decltype(view.begin())>(view.begin(), view.end());
+}
+
+}  // namespace any_span_adaptor
+
+template <typename T>
+class AnySpan;
+
+template <typename T>
+class ABSL_ATTRIBUTE_VIEW AnySpan {
+ private:
+  template <typename Iter, typename Value>
+  class IteratorBase;
+
+  template <typename U>
+  using EnableIfMutable = std::enable_if_t<!std::is_const_v<T>, U>;
+
+  template <typename U>
+  using EnableIfConst = std::enable_if_t<std::is_const_v<T>, U>;
+
+  static std::true_type CreatesATemporaryImpl(std::decay_t<T>&&);
+  static std::false_type CreatesATemporaryImpl(const T&);
+  template <typename U,
+            typename B = decltype(CreatesATemporaryImpl(std::declval<U>()))>
+  struct CreatesATemporary : B {};
+
+  // Enable if invoke(transform, element) is valid and if a reference to T can
+  // bind to its output. This prevents situations where the constructor may be
+  // ambiguous.
+  // We also verify that the conversion from TransformResult to T& does not
+  // create a temporary. Otherwise, we would get a false positive in the
+  // enabler where `const char*` looks like can be converted to
+  // `const std::string&`.
+  template <typename Transform, typename Element,
+            typename TransformResult = decltype(std::invoke(
+                std::declval<const Transform&>(), std::declval<Element>()))>
+  using EnableIfTransformIsValid =
+      std::enable_if_t<std::is_convertible_v<TransformResult, T&> &&
+                       !CreatesATemporary<TransformResult>::value>;
+
+  // Enable if Container appears to be a valid container. Just checks for size()
+  // and makes sure the class is not an AnySpan for now.
+  template <typename Container>
+  using EnableIfContainer =
+      std::enable_if_t<any_span_internal::HasSize<Container>::value &&
+                       !any_span_internal::IsAnySpan<Container>::value>;
+
+  template <typename Element>
+  using EnableIfDifferentElementType =
+      std::enable_if_t<!std::is_same_v<T, Element> &&
+                       !std::is_same_v<T, const Element>>;
+
+  template <typename Transform>
+  using EnableIfTransformIsByCopy =
+      std::enable_if_t<any_span_internal::kIsTransformCopied<Transform>, bool>;
+  template <typename Transform>
+  using EnableIfTransformIsByRef =
+      std::enable_if_t<!any_span_internal::kIsTransformCopied<Transform>, bool>;
+
+ public:
+  using element_type = T;
+  using value_type = std::remove_const_t<T>;
+  using size_type = std::size_t;
+  using difference_type = std::ptrdiff_t;
+  using absl_internal_is_view = std::true_type;
+
+  static constexpr size_type npos = static_cast<size_type>(-1);  // NOLINT
+
+  using reference = T&;
+  using const_reference = std::add_const_t<T>&;
+
+  using pointer = T*;
+  using const_pointer = std::add_const_t<T>*;
+
+  // Note that iterator will be const if T is const.
+  class iterator;
+  class const_iterator;
+
+  using reverse_iterator = std::reverse_iterator<iterator>;
+  using const_reverse_iterator = std::reverse_iterator<const_iterator>;
+
+  // Null and empty by default.
+  AnySpan() = default;
+
+  // Creates a span that wraps an initializer list. This makes it possible to
+  // pass a brace-enclosed initializer list to a function expecting an AnySpan.
+  //
+  // Example:
+  //
+  //   void Process(AnySpan<const int> x);
+  //   Process({1, 2, 3});
+  //
+  // The initializer_list must outlive this AnySpan.
+  constexpr AnySpan(  // NOLINT(google-explicit-constructor)
+      std::initializer_list<value_type> l ABSL_ATTRIBUTE_LIFETIME_BOUND)
+      : AnySpan(l.begin(), l.size()) {}
+
+  // Creates a span that wraps an initializer list of a type other than
+  // value_type, or with an explicit transform. Applies the optional transform
+  // to elements before returning them.
+  //
+  // Example:
+  //
+  //   struct Base {};
+  //   struct Derived : Base {};
+  //
+  //   void Process(AnySpan<const Base> x);
+  //   Process({Derived(a), Derived(b), Derived(c)});
+  //
+  //     where the default identity transform would apply an implicit
+  //     derived-to-base  conversion.
+  //
+  // The initializer_list must outlive this AnySpan.
+  template <typename Element, typename Transform,
+            typename = EnableIfTransformIsValid<Transform, const Element&>,
+            EnableIfTransformIsByCopy<Transform> = true>
+  constexpr AnySpan(std::initializer_list<Element> l
+                        ABSL_ATTRIBUTE_LIFETIME_BOUND,
+                    const Transform& transform)
+      : AnySpan(l.begin(), l.size(), transform) {}
+  template <typename Element,
+            typename Transform = any_span_transform::IdentityT,
+            typename = EnableIfTransformIsValid<Transform, const Element&>,
+            EnableIfTransformIsByRef<Transform> = true>
+  constexpr AnySpan(std::initializer_list<Element> l
+                        ABSL_ATTRIBUTE_LIFETIME_BOUND,
+                    const Transform& transform ABSL_ATTRIBUTE_LIFETIME_BOUND =
+                        any_span_transform::Identity())
+      : AnySpan(l.begin(), l.size(), transform) {}
+
+  // Creates a span that wraps an array. Applies the optional transform to
+  // elements before returning them.
+  //
+  // Transform must be a function object with a const operator() that takes
+  // Element as an argument and return a reference to T or compatible object.
+  //
+  // Both the transform and array must outlive this span.
+  template <typename Element, typename Transform,
+            typename = EnableIfTransformIsValid<Transform, const Element&>,
+            EnableIfTransformIsByCopy<Transform> = true>
+  constexpr AnySpan(const Element* absl_nullable ptr
+                        ABSL_ATTRIBUTE_LIFETIME_BOUND,
+                    size_type size, const Transform& transform)
+      : AnySpan(any_span_internal::MakeArrayGetter<T>(ptr, transform), size) {}
+  template <typename Element,
+            typename Transform = any_span_transform::IdentityT,
+            typename = EnableIfTransformIsValid<Transform, const Element&>,
+            EnableIfTransformIsByRef<Transform> = true>
+  constexpr AnySpan(const Element* absl_nullable ptr
+                        ABSL_ATTRIBUTE_LIFETIME_BOUND,
+                    size_type size,
+                    const Transform& transform ABSL_ATTRIBUTE_LIFETIME_BOUND =
+                        any_span_transform::Identity())
+      : AnySpan(any_span_internal::MakeArrayGetter<T>(ptr, transform), size) {}
+
+  // Creates a span that wraps an array of fixed size. Applies the optional
+  // transform to elements before returning them.
+  //
+  // Transform must be a function object with a const operator() that takes
+  // Element as an argument and return a reference to T or compatible object.
+  //
+  // Both the transform and array must outlive this span.
+  template <typename Element, size_type N, typename Transform,
+            typename = EnableIfTransformIsValid<Transform, const Element&>,
+            EnableIfTransformIsByCopy<Transform> = true>
+  constexpr AnySpan(  // NOLINT(google-explicit-constructor)
+      const Element (&array ABSL_ATTRIBUTE_LIFETIME_BOUND)[N],
+      const Transform& transform)
+      : AnySpan(array, N, transform) {}
+  template <typename Element, size_type N,
+            typename Transform = any_span_transform::IdentityT,
+            typename = EnableIfTransformIsValid<Transform, const Element&>,
+            EnableIfTransformIsByRef<Transform> = true>
+  constexpr AnySpan(  // NOLINT(google-explicit-constructor)
+      const Element (&array ABSL_ATTRIBUTE_LIFETIME_BOUND)[N],
+      const Transform& transform ABSL_ATTRIBUTE_LIFETIME_BOUND =
+          any_span_transform::Identity())
+      : AnySpan(array, N, transform) {}
+
+  // Creates a span that wraps a const container. Applies the optional transform
+  // to elements before returning them.
+  //
+  // This constructor is enabled even for mutable spans, since some
+  // container-like objects provide mutable element access even when the object
+  // itself is const (such as absl::Span)
+  //
+  // Transform must be a function object with a const operator() that takes the
+  // value type of Container as an argument and return a reference to T or
+  // compatible object.
+  //
+  // The transform, container, and the container's underlying storage must
+  // outlive this span. Any operation that may reallocate the container's
+  // storage or change its size will invalidate the span.
+  template <typename Container, typename Transform,
+            typename = EnableIfContainer<Container>,
+            typename = EnableIfTransformIsValid<
+                Transform, decltype(std::declval<const Container&>()[0])>,
+            EnableIfTransformIsByCopy<std::enable_if_t<
+                absl::type_traits_internal::IsView<Container>::value,
+                Transform>> = true>
+  constexpr AnySpan(  // NOLINT(google-explicit-constructor)
+      const Container& container, const Transform& transform)
+      : AnySpan(any_span_internal::MakeContainerGetter<T>(container, transform),
+                container.size()) {}
+  template <typename Container, typename Transform,
+            typename = EnableIfContainer<Container>,
+            typename = EnableIfTransformIsValid<
+                Transform, decltype(std::declval<const Container&>()[0])>,
+            EnableIfTransformIsByCopy<std::enable_if_t<
+                !absl::type_traits_internal::IsView<Container>::value,
+                Transform>> = true>
+  constexpr AnySpan(  // NOLINT(google-explicit-constructor)
+      const Container& container ABSL_ATTRIBUTE_LIFETIME_BOUND,
+      const Transform& transform)
+      : AnySpan(any_span_internal::MakeContainerGetter<T>(container, transform),
+                container.size()) {}
+  template <
+      typename Container, typename Transform = any_span_transform::IdentityT,
+      typename = EnableIfContainer<Container>,
+      typename = EnableIfTransformIsValid<
+          Transform, decltype(std::declval<const Container&>()[0])>,
+      EnableIfTransformIsByRef<
+          std::enable_if_t<absl::type_traits_internal::IsView<Container>::value,
+                           Transform>> = true>
+  constexpr AnySpan(  // NOLINT(google-explicit-constructor)
+      const Container& container,
+      const Transform& transform ABSL_ATTRIBUTE_LIFETIME_BOUND =
+          any_span_transform::Identity())
+      : AnySpan(any_span_internal::MakeContainerGetter<T>(container, transform),
+                container.size()) {}
+  template <typename Container,
+            typename Transform = any_span_transform::IdentityT,
+            typename = EnableIfContainer<Container>,
+            typename = EnableIfTransformIsValid<
+                Transform, decltype(std::declval<const Container&>()[0])>,
+            EnableIfTransformIsByRef<std::enable_if_t<
+                !absl::type_traits_internal::IsView<Container>::value,
+                Transform>> = true>
+  constexpr AnySpan(  // NOLINT(google-explicit-constructor)
+      const Container& container ABSL_ATTRIBUTE_LIFETIME_BOUND,
+      const Transform& transform ABSL_ATTRIBUTE_LIFETIME_BOUND =
+          any_span_transform::Identity())
+      : AnySpan(any_span_internal::MakeContainerGetter<T>(container, transform),
+                container.size()) {}
+
+  // Creates a span that wraps a mutable array. Applies the optional transform
+  // to elements before returning them.
+  //
+  // Transform must be a function object with a const operator() that takes
+  // Element as an argument and return a reference to T or compatible object.
+  //
+  // Both the transform and array must outlive this span.
+  template <typename Element, typename Transform,
+            typename = EnableIfMutable<Element>,
+            typename = EnableIfTransformIsValid<Transform, Element&>,
+            EnableIfTransformIsByCopy<Transform> = true>
+  constexpr AnySpan(Element* absl_nullable ptr ABSL_ATTRIBUTE_LIFETIME_BOUND,
+                    size_type size, const Transform& transform)
+      : AnySpan(any_span_internal::MakeArrayGetter<T>(ptr, transform), size) {}
+  template <typename Element,
+            typename Transform = any_span_transform::IdentityT,
+            typename = EnableIfMutable<Element>,
+            typename = EnableIfTransformIsValid<Transform, Element&>,
+            EnableIfTransformIsByRef<Transform> = true>
+  constexpr AnySpan(Element* absl_nullable ptr ABSL_ATTRIBUTE_LIFETIME_BOUND,
+                    size_type size,
+                    const Transform& transform ABSL_ATTRIBUTE_LIFETIME_BOUND =
+                        any_span_transform::Identity())
+      : AnySpan(any_span_internal::MakeArrayGetter<T>(ptr, transform), size) {}
+
+  // Creates a span that wraps a mutable array of fixed size. Applies the
+  // optional transform to elements before returning them.
+  //
+  // Transform must be a function object with a const operator() that takes
+  // Element as an argument and return a reference to T or compatible object.
+  //
+  // Both the transform and array must outlive this span.
+  template <typename Element, size_type N, typename Transform,
+            typename = EnableIfMutable<Element>,
+            typename = EnableIfTransformIsValid<Transform, Element&>,
+            EnableIfTransformIsByCopy<Transform> = true>
+  constexpr AnySpan(  // NOLINT(google-explicit-constructor)
+      Element (&array ABSL_ATTRIBUTE_LIFETIME_BOUND)[N],
+      const Transform& transform)
+      : AnySpan(array, N, transform) {}
+  template <typename Element, size_type N,
+            typename Transform = any_span_transform::IdentityT,
+            typename = EnableIfMutable<Element>,
+            typename = EnableIfTransformIsValid<Transform, Element&>,
+            EnableIfTransformIsByRef<Transform> = true>
+  constexpr AnySpan(  // NOLINT(google-explicit-constructor)
+      Element (&array ABSL_ATTRIBUTE_LIFETIME_BOUND)[N],
+      const Transform& transform ABSL_ATTRIBUTE_LIFETIME_BOUND =
+          any_span_transform::Identity())
+      : AnySpan(array, N, transform) {}
+
+  // Creates a span that wraps a mutable container. Only enabled if T is
+  // mutable. Applies the optional transform to elements before returning them.
+  //
+  // Transform must be a function object with a const operator() that takes the
+  // value type of Container as an argument and return a reference to T or
+  // compatible object.
+  //
+  // The transform, container, and the container's underlying storage must
+  // outlive this span. Any operation that may reallocate the container's
+  // storage or change its size will invalidate the span.
+  template <typename Container, typename Transform,
+            typename = EnableIfMutable<Container>,
+            typename = EnableIfContainer<Container>,
+            typename = EnableIfTransformIsValid<
+                Transform, decltype(std::declval<Container&>()[0])>,
+            EnableIfTransformIsByCopy<Transform> = true>
+  constexpr explicit AnySpan(  // NOLINT(google-explicit-constructor)
+      Container& container ABSL_ATTRIBUTE_LIFETIME_BOUND,
+      const Transform& transform)
+      : AnySpan(any_span_internal::MakeContainerGetter<T>(container, transform),
+                container.size()) {}
+  template <typename Container,
+            typename Transform = any_span_transform::IdentityT,
+            typename = EnableIfMutable<Container>,
+            typename = EnableIfContainer<Container>,
+            typename = EnableIfTransformIsValid<
+                Transform, decltype(std::declval<Container&>()[0])>,
+            EnableIfTransformIsByRef<Transform> = true>
+  constexpr explicit AnySpan(  // NOLINT(google-explicit-constructor)
+      Container& container ABSL_ATTRIBUTE_LIFETIME_BOUND,
+      const Transform& transform ABSL_ATTRIBUTE_LIFETIME_BOUND =
+          any_span_transform::Identity())
+      : AnySpan(any_span_internal::MakeContainerGetter<T>(container, transform),
+                container.size()) {}
+
+  // Converts a mutable span to a const span by copying the internal state
+  // (rather than wrapping the other span).
+  // TODO(b/179783710): add ABSL_ATTRIBUTE_LIFETIME_BOUND.
+  template <typename LazyT = T, typename = EnableIfConst<LazyT>>
+  constexpr AnySpan(  // NOLINT(google-explicit-constructor)
+      const AnySpan<std::remove_const_t<T>>& other)
+      : getter_(other.getter_), size_(other.size()) {}
+
+  // Creates a span that wraps around another span of different type.
+  //
+  // This has performance and lifetime consequences, and can easily happen by
+  // mistake. We make such conversions explicit here.
+  template <typename Element, typename = EnableIfDifferentElementType<Element>,
+            typename = EnableIfTransformIsValid<any_span_transform::IdentityT,
+                                                Element&>>
+  constexpr explicit AnySpan(
+      const AnySpan<Element>& other ABSL_ATTRIBUTE_LIFETIME_BOUND)
+      : AnySpan(any_span_internal::MakeContainerGetter<T>(
+                    other, any_span_transform::Identity()),
+                other.size()) {}
+
+  // Creates a span that wraps around another span. Applies the non-optional
+  // transform to elements before returning them.
+  //
+  // This has lifetime consequences, and may happen by mistake. We make it
+  // explicit here.
+  template <typename Element, typename Transform,
+            typename = EnableIfTransformIsValid<Transform, Element&>,
+            EnableIfTransformIsByCopy<Transform> = true>
+  constexpr explicit AnySpan(const AnySpan<Element>& other
+                                 ABSL_ATTRIBUTE_LIFETIME_BOUND,
+                             const Transform& transform)
+      : AnySpan(any_span_internal::MakeContainerGetter<T>(other, transform),
+                other.size()) {}
+  template <typename Element, typename Transform,
+            typename = EnableIfTransformIsValid<Transform, Element&>,
+            EnableIfTransformIsByRef<Transform> = true>
+  constexpr explicit AnySpan(
+      const AnySpan<Element>& other ABSL_ATTRIBUTE_LIFETIME_BOUND,
+      const Transform& transform ABSL_ATTRIBUTE_LIFETIME_BOUND)
+      : AnySpan(any_span_internal::MakeContainerGetter<T>(other, transform),
+                other.size()) {}
+
+  // Returns a subspan of this span. This span may become invalid before the
+  // subspan, but both the container and transform must remain valid.
+  // pos must be non-negative and <= size().
+  // len must be non-negative and <= size() - pos, or equal to npos.
+  // If len==npos, the subspan continues till the end of this span.
+  constexpr AnySpan subspan(size_type pos, size_type len = npos) const {
+    const size_t this_size = size();
+    if (len == AnySpan<T>::npos) {
+      len = this_size - pos;
+    }
+    absl::base_internal::HardeningAssertLE(pos, this_size);
+    absl::base_internal::HardeningAssertLE(
+        len, static_cast<size_type>(this_size - pos));
+    return AnySpan<T>(getter_.Offset(pos), len);
+  }
+
+  // Returns a `AnySpan` containing first `len` elements. Parameter `len` must
+  // be non-negative and <= size().
+  constexpr AnySpan first(size_type len) const {
+    absl::base_internal::HardeningAssert(len != npos);
+    return subspan(0, len);
+  }
+
+  // Returns a `AnySpan` containing last `len` elements. Parameter `len` must be
+  // non-negative and <= size().
+  constexpr AnySpan last(size_type len) const { return subspan(size() - len); }
+
+  // Size operations.
+  constexpr size_type size() const { return size_; }
+  constexpr bool empty() const { return size() == 0; }
+
+  // Element access.
+  constexpr reference operator[](size_type index) const {
+    absl::base_internal::HardeningAssertLT(index, size());
+    return getter_.Get(index);
+  }
+  constexpr reference at(size_type index) const {
+    if (ABSL_PREDICT_FALSE(index >= size())) {
+      absl::ThrowStdOutOfRange("AnySpan::at failed bounds check");
+    }
+    return getter_.Get(index);
+  }
+  constexpr reference front() const {
+    absl::base_internal::HardeningAssertGT(size(), size_type{0});
+    return (*this)[0];
+  }
+  constexpr reference back() const {
+    absl::base_internal::HardeningAssertGT(size(), size_type{0});
+    return (*this)[size() - 1];
+  }
+
+  // Iterator accessors.
+  constexpr iterator begin() const { return iterator(this, 0); }
+  constexpr iterator end() const { return iterator(this, size_); }
+  constexpr reverse_iterator rbegin() const { return reverse_iterator(end()); }
+  constexpr reverse_iterator rend() const { return reverse_iterator(begin()); }
+  constexpr const_iterator cbegin() const { return const_iterator(this, 0); }
+  constexpr const_iterator cend() const { return const_iterator(this, size_); }
+  constexpr const_reverse_iterator crbegin() const { return rbegin(); }
+  constexpr const_reverse_iterator crend() const { return rend(); }
+
+  // Constructs from a getter and size. Not for external use.
+  AnySpan(any_span_internal::Getter<T> getter, size_type size)
+      : getter_(getter), size_(size) {}
+
+  // Support for absl::Hash.
+  template <typename H>
+  friend constexpr H AbslHashValue(H state, AnySpan any_span) {
+    for (const auto& v : any_span) {
+      state = H::combine(std::move(state), v);
+    }
+    return H::combine(std::move(state), any_span.size());
+  }
+
+ private:
+  template <typename U>
+  friend class AnySpan;
+
+  template <typename U>
+  friend bool any_span_internal::IsCheap(AnySpan<U> s);
+
+  // Getter to access elements.
+  any_span_internal::Getter<T> getter_;
+
+  // The size of this span.
+  size_type size_ = 0;
+
+#if defined(__GNUC__) && !defined(__clang__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wnon-template-friend"
+#endif
+  // The technical reasons we need to declare these friends in this manner are
+  // quite subtle and confusing, but they're necessary on some toolchains to
+  // allow all mutable/const combinations with this & other range types while
+  // avoiding symbol collisions or ODR violations.
+  friend bool operator==(AnySpan<const T> a, AnySpan<const T> b);
+  friend bool operator!=(AnySpan<const T> a, AnySpan<const T> b);
+#if defined(__GNUC__) && !defined(__clang__)
+#pragma GCC diagnostic pop
+#endif
+
+  // operator==
+  friend bool operator==(AnySpan a, AnySpan b) {
+    return any_span_internal::EqualImpl<const T>(a, b);
+  }
+  friend bool operator!=(AnySpan a, AnySpan b) { return !(a == b); }
+};
+
+// Constructs an AnySpan from a container or array.
+template <int&... ExplicitArgumentBarrier, typename Container,
+          typename T = any_span_internal::ElementType<Container>>
+std::enable_if_t<
+    absl::type_traits_internal::IsView<std::remove_cv_t<Container>>::value,
+    AnySpan<T>>
+MakeAnySpan(Container& c) {
+  return AnySpan<T>(c);
+}
+template <int&... ExplicitArgumentBarrier, typename Container,
+          typename T = any_span_internal::ElementType<Container>>
+std::enable_if_t<
+    !absl::type_traits_internal::IsView<std::remove_cv_t<Container>>::value,
+    AnySpan<T>>
+MakeAnySpan(Container& c ABSL_ATTRIBUTE_LIFETIME_BOUND) {
+  return AnySpan<T>(c);
+}
+
+// Constructs an AnySpan that dereferences a container or array of pointers.
+template <int&... ExplicitArgumentBarrier, typename Container,
+          typename T = any_span_internal::DerefElementType<Container>>
+std::enable_if_t<
+    absl::type_traits_internal::IsView<std::remove_cv_t<Container>>::value,
+    AnySpan<T>>
+MakeDerefAnySpan(Container& c) {
+  return AnySpan<T>(c, any_span_transform::Deref());
+}
+template <int&... ExplicitArgumentBarrier, typename Container,
+          typename T = any_span_internal::DerefElementType<Container>>
+std::enable_if_t<
+    !absl::type_traits_internal::IsView<std::remove_cv_t<Container>>::value,
+    AnySpan<T>>
+MakeDerefAnySpan(Container& c ABSL_ATTRIBUTE_LIFETIME_BOUND) {
+  return AnySpan<T>(c, any_span_transform::Deref());
+}
+
+// Constructs an AnySpan from a pointer and size.
+template <int&... ExplicitArgumentBarrier, typename T>
+AnySpan<T> MakeAnySpan(T* absl_nullable ptr ABSL_ATTRIBUTE_LIFETIME_BOUND,
+                       std::size_t size) {
+  return AnySpan<T>(ptr, size);
+}
+
+// Constructs a const AnySpan from a container or array.
+template <int&... ExplicitArgumentBarrier, typename Container,
+          typename T = any_span_internal::ElementType<const Container>>
+std::enable_if_t<absl::type_traits_internal::IsView<Container>::value,
+                 AnySpan<const T>>
+MakeConstAnySpan(const Container& c) {
+  return AnySpan<const T>(c);
+}
+template <int&... ExplicitArgumentBarrier, typename Container,
+          typename T = any_span_internal::ElementType<const Container>>
+std::enable_if_t<!absl::type_traits_internal::IsView<Container>::value,
+                 AnySpan<const T>>
+MakeConstAnySpan(const Container& c ABSL_ATTRIBUTE_LIFETIME_BOUND) {
+  return AnySpan<const T>(c);
+}
+
+// Constructs a const AnySpan that dereferences a container or array of
+// pointers.
+template <int&... ExplicitArgumentBarrier, typename Container,
+          typename T = any_span_internal::DerefElementType<const Container>>
+std::enable_if_t<absl::type_traits_internal::IsView<Container>::value,
+                 AnySpan<const T>>
+MakeConstDerefAnySpan(const Container& c) {
+  return AnySpan<const T>(c, any_span_transform::Deref());
+}
+template <int&... ExplicitArgumentBarrier, typename Container,
+          typename T = any_span_internal::DerefElementType<const Container>>
+std::enable_if_t<!absl::type_traits_internal::IsView<Container>::value,
+                 AnySpan<const T>>
+MakeConstDerefAnySpan(const Container& c ABSL_ATTRIBUTE_LIFETIME_BOUND) {
+  return AnySpan<const T>(c, any_span_transform::Deref());
+}
+
+// Constructs an AnySpan from a pointer and size.
+template <int&... ExplicitArgumentBarrier, typename T>
+AnySpan<const T> MakeConstAnySpan(const T* absl_nullable ptr,
+                                  std::size_t size) {
+  return AnySpan<const T>(ptr, size);
+}
+
+//
+// Implementation details follow.
+//
+
+// Iterator base class. Uses CRTP (Iter should be the child class). Constness of
+// the iterator is determined by the constness of Value.
+template <typename T>
+template <typename Iter, typename Value>
+class ABSL_ATTRIBUTE_VIEW AnySpan<T>::IteratorBase {
+ private:
+  // Returns a reference to this as the child class.
+  const Iter& self() const { return static_cast<const Iter&>(*this); }
+  Iter& self() { return static_cast<Iter&>(*this); }
+
+ public:
+  using iterator_category = std::random_access_iterator_tag;
+  using value_type = std::remove_const_t<Value>;
+  using difference_type = std::ptrdiff_t;
+  using reference = Value&;
+  using pointer = Value*;
+
+  // Constructs an invalid iterator.
+  IteratorBase() = default;
+
+  reference operator*() const { return (*container_)[index_]; }
+
+  pointer absl_nonnull operator->() const { return &(*container_)[index_]; }
+
+  reference operator[](difference_type i) const {
+    return (*container_)[index_ + i];
+  }
+
+  Iter& operator+=(difference_type d) {
+    index_ += d;
+    return self();
+  }
+
+  Iter& operator-=(difference_type d) { return self() += -d; }
+
+  Iter& operator++() {
+    self() += 1;
+    return self();
+  }
+
+  Iter operator++(int) {
+    Iter copy(self());
+    ++self();
+    return copy;
+  }
+
+  Iter& operator--() {
+    self() -= 1;
+    return self();
+  }
+
+  Iter operator--(int) {
+    Iter copy(self());
+    --self();
+    return copy;
+  }
+
+  Iter operator+(difference_type d) const {
+    Iter tmp = self();
+    tmp += d;
+    return tmp;
+  }
+
+  friend Iter operator+(difference_type d, Iter i) { return i + d; }
+
+  Iter operator-(difference_type d) const { return self() + (-d); }
+
+  difference_type operator-(const Iter& other) const {
+    return index_ - other.index_;
+  }
+
+  friend bool operator==(const Iter& a, const Iter& b) {
+    return a.index_ == b.index_;
+  }
+
+  friend bool operator!=(const Iter& a, const Iter& b) {
+    return a.index_ != b.index_;
+  }
+
+  friend bool operator<(const Iter& a, const Iter& b) {
+    return a.index_ < b.index_;
+  }
+
+  friend bool operator<=(const Iter& a, const Iter& b) {
+    return a.index_ <= b.index_;
+  }
+
+  friend bool operator>(const Iter& a, const Iter& b) {
+    return a.index_ > b.index_;
+  }
+
+  friend bool operator>=(const Iter& a, const Iter& b) {
+    return a.index_ >= b.index_;
+  }
+
+ protected:
+  // Constructs an iterator that points to the given index of the given span.
+  IteratorBase(const AnySpan* absl_nullable container, size_type index)
+      : container_(container), index_(index) {}
+
+  const AnySpan* absl_nullable container_ = nullptr;
+  size_type index_ = 0;
+};
+
+// iterator implementation. This mostly just forwards to IteratorBase.
+template <typename T>
+class ABSL_ATTRIBUTE_VIEW AnySpan<T>::iterator
+    : public IteratorBase<iterator, T> {
+ private:
+  using Base = IteratorBase<iterator, T>;
+
+ public:
+  using typename Base::difference_type;
+  using typename Base::iterator_category;
+  using typename Base::pointer;
+  using typename Base::reference;
+  using typename Base::value_type;
+
+  iterator() = default;
+
+ private:
+  // Only let AnySpan construct valid instances.
+  friend class AnySpan;
+
+  iterator(const AnySpan* absl_nullable container, size_type index)
+      : Base(container, index) {}
+};
+
+// const_iterator implementation. This mostly just forwards to IteratorBase,
+// but also provides conversion from MutableIterator.
+template <typename T>
+class AnySpan<T>::const_iterator
+    : public IteratorBase<const_iterator, std::add_const_t<T>> {
+ private:
+  using Base = IteratorBase<const_iterator, std::add_const_t<T>>;
+
+ public:
+  using typename Base::difference_type;
+  using typename Base::iterator_category;
+  using typename Base::pointer;
+  using typename Base::reference;
+  using typename Base::value_type;
+
+  const_iterator() = default;
+
+  // Support conversion from mutable iterators.
+  // NOLINTNEXTLINE(google-explicit-constructor)
+  const_iterator(const iterator& other)  // NOLINT(runtime/explicit)
+      : Base(other.container_, other.index_) {}
+
+ private:
+  // Only let AnySpan construct valid instances.
+  friend class AnySpan;
+
+  const_iterator(const AnySpan* absl_nullable container, size_type index)
+      : Base(container, index) {}
+};
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_TYPES_ANY_SPAN_H_
diff --git a/absl/types/any_span_benchmark.cc b/absl/types/any_span_benchmark.cc
new file mode 100644
index 00000000000..a2c4836b6ed
--- /dev/null
+++ b/absl/types/any_span_benchmark.cc
@@ -0,0 +1,258 @@
+// Copyright 2026 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <cstddef>
+#include <cstdint>
+#include <deque>
+#include <string>
+#include <type_traits>
+#include <vector>
+
+#include "absl/base/config.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/flags/flag.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/string_view.h"
+#include "absl/types/any_span.h"
+#include "absl/types/span.h"
+#include "benchmark/benchmark.h"
+
+ABSL_FLAG(uint64_t, benchmark_container_size, 2000,
+          "The size of the containers to use for benchmarking.");
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace {
+
+//
+// Benchmarks and supporting code follows.
+//
+
+template <typename T>
+T MakeElement(int) {
+  ABSL_RAW_LOG(FATAL, "Not implemented.");
+}
+
+template <>
+std::string MakeElement<std::string>(int i) {
+  return absl::StrCat(i, i, i);
+}
+
+template <>
+int MakeElement<int>(int i) {
+  return i;
+}
+
+template <typename Container>
+Container MakeContainer() {
+  const size_t container_size = absl::GetFlag(FLAGS_benchmark_container_size);
+  Container c;
+  for (size_t i = 0; i < container_size; ++i) {
+    c.push_back(
+        MakeElement<typename Container::value_type>(static_cast<int>(i)));
+  }
+  return c;
+}
+
+template <typename Container, typename SourceContainer>
+Container MakePointerContainer(SourceContainer* source) {
+  Container c;
+  for (auto& value : *source) {
+    c.push_back(&value);
+  }
+  return c;
+}
+
+//
+// Sequential Access Benchmarks
+//
+
+// Control run, iterating original container.
+template <typename Container>
+void BM_Sequential(benchmark::State& state) {
+  auto a = MakeContainer<Container>();
+  benchmark::DoNotOptimize(a);
+  for (auto _ : state) {
+    for (auto& v : a) {
+      benchmark::DoNotOptimize(v);
+    }
+  }
+}
+
+// Wraps the container with a Span.
+template <typename Container>
+void BM_Sequential_Span(benchmark::State& state) {
+  auto container = MakeContainer<Container>();
+  absl::Span<typename Container::value_type> array_span(container);
+  benchmark::DoNotOptimize(array_span);
+  for (auto _ : state) {
+    for (auto& v : array_span) {
+      benchmark::DoNotOptimize(v);
+    }
+  }
+}
+
+// Same as BM_Sequential_Span, but uses a container of pointers and
+// dereferences it.
+template <typename Container>
+void BM_Sequential_DerefSpan(benchmark::State& state) {
+  using T = std::remove_pointer_t<typename Container::value_type>;
+  auto values = MakeContainer<std::vector<T>>();
+  const auto container = MakePointerContainer<Container>(&values);
+  absl::Span<T* const> array_span(container);
+  benchmark::DoNotOptimize(array_span);
+  for (auto _ : state) {
+    for (const auto& v : array_span) {
+      benchmark::DoNotOptimize(*v);
+    }
+  }
+}
+
+// Wraps a AnySpan<const T> around a Container. Sequentially reads the
+// entire span.
+template <typename Container>
+void BM_Sequential_AnySpan(benchmark::State& state) {
+  using T = typename Container::value_type;
+  auto container = MakeContainer<Container>();
+  AnySpan<T> span(container);
+  benchmark::DoNotOptimize(span);
+  for (auto _ : state) {
+    for (T& s : span) {
+      benchmark::DoNotOptimize(s);
+    }
+  }
+}
+
+// Same as BM_Sequential_AnySpan, but dereferences the elements.
+template <typename Container>
+void BM_Sequential_AnySpanDeref(benchmark::State& state) {
+  using T = std::remove_pointer_t<typename Container::value_type>;
+  auto values = MakeContainer<std::vector<T>>();
+  auto container = MakePointerContainer<Container>(&values);
+  AnySpan<T> span(container, any_span_transform::Deref());
+  benchmark::DoNotOptimize(span);
+  for (auto _ : state) {
+    for (auto& s : span) {
+      benchmark::DoNotOptimize(s);
+    }
+  }
+}
+
+//
+// Random Access Benchmarks
+//
+
+// Control run, accessing the original container.
+template <typename Container>
+void BM_Random(benchmark::State& state) {
+  auto a = MakeContainer<Container>();
+  auto* b = &a;
+  benchmark::DoNotOptimize(a);
+  benchmark::DoNotOptimize(b);
+  for (auto _ : state) {
+    uint64_t index_seed = 0;
+    for (size_t j = 0; j < a.size(); ++j) {
+      index_seed += 5623;
+      const uint64_t index = index_seed % a.size();
+      benchmark::DoNotOptimize((*b)[index]);
+    }
+  }
+}
+
+// Wraps a Span around a Container. Randomly reads the span.
+template <typename Container>
+void BM_Random_Span(benchmark::State& state) {
+  using T = typename Container::value_type;
+  auto v = MakeContainer<Container>();
+  absl::Span<T> array_span(v);
+  benchmark::DoNotOptimize(array_span);
+  for (auto _ : state) {
+    uint64_t index_seed = 0;
+    for (size_t j = 0; j < v.size(); ++j) {
+      index_seed += 5623;
+      const uint64_t index = index_seed % v.size();
+      benchmark::DoNotOptimize(array_span[index]);
+    }
+  }
+}
+
+// Wraps a AnySpan around a Container. Randomly reads the span.
+template <typename Container>
+void BM_Random_AnySpan(benchmark::State& state) {
+  using T = typename Container::value_type;
+  auto container = MakeContainer<Container>();
+  AnySpan<T> span(container);
+  benchmark::DoNotOptimize(span);
+  for (auto _ : state) {
+    uint64_t index_seed = 0;
+    for (size_t j = 0; j < container.size(); ++j) {
+      index_seed += 5623;
+      const uint64_t index = index_seed % span.size();
+      benchmark::DoNotOptimize(span[index]);
+    }
+  }
+}
+
+// Same as BM_Random_AnySpan, but dereferences elements.
+template <typename Container>
+void BM_Random_AnySpanDeref(benchmark::State& state) {
+  using T = std::remove_pointer_t<typename Container::value_type>;
+  auto values = MakeContainer<std::vector<T>>();
+  auto container = MakePointerContainer<Container>(&values);
+  AnySpan<T> span(container, any_span_transform::Deref());
+  benchmark::DoNotOptimize(span);
+  for (auto _ : state) {
+    uint64_t index_seed = 0;
+    for (size_t j = 0; j < container.size(); ++j) {
+      index_seed += 5623;
+      const uint64_t index = index_seed % span.size();
+      benchmark::DoNotOptimize(span[index]);
+    }
+  }
+}
+
+// Sequential access int.
+BENCHMARK_TEMPLATE(BM_Sequential, std::vector<int>);
+BENCHMARK_TEMPLATE(BM_Sequential, std::deque<int>);
+BENCHMARK_TEMPLATE(BM_Sequential_Span, std::vector<int>);
+BENCHMARK_TEMPLATE(BM_Sequential_AnySpan, std::vector<int>);
+BENCHMARK_TEMPLATE(BM_Sequential_AnySpan, std::deque<int>);
+
+// Sequential access string.
+BENCHMARK_TEMPLATE(BM_Sequential, std::vector<std::string>);
+BENCHMARK_TEMPLATE(BM_Sequential, std::deque<std::string>);
+BENCHMARK_TEMPLATE(BM_Sequential_Span, std::vector<std::string>);
+BENCHMARK_TEMPLATE(BM_Sequential_AnySpan, std::vector<std::string>);
+BENCHMARK_TEMPLATE(BM_Sequential_AnySpan, std::deque<std::string>);
+BENCHMARK_TEMPLATE(BM_Sequential_DerefSpan, std::vector<std::string*>);
+BENCHMARK_TEMPLATE(BM_Sequential_AnySpanDeref, std::vector<std::string*>);
+
+// Random access int.
+BENCHMARK_TEMPLATE(BM_Random, std::vector<int>);
+BENCHMARK_TEMPLATE(BM_Random, std::deque<int>);
+BENCHMARK_TEMPLATE(BM_Random_Span, std::vector<int>);
+BENCHMARK_TEMPLATE(BM_Random_AnySpan, std::vector<int>);
+BENCHMARK_TEMPLATE(BM_Random_AnySpan, std::deque<int>);
+
+// Random access string.
+BENCHMARK_TEMPLATE(BM_Random, std::vector<std::string>);
+BENCHMARK_TEMPLATE(BM_Random, std::deque<std::string>);
+BENCHMARK_TEMPLATE(BM_Random_Span, std::vector<std::string>);
+BENCHMARK_TEMPLATE(BM_Random_AnySpan, std::vector<std::string>);
+BENCHMARK_TEMPLATE(BM_Random_AnySpan, std::deque<std::string>);
+BENCHMARK_TEMPLATE(BM_Random_AnySpanDeref, std::vector<std::string*>);
+
+}  // namespace
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/types/any_span_test.cc b/absl/types/any_span_test.cc
new file mode 100644
index 00000000000..e5960c6aa8a
--- /dev/null
+++ b/absl/types/any_span_test.cc
@@ -0,0 +1,1208 @@
+// Copyright 2026 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/types/any_span.h"
+
+#include <cstddef>
+#include <deque>
+#include <functional>
+#include <iterator>
+#include <memory>
+#include <optional>
+#include <string>
+#include <string_view>
+#include <type_traits>
+#include <utility>
+#include <vector>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/exception_testing.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/base/macros.h"
+#include "absl/hash/hash_testing.h"
+#include "absl/meta/type_traits.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/string_view.h"
+#include "absl/types/span.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace {
+
+using testing::ElementsAre;
+
+static_assert(!absl::type_traits_internal::IsOwner<AnySpan<int>>::value &&
+                  absl::type_traits_internal::IsView<AnySpan<int>>::value,
+              "AnySpan is a view, not an owner");
+
+bool IsHardened() {
+  bool hardened = false;
+  ABSL_HARDENING_ASSERT([&hardened]() {
+    hardened = true;
+    return true;
+  }());
+  return hardened;
+}
+
+// Tests that the span points to all of the elements of the given container.
+template <typename T, typename Container>
+void ExpectSpanEqualsContainer(AnySpan<T> span, const Container& c) {
+  EXPECT_EQ(span.size(), c.size());
+  typename AnySpan<const T>::size_type index = 0;
+  for (const T& s : span) {
+    SCOPED_TRACE(index);
+    const T& expected = c[index];
+    EXPECT_EQ(expected, s);
+    EXPECT_EQ(expected, span[index]);
+    EXPECT_EQ(&expected, &span[index]);
+    ++index;
+  }
+  EXPECT_EQ(index, span.size());
+}
+
+// AnySpanTest covers parts of AnySpan that make sense for both const and
+// non-const elements. How the given tests will run is determined by whether or
+// not ElementsAreConst is true_type or false_type.
+template <typename ElementsAreConst>
+class AnySpanTest : public ::testing::Test {
+ public:
+  using IsConstTest = ElementsAreConst;
+
+  // Const T if this test instantiation is supposed to cover const AnySpans.
+  // T if this test instantiation is supposed to cover mutable AnySpans.
+  template <typename T>
+  using MaybeConst = std::conditional_t<IsConstTest::value, const T, T>;
+
+  // Initalizes a AnySpan around the given container and tests for
+  // equality in a variety of ways.
+  template <typename T, typename Container, typename Transform>
+  static void TestContainer(Container c, const Transform& t) {
+    AnySpan<MaybeConst<T>> span(c, t);
+    EXPECT_NO_FATAL_FAILURE(ExpectSpanEqualsContainer(span, c));
+  }
+
+  // Initializes a dereferencing AnySpan around the given container of pointers
+  // and tests for equality in a variety of ways.
+  template <typename T, typename Container>
+  static void TestPointerContainer(Container* c) {
+    AnySpan<const T> span(*c, any_span_transform::Deref());
+    EXPECT_EQ(span.size(), c->size());
+    int index = 0;
+    for (const T& s : span) {
+      const T& expected = *(*c)[index];
+      EXPECT_EQ(expected, s);
+      EXPECT_EQ(expected, span[index]);
+      EXPECT_EQ(&expected, &span[index]);
+      index++;
+    }
+    EXPECT_EQ(index, span.size());
+  }
+};
+
+using ElementsAreConst = ::testing::Types<std::true_type, std::false_type>;
+
+TYPED_TEST_SUITE(AnySpanTest, ElementsAreConst);
+
+TYPED_TEST(AnySpanTest, NullAndEmpty) {
+  using T = typename TestFixture::template MaybeConst<std::string>;
+  AnySpan<T> empty;
+  EXPECT_TRUE(empty.empty());
+  EXPECT_EQ(empty.size(), 0);
+  for (const std::string& a : empty) {
+    (void)a;
+    ABSL_RAW_LOG(FATAL, "Empty container should not iterate.");
+  }
+  EXPECT_EQ(empty.begin(), empty.end());
+}
+
+TYPED_TEST(AnySpanTest, Ptr) {
+  using T = typename TestFixture::template MaybeConst<std::string>;
+  std::vector<std::string> v = {"a", "b", "c", "d"};
+  AnySpan<T> span1(v.data(), v.size());
+  EXPECT_EQ(&span1[0], &v[0]);
+  AnySpan<T> span2(v.data(), v.size());
+  EXPECT_EQ(&span2[0], &v[0]);
+}
+
+TYPED_TEST(AnySpanTest, PtrPtr) {
+  using T = typename TestFixture::template MaybeConst<int>;
+  int i;
+  std::vector<int*> v = {&i};
+  AnySpan<T> span(v.data(), v.size(), any_span_transform::Deref());
+  EXPECT_EQ(&span[0], &i);
+}
+
+TYPED_TEST(AnySpanTest, DerefOptional) {
+  const std::vector<std::optional<int>> v = {17, 19};
+  const AnySpan<const int> span = absl::MakeDerefAnySpan(v);
+  EXPECT_THAT(span, ElementsAre(17, 19));
+}
+
+TYPED_TEST(AnySpanTest, ArrayOfKnownSize) {
+  using T = typename TestFixture::template MaybeConst<std::string>;
+  std::string a[] = {"a", "b", "c", "d"};
+
+  AnySpan<T> span(a);
+  EXPECT_EQ(span.size(), 4);
+  EXPECT_EQ(&span[0], &a[0]);
+}
+
+// This gets its own test because AnySpan(array, size) has the potential to be
+// ambiguous with AnySpan(array, transform).
+TYPED_TEST(AnySpanTest, ArrayWithExplicitSize) {
+  using T = typename TestFixture::template MaybeConst<std::string>;
+  std::string a[] = {"a", "b", "c", "d"};
+
+  AnySpan<T> span(a, 4);
+  EXPECT_EQ(span.size(), 4);
+  EXPECT_EQ(&span[0], &a[0]);
+}
+
+TYPED_TEST(AnySpanTest, PtrArrayOfKnownSize) {
+  using T = typename TestFixture::template MaybeConst<std::string>;
+  std::string s;
+  std::string* a[] = {&s, &s, &s, &s};
+
+  AnySpan<T> span(a, any_span_transform::Deref());
+  EXPECT_EQ(span.size(), 4);
+  EXPECT_EQ(&span[0], &s);
+}
+
+TYPED_TEST(AnySpanTest, Range) {
+  using T = typename TestFixture::template MaybeConst<std::string>;
+  std::vector<std::string> a = {"a", "b", "c", "d"};
+  auto range = any_span_adaptor::MakeAdaptorFromRange(a.begin(), a.end());
+  AnySpan<T> span(range);
+  EXPECT_NO_FATAL_FAILURE(ExpectSpanEqualsContainer(span, a));
+}
+
+TYPED_TEST(AnySpanTest, View) {
+  using T = typename TestFixture::template MaybeConst<std::string>;
+  std::vector<std::string> a = {"a", "b", "c", "d"};
+  auto range = any_span_adaptor::MakeAdaptorFromView(a);
+  AnySpan<T> span(range);
+  EXPECT_NO_FATAL_FAILURE(ExpectSpanEqualsContainer(span, a));
+}
+
+TYPED_TEST(AnySpanTest, VectorString) {
+  EXPECT_NO_FATAL_FAILURE(TestFixture::template TestContainer<std::string>(
+      std::vector<std::string>{"a", "b", "c", "d"},
+      any_span_transform::Identity()));
+}
+
+TYPED_TEST(AnySpanTest, VectorInt) {
+  EXPECT_NO_FATAL_FAILURE(TestFixture::template TestContainer<int>(
+      std::vector<int>{1, 2, 3, 4}, any_span_transform::Identity()));
+}
+
+TYPED_TEST(AnySpanTest, VectorStringPointer) {
+  std::vector<std::string> v = {"a", "b", "c", "d"};
+  std::vector<std::string*> vp;
+  for (std::string& s : v) {
+    vp.push_back(&s);
+  }
+  TestFixture::template TestPointerContainer<std::string>(&vp);
+}
+
+TYPED_TEST(AnySpanTest, VectorStringUniquePointer) {
+  std::vector<std::unique_ptr<std::string>> v;
+  for (const std::string& s : std::vector<std::string>{"a", "b", "c", "d"}) {
+    v.emplace_back(new std::string(s));
+  }
+  TestFixture::template TestPointerContainer<std::string>(&v);
+}
+
+TYPED_TEST(AnySpanTest, VectorIntPointer) {
+  std::vector<int> v = {1, 2, 3, 4};
+  std::vector<int*> vp;
+  for (int& i : v) {
+    vp.push_back(&i);
+  }
+  TestFixture::template TestPointerContainer<int>(&vp);
+}
+
+TYPED_TEST(AnySpanTest, DequeString) {
+  EXPECT_NO_FATAL_FAILURE(TestFixture::template TestContainer<std::string>(
+      std::deque<std::string>{"a", "b", "c", "d"},
+      any_span_transform::Identity()));
+}
+
+TYPED_TEST(AnySpanTest, DequeInt) {
+  EXPECT_NO_FATAL_FAILURE(TestFixture::template TestContainer<int>(
+      std::deque<int>{1, 2, 3, 4}, any_span_transform::Identity()));
+}
+
+TYPED_TEST(AnySpanTest, DequeStringPtr) {
+  std::vector<std::string> v = {"a", "b", "c", "d"};
+  std::deque<std::string*> dp;
+  for (std::string& s : v) {
+    dp.push_back(&s);
+  }
+  TestFixture::template TestPointerContainer<std::string>(&dp);
+}
+
+TYPED_TEST(AnySpanTest, ImplicitConversions) {
+  using T = typename TestFixture::template MaybeConst<std::string>;
+  std::string one = "1";
+  std::string two = "2";
+  std::vector<std::reference_wrapper<std::string>> v(
+      {std::ref(one), std::ref(two)});
+  AnySpan<T> span(v);
+
+  EXPECT_THAT(span, ElementsAre("1", "2"));
+}
+
+TYPED_TEST(AnySpanTest, TrivialSubspan) {
+  EXPECT_EQ(AnySpan<int>().subspan(0).size(), 0);
+}
+
+TYPED_TEST(AnySpanTest, TrivialFirst) {
+  using T = typename TestFixture::template MaybeConst<int>;
+  EXPECT_EQ(AnySpan<T>().first(0).size(), 0);
+}
+
+TYPED_TEST(AnySpanTest, TrivialLast) {
+  using T = typename TestFixture::template MaybeConst<int>;
+  EXPECT_EQ(AnySpan<T>().last(0).size(), 0);
+}
+
+struct Base1 {
+  virtual ~Base1() = default;
+  bool operator==(const Base1& other) const { return this == &other; }
+  int b1 = 1;
+};
+struct Base2 {
+  virtual ~Base2() = default;
+  bool operator==(const Base2& other) const { return this == &other; }
+  int b2 = 2;
+};
+struct Child : public Base1, public Base2 {
+  bool operator==(const Child& other) const { return this == &other; }
+};
+
+TYPED_TEST(AnySpanTest, VectorBaseClass) {
+  std::vector<Child> v;
+  v.resize(20);
+  EXPECT_NO_FATAL_FAILURE(TestFixture::template TestContainer<Base1>(
+      v, any_span_transform::Identity()));
+  EXPECT_NO_FATAL_FAILURE(TestFixture::template TestContainer<Base2>(
+      v, any_span_transform::Identity()));
+}
+
+TYPED_TEST(AnySpanTest, AnySpanVectorBaseClass) {
+  std::vector<Child> v;
+  v.resize(20);
+  AnySpan<Child> span(v);
+  EXPECT_NO_FATAL_FAILURE(TestFixture::template TestContainer<Base1>(
+      span, any_span_transform::Identity()));
+  EXPECT_NO_FATAL_FAILURE(TestFixture::template TestContainer<Base2>(
+      span, any_span_transform::Identity()));
+}
+
+TYPED_TEST(AnySpanTest, DerefAnySpanVectorBaseClass) {
+  Child c;
+  std::vector<Child*> v_ptrs(20, &c);
+  AnySpan<Child> span(v_ptrs, any_span_transform::Deref());
+  EXPECT_NO_FATAL_FAILURE(TestFixture::template TestContainer<Base1>(
+      span, any_span_transform::Identity()));
+  EXPECT_NO_FATAL_FAILURE(TestFixture::template TestContainer<Base2>(
+      span, any_span_transform::Identity()));
+}
+
+TYPED_TEST(AnySpanTest, AnySpanDequeBaseClass) {
+  std::deque<Child> v;
+  v.resize(20);
+  AnySpan<Child> span(v);
+  EXPECT_NO_FATAL_FAILURE(TestFixture::template TestContainer<Base1>(
+      span, any_span_transform::Identity()));
+  EXPECT_NO_FATAL_FAILURE(TestFixture::template TestContainer<Base2>(
+      span, any_span_transform::Identity()));
+}
+
+TYPED_TEST(AnySpanTest, VectorSubspan) {
+  using T = typename TestFixture::template MaybeConst<int>;
+  std::vector<int> v = {1, 2, 3, 4};
+  auto span = AnySpan<T>(v).subspan(1, 2);
+  EXPECT_EQ(span.size(), 2);
+  EXPECT_EQ(span[0], 2);
+  EXPECT_EQ(span[1], 3);
+  span = AnySpan<T>(v).subspan(1);
+  EXPECT_EQ(span.size(), 3);
+  EXPECT_EQ(span[2], 4);
+
+  std::vector<int> zero_length_vector;
+  auto zero_length_subspan = AnySpan<T>(zero_length_vector).subspan(0);
+  EXPECT_EQ(zero_length_subspan.size(), 0);
+}
+
+TYPED_TEST(AnySpanTest, DequeSubspan) {
+  using T = typename TestFixture::template MaybeConst<int>;
+  std::deque<int> v = {1, 2, 3, 4};
+  auto span = AnySpan<T>(v).subspan(1, 2);
+  EXPECT_EQ(span.size(), 2);
+  EXPECT_EQ(span[0], 2);
+  EXPECT_EQ(span[1], 3);
+}
+
+TYPED_TEST(AnySpanTest, VectorFirst) {
+  using T = typename TestFixture::template MaybeConst<int>;
+  std::vector<int> v = {1, 2, 3, 4};
+  auto span = AnySpan<T>(v).first(2);
+  EXPECT_THAT(span, ElementsAre(1, 2));
+  span = AnySpan<T>(v).first(4);
+  EXPECT_THAT(span, ElementsAre(1, 2, 3, 4));
+
+  std::vector<int> zero_length_vector;
+  auto zero_length_subspan = AnySpan<T>(zero_length_vector).first(0);
+  EXPECT_EQ(zero_length_subspan.size(), 0);
+}
+
+TYPED_TEST(AnySpanTest, DequeFirst) {
+  using T = typename TestFixture::template MaybeConst<int>;
+  std::deque<int> v = {1, 2, 3, 4};
+  auto span = AnySpan<T>(v).first(2);
+  EXPECT_THAT(span, ElementsAre(1, 2));
+}
+
+TYPED_TEST(AnySpanTest, VectorLast) {
+  using T = typename TestFixture::template MaybeConst<int>;
+  std::vector<int> v = {1, 2, 3, 4};
+  auto span = AnySpan<T>(v).last(2);
+  EXPECT_THAT(span, ElementsAre(3, 4));
+  span = AnySpan<T>(v).last(4);
+  EXPECT_THAT(span, ElementsAre(1, 2, 3, 4));
+
+  std::vector<int> zero_length_vector;
+  auto zero_length_subspan = AnySpan<T>(zero_length_vector).last(0);
+  EXPECT_EQ(zero_length_subspan.size(), 0);
+}
+
+TYPED_TEST(AnySpanTest, DequeLast) {
+  using T = typename TestFixture::template MaybeConst<int>;
+  std::deque<int> v = {1, 2, 3, 4};
+  auto span = AnySpan<T>(v).last(2);
+  EXPECT_THAT(span, ElementsAre(3, 4));
+}
+
+TYPED_TEST(AnySpanTest, LambdaTransformFromVector) {
+  using T = typename TestFixture::template MaybeConst<std::string>;
+  std::vector<std::string> v = {"b", "a", "d", "c"};
+  std::vector<int> alphabetical_order = {1, 0, 3, 2};
+  auto subscript_v = [&v](size_t i) -> std::string& { return v[i]; };
+  AnySpan<T> span(alphabetical_order, subscript_v);
+  EXPECT_EQ(span[0], "a");
+  EXPECT_EQ(span[1], "b");
+  EXPECT_EQ(span[2], "c");
+  EXPECT_EQ(span[3], "d");
+}
+
+TYPED_TEST(AnySpanTest, LambdaTransformFromSpanOfDifferentType) {
+  using T = typename TestFixture::template MaybeConst<int>;
+  using U = typename TestFixture::template MaybeConst<std::string>;
+  std::vector<std::string> v = {"b", "a", "d", "c"};
+  std::vector<int> alphabetical_order = {1, 0, 3, 2};
+  AnySpan<T> alphabetical_order_as_span(alphabetical_order);
+  auto subscript_v = [&v](size_t i) -> std::string& { return v[i]; };
+  AnySpan<U> span(alphabetical_order_as_span, subscript_v);
+  EXPECT_EQ(span[0], "a");
+  EXPECT_EQ(span[1], "b");
+  EXPECT_EQ(span[2], "c");
+  EXPECT_EQ(span[3], "d");
+}
+
+TYPED_TEST(AnySpanTest, LambdaTransformFromSpanOfSameType) {
+  using T = typename TestFixture::template MaybeConst<int>;
+  std::vector<int> v = {2, 1, 4, 3};
+  std::vector<int> sorted_order = {1, 0, 3, 2};
+  AnySpan<T> sorted_order_as_span(sorted_order);
+  auto subscript_v = [&v](size_t i) -> int& { return v[i]; };
+  AnySpan<T> span(sorted_order_as_span, subscript_v);
+  EXPECT_EQ(span[0], 1);
+  EXPECT_EQ(span[1], 2);
+  EXPECT_EQ(span[2], 3);
+  EXPECT_EQ(span[3], 4);
+}
+
+TYPED_TEST(AnySpanTest, MakeAnySpanFromVector) {
+  using V = typename TestFixture::template MaybeConst<std::vector<int>>;
+  using T = typename TestFixture::template MaybeConst<int>;
+  V v = {1, 2, 3, 4};
+  auto span = MakeAnySpan(v);
+  EXPECT_TRUE((std::is_same<decltype(span), AnySpan<T>>()));
+  EXPECT_EQ(span.size(), 4);
+  for (size_t i = 0; i < 4; ++i) {
+    EXPECT_EQ(&span[i], &v[i]);
+  }
+}
+
+TYPED_TEST(AnySpanTest, MakeAnySpanFromPtrAndSize) {
+  using V = typename TestFixture::template MaybeConst<std::vector<int>>;
+  using T = typename TestFixture::template MaybeConst<int>;
+  V v = {1, 2, 3, 4};
+  auto span = MakeAnySpan(v.data(), v.size());
+  EXPECT_TRUE((std::is_same<decltype(span), AnySpan<T>>()));
+  EXPECT_EQ(span.size(), 4);
+  for (int i = 0; i < 4; ++i) {
+    EXPECT_EQ(&span[i], &v[i]);
+  }
+}
+
+TYPED_TEST(AnySpanTest, MakeAnySpanFromArray) {
+  using T = typename TestFixture::template MaybeConst<int>;
+  T a[] = {1, 2, 3, 4};
+  auto span = MakeAnySpan(a);
+  EXPECT_TRUE((std::is_same<decltype(span), AnySpan<T>>()));
+  EXPECT_EQ(span.size(), 4);
+  for (int i = 0; i < 4; ++i) {
+    EXPECT_EQ(&span[i], &a[i]);
+  }
+}
+
+TYPED_TEST(AnySpanTest, MakeDerefAnySpanFromArray) {
+  using T = typename TestFixture::template MaybeConst<int>;
+  T v = 0;
+  T* a[] = {&v, &v, &v};
+  auto span = MakeDerefAnySpan(a);
+  EXPECT_TRUE((std::is_same<decltype(span), AnySpan<T>>()));
+  EXPECT_EQ(span.size(), 3);
+  for (int i = 0; i < 3; ++i) {
+    EXPECT_EQ(&span[i], a[i]);
+  }
+}
+
+// Tests transforms that are supported by std::invoke.
+TYPED_TEST(AnySpanTest, InvokableTransforms) {
+  using T = typename TestFixture::template MaybeConst<std::string>;
+  struct S;
+  using SRef = typename TestFixture::template MaybeConst<S>&;
+  struct S {
+    std::string a, b;
+    static T& Fun(SRef p) { return p.a; }
+    std::string& MemFun() { return a; }
+    const std::string& MemFun() const { return a; }
+  };
+
+  // For const span tests, we need a pointer to constant member function. We
+  // choose the correct type of member function here.
+  using MemFunPtr = std::conditional_t<TestFixture::IsConstTest::value,
+                                       T& (S::*)() const, T& (S::*)()>;
+
+  std::vector<S> v = {{"1", "2"}, {"3", "4"}};
+  S a[] = {{"1", "2"}, {"3", "4"}};
+  T& (*fun_ptr)(SRef) = &S::Fun;
+  MemFunPtr mem_fun_ptr = &S::MemFun;
+  T S::* mem_ptr = &S::a;
+
+  AnySpan<T> spans[] = {
+      // Function reference.
+      AnySpan<T>(v, S::Fun),
+      AnySpan<T>(a, S::Fun),
+      AnySpan<T>(a, 2, S::Fun),
+
+      // Function pointer prvalue.
+      AnySpan<T>(v, &S::Fun),
+      AnySpan<T>(a, &S::Fun),
+      AnySpan<T>(a, 2, &S::Fun),
+
+      // Function pointer lvalue.
+      AnySpan<T>(v, fun_ptr),
+      AnySpan<T>(a, fun_ptr),
+      AnySpan<T>(a, 2, fun_ptr),
+
+      // Dereferenced function pointer variable.
+      AnySpan<T>(v, *fun_ptr),
+      AnySpan<T>(a, *fun_ptr),
+      AnySpan<T>(a, 2, *fun_ptr),
+
+      // Member function pointer.
+      AnySpan<T>(v, mem_fun_ptr),
+      AnySpan<T>(a, mem_fun_ptr),
+      AnySpan<T>(a, 2, mem_fun_ptr),
+
+      // Data member.
+      AnySpan<T>(v, mem_ptr),
+      AnySpan<T>(a, mem_ptr),
+      AnySpan<T>(a, 2, mem_ptr),
+  };
+
+  for (const auto& span : spans) {
+    EXPECT_EQ(span[0], "1");
+    EXPECT_EQ(span[1], "3");
+  }
+}
+
+TYPED_TEST(AnySpanTest, VectorPtrSubspan) {
+  using T = typename TestFixture::template MaybeConst<int>;
+  std::vector<int> v = {1, 2, 3, 4};
+  std::vector<int*> vp;
+  for (int& i : v) {
+    vp.push_back(&i);
+  }
+  AnySpan<T> span = AnySpan<T>(vp, any_span_transform::Deref()).subspan(1, 2);
+  EXPECT_EQ(span.size(), 2);
+  EXPECT_EQ(span[0], 2);
+  EXPECT_EQ(span[1], 3);
+}
+
+TYPED_TEST(AnySpanTest, VectorPtrFirst) {
+  using T = typename TestFixture::template MaybeConst<int>;
+  std::vector<int> v = {1, 2, 3, 4};
+  std::vector<int*> vp;
+  for (int& i : v) {
+    vp.push_back(&i);
+  }
+  AnySpan<T> span = AnySpan<T>(vp, any_span_transform::Deref()).first(2);
+  EXPECT_THAT(span, ElementsAre(1, 2));
+}
+
+TYPED_TEST(AnySpanTest, VectorPtrLast) {
+  using T = typename TestFixture::template MaybeConst<int>;
+  std::vector<int> v = {1, 2, 3, 4};
+  std::vector<int*> vp;
+  for (int& i : v) {
+    vp.push_back(&i);
+  }
+  AnySpan<T> span = AnySpan<T>(vp, any_span_transform::Deref()).last(2);
+  EXPECT_THAT(span, ElementsAre(3, 4));
+}
+
+TYPED_TEST(AnySpanTest, Iterator) {
+  using T = typename TestFixture::template MaybeConst<std::string>;
+  std::vector<std::string> v = {"0", "1", "2", "3"};
+  AnySpan<T> span(v);
+
+  // Dereference.
+  auto it = span.begin();
+  EXPECT_EQ("0", *it);
+
+  // Postfix increment.
+  EXPECT_EQ("0", *(it++));
+  EXPECT_EQ("1", *(it++));
+
+  // Postfix decrement.
+  EXPECT_EQ("2", *(it--));
+  EXPECT_EQ("1", *(it--));
+
+  // Prefix increment.
+  EXPECT_EQ(it, span.begin());
+  EXPECT_EQ("1", *(++it));
+  EXPECT_EQ("2", *(++it));
+
+  // Postfix decrement.
+  EXPECT_EQ("1", *(--it));
+  EXPECT_EQ("0", *(--it));
+
+  // Increment by integer.
+  EXPECT_EQ(it, span.begin());
+  it += 3;
+  EXPECT_EQ("3", *it);
+
+  // Decrement by integer.
+  it = span.end();
+  it -= 4;
+  EXPECT_EQ("0", *it);
+
+  // Add and subtract.
+  EXPECT_EQ(v.begin() + 2, v.end() - 2);
+  EXPECT_EQ(2 + v.begin(), v.end() - 2);
+
+  // Index.
+  it = span.begin();
+  EXPECT_EQ("3", v[3]);
+
+  // Relational operators.
+  EXPECT_TRUE(span.begin() != span.end());
+  EXPECT_TRUE(span.begin() == span.begin());
+  EXPECT_TRUE(span.begin() < span.end());
+  EXPECT_TRUE(span.begin() <= span.end());
+  EXPECT_TRUE(span.end() > span.begin());
+  EXPECT_TRUE(span.end() >= span.begin());
+}
+
+TYPED_TEST(AnySpanTest, ConstIterator) {
+  using T = typename TestFixture::template MaybeConst<std::string>;
+  std::vector<std::string> v = {"0", "1", "2", "3"};
+  AnySpan<T> span(v);
+
+  // Dereference.
+  auto it = span.cbegin();
+  EXPECT_EQ("0", *it);
+
+  // Postfix increment.
+  EXPECT_EQ("0", *(it++));
+  EXPECT_EQ("1", *(it++));
+
+  // Postfix decrement.
+  EXPECT_EQ("2", *(it--));
+  EXPECT_EQ("1", *(it--));
+
+  // Prefix increment.
+  EXPECT_EQ(it, span.cbegin());
+  EXPECT_EQ("1", *(++it));
+  EXPECT_EQ("2", *(++it));
+
+  // Postfix decrement.
+  EXPECT_EQ("1", *(--it));
+  EXPECT_EQ("0", *(--it));
+
+  // Increment by integer.
+  EXPECT_EQ(it, span.cbegin());
+  it += 3;
+  EXPECT_EQ("3", *it);
+
+  // Decrement by integer.
+  it = span.cend();
+  it -= 4;
+  EXPECT_EQ("0", *it);
+
+  // Add and subtract.
+  EXPECT_EQ(v.cbegin() + 2, v.cend() - 2);
+  EXPECT_EQ(2 + v.cbegin(), v.cend() - 2);
+
+  // Index.
+  it = span.cbegin();
+  EXPECT_EQ("3", v[3]);
+
+  // Relational operators.
+  EXPECT_TRUE(span.cbegin() != span.cend());
+  EXPECT_TRUE(span.cbegin() == span.cbegin());
+  EXPECT_TRUE(span.cbegin() < span.cend());
+  EXPECT_TRUE(span.cbegin() <= span.cend());
+  EXPECT_TRUE(span.cend() > span.cbegin());
+  EXPECT_TRUE(span.cend() >= span.cbegin());
+}
+
+TYPED_TEST(AnySpanTest, ReverseIterator) {
+  using T = typename TestFixture::template MaybeConst<std::string>;
+  std::vector<std::string> v = {"a", "b", "c", "d"};
+  AnySpan<T> span(v);
+  auto v_it = v.rbegin();
+  for (auto it = span.rbegin(); it != span.rend(); ++it, ++v_it) {
+    EXPECT_EQ(*v_it, *it);
+  }
+}
+
+TYPED_TEST(AnySpanTest, IndexingAt) {
+  using T = typename TestFixture::template MaybeConst<std::string>;
+  std::vector<std::string> v = {"a", "b", "c", "d"};
+  AnySpan<T> span(v);
+  EXPECT_EQ(&v.at(0), &v.at(0));
+  EXPECT_EQ(&v.at(1), &v.at(1));
+  EXPECT_EQ(&v.at(3), &v.at(3));
+}
+
+TYPED_TEST(AnySpanTest, AtThrows) {
+  using T = typename TestFixture::template MaybeConst<std::string>;
+  std::vector<std::string> v = {"a", "b", "c", "d"};
+  AnySpan<T> span(v);
+  ABSL_BASE_INTERNAL_EXPECT_FAIL(span.at(4), std::out_of_range,
+                                 "failed bounds check");
+}
+
+TYPED_TEST(AnySpanTest, FrontBack) {
+  using T = typename TestFixture::template MaybeConst<std::string>;
+  std::vector<std::string> v = {"a", "b", "c", "d"};
+  AnySpan<T> span(v);
+  EXPECT_EQ(&v.front(), &span.front());
+  EXPECT_EQ("a", span.front());
+  EXPECT_EQ(&v.back(), &span.back());
+  EXPECT_EQ("d", span.back());
+}
+
+// Returns a reference to a default constructed instance of the given type.
+template <typename Container>
+Container& DefaultInstance() {
+  static auto* c = new Container;
+  return *c;
+}
+
+// This test reaches into the implementation a bit, but it buys a lot of
+// confidence that the flat-array optimizations are working correctly.
+TYPED_TEST(AnySpanTest, CheapTypes) {
+  using T = typename TestFixture::template MaybeConst<int>;
+  using any_span_internal::IsCheap;
+  EXPECT_FALSE(IsCheap(AnySpan<T>(DefaultInstance<std::deque<int>>())));
+  EXPECT_FALSE(
+      IsCheap(AnySpan<T>(DefaultInstance<std::vector<std::unique_ptr<int>>>(),
+                         any_span_transform::Deref())));
+  EXPECT_TRUE(IsCheap(AnySpan<T>(DefaultInstance<std::vector<int>>())));
+  EXPECT_TRUE(IsCheap(AnySpan<T>(DefaultInstance<absl::Span<int>>())));
+  EXPECT_TRUE(IsCheap(AnySpan<T>(DefaultInstance<std::vector<int>>())));
+  EXPECT_TRUE(IsCheap(AnySpan<T>(DefaultInstance<std::vector<int*>>(),
+                                 any_span_transform::Deref())));
+  EXPECT_TRUE(IsCheap(AnySpan<T>(DefaultInstance<std::vector<int*>>(),
+                                 any_span_transform::Deref())));
+  struct SomeContainer {
+    int& operator[](std::size_t) { ABSL_RAW_LOG(FATAL, "Not implemented"); }
+    const int& operator[](std::size_t) const {
+      ABSL_RAW_LOG(FATAL, "Not implemented");
+    }
+    std::size_t size() const { return 1; }
+  };
+  EXPECT_FALSE(IsCheap(AnySpan<T>(DefaultInstance<SomeContainer>())));
+  struct SomeCheapMutableContainer {
+    int& operator[](std::size_t) const {
+      ABSL_RAW_LOG(FATAL, "Not implemented");
+    }
+    int* data() const { return nullptr; }
+    std::size_t size() const { return 1; }
+  };
+  EXPECT_TRUE(
+      IsCheap(AnySpan<T>(DefaultInstance<SomeCheapMutableContainer>())));
+
+  struct SomeWonkyContainer {
+    int& operator[](std::size_t) const {
+      ABSL_RAW_LOG(FATAL, "Not implemented");
+    }
+    const int** data() const { return nullptr; }
+    std::size_t size() const { return 1; }
+  };
+  EXPECT_FALSE(IsCheap(AnySpan<T>(DefaultInstance<SomeWonkyContainer>())));
+}
+
+TYPED_TEST(AnySpanTest, TriviallyCopyable) {
+  using T = typename TestFixture::template MaybeConst<std::string>;
+  // Note: we could use is_trivially_copyable, but it is not implemented in
+  // Crosstool v18 which is the current version at the time of writing.
+  EXPECT_TRUE(std::is_trivially_copy_constructible_v<AnySpan<T>>);
+  EXPECT_TRUE(std::is_trivially_copy_assignable_v<AnySpan<T>>);
+  EXPECT_TRUE(std::is_trivially_destructible_v<AnySpan<T>>);
+}
+
+TYPED_TEST(AnySpanTest, IsConstructible) {
+  using T = typename TestFixture::template MaybeConst<std::string>;
+  using U = typename TestFixture::template MaybeConst<int>;
+  using VectorT =
+      typename TestFixture::template MaybeConst<std::vector<std::string>>;
+  using VectorU = typename TestFixture::template MaybeConst<std::vector<int>>;
+  EXPECT_FALSE((std::is_constructible_v<AnySpan<T>, T>));
+  EXPECT_FALSE((std::is_constructible_v<AnySpan<T>, T*>));
+  EXPECT_FALSE((std::is_constructible_v<AnySpan<T>, U>));
+  EXPECT_FALSE((std::is_constructible_v<AnySpan<T>, U(&)[5]>));
+  EXPECT_FALSE((std::is_constructible_v<AnySpan<T>, U*, std::size_t>));
+  EXPECT_FALSE((std::is_constructible_v<AnySpan<T>, VectorU&>));
+
+  EXPECT_TRUE((std::is_constructible_v<AnySpan<T>, VectorT&>));
+  EXPECT_TRUE((std::is_constructible_v<AnySpan<T>, T(&)[5]>));
+  EXPECT_TRUE((std::is_constructible_v<AnySpan<T>, T*, std::size_t>));
+}
+
+//
+// ConstAnySpanTest covers parts of AnySpan interface that are only valid for
+// AnySpans with const elements.
+//
+
+TEST(ConstAnySpanTest, ImplicitConversion) {
+  std::vector<int> v = {1, 2, 3};
+  ExpectSpanEqualsContainer<const int>(v, v);
+}
+
+TEST(ConstAnySpanTest, TemporaryArrayOfKnownSize) {
+  const int i = 5;
+  EXPECT_THAT(AnySpan<const int>({&i, &i, &i}, any_span_transform::Deref()),
+              testing::ElementsAre(5, 5, 5));
+}
+
+TEST(ConstAnySpanTest, CheapTypes) {
+  using any_span_internal::IsCheap;
+  EXPECT_TRUE(IsCheap(AnySpan<const int>(std::vector<const int*>(),
+                                         any_span_transform::Deref())));
+  EXPECT_TRUE(IsCheap(AnySpan<const int>(std::vector<const int*>(),
+                                         any_span_transform::Deref())));
+  EXPECT_TRUE(
+      IsCheap(AnySpan<const int>(DefaultInstance<absl::Span<const int>>())));
+  struct SomeCheapConstContainer {
+    const int& operator[](std::size_t) const {
+      ABSL_RAW_LOG(FATAL, "Not implemented");
+    }
+    const int* data() const { return nullptr; }
+    std::size_t size() const { return 1; }
+  };
+  EXPECT_TRUE(IsCheap(AnySpan<const int>(SomeCheapConstContainer())));
+}
+
+TEST(ConstAnySpanTest, InitializerListInt) {
+  auto test = [](AnySpan<const int> span) {
+    int expected = 1;
+    for (int i : span) {
+      EXPECT_EQ(expected++, i);
+    }
+    EXPECT_EQ(expected, 5);
+  };
+  test({1, 2, 3, 4});
+}
+
+TEST(ConstAnySpanTest, InitializerListString) {
+  auto test = [](AnySpan<const std::string> span) {
+    int expected = 1;
+    for (const std::string& i : span) {
+      EXPECT_EQ(absl::StrCat(expected++), i);
+    }
+    EXPECT_EQ(expected, 5);
+  };
+  test({"1", "2", "3", "4"});
+}
+
+TEST(ConstAnySpanTest, InitializerListDerivedToBase) {
+  // Derived to base, implicit conversion.
+  [](AnySpan<const Base2> bases) {
+    for (const Base2& b : bases) {
+      EXPECT_EQ(b.b2, 2);
+      // Make sure the value was not sliced.
+      EXPECT_EQ(typeid(b), typeid(Child));
+    }
+  }({Child{}, Child{}, Child{}});
+
+  // Derived to base with explicit transform.
+  Child children[3];
+  [](AnySpan<const Base2> bases) {
+    for (const Base2& b : bases) {
+      EXPECT_EQ(b.b2, 2);
+      // Make sure the value was not sliced.
+      EXPECT_EQ(typeid(b), typeid(Child));
+    }
+  }({{Child{}, Child{}, Child{}},
+     [](auto& obj) -> const Base2& { return obj; }});
+}
+
+TEST(ConstAnySpanTest,
+     InitializerListUsesValueTypeWhenConversionNeedsTemporaries) {
+  // If this were to call the initializer_list<Element> overload it would fail
+  // when it tries to make an lvalue transform from std::string to
+  // std::string_view.
+  // Instead, it must be calling the initializer_list<value_type> overload,
+  // doing the conversion on the call site.
+  [](AnySpan<const std::string_view> s) {
+    EXPECT_THAT(s, ElementsAre("A", "B", "C"));
+  }({"A", "B", "C"});
+}
+
+TEST(ConstAnySpanTest, MakeAnySpanFromVector) {
+  std::vector<int> v = {1, 2, 3, 4};
+  auto span = MakeConstAnySpan(v);
+  EXPECT_TRUE((std::is_same<decltype(span), AnySpan<const int>>()));
+  EXPECT_EQ(span.size(), 4);
+  for (size_t i = 0; i < 4; ++i) {
+    EXPECT_EQ(&span[i], &v[i]);
+  }
+}
+
+TEST(ConstAnySpanTest, MakeAnySpanFromPtrAndSize) {
+  std::vector<int> v = {1, 2, 3, 4};
+  auto span = MakeConstAnySpan(v.data(), v.size());
+  EXPECT_TRUE((std::is_same<decltype(span), AnySpan<const int>>()));
+  EXPECT_EQ(span.size(), 4);
+  for (size_t i = 0; i < 4; ++i) {
+    EXPECT_EQ(&span[i], &v[i]);
+  }
+}
+
+TEST(ConstAnySpanTest, MakeAnySpanFromArray) {
+  int a[] = {1, 2, 3, 4};
+  auto span = MakeConstAnySpan(a);
+  EXPECT_TRUE((std::is_same<decltype(span), AnySpan<const int>>()));
+  EXPECT_EQ(span.size(), 4);
+  for (size_t i = 0; i < 4; ++i) {
+    EXPECT_EQ(&span[i], &a[i]);
+  }
+}
+
+TEST(ConstAnySpanTest, MakeDerefAnySpanFromArray) {
+  int v = 0;
+  int* a[] = {&v, &v, &v};
+  auto span = MakeConstDerefAnySpan(a);
+  EXPECT_TRUE((std::is_same<decltype(span), AnySpan<const int>>()));
+  EXPECT_EQ(span.size(), 3);
+  for (size_t i = 0; i < 3; ++i) {
+    EXPECT_EQ(&span[i], a[i]);
+  }
+}
+
+//
+// MutableAnySpanTest covers parts of AnySpan interface that are only valid for
+// AnySpans with mutable elements.
+//
+
+TEST(MutableAnySpanTest, MutableVectorString) {
+  std::vector<std::string> v = {"a", "b", "c", "d"};
+  AnySpan<std::string> span(v);
+  span[0] = "e";
+  EXPECT_EQ(v[0], "e");
+}
+
+TEST(MutableAnySpanTest, MutableToConstConversion) {
+  std::vector<std::string> v = {"a", "b", "c", "d"};
+  AnySpan<std::string> mutable_span(v);
+  AnySpan<const std::string> const_span(mutable_span);
+  EXPECT_TRUE(any_span_internal::IsCheap(const_span));
+  mutable_span[0] = "e";
+  EXPECT_EQ(const_span[0], "e");
+  mutable_span.subspan(1)[0] = "f";
+  EXPECT_EQ(const_span[1], "f");
+
+  std::vector<std::string> v2 = {"a"};
+  mutable_span = AnySpan<std::string>(v2);
+  EXPECT_EQ(const_span[0], "e");
+}
+
+TEST(MutableAnySpanTest, DerefMutableToConstConversion) {
+  std::vector<std::string> v = {"a", "b", "c", "d"};
+  std::vector<std::string*> v_ptrs;
+  for (std::string& s : v) {
+    v_ptrs.push_back(&s);
+  }
+  AnySpan<std::string> mutable_span(v_ptrs, any_span_transform::Deref());
+  AnySpan<const std::string> const_span(mutable_span);
+  EXPECT_TRUE(any_span_internal::IsCheap(const_span));
+  mutable_span[0] = "e";
+  EXPECT_EQ(const_span[0], "e");
+  EXPECT_EQ(v[0], "e");
+  mutable_span.subspan(1)[0] = "f";
+  EXPECT_EQ(const_span[1], "f");
+}
+
+TEST(MutableAnySpanTest, MutableToConstDeque) {
+  std::deque<std::string> d = {"a", "b", "c", "d"};
+  AnySpan<std::string> mutable_span(d);
+  AnySpan<const std::string> const_span(mutable_span);
+  EXPECT_FALSE(any_span_internal::IsCheap(const_span));
+  mutable_span[0] = "e";
+  EXPECT_EQ(const_span[0], "e");
+  mutable_span.subspan(1)[0] = "f";
+  EXPECT_EQ(const_span[1], "f");
+}
+
+TEST(MutableAnySpanTest, Deref) {
+  std::string s = "a";
+  AnySpan<std::string>({&s}, any_span_transform::Deref())[0] = "b";
+  EXPECT_EQ(s, "b");
+}
+
+// Checks that recommended usage works.
+TEST(MutableAnySpanTest, AsFunctionArgument) {
+  std::vector<std::string> v = {"a", "b", "c", "d"};
+  auto set_all_to_e = [](AnySpan<std::string> span) {
+    for (std::string& s : span) {
+      s = "e";
+    }
+  };
+  set_all_to_e(AnySpan<std::string>(v));
+  for (const std::string& e : v) {
+    EXPECT_EQ(e, "e");
+  }
+}
+
+TEST(MutableAnySpanTest, MutateThroughIterator) {
+  std::vector<std::string> v = {"a", "b", "c", "d"};
+  AnySpan<std::string> span(v);
+  AnySpan<std::string>::iterator it = span.begin();
+  it[2] = "e";
+  EXPECT_EQ(v[2], "e");
+  ++it;
+  *it = "f";
+  EXPECT_EQ(v[1], "f");
+  for (std::string& s : span) {
+    s = "g";
+  }
+  for (const std::string& e : v) {
+    EXPECT_EQ(e, "g");
+  }
+}
+
+TEST(MutableAnySpanTest, MutateThroughSubSpan) {
+  std::vector<int> v = {1, 2, 3, 4};
+  AnySpan<int> span(v);
+  span = span.subspan(1, 2);
+  for (int& i : span) {
+    i = 42;
+  }
+  EXPECT_THAT(v, ElementsAre(1, 42, 42, 4));
+}
+
+TYPED_TEST(AnySpanTest, SubspanToEnd) {
+  int arr[] = {0, 1, 2};
+  AnySpan<int> span(arr);
+  EXPECT_THAT(span.subspan(0, AnySpan<int>::npos), ElementsAre(0, 1, 2));
+  EXPECT_THAT(span.subspan(1, AnySpan<int>::npos), ElementsAre(1, 2));
+  EXPECT_THAT(span.subspan(2, AnySpan<int>::npos), ElementsAre(2));
+  EXPECT_THAT(span.subspan(3, AnySpan<int>::npos), ElementsAre());
+#if GTEST_HAS_DEATH_TEST
+  if (IsHardened()) {
+    EXPECT_DEATH(span.subspan(4, AnySpan<int>::npos), "");
+    EXPECT_DEATH(span.subspan(AnySpan<int>::npos, AnySpan<int>::npos), "");
+  }
+#endif
+}
+
+TEST(MutableAnySpanTest, NonTruncatingSubspan) {
+  std::vector<int> v = {1, 2, 3, 4};
+  AnySpan<int> span(v);
+#if GTEST_HAS_DEATH_TEST
+  if (IsHardened()) {
+    EXPECT_DEATH(span.subspan(5, 0), "");
+    EXPECT_DEATH(span.subspan(5, 1), "");
+    EXPECT_DEATH(span.subspan(AnySpan<int>::npos, 0), "");
+    EXPECT_DEATH(span.subspan(AnySpan<int>::npos, 1), "");
+      EXPECT_DEATH(span.subspan(0, 5), "");
+      EXPECT_DEATH(span.first(5), "");
+      EXPECT_DEATH(span.first(AnySpan<int>::npos), "");
+  }
+#endif
+}
+
+TEST(MutableAnySpanTest, IteratorToConstIteratorConversion) {
+  std::vector<int> v = {1};
+  AnySpan<int> span(v);
+  AnySpan<int>::iterator it = span.begin();
+  AnySpan<int>::const_iterator cit = it;
+  EXPECT_EQ(&*it, &*cit);
+}
+
+// Checks that implicit conversion from a mutable view works.
+TEST(MutableAnySpanTest, ImplicitConversionFromMutableSpan) {
+  std::vector<std::string> v = {"a", "b", "c", "d"};
+  absl::Span<std::string> mutable_slice(v);
+  auto set_all_to_e = [](AnySpan<std::string> span) {
+    for (std::string& s : span) {
+      s = "e";
+    }
+  };
+  set_all_to_e(mutable_slice);
+  for (const std::string& s : v) {
+    EXPECT_EQ(s, "e");
+  }
+}
+
+bool WantsStringOrAnySpan(absl::string_view /* ignored */) { return false; }
+bool WantsStringOrAnySpan(AnySpan<std::string> /* ignored */) { return true; }
+bool WantsStringOrConstAnySpan(absl::string_view /* ignored */) {
+  return false;
+}
+bool WantsStringOrConstAnySpan(AnySpan<const std::string> /* ignored */) {
+  return true;
+}
+// Checks that AnySpan(char (&array)[N]) is not used in the overload
+// resolution when building an AnySpan<string> (it would not compile anyway).
+TEST(MutableAnySpanTest, ResolveAmbiguityWithAnySpanOfChars) {
+  std::string foo;
+  AnySpan<std::string> bar;
+  EXPECT_FALSE(WantsStringOrAnySpan("abc"));
+  EXPECT_FALSE(WantsStringOrAnySpan(foo));
+  EXPECT_TRUE(WantsStringOrAnySpan(bar));
+  EXPECT_FALSE(WantsStringOrConstAnySpan("abc"));
+  EXPECT_FALSE(WantsStringOrConstAnySpan(foo));
+  EXPECT_TRUE(WantsStringOrConstAnySpan(bar));
+}
+
+TEST(AnySpanTest, SupportsAbslHash) {
+  std::vector<size_t> v = {1, 2, 3};
+  std::vector<std::string> vs = {"1", "2", "3"};
+  std::vector<std::unique_ptr<size_t>> vu;
+  vu.push_back(std::make_unique<size_t>(1));
+  vu.push_back(std::make_unique<size_t>(2));
+  vu.push_back(std::make_unique<size_t>(3));
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly({
+      // {1, 2, 3}
+      AnySpan<const size_t>(v),
+      AnySpan<const size_t>(vu, any_span_transform::Deref()),
+      // {1, 2}
+      AnySpan<const size_t>(std::vector<size_t>({1, 2})),
+      AnySpan<const size_t>(std::deque<size_t>({1, 2})),
+      // {1, 2, 4}
+      AnySpan<const size_t>(std::vector<size_t>({1, 2, 4})),
+      AnySpan<const size_t>(std::deque<size_t>({1, 2, 4})),
+      // Empty.
+      AnySpan<const size_t>(),
+      AnySpan<const size_t>(std::vector<size_t>()),
+  }));
+}
+
+// This is necessary to avoid a bunch of lint warnings suggesting that we use
+// EXPECT_EQ/etc.
+bool Identity(bool b) { return b; }
+
+template <typename T, typename U, typename V>
+void TestEquality(T&& orig, U&& eq, V&& diff) {
+  EXPECT_TRUE(Identity(std::forward<T>(orig) == std::forward<U>(eq)));
+  EXPECT_TRUE(Identity(std::forward<U>(eq) == std::forward<T>(orig)));
+  EXPECT_FALSE(Identity(std::forward<T>(orig) != std::forward<U>(eq)));
+  EXPECT_FALSE(Identity(std::forward<U>(eq) != std::forward<T>(orig)));
+
+  EXPECT_FALSE(Identity(std::forward<T>(orig) == std::forward<V>(diff)));
+  EXPECT_FALSE(Identity(std::forward<V>(diff) == std::forward<T>(orig)));
+  EXPECT_TRUE(Identity(std::forward<T>(orig) != std::forward<V>(diff)));
+  EXPECT_TRUE(Identity(std::forward<V>(diff) != std::forward<T>(orig)));
+}
+
+TEST(AnySpanTest, Equals) {
+  int arr[] = {1, 2, 3};
+  std::vector<int> eq(std::begin(arr), std::end(arr));
+  int diff[] = {1, 2, 4};
+  TestEquality(AnySpan<int>(arr), AnySpan<int>(eq), AnySpan<int>(diff));
+  TestEquality(AnySpan<const int>(arr), AnySpan<int>(eq), AnySpan<int>(diff));
+  TestEquality(AnySpan<int>(arr), AnySpan<const int>(eq),
+               AnySpan<const int>(diff));
+  TestEquality(AnySpan<const int>(arr), AnySpan<const int>(eq),
+               AnySpan<const int>(diff));
+
+  TestEquality(AnySpan<const int>(arr), eq, diff);
+  TestEquality(AnySpan<int>(arr), eq, diff);
+}
+
+// If an object is copied, mark the bool pointed by `copied` true.
+class MarkIfCopied {
+ public:
+  explicit MarkIfCopied(bool* copied) : copied_(copied) {}
+
+  // NOLINT_NEXT_LINE(google-explicit-constructor)
+  MarkIfCopied(const MarkIfCopied& other) { *this = other; }
+
+  MarkIfCopied& operator=(const MarkIfCopied& other) {
+    copied_ = other.copied_;
+    *copied_ = true;
+    return *this;
+  }
+
+ private:
+  bool* copied_;
+};
+
+TEST(MarkIfCopied, EnsureWorks) {
+  bool copied = false;
+  MarkIfCopied item(&copied);
+  ASSERT_FALSE(copied);
+  MarkIfCopied _unused_copied_item(item);
+  EXPECT_TRUE(copied);
+}
+
+TEST(AnySpanTest, ReferenceWrapperNotCopied) {
+  bool copied1 = false;
+  bool copied2 = false;
+  MarkIfCopied item1(&copied1);
+  MarkIfCopied item2(&copied2);
+  std::vector<std::reference_wrapper<const MarkIfCopied>> items;
+  items.push_back(std::cref(item1));
+  items.push_back(std::cref(item2));
+  AnySpan<const MarkIfCopied> span(items);
+  EXPECT_EQ(2, span.size());
+  EXPECT_FALSE(copied1);
+  EXPECT_FALSE(copied2);
+}
+
+}  // namespace
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/types/any_test.cc b/absl/types/any_test.cc
deleted file mode 100644
index 4a848ae4ae8..00000000000
--- a/absl/types/any_test.cc
+++ /dev/null
@@ -1,781 +0,0 @@
-// Copyright 2017 The Abseil Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-#include "absl/types/any.h"
-
-// This test is a no-op when absl::any is an alias for std::any.
-#if !defined(ABSL_HAVE_STD_ANY)
-
-#include <initializer_list>
-#include <type_traits>
-#include <utility>
-#include <vector>
-
-#include "gtest/gtest.h"
-#include "absl/base/config.h"
-#include "absl/base/internal/exception_testing.h"
-#include "absl/base/internal/raw_logging.h"
-#include "absl/container/internal/test_instance_tracker.h"
-
-namespace {
-using absl::test_internal::CopyableOnlyInstance;
-using absl::test_internal::InstanceTracker;
-
-template <typename T>
-const T& AsConst(const T& t) {
-  return t;
-}
-
-struct MoveOnly {
-  MoveOnly() = default;
-  explicit MoveOnly(int value) : value(value) {}
-  MoveOnly(MoveOnly&&) = default;
-  MoveOnly& operator=(MoveOnly&&) = default;
-
-  int value = 0;
-};
-
-struct CopyOnly {
-  CopyOnly() = default;
-  explicit CopyOnly(int value) : value(value) {}
-  CopyOnly(CopyOnly&&) = delete;
-  CopyOnly& operator=(CopyOnly&&) = delete;
-  CopyOnly(const CopyOnly&) = default;
-  CopyOnly& operator=(const CopyOnly&) = default;
-
-  int value = 0;
-};
-
-struct MoveOnlyWithListConstructor {
-  MoveOnlyWithListConstructor() = default;
-  explicit MoveOnlyWithListConstructor(std::initializer_list<int> /*ilist*/,
-                                       int value)
-      : value(value) {}
-  MoveOnlyWithListConstructor(MoveOnlyWithListConstructor&&) = default;
-  MoveOnlyWithListConstructor& operator=(MoveOnlyWithListConstructor&&) =
-      default;
-
-  int value = 0;
-};
-
-struct IntMoveOnlyCopyOnly {
-  IntMoveOnlyCopyOnly(int value, MoveOnly /*move_only*/, CopyOnly /*copy_only*/)
-      : value(value) {}
-
-  int value;
-};
-
-struct ListMoveOnlyCopyOnly {
-  ListMoveOnlyCopyOnly(std::initializer_list<int> ilist, MoveOnly /*move_only*/,
-                       CopyOnly /*copy_only*/)
-      : values(ilist) {}
-
-  std::vector<int> values;
-};
-
-using FunctionType = void();
-void FunctionToEmplace() {}
-
-using ArrayType = int[2];
-using DecayedArray = absl::decay_t<ArrayType>;
-
-TEST(AnyTest, Noexcept) {
-  static_assert(std::is_nothrow_default_constructible<absl::any>(), "");
-  static_assert(std::is_nothrow_move_constructible<absl::any>(), "");
-  static_assert(std::is_nothrow_move_assignable<absl::any>(), "");
-  static_assert(noexcept(std::declval<absl::any&>().has_value()), "");
-  static_assert(noexcept(std::declval<absl::any&>().type()), "");
-  static_assert(noexcept(absl::any_cast<int>(std::declval<absl::any*>())), "");
-  static_assert(
-      noexcept(std::declval<absl::any&>().swap(std::declval<absl::any&>())),
-      "");
-
-  using std::swap;
-  static_assert(
-      noexcept(swap(std::declval<absl::any&>(), std::declval<absl::any&>())),
-      "");
-}
-
-TEST(AnyTest, HasValue) {
-  absl::any o;
-  EXPECT_FALSE(o.has_value());
-  o.emplace<int>();
-  EXPECT_TRUE(o.has_value());
-  o.reset();
-  EXPECT_FALSE(o.has_value());
-}
-
-TEST(AnyTest, Type) {
-  absl::any o;
-  EXPECT_EQ(typeid(void), o.type());
-  o.emplace<int>(5);
-  EXPECT_EQ(typeid(int), o.type());
-  o.emplace<float>(5.f);
-  EXPECT_EQ(typeid(float), o.type());
-  o.reset();
-  EXPECT_EQ(typeid(void), o.type());
-}
-
-TEST(AnyTest, EmptyPointerCast) {
-  // pointer-to-unqualified overload
-  {
-    absl::any o;
-    EXPECT_EQ(nullptr, absl::any_cast<int>(&o));
-    o.emplace<int>();
-    EXPECT_NE(nullptr, absl::any_cast<int>(&o));
-    o.reset();
-    EXPECT_EQ(nullptr, absl::any_cast<int>(&o));
-  }
-
-  // pointer-to-const overload
-  {
-    absl::any o;
-    EXPECT_EQ(nullptr, absl::any_cast<int>(&AsConst(o)));
-    o.emplace<int>();
-    EXPECT_NE(nullptr, absl::any_cast<int>(&AsConst(o)));
-    o.reset();
-    EXPECT_EQ(nullptr, absl::any_cast<int>(&AsConst(o)));
-  }
-}
-
-TEST(AnyTest, InPlaceConstruction) {
-  const CopyOnly copy_only{};
-  absl::any o(absl::in_place_type_t<IntMoveOnlyCopyOnly>(), 5, MoveOnly(),
-              copy_only);
-  IntMoveOnlyCopyOnly& v = absl::any_cast<IntMoveOnlyCopyOnly&>(o);
-  EXPECT_EQ(5, v.value);
-}
-
-TEST(AnyTest, InPlaceConstructionVariableTemplate) {
-  const CopyOnly copy_only{};
-  absl::any o(absl::in_place_type<IntMoveOnlyCopyOnly>, 5, MoveOnly(),
-              copy_only);
-  auto& v = absl::any_cast<IntMoveOnlyCopyOnly&>(o);
-  EXPECT_EQ(5, v.value);
-}
-
-TEST(AnyTest, InPlaceConstructionWithCV) {
-  const CopyOnly copy_only{};
-  absl::any o(absl::in_place_type_t<const volatile IntMoveOnlyCopyOnly>(), 5,
-              MoveOnly(), copy_only);
-  IntMoveOnlyCopyOnly& v = absl::any_cast<IntMoveOnlyCopyOnly&>(o);
-  EXPECT_EQ(5, v.value);
-}
-
-TEST(AnyTest, InPlaceConstructionWithCVVariableTemplate) {
-  const CopyOnly copy_only{};
-  absl::any o(absl::in_place_type<const volatile IntMoveOnlyCopyOnly>, 5,
-              MoveOnly(), copy_only);
-  auto& v = absl::any_cast<IntMoveOnlyCopyOnly&>(o);
-  EXPECT_EQ(5, v.value);
-}
-
-TEST(AnyTest, InPlaceConstructionWithFunction) {
-  absl::any o(absl::in_place_type_t<FunctionType>(), FunctionToEmplace);
-  FunctionType*& construction_result = absl::any_cast<FunctionType*&>(o);
-  EXPECT_EQ(&FunctionToEmplace, construction_result);
-}
-
-TEST(AnyTest, InPlaceConstructionWithFunctionVariableTemplate) {
-  absl::any o(absl::in_place_type<FunctionType>, FunctionToEmplace);
-  auto& construction_result = absl::any_cast<FunctionType*&>(o);
-  EXPECT_EQ(&FunctionToEmplace, construction_result);
-}
-
-TEST(AnyTest, InPlaceConstructionWithArray) {
-  ArrayType ar = {5, 42};
-  absl::any o(absl::in_place_type_t<ArrayType>(), ar);
-  DecayedArray& construction_result = absl::any_cast<DecayedArray&>(o);
-  EXPECT_EQ(&ar[0], construction_result);
-}
-
-TEST(AnyTest, InPlaceConstructionWithArrayVariableTemplate) {
-  ArrayType ar = {5, 42};
-  absl::any o(absl::in_place_type<ArrayType>, ar);
-  auto& construction_result = absl::any_cast<DecayedArray&>(o);
-  EXPECT_EQ(&ar[0], construction_result);
-}
-
-TEST(AnyTest, InPlaceConstructionIlist) {
-  const CopyOnly copy_only{};
-  absl::any o(absl::in_place_type_t<ListMoveOnlyCopyOnly>(), {1, 2, 3, 4},
-              MoveOnly(), copy_only);
-  ListMoveOnlyCopyOnly& v = absl::any_cast<ListMoveOnlyCopyOnly&>(o);
-  std::vector<int> expected_values = {1, 2, 3, 4};
-  EXPECT_EQ(expected_values, v.values);
-}
-
-TEST(AnyTest, InPlaceConstructionIlistVariableTemplate) {
-  const CopyOnly copy_only{};
-  absl::any o(absl::in_place_type<ListMoveOnlyCopyOnly>, {1, 2, 3, 4},
-              MoveOnly(), copy_only);
-  auto& v = absl::any_cast<ListMoveOnlyCopyOnly&>(o);
-  std::vector<int> expected_values = {1, 2, 3, 4};
-  EXPECT_EQ(expected_values, v.values);
-}
-
-TEST(AnyTest, InPlaceConstructionIlistWithCV) {
-  const CopyOnly copy_only{};
-  absl::any o(absl::in_place_type_t<const volatile ListMoveOnlyCopyOnly>(),
-              {1, 2, 3, 4}, MoveOnly(), copy_only);
-  ListMoveOnlyCopyOnly& v = absl::any_cast<ListMoveOnlyCopyOnly&>(o);
-  std::vector<int> expected_values = {1, 2, 3, 4};
-  EXPECT_EQ(expected_values, v.values);
-}
-
-TEST(AnyTest, InPlaceConstructionIlistWithCVVariableTemplate) {
-  const CopyOnly copy_only{};
-  absl::any o(absl::in_place_type<const volatile ListMoveOnlyCopyOnly>,
-              {1, 2, 3, 4}, MoveOnly(), copy_only);
-  auto& v = absl::any_cast<ListMoveOnlyCopyOnly&>(o);
-  std::vector<int> expected_values = {1, 2, 3, 4};
-  EXPECT_EQ(expected_values, v.values);
-}
-
-TEST(AnyTest, InPlaceNoArgs) {
-  absl::any o(absl::in_place_type_t<int>{});
-  EXPECT_EQ(0, absl::any_cast<int&>(o));
-}
-
-TEST(AnyTest, InPlaceNoArgsVariableTemplate) {
-  absl::any o(absl::in_place_type<int>);
-  EXPECT_EQ(0, absl::any_cast<int&>(o));
-}
-
-template <typename Enabler, typename T, typename... Args>
-struct CanEmplaceAnyImpl : std::false_type {};
-
-template <typename T, typename... Args>
-struct CanEmplaceAnyImpl<
-    absl::void_t<decltype(
-        std::declval<absl::any&>().emplace<T>(std::declval<Args>()...))>,
-    T, Args...> : std::true_type {};
-
-template <typename T, typename... Args>
-using CanEmplaceAny = CanEmplaceAnyImpl<void, T, Args...>;
-
-TEST(AnyTest, Emplace) {
-  const CopyOnly copy_only{};
-  absl::any o;
-  EXPECT_TRUE((std::is_same<decltype(o.emplace<IntMoveOnlyCopyOnly>(
-                                5, MoveOnly(), copy_only)),
-                            IntMoveOnlyCopyOnly&>::value));
-  IntMoveOnlyCopyOnly& emplace_result =
-      o.emplace<IntMoveOnlyCopyOnly>(5, MoveOnly(), copy_only);
-  EXPECT_EQ(5, emplace_result.value);
-  IntMoveOnlyCopyOnly& v = absl::any_cast<IntMoveOnlyCopyOnly&>(o);
-  EXPECT_EQ(5, v.value);
-  EXPECT_EQ(&emplace_result, &v);
-
-  static_assert(!CanEmplaceAny<int, int, int>::value, "");
-  static_assert(!CanEmplaceAny<MoveOnly, MoveOnly>::value, "");
-}
-
-TEST(AnyTest, EmplaceWithCV) {
-  const CopyOnly copy_only{};
-  absl::any o;
-  EXPECT_TRUE(
-      (std::is_same<decltype(o.emplace<const volatile IntMoveOnlyCopyOnly>(
-                        5, MoveOnly(), copy_only)),
-                    IntMoveOnlyCopyOnly&>::value));
-  IntMoveOnlyCopyOnly& emplace_result =
-      o.emplace<const volatile IntMoveOnlyCopyOnly>(5, MoveOnly(), copy_only);
-  EXPECT_EQ(5, emplace_result.value);
-  IntMoveOnlyCopyOnly& v = absl::any_cast<IntMoveOnlyCopyOnly&>(o);
-  EXPECT_EQ(5, v.value);
-  EXPECT_EQ(&emplace_result, &v);
-}
-
-TEST(AnyTest, EmplaceWithFunction) {
-  absl::any o;
-  EXPECT_TRUE(
-      (std::is_same<decltype(o.emplace<FunctionType>(FunctionToEmplace)),
-                    FunctionType*&>::value));
-  FunctionType*& emplace_result = o.emplace<FunctionType>(FunctionToEmplace);
-  EXPECT_EQ(&FunctionToEmplace, emplace_result);
-}
-
-TEST(AnyTest, EmplaceWithArray) {
-  absl::any o;
-  ArrayType ar = {5, 42};
-  EXPECT_TRUE(
-      (std::is_same<decltype(o.emplace<ArrayType>(ar)), DecayedArray&>::value));
-  DecayedArray& emplace_result = o.emplace<ArrayType>(ar);
-  EXPECT_EQ(&ar[0], emplace_result);
-}
-
-TEST(AnyTest, EmplaceIlist) {
-  const CopyOnly copy_only{};
-  absl::any o;
-  EXPECT_TRUE((std::is_same<decltype(o.emplace<ListMoveOnlyCopyOnly>(
-                                {1, 2, 3, 4}, MoveOnly(), copy_only)),
-                            ListMoveOnlyCopyOnly&>::value));
-  ListMoveOnlyCopyOnly& emplace_result =
-      o.emplace<ListMoveOnlyCopyOnly>({1, 2, 3, 4}, MoveOnly(), copy_only);
-  ListMoveOnlyCopyOnly& v = absl::any_cast<ListMoveOnlyCopyOnly&>(o);
-  EXPECT_EQ(&v, &emplace_result);
-  std::vector<int> expected_values = {1, 2, 3, 4};
-  EXPECT_EQ(expected_values, v.values);
-
-  static_assert(!CanEmplaceAny<int, std::initializer_list<int>>::value, "");
-  static_assert(!CanEmplaceAny<MoveOnlyWithListConstructor,
-                               std::initializer_list<int>, int>::value,
-                "");
-}
-
-TEST(AnyTest, EmplaceIlistWithCV) {
-  const CopyOnly copy_only{};
-  absl::any o;
-  EXPECT_TRUE(
-      (std::is_same<decltype(o.emplace<const volatile ListMoveOnlyCopyOnly>(
-                        {1, 2, 3, 4}, MoveOnly(), copy_only)),
-                    ListMoveOnlyCopyOnly&>::value));
-  ListMoveOnlyCopyOnly& emplace_result =
-      o.emplace<const volatile ListMoveOnlyCopyOnly>({1, 2, 3, 4}, MoveOnly(),
-                                                     copy_only);
-  ListMoveOnlyCopyOnly& v = absl::any_cast<ListMoveOnlyCopyOnly&>(o);
-  EXPECT_EQ(&v, &emplace_result);
-  std::vector<int> expected_values = {1, 2, 3, 4};
-  EXPECT_EQ(expected_values, v.values);
-}
-
-TEST(AnyTest, EmplaceNoArgs) {
-  absl::any o;
-  o.emplace<int>();
-  EXPECT_EQ(0, absl::any_cast<int>(o));
-}
-
-TEST(AnyTest, ConversionConstruction) {
-  {
-    absl::any o = 5;
-    EXPECT_EQ(5, absl::any_cast<int>(o));
-  }
-
-  {
-    const CopyOnly copy_only(5);
-    absl::any o = copy_only;
-    EXPECT_EQ(5, absl::any_cast<CopyOnly&>(o).value);
-  }
-
-  static_assert(!std::is_convertible<MoveOnly, absl::any>::value, "");
-}
-
-TEST(AnyTest, ConversionAssignment) {
-  {
-    absl::any o;
-    o = 5;
-    EXPECT_EQ(5, absl::any_cast<int>(o));
-  }
-
-  {
-    const CopyOnly copy_only(5);
-    absl::any o;
-    o = copy_only;
-    EXPECT_EQ(5, absl::any_cast<CopyOnly&>(o).value);
-  }
-
-  static_assert(!std::is_assignable<MoveOnly, absl::any>::value, "");
-}
-
-// Suppress MSVC warnings.
-// 4521: multiple copy constructors specified
-// We wrote multiple of them to test that the correct overloads are selected.
-#ifdef _MSC_VER
-#pragma warning( push )
-#pragma warning( disable : 4521)
-#endif
-
-// Weird type for testing, only used to make sure we "properly" perfect-forward
-// when being placed into an absl::any (use the l-value constructor if given an
-// l-value rather than use the copy constructor).
-struct WeirdConstructor42 {
-  explicit WeirdConstructor42(int value) : value(value) {}
-
-  // Copy-constructor
-  WeirdConstructor42(const WeirdConstructor42& other) : value(other.value) {}
-
-  // L-value "weird" constructor (used when given an l-value)
-  WeirdConstructor42(
-      WeirdConstructor42& /*other*/)  // NOLINT(runtime/references)
-      : value(42) {}
-
-  int value;
-};
-#ifdef _MSC_VER
-#pragma warning( pop )
-#endif
-
-TEST(AnyTest, WeirdConversionConstruction) {
-  {
-    const WeirdConstructor42 source(5);
-    absl::any o = source;  // Actual copy
-    EXPECT_EQ(5, absl::any_cast<WeirdConstructor42&>(o).value);
-  }
-
-  {
-    WeirdConstructor42 source(5);
-    absl::any o = source;  // Weird "conversion"
-    EXPECT_EQ(42, absl::any_cast<WeirdConstructor42&>(o).value);
-  }
-}
-
-TEST(AnyTest, WeirdConversionAssignment) {
-  {
-    const WeirdConstructor42 source(5);
-    absl::any o;
-    o = source;  // Actual copy
-    EXPECT_EQ(5, absl::any_cast<WeirdConstructor42&>(o).value);
-  }
-
-  {
-    WeirdConstructor42 source(5);
-    absl::any o;
-    o = source;  // Weird "conversion"
-    EXPECT_EQ(42, absl::any_cast<WeirdConstructor42&>(o).value);
-  }
-}
-
-struct Value {};
-
-TEST(AnyTest, AnyCastValue) {
-  {
-    absl::any o;
-    o.emplace<int>(5);
-    EXPECT_EQ(5, absl::any_cast<int>(o));
-    EXPECT_EQ(5, absl::any_cast<int>(AsConst(o)));
-    static_assert(
-        std::is_same<decltype(absl::any_cast<Value>(o)), Value>::value, "");
-  }
-
-  {
-    absl::any o;
-    o.emplace<int>(5);
-    EXPECT_EQ(5, absl::any_cast<const int>(o));
-    EXPECT_EQ(5, absl::any_cast<const int>(AsConst(o)));
-    static_assert(std::is_same<decltype(absl::any_cast<const Value>(o)),
-                               const Value>::value,
-                  "");
-  }
-}
-
-TEST(AnyTest, AnyCastReference) {
-  {
-    absl::any o;
-    o.emplace<int>(5);
-    EXPECT_EQ(5, absl::any_cast<int&>(o));
-    EXPECT_EQ(5, absl::any_cast<const int&>(AsConst(o)));
-    static_assert(
-        std::is_same<decltype(absl::any_cast<Value&>(o)), Value&>::value, "");
-  }
-
-  {
-    absl::any o;
-    o.emplace<int>(5);
-    EXPECT_EQ(5, absl::any_cast<const int>(o));
-    EXPECT_EQ(5, absl::any_cast<const int>(AsConst(o)));
-    static_assert(std::is_same<decltype(absl::any_cast<const Value&>(o)),
-                               const Value&>::value,
-                  "");
-  }
-
-  {
-    absl::any o;
-    o.emplace<int>(5);
-    EXPECT_EQ(5, absl::any_cast<int&&>(std::move(o)));
-    static_assert(std::is_same<decltype(absl::any_cast<Value&&>(std::move(o))),
-                               Value&&>::value,
-                  "");
-  }
-
-  {
-    absl::any o;
-    o.emplace<int>(5);
-    EXPECT_EQ(5, absl::any_cast<const int>(std::move(o)));
-    static_assert(
-        std::is_same<decltype(absl::any_cast<const Value&&>(std::move(o))),
-                     const Value&&>::value,
-        "");
-  }
-}
-
-TEST(AnyTest, AnyCastPointer) {
-  {
-    absl::any o;
-    EXPECT_EQ(nullptr, absl::any_cast<char>(&o));
-    o.emplace<int>(5);
-    EXPECT_EQ(nullptr, absl::any_cast<char>(&o));
-    o.emplace<char>('a');
-    EXPECT_EQ('a', *absl::any_cast<char>(&o));
-    static_assert(
-        std::is_same<decltype(absl::any_cast<Value>(&o)), Value*>::value, "");
-  }
-
-  {
-    absl::any o;
-    EXPECT_EQ(nullptr, absl::any_cast<const char>(&o));
-    o.emplace<int>(5);
-    EXPECT_EQ(nullptr, absl::any_cast<const char>(&o));
-    o.emplace<char>('a');
-    EXPECT_EQ('a', *absl::any_cast<const char>(&o));
-    static_assert(std::is_same<decltype(absl::any_cast<const Value>(&o)),
-                               const Value*>::value,
-                  "");
-  }
-}
-
-TEST(AnyTest, MakeAny) {
-  const CopyOnly copy_only{};
-  auto o = absl::make_any<IntMoveOnlyCopyOnly>(5, MoveOnly(), copy_only);
-  static_assert(std::is_same<decltype(o), absl::any>::value, "");
-  EXPECT_EQ(5, absl::any_cast<IntMoveOnlyCopyOnly&>(o).value);
-}
-
-TEST(AnyTest, MakeAnyIList) {
-  const CopyOnly copy_only{};
-  auto o =
-      absl::make_any<ListMoveOnlyCopyOnly>({1, 2, 3}, MoveOnly(), copy_only);
-  static_assert(std::is_same<decltype(o), absl::any>::value, "");
-  ListMoveOnlyCopyOnly& v = absl::any_cast<ListMoveOnlyCopyOnly&>(o);
-  std::vector<int> expected_values = {1, 2, 3};
-  EXPECT_EQ(expected_values, v.values);
-}
-
-// Test the use of copy constructor and operator=
-TEST(AnyTest, Copy) {
-  InstanceTracker tracker_raii;
-
-  {
-    absl::any o(absl::in_place_type<CopyableOnlyInstance>, 123);
-    CopyableOnlyInstance* f1 = absl::any_cast<CopyableOnlyInstance>(&o);
-
-    absl::any o2(o);
-    const CopyableOnlyInstance* f2 = absl::any_cast<CopyableOnlyInstance>(&o2);
-    EXPECT_EQ(123, f2->value());
-    EXPECT_NE(f1, f2);
-
-    absl::any o3;
-    o3 = o2;
-    const CopyableOnlyInstance* f3 = absl::any_cast<CopyableOnlyInstance>(&o3);
-    EXPECT_EQ(123, f3->value());
-    EXPECT_NE(f2, f3);
-
-    const absl::any o4(4);
-    // copy construct from const lvalue ref.
-    absl::any o5 = o4;
-    EXPECT_EQ(4, absl::any_cast<int>(o4));
-    EXPECT_EQ(4, absl::any_cast<int>(o5));
-
-    // Copy construct from const rvalue ref.
-    absl::any o6 = std::move(o4);  // NOLINT
-    EXPECT_EQ(4, absl::any_cast<int>(o4));
-    EXPECT_EQ(4, absl::any_cast<int>(o6));
-  }
-}
-
-TEST(AnyTest, Move) {
-  InstanceTracker tracker_raii;
-
-  absl::any any1;
-  any1.emplace<CopyableOnlyInstance>(5);
-
-  // This is a copy, so copy count increases to 1.
-  absl::any any2 = any1;
-  EXPECT_EQ(5, absl::any_cast<CopyableOnlyInstance&>(any1).value());
-  EXPECT_EQ(5, absl::any_cast<CopyableOnlyInstance&>(any2).value());
-  EXPECT_EQ(1, tracker_raii.copies());
-
-  // This isn't a copy, so copy count doesn't increase.
-  absl::any any3 = std::move(any2);
-  EXPECT_EQ(5, absl::any_cast<CopyableOnlyInstance&>(any3).value());
-  EXPECT_EQ(1, tracker_raii.copies());
-
-  absl::any any4;
-  any4 = std::move(any3);
-  EXPECT_EQ(5, absl::any_cast<CopyableOnlyInstance&>(any4).value());
-  EXPECT_EQ(1, tracker_raii.copies());
-
-  absl::any tmp4(4);
-  absl::any o4(std::move(tmp4));  // move construct
-  EXPECT_EQ(4, absl::any_cast<int>(o4));
-  o4 = *&o4;  // self assign
-  EXPECT_EQ(4, absl::any_cast<int>(o4));
-  EXPECT_TRUE(o4.has_value());
-
-  absl::any o5;
-  absl::any tmp5(5);
-  o5 = std::move(tmp5);  // move assign
-  EXPECT_EQ(5, absl::any_cast<int>(o5));
-}
-
-// Reset the ObjectOwner with an object of a different type
-TEST(AnyTest, Reset) {
-  absl::any o;
-  o.emplace<int>();
-
-  o.reset();
-  EXPECT_FALSE(o.has_value());
-
-  o.emplace<char>();
-  EXPECT_TRUE(o.has_value());
-}
-
-TEST(AnyTest, ConversionConstructionCausesOneCopy) {
-  InstanceTracker tracker_raii;
-  CopyableOnlyInstance counter(5);
-  absl::any o(counter);
-  EXPECT_EQ(5, absl::any_cast<CopyableOnlyInstance&>(o).value());
-  EXPECT_EQ(1, tracker_raii.copies());
-}
-
-//////////////////////////////////
-// Tests for Exception Behavior //
-//////////////////////////////////
-
-#if defined(ABSL_HAVE_STD_ANY)
-
-// If using a std `any` implementation, we can't check for a specific message.
-#define ABSL_ANY_TEST_EXPECT_BAD_ANY_CAST(...)                      \
-  ABSL_BASE_INTERNAL_EXPECT_FAIL((__VA_ARGS__), absl::bad_any_cast, \
-                                 "")
-
-#else
-
-// If using the absl `any` implementation, we can rely on a specific message.
-#define ABSL_ANY_TEST_EXPECT_BAD_ANY_CAST(...)                      \
-  ABSL_BASE_INTERNAL_EXPECT_FAIL((__VA_ARGS__), absl::bad_any_cast, \
-                                 "Bad any cast")
-
-#endif  // defined(ABSL_HAVE_STD_ANY)
-
-TEST(AnyTest, ThrowBadAlloc) {
-  {
-    absl::any a;
-    ABSL_ANY_TEST_EXPECT_BAD_ANY_CAST(absl::any_cast<int&>(a));
-    ABSL_ANY_TEST_EXPECT_BAD_ANY_CAST(absl::any_cast<const int&>(a));
-    ABSL_ANY_TEST_EXPECT_BAD_ANY_CAST(absl::any_cast<int&&>(absl::any{}));
-    ABSL_ANY_TEST_EXPECT_BAD_ANY_CAST(absl::any_cast<const int&&>(absl::any{}));
-    ABSL_ANY_TEST_EXPECT_BAD_ANY_CAST(absl::any_cast<int>(a));
-    ABSL_ANY_TEST_EXPECT_BAD_ANY_CAST(absl::any_cast<const int>(a));
-    ABSL_ANY_TEST_EXPECT_BAD_ANY_CAST(absl::any_cast<int>(absl::any{}));
-    ABSL_ANY_TEST_EXPECT_BAD_ANY_CAST(absl::any_cast<const int>(absl::any{}));
-
-    // const absl::any operand
-    ABSL_ANY_TEST_EXPECT_BAD_ANY_CAST(absl::any_cast<const int&>(AsConst(a)));
-    ABSL_ANY_TEST_EXPECT_BAD_ANY_CAST(absl::any_cast<int>(AsConst(a)));
-    ABSL_ANY_TEST_EXPECT_BAD_ANY_CAST(absl::any_cast<const int>(AsConst(a)));
-  }
-
-  {
-    absl::any a(absl::in_place_type<int>);
-    ABSL_ANY_TEST_EXPECT_BAD_ANY_CAST(absl::any_cast<float&>(a));
-    ABSL_ANY_TEST_EXPECT_BAD_ANY_CAST(absl::any_cast<const float&>(a));
-    ABSL_ANY_TEST_EXPECT_BAD_ANY_CAST(absl::any_cast<float&&>(absl::any{}));
-    ABSL_ANY_TEST_EXPECT_BAD_ANY_CAST(
-        absl::any_cast<const float&&>(absl::any{}));
-    ABSL_ANY_TEST_EXPECT_BAD_ANY_CAST(absl::any_cast<float>(a));
-    ABSL_ANY_TEST_EXPECT_BAD_ANY_CAST(absl::any_cast<const float>(a));
-    ABSL_ANY_TEST_EXPECT_BAD_ANY_CAST(absl::any_cast<float>(absl::any{}));
-    ABSL_ANY_TEST_EXPECT_BAD_ANY_CAST(absl::any_cast<const float>(absl::any{}));
-
-    // const absl::any operand
-    ABSL_ANY_TEST_EXPECT_BAD_ANY_CAST(absl::any_cast<const float&>(AsConst(a)));
-    ABSL_ANY_TEST_EXPECT_BAD_ANY_CAST(absl::any_cast<float>(AsConst(a)));
-    ABSL_ANY_TEST_EXPECT_BAD_ANY_CAST(absl::any_cast<const float>(AsConst(a)));
-  }
-}
-
-class BadCopy {};
-
-struct BadCopyable {
-  BadCopyable() = default;
-  BadCopyable(BadCopyable&&) = default;
-  BadCopyable(const BadCopyable&) {
-#ifdef ABSL_HAVE_EXCEPTIONS
-    throw BadCopy();
-#else
-    ABSL_RAW_LOG(FATAL, "Bad copy");
-#endif
-  }
-};
-
-#define ABSL_ANY_TEST_EXPECT_BAD_COPY(...) \
-  ABSL_BASE_INTERNAL_EXPECT_FAIL((__VA_ARGS__), BadCopy, "Bad copy")
-
-// Test the guarantees regarding exceptions in copy/assign.
-TEST(AnyTest, FailedCopy) {
-  {
-    const BadCopyable bad{};
-    ABSL_ANY_TEST_EXPECT_BAD_COPY(absl::any{bad});
-  }
-
-  {
-    absl::any src(absl::in_place_type<BadCopyable>);
-    ABSL_ANY_TEST_EXPECT_BAD_COPY(absl::any{src});
-  }
-
-  {
-    BadCopyable bad;
-    absl::any target;
-    ABSL_ANY_TEST_EXPECT_BAD_COPY(target = bad);
-  }
-
-  {
-    BadCopyable bad;
-    absl::any target(absl::in_place_type<BadCopyable>);
-    ABSL_ANY_TEST_EXPECT_BAD_COPY(target = bad);
-    EXPECT_TRUE(target.has_value());
-  }
-
-  {
-    absl::any src(absl::in_place_type<BadCopyable>);
-    absl::any target;
-    ABSL_ANY_TEST_EXPECT_BAD_COPY(target = src);
-    EXPECT_FALSE(target.has_value());
-  }
-
-  {
-    absl::any src(absl::in_place_type<BadCopyable>);
-    absl::any target(absl::in_place_type<BadCopyable>);
-    ABSL_ANY_TEST_EXPECT_BAD_COPY(target = src);
-    EXPECT_TRUE(target.has_value());
-  }
-}
-
-// Test the guarantees regarding exceptions in emplace.
-TEST(AnyTest, FailedEmplace) {
-  {
-    BadCopyable bad;
-    absl::any target;
-    ABSL_ANY_TEST_EXPECT_BAD_COPY(target.emplace<BadCopyable>(bad));
-  }
-
-  {
-    BadCopyable bad;
-    absl::any target(absl::in_place_type<int>);
-    ABSL_ANY_TEST_EXPECT_BAD_COPY(target.emplace<BadCopyable>(bad));
-#if defined(ABSL_HAVE_STD_ANY) && defined(__GLIBCXX__)
-    // libstdc++ std::any::emplace() implementation (as of 7.2) has a bug: if an
-    // exception is thrown, *this contains a value.
-#define ABSL_GLIBCXX_ANY_EMPLACE_EXCEPTION_BUG 1
-#endif
-#if defined(ABSL_HAVE_EXCEPTIONS) && \
-    !defined(ABSL_GLIBCXX_ANY_EMPLACE_EXCEPTION_BUG)
-    EXPECT_FALSE(target.has_value());
-#endif
-  }
-}
-
-}  // namespace
-
-#endif  // #if !defined(ABSL_HAVE_STD_ANY)
diff --git a/absl/types/bad_any_cast.h b/absl/types/bad_any_cast.h
deleted file mode 100644
index 8d020ede466..00000000000
--- a/absl/types/bad_any_cast.h
+++ /dev/null
@@ -1,71 +0,0 @@
-// Copyright 2018 The Abseil Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-//
-// -----------------------------------------------------------------------------
-// bad_any_cast.h
-// -----------------------------------------------------------------------------
-//
-// This header file defines the `absl::bad_any_cast` type.
-
-#ifndef ABSL_TYPES_BAD_ANY_CAST_H_
-#define ABSL_TYPES_BAD_ANY_CAST_H_
-
-#include <typeinfo>
-
-#include "absl/base/config.h"
-
-#ifdef ABSL_HAVE_STD_ANY
-
-#include <any>
-
-namespace absl {
-using std::bad_any_cast;
-}  // namespace absl
-
-#else  // ABSL_HAVE_STD_ANY
-
-namespace absl {
-
-// -----------------------------------------------------------------------------
-// bad_any_cast
-// -----------------------------------------------------------------------------
-//
-// An `absl::bad_any_cast` type is an exception type that is thrown when
-// failing to successfully cast the return value of an `absl::any` object.
-//
-// Example:
-//
-//   auto a = absl::any(65);
-//   absl::any_cast<int>(a);         // 65
-//   try {
-//     absl::any_cast<char>(a);
-//   } catch(const absl::bad_any_cast& e) {
-//     std::cout << "Bad any cast: " << e.what() << '\n';
-//   }
-class bad_any_cast : public std::bad_cast {
- public:
-  ~bad_any_cast() override;
-  const char* what() const noexcept override;
-};
-
-namespace any_internal {
-
-[[noreturn]] void ThrowBadAnyCast();
-
-}  // namespace any_internal
-}  // namespace absl
-
-#endif  // ABSL_HAVE_STD_ANY
-
-#endif  // ABSL_TYPES_BAD_ANY_CAST_H_
diff --git a/absl/types/bad_optional_access.h b/absl/types/bad_optional_access.h
deleted file mode 100644
index add5c452cd5..00000000000
--- a/absl/types/bad_optional_access.h
+++ /dev/null
@@ -1,74 +0,0 @@
-// Copyright 2018 The Abseil Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-//
-// -----------------------------------------------------------------------------
-// bad_optional_access.h
-// -----------------------------------------------------------------------------
-//
-// This header file defines the `absl::bad_optional_access` type.
-
-#ifndef ABSL_TYPES_BAD_OPTIONAL_ACCESS_H_
-#define ABSL_TYPES_BAD_OPTIONAL_ACCESS_H_
-
-#include <stdexcept>
-
-#include "absl/base/config.h"
-
-#ifdef ABSL_HAVE_STD_OPTIONAL
-
-#include <optional>
-
-namespace absl {
-using std::bad_optional_access;
-}  // namespace absl
-
-#else  // ABSL_HAVE_STD_OPTIONAL
-
-namespace absl {
-
-// -----------------------------------------------------------------------------
-// bad_optional_access
-// -----------------------------------------------------------------------------
-//
-// An `absl::bad_optional_access` type is an exception type that is thrown when
-// attempting to access an `absl::optional` object that does not contain a
-// value.
-//
-// Example:
-//
-//   absl::optional<int> o;
-//
-//   try {
-//     int n = o.value();
-//   } catch(const absl::bad_optional_access& e) {
-//     std::cout << "Bad optional access: " << e.what() << '\n';
-//   }
-class bad_optional_access : public std::exception {
- public:
-  bad_optional_access() = default;
-  ~bad_optional_access() override;
-  const char* what() const noexcept override;
-};
-
-namespace optional_internal {
-
-// throw delegator
-[[noreturn]] void throw_bad_optional_access();
-
-}  // namespace optional_internal
-}  // namespace absl
-
-#endif  // ABSL_HAVE_STD_OPTIONAL
-
-#endif  // ABSL_TYPES_BAD_OPTIONAL_ACCESS_H_
diff --git a/absl/types/bad_variant_access.cc b/absl/types/bad_variant_access.cc
deleted file mode 100644
index a4325c8df7d..00000000000
--- a/absl/types/bad_variant_access.cc
+++ /dev/null
@@ -1,62 +0,0 @@
-// Copyright 2017 The Abseil Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-#include "absl/types/bad_variant_access.h"
-
-#ifndef ABSL_HAVE_STD_VARIANT
-
-#include <cstdlib>
-#include <stdexcept>
-
-#include "absl/base/config.h"
-#include "absl/base/internal/raw_logging.h"
-
-namespace absl {
-
-//////////////////////////
-// [variant.bad.access] //
-//////////////////////////
-
-bad_variant_access::~bad_variant_access() = default;
-
-const char* bad_variant_access::what() const noexcept {
-  return "Bad variant access";
-}
-
-namespace variant_internal {
-
-void ThrowBadVariantAccess() {
-#ifdef ABSL_HAVE_EXCEPTIONS
-  throw bad_variant_access();
-#else
-  ABSL_RAW_LOG(FATAL, "Bad variant access");
-  abort();  // TODO(calabrese) Remove once RAW_LOG FATAL is noreturn.
-#endif
-}
-
-void Rethrow() {
-#ifdef ABSL_HAVE_EXCEPTIONS
-  throw;
-#else
-  ABSL_RAW_LOG(FATAL,
-               "Internal error in absl::variant implementation. Attempted to "
-               "rethrow an exception when building with exceptions disabled.");
-  abort();  // TODO(calabrese) Remove once RAW_LOG FATAL is noreturn.
-#endif
-}
-
-}  // namespace variant_internal
-}  // namespace absl
-
-#endif  // ABSL_HAVE_STD_VARIANT
diff --git a/absl/types/bad_variant_access.h b/absl/types/bad_variant_access.h
deleted file mode 100644
index 637db435505..00000000000
--- a/absl/types/bad_variant_access.h
+++ /dev/null
@@ -1,78 +0,0 @@
-// Copyright 2018 The Abseil Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-//
-// -----------------------------------------------------------------------------
-// bad_variant_access.h
-// -----------------------------------------------------------------------------
-//
-// This header file defines the `absl::bad_variant_access` type.
-
-#ifndef ABSL_TYPES_BAD_VARIANT_ACCESS_H_
-#define ABSL_TYPES_BAD_VARIANT_ACCESS_H_
-
-#include <stdexcept>
-
-#include "absl/base/config.h"
-
-#ifdef ABSL_HAVE_STD_VARIANT
-
-#include <variant>
-
-namespace absl {
-using std::bad_variant_access;
-}  // namespace absl
-
-#else  // ABSL_HAVE_STD_VARIANT
-
-namespace absl {
-
-// -----------------------------------------------------------------------------
-// bad_variant_access
-// -----------------------------------------------------------------------------
-//
-// An `absl::bad_variant_access` type is an exception type that is thrown in
-// the following cases:
-//
-//   * Calling `absl::get(absl::variant) with an index or type that does not
-//     match the currently selected alternative type
-//   * Calling `absl::visit on an `absl::variant` that is in the
-//     `variant::valueless_by_exception` state.
-//
-// Example:
-//
-//   absl::variant<int, std::string> v;
-//   v = 1;
-//   try {
-//     absl::get<std::string>(v);
-//   } catch(const absl::bad_variant_access& e) {
-//     std::cout << "Bad variant access: " << e.what() << '\n';
-//   }
-class bad_variant_access : public std::exception {
- public:
-  bad_variant_access() noexcept = default;
-  ~bad_variant_access() override;
-  const char* what() const noexcept override;
-};
-
-namespace variant_internal {
-
-[[noreturn]] void ThrowBadVariantAccess();
-[[noreturn]] void Rethrow();
-
-}  // namespace variant_internal
-}  // namespace absl
-
-#endif  // ABSL_HAVE_STD_VARIANT
-
-#endif  // ABSL_TYPES_BAD_VARIANT_ACCESS_H_
diff --git a/absl/types/compare.h b/absl/types/compare.h
index a213e0b0b68..e6a2a5728c3 100644
--- a/absl/types/compare.h
+++ b/absl/types/compare.h
@@ -16,56 +16,87 @@
 // compare.h
 // -----------------------------------------------------------------------------
 //
-// This header file defines the `absl::weak_equality`, `absl::strong_equality`,
-// `absl::partial_ordering`, `absl::weak_ordering`, and `absl::strong_ordering`
-// types for storing the results of three way comparisons.
+// This header file defines the `absl::partial_ordering`, `absl::weak_ordering`,
+// and `absl::strong_ordering` types for storing the results of three way
+// comparisons.
 //
 // Example:
 //   absl::weak_ordering compare(const std::string& a, const std::string& b);
 //
-// These are C++11 compatible versions of the C++20 corresponding types
-// (`std::weak_equality`, etc.) and are designed to be drop-in replacements
+// These are C++17 compatible versions of the C++20 corresponding types
+// (`std::partial_ordering`, etc.) and are designed to be drop-in replacements
 // for code compliant with C++20.
 
 #ifndef ABSL_TYPES_COMPARE_H_
 #define ABSL_TYPES_COMPARE_H_
 
+#include "absl/base/config.h"
+
+#ifdef ABSL_USES_STD_ORDERING
+
+#include <compare>  // IWYU pragma: export
+#include <type_traits>
+
+#include "absl/meta/type_traits.h"
+
+#else
+
 #include <cstddef>
 #include <cstdint>
 #include <cstdlib>
 #include <type_traits>
 
 #include "absl/base/attributes.h"
+#include "absl/base/macros.h"
 #include "absl/meta/type_traits.h"
 
+#endif
+
 namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+#ifdef ABSL_USES_STD_ORDERING
+
+using std::partial_ordering;
+using std::strong_ordering;
+using std::weak_ordering;
+
+#else
+
 namespace compare_internal {
 
 using value_type = int8_t;
 
-template <typename T>
-struct Fail {
-  static_assert(sizeof(T) < 0, "Only literal `0` is allowed.");
-};
-
-// We need the NullPtrT template to avoid triggering the modernize-use-nullptr
-// ClangTidy warning in user code.
-template <typename NullPtrT = std::nullptr_t>
-struct OnlyLiteralZero {
-  constexpr OnlyLiteralZero(NullPtrT) noexcept {}  // NOLINT
+class OnlyLiteralZero {
+ public:
+#if ABSL_HAVE_ATTRIBUTE(enable_if)
+  // On clang, we can avoid triggering modernize-use-nullptr by only enabling
+  // this overload when the value is a compile time integer constant equal to 0.
+  //
+  // In c++20, this could be a static_assert in a consteval function.
+  constexpr OnlyLiteralZero(int n)  // NOLINT
+      __attribute__((enable_if(n == 0, "Only literal `0` is allowed."))) {}
+#else  // ABSL_HAVE_ATTRIBUTE(enable_if)
+  // Accept only literal zero since it can be implicitly converted to a pointer
+  // to member type. nullptr constants will be caught by the other constructor
+  // which accepts a nullptr_t.
+  //
+  // This constructor is not used for clang since it triggers
+  // modernize-use-nullptr.
+  constexpr OnlyLiteralZero(int OnlyLiteralZero::*) noexcept {}  // NOLINT
+#endif
 
   // Fails compilation when `nullptr` or integral type arguments other than
   // `int` are passed. This constructor doesn't accept `int` because literal `0`
   // has type `int`. Literal `0` arguments will be implicitly converted to
   // `std::nullptr_t` and accepted by the above constructor, while other `int`
   // arguments will fail to be converted and cause compilation failure.
-  template <
-      typename T,
-      typename = typename std::enable_if<
-          std::is_same<T, std::nullptr_t>::value ||
-          (std::is_integral<T>::value && !std::is_same<T, int>::value)>::type,
-      typename = typename Fail<T>::type>
-  OnlyLiteralZero(T);  // NOLINT
+  template <typename T, typename = std::enable_if_t<
+                            std::is_same_v<T, std::nullptr_t> ||
+                            (std::is_integral_v<T> && !std::is_same_v<T, int>)>>
+  OnlyLiteralZero(T) {  // NOLINT
+    static_assert(sizeof(T) < 0, "Only literal `0` is allowed.");
+  }
 };
 
 enum class eq : value_type {
@@ -79,166 +110,15 @@ enum class ord : value_type { less = -1, greater = 1 };
 
 enum class ncmp : value_type { unordered = -127 };
 
-// Define macros to allow for creation or emulation of C++17 inline variables
-// based on whether the feature is supported. Note: we can't use
-// ABSL_INTERNAL_INLINE_CONSTEXPR here because the variables here are of
-// incomplete types so they need to be defined after the types are complete.
-#ifdef __cpp_inline_variables
-
-#define ABSL_COMPARE_INLINE_BASECLASS_DECL(name)
-
-#define ABSL_COMPARE_INLINE_SUBCLASS_DECL(type, name) \
-  static const type name
-
-#define ABSL_COMPARE_INLINE_INIT(type, name, init) \
-  inline constexpr type type::name(init)
-
-#else  // __cpp_inline_variables
-
-#define ABSL_COMPARE_INLINE_BASECLASS_DECL(name) \
-  ABSL_CONST_INIT static const T name
-
-#define ABSL_COMPARE_INLINE_SUBCLASS_DECL(type, name)
-
-#define ABSL_COMPARE_INLINE_INIT(type, name, init) \
-  template <typename T>                            \
-  const T compare_internal::type##_base<T>::name(init)
-
-#endif  // __cpp_inline_variables
-
-// These template base classes allow for defining the values of the constants
-// in the header file (for performance) without using inline variables (which
-// aren't available in C++11).
-template <typename T>
-struct weak_equality_base {
-  ABSL_COMPARE_INLINE_BASECLASS_DECL(equivalent);
-  ABSL_COMPARE_INLINE_BASECLASS_DECL(nonequivalent);
-};
-
-template <typename T>
-struct strong_equality_base {
-  ABSL_COMPARE_INLINE_BASECLASS_DECL(equal);
-  ABSL_COMPARE_INLINE_BASECLASS_DECL(nonequal);
-  ABSL_COMPARE_INLINE_BASECLASS_DECL(equivalent);
-  ABSL_COMPARE_INLINE_BASECLASS_DECL(nonequivalent);
-};
-
-template <typename T>
-struct partial_ordering_base {
-  ABSL_COMPARE_INLINE_BASECLASS_DECL(less);
-  ABSL_COMPARE_INLINE_BASECLASS_DECL(equivalent);
-  ABSL_COMPARE_INLINE_BASECLASS_DECL(greater);
-  ABSL_COMPARE_INLINE_BASECLASS_DECL(unordered);
-};
-
-template <typename T>
-struct weak_ordering_base {
-  ABSL_COMPARE_INLINE_BASECLASS_DECL(less);
-  ABSL_COMPARE_INLINE_BASECLASS_DECL(equivalent);
-  ABSL_COMPARE_INLINE_BASECLASS_DECL(greater);
-};
-
-template <typename T>
-struct strong_ordering_base {
-  ABSL_COMPARE_INLINE_BASECLASS_DECL(less);
-  ABSL_COMPARE_INLINE_BASECLASS_DECL(equal);
-  ABSL_COMPARE_INLINE_BASECLASS_DECL(equivalent);
-  ABSL_COMPARE_INLINE_BASECLASS_DECL(greater);
-};
-
 }  // namespace compare_internal
 
-class weak_equality
-    : public compare_internal::weak_equality_base<weak_equality> {
-  explicit constexpr weak_equality(compare_internal::eq v) noexcept
-      : value_(static_cast<compare_internal::value_type>(v)) {}
-  friend struct compare_internal::weak_equality_base<weak_equality>;
-
- public:
-  ABSL_COMPARE_INLINE_SUBCLASS_DECL(weak_equality, equivalent);
-  ABSL_COMPARE_INLINE_SUBCLASS_DECL(weak_equality, nonequivalent);
-
-  // Comparisons
-  friend constexpr bool operator==(
-      weak_equality v, compare_internal::OnlyLiteralZero<>) noexcept {
-    return v.value_ == 0;
-  }
-  friend constexpr bool operator!=(
-      weak_equality v, compare_internal::OnlyLiteralZero<>) noexcept {
-    return v.value_ != 0;
-  }
-  friend constexpr bool operator==(compare_internal::OnlyLiteralZero<>,
-                                   weak_equality v) noexcept {
-    return 0 == v.value_;
-  }
-  friend constexpr bool operator!=(compare_internal::OnlyLiteralZero<>,
-                                   weak_equality v) noexcept {
-    return 0 != v.value_;
-  }
-
- private:
-  compare_internal::value_type value_;
-};
-ABSL_COMPARE_INLINE_INIT(weak_equality, equivalent,
-                         compare_internal::eq::equivalent);
-ABSL_COMPARE_INLINE_INIT(weak_equality, nonequivalent,
-                         compare_internal::eq::nonequivalent);
-
-class strong_equality
-    : public compare_internal::strong_equality_base<strong_equality> {
-  explicit constexpr strong_equality(compare_internal::eq v) noexcept
-      : value_(static_cast<compare_internal::value_type>(v)) {}
-  friend struct compare_internal::strong_equality_base<strong_equality>;
-
- public:
-  ABSL_COMPARE_INLINE_SUBCLASS_DECL(strong_equality, equal);
-  ABSL_COMPARE_INLINE_SUBCLASS_DECL(strong_equality, nonequal);
-  ABSL_COMPARE_INLINE_SUBCLASS_DECL(strong_equality, equivalent);
-  ABSL_COMPARE_INLINE_SUBCLASS_DECL(strong_equality, nonequivalent);
-
-  // Conversion
-  constexpr operator weak_equality() const noexcept {  // NOLINT
-    return value_ == 0 ? weak_equality::equivalent
-                       : weak_equality::nonequivalent;
-  }
-  // Comparisons
-  friend constexpr bool operator==(
-      strong_equality v, compare_internal::OnlyLiteralZero<>) noexcept {
-    return v.value_ == 0;
-  }
-  friend constexpr bool operator!=(
-      strong_equality v, compare_internal::OnlyLiteralZero<>) noexcept {
-    return v.value_ != 0;
-  }
-  friend constexpr bool operator==(compare_internal::OnlyLiteralZero<>,
-                                   strong_equality v) noexcept {
-    return 0 == v.value_;
-  }
-  friend constexpr bool operator!=(compare_internal::OnlyLiteralZero<>,
-                                   strong_equality v) noexcept {
-    return 0 != v.value_;
-  }
-
- private:
-  compare_internal::value_type value_;
-};
-ABSL_COMPARE_INLINE_INIT(strong_equality, equal, compare_internal::eq::equal);
-ABSL_COMPARE_INLINE_INIT(strong_equality, nonequal,
-                         compare_internal::eq::nonequal);
-ABSL_COMPARE_INLINE_INIT(strong_equality, equivalent,
-                         compare_internal::eq::equivalent);
-ABSL_COMPARE_INLINE_INIT(strong_equality, nonequivalent,
-                         compare_internal::eq::nonequivalent);
-
-class partial_ordering
-    : public compare_internal::partial_ordering_base<partial_ordering> {
+class partial_ordering {
   explicit constexpr partial_ordering(compare_internal::eq v) noexcept
       : value_(static_cast<compare_internal::value_type>(v)) {}
   explicit constexpr partial_ordering(compare_internal::ord v) noexcept
       : value_(static_cast<compare_internal::value_type>(v)) {}
   explicit constexpr partial_ordering(compare_internal::ncmp v) noexcept
       : value_(static_cast<compare_internal::value_type>(v)) {}
-  friend struct compare_internal::partial_ordering_base<partial_ordering>;
 
   constexpr bool is_ordered() const noexcept {
     return value_ !=
@@ -246,181 +126,178 @@ class partial_ordering
   }
 
  public:
-  ABSL_COMPARE_INLINE_SUBCLASS_DECL(partial_ordering, less);
-  ABSL_COMPARE_INLINE_SUBCLASS_DECL(partial_ordering, equivalent);
-  ABSL_COMPARE_INLINE_SUBCLASS_DECL(partial_ordering, greater);
-  ABSL_COMPARE_INLINE_SUBCLASS_DECL(partial_ordering, unordered);
+  static const partial_ordering less;
+  static const partial_ordering equivalent;
+  static const partial_ordering greater;
+  static const partial_ordering unordered;
 
-  // Conversion
-  constexpr operator weak_equality() const noexcept {  // NOLINT
-    return value_ == 0 ? weak_equality::equivalent
-                       : weak_equality::nonequivalent;
-  }
   // Comparisons
-  friend constexpr bool operator==(
-      partial_ordering v, compare_internal::OnlyLiteralZero<>) noexcept {
+  friend constexpr bool operator==(partial_ordering v,
+                                   compare_internal::OnlyLiteralZero) noexcept {
     return v.is_ordered() && v.value_ == 0;
   }
-  friend constexpr bool operator!=(
-      partial_ordering v, compare_internal::OnlyLiteralZero<>) noexcept {
+  friend constexpr bool operator!=(partial_ordering v,
+                                   compare_internal::OnlyLiteralZero) noexcept {
     return !v.is_ordered() || v.value_ != 0;
   }
-  friend constexpr bool operator<(
-      partial_ordering v, compare_internal::OnlyLiteralZero<>) noexcept {
+  friend constexpr bool operator<(partial_ordering v,
+                                  compare_internal::OnlyLiteralZero) noexcept {
     return v.is_ordered() && v.value_ < 0;
   }
-  friend constexpr bool operator<=(
-      partial_ordering v, compare_internal::OnlyLiteralZero<>) noexcept {
+  friend constexpr bool operator<=(partial_ordering v,
+                                   compare_internal::OnlyLiteralZero) noexcept {
     return v.is_ordered() && v.value_ <= 0;
   }
-  friend constexpr bool operator>(
-      partial_ordering v, compare_internal::OnlyLiteralZero<>) noexcept {
+  friend constexpr bool operator>(partial_ordering v,
+                                  compare_internal::OnlyLiteralZero) noexcept {
     return v.is_ordered() && v.value_ > 0;
   }
-  friend constexpr bool operator>=(
-      partial_ordering v, compare_internal::OnlyLiteralZero<>) noexcept {
+  friend constexpr bool operator>=(partial_ordering v,
+                                   compare_internal::OnlyLiteralZero) noexcept {
     return v.is_ordered() && v.value_ >= 0;
   }
-  friend constexpr bool operator==(compare_internal::OnlyLiteralZero<>,
+  friend constexpr bool operator==(compare_internal::OnlyLiteralZero,
                                    partial_ordering v) noexcept {
     return v.is_ordered() && 0 == v.value_;
   }
-  friend constexpr bool operator!=(compare_internal::OnlyLiteralZero<>,
+  friend constexpr bool operator!=(compare_internal::OnlyLiteralZero,
                                    partial_ordering v) noexcept {
     return !v.is_ordered() || 0 != v.value_;
   }
-  friend constexpr bool operator<(compare_internal::OnlyLiteralZero<>,
+  friend constexpr bool operator<(compare_internal::OnlyLiteralZero,
                                   partial_ordering v) noexcept {
     return v.is_ordered() && 0 < v.value_;
   }
-  friend constexpr bool operator<=(compare_internal::OnlyLiteralZero<>,
+  friend constexpr bool operator<=(compare_internal::OnlyLiteralZero,
                                    partial_ordering v) noexcept {
     return v.is_ordered() && 0 <= v.value_;
   }
-  friend constexpr bool operator>(compare_internal::OnlyLiteralZero<>,
+  friend constexpr bool operator>(compare_internal::OnlyLiteralZero,
                                   partial_ordering v) noexcept {
     return v.is_ordered() && 0 > v.value_;
   }
-  friend constexpr bool operator>=(compare_internal::OnlyLiteralZero<>,
+  friend constexpr bool operator>=(compare_internal::OnlyLiteralZero,
                                    partial_ordering v) noexcept {
     return v.is_ordered() && 0 >= v.value_;
   }
+  friend constexpr bool operator==(partial_ordering v1,
+                                   partial_ordering v2) noexcept {
+    return v1.value_ == v2.value_;
+  }
+  friend constexpr bool operator!=(partial_ordering v1,
+                                   partial_ordering v2) noexcept {
+    return v1.value_ != v2.value_;
+  }
 
  private:
   compare_internal::value_type value_;
 };
-ABSL_COMPARE_INLINE_INIT(partial_ordering, less, compare_internal::ord::less);
-ABSL_COMPARE_INLINE_INIT(partial_ordering, equivalent,
-                         compare_internal::eq::equivalent);
-ABSL_COMPARE_INLINE_INIT(partial_ordering, greater,
-                         compare_internal::ord::greater);
-ABSL_COMPARE_INLINE_INIT(partial_ordering, unordered,
-                         compare_internal::ncmp::unordered);
-
-class weak_ordering
-    : public compare_internal::weak_ordering_base<weak_ordering> {
+inline constexpr partial_ordering partial_ordering::less(
+    compare_internal::ord::less);
+inline constexpr partial_ordering partial_ordering::equivalent(
+    compare_internal::eq::equivalent);
+inline constexpr partial_ordering partial_ordering::greater(
+    compare_internal::ord::greater);
+inline constexpr partial_ordering partial_ordering::unordered(
+    compare_internal::ncmp::unordered);
+
+class weak_ordering {
   explicit constexpr weak_ordering(compare_internal::eq v) noexcept
       : value_(static_cast<compare_internal::value_type>(v)) {}
   explicit constexpr weak_ordering(compare_internal::ord v) noexcept
       : value_(static_cast<compare_internal::value_type>(v)) {}
-  friend struct compare_internal::weak_ordering_base<weak_ordering>;
 
  public:
-  ABSL_COMPARE_INLINE_SUBCLASS_DECL(weak_ordering, less);
-  ABSL_COMPARE_INLINE_SUBCLASS_DECL(weak_ordering, equivalent);
-  ABSL_COMPARE_INLINE_SUBCLASS_DECL(weak_ordering, greater);
+  static const weak_ordering less;
+  static const weak_ordering equivalent;
+  static const weak_ordering greater;
 
   // Conversions
-  constexpr operator weak_equality() const noexcept {  // NOLINT
-    return value_ == 0 ? weak_equality::equivalent
-                       : weak_equality::nonequivalent;
-  }
   constexpr operator partial_ordering() const noexcept {  // NOLINT
     return value_ == 0 ? partial_ordering::equivalent
                        : (value_ < 0 ? partial_ordering::less
                                      : partial_ordering::greater);
   }
   // Comparisons
-  friend constexpr bool operator==(
-      weak_ordering v, compare_internal::OnlyLiteralZero<>) noexcept {
+  friend constexpr bool operator==(weak_ordering v,
+                                   compare_internal::OnlyLiteralZero) noexcept {
     return v.value_ == 0;
   }
-  friend constexpr bool operator!=(
-      weak_ordering v, compare_internal::OnlyLiteralZero<>) noexcept {
+  friend constexpr bool operator!=(weak_ordering v,
+                                   compare_internal::OnlyLiteralZero) noexcept {
     return v.value_ != 0;
   }
-  friend constexpr bool operator<(
-      weak_ordering v, compare_internal::OnlyLiteralZero<>) noexcept {
+  friend constexpr bool operator<(weak_ordering v,
+                                  compare_internal::OnlyLiteralZero) noexcept {
     return v.value_ < 0;
   }
-  friend constexpr bool operator<=(
-      weak_ordering v, compare_internal::OnlyLiteralZero<>) noexcept {
+  friend constexpr bool operator<=(weak_ordering v,
+                                   compare_internal::OnlyLiteralZero) noexcept {
     return v.value_ <= 0;
   }
-  friend constexpr bool operator>(
-      weak_ordering v, compare_internal::OnlyLiteralZero<>) noexcept {
+  friend constexpr bool operator>(weak_ordering v,
+                                  compare_internal::OnlyLiteralZero) noexcept {
     return v.value_ > 0;
   }
-  friend constexpr bool operator>=(
-      weak_ordering v, compare_internal::OnlyLiteralZero<>) noexcept {
+  friend constexpr bool operator>=(weak_ordering v,
+                                   compare_internal::OnlyLiteralZero) noexcept {
     return v.value_ >= 0;
   }
-  friend constexpr bool operator==(compare_internal::OnlyLiteralZero<>,
+  friend constexpr bool operator==(compare_internal::OnlyLiteralZero,
                                    weak_ordering v) noexcept {
     return 0 == v.value_;
   }
-  friend constexpr bool operator!=(compare_internal::OnlyLiteralZero<>,
+  friend constexpr bool operator!=(compare_internal::OnlyLiteralZero,
                                    weak_ordering v) noexcept {
     return 0 != v.value_;
   }
-  friend constexpr bool operator<(compare_internal::OnlyLiteralZero<>,
+  friend constexpr bool operator<(compare_internal::OnlyLiteralZero,
                                   weak_ordering v) noexcept {
     return 0 < v.value_;
   }
-  friend constexpr bool operator<=(compare_internal::OnlyLiteralZero<>,
+  friend constexpr bool operator<=(compare_internal::OnlyLiteralZero,
                                    weak_ordering v) noexcept {
     return 0 <= v.value_;
   }
-  friend constexpr bool operator>(compare_internal::OnlyLiteralZero<>,
+  friend constexpr bool operator>(compare_internal::OnlyLiteralZero,
                                   weak_ordering v) noexcept {
     return 0 > v.value_;
   }
-  friend constexpr bool operator>=(compare_internal::OnlyLiteralZero<>,
+  friend constexpr bool operator>=(compare_internal::OnlyLiteralZero,
                                    weak_ordering v) noexcept {
     return 0 >= v.value_;
   }
+  friend constexpr bool operator==(weak_ordering v1,
+                                   weak_ordering v2) noexcept {
+    return v1.value_ == v2.value_;
+  }
+  friend constexpr bool operator!=(weak_ordering v1,
+                                   weak_ordering v2) noexcept {
+    return v1.value_ != v2.value_;
+  }
 
  private:
   compare_internal::value_type value_;
 };
-ABSL_COMPARE_INLINE_INIT(weak_ordering, less, compare_internal::ord::less);
-ABSL_COMPARE_INLINE_INIT(weak_ordering, equivalent,
-                         compare_internal::eq::equivalent);
-ABSL_COMPARE_INLINE_INIT(weak_ordering, greater,
-                         compare_internal::ord::greater);
-
-class strong_ordering
-    : public compare_internal::strong_ordering_base<strong_ordering> {
+inline constexpr weak_ordering weak_ordering::less(compare_internal::ord::less);
+inline constexpr weak_ordering weak_ordering::equivalent(
+    compare_internal::eq::equivalent);
+inline constexpr weak_ordering weak_ordering::greater(
+    compare_internal::ord::greater);
+
+class strong_ordering {
   explicit constexpr strong_ordering(compare_internal::eq v) noexcept
       : value_(static_cast<compare_internal::value_type>(v)) {}
   explicit constexpr strong_ordering(compare_internal::ord v) noexcept
       : value_(static_cast<compare_internal::value_type>(v)) {}
-  friend struct compare_internal::strong_ordering_base<strong_ordering>;
 
  public:
-  ABSL_COMPARE_INLINE_SUBCLASS_DECL(strong_ordering, less);
-  ABSL_COMPARE_INLINE_SUBCLASS_DECL(strong_ordering, equal);
-  ABSL_COMPARE_INLINE_SUBCLASS_DECL(strong_ordering, equivalent);
-  ABSL_COMPARE_INLINE_SUBCLASS_DECL(strong_ordering, greater);
+  static const strong_ordering less;
+  static const strong_ordering equal;
+  static const strong_ordering equivalent;
+  static const strong_ordering greater;
 
   // Conversions
-  constexpr operator weak_equality() const noexcept {  // NOLINT
-    return value_ == 0 ? weak_equality::equivalent
-                       : weak_equality::nonequivalent;
-  }
-  constexpr operator strong_equality() const noexcept {  // NOLINT
-    return value_ == 0 ? strong_equality::equal : strong_equality::nonequal;
-  }
   constexpr operator partial_ordering() const noexcept {  // NOLINT
     return value_ == 0 ? partial_ordering::equivalent
                        : (value_ < 0 ? partial_ordering::less
@@ -432,68 +309,76 @@ class strong_ordering
                : (value_ < 0 ? weak_ordering::less : weak_ordering::greater);
   }
   // Comparisons
-  friend constexpr bool operator==(
-      strong_ordering v, compare_internal::OnlyLiteralZero<>) noexcept {
+  friend constexpr bool operator==(strong_ordering v,
+                                   compare_internal::OnlyLiteralZero) noexcept {
     return v.value_ == 0;
   }
-  friend constexpr bool operator!=(
-      strong_ordering v, compare_internal::OnlyLiteralZero<>) noexcept {
+  friend constexpr bool operator!=(strong_ordering v,
+                                   compare_internal::OnlyLiteralZero) noexcept {
     return v.value_ != 0;
   }
-  friend constexpr bool operator<(
-      strong_ordering v, compare_internal::OnlyLiteralZero<>) noexcept {
+  friend constexpr bool operator<(strong_ordering v,
+                                  compare_internal::OnlyLiteralZero) noexcept {
     return v.value_ < 0;
   }
-  friend constexpr bool operator<=(
-      strong_ordering v, compare_internal::OnlyLiteralZero<>) noexcept {
+  friend constexpr bool operator<=(strong_ordering v,
+                                   compare_internal::OnlyLiteralZero) noexcept {
     return v.value_ <= 0;
   }
-  friend constexpr bool operator>(
-      strong_ordering v, compare_internal::OnlyLiteralZero<>) noexcept {
+  friend constexpr bool operator>(strong_ordering v,
+                                  compare_internal::OnlyLiteralZero) noexcept {
     return v.value_ > 0;
   }
-  friend constexpr bool operator>=(
-      strong_ordering v, compare_internal::OnlyLiteralZero<>) noexcept {
+  friend constexpr bool operator>=(strong_ordering v,
+                                   compare_internal::OnlyLiteralZero) noexcept {
     return v.value_ >= 0;
   }
-  friend constexpr bool operator==(compare_internal::OnlyLiteralZero<>,
+  friend constexpr bool operator==(compare_internal::OnlyLiteralZero,
                                    strong_ordering v) noexcept {
     return 0 == v.value_;
   }
-  friend constexpr bool operator!=(compare_internal::OnlyLiteralZero<>,
+  friend constexpr bool operator!=(compare_internal::OnlyLiteralZero,
                                    strong_ordering v) noexcept {
     return 0 != v.value_;
   }
-  friend constexpr bool operator<(compare_internal::OnlyLiteralZero<>,
+  friend constexpr bool operator<(compare_internal::OnlyLiteralZero,
                                   strong_ordering v) noexcept {
     return 0 < v.value_;
   }
-  friend constexpr bool operator<=(compare_internal::OnlyLiteralZero<>,
+  friend constexpr bool operator<=(compare_internal::OnlyLiteralZero,
                                    strong_ordering v) noexcept {
     return 0 <= v.value_;
   }
-  friend constexpr bool operator>(compare_internal::OnlyLiteralZero<>,
+  friend constexpr bool operator>(compare_internal::OnlyLiteralZero,
                                   strong_ordering v) noexcept {
     return 0 > v.value_;
   }
-  friend constexpr bool operator>=(compare_internal::OnlyLiteralZero<>,
+  friend constexpr bool operator>=(compare_internal::OnlyLiteralZero,
                                    strong_ordering v) noexcept {
     return 0 >= v.value_;
   }
+  friend constexpr bool operator==(strong_ordering v1,
+                                   strong_ordering v2) noexcept {
+    return v1.value_ == v2.value_;
+  }
+  friend constexpr bool operator!=(strong_ordering v1,
+                                   strong_ordering v2) noexcept {
+    return v1.value_ != v2.value_;
+  }
 
  private:
   compare_internal::value_type value_;
 };
-ABSL_COMPARE_INLINE_INIT(strong_ordering, less, compare_internal::ord::less);
-ABSL_COMPARE_INLINE_INIT(strong_ordering, equal, compare_internal::eq::equal);
-ABSL_COMPARE_INLINE_INIT(strong_ordering, equivalent,
-                         compare_internal::eq::equivalent);
-ABSL_COMPARE_INLINE_INIT(strong_ordering, greater,
-                         compare_internal::ord::greater);
+inline constexpr strong_ordering strong_ordering::less(
+    compare_internal::ord::less);
+inline constexpr strong_ordering strong_ordering::equal(
+    compare_internal::eq::equal);
+inline constexpr strong_ordering strong_ordering::equivalent(
+    compare_internal::eq::equivalent);
+inline constexpr strong_ordering strong_ordering::greater(
+    compare_internal::ord::greater);
 
-#undef ABSL_COMPARE_INLINE_BASECLASS_DECL
-#undef ABSL_COMPARE_INLINE_SUBCLASS_DECL
-#undef ABSL_COMPARE_INLINE_INIT
+#endif  // ABSL_USES_STD_ORDERING
 
 namespace compare_internal {
 // We also provide these comparator adapter functions for internal absl use.
@@ -501,28 +386,29 @@ namespace compare_internal {
 // Helper functions to do a boolean comparison of two keys given a boolean
 // or three-way comparator.
 // SFINAE prevents implicit conversions to bool (such as from int).
-template <typename Bool,
-          absl::enable_if_t<std::is_same<bool, Bool>::value, int> = 0>
-constexpr bool compare_result_as_less_than(const Bool r) { return r; }
+template <typename BoolT,
+          std::enable_if_t<std::is_same_v<bool, BoolT>, int> = 0>
+constexpr bool compare_result_as_less_than(const BoolT r) {
+  return r;
+}
 constexpr bool compare_result_as_less_than(const absl::weak_ordering r) {
   return r < 0;
 }
 
 template <typename Compare, typename K, typename LK>
-constexpr bool do_less_than_comparison(const Compare &compare, const K &x,
-                                       const LK &y) {
+constexpr bool do_less_than_comparison(const Compare& compare, const K& x,
+                                       const LK& y) {
   return compare_result_as_less_than(compare(x, y));
 }
 
 // Helper functions to do a three-way comparison of two keys given a boolean or
 // three-way comparator.
 // SFINAE prevents implicit conversions to int (such as from bool).
-template <typename Int,
-          absl::enable_if_t<std::is_same<int, Int>::value, int> = 0>
+template <typename Int, std::enable_if_t<std::is_same_v<int, Int>, int> = 0>
 constexpr absl::weak_ordering compare_result_as_ordering(const Int c) {
-  return c < 0 ? absl::weak_ordering::less
-               : c == 0 ? absl::weak_ordering::equivalent
-                        : absl::weak_ordering::greater;
+  return c < 0    ? absl::weak_ordering::less
+         : c == 0 ? absl::weak_ordering::equivalent
+                  : absl::weak_ordering::greater;
 }
 constexpr absl::weak_ordering compare_result_as_ordering(
     const absl::weak_ordering c) {
@@ -531,26 +417,29 @@ constexpr absl::weak_ordering compare_result_as_ordering(
 
 template <
     typename Compare, typename K, typename LK,
-    absl::enable_if_t<!std::is_same<bool, absl::result_of_t<Compare(
-                                              const K &, const LK &)>>::value,
-                      int> = 0>
-constexpr absl::weak_ordering do_three_way_comparison(const Compare &compare,
-                                                      const K &x, const LK &y) {
+    std::enable_if_t<
+        !std::is_same_v<bool,
+                        std::invoke_result_t<Compare, const K&, const LK&>>,
+        int> = 0>
+constexpr absl::weak_ordering do_three_way_comparison(const Compare& compare,
+                                                      const K& x, const LK& y) {
   return compare_result_as_ordering(compare(x, y));
 }
 template <
     typename Compare, typename K, typename LK,
-    absl::enable_if_t<std::is_same<bool, absl::result_of_t<Compare(
-                                             const K &, const LK &)>>::value,
-                      int> = 0>
-constexpr absl::weak_ordering do_three_way_comparison(const Compare &compare,
-                                                      const K &x, const LK &y) {
-  return compare(x, y) ? absl::weak_ordering::less
-                       : compare(y, x) ? absl::weak_ordering::greater
-                                       : absl::weak_ordering::equivalent;
+    std::enable_if_t<
+        std::is_same_v<bool,
+                       std::invoke_result_t<Compare, const K&, const LK&>>,
+        int> = 0>
+constexpr absl::weak_ordering do_three_way_comparison(const Compare& compare,
+                                                      const K& x, const LK& y) {
+  return compare(x, y)   ? absl::weak_ordering::less
+         : compare(y, x) ? absl::weak_ordering::greater
+                         : absl::weak_ordering::equivalent;
 }
 
 }  // namespace compare_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_TYPES_COMPARE_H_
diff --git a/absl/types/compare_test.cc b/absl/types/compare_test.cc
index ee396fc54d7..ae73eca50a3 100644
--- a/absl/types/compare_test.cc
+++ b/absl/types/compare_test.cc
@@ -18,6 +18,7 @@
 #include "absl/base/casts.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace {
 
 // This is necessary to avoid a bunch of lint warnings suggesting that we use
@@ -25,24 +26,6 @@ namespace {
 // to an int, which can't be converted to the unspecified zero type.
 bool Identity(bool b) { return b; }
 
-TEST(Compare, WeakEquality) {
-  EXPECT_TRUE(Identity(weak_equality::equivalent == 0));
-  EXPECT_TRUE(Identity(0 == weak_equality::equivalent));
-  EXPECT_TRUE(Identity(weak_equality::nonequivalent != 0));
-  EXPECT_TRUE(Identity(0 != weak_equality::nonequivalent));
-}
-
-TEST(Compare, StrongEquality) {
-  EXPECT_TRUE(Identity(strong_equality::equal == 0));
-  EXPECT_TRUE(Identity(0 == strong_equality::equal));
-  EXPECT_TRUE(Identity(strong_equality::nonequal != 0));
-  EXPECT_TRUE(Identity(0 != strong_equality::nonequal));
-  EXPECT_TRUE(Identity(strong_equality::equivalent == 0));
-  EXPECT_TRUE(Identity(0 == strong_equality::equivalent));
-  EXPECT_TRUE(Identity(strong_equality::nonequivalent != 0));
-  EXPECT_TRUE(Identity(0 != strong_equality::nonequivalent));
-}
-
 TEST(Compare, PartialOrdering) {
   EXPECT_TRUE(Identity(partial_ordering::less < 0));
   EXPECT_TRUE(Identity(0 > partial_ordering::less));
@@ -64,6 +47,16 @@ TEST(Compare, PartialOrdering) {
   EXPECT_FALSE(Identity(0 > partial_ordering::unordered));
   EXPECT_FALSE(Identity(partial_ordering::unordered >= 0));
   EXPECT_FALSE(Identity(0 >= partial_ordering::unordered));
+  const partial_ordering values[] = {
+      partial_ordering::less, partial_ordering::equivalent,
+      partial_ordering::greater, partial_ordering::unordered};
+  for (const auto& lhs : values) {
+    for (const auto& rhs : values) {
+      const bool are_equal = &lhs == &rhs;
+      EXPECT_EQ(lhs == rhs, are_equal);
+      EXPECT_EQ(lhs != rhs, !are_equal);
+    }
+  }
 }
 
 TEST(Compare, WeakOrdering) {
@@ -77,6 +70,15 @@ TEST(Compare, WeakOrdering) {
   EXPECT_TRUE(Identity(0 < weak_ordering::greater));
   EXPECT_TRUE(Identity(weak_ordering::greater >= 0));
   EXPECT_TRUE(Identity(0 <= weak_ordering::greater));
+  const weak_ordering values[] = {
+      weak_ordering::less, weak_ordering::equivalent, weak_ordering::greater};
+  for (const auto& lhs : values) {
+    for (const auto& rhs : values) {
+      const bool are_equal = &lhs == &rhs;
+      EXPECT_EQ(lhs == rhs, are_equal);
+      EXPECT_EQ(lhs != rhs, !are_equal);
+    }
+  }
 }
 
 TEST(Compare, StrongOrdering) {
@@ -92,33 +94,19 @@ TEST(Compare, StrongOrdering) {
   EXPECT_TRUE(Identity(0 < strong_ordering::greater));
   EXPECT_TRUE(Identity(strong_ordering::greater >= 0));
   EXPECT_TRUE(Identity(0 <= strong_ordering::greater));
+  const strong_ordering values[] = {
+      strong_ordering::less, strong_ordering::equal, strong_ordering::greater};
+  for (const auto& lhs : values) {
+    for (const auto& rhs : values) {
+      const bool are_equal = &lhs == &rhs;
+      EXPECT_EQ(lhs == rhs, are_equal);
+      EXPECT_EQ(lhs != rhs, !are_equal);
+    }
+  }
+  EXPECT_TRUE(Identity(strong_ordering::equivalent == strong_ordering::equal));
 }
 
 TEST(Compare, Conversions) {
-  EXPECT_TRUE(
-      Identity(implicit_cast<weak_equality>(strong_equality::equal) == 0));
-  EXPECT_TRUE(
-      Identity(implicit_cast<weak_equality>(strong_equality::nonequal) != 0));
-  EXPECT_TRUE(
-      Identity(implicit_cast<weak_equality>(strong_equality::equivalent) == 0));
-  EXPECT_TRUE(Identity(
-      implicit_cast<weak_equality>(strong_equality::nonequivalent) != 0));
-
-  EXPECT_TRUE(
-      Identity(implicit_cast<weak_equality>(partial_ordering::less) != 0));
-  EXPECT_TRUE(Identity(
-      implicit_cast<weak_equality>(partial_ordering::equivalent) == 0));
-  EXPECT_TRUE(
-      Identity(implicit_cast<weak_equality>(partial_ordering::greater) != 0));
-  EXPECT_TRUE(
-      Identity(implicit_cast<weak_equality>(partial_ordering::unordered) != 0));
-
-  EXPECT_TRUE(implicit_cast<weak_equality>(weak_ordering::less) != 0);
-  EXPECT_TRUE(
-      Identity(implicit_cast<weak_equality>(weak_ordering::equivalent) == 0));
-  EXPECT_TRUE(
-      Identity(implicit_cast<weak_equality>(weak_ordering::greater) != 0));
-
   EXPECT_TRUE(
       Identity(implicit_cast<partial_ordering>(weak_ordering::less) != 0));
   EXPECT_TRUE(
@@ -134,24 +122,6 @@ TEST(Compare, Conversions) {
   EXPECT_TRUE(
       Identity(implicit_cast<partial_ordering>(weak_ordering::greater) >= 0));
 
-  EXPECT_TRUE(
-      Identity(implicit_cast<weak_equality>(strong_ordering::less) != 0));
-  EXPECT_TRUE(
-      Identity(implicit_cast<weak_equality>(strong_ordering::equal) == 0));
-  EXPECT_TRUE(
-      Identity(implicit_cast<weak_equality>(strong_ordering::equivalent) == 0));
-  EXPECT_TRUE(
-      Identity(implicit_cast<weak_equality>(strong_ordering::greater) != 0));
-
-  EXPECT_TRUE(
-      Identity(implicit_cast<strong_equality>(strong_ordering::less) != 0));
-  EXPECT_TRUE(
-      Identity(implicit_cast<strong_equality>(strong_ordering::equal) == 0));
-  EXPECT_TRUE(Identity(
-      implicit_cast<strong_equality>(strong_ordering::equivalent) == 0));
-  EXPECT_TRUE(
-      Identity(implicit_cast<strong_equality>(strong_ordering::greater) != 0));
-
   EXPECT_TRUE(
       Identity(implicit_cast<partial_ordering>(strong_ordering::less) != 0));
   EXPECT_TRUE(
@@ -189,10 +159,10 @@ TEST(Compare, Conversions) {
 
 struct WeakOrderingLess {
   template <typename T>
-  absl::weak_ordering operator()(const T &a, const T &b) const {
-    return a < b ? absl::weak_ordering::less
-                 : a == b ? absl::weak_ordering::equivalent
-                          : absl::weak_ordering::greater;
+  absl::weak_ordering operator()(const T& a, const T& b) const {
+    return a < b    ? absl::weak_ordering::less
+           : a == b ? absl::weak_ordering::equivalent
+                    : absl::weak_ordering::greater;
   }
 };
 
@@ -223,10 +193,10 @@ TEST(DoLessThanComparison, SanityTest) {
 }
 
 TEST(CompareResultAsOrdering, SanityTest) {
-  EXPECT_TRUE(Identity(
-      absl::compare_internal::compare_result_as_ordering(-1) < 0));
-  EXPECT_FALSE(Identity(
-      absl::compare_internal::compare_result_as_ordering(-1) == 0));
+  EXPECT_TRUE(
+      Identity(absl::compare_internal::compare_result_as_ordering(-1) < 0));
+  EXPECT_FALSE(
+      Identity(absl::compare_internal::compare_result_as_ordering(-1) == 0));
   EXPECT_FALSE(
       Identity(absl::compare_internal::compare_result_as_ordering(-1) > 0));
   EXPECT_TRUE(Identity(absl::compare_internal::compare_result_as_ordering(
@@ -236,31 +206,31 @@ TEST(CompareResultAsOrdering, SanityTest) {
   EXPECT_FALSE(Identity(absl::compare_internal::compare_result_as_ordering(
                             weak_ordering::less) > 0));
 
-  EXPECT_FALSE(Identity(
-      absl::compare_internal::compare_result_as_ordering(0) < 0));
-  EXPECT_TRUE(Identity(
-      absl::compare_internal::compare_result_as_ordering(0) == 0));
-  EXPECT_FALSE(Identity(
-      absl::compare_internal::compare_result_as_ordering(0) > 0));
+  EXPECT_FALSE(
+      Identity(absl::compare_internal::compare_result_as_ordering(0) < 0));
+  EXPECT_TRUE(
+      Identity(absl::compare_internal::compare_result_as_ordering(0) == 0));
+  EXPECT_FALSE(
+      Identity(absl::compare_internal::compare_result_as_ordering(0) > 0));
   EXPECT_FALSE(Identity(absl::compare_internal::compare_result_as_ordering(
-                           weak_ordering::equivalent) < 0));
+                            weak_ordering::equivalent) < 0));
   EXPECT_TRUE(Identity(absl::compare_internal::compare_result_as_ordering(
-                            weak_ordering::equivalent) == 0));
+                           weak_ordering::equivalent) == 0));
   EXPECT_FALSE(Identity(absl::compare_internal::compare_result_as_ordering(
                             weak_ordering::equivalent) > 0));
 
-  EXPECT_FALSE(Identity(
-      absl::compare_internal::compare_result_as_ordering(1) < 0));
-  EXPECT_FALSE(Identity(
-      absl::compare_internal::compare_result_as_ordering(1) == 0));
-  EXPECT_TRUE(Identity(
-      absl::compare_internal::compare_result_as_ordering(1) > 0));
+  EXPECT_FALSE(
+      Identity(absl::compare_internal::compare_result_as_ordering(1) < 0));
+  EXPECT_FALSE(
+      Identity(absl::compare_internal::compare_result_as_ordering(1) == 0));
+  EXPECT_TRUE(
+      Identity(absl::compare_internal::compare_result_as_ordering(1) > 0));
   EXPECT_FALSE(Identity(absl::compare_internal::compare_result_as_ordering(
-                           weak_ordering::greater) < 0));
+                            weak_ordering::greater) < 0));
   EXPECT_FALSE(Identity(absl::compare_internal::compare_result_as_ordering(
                             weak_ordering::greater) == 0));
   EXPECT_TRUE(Identity(absl::compare_internal::compare_result_as_ordering(
-                            weak_ordering::greater) > 0));
+                           weak_ordering::greater) > 0));
 }
 
 TEST(DoThreeWayComparison, SanityTest) {
@@ -307,31 +277,22 @@ TEST(DoThreeWayComparison, SanityTest) {
       absl::compare_internal::do_three_way_comparison(weak, 10, 5) > 0));
 }
 
-#ifdef __cpp_inline_variables
 TEST(Compare, StaticAsserts) {
-  static_assert(weak_equality::equivalent == 0, "");
-  static_assert(weak_equality::nonequivalent != 0, "");
-
-  static_assert(strong_equality::equal == 0, "");
-  static_assert(strong_equality::nonequal != 0, "");
-  static_assert(strong_equality::equivalent == 0, "");
-  static_assert(strong_equality::nonequivalent != 0, "");
-
-  static_assert(partial_ordering::less < 0, "");
-  static_assert(partial_ordering::equivalent == 0, "");
-  static_assert(partial_ordering::greater > 0, "");
-  static_assert(partial_ordering::unordered != 0, "");
-
-  static_assert(weak_ordering::less < 0, "");
-  static_assert(weak_ordering::equivalent == 0, "");
-  static_assert(weak_ordering::greater > 0, "");
-
-  static_assert(strong_ordering::less < 0, "");
-  static_assert(strong_ordering::equal == 0, "");
-  static_assert(strong_ordering::equivalent == 0, "");
-  static_assert(strong_ordering::greater > 0, "");
+  static_assert(partial_ordering::less < 0);
+  static_assert(partial_ordering::equivalent == 0);
+  static_assert(partial_ordering::greater > 0);
+  static_assert(partial_ordering::unordered != 0);
+
+  static_assert(weak_ordering::less < 0);
+  static_assert(weak_ordering::equivalent == 0);
+  static_assert(weak_ordering::greater > 0);
+
+  static_assert(strong_ordering::less < 0);
+  static_assert(strong_ordering::equal == 0);
+  static_assert(strong_ordering::equivalent == 0);
+  static_assert(strong_ordering::greater > 0);
 }
-#endif  // __cpp_inline_variables
 
 }  // namespace
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/types/internal/any_span.h b/absl/types/internal/any_span.h
new file mode 100644
index 00000000000..00ee7420134
--- /dev/null
+++ b/absl/types/internal/any_span.h
@@ -0,0 +1,476 @@
+// Copyright 2026 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: internal/any_span.h
+// -----------------------------------------------------------------------------
+//
+// Helper types and functions used by AnySpan. This file should only be
+// included by any_span.h.
+#ifndef ABSL_TYPES_INTERNAL_ANY_SPAN_H_
+#define ABSL_TYPES_INTERNAL_ANY_SPAN_H_
+
+#include <algorithm>
+#include <cstddef>
+#include <functional>
+#include <type_traits>
+
+#include "absl/base/config.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/base/optimization.h"
+#include "absl/meta/type_traits.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+template <typename T>
+class AnySpan;
+
+namespace any_span_transform {
+struct IdentityT;
+struct DerefT;
+}  // namespace any_span_transform
+
+namespace any_span_internal {
+
+//
+// IsAnySpan inherits from true_type if T is an instance of AnySpan.
+//
+
+template <typename T>
+struct IsAnySpan : public std::false_type {};
+
+template <typename T>
+struct IsAnySpan<AnySpan<T>> : public std::true_type {};
+
+// A type suitable for storing any function pointer. The standard allows
+// function pointers to round-trip through other function pointers, but void* is
+// implementation defined.
+using FunPtr = void (*)();
+
+// Whether a transform will be copied and therefore does not have to outlive
+// the AnySpan.
+template <typename Transform>
+constexpr bool kIsTransformCopied =
+    std::is_function_v<std::remove_pointer_t<Transform>>;
+
+// Type to pass as extra argument to TransformPtr to ensure that our
+// assumption about Transform copyability is valid.
+template <typename Transform>
+using IsTransformCopied = std::bool_constant<kIsTransformCopied<Transform>>;
+
+// A pointer to the transform function or functor that should be applied to
+// elements of the container.
+class TransformPtr {
+ public:
+  TransformPtr() = default;
+
+  // Construct from ptr to function.
+  template <typename R, typename... Args, typename CopiedTransform>
+  explicit TransformPtr(R (*f)(Args...),
+                        CopiedTransform copied_transform [[maybe_unused]])
+      : fun_ptr_(reinterpret_cast<FunPtr>(f)) {
+    static_assert(CopiedTransform::value);
+  }
+
+  // Construct from any other invokable object.
+  template <typename T, typename CopiedTransform>
+  explicit TransformPtr(const T& t,
+                        CopiedTransform copied_transform [[maybe_unused]])
+      : ptr_(&t) {
+    static_assert(!CopiedTransform::value);
+  }
+
+  // Casts the pointer to the given type.
+  template <typename Transform>
+  auto get() const {
+    if constexpr (std::is_function_v<Transform>) {
+      return reinterpret_cast<const Transform*>(fun_ptr_);
+    } else if constexpr (std::is_function_v<std::remove_pointer_t<Transform>>) {
+      return reinterpret_cast<const Transform>(fun_ptr_);
+    } else {
+      return static_cast<const Transform*>(ptr_);
+    }
+  }
+
+ private:
+  union {
+    const void* ptr_ = nullptr;
+    FunPtr fun_ptr_;
+  };
+};
+
+// A void* pointer to the container and transform functor. These are always cast
+// to the correct type since we create this object and the GetterFunction at the
+// same time (from one of the Make*Getter functions below).
+struct TransformedContainer {
+  // The container or array.
+  void* ptr;
+
+  // The transform being applied to the container.
+  TransformPtr transform;
+};
+
+// Applies transform (at the correct type) to the argument and returns the
+// result. Does some validity checking to make sure the result is not a
+// temporary (proxy containers are not allowed).
+template <typename T, typename Transform, typename U>
+T& ApplyTransform(TransformPtr transform, U& u) {  // NOLINT(runtime/references)
+  const auto t = transform.get<Transform>();
+  ABSL_RAW_DCHECK(t != nullptr, "pointer cannot be null");
+
+  // If compilation fails here due to dropping a const qualifier, it usually
+  // means you tried to wrap a const container with a non-const AnySpan.
+  //
+  // If compilation fails here due to a reference to a temporary, it usually
+  // means that the container or transform cannot be converted to a reference to
+  // T. AnySpan requires that a valid instance of T exists, conversion to a
+  // value type (such as string -> StringPiece or int -> float) is not
+  // supported.
+  //
+  // If compilation fails here due to taking the address of a temporary object,
+  // it means that the transform is returning a temporary. This is disallowed.
+  // Transforms must return a reference to T, or a reference to an object that
+  // can be safely converted to a reference to T (such as a reference_wrapper or
+  // a class that inherits from T).
+  return *&std::invoke(*t, u);  // Failed compilation? See above.
+}
+
+// The return type of GetterFunction<T>. GetterFunctions return non-const
+// references to support mutable -> const conversion of spans without additional
+// indirection.
+template <typename T>
+using GetterFunctionResult = std::remove_const_t<T>&;
+
+// A type of function pointer that can return an element of a
+// TransformedContainer.  This is used so that we can type-erase with a simple
+// function pointer instead of a vtable. Doing things this way has less pointer
+// indirection.
+template <typename T>
+using GetterFunction = GetterFunctionResult<T> (*)(const TransformedContainer&,
+                                                   std::size_t);
+
+// A GetterFunction that works on arrays.
+template <typename T, typename Element, typename Transform>
+GetterFunctionResult<T> GetFromArray(const TransformedContainer& container,
+                                     std::size_t i) {
+  ABSL_RAW_DCHECK(container.ptr != nullptr, "cannot dereference null pointer");
+  auto* array = static_cast<Element*>(container.ptr);
+  return const_cast<GetterFunctionResult<T>>(
+      ApplyTransform<T, Transform>(container.transform, array[i]));
+}
+
+// A GetterFunction that works on containers.
+template <typename T, typename Container, typename Transform>
+GetterFunctionResult<T> GetFromContainer(const TransformedContainer& container,
+                                         std::size_t i) {
+  ABSL_RAW_DCHECK(container.ptr != nullptr, "cannot dereference null pointer");
+  Container& c = *static_cast<Container*>(container.ptr);
+  return const_cast<GetterFunctionResult<T>>(
+      ApplyTransform<T, Transform>(container.transform, c[i]));
+}
+
+// A GetterFunction that crashes, indicating an invalid AnySpan has been
+// accessed..
+template <typename T>
+GetterFunctionResult<T> GetFromUninitialized(const TransformedContainer&,
+                                             std::size_t) {
+  ABSL_RAW_LOG(FATAL, "Uninitialized AnySpan access.");
+}
+
+//
+// ArrayTag and PtrArrayTag are GetterFunctions that are never called. They are
+// used as a tag so Getter::Get can access flat arrays without a function
+// pointer.
+//
+
+template <typename T>
+GetterFunctionResult<T> ArrayTag(const TransformedContainer&, std::size_t) {
+  ABSL_RAW_LOG(FATAL, "ArrayTag should never be called.");
+}
+
+template <typename T>
+GetterFunctionResult<T> PtrArrayTag(const TransformedContainer&, std::size_t) {
+  ABSL_RAW_LOG(FATAL, "PtrArrayTag should never be called.");
+}
+
+//
+// HasSize<Container> inherets from true_type if Container has a size() member.
+// false_type otherwise.
+//
+
+template <typename, typename = void>
+struct HasSize : public std::false_type {};
+
+template <class Container>
+struct HasSize<Container,
+               std::void_t<decltype(std::declval<Container&>().size())>>
+    : public std::true_type {};
+
+//
+// TypeOfData<Container>::type is the return type of data() if Container has a
+// data() member. It is NoData otherwise.
+//
+
+struct NoData {};
+
+template <typename, typename = void>
+struct TypeOfData {
+  using type = NoData;
+};
+
+template <class Container>
+struct TypeOfData<Container,
+                  std::void_t<decltype(std::declval<Container&>().data())>> {
+  using type = decltype(std::declval<Container&>().data());
+};
+
+// Element type of container based on operator[].
+template <class Container>
+using ElementType =
+    std::remove_reference_t<decltype(std::declval<Container&>()[0])>;
+
+// Element type of container when elements are dereferenced.
+template <class Container>
+using DerefElementType =
+    std::remove_reference_t<decltype(*std::declval<ElementType<Container>>())>;
+
+// DataIsValid is true_type if Container has a data() member that returns a
+// pointer to the type returned by operator[], false_type if there is no data()
+// member or if the types disagree.
+template <class Container>
+using DataIsValid =
+    std::is_same<ElementType<Container>*, typename TypeOfData<Container>::type>;
+
+// Used to access elements of a container or array.
+template <typename T>
+struct Getter {
+  Getter() {}
+
+  // Handle mutable -> const conversion.
+  template <typename LazyT = T,
+            typename = std::enable_if_t<std::is_const_v<LazyT>>>
+  explicit Getter(const Getter<std::remove_const_t<T>>& other) {
+    using MutableT = std::remove_const_t<T>;
+    if (other.fun == &ArrayTag<MutableT>) {
+      ABSL_RAW_DCHECK(other.offset == 0u, "offset must be zero");
+      fun = &ArrayTag<T>;
+      array = other.array;
+      offset = 0;
+    } else if (other.fun == &PtrArrayTag<MutableT>) {
+      ABSL_RAW_DCHECK(other.offset == 0u, "offset must be zero");
+      fun = &PtrArrayTag<T>;
+      ptr_array = other.ptr_array;
+      offset = 0;
+    } else {
+      fun = other.fun;
+      container = other.container;
+      offset = other.offset;
+    }
+  }
+
+  // Returns the element at the given index.
+  T& Get(std::size_t index) const {
+    ABSL_RAW_DCHECK(fun != nullptr, "pointer cannot be null");
+    if (ABSL_PREDICT_TRUE(fun == &ArrayTag<T>)) {
+      ABSL_RAW_DCHECK(array != nullptr, "pointer cannot be null");
+      return array[index];
+    }
+    if (fun == &PtrArrayTag<T>) {
+      ABSL_RAW_DCHECK(ptr_array != nullptr, "pointer cannot be null");
+      return *ptr_array[index];
+    }
+    return fun(container, index + offset);
+  }
+
+  // Returns a Getter offset into this one by pos.
+  Getter Offset(std::size_t pos) const {
+    // Special case when offset is zero. This safely handles empty spans and
+    // empty containers where data() can be null.
+    if (pos == 0) {
+      return *this;
+    }
+    ABSL_RAW_DCHECK(fun != nullptr, "pointer cannot be null");
+    Getter result;
+    result.fun = fun;
+    if (fun == &ArrayTag<T>) {
+      ABSL_RAW_DCHECK(array != nullptr, "pointer cannot be null");
+      ABSL_RAW_DCHECK(offset == 0u, "offset must be zero");
+      result.array = array + pos;
+      result.offset = 0;
+    } else if (fun == &PtrArrayTag<T>) {
+      ABSL_RAW_DCHECK(ptr_array != nullptr, "pointer cannot be null");
+      ABSL_RAW_DCHECK(offset == 0u, "offset must be zero");
+      result.ptr_array = ptr_array + pos;
+      result.offset = 0;
+    } else {
+      ABSL_RAW_DCHECK(container.ptr != nullptr, "pointer cannot be null");
+      result.container = container;
+      result.offset = offset + pos;
+    }
+    return result;
+  }
+
+  // A pointer to a function (or tag function) that specifies how to get an
+  // element from the array or container.
+  GetterFunction<T> fun = &GetFromUninitialized<T>;
+
+  union {
+    T* array;                        // Active if fun == ArrayTag<T>.
+    T* const* ptr_array;             // Active if fun == PtrArrayTag<T>.
+    TransformedContainer container;  // Active for all other fun.
+  };
+
+  // Offset into container. Always 0 for array or ptr_array.
+  std::size_t offset = 0;
+};
+
+//
+// MakeArrayGetter returns a Getter for an array.
+//
+// MakeArrayGetterImpl is specialised to use ArrayTag<T> when Element == T and
+// Transform == IdentityT, and to use PtrArrayTag<T> when Element == T* and
+// Transform == DerefT.
+//
+
+template <typename SpanElement, typename ArrayElement, typename Transform>
+struct MakeArrayGetterImpl {
+  template <typename U>
+  static Getter<U> Make(ArrayElement* array, const Transform& transform) {
+    Getter<U> result;
+    result.fun = &GetFromArray<U, ArrayElement, Transform>;
+    result.container.ptr = const_cast<void*>(static_cast<const void*>(array));
+    result.container.transform =
+        TransformPtr(transform, IsTransformCopied<Transform>{});
+    return result;
+  }
+};
+
+// When the span and the array are the same type.
+template <typename T>
+struct MakeArrayGetterImpl<T, T, any_span_transform::IdentityT> {
+  template <typename U>
+  static Getter<U> Make(T* array, const any_span_transform::IdentityT&) {
+    Getter<U> result;
+    result.fun = &ArrayTag<U>;
+    result.array = array;
+    return result;
+  }
+};
+
+// If we are dereferencing an array of mutable elements (T*), it is safe to add
+// constness (const T*).
+template <typename T>
+struct MakeArrayGetterImpl<const T, T, any_span_transform::IdentityT>
+    : public MakeArrayGetterImpl<const T, const T,
+                                 any_span_transform::IdentityT> {};
+
+// When the array is of pointers to elements of the same type as the span.
+template <typename T>
+struct MakeArrayGetterImpl<T, T*, any_span_transform::DerefT> {
+  template <typename U>
+  static Getter<U> Make(T* const* ptr_array,
+                        const any_span_transform::DerefT&) {
+    Getter<U> result;
+    result.fun = &PtrArrayTag<U>;
+    result.ptr_array = ptr_array;
+    return result;
+  }
+};
+
+// If we are dereferencing an array that is mutable along any extent, it is safe
+// to add constness.
+template <typename T>
+struct MakeArrayGetterImpl<const T, T*, any_span_transform::DerefT>
+    : public MakeArrayGetterImpl<const T, const T*,
+                                 any_span_transform::DerefT> {};
+
+template <typename T>
+struct MakeArrayGetterImpl<const T, T* const, any_span_transform::DerefT>
+    : public MakeArrayGetterImpl<const T, const T*,
+                                 any_span_transform::DerefT> {};
+
+template <typename T>
+struct MakeArrayGetterImpl<const T, const T* const, any_span_transform::DerefT>
+    : public MakeArrayGetterImpl<const T, const T*,
+                                 any_span_transform::DerefT> {};
+
+template <typename T, typename Element, typename Transform>
+Getter<T> MakeArrayGetter(Element* array, const Transform& transform) {
+  return MakeArrayGetterImpl<T, Element, Transform>::template Make<T>(
+      array, transform);
+}
+
+//
+// MakeContainerGetter returns a Getter for a given container. It will pass
+// container.data() to MakeArrayGetter if possible, which allows element access
+// to be inline.
+//
+// The first argument to MakeArrayGetterImpl is expected to be of type
+// DataIsValid<container>.
+//
+
+template <typename T, typename Container, typename Transform>
+Getter<T> MakeContainerGetterImpl(
+    std::true_type /* DataIsValid<Container> */,
+    Container& container,  // NOLINT(runtime/references)
+    const Transform& transform) {
+  return MakeArrayGetter<T, ElementType<Container>, Transform>(container.data(),
+                                                               transform);
+}
+
+template <typename T, typename Container, typename Transform>
+Getter<T> MakeContainerGetterImpl(
+    std::false_type /* DataIsValid<Container> */,
+    Container& container,  // NOLINT(runtime/references)
+    const Transform& transform) {
+  Getter<T> result;
+  result.fun = &GetFromContainer<T, Container, Transform>;
+  result.container.ptr =
+      const_cast<void*>(static_cast<const void*>(&container));
+  result.container.transform =
+      TransformPtr(transform, IsTransformCopied<Transform>{});
+
+  return result;
+}
+
+template <typename T, typename Container, typename Transform>
+Getter<T> MakeContainerGetter(
+    Container& container,  // NOLINT(runtime/references)
+    const Transform& transform) {
+  static_assert(std::is_reference_v<decltype(container[0])>,
+                "AnySpan only works with containers that return a reference "
+                "(no vector<bool>, or containers that return by value).");
+  return MakeContainerGetterImpl<T>(DataIsValid<Container>(), container,
+                                    transform);
+}
+
+// Used for testing. Returns true if the given AnySpan performs inline element
+// access.
+template <typename T>
+bool IsCheap(AnySpan<T> s) {
+  return s.getter_.fun == &ArrayTag<T> || s.getter_.fun == &PtrArrayTag<T>;
+}
+
+template <typename T>
+bool EqualImpl(AnySpan<T> a, AnySpan<T> b) {
+  static_assert(std::is_const_v<T>);
+  return std::equal(a.begin(), a.end(), b.begin(), b.end());
+}
+
+}  // namespace any_span_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_TYPES_INTERNAL_ANY_SPAN_H_
diff --git a/absl/types/internal/optional.h b/absl/types/internal/optional.h
deleted file mode 100644
index d41ccc75f6f..00000000000
--- a/absl/types/internal/optional.h
+++ /dev/null
@@ -1,394 +0,0 @@
-// Copyright 2017 The Abseil Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-//
-#ifndef ABSL_TYPES_INTERNAL_OPTIONAL_H_
-#define ABSL_TYPES_INTERNAL_OPTIONAL_H_
-
-#include <functional>
-#include <new>
-#include <type_traits>
-#include <utility>
-
-#include "absl/base/internal/inline_variable.h"
-#include "absl/memory/memory.h"
-#include "absl/meta/type_traits.h"
-#include "absl/utility/utility.h"
-
-// ABSL_OPTIONAL_USE_INHERITING_CONSTRUCTORS
-//
-// Inheriting constructors is supported in GCC 4.8+, Clang 3.3+ and MSVC 2015.
-// __cpp_inheriting_constructors is a predefined macro and a recommended way to
-// check for this language feature, but GCC doesn't support it until 5.0 and
-// Clang doesn't support it until 3.6.
-// Also, MSVC 2015 has a bug: it doesn't inherit the constexpr template
-// constructor. For example, the following code won't work on MSVC 2015 Update3:
-// struct Base {
-//   int t;
-//   template <typename T>
-//   constexpr Base(T t_) : t(t_) {}
-// };
-// struct Foo : Base {
-//   using Base::Base;
-// }
-// constexpr Foo foo(0);  // doesn't work on MSVC 2015
-#if defined(__clang__)
-#if __has_feature(cxx_inheriting_constructors)
-#define ABSL_OPTIONAL_USE_INHERITING_CONSTRUCTORS 1
-#endif
-#elif (defined(__GNUC__) &&                                       \
-       (__GNUC__ > 4 || __GNUC__ == 4 && __GNUC_MINOR__ >= 8)) || \
-    (__cpp_inheriting_constructors >= 200802) ||                  \
-    (defined(_MSC_VER) && _MSC_VER >= 1910)
-#define ABSL_OPTIONAL_USE_INHERITING_CONSTRUCTORS 1
-#endif
-
-namespace absl {
-
-// Forward declaration
-template <typename T>
-class optional;
-
-namespace optional_internal {
-
-// This tag type is used as a constructor parameter type for `nullopt_t`.
-struct init_t {
-  explicit init_t() = default;
-};
-
-struct empty_struct {};
-
-// This class stores the data in optional<T>.
-// It is specialized based on whether T is trivially destructible.
-// This is the specialization for non trivially destructible type.
-template <typename T, bool unused = std::is_trivially_destructible<T>::value>
-class optional_data_dtor_base {
-  struct dummy_type {
-    static_assert(sizeof(T) % sizeof(empty_struct) == 0, "");
-    // Use an array to avoid GCC 6 placement-new warning.
-    empty_struct data[sizeof(T) / sizeof(empty_struct)];
-  };
-
- protected:
-  // Whether there is data or not.
-  bool engaged_;
-  // Data storage
-  union {
-    T data_;
-    dummy_type dummy_;
-  };
-
-  void destruct() noexcept {
-    if (engaged_) {
-      data_.~T();
-      engaged_ = false;
-    }
-  }
-
-  // dummy_ must be initialized for constexpr constructor.
-  constexpr optional_data_dtor_base() noexcept : engaged_(false), dummy_{{}} {}
-
-  template <typename... Args>
-  constexpr explicit optional_data_dtor_base(in_place_t, Args&&... args)
-      : engaged_(true), data_(absl::forward<Args>(args)...) {}
-
-  ~optional_data_dtor_base() { destruct(); }
-};
-
-// Specialization for trivially destructible type.
-template <typename T>
-class optional_data_dtor_base<T, true> {
-  struct dummy_type {
-    static_assert(sizeof(T) % sizeof(empty_struct) == 0, "");
-    // Use array to avoid GCC 6 placement-new warning.
-    empty_struct data[sizeof(T) / sizeof(empty_struct)];
-  };
-
- protected:
-  // Whether there is data or not.
-  bool engaged_;
-  // Data storage
-  union {
-    T data_;
-    dummy_type dummy_;
-  };
-  void destruct() noexcept { engaged_ = false; }
-
-  // dummy_ must be initialized for constexpr constructor.
-  constexpr optional_data_dtor_base() noexcept : engaged_(false), dummy_{{}} {}
-
-  template <typename... Args>
-  constexpr explicit optional_data_dtor_base(in_place_t, Args&&... args)
-      : engaged_(true), data_(absl::forward<Args>(args)...) {}
-};
-
-template <typename T>
-class optional_data_base : public optional_data_dtor_base<T> {
- protected:
-  using base = optional_data_dtor_base<T>;
-#ifdef ABSL_OPTIONAL_USE_INHERITING_CONSTRUCTORS
-  using base::base;
-#else
-  optional_data_base() = default;
-
-  template <typename... Args>
-  constexpr explicit optional_data_base(in_place_t t, Args&&... args)
-      : base(t, absl::forward<Args>(args)...) {}
-#endif
-
-  template <typename... Args>
-  void construct(Args&&... args) {
-    // Use dummy_'s address to work around casting cv-qualified T* to void*.
-    ::new (static_cast<void*>(&this->dummy_)) T(std::forward<Args>(args)...);
-    this->engaged_ = true;
-  }
-
-  template <typename U>
-  void assign(U&& u) {
-    if (this->engaged_) {
-      this->data_ = std::forward<U>(u);
-    } else {
-      construct(std::forward<U>(u));
-    }
-  }
-};
-
-// TODO(absl-team): Add another class using
-// std::is_trivially_move_constructible trait when available to match
-// http://cplusplus.github.io/LWG/lwg-defects.html#2900, for types that
-// have trivial move but nontrivial copy.
-// Also, we should be checking is_trivially_copyable here, which is not
-// supported now, so we use is_trivially_* traits instead.
-template <typename T,
-          bool unused = absl::is_trivially_copy_constructible<T>::value&&
-              absl::is_trivially_copy_assignable<typename std::remove_cv<
-                  T>::type>::value&& std::is_trivially_destructible<T>::value>
-class optional_data;
-
-// Trivially copyable types
-template <typename T>
-class optional_data<T, true> : public optional_data_base<T> {
- protected:
-#ifdef ABSL_OPTIONAL_USE_INHERITING_CONSTRUCTORS
-  using optional_data_base<T>::optional_data_base;
-#else
-  optional_data() = default;
-
-  template <typename... Args>
-  constexpr explicit optional_data(in_place_t t, Args&&... args)
-      : optional_data_base<T>(t, absl::forward<Args>(args)...) {}
-#endif
-};
-
-template <typename T>
-class optional_data<T, false> : public optional_data_base<T> {
- protected:
-#ifdef ABSL_OPTIONAL_USE_INHERITING_CONSTRUCTORS
-  using optional_data_base<T>::optional_data_base;
-#else
-  template <typename... Args>
-  constexpr explicit optional_data(in_place_t t, Args&&... args)
-      : optional_data_base<T>(t, absl::forward<Args>(args)...) {}
-#endif
-
-  optional_data() = default;
-
-  optional_data(const optional_data& rhs) : optional_data_base<T>() {
-    if (rhs.engaged_) {
-      this->construct(rhs.data_);
-    }
-  }
-
-  optional_data(optional_data&& rhs) noexcept(
-      absl::default_allocator_is_nothrow::value ||
-      std::is_nothrow_move_constructible<T>::value)
-      : optional_data_base<T>() {
-    if (rhs.engaged_) {
-      this->construct(std::move(rhs.data_));
-    }
-  }
-
-  optional_data& operator=(const optional_data& rhs) {
-    if (rhs.engaged_) {
-      this->assign(rhs.data_);
-    } else {
-      this->destruct();
-    }
-    return *this;
-  }
-
-  optional_data& operator=(optional_data&& rhs) noexcept(
-      std::is_nothrow_move_assignable<T>::value&&
-          std::is_nothrow_move_constructible<T>::value) {
-    if (rhs.engaged_) {
-      this->assign(std::move(rhs.data_));
-    } else {
-      this->destruct();
-    }
-    return *this;
-  }
-};
-
-// Ordered by level of restriction, from low to high.
-// Copyable implies movable.
-enum class copy_traits { copyable = 0, movable = 1, non_movable = 2 };
-
-// Base class for enabling/disabling copy/move constructor.
-template <copy_traits>
-class optional_ctor_base;
-
-template <>
-class optional_ctor_base<copy_traits::copyable> {
- public:
-  constexpr optional_ctor_base() = default;
-  optional_ctor_base(const optional_ctor_base&) = default;
-  optional_ctor_base(optional_ctor_base&&) = default;
-  optional_ctor_base& operator=(const optional_ctor_base&) = default;
-  optional_ctor_base& operator=(optional_ctor_base&&) = default;
-};
-
-template <>
-class optional_ctor_base<copy_traits::movable> {
- public:
-  constexpr optional_ctor_base() = default;
-  optional_ctor_base(const optional_ctor_base&) = delete;
-  optional_ctor_base(optional_ctor_base&&) = default;
-  optional_ctor_base& operator=(const optional_ctor_base&) = default;
-  optional_ctor_base& operator=(optional_ctor_base&&) = default;
-};
-
-template <>
-class optional_ctor_base<copy_traits::non_movable> {
- public:
-  constexpr optional_ctor_base() = default;
-  optional_ctor_base(const optional_ctor_base&) = delete;
-  optional_ctor_base(optional_ctor_base&&) = delete;
-  optional_ctor_base& operator=(const optional_ctor_base&) = default;
-  optional_ctor_base& operator=(optional_ctor_base&&) = default;
-};
-
-// Base class for enabling/disabling copy/move assignment.
-template <copy_traits>
-class optional_assign_base;
-
-template <>
-class optional_assign_base<copy_traits::copyable> {
- public:
-  constexpr optional_assign_base() = default;
-  optional_assign_base(const optional_assign_base&) = default;
-  optional_assign_base(optional_assign_base&&) = default;
-  optional_assign_base& operator=(const optional_assign_base&) = default;
-  optional_assign_base& operator=(optional_assign_base&&) = default;
-};
-
-template <>
-class optional_assign_base<copy_traits::movable> {
- public:
-  constexpr optional_assign_base() = default;
-  optional_assign_base(const optional_assign_base&) = default;
-  optional_assign_base(optional_assign_base&&) = default;
-  optional_assign_base& operator=(const optional_assign_base&) = delete;
-  optional_assign_base& operator=(optional_assign_base&&) = default;
-};
-
-template <>
-class optional_assign_base<copy_traits::non_movable> {
- public:
-  constexpr optional_assign_base() = default;
-  optional_assign_base(const optional_assign_base&) = default;
-  optional_assign_base(optional_assign_base&&) = default;
-  optional_assign_base& operator=(const optional_assign_base&) = delete;
-  optional_assign_base& operator=(optional_assign_base&&) = delete;
-};
-
-template <typename T>
-struct ctor_copy_traits {
-  static constexpr copy_traits traits =
-      std::is_copy_constructible<T>::value
-          ? copy_traits::copyable
-          : std::is_move_constructible<T>::value ? copy_traits::movable
-                                                 : copy_traits::non_movable;
-};
-
-template <typename T>
-struct assign_copy_traits {
-  static constexpr copy_traits traits =
-      absl::is_copy_assignable<T>::value && std::is_copy_constructible<T>::value
-          ? copy_traits::copyable
-          : absl::is_move_assignable<T>::value &&
-                    std::is_move_constructible<T>::value
-                ? copy_traits::movable
-                : copy_traits::non_movable;
-};
-
-// Whether T is constructible or convertible from optional<U>.
-template <typename T, typename U>
-struct is_constructible_convertible_from_optional
-    : std::integral_constant<
-          bool, std::is_constructible<T, optional<U>&>::value ||
-                    std::is_constructible<T, optional<U>&&>::value ||
-                    std::is_constructible<T, const optional<U>&>::value ||
-                    std::is_constructible<T, const optional<U>&&>::value ||
-                    std::is_convertible<optional<U>&, T>::value ||
-                    std::is_convertible<optional<U>&&, T>::value ||
-                    std::is_convertible<const optional<U>&, T>::value ||
-                    std::is_convertible<const optional<U>&&, T>::value> {};
-
-// Whether T is constructible or convertible or assignable from optional<U>.
-template <typename T, typename U>
-struct is_constructible_convertible_assignable_from_optional
-    : std::integral_constant<
-          bool, is_constructible_convertible_from_optional<T, U>::value ||
-                    std::is_assignable<T&, optional<U>&>::value ||
-                    std::is_assignable<T&, optional<U>&&>::value ||
-                    std::is_assignable<T&, const optional<U>&>::value ||
-                    std::is_assignable<T&, const optional<U>&&>::value> {};
-
-// Helper function used by [optional.relops], [optional.comp_with_t],
-// for checking whether an expression is convertible to bool.
-bool convertible_to_bool(bool);
-
-// Base class for std::hash<absl::optional<T>>:
-// If std::hash<std::remove_const_t<T>> is enabled, it provides operator() to
-// compute the hash; Otherwise, it is disabled.
-// Reference N4659 23.14.15 [unord.hash].
-template <typename T, typename = size_t>
-struct optional_hash_base {
-  optional_hash_base() = delete;
-  optional_hash_base(const optional_hash_base&) = delete;
-  optional_hash_base(optional_hash_base&&) = delete;
-  optional_hash_base& operator=(const optional_hash_base&) = delete;
-  optional_hash_base& operator=(optional_hash_base&&) = delete;
-};
-
-template <typename T>
-struct optional_hash_base<T, decltype(std::hash<absl::remove_const_t<T> >()(
-                                 std::declval<absl::remove_const_t<T> >()))> {
-  using argument_type = absl::optional<T>;
-  using result_type = size_t;
-  size_t operator()(const absl::optional<T>& opt) const {
-    absl::type_traits_internal::AssertHashEnabled<absl::remove_const_t<T>>();
-    if (opt) {
-      return std::hash<absl::remove_const_t<T> >()(*opt);
-    } else {
-      return static_cast<size_t>(0x297814aaad196e6dULL);
-    }
-  }
-};
-
-}  // namespace optional_internal
-}  // namespace absl
-
-#undef ABSL_OPTIONAL_USE_INHERITING_CONSTRUCTORS
-
-#endif  // ABSL_TYPES_INTERNAL_OPTIONAL_H_
diff --git a/absl/types/internal/span.h b/absl/types/internal/span.h
index d203aad5916..f0f1af67706 100644
--- a/absl/types/internal/span.h
+++ b/absl/types/internal/span.h
@@ -22,15 +22,16 @@
 #include <type_traits>
 
 #include "absl/algorithm/algorithm.h"
-#include "absl/base/internal/throw_delegate.h"
+#include "absl/base/config.h"
 #include "absl/meta/type_traits.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
-namespace span_internal {
-// A constexpr min function
-constexpr size_t Min(size_t a, size_t b) noexcept { return a < b ? a : b; }
+template <typename T>
+class Span;
 
+namespace span_internal {
 // Wrappers for access to container data pointers.
 template <typename C>
 constexpr auto GetDataImpl(C& c, char) noexcept  // NOLINT(runtime/references)
@@ -38,12 +39,6 @@ constexpr auto GetDataImpl(C& c, char) noexcept  // NOLINT(runtime/references)
   return c.data();
 }
 
-// Before C++17, std::string::data returns a const char* in all cases.
-inline char* GetDataImpl(std::string& s,  // NOLINT(runtime/references)
-                         int) noexcept {
-  return &s[0];
-}
-
 template <typename C>
 constexpr auto GetData(C& c) noexcept  // NOLINT(runtime/references)
     -> decltype(GetDataImpl(c, 0)) {
@@ -53,7 +48,7 @@ constexpr auto GetData(C& c) noexcept  // NOLINT(runtime/references)
 // Detection idioms for size() and data().
 template <typename C>
 using HasSize =
-    std::is_integral<absl::decay_t<decltype(std::declval<C&>().size())>>;
+    std::is_integral<std::decay_t<decltype(std::declval<C&>().size())>>;
 
 // We want to enable conversion from vector<T*> to Span<const T* const> but
 // disable conversion from vector<Derived> to Span<Base>. Here we use
@@ -63,13 +58,13 @@ using HasSize =
 // which returns a reference.
 template <typename T, typename C>
 using HasData =
-    std::is_convertible<absl::decay_t<decltype(GetData(std::declval<C&>()))>*,
+    std::is_convertible<std::decay_t<decltype(GetData(std::declval<C&>()))>*,
                         T* const*>;
 
 // Extracts value type from a Container
 template <typename C>
 struct ElementType {
-  using type = typename absl::remove_reference_t<C>::value_type;
+  using type = typename std::remove_reference_t<C>::value_type;
 };
 
 template <typename T, size_t N>
@@ -81,46 +76,57 @@ template <typename C>
 using ElementT = typename ElementType<C>::type;
 
 template <typename T>
-using EnableIfMutable =
-    typename std::enable_if<!std::is_const<T>::value, int>::type;
+using EnableIfMutable = std::enable_if_t<!std::is_const_v<T>, int>;
 
 template <template <typename> class SpanT, typename T>
-bool EqualImpl(SpanT<T> a, SpanT<T> b) {
-  static_assert(std::is_const<T>::value, "");
-  return absl::equal(a.begin(), a.end(), b.begin(), b.end());
+constexpr bool EqualImpl(SpanT<T> a, SpanT<T> b) {
+  static_assert(std::is_const_v<T>);
+  return std::equal(a.begin(), a.end(), b.begin(), b.end());
 }
 
 template <template <typename> class SpanT, typename T>
-bool LessThanImpl(SpanT<T> a, SpanT<T> b) {
+constexpr bool LessThanImpl(SpanT<T> a, SpanT<T> b) {
   // We can't use value_type since that is remove_cv_t<T>, so we go the long way
   // around.
-  static_assert(std::is_const<T>::value, "");
+  static_assert(std::is_const_v<T>);
   return std::lexicographical_compare(a.begin(), a.end(), b.begin(), b.end());
 }
 
-// The `IsConvertible` classes here are needed because of the
-// `std::is_convertible` bug in libcxx when compiled with GCC. This build
-// configuration is used by Android NDK toolchain. Reference link:
-// https://bugs.llvm.org/show_bug.cgi?id=27538.
 template <typename From, typename To>
-struct IsConvertibleHelper {
+using EnableIfConvertibleTo = std::enable_if_t<std::is_convertible_v<From, To>>;
+
+// IsView is true for types where the return type of .data() is the same for
+// mutable and const instances. This isn't foolproof, but it's only used to
+// enable a compiler warning.
+template <typename T, typename = void, typename = void>
+struct IsView {
+  static constexpr bool value = false;
+};
+
+template <typename T>
+struct IsView<
+    T, std::void_t<decltype(span_internal::GetData(std::declval<const T&>()))>,
+    std::void_t<decltype(span_internal::GetData(std::declval<T&>()))>> {
  private:
-  static std::true_type testval(To);
-  static std::false_type testval(...);
+  using Container = std::remove_const_t<T>;
+  using ConstData =
+      decltype(span_internal::GetData(std::declval<const Container&>()));
+  using MutData = decltype(span_internal::GetData(std::declval<Container&>()));
 
  public:
-  using type = decltype(testval(std::declval<From>()));
+  static constexpr bool value = std::is_same_v<ConstData, MutData>;
 };
 
-template <typename From, typename To>
-struct IsConvertible : IsConvertibleHelper<From, To>::type {};
+// These enablers result in 'int' so they can be used as typenames or defaults
+// in template parameters lists.
+template <typename T>
+using EnableIfIsView = std::enable_if_t<IsView<T>::value, int>;
+
+template <typename T>
+using EnableIfNotIsView = std::enable_if_t<!IsView<T>::value, int>;
 
-// TODO(zhangxy): replace `IsConvertible` with `std::is_convertible` once the
-// older version of libcxx is not supported.
-template <typename From, typename To>
-using EnableIfConvertibleTo =
-    typename std::enable_if<IsConvertible<From, To>::value>::type;
 }  // namespace span_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_TYPES_INTERNAL_SPAN_H_
diff --git a/absl/types/internal/variant.h b/absl/types/internal/variant.h
deleted file mode 100644
index 19de2e13759..00000000000
--- a/absl/types/internal/variant.h
+++ /dev/null
@@ -1,1644 +0,0 @@
-// Copyright 2018 The Abseil Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-//
-// Implementation details of absl/types/variant.h, pulled into a
-// separate file to avoid cluttering the top of the API header with
-// implementation details.
-
-#ifndef ABSL_TYPES_variant_internal_H_
-#define ABSL_TYPES_variant_internal_H_
-
-#include <cassert>
-#include <cstddef>
-#include <cstdlib>
-#include <memory>
-#include <stdexcept>
-#include <tuple>
-#include <type_traits>
-
-#include "absl/base/config.h"
-#include "absl/base/internal/identity.h"
-#include "absl/base/internal/inline_variable.h"
-#include "absl/base/internal/invoke.h"
-#include "absl/base/macros.h"
-#include "absl/base/optimization.h"
-#include "absl/meta/type_traits.h"
-#include "absl/types/bad_variant_access.h"
-#include "absl/utility/utility.h"
-
-#if !defined(ABSL_HAVE_STD_VARIANT)
-
-namespace absl {
-
-template <class... Types>
-class variant;
-
-ABSL_INTERNAL_INLINE_CONSTEXPR(size_t, variant_npos, -1);
-
-template <class T>
-struct variant_size;
-
-template <std::size_t I, class T>
-struct variant_alternative;
-
-namespace variant_internal {
-
-// NOTE: See specializations below for details.
-template <std::size_t I, class T>
-struct VariantAlternativeSfinae {};
-
-// Requires: I < variant_size_v<T>.
-//
-// Value: The Ith type of Types...
-template <std::size_t I, class T0, class... Tn>
-struct VariantAlternativeSfinae<I, variant<T0, Tn...>>
-    : VariantAlternativeSfinae<I - 1, variant<Tn...>> {};
-
-// Value: T0
-template <class T0, class... Ts>
-struct VariantAlternativeSfinae<0, variant<T0, Ts...>> {
-  using type = T0;
-};
-
-template <std::size_t I, class T>
-using VariantAlternativeSfinaeT = typename VariantAlternativeSfinae<I, T>::type;
-
-// NOTE: Requires T to be a reference type.
-template <class T, class U>
-struct GiveQualsTo;
-
-template <class T, class U>
-struct GiveQualsTo<T&, U> {
-  using type = U&;
-};
-
-template <class T, class U>
-struct GiveQualsTo<T&&, U> {
-  using type = U&&;
-};
-
-template <class T, class U>
-struct GiveQualsTo<const T&, U> {
-  using type = const U&;
-};
-
-template <class T, class U>
-struct GiveQualsTo<const T&&, U> {
-  using type = const U&&;
-};
-
-template <class T, class U>
-struct GiveQualsTo<volatile T&, U> {
-  using type = volatile U&;
-};
-
-template <class T, class U>
-struct GiveQualsTo<volatile T&&, U> {
-  using type = volatile U&&;
-};
-
-template <class T, class U>
-struct GiveQualsTo<volatile const T&, U> {
-  using type = volatile const U&;
-};
-
-template <class T, class U>
-struct GiveQualsTo<volatile const T&&, U> {
-  using type = volatile const U&&;
-};
-
-template <class T, class U>
-using GiveQualsToT = typename GiveQualsTo<T, U>::type;
-
-// Convenience alias, since size_t integral_constant is used a lot in this file.
-template <std::size_t I>
-using SizeT = std::integral_constant<std::size_t, I>;
-
-using NPos = SizeT<variant_npos>;
-
-template <class Variant, class T, class = void>
-struct IndexOfConstructedType {};
-
-template <std::size_t I, class Variant>
-struct VariantAccessResultImpl;
-
-template <std::size_t I, template <class...> class Variantemplate, class... T>
-struct VariantAccessResultImpl<I, Variantemplate<T...>&> {
-  using type = typename absl::variant_alternative<I, variant<T...>>::type&;
-};
-
-template <std::size_t I, template <class...> class Variantemplate, class... T>
-struct VariantAccessResultImpl<I, const Variantemplate<T...>&> {
-  using type =
-      const typename absl::variant_alternative<I, variant<T...>>::type&;
-};
-
-template <std::size_t I, template <class...> class Variantemplate, class... T>
-struct VariantAccessResultImpl<I, Variantemplate<T...>&&> {
-  using type = typename absl::variant_alternative<I, variant<T...>>::type&&;
-};
-
-template <std::size_t I, template <class...> class Variantemplate, class... T>
-struct VariantAccessResultImpl<I, const Variantemplate<T...>&&> {
-  using type =
-      const typename absl::variant_alternative<I, variant<T...>>::type&&;
-};
-
-template <std::size_t I, class Variant>
-using VariantAccessResult =
-    typename VariantAccessResultImpl<I, Variant&&>::type;
-
-// NOTE: This is used instead of std::array to reduce instantiation overhead.
-template <class T, std::size_t Size>
-struct SimpleArray {
-  static_assert(Size != 0, "");
-  T value[Size];
-};
-
-template <class T>
-struct AccessedType {
-  using type = T;
-};
-
-template <class T>
-using AccessedTypeT = typename AccessedType<T>::type;
-
-template <class T, std::size_t Size>
-struct AccessedType<SimpleArray<T, Size>> {
-  using type = AccessedTypeT<T>;
-};
-
-template <class T>
-constexpr T AccessSimpleArray(const T& value) {
-  return value;
-}
-
-template <class T, std::size_t Size, class... SizeT>
-constexpr AccessedTypeT<T> AccessSimpleArray(const SimpleArray<T, Size>& table,
-                                             std::size_t head_index,
-                                             SizeT... tail_indices) {
-  return AccessSimpleArray(table.value[head_index], tail_indices...);
-}
-
-// Note: Intentionally is an alias.
-template <class T>
-using AlwaysZero = SizeT<0>;
-
-template <class Op, class... Vs>
-struct VisitIndicesResultImpl {
-  using type = absl::result_of_t<Op(AlwaysZero<Vs>...)>;
-};
-
-template <class Op, class... Vs>
-using VisitIndicesResultT = typename VisitIndicesResultImpl<Op, Vs...>::type;
-
-template <class ReturnType, class FunctionObject, class EndIndices,
-          class BoundIndices>
-struct MakeVisitationMatrix;
-
-template <class ReturnType, class FunctionObject, std::size_t... Indices>
-constexpr ReturnType call_with_indices(FunctionObject&& function) {
-  static_assert(
-      std::is_same<ReturnType, decltype(std::declval<FunctionObject>()(
-                                   SizeT<Indices>()...))>::value,
-      "Not all visitation overloads have the same return type.");
-  return absl::forward<FunctionObject>(function)(SizeT<Indices>()...);
-}
-
-template <class ReturnType, class FunctionObject, std::size_t... BoundIndices>
-struct MakeVisitationMatrix<ReturnType, FunctionObject, index_sequence<>,
-                            index_sequence<BoundIndices...>> {
-  using ResultType = ReturnType (*)(FunctionObject&&);
-  static constexpr ResultType Run() {
-    return &call_with_indices<ReturnType, FunctionObject,
-                              (BoundIndices - 1)...>;
-  }
-};
-
-template <typename Is, std::size_t J>
-struct AppendToIndexSequence;
-
-template <typename Is, std::size_t J>
-using AppendToIndexSequenceT = typename AppendToIndexSequence<Is, J>::type;
-
-template <std::size_t... Is, std::size_t J>
-struct AppendToIndexSequence<index_sequence<Is...>, J> {
-  using type = index_sequence<Is..., J>;
-};
-
-template <class ReturnType, class FunctionObject, class EndIndices,
-          class CurrIndices, class BoundIndices>
-struct MakeVisitationMatrixImpl;
-
-template <class ReturnType, class FunctionObject, class EndIndices,
-          std::size_t... CurrIndices, class BoundIndices>
-struct MakeVisitationMatrixImpl<ReturnType, FunctionObject, EndIndices,
-                                index_sequence<CurrIndices...>, BoundIndices> {
-  using ResultType = SimpleArray<
-      typename MakeVisitationMatrix<ReturnType, FunctionObject, EndIndices,
-                                    index_sequence<>>::ResultType,
-      sizeof...(CurrIndices)>;
-
-  static constexpr ResultType Run() {
-    return {{MakeVisitationMatrix<
-        ReturnType, FunctionObject, EndIndices,
-        AppendToIndexSequenceT<BoundIndices, CurrIndices>>::Run()...}};
-  }
-};
-
-template <class ReturnType, class FunctionObject, std::size_t HeadEndIndex,
-          std::size_t... TailEndIndices, std::size_t... BoundIndices>
-struct MakeVisitationMatrix<ReturnType, FunctionObject,
-                            index_sequence<HeadEndIndex, TailEndIndices...>,
-                            index_sequence<BoundIndices...>>
-    : MakeVisitationMatrixImpl<ReturnType, FunctionObject,
-                               index_sequence<TailEndIndices...>,
-                               absl::make_index_sequence<HeadEndIndex>,
-                               index_sequence<BoundIndices...>> {};
-
-struct UnreachableSwitchCase {
-  template <class Op>
-  [[noreturn]] static VisitIndicesResultT<Op, std::size_t> Run(
-      Op&& /*ignored*/) {
-#if ABSL_HAVE_BUILTIN(__builtin_unreachable) || \
-    (defined(__GNUC__) && !defined(__clang__))
-    __builtin_unreachable();
-#elif defined(_MSC_VER)
-    __assume(false);
-#else
-    // Try to use assert of false being identified as an unreachable intrinsic.
-    // NOTE: We use assert directly to increase chances of exploiting an assume
-    //       intrinsic.
-    assert(false);  // NOLINT
-
-    // Hack to silence potential no return warning -- cause an infinite loop.
-    return Run(absl::forward<Op>(op));
-#endif  // Checks for __builtin_unreachable
-  }
-};
-
-template <class Op, std::size_t I>
-struct ReachableSwitchCase {
-  static VisitIndicesResultT<Op, std::size_t> Run(Op&& op) {
-    return absl::base_internal::Invoke(absl::forward<Op>(op), SizeT<I>());
-  }
-};
-
-// The number 33 is just a guess at a reasonable maximum to our switch. It is
-// not based on any analysis. The reason it is a power of 2 plus 1 instead of a
-// power of 2 is because the number was picked to correspond to a power of 2
-// amount of "normal" alternatives, plus one for the possibility of the user
-// providing "monostate" in addition to the more natural alternatives.
-ABSL_INTERNAL_INLINE_CONSTEXPR(std::size_t, MaxUnrolledVisitCases, 33);
-
-// Note: The default-definition is for unreachable cases.
-template <bool IsReachable>
-struct PickCaseImpl {
-  template <class Op, std::size_t I>
-  using Apply = UnreachableSwitchCase;
-};
-
-template <>
-struct PickCaseImpl</*IsReachable =*/true> {
-  template <class Op, std::size_t I>
-  using Apply = ReachableSwitchCase<Op, I>;
-};
-
-// Note: This form of dance with template aliases is to make sure that we
-//       instantiate a number of templates proportional to the number of variant
-//       alternatives rather than a number of templates proportional to our
-//       maximum unrolled amount of visitation cases (aliases are effectively
-//       "free" whereas other template instantiations are costly).
-template <class Op, std::size_t I, std::size_t EndIndex>
-using PickCase = typename PickCaseImpl<(I < EndIndex)>::template Apply<Op, I>;
-
-template <class ReturnType>
-[[noreturn]] ReturnType TypedThrowBadVariantAccess() {
-  absl::variant_internal::ThrowBadVariantAccess();
-}
-
-// Given N variant sizes, determine the number of cases there would need to be
-// in a single switch-statement that would cover every possibility in the
-// corresponding N-ary visit operation.
-template <std::size_t... NumAlternatives>
-struct NumCasesOfSwitch;
-
-template <std::size_t HeadNumAlternatives, std::size_t... TailNumAlternatives>
-struct NumCasesOfSwitch<HeadNumAlternatives, TailNumAlternatives...> {
-  static constexpr std::size_t value =
-      (HeadNumAlternatives + 1) *
-      NumCasesOfSwitch<TailNumAlternatives...>::value;
-};
-
-template <>
-struct NumCasesOfSwitch<> {
-  static constexpr std::size_t value = 1;
-};
-
-// A switch statement optimizes better than the table of function pointers.
-template <std::size_t EndIndex>
-struct VisitIndicesSwitch {
-  static_assert(EndIndex <= MaxUnrolledVisitCases,
-                "Maximum unrolled switch size exceeded.");
-
-  template <class Op>
-  static VisitIndicesResultT<Op, std::size_t> Run(Op&& op, std::size_t i) {
-    switch (i) {
-      case 0:
-        return PickCase<Op, 0, EndIndex>::Run(absl::forward<Op>(op));
-      case 1:
-        return PickCase<Op, 1, EndIndex>::Run(absl::forward<Op>(op));
-      case 2:
-        return PickCase<Op, 2, EndIndex>::Run(absl::forward<Op>(op));
-      case 3:
-        return PickCase<Op, 3, EndIndex>::Run(absl::forward<Op>(op));
-      case 4:
-        return PickCase<Op, 4, EndIndex>::Run(absl::forward<Op>(op));
-      case 5:
-        return PickCase<Op, 5, EndIndex>::Run(absl::forward<Op>(op));
-      case 6:
-        return PickCase<Op, 6, EndIndex>::Run(absl::forward<Op>(op));
-      case 7:
-        return PickCase<Op, 7, EndIndex>::Run(absl::forward<Op>(op));
-      case 8:
-        return PickCase<Op, 8, EndIndex>::Run(absl::forward<Op>(op));
-      case 9:
-        return PickCase<Op, 9, EndIndex>::Run(absl::forward<Op>(op));
-      case 10:
-        return PickCase<Op, 10, EndIndex>::Run(absl::forward<Op>(op));
-      case 11:
-        return PickCase<Op, 11, EndIndex>::Run(absl::forward<Op>(op));
-      case 12:
-        return PickCase<Op, 12, EndIndex>::Run(absl::forward<Op>(op));
-      case 13:
-        return PickCase<Op, 13, EndIndex>::Run(absl::forward<Op>(op));
-      case 14:
-        return PickCase<Op, 14, EndIndex>::Run(absl::forward<Op>(op));
-      case 15:
-        return PickCase<Op, 15, EndIndex>::Run(absl::forward<Op>(op));
-      case 16:
-        return PickCase<Op, 16, EndIndex>::Run(absl::forward<Op>(op));
-      case 17:
-        return PickCase<Op, 17, EndIndex>::Run(absl::forward<Op>(op));
-      case 18:
-        return PickCase<Op, 18, EndIndex>::Run(absl::forward<Op>(op));
-      case 19:
-        return PickCase<Op, 19, EndIndex>::Run(absl::forward<Op>(op));
-      case 20:
-        return PickCase<Op, 20, EndIndex>::Run(absl::forward<Op>(op));
-      case 21:
-        return PickCase<Op, 21, EndIndex>::Run(absl::forward<Op>(op));
-      case 22:
-        return PickCase<Op, 22, EndIndex>::Run(absl::forward<Op>(op));
-      case 23:
-        return PickCase<Op, 23, EndIndex>::Run(absl::forward<Op>(op));
-      case 24:
-        return PickCase<Op, 24, EndIndex>::Run(absl::forward<Op>(op));
-      case 25:
-        return PickCase<Op, 25, EndIndex>::Run(absl::forward<Op>(op));
-      case 26:
-        return PickCase<Op, 26, EndIndex>::Run(absl::forward<Op>(op));
-      case 27:
-        return PickCase<Op, 27, EndIndex>::Run(absl::forward<Op>(op));
-      case 28:
-        return PickCase<Op, 28, EndIndex>::Run(absl::forward<Op>(op));
-      case 29:
-        return PickCase<Op, 29, EndIndex>::Run(absl::forward<Op>(op));
-      case 30:
-        return PickCase<Op, 30, EndIndex>::Run(absl::forward<Op>(op));
-      case 31:
-        return PickCase<Op, 31, EndIndex>::Run(absl::forward<Op>(op));
-      case 32:
-        return PickCase<Op, 32, EndIndex>::Run(absl::forward<Op>(op));
-      default:
-        ABSL_ASSERT(i == variant_npos);
-        return absl::base_internal::Invoke(absl::forward<Op>(op), NPos());
-    }
-  }
-};
-
-template <std::size_t... EndIndices>
-struct VisitIndicesFallback {
-  template <class Op, class... SizeT>
-  static VisitIndicesResultT<Op, SizeT...> Run(Op&& op, SizeT... indices) {
-    return AccessSimpleArray(
-        MakeVisitationMatrix<VisitIndicesResultT<Op, SizeT...>, Op,
-                             index_sequence<(EndIndices + 1)...>,
-                             index_sequence<>>::Run(),
-        (indices + 1)...)(absl::forward<Op>(op));
-  }
-};
-
-// Take an N-dimensional series of indices and convert them into a single index
-// without loss of information. The purpose of this is to be able to convert an
-// N-ary visit operation into a single switch statement.
-template <std::size_t...>
-struct FlattenIndices;
-
-template <std::size_t HeadSize, std::size_t... TailSize>
-struct FlattenIndices<HeadSize, TailSize...> {
-  template<class... SizeType>
-  static constexpr std::size_t Run(std::size_t head, SizeType... tail) {
-    return head + HeadSize * FlattenIndices<TailSize...>::Run(tail...);
-  }
-};
-
-template <>
-struct FlattenIndices<> {
-  static constexpr std::size_t Run() { return 0; }
-};
-
-// Take a single "flattened" index (flattened by FlattenIndices) and determine
-// the value of the index of one of the logically represented dimensions.
-template <std::size_t I, std::size_t IndexToGet, std::size_t HeadSize,
-          std::size_t... TailSize>
-struct UnflattenIndex {
-  static constexpr std::size_t value =
-      UnflattenIndex<I / HeadSize, IndexToGet - 1, TailSize...>::value;
-};
-
-template <std::size_t I, std::size_t HeadSize, std::size_t... TailSize>
-struct UnflattenIndex<I, 0, HeadSize, TailSize...> {
-  static constexpr std::size_t value = (I % HeadSize);
-};
-
-// The backend for converting an N-ary visit operation into a unary visit.
-template <class IndexSequence, std::size_t... EndIndices>
-struct VisitIndicesVariadicImpl;
-
-template <std::size_t... N, std::size_t... EndIndices>
-struct VisitIndicesVariadicImpl<absl::index_sequence<N...>, EndIndices...> {
-  // A type that can take an N-ary function object and converts it to a unary
-  // function object that takes a single, flattened index, and "unflattens" it
-  // into its individual dimensions when forwarding to the wrapped object.
-  template <class Op>
-  struct FlattenedOp {
-    template <std::size_t I>
-    VisitIndicesResultT<Op, decltype(EndIndices)...> operator()(
-        SizeT<I> /*index*/) && {
-      return base_internal::Invoke(
-          absl::forward<Op>(op),
-          SizeT<UnflattenIndex<I, N, (EndIndices + 1)...>::value -
-                std::size_t{1}>()...);
-    }
-
-    Op&& op;
-  };
-
-  template <class Op, class... SizeType>
-  static VisitIndicesResultT<Op, decltype(EndIndices)...> Run(
-      Op&& op, SizeType... i) {
-    return VisitIndicesSwitch<NumCasesOfSwitch<EndIndices...>::value>::Run(
-        FlattenedOp<Op>{absl::forward<Op>(op)},
-        FlattenIndices<(EndIndices + std::size_t{1})...>::Run(
-            (i + std::size_t{1})...));
-  }
-};
-
-template <std::size_t... EndIndices>
-struct VisitIndicesVariadic
-    : VisitIndicesVariadicImpl<absl::make_index_sequence<sizeof...(EndIndices)>,
-                               EndIndices...> {};
-
-// This implementation will flatten N-ary visit operations into a single switch
-// statement when the number of cases would be less than our maximum specified
-// switch-statement size.
-// TODO(calabrese)
-//   Based on benchmarks, determine whether the function table approach actually
-//   does optimize better than a chain of switch statements and possibly update
-//   the implementation accordingly. Also consider increasing the maximum switch
-//   size.
-template <std::size_t... EndIndices>
-struct VisitIndices
-    : absl::conditional_t<(NumCasesOfSwitch<EndIndices...>::value <=
-                           MaxUnrolledVisitCases),
-                          VisitIndicesVariadic<EndIndices...>,
-                          VisitIndicesFallback<EndIndices...>> {};
-
-template <std::size_t EndIndex>
-struct VisitIndices<EndIndex>
-    : absl::conditional_t<(EndIndex <= MaxUnrolledVisitCases),
-                          VisitIndicesSwitch<EndIndex>,
-                          VisitIndicesFallback<EndIndex>> {};
-
-// Suppress bogus warning on MSVC: MSVC complains that the `reinterpret_cast`
-// below is returning the address of a temporary or local object.
-#ifdef _MSC_VER
-#pragma warning(push)
-#pragma warning(disable : 4172)
-#endif  // _MSC_VER
-
-// TODO(calabrese) std::launder
-// TODO(calabrese) constexpr
-// NOTE: DO NOT REMOVE the `inline` keyword as it is necessary to work around a
-// MSVC bug. See https://github.com/abseil/abseil-cpp/issues/129 for details.
-template <class Self, std::size_t I>
-inline VariantAccessResult<I, Self> AccessUnion(Self&& self, SizeT<I> /*i*/) {
-  return reinterpret_cast<VariantAccessResult<I, Self>>(self);
-}
-
-#ifdef _MSC_VER
-#pragma warning(pop)
-#endif  // _MSC_VER
-
-template <class T>
-void DeducedDestroy(T& self) {  // NOLINT
-  self.~T();
-}
-
-// NOTE: This type exists as a single entity for variant and its bases to
-// befriend. It contains helper functionality that manipulates the state of the
-// variant, such as the implementation of things like assignment and emplace
-// operations.
-struct VariantCoreAccess {
-  template <class VariantType>
-  static typename VariantType::Variant& Derived(VariantType& self) {  // NOLINT
-    return static_cast<typename VariantType::Variant&>(self);
-  }
-
-  template <class VariantType>
-  static const typename VariantType::Variant& Derived(
-      const VariantType& self) {  // NOLINT
-    return static_cast<const typename VariantType::Variant&>(self);
-  }
-
-  template <class VariantType>
-  static void Destroy(VariantType& self) {  // NOLINT
-    Derived(self).destroy();
-    self.index_ = absl::variant_npos;
-  }
-
-  template <class Variant>
-  static void SetIndex(Variant& self, std::size_t i) {  // NOLINT
-    self.index_ = i;
-  }
-
-  template <class Variant>
-  static void InitFrom(Variant& self, Variant&& other) {  // NOLINT
-    VisitIndices<absl::variant_size<Variant>::value>::Run(
-        InitFromVisitor<Variant, Variant&&>{&self,
-                                            std::forward<Variant>(other)},
-        other.index());
-    self.index_ = other.index();
-  }
-
-  // Access a variant alternative, assuming the index is correct.
-  template <std::size_t I, class Variant>
-  static VariantAccessResult<I, Variant> Access(Variant&& self) {
-    // This cast instead of invocation of AccessUnion with an rvalue is a
-    // workaround for msvc. Without this there is a runtime failure when dealing
-    // with rvalues.
-    // TODO(calabrese) Reduce test case and find a simpler workaround.
-    return static_cast<VariantAccessResult<I, Variant>>(
-        variant_internal::AccessUnion(self.state_, SizeT<I>()));
-  }
-
-  // Access a variant alternative, throwing if the index is incorrect.
-  template <std::size_t I, class Variant>
-  static VariantAccessResult<I, Variant> CheckedAccess(Variant&& self) {
-    if (ABSL_PREDICT_FALSE(self.index_ != I)) {
-      TypedThrowBadVariantAccess<VariantAccessResult<I, Variant>>();
-    }
-
-    return Access<I>(absl::forward<Variant>(self));
-  }
-
-  // The implementation of the move-assignment operation for a variant.
-  template <class VType>
-  struct MoveAssignVisitor {
-    using DerivedType = typename VType::Variant;
-    template <std::size_t NewIndex>
-    void operator()(SizeT<NewIndex> /*new_i*/) const {
-      if (left->index_ == NewIndex) {
-        Access<NewIndex>(*left) = std::move(Access<NewIndex>(*right));
-      } else {
-        Derived(*left).template emplace<NewIndex>(
-            std::move(Access<NewIndex>(*right)));
-      }
-    }
-
-    void operator()(SizeT<absl::variant_npos> /*new_i*/) const {
-      Destroy(*left);
-    }
-
-    VType* left;
-    VType* right;
-  };
-
-  template <class VType>
-  static MoveAssignVisitor<VType> MakeMoveAssignVisitor(VType* left,
-                                                        VType* other) {
-    return {left, other};
-  }
-
-  // The implementation of the assignment operation for a variant.
-  template <class VType>
-  struct CopyAssignVisitor {
-    using DerivedType = typename VType::Variant;
-    template <std::size_t NewIndex>
-    void operator()(SizeT<NewIndex> /*new_i*/) const {
-      using New =
-          typename absl::variant_alternative<NewIndex, DerivedType>::type;
-
-      if (left->index_ == NewIndex) {
-        Access<NewIndex>(*left) = Access<NewIndex>(*right);
-      } else if (std::is_nothrow_copy_constructible<New>::value ||
-                 !std::is_nothrow_move_constructible<New>::value) {
-        Derived(*left).template emplace<NewIndex>(Access<NewIndex>(*right));
-      } else {
-        Derived(*left) = DerivedType(Derived(*right));
-      }
-    }
-
-    void operator()(SizeT<absl::variant_npos> /*new_i*/) const {
-      Destroy(*left);
-    }
-
-    VType* left;
-    const VType* right;
-  };
-
-  template <class VType>
-  static CopyAssignVisitor<VType> MakeCopyAssignVisitor(VType* left,
-                                                        const VType& other) {
-    return {left, &other};
-  }
-
-  // The implementation of conversion-assignment operations for variant.
-  template <class Left, class QualifiedNew>
-  struct ConversionAssignVisitor {
-    using NewIndex =
-        variant_internal::IndexOfConstructedType<Left, QualifiedNew>;
-
-    void operator()(SizeT<NewIndex::value> /*old_i*/
-                    ) const {
-      Access<NewIndex::value>(*left) = absl::forward<QualifiedNew>(other);
-    }
-
-    template <std::size_t OldIndex>
-    void operator()(SizeT<OldIndex> /*old_i*/
-                    ) const {
-      using New =
-          typename absl::variant_alternative<NewIndex::value, Left>::type;
-      if (std::is_nothrow_constructible<New, QualifiedNew>::value ||
-          !std::is_nothrow_move_constructible<New>::value) {
-        left->template emplace<NewIndex::value>(
-            absl::forward<QualifiedNew>(other));
-      } else {
-        // the standard says "equivalent to
-        // operator=(variant(std::forward<T>(t)))", but we use `emplace` here
-        // because the variant's move assignment operator could be deleted.
-        left->template emplace<NewIndex::value>(
-            New(absl::forward<QualifiedNew>(other)));
-      }
-    }
-
-    Left* left;
-    QualifiedNew&& other;
-  };
-
-  template <class Left, class QualifiedNew>
-  static ConversionAssignVisitor<Left, QualifiedNew>
-  MakeConversionAssignVisitor(Left* left, QualifiedNew&& qual) {
-    return {left, absl::forward<QualifiedNew>(qual)};
-  }
-
-  // Backend for operations for `emplace()` which destructs `*self` then
-  // construct a new alternative with `Args...`.
-  template <std::size_t NewIndex, class Self, class... Args>
-  static typename absl::variant_alternative<NewIndex, Self>::type& Replace(
-      Self* self, Args&&... args) {
-    Destroy(*self);
-    using New = typename absl::variant_alternative<NewIndex, Self>::type;
-    New* const result = ::new (static_cast<void*>(&self->state_))
-        New(absl::forward<Args>(args)...);
-    self->index_ = NewIndex;
-    return *result;
-  }
-
-  template <class LeftVariant, class QualifiedRightVariant>
-  struct InitFromVisitor {
-    template <std::size_t NewIndex>
-    void operator()(SizeT<NewIndex> /*new_i*/) const {
-      using Alternative =
-          typename variant_alternative<NewIndex, LeftVariant>::type;
-      ::new (static_cast<void*>(&left->state_)) Alternative(
-          Access<NewIndex>(std::forward<QualifiedRightVariant>(right)));
-    }
-
-    void operator()(SizeT<absl::variant_npos> /*new_i*/) const {
-      // This space intentionally left blank.
-    }
-    LeftVariant* left;
-    QualifiedRightVariant&& right;
-  };
-};
-
-template <class Expected, class... T>
-struct IndexOfImpl;
-
-template <class Expected>
-struct IndexOfImpl<Expected> {
-  using IndexFromEnd = SizeT<0>;
-  using MatchedIndexFromEnd = IndexFromEnd;
-  using MultipleMatches = std::false_type;
-};
-
-template <class Expected, class Head, class... Tail>
-struct IndexOfImpl<Expected, Head, Tail...> : IndexOfImpl<Expected, Tail...> {
-  using IndexFromEnd =
-      SizeT<IndexOfImpl<Expected, Tail...>::IndexFromEnd::value + 1>;
-};
-
-template <class Expected, class... Tail>
-struct IndexOfImpl<Expected, Expected, Tail...>
-    : IndexOfImpl<Expected, Tail...> {
-  using IndexFromEnd =
-      SizeT<IndexOfImpl<Expected, Tail...>::IndexFromEnd::value + 1>;
-  using MatchedIndexFromEnd = IndexFromEnd;
-  using MultipleMatches = std::integral_constant<
-      bool, IndexOfImpl<Expected, Tail...>::MatchedIndexFromEnd::value != 0>;
-};
-
-template <class Expected, class... Types>
-struct IndexOfMeta {
-  using Results = IndexOfImpl<Expected, Types...>;
-  static_assert(!Results::MultipleMatches::value,
-                "Attempted to access a variant by specifying a type that "
-                "matches more than one alternative.");
-  static_assert(Results::MatchedIndexFromEnd::value != 0,
-                "Attempted to access a variant by specifying a type that does "
-                "not match any alternative.");
-  using type = SizeT<sizeof...(Types) - Results::MatchedIndexFromEnd::value>;
-};
-
-template <class Expected, class... Types>
-using IndexOf = typename IndexOfMeta<Expected, Types...>::type;
-
-template <class Variant, class T, std::size_t CurrIndex>
-struct UnambiguousIndexOfImpl;
-
-// Terminating case encountered once we've checked all of the alternatives
-template <class T, std::size_t CurrIndex>
-struct UnambiguousIndexOfImpl<variant<>, T, CurrIndex> : SizeT<CurrIndex> {};
-
-// Case where T is not Head
-template <class Head, class... Tail, class T, std::size_t CurrIndex>
-struct UnambiguousIndexOfImpl<variant<Head, Tail...>, T, CurrIndex>
-    : UnambiguousIndexOfImpl<variant<Tail...>, T, CurrIndex + 1>::type {};
-
-// Case where T is Head
-template <class Head, class... Tail, std::size_t CurrIndex>
-struct UnambiguousIndexOfImpl<variant<Head, Tail...>, Head, CurrIndex>
-    : SizeT<UnambiguousIndexOfImpl<variant<Tail...>, Head, 0>::value ==
-                    sizeof...(Tail)
-                ? CurrIndex
-                : CurrIndex + sizeof...(Tail) + 1> {};
-
-template <class Variant, class T>
-struct UnambiguousIndexOf;
-
-struct NoMatch {
-  struct type {};
-};
-
-template <class... Alts, class T>
-struct UnambiguousIndexOf<variant<Alts...>, T>
-    : std::conditional<UnambiguousIndexOfImpl<variant<Alts...>, T, 0>::value !=
-                           sizeof...(Alts),
-                       UnambiguousIndexOfImpl<variant<Alts...>, T, 0>,
-                       NoMatch>::type::type {};
-
-template <class T, std::size_t /*Dummy*/>
-using UnambiguousTypeOfImpl = T;
-
-template <class Variant, class T>
-using UnambiguousTypeOfT =
-    UnambiguousTypeOfImpl<T, UnambiguousIndexOf<Variant, T>::value>;
-
-template <class H, class... T>
-class VariantStateBase;
-
-// This is an implementation of the "imaginary function" that is described in
-// [variant.ctor]
-// It is used in order to determine which alternative to construct during
-// initialization from some type T.
-template <class Variant, std::size_t I = 0>
-struct ImaginaryFun;
-
-template <std::size_t I>
-struct ImaginaryFun<variant<>, I> {
-  static void Run() = delete;
-};
-
-template <class H, class... T, std::size_t I>
-struct ImaginaryFun<variant<H, T...>, I> : ImaginaryFun<variant<T...>, I + 1> {
-  using ImaginaryFun<variant<T...>, I + 1>::Run;
-
-  // NOTE: const& and && are used instead of by-value due to lack of guaranteed
-  // move elision of C++17. This may have other minor differences, but tests
-  // pass.
-  static SizeT<I> Run(const H&, SizeT<I>);
-  static SizeT<I> Run(H&&, SizeT<I>);
-};
-
-// The following metafunctions are used in constructor and assignment
-// constraints.
-template <class Self, class T>
-struct IsNeitherSelfNorInPlace : std::true_type {};
-
-template <class Self>
-struct IsNeitherSelfNorInPlace<Self, Self> : std::false_type {};
-
-template <class Self, class T>
-struct IsNeitherSelfNorInPlace<Self, in_place_type_t<T>> : std::false_type {};
-
-template <class Self, std::size_t I>
-struct IsNeitherSelfNorInPlace<Self, in_place_index_t<I>> : std::false_type {};
-
-template <class Variant, class T, class = void>
-struct ConversionIsPossibleImpl : std::false_type {};
-
-template <class Variant, class T>
-struct ConversionIsPossibleImpl<
-    Variant, T,
-    void_t<decltype(ImaginaryFun<Variant>::Run(std::declval<T>(), {}))>>
-    : std::true_type {};
-
-template <class Variant, class T>
-struct ConversionIsPossible : ConversionIsPossibleImpl<Variant, T>::type {};
-
-template <class Variant, class T>
-struct IndexOfConstructedType<
-    Variant, T,
-    void_t<decltype(ImaginaryFun<Variant>::Run(std::declval<T>(), {}))>>
-    : decltype(ImaginaryFun<Variant>::Run(std::declval<T>(), {})) {};
-
-template <std::size_t... Is>
-struct ContainsVariantNPos
-    : absl::negation<std::is_same<  // NOLINT
-          absl::integer_sequence<bool, 0 <= Is...>,
-          absl::integer_sequence<bool, Is != absl::variant_npos...>>> {};
-
-template <class Op, class... QualifiedVariants>
-using RawVisitResult =
-    absl::result_of_t<Op(VariantAccessResult<0, QualifiedVariants>...)>;
-
-// NOTE: The spec requires that all return-paths yield the same type and is not
-// SFINAE-friendly, so we can deduce the return type by examining the first
-// result. If it's not callable, then we get an error, but are compliant and
-// fast to compile.
-// TODO(calabrese) Possibly rewrite in a way that yields better compile errors
-// at the cost of longer compile-times.
-template <class Op, class... QualifiedVariants>
-struct VisitResultImpl {
-  using type =
-      absl::result_of_t<Op(VariantAccessResult<0, QualifiedVariants>...)>;
-};
-
-// Done in two steps intentionally so that we don't cause substitution to fail.
-template <class Op, class... QualifiedVariants>
-using VisitResult = typename VisitResultImpl<Op, QualifiedVariants...>::type;
-
-template <class Op, class... QualifiedVariants>
-struct PerformVisitation {
-  using ReturnType = VisitResult<Op, QualifiedVariants...>;
-
-  template <std::size_t... Is>
-  constexpr ReturnType operator()(SizeT<Is>... indices) const {
-    return Run(typename ContainsVariantNPos<Is...>::type{},
-               absl::index_sequence_for<QualifiedVariants...>(), indices...);
-  }
-
-  template <std::size_t... TupIs, std::size_t... Is>
-  constexpr ReturnType Run(std::false_type /*has_valueless*/,
-                           index_sequence<TupIs...>, SizeT<Is>...) const {
-    static_assert(
-        std::is_same<ReturnType,
-                     absl::result_of_t<Op(VariantAccessResult<
-                                          Is, QualifiedVariants>...)>>::value,
-        "All visitation overloads must have the same return type.");
-    return absl::base_internal::Invoke(
-        absl::forward<Op>(op),
-        VariantCoreAccess::Access<Is>(
-            absl::forward<QualifiedVariants>(std::get<TupIs>(variant_tup)))...);
-  }
-
-  template <std::size_t... TupIs, std::size_t... Is>
-  [[noreturn]] ReturnType Run(std::true_type /*has_valueless*/,
-                              index_sequence<TupIs...>, SizeT<Is>...) const {
-    absl::variant_internal::ThrowBadVariantAccess();
-  }
-
-  // TODO(calabrese) Avoid using a tuple, which causes lots of instantiations
-  // Attempts using lambda variadic captures fail on current GCC.
-  std::tuple<QualifiedVariants&&...> variant_tup;
-  Op&& op;
-};
-
-template <class... T>
-union Union;
-
-// We want to allow for variant<> to be trivial. For that, we need the default
-// constructor to be trivial, which means we can't define it ourselves.
-// Instead, we use a non-default constructor that takes NoopConstructorTag
-// that doesn't affect the triviality of the types.
-struct NoopConstructorTag {};
-
-template <std::size_t I>
-struct EmplaceTag {};
-
-template <>
-union Union<> {
-  constexpr explicit Union(NoopConstructorTag) noexcept {}
-};
-
-// Suppress bogus warning on MSVC: MSVC complains that Union<T...> has a defined
-// deleted destructor from the `std::is_destructible` check below.
-#ifdef _MSC_VER
-#pragma warning(push)
-#pragma warning(disable : 4624)
-#endif  // _MSC_VER
-
-template <class Head, class... Tail>
-union Union<Head, Tail...> {
-  using TailUnion = Union<Tail...>;
-
-  explicit constexpr Union(NoopConstructorTag /*tag*/) noexcept
-      : tail(NoopConstructorTag()) {}
-
-  template <class... P>
-  explicit constexpr Union(EmplaceTag<0>, P&&... args)
-      : head(absl::forward<P>(args)...) {}
-
-  template <std::size_t I, class... P>
-  explicit constexpr Union(EmplaceTag<I>, P&&... args)
-      : tail(EmplaceTag<I - 1>{}, absl::forward<P>(args)...) {}
-
-  Head head;
-  TailUnion tail;
-};
-
-#ifdef _MSC_VER
-#pragma warning(pop)
-#endif  // _MSC_VER
-
-// TODO(calabrese) Just contain a Union in this union (certain configs fail).
-template <class... T>
-union DestructibleUnionImpl;
-
-template <>
-union DestructibleUnionImpl<> {
-  constexpr explicit DestructibleUnionImpl(NoopConstructorTag) noexcept {}
-};
-
-template <class Head, class... Tail>
-union DestructibleUnionImpl<Head, Tail...> {
-  using TailUnion = DestructibleUnionImpl<Tail...>;
-
-  explicit constexpr DestructibleUnionImpl(NoopConstructorTag /*tag*/) noexcept
-      : tail(NoopConstructorTag()) {}
-
-  template <class... P>
-  explicit constexpr DestructibleUnionImpl(EmplaceTag<0>, P&&... args)
-      : head(absl::forward<P>(args)...) {}
-
-  template <std::size_t I, class... P>
-  explicit constexpr DestructibleUnionImpl(EmplaceTag<I>, P&&... args)
-      : tail(EmplaceTag<I - 1>{}, absl::forward<P>(args)...) {}
-
-  ~DestructibleUnionImpl() {}
-
-  Head head;
-  TailUnion tail;
-};
-
-// This union type is destructible even if one or more T are not trivially
-// destructible. In the case that all T are trivially destructible, then so is
-// this resultant type.
-template <class... T>
-using DestructibleUnion =
-    absl::conditional_t<std::is_destructible<Union<T...>>::value, Union<T...>,
-                        DestructibleUnionImpl<T...>>;
-
-// Deepest base, containing the actual union and the discriminator
-template <class H, class... T>
-class VariantStateBase {
- protected:
-  using Variant = variant<H, T...>;
-
-  template <class LazyH = H,
-            class ConstructibleH = absl::enable_if_t<
-                std::is_default_constructible<LazyH>::value, LazyH>>
-  constexpr VariantStateBase() noexcept(
-      std::is_nothrow_default_constructible<ConstructibleH>::value)
-      : state_(EmplaceTag<0>()), index_(0) {}
-
-  template <std::size_t I, class... P>
-  explicit constexpr VariantStateBase(EmplaceTag<I> tag, P&&... args)
-      : state_(tag, absl::forward<P>(args)...), index_(I) {}
-
-  explicit constexpr VariantStateBase(NoopConstructorTag)
-      : state_(NoopConstructorTag()), index_(variant_npos) {}
-
-  void destroy() {}  // Does nothing (shadowed in child if non-trivial)
-
-  DestructibleUnion<H, T...> state_;
-  std::size_t index_;
-};
-
-using absl::internal::identity;
-
-// OverloadSet::Overload() is a unary function which is overloaded to
-// take any of the element types of the variant, by reference-to-const.
-// The return type of the overload on T is identity<T>, so that you
-// can statically determine which overload was called.
-//
-// Overload() is not defined, so it can only be called in unevaluated
-// contexts.
-template <typename... Ts>
-struct OverloadSet;
-
-template <typename T, typename... Ts>
-struct OverloadSet<T, Ts...> : OverloadSet<Ts...> {
-  using Base = OverloadSet<Ts...>;
-  static identity<T> Overload(const T&);
-  using Base::Overload;
-};
-
-template <>
-struct OverloadSet<> {
-  // For any case not handled above.
-  static void Overload(...);
-};
-
-template <class T>
-using LessThanResult = decltype(std::declval<T>() < std::declval<T>());
-
-template <class T>
-using GreaterThanResult = decltype(std::declval<T>() > std::declval<T>());
-
-template <class T>
-using LessThanOrEqualResult = decltype(std::declval<T>() <= std::declval<T>());
-
-template <class T>
-using GreaterThanOrEqualResult =
-    decltype(std::declval<T>() >= std::declval<T>());
-
-template <class T>
-using EqualResult = decltype(std::declval<T>() == std::declval<T>());
-
-template <class T>
-using NotEqualResult = decltype(std::declval<T>() != std::declval<T>());
-
-using type_traits_internal::is_detected_convertible;
-
-template <class... T>
-using RequireAllHaveEqualT = absl::enable_if_t<
-    absl::conjunction<is_detected_convertible<bool, EqualResult, T>...>::value,
-    bool>;
-
-template <class... T>
-using RequireAllHaveNotEqualT =
-    absl::enable_if_t<absl::conjunction<is_detected_convertible<
-                          bool, NotEqualResult, T>...>::value,
-                      bool>;
-
-template <class... T>
-using RequireAllHaveLessThanT =
-    absl::enable_if_t<absl::conjunction<is_detected_convertible<
-                          bool, LessThanResult, T>...>::value,
-                      bool>;
-
-template <class... T>
-using RequireAllHaveLessThanOrEqualT =
-    absl::enable_if_t<absl::conjunction<is_detected_convertible<
-                          bool, LessThanOrEqualResult, T>...>::value,
-                      bool>;
-
-template <class... T>
-using RequireAllHaveGreaterThanOrEqualT =
-    absl::enable_if_t<absl::conjunction<is_detected_convertible<
-                          bool, GreaterThanOrEqualResult, T>...>::value,
-                      bool>;
-
-template <class... T>
-using RequireAllHaveGreaterThanT =
-    absl::enable_if_t<absl::conjunction<is_detected_convertible<
-                          bool, GreaterThanResult, T>...>::value,
-                      bool>;
-
-// Helper template containing implementations details of variant that can't go
-// in the private section. For convenience, this takes the variant type as a
-// single template parameter.
-template <typename T>
-struct VariantHelper;
-
-template <typename... Ts>
-struct VariantHelper<variant<Ts...>> {
-  // Type metafunction which returns the element type selected if
-  // OverloadSet::Overload() is well-formed when called with argument type U.
-  template <typename U>
-  using BestMatch = decltype(
-      variant_internal::OverloadSet<Ts...>::Overload(std::declval<U>()));
-
-  // Type metafunction which returns true if OverloadSet::Overload() is
-  // well-formed when called with argument type U.
-  // CanAccept can't be just an alias because there is a MSVC bug on parameter
-  // pack expansion involving decltype.
-  template <typename U>
-  struct CanAccept :
-      std::integral_constant<bool, !std::is_void<BestMatch<U>>::value> {};
-
-  // Type metafunction which returns true if Other is an instantiation of
-  // variant, and variants's converting constructor from Other will be
-  // well-formed. We will use this to remove constructors that would be
-  // ill-formed from the overload set.
-  template <typename Other>
-  struct CanConvertFrom;
-
-  template <typename... Us>
-  struct CanConvertFrom<variant<Us...>>
-      : public absl::conjunction<CanAccept<Us>...> {};
-};
-
-// A type with nontrivial copy ctor and trivial move ctor.
-struct TrivialMoveOnly {
-  TrivialMoveOnly(TrivialMoveOnly&&) = default;
-};
-
-// Trait class to detect whether a type is trivially move constructible.
-// A union's defaulted copy/move constructor is deleted if any variant member's
-// copy/move constructor is nontrivial.
-template <typename T>
-struct IsTriviallyMoveConstructible:
-  std::is_move_constructible<Union<T, TrivialMoveOnly>> {};
-
-// To guarantee triviality of all special-member functions that can be trivial,
-// we use a chain of conditional bases for each one.
-// The order of inheritance of bases from child to base are logically:
-//
-// variant
-// VariantCopyAssignBase
-// VariantMoveAssignBase
-// VariantCopyBase
-// VariantMoveBase
-// VariantStateBaseDestructor
-// VariantStateBase
-//
-// Note that there is a separate branch at each base that is dependent on
-// whether or not that corresponding special-member-function can be trivial in
-// the resultant variant type.
-
-template <class... T>
-class VariantStateBaseDestructorNontrivial;
-
-template <class... T>
-class VariantMoveBaseNontrivial;
-
-template <class... T>
-class VariantCopyBaseNontrivial;
-
-template <class... T>
-class VariantMoveAssignBaseNontrivial;
-
-template <class... T>
-class VariantCopyAssignBaseNontrivial;
-
-// Base that is dependent on whether or not the destructor can be trivial.
-template <class... T>
-using VariantStateBaseDestructor =
-    absl::conditional_t<std::is_destructible<Union<T...>>::value,
-                        VariantStateBase<T...>,
-                        VariantStateBaseDestructorNontrivial<T...>>;
-
-// Base that is dependent on whether or not the move-constructor can be
-// implicitly generated by the compiler (trivial or deleted).
-// Previously we were using `std::is_move_constructible<Union<T...>>` to check
-// whether all Ts have trivial move constructor, but it ran into a GCC bug:
-// https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84866
-// So we have to use a different approach (i.e. `HasTrivialMoveConstructor`) to
-// work around the bug.
-template <class... T>
-using VariantMoveBase = absl::conditional_t<
-    absl::disjunction<
-        absl::negation<absl::conjunction<std::is_move_constructible<T>...>>,
-        absl::conjunction<IsTriviallyMoveConstructible<T>...>>::value,
-    VariantStateBaseDestructor<T...>, VariantMoveBaseNontrivial<T...>>;
-
-// Base that is dependent on whether or not the copy-constructor can be trivial.
-template <class... T>
-using VariantCopyBase = absl::conditional_t<
-    absl::disjunction<
-        absl::negation<absl::conjunction<std::is_copy_constructible<T>...>>,
-        std::is_copy_constructible<Union<T...>>>::value,
-    VariantMoveBase<T...>, VariantCopyBaseNontrivial<T...>>;
-
-// Base that is dependent on whether or not the move-assign can be trivial.
-template <class... T>
-using VariantMoveAssignBase = absl::conditional_t<
-    absl::disjunction<
-        absl::conjunction<absl::is_move_assignable<Union<T...>>,
-                          std::is_move_constructible<Union<T...>>,
-                          std::is_destructible<Union<T...>>>,
-        absl::negation<absl::conjunction<std::is_move_constructible<T>...,
-                                         // Note: We're not qualifying this with
-                                         // absl:: because it doesn't compile
-                                         // under MSVC.
-                                         is_move_assignable<T>...>>>::value,
-    VariantCopyBase<T...>, VariantMoveAssignBaseNontrivial<T...>>;
-
-// Base that is dependent on whether or not the copy-assign can be trivial.
-template <class... T>
-using VariantCopyAssignBase = absl::conditional_t<
-    absl::disjunction<
-        absl::conjunction<absl::is_copy_assignable<Union<T...>>,
-                          std::is_copy_constructible<Union<T...>>,
-                          std::is_destructible<Union<T...>>>,
-        absl::negation<absl::conjunction<std::is_copy_constructible<T>...,
-                                         // Note: We're not qualifying this with
-                                         // absl:: because it doesn't compile
-                                         // under MSVC.
-                                         is_copy_assignable<T>...>>>::value,
-    VariantMoveAssignBase<T...>, VariantCopyAssignBaseNontrivial<T...>>;
-
-template <class... T>
-using VariantBase = VariantCopyAssignBase<T...>;
-
-template <class... T>
-class VariantStateBaseDestructorNontrivial : protected VariantStateBase<T...> {
- private:
-  using Base = VariantStateBase<T...>;
-
- protected:
-  using Base::Base;
-
-  VariantStateBaseDestructorNontrivial() = default;
-  VariantStateBaseDestructorNontrivial(VariantStateBaseDestructorNontrivial&&) =
-      default;
-  VariantStateBaseDestructorNontrivial(
-      const VariantStateBaseDestructorNontrivial&) = default;
-  VariantStateBaseDestructorNontrivial& operator=(
-      VariantStateBaseDestructorNontrivial&&) = default;
-  VariantStateBaseDestructorNontrivial& operator=(
-      const VariantStateBaseDestructorNontrivial&) = default;
-
-  struct Destroyer {
-    template <std::size_t I>
-    void operator()(SizeT<I> i) const {
-      using Alternative =
-          typename absl::variant_alternative<I, variant<T...>>::type;
-      variant_internal::AccessUnion(self->state_, i).~Alternative();
-    }
-
-    void operator()(SizeT<absl::variant_npos> /*i*/) const {
-      // This space intentionally left blank
-    }
-
-    VariantStateBaseDestructorNontrivial* self;
-  };
-
-  void destroy() { VisitIndices<sizeof...(T)>::Run(Destroyer{this}, index_); }
-
-  ~VariantStateBaseDestructorNontrivial() { destroy(); }
-
- protected:
-  using Base::index_;
-  using Base::state_;
-};
-
-template <class... T>
-class VariantMoveBaseNontrivial : protected VariantStateBaseDestructor<T...> {
- private:
-  using Base = VariantStateBaseDestructor<T...>;
-
- protected:
-  using Base::Base;
-
-  struct Construct {
-    template <std::size_t I>
-    void operator()(SizeT<I> i) const {
-      using Alternative =
-          typename absl::variant_alternative<I, variant<T...>>::type;
-      ::new (static_cast<void*>(&self->state_)) Alternative(
-          variant_internal::AccessUnion(absl::move(other->state_), i));
-    }
-
-    void operator()(SizeT<absl::variant_npos> /*i*/) const {}
-
-    VariantMoveBaseNontrivial* self;
-    VariantMoveBaseNontrivial* other;
-  };
-
-  VariantMoveBaseNontrivial() = default;
-  VariantMoveBaseNontrivial(VariantMoveBaseNontrivial&& other) noexcept(
-      absl::conjunction<std::is_nothrow_move_constructible<T>...>::value)
-      : Base(NoopConstructorTag()) {
-    VisitIndices<sizeof...(T)>::Run(Construct{this, &other}, other.index_);
-    index_ = other.index_;
-  }
-
-  VariantMoveBaseNontrivial(VariantMoveBaseNontrivial const&) = default;
-
-  VariantMoveBaseNontrivial& operator=(VariantMoveBaseNontrivial&&) = default;
-  VariantMoveBaseNontrivial& operator=(VariantMoveBaseNontrivial const&) =
-      default;
-
- protected:
-  using Base::index_;
-  using Base::state_;
-};
-
-template <class... T>
-class VariantCopyBaseNontrivial : protected VariantMoveBase<T...> {
- private:
-  using Base = VariantMoveBase<T...>;
-
- protected:
-  using Base::Base;
-
-  VariantCopyBaseNontrivial() = default;
-  VariantCopyBaseNontrivial(VariantCopyBaseNontrivial&&) = default;
-
-  struct Construct {
-    template <std::size_t I>
-    void operator()(SizeT<I> i) const {
-      using Alternative =
-          typename absl::variant_alternative<I, variant<T...>>::type;
-      ::new (static_cast<void*>(&self->state_))
-          Alternative(variant_internal::AccessUnion(other->state_, i));
-    }
-
-    void operator()(SizeT<absl::variant_npos> /*i*/) const {}
-
-    VariantCopyBaseNontrivial* self;
-    const VariantCopyBaseNontrivial* other;
-  };
-
-  VariantCopyBaseNontrivial(VariantCopyBaseNontrivial const& other)
-      : Base(NoopConstructorTag()) {
-    VisitIndices<sizeof...(T)>::Run(Construct{this, &other}, other.index_);
-    index_ = other.index_;
-  }
-
-  VariantCopyBaseNontrivial& operator=(VariantCopyBaseNontrivial&&) = default;
-  VariantCopyBaseNontrivial& operator=(VariantCopyBaseNontrivial const&) =
-      default;
-
- protected:
-  using Base::index_;
-  using Base::state_;
-};
-
-template <class... T>
-class VariantMoveAssignBaseNontrivial : protected VariantCopyBase<T...> {
-  friend struct VariantCoreAccess;
-
- private:
-  using Base = VariantCopyBase<T...>;
-
- protected:
-  using Base::Base;
-
-  VariantMoveAssignBaseNontrivial() = default;
-  VariantMoveAssignBaseNontrivial(VariantMoveAssignBaseNontrivial&&) = default;
-  VariantMoveAssignBaseNontrivial(const VariantMoveAssignBaseNontrivial&) =
-      default;
-  VariantMoveAssignBaseNontrivial& operator=(
-      VariantMoveAssignBaseNontrivial const&) = default;
-
-    VariantMoveAssignBaseNontrivial&
-    operator=(VariantMoveAssignBaseNontrivial&& other) noexcept(
-        absl::conjunction<std::is_nothrow_move_constructible<T>...,
-                          std::is_nothrow_move_assignable<T>...>::value) {
-      VisitIndices<sizeof...(T)>::Run(
-          VariantCoreAccess::MakeMoveAssignVisitor(this, &other), other.index_);
-      return *this;
-    }
-
- protected:
-  using Base::index_;
-  using Base::state_;
-};
-
-template <class... T>
-class VariantCopyAssignBaseNontrivial : protected VariantMoveAssignBase<T...> {
-  friend struct VariantCoreAccess;
-
- private:
-  using Base = VariantMoveAssignBase<T...>;
-
- protected:
-  using Base::Base;
-
-  VariantCopyAssignBaseNontrivial() = default;
-  VariantCopyAssignBaseNontrivial(VariantCopyAssignBaseNontrivial&&) = default;
-  VariantCopyAssignBaseNontrivial(const VariantCopyAssignBaseNontrivial&) =
-      default;
-  VariantCopyAssignBaseNontrivial& operator=(
-      VariantCopyAssignBaseNontrivial&&) = default;
-
-    VariantCopyAssignBaseNontrivial& operator=(
-        const VariantCopyAssignBaseNontrivial& other) {
-      VisitIndices<sizeof...(T)>::Run(
-          VariantCoreAccess::MakeCopyAssignVisitor(this, other), other.index_);
-      return *this;
-    }
-
- protected:
-  using Base::index_;
-  using Base::state_;
-};
-
-////////////////////////////////////////
-// Visitors for Comparison Operations //
-////////////////////////////////////////
-
-template <class... Types>
-struct EqualsOp {
-  const variant<Types...>* v;
-  const variant<Types...>* w;
-
-  constexpr bool operator()(SizeT<absl::variant_npos> /*v_i*/) const {
-    return true;
-  }
-
-  template <std::size_t I>
-  constexpr bool operator()(SizeT<I> /*v_i*/) const {
-    return VariantCoreAccess::Access<I>(*v) == VariantCoreAccess::Access<I>(*w);
-  }
-};
-
-template <class... Types>
-struct NotEqualsOp {
-  const variant<Types...>* v;
-  const variant<Types...>* w;
-
-  constexpr bool operator()(SizeT<absl::variant_npos> /*v_i*/) const {
-    return false;
-  }
-
-  template <std::size_t I>
-  constexpr bool operator()(SizeT<I> /*v_i*/) const {
-    return VariantCoreAccess::Access<I>(*v) != VariantCoreAccess::Access<I>(*w);
-  }
-};
-
-template <class... Types>
-struct LessThanOp {
-  const variant<Types...>* v;
-  const variant<Types...>* w;
-
-  constexpr bool operator()(SizeT<absl::variant_npos> /*v_i*/) const {
-    return false;
-  }
-
-  template <std::size_t I>
-  constexpr bool operator()(SizeT<I> /*v_i*/) const {
-    return VariantCoreAccess::Access<I>(*v) < VariantCoreAccess::Access<I>(*w);
-  }
-};
-
-template <class... Types>
-struct GreaterThanOp {
-  const variant<Types...>* v;
-  const variant<Types...>* w;
-
-  constexpr bool operator()(SizeT<absl::variant_npos> /*v_i*/) const {
-    return false;
-  }
-
-  template <std::size_t I>
-  constexpr bool operator()(SizeT<I> /*v_i*/) const {
-    return VariantCoreAccess::Access<I>(*v) > VariantCoreAccess::Access<I>(*w);
-  }
-};
-
-template <class... Types>
-struct LessThanOrEqualsOp {
-  const variant<Types...>* v;
-  const variant<Types...>* w;
-
-  constexpr bool operator()(SizeT<absl::variant_npos> /*v_i*/) const {
-    return true;
-  }
-
-  template <std::size_t I>
-  constexpr bool operator()(SizeT<I> /*v_i*/) const {
-    return VariantCoreAccess::Access<I>(*v) <= VariantCoreAccess::Access<I>(*w);
-  }
-};
-
-template <class... Types>
-struct GreaterThanOrEqualsOp {
-  const variant<Types...>* v;
-  const variant<Types...>* w;
-
-  constexpr bool operator()(SizeT<absl::variant_npos> /*v_i*/) const {
-    return true;
-  }
-
-  template <std::size_t I>
-  constexpr bool operator()(SizeT<I> /*v_i*/) const {
-    return VariantCoreAccess::Access<I>(*v) >= VariantCoreAccess::Access<I>(*w);
-  }
-};
-
-// Precondition: v.index() == w.index();
-template <class... Types>
-struct SwapSameIndex {
-  variant<Types...>* v;
-  variant<Types...>* w;
-  template <std::size_t I>
-  void operator()(SizeT<I>) const {
-    type_traits_internal::Swap(VariantCoreAccess::Access<I>(*v),
-                               VariantCoreAccess::Access<I>(*w));
-  }
-
-  void operator()(SizeT<variant_npos>) const {}
-};
-
-// TODO(calabrese) do this from a different namespace for proper adl usage
-template <class... Types>
-struct Swap {
-  variant<Types...>* v;
-  variant<Types...>* w;
-
-  void generic_swap() const {
-    variant<Types...> tmp(std::move(*w));
-    VariantCoreAccess::Destroy(*w);
-    VariantCoreAccess::InitFrom(*w, std::move(*v));
-    VariantCoreAccess::Destroy(*v);
-    VariantCoreAccess::InitFrom(*v, std::move(tmp));
-  }
-
-  void operator()(SizeT<absl::variant_npos> /*w_i*/) const {
-    if (!v->valueless_by_exception()) {
-      generic_swap();
-    }
-  }
-
-  template <std::size_t Wi>
-  void operator()(SizeT<Wi> /*w_i*/) {
-    if (v->index() == Wi) {
-      VisitIndices<sizeof...(Types)>::Run(SwapSameIndex<Types...>{v, w}, Wi);
-    } else {
-      generic_swap();
-    }
-  }
-};
-
-template <typename Variant, typename = void, typename... Ts>
-struct VariantHashBase {
-  VariantHashBase() = delete;
-  VariantHashBase(const VariantHashBase&) = delete;
-  VariantHashBase(VariantHashBase&&) = delete;
-  VariantHashBase& operator=(const VariantHashBase&) = delete;
-  VariantHashBase& operator=(VariantHashBase&&) = delete;
-};
-
-struct VariantHashVisitor {
-  template <typename T>
-  size_t operator()(const T& t) {
-    return std::hash<T>{}(t);
-  }
-};
-
-template <typename Variant, typename... Ts>
-struct VariantHashBase<Variant,
-                       absl::enable_if_t<absl::conjunction<
-                           type_traits_internal::IsHashable<Ts>...>::value>,
-                       Ts...> {
-  using argument_type = Variant;
-  using result_type = size_t;
-  size_t operator()(const Variant& var) const {
-    type_traits_internal::AssertHashEnabled<Ts...>();
-    if (var.valueless_by_exception()) {
-      return 239799884;
-    }
-    size_t result = VisitIndices<variant_size<Variant>::value>::Run(
-        PerformVisitation<VariantHashVisitor, const Variant&>{
-            std::forward_as_tuple(var), VariantHashVisitor{}},
-        var.index());
-    // Combine the index and the hash result in order to distinguish
-    // std::variant<int, int> holding the same value as different alternative.
-    return result ^ var.index();
-  }
-};
-
-}  // namespace variant_internal
-}  // namespace absl
-
-#endif  // !defined(ABSL_HAVE_STD_VARIANT)
-#endif  // ABSL_TYPES_variant_internal_H_
diff --git a/absl/types/optional.h b/absl/types/optional.h
index ed8faf97a1d..a5b15b83726 100644
--- a/absl/types/optional.h
+++ b/absl/types/optional.h
@@ -16,757 +16,54 @@
 // optional.h
 // -----------------------------------------------------------------------------
 //
-// This header file defines the `absl::optional` type for holding a value which
-// may or may not be present. This type is useful for providing value semantics
-// for operations that may either wish to return or hold "something-or-nothing".
-//
-// Example:
-//
-//   // A common way to signal operation failure is to provide an output
-//   // parameter and a bool return type:
-//   bool AcquireResource(const Input&, Resource * out);
-//
-//   // Providing an absl::optional return type provides a cleaner API:
-//   absl::optional<Resource> AcquireResource(const Input&);
-//
-// `absl::optional` is a C++11 compatible version of the C++17 `std::optional`
-// abstraction and is designed to be a drop-in replacement for code compliant
-// with C++17.
+// Historical note: Abseil once provided an implementation of `absl::optional`
+// as a polyfill for `std::optional` prior to C++17. Now that C++17 is required,
+// `absl::optional` is an alias for `std::optional`.
+
 #ifndef ABSL_TYPES_OPTIONAL_H_
 #define ABSL_TYPES_OPTIONAL_H_
 
-#include "absl/base/config.h"   // TODO(calabrese) IWYU removal?
-#include "absl/utility/utility.h"
-
-#ifdef ABSL_HAVE_STD_OPTIONAL
-
-#include <optional>  // IWYU pragma: export
-
-namespace absl {
-using std::bad_optional_access;
-using std::optional;
-using std::make_optional;
-using std::nullopt_t;
-using std::nullopt;
-}  // namespace absl
-
-#else  // ABSL_HAVE_STD_OPTIONAL
-
-#include <cassert>
-#include <functional>
 #include <initializer_list>
-#include <type_traits>
-#include <utility>
+#include <optional>
 
-#include "absl/base/attributes.h"
-#include "absl/base/internal/inline_variable.h"
-#include "absl/meta/type_traits.h"
-#include "absl/types/bad_optional_access.h"
-#include "absl/types/internal/optional.h"
+#include "absl/base/config.h"
+#include "absl/base/macros.h"
+#include "absl/utility/utility.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
-// nullopt_t
-//
-// Class type for `absl::nullopt` used to indicate an `absl::optional<T>` type
-// that does not contain a value.
-struct nullopt_t {
-  // It must not be default-constructible to avoid ambiguity for opt = {}.
-  explicit constexpr nullopt_t(optional_internal::init_t) noexcept {}
-};
-
-// nullopt
-//
-// A tag constant of type `absl::nullopt_t` used to indicate an empty
-// `absl::optional` in certain functions, such as construction or assignment.
-ABSL_INTERNAL_INLINE_CONSTEXPR(nullopt_t, nullopt,
-                               nullopt_t(optional_internal::init_t()));
-
-// -----------------------------------------------------------------------------
-// absl::optional
-// -----------------------------------------------------------------------------
-//
-// A value of type `absl::optional<T>` holds either a value of `T` or an
-// "empty" value.  When it holds a value of `T`, it stores it as a direct
-// sub-object, so `sizeof(optional<T>)` is approximately
-// `sizeof(T) + sizeof(bool)`.
-//
-// This implementation is based on the specification in the latest draft of the
-// C++17 `std::optional` specification as of May 2017, section 20.6.
-//
-// Differences between `absl::optional<T>` and `std::optional<T>` include:
-//
-//    * `constexpr` is not used for non-const member functions.
-//      (dependency on some differences between C++11 and C++14.)
-//    * `absl::nullopt` and `absl::in_place` are not declared `constexpr`. We
-//      need the inline variable support in C++17 for external linkage.
-//    * Throws `absl::bad_optional_access` instead of
-//      `std::bad_optional_access`.
-//    * `make_optional()` cannot be declared `constexpr` due to the absence of
-//      guaranteed copy elision.
-//    * The move constructor's `noexcept` specification is stronger, i.e. if the
-//      default allocator is non-throwing (via setting
-//      `ABSL_ALLOCATOR_NOTHROW`), it evaluates to `noexcept(true)`, because
-//      we assume
-//       a) move constructors should only throw due to allocation failure and
-//       b) if T's move constructor allocates, it uses the same allocation
-//          function as the default allocator.
-//
-template <typename T>
-class optional : private optional_internal::optional_data<T>,
-                 private optional_internal::optional_ctor_base<
-                     optional_internal::ctor_copy_traits<T>::traits>,
-                 private optional_internal::optional_assign_base<
-                     optional_internal::assign_copy_traits<T>::traits> {
-  using data_base = optional_internal::optional_data<T>;
-
- public:
-  typedef T value_type;
-
-  // Constructors
-
-  // Constructs an `optional` holding an empty value, NOT a default constructed
-  // `T`.
-  constexpr optional() noexcept {}
-
-  // Constructs an `optional` initialized with `nullopt` to hold an empty value.
-  constexpr optional(nullopt_t) noexcept {}  // NOLINT(runtime/explicit)
-
-  // Copy constructor, standard semantics
-  optional(const optional& src) = default;
-
-  // Move constructor, standard semantics
-  optional(optional&& src) = default;
-
-  // Constructs a non-empty `optional` direct-initialized value of type `T` from
-  // the arguments `std::forward<Args>(args)...`  within the `optional`.
-  // (The `in_place_t` is a tag used to indicate that the contained object
-  // should be constructed in-place.)
-  template <typename InPlaceT, typename... Args,
-            absl::enable_if_t<absl::conjunction<
-                std::is_same<InPlaceT, in_place_t>,
-                std::is_constructible<T, Args&&...> >::value>* = nullptr>
-  constexpr explicit optional(InPlaceT, Args&&... args)
-      : data_base(in_place_t(), absl::forward<Args>(args)...) {}
-
-  // Constructs a non-empty `optional` direct-initialized value of type `T` from
-  // the arguments of an initializer_list and `std::forward<Args>(args)...`.
-  // (The `in_place_t` is a tag used to indicate that the contained object
-  // should be constructed in-place.)
-  template <typename U, typename... Args,
-            typename = typename std::enable_if<std::is_constructible<
-                T, std::initializer_list<U>&, Args&&...>::value>::type>
-  constexpr explicit optional(in_place_t, std::initializer_list<U> il,
-                              Args&&... args)
-      : data_base(in_place_t(), il, absl::forward<Args>(args)...) {
-  }
-
-  // Value constructor (implicit)
-  template <
-      typename U = T,
-      typename std::enable_if<
-          absl::conjunction<absl::negation<std::is_same<
-                                in_place_t, typename std::decay<U>::type> >,
-                            absl::negation<std::is_same<
-                                optional<T>, typename std::decay<U>::type> >,
-                            std::is_convertible<U&&, T>,
-                            std::is_constructible<T, U&&> >::value,
-          bool>::type = false>
-  constexpr optional(U&& v) : data_base(in_place_t(), absl::forward<U>(v)) {}
-
-  // Value constructor (explicit)
-  template <
-      typename U = T,
-      typename std::enable_if<
-          absl::conjunction<absl::negation<std::is_same<
-                                in_place_t, typename std::decay<U>::type>>,
-                            absl::negation<std::is_same<
-                                optional<T>, typename std::decay<U>::type>>,
-                            absl::negation<std::is_convertible<U&&, T>>,
-                            std::is_constructible<T, U&&>>::value,
-          bool>::type = false>
-  explicit constexpr optional(U&& v)
-      : data_base(in_place_t(), absl::forward<U>(v)) {}
-
-  // Converting copy constructor (implicit)
-  template <typename U,
-            typename std::enable_if<
-                absl::conjunction<
-                    absl::negation<std::is_same<T, U> >,
-                    std::is_constructible<T, const U&>,
-                    absl::negation<
-                        optional_internal::
-                            is_constructible_convertible_from_optional<T, U> >,
-                    std::is_convertible<const U&, T> >::value,
-                bool>::type = false>
-  optional(const optional<U>& rhs) {
-    if (rhs) {
-      this->construct(*rhs);
-    }
-  }
-
-  // Converting copy constructor (explicit)
-  template <typename U,
-            typename std::enable_if<
-                absl::conjunction<
-                    absl::negation<std::is_same<T, U>>,
-                    std::is_constructible<T, const U&>,
-                    absl::negation<
-                        optional_internal::
-                            is_constructible_convertible_from_optional<T, U>>,
-                    absl::negation<std::is_convertible<const U&, T>>>::value,
-                bool>::type = false>
-  explicit optional(const optional<U>& rhs) {
-    if (rhs) {
-      this->construct(*rhs);
-    }
-  }
-
-  // Converting move constructor (implicit)
-  template <typename U,
-            typename std::enable_if<
-                absl::conjunction<
-                    absl::negation<std::is_same<T, U> >,
-                    std::is_constructible<T, U&&>,
-                    absl::negation<
-                        optional_internal::
-                            is_constructible_convertible_from_optional<T, U> >,
-                    std::is_convertible<U&&, T> >::value,
-                bool>::type = false>
-  optional(optional<U>&& rhs) {
-    if (rhs) {
-      this->construct(std::move(*rhs));
-    }
-  }
-
-  // Converting move constructor (explicit)
-  template <
-      typename U,
-      typename std::enable_if<
-          absl::conjunction<
-              absl::negation<std::is_same<T, U>>, std::is_constructible<T, U&&>,
-              absl::negation<
-                  optional_internal::is_constructible_convertible_from_optional<
-                      T, U>>,
-              absl::negation<std::is_convertible<U&&, T>>>::value,
-          bool>::type = false>
-  explicit optional(optional<U>&& rhs) {
-    if (rhs) {
-      this->construct(std::move(*rhs));
-    }
-  }
-
-  // Destructor. Trivial if `T` is trivially destructible.
-  ~optional() = default;
-
-  // Assignment Operators
-
-  // Assignment from `nullopt`
-  //
-  // Example:
-  //
-  //   struct S { int value; };
-  //   optional<S> opt = absl::nullopt;  // Could also use opt = { };
-  optional& operator=(nullopt_t) noexcept {
-    this->destruct();
-    return *this;
-  }
-
-  // Copy assignment operator, standard semantics
-  optional& operator=(const optional& src) = default;
-
-  // Move assignment operator, standard semantics
-  optional& operator=(optional&& src) = default;
-
-  // Value assignment operators
-  template <
-      typename U = T,
-      typename = typename std::enable_if<absl::conjunction<
-          absl::negation<
-              std::is_same<optional<T>, typename std::decay<U>::type>>,
-          absl::negation<
-              absl::conjunction<std::is_scalar<T>,
-                                std::is_same<T, typename std::decay<U>::type>>>,
-          std::is_constructible<T, U>, std::is_assignable<T&, U>>::value>::type>
-  optional& operator=(U&& v) {
-    this->assign(std::forward<U>(v));
-    return *this;
-  }
-
-  template <
-      typename U,
-      typename = typename std::enable_if<absl::conjunction<
-          absl::negation<std::is_same<T, U>>,
-          std::is_constructible<T, const U&>, std::is_assignable<T&, const U&>,
-          absl::negation<
-              optional_internal::
-                  is_constructible_convertible_assignable_from_optional<
-                      T, U>>>::value>::type>
-  optional& operator=(const optional<U>& rhs) {
-    if (rhs) {
-      this->assign(*rhs);
-    } else {
-      this->destruct();
-    }
-    return *this;
-  }
-
-  template <typename U,
-            typename = typename std::enable_if<absl::conjunction<
-                absl::negation<std::is_same<T, U>>, std::is_constructible<T, U>,
-                std::is_assignable<T&, U>,
-                absl::negation<
-                    optional_internal::
-                        is_constructible_convertible_assignable_from_optional<
-                            T, U>>>::value>::type>
-  optional& operator=(optional<U>&& rhs) {
-    if (rhs) {
-      this->assign(std::move(*rhs));
-    } else {
-      this->destruct();
-    }
-    return *this;
-  }
-
-  // Modifiers
-
-  // optional::reset()
-  //
-  // Destroys the inner `T` value of an `absl::optional` if one is present.
-  ABSL_ATTRIBUTE_REINITIALIZES void reset() noexcept { this->destruct(); }
-
-  // optional::emplace()
-  //
-  // (Re)constructs the underlying `T` in-place with the given forwarded
-  // arguments.
-  //
-  // Example:
-  //
-  //   optional<Foo> opt;
-  //   opt.emplace(arg1,arg2,arg3);  // Constructs Foo(arg1,arg2,arg3)
-  //
-  // If the optional is non-empty, and the `args` refer to subobjects of the
-  // current object, then behaviour is undefined, because the current object
-  // will be destructed before the new object is constructed with `args`.
-  template <typename... Args,
-            typename = typename std::enable_if<
-                std::is_constructible<T, Args&&...>::value>::type>
-  T& emplace(Args&&... args) {
-    this->destruct();
-    this->construct(std::forward<Args>(args)...);
-    return reference();
-  }
-
-  // Emplace reconstruction overload for an initializer list and the given
-  // forwarded arguments.
-  //
-  // Example:
-  //
-  //   struct Foo {
-  //     Foo(std::initializer_list<int>);
-  //   };
-  //
-  //   optional<Foo> opt;
-  //   opt.emplace({1,2,3});  // Constructs Foo({1,2,3})
-  template <typename U, typename... Args,
-            typename = typename std::enable_if<std::is_constructible<
-                T, std::initializer_list<U>&, Args&&...>::value>::type>
-  T& emplace(std::initializer_list<U> il, Args&&... args) {
-    this->destruct();
-    this->construct(il, std::forward<Args>(args)...);
-    return reference();
-  }
-
-  // Swaps
-
-  // Swap, standard semantics
-  void swap(optional& rhs) noexcept(
-      std::is_nothrow_move_constructible<T>::value&&
-          type_traits_internal::IsNothrowSwappable<T>::value) {
-    if (*this) {
-      if (rhs) {
-        type_traits_internal::Swap(**this, *rhs);
-      } else {
-        rhs.construct(std::move(**this));
-        this->destruct();
-      }
-    } else {
-      if (rhs) {
-        this->construct(std::move(*rhs));
-        rhs.destruct();
-      } else {
-        // No effect (swap(disengaged, disengaged)).
-      }
-    }
-  }
-
-  // Observers
-
-  // optional::operator->()
-  //
-  // Accesses the underlying `T` value's member `m` of an `optional`. If the
-  // `optional` is empty, behavior is undefined.
-  //
-  // If you need myOpt->foo in constexpr, use (*myOpt).foo instead.
-  const T* operator->() const {
-    assert(this->engaged_);
-    return std::addressof(this->data_);
-  }
-  T* operator->() {
-    assert(this->engaged_);
-    return std::addressof(this->data_);
-  }
-
-  // optional::operator*()
-  //
-  // Accesses the underlying `T` value of an `optional`. If the `optional` is
-  // empty, behavior is undefined.
-  constexpr const T& operator*() const& {
-    return ABSL_ASSERT(this->engaged_), reference();
-  }
-  T& operator*() & {
-    assert(this->engaged_);
-    return reference();
-  }
-  constexpr const T&& operator*() const && {
-    return absl::move(reference());
-  }
-  T&& operator*() && {
-    assert(this->engaged_);
-    return std::move(reference());
-  }
-
-  // optional::operator bool()
-  //
-  // Returns false if and only if the `optional` is empty.
-  //
-  //   if (opt) {
-  //     // do something with opt.value();
-  //   } else {
-  //     // opt is empty.
-  //   }
-  //
-  constexpr explicit operator bool() const noexcept { return this->engaged_; }
-
-  // optional::has_value()
-  //
-  // Determines whether the `optional` contains a value. Returns `false` if and
-  // only if `*this` is empty.
-  constexpr bool has_value() const noexcept { return this->engaged_; }
-
-// Suppress bogus warning on MSVC: MSVC complains call to reference() after
-// throw_bad_optional_access() is unreachable.
-#ifdef _MSC_VER
-#pragma warning(push)
-#pragma warning(disable : 4702)
-#endif  // _MSC_VER
-  // optional::value()
-  //
-  // Returns a reference to an `optional`s underlying value. The constness
-  // and lvalue/rvalue-ness of the `optional` is preserved to the view of
-  // the `T` sub-object. Throws `absl::bad_optional_access` when the `optional`
-  // is empty.
-  constexpr const T& value() const & {
-    return static_cast<bool>(*this)
-               ? reference()
-               : (optional_internal::throw_bad_optional_access(), reference());
-  }
-  T& value() & {
-    return static_cast<bool>(*this)
-               ? reference()
-               : (optional_internal::throw_bad_optional_access(), reference());
-  }
-  T&& value() && {  // NOLINT(build/c++11)
-    return std::move(
-        static_cast<bool>(*this)
-            ? reference()
-            : (optional_internal::throw_bad_optional_access(), reference()));
-  }
-  constexpr const T&& value() const && {  // NOLINT(build/c++11)
-    return absl::move(
-        static_cast<bool>(*this)
-            ? reference()
-            : (optional_internal::throw_bad_optional_access(), reference()));
-  }
-#ifdef _MSC_VER
-#pragma warning(pop)
-#endif  // _MSC_VER
-
-  // optional::value_or()
-  //
-  // Returns either the value of `T` or a passed default `v` if the `optional`
-  // is empty.
-  template <typename U>
-  constexpr T value_or(U&& v) const& {
-    static_assert(std::is_copy_constructible<value_type>::value,
-                  "optional<T>::value_or: T must be copy constructible");
-    static_assert(std::is_convertible<U&&, value_type>::value,
-                  "optional<T>::value_or: U must be convertible to T");
-    return static_cast<bool>(*this)
-               ? **this
-               : static_cast<T>(absl::forward<U>(v));
-  }
-  template <typename U>
-  T value_or(U&& v) && {  // NOLINT(build/c++11)
-    static_assert(std::is_move_constructible<value_type>::value,
-                  "optional<T>::value_or: T must be move constructible");
-    static_assert(std::is_convertible<U&&, value_type>::value,
-                  "optional<T>::value_or: U must be convertible to T");
-    return static_cast<bool>(*this) ? std::move(**this)
-                                    : static_cast<T>(std::forward<U>(v));
-  }
-
- private:
-  // Private accessors for internal storage viewed as reference to T.
-  constexpr const T& reference() const { return this->data_; }
-  T& reference() { return this->data_; }
+using bad_optional_access ABSL_REFACTOR_INLINE
+    = std::bad_optional_access;
 
-  // T constraint checks.  You can't have an optional of nullopt_t, in_place_t
-  // or a reference.
-  static_assert(
-      !std::is_same<nullopt_t, typename std::remove_cv<T>::type>::value,
-      "optional<nullopt_t> is not allowed.");
-  static_assert(
-      !std::is_same<in_place_t, typename std::remove_cv<T>::type>::value,
-      "optional<in_place_t> is not allowed.");
-  static_assert(!std::is_reference<T>::value,
-                "optional<reference> is not allowed.");
-};
-
-// Non-member functions
-
-// swap()
-//
-// Performs a swap between two `absl::optional` objects, using standard
-// semantics.
-template <typename T, typename std::enable_if<
-                          std::is_move_constructible<T>::value &&
-                              type_traits_internal::IsSwappable<T>::value,
-                          bool>::type = false>
-void swap(optional<T>& a, optional<T>& b) noexcept(noexcept(a.swap(b))) {
-  a.swap(b);
-}
-
-// make_optional()
-//
-// Creates a non-empty `optional<T>` where the type of `T` is deduced. An
-// `absl::optional` can also be explicitly instantiated with
-// `make_optional<T>(v)`.
-//
-// Note: `make_optional()` constructions may be declared `constexpr` for
-// trivially copyable types `T`. Non-trivial types require copy elision
-// support in C++17 for `make_optional` to support `constexpr` on such
-// non-trivial types.
-//
-// Example:
-//
-//   constexpr absl::optional<int> opt = absl::make_optional(1);
-//   static_assert(opt.value() == 1, "");
 template <typename T>
-constexpr optional<typename std::decay<T>::type> make_optional(T&& v) {
-  return optional<typename std::decay<T>::type>(absl::forward<T>(v));
+ABSL_REFACTOR_INLINE constexpr decltype(std::make_optional(
+    std::declval<T>())) make_optional(T&& value) {
+  return std::make_optional(std::forward<T>(value));
 }
 
 template <typename T, typename... Args>
-constexpr optional<T> make_optional(Args&&... args) {
-  return optional<T>(in_place_t(), absl::forward<Args>(args)...);
+constexpr decltype(std::make_optional<T>(
+    std::declval<Args>()...)) make_optional(Args&&... args) {
+  return std::make_optional<T>(std::forward<Args>(args)...);
 }
 
 template <typename T, typename U, typename... Args>
-constexpr optional<T> make_optional(std::initializer_list<U> il,
-                                    Args&&... args) {
-  return optional<T>(in_place_t(), il,
-                     absl::forward<Args>(args)...);
-}
-
-// Relational operators [optional.relops]
-
-// Empty optionals are considered equal to each other and less than non-empty
-// optionals. Supports relations between optional<T> and optional<U>, between
-// optional<T> and U, and between optional<T> and nullopt.
-//
-// Note: We're careful to support T having non-bool relationals.
-
-// Requires: The expression, e.g. "*x == *y" shall be well-formed and its result
-// shall be convertible to bool.
-// The C++17 (N4606) "Returns:" statements are translated into
-// code in an obvious way here, and the original text retained as function docs.
-// Returns: If bool(x) != bool(y), false; otherwise if bool(x) == false, true;
-// otherwise *x == *y.
-template <typename T, typename U>
-constexpr auto operator==(const optional<T>& x, const optional<U>& y)
-    -> decltype(optional_internal::convertible_to_bool(*x == *y)) {
-  return static_cast<bool>(x) != static_cast<bool>(y)
-             ? false
-             : static_cast<bool>(x) == false ? true
-                                             : static_cast<bool>(*x == *y);
+constexpr decltype(std::make_optional<T>(
+    std::declval<std::initializer_list<U>>(),
+    std::declval<Args>()...)) make_optional(std::initializer_list<U> il,
+                                            Args&&... args) {
+  return std::make_optional<T>(il, std::forward<Args>(args)...);
 }
 
-// Returns: If bool(x) != bool(y), true; otherwise, if bool(x) == false, false;
-// otherwise *x != *y.
-template <typename T, typename U>
-constexpr auto operator!=(const optional<T>& x, const optional<U>& y)
-    -> decltype(optional_internal::convertible_to_bool(*x != *y)) {
-  return static_cast<bool>(x) != static_cast<bool>(y)
-             ? true
-             : static_cast<bool>(x) == false ? false
-                                             : static_cast<bool>(*x != *y);
-}
-// Returns: If !y, false; otherwise, if !x, true; otherwise *x < *y.
-template <typename T, typename U>
-constexpr auto operator<(const optional<T>& x, const optional<U>& y)
-    -> decltype(optional_internal::convertible_to_bool(*x < *y)) {
-  return !y ? false : !x ? true : static_cast<bool>(*x < *y);
-}
-// Returns: If !x, false; otherwise, if !y, true; otherwise *x > *y.
-template <typename T, typename U>
-constexpr auto operator>(const optional<T>& x, const optional<U>& y)
-    -> decltype(optional_internal::convertible_to_bool(*x > *y)) {
-  return !x ? false : !y ? true : static_cast<bool>(*x > *y);
-}
-// Returns: If !x, true; otherwise, if !y, false; otherwise *x <= *y.
-template <typename T, typename U>
-constexpr auto operator<=(const optional<T>& x, const optional<U>& y)
-    -> decltype(optional_internal::convertible_to_bool(*x <= *y)) {
-  return !x ? true : !y ? false : static_cast<bool>(*x <= *y);
-}
-// Returns: If !y, true; otherwise, if !x, false; otherwise *x >= *y.
-template <typename T, typename U>
-constexpr auto operator>=(const optional<T>& x, const optional<U>& y)
-    -> decltype(optional_internal::convertible_to_bool(*x >= *y)) {
-  return !y ? true : !x ? false : static_cast<bool>(*x >= *y);
-}
-
-// Comparison with nullopt [optional.nullops]
-// The C++17 (N4606) "Returns:" statements are used directly here.
-template <typename T>
-constexpr bool operator==(const optional<T>& x, nullopt_t) noexcept {
-  return !x;
-}
-template <typename T>
-constexpr bool operator==(nullopt_t, const optional<T>& x) noexcept {
-  return !x;
-}
-template <typename T>
-constexpr bool operator!=(const optional<T>& x, nullopt_t) noexcept {
-  return static_cast<bool>(x);
-}
-template <typename T>
-constexpr bool operator!=(nullopt_t, const optional<T>& x) noexcept {
-  return static_cast<bool>(x);
-}
-template <typename T>
-constexpr bool operator<(const optional<T>&, nullopt_t) noexcept {
-  return false;
-}
-template <typename T>
-constexpr bool operator<(nullopt_t, const optional<T>& x) noexcept {
-  return static_cast<bool>(x);
-}
-template <typename T>
-constexpr bool operator<=(const optional<T>& x, nullopt_t) noexcept {
-  return !x;
-}
-template <typename T>
-constexpr bool operator<=(nullopt_t, const optional<T>&) noexcept {
-  return true;
-}
-template <typename T>
-constexpr bool operator>(const optional<T>& x, nullopt_t) noexcept {
-  return static_cast<bool>(x);
-}
-template <typename T>
-constexpr bool operator>(nullopt_t, const optional<T>&) noexcept {
-  return false;
-}
-template <typename T>
-constexpr bool operator>=(const optional<T>&, nullopt_t) noexcept {
-  return true;
-}
-template <typename T>
-constexpr bool operator>=(nullopt_t, const optional<T>& x) noexcept {
-  return !x;
-}
+using std::nullopt ABSL_REFACTOR_INLINE;
 
-// Comparison with T [optional.comp_with_t]
+using nullopt_t ABSL_REFACTOR_INLINE
+    = std::nullopt_t;
 
-// Requires: The expression, e.g. "*x == v" shall be well-formed and its result
-// shall be convertible to bool.
-// The C++17 (N4606) "Equivalent to:" statements are used directly here.
-template <typename T, typename U>
-constexpr auto operator==(const optional<T>& x, const U& v)
-    -> decltype(optional_internal::convertible_to_bool(*x == v)) {
-  return static_cast<bool>(x) ? static_cast<bool>(*x == v) : false;
-}
-template <typename T, typename U>
-constexpr auto operator==(const U& v, const optional<T>& x)
-    -> decltype(optional_internal::convertible_to_bool(v == *x)) {
-  return static_cast<bool>(x) ? static_cast<bool>(v == *x) : false;
-}
-template <typename T, typename U>
-constexpr auto operator!=(const optional<T>& x, const U& v)
-    -> decltype(optional_internal::convertible_to_bool(*x != v)) {
-  return static_cast<bool>(x) ? static_cast<bool>(*x != v) : true;
-}
-template <typename T, typename U>
-constexpr auto operator!=(const U& v, const optional<T>& x)
-    -> decltype(optional_internal::convertible_to_bool(v != *x)) {
-  return static_cast<bool>(x) ? static_cast<bool>(v != *x) : true;
-}
-template <typename T, typename U>
-constexpr auto operator<(const optional<T>& x, const U& v)
-    -> decltype(optional_internal::convertible_to_bool(*x < v)) {
-  return static_cast<bool>(x) ? static_cast<bool>(*x < v) : true;
-}
-template <typename T, typename U>
-constexpr auto operator<(const U& v, const optional<T>& x)
-    -> decltype(optional_internal::convertible_to_bool(v < *x)) {
-  return static_cast<bool>(x) ? static_cast<bool>(v < *x) : false;
-}
-template <typename T, typename U>
-constexpr auto operator<=(const optional<T>& x, const U& v)
-    -> decltype(optional_internal::convertible_to_bool(*x <= v)) {
-  return static_cast<bool>(x) ? static_cast<bool>(*x <= v) : true;
-}
-template <typename T, typename U>
-constexpr auto operator<=(const U& v, const optional<T>& x)
-    -> decltype(optional_internal::convertible_to_bool(v <= *x)) {
-  return static_cast<bool>(x) ? static_cast<bool>(v <= *x) : false;
-}
-template <typename T, typename U>
-constexpr auto operator>(const optional<T>& x, const U& v)
-    -> decltype(optional_internal::convertible_to_bool(*x > v)) {
-  return static_cast<bool>(x) ? static_cast<bool>(*x > v) : false;
-}
-template <typename T, typename U>
-constexpr auto operator>(const U& v, const optional<T>& x)
-    -> decltype(optional_internal::convertible_to_bool(v > *x)) {
-  return static_cast<bool>(x) ? static_cast<bool>(v > *x) : true;
-}
-template <typename T, typename U>
-constexpr auto operator>=(const optional<T>& x, const U& v)
-    -> decltype(optional_internal::convertible_to_bool(*x >= v)) {
-  return static_cast<bool>(x) ? static_cast<bool>(*x >= v) : false;
-}
-template <typename T, typename U>
-constexpr auto operator>=(const U& v, const optional<T>& x)
-    -> decltype(optional_internal::convertible_to_bool(v >= *x)) {
-  return static_cast<bool>(x) ? static_cast<bool>(v >= *x) : true;
-}
+using std::optional ABSL_REFACTOR_INLINE;
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
-namespace std {
-
-// std::hash specialization for absl::optional.
-template <typename T>
-struct hash<absl::optional<T> >
-    : absl::optional_internal::optional_hash_base<T> {};
-
-}  // namespace std
-
-#undef ABSL_MSVC_CONSTEXPR_BUG_IN_UNION_LIKE_CLASS
-
-#endif  // ABSL_HAVE_STD_OPTIONAL
-
 #endif  // ABSL_TYPES_OPTIONAL_H_
diff --git a/absl/types/optional_exception_safety_test.cc b/absl/types/optional_exception_safety_test.cc
deleted file mode 100644
index f99e35c0544..00000000000
--- a/absl/types/optional_exception_safety_test.cc
+++ /dev/null
@@ -1,290 +0,0 @@
-// Copyright 2017 The Abseil Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-#include "absl/types/optional.h"
-
-#include "absl/base/config.h"
-
-// This test is a no-op when absl::optional is an alias for std::optional and
-// when exceptions are not enabled.
-#if !defined(ABSL_HAVE_STD_OPTIONAL) && defined(ABSL_HAVE_EXCEPTIONS)
-
-#include "gtest/gtest.h"
-#include "absl/base/internal/exception_safety_testing.h"
-
-namespace absl {
-
-namespace {
-
-using ::testing::AssertionFailure;
-using ::testing::AssertionResult;
-using ::testing::AssertionSuccess;
-using ::testing::MakeExceptionSafetyTester;
-
-using Thrower = testing::ThrowingValue<testing::TypeSpec::kEverythingThrows>;
-using Optional = absl::optional<Thrower>;
-
-using MoveThrower = testing::ThrowingValue<testing::TypeSpec::kNoThrowMove>;
-using MoveOptional = absl::optional<MoveThrower>;
-
-constexpr int kInitialInteger = 5;
-constexpr int kUpdatedInteger = 10;
-
-template <typename OptionalT>
-bool ValueThrowsBadOptionalAccess(const OptionalT& optional) try {
-  return (static_cast<void>(optional.value()), false);
-} catch (const absl::bad_optional_access&) {
-  return true;
-}
-
-template <typename OptionalT>
-AssertionResult OptionalInvariants(OptionalT* optional_ptr) {
-  // Check the current state post-throw for validity
-  auto& optional = *optional_ptr;
-
-  if (optional.has_value() && ValueThrowsBadOptionalAccess(optional)) {
-    return AssertionFailure()
-           << "Optional with value should not throw bad_optional_access when "
-              "accessing the value.";
-  }
-  if (!optional.has_value() && !ValueThrowsBadOptionalAccess(optional)) {
-    return AssertionFailure()
-           << "Optional without a value should throw bad_optional_access when "
-              "accessing the value.";
-  }
-
-  // Reset to a known state
-  optional.reset();
-
-  // Confirm that the known post-reset state is valid
-  if (optional.has_value()) {
-    return AssertionFailure()
-           << "Optional should not contain a value after being reset.";
-  }
-  if (!ValueThrowsBadOptionalAccess(optional)) {
-    return AssertionFailure() << "Optional should throw bad_optional_access "
-                                 "when accessing the value after being reset.";
-  }
-
-  return AssertionSuccess();
-}
-
-template <typename OptionalT>
-AssertionResult CheckDisengaged(OptionalT* optional_ptr) {
-  auto& optional = *optional_ptr;
-
-  if (optional.has_value()) {
-    return AssertionFailure()
-           << "Expected optional to not contain a value but a value was found.";
-  }
-
-  return AssertionSuccess();
-}
-
-template <typename OptionalT>
-AssertionResult CheckEngaged(OptionalT* optional_ptr) {
-  auto& optional = *optional_ptr;
-
-  if (!optional.has_value()) {
-    return AssertionFailure()
-           << "Expected optional to contain a value but no value was found.";
-  }
-
-  return AssertionSuccess();
-}
-
-TEST(OptionalExceptionSafety, ThrowingConstructors) {
-  auto thrower_nonempty = Optional(Thrower(kInitialInteger));
-  testing::TestThrowingCtor<Optional>(thrower_nonempty);
-
-  auto integer_nonempty = absl::optional<int>(kInitialInteger);
-  testing::TestThrowingCtor<Optional>(integer_nonempty);
-  testing::TestThrowingCtor<Optional>(std::move(integer_nonempty));  // NOLINT
-
-  testing::TestThrowingCtor<Optional>(kInitialInteger);
-  using ThrowerVec = std::vector<Thrower, testing::ThrowingAllocator<Thrower>>;
-  testing::TestThrowingCtor<absl::optional<ThrowerVec>>(
-      absl::in_place,
-      std::initializer_list<Thrower>{Thrower(), Thrower(), Thrower()},
-      testing::ThrowingAllocator<Thrower>());
-}
-
-TEST(OptionalExceptionSafety, NothrowConstructors) {
-  // This constructor is marked noexcept. If it throws, the program will
-  // terminate.
-  testing::TestThrowingCtor<MoveOptional>(MoveOptional(kUpdatedInteger));
-}
-
-TEST(OptionalExceptionSafety, Emplace) {
-  // Test the basic guarantee plus test the result of optional::has_value()
-  // is false in all cases
-  auto disengaged_test = MakeExceptionSafetyTester().WithContracts(
-      OptionalInvariants<Optional>, CheckDisengaged<Optional>);
-  auto disengaged_test_empty = disengaged_test.WithInitialValue(Optional());
-  auto disengaged_test_nonempty =
-      disengaged_test.WithInitialValue(Optional(kInitialInteger));
-
-  auto emplace_thrower_directly = [](Optional* optional_ptr) {
-    optional_ptr->emplace(kUpdatedInteger);
-  };
-  EXPECT_TRUE(disengaged_test_empty.Test(emplace_thrower_directly));
-  EXPECT_TRUE(disengaged_test_nonempty.Test(emplace_thrower_directly));
-
-  auto emplace_thrower_copy = [](Optional* optional_ptr) {
-    auto thrower = Thrower(kUpdatedInteger, testing::nothrow_ctor);
-    optional_ptr->emplace(thrower);
-  };
-  EXPECT_TRUE(disengaged_test_empty.Test(emplace_thrower_copy));
-  EXPECT_TRUE(disengaged_test_nonempty.Test(emplace_thrower_copy));
-}
-
-TEST(OptionalExceptionSafety, EverythingThrowsSwap) {
-  // Test the basic guarantee plus test the result of optional::has_value()
-  // remains the same
-  auto test =
-      MakeExceptionSafetyTester().WithContracts(OptionalInvariants<Optional>);
-  auto disengaged_test_empty = test.WithInitialValue(Optional())
-                                   .WithContracts(CheckDisengaged<Optional>);
-  auto engaged_test_nonempty = test.WithInitialValue(Optional(kInitialInteger))
-                                   .WithContracts(CheckEngaged<Optional>);
-
-  auto swap_empty = [](Optional* optional_ptr) {
-    auto empty = Optional();
-    optional_ptr->swap(empty);
-  };
-  EXPECT_TRUE(engaged_test_nonempty.Test(swap_empty));
-
-  auto swap_nonempty = [](Optional* optional_ptr) {
-    auto nonempty =
-        Optional(absl::in_place, kUpdatedInteger, testing::nothrow_ctor);
-    optional_ptr->swap(nonempty);
-  };
-  EXPECT_TRUE(disengaged_test_empty.Test(swap_nonempty));
-  EXPECT_TRUE(engaged_test_nonempty.Test(swap_nonempty));
-}
-
-TEST(OptionalExceptionSafety, NoThrowMoveSwap) {
-  // Tests the nothrow guarantee for optional of T with non-throwing move
-  {
-    auto empty = MoveOptional();
-    auto nonempty = MoveOptional(kInitialInteger);
-    EXPECT_TRUE(testing::TestNothrowOp([&]() { nonempty.swap(empty); }));
-  }
-  {
-    auto nonempty = MoveOptional(kUpdatedInteger);
-    auto empty = MoveOptional();
-    EXPECT_TRUE(testing::TestNothrowOp([&]() { empty.swap(nonempty); }));
-  }
-  {
-    auto nonempty_from = MoveOptional(kUpdatedInteger);
-    auto nonempty_to = MoveOptional(kInitialInteger);
-    EXPECT_TRUE(
-        testing::TestNothrowOp([&]() { nonempty_to.swap(nonempty_from); }));
-  }
-}
-
-TEST(OptionalExceptionSafety, CopyAssign) {
-  // Test the basic guarantee plus test the result of optional::has_value()
-  // remains the same
-  auto test =
-      MakeExceptionSafetyTester().WithContracts(OptionalInvariants<Optional>);
-  auto disengaged_test_empty = test.WithInitialValue(Optional())
-                                   .WithContracts(CheckDisengaged<Optional>);
-  auto engaged_test_nonempty = test.WithInitialValue(Optional(kInitialInteger))
-                                   .WithContracts(CheckEngaged<Optional>);
-
-  auto copyassign_nonempty = [](Optional* optional_ptr) {
-    auto nonempty =
-        Optional(absl::in_place, kUpdatedInteger, testing::nothrow_ctor);
-    *optional_ptr = nonempty;
-  };
-  EXPECT_TRUE(disengaged_test_empty.Test(copyassign_nonempty));
-  EXPECT_TRUE(engaged_test_nonempty.Test(copyassign_nonempty));
-
-  auto copyassign_thrower = [](Optional* optional_ptr) {
-    auto thrower = Thrower(kUpdatedInteger, testing::nothrow_ctor);
-    *optional_ptr = thrower;
-  };
-  EXPECT_TRUE(disengaged_test_empty.Test(copyassign_thrower));
-  EXPECT_TRUE(engaged_test_nonempty.Test(copyassign_thrower));
-}
-
-TEST(OptionalExceptionSafety, MoveAssign) {
-  // Test the basic guarantee plus test the result of optional::has_value()
-  // remains the same
-  auto test =
-      MakeExceptionSafetyTester().WithContracts(OptionalInvariants<Optional>);
-  auto disengaged_test_empty = test.WithInitialValue(Optional())
-                                   .WithContracts(CheckDisengaged<Optional>);
-  auto engaged_test_nonempty = test.WithInitialValue(Optional(kInitialInteger))
-                                   .WithContracts(CheckEngaged<Optional>);
-
-  auto moveassign_empty = [](Optional* optional_ptr) {
-    auto empty = Optional();
-    *optional_ptr = std::move(empty);
-  };
-  EXPECT_TRUE(engaged_test_nonempty.Test(moveassign_empty));
-
-  auto moveassign_nonempty = [](Optional* optional_ptr) {
-    auto nonempty =
-        Optional(absl::in_place, kUpdatedInteger, testing::nothrow_ctor);
-    *optional_ptr = std::move(nonempty);
-  };
-  EXPECT_TRUE(disengaged_test_empty.Test(moveassign_nonempty));
-  EXPECT_TRUE(engaged_test_nonempty.Test(moveassign_nonempty));
-
-  auto moveassign_thrower = [](Optional* optional_ptr) {
-    auto thrower = Thrower(kUpdatedInteger, testing::nothrow_ctor);
-    *optional_ptr = std::move(thrower);
-  };
-  EXPECT_TRUE(disengaged_test_empty.Test(moveassign_thrower));
-  EXPECT_TRUE(engaged_test_nonempty.Test(moveassign_thrower));
-}
-
-TEST(OptionalExceptionSafety, NothrowMoveAssign) {
-  // Tests the nothrow guarantee for optional of T with non-throwing move
-  {
-    auto empty = MoveOptional();
-    auto nonempty = MoveOptional(kInitialInteger);
-    EXPECT_TRUE(testing::TestNothrowOp([&]() { nonempty = std::move(empty); }));
-  }
-  {
-    auto nonempty = MoveOptional(kInitialInteger);
-    auto empty = MoveOptional();
-    EXPECT_TRUE(testing::TestNothrowOp([&]() { empty = std::move(nonempty); }));
-  }
-  {
-    auto nonempty_from = MoveOptional(kUpdatedInteger);
-    auto nonempty_to = MoveOptional(kInitialInteger);
-    EXPECT_TRUE(testing::TestNothrowOp(
-        [&]() { nonempty_to = std::move(nonempty_from); }));
-  }
-  {
-    auto thrower = MoveThrower(kUpdatedInteger);
-    auto empty = MoveOptional();
-    EXPECT_TRUE(testing::TestNothrowOp([&]() { empty = std::move(thrower); }));
-  }
-  {
-    auto thrower = MoveThrower(kUpdatedInteger);
-    auto nonempty = MoveOptional(kInitialInteger);
-    EXPECT_TRUE(
-        testing::TestNothrowOp([&]() { nonempty = std::move(thrower); }));
-  }
-}
-
-}  // namespace
-
-}  // namespace absl
-
-#endif  // #if !defined(ABSL_HAVE_STD_OPTIONAL) && defined(ABSL_HAVE_EXCEPTIONS)
diff --git a/absl/types/optional_ref.h b/absl/types/optional_ref.h
new file mode 100644
index 00000000000..920fe560dda
--- /dev/null
+++ b/absl/types/optional_ref.h
@@ -0,0 +1,295 @@
+// Copyright 2026 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: optional_ref.h
+// -----------------------------------------------------------------------------
+//
+// `optional_ref<T>` provides a `std::optional`-like interface around `T*`.
+// It is similar to C++26's `std::optional<T&>`, but with slight enhancements,
+// such as the fact that it permits construction from rvalues. That is, it
+// relaxes the std::reference_constructs_from_temporary constraint. Its intent
+// is to make it easier for functions to accept nullable object addresses,
+// regardless of whether or not they point to temporaries.
+//
+// It can be constructed in the following ways:
+//   * optional_ref<T> ref;
+//   * optional_ref<T> ref = std::nullopt;
+//   * T foo; optional_ref<T> ref = foo;
+//   * std::optional<T> foo; optional_ref<T> ref = foo;
+//   * T* foo = ...; optional_ref<T> ref = foo;
+//   * optional_ref<T> foo; optional_ref<const T> ref = foo;
+//
+// Since it is trivially copyable and destructible, it should be passed by
+// value.
+//
+// Other properties:
+//   * Assignment is not allowed. Example:
+//       optional_ref<int> ref;
+//       // Compile error.
+//       ref = 2;
+//
+//   * operator bool() is intentionally not defined, as it would be error prone
+//     for optional_ref<bool>.
+//
+// Example usage, assuming some type `T` that is expensive to copy:
+//   void ProcessT(optional_ref<const T> input) {
+//     if (!input.has_value()) {
+//       // Handle empty case.
+//       return;
+//     }
+//     const T& val = *input;
+//     // Do something with val.
+//   }
+//
+//   ProcessT(std::nullopt);
+//   ProcessT(BuildT());
+
+#ifndef ABSL_TYPES_OPTIONAL_REF_H_
+#define ABSL_TYPES_OPTIONAL_REF_H_
+
+#include <cstddef>
+#include <memory>
+#include <optional>
+#include <type_traits>
+#include <utility>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/hardening.h"
+#include "absl/base/macros.h"
+#include "absl/base/optimization.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+template <typename T>
+class optional_ref {
+  template <typename U>
+  using EnableIfConvertibleFrom =
+      std::enable_if_t<std::is_convertible_v<U*, T*>>;
+
+ public:
+  using value_type = T;
+
+  constexpr optional_ref() : ptr_(nullptr) {}
+  constexpr optional_ref(  // NOLINT(google-explicit-constructor)
+      std::nullopt_t)
+      : ptr_(nullptr) {}
+
+  // Constructor given a concrete value.
+  constexpr optional_ref(  // NOLINT(google-explicit-constructor)
+      T& input ABSL_ATTRIBUTE_LIFETIME_BOUND)
+      : ptr_(std::addressof(input)) {}
+
+  // Constructors given an existing std::optional value.
+  // Templated on the input optional's type to avoid creating a temporary.
+  template <typename U, typename = EnableIfConvertibleFrom<const U>>
+  constexpr optional_ref(  // NOLINT(google-explicit-constructor)
+      const std::optional<U>& input ABSL_ATTRIBUTE_LIFETIME_BOUND)
+      : ptr_(input.has_value() ? std::addressof(*input) : nullptr) {}
+  template <typename U, typename = EnableIfConvertibleFrom<U>>
+  constexpr optional_ref(  // NOLINT(google-explicit-constructor)
+      std::optional<U>& input ABSL_ATTRIBUTE_LIFETIME_BOUND)
+      : ptr_(input.has_value() ? std::addressof(*input) : nullptr) {}
+
+  // Constructor given a T*, where nullptr indicates empty/absent.
+  constexpr optional_ref(  // NOLINT(google-explicit-constructor)
+      T* input ABSL_ATTRIBUTE_LIFETIME_BOUND)
+      : ptr_(input) {}
+
+  // Don't allow naked nullptr as input, as this creates confusion in the case
+  // of optional_ref<T*>. Use std::nullopt instead to create an empty
+  // optional_ref.
+  constexpr optional_ref(  // NOLINT(google-explicit-constructor)
+      std::nullptr_t) = delete;
+
+  // Copying is allowed.
+  optional_ref(const optional_ref<T>&) = default;
+  // Assignment is not allowed.
+  optional_ref<T>& operator=(const optional_ref<T>&) = delete;
+
+  // Conversion from optional_ref<U> is allowed iff U* is convertible to T*.
+  // (Note this also allows non-const to const conversions.)
+  template <typename U, typename = EnableIfConvertibleFrom<U>>
+  constexpr optional_ref(  // NOLINT(google-explicit-constructor)
+      optional_ref<U> input)
+      : ptr_(input.as_pointer()) {}
+
+  // Determines whether the `optional_ref` contains a value. Returns `false` if
+  // and only if `*this` is empty.
+  constexpr bool has_value() const { return ptr_ != nullptr; }
+
+  // Returns a reference to an `optional_ref`s underlying value. The constness
+  // and lvalue/rvalue-ness of the `optional_ref` is preserved to the view of
+  // the `T` sub-object. Throws the same error as `std::optional`'s `value()`
+  // when the `optional_ref` is empty.
+  constexpr T& value() const {
+    return ABSL_PREDICT_TRUE(ptr_ != nullptr)
+               ? *ptr_
+               // Replicate the same error logic as in `std::optional`'s
+               // `value()`. It either throws an exception or aborts the
+               // program. We intentionally ignore the return value of
+               // the constructed optional's value as we only need to run
+               // the code for error checking.
+               : ((void)std::optional<T>().value(), *ptr_);
+  }
+
+  // Returns the value iff *this has a value, otherwise returns `default_value`.
+  template <typename U>
+  constexpr T value_or(U&& default_value) const {
+    // Instantiate std::optional<T>::value_or(U) to trigger its static_asserts.
+    if (false) {
+      // We use `std::add_const_t` here since just using `const` makes MSVC
+      // complain about the syntax.
+      (void)std::add_const_t<std::optional<T>>{}.value_or(
+          std::forward<U>(default_value));
+    }
+    return ptr_ != nullptr ? *ptr_
+                           : static_cast<T>(std::forward<U>(default_value));
+  }
+
+  // Accesses the underlying `T` value of an `optional_ref`. If the
+  // `optional_ref` is empty, behavior is undefined.
+  constexpr T& operator*() const {
+    absl::base_internal::HardeningAssertNonNull(ptr_);
+    return *ptr_;
+  }
+  constexpr T* operator->() const {
+    absl::base_internal::HardeningAssertNonNull(ptr_);
+    return ptr_;
+  }
+
+  // Convenience function to represent the `optional_ref` as a `T*` pointer.
+  constexpr T* as_pointer() const { return ptr_; }
+  // Convenience function to represent the `optional_ref` as an `optional`,
+  // which incurs a copy when the `optional_ref` is non-empty. The template type
+  // allows for implicit type conversion; example:
+  //   optional_ref<std::string> a = ...;
+  //   std::optional<std::string_view> b = a.as_optional<std::string_view>();
+  template <typename U = std::decay_t<T>>
+  constexpr std::optional<U> as_optional() const {
+    if (ptr_ == nullptr) return std::nullopt;
+    return *ptr_;
+  }
+
+ private:
+  T* const ptr_;
+
+  // T constraint checks.  You can't have an optional of nullopt_t or
+  // in_place_t.
+  static_assert(!std::is_same_v<std::nullopt_t, std::remove_cv_t<T>>,
+                "optional_ref<nullopt_t> is not allowed.");
+  static_assert(!std::is_same_v<std::in_place_t, std::remove_cv_t<T>>,
+                "optional_ref<in_place_t> is not allowed.");
+};
+
+// Template type deduction guides:
+
+template <typename T>
+optional_ref(const T&) -> optional_ref<const T>;
+template <typename T>
+optional_ref(T&) -> optional_ref<T>;
+
+template <typename T>
+optional_ref(const std::optional<T>&) -> optional_ref<const T>;
+template <typename T>
+optional_ref(std::optional<T>&) -> optional_ref<T>;
+
+template <typename T>
+optional_ref(T*) -> optional_ref<T>;
+
+namespace optional_ref_internal {
+
+// This is a C++-11 compatible version of std::equality_comparable_with that
+// only requires `t == u` is a valid boolean expression.
+//
+// We still need this for a couple reasons:
+// -  As of 2026-02-13, Abseil supports C++17.
+//  - Even for targets that are built with the default toolchain, using
+//    std::equality_comparable_with gives us an error due to mutual recursion
+//    between its definition and our definition of operator==.
+//
+template <typename T, typename U>
+using enable_if_equality_comparable_t = std::enable_if_t<std::is_convertible_v<
+    decltype(std::declval<T>() == std::declval<U>()), bool>>;
+
+}  // namespace optional_ref_internal
+
+// Compare an optional referenced value to std::nullopt.
+
+template <typename T>
+constexpr bool operator==(optional_ref<T> a, std::nullopt_t) {
+  return !a.has_value();
+}
+template <typename T>
+constexpr bool operator==(std::nullopt_t, optional_ref<T> b) {
+  return !b.has_value();
+}
+template <typename T>
+constexpr bool operator!=(optional_ref<T> a, std::nullopt_t) {
+  return a.has_value();
+}
+template <typename T>
+constexpr bool operator!=(std::nullopt_t, optional_ref<T> b) {
+  return b.has_value();
+}
+
+// Compare two optional referenced values. Note, this does not test that the
+// contained `ptr_`s are equal. If the caller wants "shallow" reference equality
+// semantics, they should use `as_pointer()` explicitly.
+
+template <typename T, typename U>
+constexpr bool operator==(optional_ref<T> a, optional_ref<U> b) {
+  return a.has_value() ? *a == b : !b.has_value();
+}
+
+// Compare an optional referenced value to a non-optional value.
+
+template <
+    typename T, typename U,
+    typename = optional_ref_internal::enable_if_equality_comparable_t<T, U>>
+constexpr bool operator==(const T& a, optional_ref<U> b) {
+  return b.has_value() && a == *b;
+}
+template <
+    typename T, typename U,
+    typename = optional_ref_internal::enable_if_equality_comparable_t<T, U>>
+constexpr bool operator==(optional_ref<T> a, const U& b) {
+  return b == a;
+}
+
+// Inequality operators, as above.
+
+template <typename T, typename U>
+constexpr bool operator!=(optional_ref<T> a, optional_ref<U> b) {
+  return !(a == b);
+}
+template <
+    typename T, typename U,
+    typename = optional_ref_internal::enable_if_equality_comparable_t<T, U>>
+constexpr bool operator!=(optional_ref<T> a, const U& b) {
+  return !(a == b);
+}
+template <
+    typename T, typename U,
+    typename = optional_ref_internal::enable_if_equality_comparable_t<T, U>>
+constexpr bool operator!=(const T& a, optional_ref<U> b) {
+  return !(a == b);
+}
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_TYPES_OPTIONAL_REF_H_
diff --git a/absl/types/optional_ref_test.cc b/absl/types/optional_ref_test.cc
new file mode 100644
index 00000000000..c6f2e761746
--- /dev/null
+++ b/absl/types/optional_ref_test.cc
@@ -0,0 +1,370 @@
+// Copyright 2026 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/types/optional_ref.h"
+
+#include <cstddef>
+#include <memory>
+#include <optional>
+#include <string>
+#include <string_view>
+#include <type_traits>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+#include "absl/log/log.h"
+#include "absl/strings/str_cat.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace {
+
+using ::testing::Optional;
+using ::testing::Pointee;
+
+TEST(OptionalRefTest, SimpleType) {
+  int val = 5;
+  optional_ref<int> ref = optional_ref(val);
+  optional_ref<int> empty_ref = std::nullopt;
+  EXPECT_THAT(ref, Optional(5));
+  EXPECT_TRUE(ref.has_value());
+  EXPECT_EQ(*ref, val);
+  EXPECT_EQ(ref.value(), val);
+  EXPECT_EQ(ref, ref);
+  EXPECT_EQ(ref, val);
+  EXPECT_EQ(val, ref);
+  EXPECT_NE(ref, empty_ref);
+  EXPECT_NE(empty_ref, ref);
+}
+
+TEST(OptionalRefTest, SimpleConstType) {
+  const int val = 5;
+  optional_ref<const int> ref = optional_ref(val);
+  EXPECT_THAT(ref, Optional(5));
+}
+
+TEST(OptionalRefTest, DefaultConstructed) {
+  optional_ref<int> ref;
+  EXPECT_EQ(ref, std::nullopt);
+  EXPECT_EQ(std::nullopt, ref);
+}
+
+TEST(OptionalRefTest, EmptyOptional) {
+  auto ref = optional_ref<int>(std::nullopt);
+  EXPECT_EQ(ref, std::nullopt);
+  EXPECT_EQ(std::nullopt, ref);
+}
+
+TEST(OptionalRefTest, OptionalType) {
+  const std::optional<int> val = 5;
+  optional_ref<const int> ref = val;
+  EXPECT_THAT(ref, Optional(5));
+  EXPECT_EQ(ref.as_pointer(), &*val);
+
+  const std::optional<int> empty;
+  optional_ref<const int> empty_ref = empty;
+  EXPECT_EQ(empty_ref, std::nullopt);
+
+  // Cannot make non-const reference to const optional.
+  static_assert(
+      !std::is_convertible_v<const std::optional<int>&, optional_ref<int>>);
+}
+
+TEST(OptionalRefTest, NonConstOptionalType) {
+  std::optional<int> val = 5;
+  optional_ref<int> ref = val;
+  EXPECT_THAT(ref, Optional(5));
+  EXPECT_EQ(ref.as_pointer(), &*val);
+
+  std::optional<int> empty;
+  optional_ref<int> empty_ref = empty;
+  EXPECT_EQ(empty_ref, std::nullopt);
+}
+
+TEST(OptionalRefTest, NonConstOptionalTypeToConstRef) {
+  std::optional<int> val = 5;
+  optional_ref<const int> ref = val;
+  EXPECT_THAT(ref, Optional(5));
+  EXPECT_EQ(ref.as_pointer(), &*val);
+
+  std::optional<int> empty;
+  optional_ref<const int> empty_ref = empty;
+  EXPECT_EQ(empty_ref, std::nullopt);
+}
+
+TEST(OptionalRefTest, NonConstOptionalWithConstValueType) {
+  std::optional<const int> val = 5;
+  optional_ref<const int> ref = val;
+  EXPECT_THAT(ref, Optional(5));
+  EXPECT_EQ(ref.as_pointer(), &*val);
+
+  std::optional<const int> empty;
+  optional_ref<const int> empty_ref = empty;
+  EXPECT_EQ(empty_ref, std::nullopt);
+
+  // Not possible to convert to non-const reference.
+  static_assert(
+      !std::is_convertible_v<std::optional<const int>&, optional_ref<int>>);
+}
+
+class TestInterface {};
+class TestDerivedClass : public TestInterface {};
+
+TEST(OptionalRefTest, BaseDerivedConvertibleOptionalType) {
+  const std::optional<TestDerivedClass> dc = TestDerivedClass{};
+  optional_ref<const TestInterface> ref = dc;
+  EXPECT_NE(ref, std::nullopt);
+  EXPECT_EQ(ref.as_pointer(), &*dc);
+
+  const std::optional<TestDerivedClass> empty;
+  optional_ref<const TestInterface> empty_ref = empty;
+  EXPECT_EQ(empty_ref, std::nullopt);
+
+  // Not possible in the other direction.
+  static_assert(!std::is_convertible_v<const std::optional<TestInterface>&,
+                                       optional_ref<const TestDerivedClass>>);
+  static_assert(!std::is_convertible_v<const std::optional<TestDerivedClass>&,
+                                       optional_ref<TestInterface>>);
+}
+
+TEST(OptionalRefTest, NonConstBaseDerivedConvertibleOptionalType) {
+  std::optional<TestDerivedClass> dc = TestDerivedClass{};
+  optional_ref<TestInterface> ref = dc;
+  EXPECT_NE(ref, std::nullopt);
+  EXPECT_EQ(ref.as_pointer(), &*dc);
+
+  std::optional<TestDerivedClass> empty;
+  optional_ref<TestInterface> empty_ref = empty;
+  EXPECT_EQ(empty_ref, std::nullopt);
+
+  // Not possible in the other direction.
+  static_assert(!std::is_convertible_v<std::optional<TestInterface>&,
+                                       optional_ref<TestDerivedClass>>);
+}
+
+TEST(OptionalRefTest, NonConstBaseDerivedConvertibleOptionalTypeToConstRef) {
+  std::optional<TestDerivedClass> dc = TestDerivedClass{};
+  optional_ref<const TestInterface> ref = dc;
+  EXPECT_NE(ref, std::nullopt);
+  EXPECT_EQ(ref.as_pointer(), &*dc);
+
+  std::optional<TestDerivedClass> empty;
+  optional_ref<const TestInterface> empty_ref = empty;
+  EXPECT_EQ(empty_ref, std::nullopt);
+
+  // Not possible in the other direction.
+  static_assert(!std::is_convertible_v<std::optional<TestInterface>&,
+                                       optional_ref<const TestDerivedClass>>);
+  static_assert(!std::is_convertible_v<const std::optional<TestInterface>&,
+                                       optional_ref<const TestDerivedClass>>);
+}
+
+TEST(OptionalRefTest, PointerCtor) {
+  int val = 5;
+  optional_ref<const int> ref = &val;
+  EXPECT_THAT(ref, Optional(5));
+
+  auto auto_ref = optional_ref(&val);
+  static_assert(std::is_same_v<decltype(auto_ref), optional_ref<int>>,
+                "optional_ref(T*) should deduce to optional_ref<T>.");
+  EXPECT_THAT(auto_ref, Optional(5));
+
+  int* foo = nullptr;
+  optional_ref<const int> empty_ref = foo;
+  EXPECT_EQ(empty_ref, std::nullopt);
+
+  optional_ref<int*> ptr_ref = foo;
+  EXPECT_THAT(ptr_ref, Optional(nullptr));
+  static_assert(
+      !std::is_constructible_v<optional_ref<int*>, std::nullptr_t>,
+      "optional_ref should not be constructible with std::nullptr_t.");
+
+  // Pointer polymorphism works.
+  TestDerivedClass dc;
+  optional_ref<const TestInterface> dc_ref = &dc;
+  EXPECT_NE(dc_ref, std::nullopt);
+}
+
+TEST(OptionalRefTest, ValueDeathWhenEmpty) {
+  optional_ref<int> ref;
+#ifdef ABSL_HAVE_EXCEPTIONS
+  EXPECT_THROW(ref.value(), std::bad_optional_access);
+#else
+  EXPECT_DEATH_IF_SUPPORTED(ref.value(), "");
+#endif
+}
+
+TEST(OptionalRefTest, ImplicitCtor) {
+  const int val = 5;
+  optional_ref<const int> ref = val;
+  EXPECT_THAT(ref, Optional(5));
+}
+
+TEST(OptionalRefTest, DoesNotCopy) {
+  // Non-copyable type.
+  auto val = std::make_unique<int>(5);
+  optional_ref<std::unique_ptr<int>> ref = optional_ref(val);
+  EXPECT_THAT(ref, Optional(Pointee(5)));
+}
+
+TEST(OptionalRefTest, DoesNotCopyConst) {
+  // Non-copyable type.
+  const auto val = std::make_unique<int>(5);
+  optional_ref<const std::unique_ptr<int>> ref = optional_ref(val);
+  EXPECT_THAT(ref, Optional(Pointee(5)));
+}
+
+TEST(OptionalRefTest, RefCopyable) {
+  auto val = std::make_unique<int>(5);
+  optional_ref<std::unique_ptr<int>> ref = optional_ref(val);
+  optional_ref<std::unique_ptr<int>> copy = ref;
+  EXPECT_THAT(copy, Optional(Pointee(5)));
+}
+
+TEST(OptionalRefTest, ConstConvertible) {
+  auto val = std::make_unique<int>(5);
+  optional_ref<std::unique_ptr<int>> ref = optional_ref(val);
+  optional_ref<const std::unique_ptr<int>> converted = ref;
+  EXPECT_THAT(converted, Optional(Pointee(5)));
+  EXPECT_EQ(converted.as_pointer(), &val);
+
+  // Not possible in the other direction.
+  static_assert(!std::is_convertible_v<optional_ref<const std::unique_ptr<int>>,
+                                       optional_ref<std::unique_ptr<int>>>);
+}
+
+TEST(OptionalRefTest, BaseDerivedConvertible) {
+  TestDerivedClass dc;
+  optional_ref<TestDerivedClass> dc_ref = dc;
+  optional_ref<TestInterface> converted = dc_ref;
+  EXPECT_NE(converted, std::nullopt);
+  EXPECT_EQ(converted.as_pointer(), &dc);
+
+  // Not possible in the other direction.
+  static_assert(!std::is_convertible_v<optional_ref<TestInterface>,
+                                       optional_ref<TestDerivedClass>>);
+}
+
+TEST(OptionalRefTest, BaseDerivedConstConvertible) {
+  TestDerivedClass dc;
+  optional_ref<TestDerivedClass> dc_ref = dc;
+  optional_ref<const TestInterface> converted = dc_ref;
+  EXPECT_NE(converted, std::nullopt);
+  EXPECT_EQ(converted.as_pointer(), &dc);
+
+  // Not possible in the other direction.
+  static_assert(!std::is_convertible_v<optional_ref<const TestInterface>,
+                                       optional_ref<TestDerivedClass>>);
+  static_assert(!std::is_convertible_v<optional_ref<const TestDerivedClass>,
+                                       optional_ref<TestInterface>>);
+}
+
+TEST(OptionalRefTest, BaseDerivedBothConstConvertible) {
+  TestDerivedClass dc;
+  optional_ref<const TestDerivedClass> dc_ref = dc;
+  optional_ref<const TestInterface> converted = dc_ref;
+  EXPECT_NE(converted, std::nullopt);
+  EXPECT_EQ(converted.as_pointer(), &dc);
+
+  // Not possible in the other direction.
+  static_assert(!std::is_convertible_v<optional_ref<const TestInterface>,
+                                       optional_ref<const TestDerivedClass>>);
+}
+
+TEST(OptionalRefTest, TriviallyCopyable) {
+  static_assert(
+      std::is_trivially_copyable_v<optional_ref<std::unique_ptr<int>>>);
+}
+
+TEST(OptionalRefTest, TriviallyDestructible) {
+  static_assert(
+      std::is_trivially_destructible_v<optional_ref<std::unique_ptr<int>>>);
+}
+
+TEST(OptionalRefTest, RefNotAssignable) {
+  static_assert(!std::is_copy_assignable_v<optional_ref<int>>);
+  static_assert(!std::is_move_assignable_v<optional_ref<int>>);
+}
+
+struct TestStructWithCopy {
+  TestStructWithCopy() = default;
+  TestStructWithCopy(TestStructWithCopy&&) {
+    LOG(FATAL) << "Move constructor should not be called";
+  }
+  TestStructWithCopy(const TestStructWithCopy&) {
+    LOG(FATAL) << "Copy constructor should not be called";
+  }
+  TestStructWithCopy& operator=(const TestStructWithCopy&) {
+    LOG(FATAL) << "Assign operator should not be called";
+  }
+};
+
+TEST(OptionalRefTest, DoesNotCopyUsingFatalCopyAssignOps) {
+  TestStructWithCopy val;
+  optional_ref<TestStructWithCopy> ref = optional_ref(val);
+  EXPECT_NE(ref, std::nullopt);
+  EXPECT_NE(optional_ref(TestStructWithCopy{}), std::nullopt);
+}
+
+std::string AddExclamation(optional_ref<const std::string> input) {
+  if (!input.has_value()) {
+    return "";
+  }
+  return absl::StrCat(*input, "!");
+}
+
+TEST(OptionalRefTest, RefAsFunctionParameter) {
+  EXPECT_EQ(AddExclamation(std::nullopt), "");
+  EXPECT_EQ(AddExclamation(std::string("abc")), "abc!");
+  std::string s = "def";
+  EXPECT_EQ(AddExclamation(s), "def!");
+  EXPECT_EQ(AddExclamation(std::make_optional<std::string>(s)), "def!");
+}
+
+TEST(OptionalRefTest, ValueOrWhenHasValue) {
+  std::optional<int> val = 5;
+  EXPECT_EQ(optional_ref(val).value_or(2), 5);
+}
+
+TEST(OptionalRefTest, ValueOrWhenEmpty) {
+  std::optional<int> val = std::nullopt;
+  EXPECT_EQ(optional_ref(val).value_or(2), 2);
+}
+
+TEST(OptionalRefTest, AsOptional) {
+  EXPECT_EQ(optional_ref<int>().as_optional(), std::nullopt);
+  std::string val = "foo";
+  optional_ref<const std::string> ref = val;
+  static_assert(
+      std::is_same_v<decltype(ref.as_optional()), std::optional<std::string>>,
+      "The type parameter of optional_ref should decay by default for the "
+      "return type in as_optional().");
+  std::optional<std::string> opt_string = ref.as_optional();
+  EXPECT_THAT(opt_string, Optional(val));
+
+  std::optional<std::string_view> opt_view =
+      ref.as_optional<std::string_view>();
+  EXPECT_THAT(opt_view, Optional(val));
+}
+
+TEST(OptionalRefTest, Constexpr) {
+  static constexpr int foo = 123;
+  constexpr optional_ref<const int> ref(foo);
+  static_assert(ref.has_value() && *ref == foo && ref.value() == foo);
+}
+
+}  // namespace
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/types/optional_test.cc b/absl/types/optional_test.cc
deleted file mode 100644
index 35efa863507..00000000000
--- a/absl/types/optional_test.cc
+++ /dev/null
@@ -1,1661 +0,0 @@
-// Copyright 2017 The Abseil Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-#include "absl/types/optional.h"
-
-// This test is a no-op when absl::optional is an alias for std::optional.
-#if !defined(ABSL_HAVE_STD_OPTIONAL)
-
-#include <string>
-#include <type_traits>
-#include <utility>
-
-#include "gtest/gtest.h"
-#include "absl/base/config.h"
-#include "absl/base/internal/raw_logging.h"
-#include "absl/meta/type_traits.h"
-#include "absl/strings/string_view.h"
-
-struct Hashable {};
-
-namespace std {
-template <>
-struct hash<Hashable> {
-  size_t operator()(const Hashable&) { return 0; }
-};
-}  // namespace std
-
-struct NonHashable {};
-
-namespace {
-
-std::string TypeQuals(std::string&) { return "&"; }
-std::string TypeQuals(std::string&&) { return "&&"; }
-std::string TypeQuals(const std::string&) { return "c&"; }
-std::string TypeQuals(const std::string&&) { return "c&&"; }
-
-struct StructorListener {
-  int construct0 = 0;
-  int construct1 = 0;
-  int construct2 = 0;
-  int listinit = 0;
-  int copy = 0;
-  int move = 0;
-  int copy_assign = 0;
-  int move_assign = 0;
-  int destruct = 0;
-  int volatile_copy = 0;
-  int volatile_move = 0;
-  int volatile_copy_assign = 0;
-  int volatile_move_assign = 0;
-};
-
-// Suppress MSVC warnings.
-// 4521: multiple copy constructors specified
-// 4522: multiple assignment operators specified
-// We wrote multiple of them to test that the correct overloads are selected.
-#ifdef _MSC_VER
-#pragma warning( push )
-#pragma warning( disable : 4521)
-#pragma warning( disable : 4522)
-#endif
-struct Listenable {
-  static StructorListener* listener;
-
-  Listenable() { ++listener->construct0; }
-  explicit Listenable(int /*unused*/) { ++listener->construct1; }
-  Listenable(int /*unused*/, int /*unused*/) { ++listener->construct2; }
-  Listenable(std::initializer_list<int> /*unused*/) { ++listener->listinit; }
-  Listenable(const Listenable& /*unused*/) { ++listener->copy; }
-  Listenable(const volatile Listenable& /*unused*/) {
-    ++listener->volatile_copy;
-  }
-  Listenable(volatile Listenable&& /*unused*/) { ++listener->volatile_move; }
-  Listenable(Listenable&& /*unused*/) { ++listener->move; }
-  Listenable& operator=(const Listenable& /*unused*/) {
-    ++listener->copy_assign;
-    return *this;
-  }
-  Listenable& operator=(Listenable&& /*unused*/) {
-    ++listener->move_assign;
-    return *this;
-  }
-  // use void return type instead of volatile T& to work around GCC warning
-  // when the assignment's returned reference is ignored.
-  void operator=(const volatile Listenable& /*unused*/) volatile {
-    ++listener->volatile_copy_assign;
-  }
-  void operator=(volatile Listenable&& /*unused*/) volatile {
-    ++listener->volatile_move_assign;
-  }
-  ~Listenable() { ++listener->destruct; }
-};
-#ifdef _MSC_VER
-#pragma warning( pop )
-#endif
-
-StructorListener* Listenable::listener = nullptr;
-
-// ABSL_HAVE_NO_CONSTEXPR_INITIALIZER_LIST is defined to 1 when the standard
-// library implementation doesn't marked initializer_list's default constructor
-// constexpr. The C++11 standard doesn't specify constexpr on it, but C++14
-// added it. However, libstdc++ 4.7 marked it constexpr.
-#if defined(_LIBCPP_VERSION) && \
-    (_LIBCPP_STD_VER <= 11 || defined(_LIBCPP_HAS_NO_CXX14_CONSTEXPR))
-#define ABSL_HAVE_NO_CONSTEXPR_INITIALIZER_LIST 1
-#endif
-
-struct ConstexprType {
-  enum CtorTypes {
-    kCtorDefault,
-    kCtorInt,
-    kCtorInitializerList,
-    kCtorConstChar
-  };
-  constexpr ConstexprType() : x(kCtorDefault) {}
-  constexpr explicit ConstexprType(int i) : x(kCtorInt) {}
-#ifndef ABSL_HAVE_NO_CONSTEXPR_INITIALIZER_LIST
-  constexpr ConstexprType(std::initializer_list<int> il)
-      : x(kCtorInitializerList) {}
-#endif
-  constexpr ConstexprType(const char*)  // NOLINT(runtime/explicit)
-      : x(kCtorConstChar) {}
-  int x;
-};
-
-struct Copyable {
-  Copyable() {}
-  Copyable(const Copyable&) {}
-  Copyable& operator=(const Copyable&) { return *this; }
-};
-
-struct MoveableThrow {
-  MoveableThrow() {}
-  MoveableThrow(MoveableThrow&&) {}
-  MoveableThrow& operator=(MoveableThrow&&) { return *this; }
-};
-
-struct MoveableNoThrow {
-  MoveableNoThrow() {}
-  MoveableNoThrow(MoveableNoThrow&&) noexcept {}
-  MoveableNoThrow& operator=(MoveableNoThrow&&) noexcept { return *this; }
-};
-
-struct NonMovable {
-  NonMovable() {}
-  NonMovable(const NonMovable&) = delete;
-  NonMovable& operator=(const NonMovable&) = delete;
-  NonMovable(NonMovable&&) = delete;
-  NonMovable& operator=(NonMovable&&) = delete;
-};
-
-struct NoDefault {
-  NoDefault() = delete;
-  NoDefault(const NoDefault&) {}
-  NoDefault& operator=(const NoDefault&) { return *this; }
-};
-
-struct ConvertsFromInPlaceT {
-  ConvertsFromInPlaceT(absl::in_place_t) {}  // NOLINT
-};
-
-TEST(optionalTest, DefaultConstructor) {
-  absl::optional<int> empty;
-  EXPECT_FALSE(empty);
-  constexpr absl::optional<int> cempty;
-  static_assert(!cempty.has_value(), "");
-  EXPECT_TRUE(
-      std::is_nothrow_default_constructible<absl::optional<int>>::value);
-}
-
-TEST(optionalTest, nulloptConstructor) {
-  absl::optional<int> empty(absl::nullopt);
-  EXPECT_FALSE(empty);
-  constexpr absl::optional<int> cempty{absl::nullopt};
-  static_assert(!cempty.has_value(), "");
-  EXPECT_TRUE((std::is_nothrow_constructible<absl::optional<int>,
-                                             absl::nullopt_t>::value));
-}
-
-TEST(optionalTest, CopyConstructor) {
-  {
-    absl::optional<int> empty, opt42 = 42;
-    absl::optional<int> empty_copy(empty);
-    EXPECT_FALSE(empty_copy);
-    absl::optional<int> opt42_copy(opt42);
-    EXPECT_TRUE(opt42_copy);
-    EXPECT_EQ(42, *opt42_copy);
-  }
-  {
-    absl::optional<const int> empty, opt42 = 42;
-    absl::optional<const int> empty_copy(empty);
-    EXPECT_FALSE(empty_copy);
-    absl::optional<const int> opt42_copy(opt42);
-    EXPECT_TRUE(opt42_copy);
-    EXPECT_EQ(42, *opt42_copy);
-  }
-  {
-    absl::optional<volatile int> empty, opt42 = 42;
-    absl::optional<volatile int> empty_copy(empty);
-    EXPECT_FALSE(empty_copy);
-    absl::optional<volatile int> opt42_copy(opt42);
-    EXPECT_TRUE(opt42_copy);
-    EXPECT_EQ(42, *opt42_copy);
-  }
-  // test copyablility
-  EXPECT_TRUE(std::is_copy_constructible<absl::optional<int>>::value);
-  EXPECT_TRUE(std::is_copy_constructible<absl::optional<Copyable>>::value);
-  EXPECT_FALSE(
-      std::is_copy_constructible<absl::optional<MoveableThrow>>::value);
-  EXPECT_FALSE(
-      std::is_copy_constructible<absl::optional<MoveableNoThrow>>::value);
-  EXPECT_FALSE(std::is_copy_constructible<absl::optional<NonMovable>>::value);
-
-  EXPECT_FALSE(
-      absl::is_trivially_copy_constructible<absl::optional<Copyable>>::value);
-#if defined(ABSL_HAVE_STD_OPTIONAL) && defined(__GLIBCXX__)
-  // libstdc++ std::optional implementation (as of 7.2) has a bug: when T is
-  // trivially copyable, optional<T> is not trivially copyable (due to one of
-  // its base class is unconditionally nontrivial).
-#define ABSL_GLIBCXX_OPTIONAL_TRIVIALITY_BUG 1
-#endif
-#ifndef ABSL_GLIBCXX_OPTIONAL_TRIVIALITY_BUG
-  EXPECT_TRUE(
-      absl::is_trivially_copy_constructible<absl::optional<int>>::value);
-  EXPECT_TRUE(
-      absl::is_trivially_copy_constructible<absl::optional<const int>>::value);
-#ifndef _MSC_VER
-  // See defect report "Trivial copy/move constructor for class with volatile
-  // member" at
-  // http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#2094
-  // A class with non-static data member of volatile-qualified type should still
-  // have a trivial copy constructor if the data member is trivial.
-  // Also a cv-qualified scalar type should be trivially copyable.
-  EXPECT_TRUE(absl::is_trivially_copy_constructible<
-              absl::optional<volatile int>>::value);
-#endif  // _MSC_VER
-#endif  // ABSL_GLIBCXX_OPTIONAL_TRIVIALITY_BUG
-
-  // constexpr copy constructor for trivially copyable types
-  {
-    constexpr absl::optional<int> o1;
-    constexpr absl::optional<int> o2 = o1;
-    static_assert(!o2, "");
-  }
-  {
-    constexpr absl::optional<int> o1 = 42;
-    constexpr absl::optional<int> o2 = o1;
-    static_assert(o2, "");
-    static_assert(*o2 == 42, "");
-  }
-  {
-    struct TrivialCopyable {
-      constexpr TrivialCopyable() : x(0) {}
-      constexpr explicit TrivialCopyable(int i) : x(i) {}
-      int x;
-    };
-    constexpr absl::optional<TrivialCopyable> o1(42);
-    constexpr absl::optional<TrivialCopyable> o2 = o1;
-    static_assert(o2, "");
-    static_assert((*o2).x == 42, "");
-#ifndef ABSL_GLIBCXX_OPTIONAL_TRIVIALITY_BUG
-    EXPECT_TRUE(absl::is_trivially_copy_constructible<
-                absl::optional<TrivialCopyable>>::value);
-    EXPECT_TRUE(absl::is_trivially_copy_constructible<
-                absl::optional<const TrivialCopyable>>::value);
-#endif
-    // When testing with VS 2017 15.3, there seems to be a bug in MSVC
-    // std::optional when T is volatile-qualified. So skipping this test.
-    // Bug report:
-    // https://connect.microsoft.com/VisualStudio/feedback/details/3142534
-#if defined(ABSL_HAVE_STD_OPTIONAL) && defined(_MSC_VER) && _MSC_VER >= 1911
-#define ABSL_MSVC_OPTIONAL_VOLATILE_COPY_BUG 1
-#endif
-#ifndef ABSL_MSVC_OPTIONAL_VOLATILE_COPY_BUG
-    EXPECT_FALSE(std::is_copy_constructible<
-                 absl::optional<volatile TrivialCopyable>>::value);
-#endif
-  }
-}
-
-TEST(optionalTest, MoveConstructor) {
-  absl::optional<int> empty, opt42 = 42;
-  absl::optional<int> empty_move(std::move(empty));
-  EXPECT_FALSE(empty_move);
-  absl::optional<int> opt42_move(std::move(opt42));
-  EXPECT_TRUE(opt42_move);
-  EXPECT_EQ(42, opt42_move);
-  // test movability
-  EXPECT_TRUE(std::is_move_constructible<absl::optional<int>>::value);
-  EXPECT_TRUE(std::is_move_constructible<absl::optional<Copyable>>::value);
-  EXPECT_TRUE(std::is_move_constructible<absl::optional<MoveableThrow>>::value);
-  EXPECT_TRUE(
-      std::is_move_constructible<absl::optional<MoveableNoThrow>>::value);
-  EXPECT_FALSE(std::is_move_constructible<absl::optional<NonMovable>>::value);
-  // test noexcept
-  EXPECT_TRUE(std::is_nothrow_move_constructible<absl::optional<int>>::value);
-#ifndef ABSL_HAVE_STD_OPTIONAL
-  EXPECT_EQ(
-      absl::default_allocator_is_nothrow::value,
-      std::is_nothrow_move_constructible<absl::optional<MoveableThrow>>::value);
-#endif
-  EXPECT_TRUE(std::is_nothrow_move_constructible<
-              absl::optional<MoveableNoThrow>>::value);
-}
-
-TEST(optionalTest, Destructor) {
-  struct Trivial {};
-
-  struct NonTrivial {
-    NonTrivial(const NonTrivial&) {}
-    NonTrivial& operator=(const NonTrivial&) { return *this; }
-    ~NonTrivial() {}
-  };
-
-  EXPECT_TRUE(std::is_trivially_destructible<absl::optional<int>>::value);
-  EXPECT_TRUE(std::is_trivially_destructible<absl::optional<Trivial>>::value);
-  EXPECT_FALSE(
-      std::is_trivially_destructible<absl::optional<NonTrivial>>::value);
-}
-
-TEST(optionalTest, InPlaceConstructor) {
-  constexpr absl::optional<ConstexprType> opt0{absl::in_place_t()};
-  static_assert(opt0, "");
-  static_assert((*opt0).x == ConstexprType::kCtorDefault, "");
-  constexpr absl::optional<ConstexprType> opt1{absl::in_place_t(), 1};
-  static_assert(opt1, "");
-  static_assert((*opt1).x == ConstexprType::kCtorInt, "");
-#ifndef ABSL_HAVE_NO_CONSTEXPR_INITIALIZER_LIST
-  constexpr absl::optional<ConstexprType> opt2{absl::in_place_t(), {1, 2}};
-  static_assert(opt2, "");
-  static_assert((*opt2).x == ConstexprType::kCtorInitializerList, "");
-#endif
-
-  EXPECT_FALSE((std::is_constructible<absl::optional<ConvertsFromInPlaceT>,
-                                      absl::in_place_t>::value));
-  EXPECT_FALSE((std::is_constructible<absl::optional<ConvertsFromInPlaceT>,
-                                      const absl::in_place_t&>::value));
-  EXPECT_TRUE(
-      (std::is_constructible<absl::optional<ConvertsFromInPlaceT>,
-                             absl::in_place_t, absl::in_place_t>::value));
-
-  EXPECT_FALSE((std::is_constructible<absl::optional<NoDefault>,
-                                      absl::in_place_t>::value));
-  EXPECT_FALSE((std::is_constructible<absl::optional<NoDefault>,
-                                      absl::in_place_t&&>::value));
-}
-
-// template<U=T> optional(U&&);
-TEST(optionalTest, ValueConstructor) {
-  constexpr absl::optional<int> opt0(0);
-  static_assert(opt0, "");
-  static_assert(*opt0 == 0, "");
-  EXPECT_TRUE((std::is_convertible<int, absl::optional<int>>::value));
-  // Copy initialization ( = "abc") won't work due to optional(optional&&)
-  // is not constexpr. Use list initialization instead. This invokes
-  // absl::optional<ConstexprType>::absl::optional<U>(U&&), with U = const char
-  // (&) [4], which direct-initializes the ConstexprType value held by the
-  // optional via ConstexprType::ConstexprType(const char*).
-  constexpr absl::optional<ConstexprType> opt1 = {"abc"};
-  static_assert(opt1, "");
-  static_assert(ConstexprType::kCtorConstChar == (*opt1).x, "");
-  EXPECT_TRUE(
-      (std::is_convertible<const char*, absl::optional<ConstexprType>>::value));
-  // direct initialization
-  constexpr absl::optional<ConstexprType> opt2{2};
-  static_assert(opt2, "");
-  static_assert(ConstexprType::kCtorInt == (*opt2).x, "");
-  EXPECT_FALSE(
-      (std::is_convertible<int, absl::optional<ConstexprType>>::value));
-
-  // this invokes absl::optional<int>::optional(int&&)
-  // NOTE: this has different behavior than assignment, e.g.
-  // "opt3 = {};" clears the optional rather than setting the value to 0
-  // According to C++17 standard N4659 [over.ics.list] 16.3.3.1.5, (9.2)- "if
-  // the initializer list has no elements, the implicit conversion is the
-  // identity conversion", so `optional(int&&)` should be a better match than
-  // `optional(optional&&)` which is a user-defined conversion.
-  // Note: GCC 7 has a bug with this overload selection when compiled with
-  // `-std=c++17`.
-#if defined(__GNUC__) && !defined(__clang__) && __GNUC__ == 7 && \
-    __cplusplus == 201703L
-#define ABSL_GCC7_OVER_ICS_LIST_BUG 1
-#endif
-#ifndef ABSL_GCC7_OVER_ICS_LIST_BUG
-  constexpr absl::optional<int> opt3({});
-  static_assert(opt3, "");
-  static_assert(*opt3 == 0, "");
-#endif
-
-  // this invokes the move constructor with a default constructed optional
-  // because non-template function is a better match than template function.
-  absl::optional<ConstexprType> opt4({});
-  EXPECT_FALSE(opt4);
-}
-
-struct Implicit {};
-
-struct Explicit {};
-
-struct Convert {
-  Convert(const Implicit&)  // NOLINT(runtime/explicit)
-      : implicit(true), move(false) {}
-  Convert(Implicit&&)  // NOLINT(runtime/explicit)
-      : implicit(true), move(true) {}
-  explicit Convert(const Explicit&) : implicit(false), move(false) {}
-  explicit Convert(Explicit&&) : implicit(false), move(true) {}
-
-  bool implicit;
-  bool move;
-};
-
-struct ConvertFromOptional {
-  ConvertFromOptional(const Implicit&)  // NOLINT(runtime/explicit)
-      : implicit(true), move(false), from_optional(false) {}
-  ConvertFromOptional(Implicit&&)  // NOLINT(runtime/explicit)
-      : implicit(true), move(true), from_optional(false) {}
-  ConvertFromOptional(
-      const absl::optional<Implicit>&)  // NOLINT(runtime/explicit)
-      : implicit(true), move(false), from_optional(true) {}
-  ConvertFromOptional(absl::optional<Implicit>&&)  // NOLINT(runtime/explicit)
-      : implicit(true), move(true), from_optional(true) {}
-  explicit ConvertFromOptional(const Explicit&)
-      : implicit(false), move(false), from_optional(false) {}
-  explicit ConvertFromOptional(Explicit&&)
-      : implicit(false), move(true), from_optional(false) {}
-  explicit ConvertFromOptional(const absl::optional<Explicit>&)
-      : implicit(false), move(false), from_optional(true) {}
-  explicit ConvertFromOptional(absl::optional<Explicit>&&)
-      : implicit(false), move(true), from_optional(true) {}
-
-  bool implicit;
-  bool move;
-  bool from_optional;
-};
-
-TEST(optionalTest, ConvertingConstructor) {
-  absl::optional<Implicit> i_empty;
-  absl::optional<Implicit> i(absl::in_place);
-  absl::optional<Explicit> e_empty;
-  absl::optional<Explicit> e(absl::in_place);
-  {
-    // implicitly constructing absl::optional<Convert> from
-    // absl::optional<Implicit>
-    absl::optional<Convert> empty = i_empty;
-    EXPECT_FALSE(empty);
-    absl::optional<Convert> opt_copy = i;
-    EXPECT_TRUE(opt_copy);
-    EXPECT_TRUE(opt_copy->implicit);
-    EXPECT_FALSE(opt_copy->move);
-    absl::optional<Convert> opt_move = absl::optional<Implicit>(absl::in_place);
-    EXPECT_TRUE(opt_move);
-    EXPECT_TRUE(opt_move->implicit);
-    EXPECT_TRUE(opt_move->move);
-  }
-  {
-    // explicitly constructing absl::optional<Convert> from
-    // absl::optional<Explicit>
-    absl::optional<Convert> empty(e_empty);
-    EXPECT_FALSE(empty);
-    absl::optional<Convert> opt_copy(e);
-    EXPECT_TRUE(opt_copy);
-    EXPECT_FALSE(opt_copy->implicit);
-    EXPECT_FALSE(opt_copy->move);
-    EXPECT_FALSE((std::is_convertible<const absl::optional<Explicit>&,
-                                      absl::optional<Convert>>::value));
-    absl::optional<Convert> opt_move{absl::optional<Explicit>(absl::in_place)};
-    EXPECT_TRUE(opt_move);
-    EXPECT_FALSE(opt_move->implicit);
-    EXPECT_TRUE(opt_move->move);
-    EXPECT_FALSE((std::is_convertible<absl::optional<Explicit>&&,
-                                      absl::optional<Convert>>::value));
-  }
-  {
-    // implicitly constructing absl::optional<ConvertFromOptional> from
-    // absl::optional<Implicit> via
-    // ConvertFromOptional(absl::optional<Implicit>&&) check that
-    // ConvertFromOptional(Implicit&&) is NOT called
-    static_assert(
-        std::is_convertible<absl::optional<Implicit>,
-                            absl::optional<ConvertFromOptional>>::value,
-        "");
-    absl::optional<ConvertFromOptional> opt0 = i_empty;
-    EXPECT_TRUE(opt0);
-    EXPECT_TRUE(opt0->implicit);
-    EXPECT_FALSE(opt0->move);
-    EXPECT_TRUE(opt0->from_optional);
-    absl::optional<ConvertFromOptional> opt1 = absl::optional<Implicit>();
-    EXPECT_TRUE(opt1);
-    EXPECT_TRUE(opt1->implicit);
-    EXPECT_TRUE(opt1->move);
-    EXPECT_TRUE(opt1->from_optional);
-  }
-  {
-    // implicitly constructing absl::optional<ConvertFromOptional> from
-    // absl::optional<Explicit> via
-    // ConvertFromOptional(absl::optional<Explicit>&&) check that
-    // ConvertFromOptional(Explicit&&) is NOT called
-    absl::optional<ConvertFromOptional> opt0(e_empty);
-    EXPECT_TRUE(opt0);
-    EXPECT_FALSE(opt0->implicit);
-    EXPECT_FALSE(opt0->move);
-    EXPECT_TRUE(opt0->from_optional);
-    EXPECT_FALSE(
-        (std::is_convertible<const absl::optional<Explicit>&,
-                             absl::optional<ConvertFromOptional>>::value));
-    absl::optional<ConvertFromOptional> opt1{absl::optional<Explicit>()};
-    EXPECT_TRUE(opt1);
-    EXPECT_FALSE(opt1->implicit);
-    EXPECT_TRUE(opt1->move);
-    EXPECT_TRUE(opt1->from_optional);
-    EXPECT_FALSE(
-        (std::is_convertible<absl::optional<Explicit>&&,
-                             absl::optional<ConvertFromOptional>>::value));
-  }
-}
-
-TEST(optionalTest, StructorBasic) {
-  StructorListener listener;
-  Listenable::listener = &listener;
-  {
-    absl::optional<Listenable> empty;
-    EXPECT_FALSE(empty);
-    absl::optional<Listenable> opt0(absl::in_place);
-    EXPECT_TRUE(opt0);
-    absl::optional<Listenable> opt1(absl::in_place, 1);
-    EXPECT_TRUE(opt1);
-    absl::optional<Listenable> opt2(absl::in_place, 1, 2);
-    EXPECT_TRUE(opt2);
-  }
-  EXPECT_EQ(1, listener.construct0);
-  EXPECT_EQ(1, listener.construct1);
-  EXPECT_EQ(1, listener.construct2);
-  EXPECT_EQ(3, listener.destruct);
-}
-
-TEST(optionalTest, CopyMoveStructor) {
-  StructorListener listener;
-  Listenable::listener = &listener;
-  absl::optional<Listenable> original(absl::in_place);
-  EXPECT_EQ(1, listener.construct0);
-  EXPECT_EQ(0, listener.copy);
-  EXPECT_EQ(0, listener.move);
-  absl::optional<Listenable> copy(original);
-  EXPECT_EQ(1, listener.construct0);
-  EXPECT_EQ(1, listener.copy);
-  EXPECT_EQ(0, listener.move);
-  absl::optional<Listenable> move(std::move(original));
-  EXPECT_EQ(1, listener.construct0);
-  EXPECT_EQ(1, listener.copy);
-  EXPECT_EQ(1, listener.move);
-}
-
-TEST(optionalTest, ListInit) {
-  StructorListener listener;
-  Listenable::listener = &listener;
-  absl::optional<Listenable> listinit1(absl::in_place, {1});
-  absl::optional<Listenable> listinit2(absl::in_place, {1, 2});
-  EXPECT_EQ(2, listener.listinit);
-}
-
-TEST(optionalTest, AssignFromNullopt) {
-  absl::optional<int> opt(1);
-  opt = absl::nullopt;
-  EXPECT_FALSE(opt);
-
-  StructorListener listener;
-  Listenable::listener = &listener;
-  absl::optional<Listenable> opt1(absl::in_place);
-  opt1 = absl::nullopt;
-  EXPECT_FALSE(opt1);
-  EXPECT_EQ(1, listener.construct0);
-  EXPECT_EQ(1, listener.destruct);
-
-  EXPECT_TRUE((
-      std::is_nothrow_assignable<absl::optional<int>, absl::nullopt_t>::value));
-  EXPECT_TRUE((std::is_nothrow_assignable<absl::optional<Listenable>,
-                                          absl::nullopt_t>::value));
-}
-
-TEST(optionalTest, CopyAssignment) {
-  const absl::optional<int> empty, opt1 = 1, opt2 = 2;
-  absl::optional<int> empty_to_opt1, opt1_to_opt2, opt2_to_empty;
-
-  EXPECT_FALSE(empty_to_opt1);
-  empty_to_opt1 = empty;
-  EXPECT_FALSE(empty_to_opt1);
-  empty_to_opt1 = opt1;
-  EXPECT_TRUE(empty_to_opt1);
-  EXPECT_EQ(1, empty_to_opt1.value());
-
-  EXPECT_FALSE(opt1_to_opt2);
-  opt1_to_opt2 = opt1;
-  EXPECT_TRUE(opt1_to_opt2);
-  EXPECT_EQ(1, opt1_to_opt2.value());
-  opt1_to_opt2 = opt2;
-  EXPECT_TRUE(opt1_to_opt2);
-  EXPECT_EQ(2, opt1_to_opt2.value());
-
-  EXPECT_FALSE(opt2_to_empty);
-  opt2_to_empty = opt2;
-  EXPECT_TRUE(opt2_to_empty);
-  EXPECT_EQ(2, opt2_to_empty.value());
-  opt2_to_empty = empty;
-  EXPECT_FALSE(opt2_to_empty);
-
-  EXPECT_FALSE(absl::is_copy_assignable<absl::optional<const int>>::value);
-  EXPECT_TRUE(absl::is_copy_assignable<absl::optional<Copyable>>::value);
-  EXPECT_FALSE(absl::is_copy_assignable<absl::optional<MoveableThrow>>::value);
-  EXPECT_FALSE(
-      absl::is_copy_assignable<absl::optional<MoveableNoThrow>>::value);
-  EXPECT_FALSE(absl::is_copy_assignable<absl::optional<NonMovable>>::value);
-
-  EXPECT_TRUE(absl::is_trivially_copy_assignable<int>::value);
-  EXPECT_TRUE(absl::is_trivially_copy_assignable<volatile int>::value);
-
-  struct Trivial {
-    int i;
-  };
-  struct NonTrivial {
-    NonTrivial& operator=(const NonTrivial&) { return *this; }
-    int i;
-  };
-
-  EXPECT_TRUE(absl::is_trivially_copy_assignable<Trivial>::value);
-  EXPECT_FALSE(absl::is_copy_assignable<const Trivial>::value);
-  EXPECT_FALSE(absl::is_copy_assignable<volatile Trivial>::value);
-  EXPECT_TRUE(absl::is_copy_assignable<NonTrivial>::value);
-  EXPECT_FALSE(absl::is_trivially_copy_assignable<NonTrivial>::value);
-
-  // std::optional doesn't support volatile nontrivial types.
-#ifndef ABSL_HAVE_STD_OPTIONAL
-  {
-    StructorListener listener;
-    Listenable::listener = &listener;
-
-    absl::optional<volatile Listenable> empty, set(absl::in_place);
-    EXPECT_EQ(1, listener.construct0);
-    absl::optional<volatile Listenable> empty_to_empty, empty_to_set,
-        set_to_empty(absl::in_place), set_to_set(absl::in_place);
-    EXPECT_EQ(3, listener.construct0);
-    empty_to_empty = empty;  // no effect
-    empty_to_set = set;      // copy construct
-    set_to_empty = empty;    // destruct
-    set_to_set = set;        // copy assign
-    EXPECT_EQ(1, listener.volatile_copy);
-    EXPECT_EQ(0, listener.volatile_move);
-    EXPECT_EQ(1, listener.destruct);
-    EXPECT_EQ(1, listener.volatile_copy_assign);
-  }
-#endif  // ABSL_HAVE_STD_OPTIONAL
-}
-
-TEST(optionalTest, MoveAssignment) {
-  {
-    StructorListener listener;
-    Listenable::listener = &listener;
-
-    absl::optional<Listenable> empty1, empty2, set1(absl::in_place),
-        set2(absl::in_place);
-    EXPECT_EQ(2, listener.construct0);
-    absl::optional<Listenable> empty_to_empty, empty_to_set,
-        set_to_empty(absl::in_place), set_to_set(absl::in_place);
-    EXPECT_EQ(4, listener.construct0);
-    empty_to_empty = std::move(empty1);
-    empty_to_set = std::move(set1);
-    set_to_empty = std::move(empty2);
-    set_to_set = std::move(set2);
-    EXPECT_EQ(0, listener.copy);
-    EXPECT_EQ(1, listener.move);
-    EXPECT_EQ(1, listener.destruct);
-    EXPECT_EQ(1, listener.move_assign);
-  }
-  // std::optional doesn't support volatile nontrivial types.
-#ifndef ABSL_HAVE_STD_OPTIONAL
-  {
-    StructorListener listener;
-    Listenable::listener = &listener;
-
-    absl::optional<volatile Listenable> empty1, empty2, set1(absl::in_place),
-        set2(absl::in_place);
-    EXPECT_EQ(2, listener.construct0);
-    absl::optional<volatile Listenable> empty_to_empty, empty_to_set,
-        set_to_empty(absl::in_place), set_to_set(absl::in_place);
-    EXPECT_EQ(4, listener.construct0);
-    empty_to_empty = std::move(empty1);  // no effect
-    empty_to_set = std::move(set1);      // move construct
-    set_to_empty = std::move(empty2);    // destruct
-    set_to_set = std::move(set2);        // move assign
-    EXPECT_EQ(0, listener.volatile_copy);
-    EXPECT_EQ(1, listener.volatile_move);
-    EXPECT_EQ(1, listener.destruct);
-    EXPECT_EQ(1, listener.volatile_move_assign);
-  }
-#endif  // ABSL_HAVE_STD_OPTIONAL
-  EXPECT_FALSE(absl::is_move_assignable<absl::optional<const int>>::value);
-  EXPECT_TRUE(absl::is_move_assignable<absl::optional<Copyable>>::value);
-  EXPECT_TRUE(absl::is_move_assignable<absl::optional<MoveableThrow>>::value);
-  EXPECT_TRUE(absl::is_move_assignable<absl::optional<MoveableNoThrow>>::value);
-  EXPECT_FALSE(absl::is_move_assignable<absl::optional<NonMovable>>::value);
-
-  EXPECT_FALSE(
-      std::is_nothrow_move_assignable<absl::optional<MoveableThrow>>::value);
-  EXPECT_TRUE(
-      std::is_nothrow_move_assignable<absl::optional<MoveableNoThrow>>::value);
-}
-
-struct NoConvertToOptional {
-  // disable implicit conversion from const NoConvertToOptional&
-  // to absl::optional<NoConvertToOptional>.
-  NoConvertToOptional(const NoConvertToOptional&) = delete;
-};
-
-struct CopyConvert {
-  CopyConvert(const NoConvertToOptional&);
-  CopyConvert& operator=(const CopyConvert&) = delete;
-  CopyConvert& operator=(const NoConvertToOptional&);
-};
-
-struct CopyConvertFromOptional {
-  CopyConvertFromOptional(const NoConvertToOptional&);
-  CopyConvertFromOptional(const absl::optional<NoConvertToOptional>&);
-  CopyConvertFromOptional& operator=(const CopyConvertFromOptional&) = delete;
-  CopyConvertFromOptional& operator=(const NoConvertToOptional&);
-  CopyConvertFromOptional& operator=(
-      const absl::optional<NoConvertToOptional>&);
-};
-
-struct MoveConvert {
-  MoveConvert(NoConvertToOptional&&);
-  MoveConvert& operator=(const MoveConvert&) = delete;
-  MoveConvert& operator=(NoConvertToOptional&&);
-};
-
-struct MoveConvertFromOptional {
-  MoveConvertFromOptional(NoConvertToOptional&&);
-  MoveConvertFromOptional(absl::optional<NoConvertToOptional>&&);
-  MoveConvertFromOptional& operator=(const MoveConvertFromOptional&) = delete;
-  MoveConvertFromOptional& operator=(NoConvertToOptional&&);
-  MoveConvertFromOptional& operator=(absl::optional<NoConvertToOptional>&&);
-};
-
-// template <typename U = T> absl::optional<T>& operator=(U&& v);
-TEST(optionalTest, ValueAssignment) {
-  absl::optional<int> opt;
-  EXPECT_FALSE(opt);
-  opt = 42;
-  EXPECT_TRUE(opt);
-  EXPECT_EQ(42, opt.value());
-  opt = absl::nullopt;
-  EXPECT_FALSE(opt);
-  opt = 42;
-  EXPECT_TRUE(opt);
-  EXPECT_EQ(42, opt.value());
-  opt = 43;
-  EXPECT_TRUE(opt);
-  EXPECT_EQ(43, opt.value());
-  opt = {};  // this should clear optional
-  EXPECT_FALSE(opt);
-
-  opt = {44};
-  EXPECT_TRUE(opt);
-  EXPECT_EQ(44, opt.value());
-
-  // U = const NoConvertToOptional&
-  EXPECT_TRUE((std::is_assignable<absl::optional<CopyConvert>&,
-                                  const NoConvertToOptional&>::value));
-  // U = const absl::optional<NoConvertToOptional>&
-  EXPECT_TRUE((std::is_assignable<absl::optional<CopyConvertFromOptional>&,
-                                  const NoConvertToOptional&>::value));
-  // U = const NoConvertToOptional& triggers SFINAE because
-  // std::is_constructible_v<MoveConvert, const NoConvertToOptional&> is false
-  EXPECT_FALSE((std::is_assignable<absl::optional<MoveConvert>&,
-                                   const NoConvertToOptional&>::value));
-  // U = NoConvertToOptional
-  EXPECT_TRUE((std::is_assignable<absl::optional<MoveConvert>&,
-                                  NoConvertToOptional&&>::value));
-  // U = const NoConvertToOptional& triggers SFINAE because
-  // std::is_constructible_v<MoveConvertFromOptional, const
-  // NoConvertToOptional&> is false
-  EXPECT_FALSE((std::is_assignable<absl::optional<MoveConvertFromOptional>&,
-                                   const NoConvertToOptional&>::value));
-  // U = NoConvertToOptional
-  EXPECT_TRUE((std::is_assignable<absl::optional<MoveConvertFromOptional>&,
-                                  NoConvertToOptional&&>::value));
-  // U = const absl::optional<NoConvertToOptional>&
-  EXPECT_TRUE(
-      (std::is_assignable<absl::optional<CopyConvertFromOptional>&,
-                          const absl::optional<NoConvertToOptional>&>::value));
-  // U = absl::optional<NoConvertToOptional>
-  EXPECT_TRUE(
-      (std::is_assignable<absl::optional<MoveConvertFromOptional>&,
-                          absl::optional<NoConvertToOptional>&&>::value));
-}
-
-// template <typename U> absl::optional<T>& operator=(const absl::optional<U>&
-// rhs); template <typename U> absl::optional<T>& operator=(absl::optional<U>&&
-// rhs);
-TEST(optionalTest, ConvertingAssignment) {
-  absl::optional<int> opt_i;
-  absl::optional<char> opt_c('c');
-  opt_i = opt_c;
-  EXPECT_TRUE(opt_i);
-  EXPECT_EQ(*opt_c, *opt_i);
-  opt_i = absl::optional<char>();
-  EXPECT_FALSE(opt_i);
-  opt_i = absl::optional<char>('d');
-  EXPECT_TRUE(opt_i);
-  EXPECT_EQ('d', *opt_i);
-
-  absl::optional<std::string> opt_str;
-  absl::optional<const char*> opt_cstr("abc");
-  opt_str = opt_cstr;
-  EXPECT_TRUE(opt_str);
-  EXPECT_EQ(std::string("abc"), *opt_str);
-  opt_str = absl::optional<const char*>();
-  EXPECT_FALSE(opt_str);
-  opt_str = absl::optional<const char*>("def");
-  EXPECT_TRUE(opt_str);
-  EXPECT_EQ(std::string("def"), *opt_str);
-
-  // operator=(const absl::optional<U>&) with U = NoConvertToOptional
-  EXPECT_TRUE(
-      (std::is_assignable<absl::optional<CopyConvert>,
-                          const absl::optional<NoConvertToOptional>&>::value));
-  // operator=(const absl::optional<U>&) with U = NoConvertToOptional
-  // triggers SFINAE because
-  // std::is_constructible_v<MoveConvert, const NoConvertToOptional&> is false
-  EXPECT_FALSE(
-      (std::is_assignable<absl::optional<MoveConvert>&,
-                          const absl::optional<NoConvertToOptional>&>::value));
-  // operator=(absl::optional<U>&&) with U = NoConvertToOptional
-  EXPECT_TRUE(
-      (std::is_assignable<absl::optional<MoveConvert>&,
-                          absl::optional<NoConvertToOptional>&&>::value));
-  // operator=(const absl::optional<U>&) with U = NoConvertToOptional triggers
-  // SFINAE because std::is_constructible_v<MoveConvertFromOptional, const
-  // NoConvertToOptional&> is false. operator=(U&&) with U = const
-  // absl::optional<NoConverToOptional>& triggers SFINAE because
-  // std::is_constructible<MoveConvertFromOptional,
-  // absl::optional<NoConvertToOptional>&&> is true.
-  EXPECT_FALSE(
-      (std::is_assignable<absl::optional<MoveConvertFromOptional>&,
-                          const absl::optional<NoConvertToOptional>&>::value));
-}
-
-TEST(optionalTest, ResetAndHasValue) {
-  StructorListener listener;
-  Listenable::listener = &listener;
-  absl::optional<Listenable> opt;
-  EXPECT_FALSE(opt);
-  EXPECT_FALSE(opt.has_value());
-  opt.emplace();
-  EXPECT_TRUE(opt);
-  EXPECT_TRUE(opt.has_value());
-  opt.reset();
-  EXPECT_FALSE(opt);
-  EXPECT_FALSE(opt.has_value());
-  EXPECT_EQ(1, listener.destruct);
-  opt.reset();
-  EXPECT_FALSE(opt);
-  EXPECT_FALSE(opt.has_value());
-
-  constexpr absl::optional<int> empty;
-  static_assert(!empty.has_value(), "");
-  constexpr absl::optional<int> nonempty(1);
-  static_assert(nonempty.has_value(), "");
-}
-
-TEST(optionalTest, Emplace) {
-  StructorListener listener;
-  Listenable::listener = &listener;
-  absl::optional<Listenable> opt;
-  EXPECT_FALSE(opt);
-  opt.emplace(1);
-  EXPECT_TRUE(opt);
-  opt.emplace(1, 2);
-  EXPECT_EQ(1, listener.construct1);
-  EXPECT_EQ(1, listener.construct2);
-  EXPECT_EQ(1, listener.destruct);
-
-  absl::optional<std::string> o;
-  EXPECT_TRUE((std::is_same<std::string&, decltype(o.emplace("abc"))>::value));
-  std::string& ref = o.emplace("abc");
-  EXPECT_EQ(&ref, &o.value());
-}
-
-TEST(optionalTest, ListEmplace) {
-  StructorListener listener;
-  Listenable::listener = &listener;
-  absl::optional<Listenable> opt;
-  EXPECT_FALSE(opt);
-  opt.emplace({1});
-  EXPECT_TRUE(opt);
-  opt.emplace({1, 2});
-  EXPECT_EQ(2, listener.listinit);
-  EXPECT_EQ(1, listener.destruct);
-
-  absl::optional<Listenable> o;
-  EXPECT_TRUE((std::is_same<Listenable&, decltype(o.emplace({1}))>::value));
-  Listenable& ref = o.emplace({1});
-  EXPECT_EQ(&ref, &o.value());
-}
-
-TEST(optionalTest, Swap) {
-  absl::optional<int> opt_empty, opt1 = 1, opt2 = 2;
-  EXPECT_FALSE(opt_empty);
-  EXPECT_TRUE(opt1);
-  EXPECT_EQ(1, opt1.value());
-  EXPECT_TRUE(opt2);
-  EXPECT_EQ(2, opt2.value());
-  swap(opt_empty, opt1);
-  EXPECT_FALSE(opt1);
-  EXPECT_TRUE(opt_empty);
-  EXPECT_EQ(1, opt_empty.value());
-  EXPECT_TRUE(opt2);
-  EXPECT_EQ(2, opt2.value());
-  swap(opt_empty, opt1);
-  EXPECT_FALSE(opt_empty);
-  EXPECT_TRUE(opt1);
-  EXPECT_EQ(1, opt1.value());
-  EXPECT_TRUE(opt2);
-  EXPECT_EQ(2, opt2.value());
-  swap(opt1, opt2);
-  EXPECT_FALSE(opt_empty);
-  EXPECT_TRUE(opt1);
-  EXPECT_EQ(2, opt1.value());
-  EXPECT_TRUE(opt2);
-  EXPECT_EQ(1, opt2.value());
-
-  EXPECT_TRUE(noexcept(opt1.swap(opt2)));
-  EXPECT_TRUE(noexcept(swap(opt1, opt2)));
-}
-
-template <int v>
-struct DeletedOpAddr {
-  int value = v;
-  constexpr DeletedOpAddr() = default;
-  constexpr const DeletedOpAddr<v>* operator&() const = delete;  // NOLINT
-  DeletedOpAddr<v>* operator&() = delete;                        // NOLINT
-};
-
-// The static_assert featuring a constexpr call to operator->() is commented out
-// to document the fact that the current implementation of absl::optional<T>
-// expects such usecases to be malformed and not compile.
-TEST(optionalTest, OperatorAddr) {
-  constexpr int v = -1;
-  {  // constexpr
-    constexpr absl::optional<DeletedOpAddr<v>> opt(absl::in_place_t{});
-    static_assert(opt.has_value(), "");
-    // static_assert(opt->value == v, "");
-    static_assert((*opt).value == v, "");
-  }
-  {  // non-constexpr
-    const absl::optional<DeletedOpAddr<v>> opt(absl::in_place_t{});
-    EXPECT_TRUE(opt.has_value());
-    EXPECT_TRUE(opt->value == v);
-    EXPECT_TRUE((*opt).value == v);
-  }
-}
-
-TEST(optionalTest, PointerStuff) {
-  absl::optional<std::string> opt(absl::in_place, "foo");
-  EXPECT_EQ("foo", *opt);
-  const auto& opt_const = opt;
-  EXPECT_EQ("foo", *opt_const);
-  EXPECT_EQ(opt->size(), 3);
-  EXPECT_EQ(opt_const->size(), 3);
-
-  constexpr absl::optional<ConstexprType> opt1(1);
-  static_assert((*opt1).x == ConstexprType::kCtorInt, "");
-}
-
-// gcc has a bug pre 4.9.1 where it doesn't do correct overload resolution
-// when overloads are const-qualified and *this is an raluve.
-// Skip that test to make the build green again when using the old compiler.
-// https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59296 is fixed in 4.9.1.
-#if defined(__GNUC__) && !defined(__clang__)
-#define GCC_VERSION (__GNUC__ * 10000 \
-                     + __GNUC_MINOR__ * 100 \
-                     + __GNUC_PATCHLEVEL__)
-#if GCC_VERSION < 40901
-#define ABSL_SKIP_OVERLOAD_TEST_DUE_TO_GCC_BUG
-#endif
-#endif
-
-// MSVC has a bug with "cv-qualifiers in class construction", fixed in 2017. See
-// https://docs.microsoft.com/en-us/cpp/cpp-conformance-improvements-2017#bug-fixes
-// The compiler some incorrectly ingores the cv-qualifier when generating a
-// class object via a constructor call. For example:
-//
-// class optional {
-//   constexpr T&& value() &&;
-//   constexpr const T&& value() const &&;
-// }
-//
-// using COI = const absl::optional<int>;
-// static_assert(2 == COI(2).value(), "");  // const &&
-//
-// This should invoke the "const &&" overload but since it ignores the const
-// qualifier it finds the "&&" overload the best candidate.
-#if defined(_MSC_VER) && _MSC_VER < 1910
-#define ABSL_SKIP_OVERLOAD_TEST_DUE_TO_MSVC_BUG
-#endif
-
-TEST(optionalTest, Value) {
-  using O = absl::optional<std::string>;
-  using CO = const absl::optional<std::string>;
-  using OC = absl::optional<const std::string>;
-  O lvalue(absl::in_place, "lvalue");
-  CO clvalue(absl::in_place, "clvalue");
-  OC lvalue_c(absl::in_place, "lvalue_c");
-  EXPECT_EQ("lvalue", lvalue.value());
-  EXPECT_EQ("clvalue", clvalue.value());
-  EXPECT_EQ("lvalue_c", lvalue_c.value());
-  EXPECT_EQ("xvalue", O(absl::in_place, "xvalue").value());
-  EXPECT_EQ("xvalue_c", OC(absl::in_place, "xvalue_c").value());
-#ifndef ABSL_SKIP_OVERLOAD_TEST_DUE_TO_GCC_BUG
-  EXPECT_EQ("cxvalue", CO(absl::in_place, "cxvalue").value());
-#endif
-  EXPECT_EQ("&", TypeQuals(lvalue.value()));
-  EXPECT_EQ("c&", TypeQuals(clvalue.value()));
-  EXPECT_EQ("c&", TypeQuals(lvalue_c.value()));
-  EXPECT_EQ("&&", TypeQuals(O(absl::in_place, "xvalue").value()));
-#if !defined(ABSL_SKIP_OVERLOAD_TEST_DUE_TO_MSVC_BUG) && \
-    !defined(ABSL_SKIP_OVERLOAD_TEST_DUE_TO_GCC_BUG)
-  EXPECT_EQ("c&&", TypeQuals(CO(absl::in_place, "cxvalue").value()));
-#endif
-  EXPECT_EQ("c&&", TypeQuals(OC(absl::in_place, "xvalue_c").value()));
-
-  // test on volatile type
-  using OV = absl::optional<volatile int>;
-  OV lvalue_v(absl::in_place, 42);
-  EXPECT_EQ(42, lvalue_v.value());
-  EXPECT_EQ(42, OV(42).value());
-  EXPECT_TRUE((std::is_same<volatile int&, decltype(lvalue_v.value())>::value));
-  EXPECT_TRUE((std::is_same<volatile int&&, decltype(OV(42).value())>::value));
-
-  // test exception throw on value()
-  absl::optional<int> empty;
-#ifdef ABSL_HAVE_EXCEPTIONS
-  EXPECT_THROW((void)empty.value(), absl::bad_optional_access);
-#else
-  EXPECT_DEATH((void)empty.value(), "Bad optional access");
-#endif
-
-  // test constexpr value()
-  constexpr absl::optional<int> o1(1);
-  static_assert(1 == o1.value(), "");  // const &
-#if !defined(ABSL_SKIP_OVERLOAD_TEST_DUE_TO_MSVC_BUG) && \
-    !defined(ABSL_SKIP_OVERLOAD_TEST_DUE_TO_GCC_BUG)
-  using COI = const absl::optional<int>;
-  static_assert(2 == COI(2).value(), "");  // const &&
-#endif
-}
-
-TEST(optionalTest, DerefOperator) {
-  using O = absl::optional<std::string>;
-  using CO = const absl::optional<std::string>;
-  using OC = absl::optional<const std::string>;
-  O lvalue(absl::in_place, "lvalue");
-  CO clvalue(absl::in_place, "clvalue");
-  OC lvalue_c(absl::in_place, "lvalue_c");
-  EXPECT_EQ("lvalue", *lvalue);
-  EXPECT_EQ("clvalue", *clvalue);
-  EXPECT_EQ("lvalue_c", *lvalue_c);
-  EXPECT_EQ("xvalue", *O(absl::in_place, "xvalue"));
-  EXPECT_EQ("xvalue_c", *OC(absl::in_place, "xvalue_c"));
-#ifndef ABSL_SKIP_OVERLOAD_TEST_DUE_TO_GCC_BUG
-  EXPECT_EQ("cxvalue", *CO(absl::in_place, "cxvalue"));
-#endif
-  EXPECT_EQ("&", TypeQuals(*lvalue));
-  EXPECT_EQ("c&", TypeQuals(*clvalue));
-  EXPECT_EQ("&&", TypeQuals(*O(absl::in_place, "xvalue")));
-#if !defined(ABSL_SKIP_OVERLOAD_TEST_DUE_TO_MSVC_BUG) && \
-    !defined(ABSL_SKIP_OVERLOAD_TEST_DUE_TO_GCC_BUG)
-  EXPECT_EQ("c&&", TypeQuals(*CO(absl::in_place, "cxvalue")));
-#endif
-  EXPECT_EQ("c&&", TypeQuals(*OC(absl::in_place, "xvalue_c")));
-
-  // test on volatile type
-  using OV = absl::optional<volatile int>;
-  OV lvalue_v(absl::in_place, 42);
-  EXPECT_EQ(42, *lvalue_v);
-  EXPECT_EQ(42, *OV(42));
-  EXPECT_TRUE((std::is_same<volatile int&, decltype(*lvalue_v)>::value));
-  EXPECT_TRUE((std::is_same<volatile int&&, decltype(*OV(42))>::value));
-
-  constexpr absl::optional<int> opt1(1);
-  static_assert(*opt1 == 1, "");
-#if !defined(ABSL_SKIP_OVERLOAD_TEST_DUE_TO_MSVC_BUG) && \
-    !defined(ABSL_SKIP_OVERLOAD_TEST_DUE_TO_GCC_BUG)
-  using COI = const absl::optional<int>;
-  static_assert(*COI(2) == 2, "");
-#endif
-}
-
-TEST(optionalTest, ValueOr) {
-  absl::optional<double> opt_empty, opt_set = 1.2;
-  EXPECT_EQ(42.0, opt_empty.value_or(42));
-  EXPECT_EQ(1.2, opt_set.value_or(42));
-  EXPECT_EQ(42.0, absl::optional<double>().value_or(42));
-  EXPECT_EQ(1.2, absl::optional<double>(1.2).value_or(42));
-
-  constexpr absl::optional<double> copt_empty, copt_set = {1.2};
-  static_assert(42.0 == copt_empty.value_or(42), "");
-  static_assert(1.2 == copt_set.value_or(42), "");
-#ifndef ABSL_SKIP_OVERLOAD_TEST_DUE_TO_MSVC_BUG
-  using COD = const absl::optional<double>;
-  static_assert(42.0 == COD().value_or(42), "");
-  static_assert(1.2 == COD(1.2).value_or(42), "");
-#endif
-}
-
-// make_optional cannot be constexpr until C++17
-TEST(optionalTest, make_optional) {
-  auto opt_int = absl::make_optional(42);
-  EXPECT_TRUE((std::is_same<decltype(opt_int), absl::optional<int>>::value));
-  EXPECT_EQ(42, opt_int);
-
-  StructorListener listener;
-  Listenable::listener = &listener;
-
-  absl::optional<Listenable> opt0 = absl::make_optional<Listenable>();
-  EXPECT_EQ(1, listener.construct0);
-  absl::optional<Listenable> opt1 = absl::make_optional<Listenable>(1);
-  EXPECT_EQ(1, listener.construct1);
-  absl::optional<Listenable> opt2 = absl::make_optional<Listenable>(1, 2);
-  EXPECT_EQ(1, listener.construct2);
-  absl::optional<Listenable> opt3 = absl::make_optional<Listenable>({1});
-  absl::optional<Listenable> opt4 = absl::make_optional<Listenable>({1, 2});
-  EXPECT_EQ(2, listener.listinit);
-
-  // Constexpr tests on trivially copyable types
-  // optional<T> has trivial copy/move ctors when T is trivially copyable.
-  // For nontrivial types with constexpr constructors, we need copy elision in
-  // C++17 for make_optional to be constexpr.
-  {
-    constexpr absl::optional<int> c_opt = absl::make_optional(42);
-    static_assert(c_opt.value() == 42, "");
-  }
-  {
-    struct TrivialCopyable {
-      constexpr TrivialCopyable() : x(0) {}
-      constexpr explicit TrivialCopyable(int i) : x(i) {}
-      int x;
-    };
-
-    constexpr TrivialCopyable v;
-    constexpr absl::optional<TrivialCopyable> c_opt0 = absl::make_optional(v);
-    static_assert((*c_opt0).x == 0, "");
-    constexpr absl::optional<TrivialCopyable> c_opt1 =
-        absl::make_optional<TrivialCopyable>();
-    static_assert((*c_opt1).x == 0, "");
-    constexpr absl::optional<TrivialCopyable> c_opt2 =
-        absl::make_optional<TrivialCopyable>(42);
-    static_assert((*c_opt2).x == 42, "");
-  }
-}
-
-template <typename T, typename U>
-void optionalTest_Comparisons_EXPECT_LESS(T x, U y) {
-  EXPECT_FALSE(x == y);
-  EXPECT_TRUE(x != y);
-  EXPECT_TRUE(x < y);
-  EXPECT_FALSE(x > y);
-  EXPECT_TRUE(x <= y);
-  EXPECT_FALSE(x >= y);
-}
-
-template <typename T, typename U>
-void optionalTest_Comparisons_EXPECT_SAME(T x, U y) {
-  EXPECT_TRUE(x == y);
-  EXPECT_FALSE(x != y);
-  EXPECT_FALSE(x < y);
-  EXPECT_FALSE(x > y);
-  EXPECT_TRUE(x <= y);
-  EXPECT_TRUE(x >= y);
-}
-
-template <typename T, typename U>
-void optionalTest_Comparisons_EXPECT_GREATER(T x, U y) {
-  EXPECT_FALSE(x == y);
-  EXPECT_TRUE(x != y);
-  EXPECT_FALSE(x < y);
-  EXPECT_TRUE(x > y);
-  EXPECT_FALSE(x <= y);
-  EXPECT_TRUE(x >= y);
-}
-
-
-template <typename T, typename U, typename V>
-void TestComparisons() {
-  absl::optional<T> ae, a2{2}, a4{4};
-  absl::optional<U> be, b2{2}, b4{4};
-  V v3 = 3;
-
-  // LHS: absl::nullopt, ae, a2, v3, a4
-  // RHS: absl::nullopt, be, b2, v3, b4
-
-  // optionalTest_Comparisons_EXPECT_NOT_TO_WORK(absl::nullopt,absl::nullopt);
-  optionalTest_Comparisons_EXPECT_SAME(absl::nullopt, be);
-  optionalTest_Comparisons_EXPECT_LESS(absl::nullopt, b2);
-  // optionalTest_Comparisons_EXPECT_NOT_TO_WORK(absl::nullopt,v3);
-  optionalTest_Comparisons_EXPECT_LESS(absl::nullopt, b4);
-
-  optionalTest_Comparisons_EXPECT_SAME(ae, absl::nullopt);
-  optionalTest_Comparisons_EXPECT_SAME(ae, be);
-  optionalTest_Comparisons_EXPECT_LESS(ae, b2);
-  optionalTest_Comparisons_EXPECT_LESS(ae, v3);
-  optionalTest_Comparisons_EXPECT_LESS(ae, b4);
-
-  optionalTest_Comparisons_EXPECT_GREATER(a2, absl::nullopt);
-  optionalTest_Comparisons_EXPECT_GREATER(a2, be);
-  optionalTest_Comparisons_EXPECT_SAME(a2, b2);
-  optionalTest_Comparisons_EXPECT_LESS(a2, v3);
-  optionalTest_Comparisons_EXPECT_LESS(a2, b4);
-
-  // optionalTest_Comparisons_EXPECT_NOT_TO_WORK(v3,absl::nullopt);
-  optionalTest_Comparisons_EXPECT_GREATER(v3, be);
-  optionalTest_Comparisons_EXPECT_GREATER(v3, b2);
-  optionalTest_Comparisons_EXPECT_SAME(v3, v3);
-  optionalTest_Comparisons_EXPECT_LESS(v3, b4);
-
-  optionalTest_Comparisons_EXPECT_GREATER(a4, absl::nullopt);
-  optionalTest_Comparisons_EXPECT_GREATER(a4, be);
-  optionalTest_Comparisons_EXPECT_GREATER(a4, b2);
-  optionalTest_Comparisons_EXPECT_GREATER(a4, v3);
-  optionalTest_Comparisons_EXPECT_SAME(a4, b4);
-}
-
-struct Int1 {
-  Int1() = default;
-  Int1(int i) : i(i) {}  // NOLINT(runtime/explicit)
-  int i;
-};
-
-struct Int2 {
-  Int2() = default;
-  Int2(int i) : i(i) {}  // NOLINT(runtime/explicit)
-  int i;
-};
-
-// comparison between Int1 and Int2
-constexpr bool operator==(const Int1& lhs, const Int2& rhs) {
-  return lhs.i == rhs.i;
-}
-constexpr bool operator!=(const Int1& lhs, const Int2& rhs) {
-  return !(lhs == rhs);
-}
-constexpr bool operator<(const Int1& lhs, const Int2& rhs) {
-  return lhs.i < rhs.i;
-}
-constexpr bool operator<=(const Int1& lhs, const Int2& rhs) {
-  return lhs < rhs || lhs == rhs;
-}
-constexpr bool operator>(const Int1& lhs, const Int2& rhs) {
-  return !(lhs <= rhs);
-}
-constexpr bool operator>=(const Int1& lhs, const Int2& rhs) {
-  return !(lhs < rhs);
-}
-
-TEST(optionalTest, Comparisons) {
-  TestComparisons<int, int, int>();
-  TestComparisons<const int, int, int>();
-  TestComparisons<Int1, int, int>();
-  TestComparisons<int, Int2, int>();
-  TestComparisons<Int1, Int2, int>();
-
-  // compare absl::optional<std::string> with const char*
-  absl::optional<std::string> opt_str = "abc";
-  const char* cstr = "abc";
-  EXPECT_TRUE(opt_str == cstr);
-  // compare absl::optional<std::string> with absl::optional<const char*>
-  absl::optional<const char*> opt_cstr = cstr;
-  EXPECT_TRUE(opt_str == opt_cstr);
-  // compare absl::optional<std::string> with absl::optional<absl::string_view>
-  absl::optional<absl::string_view> e1;
-  absl::optional<std::string> e2;
-  EXPECT_TRUE(e1 == e2);
-}
-
-
-TEST(optionalTest, SwapRegression) {
-  StructorListener listener;
-  Listenable::listener = &listener;
-
-  {
-    absl::optional<Listenable> a;
-    absl::optional<Listenable> b(absl::in_place);
-    a.swap(b);
-  }
-
-  EXPECT_EQ(1, listener.construct0);
-  EXPECT_EQ(1, listener.move);
-  EXPECT_EQ(2, listener.destruct);
-
-  {
-    absl::optional<Listenable> a(absl::in_place);
-    absl::optional<Listenable> b;
-    a.swap(b);
-  }
-
-  EXPECT_EQ(2, listener.construct0);
-  EXPECT_EQ(2, listener.move);
-  EXPECT_EQ(4, listener.destruct);
-}
-
-TEST(optionalTest, BigStringLeakCheck) {
-  constexpr size_t n = 1 << 16;
-
-  using OS = absl::optional<std::string>;
-
-  OS a;
-  OS b = absl::nullopt;
-  OS c = std::string(n, 'c');
-  std::string sd(n, 'd');
-  OS d = sd;
-  OS e(absl::in_place, n, 'e');
-  OS f;
-  f.emplace(n, 'f');
-
-  OS ca(a);
-  OS cb(b);
-  OS cc(c);
-  OS cd(d);
-  OS ce(e);
-
-  OS oa;
-  OS ob = absl::nullopt;
-  OS oc = std::string(n, 'c');
-  std::string sod(n, 'd');
-  OS od = sod;
-  OS oe(absl::in_place, n, 'e');
-  OS of;
-  of.emplace(n, 'f');
-
-  OS ma(std::move(oa));
-  OS mb(std::move(ob));
-  OS mc(std::move(oc));
-  OS md(std::move(od));
-  OS me(std::move(oe));
-  OS mf(std::move(of));
-
-  OS aa1;
-  OS ab1 = absl::nullopt;
-  OS ac1 = std::string(n, 'c');
-  std::string sad1(n, 'd');
-  OS ad1 = sad1;
-  OS ae1(absl::in_place, n, 'e');
-  OS af1;
-  af1.emplace(n, 'f');
-
-  OS aa2;
-  OS ab2 = absl::nullopt;
-  OS ac2 = std::string(n, 'c');
-  std::string sad2(n, 'd');
-  OS ad2 = sad2;
-  OS ae2(absl::in_place, n, 'e');
-  OS af2;
-  af2.emplace(n, 'f');
-
-  aa1 = af2;
-  ab1 = ae2;
-  ac1 = ad2;
-  ad1 = ac2;
-  ae1 = ab2;
-  af1 = aa2;
-
-  OS aa3;
-  OS ab3 = absl::nullopt;
-  OS ac3 = std::string(n, 'c');
-  std::string sad3(n, 'd');
-  OS ad3 = sad3;
-  OS ae3(absl::in_place, n, 'e');
-  OS af3;
-  af3.emplace(n, 'f');
-
-  aa3 = absl::nullopt;
-  ab3 = absl::nullopt;
-  ac3 = absl::nullopt;
-  ad3 = absl::nullopt;
-  ae3 = absl::nullopt;
-  af3 = absl::nullopt;
-
-  OS aa4;
-  OS ab4 = absl::nullopt;
-  OS ac4 = std::string(n, 'c');
-  std::string sad4(n, 'd');
-  OS ad4 = sad4;
-  OS ae4(absl::in_place, n, 'e');
-  OS af4;
-  af4.emplace(n, 'f');
-
-  aa4 = OS(absl::in_place, n, 'a');
-  ab4 = OS(absl::in_place, n, 'b');
-  ac4 = OS(absl::in_place, n, 'c');
-  ad4 = OS(absl::in_place, n, 'd');
-  ae4 = OS(absl::in_place, n, 'e');
-  af4 = OS(absl::in_place, n, 'f');
-
-  OS aa5;
-  OS ab5 = absl::nullopt;
-  OS ac5 = std::string(n, 'c');
-  std::string sad5(n, 'd');
-  OS ad5 = sad5;
-  OS ae5(absl::in_place, n, 'e');
-  OS af5;
-  af5.emplace(n, 'f');
-
-  std::string saa5(n, 'a');
-  std::string sab5(n, 'a');
-  std::string sac5(n, 'a');
-  std::string sad52(n, 'a');
-  std::string sae5(n, 'a');
-  std::string saf5(n, 'a');
-
-  aa5 = saa5;
-  ab5 = sab5;
-  ac5 = sac5;
-  ad5 = sad52;
-  ae5 = sae5;
-  af5 = saf5;
-
-  OS aa6;
-  OS ab6 = absl::nullopt;
-  OS ac6 = std::string(n, 'c');
-  std::string sad6(n, 'd');
-  OS ad6 = sad6;
-  OS ae6(absl::in_place, n, 'e');
-  OS af6;
-  af6.emplace(n, 'f');
-
-  aa6 = std::string(n, 'a');
-  ab6 = std::string(n, 'b');
-  ac6 = std::string(n, 'c');
-  ad6 = std::string(n, 'd');
-  ae6 = std::string(n, 'e');
-  af6 = std::string(n, 'f');
-
-  OS aa7;
-  OS ab7 = absl::nullopt;
-  OS ac7 = std::string(n, 'c');
-  std::string sad7(n, 'd');
-  OS ad7 = sad7;
-  OS ae7(absl::in_place, n, 'e');
-  OS af7;
-  af7.emplace(n, 'f');
-
-  aa7.emplace(n, 'A');
-  ab7.emplace(n, 'B');
-  ac7.emplace(n, 'C');
-  ad7.emplace(n, 'D');
-  ae7.emplace(n, 'E');
-  af7.emplace(n, 'F');
-}
-
-TEST(optionalTest, MoveAssignRegression) {
-  StructorListener listener;
-  Listenable::listener = &listener;
-
-  {
-    absl::optional<Listenable> a;
-    Listenable b;
-    a = std::move(b);
-  }
-
-  EXPECT_EQ(1, listener.construct0);
-  EXPECT_EQ(1, listener.move);
-  EXPECT_EQ(2, listener.destruct);
-}
-
-TEST(optionalTest, ValueType) {
-  EXPECT_TRUE((std::is_same<absl::optional<int>::value_type, int>::value));
-  EXPECT_TRUE((std::is_same<absl::optional<std::string>::value_type,
-                            std::string>::value));
-  EXPECT_FALSE(
-      (std::is_same<absl::optional<int>::value_type, absl::nullopt_t>::value));
-}
-
-template <typename T>
-struct is_hash_enabled_for {
-  template <typename U, typename = decltype(std::hash<U>()(std::declval<U>()))>
-  static std::true_type test(int);
-
-  template <typename U>
-  static std::false_type test(...);
-
-  static constexpr bool value = decltype(test<T>(0))::value;
-};
-
-TEST(optionalTest, Hash) {
-  std::hash<absl::optional<int>> hash;
-  std::set<size_t> hashcodes;
-  hashcodes.insert(hash(absl::nullopt));
-  for (int i = 0; i < 100; ++i) {
-    hashcodes.insert(hash(i));
-  }
-  EXPECT_GT(hashcodes.size(), 90);
-
-  static_assert(is_hash_enabled_for<absl::optional<int>>::value, "");
-  static_assert(is_hash_enabled_for<absl::optional<Hashable>>::value, "");
-  static_assert(
-      absl::type_traits_internal::IsHashable<absl::optional<int>>::value, "");
-  static_assert(
-      absl::type_traits_internal::IsHashable<absl::optional<Hashable>>::value,
-      "");
-  absl::type_traits_internal::AssertHashEnabled<absl::optional<int>>();
-  absl::type_traits_internal::AssertHashEnabled<absl::optional<Hashable>>();
-
-#if ABSL_META_INTERNAL_STD_HASH_SFINAE_FRIENDLY_
-  static_assert(!is_hash_enabled_for<absl::optional<NonHashable>>::value, "");
-  static_assert(!absl::type_traits_internal::IsHashable<
-                    absl::optional<NonHashable>>::value,
-                "");
-#endif
-
-  // libstdc++ std::optional is missing remove_const_t, i.e. it's using
-  // std::hash<T> rather than std::hash<std::remove_const_t<T>>.
-  // Reference: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82262
-#ifndef __GLIBCXX__
-  static_assert(is_hash_enabled_for<absl::optional<const int>>::value, "");
-  static_assert(is_hash_enabled_for<absl::optional<const Hashable>>::value, "");
-  std::hash<absl::optional<const int>> c_hash;
-  for (int i = 0; i < 100; ++i) {
-    EXPECT_EQ(hash(i), c_hash(i));
-  }
-#endif
-}
-
-struct MoveMeNoThrow {
-  MoveMeNoThrow() : x(0) {}
-  [[noreturn]] MoveMeNoThrow(const MoveMeNoThrow& other) : x(other.x) {
-    ABSL_RAW_LOG(FATAL, "Should not be called.");
-    abort();
-  }
-  MoveMeNoThrow(MoveMeNoThrow&& other) noexcept : x(other.x) {}
-  int x;
-};
-
-struct MoveMeThrow {
-  MoveMeThrow() : x(0) {}
-  MoveMeThrow(const MoveMeThrow& other) : x(other.x) {}
-  MoveMeThrow(MoveMeThrow&& other) : x(other.x) {}
-  int x;
-};
-
-TEST(optionalTest, NoExcept) {
-  static_assert(
-      std::is_nothrow_move_constructible<absl::optional<MoveMeNoThrow>>::value,
-      "");
-#ifndef ABSL_HAVE_STD_OPTIONAL
-  static_assert(absl::default_allocator_is_nothrow::value ==
-                    std::is_nothrow_move_constructible<
-                        absl::optional<MoveMeThrow>>::value,
-                "");
-#endif
-  std::vector<absl::optional<MoveMeNoThrow>> v;
-  for (int i = 0; i < 10; ++i) v.emplace_back();
-}
-
-struct AnyLike {
-  AnyLike(AnyLike&&) = default;
-  AnyLike(const AnyLike&) = default;
-
-  template <typename ValueType,
-            typename T = typename std::decay<ValueType>::type,
-            typename std::enable_if<
-                !absl::disjunction<
-                    std::is_same<AnyLike, T>,
-                    absl::negation<std::is_copy_constructible<T>>>::value,
-                int>::type = 0>
-  AnyLike(ValueType&&) {}  // NOLINT(runtime/explicit)
-
-  AnyLike& operator=(AnyLike&&) = default;
-  AnyLike& operator=(const AnyLike&) = default;
-
-  template <typename ValueType,
-            typename T = typename std::decay<ValueType>::type>
-  typename std::enable_if<
-      absl::conjunction<absl::negation<std::is_same<AnyLike, T>>,
-                        std::is_copy_constructible<T>>::value,
-      AnyLike&>::type
-  operator=(ValueType&& /* rhs */) {
-    return *this;
-  }
-};
-
-TEST(optionalTest, ConstructionConstraints) {
-  EXPECT_TRUE((std::is_constructible<AnyLike, absl::optional<AnyLike>>::value));
-
-  EXPECT_TRUE(
-      (std::is_constructible<AnyLike, const absl::optional<AnyLike>&>::value));
-
-  EXPECT_TRUE((std::is_constructible<absl::optional<AnyLike>, AnyLike>::value));
-  EXPECT_TRUE(
-      (std::is_constructible<absl::optional<AnyLike>, const AnyLike&>::value));
-
-  EXPECT_TRUE((std::is_convertible<absl::optional<AnyLike>, AnyLike>::value));
-
-  EXPECT_TRUE(
-      (std::is_convertible<const absl::optional<AnyLike>&, AnyLike>::value));
-
-  EXPECT_TRUE((std::is_convertible<AnyLike, absl::optional<AnyLike>>::value));
-  EXPECT_TRUE(
-      (std::is_convertible<const AnyLike&, absl::optional<AnyLike>>::value));
-
-  EXPECT_TRUE(std::is_move_constructible<absl::optional<AnyLike>>::value);
-  EXPECT_TRUE(std::is_copy_constructible<absl::optional<AnyLike>>::value);
-}
-
-TEST(optionalTest, AssignmentConstraints) {
-  EXPECT_TRUE((std::is_assignable<AnyLike&, absl::optional<AnyLike>>::value));
-  EXPECT_TRUE(
-      (std::is_assignable<AnyLike&, const absl::optional<AnyLike>&>::value));
-  EXPECT_TRUE((std::is_assignable<absl::optional<AnyLike>&, AnyLike>::value));
-  EXPECT_TRUE(
-      (std::is_assignable<absl::optional<AnyLike>&, const AnyLike&>::value));
-  EXPECT_TRUE(std::is_move_assignable<absl::optional<AnyLike>>::value);
-  EXPECT_TRUE(absl::is_copy_assignable<absl::optional<AnyLike>>::value);
-}
-
-#if !defined(__EMSCRIPTEN__)
-struct NestedClassBug {
-  struct Inner {
-    bool dummy = false;
-  };
-  absl::optional<Inner> value;
-};
-
-TEST(optionalTest, InPlaceTSFINAEBug) {
-  NestedClassBug b;
-  ((void)b);
-  using Inner = NestedClassBug::Inner;
-
-  EXPECT_TRUE((std::is_default_constructible<Inner>::value));
-  EXPECT_TRUE((std::is_constructible<Inner>::value));
-  EXPECT_TRUE(
-      (std::is_constructible<absl::optional<Inner>, absl::in_place_t>::value));
-
-  absl::optional<Inner> o(absl::in_place);
-  EXPECT_TRUE(o.has_value());
-  o.emplace();
-  EXPECT_TRUE(o.has_value());
-}
-#endif  // !defined(__EMSCRIPTEN__)
-
-}  // namespace
-
-#endif  // #if !defined(ABSL_HAVE_STD_OPTIONAL)
diff --git a/absl/random/internal/seed_salting_sequence_generator_empty_sequence.cc b/absl/types/source_location.cc
similarity index 54%
rename from absl/random/internal/seed_salting_sequence_generator_empty_sequence.cc
rename to absl/types/source_location.cc
index 8797e2e7a6f..47028a542aa 100644
--- a/absl/random/internal/seed_salting_sequence_generator_empty_sequence.cc
+++ b/absl/types/source_location.cc
@@ -1,10 +1,10 @@
-// Copyright 2017 The Abseil Authors.
+// Copyright 2026 The Abseil Authors
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
 // You may obtain a copy of the License at
 //
-//      https://www.apache.org/licenses/LICENSE-2.0
+//     https://www.apache.org/licenses/LICENSE-2.0
 //
 // Unless required by applicable law or agreed to in writing, software
 // distributed under the License is distributed on an "AS IS" BASIS,
@@ -12,19 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-#include <iostream>
-#include <random>
+#include "absl/types/source_location.h"
 
-#include "absl/random/random.h"
-
-// This program is used in integration tests.
-
-int main() {
-  std::seed_seq seed_seq{};
-  absl::BitGen rng(seed_seq);
-  constexpr size_t kSequenceLength = 8;
-  for (size_t i = 0; i < kSequenceLength; i++) {
-    std::cout << rng() << "\n";
-  }
-  return 0;
-}
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
diff --git a/absl/types/source_location.h b/absl/types/source_location.h
new file mode 100644
index 00000000000..6e14836d66f
--- /dev/null
+++ b/absl/types/source_location.h
@@ -0,0 +1,172 @@
+// Copyright 2017 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: source_location.h
+// -----------------------------------------------------------------------------
+//
+// absl::SourceLocation provides source-code location info for C++17 and later.
+//
+// Critically, it is a **view type** (like std::string_view). Unlike
+// std::source_location, it is not permanently valid and must not outlive its
+// source. Using an invalid location is an error and may result in warnings
+// or crashes.
+//
+// Additionally, it does not guarantee the retention of all caller information
+// (e.g. column or function name) and may e.g. return unspecified values for
+// performance reasons.
+//
+// To define a function that has access to the source location of the
+// callsite, define it with a parameter of type `absl::SourceLocation`. The
+// caller can then invoke the function, passing
+// `absl::SourceLocation::current()` as the argument.
+//
+// If at all possible, make the `absl::SourceLocation` parameter be the
+// function's last parameter. That way, when `std::source_location` is
+// available, you will be able to switch to it, and give the parameter a default
+// argument of `std::source_location::current()`. Users will then be able to
+// omit that argument, and the default will automatically capture the location
+// of the callsite.
+
+#ifndef ABSL_TYPES_SOURCE_LOCATION_H_
+#define ABSL_TYPES_SOURCE_LOCATION_H_
+
+#include <cstdint>
+#include <type_traits>
+
+#include "absl/base/config.h"
+#include "absl/base/nullability.h"
+
+// This needs to come after absl/base/config.h, which is responsible for
+// defining ABSL_HAVE_STD_SOURCE_LOCATION
+#ifdef ABSL_HAVE_STD_SOURCE_LOCATION
+#include <source_location>  // NOLINT(build/c++20)
+#endif
+
+// For OSS release, whether to alias to std::source_location is configurable via
+// config.h/options.h, similar to std::string_view/variant/etc.
+#if defined(ABSL_USES_STD_SOURCE_LOCATION) && \
+    defined(ABSL_HAVE_STD_SOURCE_LOCATION)
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+using SourceLocation = std::source_location;
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#else  // ABSL_HAVE_STD_SOURCE_LOCATION
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+// C++17-compatible class representing a specific location in the source code of
+// a program. Similar to std::source_location, but with a few key differences
+// explained above.
+class SourceLocation {
+  struct PrivateTag {
+   private:
+    explicit PrivateTag() = default;
+    friend class SourceLocation;
+  };
+
+ public:
+  // Avoid this constructor; it populates the object with dummy values.
+  SourceLocation() = default;
+
+#ifdef ABSL_HAVE_STD_SOURCE_LOCATION
+  constexpr SourceLocation(  // NOLINT(google-explicit-constructor)
+      std::source_location loc)
+      : SourceLocation(loc.line(), loc.file_name()) {}
+#endif
+
+#ifdef ABSL_INTERNAL_HAVE_BUILTIN_LINE_FILE
+  // SourceLocation::current
+  //
+  // Creates a `SourceLocation` based on the current source location. Currently,
+  // it only captures file and line information for efficiency purposes, but
+  // that is subject to change. APIs that accept a `SourceLocation` as a default
+  // parameter can use this to capture their caller's locations.
+  //
+  // Example:
+  //
+  //   void TracedAdd(int i, SourceLocation loc = SourceLocation::current()) {
+  //     std::cout << loc.file_name() << ":" << loc.line() << " added " << i;
+  //     ...
+  //   }
+  //
+  //   void UserCode() {
+  //     TracedAdd(1);
+  //     TracedAdd(2);
+  //   }
+  static constexpr SourceLocation current(
+      PrivateTag = PrivateTag{}, std::uint_least32_t line = __builtin_LINE(),
+      const char* absl_nonnull file_name = __builtin_FILE()) {
+    return SourceLocation(line, file_name);
+  }
+#else
+  // Creates a dummy `SourceLocation` of "<source_location>" at line number 1,
+  // if no `SourceLocation::current()` implementation is available.
+  static constexpr SourceLocation current() {
+    return SourceLocation(1, "<source_location>");
+  }
+#endif
+  // The line number of the captured source location, or an unspecified value
+  // if this information is not available.
+  constexpr std::uint_least32_t line() const noexcept { return line_; }
+
+  // The column number of the captured source location, or an unspecified value
+  // if this information is not available.
+  constexpr std::uint_least32_t column() const noexcept { return 0; }
+
+  // The file name of the captured source location, or an unspecified string
+  // if this information is not available. Guaranteed to never be NULL.
+  constexpr const char* absl_nonnull file_name() const noexcept {
+    return file_name_;
+  }
+
+  // The function name of the captured source location, or an unspecified string
+  // if this information is not available. Guaranteed to never be NULL.
+  //
+  // NOTE: Currently, we deliberately avoid providing the function name, as it
+  // can bloat binary sizes and is non-critical. This may change in the future.
+  constexpr const char* absl_nonnull function_name() const noexcept {
+    return "";
+  }
+
+ private:
+  // `file_name` must outlive all copies of the `absl::SourceLocation` object,
+  // so in practice it should be a string literal.
+  constexpr SourceLocation(std::uint_least32_t line,
+                           const char* absl_nonnull file_name)
+      : line_(line), file_name_(file_name) {}
+
+  // We would use [[maybe_unused]] here, but it doesn't work on all supported
+  // toolchains at the moment.
+  friend constexpr int UseUnused() {
+    static_assert(SourceLocation(0, nullptr).unused_column_ == 0,
+                  "Use the otherwise-unused member.");
+    return 0;
+  }
+
+  // "unused" members are present to minimize future changes in the size of this
+  // type.
+  std::uint_least32_t line_ = 0;
+  std::uint_least32_t unused_column_ = 0;
+  const char* absl_nonnull file_name_ = "";
+};
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+#endif  // ABSL_HAVE_STD_SOURCE_LOCATION
+
+#endif  // ABSL_TYPES_SOURCE_LOCATION_H_
diff --git a/absl/types/source_location_test.cc b/absl/types/source_location_test.cc
new file mode 100644
index 00000000000..a312aa97642
--- /dev/null
+++ b/absl/types/source_location_test.cc
@@ -0,0 +1,139 @@
+// Copyright 2017 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/types/source_location.h"
+
+#include "absl/base/config.h"
+
+#ifdef ABSL_HAVE_STD_SOURCE_LOCATION
+#include <source_location>  // NOLINT(build/c++20)
+#endif
+#include <type_traits>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/strings/string_view.h"
+
+namespace {
+
+using ::absl::SourceLocation;
+using ::testing::EndsWith;
+
+TEST(SourceLocationTest, DefaultConstructionWorks) {
+  static_assert(!std::is_trivially_default_constructible_v<SourceLocation>);
+  static_assert(std::is_nothrow_default_constructible_v<SourceLocation>);
+  constexpr SourceLocation loc1 [[maybe_unused]];
+  SourceLocation loc2 [[maybe_unused]]{};
+  EXPECT_EQ(loc1.line(), loc2.line());
+}
+
+#ifdef ABSL_INTERNAL_HAVE_BUILTIN_LINE_FILE
+
+TEST(SourceLocationTest, ConstexprMembers) {
+  constexpr SourceLocation loc1 = absl::SourceLocation::current();
+  const SourceLocation loc2 = absl::SourceLocation::current();
+  EXPECT_EQ(loc1.line(), loc2.line() - 1);
+  EXPECT_EQ(absl::string_view(loc1.file_name()), loc2.file_name());
+}
+
+TEST(SourceLocationTest, ConversionFromStdSourceLocationWorks) {
+#ifndef ABSL_HAVE_STD_SOURCE_LOCATION
+  GTEST_SKIP() << "std::source_location is not available";
+#else
+  constexpr SourceLocation loc1 = std::source_location::current();
+  const std::source_location loc2 = std::source_location::current();
+  EXPECT_EQ(loc1.line(), loc2.line() - 1);
+  EXPECT_EQ(absl::string_view(loc1.file_name()), loc2.file_name());
+#endif
+}
+
+TEST(SourceLocationTest, CopyConstructionWorks) {
+  constexpr SourceLocation location = absl::SourceLocation::current();
+  constexpr int line = __LINE__ - 1;
+
+  EXPECT_EQ(location.line(), line);
+  EXPECT_THAT(location.file_name(), EndsWith("source_location_test.cc"));
+}
+
+TEST(SourceLocationTest, CopyAssignmentWorks) {
+  SourceLocation location;
+  location = absl::SourceLocation::current();
+  constexpr int line = __LINE__ - 1;
+
+  EXPECT_EQ(location.line(), line);
+  EXPECT_THAT(location.file_name(), EndsWith("source_location_test.cc"));
+}
+
+SourceLocation Echo(const SourceLocation& location) { return location; }
+
+TEST(SourceLocationTest, ExpectedUsageWorks) {
+  SourceLocation location = Echo(absl::SourceLocation::current());
+  constexpr int line = __LINE__ - 1;
+
+  EXPECT_EQ(location.line(), line);
+  EXPECT_THAT(location.file_name(), EndsWith("source_location_test.cc"));
+}
+
+TEST(SourceLocationTest, CurrentWorks) {
+  constexpr SourceLocation location = SourceLocation::current();
+  constexpr int line = __LINE__ - 1;
+
+  EXPECT_EQ(location.line(), line);
+  EXPECT_THAT(location.file_name(), EndsWith("source_location_test.cc"));
+}
+
+SourceLocation FuncWithDefaultParam(
+    SourceLocation loc = SourceLocation::current()) {
+  return loc;
+}
+
+TEST(SourceLocationTest, CurrentWorksAsDefaultParam) {
+  SourceLocation location = FuncWithDefaultParam();
+  constexpr int line = __LINE__ - 1;
+
+  EXPECT_EQ(location.line(), line);
+  EXPECT_THAT(location.file_name(), EndsWith("source_location_test.cc"));
+}
+
+#endif
+
+template <typename T>
+bool TryPassLineAndFile(decltype(T::current(0, ""))*) {
+  return true;
+}
+template <typename T>
+bool TryPassLineAndFile(decltype(T::current({}, 0, ""))*) {
+  return true;
+}
+template <typename T>
+bool TryPassLineAndFile(decltype(T::current(typename T::Tag{}, 0, ""))*) {
+  return true;
+}
+template <typename T>
+bool TryPassLineAndFile(...) {
+  return false;
+}
+
+TEST(SourceLocationTest, CantPassLineAndFile) {
+#ifdef ABSL_HAVE_STD_SOURCE_LOCATION
+  using StdSourceLocation = std::source_location;
+#else
+  using StdSourceLocation = void;
+#endif
+  if constexpr (!std::is_same_v<absl::SourceLocation, StdSourceLocation>) {
+    EXPECT_FALSE(TryPassLineAndFile<absl::SourceLocation>(nullptr));
+  }
+}
+
+}  // namespace
diff --git a/absl/types/span.h b/absl/types/span.h
index b007fc1fd3d..41aba0aea5b 100644
--- a/absl/types/span.h
+++ b/absl/types/span.h
@@ -17,39 +17,37 @@
 // span.h
 // -----------------------------------------------------------------------------
 //
-// This header file defines a `Span<T>` type for holding a view of an existing
-// array of data. The `Span` object, much like the `absl::string_view` object,
-// does not own such data itself. A span provides a lightweight way to pass
-// around view of such data.
+// This header file defines a `Span<T>` type for holding a reference to existing
+// array data. The `Span` object, much like the `absl::string_view` object,
+// does not own such data itself, and the data being referenced by the span must
+// outlive the span itself. Unlike `view` type references, a span can hold a
+// reference to mutable data (and can mutate it for underlying types of
+// non-const T.) A span provides a lightweight way to pass a reference to such
+// data.
 //
 // Additionally, this header file defines `MakeSpan()` and `MakeConstSpan()`
 // factory functions, for clearly creating spans of type `Span<T>` or read-only
 // `Span<const T>` when such types may be difficult to identify due to issues
 // with implicit conversion.
 //
-// The C++ standards committee currently has a proposal for a `std::span` type,
-// (http://wg21.link/p0122), which is not yet part of the standard (though may
-// become part of C++20). As of August 2017, the differences between
-// `absl::Span` and this proposal are:
-//    * `absl::Span` uses `size_t` for `size_type`
-//    * `absl::Span` has no `operator()`
-//    * `absl::Span` has no constructors for `std::unique_ptr` or
-//      `std::shared_ptr`
+// The C++20 standard includes a `std::span` type. As of January 2026, the
+// differences between `absl::Span` and `std::span` are:
+//    * `absl::Span` has `operator==` (which is likely a design bug,
+//       per https://abseil.io/blog/20180531-regular-types)
 //    * `absl::Span` has the factory functions `MakeSpan()` and
 //      `MakeConstSpan()`
-//    * `absl::Span` has `front()` and `back()` methods
 //    * bounds-checked access to `absl::Span` is accomplished with `at()`
+//      however `std::span` now supports the same as of the draft C++26 standard
 //    * `absl::Span` has compiler-provided move and copy constructors and
-//      assignment. This is due to them being specified as `constexpr`, but that
-//      implies const in C++11.
-//    * `absl::Span` has no `element_type` or `index_type` typedefs
-//    * A read-only `absl::Span<const T>` can be implicitly constructed from an
-//      initializer list.
+//      assignment.
 //    * `absl::Span` has no `bytes()`, `size_bytes()`, `as_bytes()`, or
-//      `as_mutable_bytes()` methods
+//      `as_writable_bytes()` methods
 //    * `absl::Span` has no static extent template parameter, nor constructors
 //      which exist only because of the static extent parameter.
 //    * `absl::Span` has an explicit mutable-reference constructor
+//    * `absl::Span::subspan(pos, len)` always truncates `len` to
+//      `size() - pos`, whereas `std::span::subspan()` only truncates when the
+//      `len` parameter is defaulted.
 //
 // For more information, see the class comments below.
 #ifndef ABSL_TYPES_SPAN_H_
@@ -63,22 +61,52 @@
 #include <type_traits>
 #include <utility>
 
-#include "absl/base/internal/throw_delegate.h"
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/hardening.h"
 #include "absl/base/macros.h"
+#include "absl/base/nullability.h"
 #include "absl/base/optimization.h"
-#include "absl/base/port.h"    // TODO(strel): remove this include
+#include "absl/base/port.h"  // TODO(strel): remove this include
+#include "absl/base/throw_delegate.h"
+#include "absl/hash/internal/weakly_mixed_integer.h"
 #include "absl/meta/type_traits.h"
 #include "absl/types/internal/span.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+template <typename T>
+class Span;
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+// If std::ranges is available, mark Span as satisfying the `view` and
+// `borrowed_range` concepts, just like std::span.
+#if __has_include(<version>)
+#include <version>  // NOLINT(misc-include-cleaner)
+#endif
+#if defined(__cpp_lib_ranges) && __cpp_lib_ranges >= 201911L
+#include <ranges>  // NOLINT(build/c++20)
+template <typename T>
+// NOLINTNEXTLINE(build/c++20)
+inline constexpr bool std::ranges::enable_view<absl::Span<T>> = true;
+template <typename T>
+// NOLINTNEXTLINE(build/c++20)
+inline constexpr bool std::ranges::enable_borrowed_range<absl::Span<T>> = true;
+#endif
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 //------------------------------------------------------------------------------
 // Span
 //------------------------------------------------------------------------------
 //
-// A `Span` is an "array view" type for holding a view of a contiguous data
-// array; the `Span` object does not and cannot own such data itself. A span
-// provides an easy way to provide overloads for anything operating on
+// A `Span` is an "array reference" type for holding a reference of contiguous
+// array data; the `Span` object does not and cannot own such data itself. A
+// span provides an easy way to provide overloads for anything operating on
 // contiguous sequences without needing to manage pointers and array lengths
 // manually.
 
@@ -96,7 +124,8 @@ namespace absl {
 // constructors.
 //
 // A `Span<T>` is somewhat analogous to an `absl::string_view`, but for an array
-// of elements of type `T`. A user of `Span` must ensure that the data being
+// of elements of type `T`, and unlike an `absl::string_view`, a span can hold a
+// reference to mutable data. A user of `Span` must ensure that the data being
 // pointed to outlives the `Span` itself.
 //
 // You can construct a `Span<T>` in several ways:
@@ -126,7 +155,7 @@ namespace absl {
 // Note that `Span` objects, in addition to requiring that the memory they
 // point to remains alive, must also ensure that such memory does not get
 // reallocated. Therefore, to avoid undefined behavior, containers with
-// associated span views should not invoke operations that may reallocate memory
+// associated spans should not invoke operations that may reallocate memory
 // (such as resizing) or invalidate iterators into the container.
 //
 // One common use for a `Span` is when passing arguments to a routine that can
@@ -150,29 +179,32 @@ namespace absl {
 //   int* my_array = new int[10];
 //   MyRoutine(absl::Span<const int>(my_array, 10));
 template <typename T>
-class Span {
+class ABSL_ATTRIBUTE_VIEW Span {
  private:
   // Used to determine whether a Span can be constructed from a container of
   // type C.
   template <typename C>
   using EnableIfConvertibleFrom =
-      typename std::enable_if<span_internal::HasData<T, C>::value &&
-                              span_internal::HasSize<C>::value>::type;
+      std::enable_if_t<!std::is_same_v<Span, std::remove_reference_t<C>> &&
+                       span_internal::HasData<T, C>::value &&
+                       span_internal::HasSize<C>::value>;
 
   // Used to SFINAE-enable a function when the slice elements are const.
   template <typename U>
-  using EnableIfConstView =
-      typename std::enable_if<std::is_const<T>::value, U>::type;
+  using EnableIfValueIsConst = std::enable_if_t<std::is_const_v<T>, U>;
 
   // Used to SFINAE-enable a function when the slice elements are mutable.
   template <typename U>
-  using EnableIfMutableView =
-      typename std::enable_if<!std::is_const<T>::value, U>::type;
+  using EnableIfValueIsMutable = std::enable_if_t<!std::is_const_v<T>, U>;
 
  public:
-  using value_type = absl::remove_cv_t<T>;
-  using pointer = T*;
-  using const_pointer = const T*;
+  using element_type = T;
+  using value_type = std::remove_cv_t<T>;
+  // TODO(b/316099902) - pointer should be absl_nullable, but this makes it hard
+  // to recognize foreach loops as safe. absl_nullability_unknown is currently
+  // used to suppress -Wnullability-completeness warnings.
+  using pointer = T* absl_nullability_unknown;
+  using const_pointer = const T* absl_nullability_unknown;
   using reference = T&;
   using const_reference = const T&;
   using iterator = pointer;
@@ -181,29 +213,53 @@ class Span {
   using const_reverse_iterator = std::reverse_iterator<const_iterator>;
   using size_type = size_t;
   using difference_type = ptrdiff_t;
+  using absl_internal_is_view = std::true_type;
 
-  static const size_type npos = ~(size_type(0));
+  // NOLINTNEXTLINE
+  static constexpr size_type npos = static_cast<size_type>(-1);
 
   constexpr Span() noexcept : Span(nullptr, 0) {}
-  constexpr Span(pointer array, size_type length) noexcept
+  constexpr Span(pointer array ABSL_ATTRIBUTE_LIFETIME_BOUND,
+                 size_type length) noexcept
       : ptr_(array), len_(length) {}
 
   // Implicit conversion constructors
   template <size_t N>
-  constexpr Span(T (&a)[N]) noexcept  // NOLINT(runtime/explicit)
+  constexpr Span(T(  // NOLINT(google-explicit-constructor)
+      &a ABSL_ATTRIBUTE_LIFETIME_BOUND)[N]) noexcept
       : Span(a, N) {}
 
   // Explicit reference constructor for a mutable `Span<T>` type. Can be
   // replaced with MakeSpan() to infer the type parameter.
   template <typename V, typename = EnableIfConvertibleFrom<V>,
-            typename = EnableIfMutableView<V>>
-  explicit Span(V& v) noexcept  // NOLINT(runtime/references)
+            typename = EnableIfValueIsMutable<V>,
+            typename = span_internal::EnableIfNotIsView<V>>
+  explicit Span(
+      V& v
+          ABSL_ATTRIBUTE_LIFETIME_BOUND) noexcept  // NOLINT(runtime/references)
       : Span(span_internal::GetData(v), v.size()) {}
 
   // Implicit reference constructor for a read-only `Span<const T>` type
   template <typename V, typename = EnableIfConvertibleFrom<V>,
-            typename = EnableIfConstView<V>>
-  constexpr Span(const V& v) noexcept  // NOLINT(runtime/explicit)
+            typename = EnableIfValueIsConst<V>,
+            typename = span_internal::EnableIfNotIsView<V>>
+  // NOLINTNEXTLINE(google-explicit-constructor)
+  constexpr Span(const V& v ABSL_ATTRIBUTE_LIFETIME_BOUND) noexcept
+      : Span(span_internal::GetData(v), v.size()) {}
+
+  // Overloads of the above two functions that are only enabled for view types.
+  // This is so we can drop the ABSL_ATTRIBUTE_LIFETIME_BOUND annotation. These
+  // overloads must be made unique by using a different template parameter list
+  // (hence the = 0 for the IsView enabler).
+  template <typename V, typename = EnableIfConvertibleFrom<V>,
+            typename = EnableIfValueIsMutable<V>,
+            span_internal::EnableIfIsView<V> = 0>
+  explicit Span(V& v) noexcept  // NOLINT(runtime/references)
+      : Span(span_internal::GetData(v), v.size()) {}
+  template <typename V, typename = EnableIfConvertibleFrom<V>,
+            typename = EnableIfValueIsConst<V>,
+            span_internal::EnableIfIsView<V> = 0>
+  constexpr Span(const V& v) noexcept  // NOLINT(google-explicit-constructor)
       : Span(span_internal::GetData(v), v.size()) {}
 
   // Implicit constructor from an initializer list, making it possible to pass a
@@ -242,9 +298,9 @@ class Span {
   //   Process(ints);
   //
   template <typename LazyT = T,
-            typename = EnableIfConstView<LazyT>>
-  Span(
-      std::initializer_list<value_type> v) noexcept  // NOLINT(runtime/explicit)
+            typename = EnableIfValueIsConst<LazyT>>
+  Span(std::initializer_list<value_type> v
+           ABSL_ATTRIBUTE_LIFETIME_BOUND) noexcept  // NOLINT(runtime/explicit)
       : Span(v.begin(), v.size()) {}
 
   // Accessors
@@ -274,8 +330,8 @@ class Span {
   //
   // Returns a reference to the i'th element of this span.
   constexpr reference operator[](size_type i) const noexcept {
-    // MSVC 2015 accepts this as constexpr, but not ptr_[i]
-    return *(data() + i);
+    absl::base_internal::HardeningAssertLT(i, size());
+    return ptr_[i];
   }
 
   // Span::at()
@@ -284,67 +340,82 @@ class Span {
   constexpr reference at(size_type i) const {
     return ABSL_PREDICT_TRUE(i < size())  //
                ? *(data() + i)
-               : (base_internal::ThrowStdOutOfRange(
-                      "Span::at failed bounds check"),
+               : (ThrowStdOutOfRange("Span::at failed bounds check"),
                   *(data() + i));
   }
 
   // Span::front()
   //
-  // Returns a reference to the first element of this span.
+  // Returns a reference to the first element of this span. The span must not
+  // be empty.
   constexpr reference front() const noexcept {
-    return ABSL_ASSERT(size() > 0), *data();
+    absl::base_internal::HardeningAssertGT(size(), static_cast<size_t>(0));
+    return *data();
   }
 
   // Span::back()
   //
-  // Returns a reference to the last element of this span.
+  // Returns a reference to the last element of this span. The span must not
+  // be empty.
   constexpr reference back() const noexcept {
-    return ABSL_ASSERT(size() > 0), *(data() + size() - 1);
+    absl::base_internal::HardeningAssertGT(size(), static_cast<size_t>(0));
+    return *(data() + size() - 1);
   }
 
   // Span::begin()
   //
-  // Returns an iterator to the first element of this span.
+  // Returns an iterator pointing to the first element of this span, or `end()`
+  // if the span is empty.
   constexpr iterator begin() const noexcept { return data(); }
 
   // Span::cbegin()
   //
-  // Returns a const iterator to the first element of this span.
+  // Returns a const iterator pointing to the first element of this span, or
+  // `end()` if the span is empty.
   constexpr const_iterator cbegin() const noexcept { return begin(); }
 
   // Span::end()
   //
-  // Returns an iterator to the last element of this span.
+  // Returns an iterator pointing just beyond the last element at the
+  // end of this span. This iterator acts as a placeholder; attempting to
+  // access it results in undefined behavior.
   constexpr iterator end() const noexcept { return data() + size(); }
 
   // Span::cend()
   //
-  // Returns a const iterator to the last element of this span.
+  // Returns a const iterator pointing just beyond the last element at the
+  // end of this span. This iterator acts as a placeholder; attempting to
+  // access it results in undefined behavior.
   constexpr const_iterator cend() const noexcept { return end(); }
 
   // Span::rbegin()
   //
-  // Returns a reverse iterator starting at the last element of this span.
+  // Returns a reverse iterator pointing to the last element at the end of this
+  // span, or `rend()` if the span is empty.
   constexpr reverse_iterator rbegin() const noexcept {
     return reverse_iterator(end());
   }
 
   // Span::crbegin()
   //
-  // Returns a reverse const iterator starting at the last element of this span.
+  // Returns a const reverse iterator pointing to the last element at the end of
+  // this span, or `crend()` if the span is empty.
   constexpr const_reverse_iterator crbegin() const noexcept { return rbegin(); }
 
   // Span::rend()
   //
-  // Returns a reverse iterator starting at the first element of this span.
+  // Returns a reverse iterator pointing just before the first element
+  // at the beginning of this span. This pointer acts as a placeholder;
+  // attempting to access its element results in undefined behavior.
   constexpr reverse_iterator rend() const noexcept {
     return reverse_iterator(begin());
   }
 
   // Span::crend()
   //
-  // Returns a reverse iterator starting at the first element of this span.
+  // Returns a reverse const iterator pointing just before the first element
+  // at the beginning of this span. This pointer acts as a placeholder;
+  // attempting to access its element results in undefined behavior.
   constexpr const_reverse_iterator crend() const noexcept { return rend(); }
 
   // Span mutations
@@ -353,7 +424,7 @@ class Span {
   //
   // Removes the first `n` elements from the span.
   void remove_prefix(size_type n) noexcept {
-    assert(size() >= n);
+    absl::base_internal::HardeningAssertGE(size(), n);
     ptr_ += n;
     len_ -= n;
   }
@@ -362,7 +433,7 @@ class Span {
   //
   // Removes the last `n` elements from the span.
   void remove_suffix(size_type n) noexcept {
-    assert(size() >= n);
+    absl::base_internal::HardeningAssertGE(size(), n);
     len_ -= n;
   }
 
@@ -374,6 +445,11 @@ class Span {
   // will be trimmed to at most size() - `pos`. A default `len` value of `npos`
   // ensures the returned subspan continues until the end of the span.
   //
+  // Note that trimming behavior differs from `std::span::subspan()`.
+  // `std::span::subspan()` requires `len == npos || pos + len <= size()`.
+  // In other words, `std::span::subspan()` only trims `len` when its value is
+  // defaulted.
+  //
   // Examples:
   //
   //   std::vector<int> vec = {10, 11, 12, 13};
@@ -383,9 +459,8 @@ class Span {
   //   absl::MakeSpan(vec).subspan(4);     // {}
   //   absl::MakeSpan(vec).subspan(5);     // throws std::out_of_range
   constexpr Span subspan(size_type pos = 0, size_type len = npos) const {
-    return (pos <= size())
-               ? Span(data() + pos, span_internal::Min(size() - pos, len))
-               : (base_internal::ThrowStdOutOfRange("pos > size()"), Span());
+    return (pos <= size()) ? Span(data() + pos, (std::min)(size() - pos, len))
+                           : (ThrowStdOutOfRange("pos > size()"), Span());
   }
 
   // Span::first()
@@ -400,9 +475,8 @@ class Span {
   //   absl::MakeSpan(vec).first(3);  // {10, 11, 12}
   //   absl::MakeSpan(vec).first(5);  // throws std::out_of_range
   constexpr Span first(size_type len) const {
-    return (len <= size())
-               ? Span(data(), len)
-               : (base_internal::ThrowStdOutOfRange("len > size()"), Span());
+    return (len <= size()) ? Span(data(), len)
+                           : (ThrowStdOutOfRange("len > size()"), Span());
   }
 
   // Span::last()
@@ -417,16 +491,14 @@ class Span {
   //   absl::MakeSpan(vec).last(3);  // {11, 12, 13}
   //   absl::MakeSpan(vec).last(5);  // throws std::out_of_range
   constexpr Span last(size_type len) const {
-    return (len <= size())
-               ? Span(size() - len + data(), len)
-               : (base_internal::ThrowStdOutOfRange("len > size()"), Span());
+    return (len <= size()) ? Span(size() - len + data(), len)
+                           : (ThrowStdOutOfRange("len > size()"), Span());
   }
 
   // Support for absl::Hash.
   template <typename H>
   friend H AbslHashValue(H h, Span v) {
-    return H::combine(H::combine_contiguous(std::move(h), v.data(), v.size()),
-                      v.size());
+    return H::combine_contiguous(std::move(h), v.data(), v.size());
   }
 
  private:
@@ -434,9 +506,6 @@ class Span {
   size_type len_;
 };
 
-template <typename T>
-const typename Span<T>::size_type Span<T>::npos;
-
 // Span relationals
 
 // Equality is compared element-by-element, while ordering is lexicographical.
@@ -453,157 +522,157 @@ const typename Span<T>::size_type Span<T>::npos;
 
 // operator==
 template <typename T>
-bool operator==(Span<T> a, Span<T> b) {
+constexpr bool operator==(Span<T> a, Span<T> b) {
   return span_internal::EqualImpl<Span, const T>(a, b);
 }
 template <typename T>
-bool operator==(Span<const T> a, Span<T> b) {
+constexpr bool operator==(Span<const T> a, Span<T> b) {
   return span_internal::EqualImpl<Span, const T>(a, b);
 }
 template <typename T>
-bool operator==(Span<T> a, Span<const T> b) {
+constexpr bool operator==(Span<T> a, Span<const T> b) {
   return span_internal::EqualImpl<Span, const T>(a, b);
 }
 template <
     typename T, typename U,
     typename = span_internal::EnableIfConvertibleTo<U, absl::Span<const T>>>
-bool operator==(const U& a, Span<T> b) {
+constexpr bool operator==(const U& a, Span<T> b) {
   return span_internal::EqualImpl<Span, const T>(a, b);
 }
 template <
     typename T, typename U,
     typename = span_internal::EnableIfConvertibleTo<U, absl::Span<const T>>>
-bool operator==(Span<T> a, const U& b) {
+constexpr bool operator==(Span<T> a, const U& b) {
   return span_internal::EqualImpl<Span, const T>(a, b);
 }
 
 // operator!=
 template <typename T>
-bool operator!=(Span<T> a, Span<T> b) {
+constexpr bool operator!=(Span<T> a, Span<T> b) {
   return !(a == b);
 }
 template <typename T>
-bool operator!=(Span<const T> a, Span<T> b) {
+constexpr bool operator!=(Span<const T> a, Span<T> b) {
   return !(a == b);
 }
 template <typename T>
-bool operator!=(Span<T> a, Span<const T> b) {
+constexpr bool operator!=(Span<T> a, Span<const T> b) {
   return !(a == b);
 }
 template <
     typename T, typename U,
     typename = span_internal::EnableIfConvertibleTo<U, absl::Span<const T>>>
-bool operator!=(const U& a, Span<T> b) {
+constexpr bool operator!=(const U& a, Span<T> b) {
   return !(a == b);
 }
 template <
     typename T, typename U,
     typename = span_internal::EnableIfConvertibleTo<U, absl::Span<const T>>>
-bool operator!=(Span<T> a, const U& b) {
+constexpr bool operator!=(Span<T> a, const U& b) {
   return !(a == b);
 }
 
 // operator<
 template <typename T>
-bool operator<(Span<T> a, Span<T> b) {
+constexpr bool operator<(Span<T> a, Span<T> b) {
   return span_internal::LessThanImpl<Span, const T>(a, b);
 }
 template <typename T>
-bool operator<(Span<const T> a, Span<T> b) {
+constexpr bool operator<(Span<const T> a, Span<T> b) {
   return span_internal::LessThanImpl<Span, const T>(a, b);
 }
 template <typename T>
-bool operator<(Span<T> a, Span<const T> b) {
+constexpr bool operator<(Span<T> a, Span<const T> b) {
   return span_internal::LessThanImpl<Span, const T>(a, b);
 }
 template <
     typename T, typename U,
     typename = span_internal::EnableIfConvertibleTo<U, absl::Span<const T>>>
-bool operator<(const U& a, Span<T> b) {
+constexpr bool operator<(const U& a, Span<T> b) {
   return span_internal::LessThanImpl<Span, const T>(a, b);
 }
 template <
     typename T, typename U,
     typename = span_internal::EnableIfConvertibleTo<U, absl::Span<const T>>>
-bool operator<(Span<T> a, const U& b) {
+constexpr bool operator<(Span<T> a, const U& b) {
   return span_internal::LessThanImpl<Span, const T>(a, b);
 }
 
 // operator>
 template <typename T>
-bool operator>(Span<T> a, Span<T> b) {
+constexpr bool operator>(Span<T> a, Span<T> b) {
   return b < a;
 }
 template <typename T>
-bool operator>(Span<const T> a, Span<T> b) {
+constexpr bool operator>(Span<const T> a, Span<T> b) {
   return b < a;
 }
 template <typename T>
-bool operator>(Span<T> a, Span<const T> b) {
+constexpr bool operator>(Span<T> a, Span<const T> b) {
   return b < a;
 }
 template <
     typename T, typename U,
     typename = span_internal::EnableIfConvertibleTo<U, absl::Span<const T>>>
-bool operator>(const U& a, Span<T> b) {
+constexpr bool operator>(const U& a, Span<T> b) {
   return b < a;
 }
 template <
     typename T, typename U,
     typename = span_internal::EnableIfConvertibleTo<U, absl::Span<const T>>>
-bool operator>(Span<T> a, const U& b) {
+constexpr bool operator>(Span<T> a, const U& b) {
   return b < a;
 }
 
 // operator<=
 template <typename T>
-bool operator<=(Span<T> a, Span<T> b) {
+constexpr bool operator<=(Span<T> a, Span<T> b) {
   return !(b < a);
 }
 template <typename T>
-bool operator<=(Span<const T> a, Span<T> b) {
+constexpr bool operator<=(Span<const T> a, Span<T> b) {
   return !(b < a);
 }
 template <typename T>
-bool operator<=(Span<T> a, Span<const T> b) {
+constexpr bool operator<=(Span<T> a, Span<const T> b) {
   return !(b < a);
 }
 template <
     typename T, typename U,
     typename = span_internal::EnableIfConvertibleTo<U, absl::Span<const T>>>
-bool operator<=(const U& a, Span<T> b) {
+constexpr bool operator<=(const U& a, Span<T> b) {
   return !(b < a);
 }
 template <
     typename T, typename U,
     typename = span_internal::EnableIfConvertibleTo<U, absl::Span<const T>>>
-bool operator<=(Span<T> a, const U& b) {
+constexpr bool operator<=(Span<T> a, const U& b) {
   return !(b < a);
 }
 
 // operator>=
 template <typename T>
-bool operator>=(Span<T> a, Span<T> b) {
+constexpr bool operator>=(Span<T> a, Span<T> b) {
   return !(a < b);
 }
 template <typename T>
-bool operator>=(Span<const T> a, Span<T> b) {
+constexpr bool operator>=(Span<const T> a, Span<T> b) {
   return !(a < b);
 }
 template <typename T>
-bool operator>=(Span<T> a, Span<const T> b) {
+constexpr bool operator>=(Span<T> a, Span<const T> b) {
   return !(a < b);
 }
 template <
     typename T, typename U,
     typename = span_internal::EnableIfConvertibleTo<U, absl::Span<const T>>>
-bool operator>=(const U& a, Span<T> b) {
+constexpr bool operator>=(const U& a, Span<T> b) {
   return !(a < b);
 }
 template <
     typename T, typename U,
     typename = span_internal::EnableIfConvertibleTo<U, absl::Span<const T>>>
-bool operator>=(Span<T> a, const U& b) {
+constexpr bool operator>=(Span<T> a, const U& b) {
   return !(a < b);
 }
 
@@ -643,24 +712,42 @@ bool operator>=(Span<T> a, const U& b) {
 //     return absl::MakeSpan(&array[0], num_elements_);
 //   }
 //
+// NOTE: To avoid undefined behavior if the container is empty, use `.data()`
+// or pass the container directly instead of using `&v[0]` or `&v[v.size()]`.
+//
 template <int&... ExplicitArgumentBarrier, typename T>
-constexpr Span<T> MakeSpan(T* ptr, size_t size) noexcept {
+constexpr Span<T> MakeSpan(T* absl_nullable ptr ABSL_ATTRIBUTE_LIFETIME_BOUND,
+                           size_t size) noexcept {
   return Span<T>(ptr, size);
 }
 
 template <int&... ExplicitArgumentBarrier, typename T>
-Span<T> MakeSpan(T* begin, T* end) noexcept {
-  return ABSL_ASSERT(begin <= end), Span<T>(begin, end - begin);
+Span<T> MakeSpan(T* absl_nullable begin ABSL_ATTRIBUTE_LIFETIME_BOUND,
+                 T* absl_nullable end) noexcept {
+  absl::base_internal::HardeningAssertLE(begin, end);
+  return Span<T>(begin, static_cast<size_t>(end - begin));
 }
 
 template <int&... ExplicitArgumentBarrier, typename C>
 constexpr auto MakeSpan(C& c) noexcept  // NOLINT(runtime/references)
-    -> decltype(absl::MakeSpan(span_internal::GetData(c), c.size())) {
+    -> std::enable_if_t<span_internal::IsView<C>::value,
+                        decltype(absl::MakeSpan(span_internal::GetData(c),
+                                                c.size()))> {
+  return MakeSpan(span_internal::GetData(c), c.size());
+}
+
+template <int&... ExplicitArgumentBarrier, typename C>
+constexpr auto MakeSpan(
+    C& c ABSL_ATTRIBUTE_LIFETIME_BOUND) noexcept  // NOLINT(runtime/references)
+    -> std::enable_if_t<!span_internal::IsView<C>::value,
+                        decltype(absl::MakeSpan(span_internal::GetData(c),
+                                                c.size()))> {
   return MakeSpan(span_internal::GetData(c), c.size());
 }
 
 template <int&... ExplicitArgumentBarrier, typename T, size_t N>
-constexpr Span<T> MakeSpan(T (&array)[N]) noexcept {
+constexpr Span<T> MakeSpan(
+    T (&array ABSL_ATTRIBUTE_LIFETIME_BOUND)[N]) noexcept {
   return Span<T>(array, N);
 }
 
@@ -689,23 +776,38 @@ constexpr Span<T> MakeSpan(T (&array)[N]) noexcept {
 //   ProcessInts(absl::MakeConstSpan(std::vector<int>{ 0, 0, 0 }));
 //
 template <int&... ExplicitArgumentBarrier, typename T>
-constexpr Span<const T> MakeConstSpan(T* ptr, size_t size) noexcept {
+constexpr Span<const T> MakeConstSpan(
+    T* absl_nullable ptr ABSL_ATTRIBUTE_LIFETIME_BOUND, size_t size) noexcept {
   return Span<const T>(ptr, size);
 }
 
 template <int&... ExplicitArgumentBarrier, typename T>
-Span<const T> MakeConstSpan(T* begin, T* end) noexcept {
-  return ABSL_ASSERT(begin <= end), Span<const T>(begin, end - begin);
+Span<const T> MakeConstSpan(T* absl_nullable begin
+                                ABSL_ATTRIBUTE_LIFETIME_BOUND,
+                            T* absl_nullable end) noexcept {
+  absl::base_internal::HardeningAssertLE(begin, end);
+  return Span<const T>(begin, end - begin);
+}
+
+template <int&... ExplicitArgumentBarrier, typename C>
+constexpr auto MakeConstSpan(const C& c) noexcept
+    -> std::enable_if_t<span_internal::IsView<C>::value,
+                        decltype(MakeSpan(c))> {
+  return MakeSpan(c);
 }
 
 template <int&... ExplicitArgumentBarrier, typename C>
-constexpr auto MakeConstSpan(const C& c) noexcept -> decltype(MakeSpan(c)) {
+constexpr auto MakeConstSpan(const C& c ABSL_ATTRIBUTE_LIFETIME_BOUND) noexcept
+    -> std::enable_if_t<!span_internal::IsView<C>::value,
+                        decltype(MakeSpan(c))> {
   return MakeSpan(c);
 }
 
 template <int&... ExplicitArgumentBarrier, typename T, size_t N>
-constexpr Span<const T> MakeConstSpan(const T (&array)[N]) noexcept {
+constexpr Span<const T> MakeConstSpan(
+    const T (&array ABSL_ATTRIBUTE_LIFETIME_BOUND)[N]) noexcept {
   return Span<const T>(array, N);
 }
+ABSL_NAMESPACE_END
 }  // namespace absl
 #endif  // ABSL_TYPES_SPAN_H_
diff --git a/absl/types/span_test.cc b/absl/types/span_test.cc
index 22467a0a767..5a8b82e61a0 100644
--- a/absl/types/span_test.cc
+++ b/absl/types/span_test.cc
@@ -27,13 +27,34 @@
 #include "absl/base/attributes.h"
 #include "absl/base/config.h"
 #include "absl/base/internal/exception_testing.h"
+#include "absl/base/options.h"
 #include "absl/container/fixed_array.h"
 #include "absl/container/inlined_vector.h"
+#include "absl/hash/hash.h"
 #include "absl/hash/hash_testing.h"
+#include "absl/meta/type_traits.h"
 #include "absl/strings/str_cat.h"
 
 namespace {
 
+static_assert(!absl::type_traits_internal::IsOwner<absl::Span<int>>::value &&
+                  absl::type_traits_internal::IsView<absl::Span<int>>::value,
+              "Span is a view, not an owner");
+
+using S = absl::Span<int>;
+
+static_assert(
+    std::is_trivially_destructible_v<S> && std::is_trivially_copyable_v<S> &&
+        std::is_trivially_assignable_v<S, S&> &&
+        std::is_trivially_copy_assignable_v<S> &&
+        std::is_trivially_move_assignable_v<S> &&
+        std::is_trivially_assignable_v<S, const S&&> &&
+        std::is_trivially_constructible_v<S, S&> &&
+        std::is_trivially_copy_constructible_v<S> &&
+        std::is_trivially_move_constructible_v<S> &&
+        std::is_trivially_constructible_v<S, const S&&>,
+    "Span should be trivial in everything except default-constructibility");
+
 MATCHER_P(DataIs, data,
           absl::StrCat("data() ", negation ? "isn't " : "is ",
                        testing::PrintToString(data))) {
@@ -73,13 +94,11 @@ TEST(IntSpan, ArrayCtor) {
   absl::Span<int> s(a);
   EXPECT_THAT(s, SpanIs(a, 3));
 
-  EXPECT_TRUE((std::is_constructible<absl::Span<const int>, int[3]>::value));
-  EXPECT_TRUE(
-      (std::is_constructible<absl::Span<const int>, const int[3]>::value));
-  EXPECT_FALSE((std::is_constructible<absl::Span<int>, const int[3]>::value));
-  EXPECT_TRUE((std::is_convertible<int[3], absl::Span<const int>>::value));
-  EXPECT_TRUE(
-      (std::is_convertible<const int[3], absl::Span<const int>>::value));
+  EXPECT_TRUE((std::is_constructible_v<absl::Span<const int>, int[3]>));
+  EXPECT_TRUE((std::is_constructible_v<absl::Span<const int>, const int[3]>));
+  EXPECT_FALSE((std::is_constructible_v<absl::Span<int>, const int[3]>));
+  EXPECT_TRUE((std::is_convertible_v<int[3], absl::Span<const int>>));
+  EXPECT_TRUE((std::is_convertible_v<const int[3], absl::Span<const int>>));
 }
 
 template <typename T>
@@ -104,9 +123,8 @@ TEST(IntSpan, ContainerCtor) {
   EXPECT_THAT(const_from_span, SpanIs(s_filled));
 
   EXPECT_TRUE(
-      (std::is_convertible<std::vector<int>&, absl::Span<const int>>::value));
-  EXPECT_TRUE(
-      (std::is_convertible<absl::Span<int>&, absl::Span<const int>>::value));
+      (std::is_convertible_v<std::vector<int>&, absl::Span<const int>>));
+  EXPECT_TRUE((std::is_convertible_v<absl::Span<int>&, absl::Span<const int>>));
 
   TakesGenericSpan(absl::Span<int>(filled));
 }
@@ -138,22 +156,20 @@ TEST(CharSpan, StringCtor) {
   absl::Span<const char> s_const_abc = abc;
   EXPECT_THAT(s_const_abc, SpanIs(abc));
 
-  EXPECT_FALSE((std::is_constructible<absl::Span<int>, std::string>::value));
-  EXPECT_FALSE(
-      (std::is_constructible<absl::Span<const int>, std::string>::value));
-  EXPECT_TRUE(
-      (std::is_convertible<std::string, absl::Span<const char>>::value));
+  EXPECT_FALSE((std::is_constructible_v<absl::Span<int>, std::string>));
+  EXPECT_FALSE((std::is_constructible_v<absl::Span<const int>, std::string>));
+  EXPECT_TRUE((std::is_convertible_v<std::string, absl::Span<const char>>));
 }
 
 TEST(IntSpan, FromConstPointer) {
-  EXPECT_TRUE((std::is_constructible<absl::Span<const int* const>,
-                                     std::vector<int*>>::value));
-  EXPECT_TRUE((std::is_constructible<absl::Span<const int* const>,
-                                     std::vector<const int*>>::value));
-  EXPECT_FALSE((
-      std::is_constructible<absl::Span<const int*>, std::vector<int*>>::value));
-  EXPECT_FALSE((
-      std::is_constructible<absl::Span<int*>, std::vector<const int*>>::value));
+  EXPECT_TRUE((std::is_constructible_v<absl::Span<const int* const>,
+                                       std::vector<int*>>));
+  EXPECT_TRUE((std::is_constructible_v<absl::Span<const int* const>,
+                                       std::vector<const int*>>));
+  EXPECT_FALSE(
+      (std::is_constructible_v<absl::Span<const int*>, std::vector<int*>>));
+  EXPECT_FALSE(
+      (std::is_constructible_v<absl::Span<int*>, std::vector<const int*>>));
 }
 
 struct TypeWithMisleadingData {
@@ -170,9 +186,9 @@ struct TypeWithMisleadingSize {
 
 TEST(IntSpan, EvilTypes) {
   EXPECT_FALSE(
-      (std::is_constructible<absl::Span<int>, TypeWithMisleadingData&>::value));
+      (std::is_constructible_v<absl::Span<int>, TypeWithMisleadingData&>));
   EXPECT_FALSE(
-      (std::is_constructible<absl::Span<int>, TypeWithMisleadingSize&>::value));
+      (std::is_constructible_v<absl::Span<int>, TypeWithMisleadingSize&>));
 }
 
 struct Base {
@@ -183,14 +199,14 @@ struct Base {
 struct Derived : Base {};
 
 TEST(IntSpan, SpanOfDerived) {
-  EXPECT_TRUE((std::is_constructible<absl::Span<int>, Base&>::value));
-  EXPECT_TRUE((std::is_constructible<absl::Span<int>, Derived&>::value));
+  EXPECT_TRUE((std::is_constructible_v<absl::Span<int>, Base&>));
+  EXPECT_TRUE((std::is_constructible_v<absl::Span<int>, Derived&>));
   EXPECT_FALSE(
-      (std::is_constructible<absl::Span<Base>, std::vector<Derived>>::value));
+      (std::is_constructible_v<absl::Span<Base>, std::vector<Derived>>));
 }
 
 void TestInitializerList(absl::Span<const int> s, const std::vector<int>& v) {
-  EXPECT_TRUE(absl::equal(s.begin(), s.end(), v.begin(), v.end()));
+  EXPECT_TRUE(std::equal(s.begin(), s.end(), v.begin(), v.end()));
 }
 
 TEST(ConstIntSpan, InitializerListConversion) {
@@ -198,10 +214,10 @@ TEST(ConstIntSpan, InitializerListConversion) {
   TestInitializerList({1}, {1});
   TestInitializerList({1, 2, 3}, {1, 2, 3});
 
-  EXPECT_FALSE((std::is_constructible<absl::Span<int>,
-                                      std::initializer_list<int>>::value));
-  EXPECT_FALSE((
-      std::is_convertible<absl::Span<int>, std::initializer_list<int>>::value));
+  EXPECT_FALSE(
+      (std::is_constructible_v<absl::Span<int>, std::initializer_list<int>>));
+  EXPECT_FALSE(
+      (std::is_convertible_v<absl::Span<int>, std::initializer_list<int>>));
 }
 
 TEST(IntSpan, Data) {
@@ -232,6 +248,11 @@ TEST(IntSpan, ElementAccess) {
 
   EXPECT_EQ(s.front(), s[0]);
   EXPECT_EQ(s.back(), s[9]);
+
+#if !defined(NDEBUG) || ABSL_OPTION_HARDENED
+  EXPECT_DEATH_IF_SUPPORTED(s[-1], "");
+  EXPECT_DEATH_IF_SUPPORTED(s[10], "");
+#endif
 }
 
 TEST(IntSpan, AtThrows) {
@@ -268,6 +289,13 @@ TEST(IntSpan, RemovePrefixAndSuffix) {
   EXPECT_EQ(s.size(), 0);
 
   EXPECT_EQ(v, MakeRamp(20, 1));
+
+#if !defined(NDEBUG) || ABSL_OPTION_HARDENED
+  absl::Span<int> prefix_death(v);
+  EXPECT_DEATH_IF_SUPPORTED(prefix_death.remove_prefix(21), "");
+  absl::Span<int> suffix_death(v);
+  EXPECT_DEATH_IF_SUPPORTED(suffix_death.remove_suffix(21), "");
+#endif
 }
 
 TEST(IntSpan, Subspan) {
@@ -635,19 +663,20 @@ TEST_F(IntSpanOrderComparisonTest, EmptySpans) {
 TEST(IntSpan, ExposesContainerTypesAndConsts) {
   absl::Span<int> slice;
   CheckType<absl::Span<int>::iterator>(slice.begin());
-  EXPECT_TRUE((std::is_convertible<decltype(slice.begin()),
-                                   absl::Span<int>::const_iterator>::value));
+  EXPECT_TRUE((std::is_convertible_v<decltype(slice.begin()),
+                                     absl::Span<int>::const_iterator>));
   CheckType<absl::Span<int>::const_iterator>(slice.cbegin());
-  EXPECT_TRUE((std::is_convertible<decltype(slice.end()),
-                                   absl::Span<int>::const_iterator>::value));
+  EXPECT_TRUE((std::is_convertible_v<decltype(slice.end()),
+                                     absl::Span<int>::const_iterator>));
   CheckType<absl::Span<int>::const_iterator>(slice.cend());
   CheckType<absl::Span<int>::reverse_iterator>(slice.rend());
-  EXPECT_TRUE(
-      (std::is_convertible<decltype(slice.rend()),
-                           absl::Span<int>::const_reverse_iterator>::value));
+  EXPECT_TRUE((std::is_convertible_v<decltype(slice.rend()),
+                                     absl::Span<int>::const_reverse_iterator>));
   CheckType<absl::Span<int>::const_reverse_iterator>(slice.crend());
   testing::StaticAssertTypeEq<int, absl::Span<int>::value_type>();
   testing::StaticAssertTypeEq<int, absl::Span<const int>::value_type>();
+  testing::StaticAssertTypeEq<int, absl::Span<int>::element_type>();
+  testing::StaticAssertTypeEq<const int, absl::Span<const int>::element_type>();
   testing::StaticAssertTypeEq<int*, absl::Span<int>::pointer>();
   testing::StaticAssertTypeEq<const int*, absl::Span<const int>::pointer>();
   testing::StaticAssertTypeEq<int&, absl::Span<int>::reference>();
@@ -767,9 +796,9 @@ TEST(IntSpan, NoexceptTest) {
 template <int i>
 struct ConstexprTester {};
 
-#define ABSL_TEST_CONSTEXPR(expr)                       \
-  do {                                                  \
-    ABSL_ATTRIBUTE_UNUSED ConstexprTester<(expr, 1)> t; \
+#define ABSL_TEST_CONSTEXPR(expr)                                          \
+  do {                                                                     \
+    ABSL_ATTRIBUTE_UNUSED ConstexprTester<(static_cast<void>(expr), 1)> t; \
   } while (0)
 
 struct ContainerWithConstexprMethods {
@@ -806,6 +835,41 @@ TEST(ConstIntSpan, ConstexprTest) {
   ABSL_TEST_CONSTEXPR(span[0]);
 }
 
+#if defined(ABSL_INTERNAL_CPLUSPLUS_LANG) && \
+    ABSL_INTERNAL_CPLUSPLUS_LANG >= 202002L
+
+TEST(ConstIntSpan, ConstexprRelOpsTest) {
+  static constexpr int lhs_data[] = {1, 2, 3};
+  static constexpr int rhs_data[] = {1, 2, 3};
+
+  constexpr absl::Span<const int> lhs = absl::MakeConstSpan(lhs_data, 3);
+  constexpr absl::Span<const int> rhs = absl::MakeConstSpan(rhs_data, 3);
+
+  ABSL_TEST_CONSTEXPR(lhs_data == rhs);
+  ABSL_TEST_CONSTEXPR(lhs_data != rhs);
+  ABSL_TEST_CONSTEXPR(lhs_data < rhs);
+  ABSL_TEST_CONSTEXPR(lhs_data <= rhs);
+  ABSL_TEST_CONSTEXPR(lhs_data > rhs);
+  ABSL_TEST_CONSTEXPR(lhs_data >= rhs);
+
+  ABSL_TEST_CONSTEXPR(lhs == rhs);
+  ABSL_TEST_CONSTEXPR(lhs != rhs);
+  ABSL_TEST_CONSTEXPR(lhs < rhs);
+  ABSL_TEST_CONSTEXPR(lhs <= rhs);
+  ABSL_TEST_CONSTEXPR(lhs > rhs);
+  ABSL_TEST_CONSTEXPR(lhs >= rhs);
+
+  ABSL_TEST_CONSTEXPR(lhs == rhs_data);
+  ABSL_TEST_CONSTEXPR(lhs != rhs_data);
+  ABSL_TEST_CONSTEXPR(lhs < rhs_data);
+  ABSL_TEST_CONSTEXPR(lhs <= rhs_data);
+  ABSL_TEST_CONSTEXPR(lhs > rhs_data);
+  ABSL_TEST_CONSTEXPR(lhs >= rhs_data);
+}
+
+#endif  // defined(ABSL_INTERNAL_CPLUSPLUS_LANG) &&
+        //  ABSL_INTERNAL_CPLUSPLUS_LANG >= 202002L
+
 struct BigStruct {
   char bytes[10000];
 };
@@ -830,4 +894,16 @@ TEST(Span, Hash) {
        T(array + 1, 2), T(array + 2, 2)}));
 }
 
+// std::vector is implicitly convertible to absl::Span.
+// There are real life cases where clients rely on this consistency in order to
+// implement heterogeneous lookup.
+TEST(Span, HashConsistentWithVectorLike) {
+  EXPECT_EQ(absl::HashOf(absl::Span<const int>({1, 2, 3})),
+            absl::HashOf(std::vector<int>{1, 2, 3}));
+  EXPECT_EQ(absl::HashOf(absl::Span<const int>({1, 2, 3})),
+            absl::HashOf(absl::InlinedVector<int, 2>{1, 2, 3}));
+  EXPECT_EQ(absl::HashOf(absl::Span<const int>({1, 2, 3})),
+            absl::HashOf(absl::FixedArray<int>{1, 2, 3}));
+}
+
 }  // namespace
diff --git a/absl/types/variant.h b/absl/types/variant.h
index 7ef7f88ca11..89ff8dd5102 100644
--- a/absl/types/variant.h
+++ b/absl/types/variant.h
@@ -16,807 +16,91 @@
 // variant.h
 // -----------------------------------------------------------------------------
 //
-// This header file defines an `absl::variant` type for holding a type-safe
-// value of some prescribed set of types (noted as alternative types), and
-// associated functions for managing variants.
-//
-// The `absl::variant` type is a form of type-safe union. An `absl::variant`
-// should always hold a value of one of its alternative types (except in the
-// "valueless by exception state" -- see below). A default-constructed
-// `absl::variant` will hold the value of its first alternative type, provided
-// it is default-constructible.
-//
-// In exceptional cases due to error, an `absl::variant` can hold no
-// value (known as a "valueless by exception" state), though this is not the
-// norm.
-//
-// As with `absl::optional`, an `absl::variant` -- when it holds a value --
-// allocates a value of that type directly within the `variant` itself; it
-// cannot hold a reference, array, or the type `void`; it can, however, hold a
-// pointer to externally managed memory.
-//
-// `absl::variant` is a C++11 compatible version of the C++17 `std::variant`
-// abstraction and is designed to be a drop-in replacement for code compliant
-// with C++17.
+// Historical note: Abseil once provided an implementation of `absl::variant`
+// as a polyfill for `std::variant` prior to C++17. Now that C++17 is required,
+// `absl::variant` is an alias for `std::variant`.
 
 #ifndef ABSL_TYPES_VARIANT_H_
 #define ABSL_TYPES_VARIANT_H_
 
-#include "absl/base/config.h"
-#include "absl/utility/utility.h"
-
-#ifdef ABSL_HAVE_STD_VARIANT
-
-#include <variant>  // IWYU pragma: export
-
-namespace absl {
-using std::bad_variant_access;
-using std::get;
-using std::get_if;
-using std::holds_alternative;
-using std::monostate;
-using std::variant;
-using std::variant_alternative;
-using std::variant_alternative_t;
-using std::variant_npos;
-using std::variant_size;
-using std::variant_size_v;
-using std::visit;
-}  // namespace absl
-
-#else  // ABSL_HAVE_STD_VARIANT
+#include <stddef.h>
 
-#include <functional>
-#include <new>
-#include <type_traits>
-#include <utility>
+#include <variant>
 
+#include "absl/base/config.h"
 #include "absl/base/macros.h"
-#include "absl/base/port.h"
-#include "absl/meta/type_traits.h"
-#include "absl/types/internal/variant.h"
+#include "absl/utility/utility.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
-// -----------------------------------------------------------------------------
-// absl::variant
-// -----------------------------------------------------------------------------
-//
-// An `absl::variant` type is a form of type-safe union. An `absl::variant` --
-// except in exceptional cases -- always holds a value of one of its alternative
-// types.
-//
-// Example:
-//
-//   // Construct a variant that holds either an integer or a std::string and
-//   // assign it to a std::string.
-//   absl::variant<int, std::string> v = std::string("abc");
-//
-//   // A default-constructed variant will hold a value-initialized value of
-//   // the first alternative type.
-//   auto a = absl::variant<int, std::string>();   // Holds an int of value '0'.
-//
-//   // variants are assignable.
-//
-//   // copy assignment
-//   auto v1 = absl::variant<int, std::string>("abc");
-//   auto v2 = absl::variant<int, std::string>(10);
-//   v2 = v1;  // copy assign
-//
-//   // move assignment
-//   auto v1 = absl::variant<int, std::string>("abc");
-//   v1 = absl::variant<int, std::string>(10);
-//
-//   // assignment through type conversion
-//   a = 128;         // variant contains int
-//   a = "128";       // variant contains std::string
-//
-// An `absl::variant` holding a value of one of its alternative types `T` holds
-// an allocation of `T` directly within the variant itself. An `absl::variant`
-// is not allowed to allocate additional storage, such as dynamic memory, to
-// allocate the contained value. The contained value shall be allocated in a
-// region of the variant storage suitably aligned for all alternative types.
-template <typename... Ts>
-class variant;
+using bad_variant_access ABSL_REFACTOR_INLINE
+    = std::bad_variant_access;
 
-// swap()
-//
-// Swaps two `absl::variant` values. This function is equivalent to `v.swap(w)`
-// where `v` and `w` are `absl::variant` types.
-//
-// Note that this function requires all alternative types to be both swappable
-// and move-constructible, because any two variants may refer to either the same
-// type (in which case, they will be swapped) or to two different types (in
-// which case the values will need to be moved).
-//
-template <
-    typename... Ts,
-    absl::enable_if_t<
-        absl::conjunction<std::is_move_constructible<Ts>...,
-                          type_traits_internal::IsSwappable<Ts>...>::value,
-        int> = 0>
-void swap(variant<Ts...>& v, variant<Ts...>& w) noexcept(noexcept(v.swap(w))) {
-  v.swap(w);
+template <size_t I, typename... Args>
+[[deprecated]] constexpr auto get(Args&&... args)
+    -> decltype(std::get<I>(std::forward<Args>(args)...)) {
+  return std::get<I>(std::forward<Args>(args)...);
 }
 
-// variant_size
-//
-// Returns the number of alternative types available for a given `absl::variant`
-// type as a compile-time constant expression. As this is a class template, it
-// is not generally useful for accessing the number of alternative types of
-// any given `absl::variant` instance.
-//
-// Example:
-//
-//   auto a = absl::variant<int, std::string>;
-//   constexpr int num_types =
-//       absl::variant_size<absl::variant<int, std::string>>();
-//
-//   // You can also use the member constant `value`.
-//   constexpr int num_types =
-//       absl::variant_size<absl::variant<int, std::string>>::value;
-//
-//   // `absl::variant_size` is more valuable for use in generic code:
-//   template <typename Variant>
-//   constexpr bool IsVariantMultivalue() {
-//       return absl::variant_size<Variant>() > 1;
-//   }
-//
-// Note that the set of cv-qualified specializations of `variant_size` are
-// provided to ensure that those specializations compile (especially when passed
-// within template logic).
-template <class T>
-struct variant_size;
-
-template <class... Ts>
-struct variant_size<variant<Ts...>>
-    : std::integral_constant<std::size_t, sizeof...(Ts)> {};
-
-// Specialization of `variant_size` for const qualified variants.
-template <class T>
-struct variant_size<const T> : variant_size<T>::type {};
-
-// Specialization of `variant_size` for volatile qualified variants.
-template <class T>
-struct variant_size<volatile T> : variant_size<T>::type {};
-
-// Specialization of `variant_size` for const volatile qualified variants.
-template <class T>
-struct variant_size<const volatile T> : variant_size<T>::type {};
-
-// variant_alternative
-//
-// Returns the alternative type for a given `absl::variant` at the passed
-// index value as a compile-time constant expression. As this is a class
-// template resulting in a type, it is not useful for access of the run-time
-// value of any given `absl::variant` variable.
-//
-// Example:
-//
-//   // The type of the 0th alternative is "int".
-//   using alternative_type_0
-//     = absl::variant_alternative<0, absl::variant<int, std::string>>::type;
-//
-//   static_assert(std::is_same<alternative_type_0, int>::value, "");
-//
-//   // `absl::variant_alternative` is more valuable for use in generic code:
-//   template <typename Variant>
-//   constexpr bool IsFirstElementTrivial() {
-//       return std::is_trivial_v<variant_alternative<0, Variant>::type>;
-//   }
-//
-// Note that the set of cv-qualified specializations of `variant_alternative`
-// are provided to ensure that those specializations compile (especially when
-// passed within template logic).
-template <std::size_t I, class T>
-struct variant_alternative;
-
-template <std::size_t I, class... Types>
-struct variant_alternative<I, variant<Types...>> {
-  using type =
-      variant_internal::VariantAlternativeSfinaeT<I, variant<Types...>>;
-};
-
-// Specialization of `variant_alternative` for const qualified variants.
-template <std::size_t I, class T>
-struct variant_alternative<I, const T> {
-  using type = const typename variant_alternative<I, T>::type;
-};
-
-// Specialization of `variant_alternative` for volatile qualified variants.
-template <std::size_t I, class T>
-struct variant_alternative<I, volatile T> {
-  using type = volatile typename variant_alternative<I, T>::type;
-};
-
-// Specialization of `variant_alternative` for const volatile qualified
-// variants.
-template <std::size_t I, class T>
-struct variant_alternative<I, const volatile T> {
-  using type = const volatile typename variant_alternative<I, T>::type;
-};
-
-// Template type alias for variant_alternative<I, T>::type.
-//
-// Example:
-//
-//   using alternative_type_0
-//     = absl::variant_alternative_t<0, absl::variant<int, std::string>>;
-//   static_assert(std::is_same<alternative_type_0, int>::value, "");
-template <std::size_t I, class T>
-using variant_alternative_t = typename variant_alternative<I, T>::type;
-
-// holds_alternative()
-//
-// Checks whether the given variant currently holds a given alternative type,
-// returning `true` if so.
-//
-// Example:
-//
-//   absl::variant<int, std::string> foo = 42;
-//   if (absl::holds_alternative<int>(foo)) {
-//       std::cout << "The variant holds an integer";
-//   }
-template <class T, class... Types>
-constexpr bool holds_alternative(const variant<Types...>& v) noexcept {
-  static_assert(
-      variant_internal::UnambiguousIndexOfImpl<variant<Types...>, T,
-                                               0>::value != sizeof...(Types),
-      "The type T must occur exactly once in Types...");
-  return v.index() ==
-         variant_internal::UnambiguousIndexOf<variant<Types...>, T>::value;
+template <typename T, typename... Args>
+[[deprecated]] constexpr decltype(std::get<T>(std::declval<Args>()...)) get(
+    Args&&... args) {
+  return std::get<T>(std::forward<Args>(args)...);
 }
 
-// get()
-//
-// Returns a reference to the value currently within a given variant, using
-// either a unique alternative type amongst the variant's set of alternative
-// types, or the variant's index value. Attempting to get a variant's value
-// using a type that is not unique within the variant's set of alternative types
-// is a compile-time error. If the index of the alternative being specified is
-// different from the index of the alternative that is currently stored, throws
-// `absl::bad_variant_access`.
-//
-// Example:
-//
-//   auto a = absl::variant<int, std::string>;
-//
-//   // Get the value by type (if unique).
-//   int i = absl::get<int>(a);
-//
-//   auto b = absl::variant<int, int>;
-//
-//   // Getting the value by a type that is not unique is ill-formed.
-//   int j = absl::get<int>(b);     // Compile Error!
-//
-//   // Getting value by index not ambiguous and allowed.
-//   int k = absl::get<1>(b);
-
-// Overload for getting a variant's lvalue by type.
-template <class T, class... Types>
-constexpr T& get(variant<Types...>& v) {  // NOLINT
-  return variant_internal::VariantCoreAccess::CheckedAccess<
-      variant_internal::IndexOf<T, Types...>::value>(v);
+template <size_t I, typename... Args>
+[[deprecated]] constexpr decltype(std::get_if<I>(std::declval<Args>()...))
+get_if(Args&&... args) {
+  return std::get_if<I>(std::forward<Args>(args)...);
 }
 
-// Overload for getting a variant's rvalue by type.
-// Note: `absl::move()` is required to allow use of constexpr in C++11.
-template <class T, class... Types>
-constexpr T&& get(variant<Types...>&& v) {
-  return variant_internal::VariantCoreAccess::CheckedAccess<
-      variant_internal::IndexOf<T, Types...>::value>(absl::move(v));
+template <typename T, typename... Args>
+[[deprecated]] constexpr decltype(std::get_if<T>(std::declval<Args>()...))
+get_if(Args&&... args) {
+  return std::get_if<T>(std::forward<Args>(args)...);
 }
 
-// Overload for getting a variant's const lvalue by type.
-template <class T, class... Types>
-constexpr const T& get(const variant<Types...>& v) {
-  return variant_internal::VariantCoreAccess::CheckedAccess<
-      variant_internal::IndexOf<T, Types...>::value>(v);
+template <typename T, typename... Args>
+[[deprecated]] constexpr decltype(std::holds_alternative<T>(
+    std::declval<Args>()...))
+holds_alternative(Args&&... args) {
+  return std::holds_alternative<T>(std::forward<Args>(args)...);
 }
 
-// Overload for getting a variant's const rvalue by type.
-// Note: `absl::move()` is required to allow use of constexpr in C++11.
-template <class T, class... Types>
-constexpr const T&& get(const variant<Types...>&& v) {
-  return variant_internal::VariantCoreAccess::CheckedAccess<
-      variant_internal::IndexOf<T, Types...>::value>(absl::move(v));
-}
-
-// Overload for getting a variant's lvalue by index.
-template <std::size_t I, class... Types>
-constexpr variant_alternative_t<I, variant<Types...>>& get(
-    variant<Types...>& v) {  // NOLINT
-  return variant_internal::VariantCoreAccess::CheckedAccess<I>(v);
-}
-
-// Overload for getting a variant's rvalue by index.
-// Note: `absl::move()` is required to allow use of constexpr in C++11.
-template <std::size_t I, class... Types>
-constexpr variant_alternative_t<I, variant<Types...>>&& get(
-    variant<Types...>&& v) {
-  return variant_internal::VariantCoreAccess::CheckedAccess<I>(absl::move(v));
-}
+using monostate ABSL_REFACTOR_INLINE
+    = std::monostate;
 
-// Overload for getting a variant's const lvalue by index.
-template <std::size_t I, class... Types>
-constexpr const variant_alternative_t<I, variant<Types...>>& get(
-    const variant<Types...>& v) {
-  return variant_internal::VariantCoreAccess::CheckedAccess<I>(v);
-}
-
-// Overload for getting a variant's const rvalue by index.
-// Note: `absl::move()` is required to allow use of constexpr in C++11.
-template <std::size_t I, class... Types>
-constexpr const variant_alternative_t<I, variant<Types...>>&& get(
-    const variant<Types...>&& v) {
-  return variant_internal::VariantCoreAccess::CheckedAccess<I>(absl::move(v));
-}
-
-// get_if()
-//
-// Returns a pointer to the value currently stored within a given variant, if
-// present, using either a unique alternative type amongst the variant's set of
-// alternative types, or the variant's index value. If such a value does not
-// exist, returns `nullptr`.
-//
-// As with `get`, attempting to get a variant's value using a type that is not
-// unique within the variant's set of alternative types is a compile-time error.
-
-// Overload for getting a pointer to the value stored in the given variant by
-// index.
-template <std::size_t I, class... Types>
-constexpr absl::add_pointer_t<variant_alternative_t<I, variant<Types...>>>
-get_if(variant<Types...>* v) noexcept {
-  return (v != nullptr && v->index() == I)
-             ? std::addressof(
-                   variant_internal::VariantCoreAccess::Access<I>(*v))
-             : nullptr;
-}
-
-// Overload for getting a pointer to the const value stored in the given
-// variant by index.
-template <std::size_t I, class... Types>
-constexpr absl::add_pointer_t<const variant_alternative_t<I, variant<Types...>>>
-get_if(const variant<Types...>* v) noexcept {
-  return (v != nullptr && v->index() == I)
-             ? std::addressof(
-                   variant_internal::VariantCoreAccess::Access<I>(*v))
-             : nullptr;
-}
-
-// Overload for getting a pointer to the value stored in the given variant by
-// type.
-template <class T, class... Types>
-constexpr absl::add_pointer_t<T> get_if(variant<Types...>* v) noexcept {
-  return absl::get_if<variant_internal::IndexOf<T, Types...>::value>(v);
-}
-
-// Overload for getting a pointer to the const value stored in the given variant
-// by type.
-template <class T, class... Types>
-constexpr absl::add_pointer_t<const T> get_if(
-    const variant<Types...>* v) noexcept {
-  return absl::get_if<variant_internal::IndexOf<T, Types...>::value>(v);
-}
-
-// visit()
-//
-// Calls a provided functor on a given set of variants. `absl::visit()` is
-// commonly used to conditionally inspect the state of a given variant (or set
-// of variants).
-//
-// The functor must return the same type when called with any of the variants'
-// alternatives.
-//
-// Example:
-//
-//   // Define a visitor functor
-//   struct GetVariant {
-//       template<typename T>
-//       void operator()(const T& i) const {
-//         std::cout << "The variant's value is: " << i;
-//       }
-//   };
-//
-//   // Declare our variant, and call `absl::visit()` on it.
-//   // Note that `GetVariant()` returns void in either case.
-//   absl::variant<int, std::string> foo = std::string("foo");
-//   GetVariant visitor;
-//   absl::visit(visitor, foo);  // Prints `The variant's value is: foo'
-template <typename Visitor, typename... Variants>
-variant_internal::VisitResult<Visitor, Variants...> visit(Visitor&& vis,
-                                                          Variants&&... vars) {
-  return variant_internal::
-      VisitIndices<variant_size<absl::decay_t<Variants> >::value...>::Run(
-          variant_internal::PerformVisitation<Visitor, Variants...>{
-              std::forward_as_tuple(absl::forward<Variants>(vars)...),
-              absl::forward<Visitor>(vis)},
-          vars.index()...);
-}
-
-// monostate
-//
-// The monostate class serves as a first alternative type for a variant for
-// which the first variant type is otherwise not default-constructible.
-struct monostate {};
-
-// `absl::monostate` Relational Operators
-
-constexpr bool operator<(monostate, monostate) noexcept { return false; }
-constexpr bool operator>(monostate, monostate) noexcept { return false; }
-constexpr bool operator<=(monostate, monostate) noexcept { return true; }
-constexpr bool operator>=(monostate, monostate) noexcept { return true; }
-constexpr bool operator==(monostate, monostate) noexcept { return true; }
-constexpr bool operator!=(monostate, monostate) noexcept { return false; }
-
-
-//------------------------------------------------------------------------------
-// `absl::variant` Template Definition
-//------------------------------------------------------------------------------
-template <typename T0, typename... Tn>
-class variant<T0, Tn...> : private variant_internal::VariantBase<T0, Tn...> {
-  static_assert(absl::conjunction<std::is_object<T0>,
-                                  std::is_object<Tn>...>::value,
-                "Attempted to instantiate a variant containing a non-object "
-                "type.");
-  // Intentionally not qualifying `negation` with `absl::` to work around a bug
-  // in MSVC 2015 with inline namespace and variadic template.
-  static_assert(absl::conjunction<negation<std::is_array<T0> >,
-                                  negation<std::is_array<Tn> >...>::value,
-                "Attempted to instantiate a variant containing an array type.");
-  static_assert(absl::conjunction<std::is_nothrow_destructible<T0>,
-                                  std::is_nothrow_destructible<Tn>...>::value,
-                "Attempted to instantiate a variant containing a non-nothrow "
-                "destructible type.");
-
-  friend struct variant_internal::VariantCoreAccess;
-
- private:
-  using Base = variant_internal::VariantBase<T0, Tn...>;
-
- public:
-  // Constructors
-
-  // Constructs a variant holding a default-initialized value of the first
-  // alternative type.
-  constexpr variant() /*noexcept(see 111above)*/ = default;
-
-  // Copy constructor, standard semantics
-  variant(const variant& other) = default;
-
-  // Move constructor, standard semantics
-  variant(variant&& other) /*noexcept(see above)*/ = default;
-
-  // Constructs a variant of an alternative type specified by overload
-  // resolution of the provided forwarding arguments through
-  // direct-initialization.
-  //
-  // Note: If the selected constructor is a constexpr constructor, this
-  // constructor shall be a constexpr constructor.
-  //
-  // NOTE: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0608r1.html
-  // has been voted passed the design phase in the C++ standard meeting in Mar
-  // 2018. It will be implemented and integrated into `absl::variant`.
-  template <
-      class T,
-      std::size_t I = std::enable_if<
-          variant_internal::IsNeitherSelfNorInPlace<variant,
-                                                    absl::decay_t<T>>::value,
-          variant_internal::IndexOfConstructedType<variant, T>>::type::value,
-      class Tj = absl::variant_alternative_t<I, variant>,
-      absl::enable_if_t<std::is_constructible<Tj, T>::value>* =
-          nullptr>
-  constexpr variant(T&& t) noexcept(std::is_nothrow_constructible<Tj, T>::value)
-      : Base(variant_internal::EmplaceTag<I>(), absl::forward<T>(t)) {}
-
-  // Constructs a variant of an alternative type from the arguments through
-  // direct-initialization.
-  //
-  // Note: If the selected constructor is a constexpr constructor, this
-  // constructor shall be a constexpr constructor.
-  template <class T, class... Args,
-            typename std::enable_if<std::is_constructible<
-                variant_internal::UnambiguousTypeOfT<variant, T>,
-                Args...>::value>::type* = nullptr>
-  constexpr explicit variant(in_place_type_t<T>, Args&&... args)
-      : Base(variant_internal::EmplaceTag<
-                 variant_internal::UnambiguousIndexOf<variant, T>::value>(),
-             absl::forward<Args>(args)...) {}
-
-  // Constructs a variant of an alternative type from an initializer list
-  // and other arguments through direct-initialization.
-  //
-  // Note: If the selected constructor is a constexpr constructor, this
-  // constructor shall be a constexpr constructor.
-  template <class T, class U, class... Args,
-            typename std::enable_if<std::is_constructible<
-                variant_internal::UnambiguousTypeOfT<variant, T>,
-                std::initializer_list<U>&, Args...>::value>::type* = nullptr>
-  constexpr explicit variant(in_place_type_t<T>, std::initializer_list<U> il,
-                             Args&&... args)
-      : Base(variant_internal::EmplaceTag<
-                 variant_internal::UnambiguousIndexOf<variant, T>::value>(),
-             il, absl::forward<Args>(args)...) {}
-
-  // Constructs a variant of an alternative type from a provided index,
-  // through value-initialization using the provided forwarded arguments.
-  template <std::size_t I, class... Args,
-            typename std::enable_if<std::is_constructible<
-                variant_internal::VariantAlternativeSfinaeT<I, variant>,
-                Args...>::value>::type* = nullptr>
-  constexpr explicit variant(in_place_index_t<I>, Args&&... args)
-      : Base(variant_internal::EmplaceTag<I>(), absl::forward<Args>(args)...) {}
-
-  // Constructs a variant of an alternative type from a provided index,
-  // through value-initialization of an initializer list and the provided
-  // forwarded arguments.
-  template <std::size_t I, class U, class... Args,
-            typename std::enable_if<std::is_constructible<
-                variant_internal::VariantAlternativeSfinaeT<I, variant>,
-                std::initializer_list<U>&, Args...>::value>::type* = nullptr>
-  constexpr explicit variant(in_place_index_t<I>, std::initializer_list<U> il,
-                             Args&&... args)
-      : Base(variant_internal::EmplaceTag<I>(), il,
-             absl::forward<Args>(args)...) {}
-
-  // Destructors
-
-  // Destroys the variant's currently contained value, provided that
-  // `absl::valueless_by_exception()` is false.
-  ~variant() = default;
-
-  // Assignment Operators
-
-  // Copy assignment operator
-  variant& operator=(const variant& other) = default;
-
-  // Move assignment operator
-  variant& operator=(variant&& other) /*noexcept(see above)*/ = default;
-
-  // Converting assignment operator
-  //
-  // NOTE: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0608r1.html
-  // has been voted passed the design phase in the C++ standard meeting in Mar
-  // 2018. It will be implemented and integrated into `absl::variant`.
-  template <
-      class T,
-      std::size_t I = std::enable_if<
-          !std::is_same<absl::decay_t<T>, variant>::value,
-          variant_internal::IndexOfConstructedType<variant, T>>::type::value,
-      class Tj = absl::variant_alternative_t<I, variant>,
-      typename std::enable_if<std::is_assignable<Tj&, T>::value &&
-                              std::is_constructible<Tj, T>::value>::type* =
-          nullptr>
-  variant& operator=(T&& t) noexcept(
-      std::is_nothrow_assignable<Tj&, T>::value&&
-          std::is_nothrow_constructible<Tj, T>::value) {
-    variant_internal::VisitIndices<sizeof...(Tn) + 1>::Run(
-        variant_internal::VariantCoreAccess::MakeConversionAssignVisitor(
-            this, absl::forward<T>(t)),
-        index());
-
-    return *this;
-  }
-
-
-  // emplace() Functions
-
-  // Constructs a value of the given alternative type T within the variant.
-  //
-  // Example:
-  //
-  //   absl::variant<std::vector<int>, int, std::string> v;
-  //   v.emplace<int>(99);
-  //   v.emplace<std::string>("abc");
-  template <
-      class T, class... Args,
-      typename std::enable_if<std::is_constructible<
-          absl::variant_alternative_t<
-              variant_internal::UnambiguousIndexOf<variant, T>::value, variant>,
-          Args...>::value>::type* = nullptr>
-  T& emplace(Args&&... args) {
-    return variant_internal::VariantCoreAccess::Replace<
-        variant_internal::UnambiguousIndexOf<variant, T>::value>(
-        this, absl::forward<Args>(args)...);
-  }
-
-  // Constructs a value of the given alternative type T within the variant using
-  // an initializer list.
-  //
-  // Example:
-  //
-  //   absl::variant<std::vector<int>, int, std::string> v;
-  //   v.emplace<std::vector<int>>({0, 1, 2});
-  template <
-      class T, class U, class... Args,
-      typename std::enable_if<std::is_constructible<
-          absl::variant_alternative_t<
-              variant_internal::UnambiguousIndexOf<variant, T>::value, variant>,
-          std::initializer_list<U>&, Args...>::value>::type* = nullptr>
-  T& emplace(std::initializer_list<U> il, Args&&... args) {
-    return variant_internal::VariantCoreAccess::Replace<
-        variant_internal::UnambiguousIndexOf<variant, T>::value>(
-        this, il, absl::forward<Args>(args)...);
-  }
-
-  // Destroys the current value of the variant (provided that
-  // `absl::valueless_by_exception()` is false, and constructs a new value at
-  // the given index.
-  //
-  // Example:
-  //
-  //   absl::variant<std::vector<int>, int, int> v;
-  //   v.emplace<1>(99);
-  //   v.emplace<2>(98);
-  //   v.emplace<int>(99);  // Won't compile. 'int' isn't a unique type.
-  template <std::size_t I, class... Args,
-            typename std::enable_if<
-                std::is_constructible<absl::variant_alternative_t<I, variant>,
-                                      Args...>::value>::type* = nullptr>
-  absl::variant_alternative_t<I, variant>& emplace(Args&&... args) {
-    return variant_internal::VariantCoreAccess::Replace<I>(
-        this, absl::forward<Args>(args)...);
-  }
-
-  // Destroys the current value of the variant (provided that
-  // `absl::valueless_by_exception()` is false, and constructs a new value at
-  // the given index using an initializer list and the provided arguments.
-  //
-  // Example:
-  //
-  //   absl::variant<std::vector<int>, int, int> v;
-  //   v.emplace<0>({0, 1, 2});
-  template <std::size_t I, class U, class... Args,
-            typename std::enable_if<std::is_constructible<
-                absl::variant_alternative_t<I, variant>,
-                std::initializer_list<U>&, Args...>::value>::type* = nullptr>
-  absl::variant_alternative_t<I, variant>& emplace(std::initializer_list<U> il,
-                                                   Args&&... args) {
-    return variant_internal::VariantCoreAccess::Replace<I>(
-        this, il, absl::forward<Args>(args)...);
-  }
-
-  // variant::valueless_by_exception()
-  //
-  // Returns false if and only if the variant currently holds a valid value.
-  constexpr bool valueless_by_exception() const noexcept {
-    return this->index_ == absl::variant_npos;
-  }
-
-  // variant::index()
-  //
-  // Returns the index value of the variant's currently selected alternative
-  // type.
-  constexpr std::size_t index() const noexcept { return this->index_; }
-
-  // variant::swap()
-  //
-  // Swaps the values of two variant objects.
-  //
-  void swap(variant& rhs) noexcept(
-      absl::conjunction<
-          std::is_nothrow_move_constructible<T0>,
-          std::is_nothrow_move_constructible<Tn>...,
-          type_traits_internal::IsNothrowSwappable<T0>,
-          type_traits_internal::IsNothrowSwappable<Tn>...>::value) {
-    return variant_internal::VisitIndices<sizeof...(Tn) + 1>::Run(
-        variant_internal::Swap<T0, Tn...>{this, &rhs}, rhs.index());
-  }
-};
-
-// We need a valid declaration of variant<> for SFINAE and overload resolution
-// to work properly above, but we don't need a full declaration since this type
-// will never be constructed. This declaration, though incomplete, suffices.
-template <>
-class variant<>;
-
-//------------------------------------------------------------------------------
-// Relational Operators
-//------------------------------------------------------------------------------
-//
-// If neither operand is in the `variant::valueless_by_exception` state:
-//
-//   * If the index of both variants is the same, the relational operator
-//     returns the result of the corresponding relational operator for the
-//     corresponding alternative type.
-//   * If the index of both variants is not the same, the relational operator
-//     returns the result of that operation applied to the value of the left
-//     operand's index and the value of the right operand's index.
-//   * If at least one operand is in the valueless_by_exception state:
-//     - A variant in the valueless_by_exception state is only considered equal
-//       to another variant in the valueless_by_exception state.
-//     - If exactly one operand is in the valueless_by_exception state, the
-//       variant in the valueless_by_exception state is less than the variant
-//       that is not in the valueless_by_exception state.
-//
-// Note: The value 1 is added to each index in the relational comparisons such
-// that the index corresponding to the valueless_by_exception state wraps around
-// to 0 (the lowest value for the index type), and the remaining indices stay in
-// the same relative order.
-
-// Equal-to operator
 template <typename... Types>
-constexpr variant_internal::RequireAllHaveEqualT<Types...> operator==(
-    const variant<Types...>& a, const variant<Types...>& b) {
-  return (a.index() == b.index()) &&
-         variant_internal::VisitIndices<sizeof...(Types)>::Run(
-             variant_internal::EqualsOp<Types...>{&a, &b}, a.index());
-}
+using variant ABSL_REFACTOR_INLINE
+    = std::variant<Types...>;
 
-// Not equal operator
-template <typename... Types>
-constexpr variant_internal::RequireAllHaveNotEqualT<Types...> operator!=(
-    const variant<Types...>& a, const variant<Types...>& b) {
-  return (a.index() != b.index()) ||
-         variant_internal::VisitIndices<sizeof...(Types)>::Run(
-             variant_internal::NotEqualsOp<Types...>{&a, &b}, a.index());
-}
+template <size_t I, typename T>
+using variant_alternative ABSL_REFACTOR_INLINE
+    = std::variant_alternative<I, T>;
 
-// Less-than operator
-template <typename... Types>
-constexpr variant_internal::RequireAllHaveLessThanT<Types...> operator<(
-    const variant<Types...>& a, const variant<Types...>& b) {
-  return (a.index() != b.index())
-             ? (a.index() + 1) < (b.index() + 1)
-             : variant_internal::VisitIndices<sizeof...(Types)>::Run(
-                   variant_internal::LessThanOp<Types...>{&a, &b}, a.index());
-}
+template <size_t I, typename T>
+using variant_alternative_t ABSL_REFACTOR_INLINE
+    = std::variant_alternative_t<I, T>;
 
-// Greater-than operator
-template <typename... Types>
-constexpr variant_internal::RequireAllHaveGreaterThanT<Types...> operator>(
-    const variant<Types...>& a, const variant<Types...>& b) {
-  return (a.index() != b.index())
-             ? (a.index() + 1) > (b.index() + 1)
-             : variant_internal::VisitIndices<sizeof...(Types)>::Run(
-                   variant_internal::GreaterThanOp<Types...>{&a, &b},
-                   a.index());
-}
+inline constexpr size_t variant_npos ABSL_REFACTOR_INLINE
+    = std::variant_npos;
 
-// Less-than or equal-to operator
-template <typename... Types>
-constexpr variant_internal::RequireAllHaveLessThanOrEqualT<Types...> operator<=(
-    const variant<Types...>& a, const variant<Types...>& b) {
-  return (a.index() != b.index())
-             ? (a.index() + 1) < (b.index() + 1)
-             : variant_internal::VisitIndices<sizeof...(Types)>::Run(
-                   variant_internal::LessThanOrEqualsOp<Types...>{&a, &b},
-                   a.index());
-}
+template <typename T>
+using variant_size ABSL_REFACTOR_INLINE
+    = std::variant_size<T>;
 
-// Greater-than or equal-to operator
-template <typename... Types>
-constexpr variant_internal::RequireAllHaveGreaterThanOrEqualT<Types...>
-operator>=(const variant<Types...>& a, const variant<Types...>& b) {
-  return (a.index() != b.index())
-             ? (a.index() + 1) > (b.index() + 1)
-             : variant_internal::VisitIndices<sizeof...(Types)>::Run(
-                   variant_internal::GreaterThanOrEqualsOp<Types...>{&a, &b},
-                   a.index());
-}
+template <typename T>
+inline constexpr size_t variant_size_v ABSL_REFACTOR_INLINE
+    = std::variant_size_v<T>;
 
-}  // namespace absl
-
-namespace std {
-
-// hash()
-template <>  // NOLINT
-struct hash<absl::monostate> {
-  std::size_t operator()(absl::monostate) const { return 0; }
-};
-
-template <class... T>  // NOLINT
-struct hash<absl::variant<T...>>
-    : absl::variant_internal::VariantHashBase<absl::variant<T...>, void,
-                                              absl::remove_const_t<T>...> {};
-
-}  // namespace std
-
-#endif  // ABSL_HAVE_STD_VARIANT
+template <typename... Args>
+[[deprecated]] constexpr decltype(std::visit(std::declval<Args>()...)) visit(
+    Args&&... args) {
+  return std::visit(std::forward<Args>(args)...);
+}
 
-namespace absl {
 namespace variant_internal {
-
 // Helper visitor for converting a variant<Ts...>` into another type (mostly
 // variant) that can be constructed from any type.
 template <typename To>
@@ -826,30 +110,30 @@ struct ConversionVisitor {
     return To(std::forward<T>(v));
   }
 };
-
 }  // namespace variant_internal
 
 // ConvertVariantTo()
 //
-// Helper functions to convert an `absl::variant` to a variant of another set of
+// Helper functions to convert an `std::variant` to a variant of another set of
 // types, provided that the alternative type of the new variant type can be
 // converted from any type in the source variant.
 //
 // Example:
 //
-//   absl::variant<name1, name2, float> InternalReq(const Req&);
+//   std::variant<name1, name2, float> InternalReq(const Req&);
 //
 //   // name1 and name2 are convertible to name
-//   absl::variant<name, float> ExternalReq(const Req& req) {
-//     return absl::ConvertVariantTo<absl::variant<name, float>>(
+//   std::variant<name, float> ExternalReq(const Req& req) {
+//     return absl::ConvertVariantTo<std::variant<name, float>>(
 //              InternalReq(req));
 //   }
 template <typename To, typename Variant>
 To ConvertVariantTo(Variant&& variant) {
-  return absl::visit(variant_internal::ConversionVisitor<To>{},
-                     std::forward<Variant>(variant));
+  return std::visit(variant_internal::ConversionVisitor<To>{},
+                    std::forward<Variant>(variant));
 }
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_TYPES_VARIANT_H_
diff --git a/absl/types/variant_benchmark.cc b/absl/types/variant_benchmark.cc
deleted file mode 100644
index a5f52164414..00000000000
--- a/absl/types/variant_benchmark.cc
+++ /dev/null
@@ -1,220 +0,0 @@
-// Copyright 2017 The Abseil Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-// Unit tests for the variant template. The 'is' and 'IsEmpty' methods
-// of variant are not explicitly tested because they are used repeatedly
-// in building other tests. All other public variant methods should have
-// explicit tests.
-
-#include "absl/types/variant.h"
-
-#include <cstddef>
-#include <cstdlib>
-#include <string>
-#include <tuple>
-
-#include "benchmark/benchmark.h"
-#include "absl/utility/utility.h"
-
-namespace absl {
-namespace {
-
-template <std::size_t I>
-struct VariantAlternative {
-  char member;
-};
-
-template <class Indices>
-struct VariantOfAlternativesImpl;
-
-template <std::size_t... Indices>
-struct VariantOfAlternativesImpl<absl::index_sequence<Indices...>> {
-  using type = absl::variant<VariantAlternative<Indices>...>;
-};
-
-template <std::size_t NumAlternatives>
-using VariantOfAlternatives = typename VariantOfAlternativesImpl<
-    absl::make_index_sequence<NumAlternatives>>::type;
-
-struct Empty {};
-
-template <class... T>
-void Ignore(T...) noexcept {}
-
-template <class T>
-Empty DoNotOptimizeAndReturnEmpty(T&& arg) noexcept {
-  benchmark::DoNotOptimize(arg);
-  return {};
-}
-
-struct VisitorApplier {
-  struct Visitor {
-    template <class... T>
-    void operator()(T&&... args) const noexcept {
-      Ignore(DoNotOptimizeAndReturnEmpty(args)...);
-    }
-  };
-
-  template <class... Vars>
-  void operator()(const Vars&... vars) const noexcept {
-    absl::visit(Visitor(), vars...);
-  }
-};
-
-template <std::size_t NumIndices, std::size_t CurrIndex = NumIndices - 1>
-struct MakeWithIndex {
-  using Variant = VariantOfAlternatives<NumIndices>;
-
-  static Variant Run(std::size_t index) {
-    return index == CurrIndex
-               ? Variant(absl::in_place_index_t<CurrIndex>())
-               : MakeWithIndex<NumIndices, CurrIndex - 1>::Run(index);
-  }
-};
-
-template <std::size_t NumIndices>
-struct MakeWithIndex<NumIndices, 0> {
-  using Variant = VariantOfAlternatives<NumIndices>;
-
-  static Variant Run(std::size_t /*index*/) { return Variant(); }
-};
-
-template <std::size_t NumIndices, class Dimensions>
-struct MakeVariantTuple;
-
-template <class T, std::size_t /*I*/>
-using always_t = T;
-
-template <std::size_t NumIndices>
-VariantOfAlternatives<NumIndices> MakeVariant(std::size_t dimension,
-                                              std::size_t index) {
-  return dimension == 0
-             ? MakeWithIndex<NumIndices>::Run(index % NumIndices)
-             : MakeVariant<NumIndices>(dimension - 1, index / NumIndices);
-}
-
-template <std::size_t NumIndices, std::size_t... Dimensions>
-struct MakeVariantTuple<NumIndices, absl::index_sequence<Dimensions...>> {
-  using VariantTuple =
-      std::tuple<always_t<VariantOfAlternatives<NumIndices>, Dimensions>...>;
-
-  static VariantTuple Run(int index) {
-    return std::make_tuple(MakeVariant<NumIndices>(Dimensions, index)...);
-  }
-};
-
-constexpr std::size_t integral_pow(std::size_t base, std::size_t power) {
-  return power == 0 ? 1 : base * integral_pow(base, power - 1);
-}
-
-template <std::size_t End, std::size_t I = 0>
-struct VisitTestBody {
-  template <class Vars, class State>
-  static bool Run(Vars& vars, State& state) {
-    if (state.KeepRunning()) {
-      absl::apply(VisitorApplier(), vars[I]);
-      return VisitTestBody<End, I + 1>::Run(vars, state);
-    }
-    return false;
-  }
-};
-
-template <std::size_t End>
-struct VisitTestBody<End, End> {
-  template <class Vars, class State>
-  static bool Run(Vars& /*vars*/, State& /*state*/) {
-    return true;
-  }
-};
-
-// Visit operations where branch prediction is likely to give a boost.
-template <std::size_t NumIndices, std::size_t NumDimensions = 1>
-void BM_RedundantVisit(benchmark::State& state) {
-  auto vars =
-      MakeVariantTuple<NumIndices, absl::make_index_sequence<NumDimensions>>::
-          Run(static_cast<std::size_t>(state.range(0)));
-
-  for (auto _ : state) {  // NOLINT
-    benchmark::DoNotOptimize(vars);
-    absl::apply(VisitorApplier(), vars);
-  }
-}
-
-// Visit operations where branch prediction is unlikely to give a boost.
-template <std::size_t NumIndices, std::size_t NumDimensions = 1>
-void BM_Visit(benchmark::State& state) {
-  constexpr std::size_t num_possibilities =
-      integral_pow(NumIndices, NumDimensions);
-
-  using VariantTupleMaker =
-      MakeVariantTuple<NumIndices, absl::make_index_sequence<NumDimensions>>;
-  using Tuple = typename VariantTupleMaker::VariantTuple;
-
-  Tuple vars[num_possibilities];
-  for (std::size_t i = 0; i < num_possibilities; ++i)
-    vars[i] = VariantTupleMaker::Run(i);
-
-  while (VisitTestBody<num_possibilities>::Run(vars, state)) {
-  }
-}
-
-// Visitation
-//   Each visit is on a different variant with a different active alternative)
-
-// Unary visit
-BENCHMARK_TEMPLATE(BM_Visit, 1);
-BENCHMARK_TEMPLATE(BM_Visit, 2);
-BENCHMARK_TEMPLATE(BM_Visit, 3);
-BENCHMARK_TEMPLATE(BM_Visit, 4);
-BENCHMARK_TEMPLATE(BM_Visit, 5);
-BENCHMARK_TEMPLATE(BM_Visit, 6);
-BENCHMARK_TEMPLATE(BM_Visit, 7);
-BENCHMARK_TEMPLATE(BM_Visit, 8);
-BENCHMARK_TEMPLATE(BM_Visit, 16);
-BENCHMARK_TEMPLATE(BM_Visit, 32);
-BENCHMARK_TEMPLATE(BM_Visit, 64);
-
-// Binary visit
-BENCHMARK_TEMPLATE(BM_Visit, 1, 2);
-BENCHMARK_TEMPLATE(BM_Visit, 2, 2);
-BENCHMARK_TEMPLATE(BM_Visit, 3, 2);
-BENCHMARK_TEMPLATE(BM_Visit, 4, 2);
-BENCHMARK_TEMPLATE(BM_Visit, 5, 2);
-
-// Ternary visit
-BENCHMARK_TEMPLATE(BM_Visit, 1, 3);
-BENCHMARK_TEMPLATE(BM_Visit, 2, 3);
-BENCHMARK_TEMPLATE(BM_Visit, 3, 3);
-
-// Quaternary visit
-BENCHMARK_TEMPLATE(BM_Visit, 1, 4);
-BENCHMARK_TEMPLATE(BM_Visit, 2, 4);
-
-// Redundant Visitation
-//   Each visit consistently has the same alternative active
-
-// Unary visit
-BENCHMARK_TEMPLATE(BM_RedundantVisit, 1)->Arg(0);
-BENCHMARK_TEMPLATE(BM_RedundantVisit, 2)->DenseRange(0, 1);
-BENCHMARK_TEMPLATE(BM_RedundantVisit, 8)->DenseRange(0, 7);
-
-// Binary visit
-BENCHMARK_TEMPLATE(BM_RedundantVisit, 1, 2)->Arg(0);
-BENCHMARK_TEMPLATE(BM_RedundantVisit, 2, 2)
-    ->DenseRange(0, integral_pow(2, 2) - 1);
-BENCHMARK_TEMPLATE(BM_RedundantVisit, 4, 2)
-    ->DenseRange(0, integral_pow(4, 2) - 1);
-
-}  // namespace
-}  // namespace absl
diff --git a/absl/types/variant_exception_safety_test.cc b/absl/types/variant_exception_safety_test.cc
deleted file mode 100644
index fd7e6c7fe80..00000000000
--- a/absl/types/variant_exception_safety_test.cc
+++ /dev/null
@@ -1,530 +0,0 @@
-// Copyright 2017 The Abseil Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-#include "absl/types/variant.h"
-
-#include "absl/base/config.h"
-
-// This test is a no-op when absl::variant is an alias for std::variant and when
-// exceptions are not enabled.
-#if !defined(ABSL_HAVE_STD_VARIANT) && defined(ABSL_HAVE_EXCEPTIONS)
-
-#include <iostream>
-#include <memory>
-#include <utility>
-#include <vector>
-
-#include "gmock/gmock.h"
-#include "gtest/gtest.h"
-#include "absl/base/internal/exception_safety_testing.h"
-#include "absl/memory/memory.h"
-
-// See comment in absl/base/config.h
-#if !defined(ABSL_INTERNAL_MSVC_2017_DBG_MODE)
-
-namespace absl {
-namespace {
-
-using ::testing::MakeExceptionSafetyTester;
-using ::testing::strong_guarantee;
-using ::testing::TestNothrowOp;
-using ::testing::TestThrowingCtor;
-
-using Thrower = testing::ThrowingValue<>;
-using CopyNothrow = testing::ThrowingValue<testing::TypeSpec::kNoThrowCopy>;
-using MoveNothrow = testing::ThrowingValue<testing::TypeSpec::kNoThrowMove>;
-using ThrowingAlloc = testing::ThrowingAllocator<Thrower>;
-using ThrowerVec = std::vector<Thrower, ThrowingAlloc>;
-using ThrowingVariant =
-    absl::variant<Thrower, CopyNothrow, MoveNothrow, ThrowerVec>;
-
-struct ConversionException {};
-
-template <class T>
-struct ExceptionOnConversion {
-  operator T() const {  // NOLINT
-    throw ConversionException();
-  }
-};
-
-// Forces a variant into the valueless by exception state.
-void ToValuelessByException(ThrowingVariant& v) {  // NOLINT
-  try {
-    v.emplace<Thrower>();
-    v.emplace<Thrower>(ExceptionOnConversion<Thrower>());
-  } catch (const ConversionException&) {
-    // This space intentionally left blank.
-  }
-}
-
-// Check that variant is still in a usable state after an exception is thrown.
-testing::AssertionResult VariantInvariants(ThrowingVariant* v) {
-  using testing::AssertionFailure;
-  using testing::AssertionSuccess;
-
-  // Try using the active alternative
-  if (absl::holds_alternative<Thrower>(*v)) {
-    auto& t = absl::get<Thrower>(*v);
-    t = Thrower{-100};
-    if (t.Get() != -100) {
-      return AssertionFailure() << "Thrower should be assigned -100";
-    }
-  } else if (absl::holds_alternative<ThrowerVec>(*v)) {
-    auto& tv = absl::get<ThrowerVec>(*v);
-    tv.clear();
-    tv.emplace_back(-100);
-    if (tv.size() != 1 || tv[0].Get() != -100) {
-      return AssertionFailure() << "ThrowerVec should be {Thrower{-100}}";
-    }
-  } else if (absl::holds_alternative<CopyNothrow>(*v)) {
-    auto& t = absl::get<CopyNothrow>(*v);
-    t = CopyNothrow{-100};
-    if (t.Get() != -100) {
-      return AssertionFailure() << "CopyNothrow should be assigned -100";
-    }
-  } else if (absl::holds_alternative<MoveNothrow>(*v)) {
-    auto& t = absl::get<MoveNothrow>(*v);
-    t = MoveNothrow{-100};
-    if (t.Get() != -100) {
-      return AssertionFailure() << "MoveNothrow should be assigned -100";
-    }
-  }
-
-  // Try making variant valueless_by_exception
-  if (!v->valueless_by_exception()) ToValuelessByException(*v);
-  if (!v->valueless_by_exception()) {
-    return AssertionFailure() << "Variant should be valueless_by_exception";
-  }
-  try {
-    auto unused = absl::get<Thrower>(*v);
-    static_cast<void>(unused);
-    return AssertionFailure() << "Variant should not contain Thrower";
-  } catch (const absl::bad_variant_access&) {
-  } catch (...) {
-    return AssertionFailure() << "Unexpected exception throw from absl::get";
-  }
-
-  // Try using the variant
-  v->emplace<Thrower>(100);
-  if (!absl::holds_alternative<Thrower>(*v) ||
-      absl::get<Thrower>(*v) != Thrower(100)) {
-    return AssertionFailure() << "Variant should contain Thrower(100)";
-  }
-  v->emplace<ThrowerVec>({Thrower(100)});
-  if (!absl::holds_alternative<ThrowerVec>(*v) ||
-      absl::get<ThrowerVec>(*v)[0] != Thrower(100)) {
-    return AssertionFailure()
-           << "Variant should contain ThrowerVec{Thrower(100)}";
-  }
-  return AssertionSuccess();
-}
-
-template <typename... Args>
-Thrower ExpectedThrower(Args&&... args) {
-  return Thrower(42, args...);
-}
-
-ThrowerVec ExpectedThrowerVec() { return {Thrower(100), Thrower(200)}; }
-ThrowingVariant ValuelessByException() {
-  ThrowingVariant v;
-  ToValuelessByException(v);
-  return v;
-}
-ThrowingVariant WithThrower() { return Thrower(39); }
-ThrowingVariant WithThrowerVec() {
-  return ThrowerVec{Thrower(1), Thrower(2), Thrower(3)};
-}
-ThrowingVariant WithCopyNoThrow() { return CopyNothrow(39); }
-ThrowingVariant WithMoveNoThrow() { return MoveNothrow(39); }
-
-TEST(VariantExceptionSafetyTest, DefaultConstructor) {
-  TestThrowingCtor<ThrowingVariant>();
-}
-
-TEST(VariantExceptionSafetyTest, CopyConstructor) {
-  {
-    ThrowingVariant v(ExpectedThrower());
-    TestThrowingCtor<ThrowingVariant>(v);
-  }
-  {
-    ThrowingVariant v(ExpectedThrowerVec());
-    TestThrowingCtor<ThrowingVariant>(v);
-  }
-  {
-    ThrowingVariant v(ValuelessByException());
-    TestThrowingCtor<ThrowingVariant>(v);
-  }
-}
-
-TEST(VariantExceptionSafetyTest, MoveConstructor) {
-  {
-    ThrowingVariant v(ExpectedThrower());
-    TestThrowingCtor<ThrowingVariant>(std::move(v));
-  }
-  {
-    ThrowingVariant v(ExpectedThrowerVec());
-    TestThrowingCtor<ThrowingVariant>(std::move(v));
-  }
-  {
-    ThrowingVariant v(ValuelessByException());
-    TestThrowingCtor<ThrowingVariant>(std::move(v));
-  }
-}
-
-TEST(VariantExceptionSafetyTest, ValueConstructor) {
-  TestThrowingCtor<ThrowingVariant>(ExpectedThrower());
-  TestThrowingCtor<ThrowingVariant>(ExpectedThrowerVec());
-}
-
-TEST(VariantExceptionSafetyTest, InPlaceTypeConstructor) {
-  TestThrowingCtor<ThrowingVariant>(absl::in_place_type_t<Thrower>{},
-                                    ExpectedThrower());
-  TestThrowingCtor<ThrowingVariant>(absl::in_place_type_t<ThrowerVec>{},
-                                    ExpectedThrowerVec());
-}
-
-TEST(VariantExceptionSafetyTest, InPlaceIndexConstructor) {
-  TestThrowingCtor<ThrowingVariant>(absl::in_place_index_t<0>{},
-                                    ExpectedThrower());
-  TestThrowingCtor<ThrowingVariant>(absl::in_place_index_t<3>{},
-                                    ExpectedThrowerVec());
-}
-
-TEST(VariantExceptionSafetyTest, CopyAssign) {
-  // variant& operator=(const variant& rhs);
-  // Let j be rhs.index()
-  {
-    // - neither *this nor rhs holds a value
-    const ThrowingVariant rhs = ValuelessByException();
-    ThrowingVariant lhs = ValuelessByException();
-    EXPECT_TRUE(TestNothrowOp([&]() { lhs = rhs; }));
-  }
-  {
-    // - *this holds a value but rhs does not
-    const ThrowingVariant rhs = ValuelessByException();
-    ThrowingVariant lhs = WithThrower();
-    EXPECT_TRUE(TestNothrowOp([&]() { lhs = rhs; }));
-  }
-  // - index() == j
-  {
-    const ThrowingVariant rhs(ExpectedThrower());
-    auto tester =
-        MakeExceptionSafetyTester()
-            .WithInitialValue(WithThrower())
-            .WithOperation([&rhs](ThrowingVariant* lhs) { *lhs = rhs; });
-    EXPECT_TRUE(tester.WithContracts(VariantInvariants).Test());
-    EXPECT_FALSE(tester.WithContracts(strong_guarantee).Test());
-  }
-  {
-    const ThrowingVariant rhs(ExpectedThrowerVec());
-    auto tester =
-        MakeExceptionSafetyTester()
-            .WithInitialValue(WithThrowerVec())
-            .WithOperation([&rhs](ThrowingVariant* lhs) { *lhs = rhs; });
-    EXPECT_TRUE(tester.WithContracts(VariantInvariants).Test());
-    EXPECT_FALSE(tester.WithContracts(strong_guarantee).Test());
-  }
-  // libstdc++ std::variant has bugs on copy assignment regarding exception
-  // safety.
-#if !(defined(ABSL_HAVE_STD_VARIANT) && defined(__GLIBCXX__))
-  // index() != j
-  // if is_nothrow_copy_constructible_v<Tj> or
-  // !is_nothrow_move_constructible<Tj> is true, equivalent to
-  // emplace<j>(get<j>(rhs))
-  {
-    // is_nothrow_copy_constructible_v<Tj> == true
-    // should not throw because emplace() invokes Tj's copy ctor
-    // which should not throw.
-    const ThrowingVariant rhs(CopyNothrow{});
-    ThrowingVariant lhs = WithThrower();
-    EXPECT_TRUE(TestNothrowOp([&]() { lhs = rhs; }));
-  }
-  {
-    // is_nothrow_copy_constructible<Tj> == false &&
-    // is_nothrow_move_constructible<Tj> == false
-    // should provide basic guarantee because emplace() invokes Tj's copy ctor
-    // which may throw.
-    const ThrowingVariant rhs(ExpectedThrower());
-    auto tester =
-        MakeExceptionSafetyTester()
-            .WithInitialValue(WithCopyNoThrow())
-            .WithOperation([&rhs](ThrowingVariant* lhs) { *lhs = rhs; });
-    EXPECT_TRUE(tester
-                    .WithContracts(VariantInvariants,
-                                   [](ThrowingVariant* lhs) {
-                                     return lhs->valueless_by_exception();
-                                   })
-                    .Test());
-    EXPECT_FALSE(tester.WithContracts(strong_guarantee).Test());
-  }
-#endif  // !(defined(ABSL_HAVE_STD_VARIANT) && defined(__GLIBCXX__))
-  {
-    // is_nothrow_copy_constructible_v<Tj> == false &&
-    // is_nothrow_move_constructible_v<Tj> == true
-    // should provide strong guarantee because it is equivalent to
-    // operator=(variant(rhs)) which creates a temporary then invoke the move
-    // ctor which shouldn't throw.
-    const ThrowingVariant rhs(MoveNothrow{});
-    EXPECT_TRUE(MakeExceptionSafetyTester()
-                    .WithInitialValue(WithThrower())
-                    .WithContracts(VariantInvariants, strong_guarantee)
-                    .Test([&rhs](ThrowingVariant* lhs) { *lhs = rhs; }));
-  }
-}
-
-TEST(VariantExceptionSafetyTest, MoveAssign) {
-  // variant& operator=(variant&& rhs);
-  // Let j be rhs.index()
-  {
-    // - neither *this nor rhs holds a value
-    ThrowingVariant rhs = ValuelessByException();
-    ThrowingVariant lhs = ValuelessByException();
-    EXPECT_TRUE(TestNothrowOp([&]() { lhs = std::move(rhs); }));
-  }
-  {
-    // - *this holds a value but rhs does not
-    ThrowingVariant rhs = ValuelessByException();
-    ThrowingVariant lhs = WithThrower();
-    EXPECT_TRUE(TestNothrowOp([&]() { lhs = std::move(rhs); }));
-  }
-  {
-    // - index() == j
-    // assign get<j>(std::move(rhs)) to the value contained in *this.
-    // If an exception is thrown during call to Tj's move assignment, the state
-    // of the contained value is as defined by the exception safety guarantee of
-    // Tj's move assignment; index() will be j.
-    ThrowingVariant rhs(ExpectedThrower());
-    size_t j = rhs.index();
-    // Since Thrower's move assignment has basic guarantee, so should variant's.
-    auto tester = MakeExceptionSafetyTester()
-                      .WithInitialValue(WithThrower())
-                      .WithOperation([&](ThrowingVariant* lhs) {
-                        auto copy = rhs;
-                        *lhs = std::move(copy);
-                      });
-    EXPECT_TRUE(tester
-                    .WithContracts(
-                        VariantInvariants,
-                        [&](ThrowingVariant* lhs) { return lhs->index() == j; })
-                    .Test());
-    EXPECT_FALSE(tester.WithContracts(strong_guarantee).Test());
-  }
-  {
-    // libstdc++ introduced a regression between 2018-09-25 and 2019-01-06.
-    // The fix is targeted for gcc-9.
-    // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87431#c7
-    // https://gcc.gnu.org/viewcvs/gcc?view=revision&revision=267614
-#if !(defined(ABSL_HAVE_STD_VARIANT) && \
-      defined(_GLIBCXX_RELEASE) && _GLIBCXX_RELEASE == 8)
-    // - otherwise (index() != j), equivalent to
-    // emplace<j>(get<j>(std::move(rhs)))
-    // - If an exception is thrown during the call to Tj's move construction
-    // (with j being rhs.index()), the variant will hold no value.
-    ThrowingVariant rhs(CopyNothrow{});
-    EXPECT_TRUE(MakeExceptionSafetyTester()
-                    .WithInitialValue(WithThrower())
-                    .WithContracts(VariantInvariants,
-                                   [](ThrowingVariant* lhs) {
-                                     return lhs->valueless_by_exception();
-                                   })
-                    .Test([&](ThrowingVariant* lhs) {
-                      auto copy = rhs;
-                      *lhs = std::move(copy);
-                    }));
-#endif  // !(defined(ABSL_HAVE_STD_VARIANT) &&
-        //   defined(_GLIBCXX_RELEASE) && _GLIBCXX_RELEASE == 8)
-  }
-}
-
-TEST(VariantExceptionSafetyTest, ValueAssign) {
-  // template<class T> variant& operator=(T&& t);
-  // Let Tj be the type that is selected by overload resolution to be assigned.
-  {
-    // If *this holds a Tj, assigns std::forward<T>(t) to the value contained in
-    // *this. If  an exception is thrown during the assignment of
-    // std::forward<T>(t) to the value contained in *this, the state of the
-    // contained value and t are as defined by the exception safety guarantee of
-    // the assignment expression; valueless_by_exception() will be false.
-    // Since Thrower's copy/move assignment has basic guarantee, so should
-    // variant's.
-    Thrower rhs = ExpectedThrower();
-    // copy assign
-    auto copy_tester =
-        MakeExceptionSafetyTester()
-            .WithInitialValue(WithThrower())
-            .WithOperation([rhs](ThrowingVariant* lhs) { *lhs = rhs; });
-    EXPECT_TRUE(copy_tester
-                    .WithContracts(VariantInvariants,
-                                   [](ThrowingVariant* lhs) {
-                                     return !lhs->valueless_by_exception();
-                                   })
-                    .Test());
-    EXPECT_FALSE(copy_tester.WithContracts(strong_guarantee).Test());
-    // move assign
-    auto move_tester = MakeExceptionSafetyTester()
-                           .WithInitialValue(WithThrower())
-                           .WithOperation([&](ThrowingVariant* lhs) {
-                             auto copy = rhs;
-                             *lhs = std::move(copy);
-                           });
-    EXPECT_TRUE(move_tester
-                    .WithContracts(VariantInvariants,
-                                   [](ThrowingVariant* lhs) {
-                                     return !lhs->valueless_by_exception();
-                                   })
-                    .Test());
-
-    EXPECT_FALSE(move_tester.WithContracts(strong_guarantee).Test());
-  }
-  // Otherwise (*this holds something else), if is_nothrow_constructible_v<Tj,
-  // T> || !is_nothrow_move_constructible_v<Tj> is true, equivalent to
-  // emplace<j>(std::forward<T>(t)).
-  // We simplify the test by letting T = `const Tj&`  or `Tj&&`, so we can reuse
-  // the CopyNothrow and MoveNothrow types.
-
-  // if is_nothrow_constructible_v<Tj, T>
-  // (i.e. is_nothrow_copy/move_constructible_v<Tj>) is true, emplace() just
-  // invokes the copy/move constructor and it should not throw.
-  {
-    const CopyNothrow rhs;
-    ThrowingVariant lhs = WithThrower();
-    EXPECT_TRUE(TestNothrowOp([&]() { lhs = rhs; }));
-  }
-  {
-    MoveNothrow rhs;
-    ThrowingVariant lhs = WithThrower();
-    EXPECT_TRUE(TestNothrowOp([&]() { lhs = std::move(rhs); }));
-  }
-  // if is_nothrow_constructible_v<Tj, T> == false &&
-  // is_nothrow_move_constructible<Tj> == false
-  // emplace() invokes the copy/move constructor which may throw so it should
-  // provide basic guarantee and variant object might not hold a value.
-  {
-    Thrower rhs = ExpectedThrower();
-    // copy
-    auto copy_tester =
-        MakeExceptionSafetyTester()
-            .WithInitialValue(WithCopyNoThrow())
-            .WithOperation([&rhs](ThrowingVariant* lhs) { *lhs = rhs; });
-    EXPECT_TRUE(copy_tester
-                    .WithContracts(VariantInvariants,
-                                   [](ThrowingVariant* lhs) {
-                                     return lhs->valueless_by_exception();
-                                   })
-                    .Test());
-    EXPECT_FALSE(copy_tester.WithContracts(strong_guarantee).Test());
-    // move
-    auto move_tester = MakeExceptionSafetyTester()
-                           .WithInitialValue(WithCopyNoThrow())
-                           .WithOperation([](ThrowingVariant* lhs) {
-                             *lhs = ExpectedThrower(testing::nothrow_ctor);
-                           });
-    EXPECT_TRUE(move_tester
-                    .WithContracts(VariantInvariants,
-                                   [](ThrowingVariant* lhs) {
-                                     return lhs->valueless_by_exception();
-                                   })
-                    .Test());
-    EXPECT_FALSE(move_tester.WithContracts(strong_guarantee).Test());
-  }
-  // Otherwise (if is_nothrow_constructible_v<Tj, T> == false &&
-  // is_nothrow_move_constructible<Tj> == true),
-  // equivalent to operator=(variant(std::forward<T>(t)))
-  // This should have strong guarantee because it creates a temporary variant
-  // and operator=(variant&&) invokes Tj's move ctor which doesn't throw.
-  // libstdc++ std::variant has bugs on conversion assignment regarding
-  // exception safety.
-#if !(defined(ABSL_HAVE_STD_VARIANT) && defined(__GLIBCXX__))
-  {
-    MoveNothrow rhs;
-    EXPECT_TRUE(MakeExceptionSafetyTester()
-                    .WithInitialValue(WithThrower())
-                    .WithContracts(VariantInvariants, strong_guarantee)
-                    .Test([&rhs](ThrowingVariant* lhs) { *lhs = rhs; }));
-  }
-#endif  // !(defined(ABSL_HAVE_STD_VARIANT) && defined(__GLIBCXX__))
-}
-
-TEST(VariantExceptionSafetyTest, Emplace) {
-  // If an exception during the initialization of the contained value, the
-  // variant might not hold a value. The standard requires emplace() to provide
-  // only basic guarantee.
-  {
-    Thrower args = ExpectedThrower();
-    auto tester = MakeExceptionSafetyTester()
-                      .WithInitialValue(WithThrower())
-                      .WithOperation([&args](ThrowingVariant* v) {
-                        v->emplace<Thrower>(args);
-                      });
-    EXPECT_TRUE(tester
-                    .WithContracts(VariantInvariants,
-                                   [](ThrowingVariant* v) {
-                                     return v->valueless_by_exception();
-                                   })
-                    .Test());
-    EXPECT_FALSE(tester.WithContracts(strong_guarantee).Test());
-  }
-}
-
-TEST(VariantExceptionSafetyTest, Swap) {
-  // if both are valueless_by_exception(), no effect
-  {
-    ThrowingVariant rhs = ValuelessByException();
-    ThrowingVariant lhs = ValuelessByException();
-    EXPECT_TRUE(TestNothrowOp([&]() { lhs.swap(rhs); }));
-  }
-  // if index() == rhs.index(), calls swap(get<i>(*this), get<i>(rhs))
-  // where i is index().
-  {
-    ThrowingVariant rhs = ExpectedThrower();
-    EXPECT_TRUE(MakeExceptionSafetyTester()
-                    .WithInitialValue(WithThrower())
-                    .WithContracts(VariantInvariants)
-                    .Test([&](ThrowingVariant* lhs) {
-                      auto copy = rhs;
-                      lhs->swap(copy);
-                    }));
-  }
-  // Otherwise, exchanges the value of rhs and *this. The exception safety
-  // involves variant in moved-from state which is not specified in the
-  // standard, and since swap is 3-step it's impossible for it to provide a
-  // overall strong guarantee. So, we are only checking basic guarantee here.
-  {
-    ThrowingVariant rhs = ExpectedThrower();
-    EXPECT_TRUE(MakeExceptionSafetyTester()
-                    .WithInitialValue(WithCopyNoThrow())
-                    .WithContracts(VariantInvariants)
-                    .Test([&](ThrowingVariant* lhs) {
-                      auto copy = rhs;
-                      lhs->swap(copy);
-                    }));
-  }
-  {
-    ThrowingVariant rhs = ExpectedThrower();
-    EXPECT_TRUE(MakeExceptionSafetyTester()
-                    .WithInitialValue(WithCopyNoThrow())
-                    .WithContracts(VariantInvariants)
-                    .Test([&](ThrowingVariant* lhs) {
-                      auto copy = rhs;
-                      copy.swap(*lhs);
-                    }));
-  }
-}
-
-}  // namespace
-}  // namespace absl
-
-#endif  // !defined(ABSL_INTERNAL_MSVC_2017_DBG_MODE)
-
-#endif  // #if !defined(ABSL_HAVE_STD_VARIANT) && defined(ABSL_HAVE_EXCEPTIONS)
diff --git a/absl/types/variant_test.cc b/absl/types/variant_test.cc
index b5912745714..fafae8945a3 100644
--- a/absl/types/variant_test.cc
+++ b/absl/types/variant_test.cc
@@ -12,62 +12,14 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-// Unit tests for the variant template. The 'is' and 'IsEmpty' methods
-// of variant are not explicitly tested because they are used repeatedly
-// in building other tests. All other public variant methods should have
-// explicit tests.
-
 #include "absl/types/variant.h"
 
-// This test is a no-op when absl::variant is an alias for std::variant.
-#if !defined(ABSL_HAVE_STD_VARIANT)
-
-#include <algorithm>
-#include <cstddef>
-#include <functional>
-#include <initializer_list>
 #include <memory>
-#include <ostream>
-#include <queue>
-#include <type_traits>
-#include <unordered_set>
-#include <utility>
-#include <vector>
+#include <string>
+#include <variant>
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
-#include "absl/base/config.h"
-#include "absl/base/port.h"
-#include "absl/memory/memory.h"
-#include "absl/meta/type_traits.h"
-#include "absl/strings/string_view.h"
-
-#ifdef ABSL_HAVE_EXCEPTIONS
-
-#define ABSL_VARIANT_TEST_EXPECT_FAIL(expr, exception_t, text) \
-  EXPECT_THROW(expr, exception_t)
-
-#else
-
-#define ABSL_VARIANT_TEST_EXPECT_FAIL(expr, exception_t, text) \
-  EXPECT_DEATH(expr, text)
-
-#endif  // ABSL_HAVE_EXCEPTIONS
-
-#define ABSL_VARIANT_TEST_EXPECT_BAD_VARIANT_ACCESS(...)                 \
-  ABSL_VARIANT_TEST_EXPECT_FAIL((void)(__VA_ARGS__), absl::bad_variant_access, \
-                                "Bad variant access")
-
-struct Hashable {};
-
-namespace std {
-template <>
-struct hash<Hashable> {
-  size_t operator()(const Hashable&);
-};
-}  // namespace std
-
-struct NonHashable {};
 
 namespace absl {
 namespace {
@@ -76,2190 +28,6 @@ using ::testing::DoubleEq;
 using ::testing::Pointee;
 using ::testing::VariantWith;
 
-struct MoveCanThrow {
-  MoveCanThrow() : v(0) {}
-  MoveCanThrow(int v) : v(v) {}  // NOLINT(runtime/explicit)
-  MoveCanThrow(const MoveCanThrow& other) : v(other.v) {}
-  MoveCanThrow& operator=(const MoveCanThrow& /*other*/) { return *this; }
-  int v;
-};
-
-bool operator==(MoveCanThrow lhs, MoveCanThrow rhs) { return lhs.v == rhs.v; }
-bool operator!=(MoveCanThrow lhs, MoveCanThrow rhs) { return lhs.v != rhs.v; }
-bool operator<(MoveCanThrow lhs, MoveCanThrow rhs) { return lhs.v < rhs.v; }
-bool operator<=(MoveCanThrow lhs, MoveCanThrow rhs) { return lhs.v <= rhs.v; }
-bool operator>=(MoveCanThrow lhs, MoveCanThrow rhs) { return lhs.v >= rhs.v; }
-bool operator>(MoveCanThrow lhs, MoveCanThrow rhs) { return lhs.v > rhs.v; }
-
-// This helper class allows us to determine if it was swapped with std::swap()
-// or with its friend swap() function.
-struct SpecialSwap {
-  explicit SpecialSwap(int i) : i(i) {}
-  friend void swap(SpecialSwap& a, SpecialSwap& b) {
-    a.special_swap = b.special_swap = true;
-    std::swap(a.i, b.i);
-  }
-  bool operator==(SpecialSwap other) const { return i == other.i; }
-  int i;
-  bool special_swap = false;
-};
-
-struct MoveOnlyWithListConstructor {
-  MoveOnlyWithListConstructor() = default;
-  explicit MoveOnlyWithListConstructor(std::initializer_list<int> /*ilist*/,
-                                       int value)
-      : value(value) {}
-  MoveOnlyWithListConstructor(MoveOnlyWithListConstructor&&) = default;
-  MoveOnlyWithListConstructor& operator=(MoveOnlyWithListConstructor&&) =
-      default;
-
-  int value = 0;
-};
-
-#ifdef ABSL_HAVE_EXCEPTIONS
-
-struct ConversionException {};
-
-template <class T>
-struct ExceptionOnConversion {
-  operator T() const {  // NOLINT(runtime/explicit)
-    throw ConversionException();
-  }
-};
-
-// Forces a variant into the valueless by exception state.
-template <class H, class... T>
-void ToValuelessByException(absl::variant<H, T...>& v) {  // NOLINT
-  try {
-    v.template emplace<0>(ExceptionOnConversion<H>());
-  } catch (ConversionException& /*e*/) {
-    // This space intentionally left blank.
-  }
-}
-
-#endif  // ABSL_HAVE_EXCEPTIONS
-
-// An indexed sequence of distinct structures holding a single
-// value of type T
-template<typename T, size_t N>
-struct ValueHolder {
-  explicit ValueHolder(const T& x) : value(x) {}
-  typedef T value_type;
-  value_type value;
-  static const size_t kIndex = N;
-};
-template<typename T, size_t N>
-const size_t ValueHolder<T, N>::kIndex;
-
-// The following three functions make ValueHolder compatible with
-// EXPECT_EQ and EXPECT_NE
-template<typename T, size_t N>
-inline bool operator==(const ValueHolder<T, N>& left,
-                       const ValueHolder<T, N>& right) {
-  return left.value == right.value;
-}
-
-template<typename T, size_t N>
-inline bool operator!=(const ValueHolder<T, N>& left,
-                       const ValueHolder<T, N>& right) {
-  return left.value != right.value;
-}
-
-template<typename T, size_t N>
-inline std::ostream& operator<<(
-    std::ostream& stream, const ValueHolder<T, N>& object) {
-  return stream << object.value;
-}
-
-// Makes a variant holding twelve uniquely typed T wrappers.
-template<typename T>
-struct VariantFactory {
-  typedef variant<ValueHolder<T, 1>, ValueHolder<T, 2>, ValueHolder<T, 3>,
-                  ValueHolder<T, 4>>
-      Type;
-};
-
-// A typelist in 1:1 with VariantFactory, to use type driven unit tests.
-typedef ::testing::Types<ValueHolder<size_t, 1>, ValueHolder<size_t, 2>,
-                         ValueHolder<size_t, 3>,
-                         ValueHolder<size_t, 4>> VariantTypes;
-
-// Increments the provided counter pointer in the destructor
-struct IncrementInDtor {
-  explicit IncrementInDtor(int* counter) : counter(counter) {}
-  ~IncrementInDtor() { *counter += 1; }
-  int* counter;
-};
-
-struct IncrementInDtorCopyCanThrow {
-  explicit IncrementInDtorCopyCanThrow(int* counter) : counter(counter) {}
-  IncrementInDtorCopyCanThrow(IncrementInDtorCopyCanThrow&& other) noexcept =
-      default;
-  IncrementInDtorCopyCanThrow(const IncrementInDtorCopyCanThrow& other)
-      : counter(other.counter) {}
-  IncrementInDtorCopyCanThrow& operator=(
-      IncrementInDtorCopyCanThrow&&) noexcept = default;
-  IncrementInDtorCopyCanThrow& operator=(
-      IncrementInDtorCopyCanThrow const& other) {
-    counter = other.counter;
-    return *this;
-  }
-  ~IncrementInDtorCopyCanThrow() { *counter += 1; }
-  int* counter;
-};
-
-// This is defined so operator== for ValueHolder<IncrementInDtor> will
-// return true if two IncrementInDtor objects increment the same
-// counter
-inline bool operator==(const IncrementInDtor& left,
-                       const IncrementInDtor& right) {
-  return left.counter == right.counter;
-}
-
-// This is defined so EXPECT_EQ can work with IncrementInDtor
-inline std::ostream& operator<<(
-    std::ostream& stream, const IncrementInDtor& object) {
-  return stream << object.counter;
-}
-
-// A class that can be copied, but not assigned.
-class CopyNoAssign {
- public:
-  explicit CopyNoAssign(int value) : foo(value) {}
-  CopyNoAssign(const CopyNoAssign& other) : foo(other.foo) {}
-  int foo;
- private:
-  const CopyNoAssign& operator=(const CopyNoAssign&);
-};
-
-// A class that can neither be copied nor assigned. We provide
-// overloads for the constructor with up to four parameters so we can
-// test the overloads of variant::emplace.
-class NonCopyable {
- public:
-  NonCopyable()
-      : value(0) {}
-  explicit NonCopyable(int value1)
-      : value(value1) {}
-
-  NonCopyable(int value1, int value2)
-      : value(value1 + value2) {}
-
-  NonCopyable(int value1, int value2, int value3)
-      : value(value1 + value2 + value3) {}
-
-  NonCopyable(int value1, int value2, int value3, int value4)
-      : value(value1 + value2 + value3 + value4) {}
-  NonCopyable(const NonCopyable&) = delete;
-  NonCopyable& operator=(const NonCopyable&) = delete;
-  int value;
-};
-
-// A typed test and typed test case over the VariantTypes typelist,
-// from which we derive a number of tests that will execute for one of
-// each type.
-template <typename T>
-class VariantTypesTest : public ::testing::Test {};
-TYPED_TEST_SUITE(VariantTypesTest, VariantTypes);
-
-////////////////////
-// [variant.ctor] //
-////////////////////
-
-struct NonNoexceptDefaultConstructible {
-  NonNoexceptDefaultConstructible() {}
-  int value = 5;
-};
-
-struct NonDefaultConstructible {
-  NonDefaultConstructible() = delete;
-};
-
-TEST(VariantTest, TestDefaultConstructor) {
-  {
-    using X = variant<int>;
-    constexpr variant<int> x{};
-    ASSERT_FALSE(x.valueless_by_exception());
-    ASSERT_EQ(0, x.index());
-    EXPECT_EQ(0, absl::get<0>(x));
-    EXPECT_TRUE(std::is_nothrow_default_constructible<X>::value);
-  }
-
-  {
-    using X = variant<NonNoexceptDefaultConstructible>;
-    X x{};
-    ASSERT_FALSE(x.valueless_by_exception());
-    ASSERT_EQ(0, x.index());
-    EXPECT_EQ(5, absl::get<0>(x).value);
-    EXPECT_FALSE(std::is_nothrow_default_constructible<X>::value);
-  }
-
-  {
-    using X = variant<int, NonNoexceptDefaultConstructible>;
-    X x{};
-    ASSERT_FALSE(x.valueless_by_exception());
-    ASSERT_EQ(0, x.index());
-    EXPECT_EQ(0, absl::get<0>(x));
-    EXPECT_TRUE(std::is_nothrow_default_constructible<X>::value);
-  }
-
-  {
-    using X = variant<NonNoexceptDefaultConstructible, int>;
-    X x{};
-    ASSERT_FALSE(x.valueless_by_exception());
-    ASSERT_EQ(0, x.index());
-    EXPECT_EQ(5, absl::get<0>(x).value);
-    EXPECT_FALSE(std::is_nothrow_default_constructible<X>::value);
-  }
-  EXPECT_FALSE(
-      std::is_default_constructible<variant<NonDefaultConstructible>>::value);
-  EXPECT_FALSE((std::is_default_constructible<
-                variant<NonDefaultConstructible, int>>::value));
-  EXPECT_TRUE((std::is_default_constructible<
-               variant<int, NonDefaultConstructible>>::value));
-}
-
-// Test that for each slot, copy constructing a variant with that type
-// produces a sensible object that correctly reports its type, and
-// that copies the provided value.
-TYPED_TEST(VariantTypesTest, TestCopyCtor) {
-  typedef typename VariantFactory<typename TypeParam::value_type>::Type Variant;
-  using value_type1 = absl::variant_alternative_t<0, Variant>;
-  using value_type2 = absl::variant_alternative_t<1, Variant>;
-  using value_type3 = absl::variant_alternative_t<2, Variant>;
-  using value_type4 = absl::variant_alternative_t<3, Variant>;
-  const TypeParam value(TypeParam::kIndex);
-  Variant original(value);
-  Variant copied(original);
-  EXPECT_TRUE(absl::holds_alternative<value_type1>(copied) ||
-              TypeParam::kIndex != 1);
-  EXPECT_TRUE(absl::holds_alternative<value_type2>(copied) ||
-              TypeParam::kIndex != 2);
-  EXPECT_TRUE(absl::holds_alternative<value_type3>(copied) ||
-              TypeParam::kIndex != 3);
-  EXPECT_TRUE(absl::holds_alternative<value_type4>(copied) ||
-              TypeParam::kIndex != 4);
-  EXPECT_TRUE((absl::get_if<value_type1>(&original) ==
-               absl::get_if<value_type1>(&copied)) ||
-              TypeParam::kIndex == 1);
-  EXPECT_TRUE((absl::get_if<value_type2>(&original) ==
-               absl::get_if<value_type2>(&copied)) ||
-              TypeParam::kIndex == 2);
-  EXPECT_TRUE((absl::get_if<value_type3>(&original) ==
-               absl::get_if<value_type3>(&copied)) ||
-              TypeParam::kIndex == 3);
-  EXPECT_TRUE((absl::get_if<value_type4>(&original) ==
-               absl::get_if<value_type4>(&copied)) ||
-              TypeParam::kIndex == 4);
-  EXPECT_TRUE((absl::get_if<value_type1>(&original) ==
-               absl::get_if<value_type1>(&copied)) ||
-              TypeParam::kIndex == 1);
-  EXPECT_TRUE((absl::get_if<value_type2>(&original) ==
-               absl::get_if<value_type2>(&copied)) ||
-              TypeParam::kIndex == 2);
-  EXPECT_TRUE((absl::get_if<value_type3>(&original) ==
-               absl::get_if<value_type3>(&copied)) ||
-              TypeParam::kIndex == 3);
-  EXPECT_TRUE((absl::get_if<value_type4>(&original) ==
-               absl::get_if<value_type4>(&copied)) ||
-              TypeParam::kIndex == 4);
-  const TypeParam* ovalptr = absl::get_if<TypeParam>(&original);
-  const TypeParam* cvalptr = absl::get_if<TypeParam>(&copied);
-  ASSERT_TRUE(ovalptr != nullptr);
-  ASSERT_TRUE(cvalptr != nullptr);
-  EXPECT_EQ(*ovalptr, *cvalptr);
-  TypeParam* mutable_ovalptr = absl::get_if<TypeParam>(&original);
-  TypeParam* mutable_cvalptr = absl::get_if<TypeParam>(&copied);
-  ASSERT_TRUE(mutable_ovalptr != nullptr);
-  ASSERT_TRUE(mutable_cvalptr != nullptr);
-  EXPECT_EQ(*mutable_ovalptr, *mutable_cvalptr);
-}
-
-template <class>
-struct MoveOnly {
-  MoveOnly() = default;
-  explicit MoveOnly(int value) : value(value) {}
-  MoveOnly(MoveOnly&&) = default;
-  MoveOnly& operator=(MoveOnly&&) = default;
-  int value = 5;
-};
-
-TEST(VariantTest, TestMoveConstruct) {
-  using V = variant<MoveOnly<class A>, MoveOnly<class B>, MoveOnly<class C>>;
-
-  V v(in_place_index<1>, 10);
-  V v2 = absl::move(v);
-  EXPECT_EQ(10, absl::get<1>(v2).value);
-}
-
-// Used internally to emulate missing triviality traits for tests.
-template <class T>
-union SingleUnion {
-  T member;
-};
-
-// NOTE: These don't work with types that can't be union members.
-//       They are just for testing.
-template <class T>
-struct is_trivially_move_constructible
-    : std::is_move_constructible<SingleUnion<T>>::type {};
-
-template <class T>
-struct is_trivially_move_assignable
-    : absl::is_move_assignable<SingleUnion<T>>::type {};
-
-TEST(VariantTest, NothrowMoveConstructible) {
-  // Verify that variant is nothrow move constructible iff its template
-  // arguments are.
-  using U = std::unique_ptr<int>;
-  struct E {
-    E(E&&) {}
-  };
-  static_assert(std::is_nothrow_move_constructible<variant<U>>::value, "");
-  static_assert(std::is_nothrow_move_constructible<variant<U, int>>::value, "");
-  static_assert(!std::is_nothrow_move_constructible<variant<U, E>>::value, "");
-}
-
-// Test that for each slot, constructing a variant with that type
-// produces a sensible object that correctly reports its type, and
-// that copies the provided value.
-TYPED_TEST(VariantTypesTest, TestValueCtor) {
-  typedef typename VariantFactory<typename TypeParam::value_type>::Type Variant;
-  using value_type1 = absl::variant_alternative_t<0, Variant>;
-  using value_type2 = absl::variant_alternative_t<1, Variant>;
-  using value_type3 = absl::variant_alternative_t<2, Variant>;
-  using value_type4 = absl::variant_alternative_t<3, Variant>;
-  const TypeParam value(TypeParam::kIndex);
-  Variant v(value);
-  EXPECT_TRUE(absl::holds_alternative<value_type1>(v) ||
-              TypeParam::kIndex != 1);
-  EXPECT_TRUE(absl::holds_alternative<value_type2>(v) ||
-              TypeParam::kIndex != 2);
-  EXPECT_TRUE(absl::holds_alternative<value_type3>(v) ||
-              TypeParam::kIndex != 3);
-  EXPECT_TRUE(absl::holds_alternative<value_type4>(v) ||
-              TypeParam::kIndex != 4);
-  EXPECT_TRUE(nullptr != absl::get_if<value_type1>(&v) ||
-              TypeParam::kIndex != 1);
-  EXPECT_TRUE(nullptr != absl::get_if<value_type2>(&v) ||
-              TypeParam::kIndex != 2);
-  EXPECT_TRUE(nullptr != absl::get_if<value_type3>(&v) ||
-              TypeParam::kIndex != 3);
-  EXPECT_TRUE(nullptr != absl::get_if<value_type4>(&v) ||
-              TypeParam::kIndex != 4);
-  EXPECT_TRUE(nullptr != absl::get_if<value_type1>(&v) ||
-              TypeParam::kIndex != 1);
-  EXPECT_TRUE(nullptr != absl::get_if<value_type2>(&v) ||
-              TypeParam::kIndex != 2);
-  EXPECT_TRUE(nullptr != absl::get_if<value_type3>(&v) ||
-              TypeParam::kIndex != 3);
-  EXPECT_TRUE(nullptr != absl::get_if<value_type4>(&v) ||
-              TypeParam::kIndex != 4);
-  const TypeParam* valptr = absl::get_if<TypeParam>(&v);
-  ASSERT_TRUE(nullptr != valptr);
-  EXPECT_EQ(value.value, valptr->value);
-  const TypeParam* mutable_valptr = absl::get_if<TypeParam>(&v);
-  ASSERT_TRUE(nullptr != mutable_valptr);
-  EXPECT_EQ(value.value, mutable_valptr->value);
-}
-
-TEST(VariantTest, AmbiguousValueConstructor) {
-  EXPECT_FALSE((std::is_convertible<int, absl::variant<int, int>>::value));
-  EXPECT_FALSE((std::is_constructible<absl::variant<int, int>, int>::value));
-}
-
-TEST(VariantTest, InPlaceType) {
-  using Var = variant<int, std::string, NonCopyable, std::vector<int>>;
-
-  Var v1(in_place_type_t<int>(), 7);
-  ASSERT_TRUE(absl::holds_alternative<int>(v1));
-  EXPECT_EQ(7, absl::get<int>(v1));
-
-  Var v2(in_place_type_t<std::string>(), "ABC");
-  ASSERT_TRUE(absl::holds_alternative<std::string>(v2));
-  EXPECT_EQ("ABC", absl::get<std::string>(v2));
-
-  Var v3(in_place_type_t<std::string>(), "ABC", 2);
-  ASSERT_TRUE(absl::holds_alternative<std::string>(v3));
-  EXPECT_EQ("AB", absl::get<std::string>(v3));
-
-  Var v4(in_place_type_t<NonCopyable>{});
-  ASSERT_TRUE(absl::holds_alternative<NonCopyable>(v4));
-
-  Var v5(in_place_type_t<std::vector<int>>(), {1, 2, 3});
-  ASSERT_TRUE(absl::holds_alternative<std::vector<int>>(v5));
-  EXPECT_THAT(absl::get<std::vector<int>>(v5), ::testing::ElementsAre(1, 2, 3));
-}
-
-TEST(VariantTest, InPlaceTypeVariableTemplate) {
-  using Var = variant<int, std::string, NonCopyable, std::vector<int>>;
-
-  Var v1(in_place_type<int>, 7);
-  ASSERT_TRUE(absl::holds_alternative<int>(v1));
-  EXPECT_EQ(7, absl::get<int>(v1));
-
-  Var v2(in_place_type<std::string>, "ABC");
-  ASSERT_TRUE(absl::holds_alternative<std::string>(v2));
-  EXPECT_EQ("ABC", absl::get<std::string>(v2));
-
-  Var v3(in_place_type<std::string>, "ABC", 2);
-  ASSERT_TRUE(absl::holds_alternative<std::string>(v3));
-  EXPECT_EQ("AB", absl::get<std::string>(v3));
-
-  Var v4(in_place_type<NonCopyable>);
-  ASSERT_TRUE(absl::holds_alternative<NonCopyable>(v4));
-
-  Var v5(in_place_type<std::vector<int>>, {1, 2, 3});
-  ASSERT_TRUE(absl::holds_alternative<std::vector<int>>(v5));
-  EXPECT_THAT(absl::get<std::vector<int>>(v5), ::testing::ElementsAre(1, 2, 3));
-}
-
-TEST(VariantTest, InPlaceTypeInitializerList) {
-  using Var =
-      variant<int, std::string, NonCopyable, MoveOnlyWithListConstructor>;
-
-  Var v1(in_place_type_t<MoveOnlyWithListConstructor>(), {1, 2, 3, 4, 5}, 6);
-  ASSERT_TRUE(absl::holds_alternative<MoveOnlyWithListConstructor>(v1));
-  EXPECT_EQ(6, absl::get<MoveOnlyWithListConstructor>(v1).value);
-}
-
-TEST(VariantTest, InPlaceTypeInitializerListVariabletemplate) {
-  using Var =
-      variant<int, std::string, NonCopyable, MoveOnlyWithListConstructor>;
-
-  Var v1(in_place_type<MoveOnlyWithListConstructor>, {1, 2, 3, 4, 5}, 6);
-  ASSERT_TRUE(absl::holds_alternative<MoveOnlyWithListConstructor>(v1));
-  EXPECT_EQ(6, absl::get<MoveOnlyWithListConstructor>(v1).value);
-}
-
-TEST(VariantTest, InPlaceIndex) {
-  using Var = variant<int, std::string, NonCopyable, std::vector<int>>;
-
-  Var v1(in_place_index_t<0>(), 7);
-  ASSERT_TRUE(absl::holds_alternative<int>(v1));
-  EXPECT_EQ(7, absl::get<int>(v1));
-
-  Var v2(in_place_index_t<1>(), "ABC");
-  ASSERT_TRUE(absl::holds_alternative<std::string>(v2));
-  EXPECT_EQ("ABC", absl::get<std::string>(v2));
-
-  Var v3(in_place_index_t<1>(), "ABC", 2);
-  ASSERT_TRUE(absl::holds_alternative<std::string>(v3));
-  EXPECT_EQ("AB", absl::get<std::string>(v3));
-
-  Var v4(in_place_index_t<2>{});
-  EXPECT_TRUE(absl::holds_alternative<NonCopyable>(v4));
-
-  // Verify that a variant with only non-copyables can still be constructed.
-  EXPECT_TRUE(absl::holds_alternative<NonCopyable>(
-      variant<NonCopyable>(in_place_index_t<0>{})));
-
-  Var v5(in_place_index_t<3>(), {1, 2, 3});
-  ASSERT_TRUE(absl::holds_alternative<std::vector<int>>(v5));
-  EXPECT_THAT(absl::get<std::vector<int>>(v5), ::testing::ElementsAre(1, 2, 3));
-}
-
-TEST(VariantTest, InPlaceIndexVariableTemplate) {
-  using Var = variant<int, std::string, NonCopyable, std::vector<int>>;
-
-  Var v1(in_place_index<0>, 7);
-  ASSERT_TRUE(absl::holds_alternative<int>(v1));
-  EXPECT_EQ(7, absl::get<int>(v1));
-
-  Var v2(in_place_index<1>, "ABC");
-  ASSERT_TRUE(absl::holds_alternative<std::string>(v2));
-  EXPECT_EQ("ABC", absl::get<std::string>(v2));
-
-  Var v3(in_place_index<1>, "ABC", 2);
-  ASSERT_TRUE(absl::holds_alternative<std::string>(v3));
-  EXPECT_EQ("AB", absl::get<std::string>(v3));
-
-  Var v4(in_place_index<2>);
-  EXPECT_TRUE(absl::holds_alternative<NonCopyable>(v4));
-
-  // Verify that a variant with only non-copyables can still be constructed.
-  EXPECT_TRUE(absl::holds_alternative<NonCopyable>(
-      variant<NonCopyable>(in_place_index<0>)));
-
-  Var v5(in_place_index<3>, {1, 2, 3});
-  ASSERT_TRUE(absl::holds_alternative<std::vector<int>>(v5));
-  EXPECT_THAT(absl::get<std::vector<int>>(v5), ::testing::ElementsAre(1, 2, 3));
-}
-
-TEST(VariantTest, InPlaceIndexInitializerList) {
-  using Var =
-      variant<int, std::string, NonCopyable, MoveOnlyWithListConstructor>;
-
-  Var v1(in_place_index_t<3>(), {1, 2, 3, 4, 5}, 6);
-  ASSERT_TRUE(absl::holds_alternative<MoveOnlyWithListConstructor>(v1));
-  EXPECT_EQ(6, absl::get<MoveOnlyWithListConstructor>(v1).value);
-}
-
-TEST(VariantTest, InPlaceIndexInitializerListVariableTemplate) {
-  using Var =
-      variant<int, std::string, NonCopyable, MoveOnlyWithListConstructor>;
-
-  Var v1(in_place_index<3>, {1, 2, 3, 4, 5}, 6);
-  ASSERT_TRUE(absl::holds_alternative<MoveOnlyWithListConstructor>(v1));
-  EXPECT_EQ(6, absl::get<MoveOnlyWithListConstructor>(v1).value);
-}
-
-////////////////////
-// [variant.dtor] //
-////////////////////
-
-// Make sure that the destructor destroys the contained value
-TEST(VariantTest, TestDtor) {
-  typedef VariantFactory<IncrementInDtor>::Type Variant;
-  using value_type1 = absl::variant_alternative_t<0, Variant>;
-  using value_type2 = absl::variant_alternative_t<1, Variant>;
-  using value_type3 = absl::variant_alternative_t<2, Variant>;
-  using value_type4 = absl::variant_alternative_t<3, Variant>;
-  int counter = 0;
-  IncrementInDtor counter_adjuster(&counter);
-  EXPECT_EQ(0, counter);
-
-  value_type1 value1(counter_adjuster);
-  { Variant object(value1); }
-  EXPECT_EQ(1, counter);
-
-  value_type2 value2(counter_adjuster);
-  { Variant object(value2); }
-  EXPECT_EQ(2, counter);
-
-  value_type3 value3(counter_adjuster);
-  { Variant object(value3); }
-  EXPECT_EQ(3, counter);
-
-  value_type4 value4(counter_adjuster);
-  { Variant object(value4); }
-  EXPECT_EQ(4, counter);
-}
-
-#ifdef ABSL_HAVE_EXCEPTIONS
-
-// See comment in absl/base/config.h
-#if defined(ABSL_INTERNAL_MSVC_2017_DBG_MODE)
-TEST(VariantTest, DISABLED_TestDtorValuelessByException)
-#else
-// Test destruction when in the valueless_by_exception state.
-TEST(VariantTest, TestDtorValuelessByException)
-#endif
-{
-  int counter = 0;
-  IncrementInDtor counter_adjuster(&counter);
-
-  {
-    using Variant = VariantFactory<IncrementInDtor>::Type;
-
-    Variant v(in_place_index<0>, counter_adjuster);
-    EXPECT_EQ(0, counter);
-
-    ToValuelessByException(v);
-    ASSERT_TRUE(v.valueless_by_exception());
-    EXPECT_EQ(1, counter);
-  }
-  EXPECT_EQ(1, counter);
-}
-
-#endif  // ABSL_HAVE_EXCEPTIONS
-
-//////////////////////
-// [variant.assign] //
-//////////////////////
-
-// Test that self-assignment doesn't destroy the current value
-TEST(VariantTest, TestSelfAssignment) {
-  typedef VariantFactory<IncrementInDtor>::Type Variant;
-  int counter = 0;
-  IncrementInDtor counter_adjuster(&counter);
-  absl::variant_alternative_t<0, Variant> value(counter_adjuster);
-  Variant object(value);
-  object.operator=(object);
-  EXPECT_EQ(0, counter);
-
-  // A std::string long enough that it's likely to defeat any inline representation
-  // optimization.
-  const std::string long_str(128, 'a');
-
-  std::string foo = long_str;
-  foo = *&foo;
-  EXPECT_EQ(long_str, foo);
-
-  variant<int, std::string> so = long_str;
-  ASSERT_EQ(1, so.index());
-  EXPECT_EQ(long_str, absl::get<1>(so));
-  so = *&so;
-
-  ASSERT_EQ(1, so.index());
-  EXPECT_EQ(long_str, absl::get<1>(so));
-}
-
-// Test that assigning a variant<..., T, ...> to a variant<..., T, ...> produces
-// a variant<..., T, ...> with the correct value.
-TYPED_TEST(VariantTypesTest, TestAssignmentCopiesValueSameTypes) {
-  typedef typename VariantFactory<typename TypeParam::value_type>::Type Variant;
-  const TypeParam value(TypeParam::kIndex);
-  const Variant source(value);
-  Variant target(TypeParam(value.value + 1));
-  ASSERT_TRUE(absl::holds_alternative<TypeParam>(source));
-  ASSERT_TRUE(absl::holds_alternative<TypeParam>(target));
-  ASSERT_NE(absl::get<TypeParam>(source), absl::get<TypeParam>(target));
-  target = source;
-  ASSERT_TRUE(absl::holds_alternative<TypeParam>(source));
-  ASSERT_TRUE(absl::holds_alternative<TypeParam>(target));
-  EXPECT_EQ(absl::get<TypeParam>(source), absl::get<TypeParam>(target));
-}
-
-// Test that assisnging a variant<..., T, ...> to a variant<1, ...>
-// produces a variant<..., T, ...> with the correct value.
-TYPED_TEST(VariantTypesTest, TestAssignmentCopiesValuesVaryingSourceType) {
-  typedef typename VariantFactory<typename TypeParam::value_type>::Type Variant;
-  using value_type1 = absl::variant_alternative_t<0, Variant>;
-  const TypeParam value(TypeParam::kIndex);
-  const Variant source(value);
-  ASSERT_TRUE(absl::holds_alternative<TypeParam>(source));
-  Variant target(value_type1(1));
-  ASSERT_TRUE(absl::holds_alternative<value_type1>(target));
-  target = source;
-  EXPECT_TRUE(absl::holds_alternative<TypeParam>(source));
-  EXPECT_TRUE(absl::holds_alternative<TypeParam>(target));
-  EXPECT_EQ(absl::get<TypeParam>(source), absl::get<TypeParam>(target));
-}
-
-// Test that assigning a variant<1, ...> to a variant<..., T, ...>
-// produces a variant<1, ...> with the correct value.
-TYPED_TEST(VariantTypesTest, TestAssignmentCopiesValuesVaryingTargetType) {
-  typedef typename VariantFactory<typename TypeParam::value_type>::Type Variant;
-  using value_type1 = absl::variant_alternative_t<0, Variant>;
-  const Variant source(value_type1(1));
-  ASSERT_TRUE(absl::holds_alternative<value_type1>(source));
-  const TypeParam value(TypeParam::kIndex);
-  Variant target(value);
-  ASSERT_TRUE(absl::holds_alternative<TypeParam>(target));
-  target = source;
-  EXPECT_TRUE(absl::holds_alternative<value_type1>(target));
-  EXPECT_TRUE(absl::holds_alternative<value_type1>(source));
-  EXPECT_EQ(absl::get<value_type1>(source), absl::get<value_type1>(target));
-}
-
-// Test that operator=<T> works, that assigning a new value destroys
-// the old and that assigning the new value again does not redestroy
-// the old
-TEST(VariantTest, TestAssign) {
-  typedef VariantFactory<IncrementInDtor>::Type Variant;
-  using value_type1 = absl::variant_alternative_t<0, Variant>;
-  using value_type2 = absl::variant_alternative_t<1, Variant>;
-  using value_type3 = absl::variant_alternative_t<2, Variant>;
-  using value_type4 = absl::variant_alternative_t<3, Variant>;
-
-  const int kSize = 4;
-  int counter[kSize];
-  std::unique_ptr<IncrementInDtor> counter_adjustor[kSize];
-  for (int i = 0; i != kSize; i++) {
-    counter[i] = 0;
-    counter_adjustor[i] = absl::make_unique<IncrementInDtor>(&counter[i]);
-  }
-
-  value_type1 v1(*counter_adjustor[0]);
-  value_type2 v2(*counter_adjustor[1]);
-  value_type3 v3(*counter_adjustor[2]);
-  value_type4 v4(*counter_adjustor[3]);
-
-  // Test that reassignment causes destruction of old value
-  {
-    Variant object(v1);
-    object = v2;
-    object = v3;
-    object = v4;
-    object = v1;
-  }
-
-  EXPECT_EQ(2, counter[0]);
-  EXPECT_EQ(1, counter[1]);
-  EXPECT_EQ(1, counter[2]);
-  EXPECT_EQ(1, counter[3]);
-
-  std::fill(std::begin(counter), std::end(counter), 0);
-
-  // Test that self-assignment does not cause destruction of old value
-  {
-    Variant object(v1);
-    object.operator=(object);
-    EXPECT_EQ(0, counter[0]);
-  }
-  {
-    Variant object(v2);
-    object.operator=(object);
-    EXPECT_EQ(0, counter[1]);
-  }
-  {
-    Variant object(v3);
-    object.operator=(object);
-    EXPECT_EQ(0, counter[2]);
-  }
-  {
-    Variant object(v4);
-    object.operator=(object);
-    EXPECT_EQ(0, counter[3]);
-  }
-
-  EXPECT_EQ(1, counter[0]);
-  EXPECT_EQ(1, counter[1]);
-  EXPECT_EQ(1, counter[2]);
-  EXPECT_EQ(1, counter[3]);
-}
-
-// This tests that we perform a backup if the copy-assign can throw but the move
-// cannot throw.
-TEST(VariantTest, TestBackupAssign) {
-  typedef VariantFactory<IncrementInDtorCopyCanThrow>::Type Variant;
-  using value_type1 = absl::variant_alternative_t<0, Variant>;
-  using value_type2 = absl::variant_alternative_t<1, Variant>;
-  using value_type3 = absl::variant_alternative_t<2, Variant>;
-  using value_type4 = absl::variant_alternative_t<3, Variant>;
-
-  const int kSize = 4;
-  int counter[kSize];
-  std::unique_ptr<IncrementInDtorCopyCanThrow> counter_adjustor[kSize];
-  for (int i = 0; i != kSize; i++) {
-    counter[i] = 0;
-    counter_adjustor[i].reset(new IncrementInDtorCopyCanThrow(&counter[i]));
-  }
-
-  value_type1 v1(*counter_adjustor[0]);
-  value_type2 v2(*counter_adjustor[1]);
-  value_type3 v3(*counter_adjustor[2]);
-  value_type4 v4(*counter_adjustor[3]);
-
-  // Test that reassignment causes destruction of old value
-  {
-    Variant object(v1);
-    object = v2;
-    object = v3;
-    object = v4;
-    object = v1;
-  }
-
-  // libstdc++ doesn't pass this test
-#if !(defined(ABSL_HAVE_STD_VARIANT) && defined(__GLIBCXX__))
-  EXPECT_EQ(3, counter[0]);
-  EXPECT_EQ(2, counter[1]);
-  EXPECT_EQ(2, counter[2]);
-  EXPECT_EQ(2, counter[3]);
-#endif
-
-  std::fill(std::begin(counter), std::end(counter), 0);
-
-  // Test that self-assignment does not cause destruction of old value
-  {
-    Variant object(v1);
-    object.operator=(object);
-    EXPECT_EQ(0, counter[0]);
-  }
-  {
-    Variant object(v2);
-    object.operator=(object);
-    EXPECT_EQ(0, counter[1]);
-  }
-  {
-    Variant object(v3);
-    object.operator=(object);
-    EXPECT_EQ(0, counter[2]);
-  }
-  {
-    Variant object(v4);
-    object.operator=(object);
-    EXPECT_EQ(0, counter[3]);
-  }
-
-  EXPECT_EQ(1, counter[0]);
-  EXPECT_EQ(1, counter[1]);
-  EXPECT_EQ(1, counter[2]);
-  EXPECT_EQ(1, counter[3]);
-}
-
-///////////////////
-// [variant.mod] //
-///////////////////
-
-TEST(VariantTest, TestEmplaceBasic) {
-  using Variant = variant<int, char>;
-
-  Variant v(absl::in_place_index<0>, 0);
-
-  {
-    char& emplace_result = v.emplace<char>();
-    ASSERT_TRUE(absl::holds_alternative<char>(v));
-    EXPECT_EQ(absl::get<char>(v), 0);
-    EXPECT_EQ(&emplace_result, &absl::get<char>(v));
-  }
-
-  // Make sure that another emplace does zero-initialization
-  absl::get<char>(v) = 'a';
-  v.emplace<char>('b');
-  ASSERT_TRUE(absl::holds_alternative<char>(v));
-  EXPECT_EQ(absl::get<char>(v), 'b');
-
-  {
-    int& emplace_result = v.emplace<int>();
-    EXPECT_TRUE(absl::holds_alternative<int>(v));
-    EXPECT_EQ(absl::get<int>(v), 0);
-    EXPECT_EQ(&emplace_result, &absl::get<int>(v));
-  }
-}
-
-TEST(VariantTest, TestEmplaceInitializerList) {
-  using Var =
-      variant<int, std::string, NonCopyable, MoveOnlyWithListConstructor>;
-
-  Var v1(absl::in_place_index<0>, 555);
-  MoveOnlyWithListConstructor& emplace_result =
-      v1.emplace<MoveOnlyWithListConstructor>({1, 2, 3, 4, 5}, 6);
-  ASSERT_TRUE(absl::holds_alternative<MoveOnlyWithListConstructor>(v1));
-  EXPECT_EQ(6, absl::get<MoveOnlyWithListConstructor>(v1).value);
-  EXPECT_EQ(&emplace_result, &absl::get<MoveOnlyWithListConstructor>(v1));
-}
-
-TEST(VariantTest, TestEmplaceIndex) {
-  using Variant = variant<int, char>;
-
-  Variant v(absl::in_place_index<0>, 555);
-
-  {
-    char& emplace_result = v.emplace<1>();
-    ASSERT_TRUE(absl::holds_alternative<char>(v));
-    EXPECT_EQ(absl::get<char>(v), 0);
-    EXPECT_EQ(&emplace_result, &absl::get<char>(v));
-  }
-
-  // Make sure that another emplace does zero-initialization
-  absl::get<char>(v) = 'a';
-  v.emplace<1>('b');
-  ASSERT_TRUE(absl::holds_alternative<char>(v));
-  EXPECT_EQ(absl::get<char>(v), 'b');
-
-  {
-    int& emplace_result = v.emplace<0>();
-    EXPECT_TRUE(absl::holds_alternative<int>(v));
-    EXPECT_EQ(absl::get<int>(v), 0);
-    EXPECT_EQ(&emplace_result, &absl::get<int>(v));
-  }
-}
-
-TEST(VariantTest, TestEmplaceIndexInitializerList) {
-  using Var =
-      variant<int, std::string, NonCopyable, MoveOnlyWithListConstructor>;
-
-  Var v1(absl::in_place_index<0>, 555);
-  MoveOnlyWithListConstructor& emplace_result =
-      v1.emplace<3>({1, 2, 3, 4, 5}, 6);
-  ASSERT_TRUE(absl::holds_alternative<MoveOnlyWithListConstructor>(v1));
-  EXPECT_EQ(6, absl::get<MoveOnlyWithListConstructor>(v1).value);
-  EXPECT_EQ(&emplace_result, &absl::get<MoveOnlyWithListConstructor>(v1));
-}
-
-//////////////////////
-// [variant.status] //
-//////////////////////
-
-TEST(VariantTest, Index) {
-  using Var = variant<int, std::string, double>;
-
-  Var v = 1;
-  EXPECT_EQ(0, v.index());
-  v = "str";
-  EXPECT_EQ(1, v.index());
-  v = 0.;
-  EXPECT_EQ(2, v.index());
-
-  Var v2 = v;
-  EXPECT_EQ(2, v2.index());
-  v2.emplace<int>(3);
-  EXPECT_EQ(0, v2.index());
-}
-
-TEST(VariantTest, NotValuelessByException) {
-  using Var = variant<int, std::string, double>;
-
-  Var v = 1;
-  EXPECT_FALSE(v.valueless_by_exception());
-  v = "str";
-  EXPECT_FALSE(v.valueless_by_exception());
-  v = 0.;
-  EXPECT_FALSE(v.valueless_by_exception());
-
-  Var v2 = v;
-  EXPECT_FALSE(v.valueless_by_exception());
-  v2.emplace<int>(3);
-  EXPECT_FALSE(v.valueless_by_exception());
-}
-
-#ifdef ABSL_HAVE_EXCEPTIONS
-
-TEST(VariantTest, IndexValuelessByException) {
-  using Var = variant<MoveCanThrow, std::string, double>;
-
-  Var v(absl::in_place_index<0>);
-  EXPECT_EQ(0, v.index());
-  ToValuelessByException(v);
-  EXPECT_EQ(absl::variant_npos, v.index());
-  v = "str";
-  EXPECT_EQ(1, v.index());
-}
-
-TEST(VariantTest, ValuelessByException) {
-  using Var = variant<MoveCanThrow, std::string, double>;
-
-  Var v(absl::in_place_index<0>);
-  EXPECT_FALSE(v.valueless_by_exception());
-  ToValuelessByException(v);
-  EXPECT_TRUE(v.valueless_by_exception());
-  v = "str";
-  EXPECT_FALSE(v.valueless_by_exception());
-}
-
-#endif  // ABSL_HAVE_EXCEPTIONS
-
-////////////////////
-// [variant.swap] //
-////////////////////
-
-TEST(VariantTest, MemberSwap) {
-  SpecialSwap v1(3);
-  SpecialSwap v2(7);
-
-  variant<SpecialSwap> a = v1, b = v2;
-
-  EXPECT_THAT(a, VariantWith<SpecialSwap>(v1));
-  EXPECT_THAT(b, VariantWith<SpecialSwap>(v2));
-
-  a.swap(b);
-  EXPECT_THAT(a, VariantWith<SpecialSwap>(v2));
-  EXPECT_THAT(b, VariantWith<SpecialSwap>(v1));
-  EXPECT_TRUE(absl::get<SpecialSwap>(a).special_swap);
-
-  using V = variant<MoveCanThrow, std::string, int>;
-  int i = 33;
-  std::string s = "abc";
-  {
-    // lhs and rhs holds different alternative
-    V lhs(i), rhs(s);
-    lhs.swap(rhs);
-    EXPECT_THAT(lhs, VariantWith<std::string>(s));
-    EXPECT_THAT(rhs, VariantWith<int>(i));
-  }
-#ifdef ABSL_HAVE_EXCEPTIONS
-  V valueless(in_place_index<0>);
-  ToValuelessByException(valueless);
-  {
-    // lhs is valueless
-    V lhs(valueless), rhs(i);
-    lhs.swap(rhs);
-    EXPECT_THAT(lhs, VariantWith<int>(i));
-    EXPECT_TRUE(rhs.valueless_by_exception());
-  }
-  {
-    // rhs is valueless
-    V lhs(s), rhs(valueless);
-    lhs.swap(rhs);
-    EXPECT_THAT(rhs, VariantWith<std::string>(s));
-    EXPECT_TRUE(lhs.valueless_by_exception());
-  }
-  {
-    // both are valueless
-    V lhs(valueless), rhs(valueless);
-    lhs.swap(rhs);
-    EXPECT_TRUE(lhs.valueless_by_exception());
-    EXPECT_TRUE(rhs.valueless_by_exception());
-  }
-#endif  // ABSL_HAVE_EXCEPTIONS
-}
-
-//////////////////////
-// [variant.helper] //
-//////////////////////
-
-TEST(VariantTest, VariantSize) {
-  {
-    using Size1Variant = absl::variant<int>;
-    EXPECT_EQ(1, absl::variant_size<Size1Variant>::value);
-    EXPECT_EQ(1, absl::variant_size<const Size1Variant>::value);
-    EXPECT_EQ(1, absl::variant_size<volatile Size1Variant>::value);
-    EXPECT_EQ(1, absl::variant_size<const volatile Size1Variant>::value);
-  }
-
-  {
-    using Size3Variant = absl::variant<int, float, int>;
-    EXPECT_EQ(3, absl::variant_size<Size3Variant>::value);
-    EXPECT_EQ(3, absl::variant_size<const Size3Variant>::value);
-    EXPECT_EQ(3, absl::variant_size<volatile Size3Variant>::value);
-    EXPECT_EQ(3, absl::variant_size<const volatile Size3Variant>::value);
-  }
-}
-
-TEST(VariantTest, VariantAlternative) {
-  {
-    using V = absl::variant<float, int, const char*>;
-    EXPECT_TRUE(
-        (std::is_same<float, absl::variant_alternative_t<0, V>>::value));
-    EXPECT_TRUE((std::is_same<const float,
-                              absl::variant_alternative_t<0, const V>>::value));
-    EXPECT_TRUE(
-        (std::is_same<volatile float,
-                      absl::variant_alternative_t<0, volatile V>>::value));
-    EXPECT_TRUE((
-        std::is_same<const volatile float,
-                     absl::variant_alternative_t<0, const volatile V>>::value));
-
-    EXPECT_TRUE((std::is_same<int, absl::variant_alternative_t<1, V>>::value));
-    EXPECT_TRUE((std::is_same<const int,
-                              absl::variant_alternative_t<1, const V>>::value));
-    EXPECT_TRUE(
-        (std::is_same<volatile int,
-                      absl::variant_alternative_t<1, volatile V>>::value));
-    EXPECT_TRUE((
-        std::is_same<const volatile int,
-                     absl::variant_alternative_t<1, const volatile V>>::value));
-
-    EXPECT_TRUE(
-        (std::is_same<const char*, absl::variant_alternative_t<2, V>>::value));
-    EXPECT_TRUE((std::is_same<const char* const,
-                              absl::variant_alternative_t<2, const V>>::value));
-    EXPECT_TRUE(
-        (std::is_same<const char* volatile,
-                      absl::variant_alternative_t<2, volatile V>>::value));
-    EXPECT_TRUE((
-        std::is_same<const char* const volatile,
-                     absl::variant_alternative_t<2, const volatile V>>::value));
-  }
-
-  {
-    using V = absl::variant<float, volatile int, const char*>;
-    EXPECT_TRUE(
-        (std::is_same<float, absl::variant_alternative_t<0, V>>::value));
-    EXPECT_TRUE((std::is_same<const float,
-                              absl::variant_alternative_t<0, const V>>::value));
-    EXPECT_TRUE(
-        (std::is_same<volatile float,
-                      absl::variant_alternative_t<0, volatile V>>::value));
-    EXPECT_TRUE((
-        std::is_same<const volatile float,
-                     absl::variant_alternative_t<0, const volatile V>>::value));
-
-    EXPECT_TRUE(
-        (std::is_same<volatile int, absl::variant_alternative_t<1, V>>::value));
-    EXPECT_TRUE((std::is_same<const volatile int,
-                              absl::variant_alternative_t<1, const V>>::value));
-    EXPECT_TRUE(
-        (std::is_same<volatile int,
-                      absl::variant_alternative_t<1, volatile V>>::value));
-    EXPECT_TRUE((
-        std::is_same<const volatile int,
-                     absl::variant_alternative_t<1, const volatile V>>::value));
-
-    EXPECT_TRUE(
-        (std::is_same<const char*, absl::variant_alternative_t<2, V>>::value));
-    EXPECT_TRUE((std::is_same<const char* const,
-                              absl::variant_alternative_t<2, const V>>::value));
-    EXPECT_TRUE(
-        (std::is_same<const char* volatile,
-                      absl::variant_alternative_t<2, volatile V>>::value));
-    EXPECT_TRUE((
-        std::is_same<const char* const volatile,
-                     absl::variant_alternative_t<2, const volatile V>>::value));
-  }
-}
-
-///////////////////
-// [variant.get] //
-///////////////////
-
-TEST(VariantTest, HoldsAlternative) {
-  using Var = variant<int, std::string, double>;
-
-  Var v = 1;
-  EXPECT_TRUE(absl::holds_alternative<int>(v));
-  EXPECT_FALSE(absl::holds_alternative<std::string>(v));
-  EXPECT_FALSE(absl::holds_alternative<double>(v));
-  v = "str";
-  EXPECT_FALSE(absl::holds_alternative<int>(v));
-  EXPECT_TRUE(absl::holds_alternative<std::string>(v));
-  EXPECT_FALSE(absl::holds_alternative<double>(v));
-  v = 0.;
-  EXPECT_FALSE(absl::holds_alternative<int>(v));
-  EXPECT_FALSE(absl::holds_alternative<std::string>(v));
-  EXPECT_TRUE(absl::holds_alternative<double>(v));
-
-  Var v2 = v;
-  EXPECT_FALSE(absl::holds_alternative<int>(v2));
-  EXPECT_FALSE(absl::holds_alternative<std::string>(v2));
-  EXPECT_TRUE(absl::holds_alternative<double>(v2));
-  v2.emplace<int>(3);
-  EXPECT_TRUE(absl::holds_alternative<int>(v2));
-  EXPECT_FALSE(absl::holds_alternative<std::string>(v2));
-  EXPECT_FALSE(absl::holds_alternative<double>(v2));
-}
-
-TEST(VariantTest, GetIndex) {
-  using Var = variant<int, std::string, double, int>;
-
-  {
-    Var v(absl::in_place_index<0>, 0);
-
-    using LValueGetType = decltype(absl::get<0>(v));
-    using RValueGetType = decltype(absl::get<0>(absl::move(v)));
-
-    EXPECT_TRUE((std::is_same<LValueGetType, int&>::value));
-    EXPECT_TRUE((std::is_same<RValueGetType, int&&>::value));
-    EXPECT_EQ(absl::get<0>(v), 0);
-    EXPECT_EQ(absl::get<0>(absl::move(v)), 0);
-
-    const Var& const_v = v;
-    using ConstLValueGetType = decltype(absl::get<0>(const_v));
-    using ConstRValueGetType = decltype(absl::get<0>(absl::move(const_v)));
-    EXPECT_TRUE((std::is_same<ConstLValueGetType, const int&>::value));
-    EXPECT_TRUE((std::is_same<ConstRValueGetType, const int&&>::value));
-    EXPECT_EQ(absl::get<0>(const_v), 0);
-    EXPECT_EQ(absl::get<0>(absl::move(const_v)), 0);
-  }
-
-  {
-    Var v = std::string("Hello");
-
-    using LValueGetType = decltype(absl::get<1>(v));
-    using RValueGetType = decltype(absl::get<1>(absl::move(v)));
-
-    EXPECT_TRUE((std::is_same<LValueGetType, std::string&>::value));
-    EXPECT_TRUE((std::is_same<RValueGetType, std::string&&>::value));
-    EXPECT_EQ(absl::get<1>(v), "Hello");
-    EXPECT_EQ(absl::get<1>(absl::move(v)), "Hello");
-
-    const Var& const_v = v;
-    using ConstLValueGetType = decltype(absl::get<1>(const_v));
-    using ConstRValueGetType = decltype(absl::get<1>(absl::move(const_v)));
-    EXPECT_TRUE((std::is_same<ConstLValueGetType, const std::string&>::value));
-    EXPECT_TRUE((std::is_same<ConstRValueGetType, const std::string&&>::value));
-    EXPECT_EQ(absl::get<1>(const_v), "Hello");
-    EXPECT_EQ(absl::get<1>(absl::move(const_v)), "Hello");
-  }
-
-  {
-    Var v = 2.0;
-
-    using LValueGetType = decltype(absl::get<2>(v));
-    using RValueGetType = decltype(absl::get<2>(absl::move(v)));
-
-    EXPECT_TRUE((std::is_same<LValueGetType, double&>::value));
-    EXPECT_TRUE((std::is_same<RValueGetType, double&&>::value));
-    EXPECT_EQ(absl::get<2>(v), 2.);
-    EXPECT_EQ(absl::get<2>(absl::move(v)), 2.);
-
-    const Var& const_v = v;
-    using ConstLValueGetType = decltype(absl::get<2>(const_v));
-    using ConstRValueGetType = decltype(absl::get<2>(absl::move(const_v)));
-    EXPECT_TRUE((std::is_same<ConstLValueGetType, const double&>::value));
-    EXPECT_TRUE((std::is_same<ConstRValueGetType, const double&&>::value));
-    EXPECT_EQ(absl::get<2>(const_v), 2.);
-    EXPECT_EQ(absl::get<2>(absl::move(const_v)), 2.);
-  }
-
-  {
-    Var v(absl::in_place_index<0>, 0);
-    v.emplace<3>(1);
-
-    using LValueGetType = decltype(absl::get<3>(v));
-    using RValueGetType = decltype(absl::get<3>(absl::move(v)));
-
-    EXPECT_TRUE((std::is_same<LValueGetType, int&>::value));
-    EXPECT_TRUE((std::is_same<RValueGetType, int&&>::value));
-    EXPECT_EQ(absl::get<3>(v), 1);
-    EXPECT_EQ(absl::get<3>(absl::move(v)), 1);
-
-    const Var& const_v = v;
-    using ConstLValueGetType = decltype(absl::get<3>(const_v));
-    using ConstRValueGetType = decltype(absl::get<3>(absl::move(const_v)));
-    EXPECT_TRUE((std::is_same<ConstLValueGetType, const int&>::value));
-    EXPECT_TRUE((std::is_same<ConstRValueGetType, const int&&>::value));
-    EXPECT_EQ(absl::get<3>(const_v), 1);
-    EXPECT_EQ(absl::get<3>(absl::move(const_v)), 1);  // NOLINT
-  }
-}
-
-TEST(VariantTest, BadGetIndex) {
-  using Var = variant<int, std::string, double>;
-
-  {
-    Var v = 1;
-
-    ABSL_VARIANT_TEST_EXPECT_BAD_VARIANT_ACCESS(absl::get<1>(v));
-    ABSL_VARIANT_TEST_EXPECT_BAD_VARIANT_ACCESS(absl::get<1>(std::move(v)));
-
-    const Var& const_v = v;
-    ABSL_VARIANT_TEST_EXPECT_BAD_VARIANT_ACCESS(absl::get<1>(const_v));
-    ABSL_VARIANT_TEST_EXPECT_BAD_VARIANT_ACCESS(
-        absl::get<1>(std::move(const_v)));  // NOLINT
-  }
-
-  {
-    Var v = std::string("Hello");
-
-    ABSL_VARIANT_TEST_EXPECT_BAD_VARIANT_ACCESS(absl::get<0>(v));
-    ABSL_VARIANT_TEST_EXPECT_BAD_VARIANT_ACCESS(absl::get<0>(std::move(v)));
-
-    const Var& const_v = v;
-    ABSL_VARIANT_TEST_EXPECT_BAD_VARIANT_ACCESS(absl::get<0>(const_v));
-    ABSL_VARIANT_TEST_EXPECT_BAD_VARIANT_ACCESS(
-        absl::get<0>(std::move(const_v)));  // NOLINT
-  }
-}
-
-TEST(VariantTest, GetType) {
-  using Var = variant<int, std::string, double>;
-
-  {
-    Var v = 1;
-
-    using LValueGetType = decltype(absl::get<int>(v));
-    using RValueGetType = decltype(absl::get<int>(absl::move(v)));
-
-    EXPECT_TRUE((std::is_same<LValueGetType, int&>::value));
-    EXPECT_TRUE((std::is_same<RValueGetType, int&&>::value));
-    EXPECT_EQ(absl::get<int>(v), 1);
-    EXPECT_EQ(absl::get<int>(absl::move(v)), 1);
-
-    const Var& const_v = v;
-    using ConstLValueGetType = decltype(absl::get<int>(const_v));
-    using ConstRValueGetType = decltype(absl::get<int>(absl::move(const_v)));
-    EXPECT_TRUE((std::is_same<ConstLValueGetType, const int&>::value));
-    EXPECT_TRUE((std::is_same<ConstRValueGetType, const int&&>::value));
-    EXPECT_EQ(absl::get<int>(const_v), 1);
-    EXPECT_EQ(absl::get<int>(absl::move(const_v)), 1);
-  }
-
-  {
-    Var v = std::string("Hello");
-
-    using LValueGetType = decltype(absl::get<1>(v));
-    using RValueGetType = decltype(absl::get<1>(absl::move(v)));
-
-    EXPECT_TRUE((std::is_same<LValueGetType, std::string&>::value));
-    EXPECT_TRUE((std::is_same<RValueGetType, std::string&&>::value));
-    EXPECT_EQ(absl::get<std::string>(v), "Hello");
-    EXPECT_EQ(absl::get<std::string>(absl::move(v)), "Hello");
-
-    const Var& const_v = v;
-    using ConstLValueGetType = decltype(absl::get<1>(const_v));
-    using ConstRValueGetType = decltype(absl::get<1>(absl::move(const_v)));
-    EXPECT_TRUE((std::is_same<ConstLValueGetType, const std::string&>::value));
-    EXPECT_TRUE((std::is_same<ConstRValueGetType, const std::string&&>::value));
-    EXPECT_EQ(absl::get<std::string>(const_v), "Hello");
-    EXPECT_EQ(absl::get<std::string>(absl::move(const_v)), "Hello");
-  }
-
-  {
-    Var v = 2.0;
-
-    using LValueGetType = decltype(absl::get<2>(v));
-    using RValueGetType = decltype(absl::get<2>(absl::move(v)));
-
-    EXPECT_TRUE((std::is_same<LValueGetType, double&>::value));
-    EXPECT_TRUE((std::is_same<RValueGetType, double&&>::value));
-    EXPECT_EQ(absl::get<double>(v), 2.);
-    EXPECT_EQ(absl::get<double>(absl::move(v)), 2.);
-
-    const Var& const_v = v;
-    using ConstLValueGetType = decltype(absl::get<2>(const_v));
-    using ConstRValueGetType = decltype(absl::get<2>(absl::move(const_v)));
-    EXPECT_TRUE((std::is_same<ConstLValueGetType, const double&>::value));
-    EXPECT_TRUE((std::is_same<ConstRValueGetType, const double&&>::value));
-    EXPECT_EQ(absl::get<double>(const_v), 2.);
-    EXPECT_EQ(absl::get<double>(absl::move(const_v)), 2.);
-  }
-}
-
-TEST(VariantTest, BadGetType) {
-  using Var = variant<int, std::string, double>;
-
-  {
-    Var v = 1;
-
-    ABSL_VARIANT_TEST_EXPECT_BAD_VARIANT_ACCESS(absl::get<std::string>(v));
-    ABSL_VARIANT_TEST_EXPECT_BAD_VARIANT_ACCESS(
-        absl::get<std::string>(std::move(v)));
-
-    const Var& const_v = v;
-    ABSL_VARIANT_TEST_EXPECT_BAD_VARIANT_ACCESS(
-        absl::get<std::string>(const_v));
-    ABSL_VARIANT_TEST_EXPECT_BAD_VARIANT_ACCESS(
-        absl::get<std::string>(std::move(const_v)));  // NOLINT
-  }
-
-  {
-    Var v = std::string("Hello");
-
-    ABSL_VARIANT_TEST_EXPECT_BAD_VARIANT_ACCESS(absl::get<int>(v));
-    ABSL_VARIANT_TEST_EXPECT_BAD_VARIANT_ACCESS(absl::get<int>(std::move(v)));
-
-    const Var& const_v = v;
-    ABSL_VARIANT_TEST_EXPECT_BAD_VARIANT_ACCESS(absl::get<int>(const_v));
-    ABSL_VARIANT_TEST_EXPECT_BAD_VARIANT_ACCESS(
-        absl::get<int>(std::move(const_v)));  // NOLINT
-  }
-}
-
-TEST(VariantTest, GetIfIndex) {
-  using Var = variant<int, std::string, double, int>;
-
-  {
-    Var v(absl::in_place_index<0>, 0);
-    EXPECT_TRUE(noexcept(absl::get_if<0>(&v)));
-
-    {
-      auto* elem = absl::get_if<0>(&v);
-      EXPECT_TRUE((std::is_same<decltype(elem), int*>::value));
-      ASSERT_NE(elem, nullptr);
-      EXPECT_EQ(*elem, 0);
-      {
-        auto* bad_elem = absl::get_if<1>(&v);
-        EXPECT_TRUE((std::is_same<decltype(bad_elem), std::string*>::value));
-        EXPECT_EQ(bad_elem, nullptr);
-      }
-      {
-        auto* bad_elem = absl::get_if<2>(&v);
-        EXPECT_TRUE((std::is_same<decltype(bad_elem), double*>::value));
-        EXPECT_EQ(bad_elem, nullptr);
-      }
-      {
-        auto* bad_elem = absl::get_if<3>(&v);
-        EXPECT_TRUE((std::is_same<decltype(bad_elem), int*>::value));
-        EXPECT_EQ(bad_elem, nullptr);
-      }
-    }
-
-    const Var& const_v = v;
-    EXPECT_TRUE(noexcept(absl::get_if<0>(&const_v)));
-
-    {
-      auto* elem = absl::get_if<0>(&const_v);
-      EXPECT_TRUE((std::is_same<decltype(elem), const int*>::value));
-      ASSERT_NE(elem, nullptr);
-      EXPECT_EQ(*elem, 0);
-      {
-        auto* bad_elem = absl::get_if<1>(&const_v);
-        EXPECT_TRUE(
-            (std::is_same<decltype(bad_elem), const std::string*>::value));
-        EXPECT_EQ(bad_elem, nullptr);
-      }
-      {
-        auto* bad_elem = absl::get_if<2>(&const_v);
-        EXPECT_TRUE((std::is_same<decltype(bad_elem), const double*>::value));
-        EXPECT_EQ(bad_elem, nullptr);
-      }
-      {
-        auto* bad_elem = absl::get_if<3>(&const_v);
-        EXPECT_EQ(bad_elem, nullptr);
-        EXPECT_TRUE((std::is_same<decltype(bad_elem), const int*>::value));
-      }
-    }
-  }
-
-  {
-    Var v = std::string("Hello");
-    EXPECT_TRUE(noexcept(absl::get_if<1>(&v)));
-
-    {
-      auto* elem = absl::get_if<1>(&v);
-      EXPECT_TRUE((std::is_same<decltype(elem), std::string*>::value));
-      ASSERT_NE(elem, nullptr);
-      EXPECT_EQ(*elem, "Hello");
-      {
-        auto* bad_elem = absl::get_if<0>(&v);
-        EXPECT_TRUE((std::is_same<decltype(bad_elem), int*>::value));
-        EXPECT_EQ(bad_elem, nullptr);
-      }
-      {
-        auto* bad_elem = absl::get_if<2>(&v);
-        EXPECT_TRUE((std::is_same<decltype(bad_elem), double*>::value));
-        EXPECT_EQ(bad_elem, nullptr);
-      }
-      {
-        auto* bad_elem = absl::get_if<3>(&v);
-        EXPECT_TRUE((std::is_same<decltype(bad_elem), int*>::value));
-        EXPECT_EQ(bad_elem, nullptr);
-      }
-    }
-
-    const Var& const_v = v;
-    EXPECT_TRUE(noexcept(absl::get_if<1>(&const_v)));
-
-    {
-      auto* elem = absl::get_if<1>(&const_v);
-      EXPECT_TRUE((std::is_same<decltype(elem), const std::string*>::value));
-      ASSERT_NE(elem, nullptr);
-      EXPECT_EQ(*elem, "Hello");
-      {
-        auto* bad_elem = absl::get_if<0>(&const_v);
-        EXPECT_TRUE((std::is_same<decltype(bad_elem), const int*>::value));
-        EXPECT_EQ(bad_elem, nullptr);
-      }
-      {
-        auto* bad_elem = absl::get_if<2>(&const_v);
-        EXPECT_TRUE((std::is_same<decltype(bad_elem), const double*>::value));
-        EXPECT_EQ(bad_elem, nullptr);
-      }
-      {
-        auto* bad_elem = absl::get_if<3>(&const_v);
-        EXPECT_EQ(bad_elem, nullptr);
-        EXPECT_TRUE((std::is_same<decltype(bad_elem), const int*>::value));
-      }
-    }
-  }
-
-  {
-    Var v = 2.0;
-    EXPECT_TRUE(noexcept(absl::get_if<2>(&v)));
-
-    {
-      auto* elem = absl::get_if<2>(&v);
-      EXPECT_TRUE((std::is_same<decltype(elem), double*>::value));
-      ASSERT_NE(elem, nullptr);
-      EXPECT_EQ(*elem, 2.0);
-      {
-        auto* bad_elem = absl::get_if<0>(&v);
-        EXPECT_TRUE((std::is_same<decltype(bad_elem), int*>::value));
-        EXPECT_EQ(bad_elem, nullptr);
-      }
-      {
-        auto* bad_elem = absl::get_if<1>(&v);
-        EXPECT_TRUE((std::is_same<decltype(bad_elem), std::string*>::value));
-        EXPECT_EQ(bad_elem, nullptr);
-      }
-      {
-        auto* bad_elem = absl::get_if<3>(&v);
-        EXPECT_TRUE((std::is_same<decltype(bad_elem), int*>::value));
-        EXPECT_EQ(bad_elem, nullptr);
-      }
-    }
-
-    const Var& const_v = v;
-    EXPECT_TRUE(noexcept(absl::get_if<2>(&const_v)));
-
-    {
-      auto* elem = absl::get_if<2>(&const_v);
-      EXPECT_TRUE((std::is_same<decltype(elem), const double*>::value));
-      ASSERT_NE(elem, nullptr);
-      EXPECT_EQ(*elem, 2.0);
-      {
-        auto* bad_elem = absl::get_if<0>(&const_v);
-        EXPECT_TRUE((std::is_same<decltype(bad_elem), const int*>::value));
-        EXPECT_EQ(bad_elem, nullptr);
-      }
-      {
-        auto* bad_elem = absl::get_if<1>(&const_v);
-        EXPECT_TRUE(
-            (std::is_same<decltype(bad_elem), const std::string*>::value));
-        EXPECT_EQ(bad_elem, nullptr);
-      }
-      {
-        auto* bad_elem = absl::get_if<3>(&const_v);
-        EXPECT_EQ(bad_elem, nullptr);
-        EXPECT_TRUE((std::is_same<decltype(bad_elem), const int*>::value));
-      }
-    }
-  }
-
-  {
-    Var v(absl::in_place_index<0>, 0);
-    v.emplace<3>(1);
-    EXPECT_TRUE(noexcept(absl::get_if<3>(&v)));
-
-    {
-      auto* elem = absl::get_if<3>(&v);
-      EXPECT_TRUE((std::is_same<decltype(elem), int*>::value));
-      ASSERT_NE(elem, nullptr);
-      EXPECT_EQ(*elem, 1);
-      {
-        auto* bad_elem = absl::get_if<0>(&v);
-        EXPECT_TRUE((std::is_same<decltype(bad_elem), int*>::value));
-        EXPECT_EQ(bad_elem, nullptr);
-      }
-      {
-        auto* bad_elem = absl::get_if<1>(&v);
-        EXPECT_TRUE((std::is_same<decltype(bad_elem), std::string*>::value));
-        EXPECT_EQ(bad_elem, nullptr);
-      }
-      {
-        auto* bad_elem = absl::get_if<2>(&v);
-        EXPECT_TRUE((std::is_same<decltype(bad_elem), double*>::value));
-        EXPECT_EQ(bad_elem, nullptr);
-      }
-    }
-
-    const Var& const_v = v;
-    EXPECT_TRUE(noexcept(absl::get_if<3>(&const_v)));
-
-    {
-      auto* elem = absl::get_if<3>(&const_v);
-      EXPECT_TRUE((std::is_same<decltype(elem), const int*>::value));
-      ASSERT_NE(elem, nullptr);
-      EXPECT_EQ(*elem, 1);
-      {
-        auto* bad_elem = absl::get_if<0>(&const_v);
-        EXPECT_TRUE((std::is_same<decltype(bad_elem), const int*>::value));
-        EXPECT_EQ(bad_elem, nullptr);
-      }
-      {
-        auto* bad_elem = absl::get_if<1>(&const_v);
-        EXPECT_TRUE(
-            (std::is_same<decltype(bad_elem), const std::string*>::value));
-        EXPECT_EQ(bad_elem, nullptr);
-      }
-      {
-        auto* bad_elem = absl::get_if<2>(&const_v);
-        EXPECT_EQ(bad_elem, nullptr);
-        EXPECT_TRUE((std::is_same<decltype(bad_elem), const double*>::value));
-      }
-    }
-  }
-}
-
-//////////////////////
-// [variant.relops] //
-//////////////////////
-
-TEST(VariantTest, OperatorEquals) {
-  variant<int, std::string> a(1), b(1);
-  EXPECT_TRUE(a == b);
-  EXPECT_TRUE(b == a);
-  EXPECT_FALSE(a != b);
-  EXPECT_FALSE(b != a);
-
-  b = "str";
-  EXPECT_FALSE(a == b);
-  EXPECT_FALSE(b == a);
-  EXPECT_TRUE(a != b);
-  EXPECT_TRUE(b != a);
-
-  b = 0;
-  EXPECT_FALSE(a == b);
-  EXPECT_FALSE(b == a);
-  EXPECT_TRUE(a != b);
-  EXPECT_TRUE(b != a);
-
-  a = b = "foo";
-  EXPECT_TRUE(a == b);
-  EXPECT_TRUE(b == a);
-  EXPECT_FALSE(a != b);
-  EXPECT_FALSE(b != a);
-
-  a = "bar";
-  EXPECT_FALSE(a == b);
-  EXPECT_FALSE(b == a);
-  EXPECT_TRUE(a != b);
-  EXPECT_TRUE(b != a);
-}
-
-TEST(VariantTest, OperatorRelational) {
-  variant<int, std::string> a(1), b(1);
-  EXPECT_FALSE(a < b);
-  EXPECT_FALSE(b < a);
-  EXPECT_FALSE(a > b);
-  EXPECT_FALSE(b > a);
-  EXPECT_TRUE(a <= b);
-  EXPECT_TRUE(b <= a);
-  EXPECT_TRUE(a >= b);
-  EXPECT_TRUE(b >= a);
-
-  b = "str";
-  EXPECT_TRUE(a < b);
-  EXPECT_FALSE(b < a);
-  EXPECT_FALSE(a > b);
-  EXPECT_TRUE(b > a);
-  EXPECT_TRUE(a <= b);
-  EXPECT_FALSE(b <= a);
-  EXPECT_FALSE(a >= b);
-  EXPECT_TRUE(b >= a);
-
-  b = 0;
-  EXPECT_FALSE(a < b);
-  EXPECT_TRUE(b < a);
-  EXPECT_TRUE(a > b);
-  EXPECT_FALSE(b > a);
-  EXPECT_FALSE(a <= b);
-  EXPECT_TRUE(b <= a);
-  EXPECT_TRUE(a >= b);
-  EXPECT_FALSE(b >= a);
-
-  a = b = "foo";
-  EXPECT_FALSE(a < b);
-  EXPECT_FALSE(b < a);
-  EXPECT_FALSE(a > b);
-  EXPECT_FALSE(b > a);
-  EXPECT_TRUE(a <= b);
-  EXPECT_TRUE(b <= a);
-  EXPECT_TRUE(a >= b);
-  EXPECT_TRUE(b >= a);
-
-  a = "bar";
-  EXPECT_TRUE(a < b);
-  EXPECT_FALSE(b < a);
-  EXPECT_FALSE(a > b);
-  EXPECT_TRUE(b > a);
-  EXPECT_TRUE(a <= b);
-  EXPECT_FALSE(b <= a);
-  EXPECT_FALSE(a >= b);
-  EXPECT_TRUE(b >= a);
-}
-
-#ifdef ABSL_HAVE_EXCEPTIONS
-
-TEST(VariantTest, ValuelessOperatorEquals) {
-  variant<MoveCanThrow, std::string> int_v(1), string_v("Hello"),
-      valueless(absl::in_place_index<0>),
-      other_valueless(absl::in_place_index<0>);
-  ToValuelessByException(valueless);
-  ToValuelessByException(other_valueless);
-
-  EXPECT_TRUE(valueless == other_valueless);
-  EXPECT_TRUE(other_valueless == valueless);
-  EXPECT_FALSE(valueless == int_v);
-  EXPECT_FALSE(valueless == string_v);
-  EXPECT_FALSE(int_v == valueless);
-  EXPECT_FALSE(string_v == valueless);
-
-  EXPECT_FALSE(valueless != other_valueless);
-  EXPECT_FALSE(other_valueless != valueless);
-  EXPECT_TRUE(valueless != int_v);
-  EXPECT_TRUE(valueless != string_v);
-  EXPECT_TRUE(int_v != valueless);
-  EXPECT_TRUE(string_v != valueless);
-}
-
-TEST(VariantTest, ValuelessOperatorRelational) {
-  variant<MoveCanThrow, std::string> int_v(1), string_v("Hello"),
-      valueless(absl::in_place_index<0>),
-      other_valueless(absl::in_place_index<0>);
-  ToValuelessByException(valueless);
-  ToValuelessByException(other_valueless);
-
-  EXPECT_FALSE(valueless < other_valueless);
-  EXPECT_FALSE(other_valueless < valueless);
-  EXPECT_TRUE(valueless < int_v);
-  EXPECT_TRUE(valueless < string_v);
-  EXPECT_FALSE(int_v < valueless);
-  EXPECT_FALSE(string_v < valueless);
-
-  EXPECT_TRUE(valueless <= other_valueless);
-  EXPECT_TRUE(other_valueless <= valueless);
-  EXPECT_TRUE(valueless <= int_v);
-  EXPECT_TRUE(valueless <= string_v);
-  EXPECT_FALSE(int_v <= valueless);
-  EXPECT_FALSE(string_v <= valueless);
-
-  EXPECT_TRUE(valueless >= other_valueless);
-  EXPECT_TRUE(other_valueless >= valueless);
-  EXPECT_FALSE(valueless >= int_v);
-  EXPECT_FALSE(valueless >= string_v);
-  EXPECT_TRUE(int_v >= valueless);
-  EXPECT_TRUE(string_v >= valueless);
-
-  EXPECT_FALSE(valueless > other_valueless);
-  EXPECT_FALSE(other_valueless > valueless);
-  EXPECT_FALSE(valueless > int_v);
-  EXPECT_FALSE(valueless > string_v);
-  EXPECT_TRUE(int_v > valueless);
-  EXPECT_TRUE(string_v > valueless);
-}
-
-#endif
-
-/////////////////////
-// [variant.visit] //
-/////////////////////
-
-template <typename T>
-struct ConvertTo {
-  template <typename U>
-  T operator()(const U& u) const {
-    return u;
-  }
-};
-
-TEST(VariantTest, VisitSimple) {
-  variant<std::string, const char*> v = "A";
-
-  std::string str = absl::visit(ConvertTo<std::string>{}, v);
-  EXPECT_EQ("A", str);
-
-  v = std::string("B");
-
-  absl::string_view piece = absl::visit(ConvertTo<absl::string_view>{}, v);
-  EXPECT_EQ("B", piece);
-
-  struct StrLen {
-    int operator()(const char* s) const { return strlen(s); }
-    int operator()(const std::string& s) const { return s.size(); }
-  };
-
-  v = "SomeStr";
-  EXPECT_EQ(7, absl::visit(StrLen{}, v));
-  v = std::string("VeryLargeThisTime");
-  EXPECT_EQ(17, absl::visit(StrLen{}, v));
-}
-
-TEST(VariantTest, VisitRValue) {
-  variant<std::string> v = std::string("X");
-  struct Visitor {
-    bool operator()(const std::string&) const { return false; }
-    bool operator()(std::string&&) const { return true; }  // NOLINT
-
-    int operator()(const std::string&, const std::string&) const { return 0; }
-    int operator()(const std::string&, std::string&&) const {
-      return 1;
-    }  // NOLINT
-    int operator()(std::string&&, const std::string&) const {
-      return 2;
-    }                                                                 // NOLINT
-    int operator()(std::string&&, std::string&&) const { return 3; }  // NOLINT
-  };
-  EXPECT_FALSE(absl::visit(Visitor{}, v));
-  EXPECT_TRUE(absl::visit(Visitor{}, absl::move(v)));
-
-  // Also test the variadic overload.
-  EXPECT_EQ(0, absl::visit(Visitor{}, v, v));
-  EXPECT_EQ(1, absl::visit(Visitor{}, v, absl::move(v)));
-  EXPECT_EQ(2, absl::visit(Visitor{}, absl::move(v), v));
-  EXPECT_EQ(3, absl::visit(Visitor{}, absl::move(v), absl::move(v)));
-}
-
-TEST(VariantTest, VisitRValueVisitor) {
-  variant<std::string> v = std::string("X");
-  struct Visitor {
-    bool operator()(const std::string&) const& { return false; }
-    bool operator()(const std::string&) && { return true; }
-  };
-  Visitor visitor;
-  EXPECT_FALSE(absl::visit(visitor, v));
-  EXPECT_TRUE(absl::visit(Visitor{}, v));
-}
-
-TEST(VariantTest, VisitResultTypeDifferent) {
-  variant<std::string> v = std::string("X");
-  struct LValue_LValue {};
-  struct RValue_LValue {};
-  struct LValue_RValue {};
-  struct RValue_RValue {};
-  struct Visitor {
-    LValue_LValue operator()(const std::string&) const& { return {}; }
-    RValue_LValue operator()(std::string&&) const& { return {}; }  // NOLINT
-    LValue_RValue operator()(const std::string&) && { return {}; }
-    RValue_RValue operator()(std::string&&) && { return {}; }  // NOLINT
-  } visitor;
-
-  EXPECT_TRUE(
-      (std::is_same<LValue_LValue, decltype(absl::visit(visitor, v))>::value));
-  EXPECT_TRUE(
-      (std::is_same<RValue_LValue,
-                    decltype(absl::visit(visitor, absl::move(v)))>::value));
-  EXPECT_TRUE((
-      std::is_same<LValue_RValue, decltype(absl::visit(Visitor{}, v))>::value));
-  EXPECT_TRUE(
-      (std::is_same<RValue_RValue,
-                    decltype(absl::visit(Visitor{}, absl::move(v)))>::value));
-}
-
-TEST(VariantTest, VisitVariadic) {
-  using A = variant<int, std::string>;
-  using B = variant<std::unique_ptr<int>, absl::string_view>;
-
-  struct Visitor {
-    std::pair<int, int> operator()(int a, std::unique_ptr<int> b) const {
-      return {a, *b};
-    }
-    std::pair<int, int> operator()(absl::string_view a,
-                                   std::unique_ptr<int> b) const {
-      return {static_cast<int>(a.size()), static_cast<int>(*b)};
-    }
-    std::pair<int, int> operator()(int a, absl::string_view b) const {
-      return {a, static_cast<int>(b.size())};
-    }
-    std::pair<int, int> operator()(absl::string_view a,
-                                   absl::string_view b) const {
-      return {static_cast<int>(a.size()), static_cast<int>(b.size())};
-    }
-  };
-
-  EXPECT_THAT(absl::visit(Visitor(), A(1), B(std::unique_ptr<int>(new int(7)))),
-              ::testing::Pair(1, 7));
-  EXPECT_THAT(absl::visit(Visitor(), A(1), B(absl::string_view("ABC"))),
-              ::testing::Pair(1, 3));
-  EXPECT_THAT(absl::visit(Visitor(), A(std::string("BBBBB")),
-                          B(std::unique_ptr<int>(new int(7)))),
-              ::testing::Pair(5, 7));
-  EXPECT_THAT(absl::visit(Visitor(), A(std::string("BBBBB")),
-                          B(absl::string_view("ABC"))),
-              ::testing::Pair(5, 3));
-}
-
-TEST(VariantTest, VisitNoArgs) {
-  EXPECT_EQ(5, absl::visit([] { return 5; }));
-}
-
-struct ConstFunctor {
-  int operator()(int a, int b) const { return a - b; }
-};
-
-struct MutableFunctor {
-  int operator()(int a, int b) { return a - b; }
-};
-
-struct Class {
-  int Method(int a, int b) { return a - b; }
-  int ConstMethod(int a, int b) const { return a - b; }
-
-  int member;
-};
-
-TEST(VariantTest, VisitReferenceWrapper) {
-  ConstFunctor cf;
-  MutableFunctor mf;
-  absl::variant<int> three = 3;
-  absl::variant<int> two = 2;
-
-  EXPECT_EQ(1, absl::visit(std::cref(cf), three, two));
-  EXPECT_EQ(1, absl::visit(std::ref(cf), three, two));
-  EXPECT_EQ(1, absl::visit(std::ref(mf), three, two));
-}
-
-// libstdc++ std::variant doesn't support the INVOKE semantics.
-#if !(defined(ABSL_HAVE_STD_VARIANT) && defined(__GLIBCXX__))
-TEST(VariantTest, VisitMemberFunction) {
-  absl::variant<std::unique_ptr<Class>> p(absl::make_unique<Class>());
-  absl::variant<std::unique_ptr<const Class>> cp(
-      absl::make_unique<const Class>());
-  absl::variant<int> three = 3;
-  absl::variant<int> two = 2;
-
-  EXPECT_EQ(1, absl::visit(&Class::Method, p, three, two));
-  EXPECT_EQ(1, absl::visit(&Class::ConstMethod, p, three, two));
-  EXPECT_EQ(1, absl::visit(&Class::ConstMethod, cp, three, two));
-}
-
-TEST(VariantTest, VisitDataMember) {
-  absl::variant<std::unique_ptr<Class>> p(absl::make_unique<Class>(Class{42}));
-  absl::variant<std::unique_ptr<const Class>> cp(
-      absl::make_unique<const Class>(Class{42}));
-  EXPECT_EQ(42, absl::visit(&Class::member, p));
-
-  absl::visit(&Class::member, p) = 5;
-  EXPECT_EQ(5, absl::visit(&Class::member, p));
-
-  EXPECT_EQ(42, absl::visit(&Class::member, cp));
-}
-#endif  // !(defined(ABSL_HAVE_STD_VARIANT) && defined(__GLIBCXX__))
-
-/////////////////////////
-// [variant.monostate] //
-/////////////////////////
-
-TEST(VariantTest, MonostateBasic) {
-  absl::monostate mono;
-  (void)mono;
-
-  // TODO(mattcalabrese) Expose move triviality metafunctions in absl.
-  EXPECT_TRUE(absl::is_trivially_default_constructible<absl::monostate>::value);
-  EXPECT_TRUE(is_trivially_move_constructible<absl::monostate>::value);
-  EXPECT_TRUE(absl::is_trivially_copy_constructible<absl::monostate>::value);
-  EXPECT_TRUE(is_trivially_move_assignable<absl::monostate>::value);
-  EXPECT_TRUE(absl::is_trivially_copy_assignable<absl::monostate>::value);
-  EXPECT_TRUE(absl::is_trivially_destructible<absl::monostate>::value);
-}
-
-TEST(VariantTest, VariantMonostateDefaultConstruction) {
-  absl::variant<absl::monostate, NonDefaultConstructible> var;
-  EXPECT_EQ(var.index(), 0);
-}
-
-////////////////////////////////
-// [variant.monostate.relops] //
-////////////////////////////////
-
-TEST(VariantTest, MonostateComparisons) {
-  absl::monostate lhs, rhs;
-
-  EXPECT_EQ(lhs, lhs);
-  EXPECT_EQ(lhs, rhs);
-
-  EXPECT_FALSE(lhs != lhs);
-  EXPECT_FALSE(lhs != rhs);
-  EXPECT_FALSE(lhs < lhs);
-  EXPECT_FALSE(lhs < rhs);
-  EXPECT_FALSE(lhs > lhs);
-  EXPECT_FALSE(lhs > rhs);
-
-  EXPECT_LE(lhs, lhs);
-  EXPECT_LE(lhs, rhs);
-  EXPECT_GE(lhs, lhs);
-  EXPECT_GE(lhs, rhs);
-
-  EXPECT_TRUE(noexcept(std::declval<absl::monostate>() ==
-                       std::declval<absl::monostate>()));
-  EXPECT_TRUE(noexcept(std::declval<absl::monostate>() !=
-                       std::declval<absl::monostate>()));
-  EXPECT_TRUE(noexcept(std::declval<absl::monostate>() <
-                       std::declval<absl::monostate>()));
-  EXPECT_TRUE(noexcept(std::declval<absl::monostate>() >
-                       std::declval<absl::monostate>()));
-  EXPECT_TRUE(noexcept(std::declval<absl::monostate>() <=
-                       std::declval<absl::monostate>()));
-  EXPECT_TRUE(noexcept(std::declval<absl::monostate>() >=
-                       std::declval<absl::monostate>()));
-}
-
-///////////////////////
-// [variant.specalg] //
-///////////////////////
-
-TEST(VariantTest, NonmemberSwap) {
-  using std::swap;
-
-  SpecialSwap v1(3);
-  SpecialSwap v2(7);
-
-  variant<SpecialSwap> a = v1, b = v2;
-
-  EXPECT_THAT(a, VariantWith<SpecialSwap>(v1));
-  EXPECT_THAT(b, VariantWith<SpecialSwap>(v2));
-
-  std::swap(a, b);
-  EXPECT_THAT(a, VariantWith<SpecialSwap>(v2));
-  EXPECT_THAT(b, VariantWith<SpecialSwap>(v1));
-#ifndef ABSL_HAVE_STD_VARIANT
-  EXPECT_FALSE(absl::get<SpecialSwap>(a).special_swap);
-#endif
-
-  swap(a, b);
-  EXPECT_THAT(a, VariantWith<SpecialSwap>(v1));
-  EXPECT_THAT(b, VariantWith<SpecialSwap>(v2));
-  EXPECT_TRUE(absl::get<SpecialSwap>(b).special_swap);
-}
-
-//////////////////////////
-// [variant.bad.access] //
-//////////////////////////
-
-TEST(VariantTest, BadAccess) {
-  EXPECT_TRUE(noexcept(absl::bad_variant_access()));
-  absl::bad_variant_access exception_obj;
-  std::exception* base = &exception_obj;
-  (void)base;
-}
-
-////////////////////
-// [variant.hash] //
-////////////////////
-
-TEST(VariantTest, MonostateHash) {
-  absl::monostate mono, other_mono;
-  std::hash<absl::monostate> const hasher{};
-  static_assert(std::is_same<decltype(hasher(mono)), std::size_t>::value, "");
-  EXPECT_EQ(hasher(mono), hasher(other_mono));
-}
-
-TEST(VariantTest, Hash) {
-  static_assert(type_traits_internal::IsHashable<variant<int>>::value, "");
-  static_assert(type_traits_internal::IsHashable<variant<Hashable>>::value, "");
-  static_assert(type_traits_internal::IsHashable<variant<int, Hashable>>::value,
-                "");
-
-#if ABSL_META_INTERNAL_STD_HASH_SFINAE_FRIENDLY_
-  static_assert(!type_traits_internal::IsHashable<variant<NonHashable>>::value,
-                "");
-  static_assert(
-      !type_traits_internal::IsHashable<variant<Hashable, NonHashable>>::value,
-      "");
-#endif
-
-// MSVC std::hash<std::variant> does not use the index, thus produce the same
-// result on the same value as different alternative.
-#if !(defined(_MSC_VER) && defined(ABSL_HAVE_STD_VARIANT))
-  {
-    // same value as different alternative
-    variant<int, int> v0(in_place_index<0>, 42);
-    variant<int, int> v1(in_place_index<1>, 42);
-    std::hash<variant<int, int>> hash;
-    EXPECT_NE(hash(v0), hash(v1));
-  }
-#endif  // !(defined(_MSC_VER) && defined(ABSL_HAVE_STD_VARIANT))
-
-  {
-    std::hash<variant<int>> hash;
-    std::set<size_t> hashcodes;
-    for (int i = 0; i < 100; ++i) {
-      hashcodes.insert(hash(i));
-    }
-    EXPECT_GT(hashcodes.size(), 90);
-
-    // test const-qualified
-    static_assert(type_traits_internal::IsHashable<variant<const int>>::value,
-                  "");
-    static_assert(
-        type_traits_internal::IsHashable<variant<const Hashable>>::value, "");
-    std::hash<absl::variant<const int>> c_hash;
-    for (int i = 0; i < 100; ++i) {
-      EXPECT_EQ(hash(i), c_hash(i));
-    }
-  }
-}
-
-////////////////////////////////////////
-// Miscellaneous and deprecated tests //
-////////////////////////////////////////
-
-// Test that a set requiring a basic type conversion works correctly
-#if !defined(ABSL_HAVE_STD_VARIANT)
-TEST(VariantTest, TestConvertingSet) {
-  typedef variant<double> Variant;
-  Variant v(1.0);
-  const int two = 2;
-  v = two;
-  EXPECT_TRUE(absl::holds_alternative<double>(v));
-  ASSERT_TRUE(nullptr != absl::get_if<double>(&v));
-  EXPECT_DOUBLE_EQ(2, absl::get<double>(v));
-}
-#endif  // ABSL_HAVE_STD_VARIANT
-
-// Test that a vector of variants behaves reasonably.
-TEST(VariantTest, Container) {
-  typedef variant<int, float> Variant;
-
-  // Creation of vector should work
-  std::vector<Variant> vec;
-  vec.push_back(Variant(10));
-  vec.push_back(Variant(20.0f));
-
-  // Vector resizing should work if we supply a value for new slots
-  vec.resize(10, Variant(0));
-}
-
-// Test that a variant with a non-copyable type can be constructed and
-// manipulated to some degree.
-TEST(VariantTest, TestVariantWithNonCopyableType) {
-  typedef variant<int, NonCopyable> Variant;
-  const int kValue = 1;
-  Variant v(kValue);
-  ASSERT_TRUE(absl::holds_alternative<int>(v));
-  EXPECT_EQ(kValue, absl::get<int>(v));
-}
-
-// Test that a variant with a non-copyable type can be transformed to
-// the non-copyable type with a call to `emplace` for different numbers
-// of arguments. We do not need to test this for each of T1 ... T8
-// because `emplace` does not overload on T1 ... to T8, so if this
-// works for any one of T1 ... T8, then it works for all of them. We
-// do need to test that it works with varying numbers of parameters
-// though.
-TEST(VariantTest, TestEmplace) {
-  typedef variant<int, NonCopyable> Variant;
-  const int kValue = 1;
-  Variant v(kValue);
-  ASSERT_TRUE(absl::holds_alternative<int>(v));
-  EXPECT_EQ(kValue, absl::get<int>(v));
-
-  // emplace with zero arguments, then back to 'int'
-  v.emplace<NonCopyable>();
-  ASSERT_TRUE(absl::holds_alternative<NonCopyable>(v));
-  EXPECT_EQ(0, absl::get<NonCopyable>(v).value);
-  v = kValue;
-  ASSERT_TRUE(absl::holds_alternative<int>(v));
-
-  // emplace with one argument:
-  v.emplace<NonCopyable>(1);
-  ASSERT_TRUE(absl::holds_alternative<NonCopyable>(v));
-  EXPECT_EQ(1, absl::get<NonCopyable>(v).value);
-  v = kValue;
-  ASSERT_TRUE(absl::holds_alternative<int>(v));
-
-  // emplace with two arguments:
-  v.emplace<NonCopyable>(1, 2);
-  ASSERT_TRUE(absl::holds_alternative<NonCopyable>(v));
-  EXPECT_EQ(3, absl::get<NonCopyable>(v).value);
-  v = kValue;
-  ASSERT_TRUE(absl::holds_alternative<int>(v));
-
-  // emplace with three arguments
-  v.emplace<NonCopyable>(1, 2, 3);
-  ASSERT_TRUE(absl::holds_alternative<NonCopyable>(v));
-  EXPECT_EQ(6, absl::get<NonCopyable>(v).value);
-  v = kValue;
-  ASSERT_TRUE(absl::holds_alternative<int>(v));
-
-  // emplace with four arguments
-  v.emplace<NonCopyable>(1, 2, 3, 4);
-  ASSERT_TRUE(absl::holds_alternative<NonCopyable>(v));
-  EXPECT_EQ(10, absl::get<NonCopyable>(v).value);
-  v = kValue;
-  ASSERT_TRUE(absl::holds_alternative<int>(v));
-}
-
-TEST(VariantTest, TestEmplaceDestroysCurrentValue) {
-  typedef variant<int, IncrementInDtor, NonCopyable> Variant;
-  int counter = 0;
-  Variant v(0);
-  ASSERT_TRUE(absl::holds_alternative<int>(v));
-  v.emplace<IncrementInDtor>(&counter);
-  ASSERT_TRUE(absl::holds_alternative<IncrementInDtor>(v));
-  ASSERT_EQ(0, counter);
-  v.emplace<NonCopyable>();
-  ASSERT_TRUE(absl::holds_alternative<NonCopyable>(v));
-  EXPECT_EQ(1, counter);
-}
-
-TEST(VariantTest, TestMoveSemantics) {
-  typedef variant<std::unique_ptr<int>, std::unique_ptr<std::string>> Variant;
-
-  // Construct a variant by moving from an element value.
-  Variant v(absl::WrapUnique(new int(10)));
-  EXPECT_TRUE(absl::holds_alternative<std::unique_ptr<int>>(v));
-
-  // Construct a variant by moving from another variant.
-  Variant v2(absl::move(v));
-  ASSERT_TRUE(absl::holds_alternative<std::unique_ptr<int>>(v2));
-  ASSERT_NE(nullptr, absl::get<std::unique_ptr<int>>(v2));
-  EXPECT_EQ(10, *absl::get<std::unique_ptr<int>>(v2));
-
-  // Moving from a variant object leaves it holding moved-from value of the
-  // same element type.
-  EXPECT_TRUE(absl::holds_alternative<std::unique_ptr<int>>(v));
-  ASSERT_NE(nullptr, absl::get_if<std::unique_ptr<int>>(&v));
-  EXPECT_EQ(nullptr, absl::get<std::unique_ptr<int>>(v));
-
-  // Assign a variant from an element value by move.
-  v = absl::make_unique<std::string>("foo");
-  ASSERT_TRUE(absl::holds_alternative<std::unique_ptr<std::string>>(v));
-  EXPECT_EQ("foo", *absl::get<std::unique_ptr<std::string>>(v));
-
-  // Move-assign a variant.
-  v2 = absl::move(v);
-  ASSERT_TRUE(absl::holds_alternative<std::unique_ptr<std::string>>(v2));
-  EXPECT_EQ("foo", *absl::get<std::unique_ptr<std::string>>(v2));
-  EXPECT_TRUE(absl::holds_alternative<std::unique_ptr<std::string>>(v));
-}
-
-variant<int, std::string> PassThrough(const variant<int, std::string>& arg) {
-  return arg;
-}
-
-TEST(VariantTest, TestImplicitConversion) {
-  EXPECT_TRUE(absl::holds_alternative<int>(PassThrough(0)));
-
-  // We still need the explicit cast for std::string, because C++ won't apply
-  // two user-defined implicit conversions in a row.
-  EXPECT_TRUE(
-      absl::holds_alternative<std::string>(PassThrough(std::string("foo"))));
-}
-
 struct Convertible2;
 struct Convertible1 {
   Convertible1() {}
@@ -2280,435 +48,117 @@ struct Convertible2 {
 };
 
 TEST(VariantTest, TestRvalueConversion) {
-#if !defined(ABSL_HAVE_STD_VARIANT)
-  variant<double, std::string> var(
-      ConvertVariantTo<variant<double, std::string>>(
-          variant<std::string, int>(0)));
-  ASSERT_TRUE(absl::holds_alternative<double>(var));
-  EXPECT_EQ(0.0, absl::get<double>(var));
-
-  var = ConvertVariantTo<variant<double, std::string>>(
-      variant<const char*, float>("foo"));
-  ASSERT_TRUE(absl::holds_alternative<std::string>(var));
-  EXPECT_EQ("foo", absl::get<std::string>(var));
+  std::variant<Convertible1, Convertible2> v(
+      ConvertVariantTo<std::variant<Convertible1, Convertible2>>(
+          (std::variant<Convertible2, Convertible1>(Convertible1()))));
+  ASSERT_TRUE(std::holds_alternative<Convertible1>(v));
 
-  variant<double> singleton(
-      ConvertVariantTo<variant<double>>(variant<int, float>(42)));
-  ASSERT_TRUE(absl::holds_alternative<double>(singleton));
-  EXPECT_EQ(42.0, absl::get<double>(singleton));
-
-  singleton = ConvertVariantTo<variant<double>>(variant<int, float>(3.14f));
-  ASSERT_TRUE(absl::holds_alternative<double>(singleton));
-  EXPECT_FLOAT_EQ(3.14f, static_cast<float>(absl::get<double>(singleton)));
-
-  singleton = ConvertVariantTo<variant<double>>(variant<int>(0));
-  ASSERT_TRUE(absl::holds_alternative<double>(singleton));
-  EXPECT_EQ(0.0, absl::get<double>(singleton));
-
-  variant<int32_t, uint32_t> variant2(
-      ConvertVariantTo<variant<int32_t, uint32_t>>(variant<int32_t>(42)));
-  ASSERT_TRUE(absl::holds_alternative<int32_t>(variant2));
-  EXPECT_EQ(42, absl::get<int32_t>(variant2));
-
-  variant2 = ConvertVariantTo<variant<int32_t, uint32_t>>(variant<uint32_t>(42));
-  ASSERT_TRUE(absl::holds_alternative<uint32_t>(variant2));
-  EXPECT_EQ(42, absl::get<uint32_t>(variant2));
-#endif  // !ABSL_HAVE_STD_VARIANT
-
-  variant<Convertible1, Convertible2> variant3(
-      ConvertVariantTo<variant<Convertible1, Convertible2>>(
-          (variant<Convertible2, Convertible1>(Convertible1()))));
-  ASSERT_TRUE(absl::holds_alternative<Convertible1>(variant3));
-
-  variant3 = ConvertVariantTo<variant<Convertible1, Convertible2>>(
-      variant<Convertible2, Convertible1>(Convertible2()));
-  ASSERT_TRUE(absl::holds_alternative<Convertible2>(variant3));
+  v = ConvertVariantTo<std::variant<Convertible1, Convertible2>>(
+      std::variant<Convertible2, Convertible1>(Convertible2()));
+  ASSERT_TRUE(std::holds_alternative<Convertible2>(v));
 }
 
 TEST(VariantTest, TestLvalueConversion) {
-#if !defined(ABSL_HAVE_STD_VARIANT)
-  variant<std::string, int> source1 = 0;
-  variant<double, std::string> destination(
-      ConvertVariantTo<variant<double, std::string>>(source1));
-  ASSERT_TRUE(absl::holds_alternative<double>(destination));
-  EXPECT_EQ(0.0, absl::get<double>(destination));
-
-  variant<const char*, float> source2 = "foo";
-  destination = ConvertVariantTo<variant<double, std::string>>(source2);
-  ASSERT_TRUE(absl::holds_alternative<std::string>(destination));
-  EXPECT_EQ("foo", absl::get<std::string>(destination));
-
-  variant<int, float> source3(42);
-  variant<double> singleton(ConvertVariantTo<variant<double>>(source3));
-  ASSERT_TRUE(absl::holds_alternative<double>(singleton));
-  EXPECT_EQ(42.0, absl::get<double>(singleton));
+  std::variant<Convertible2, Convertible1> source((Convertible1()));
+  std::variant<Convertible1, Convertible2> v(
+      ConvertVariantTo<std::variant<Convertible1, Convertible2>>(source));
+  ASSERT_TRUE(std::holds_alternative<Convertible1>(v));
 
-  source3 = 3.14f;
-  singleton = ConvertVariantTo<variant<double>>(source3);
-  ASSERT_TRUE(absl::holds_alternative<double>(singleton));
-  EXPECT_FLOAT_EQ(3.14f, static_cast<float>(absl::get<double>(singleton)));
-
-  variant<int> source4(0);
-  singleton = ConvertVariantTo<variant<double>>(source4);
-  ASSERT_TRUE(absl::holds_alternative<double>(singleton));
-  EXPECT_EQ(0.0, absl::get<double>(singleton));
-
-  variant<int32_t> source5(42);
-  variant<int32_t, uint32_t> variant2(
-      ConvertVariantTo<variant<int32_t, uint32_t>>(source5));
-  ASSERT_TRUE(absl::holds_alternative<int32_t>(variant2));
-  EXPECT_EQ(42, absl::get<int32_t>(variant2));
-
-  variant<uint32_t> source6(42);
-  variant2 = ConvertVariantTo<variant<int32_t, uint32_t>>(source6);
-  ASSERT_TRUE(absl::holds_alternative<uint32_t>(variant2));
-  EXPECT_EQ(42, absl::get<uint32_t>(variant2));
-#endif
-
-  variant<Convertible2, Convertible1> source7((Convertible1()));
-  variant<Convertible1, Convertible2> variant3(
-      ConvertVariantTo<variant<Convertible1, Convertible2>>(source7));
-  ASSERT_TRUE(absl::holds_alternative<Convertible1>(variant3));
-
-  source7 = Convertible2();
-  variant3 = ConvertVariantTo<variant<Convertible1, Convertible2>>(source7);
-  ASSERT_TRUE(absl::holds_alternative<Convertible2>(variant3));
+  source = Convertible2();
+  v = ConvertVariantTo<std::variant<Convertible1, Convertible2>>(source);
+  ASSERT_TRUE(std::holds_alternative<Convertible2>(v));
 }
 
 TEST(VariantTest, TestMoveConversion) {
-  using Variant =
-      variant<std::unique_ptr<const int>, std::unique_ptr<const std::string>>;
+  using Variant = std::variant<std::unique_ptr<const int>,
+                               std::unique_ptr<const std::string>>;
   using OtherVariant =
-      variant<std::unique_ptr<int>, std::unique_ptr<std::string>>;
+      std::variant<std::unique_ptr<int>, std::unique_ptr<std::string>>;
 
   Variant var(
-      ConvertVariantTo<Variant>(OtherVariant{absl::make_unique<int>(0)}));
-  ASSERT_TRUE(absl::holds_alternative<std::unique_ptr<const int>>(var));
-  ASSERT_NE(absl::get<std::unique_ptr<const int>>(var), nullptr);
-  EXPECT_EQ(0, *absl::get<std::unique_ptr<const int>>(var));
+      ConvertVariantTo<Variant>(OtherVariant{std::make_unique<int>(0)}));
+  ASSERT_TRUE(std::holds_alternative<std::unique_ptr<const int>>(var));
+  ASSERT_NE(std::get<std::unique_ptr<const int>>(var), nullptr);
+  EXPECT_EQ(0, *std::get<std::unique_ptr<const int>>(var));
 
   var = ConvertVariantTo<Variant>(
-      OtherVariant(absl::make_unique<std::string>("foo")));
-  ASSERT_TRUE(absl::holds_alternative<std::unique_ptr<const std::string>>(var));
-  EXPECT_EQ("foo", *absl::get<std::unique_ptr<const std::string>>(var));
+      OtherVariant(std::make_unique<std::string>("foo")));
+  ASSERT_TRUE(std::holds_alternative<std::unique_ptr<const std::string>>(var));
+  EXPECT_EQ("foo", *std::get<std::unique_ptr<const std::string>>(var));
 }
 
 TEST(VariantTest, DoesNotMoveFromLvalues) {
   // We use shared_ptr here because it's both copyable and movable, and
   // a moved-from shared_ptr is guaranteed to be null, so we can detect
   // whether moving or copying has occurred.
-  using Variant =
-      variant<std::shared_ptr<const int>, std::shared_ptr<const std::string>>;
+  using Variant = std::variant<std::shared_ptr<const int>,
+                               std::shared_ptr<const std::string>>;
   using OtherVariant =
-      variant<std::shared_ptr<int>, std::shared_ptr<std::string>>;
+      std::variant<std::shared_ptr<int>, std::shared_ptr<std::string>>;
 
   Variant v1(std::make_shared<const int>(0));
 
   // Test copy constructor
   Variant v2(v1);
-  EXPECT_EQ(absl::get<std::shared_ptr<const int>>(v1),
-            absl::get<std::shared_ptr<const int>>(v2));
+  EXPECT_EQ(std::get<std::shared_ptr<const int>>(v1),
+            std::get<std::shared_ptr<const int>>(v2));
 
   // Test copy-assignment operator
   v1 = std::make_shared<const std::string>("foo");
   v2 = v1;
-  EXPECT_EQ(absl::get<std::shared_ptr<const std::string>>(v1),
-            absl::get<std::shared_ptr<const std::string>>(v2));
+  EXPECT_EQ(std::get<std::shared_ptr<const std::string>>(v1),
+            std::get<std::shared_ptr<const std::string>>(v2));
 
   // Test converting copy constructor
   OtherVariant other(std::make_shared<int>(0));
   Variant v3(ConvertVariantTo<Variant>(other));
-  EXPECT_EQ(absl::get<std::shared_ptr<int>>(other),
-            absl::get<std::shared_ptr<const int>>(v3));
+  EXPECT_EQ(std::get<std::shared_ptr<int>>(other),
+            std::get<std::shared_ptr<const int>>(v3));
 
   other = std::make_shared<std::string>("foo");
   v3 = ConvertVariantTo<Variant>(other);
-  EXPECT_EQ(absl::get<std::shared_ptr<std::string>>(other),
-            absl::get<std::shared_ptr<const std::string>>(v3));
+  EXPECT_EQ(std::get<std::shared_ptr<std::string>>(other),
+            std::get<std::shared_ptr<const std::string>>(v3));
 }
 
 TEST(VariantTest, TestRvalueConversionViaConvertVariantTo) {
-#if !defined(ABSL_HAVE_STD_VARIANT)
-  variant<double, std::string> var(
-      ConvertVariantTo<variant<double, std::string>>(
-          variant<std::string, int>(3)));
-  EXPECT_THAT(absl::get_if<double>(&var), Pointee(3.0));
-
-  var = ConvertVariantTo<variant<double, std::string>>(
-      variant<const char*, float>("foo"));
-  EXPECT_THAT(absl::get_if<std::string>(&var), Pointee(std::string("foo")));
-
-  variant<double> singleton(
-      ConvertVariantTo<variant<double>>(variant<int, float>(42)));
-  EXPECT_THAT(absl::get_if<double>(&singleton), Pointee(42.0));
+  std::variant<Convertible1, Convertible2> v(
+      ConvertVariantTo<std::variant<Convertible1, Convertible2>>(
+          (std::variant<Convertible2, Convertible1>(Convertible1()))));
+  ASSERT_TRUE(std::holds_alternative<Convertible1>(v));
 
-  singleton = ConvertVariantTo<variant<double>>(variant<int, float>(3.14f));
-  EXPECT_THAT(absl::get_if<double>(&singleton), Pointee(DoubleEq(3.14f)));
-
-  singleton = ConvertVariantTo<variant<double>>(variant<int>(3));
-  EXPECT_THAT(absl::get_if<double>(&singleton), Pointee(3.0));
-
-  variant<int32_t, uint32_t> variant2(
-      ConvertVariantTo<variant<int32_t, uint32_t>>(variant<int32_t>(42)));
-  EXPECT_THAT(absl::get_if<int32_t>(&variant2), Pointee(42));
-
-  variant2 = ConvertVariantTo<variant<int32_t, uint32_t>>(variant<uint32_t>(42));
-  EXPECT_THAT(absl::get_if<uint32_t>(&variant2), Pointee(42));
-#endif
-
-  variant<Convertible1, Convertible2> variant3(
-      ConvertVariantTo<variant<Convertible1, Convertible2>>(
-          (variant<Convertible2, Convertible1>(Convertible1()))));
-  ASSERT_TRUE(absl::holds_alternative<Convertible1>(variant3));
-
-  variant3 = ConvertVariantTo<variant<Convertible1, Convertible2>>(
-      variant<Convertible2, Convertible1>(Convertible2()));
-  ASSERT_TRUE(absl::holds_alternative<Convertible2>(variant3));
+  v = ConvertVariantTo<std::variant<Convertible1, Convertible2>>(
+      std::variant<Convertible2, Convertible1>(Convertible2()));
+  ASSERT_TRUE(std::holds_alternative<Convertible2>(v));
 }
 
 TEST(VariantTest, TestLvalueConversionViaConvertVariantTo) {
-#if !defined(ABSL_HAVE_STD_VARIANT)
-  variant<std::string, int> source1 = 3;
-  variant<double, std::string> destination(
-      ConvertVariantTo<variant<double, std::string>>(source1));
-  EXPECT_THAT(absl::get_if<double>(&destination), Pointee(3.0));
-
-  variant<const char*, float> source2 = "foo";
-  destination = ConvertVariantTo<variant<double, std::string>>(source2);
-  EXPECT_THAT(absl::get_if<std::string>(&destination),
-              Pointee(std::string("foo")));
-
-  variant<int, float> source3(42);
-  variant<double> singleton(ConvertVariantTo<variant<double>>(source3));
-  EXPECT_THAT(absl::get_if<double>(&singleton), Pointee(42.0));
-
-  source3 = 3.14f;
-  singleton = ConvertVariantTo<variant<double>>(source3);
-  EXPECT_FLOAT_EQ(3.14f, static_cast<float>(absl::get<double>(singleton)));
-  EXPECT_THAT(absl::get_if<double>(&singleton), Pointee(DoubleEq(3.14f)));
+  std::variant<Convertible2, Convertible1> source((Convertible1()));
+  std::variant<Convertible1, Convertible2> v(
+      ConvertVariantTo<std::variant<Convertible1, Convertible2>>(source));
+  ASSERT_TRUE(std::holds_alternative<Convertible1>(v));
 
-  variant<int> source4(3);
-  singleton = ConvertVariantTo<variant<double>>(source4);
-  EXPECT_THAT(absl::get_if<double>(&singleton), Pointee(3.0));
-
-  variant<int32_t> source5(42);
-  variant<int32_t, uint32_t> variant2(
-      ConvertVariantTo<variant<int32_t, uint32_t>>(source5));
-  EXPECT_THAT(absl::get_if<int32_t>(&variant2), Pointee(42));
-
-  variant<uint32_t> source6(42);
-  variant2 = ConvertVariantTo<variant<int32_t, uint32_t>>(source6);
-  EXPECT_THAT(absl::get_if<uint32_t>(&variant2), Pointee(42));
-#endif  // !ABSL_HAVE_STD_VARIANT
-
-  variant<Convertible2, Convertible1> source7((Convertible1()));
-  variant<Convertible1, Convertible2> variant3(
-      ConvertVariantTo<variant<Convertible1, Convertible2>>(source7));
-  ASSERT_TRUE(absl::holds_alternative<Convertible1>(variant3));
-
-  source7 = Convertible2();
-  variant3 = ConvertVariantTo<variant<Convertible1, Convertible2>>(source7);
-  ASSERT_TRUE(absl::holds_alternative<Convertible2>(variant3));
+  source = Convertible2();
+  v = ConvertVariantTo<std::variant<Convertible1, Convertible2>>(source);
+  ASSERT_TRUE(std::holds_alternative<Convertible2>(v));
 }
 
 TEST(VariantTest, TestMoveConversionViaConvertVariantTo) {
-  using Variant =
-      variant<std::unique_ptr<const int>, std::unique_ptr<const std::string>>;
+  using Variant = std::variant<std::unique_ptr<const int>,
+                               std::unique_ptr<const std::string>>;
   using OtherVariant =
-      variant<std::unique_ptr<int>, std::unique_ptr<std::string>>;
+      std::variant<std::unique_ptr<int>, std::unique_ptr<std::string>>;
 
   Variant var(
-      ConvertVariantTo<Variant>(OtherVariant{absl::make_unique<int>(3)}));
-  EXPECT_THAT(absl::get_if<std::unique_ptr<const int>>(&var),
+      ConvertVariantTo<Variant>(OtherVariant{std::make_unique<int>(3)}));
+  EXPECT_THAT(std::get_if<std::unique_ptr<const int>>(&var),
               Pointee(Pointee(3)));
 
   var = ConvertVariantTo<Variant>(
-      OtherVariant(absl::make_unique<std::string>("foo")));
-  EXPECT_THAT(absl::get_if<std::unique_ptr<const std::string>>(&var),
+      OtherVariant(std::make_unique<std::string>("foo")));
+  EXPECT_THAT(std::get_if<std::unique_ptr<const std::string>>(&var),
               Pointee(Pointee(std::string("foo"))));
 }
 
-// If all alternatives are trivially copy/move constructible, variant should
-// also be trivially copy/move constructible. This is not required by the
-// standard and we know that libstdc++ variant doesn't have this feature.
-// For more details see the paper:
-// http://open-std.org/JTC1/SC22/WG21/docs/papers/2017/p0602r0.html
-#if !(defined(ABSL_HAVE_STD_VARIANT) && defined(__GLIBCXX__))
-#define ABSL_VARIANT_PROPAGATE_COPY_MOVE_TRIVIALITY 1
-#endif
-
-TEST(VariantTest, TestCopyAndMoveTypeTraits) {
-  EXPECT_TRUE(std::is_copy_constructible<variant<std::string>>::value);
-  EXPECT_TRUE(absl::is_copy_assignable<variant<std::string>>::value);
-  EXPECT_TRUE(std::is_move_constructible<variant<std::string>>::value);
-  EXPECT_TRUE(absl::is_move_assignable<variant<std::string>>::value);
-  EXPECT_TRUE(std::is_move_constructible<variant<std::unique_ptr<int>>>::value);
-  EXPECT_TRUE(absl::is_move_assignable<variant<std::unique_ptr<int>>>::value);
-  EXPECT_FALSE(
-      std::is_copy_constructible<variant<std::unique_ptr<int>>>::value);
-  EXPECT_FALSE(absl::is_copy_assignable<variant<std::unique_ptr<int>>>::value);
-
-  EXPECT_FALSE(
-      absl::is_trivially_copy_constructible<variant<std::string>>::value);
-  EXPECT_FALSE(absl::is_trivially_copy_assignable<variant<std::string>>::value);
-#if ABSL_VARIANT_PROPAGATE_COPY_MOVE_TRIVIALITY
-  EXPECT_TRUE(absl::is_trivially_copy_constructible<variant<int>>::value);
-  EXPECT_TRUE(absl::is_trivially_copy_assignable<variant<int>>::value);
-  EXPECT_TRUE(is_trivially_move_constructible<variant<int>>::value);
-  EXPECT_TRUE(is_trivially_move_assignable<variant<int>>::value);
-#endif  // ABSL_VARIANT_PROPAGATE_COPY_MOVE_TRIVIALITY
-}
-
-TEST(VariantTest, TestVectorOfMoveonlyVariant) {
-  // Verify that variant<MoveonlyType> works correctly as a std::vector element.
-  std::vector<variant<std::unique_ptr<int>, std::string>> vec;
-  vec.push_back(absl::make_unique<int>(42));
-  vec.emplace_back("Hello");
-  vec.reserve(3);
-  auto another_vec = absl::move(vec);
-  // As a sanity check, verify vector contents.
-  ASSERT_EQ(2, another_vec.size());
-  EXPECT_EQ(42, *absl::get<std::unique_ptr<int>>(another_vec[0]));
-  EXPECT_EQ("Hello", absl::get<std::string>(another_vec[1]));
-}
-
-TEST(VariantTest, NestedVariant) {
-#if ABSL_VARIANT_PROPAGATE_COPY_MOVE_TRIVIALITY
-  static_assert(absl::is_trivially_copy_constructible<variant<int>>(), "");
-  static_assert(absl::is_trivially_copy_assignable<variant<int>>(), "");
-  static_assert(is_trivially_move_constructible<variant<int>>(), "");
-  static_assert(is_trivially_move_assignable<variant<int>>(), "");
-
-  static_assert(absl::is_trivially_copy_constructible<variant<variant<int>>>(),
-                "");
-  static_assert(absl::is_trivially_copy_assignable<variant<variant<int>>>(),
-                "");
-  static_assert(is_trivially_move_constructible<variant<variant<int>>>(), "");
-  static_assert(is_trivially_move_assignable<variant<variant<int>>>(), "");
-#endif  // ABSL_VARIANT_PROPAGATE_COPY_MOVE_TRIVIALITY
-
-  variant<int> x(42);
-  variant<variant<int>> y(x);
-  variant<variant<int>> z(y);
-  EXPECT_TRUE(absl::holds_alternative<variant<int>>(z));
-  EXPECT_EQ(x, absl::get<variant<int>>(z));
-}
-
-struct TriviallyDestructible {
-  TriviallyDestructible(TriviallyDestructible&&) {}
-  TriviallyDestructible(const TriviallyDestructible&) {}
-  TriviallyDestructible& operator=(TriviallyDestructible&&) { return *this; }
-  TriviallyDestructible& operator=(const TriviallyDestructible&) {
-    return *this;
-  }
-};
-
-struct TriviallyMovable {
-  TriviallyMovable(TriviallyMovable&&) = default;
-  TriviallyMovable(TriviallyMovable const&) {}
-  TriviallyMovable& operator=(const TriviallyMovable&) { return *this; }
-};
-
-struct TriviallyCopyable {
-  TriviallyCopyable(const TriviallyCopyable&) = default;
-  TriviallyCopyable& operator=(const TriviallyCopyable&) { return *this; }
-};
-
-struct TriviallyMoveAssignable {
-  TriviallyMoveAssignable(TriviallyMoveAssignable&&) = default;
-  TriviallyMoveAssignable(const TriviallyMoveAssignable&) {}
-  TriviallyMoveAssignable& operator=(TriviallyMoveAssignable&&) = default;
-  TriviallyMoveAssignable& operator=(const TriviallyMoveAssignable&) {
-    return *this;
-  }
-};
-
-struct TriviallyCopyAssignable {};
-
-#if ABSL_VARIANT_PROPAGATE_COPY_MOVE_TRIVIALITY
-TEST(VariantTest, TestTriviality) {
-  {
-    using TrivDestVar = absl::variant<TriviallyDestructible>;
-
-    EXPECT_FALSE(is_trivially_move_constructible<TrivDestVar>::value);
-    EXPECT_FALSE(absl::is_trivially_copy_constructible<TrivDestVar>::value);
-    EXPECT_FALSE(is_trivially_move_assignable<TrivDestVar>::value);
-    EXPECT_FALSE(absl::is_trivially_copy_assignable<TrivDestVar>::value);
-    EXPECT_TRUE(absl::is_trivially_destructible<TrivDestVar>::value);
-  }
-
-  {
-    using TrivMoveVar = absl::variant<TriviallyMovable>;
-
-    EXPECT_TRUE(is_trivially_move_constructible<TrivMoveVar>::value);
-    EXPECT_FALSE(absl::is_trivially_copy_constructible<TrivMoveVar>::value);
-    EXPECT_FALSE(is_trivially_move_assignable<TrivMoveVar>::value);
-    EXPECT_FALSE(absl::is_trivially_copy_assignable<TrivMoveVar>::value);
-    EXPECT_TRUE(absl::is_trivially_destructible<TrivMoveVar>::value);
-  }
-
-  {
-    using TrivCopyVar = absl::variant<TriviallyCopyable>;
-
-    EXPECT_TRUE(is_trivially_move_constructible<TrivCopyVar>::value);
-    EXPECT_TRUE(absl::is_trivially_copy_constructible<TrivCopyVar>::value);
-    EXPECT_FALSE(is_trivially_move_assignable<TrivCopyVar>::value);
-    EXPECT_FALSE(absl::is_trivially_copy_assignable<TrivCopyVar>::value);
-    EXPECT_TRUE(absl::is_trivially_destructible<TrivCopyVar>::value);
-  }
-
-  {
-    using TrivMoveAssignVar = absl::variant<TriviallyMoveAssignable>;
-
-    EXPECT_TRUE(is_trivially_move_constructible<TrivMoveAssignVar>::value);
-    EXPECT_FALSE(
-        absl::is_trivially_copy_constructible<TrivMoveAssignVar>::value);
-    EXPECT_TRUE(is_trivially_move_assignable<TrivMoveAssignVar>::value);
-    EXPECT_FALSE(absl::is_trivially_copy_assignable<TrivMoveAssignVar>::value);
-    EXPECT_TRUE(absl::is_trivially_destructible<TrivMoveAssignVar>::value);
-  }
-
-  {
-    using TrivCopyAssignVar = absl::variant<TriviallyCopyAssignable>;
-
-    EXPECT_TRUE(is_trivially_move_constructible<TrivCopyAssignVar>::value);
-    EXPECT_TRUE(
-        absl::is_trivially_copy_constructible<TrivCopyAssignVar>::value);
-    EXPECT_TRUE(is_trivially_move_assignable<TrivCopyAssignVar>::value);
-    EXPECT_TRUE(absl::is_trivially_copy_assignable<TrivCopyAssignVar>::value);
-    EXPECT_TRUE(absl::is_trivially_destructible<TrivCopyAssignVar>::value);
-  }
-}
-#endif  // ABSL_VARIANT_PROPAGATE_COPY_MOVE_TRIVIALITY
-
-// To verify that absl::variant correctly use the nontrivial move ctor of its
-// member rather than use the trivial copy constructor.
-TEST(VariantTest, MoveCtorBug) {
-  // To simulate std::tuple in libstdc++.
-  struct TrivialCopyNontrivialMove {
-    TrivialCopyNontrivialMove() = default;
-    TrivialCopyNontrivialMove(const TrivialCopyNontrivialMove&) = default;
-    TrivialCopyNontrivialMove(TrivialCopyNontrivialMove&&) { called = true; }
-    bool called = false;
-  };
-  {
-    using V = absl::variant<TrivialCopyNontrivialMove, int>;
-    V v1(absl::in_place_index<0>);
-    // this should invoke the move ctor, rather than the trivial copy ctor.
-    V v2(std::move(v1));
-    EXPECT_TRUE(absl::get<0>(v2).called);
-  }
-  {
-    // this case failed to compile before our fix due to a GCC bug.
-    using V = absl::variant<int, TrivialCopyNontrivialMove>;
-    V v1(absl::in_place_index<1>);
-    // this should invoke the move ctor, rather than the trivial copy ctor.
-    V v2(std::move(v1));
-    EXPECT_TRUE(absl::get<1>(v2).called);
-  }
-}
-
 }  // namespace
 }  // namespace absl
-
-#endif  // #if !defined(ABSL_HAVE_STD_VARIANT)
diff --git a/absl/utility/BUILD.bazel b/absl/utility/BUILD.bazel
index 6881f939c51..85e18178ed2 100644
--- a/absl/utility/BUILD.bazel
+++ b/absl/utility/BUILD.bazel
@@ -14,17 +14,23 @@
 # limitations under the License.
 #
 
-load("@rules_cc//cc:defs.bzl", "cc_library", "cc_test")
+load("@rules_cc//cc:cc_library.bzl", "cc_library")
 load(
     "//absl:copts/configure_copts.bzl",
     "ABSL_DEFAULT_COPTS",
     "ABSL_DEFAULT_LINKOPTS",
-    "ABSL_TEST_COPTS",
 )
 
-package(default_visibility = ["//visibility:public"])
+package(
+    default_visibility = ["//visibility:private"],
+    features = [
+        "header_modules",
+        "layering_check",
+        "parse_headers",
+    ],
+)
 
-licenses(["notice"])  # Apache 2.0
+licenses(["notice"])
 
 cc_library(
     name = "utility",
@@ -33,23 +39,10 @@ cc_library(
     ],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
     deps = [
-        "//absl/base:base_internal",
         "//absl/base:config",
-        "//absl/meta:type_traits",
-    ],
-)
-
-cc_test(
-    name = "utility_test",
-    srcs = ["utility_test.cc"],
-    copts = ABSL_TEST_COPTS,
-    linkopts = ABSL_DEFAULT_LINKOPTS,
-    deps = [
-        ":utility",
         "//absl/base:core_headers",
-        "//absl/memory",
-        "//absl/strings",
-        "@com_google_googletest//:gtest_main",
+        "//absl/meta:type_traits",
     ],
 )
diff --git a/absl/utility/CMakeLists.txt b/absl/utility/CMakeLists.txt
index e1edd19aa01..ece42de2d72 100644
--- a/absl/utility/CMakeLists.txt
+++ b/absl/utility/CMakeLists.txt
@@ -22,23 +22,8 @@ absl_cc_library(
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
-    absl::base_internal
     absl::config
+    absl::core_headers
     absl::type_traits
   PUBLIC
 )
-
-absl_cc_test(
-  NAME
-    utility_test
-  SRCS
-    "utility_test.cc"
-  COPTS
-    ${ABSL_TEST_COPTS}
-  DEPS
-    absl::utility
-    absl::core_headers
-    absl::memory
-    absl::strings
-    gmock_main
-)
diff --git a/absl/utility/utility.h b/absl/utility/utility.h
index eef8fb41fe8..3cb6940f182 100644
--- a/absl/utility/utility.h
+++ b/absl/utility/utility.h
@@ -11,31 +11,6 @@
 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 // See the License for the specific language governing permissions and
 // limitations under the License.
-//
-// This header file contains C++11 versions of standard <utility> header
-// abstractions available within C++14 and C++17, and are designed to be drop-in
-// replacement for code compliant with C++14 and C++17.
-//
-// The following abstractions are defined:
-//
-//   * integer_sequence<T, Ints...>  == std::integer_sequence<T, Ints...>
-//   * index_sequence<Ints...>       == std::index_sequence<Ints...>
-//   * make_integer_sequence<T, N>   == std::make_integer_sequence<T, N>
-//   * make_index_sequence<N>        == std::make_index_sequence<N>
-//   * index_sequence_for<Ts...>     == std::index_sequence_for<Ts...>
-//   * apply<Functor, Tuple>         == std::apply<Functor, Tuple>
-//   * exchange<T>                   == std::exchange<T>
-//   * make_from_tuple<T>            == std::make_from_tuple<T>
-//
-// This header file also provides the tag types `in_place_t`, `in_place_type_t`,
-// and `in_place_index_t`, as well as the constant `in_place`, and
-// `constexpr` `std::move()` and `std::forward()` implementations in C++11.
-//
-// References:
-//
-//  https://en.cppreference.com/w/cpp/utility/integer_sequence
-//  https://en.cppreference.com/w/cpp/utility/apply
-//  http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3658.html
 
 #ifndef ABSL_UTILITY_UTILITY_H_
 #define ABSL_UTILITY_UTILITY_H_
@@ -43,306 +18,134 @@
 #include <cstddef>
 #include <cstdlib>
 #include <tuple>
+#include <type_traits>
 #include <utility>
 
+#include "absl/base/attributes.h"
 #include "absl/base/config.h"
-#include "absl/base/internal/inline_variable.h"
-#include "absl/base/internal/invoke.h"
-#include "absl/meta/type_traits.h"
+#include "absl/base/macros.h"
 
-namespace absl {
-
-// integer_sequence
-//
-// Class template representing a compile-time integer sequence. An instantiation
-// of `integer_sequence<T, Ints...>` has a sequence of integers encoded in its
-// type through its template arguments (which is a common need when
-// working with C++11 variadic templates). `absl::integer_sequence` is designed
-// to be a drop-in replacement for C++14's `std::integer_sequence`.
-//
-// Example:
-//
-//   template< class T, T... Ints >
-//   void user_function(integer_sequence<T, Ints...>);
-//
-//   int main()
-//   {
-//     // user_function's `T` will be deduced to `int` and `Ints...`
-//     // will be deduced to `0, 1, 2, 3, 4`.
-//     user_function(make_integer_sequence<int, 5>());
-//   }
-template <typename T, T... Ints>
-struct integer_sequence {
-  using value_type = T;
-  static constexpr size_t size() noexcept { return sizeof...(Ints); }
-};
-
-// index_sequence
-//
-// A helper template for an `integer_sequence` of `size_t`,
-// `absl::index_sequence` is designed to be a drop-in replacement for C++14's
-// `std::index_sequence`.
-template <size_t... Ints>
-using index_sequence = integer_sequence<size_t, Ints...>;
-
-namespace utility_internal {
+// TODO(b/290784225): Include what you use cleanup required.
+#include "absl/meta/type_traits.h"
 
-template <typename Seq, size_t SeqSize, size_t Rem>
-struct Extend;
+// TODO(b/509512528): Deprecate the C++14/C++17 symbols publicly, in all files.
 
-// Note that SeqSize == sizeof...(Ints). It's passed explicitly for efficiency.
-template <typename T, T... Ints, size_t SeqSize>
-struct Extend<integer_sequence<T, Ints...>, SeqSize, 0> {
-  using type = integer_sequence<T, Ints..., (Ints + SeqSize)...>;
-};
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+// Historical note: Abseil once provided implementations of these
+// abstractions for platforms that had not yet provided them. Those
+// platforms are no longer supported. New code should simply use the
+// the ones from std directly.
+template <class F, class T>
+ABSL_DEPRECATE_AND_INLINE()
+constexpr decltype(auto)
+    apply(F&& f, T&& t) noexcept(noexcept(std::apply(std::declval<F>(),
+                                                     std::declval<T>()))) {
+  return std::apply(std::forward<F>(f), std::forward<T>(t));
+}
 
-template <typename T, T... Ints, size_t SeqSize>
-struct Extend<integer_sequence<T, Ints...>, SeqSize, 1> {
-  using type = integer_sequence<T, Ints..., (Ints + SeqSize)..., 2 * SeqSize>;
-};
+template <class T1, class T2 = T1>
+ABSL_DEPRECATE_AND_INLINE()
+constexpr T1 exchange(T1& obj, T2&& new_value) noexcept(
+    noexcept(std::exchange(std::declval<T1&>(), std::declval<T2>()))) {
+  return std::exchange(obj, std::forward<T2>(new_value));
+}
 
-// Recursion helper for 'make_integer_sequence<T, N>'.
-// 'Gen<T, N>::type' is an alias for 'integer_sequence<T, 0, 1, ... N-1>'.
-template <typename T, size_t N>
-struct Gen {
-  using type =
-      typename Extend<typename Gen<T, N / 2>::type, N / 2, N % 2>::type;
-};
+template <class T>
+[[deprecated("Use std::forward instead.")]] [[nodiscard]] constexpr T&& forward(
+    std::remove_reference_t<T>& arg ABSL_ATTRIBUTE_LIFETIME_BOUND) noexcept {
+  // NOLINTNEXTLINE: Avoid warnings about T not being the spelled type of arg.
+  return std::forward<T>(arg);
+}
 
-template <typename T>
-struct Gen<T, 0> {
-  using type = integer_sequence<T>;
-};
+template <class T>
+[[deprecated("Use std::forward instead.")]] [[nodiscard]] constexpr T&& forward(
+    std::remove_reference_t<T>&& arg ABSL_ATTRIBUTE_LIFETIME_BOUND) noexcept {
+  // NOLINTNEXTLINE: Avoid warnings about T not being the spelled type of arg.
+  return std::forward<T>(arg);
+}
 
-template <typename T>
-struct InPlaceTypeTag {
-  explicit InPlaceTypeTag() = delete;
-  InPlaceTypeTag(const InPlaceTypeTag&) = delete;
-  InPlaceTypeTag& operator=(const InPlaceTypeTag&) = delete;
-};
+inline constexpr const std::in_place_t& in_place ABSL_DEPRECATE_AND_INLINE() =
+    std::in_place;
 
 template <size_t I>
-struct InPlaceIndexTag {
-  explicit InPlaceIndexTag() = delete;
-  InPlaceIndexTag(const InPlaceIndexTag&) = delete;
-  InPlaceIndexTag& operator=(const InPlaceIndexTag&) = delete;
-};
-
-}  // namespace utility_internal
-
-// Compile-time sequences of integers
-
-// make_integer_sequence
-//
-// This template alias is equivalent to
-// `integer_sequence<int, 0, 1, ..., N-1>`, and is designed to be a drop-in
-// replacement for C++14's `std::make_integer_sequence`.
-template <typename T, T N>
-using make_integer_sequence = typename utility_internal::Gen<T, N>::type;
-
-// make_index_sequence
-//
-// This template alias is equivalent to `index_sequence<0, 1, ..., N-1>`,
-// and is designed to be a drop-in replacement for C++14's
-// `std::make_index_sequence`.
-template <size_t N>
-using make_index_sequence = make_integer_sequence<size_t, N>;
-
-// index_sequence_for
-//
-// Converts a typename pack into an index sequence of the same length, and
-// is designed to be a drop-in replacement for C++14's
-// `std::index_sequence_for()`
-template <typename... Ts>
-using index_sequence_for = make_index_sequence<sizeof...(Ts)>;
+inline constexpr const std::in_place_index_t<I>& in_place_index
+    [[deprecated("Use std::in_place_index<I> instead.")]] =
+        std::in_place_index<I>;
 
-// Tag types
-
-#ifdef ABSL_HAVE_STD_OPTIONAL
-
-using std::in_place_t;
-using std::in_place;
-
-#else  // ABSL_HAVE_STD_OPTIONAL
-
-// in_place_t
-//
-// Tag type used to specify in-place construction, such as with
-// `absl::optional`, designed to be a drop-in replacement for C++17's
-// `std::in_place_t`.
-struct in_place_t {};
-
-ABSL_INTERNAL_INLINE_CONSTEXPR(in_place_t, in_place, {});
-
-#endif  // ABSL_HAVE_STD_OPTIONAL
-
-#if defined(ABSL_HAVE_STD_ANY) || defined(ABSL_HAVE_STD_VARIANT)
-using std::in_place_type;
-using std::in_place_type_t;
-#else
-
-// in_place_type_t
-//
-// Tag type used for in-place construction when the type to construct needs to
-// be specified, such as with `absl::any`, designed to be a drop-in replacement
-// for C++17's `std::in_place_type_t`.
-template <typename T>
-using in_place_type_t = void (*)(utility_internal::InPlaceTypeTag<T>);
-
-template <typename T>
-void in_place_type(utility_internal::InPlaceTypeTag<T>) {}
-#endif  // ABSL_HAVE_STD_ANY || ABSL_HAVE_STD_VARIANT
-
-#ifdef ABSL_HAVE_STD_VARIANT
-using std::in_place_index;
-using std::in_place_index_t;
-#else
-
-// in_place_index_t
-//
-// Tag type used for in-place construction when the type to construct needs to
-// be specified, such as with `absl::any`, designed to be a drop-in replacement
-// for C++17's `std::in_place_index_t`.
 template <size_t I>
-using in_place_index_t = void (*)(utility_internal::InPlaceIndexTag<I>);
+using in_place_index_t [[deprecated("Use std::in_place_index_t<I> instead.")]] =
+    std::in_place_index_t<I>;
 
-template <size_t I>
-void in_place_index(utility_internal::InPlaceIndexTag<I>) {}
-#endif  // ABSL_HAVE_STD_VARIANT
+using in_place_t ABSL_DEPRECATE_AND_INLINE() = std::in_place_t;
 
-// Constexpr move and forward
+template <class T>
+inline constexpr const std::in_place_type_t<T>& in_place_type
+ABSL_DEPRECATE_AND_INLINE() = std::in_place_type<T>;
 
-// move()
-//
-// A constexpr version of `std::move()`, designed to be a drop-in replacement
-// for C++14's `std::move()`.
-template <typename T>
-constexpr absl::remove_reference_t<T>&& move(T&& t) noexcept {
-  return static_cast<absl::remove_reference_t<T>&&>(t);
-}
+template <class T>
+using in_place_type_t ABSL_DEPRECATE_AND_INLINE() = std::in_place_type_t<T>;
 
-// forward()
-//
-// A constexpr version of `std::forward()`, designed to be a drop-in replacement
-// for C++14's `std::forward()`.
-template <typename T>
-constexpr T&& forward(
-    absl::remove_reference_t<T>& t) noexcept {  // NOLINT(runtime/references)
-  return static_cast<T&&>(t);
-}
+template <size_t... I>
+using index_sequence [[deprecated("Use std::index_sequence instead.")]] =
+    std::index_sequence<I...>;
 
-namespace utility_internal {
-// Helper method for expanding tuple into a called method.
-template <typename Functor, typename Tuple, std::size_t... Indexes>
-auto apply_helper(Functor&& functor, Tuple&& t, index_sequence<Indexes...>)
-    -> decltype(absl::base_internal::Invoke(
-        absl::forward<Functor>(functor),
-        std::get<Indexes>(absl::forward<Tuple>(t))...)) {
-  return absl::base_internal::Invoke(
-      absl::forward<Functor>(functor),
-      std::get<Indexes>(absl::forward<Tuple>(t))...);
-}
+template <class T, T... I>
+using integer_sequence [[deprecated("Use std::integer_sequence instead.")]] =
+    std::integer_sequence<T, I...>;
 
-}  // namespace utility_internal
+template <class... T>
+using index_sequence_for
+    [[deprecated("Use std::index_sequence_for instead.")]] =
+        std::index_sequence_for<T...>;
 
-// apply
-//
-// Invokes a Callable using elements of a tuple as its arguments.
-// Each element of the tuple corresponds to an argument of the call (in order).
-// Both the Callable argument and the tuple argument are perfect-forwarded.
-// For member-function Callables, the first tuple element acts as the `this`
-// pointer. `absl::apply` is designed to be a drop-in replacement for C++17's
-// `std::apply`. Unlike C++17's `std::apply`, this is not currently `constexpr`.
-//
-// Example:
-//
-//   class Foo {
-//    public:
-//     void Bar(int);
-//   };
-//   void user_function1(int, std::string);
-//   void user_function2(std::unique_ptr<Foo>);
-//   auto user_lambda = [](int, int) {};
-//
-//   int main()
-//   {
-//       std::tuple<int, std::string> tuple1(42, "bar");
-//       // Invokes the first user function on int, std::string.
-//       absl::apply(&user_function1, tuple1);
-//
-//       std::tuple<std::unique_ptr<Foo>> tuple2(absl::make_unique<Foo>());
-//       // Invokes the user function that takes ownership of the unique
-//       // pointer.
-//       absl::apply(&user_function2, std::move(tuple2));
-//
-//       auto foo = absl::make_unique<Foo>();
-//       std::tuple<Foo*, int> tuple3(foo.get(), 42);
-//       // Invokes the method Bar on foo with one argument, 42.
-//       absl::apply(&Foo::Bar, tuple3);
-//
-//       std::tuple<int, int> tuple4(8, 9);
-//       // Invokes a lambda.
-//       absl::apply(user_lambda, tuple4);
-//   }
-template <typename Functor, typename Tuple>
-auto apply(Functor&& functor, Tuple&& t)
-    -> decltype(utility_internal::apply_helper(
-        absl::forward<Functor>(functor), absl::forward<Tuple>(t),
-        absl::make_index_sequence<std::tuple_size<
-            typename std::remove_reference<Tuple>::type>::value>{})) {
-  return utility_internal::apply_helper(
-      absl::forward<Functor>(functor), absl::forward<Tuple>(t),
-      absl::make_index_sequence<std::tuple_size<
-          typename std::remove_reference<Tuple>::type>::value>{});
+template <class T, class Tuple>
+ABSL_DEPRECATE_AND_INLINE()
+[[nodiscard]] constexpr decltype(std::make_from_tuple<T>(std::declval<Tuple>()))
+    make_from_tuple(Tuple&& arg) noexcept(
+        noexcept(std::make_from_tuple<T>(std::declval<Tuple>()))) {
+  return std::make_from_tuple<T>(std::forward<Tuple>(arg));
 }
 
-// exchange
-//
-// Replaces the value of `obj` with `new_value` and returns the old value of
-// `obj`.  `absl::exchange` is designed to be a drop-in replacement for C++14's
-// `std::exchange`.
-//
-// Example:
-//
-//   Foo& operator=(Foo&& other) {
-//     ptr1_ = absl::exchange(other.ptr1_, nullptr);
-//     int1_ = absl::exchange(other.int1_, -1);
-//     return *this;
-//   }
-template <typename T, typename U = T>
-T exchange(T& obj, U&& new_value) {
-  T old_value = absl::move(obj);
-  obj = absl::forward<U>(new_value);
-  return old_value;
+template <size_t N>
+using make_index_sequence
+    [[deprecated("Use std::make_index_sequence instead.")]] =
+        std::make_index_sequence<N>;
+
+template <class T, T N>
+using make_integer_sequence
+    [[deprecated("Use std::make_integer_sequence instead.")]] =
+        std::make_integer_sequence<T, N>;
+
+template <class It, class OutIt>
+[[deprecated("Use std::move instead.")]]
+constexpr OutIt move(It&& begin, It&& end, OutIt&& output) {
+  return std::move(std::forward<It>(begin), std::forward<It>(end),
+                   std::forward<OutIt>(output));
 }
 
-namespace utility_internal {
-template <typename T, typename Tuple, size_t... I>
-T make_from_tuple_impl(Tuple&& tup, absl::index_sequence<I...>) {
-  return T(std::get<I>(std::forward<Tuple>(tup))...);
+template <class T>
+[[deprecated("Use std::move instead.")]]
+[[nodiscard]] constexpr std::remove_reference_t<T>&&
+move(T&& arg ABSL_ATTRIBUTE_LIFETIME_BOUND) noexcept {
+  return std::move(arg);  // NOLINT(bugprone-move-forwarding-reference)
 }
-}  // namespace utility_internal
 
-// make_from_tuple
-//
-// Given the template parameter type `T` and a tuple of arguments
-// `std::tuple(arg0, arg1, ..., argN)` constructs an object of type `T` as if by
-// calling `T(arg0, arg1, ..., argN)`.
-//
-// Example:
-//
-//   std::tuple<const char*, size_t> args("hello world", 5);
-//   auto s = absl::make_from_tuple<std::string>(args);
-//   assert(s == "hello");
-//
-template <typename T, typename Tuple>
-constexpr T make_from_tuple(Tuple&& tup) {
-  return utility_internal::make_from_tuple_impl<T>(
-      std::forward<Tuple>(tup),
-      absl::make_index_sequence<
-          std::tuple_size<absl::decay_t<Tuple>>::value>{});
-}
+#if ABSL_INTERNAL_CPLUSPLUS_LANG >= 202002L
+// Backfill for std::nontype_t. An instance of this class can be provided as a
+// disambiguation tag to `absl::function_ref` to pass the address of a known
+// callable at compile time.
+// Requires C++20 due to `auto` template parameter.
+template <auto>
+struct nontype_t {
+  explicit nontype_t() = default;
+};
+template <auto V>
+constexpr nontype_t<V> nontype{};
+#endif
 
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_UTILITY_UTILITY_H_
diff --git a/absl/utility/utility_test.cc b/absl/utility/utility_test.cc
deleted file mode 100644
index f044ad644a4..00000000000
--- a/absl/utility/utility_test.cc
+++ /dev/null
@@ -1,376 +0,0 @@
-// Copyright 2017 The Abseil Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-#include "absl/utility/utility.h"
-
-#include <sstream>
-#include <string>
-#include <tuple>
-#include <type_traits>
-#include <vector>
-
-#include "gmock/gmock.h"
-#include "gtest/gtest.h"
-#include "absl/base/attributes.h"
-#include "absl/memory/memory.h"
-#include "absl/strings/str_cat.h"
-
-namespace {
-
-#ifdef _MSC_VER
-// Warnings for unused variables in this test are false positives.  On other
-// platforms, they are suppressed by ABSL_ATTRIBUTE_UNUSED, but that doesn't
-// work on MSVC.
-// Both the unused variables and the name length warnings are due to calls
-// to absl::make_index_sequence with very large values, creating very long type
-// names. The resulting warnings are so long they make build output unreadable.
-#pragma warning( push )
-#pragma warning( disable : 4503 )  // decorated name length exceeded
-#pragma warning( disable : 4101 )  // unreferenced local variable
-#endif  // _MSC_VER
-
-using ::testing::ElementsAre;
-using ::testing::Pointee;
-using ::testing::StaticAssertTypeEq;
-
-TEST(IntegerSequenceTest, ValueType) {
-  StaticAssertTypeEq<int, absl::integer_sequence<int>::value_type>();
-  StaticAssertTypeEq<char, absl::integer_sequence<char>::value_type>();
-}
-
-TEST(IntegerSequenceTest, Size) {
-  EXPECT_EQ(0, (absl::integer_sequence<int>::size()));
-  EXPECT_EQ(1, (absl::integer_sequence<int, 0>::size()));
-  EXPECT_EQ(1, (absl::integer_sequence<int, 1>::size()));
-  EXPECT_EQ(2, (absl::integer_sequence<int, 1, 2>::size()));
-  EXPECT_EQ(3, (absl::integer_sequence<int, 0, 1, 2>::size()));
-  EXPECT_EQ(3, (absl::integer_sequence<int, -123, 123, 456>::size()));
-  constexpr size_t sz = absl::integer_sequence<int, 0, 1>::size();
-  EXPECT_EQ(2, sz);
-}
-
-TEST(IntegerSequenceTest, MakeIndexSequence) {
-  StaticAssertTypeEq<absl::index_sequence<>, absl::make_index_sequence<0>>();
-  StaticAssertTypeEq<absl::index_sequence<0>, absl::make_index_sequence<1>>();
-  StaticAssertTypeEq<absl::index_sequence<0, 1>,
-                     absl::make_index_sequence<2>>();
-  StaticAssertTypeEq<absl::index_sequence<0, 1, 2>,
-                     absl::make_index_sequence<3>>();
-}
-
-TEST(IntegerSequenceTest, MakeIntegerSequence) {
-  StaticAssertTypeEq<absl::integer_sequence<int>,
-                     absl::make_integer_sequence<int, 0>>();
-  StaticAssertTypeEq<absl::integer_sequence<int, 0>,
-                     absl::make_integer_sequence<int, 1>>();
-  StaticAssertTypeEq<absl::integer_sequence<int, 0, 1>,
-                     absl::make_integer_sequence<int, 2>>();
-  StaticAssertTypeEq<absl::integer_sequence<int, 0, 1, 2>,
-                     absl::make_integer_sequence<int, 3>>();
-}
-
-template <typename... Ts>
-class Counter {};
-
-template <size_t... Is>
-void CountAll(absl::index_sequence<Is...>) {
-  // We only need an alias here, but instantiate a variable to silence warnings
-  // for unused typedefs in some compilers.
-  ABSL_ATTRIBUTE_UNUSED Counter<absl::make_index_sequence<Is>...> seq;
-}
-
-// This test verifies that absl::make_index_sequence can handle large arguments
-// without blowing up template instantiation stack, going OOM or taking forever
-// to compile (there is hard 15 minutes limit imposed by forge).
-TEST(IntegerSequenceTest, MakeIndexSequencePerformance) {
-  // O(log N) template instantiations.
-  // We only need an alias here, but instantiate a variable to silence warnings
-  // for unused typedefs in some compilers.
-  ABSL_ATTRIBUTE_UNUSED absl::make_index_sequence<(1 << 16) - 1> seq;
-  // O(N) template instantiations.
-  CountAll(absl::make_index_sequence<(1 << 8) - 1>());
-}
-
-template <typename F, typename Tup, size_t... Is>
-auto ApplyFromTupleImpl(F f, const Tup& tup, absl::index_sequence<Is...>)
-    -> decltype(f(std::get<Is>(tup)...)) {
-  return f(std::get<Is>(tup)...);
-}
-
-template <typename Tup>
-using TupIdxSeq = absl::make_index_sequence<std::tuple_size<Tup>::value>;
-
-template <typename F, typename Tup>
-auto ApplyFromTuple(F f, const Tup& tup)
-    -> decltype(ApplyFromTupleImpl(f, tup, TupIdxSeq<Tup>{})) {
-  return ApplyFromTupleImpl(f, tup, TupIdxSeq<Tup>{});
-}
-
-template <typename T>
-std::string Fmt(const T& x) {
-  std::ostringstream os;
-  os << x;
-  return os.str();
-}
-
-struct PoorStrCat {
-  template <typename... Args>
-  std::string operator()(const Args&... args) const {
-    std::string r;
-    for (const auto& e : {Fmt(args)...}) r += e;
-    return r;
-  }
-};
-
-template <typename Tup, size_t... Is>
-std::vector<std::string> TupStringVecImpl(const Tup& tup,
-                                          absl::index_sequence<Is...>) {
-  return {Fmt(std::get<Is>(tup))...};
-}
-
-template <typename... Ts>
-std::vector<std::string> TupStringVec(const std::tuple<Ts...>& tup) {
-  return TupStringVecImpl(tup, absl::index_sequence_for<Ts...>());
-}
-
-TEST(MakeIndexSequenceTest, ApplyFromTupleExample) {
-  PoorStrCat f{};
-  EXPECT_EQ("12abc3.14", f(12, "abc", 3.14));
-  EXPECT_EQ("12abc3.14", ApplyFromTuple(f, std::make_tuple(12, "abc", 3.14)));
-}
-
-TEST(IndexSequenceForTest, Basic) {
-  StaticAssertTypeEq<absl::index_sequence<>, absl::index_sequence_for<>>();
-  StaticAssertTypeEq<absl::index_sequence<0>, absl::index_sequence_for<int>>();
-  StaticAssertTypeEq<absl::index_sequence<0, 1, 2, 3>,
-                     absl::index_sequence_for<int, void, char, int>>();
-}
-
-TEST(IndexSequenceForTest, Example) {
-  EXPECT_THAT(TupStringVec(std::make_tuple(12, "abc", 3.14)),
-              ElementsAre("12", "abc", "3.14"));
-}
-
-int Function(int a, int b) { return a - b; }
-
-int Sink(std::unique_ptr<int> p) { return *p; }
-
-std::unique_ptr<int> Factory(int n) { return absl::make_unique<int>(n); }
-
-void NoOp() {}
-
-struct ConstFunctor {
-  int operator()(int a, int b) const { return a - b; }
-};
-
-struct MutableFunctor {
-  int operator()(int a, int b) { return a - b; }
-};
-
-struct EphemeralFunctor {
-  EphemeralFunctor() {}
-  EphemeralFunctor(const EphemeralFunctor&) {}
-  EphemeralFunctor(EphemeralFunctor&&) {}
-  int operator()(int a, int b) && { return a - b; }
-};
-
-struct OverloadedFunctor {
-  OverloadedFunctor() {}
-  OverloadedFunctor(const OverloadedFunctor&) {}
-  OverloadedFunctor(OverloadedFunctor&&) {}
-  template <typename... Args>
-  std::string operator()(const Args&... args) & {
-    return absl::StrCat("&", args...);
-  }
-  template <typename... Args>
-  std::string operator()(const Args&... args) const& {
-    return absl::StrCat("const&", args...);
-  }
-  template <typename... Args>
-  std::string operator()(const Args&... args) && {
-    return absl::StrCat("&&", args...);
-  }
-};
-
-struct Class {
-  int Method(int a, int b) { return a - b; }
-  int ConstMethod(int a, int b) const { return a - b; }
-
-  int member;
-};
-
-struct FlipFlop {
-  int ConstMethod() const { return member; }
-  FlipFlop operator*() const { return {-member}; }
-
-  int member;
-};
-
-TEST(ApplyTest, Function) {
-  EXPECT_EQ(1, absl::apply(Function, std::make_tuple(3, 2)));
-  EXPECT_EQ(1, absl::apply(&Function, std::make_tuple(3, 2)));
-}
-
-TEST(ApplyTest, NonCopyableArgument) {
-  EXPECT_EQ(42, absl::apply(Sink, std::make_tuple(absl::make_unique<int>(42))));
-}
-
-TEST(ApplyTest, NonCopyableResult) {
-  EXPECT_THAT(absl::apply(Factory, std::make_tuple(42)),
-              ::testing::Pointee(42));
-}
-
-TEST(ApplyTest, VoidResult) { absl::apply(NoOp, std::tuple<>()); }
-
-TEST(ApplyTest, ConstFunctor) {
-  EXPECT_EQ(1, absl::apply(ConstFunctor(), std::make_tuple(3, 2)));
-}
-
-TEST(ApplyTest, MutableFunctor) {
-  MutableFunctor f;
-  EXPECT_EQ(1, absl::apply(f, std::make_tuple(3, 2)));
-  EXPECT_EQ(1, absl::apply(MutableFunctor(), std::make_tuple(3, 2)));
-}
-TEST(ApplyTest, EphemeralFunctor) {
-  EphemeralFunctor f;
-  EXPECT_EQ(1, absl::apply(std::move(f), std::make_tuple(3, 2)));
-  EXPECT_EQ(1, absl::apply(EphemeralFunctor(), std::make_tuple(3, 2)));
-}
-TEST(ApplyTest, OverloadedFunctor) {
-  OverloadedFunctor f;
-  const OverloadedFunctor& cf = f;
-
-  EXPECT_EQ("&", absl::apply(f, std::tuple<>{}));
-  EXPECT_EQ("& 42", absl::apply(f, std::make_tuple(" 42")));
-
-  EXPECT_EQ("const&", absl::apply(cf, std::tuple<>{}));
-  EXPECT_EQ("const& 42", absl::apply(cf, std::make_tuple(" 42")));
-
-  EXPECT_EQ("&&", absl::apply(std::move(f), std::tuple<>{}));
-  OverloadedFunctor f2;
-  EXPECT_EQ("&& 42", absl::apply(std::move(f2), std::make_tuple(" 42")));
-}
-
-TEST(ApplyTest, ReferenceWrapper) {
-  ConstFunctor cf;
-  MutableFunctor mf;
-  EXPECT_EQ(1, absl::apply(std::cref(cf), std::make_tuple(3, 2)));
-  EXPECT_EQ(1, absl::apply(std::ref(cf), std::make_tuple(3, 2)));
-  EXPECT_EQ(1, absl::apply(std::ref(mf), std::make_tuple(3, 2)));
-}
-
-TEST(ApplyTest, MemberFunction) {
-  std::unique_ptr<Class> p(new Class);
-  std::unique_ptr<const Class> cp(new Class);
-  EXPECT_EQ(
-      1, absl::apply(&Class::Method,
-                     std::tuple<std::unique_ptr<Class>&, int, int>(p, 3, 2)));
-  EXPECT_EQ(1, absl::apply(&Class::Method,
-                           std::tuple<Class*, int, int>(p.get(), 3, 2)));
-  EXPECT_EQ(
-      1, absl::apply(&Class::Method, std::tuple<Class&, int, int>(*p, 3, 2)));
-
-  EXPECT_EQ(
-      1, absl::apply(&Class::ConstMethod,
-                     std::tuple<std::unique_ptr<Class>&, int, int>(p, 3, 2)));
-  EXPECT_EQ(1, absl::apply(&Class::ConstMethod,
-                           std::tuple<Class*, int, int>(p.get(), 3, 2)));
-  EXPECT_EQ(1, absl::apply(&Class::ConstMethod,
-                           std::tuple<Class&, int, int>(*p, 3, 2)));
-
-  EXPECT_EQ(1, absl::apply(&Class::ConstMethod,
-                           std::tuple<std::unique_ptr<const Class>&, int, int>(
-                               cp, 3, 2)));
-  EXPECT_EQ(1, absl::apply(&Class::ConstMethod,
-                           std::tuple<const Class*, int, int>(cp.get(), 3, 2)));
-  EXPECT_EQ(1, absl::apply(&Class::ConstMethod,
-                           std::tuple<const Class&, int, int>(*cp, 3, 2)));
-
-  EXPECT_EQ(1, absl::apply(&Class::Method,
-                           std::make_tuple(absl::make_unique<Class>(), 3, 2)));
-  EXPECT_EQ(1, absl::apply(&Class::ConstMethod,
-                           std::make_tuple(absl::make_unique<Class>(), 3, 2)));
-  EXPECT_EQ(
-      1, absl::apply(&Class::ConstMethod,
-                     std::make_tuple(absl::make_unique<const Class>(), 3, 2)));
-}
-
-TEST(ApplyTest, DataMember) {
-  std::unique_ptr<Class> p(new Class{42});
-  std::unique_ptr<const Class> cp(new Class{42});
-  EXPECT_EQ(
-      42, absl::apply(&Class::member, std::tuple<std::unique_ptr<Class>&>(p)));
-  EXPECT_EQ(42, absl::apply(&Class::member, std::tuple<Class&>(*p)));
-  EXPECT_EQ(42, absl::apply(&Class::member, std::tuple<Class*>(p.get())));
-
-  absl::apply(&Class::member, std::tuple<std::unique_ptr<Class>&>(p)) = 42;
-  absl::apply(&Class::member, std::tuple<Class*>(p.get())) = 42;
-  absl::apply(&Class::member, std::tuple<Class&>(*p)) = 42;
-
-  EXPECT_EQ(42, absl::apply(&Class::member,
-                            std::tuple<std::unique_ptr<const Class>&>(cp)));
-  EXPECT_EQ(42, absl::apply(&Class::member, std::tuple<const Class&>(*cp)));
-  EXPECT_EQ(42,
-            absl::apply(&Class::member, std::tuple<const Class*>(cp.get())));
-}
-
-TEST(ApplyTest, FlipFlop) {
-  FlipFlop obj = {42};
-  // This call could resolve to (obj.*&FlipFlop::ConstMethod)() or
-  // ((*obj).*&FlipFlop::ConstMethod)(). We verify that it's the former.
-  EXPECT_EQ(42, absl::apply(&FlipFlop::ConstMethod, std::make_tuple(obj)));
-  EXPECT_EQ(42, absl::apply(&FlipFlop::member, std::make_tuple(obj)));
-}
-
-TEST(ExchangeTest, MoveOnly) {
-  auto a = Factory(1);
-  EXPECT_EQ(1, *a);
-  auto b = absl::exchange(a, Factory(2));
-  EXPECT_EQ(2, *a);
-  EXPECT_EQ(1, *b);
-}
-
-TEST(MakeFromTupleTest, String) {
-  EXPECT_EQ(
-      absl::make_from_tuple<std::string>(std::make_tuple("hello world", 5)),
-      "hello");
-}
-
-TEST(MakeFromTupleTest, MoveOnlyParameter) {
-  struct S {
-    S(std::unique_ptr<int> n, std::unique_ptr<int> m) : value(*n + *m) {}
-    int value = 0;
-  };
-  auto tup =
-      std::make_tuple(absl::make_unique<int>(3), absl::make_unique<int>(4));
-  auto s = absl::make_from_tuple<S>(std::move(tup));
-  EXPECT_EQ(s.value, 7);
-}
-
-TEST(MakeFromTupleTest, NoParameters) {
-  struct S {
-    S() : value(1) {}
-    int value = 2;
-  };
-  EXPECT_EQ(absl::make_from_tuple<S>(std::make_tuple()).value, 1);
-}
-
-TEST(MakeFromTupleTest, Pair) {
-  EXPECT_EQ(
-      (absl::make_from_tuple<std::pair<bool, int>>(std::make_tuple(true, 17))),
-      std::make_pair(true, 17));
-}
-
-}  // namespace
-
diff --git a/ci/absl_alternate_options.h b/ci/absl_alternate_options.h
new file mode 100644
index 00000000000..54a1b78cd9f
--- /dev/null
+++ b/ci/absl_alternate_options.h
@@ -0,0 +1,30 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//    https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Alternate options.h file, used in continuous integration testing to exercise
+// option settings not used by default.
+
+// SKIP_ABSL_INLINE_NAMESPACE_CHECK
+
+#ifndef ABSL_CI_ABSL_ALTERNATE_OPTIONS_H_
+#define ABSL_CI_ABSL_ALTERNATE_OPTIONS_H_
+
+#define ABSL_OPTION_USE_STD_SOURCE_LOCATION 0
+#define ABSL_OPTION_USE_STD_ORDERING 0
+#define ABSL_OPTION_USE_INLINE_NAMESPACE 1
+#define ABSL_OPTION_INLINE_NAMESPACE_NAME ns
+#define ABSL_OPTION_HARDENED 1
+#define ABSL_OPTION_INLINE_HW_ACCEL_STRATEGY 2
+
+#endif  // ABSL_CI_ABSL_ALTERNATE_OPTIONS_H_
diff --git a/ci/cmake_common.sh b/ci/cmake_common.sh
new file mode 100644
index 00000000000..1804e148071
--- /dev/null
+++ b/ci/cmake_common.sh
@@ -0,0 +1,19 @@
+# Copyright 2020 The Abseil Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# The commit of GoogleTest to be used in the CMake tests in this directory.
+# Keep this in sync with the commit in the MODULE.bazel file.
+readonly ABSL_GOOGLETEST_VERSION="1.18.0"
+
+ABSL_GOOGLETEST_DOWNLOAD_URL="https://github.com/google/googletest/releases/download/v${ABSL_GOOGLETEST_VERSION}/googletest-${ABSL_GOOGLETEST_VERSION}.tar.gz"
diff --git a/ci/cmake_install_test.sh b/ci/cmake_install_test.sh
index 03eb04329ed..5e3e07b3d9b 100755
--- a/ci/cmake_install_test.sh
+++ b/ci/cmake_install_test.sh
@@ -16,18 +16,49 @@
 
 set -euox pipefail
 
-if [ -z ${ABSEIL_ROOT:-} ]; then
+if [[ -z ${ABSEIL_ROOT:-} ]]; then
   ABSEIL_ROOT="$(realpath $(dirname ${0})/..)"
 fi
 
+if [[ -z ${LINK_TYPE:-} ]]; then
+  LINK_TYPE="STATIC DYNAMIC"
+fi
+
+source "${ABSEIL_ROOT}/ci/cmake_common.sh"
+
+# Avoid depending on GitHub by looking for a cached copy of GoogleTest.
+if [[ -r "${KOKORO_GFILE_DIR:-}/distdir/googletest-${ABSL_GOOGLETEST_VERSION}.tar.gz" ]]; then
+  ABSL_GOOGLETEST_DOWNLOAD_URL="file:///distdir/googletest-${ABSL_GOOGLETEST_VERSION}.tar.gz"
+  DOCKER_EXTRA_ARGS="--mount type=bind,source=${KOKORO_GFILE_DIR}/distdir,target=/distdir,readonly ${DOCKER_EXTRA_ARGS:-}"
+fi
+
+source "${ABSEIL_ROOT}/ci/linux_docker_containers.sh"
+readonly DOCKER_CONTAINER=${LINUX_GCC_LATEST_CONTAINER}
+
+# Verify that everything works with the standard "cmake && make && make install"
+# without building tests or requiring GoogleTest.
 time docker run \
-    --volume="${ABSEIL_ROOT}:/abseil-cpp:ro" \
-    --workdir=/abseil-cpp \
+    --mount type=bind,source="${ABSEIL_ROOT}",target=/abseil-cpp-ro,readonly \
     --tmpfs=/buildfs:exec \
-    --cap-add=SYS_PTRACE \
+    --workdir=/buildfs \
     --rm \
-    -e CFLAGS="-Werror" \
-    -e CXXFLAGS="-Werror" \
-    gcr.io/google.com/absl-177019/linux_gcc-latest:20190703 \
-    /bin/bash CMake/install_test_project/test.sh $@
+    ${DOCKER_EXTRA_ARGS:-} \
+    ${DOCKER_CONTAINER} \
+    /bin/bash -c "cmake /abseil-cpp-ro && make -j$(nproc) && make install"
 
+# Verify that a more complicated project works.
+for link_type in ${LINK_TYPE}; do
+  time docker run \
+    --mount type=bind,source="${ABSEIL_ROOT}",target=/abseil-cpp-ro,readonly \
+    --tmpfs=/buildfs:exec \
+    --tmpfs=/abseil-cpp:exec \
+    --workdir=/abseil-cpp \
+    --cap-add=SYS_PTRACE \
+    -e "ABSL_GOOGLETEST_VERSION=${ABSL_GOOGLETEST_VERSION}" \
+    -e "ABSL_GOOGLETEST_DOWNLOAD_URL=${ABSL_GOOGLETEST_DOWNLOAD_URL}" \
+    -e "LINK_TYPE=${link_type}" \
+    --rm \
+    ${DOCKER_EXTRA_ARGS:-} \
+    ${DOCKER_CONTAINER} \
+    /bin/bash -c "cp -r /abseil-cpp-ro/* . && CMake/install_test_project/test.sh"
+done
diff --git a/ci/linux_arm_clang-latest_libcxx_bazel.sh b/ci/linux_arm_clang-latest_libcxx_bazel.sh
new file mode 100755
index 00000000000..24d5e48ede7
--- /dev/null
+++ b/ci/linux_arm_clang-latest_libcxx_bazel.sh
@@ -0,0 +1,106 @@
+#!/bin/bash
+#
+# Copyright 2019 The Abseil Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# This script that can be invoked to test abseil-cpp in a hermetic environment
+# using a Docker image on Linux. You must have Docker installed to use this
+# script.
+
+set -euox pipefail
+
+if [[ -z ${ABSEIL_ROOT:-} ]]; then
+  ABSEIL_ROOT="$(realpath $(dirname ${0})/..)"
+fi
+
+if [[ -z ${STD:-} ]]; then
+  STD="c++17 c++20"
+fi
+
+if [[ -z ${COMPILATION_MODE:-} ]]; then
+  COMPILATION_MODE="fastbuild opt"
+fi
+
+if [[ -z ${EXCEPTIONS_MODE:-} ]]; then
+  EXCEPTIONS_MODE="-fno-exceptions -fexceptions"
+fi
+
+source "${ABSEIL_ROOT}/ci/linux_docker_containers.sh"
+readonly DOCKER_CONTAINER=${LINUX_ARM_CLANG_LATEST_CONTAINER}
+
+# Print information about the environment.
+docker run "${DOCKER_CONTAINER}" /usr/bin/fastfetch -c ci
+docker run "${DOCKER_CONTAINER}" /opt/llvm/bin/clang -v
+docker run "${DOCKER_CONTAINER}" cat /root/cached_bazel_versions
+
+# USE_BAZEL_CACHE=1 only works on Kokoro.
+# Without access to the credentials this won't work.
+if [[ ${USE_BAZEL_CACHE:-0} -ne 0 ]]; then
+  DOCKER_EXTRA_ARGS="--mount type=bind,source=${KOKORO_KEYSTORE_DIR},target=/keystore,readonly ${DOCKER_EXTRA_ARGS:-}"
+  # Bazel doesn't track changes to tools outside of the workspace
+  # (e.g. /usr/bin/gcc), so by appending the docker container to the
+  # remote_http_cache url, we make changes to the container part of
+  # the cache key. Hashing the key is to make it shorter and url-safe.
+  container_key=$(echo ${DOCKER_CONTAINER} | sha256sum | head -c 16)
+  BAZEL_EXTRA_ARGS="--remote_cache=https://storage.googleapis.com/absl-bazel-remote-cache/${container_key} --google_credentials=/keystore/73103_absl-bazel-remote-cache ${BAZEL_EXTRA_ARGS:-}"
+fi
+
+# Use Bazel Vendor mode to reduce reliance on external dependencies.
+# See https://bazel.build/external/vendor and the Dockerfile for
+# an explaination of how this works.
+if [[ ${KOKORO_GFILE_DIR:-} ]] && [[ -f "${KOKORO_GFILE_DIR}/distdir/abseil-cpp_vendor.tar.gz" ]]; then
+  DOCKER_EXTRA_ARGS="--mount type=bind,source=${KOKORO_GFILE_DIR}/distdir,target=/distdir,readonly --env=BAZEL_VENDOR_ARCHIVE=/distdir/abseil-cpp_vendor.tar.gz ${DOCKER_EXTRA_ARGS:-}"
+  BAZEL_EXTRA_ARGS="--vendor_dir=/abseil-cpp_vendor ${BAZEL_EXTRA_ARGS:-}"
+fi
+
+for std in ${STD}; do
+  for compilation_mode in ${COMPILATION_MODE}; do
+    for exceptions_mode in ${EXCEPTIONS_MODE}; do
+      echo "--------------------------------------------------------------------"
+      time docker run \
+        --env="USE_BAZEL_VERSION=9.1.0" \
+        --env="CC=/opt/llvm/bin/clang" \
+        --env="BAZEL_CXXOPTS=-std=${std}:-nostdinc++" \
+        --env="BAZEL_LINKOPTS=-L/opt/llvm/lib/aarch64-unknown-linux-gnu:-lc++:-lc++abi:-lm:-Wl,-rpath=/opt/llvm/lib/aarch64-unknown-linux-gnu" \
+        --env="CPLUS_INCLUDE_PATH=/opt/llvm/include/c++/v1:/opt/llvm/include/aarch64-unknown-linux-gnu/c++/v1/" \
+        --mount type=bind,source="${ABSEIL_ROOT}",target=/abseil-cpp-ro,readonly \
+        --tmpfs=/abseil-cpp \
+        --workdir=/abseil-cpp \
+        --cap-add=SYS_PTRACE \
+        --rm \
+        ${DOCKER_EXTRA_ARGS:-} \
+        ${DOCKER_CONTAINER} \
+        /bin/bash --login -c "
+          cp -r /abseil-cpp-ro/* /abseil-cpp/
+          if [ -n \"${ALTERNATE_OPTIONS:-}\" ]; then
+            cp ${ALTERNATE_OPTIONS:-} absl/base/options.h || exit 1
+          fi
+          /usr/local/bin/bazel test ... \
+            --compilation_mode=\"${compilation_mode}\" \
+            --copt=\"${exceptions_mode}\" \
+            --copt=\"-DGTEST_REMOVE_LEGACY_TEST_CASEAPI_=1\" \
+            --copt=-Werror \
+            --define=\"absl=1\" \
+            --features=external_include_paths \
+            --keep_going \
+            --per_file_copt=external/.*@-w \
+            --show_timestamps \
+            --test_env=\"GTEST_INSTALL_FAILURE_SIGNAL_HANDLER=1\" \
+            --test_env=\"TZDIR=/abseil-cpp/absl/time/internal/cctz/testdata/zoneinfo\" \
+            --test_output=errors \
+            --test_tag_filters=-benchmark \
+            ${BAZEL_EXTRA_ARGS:-}"
+    done
+  done
+done
diff --git a/ci/linux_clang-latest_libcxx_asan_bazel.sh b/ci/linux_clang-latest_libcxx_asan_bazel.sh
index cc8c695aabd..b8d2b5f92a8 100755
--- a/ci/linux_clang-latest_libcxx_asan_bazel.sh
+++ b/ci/linux_clang-latest_libcxx_asan_bazel.sh
@@ -20,74 +20,95 @@
 
 set -euox pipefail
 
-if [ -z ${ABSEIL_ROOT:-} ]; then
+if [[ -z ${ABSEIL_ROOT:-} ]]; then
   ABSEIL_ROOT="$(realpath $(dirname ${0})/..)"
 fi
 
-if [ -z ${STD:-} ]; then
-  STD="c++11 c++14 c++17"
+if [[ -z ${STD:-} ]]; then
+  STD="c++17 c++20 c++23"
 fi
 
-if [ -z ${COMPILATION_MODE:-} ]; then
+if [[ -z ${COMPILATION_MODE:-} ]]; then
   COMPILATION_MODE="fastbuild opt"
 fi
 
-if [ -z ${EXCEPTIONS_MODE:-} ]; then
+if [[ -z ${EXCEPTIONS_MODE:-} ]]; then
   EXCEPTIONS_MODE="-fno-exceptions -fexceptions"
 fi
 
-readonly DOCKER_CONTAINER="gcr.io/google.com/absl-177019/linux_clang-latest:20190813"
+source "${ABSEIL_ROOT}/ci/linux_docker_containers.sh"
+readonly DOCKER_CONTAINER=${LINUX_CLANG_LATEST_CONTAINER}
+
+# Print information about the environment.
+docker run "${DOCKER_CONTAINER}" /usr/bin/fastfetch -c ci
+docker run "${DOCKER_CONTAINER}" /opt/llvm/bin/clang -v
+docker run "${DOCKER_CONTAINER}" cat /root/cached_bazel_versions
 
 # USE_BAZEL_CACHE=1 only works on Kokoro.
 # Without access to the credentials this won't work.
-if [ ${USE_BAZEL_CACHE:-0} -ne 0 ]; then
-  DOCKER_EXTRA_ARGS="--volume=${KOKORO_KEYSTORE_DIR}:/keystore:ro ${DOCKER_EXTRA_ARGS:-}"
+if [[ ${USE_BAZEL_CACHE:-0} -ne 0 ]]; then
+  DOCKER_EXTRA_ARGS="--mount type=bind,source=${KOKORO_KEYSTORE_DIR},target=/keystore,readonly ${DOCKER_EXTRA_ARGS:-}"
   # Bazel doesn't track changes to tools outside of the workspace
   # (e.g. /usr/bin/gcc), so by appending the docker container to the
   # remote_http_cache url, we make changes to the container part of
   # the cache key. Hashing the key is to make it shorter and url-safe.
   container_key=$(echo ${DOCKER_CONTAINER} | sha256sum | head -c 16)
-  BAZEL_EXTRA_ARGS="--remote_http_cache=https://storage.googleapis.com/absl-bazel-remote-cache/${container_key} --google_credentials=/keystore/73103_absl-bazel-remote-cache ${BAZEL_EXTRA_ARGS:-}"
+  BAZEL_EXTRA_ARGS="--remote_cache=https://storage.googleapis.com/absl-bazel-remote-cache/${container_key} --google_credentials=/keystore/73103_absl-bazel-remote-cache ${BAZEL_EXTRA_ARGS:-}"
+fi
+
+# Avoid depending on external sites like GitHub by checking --distdir for
+# external dependencies first.
+# https://docs.bazel.build/versions/master/guide.html#distdir
+if [[ ${KOKORO_GFILE_DIR:-} ]] && [[ -d "${KOKORO_GFILE_DIR}/distdir" ]]; then
+  DOCKER_EXTRA_ARGS="--mount type=bind,source=${KOKORO_GFILE_DIR}/distdir,target=/distdir,readonly ${DOCKER_EXTRA_ARGS:-}"
+  BAZEL_EXTRA_ARGS="--distdir=/distdir ${BAZEL_EXTRA_ARGS:-}"
 fi
 
+# https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html#available-checks
+readonly UBSAN_CHECKS="float-divide-by-zero,nullability,undefined"
+
 for std in ${STD}; do
   for compilation_mode in ${COMPILATION_MODE}; do
     for exceptions_mode in ${EXCEPTIONS_MODE}; do
       echo "--------------------------------------------------------------------"
       time docker run \
-        --volume="${ABSEIL_ROOT}:/abseil-cpp:ro" \
+        --env="USE_BAZEL_VERSION=9.1.0" \
+        --env="CC=/opt/llvm/bin/clang" \
+        --env="BAZEL_CXXOPTS=-std=${std}:-nostdinc++" \
+        --env="BAZEL_LINKOPTS=-L/opt/llvm/lib/x86_64-unknown-linux-gnu:-lc++:-lc++abi:-lm:-Wl,-rpath=/opt/llvm/lib/x86_64-unknown-linux-gnu" \
+        --env="CPLUS_INCLUDE_PATH=/opt/llvm/include/c++/v1:/opt/llvm/include/x86_64-unknown-linux-gnu/c++/v1/" \
+        --mount type=bind,source="${ABSEIL_ROOT}",target=/abseil-cpp,readonly \
         --workdir=/abseil-cpp \
         --cap-add=SYS_PTRACE \
         --rm \
-        -e CC="/opt/llvm/clang/bin/clang" \
-        -e BAZEL_COMPILER="llvm" \
-        -e BAZEL_CXXOPTS="-std=${std}:-nostdinc++" \
-        -e BAZEL_LINKOPTS="-L/opt/llvm/libcxx/lib:-lc++:-lc++abi:-lm:-Wl,-rpath=/opt/llvm/libcxx/lib" \
-        -e CPLUS_INCLUDE_PATH="/opt/llvm/libcxx/include/c++/v1" \
         ${DOCKER_EXTRA_ARGS:-} \
         ${DOCKER_CONTAINER} \
+        /bin/bash --login -c "
         /usr/local/bin/bazel test ... \
-          --compilation_mode="${compilation_mode}" \
-          --copt="${exceptions_mode}" \
-          --copt="-DDYNAMIC_ANNOTATIONS_ENABLED=1" \
-          --copt="-DADDRESS_SANITIZER" \
-          --copt="-DUNDEFINED_BEHAVIOR_SANITIZER" \
-          --copt="-fsanitize=address" \
-          --copt="-fsanitize=float-divide-by-zero" \
-          --copt="-fsanitize=nullability" \
-          --copt="-fsanitize=undefined" \
+          --compilation_mode=\"${compilation_mode}\" \
+          --copt=\"${exceptions_mode}\" \
+          --copt=\"-D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_DEBUG\" \
+          --copt=\"-DGTEST_REMOVE_LEGACY_TEST_CASEAPI_=1\" \
+          --copt=\"-fsanitize=address\" \
+          --copt=\"-fsanitize=${UBSAN_CHECKS}\" \
+          --copt=\"-fno-sanitize-recover=${UBSAN_CHECKS}\" \
+          --copt=\"-fno-sanitize-blacklist\" \
           --copt=-Werror \
+          --define=\"absl=1\" \
+          --enable_bzlmod=true \
+          --features=external_include_paths \
           --keep_going \
-          --linkopt="-fsanitize=address" \
-          --linkopt="-fsanitize-link-c++-runtime" \
+          --linkopt=\"-fsanitize=address\" \
+          --linkopt=\"-fsanitize-link-c++-runtime\" \
+          --per_file_copt=external/.*@-w \
           --show_timestamps \
-          --test_env="ASAN_SYMBOLIZER_PATH=/opt/llvm/clang/bin/llvm-symbolizer" \
-          --test_env="TZDIR=/abseil-cpp/absl/time/internal/cctz/testdata/zoneinfo" \
-          --test_env="UBSAN_OPTIONS=print_stacktrace=1" \
-          --test_env="UBSAN_SYMBOLIZER_PATH=/opt/llvm/clang/bin/llvm-symbolizer" \
+          --test_env=\"ASAN_SYMBOLIZER_PATH=/opt/llvm/bin/llvm-symbolizer\" \
+          --test_env=\"TZDIR=/abseil-cpp/absl/time/internal/cctz/testdata/zoneinfo\" \
+          --test_env=\"UBSAN_OPTIONS=print_stacktrace=1\" \
+          --test_env=\"UBSAN_SYMBOLIZER_PATH=/opt/llvm/bin/llvm-symbolizer\" \
           --test_output=errors \
-          --test_tag_filters="-benchmark,-noasan" \
-          ${BAZEL_EXTRA_ARGS:-}
+          --test_tag_filters=\"-benchmark,-noasan\" \
+          ${BAZEL_EXTRA_ARGS:-}"
     done
   done
 done
diff --git a/ci/linux_clang-latest_libcxx_bazel.sh b/ci/linux_clang-latest_libcxx_bazel.sh
index 5849a84eda8..99e92b9152b 100755
--- a/ci/linux_clang-latest_libcxx_bazel.sh
+++ b/ci/linux_clang-latest_libcxx_bazel.sh
@@ -20,34 +20,48 @@
 
 set -euox pipefail
 
-if [ -z ${ABSEIL_ROOT:-} ]; then
+if [[ -z ${ABSEIL_ROOT:-} ]]; then
   ABSEIL_ROOT="$(realpath $(dirname ${0})/..)"
 fi
 
-if [ -z ${STD:-} ]; then
-  STD="c++11 c++14 c++17"
+if [[ -z ${STD:-} ]]; then
+  STD="c++17 c++20 c++23"
 fi
 
-if [ -z ${COMPILATION_MODE:-} ]; then
+if [[ -z ${COMPILATION_MODE:-} ]]; then
   COMPILATION_MODE="fastbuild opt"
 fi
 
-if [ -z ${EXCEPTIONS_MODE:-} ]; then
+if [[ -z ${EXCEPTIONS_MODE:-} ]]; then
   EXCEPTIONS_MODE="-fno-exceptions -fexceptions"
 fi
 
-readonly DOCKER_CONTAINER="gcr.io/google.com/absl-177019/linux_clang-latest:20190813"
+source "${ABSEIL_ROOT}/ci/linux_docker_containers.sh"
+readonly DOCKER_CONTAINER=${LINUX_CLANG_LATEST_CONTAINER}
+
+# Print information about the environment.
+docker run "${DOCKER_CONTAINER}" /usr/bin/fastfetch -c ci
+docker run "${DOCKER_CONTAINER}" /opt/llvm/bin/clang -v
+docker run "${DOCKER_CONTAINER}" cat /root/cached_bazel_versions
 
 # USE_BAZEL_CACHE=1 only works on Kokoro.
 # Without access to the credentials this won't work.
-if [ ${USE_BAZEL_CACHE:-0} -ne 0 ]; then
-  DOCKER_EXTRA_ARGS="--volume=${KOKORO_KEYSTORE_DIR}:/keystore:ro ${DOCKER_EXTRA_ARGS:-}"
+if [[ ${USE_BAZEL_CACHE:-0} -ne 0 ]]; then
+  DOCKER_EXTRA_ARGS="--mount type=bind,source=${KOKORO_KEYSTORE_DIR},target=/keystore,readonly ${DOCKER_EXTRA_ARGS:-}"
   # Bazel doesn't track changes to tools outside of the workspace
   # (e.g. /usr/bin/gcc), so by appending the docker container to the
   # remote_http_cache url, we make changes to the container part of
   # the cache key. Hashing the key is to make it shorter and url-safe.
   container_key=$(echo ${DOCKER_CONTAINER} | sha256sum | head -c 16)
-  BAZEL_EXTRA_ARGS="--remote_http_cache=https://storage.googleapis.com/absl-bazel-remote-cache/${container_key} --google_credentials=/keystore/73103_absl-bazel-remote-cache ${BAZEL_EXTRA_ARGS:-}"
+  BAZEL_EXTRA_ARGS="--remote_cache=https://storage.googleapis.com/absl-bazel-remote-cache/${container_key} --google_credentials=/keystore/73103_absl-bazel-remote-cache ${BAZEL_EXTRA_ARGS:-}"
+fi
+
+# Use Bazel Vendor mode to reduce reliance on external dependencies.
+# See https://bazel.build/external/vendor and the Dockerfile for
+# an explaination of how this works.
+if [[ ${KOKORO_GFILE_DIR:-} ]] && [[ -f "${KOKORO_GFILE_DIR}/distdir/abseil-cpp_vendor.tar.gz" ]]; then
+  DOCKER_EXTRA_ARGS="--mount type=bind,source=${KOKORO_GFILE_DIR}/distdir,target=/distdir,readonly --env=BAZEL_VENDOR_ARCHIVE=/distdir/abseil-cpp_vendor.tar.gz ${DOCKER_EXTRA_ARGS:-}"
+  BAZEL_EXTRA_ARGS="--vendor_dir=/abseil-cpp_vendor ${BAZEL_EXTRA_ARGS:-}"
 fi
 
 for std in ${STD}; do
@@ -55,29 +69,39 @@ for std in ${STD}; do
     for exceptions_mode in ${EXCEPTIONS_MODE}; do
       echo "--------------------------------------------------------------------"
       time docker run \
-        --volume="${ABSEIL_ROOT}:/abseil-cpp:ro" \
+        --env="USE_BAZEL_VERSION=9.1.0" \
+        --env="CC=/opt/llvm/bin/clang" \
+        --env="BAZEL_CXXOPTS=-std=${std}:-nostdinc++" \
+        --env="BAZEL_LINKOPTS=-L/opt/llvm/lib/x86_64-unknown-linux-gnu:-lc++:-lc++abi:-lm:-Wl,-rpath=/opt/llvm/lib/x86_64-unknown-linux-gnu" \
+        --env="CPLUS_INCLUDE_PATH=/opt/llvm/include/c++/v1:/opt/llvm/include/x86_64-unknown-linux-gnu/c++/v1/" \
+        --mount type=bind,source="${ABSEIL_ROOT}",target=/abseil-cpp-ro,readonly \
+        --tmpfs=/abseil-cpp \
         --workdir=/abseil-cpp \
         --cap-add=SYS_PTRACE \
         --rm \
-        -e CC="/opt/llvm/clang/bin/clang" \
-        -e BAZEL_COMPILER="llvm" \
-        -e BAZEL_CXXOPTS="-std=${std}:-nostdinc++" \
-        -e BAZEL_LINKOPTS="-L/opt/llvm/libcxx/lib:-lc++:-lc++abi:-lm:-Wl,-rpath=/opt/llvm/libcxx/lib" \
-        -e CPLUS_INCLUDE_PATH="/opt/llvm/libcxx/include/c++/v1" \
         ${DOCKER_EXTRA_ARGS:-} \
         ${DOCKER_CONTAINER} \
-        /usr/local/bin/bazel test ... \
-          --compilation_mode="${compilation_mode}" \
-          --copt="${exceptions_mode}" \
-          --copt=-Werror \
-          --define="absl=1" \
-          --keep_going \
-          --show_timestamps \
-          --test_env="GTEST_INSTALL_FAILURE_SIGNAL_HANDLER=1" \
-          --test_env="TZDIR=/abseil-cpp/absl/time/internal/cctz/testdata/zoneinfo" \
-          --test_output=errors \
-          --test_tag_filters=-benchmark \
-          ${BAZEL_EXTRA_ARGS:-}
+        /bin/bash --login -c "
+          cp -r /abseil-cpp-ro/* /abseil-cpp/
+          if [ -n \"${ALTERNATE_OPTIONS:-}\" ]; then
+            cp ${ALTERNATE_OPTIONS:-} absl/base/options.h || exit 1
+          fi
+          /usr/local/bin/bazel test ... \
+            --compilation_mode=\"${compilation_mode}\" \
+            --copt=\"${exceptions_mode}\" \
+            --copt=\"-DGTEST_REMOVE_LEGACY_TEST_CASEAPI_=1\" \
+            --copt=-Werror \
+            --define=\"absl=1\" \
+            --enable_bzlmod=true \
+            --features=external_include_paths \
+            --keep_going \
+            --per_file_copt=external/.*@-w \
+            --show_timestamps \
+            --test_env=\"GTEST_INSTALL_FAILURE_SIGNAL_HANDLER=1\" \
+            --test_env=\"TZDIR=/abseil-cpp/absl/time/internal/cctz/testdata/zoneinfo\" \
+            --test_output=errors \
+            --test_tag_filters=-benchmark \
+            ${BAZEL_EXTRA_ARGS:-}"
     done
   done
 done
diff --git a/ci/linux_clang-latest_libcxx_tsan_bazel.sh b/ci/linux_clang-latest_libcxx_tsan_bazel.sh
index 8721ada2ed7..56c3c412e9a 100755
--- a/ci/linux_clang-latest_libcxx_tsan_bazel.sh
+++ b/ci/linux_clang-latest_libcxx_tsan_bazel.sh
@@ -20,34 +20,48 @@
 
 set -euox pipefail
 
-if [ -z ${ABSEIL_ROOT:-} ]; then
+if [[ -z ${ABSEIL_ROOT:-} ]]; then
   ABSEIL_ROOT="$(realpath $(dirname ${0})/..)"
 fi
 
-if [ -z ${STD:-} ]; then
-  STD="c++11 c++14 c++17"
+if [[ -z ${STD:-} ]]; then
+  STD="c++17 c++20 c++23"
 fi
 
-if [ -z ${COMPILATION_MODE:-} ]; then
+if [[ -z ${COMPILATION_MODE:-} ]]; then
   COMPILATION_MODE="fastbuild opt"
 fi
 
-if [ -z ${EXCEPTIONS_MODE:-} ]; then
+if [[ -z ${EXCEPTIONS_MODE:-} ]]; then
   EXCEPTIONS_MODE="-fno-exceptions -fexceptions"
 fi
 
-readonly DOCKER_CONTAINER="gcr.io/google.com/absl-177019/linux_clang-latest:20190813"
+source "${ABSEIL_ROOT}/ci/linux_docker_containers.sh"
+readonly DOCKER_CONTAINER=${LINUX_CLANG_LATEST_CONTAINER}
+
+# Print information about the environment.
+docker run "${DOCKER_CONTAINER}" /usr/bin/fastfetch -c ci
+docker run "${DOCKER_CONTAINER}" /opt/llvm/bin/clang -v
+docker run "${DOCKER_CONTAINER}" cat /root/cached_bazel_versions
 
 # USE_BAZEL_CACHE=1 only works on Kokoro.
 # Without access to the credentials this won't work.
-if [ ${USE_BAZEL_CACHE:-0} -ne 0 ]; then
-  DOCKER_EXTRA_ARGS="--volume=${KOKORO_KEYSTORE_DIR}:/keystore:ro ${DOCKER_EXTRA_ARGS:-}"
+if [[ ${USE_BAZEL_CACHE:-0} -ne 0 ]]; then
+  DOCKER_EXTRA_ARGS="--mount type=bind,source=${KOKORO_KEYSTORE_DIR},target=/keystore,readonly ${DOCKER_EXTRA_ARGS:-}"
   # Bazel doesn't track changes to tools outside of the workspace
   # (e.g. /usr/bin/gcc), so by appending the docker container to the
   # remote_http_cache url, we make changes to the container part of
   # the cache key. Hashing the key is to make it shorter and url-safe.
   container_key=$(echo ${DOCKER_CONTAINER} | sha256sum | head -c 16)
-  BAZEL_EXTRA_ARGS="--remote_http_cache=https://storage.googleapis.com/absl-bazel-remote-cache/${container_key} --google_credentials=/keystore/73103_absl-bazel-remote-cache ${BAZEL_EXTRA_ARGS:-}"
+  BAZEL_EXTRA_ARGS="--remote_cache=https://storage.googleapis.com/absl-bazel-remote-cache/${container_key} --google_credentials=/keystore/73103_absl-bazel-remote-cache ${BAZEL_EXTRA_ARGS:-}"
+fi
+
+# Use Bazel Vendor mode to reduce reliance on external dependencies.
+# See https://bazel.build/external/vendor and the Dockerfile for
+# an explaination of how this works.
+if [[ ${KOKORO_GFILE_DIR:-} ]] && [[ -f "${KOKORO_GFILE_DIR}/distdir/abseil-cpp_vendor.tar.gz" ]]; then
+  DOCKER_EXTRA_ARGS="--mount type=bind,source=${KOKORO_GFILE_DIR}/distdir,target=/distdir,readonly --env=BAZEL_VENDOR_ARCHIVE=/distdir/abseil-cpp_vendor.tar.gz ${DOCKER_EXTRA_ARGS:-}"
+  BAZEL_EXTRA_ARGS="--vendor_dir=/abseil-cpp_vendor ${BAZEL_EXTRA_ARGS:-}"
 fi
 
 for std in ${STD}; do
@@ -55,34 +69,38 @@ for std in ${STD}; do
     for exceptions_mode in ${EXCEPTIONS_MODE}; do
       echo "--------------------------------------------------------------------"
       time docker run \
-        --volume="${ABSEIL_ROOT}:/abseil-cpp:ro" \
+        --env="USE_BAZEL_VERSION=9.1.0" \
+        --env="CC=/opt/llvm/bin/clang" \
+        --env="BAZEL_CXXOPTS=-std=${std}:-nostdinc++" \
+        --env="BAZEL_LINKOPTS=-L/opt/llvm-tsan/lib:-lc++:-lc++abi:-lgcc_s:-lm:-Wl,-rpath=/opt/llvm-tsan/lib" \
+        --env="CPLUS_INCLUDE_PATH=/opt/llvm-tsan/include/c++/v1" \
+        --mount type=bind,source="${ABSEIL_ROOT}",target=/abseil-cpp,readonly \
         --workdir=/abseil-cpp \
         --cap-add=SYS_PTRACE \
         --rm \
-        -e CC="/opt/llvm/clang/bin/clang" \
-        -e BAZEL_COMPILER="llvm" \
-        -e BAZEL_CXXOPTS="-std=${std}:-nostdinc++" \
-        -e BAZEL_LINKOPTS="-L/opt/llvm/libcxx-tsan/lib:-lc++:-lc++abi:-lm:-Wl,-rpath=/opt/llvm/libcxx-tsan/lib" \
-        -e CPLUS_INCLUDE_PATH="/opt/llvm/libcxx-tsan/include/c++/v1" \
         ${DOCKER_EXTRA_ARGS:-} \
         ${DOCKER_CONTAINER} \
+        /bin/bash --login -c "
         /usr/local/bin/bazel test ... \
-          --build_tag_filters="-notsan" \
-          --compilation_mode="${compilation_mode}" \
-          --copt="${exceptions_mode}" \
-          --copt="-DDYNAMIC_ANNOTATIONS_ENABLED=1" \
-          --copt="-DTHREAD_SANITIZER" \
-          --copt="-fsanitize=thread" \
+          --build_tag_filters=\"-notsan\" \
+          --compilation_mode=\"${compilation_mode}\" \
+          --copt=\"${exceptions_mode}\" \
+          --copt=\"-DGTEST_REMOVE_LEGACY_TEST_CASEAPI_=1\" \
+          --copt=\"-fsanitize=thread\" \
+          --copt=\"-fno-sanitize-blacklist\" \
           --copt=-Werror \
+          --define=\"absl=1\" \
+          --enable_bzlmod=true \
+          --features=external_include_paths \
           --keep_going \
-          --linkopt="-fsanitize=thread" \
+          --linkopt=\"-fsanitize=thread\" \
+          --per_file_copt=external/.*@-w \
           --show_timestamps \
-          --test_env="TSAN_OPTIONS=report_atomic_races=0" \
-          --test_env="TSAN_SYMBOLIZER_PATH=/opt/llvm/clang/bin/llvm-symbolizer" \
-          --test_env="TZDIR=/abseil-cpp/absl/time/internal/cctz/testdata/zoneinfo" \
+          --test_env=\"TSAN_SYMBOLIZER_PATH=/opt/llvm/bin/llvm-symbolizer\" \
+          --test_env=\"TZDIR=/abseil-cpp/absl/time/internal/cctz/testdata/zoneinfo\" \
           --test_output=errors \
-          --test_tag_filters="-benchmark,-notsan" \
-          ${BAZEL_EXTRA_ARGS:-}
+          --test_tag_filters=\"-benchmark,-notsan\" \
+          ${BAZEL_EXTRA_ARGS:-}"
     done
   done
 done
diff --git a/ci/linux_clang-latest_libstdcxx_bazel.sh b/ci/linux_clang-latest_libstdcxx_bazel.sh
index 78ce0b32e49..2fc8210f5fb 100755
--- a/ci/linux_clang-latest_libstdcxx_bazel.sh
+++ b/ci/linux_clang-latest_libstdcxx_bazel.sh
@@ -20,34 +20,49 @@
 
 set -euox pipefail
 
-if [ -z ${ABSEIL_ROOT:-} ]; then
+if [[ -z ${ABSEIL_ROOT:-} ]]; then
   ABSEIL_ROOT="$(realpath $(dirname ${0})/..)"
 fi
 
-if [ -z ${STD:-} ]; then
-  STD="c++11 c++14 c++17"
+if [[ -z ${STD:-} ]]; then
+  STD="c++17 c++20 c++23"
 fi
 
-if [ -z ${COMPILATION_MODE:-} ]; then
+if [[ -z ${COMPILATION_MODE:-} ]]; then
   COMPILATION_MODE="fastbuild opt"
 fi
 
-if [ -z ${EXCEPTIONS_MODE:-} ]; then
+if [[ -z ${EXCEPTIONS_MODE:-} ]]; then
   EXCEPTIONS_MODE="-fno-exceptions -fexceptions"
 fi
 
-readonly DOCKER_CONTAINER="gcr.io/google.com/absl-177019/linux_clang-latest:20190813"
+source "${ABSEIL_ROOT}/ci/linux_docker_containers.sh"
+readonly DOCKER_CONTAINER=${LINUX_CLANG_LATEST_CONTAINER}
+
+# Print information about the environment.
+docker run "${DOCKER_CONTAINER}" /usr/bin/fastfetch -c ci
+docker run "${DOCKER_CONTAINER}" /usr/local/bin/gcc -v
+docker run "${DOCKER_CONTAINER}" /opt/llvm/bin/clang -v
+docker run "${DOCKER_CONTAINER}" cat /root/cached_bazel_versions
 
 # USE_BAZEL_CACHE=1 only works on Kokoro.
 # Without access to the credentials this won't work.
-if [ ${USE_BAZEL_CACHE:-0} -ne 0 ]; then
-  DOCKER_EXTRA_ARGS="--volume=${KOKORO_KEYSTORE_DIR}:/keystore:ro ${DOCKER_EXTRA_ARGS:-}"
+if [[ ${USE_BAZEL_CACHE:-0} -ne 0 ]]; then
+  DOCKER_EXTRA_ARGS="--mount type=bind,source=${KOKORO_KEYSTORE_DIR},target=/keystore,readonly ${DOCKER_EXTRA_ARGS:-}"
   # Bazel doesn't track changes to tools outside of the workspace
   # (e.g. /usr/bin/gcc), so by appending the docker container to the
   # remote_http_cache url, we make changes to the container part of
   # the cache key. Hashing the key is to make it shorter and url-safe.
   container_key=$(echo ${DOCKER_CONTAINER} | sha256sum | head -c 16)
-  BAZEL_EXTRA_ARGS="--remote_http_cache=https://storage.googleapis.com/absl-bazel-remote-cache/${container_key} --google_credentials=/keystore/73103_absl-bazel-remote-cache ${BAZEL_EXTRA_ARGS:-}"
+  BAZEL_EXTRA_ARGS="--remote_cache=https://storage.googleapis.com/absl-bazel-remote-cache/${container_key} --google_credentials=/keystore/73103_absl-bazel-remote-cache ${BAZEL_EXTRA_ARGS:-}"
+fi
+
+# Use Bazel Vendor mode to reduce reliance on external dependencies.
+# See https://bazel.build/external/vendor and the Dockerfile for
+# an explaination of how this works.
+if [[ ${KOKORO_GFILE_DIR:-} ]] && [[ -f "${KOKORO_GFILE_DIR}/distdir/abseil-cpp_vendor.tar.gz" ]]; then
+  DOCKER_EXTRA_ARGS="--mount type=bind,source=${KOKORO_GFILE_DIR}/distdir,target=/distdir,readonly --env=BAZEL_VENDOR_ARCHIVE=/distdir/abseil-cpp_vendor.tar.gz ${DOCKER_EXTRA_ARGS:-}"
+  BAZEL_EXTRA_ARGS="--vendor_dir=/abseil-cpp_vendor ${BAZEL_EXTRA_ARGS:-}"
 fi
 
 for std in ${STD}; do
@@ -55,28 +70,35 @@ for std in ${STD}; do
     for exceptions_mode in ${EXCEPTIONS_MODE}; do
       echo "--------------------------------------------------------------------"
       time docker run \
-        --volume="${ABSEIL_ROOT}:/abseil-cpp:ro" \
+        --env="USE_BAZEL_VERSION=9.1.0" \
+        --env="CC=/opt/llvm/bin/clang" \
+        --env="BAZEL_CXXOPTS=-std=${std}" \
+        --mount type=bind,source="${ABSEIL_ROOT}",target=/abseil-cpp,readonly \
         --workdir=/abseil-cpp \
         --cap-add=SYS_PTRACE \
         --rm \
-        -e CC="/opt/llvm/clang/bin/clang" \
-        -e BAZEL_COMPILER="llvm" \
-        -e BAZEL_CXXOPTS="-std=${std}" \
-        -e CPLUS_INCLUDE_PATH="/usr/include/c++/6" \
         ${DOCKER_EXTRA_ARGS:-} \
         ${DOCKER_CONTAINER} \
+        /bin/bash --login -c "
         /usr/local/bin/bazel test ... \
-          --compilation_mode="${compilation_mode}" \
-          --copt="${exceptions_mode}" \
+          --compilation_mode=\"${compilation_mode}\" \
+          --copt=\"--gcc-toolchain=/usr/local\" \
+          --copt=\"-DGTEST_REMOVE_LEGACY_TEST_CASEAPI_=1\" \
+          --copt=\"${exceptions_mode}\" \
+          --copt=\"-march=haswell\" \
           --copt=-Werror \
-          --define="absl=1" \
+          --define=\"absl=1\" \
+          --enable_bzlmod=true \
+          --features=external_include_paths \
           --keep_going \
+          --linkopt=\"--gcc-toolchain=/usr/local\" \
+          --per_file_copt=external/.*@-w \
           --show_timestamps \
-          --test_env="GTEST_INSTALL_FAILURE_SIGNAL_HANDLER=1" \
-          --test_env="TZDIR=/abseil-cpp/absl/time/internal/cctz/testdata/zoneinfo" \
+          --test_env=\"GTEST_INSTALL_FAILURE_SIGNAL_HANDLER=1\" \
+          --test_env=\"TZDIR=/abseil-cpp/absl/time/internal/cctz/testdata/zoneinfo\" \
           --test_output=errors \
           --test_tag_filters=-benchmark \
-          ${BAZEL_EXTRA_ARGS:-}
+          ${BAZEL_EXTRA_ARGS:-}"
     done
   done
 done
diff --git a/ci/linux_docker_containers.sh b/ci/linux_docker_containers.sh
new file mode 100644
index 00000000000..7b6e0a8489e
--- /dev/null
+++ b/ci/linux_docker_containers.sh
@@ -0,0 +1,22 @@
+# Copyright 2019 The Abseil Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# The file contains Docker container identifiers currently used by test scripts.
+# Test scripts should source this file to get the identifiers.
+
+readonly LINUX_ALPINE_CONTAINER="gcr.io/google.com/absl-177019/alpine:20230612"
+readonly LINUX_CLANG_LATEST_CONTAINER="gcr.io/google.com/absl-177019/linux_hybrid-latest@sha256:baa99e65a3cc837bab8e5246f1870d25c76d0adf2b1d598ee5a3ec77fd2e19bd"
+readonly LINUX_ARM_CLANG_LATEST_CONTAINER="gcr.io/google.com/absl-177019/linux_arm_hybrid-latest@sha256:04479ae67993e1a70dc1730bcddfe3233ac3af178ede0b33044dd9c31b889ac8"
+readonly LINUX_GCC_LATEST_CONTAINER="gcr.io/google.com/absl-177019/linux_hybrid-latest@sha256:baa99e65a3cc837bab8e5246f1870d25c76d0adf2b1d598ee5a3ec77fd2e19bd"
+readonly LINUX_GCC_FLOOR_CONTAINER="gcr.io/google.com/absl-177019/linux_hybrid-latest@sha256:baa99e65a3cc837bab8e5246f1870d25c76d0adf2b1d598ee5a3ec77fd2e19bd"
diff --git a/ci/linux_gcc-4.9_libstdcxx_bazel.sh b/ci/linux_gcc-floor_libstdcxx_bazel.sh
similarity index 55%
rename from ci/linux_gcc-4.9_libstdcxx_bazel.sh
rename to ci/linux_gcc-floor_libstdcxx_bazel.sh
index c3b936bc209..765b784f553 100755
--- a/ci/linux_gcc-4.9_libstdcxx_bazel.sh
+++ b/ci/linux_gcc-floor_libstdcxx_bazel.sh
@@ -1,6 +1,6 @@
 #!/bin/bash
 #
-# Copyright 2019 The Abseil Authors.
+# Copyright 2020 The Abseil Authors.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -20,27 +20,33 @@
 
 set -euox pipefail
 
-if [ -z ${ABSEIL_ROOT:-} ]; then
+if [[ -z ${ABSEIL_ROOT:-} ]]; then
   ABSEIL_ROOT="$(realpath $(dirname ${0})/..)"
 fi
 
-if [ -z ${STD:-} ]; then
-  STD="c++11 c++14"
+if [[ -z ${STD:-} ]]; then
+  STD="c++17"
 fi
 
-if [ -z ${COMPILATION_MODE:-} ]; then
+if [[ -z ${COMPILATION_MODE:-} ]]; then
   COMPILATION_MODE="fastbuild opt"
 fi
 
-if [ -z ${EXCEPTIONS_MODE:-} ]; then
+if [[ -z ${EXCEPTIONS_MODE:-} ]]; then
   EXCEPTIONS_MODE="-fno-exceptions -fexceptions"
 fi
 
-readonly DOCKER_CONTAINER="gcr.io/google.com/absl-177019/linux_gcc-4.9:20190702"
+source "${ABSEIL_ROOT}/ci/linux_docker_containers.sh"
+readonly DOCKER_CONTAINER="${LINUX_GCC_FLOOR_CONTAINER}"
+
+# Print information about the environment.
+docker run "${DOCKER_CONTAINER}" /usr/bin/fastfetch -c ci
+docker run "${DOCKER_CONTAINER}" /opt/gcc-10/bin/gcc -v
+docker run "${DOCKER_CONTAINER}" cat /root/cached_bazel_versions
 
 # USE_BAZEL_CACHE=1 only works on Kokoro.
 # Without access to the credentials this won't work.
-if [ ${USE_BAZEL_CACHE:-0} -ne 0 ]; then
+if [[ ${USE_BAZEL_CACHE:-0} -ne 0 ]]; then
   DOCKER_EXTRA_ARGS="--volume=${KOKORO_KEYSTORE_DIR}:/keystore:ro ${DOCKER_EXTRA_ARGS:-}"
   # Bazel doesn't track changes to tools outside of the workspace
   # (e.g. /usr/bin/gcc), so by appending the docker container to the
@@ -50,31 +56,45 @@ if [ ${USE_BAZEL_CACHE:-0} -ne 0 ]; then
   BAZEL_EXTRA_ARGS="--remote_http_cache=https://storage.googleapis.com/absl-bazel-remote-cache/${container_key} --google_credentials=/keystore/73103_absl-bazel-remote-cache ${BAZEL_EXTRA_ARGS:-}"
 fi
 
+# Use Bazel Vendor mode to reduce reliance on external dependencies.
+# See https://bazel.build/external/vendor and the Dockerfile for
+# an explaination of how this works.
+if [[ ${KOKORO_GFILE_DIR:-} ]] && [[ -f "${KOKORO_GFILE_DIR}/distdir/abseil-cpp_vendor.tar.gz" ]]; then
+  DOCKER_EXTRA_ARGS="--mount type=bind,source=${KOKORO_GFILE_DIR}/distdir,target=/distdir,readonly --env=BAZEL_VENDOR_ARCHIVE=/distdir/abseil-cpp_vendor.tar.gz ${DOCKER_EXTRA_ARGS:-}"
+  BAZEL_EXTRA_ARGS="--vendor_dir=/abseil-cpp_vendor ${BAZEL_EXTRA_ARGS:-}"
+fi
+
 for std in ${STD}; do
   for compilation_mode in ${COMPILATION_MODE}; do
     for exceptions_mode in ${EXCEPTIONS_MODE}; do
       echo "--------------------------------------------------------------------"
       time docker run \
+        --env="USE_BAZEL_VERSION=9.1.0" \
+        --env="CC=/opt/gcc-10/bin/gcc" \
+        --env="BAZEL_CXXOPTS=-std=${std}" \
+        --env="BAZEL_LINKOPTS=-L/opt/gcc-10/lib64:-Wl,-rpath=/opt/gcc-10/lib64" \
         --volume="${ABSEIL_ROOT}:/abseil-cpp:ro" \
         --workdir=/abseil-cpp \
         --cap-add=SYS_PTRACE \
         --rm \
-        -e CC="/usr/bin/gcc-4.9" \
-        -e BAZEL_CXXOPTS="-std=${std}" \
         ${DOCKER_EXTRA_ARGS:-} \
         ${DOCKER_CONTAINER} \
+        /bin/bash --login -c "
         /usr/local/bin/bazel test ... \
-          --compilation_mode="${compilation_mode}" \
-          --copt="${exceptions_mode}" \
+          --compilation_mode=\"${compilation_mode}\" \
+          --copt=\"${exceptions_mode}\" \
+          --copt=\"-DGTEST_REMOVE_LEGACY_TEST_CASEAPI_=1\" \
           --copt=-Werror \
-          --define="absl=1" \
+          --define=\"absl=1\" \
+          --features=external_include_paths \
           --keep_going \
+          --per_file_copt=external/.*@-w \
           --show_timestamps \
-          --test_env="GTEST_INSTALL_FAILURE_SIGNAL_HANDLER=1" \
-          --test_env="TZDIR=/abseil-cpp/absl/time/internal/cctz/testdata/zoneinfo" \
+          --test_env=\"GTEST_INSTALL_FAILURE_SIGNAL_HANDLER=1\" \
+          --test_env=\"TZDIR=/abseil-cpp/absl/time/internal/cctz/testdata/zoneinfo\" \
           --test_output=errors \
           --test_tag_filters=-benchmark \
-          ${BAZEL_EXTRA_ARGS:-}
+          ${BAZEL_EXTRA_ARGS:-}"
     done
   done
 done
diff --git a/ci/linux_gcc-latest_libstdcxx_bazel.sh b/ci/linux_gcc-latest_libstdcxx_bazel.sh
index 10f6be2135a..509aeb919d4 100755
--- a/ci/linux_gcc-latest_libstdcxx_bazel.sh
+++ b/ci/linux_gcc-latest_libstdcxx_bazel.sh
@@ -20,34 +20,48 @@
 
 set -euox pipefail
 
-if [ -z ${ABSEIL_ROOT:-} ]; then
+if [[ -z ${ABSEIL_ROOT:-} ]]; then
   ABSEIL_ROOT="$(realpath $(dirname ${0})/..)"
 fi
 
-if [ -z ${STD:-} ]; then
-  STD="c++11 c++14 c++17"
+if [[ -z ${STD:-} ]]; then
+  STD="c++17 c++20 c++23"
 fi
 
-if [ -z ${COMPILATION_MODE:-} ]; then
+if [[ -z ${COMPILATION_MODE:-} ]]; then
   COMPILATION_MODE="fastbuild opt"
 fi
 
-if [ -z ${EXCEPTIONS_MODE:-} ]; then
+if [[ -z ${EXCEPTIONS_MODE:-} ]]; then
   EXCEPTIONS_MODE="-fno-exceptions -fexceptions"
 fi
 
-readonly DOCKER_CONTAINER="gcr.io/google.com/absl-177019/linux_gcc-latest:20190703"
+source "${ABSEIL_ROOT}/ci/linux_docker_containers.sh"
+readonly DOCKER_CONTAINER=${LINUX_GCC_LATEST_CONTAINER}
+
+# Print information about the environment.
+docker run "${DOCKER_CONTAINER}" /usr/bin/fastfetch -c ci
+docker run "${DOCKER_CONTAINER}" /usr/local/bin/gcc -v
+docker run "${DOCKER_CONTAINER}" cat /root/cached_bazel_versions
 
 # USE_BAZEL_CACHE=1 only works on Kokoro.
 # Without access to the credentials this won't work.
-if [ ${USE_BAZEL_CACHE:-0} -ne 0 ]; then
-  DOCKER_EXTRA_ARGS="--volume=${KOKORO_KEYSTORE_DIR}:/keystore:ro ${DOCKER_EXTRA_ARGS:-}"
+if [[ ${USE_BAZEL_CACHE:-0} -ne 0 ]]; then
+  DOCKER_EXTRA_ARGS="--mount type=bind,source=${KOKORO_KEYSTORE_DIR},target=/keystore,readonly ${DOCKER_EXTRA_ARGS:-}"
   # Bazel doesn't track changes to tools outside of the workspace
   # (e.g. /usr/bin/gcc), so by appending the docker container to the
   # remote_http_cache url, we make changes to the container part of
   # the cache key. Hashing the key is to make it shorter and url-safe.
   container_key=$(echo ${DOCKER_CONTAINER} | sha256sum | head -c 16)
-  BAZEL_EXTRA_ARGS="--remote_http_cache=https://storage.googleapis.com/absl-bazel-remote-cache/${container_key} --google_credentials=/keystore/73103_absl-bazel-remote-cache ${BAZEL_EXTRA_ARGS:-}"
+  BAZEL_EXTRA_ARGS="--remote_cache=https://storage.googleapis.com/absl-bazel-remote-cache/${container_key} --google_credentials=/keystore/73103_absl-bazel-remote-cache ${BAZEL_EXTRA_ARGS:-}"
+fi
+
+# Use Bazel Vendor mode to reduce reliance on external dependencies.
+# See https://bazel.build/external/vendor and the Dockerfile for
+# an explaination of how this works.
+if [[ ${KOKORO_GFILE_DIR:-} ]] && [[ -f "${KOKORO_GFILE_DIR}/distdir/abseil-cpp_vendor.tar.gz" ]]; then
+  DOCKER_EXTRA_ARGS="--mount type=bind,source=${KOKORO_GFILE_DIR}/distdir,target=/distdir,readonly --env=BAZEL_VENDOR_ARCHIVE=/distdir/abseil-cpp_vendor.tar.gz ${DOCKER_EXTRA_ARGS:-}"
+  BAZEL_EXTRA_ARGS="--vendor_dir=/abseil-cpp_vendor ${BAZEL_EXTRA_ARGS:-}"
 fi
 
 for std in ${STD}; do
@@ -55,26 +69,37 @@ for std in ${STD}; do
     for exceptions_mode in ${EXCEPTIONS_MODE}; do
       echo "--------------------------------------------------------------------"
       time docker run \
-        --volume="${ABSEIL_ROOT}:/abseil-cpp:ro" \
+        --env="USE_BAZEL_VERSION=9.1.0" \
+        --env="CC=/usr/local/bin/gcc" \
+        --env="BAZEL_CXXOPTS=-std=${std}" \
+        --mount type=bind,source="${ABSEIL_ROOT}",target=/abseil-cpp-ro,readonly \
+        --tmpfs=/abseil-cpp \
         --workdir=/abseil-cpp \
         --cap-add=SYS_PTRACE \
         --rm \
-        -e CC="/usr/local/bin/gcc" \
-        -e BAZEL_CXXOPTS="-std=${std}" \
         ${DOCKER_EXTRA_ARGS:-} \
         ${DOCKER_CONTAINER} \
-        /usr/local/bin/bazel test ... \
-          --compilation_mode="${compilation_mode}" \
-          --copt="${exceptions_mode}" \
-          --copt=-Werror \
-          --define="absl=1" \
-          --keep_going \
-          --show_timestamps \
-          --test_env="GTEST_INSTALL_FAILURE_SIGNAL_HANDLER=1" \
-          --test_env="TZDIR=/abseil-cpp/absl/time/internal/cctz/testdata/zoneinfo" \
-          --test_output=errors \
-          --test_tag_filters=-benchmark \
-          ${BAZEL_EXTRA_ARGS:-}
+        /bin/bash --login -c "
+          cp -r /abseil-cpp-ro/* /abseil-cpp/
+          if [ -n \"${ALTERNATE_OPTIONS:-}\" ]; then
+            cp ${ALTERNATE_OPTIONS:-} absl/base/options.h || exit 1
+          fi
+          /usr/local/bin/bazel test ... \
+            --compilation_mode=\"${compilation_mode}\" \
+            --copt=\"${exceptions_mode}\" \
+            --copt=\"-DGTEST_REMOVE_LEGACY_TEST_CASEAPI_=1\" \
+            --copt=-Werror \
+            --define=\"absl=1\" \
+            --enable_bzlmod=true \
+            --features=external_include_paths \
+            --keep_going \
+            --per_file_copt=external/.*@-w \
+            --show_timestamps \
+            --test_env=\"GTEST_INSTALL_FAILURE_SIGNAL_HANDLER=1\" \
+            --test_env=\"TZDIR=/abseil-cpp/absl/time/internal/cctz/testdata/zoneinfo\" \
+            --test_output=errors \
+            --test_tag_filters=-benchmark \
+            ${BAZEL_EXTRA_ARGS:-}"
     done
   done
 done
diff --git a/ci/linux_gcc-latest_libstdcxx_cmake.sh b/ci/linux_gcc-latest_libstdcxx_cmake.sh
index 3e831c14b86..2f69bba7178 100755
--- a/ci/linux_gcc-latest_libstdcxx_cmake.sh
+++ b/ci/linux_gcc-latest_libstdcxx_cmake.sh
@@ -14,48 +14,72 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-# TODO(absl-team): This script isn't fully hermetic because
-# -DABSL_USE_GOOGLETEST_HEAD=ON means that this script isn't pinned to a fixed
-# version of GoogleTest. This means that an upstream change to GoogleTest could
-# break this test. Fix this by allowing this script to pin to a known-good
-# version of GoogleTest.
-
 set -euox pipefail
 
-if [ -z ${ABSEIL_ROOT:-} ]; then
+if [[ -z ${ABSEIL_ROOT:-} ]]; then
   ABSEIL_ROOT="$(realpath $(dirname ${0})/..)"
 fi
 
-if [ -z ${ABSL_CMAKE_CXX_STANDARDS:-} ]; then
-  ABSL_CMAKE_CXX_STANDARDS="11 14 17"
+source "${ABSEIL_ROOT}/ci/cmake_common.sh"
+
+# Avoid depending on GitHub by looking for a cached copy of GoogleTest.
+if [[ -r "${KOKORO_GFILE_DIR:-}/distdir/googletest-${ABSL_GOOGLETEST_VERSION}.tar.gz" ]]; then
+  ABSL_GOOGLETEST_DOWNLOAD_URL="file:///distdir/googletest-${ABSL_GOOGLETEST_VERSION}.tar.gz"
+  DOCKER_EXTRA_ARGS="--mount type=bind,source=${KOKORO_GFILE_DIR}/distdir,target=/distdir,readonly ${DOCKER_EXTRA_ARGS:-}"
 fi
 
-if [ -z ${ABSL_CMAKE_BUILD_TYPES:-} ]; then
+if [[ -z ${ABSL_CMAKE_CXX_STANDARDS:-} ]]; then
+  ABSL_CMAKE_CXX_STANDARDS="17 20"
+fi
+
+if [[ -z ${ABSL_CMAKE_BUILD_TYPES:-} ]]; then
   ABSL_CMAKE_BUILD_TYPES="Debug Release"
 fi
 
+if [[ -z ${ABSL_CMAKE_BUILD_SHARED:-} ]]; then
+  ABSL_CMAKE_BUILD_SHARED="OFF ON"
+fi
+
+if [[ -z ${ABSL_CMAKE_BUILD_MONOLITHIC_SHARED_LIBS:-} ]]; then
+  ABSL_CMAKE_BUILD_MONOLITHIC_SHARED_LIBS="OFF ON"
+fi
+
+source "${ABSEIL_ROOT}/ci/linux_docker_containers.sh"
+readonly DOCKER_CONTAINER=${LINUX_GCC_LATEST_CONTAINER}
+
 for std in ${ABSL_CMAKE_CXX_STANDARDS}; do
   for compilation_mode in ${ABSL_CMAKE_BUILD_TYPES}; do
-    echo "--------------------------------------------------------------------"
-    echo "Testing with CMAKE_BUILD_TYPE=${compilation_mode} and -std=c++${std}"
-
-    time docker run \
-      --volume="${ABSEIL_ROOT}:/abseil-cpp:ro" \
-      --workdir=/abseil-cpp \
-      --tmpfs=/buildfs:exec \
-      --cap-add=SYS_PTRACE \
-      --rm \
-      -e CFLAGS="-Werror" \
-      -e CXXFLAGS="-Werror" \
-      gcr.io/google.com/absl-177019/linux_gcc-latest:20190703 \
-      /bin/bash -c "
-        cd /buildfs && \
-        cmake /abseil-cpp \
-          -DABSL_USE_GOOGLETEST_HEAD=ON \
-          -DABSL_RUN_TESTS=ON \
-          -DCMAKE_BUILD_TYPE=${compilation_mode} \
-          -DCMAKE_CXX_STANDARD=${std} && \
-        make -j$(nproc) && \
-        ctest -j$(nproc) --output-on-failure"
+    for build_shared in ${ABSL_CMAKE_BUILD_SHARED}; do
+      if [[ $build_shared == "OFF" ]]; then
+        monolithic_shared_options="OFF"
+      else
+        monolithic_shared_options="$ABSL_CMAKE_BUILD_MONOLITHIC_SHARED_LIBS"
+      fi
+
+      for monolithic_shared in $monolithic_shared_options; do
+        time docker run \
+          --mount type=bind,source="${ABSEIL_ROOT}",target=/abseil-cpp,readonly \
+          --tmpfs=/buildfs:exec \
+          --workdir=/buildfs \
+          --cap-add=SYS_PTRACE \
+          --rm \
+          -e CFLAGS="-Werror" \
+          -e CXXFLAGS="-Werror" \
+          ${DOCKER_EXTRA_ARGS:-} \
+          "${DOCKER_CONTAINER}" \
+          /bin/bash -c "
+            cmake /abseil-cpp \
+              -DABSL_GOOGLETEST_DOWNLOAD_URL=${ABSL_GOOGLETEST_DOWNLOAD_URL} \
+              -DBUILD_SHARED_LIBS=${build_shared} \
+              -DABSL_BUILD_TESTING=ON \
+              -DCMAKE_BUILD_TYPE=${compilation_mode} \
+              -DCMAKE_CXX_STANDARD=${std} \
+              -DABSL_BUILD_MONOLITHIC_SHARED_LIBS=${monolithic_shared} \
+              -DCMAKE_MODULE_LINKER_FLAGS=\"-Wl,--no-undefined\" && \
+            make -j$(nproc) && \
+            TZDIR=/abseil-cpp/absl/time/internal/cctz/testdata/zoneinfo \
+            ctest -j$(nproc) --output-on-failure"
+        done
+    done
   done
 done
diff --git a/ci/linux_gcc_alpine_cmake.sh b/ci/linux_gcc_alpine_cmake.sh
new file mode 100755
index 00000000000..1d64a95acdf
--- /dev/null
+++ b/ci/linux_gcc_alpine_cmake.sh
@@ -0,0 +1,85 @@
+#!/bin/bash
+#
+# Copyright 2019 The Abseil Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+set -euox pipefail
+
+if [[ -z ${ABSEIL_ROOT:-} ]]; then
+  ABSEIL_ROOT="$(realpath $(dirname ${0})/..)"
+fi
+
+source "${ABSEIL_ROOT}/ci/cmake_common.sh"
+
+# Avoid depending on GitHub by looking for a cached copy of GoogleTest.
+if [[ -r "${KOKORO_GFILE_DIR:-}/distdir/googletest-${ABSL_GOOGLETEST_VERSION}.tar.gz" ]]; then
+  ABSL_GOOGLETEST_DOWNLOAD_URL="file:///distdir/googletest-${ABSL_GOOGLETEST_VERSION}.tar.gz"
+  DOCKER_EXTRA_ARGS="--mount type=bind,source=${KOKORO_GFILE_DIR}/distdir,target=/distdir,readonly ${DOCKER_EXTRA_ARGS:-}"
+fi
+
+if [[ -z ${ABSL_CMAKE_CXX_STANDARDS:-} ]]; then
+  ABSL_CMAKE_CXX_STANDARDS="17"
+fi
+
+if [[ -z ${ABSL_CMAKE_BUILD_TYPES:-} ]]; then
+  ABSL_CMAKE_BUILD_TYPES="Debug Release"
+fi
+
+if [[ -z ${ABSL_CMAKE_BUILD_SHARED:-} ]]; then
+  ABSL_CMAKE_BUILD_SHARED="OFF ON"
+fi
+
+if [[ -z ${ABSL_CMAKE_BUILD_MONOLITHIC_SHARED_LIBS:-} ]]; then
+  ABSL_CMAKE_BUILD_MONOLITHIC_SHARED_LIBS="OFF ON"
+fi
+
+source "${ABSEIL_ROOT}/ci/linux_docker_containers.sh"
+readonly DOCKER_CONTAINER=${LINUX_ALPINE_CONTAINER}
+
+for std in ${ABSL_CMAKE_CXX_STANDARDS}; do
+  for compilation_mode in ${ABSL_CMAKE_BUILD_TYPES}; do
+    for build_shared in ${ABSL_CMAKE_BUILD_SHARED}; do
+      if [[ $build_shared == "OFF" ]]; then
+        monolithic_shared_options="OFF"
+      else
+        monolithic_shared_options="$ABSL_CMAKE_BUILD_MONOLITHIC_SHARED_LIBS"
+      fi
+
+      for monolithic_shared in $monolithic_shared_options; do
+        time docker run \
+          --mount type=bind,source="${ABSEIL_ROOT}",target=/abseil-cpp,readonly \
+          --tmpfs=/buildfs:exec \
+          --workdir=/buildfs \
+          --cap-add=SYS_PTRACE \
+          --rm \
+          -e CFLAGS="-Werror" \
+          -e CXXFLAGS="-Werror" \
+          ${DOCKER_EXTRA_ARGS:-} \
+          "${DOCKER_CONTAINER}" \
+          /bin/sh -c "
+            cmake /abseil-cpp \
+              -DABSL_GOOGLETEST_DOWNLOAD_URL=${ABSL_GOOGLETEST_DOWNLOAD_URL} \
+              -DBUILD_SHARED_LIBS=${build_shared} \
+              -DABSL_BUILD_TESTING=ON \
+              -DCMAKE_BUILD_TYPE=${compilation_mode} \
+              -DCMAKE_CXX_STANDARD=${std} \
+              -DABSL_BUILD_MONOLITHIC_SHARED_LIBS=${monolithic_shared} \
+              -DCMAKE_MODULE_LINKER_FLAGS=\"-Wl,--no-undefined\" && \
+            make -j$(nproc) && \
+            TZDIR=/abseil-cpp/absl/time/internal/cctz/testdata/zoneinfo \
+            ctest -j$(nproc) --output-on-failure"
+        done
+    done
+  done
+done
diff --git a/ci/macos_xcode_bazel.sh b/ci/macos_xcode_bazel.sh
index 3e13b1538c7..54d06eb6654 100755
--- a/ci/macos_xcode_bazel.sh
+++ b/ci/macos_xcode_bazel.sh
@@ -19,19 +19,28 @@
 
 set -euox pipefail
 
-if [ -z ${ABSEIL_ROOT:-} ]; then
+# Use Xcode 26.2
+sudo xcode-select -s /Applications/Xcode_26.2.app/Contents/Developer
+
+if [[ -z ${ABSEIL_ROOT:-} ]]; then
   ABSEIL_ROOT="$(realpath $(dirname ${0})/..)"
 fi
 
 # If we are running on Kokoro, check for a versioned Bazel binary.
-KOKORO_GFILE_BAZEL_BIN="bazel-0.28.1-darwin-x86_64"
-if [ ${KOKORO_GFILE_DIR:-} ] && [ -f ${KOKORO_GFILE_DIR}/${KOKORO_GFILE_BAZEL_BIN} ]; then
+KOKORO_GFILE_BAZEL_BIN="bazel-9.1.0-darwin-x86_64"
+if [[ ${KOKORO_GFILE_DIR:-} ]] && [[ -f ${KOKORO_GFILE_DIR}/${KOKORO_GFILE_BAZEL_BIN} ]]; then
   BAZEL_BIN="${KOKORO_GFILE_DIR}/${KOKORO_GFILE_BAZEL_BIN}"
   chmod +x ${BAZEL_BIN}
 else
   BAZEL_BIN="bazel"
 fi
 
+# Use Bazel Vendor mode to reduce reliance on external dependencies.
+if [[ ${KOKORO_GFILE_DIR:-} ]] && [[ -f "${KOKORO_GFILE_DIR}/distdir/abseil-cpp_vendor.tar.gz" ]]; then
+  tar -xf "${KOKORO_GFILE_DIR}/distdir/abseil-cpp_vendor.tar.gz" -C "${HOME}/"
+  BAZEL_EXTRA_ARGS="--vendor_dir=${HOME}/abseil-cpp_vendor ${BAZEL_EXTRA_ARGS:-}"
+fi
+
 # Print the compiler and Bazel versions.
 echo "---------------"
 gcc -v
@@ -41,10 +50,21 @@ echo "---------------"
 
 cd ${ABSEIL_ROOT}
 
+if [[ -n "${ALTERNATE_OPTIONS:-}" ]]; then
+  cp ${ALTERNATE_OPTIONS:-} absl/base/options.h || exit 1
+fi
+
 ${BAZEL_BIN} test ... \
-  --copt=-Werror \
+  --copt="-DGTEST_REMOVE_LEGACY_TEST_CASEAPI_=1" \
+  --copt="-Werror" \
+  --define="absl=1" \
+  --cxxopt="-std=c++17" \
+  --enable_bzlmod=true \
+  --features=external_include_paths \
   --keep_going \
+  --per_file_copt=external/.*@-w \
   --show_timestamps \
   --test_env="TZDIR=${ABSEIL_ROOT}/absl/time/internal/cctz/testdata/zoneinfo" \
   --test_output=errors \
-  --test_tag_filters=-benchmark
+  --test_tag_filters=-benchmark \
+  ${BAZEL_EXTRA_ARGS:-}
diff --git a/ci/macos_xcode_cmake.sh b/ci/macos_xcode_cmake.sh
index 75b0f2d1909..7a6ccd59df0 100755
--- a/ci/macos_xcode_cmake.sh
+++ b/ci/macos_xcode_cmake.sh
@@ -14,30 +14,65 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-# This script is invoked on Kokoro to test Abseil on macOS.
-# It is not hermetic and may break when Kokoro is updated.
-
 set -euox pipefail
 
-if [ -z ${ABSEIL_ROOT:-} ]; then
+# Use Xcode 26.2
+sudo xcode-select -s /Applications/Xcode_26.2.app/Contents/Developer
+
+brew install cmake
+
+export CMAKE_BUILD_PARALLEL_LEVEL=$(sysctl -n hw.ncpu)
+export CTEST_PARALLEL_LEVEL=$(sysctl -n hw.ncpu)
+
+if [[ -z ${ABSEIL_ROOT:-} ]]; then
   ABSEIL_ROOT="$(dirname ${0})/.."
 fi
 ABSEIL_ROOT=$(realpath ${ABSEIL_ROOT})
 
-if [ -z ${ABSL_CMAKE_BUILD_TYPES:-} ]; then
+source "${ABSEIL_ROOT}/ci/cmake_common.sh"
+
+# Avoid depending on GitHub by looking for a cached copy of GoogleTest.
+if [[ -r "${KOKORO_GFILE_DIR:-}/distdir/googletest-${ABSL_GOOGLETEST_VERSION}.tar.gz" ]]; then
+  ABSL_GOOGLETEST_DOWNLOAD_URL="${KOKORO_GFILE_DIR}/distdir/googletest-${ABSL_GOOGLETEST_VERSION}.tar.gz"
+fi
+
+if [[ -z ${ABSL_CMAKE_BUILD_TYPES:-} ]]; then
   ABSL_CMAKE_BUILD_TYPES="Debug"
 fi
 
+if [[ -z ${ABSL_CMAKE_BUILD_SHARED:-} ]]; then
+  ABSL_CMAKE_BUILD_SHARED="OFF ON"
+fi
+
+if [[ -z ${ABSL_CMAKE_BUILD_MONOLITHIC_SHARED_LIBS:-} ]]; then
+  ABSL_CMAKE_BUILD_MONOLITHIC_SHARED_LIBS="OFF ON"
+fi
+
 for compilation_mode in ${ABSL_CMAKE_BUILD_TYPES}; do
-  BUILD_DIR=$(mktemp -d ${compilation_mode}.XXXXXXXX)
-  cd ${BUILD_DIR}
-
-  # TODO(absl-team): Enable -Werror once all warnings are fixed.
-  time cmake ${ABSEIL_ROOT} \
-    -GXcode \
-    -DCMAKE_BUILD_TYPE=${compilation_mode} \
-    -DABSL_USE_GOOGLETEST_HEAD=ON \
-    -DABSL_RUN_TESTS=ON
-  time cmake --build .
-  time ctest -C ${compilation_mode} --output-on-failure
+  for build_shared in ${ABSL_CMAKE_BUILD_SHARED}; do
+    if [[ $build_shared == "OFF" ]]; then
+      monolithic_shared_options="OFF"
+    else
+      monolithic_shared_options="$ABSL_CMAKE_BUILD_MONOLITHIC_SHARED_LIBS"
+    fi
+
+    for monolithic_shared in $monolithic_shared_options; do
+      BUILD_DIR=$(mktemp -d ${compilation_mode}.XXXXXXXX)
+      cd ${BUILD_DIR}
+
+      # TODO(absl-team): Enable -Werror once all warnings are fixed.
+      time cmake ${ABSEIL_ROOT} \
+        -GXcode \
+        -DBUILD_SHARED_LIBS=${build_shared} \
+        -DABSL_BUILD_TESTING=ON \
+        -DCMAKE_BUILD_TYPE=${compilation_mode} \
+        -DCMAKE_CXX_STANDARD=17 \
+        -DCMAKE_MODULE_LINKER_FLAGS="-Wl,--no-undefined" \
+        -DABSL_BUILD_MONOLITHIC_SHARED_LIBS=${monolithic_shared} \
+        -DABSL_GOOGLETEST_DOWNLOAD_URL="${ABSL_GOOGLETEST_DOWNLOAD_URL}"
+      time cmake --build .
+      time TZDIR=${ABSEIL_ROOT}/absl/time/internal/cctz/testdata/zoneinfo \
+        ctest -C ${compilation_mode} --output-on-failure
+    done
+  done
 done
diff --git a/ci/windows_clangcl_bazel.bat b/ci/windows_clangcl_bazel.bat
new file mode 100755
index 00000000000..42be0f71bb0
--- /dev/null
+++ b/ci/windows_clangcl_bazel.bat
@@ -0,0 +1,70 @@
+:: Copyright 2023 The Abseil Authors
+::
+:: Licensed under the Apache License, Version 2.0 (the "License");
+:: you may not use this file except in compliance with the License.
+:: You may obtain a copy of the License at
+::
+::     https://www.apache.org/licenses/LICENSE-2.0
+::
+:: Unless required by applicable law or agreed to in writing, software
+:: distributed under the License is distributed on an "AS IS" BASIS,
+:: WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+:: See the License for the specific language governing permissions and
+:: limitations under the License.
+
+SETLOCAL ENABLEDELAYEDEXPANSION
+
+:: Set LLVM directory.
+SET BAZEL_LLVM=C:\Program Files\LLVM
+
+:: Change directory to the root of the project.
+CD %~dp0\..
+if %errorlevel% neq 0 EXIT /B 1
+
+:: Use Bazel Vendor mode to reduce reliance on external dependencies.
+IF EXIST "%KOKORO_GFILE_DIR%\distdir\abseil-cpp_vendor.tar.gz" (
+  tar --force-local -xf "%KOKORO_GFILE_DIR%\distdir\abseil-cpp_vendor.tar.gz" -C c:\
+  SET VENDOR_FLAG=--vendor_dir=c:\abseil-cpp_vendor
+) ELSE (
+  SET VENDOR_FLAG=
+)
+
+:: Set the standard version, [c++17|c++20|c++latest]
+:: https://msdn.microsoft.com/en-us/library/mt490614.aspx
+:: The default is c++17 if not set on command line.
+IF "%STD%"=="" SET STD=c++17
+
+:: Set the compilation_mode (fastbuild|opt|dbg)
+:: https://docs.bazel.build/versions/master/user-manual.html#flag--compilation_mode
+:: The default is fastbuild
+IF "%COMPILATION_MODE%"=="" SET COMPILATION_MODE=fastbuild
+
+:: Copy the alternate option file, if specified.
+IF NOT "%ALTERNATE_OPTIONS%"=="" copy %ALTERNATE_OPTIONS% absl\base\options.h
+
+:: To upgrade Bazel, first download a new binary from
+:: https://github.com/bazelbuild/bazel/releases and copy it to
+:: /google/data/rw/teams/absl/kokoro/windows.
+::
+:: TODO(absl-team): Remove -Wno-microsoft-cast
+%KOKORO_GFILE_DIR%\bazel-9.1.0-windows-x86_64.exe ^
+  test ... ^
+  --compilation_mode=%COMPILATION_MODE% ^
+  --compiler=clang-cl ^
+  --copt=/WX ^
+  --copt=-Wno-microsoft-cast ^
+  --cxxopt=/std:%STD% ^
+  --define=absl=1 ^
+  --enable_bzlmod=true ^
+  --extra_execution_platforms=//:x64_windows-clang-cl ^
+  --extra_toolchains=@local_config_cc//:cc-toolchain-x64_windows-clang-cl ^
+  --keep_going ^
+  --per_file_copt=external/.*@/w ^
+  --test_env="GTEST_INSTALL_FAILURE_SIGNAL_HANDLER=1" ^
+  --test_env=TZDIR="%CD%\absl\time\internal\cctz\testdata\zoneinfo" ^
+  --test_output=errors ^
+  --test_tag_filters=-benchmark ^
+  %VENDOR_FLAG%
+
+if %errorlevel% neq 0 EXIT /B 1
+EXIT /B 0
diff --git a/ci/windows_msvc_bazel.bat b/ci/windows_msvc_bazel.bat
new file mode 100755
index 00000000000..a4bd3d79959
--- /dev/null
+++ b/ci/windows_msvc_bazel.bat
@@ -0,0 +1,61 @@
+:: Copyright 2023 The Abseil Authors
+::
+:: Licensed under the Apache License, Version 2.0 (the "License");
+:: you may not use this file except in compliance with the License.
+:: You may obtain a copy of the License at
+::
+::     https://www.apache.org/licenses/LICENSE-2.0
+::
+:: Unless required by applicable law or agreed to in writing, software
+:: distributed under the License is distributed on an "AS IS" BASIS,
+:: WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+:: See the License for the specific language governing permissions and
+:: limitations under the License.
+
+SETLOCAL ENABLEDELAYEDEXPANSION
+
+:: Change directory to the root of the project.
+CD %~dp0\..
+if %errorlevel% neq 0 EXIT /B 1
+
+:: Use Bazel Vendor mode to reduce reliance on external dependencies.
+IF EXIST "%KOKORO_GFILE_DIR%\distdir\abseil-cpp_vendor.tar.gz" (
+  tar --force-local -xf "%KOKORO_GFILE_DIR%\distdir\abseil-cpp_vendor.tar.gz" -C c:\
+  SET VENDOR_FLAG=--vendor_dir=c:\abseil-cpp_vendor
+) ELSE (
+  SET VENDOR_FLAG=
+)
+
+:: Set the standard version, [c++17|c++latest]
+:: https://msdn.microsoft.com/en-us/library/mt490614.aspx
+:: The default is c++17 if not set on command line.
+IF "%STD%"=="" SET STD=c++17
+
+:: Set the compilation_mode (fastbuild|opt|dbg)
+:: https://docs.bazel.build/versions/master/user-manual.html#flag--compilation_mode
+:: The default is fastbuild
+IF "%COMPILATION_MODE%"=="" SET COMPILATION_MODE=fastbuild
+
+:: Copy the alternate option file, if specified.
+IF NOT "%ALTERNATE_OPTIONS%"=="" copy %ALTERNATE_OPTIONS% absl\base\options.h
+
+:: To upgrade Bazel, first download a new binary from
+:: https://github.com/bazelbuild/bazel/releases and copy it to
+:: /google/data/rw/teams/absl/kokoro/windows.
+"%KOKORO_GFILE_DIR%\bazel-9.1.0-windows-x86_64.exe" ^
+  test ... ^
+  --compilation_mode=%COMPILATION_MODE% ^
+  --copt=/WX ^
+  --cxxopt=/std:%STD% ^
+  --define=absl=1 ^
+  --enable_bzlmod=true ^
+  --keep_going ^
+  --per_file_copt=external/.*@/w ^
+  --test_env="GTEST_INSTALL_FAILURE_SIGNAL_HANDLER=1" ^
+  --test_env=TZDIR="%CD%\absl\time\internal\cctz\testdata\zoneinfo" ^
+  --test_output=errors ^
+  --test_tag_filters=-benchmark ^
+  %VENDOR_FLAG%
+
+if %errorlevel% neq 0 EXIT /B 1
+EXIT /B 0
diff --git a/ci/windows_msvc_cmake.bat b/ci/windows_msvc_cmake.bat
new file mode 100755
index 00000000000..6fbc324035d
--- /dev/null
+++ b/ci/windows_msvc_cmake.bat
@@ -0,0 +1,69 @@
+:: Copyright 2023 The Abseil Authors
+::
+:: Licensed under the Apache License, Version 2.0 (the "License");
+:: you may not use this file except in compliance with the License.
+:: You may obtain a copy of the License at
+::
+::     https://www.apache.org/licenses/LICENSE-2.0
+::
+:: Unless required by applicable law or agreed to in writing, software
+:: distributed under the License is distributed on an "AS IS" BASIS,
+:: WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+:: See the License for the specific language governing permissions and
+:: limitations under the License.
+
+SETLOCAL ENABLEDELAYEDEXPANSION
+
+:: The version of GoogleTest to be used in the CMake tests in this directory.
+:: Keep this in sync with the version in the MODULE.bazel file.
+SET ABSL_GOOGLETEST_VERSION=1.18.0
+
+IF EXIST %KOKORO_GFILE_DIR%\distdir\googletest-%ABSL_GOOGLETEST_VERSION%.tar.gz (
+  SET ABSL_GOOGLETEST_DOWNLOAD_URL=file://%KOKORO_GFILE_DIR%\distdir\googletest-%ABSL_GOOGLETEST_VERSION%.tar.gz
+) ELSE (
+  SET ABSL_GOOGLETEST_DOWNLOAD_URL=https://github.com/google/googletest/releases/download/v%ABSL_GOOGLETEST_VERSION%/googletest-%ABSL_GOOGLETEST_VERSION%.tar.gz
+)
+
+:: Replace '\' with '/' in Windows paths for CMake.
+:: Note that this cannot go inside the IF block above, because BAT files are weird.
+SET ABSL_GOOGLETEST_DOWNLOAD_URL=%ABSL_GOOGLETEST_DOWNLOAD_URL:\=/%
+
+IF EXIST "C:\Program Files\CMake\bin\" (
+  SET CMAKE_BIN="C:\Program Files\CMake\bin\cmake.exe"
+  SET CTEST_BIN="C:\Program Files\CMake\bin\ctest.exe"
+) ELSE (
+  SET CMAKE_BIN="cmake.exe"
+  SET CTEST_BIN="ctest.exe"
+)
+
+SET CTEST_OUTPUT_ON_FAILURE=1
+SET CMAKE_BUILD_PARALLEL_LEVEL=16
+SET CTEST_PARALLEL_LEVEL=16
+
+:: Change directory to the root of the project.
+CD %~dp0\..
+if %errorlevel% neq 0 EXIT /B 1
+
+SET TZDIR=%CD%\absl\time\internal\cctz\testdata\zoneinfo
+
+MKDIR "build"
+CD "build"
+
+SET CXXFLAGS="/WX"
+
+%CMAKE_BIN% ^
+  -DABSL_BUILD_TESTING=ON ^
+  -DABSL_GOOGLETEST_DOWNLOAD_URL=%ABSL_GOOGLETEST_DOWNLOAD_URL% ^
+  -DBUILD_SHARED_LIBS=%ABSL_CMAKE_BUILD_SHARED% ^
+  -DCMAKE_CXX_STANDARD=%ABSL_CMAKE_CXX_STANDARD% ^
+  -G "%ABSL_CMAKE_GENERATOR%" ^
+  ..
+IF %errorlevel% neq 0 EXIT /B 1
+
+%CMAKE_BIN% --build . --target ALL_BUILD --config %ABSL_CMAKE_BUILD_TYPE%
+IF %errorlevel% neq 0 EXIT /B 1
+
+%CTEST_BIN% -C %ABSL_CMAKE_BUILD_TYPE% -E "absl_lifetime_test|absl_symbolize_test"
+IF %errorlevel% neq 0 EXIT /B 1
+
+EXIT /B 0
diff --git a/conanfile.py b/conanfile.py
old mode 100644
new mode 100755
index cd124aaee4b..4bbc62eedd6
--- a/conanfile.py
+++ b/conanfile.py
@@ -30,7 +30,7 @@ def configure(self):
             raise ConanInvalidConfiguration("Abseil does not support MSVC < 14")
 
     def build(self):
-        tools.replace_in_file("CMakeLists.txt", "project(absl)", "project(absl)\ninclude(conanbuildinfo.cmake)\nconan_basic_setup()")
+        tools.replace_in_file("CMakeLists.txt", "project(absl LANGUAGES CXX)", "project(absl LANGUAGES CXX)\ninclude(conanbuildinfo.cmake)\nconan_basic_setup()")
         cmake = CMake(self)
         cmake.definitions["BUILD_TESTING"] = False
         cmake.configure()
diff --git a/create_lts.py b/create_lts.py
new file mode 100755
index 00000000000..c20577a743a
--- /dev/null
+++ b/create_lts.py
@@ -0,0 +1,139 @@
+#!/usr/bin/env python3
+#
+# Copyright 2021 The Abseil Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+"""A script to do source transformations to create a new LTS release.
+
+   Usage: ./create_lts.py YYYYMMDD
+"""
+
+import sys
+
+
+def ReplaceStringsInFile(filename, replacement_dict):
+  """Performs textual replacements in a file.
+
+  Rewrites filename with the keys in replacement_dict replaced with
+  their values. This function assumes the file can fit in memory.
+
+  Args:
+    filename: the filename to perform the replacement on
+    replacement_dict: a dictionary of key strings to be replaced with their
+      values
+
+  Raises:
+    Exception: A failure occurred
+  """
+  f = open(filename, 'r')
+  content = f.read()
+  f.close()
+
+  for key, value in replacement_dict.items():
+    original = content
+    content = content.replace(key, value)
+    if content == original:
+      raise Exception('Failed to find {} in {}'.format(key, filename))
+
+  f = open(filename, 'w')
+  f.write(content)
+  f.close()
+
+
+def StripContentBetweenTags(filename, strip_begin_tag, strip_end_tag):
+  """Strip contents from a file.
+
+  Rewrites filename with by removing all content between
+  strip_begin_tag and strip_end_tag, including the tags themselves.
+
+  Args:
+    filename: the filename to perform the replacement on
+    strip_begin_tag: the start of the content to be removed
+    strip_end_tag: the end of the content to be removed
+
+  Raises:
+    Exception: A failure occurred
+  """
+  f = open(filename, 'r')
+  content = f.read()
+  f.close()
+
+  while True:
+    begin = content.find(strip_begin_tag)
+    if begin == -1:
+      break
+    end = content.find(strip_end_tag, begin + len(strip_begin_tag))
+    if end == -1:
+      raise Exception('{}: imbalanced strip begin ({}) and '
+                      'end ({}) tags'.format(filename, strip_begin_tag,
+                                             strip_end_tag))
+    content = content.replace(content[begin:end + len(strip_end_tag)], '')
+
+  f = open(filename, 'w')
+  f.write(content)
+  f.close()
+
+
+def main(argv):
+  if len(argv) != 2:
+    print('Usage: {} YYYYMMDD'.format(sys.argv[0], file=sys.stderr))
+    sys.exit(1)
+
+  datestamp = sys.argv[1]
+  if len(datestamp) != 8 or not datestamp.isdigit():
+    raise Exception(
+        'datestamp={} is not in the YYYYMMDD format'.format(datestamp))
+
+  # Replacement directives go here.
+  ReplaceStringsInFile(
+      'MODULE.bazel', {
+          'version = "head"':
+              'version = "{}.0"'.format(datestamp)
+      })
+  ReplaceStringsInFile(
+      'absl/base/config.h', {
+          '#undef ABSL_LTS_RELEASE_VERSION':
+              '#define ABSL_LTS_RELEASE_VERSION {}'.format(datestamp),
+          '#undef ABSL_LTS_RELEASE_PATCH_LEVEL':
+              '#define ABSL_LTS_RELEASE_PATCH_LEVEL 0'
+      })
+  ReplaceStringsInFile(
+      'absl/base/options.h', {
+          '#define ABSL_OPTION_USE_INLINE_NAMESPACE 0':
+              '#define ABSL_OPTION_USE_INLINE_NAMESPACE 1',
+          '#define ABSL_OPTION_INLINE_NAMESPACE_NAME head':
+              '#define ABSL_OPTION_INLINE_NAMESPACE_NAME lts_{}'.format(
+                  datestamp)
+      })
+  ReplaceStringsInFile(
+      'CMakeLists.txt',
+      {
+          'project(absl LANGUAGES CXX)': (
+              'project(absl LANGUAGES CXX VERSION {})'.format(datestamp)
+          ),
+          # Set the SOVERSION to YYMM.0.0 - The first 0 means we only have ABI
+          # compatible changes, and the second 0 means we can increment it to
+          # mark changes as ABI-compatible, for patch releases.  Note that we
+          # only use the last two digits of the year and the month because the
+          # MacOS linker requires the first part of the SOVERSION to fit into
+          # 16 bits.
+          # https://www.sicpers.info/2013/03/how-to-version-a-mach-o-library/
+          'ABSL_SOVERSION 0': 'ABSL_SOVERSION "{}.0.0"'.format(datestamp[2:6]),
+      },
+  )
+  StripContentBetweenTags('CMakeLists.txt', '# absl:lts-remove-begin',
+                          '# absl:lts-remove-end')
+
+
+if __name__ == '__main__':
+  main(sys.argv)